Lignites of North America

LIGNITES OF NORTH AMERICA COAL SCIENCE AND TECHNOLOGY Series Editor: Larry L. Anderson Department of Fuels Engineeri...

Author: Harold H. Schobert

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LIGNITES OF NORTH AMERICA

COAL SCIENCE AND TECHNOLOGY Series Editor:

Larry L. Anderson Department of Fuels Engineering, University of Utah, Salt Lake City, UT 84112, U.S.A. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol. Vol.

l: Geochemistry of Coal (Bou~ka) 2: Fundamentals of Coal Benefication and Utilization (Tsai) 3: Coal: Typology, Chemistry, Physics and Constitution (Van Krevelen) 4: Coal Pyrolysis (Gavalas) 5: Free Radicals in Coals and Synthetics Fuels (Petrakis and Grandy) 6: Coal Combustion Chemistry-Correlation Aspects (Badin) 7: The Chemistry of Coal (Berkowitz) 8: Natural Gas Substitutes from Coal and Oil (Qader) 9: Processing and Utilization of High-Sulfur Coals (Attia, Editor) 10: Coal Science and Chemistry (Volborth, Editor) 11:1987 International Conference on Coal Science (Moulijn, Nater and Chermin, Editors) Vol. 12: Spectroscopic Analysis of Coal Liquids (Kershaw, Editor) Vol. 13: Energy Recovery from Lignin, Peat and Lower Rank Coals (Trantolo and Wise, Editors) Vol. 14: Chemistry of Coal Weathering (Nelson, Editor) Vol. 15: Advanced Methodologies in Coal Characterization (Charcosset, Editorassisted by Nickel-Pepin-Donat) Vol. 16: Processing and Utilization of High-Sulfur Coals III (Markuszewski and Wheelock, Editors) Vol. 17: Chlorine in Coal (Stringer and Banerjee, Editors) Vol. 18: Processing and Utilization of High-Sulfur Coals IV (Dugan, Quigley and Attia, Editors) Vol. 19: Coal Quality and Combustion Performance: An International Perspective (Unsworth, Barratt and Roberts) Vol. 20: Fundamentals of Coal Combustion for Clean and Efficient Use (Smoot, Editor) Vol. 21: Processing and Utilization of High-Sulfur Coals V (Parekh and Groppo, Editors) Vol. 22: Atmospheric Fluidized Bed Coal Combustion. Research, Development and Application (Valk, Editor) Vol. 23: Lignites of North America (Schobert)

COALSCIENCEAND TECHNOLOGY23

L/GNITES OF NORTH AMERICA HAROLD H. SCHOBERT Fuel Science Program, The Pennsylvania State University, University Park, PA 16802-2303, U.S.A.

1995 ELSEVIER Amsterdam - Lausanne - New York-

Oxford-

Shannon - Tokyo

ELSEVIER SCIENCE B.V. Sara B urgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands

ISBN: 0-444-89823-9

9 1995 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the USA. This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copyright questions, including photocopying outside of the USA, should be referred to the publisher No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands

This book is dedicated to the memory of two lignite researchers who made numerous contributions to lignite science and technology, who freely gave of their time and knowledge to answer questions and to help me, and, most importantly, who were my friends--

Wayne Kube Merle Fegley

This Page Intentionally Left Blank

vii

PREFACE

This book is intended to provide a comprehensive survey of the origin, the fundamental properties, and the technology of utilization of the lignites of North America. I have presumed that the most likely users of this book will be professional scientists and engineers working in some field of coal research or coal technology. Thus I have made no attempt to explain terms that I consider likely to be part of the working vocabulary of such persons. I hope that most persons just coming into the field, with little or no prior exposure to the terminology and conventions of the field, will nevertheless find most of the material reasonably accessible. Coals of course display a continuum of properties, often with no sharp, step change between ranks. In writing about lignites of North America, it would have been all too easy to allow the manuscript to balloon to even larger size by including throughout the text comparisons of the properties and behavior of lignites with those for coals of other ranks. In an effort to keep an enormous amount of information under some sort of control, I have, to a great extent, restricted the discussion strictly to lignites, although I have included some occasional comparisons with other coals, especially brown coals and subbituminous coals. Although there are some interesting distinctions among coals based on their geographic distribution, in many situations coals of comparable rank and composition show similar behavior regardless of their geographic source. A second constraint, therefore, in writing about lignites of North America has been the exclusion of most (but not all) information about lignites from other parts of the world. As I have done in comparing lignites with other ranks of coals, I have occasionally made reference to results on lignites from elsewhere in the world, mainly to point out how North American lignites often show similarities to other lignites. Much excellent work on lignites is being done around the world, and the North American coal scientist or engineer could profit from following it. As this book began to take shape, it seemed to me that only the most ardent of lignitophiles would ever be likely to read it straight through. Rather, I suspect that many persons will dip into the selected chapters or sections most relevant to their immediate interests. For that reason, I have tried to make extensive references to the original work. This book has 1,830 citations to the literature. Unfortunately, much of that original work has often appeared previously only in "obscure government reports" (to borrow a charming phrase coined by a reviewer of one of my papers). I have tried to include sufficient information on experimental conditions and scale of work (that is, whether bench-, pilot-, or commercial-scale) so that the reader can judge whether or not the material is pertinent to his or her interest, and, if appropriate, follow it further into the original literature. The very extensive index is a further effort to make the contents of the book easily

viii accessible to the reader. We who work on coal suffer from having no systematic nomenclature for naming coals. Some authors name the lignite on the basis of seam, others on the basis of formation, others on the basis of its county of origin, and still others on the basis of the mine. Furthermore, some mines have been closed since the lignite was studied and reported on, and in some states or regions the names of geological formations and units have been changed over the years. In some papers, the authors refer simply to, e.g., "a North Dakota lignite," or to the even more awful "a lignite." As a result, the coal literature is a welter of confusing, occasionally colorful, and sometimes bizarre, names. One option for dealing with this mess would have been to impose my own system of order, to decide, for example, that all lignites would be named on the basis of the mine, or, say, on the county from which they come. It seemed to me, however, that the potential for the inadvertent introduction of error was enormous. I have instead reported the name of the lignite as it was used in the original literature. To be sure, this approach perpetuates a mess, but obviates the potential for generating an even more colossal mess by giving some lignites the names of things they are not. I have reported numerical data almost entirely in SI units, except for temperatures, which are generally given as degrees Celsius rather than kelvins. For readers who have been steeped in English units and find SI units strange, I would say simply that it's time to learn. The only situations in which English units have been retained are equations that have constants or coefficients that were derived from the fitting of original data that also had English units. In such cases I felt as I did with the issue of nomenclature--the prospects for inadvertent introduction of error by an attempted "correction" outweighed the discomfort of retaining the obsolete units. In those instances I have tried clearly to state in the accompanying text the appropriate units to use. The writing of this book has been a long and arduous undertaking. Throughout, I have been helped greatly by many persons who have given me access to materials in their personal collections, who have answered questions, and who have given advice on miscellaneous computer problems. I particularly thank my present and former colleagues at Penn State, and the members of what used to be the Coal Science Division ( a group for whom the term used above, "occasionally colorful, and sometimes bizarre," is most apt) at what used to be the University of North Dakota Energy Research Center. I am pleased to acknowledge a pleasant and, to me, quite helpful correspondence with Olav Schmitz of Elsevier, and to several generations of patient Elsevier editors, several of whom, I think, gave up in despair. This book has been prepared entirely on my own time with my own resources. Any statements of opinion, or any editorializing, are mine alone, and do not necessarily reflect the positions either of The Pennsylvania State University or of Elsevier. Of course I take full responsibility for any errors in the book. Harold H. Schobert

University Park, Pennsylvania

iX

CONTENTS

Preface .........................................................................................

C h a p t e r 1. T h e p r i n c i p a l lignite d e p o s i t s o f N o r t h A m e r i c a ............................. 1.1. 1.2.

1.3.

Introduction The

Gulf

....................................................................

region

1.2.1.

Texas

1.2.2. 1.2.3.

lignites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 4

.................................................................

4

Alabama

..............................................................

12

Arkansas

.............................................................

15

1.2.4.

Louisiana

.............................................................

16

1.2.5.

Mississippi

...........................................................

18

1.2.6.

Tennessee

The

Fort

Union

............................................................

region

1.3.1.

North

1.3.2.

Montana

Dakota

.........................................................

19 19

........................................................

19

..............................................................

28

1.3.3.

Saskatchewan

........................................................

34

1.3.4.

South

........................................................

35

Dakota

1.4. O t h e r l i g n i t e s o f the U n i t e d S t a t e s ............................................ 1.4.1.

California

.............................................................

1.4.2.

The

Rocky

Mountains

1.4.3.

The

Plains

states

The

Northwest

1.4.4. 1.5.

vii

Other

lignites

Ontario

1.5.2.

British

1.5.3.

The

1.5.4.

Nova

References

..............................................

41 42

.......................................................

42

.......................................................

43

................................................................

43

Columbia

Canadian Scotia

....................................................

Arctic

................................................

44 45

C h a p t e r 2. T h e d e p o s i t i o n a n d f o r m a t i o n o f lignite ........................................ Introduction

2.2.

Paleoflora

44

..........................................................

.............................................................................

2.1.

39

.....................................................

of Canada

1.5.1.

39

46

51

......................................................................

51

........................................................................

51

2.2.1. N o r t h e r n G r e a t P l a i n s p a l e o f l o r a ..................................

51

2.2.2.

52

Gulf Coast

paleoflora

...............................................

2.3.

Environments

of deposition

...................................................

2.3.1.

Fluvial

environments

...............................................

55

2.3.2.

Deltaic

environments

................................................

58

.............................................

59

2.3.3.

Lagoonal

2.3.4.

Backswamp

environments

2.3.5.

Marsh

2.3.6.

Lacustrine

environments

environments

.........................................

59

................................................

61

environments

...........................................

2.4. T r a n s f o r m a t i o n o f p l a n t material to lignite ................................... 2.4.1.

Introduction

2.4.2.

Loss

..........................................................

of cellulose

....................................................

2.4.3. S t r u c t u r a l e v o l u t i o n o f l i g n i n r e s i d u e s ............................ 2.4.4. E v o l u t i o n o f the c a r b o n s k e l e t o n .................................

63 65 66 68

2.4.6.

71

Sulfur

................................................................. o f m e t a l i o n s ........................................

.............................................................................

C h a p t e r 3. T h e o r g a n i c s t r u c t u r e o f l i g n i t e s ................................................

3.3.

63

7O

References

3.2.

62

2.4.5. T r a n s f o r m a t i o n s o f o x y g e n f u n c t i o n a l g r o u p s ...................

2.4.7. Accumulation

3.1.

54

Petrography

of lignites

3.1.1.

Lithologic

3.1.2.

Lithotypes

3.1.3.

Macerals

The

carbon

.........................................................

72 74

79 79

..................................................

79

............................................................

81

..............................................................

85

layering

skeleton

............................................................

92

3.2.1.

Aromatic

carbon

.....................................................

92

3.2.2.

Aliphatic

carbon

.....................................................

101

.....................................................................

109

Heteroatoms 3.3.1.

Introduction

..........................................................

109

.............................................

109

..........................................................

113

3.3.2.

Carboxylic

3.3.3.

Humic

acid groups

3.3.4.

Methoxyl

....................................................

114

3.3.5.

Phenolic

groups

.....................................................

115

3.3.6.

Carbonyl

groups

....................................................

116

acids

groups

.........................................

116

..........................................................

118

3.3.7. Ethers other than methoxyl 3.3.8. 3.3.9.

Ester

groups

C e l l u l o s e a n d l i g n i n r e s i d u e s .......................................

3.3.10.

Nitrogen

groups

3.3.11.

Sulfur

3.3.12.

Chlorine

118

....................................................

121

.......................................................

122

.............................................................

123

3.4. T h e t h r e e - d i m e n s i o n a l s t r u c t u r e o f lignite ...................................

123

groups

xi 3.4.1. E v i d e n c e for a colloidal structure ..................................

123

3.4.2. C r o s s l i n k i n g in lignite structures ..................................

124

3.4.3. M o l e c u l e s trapped in the lignite structure .........................

126

3.4.4.

D e p o l y m e r i z a t i o n r e a c t i o n s .........................................

129

.............................................................................

130

References

C h a p t e r 4. F u n d a m e n t a l organic reaction chemistry of lignites ...........................

139

4.1. R e a c t i o n s w i t h c a r b o n m o n o x i d e ..............................................

139

4.1.1. Reactions of c a r b o n m o n o x i d e with lignite .......................

139

4.1.2. S u p p o r t i n g m o d e l c o m p o u n d studies .............................

142

4.1.3. C a r b o n m o n o x i d e - h y d r o g e n m i x t u r e s ..........................

144

4.1.4. C a r b o n m o n o x i d e - steam m i x t u r e s ...............................

145

4.2.

Reactions

with

hydrogen

.......................................................

4.2.1. H y d r o p y r o l y s i s and hydrogasification reactions .................

149

4.2.2. R e a c t i o n s u n d e r liquefaction conditions ...........................

158

4.3. R e a c t i o n s in o t h e r g a s e o u s a t m o s p h e r e s ...................................... 4.3.1.

4.4.

149

Oxidation

160

...................................................

160

4.3.2. R e a c t i o n s with w a t e r or steam ......................................

164

4.3.3. R e a c t i o n s with sulfur c o m p o u n d s ..................................

169

Pyrolysis

reactions

of lignites

4.4.1.

Heats

4.4.2.

Kinetics

.............................................................

of reaction

.....................................................

of pyrolysis

172 172

................................................

173

4.4.3. C o m p a r a t i v e effects of gaseous a t m o s p h e r e s ....................

177

4.4.4. Effects of lignite moisture content on pyrolysis .................

179

4.4.5. Effects of cations on lignite pyrolysis ............................

179

4.4.6. E f f e c t s o f p e t r o g r a p h i c c o m p o s i t i o n ..............................

184

4.4.7. Changes in physical structure a c c o m p a n y i n g pyrolysis ........

186

4.4.8. C h a n g e s in functional groups during pyrolysis .................

188

4.4.9. E v o l u t i o n of products during pyrolysis ..........................

194

References

.............................................................................

207

C h a p t e r 5. T h e i n o r g a n i c c o n s t i t u e n t s o f lignites ..........................................

218

5.1. I n c o r p o r a t i o n o f m a j o r e l e m e n t s ..............................................

218

5.1.1. A c c u m u l a t i o n of inorganic c o m p o n e n t s ..........................

218

5.1.2.

Chemical

..............................................

220

5.1.3.

The

5.2.

Minerals

in

fractionation

major elements lignites

5.2.1.

Introductory

5.2.2.

Sulfide

.................................................

230

.............................................................

239

overview

..............................................

239

.....................................................

242

minerals

xii

5.3.

minerals

.......................................................

244

5.2.3.

Oxide

5.2.4.

Carbonate

minerals

..................................................

245

5.2.5.

Phosphate

minerals

.................................................

246

5.2.6.

Sulfate

minerals

.....................................................

246

5.2.7.

Silicate

minerals

.....................................................

247

5.2.8.

Oxalate

minerals

.....................................................

250

..................................................................

250

Trace

elements

5.3.1.

Introduction

5.3.2.

Alkali

5.3.3.

Alkaline

..........................................................

250

.........................................................

251

metals

.......................................................

253

5.3.4. F i r s t - r o w t r a n s i t i o n m e t a l s .........................................

254

5.3.5.

257

5.3.6.

earths

H e a v y t r a n s i t i o n m e t a l s ............................................. ..........................................................

258

.............................................................

259

Lanthanides

5.3.7.

Actinides

5.3.8.

Group

lib elements

.................................................

263

5.3.9.

Group

III e l e m e n t s

..................................................

263

5.3.10.

Group

IV e l e m e n t s

................................................

263

V elements

.................................................

265

.........................................................

265

............................................................

265

5.4. V a r i a b i l i t y o f i n o r g a n i c c o m p o s i t i o n .........................................

266

5.3.11.

Group

5.3.12.

Chalcogens

5.3.13.

Halogens

5.4.1.

Introduction

..........................................................

266

5.4.2. Factors affecting the inorganic composition of lignites .........

266

5.4.3. Vertical v a r i a b i l i t y within s e a m s ...................................

271

5.4.4.

...................................................

275

5.4.5. State and regional v a r i a b i l i t y .......................................

278

References

In-mine

variability

.............................................................................

C h a p t e r 6. B e h a v i o r o f inorganic c o m p o n e n t s of lignites .................................

281

290

........................................................

290

6.2. R e a c t i o n s o f e x c h a n g e a b l e cations .............................................

293

6.1.

Low-temperature

ashing

6.2.1. R e a c t i o n s during ashing or c o m b u s t i o n ...........................

293

6.2.2. B e h a v i o r of c a l c i u m in lignite liquefaction ........................

297

6.3. M i n e r a l reactions at e l e v a t e d t e m p e r a t u r e s ....................................

298

6.3.1.

Anhydrite

..............................................................

298

6.3.2.

Bredigite

...............................................................

299

..................................................................

299

6.3.3.

Calcite

..................................................

299

6.3.5. G e h l e n i t e - a k e r m a n i t e series ........................................

301

6.3.4.

Feldspathic

minerals

xiii 6.3.6.

Hematite

................................................................

301

6.3.7.

Hercynite

...............................................................

302

6.3.8.

Kaolinite

.................................................................

302

6.3.9.

Magnetite

...............................................................

303

6.3.10.

Pyrite

..................................................................

303

6.3.11.

Quartz

.................................................................

304

silicates

6.3.12.

Sodium

.....................................................

305

6.3.13.

Sulfur

......................................................

305

6.3.14.

V o l a t i l i z a t i o n d u r i n g a s h i n g ........................................

306

6.4. T h e r m a l p r o p e r t i e s o f ashes and slags .........................................

307

retention

6.4.1.

Specific

heat

6.4.2.

Radiative

heat transfer

6.5.

Sintering

6.6.

Ash

........................................................... ................................................

307 307

...........................................................................

309

.........................................................................

316

fusion

6.7. S l a g v i s c o s i t y a n d s u r f a c e tension ..............................................

320

6.7.1. L a b o r a t o r y data on slag viscosity ...................................

320

6.7.2. C a l c u l a t i o n o f viscosity f r o m c o m p o s i t i o n .........................

327

6.7.3. A p p l i c a t i o n s o f viscosity data in lignite processing ...............

335

6.7.4.

335

References

Surface

tension

........................................................

..............................................................................

337

C h a p t e r 7. P h y s i c a l p r o p e r t i e s o f lignites ...................................................

343

7.1.

Bulk

mechanical

7.1.1. 7.1.2.

Friability

Grindability

7.1.5.

7.3.

7.4.

Density

.....................................................

shrinkage

7.1.3. 7.1.4.

7.2.

properties

Volumetric

................................................

..............................................................

343 343 345

..........................................................

351

.............................................................

354

F r e e sliding s h e a r s t r e n g t h .........................................

355

Hardness

...........................................................................

357

7.2.1. D e n s i t y m e a s u r e m e n t s by i m m e r s i o n in fluids ...................

357

7.2.2.

Bulk

357

7.2.3.

Mercury

7.2.4.

Helium

Thermal

density

..........................................................

densities

....................................................

357

.....................................................

358

..............................................................

358

densities

properties

7.3.1.

Specific

heat

7.3.2.

Thermal

conductivity

Surface

358

................................................

359

.......................................................

360

o f s u r f a c e a r e a ........................................

360

area and porosity

7.4.1. M e a s u r e m e n t 7.4.2.

..........................................................

Porosity and pore volume

...........................................

365

xiv 7.4.3.

H y d r a u l i c conductivity ...............................................

368

7.4.4. Effects of heating on surface area and porosity ...................

368

7.4.5. The fractal structure of lignite .......................................

372

References

..............................................................................

C h a p t e r 8. Moisture in lignite .................................................................. 8.1.

Introduction

8.2.

Equilibrium

....................................................................... moisture

............................................................

378

383 383 385

8.3. Measurement of moisture by elevated-temperature methods ................

387

8.4. Evidence for bimodal incorporation of water .................................

391

8.5. Adsorption and desorption of moisture ........................................

396

8.6. Heat of wetting in water .........................................................

405

References

4O6

..............................................................................

Chapter 9. Mining, transportation, and storage ............................................. 9.1.

Lignite mining

....................................................................

410 410

9.1.1. Historical development of lignite mining ..........................

410

9.1.2. Overview of surface mining .........................................

412

9.1.3. Preliminary steps in opening a mine ...............................

415

9.1.4.

O v e r b u r d e n removal

.................................................

416

9.1.5.

L i g n i t e removal

.......................................................

421

9.1.6. M i n e d - l a n d reclamation ..............................................

423

9.1.7. Specific examples of mining practices ..............................

424

9.2.

Transportation

...................................................................

429

9.3.

H a n d l i n g and storage ...........................................................

432

9.3.1.

Handling of lignite ..................................................

432

9.3.2.

Stockpiling

433

...........................................................

9.3.3. Low-temperature air oxidation .....................................

436

9.3.4. Autogenous heating and combustion ..............................

436

9.3.5. Storage of dried lignite ..............................................

AAA

9.3.6.

446

Dust explosions

......................................................

.............................................................................

446

C h a p t e r 10. Beneficiation of lignite .........................................................

451

References

10.1.

Drying

..........................................................................

451

......................................................

451

10.1.2. Entrained and fluidized drying ...................................

457

10.1.3.

H o t - w a t e r drying ..................................................

457

10.1.4. Steam and hot-water drying ......................................

463

10.1.1.

Rotary

drying

xv 10.1.5.

Steam

drying

.......................................................

465

..................................................

467

in hot oil ...................................................

473

10.2. R e m o v a l o f m i n e r a l m a t t e r ...................................................

474

10.1.6.

Fleissner

10.1.7.

Drying

process

10.2.1.

Introduction

........................................................

10.2.2.

Gulf Coast lignites

474

.................................................

475

10.2.3. N o r t h e r n G r e a t Plains lignites ....................................

477

10.3. R e m o v a l of inorganic constituents by ion e x c h a n g e ......................

480

10.3.1.

Introduction

480

10.3.3.

Bench-scale

...................................................

483

10.3.4.

Pilot-scale

Desulfurization

10.5.

Size

10.7.

480

10.3.2. F u n d a m e n t a l s o f ion e x c h a n g e with lignites ....................

10.4.

10.6.

........................................................

tests

.....................................................

486

.................................................................

487

tests

................................................................

489

10.5.1. Size r e d u c t i o n at the m i n e .........................................

489

10.5.2. P u l v e r i z a t i o n for c o m b u s t i o n .....................................

491

10.5.3.

492

preparation

Micropulverization

Briquetting

.................................................

.....................................................................

493

10.6.1. L a b o r a t o r y and pilot-scale studies ................................

493

10.6.2.

............................................

496

..........................................................

497

Commercial

Lignite/water

briquetting

slurries

.............................................

497

10.7.2. R h e o l o g y of l i g n i t e / w a t e r slurries ................................

499

10.7.3. Solids loadings of lignite/water slurries .........................

499

10.7.1.

References

Pilot-scale preparation

.............................................................................

499

o f lignites .........................................................

505

11.1. F u n d a m e n t a l s of lignite c o m b u s t i o n ........................................

505

11.1.1. C a l o r i f i c v a l u e s of lignites ........................................

505

11.1.2. M e c h a n i s m s o f c o m b u s t i o n ......................................

507

11.1.3. I n o r g a n i c transformations during c o m b u s t i o n .................

515

Chapter

11. C o m b u s t i o n

11.2. S m a l l - s c a l e c o m b u s t i o n 11.2.1.

Historical

s y s t e m s ............................................ ............................................

521

11.2.2. D o m e s t i c use o f lignite ............................................

521

11.2.3.

........................................................

522

p r a c t i c e .................................................

527

Stoker

development

521

firing

11.3. C u r r e n t c o m b u s t i o n

11.3.1. Historical d e v e l o p m e n t of pulverized firing of lignite .........

527

11.3.2. Evaluation of lignites as potential fuels for p o w e r plants .....

528

11.3.3. C o m m e r c i a l firing for p o w e r generation ........................

531

xvi 11.3.4. S u r v e y of current commercial practice ..........................

534

11.3.5. Emission control technology

.......................................

543

11.4. A s h deposition, slagging and corrosion ....................................

544

11.4.1.

11.5.

........................................................

544

11.4.2. Factors affecting fouling and slagging ..........................

Introduction

546

11.4.3. T h e m e c h a n i s m of deposition ....................................

56O

11.4.4. G r o w t h of strength in deposits ...................................

572

11.4.5. Remedial measures for combatting fouling and slagging .....

575

11.4.6.

..............................................

582

....................................................

583

.........................................................

583

Ash-related corrosion

Fluidized-bed 11.5.1.

combustion

Introduction

11.5.2.

Combustion

behavior

..............................................

585

11.5.3.

I n - b e d s u l f u r capture

...............................................

585

11.5.4.

NOx

......................................................

588

11.5.5. References

Bed

formation

agglomeration

..................................................

589

.............................................................................

591

C h a p t e r 12. A l t e r n a t i v e uses of lignites .....................................................

602

12.1.

o f lignite .........................................................

602

12.1.1. F u n d a m e n t a l s of gasification .....................................

602

12.1.2. C o m m e r c i a l gasification in North A m e r i c a .....................

614

12.1.3. C o m m e r c i a l gasification outside North A m e r i c a ...............

622

Gasification

12.1.4. Pilot-scale gasification of lignites ................................

624

12.1.5. U n d e r g r o u n d gasification of lignite .............................

637

12.1.6. Biological gasification of lignites ................................

638

12.2. Direct l i q u e f a c t i o n of lignites ................................................

639

12.2.1. T h e E x x o n D o n o r Solvent process ..............................

639

12.2.2.

643

Project

Lignite

12.2.3. T h e C O - S t e a m

...................................................... process ............................................

648

............................................

651

12.2.5. Bench-scale studies for process i m p r o v e m e n t .................

652

12.3. C h e m i c a l products from lignites .............................................

655

12.2.4. T w o - s t a g e

12.3.1.

Montan

12.3.2.

Activated

liquefaction

wax

........................................................

655

...................................................

656

12.3.3. Direct uses of lignites in water treatment ........................

658

12.3.4.

658

Charcoal

carbon

............................................................

12.3.5. Applications in extractive m e t a l l u r g y ............................

659

12.3.6. R e c o v e r y of uranium from lignites ..............................

661

12.3.7.

663

Leonardite

..........................................................

xvii 12.3.8. Prospects for other chemical products from lignites ............

665

12.3.9. A p p l i c a t i o n s of lignite ashes .......................................

666

12.4. C o m b u s t i o n of lignite-water slurries .........................................

666

12.4.1. Slurry preparation for c o m b u s t i o n .................................

666

12.4.2.

Combustion

...........................................

667

12.4.3.

Ash

.........................................................

667

References

Index

performance

behavior

..............................................................................

669

............................................................................................

679

This Page Intentionally Left Blank

Chapter 1

THE P R I N C I P A L LIGNITE DEPOSITS OF NORTH A M E R I C A

1.1 I N T R O D U C T I O N This chapter describes the lignite deposits of North America, providing information on the estimated reserves and resources, the geological setting of the deposits, and the quality of the lignites. Two major deposits of lignites exist in North America. The Gulf Coast lignites occur in the southeastern United States, in deposits lying in a band roughly parallel to the coastline of the Gulf of Mexico, forming an arc stretching from southern Texas across Louisiana and Mississippi into Alabama. The second major deposit is the Fort Union lignite field, lying in the northern Great Plains in portions of North Dakota, Montana, and Saskatchewan. The Gulf Coast and Fort Union lignites represent the largest proportion of reserves and resources of lignites in North America, are the most important commercial sources of lignites, and are those which have been most extensively studied. Hence most of this chapter is devoted to a discussion of these lignites. Smaller deposits of lignites occur outside the Gulf and Fort Union regions. Some information on these lignites is also presented in this chapter. Even though the reserve base of these lignites is much smaller and in many cases the deposits have not been as well studied, some have commercial potential and all are worthy of additional investigation. Estimated lignite reserves and resources in the United States are shown in Table 1.1 [ 1]. In addition, the principal Canadian lignite deposit, in Saskatchewan, includes 35.2 Gt of in-place resources [2]. The remaining reserves of U. S. lignite able to be recovered by strip mining are shown, by state, in Table 1.2 [3]. The same information is shown by coal province in Table 1.3 [3].

TABLE 1.1 U.S. lignite reserves and resources, gigatonnes [1].

Region

Strippable Reserve Base

Fort Union North Dakota Montana South Dakota Total Gulf Texas Alabama Arkansas Louisiana Total Denver Other states Total ........................

Identified Resources

9.2 14.3 0.4 23.9

318.1 102.1 2.0 422.2

7.2 1.0 1.8 0.5 10.5 2.6

47.4 1.8 12.2 0.5 61.9 9.1 0.2 37.0 ............. 493.4

TABLE 1.2 Remaining U. S. lignite reserves recoverable by strip mining, Mt [3]. State Reserves California 23 Colorado 907 Montana 15,168 North Dakota 11,839 South Dakota 300 Texas 8,618 Washington 5 Wyoming 454 Total ............... 37,314

TABLE 1.3 Remaining lignite reserves in coal provinces recoverable by strip mining, Mt [3]. Province Reserves Coastal Plains 8,618 Great Plains 19,456 Pacific Coast- Sierra Nevada 27 Wyoming Basins 9,213 T otal ............................... 37,314

About 10% of all U. S. low-rank coals (including subbituminous) are classified as the strippable

reserve base [4]. For lignites, the strippable reserve base is 37.0 Gt, and the total identified resources are 493.4 Gt [4]. Data on the reserve base of lignites, organized by state, county, bed, and thickness, is available in [5]. The identified resources of U. S. lignites are classified by depth of seam in Table 1.4 and by seam thickness in Table 1.5 [6]. Identified resources are classified by sulfur content in Table 1.6 and by ash in Table 1.7 [6]. In both tables, the data are shown as weight percent on an as-received basis. In the sections which follow, the principal lignite deposits of each state or province are discussed. The first two sections discuss the Gulf and Fort Union region lignites, with shorter sections treating some of the other lignite deposits. Most of the discussion is devoted to the lignites of Texas, North Dakota, and Montana, since these are the most important deposits in commercial use. A few very small deposits of lignites, having no evident commercial potential but of interest for the geochemistry and organic structure of lignites, are treated briefly in other chapters. T A B L E 1.4 U.S. identified lignite resources (gigatonnes) classified by depth [6].

Depth of Lignite Seam (meters) Region 0-305 305-610 Unknown Total Fort Union 333.6 88.5 422.1 Gulf 39.0 23.0 62.0 Denver 9.1 9.1 Other states 0.2 0.2 Totals ..... 381.7 ............. 23.0 ........... 88.7 .......... 493.4

T A B L E 1.5 U.S. identified lignite resources (gigatonnes) classified by seam thickness [6].

Seam Thickness (meters) Region 0-3 3--6 >6 Unknown Total Fort Union 6.1 11.8 10.2 394.1 422.1 Gulf 21.5 2.4 0.2 37.9 62.0 Denver 9.1 9.1 Other states 0.2 0.2 Totals .......... 27.6 ........... 14.2 ........... 10.4 ........... 441.3 ......... 493.4

TABLE 1.6 U.S. identified lignite resources (gigatonnes) classified by sulfur content (weight percent asreceived basis) [6].

Region Fort Union Gulf Denver Other states Totals .........

0-- 1 422.1 58.4 9.1 489.6

Sulfur Content, as-received 1-3 >3 1.8

Unknown

1.8

........ 1.8 ............. 1.8 ...........

0.2 0.2 ...........

Total 422.1 62.0 9.1 0.2 493.4

TABLE 1.7 U.S. identified lignite resources (gigatonnes) classified by ash value (weight percent as-received basis) [6]. ...... Ash, as-received Region 5-10 > 10 Unknown Total Fort Union 422.1 422.1 Gulf 18.2 43.8 62.0 Denver 9.1 9.1 Other states 0.2 0.2 Totals .............. 440.3 ......... 43.8 .......... 9.3 ............ 493.4

1.2 T H E G U L F R E G I O N

LIGNITES

1.2.1 Texas (i) History. The first scientific description of lignite in Texas was published in 1839 [6]. Small-scale production for local use began in the 1850's [7]. Production reached 1.4 Mt in 1914 [8], but then declined steadily to 1950, when it dropped below 18,000 tonnes. Before World War II, lignite was mined in many counties, in a band from Medina County northeastward to Louisiana. Around the turn of the century, lignite mined near Calvert Bluff was blended with bituminous coal as a locomotive fuel [9]. A use of lignite probably unique to Texas was combustion for process heat needed in the production of salt [10]. Mining in the vicinity of Darco (Harrison County) for activated carbon production started in 1931. Over the years lignite was gradually supplanted as a fuel source by natural gas and petroleum. Mining of lignite as fuel ceased in 1946 [11]. Some mining continued, however, for lignite used as a feedstock for activated carbon production. Mining for fuel use resumed in 1955 with the opening of a large power plant at Sandow (Milam County) to supply power to Alcoa for aluminum refining. Production began increasing sharply in the 1960's. (ii) Occurrence. Texas possesses the largest lignite resources of the Gulf Coast states, as shown in Table 1.8 [ 12]:

TABLE 1.8 Gulf Coast lignite resources by state, gigatonnes [12].

State Alabama Arkansas Louisiana Mississippi Tennessee Texas

Resources* 1.27 2.27 1.54 4.54 0.91 21.23

*Estimated in-place tonnage of seams greater than 1 m thick to 60 m deep. The resources of Texas lignite are summarized by geologic unit and region in Table 1.9 [13]. In Texas, lignite occurs in three stratigraphic units of Eocene age: the Wilcox and Jackson Groups and the Yegua Formation [ 14,15]. TABLE 1.9 Texas lignite resources (megatonnes) [ 13]. Geologic Unit Wilcox

Resources Re~ion Near-surface Deep-basin East-Central 5,880 5~,940 Northeast 4,626 0 Sabine Uplift 3,549 4,959 South 978 4,201 subtotal .............. 15,032 ........ 15,100 ....... Jackson East 4,081 5,080 South 687 4,785 subtotal ................ 4,768 ....... 9,124 ....... Yegua East 1,407 0 T O T A L ........................ 21,208 ....... 24,966 .......

Total 11,820 4,626 8,508 5,179 30,133 9,162 5,472 14,634 1,407 46,174

The potential near-surface resources at depths less than 60 m [14], amenable to conventional surface mining, and in seams 0.9 m or thicker, are 21.2 Gt [15]. About 70% of the near-surface lignite occurs in the Wilcox, about 23% in the Jackson and 7% in the Yegua [14]. More than 90% of this lignite occurs in the Wilcox and Jackson Groups north of the Colorado River [ 14]. The average seam thickness is less than 1.5 m, with a seam of 3 m being exceptional [15]. Wilcox lignite is the best quality, having a calorific value of 15.1 MJ/kg, 33% moisture, 1% sulfur, and 15% ash [ 15]. Yegua lignite is intermediate in quality and Jackson is the poorest. Near-surface lignite occurs in three areas: a continuous band running approximately southwest to northeast from the Rio Grande River in Webb County, passing south of San Antonio, Austin and Waco to Texarkana; a second band which is roughly parallel to the first, from the Rio Grande in Starr County to the Angelina River; and an irregularly shaped deposit in Panola County

and the surrounding region [16]. Near-surface lignite occurs in the main lignite-bearing rocks in an area centered on Panola and adjacent counties of the Sabine Uplift. Near-surface lignite also occurs in two bands, a northern band continuous from the Rio Grande River in Webb County to the Red River in Bowie County; and a parallel, but discontinuous, band lying coastward and stretching from the Rio Grande River in Starr County to the Angelina River in Angelina County. The northern band is divided into three regions [17]: South, from Webb to Caldwell counties; Central, from Bastrop through Freestone counties; and East, from Henderson through Bowie counties and the Sabine Uplift. The coastward band has two regions [ 17]: South, Atascosa, LaSalle, McMullen, Starr, and Zapata counties; and Southeast, from Fayette County through Angelina County. The near-surface lignite resource strata are divided into seven units on a geographic basis; resources according to these units are shown in Table 1.10 [18]. The Wilcox Group accounts for 71% of the total tonnage and 77% of the resource on a calorific value basis. TABLE 1.10 Resources of near-surface lignite in Texas, megatonnes [18]. Region Central Wilcox Northeast Wilcox Sabine Uplift Wilcox South Wilcox East Jackson South Jackson East Claiborne Total ...........................

Resource 5,880 4,626 3,549 978 4,081 687 1,407 21,208

The thickest, most laterally continuous deposits are in the Wilcox of central Texas and the central region of the Sabine Uplift, and in the east Jackson. Some Wilcox seams are laterally continuous for 27 km; some Jackson seams, for up to 48 km [18]. Typical seam thickness is about 3 m, though in some areas seams may reach 4.6 m [ 18]. The largest resource blocks are 900 Mt in the east Jackson and 450 Mt in the central Wilcox. Seams in the northeast Wilcox and the northern Sabine Uplift are thinner, usually less than 2.4 m, but resource blocks of up to 450 Mt are present [18]. In the Yegua Formation seams are usually less than 1.8 m and are discontinuous; resource blocks are 136 Mt or less. South of the Colorado River the Jackson Group has resource blocks of up to 272 Mt [ 18]. Deep-basin lignite is that which occurs under more than 60 m of cover [14]. The resources lying between 60 and 1,524 m are 90 Gt [17]. About 32 Gt lie between 60 and 600 m [17]. Of this, 69% is in the Wilcox and 31% in the Jackson [18]. Deep-basin lignite is unimportant in the Yegua. Deep-basin lignite occurs coastward and downdip from the near-surface deposits. The principal deep-basin lignite occurs in central and eastern Texas, in an area running from Gonzales County to Cherokee County. In Cherokee County the deep-basin lignite splits into two bands

which curl around the sides of the Sabine Uplift. The deep-basin lignite in south Texas extends in a band from the common border of Jim Hogg, Starr, and Zapata counties to McMullen County [ 16]. Six small isolated areas of deep-basin lignite have also been mapped. The largest is in Washington County. The largest and most extensive deposits of deep-basin lignite are in the Wilcox Group of central Texas. Deep-basin lignites are thickest and most numerous in Bastrop, Fayette, Houston, Lee, Leon, and Madison counties. The eastern half of Texas is underlain by lignite deposits, which dip very slightly toward the coast. The lignite can be mined by stripping in regions close to the outcrops. The amount economically recoverable to 46 m depths is 6.1 Gt, and to 60 m is 8.1 Gt [19]. Although the grade of the lignite is variable, an average calorific value is about 15.1 MJ/kg [19]. Ash and moisture contents are both usually high. The strippable lignite lies between the two major metropolitan belts in Texas, and has potential for future use as a fuel for industrial process heat, in addition to its present use as a fuel for electric power generation. Strippable reserves are estimated at 1.2 Gt [8]. Lignite occurs in the Coastal Plain of Texas, in a band 965 km long and 80 to 160 km wide running southwest to northeast from the Rio Grande to the Sabine rivers [20]. The band tends to parallel the coast line, lying 160 to 240 km inland. The main lignite deposits in Texas occur in the Wilcox Group. Smaller deposits occur in the Y egua Formation and the Jackson Group. The most extensive resources occur in the Wilcox Group in eastern and central Texas generally north of the Colorado River. Essentially all of the lignites are Eocene age, although a few of the Wilcox lignites south of the Colorado River and in the southern part of the Sabine Uplift may be Paleocene [21]. The lignites occur throughout the lower Tertiary but are especially abundant in the lower and middle Eocene. Lignite occurs in 49 counties of eastern and central Texas [22]. The strata associated with the lignite beds are largely clays and unconsolidated sands. The region has a thick covering of soil, so that exposures of bedrock are rare; consequently, the extent and thickness of the lignite beds is usually determined from shallow drill holes or pits. Individual seams vary widely in thickness and quality. Thicknesses range up to 6 m [8]. Virtually all of the lignites contain less than 2% sulfur; most are below 1% [8]. Calorific values range from 16.3 to 20.9 MJ/kg (as-received basis) [8]. (iii)

Quality. Generally

the differences in grade and continuity of seams can be correlated

with the depositional environment [14]. North of the Trinity River the Wilcox lignite occurs in lenticular seams of moderately high ash value but low sulfur, and a maximum thickness of 4.6 m. This lignite accumulated on an ancient alluvial plain. Stream courses were closely spaced, and interchannel basins were fairly small. The associated peat swamps were of restricted area and were vulnerable to flooding, with attendant influx of sediments. South of the Trinity River and on the Sabine Uplift the interchannel basins were formed lower on the alluvial plain and thus were larger and less likely to experience flooding. The Jackson Group lignites accumulated in marshes on the lower delta plain. Blanket peats could form over extensive areas of the abandoned delta lobes. Because these blanket peats lay more to the seaward, they were more likely to be permeated by sulfur-rich waters. These factors result in the Jackson lignites being more laterally continuous and

of higher sulfur content than the Wilcox lignites. The highest quality lignite occurs in the Lower Eocene Wilcox Group north of the Colorado River. In this region the individual seams are thicker, of more uniform quality, and more persistent than in other locations in Texas. Lower quality lignite of the Wilcox, as well as Upper Eocene Yegua and Manning Formations, is found south of the Colorado River. Most Texas lignite is xyloid; over 50% derives from the woody parts of plants [7]. However, Texas lignite commonly contains considerable amounts of attrital material, from residual remains of plant matter chemically or mechanically broken down to microscopic fragments during the early stages of coalification, as well as some decay-resistant parts of plants. On a dry basis, the ash value of Texas lignites ranges from less than 10% to over 40% [7]. The average ash value is about 16% [7,23]. North of the Colorado River the average is less than 15%; some Wilcox lignites in the southern part of the Sabine Uplift yield less than 10% [7]. Lignites with very high ash, occasionally exceeding 40%, occur south of the Colorado River. The Upper Eocene lignites also show a geographic trend in ash value, ranging from 15-20% ash in east Texas to 35% in south Texas [7]. The average calorific value of Texas lignites is 24.2 MJ/kg on a dry basis [7]; it ranges from 27.9 MJ/kg for east Texas lignites to less than 23.2 MJ/kg for lignite in south Texas [7,23]. This change in calorific value tracks the trends discussed above for the geographic distribution of ash. As a rule, if lignites of comparable ash value are compared, those having the higher calorific value have higher fixed carbon and higher carbon and hydrogen contents. Lignites with calorific values above 25.6 MJ/kg are found in the Sabine Uplift, in Lee County to the Trinity River and in Houston and Trinity counties. Medium calorific values (23.2 to 25.2 MJ/kg) are found in lignites from Trinity River to Bowie County, and in Medina, Bastrop, and Fayette counties. South Texas lignites of Eocene age generally have calorific values less than 23.2 M.l/kg. The average fixed carbon content of Texas lignites is 37%, on a dry basis [7]. North of the Colorado River the fixed carbon of Wilcox lignites ranges from 30 to 50%, but south of the fiver these lignites contain less than 30% fixed carbon [7]. For lignites of Upper Eocene age, the fixed carbon is generally 30-40% in east Texas and decreases to less than 30% in south Texas [7]. The volatile matter content decreases toward the south, consistent with the trend for increasing ash content. Lignites having volatile matter exceeding 50% on a dry basis occur from the Trinity River to the Sabine River, in the southern part of the Sabine Uplift, and in Franklin, Bowie, Bostrop, Houston, and Trinity counties. Volatile matter contents in the range of 40 to 50% (dry basis) are found in lignites of Rains, Wood, Hopkins, Fayette, Medina and Caldwell counties, in the northern part of the Sabine Uplift, and from Lee County to the Trinity River. The South Texas lignites of Eocene age generally have less than 40% volatile matter (dry basis) [7]. Most Texas lignites have sulfur contents below 1.5% [23], although south of the Colorado River the sulfur content of the Wilcox lignite reaches 2% [7]. As a rule the Wilcox lignites north of the Colorado River contain less than 1% sulfur (as-received basis) [7]. Lignites occurring between the Brazos and Trinity Rivers may have sulfur contents in the range of 1-1.5% [7]. Sulfur is

lowest in the northeast, where lignite accumulated in ancient alluvial plains in fresh water swamps. Explanations for the higher sulfur content of the lignites of the Sabine Uplift and east central Texas include a greater degree of marine influence in the depositional environment, a slower rate of accumulation, or a deeper burial and consequent higher temperatures during coalification [24]. For lignites of Upper Eocene age, the sulfur content varies from under 1% in east Texas to 2% in south Texas [7]. On an as-received basis the average moisture content of Texas lignites is about 30% [7]. There is little regional variation in the moisture content. Several useful surveys of Texas lignites, which include extensive analytical data, are available in [7,17,20,24]. (iv) Wilcox Group lignites. The most extensive deposits of lignites, and those of greatest commercial importance, occur in the Wilcox Group. In central Texas and the northeastern Gulf Coastal Plain the Wilcox Group is divided into the Sabinetown, Calvert Bluff, Simsboro, Hooper, and Seguin Formations. The principal lignite deposits occur in the lower part of the Calvert Bluff. The Wilcox Group contains lignite interbedded with sand and clay. The Wilcox is early Eocene in age and non-marine in origin. The total thickness of the strata ranges from 183 to 457 m [25]. Lignite occurs in the upper two-thirds of the Wilcox [26], where it accumulated in hardwood swamps between alluvial ridges [27]. The commercial deposits, in seams 0.6 to 4 m thick, range from 23 to 363 Mt [26]. Generally the lignite seams are lenticular. Few seams extend over more than 26 km 2 [7]. In south Texas the seams are thinner and have more partings than the seams in east Texas. The partings consist of clay, shale, or very poor, highly mineralized grades of lignite called blackjack, bone, or rotten coal. The thicker and more extensive seams are generally more uniform and of better quality than the thinner, irregular seams. This same relationship between extent and thickness of seams and uniformity and quality of the lignite is observed for Montana lignites. The best grade of lignite, and the largest deposits, occur in the Wilcox Group north of the Colorado River [14]. The seam thickness ranges from 0.6 to 6.7 m (typically 0.9-3 m) and may be laterally continuous for up to 24 km [ 14]. North of the Trinity River the Wilcox lignite is of poorer grade and in seams which are both thinner (typically 0.6-1.2 m) and lenticular. Over the years there have probably been over a hundred mines working the Wilcox lignite [ 16]. In central Texas, Wilcox Group lignite is commercially mined at two large strip mines, one at Alcoa in Milam County and the other at the Big Brown electric generating plant in Freestone County. In this region the lignites were deposited in deltaic environments. Wilcox lignite is being mined in east Texas at Darco (Harrison County) for conversion to activated carbon. Here the Wilcox lignite is of fluvial origin. In east-central Texas, the Wilcox is divided into three formations, which are, in ascending order, the Hooper, the Simsboro, and the Calvert Bluff. Lignite is found at the top of the Hooper Formation, and at the bottom and top of the Calvert Bluff Formation. The estimated lignite in the lower and upper Calvert Bluff in east-central Texas is 5.4 Gt

10 [24], based on seams greater than 1 m thick with less than 60 m overburden. Calvert Bluff seams are typically 0.6-3 m thick with a maximum of about 6 m [24]. Some seams have been correlated for 32 km. Sixteen lignite seams are found in the Calvert Bluff [28]. The principal seams (from an economic point of view) are found in the lower Calvert Bluff within 91 m of the Simsboro Formation, and in upper 61 m of the Calvert Bluff beneath the Carrizo Sand Formation. In the lower Calvert Bluff, the thick seams tend to be both most numerous and laterally continuous in the region between the Navasota and the Colorado Rivers. An example is the commercially important Sandow deposit in Milam County. In the upper Calvert Bluff the seams are thickest and most extensive northeast of the Brazos River. The Big Brown deposit in Freestone county is an example. The important lignite deposits in Bastrop County are in the lower part of the Calvert Bluff. In the western part of Bastrop County the Hooper Formation contains numerous beds of lignite, although the beds are thin and lenticular and are unlikely to be of commercial importance. The principal lignite bed in Leon County is 2-3.3 m in thickness [7]. The upper 30--60 cm are rotten coal, lignite which is extremely friable and of otherwise poor quality. The most important of the Texas lignites occur in Milam County. The Calvert Bluff lignites in Milam County extend in a band running northeastward to the Brazos River. In some cases the seams may be up to 6 m thick, but the average thickness is 2.1 m [7]. Between the Colorado and Trinity Rivers in east-central Texas the Wilcox Group is 366 to 1067 m thick [26]. The only significant lignite in this region is in the Calvert Bluff Formation. The lignite occurs in the lower part of this formation above the Simsboro Sand. Irregular lignite occurrences are reported for the upper Calvert Bluff. The seams range in thickness from 0.6 to 7.6 m, although 1.5-3 m is typical [26]. Some are continuous for about 22 km [26]. Lignite in east-central Texas occurs between the Colorado and Trinity Rivers [24]. In this region the lignite is in the lower Calvert Bluff. The Calvert Bluff lignite occurs between belts of channel sand. In the Calvert Bluff lignites, the number of individual lignites increases and the thickness decreases in the direction of the ancient interchannel flood basins; the best lignite deposits occur at the juncture of the ancient alluvial and deltaic plains [24]. In northeast Texas near-surface lignite occurs in the upper two thirds of the Wilcox [24]. Seams in the Wilcox are less continuous than in the Calvert Bluff. This difference reflects the fact that the Wilcox lignites accumulated in smaller basins which did not remain intact over as long a time as in the deposition of the Calvert Bluff lignites. The lignite seams of greatest thickness and lateral continuity occur on the Sabine Uplift, the area of transition from the ancient alluvial to deltaic plain [24]. Simsboro lignites are found principally in sand-deficient interchannel areas [15]. Hooper lignites are most numerous and thickest in the upper portion of the formation, immediately below the Simsboro. Lignite is most common in the upper two-thirds of the Wilcox Group. The Wilcox Mount Pleasant Fluvial System occurs over a large area on the Texas-Louisiana border [29]. The lignite mined near Darco belongs to this Mount Pleasant Fluvial System. The Darco lignite is fairly

11 thick (2-3 m) and laterally extensive. The lower portion of the Darco lignite is primarily attrital lignite; a woody lignite constituting the second (and smaller) portion of the seam overlies the attrital lignite. (v) Yegua Formation lignite. Commercially, the Yegua lignite is second in importance to the Wilcox Group lignites. The Yegua lignites are not as extensive and are of poorer quality than the Wilcox lignites. Most of the important deposits of Yegua lignite occur north of the Brazos River, and tend to be concentrated in the middle and upper parts of the formation [7]. Lignite occurs in the upper part of the Y egua Formation and the lower Jackson Group in south Texas, south of the Colorado River. The deep-basin lignite occurs in a band running toward the northeast. At the northeast extremity, the lignite outcrops in central McMullen and Atascosa Counties, where it is currently being mined for the San Miguel electric power station. To the south, it occurs in strippable deposits in Zapata and Starr Counties in the Rio Grande valley. The lignites in south Texas amount to more than 180 Mt in seams of 0.6 to 4 m thickness [26]. Some of the seams extend for 22 km [26]. In Houston County the beds of Yegua lignite range from 0.6 to 1.8 m in thickness (averaging about 1.5 m) and generally occur at depths of 9-18 m [23]. These lignites are of commercial quality and have sufficient seam thickness for commercial exploitation. In south Texas lignite of a lagoonal origin is sometimes grouped as "Yegua-Jackson" lignite. The Caddell Formation (the lowest in the Jackson Group) is nominally the marker separating the Jackson and the Yegua, but in regions of south Texas the Caddell is either absent or, at best, difficult to recognize. The boundary between the Jackson and the Yegua becomes somewhat arbitrary and has to be established by paleontological evidence [ 16]. This lignite has been used commercially as drilling mud additive. (v) Jackson Group lignite. The poorest grade Texas lignite occurs in the Jackson Group [ 14]. Many of the seams are less than 1.5 m thick, though some may be laterally continuous for up to 48 km [ 14]. Most lignite beds in the Jackson Group are too local, too thin, of too poor quality, or suffer some combination of these factors, to be mined. In east Texas, the Jackson Group has been divided into four formations: the Caddell, Wellborn, Manning, and Whitsett [28]. The thickest lignite seams occur primarily in the Manning Formation. The more important lignites of the Jackson Group occur near the middle of the group and in the lower part of the Manning Formation. In Fayette County the Manning Formation lignites are notable for a very high ash value. Manning lignites have sulfur contents greater than 1.5% [ 17]. Compared to Wilcox lignite, Jackson lignites generally have higher moisture, ash, and sulfur contents, and lower calorific values [24]. Generally the older seams have the lower calorific values. The high sulfur contents and ash values are a result of the accumulation of lignite in swamps or marshes in the standplain-lagoonal regions or on the delta plain. The sulfur content also results from accumulation closer to the sea, the organic matter being affected by sulfate-rich marine waters. The high ash value reflects a greater contamination by clastics, either water-borne sediment spreading over and into the delta plain or strandplain, or airborne volcanic ash. The San

12 Miguel lignite interval contains four to six partings each 15-30 cm thick, which are believed to be of volcanic origin [24]. The best Jackson lignite occurs in east Texas. In southeast Texas, the Jackson Group lying between the Colorado and Neches rivers is about 300 m thick [26]. It consists of four formations: Caddell (at the base), Wellborn, Manning, and Whitsett. Lignite occurs in the upper Wellbom and upper Manning Formations. The Manning is the more important. Outcrops of Jackson lignites are numerous, but generally the seams are thin, usually less than 0.9 m [26]. In a 60 m section at Somerville, eighteen lignites are present, but only four are of greater thickness than 0.9 m [26]. In east Texas, lignite occurs as a continuous band in the Manning and Upper Wellborn Formations. In the Manning Formation seams are typically 0.9-2.4 m thick with a maximum of about 3.7 m. The seams are laterally continuous, commonly for 32 km and possibly up to 64 km [24]. The San Miguel lignite (lower Jackson) extends for about 40 km; the interval is typically 3-4.6 m thick [24]. Between the Colorado and Angelena rivers in east Texas, deltaic lignite occurs in the Jackson Group (upper Eocene) [24]. Lignite found in outcrops occurs in the Manning and upper Wellborn Formations [24]. The Jackson lignites have not been characterized by petrographic or palynological studies to the same extent as the Wilcox lignites, but arguments have been made for either marshy [27] or swampy [30] environments of deposition. The San Miguel lignite (upper Yegua or lower Jackson) ranges from reddish brown to brownish black in color with a dull to waxy luster [31]. Cores show no evident banding, and there is little or no hard or woody material. The Hardgrove grindability is 89 (at 28.7% moisture) and the lignite can generally be cut smoothly with a knife [31]. Pyritic sulfur averages 0.72% [31]. Seam partings range from over 60 cm in thickness down to the thickness of a knife edge [31 ]. Partings tend to be clays or carbonaceous shales. 1.2.2 Alabama (i) Occurrence. Alabama has a demonstrated reserve of lignite of 262 Mt lying under less than 76 m of overburden [32]. The inferred reserve is 1.8 Gt [32]. For the deep-basin lignites, which lie under 76-1,830 m of overburden, the demonstrated resources amount to 426 Mt, and the inferred resources, 3.4 Gt [32]. Over half the total estimated lignite resources are located in western Alabama in the Naheola Formation of the Midway Group. About 1.8 Gt occurs in nearsurface deposits of the Oak Hill lignite bed [33]. The near-surface Oak Hill lignite demonstrated resource is 925 Mt, and 4.3 Gt for the deep-basin resource [54]. The demonstrated reserves of Oak Hill lignite amount to 256 Mt, assuming an 85% recovery factor [33]. The subeconomic resources add another 205 Mt [33]. The total hypothetical resource in southwestern Alabama is 7 Gt [34]. Alabama lignites are Tertiary and late Cretaceous in age. The most important are of Eocene age (Wilcox Group). In Alabama the Wilcox is a band roughly 40 km wide running northwest to southeast across the state. This deposit includes almost the whole of Wilcox, Butler, and Crenshaw counties, and lesser portions of twelve other counties. The total demonstrated and inferred resources of Gravel Creek lignite are 386 Mt [34].

13 The oldest Gulf Coast lignites occur in the Naheola Formation (Upper Paleocene) of Alabama and Mississippi. The high sulfur contents of these lignites suggest that they may have been deposited under marine conditions [35]. The Wilcox and Claiborne Groups, which extend into Alabama from Texas, are Upper Paleocene to Lower Eocene. These lignites were deposited in a variety of environments, which ranged from fresh through brackish to saline [ 17]. Lignite is widely distributed in Paleocene and Eocene formations along the Gulf Coastal Plain in Alabama. In the Alabama-Tombigbee Rivers Region, the main lignite deposits occur in the Midway, Wilcox, and Claiborne Groups [33]. In the Midway, the lignite deposits occur in the Oak Hill Member of the Naheola Formation. The Naheola Formation is divided into the Oak Hill (lower) and Coal Bluff (upper) Members. Lignite seams ranging from 0.3 to 4.3 m thick are present near the top of the Oak Hill [33]. The Tertiary lignites run in a west-to-east band across the southern part of the state. The thickest and most persistent bed is the Oak Hill lignite. The Oak Hill Member of the Naheola Formation contains a remarkable lignite seam that can be traced for 120 km from the MississippiAlabama border in a southeastern direction to Wilcox County [18]. Over this distance the seam thickness varies from 0.3 to 4 m [18]. The inferred resources in Wilcox County are 188 Mt, underlying an area of 175 km2 [36]. Some unusual lignite deposits nearly 15 m thick have been formed in depressions created by the collapse of underground caverns. These deposits are highly lenticular (less than 1.6 km in width) and are of interest only as geological oddities. They have no evident commercial value. (ii)

Quality.

Typically the sulfur content of Alabama lignites is 1.6-6.3%, the ash value

is less than 16%, and the calorific value ranges from 9.35 to 26.1 MJ/kg (average 20.9), on a moisture-free basis [34]. An unusual aspect of the eastern Alabama lignites is a trend for increasing oxygen content with increasing depth [37]. Since it is normal to presume an increase in coalification, and hence a decrease of oxygen, with increasing depth, this finding is surprising, but appears to be real, despite the possible errors associated with calculating oxygen content by difference. A study of drill core samples has shown wide differences in sulfur contents [37], suggesting major differences in the accumulation of the precursor peats. The generally high levels of sulfur suggest a saline deposition, comparable to the environment of the Florida Everglades

(e.g. [38]) where peat is now accumulating. Analyses of some specific lignite samples have been tabulated in [20]. The Oak Hill lignite has an average calorific value of 20.9 MJ/kg (moisture-free basis), with a range of 9.35-26.0 MJ/kg [33]. Most Oak Hill lignite yields less than 16% ash; ash increases toward the pinchout in Wilcox County, and consequently the calorific value decreases in this area. Moisture ranges from 45 to 53%; sulfur, from 1.6 to 6.3% (moisture-free basis) [33]. The sulfatic sulfur amounts to 12% of the total sulfur; the balance being almost equally divided between organic and pyritic [33]. The lignite of the Coal Bluff seam is of unusually high ash. On a moisture-free basis, this lignite contains 23.1% carbon, 2.5% hydrogen, 0.6% nitrogen, 2.3% sulfur, 10.1% oxygen, and 61.4% ash [36]. The ash fusion properties are 1560 ~ initial

14 deformation, 1599" softening, and 1599" C fluid temperatures [36]. The average composition of Gravel Creek lignite is 44.05% moisture, 14.9% fixed carbon, 26.2% volatile matter, 19.79% ash, and 1.6% sulfur [34]. The average calorific value is 11.05 MJ/kg (as-received basis) and 22.4 MJ/kg (maf basis) [34]. Tuscahoma lignite (Wilcox) has an average 41% ash, 2.9% sulfur, and calorific value of 13.5 MJ/kg [33,34]. The range of calorific values of Tuscahoma lignites is 5.6 to 22.6 MJ/kg. The range of sulfur contents is 2.0-6.1%; of this, sulfatic sulfur has the uncommonly high proportion of 24.8%, while the organic sulfur is 44.6% and the pyritic, 30.6% [33]. (iii)

Midway Group. Lignite having possible commercial value occurs in the Oak Hill

Member of the Naheola Formation (middle Paleocene). The bed is generally 0.5--4 m in thickness, and represents the most extensive lignite deposit in southwestern Alabama [39]. This lignite accumulated in the interchannel areas of coastal marshes lying on a delta plain. The marshes lay behind a barrier system. The lignite itself occurs in a sequence dominated by clay. The upper portion of the Tuscahoma Sand Member (upper Paleocene) contains lignite having no commercial value. This lignite does not occur over wide areas, and the beds are generally less than 1 m thick [39]. Up to six lignite seams occur in the Tuscahoma Sand, but their combined thickness is only about 1.5 m [39]. The Oak Hill Member is 25--50 m thick. It contains fine-grained sand, sandy silts, and silty clays [40]. The lignites typically occur near the top of the Oak Hill, and are enclosed in clays. In some localities the Oak Hill seams may have been removed, wholly or in part, by erosion. The lignite is 1.2-1.5 m thick, and extends from Kemper County in eastern Mississippi to the Alabama River (Wilcox County, Alabama) [35]. Two seams of lignite occur in the upper Oak Hill Member. The upper, which is not everywhere present, is typically about 15 cm thick [33]. The interseam parting, about 1 m thick, is composed of carbonaceous sand, clay, and silt. The lower seam is 0.3 to 4.2 m thick and is the most continuous lignite seam in Alabama [33]. It extends in a belt up to 13 km wide from the Mississippi border southeasterly through Sumter, Choctaw, Marengo, and Wilcox counties. The bed finally pinches out in Wilcox County on the east side of the Alabama River, near Camden. Oak Hill lignite is enclosed in nonmarine sediments. The low ash, moderate to high sulfur, large areal extent, and tabular shape of the lignite suggest that deposition was not influenced by active streams or by the Gulf of Mexico, although a high sulfur content suggests brackish water [33]. A depositional environment having these characteristics is an inactive delta lobe. The Coal Bluff seam, in the Naheola Formation, averages 1.2 to 2.4 m in thickness [36]. In Wilcox County it averages 1.8 to 2.1 m [36]. It underlies an area of about 28 km2 between County Line and Kimbrough [36]. The measured and indicated lignite reserves in this area amount to about 66 Mt [36]. The overburden is unconsolidated sands and clays. Deep-basin lignites occur at the top of the Midway Group, in the southern portion of the Alabama-Tombigbee Rivers Region. Throughout most of the region the cover is less than 610 m thick, but in the southwestern portion of this region the Midway lignite lies beneath 910-1,200 m

15 of sediments [33]. The Midway lignites in Choctaw, Clarke, Monroe, and Washington counties comprise the most extensive and largest of the deep-basin lignites. Seam thickness can exceed 4.3 m [33]. The demonstrated reserves are 426 Mt, with inferred resources of over 3.4 Gt [33]. (iv) Wilcox Group. In Alabama the Wilcox is divided into the Nanafalia, Tuscahoma Sand, and Hatchetigbee Formations. Thin seams of lignite occur in the Nanafalia Formation, the Tuscahoma Sand, and the Hatchetigbee Formation. The principal occurrences of Wilcox lignite are in the upper (unnamed) member of the Tuscahoma Sand. This member contains six seams of fairly thin lignite. The Tuscahoma lignite occurs in a belt roughly paralleling the Oak Hill lignite, but about 27 km to the south [33]. The Tuscahoma lignite is continuous under large areas of Choctaw, Clarke, and the western portion of Wilcox counties, and in some cases may be over 1.5 m thick [33]. The demonstrated reserves of Tuscahoma lignite amount to 6.6 Mt, assuming an 85% recovery factor [33,34]. The demonstrated subeconomic resources are about 43 Mt [33]. The stacked, lenticular beds (also described as thin and tabular), high ash value, and low sulfur content all suggest deposition between distributary channels of an active delta lobe [33]. Thin seams of lignite also occur in the upper member of the Hatchetigbee Formation. The Tallahatta Formation is the lowest formation in the Claiborne Group; the Lisbon Formation overlies the Tallahatta and contains some deposits of lignite and lignitic clay. The Gosport Sand overlies the Lisbon and contains two very thin, lenticular beds of lignite. The lignites of eastern Alabama are in the Gravel Creek Sand Member of the Lower Nanafalia Formation (Wilcox Group). These lignites are younger than those of western Alabama. In contrast to the western Alabama lignite which is laterally extensive, the eastern Alabama lignite tends to occur in "pods" which may be fairly thick but are of limited areal extent [35]. The seams are enclosed by silty or micaceous sands. These lignite pods may have arisen from peat deposition in abandoned stream channels [41]. Deep-basin lignites also occur in the Wilcox Group. However, they appear to be less extensive than the Midway deep-basin lignites, as well as somewhat thinner (rarely exceeding 1.5 m) [33]. The deposits are thin and lenticular. 1.2.3 Arkansas (i) History. Arkansas lignites have been used as sources of montan wax, dyes, and heating oils, as well as fuel for local domestic heating. The lignite makes an acceptable domestic and industrial fuel when dehydrated and shaped into briquettes. Early attempts to use the lignite as a fuel for blacksmith forges were unsuccessful [42]. (ii) Occurrence. Lignites occur in the southwest and northeast corners of Arkansas and in the south-central region, in an area of about 15,900 km2 [43]. The major deposit lies in a 8-32 km wide belt running from Little Rock to the Texas border [25]. The total lignite to a depth of 45 m is estimated to be 12.2 Gt [18], irrespective of seam thickness. In some cases, lignite deposits exceeding 1 m of thickness are encountered at depths of less than 45 m [1]. Most seams are less than 2 m thick and are lenticular. In addition, the seams may be broken up by faults or ancient

16 channels. The total resources of Wilcox lignite in Arkansas are 3.9 Gt [34]. Pulaski County has the largest reserves, with approximately 40% of the estimated total. Other counties containing large deposits, expressed as a percentage of the total state reserves, are Saline, 20%; Ouachita, 16%; and Dallas, 15%. The most extensive lignite area in Arkansas is about 155 km2, northwest of Camden in Ouachita County [44]. The lignite is exposed along the banks of the Ouachita River and its tributaries. The average seam thickness is about 75-90 cm, with a maximum of 1.8 m [44]. The deposit contains about 68 Mt [44]. A single deposit of an extent of about 24 hectares, but containing a lignite seam 2.5 m thick, occurs near Manning in Dallas County [44]. This lignite was used to manufacture brown pigment. Lignite near Sweet Home in Pulaski County has a maximum thickness of 8 m [44]. Arkansas lignites are found in the Wilcox, Claiborne, and Jackson Groups (Tertiary). Most of the lignite occurs in the Wilcox Formation (Eocene) in the Gulf Coastal Plain. The lignite is present in Cleveland, Dallas, Grant, Hot Spring, Ouachita, Pulaski, and Saline counties. Many of the beds are thin and lenticular, with no real commercial value. Over 85% of the strippable reserves are in the Wilcox Group, which contains about 19.6 Mt [8]. The Crowley's Ridge area contains lignite of the Wilcox Group in Clay, Craighead, Greene, and Poinsett counties. The most abundant lignite and the thickest seams generally occur in the Wilcox Group. The average thickness of Wilcox Group strata in this region is about 275 m. Lignite occurs in the Wilcox in beds up to 4 m thick within 45 m of the surface [34]. The thickest seams exceed 7 m. The beds are generally lenticular. The most important beds in the group occur in the lower and middle Wilcox. These lignites were deposited in a fluvial environment. The lignite has been most extensively studied in Pulaski and Saline counties. The Claiborne Group overlies the Wilcox. Claiborne lignites occur in lenticular beds of limited extent, with maximum thickness of 3 m [34]. The estimated resources are 4.3 Gt [34]. The lignites originated as attrital deposits in deltaic and shallow marine environments [34]. Some lignite occurs in the Tokio Formation [42]. (iii)

Quality. The

calorific value of Arkansas lignite is about 14 MJ/kg [44]. The lignites are

high in ash, and therefore have low calorific values on an as-received basis. The sulfur content of Wilcox Group lignites is less than 1% [8]. The calorific value ranges from 6.44 to 17.6 MJ/kg [8]. The lignites of the Wilcox Group average 35.7% moisture, 17.6% ash, 0.57% sulfur, and 13.4 MJ/kg calorific value on an as-received basis [34]. On a moist, mineral-matter-free basis the Wilcox lignites average 16.5 MJ/kg, corresponding to an ASTM rank of lignite A [34]. These lignites contain 64-90% translucent attritus [25]. The average composition of Claiborne lignites, on an as-received basis, is 38.7% moisture, 17.9% ash, 0.62% sulfur, with a calorific value of 12.7 MJ/kg [34]. The ASTM rank is lignite A, based on a moist, mineral-matter-free calorific value of 15.7 M.l/kg. 1.2.4 Louisiana (i) History. A lignite bed about 1.5 m thick is exposed on the east bank of the Sabine River

17 [45]. The first attempt at mining this lignite occurred in the 1870's, but the barge transporting the lignite downriver to market sank, and the attempt was apparently never repeated. The lignite was considered to be a marketable fuel only in the local area, where it was in competition with pine wood. A 5.5 m thick bed of lignite is exposed at Iron Mine Run on Avery Island [46]. An attempt to exploit the lignite for local fuel use never came to fruition. (ii) Occurrence. The lignite in Louisiana is early Tertiary in age. Outcrops occur north of a line running from Natchitoches to Sabinetown and from the Red to the Sabine River. Underground deposits occur between the Red and the Ouachita rivers, along a line from Natchitoches to Catahoula Parish. The area of the state underlain by lignite is estimated to be 23,000 kin2 [20]. The resources likely to be of commercial significance are located in the Sabine Uplift area in the northwestern portion of the state [47], and are of the same age as the Wilcox lignites of Texas [18]. The estimated lignite resources are 1.5 Gt, but, because of the limited amount of data available for some areas, the actual total may be higher by as much as 450 Mt [18]. Most of the seams are less than 2.5 m thick [ 18]; many are less than 1.5 m thick [8], and tend to be lenticular. The Chemard Lake Lentil (DeSoto Parish) is continuous for a distance of 24 km. This single deposit may contain over 450 Mt. It is about 3 m thick and contains lignites and lignitic clays [48]. The Lentil marks the contact of the Naborton and Dolet Hills Formations, and is also sometimes known as the Blue Bed. A second bed of lignite, sometimes called the Red Bed, lies 8-15 m below the Chemard Lake Lentil. The two beds are separated by shales, clays, siltstones, and sandstones. The Green Bed is about 34 m below the Chemard Lake Lentil. The Green Bed has a maximum thickness of 1.5 m [48]. The Sabine Uplift is centered in De Soto Parish and extends from there into Louisiana and Texas. Lignite occurs principally in the Wilcox Group (Eocene); it is found to a lesser extent in the overlying Claiborne and Jackson Groups. The principal lignite bed is the Chemard Lake, which occurs at the top of the Naborton Formation of the Wilcox Group. The Naborton Formation includes lignitic silts, as well as clays and calcareous sands [48]. Chemard Lake lignite occurs over a fairly wide area of Louisiana. It is laterally extensive for over 80 km [49]. The lignite thickness ranges from 2.4 m to less than 30 cm; the overburden averages about 23 m [47]. The estimated reserves of Chemard Lake lignite amount to 495 Mr, based on an assumption of 1.8 m average seam thickness and specific gravity of 1.29 [47]. Chemard Lake lignite has a woody structure, low sulfur content, moderate calorific value, and high ash value, characteristics which suggest deposition from a fresh water environment. In Sabine Parish an area of about 40 km2 is underlain with lignite having an average seam thickness of 1 m [47]. The reserves are estimated to be 45 Mt [47]. Lignite outcrops at the surface in nearly all the formations of the Wilcox and Claiborne groups in Sabine Parish. However, the only ones which seem to have commercial potential are the Bayou San Miguel and the Sabine River Stone Coal Bluff beds [45]. The Bayou San Miguel bed is typically 1 m thick; the lignite is hard and black with little woody structure [50]. The Sabine River - Stone Coal Bluff lignite seam is of comparable thickness. Other lignite deposits of potential commercial value occur in the Beinville,

18 Bossier, Caddo, Natchitoches, and Red River parishes. The Porters Creek Formation is the topmost formation in the Midway Group. It consists of lignitic shales as well as calcareous shales and clays [48]. The Porters Creek Formation directly underlies the Naborton Formation in the Wilcox Group. (iii) Quality. Chemard Lake lignite has a low sulfur content (0.53%) and a calorific value of 14.7 MJ/kg [49]. The average maceral content is 60.8% vitrinite, 26.3% exinite, and 12.9% inertinite [49]. This lignite consists of bright, banded, and banded-bright lithotypes deriving mainly from forest-moor paleoflora which accumulated on an upper deltaic plain. The presence of duller lithotypes correlates with increased pyrite content. Sunrise lignite (also called the Green Seam or Green Bed) lies about 30 m below the Chemard Lake lignite. The sulfur and calorific values of the Sunrise lignite are slightly higher than the Chemard Lake lignite, 0.62% and 16 MJ/kg, respectively [49]. Lignites from De Soto and Sabine parishes have an average calorific value of 16.6 MJ/kg, with 0.63% sulfur, 16.1% ash, and 30% moisture [47]. Analyses of lignite and ash samples have been tabulated in [20]. 1.2.5 Mississippi (i) Occurrence. Lignite in Mississippi is found mainly in the area north of a line through Meridian, Jackson, and Vicksburg and to the east of the "bluff." In Mississippi the so-called bluff is actually a line of bluffs paralleling the Mississippi River and running on the east side of the river from Kentucky to Louisiana. The seams tend to be lenticular and generally less than 1.5 m thick [8]. Lignites outcrop in an area starting at the border with Tennessee in the north, curving through the central part of Mississippi, and then extending to the border with Alabama in the east [51]. Lignite occurrences cover 32,600 km2 in the state, with outcrops in 33 of the 82 counties [34]. The lignite beds are generally tabular and discontinuous. Mississippi lignite reserves are found in the Wilcox, Claiborne and Midway Groups. The total estimated resource is estimated to be 4.5 Gt [ 18], predominantly in the Wilcox, and is of Eocene age. The principal lignite beds occur in the Ackerman Member of the Wilcox Group. The Wilcox lignites are of principal significance in east-central Mississippi, in a belt running from Calhoun through Lauderdale counties. The best deposits lie in Webster, Calhoun, and Lafayette counties. Other counties containing appreciable deposits are Yalobusha, Choctaw, and Jasper. Lignites of commercial potential in the Wilcox were deposited in fluvial or delta-plain environments. These lignites are irregular in shape, erratic in thickness, and of limited extent [ 1]. The shapes vary from narrow deposits typical of meandering, fluvial systems to elliptical deposits of deltaic origin. At the outer edges, the seams may be only a few centimeters thick, but at the center of the deposit the seams may be up to 3.7 m thick [1]. Generally the Claiborne lignites are similar to those of the Wilcox Group. The Claiborne lignites are found mainly in the Cockfield (Yegua) Formation. Lignite is abundant in this formation. Seams up to 1.5 m thick have been observed [51]. Some sands in the

19

Cockfield are lignitic, and some lignitic clays are also observed. Some lignites are also found in the Cook Mountain, Kosciusko, and Zilpha Formations. The depositional environment for the Claiborne lignites was similar to that for the Wilcox lignites. (ii) Quality. The moisture content of Mississippi lignite is in the range of 40-45%, and may exceed 50% [34]. The sulfur content runs up to 4%, but is generally less than 2% [34]. On a moisture-free basis the calorific value is in the range of 16 to 26 MJ/kg [34]. A summary of analyses of some Mississippi lignite samples has been compiled in [20]. The Wilcox lignites average about 16% ash and less than 1% sulfur [51]. The sulfur content increases geographically as lignite is sampled more and more closely to the ancient marine environment of Kemper and Lauderdale counties. The moisture is in the range of 40--45%; the calorific value, 11-13 MJ/kg (as-received basis) [51]. The Claiborne lignites are generally high in ash and low in calorific value. 1.2.6 Tennessee Lignite is found in western Tennessee in the Wilcox and Claiborne Groups. The total resource is estimated to be 900 Mt [18]. The seam thickness is generally less than 2.7 m [18]. 1.3 THE F O R T UNION REGION

1.3.1 North Dakota (i) History. The first use of North Dakota lignite as a fuel was made by the members of the Lewis and Clark expedition, which wintered in North Dakota in 1804 [52,53]. These explorers reported on the abundant deposits of lignite observed along the Missouri River when they passed through the area in 1805. Extensive exploration of the lignite deposits, particularly in the Missouri Valley, was conducted by the Hayden expedition in 1854 [54]. The lignite-bearing strata were referred to as the Great Lignite Group [55]. Hayden's chief colleague, F. B. Meek, substituted the name Fort Union for Great Lignite [56]. The name Fort Union was taken from a United States army fort on the banks of the Missouri River near the Montana-North Dakota border [55], although the fort was in fact in Montana. The first commercial mining was carried out by the Northern Pacific Railroad in 1884, across the Little Missouri River from Medora [52]. All North Dakota lignite is currently recovered by strip mining. The production was 3 Mt in 1950 and then gradually declined to 2.1 Mt by 1958; however, the production increased again through the 1960's and reached 4.1 Mt in 1968 [8]. (ii) Occurrence. The largest reserves of lignite in the United States occur in western North Dakota and adjacent areas of eastern Montana and northwestern South Dakota. The age of the lignite ranges from late Cretaceous through Eocene [57]. Lignite occurs in the Hell Creek Formation (Upper Cretaceous), the Fort Union Group (Paleocene), and the Golden Valley Formation (Eocene). The lignite deposits of North Dakota generally lie west of the 100th meridian. The lignite field can be divided into three provinces, based on the overburden characteristics [58].

20 North of the Missouri River the overburden is primarily glacial drift. Between the Missouri River and the limit of glaciation, the overburden is mainly fine-grained clastic sediments with some scattered areas of glacial drift. In the unglaciated area (the southwestern region of the state) the overburden is clastic rocks. The overburden on most of the lignite in North Dakota contains higher quantities of sodium than does that on the subbituminous coals further to the west in Montana and Wyoming [58]. The high sodium content has consequences for possible sodium toxicity problems during mined land reclamation, and may also represent the source of sodium in the lignite, which is related to ash deposition problems during combustion in power plant boilers (Chapter 11). More than one hundred lignite beds have been described in western North Dakota, but because few have been traced laterally for more than 40 km, it is possible that many of these hundred beds may be at the same, or nearly the same, horizon as others [59]. Exposures of lignite beds are particularly good southwest of the Missouri River, since overlying glacial deposits are fairly thin and discontinuous. In contrast, northeast of the Missouri the exposures of lignitebearing series are very poor because of a widespread cover of glacial deposits, which in some places are 60 m thick [59]. Good exposures of the lignite beds are observed in bluffs along stream valleys. An excellent example is the Little Missouri River in Slope, Billings, and Golden Valley counties, where the relief in places reaches 300 m, exposing the entire section of lignite-beating rocks [59]. A total of 22 lignite beds has been noted in these counties; the thickness of individual beds ranges from 1 to 11 m, and the aggregate thickness of all the beds is 58 m [59]. The lignite deposits of North Dakota cover an area of about 83,000 km2, of which about 72,000 km2 are underlain by beds over 1.4 m thick [58,60]. However, only about 2800 km2 contain lignite recoverable by surface mining [58]. The demonstrated strippable reserve base of lignite in North Dakota is 11.8 Gt [3], based on the criteria of a minimum seam thickness of 1.5 m, a maximum overburden of 30 m, and a maximum stripping ratio of 10:1 [ 1]. (The economic limits for a reserve to be considered strippable are a minimum seam thickness of 1.5 m, 15 m of overburden, maximum stripping depth of 67 m, and minimum calorific value of 14 MJ/kg [8]). The total estimated resources amount to 318 Gt [58,60-62] (other sources have estimated 544 Gt [59]). In addition, there are an estimated 163 Gt of lignite in the category of hypothetical resources

(i.e., resources in unexplored and unmapped areas) [58,60,62]. Of the total identified resource, 89.3% is subeconomic, a category which includes the inferred resources and any indicated or measured resources in seams less than 1.5 m thick [58]. The remainder (10.7%) represents demonstrated, measured, or indicated resources in seams 1.5 m or greater in thickness [58]. Approximately 14 Gt lie within 30 m of the surface [58]. If a 90% recovery factor is allowed for strip mining, the recoverable reserve is 13.1 Gt [58]. Future improvements in mining technology, to increase the depth at which strip mining is economically feasible and to increase the recovery factor, will bring more of the total resource into the category of recoverable reserves. The largest amounts of recoverable reserves, in units of megatonnes, occur in the following counties: Slope, 1825; Dunn, 1633; Mercer, 1621; Stark, 1041; and Williams, 923 [58]. The remainder is distributed among eighteen other counties.

71 Resource estimates come mainly from outcrop information. Lignite present at some distance from the exposure might not be included in the estimate. In some areas only the most prominent bed is included in the estimate, neglecting lignite in other beds. Furthermore, only lignite in beds at least 75 cm thick is included in the estimate. Thus the actual amount of lignite in the ground is likely to be somewhat greater than the estimated amount. Lignite recoverable by strip mining has been estimated to be 17 Gt [61]. A useful summary, with measured, indicated, and inferred resources divided by seam thickness (0.75-1.5, 1.5-3, and >3 m) for each county has been published [62]. Reserves recoverable by strip mining are summarized in Table 1.11 [3]. TABLE 1.11 North Dakota lignite reserves recoverable by strip mining [3]. Area Reserves Mt Adams County 91 Beach- Wibaux 454 Beaver Creek 136 Beulah-Zap 816 Bowman 1,089 Center 635 Dickinson 1,633 Dunn Center 1,633 Falkirk- Washburn 1,089 Gascoyne- Scranton 272 Grant County 272 Hazen 726 Hettinger County 181 Niobe 272 Noonan - Columbus 136 Renners Cove 907 Stanton 272 Stony Creek 635 Velva 544 Wilton 45 Total .................. 11,838

The oldest lignite-bearing unit is the Hell Creek Formation, consisting mainly of about 150 m of nonmarine sandstones, siltstones, and mudstones [58]. Lignites in the Hell Creek are generally too thin for successful commercial mining, discontinuous, and of poor quality. The Hell Creek is the youngest of the Cretaceous formations, and contacts the Ludlow, the oldest of the Tertiary formations. The top of the Hell Creek was recognized as the base of the Paleocene sediments [63]. Some lignite also occurs in the Lance Formation (Cretaceous), which underlies the Fort Union. The Lance Formation outcrops in a belt running southwestward from central North Dakota to northwestern South Dakota; the belt of outcrops is roughly 80 km [59]. The Fort Union Group contains essentially all of the lignite included in the estimated North

22 Dakota resources. The Fort Union Group is included in the rocks of the Zuni Sequence (Cenozoic). The Fort Union Group derived from erosion of the Laramide Rocky Mountains [64]. Both the lignites and the elastic sediments in the Fort Union are predominantly derived from terrestrial origins [65], except for the Cannonball Formation in the lower part of the Fort Union. (The Cannonball is a marine deposit, while the other formations are non-marine fluvial, alluvial, or swampy origin.) Lignites occur in the Ludlow, Slope, Bullion Creek, and Sentinel Butte Formations [1], all but the Cannonball. Most of the lignite recoverable by strip mining is in the Bullion Creek and Sentinel Butte Formations [28]. The reserves and resources of the lignite in the Fort Union have been estimated at, respectively, 18 Gt and 318 Gt [66]. The vertical range of beds in the Fort Union is about 400 m [20]. In North Dakota the eastern border of the Fort Union runs southeastward from Renville County on the northern boundary roughly to the center of the state. In mid-state the border begins running southwesterly to Adams County on the southern boundary. South and east from Bismarck the Fort Union is thin, and in many places entirely missing. Some of the lignite beds extend over 3,900 km2, with thicknesses ranging from 1.5 to 12 m [93]. However, the lignites vary considerably in thickness, down to a few millimeters [68]. Lignite seams comprise less than 5% of the total thickness of these formations, the majority of the thickness being interbedded silt and clay and silty fine- to medium-grained sand [73]. Most of the strippable lignite deposits are found in the Williston Basin, the largest occurring on the southwestern flank of the Basin, and smaller deposits on the eastern flank. Here the bed thickness ranges widely, from less than 25 mm to over 7.6 m [8]. In some cases two or more beds of thickness appropriate for commercial mining may occur in the same area. Most of the mineable lignite is under less than 300 m of overburden, and 70% is under less than 150 m [60]. There is very little structural variation in the lignite in the Fort Union. The beds are generally flat, with an estimated dip of only 4 m/km [69]. Both the Cannonball and Ludlow Formations are about 90 m thick [67]. The Ludlow contains lignites that range in thickness from less than 30 cm to about 7 m [1], with aggregate thickness up to 12 m [62,70]. The Ludlow is best exposed in southwestern North Dakota, especially in Bowman County. The Ludlow underlies the Cannonball in the western portions of North Dakota. The Ludlow Formation is the lowermost of the Fort Union Group, and is similar to the underlying Hell Creek Formation. The Slope Formation contains strata that had previously been assigned to the upper Ludlow or to the Tongue River. Lignite makes up about 10% of the rocks in the Slope Formation, and occurs in beds having a maximum thickness of 4 m [1]. Like the Slope Formation, the Bullion Creek has recently been defined and contains many strata formerly assigned to the Tongue River Formation. The Bullion Creek is also similar to the Slope Formation in that lignite beds comprise about 10% of the strata [1]. The lignites average about 1 m in thickness, with a maximum bed thickness of 4 m [1]. The Harmon and Hansen beds are the lowest lignite members of the Bullion Creek Formation. The Harmon bed underlies small areas in themnorthern part of Bowman County. It reaches a maximum thickness of 10 m [60].

23 Overburden is 36 m or less [3]. At the Gascoyne mine (Bowman County) two seams are separated by about 90 cm of partings. The Gascoyne mine supplies lignite for use in a power plant in Big Stone City, South Dakota. The Weller Slough bed is also in the Bullion Creek Formation. It is about 3.7 m thick in Mercer County [71 ]. The Coal Lake Coulee bed generally ranges from 60 to 120 cm thick, with a maximum of 2.4 m [71]. The Tavis Creek bed, a relatively thin (ca. 90cm) bed, lies just below the contact between the Bullion Creek and Sentinel Butte Formations. Lignite beds in the Sentinel Butte range in thickness from less than 30 cm to, in some instances, over 6 m [ 1]. Sentinel Butte lignites are less continuous than those of the Bullion Creek. Sentinel Butte sediments tend to be darker, relative to the Bullion Creek, and are described as "somber" [58]. Generally, lignite seams occurring in the Sentinel Butte are lenticular and often split into multiples. The Sentinel Butte Formation is primarily silt, clay, and lignite, with sands at both base and top. The Hagel bed is the lowest lignite bed in the Sentinel Butte [71]. The average thickness in Dunn County is about 2.4 m, although in some locations the Hagel bed splits into several minor seams separated by inter-seam partings up to about 1 m thick [71]. The Hagel bed dates to the late Paleocene. It is extensive throughout the central and south-central portions of Oliver County along Square Butte Creek and its various tributaries [72]. The Hagel bed occurs throughout the Knife River drainage basin and into adjacent McClean and Oliver counties [71,73]. Currently the Hagel bed is mined at the Center mine near Center (Oliver County), the Glenharold mine near Stanton (Mercer County), and the Falkirk mine near Falkirk (McClean County). At the Center mine the seam is about 3.4 m thick under 15 m of overburden [3]. Here the Hagel seam contains partings of dark carbonaceous clay or gray clays. The underclay is a graygreen clay with some fragments of lignite; the overburden is gray and consists mainly of clayey silts and silty clays [74]. This lignite is burned at the Square Butte power plant. The BaukolNoonan mine supplies lignite for an adjacent power plant. The seam thickness ranges from 3 to 4.3 m [75]. The Keuther seam, about 11-12 m above the Hagel in this location [58,75], is mined if local conditions permit; however, much of the seam has been removed by erosion or has oxidized to leonardite. Baukol-Noonan lignite is generally of low sodium content. This characteristic is related to the permeable overburden containing abundant carbonate minerals; sodium in groundwater could be replaced by calcium through ion exchange. The overburden is mainly glacial till and poorly consolidated sandstones and shale. At the Glenharold mine the seam thickness ranges from 60 cm to 2.4 m, under 12 m of overburden [3]. The overburden is primarily soft sandstone, shale, and clay with a veneer of glacial drift. However, a layer of a hard, well cemented sandstone is immediately above the Stanton lignite. This lignite is also used as fuel for an electric power plant. The Falkirk mine operates in two seams, one of 90 cm and the other of 2.4 m thickness, separated by a 1.5 m parting [3]. The overburden thickness is 24 m [3]. The lignite is used as fuel in a power plant in nearby Underwood. The Kinneman Creek bed is one of the most persistent lignites throughout the Knife River Basin. The main seam is about 2.4 m thick [71]. Associated seams about half as thick lie within 2.4 to 12 m of the main seam. Lignites along the Little Missouri River northward from the vicinity of Yule are sometimes

24 referred to as the Great Bend group of beds [76]. Of these, Bed I (also called the Sand Creek bed) is the thickest bed (about 11 m) that outcrops anywhere in North Dakota. This outcrop was located about 8 km southwest of the town of Amidon [76]. The Sand Creek bed can be traced for over 64 km, from the old Yule Post Office at the mouth of Williams Creek nearly to the Harmon ranch, 13 km south of Medora. The Antelope Creek bed usually occurs as two seams separated by a clay parting, but in some locations may consist of several seams separated by clay, sand, or silt. The sequence of multiple seams with interbedded sand, silt, or clay may be up to 15 m [71]. It is generally traceable throughout the Knife River Basin. The Jim Creek bed is a thin seam generally less than 1 m thick which occasionally splits into two [71]. The Spaer bed has a maximum thickness of about 1.8 m in the vicinity of Beulah and Zap and generally is of 60 cm to 1.8 m thickness in that region [71]. Elsewhere in the Knife River Basin it is thinner and even pinches out in places. In some locations it separates into four very thin beds separated by silt, sand, or clay. In Billings County the D and E seams range up to 4 m in thickness [3]. This lignite is notable in being the first commercial discovery of uraniferous lignite in the United States, the discovery occurring in the vicinity of Rocky Ridge in Billings County. Mining of lignite for its uranium content was carried out in 1956-8 and again in 1966-7 [3]. Northeast of Dickinson, the D and E seams are 2.7 to 5.5 m thick, under a maximum of 30 m of overburden. The Beulah-Zap deposit in Mercer and Oliver counties is comprised of three separate lignite fields: North Beulah, South Beulah, and Zap. In the North Beulah and Zap fields lignite is found in the Zap bed. In the South Beulah field the lignite is in the Schoolhouse bed. Both beds are part of the Sentinel Butte Formation. At the Indian Head mine the Zap bed is typically 3-3.7 m thick [58]. The maximum thickness is 5.2 m [28]. The overburden is mainly composed of clay and sandstone, and averages 23 m in thickness [58]. The Beulah-Zap bed averages about 3.6 m in thickness; however, it frequently splits into as many as five seams with clay, silt, or sand partings and in such cases the thickness of the entire interval may be as much as 9 m [71]. North of Beulah the Beulah-Zap merges with the Spaer to form a single seam about 6 m thick [71]; in western Dunn County the Beulah-Zap merges with the Schoolhouse bed. The Beulah-Zap bed underlies a large area of Mercer County. In places it reaches a maximum thickness of 6.7 m [60]. The Beulah-Zap also occurs in southeastern Dunn County, where it crops out along both sides of the Little Missouri River. The maximum thickness of the Beulah-Zap in Dunn County is 4.3 m [60]. In the Renner's Cove area the Beulah-Zap is 5.5 m thick, with overburden ranging from 15-30 m [3]. Currently active mining occurs at the Indian Head and Beulah mines. The latter enjoys a thicker seam, 4.9 overburden, 15

vs.

vs.

2.7 m on average, and less

20 m. At the Indian Head mine the Beulah-Zap seam has an average thickness

of about 4.3 m [75]. The Freedom mine was established to supply lignite to the Antelope Valley power station and the adjacent coal gasification plant near Beulah. The seam thickness here is 4.9 m, with maximum overburden of 30 m [3]. An extension of the Zap bed, running northward from

25 the Zap lignite field, is found south of Lake Sakakawea in Mercer County. The average thickness here is 5.5 m, with up to 30 m of overburden [58]. The Zap bed also makes up the Hazen deposit in Mercer County. The average thickness of the bed is about 3.7 m, but is often split by a shale parting [58]. The overburden is mainly sandstone and clay with a thin cap of glacial till. The Dakota Star mine was operated in this deposit until 1966. The Schoolhouse bed in the South Beulah field consists of three seams which average 3.7, 3.4, and 1.4 m in thickness [58]. The overburden thickness is an average 19 m and is primarily clay and sandstone [58]. The Schoolhouse bed splits into two seams separated by 2.4-3.7 m of clay, sand, and silt; the two seams themselves may be in the range of 1.5 m in thickness [71]. The Twin Buttes bed is of highly variable quality, ranging from a fairly thick single seam of good quality lignite to a thin seam of a carbonaceous clay. In some locations it consists of a single, 1.5 m thick seam while in others it occurs as two seams of this thickness separated by clay up to 1.2 m thick [71]. The Dunn Center deposit in Dunn County ranges from 3.4 to 7.3 m thick with less than 15 m of shale overburden [58]. The Stanton bed forms the Washburn deposit in McLean County. The average thickness of the Stanton bed is 2.4 m, with an average overburden thickness of 15 m [58]. The overburden is mainly clays, sands, and sandy clays with a thin veneer of glacial drift. The Stanton bed also comprises the Stanton deposit in Mercer and Oliver counties. In Burke County the Bonus seam is about 2.6 m thick, and overlies a 1.8 m thick Niobe seam. The overburden covering the Bonus seam is 46 m or less [3]. In the Noonan-Columbus area, the Noonan seam is 2-3 m thick [3]. Lignite is actively mined at the Baukol-Noonan or Larsen mine, where the seam is 2.4 m feet thick with a maximum overburden thickness of 15 m [3]. In the Dickinson area a small amount of lignite is mined from the Lehigh seam at the Husky No. 2 mine. This lignite is used for the production of charcoal briquettes. The seam is about 3 m thick, under 27 m of overburden [3]. Lignite is mined from the Coteau seam in Ward County at the Velva mine. Seam thickness at the mine is about 3.7 m [3]. The maximum overburden thickness in Ward County is 30 m, with the lignite ranging from 3 to 4.6 m thick [3]. The Coteau bed was named because of its proximity to the rough, hilly moraine belt known as the Missouri Coteau [76]. The Harnisch bed is the uppermost lignite in the Sentinel Butte. It occurs just below the contact with the Golden Valley Formation. It is typically 1.2 m thick. In Dunn County the bed separates into a series of seams separated by clay; the entire interval may be up to 5.5 m thick [71]. The Golden Valley Formation (Eocene) lies above the Fort Union. The Golden Valley contains thin beds of lignite, but the amount is insignificant, and the lignite has little commercial potential [67]. The principal members are a kaolinitic claystone interbedded with sandstone and a micaceous sandstone/siltstone. The former is capped with a thin layer of lignite which seldom exceeds 1.8 m in thickness [58]. The Golden Valley is also included in the Zuni Sequence, which terminated following deposition of the Golden Valley. fiii)

Quality.

North Dakota lignite is mainly brown, though in some deposits it appears

26 black and lustrous. Many samples are conspicuously woody in appearance, often exhibiting the grain or cells of the wood. Breakage along the grain occurs readily, but less easily in other directions. Sometimes flattened trunks or branches are found in the lignite. The extent of the "woodiness" varies with location in the seam, with some portions of the seam consisting of alternating layers of a tough, brown lignite and a black, lustrous, brittle material. Weathered surfaces are generally black, with a bright, vitreous luster. The luster of the weathered surfaces seems inversely related to the ash value; as the yield of ash increases, the luster of the weathered surface is duller. The high moisture content, which may exceed 40% in some locations, makes freshly mined pieces sometimes feel moist. The lignite normally cleaves parallel to the bedding plane but seldom cleaves vertically. Consequently the lignite can sometimes be mined in thick slabs often more than 1 m long. The lignite degrades rapidly on exposure to the air (a process called slacking or slackening). Storage of the lignite occasionally leads to spontaneous combustion. The beds are sometimes marked by clinkers where the lignite has bumed from natural causes (the characteristic color of this material leads to the term "red dog"). Severely weathered exposures of lignite contain a soft, earthy, medium-brown material known as leonardite. Extensive compilations of analyses of North Dakota lignites have been published [20,58,7779]. Moisture and ash values are the most variable. For 413 samples, the average moisture content was 37.5%, with a standard deviation of 2.3 and range of 28.1-45.1% [79]. The average ash value was 6.4% on an as-received basis, with standard deviation of 1.9 [79]. The ash value increases in a westerly direction. Calorific values are virtually constant, 28 MJ/kg on an maf basis [79]. The average sulfur content was 1.1% (maf basis), with a range of 0.3-4.5% [79]. The average CaO content of the ash 31% [79]. The average Na20 content is 6.5%, but the standard deviation is 5.0 and the range is a remarkable 0.1-27.0%. SiO2 is highly variable and averages 27% [79]. Both A1203 and Fe203 show little variability; they average 14% and 12%, respectively [79]. Structural deformation in the Fort Union has been very slight except in the extreme southwestern part of North Dakota (the Badlands) and in the Black Hills region of western South Dakota. Consequently there is little variation in rank throughout the Fort Union, particularly in North Dakota. In eastern Montana the coals of the Fort Union become subbituminous in rank, but the change is so gradual with westward distance that there is no apparent change in North Dakota. In fact, the variation of lignite composition and properties from one horizon to another is about the same magnitude as the variation within a single lignite bed. North Dakota lignites show large within-mine variability of most components. An extreme case is the Gascoyne mine (Harmon Bed, Bullion Creek Formation), for which the following ranges are reported [79]: moisture, 32.444.8%; ash, 5.0-14.3%; SiO 2, 18.0-61.6%; A1203, I0.1-16.2%; Fe203, 2.8-15.1%; CaO, 14.7-44.7%; and Na20, 1.1-10.2%. The average quality of Gascoyne lignite, on an as-received basis, is a calorific value of 14.4 MJ/kg, with a proximate analysis of 40% moisture, 25% volatile matter, 26% fixed carbon, and 6% ash [3]. The average as-received sulfur content is 0.75% [3].

27 The quality of lignite from the Center mine (Hagel bed, Sentinel Butte Formation), on an asreceived basis, is calorific value 15.3-16.4 MJ/kg, 6--7% ash, 0.6-0.9% sulfur, 35% moisture, 29% volatile matter, and 29% fixed carbon [3,58]. The lignite is black to brownish black and slacks rapidly when exposed to air [74]. Lignite from the Glenharold mine in this seam is slightly better with regard to calorific value, ash, and sulfur, being, on an as-received basis, 16-16.5 M.l/kg calorific value; 38% moisture, 26-27% volatile matter, 30-31% fixed carbon, and 4-5% ash; and 0.4-0.5% sulfur [3,58]. From the Falkirk mine, also operating in the Hagel seam, the average quality is 14.9 MJ/kg calorific value; proximate analysis of 39% moisture, 29% volatile matter, 28% fixed carbon, and 7% ash; and a sulfur content of 0.6% (as-received basis). Typically the lignite of the Dunn Center deposit of the Sentinel Butte is 38-39% moisture, 27% volatile matter, 27% fixed carbon, 7-8% ash, 0.8-1% sulfur with a calorific value of 14.6 MJ/kg, on an as-received basis [58]. The average quality of lignite from the Leigh seam, used commercially for the manufacture of briquettes, is, on an as-received basis, 15.1 MJ/kg calorific value; 35% moisture, 25% volatile matter, 32% fixed carbon, and 7% ash; and 1.2% sulfur. The average qualities of lignites from the Dakota Star, Indian Head, and Beulah mines (Beulah-Zap bed) are quite similar. On an as-received basis the calorific value is 16-16.5 MJ/kg. The proximate analysis is 35-37% moisture, 26-29% volatile matter, 28-31% fixed carbon, 6 8% ash, and a sulfur content ranging from 0.6% at the Indian Head mine to 0.8% at Beulah [3,58]. Indian Head lignite is characterized by abundant, thick vitrain. Coalified tree stumps near the top of the seam have been observed, ranging in diameter from 15 to 75 cm [75]. Indian Head lignite is high in sodium, containing typically 8.5% Na20 in the ash [75]. The overburden is a highly impermeable clay low in carbonate minerals, the reverse of the situation for the low-sodium Hagel seam at the Baukol-Noonan mine. The composition of lignite from the Zap bed south of Lake Sakakawea, on an as-received basis, is 34% moisture, 30% volatile matter, 30% fixed carbon, 6% ash, 0.5% sulfur with a calorific value of 16.4 MJ/kg [58]. Velva lignite, mined from the Coteau seam, on an as-received basis, averages 15.8 M,l/kg calorific value; 37% moisture, 27% volatile matter, 31% fixed carbon, and 5% ash; with 0.2% sulfur [3]. It is remarkable for a very high calcium content, sometimes exceeding 40% reported as calcium oxide in the ash. The lignite in the Coteau bed is black and less woody in appearance than most of the lignite elsewhere in North Dakota. The lignite is intermediate in appearance between a cannel coal and the brown lignite typical of much of the North Dakota lignite [76]. It readily splits along the bedding plane, and tends to slack when exposed to sun and air movement, or other storage conditions that allow the moisture to evaporate. The lignites in the Fort Union Group frequently serve as aquifers. (In fact, any rock with 35% moisture arguably is an aquifer [58].) The yield of water is generally 4 to 40 L/min [58]. The chemical quality of the water from the lignite aquifers varies greatly in hardness, being extremely hard in some cases, and frequently shows a reddish-brown coloration due to dissolved or suspended organic materials. The water is used for both for domestic purposes and for watering livestock. Groundwater flowing into the pits can be a problem in many of the lignite mines in the

28 region. Bedrock aquifers consist mainly of consolidated sandstones and limestones with some interbedding of shales, siltstones, or claystones. However, some of the sands and clays are associated with lignites. Both the Fort Union and Hell Creek Formations contain major lignite beds that are aquifers. 1.3.2 Montana (i) Occurrence. About 200 Gt of mineable coal are available in Montana, and, of this, about three-fourths is lignite or subbituminous in rank. The lignite reserves of Montana are estimated to be 79 Gt [80,81], generally found in the northeastern part of the state. The area underlain by lignite exceeds 18,000 km2 [20]. Many of the strippable coal beds are up to 12 m thick. Six continuous lignite beds are present in northeastern Montana; four are fairly thick [82,83]. The seams range from a few cm to over 6 m in thickness [20], but tend to be in the range of 4 to 6 m, with a maximum of about 26 m [8]. The vertical range of lignite seams exceeds 450 m [20]. The Fort Union Formation contains over 90% of the coal reserves of Montana [81]. The formation derives its name from Old Fort Union, which was located near the confluence of the Missouri and Yellowstone Rivers [84]. In Montana the Fort Union Formation has been divided into three members, which are, from oldest to youngest, the Tullock, Lebo, and Tongue River Members [85]. The three members are of Paleocene age. The base of the Tullock was defined to be the first persistent bed of lignite above the Cretaceous dinosaur-bearing beds [63]. The contact with the Lebo Member is taken to be the base of the Big Dirty lignite seam [85]. The Lebo shale member is distinguished from the Tongue River on the basis of lithologic differences [81]. Near the North Dakota border the rocks occurring at the same stratigraphic position as the Lebo are known as the Ludlow Formation. The coals of the Tongue River Member are lignite or subbituminous in rank. In the eastern part of the state the Tongue River is 335 m thick [81]. Some lignites of Tertiary age occur in former lake beds which are now basins lying between mountain ranges in the southwestern portion of the state [81]. Strippable deposits of lignite occur in the eastern part of the state, in the Tongue River Member. The demonstrated reserve of strippable lignite in Montana is 14.3 Gt, based on the criteria of a minimum seam thickness of 1.5 m, a maximum overburden of 38 m, and a maximum stripping ratio of 8:1 [1]. The lignites occur in the Tongue River member are found in the Powder River and Williston Basins, which respectively contain 8.8 and 6.4 Gt of reserves recoverable by strip mining [3]. The reserves in the Powder River Basin are shown in Table 1.12 [3]. The Tongue River contains most of the economically important coal deposits of eastern Montana, with total strippable reserves of all ranks estimated to be 25 Gt [81,85]. The Knobloch Bed contains the largest strippable reserves in the Tongue River, 6.8 Gt [85]. Here the lignite seams may be up to 12 m in thickness, and much of the lignite is within 30 m of the surface [3]. A summary of the reserves recoverable by strip mining is given in Table 1.13 [3].

29 TABLE 1.12 Powder River Basin lignite reserves recoverable by strip mining [3]. Area Reserves, Mt Ashland 1633 Beaver-Liscom Creek 272 Broadus 590 Cache Creek 18 Diamond Butte 227 East Moorhead 318 Fire Creek - Pinto Creek 136 Foster Creek 984 Goodspeed Butte 272 Pumpkin Creek 1452 Sand Creek 168 Sonnette 454 Threemile Butte 91 West Moorhead 1361 Yager Butte 816 Total ..................................... 8792

TABLE 1.13 Williston Basin reserves recoverable by strip mining [3].

Area Reserves, Mt Beach- Wibaux 544 Breezy Flat 272 Burns Creek 1360 Coalridge 72 Ekalaka 18 Fort Kipp 113 Four Buttes 45 Fox Lake 136 Glendive 726 Hardscrabble Creek 181 Knowlton 454 Lame Jones - Milk Creeks 91 Lanark 45 Lane 181 Little Beaver Creek 82 Little Sheep Mountain 109 North Fork- Thirteenmile Creeks 544 O'Brien - Alkali Creeks 318 Pine Hills 45 Poplar River 45 Redwater Creek 454 Reserve 27 Smith - D r y - Parsons Creeks 91 Weldon - Timber Creeks 318 Wolf Creek 136 Total .................................... 6407

30 Cii) Quality. The coals of Montana show a progressive increase in rank westward from the border with North Dakota. Generally the lowest rank occurs along the North Dakota border and the highest, along the Wyoming border. In easternmost Montana the coal is lignite; near Miles City it is subbituminous; west of Miles City it becomes subbituminous C in rank; and reaches bituminous rank near Red Lodge. The coal in the Fort Union Formation in eastern Montana ranges in calorific value from 16.3 to 20.9 MJ/kg on an as-received basis [86]. With the westward increase in rank is a concomitant decrease in moisture content. Montana lignites generally have as-mined moisture contents in excess of 35%, with about 30% fixed carbon and 25-30% volatile matter [79]. Lignite in eastern Montana contains up to 43% moisture [81]. The ratio of fixed carbon to volatile matter averages 1.18 [79]. Ash values are typically in the range of 5-14%; and sulfur, 0.5-1.0%, although sulfur contents up to 1.7% have been reported [79,81]. On an maf basis the sulfur content averages 1.0% [79]. The average as-received calorific value is 15.8 MJ/kg [79]. The heating values show a tendency to increase from east to west. In the ash the CaO content averages 27%; MgO, 10%. The SiO2 content averaged 31%; A1203, 20%; and Fe203, 8% [79]. The iron content of the ash is very variable. Lignites formed in lake bed deposits of Tertiary age are much higher in ash value, about 20%, but of comparable sulfur content, relative to the Fort Union lignites [81]. As a rule, beds of thickness greater than 1.5 m have lower ash and are more uniform in quality than the smaller beds [87]. However, the quality is highly variable laterally; some lignite seams up to 4.6 m thick which are low ash may become high in ash value in a lateral distance of a few hundred meters to a kilometer [87]. The lateral variability originates from unstable conditions of deposition in the bogs or marshes where the original organic matter was accumulating. Lignites from Richland County show subconchoidal fractures and a brown streak [87]. Transverse to the bedding plane, a fresh surface usually shows compact, alternating layers of dull and shiny material. The woody texture of the lignite is preserved, with only a moderate flattening of the cells. On exposure to air, the lignite loses moisture, and the surface luster dulls; small cracks appear, which gradually become deeper and more numerous until the sample slacks to a powder. Fresh samples of lignite from the Knobloch bed show a bright luster, attributed to previtrain, a material derived from the coalification of single, relatively large fragments of ancient plants. Lignite occurs along Coal Creek near the North Fork of the Flathead River in Flathead County, as well as along the Middle and South Forks of the Flathead River [88]. Only the deposit along the North Fork of the fiver was actively worked. The lignite from the Coal Creek region contains nodules of mineral resin [89]. Tables of analyses of Montana lignites have been published [20,79,90] including data on ash composition [79]. A compilation of analyses of samples of the major deposits in seven Tongue River coal beds has been published [85]. (iii) Powder River Basin lignites. In the Moorhead area the Anderson, Dietz, and Canyon beds have commercial potential [91]. Dietz coal generally ranks as subbituminous C [91], but is classified as lignite in the regions of Decker and West Moorhead. In the Decker area the Dietz

31 seams combine with the Anderson to produce a multiple seam up to 24 m in thickness [3]. The lignite contains about 5% ash and 0.35% sulfur and has a calorific value of 17.9 MJ/kg, on an asreceived basis [3]. The quality is very similar in the West Moorhead region, where the seam thickness is about 5 m [3]. The West Moorhead coal is classified at the exact upper limit of lignite, having a calorific value of 19.3 MJ/kg on a moist, mineral-matter-free basis [3]. The Canyon, one of the most widespread beds in Montana, is not commercially mined. Much of the coal is classified as subbituminous C, but some lignite and some subbituminous B coals have been reported [91]. Lignite occurs near Diamond Butte and Threemile Butte. At Diamond Butte, the average seam thickness is 3.5 m [3]. On an as-received basis, the lignite quality is 4 - 5 % ash, 0.3-0.4% sulfur, and 17-17.4 MJ/kg calorific value [3]. In the Threemile Butte area, the seam has an average thickness of 2.6 m and the lignite is of lower quality: 6.2% ash, 0.9% sulfur, and 15.8 MJ/kg (as-received basis) [3]. Of the Tongue River coals in Montana, the Canyon bed shows both the greatest ash value and the greatest variation in ash value. The maximum thickness of the Knobloch bed is 20 m in the Otter Creek and Ashland deposits. The Knobloch is not currently mined. The rank varies from lignite to subbituminous. Lignite is found in the Foster and Sand Creeks areas. The Sand Creek lignite has a calorific value of 17 MJ/kg, with 6.5% ash and 0.3% sulfur [3]; Foster Creek, a calorific value of 17.7 MJ/kg with 7.5% ash and 0.9% sulfur [3]. At Foster Creek the seam is about 3.4 m thick [3]. The magnitude of the Foster Creek deposit is illustrated by the estimate that sufficient reserves exist to fuel thirty 200 MW electric generating stations (i.e., 6000 MW total) for forty years [92]. In the Beaver Creek-Liscom Creek area, the Knobloch is about 4.6 m thick; the lignite is about 9% ash and 1% sulfur [3]. The coal in the Knobloch bed reaches a thickness of 14 m on Otter Creek; the rank ranges from lignite to subbituminous C [86]. The Otter Creek deposit is low in ash and sulfur. Characteristics of the Knobloch coals are summarized in Table 1.14 [90]. TABLE 1.14 Characteristics of Knobloch bed coals [90]. Coal deposit Ashland Beaver - Liscom Creeks Foster Creek Otter Creek Poker Jim O'Dell Creeks Sand Creek

Cal. value, MJ/kg 17.8-21.1 17.1-19.6 17.2-18.2 18.6-21.2 19.5--21.2 16.8-17.3

Sulfur,% 0.1-0.5 0.2-0.9 0.3-1.6 0.1-0.4 0.1-0.6 0.3

Ash, % 3.7--6.8 5.1-13.8 6,7-8.7 3.0-10.6 3.7-6.4 5.1-8.3

The Roland seam represents the top of the Tongue River Member of the Fort Union Formation. The rank ranges from lignite in the Squirrel Creek area to subbituminous in the Roland area. The seam thickness ranges from 1.8 to 4.3 m [3]. In general the coal in the Roland deposit ranges in calorific value from 16.3 to 21.2 MJ/kg, with sulfur content of 0.2-0.7% and ash value

32 of 3.8--9.7% [90]. In the Squirrel Creek deposit the calorific value ranges from 15.4 to 19.3 MJ/kg; sulfur, 0.2-0.6%; and ash, 3.0-14.2% [90]. The Roland bed is about 60 m above the Smith bed [91]. The Cook bed lies below the Canyon and above the Wall beds. The bed occurs in Powder River County in the regions around Goodspeed Butte, Sonnette, and Yager Butte. The seam occurs as two benches. In all three areas the calorific value is 15.6-15.8 MJ/kg and the ash content is 10-12% on an as-received basis [3]. In the Yager Butte deposit the benches are 12-23 m apart, and thicknesses are 0-5.8 m for the upper bench and 2-3.4 m for the lower [90]. The as-received sulfur content is 0.5% [3]. The Goodspeed Butte lignite benches average 5 and 4 m, but the sulfur content is 1.5% [3]. The benches lie 10-14 m apart [90]. At Sonette the bench thicknesses and the sulfur content are intermediate between these extremes. Here the benches are 6.6 to 12 m apart [90]. Characteristics of the Cook Bed coals are shown in Table 1.15 [90]. TABLE 1.15 Characteristics of Cook bed coals [90]. Coal deposit Goodspeed Butte Sonnette Yager Butte

Cal. value, M.l/kg 15.5-16.0 15.2-16.7 13.7-17.9

Sulfur,% 1.2-2.1 0.7-1.9 0.3-0.7

Ash,% 8.9-12.4 6.5-13.3 3.8-20.7

The Elk bed is also in the Yager Butte area. It is about 4.6 m thick [3], with a range of 3 to 6.4 m [90]. Lignite quality, on an as-received basis, is 17.4 MJ/kg calorific value, 5.5% ash, and 0.35% sulfur [3]. The Dunning bed lies below the Elk. The average thickness of the Dunning is 5 m [3], with a range of 4.3-6 m [90]. On an as-received basis, the calorific value is 17.9 MJ/kg; ash, 5%; and sulfur, 0.3% [3]. The Sawyer bed occurs in Powder River County around Ashland and Pumpkin and Little Pumpkin Creeks. The thickness reaches 11 m [90]. Sulfur content is low, 0.4-4).6%. In the Ashland area the ash is 5% and the calorific value is 18.1 MJ/kg; the ash value is higher near Pumpkin Creek, 8.5%, and thus the calorific value is somewhat lower, 17 MJ/kg [3]. In Powder River County the Broadus bed ranges up to 7.6 m thick [3,90]. The maximum thickness is 8 m at the Peerless mine [86]. The calorific value is 17.2 MJ/kg; ash, 7%; and sulfur, 0.3% [3]. The reserve estimated to be recoverable by strip mining is 590 Mt [3]. The Broadus bed lies 30 m above the base of the Tongue River [90]. The coal in the Flowers-Goodale seam varies in rank. In the Foster Creek area the rank is lignite, with a calorific value of 17.7 MJ/kg, 8% ash and 0.5% sulfur [3]. The seam thickness in the Foster Creek area ranges from 60 cm to 4.3 m [92]. In the Beaver Creek-Liscom Creek area the Terret seam is about 2.4 m thick; the average quality is 16 MJ/kg, 9% ash, and 1.1% sulfur, on an as-received basis [3]. In the Foster Creek area the coal is at the border of classification between lignite A and subbituminous C, with a moist, mineral-matter-free calorific value of 19.4 MJ/kg. This coal has a 6% ash value and a remarkably

33 low sulfur content of 0.2% [3]. Here the seam is about 2.7 m thick [3,92]. In the East Moorhead area the T seam ranges in thickness to 7.6 m; the lignite quality is 6% ash, 0.6% sulfur, and 16.5 MJ/kg on an as-received basis [3]. At East Moorhead the ash ranges up to 13.2% and the sulfur to 1.2% [90]. The T bed lies about 80 m above the Broadus [90]. The Pawnee bed is about 6 m thick in the regions of Cache Creek and Pumpkin Creek near Sonnette [86,90]. The Pawnee forms two benches which may be up to 14 m apart [79]. Characteristics of the coal in the Sonnette field are 12.9-18.4 MJ/kg calorific value, 0.2-2.7% sulfur, and 3.925.3% ash [90]. In the Fire Gulch deposit, the calorific value is 17.8 MJ/kg with 0.2% sulfur and 6.0% ash [3]. The Brewster-Arnold bed lies 72-84 m below the Wall bed [90]. The maximum thickness is 6 m [90]. In the Birney field the characteristics of the Brewster-Arnold coal are 18.6-21.9 MJ/kg calorific value, 0 . 2 - 0 . 7 % sulfur, and 3.1-8.2% ash [90]. (iv) Williston Basin lignites. The Dominy bed contains two or three benches, the lower averaging 6 m and the upper 2 m in thickness [90]. In the Knowlton deposit the coal quality is 14.6-15.9 MJ/kg calorific value, 0.2-0.9% sulfur, and 3.8-10.5% ash [3,90]. Comparable values for the Pine Hills deposit are 16.9-17.2 M.l/kg, 0.4-0.6% sulfur, and 6.6-8.1% ash [3,90]. In the Knowlton area the bed occurs as three benches, the upper being 8 m; the middle, 4 m; and the bottom, 3 m. In the Pine Hills area the lower bench is 6 m thick [3]. In the Poplar River area these seams are 2.5 to 3 m thick. The calorific value is low, 13.7 MJ/kg on an as-received basis, with 9% ash and 0.5% sulfur. In the Beach-Wibaux area, the C seam ranges to 9 m in thickness, with 36 m or less of overburden. The maximum thickness is 12 m, at the Black Diamond mine [3]. In some places the overburden is less than 18 m thick. The lignite quality, on an as-received basis, is 14 MJ/kg calorific value, 8% ash, and 0.9% sulfur [3]. In the Four Buttes area, the seam is much thinner, only about 3 m on average. The lignite quality is much the same, being about 14.2 MJ/kg calorific value, 10% ash, and 1.1% sulfur on an asreceived basis [3]. In the Burns Creek area the Pust seam ranges up to 6 m in thickness, with less than 60 m of overburden [3]. The lignite quality is 14.2 MJ/kg calorific value, 8% ash, and 0.65% sulfur [3]. In the North Fork-Thirteenmile Creek area, the seam is 8 m thick, and the quality has improved. Here the average calorific value is 16 M.l/kg, with 7% ash and 0.5% sulfur [3]. Seam thickness is 3.4 m in the Fox Lake area, with 15.8 MJ/kg calorific value, 6% ash, and 0.5% sulfur. In the Redwater Creek area the S seam can reach 6 m in thickness, usually under less than 46 m of overburden [3]. On an as-received basis the lignite has a calorific value of 15.8 MJ/kg, with 11% ash and 0.4% sulfur [3]. In the Weldon-Timber Creek area, the seam thickness is only 3.6 m, but the quality has improved to 17.4 MJ/kg calorific value, 6% ash, and 0.3% sulfur [3]. In the Breezy Flat area, lignite is mined commercially at the Savage mine, the output supplying a power plant at Sidney. At the Savage mine the seam thickness is about 6 m. The calorific value on an as-received basis is 15.1 MJ/kg. The proximate analysis averages 38% moisture, 27% volatile matter, 27% fixed carbon, and 7% ash. The sulfur content is 0.5% [3]. In the Hardscrabble Creek area the F and H seams range from 2 to 3 m in thickness, with

34 overburden less than 60 m [3]. On an as-received basis the lignites have a calorific value of 15.1 MJ/kg, ash of 7% and sulfur content of 0.6% [3]. The Fort Kipp and Peck seams are each about 1.5 m thick, separated by a parting of about 6 m. In the Fort Kipp area the average calorific value is 15.8 MJ/kg on an as-received basis, with 5% ash and 0.3% sulfur. In the Reserve area the Timber Coulee seam is about 3 m thick. Quality is 15.8 MJ/kg calorific value, 7% ash, and 1% sulfur on an as-received basis [3]. The Smith bed lies about 34-46 m above the Anderson bed [91]. The Smith is generally thin, but is remarkable for the large number of petrified tree stumps it contains. Many of the stumps remain in an uptight position. Two samples of the Smith bed lignite from the Decker deposit have calorific values of 17.7 and 19.2 MJ/kg, sulfur 0.6 and 1.0%, and ash 6.8 and 30.2% [90]. Silica was a major constituent of the ash, amounting to 38.5 and 78.3% of the ash. 1.3.3 Saskatchewan Lignite provides about a third of Canada's recoverable coal reserves on a weight basis, and about a fourth when the amount of coal is expressed on a calorific value basis [93]. The province of Saskatchewan contains the major Canadian lignite deposits, which are of Tertiary age. The resources amount to 3.8 Gt, of which 1.36 Gt are measured and the remaining 2.43 Gt are indicated [94]. The total recoverable reserves of Saskatchewan lignite are estimated to be 2.1-2.4 Gt, all extractable by surface mining [93,95]. On an as-mined basis, the Saskatchewan lignites tend to fall in the range of 20-32% moisture, 15-25% volatile matter, and 13-35% ash [94]. Sulfur contents are low, in the range of 0.3-0.8% (daf basis). The calorific value ranges from 12-15.4 MJ/kg. Some of the properties of lignites from the major areas in Saskatchewan are summarized in Table 1.16 [94]. TABLE 1.16 Quality of Saskatchewan lignites [94].

Area Cypress Estevan Willow Bunch Wood Mountain

Proximate Analysis Moisture,% Vol. Mat.,% 20-28 22-45 27-32 22-25 25-32 20-25 24-28 15-23

Ash,% 15-23 13-25 14-30 22-35

Calorific val. MJ/kg, moist 8.8--13.7 13.0-15.4 12.1-15.1 10.7-13.7

The sulfur content of the lignites from all four areas lies in the range 0.2-0.6% [94]. Data on the quality of lignites in Saskatchewan is available in [96]. Lignite occurs principally in four basins: Cypress, Estevan, Willow Bunch, and Wood Mountain; these deposits are collectively known as the Ravenscrag Formation. A small deposit, in seams too thin for profitable mining (i.e., less than about 1.5 m), occurs in the area of Wapawekka near Lac La Ronge. Not only are the seams thin, but this lignite is very high ash.

35 Seams in the Ravenscrag Formation range from 1 to 3 m; generally the lignite is shallow and level [95]. Lignite of the Ravenscrag Formation is mined in a 2.5 m seam in the Klimax Mine of the Butte River Coal Company near Estevan [97]. The Ravenscrag Formation is equivalent in age (Paleocene) to the Fort Union lignites in the United States. At Estevan there are eight lignite seams in a stratigraphic interval of 230 m [97]. Of these, the four upper seams amount collectively to 8 m of lignite, and occur in an interval of 38 m [97]. The annual production of Saskatchewan lignite is about 10 Mt [95], used almost entirely for electric power generation. Most is burned in minemouth plants, but about 1 Mt per year is exported to Ontario. The lignite in Saskatchewan is a major contributor to the electric power production in that province. In 1982 the Saskatchewan Power Corporation consumed 71% of the lignite produced in the province [98]. Saskatchewan Power operates the Souris Valley and Poplar River mines. Other mines which provide lignite to Saskatchewan Power are the Utility and Costello mines (Manalta Coal Ltd.) and the Boundary and Bienfait mines (Luscar Ltd.). The Saskatchewan lignite deposit extends into the southwestern portion of Manitoba, but in Manitoba the seams are so thin as to not be currently considered for mining. 1.3.4 South Dakota (i) History. The total recorded lignite production in South Dakota is only 1.23 Mt, through January 1, 1964 [ 103]. Production peaked in 1941, when 63.5 kt of lignite were mined [79,103]. Most of the production has come from strip mines in Dewey and Corson counties. Virtually all the lignite mined in South Dakota for fuel use was used for domestic heating. During the settlement of the Dakotas, the lignite deposits in the southeastern quarter of South Dakota were of considerable local interest, because of the great distances to other supplies of fuel. The only gun battle ever reported in the lignite literature is said to have occurred as a result of a dispute over a lignite seam in Big Sioux Valley [ 104]. The lignites of South Dakota have been of some commercial interest due to their uranium content. During active mining in 1955-8 and again in 1962--6, 320 t of uranium were recovered from lignites of the Slim Buttes and Cave Hills areas of Harding County. At one time, the most important lignite mining area of South Dakota was the Isabel-Firesteel district in Dewey and Ziebach counties. The lignite in this district is neither as thick nor as extensive as other deposits in the state, but commercial activity was greatly facilitated by the availability of a railroad line which allowed the lignite to be transported and marketed away from the immediate area of mining [3]. (ii) Occurrence. Lignite occurs in a rectangular area in the northwestern part of the state, primarily in Perkins and Harding counties. Lignite having commercial potential occurs in Harding, Dewey, Perkins, and Corson counties, and the northern portion of Meade County; small portions of five other counties may contain lignite. The lignites are an extension of the larger deposits in North Dakota. They occur in the Lance and Fort Union Formations (Tertiary). Bed thickness is typically 1.2 to 1.8 m [8]. The Fort Union Formation does not extend very far into South Dakota, and little lignite is found among the Fort Union sediments in this state. The predominant lignite

36 beds occur in the Lance Formation, particularly in the Ludlow and Hell Creek members [18]. The lignite-bearing area of South Dakota is at the southern edge of the Fort Union region, near the edges of the areas in which these lignites were deposited. Thus the beds tend to be thinner and more lenticular than the lignite deposits of North Dakota and Montana. The oldest lignite-bearing formation is the Fox Hills. The Fox Hills Formation is the first shore deposit of the Cretaceous sea as it began its retreat. The Stoneville lignite-bearing member lies in the upper third of the Fox Hills [ 104]. The Hell Creek Member (late Cretaceous) of the Lance Formation overlies the Fox Hills. The Hell Creek underlies most of the northwestern quarter of the state [103]. It contains much of the South Dakota lignite. The lignite outcrops form a semicircle in Corson, Dewey, Harding, and Perkins counties. The Hell Creek lignite generally occurs in thin, lenticular beds less than 300 m in diameter and 60-100 cm thick [105], and in fact it is better known as a good source of dinosaur fossils than as a lignite source. Some of the Hell Creek lignite occurs in beds of 1.2 to 1.8 m in thickness [ 104]. The lignite beds generally occur 12-24 m above the base of the formation. Seams thick enough to support commercial mining are found in the Isabel-Firesteel area, Dewey and Ziebach counties; Gopher, Corson County; and the Slim Buttes area of Harding and Perkins counties. Mining has also occurred in northern Meade County. The Fort Union Formation comprises, in ascending order, the Ludlow, Cannonball, and Tongue River Members. The Cannonball Member is a marine unit and does not contain lignite; however, the Ludlow and Tongue River Members are non-marine and are lignite-beating. The Ludlow is the most prolific lignite-bearing unit in South Dakota [106]. The Tongue River is present only in the northern parts of Harding and Perkins Counties. Although the seams range up to 2.7 m in thickness [103], the total amount of lignite in this member in South Dakota is small because of its limited areal extent. The Ludlow Member of the Fort Union Formation overlies the Hell Creek. In South Dakota the Ludlow contains the greatest amount of lignite. In Harding County the Ludlow may be up to 107 m thick. The Ludlow is thought to be equivalent to the Lebo Shale and the Tullock Member of the Fort Union in Montana [106]. The beds containing the largest reserves of South Dakota lignite are the Widow Clark and the Giannonatti, both of which occur in the Ludlow member of the Fort Union in Harding County [ 106]. The abundant lignite seams in Harding County occur in terrestrial and shore-line deposits of sand and sandy shales. These lignite are the most persistent and thickest in South Dakota, with a maximum thickness of 4.3 m [ 104]. The two beds make up the Cave Hills field. The Giannonatti is the thickest bed in South Dakota, having a maximum thickness of 4.2 m [ 106], but it tends to thin rapidly. The Giannonatti seam has an average thickness of 1.8 m and a maximum of 4 m [3]. The maximum overburden thickness is 37 m [3]. The lignite in the Slim Buttes area averages about 1 m in thickness [3]. The youngest lignite is in the Tongue River Member of the Fort Union. The Tongue River overlies the Ludlow. Despite the importance of the Tongue River as a source of lignite in North

37 Dakota, Montana, and Wyoming, it contains little lignite in South Dakota. In Perkins County the thickest bed is the Lodgepole, one of several beds of lignite in the Tongue River Member, which ranges from 1.5 to 2.7 m thick [3,106]. The lignite reserves of South Dakota are estimated to be 1.8 Gt [80,106]. The area of the state underlain by known lignite deposits is 20,000 km2 [103,106]. About 84% of the reserves are found in Harding County; most of the rest in Perkins County (about 9%) and Dewey County (about 6%). Very small amounts of lignite are found in Corson, Meade, and Ziebach counties. The demonstrated strippable reserve base in South Dakota is 360 Mt, based on the criteria of a minimum seam thickness of 1.5 m, a maximum overburden of 30 m, and a maximum stripping ratio of 12:1 [1]. The major reserves recoverable by strip mining are shown in Table 1.17 [3]. TABLE 1.17 Reserves of South Dakota lignite recoverable by strip mining [3]. Area Reserves, Mt Cave Hills 159 Isabel - Firesteel 91 Lodgepole 27 Slim Buttes 23 Total .............................. 300

The total resources have been estimated at 2.7 Gt [8]. Over 900 Mt of lignite are estimated to occur in seams more than 86 cm thick [107]. This estimate does not take into account the entire lignitebearing region of South Dakota [ 104]. Based on the original known resources estimate of 1.8 Gt [106], 63% occurs in seams of 0.8-1.5 m thickness and only 3% in seams greater than 3 m [103]. The Stoneville area is underlain by at least 45 Mt of lignite [108]. However, only 1.1 Mt occurs in deposits thick enough, and accessible enough, for mining [104]. Perkins and Harding counties contain 995 kt of mineable lignite, excluding the Hell Creek and Fox Hills lignites [ 107]. In the southeastern quarter of South Dakota some lignite seams 5-10 cm in thickness are exposed in the Dakota Formation of Big Sioux Valley and in the Niobrara Formation near Springfield [ 104]. Some lignite has been encountered by water well drillers at depths of 18-30 m from the surface [104]. These occurrences have been reported near Stickney, Aurora County; Geddes, Charles Mix County; and Fulton, Hanson County. These lignites occur in or near the top of the Dakota Sandstone and the base of the sand in the Graneros Formation. (iii)

Quality.

The lignite beds range from a few cm to over 5 m in thickness [20]. In

general, the seams in South Dakota are not as thick as lignites of the same strata in Montana and North Dakota. Most of the lignite is close to the surface, with overburden thickness typically in the range of 1.8 to 6 m [20]. The lignite beds are generally flat-lying in regions of little to moderate topographic relief. The average calorific value of the lignites in the Lance and Fort Union Formations is 14-16.3 MJ/kg and the average sulfur content is less than 1% [8,104]. On an as-

38 received basis, the Fort Union lignite is 34.3% moisture, 29.1% volatile matter, 29.5% fixed carbon, and 7.1% ash [ 104]. Proximate analyses of some South Dakota lignite samples have been tabulated in [20]. The average analyses of Ludlow lignites are shown in Table 1.18 [ 104]. TABLE 1.18 Average analyses of lignite from Ludlow member, Lance Formation, South Dakota [ 104].

Proximate, % Moisture Volatile Matter Fixed Carbon Ash Ultimate, % Carbon Hydrogen Nitrogen Sulfur Oxygen Calorific Value, MJ/kg

As-received

Moisture-and-ash-free

37.92 28.43 26.25 10.55

-49.59 50.41 --

3 5.01 6.33 0.50 1.25 45.20 13.4

68.68 4.16 0.99 2.58 23.25 25.7

The average quality of the lignite in the Cave Hills field, on an as-received basis, is a calorific value of 13.5 MJ/kg; proximate analysis of 41% moisture, 24% volatile matter, 26% fixed carbon, and 9% ash; with 0.9% sulfur and 0.1% U308 [3]. Average as-received quality of lignite in the Lxxtgepole seam is 16.3 MJ/kg calorific value, 11% ash, and 0.75% sulfur [3]. The Firesteel seam is 1 to 2 m in thickness [3]. Average as-received quality is 16.3 MJ/kg calorific value, 6% ash, and 0.5% sulfur [3]. The Hell Creek Formation lignite is close to subbituminous in rank. It has been classified as lignite by virtue of the high moisture content, which is sufficient to cause slacking on air drying, and a brown streak test [ 105]. Distinct horizontal laminations are observed in the Hell Creek lignite exposed on the Standing Rock and Cheyenne River Reservations. These laminations are a result of alternation of a dull, dark brown lignite which constitutes most of the seam with a bright, black socalled glance coal occurring in laminae or lenses of about 2.5 cm in thickness. The Stoneville lignite, and the Hell Creek lignite found near Isabel, contain unusual amounts of transparent pale lemon-yellow, brittle, irregularly shaped resin bodies. These resin bodies range from the size of specks to about 6 mm in diameter [105], with reports of some up to 12 mm across [104]. The average as-mined analysis of the Hell Creek lignite is 36.8% moisture, 26.6% volatile matter, 7.1% ash, and 29.4% fixed carbon [104]. The maximum moisture content is about 42% [104]. The average sulfur content, on an as-received basis, is 0.52% [ 104]. The calorific value averages 15.7 MJ/kg on an as-received basis, and 24.9 MJ/kg air-dried [104]. The lignite quality in the Slim Buttes area averages, on an as-received basis, 14.9 MJ/kg

39 calorific value, 7% ash, and 2% sulfur [3]. The ultimate analysis of lignite from the Phillips mine, near Strool, is 72.6% carbon, 4.84% hydrogen, 1.28% nitrogen, 2.39% sulfur, and 18.9% oxygen, on a moisture-and-ash-free basis [104]. The calorific value, on the same basis, is 28.5 MJ/kg. As a rule, the Fox Hills lignites are the thinnest of any of the South Dakota lignites. The seam thickness ranges from a few centimeters up to 1.4 meters, although over most of the area the average is less than 60 cm [ 104]. 1.4 O T H E R L I G N I T E S OF T H E UNITED STATES

1.4.1 California (i) History. The discovery of lignite in California dates from the placer mining of the gold rush days. The first discoveries were in the areas of Big Bar, Cox Bar, and along Hayfork Creek. The existence of the Big Bar deposit on the Trinity River was first noticed in 1896 [109]. For many years the lignite was used locally as a fuel in blacksmith forges in the mining camps. A lignite interbedded with fine-grained sandstone and shale was mined near Big Bar from 1929 to 1934 and was used locally as a fuel [110]. The lignite of the Carmel Mine (Monterey County) was discovered in 1874 [111]. Some of the lignite was shipped to San Francisco for use as a fuel. In 1879 a railroad was constructed to transport the lignite from the mine to the coast (a distance of about 8 km) for loading onto ships [112]. Nevertheless, mining ceased the next year, and then continued in an on-again, off-again manner, apparently subject to various law suits. The total production through 1893 amounted to only 18 kt [113]. By 1925 the mine had been idle for many years [114]. Lignite reserves recoverable by strip mining in Amador County in the region of lone were used a century ago in the locality as a domestic fuel. Occasional use of the lignite in this way continued sporadically until about the time of World War II. Since the late 1940's, the lignite has been mined as a source of montan wax, rather than as a fuel. A lignite seam 1.4 m thick was uncovered by the gold mining operations of the Wiltshire mine, which was a hydraulic mine. Some of this lignite was recovered in 1926 by driving adits into the seam and was used locally as a fuel for blacksmith forges [115]. By 1929 some of the lignite was being transported to Redding and Eureka for use as domestic fuel, but the mine closed long before the onset of World War II [ 116]. The Reese deposit (Trinity County) was mined commercially in 1936-38, the lignite being used locally and transported to Redding. In 1949 the feasibility of using the Reese lignite for the manufacture of barbecue briquettes was investigated, but commercial production was never begun. (ii) Occurrence. The area of the state underlain by lignite is estimated to be 1,300 km 2, with a total of about 900 Mt of lignite [20]. The most extensive deposit of lignite occurs in Amador County. The lignite occurs in the Ione Formation (Eocene) [3]; lignite beds range from a few cm to 9 m in thickness [25]. Only one mine is currently in operation in the area of Ione. The lignite beds

40 and associated clay and sand strata comprise the Ione Formation. The lignites are lenticular and discontinuous. They are believed to have formed in manner similar to the Gulf Coast lignites [ 117]. Interest in these lignites stems from the fact that they are commercial sources of waxes. Small lignite deposits occur near Covelo (Mendocino County), 220 km north of San Francisco, and near Hayfork and Douglas City in Trinity County. The Covelo lignite is Eocene, and the others are probably Pliocene coals [ 118]. Lignite of Eocene or Oligocene age is found in Hayfork Valley and Hyampom Valley, at Poison Camp, and near Redding Creek. The commercial development of these lignites has suffered because of the distance to markets, as well as competition from other fuels and hydroelectric power. (iii)

Quality.

On an as-received basis, the lignite of the Ione Formation is 42% moisture,

32% volatile matter, 14% fixed carbon, and 11% ash, with 1 to 1.3% sulfur and a calorific value ranging from 13 to 14.2 MJ/kg [3]. A discussion of California lignite on a county-by-county basis has been published, with a tabulation of a few analyses [20]. The Ione lignites occur in an Eocene sequence which is mainly sandstone [118]. The lignites have very high in liptinite content, derived from spores, cuticles, resins, and waxes. Much of the liptinite consists of a fine, broken detritus. Cutinite occurs in both thin and massive bands. Blebs of cuticular wax and dispersed microspores can be observed. Resin occurs both in isolated, oval-shaped bodies and in vesiculated rodlets. Although there is a distinct microscopic structure parallel to the contacts, the Ione lignite does not contain massive amounts of woody lignite nor fine attritus, both of which are characteristic of other lignite deposits. Layers of huminite derived from wood, bark, or root material are few. Thus most of the plant organs and tissues have been preserved, but most of the humic material has experienced severe decay. The lignite is still highly liptinitic (as indicated by a high hydrogen index on programmed pyrolysis) even after wax extraction. Furthermore, the liptinitic fragments are not visibly altered by the extraction process. These observations suggest that the wax product is derived from cuticular waxes and not from 9resins or spores [ 118]. The lignites in Trinity County are entirely woody. The Covelo lignite contains abundant remains of fungal sclerotia. The lignite in the Reese deposit (Trinity County) has an ash value of about 15- 20% and a calorific value of 15.8-16 MJ/kg [116]. The seam thickness in the area of Ione is about 3.7 m with about 18 m of overburden [3]. Thicker, lenticular seams reaching 7.6 m in thickness occur at depths to 46 m [3]. The lignite beds near Ione and Buena Vista in Amador County vary in thickness from 1.2 to 9 m [ 154]. The Reese Deposit in Trinity County is 2.6 m thick [155]. The Temblor Formation (Miocene) contains four thin beds of lignite, each less than 90 cm thick [156]. The lignite seams lie in the upper 46 m of the formation. Despite the fact that California lignite deposits are small, the wax industry based on the solvent extraction of these lignites is an important one. The wax-from-lignite operation in California serves as an important reminder that lignites have potential uses other than combustion,

41 and that an assessment of their potential as sources of chemicals rather than as boiler fuels is overdue. 1.4.2 The Rocky Mountains (i) Colorado. Years ago lignite was obtained from underground mines in the areas around Ramah and Scranton. In some of the lignite-bearing areas east of Denver, mining started in 1887 [8]. Lignites occur about 24 km east of Denver, and continue southward into Adams, Arapahoe, and Elbert counties. Reserves of lignite accessible by strip mining are present in the Denver and Cheyenne basins [3]. The reserves remaining in the Laramie Formation are estimated at 18 Gt, and, in the Denver Formation, 9 Gt [1]. The Denver Formation may also contain up to another 900 Mt at depths greater than 300 m [1]. Large reserves of lignite occur in the Denver Formation (Paleocene) beneath the Dawson Arkose [3]. Adams, Arapahoe, Elbert and E1 Paso counties contain an estimated 900 Mt of lignite in areas with less than 60 m of overburden [3]. In the Denver Basin east and southeast of Denver there are estimated to be several Gt of lignite [120]. The Laramie Formation (Cretaceous) contains seven seams, of which No. 6 is the one most readily exploited. Strip mining potentially could occur in portions of Adams, Arapahoe, Boulder, and Weld counties, in a region roughly bounded by Eldorado Springs, Frederick, and Henderson. The coal seams in the Laramie Formation are lenticular, but may range up to 3 m in thickness [3]. The No. 6 seam contains coal which is at the lignite A / subbituminous C boundary, having a calorific value of 17.4-20 MJ/kg (as-received basis) [3]. Other properties of this coal, on an asreceived basis, are 22% moisture, 35% volatile matter, 8% ash, and 0.5% sulfur [3]. The lignite in the Denver Formation is brownish black. Like most lignites, it readily undergoes weathering, slacking, and disintegration. The moisture content is 22-40% and the calorific value is 9.3-17.4 MJ/kg [1]. Most of the coal is classified as lignite A, but some of the beds contain intervals that rank as subbituminous C. The lignite quality in the Denver Formation shows a wide range, having, on an as-received basis, 22-40% moisture, 8-30% ash, and 0.2-0.6% sulfur [3]. Lignite also occurs in the Denver Formation (Upper Cretaceous to Paleocene) [ 1]. Denver Formation lignites were deposited in swampy areas, and occur in sequences with shales, claystones, and sandstones. A useful compilation of lignite analyses from the Denver Region is available [ 121]. The overburden is composed of thick sandstones and conglomerates. Many of the lignite beds contain partings of kaolin which may range from about one cm in thickness to over 60 cm [ 1]. The seams become thinner southward in the Denver Formation, being 3-4.5 m thick in the north, and about 1.5-3 m thick in the south [3]. The multiple beds in the Denver Basin are relatively thick. The total thickness of lignite is 18 to 24 m [ 120], with the lignite beds separated by mudstones. The lignites lie at depths of 76 to 460 m [120]. (ii) Idaho. Lignites of poor quality occur in Cassia County southwest of Oakley [3]. The lower Salt Lake Formation (Tertiary) contains six seams, each 0.6 to 1.5 m thick, under less than 24 m of overburden [3]. The lignite quality, on an as-received basis, shows 20-35% moisture,

42 30- 60% ash, and 1-1.3% sulfur, with calorific values less than 11.6 MJ/kg [3]. 1.4.3 The Plains States (i) Kansas. The lignite reserves in Kansas are estimated to be 179 Mt [80]. The estimate is somewhat uncertain because the lignite beds are less than 76 cm in thickness, the minimum bed thickness accepted by the U. S. Geological Survey for estimating lignite reserves. Consequently the estimate is derived from the State Geological Survey of Kansas [ 122]. Lignite occurs in the Dakota Formation (Cretaceous) in the north-central part of the state. Lignite occurs in 19 counties. The lignite has been mined in at least 12 counties in the years 18751940. The original reserve of lignite in these 12 counties was 180 Mt [8]. The last active lignite mine was in Cloud County, and closed in 1940 [8]. The lignite was used for domestic heating and as a locomotive fuel. The lignite beds in Kansas are extremely variable in thickness. Seam thickness ranges from 5 cm to 3.7 m [8]. The sulfur content ranges from 0.48 to 6.28% [8]. The highest reported calorific value is 17 MJ/kg. (ii) Oklahoma. Lignite is found in the Purgatoire Formation in Cimmaron County. The Purgatoire Formation runs along the northwestern portion of the county, where the lignite seams are exposed in areas about 5 km apart [8]. The seam thickness is about 46 cm. The lignite has been mined on a local basis for domestic fuel. Two analyses of Oklahoma lignite indicate sulfur contents of 0.40-4).50% and calorific values of 17.6-19.8 MJ/kg [8]. 1.4.4 The Northwest (i) Washington. Small reserves of lignite occur in the Cedar Creek area of Lewis County. The reserves that could be recovered by strip mining amount to about 5 Mt [3]. The total identified resources of lignite in the Kelso-Castle Rock area are 114 Mt [1]. Some lignite occurs in the Skookumchuck Formation (late Eocene) in the Centralia-Chehalis area. Most of the coal in this formation is subbituminous C, and some is classified as subbituminous B. Most seams in the Skookumchuck Formation are 1.8-2.4 m thick, although the range of thickness runs from less than 30 cm to over 12 m [1]. The total identified resource in the Centralia-Chehalis Area (that is, including both lignite and subbituminous coal) is 3.4 Gt [ 1]. Coal occurs in the Cowlitz Formation (Eocene) and Toutle Formation (Oligocene) in the Kelso-Castle Rock Area. Most of the coal in the Cowlitz Formation is subbituminous C rank, though some lignite also occurs. All of the coal in the Toutle is lignite. Small reserves of lignites occur in the Cedar Creek No. 1 and No. 2 seams of the Toutle Formation. The seam thickness is in the range of 1.5 to 3 m [3]. The overburden thickness is 18 m, and the parting between the seams is about 3 m thick [3]. The analysis of one sample of Toutle Formation lignite is reported as 27.9% moisture, 28.0% volatile matter, 27.6% fixed carbon, 16.5% ash, 1.4% sulfur, and a calorific value of 15.7 MJ/kg, all on an as-received basis [ 1]. (ii) Alaska. The Kenai Formation in the Homer district includes about 30 coal beds that

43 range in thickness from 0.9 to 2 m, as well as additional, thinner beds [ 1]. The rank ranges from lignite through subbituminous B; most is subbituminous C. The Broad Pass coal field contains two districts, Broad Pass Station and Costello Creek. The latter contains subbituminous coal; the Broad Pass Station coal is lignite. Mine samples from the Broad Pass field show the following ranges of composition: 8.7-18.8% moisture, 32.043.4% volatile matter, 23.3--42.2% fixed carbon, 6.0--21.2% ash, 0.3-0.6% sulfur, and calorific values of 18.6-24.6 MJ/kg, all expressed on an as-received basis [1]. Alaskan lignites are generally of low sulfur content [ 123]. 1.5 O T H E R L I G N I T E S OF C A N A D A

1.5.1 Ontario Early reports, dating from the late 19th century, mention deposits of lignite and peat along the Missinaibi, Mattagami, and Abitibi rivers in the Moose River Basin, as well as some outlying deposits along the Kwataboahegan River. The lignite deposits near Coal River and at Blacksmith Rapids were considered to be of superior quality [ 134]. Ontario possesses 198 Mt of measured lignite resources [94]. The lignite deposit in the Mattagami Formation is estimated at 180 Mt [125]. The proven resources of the Onakawana field are 190 Mt [ 125]. A lignite deposit estimated to be less than 450 Mt is found in the James Bay area of northern Ontario [2]. A lignite deposit occurs in the southern Moose River Basin in the Onakawana field. It is not well studied; there is not as yet even a reliable estimate of reserves. In the Onakawana coal field the various lignite beds are fairly continuous, and are enclosed by clay. The lignite in the Onakawana field is low in ash and sulfur, but contains about 50% moisture [125]. The lignite in the Onakawana field between the Abitibi and Mattagami rivers seems to have the greatest potential for exploitation. The lignite-containing sequence includes a basal clay, a lower lignite seam of relatively constant thickness, a seam parting of clay, and an upper seam of variable thickness, overlain by clay. The average thickness of the lower seam is 4.2 m, reaching a maximum of 6 m along its southeastern limit near the Onakawana River [125]. The average thickness of the upper seam is 5.4 m [125]. The lignite consists of coalified wood, including recognizable tree trunks or branches, as well as more earthy lignite containing spores and resin inclusions. Some lignite may also occur on the east side of the Mattagami River, in the region running from the southeastern boundary of the Onakawana field toward Grand Rapids, with another potential deposit running westward from Onakawana. The Mattagami Formation (lower Cretaceous) underlies the southern part of the Moose River Basin adjacent to the Canadian Shield in northern Ontario. The lignite is relatively thick and of low sulfur content, but generally of high moisture. The age of the lignite in the Moose River Basin (Mesozoic) was established from fossil plants [126]. The Mattagami Formation is 119 meters thick [ 125]. Limited data suggest that the lignite in the southern Moose River Basin is about

44 50% moisture, 22% volatile matter and 6% ash on an as-mined basis, 0.9% sulfur (daf basis) and provides 14 MJ/kg calorific value (moist basis) [94]. The principal lignite beds in the Mattagami Formation occur on Coal River at the juncture with the Missinaibi River, near the confluence of Adam Creek with the Mattagami River, at Blacksmith Rapids on the Abitibi River near Onakawana, and at Portage Island at the confluence of the Missinaibi and Mattagami rivers. Early studies of the lignite exposures at Coal River indicated a bed 0.9 m thick [127]. laalynological studies of the lignite confirmed the early Cretaceous age, either Early Albian or Middle Albian or Aptian [128]. 1.5.2 British Columbia Measured resources of lignite in British Columbia and the Yukon are 1,670 Mt, classified as indicated resources [94]. The reserves recoverable by surface mining are 360 Mt [94]. The coal reserves in the Hat Creek deposit amount to 360 Mt [94]. The Hat Creek deposit contains large reserves of low-rank coal, both lignite and subbituminous. The lignite is variable in quality. The ranges for the proximate analyses are 20-23% moisture, 24-30% volatile matter, and 8-23% ash, on an as-mined basis [94]. Sulfur ranges from 0.5-0.8% on a daf basis. The calorific value is 11.6-22.1 M.l/kg on a moist basis. The lignite ash has high fusion temperatures [94].

1.5.3 The Canadian Arctic The existence of coal-bearing strata in the Tintina Trench has been known since the gold rush era in the late nineteenth century [ 129]. Early in the 20th century, the coal near Dawson was mined, some of the coal being used in an electric power plant. The Gates Mine worked the lignite along Coal Creek in 1898 to 1903, and again briefly in 1937 [130]. Another mine on Coal Creek supplied fuel until 1914 for a power station providing electricity for the gold dredges operating in the Klondike. Mining also occurred along Cliff Creek from 1898 to 1903. More recently, however, there has been little interest in mining the lignite because of its low rank, the complexity of the geologic structures, and the distance from potential markets. Some interest was sparked in the 1970's by the proposed construction of a natural gas pipeline from the Mackenzie Delta and Prudhoe Bay gas fields. It was thought that the lignite could be used to fire local electric power stations which would provide the necessary power for the gas pumping stations. Lignite in the Yukon and Northwest Territories exists in seams up to 9 m thick [95]; however, most of the data comes only from outcrops. Exploration of the lignite resource has so far been limited. The strata of the Eureka Sound Formation contain major deposits of lignite. This formation occurs in the Canadian Arctic Archipelago, running northward from Banks and Baffin Islands to Ellesmere Island. The Remus Basin on Ellesmere Island contains over 90 seams of coal in a section approximately 3,200 m thick [131]. The coals in this section are lignites and subbituminous and high volatile bituminous coals. Some lignite seams on Axel Heiberg Island are 7 m thick. In the southern part of the Remus Basin the lignite is the most resistant unit in the stratigraphic

45 sequence. In some places ridges are capped with lignite seams up to 25 m thick [ 131 ]. The total inferred coal resources of the Eureka Sound Formation are about 50 Gt [131]. On the basis of limited analytical data, the coal of the Eureka Sound Formation appears to be low sulfur (<1%) with highly variable ash content, although seams with less than 10% ash occur. The range of calorific values is 23-33 MJ/kg (moist, mineral-matter-free basis). Huminite or vitrinite macerals account for 70-90% of the coal [131 ]. Coal-bearing strata of Early Tertiary age (Paleocene-Eocene) occur in the Tintina Trench in southern and central Yukon Territory [130]. The coal ranges in rank from lignite to semianthracite. The seams occur near Watson Lake, Ross River, and in the region between Dawson and the border with the United States. (The very high rank coals were formed by thermal upranking by magmatic intrusions.) Some thin seams of poorly characterized coal also occur in the areas around Dawson and Watson Lake. The complex geological structure of the Tintina Trench (lateral movement along some 450 km during the Cretaceous, and possible further movement more recently than the Eocene, have been postulated), as well as the lateral variability of the coal and the restricted extent of the coalbearing strata make it difficult to evaluate the potential of the resource. The coal having some potential for mining occurs in the vicinity of Dawson and Ross River [ 130]. The only practical approach to mining the coal would be surface mining, and that approach may be made difficult by the thickness of the surface deposits. The thickest exposure of coal occurs along the east side of the Laird River, about 2 km from the confluence with the Rancheria River [ 130]. Here five seams occur, ranging in thickness from 0.4 to 2.1 m [ 130]. Several thinner beds have also been reported. The coal is of lignite A and lignite B rank. Abundant plant remains are evident. When the lignite is weathered, the annular rings of logs can be separated easily. Resin occurs in nodules of up to 1 cm maximum thickness, as well as in lenticular layers up to 2 cm thick [168]. The ash content of the lignite in the thickest seam is about 50% [ 130]. 1.5.4 Nova Scotia Based on samples from two bore holes for Nova Scotia lignites, the as-received moisture and ash contents range from 5.02 to 10.13% and 8.67 to 49.85%, respectively. As-received calorific values were 11.3 and 23.1 MJ/kg. Equilibrium moistures were 30.13 and 34.79%. The ASTM rank classification for both samples was lignite A, with corroboration by vitrinite reflectance [ 132].

46 REFERENCES

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51

Chapter 2

THE DEPOSITION AND FORMATION OF LIGNITE 2.1 I N T R O D U C T I O N This chapter treats the origin of lignite. Three major topics are discussed: first, the plant material that accumulated and eventually transformed to lignite; second, the environments in which the plant material was deposited; and third, the chemical changes occurring in the plant material leading to the formation of lignite. The third section, on coalification reactions, provides a connection to the material presented in Chapter 3, which treats in detail the organic structure of lignites at the macroscopic, microscopic and molecular levels. 2.2 PALEOFLORA Conifers predominated in the paleofloral assemblages giving rise to the lignites of the northern Great Plains [1]. Consequently, these lignites consist largely of woody material with relatively small amounts of attritus or plant debris. In contrast, lignites of the Gulf Coast region largely formed from angiosperms. Conifers were a minor component of the paleoflora of the Texas lignites. For these reasons, the discussion which follows is divided to treat separately the lignites of these two regions. 2.2.1 Northern Great Plains paleoflora The flora of the Golden Valley Formation (Paleocene-Eocene, western North Dakota) have been divided into three general associations: aquatic and marsh vegetation, lowland forest flora, and upland plants, with a fourth category of plants of uncertain classification [2]. Palynoflora of the Hagel bed (North Dakota) were dominated by gymnosperms, with fewer angiosperms and pteridophytes [3], based on samples collected at the Falkirk, Glenharold, and Center mines. Angiosperms make a much higher contribution to the Falkirk lignite than to the Center or Glenharold lignites. Center lignite contained the greatest amount of pteridophytes, which relates to its also having the greatest number of claystone-lignite contacts. Such contacts typically display high percentages of pteridophytes. Vertical variations in the three mines show similar patterns: the

Taxodiaceae-Cupressaceaegymnosperms always dominate; angiosperms usually are

more abundant near the seam margins; and ptreidophytes have greater abundance at the claystonelignite contacts.

52 Gymnosperms dominate throughout the Center mine lignite. An increasing abundance of angiosperms near the seam margins suggests their relationship with detrital material, as discussed below. Angiosperm abundance also increases near the seam margins in the Glenharold lignite. Angiosperms were more consistently a major element in Falkirk lignite relative to the Center and Glenharold lignites, whereas pteridophytes were comparatively reduced in abundance. The abundances of various palynomorphs, expressed by paleoenvironmental groups, show that fern-conifer swamp taxa dominate almost completely; in general, the palynoflora are consistent and relatively constant [3]. An increase in angiosperm pollen in lignite near claystone and siltstone layers, and angiosperm pollen in the claystones, indicates the riparian character of the angiosperms. The riparian characteristic and the association with the clastics both suggest that angiosperm pollen mainly transported into the depositional environment as detrital material. The increase of pteridophytes at the claystone-lignite contacts represents a successional change with changing environment; the pteridophytes may have been the first flora to colonize the clay surface with a change in the paleoenvironment to favor peat deposition. Such a change represents the beginning of autochthonous deposition of organic matter. Fern spore palynomorphs have been related to open swamp and lake fringing environments [4]. Palynomorphs indicative of a cypress forest - swamp environment dominate throughout the Center, Falkirk, and Glenharold lignites. Fem types such as Osmunda, indicative of swampy conditions, are also common constituents. Steriosporites antiquasporites suggest floating peat islands similar to those in the modem Okefenokee Swamp [5]. Riparian species dominate the deciduous flora. Preserved plant tissues, both macroscopic xylites and microscopically observable coalified wood, indicate deposition in a watery environment where the water provided protection from decay. This environment is common in forest swamps with abundant standing water [6]. A fluctuating variation between the conifers (Taxodiaceae--Cupressaceae)and the angiosperms (Myriceae--Betulaceae)occurred in lignites from Harding County, South Dakota and Billings County, North Dakota [7]. Water depth controlled the fluctuation between conifers and angiosperms [7]. Lignite in those portions of the Harding County seam with high concentrations of

Taxodiaceae-Cupressaceae pollen also had high amounts of anthraxylous material [8]. In comparison, high concentrations of cuticular debris and attrital resins characterized those portions of the seam with correspondingly high concentrations of Myricaceae--Betuilaceae pollen [7]. The near-surface lignites of Late Cretaceous-Early Tertiary age in Alberta derive from

Taxodiaceae (particularly bald cypress trees) [9]. Older lignites (Early Cretaceous) derive from pines of the family Podocarpaceaeas well as extinct conifers such as Cheirolepidiaceae. Resin inclusions are attributable to the dominance of conifers in the paleoflora [ 1]. Lignites near Gore, New Zealand contain remarkable resin inclusions the size of a human head [ 10]. 2.2.2 Gulf Coast paleoflora Wilcox (Texas) lignites and related sediments contain fossils representative of tropical, subtropical, and temperate flora; however, the dominant climate was subtropical to tropical [ 11].

53 The maximum climatic warming occurred during deposition of the Claiborne sediments, which overlie the Wilcox [ 12]. By the time the Jackson sediments were deposited, the climate had cooled, resulting in a shift to more temperate conditions. The presence of oak pollen in the Jackson, but not in the Wilcox, supports this concept [13,14].

Normapolle and Engelhardia pollen predominate in the Midway and Wilcox lignites and associated sediments of the Gulf Coast, along with pollen of extinct Juglandaceae [4,13]. Other distinctive pollen in these sediments include Engelhardia dilatus, Sernapollenites, and

Thomsonipollis [4]. Thomsonipollis is common throughout the Wilcox. The Wilcox lignites of Mississippi show a pollen and spore assemblage typical of lowland swamp flora occupying a subtropical coastal plain, with contributions from the temperate flora of the foothills of the Southern Appalachians. The first discovery of Nipadites (the fruit of a nipa palm) in the paleoflora of the Western Hemisphere, Nipadites burtini umbonatus Bowerbank, was in lignite from Grenada County, Mississippi [15]. In the Cretaceous McNairy Formation lignites of Kentucky, the most abundant taxa were

Cupuliferoidaepollinites and Taxodiaceaepollinites [16]. The latter were more important in Cretaceous than in Eocene lignites. The most abundant spore in the McNairy lignites was

Stereisporites, which indicates Sphagnum peat [ 16]. Angiosperm flora dominated the Claiborne lignites in Kentucky. Important, and relatively wide-ranging, flora included Momipites-Engelhardia, TricolporopoUenites, Cupuliferoipollinites, and Cupuliferoidaepollenites liblarensis [161. Calvert Bluff (Wilcox) lignites consist of woody tissue, with the woody structure still evident, some cellular material which is probably epidermal, and palynomorphs which are sparse to common [4]. Swamp tree pollen dominates the palynomorphs, the Betulaceae--Myricaceae,

Nyssa, and Thomsonipollis, consistent with this lignite having formed in a swamp between a low alluvial plain and a higher delta plain [ 17]. Sphagnum is common in some Calvert Bluff lignites. Palynomorphs indicative of a marshy environment--Calamuspollenites-Arecipites, Carex, and Liliacidites--occur at the beginning of the Calvert Bluff and then only infrequently throughout the deposit. Engelhardia are also common in the Calvert Bluff; however Betulaceae--Myricaceae pollen and Sphagnum spores dominate typical Calvert Bluff lignites. (The latter flora are also common in Alabama lignites.) The Betulaceae--Myricaceae and Sphagnum indicate a swampy environment. A high percentage of vitrite and clarite microlithotypes occurs in the lower two-thirds of the Chemard Lake Lentil lignite (Louisiana) [18]. Carya and TriporopoUenites bituitus are abundant in the associated paleofloral assemblage. Durite and clarodurite increase toward the top of the seam. The duroclarite and carbominerite macerals contain pollen of Engelhardia

and

Microreticulatosporites. The lignite was deposited in a fresh water swamp on an upper deltaic plain dominated by hardwoods. The Red Seam lignite was deposited in the same environment as the Sunrise lignite [ 18]. The composite pollen assemblage for the Sunrise and Chemard Lake lignites shows three main elements: ferns (Triplanosporite--Deltoidosporite group), conifers (Taxodium),

54

and deciduous arboreal pollen such as the hazelnut (Bemlaceae) and hickory (Juglandaceae) [ 19]. Palynomorphs recovered from the Chemard Lake lignite included 51 genera and 67 species. The palynomorph assemblage indicated an upper deltaic swamp dominated by angiosperms such as

Tricolpopollenites h i ans , Triporopollenites bituitus, Triatriopollenites arboratus, and Thomsonipollis magnificus [19]. Several genera and species of the Nomapollis group occur in older Wilcox sediments, but only one is found in the Claiborne [11]. A marked change in flora occurred between the Wilcox and Claiborne. Palynomorphs that either reach a maximum in, or are unique to, the Claiborne include Amonoa, Nymphaea, Bombapollis, Bombacacidites claibornensis, Nyssapollenites,

Enopodios, Yeguapollis, and Nuxpollenites, as well as the tropical or subtropical Burseraceae, Sapotaceae, Sapindaceae, Alangiaceae, and Bombacaceae [4,11]. Yegua lignites contain structured woody tissue, and in addition commonly contain a coarse, amorphous material. Both Engelhardia and Nyssa pollen are common to abundant; Amanoa is sparse to common [4]. These palynomorphs indicate deposition in a swampy environment.

Polypodium spores are common in Yegua and Manning lignites; ferns of this type grow in open areas of the swamp or in marshy conditions. Yegua lignites are primarily swamp or marsh lignites. (Aquatic ferns have also been noted in Tertiary lignites in the Rhine region of Germany [20].) Organic material in Manning lignites is more finely divided and more degraded to an amorphous state than in Calvert Bluff or Yegua lignites, indicating a marshy origin of the Manning lignites [4]. Pollen of Engelhardia and Nyssa are abundant, along with occasional examples of

Pollenites laesius. Throughout the Gulf Coast the upper Jackson is marked by very abundant Engelhardia. At the end of the Eocene, Engelhardia became less common and were replaced by the more abundant oak, Quercus [4]. Gulf Coast lignites do not contain Taxodium or Liquidambar (red gum) pollen in amounts or distributions characteristic of modern peats from inland swamps of levee flanks of the Mississippi delta [4]. (Taxodium is abundant in clastic Paleocene or Lower Eocene sediments of the Gulf Coast, and occurs in Calvert Bluff lignites, but mainly in association with clay partings.) Swamp-derived lignites of the Gulf Coast region show palynomorphs of both Engelhardia and

Nyssapollenites, as well as other triporate pollen from deciduous hardwoods [21]. Cypress pollen has not been found in any of the Wilcox, Claiborne, or Jackson lignites, but such pollen abounds in modern sediments of the Mississippi delta. Herbaceous plants in the marsh environment included the Carex, Arecipites, Calamuspollenites, and Liliacidites [21]. 2.3 E N V I R O N M E N T S OF DEPOSITION

This section discusses the major types of depositional environments in which plant debris accumulated and eventually transformed to lignite. The paleoflora that contributed to the major lignite deposits have been introduced; the section that follows will describe some of the chemical pathways of coalification in the conversion of accumulated plant matter to lignite. The subsections

55 below are arranged by the major types of depositional environments. There is, however, some inevitable overlap in the classification of these environments; for example, a swamp existing on a delta might be interpreted as a deltaic or a swampy environment. Furthermore, in some instances differing interpretations have been advanced for the depositional environment of a particular lignite. 2.3.1 Fluvial environments Fluvial lignite is characterized by a high percentage of woody material, low sulfur content, and palynoflora typical of a forested, fresh-water swamp [22]. For example, pyritic sulfur contents of 0.03--0.50% reported for some Wilcox, Jackson, and Yegua lignites [11] are consistent with fresh-water peats [23]. Tree stumps and trunks 6 m long have been observed in fluvial Texas lignites [24]. The commercial deposits of Texas lignite formed as backswamp peat deposits on broad floodplains [22]. Fluvial or fluvial-dominated deltaic environments are typical lignite facies of East Texas [25]. Fluvial lignite in east-central Texas is found between the Colorado and Trinity rivers in the lower Calvert Bluff Formation of the Wilcox Group (Lower Eocene). About three-fourths of the lignite resources in Texas are fluvial [25]; the commercial deposits tend to occur in areas between paleochannels. Layered sequences that result from cycles of fluvial sedimentation characterize the deposition pattern. The layers tend to be thicker at the bottom of the sequence and become thinner as one moves up the sequence. Thick beds of lignite in the Upper Wilcox (> 1.5 m) occur between fluvial channel sand belts [26]. These lignites cap crevasse splays that are 9 to 12 m thick and coarsen upward. The peat deposits accumulating in Borneo between dendritic stream courses may be a modern analogue of the East Texas Wilcox lignite depositional environments [3]. Interpretations of the deposition of the Yegua lignites suggest a wave-dominated delta [27] or a fluvial-dominated system [28]. Both interpretations suggest that the lignite deposited in interchannel positions [27], accumulating in hardwood swamps, and that the original peat accumulated in a fluvial environment on an ancient coastal deltaic plain. Thin marine deposits, glauconitic sands, marls, or muds separate the thick lignite-bearing units [27]. Compared to the lignites in the northeast, accumulation in east-central Texas was lower on the coastal plain and the deposit was consequently subjected to greater marine influence. This accounts for the higher sulfur contents of the east-central lignites compared to those of the northeast. (Other factors causing a difference in sulfur contents may have been higher temperatures resulting from deeper burial, or a slower rate of accumulation [25].) The Calvert Bluff and Yegua lignites were deposited in a fluvial-deltaic system [11,29]. The palynology of the Calvert Bluff suggests primary accumulation in hardwood swamp environments, and to a lesser extent in marshes [4,27,30-32]. The Mississippi delta system is a modem analogue for the Calvert Bluff depositional environment [27,33,34]. The relationship between swamps and marshes to channels on the delta is similar to the Calvert Bluff lignite and its associated channel facies. The A tchafalaya and Des Allemands-Barataria basins are modern analogues of the interchannel basins in the Calvert Bluff [17,27,35]. The Des Allemands-Barataria

56 Basin is part of the fluvial-deltaic complex of the Mississippi, shallow with low relief, broadening and opening to the south. The Des Allemands-Barataria Basin lies between levee and meanderbelt deposits and an older, abandoned course of the Mississippi. The basin contains a distinct zonation which includes, proceeding coastward, a cypress-gum swamp, freshwater marsh, saline marsh, and interconnected lake system. Moving toward the Gulf, the number of basins increases but their size diminishes. Modem peat deposits in the Mississippi delta are thickest and most extensive where the alluvial and delta plains meet. The Calvert Bluff lies just above the Simsboro Sand. The lignite shows no relationship to the underlying sand; however, the sand was the platform for accumulation of the lower Calvert Bluff lignites, as well as being a fresh-water conduit and aquifer

[25]. Wilcox Group lignites in the Sabine Uplift area were deposited by rivers flowing southwesterly from the Ouachita Mountains [36]. These lignites are divided into two informal units: the Upper Wilcox, an aggradational, fluvial unit, overlying the Lower Wilcox progradational, deltaic unit. Lower Wilcox lignites of Texas extend eastward into Louisiana. These lignites formed in a very large interdeltaic area which itself formed as fluvial sedimentation moved both to the east and west of the Sabine Uplift [36]. Abundant kaolinite and quartz, with minor amounts of dickite, suggest a fresh water fluvial environment of deposition for the Chemard Lake lignite (Louisiana) [19]. Taxodium and Thomsonipollis palynomorphs also indicate a fluvial environment between a lower fluvial plain and an upper delta plain. Upper Wilcox lignites are found in small floodbasins between sand belts filling fluvial channels in large alluvial floodplains, analogous to the modern Des Allemands-Barataria Basin. In the Wilcox Mount Pleasant Fluvial System on the Texas-Louisiana border the environment was one of greatly meandering fresh water streams, and peat was deposited in, or adjacent to, abandoned stream channels, along stream terraces, or between overbank deposits. South and east from this region (i.e., toward the sea) peat also accumulated in the Rockdale delta system, which was characterized by marsh distributary channels. Wilcox lignites of Mississippi were deposited in a low-lying environment near the coast [37]. During the Eocene, the embayment area was a humid, subtropical coastal plain supporting strand and lowland swamp flora [38]. Further inland, the Appalachian foothills supported a temperate, upland forest. These lignites are characterized by small deposits of erratic seam thickness, and by multiple seams. The seams, generally of irregular shape, range from long, narrow fluvial deposits to more elliptical deposits typical of a deltaic origin. The seams may be up to 3.7 m thick, but at the margins of the deposits the thickness may be only a few centimeters [37]. Mississippi lignites deposited in an area created by southward movement of the Mississippi Embayment toward the Gulf of Mexico. Many features of the coastal plain--swamps and marshes, estuaries, landlocked bays, and lagoons--provided basins for accumulation of plant remains. These lignites occur as small, stacked deposits in contrast to large, blanket-like seams. The configurations of the ancient swamps, bays, and other features of the coastal plain determined the shapes of Mississippi Wilcox lignite deposits.

57 The Golden Valley Formation (Early Tertiary) in North Dakota deposited in a fluvial environment [2]. The existence of sandstone lenses typical of channel facies, interbedded with deposits typical of backswamp or overbank conditions, provides the basis for this deduction [2]. The Fort Union Group in southwestern North Dakota consists of deltaic (the Slope and Ludlow Formations) and marine (the Cannonball Formation) deposits overlain by alluvial plain deposits (the Bullion Creek and Sentinel Butte Formations). The lower Sentinel Butte in the Knife River area of west central North Dakota was deposited in swamp and fluvio-lacustrine conditions [39]. The lignite was deposited in the swamp environments and the elastic sediments in the fluviolacustrine conditions. These two systems alternately dominated an alluvial plain that drained to the east and southeast toward the Cannonball Sea. The continuity of the lignite suggests that similar environmental conditions prevailed across wide areas of the Williston Basin during the time of deposition. The thickness of the lignites shows that the environmental conditions prevailed for long periods of time. The times required for deposition were about 4,500 years for lignite 1 m thick to 14,000 years for 3 m thick seams [39]. An alluvial flood plain depositional model has been proposed to fit the Tongue River and Sentinel Butte [40]. The sand beds represent channel fills or deposits formed by migration of the channels. Sandy and clayey silt beds deposited as natural stream levees, while other clays and silts deposited in back basin areas of the flood plain. Because the lignite beds in the Bullion Creek and Sentinel Butte are relatively thick and laterally continuous for long distances, there must have been lengthy periods of fairly uniform conditions in the late Paleocene in North Dakota. The environment of deposition was similar to the modern Mississippi River delta [41]. Tongue River lignites were deposited by eastward-flowing, sluggish streams draining a source area to the west [42]. A stable fluvial system allowed development of protected back-swamps. As this depositional episode ended, elevation of the source area was reduced to the extent that the rate of subsidence exceeded the rate of sedimentation. Swampy conditions then became widespread throughout western North Dakota. Here the lignite beds are thin without extensive lateral persistence. The proximity of the lignite to other sediments of clearly fluvial origin strongly indicates that the lignites were deposited in fluvial conditions. These lignites are generally high in moisture and ash. In central and eastern Montana, and adjoining areas of the Dakotas and Wyoming, a broad, low-lying coastal plain existed during the Cretaceous and Early Tertiary. Extensive peat swamps formed on the plain. The peat beds were subjected to recurrent burial by sediments spreading out from the uplifting of the Rocky Mountains. Continual repetition of this process resulted in numerous buried beds of peat that eventually became beds of lignite. For example, near Roundup twenty-six beds of coal have been identified [43]. Moose River Basin (Ontario) lignite was deposited in a floodplain environment [44]. Freshwater alluvial floodplain deposition of this lignite is reflected in its syngenetic mineral content; the primary clays are kaolinite and illite, and pyrite is less common than in coals deposited in marine or marine-influenced environments [45]. The Ravenscrag Formation in Saskatchewan is equivalent of the upper Fort Union in North

58 Dakota [46]. The lignite was deposited in an alluvial plain-fluvial environment [46]. This lignite shows significant banding, which, together with the variations in petrography, suggest the accumulation of the precursor peat in a forest moor environment. Water level variations in the swamp contributed to a diverse sequence of plant communities, which in turn resulted in a diversity in the peat. Periodic low water levels may expose the peat to air, with oxidation forming inertinite. At other times, deep water levels contributed to the accumulation of attrital material. During periods in which the water level was relatively stable, preservation of woody plant parts was favored. These combined effects lead to formation of a banded lignite seam with a variety of macerals [46]. Coal of the Tintina Trench in the Canadian Arctic was deposited in fluvial and lacustrine environments [47]. No influence of marine sources is apparent. Sub-environments included lakes, alluvial fans, fan deltas, braided and meandering streams, and associated flood plain environments [47]. Variations in the sedimentary loading, and particularly variations in the geothermal gradient, complicated the coalification processes. The geothermal gradient was abnormally high during much of the burial period, a result of the emplacement of shallow magmatic intrusions. Coals of the Tintina Trench display the highest rank gradients--0.1 to 0.2% Ro/100 m - - o f any coals of western Canada [47]. A further complication of the stratigraphic pattern is strike-slip faulting, which has juxtaposed areas of different thermal histories. The Alaskan lignites and subbituminous coals in the Beluga, Nenana, and Yentra coal fields formed in various environments, including alluvial plains of non-marine continental-fluvial systems, and forest moor backswamp environments on valley flats [48]. Peats formed mainly from tree vegetation. As in the case of the Saskatchewan lignites [46], drier conditions led to coals with higher amounts of inertinites [48]. 2.3.2 Deltaic environments Upper deltaic plain coals were deposited in swamps, and formed mainly from the arborescent plants growing in the region [49]. As a result, large amounts of massive vitrinite formed, with lesser amounts of cutinite, inertinite, resinite, and sporinite. In comparison, vegetation characterizing bay or estuarine facies is generally less arborescent [49]. Abundant amounts of amorphinite, as well as framboidal pyrite, reflect a significant contribution of microbial action to degradation of the plant remains. Deltaic lignite probably originated in marshes. Although deltaic lignite is non-woody (particularly in comparison with fluvial lignite), woody material occurs with some deltaic lignites. The Alcoa seam (Milam County, Texas) palynology suggests an alternation of flesh-water marsh and hardwood swamp environments. Deltaic Wilcox lignites are characterized by low ash, moderate sulfur content, and a wide lateral extent, consistent with original deposition as a blanket peat, that is, peat which formed on inactive delta lobes [50]. Wave-dominated deltaic facies are characteristic of the Eocene lignites of South Texas [25,51]. A modern analogue is found in Malaysia, in the delta of the Klang and Langat rivers.

59 Deltaic lignite is found in the Wilcox in Central Texas and in the Yegua and Manning formations in the southeast. Three types of sedimentation patterns have been identified in deltaic lignite: sedimentation alternating between distributary channels and interchannel areas, repetitive delta front sequences, and meanderbelt deposits [22]. Commercially important deltaic lignites occur with interdistributary deposits in delta plains. East Texas deltaic lignite occurs in the Jackson Group between the Colorado and Angelina Rivers [22]. Jackson Group lignite in Southeast Texas also accumulated in deltaic environments, originating from blanket peats that spread across abandoned distributary channels in lower delta plain environments [27]. Lignite in the Manning and upper Wellborn Formations accumulated in either swamps or marshes [4,52]. Laterally extensive and relatively thick lignites derive from blanket peats originating in a variety of inactive environments from distributory channel fills to bay or lake fills [25]. Lignites that are more restricted areally may have accumulated in interdistributary basins while deltation was active [25]. 2.3.3 Lagoonal environments Lagoonal lignite is characterized by high sulfur contents and high ash values. These characteristics suggest an origin in a salt marsh that experienced frequent introduction of clastics [22]. Deposits of lagoonal lignite in Texas occur mainly in the Wilcox and Jackson Groups and Yegua Formation in the south, and in the Wilcox in East Texas. The sedimentation pattern coarsens upward; lignites are associated with lagoonal muds. Lagoonal lignite occurs in South Texas in the Jackson Group south of the Atascosa River [25]. Lignite occurrences are elongate, extending in a belt roughly 16 to 24 km wide and 200 km long [25]. South of the Atascosa River, Jackson lignite is strandplain or lagoonal [25]. Modem analogues of this system occur in western Mexico, along the Nayarit coast. There the peats are accumulating in a strandplain-lagoonal system in which peat is being laid down on top of a strandplain-barrier bar clastic sediment [53]. The resulting stratigraphic sequence of clastics topped by peat is similar to sequences that can be recognized in the lower Jackson [25]. 2.3.4 Backswamp environments Martin Lake (Texas) lignite was deposited in an arboreal (Nyssa) swamp, as indicated by high ulminite and terrestrially derived exinites [54]. Areas in the Martin Lake seam that show high inertinite contents indicate regions where the peat swamp was drying. Darco (Texas) lignite formed from backswamp peats on flood plains that were separated by meander belts [22]. The depositional environment was a forested, fresh-water swamp, inferred on the basis of a low sulfur content [22], woody appearance, and accumulated palynoflora [55]. Chemard Lake lignite is the product of upper deltaic arborescent flora that accumulated in a fresh-water swamp [56]. The forest vegetation is indicated by bright, banded, and banded-bright lithotypes. Clarains are the dominant lithotypes with clarite-V and vitrite the dominant

60 microlithotypes [19]. These lithotypes contain significant amounts of pollen of Carya and

Triporopollenites bituitus. Dull macerals such as duroclarite and carbominerite occur in association with the floral assemblage containing higher amounts of Engelhardia and Microreticulatosporites [19]. The vitrinite (60.8% average, moisture-and-ash-free (maf) basis) and inertinite (12.9% average, maf basis) contents also indicate an origin from woody vegetation. This lignite has a low content of pyrite, an indication of deposition in a fresh-water environment. Some clarite bands contain high pyrite, indicative of a marine transgression that covered parts of the peat [57]. Chemard Lake lignite palynology suggests an upper deltaic interdistributary environment [57]. The lignite is dominated by arborescent paleovegetation that accumulated in a fresh-water swamp on the upper deltaic plain [ 19]; the lignite contains minor amounts of hardwood pollen grains. Lignite in the Knife River Basin of North Dakota formed in swamps in the vicinity of the Cannonball Sea, which was in retreat during the late Paleocene [41]. The environment of deposition of the Hagel bed, inferred from palynomorphs, was a laterally extensive, aborescentdominated swamp forest characterized by cypress trees and associated herbaceous ferns and mosses, with a minor deciduous flora [58]. The dominance of Taxodiaceae--Cupressaceae pollen indicates the swampy terrain with abundant taxodium trees. Abundant xylitic material in the lignite indicates that plant remains accumulated under standing water, and that the surface was covered by standing water for most of the year. This environment was similar floristically and physically to the present Okenfenokee Swamp [59]. The Okefenokee currently has an area of 1700 km2 depositing a thick, continuous layer of peat [60]. The Beulah-Zap (North Dakota) depositional swamp was about 1050 km2 [61]. Evidence for lacustrine deposition of the Beulah-Zap lignite includes fine-grained (clay and silt size) sediments in finely laminated underclays and partings within the lignite [62]. The fine grained, finely laminated partings represent transgression-regression sequences in a marsh-lake system. No evidence of a meandering fluvial system, such as sand deposits, peat erosion, or channel structures, exists. The rate of peat deposition in the Okefenokee is about 1 cm per 20 years [63]. The compaction ratio of peat to soft brown coal is about 2 [6]. These data applied to the Hagel seam suggest a rate of deposition of approximately 4000 yr/m [58], which is very rapid on a geological time scale. The alternation of lignite and clastic deposition also took place rapidly, probably controlled by shifting of associated lacustrine and fluvial depositional environments. Pollen grains from Falkirk lignite indicate a more upland flora and are, in part, detritus. The Falkirk also has the coarsest grained clastics of three mines (Falkirk, Center, and Glenharold) at which the Hagel was studied. Lignite of the Falkirk mine was more closely associated with fluvial environments, possibly the system that supplied water to the taxodium swamp and lake environments of the Center and Glenharold mines [58]. The Center and Glenharold mines were more likely associated with lacustrine deposition [3]. The lower sodium content of the Falkirk relative to the Center and Glenharold lignites may be due to a higher mobility of the sodium ion in the hydrologically more dynamic fluvial system [58].

61 In Montana, the Tongue River Member of the Fort Union Formation was deposited by slow-moving streams on a low gradient. Sediments were transported eastward in suspension. Near the close of the depositional event, the altitude of the sediment source to the west was reduced. As a result, the rate of subsidence exceeded that of deposition, and a swamp was created. The coal beds were deposited in this backswamp [64]. 2.3.5 Marsh environments Texas lignites in the Manning Formation (Jackson Group) accumulated in marshes on the lower delta plain of a fluvial-deltaic environment [35]. These lignites derive from blanket peats that accumulated on foundering lobes of the delta and spread into inactive environments such as lake or bay fills and distributary channels [28]. Lignites of the lower seam at the Big Brown mine and the lower and middle portions of the Sandow mine were deposited in reed marsh complexes, as indicated by high humodetrinite and liptodetrinite contents [54]. Lignite from the San Miguel mine also deposited in a reed marsh. Gelification is more pronounced than in the Big Brown or Sandow lignites, and there is greater evidence of bacterial activity, suggesting that the conditions in the marsh that produced the San Miguel lignite were at various times both oxic and anoxic [54]. Deposition of some organic material in regions of subaquatic bacterial activity is indicated by the presence of dinoflagellates and sapropelinites. The high gelinite content indicates an oxic environment, which may have derived from aqueous oxidation of peat. The presence of eugelinite indicates that the water was at times brackish. Martin Lake lignite formed in fresh or slightly brackish water. The lignite has a low sulfur content, and a sulfur isotope ratio of +9.5 [65], indicative of a freshwater or brackish environment. The coarsening-upward sequence in the overburden contains pyrite nodules and siderite bands and concretions, indicative of highly reducing, anoxic, non-sulfidic conditions in the bottom sediments, and further indicating that the pore fluid in the sediments was not sea water [66,67]. Coexistence of siderite and pyrite indicates brackish water deposition. North Dakota lignites have been suggested to have been deposited from peat bogs similar to the modern swamps of Wisconsin and Michigan [68]. Beulah-Zap lignite was deposited in a marshdominated lacustrine system [62]. The depositional basin had extremely low relief. The rates of peat accumulation and basin subsidence were in equilibrium, resulting in a thick and laterally continuous lignite free of carbonaceous shales. Beulah-Zap deposition started in moderately deep water. Vegetation growth was halted twice by major increases in the water level that also resulted in deposition of clay and silt in the southern portion of the basin. Deposition ended with a major drying episode, followed by extensive flooding that terminated peat accumulation throughout the depositional basin. The sequence of depositional conditions was deduced from the distribution and abundance of the lithotypes, using the technique of seam formation analysis [62]. The lowest seam formed in deep water, with a gradually, but steadily, shallowing water level. Deposition of the middle seam began in relatively shallow water. The water level then increased throughout the depositional basin, so that the middle portion of the middle seam formed in moderate to deep

62 water. As deposition continued, a sequence of deepening followed by shallowing occurred. The uppermost Beulah-Zap seam fluctuates in thickness and is not laterally extensive. The depositional environment was unstable, with the majority of deposition occurring in moderate to shallow water with an overall shallowing trend preceding the end of deposition. The most important coalification process in the Beulah-Zap lignite was humification, as indicated by the dominance of huminite macerals in the lignite [62]. Gelification was important in horizons overlying inorganic-rich zones or carbonaceous shales. Fusinitization was a minor process occurring mainly at the end of deposition as the shallowing water allowed periodic subaerial exposure. Good preservation of fusinite macerals suggests that more fusinitization occurred by fire than by subaerial exposure. 2.3.6 Lacustrine environments In a lacustrine setting, marshes and swamps will form along the shores of the lake [49]. Vitrinite will form in abundance from the woody parts of plants. Alginite will occur to a lesser extent than in lignites deposited at the depocenter, because algae would be the dominant source of plant material only toward the center of the lake. In lignites formed near the depocenter, alginite will be more dominant, along with amorphinite, the degradation product of alginite [49]. The Chemard Lake lignite paleoflora have been interpreted as representing peat accumulation in fresh water lakes of an upper deltaic environment [19]. The fresh water environment is indicated by spores of fresh water algae, Schizosporis texus and Schizosporis

parvus. Other vegetation indicative of a lake or swamp environment included algae, fungi, ferns, Sphagnum, and hardwood trees such as Carya and Engelhardia [ 19]. Similar assemblages have been reported for the Mount Pleasant and Rockdale Delta systems of Texas [31]. Infilling of the swamp is demonstrated by the vertical distribution of palynomorphs. Algal spores are dominant near the base of the seam. In the middle of the seam the spores of ferns and mosses are abundant. Hardwood tree pollen becomes more abundant near the top of the seam, the development of deeply rooted vegetation being consistent with a reduction of surface water. Taxodium is rare in Chemard Lake lignite. Evidence for deposition in an ancient oxbow lake exists for Claiborne Group lignite in Tennessee [4]. Arboreal pollen is common at the base of the lignite. Herbaceous pollen becomes increasingly abundant upward. As the source of clastic sediments to the lake was cut off, the lake filled with organic debris from the surrounding area, which was wooded. However, herbaceous plants became established around the lake margins and gradually came to dominate the flora as the lake filled with organic debris. Similarly, lignites in the Jackson Purchase region of Kentucky may have formed in abandoned oxbows of Eocene rivers [ 16]. Large changes in peat thickness and paleofloral content over relatively short distances would occur in such situations, and produce significant variations in petrographic and palynological composition of the lignite.

63 2.3.7 Summary Figure 2.1, adapted from [69], summarizes some of the principal features of paleoflora, environments and depositional settings, and the resulting microscopic features of the lignites.

Paleoflora

Sequoia

Environment

Setting

Microscopic features

Lithotype

NyssaTaxodium

Swampmarsh complex

Marsh

Aquatic

Upper delta plain

Junction of upper and lower delta plain or lower delta plain

Lakes on delta plain

Mixed humotelinite (ulminite), Abundant humodetrinhumotelinite ite, lipto(mainly uldetrinite, minite), sporinite, sporinite, cutinite, resinite, and alginite, and suberinite resinite

Mainly humodetrinite and liptodetrinite with minor humotelinite alginite

Mainly humocollinite, humodetrinite, sapropelite and major alginite

Moderate to finely banded xylitic to non-xylitic coal

Unbanded non-xylitic to unbanded detrital coal

Unbanded, tough non-xylitic or detrital coal

Swamp

Alluvial plain backswamp or swamp at junction of lower alluvial plain and upper delta plain

Mainly humotelinite with textinite A, suberinite, resinite

Moderate to highly banded xylitic coal

Nyssa BetalaceaeMyricaceae flotant

Botyrococcus Schizoporis, Pediastrum Engelhardtia Degraded stem, Arecipites bark tissues, Liliacidites marsh plants

Moderately banded to unbanded non-xylitic coal

Fig. 2.1. Summary of relationships among paleoflora, depositional environment and setting, microscopic and macroscopic features of resulting coal. Adapted from [69].

2.4 TRANSFORMATION OF PLANT MATERIAL TO LIGNITE 2.4.1 Introduction Peat represents the first stage of coal formation. At some point a change in the environment results in the end of peat formation and burial of the existing peat with clay, sand, or other sediments. This point represents the transition from diagenesis, or the biochemical phase of coal

64 formation, to catagenesis, or the geochemical phase. A typical peat contains 80--90% moisture and, on a dry basis, 50--60% volatile matter and 40-50% carbon. The transition from peat to lignite is marked by reduction in moisture content and volatile matter and by an increase in carbon content. North Dakota lignite contains about 35% moisture and, on a dry basis, 45% volatile matter and 70% carbon. These changes are accompanied by an increased calorific value, from 16-21 MJ/kg (dry basis) for peat up to 28 MJ/kg (dry) for North Dakota lignite [ 18]. Pollen metamorphism in lignites suggests that temperature is the primary factor in coalification, and that pressure may retard coalification [73]. At slow heating rates lignites begin to evolve volatiles at temperatures as low as 200~ [74], indicating that lignites could not have been exposed to temperatures higher than this during coalification. Hydrocarbons evolved from lignites on controlled pyrolysis reflect the geochemical origin of the coal. Sapropelic lignites from the Wilcox, Claiborne, and Jackson Groups have a bimodal nalkane distribution with domination by n-alkanes >C 25, whereas humic lignites show n-alkanes
65 gelification, indicating that gelification progresses from the outer part of the log toward the interior. Cellulose, mineral matter filling cell cavities, or other cell inclusions all retard the development of homogeneity, thus impeding gelification. At least three stages of coalification occur as wood is transformed to bituminous coal [70,71]. In the first stage, cellulose is hydrolyzed and lost from the wood, whereas the more resistant lignin is retained and concentrated. Lignin residues subsequently change to lignitic coalified wood in the second stage. During this stage, the oxygen content remains relatively constant while the hydrogen content drops. A compensating increase in the carbon content accompanies the reduction in hydrogen. The principal chemical changes in the second stage are loss of water and methoxyl groups, and loss of C3 side chains from aromatic rings in lignin. The third stage represents an upranking of the coalified wood beyond the lignite rank to subbituminous and bituminous ranks. Coalification of organic matter to form lignites is principally a loss of functional groups and condensation of small molecular units into larger ones [79]. The aromatic portion of lignite derives from the lignin of vascular plants [71]. 2.4.2 Loss of cellulose The first stage of microbial degradation of wood is the loss of cellulose; the lignin-derived structures are preserved during this stage [72,80-82]. Cellulose is inferred not to exist in lignite [83]. Solid state nuclear magnetic resonance (NMR) and pyrolysis/gas chromatography/mass spectrometry (Py-GC-MS) both show that peat and degraded wood have lost most of the cellulosic material but have retained lignin fairly intact [84]. Thus cellulose has, at best, a minor role in coalification. NMR and infrared (IR) spectroscopy of wood samples buried for various lengths of time, including some as old as Eocene age, show that lignin was preserved while cellulose was degraded and lost [85]. Birefringence of huminitic materials, an indicator of the presence of crystalline cellulose, drops rapidly during peatification and the early stages of coalification [86]. Buffed logs coalified to lignite rank have lost essentially all the cellulosic components [78]. Cellulose is not present to an appreciable extent in coalified xylem tissue, even of brown coal rank. Cellulose or thermally altered cellulose produces m-hydroxybenzoic acid (1), a compound not produced from the oxidation of substances derived from lignin, upon alkaline copper(II) oxide oxidation. Wyoming lignite and Soya (Japanese) lignite gave conversions of 43-47% to products soluble in organic solvents; m-hydroxybenzoic acid was not found [87]. The products did include p-hydroxybenzoic acid (2), vanillic acid (3), and various hydroxybenzene polycarboxylic acids and benzenepolycarboxylic acids characteristic of the oxidation products of softwood lignin. Cellulose has been isolated from early Tertiary lignites; however, this isolated cellulose is about one-tenth the molecular size of modern cellulose [76]. An argument of the importance of cellulose as a precursor to oxygen functional groups has been made for New Zealand lignites, on the basis of 180/160 isotope ratio measurements [88]. These lignites are considerably younger (Upper Eocene) than most North American lignites. The isotope ratios for lignite components were compared with compounds extracted from peat and modern wood.

66 COOH

COOH

~ , O H

COOH

(

OCH3 OH

1

2

OH

3

Sporopollenin reacts at 1500C in the presence of clays to produce a synthetic sporinite that has a 13C NMR spectrum similar to that of sporinite from North Dakota lignite [89]. NMR spectra show a reduction in aliphatic C-O groups with a simultaneous increase in unsaturated carbon (at 110-155 ppm). Structural similarities between the synthetic and the authentic sporinite, as well as similar behavior of the two materials in oxidation and pyrolysis reactions, show that carbohydrates and other easily hydrolyzable material in the original spores was eliminated during diagenesis. Microbial degradation was the major pathway for the elimination of these geochemically labile materials; chemical reactions played a minor role. 2.4.3 Structural evolution of lignin residues Huminitic materials derived from cell walls are predominately modified lignin with essentially no contribution from carbohydrates [90]. The characteristic thermal behavior of lignin is present in thermograms of lignite, but is much less pronounced in those of bituminous coals and is absent from anthracite thermograms [91], suggesting that the structure of lignite resembles that of lignin. The lignified fiber-tracheid and fiber cell walls vitrinize in the early stages of coalification [92]. After loss of the cellulosic components, significant structural changes must then ensue as the preserved lignin eventually coalifies to lignite. These changes can be inferred, at least broadly, by recognizing that the atomic H/C ratio drops from 1.2 in lignin to 0.8 in lignite, while the corresponding change in the atomic O/C ratio is 0.45 to 0.19 [ 15]. Thus lignin is not preserved intact; rather, loss of methoxyl and some C3 side chains occurs [85]. IR analyses of samples beginning with lignin and proceeding through the various ranks of coal have shown a consistent disappearance of bands in the region of 1265-1290 cm-1 [83]. The bands in this region are from the principal oxygen-containing structures in lignin. Xylem tissue still retains large amounts of lignin on coalification to brown coal rank, although modification of the lignin structures begins to occur. Lignin becomes converted to humic acids while cellulose disappears [93]. Py-GC-MS of lignitic wood shows the presence of guaiacol (4), 4-methylguaiacol (5), and

trans-isoeugenol (6), which are lignin-derived and which are also observed in modem buried woods [84]. Pyrolysis-GC of coalified logs [94] also produces methoxyphenols, such as 4vinylguaiacol (7), in addition to the compounds previously named [84]. Eugenol (8) and isoeugenol in lignitic wood indicate that lignin has survived the coalification process reasonably

67

]~Y OH '-

OH

OH OCH

jOCH3

CH : C H C H 3

CH 3

OH

y

OCH

OH CH3

CH :CH 2

<

OCH3

C H 2C H --CH 2

intact, and in particular that the characteristic propyl side chain has been preserved. Py-GC-MS of lignite collected from a clay pit of Pleistocene age (Long Island, New York) showed as major products 4, 5, 7, 4-ethylguaiacol (9), 8, cis-isoeugenol, t6, vanillin (10), and acetoguaiacone (11) [95]. These compounds derive from lignin or partially altered lignin. Eugenol and isoeugenols indicate that some lignin exists virtually unaltered in lignitic wood [84]. In particular, their presence suggests that the characteristic propyl side chain of lignin has been preserved [84]. In the Long Island clay pit lignite, the lignin is very little altered from lignin in modern wood. Compounds derived from extensively altered lignin are minor components of the total pyrolysis products. In comparison, the dominant products from a lignitic coalified wood from the Upper Cretaceous Magothy Formation (Bethel, Maryland) were phenols and cresols, rather than the methoxyphenols obtained from the Long Island lignite [95]. This difference suggests a more extensive alteration of the lignin in the Magothy lignite. Phenol, cresols, and dimethylphenols are more significant in the GC-MS traces for wood coalified to lignite rank than for brown coal xylite or degraded modern woods. Methoxyphenols account for about half the total phenols in the pyrolysis products. However, simple phenols such as phenol itself, the cresols, and the dimethylphenols are also observed. The presence of these compounds indicates that coalification eventually leads to the production of phenolic structures from the methoxyphenols [84]. Phenols and catechols amount to about 25% of the Py-GC-MS products from lignites, but comprise only about 5% of the products from degraded modern woods [15]. Phenol, cresols, and dimethylphenols identified in the F'y-GC of coalified wood are products of coalification and arise from demethylation of methoxyl groups in the original lignin. Coalification leads to the production

68 OH

OH

OH

CHECH 3

CHO

COCH 3

9

10

11

of phenolic structures that derive from the methoxyphenols (e.g., the formation of catechol-like structures from guaiacyl units). Wood coalified to subbituminous rank shows no methoxyphenols at all in the reaction products. Lignin structural changes involve reduction of the number of methoxyl groups per ring by about half, with consequent diminution of lignin pyrolysis products relative to the total pyrolysis products. The reactions represent decomposition of lignin structures by loss of methoxyl and C3 side chain groups and loss of water [96]. The concentrations of methoxyl, carboxyl, and carbonyl groups decrease as carbon content increases, even in brown coals [96]. However, hydroxyl decreases sharply at the onset of brown coal formation and then remains relatively constant until the bituminous coals form. NMR of coalified gymnosperm wood shows a progressive loss of methoxyl carbon from lignin to brown coal xylite to wood coalified to lignite rank [15,84]. The 56 ppm peak (methoxyl carbon) is small relative to that at 146 ppm (aryl carbon bonded to oxygen), indicating that methoxylated phenols are minor compared to other types of phenols. The characteristic NMR peak at 146 ppm indicates adjacent aryl oxygen structures, as in catechol or methoxyphenols. The peak ratios for the 146 ppm peak and the total aromatic carbon (100-160 ppm) show about two aryl-O carbons per aromatic ring. In this regard the lignite spectra resemble those from modem degraded wood. Thus the proportion of aromatic carbons that have an attached oxygen atom (two aryl carbon-oxygen bonds per aromatic ring, i.e., about one-third of the total aromatic carbon) does not change in conversion of degraded wood to lignite, even though methoxyl carbon does decrease [84]. Loss of methoxyl carbon but retention of aryl oxygen indicates demethylation reactions, rather than demethoxylation [84]. The aromatic structures that result from demethylation are like catechols, linked by aryl ethers as in lignin. Loss of CH3- rather than C H 3 0 - forms the catechollike structures in the lignite. 2.4.4 Evolution of the carbon skeleton In lignites the carbon structure experiences aromatization (e.g., of terpenoids), loss of functional groups, and increasing condensation as coalification proceeds [79]. Benzenoid hydrocarbon structures may derive from aromatic amino acids such as phenylalanine (12) and tyrosine (13), known to be present in recent sediments; other sources of benzenoid hydrocarbons

69

~

-'-CH2~HCOOH

HO~CHz~HCOOH NH2

NH 2

12

13

may be among the families of coumarins, flavones, and anthrocyanins [76]. These compounds thermally degrade to aromatic hydrocarbons which are still reactive under the prevailing thermal conditions and may then polymerize. Pyrolysis/mass spectrometry of lignites shows that retene (14) is a characteristic biomarker for the Northern Great Plains lignites, and an unidentified compound having mass/charge ratio of 194 is characteristic of the Gulf province lignites [97].

14

In both lignin and decomposed modem wood half of the aromatic carbons are protonated (that is, three of the six carbons in an aromatic ring), but in lignite the fraction of protonated aromatic carbons drops to 0.37, suggesting that in the lignite only two of the six aromatic carbons are protonated [98]. With no change in the extent of oxygen substitution during the conversion of lignin to lignite, the reduction in the number of Car-H bonds can only be effected by increasing the amount of Car-C bonds. NMR evidence also indicates that further ring substitution at the C-2, C5, or C-6 positions accompanies the demethylation [15]. Increased ring substitution arises from condensation of guaiacyl units. The position of the aryl carbon-oxygen peak remains nearly constant at 146-148 ppm [15], indicating that the two oxygens on the ring remain adjacent throughout demethylation and condensation. The 1515 cm-1 band in the IR spectra of lignin, wood, peat, and brown coal is weak in a lignite spectrum and absent from the spectra of higher rank coals [99]. This band is also absent from the spectra of polycyclic aromatic hydrocarbons and coal tar pitches. The increasing condensation of the single aromatic rings typical of the lignin structure into polycyclic aromatic systems in the transition from brown coal to lignite and then to higher rank coals can be followed by infrared spectral changes. In early catagenesis, lignin-like structures experience an increase in crosslinking and condensation of smaller molecular fragments into larger ones [100]. The reactions that occur include dealkylation [100], demethylation [72,80,94,100], demethoxylation [80,100], oxidation [100], ring cleavage [100], and condensation [72,100]. These processes produce an aromatic-rich material as the major portion of

70 the macromolecular structure of lignite. Alkylation of aromatic rings, following cleavage of the 13-O4 aryl ether linkage in the lignin structure (the numbering scheme is shown as structure 15), may be more important than ring condensation to form naphthalenic moieties [98].

C~C--C

OCH 3 15

O

I

2.4.5 Transformations of oxygen functional groups After lignin in wood first loses methoxyl groups it then loses aryl ether and phenolic groups [15]. In macerals derived from lignin-like structures, ether bridges may be lost during coalification by elimination of carbon monoxide [ 100]. CO elimination results in the loss of oxygen from the bridge, leaving o n l y - C H 2 - groups in methylene bridges. In liptinite macerals the methylene bridges are present in the original organic matter as it accumulated. The single, homogeneous deposit in the Mahakam delta, Kalimantan, Indonesia contains all of the coalification stages from modern plant material through peat and lignite to bituminous coal. The transformations of oxygen functional groups have been studied by IR and 13C NMR [102]. Up to a vitrinite reflectance of 0.20% (an atomic O/C ratio of about 0.50) the principal mode of decrease of oxygen is dehydroxylation. For Ro in the range of 0.20-0.60% (corresponding O/C 0.504). 12) loss of oxygen primarily occurs via decarboxylation. The major product of alkaline silver oxide oxidation of lignin methylated with dimethyl-d6 sulfate is 3-methoxy-4-methoxy-da-benzoic acid (15), whereas oxidation of methylated Beulah lignite produces large amounts of the 3,4-dimethoxy-d6-benzoic acid (16) [57]. Phenol polycarboxylic acids and benzene polycarboxylic acids produced in significant amounts from lignite oxidation are found in low to negligible concentrations among the oxidation products of lignin. The phenolic structural framework of lignin is altered early in the coalification process. Dehydroxylation of phenolic structures in lignin to benzene, naphthalene, or tetralin structures in lignite may occur by hydrogen transfer between the lignin and terpenoids, catalyzed by clay minerals. For example, the reaction of poly(p-hydroxystyrene) with d-limonene (17) in the presence of montmorillonite produces alkyl benzenes, alkyl phenols, and p-cymene (18) as products [57]. Dehydration also occurs during coalification of peat through brown coal to lignite. Consideration of coals and coal precursors as a C-H-O ternary system allows the plotting of reaction trajectories on an appropriate ternary diagram [103]. Extrapolation of reaction trajectories

71 COOH

COOH

CH 3

CH 3

) OCH 3

OCD3

OCD 3

OCD 3

/C~ H3C

16

17

18

CH 2

/ CH x H3C CH 3

19

connecting peat, brown coal (Morwell, Victoria, Australia), and lignite passes almost directly through the point representing water on the hydrogen-oxygen axis, indicating that dehydration has made a significant contribution to the coalification process. Thermal degradation of sporopollenin in the presence of clays proceeds through a variety of reaction pathways, including dehydration, ether cleavage, aromatization and condensation. Oxygen is lost by dehydration, since the decrease in aliphatic C-O is accompanied by an increase in unsaturated carbon [89]. 2.4.6 Sulfur In many bituminous coals, pyrite is the dominant form of sulfur, with organic sulfur being the second largest proportion and sulfatic sulfur a minor constituent. In peat and in some lignites, organic sulfur dominates form and pyrite is minor. Potential sources of organic sulfur include sulfur-containing amino acids (in proteins) such as cysteine (20), cystine (21), glutathione (22), and thionine (23); and sulfate esters of polysaccharides. The change of the relative importance in the forms of sulfur with increasing rank suggests that pyritic sulfur increases at the expense of the organic or sulfate sulfur, or both. Formation of pyrite depends on the existence of organic sulfur in the Everglades peats [ 104], some samples of which approach lignite in terms of composition and heating value on a moisture-free basis. Much pyrite in sediments in the early diagenetic stage is the so-called framboidal pyrite, frequently found in the remains of plant tissues, growing, for example, in cell cavities. Formation of the framboidal pyrite relates to release of hydrogen sulfide from the tissues; the hydrogen sulfide in turn derives from the organic sulfur. Reduction of organic sulfates (e.g. sulfate esters of polysaccharides) by bacteria such as

Desulfovibrio generates hydrosulfide ions or organic sulfides. These species react with iron(II) ions to form pyrite. Iron(II) sulfide has a concentration two to three orders of magnitude less than that of pyritic sulfur, and does not show variations in depth comparable to those observed for either sulfatic sulfur or pyritic sulfur. Formation of iron(II) sulfide is not important in the production of pyrite, which may form by direct reaction of reduced sulfur forms with the iron(II) ion. (These observations do not necessarily rule out the possible role of iron(II) sulfide as a transitory intermediate.) Pyrite formation in organic-rich swamps develops from the use of organic

72 NH2 i HSCH 2CHCOOH

20

NH 2 NH 2 I i HOOCCHCH2--S ~ S ~ C H 2CHCOOH

21

COOH CH2SH ] I HzNCHCHzCH2CONHCHCONHCHzCOOH H2N

22

23

oxysulfur compounds in the respiration processes of sulfur-reducing bacteria [ 104]. Reduced sulfur species such as methanethiol and hydrogen sulfide correlate with the Wildcat (Texas) lignites that were deposited in marine lagoonal environments, while oxidized sulfur products such as sulfur dioxide and carbon disulfide correlate with Montana lignites, which had no evident marine influence during deposition [97]. Oxidized sulfur compounds of the Montana lignite may correlate with the high inertinite content of the samples analyzed. 2.4.7 Accumulation of metal ions The sorption behavior of cations on Enderlin (North Dakota) leonardite was very similar to the sorption of cations on fulvic acids extracted from a podzol [ 105]. Fe+3 had the highest sorption (56 meq/100 g); sorption capacities decreased in the order Cu+2 > Al+3 > Zn+2 > Mn+2, with the sorption of Mn+2 being 35 meq/100 g [105]. Extraction of the leonardite after sorption of these cations, using water, hydrochloric acid, or ammonium acetate solution showed that Fe+3 was the most tightly bound, followed the order Al+3 > Cu+2 > Zn+2 > Mn+2. Extraction with ethylenediaminetetraacetic acid (EDTA) removed about 50% of each cation, suggesting that the strength of bonding to sites in the leonardite about equals that of the bonding of cations to EDTA. Since EDTA is a strong chelating agent, these ions, once sorbed, are fairly tightly held in the leonardite. Although leonardite is a weathering product of lignite, the similarity of leonardite behavior to that of fulvic acids in soils suggests that a similar sorption of cations from ground water into organic matter undergoing diagenesis may be responsible for the incorporation of some of these metals into lignite. Spanish lignites provide insights into the mechanisms of accumulation of heavy metal cations [106]. The adsorption of Sr, Pb, U, and Th was studied by measuring the adsorption isotherms of these ions from aqueous solutions of their nitrates onto lignite samples. The adsorption behavior was described in terms of the Langmuir equation

73 N = Noac/(l+ac) where N is the concentration of the metal in the lignite, c the equilibrium concentration of the metal in the aqueous phase, and a the ratio of adsorption and desorption constants. No is the saturation capacity. The slope of the tangent of the isotherm at low concentrations, Noa, is the geochemical enrichment factor (GEF) and, for very low concentrations, represents the equilibrium ratio of the metal in the solution and the lignite. The saturation capacity ranged from 0.18 for thorium to 1.82 for lead, and the GEF values, 65 for strontium to 6270 for lead [106]. The saturation capacities of these lignites increase with the humic acid contents. The metal retention ability of the humic acids isolated from the lignites (by sodium hydroxide extraction [ 106]) increases with pH and atomic weight and valence of the metal. The humic acids can act to concentrate heavy metal ions in the lignite. Low-rank coals are much more effective at extracting uranium from aqueous solutions than are high rank coals [107]. With Estevan (Saskatchewan) lignite contacted with an aqueous solution of uranyl sulfate, U02804, (155 ppm U, pH 2.65), the uranium content dropped to less than 2 ppm in 24 hours [108]. Tests of vitrinite-rich and fusinite-rich samples show that petrographic composition is only a secondary effect in the ability of the lignite to sorb uranium. For example, the uranium content of the vitrinite-rich sample before the experiment was 2 ppm, while after the experiment the uranium content had increased to 2031 ppm [ 108]. Comparable data for the fusiniterich sample are, respectively, 2 and 2288 ppm [ 108]. Contacting the uranyl sulfate solution with the fusinite-rich sample resulted in a higher pH of the solution at the end of the experiment than did the vitrinite-rich sample. The greater increase in pH may be an effect of leaching alkalies from the relatively more inorganic-rich fusinite. An increase in the pH of uranium solutions precipitates hydrous uranium oxides. The petrographic composition of the lignite may affect the pH of the solution or groundwater in contact with the lignite, and pH in turn affects uranium precipitation. A positive correlation was found between the ash content of Hagel bed lignite and several palynomorphs of detrital origin [ 109]. Hydrogeochemical removal of calcium by dissolution of carbonates from the overburden helped concentrate calcium in the central parts of lignite seams in North Dakota, and also increased the Ca/Na ratio in the upper portions of the seams [ 110]. Oxidation and reduction processes are also influenced by groundwater. The development of pyrite in fractures in the lignite seam, followed by the growth of gypsum crystals on the surface of the pyrite, indicates that the groundwater transported iron and sulfur under reducing conditions, and at a later time transported calcium under oxidizing conditions [ 110]. Chapter 5 provides some additional information on the accumulation of metals in lignites, primarily from the point of view of the influence of accumulation on the observed concentrations of metallic elements in lignites.

74 REFERENCES

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

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79

Chapter 3

THE ORGANIC STRUCTURE OF LIGNITES This Chapter discusses the organic structural features of lignites, beginning with the lithologic layering in lignite seams, proceeding to characteristics of lithotypes and macerals, and then to details of structure at the molecular level. This Chapter follows in part from the discussion in Section 2.4 on the coalification of plant matter. It then sets the stage for the discussion of organic reactions of lignites in Chapter 4. 3.1 P E T R O G R A P H Y OF LIGNITES 3.1.1 Lithologic Layering Early studies of northern Great Plains lignites recognized the existence of distinct layers discernable in the beds [1]. More recent work has focused on the lithologic layers, also called lithobodies, observed in the Beulah-Zap lignite (North Dakota) at the Freedom Mine [2,3]. The subdivision of the seam into distinct lithologic layers is based on megascopic characteristics: appearance of the broken surfaces in the highwall; luster; fracture and hardness; and the presence of lithologically distinct units such as thin layers of clay or silt. The pattern of layering observed in the Freedom mine persists at least into the neighboring Beulah mine, a distance of 16 km [2]. The top three layers are similar petrographically, with the fourth layer substantially different. The relationship of these petrographic differences to pyrolysis behavior is discussed in Chapter 4. Table 3.1 provides the petrographic and chemical analyses of the four lithologic layers in the Freedom mine [4]. Lithologic layering is also evident in both the Beulah and Center (North Dakota) mines (which are in the Beulah-Zap and Hagel beds, respectively). In both cases, huminite- and liptinitegroup macerals are more abundant near the base of the seam, and inertinite content is inversely related to huminite content [5]. The huminite-group macerals are more abundant in the Hagel lignite (83%) than in the Beulah-Zap (65%). Fusinite macerals, particularly semifusinite, are more abundant in the Beulah-Zap than in the Hagel bed. Lithotype abundance is the basis for classification of lithobodies in the Beulah-Zap seam at the Beulah, Indian Head, and Freedom mines; six major types are recognized on this basis [6]. This classification scheme was established on the basis of principal factor analysis and cluster analysis. The classification system is shown in Table 3.2 [6]. The lithobodies range from 5 to 85 cm in thickness [7]. As a rule, attritus-dominated lithobodies occur more frequently near the top of

80 TABLE3.1 Petrographic and chemical analyses of the lithologic layers in the Freedom mine [4]. Layer: Maceral Analysis (a) Huminite Group Ulminite Humodetrinite Gelinite Corpohuminite Liptinite Group Sporinite Cutinite Resinite Suberinite Alginite Liptodetrinite Fluorinite Bituminite Inertinite Group Fusinite Semifusinite Macrinite Sclerotinite Inertodetrinite Micrinite Minerals Pyrite Quartz Clays Proximate Analysis (b) Volatile Matter Fixed Carbon Ash Ultimate Analysis (b) Hydrogen Carbon Nitrogen Sulfur Oxygen (c) Calorific Value MJ/kg (b)

1 (Top)

2

3

4 (Bottom)

35.5 25.2 0.5 1.0

38.9 23.1 0.4 2.6

38.2 21.9 1.4 2.0

42.8 18.0 1.3 6.2

0.9 0.7 2.7 0.0 1.2 5.0 0.0 0.0

2.4 O.5 1.9 0.5 0.4 5.9 0.4 0.0

1.4 O.5 0.9 0.4 0.9 3.8 0.0 0.0

3.3 O.5 1.7 1.5 1.2 5.3 0.0 0.0

4.5 6.8 0.7 0.3 8.9 1.3

4.6 8.1 0.5 0.5 7.6 0.7

8.5 7.2 0.2 0.4 8.5 1.8

2.7 5.3 0.0 0.3 4.2 1.3

1.2 2.7 0.5

0.4 0.2 0.2

1.1 0.4 0.4

0.8 1.0 0.5

38.7 46.8 14.6

43.4 45.4 11.1

43.5 50.1 6.5

42.5 50.9 6.5

3.8 60.9 1.0 1.8 17.9 23.2

4.1 62.9 1.1 0.7 20.0 24.1

4.5 66.2 1.1 0.6 21.2 25.8

4.3 66.3 1.5 0.8 20.6 25.9

Notes: (a) percent volume, (b) moisture flee basis, (c) by difference. the lignite, whereas vitrain-dominated bodies occur more frequently in the center and near the base. Lithobody 6 has a very high ash value (32%, dry basis), and, on an maf basis, has higher volatile matter (54.8%) and oxygen (24.0%), and lower fixed carbon (45.3%), carbon (68.8%), and calorific value (25.6 MJ/kg) than the other lithobodies [6]. The proximate and ultimate analyses of the other five lithobodies are remarkably similar. For example, on an maf basis the volatile matter is 44.8--48.7%; carbon, 71.1-72.6%; and calorific value, 27.2-27.7 MJ/kg [6]. Division of the Beulah-Zap bed into ten lithobodies has also been proposed [7]. Lithologic layering in the Gascoyne mine (Harmon bed, North Dakota) but has not been

81 TABLE 3.2 Average lithotype abundance (volume percent) in Beulah-Zap lithobodies [6]. Lithobody 1) Attritus dominated, abundant fusain 2) Vitrain dominated 3) Vitrain dominated, with attritus 4) Vitrain and attritus equal 5) Attritus dominated, with vitrain 6) Attritus dominated

Fusain 19.0 1.5 3.4 6.3 3.1 0.6

Vitrain 25.O 91.5 76.0 50.7 32.3 18.0

Attritus 56.O 7.7 21.5 43.0 64.7 81.4

studied in as much detail as the Beulah-Zap bed at the Freedom mine. Five layers were identified, and were partially differentiated on the basis of fusain content [8]. The ranges of fusain are Layer 1 (bottom), 2-6%; Layer 2, 3.5-13%; Layer 3, 3.5-10%; Layer 4, 19%; and Layer 5 (top), 5.5-7.5% [8]. Fusain forms horizontal planes of weakness in lignite seams. Since horizontal fracture helps differentiate layers in the seam, a classification scheme for distinguishing layers on the basis of fusain content could be developed in the future. 3.1.2 Lithotypes Lithotypes are megascopically observable components of the lithologic layers, and are distinguished on the basis of physical characteristics. For North Dakota lignites, vitrain and fusain have been described according to the Stopes-Heerlen system, and attritus by the Thiessen-Bureau of Mines system [9]. Fusain is composed of fragmental chips and fibers. It occurs in lenses up to 2 cm thick on bedding plane surfaces [2,8,10]. Typical dimensions of fusain are 2--4 mm thickness and about 2 cm2 in area [8]. Thicker lenses may be continuous for up to 3 m [8,10]. Discontinuous fusain fragments are typically 2-5 cm in length, although one fusain horizon was traceable for 30 m [10]. Fusain displayed a characteristic extreme friability, which was responsible for many horizontal partings in the seam. Crumbling fusain in the hand produces a very fine, "dirty" powdery residue [11]. Fusain is opaque in thin sections but easily recognized by the porous structure remaining from its woody origin. Generally fusain is less important than the attritus or vitrain. Vitrain occurs as discontinuous layers within the attritus. The distinguishing characteristics of vitrain are a bright luster, smooth surface, and fractures 90 ~ to the bedding plane (i.e., bidirectional cleating); the most typical physical property was its brittleness [2,10]. A fracture pattern along two perpendicular planes produces regular, blocky fragments. Vitrain does not produce much dust on fracturing. The minimum thickness of vitrain lenses is about 5 mm, typically forming fiat lenticular bodies about 10-30 mm long [8]. The maximum horizontal extent of vitrain lenses is less than 25 cm [10]. Discontinuous lenses 5-30 mm thick are contained within a dull granular matrix [ 10]. Some vitrain lenses, usually less than half of the total vitrain, contain obvious plant structures, such as concentric growth tings. Such structures are typical of a second form of

82 vitrain, anthraxylon [ 12], which has a silky luster, conchoidal fracture, and is very hard but brittle. The anthraxylon form of vitrain often occurs in bodies of 5--50 mm thickness. A type of vitrain with a dull texture in the Beulah-Zap lignite occurred most frequently in massive attrital lithobodies and usually contained significant amounts of well-preserved plant structures. Among the Fort Union lignites, vitrain bands > 12 mm in thickness may make up more than 15% of the lignite [ 13], with a total huminite content of about 80% or more. Large vitrain lenses can be seen in the working faces of mines, where they may occur in the form of compressed tree stumps or logs of length and diameter each > 1 m [ 13]. Attritus is a collective term for layers of dull to moderately bright lignite interlaminated with vitrain and fusain lenses [10]. Attritus has characteristic granular texture [2,14]. Attritus flakes if scraped, but shows extreme resistance when struck. Attrital layers 10 cm thick can often be traced laterally for several meters [ 10]. Attritus occurs as granular massive layers of 5--40 mm thickness with dull luster, irregular fracture along horizontal planes, and some 90* cleating (though not as extensive as in vitrain) [8]. This behavior is particularly noticeable in the field, in that lithologic layers in which attritus is the major lithotype appear as bulges or protrusions from the exposed lignite face, showing fewer ~ffects of weathering and the absence of desiccation cracks. Attrital lignite breaks easily along irregular horizontal planes when struck with a hammer. Woody vitrain is much more resistant. Attrital lignites are mainly composed of plant debris, much of which was macerated or otherwise decomposed (e.g. by bacterial action), probably during the peat stage of coalification [ 15]. Most of the attritus is translucent, often with a yellowish coloration, when examined in thin section, but up to 15% may be opaque [15]. The plant debris derives from plant fibers; decayresistant reproductive parts such as pollen, spores, and seeds; and cuticles of leaves and fruit. These materials are generally translucent orange in thin section. (Completely opaque attritus has not been related to specific plant components [ 15].) Attrital lignites have a uniform grainy texture and light to dark brown coloration. They are not banded in appearance. Translucent and opaque attritus are differentiated on the basis of optical behavior in thin section [16]. Translucent attritus generally consists of pollen, spores, and leaf cuticles. Opaque attritus is amorphous or may show some cellular structure. (Fusain is also opaque and may show a woody cellular structure, but is differentiated from opaque attritus by being thicker in vertical section in the seam [ 16].) Attrital lignite originated in a swampy environment that provided for very rapid decay of plant material [15]. In such an environment, only the most decay-resistant portions of plant material could be preserved reasonably intact. Any woody tissue that would survive in this environment would be very finely macerated. Some attrital lignites may be allochthonous; that is, they accumulated in shallow water by the influx of water- or air-borne plant debris. Arkansas lignites may have accumulated in this fashion [15]. Lignites that give high yields of extractable waxes are predominantly attrital [ 15]. Woody lignites, in contrast, give low yields of waxes. Nonbanded attrital lignites having high concentrations of waxy plant debris may be "canneloid," i.e., the low-rank homolog of cannel

83 coals [ 1]. The average concentrations of the lithotypes in Beulah-Zap lignite are 35-45% vitrain, 5% fusain, and 40--60% attritus [3,17]. Attritus is more abundant directly overlying clay partings and at the base of the seam, whereas fusain was essentially absent at these locations [3,10,17]. This distribution is generally comparable to observations made on bituminous coal seams [ 18]. Vitrain occurs as discontinuous lenses less than 5 mm thick within the attritus. Lithobodies dominated by vitrain or fusain occur more frequently near the top of the seam. The horizontal variation of lithotypes in six samples from various locations in the Gascoyne mine shows vitrain ranges of 28-37%, attritus 59-66%, and fusain 2-7% [8]. The results are fairly consistent from one sampling location to another, and show no obvious trend with horizontal direction. Characteristics of the lithotypes separated from Beulah-Zap lignite are summarized in Table 3.3. The data in Table 3.3 are averages calculated from more extensive tabulations in [10]. Proximate and ultimate analyses for one set of lithotypes of Beulah lignite are given in Table 3.4 [ 19]. The petrographic analyses of the samples used to obtain the data in Table 3.4 are given in Table 3.13. TABLE 3.3 Average analyses of Beulah-Zap lithotypes (moisture-free) Vitrain Attritus Fusain Proximate (a) Volatile Matter Fixed Carbon Ash Ultimate (b) Hydrogen Carbon Nitrogen Sulfur Oxygen Calorific Value, MJ/kg (b)

48.2 45.4 6.4

46.5 45.4 8.2

39.9 45.1 15.0

4.42 64.66 0.94 0.81 22.79 25.5

4.09 64.17 1.10 0.67 21.89 24.9

3.40 58.12 0.75 0.69 20.48 22.9

(a) Based on five fusain and seventeen attritus and vitrain samples. (b) Based on four fusain and sixteen attritus and vitrain samples.

The proximate analysis of unseparated lignite agrees fairly well with a proximate analysis calculated from weighted results of the proximate analyses of the separated lithotypes. The proximate analysis of lithotypes separated from Beulah lignite, and the agreement with the reconstructed analysis of the unseparated lignite, are shown in Table 3.5 [ 13,20]. The data in Table 3.5 were obtained by thermogravimetric analysis and are shown on a moisture-free basis.

84 TABLE 3.4 Proximate and ultimate analyses of Beulah lithotypes, moisture-free basis [19]. Vitrain Proximate Volatile Matter Fixed Carbon Ash Ultimate Hydrogen Carbon Nitrogen Sulfur Ash Calorific value (MJ/kg)

Attritus

Fusain

45.9 47.5 6.6

43.9 46.5 9.6

40.3 48.4 11.3

4.70 66.93 0.81 1.05 6.6 26.1

4.01 65.14 1.03 0.76 9.6 24.8

3.85 65.30 0.92 1.61 11.3 25.1

TABLE 3.5 Proximate analyses of lithotypes and unseparated Beulah lignite [13,20].

Volatile Matter Fixed Carbon Ash

Vitrain Attritus Fusain 41 43 33 52 50 59 7 7 8

Whole Lignite Calcd.* Exptl. 42 42 51 49 7 8

*Calculated from the average of the values for the lithotypes, weighted for the amount of each li thotype in the unseparated lignite.

A compositional relationship is found among the medium dark, medium light, and dark lithotypes of Victorian brown coal [21] and attritus and fusain in Beulah-Zap lignite [2] by expressing elemental compositions on a ternary bond equivalence diagram [21,22]. The five lithotypes are related in order of decreasing quantity of cellulose (or, in other words, by increasing loss of cellulose): medium dark > medium light > dark > attritus > fusain. Further, a linear correlation (r = 0.85) exists between the aromaticity of the lignites and the carbon bond equivalence [21. The most thorough investigation of separated lithotypes has been performed with those from Beulah lignite. Some properties of hand-separated samples of lithotypes from the Beulah mine are provided in Table 3.6 [2]. Vitrain shows evidence of significant biochemical activity. The very low cellulose content suggests bacterial degradation during diagenesis. The principal ulminite maceral in the vitrain is euulminite, in which very little cell wall structure remains. The attritus is less well compacted than the vitrain, which accounts in part for its much higher relative mechanical friability. Fusain is charcoallike in appearance, indicative of an origin in bog or forest fires. The large amount of residual

85 TABLE 3.6 Characteristics of lithotypes of Beulah lignite [2]. Proper t3, Carbon, %maf Hydrogen, %maf Nitrogen, %maf Sulfur, %maf Aromaticity (fa) Cellulose, %maf Methoxy, %maf Carboxyl, meq/g maf Inorganics, % mf (a) Minerals, %mf (a) Mechanical friability, %(b) Fusinite, % Semifusinite, % Ulminite, % Humodetrinite, % Inertodetrinite, %

V i train 71.66 5.03 0.87 1.12 0.65 0.005 2.08 2.39 2.90 1.35 18.4 1 1 69 10 2

A ttri tus 72.06 4.44 1.14 0.84 0.66 0.025 0.68 2.83 4.01 3.17 36.1 2 5 31 34 10

Fusain 73.62 4.34 1.04 1.82 0.80 0.023 0.72 2.47 3.34 4.06 32.3 45 20 10 5 8

(a) follows Australian brown coal practice (e.g., [24]) (b) discussed in Chapter 7 cellulose, relative to vitrain, suggests interruption of biochemical processes by the fire. Since decarboxylation occurs readily during pyrolysis, the similarity of carboxyl contents among the lithotypes indicates that they formed by later oxidation of the coal and are not relics of the original plant material. A simplistic approach based on separation of Beulah lignite into light and dark lithotypes has produced some insights on differences between lithotypes [23]. The darker lithotype was the more aromatic (fa = 0.71) and contained fewer carboxyl groups, as estimated from X-ray photoelectron spectra. The dark lithotype also had the more pronounced 1600 cm-1 band in the Raman spectrum, indicative of a somewhat better ordered or more graphitic structure of the dark lithotype. Consistent with a higher carboxyl content, the light lithotype had a higher sodium content. The better structural ordering in the dark lithotype was due to a lack of steric effects that would be associated with the relatively bulky carboxyl groups and their sodium counterions. Six classes of lithotypes have been determined for Texas lignites, shown in Table 3.7 [25]. 3.1.3 Macerals (i) Within-seam variation. In the Beulah-Zap bed inertinite macerals are more abundant in the top-most meter of the seam [26]. Inertodetrinite, desinite, gelinite, and semifusinite concentrated near the top of the seam [27], although in some locations semifusinite is relatively constant throughout the seam [17]. The dominant inertinite maceral at the top of the seam is inertodetrinite [ 17]. Thicker fusain lenses contain fusinite and semifusinite. (In some cases discrete fusain lenses are observed toward the base of the seam.) Megascopically the lignite in this region

86 TABLE 3.7 Lithotype classes, lithotypes, and inclusions for Texas lignites [25]. Lithotype Class Pure coal, detrital

Lithotypes* Banded

Pure coal, non-xylitic

Unbanded Finely Moderately Moderately Highly Unbanded

Pure coal, xylitic Impure coal, detrital Impure coal, non-xylitic Impure coal, xylitic

Unbanded Finely Moderately Highly Moderately

Inclusions Gel particles Resin bodies Gelified groundmass Resin bodies Charcoal Gelified tissues Cuticles Gel particles Resin bodies Gelified groundmass Charcoal Resin bodies Charcoal

*Banding is classified as follows: finely, <2mm; moderately, >2mm, <5mm; and highly, >5mm.

of the seam appears to be more fragmental than lignite lower in the seam, and has a dull luster. Huminites dominate throughout the middle and lower portions of the Beulah-Zap lignite. Ulminite that is not extensively gelified associates with inertinites near the top of the seam, and may incorporate corpohuminite in the preserved cell lumens. The more highly gelified huminites are more abundant near the base of the seam. Gelinite and gelified ulminite occur in association with attritus above the underclay and clay partings [ 17]. Attrinite and desinite are more abundant near the base of the seam. The huminite macerals become more abundant, relative to the inertinites, toward the base of the seam, with ulminite, corpohuminite, and fusinite concentrating near the center [27]. The seam becomes more massive and displays a brighter luster with increase in huminites. No obvious pattern of liptinite distribution occurs [ 17,26], although attrinite, sporinite, and cutinite concentrate near the base [27]. Liptinites generally show very little vertical variation [ 10]. Maceral composition does not vary significantly in lateral directions [10]. Distributions of huminite and inertinite maceral groups in Beulah-Zap bed at the Freedom mine show a similar pattern to the Beulah mine, with inertinites more abundant near the top of the seam and the huminites more abundant toward the bottom [28]. A vertical variation in mean reflectance values in two stratigraphic sequences of Beulah-Zap lignite [29] shows the influences of the original environment of deposition. In Estevan (Saskatchewan) lignite, fusinite and semifusinite averaged 19% for the seam, but were concentrated in the lower two-thirds of the seam [30]. Exinite was distributed throughout the seam; the highest concentration was 13-20 cm from the top. Reflectance of eu-ulminites A and B increases downward for both the Estevan and Willow Branch (Saskatchewan) lignites [31 ]. This trend is not observed for corpohuminite [31]. Eu-ulminites of low reflectance also tend to have low

87 calorific values [31]. In Sandow (Texas) lignite, attrinite and desinite contents exceed ulminite in the upper part of the seam [25]. The upper portion also includes high concentrations of sporinite and thin-walled cutinite. Fusinite and semifusinite contents are low everywhere except in the upper portion. The middle of the seam contains abundant attrinite, desinite, liptodetrinite, and resinite. In the lower portion of the seam, a high huminite content is due to attrinite, desinite, and levigelinite. Sunrise (Louisiana) lignite consists of bright lithotypes and vitrite and clarite microlithotypes near the base, while the upper two-thirds is semibright with exinite, inertininte and mineral matter increasing while vitrinite decreases [32]. Column sections from the Dakota Colleries mine (Mercer County, North Dakota) show wide variations of petrographic composition with respect to depth [33]. (ii) Association with lithotypes. In Beulah-Zap lignites, huminite and inertinite macerals are more abundant in the vitrain and fusain lithotypes, respectively [10]. Liptinites are not predominantly associated with any particular lithotype, but tend to associate with the huminitegroup macerals. Generally ulminite is most abundant in vitrain; fusinite and semifusinite are abundant in fusain; and macerals with detrital origin (attrinite, desinite, liptodetrinite, and inertodetrinite) are common in attritus [6]. Petrographic data on the Freedom lignite has been shown in Table 3.1. Petrographic analysis of the lithotypes from the Beulah mine is shown in Table 3.8 [3,19]. TABLE 3.8 Petrographic analysis of Beulah lithotypes [3,19]. Vitrain Huminite Textinite Ulminite Attrinite Desinite Gelinite Corpohuminite Total Huminite ................. Liptinite. Sporinite Cutinite Resinite Suberinite Liptodetrinite Total Liptinite ................... Inertinite Fusinite Semifusinite Sclerotinite Inertodetrinite Micrinite Total Inertinite ................. *tr - trace

Durain

Fusain

4 0 tr* 64 34 27 10 16 9 4 5 3 1 2 1 4 2 2 84 .............. 59 .............. 42 3 2 1 1 1 9 ..............

2 1 1 2 2 7 ..............

1 1 1 2 2 5

tr 2 tr 2 1 5 .............

tr 10 1 20 tr 31 ..............

15 25 1 9 tr 49

88 As an indication of the variability in these same lithotypes, average maceral contents have been reported as follows: total huminite, 87% in vitrain, 71% in attritus, 16% in fusain; total liptinite, 7% in vitrain, 12% in attritus, 5% in fusain; and total inertinite, 5% in vitrain, 17% in attritus, and 63% in fusain [10]. Relative standard deviations in the range of 36-83% for the major macerals such as ulminite, attrinite, desinite, liptodetrinite, fusinite, semifusinite, and inertodetrinite [ 10], and relative standard deviations in many cases over 100% for the minor macerals [34], were observed for a suite of 96 samples of Beulah-Zap lignites. Comparable differences in maceral composition of lithotypes of Savage (Montana) lignite have been observed [35]. Ulminite is the major constituent of the vitrain lithotypes in the Beulah-Zap lignite, the vitrain containing up to 70% ulminite and 93% of all the huminite group macerals [ 10]. Eu-ulminite predominates among the huminite group macerals. Both the A and B varieties were observed, but A was the major constituent. Corpohuminite associates closely with ulminite. In a few cases, gelinite and highly gelified ulminite were the most abundant macerals. Gelinite occurs in thin, vitreous vitrain layers associated with a humodetrinite matrix. Most vitrain lenses have low concentrations of macerals other than the huminites [17]. Fusinite exhibited very well preserved cell structure. Inertinite macerals are the dominant constituent of fusain. Fusinite and semifusinite make up 61% of the total maceral content [10,17]. Ulminite occurs in all fusain samples, often with semifusinite at the boundaries of the fragments. Liptinite macerals associate more intimately with the huminites than with the inertinites. Humodetrinite macerals contribute the majority of the macerals that represent the detrital fragments in attritus [17]. Detrital macerals make up about 55% of the attritus [10,17], with humodetrinite constituting the majority [10]. However, the single most abundant maceral associated with attritus is ulminite [17]. Inertodetrinite occurs as elongated, angular fragments. (iii) Characteristics of lignite macerals. Low-rank vitrinites having a reflectance less than 0.5% are called xylinoids [36]. Several xylinoids may develop from a single chip of wood [36]. Xylinoids are the coalified remains of wood or bark. They belong to the vitrinite group of macerals [35]. (The term xylinoid has been used, mainly in the early literature, to refer to the macerals of the vitrinite group normally found in lignites [35]). In the lignite and subbituminous ranks, vitrinite reflectance varies relatively little as a function of rank. The maximum reflectance in oil changes from about 0.3% at 16.3 MJ/kg (m,mmf basis) to about 0.5% at 25.6 MJ/kg [37]. Consequently, vitrinite reflectance is questionable as an effective method of rank differentiation for the low-rank coals. The maxima in the fluorescence spectra of sporinite show a much greater change with rank, shifting from 40(0500 nm for peats to 630-670 nm for high volatile B bituminous [38]; sporinite fluorescence spectra provide a useful means of rank differentiation. In Canadian low-rank coals, fluorescence intensities decrease to a minimum at about 0.40% Rrandomand then increase again (as a function of reflectance) [39]. Eight northern Great Plains lignites from North Dakota, Montana, and Wyoming showed random reflectances ranging from 0.27 to 0.38% [40], the median value was 0.32%; the mean, 0.32%; and the standard deviation, 0.046. The ulminite reflectance of Freedom lignite was 0.36

89 (Rmax) [8]. Although this value was said to be low [8], it is in fact at the high end of data measured elsewhere [40]. Average random reflectance values for Texas lignites are higher than for North Dakota lignites [41]. Saskatchewan lignites show Rrandom of 0.27-0.33% [39]. At vitrinite reflectance values higher than 0.6% it is no longer possible to distinguish suberinite [42]. Similarly, as rank increases it becomes increasingly difficult to distinguish attrinite and desinite from collinite or desmocollinite [42]. Vitrinites can be characterized as structured, structureless, or groundmass vitrinite [30]. Structured vitrinite has, as the name implies, as well-defined cell structure. It is sometimes difficult to distinguish from semifusinite. Structured vitrinite may grade into the structureless form. Structureless vitrinite has few or no cell structures. The texture of structureless vitrinite is smooth, though sometimes cracked, and it has a higher reflectance than structured vitrinite. Groundmass vitrinite forms a groundmass for particles of other macerals. Groundmass vitrinite has a lower reflectivity than structured vitrinite. Vitrinite makes up, on average, 64% of the whole seam of Estevan lignite [30]. The range of vitrinite contents in individual samples was 48-80%. The vitrinite was about evenly divided among structured, structureless, and groundmass types. Structured vitrinite was most abundant at the top and bottom of the seam, as well as an interval about 43-71 cm from the top. Groundmass vitrinite was more abundant in the top half of the seam. Structureless vitrinite was about evenly divided in all seam intervals. Xylinite, detrinite, and dopplerinite are the counterparts of the vitrinite in higher rank coals [30]. As rank increases, these macerals become less diverse in both optical and chemical properties; eventually they form the vitrinite group. Xylinites are humic constituents of lignites whose most important characteristic is a well preserved cell structure. In detrinites the cell structure is discernable, though not as prominent as in xylinites. Detrinite may sometimes be observed as a ground mass containing spore or pollen remains, resin bodies, or particles of inertinites. Dopplerinite has undergone gelification and consequently has little or no discernable cell structure. Thus dopplerinite is more homogeneous than xylinite or detrinite. Ulminite in Hagel (North Dakota) lignite has a higher degree of gelification than Beulah-Zap lignite [43]. This difference suggests that these two Paleocene lignites may have had somewhat different depositional origins. The abundant woody material in the Hagel lignite, together with corroborative palynological data [44], suggests that the environment of deposition of the Hagel lignite was dominated by arboreal vegetation. The Beulah-Zap depositional environment was dominated by more abundant herbaceous plants. Ulminite dominates maceral the huminite group macerals, and in Beulah-Zap lignite accounts for 62% of the huminites [ 10 ]. The submaceral varieties of ulminite depend on the degree of gelification of the colloidal humic mass of degraded cellulose and lignin. Most ulminite in Beulah-Zap lignite is eu-ulminite, which is highly gelified and contains few preserved plant structures. Texto-ulminite is found in minor quantities near the top of the seam. In comparison, Martin Lake lignites (Texas) show well-preserved cell structure as texto-ulminite [25]. Many

90 huminites in deep (70-315 m) Wilcox (Texas) lignites have experienced partial or complete gelification [45]. Some are granular with a low reflectance and a weak orange fluorescence, possibly representing an early stage in the formation of micrinite. Micrinite is indeed found in close proximity to this type of huminite. The humodetrinite subgroup contains attrinite and desinite. These two macerals account for 35% of the huminite group and 20% of all macerals in the Beulah-Zap lignite [ 10]. The distinction between the two is based on size; humodetrinite <10 0rn is classed as attrinite. Attrinite is the most abundant humodetrinite maceral in Beulah-Zap lignite [ 10]. Attrinite, desinite, and levigeliniteare the principal huminites in San Miguel (Texas) lignite. Other huminite macerals in Beulah-Zap lignite include gelinite and corpohuminite, which together account for about 2% of the huminites [ 10]. Corpohuminite has a higher reflectance than normal for a huminite group maceral, due to its high hydrogen content, and is distinguished from inertinites by a characteristic bright oval or circular morphology in cross section. Corpohuminite is commonly associated with ulminite; differentiation between them is difficult in highly gelified samples. Anthraxylon derives from bark and woody tissues. During coalification, anthraxylon develops a homogeneous appearance and smooth texture. Anthraxylon is also characterized by a bright luster, conchoidal fracture, and brittleness. In thin section, anthraxylon has a translucent reddish color and often shows the structure of the woody tissue [ 15,16,46]. In thin sections cut normal to the bedding planes, anthraxylon appears as well-defined homogeneous bands running across the section. The so-called "woody" lignites are composed mainly of anthraxylon. The two most abundant inertinite macerals in Beulah-Zap lignite are fusinite and semifusinite [10]. Together they account for 55% of the total inertinites [10]. The reflectance of fusain from Beulah lignite is 1.73 [47]. Semifusinite differs from fusinite by having a lower reflectance and thicker cell wall structures. Both are extremely brittle. North Dakota lignite is richer in semifusinite than Texas lignite [41]. Fragments of detrital inertinites too small to determine a maceral type are classified as inertodetrinite. In the Beulah-Zap lignite most inertodetrinite particles are <25 ~tm in diameter [ 10]. Inertodetrinite comprises about 40% of inertinites in Beulah-Zap lignite [ 10]. Both macrinite and sclerotinite occur in trace amounts ( i.e., less than 1%) in Beulah-Zap lignite [ 10]. In Martin Lake lignite, fusinite and semifusinite show well preserved cell structure [25]. These two macerals, along with inertodetrinite, account for most of the inertinites in this lignite. Some inertodetrinites show rounded or corroded grain boundaries, indicative of transportation from outside the site of deposition. In comparison, inertinites in Big Brown (Texas) lignite are a mixture of sclerotinite, inertodetrinite, and macrinite. The mean inertinite reflectance for Martin Lake lignite exceeds that of other Wilcox Group lignites in Texas, suggesting the possible deposition of this lignite in a more highly oxygenated environment. Sporinite is distinguished from other liptinites by a characteristic elongated morphology. In Beulah-Zap lignite sporinite commonly occurs in the humodetrinite matrix and may range up to 200 ~tm in length [ 10]. Cutinite occurs as long, thin layers in a matrix of attritus. It comprises less than

91 2% of Beulah-Zap lignite [10]. Resinite also occurs in the attrital matrix, in bodies ranging from tens to hundreds of microns in diameter [10]. Texas lignite is richer in liptinites than North Dakota lignite [41]. In Martin Lake lignite the principal liptinites are sporinite and cutinite. Sporinite and cutinite are also the principal liptinites in San Miguel lignite, along with liptodetrinite. However, in Big Brown lignite, liptodetrinite is equal or higher in content to the sporinite and cutinite. Resinite occurs in Big Brown lignite in globular form and in exsudatinite form. (iv)

Comparative petrographic compositions of lignites. Lignites

are more petrographically

heterogeneous than higher rank coals [41]. The petrographic compositions of six North Dakota lignites are summarized in Table 3.9 [10]. Compilations of petrographic analyses of lignites prior to 1950 have been published [46,48], including some data on the vertical variability of petrographic composition within a seam [46]. In addition, the report [46] is an excellent guide to the literature published prior to 1950 on the occurrence and petrography of lignites. TABLE 3.9 Bulk maceral analyses of North Dakota lignites (Vol. pct.) [ 10]. Lignite*

CEN

FAL

FRE

GAS

IND

VEL

52 24 tr tr 1

41 29 2 1 2

37 25 3 1 1

42 22 3 1 1

38 35 2 1 1

39 27 3 1 2

Sporinite Cutinite Resinite Suberinite Liptodetrinite

1 1 1 0 3

3 1 1 0 3

2 1 1 0 7

1 1 1 0 4

2 1 1 tr 6

tr 1 2 tr 4

Fusinite Semifusinite Macrinite Sclerotinite Inertodetrinite

1 10 0 tr 3

1 6 0 0 7

3 4 0 tr 10

1 2 0 tr 4

3 2 0 tr 7

7 5 tr tr 8

3

3

3

3

2

tr

Ulminite Attrinite Desinite Gelinite Corpohuminite

Minerals

*CEN - Center; F A L - Falkirk; FRE- Freedom; GAS - Gascoyne, Blue Pit; IND- Indian Head; V E L - Velva; tr- trace.

92 3.2 THE CARBON SKELETON

This section begins a discussion of the molecular architecture of lignites, with specific concern for the carbon framework. The two subsections treat the aromatic carbon and the aliphatic carbon, respectively. In each of these subsections the discussion is divided, for convenience, according to the principal experimental technique used. The macromolecular or three-dimensional structural arrangements in lignites are treated separately in Section 3.4. 3.2.1 Aromatic carbon (i) Oxidation studies. Oxidation of lignite with a variety of oxidants indicates that the predominant aromatic structures are benzene and phenol rings [49,50]. Hydroaromatic structures, such as tetralin fragments, may also be important [49,50]. Structures containing more than one ring are fairly rare. Polycyclic and heterocyclic ring systems are not important components of the structure [49]. Oxidation of Decker (Wyoming) lignite with 0.4 M sodium dichromate at 250~ for 40 h produced no aromatic products containing more than two fused rings [51,52]. (In contrast to some other oxidizing agents, sodium dichromate tends to preserve polynuclear aromatic systems, rather than degrading them to benzenecarboxylic acids.) Naphthalenecarboxylic acids were only about 10% as abundant as benzenecarboxylic acids from the dichromate oxidation of Sheridan (Wyoming) lignite [52]. Zap lignite produced benzenecarboxylic acids as the major products (accounting for about 82% of all acids identified) with very minor amounts of naphthalenecarboxylic acids (about 3%) and no phenanthrenecarboxylic acids [53]. In terms of relative abundance, the major aromatic units (identified as methyl esters of carboxylic acids or as neutral compounds) were benzene, 100; naphthalene, about 11; anthraquinone (1), about 2; and 9fluorenone (2), about 5 [54]. O

O

O 1

2

Oxidation of Estevan lignite with aqueous potassium permanganate at 60-70~

produced

47.3% yield of benzoic acids, with 21.4% oxalic acid and 6.8% acetic acid [55]. Expressed in terms of the carbon content of the original lignite, 1.7-3.8% of the carbon appeared as acetic acid, 6-8% as oxalic acid, and 45-50% as benzoic acids [55]. The benzoic acids identified, but not quantified, included phthalic (3), isophthalic (4), terephthalic (5), hemimellitic (6), trimellitic (7),

93 COOH

COOH

~L~COOH

4

~

~

COOH

COOH

COOH COOH COOH 7

~

5

COOH COOH

HOOC ~

COOH

COOH

~,

ZCOOH 8 COOH

COOH COOH

COOH

COOH HOOC

HOOC ~COOH HOOC~

COOH

COOH

COOH 10

~COOH

11

trimesic (8), mellophanic (9), pyromellitic (10), benzenepentacarboxylic, and mellitic (11) acids. Formation of polycarboxylic acids having adjacent carboxyl groups (i.e., all of the polycarboxylic acids except trimesic) implies existence of larger fused ring systems. Potassium permanganate oxidation of lignite from Oliver County, North Dakota produced mainly benzenedi- and tricarboxylic acids as the aromatic oxidation products [56]. Permanganate oxidation of sporinite from North Dakota lignite produced methoxy- and hydroxybenzene carboxylic acids [57]. Phthalic acid was the most abundant aromatic acid in the permanganate oxidation products of methylated Sheridan lignite [58]. Aromatic acids from the oxidation of Beulah lignite with ruthenium tetroxide are listed with the aliphatic acid products in Table 3.14; tetra- and pentacarboxylic acids predominated among the benzenecarboxylic acids [59]. The relatively large yields of benzenetri-, tetra-, and pentacarboxylic acids suggest a preponderance of fused ring systems [60], in general agreement with permanganate oxidation of Estevan lignite, although such interpretation conflicts with other oxidation studies [49,50] and calorimetry studies [61 ]. The relative unimportance of naphthalene rings is indicated by a low yield of phthalic acid. Oxidation of Rockdale (Texas) lignite with ruthenium tetroxide produced about 10-15 mole percent of benzenecarboxylic acids among the products [62]. The most abundant was phthalic acid, in contrast to the results obtained with Beulah lignite. It has not been established whether this distinction has any implications for the organic geochemistry of these two lignites. Neither 4 nor 5 were found in the Rockdale lignite oxidation products, nor was 8.

94 The product ratios of the acids 3, 6, 7, and 10 were 20:20:10:1. Some methylbenzenecarboxylic acids were also produced. These results indicate that ortho-substitufion patterns on the aromatic rings are important in the structure of this lignite, and that biphenyl-type structures are insignificant. The di-acids and tri-acids result from oxidation of such structures as naphthalene, naphthols, and heterocyclic compounds of oxygen. The low yield of the tetra-acid relative to the diand tri-acids indicates that condensed aromatic structures, such as anthracenes and phenanthrenes, are not important components of the structure of this lignite. The high yield of phthalic acid (3) relative to the 1,3- and 1,4- diacids is evidence for 1,2- fusion patterns in this lignite [63]. This type of ring fusion is further corroborated by the higher yields of 6 and 7 relative to 8. The formation of trimellitic acid (7) suggests that 2-arylnaphthalene structures might be important in the structure of this lignite. Oxidation of Sheridan lignite with acetic acid and hydrogen peroxide at 400C for 10 days caused 80% conversion to methanol-soluble acids [54]. The active oxidant in this system is peroxyacetic acid, which oxidizes naphthols to phthalic acid, but does not convert phenols to muconic acids (oxidation products of phenols that form by oxidative destruction of the aromatic ring; e.g., phenol itself is converted to the parent muconic acid, 2,4-hexadienedioic acid). Of the aromatic products, 69% were benzenecarboxylic acids and 31% methoxybenzenecarboxylic acids [541. Oxidation of Sheridan lignite with 70% nitric acid at reflux for 16-24 hours resulted in a 70% recovery of benzenecarboxylic acids [52]. The approximate percentages of the acid products were di-acids, 3%; tri-acids, 37%; tetra-acids, 39%; benzenepentacarboxylic acid, 15%, and mellitic acid (11), 4%. The low yield of mellitic acid, whose formation indicates extensive ring condensation, suggests that highly condensed ring structures are not important in this lignite. In comparison, nitric acid oxidation of three Chinese lignites formed alkylated aromatic dicarboxylic acids, in which the aromatic moiety was benzene, naphthalene, anthracene, or phenanthrene [64]. Benzenecarboxylic acids were the primary products from oxidation of Sheridan lignite by bubbling air through an HC1 solution while irradiating with ultraviolet light [52]. Photochemical oxidation of Decker lignite for eight days with a mercury lamp also produced a series of benzenecarboxylic acids as the primary products [51]. Benzenepolycarboxylic acids were the major aromatic products of oxidation of Wyoming lignite with, six oxidizing agents: potassium permanganate, alkaline cupric oxide, nitric acid, sodium dichromate, peroxytrifluoroacetic acid, and cesium fluorosulfonate [49]. Reaction with alkaline copper(II) oxide produced phenolic acids similar to typical oxidation products of lignin. Formation of the pyridinium iodide of lignite, followed by alkaline silver oxide oxidation, yields aliphatic- and aromatic-rich fractions [65]. Application of this reaction sequence to a Wyoming lignite forms alkylbenzenes as the major aromatic components. Lesser amounts of naphthalenes, aromatized sesquiterpenoids, indanes, tetralins, and fluorenes were produced. No evidence was found for aromatics of more than four tings. Among the phenolic compounds, phenol, cresols, anisole, and xylenols were abundant.

95 (ii) Nuclear magnetic resonance. The distribution of aromatic carbon types in two samples of Beulah-Zap lignite is summarized in Table 3.10 [66]. TABLE 3.10 Distribution of aromatic carbon types in Beulah-Zap lignite* [66] Sample Carbon type Total aromatic carbon (Ira) Aromatic carbon in an aromatic ring (fa') Carbonyl carbon (fac) F'rotonated aromatic carbon (faH) Nonprotonated aromatic carbon (faN) Phenolic or phenolic ether carbon (faP) Alkylated aromatic carbon (fas) Aromatic bridgeheads (faR)

A 0.61 0.54 0.07 0.26 0.28 0.06 0.13 0.09

O 0.66 0.58 0.08 0.21 0.37 0.08 0.16 0.13

*A = Argonne premium sample; O = sample presumed to be oxidized,

The value for protonated aromatic carbon, faH, observed in this experiment is similar to that for high volatile bituminous coals [66]. This behavior is unusual in that generally, for coals ranging from lignite through low volatile bituminous, faH increases with rank. The North Dakota lignite is anomalous in this respect. Considerable debate exists about appropriate methods and techniques for obtaining accurate, quantitative NMR data on coals. Values of fa and other structural parameters are clearly sensitive to the experimental method used, even when different methods are applied to portions of the same sample of a given lignite. For Beulah-Zap lignite, fa determined by cross-polarization magic angle spinning (CPMAS) at a magnetic field strength of 2.3 Tesla is 0.66. The same sample, analyzed at 4.7 Tesla in an experiment designed to suppress spinning side bands in the spectrum, showed an fa of 0.69 [67]. CPMAS fa measurements for Beulah-Zap lignite made at another laboratory were 0.67-0.70 [68], with some dependence on frequency, measurements being made at 25 and 75 MHz. Single-pulse excitation spectra of the same lignite indicated fa of 0.74-0.76 at 25 MHz and 0.77 at 75 MHz [68]. About 95% of the carbon in the lignite was observed in singlepulse excitation at 25 MHz. Treatment with samarium iodide gave fa of 0.68-0.70 in a CPMAS experiment, with 55% of the carbon observed [69]. Applying the Bloch-decay method to the SmI2treated lignite allowed observation of 65% of the carbon, with fa found to be 0.74. Depending on the method of choice, fa of Beulah-Zap lignite can be measured as 0.61-0.77. In somewhat similar work, the aromaticity of a Powder River Basin lignite was 0.55 when measured by cross polarization, and 0.58 determined by Bloch decay [70]. Using hexamethylbenzene as an internal standard, the Bloch decay measurement observed 65% of the carbon, while the cross polarization

96 experiment observed 55%. Distribution of carbon types in two Canadian lignites, an Ontario lignite and Bienfait (Saskatchewan) lignite, is summarized in Table 3.11 [71]. The data were obtained by dipolar dephasing techniques. Some data on the aliphatic carbons are shown also for completeness; aliphatic carbon is discussed in the following subsection. TABLE3.11 Distribution of carbon types in Canadian lignites [71]. Carbon type Total aromatic carbon (fa) Fraction of aromatic carbon nonprotonated (faa,N) Fraction of aromatic carbon protonated (faa,H) Fraction of all carbon protonated and aromatic (faH) Methyl carbon mass fraction (fMe) Lowest value reported Highest value reported Mobile methine and methylene; quaternary carbon, rotating methyl (fw) Rigid methyl, methine and methylene (fs)

Ontario 0.61

Bienfait 0.70

0.59 0.41

0.85 0.15

0.25

0.11

0.19 0.31

0.18 0.28

0.14 0.86

0.39 0.61

For comparison, the fa of Estevan lignite is 0.58-0.60 [39]. Values of fa of 0.61 and 0.62 are observed for an Ontario lignite, the higher value obtained by CPMAS at 2.3 Tesla, and the lower in an experiment at 4.7 Tesla to suppress spinning side bands [67]. Structural parameters for a variety of lignites, as determined by cross polarization - magic angle spinning NMR, are shown in Table 3.12 [39]. The table headings are defined as follows: fa, fraction of aromatic carbons; fall, protonated aromatic carbons; faN, non-protonated aromatic carbons; fsH, protonated aliphatic carbons; and fsN, non-protonated and methyl carbons. TABLE3.12 CP/MAS NMR Structural parameters of lignites [39] Sample* 1 2 3 4 5 6

%C 67.5 69.9 70.3 68.0 72.9 ....

H/C 0.94 0.89 1.12 0.65 0.87

f_a__ 0.64 0.55 0.37 0.43 0.64 0.57

fall 0.17 0.40 0.30 0.29 0.49 0.38

0.47 0.15 0.07 0.14 0.15 0.19

0.25 .... .... 0.46 .... ....

0.11 0.11

*Samples are identified as follows: 1, Beluga, Alaska; 2, Chester, Mississippi; 3, Henning, Tennessee; 4, King Cannel, Utah; 5, Oxbow, Louisiana; and 6, Thermo, Texas. Table headings are defined in text.

97 Although there is certainly a spread in values of fa, for the ten lignites characterized by the data in Tables 3.10 to 3.12, eight of the ten fa values lie in the range 0.55-0.70, and six are in the narrower range 0.57-0.66. The values for faH and faN vary more widely, although seven of the ten values of faHlie in the range 0.20--0.40. A variety of other lignites has been studied at least sufficiently to obtain a value of fa. Reported values include 0.57 for both a Canadian lignite [72] and Pust (Montana) lignite [73], 0.55 for a Wyoming lignite [74], and 0.44 for Rockdale [73] and Big Brown lignites [75]. In comparison with the data on North American lignites, fa values of 0.58 and 0.64 have been determined for, respectively, Central Otago and Southland (both New Zealand) lignites [76]. (iii) Calorimetric measurements. The estimation of aromaticity, fa, can be performed using a pressure differential scanning calorimeter (PDSC) [61,77]. A 1-1.5 mg sample o f - 1 0 0 mesh coal is heated in a 3.5 MPa atmosphere of oxygen between 150 and 600~

at a linear rate of

20*/min. The thermograms usually consist of two exothermic peaks; the lower temperature peak is assigned to the aliphatic carbon and the higher temperature peak to aromatic carbon. An example of the PDSC thermogram for Ledbetter (Texas) lignite is shown as Fig. 3.1 [78]. The assignment of the low-temperature peak to aliphatic carbon and the high temperature peak to aromatic carbon is based on model compound studies [61]. A wide variety of model compounds were studied by PDSC, ranging from the purely aliphatic to purely aromatic, nTetracontane (n-C4oH82) has a single, low-temperature peak at 215~

a temperature lower even

than the low temperature peak of a sample of Minnesota peat. In contrast, phenanthrene produces a single peak at 410~

in the temperature range characteristic of the high temperature peak in the

thermograms of subbituminous coals. Retene (7-isopropyl-l-methylphenanthrene, fa = 0.78) produced a thermogram with a small low-temperature peak and a more prominent high temperature peak, remarkably similar to that of Rosebud (Montana) subbituminous coal as shown in Fig. 3.2 [61]. An "apparent aromaticity" is determined by taking the ratio of the high temperature (presumably aromatic) peak height to the sum of the heights of both peaks. For run-of-mine coals, the aromaticity is then calculated from the equation fa = 0.263 + 0.868x where x is the apparent aromaticity [77]. The linear least squares coefficient of determination r2 is 0.85. The original basis on which this relationship was established required knowledge of the "true" aromaticity of the samples; this was determined from the Teichmuller plot of fa vs. maf carbon content [79]. Thus for example an aromaticity of 0.60 was derived from PDSC data for a Beulah lignite [23]. This value is at the low end of those obtained from NMR measurements discussed above, but measurements were not made on the same samples. Separated lithotypes of Beulah lignite gave values of 0.94 for fusain, 0.64 for vitrain, and

98 100

80

60

40

20

0

,

100

I

200

,

I

300

,

I

,

400

I

500

600

Temperature, *C Figure 3.1. PDSC thermogram of Ledbetter lignite [78].

0.63 for attritus [3,77,79]. The remarkably high value for the fusain lithotype of this sample was not observed for fusains of other lignites, but the data for vitrain and attritus are generally comparable with other observations shown in Table 3.13 [80,81]. TABLE 3.13 Aromaticities of lithotypes of Fort Union lignites [80,81].

Lignite Beulah Gascoyne Glenharold Indian Head Velva

Vitrain Attritus Fusain 0.65 0.66 0.80 0.54 0.71 0.78 0.63 0.62 0.67 0.71 0.71 0.77 0.66 0.65 0.86

Whole Coal Calcd.* Exptl. 0.66 0.66 0.64 0.57 0.63 0.63 0.71 0.68 0.66 0.70

*Calculated from the aromaticities of the individual lithotypes, weighted for the amount of each present in the unseparated samples. The aromaticities of the vitrain and attritus are very similar, with the single exception of Gascoyne, and are always lower than the value for fusain. Core samples of Ledbetter (Texas) lignite showed a slight decrease in aromaticity with

99 200

I

'

'

I

'

I

'

I

,

P.

,/" "'~,

I00

0 100

,

I

200

,

I

300

/

,

!

I

400

,

I ~

500

600

Temperature, ~ Figure 3.2. PDSC thermogram of retene (solid line) and Rosebud subbituminous coal (dashed line) [61].

depth, ranging from 0.65 for the Brown seam at 21-28 m to 0.56 for the Green seam at 55-58 m [20,77,79]. The thermograms were very similar in shape and positions of the peak maxima on the temperature scale to those of poly(vinyltoluene), for which fa is 0.66. A comparable study of Baukol-Noonan (North Dakota) lignite showed less variability, albeit over a much narrower vertical span. Over 2 m the aromaticity varied from 0.66 to 0.71 with a standard deviation of 0.02. Nevertheless it did decrease slightly with depth, from 0.71 two meters above the base of the unit to 0.68 at the base. Petrographic data were not available for either set of samples. (iv) Other methods of determining aromaticity. An aromaticity of 0.60 was estimated for a Montana lignite by X-ray diffraction studies [82], based on a resolution of two peaks, the (002) band typical of the interplanar spacing in highly condensed aromatic systems (most notably graphite) and the y band observed, for example, in paraffin and attributed to the spacing between disordered aliphatic or alicyclic structures. An innovative but seldom-used method involving reaction of coal with fluorine indicated a value of 0.52 for a Wyoming lignite [83]. Fourier transform infrared (FFIR) spectroscopy of Beulah lignite indicated fa of 0.84 [84]; well above the aromaticities usually found for lignites, unless the sample happened to be fusain-rich. FTIR examination of Titus (Texas) lignite showed 0.7% aromatic C-H and 4.1% aliphatic C-H [85]. (v) Ring condensation. For an aromatic ring system with a given number of condensed rings, the H/C atomic ratio will vary depending on the type of condensation. That is, for

1 O0 pericondensation, where the rings are as condensed as completely as possible, the H/C ratio will be lower than for a cata- condensed system, in which condensation is minimal. Examples of peri- and catacondensed four-ring systems would be pyrene (12) and chrysene (13), respectively. The relationship between the H/C ratio and the number of rings for peri- and catacondensation has been shown graphically [86]. The relationship between atomic H/C ratio and carbon content has also been shown graphically [87]. Thus for a known carbon content it is possible to estimate the H/C ratio and, from that, the number of condensed rings. In the case of coals having 70--83% carbon, the average ring number found in this way is two [86]. (For two condensed rings there is no difference between peri- and catacondensation, since the only possible structure is naphthalene.) Indeed, most studies of ring condensation suggest that the aromatic ring number of "younger coals" is 1-2 [88]. The average aromatic unit in lignites is estimated to contain nine atoms [89], again implying a mixture of one- and two-ring systems.

12

13

Many oxidation studies also suggest that the aromatic ring systems in lignites are small, with benzene and naphthalene structures predominating. Degradation of lignite with alkaline copper(II) oxide, a reagent used for lignin depolymerization, showed that benzene and phenol structures are the predominant aromatic components [49]. NMR of Beulah-Zap lignite indicates that the aromatic cluster size is nine carbon atoms [66]. The mole fraction of bridgehead carbons is 0.17, with three attachments per cluster and an average molecular weight of 277 [66]. NMR of pyridine extracts from North Dakota lignites that had been treated with sodium hydroxide and ethanol at 300"C for up to two hours shows a total ring number per structural unit of 3.0-3.5, with an aromatic ring number per cluster of 1.5-2.0 [90]. There are on average about 1.5 naphthenic rings per cluster. The aromaticities of these extracts were in the range 0.60-0.66. NMR of Nakayama (Japan) lignite showed that the petroleum-ether-soluble portion of the pyridine extract had single aromatic or quinone tings [91]. The portion of the pyridine extract soluble in chloroform but insoluble in petroleum ether had both single and double ring systems. The temperature of the maximum in the aromatic (high-temperature) peak in the PDSC thermogram was determined for model compounds containing one to five fused aromatic tings.

I01 Generally the temperature maximum of the high temperature peak increases with increasing ring condensation. Similarly, the high temperature peak maximum increases for coals of increasing rank. Although the data have sufficient scatter to warrant plotting as a band rather than as a line, nevertheless in the rank range peat-bituminous the band for the coal samples is remarkably congruent with the band for the model compounds [61]. This similarity of behavior was used to infer that the average ring condensation in lignites is about 1.5 to 2 fused rings per aromatic cluster [23,61]. X-ray diffraction shows that the average diameter of aromatic layers in lignites is about 0.5 nm, consistent with, on average, 7 to 8 atoms per layer [92]. (Considering the differences in techniques used, this is in good agreement with other estimates [66,89] of 9 atoms per aromatic cluster.) The average bond length between atoms in the layers is about 0.1387 nm, for comparison, the length of the C-C bonds in benzene is 0.139 nm [93]. The spacing between layers in lignites is 0.37-0.435 nm, compared with the interlayer spacing in graphite of 0.335 nm. Lignites at the upper end of this rank range show the beginnings of the characteristic (002) band of graphite in the x-ray diffractograms. Pyrolysis/mass spectrometry of Texas lignite indicated the presence of naphthalene-derived structures such as tetralin, dihydronaphthalene, and alkylated naphthalenes, as well as tetrahydroanthracene [94]. Single ring structures predominate among the aromatic compounds in a supercritical fluid extract of lignite [95]. 3.2.2 Aliphatic carbon This subsection discusses the principal forms of aliphatic carbon in lignites, the methods by which they have been studied, and estimates of the amounts of the various aliphatic structures. (i) Oxidation studies. Oxidation of Beulah lignite with ruthenium tetroxide gave relatively high yields of succinic and glutaric acids (totalling 18.3 area % of the gas chromatogram of the methylated products) [60]. This result implies that the major links between aromatic units in this lignite are dimethylene and trimethylene bridges. (Some keto structures, such as tetralone, could also contribute to the formation of these acids.) Succinic acid is the aliphatic dicarboxylic acid produced in greatest amount [96]. This suggests that either dimethylene bridges or hydroaromatic structures such as 4,5-dihydropyrene are major contributors to the structure of this lignite. Oxidation at 27~

in the presence of a phase-transfer catalyst yielded 2.7% succinic, 2.5%

glutaric, and 1.2% adipic acids (maf basis) [97]. The percentage of the carbon in this lignite present in each of the types of aliphatic bridging groups is dimethylene (and possibly 2-tetralone structures), 1.5%; trimethylene, 1.5%; and tetramethylene (and possibly tetralin), 0.7% [3]. Only small amounts of aliphatic monocarboxylic acids were produced from this lignite during ruthenium tetroxide oxidation. A complete analysis of acids observed in the ruthenium tetroxide oxidation of Beulah lignite is shown in Table 3.14 [59,98,99]. The major products are aliphatic dicarboxylic acids and benzene polycarboxylic acids [ 100].

102 TABLE 3.14 Yield of acids from ruthenium tetroxide oxidation of Beulah lignite, % maf [59,98,99]. Acid Succinic 2-Methylsuccinic Glutaric 2-Methylglutaric Adipic 2-Methyladipic Pimelic Suberic Azeleic Sebacic Octanoic Nonanoic Lauric

Yield 2.38 0.41 0.59 0.26 0.11 0.07 0.04 0.12 0.04 0.01 0.02 0 <0.01

Acid Yield Myristic 0.01 Palmitic <0.01 1,2,3-Propanetricarboxylic 0.70 1,2,4-Butanetricarboxylic 0.31 Phthalic 0.07 1,2,3-Benzenetricarboxylic 0.23 1,2,4-Benzenetricarboxylic 0.30 1,3,5- B enzenetricarboxyli c 0.01 1,2,3,4-Benzenetetracarboxylic 1.5 1,2,4,5-Benzenetetracarboxylic 1.53 1,2,3,5-Benzenetetracarboxylic 1.5 Benzenepentacarboxylic 1.5 B enzenehexacarboxylic 0.01

Some ambiguity arises because certain of the aliphatic diacids could be produced from two structures: polymethylene bridging groups or hydroaromatic structures. A suspension of Beulah lignite in xylene was dehydrogenated by refluxing with dichlorodicyanoquinone [59,98]. The yield of succinic acid from subsequent ruthenium tetroxide oxidation decreased from 2.38% in the untreated lignite to 1.42% in the dehydrogenated lignite. This result suggests that about half the succinic acid may derive from dihydrophenanthrene or dihydropyrene structures. Further corroboration is an increase in yield of phthalic acid, from 0.07 to 0.22%. The increased yield of phthalic acid from the dehydrogenated lignite is attributable to dehydrogenation of dihydrophenanthrene or dihydropyrene structures. In comparison, the yields of adipic acid from the untreated and dehydrogenated lignite were 0.11 and 0.12%, respectively. Thus very little of the adipic acid arises from tetralin-like hydroaromatic structures; only about 10% of the C4 units are present in tetralin structures [101]. Dehydrogenation of a fairly aliphatic Big Brown lignite (fa of 0.44) with dicyanodichloroquinone also reduced the succinic acid yield by about 50%, from 2.06% in the untreated lignite to 1.01% in the dehydrogenated lignite [75], again suggesting that about half of the aliphatic precursors to succinic acid are present in hydroaromatic structures based on dihydropyrene or dihydrophenanthrene moieties. Similar reductions, of about 50%, were observed in yields of glutaric acid and adipic acid, suggesting that about the half the total yield of these acids results from the oxidation of indan and tetralin (respectively) moieties. Comparison of the yields of major acids before and after dehydrogenation suggests that tetralin-like structures

(i.e., tetrahydroaromatics) are only about 10% as plentiful as the dihydroaromatics in this lignite. Interpretation of data from the ruthenium tetroxide oxidation is complicated because a given acid product many not be related unequivocally to a particular structure in the lignite. For example, 1,5-pentanedicarboxylic acid arises from at least four types of structures [62], trimethylene bridges between aromatic units, indan, 1-aryltetralins, and structures of the type Ar(CH2)3(CH)(Ar)2.

103 Malonic acid is a very minor product in the ruthenium tetroxide oxidation of diphenylmethane. Consequently, absence of malonic acid from the aliphatic dicarboxylic acid products is not necessarily an indication of the absence of single methylene carbon bridges from the lignite structure. The formation of benzene polycarboxylic acids implies extensive crosslinking of the structure, though some carboxyl groups are already present in the lignite even before oxidation. Methylation of Beulah lignite with methyl-d3 iodide prior to oxidation with ruthenium tetroxide showed that about 10% of the aliphatic diacids were produced from the oxidation of arylalkanoic acids [59]. An example would be the formation of succinic acid from 3-arylpropanoic acid structures. During oxidation with ruthenium tetroxide acetic, propionic, and butyric acids (and, in principle, higher acids) are produced from alkyl side chains. Since oxidation preserves the ring carbon to which the side chain was attached, a methyl side chain produces acetic acid; ethyl, propionic acid, etc. This method showed the following percentages of carbon incorporated in each type of side chain in Beulah lignite: methyl, 1.8%; ethyl, 0.14%; propyl, 0.04% [3,97]. Oxidation of Rockdale lignite with ruthenium tetroxide produced about sixty aliphatic acids [62]. All of the dicarboxylic acids from four to eleven carbon atoms were found. Virtually every isomer of the diacids containing five, six, or seven carbon acids was found, as well as a variety of tri- and tetraacids. The predominant acids among the products included butanedioic, pentanedioic, 2-methyl-l,4-butanedioic, hexanedioic, 2-methyl-1,5-pentanedioic, heptanedioic, octanedioic, nonanedioic, 1,2,3-propanetricarboxylic, phthalic (3), and 1,2,4-benzenetricarboxylic (7) acids [63]. These results show that branched aliphatic structures are relatively abundant in this lignite. About twenty aliphatic tri- and tetracarboxylic acids were observed in the oxidation products from Rockdale lignite [63]. Although unambiguous structures were not assigned, the results suggest the presence of branch points such as

~

H 2 ---CH ---CH 2 - - - ~

D Oxidation of various lignites with ruthenium tetroxide provided evidence for methylene bridges of two to twelve carbons [ 102]. As discussed above, the method is not a satisfactory test for single methylene carbons. The relative amounts of the various methylene bridges decrease with increasing length of the bridge. Oxidation of various lignites with ruthenium tetroxide in the presence of a phase-transfer

104 catalyst [100] was used to compare the yields of six acids: succinic, glutaric, adipic, 1,2,3propanetricarboxylic, phthalic (3), and 1,2,4,5-benzenetetracarboxylic (10). The results are shown in Table 3.15 [59,98,99]. The results of ruthenium tetroxide oxidation of lithotypes of Beulah lignite are shown in Table 3.16 [59,98,99]. TABLE 3.15 Comparative yields of acids* from ruthenium tetroxide oxidation of lignites, % maf. [59,98,99] Lignite Beulah Gascoyne Velva Martin Lake SanMiguel

SU 2.38 2.58 2.71 2.50 2.94

GL 0.59 0.56 0.56 0.96 0.61

AD 0.11 0.13 0.16 0.27 0.12

l:rI" PH 0.70 0.07 0.62 0.07 0.73 0.05 0.76 0.09 0.62 0.04

BT 1.53 1.60 1.37 1.55 0.73

*SU, succinic; GL, glutaric; AD, adipic; FrF, 1,2,3-propanetricarboxylic; PH, phthalic; BT, 1,2,4,5-benzenetetracarboxylic.

TABLE 3.16 Yields of acids* from ruthenium tetroxide oxidation of Beulah lithotypes, % maf [59,98] Lithotype Vitrain Attritus Fusain

SU 2.41 2.10 1.38

GL 0.60 0.50 0.33

AD 0.08 0.12 0.10

PT 0.58 0.52 0.27

PH 0.02 0.08 0.15

BT 0.71 1.38 1.40

*See note for Table 3.15 above.

Comparison of Tables 3.15 and 3.16 shows a wider variation in the amounts of these six acids among the lithotypes of a single lignite than among "whole" samples of different lignites. This finding highlights the great variability in samples from a single mine, and indicates the need for reporting petrographic data (whenever it exists!) with other analytical results. Furthermore, correlation of data from different experiments with the same lignite may be questionable unless all of the experiments have been done with the same sample. Reaction of a North Dakota lignite with peroxytrifluoroacetic acid indicated that 5.5% of the hydrogen originally in the lignite was identified in succinic acid, and 1.2% in malonic acid [103]. Succinic acid is produced from bibenzyl-type linkages, including 9,10-dihydrophenanthrene- and acenaphthene-type structures, -CH2CH2CO- linkages, and indans [103]. Malonic acid presumably arises from single methylene bridges, such as diphenylmethane-type structures. Oxidation converted 3.9% of the hydrogen originally present in the lignite to acetic acid [ 103]. This

105 reaction should convert methyl groups attached to aromatic rings to acetic acid. Reaction of Hagel lignite showed that 0.20% of the carbon was present as arylmethyl groups and 0.06% as arylethyl groups [ 104]. There was no evidence for the existence of arylpropyl structures [ 104]. In contrast, sodium hypochlorite oxidation indicates that about 1% of the total carbon in low-rank coals is present in n-butyl groups and 3-5% is in n-propyl groups [ 105]. Sheridan lignite, methylated with dimethyl sulfate to protect phenolic rings from attack, was oxidized with sodium dichromate. No long-chain acids were produced; but they were formed from a cannel coal containing 19.2% exinite [ 106]. (Sheridan lignite contains about 1% exinite.) The difference in behavior between the cannel coal and Sheridan lignite suggests that the lignite structures responsible for the formation of long-chain acids during oxidation are not the same as the aliphatic structures in cannel coal [ 106]. The latter derive from spores or pollen. Lignite can be separated into aliphatic- and aromatic-rich fractions by treatment with iodine in pyridine, followed by alkaline silver oxide oxidation of the resulting pyridinium iodides [65]. The aliphatic-rich fraction contained materials of molecular weights 60(05000, and was similar in composition to alginite or Type I kerogen, in agreement with results from dichromate oxidation [ 107]. The major fragments observed in solid probe mass spectrometry were alkanes and alkenes up to C 20, alkylated monocyclic alkanes and alkenes, sterane (14), sterene (15), 4-methylsterene (16), tri-, tetra- and pentacyclic terpanes, pentacyclic terpenes, and benzenes and phenols having long chain alkyl substituents.

C 14

15

16 Sheridan lignite contains aliphatic-rich materials similar to Type I kerogen, but not directly

106 related to exinite macerals [ 108]. Oxidation with potassium permanganate produces large amounts of aliphatic dicarboxylic acids in the range C4 to C21, with the C9 acid being most abundant [49,58,108]. Comparable results were obtained in the oxidation of Soya (Japanese) lignite [49]. Branched dicarboxylic acids in the range C5 to C10 and tricarboxylic acids in the range C6 to C8 were also identified in the oxidation products from the Sheridan lignite, but in much lower concentrations than the dicarboxylic acids [58,108]. These compounds might derive from material similar to Type I kerogen; that is, from lipid-rich organic material. Phthalic acid and 1,2,4- and 1,2,3-benzenetricarboxylic acids were abundant among the aromatic products of oxidation [ 108]. Oxidation products from methylated Sheridan lignite are similar to those produced from Green River oil shale [ 107]. Specifically, straight-chain aliphatic diacids were abundant in the range C3 to C15, with a maximum at C9 [107]. Diacids with branched chains or cyclic structures were observed only in much lower quantity. Treatment of Sheridan lignite with 15% nitric acid at 80"C for 14 h yields 11-13% aliphatic diacids in the range C4_C12[49]. Aliphatic diacids in the range C4-C13 were produced by oxidizing this lignite with CsFSO4 [49]. Potassium permanganate oxidation of sporinite isolated from a North Dakota lignite yielded straight-chain dicarboxylic acids as the major class of product [57]. Branched-chain diacids and tricarboxylic acids were minor products. Oxidation with nitric acid showed diacids in the range of C a to C13 [57].

(ii) Nuclear magnetic resonance. NMR of Big Brown lignite shows it to be more aliphatic than typical North Dakota lignites, having an aromaticity of 0.44 [75]. NMR suggests that much of the aliphatic carbon is present is polymethylene chains. Polymethylene structures containing at least four --CH2-- units have been observed by liquefaction in tetrahydroquinoline followed by NMR analysis of the deuterochloroform-solubles [ 109,110]. Application of this technique to a series of Texas lignites showed contents of --(CH2)n-- of 6.53-10.52% for Wilcox Group lignites, 7.83-14.51% for Yegua Formation li gnites, and 8.05-13.89% for Manning Formation lignites (weight percent, maf basis) [ 109]. The average for ten Wilcox Group samples was 8.23%, with a standard deviation of 1.14. For Alcoa (Texas) lignite, 6.6% of the lignite is found as polymethylene groups by this technique [110]. Despite the fact that the Wilcox stretches for hundreds of miles, the uniformity of polymethylene content suggests a relatively constant paleoflora and environment of deposition over a large area [ 109]. NMR of Rockdale lignite of (fa 0.44) showed that the amount of methylene groups present as part of the macromolecular network structure of the coal is greater than that trapped in the coal as extractable fatty acids [111]. Proton NMR spectroscopy of the 3:1 benzene:methanol extracts from seven Texas lignites showed an average CH2/CH3 ratio of 3.49 [111]. (The extract yields were in the range 2.3-7.6%.) By comparison, a German brown coal treated in the same fashion yielded 3.3% extract with a CH 2/CH3 of 5.63. This difference might be attributable to a shortening of aliphatic chains with increasing maturation, or a greater amount of chain branching or methyl substituents in the Texas lignites [ 111]. In comparing these two studies [ 109,111 ] it should be borne in mind that the

107 former observes polymethylene units liberated from the macromolecular structure by breaking covalent bonds, whereas the latter is a characterization of compounds produced by solvent extraction. Supercritical toluene extracts of lignites have sharp bands at 1.3 ppm in the 1H spectra and at 29.7 ppm in the 13C spectra, assigned to methylene groups in chains containing at least eight carbon atoms [95]. The average chain length of the extracts is between 2.5 and 4 carbon atoms [95]. Aromatic structures are predominantly single-ring moieties. Brandon (Vermont) lignite shows a sharp, intense peak for aliphatic carbons indicative of material derived from algal residues [ 112]. This observation compares with results of oxidation studies which suggested the existence of material like Type I kerogen in lignite [65,107]. Two samples of Beulah-Zap lignite provided the 13C NMR data on aliphatic carbon summarized in Table 3.17 [66]. TABLE 3.17 Aliphatic carbon types in Beulah-Zap lignites* [66]. Sample Carbon type Total aliphatic carbon (fai) Aliphatic carbon as CH or CH2 (falH) CH3 or non-protonated aliphatic carbon (fai*) Aliphatic carbon bonded to oxygen (faio)

A 0.39 0.25 0.14 0.12

O 0.34 0.21 0.13 0.10

*A = sample from Argonne National Laboratory premium coal sample bank; O = non-premium sample believed to have been oxidized. (iii) Other instrumental methods. Fourier transform infrared spectroscopy (FTIR) of Bienfait lignite shows an absorption at 693 cm-1 attributed to a double bond in a side chain, or possibly in a trapped long-chain compound [ 113]. This band could also arise from substitution of dissimilar groups onto an aromatic ring [ 113]. Examination of the asymmetric C-H stretch in the FFIR spectra shows very few --CH3 bands in Beulah lignite [114]. T h e - C H 3 / - C H 2 - r a t i o increases with rank. FTIR of Titus lignite showed 4.1% by weight of aliphatic C-H, compared to 0.7% aromatic C-H (both on a dmmf basis) [85]. Secondary ion mass spectrometry (SIMS) shows that, in general, low-rank coals contain more methyl groups than higher rank coals, on the basis of comparison of intensities of the peak at 15 daltons [ 115]. The complete SIMS spectra of low-rank coals are similar to those of wood. The material considered to be amorphous in X-ray diffraction includes aliphatic side chains on aromatic ring systems, phenolic oxygen, and other functional groups [92]. The amount of amorphous material decreases with increasing carbon content. In the range of 60-70% carbon, the amorphous material decreases from about 38% of the coal to about 34% [92]. (iv) Studies of lignite extracts. Henning lignite, Seyitomer (Turkey) lignite and three British

108 bituminous coals, were studied by a series of pyrolysis and extraction methods [ 116]. Extractions were done using supercritical toluene and a hydrogen-donor solvent. After separation of the products into preasphaltenes, asphaltenes, and pentane-solubles, analysis was done using 13C NMR and GC/MS. About 30% of the total carbon in the two lignites is present as aliphatic carbon. Less than half of the aliphatic carbon is present in hydroaromatic structures or bridges between aromatic ring systems. About 20% is present as methyl groups. Alkyl chains of eight or more carbons account for about 35% of the aliphatic carbon (equivalent to 10% of all the carbon in the lignite). Long-chain n-alkanes and high-molecular-weight aliphatic acids or waxes are minor components. This suggests that many of the long aliphatic chains are bonded to aromatic structures, and contrasts with the view (e.g., [ 112]) that aromatic and long-chain aliphatic materials represent separate domains in lignite. Aliphatic structures of 23-25 carbon atoms, possibly remnants of the original plant material, have been isolated from Novodmitrovskoe (Ukrainian) lignite [ 117]. Sequential solvent extraction of Beulah-Zap lignite shows terpenoid hydrocarbons to be the major components of the extract, with tricyclic diterpenoids the most abundant [118. n-Alkanes are relatively minor components. In the range C16-C32the alkanes with odd numbers of carbon atoms predominate, with a maximum at C27 [118]. (v) Characterization of reaction products. Straight-chain hydrocarbons in the C24-C28 range were isolated from the neutral fraction of lignite low-temperature tar [33]. The largest alkyl substituent on an aromatic ring was found to be C4. The largely aromatic structures were present in a dispersed phase in an aliphatic matrix of high-molecular-weight hydrocarbons, forming a solid colloid. Additional structural integrity is provided by crosslinking in the dispersed aromatic phase and by hydrogen bonding. This structural model accounts for the existence of long-chain aliphatic molecules. Compounds making up the aliphatic matrix, which is not necessarily the major portion of the lignite structure, could derive from the waxes and resins of plant debris. A contribution of the phytyl chain from chlorophyll was also suggested [33]. Flash pyrolysis indicates that lignites may contain up to 10% polymethylene units of the type --(CH2)n-- [ 119]. Pyrolysis at 600~ drives off some of these units as alkanes or alkenes in the range C17 at least to C24 [119]. Flash pyrolysis of Alcoa lignite produced various light hydrocarbon gases, including 3.2% ethylene, 0.74% propylene, and 0.41% butadiene [120]. These products arise from polymethylene precursors having more than four --CH2-- units in the chain [120]. The amount of --(CH2)n--, where n>4, in this lignite amounted to 5.3% based on the flash pyrolysis results [ 110]. This is somewhat lower than the estimate of 6.6% obtained from NMR analysis of products of liquefaction of this lignite in tetrahydroquinoline [ 110]. Lignite liquefaction products contain a homologous series of alkanes, n-Alkanes with up to 32 carbons are obtained in yields of 3-5% (maf basis) during liquefaction with synthesis gas. Since there seems no plausible way in which these compounds could be synthesized in situ or be an artifact of the experiment, these alkanes are present in the lignite and are liberated during liquefaction [121,122].

109 Reaction of Texas lignite in pyrolysis, in hydrogen, or in the presence of a hydrogen donor solvent (tetralin) produces in each case products composed predominantly of long-chain aliphatic compounds [ 121 ]. The reaction temperatures were kept below 400~ to avoid secondary reactions of the products. The relative proportions of the alkanes from each type of experiment were essentially the same, although pyrolysis in inert gas gave the smallest yield of alkanes and reaction in tetralin the highest. The constant proportion of alkanes released in each experiment suggests that these products are indeed constituents of the lignite structure, and presumably derive from the original plant material. Soxhlet extraction of Beulah-Zap lignite with tetrahydrofuran yields cyclic and branched alkanes, diterpanoids, tetracyclic tel-panes, and pentacyclic triterpanes [ 122]. Subsequent reaction with hydrogen at or below 290~ in methanol (290~

followed by reaction in a 10% solution of potassium hydroxide

nitrogen atmosphere, 1 h) gave a product containing n-alkanes, alkyl

aromatics, and heterocyclic compounds [122].The aliphatic portion is 4.5% of the product, equivalent to 3.2% of the lignite [122]. The results indicate that alkyl groups and methylene bridges are part of the macromolecular network of the lignite. A smooth distribution of n-alkanes in the range C 12-C40 occurs, with no preference for odd- or even-numbered carbon chains. Some branched alkanes and alkenes were also observed. Dehydrogenation of vitrains has shown that 25-30% of the hydrogen content of lignites is present in hydroaromatic structures [123]. The reactions were carried out using 1% palladium dispersed on calcium carbonate as the dehydrogenation reagent in phenanthridine as the vehicle [123,124]. After depolymerization with phenol and boron trifluoride at 100~

the yield of phenol-

soluble products was proportional to the number of cleaved methylene bridges [ 125]. Lignite gave a 72.8% net yield of phenol-solubles, 2.5 to 8 times as great as yields observed from bituminous coals. 3.3 H E T E R O A T O M S

3.3.1 Introduction Oxygen is by far the most important of the heteroatoms in lignite. North American lignites can contain over 20% oxygen on an maf basis. Most of this section is a discussion of the distribution of oxygen among various functional groups. Much less is known about the organic sulfur and the nitrogen in lignites. Each accounts for about 1% of the lignite (maf basis). Few thorough investigations have been conducted on the contribution of these two elements to the structure of the lignite, or on the functional groups in which they are incorporated. 3.3.2 Carboxylic acid groups The carboxylic acid group is one of the most important of the oxygen-containing groups in lignites. In North Dakota lignites up to 65% of the oxygen occurs in these groups [ 126]. However,

110 this value is quite high compared to most of the measurements reported in the recent literature. A Beulah-Zap lignite in which the total organic oxygen content was 17.21% (dry basis), as measured by neutron activation analysis, contains 3.81-3.94% oxygen as carboxyl oxygen [127]. This is equivalent to 22-23% of the total oxygen being present in carboxyl groups. A value of 18% was observed for Estevan lignite vacuum-dried at 100*C [128]. Some estimates give values even lower, 10-13% of the oxygen as carboxyl, for example [129]. The carboxylic acid group is labile under mild thermal conditions, decomposing to carbon dioxide well below 500* C. Although the volatile matter content of lignites is higher than of bituminous coals, much of the material lost from lignite as volatiles is carbon dioxide rather than hydrocarbon gases, oils, or tars. Thus only about 17% of the calorific yield of North Dakota lignite occurs in the volatile products whereas about 30% occurs in the volatiles from bituminous coals [ 130]. The ability of the acid groups to bind cations on ion exchange sites is important in the inorganic chemistry of lignites (Chapter 5). A carboxyl-metal cation-carboxyl linkage may also have a role in crosslinking of the lignite structure [89]. A general procedure for the determination of carboxylic acid groups has been developed [131,132]. The basis of the method is that the carboxyl groups, which have all been put into the acid form by a preliminary demineralization step, are reacted with barium acetate solution. Barium ions displace acidic protons from the carboxyl groups. Titration of the now-acidic solution with standard base determines the quantity of protons released, equivalent to the number of acidic functional groups in the coal. Experimentation with several variations of the general method led to the recommendation of a single reflux of 4 h with barium acetate at pH 8.25 followed by titration of the acetic acid formed as the most reliable method [131]. North Dakota and Texas lignites contain 2.2-2.3 meq/g (dmmf) carboxyl [131]. Total acid group contents (i.e., carboxyl plus phenolic hydroxyl) range from 7.5 for a South Dakota lignite and 6.9 for a North Dakota lignite, to 6.1 for a Texas lignite, all reported as meq/g, dry basis [132]. Demineralization preliminary to the determination of carboxyl groups involves reaction with 5M hydrochloric acid, concentrated hydrofluoric acid, and then concentrated hydrochloric acid, all at 55--60~ for 1 hour, followed by extensive washing with distilled, CO2-free water [133]. The carboxyl content can then be determined by reaction with 1N barium acetate at pH 8.25-8.30, followed by titration with 0.05N barium hydroxide to restore the pH to the initial value [ 133]. Carboxyl contents of various lignites are shown in Table 3.18. The percentage of the total oxygen content of the coal incorporated in the carboxyl groups is also given, when that information was provided in the original literature. A modification to the barium-exchange method involves ion exchange with barium chloride solution at pH 8.3 in a blender under a nitrogen, followed by titration [75]. For Big Brown lignite, the carboxyl content was found by this method to be 3.6 meq/g (maf basis). Data for Beulah lignite highlight the variability of carboxyl content [23,134,135,138]. Six samples show carboxyl contents ranging from 1.46 to 2.97 meq/g with a median value of 2.31 meq/g. Four samples identified as "Seam 2" Beulah lignite ranged from 2.25 to 3.67 meq/g, with a median of 2.72. It is likely that some of the spread in the data may be accounted for by variations

111 TABLE 3.18 Carboxylic acid contents for lignites Source of lignite Carboxyl State Seam or Mine meq/~ (dmmt) Alabama Choctaw 2.83 Alabama Pike County 2.04 Montana Fort Union 3.00 N. Dakota Baukol Noonan 2.74 N. Dakota Beulah 2.78 N. Dakota Beulah 2.30 N. Dakota Center 3.58 N. Dakota Freedom 2.60 N. Dakota Gascoyne Blue 2.68 N. Dakota Gascoyne Red 2.46 N. Dakota Gascoyne White 2.62 N. Dakota Gascoyne Yell. 2.67 N. Dakota Hagel 3.13 N. Dakota Indian Head 2.45 N. Dakota Kincaid 2.89 N. Dakota (unident.) 2.2 N. Dakota Velva 2.55 Texas Big Brown 2.26 Texas Bryan 2.25 Texas Darco 2.11 Texas Ledbetter 1.82 Texas San Miguel 1.4 Texas (unident.) 2.3 (unident) 1.1

Percent of oxygen 42 38

46

37

Ref. [ 134,135] [134,135] [133, 136,137] [134,135] [138] [23] [134,135] [139] [138] [26] [26] [138] [133,136,137] [134,135] [33] [131] [134,135] [ 134,135] [134,135] [ 133,136,137] [ 134,135] [ 134] [ 131 ] [ 138]

in experimental technique; furthermore, no information was provided on how long the samples might have been exposed to air or on the petrographic composition of the samples. (The carboxyl contents of vitrain and fusain from Beulah lignite are 2.28 and 2.49 meq/g, respectively [140], and attritus, 2.83 meq/g [139].) Nevertheless, the carboxyl content likely varies within a seam. Certainly the amount of inorganic material in the lignite, including the ion-exchangeable cations (Chapter 5) varies throughout the seam. The variation in carboxyl content may relate directly to the observed variation in inorganic composition. Values for two samples of Gascoyne "Blue Pit" lignite are 2.71 and 2.90 meq/g; for two samples of "Red Pit" lignite, 2.46 and 2.57 meq/g [26]. (These designations refer to different working pits within the mine.) The data for Gascoyne lignite in Table 3.18 also indicate how carboxyl content may vary from one working pit to another within the same mine. The total oxygen incorporated in carboxyl groups for three lignites [ 136] is lower than the reported maximum of 65% of total oxygen incorporated as carboxyl groups [ 126]. For these three samples, the percentage of carbon in the carboxyl groups are 7.6% for Hagel, 3.4% for Darco (Texas), and 5.2% for Montana lignite [133]. Approximately 50% of the total surface area measured by CO2 adsorption is covered with carboxyl groups, of which about half are in the

112 cationic form [137,141]. Acidic hydrogen in six Texas lignites was determined by introducing a pulverized sample into a solution of lithium aluminum hydride in diethyl ether, and measuring the total volume of hydrogen gas evolved. This method will liberate hydrogen attached to any group acidic enough to react with lithium aluminum hydride; thus it does not differentiate between carboxyl and hydroxyl groups. Active hydrogen ranged from 1.52 to 2.77 meq/g (daf basis), averaging 2.22 meq/g [111]. Assuming that the active hydrogen represents 2.22 meq/g of carboxyl, this carboxyl content is equivalent to 71 mg/g of oxygen, or about 7.1%. The total oxygen content of these six samples, as determined by neutron activation analysis, was 26.09%-36.15% [111]. Thus a significant fraction, roughly four-fifths, of the oxygen in these lignites must be present in carbonyl, methoxyl, and other ether groups. The reaction of lignite with sulfur tetrafluoride has been suggested as a method for determining carboxyl groups [ 142]. Carboxyl groups are converted to acyl fluorides, which can then be determined by high-resolution solid-state NMR. IR evidence exists for the conversion of --COOH to --COF by reaction with sulfur tetrafluoride at 110~ for 1 h [143]. The FFIR spectrum of Beulah lignite shows a major band at 1562 cm-1, assigned to carboxylate groups [114]. Major bands at 1655 and 1520 cm-1 in the spectrum of Beulah-Zap lignite are assigned to conjugated carbonyl and carboxyl groups [ 114]. A concern about reliance on the barium-exchange method for carboxyl determination is that the exchange reaction might not be complete, and therefore the results might underestimate the total quantity of carboxyl [144]. Conversion factors have been developed to relate the intensity or area of the 1700 cm-1 FFIR peak to the percentage of oxygen in the sample present as --COOH [144]. X-ray photoelectron spectroscopy of Beulah lignite, using poly(ethylene terephthalate) as a calibration standard, estimated the molar ratio of carboxylate to all C-O single bonds to be 0.625 [23]. The spectroscopic data could not be refined to distinguish between phenolic and ether C-O. Little work has been done on applying XPS to oxygen functional group analysis of lignites. However, this result is high relative to the ratio of [-COOH]/{[-C-OH] + I-C-O-C-]} calculated from data obtained from other methods. The value of this ratio is 0.33 for a Beulah-Zap lignite calculated from the data in [127], and 0.40 calculated from the data in [128]. Ruthenium tetroxide oxidation provides information on the structural arrangement of carboxyl groups [97]. The products include three tricarboxylic acids, oxalomalonic (17), 7.7% of the organic products of the reaction; oxalosuccinic (18), 3.2%; and oxaloglutaric (19), 1%. The benzoic, phenylacetic, and phenylpropionic acid structures of this lignite are in the proportion 65:27:8. Oxidation of Beulah lignite methylated with methyl-d3 iodide indicates that about 10% of the aliphatic diacids produced in the reaction derive from arylalkanoic acids such as 3arylpropanoic acid [59]. This result indicates that at least some of the carboxyl groups in lignite are at the ends of aliphatic chains connected to aromatic rings. The 1600 cm-1 band in the Raman spectrum (assigned to the E2g mode of a graphite-like structure) of two Beulah lignites differing in carboxyl contents is less intense in the sample of higher carboxyl content [23]. This suggests that

113 HOOC CHCOOH I CO I COOH 17

HOOCCHCH 2COOH I CO I COOH 18

HOOCCH ,,CHCH 2COOH "1 CO I COOH 19

the steric requirements of the relatively bulky carboxyl groups, with their associated cationic counterions, may prevent alignment of aromatic ring systems, affecting long-range structural order in the lignite. Fatty acids have been isolated from lignites [145]. In contrast to modern biological systems, in which fatty acids with even numbers of carbon atoms predominate, the isolated acids include ones with odd numbers of carbon atoms [145]. Microbes may preferentially use the evennumbered acids, so that even if they far exceed those with odd-numbered chains in modern systems, microbial consumption of the even-numbered chains increases the proportion of oddnumbered acids among the total [ 146]. Waxes isolated from lignites also contain both even- and odd-numbered fatty acids and alcohols [145]. Acidic fractions isolated by sequential extraction of Beulah-Zap lignite consist mainly of even-numbered acids in the range C22-C30, with a maximum at C26 [118]. Field ionization mass spectrometry signals from Beulah-Zap lignite also demonstrate the existence of fatty acids [147]. 3.3.3 Humic acids Extraction of fresh lignite with dilute aqueous alkali yields 2-4% of so-called natural humic acids [33]. Up to 90% of the original dry, ash free organic material can be extracted from Dow (Texas) lignite after reaction with 0.25N sodium hydroxide solution for 5 h at 433 K [148]. Extraction of German brown coals with 1% ammonia, followed by precipitation with HCI, yields 0.5-3% humic acids [ 149]. Since mild atmospheric oxidation of the base-extracted residue yields more humic acids on repeated extraction, it is not clear whether the natural humic acids were present in the lignite or formed as a result of initial oxidation as soon as the lignite is first exposed to the atmosphere. Nevertheless the extracted humic acids represent only a minor modification of the lignite structure. Lignites can be converted to sodium humates by high speed blending in aqueous sodium hydroxide [150]. The yield increases in the presence of small amounts of air (i.e., air leaking into the apparatus--a kitchen blender--which had initially been purged with nitrogen). Humic acids from Big Brown and Beulah lignites and Wyodak (Wyoming) subbituminous coal show similar aromatic/aliphatic peak intensity ratios in 13C NMR spectra, suggesting similarities in the chemical nature of the acids. High speed blending (about 24,000 rpm) of Beulah lignite in 5% aqueous sodium hydroxide gave 89% conversion to humic acids. The acids had a weight-average molecular weight of 4.3 x 105. The weight-average molecular weight is inversely proportional to the rotational speed

114 of the blending, suggesting that higher shear rates effect greater disruption of the coal structure, giving higher yields of a lower molecular weight product. The effect of blending conditions on Russian low-rank coals has been demonstrated [151]; amorphous humic acids are recovered, but the properties of the products depend on the conditions of blending (i.e., choice of reagents, and use of an air or inert atmosphere). Very low conversions were obtained when the reaction was attempted with 30% aqueous ammonia (12%) or with pyridine (8%), suggesting that the liberation of humic acids is neither the result of cleavage of hydrogen bonds nor of ester hydrolysis [ 150]. Humic acids isolated by high speed blending of Beulah lignite in nitrogen have a composition of 67.5% C, 4.2% H, 0.86% N, and 26.7% O (maf basis) [150]. The inorganic content of the humic acid is also low, reported as 0.83% ash [ 150]. The integrated peak intensity for the carboxyl carbons in the 13C NMR spectrum was about 11% of the total integrated intensity. Some differences in the elemental composition and the carboxyl content are observed depending on the specific isolation technique. Blending Big Brown lignite with 5% sodium hydroxide gave a 81% yield of a humate latex [75]. The humic acids were more aromatic than the original lignite. The molecular weights of reduced, methylated derivatives of the acids were 1,300,000, similar to humic acids from North Dakota lignites. (Humic acid extracted from Beulah lignite had a molecular weight of 1,300,000, as measured by low angle laser light scattering [ 152].) Alkaline digestion of Australian brown coal results from disruption of the structure by electrical double layer repulsion and migration of water molecules into the hydrogen-bonded gel structure of the coal [153]. The mechanism involves interaction of hydroxide with the acidic functional groups, followed by chemisorption of alkali metal cations by the coal structure. 3.3.4 Methoxyl groups Arylmethoxyl groups are important in the structure of North Dakota lignite, but may be insignificant in Wyoming lignite [103]. Oxidation of North Dakota lignite with peroxytrifluoroacetic acid indicated that about 15% of the total hydrogen was present in arylmethoxyl groups, comparable to lignin [103]. This oxidation converts arylmethoxyls to methyl trifluoroacetate, which was measured by NMR. A simplified methoxyl determination involves refluxing lignite in concentrated sulfuric acid, followed by distillation and gas chromatographic measurement of the liberated methanol in the distillate [102], a value of 0.71% being found for Indian Head (North Dakota) lignite [154]. Kincaid (North Dakota) lignite has a methoxyl content of 0.90%, or 0.29 meq/g, as determined using the Zeisel method [33]. The methoxyl content of Indian Head lignite was determined to be 0.7% (as-received basis) by oxidation with peroxytrifluoroacetic acid, followed by hydrolysis of the methyl ester products and gas chromatographic measurement of the liberated methanol [155]. This gives quite good agreement with the method using sulfuric acid reaction [102]. The methoxyl groups are very labile in hot water drying (Chapter 10) and extraction with supercritical water. The methanol yield during

115 hot water drying of the same lignite at 325~ was 0.5%, and during supercritical water extraction at 390"C, 0.6% (as-received basis) [ 155]. Oxidative degradation with sodium periodate is specific for o-methoxyphenols (the products are methanol and o-benzoquinone). Determination of the methanol provides a measure of guaiacol groups not etherified at the 4-0 position. This procedure applied to Beulah lignite showed at least a third of the methoxyl groups present in free guaiacol structures [3,155]. A similar proportion is observed in sodium periodate oxidation of wood [ 155]. Under reflux, the methanol yield increases from 0.25 to 0.44% (as-received basis), due either to the depolymerization of the lignite structure making more guaiacol groups accessible to periodate, or to hydrolysis of omethoxyphenyl linkages generating new guaiacol units [155]. The methoxyl contents of Beulah lignite lithotypes, determined by the peroxytrifluoroacetic method, were 1.3% for the vitrain and 0.7% for attritus (maf basis) [3]. The composite sample

(i.e., not separated into lithotypes) contained 1.3%. A value of 1.8% (maf basis) has also been reported for methoxyl in Beulah vitrain [156]. Methoxyhydroxybenzene structures in the vitrinitic components of lignite derive from fossil lignin moieties [ 147]. Fusain contains 1.1% methoxy on an as-received basis [155]. A composite sample of Indian Head lignite treated in a similar manner had 1.2% methoxyl. The methoxyl content, determined by this method, for a coalified tree stump from the Beulah mine was found to be 0.2% on an as-received basis [ 155]. (Fusain also had the highest amount of residual cellulose, discussed below. Whatever geochemical process interrupted the degradation of cellulose also interrupted lignin degradation. The low value obtained for the coalified tree stump may reflect an easier access of fungal enzymes to the isolated stump than to highly compressed lignite in the interior of a seam.) Potassium permanganate oxidation of sporinite, followed by methylation of the products with dimethyl sulfate-d6, produces both methoxy-d3- and methoxybenzenecarboxylic acids [157]. This confirms the existence of both hydroxy- and methoxybenzene structures in sporinite. 3.3.5 Phenolic groups Many of the aromatic tings in lignites have an --OH group, and some have either a second --OH group or one or two methoxyl groups [158]. Beulah-Zap lignite contains 9.16% oxygen (dry basis) as phenolic oxygen, amounting to 53% of the total organic oxygen in the sample [ 127]. For Estevan lignite, the corresponding data are 7.2% and 38%, respectively [128]. A phenolic content of 2.97 meq/g has been determined in Kincaid lignite [33]. Extraction of Estevan lignite with benzene at 4.8 MPa (approximately 280~

for 3-10 days

gave an extract separable into acidic and non-acidic portions by treatment with 5% aqueous potassium hydroxide. Phenols were liberated by acidification and subsequently identified as their brominated derivatives. The total phenolic content of this lignite was 2.55%, the principal phenols being phenol, p-cresol, and catechol [55]. The same phenols, in the same proportion, were extracted from Morwell (Victoria, Australia) brown coal. Extraction of mono- and dihydric phenols from lignite with benzene suggested that the phenols exist as such in the lignite, either as free

116 phenols or in some weakly bonded association [33]. The yield of phenols increases with increasing pressure at least to 280~ [33]. Extractions at high temperature are ambiguous in that one is not sure whether the increased yield is due to improved extraction of species already existing in the lignite or is due to species liberated by thermal decomposition. In this case, 280 ~was presumed to be below the onset of thermal decomposition. Reaction with triethylborane has been proposed as a means of determining hydroxyl groups [ 159]. The reagent reacts with hydroxyl groups to form the O-diethylborylate and liberate ethane. The ethane can be determined volumetrically. No data on North American lignites have been published, but for three German brown coals containing 67% carbon and 26-27% oxygen (daf basis) the hydroxyl contents determined by O-diethylborylation ranged from 6.20 to 7.83 mmol/g, equivalent to the incorporation of 36 to 46% of the total oxygen in hydroxyl groups [ 159]. This is in reasonable agreement with results mentioned above, but obtained by greatly different methods, for Beulah-Zap [ 127] and Estevan [128] lignites. Potassium permanganate oxidation of sporinite from North Dakota lignite and methylation of the oxidation products with dimethyl-d6 sulfate produced a mixture of methoxy-d3 and natural methoxybenzenecarboxylic acids [57,157]. This indicates that the aromatic units contain both phenolic and methoxyl functional groups. GC-MS analysis of the product mixture indicates that the phenolic groups are more prevalent than the methoxy by a factor of about 9.4 [57]. 3.3.6 Carbonyl groups The carbonyl oxygen content of Kincaid lignite was determined to be 4.4% (maf basis) [33], equivalent to 2.7 meq/g of oxygen in ketone, quinone, and aldehyde groups. Beulah-Zap lignite contains 1.96% oxygen in carbonyl structures, equivalent to 11% of the total organic oxygen [ 127]. Results for Estevan lignite are somewhat higher, 3.6% oxygen in carbonyl groups, or 19% of the total oxygen [128]. NMR of Estevan lignite suggests 29-32% of the structural groups in the coal contain C=O structures [39], presumably including carbonyl and carboxyl. The existence of carbonyls in Beulah lignite has been suggested on the basis of the band at 1655 cm-1 in the FFIR spectrum [114]. 3.3.7 Ethers other than methoxyl Ethers in Beulah-Zap lignite, estimated by difference after determination of carboxyl, carbonyl, and phenolic groups, account for 2.28% oxygen, or about 13% of all the oxygen in the lignite [ 127]. The comparable figure for Estevan lignite is 25% of the total oxygen in "residual" oxygen functional groups [ 128]. 4-Nitroperbenzoic acid cleaves benzyl ethers [160]. Benzyl ether linkages are an important component of lignin structures and may persist from the lignin into coals. Reaction of Beulah lignite with 4-nitroperbenzoic acid in refluxing chloroform, followed by extraction of the residue with base, gave yields of humic acids of up to 90% (maf basis) [152,161]. The soluble material resembled the waxy material removed from lignite by solvent extraction prior to oxidation.

117 Cleavage of benzyl ether crosslinks generated base-soluble carboxylic acid or phenolic functional groups. (This reaction could involve other structural featrues which are reactive toward 4nitroperbenzoic acid; carbon bridges between aromatic units, as in diphenylmethane, are unreactive, but phenanthrene and anthracene structures could be oxidized at least as far as quinones.) The molecular weight of the base-soluble product was determined by low angle laser light scattering to be 1,300,000 [152,161], comparable to that of humic acids extracted directly from the lignite by base. Thus oxidative cleavage of benzyl ethers releases structural units of very large size, comparable to humic acids. Since the yield is much larger from the 4-nitroperbenzoic acid oxidation than from base extraction without prior oxidation, the results imply that large macromolecular humic acid-like structural units are present in the lignite, held in place by benzyl ether groups. Reaction with phenol and boron trifluoride, a reagent pair well known to depolymerize coal [ 125], showed that lignite produced the highest yield (75%) of phenol-soluble products of any of the ranks of coal tested [ 162]. Substitution of p-toluenesulfonic acid (pTSA) for boron trifluoride converted over 90% of low-rank coals to pyridine-soluble products [162]. For lignites, a major contribution to this extensive depolymerization is cleavage of aliphatic and benzyl ethers. Extensive depolymerization of lignite in these reagents suggests the presence of abundant ether linkages. However, aliphatic bridges linked to single phenolic rings are also reactive enough to participate in depolymerization by phenol-BF3 or phenol-pTSA. The well-known ability of aqueous alkali to solubilize large amounts of lignite has been attributed to the cleavage of ether linkages [163]. The benzyl aryl ethers of the 1~-O-4, ~x-O-4, andy0-4 type appear to be especially labile. (The structural notation for lignin is given on page 70.) The reaction occurring upon blending lignite in aqueous sodium hydroxide proceeds by base-catalyzed guaiacol ether cleavage similar to the alkaline pulping of lignin. Humic acids have been isolated from brown coals by treatment with sodium in liquid ammonia [ 164], a reagent sufficiently basic to convert ethers to alkenes [165]. Benzyl ethers in lignin-derived structures may be sterically hindered, and thus high base concentrations and high shear rates may be important in breaking the lignite structure down to colloidal dimensions. Humic acid derivatives obtained by high speed blending have a weight average molecular weight of 4.3 x 105, lower by a factor of two than obtained by medium-speed blending, and lower by a factor of three compared to magnetic stirring [150]. The yield was highest after high-speed blending. The more severe blending breaks more crosslinks in the lignite structure, giving a humic acid yield that is higher but in fragments of lower molecular weight. Oxidation of Sheridan lignite with potassium permanganate forms, among many other acids, furan-, benzofuran-, dibenzofuran-, and xanthonecarboxylic acids, indicating the presence of some oxygen in heterocyclic ring structures [ 108]. Oxidation with sodium dichromate at 2.50~ for 36-40 h produced cyclic ethers having the following approximate relative abundances: benzofuran (20), 7%; isochroman (21), 2.5%; dibenzodioxane (22), 2%; xanthone (23), 17%; benzoxanthone (24), 1%; and dibenzofuran (25), 2.5% [52]. Some oxidation studies suggest that

118

20

21 O

23

22 O

24

25

the contribution of cyclic structures such as furan, 20, 23, and 25 is minor [50]. Reactions of Texas lignites with mixed carboxylic-sulfonic acid anhydrides indicate that few ether linkages occur [ 111]. 3.3.8 Ester groups Little seems to be known about the contribution of esters to lignite structure, although there seems to be a consensus that these groups are not of great importance. Reaction of Beulah lignite with sodium methoxide in methanol produced a waxy material with no spectroscopic evidence of incorporation of methoxide into the residue, indicating that no transesterification with methoxide had occurred. This observation suggests that esters are probably not an important component of the structure of lignite [150]. Some mono- esters and aromatic di-esters have been observed in BeulahZap lignite by field ionization mass spectrometry [ 147]. 3.3.9 Cellulose and lignin residues Plant lignin is first concentrated, and then modified, in the conversion to lignite [166]. Formation of methanol during pyrolysis indicates the presence of guaiacol units [3]. They may be present as plant lignin that has survived coalification as relatively unaltered lignin substructures, or as isolated guaiacols liberated by a decomposition process that did not affect the methoxy or side chain carbons. Since thermal alteration of lignins generally involves loss of methoxy and conversion of side chains to ketones or carboxylic acids, it seems more likely that relatively unaltered lignin is present. Guaiacol, 4-methylguaiacol, 6-methylguaiacol, 4-ethylguaiacol, and 4propylguaiacol have been identified as pyrolysis by-products in the gasification of lignite [167]. (Examples of several of these structures are shown in Chapter 2.) Lignin-like polymers are an important component of the structure of low-rank coals. Oxidation of low-rank coals with alkaline copper(II) oxide produced large quantities of p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and 4-hydroxybenzenedicarboxylic acids [ 108]. Lignin

119 polymers are converted to lignin-like materials during early stages of coalification, and these ligninlike polymers become the foundation of the macromolecular structure of the lignite. Preferential concentration of these lignin-like polymers, along with phenolic materials and lipids, then results in the formation of peat and, eventually, lignite. Pyrolysis/mass spectrometry of Texas lignite shows lignin peaks diminished relative to modern biomass samples [102]. Similar experiments with lowmoor peats also show a diminution of lignin monomer peaks in the mass spectrum relative to modem biomass. Formation of p-hydroxybenzoic acid during lignite oxidation indicates the existence of lignin-like structures [50]. Oxidation of Sheridan lignite with alkaline copper(II) oxide produced large amounts of p-hydroxy- and 3,4-dihydroxybenzoic acids [ 168]. Both are known oxidation products of lignin. No o- or m-hydroxybenzoic acids were found. Conversion of the hydroxy acids to deutromethoxy ethers and subsequent mass spectral analysis showed that the phenolic acids derived from cleavage of alkyl ether structures via the reaction

ROCH 2@

O

R

'

[~ Hooc oH

rather than methoxyaryl ethers. No significant amounts of methoxyaryl ethers formed. Hydroxybenzene polycarboxylic acids and hydroxynaphthalene carboxylic acids in the products indicate that lignin-derived structures are more aromatic and more highly crosslinked in the lignite than in natural lignin. Taylerton (Saskatchewan) lignite exhibits thermal decomposition behavior (differential thermal analysis in a nitrogen atmosphere, heating at 6~

similar to that of a blend of 80%

Klason lignin and 20% cellulose [169]. The lignite has exothermic peak maxima around 300 ~and 455~ the cellulose:lignin mixture, around 320 ~and 420~ A lignite-like thermogram is also obtained from synthetic humic acids produced by the oxidative polymerization of hydroquinone with sodium peroxodisulfate [169]. Determination of methoxyl groups provides an estimate of the lignin content of lignite. A methoxyl content of 0.75% was determined for Beulah lignite by oxidation with peroxytrifluoroacetic acid, followed by hydrolysis of the methyl trifluoroacetate and gas chromatographic determination of methanol [ 170,171]. If an average structure for lignin equivalent to poly(coniferyl alcohol) were assumed, this result corresponds to a lignin content of 5% in this lignite sample [ 170,171 ]. For various lignites the calculated lignin concentration ranged between 5% and 10% [102]. These same lignites contained only traces of cellulose residues. The results again substantiate the concept that cellulose structures break down early in coalification whereas lignin endures for longer times. Decomposition of the partially coalified lignin in reactions such as in liquefaction produces a mixture of phenols and aromatic hydrocarbons in ratio of about 1:1 [1721.

120 Few arguments have been raised in opposition to the presence of lignin residues in lignite. Degradation of Texas lignites with thioacetic acid and boron trifluoride etherate (Nimz's procedure) produced no monomeric phenols typical of lignin degradation products [ 111 ]. This conclusion is based on a search for characteristic methyl ether and aromatic ring protons in the 1H NMR spectrum and methyl ether resonances in the 13C NMR spectrum of the products. These negative results suggest that polymeric phenolic linkages typical of lignin are not present in the Texas lignites; however, no palynologicat or petrographic characterization of the samples was published. The 180/160 ratios of New Zealand lignite suggest that the major source of oxygen in the lignite was cellulose, and that lignin and plant resins provided insignificant contributions [173]. The fibrous lignites of Central Europe are considered to contain relatively high proportions of cellulose [174], although that view has been challenged with the argument that the fibrous structure in fact represents lignin [149]. The presence of appreciable lignin residues in fibrous lignite increases the amount of phenols and pitch in the carbonization tars [149]. The dark color of euulminite A in Saskatchewan lignites may indicate the presence of residues from cellulose decomposition [31]. These products disappear early in coalification of the Canadian lignites [31]. Treatment with a solution of cadmium oxide in aqueous ethylenediamine (cadoxen) effectively extracted cellulose from lignite [175]. Extraction with cadoxen gave a dark humic material from which small amounts (e.g., 0.01%) of glucose and other monosaccharides were obtained by hydrolysis. Quantitative extraction could not be obtained, but the results probably give an order-of-magnitude estimate of the cellulose content of North Dakota lignites. The method applied to a sample from a coalified tree stump from the Beulah mine showed a cellulose content of 0.01% [98,176], despite the fact that the sample retained remarkable physical similarity to a modern tree stump. This observation is consistent with the idea that cellulose is destroyed early in the coalification process. Monosaccharides from the hydrolysis of cadoxen-extracted polysaccharides in Beulah lignite, expressed as ratios to glucose, are glucose, 1.0; galactose, 0.5; xylose, 0.2; arabinose, 0.03; and ribose, 0.02 [175]. These were determined by gas chromatography of the alditol acetates, formed by reduction with sodium borohydride and subsequent esterification with acetic anhydride [175]. Proof that the isolated glucose derives from cellulose was obtained by methylation of the cadoxen-extracted material. The O-methylated material subsequently was hydrolyzed with dilute sulfuric acid and the hydrolysis products were converted to alditol acetate derivatives. GC/MS of the alditol acetates showed that the product was 2,3,6-tri-O-methylglucitol triacetate, identical to the product obtained from the same sequence of reactions applied to cellulose [11]. That no tetra-O-methylglucitol diacetate was observed implies that the glucose chains in cellulose must be very long. The cellulose contents of lithotypes of Beulah lignite were determined by the cadoxen method: vitrain, 0.003%; attritus, 0.015%; fusain, 0.014%, on an as-received basis [98]; and, on an maf basis, are vitrain, 0.005%; attritus, 0.025%; and fusain, 0.023% [3,175]. The cellulose content of a composite (i.e., not separated into lithotypes) sample of the same lignite was 0.003%

121 (as-received) [98]. The dominance of vitrain is probably responsible for the value for the "whole" sample [ 170]. These very low values of cellulose content show that whatever chemical differences exist among the lithotypes must be due to structures other than cellulose residues. Cellulose has long been thought to be the more easily degraded component of wood [177]. Low amounts of cellulose in North Dakota lignites are consistent with the hypothesis that cellulose was broken down by microbiological oxidation at an early stage of coalification. The higher values for attritus and fusain suggest some interruption of the biochemical degradation process. Although the extraction of cellulose with cadoxen may be incomplete, and thus the absolute numbers are questionable, the relative rankings of the lithotypes may be useful. The existence of some low-rank coals with high cellulose contents may be indicative of alternative, anaerobic coalification processes [178]. (Lignin and cellulose contents in Polish lignites have been reported to be as high as 61% and 44%, respectively [179]; these results seem remarkably high in light of the work reported for North American lignites.) The nutrient supply in the early stages of coalification may be a factor in determining the extent to which cellulose is degraded. In low-moor peat samples, low cellulose levels result from more extensive microbiological degradations permitted by the higher nutrient levels [ 180]. 3.3.10 Nitrogen groups Small quantities of porphyrins can be isolated from lignites. Extraction of Canakkale-~an (Turkey) lignite with methanol and sulfuric acid, followed by ultraviolet and mass spectral analysis of the extract, showed the presence of iron, gallium, and manganese metalloporphyrins [ 181]. The iron porphyrins were etioporphyrin, carboxyetioporphyrin, and dicarboxyetioporphyrin, with 28-30 carbon atoms. Isolation of tetrapyrrole pigments is accomplished by extraction with methanesulfonic acid at 100~ neutralization with sodium carbonate, and re-extraction with ether. Texas, Montana, and Baukol-Noonan lignites contained these pigments at a concentration of 0.005 ~tg per gram [ 182]. Hagel (North Dakota) lignite contained 0.01 ~tg/g. Examination of 42 coals of all ranks showed that the highest concentration of these nitrogen compounds appears be in the hvB bituminous coals; however, the decreased yield in the lower rank coals may reflect a lower extraction efficiency resulting from the inability of the solvent to swell the coal. Extraction of a Mississippi lignite with 7% sulfuric acid in methanol at room temperature for 36 h, followed by separation of products by thin layer chromatography identified an iron complex of a dicarboxyetioporphyrin, C36I--~N404Fe, possibly mesoheme [183]. Pyridine and pyrrole tings, as well as nitrogen-containing aliphatic linkages, are minor contributors to the structure of lignite, as concluded from oxidation studies [50]. In some cases the presence of pyrrole rings could not be confirmed [50]. Oxidation of Sheridan lignite with aqueous sodium dichromate at 250~ for 36--40 h produced pyridinecarboxylic acids in relative abundance of about 2% [52]. In comparison, the relative yield of all acids with heterocyclic oxygen was about 32%. No evidence was obtained for pyrrole ring systems, nor for aliphatic amines, by oxidative

122 degradation of Wyoming lignite [49]. This work used six different oxidizing agents. Humic acids isolated from a Spanish lignite produced small amounts of thirteen amino acids upon hydrolysis in 6 N HCI [184]. 3.3.11 Sulfur groups Ultimate analyses of lignites from the Center, Falkirk, and Glenharold mines (Hagel seam) have shown an inverse relationship between sulfur and nitrogen [44]. This observation agrees with an earlier proposition that as the available nitrogen is biochemically fixed, the fixation of sulfur by microorganisms decreases [185]. Asphaltenes and preasphaltenes from liquefaction of coals of different ranks show a dependence of the sulfur content of these heavy products on rank [ 186]. The results suggest that sulfur is more tightly bound to the coal structure in higher rank coals. Sulfur forms determined in Beulah lignite by X-ray absorption fine structure (XAFS) spectroscopy are shown in Table 3.19 [187]. The sulfur forms in a Texas lignite and some of TABLE 3.19. Forms of sulfur in Beulah lignite, determined by XAFS [187].

Total sulfur,weight percent Sulfur forms, percent of total Pyrite Sulfide Thiophene Sulfoxide Sulfone Sulfate

Sample 1 (fresh) 0.80

Sample 2 0.80

29 28 30 2 0 12

37 24 30 2 0 7

its macerals, also determined by XAFS, are shown in Table 3.20 [187]. TABLE 3.20 Forms of sulfur in a Texas lignite and some of its macerals, determined by XAFS [187]. "Whole lignite" Sulfur forms, percent of total Pyrite Sulfide Thiophene Sulfoxide Sulfone Sulfate

49 18 30 0 1 2

Sporinite 46 54 0 1 0

Vitrinite 12 28 55 0 4 0

Inertinite 73 10 14 0 3 1

The data in Table 3.19 can be compared with the results of a programmed-temperature oxidation of

123 the same lignite, which indicated that, as a percent of the total sulfur (which itself amounted to 0.80%), pyrite is 31%, aromatic sulfur (thiophenic structures, aryl sulfides, and aryl sulfones) 20%, non-aromatic sulfur 34%, and sulfate not detected [ 143]. Organic sulfur species in Alcoa (Texas) lignite were examined by flash pyrolysis [188]. This lignite contained 1.30% total sulfur, with 0.73% organic sulfur. The observed distribution of sulfur types, expressed as a percentage of the organic sulfur, was 82% aliphatic sulfides and mercaptans, <1% aromatic sulfides and mercaptans, and 18% stable structures such as thiophenic compounds [ 188]. Thioethers represent up to 76% of the organic sulfur in Central European lowrank coals [ 189]. A general estimate is that 40-50% of the total organic sulfur in lignites is present in aliphatic sulfide structures [89]. Thiophene moieties, as well as sulfur groups in aliphatic chains, are not important contributors to the structure of lignite, on the basis of oxidation studies [50]. Thiophenic structures amount to about 24% of the organic sulfur in the low-rank coals of Central Europe [ 189]. A rule-ofthumb estimate for lignites suggests that about 30% of the total organic sulfur is present in thiophenic structures [89]. In contrast to the oxidation results [50], X-ray absorption near edge spectroscopy (XANES) indicates that 46 mol% of the sulfur in Beulah-Zap lignite is present in thiophenic structures [190]. X-ray photoelectron spectroscopy (XPS) of the sulfur 2p binding energies indicates that the environment of organic sulfur in a Polish brown coal is heterocyclic (binding energy of 163.3 eV) [191]. XPS of Beulah-Zap lignite indicates that 55 mol% of the sulfur in Beulah-Zap lignite is in thiophenic structures, which seems reasonable agreement with the XANES results, but is much higher than results from XAFS [ 190]. 3.3.12 Chlorine The chlorine content of most North American lignites is _<0.1%, which is considered to be negligible in terms of any impact on processing of the lignite [192]. XAFS examination of BeulahZap lignite indicated a chlorine content of 0.04%, which is possibly present as chlorine atoms bound to aromatic rings [ 193], though it may be premature to attach significance to these results. Most coals of higher rank examined by XAFS contain chlorine as aqueous chloride ion in microcracks in the coal particles [ 193]. 3.4 T H E T H R E E - D I M E N S I O N A L

S T R U C T U R E OF L I G N I T E

This section discusses aspects of the three-dimensional structure of lignites, particularly the crosslinking to form a macromolecular network, swelling of the structure, and molecules apparently trapped inside the network. 3.4.1 Evidence for a colloidal structure Several lines of evidence have been adduced to support the concept of a colloidal structure for lignite. Colloids carry an electric charge; passage of 110 v through an aqueous suspension of

124 lignite causes immediate flocculation [ 194]. Lignite is peptized by treatment with aqueous alkali. This property is shared with subbituminous coals but not with bituminous coals or anthracites [ 194]. The adsorption behavior of water vapor and other gases onto lignites is typical of a colloid. In this regard, lignite shows a strong similarity of behavior to silica gel. 3.4.2 Crosslinking in lignite structures The glass transition temperature, Tg, of Titus lignite is 580 K, as indicated by differential scanning calorimetry [195]. Swelling this lignite with pyridine caused a very large decrease of T g in the early portion of the swelling. With a pyridine uptake of 0.265 g per gram of lignite (48% of the pyridine uptake achieved at equilibrium), the glass transition temperature dropped to 498 K. When equilibrium had been achieved, T g had decreased to 423 K. This dependence of Tg on the weight fraction of the solvent (i.e., pyridine) is similar to the effects observed with glassy crosslinked polymers. Field ionization mass spectrometry of Beulah-Zap lignite showed a very low abundance of signals above m/z = 200 [196], indicative of a very highly crosslinked macromolecular structure of this lignite. Oxidation of Texas lignite with ruthenium tetroxide produces twenty aliphatic tri- and tetracarboxylic acids [63]. One source of such acids could be the oxidation of structures of the type

~

~/~----CH 2 ~CH

---CH 2

(as discussed previously on page 103) which, in this example, would form propane-l,2,3tricarboxylic acid. The large number of tri- and tetraacids formed implies that these branching or crosslinking points must exist in a variety of configurations. This work suggested the existence of one to two crosslinks per 100 carbon atoms [63]. Transalkylation

of coal using

1,1-diphenylethane

in toluene solution,

with

trifluoromethanesulfonic acid as catalyst, converts the methine carbon -CH- crosslinking site to tritolylmethane (26) [ 197]. The number of methine carbons was 0.38 per 100 carbon atoms in Hagel, and 0.104 in a Wilcox Group lignite [197]. In comparison, the value for a Pittsburgh seam (Pennsylvania) bituminous coal was 0.0076 [197]. The number of crosslink sites correlates roughly with solubility of the coal in tetrahydrofuran. Assuming a molecular weight of 130 for a hypothetical repeating unit in the structure (the molecular weight of naphthalene is 128 and of propylbenzene, 120), the number of repeating units between crosslinks was about 10, ranging from 9.7 for Titus to 10.8 for Darco (Texas) lignite,

125

26

CH3

based on pyridine swelling studies [198]. The average molecular weight between crosslinks for these two lignites was 1261 and 1404, respectively; for lignite from the Titus A Pit seam, 1313 [198]. These values were corrected for pore volume, mineral matter, and adsorbed pyridine. In contrast, the average molecular weight between crosslinks in Hagel lignite was estimated to be 70 on the basis of methanol sorption [ 199]. A structure with molecular weight in this range is --CH2--CH2--O--CH2--CH2-- (molecular weight 72). Solvent-swelling ratios measured in pyridine are Indian Head, 1.6; Gascoyne (Blue pit), 1.3; and Center, 1.3 [154]. Values of 2.1-2.2 were observed for three Texas lignites at 35 ~ [198]. The swelling ratios observed for the Texas lignites increased with temperature, so that at at 80 ~the values for the same lignites were in the range 3.2-3.5 [198]. This behavior with temperature indicates that calculated values of the number of repeat units between crosslinks and the average molecular weight between crosslinks both increase with temperature. Possibly at the higher temperature pyridine vapor more effectively breaks down the hydrogen-bonded structure of the lignite [ 198]. Solvent-swelling ratios of Big Brown lignite were measured perpendicular and parallel to the bedding plane for three solvents [200]. These ratios, perpendicular-to-parallel, are 1.08 in chlorobenzene, 1.07 in tetrahydrofuran, and 1.14 in pyridine. Thus the lignites, as is also true for coals of higher rank, are anisotropic. The anisotropy reflects geological pressures exerted during coalification. Because anisotropy observed by solvent swelling is a bulk effect, it must arise from molecular structures that have a greater bonding density in the bedding plane rather than perpendicular to the plane. One configuration that fulfills this requirement is disk-shaped micelles more strongly bonded around the edges than at the surfaces. Anisotropic swelling must reflect anisotropic distribution of crosslinks. Furthermore, if the crosslink density is anisotropic, it is reasonable to assume that mechanical properties will also be anisotropic. In the absence of gross heterogeneity and any major faults such as cracks or cleats, this lignite should therefore be stronger in the direction of the bedding plane than perpendicular to it. Chlorobenzene is not capable of breaking hydrogen bonds. On the other hand, pyridine probably breaks most of the hydrogen bonds in the lignite. Since the perpendicular-to-parallel swelling ratio is highest in pyridine and lowest in chlorobenzene, the distribution of covalent bonds must be highly anisotropic [200] because the anisotropy is greatest in pyridine, where the effects of

126 hydrogen bonding are least. It follows further that the density of hydrogen bonds must be greater perpendicular rather than parallel to the bedding plane. Hydrogen bonding of water to carboxyl or phenolic groups establishes a supramolecular structure in which the polymeric network has been expanded or plasticized by the water molecules [201]. Evidence for hydrogen bonding in Beinfait lignite has been obtained by FFIR [113]. Carboxyl groups might also participate in crosslinking if polyvalent metal cations are present as the counterions, in structures of the type -- COO--- M+2-- COO- [89]. Spin lattice relaxation times, determined by pulsed electron paramagnetic resonance, suggest that Beulah-Zap lignite has a more rigid structure, in terms of molecular flexing of the network, than bituminous coals [202]. A Wyodak subbituminous coal is similar to the lignite. 3.4.3 Molecules trapped in the lignite structure This subsection discusses some of the species that can be removed from lignite by mild solvent extraction. If extractions are carried out at temperatures below a point at which thermally induced cleavage of covalent bonds would occur, it can be presumed that the compounds isolated were trapped in the macromolecular network of the lignite, and were not part of the macromolecular structure itself. In a sense they represent a separate phase from the network. The isolation of waxes from lignite by mild solvent extraction has been carried out commercially; for that reason, a discussion of wax extraction is deferred to Chapter 12. The yields of extract from the Soxhlet extraction of vacuum-dried Indian Head lignite with various solvents are shown in Table 3.21 [203-205]. TABLE 3.21 Soxhlet extract yields from vacuum-dried Indian Head lignite [203-205]. Solvent Benzene Cyclohexane Heptane Hexamethyleneimine Hexane Methanol Octane Pentane Pyridine

Yield, % 0.75 0.51 0.21 27.0 0.17 1.5 0.86 0.26 8.3

Reference [203] [204] [205] [205] [205] [204] [203] [203] [203]

It was not determined whether the remarkable extraction yield achieved with hexamethyleneimine is an artifact reflecting incorporation of the solvent in the extract, or is a preliminary indication that this compound could be an outstanding solvent for the extraction of lignites. Beulah and Big Brown lignites were examined by sequential Soxhlet extraction, beginning with chloroform [206]. Each extract was divided into hexane-soluble and hexane-insoluble

127 fractions. The total chloroform-soluble material was 2.8% of the Beulah and 3.7% of Big Brown (maf basis). The hexane-soluble portions of the chloroform extracts represented 1.0 and 1.7% of the lignites, respectively. For Beulah lignite, the hexane-solubles are 36% of the chloroform extract, and for Big Brown lignite, 46%. The residue from the chloroform extraction was then further extracted with a chloroform:acetone:methanol azeotrope (in proportions 47:30:23). The azeotrope-soluble material was 1.6% of the Beulah residue and 2.4% of the Big Brown. The hexane-soluble portion of the azeotrope extract amounted to 0.13% of the Beulah and 0.19% of the Big Brown residues. Expressing the hexane-solubles as a percentage of the total azeotrope extract shows remarkable agreement: 8.1% for Beulah and 7.9% for Big Brown. Among the compounds identified in the extracts were sesquiterpenes tentatively assigned a bicyclic C 10 structure with five substituent carbon atoms. Compounds identified by capillary GC analyses of the extracts of Beulah lignite are listed in Table 3.22 [206]. TABLE 3.22 Compounds identified in extracts of Beulah lignite [206]. Compound GC Area Percent Pristane 0.15 n-Alkanes C-14 0.82 C-19 0.74 C-20 0.99 C-21 1.38 C-22 1.22 C-23 0.69 C-24 0.67 C-25 1.89 C-26 1.55 C-27 2.92 C-28 0.42 C-29 0.76 C-30 0.57 C-31 0.29

Compound GC Area Percent Cadalene 0.31 Alicyclic terpenoids C15H26 c 1.9 C15H26 d 1.12 C15H26 e 0.24 C15H26 g 2.3 C15H26 h 0.10 C20H36 tricyclic alkane 1.7 C17H30 tricyclic alkene 1.2 C33H58 0.07

Remarkable differences were found between the extracts of the two lignites for the terpenoid and related compounds. None of the C 15H26 sesquiterpenes, cadalene, the C20H36, nor C17H30 were found in the extract from Big Brown lignite. However, compounds that were found in the Big Brown extract included C31H54 and C32H56 hydrocarbons (3.3 and 1.1 area %, respectively) and an unidentified compound of retention time intermediate between C17H30 and C29H50 (0.66 area %). In addition, the C 33H58hydrocarbon occurred in the Big Brown extract at 3.5 area % (compared with 0.07% for the Beulah extract). Decker lignite was extracted with refluxing 3:1 benzene:methanol for 24-48 h [51,54]. The products were analyzed by GC/MS and MS, after separation into fractions by alumina column

128 chromatography. The major components of the hydrocarbon fraction were sesqui- and diterpenoids. Three compounds, eudalene, cadalene, and simonellite, indicate a significant input of coniferous plant material. Eudalene and cadalene have been isolated from Siberian conifers, and simonellite has been extracted from lignite of coniferous origin [51]. Diterpenoids found in this extract have been isolated from conifer resins [207,208]. (A German lignite extracted with benzeneethanol yielded 5-7.5% "bitumen" that consisted mainly of resins [ 149].) On a relative basis (with the predominant compound(s) in the gas chromatogram set equal to 100), the compounds identified were sesquiterpenoids, 75.1; diterpenoids, 100.0; indan/tetralin derivatives, 44.1; and naphthalene derivatives, 6.4 [54]. Compounds observed included C 15Hx, where x ranges from 28 to 30, and three C 10H2z-- derivatives of tetralin [54]. Other types of compounds identified included steranes, ~,-pyrones, triterpenoids, and oxygen-containing triterpenoids [51]. The benzene:methanol extract contained 67% of waxes containing 24-32 carbon atoms [51]. Benzene:methanol (40:60) extracts of Beulah-Zap lignite contained carboxylic acids as large as (:33, as well as a series of esters [209]. Field ionization mass spectrometry of Beulah-Zap lignite identified fatty acids and esters having m/z of 368, 396, 424, 452, and 480, which are typical of plant-derived compounds and exist in the lignite as extractable biomarkers [ 196]. Extraction of North Dakota lignite sporinite, using 3:1 benzene:methanol, yielded C 12-C30 aliphatic monocarboxylic acids as major products [57]. Carbon chains of even numbers of carbon atoms predominated; the maximum concentration was observed at C16, with a secondary maximum at C28. Rockdale lignite provided a 1.3% extract yield in hexane [73]. After methylating the extract with dimethylformamide dimethyl acetal in pyridine, gas chromatographic analysis indicated that the principal components of the extract were aliphatic carboxylic acids in the range C 24-C34. Acids in the ranges C13-C23 and C35-C36 were very minor components. The even-numbered carbon chains predominated over the odd-numbered chains. These characteristics resemble those of modern plant waxes. The total acids represented 7% of the weight of the hexane extract. Other prominent components were the C23-C33hydrocarbons. Three substituted aromatic dicarboxylic acids were observed in the gas chromatogram, but were not fully identified. Soxhlet extraction of Big Brown lignite with tetrahydrofuran provided a 7.2% yield of extract [210]. The major components included cyclic aliphatic compounds such as mortanes and hopanes, i.e., hydroaromatic isoprenes. Polycyclic aromatic compounds were not important components of the extract. Sheridan lignite was treated with 5% hydrochloric acid to release any organic acids associated with minerals, and then extracted with 2.5% sodium hydroxide solution at 35~ for 16 h [211]. The yield of acids from this experiment was 2.6%; yields under the same conditions for a bituminous coal and an anthracite were 0.23% and zero, respectively. The acids were identified by GC-MS and high resolution MS of their methyl esters; 36 acids were identified, ranging from succinic and glutaric through methoxymethyldibenzofurancarboxylic acid. Comparison of the acids

129 extracted from the lignite with similar extractions of shale, lignin, peat, and humic acids showed the following [211]: No benzenecarboxylic acids with C3 side chains were isolated from the lignite, but are common in extracts of the other materials. Benzene di- and tricarboxylic acids are abundant in the lignite extract, but not in extracts of the other materials. Phenolic acids extracted from the lignite contain a single hydroxy group and no methoxy groups, whereas the other materials yield acids with two or more hydroxy or methoxy groups. Two diterpenoid acids, dehydroabietic acid and its methyl homolog, presumably derive from abietic acid, which is a common constituent of the resins of conifers [211]. These acids may be the precursors of diterpenoid hydrocarbons isolated from benzene-methanol extracts of the lignite. Furan, benzofuran, and dibenzofuran acids extracted from the lignite may derive from two possible sources: from furoguaiacin and pinoresinol lignins in wood, or from degradation of dibenzofuran compounds formed from condensation reactions of phenols during coalification [211]. The acids trapped in this lignite are similar to acids produced during oxidation with alkaline cupric oxide [168], suggesting that the trapped acids are oxidation products from the degradation of ligninderived materials during coalification. Extraction of Boundary Dam and Coronach lignites (both Saskatchewan) with pentane, toluene, and tetrahydrofuran show removal of up to 20% of the organic substance, but no change in aromaticity (as inferred from measurements of mean random reflectance) of the extracted coal [212]. 3.4.4 Depolymerization reactions Phenol softens lignite [213]. Treatment with phenol and boron trifluoride at 100~ results in depolymerization via cleavage of aliphatic-aromatic carbon-carbon bonds and exchange of the aromatic structures with phenol. Depolymerization was substantially more effective for Velva (North Dakota) lignite than for five other coals of higher rank [214], based on the total of the yields of benzene-, benzene/methanol-, and phenol-soluble fractions produced in the reaction. This sum for Velva lignite was 75.2%. By contrast, comparable data for other coals studied ranged from 47.4% for Ireland (West Virginia) high volatile bituminous to 9.8% for Itmann (West Virginia) low volatile bituminous. A blank extract with phenol showed only 2.4% solubility, lower than all other coals except the Itmann lvb. These results were interpreted in terms of a structure having small aromatic units extensively crosslinked with bridging groups to result in a rigid polymer [214]. A rigid, highly crosslinked polymer should give a low extraction yield in phenol. Extensive depolymerization in phenol-boron trifluoride indicates both a large number of bridges and of their aliphatic character. Petrographic analysis of the residue after solvent extraction of the phenol-boron trifluoride reaction product showed 98% (semifusinite + micrinite) and 2% fusinite. The vitrinite and exinites had been completely depolymerized. Depolymerization of Beulah lignite with phenol, catalyzed by p-toluenesulfonic acid, was used to obtain structural information on the methanol-soluble and -insoluble products [215]. The results were not unequivocal, because of the incorporation of some phenol into the lignite

130 a structure characterized by high phenolic or hydroxy OH and low carbonyl content, with a predominantly aromatic carbon framework. Depolymerization decreases the oxygen content, attributed to dehydration of phenolic structures. Long-chain aliphatics, alkenes, or carbonyls were not detected in the 13C NMR spectra of the extracts. REFERENCES

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135 118 119 120 121 122 123 124

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138 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217

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139

Chapter 4

FUNDAMENTAL ORGANIC REACTION CHEMISTRY OF LIGNITES

This chapter treats the organic chemistry of lignites in a variety of reaction conditions. The principal focus is on fundamental reaction chemistry of the carbonaceous portion of the lignite, particularly, though not exclusively, with regard to reaction mechanisms and kinetics gained through bench-scale studies. Sections 4.1-4.3 treat the reactions of lignites with various reagents. Section 4.4 covers the reactions of lignites in pyrolysis conditions The reactions of the inorganic constituents are treated in Chapter 6. The reactions of lignite chars, as opposed to the otherwise untreated lignites themselves, are of course of crucial importance in the combustion and gasification of lignites. Char reactions in oxidizing atmospheres are treated in Chapter 11, and char reactions under gasification conditions are discussed in Chapter 12.

4.1 R E A C T I O N S W I T H C A R B O N M O N O X I D E

4.1.1 Reactions of carbon monoxide with lignite Reaction of lignites with carbon monoxide involves a direct attack on the lignite structure, presumably at carbonyl groups, and reaction with moisture to generate hydrogen in situ. The reactions can be represented as

CO + H20 ~ CO2 + H2 CO + Lignite-O ---, CO2 + Lignite CO + Lignite + I-t20 ---, CO2 + Lignite-2H

where the symbols Lignite-O and Lignite-2H refer respectively to an oxygen functional group in lignite and partially hydrogenated lignite [1]. The reaction with oxygen linkages occurs above 425~ [ 1]. In excess base, the mechanism is [2]

OH- + CO ~ H-O-C:OLignite + H-O-C:O- ---, Lignite-H-+ CO2 Lignite-H- + H20 ---, Lignite-2H + OH-

140 Lignites react preferentially with carbon monoxide, even in the presence of hydrogen, removing oxygen as carbon dioxide and forming hydrogen in situ via the water gas shift reaction. This implies the potential of developing a liquefaction process using synthesis gas rather than hydrogen as the reactant. Such a process would have a lower overall hydrogen consumption. Carbon monoxide or carbon monoxide - water mixtures are excellent reagents for lignite liquefaction, with high conversions and high yields of benzene-soluble oils [3]. This observation, substantiated in several types of reactors in several laboratories, suggests that conversion involves attack by carbon monoxide on one or more structural features of the lignite. CO functions as a reductant in the temperature range 350-M800C [4]. Conversions of 90-95% at 380--4000C have been reported for lignites [5,6]. The mechanism of reaction of carbon monoxide with carbonyls involves formation of formate by reaction of CO with alkaline species in the lignite, followed by a crossed Cannizzaro reaction between hydrogen in the formate and carbonyl in the lignite [6]: O M++OH- + CO

[I ~

~

_

H

0

II

O

II

H\go-M+

o

oO- M+

\/ e\ /

M§

/c\

/\ H

+

CO 2

/OH

.- "c /\

+ OH-

+ M+

As supporting evidence, sodium formate has been shown to have a catalytic effect in the liquefaction of subbituminous coals [7]. Further, formic acid or formate ion generated in situ promotes liquefaction of low-rank coals such as Texas lignites [8]. The crossed Cannizzaro reaction proceeds via donation of aldehydic hydrogen from the formate ion to the carbonyl carbon. Addition of hydride to the carbonyl carbon forms an alkoxide, which rapidly reacts with water to generate hydroxide; the hydroxide in turn reacts with carbon monoxide to regenerate formate [6]. The competing decomposition of sodium formate to sodium carbonate, CO, and hydrogen dominates above 400~

The instability of formate above this temperature favors a different

product slate [9]. This reaction is undesirable because hydrogen liberated in this way is not as reactive as hydride donated from formate. The side reaction becomes increasingly important when alkaline catalysts are present, or at temperatures above 400~

[6]. Thus, for example, CO

141 pretreatment of Texas lignite for improved liquefaction is conducted at 290-3700C [8]. Under liquefaction conditions, the highest conversions and liquid yields in carbon monoxide are attained in 10-15 minutes at maximum temperatures of 375~

[10]. This conclusion derives from

experiments with Indian Head (North Dakota) and Big Brown (Texas) lignites in the presence of anthracene oil or hydrotreated anthracene oil as solvents. In liquefaction systems, carbon monoxide also participates in methylation reactions. After liquefaction in a solvent mixture of phenanthrene and 1-naphthol, methylnaphthol occurs among the products, as do methylnaphthalene and methyl derivatives of dinaphthofuran [11]. (Dinaphthofuran forms by oxidation of 1-naphthol in the presence of coal [12].) In addition, detection of the methylene-bridged compound di(hydroxynaphthyl)methane suggests that some formaldehyde forms in the reaction system, producing (hydroxymethyl)naphthol which then condenses with another naphthol to give di(hydroxynaphthyl)methane [11]. Methylation during liquefaction may contribute to increased oil yields at higher pressures [6]. The role of methylation in inhibiting retrogressive cross-linking is discussed in later in this Chapter. Carbon monoxide preferentially hydrogenates the Ho position, i.e., positions on aliphatic structures or on alkyl side chains on aromatic tings [13]. To account for the increase in Ho by reaction with carbon monoxide, functional groups that would decompose to form water must be located on positions which would, upon hydrogenation, become Ho-type positions. For example, hydrogenation at a phenolic-OH would not increase the Ho-type hydrogens. Thus in the absence of carbon monoxide one expects the reaction H

OH

I I R--C ~C~R I HI H

~

R---CH---CH ~ R

+H20

whereas in the presence of carbon monoxide one obtains the reaction H CO +

OH

I I R~C ~C~R I HI H

H ~

H

I I HI H

R--C - - - C ~ R

+ CO2

In this connection, alcohols such as cyclohexanol do not undergo dehydration reaction around 360~

but glycols derived from cellulose structures do. The -OH functional groups responsible

for facile reaction with carbon monoxide at relatively low temperatures and the resulting hydrogenation at Ho positions may be remnants of cellulose. The heat of reaction of carbon monoxide with Indian Head lignite was measured by differential scanning calorimetry [ 14]. An exothermic reaction began at 270~ (at 2.72 MPa) and

142 continued to the upper limit of measurement at 605~

At a heating rate of 20~

the heat of

pyrolysis was 160 J/g, and the temperature of maximum rate of heat evolution (TMRHE) was 605~

An increase in heating rate to 50~

resulted in a heat of pyrolysis of 100 J/g and

TMRHE of 4360C. At 2.8 MPa the heat flow curves have unusual characteristics above 5000C [ 15]. A distinct primary pyrolysis peak is evident, as in similar experiments performed in argon. However, for reactions conducted in argon the exothermicity decreases above 470~

whereas in

carbon monoxide the exothermicity continues to increase above this temperature. This effect arises from reaction of carbon monoxide with activated hydrogen [ 14]. Hydrogen evolution from North Dakota lignites begins at 4500C [16], about the same temperature as the first indications of exothermicity in the pyrolysis in a carbon monoxide atmosphere. Thus a sequence of reactions might be

CO + R1CH2CH2R2 ~ RICH=CHR2 + H2C=O H2CO + 2H ~ CH3OH

possibly followed by

CH3OH + 2H ~ CI-I4 + H20

(In support of this postulated mechanism, no formaldehyde was detected in liquefaction of lignites with carbon monoxide, water, and alkaline catalysts, but small amounts of methanol were observed [6].) Summing the three preceding equations gives a net reaction CO + 6H --~ CH4 + H20

for which the heat of reaction is -220 kJ/mol CO. Assuming a 50% conversion of evolved hydrogen provides good agreement with the heat effects observed in the pyrolysis of lignite in CO atmospheres.

4.1.2 Supporting model compound studies Carbon monoxide reduces butyraldehyde; however, the yield of the expected product, n-butanol, is low because of the competing aldol condensation reaction. The significance of the aldol condensation [17] derives from the condensation of a carbonyl with a second compound having "active" hydrogens being a potential contribution to retrogressive reactions forming unreactive solid structures rather than the desired liquids. The reduction should be carried out as rapidly as possible to minimize aldol or analogous condensation reactions. With coals, access of CO to some of the carbonyl sites will likely be hindered, making it essentially impossible to reduce

143 all of the available carbonyls at practical reaction conditions. Thus some condensation reactions are likely to occur. Model compound reactions highlight the role of formic acid and formate in potential reactions of lignites. The reaction of diphenyl ether proceeds at 315~

in 15% aqueous sodium

formate via acid-catalyzed attack of formate on the ether protonated in the ortho position, forming two molecules of phenol [18]. Diphenyl ether does not react with water or 15% aqueous formic acid at these conditions. 1-Phenoxynaphthalene is hydrolyzed with 4.5% conversion in water at 315~ in two hours, but the conversion in 15% aqueous formic acid is 36.6% at the same reaction time [18]. The main reaction in formic acid is hydrolysis, rather than reduction. This compound reacts much more slowly in aqueous sodium formate (24.6% conversion in 3 days), but reduction becomes more important, with products such as 1,2-dihydronaphthalene forming. Reaction of 9phenoxyphenanthrene was also slower in sodium formate than in formic acid. Either of these two reagents slowly converts phenanthrene to 9,10-dihydrophenanthrene (e.g., yields of 0.5-1.0% after 3 days at 3150C) [ 18]. Reaction of graphite with a mixture of carbon monoxide and steam goes to completion in 18 h at 400~

the main product being carbon dioxide [ 19]. The reaction proceeds via a benzenoid

graphite structure and is represented as

o

+ 24 CO + 1 2 H 2 0

o o

o

o

o

o

I-

H

H

HO

+ 24 CO 2

H

H H

OH

H

OH

OH

144 Carbon monoxide promotes the reaction by reacting with quinone functional groups at the edges of the aromatic sheets. The process may be autocatalytic. Reaction of carbon monoxide with sodium or potassium hydroxide can produce the series of carboxylates from acetate to caproate [19]. This reaction can be carried out with carbonaceous reactants other than carbon monoxide, but only with carbon monoxide is formate detected among the products. Addition of hydrogen to the carbon monoxide prevents any measurable reaction in this system. Increasing the partial pressure of water may also increase the partial pressure of hydrogen in the system via CO + OH- + H20 --~ H2 + HCO3[ 19]. This process can indirectly favor formation of additional formate because the hydrogen can shift the equilibrium in the reaction [ 19] 2 HCOO" -"" -OOC-COO" + I-I2

In the absence of catalysts, hydrogen is more effective than carbon monoxide for reduction of alcohols, ketones, aldehydes, ethers, and alkenes. Carbon monoxide was superior only for carbohydrates [6,11]. However, in the presence of alkaline catalysts such as sodium carbonate, carbon monoxide became more active than hydrogen for the reduction of carbonyls or compounds which might be precursors to carbonyls, such as vicinal glycols [6]. For example, at 250"C and 10.4 MPa CO, a 91% conversion of benzaldehyde to benzyl alcohol was effected in an hour. Using hydrogen at the same conditions, a 6% yield was obtained, of which half was estimated to have come from disproportionation (i.e., the Cannizzaro reaction) rather than from direct reduction.

4.1.3 Carbon monoxide- hydrogen mixtures When carbon monoxide and hydrogen are both present in a liquefaction reactor, carbon monoxide is usually consumed in larger amounts [20], and will react more rapidly than the hydrogen [21 ]. Developmental work on the Exxon Donor Solvent process showed that synthesis gas - water mixtures were as effective liquefaction reagents as hydrogen when used in the presence of a donor solvent [1]. In fact, the hydrogen donated by the solvent was greater in the synthesis gas atmosphere than in hydrogen. In the Solvent Refined Coal (SRC I) pilot plant using carbon monoxide - hydrogen mixtures, conversion increased as the CO concentration in the gas increased

[1]. Since CO reacts preferentially to H2 at temperatures up to 4800C, some CO (or an intermediate derived from CO) must be reacting directly with the lignite to form volatile products, as for example [22]

145 CO + Lignite-OH ---, Lignite-H + CO2 or

CO + H20 + Lignite-O-Lignite ---, 2 Lignite-H + CO2

Reactions of Indian Head lignite in batch autoclaves showed an increase in both conversion and yield of solvent-refined lignite (SRL) for reaction in the presence of an equimolar mixture of CO and H2 when compared to reaction at similar conditions in H2 [23]. This was attributed to a better ability of a CO-H2 mixture, compared to gaseous hydrogen, to donate hydrogen and stabilize free radicals at short reaction times. Reactions of Estevan (Saskatchewan) lignite with CO-H2 mixtures indicated a relationship between carbon monoxide consumption and oil yield [24]. Pilot-scale reactions of Indian Head lignite with carbon monoxide - hydrogen mixtures having 25-75 mol% hydrogen in the gas showed little change in the yields of major products as the hydrogen concentration was varied over this range [21]. (The reactions were carried out at 440~ 17.5 MPa, 1.35 liquid hourly space velocity, and a solvent/lignite ratio of 1.97.) For example, the yield of oils plus SLR was about 70% (maf lignite basis) with 25 mol% in the gas, and about 65% with 75 mol% hydrogen [21]. Batch liquefaction in pure carbon monoxide, pure hydrogen, and 1:1 CO-H2 showed that the highest yields of liquid products (light oil plus SLR) were obtained using the 1:1 CO-H2 atmosphere and lignite that retained its natural moisture content [21]. Gas yields increased as the proportion of carbon monoxide in the gas mixture increased, being highest for the tests using pure carbon monoxide. Reactions of North Dakota lignite in gas mixtures ranging from 100% CO to 100% H2 in 25% increments showed that conversions and yield of total liquids decreased as the fraction of CO in the feed gas decreased [25]. Lignites react readily with CO-H2 mixtures without added catalyst [6]. However, addition of pyrite improved conversion. Sodium, potassium, iron, and nickel cations back-exchanged onto lignite that had been treated with 1M hydrochloric acid all showed catalytic activity for reaction with synthesis gas [26]. Excess alkali may cause too much hydrogen (i.e., more than can be used efficiently) to be formed by the water gas shift reaction. Hydrogen produced from the water gas shift as elemental molecular H2 is less reactive than hydrogen donated as hydride from formate. Pyrite, added in amounts to 10% of the weight of the coal, improves conversion and oil yield and reduces the viscosity of the product [6]. It also catalyzes the water gas shift reaction, increasing the apparent consumption of carbon monoxide [6]. Hydrogen sulfide is a good catalyst for the reaction of synthesis gas with lignite, particularly in conjunction with iron compounds [6].

4.1.4 Carbon monoxide- steam mixtures Conversions of 90-95% for lignite in a CO-H20 mixture have been obtained at 380--4(K)~

146 [26]. Reaction is complete in 10 minutes at 380~ (although approximately an hour of heat-up time was required to reach this temperature, and some reactions likely occurred before the 380 ~ temperature was reached). The lignite reactivity was decreased by aging (conversion decreasing from 90% to 77% after 4 weeks' exposure to air) and by drying (conversion dropped to 54% after 24 hours at 1050C). Drying low-rank coals by conventional processes reduces their reactivity in liquefaction, the loss being especially noteworthy when synthesis gas, rather than hydrogen, is used as the reactant. The effectiveness of the carbon monoxide- water mixture may be due to the formation of an activated hydrogen in the water gas shift reaction, this hydrogen then intervening to inhibit recondensation reactions leading to insoluble chars. The formation of benzene-solubles from lignite is much more rapid in reaction with carbon monoxide and water than in the presence of hydrogen [27]. At short reaction times the rate in CO-H20 is about double that in hydrogen. In the presence of a good solvent, such as mixtures of a-naphthol and phenanthrene or isoquinoline, the reaction is essentially complete in 15-20 min. at 380-4(O~

Phenanthrene is not a good solvent in the presence of carbon monoxide and water, but

good results have been obtained using 1:1 phenanthrene : 1-naphthol [6]. About 10% of the phenanthrene is converted to dihydrophenanthrene. After 2 h, the yield of benzene-solubles from hydrogen still does not exceed that from reaction with carbon monoxide-water, even when the pressure of the hydrogen exceeds the combined partial pressures of carbon monoxide and steam. At 3800C carbon monoxide is superior to synthesis gas for conversion of lignite [6]. Since hydrogen does not react at this temperature in the absence of a hydrogenation catalyst, generation of hydrogen from the carbon monoxide:steam mixture only dilutes the reacting carbon monoxide. The rate of formation of benzene-solubles in reaction with carbon monoxide and water relates inversely to rank, being greatest with lignite and much less for a hvA bituminous. Both carbon monoxide and water must be present to obtain good conversions. Increasing the carbon monoxide partial pressure has a greater effect than increasing the steam partial pressure. The amount of carbon dioxide in the product gas provides a rough indication of the extent of the water gas shift reaction, and hence of the amount of hydrogen formed from the carbon monoxide and steam. If none of the hydrogen produced in the water gas shift reaction were used in reaction with the lignite, the amounts of carbon dioxide and hydrogen in the product gas should be equal. The difference between the amount of carbon dioxide and the amount of hydrogen actually found in the gas thus provides an indication of the hydrogen consumed. No correlation was found between the hydrogen utilization, expressed as the quotient of hydrogen consumed by hydrogen produced in the reaction (deduced from the carbon dioxide), and conversion in a series of six experiments [27]. However, these six experiments used five different solvents, four temperatures, and two reaction times, and in addition one of the experiments used lignite treated with potassium carbonate as catalyst, so it is questionable whether correlations might reasonably be expected in any case. With carbon monoxide present in a reacting gas mixture, the molar amount of carbon monoxide consumed approximately equals the amount of water consumed, when measured as the

147 sum of moisture in the lignite plus "combined water" [13]. Above 360~ the combined water is liberated rapidly, but without the consumption of external hydrogen. Thus the hydrogen atoms needed to make the water molecules must be abstracted from the lignite structure. If this occurs, the structure would become more aromatic in the early stages of reaction. However, any carbon monoxide present reacts either with the combined water via the shift reaction, or directly with the functional group that is the precursor of the water. In this case, dehydrogenation of the structure to provide the necessary hydrogen atoms to make water does not occur, and in fact a net hydrogenation of the structure takes place. The net hydrogenation when CO is the reacting gas requires the presence of functional groups that are capable of decomposing to water at relatively low temperatures. Carbon monoxide does not facilitate cleavage of the lignite structure into liquid products, but rather reduces the number of carbonyl groups so that products formed by thermal cracking do not recondense to high molecular weight liquids or solids. Virtually no evidence suggests participation of carbon monoxide in cleavage reactions [6]. The reaction of isotopically labelled (13CO) synthesis gas with Big Brown lignite at 450"C and 24--26 MPa showed no incorporation of 13C into the distilled products or the hydrocarbon gases [28]. This indicates that neither Fischer-Tropsch reactions nor carbonylation took place. The only isotopically labelled product found was 13CO2. The 13C NMR spectra of CH2Cl2-soluble and -insoluble products of reaction with CO showed no differences for reaction with ordinary CO (in which about 1% of the carbon atoms would be 13C), and labelled CO (in which half the molecules were 13CO) for reactions at 300, 340, and 380~

[27]. Thus CO participates in the water gas shift reaction with water in the

lignite. Since equilibrium favors hydrogen and carbon dioxide relative to water and carbon monoxide at low temperatures, carbon monoxide should be an effective reactant with lignite at low temperatures. The utility of carbon monoxide- steam mixtures derives from the ability of carbon monoxide to reduce carbonyl groups while the hydrogen from the water gas shift reaction effects hydrogenolysis or hydrocracking. The combination of carbon monoxide and hydrogen can be effective in liquefaction because each component plays a different role, each complimenting the action of the other. Carbon monoxide may also participate in hydrogenation of alkenes through the formate ion intermediate. For example, 1-octene has been hydrogenated in a carbon monoxide:water mixture in the presence of a potassium carbonate catalyst [6]. In the reaction of Martin Lake (Texas) lignite with D20 and 13CO (5 MPa, 2 hours, 315~

most of the deuterium is transferred into the lignite;

only about 10% is converted to D2 via the water gas shift reaction [29]. Incorporation of 13C into the lignite did not occur; however, the existence of deuteroformate (DCOO-) was demonstrated by nuclear magnetic resonance spectroscopy [29]. The reactions favored at low temperatures are of the type

CO + Lignite-O

~

CO 2 + Lignite

148 and

CO + Lignite-H20 ---, CO2 + Lignite-2H

where Lignite-H20 refers to moisture in lignite, the other symbols have been defined previously [4]. In hydrogen--carbon monoxide-water systems the hydrogen concentration does not increase, although it might be expected to do so as a result of the shift reaction. This increase indicates consumption of hydrogen in the reaction [4]. The net consumption of hydrogen in such systems is small relative to that of carbon monoxide, suggesting that the reaction of the lignite with carbon monoxide is preferential to that with hydrogen, in the temperature range 350-480~ [4]. Lignite, along with many other carbonaceous materials, rapidly reacts to produce hydrogen when heated with strong base in the presence of water, viz.,

C + 2 KOH + H20 ~ K2CO3 + 2 H2

[19]. Sodium and calcium in lignite can serve as natural catalysts for the water gas shift reaction, and also appear to catalyze the reaction of lignite with carbon monoxide and steam [27]. The alkali or alkaline earth carbonate catalyzed water gas shift reaction proceeds via formate intermediates. Alkali or alkaline earth salts of acetic, propanoic, and lactic acids have also been suggested as promoters for the carbon monoxide pretreatment prior to liquefaction [8]. In contrast, transition metal catalysis of the water gas shift reaction proceeds via metal oxides. Transition metals do not catalyze the reaction of lignite with carbon monoxide and steam, further evidence that the lignite reaction proceeds via formate intermediates [27]. Asphaltenes from lignite liquefaction in synthesis gas contain more aliphatic hydrogen than asphaltenes from comparable experiments in hydrogen. Hydrogen sulfide catalysis facilitates the increase in aliphatic hydrogen. Hydrogen sulfide also promotes reaction of carbon monoxide with low-rank coals [8]. Phenyl glycol converts readily to toluene and ethylbenzene at 300"C in a CO/H 2S atmosphere [30]. Complete conversion could be obtained at temperatures as low as 240* in CO/H20/H2S/H2 in the presence of Co-Fe or Fe catalysts on silica. At low temperatures, HzS must be present in the reaction mixture to produce non-oxygenated products. In the absence of H2S, polymeric carbonyl compounds formed. High temperature reactions of cellulose with CO produce chars, tars, and phenol, but with added H2S the reaction proceeds via a thiol intermediate to hydrocarbons [31]. Analogous processes may occur in the reaction of lignites with CO, although North American lignites have vanishingly small cellulose contents (Chapter 3). In a carbon monoxide - steam mixture, any compound that reacts with hydrogen more readily than with carbon dioxide can be hydrogenated as a result of the water gas shift equilibrium [ 19]. At 300"C a carbon monoxide - steam mixture results in 43% conversion of acetophenone to

149 1-phenylethanol. By comparison, hydrogen at 380 ~ effects only a 24% conversion, with the products being 10% ethylbenzene, 9% benzene, and 5% toluene [6]. These results show that carbon monoxide is superior for reduction, whereas hydrogen is the more effective at cracking. However, at higher reaction temperatures the reducing ability of hydrogen increases with respect to carbon monoxide. At 425~ the effectiveness for the reduction of benzophenone increased in the order hydrogen < carbon monoxide < synthesis gas [32]. Phenols and methylated phenols are most likely to survive reaction with carbon monoxide and steam [19]. Reaction of carbon monoxide in the presence of limited amounts of water produces benzene, toluene, xylenes, as well as possibly furans, pyrans, and their methylated derivatives [19]. Reaction of carbon monoxide and steam with thiophenol, thioanisole, diphenyl disulfide, and diphenyl sulfide at 450~ produces benzoic acid. The reaction mechanism is [22]

s + co

~__ ~ s - - c o

O

O

Carbon monoxide reacts with diphenyl sulfide to produce toluene (2.8% yield), with the methyl group in the toluene originating from the carbon atom in the CO [33]. In the absence of water, thiophenol produces benzene via an intermediate PhSCO [22]. 4.2 R E A C T I O N S W I T H H Y D R O G E N

4.2.1 Hydropyrolysis and hydrogasification reactions This subsection discusses reactions of raw, or otherwise unreacted, lignites in hydrogen atmospheres, and reaction temperatures up to 1000~

The discussion proceeds in order of

increasing reaction temperature. The reactions of lignite chars with hydrogen and other reducing gases are treated in Chapter 12. Some aspects of the fundamental reaction chemistry of lignites with hydrogen at liquefaction conditions (i.e., in the presence of a liquid vehicle, high hydrogen pressures, and temperatures generally below 500~

are discussed in the next subsection. A useful

review of the lignite hydropyrolysis literature through 1977 has been published [34]. (i) Reaction fundamentals. Thermogravimetric analysis of five Louisiana lignites indicated that hydropyrolysis begins around 200~ [35], in reasonable agreement with temperature of onset

150 of reaction in "differential scanning calorimetry [36]. The temperature maximum for the reaction and the kinetic parameters are shown in Table 4.1, with data also for reaction in nitrogen for comparison. The temperature range for these experiments was I(D-800~ TABLE 4.1 Kinetic parameters for hydropyrolysis of Louisiana lignites in hydrogen and nitrogen atmospheres [35]. Heating rate __*C/min . 435 25 11

Tm, ~ ~ N9 452 455 417 452 408 441

Eo, kJ/mol H~ __.~N2_ 221 198 205 217 209 218

o, kJ/mol H~ 36 30 35 28 39 31

Four lignites--Richland County (Montana), Darco (Texas), Zap, and Harmon (the latter two both North Dakota)--studied by differential scanning calorimetry [37,38] with hydrogen pressure 5.5 MPa and maximum temperature 570~

displayed exothermic heats of hydrogenation

in the range -503 to -640 J/g (daf basis). These are the highest values for any rank of coal studied; values for subbituminous coals were fairly close to those for lignites, and tended to decrease with increasing rank. A portion of the heat release arises from hydrogenation of oxygen functional groups or carbon-oxygen surface complexes, producing water and methane [36]. The weight loss at 570* was 56.6-74.5% (daf basis). The onset of exothermic reaction occurred in the range 232-266~

the lowest values for any rank of coal. Small endothermic heats, less than +8 J/g,

were observed below 230~ [36]. Differential thermal analysis (DTA) of New Zealand lignites in 8.0 MPa hydrogen produced two exothermic peaks; for Morton Mains lignite the temperatures of the maxima are 360 and 480~

[38]. Similarly, Indian Head lignite has a strong exotherm in hydrogen, beginning

about 4000C and reaching a maximum at 540~ [39]. Acid washing to remove cations produces DTA curves virtually identical with the untreated lignite, indicating that the inherent cations seem to have no effect on the reaction with hydrogen. Various metal ions back-exchanged onto the lignite did alter the positions, and in some cases the numbers, of the peak maxima. For example, untreated Mataura (New Zealand) lignite has peak maxima at 360 and 525~

exchange of nickel

results in three DTA peaks with maxima at 250, 350, and 410" C [38]. The reaction of Indian Head lignite, studied by differential scanning calorimetry [14] in 2.72 MPa H2, shows a sharp exothermic reaction, due to hydrogenolysis of hydroxyl and ether groups, beginning at 460~

Support for the proposed reaction of these functional groups comes

from studies of hydropyrolysis of subbituminous coal, in which infrared absorptions characteristic of phenolic groups decreased at reaction temperatures above 470~

[40]. As an alternative

explanations, gaseous hydrogen chemisorbed on an active sites on the char surface reacts to

151 produce methane and regenerate active sites [15], or hydrogen reacts with n bond systems to produce successively methylene, methyl, and then methane, with regeneration of a new double bond for each one consumed [41 ]. Hydrogen also may stabilize reactive volatiles, acting to prevent polymerization and condensation of solids on the surface of the char. Any of these proposed mechanisms would increase the volatiles yield, as has been observed for the hydropyrolysis of a Louisiana lignite [35]. The reaction scheme for flash hydropyrolysis of lignite follows the sequence of reactions [42], 1

Lignite

+ H2 ~

2

BTX + H2 --* CH4

+ C2H6

3 4 Lignite + H2 ---" CH4 --* C + H2 5 6 Lignite + H2 ~ C2H6 ~ C + H2 where BTX represents benzene, toluene, and xylenes. The approximate rate constants calculated for these reactions are shown in Table 4.2 [42], for pressures of hydrogen (represented as PH) of 10.3-17.2 MPa and temperatures of 973-1173 K. Heating rates of 20,000 to 30,000~

cause

cracking of polycyclic structures, with stabilization of the radicals by hydrogen [42]. Flash hydrogenation of a North Dakota lignite yields highly oxygenated liquids as primary products; these are converted to the desired benzene, toluene, and xylene in subsequent gas phase reactions with hydrogen [43]. TABLE 4.2 Rate constants for flash hydropyrolysis of lignites [42]. Reaction* 3.16 1.33 3.94 97.5 1.03 3.30

Calculated rate constant x 1012 PH0.137 exp(-68700/RT) x 1014 PH0.004 exp(-71700/RT) x 104 PH0.07 exp(-29700/RT) Pr_l-0.043exp (-15100/RT) x 108 Prr0.12 exp (-44000/RT) x 1014 PH1.17 exp (-85700/RT)

*In reference to reaction scheme shown above; hydrogen pressures in pounds per square inch. All of the reactions in this scheme are rate limited rather than diffusion limited. Rate constants for the reaction of Velva (North Dakota) lignite with hydrogen at 925~ and 10.4 MPa are shown in Fig. 4.1 as a function of the percentage of the total carbon gasified [43]. The data were obtained in a bench scale reactor with 5-10 g of lignite preheated before entering the reactor. The hydrogen balance as a function of conversion is shown in Figs. 4.2 and 4.3 [43]. The shape of the curve in Fig. 4.3 indicates an initial net hydrogen production, resulting from attendant

152 1

o ..o KJ

0.1

0.01

,,,,I,,,,I,,,,i,,,~1,,,,I,,,,i,,,,

0

10

20

30

40

50

60

70

Total carbon gasified, % Figure 4.1. Reaction rate for hydrogasification of Velva lignite at 925~ and 10.4 MPa [53]. Reaction rate is expressed as kg carbon in product methane per kg carbon in bed, per hour per atmosphere.

o

0.9

~" 0.8 o

0 0.7

''''

I ' ' ' '

20

I ' ' ' '

40

I '~''

60

I ~'''

80

100

Total carbon gasified, % Figure 4.2. Gaseous hydrogen consumption for hydrogasification of Velva lignite at 925~ 10.4 MPa and 2.83 m3/h hydrogen rate [53]. Hydrogen consumption is expressed as ratio of H2 in exit gas to HE in feed.

pyrolysis reactions" that is, the maximum consumption of hydrogen occurs at a higher carbon gasification than does the maximum evolution of hydrogen. Flash hydropyrolysis of coals of various ranks, including Hagel (North Dakota) lignite,

153 1.2

Ix0

'10

1.1~

-

laO

"

,'4 9

1--

X

0.9 0

' ' ' '

I''''

20

I''''

40

I''''

60

I''''

80

lOO

Total carbon gasified, % Figure 4.3. Gaseous hydrogen balance as a function of conversion for hydrogasification of Velva lignite at 925~ 10.4 MPa and 2.83 m3/h hydrogen rate [53]. Hydrogen balance expressed as ratio of HE in exit gas to H2 in feed.

showed the following relationships between product formation and the petrographic composition: Methane = 0.17 V + 0.58 (PV + E + R) Ethane = 0.08 V + 0.05 (PV + E + R) BTX = 0.08 V + 0.127 (PV + E + R) [44]. In these equations V is vitrinite, PV pseudovitrinite, E exinite, and R resinite. The correlation coefficients are 0.981 for methane formation, 0.766 for ethane, and 0.427 for BTX [44]. (ii) Effects of temperature. Total carbon oxide yields increase with increasing maximum temperature of reaction for Montana and North Dakota lignites in an entrained flow reactor [45]. The primary yield of carbon dioxide occurs below 480~

and remains constant at higher

temperatures. Increasing the temperature of hydropyrolysis of Rockdale (Texas) lignite above 750~ significantly reduces the yield of heavy liquids, to about 0.5% at 850~

At the higher hydrogen

pressures the maximum yield of BTX depends much less on temperature than does that of the heavier liquids. Consequently, the maximum in the yield of all liquids (i.e., BTX and heavier compounds) will occur at 750~

the temperature of maximum heavy liquids formation, and

amounts to about 18% [42]. Hydrogenation of Hagel lignite at heating rates over 104 ~

shows an increase in total gas

yield increasing reaction temperature, but the liquid products reach a maximum at 750-8(OOC [46].

154 These experiments were performed for hydrogen pressures in the range 1.03-27.6 MPa. At 3.4 MPa the maximum yield of liquid product occurs at 800"C; with increasing hydrogen pressure the temperature at which maximum liquid yield occurs drops, so that at 27.6 MPa the maximum yield occurs around 750"C [46]. At high temperatures, e.g., 850~

benzene is the only liquid product,

but at 650~ only about half the liquid is benzene. Methane is almost the sole gaseous product at 850 ~ but is only half the total gases at 650~

At 1.03 MPa hydrogen, the amount of ethylene is

30% of the amount of ethane formed, while at 27.6 MPa, ethylene amounts only to about 0.5% of ethane [46]. An increase in hydrogen pressure suppresses formation of ethylene [47]. (iii) Effects of hydrogen pressure. Evolution of carbon from Montana lignite in hydrogen, hydrogen-helium mixtures, and helium increases with temperature and with hydrogen partial pressure [48]. Oxygen evolution increases with temperature. Conversions in hydrogen exceed those in helium, but hydrogen partial pressure has no effect in the range of 1.8-5.2 MPa [48]. Loss of hydrogen from the lignite increases with temperature, but does not depend on the hydrogen partial pressure. Results obtained in a hydrogen-helium mixture with hydrogen partial pressure 1.8 MPa are essentially the same for reaction in pure hydrogen at 1.8 MPa. Initial gasification depends mainly on hydrogen partial pressure and not the total pressure. This behavior differs from that of bituminous coals, because lignites do not pass through a plastic stage during devolatilization. The evolution of carbon dioxide is the same in hydrogen or helium atmosphere and does not depend on hydrogen partial pressure. The yield of methane plus ethane depends strongly on partial pressure. Above 700~ significant increases in yields occur with increasing temperature. Between 540 and 650~ the increase in methane plus ethane yields is the same for any hydrogen partial pressure in the range 1.8-5.2 MPa; but above 650~ the yields increase with increasing hydrogen partial pressure. The yields of oils, tars, and light aliphatic gases are generally similar in hydrogen or helium. At any hydrogen partial pressure, the evolution of methane and ethane relates directly to the total amount of hydrogen removed from the lignite. This proportionality increases significantly with increasing pressure. However, the removal of hydrogen from the lignite does not depend on partial pressure of hydrogen, being essentially identical in hydrogen or helium atmospheres. During the process of hydrogen release from the lignite, active sites form, which catalyze methane and ethane formation in the presence of gaseous hydrogen. Montana lignite pyrolyzed in 0.1 MPa helium yields 1% of methane at 800~

but the

methane yield increases to 5% in 6.9 MPa hydrogen [47]. In hydrogen, methane evolution begins below 600~

whereas in helium, methane evolution does not start until temperatures exceed 750~

[47]. Hydropyrolysis in 6.9 MPa hydrogen increases the total volatiles evolution by about 7.5% relative to reaction in 0.1 MPa hydrogen, but only a very small increase in tar yield occurs. The maximum total yield of tar and hydrocarbon liquids is 10% in 6.9 MPa hydrogen. Hydropyrolysis at elevated pressures enhances the yields of light hydrocarbon gases (except ethylene). Most of the incremental yield achieved with increasing pressure is in the form of methane. Flash hydropyrolysis of a North Dakota lignite at hydrogen pressures of 3.4 to 13.7 MPa

155 and reaction temperatures of 7(K)-800~ showed maximum yields of BTX in the range 775-8000C [42]. At 17.2 MPa hydrogen pressure, the yield of BTX was constant at 9% over the temperature interval 725-775"C [42]. An increase in pressure from 3.4 to 6.9 MPa increased the yield of BTX from 4.5 to 7% [42]. However, further substantial increases in pressure, 13.7 MPa, only managed to increase the BTX yield from 7 to 9%. A further rise in pressure to 17.2 MPa has no effect on increasing yield of BTX, but does reduce the temperature at which the maximum yield can be obtained. At 17.2 MPa hydrogen pressure, the residence time necessary to obtain the maximum BTX yield drops from 9 s at 7250C to 2 s at 850~ necessary to achieve the maximum BTX yield

[42]. At residence times greater than that

(e.g., >9 s at 725 ~ decomposition of the BTX

occurs. Reaction above 850~ results in essentially no liquid products being formed, the products being almost entirely methane and ethane. The yield of these compounds is a direct function of the hydrogen pressure. Increasing residence time beyond 7 s results in decomposition of the methane, with a consequent reduction in total yields. At 18 MPa and 8750C the conversion of lignite to methane and ethane was 88% [42]. Reduction of the hydrogen/lignite ratio from 1 to 0.25 reduces the yield of gases. The methane yield at 9550C is shown as a function of hydrogen partial pressure in Fig. 4.4 [49]. The data were obtained for an unidentified lignite in a 2.3 kg/h bench scale reactor in support of the development of the BI-GAS gasification process. Hydropyrolysis of Mercer County (North Dakota) lignite in 1.5-2 MPa hydrogen has produced methane yields in the range 15-25% [50]. (iv)

Evolution of products. Reactions of Rcckdale lignite below 540~ are mainly the

evolution of carbon dioxide, production of some water and carbon monoxide, formation of some low molecular weight aliphatic compounds, and formation of oils and tars [48]. All are essentially pyrolysis reactions. In the temperature range 540-700~

the remaining oxygen in the lignite

evolves as carbon monoxide and water. More water forms in hydrogen than in helium. Gasification of Montana and North Dakota lignites in hydrogen produces an increased yield of steam, but a lower amount of carbon dioxide than in helium [45]. The total amount of oxygen derived from the lignite is essentially the same for reaction in either gas; reaction in a hydrogen atmosphere results in less oxygen being evolved as carbon dioxide and more as steam. The measured kinetic parameters for the evolution of oxygenated species are shown in Table 4.3 [45] for assumed first-order reactions. TABLE 4.3 Kinetic parameters for evolution of oxygenated species [45].

Component C02

H20 CO

Pre-exponential (s-l) 2.0 x 105 5.4 x 109 5.5 x 105

Activation Energy (kcal/mol) 27.83 38.89 25.80

156 lOO ,Iv ~ ,Iv

~"

90-

80-

'~ 70. ro 60

,%

~9 40

.Y

20 ,~ 10''"

o

I"''1'"'

5

10

I"''1'"'

15

20

I''"

25

I"''1'"'1"''1'"'

30

35

40

45

50

Hydrogen partial pressure, MPa Figure 4.4. Correlation of methane yield with hydrogen partial pressure at 950~ [56]. Methane yield is expressed as the amount of carbon in methane as a percentage of the amount of carbon in the coal. Data points include several bituminous coals in addition to lignite.

The maximum carbon dioxide yield (expressed as gram-atoms of oxygen per gram-atom of feed carbon) is equal to the carboxyl oxygen in the raw lignite for total oxygen content below 0.1 (in each case, the parameters are expressed in terms of gram-atoms of oxygen per gram-atom of carbon in the feed lignite). When the total oxygen content exceeds 0.1 on this basis, the maximum carbon dioxide yield is 0.4(no - 0.1) where no is the total oxygen in the lignite [45]. For total oxygen below 0.23, the steam yield is equal to the hydroxyl oxygen in the feed lignite; for oxygen contents greater than this value, the steam yield is 0.13 + 2(no- 0.23) [45]. The carbon monoxide yield is given by 0.3 l ( n o - 0.1) [45]. Lignite carbonized to reduce the oxygen content prior to reaction consumes less hydrogen, reduction of hydrogen consumption up to 50% for a North Dakota lignite and 25% for Soya Koishi (Japanese) lignite [51]. Above 650~ the amount of hydrogen evolved from Rockdale lignite begins to increase rapidly with increasing temperature [48]. In dense-phase hydropyrolysis, the highest total carbon conversion (50%) occurred at 13.7 MPa and 785~ [52]. The products included methane (14%), ethane (8%), and hydrocarbon liquids including BTX (16.5%). During hydropyrolysis of three Louisiana lignites at 500, 600, and 800~

alkanes and

alkenes are the compound classes produced in largest amount, benzene derivatives second, and phenols third [53]. Little variation in the composition of the products occurs, both among the different lignites and as a function of temperature. Pyrolysis under otherwise similar conditions but in a nitrogen atmosphere results in the phenols being produced in second largest amount with benzene derivatives third.

157 Hydropyrolysis of North Dakota lignite in a free-fall, dilute phase reactor system achieved carbon conversions of up to 80% and liquid yields as high as 16% (maf basis) [54]. The reaction conditions were temperatures of 620-850~ 28,000-80,000~

residence times of 10003000 ms; and heating rates of

At the 16% liquid yield, the liquid composition was 94% benzene, with small

amounts of naphthalene and anthracene. Only light aromatics are in the liquid produced above 650~

Methane and ethane predominate among the gases. At carbon conversions of 45% and

residence times of 500 ms, the selectivity to liquids is 43% [54]. At higher carbon conversions, the lignite increasingly hydrogasifies to methane. Carbon conversion increased as a function of both temperature and heating rate. If the residence time increases, the BTX increasingly crack to coke and hydrogen. Lignite reacting with hydrogen in an entrained downward flow reactor yields 10% BTX (based on the carbon in the lignite) at 13.7-17.2 MPa and 750-775~

[55]. Yields of 13%

BTX are achieved with subbituminous coal under the same conditions. Methane yields over 80% have been achieved. In dilute-phase flash hydropyrolysis of a North Dakota lignite, BTX yields reached a maximum in the range of 10.3-13.7 MPa and 700-800~

[56]. The maximum yield of

methane (54%) occurred at 750~ and 27.5 MPa, produced with a 14% yield of ethane and 10% yield of total hydrocarbon liquids (including BTX). Total carbon conversions of 80% were achieved at 27.5 MPa and 750~

Total yield increased as a function of residence time, but

increased residence time reduced the BTX yield, as a result of increased cracking. During reaction of hydrogen with Rockdale lignite at temperatures to 800~ and pressures to 41.2 MPa, oil formation begins at about 400 ~ and increases rapidly to 525 ~ [57]. At about 525~ the net production of oils decreases because of cracking reactions. Other than oils and tars, the total amounts of materials formed do not depend on gas atmosphere or heating rate, suggesting a direct stoichiometric relationship between these species and the amounts of appropriate precursor functional groups in the lignite. Oil and tar formation is about the same in hydrogen or helium and increases with increasing heat-up rate, reflecting competition between stabilization and repolymerization of radical fragments. The heavier compounds produced in hydropyrolysis of Rockdale lignite are polynuclear aromatic hydrocarbons, about 40% of this fraction being naphthalene [42]. The maximum yield of heavy liquidsis about the same as that of BTX, but the maximum in yield for the heavier compounds occurs at lower temperature. At 13.7 MPa the maximum yield of BTX, 10%, occurs at 800~ whereas the maximum yield of heavy liquids, 9%, occurs at 750~

[42].

Chars remaining after reaction of Indian Head lignite with hydrogen contained 2.87% hydrogen, compared with 2.36% in char from a similar experiment in argon [15]. In flash hydropyrolysis of lignite, about 50% of the sulfur in the lignite remains in the char, along with most of the nitrogen [42]. Only small quantities of these two elements occur in the liquid products. Sulfur and nitrogen compounds were present in the hydropyrolysis products of Louisiana lignites to greater extent than observed when the same experiments are performed in nitrogen [53]. Pyrolysis in nitrogen may result in nitrogen- or sulfur-containing radicals recombining to remain in the char, whereas in hydrogen these radicals could be capped and produce stable thiophene,

158 pyrrole, or pyridine derivatives. Reaction of Onakawana (Ontario) lignite with hydrogen at 13.9 MPa and 4500C for 3 h produced slight caking tendencies [58]. Prior to this treatment, the lignite showed no dilatation, and an 8% contraction at 478~ 331~

After treatment in hydrogen, an 11% contraction was observed at

and a 6% dilatation at 414~ (these temperatures being the temperatures of the maxima),

with a 1.5 free swelling index. Hard, agglomerated residues remained after the dilatometry measurements; mesophase spheres were observed by microscopy. 4.2.2 Reactions under liquefaction conditions This subsection focuses on some of the fundamental chemistry of high-pressure hydrogenation lignites. Liquefaction processing of lignites is treated in Chapter 12. Early work on lignite hydrogenation indicated that lignites are much more sensitive to reaction conditions than are bituminous coals [59,60]. The yield of acetone-insoluble residue in hydrogenation of North Dakota lignites (together with subbituminous and high volatile A bituminous coals of Utah, Montana, and Wyoming) could be predicted reasonably well from petrographic analysis on the assumptions that the mineral matter (including added catalysts) and fusain were completely inert and that only 60% of opaque attritus would convert to soluble products [61]. High-oxygen coals did not appear to consume more hydrogen than coals of lower oxygen content. In contrast, other work suggests that the petrographic composition of lignite does not appear to have a significant effect on the liquefaction reactivity [62]. A nearly linear relationship between soluble products of liquefaction and percent huminite expressed on a daf basis [63,64] exists for North Dakota lignites. However, adding data for Texas lignites to the set produces significant scatter and any broad correlations seem dubious. The linear relationship of solubility and huminite for the North Dakota lignites suggests that other macerals in the lignite (including liptinites) have little contribution to the solubility yield, whereas in Texas lignites macerals other than huminites may have an active role in the hydrogenation. The reactions were conducted in 130 mL reactors using 10 g dry,-100 mesh lignite, 20 g tetralin, and an initial hydrogen pressure of 5.5 MPa, reacting at 430~ for 30 minutes. Data for both lignites can be fitted to a single straight line, but there is much less scatter for the data from the North Dakota lignite than for the Texas lignite. At 40(0-450~ and 20-30 MPa hydrogen, all petrographic constituents of lignite react virtually completely, with the exceptions of fusain and opaque attritus [65]. Reaction parameters affecting lignite liquefaction include heating rate, catalyst, and solvent. Fast heating of Estevan lignite gives slightly better liquefaction results relative to slow (5~ heating [66]. For reactions with a 3:1 tetralin/lignite ratio at 4000C in 14 MPa nitrogen, rapid heating obtained by injection of the lignite into the preheated autoclave provided 57% conversion, while the slow heating of lignite in an autoclave from room temperature to the reaction temperature resulted in conversions of about 47% [66]. A survey of potential catalysts showed that the best hydrogenation yield from U.S. lignites was obtained with ammonium molybdate [67]. The

159 reaction conditions were 10 MPa hydrogen and 500~ the best conversion was 82%, with a liquid yield of 50%. Low-rank coals, such as Hagel lignite, have a greater tendency to undergo retrogressive condensation in solvent-free liquefaction than in more conventional processes [68]. Heating Beulah (Noah Dakota) lignite in hydrogen increases the free radical concentration [69]. On a dmmf basis the original lignite has a radical population of 0.72 x 1019 spins/gram. On heating in hydrogen in the range 406-506~

the radical population increased to 0.97 x 1019

spins/gram. No change occurred as a function of temperature in this range (this behavior contrasting with subbituminous and bituminous coals), nor was there an effect of hydrogen pressure. The g-value of this lignite was 2.00379; the sample aromaticity, 0.70. Cleavage of bibenzyl linkages may be a key feature in the reaction of lignites with hydrogen. The ease of liquefaction has been correlated with the yield of succinic acid produced during oxidation with peroxytrifluoroacetic acid [62]. The succinic acid presumably arises from bibenzyl structures (including 9,10-dihydrophenanthrene and acenaphthenes). Cracking of the aliphatic groups, together with decarboxylation and loss of other oxygen functional groups, comprise the significant structural transformations in the liquefaction of Martin Lake lignite [70]. Big Brown and Indian Head lignites showed a remarkable temperature dependence for methane formation [10]. Below 3630C, all of the CH4 yield arises during the first few minutes, and the total CH4 yield is independent of reaction time in the range of about 5 to 60 minutes. At 3750C some dependence on time is observed; with Indian Head lignite the methane yield ranges from about 0.25% (of the lignite on an maf basis) at 5 minutes to about 0.35% at 60 minutes. (This compares with 80% methane yield from the same lignite in dilute-phase hydropyrolysis at 750-775~

[55].) With further increase in reaction temperature, the dependence of CH4 yield on

reaction time becomes pronounced, now over the same time range increasing from about 0.32 to 1.5%.

Indian Head lignite, hydrogenated at 450~

in 9 MPa hydrogen for 1 h, produced the

largest amount of gas and had the highest hydrogen consumption among several low-rank coals [71]. Of the coals tested, Indian Head had the largest substitutional index of the aromatic clusters, the lowest aromaticity, the largest naphthenic ring unit per structural unit, and the greatest number of carbon atoms per structural unit. Loss of oxygen proceeds easily up to a point, but a residue of oxygen remains fairly inert to reaction with hydrogen [72]. This suggests that two general types of oxygen occur in low-rank coals, "reactive oxygen," present in unstable structures, though not specifically identified, and "inactive oxygen" in stable functional groups [72]. The reactive oxygen may be present as aliphatic ethers, whereas the inactive oxygen could be in the form of phenols and aromatic ethers such as diphenyl ether and diphenylene oxide [73]. The rate of removal of oxygen decreases after about 80% of the oxygen originally present has been removed; when oxygen drops to about 3%, further removal is very difficult [72]. Liquefaction conversion correlates with removal of oxygen for conversions up to about 90% [73]. High conversions of Indian Head lignite resulted from relatively easy cleavage of ether

160 groups and from the presence of abundant polar functional groups such as carboxyl and phenolic, the loss of which can contribute to the cleavage of C-C bonds. Conversions of 80% to benzenesolubles and 55% to hexane-solubles, were obtained at 400"C for 15 min. in 10 MPa (cold) hydrogen with a nickel catalyst [74]. Non-donor solvents such as naphthalene or phenanthrene were used. When a donor solvent is used, the solubility of Texas lignite in tetralin seems independent of whether hydrogen or helium is used as the gas atmosphere, for reactions at 375"C [75]. Differential thermal analysis of New Zealand lignites showed an endothermic reaction with tetralin in the absence of hydrogen [38]. Further demonstrating the role of ether groups, pretreatment of Zap (North Dakota) lignite with 20-30% aqueous sodium hydroxide enhanced its reactivity [76]. Pretreatment in 2.07 MPa nitrogen for 1 h at 150-300~ with 20% NaOH solubilizes >70% of the lignite. Upon subsequent liquefaction at 410~

for 1 h in 6.7 MPa hydrogen, oil yields increase and preasphaltenes and

residue decrease. An increase in liquefaction reactivity relates to increased severity of pretreatment. The hydrolysis presumably occurs at ether linkages. If the liquefaction reaction is hydrogendeficient, condensation reactions between phenols rapidly increase the number of ether crosslinks. Formation of phenols may result from reaction of hydrogen with lignin structures. The relative concentrations of six phenols--o-cresol, m-cresol, p-cresol, 2-ethylphenol, 2,4dimethylphenol, and 4-ethylphenol -- produced in the liquefaction of Beulah lignite [77] showed the same pattern as observed for pine lignin [78], Loy Yang (Victoria, Australia) brown coal [78], and Wyodak (Wyoming) subbituminous coal [77]. Reduction of the phenols is favored by high partial pressures of hydrogen or high concentrations of hydrogen donors [79]. Hydrogenation of lignin residues produces phenols and aromatic compounds in ratio of about 1:1 [80]. Cleavage of ether bonds to form phenols and alkylindans is favored over cleavage to alkylbenzenes and alkylindanols. The liquid yield from Hagel lignite, as well as a Turkish and a Spanish lignite, increases as a function of organic sulfur content, regardless of whether the lignite had been impregnated with a catalyst [81,82]. Furthermore, the increase in liquids yield obtained in a hydrogen atmosphere relative to that obtained by thermolysis of the same lignite in nitrogen, was also proportional to the organic sulfur content of the lignite. These experiments were carried out in the absence of a liquid hydrogen donor vehicle. 4.3 R E A C T I O N S IN O T H E R G A S E O U S A T M O S P H E R E S

4.3.1 Oxidation reactions This subsection treats reactions of lignites with air or oxygen at relatively mild conditions. Further discussions of oxidation of lignites are found in Chapter 9, relating to spontaneous heating and combustion, and in Chapter 11, on combustion processes. Reactions of lignites with oxidants in the liquid phase, primarily for structural studies, have been treated in Chapter 3. Chemisorption of oxygen and formation of peroxides is the first step in the low-

161 temperature oxidation of lignites [83-85]. Subsequent formation of carboxyl groups may be responsible for reducing the rate as oxidation progresses [16]. At low temperatures oxygen accumulates on the lignite, whereas above 70-80~

gaseous oxidation products evolve [16,86].

Using tetralin and diphenylmethane as examples, the initial reactions can be written: OOH

OOH

I

+ 0 2

[84]. These equations highlight the importance of reaction at the benzylic carbon. Once the peroxide has formed, free radical chain reactions can proceed, leading ultimately to the formation of carbon monoxide, carbon dioxide and water [84]. An increase in the number of crosslinks in the lignite accompanies these changes [86]. A reaction sequence, shown on the top of page 162, first elucidated for phenol-formaldehyde resins [87] demonstrates formation of the reaction products and increased crosslinking. Increased crosslinking affects other properties of the lignite, including the amount of extractable material and the volatile matter content [84]. A sequence of oxidation reactions has been proposed as follows (here Ar represents a general aromatic system and ArH2 a general hydroaromatic system): 4 Lignite-COH + 2 Ar + 02 + 2 H20 ~ 4 Lignite-COOH + 2 ArH2 4 Lignite-COOH + 2 Ar ----4 Lignite + 2 ArH2 + 4CO2 4 Lignite + O2 + 2 I-I20--* 4 Lignite-OH (i.e., Lignite-CHzOH) 4 Lignite-CHzOH + 4 Ar ---, 4 Lignite-COH + 4 ArH2

[85,88]. Reaction of lignite with oxygen produces -OH groups (as in the third equation above) that oxidize sequentially to aldehydes and acids; the acids decarboxylate, resulting in loss of some of the carbon from the lignite structure and generating sites at which the reaction sequence could begin anew. Although the initial stage of oxygen reaction with lignite is a chemisorption [85], in later stages diffusion becomes important [89]. The rate of oxygen uptake at a given temperature decreases as a function of time. The abundant phenolic structures enhance the susceptibility of lowrank coals to oxidation [90]. X-ray photoelectron studies of air oxidation of Beulah lignite vitrain provided results summarized in Table 4.4 [91]. The data are shown as the ratio of the intensity of the indicated peak

162 OOH I

O II +

0 II

H20

0 II ,C ~

+ 0

il

C9 ~-

~

o

+

O II 1C ~

co O II

4- "OH

+

~

~

C~OH

+ CO 2

r

to the total intensity of all carbon ls peaks. Air oxidation of Pust (Montana) lignite increases the number of hydroxyl groups [92], suggesting that autoxidation proceeds via alkoxy and hydroxyl radical intermediates [93]. Diffuse reflectance infrared Fourier transform (DRIFT) spectra TABLE 4.4 Oxidation of vitrain in air, expressed as ratio of peak intensity to total carbon ls peak intensities [91]

Time Temp., ~ (start) (ambient) 24 hr 95 24 hr 150 24 hr 200 5 days 25

Ether or Hydroxyl 0.04 0.03 0.04 0.27 0.08

Carbonyl 0.00 0.03 0.04 0.01 0.03

.Carboxyl 0.20 0.01 0.02 0.09 0.00

163 of Beulah lignite that had been vacuum dried and then exposed to water-saturated air at 220C for 1 week and 1 month showed large increases in - - O H [94]. A slight increase in absorption around 1100 cm-1 was attributed to ethers or alcohols. Absorption in the carbonyl region of 1500-1800 cm-1 also increased. Infrared evidence indicated chemisorption of water onto the oxygen functional groups by hydrogen bonding. During oxidation of a North Dakota lignite in a fluid bed reactor at 2000C, total acidity (phenolic plus carboxylic, measured by barium cation exchange at pH 12.5) increased from 5 meq/g to an asymptotic value of 9 meq/g after 12 hours [95]. At 230~

the total acidity had

increased to 13 meq/g after 12 hours, but had not yet reached an asymptote. The formation of carbon oxides and phenolic and carboxylic groups represent parallel processes that occur at similar rates.

J

CO + CO2

Lignite --COOH + -OH

The rate of oxidation depends on many many factors, which include rank, temperature, oxygen concentration, particle size, moisture in the coal and the oxidizing atmosphere, and the extent of any prior oxidation. Oxidation is first order with respect to oxygen partial pressure at 25, 65, and 95~

[96]. The rate increases with temperature; in the range of 5--33% moisture, the

average activation energy for five lignites was 11.8 kcal/mol 0 2 [96]. Over the range 25-950C oxidation rates are a maximum at moisture contents near the equilibrium moisture of 20%. Rates at 5 and 36% moisture are similar to each other and are about half the value at 20% moisture [96]. For lignites dried in an inert atmosphere, the effect of moisture on oxidation rate is proportional to 1 + 0.674M - 0.01634M2

where M is the moisture content in percent [96]. The oxidation rate correlates with the cube root of the specific surface area S, where S is given by S = 1.22/m

and m is the mean sieve size in inches [96]. Oxidation starts at the surface and falls off exponentially with depth from the particle surface, in which case r = (k/m)[ 1 - exp(-5.5m)]

164 [96]. Here r is the oxidation rate in ppm O2/hr, k the rate constant, and m is defined as above. At a constant temperature below 70~

the oxidation rate decreases with time and with

cumulative oxidation [96]. For different lignites, the initial rates and the time decay vary randomly with respect to source of the lignite and the position in the seam from which the samples were obtained. (Apparently no petrographic data to accompanied these samples.) The decrease in rate of oxidation does not occur above 70~

conditions at which appreciable amounts of carbon dioxide

form [96]. The activation energy for reaction with oxygen is 50 kJ/mol [96]. For comparison, the activation energy for oxidation of lignite with potassium permanganate solution is 19 kJ/mol, based on tests using 0.1N KMnO4 solution, 6x8 mesh lignite, and temperatures of 24, 57, and 100~ [96]. The heat of reaction increased from 314 kJ/mol O2 at 20~ to 377 ld/mol O2 at 900C [96]. Moisture content, particle size, and extent of cumulative oxidation had no apparent effect on the heat of reaction. The effect of temperature on heat of reaction and oxygen consumption rate is shown in Figs. 4.5 and 4.6 [96]. The incremental carbon conversion of Onakawana lignite in air decreased with increasing temperature [95]. That is, the increased carbon conversion obtained by raising the reaction temperature from 715 to 805~ was essentially the same as obtained from the much larger increase in reaction temperature from 805 to 1000~

As reaction temperature increases from 8050C to

1000~ carbon monoxide formation increases while carbon dioxide formation slows. Both untreated and acid-washed Fort Union (Montana) lignite lost about 90% of its initial weight on reaction in air [97]. The presence of the metal cations decreases the rate of weight loss in air, relative to the acid-washed sample from which the cations had been removed. Oxidation of lignite by air during storage affects reactivity in subsequent processing. Lignite stored in air for 70 weeks showed a decreased yield of solvent refined lignite (37%, maf basis) compared to fresh lignite (45%) or lignite stored under nitrogen or under water for the same time [21]. The gas yields (25%), and light oil yields (21%) were identical regardless of whether fresh lignite or lignite exposed to air for 70 weeks were tested, and the conversions were very similar (91% for fresh, and 88% for air-oxidized lignite) [21]. 4.3.2 Reactions with water or steam This subsection discusses results of reactivity experiments in which the starting material was a raw lignite; in general, the discussion proceeds in order of increasing reaction temperature. The gasification reactions of lignite chars with steam are discussed in Chapter 12. Catechol-like structures decompose at relatively low-severity conditions. This effect has been observed in the hydrous pyrolysis reaction of Patapsco (Maryland) lignite at 100-350~

for

reaction times of 30 minutes to 10 days [98]. The catechol structures, characteristic of lignin remnants, transform to phenol structures. Loss of aliphatic components also occurs [98]. Alkanes appear among the reaction products of Rapponmatsu (Japanese) lignite treated at

165 500

x o o

400

fl

300

"~ 2oo o

~

lOO

m ''''

I ' ' ' '

20

I''''

40

I''''

60

I''''

80

100

Temperature, ~ Figure 4.5. The heat of reaction (IO/mol oxygen consumed) as a function of temperature for reaction of Baukol-Noonan lignite in oxygen (adapted from[97]). The curve shown is the best visual fit of data representing samples of various particle sizes, states of activation, and moisture contents.

a~ 0.08

~9 0 . 0 7 ~

0.06

0.05" .~" 0 . 0 4 " 4","

.

~'

0.03

oO

0.02

a~ 0.01 " X

o

o

'''~

I'''' 20

I '''~ 40

I'''' 60

I'''' 80

100

Temperature, ~ Figure 4.6. Moles of oxygen consumed per hour, per kilogram of maf lignite, as a function of temperature, for the reaction of Baukol-Noonan lignite with oxygen (adapted from [97]). The curve shown is the best visual fit of data representing samples of various particle sizes, states of activation, and moisture contents.

166 20(O500C in 0--4 M aqueous sodium hydroxide [99]. The other gaseous products were carbon dioxide and hydrogen. Hydrogen and the alkanes increase with increasing reaction temperature, above 350"C. Phenols, along with cresols, naphthols, and other aromatics, appear in the liquid products of the reaction. Hydrolysis reactions are important below 300"C. The alkanes form by decarboxylation reactions, the rate of which begins to increase around 350~ pitch, and significant gas yields, becomes important above 400~

Formation of tar or

as a result of dehydration

reactions [99]. The reduction of oxygen functional groups also accompanies aqueous reaction of Zap lignite (250-350~

28 MPa, and 5-300 min reaction time) [100]. The methane yield increases

with increasing reaction time. A noticeable increase in tar yield occurs at short reaction times under these conditions, but does not persist at longer times. Beulah-Zap lignite shows a maximum in oil yield at 340~ 250-360~

for hydrous pyrolysis at

72 h [101]i The yield amounts to 30 mg/g [101]. In comparison, a lignite of Miocene

age from the Far East had a much higher oil yield (101 mg/g) with maximum oil production occurring above 360~

[ 101]. The distinction reflects the different petrographic compositions of

the two samples, the Beulah-Zap containing about 3% liptinite (so-called liptinite-poor), but the Far Eastern lignite being liptinite-rich with about 32% liptinite. Reaction of the Beulah-Zap lignite below 310~ forms an oil containing low molecular weight aromatics (e.g., alkyltetralins and alkylnaphthalenes), phenols, and n-alkanes in the range C14-C35 [101]. The dominant n-alkane is C 25. The alkanes and aromatics occur in comparable amounts. Increasing the reaction temperature to 350-3600C increases the yield of n-alkanes, and they become more abundant than the aromatics. The range of n-alkanes extends from
All of the sulfur removed was organic sulfur.

Montana lignite pyrolyzed in wet nitrogen initially displayed a lower weight loss relative to pyrolysis in dry nitrogen [97]. This difference is a result of the slower heating rate of particles in an atmosphere of wet nitrogen. There was no difference in the total weight loss achieved at the end of the reaction for pyrolysis in wet nitrogen, dry nitrogen, or carbon dioxide, indicating that there was no reaction between the pyrolyzing lignite and the surrounding gaseous atmosphere. Untreated

167 lignite showed lower total weight loss than lignite from which the cations had been removed by acid-washing. Both Indian Head and Velva lignites were more reactive in steam at 7000C than Martin Lake lignite, which in turn showed about the same reactivity as a Wyoming subbituminous coal [ 104]. The gas composition varied only slightly among the lignites, being 63--64% H2, 31-34% CO2, 2-5% CO, and 0.3-0.7% CI--I4 [104]. Reaction of Canadian coals of various ranks in steam showed lignite to be the most reactive [105]. The reaction could be represented by a shrinking core model. In general the reactivity of lignite toward steam is substantially superior to that of bituminous coal, even when reaction of the latter is catalyzed by potassium carbonate. Velva lignite with no added catalyst has about 65% of its original fixed carbon remaining after 10 minutes of reaction at 750~

the comparable datum for River King (Illinois) bituminous coal with 10%

potassium carbonate addition is 87%. Indian Head lignite reacted with steam in a thermogravimetric analyzer at 7000C had a carbon conversion rate of 0.38 g/h.g [106]. When 10% by weight potassium carbonate was added as a catalyst, the carbon conversion rate increased to 2.7 g/h.g. In comparison, rates of 0.3-0.4 g/h.g were reported for Illinois No.6 bituminous coal in the Exxon catalytic gasification process and 0.19 g/h.g for Velva lignite in the CO2 Acceptor Process (Chapter 12). Reactivity parameters calculated on the rate of fixed carbon loss, normalized to the initial mass of fixed carbon, in the range 0-50% conversion for Velva lignite reacting with steam at 7500C are shown in Table 4.5 [107]. TABLE 4.5 Effect of catalysts on reactivity of Velva lignite in steam at 7500C [107].

Catalyst None Sunflower hull ash Sodium carbonate Potassium carbonate Nahcolite Trona

Catalyst Loading, Wt % As-received 0 20 10 10 10 10

Reactivity Parameter 50% Conversion, g/hr.g 2.0 3.5 5.5 5.5 6.2 6.8

The reactivity at 750~ of Velva lignite loaded with potassium carbonate in the range of 220% (as-received basis) increased with catalyst loading in the range 2-10% K2CO3, the reactivity parameter increasing from 3.3 g/h.g at 2% loading to 5.5 g/h.g at 10% loading [107]. Higher catalyst loadings had no effect on reactivity, the average reactivity parameter being 5.4 g/h.g through the range 10-20% K2CO3 [ 107]. Reactivity of Velva lignite in steam at 750~

increases slowly with increasing sodium

168 carbonate loading. The reactivity parameter at 50% conversion increases from 2.0 g/h.g with no added catalyst to 6.1 g/h.g with 20% (by weight, as-received basis) added sodium carbonate [ 108]. At relatively low catalyst loadings the effect of increasing potassium carbonate loading is more pronounced. For example, after 10 minutes of reaction the amount of fixed carbon remaining is 11-12% with 10% addition of either sodium or potassium carbonate; however, with 2% loading of catalyst, the percentages of fixed carbon remaining after 10 minutes are 43% for potassium and about 25% for the sodium carbonate case. Thus increasing the potassium level produces a larger increase in reactivity than is obtained for comparable changes in sodium. Both trona and nahcolite provided a more rapid carbon conversion, for Velva lignite at 7500C, than potassium or sodium carbonate [107]. With trona or nahcolite, 90% carbon conversion (as measured by remaining fixed carbon) was achieved in eight minutes, whereas with the sodium or potassium carbonate the 90% conversion was achieved in ten minutes [ 107]. The carbon conversion of Onakawana lignite with steam doubled in the range 700 to 822~ [95]. The yields of CO and H2 tripled in this range, whereas CO 2 production increased much more slowly and in fact decreased at even higher temperatures. The H2/CO ratio decreased with increasing reaction temperature. An increase in reaction temperature from 700 to 800~ reduced the time required to achieve 80% conversion of the fixed carbon from 105 to 80 minutes for Indian Head lignite with potassium carbonate catalyst [ 104]. The principal effect of increasing temperature on gas composition is to increase the proportion of CO in the product. Because the steam-carbon reaction is endothermic whereas the water gas shift reaction is exothermic, an increase in reaction temperature displaces the equilibrium of the steam-carbon reaction to the fight, but equilibrium moves to the left for the water gas shift. Thus the amount of CO increases. Since hydrogen is a reaction product in either case, temperature changes have little effect on the amount of hydrogen in the gas. For samples impregnated with sodium or potassium carbonate and reacted with steam at 700 ~ or 7500C, sodium or potassium compounds were, respectively, the major constituents of the ash. At 8000C, sodium compounds were again a major constituent of the ash for the sodium carbonate catalyzed reaction, but potassium compounds were not detected in the ash when potassium carbonate was used at this temperature. The reaction mechanism for catalysts involves reduction of the carbonate to the alkali metal:

M2CO3 + 2C ---* 2M + 3CO

where M is an alkali metal [107]. Since sodium vaporizes at 883~ and potassium at 774~

the

absence of potassium compounds from the ash of lignite reacted at 800~ is consistent with this proposed reaction. Furthermore, potassium vapor has been observed in equilibrium with char [109] and potassium has been found condensed in the cooler portions of bench-scale reactors [110]. The reactivity parameter calculated at the 50% carbon conversion level could be used as a

169 rate constant in the Arrhenius equation, thus allowing calculation of activation energies and preexponential factors. Kinetic data for Indian Head, Velva and Martin Lake lignites are summarized in Table 4.6 [107]. TABLE 4.6 Kinetic parameters for reactions of lignites with steam [ 107].

As Rec'd. 10% K2CO3 10% NazCO3

Apparent Activation Energies, kcal/mol*_ IH V ML 28.8 29.8 25.8 18.4 11.5 19.4 15.9 11.9

F're-exponential Factors, h r - 1 IH V ML 1.78 x 106 4.11 x 106 5.11 x 105 4.26 x 104 1.85 x 103 6.0 x 104 1.31 x 104 2.25 x 103

*Per mole of fixed carbon. IH - Indian Head" V - Velva; ML - Martin Lake. The alkali carbonates can decrease apparent activation energies by up to 60% [ 107]. Since sodium and potassium carbonates provide similar reductions in apparent activation energies when used on an equal mass basis, potassium carbonate, by virtue of its higher molecular weight, is more effective on a molar basis. However, there is a cost advantage in favor of using the sodium carbonate if this work were to be scaled up. 4.3.3 Reactions with sulfur compounds Hydrogen sulfide promotes liquefaction of benzophenone, bibenzyl, and diphenyl sulfide more effectively than does hydrogen [22]. Hydrogen sulfide (or sulfur) promotes cleavage of the aliphatic C-N bond in N,N-dimethylaniline, whereas hydrogen cleaves the aromatic C-N bond to produce benzene and toluene. Hydrogen sulfide is superior to hydrogen for hydrocracking bibenzyl and diphenylmethane [ 111]. Hydrogen sulfide acts as a hydrogen donor in the bibenzyl reaction. Hydrogen sulfide also forms aromatic-sulfur bonds; for example, the reaction of hydrogen sulfide with diphenylmethane at 425~ forms equimolar amounts of thiophenol and toluene [ 111]. The role of hydrogen sulfide can be explained in terms of bond energies, particularly that the bond dissociation energy of HzS is about the same as that of a C-H bond while the bond dissociation energy of H2 is much larger [ 111 ]. This comparison indicates the need for a catalyst to help effect dissociation of hydrogen, unless temperatures high enough to dissociate the hydrogen molecule thermally are employed. Hydrogen sulfide serves as a hydrogen donor for methylene bond cleavages, as in diphenylmethane-like structures [112]. Hydrogen sulfide does not catalyze lignite hydrogenation, but rather enhances the reaction of lignite with carbon monoxide and water, possibly by catalyzing the water gas shift reaction. Increased carbon dioxide and hydrocarbon gas formation during liquefaction of Zap lignite may be a result of hydrogen sulfide catalysis of the water gas shift reaction, hence forming additional hydrogen which in turn is the species actually responsible for

170 enhancing hydrocarbon gas formation [113]. Sulfur facilitates conversion of diphenylmethane better than does hydrogen sulfide [ 111]. However, the presence of hydrogen sulfide with sulfur enhances formation of toluene and thiophenol at the expense of products of larger molecular size. Addition of hydrogen sulfide to a liquefaction reactor enables liquefaction to be carried out at temperatures below those needed for non-catalyzed reactions [112,114,115]. Furthermore, addition of hydrogen sulfide may reduce net hydrogen consumption, by reducing production of byproduct hydrocarbon gases, the formation of which would have used some of the available hydrogen. Hydrogen sulfide may improve hydrogen donation, hydrocracking, aromatic ring opening, as well as catalyze the water gas shift reaction [9]. The major reactions of hydrogen sulfide and sulfur in lignite liquefaction are hydrocracking (as, for example, the cleavage of bibenzyl), action as a hydrogen donor, attack on aromatic rings (as in the conversion of diphenylmethane to toluene and thiophenol), and catalysis of the water gas shift reaction via a carbonyl sulfide intermediate [22]. Addition of H2S during reactions of lignite with CO/H 2 mixtures increases both conversion and gas production as the proportion of CO in the gas mixture increases. Addition of H2S to syngas for liquefaction of Indian Head lignite increases conversion by about 10% at 300"C and by about 25% at 440~

[116]. Liquefaction of Big Brown lignite in anthracene oil showed increased

conversion in the presence of H2S at 420* C [117]. Small quantities of hydrogen sulfide enhance conversions of Indian Head, Beulah, and Big Brown lignites regardless of whether the solvent vehicle is water or anthracene oil [118]. Addition of hydrogen sulfide to hydrogen, or to mixtures of carbon monoxide and hydrogen results in improved lignite conversions in several solvent systems [119]. This has been verified for three lignites--Indian Head, Beulah and Big Brown--and three solvents--water, dihydrophenanthrene, and anthracene oil blended with middle distillate from the Solvent Refined Coal process. For example, with Indian Head lignite in water for 1 h at 4200C, conversion increases from 37.4% to 42.8% by changing the gas atmosphere from 1:1 hydrogen:carbon monoxide to a 2:2:1 H2:CO:H2S mixture. Similarly, for Big Brown lignite in dihydrophenanthrene, at otherwise the same conditions, conversion improved from 50.7% to 65.3%. Hydrogen sulfide is more beneficial in a hydrogen-carbon monoxide-steam atmosphere than in pure hydrogen [ 118]. Both increases [22,113] and decreases [117] in oil yield are claimed for addition of hydrogen sulfide. Addition of hydrogen sulfide (or sulfur) enhances the yield of oils and reduces gas consumption in liquefaction of Big Brown lignite [22]. The highest conversion of Beulah or Big Brown lignite to volatiles was achieved with H2S partial pressures in the range 344-687 kPa [ 120]. This partial pressure corresponds to about 4-10 % by weight of the daf lignite. Addition of hydrogen sulfide to synthesis gas results in higher total oil yield at 400~ than is obtained at 460~ in hydrogen [22]. A lower reductant consumption is achieved in the hydrogen sulfide-synthesis gas system, despite the higher oil yield, because the formation of unwanted light hydrocarbon gases is reduced. In addition, the reductant consumed with added H2S was only about a third the

171 amount used in the absence of HzS. Addition of HzS allows temperatures to be reduced by 20~ with no penalty in yields of distillate or solvent refined lignite [ 117]. The benefits of reduced temperature include reduction of hydrogen demand and decreased formation of by-product hydrocarbon gases. When North Dakota lignite is liquefied in bottoms-recycle operation, addition of HzS decreases the viscosity of the recycle slurry [ 117]. Addition of hydrogen sulfide in the co-processing of Estevan lignite in vacuum bottoms improved tetrahydrofuran solubles, pitch conversion, and distillate yield [121]. The gas atmosphere was 1:1 H2:CO. The improvement in product yields increased almost linearly with addition of hydrogen sulfide; the effect was attributed to the interaction of hydrogen sulfide with carbon monoxide to promote the water gas shift reaction. Evidence supporting this role of hydrogen sulfide was an increase in the yield of carbon dioxide with an increase in hydrogen sulfide addition. The C - - S bond is considerably weaker than C--C or C - - O bonds. Sulfur functional groups in the lignite thus become places where the macromolecular structure can undergo facile thermal decomposition, or decomposition facilitated by reaction with hydrogen [ 122]. Reaction of organic sulfur with hydrogen will produce hydrogen sulfide. Since hydrogen sulfide facilitates liquefaction reactions, a benefit is obtained from this newly formed hydrogen sulfide even in cases in which no deliberate addition of hydrogen sulfide was made to the gases charged to the reactor. The liquefaction of lignites of high organic sulfur content seems to be autocatalytic [82]. The effect is more pronounced for lignites having very high sulfur contents (such as Turkish or Spanish lignites, in which the organic sulfur can exceed 4%, and in extreme cases even exceed 10%), rather than for North American lignites. North Dakota lignite reacts readily with sulfur dioxide at 60(O650~

[ 123]. Conversion of

sulfur dioxide to sulfur is achieved to about 90% in a single stage reaction. The major gaseous products of this reaction are carbon dioxide, 34-41%; hydrogen sulfide, 2-5%; carbonyl sulfide, 1-2%; sulfur dioxide, 1-4%; carbon disulfide, 0.1-1%; and water, 16-17% [123]. The mechanism involves the reaction of volatile hydrocarbons from the lignite with sulfur dioxide inside the lignite pore structure, resulting in complete conversion of the hydrocarbons with no byproduct tar formation. (Some by-product tars are formed when the reaction is run with bituminous coal.) Transmission electron microscopy of partially reacted lignite shows a large increase in porosity in the 5-20 nm range [123]. Sulfur dioxide is initially chemisorbed on the char surface, with subsequent formation of carbon dioxide and surface carbon-sulfur species [124]. The C--S complex then reacts with sulfur dioxide. The reaction occurs in two stages. The first stage is rapid, and is controlled mainly by the devolatilization rate of the lignite [ 124]. The second stage, which actually limits the overall rate, is controlled by the surface properties of the char. In this stage the rate of SO2 reduction was higher for lignite than for any other rank of coal. The activation energy for reaction with lignite is about 134 k.J/mol, compared with 150 kJ/mol for hvB bituminous coal [124].

172 4.4 P Y R O L Y S I S

OF LIGNITES

4.4.1 Heats of reaction Taylerton (Saskatchewan) lignite displays two exothermic reactions when heated in nitrogen (heating rate 6*C/min) [125]. A broad peak (in differential thermal analysis) occurs at lower temperatures, with a maximum around 300~

A second, better defined peak, occurs with a

m a x i m u m at about 450 ~. The onset of active thermal decomposition is around 180". Indian Head lignite, studied using differential scanning calorimetry in argon, has exothermic heats of pyrolysis varying from 54 to 130 J/g over the range of conditions studied [14]. The effects of pressure, heating rate, and particle size are summarized in Table 4.7 [ 14].

T A B L E 4.7 Heats of pyrolysis and vaporization of Indian Head lignite determined by differential scanning calorimetry, as functions of particle size, heating rate, and atmosphere [14]. Adjusted heats are calculated by assuming zero heat flow at 300~ Exothermic temperature Heat of Mesh size Heating rate range TMHRE vaporization (rvler) (~ (~ (~ (J/z) Atmosphere 0.1MPaAr

-8 +14

20 50 -28 +48 20 50 - 100+200 20 50 2.72 MPa Ar -28 +48 20 50 2.72 MPa CO -28 +48 20 50 2.72 MPa H2 -28 +48 20 50

256-571 (a) 370-548 340-479 382-490 203-570(a) 368-518 360-601(a 374-570(a) 266-606(a) 270-604(a) 294-544(a) 344-556(a)

411 432 408 424 394 440 414 436 605 436 544 550

140 72.6 35.5 33.9 161 35.8 53.6 107 225 183 160 164

Heat of pyrolysis (J/u)

-105 -222 -98.6 -140 -103 -206 -90.8 -111 -68.5 -22.7 -18.4 -55.5

Adjusted Adjusted heat of heat of vaporization pyrolysis (J/u) (J/u) 95 130 60 64 (b) (b) 54 (b) 160 100 (b) (b)

130 160 61 86 (b) (b) 91 91 100 72 (b) (b)

a Continued exothermic behavior beyond highest measured temperature, b No adjustment made because of non-constant heat flow in the region of 300~

An exothermic reaction with a temperature of maximum heat release (TMRHE) in the approximate range of 4(D--440"C, as indicated in Table 4.7, is in agreement with earlier work [ 126]. Heating in 2.8 MPa argon (particle size 28x48 mesh, heating rate 20~

shifts the endothermic peaks to

higher temperatures compared with an otherwise identical experiment at ambient pressure [ 15]. The peak shift corresponds to an increased boiling point of water to 2290C at 2.8 MPa. However the T M R H E is independent of pressure, which suggests that the thermal decomposition reactions are zeroth or first order. Other work indicates the heat of pyrolysis of Indian Head lignite in argon to be 113-118

173 J/g [ 15,127]. When using such data for practical applications (e.g., feeding as-received lignite to a fixed-bed gasifier) it should be remembered that the heat requirements for the vaporization of the moisture are enormous compared to the heats of pyrolysis. At atmospheric pressure the heat of vaporization of pure water would be 2235 J/g, and values as high as 4400 J/g have been reported for the removal of moisture from lignite [ 120]. 4.4.2 Kinetics of pyrolysis (i) Reaction order. Kinetics of product evolution from lignite pyrolysis in a heated grid reactor were modelled by first order decomposition reactions [ 128]. The experimental conditions included heating rates of 270-10000~

peak temperatures up to 1100~

pressures of 0.01-100

kPa, particle sizes of 74--1000 ~t, and holding times of 0--30 s at peak temperature. Evolution of volatiles proceeds with release of products in overlapping but sequential intervals. Pyrolysis of San Miguel (Texas) lignite at 650-800~ and ambient pressure into char, tar, and gas can be described by three parallel first order reactions [ 129]. Weight loss data collected at different heating rates can be represented by the multiple parallel independent reaction model (MPIR). Thus, for Montana lignite, the maximum yield of volatiles, V*, is independent of heating rate [130]; the maximum volatiles yield being virtually constant at about 40% over a range of heating rates from 0.1 and 10,000~

The rate of volatiles

evolution is the sum of contributions from each of the individual first-order reactions proceeding in parallel. Thus

dV dt-Xkiexp

-Ei * (~-)(Vi-Vi)

where the subscript i denotes a single ("i-th") reaction [130]. All reactions are assumed to have the same pre-exponential factor. Activation energies are represented by a Gaussian distribution for which the mean is Eo and the standard deviation is o. Therefore

= f(E)

)0.5 [o(2~t

1[ ] exp

- (E-Eo)2 22

]

where f(E) is Vi*/V* and V* is the sum of Vi* for all i. Integration then gives

(V**V) = V

exp - k ~

exp

~

dt f(E) dE

[ 130]. For any single reaction (a parameter for a single reaction being denoted by the subscript s) it

174 is desirable to be able to relate both the activation energy Es and the rate constant kos to the heating rate. Representing the heating rate by m and using available data [ 131,132] provides, by empirical fitting, the equations [130]: log (kos) = -3.16514 + 0.941867 [3+ log (m)] Es = -5909.411 + 182.7911 [3 + log (m)] + 66.80278 [3 + log (m)]2

Reaction of a North Dakota lignite in hydrogen showed apparent reaction orders of 2.9 for thermal decomposition at 450~ and 4.7 at 575* [ 133]. (ii) Rate constants. Evolution of primary pyrolysis species has been modelled using a functional group method that is essentially rank independent [ 134]. The model is insensitive to the type of reactor, the pyrolysis temperature, and the heating rate. In particular, the model seems able to accommodate heating rates in the range 0.5--20000"C/s. The kinetic parameters for this model have been tabulated [ 134]. The total weight loss is modeled with a rate constant given by k=4.28 x 1014 exp (-54570/RT) sec-1

Pyrolysis of Texas lignite in the range 700-1000~

followed first-order kinetics with a rate

constant k = 600 exp(-5400/T) for the first 50% of the weight loss [135]. A second stage of pyrolysis, involving tar cracking and hydrogen release, was slower, with a rate constant given by k = 90 exp(-3850/T) [135]. For particles up to 200 ~m there seemed to be no effects of particle size, although earlier work on North Dakota and Montana lignites at 808~ showed an internal diffusion influence in the first stage for particles larger than 50 ~tm [136]. Pyrolysis of Darco lignite in an entrained flow reactor in the range 700-10000C proceeds in two stages: an initial, rapid devolatilization accounting for about 50% of the weight loss, followed by a second, slower devolatilization. The rate of devolatilization in the first stage can be described by a first order expression, the rate constant being given by 0.6x103 exp(-45kJ/mol RT) [135,137]. If the entire process of two stages is modeled by a single first order expression, the rate constant is given by 0.9x102 exp(-32kJ/mol RT) [135]. The apparent rate constant for volatiles production for an assumed first order pyrolysis of Indian Head lignite in a slagging, fixed-bed gasifier is 0.011 min-1 [138]. Kinetic data for

175 pyrolysis of 200x400 mesh Monticello (Texas) lignite in nitrogen are shown in Table 4.8 [ 139]. TABLE 4.8 Kinetic data for pyrolysis of Monticello lignite [ 139]. Average Temperature, K 1035 1112 1295 1585 1674

k, sec-l_ 1.101 1.365 2.207 3.017 6.385

Kinetic Parameters* ...... E, cal/mole __1_1~,sec-l_

7980

50.8

* k - rate constant" E - activation energy; ko- frequency factor; correlation coefficient of In k vs. 1/T - -0.95

(iii) Apparent activation energies. The apparent activation energy for pyrolysis of Monticello lignite in a drop tube furnace at 1730 K was 33.4 kJ/mol with a frequency factor of 50.8 s-1 [ 139,140]. Reaction of Darco lignite in an entrained flow isothermal reactor displayed an activation energy of 32 kJ/mol, and a frequency factor of 90 s-1 [135]. However, low apparent activation energies are not necessarily indicative of physical rate control [135]. Devolatilization of Carbon County (Utah) lignite in a fixed bed reactor in 0.1 MPa steamoxygen mixture at 350-550~

for particle sizes of 2xl, 3x2, and 4x3 mm, showed an apparent

activation energy for first-order devolatilization in the range 63-75 kJ/mol [ 141]. The activation energies were not sensitive to particle size for the range of sizes studied, but rates of devolatilization depended on particle size and temperature. Montana lignite pyrolyzed in an entrained flow reactor in the range 973-1173 K showed apparent first order kinetics for weight loss, with an activation energy of 58 kJ/mol and a preexponential of 8 x 103 s-1 [142]. Acid washing to remove the exchangeable cations results in an activation energy of 148 kJ/mol and preexponential of 2 x 108 s-l, whereas back-exchange of calcium cations onto the lignite gives values of, respectively, 99 kJ/mol and 5 x 105 s-1. These data represent more a relative ranking of the effects of the treatment of the lignite than indicative of the absolute values of the kinetic parameters [ 142]. Since metal cations can cause significant changes in amount and rate of volatile release (discussed later in this section), the mechanisms of volatile release may be affected by the cations. The activation energies for pyrolysis of Louisiana lignite are 211 kJ/mol for reaction in nitrogen and 224-230 kJ/mol for reaction in hydrogen [35]. These measurements were made in a thermogravimetric analyzer. Activation energies for reaction of a North Dakota lignite in hydrogen at 450* and 5750C were 122 and 295 kJ/mol [ 133]. Pyrolysis of Indian Head lignite in a thermogravimetric analyzer was described by the equation

176

- V = 1 ] o exp V* o(23t) 05

exp mE

2

~

-

dE

20

where the variables are defined as follows: V, volatiles content; V*, potential volatiles content; E, activation energy, cal/mol; Eo, mean activation energy, cal/mol" ko, pre-exponential factor, min-1; m, heating rate, ~

and o, the standard deviation of Eo. The integral was evaluated over the

limits 1000 to 99,000 cal/mol, which corresponds to Eo _+4o when Eo is 50,000 cal/mol (209 kJ/mol), thus including 99% of the possible values of E. Assuming a pre-exponential of 1015 [143], values of Eo ranged from 52,700 cal/mol to 56,900 cal/mol (221 to 238 kJ/mol) for various combinations of particle size, heating rate and argon flow rate [ 15]. Similar studies have reported values in the range 205 to 230 kJ/mol [35,131]. The apparent activation energy for the release of volatile matter by the thermal scission of oxygen functional groups is 230 kJ/mol [ 136]. (iv) Temperature ofmaxhnum rate of weight loss. The temperature of the maximum rate of weight loss (TMRWL) is a useful descriptor of pyrolysis kinetics [ 144]. For Indian Head lignite, heating rate had the greatest effect on TMRWL, since an increase in heating rate invariably increased TMRWL [ 15]. The TMRWL was 4300C for a heating rate of 5~ pressure, but an increase in heating rate to 100~

in argon at ambient

shifted the TMRWL to 4630C [ 15]. A second

North Dakota lignite (440 and 490~ at 40 and 160~

respectively) and a Louisiana lignite

(441 and 4520C at 11 and 250C/min, respectively) behave similarly [35]. The relationship between TMRWL and heating rate was TMRWL = 412.7 + 10.53 In (m) where m is the heating rate in ~

[15]. This empirical relationship is valid only for the

apparatus and experimental conditions (thermogravimetric analyzer with 30 mg sample of 28x48 (Tyler) mesh, 50 cm3/min argon flow) from which the data were derived; however, it suggests a logarithmic dependence on heating rate. The argon flow rate also affected TMRWL, with a large rate decreasing TMRWL. The effect of flow rate is a mass transfer effect, since the increase in volumetric flow rate also increases the linear velocity of the gas (e.g. in the apparatus used, an increase in flow rate from 20 to 200 cm3/min increased the linear velocity from 4.1 to 41 cm/min) [151. Two distinct temperature regions of pyrolysis behavior represent loss of water followed by primary devolatilization (presumed to form CO and CO2) [39]. A TMRWL can be assigned to each; values for four lignites are shown in Table 4.9, along with the mass loss observed at 500~ [39].

(v)Rate-limiting factors. The low value of the activation energy for pyrolysis of Wilcox (Texas) lignite in a drop tube furnace at 1730 K suggests a physical rather than a chemical control of the pyrolysis in this system [140]. Intraparticle mass transport is the rate-limiting factor in

177 TABLE 4.9 TMRWL for loss of water and primary devolatilization of four lignites [39].

Lignite Beulah Big Brown Indian Head Velva

TMRWL,~ Low temp. High temp. 118 405 -400 110 395 85 380

pyrolysis of San Miguel lignite (650-800~

Mass loss at 500~ maf basis 34.8% 34.4% 36.5% 38.9%

ambient pressure) [129].

Pyrolysis of Darco lignite in an entrained flow reactor showed that for residence times of up to 0.4 s weight loss was independent of the particle size (for a range of particle sizes of 41-201 ~tm) [135]. This indicates that internal heat and mass transfer control does not govern the pyrolysis. Mass transfer control in the micropores of the lignite can not be entirely ruled out, but the opening of cracks and fissures by the violent pyrolysis processes likely obviates significant control by micropores. For these conditions, pyrolysis occurs in the chemically controlled reaction regime. 4.4.3 Comparative effects of gaseous atmosphere (i) Nitrogen, wet nitrogen, or carbon dioxide. Behavior of a Montana lignite in an entrained flow reactor at 1173 K depended on whether the atmosphere was nitrogen, wet nitrogen, or carbon dioxide. The initial weight loss is greater in wet nitrogen (2.7% water by volume) than in dry nitrogen, presumably because of a slower heating rate of the lignite particles [97]. The maximum weight loss varied from 31% in dry nitrogen to 39% in carbon dioxide. The increased wight loss in carbon dioxide and wet nitrogen is a result of the reaction of volatile material from the lignite with the water or carbon dioxide present in the reactor preventing redeposition of primary volatile species. The initial weight loss of Richland County lignite in an entrained flow reactor is greater for pyrolysis in nitrogen relative to wet nitrogen because of the slower heating of particles in wet nitrogen [97]. There was little effect on final weight loss from Richland County lignite in an entrained flow reactor heating in dry nitrogen, wet nitrogen, or carbon dioxide, suggesting that no significant reactions occur between the water or carbon dioxide and the lignite undergoing pyrolysis [97]. Reactions of Hagel lignite in nitrogen, wet nitrogen, and carbon dioxide showed no significant effects of atmosphere on primary devolatilization atmospheric pressure [145]. The experimental conditions covered various size ranges (35x48 mesh to 80x100 mesh), temperatures of 800-1600~

and heating rates of 102-103 s-1. Increasing the pressure resulted in the smaller

sized particles showing a higher weight loss, the smaller size providing less diffusion resistance to the volatiles escaping from the particles, and less opportunity for volatiles to decompose (i.e.,

178 participate in secondary char-forming reactions) inside the particle. Higher carbon dioxide pressures appear to reduce the extent of pyrolysis. (ii) Nitrogen vs. hydrogen. The largest product group from pyrolysis of three Louisiana lignites in nitrogen at 500", 600", and 800"C was alkanes and alkenes, while phenols were second in importance and benzenes third most abundant [53]. The product composition showed little variation among the lignites and little variation as a function of temperature. When the pyrolysis was conducted in hydrogen, the order of abundance of the product groups was alkanes and alkenes > benzenes > phenols. Besides changing the relative order of the benzenes and phenols, nitrogen and sulfur compounds (e.g., pyridines, pyrroles, and thiophenes) were produced in greater quantities during hydropyrolysis, suggesting that the hydrogen acts to stabilize such compounds which would otherwise, in nitrogen, deposit as char. The effects of heating rate on volatile losses are essentially reversed for nitrogen and hydrogen atmospheres. Five Louisiana lignites heated in nitrogen at 11, 25, and 435~

in the

range 10(0-800~ showed that the volatile losses are greater the higher the heating rate [35]. The trend is exactly opposite in hydrogen. The total volatiles yield is always greater in hydrogen than in nitrogen, for comparable heating rates and reaction temperatures. There is little variation in activation energy among various combinations of heating rates and atmospheres; for example, in nitrogen the activation energy ranges from 198 kJ/mol at 435~

to 218 kJ/mol at 1 l~

while in hydrogen the comparable values are 221 and 209 kJ/mol, respectively. (iii) Nitrogen vs. air. Pyrolysis of Richland County lignite in an entrained flow reactor gave a 30% maximum weight loss in nitrogen [97]. In comparison, pyrolysis of either untreated or acid-washed samples in air, in otherwise similar reaction conditions, resulted in a 90% weight loss for both. (iv) Nitrogen vs. steam. Pyrolysis of Alcoa (Texas) lignite falling freely through a countercurrent stream of nitrogen or steam causes no diminution of the sulfur content (originally 1.09% in the untreated lignite) for pyrolysis in nitrogen, but a reduction in sulfur as function of pyrolysis temperature when a steam atmosphere is used [107]. In steam, no change in sulfur is noted for pyrolysis temperatures up to 565~ char drops, reaching 0.66% at 868~

but at higher temperatures the sulfur content of the

Production of gases is a linear function of temperature for

pyrolysis in nitrogen; in steam, a sharp increase in gas production as a function of temperature occurs above 700~ Vacuum pyrolysis of a North Dakota lignite which had been treated with steam at 320~ and 7.6 MPa for 15 minutes showed that fewer total volatiles were emitted on pyrolysis to 7400C than in the case of the untreated lignite [146]. However, the tar yield increased slightly, from 2.1 to 4.2%. (v) Hydrogen vs. synthesis gas. Reaction of a North Dakota lignite in a CO-H2 mixture showed higher reaction rates than in hydrogen showed apparent reaction orders of 2.9 for thermal decomposition at 450~ and 4.7 at 575 ~ [133]. Comparable experiments showed essentially no change for the reaction of a bituminous coal in the two atmospheres [133].

179 4.4.4 Effects of lignite moisture content on pyrolysis The necessary air/lignite ratio doubles for carbonizing high-moisture lignite, without an increase in the carbonization temperature, for entrained flow carbonization in an internally heated reactor [147]. For example, carbonization of Sandow (Texas) lignite dried to 2.9% moisture was effected at an air/lignite ratio of 2.35 x 10-4 m3/kg at 525~

for Glenharold (North Dakota) lignite

at 20.6% moisture the corresponding data are 4.33 x 10-4 m3/kg and 593~ [147]. The higher air rate is required by the additional thermal requirement associated with drying the moist lignite. The practical implication is that operating problems with carbonization of lignites at their full moisture content--specifically, an increase in gas volume and velocity due to the additional steam, and condensation with consequent bridging in storage and feed bins--show that all lignites should be partially dried before carbonizing [147]. Thermogravimetric analyses of Indian Head lignite in argon demonstrated effects of airdrying and sample aging [148]. Freshly air-dried lignite showed a variety of transitions in the first derivative TGA curve, with reactions occurring at 200, 400, 500, and 750~ occurring in the regions of 600-700 and 800-1000~

and smaller changes

After three to four days of the lignite's

standing in the laboratory, the first-derivative TGA indicated much simpler pyrolysis behavior, with major reactions only at 200 ~ and 450~

and small changes at 550 ~ and 850~

weeks, the only evident changes occurred at 200 ~ and 450~ the first derivative TGA plot up to 1100~

After two

with no other reactions apparent in

The reasons for this behavior were not elucidated, nor,

insofar as is known, was this work ever followed up. However, these observations show that drying and subsequent aging may significantly affect the pyrolysis behavior of lignite. Moisture can have a significant effect on the extent of pyrolysis. An increased weight loss observed for dried Hagel lignite, relative to the moist lignite, is due to faster heating of the dried particles and faster release of volatiles away from the particles [ 145]. As-received and vacuum-dried Gascoyne (North Dakota) lignite produce similar yields of water-soluble organics upon pyrolysis [ 149]. The conditions of the experiment were a heating rate of 450C/min to 850~

in a helium atmosphere. Although the yields of organic compounds were

similar, substantial differences in the rate of gas evolution were noticed over a wide temperature range. 4.4.5 Effects of cations on lignite pyrolysis An important feature distinguishing low-rank coals from higher rank coals is the presence, in low-rank coals, of abundant cations associated with the carboxyl groups. (The association of inorganic components with the carbonaceous portion of the coal is treated in Chapter 5.) The cations affect many aspects of lignite behavior. They can have chemical or physical roles in affecting pyrolysis of lignites [ 150]. Cations catalyze secondary reactions that increase the amount of char formed. The cations also catalyze condensation of tar species into higher molecular weight material (even if this increased molecular weight material is not char), reducing the volatility of these species. Either the cations themselves or their reaction products formed in early stages of

180 pyrolysis can block the apertures of pores, impeding escape of volatiles. (i) Effects of cations on weight loss. The effect of cations on entrained flow pyrolysis of Montana lignite, in helium and nitrogen atmospheres at 973 and 1173 K, was studied using raw lignite, lignite acid washed to remove exchangeable cations, and acid-washed lignites backexchanged with calcium, magnesium, or sodium [97,142,151]. All forms showed the same qualitative behavior, in that as a function of time a rapid initial weight loss is followed by a period in which there is essentially no further weight loss [142]. Further, the maximum weight loss increased with pyrolysis temperature. Despite these qualitative similarities, substantial quantitative differences were observed between the raw and acid-washed lignites. The former showed a weight loss of 30% (on a dry, inorganic-cation-free basis) in 0.15 s, while the acid-washed lignite showed a weight loss of 50% in 0.05 s at 1173 K [97,142]. The calcium-loaded and raw lignites behaved alike. The calcium-loaded lignite had a cation content of 3.1%; the raw lignite, 2.8%. In 0.15 s at 1173 K, the calcium-loaded sample lost 30% of its weight while the raw lignite lost 29% [142]. (The similarity of behavior indicates the reversibility of the ion exchange process; the acid washing and back-exchange of calcium induced no alterations that affected the pyrolysis behavior.) The maximum weight loss decreased as calcium content increased. Cation loading has no significant effect on ASTM volatile matter content. In comparison, at extended residence times in an entrained flow reactor, the presence of cations decreased weight loss for all temperatures, cations, and degrees of cation loading tested [ 142]. This difference in behavior derives from the degree of secondary char-forming reactions, which are favored in fixed beds, but have a much lower likelihood in entrained flow reactors. The cations, in entrained flow pyrolysis, promote formation of secondary pyrolysis products that deposit on the char, either by cracking of primary products to carbon-rich solids and light gases or by polymerizing tars to molecular sizes too large to remain volatile. Thus in entrained flow pyrolysis (1173 K, 0.03-0.3 s residence time) a lignite treated with hydrochloric acid to remove the exchangeable cations and put all of the carboxyl groups into the acid form experiences about a 50% weight loss. In comparison, acid-treated lignite back-exchanged with calcium or magnesium experiences only a 30% weight loss at similar experimental conditions [ 152]. Cations also reduce weight loss at residence times longer than obtained in entrained flow reactors [ 150]. (ii) Effects of cations on pyrolysis kinetics. The rate of volatile release, the apparent activation energy for volatile release, and the tar yield are reduced by the cations [150]. Cations also reduce the rate of decomposition of the carboxyl groups. For pyrolysis in air, the initial rate of weight loss is lower for lignite containing cations than for comparable acid-washed samples [ 150]. As examples of the effect of cations on pyrolysis rate, the activation energy and preexponential factors for entrained flow pyrolysis of Montana lignite were 58 kJ/mol and 8 x 103 s-l, respectively [142]. Acid washing increased these values to 148 kJ/mol and 2 x 108 s-l, while backexchange of calcium resulted in activation energy of 99 kJ/mol with preexponential of 5 x 105 s-1 [1421. The rate constants for decarboxylation of Montana lignite in entrained flow pyrolysis

181 increase from 0.91 s-1 at 973 K to 9.1 s-1 at 1173 K [151]. Acid washing increases these values by roughly a factor of four, to 3.9 s-1 at 973 K to 31.5 s-1 at 1173 K [151]. Decarboxylation of the raw lignite has an activation energy of 110 "lcJ/mol and pre-exponential factor of 7 x 105 s-l; the respective values for acid-washed lignite are 97 kJ/mol and 6 x 105 s-1 [151]. The temperature of maximum rate of hydrogen evolution is lowered in the presence of cations, whereas the maximum temperature of carbon monoxide release in increased during slow heating rate pyrolysis of Darco lignite in nitrogen [153]. Rates of gas evolution from raw and calcium-loaded samples of Darco lignite are similar [ 154]. The maximum rate of hydrogen evolution from Darco lignite shifts to lower temperatures for cation-loaded lignite [154]. (iii) Effects of cations on volatiles yields and quality. Pyrolysis of demineralized lignites produces a carbon dioxide yield equivalent to the concentration of carboxyl groups in the lignite. However, back-exchange of cations onto demineralized lignite enhances carbon dioxide evolution relative to that of demineralized lignite. The maximum increase in carbon dioxide production amounted to 67%, for lignite back-exchanged with potassium [153], due to catalysis of the water gas shift reaction. Volatilization of water and carbon dioxide decreases the average number of oxygens around a calcium ion, but bulk species containing calcium do not form under these conditions. In Falkirk (North Dakota) lignite loaded with calcium by ion exchange, the radial structure function (RSF) of the calcium, as observed by x-ray absorption fine structure, is typical of a non-crystalline and highly dispersed calcium [155]. The RSF does not change on rapid pyrolysis, indicating that the calcium is still in a highly dispersed, non-crystalline state [ 155,156]. The calcium remains bound to oxygen atoms from the original carboxyl groups. As the pyrolysis severity is increased, by increasing either the time, temperature, or both, long-range order begins to develop. The formation of long-range order presumes some mobility of the calcium atoms. The RSF begins to resemble that of calcium oxide, although some calcium remains in a highly dispersed state. Significant amounts of bulk calcium oxide form in slow heating pyrolysis. Mild pyrolysis of Richland County lignite shows no change in the immediate coordination sphere around the calcium [157]. The calcium-oxygen bond distance decreases by 2-3 % from 25 ~ to 420~ Hydrogen production is enhanced during slow heating pyrolysis of cation-loaded lignites. The maximum enhancement relative to demineralized lignite was obtained for sodium-loaded lignite, a 42% increase [ 153]. The increased hydrogen may result from the water gas shift reaction or catalysis of the steam-carbon reaction. A reduction in the water yield accompanies the increased production of hydrogen in the presence of cations. The reduced water yield is consistent with the occurrence of the water gas shift reaction, carbon-steam reaction, or both. A slight decrease in hydrogen yield accompanies acid washing of Estevan lignite [ 158]. Untreated, ion-exchanged, and demineralized Zap lignite all evolve carbon monoxide between 400 ~ and 800~

[ 159]. Evolution of carbon monoxide is lower below 750 ~ but is higher

above this temperature, for the untreated and ion-exchanged samples relative to the demineralized

182 lignite. The fraction of total carbon monoxide evolving below 750~ increases with increasing tar yield [ 159]. The increased CO evolution below 750~ from the demineralized lignite, and greater tar yield, indicate a greater proportion of oxygen functional groups decomposing to CO without attendant cross-linking. On the other hand, the reduced CO evolution below 750~ for the ionexchanged sample results from a retention of oxygen in cross-links. Acid-washing Velva lignite gave comparable char and tar yields for the acid-washed and untreated lignites, 64.6 vs 62.5% char yields (daf basis) and 3.48 vs 3.65% tar yields, respectively [ 160]. The gas composition from the acid-washed lignite showed increased carbon monoxide and ethane and decreased carbon dioxide and hydrogen compared to the gas from the untreated lignite. A higher yield of carbon monoxide from demineralized samples could relate to an increase in the number of oxygen functional groups able to evolve as CO without instead being incorporated into cross-links [159]. In comparison, more tar and less carbon monoxide and dioxide were evolved from demineralized Zap lignite than from untreated lignite [ 161]. A higher tar yield with accompanying lower gas yield (mostly resulting from reduced carbon dioxide yield) is also observed for acid-washed Estevan lignite, relative to the untreated material [158]. Higher tar yields, with lower carbon dioxide yields, similarly result from acid washing of Coronach (Saskatchewan) lignite [158]. Cation-loaded lignites generally seem to give greater amounts of carbon dioxide, and hydrogen, than acid-washed lignites [154]. Pyrolysis of Velva lignite with and without added potassium carbonate or trona showed significant differences in the production of catechol, methylcatechols, guaiacol, and methylguaiacols [155]. Samples treated with potassium carbonate or trona resulted in a >90% reduction of catechol and a >80% reduction in methylcatechols. Similarly, the amounts of catechol and methylcatechol increase in tars from acid-washed Coronach lignite, relative to the untreated lignite [ 158]. (Phenols are much less affected.) The removal of the cations somehow affects the reactivity of the structures that are the precursors to catechol and its derivatives. Demineralized lignite generally gives a higher tar yield [159,161]. Tar yield from Zap lignite increased with increasing extent of ion exchange removal of cations [ 159]. Metal cations reduced tar yield during entrained flow pyrolysis of Montana lignite by 70 to 94% [151]. With Estevan lignite, tar yield drops as severity of acid washing is increased [158]. Tars from untreated lignite are more aliphatic than those from comparable experiments using acid-washed lignite [ 150]. Slow heating rate pyrolysis of Darco lignite in a nitrogen atmosphere showed little effect, up to 1000~

of metal cations on weight of char produced [153]. The maximum char yield for

lignites back-exchanged with various cations was only 3.4% greater than the char obtained from demineralized lignite. However, the cations increased the total weight of gases formed, by up to 17% with calcium-loaded lignite [153]. Acid washing reduced char yields from Saskatchewan lignites [ 158]. The role of the cations in promotion of secondary char-forming reactions can be shown in part by the difference in physical properties and chemical constitution (as deduced from F r l R spectra) of tar from untreated and acid-washed Montana lignite [151]. The tar from the untreated

183 lignite was a black, gummy material with about three times as much aliphatic hydrogen as the brown, powdery tar from the acid-washed lignite. Removal of the cations from lignite significantly reduced the deposition of carbon in the lignite char from methane cracking reactions [ 162]. Carbon deposition reduces the surface area and open pore volumes, and therefore adversely affects the reactivity of the char. Cross-linking behavior of ion-exchanged Zap lignite resembles that of untreated lignite [161]. However, the cross-linking behavior of the demineralized lignite differs; specifically, shifting to higher temperatures for the same loss in solvent-swelling capability (which would presumably represent the same extent of cross-linking). (iv) Comparative effects of different cations. When compared on a similar molar basis, calcium and magnesium had nearly identical effects on the maximum weight loss in entrained flow pyrolysis of Montana lignite (helium and nitrogen atmospheres, 973 and 1173 K) [97,142,151]. Sodium-loaded lignite that contained about two-thirds of the amount of calcium or magnesium (in meq/g) showed the highest or second highest maximum weight losses of all the cation-form lignites. The important factor is the number of carboxyl groups involved in bonding to the cations (or, from the other perspective, the equivalents of cations present). The effect of cations on carbon monoxide production, relative to demineralized lignite, depends on the type of cation. Potassium- and sodium-loaded lignites form less carbon monoxide, and less methane, than lignites loaded with divalent cations [154]. Back-exchanging lignite with calcium or barium increases carbon monoxide formation, while sodium or potassium decrease it [153]. This effect may be a result of the relative activities of these cations to catalyze the water gas shift reaction vs. activities for the carbon-steam reaction. Divalent cations, Ca, Mg, and Ba, produced greater gas yields than the monovalent K or Na during slow heating rate pyrolysis of Darco lignite in nitrogen [153]. Methane formation is reduced when the lignite is loaded with sodium or potassium cations, but is enhanced if the lignite is back-exchanged with divalent cations [153,154]. This effect represents a balance between methane cracking and methane generation via carbon gasification or gas-phase methanation. Cation-loaded lignites produce more carbon dioxide and hydrogen than demineralized lignite [ 154]. The order of cations in enhancing carbon dioxide and hydrogen yields is K ~ Na > Ba > Ca > Mg > demineralized lignite [154]. A similar order exists for ranking the reduction in carbon monoxide yield. Release of water during pyrolysis is less from lignite loaded with divalent cations than for the demineralized lignite, or lignite loaded with monovalent cations [154]. Any cation-loaded lignite releases less total amount of water than the demineralized lignite. Treating Velva lignite with aqueous magnesium chloride decreased tar yield and increased char formation during carbonization [160]. Similar treatment with iron(III) chloride also decreased tar yield, which in this instance was accompanied by increases in both char and gas. With aluminum chloride, a reduction of tar yield again occurs, but the compensating increase in gas and char was mainly as increased gas yield. In comparison, calcium carbonate as an additive during

184 carbonization gave a very small decrease in tar yield (2.6% vs. 10.3% for aluminum chloride and 38.5% for iron(Ill) chloride) but a significant increase in gas yield (17.9%). The effects of added potassium carbonate or trona on production of catechols and guaiacols from Velva lignite could not be observed for treatment of the lignite with calcium carbonate [155]. Tar yield is higher from potassium-exchanged Zap lignite than from samples exchanged with calcium or barium [ 159] The divalent cations may "tighten" the lignite structure by enhancing the cross-linking of molecular fragments in the structure, increasing the difficulty of tar formation. Treatment of Montana lignite with aqueous solutions of sodium, calcium, iron, magnesium, and aluminum salts all increased char yield [ 160]. In these experiments the lignite-salt mixture was heated to 500~ in 15 minutes (i.e., a heating rate of 320/min) and held at 50(O510 ~ for 1.5 h. The largest increase in char yield was obtained when aluminum sulfate was used as the additive. 4.4.6 Effects of petrographic composition For coals of various ranks shows that the initial volatilization temperature, T i, and the average volatilization rate, kv, measured by thermogravimetry, correlates with vitrinite reflectance (as well as volatile matter) [137]. For lignite, Ti is lowest (about 250~

and kv is at a maximum

compared to other ranks of coal. Durain, fusain and vitrain lithotypes separated from Beulah lignite were pyrolyzed in helium at a rate of 450C/min to a final temperature of 850~

Gas chromatographic analysis of the

water-soluble organic products gave the results shown in Table 4.10 [163]: TABLE 4.10 Yields* of water-soluble organics from three Beulah lithotypes [ 163]. Compound Methanol Acetone Acetonitrile 2-Butanone Propionitrile Phenol o-Cresol p-Cresol m-Cresol Catechol

Vitrain 2680 1910 280 480 110 2680 590 720 680 6940

Attritus 580 1340 310 380 160 1690 340 460 420 990

Fusain 730 1390 270 380 370 3040 780 850 880 1010

*Yields are reported in micrograms per gram of lignite (mat" basis)

In addition to the variations in yields from the various lithotypes, most notable for methanol, acetone, phenol, the cresols, and catechol, the total yield of compounds from the vitrain was about 2.5 times that from durain (17,070

vs.

6,670 ~tg/g, respectively) and almost double that from

fusain (9700 l~g/g). The results show that the yields vary from one lithotype to another, and

185 suggest that a petrographic analysis of lignite might be useful in helping to predict pyrolysis yields. Since the proportions of the different lithotypes vary within a seam, the composition of pyrolysis products in a commercial operation (e.g., gasification) may vary depending on the place in the mine from which the specific batch of lignite had been extracted. Four distinct lithologic layers have been observed in the Freedom (North Dakota) mine [164]. Pyrolysis of samples of each of the four lithologic layers under the same conditions described above also results in significant differences in the water-soluble organics [ 165-167]. TABLE4.11 Yields* of water-soluble organics from lithologic layers, Freedom mine, North Dakota [ 165-167] Compound Methanol Acetone Acetonitrile 2-Butanone Propionitrile Phenol o-Cresol p-Cresol m-Cresol Catechol

Layer 1 990 1350 240 360 70 2110 610 680 710 990

Layer 2 1010 1320 250 340 130 1720 520 570 630 1010

Layer 3 940 1490 260 420 280 1800 580 600 720 1200

Layer 4 1590 1420 190 350 190 3820 980 1190 1420 31 50

*Yields are in micrograms per gram of lignite (maf basis).

In general, the yields from the top three layers (i.e., layers 1-3) are reasonably similar, while the fourth differs considerably, particularly in the yields of phenol, the cresols, and catechol. This distinction among layers is also borne out in the petrography, the first three layers being similar while the fourth shows a much larger amount of corpohuminite (the submaceral phlobaphenite), present as 6.4 volume percent in layer 4 but ranging from 1.0 to 2.6% in the other three layers [167,168]. Phlobaphenite derives from catechoi and tannins and may be responsible for the substantially higher yields of the phenolic compounds. Catechol yields correlate with the amount of corpohuminite in each layer, with a linear least squares correlation coefficient of 0.92 [167]. The total yields of water-soluble organics and catechol correlate directly with the volatile matter (maf basis) of the four samples. The four layers have also been examined by Fischer assay. The data are shown in Table 4.12 [167,169]. Pyrolysis of Zap and Darco lignites in a steam-nitrogen mixture to temperatures up to 2400 K and pressures to 1.3 MPa showed only a weak correlation between petrographic composition and volatiles yield [ 170].

186 TABLE4.12 Fischer assay yields (as-received basis) for lithologic layers in Freedom lignite [167,169].

Tar

(kg/t) (l_Jt) API gravity Gas (kg/t) (m3/t) Water (kg/t) (L/t) Char (kg/t)

Layer 1 49.4 50.5 12.9 142.7 85.6 225.3 225.6 662.9

Layer 2 28.2 28.8 12.9 139.9 94.2 241.6 241.9 590.3

Layer 3 40.1 40.9 12.9 127.2 87.5 288.2 288.2 544.6

Layer 4 32.8 33.4 12.9 113.0 78.3 333.0 333.2 521.0

4.4.7 Changes in physical structure accompanying pyrolysis Pyrolysis of Monticello lignite in a drop tube reactor, in nitrogen, increased the open pore volume by about an order of magnitude [ 139]. The unreacted sample had a total open pore volume of 0.078 cm3/g; at 53% pyrolysis weight loss in a 1450"C atmosphere, the total open pore volume increased to 0.980 cm3/g [ 139]. The BET surface area increased from 3 to 117 m2/g. As temperature increases, the loss of volatile constituents increases the microporosity, making the resulting char readily accessible to reactants. While the volatiles are being driven off, realignment of aromatic regions occurs concomitantly. The rearrangement of aromatic structures decreases pore volume. Up to about 900"C volatile loss predominates relative to pore volume loss, so that the net effect is an "opening up" of the char structure [171]. At higher temperatures, the pore volume loss accompanying structural realignment becomes predominant. Formation of carbon in the pores by thermal decomposition of gaseous pyrolysis products on carbonaceous or mineral matter surfaces may also result in loss of pore volume. Lignite chars are aperture-cavity materials [172]. The pores have a narrow opening leading to a cavity of larger volume inside the material. Deposition of 2.6% (by weight) of carbon via chemical vapor decomposition of methane (as would be representative of cracking of hydrocarbon volatiles on the carbon surface) decreased the open porosity in the char from 35.6 to 25.3% [171]. Thus some of the apertures become so reduced in size that helium can not penetrate at room temperature. The amount of carbon deposited by cracking methane is much less than anticipated from the open pore volume of the char; this is consistent with the existence of an aperture-cavity pore structure [171]. Deposition of carbon can reduce subsequent reactivity both by decreasing the accessible active surface area and by coating the surfaces of potentially catalytically active inorganic species. Specific surface areas measured by nitrogen and carbon dioxide adsorption both increase as a function of isothermal pyrolysis time. The largest changes occur after about 200 ms [136]. Using Savage (Montana) lignite with an isothermal pyrolysis time of 0.16 s as an example, the measured surface areas (daf basis) were 9.5 m2/g in nitrogen and 271 m2/g in carbon dioxide. This

187 difference is attributed to a molecular sieve effect, suggesting the presence of large amounts of pores with apertures in the range 0.49--0.52 nm [136]. Specific

surface

areas

of

chars

of

three

lignites--Darco,

Savage,

and

Glenharold--pyrolyzed in ambient pressure nitrogen in an entrained flow reactor at heating rate 8000~

temperature of 808~

and isothermal pyrolysis times between 18 and 1025 ms

increased with increasing pyrolysis time, and the increase in surface area became more appreciable with times greater than 200 ms [173]. Helium density of the chars gradually increases with pyrolysis time, but the mercury density decreases. The decrease in mercury density is initially rapid, but becomes slower at longer times. The changes in both densities become nil with pyrolysis times greater than 600 ms. The total open pore volume and the porosity both increase rapidly with time, but the change is minimal above 600 ms. Release of volatile matter develops of internal porosity as a result of the opening of previously closed pores, forming new pores, or enlarging either existing or new pores, or through some combination of these factors. Helium and mercury densities become constant after about 600 ms of isothermal pyrolysis; however, at short times the helium density increases and the mercury density decreases. These changes are consistent with an increase in porosity and total pore volume [ 136]. This behavior contrasts with that of chars produced in a relatively slow heating fluidized bed, where there is negligible difference between the mercury densities of the lignite and char, but the helium density of the char is much higher than that of the lignite. The pore volumes of chars from the slow heating are less than half those of chars from rapid heating [ 136]. Whether a more open or less open pore structure than that of the original lignite is produced in pyrolysis depends on the balance of two processes: creation of additional pore volume by release of volatiles, and removal of pore volume by the alignment and growth of aromatic and hydroaromatic structural units. Rank, heating rate, maximum heat treatment temperature, and time held at the maximum temperature affect this balance. With lignites, volatiles release dominates, a situation typical of low-temperature pyrolysis [136]. Formation of high-temperature (12000C) semicoke from Beulah-Zap lignite, as well as four coals of higher rank, in a flat-flame burner shows the effects of pyrolysis on a variety of the char properties [ 174]. Micropores of <1.5 nm radius increased two- to three-fold, whereas mesopores in the 1.5--20 nm range increased by factors of 20-200. The densities of the carbonaceous material in the semicoke increased by about 25% as a result of improved alignment of aromatic layers; however, the densities of the particles decreased by about 50% because of the increased porosity, which increased three- to four-fold, mostly in the macropore (>20 nm) range. Pore volumes increased by factors of five to ten. Rosin-Rammler parameters [ 175] for lignite chars show that pyrolysis produces a broader distribution of the particle size than existed in the lignite before pyrolysis [136]. In addition, pyrolysis decreases the weight-mean particle size. Particle shrinkage and density changes occur simultaneously during entrained flow pyrolysis [ 176]. Reduction in particle size is independent of temperature, but significant particle decrepitation does not occur. Wilcox lignite particles of 50x60

188 mesh pyrolyzed at heating rates of 0.1, 10, and 1000~

showed no visible change in the char

particles as heating rate was increased, other than formation of a few relatively large cracks or fractures [177,178]. Similarly, heating rate had little effect on porosity and macropore surface area. A decrease in average particle size radius accompanies increasing heating rate, a result of increased fragmentation of the particles as they undergo devolatilization [178] 4.4.8 Changes in functional groups during pyrolysis (i) Carboxyl groups. Carbon dioxide arises mainly from thermal decarboxylation; formation of some carbon dioxide from carbonyl groups has been suggested [ 179]. The carboxyl content of char from Beulah lignite at 225~ is comparable to that of unheated lignite; for char produced at 500~

the carboxyl content is equivalent to loss of about 55--60% of the original

carboxyl groups [180]. Decarboxylation of Russian brown coals begins at 3500C, along with the loss of some phenolic --OH [181]. In the presence of traces of oxygen, these reactions start about 100~ lower. Decarboxylation and loss of phenolic --OH are complete by about 5000C. Pyrolysis of 17 coals of various ranks, including Velva lignite, by Curie point pyrolysis at 3000*C/s shows a good linear relationship (correlation coefficient 0.93) between the carbon dioxide yield and the stretching vibration of the carbonyl band in the carboxyl group at 1760 cm-1 [ 182]. Observations of the yield of carbon dioxide as a function of time for pyrolysis of a North Dakota and an Montana lignite, and of the rate of carbon dioxide formation as a function of temperature for a Texas lignite, suggest that carbon dioxide formed during lignite pyrolysis has two sources, one weakly bound to the structure and the other tightly bound [183]. In an entrained flow reactor 35% of the total weight loss from Montana lignite could be attributed to decarboxylation [151]. Loss of carboxyl occurs both by direct thermal scission of carboxyl groups and by loss of tar molecules which themselves may contain some carboxyls. Decarboxylation kinetics are first order. The rate constants ranged from 0.91 s-1 at 973 K to 9.1 s-1 at 1173 K [151]. The activation energy was 110 k.l/mole with a pre-exponential of 7 x 10-5. The activation energy is lower than might be expected for chemical bond breaking, and reflects the fact that a significant portion of the carboxyls are actually released with the tar, the tar in turn being held in the lignite by bonds of low strength with low activation energies for cleavage. The loss of carboxyl groups on mild pyrolysis is catalyzed by copper [ 184]. For example, loading 1% copper onto Megalopolis (Greek) lignite by treatment with aqueous copper sulfate results in a 40% increase in calorific value, from 24.25 to 34.07 MJ/kg, after mild pyrolysis to 300~

So far as is known, this work has not been extended to North American lignites. Entrained flow pyrolysis of Savage lignite at 800~

showed rapid reduction of carboxyl,

along with hydroxyl and aliphatic hydrogen, by FTIR analysis of the char [176]. C - - O bonds are retained and aromatic hydrogen increases. Water and carbon dioxide are the primary products below 400~ [179]. This is in contrast to reactions of bituminous coal under the same conditions, in which case hydrocarbons are the dominant product. The yields from lignite are not affected greatly by pressure or particle size.

189 The first evidence of tar evolution during pyrolysis of a North Dakota lignite on a wire grid heater coincides with the first evidence of weight loss and the first evidence of carbon dioxide release [ 185]. This reaction system used 5 lxm particles heated at 950-1300~

in 172 kPa helium.

The decomposition of functional groups leading to the formation of carbon dioxide occurs essentially simultaneously with the cleavage of bonds leading to tar formation under these conditions. (ii) Hydroxyl groups. Some evolved water comes from the moisture in the lignite; however, additional water can be liberated by thermal dehydration of hydroxyl groups [176,186]. Pyrolysis of Beulah lignite in a heated grid cell at various temperature increments from 350 ~ to 9000C showed a decrease in --OH, as indicated by decreases in the broad FFIR peak in the region 3600-2200 cm-1 and sharp peak at 1600 cm-] [187]. Formation of water on pyrolysis of hydroxyl groups occurs via the reactions R-OH + R'-OH ---, R-O-R' + H20 R-OH + R"-H ~ R-R" + H20

where R, R', and R" are various organic structural fragments of the lignite [ 154]. In the presence of water, dehydration and decarboxylation of German brown coal above 300~ are suppressed in favor of release of humic acids [ 188]. However, other work has indicated that the changes in functional group composition were independent of whether the lignite was pyrolyzed in a wet or dry condition [ 189]. Beulah lignite exposed to D20 vapor for about 15 minutes readily undergoes deuterium exchange [ 183]. Pyrolysis of the deuterated lignite to 5000C produced deuterated gas species D20 and DHO. D - - O bonds were also present in the tar. These results suggest that the water produced during pyrolysis and the hydroxy groups found in the tar derive from hydroxy groups in the lignite. If the hydroxy groups were abstracting hydrogen and leaving as water, with no other structural changes, a comparable amount of D - - O and H - - O bonds would be expected in the pyrolysis products of the deuterated lignite. In fact, the amount of D - - O bonds was greater than H - - O bonds, suggesting that some ether formation takes place between two --OD functional groups with the liberation of D20 and retention of an ether group in the char. Water liberated on pyrolysis of Spanish lignite correlates with the amount of evolved carbon dioxide [ 190]. Several water molecules form per CO2 molecule, showing that the hydroxyl groups other than those that are part of the carboxyl functional group contribute significantly to water production. The correlation between water and carbon dioxide fails for pyrolysis temperatures above 5000C; some of the CO2 produced above 5000C comes from decomposition of other functional groups without concomitant water formation. Polyhydroxy aromatic compounds readily undergo self-condensation reactions; examples are 1,3-dihydroxynaphthalene and resorcinol. The existence of polyhydroxy aromatic structures in

190

lignite may also contribute, along with the thermal decarboxylation, to the facile crosslinking of lignites. (iii) Other oxygen functional groups. The quinone concentration in Russian brown coals decreases steadily over the temperature range 350--500"C [181]. Pyrolysis of guaiacol yields methane, carbon monoxide, catechol, and phenol as the only products at low conversions [191]. At higher conversions a solid char is formed, accompanied by reduced yields of catechol. Guaiacol decomposition proceeds via two parallel reaction pathways:

q- CH 4 OH

OH

+ OH

CO

OH

The major products of pyrolysis of benzaldehyde are carbon monoxide and benzene.

+

CO

2,6-Dimethoxyphenol and isoeugenol decompose analogously to produce methane, carbon monoxide, and liquid products. Vanillin produces carbon monoxide and methane. At low conversions of vanillin, the major products are guaiacol and dihydroxybenzaldehyde. At higher conversions the products include catechol and phenol, and, with extensive conversion, a carbonaceous char is produced. The decomposition of vanillin is illustrated by HC d'

O~

"-

+ OH

OH

HC ~0~

o"

OH

co

HC ~OH

o*

+

CH 4

OH

Guaiacyl substituents enhance decarbonylation of aryl aldehydes, but demethanation of guaiacyl structures seems unaffected by the presence of the carbonyl group.

191 A summary of the functional group composition of Beulah lignite and the rate expressions for the decomposition of the functional groups or formation of pyrolysis products using a heated grid cell in a Fourier transform infrared spectrometer is given in Table 4.13 [187]. TABLE 4.13 Kinetic rates and functional group composition for Beulah lignite [187]. Composition parameter (dmmf) Carbon Hydrogen Nitrogen Sulfur (organic) Oxygen Sulfur (mineral) Carboxyl Hydroxyl Ether loose Ether tight Nitrogen loose Nitrogen tight Aliphatic hydrogen Aromatic hydrogen Nonvolatile carbon Organic sulfur Total Tar Olefins Acetylene Soot

Composition 0.726 0.044 0.010 0.001 0.219 0.028* 0.050 0.045 0.02 0.23 0.011 0.009 0.129 0.015 0.49 0.001 1.000 0.07

Kinetic expressions

kl = 5400 exp (-8850 / T) k2 = 5400 exp (-8850 / T) k3 = 5400 exp (-8850 / T) k4 = 2.15 ~ 1016 exp (-57000 / T) k5 = 5400 exp (-8850 / T) k6 = 290 exp (-13000 / T) k7 19000 e x p (- 11000 / T) k8 = 40644 exp (- 14085 / T) k9=0 "

-

kt = 5400 exp (-8850 / T) ko = 2 9 109 exp (-24000 / T) ka = 1~ 1016 exp (-50000 / T) ks = 4 ~ 1019 exp (-60(900 / T)

*Dry basis A large concentration of ether groups results in the large temperature dependence of volatiles at high temperatures. This effect is not observed in pyrolysis of bituminous coals under comparable conditions. Pyrolysis of Center (North Dakota) lignite (helium atmosphere, 45~ 850~

heating rate,

maximum temperature) was compared with that of a second sample of the same lignite

treated with potassium hydroxide to form non-volatile potassium phenoxides, to discriminate between phenols arising from cleavage of alkylaryl ethers and those liberated from pre-existing phenolic structures. The yields of the three cresols were essentially the same from the two samples, while the phenol yield was reduced from 1310 to 630 lag/g (maf basis) [165,192]. About half the phenol yield from untreated lignite is due to cleavage of alkylaryl ethers and the remainder arises from phenolic structures already in the lignite. Dialkyl ethers form hydrocarbons and carbon monoxide on pyrolysis, and diaryl ethers are stable at high temperatures [193]. Thus the only type

192 of ether groups likely to contribute to phenol formation during pyrolysis under these conditions would be the alkylaryl ethers. This experiment resulted in about a 50% increase in the yield of acetone (from 850 to 1310 rtg/g) from the KOH-treated sample compared to the untreated sample [ 165]. No mechanism was offered for this change. Anisole pyrolyzes to form methane, carbon monoxide, and hydrogen [191]. The major liquid products include o-cresol, phenol, and benzene, with some toluene, xylenes, and xylenols formed as well. Anisole undergoes rearrangement to o-cresol, to form methane plus phenol, and to form benzene plus carbon monoxide. Methoxyl groups may be a source of some of the methane produced during pyrolysis [154]. The formation of water from hydroxyl groups, shown by the reactions discussed above, does not account for all of the water evolved during lignite pyrolysis. Water not accounted for by hydroxyl group reactions may arise from cleavage of ethers, postulated to produce aldehydes as intermediates [ 154]. In a subsequent step, the aldehydes could react with mobile hydrogens in the lignite to form water. Analogously, the mobile hydrogens could also react with carbonyl groups in the lignite structure to produce water. The yield of methanol from eleven samples of coals of ranks from lignite through high volatile C bituminous correlates with the methoxyl content of these coals. The correlation coefficient is 0.95 [163]. The yields of both 2-butanone and acetone for six coals, lignite through hvC bituminous, correlated inversely with the carbon/oxygen ratios. In both cases the correlation coefficient was 0.99 [163]. Methanol yields from the separated lithotypes of Beulah lignite, pyrolyzed at 850"C (helium atmosphere, 45~

heating rate) show a good correlation with the methoxyl contents of the

lithotypes, a linear least squares fit having a correlation coefficient of 0.95 [ 163]. Both acetone and 2-butanone, correlated with the C/O ratios of the lithotypes, with least squares correlation coefficients of 0.99 [163]. Carbon monoxide can be formed in a cheletropic extrusion of a carbonyl unit from such structures as coniferaldehyde [191 ]. Carbon dioxide can be formed from cycloreversion of lactones and aryl carboxylic acids. Elimination of water can result from the retro-ene reactions of guaiacylglycerol units in residual lignin structures. Pyrolytic decomposition of trans-cinnamaldehyde produces carbon monoxide and smaller amounts of hydrogen, methane, and acetylene [191]. A wide variety of liquid products form, including phenols, cresols, styrene, toluene, benzene, biphenyl, alkylated benzenes, and dimers produced in condensation reactions. The reaction proceeds via CH :CH

0

+

CO

The major gaseous products of acetophenone pyrolysis are carbon monoxide and methane, in

1 93 approximately 2:1 ratio, along with some hydrogen [191]. A very diverse mixture of liquid products was formed, including benzene, toluene, xylenes, styrene, cresols, benzaldehyde, biphenyl, as well as an assortment of aromatic ethers and dimers. Carbon monoxide is formed in benzaldehyde pyrolysis via cheletropic extrusion reactions [191]. Elimination of methane from guaiacol during pyrolysis occurs via a pericyclic group transfer elimination reaction. (iv) Aliphatic and aromatic hydrogen. Pyrolysis of Beulah lignite in a heated grid cell at various temperature increments from 350 ~ to 900~

showed (by FFIR analysis of the chars)

decreasing concentration of aliphatic C-H bonds, as noted by a decrease in the aliphatic stretch at 2900 cm-1 [187]. Loss of aromatic C-H begins only above 500~ Entrained flow pyrolysis of Beulah lignite in 0.1 MPa N2 showed that aliphatic species (and hydroxyl groups) were removed during primary pyrolysis, followed by char condensation with loss of aromatic hydrogen during secondary pyrolysis [ 189]. Aliphatic hydrogen occurs in labile methylene or polymethylene bridges between aromatic units in the structure. The labile bridges are key reaction centers during pyrolysis [194]. Their abundant occurrence in lignites facilitates conversion of lignites to high yields of non-condensible products [ 194]. (v) Sulfur and nitrogen. The distribution of sulfur in the flash pyrolysis products of Alcoa lignite is summarized in Table 4.14 [195]. The pyrolysis experiments were conducted at 850~ TABLE 4.14 Distribution of sulfur in flash pyrolysis products of Alcoa lignite [195]. Fraction Gas Tar Char Trapped on sand and reactor

Yield, wt. % 36.3 5.5 31.1 29.1

% of total sulfur 48.4 8.8 31.4 1.3

A correlation between desulfurization occurring during pyrolysis and the relative amounts of the different sulfur forms in the lignites could not be developed for Turkish lignites containing high concentrations of organic sulfur [ 196]. The first trace of hydrogen sulfide is observed at 200~

during low-temperature

carbonization of Sandow lignite, but formation of this gas does not become significant until temperatures of 300 ~ are reached [ 197]. The temperature dependence of the amount of sulfur liberated as hydrogen sulfide as a percentage of the total sulfur in the lignite is shown in Table 4.15 [197]. Isothermal pyrolysis of lignite over 300--1000~ and residence times of 5--80 s showed that the nitrogen was initially released almost entirely into the tar [198]. The composition and 13C NMR and FFIR spectra of the tar closely resemble those of the parent lignite. The nitrogen structures in the tar are very similar to those of the lignite. The initial release of nitrogen is described by the tar

194 TABLE 4.15 Sulfur liberation (sulfur in H2S as percentage of total sulfur) from Sandow lignite as a function of temperature [ 197]. Temp., ~ 150 200 250 300 400 500

% of Total Sulfur as S in H2~_S 0.00 0.31 0.47 2.51 20.1 37.4

release rate constant, given by [ 198] k - 81 exp (-5800/T) s-k The observed rate is smaller than those measured for nitrogen-containing heterocycles such as pyridine and pyrrole derivatives. Thus the nitrogen is likely contained in thermally stable heterocyclic ring systems that resist cleavage during pyrolysis. At higher temperatures a secondary release of nitrogen into the gases occurs, presumably the result of ring-opening reactions at high temperature. Pyrolysis of Montana lignite in an electrically heated grid reactor also shows that the nitrogen exists in strongly bonded compounds that are among the most thermally stable structures in the lignite [198]. Nitrogen compounds are similarly released without cleavage of the C - - N bonds into the tar during initial devolatilization, the amount of nitrogen released from the lignite being proportional to the amount of tar released. At high temperatures, decomposition of nitrogencontaining structures releases nitrogen from the char into the gas. The rate for this decomposition of nitrogen compounds is about two orders of magnitude lower than the rate of release of nitrogen compounds into the tar. 4.4.9 Evolution of products during pyrolysis (i)

Effects of pyrolysis temperature. Devolatilization of Montana lignite occurs in an

electrically heated grid apparatus in five main phases: evolution of moisture at 100~ carbon dioxide and a small amount of tar around 450~

evolution of water (from thermal

dehydration reactions) and carbon dioxide in the range 500-700~ hydrogen, hydrocarbon gases and tar at 700-900~

evolution of

evolution of the carbon oxides,

and finally formation of additional amounts of

the carbon oxides at very high temperatures [199]. In helium at rates of 1000~

the tar yield

increased with temperature to an asymptotic maximum at 5.4% above 8000C. The maximum yield of hydrogen and light hydrocarbon gases was 3.3%. The principal components of this product fraction are methane (1.3% yield), ethylene (0.6%), and hydrogen (0.5%), along with smaller amounts of ethane, propane, propylene, and benzene. Methane and ethylene yields reach an

195 asymptotic value in the range 600-7000C, but upon further heating the yields of these gases again increase, with a second asymptote around 900~

In contrast, the hydrogen yield shows only a

"one-step" behavior with the major evolution occurring at high temperatures. The yields of water, carbon monoxide, and carbon dioxide reach asymptotic values of 16.5%, 7.1%, and 8.4%, respectively. By holding at 1000~ for 5-10 s, instead of immediately cooling the sample when the desired pyrolysis temperature had been reached, the yields of the carbon oxides were increased to 9.4% for the monoxide and 9.5% for the dioxide. The occurrence of pyrolysis reactions at high temperatures independently of those taking place at lower temperatures indicates the existence of multiple parallel independent reactions. Assay yields and gas compositions for low-temperature carbonization of Sandow lignite are shown in Table 4.16 [197]. TABLE 4.16 Low temperature carbonization yields and gas compositions for Sandow lignite [197].

Yields, % maf Char Water Tar Light oil Gas Hydrogen Sulfide Gas Composition, % Carbon Dioxide Illuminants Carbon Monoxide Hydrogen Methane Ethane

Temp. ~ 250 300

150

200

400

99.5 0 0 0 0.5 0

99.2 0 0 0.2 0.7 0

98.0 0 0 0.7 1.4 0

93.8 1.8 0.2 1.4 3.0 0.1

74.6 6.8 7.3 1.6 9.4 0.6

95.9 0 0 0 4.1 0

90.3 0.3 6.5 0 2.9 0

88.7 0.4 8.8 0 2.1 0

78.2 0.8 12.7 0.8 7.3 0.2

67.6 1.0 13.3 0.8 16.9 0.4

Elbistan (Turkish) lignite pyrolyzed in a Fisher retort (220-600~ Abundant gas formation starts above 300~

N2) behaves similarly.

increasing temperature enhances gas formation [200].

North Dakota lignite pyrolyzed at 450 ~ and 600~

in a Parr reactor shows only minor

change in the distribution of n-alkanes [201]. This suggests that there is little additional cracking of the alkanes at 6000C relative to 450 ~ and that these alkanes are products of volatilization (or "thermal extraction") rather than cracking. The major n-alkanes were between C14 and C21 [201]. The effect of carbonization temperature on gas composition is shown in Table 4.17, for Sandow lignite [197]. In low-temperature reactions of Sandow lignite, the first indication of disruption of the coal structure occurred at 200~

a yield of 0.2% of light oils, along with small

amounts of carbon monoxide and illuminants in the gas [197]. At this temperature, 3.1% of the original amount of moisture in the lignite was still present, determined by xylene distillation (the

196 TABLE 4.17 Effect of carbonization temperature on gas composition from Sandow lignite [ 197].

Gas yield, % maf Gas composition, % Carbon dioxide Illuminants Carbon monoxide Hydrogen Methane Ethane

5OO 12.6

Temp., ~ 600 900 18.1 25.3

44.5 1.5 10.7 14.7 27.0 1.6

33.3 1.2 12.1 23.2 28.3 1.9

20.2 0.4 16.5 42.2 19.9 0.8

untreated lignite contained 32.1% moisture). At 250"C the first trace of tar was observed. A small amount of moisture, 1.2% of the original amount, still remained. At 3000C the amounts of gas and light oil were doubled compared to yields at 250 ~, and the first significant amounts of tar were noted. The maximum increase in yields occurs between 300 and 4000C. Tar yield is maximized at 500~

above that temperature the main carbonization products are gases. Heavy hydrocarbons

volatilized from a North Dakota lignite, treated by pyrolysis-GC, between 500 ~ and 7000C [201]. Maximum yield of volatiles was below 600 ~ [201]. Weight loss during pyrolysis of Darco lignite in an entrained flow reactor increased with temperature in the range 700-10000C for a given residence time [135]. The maximum yield of volatiles was 66.7% on a dry, ash-free basis, a factor of 1.3 greater than the ASTM proximate analysis volatile matter content [ 135]. A similar factor of 1.3 has been observed for RWE-Ki31n (German) lignite pyrolyzed at 80(01300 K [202]. The increased volatiles yield is a consequence of the reduction of secondary char-forming reactions and not a result of the substantially greater heating rate in the entrained flow reactor (over 104 *C/s) relative to the heating rate of the ASTM volatile matter test (about 20~ The effect of maximum pyrolysis temperature was determined by pyrolysis of -60 mesh Indian Head lignite at heating rates of 45~

to final temperatures of 850* and l l00~

in

helium. The yields of the smaller aliphatic compounds--methanol, acetone, acetonitrile, 2butanone, and propionitrile--were similar in the two experiments methanol at 850 ~ and 1100~

(e.g.,

1490 vs. 1460 tag/g for

respectively) [195,203]. However, the yields of phenol and o- and p-

cresols were higher, by 40--47%, at 1100~ (e.g., 2300 vs 3220 ~tg/g phenol) [ 195,203]. The mcresol yield at 1100~

was 79% greater than the yield at 850~

1270 vs. 710 ~tg/g respectively

[ 195]. This suggests that pyrolytic evolution of phenol and the cresols is probably not complete at the lower temperature. The phenol and cresols evolved at the lower temperatures may originate from cleavage of alkylaryl ethers [203]. The additional phenol and cresols formed at 1100~ may originate from cleavage of diaryl ethers, which have higher C - - O bond energies than the alkylaryl ethers [ 163].

197 As the pyrolysis temperature of Alcoa lignite is increased, in flash pyrolysis experiments, the polymethylene content in the tar decreases [204]. Hydroaromatic and benzylic species behave similarly. The aromatic species in the tar increase rapidly as a function of temperature, becoming the dominant components at high temperatures. At 2127 ~

rapid pyrolysis in nitrogen-steam atmospheres showed the total volatile yield

from all ranks of coal to be much higher than the proximate analysis volatile matter yield [170]. At 1327 ~

this is true only for lignite. (ii) Effects of heating rate. During pyrolysis of Center lignite to 380 ~ and 8500C at rates of 5

and 450C/min the yields of phenol and the cresols were higher with slower heating rate [163], regardless of the final pyrolysis temperature. However, the slower heating rate experiments are also substantially longer (170 vs. 20 minutes for a final pyrolysis temperature of 850~

the

effects attributed to heating rate may be confounded by residence time effects. The data are summarized in Table 4.18. TABLE4.18 Effects of heating rate on yields* of water-soluble organics from Center lignite [ 163].

Compound Acetone Acetonitrile 2-B utanone o-Cresol m-Cresol p-Cresol Methanol Phenol F'ropionitrile

380~ Maximum Temp. 5~ 45~C/min 110 40 ND ND 20 10 50 10 80 10 180 40 570 270 460 100 ND ND

850~ Maximum Temp. 5~ 45~C/min 1190 1490 300 280 310 440 1300 780 1920 1400 1740 1360 1590 1700 4380 3460 30 380

*Micrograms/Gram maf lignite. ND = not detected.

For Indian Head lignite in argon, the rate of mass loss per degree above 700~ was higher for the sample heated at 5~

than for a comparable sample heated at 100~

suggests that above 700 ~ (at least up to 1100~

[15]. This

pyrolysis weight loss is not only a function of

temperature but also of time. Louisiana lignites pyrolyzed in a thermogravimetric analyzer (in nitrogen at atmospheric pressure) show higher volatile losses at higher heating rates [35]. TMRWL also increased with increasing heating rate. In hydrogen, heating rate effects are the opposite of those in nitrogen, so that volatiles production is greater at lower heating rates. However, the temperature of maximum rate of weight loss in hydrogen increases with temperature as in nitrogen. The total volatiles produced are greater in hydrogen for similar reaction conditions. Hydropyrolysis begins at about 2000C [35]. The increased weight loss in hydrogen relative to nitrogen ranged from 18.9% at heating rates of 11 ~

to 5.8% at 4350C/min [35].

198 Heating rate was the most important variable affecting the weight loss behavior from North Dakota lignite for pyrolysis in a thermogravimetric analyzer to 1000"C in helium [131]. Pyrolysis at 160*C/min showed a TMRWL 25-50"C higher than for pyrolysis at 40~

However,

heating rate did not have a significant effect on volatiles yield. Heating rate had no evident effect on activation energies for pyrolysis of a North Dakota lignite, values of 224 kJ/mol and 230 kJ/mol being observed at heating rates of 160~

and 40~

respectively. With Montana lignite,

some differences in kinetic parameters were observed at the different heating rates. At 160~ the activation energy was 223 kJ/mol with a pre-exponential of 1.67 x 1013; whereas at the lower heating rate, the respective values were 175 kJ/mol and 1.07 x 1010 [131]. Tar yield increases with increased heating rate [205,206]. Tar yields for rapid heating rates at atmospheric pressure are 10-20% [207]. Field ionization mass spectrometry (FIMS) of tars produced at low heating rates indicates a marked fall-off at higher molecular weights [208]. For higher heating rates, FIMS shows an increased contribution from larger molecules, consistent with a reduction in the extent of crosslinking during pyrolysis [208]. High values of number average molecular weights of tars reflect suppressed fragmentation of molecules [207]. (iii) Effects o f residence time. Weight loss is a function of both isothermal pyrolysis time and particle size for Savage and Glenharold lignites pyrolyzed in an entrained flow reactor [209] in nitrogen at 808~ [136]. For example, the weight loss from Savage lignite after 0.16 s ranged from 22% (daf basis) of the original coal for 200x270 mesh lignite to 6% for 70x100 mesh. The weight loss depended on the isothermal exposure time (that is, the time during which the sample was at 808~

with no significant weight loss occurring during either very rapid heating or very rapid

cooling. Pyrolysis of Monticello lignite in a drop tube reactor illustrates the effects of residence time and temperature. At 0.2 s residence time in nitrogen, the pyrolysis weight loss increased from 16% to 51% with an increase in temperature from 788~ to 1455~ [139]. At 788~

pyrolysis

weight loss increases from 3% to 34% as residence time increases from 0.05 to 0.8 s. At 1455~ pyrolysis is essentially complete within 0.2 s. The difference in weight loss vs. time for various particle sizes persists to longer heating times (to 1.0 s), but with a distinct change in the slope of the weight loss vs. time curves. For Glenharold lignite the time at which the slope change occurred, 0.2 s, was independent of particle size [136]. These sharp breaks in the curve are characteristic of the occurrence of two parallel reactions: rapid release of volatiles by decomposition of oxygen-containing functional groups and methyl substituents, and slower release of volatiles as the hydroaromatic structures convert first to aromatic structures and then to small carbon crystallites. Loss of oxygen functional groups and side chain methyl groups produces only volatiles. However, decomposition of hydroaromatic structures and subsequent formation of carbon crystallites produces significant amounts of char. The relative proportions of volatiles and char depend on the amount of functional groups and the size and shape of the aromatic and hydroaromatic structures--which are governed by the rank of the coal--and by the pyrolysis conditions.

199 Pyrolysis of Wilcox lignite in a drop tube furnace at temperatures of 1460~

showed

pyrolysis was complete in 0.2 s [140]. In this reactor system, volatile evolution was 12% higher than the ASTM volatile matter yield. Weight loss during entrained flow pyrolysis of Glenharold, Darco, and Savage lignites depends both on the particle size and the residence time in the reactor, with smaller particle sizes and longer residence times producing greater weight losses [173]. Prolonged heating at 1000~

in a heated grid reactor gave a total volatiles yield from

Montana lignite of 44.0% [199]. Only about 22% of the carbon was volatilized, indicating that most of the volatiles result from loss of hydrogen and oxygen. The reduction in sulfur was about 70%; that of nitrogen, 25%. At a heating rate of 1000~

thermal dehydration reactions leading to

the formation of water are about 90% complete before significant tar formation begins. The mass lost from Indian Head lignite in a thermogravimetric analyzer at 1100~ in argon at ambient pressure agrees quite well with the standard ASTM volatile matter determination [ 15]. The range of values observed in pyrolysis in the TGA was 38.2 to 42.0%, while the ASTM volatile matter determination on the same samples indicated 39.0 and 42.2%, respectively. Values from another laboratory, obtained by heating to 1000~

were 38.6 and 41.2% [131]. The amount

of volatiles determined by this TGA method depends on heating rate, since at lower heating rates the residence time will be longer and the volatiles release will be higher. The effect of residence time at maximum temperature was evaluated for Indian Head lignite at two residence times, 0 and 30 minutes, and at two temperatures, 380* and 850~ lignite pyrolyzed in helium at a heating rate of 45~

for --60 mesh

At 380~ the effect of residence time was

marked, with yields for a holding time of 0 minutes being below the limits of detection by gas chromatography, but with significant yields observed at 30 minutes [ 163]. In contrast, at 850~ there was essentially no difference in yields between the two reaction times. The results are summarized in Table 4.19 [163]. TABLE4.19 Effects of time at temperature on yield* of water-soluble organics from Indian Head lignite [163].

Compound Acetone Acetonitrile 2-Butanone Catechol o-Cresol m-Cresol p-Cresol Methanol Phenol Propionitrile

380"C Maximum Temp. 0 Minutes 30 Minutes ND 480 ND 30 ND 140 ND 1480 ND 350 ND 450 ND 550 ND 900 ND 1370 ND ND

*Micrograms/Gram maf lignite. ND = not detected.

850~ Maximum Temp. 0 Minutes 30 Minutes 1420 1460 230 240 400 370 2210 2120 510 530 710 820 760 820 1490 1740 2300 2360 130 270

200 The amount of light hydrocarbon gases formed during carbonization reactions in slagging fixed-bed gasification increases with increased the gas residence time in the reactor [210]. (The residence time was increased by increasing the pressure or by decreasing the oxygen feed rate to the gasification reactions.) In either case, a mass balance showed increased weight of gases equivalent to a decrease in tars. This suggests that at least portions of the methane, ethane, propane, and butane derive from cracking or dealkylation reactions of the tar components. Pyrolysis of Elbistan lignite for 60-180 min shows that liberation of both tar and gas slows with increasing residence time [200]. The residence time at which char yield becomes constant is in this range, and depends on temperature. Two factors account for the tar formation occurring during the early stages of pyrolysis [207]. First, the tar precursors initially present in the lignite are readily swept away by the flux of non-condensible gases being produced at the same time. Second, however, as pyrolysis--and associated cross-linking--proceed, fewer and fewer bridges remain to be cleaved to replenish the supply of tar precursors. (iv) Effects of pressure. Pressure effects diminish for lignites, compared to coals of higher rank, because few tar precursors are generated on decomposition of the lignite structure [207], and because the tar yield is low [211]. As an example, a lignite giving a 7% tar yield (daf basis) at 1 Pa still produced a yield of 5% at 0.1 MPa [211]. Although the tar yield decreases slightly with pressure, the gas yield increases to compensate [211]. Montana lignite, pyrolyzed in vacuum to temperatures of 800~ in a heated grid reactor, displayed no differences from pyrolysis to the same temperatures in 0.1 MPa helium [ 199]. Some differences were observed above 800~

Vacuum pyrolysis results in higher yields of heavy

hydrocarbons and lower yields of light gases, as opposed to pyrolysis in 0.1 MPa helium [ 199]. The total weight loss during vacuum pyrolysis exceeds that in helium, suggesting that secondary cracking reactions and char-forming reactions influence the observed yields. The effect of pressure on yields of water-soluble organics was minimal with Gascoyne and Center lignites pyrolyzed at 8500C (heating rate of 230C/min) in a 50 cm3/min stream of nitrogen. Little difference in the yields of methanol, acetone, acetonitrile, 2-butanone, propionitrile, phenol, and the three cresols occurred at ambient and 2.8 MPa pressure [212]. The yield of catechol from Center lignite decreased from 1860 to 210 ~tg/g. (v) Effects ofparticle size. Entrained flow pyrolysis of Darco lignite in the temperature range 700-10000C shows that weight loss is independent of particle size in the range 41-201 ~tm [176]. At heating rates of 10,000~

pyrolysis rate is independent of particle size in the range

41-201 ~m [176]. This observation suggests that physical factors do not control the pyrolysis. Particle size in the range 60-1000 ~tm does not influence total pyrolysis yield from Greek lignites [213]. The maximum potential weight loss to be expected from Darco lignite pyrolysis in an entrained flow reactor is 66% [176]. This value is much higher than the volatile matter content obtained during the proximate analysis (51%, daf basis). The maximum potential weight loss is independent of particle size, suggesting that secondary reactions leading to carbon or char

201 deposition inside the pores are not important. The effects of experimental conditions on pyrolysis of Indian Head lignite in a thermogravimetric analyzer are summarized in Table 4.20 [ 15]. TABLE 4.20 Effect of experimental conditions on pyrolysis of Indian Head lignite [ 15].

Increasing Particle size Sample weight Gas flow rate Heating rate

(vi)

TMRWL No trend No trend Decrease Increase

Effect on , ~ Decrease Decrease No trend Decrease

V* No trend Decrease Decrease Decrease

Comparative pyrolysis of different lignites. A comparison of product yields from three

North Dakota lignites is shown in Table 4.21 [163,195]. The experimental conditions were a heating rate of 45~

maximum temperature of 8500C, -60 mesh particles, and helium

atmosphere. TABLE 4.21 Yields* of polar organics from three North Dakota lignites [163,195]. Compound Methanol Acetone Acetonitrile 2-Butanone Propionitrile Phenol o-Cresol p-Cresol m-Cresol

Indian Head 1490 1420 230 400 130 2300 510 760 710

Gascoyn.e 730 1600 220 510 230 2900 590 1080 910

Center 1700 1480 280 440 370 3440 780 1360 1400

*Yields are reported in micrograms per gram of lignite (maf basis).

(vii)

Evolution of pyrolysis products. Carbon dioxide and water are the major products

during the initial stages of pyrolysis [206]. As the number of methyl groups in phenol-formaldehyde resins or in methylated phenols increases, the temperature at which methane formation is at a maximum shifts to lower temperatures [214]. As would be expected, the methane yield increases with increasing methyl groups. Japanese lignites show a similar carbonization behavior to that of resins of methylated phenols. Methane could be formed by a concerted group transfer elimination from a guaiacyl structure [191]:

202 %

~

0

O~CH3 ~--

-t- CH 4

OH

Pyrolysis of various materials--including lignin, methylated and perdeuteromethylated coals, and polymers with methoxy or methyl groups--has shown that when methoxyl groups are present, methane evolution occurs consistently at 4500C, independently of other reactions involved with tar formation [215]. Cleavage of the C--O bond forms methyl radicals, which then produce methane via hydrogen abstraction reactions. For substances containing methyl, rather than methoxyl groups, methane evolution coincides with tar formation at 500oC, and results from cleavage of methyl groups from the structure. Heated grid pyrolysis of a North Dakota lignite, at heating rates of 950-1300 ~

and

particle sizes of 54-74 ~tm in helium, indicated that all hydrocarbon gases appear at higher temperatures than carbon dioxide [185]. Some carbon monoxide was observed below 577~ [185]. The appearance of CO suggests the presence of some of the oxygen in the lignite in thermally labile functional groups other than carboxyl. When lignites are heated in hydrogen-containing gases the initial reactions involve loss the oxides of carbon, water, tars and oils, and some gaseous hydrocarbons [216]. The solid product of this reaction has been referred to as an "intermediate semi-char" [216]. In a second stage, decomposition of the semi-char with evolution of hydrogen forms a comparatively unreactive char. The yields of methane and ethane (if corrected for those deriving directly from the decomposition of the lignite and for those obtained cracking of the other light hydrocarbon gases) relate stoichiometrically to the amount of hydrogen evolved during the second stage of devolatilization. The transition from semi-char to the final, relatively unreactive char passes through an intermediate stage in which the solid is potentially quite reactive. The ratio of methane and ethane to char is proportional to the hydrogen partial pressure. Pyrolysis of Beulah-Zap lignite in the Rock-Eval test showed the following results [217]: free hydrocarbons evolved during the initial 3 minutes at 300~

1.3 mg/g (as received basis); total

amount of free hydrocarbon evolved during pyrolysis, 60 mg/g; carbon dioxide yield below 390~

13.9 mg/g; and temperature of maximum rate of hydrocarbon evolution, 1150C. The kinetic

parameters for this lignite in the Rock-Eval test, as determined by nonlinear regression, were a preexponential factor of 1.2 x 1015, and an activation energy of 232 kJ/mol with o of 6.1%. For coals ranging in rank through low volatile bituminous, carbon dioxide evolution is highest and total hydrocarbon evolution lowest for the lignite. Pyrolysis of this same lignite in a fluidized bed reactor at a heating rate of 4~

in argon atmosphere resulted in the following gas yields

(expressed as mg/g, daf basis): 14.6 H2, 24.1 CH4, 2.4 C2H4, 3.5 C2H6, 2.6 C3H8, 4.7 C4H10, 93 CO2, 82 CO, and 120 H20 (above 2000C) [202]. Carbon dioxide evolution below 520~ was

203 70 mg/g. Carbonization of Dakota (South Dakota) lignite at 500~ at a heating rate of 6-7~ produced gas containing 89.8% CO2, 4.2% CH4, 4.5% C2H6, and 1.5% illuminants [197]. Flash pyrolysis of Alcoa lignite at 8500C for 0.5-1 s produced the following gas yields (on a moisture-free basis): methane, 2.4%; ethylene, 3.2%; ethane, 0.33%; propylene, 0.74%;, propane, 0.04%; 1,3-butadiene, 0.41%, 1-butene, 0.13%; butane, nil; and benzene, 0.8% [218]. The concentration of polymethylene groups was about 5.3%, based on evolution of ethylene and other hydrocarbons [219]. Flash pyrolysis of lignite at temperatures of 700~ or above produces low molecular weight aliphatic hydrocarbons from cracking longer polymethylene chains. The polymethylene units are part of the coal structure or, possibly, molecules trapped in the coal matrix [220]. Reaction at lower temperatures (e.g., 600~

preserves some of the long chain (C 17 and higher) alkanes and alkenes,

which form part of the tar. Some lignite and subbituminous coals may contain as much as 10% (CHz)x [220]. Pyrolysis of Onakawana lignite in a spouted bed reactor, in nitrogen, showed a maximum liquid yield occurring about 5000C [221 ]. The liquid, however, amounted to about 80% water; the yield of dry tar was 6 - 9 % . Above 400~ the volatile content of the char decreases slowly with increasing temperature, but above 525~ a rapid decrease begins. The methane yield is about 1% at temperatures to 5500C, but increases rapidly above that temperature. About 50% of the gas yield is carbon dioxide, with much smaller amounts of methane, ethylene, and hydrogen. Production of alkenes during carbonization in a slagging fixed-bed gasifier decreases with increasing operating pressure of the gasifier [ 138]. Alkylphenols, alkyldihydroxybenzenes, and alkylmethoxyphenols dominate the products from Curie point pyrolysis of lignites [222]. For laser pyrolysis, major products include toluene, phenol, the cresols, C2-phenols, naphthalene, and methylguaiacol [223]. Most of the tar generation in lignite pyrolysis precedes evolution of light liquids [217]. Tar generation seems to be associated with thermal breakdown of oxygen and sulfur functional groups; the light liquids are formed by the cleavage of stronger bonds. Elimination of carbon dioxide, carbon monoxide and water may occur during crosslinking reactions, the extent being affected by heating rate [134]. The amount of tar formed in devolatilization correlates with the daf volatile matter content by X = 0.95 VM + 0.025 [224]. This equation is applicable for the following conditions: temperatures 400-1000~ sizes 25-1000 ~tm, heating rates 5.6x10-3-104 ~

particle

and pressures 10-3-10 MPa. The ultimate

weight loss was unaffected by heating rate in the range 650 to 104 ~

[224].

Evolution of tar from Texas lignite (pyrolyzed at maximum temperature of 400~

began at

204 about 300" [225]. Alkanes as large as C30 were observed in the products, along with some alkenes. Comparison of these results for lignite with model compound studies suggests that the alkanes and attendant alkenes are formed from cleavage of alkylaromatics. Mass spectroscopic analyses of the tars produced from Indian Head and Baukol-Noonan (North Dakota) lignites during slow heating rate carbonization in a slagging fixed-bed gasifier were compared with analyses of benzene:methanol extracts of lignite [226,227]. Seventeen of the same compound types, based on Z-number and molecular weights, occur in both the tar and the extract [227]. The major difference was that highly alkylated compounds found in the extract, such as C 5 tetralins and C 10 benzenes [228], were missing from the tar. For thirteen compounds found in the tars, the Spearman rank correlation between their concentration in the tar and their thermal stabilities, as measured by liquid phase pyrolysis reactions [229], was 0.725 (the probability of r = 0.725 occurring by chance is less than 0.01). During carbonization the long alkyl groups were thermally cleaved. Solvent extraction had shown that pyridine was the only heteroaromatic nitrogen or sulfur structure [226]. Quinoline derivatives were also identified in the tar, but it was not determined whether the quinoline structures were present in the original lignite or were artifacts of the carbonization process [228]. Methylation of the carboxyl groups increases tar yield on pyrolysis [205]. The molecular weight distribution of the tar resembles that from pyrolysis of bituminous coals [205]. Texas lignite reacted with tetrabutylammonium hydroxide followed by methyl iodide, and then pyrolyzed by heating at 5-10~

to 620~

showed an increased tar yield well above the amount calculated

simply from the added methyl groups [230]. That is, methylation of the lignite made the tars more volatile. Evolution of tars from untreated lignite could be inhibited by crosslinking reactions of the type Lignite-OH + HO-Lignite ---, Lignite-O-Lignite + H20 Methylation eliminates inhibition of volatiles release by preventing formation of ether crosslinks. Methanol evolution from the methylated lignite is significant, and suggests methylation of the carboxyl groups as well as aliphatic hydroxyl groups. (viii)

Crosslinking during pyrolysis. Low-rank coals are non-caking, at least at heating

rates typical of fixed-bed carbonization or the free swelling index test. Humovitrains of lignites do exhibit plasticity under normal carbonization conditions [231]. The high oxygen content of lignite provides the possibility of crosslinking to occur as the oxygen-containing functional groups react during pyrolysis [232]. The extent of crosslinking during pyrolysis can be reduced by using very high heating rates. Such heating rates also produce a melting and swelling of the char, as well as higher yields of soluble products. Onset of crosslinking in lignite pyrolysis coincides with the release of carbon dioxide [ 161,232] and is related to the decomposition of carboxyl groups [ 161,190,205]. A decrease in the ability of lignites to swell in organic solvents, observed during early stages of pyrolysis, correlates

205

with the evolution of carbon dioxide [233]. Big Brown lignite shows a slower rate of crosslinking (as determined by measurements of the volumetric swelling ratio) than does Indian Head lignite; Big Brown evolves less carbon dioxide under identical conditions than Indian Head, and the area of the carbonyl peak in the FTIR spectrum of Big Brown is also less than for Indian Head. The ability to swell in solvents is inversely related to extent of cross-linking [233]. These observations suggest that crosslinking relates directly to the loss of carboxyl groups [205]. Cross-linking of Zap lignite accompanied a decrease of carboxyl and hydroxyl groups in the lignite structure [ 161 ]. An increase in cross-linking for oxidized lignites also demonstrates the relationship between crosslinking and oxygen functional groups [161]. Furthermore, cross-linking begins almost immediately upon drying [233]. During pyrolysis, low-rank coals crosslink at a much lower temperature than high volatile bituminous coals [234]. Zap lignite crosslinks before tar evolution becomes significant [161,235] and before any rapid loss of aliphatic hydrogen [ 161 ]. Lignite crosslinking occurs at temperatures around 380~

[205], and possibly as low as 200~

[161,236], much lower than that at which

crosslinking takes place in bituminous coals. The tendency of lignites to crosslink under relatively slow heating conditions may be a reason that lignites are sometimes thought (erroneously) to be less desirable for liquefaction than bituminous coals. However, both volatile release and crosslinking depend on heating rate. With slow heating, e.g., 0.5~

lignite crosslinking begins

before tar evolution becomes significant. If the heating rate is very high, e.g., 20,000~

tar

evolution coincides with crosslinking and, as a consequence, the tar yield is much higher. These very high heating rates also provide visual evidence for the development of thermoplasticity in the lignite. The char shows definite visual evidence of fluidity, bubbling, and swelling, consequences of the reduced crosslinking.The occurrence of thermoplasticity indicates that the rate of bond cleavage must be competitive with the rate of crosslinking. Furthermore, this so-called ultra-rapid heating substantially decreases the crosslinking [ 161 ]. The tendency to crosslink at lower temperatures prevents softening of the coal, and is in part responsible for the relatively low tar yields from low-rank coals, compared with tar yields of bituminous coals [237]. Even though the bridges between aromatic ring systems in lignites are labile, to propensity of oxygen functional groups to enhance cross-linking results in the formation strong linkages in the char [238]. The molecular species that would be the precursors to tar participate in the crosslinking before they are able to escape into the vapor phase and consequently become part of the char. For pyrolysis of 53-88 ~tm North Dakota lignite in a heated grid reactor at heating rates of 1000 ~

(and subsequent cooling rates of 200-400 ~

in vacuum or helium,

the number average molecular weight of the tar produced at 464~ is 323; for maximum pyrolysis temperatures of 860~

the value is 279 [234, 237]. This decrease in molecular weight of the tar

with increasing pyrolysis temperature is consistent with a higher degree of crosslinking accompanying more severe thermal treatment. Structures that would be tar precursors participate in crosslinking so rapidly that they are incorporated into the char before being able to escape. Heated grid pyrolysis of Beulah lignite at heating rates of 1000 ~

to 464~ produced a 3.0% yield of

206 tar. Retrogressive reactions leading to char formation are quite facile in lignite. The crosslinking behavior can be addressed without even necessarily considering specific functional groups. In the FLASHCHAIN theory of pyrolysis, coal components are classified into four pseudo-components on the basis of the ultimate analysis, carbon aromaticity, proton aromaticity, aromatic cluster size, and pyridine extract yield [ 194]. All of the aliphatics and oxygen are assigned to labile bridges. Such labile bridges abound in lignites, large yields of noncondensible products arise from lignite pyrolysis. However, the abundant oxygen sites promote cross-linking, which in turn suppresses tar formation [194,207]. Cleavage of the labile bridges and associated cross-linking produces the non-condensible gases, but does not significantly break down the lignite macromolecule into tar precursors [207]. Hence most of the aromatic structures remain associated with the char. Methane yields from various ion-exchanged samples of Zap lignite relate inversely to tar yields [ 159]; as potential tar precursors are incorporated into char structures, they can yield volatiles only by cracking off relatively small molecular fragments. With an increase in temperature, residence time, or both, the aromaticity of the char relative to the parent lignite increases, and the amount of aliphatic carbon remaining in the char decreases [239]. For Beulah-Zap lignite, the size of aromatic clusters increases monotonically with final temperature, for slow heating. The effect of rapid heating (104 ~

on char structure is

summarized in Table 4.22 [239]. TABLE 4.22 Changes in aromatic cluster size with pyrolysis conditions, BeulahZap lignite, 104 ~ heating [239] Final temp.,~

Time at temp.

Average carbons per cluster 8.5 0.5 min 11.4 2.4 s 13.3 60 ms 9.8 160 ms 17.8 -

800 800 1600 1600

-

Methylation makes lignite behavior during pyrolysis similar to that of high-rank caking coals [208]. Methylation decreases retrogressive reactions during pyrolysis (and liquefaction) [240]. The yields and molecular weight distributions of the tar, as well as visual evidence that the lignite softened and passed through a plastic stage, are consistent with a substantial reduction in crosslinking. Methylation of hydroxyl and carboxyl groups prevents their participation in crosslinking. The molecular weight distributions are comparable to those of tars from bituminous coals. FTIR analysis of the tars from very rapid heating rate pyrolysis indicates that the tar molecules have not lost the oxygen functional groups. At lower heating rates, the oxygen functional groups crack to carbon dioxide, carbon monoxide, or water. The oxygen groups thus do not appear in the tar components. The reactions that cleave these light gaseous products appear to

207 be those accompanied by crosslinking. Cross-linking of ion-exchanged Zap lignite resembles that of untreated lignite [161]. Loading lignite with calcium by ion exchange and subsequent pyrolysis shows no change relative to the untreated lignite [205]. However, pyrolysis of demineralized Indian Head lignite shows that a shift to higher temperatures is needed to achieve the same extent of crosslinking. Demineralization reduces cross-linking [240]. Compared to the untreated lignite, tar evolution was higher from the demineralized lignite, with less carbon monoxide and carbon dioxide evolution. These results suggest that the ion-exchangeable cations may have an important role in pyrolysis reactions. Demineralized Zap lignite behaves in the same way as the demineralized Indian Head [161]. REFERENCES

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Fuel Soc. Japan, 53 (1974) 1064-1072. R.M. Carangelo, P.R. Solomon, and D.J. Gerson, Application of TG-FT-i.r. to study hydrocarbon structure and kinetics, Fuel, 66 (1987) 960-967. J.L. Johnson, Kinetics of initial coal hydrogasification stages, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 22(1) (1977) 17-29. A.K. Bumham, M.S. Oh, R.W. Crawford, and A.M. Samoun, Pyrolysis of Argonne premium coals: Activation energy distributions and related chemistry, Energy Fuels, 3 (1989) 42-55. W.H. Calkins, E. Hagaman, and H. Zeldes, Coal flash pyrolysis. 1. An indication of the olefin precursors in coal by CP/MAS 13C nmr spectroscopy, Fuel, 63 (1984) 1113-1118. W.H. Calkins, Coal flash pyrolysis. 3. An analytical method for polymethylene moieties in coal, Fuel, 63 (1984) 1125-1129. W.H. Calkins, Coal structure vs. flash pyrolysis products, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 28(5) (1983) 85-105. K.C. Teo and A.P. Watkinson, Rapid pyrolysis of Canadian coals in miniature spouted bed reactor, Fuel, 65 (1986) 949-959. H. Huai, R. Lo, Y. Yun, and H.L.C. Meuzelaar, A comparative study of 8 U.S. coals by several different pyrolysis / mass spectrometry techniques, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 35 (1990) 816-823. W.S. Maswadeli, Y. Fu, J. Dubow, and H.L.C. Meuzelaar, Structure / reactivity studies of single coal particles at very high heating rates by laser pyrolysis GC/MS, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 37 (1992) 699-706. L.H. Chen and C.Y. Wen, A model for coal pyrolysis, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 24(3) (1979) 141-146. R.M. Roberts and K.M. Sweeney, Low-temperature pyrolysis of Texas lignite, basic extracts, and some related model compounds, Fuel, 63 (1984) 904-908. R. Hayatsu, R.E. Winans, R.G. Scott, L.P. Moore, and M.H. Studier, Trapped organic compounds and aromatic units in coal, Fuel, 57 (1978) 541-548. D.J. Miller, J.K. Olson, and H.H. Schobert, Organic structural studies of lignite coal tars, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 25(1) (1980) 256-263. D.J. Miller, J.K. Olson, and H.H. Schobert, Mass spectroscopic characterization of tars from the gasification of low-rank coals, Fuel, 60 (1981) 370-374. A.G. Sharkey Jr., J.L. Schultz, and R.A. Friedel, Mass spectra of pyrolyzates of several aromatic structures identified in coal extracts, Carbon, 4 (1966) 365-374. C.J. Chu, S.A. Cannon, R.H. Hauge, and J.L. Margrave, Studies of the effects of methylation on the pyrolysis behavior of four brown coals, Fuel, 65 (1986) 1740-1749. F.T.C. Ting, The characterization of coking properties of selected components of lignite and subbituminous coals, Univ. North Dakota Dept. of Geol., Unpublished report, 1972. F.J. Derbyshire, A. Davis, and R. Lin, Considerations of physicochemical phenomena in coal processing, Energy Fuels, 3 (1989) 431-437. E.M. Suuberg, Y. Otake, and S.C. Deevi, Solvent swelling as a measure of the breakdown of the macromolecular structure of coal, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 36 (1991) 258-266. E.M. Suuberg, P.E. Unger, and J.W. Larsen, The relation between tar and extractables formation and crosslinking during coal pyrolysis, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 30(4) (1985) 291-295. P.R. Solomon, D.G. Hamblen, G.V. Deshpande, and M.A. Serio, A general model of coal devolatilization, Proceedings 1987 International Conference on Coal Science, pp. 601604. P.R. Solomon, S. Charpenay, Z.Z. Yu, M.A. Serio, E. Kroo, M.S. Solum, and R.J. Pugmire, Network changes during coal pyrolysis: Experiment and theory, Proceedings 1991 International. Conference on Coal Science, pp. 484-487. E.M. Suuberg, P.E. Unger, and J.W. Larsen, Relation between tar and extractables formation and cross-linking during coal pyrolysis, Energy Fuels, 1 (1987) 305-308. S. Niksa and A.R. Kerstein, Modeling the devolatilization behavior of various coals, Proceedings 1991 International Conference on Coal Science, pp. 488-491. M.S. Solum, R.J. Pugmire, D.M. Grant, T.H. Fletcher, and P.R. Solomon, Solid state 13C NMR studies of coal char structure evolution, Amer. Chem. Soc. Div. Fuel Chem.

217 240

Preprints, 34 (1989) 1337-1344. M.A. Serio, P.R. Solomon, E. Kroo, R. Bassilakis, R. Malhotra, and D. McMillen, Studies of retrogressive reactions in direct liquefaction, Proceedings 1991 International Conference on Coal Science, pp. 656-659.

218

Chapter 5

THE I N O R G A N I C CONSTITUENTS OF LIGNITES

5.1 INCORPORATION OF MAJOR ELEMENTS 5.1.1 Accumulation of inorganic components The average ash value of peat, on a dry basis, is 8.8% [1]. On the same basis, the ash value of Fort Union lignites ranges from 8 to 9% [1]. The similarity suggests that, for these lignites, most of the accumulation of inorganic constituents occurred during the formation of the precursor peat. Separated samples of anthraxylon yielded 2.5-4% ash on an as-received basis. The mean value of ash of common tree woods is 3.6% [2]. The agreement of ash values for lignitederived anthraxylon and modern wood, and the fact that most lignites have ash values well above 4%, suggest that most of the inorganic components associate with the non-anthraxylous (i.e., nonwoody) portion of the lignite. Silicified samples of lignites were found in the Wood Mountain uplands of south-central Saskatchewan [3]. The silicified plant debris consists mainly of compressed stems and grass blades. In Rosenbach (German) lignite, silicification arises from colloidal silica derived from sedimentary rocks [4]. The variation of elemental concentration as function of ash value indicates both the source and the fate of elements in the depositional system. Coals appear to be closed systems for most inorganic elements; that is, once an element enters the depositional system, it tends to stay and become incorporated in the coal [5,6]. Comparison of elemental concentrations in lignites with those in higher rank coals determines the elements for which the concept of a closed system is valid. Compared to higher rank coals, lignites are relatively enriched in Ba, B, Ca, Mg, Na, and Sr [5,7]. These elements are associated with the organic portion of the lignite and their concentrations in coals decrease as a consequence of the increase in rank. All of these elements, except boron, associate largely or wholly with carboxyl groups in lignite; loss of these groups on coalification to higher ranks removes the ion-exchange sites that are the principal means of incorporating these elements. As a cautionary note, the organic affinity of a given element may vary among coals from the eastern United States, Illinois Basin, and the western United States [8]; a generalization about the organic affinity of an element in a coal sample that has not been analyzed may be inaccurate. Data for a wide range of elements, including such chemically diverse species as Ba, Cu, Ga, Hf, Pb, Mo, K, Sc, Ag, Ta, Th, Ti, V, Y, Zr, and the rare earths, show that elemental concentration increases with ash value. This relationship indicates a detrital source for these

219 elements [5]. Calcium and strontium, on the other hand, decrease in concentration with increasing ash value. These two elements derive from the original plant material; consequently, their original concentration in the coalifying organic matter becomes "diluted" with increasing influx of detrital inorganic matter [5]. In some South Australian lignites, the ash value and mineral matter content are highest where the detrital inertinite is highest [9]. In this instance, the local depositional environment had a direct influence on the accumulation of inorganic species by the lignite, particularly the inherent mineral matter and syngenetic minerals. The organic functional groups in lignite act as traps in concentrating elements from groundwater, mainly by ion-exchange processes. In addition, some coordination or chelation occurs, as evidenced by the presence of acid-soluble forms of elements which cannot be accounted for as carbonates or other acid-soluble minerals. The association of exchangeable cations with carboxyl groups can be inferred from pKa data. Lignites contain two acidic functional groups: carboxyl and phenolic. The pKa values of the carboxyl groups range from 4 to 5, and thus they undergo ion-exchange, whereas the pKa's of phenols are in the range of 10.5-12, hence phenols more likely participate in complex formation [ 10]. Ion exchange of cations in groundwater with carboxylate groups in lignite may be affected by the specificity of cations for coordinating with carboxylate as a ligand [ 11]. If two cations occur in solution (e.g., groundwater) at equal activities, one will be preferred to the other; consequently the amount of one held in association with the carboxylate groups will be greater than the amount of the other. The ionic potential (charge to radius ratio) of the cations governs this selectivity. A cation of large ionic potential will displace one having smaller ionic potential. The practical implication is that, assuming equal activities in solution, a higher quantity of an alkaline earth cation, such as Ca+2, will be incorporated relative to an alkali cation such as Na+. If only a series of alkali metal cations, or only alkaline earth cations, are compared (so that ionic charge is not a factor) then the cations undergo exchange in order of radii, i.e., Li+
220 capacities determined in this way are about twice the amount of cations actually extracted from the lignite with ammonium acetate, then it appears that a large fraction (roughly half) of the carboxyl groups in the lignite exist in the free acid form. Electron microprobe analysis of Baukol-Noonan (North Dakota) lignite, tracking elemental concentrations across a strip of the sample, indicated that some of the iron, aluminum, and magnesium were associated with the organic portion, presumably as carboxylate salts [ 13]. The deduction is based on the existence of a uniform background signal for these elements across the sample; by contrast, elements present in discrete mineral grains would show signal spikes at locations in which the particular mineral grain was present. 5.1.2 Chemical fractionation The inorganic components of lignite occur in several forms: discrete minerals, ions bonded to carboxylate groups in the coal structure, ions associated on the ionic sites of clays, or coordinated to oxygen-, nitrogen-, or sulfur-containing functional groups in the lignite structure. Chemical fractionation is the procedure for determining the proportions of an element present in each of these modes of occurrence [ 14]. Chemical fractionation uses extraction with 1M ammonium acetate solution to remove cations present on ion-exchange sites, followed by extraction with 1M HC1. Hydrochloric acid removes elements present in acid-soluble minerals, such as carbonates and hydrous oxides, and may also remove some elements present as coordination complexes. The elements remaining after treatment with both reagents are considered to be present in insoluble minerals such as quartz or pyrite. Analysis of the extracts and of the residue at each step provides the data needed to determine the proportions of a given element present in each form. The specific details of chemical fractionation have been modified from time to time since the original publication; Fig. 5.1 is a flow sheet of chemical fractionation as it has now evolved [15]. Detailed procedures have been published [ 16]. Specific gravity fractionation in combination with chemical fractionation distinguishes between organically and inorganically associated elements. Sodium is almost completely ionexchangeable and virtually no sodium is found in the heavy fractions [14]. In comparison, potassium in the lighter fractions is ion-exchangeable, hence likely bound to carboxyl groups, while the potassium in heavy fractions is acid-insoluble, occurring in illite [ 14]. The trace elements Be, V, Y, Yb, Sc, Cr, Cu, and Zn, as well as the acid-soluble portion of the Ti, concentrate in the lower specific gravity fractions [ 14]. This behavior suggests that these elements are likely held in the lignite as coordination complexes. A portion of the aluminum showed similar behavior, suggesting that some of the aluminum in lignite might also be present as a coordination complex. Susceptibility to removal of cations by ammonium acetate treatment relates inversely to the ionic radii, within an isovalent group. The susceptibility to removal of a suite of five elements from Gascoyne (North Dakota) lignite is Na§ > Mg+2 > Ca+2 = Sr+2 > Ba+2 [17]. For the divalent elements, the percentages remaining in the lignite after three extractions with ammonium acetate are

221 coal

-325 mesh coal Vacuum dried 48h

Analysis

~ cc~al Analysis

~,M filtrate

1 extraction 100 m L H20

250C - 24 h ~ residue

Analysis

~,9 filtrate t

3 extractions 100 mL 1M NH4OAc 70~ 24 h Analysis

residue~ 2 extractions 100 m L 1M HCI

70~ - 24 h I

Analysis ~

filtrate

I

I

I

v

v_

residue

Analysis

Figure 5.1. Flow chart for chemical fractionation procedure [15].

Mg, 3%; Ca, 10%, Sr, 9%; and Ba, 50%. Removal of alkaline earth elements by extraction of three lignites with 1Mammonium acetate produced the results shown in Table 5.1, where the data show the percentage of the initial concentration of each element extracted from the lignite by the ammonium acetate solution [ 17]. Assuming that two carboxyl groups associate with each divalent cation and one with each monovalent cation, the percentage of total carboxyl groups associated with cations ranges from 43% for the Hagel (North Dakota) lignite to 54-60% for the Montana lignite [16,18]. The effect of ammonium acetate extraction on both major and trace elements in BaukolNoonan lignite is shown in Table 5.2 [19]. The twelve elements shown in Table 5.2 are those for which the greatest extractability in ammonium acetate solution was observed; data for 31 other major and trace elements are given in the original literature [19]. A comparison of the amounts of ion-exchangeable cations in lignite sampled from two pits of the same mine (Gascoyne) is provided in Table 5.3 [20]. (Additional data on these samples are shown later in this chapter.)

222 TABLE 5.1 Percentage of elements extracted by aqueous ammonium acetate [ 17]. Source of lignite State Seam N. Dakota Hagel Texas Darco Montana Fort Union

Percentage extracted C___~a M_M_g B___~a Sl" 70 60 100 78 79 62 68 82 79 84 71 74

TABLE 5.2 Percentage of elements extracted from Baukol-Noonan lignite [19]. Element % Extracted AI 10.6 Ba 20.0 Br 59.8 Ca 63.4 K 33.3 Mg 80.8

Element Mn Na Ni P Rb Sr

% Extracted 27.1 98.5 62.5 27.5 29.6 58.3

TABLE 5.3 Comparison of exchangeable cations in two Gascoyne lignite samples from different pits [20]

Element Aluminum Barium Calcium Iron Magnesium Manganese Potassium Silicon Sodium Titanium Carboxyl, meq/g

Red Pit Blue Pit Initial conc. % Removed Initial conc. % Removed lxg/g mf NH4Cg_H_3Q2 ~ / g mf N H 4 ~ Q 2 8740 0 7300 0 593 39 1268 61 17370 78 22790 85 3890 0 2540 0 2991 74 2588 75 123 34 163 39 1260 22 1430 19 28640 0 10920 1 1317 100 2694 100 1180 0 546 0 2.46

2.65

A concern in chemical fractionation is the potential solubility of various minerals in ammonium acetate solution, since cations contributed by these soluble minerals would be counted as exchangeable cations. Observed weight losses for common lignite minerals treated with 1M

223 ammonium acetate solution are shown in Table 5.4 [ 16]. Furthermore, exchangeable ions occur in clay minerals. The potential contribution of exchangeable cations from clays is shown in Table 5.5 [16]. TABLE 5.4 Weight loss of minerals treated in ammonium acetate solution [ 16]. Mineral Calcite Dolomite Gypsum Illite Kaolinite Montmorillonite Pyrite Quartz

% Weight loss 9 10

100 4 6 18 1 0

TABLE 5.5 Exchangeable cations in clays [ 16].

Clay Kaolinite Illite Montmorillonite

Exchange capacity 10-3 mol/g clay 0.02-0.10 0.13-0.42 0.80-1.50

Contribution to exchangeable cation content l_._Q0-6mol/g dmmf coal 0.50-2.50 3.25-10.5 20.0-37.5

Optical microscopy and mineralogical analysis of three lignites indicated that gypsum amounted to less than 10% of the total mineral matter [ 16]. The possible error contributed by the complete solubilization of gypsum in ammonium acetate solution for a lignite of 10% mineral matter and 1.5% actual exchangeable calcium is about 10% of the measured exchangeable calcium [16]. Since gypsum forms during lignite weathering, the amount of exchangeable calcium contributed by gypsum in unweathered lignite would be less than this estimate of 10%. The role of 0.1M hydrochloric acid in extracting elements from co3rdination complexes is not clear. The suitability of hydrochloric acid in this role depends on the stability constants of the complexes to be disrupted. Some complexes may not even dissociate at pH 1 [ 10]. Acid extraction is by no means selective for complexed metal ions, since dilute hydrochloric acid will also dissolve carbonates and some oxides and sulfides, as well as leach cations from clays. Chemical fractionation data for a high-sodium Beulah (North Dakota) lignite and its constituent lithotypes are shown in Table 5.6 [21]. Vitrains from a single layer in the Beulah-Zap seam have uniform elemental composition;

224 TABLE 5.6 Chemical fractionation data for Beulah lignite and its constituent lithotypes, ~g/g dry basis [21].

Element Na Mg A1 Si K Ca Ti Mn Fe Ni Ba

Lignite Removed by NH4OAc HCI 5412 26 1913 179 0 2323 13 423 31 166 3135 7136 0 11 13 32 5 2144 0 1 346 428

Attritus Removed by NH4OAc HCI 2685 18 1644 410 0 3064 66 1418 0 0 5434 6536 0 23 14 70 5 1659 0 1 156 668

Fusain Removed by NH4OAc HCI 3854 28 2355 312 0 2292 53 963 31 0 8041 4876 0 7 17 60 3 1613 0 2 299 451

Vitrain Removed by NH4OAc HCI 5569 80 2095 104 0 3044 42 318 31 0 7132 2747 0 3 13 37 0 1411 0 2 338 350

but samples of vitrain from different levels in the seam have different inorganic compositions, especially with regard to calcium, magnesium, and sulfur [22]. Fusain and attritus have inorganic compositions significantly different from the vitrain, particularly higher quantities of calcium, magnesium, and silicon, and lower amounts of aluminum, iron, and sulfur. Microprobe analysis of ulminite from the Beulah-Zap bed showed 1.2% S, 0.7% Ca, 0.4% Na, 0.3% Fe, 0.3% Mg and smaller amounts of Si, K, Ba, and Sr [23]. The lithotypes of Beulah lignite show strong differences in inorganic composition [24]. Vitrain contains major amounts of calcium and sulfur, significant sodium and magnesium, and lesser amounts of aluminum, silicon, potassium, strontium, and barium. Fusain also contains major amounts of calcium, but with significantly lesssodium and sulfur compared to vitrain. Attritus contains major amounts of calcium, magnesium, and sodium. Various grains of vitrain from Beulah lignite showed a consistent chemical pattern of high sulfur and calcium, moderate sodium, magnesium, and aluminum, and low silicon, potassium, iron, and strontium [25]. Several subtypes of vitrain grains have been identified on the basis of S/Ca and Mg/AI ratios. Major differences exist among ulminite macerals, both from within a particular lignite seam, as well as in different seams. The data in Table 5.7 illustrate these differences [22]. Generally, the ulminites of the Beulah-Zap have high iron and sodium and low sulfur. Hagel seam ulminites have high calcium but low sodium and sulfur. Martin Lake (Texas) ulminites have low sodium and high magnesium and sulfur. These differences reflect the effects of hydrogeochemical processes during diagenesis. Maceral samples from eastern Alabama lignite showed definite changes in inorganic composition [11]. Calcium and sulfur were homogeneously dispersed, suggesting that both elements are organically associated. Humodetrinites showed S/Ca ratios of 2. Fusinites showed S/Ca of nearly 1, but with some variations in X-ray intensity for Ca from one particle to another.

225 TABLE 5.7 Inorganic compositions of ulminites (weight percent) determined by electron microprobe analysis [22]. Lignite Beulah-Zap Hagel Martin Lake

Sample 1 2 1 2 1 2

AI 0.22 0.18 0.16 0.24 0.32 0.16

Ca 0.49 1.54 1.03 2.40 1.69 1.67

Fe 0.51 0.79 0.24 0.68 0.45 0.36

Mg 0.11 0.38 0.15 0.42 0.38 0.42

Na 0.32 0.54 0.06 0.07 0.15 0.06

S 0.45 0.43 0.59 0.51 1.25 1.16

Si 0.10 0.12 0.08 0.17 0.28 0.07

Sr 0.04 0.06 0.01 0.05 0.06

High-calcium fusinites displayed Ca intensities exceeding any of the humodetrinites. Gelinite gave the highest observed S/Ca ratio, 3, and a calcium intensity comparable to humodetrinite. Inertodetrinite gave the highest calcium intensity (about triple that of humodetrinite) but the lowest S/Ca ratio, 0.5. The content of organically bound calcium varies from one maceral to another and can be high in the inert macerals. Variations in the S/Ca ratio indicate chemical differences among macerals. A high concentration of ion-exchangeable calcium in fusinite is unusual because fusinite is usually considered to have low oxygen content [26], so it would be expected that fusinite would have few functional groups capable of acting as ion-exchange sites. (This observation relates to suggestions that lignites may contain highly reflecting materials that may be identified as fusinite or inertodetrinite, but that are also highly oxidized, thus containing large amounts of oxygen functional groups [27,28].) Chemical fractionation of two samples of Beulah lignite, a high-sodium lignite with 2.32 meq/g carboxyl, and a low-sodium lignite with 1.46 meq/g carboxyl, are shown in Tables 5.8 and 5.9, respectively [29]. Although the absolute amounts of the various elements change markedly from one lignite to the other, the percentages of each element in exchangeable, acid-soluble, or residual forms are quite similar. TABLE 5.8 Chemical fractionation of high-sodium Beulah lignite [29].

Element Aluminum Barium Calcium Iron Magnesium Manganese Silicon Sodium Titanium

Initial conc. opm (dry) 2880 720 7770 4880 980 24 7760 4620 104

% Removed by .NH4_Q2_~O~ HCI 0 81 48 40 73 18 0 45 85 15 22 75 0 6 100 0 0 11

% in Residue 19 12 8 55 0 3 94 0 89

226 TABLE 5.9 Chemical fractionation of low-sodium Beulah lignite [29].

Element Barium Calcium Iron Magnesium Manganese Silicon Sodium Titanium

Initial conc. oom (dry) 370 7500 10560 1480 52 7990 1380 280

% Removed by NH4_~2_~Q2 HCI 27 24 78 11 0 36 77 16 29 66 0 16 97 1 0 9

% in Residue 49 11 64 7 5 84 2 91

Additional data on the comparative behavior of high- and low-sodium samples of the same lignites (Beulah and Gascoyne) is presented in [30]. A higher percentage of barium was removed by ammonium acetate from the high-sodium lignites than from the low-sodium lignites. With hydrochloric acid, a higher percentage of aluminum was removed from the high-sodium than from the low-sodium lignites. The difference in the behavior of aluminum seems related to the presence of micaceous clays, with the aluminum removed by hydrochloric acid resulting from selective attack on gibbsite layers in the micaceous clays. However, in comparing the two pairs of lignites, no significant differences exist between the modes of occurrence of the inorganic constituents, the major differences being in the amounts of the inorganic elements present in each lignite. Chemical fractionation data for lignite from the Blue and Red pits of the Gascoyne mine are shown in Tables 5.10 and 5.11, respectively [31]. TABLE 5.10 Chemical fractionation of Gascoyne lignite, Blue pit [31].

Element Aluminum Calcium Copper Iron Magnesium Manganese Potassium Silicon Strontium Titanium

Initial conc. % Extracted % Extracted ppm, mf by NH4OAc by HC1 7300 22.6 38.8 22790 85.1 14.3 11 0 0 2540 0 42.5 17890 98.0 2.0 25 28.0 72.0 1430 89.0 0 10920 11.6 16.7 313 94.6 5.4 546 0 57.0

% Remaining in Lignite 38.6 0.6 100 57.5 0 0 11.0 71.7 0 43.0

227 TABLE 5.11 Chemical fractionation of Gascoyne lignite, Red pit [31].

Element Aluminum Calcium Copper Iron Magnesium Manganese Potassium Silicon Strontium Titanium

Initial conc., ppm, mf 8740 17370 10 3890 20540 18 1260 28640 276 1180

% Extracted % Extracted by NH4OAc by HCI 22.2 27.6 78.1 19.8 0 0 0 55.8 95.4 4.6 22.0 78.0 62.8 3.7 17.2 8.1 77.5 22.5 0 13.6

% Remaining in Lignite 50.2 2.1 100 44.2 0 0 33.5 74.7 0 86.4

Chemical fractionation data for Choctaw (Alabama) lignite are shown in Table 5.12 [32]. This is one of the most extensive sets of data available on chemical fractionation results for trace elements. This sample of Choctaw lignite had been washed prior to shipment, but details of the washing process are not known. TABLE 5.12 Chemical fractionation of Choctaw lignite (dry basis) [32].

Element Aluminum Antimony Barium Bromine Calcium Cesium Cobalt Europium Gold Iodine Iron Lanthanum Magnesium Manganese Molybdenum Potassium Rubidium Samarium Scandium Selenium Silicon Sodium Thorium Titanium Vanadium

Initial conc. % Removed by ppm (dry) NI-t4_~2_H~Q2_._ HC1 5907 0 37 0.87 5 54 87 80 0 6 15 0 13958 48 44 0.15 0 0 3 52 36 0.17 6 24 0.0011 36 11 0.81 0 12 14092 8 70 2 0 22 1098 34 27 112 14 85 10 23 46 746 62 0 4 0 0 0.46 4 15 3 41 0 2 4 0 11305 0 0 864 87 1 0.80 0 0 356 0 0 13 0 42

% in Residue 63 42 20 85 8 100 12 70 54 88 22 78 38 1 31 38 100 81 59 96 100 12 100 100 58

228 Chemical fractionation data for Center (North Dakota) lignite are provided in Table 5.13 [20]. These data illustrate the effect of a water leaching prior to ammonium acetate extraction. This source [20] also contains additional data on Beulah and Gascoyne (Yellow pit) lignites. TABLE 5.13 Chemical fractionation of Center lignite (moisture-free basis) [20].

Element Barium Calcium Iron Magnesium Potassium Silicon Sodium Strontium Titanium

Initial conc. ~t~/lz mf 40 27301 19754 3747 691 11630 2329 400 138 -

v

_

H:,0 0 0 0 0 4 1 21 0 0

% Removed by

N'H4C~HH~O~ 32 82 0 78 21 1 76 94 0

HCI 68 18 26 8 15 21 1 16 8

% in Residue 0 0 74 14 60 77 2 0 92

The chemical fractionation data for Indian Head (North Dakota) lignite are shown in Table 5.14 [21]. TABLE 5.14 Chemical fractionation of Indian Head lignite [21].

Element Aluminum Barium Calcium Iron Magnesium Nickel Potassium Silicon Sodium Titanium

Initial conc .... % Removed by ~t~/~z, mf NHaOAc HC1 4940 0 51 519 38 61 15560 39 12 6810 0 40 9690 19 1 5 0 18 1150 2 1 7060 1 14 6221 76 0 402 0 2

% in Residue 49 1 49 60 80 82 97 85 24 98

Chemical fractionation results for Bryan (Texas) lignite are given in Table 5.15 [33]. Additional data on Beulah lignite are available in this source [33]. Despite differences in geographical source and ash content, extraction of alkali and alkaline earth elements is reasonably consistent between the Bryan lignite data shown in Table 5.15 and Beulah lignite. Larger differences between the lignites are observed when comparing the results of the hydrochloric acid extraction. The solubility of aluminum may reflect its occurrence as a complex with humic acid structures in ways analogous to the reported behavior of aluminum in peat [34] or soil [3 5].

229 TABLE 5.15 Chemical fractionation of Bryan lignite [33].

Element Aluminum Barium Calcium Chromium Copper Iron Magnesium Manganese Nickel Potassium Silicon Sodium Strontium Sulfur Titanium

Initial conc. ~ / ~ , mf 12360 190 7130 21 24 20950 2000 300 40 1970 69400 310 80 19500 1410

% Removed by NI-Lt.OAc HC1 0 22 28 55 62 18 14 43 0 42 0 68 94 6 43 51 0 52 9 8 0 0 75 3 81 19 0 0 0 29

% in Residue 78 17 20 43 58 32 0 6 48 83 100 22 0 100 71

Data for another Texas lignite, Monticello, are given in Table 5.16 [36]. TABLE 5.16 Chemical fractionation of Monticello lignite [36].

Element Aluminum Barium Calcium Iron Magnesium Potassium Silicon Sodium Strontium Titanium

Initial conc. ~t~,l~, mf 9110 100 7320 2200 1180 335 23600 315 130 395

HgO 0 0 0 0 0 21 0 31 0 0

% Removed by NH4OAc 0 52 77 0 70 5 0 31 72 0

HC1 5 26 13 53 4 0 1 0 11 15

% in Residue 95 22 10 47 26 74 99 18 17 85

A procedure for estimating the amount of alkali and alkaline earth oxides in ash which derived from cations associated with the organic matter defines [37] "net calcium oxide" as the amount of acid-soluble calcium not stoichiometrically equivalent to the amount of carbonate estimated from the CO2 yield, viz.,

CaOnet = CaOa.s.- 1.274 CO2

230 where the calcium oxide term on the right-hand side is determined from the acid-soluble calcium. Then the organically derived alkali and alkaline earth oxides is found from ALK = CaOnet + (MgO + Na20 + K20)a.s. where again the subscript a.s. indicates the acid-soluble amounts of the respective species. When a low-rank coal containing significant alkali or alkaline earth elements associated with the carboxyl groups is ashed, some of the oxygen in the ash will derive from the organic functional groups. The amount of this oxygen can be estimated by calculating the amount that would be associated with the exchangeable cations. Oxygen associated with ion-exchangeable elements, Oie, is found from

Ole- (Ocao)ne t + (OMgO + ONa20 + OK20)a.s.

[37] where the subscripts have the same meaning as before. 5.1.3 The major elements (i) Aluminum. A portion of the aluminum in Beulah and Savage (Montana) lignites was found to be organically bound, on the basis of electron microprobe line scans [38]. In a North Dakota lignite, approximately one-third of the aluminum was ion-exchangeable, the balance present in clays [ 11 ]. 27AI nuclear magnetic resonance may provide an opportunity for determining how aluminum is associated with the organic portion of the lignite. The data reported so far indicate the possibility of distinguishing between tetrahedral and octahedral coordination, but no specific details of the organic environment [39]. Chemical fractionation of specific gravity fractions of Glenharold lignite showed about a third of the total aluminum to be extractable with acid, the amount increasing as the gravity of the fraction decreased [10]. This suggests some aluminum complexation by the organic matter, since the aluminum leached from clays with dilute hydrochloric acid is small, and acid-soluble aluminum minerals such as gibbsite and boehmite have specific gravities in the range of 2.8-3.0 (thus concentrating in the heavier fractions). In some Alabama lignites the acid-soluble aluminum amounts to 50% of the total [ 11]. Acid-soluble aluminum does not correlate with acid-insoluble aluminum, indicating that the aluminum dissolving in acid does not represent acid leaching of the clays. Gascoyne lignite (previously treated with ammonium acetate to remove ion-exchangeable cations) was leached with 0.1M disodium EDTA at pH's of 4.5, 5.5, and 7.0. In addition, kaolinite, montmorillonite, halloysite, and illite were also treated with this reagent. The lignite extracts contained up to ten times more aluminum than the mineral extracts [40,41], with little variation due to pH. The higher levels of aluminum removed from the lignite represent aluminum contained in some type of complex with the organic structure. (Some potassium, calcium, and iron

231 were also removed from the lignite by the EDTA procedure, but the nature of the association of these elements was not elucidated.) Aluminum is associated with detrital minerals in Alabama lignite [ 11]. Aluminum occurs in clays in the Calvert Bluff (Texas) lignites [42] and Monticello lignite [36]. An electron microprobe study of Gascoyne and Beulah lignites showed aluminum present only in combination with silica in kaolinite, illite, and mica [30]. As in the case of silicon, some acid-insoluble aluminum is present in all specific gravity fractions, but occurs in maximum amount in the 1.5-1.8 sp. gr. fraction [ 10]. This fraction also contains the highest amount of clays. The acid-insoluble aluminum is accounted for by the kaolinite content [ 11]. (ii) Calcium. Electron microprobe line scans of Beulah and Savage lignites show the calcium to be predominantly organically bound [38]. The dispersion of calcium through lignite as salts of carboxyl groups has been confirmed by extended X-ray absorption fine structure (EXAFS) spectroscopy [43--45] and X-ray absorption near-edge spectroscopy (XANES) [45,46]. Infrared spectra of humic acids extracted from Spanish lignites show calcium ions complexed with the carboxyl groups [47]. The average bond distances in the calcium sites are 0.24 nm [45]. The calcium is coordinated by six oxygen atoms. There is little long-range order around the calcium ions, suggesting that the calcium sites are separated from each other and are well dispersed throughout the samples [48]. Calcium is present in ion-exchangeable form in an Alabama lignite, and is the major exchangeable cation in these lignites [11]. Among Alabama lignites the Ca+2/Mg+2 molar ratios vary from 2.0 to 9.1 [11]. Chemical fractionation of three lignites showed calcium to be the predominant cation [ 16]. This observation was substantiated in studies on a larger number of lowrank coals; calcium represented 1-2.5% of the dry coal, and 75-95% was exchangeable with ammonium acetate [ 14]. About 90% of the calcium in the lignite of the Calvert Bluff Formation occurs in organic association [42]. The vertical distribution of ion-exchangeable calcium in Alabama lignite varies little with position in the seam [14], but correlates with variations in petrographic composition. Specific gravity fractions of Glenharold lignite show that up to 92% of the calcium was extracted into ammonium acetate [ 11 ], but the proportion of calcium soluble in ammonium acetate decreased with increasing specific gravity. Some calcium in Alabama lignite is present in carbonate minerals (calcite) [11,16] and associated with detrital minerals. Although most of the ion-exchangeable calcium is present as the counterion of carboxyl groups, some exchangeable calcium may also be present in gypsum, which is partly soluble in ammonium acetate solution. The non-exchangeable calcium in Calvert Bluff lignites [42] occurs as calcite, clays and phosphates. About 8% of the total calcium in Glenharold lignite is present as gypsum or calcite. The acid-soluble and acid-insoluble calcium may not be different forms of the element, but rather residual material soluble, but incompletely dissolved, in ammonium acetate. (iii) Iron. Some ion-exchangeable iron was observed in a North Dakota lignite [11]. Analyses of Beulah and Savage lignites by electron microprobe line suggest that a portion of the

232 iron in these samples may be organically bound, on the basis of line scans that indicate an appreciable, nearly constant background in addition to occasional spikes indicative of discrete mineral grains [38]. No iron was detected in ammonium acetate extracts of specific gravity fractions of Glenharold lignite [ 11 ]. Supporting evidence also comes from studies of iron salts ionexchanged onto lignite, in investigations of dispersed iron catalysts for direct liquefaction [49]. M6ssbauer spectroscopy of lignite exchanged with iron(II) chloride solution shows about 55% of the iron in clusters of 0-2 nm [49]. After exchange with iron(II) acetate, about 45% of the iron is in clusters >6.5 nm, and about 30% in 0-2 nm clusters. XANES indicates a coordination number of 4.8_+1 in the third shell of atoms around a central iron atom, and 7.6-2_1 in the fourth [49]. Iron soluble in 2M HCI increases with increasing specific gravity of fractions of Glenharold lignite, and may represent dissolution of goethite, jarosite, and melanterite. Some acidsoluble iron, especially in the lighter specific gravity fractions, may exist as complexes which are dissociated by acid [11]. In the Calvert Bluff lignites iron is found as siderite, as well as pyrite and marcasite [42]. Chemical fractionation of an Alabama lignite showed iron to be associated with pyrite [11]. The iron in the 1.80 sp. gr. sink fraction of Glenharold lignite is more than triple the amount calculated from the pyritic sulfur (1854 and 560 ppm, respectively). This implies a second, highspecific gravity iron mineral in the lignite, but it was not specifically identified. Iron is present in this lignite in minerals that concentrate in the heaviest specific gravity fractions [ 11 ]. Most of the iron in Beulah and San Miguel (Texas) lignites is present as pyrite, as indicated by MOssbauer spectroscopy [46]; the same technique showed 100% of the iron in a sample of Beulah-Zap lignite to be present as pyrite, with no indication of iron sulfates, iron-bearing clays, or siderite [50]. Determination of pyritic sulfur by extraction with dilute nitric acid, evaporation of the extract to dryness, dissolution of the solid in hydrochloric acid, and analysis of the resulting solution for iron and sulfur [51] allows calculation of pyritic sulfur from the equation

Spy-" 1.145(FEN- Fell) where Spy is the pyritic sulfur, FeN the percentage of iron in the nitric acid extraction, and Fe H the percentage of iron soluble in the hydrochloric acid. For most coals the sulfur determined directly as pyritic sulfur and that calculated from the so-called pyritic iron agreed quite well. For lignites, however, the difference between determined and calculated pyritic sulfur was greater than for any other coals, leading to the inference that not all of the iron in lignite was in the form of pyrite, but that a portion of the iron is present in some other form soluble in dilute HNO3. This observation was not followed up to investigate the nature of the acid-soluble iron. In Alabama lignite the acid-soluble iron correlated with the pyrite, suggesting that the acidsoluble material was oxidation products of the pyrite [11 ]. However, the correlation does not exist for Darco (Texas) lignite, suggesting that in this lignite some other form of acid-soluble iron

Occurs.

233 (iv)

Magnesium. Electron microprobe line scans of Beulah and Savage lignites suggest the

magnesium to be present in organically bound form, based on a relatively constant composition across the sample rather than sharp peaks indicative of mineral grains [38]. Chemical fractionation of three lignites showed magnesium to be the second-most dominant cation [16]. Magnesium is present primarily in ion-exchangeable form in Alabama lignite, second only to exchangeable calcium in these lignites [11]. In a North Dakota lignite the ion-exchangeable form predominated [11]. Most of the magnesium in specific gravity fractions of Glenharold lignite is present as ionexchangeable form. In some lignites magnesium is present totally in an ion-exchangeable form [16]. Electron microprobe analyses of Gascoyne and Beulah lignites showed magnesium in combination with aluminosilicates and with calcium carbonate [30]. Some magnesium in Alabama lignites also occurred in carbonate minerals and with detrital minerals [11]. The presence of magnesium in detrital minerals results from its substitution in the clay structures. The partial insolubility of magnesium and its presence in heavier specific gravity fractions was also attributed to its presence in the lattices of clays [10]. The acid-insoluble magnesium in specific gravity fractions of Glenharold lignite had a similar distribution as the acid-insoluble silicon and aluminum, suggesting that it is tightly bound in clays. In Alabama lignite the acid-insoluble magnesium correlated with the clays [ 11]. Magnesium occurs mainly with clays and micas in the lignite of the Calvert Bluff [42]. (v)

Nitrogen. The possible occurrence of inorganic nitrogen as nitrate has been a subject of

controversy. Nitrates can be extracted from lignites with water. The extraction process appears to be complex, and depends on the interactions among the ion-exchange behavior and concentrations of the various ions present in the lignite, and on the adsorption properties of the lignite itself [52]. The difficulty of water extraction of nitrates suggests some special interaction, such as preferential adsorption of nitrate on clays or encapsulation of nitrates in coalified plant structures. X-ray powder patterns of Beulah lignite include several lines characteristic of calcium nitrate, corroborating the presence of alkaline earth nitrates in water extracts of Beulah lignites [52]. Nitrate has been reported in the low-temperature ash of lignites [53,54]. Sodium and alkaline earth nitrates have been identified in the low temperature ashes of lignites, but not in lowtemperature ashes of bituminous coals or anthracites [55]. The possibility of this nitrate being mainly an artifact of the ashing process was first raised because of the inability to detect sodium nitrate in the water extracts of North Dakota lignites [.56]. Fourier transform infrared spectroscopy (FTIR) of the low-temperature ashes of lignites, washed lignites, lignite extracts, and model compounds led to the assignment of the band at 1384 cm-1 as evidence that the fixation of organic nitrogen occurs during the ashing process to produce nitrate. The formation of nitrate from organic nitrogen during low temperature ashing was demonstrated by ashing a mixture of 3-hydroxyl-6methylpyridine, sodium carbonate, and graphite. The presence of nitrate in the resulting ash was confirmed by FTIR analysis, specifically by the presence of the 1384 cm-1 peak [57]. Fixation depends on the presence of the carboxyl groups in the lignite and is affected by the type of metal

234 ion associated with the acid groups. Spectra of water extracts show that a small quantity of nitrate may be present in the parent lignite, although most of the nitrate in the ash is formed as an artifact of the ashing process [57]. (vi) Phosphorus. The average phosphorus content in five Montana lignite samples was 0.32% on a dry basis, with a range of values of 0.004% to 0.058% [58]. For eight North Dakota lignites, the comparable values were 0.009% and 0.002-0.017%, respectively [58]. The low concentration of phosphorus in most lignites, and its evident lack of significant affect on any lignite technology, likely are the reasons why the geochemistry of phosphorus in North American lignites has been studied so little. (vii) Potassium. In many lignites potassium is present both in detrital minerals and in ionexchangeable form [11,16]. EXAFS shows that some of the potassium in lignite is bonded to carboxyl groups [48]. Chemical fractionation of specific gravity fractions of Glenharold lignite showed that ion-exchangeable potassium decreases sharply with increasing gravity, but the amount of acid-soluble potassium increased with gravity [10]. In this lignite about 44% of the total potassium was ion-exchangeable and 34% bound in minerals such as illite. Potassium is associated primarily with the detrital minerals in an Alabama lignite [ 11]. Acidinsoluble potassium in Alabama lignite correlated with the clays, suggesting that potassium is present in illite or in a mixed-layer clay [11]. Potassium also associates with clay minerals in Monticello lignite [36]. In the Calvert Bluff lignites potassium is found in clays, mica, and feldspars [42]. XANES of San Miguel and Beulah lignites shows potassium present almost entirely as illite clays [46] Potassium in Gascoyne and Beulah lignites was present in aluminosilicates, illite, mica, and potassium feldspar [30]. (viii) Silicon. Chemical fractionation of specific gravity fractions of Glenharold lignite showed some acid-insoluble silicon present in all gravity fractions [ 10]. The highest concentration of acid-insoluble silicon occurred in the 1.5-1.8 sp. gr. fraction; this fraction also contains the highest concentration of clays. The highest concentration of quartz was found in the 1.80 sp. gr. sink fraction. The acid-insoluble silicon is contained in quartz and kaolinite [11]. Silicon is associated with detrital minerals in an Alabama lignite [11]. Most of the silicon in Calvert Bluff lignites occurs in quartz and clays [42], as is also true for Monticello lignite [36]. (ix) Sodium. Sodium is the only element extracted from lignites in appreciable quantities by water leaching [59]. It was assumed to be present as the chloride, sulfate, or as water-soluble humate salts. Some data on the amounts of the total sodium extracted by water are shown in Table 5.17 [59]. Press dewatering of Falkirk (North Dakota) lignite at 1 l0 MPa expressed 20 mL of water from a 550 g sample. The water had a pH of 8.1 with sodium the dominant cation, at 3600 ppm [60--62]. (Other cations observed were Ca, at 430 ppm, and Mg, 210 ppm.) The dominant anion was sulfate (10480 ppm); carbonate was not analyzed. A similar experiment with Gascoyne lignite expressed water containing 3200 ppm Na, 510 ppm Ca, and 360 ppm Mg [63]. These water analyses, which presumably reflect the composition of water contained in the pores of the lignite,

235 TABLE 5.17 Water-extractable sodium in lignites [59]. Lignite Beulah Center Gascoyne (White) Gascoyne (Yellow)

Initial Na Content, ppm 5800 2300 5100 700

% Extracted by Water 14 21 18 42

are remarkably similar, especially in light of very significant differences in the ash value and ash composition between the lignites. Falkirk lignite had 8.2% ash containing 30.3% CaO, and supposedly 0.0% reported Na20, whereas the Gascoyne sample had 13.5% ash containing 12.4% CaO and 3.6% Na20. The complete analyses of the expressed pore water from the Falkirk and Gascoyne lignites are shown in Table 5.18 [59]. TABLE 5.18 Analyses of water expressed from Falkirk and Gascoyne lignites, ~tg/mL [62].

Aluminum Barium Boron Calcium Chloride Copper Iron

Magnesium Manganese Molybdenum Nitrate Potassium Silicon Sodium Strontium Sulfate Zinc pH

Falkirk 2 0.1 10 430 13 0.1

Gascoyne <0.3 0.1 24 510 17 0.1

<0.1

<0.1

210 0.3 0.7 4 25 19 3600 8 10500 0.1 8

360 0.7 0.7 * 26 22 3200 13 10080 0.2 7.5

*Not reported Groundwater compositions in the Kinneman Creek (North Dakota) and Hagel beds as well as sand and silt units below the Hagel show a genetic similarity among the water compositions in the various units [64]. The groundwater types range from Na to Na,Ca-HCO3 and from SO4 to SO4-HCO3 types. Distilled, demineralized water removed essentially all of the sodium from an unidentified lignite from the area of Minot, North Dakota [52]. Sodium is entirely or very largely present in ion-exchangeable form in lignites [11,16,56].

236 Electron microprobe analyses of Beulah and Savage lignites show the sodium to be organically bound [38]. 23Na nuclear magnetic resonance has shown that the sodium ion is strongly hydrated and associated with a single organic ligand [39]. The major mode of occurrence of sodium in lignite is as the salt of carboxylic acids. This sodium likely entered the lignite by ion exchange with groundwater, either in the original coal-forming environment or after burial. Distinct differences among vitrain, attritus, and fusain are shown in Table 5.19 [65]. TABLE 5.19 Sodium content of ashes from lignite lithotypes [65]. Lignite Glenharold

Baukol-Noonan

Lithotype Vitmin Vitmin Attritus Fusain Vitrain Attritus*

% Na~O 10.3 14.8 7.4 5.0 3.9 1.5

* Fusain-rich

Generally vitrain tends to contain more sodium than the other lithotypes. Since vitrain also usually has the lowest ash value, these observations provide further evidence for the organic association of sodium. Comparisons of the infrared spectra of a high-sodium vitrain and a low-sodium fusain show peaks in the spectrum of the vitrain at 1450 and 1520 cm-1, which are assigned respectively to the symmetrical and asymmetrical vibrations of the ionized carboxyl group, --COO- [65]. The observation suggests that a higher proportion of the carboxyl groups in the vitrain may be in the ionized (i.e., salt) form. Chemical fractionation of specific gravity fractions of Glenharold lignite showed a marked concentration of sodium in the lighter fractions [10]. Only about 1% of the total sodium occurred in the 1.80 sp. gr. sink fraction. About 96% of the total sodium was extractable in ammonium acetate, but for a series of specific gravity fractions the proportion soluble in ammonium acetate decreased as specific gravity increased [11]. Nevertheless, only about 2% of the sodium is acidinsoluble, and, of that, most was in the heaviest specific gravity fraction. The observation that 96% of the sodium is extracted into ammonium acetate is consistent with the virtual absence of sodiumcontaining minerals (except for traces of plagioclase) from this lignite. Small amounts of sodium (generally less than 5% of the total sodium) were observed by scanning electron microscopy to be in association with aluminosilicates in Gascoyne and Beulah lignites [30]. Sodium in mineral matter occurs principally in association with clays, principally smectites, although some may be bound in tectosilicates such as albite, NaAISi308, or analcime, NaAISi206.H20. In clays, sodium is predominantly associated with ion-exchange sites, so its concentration in the clay is affected by the composition of groundwater. In tectosilicates, sodium is

237 much more tightly held so is not affected by groundwater. Sodium in tectosilicates arrived in the coal-forming environment as part of detrital tectosilicate particles. (x)

Sulfur. Electron microprobe analyses of Beulah and Savage lignites show that the

sulfur is largely well dispersed through the lignite particles, on the basis of line scans taken across the samples [38]. Unfortunately the data were not correlated with the relative amounts of organic and pyritic sulfur in the samples. Data on the forms of sulfur in a suite of North Dakota lignites are presented in Table 5.20 [66]. TABLE 5.20 Forms of sulfur in North Dakota lignites [66]. Lignite Gascoyne Dunn County Beulah-Zap Center Stanton Dickinson Velva Williams County

Sulfatic 0.04 0.18 0.05 0.02 0.01 0.08 0.02 0.32

F'yritic 0.71 0.38 0.72 0.21 0.04 4.73 0.11 2.82

Organic 1.29 1.01 0.42 0.47 0.65 0.78 0.34 0.66

Distribution of sulfur forms in Dakota Star (North Dakota) lignite is shown in Table 5.21, on an asanalyzed basis, to illustrate the variability among different samples from the same mine [67]. TABLE 5.21 Variation of sulfur forms for lignite samples from the same mine [67]. Sample 1 2 3

Total,% 0.99 1.00 0.88

Sulfides

Pyritic, % 0.28 0.38 0.42

Sulfatic, % 0.38 0.34 0.08

Organic, % 0.32 0.32 0.27

(i.e., as distinct from pyrite) were not found in the untreated lignite. Sulfatic sulfur is a

minor component of the sulfur in Texas lignites [68]. Retention of sulfur during ashing complicates both the determination of ash and of sulfur. German lignites heated with the Eschka mixture (a 2:1 mixture of magnesium carbonate or oxide and sodium oxide) lost both organic and inorganic sulfur compounds due to smoldering during heating [69]. Uniform heating in an electric muffle furnace eliminated the problem. Only the sodium carbonate in the Eschka mixture was active in retaining sulfur, and indeed accurate sulfur determinations could be performed using sodium carbonate alone with slightly modified technique.

238 Magnesium oxide in the Eschka mixture appears simply to be helping provide a more uniform melt. Beulah lignite contains 128+4 ~tg of elemental sulfur per gram of lignite (dry basis) [70]. The quantity represents about 1.6% of the total sulfur in the sample, or 2.5% of the organic and 3.7% of the inorganic sulfur. (xi) Titanium. Titanium has been found to be present in organic complexes in lignites. Acidsoluble titanium, presumed to be present as a coordination complex, was enriched at the seam margins. This enrichment is the basis for presuming that at least this form of titanium came in to the lignite in solution and was trapped at the seam margins by reaction with the organic functional groups in the lignite [ 11]. Principal components analysis for data from Alabama lignites has shown that acid-soluble and acid-insoluble titanium both load on the same factor, suggesting that the acidsoluble titanium arose from alteration of ilmenite [ 11 ]. Elemental associations in five lignite drill cores showed that 30-50% of the titanium was extractable by dilute acid [ 11]. This extractable portion of the titanium was inferred to be incorporated in an organic complex. A major portion of the titanium was acid-soluble in four low-rank coals, including Texas, Alabama, and North Dakota lignites [ 14]. The acid-soluble titanium concentration peaks at seam margins, but is low or even zero in floor and roof rocks. In contrast, acid-insoluble titanium also shows peak concentrations at seam margins, but has high concentrations in the floor and roof as well, further substantiating the idea that acid-soluble titanium has been complexed by the organic portion of the lignite. Specific gravity fractions of Glenharold lignite showed no ion-exchangeable or acid-soluble titanium [10]. Common titanium minerals--rutile, brookite, anatase, and ilmenite--are all insoluble in dilute hydrochloric acid. However, the amount of titanium increased with decreasing specific gravity of the fraction, which could not be the case if the principal source of titanium were one or more of these minerals. The inverse relationship of acid-insoluble titanium concentration was attributed to the existence of a stable titanium complex with the organic matter. A very small fraction of the titanium in Texas lignite (Zavala County) can be extracted into various organic solvents [71]. The amount ranges from 0.03% of the total titanium extracted into dioxane to 0.47% taken into dimethyl sulfoxide. The fraction present in ion-exchangeable form is less than 0.2% of the total, suggesting that only a very small portion of the titanium is present as carboxylate salts. When this lignite is separated into density fractions, the titanium concentrations increase with increasing density, such that 93% of the titanium is present (cumulatively) in the fractions of density > 1.86. These observations suggest that titanium is associated with the mineral matter. Titanium is associated with detrital minerals in Alabama lignite [11]. Titanium can replace aluminum in aluminosilicate minerals, and was observed combined with aluminosilicates in Beulah and Gascoyne lignites, though it was thought that the association may have arisen from rutile intrusions [30]. Titanium is found in rutile and anatase in the lignite of the Calvert Bluff [42], and in rutile in Monticello lignite [36]. Titanium occurs as elongated grains of TiO2 less than 10 ~tm long, and associated with quartz and other minerals. A significant amount of the titanium was

239 associated with the aluminosilicates in the low temperature ash, associated with the aluminosilicates as finely dispersed TiO2. This finely dispersed TiO2 represents anatase produced authigenically, while the discrete grains represent detrital rutile. 5.2 MINERALS IN LIGNITES

5.2.1 Introductory overview Float-sink separation of fourteen low-rank coals--lignite and subbituminous--using 50x140 mesh material in carbon tetrachloride (specific gravity 1.55) allows identification of minerals by optical microscopic point count analysis of the sink fraction. Commonly occurring minerals were quartz, calcite, kaolinite, pyrite and gypsum; minor constituents included illite, montmorillonite, jarosite, hornblende, muscovite, barite, oligoclase, andesine, albite, orthoclase, dolomite, aragonite, hematite, biotite, szomolnokite, and marcasite [72]. The relationship of minerals in lignites of the Sentinel Butte Formation (North Dakota) lignite to the minerals in overburden and underclay is illustrated by the data in Table 5.22 [73,74]. TABLE 5.22 Minerals in overburden, lignite, and underclay of Sentinel Butte Formation [73,7"

Alkali feldspars Augite Barite Biotite Calcite/dolomite Chlorite Gypsum Hematite Hornblende Illite Kaolinite Montmorillonite Plagioclase Pyrite Quartz Volcanic glass

Overburden Common Minor Common Common Minor Common Common Abundant Abundant Abundant Minor

Lignite Minor Minor Minor Minor Minor Abundant Common Minor Abundant Minor Minor Common Abundant

Underclay Minor

Minor

Minor Abundant Abundant Minor Abundant

The major detrital minerals include montmorillonite, quartz, plagioclase, alkali feldspar, biotite, chlorite, and volcanic glass. Pyrite, gypsum, hematite, siderite, and possibly calcite formed via post-depositional processes. Minerals observed most frequently in Beulah lignite (using scanning electron microscopy, SEM) were quartz, clays (particularly kaolinite, montmorillonite, and illite) and pyrite [75]. Less common minerals were magnesium silicates (talc), rutile, hematite, and various carbonates. Rare occurrences included zircon, feldspars, barite, and gypsum. Minerals observed to occur in four lignites, on the basis of SEM examination [5], are listed in Table 5.23.

240

TABLE 5.23 Minerals observed in four lignite samples by scanning electron microscopy [5]. Anatase Apailte Barite Calcite Ca-Montmorillonite Cassiterite Celestite Chalcopyrite

Clausthalite Crandallite Diaspore Epidote Feldspars Galena Gypsum Illite

Ilmenite Kaolinite Marcasite Micas Mixed layer clays Monazite Pyrite Quartz

Rutile Siderite Sphalerite Sphene Tourmaline Xenotime Zircon

Lignites are notoriously difficult to decompose during low-temperature ashing. Suggested reasons include the possibility of surface water destroying peroxides which are responsible for oxidation of the organic structure, or the possible blocking of reactive sites by water molecules [76]. Identification of minerals in the low-temperature ash can be performed by combining X-ray diffraction and infrared spectroscopy. However, the bassanite and calcite X-ray peaks, at 29.8 ~ and 29.4* 2 0 respectively, overlap. This problem can be remedied by heating the low-temperature ash to 500 ~ to convert bassanite to anhydrite. Quartz, calcite, and pyrite can be determined by X-ray diffraction; kaolinite and anhydrite by infrared spectroscopy. Other clay minerals (e.g., illite and montmorillonite) can be estimated by a normative calculation. The amounts of SiO2 and A1203 in kaolinite and quartz are calculated and subtracted from the total amounts; the balance remaining can then be apportioned among the other clay minerals by presuming that they contain 25% A1203 and 50% SiO2. Even after the normative determination of these so-called "other clays," there may still remain a portion of the low-temperature ash unaccounted for. Results of applying this procedure to three lignites are shown in Table 5.24 [ 16,17]. TABLE 5.24 Mineral matter in low-temperature ashes [ 16,17]. State Seam LTA (% of dry coal) Kaolinite Quartz Pyrite Calcite Anhydrite Other clays Unaccounted for

N. Dakota Hagel 11.5 5 9 * 11 21 17 37

Texas Darco 20.5 41 12 * 2 14 7 14

*Pyrite was detected in amounts too small to quantify.

Montana Fort Union 17.5 20 19 * 16 l0 8 27

241 Qualitative analysis of the low-temperature ash of Baukol-Noonan lignite showed the following minerals: kaolinite, illite, chlorite, calcite, coquimbite, barite, bassanite, quartz, and plagioclase [ 19]. Computer-controlled scanning electron microscopy analysis of Beulah lignite indicated the minerals listed in Table 5.25, shown as a percentage of the total minerals in the lignite [77]. The total mineral content was 4.8%. TABLE 5.25 Minerals identified in Beulah lignite by computer-controlled scanning electron microscopy [77]. Quartz Iron oxide Aluminosilicates Ca-aluminosilicates Fe-aluminosilicates K-aluminosilicates Pyrite

17.5% 1.6 40.8 0.2 0.1 0.9 27.5

Gypsum Barite Calcite Rutile Pyrrhotite Si-rich minerals Unknown

1.6% 0.9 0.1 0.3 0.7 0.4 6.7

Minerals identified in a sample of Scranton (North Dakota) lignite by Fourier transform infrared analysis of the low-temperature ash are shown in Table 5.26 [78], The data are shown as weight percent of the low-temperature ash. TABLE 5.26 Minerals identified in Scranton lignite by FrlR of low-temperature ash [78]. Kaolin Mica Illite Mixed layer clays Montmorillonite Feldspar Chlorite Miscellaneous clays

1.1-2.9% 0 0.0-1.2 0 15.5-20.4 0-13.5 0 0.0-2.7

Quartz Fe sulfide Fe oxide Fe sulfate Siderite Calcite Gypsum Dolomite

15.5-20.9% 4.8-13.1 0.0-0.3 0 2.4-5.8 0 34.4-49.2 0

A survey of 86 coals was used to assess the potential of calculating the distribution of minerals from the high temperature (i.e., 750"C) ash composition, the results being compared with mineralogical analysis by X-ray diffraction and infrared spectrometry on the low-temperature ash [52]. It was concluded that the process is not applicable to lignites (or, for that matter, to subbituminous coals) because 30 to 50% of the ash-forming constituents are present as cations associated with carboxyl groups or are present as coordination complexes. In the discussion which follows, the order of treatment of mineral families follows standard

242 mineralogical texts (e.g., [79].) 5.2.2 Sulfide minerals (i) Pyrite. Pyrite is the predominant sulfide mineral in lignites. Pyrite contents are low in North American lignites [16], relative to coals of higher rank. Pyrite begins forming syngenetically as small framboids during the peat stage of coal formation [80,81]. The framboids occur as isolated blebs and do not follow cracks or fissures. Epigenetic pyrite mineralization occurs after the coal has been formed, and is brought about by ironcontaining solutions passing through the coal. Some evidence for epigenetic pyrite mineralization is evident as pyrite deposited in woody tissue in the lignite [31,75]. Epigenetic pyrite is concentrated in the basal sections of the seam, is massive in form, and follows cracks and fissures in the lignite [75]. Pyrite nodules in the 6 to 13 ~tm size range have been observed in Beulah lignite; these nodules had grown between layers in the lignite [82]. Massive epigenetic pyrite occurs also in Ravenscrag lignite [83]. Pyrite occurs in Fort Union lignites as material deposited in cracks or cleats. The size of pyrite particles varies widely, from microscopic crystals to nodules of more than 2.5 cm diameter [1]. The pyrite originates from bacterial action on solutions containing iron and sulfur. Sulfur balls, which contain pyrite, gypsum, and lignite, range from 2.5 to over 15 cm in diameter [1]. The Fort Union lignites developed primarily in fresh water environments and, consequently, contain relatively small amounts of pyrite [84]. Pyrite is an authigenic component of the lignites of the Sentinel Butte Formation because it is not present in the overburden or underclay (cf. Table 5.22) [73]. Pyrite was frequently observed in the 1.55 sp. gr. sink fractions Beulah, Glenharold, Savage, and Gascoyne lignites [10,85-88]. Nearly all of the pyrite observed (by scanning electron microscopy with electron microprobe, SEM-EDX) was syngenetic [80,81], seen as small rounded blebs or isolated framboids throughout the seam. Similarly, most of the pyrite in Hat Creek (British Columbia) lignites is syngenetic [89]. Pyrite was found primarily as 20-30 ~m diameter framboids in the North Dakota lignites [29,31,88]. Most of the framboids are well dispersed through the lignite, so it is likely that framboidal pyrite is authigenic. Epigenetic pyrite occurring in cracks, crevices, and cell cavities exists in both microscopic (30-300 ~tm) and megascopic (10-50 mm) sizes. Some bands of massive pyrite occur in basal sections of the Beulah-Zap seam, where the pyrite follows the woody structure of the lignite [31]. Pyrite was most commonly found associated with vitrain, as cell fillings or 20-60 lxm diameter framboids. In vitrain, pyrite is predominant among the Minerals, accounting for about 60% of the total Minerals [90]. (The capitalized terms Minerals and Inorganics follow Australian brown coal nomenclature, e.g., [91].) Pyrite in vitrain is framboidal and arose syngenetically in the peat or early diagenesis stage [80]. In fusain, epigenetic pyrite is found filling cell cavities, and formed from aqueous solutions migrating through the relatively porous fusain [90]. Pyrite is observed filling cell voids in fusinite in Saskatchewan lignites [92], and, in Moose River Basin (Ontario) lignite, not only infilling cell lumens but also replacing cell

243 walls and infilling cracks [93]. Pyrite distributes in the Beulah lignite fairly evenly throughout the seam [75,88], with occasional spikes of concentration, as, in this case, a very high frequency of pyrite 1 m above the base of the seam [75]. In samples collected from the Freedom (North Dakota) mine, pyrite in the uppermost lithologic layer was found as small framboidal masses about 50 ~tm in diameter [41]. This pyrite may be the result of microbial activity [41]. Near the base of the seam, pyrite is found as fillings in cell cavities and fissures, suggesting possible precipitation from groundwater. Pyrite was a major mineral in the 1.80 sp. gr. fraction of Glenharold lignite [11]. In this sample, pyrite occurred predominantly as impregnations in the lignite. Many of the lignite particles impregnated with pyrite were coated with melanterite (FeSO4"7H20), formed by weathering of the pyrite. The melanterite may be an artifact of the sampling process, since it was observed to form rapidly in the laboratory. In comparison, jarosite and goethite likely arose from oxidation of the pyrite in situ. The Hagel seam lignite at the Baukol-Noonan mine is characterized by abundant, thick vitrain lenses which derive from coalified logs and stumps of conifers. Often the tree rings in these coalified pieces are characterized by pyrite mineralization. The pyrite usually associates with wood or ray cells. Iron sulfides can generally occur as joint fillings, nodules, or irregularly shaped bodies in the seam. On a microscopic level some iron sulfides are observed in association with medullary ray cells and summer wood of conifers [94]. Infilling of medullary ray cells by pyrite may represent a significant fraction of the total pyrite accumulated in a lignite seam. Pyrite occurs in less than half the samples of Moose River Basin lignite [93]. In pyrite-rich samples of this lignite, the pyrite may replace the whole wood structure, infill cell cavities, coat organic surfaces, or occur in cracks. Pyrite is commonly associated with gypsum. Authigenic pyrite occurs as syngenetic deposits replacing the original plant structure by infilling cell cavities. Pyrite may also occur as infillings of shrinkage cracks or fractures, in which case there is no relationship to cell morphology or structure of the organic portion of the lignite. In such instances the pyrite is rather coarse grained compared to syngenetic pyrite. The Sabine River--Stone Coal Bluff lignites of Louisiana contain abundant pyrite in the form of nodules and pyritized wood. A pyritized log 90 cm long and 30 cm in diameter was found in this seam [95]. (ii) Pyrrhotite. Pyrrhotite has been identified by SEM-EDX analysis in Martin Lake lignite [96]. This mineral generally occurred in the vicinity of pyrite and was found as lenticular masses, spherical nodules, or framboids. An unusual occurrence was a framboid having a pyrite core rimmed with pyrrhotite. (iii) Minor sulfides and selenides. Millerite was reported in isolated samples of Moose River Basin lignite [93]. Sulfide or selenide minerals tentatively identified in Calvert Bluff lignites included chalcopyrite, clausthalite, galena, marcasite, and sphalerite [42]. Greigite, Fe3S4, sometimes called the thiospinel of iron, occurs as a trace component in one of nineteen samples of Poplar River (Hart seam, Saskatchewan) lignite [92].

244 5.2.3 Oxide minerals Although quartz is of course an oxide, it is discussed below in the subsection on silicate minerals. (i) Hematite and magnetite. Hematite was observed in the 1.55 sp. gr. sink fractions of Beulah and Velva (North Dakota) lignites [85,86]. It was not found in comparable samples from four other Northern Great Plains lignites. Thin plates of hematite, about 15 ~tm diameter, were found as a minor constituent of Beulah-Zap lignite [29,31]. Some hematite particles were seen lining pores and cell cavities, an occurrence which suggests an authigenic origin. Hematite occurs in significant quantities in Beulah lignite [88]. Magnetite occurs with high frequency in the CC14 sink fraction of Beulah lignite [86]. (ii) Gibbsite and boehmite. The presence of gibbsite in Moose River Basin lignite was inferred from unusually high values of AI:Si (8:1) determined in the X-ray photoelectron spectroscopic examination of some samples [93]. Gibbsite may be an indicator of intense weathering. Gibbsite is found only in isolated samples, associated with quartz [93]. It appears to be an authigenic coating on the quartz grain surface [93]. Boehmite has been reported in some samples of Estevan (Saskatchewan) lignite, associated with the calcium oxalate mineral weddellite in six of seventeen samples analyzed, but at appreciable amounts (4-6% of ash) in only three [83,92]. (iii) Rutile and anatase. Small amounts of rutile and rutilated quartz were observed as weathered grains 5-25 l~m diameter in Beulah-Zap lignite [29,31]. The rutile grains are probably heavy mineral detritus. Rutile has been reported as a trace mineral in some Beulah-Zap lignite samples [87] and as a significant occurrence in others [88]. Rutile usually occurs as an accessory mineral in detrital mineral matter [88]. Needle-shaped crystals of rutile (and possibly anatase) were observed in Texas lignite samples obtained from drill cores in four Texas counties [97]. Rutile was observed in the Calvert Bluff lignites of Texas [42]. Discrete grains of detrital rutile, of about 10 ~tm size, have been observed in Wilcox Group lignite from Zavala County, Texas [71]. Anatase was observed in Alabama lignite [11]. Its formation may be due to the removal of iron from ilmenite (which, for example, has been observed in the roof of Darco lignite) by leaching with mildly acidic water, leaving a hydrated titanium dioxide which is then converted to anatase. Anatase was tentatively identified in a Texas lignite from the Calvert Bluff formation [42]. Finely disseminated particles of authigenic anatase are associated with the clays and biogenic structures in Zavala County lignite [71]. Anatase occurs infrequently in Saskatchewan lignites. In seventeen samples of Estevan lignite, anatase was observed as a trace component of the ash in one sample, and at 7% in a second [83,92]. It was also observed at 7% of the ash in one of thirteen samples from the Ferris seam outcrop [92]. (iv) Zircon. Zircon is a rare occurrence in Beulah lignite [88]. Small (10-25 0m) weathered grains of zircon were observed in Beulah-Zap lignite [29], including a 17 ~tm particle that still retained its evident crystal morphology [31]. Zircon is well known to have a high-temperature igneous origin [98]. Euhedral zircon crystals are found as a component of volcanic ash in coal

245 partings, and it is usually associated with sedimentary rocks as a heavy mineral detrital constituent. Thus the zircon observed in the Beulah-Zap lignite is almost certainly detrital. Zircon was tentatively identified in the lignite of the Calvert Bluff [42]. Zircon has also been observed in Tertiary brown coals of the Weisse Elster Basin in Germany [99]. 5.2.4 Carbonate minerals (i) Calcite. Calcite is the dominant carbonate mineral in most North American lignites [100]. For example, it constitutes 10--48% of the ash of Estevan lignites, and 4--42% of the ash of Poplar River lignites [92]. Exceptions exist, however. Thus, no calcite was observed in the lowtemperature ash of Scranton lignite [78], nor was it detected in a sample of Beulah-Zap [ 101] or in most samples of Moose River Basin lignites [93]. Calcite occurs in Fort Union lignites in cracks or cleats in the lignite. It originated by precipitation from solution after the lignite had been formed [1]. Carbonate minerals in Beulah lignite are authigenic because they can be seen lining cracks in the lignite or in cavities and pores in the carbonaceous structure [31,75,88]. Similarly, calcite, when it occurs, infills cell lumens in Moose River Basin lignite [93]. Calcite is the predominant mineral phase in the 1.55 sp. gr. sink fraction of Baukol-Noonan lignite, and also occurs in Glenharold lignite [85]. It was not observed in four other lignites in the same study. Large calcite nodules are present in Freedom lignite [ 102]. Calcite occurs as plates 25-40 gm in diameter in crevices or cracks in Beulah lignite, and in massive patches on the surface of the lignite [88]. Calcite was observed in a lignite of the Calvert Bluff formation [42]. Small amounts of calcite occur in Moose River Basin lignites, in isolated samples [93]. Detrital calcite and dolomite were observed in Glenharold lignite [10]. Calcite and dolomite both occur with low frequency in the CC14 sink fraction of Beulah lignite, and with medium frequency in the same fraction of Savage lignite [86]. Calcite, magnesite, and dolomite particles of 25-40 ~m diameter were found as crevice or cleat growths in Beulah-Zap lignite [29,31]. The textural form suggests an authigenic origin. Trace amounts of aragonite occur in samples of Estevan lignite [83,92]. (ii) Dolomite. Dolomite was found as a major constituent of the low-temperature ash of a Montana lignite [103]. Large lenticular inclusions of dolomite have been observed in European lignites, with MgO contents of 12-38% and CaO/MgO ratios of 1.30-1.60 [104]. Calcite, magnesite, and dolomite occur in small amounts in Beulah lignite. Magnesite and dolomite occur mainly as crevice or cleat growths [88]. Dolomite is seen in only a few samples of Saskatchewan lignites, but in those cases it accounts for a large percentage of the ash, e.g., 28% of the ash in one of seventeen samples of Estevan lignite, and 50% of the ash in one of nineteen samples of Poplar River lignite [83,92]. Dolomite is also observed in some Hat Creek lignites [89]. (iii) Siderite. Siderite occurs as a trace component of Moose River Basin lignite, being found in isolated samples [93]. It also occurs in a few samples of Estevan and Poplar River lignites, accounting for up to 7% of the ash in occasional samples of Estevan lignite [83,92].

246 Siderite is also occasionally a major constituent of Hat Creek lignite [89]. Siderite was observed by X-ray diffraction in samples of overburden from the Beulah mine; its presence in the lignite itself would account for the observation of iron soluble in dilute hydrochloric acid during chemical fractionation [82]. Siderite was observed in the lignite of the Calvert Bluff formation of Texas [42]. (iv) Minor carbonates. Witherite [83,92], rhodocrosite [89], and manganoan calcite [89] have been reported to occur in Canadian lignites. 5.2.5 Phosphate minerals Monazite has been reported as a trace mineral in Beulah-Zap lignites [87]. It was tentatively identified in the lignite of the Calvert Bluff formation of Texas, as were the phosphate minerals crandallite and xenotime [42]. Apatite was found filling the cell voids in a sample of Magothy lignite [105]. In this particular lignite, the amount of apatite appears to be significant. 5.2.6 Sulfate minerals (i) Gypsum, bassanite, anhydrite, and selenite. In the Fort Union lignite, gypsum and selenite are among the most abundant of minerals deposited by precipitation from solution into cracks or cleats in the lignite after coalification was well advanced. Gypsum crystals of 5-20 mm are common occurrences at the Beulah mine [29]. The development of gypsum is authigenic, via conversion of organic sulfur and calcium carboxylates. Gypsum is commonly encountered in the North Dakota lignites [84], though occasional reports have asserted the contrary [88]. Gypsum has been observed by X-ray diffraction in the sink fraction of CC14float-sink separation of Glenharold lignite [85]. It is also the most common sulfate in Saskatchewan lignites [92], and is common in Hat Creek lignites [89]. Gypsum was observed in samples of lignite from the Calvert Bluff Formation [42]. Gypsum occurs rarely in Moose River Basin lignite, being found in less than half the samples examined. Its presence is attributed to recent weathering of pyrite [93]. Because of this origin, the occurrences of gypsum and pyrite are spatially related. The origin of gypsum by pyrite weathering may account for the very high concentrations of gypsum occasionally reported. For example, the minerals of the Ferris seam outcrop contain up to 81% gypsum [92]. Selenite occurs in sediments when opportunities exist for slow crystallization from solution, the slow crystallization thus providing the necessary time for the large, elongate crystals to form. The occurrence of selenite in Glenharold lignite was considered inferential evidence that during the development of this particular seam (part of the Hagel bed) extensive dehydration had occurred [ 10]. Selenite crystals 7.5 cm long have been reported from the Harmon (North Dakota) bed [I]. The upper portion of the lignite in the Freedom mine contains unusually large amounts of selenite in vertical fractures, though selenite deposits occur throughout the mine [ 102]. Bassanite, which may be an artifact of the low-temperature ashing of gypsum, was identified as a major component of 27 Texas lignite samples taken from four drill cores [97].

247 Bassanite was also observed in the low-temperature ash of a Montana lignite, again formed from the parent gypsum [ 103]. Bassanite is reported along with gypsum in Poplar River lignite [92]. On the other hand, no bassanite was reported from the Hat Creek lignites, even though gypsum is a common constituent [89]. Anhydrite was reported in one of thirteen samples of the Ferris seam outcrop; gypsum is common in these samples [92]. (ii) Sodium sulfate. Lignite from Ward County, North Dakota contains extensive pockets of a white material which contained 47% sodium sulfate [ 106]. The sodium sulfate was deposited by groundwater percolation; an excellent correlation was observed between the elevation of this particular lignite bed and the sodium oxide content of the ash, with samples taken from lower elevations having higher sodium contents [106]. Thenardite has been reported in North Dakota lignites [65], forming epigenetically by evaporation of groundwaters having high concentrations of sulfate ions. (iii) Barite. Barite was a minor component in the 1.55 sp. gr. sink fractions of Beulah, Gascoyne, and Velva lignites [85], and is generally rare in Beulah lignite [88]. Barite was tentatively identified in a Calvert Bluff formation lignite [42]. Scanning transmission electron microscopy of sub-micron mineral matter in Hagel lignite tentatively identified barite as a major component of the mineral matter below 100 nm in diameter [107]. This finding would not have been predicted from the composition of the ash of the lignite, since barium is always a minor to trace component of the ash. This also shows that the distribution of elements among different particle sizes in the mineral matter is not uniform. Barite generally occurs only as a trace constituent of Saskatchewan lignites, though in occasional samples it can be an appreciable contributor to the mineral content, up to 16% in one sample of Estevan lignite [83,92]. (iv) Minor sulfates. Jarosite is commonly encountered in North Dakota lignites [84]. Celestite was tentatively identified in Calvert Bluff lignite [42] and was observed in the low temperature ash of Zavala County lignite [108]. 5.2.7 Silicate minerals (i) Quartz. Along with clays, quartz is one of the most significant components of mineral matter in American lignites [16]. Quartz was the most frequently encountered mineral in Beulah lignite [29,88], in Monticello lignite [36], and in Saskatchewan lignites [92]. It is a major mineral constituent throughout the Beulah-Zap seam [87], and was a ubiquitous component of the 1.55 sp. gr. sink fractions of six Northern Great Plains lignites [85]. It occurs with medium frequency in the CC14sink fraction of Beulah lignite, and with high frequency in the same fraction of Savage lignite [86]. Quartz was found as a major component of the low temperature ashes of Texas lignites [42,97] About 80% of the Moose River Basin lignite samples contain quartz, and it is a dominant component in these lignites [93]. In Beulah lignite the quartz is fairly well size-sorted with a slight skew to the larger sizes [31,75,88]. The distribution is similar to that expected for detrital quartz transported by wind or

248 water. Typical quartz groins were 20-30 ~tm in diameter with subrounded to angular form [88]. The sub-rounded to angular nature indicates a detrital origin [31,75,88]. Authigenic quartz was the primary mineral, with smaller amounts of detrital quartz, in 27 samples of Texas lignites from four drill cores from Nacogdoches, Panola, Rusk, and Shelby counties. Quartz in Moose River Basin lignite occurs mainly as detrital grains, in grains less than 190 ~tm, subrounded to subangular [93]. In a single sample of laminated fusain and sandstone, cell infillings of authigenic silica were noted. The grain size of authigenic silica depends on the size of the cell cavities, but averages about 125 ~tm [93]. Authigenic silica has no crystal faces. Detrital quartz is associated with kaolinite and mica, usually in mineral-rich bands in the lignite. Quartz in Saskatchewan lignites is also mainly detrital [83,92]. Other evidence for the detrital origin of quartz in Beulah-Zap lignite is the presence of rutile inclusions [29,31]. Rutile associated with quartz is usually a high-temperature polymorph [ 109]. The high-temperature origin also indicates that the rutilated quartz is detrital. Some quartz grains showed distinct euhedral faces; these crystalline quartz grains may represent wind-blown volcanic ash. A possible volcanic origin is also ascribed to cristobalite and tridymite in Hat Creek lignite [89]. In the Freedom mine in the Beulah-Zap bed, quartz is more abundant in the uppermost lithologic layer than lower in the seam. All of the quartz grains observed were less than 30 ~m diameter; these quartz grains are a result of wind-blown sedimentation [41]. Detrital quartz was observed in Glenharold lignite [10]. The quartz distribution shows higher concentrations in the middle and upper portions of the Beulah-Zap seam [88]. Quartz (cristobalite and tridymite) also shows concentration in the upper half of the Hat Creek lignite [89]. It was most abundant in attritus, as angular to sub-rounded grains of 10-60 ~m size. Quartz makes up about 20% of the Minerals present in vitrain in the Beulah-Zap lignite [90]. Quartz and other phases of SiO2 are present as a result of inclusion of attrital quartz and of preferential silicification of woody material. Quartz constitutes about 20% of the Minerals in the vitrain of Beulah-Zap lignite [90], present as a result both of preferential silicification of woody material and of inclusion of attrital quartz. (ii) Kaolinite. Clays in general are one of the most significant fractions of mineral matter in American lignites [ 16]. Kaolinite represents virtually all of the clay occurrences in Beulah lignite [29,87,88], along with small amounts of halloysite, a hydrated form of kaolinite. It is a major constituent of the Hat Creek lignites, occurring in 74 of 76 samples analyzed, and increasing with depth [89]. About 50% of the low-temperature ash of a Montana lignite was kaolinite [103]. Kaolinite is a major component of the low-temperature ashes of 27 Texas lignite samples selected from four drill cores [97]. Kaolinite has been reported in the lignite of the Calvert Bluff formation [42]. Clay minerals in Beulah lignite tend to occur as massive mineralized areas some 30-400 ~m in diameter and flat to platy in form [88]. In the 13eulah-Zap bed kaolinite generally forms dull, massive bands tens of microns thick. The banded appearance implies a detrital origin. Clay minerals tend to concentrate at the margins of the seam in Beulah lignite [75,88]. Kaolinite and

249 halloysite occur most often near the top of the seam in fusinite cell cavities. Kaolinite occurs as irregular aggregates or in structures described as "book-like" [87], whereas halloysite typically occurs with 1 ~n diameter circular cross sections. Clays of detrital origin predominate among the Minerals in attritus [90]. Kaolinite is a major component of some Moose River Basin lignites, occurring both in association with the organic components and in clastic horizons in the lignite [93]. About 80% of the samples had kaolinite scattered throughout the organic portion. Kaolinite is the most common clay mineral in this lignite. Authigenic kaolinite in crystals of about 5 ~m in diameter is observed in fusinite cell lumens [93]. It likely arises from degradation of feldspar. Kaolinite in Saskatchewan lignites may have been introduced in situ by flocculation from ground waters in the peat swamp, or by breakdown of feldspars [83]. (iii) Mica. In Moose River Basin lignite, mica usually occurs with quartz [93]. It is a minor accessory mineral in this lignite [93]. Mica is mainly detrital [93]. Mica is sometimes observed in Saskatchewan lignites, occurring in measurable quantities in three of nineteen samples of Poplar River lignite, and two of eleven of Ferris lignite [92], and two of seventeen of Estevan lignite [83,92]. Mica has also been reported in the lignite of the Calvert Bluff formation of Texas [42]. (iv) lllite. Illite occurs as a minor accessory detrital mineral in relatively low amounts in Moose River Basin lignite [93]. It is observed in variable amounts in Beulah-Zap lignite, reported occurrences ranging from very low [29] to frequent [88]. It is similarly observed in variable amounts in Hat Creek lignites, being present in only twenty of 76 samples, and in major concentration in only three of those twenty [89]. In comparison, illite occurred, in trace amounts, in only one of seventeen samples of Estevan lignite [83], and at most in trace amounts in Moose River Basin lignite [93]. Illite has been reported in the Calvert Bluff lignite of Texas [42]. Potassium aluminosilicate grains in Monticello lignite are considered to be illite [36]. Since illites are weathering products of feldspars [110], illite in lignite may represent weathered feldspars. (v) Nacrite. Nacrite is an uncommon clay mineral of the kaolin group. Its composition is the same as kaolinite, AlzSizO5(OH)4, but it has a distinct structure. Nacrite has been reported, on the basis of X-ray diffraction and electron microprobe analyses, in the 1.55 sp. gr. sink fractions of six Northern Great Plains lignites: Beulah, Baukol-Noonan, Glenharold, Savage, Velva, and Gascoyne [85]. Nacrite also occurs in association with quartz. Nacrite occurs with "medium-high" frequency in the CC14 sink fraction of Beulah lignite and with high frequency in this fraction of Savage lignite [86]. (vi) Volcanic ash. Volcanic ash can be a significant component of low-rank coal deposits. The deposits are recognizable as distinct, light colored partings, but in some cases the volcanic ash may be disseminated throughout the coal [5]. Nineteenth century big-game hunting expeditions in Africa revealed some lignitic coal associated with massive deposits of volcanic ash [ 111 ]. A useful monograph on volcanic ash is available [ 112]. (vii) Minor silicates. Halloysite was a major component of the low-temperature ashes of 27 Texas lignite samples, selected from four drill cores [97]. Albite, NaA1Si308, occurs with very

250 low frequency in the CC14sink fraction of Beulah lignite [86]. Lawsonite, CaAI 2SIO4" H20, was found in the low-temperature ash of a Montana lignite [ 103]. Epidote, feldspars, pyroxenes, and tourmaline have been identified in lignite from the Calvert Bluff formation [42]. In addition, tentative identification was made of Ca-montmorillonite. Iron- and calcium-containing montmorillonite have been identified in Monticello lignite [36]. Montmorillonite was frequently encountered in Beulah lignite [88], but is essentially absent from Hat Creek lignites [89]. Talc was occasionally encountered in Beulah lignite [88] as thin flakes 15-35 ~n in diameter. Feldspars are rare in Beulah lignite [88], and are encountered in some samples of Saskatchewan lignites [83,92]. 5.2.8 Oxalate minerals Two forms of calcium oxalate, weddellite and whewellite, have reported as minor components of Saskatchewan lignites [83,92]. Weddellite is usually associated with deep-sea deposits. 5.3 T R A C E E L E M E N T S

5.3.1 Introduction The measurement of trace element concentrations in lignite is of interest for at least three reasons. First, the data are important in developing an understanding of the inorganic geochemistry of lignite

(e.g., [74]). Second, at least seven trace elements--As, Be, Cd, F, Pb, Hg, and

Se--may be toxic. Their release to the environment is of concern during lignite utilization or by extraction from products (such as groundwater leaching of ash used as mine fill). Third, interest in occasionally expressed in recovering valuable elements from lignite or its ash. At one time, the prospect of recovering uranium from lignite ash received serious consideration. More recently some attention has shifted to the potential extraction of gallium and germanium. Much of the early work on trace element concentrations in lignite was done on ashes. This approach was dictated in part by the detection limits of the analytical methods used. Since ashing causes roughly a ten-fold increase in concentration of the elements, in some cases it became possible to determine an element in the ash, even if its concentration in the lignite itself was below the detection limit of the analytical method. When the purpose of analyzing ash is to determine the concentration of the element in lignite, one must presume that no volatilization loss occurred during ashing, a presumption not necessarily valid. The determination of trace element concentrations in ash is important in its own fight for environmental studies, as, for example, to assess the potential for leaching of toxic elements by groundwater from ash. Trace element concentrations in North Dakota lignite and lignite ash have been well studied [8,54,74,113-121], because of the state's already extensive electric power industry and a perceived potential for eventually supporting an equally extensive lignite gasification industry. Useful summaries of the data available prior to 1975 have been published [121,122]. About sixty trace elements have been detected in North Dakota lignite. (Another twelve

251 elements occur in larger concentrations in the lignite or its inorganic components; disregarding the noble gases and the highly unstable radioactive elements, lignite contains in some quantity almost 90% of all the known elements.) The concentrations of different trace elements in lignite vary greatly. A difference of an order of magnitude between the lowest and highest reported values can be encountered frequently for some elements. Differences of two orders of magnitude are not uncommon. Even in the case of a single vertical slice through one seam, the concentration of trace elements can vary by at least an order of magnitude [74,113]. For example, within 2 m in the Center mine, the uranium concentration varied from 0.35 to 6.1 ppm [74]. Data for concentrations of trace elements in eastern and western Alabama and Darco lignites are presented in Table 5.27 [11]. Six Fort Union lignites, five from North Dakota and one from Montana (but otherwise unidentified as to source) were included in a major survey of the trace elements in coal [8]. The ranges of values observed are shown in Table 5.28. Trace element concentrations of three Saskatchewan lignites (from the Bienfait, Hart, and Shaunavon seams) are shown in Table 5.29 [92].

TABLE 5.27 Trace element concentrations* in Gulf Coast lignites [11]. Element Beryllium Chromium Gallium Lanthanum Nickel Scandium Vanadium Yttrium Ytterbium Zinc

Eastern Alabama 0.4-1.3 4-12 0.8-3 9-17 2--6 2-5 16-39 7-11 0.5-1.2 11--49

Western Alabama 1- 4 2- 24 3- 9 2-7 1-25 0.7-5 1-78 7-21 2--4 4--43

Darco 0.04-2.7 2-16 0.3-6 0.7-12 2-22 0.8-8 5--50 3-13 0.3-1.1 7-55

*ppm, dry coal basis

Although the order of magnitude of concentration of a given element is often the same from one lignite to another, there can nevertheless be significant exceptions. For example, the highest concentrations of germanium, cesium, and molybdenum are greater than the lowest concentrations by factors of 10, 15, and 20, respectively. Generalizations about the concentration of a particular trace element in lignite should at best be made and used only with great caution. 5.3.2 Alkali metals Lithium occurs at 28-30 vtg/g in Beulah-Zap lignite ash, and 2.7-2.9 ~tg/g on a "whole coal" basis [123]. Lithium, rubidium, and cesium are inorganically associated in Ravenscrag lignites [83].

252 TABLE 5.28 Trace element concentrations* in six Fort Union lignites [8]. Element Ag As B Ba Be Br Cd Ce Co Cr Cs Cu Dy Eu F

Range*_ Element 0.02-0.07 Ga 1.8-9.8 Ge 44-100 Hf 480-1600 Hg 0.12-0.70 I 1.0-1.9 In <0.10 La 3.3-11 Lu 0.80-1.1 Mn 6.0-17 Mo 0.02-0.30 Ni 3.1-6.1 P 0.36-4).61 Pb 0.07-0.13 Rb 19---64 Sb

Range*_ 0.90--4.2 0.10-0.95 0.37-1.2 0.04-0.16 <0.30-0.95 <0.01-0.25 2.0-5.7 <0.01-0.06 33--86 0.10-2.0 2.2-5.6 27-200 1.1-5.5 <1.0-1.9 0.20-0.75

Element Sc Se Sm Sn Sr Ta Tb Th U V W Yb Zn Zr

Range*_ 0.70-1.8 0.40-1.5 0.30-0.50 <0.30 240-500 0.05-0.18 0.10-0.23 0.82-3.1 0.74-1.7 4.8-7.7 0.24-1.1 0.16-0.55 <0.30-4.0 14-36

*Data reported as parts per million, moisture-free whole coal basis. Only three samples were analyzed for Tb.

TABLE 5.29 Trace element concentrations* in three Saskatchewan lignites [92]. Element As B Ba Br Ce C1 Co Cr Cs Cu Dy Eu

Range*_ 0.5-9.9 27-233 577-969 1.4-4.1 10.4-51.9 39-65 1-11 6-13 0.1-1.3 7-13 1.2-4.3 0.2-1.0

Element Hf Ho La Lu Mn Mo Nb Nd Ni Pb Rb Sb

Range*_ 1-3.6 0.2-0.9 6.3-28.6 0.1-0.4 13-145 1.9-12.9 2-13 5-18 4-20 6.4-17.3 11-14 0.8-1.5

Element Sc Se Sm Sr Ta Th Tm U V W Yb Zn

Range*_ 1.7-2.2 2.3-4.1 0.7-5.3 288-537 0.2-0.8 2.9-11.6 0.3-0.6 1.6-7.1 9-28 1.0-2.2 0.4-2.8 62

*Data reported as parts per million, whole coal basis.

The mean rubidium content of Moose River lignites is 5.9 ppm [ 124], with a range of 0.0-20.0. In these lignites rubidium is associated with the detrital minerals [93]. Rubidium is associated with clays in Calvert Bluff formation lignite of Texas [42]. Roughly the same rubidium concentration, 6.3 ppm, is seen in the Hat Creek No. 2 lignite [125]; a value of 5.0 ppm is reported for Ravenscrag lignite [83]. Higher rubidium contents, 11-14 ppm, are seen in

253 Saskatchewan lignites [92]. The rubidium content of Beulah-Zap lignite is 0.9_+0.2 lag/g on a whole coal basis [126]; an independent determination gives the value 0.90_+0.065 [127]. The possibility of rubidium occurring on ion-exchange sites in clay structures has been suggested [11 ]. Cesium was also associated with clays in Calvert Bluff lignite [42]. The cesium concentration of Beulah-Zap lignite is 0.09_+0.032 ~tg/g [ 127]; the Saskatchewan lignites show cesium concentrations in the range 0.1-1.3 ppm [92]. Hat Creek lignite is at the high end of this range, 1.0 ppm [125] 5.3.3. Alkaline earths Beryllium in Glenharold lignite is clearly associated with the organic portion of the lignite [ 10]. The lighter specific gravity fractions of this lignite are enriched with beryllium in the same manner as they are with the organically associated major elements. In fact, beryllium occurs in Bulgarian lignites as a complex with humic and fulvic acids [128]. However, beryllium seems to be associated with inorganic components of Bohemian lignites, based on a correlation of beryllium content and ash value [ 129]. Beryllium accumulation appears to occur via a variety of processes [1291. The beryllium content of Beulah-Zap lignite is 0.17-0.18 ~tg/g on a whole coal basis, and is 1.8-1.9 ~g/g in the ash [123]. Other work reports a beryllium content of 2.4-3.5 ppm for this ash [ 130]. A survey of numerous samples of North and South Dakota lignites indicates a beryllium content of the ash in the range <1-18 ppm [131]. Both strontium and barium have high concentrations in low-rank coals, and were especially pronounced in North Dakota lignites [54]. Strontium generally occurs largely [11] or almost totally [16] in ion-exchangeable form in lignites. This observation has been made for Alabama [11], Texas [ 11,36], and North Dakota [11] lignites. Strontium has a dominantly organic association in Calvert Bluff lignite, but is also found in celestite [42]. It is also organically bound in Ravenscrag lignite [83]. The mean strontium content of 17 Moose River samples is 109 ppm [124]. Strontium has several associations--organically bound, with the clays, and as a carbonate [93]. The strontium concentration of Beulah-Zap lignite is 650+_50 ~g/g [ 126]; also reported as 600_+67 ~g/g [127]. Other workers report a somewhat lower value, 500-510 ~g/g in the lignite, and 5300-5400 ~g/g in the ash [123]. A range of 6200-7300 is also reported for the ash of this lignite [130], in reasonable agreement with other work [132]. Saskatchewan lignites have strontium contents in the range 288-537 ppm [92], with the high end of the range (537.7 ppm) observed in Ravenscrag lignite [83]. An even lower concentration, 116 ppm, is seen in Hat Creek No. 2 lignite [125]. Strontium ranges from 0 to 1000 ppm in Moose River Basin lignites, with a mean value of 108.7 [93].In the ashes of lignites from North and South Dakota, strontium concentrations are in the range 100--4700 ppm [ 131]. The distribution of strontium in specific gravity fractions of Glenharold lignite follows the behavior of calcium [10]. Most is ion-exchangeable with ammonium acetate, and the amount is generally higher in the lighter fractions. About 2-3% of the total strontium in this lignite

254 concentrated in the acid-insoluble portion of the 1.80 sp. gr. sink fraction, presumably occurring as strontium sulfate. Strontium is associated with clays and carbonates as well as with the organic fraction of Moose River Basin lignite [93]. In Hicas (Hungary) lignite, strontium is absorbed into the crystal lattice of aragonite [133]. Strontium is believed to be syngenetic. In many lignites barium is present primarily in ion-exchangeable form. Similarly, both strontium and barium are associated with organic matter in Indian lignites [134]. However, in Ravenscrag lignite barium is associated with both inorganic and organic materials [83]. In a North Dakota lignite it was present as a carbonate tentatively identified as witherite [ 11]. In Monticello lignite, 52% of the barium was removed as an exchangeable cation, 26% was removed by hydrochloric acid, and 22% was insoluble in acid [36]. Barium was found associated with detrital minerals in an Alabama lignite. The barium content of Beulah-Zap lignite is 4 8 ~

~tg/g on a whole coal basis [126]; other

workers give a range 390-450 ~tg/g in the coal, and 410(O4700 ~tg/g in the ash [123]. Higher ranges, 4700-5(K~ [132] and 5300-7400 ppm [ 130], are also reported for the ash; and a higher value, 680+-50, for the lignite [127]. Saskatchewan lignites have barium contents in the range 577-969 ppm [92]. In various lignite samples from the Dakotas, the barium content of the ash ranges from less than 10 ppm to a high of 1.22% [131]. In Glenharold lignite, a small amount (15 ppm) of ion-exchangeable barium was found in all specific gravity fractions [10]. Most of occurred in the 1.80 sp. gr. sink fraction as the acidsoluble carbonate and the acid-insoluble sulfate, with the latter predominating. In this lignite the prevalence of barium in mineral forms is different from the behavior of strontium, which was mostly found in the ion-exchangeable form [ 11]. Acid-insoluble barium was detected by neutron activation analysis in gypsum, suggesting that the barium is present as a sulfate [11 ]. Barium-containing minerals predominate among the submicron mineral matter in a North Dakota lignite examined by scanning transmission electron microscopy [ 135]. Barium occurs in barite in Calvert Bluff lignite [42]. 5.3.4 First-row transition metals Scandium was found with detrital minerals in an Alabama lignite [11]. It has a strong inorganic association in Calvert Bluff lignites, being associated with clays [42]. In contrast, scandium has a predominantly organic association with Glenharold lignite [10]. In Ravenscrag lignite it is associated both with minerals and organic structures [83]. The scandium content of Beulah-Zap lignite ash is 9.1-10 ppm [130]; the value in the lignite is 0.85+_0.047 ~tg/g [127]. Scandium contents of Saskatchewan lignites are higher, in the range 1.7-7.2 ppm [92]. Hat Creek No. 2 lignite contains 6.6 ppm scandium [ 125]. The occurrence of vanadium is believed to be due to its ability to exist in multiple oxidation states, this ability facilitating its inclusion in biogeochemical reactions of oxidizing or reducing nature [136]. Vanadium is well known to occur in petroleum as vanadyl porphyrins, but these compounds are not thought to be present in coals [14]. The porphyrin contents of coals are

255 generally very low [ 137,138], and probably too low to account for the vanadium content [139]. Thus the organically associated vanadium must be present in some other complex than porphyrins [139]. Vanadium is enriched at the seam margins in lignites, suggesting that it may have been trapped at the seam margins by the formation of coordination complexes with functional groups in the lignite structure [11]. Vanadium is associated with inorganic components of Ravenscrag lignites [83]. The vanadium content of Beulah-Zap lignite is 3.4--3.6 ~tg/g on a whole basis, and 36-38 ~tg/g in the ash [ 123,130]. Higher vanadium contents are seen in Saskatchewan lignites, 9-28 ppm [92]. Very high (comparatively speaking!) vanadium is observed in Hat Creek No. 2 lignite, 122.2 ppm [125]. The vanadium content of the ashes of a variety of Dakota lignites is 10-550 ppm [131]. Vanadium has a mainly organic association in Glenharold lignite [10]. Vanadium is also primarily organically associated in Indian lignites [134]. Organically associated vanadium could be present as a coordination complex or as ion-exchangeable V§ ions [ 11 ]. Vanadium in the highest gravity fraction of Glenharold lignite is present in clay minerals [ 11]. Vanadium occurs with the clays in Calvert Bluff lignite [42], and with the inorganic fraction in the Tertiary lignites of Northern Ireland [ 140]. The mean chromium content of Moose River lignite is 65.6 ppm [124], with a range of 0--4975 [93]. The chromium is associated with heavy minerals, possibly as grains of chromite [93]. Saskatchewan lignites have less chromium, 6--13 ppm [92]. A lower value, 4_+1 ~tg/g, is reported for Beulah-Zap lignite [126]; other work reports 2.2-2.6 0g/g in the lignite [123,127] and 24--29 ~tg/g [123,130,132] in the ash. The chromium content in Hat Creek No. 2 lignite is about the same, 30 ppm [ 125]. Chromium in Glenharold lignite has a predominantly organic association [ 10], as it does in Indian lignites [ 134]. Chromium in the Calvert Bluff lignite occurs with the clays and oxide minerals [42]. In the Tertiary lignites of Northern Ireland chromium was introduced with the inorganic fraction [ 140]. Manganese was found primarily in ion-exchangeable form in an Alabama lignite, but also in carbonate minerals and associated with pyrite [11]. Manganese also associates with both the organic and inorganic portions of Indian lignites [ 134]. Manganese-containing metalloporphyrins have been isolated from a Turkish lignite [141]. Manganese is inorganically combined in Ravenscrag lignite [83]. The manganese content of Beulah-Zap lignite is 85_+9 ~tg/g [ 126]. Other work gives a comparable range, 79-81 ~tg/g in the lignite, and 830-850 in its ash [123]. The Saskatchewan lignites bracket that range, having 13-145 ppm manganese on a whole coal basis [92]. Other Dakota lignites have manganese contents in the ash from <10 ppm to 700 ppm [131]. In Glenharold lignite, 25% of the total manganese was exchangeable with ammonium acetate, 72% was soluble in dilute hydrochloric acid, and 3% remained in the insoluble residue [10]. Chemical fractionation of the specific gravity fractions of this lignite showed that the ion-exchangable manganese first decreased and then increased with increasing gravity, suggesting that ionexchangable manganese may be held in the lignite both as a counterion to the carboxyl groups and

256 in clays. Acid-soluble manganese was presumed present in Glenharold lignite as the carbonate, and occurs in siderite in lignite of the Calvert Bluff formation [42]. Manganese may be present in Alabama lignites in coordination complexes. If manganese were present as the sulfide, carbonate or as an acid-soluble form in clay, the amount of acid-soluble manganese would increase in samples from horizons with high pyrite or high clay concentrations, but in fact acid-soluble manganese is not increased in such samples [11]. Manganese was associated with iron oxides in spherical particles of about 20 nm [97] in lignite cores from four counties in Texas. This manganese would report to the acid-soluble fraction on chemical fractionation. Insoluble manganese may be present in pyrolusite, or associated with iron sulfides or oxides. Cobalt adsorption on lignite proceeds more slowly than that of nickel, but 99% of the cobalt in ammoniacal solutions can be absorbed on Indian lignites [142]. Cobalt has a strong organic association with lignite of the Calvert Bluff formation, and occurs with the sulfide minerals as well [42]. Similarly, cobalt is associated both with the mineral and organic portions of Ravenscrag lignite [83]. It is organically associated in Indian lignites [134]. The mean cobalt content of Moose River lignite is 35.9 ppm [124], associated with sulfides and the clay minerals [93]. The range of cobalt concentrations in this lignite is 5.0-197.5 ppm [93]. Cobalt occurs at 0.78_+0.5 ~tg/g in Beulah-Zap lignite [127], with 5.7-6.0 ppm in the ash [ 130]. A wide range of values, <1-200 ppm, is observed for ashes of various Dakota lignites [131]. Saskatchewan lignites show somewhat higher cobalt contents, 1-11 ppm [92]. Hat Creek No. 2 lignite is in this range, with 9.1 ppm cobalt [ 125]. The mean nickel content of Moose River lignites is 47.4 ppm [124]. The range is 4.2-190.7 ppm [93]. It occurs in several modes: associated with the organic matter, with the clay minerals, and with pyrite [93]. The nickel associated with sulfides may also occur as millerite [93]. Nickel is inorganically associated in Ravenscrag lignites [83]. Nickel is organically associated in Indian lignites [134]. Saskatchewan lignites have less nickel, 4-20 ppm [83]. A lower value, 3.3_+0.1 ~tg/g, is observed in Beulah-Zap lignite [ 126]. Other work puts the nickel content of this lignite even lower, 1.1-1.4 ~tg/g, and 12-15 ~tg/g in the ash [123]. Ranges of 17-22 [130] and 17-25 [132] ppm are also reported for this ash. An enormous range of nickel contents, 2-1500 ppm, has been reported for a variety of other Dakota lignites [131]. In Glenharold lignite, nickel has a mainly organic association [10,11]. Indian lignites are very effective at adsorbing nickel from solutions containing about 1% nickel [143], suggesting that nickel might enter lignites by concentration from groundwater percolating through the lignite. Copper is enriched at seam margins of some lignites, implying that it had come into the seam in solution and had been trapped by the functional groups of the lignite. In a North Dakota lignite copper was found to be mainly present as a coordination complex [11]. In Glenharold lignite, copper was found primarily in an acid-soluble form, and particularly in the lighter specific gravity fractions [10]. This behavior suggests an organic association, presumably in the form of coordination complexes. The mean copper content of Moose River lignite samples was 20.0 ppm

257 [124], with a range 2.0-44.0 [93]. In this lignite, copper associates with the clays [93]. A value of 4.2_+1.1 ~tg/g is reported for Beulah-Zap lignite [126]. Other work gives a comparable range, 3.5-5.6 ~tg/g in the lignite, and 37-59 ~tg/g in its ash [123]. The ash analysis is also reported as 34--36 [ 132] and 40-75 ppm copper [ 130]. The copper content of ashes of other Dakota lignites ranges widely, from <1 ppm to 700 ppm [131]. As seen with other elements, the copper content of Saskatchewan lignites seems intermediate between the northern Ontario Moose River Basin lignite and the North Dakota Beulah-Zap lignite. Saskatchewan lignites contain 7-13 ppm copper [92]. Copper associates with clay minerals [93]. Copper was found in chalcopyrite in lignite of the Calvert Bluff formation [42]. 5.3.5 Heavy transition metals The yttrium content of Beulah-Zap lignite is 2.4_+1.3 ~tg/g on a whole coal basis [ 126]. A slightly lower range, 1.8-1.9 ~tg/g on a whole coal basis, corresponding to 19-20 ~tg/g in the ash, has been reported [ 123]. Other work places the yttrium content of the ash as 24--27 ppm [ 130]. In Glenharold lignite, yttrium has mainly an organic association [10], as it does in Indian lignites [ 134]. However, in the Calvert Bluff lignite of Texas, yttrium has a strong inorganic association in the mineral xenotime [42]. Zirconium was found with detrital minerals in an Alabama lignite, and in many other lignites, but has an organic affinity in a North Dakota lignite [11]. In a suite of lignites from the Dakotas, the zirconium content of the ash is 40-3000 ppm [131]. The zirconium content of BeulahZap lignite is 16_+5 ~tg/g on a whole coal basis [126]. The zirconium content of the ash of this lignite is 130-140 ppm [130], though a lower range, 47-74 ppm, has also been reported [132]. In Glenharold lignite, zirconium was enriched in the lighter specific gravity fractions [10]. This behavior suggests an organic association for zirconium in this lignite [11]. The mean zirconium concentration in Moose River lignite is 93.5 ppm [124], with a range of 0-530 ppm [93] associated with the pyrite [93]. Zirconium occurs as zircon in the Calvert Bluff lignites [42]. Hafnium also occurs in zircon in these lignites [42]. Hafnium is associated with clays in Saskatchewan lignites [83]. The hafnium content of Beulah-Zap lignite is 0.341_+0.0025 ~tg/g [127]. Higher values, 1-3.6 ppm, are seen in Saskatchewan lignites [92]; Hat Creek No. 2 lignite is also in this range, with a hafnium content of 2.3 ppm [ 125]. The niobium content of Beulah-Zap lignite ash is 5.3--6.6 ppm [ 130]. Little is known about the association of niobium in North American lignites; in Indian lignites it is thought to have an organic association [ 134]. In Calvert Bluff lignites, tantalum occurs with oxide minerals [42]. The tantalum content of Beulah-Zap lignite is very low, being 0.093-+0.001 ~tg/g [127]. Tantalum in Saskatchewan lignites is in the range 0.2--0.8 ppm, on a whole coal basis [92]. In core samples of North and South Dakota lignites molybdenum was more concentrated near the tops of the lignite beds than in any other sections [144]. In this respect the behavior of molybdenum parallels that of uranium. Molybdenum is associated with pyrite in some lignites [14]. The sulfur in pyrite originates with the bacterial reduction of aqueous sulfate to H2S. The

258 H2S converts molybdate ions to thiomolybdate, which eventually decomposes to MoS 3 and then to MoS2. In a North Dakota lignite, molybdenum and pyrite were concentrated in the heaviest specific gravity fraction [14]. Molybdenum also occurs with the sulfide minerals in the Calvert Bluff lignites [42]. The mean molybdenum content of Moose River lignite samples is 5.9 ppm [124], associated with the organic portion of the lignite [93]. The range of molybdenum values for this lignite is 5--15 ppm [93]. In Bulgarian lignites, molybdenum also occurs in association with humic and fulvic acids [ 128]. Molybdenum associates with both the organic and mineral portions of the Saskatchewan lignites [83]. A suite of lignite ashes from the Dakotas have molybdenum concentrations in the very wide range 2-7200 ppm [131].The molybdenum content of Beulah-Zap lignite ash is 8.2-8.7 ppm [130]. The Saskatchewan lignites contain 1.9-12.9 ppm molybdenum on a whole coal basis [92]. Hat Creek No. 2 lignite has a molybdenum content of 3.5 ppm [ 125]. Tungsten in Moose River Basin lignites occurs with the sulfide minerals; a portion may be associated with the organic structure [124]. It may also associate with carbonates [93]. The tungsten values range from 1.0 to 3.0 ppm, with a mean of 1.7 ppm [93]. Tungsten associates with both organic and inorganic portions of the Saskatchewan lignites [83]. In Bulgarian lignites tungsten is associated with the humic and fulvic acids [ 128]. The tungsten content of Beulah-Zap lignite is 0.35•

~tg/g [127]. Higher values, 1.0-2.2 ppm, occur in the Saskatchewan lignites

[92]. The platinum group elements (Pt, Pd, Rh and Ir, collectively indicated as PGE) show increasing concentrations with decreasing ash contents in Moose River Basin lignite [ 124]. This is an indication of organic association of these elements. In addition, some of the PGE's may occur with the detrital minerals [93]. Concentrations of PGE's are very low, being, e.g., 13.3 ppb for platinum, 16.1 ppb for palladium, and 0.07 ppb for iridium [93]. PGE's also occur at very low concentrations in Beulah-Zap lignite ash [ 130]. Silver in the ash of North and South Dakota lignites ranges from a high of 1.7 ppm to less than 0.1 ppm [131]. The mean gold content of Moose River Basin lignite is 13.6 ppb, with a range of 1-96 ppb [93]. 5.3.6 Lanthanides The lanthanum concentration in Beulah-Zap lignite ash is 37--40 ppm [130]; the concentration in the lignite, on a whole coal basis, is 2.82•

~tg/g [ 127]. A somewhat lower

range of concentrations, 6.3-28.6 ppm on a whole coal basis, is seen in Saskatchewan lignites [92]. The cerium concentration of Beulah-Zap lignite is 4.4•

ppm, on a whole coal basis

[127]. Much higher cerium concentrations are seen in the Saskatchewan lignites, in the range 10.4-51.9 ppm, on the same basis [92]. In Beulah-Zap lignite ash, the praseodymium concentration ranges from 7.3 ppm to less than 6.8 [ 130]. The neodymium concentration of Beulah-Zap lignite ash is 5.3-6.6 ppm [130]; in the

259 lignite itself, on a whole coal basis, the concentration is 2.3+0.8 ppm [127]. These values are at the low end of the range seen for Saskatchewan lignites, which is 5-18 ppm, on a whole coal basis [92]. Samarium, in Beulah-Zap lignite ash, ranges from less than 3.2 ppm to 4.9 ppm [130]. The concentration in the whole lignite is 0.41+0.046 l*g/g [127]. The samarium content of the lignite is within the range reported for Saskatchewan lignites, 0.7-5.3 ppm [92]. Europium is present in Beulah-Zap lignite at a concentration of 0.081+0.012 ~g/g, on a whole coal basis [ 127]. Europium occurs at similar concentration in the Saskatchewan lignites, 0.2-1.0 ppm [92]. Terbium, along with lutetium, seem to be the least plentiful of the lanthanides. The terbium concentration in Beulah-Zap lignite is 0.056-~0.014 ~g/g on a whole coal basis [ 127]. Dysprosium occurs at concentrations of 1.2--4.3 ppm in Saskatchewan lignites [92]. The holmium content is 0.2-0.9 ppm, on a whole coal basis [92]. Thulium is found in concentrations in the range of 0.3--0.6 ppm, on a whole coal basis [92]. The enrichment of ytterbium at seam margins implies trapping by the functional groups in the lignite structure [ 11]. In Glenharold lignite, ytterbium has predominantly an organic association [10]. The ytterbium content of Beulah-Zap lignite is 0.29+0.092 l*g/g [127]. Ytterbium concentrations in Saskatchewan lignites are higher, being in the range 0.4-2.8 ppm on a whole coal basis [92]. Lutetium in Beulah-Zap lignite occurs in concentration 0.036_+0.005 ~g/g [127]. Higher lutetium concentrations are observed in the Saskatchewan lignites, 0.1-0.4 ppm [92]. All of the data on lutetium are on a whole coal basis. Most of the rare earth elements have inorganic associations with the Saskatchewan lignites [83]. However, cerium, dysprosium, and lutetium are associated with both the minerals and the organic structure [83]. 5.3.7 Actinides The thorium content of Moose River outcrop and drill core samples [ 145] ranges from <0.3 ppm to I 1.0 ppm, the latter being recorded for a black earthy and woody lignite. The median value is 2.5 ppm. Thorium in this lignite occurs both in clay minerals and the heavy minerals [93]. Thorium in Beulah-Zap lignite occurs at 1.07_+0.11 ~tg/g, on a whole coal basis [127]. It has a higher range in Saskatchewan lignites, 2.9-11.6 ppm [92], where it is associated with clays [83]. The Hat Creek No. 2 lignite is at the low end of this range, 2.4 ppm [125]. Thorium occurs in clays and monazite in the lignites of the Calvert Bluff formation [42]. Lignites of the Rhineland contain 0.4-2.7 ppm thorium, with an average of 1.1 ppm [ 146]. One concept of the origin of uranium in Fort Union lignites is that it was brought into the lignite from the precursor peat [ 147]. No correlation of uranium with the petrographic constituents was observed for samples of Slim Buttes (South Dakota) lignites, nor is there a significant correlation of uranium with degraded humic matter [148]; however, layers of a lignite bed with

260 highest uranium concentration also contained particulate plant debris and decayed plant matter [149]. Furthermore, the high-uranium layers were usually overlain by a layer high in detrital material [149]. The highest concentrations of waxy material in Goose Creek (South Dakota) lignite correlated with the highest radioactivity [ 148]. An alternative theory of the origin of uranium in lignite is that it brought in with percolating groundwater [147,150-153], possibly by lateral movement of the water [154], but more likely by leaching from overlying tuffs followed by epigenetic mineralization in the lignite [151,155]. Support for this concept is based on the following observations: 1) in all of areas studied in North and South Dakota, Wyoming, Idaho, and New Mexico, uraniferous lignite beds were overlain by radioactive volcanic rocks; 2) springs from these volcanic rocks show high levels of uranium in the water; 3) the extent of mineralization of the lignite can be correlated with variations in the permeability of the overlying rock; 4) the topographically higher beds in a series are more radioactive than lower beds in the same series; and 5) the uranium content is independent of the age of the coal but correlates with topographic location and the permeability of the overlying rocks [150]. Furthermore, twenty cores of North and South Dakota lignites showed uranium more highly concentrated at the tops of the beds than in other sections [ 144,148]. Uranium is trapped in lignite by reduction of hexavalent uranium in solution to insoluble tetravalent uranium by the lignite. A consequence of the concentration of uranium in this manner is that some lignites will give unusually high readings during 7-ray logging (usually most coals will show very low radioactivity levels, comparable, for example, to limestone). South Dakota lignites are an example of unusually high 7 radiation [ 156]. In North Dakota, uranium generally occurs in lignite or carbonaceous shales overlain or underlain by a sandstone aquifer. Uranium in the groundwater of these aquifers derives from leaching from volcanic ash by meteoric waters. Water of meteoric origin leaches uranium from the ash, the uranium-bearing water then flows downward and laterally until it contacts lignite or other organic sediments which capture the uranium as coordination complexes. The ability of uranium to exist in multiple oxidation states enhances its ability to participate in either oxidation or reduction reactions with the lignite. Uranium in Wilcox and Jackson lignites appears concentrated near the contact of the lignite with a sandstone or shale [157]. This again suggests that uranium, being transported by groundwater as soluble complexes, accumulates at the contact via reaction with the organic portion of the lignite. A variation of the pyrite or silicates content of lignite will cause a similar variation in the amount of uranium present; however, a change in the carbon content of the lignite will cause an even greater change in uranium [158]. This implies that the geochemical control of uranium content through some factor common to uranium and carbon (i.e., the organic portion of the lignite) is more important than control through factors common to uranium and the minerals. Carbon is the most important component in relating the uranium content to the bulk composition of the sample. A multiple linear regression equation originally worked out for black shales is

261 0 = -107.1686 + 1.2182 X 1 + 2.6168 X2 + 4.4610 X3 where 0 is the uranium content in parts per million, X: the percent silicates, X 2 the percent pyrite, and X3 the percent carbon in the sample [ 158]. The uncertainty in the predicted value of uranium is +12.4 ppm. French

(e.g., Arjuzanx) lignites having high concentrations of carboxyl groups will rapidly

remove uranium from aqueous solution by ion exchange [159]. Infrared spectral evidence suggests that the uranium forms complexes with the carboxyl groups. Such complexes subsequently undergo thermal decomposition, forming uranitite upon decarboxylation of the lignite structure. About half the uranyl ion in solution can be precipitated as uranitite in about 130 days [160]. For French lignites that do not have high concentrations of carboxyl groups

(e.g, Gardanne) other

uranium complexes form without ion exchange [159]. The overall mechanism of uranium incorporation involves complexation of uranyl species without reduction in low-temperature environments, followed later by reduction in diagenetic or hydrothermal environments. The ratedetermining step at 100-200~ may be the formation of uranium oxides [ 160]. Uranium can incorporated in the lignite as uranitite or coffinite at favorable Eh-pH conditions [161]. These minerals occur together with the uranium complexed to the organic structure. When groundwater containing [UO2(CO3)2]-2 enters an acidic environment, the uranyl ions are scavenged by the organic material, or are precipitated as minerals such as autunite. Some microenvironments may have an Eh sufficiently negative to permit formation of minerals containing U(IV). Any U(IV) minerals that precipitate in reducing zones might be recycled by subsequent oxidation and returned to the system as uranyl ions, which then may be trapped by the organic matter. Uranium is present in many low-rank coals as an organic complex [157,159,162]. About 60--80% of the total uranium in Texas lignite is bound to the humic portion [161]. The remainder is associated with fine-grained minerals. Uranium has a dominant organic association in Calvert Bluff lignite, though some may also be found with zircon [42]. Density gradient fractionation of a Texas lignite containing about 2500 ppm uranium suggests that the uranium is associated with the organic matter [ 157]. X-ray mapping of a Texas lignite shows a homogeneous distribution of the uranium through the lignite [157]. Extraction of this lignite with dimethyl sulfoxide removes 40-50% of the uranium in molecular structures which have molecular weights below 1500 [157]. Addition of benzene, water, or acetone to the dimethyl sulfoxide extracts results in formation of a precipitate having infrared spectra similar to lignite humic acids. This humic acid-like precipitate is enriched in uranium. The uranium content of Moose River lignite correlates with the nickel content. Since nickel is thought to be associated with humic acids in coals (e.g., [163]) the correlation of uranium with nickel may reflect in turn an organic association for the uranium. Uranium in a weathered sample of Mendenhall (South Dakota) lignite was associated with the organic matter as a coordination complex [ 164]. Some uranium minerals, such as metaautunite, metatorbernite, metazeunerite, novacekite,

262 saleeite, and abernathyite, were found as coatings on fracture surfaces of South Dakota lignites [154]. Megascopically visible uranium minerals, mostly phosphates and arsenates such as metatorbernite, have been reported in North and South Dakota lignites [165]. However, in a Jackson lignite from Karnes County, Texas, about 10% of the lignite is present in the form of 10-30 ~tm minerals in the >2.90 sp. gr. fraction of the low-temperature ash [ 161]. These particles are coffinite, or products of its alteration. Other minerals included sodium autunite, metauranocirite and sabugalite [ 165]. The uranium may be associated with clay minerals or carbonates in Moose River Basin lignite [93]. Typical uranium-bearing lignites are very high in ash. For example, samples from former National Lead Co. holdings in Slope and Billings Counties, North Dakota, have ash contents of 30-60% with uranium contents, expressed as U308 on a dry basis, as high as 2% [166]. However, some lignite samples of unknown provenance contained only 9% ash on a dry basis with up to 0.5% U308 [166]. These data contrast with substantially lower values for many other North Dakota lignites [74,113,117,119,120], although most lignites for which uranium data have been obtained were not specially selected to be uraniferous. A Saskatchewan lignite containing 0.08% U (on a coal basis) was considered to be of comparable quality to uranium ores [167]. Montana lignites contain an average of 0.005% uranium, reported as U308 [155]. The average uranium content of Moose River lignite is 1.63 ppm [145]; the range is 0.14-7.20 ppm [93]. A similar value, 1.4 ppm, is seen in Hat Creek No. 2 lignite [ 125]. These values are at the low end of the range seen in Saskatchewan lignites, 1.6-7.1 ppm [92]. The value for Beulah-Zap lignite is 0.49+_0.06 ~tg/g [ 127]. Analyses of three lignites, Savage, Gascoyne and Beulah, show average contents of 238U of 6.0, 8.4, and 3.7 mBq/g, respectively. The content of 234U in the same samples was 6.1, 8.9, and 3.9 mBq/g, respectively [168]. The maximum concentrations of uranium in Texas lignites are 20 ppm in the Wilcox formation to 7800 ppm in the upper Jackson formation [157]. The uranium content of Keuper (Vosges region of France) lignite of Triassic age is 12.7-18.8 ppm [136]. Lignites of the Rhineland have uranium contents in the range of 0.2-1.4 ppm, averaging about 0.4 ppm [ 146]. An extensive investigation of United States lignites, involving over 500 samples from the western U.S., shows that, for 423 lignites in the Dakotas, the uranium content of the ash ranged from a low of 0.001% to a high of 6.3862% [131]. Sixteen of the 423 ashes contained uranium in excess of 1%. The highest value occurred in a sample from the Lower Ludlow Formation, Slim Buttes area, Harding County, South Dakota [131]. California lignites, in contrast, generally have much lower uranium contents. Of 171 samples analyzed, only 28 contained uranium in concentration greater than 0.001% in the ash, and the highest value was about 0.05% [131]. Lowrank coals of the southwestern United States also contain, in some cases, high uranium contents. Fourteen ash samples contained from 0.0008% to 5.5880% [131]. The highest value was observed in a sample of woody lignite from the Big Bend strip mine, near San Rafael, New Mexico [ 131 ].

263 5.3.8 Group lib elements Zinc is present primarily in complexes in a North Dakota lignite [11]. In Glenharold lignite most of the zinc is present in an acid-soluble form and concentrates in the lighter specific gravity fractions [ 10], presumably as a coordination compound. In western Alabama lignite, much of the zinc is found in acid-soluble form, but the abundance of pyrite in this lignite suggests that zinc is present as sphalerite [11]. Zinc is associated with the inorganic components of Saskatchewan lignites [83]. In Beulah-Zap lignite, the zinc concentration is 5.7_+2.5 ~tg/g on a whole coal basis [126,127]; other work shows comparable, albeit slightly lower values, 4.5-5.1 ~tg/g on a whole coal basis, equivalent to 47-54 ~tg/g in the ash [123]. A range of 54-61 ppm in the ash has also been reported [132]. A suite of lignites from the Dakotas contain 2-510 ppm zinc in the ash [131]. Zinc occurs in Hat Creek No. 2 lignite at 26.4 ppm [ 125]. Even higher zinc concentrations occur in Saskatchewan lignites, about 62 ppm [92]. Zinc has an organic association in Moose River lignite [93,124]; its mean concentration is 159.2 ppm [93], with a range of 5.2-311.2 ppm [93]. In soils, cadmium is incorporated in humic acids, with about half the cadmium present in exchangeable form and the remainder present in coordination complexes [169], suggesting that coalifying organic matter could trap and retain cadmium during its conversion to lignite. The mean cadmium content of Moose River lignites is 0.76 ppm [93,124], present in sphalerite [93]. The range of cadmium concentrations in this lignite is 0--4.77 ppm [93]. Cadmium occurs in quite low concentrations in Beulah-Zap lignite, 0.044,4).048 ~tg/g on a whole coal basis, or 0.46-0.50 ~tg/g in the ash [ 123]. 5.3.9 Group III elements Boron has a dominantly organic association with the lignite of the Calvert Bluff formation, and also occurs with tourmaline [42]. Boron is organically bound in Saskatchewan lignites; high concentrations were likely introduced into the lignite by circulating groundwater [83]. In BeulahZap lignite, the boron content is 50(O810 ppm [130]. Somewhat lower values, 27-233 ppm on a whole coal basis, are seen in the Saskatchewan lignites [92], and an even lower value, 9.4 ppm, in Hat Creek No. 2 lignite [ 125]. Boron contents of the ashes of a variety of North and South Dakota lignites are in the range 9-500 ppm [ 131]. Gallium has been observed to be enriched at seam margins in lignites, implying that it was trapped there by interactions with the functional groups in the lignite structure [11]. Galliumcontaining metalloporphyrins have been isolated from a Turkish lignite [ 141 ]. However, gallium might also substitute for aluminum in clay structures. Gallium in the Calvert Bluff lignites is associated with the clays [42], and is associated exclusively with mineral matter in Indian lignites [134]. The gallium content of Beulah-Zap lignite is 16--20 ppm [130]. In the ash of a suite of Dakota lignites, gallium ranges from 1 ppm to 38 ppm [131]. 5.3.10 Group IV elements The retention of germanium is due to interaction with the hydroxyl groups in humic acids

264 [ 170]. In lignites of the former Soviet Union, 12% of the germanium content is adsorbed, and the remainder is chemically bonded to the lignite [171]. With these lignites also interaction with the humic acids is responsible for chemical incorporation of the germanium [171]. Germanium is introduced from solution during coalification. It interacts with lignin structures in the early stages of coalification [ 172]. Acid-soluble germanium is likely present as organic complexes [139]. Germanium is enriched in the lighter specific gravity fractions of Indian lignites, suggesting that it is largely associated with the organic matter [ 173] in coordination complexes. There was no correlation of germanium content with vertical distribution in the seam [ 173], though with Russian brown coals a high germanium content is noticed at the seam margins [ 174]. An inverse relationship between iron and germanium was noted in Russian brown coals, the presence of siderite indicating an absence of germanium, and marcasite and pyrite indicating a decreased germanium content. In ashes, a high germanium content correlates with high calcium and aluminum and low iron. In addition, germanium correlated directly with gallium and vanadium and inversely with nickel, cobalt, and lanthanum. In these coals only 0.6-2.3% of the germanium was bonded to humic acids [174]. However, in some Bulgarian lignites the germanium is associated with the humic and fulvic acids [128]. Occasional reports have indicated extremely high concentrations of germanium in some lignite ashes. Patuxent (District of Columbia) lignite ash contains 6.0% Ge [175], while other lignitic logs of similar age (Cretaceous) along the Atlantic seaboard are have as much as 7.5% Ge in the ash [ 176]. Patuxent lignite ash is well endowed with rare elements which, in this instance, scarcely merit the term "trace." The ash of this lignite can contain up to 5.0% vanadium, 0.8% chromium, and 0.2% gallium [175]. North Dakota lignite ashes used for comparison show maximum values, on a percentage basis, of .0065% Ge, .02% V, .088% Cr, and .006% Ga. A suite of lignites from the Dakotas have germanium contents, in the ash, in the range <1-90 ppm [1311. Tin concentrations in Beulah-Zap lignite ash range from 13 ppm to less than 4.6 ppm [130]. In other Dakota lignites similarly low concentrations occur, in the range <1-9 ppm [131]. Lead is associated with selenium in small particles of clausthalite in drill cores from four counties in Texas [97]. In addition, some 30 nm spherical particles rich in lead and tin were observed. Lead occurs as galena in the Calvert Bluff lignites [42]. The mean lead content of Moose River lignite is 25.1 ppm [93,124], with a range 5-155 ppm [93], possibly associated with the organic structure. A much lower lead concentration occurs in Beulah-Zap lignite, 0.2•

~g/g

[ 126], although other work indicates a lead content of 1.5 ~g/g on a whole coal basis, or 16 ~g/g in the ash [123]. An even higher lead concentration in the ash, 22-32 ppm, has also been reported [130]. These results with Beulah-Zap lignite fall in the range observed for other Dakota lignites, <1-57 ppm in the ash [131]. Saskatchewan lignites contain 6.4-17.3 ppm lead [92], inorganically associated [83].

265 5.3.11 Group V elements In Texas lignites, arsenic contents vary only slightly as a function of depth in the seam [161]. Arsenic is associated with the organic fraction, as indicated by its concentrating in the lighter density fractions. The arsenic content of these samples ranged from 1 to 5.5 ppm [177]. BeulahZap lignite has an arsenic content in this range, 3.3_+0.1 ~g/g [126]. Other work places the value at 2.63_+0.19 ~tg/g [127]. A wide range of arsenic concentrations is seen in ashes of various Dakota lignites, ranging from less than 0.001% to a high of 0.24% [131]. The range of arsenic concentrations in Saskatchewan lignites, 0.5-9.9 ppm, is somewhat wider, but otherwise comparable to, the values seen for the Texas and North Dakota lignites [92]. A higher value is observed for the Hat Creek No. 2 lignite, 12.7 ppm [ 125]. In Moose River Basin lignites arsenic is associated with sulfide minerals [93,124]. The mean arsenic content in Moose River Basin lignite is 4.0 ppm, with a range 3.0--6.0 [93]. In the Calvert Bluff lignites, arsenic is strongly associated with the organic fraction of the lignite, as well as with sulfides [42]. Both antimony and arsenic are associated with the inorganic portion of the Saskatchewan lignites [83]. The antimony content of Beulah-Zap lignite is 0.154_+0.012 ~tg/g on a whole coal basis [ 127]. Ravenscrag lignite has a higher antimony concentration, 0.8 ppm [83]. 5.3.12 Chalcogens The selenium content of some Texas lignite samples ranged from 3.9 to 22.9 ppm [177]. The association of selenium with the lower density fractions suggests that it is incorporated in the lignite with the organic portion. However, in the Calvert Bluff lignite, selenium has both organic and inorganic associations, being found in clausthalite as well as with the organic portion [42]. Selenium has been observed in other Texas lignites in association with lead in particles of clausthalite [97]. Selenium in Beulah-Zap lignite has a concentration of 2.2_+1.2 ~tg/g [ 126]; other work places the value somewhat lower, at 0.58_+0.15 0.g/g [127]. The range of concentrations in Saskatchewan lignites is 2.3-4.1 ppm [92]. The polonium in samples of Savage, Gascoyne, and Beulah lignites averaged, for two determinations, respectively 7.4, 10.2, and 4.5 mBq/g [168]. 5.3.13 Halogens Fluorine concentration in Beulah-Zap lignite is 35_+6 ~tg/g on a whole coal basis [126]. Bromine has a dominantly organic association in Calvert Bluff lignite [42], as it does in Saskatchewan lignites [83]. Chlorine also has a dominantly organic association in Calvert Bluff lignite [42] and in Moose River Basin lignites [93], though associates with both mineral and organic components in Saskatchewan lignites [92]. In Saskatchewan lignites, the chlorine content ranges from 39 to 65 ppm [92]. A similar concentration, 42.3 ppm, occurs in Hat Creek No. 2 lignite [125]. The bromine content of Beulah-Zap lignite is 0.9_+0.4 ~tg/g [126], also reported as 1.42_+0.19 ~tg/g [127]. This is at the low end of the range seen for Saskatchewan lignites, 1.4--4.1 ppm [92]. The bromine concentration is higher in Hat Creek No. 2 lignite, 11.8 ppm [125].

266 5.4 VARIABILITY OF INORGANIC C O M P O S I T I O N 5.4.1 Introduction As the foregoing sections have shown, the inorganic chemistry of lignites is very complex. Many of the major inorganic elements in coal distribute among two or more possible modes of occurrence, including ion-exchange sites on carboxyl groups, coordination sites on heteroatoms, and in a variety of minerals. Analysis of lignites to the trace level shows that virtually all elements, except the noble gases and highly unstable radioactive elements, occur at least to some extent. Furthermore, surveys of the variation of elemental concentrations vertically or horizontally within a seam or comparing one seam with another show that the inorganic composition of lignites is highly variable. Nevertheless, despite this complexity, it is important to realize that all of these aspects of the inorganic chemistry of lignite--the concentration of elements, their distribution among possible modes of occurrence, and their spatial distribution--are all governed by, and consequences of, the laws of chemistry and geology. Geochemical factors such as source rocks, tectonics, depositional environment, nature of the plant material and its degradation or alteration, and pH, Eh, and composition of ground water will vary over some range of possible conditions or values. It would not be unreasonable to expect, consequently, that an extraordinary variability in inorganic composition and mineralogy would be observed for coals. Remarkably, even if one considers coals of all ranks, geological ages, and geographic origins, rather than just lignites, the differences in mineralogy and in trace element concentrations are primarily differences of amount or degree rather than fundamental differences of kinds [5]. In fact, virtually all major minerals in all coals belong to one of four categories: oxides, carbonates, sulfides, and silicates [178]. The minerals that constitute the major components of all North American coals are in the relatively short list of pyrite, calcite, quartz, kaolinite, illite and mixed-layer clays, and siderite [5]. Even among accessory minerals, the only significant differences observed in a suite of 100 coals was a slightly higher frequency of occurrence of apatite and barite in coals from the western U. S. [ 179]. Trace element data for U. S. coals [7] show that the only significant difference among ranks is a decrease in the concentrations of six elements--Ba, B, Ca, Mg, Na, and Sr--as rank increases [5]. The practical implication of the variability of elemental concentrations in lignite is that if it is desired to know the average concentration of an element in a lignite seam, it is vital that the analysis sample be representative of the whole seam. 5.4.2 Factors affecting the inorganic composition of lignites (i)

Geologicalfactors.

The first factor affecting the observed inorganic composition of

lignite is the nature of the geological conditions prevailing in the depositional setting. These conditions may include the kind of depositional environment, the kinds of source rocks, and the tectonic characteristics of the region. The amount of detrital material brought into and deposited in the region where lignite is being laid down will be determined by these geological conditions. At

267 the same time, the chemical nature of the plant material and the extent to which the original molecular structures of the plant components are altered or degraded during coalification may affect the ability of the organic material to trap inorganic species in coordination complexes. The formation of authigenic material will be affected by the characteristics of the groundwater coming into the coalifying plant material. The important groundwater characteristics include its composition, pH, and Eh. The locations of the inorganic constituents in a lignite seam are determined by the ways in which they were accumulated in the lignite. Detrital constituents, carried in by water or wind, will most likely be enriched at the margins of the seam and at any partings within the seam. Examples of detrital constituents include quartz, feldspar, mica, clays, and volcanic ash [43]. Authigenic minerals are mainly formed by precipitation from solution. As groundwater moves through the accumulating plant debris or through the lignite, ion-exchange processes with carboxylic acid groups or other minerals can also occur. Depending on the composition of the groundwater, changes in its composition over time, and changes in patterns of groundwater movement, distribution patterns reflecting the influence of groundwater could involve concentration of elements at the seam margins or enrichment at the center of the seam. Some inorganic elements may also be contributed by the plant matter; the vertical distribution patterns of such elements may be related to the depositional history of the original swamp. The variations in elemental concentrations in a particular lignite deposit will result from the combined effects of several factors. One is the geochemical behavior of the element, i.e., its likelihood of occurring as, for example, an insoluble hydrolyzate or a soluble cation, as determined by ionic potential. A second factor is the characteristics of the lignite, such as the availability of ionexchange sites for binding cations. A third factor is the geological setting of the deposit, which might determine, for example, the potential for, and extent of, authigenic mineralization. The observed patterns of distribution can be explained by changes in depositional conditions during accumulation, changes during diagenesis and post-diagenetic processes. For example, the addition of detrital clay and silt at the beginning and end of peat deposition would increase Si and A1 concentrations at the margins of the seam. Other depositional factors include changes of Eh or pH, influx of volcanic ash, or changes in the flora during peat accumulation. Lateral flow of water through the deposit after deposition could selectively concentrate elements in the margins of the seam. Similarly, vertical flow of water might concentrate elements at either the upper or lower margin. Other post-depositional factors which might affect the distribution of the inorganic components include the extent of degradation of plant material, degree of compaction (influencing the permeability of the lignite or its surrounding sediments), and changes in groundwater chemistry. (ii) Organic vs. inorganic affinity. The organic or inorganic affinity of an element is determined by the relationship between the concentration of that element in the moisture-free coal and the ash value of the coal [8,180]. For lignites having an ash value greater than about 5%, an increase in mineral matter content generally reduces proportion of the trace element content that is

268 organically bound [ 181]. Linear least squares analysis of data from the Center mine [ 113] showed that Na, Ca, Mn, Br, Sr, Y, and Ba had organic affinity; Mg, K, Cu, As, Rb, Ce, and Eu showed both organic and inorganic affinity; and all the remaining elements had inorganic affinity. In the Calvert Bluff Formation lignite the concentrations of most elements correlate with the ash, suggesting a detrital origin. Elements displaying this behavior are AI, Ba, Cr, Mg, Ti, V, Si, Dy, Hf, La, Sc, Se, Ta, and Th [42]. Ca and Sr show a negative correlation with ash, indicating that they associate with the organic portion of the lignite. Co and As were also thought to have an organic association. A correlation of elemental concentration with specific gravity of densityseparated samples indicates the elemental association with the mineral constituents. The elements in these samples that show this behavior are Na, AI, Cr, Fe, Mo, Sb, Ba, W, U, Th, Sc, Hf and most of the rare earths [42]. The only elements whose concentrations decrease with increasing specific gravity are Ca and Co. No trend was evident for V, As, Se, Cs, Ag, Ta, C1, and Br. Samples of high ash tended to have a higher proportion of the detrital minerals quartz and illite and a lower proportion of authigenic minerals kaolinite, calcite, siderite, and pyrite [42]. About 10% of the rare earth elements associate with the organic fraction [5]. In Saskatchewan lignites Na, Sr, S, Br, and B are entirely organically bound [92]. Ba, Ca, Co, Mo, Ce, Dy, Lu, C1, Sc, and W associate with both inorganic and organic components. The remainder of the elements are almost entirely inorganically bound. Generally, patterns of elemental distribution showing little vertical variation (sometimes called regular or even distributions within the seam) are observed for those elements associated with the organic components of lignite. More irregular pattems, reflecting concentration at seam margins or at one or more locations within the seam, may be characteristic of elements related to detrital and authigenic minerals. Elements displaying an even distribution are characterized by ion-exchangeable behavior during chemical fractionation, an organic affinity, and an ionic potential less than 3 [30]. Generally these elements include Ba, Ca, Mg, Mn, Na, and Sr. Elements showing enrichment at one or both margins are characterized by association with the acid-soluble fraction or the residue in chemical fractionation, an inorganic affinity, and ionic potential between 3 and 12 [30]. Elements which usually fall into this group include Al, Br, Ce, Cl, Cr, Eu, Rb, Sm, Sc, Si, Th, Ti, U, V, Yb, and Zr. The elements having a random or irregular distribution remain in the residue during chemical fractionation, have an inorganic affinity, and tend to form by authigenic mineralization. In addition, most are chalcophilic. These elements include Sb, As, Cd, Fe, Ni, Se, and Zn. The irregular distribution of these elements is a result of the syngenetic and epigenetic mineralization by formation of sulfides. (iii)

Effects of groundwater. In

North Dakota, the most common subsurface water

composition in Tertiary deposits has dominant concentrations of Na+, HCO3-, and SO4-2 [ 182]. As meteoric water infiltrates below the surface, mineral dissolution and ion-exchange processes occur. The composition of the water at equilibrium, and the concentrations of the inorganic species in it, depend on the initial pH, the extent of calcium (from dissolution of calcite, dolomite, or gypsum)

269 exchange for sodium on ion-exchange sites in clays, and whether pyrite is dissolved. As the meteoric water first infiltrates below the surface, it becomes charged with CO2. Dissolution of calcite in this water results in the dominant ions being Ca+2 and HCO3--. Some Mg+2 may be added from dissolution of dolomite. Ion exchange of Ca+2 for Na+ increases the concentration of Na + in the water. Dissolution of pyrite or gypsum provides a source of SO4-2. In North Dakota, subsurface water movement is generally slow. The hydraulic conductivity of Tertiary sand is 10-5 to 10-6m/s; of glacial till, 10-8 to 10-10 m/s, and of lignite, 10-5 to 10-7 m/s [182]. The average compositions of pore water from the spoils of two North Dakota lignite mines are shown in Table 5.30 (calculated from data in [ 182]). The composition of the pore water in the spoils may be affected by hydrogeological processes accompanying mining and thus may not be representative of groundwater percolating through undisturbed overburden. TABLE 5.30 Average compositions of pore water from spoils at two North Dakota lignite mines, mg/L [ 182].

No. of samples Magnesium Sodium Potassium Bicarbonate Sulfate

Center Average Std. Dev. 8* 215 105 433 355 34 22 854 421 1775 755

Indian Head Average Std. Dev. 4 172 35 1083 92 36 13 1286 46 2493 315

*Seven samples analyzed for potassium, six for the anions.

The ratio of sodium to (calcium + magnesium) in the groundwater determines the sodium distribution in the lignites near Underwood, North Dakota [183]. The factors controlling the Na/(Ca+Mg) ratio include the texture, lithologic composition, and the thickness of the overburden. Ion-exchange processes can only occur when the lignite is saturated with water. Consequently, the position of the water table relative to the lignite is also important. If the lignite becomes unsaturated, the elemental distribution (particularly the sodium distribution) established during saturation will be preserved into the unsaturated condition. Sodium-rich lignites are characterized by low hydraulic conductivity in the lignite and the overburden, and by a clayey overburden rich in exchangeable sodium, whereas low sodium lignites have thin, coarse-textured overburden [ 183]. The correlation of the sodium content of the lignite with the Na/(Ca+Mg) ratio of the groundwater suggests that groundwater establishes an ion-exchange equilibrium with the lignite [183]. It is important to determine whether the composition of the groundwater is established before it enters the lignite, or whether the groundwater composition is established after being in contact with the lignite. That is, does the groundwater composition establish the inorganic content

270 of the lignite, or does the inorganic content of the lignite establish the groundwater composition? A linear correlation between the percentage of sodium in dry lignite and the Na/(Ca + Mg) ratio of a saturated paste extract of the sediment above the lignite establishes the former case: the composition of the groundwater is determined before it enters the lignite and therefore controls the inorganic composition of the lignite, and not vice versa. Sodium in groundwater most likely derives from clay in the overburden. Water moving downward first becomes enriched in calcium and magnesium by dissolution of calcite and dolomite, followed by dissolution of gypsum. As this calcium- and magnesium-rich water then moves through clayey sediments, exchange of calcium and magnesium with clays enriches the water in sodium. The dominant clay in the lignite overburdens in the Underwood area is sodium smectite. High-sodium lignites are almost invariably overlain by these clayey sediments. High-sodium lignite also occurs in areas of thick overburden [ 183]. An inverse relationship exists between the thickness of the overburden and the hydraulic conductivity of the lignite; the hydraulic conductivity decreasing by an order of magnitude for each 17 m increase in depth [183]. The reduction of hydraulic conductivity could reduce the groundwater flow, and in turn the reduction in groundwater flow could reduce the rate of sodium removal from the overburden and sodium exchange with the lignite. However, thickness of overburden alone does not guarantee a high sodium lignite. If the overburden is not a significant reservoir of sodium (e.g., a sand overburden), the lignite will be low sodium regardless of overburden thickness. Several factors affect the Na/(Ca+Mg) ratio. A reservoir of sodium-rich clay in the overburden is necessary to provide a large Na/(Ca+Mg) ratio in the groundwater and thus to prevent a calcium- and magnesium-rich groundwater from removing sodium from the lignite. A high-sodium lignite would require a clay overburden several hundred meters wide and several meters thick [ 183]. If the lignite is above the water table, no exchange will occur. Taken in combination, these factors affect the sodium content of the lignite in the following ways [183]: A lignite below the water table, with clayey overburden greater than 25 m thick, will be high sodium. A similar lignite but with a sandy overburden will be low in sodium, because the groundwater reaching the lignite will have a high content of calcium and magnesium and will, therefore, exchange sodium from, rather than into, the lignite. A lignite below the watertable but overlain by thin sandy, silty, or clayey overburden will be low in sodium. A lignite above the water table, regardless of the thickness and lithology of the overburden, will have whatever sodium content was established at the time the lignite was beneath the water table. At depths below 60 m the water flow in the lignite will be very sluggish. The Na/(Ca+Mg) ratio in the groundwater will be high [183]. An inverse relationship between sodium and calcium, and between sodium and (calcium + magnesium) occurs in North Dakota lignites and their lithotypes, suggesting some petrographic control or relationship with inorganic composition [ 184]. Low-sodium lignite of the Williston basin is generally overlain by loosely consolidated sand or silt [ 184]. More generally, low-sodium lignite is always overlain by immediately (i.e., within about 60 cm) sandy or silty roof rock or by thin, weathered overburden of any lithology [184].

271 5.4.3 Vertical variability within seams (i) Introduction. Elements with inorganic association generally show distribution patterns having a concentration at one of both margins of the seam. In a few cases, mainly with minor elements, an irregular or undefinable distribution pattern is observed. These elements are primarily unaffected by chemical fractionation, remaining in the residue. Such elements also have ionic potentials in the range of 3-12, characteristic of elements normally expected to form insoluble hydrolysates in geochemical processes [185]. This group of elements includes Si, Sc, Ti, V, Cr, Co, Zr, Sb, Cs, La, Sm, Eu, Yb, Th, and U. They are frequently found to occur in detrital or authigenic mineral grains in the upper and lower margins of the seams. In comparison, elements having an organic affinity are ones having ionic potentials less than 3, and expected to occur as hydrated cations [ 185]. The distribution patterns show a slight concentration in the center of the seam, or, again in some cases, an irregular distribution. During chemical fractionation, they are almost completely removed either in the ion-exchange step or by ion exchange and HC1 extraction. Electron microprobe analysis of individual lithotype or maceral samples shows that the elements having organic affinity are disseminated through the organic components with no evidence of occurrence as discrete mineral grains. The main elements in this category are Na, Ca, Sr, and Ba. Many trace elements (e.g., Be, Ge, Yb, Y, and Sc) tend to concentrate at the seam margins [14]. Some others have fairly even distribution profiles (e.g., Cu and V) [14]. The concentrations of trace elements appear to vary widely from one seam to another. This variability reflects differences in the availability of the elements from nearby source rocks. Since many trace elements are in low concentration or even undetectable in roof rocks and seam partings, the organic material of the lignite likely had a role in trapping these elements from circulating groundwaters. (ii) Northern Great Plains lignites. Beulah-Zap lignite typically has a higher ash value at the seam margins compared to the inner portions of the seam [87]. This relationship persists laterally through a wide area encompassing the Beulah, Indian Head, and Freedom mines. The distributions in a vertical section of the Beulah seam follow the following trends: Co, Se, Eu, Sm, Sb, and Br are concentrated at the margins; V, Cr, and Sb are concentrated at the base; Ba, Yb, U, V, Cr are concentrated at the top; Mg, Ti, Ru, Cu, Zn, Ni, K, A1 and Si have an even distribution throughout the section; and Na, Ca, and Se are concentrated in the center of the seam [31 ]. Other studies of the Beulah-Zap bed show that the elements concentrated at one or both margins include Sb, Br, Ce, Cr, Co, Eu, La, P, Sm, Sc, Si, Th, Ti, U, V, Yb, Y, and Zr [41 ]. Elements showing concentration in the central portion of the seam, sometimes somewhat irregularly, are Ba, Ca, Mg, Na, and Sr [41 ]. Elements with irregular distribution patterns (which may in fact be a superposition of two or more patterns) include A1, As, Cd, Cu, Ge, Fe, Mn, K, Se, S, and Zn [41 ]. Patterns of elemental distributions for 36 elements in three sections of Beulah and one of Center lignites have been published [30,41]. For the four sets of data, the pattern of elemental distribution (e.g., enrichment at both margins, enrichment at the center, etc.) for any given element

272

is almost never the same in the four sections. Only Eu and Sc consistently show enrichment at both seam margins. A larger group of elements--A1, Sb, Cr, Co, Cu, Sm, Si, S, Ti, U, V, and Yb---are enriched in all cases either at one of the margins or at both. Iron consistently shows an irregular distribution, presumably reflecting concentration of pyrite in different portions of each of the seams. The variability of inorganic composition of stratigraphic sequences in the Beulah and Center mines was determined by sampling overburden, lignite, seam partings, and underclay, with analyses for 35 elements were by neutron activation or X-ray fluorescence techniques. Extensive tables of the data from this study have been published [74,113]. Examples of the patterns of distribution in the Beulah mine (Beulah-Zap bed) are shown in Figure 5.2, and patterns in the Center mine are shown in Figure 5.3. '

3.5

~, . . .

.

I

.

.

i

.

3

,,"

~

2.5

,=..,,.,

Z !

1.5

o

1

.~

0.5

m I-! 17/1 !i i II

i w=

0

IRON CALCIUM SILICON ALUMINUM MAGNESIUM SODIUM

-0.5 9

0

,

I

,

I

,

I

,

10000

Elemental concentration, ppm dry basis Figure 5.2. Vertical distribution of major elements in Orange pit, Beulah lignite [74, 113]. In Center lignite the elements concentrating at or near the margins of the seam are A1, Ti, Fe, C1, Sc, Cr, Co, Ni, Zn, As, Ru, Ag, Cs, Ba, La, Ce, Sm, Eu, and U [186]. These elements are, in most cases, associated with detrital constituents such as clays. These elements have ionic potentials in the range of 3 to 12, expected geochemically to form insoluble hydrolyzates, and usually occur as acid-insoluble species in chemical fractionation. Elements with ionic potentials less than 3 occur as hydrated cations and generally show even distribution through the seam.These elements usually appear in the ion-exchangeable fraction during chemical fractionation. They are Cd, Mn, Mg, Na, and Ca [186]. These elements are associated with the organic portion of the

273

2.15 1.85 1.55 1.25

am 1"7 !~1 !~ WI II

0.95 0

0.65

Iron Calcium Silicon Aluminum Magnesium Sodium

0.35 0.05

0

10000 Elemental concentration, ppm dry basis

20000

Figure 5.3. Vertical distribution of major elements in Center lignite [74,1113].

lignite or with authigenic minerals. V, K, and Sb increase toward the base of the seam [186]. Elements having no clear pattern of distribution are Se, Br, Cs, and Yb. The concentrations of the rare earth elements in this lignite agree with the rare earth abundance pattern in sedimentary rocks. The addition of detrital clay and silt at the beginning and end of peat deposition would increase Si, AI, Mg, Ca, Na, and K at the margins of the lignite. Elemental redistribution (particularly of those elements which are exchangeable) might then result from the flows of meteoric water or groundwater. Lignite of the Kinneman Creek bed shows the same major trends of elemental distribution: concentration at the margins, concentration in the lower part of the seam, an even distribution, and irregular distribution [74]. A1, Si, S, Sc, Fe, Co, Ni, Zn, As, Rb, Y, Zr, Ag, Ba, Ce, Sm, Eu, Yb, Th, and U concentrate at the margins. Elements concentrated in the lower part of the seam are C1, K, Ti, V, Cr, Cu, Ge, Se, Ru, Sb, Cs, and La. Elements showing an even distribution (sometimes tending toward a slight increase in concentration at the center of the seam) include Na, Mg, Ca, Sr, and Mn. Elements with irregular distribution are P, Br, and Cd. In the Baukol-Noonan mine sodium, which exists predominantly or wholly in ionexchangeable form, tends to concentrate near the center of the seam [187]. In contrast, arsenic concentrates at the seam margins. Estevan (Saskatchewan) lignite shows higher uranium values at the top and bottom of the

274 seam [ 188]. This behavior suggests uranium sorption by interaction of the organic matter in the lignite with circulating groundwaters after the lignite had been buried. Vanadium concentrates at the top of the seam [188], similar to North Dakota lignites [189]. Other work with Saskatchewan lignites shows that Zn, S, and Se concentrate near the top of the seam; Sb and Be show concentration toward the bottom [83]. (iii) Gulf Coast lignites. Ion-exchangeable barium and manganese decrease by a factor of two with increase in depth, whereas calcium, magnesium, strontium, and sodium do not show this behavior in Darco lignite [11]. The concentration gradient for barium and manganese may be a result of a chromatographic effect caused by the ions being carried downward by groundwater. Both barium and manganese have a high degree of ion exchange specificity. The molar ratio of ionexchangeable Ca/Ba increases from 64 at the upper margin of the seam to 100 at the lower margin [ 11]. This is typical of an ion-exchange process from groundwater, since Ba§ should replace Ca§ as long as the concentration of Ba§ in solution is adequate. The sodium content of two Texas lignites increases with increasing depth from the surface

(i.e., sodium content increases near the bottom margin of the seam) [ 190]. The percentage of the total sodium in a water-soluble form decreases as depth increases. The patterns of vertical variability of minerals in Martin Lake lignite reflect changes in the depositional conditions during accumulation of the lignite, and subsequent chemical changes during and after diagenesis [96]. The environment of deposition was characterized by abundant quartz (sand) prior to the development of the peat swamp. Clay was deposited at the end of peat accumulation and afterward. An increase of pyrite in the center of the seam may reflect changes caused by flow of meteoric water. The vertical variability of elements in Darco, Wall (Wyoming), and eastern Alabama lignite shows an even distribution for the ion-exchangeable portions of calcium (all three seams), magnesium (Wall and eastern Alabama), sodium (Wall), strontium (eastern Alabama), barium (Darco), and manganese (Darco) [191]. In comparison, the acid-insoluble portions of aluminum, potassium, silicon, and magnesium in the Darco seam showed high concentrations at the margins and near partings, with a sharp increase in concentration in approximately the center of the thickest lignite [ 191 ]. In four lignites, three from the Gulf Coast and one from the Northern Great Plains [11], some trace elements are enriched with respect to at least one seam boundary, i.e., a roof, floor, or parting. All four lignites showed enrichment of Be, Cu, Ga, Sc, Y, and Yb. Three of the lignites showed enrichment of Cr, Ni, and V; two, Ge, La, Pb, and Zr; and one lignite showed enrichment of Ce relative to surrounding sediments. These four coals, including Texas, Alabama, and North Dakota lignites, show little variation of ion-exchangeable calcium with seam height [14]. The variation in calcium correlated with petrographic composition. Similar behavior was observed for the ion-exchangeable portions of magnesium, strontium, barium, manganese, sodium, and potassium. This behavior is attributed to the influence of circulating groundwater on establishing the concentrations of the ion-

275 exchangeable cations. In particular, calcium is the major cation in most groundwaters, and the divalent calcium cation will preferentially replace monovalent cations in ion-exchange processes; calcium is the predominant cation in the coals studied [ 14]. Little vertical variation was observed for the trace elements in an Alabama lignite [139]. In eastern Alabama lignite the exchangeable elements showed minor fluctuations with depth [11]. Zones of high concentrations of fusinite and inertodetrinite also showed higher concentrations of exchangeable elements. 5.4.4 In-mine variability The variation of total sulfur between different sections of the same coal bed is very pronounced, except for cases of very low sulfur lignites [ 192]. This variability is attributed to an irregular distribution of pyrite [192]. Data from a survey of the variability of ash composition and fusibility of lignites from ten major mines in western North Dakota and eastern Montana [ 106,193] were treated statistically by determining the range and 2a (twice the estimated standard deviation) limits. For samples taken from a single mine, either at 6.1 m horizontal intervals or vertically through the bed, the 2a limits were of about the same order of magnitude as the 2~ limits for all the samples (i.e., representing the entire mine). This result suggests that much of the variation within a mine occurs within relatively short distances. In commercial operation, however, much of this variability may be eliminated as blending occurs during mining and subsequent handling and transportation. One exception to these findings is sodium, where samples taken close together showed smaller limits of variation than did the data for the entire mine [193]. Within a single vertical section much of the apparent variability of sodium content could be eliminated by expressing the sodium content as a percentage of coal rather than of ash. The observation is in keeping with more recent work [ 11,16,56] which shows that for most lignites most of the sodium is associated with the carbonaceous portion on ion-exchange sites. The organic association of sodium indicates that the concentration of sodium does not correlate directly with ash content. The composition of ash also varied significantly between mines. The variability within a mine was sufficiently unique that it could be considered to be a characteristic of that mine. Lignite samples from three mines in the Hagel bed showed a correlation between the percentage of ash and several kinds of colpate and porate palynomorphs [194]. These palynomorphs are of detrital origin, suggesting that much of the inorganic material contributing to ash is also of detrital origin. An extensive study has been made of the variability in inorganic composition and mineralogy of lignite from three pits in the Gascoyne mine [ 195], identified as the Red, White, and Blue pits. The elemental composition of the lignites is shown in Table 5.31 [195]. A qualitative comparison of the mineral components of these lignites is shown in Table 5.32; the major and minor minerals are listed according to apparent decreasing abundance, based on peak heights in the TABLE 5.31

276 TABLE 5.31 Variability of inorganic composition of lignite from three pits of the Gascoyne mine. (Data are shown as ~tg/g, dry basis) [195]. Element Aluminum Calcium Chromium Copper Iron Magnesium Molybdenum Potassium Silicon Sodium Strontium Titanium Zinc

Red Pit 5240 15290 9.4 5.6 5300 4719 13.1 367 13690 2041 172 429 10

Blue Pit White Pit 11480 8140 14070 19550 13.4 13.9 13.4 9.9 2810 3420 4885 6000 23.0 22 1299 358 33060 12550 5479 6119 129 94 1060 370 <20 9

X-ray diffractogram of the low-temperature ash.

TABLE 5.32 Variability of mineral constituents of lignite from three pits of the Gascoyne mine [195]. Red Pit Quartz Kaolinite*

Blue Pit Quartz Bassanite Kaolinite*

White Pit Quartz Bassanite Kaolinite*

Minor

Pyrite Bassanite Illite ? Barite

Pyrite Illite ? Barite

Pyrite Calcite

Rare

Muscovite Zircon

Siderite Chromite Zircon SrSO4

Barite Rutile SrSO4

Major

*Identified as the hydrated form, halloysite.

Carboxyl contents of the three lignites are similar, ranging from 2.2 meq/g for the White Pit lignite to 2.6 meq/g for the Blue Pit lignite. Generally the percentage of each element removed in chemical fractionation by ammonium acetate extraction is similar among the three lignites. Furthermore, those elements customarily removed in large percentages by ammonium acetate solution, such as sodium and magnesium, are similarly removed in large percentage from these

277 52% for the White. Extraction with disodium EDTA removes 40% of the aluminum from the Red Pit lignite, but only 16-18% from the other two lignites, suggesting that a much higher percentage of the aluminum may be present in coordination complexes in the Red Pit lignite. Chemical fractionation results are summarized in Table 5.33 [195]. In general the behavior of the major elements is similar among the three samples, and, of the minor elements, only strontium shows marked differences (compare the White Pit lignite with the other two). TABLE 5.33 Chemical fractionation results for lignite from three pits of the Gascoyne mine. (Results expressed as percent of element originally in the lignite.) [195].

Element Aluminum Calcium Chromium Copper Iron Magnesium Molybdenum Potassium Silicon Sodium Strontium

Red Pit NH4OAc HCI 0 52 74 14 12 38 5 0 0 38 97 0 23 52 10 0 0 2 82 1 73 21

Blue Pit White Pit Res. NH4.,OAc HCI Res. NIqa.OAc HCI 48 0 31 69 0 32 12 75 24 1 71 29 50 18 22 60 15 14 95 0 16 84 0 0 62 0 29 71 0 32 3 91 5 4 80 6 25 13 30 57 15 21 90 4 0 96 14 0 98 0 2 98 0 5 17 100 0 0 91 0 6 17 83 0 69 31

Res.* 68 0 71 100 68 14 64 86 95 9 0

*Percentage of element remaining in residue (i.e., not extracted by either reagent)

The comparison of the compositions of low- and high-sodium lignites from the Gascoyne and Beulah mines is shown in Table 5.34 [196]. In the high-sodium lignites the carboxyl, calcium, and barium are higher than in the lowsodium lignites from the same mine. Magnesium, aluminum, silicon, iron, and ash contents are lower in the high-sodium lignites than in the corresponding low-sodium lignites. The low temperature ashes of the lignites are similar. Chemical fractionation of these four samples shows a higher removal of aluminum by hydrochloric acid from the high-sodium lignites than from the low-sodium lignites from the same mine [196]. In all four samples the ratio of the amounts of aluminum removed to silicon removed range from 4:1 to 14:1 [ 196]. This value does not correspond to the AI/Si ratio in any common clay mineral, and suggests that hydrochloric acid selectively attacks the aluminum-containing layer in the clay. In the residue insoluble potassium is present in micaceous clays, titanium in futile, and barium as barite.

278 TABLE 5.34 Comparison of compositions (dry basis) of low- and high-sodium lignites from Gascoyne and Beulah mines [196].

Component Aluminum (a) Barium Calcium Iron Magnesium Manganese Potassium Silicon Sodium Ti tani um Carbon (c) Hydrogen Nitrogen Sulfur Oxygen Ash Carboxyl (d)

Gascoyne Low-sodium High-sodium 8740 7300 593 1268 17370 22790 3890 2540 2991 2588 123 163 1260 1430 28640 10920 1317 2694 1180 546 58.3 54.5 4.0 5.2 0.88 0.84 1.7 1.4 19.6 19.3 15.5 8.8 2.46 2.54

Beulah Low-sodium High-sodium 5420 2890 179 397 15610 18110 10240 5160 1476 979 52 24 916 ND (b) 8950 3530 1379 4625 503 583 61.4 66.4 4.1 3.6 0.42 0.87 3.2 1.2 18.2 19.5 12.6 8.4 2.47 2.76

Notes: a) Elements aluminum through titanium reported as ~tg/g, dry basis; b) Not determined; c) Elements C through O and Ash reported as weight percent, dry basis (oxygen by difference); d) meq/g dry basis.

5.4.5 State and regional variability Data on the composition of 413 samples of ashes from North Dakota lignites have been tabulated [ 197]. Average CaO content is high, at 31%. The highest concentrations occur in samples from the eastern part of the state's lignite fields (east of Range 97W). Na20 averages 6.5% but with a very high variability, having a standard deviation of 5, and a range of 0.1-27%. The highest SiO2 contents occur west of Range 97W. The average for the state is 27%; variability is high. The average A1203 content is 14%; that of Fe203, 12%. Neither of these components shows great variability or evident geographic trends. The variability among samples from a given mine was large for all of the mines sampled in the study. The variability of ash composition for samples taken from the Gascoyne mine was remarkable. For example, SiO2 ranged from 61.6% to 18.0%, CaO from 44.7% to 14.7%, and Na20 from 10.2% to 1.1%. Similar data for 82 samples of Montana lignite ash has been published [197]. The CaO content averaged 27%, with high variability but no evident trend with location. The average SiO2 content was 31%. The A1203 content averaged 20% with no location trend. Fe203 averaged 8% but with high variability. The average Na20 content was 2.2% with high variability. Even allowing for variability, this sodium content is markedly lower than that found in North Dakota lignites. Analyses of eighteen Northern Great Plains province samples show that this province has the highest mean values of Mg, Ca, Ba, and Na, and the lowest mean values of Si, Ti, K, Cr, Cu,

279 the highest mean values of Mg, Ca, Ba, and Na, and the lowest mean values of Si, Ti, K, Cr, Cu, Ga, La, Ni, Rb, Sc, Yb, and Zn of coals from the western coal provinces (Northern Great Plains, Gulf, Rocky Mountain, and Pacific) in the United States [198], when the data are expressed on an ash basis. On a whole-coal basis, this province has the highest means of Mg, Ca, Na, and Ba, and lowest means of Si, Al, K, Cr, Ga, La, Ni, Sc, U, Y, Yb, Zn, and Zr; pyritic and organic sulfur; and high-temperature ash. (In this work, high-temperature ash is prepared by heating a 10 g sample of coal slowly to 750"C, holding at that temperature over night, grinding the ash to -200 mesh, and drying at 750* for three hours [ 198].) A ternary composition diagram for Northern Great Plains province ashes is shown in Fig. 5.4 [ 198]. This diagram is prepared by normalizing SiO2, A1203, and the sum of Fe203, CaO, and MgO to 100%. These samples have SIO2/A1203 ranging from 0.73 to 3.76 and 20-59% of the other elements, which is a moderately high amount in comparison to the ashes of coals from other provinces in the United States.A survey of the inorganic composition of American coals includes data on the major element concentrations in six Fort Union lignites [8]. Five of these lignites were from North Dakota and the other was from Montana, but the origin of the samples was not otherwise specified. The ranges of concentrations are shown in Table 5.35. (Trace element data on the same samples were provided in Table 5.28.) TABLE 5.35 Major element concentrations in six Fort Union lignites [8]. Element Aluminum Calcium Chlorine Iron Magnesium

Range 0.31-0.89 1.70-3.80 0.01-0.03 0.40-O. 60 0.18-0.3 9

Element Potassium Silicon Sodium Titanium

Range 0.01-0.07 0.58-1.30 0.02-0.46 0.02-0.04

Analyses of 18 Gulf province samples show that on a whole-coal basis, the Gulf province coals have the highest mean values of Be, Ga, Mn, Sc, U, and pyritic, organic, and sulfate sulfur, and the lowest means of P and Ba [ 198] among the four western coal provinces. A ternary composition diagram for Gulf province ashes is shown in Fig. 5.5 [198]. The SIO2/A1203 ratio is fairly high, ranging from 1.78 to 4.88, and the other elements have a very wide range, 3-52%. For most major and trace elements, the ranges of concentrations in the Fort Union and Gulf Coast lignites overlap [21]. Comparison of mean concentrations plus or minus one standard deviation shows that aluminum, bromine, europium, lanthanum, nickel, scandium, selenium, and titanium concentrations tend to be higher for the Gulf Coast lignites in this data set, while the silver concentration is higher in the Fort Union lignites [199]. The ranges of concentrations for the other elements overlapped between the two sets of lignites.

280

SiO2

5O

A1203

Fe203 + CaO + MgO

Figure 5.4. Ternary composition diagram for ashes of Northern Great Plains province coals, normalized to 100% [198]. Compositions of 18 ash samples lie within shaded region.

SiO2

A1203

Fe203 + CaO + MgO

Figure 5.5. Ternary composition diagram for ashes of Gulf province coals, normalized to 100% [198]. Compositions of 18 ash samples lie within shaded region.

Useful surveys of data on ash composition have been published [115,131,200]. An extensive compilation of data on variability in the Beulah mine for about 30 elements has been

281 published [41]. A survey of variability of ash composition and fusibility of lignites from ten major mines in North Dakota and Montana, together with supplemental data on ash fusibility and information on the location and mining practices of each of the ten mines, has been published [106,193]. REFERENCES

10 11 12 13 14 15 16 17 18 19 20 21

U.S. Bureau of Mines, Technology of Lignitic Coals, U.S. Bur. Mines Inf. Circ. 7691, (1954). G.A. Richter, Cellulose from hardwood, Ind. Eng. Chem., 33 (1941) 75-83. P.L. Broughton, Silicified lignite from the Tertiary of south-central Saskatchewan, Can. J. Earth Sci. 13 (1976) 1719-1722. K. Hoehne, Late Tertiary silicified wood in the Rosenback coal seams near Latschach, Lake Faaker, Carinthia, Geologie 2 (1953) 185-189. R.B. Finkleman, The origin, occurrence, and distribution of the inorganic constituents in low-rank coals, in: H.H. Schobert (Ed.), Proceedings of the Low-rank Coal Basic Coal Science Workshop, U.S. Dept. Energy Rept. CONF-811268, (1982), pp. 69-90. R.B. Finkelman, Mode of occurrence of accessory sulfide and selenide minerals in coal, Proc. IX Intl. Carboniferous Conf., 1982. V.E. Swanson, J.H. Medlin, J.R. Hatch, S.L. Coleman, G.H. Wood Jr., S.D. Woodruff, and R.T. Hildebrand, Collection, chemical analysis, and evaluation of coal samples in 1975, U.S. Geol. Surv. Rept. 76-468, (1976). H.J. Gluskoter, R.R. Ruchm W.G. Miller, R.A. Cahill, G.B. Dreher, and J.K. Kuhn, Trace elements in coal: Occurrence and distribution. Ill. State Geol. Surv. Circ. 499, (1977). D. Readett, G. Springbett, L. Green, K. Quast and S. Hall, Borecore analysis of South Australian lignites, Proc. 2d Coal Res. Conf. New Zealand, 1987, Paper R5.3. R.N. Miller and P.H. Given, The association of major, minor and trace inorganic elements with lignites. I. Experimental approach and study of a North Dakota lignite, Geochim. Cosmochim. Acta, 50 (1986) 2033-2043. R.N. Miller and P.H. Given, A geochemical study of the inorganic constituents in some low-rank coals, U.S. Energy Res.Devel. Admin. Rept. FE-2494-TR-1, (1978). H.N.S. Schafer, Carboxyl groups and ion exchange in low-rank coals, Fuel, 49 (1970) 197-213. W. Beckering, H.L. Haight, and W.W. Fowkes, Examination of coal and coal ash by xray techniques, in: J.L. Elder and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Bur. Mines Inf. Circ. 8471, (1970), pp. 89-102. R.N. Miller and P.H. Given, Variations in organic [sic.] constituents of some low rank goals [sic.], in: R.H. Bryers (Ed.), Ash Deposits and Corrosion Due to Impurities in Combustion Gases, Hemisphere, Washington, 1977, pp. 39-50. .1.19. Hurley, personal communication, Grand Forks, ND, May 1986. M.E. Morgan, R.G. Jenkins, and P.L. Walker Jr., Inorganic constituents in American lignites, U.S. Dept. Energy Rept. FE-2030-TR21, (1980). M.E. Morgan, R.G. Jenkins, and P.L. Walker Jr., Inorganic constituents in American lignites, Fuel, 60 ( 1981) 189-193. M.E. Morgan, R.G. Jenkins, and P.L. Walker Jr., Analysis of the inorganic constituents in American lignites, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 25(1) (1980) 219222. J.K. Kuhn, F.L. Fiene, R.A. Cahill, H.J. Gluskoter, and N.F. Shimp, Abundance of trace and minor elements in organic and mineral fractions of coal, Ill. State Geol. Surv. Environ. Geol. Notes EGN88, (1980). J.P. Hurley, Oral presentation, Project Sodium semiannual sponsors' meeting, Grand Forks, N.D., November 1985. S.A. Benson, J.P. Hurley, E.N. Steadman, Distribution of inorganics, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1642, (1984),

282 22 23 24 25 26 27 28 29

30 31 32 33 34 35 36 37 38

39 40 41 42 43 44

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Univ. North Dakota Chem. Eng. Dept. Rept., January 1958. A.R. Cameron and T.F. Birmingham, Radioactivity in weatern Canadian coals, Geol. Surv. Canada Paper 70-72, (1970). D.T. Abbott, C.E, Styron, and V.R, Casella, Radionuclides in western coal, U.S. Dept. Energy Rept. MLM-3026, 1983. R. Riffaldi and R. Levi-Minzi, Adsorption and desorption of Cd on humic acid fraction of soils, Water Air Soil Pollut. 5 (1975) 179-184. S.A. Gordon, Adsorption as a means of accumulating germanium in coals. Nauch. Trudy Moscov. Gore. Inst. Sbornik, 27 (1959) 47-56. T.G. Komienko, Investigation of ways of entry of germanium into brown coals, Azerb. Khim. Zh., 3 (1962) 125-131. S.M. Manskaya, L.A. Kodina, V.N. Generalova, and R.P. Kravtsova, Interaction between germanium and lignin structures in the early stages of formation of coal, Geochem. Int., 9 (1972) 385-394. N.N. Banerjee, H.S. Rao, and A. Lahiri, Germanium in Indian coals, Indian J. Technol. 12 (1974) 353-358. V.V. Kuryukov and S.V. Paradeev, Germanium in some brown coals, Zap. Leningr. Gore. Inst. 45 (1963) 39-44. T. Stadnichenko, K.J. Murata, and J.N. Axelrod, Germaniferous lignite from the District of Columbia and vicinity, Science, 112 (1950), 209-214. T. Stadnichenko, K.J. Murata, P. Zubovic, and E.L. Hufschmidt, Concentration of germanium in the ash of American coal, U.S. Geol. Surv. Circ. 272, (1953). P.J. Clark, R.A. Zingaro, K.J. Irgolic, and A.N. McGinley, Arsenic and selenium in Texas lignite, Int. J. Environ. Anal. Chem., 7 (1980) 295-314. H.J. Gluskoter, Mineral matter and trace elements in coal, in: S.P. Babu (Ed.), Trace Elements in Fuel, American Chemical Society, Washington, 1975, pp. 1-22. R.B. Finkelman, Modes of occurrence of trace elements in coal, U.S. Geol. Surv. Rept. OF-81-89, ( 1981). G.D. Nicholls, Trace elements in sediments. Assessment of their possible utility as depth indicators, in: D. Murchison and T.S. Westoll (Eds.), Coal and Coal-bearing Strata, Elsevier, New York, 1968, pp. 269-307. D.J. Swaine, The organic association of elements in coals, Org. Geochem., 18 (1992) 259261. G.H. Groenewold, R.D. Koob, G.J. McCarthy, B.W. Rehm, and W.M. Peterson, Geological and geochemical controls on the chemical evolution of subsurface water in undisturbed and surface-mined landscapes in western North Dakota, N.D. Geol. Survey Rept. of Invest. 79, (1983). C.S. Fulton, The geological, geochemical, and hydrological factors affecting the distribution of sodium in lignite in west central North Dakota, in: M.L.Jones (Ed.), Technology and Utilization of Low-rank Coal, U.S. Dept. Energy Rept. DOE/METC86/6036(Vol.2), (1986), pp. 559-588. F.T.C. Ting, Geochemistry of sodium in North Dakota lignite, in: M.L.Jones (Ed.), Technology and Utilization of Low-rank Coal, U.S. Dept. of Energy Rept. DOE/METC86/6036(Voi.2), (1986), pp. 589-599. B. Mason, Principles of Geochemistry, Wiley, New York, 1958, pp. 155-157. R.G. Roaldson, Inorganic constituents of a stratigraphic section of Center mine seam #1, University of North Dakota Energy Research Center Rept. UNDERC-IR-3, (1983). H.H. Schobert, S.A. Benson, and W.B. Hauserman, Inorganics, physical properties, and moisture in low-rank coals, Grand Forks Energy Technology Center monthly report, November 1982. A.R. Cameron and T.F. Birmingham, Petrographic and chemical properties of a lignite from Estevan, Saskatchewan, Geol. Surv. Canada Paper 71-8, (1971). P. Zubovic, Chemical basis of minor-element associations in coal and other carbonaceous sediments, U.S. Geol. Surv. Prof. Pap. 424-D (1961). A.F. Duzy, M.P. Corriveau, R. Byrom, and R.E. Zimmerman, Western coal deposits-pertinent qualitative evaluations prior to mining and utilization, in: G.H. Gronhovd and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Dept. of Energy Rept. GFERC/IC-77/1, (1978), pp. 13-42.

289 191 192 193 194 195

196 197 198 199 200

R.N. Miller and P.H. Given, The association of major, minor and trace inorganic elements with lignites. II. Minerals, and major and minor element profiles, in four seams, Geochim. Cosmochim. Acta, 51 (1987) 1311-1322. A. Magnusson, Sulfur in North Dakota lignite, Unpublished manuscript, University of North Dakota, 1950(?). E.A. Sondreal, W.R. Kube, and J.L. Elder, Analysis of the northern Great Plains lignites and their ash: A study of variability, U.S. Bur. Mines Rept. Invest. 7158, (1968). E.N. Steadman, Palynology of the Hagel lignite bed and associated strata, Sentinel Butte Formation (Paleocene), in central North Dakota, M.A Thesis, University of North Dakota, Grand Forks, ND, 1985. J.P. Hurley, B.G. Miller, M.L. Jones, R.B. Finkelman, and J.D. Yeakel, Correlation of coal characteristics and fouling tendencies of various coals from the Gascoyne mine, in: M.L. Jones (Ed.), Technology and Utilization of Low-rank Coal, U.S. Dept. Energy Rept. DOE/METC-86/6036(Vol. 1), (1986), pp. 76-85. J.P. Hurley and S.A. Benson, Comparative elemental associations in lignites having significant within-mine variability of sodium content, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 29(6) (1984) 210-214. S.A. Cooley and R.C. Ellman, Analyses of the northern Great Plains province lignites and subbituminous coals and their ash, U.S. Dept. Energy Rept. DOE/GFETC/RI-81/2, (1981). D.C. Glick and A. Davis, Variability in the inorganic content of United States coals--a multivariate statistical study, U.S. Dept. of Energy Rept. DOE/PC/30013-F10, (1984). H.H. Schobert, Unpublished data, University Park, PA, 1988. W.A. Selvig and F.H. Gibson, Analyses of ash from United States coals, U.S. Bur. Mines Bull. 567, (1956).

290

Chapter 6

B E H A V I O R OF I N O R G A N I C C O M P O N E N T S OF LIGNITES

This chapter treats the fundamentals of the behavior of the inorganic components of lignites: inorganic properties and reactions that would be measured in the laboratory, and methods for predicting properties of ashes and slags. This chapter thus bears the same relationship to Chapter 5, which treated the inorganic composition of lignites, as Chapter 4 related to Chapter 3. The behavior of the inorganic portion of the lignite during utilization may well be, as numerous authors have claimed, just as important, or perhaps even more important, than the behavior of the carbonaceous portion. This is especially so in the dominant commercial application of lignites, combustion in electric power stations. Approaches to the reduction or removal of inorganics are discussed in Chapter 10, behavior of inorganics in combustion in Chapter 11, and behavior during other potential utilization processes is discussed in Chapter 12. 6.1 LOW-TEMPERATURE

ASHING

The objective of low-temperature ashing is to isolate the inorganic constituents --particularly minerals--from the carbonaceous part of the coal without altering the composition or phase relationships of the minerals. Low-temperature ashing is usually performed at temperatures in the range 125-150~

which are sufficiently low so that it might be presumed that no thermal

alteration of the coal minerals would occur. (In fact, this presumption is not entirely correct.) An oxygen plasma, generated by subjecting a relatively low pressure of oxygen to a radio frequency field, is used to effect reaction at these temperatures. Lignites require very long times to complete low-temperature ashing. One of the limiting factors, not unique to lignites, is the surface area of the sample exposed to the oxygen plasma. Ashing occurs on the upper surface of the sample; consequently, the ash accumulates above the unreacted coal and may restrict access of the oxygen. Coals of higher mineral matter content require longer times for complete ashing [1], perhaps in part because so great a volume of ash builds up atop the unreacted coal. The build-up of ash can be counteracted to some extent by occasionally interrupting the process to stir the partially ashed samples. Ten samples from the Beulah-Zap (North Dakota) seam required ashing times of 2(X)-250 h with an oxygen pressure of 133 Pa and oxygen flow of 270 cm3/min [2]. A strategy for reducing ashing time is to reduce the size of the sample. This approach has the obvious drawback that the

291 quantity of ash produced is reduced accordingly; this is particularly serious if the ash is being generated for subsequent experiments. Low-temperature ashing may in fact not even be suitable for studying the mineralogy of lignites without appropriate sample pretreatment [3,4]. The cations associated with the carboxyl groups may be responsible for the long (relative to bituminous coals) ashing times; the longer the ashing time, the greater the chances of altering the original minerals by oxidation. During the first 24 h of reaction, 55-68% of organic matter is removed from low-rank coals, but 77-99% from high-rank coals [ 1]. Demineralizing the lignite with hydrochloric and hydrofluoric acids converted it to a form in which ashing was essentially complete in 24 h and no sulfur was fixed in the ash. Significant reductions of ashing time were achieved lbr eleven lignites (nine North Dakota and two Gulf region) by removing the cations. The lignites were treated three times with 1M ammonium acetate solution at 70~ for 24 h, vacuum dried, and ashed at 150 W rf power, 100 cm3/min oxygen flow, and 132 Pa pressure [5]. X-ray diffraction analysis of the ashes produced by this procedure showed no difference from ashes produced in control experiments without the ammonium acetate pretreatment, except for the presence of bassanite in the control samples. Ammonium acetate pretreatment reduced the ashing time for Indian Head (North Dakota) lignite from 420 to 265 h [6]. Details of the experimental procedure have been published in various sources [ 1,7]. The low-temperature ash yield from low-rank coals is much greater than the calculated mineral matter content [8]. The alkali and alkaline earth cations inhibit the ashing process such that in some cases complete ashing is not achieved, providing an artificially high ash weight. Furthermore, some of the combustion products--oxides of carbon, nitrogen and sulfur--can react with the highly dispersed cations and become incorporated in the ash, again increasing the weight relative to that expected from the mineral matter. The cations give rise to products, such as carbonates, sulfates, or oxides, that were not present in the original lignite [1]. Up to 50% of the low-temperature ash produced from untreated lignites is an artifact of the ashing procedure, as a result of fixation of carbon dioxide and sulfur dioxide by alkali or alkaline earth cations to form carbonates and sulfates [9]. Their presence interferes with determinations of carbonate or sulfate minerals which in fact originally in the lignite. These problems can be eliminated by removing the cations before ashing. The best direct measure of mineral matter content is obtained by adding the low-temperature ash obtained from acid-washed lignite and the weight of the acid-soluble cations. Thus MM = CLTA [(100 - A) / 100] + A where MM is the mineral matter content, CLTA the low-temperature ash from dry acid-washed lignite, and A the weight loss observed on acid washing [8]. Despite these concerns, lowtemperature ashing is the best method currently available for determining the pyrite and detrital minerals in lignite [ 1].

292 Exposure of ash samples to the laboratory atmosphere for 10 min. causes a substantial increase in weight [1]. For a suite of thirty samples, this increase in weight resulted in a corresponding mean increase in calculated mineral matter content of the coal by 0.60. This error can be avoided by using a desiccant in the purge system for the asher, keeping the samples covered when they are first removed from the asher, and storing the samples in a desiccator until they are cool [ 1]. Low-temperature ashing of a suite of ten lignites showed quartz and bassanite as major phases in all samples [2]. Pyrite and calcite occurred in eight of the ten [2]. Kaolinite was the only clay mineral identified, observed in nine samples [2]. Those lignites containing sodium greater than 8.6% Na20 in the ash (i.e., the ash as customarily prepared at high temperature for determination of ash yield and ash composition) contained sodium nitrate as a major phase in the low-temperature ash [2]. Bassanite, the hemihydrate of calcium sulfate, is frequently detected in the low-temperature ashes of lignites. Its presence is an artifact of the experiment, and not an indication of its occurrence in the lignite. Two mechanisms explain the formation of bassanite during lowtemperature ashing. The first is partial dehydration of gypsum [ 10]: CaSO4"2H20 ~ CaSO4"0.5H20 + 1.5 H20 Fixation of sulfur by calcium provides a second route to bassanite. In most lignites much of the calcium is present as cations associated with the carboxylate groups. As the lignite structure is decomposed during ashing, the calcium cations become free to react with other available species. At the same time, the organic sulfur is being oxidized. Thus the potential exists for the reaction [2,5]: Ca +2 + 8 0 2 + 0 2 + 0.5 H 2 0 ~

CaSO4"0.5H20

Nine North Dakota and two Gulf Coast showed no evidence for the presence of gypsum (by X-ray diffraction), but bassanite was found in the low-temperature ashes of those samples from which the exchangeable calcium cations had not been removed [5]. Unusually high amounts of sodium and calcium sulfates in lignite ashes derive from fixation of organic sulfur [11]. Increasing the power level of the asher decreases sulfur fixation [ 1]. Organic sulfur first converts to sulfur trioxide, which may then be fixed if carbonate minerals occur in the lignite, or if carbonates can be formed in the ashing process. Ashing a lignite demineralized with hydrochloric and hydrofluoric acids resulted in no sulfur retention [1]. However, when sodium or calcium were back-exchanged onto the demineralized lignite, 80--100% of the sulfur was fixed as sulfates [ 1].

293 6.2 R E A C T I O N S OF E X C H A N G E A B L E CATIONS

6.2.1 Reactions during ashing or combustion Even though large percentages of the alkali and alkaline earth elements in lignite are present as ion-exchangeable cations, possibly dispersed at an atomic level, rather than as discrete mineral particles, the role of these elements in the high temperature inorganic chemistry of lignites cannot be neglected. As the lignite is heated or burned, the cations will at some point become liberated by decarboxylation of the lignite structure and eventually contribute in some fashion to the formation of ash. Furthermore, the cations can also catalyze further reactions of the char (e.g., [ 12]) and participate in sulfur fixation (e.g., [13]). In general, those low-rank coals with high concentrations of alkali elements tend to produce much more complex mineral assemblages upon ashing at 1000~ than do coals with lower alkali concentrations [ 14]. Alkalies participate in formation of aluminosilicate structures, rather than, for example, forming simple oxides or sulfates. Calcium, however, may often be found as calcium sulfate, which forms by reaction of organically-bound calcium with pyritic or organic sulfur. The principal transformations in the ashing of low-rank coals to 1000~ are oxidation of pyrite to hematite and magnetite, fixation of sulfur by organically bound alkali and alkaline earth elements, and dehydration and reordering of clay structures accompanied by interstitial infilling by alkali and alkaline earth cations released during decomposition of carboxyl groups. Quartz appears to be unreactive to 1000~ Slags obtained under controlled laboratory conditions from ion-exchanged, low-sodium, and high-sodium Beulah lignites show that when a higher percentage of alkali or alkaline earth cations are present in the lignite structure, a larger percentage of complex calcium or sodium aluminosilicates will be found in the slag [ 15]. Slag from the low-sodium lignite contained spinel and pyroxene but none of the feldspathoid minerals commonly found in slag or ash deposits. However, slag from high-sodium lignite contained melilites, hauyne, and nepheline, feldspathoids typically found in ash deposits from high-fouling lignites (Chapter 11). The availability of organically bound sodium and calcium cations for substitution into aluminosilicate mineral structures is a prerequisite for producing minerals typical of boiler tube fouling deposits. The effect of the exchangeable cations on altering the ash chemistry, especially at high temperatures, is illustrated by the data in Table 6.1 [16], X-ray diffraction analyses of ashes produced at 150 ~ 750 ~ and 1000 ~ C from samples of Gascoyne (North Dakota) lignite taken from two pits in the mine, with and without ammonium acetate treatment to remove the cations. The minerals are listed in Table 6.1 in decreasing order of the intensity of the diffraction peaks. For the Red pit lignite 100% of the sodium, 96% of the magnesium, 78% of the calcium, and 22% of the potassium were removed by ion exchange [17]. Comparable data for the Blue pit lignite were 100, 96, 85, and 19%, respectively. In the untreated sample of the Blue pit lignite, melilites and nosean formed at 750 ~ These feldspathoids are typical of materials found in ash deposits on boiler tubes (Chapter 11). They contain alkali or alkaline earth cations in a silica-

294 TABLE 6.1 Compositions of ash from Gascoyne lignite with and without cation removal [ 16]. Ashing Temperature, ~ 750 ~ Quartz Anhydrite Hematite Hematite Hauyne

Lignite (and treatment) Red pit (untreated)

150~ Quartz Kaolinite Pyrite Magnetite

1000~ Quartz Anhydrite Pyroxene

Red pit (ion-exchanged)

Quartz Kaolinite Pyrite

Quartz Anhydrite Hematite Magnetite

Quartz Hematite Magnetite Anhydrite

Blue pit (untreated)

Quartz Kaolinite Pyrite Bassanite*

Anhydrite Quartz Hematite Magnetite Nosean Melilite

Anhydrite Melilite Hauyne Quartz

Blue pit (ion exchanged)

Quartz Kaolinite Pyrite

Quartz Anhydrite Hematite Magnetite

Quartz Anhydrite Melilite*

*Identification questionable

deficient aluminosilicate structure. In comparison, feldspathoids did not form at 750~ in the ionexchanged sample. Similar behavior is seen for the Red pit lignite. Abundant sodium encourages the formation of feldspathoids, particularly melilites. The removal of sodium and large reductions in calcium content either eliminate the formation of feldspathoids or increase the temperature at which they form. Data for eight other lignites, showing the mineralogical composition as determined for ash and slag samples by X-ray diffraction, are given in Table 6.2 [18]. The behavior of the exchangeable ions in Savage (Montana) lignite was studied in a droptube furnace having a combustion temperature of approximately 1723~ [19]. Partially burned particles were examined by scanning electron microscopy with energy-dispersive X-ray analysis. Devolatilized lignite (corresponding to a 100 ms residence time and 47% weight loss) had magnesium, aluminum, calcium and sulfur still dispersed throughout the char. At 115 ms (57% weight loss), submicron grains of ash formed as a result of the release of inorganics from the carbonaceous material. No silicon was detected in the grains, a fact which rules out clay minerals as their possible origin. The grains may be oxides, sulfides, or carbides. Some vaporization loss occurs, amounting to about 20% of the magnesium, a small amount of the calcium, and less than 1% of the aluminum. With extensive char burnout (130 ms, 72% weight loss), ash on the surface

295 TABLE 6.2 Mineralogical composition of ash and slag samples by X-ray diffraction* [18].

Beulah (low sodium)

125" Quartz pyrite Kaolinite Bassanite

Beulah (hi gh sodium)

Quartz Bassanite Kaolinite

Pyrite

Center

Quartz Bassanite

Pyrite

Ashes 750* Quartz Hematite Magnetite Anhydrite Anhydrite Hematite Magnetite Quartz Melilite Hauyne Anhydrite Hematite Quartz

Kaolinite Choctaw

Quartz

Pyrite

Kaolinite Bassanite Plagioclase Falkirk

Quartz Kaolinite

Pyrite

Magnetite Indian Head

Quartz

Pyrite

Kaolinite Bassanite

Pike County

Quartz

Pyrite

Kaolinite San Miguel

Heulandite

Quartz Kaolinite

Pyrite Bassanite Plagioclase

Anhydrite Quartz Hematite Magnetite Plagioclase Anhydrite Quartz Hematite Hematite Melilite Anhydrite Quartz Hematite Nosean Melilite Hauyne Anhydrite Quartz Pyrite Heulandite Anhydrite Hematite Quartz Anorthite Melilite

*Listed in decreasing order of x-ray peak intensity.

1000" Anhydrite Pyroxene Magnetite Hauyne Hematite Quartz Anhydrite Melilite Magnetite Hematite Hauyne

Quartz

Slag 1300" Anhydrite Pyroxene Magnetite

Melilite Hauyne Nepheline Magnetite Quartz Corundum

Corundum Anhydrite Hauyne Pyroxene Melilite Hematite Quartz Anhydrite Hematite

Quartz

Magnetite Plagioclase Pyroxene Anhydrite Akermanite Quartz Anhydrite Pyroxene Melilite Hematite Magnetite Hauyne Melilite (Amorphous) Hematite Anhydrite Hauyne Magnetite Pyroxene Sodium Sulfate Anhydrite Hematite Melilite Anorthite Quartz Anorthite (Amorphous) Hematite Quartz Magnetite Anhydrite

296 of the char grows both in terms of particle size and of coverage of the char surface. At this point silicon is observed as a component of the ash. It may derive from two sources: incorporation of micron-sized clay mineral particles into alkaline earth-rich ash, or reduction of silica to silicon monoxide in the interior of the char and transport of SiO to the surface. At a late stage of char burnout (150 ms, 82% weight loss) agglomeration and growth of ash particles has occurred. Whether the particles melt to form glassy spheres or remain as solid aggregates depends on whether or not the released cations can react with available mineral matter such as silica or clay particles. Sodium acetate reacts at 1000~ with kaolinite to produce nepheline as the sole product, but sodium sulfate reacts to form both carnegieite and nepheline [ 15]. The chemical formulas of these two materials are identical, but their crystal structures differ, and consequently their physical properties are also different. Carnegieite is not present in ashes produced from lignites under laboratory conditions at 1000~

but has been identified as a reaction product of mullite refractory

in the walls of a slagging gasifier. Reactions of various mixtures of calcium acetate, calcite, kaolinite, pyrite, sodium acetate, sodium sulfate, and quartz were used to model inorganic reactions during ashing [15]. When calcite was used as a calcium source, calcium oxide was present as a decomposition product in mixtures heated to 750~

but when calcium acetate was used as a calcium source, no calcium

oxide was observed. This suggests that the source of calcium for the formation of complex calcium aluminosilicates is primarily organically bound calcium, with only minor amounts contributed by any calcite that might be present. In the competitive reaction of sodium and calcium acetates with kaolinite, nepheline and gehlenite formed at 1000~

[ 15]. Since the former contains sodium but no calcium, whereas the

latter contains calcium and no sodium, it appears that there is no mutual substitution of sodium and calcium into the same aluminosilicate framework. The reaction of kaolinite with sodium sulfate and calcium acetate also produces hauyne in addition to gehlenite and nepheline. Hauyne, (Na, Ca)8(Si,A1)12024(SO4)2, is a silica-deficient feldspathoid. Hauyne is also a product of the reaction of sodium acetate, calcium acetate, pyrite, kaolinite, and quartz. The silica-deficient structure forms in this case even in the presence of quartz. This suggests that at 1000~ quartz is unreactive toward alkali or alkaline earth cations or other aluminosilicates. Calcium acetate reacts with pyrite to produce anhydrite at both 750 ~ and 1000~

[15].

Based on comparison of X-ray diffraction peak intensities, sulfur fixation by reaction of calcium acetate with pyrite is greater than in the reaction of calcite with pyrite. Pyrite oxidation and thermal decomposition of calcium acetate both occurred in the temperature range 400-500~

whereas

calcite decomposition does not begin until about 7500C. Consequently, a reactive form of calcium available for sulfur fixation is not as readily available in the calcite-pyrite system as in the calcium acetate-pyrite system. To evaluate the relative contributions of sodium and calcium to feldspathoid formation, mixtures of sodium acetate with kaolinite, calcium acetate with kaolinite, and sodium and calcium

297 acetates with kaolinite were heated under similar conditions as actual lignite ashes. Neither the sodium acetate - kaolinite nor the calcium acetate - kaolinite system showed evidence of substitution into the kaolinite structure until a temperature of 1000~

was used. The reaction

products were, respectively, nepheline and gehlenite [ 17]. In the system containing both acetates, nepheline formed at 750~

This may indicate a cooperative effect of sodium and calcium, but the

fact that nepheline was the observed product indicates that sodium is more mobile in filling the void spaces in the kaolinite structure. This agrees with pilot- and full-scale combustion practice, which show that lignites of high sodium and moderate calcium content are very likely to produce severe ash deposition (Chapter 11). 6.2.2 Behavior of calcium in lignite liquefaction The formation of calcium carbonate deposits during liquefaction of low-rank coals is a widely experienced operating problem [20,21], reported for the Exxon Donor Solvent process, the Solvent Refined Coal (SRC I) process, and Project Lignite. Formation of solid deposits during liquefaction can cause several problems, including a reduction of reactor volume, which in turn reduces slurry residence time; reduced conversions and a higher viscosity of the bottoms; and, in severe cases, plugging of the reactor or the downstream process equipment. (Some of the solids which form during liquefaction are carbonaceous coke-like materials, rather than inorganic materials.) Liquefaction of coals of any rank can also lead to inorganic deposits; in lignites these are predominantly composed of calcium carbonate, which grows in layers on the reactor walls or around inert particles in the reactor. The latter type of growth is sometimes referred to as "oolites." The mechanism for calcium carbonate formation is not known. Oolites tend to form in sizes one mesh size larger than the coal being liquefied; that is, feeding -100 mesh coal produces oolites of 50x100 mesh [21]. The oolites are not swept out of the reactor by normal fluid flow. Deposit accumulation is not affected by temperature, space velocity in the reactor, solvent quality or boiling range, or hydrogen gas rate. Both oolite formation and wall scale accumulation increase with pressure from 10 to 17 MPa [21]. No build-up of calcium carbonate was observed during liquefaction of Beulah lignite in a stirred autoclave [22]. Thermogravimetric analysis indicated about 5.7% calcium carbonate in the solids. High concentrations of carbon dioxide and water in the autoclave may help retard formation of agglomerates of the carbonates. The growth rate of calcium carbonate is a function of the amount of ion-exchangeable calcium in the coal [20]. Decomposition of the carboxylate salts in the coal triggers the deposition of calcium carbonate. Growth of oolites or wall deposits is unaffected by temperature, space velocity in the reactor, solvent donor quality, or hydrogen consumption. However, the rate of formation does increase with increase in reactor pressure [20]. During liquefaction of a North Dakota lignite, the wall scale forms in the first stages of the reactor and the oolites are calcite. In the later stages of the reactor the deposits are not calcium carbonate but rather a sodium magnesium carbonate, eitelite [21]. During liquefaction of Texas lignite, the wall scale is rare form of calcium carbonate, vaterite, but the oolites are mainly calcite.

298 Most oolites have surfaces of calcium carbonate, although some have surfaces of iron sulfide. The calcium carbonate grows around "seeds" of mineral matter. For wall scale, the surface next to the reactor wall itself is primarily iron sulfide with small amounts of nickel sulfide, suggesting that sulfidation products of the reactor wall provide growth sites for calcium carbonate. The surface of the wall scale exposed to the reactor contents is very similar to the composition of the oolites. Various control techniques include removal of any free-flowing solids, washing the reactor walls with acid during shutdowns, or removal of calcium from the lignite by ion exchange. In addition, the pretreatment of the lignite with sulfuric acid, sulfur dioxide, or iron(II) sulfate, to form calcium sulfate, has been an effective strategy. Oolite growth can be controlled if the withdrawal rate of the solids can be kept sufficiently high to insure that the residence time is not adequate for growth. Tests with Wyoming subbituminous coal showed that in the absence of any control strategy 40% of the ash formed from the reactor solids was calcium carbonate [21]. The fraction of reactor solids ash as calcium carbonate for various pretreatments of the feed coal was 20% for ion-exchange with sodium sulfate, 8% for treatment with sulfur dioxide, and 1% for treatment with iron(II) sulfate and sulfuric acid [21 ]. 6.3 M I N E R A L R E A C T I O N S AT E L E V A T E D T E M P E R A T U R E S

This section discusses high-temperature reactions of some of the minerals occurring in lignites and their ashes. A further discussion of mineral reactions, as they specifically pertain to boiler fouling and slagging, is given in Chapter 11. 6.3.1 Anhydrite Anhydrite was a major mineral phase in ash samples prepared in the temperature range 600--800"C from ten lignites [2]. In some ashes anhydrite appeared at temperatures as low as 2000C. Anhydrite is produced by the total dehydration of gypsum. The formation of the hemihydrate, bassanite, has been discussed in Section 6.1. Anhydrite forms by dehydration of bassanite at 190-200~

[2]. The dehydration of bassanite forms anhydrite found in ashes of Greek

lignites prepared at 300-500"C [23]. Another potential source of calcium sulfate is fixation of organic sulfur by organically bound calcium cations [18,24,25]. Anhydrite is often a major mineral phase in ashes produced at 750 ~ or 1000~ and is present in most lignite ash slags [ 18]. Above 850~

anhydrite can react with calcium sulfide to form calcium oxide:

3CASO4 + CaS ~ 4CaO + 4S02

In ash systems it is likely that any calcium oxide that forms will react with other available species, such as silicates or aluminosilicates. Calcium made available by decomposition of anhydrite in

299 ashes of Beulah-Zap lignites reacted to form members of the gehlenite- akermanite solid solution series [2]. Samples which contained less than 0.98% Na20 also formed bredigite between 1100 and 1200~ Anhydrite observed in ashes of Greek lignites may also arise from reactions of pyrite with calcite [23]. 6.3.2 Bredigite Bredigite, Ca2SiO4, forms between 1000 and 1200~

It formed in ashes of Beulah-Zap

lignites which had less than 0.98% NaeO, but not in ashes with greater than 7.35% Na20 [2]. Bredigite forms after the gehlenite - akermanite minerals. Since the latter derive their silica and alumina from kaolinite, the silica in bredigite likely derives from quartz. Lignite ashes of high sodium content tended to form sodium sulfates when heated to intermediate temperatures but sodium silicates at higher temperatures [2]. However, low-sodium ashes formed bredigite at the higher temperatures [2]. 6.3.3 Calcite Calcite begins to decompose at 6750C, liberating calcium oxide. The calcium oxide can participate in sulfur fixation reactions, the specific products depending on the temperature. Below 650 ~ calcium sulfite is stable. Between 650 ~and 850~the sulfide and sulfate are stable: 4CASO3 ---- 3CASO4 + CaS Above 850 ~ the anhydrite decomposes as shown previously 3CASO4 + CaS --* 4CaO + 4S02

with the oxide then reacting with available alumina and silica to form the gehlenite - akermanite series. Although the decomposition of calcite can supply reactive calcium to the ash system, liberation of calcium from calcite requires higher temperatures than does the formation of reactive calcium species from organically bound calcium. For example, the reaction of calcium acetate with kaolinite produced an amorphous phase and CaO at 750~ and gehlenite at 10000; in comparison, the reaction of calcite with kaolinite produced an amorphous phase and unreacted calcite at 750 ~ and a mixture of gehlenite, mullite, and CaO at 1000~ [4]. 6.3.4 Feldspathic minerals The

high-temperature

formation

of

feldspathic

minerals

(e.g., melilites,

(Na,Ca)2(Mg,Fe,AI)(Si,AI)207, nosean, Na8AI6Si6024SO4, and hauyne) is of concern because of

300 the occurrence of these species in troublesome ash deposits on boiler tubes. These minerals contain calcium and sodium cations in a silica-deficient aluminosilicate structure. Their formation is a result of the reordering of clay structures and the filling of interstitial spaces in the rearranging clay structures by relatively mobile cations. A comparison of two lignites from the Gascoyne mine, one from the Red pit and the other from the Blue pit, showed that the latter, which is higher in sodium (2694 ~tg/g Na vs. 1317 ~g/g, dry basis) formed abundant feldspathic minerals both at ASTM ashing conditions (7500C) and at 1000*C [26]. In contrast, hauyne was the only feldspathic mineral produced from the Red pit lignite, and then only by ashing at 10000 [26]. In general, lignites containing higher amounts of sodium tend to form complex aluminosilicates--melilites, hauyne, nepheline, nosean, and pyroxenes--at lower temperatures

(e.g., 7500C) than samples high in calcium [18]. High-sodium coals are notorious for producing severe deposition of ash in boilers (Chapter 11), and these complex aluminosilicates are commonly found in steam-tube deposits from combustion of most lignites. A proposed mechanism to explain these observations is based on the similarity of the temperature at which clay structures begin to rearrange and the alkali or alkaline earth cations become available for reaction [26]. The dehydration of kaolinite and other clays occurs around 4000C [27]. Interstitial voids are created as a result of removal of --OH during dehydration; the interstitial spaces could result in partial or complete collapse of the crystalline structure of the clay. However, at a temperature roughly the same as that at which clays dehydrate, sodium and other cations become available for reaction as they are freed by thermal decarboxylation of the lignite structure. At higher temperatures, say about 900~

the additional energy provided to the system

expands the dehydrated clay structure, thereby increasing interstitial void spaces. The increased temperature also increases the mobility of the sodium and calcium cations, affording them greater ability to penetrate the clay structure. In the case of a low-sodium lignite, fewer sodium cations are available to be released during decarboxylation. Therefore, the probability of sufficient sodium cations entering the collapsed, dehydrated clay structures and forming new aluminosilicate phases at low temperatures is limited. Thus feldspathoids are not detected in the ASTM ash of the Red pit Gascoyne lignite, for example. In a high-sodium lignite, abundant sodium cations are released during decarboxylation, saturating the clay structure even in its collapsed state at temperatures below 900~

Thus complex aluminosilicates form even at the comparatively low temperatures of

the ASTM ashing, as evidenced by the results obtained with the Blue pit Gascoyne ash. Further verification of this proposed mechanism is afforded by ashing the lignites from which most of the exchangeable cations had been removed by treatment with aqueous ammonium acetate. (This treatment removes 100% of the sodium and 75--85% of the calcium and magnesium [26].) No new aluminosilicate mineral phases were observed in the ASTM ash of either Gascoyne sample, and at 1000~ the only mineral identified, and that questionably, by X-ray diffraction was melilite and only from the Blue pit lignite [26].

301 6.3.5 Gehlenite - Akermanite Series Gehlenite, Ca2A12SiOT, and akermanite, CazMgSi2OT, form a solid solution series. Their X-ray patterns are very similar, making them difficult to distinguish. Members of this series are likely to form when the CaO content of the system is greater than A1203. Formation of these species rarely occurs below about 900~

since the formation of the gehlenite-akermanite minerals

occurs at temperatures comparable to the decomposition temperature of anhydrite, these minerals may form at the expense of anhydrite [2]. The alumina and silica derive from kaolinite, since at 900* kaolinite is amorphous and more likely to undergo reaction than is quartz, which is still crystalline at 900 ~ [2]. The gehlenite- akermanite series was found in all of a suite of ten lignite ashes in the temperature range l l00-1200oc [2]. In some samples these minerals appeared at temperatures as low as 900~ Under laboratory conditions, the aluminosilicate solid solutions appear in ashes formed at 750~

[28]. The intensity of characteristic peaks in the X-ray diffractogram increases for ashes

produced at 1000~ relative to ashes produced at 750~

while at the same time the intensities of

quartz peaks decrease and the kaolinite peaks disappear. Melilite solid solution phases are common in slags poor in A1203 and rich in CaO [28]. A source of these phases may be the reaction of bassanite with kaolinite [28]. The specific aluminosilicates which form at high temperature may be dependent on the types of cations available for substitution into the clay structures. For example, lignite from the Blue pit of the Gascoyne mine is known to be high fouling, with abundant organically bound sodium. The phases observed in the X-ray diffractogram of ash produced from this lignite at 1030~ include gehlenite, anhydrite, hematite, lazurite, and nosean [28]. In comparison, the relatively low-fouling lignite from the Red pit of the Gascoyne mine formed quartz, anhydrite, augite, and hematite under the same conditions. 6.3.6 Hematite Hematite was a major phase in seven of ten lignite ashes [2]. Hematite is present in the temperature range 300-1000~

[2]. Magnetite forms in the range 800-1200~

with hercynite

forming after magnetite at high temperatures [2]. Oxidation of iron compounds to hematite occurs during ashing of lignites at 750~ Hematite has been reported to form at temperatures as low as 500~

[28].

and may in some cases be

accompanied by anhydrite from reactions of the pyritic sulfur with calcite [29]. The reduction of iron compounds to iron metal has been observed during the slagging fixedbed gasification of lignite [30]. In the presence of an atmosphere typically 56-58% CO and 2829% H2, about 25% of the iron charged as "Fe203" in the ash would be reduced to the metal. A material balance performed on the inorganic constituents of Indian Head lignite in a slagging gasifier has shown that about 33% of the iron charged with the lignite is reduced to the metal [31].

302 6.3.7 Hercynite Hercynite, FeAI204, forms at high temperatures (about 1200 ~ from the direct reaction of magnetite with alumina. The alumina probably derives from kaolinite, and the magnetite from hematite (which in turn could be formed from pyrite originally present in the lignite). 6.3.8 Kaolinite (i) Decomposition. Kaolinite, A12Si2Os(OH)4, is stable to about 400*. Above that temperature kaolinite begins to decompose via loss of its hydroxyl groups as water. Kaolinite dehydration occurs in the range 400--525* [29]. Dehydration of kaolinite was observed in the ashing of Greek lignite at 400"C [23]. Dehydration is complete by 550 ~ the products being metakaolinite from well-crystallized kaolinite samples, or amorphous alumina and silica from poorly crystallized material. These products are stable to about 10000, provided that they do not undergo chemical attack. At increasing temperatures the collapsed kaolinite structure forms corundum (y-A1203) [18]); mullite and cristobalite, reported to form in bituminous coal ashes at this temperature [29], were not observed in high-temperature ashes of lignites. This distinction may reflect the fact that the original kaolinite structure in the lignites was poorly defined (e.g., [27]), or that alkali cations retarded the development of the mullite and cristobalite [27]. Above 10000 metakaolinite or amorphous alumina and silica begin to convert to mullite. However, these species are more likely to undergo chemical reactions than is kaolinite, particularly in lignite ash systems where other concurrent reactions are generating reactive species such as CaO or Na20. (ii) Reactions with sodium. The collapsed kaolinitic structure can serve as a framework for the formation of a variety of aluminosilicates. Interstitial in-filling of alkali or alkaline earth cations occurs in the dehydrated kaolinite structure with increasing temperature, due to thermal expansion and reordering of the collapsed clay structure. This process represents one of the most important of the reactions of lignite minerals, because some of the resulting alkali or alkaline earth aluminosilicates have relatively low melting points and may play a significant role in the formation of deposits on boiler walls or steam tubes. Using sodium acetate to simulate sodium carboxylates in lignite, the reaction with kaolinite at 750"C produced an amorphous phase, probably by dehydration, and carnegieite by interstitial infilling in the reordered structure. At 1000~

the reaction product is nepheline [26]. Another

potential source of sodium is sodium sulfate. At 750~ an amorphous phase and unreacted sodium sulfate are present (the melting point of sodium sulfate is 884~

and it decomposes at higher

temperatures); at 1000*C the product is nepheline [5]. Reaction of sodium carbonate with kaolinite at 800~

for 30 minutes yielded a glass

containing nepheline and minor amounts of carnegieite [32]. Reaction with sodium bicarbonate under the same conditions produced nepheline but not carnegieite. (iii) Reactions with calcium. The reaction of calcium acetate with kaolinite at 750"C produces an amorphous phase and calcium oxide. At 1000~ the reaction product is gehlenite [26]. The reaction of calcite with kaolinite is somewhat similar to that of calcium acetate, but reflects the

303 fact that calcite decomposes at a much higher temperature than calcium carboxylates. At 750~ a mixture of calcite and kaolinite results in an amorphous phase and unreacted calcite. At 1000~ the products are gehlenite, mullite, and CaO [5]. (iv) C o m p e t i t i v e s o d i u m - c a l c i u m reactions. During the ashing of lignite, both sodium and calcium cations become available from decarboxylation or from inorganic sodium or calcium species originally in the lignite. The nature of the aluminosilicates which form, and the subsequent slag or ash deposition behavior, will be determined by the relative success of the sodium and calcium cations in penetrating and in-filling the reordered clay structure. The reaction of calcium acetate, sodium acetate, and kaolinite at 750~ produced carnegieite as the only phase identifiable by X-ray diffraction. At 1000~ the reaction products are gehlenite and nepheline [5]. That both these phases form is an indication that no mutual interstitial void filling by sodium and calcium occurs within the same aluminosilicate structure. This is a reflection of the preferred oxide coordination behavior of the two cations. Calcium prefers tetrahedral coordination, so that reaction of calcium cations with kaolinite would be expected to form gehlenite, in which the calcium is tetrahedrally coordinated. In comparison, sodium favors octahedral coordination, which can be attained by in-filling octahedral sites to produce the nepheline structure. The reaction of sodium sulfate, calcium acetate, and kaolinite at 7500C forms an amorphous phase by dehydration of the kaolinite and leaves unreacted sodium sulfate. At 1000~ the reaction products are gehlenite, nepheline, and hauyne [5]. In this case sulfur becomes involved in the formation of new high-temperature phases. 6.3.9 Magnetite Magnetite, Fe304, is the iron-containing compound most frequently formed at high temperatures. Thus it is often identified in lignite slags, for example. Magnetite can form from the thermal decomposition of hematite at 14000C. It can also form in the reduction of hematite by troilite, discussed below in the subsection dealing with pyrite. 6.3.10 Pyrite (i) O x i d a t i o n . Pyrite can decompose during low-temperature ashing, being oxidized to hematite. Up to 30% of the pyrite initially present in a sample can be so oxidized, the exact extent of conversion depending upon crystallinity of the pyrite and the wattage used to generate the oxygen plasma in the asher [3]. At higher temperatures the further reactions of pyrite are dependent on the atmosphere. At 500~ pyrite will be oxidized to hematite and sulfur dioxide: 4FeS2 + 502 ~ 2Fe203 + 8SO2

The sulfur dioxide can be captured by CaO. Capture of sulfur by calcium originally associated with carboxyl groups is illustrated by the reaction of calcium acetate with pyrite. At 750~ the reaction products are anhydrite and magnetite;

304 at 1000 ~ a mixture of anhydrite, magnetite, and hematite [5]. At temperatures in the range of 800-1000~

pyrite can react with hematite to produce

troilite: 7FeS2 + 2Fe203 ~ 11FeS + 3802

In subsequent reactions the troilite can reduce hematite: FeS + 10Fe203 ~ 7Fe304 + SO2

The decomposition of pyrite in lignite may be different from that in bituminous coals [33]. Heating Merta Road (India) lignite in air at 3000C converted about 77% of the iron in the lignite into ~t-Fe203, and another 17% to super-paramagnetic clusters of a-Fe203 [33]. After heating at 5(X)~ the small grains of a-Fe203 that exhibited super-paramagnetic behavior were not present. (ii) Reduction. During gasification of lignite, a so-called "sulfide sulfur" form is produced in the char. The sulfide sulfur is determined by digestion of the sample with 5M hydrochloric acid and analysis of the evolved gas for hydrogen sulfide. Examination of gasification chars produced from Dakota Star (North Dakota) lignite led to a proposed mechanism of sulfide sulfur formation via reduction of pyrite [34]: C + H20 ~ C O + 2H FeS 2 +

2H ~ FeS + H2S Pyrolysis of Texas lignites showed complete decomposition of pyrite to FeS [35].

Pyrolysis of Indian Head lignite at 380~ in a helium atmosphere (heating rate 20~

resulted

in the reduction of pyrite to pyrrhotite (identified as Fe:.4S) [36]. Pyrite in Beulah and Big Brown (Texas) lignites was reduced to pyrrhotite during liquefaction [37]. 6.3.11 Quartz Quartz undergoes a sequence of thermally induced structural transformations, converting at 5730C to so-called high-temperature quartz, at 876 ~ to tridymite, and at 1470 ~ to cristobalite. Aside from the structural transformations, quartz appears to be chemically stable to at least 1000" [ 18]. In a suite of ten lignite ashes, quartz was stable at 1000~ in all samples, and was still present at 1200~ in five [2]. However, with increasing temperatures the intensity of quartz peaks in the Xray diffractogram decreases while diffraction peaks of aluminosilicates increase. This change may be a result of volatized sodium reacting with quartz, forming sodium silicates and fixing the sodium [38].

305 6.3.12 Sodium silicates Sodium metasilicate, Na2SiO3, sodium disilicate, Na2Si2Os, and sodium trisilicate, Na2Si3OT, have very similar X-ray patterns, and are not easily distinguished by X-ray diffraction analysis of lignite ashes or slags. The silicates can form when sodium oxide, liberated for example by decomposition of sodium sulfate, reacts with quartz. The silicates were the major sodiumcontaining species observed in a suite of Beulah-Zap lignite ashes [2]. Sodium silicates are known to form glassy phases [39]; the significance for the formation of strong ash deposits during lignite combustion will be discussed in Chapter 11. Sodium trisilicate forms a solid solution with gehlenite, the maximum solubility of the trisilicate being 15%. 6.3.13 Sulfur retention During ashing in the laboratory, a significant portion of the total sulfur originally in the lignite is retained in the ash. The percentage of total sulfur retained ranges from 60% to 90% [40]. In comparison, only about 10-30% of the sulfur is retained in the fly ash in a lignite-fired pulverized coal boiler [40]. Anhydrite observed in ashes produced at high temperatures may arise either from dehydration of gypsum or from reaction of sulfur released from sulfides

(e.g., pyrite)

with calcite or organically bound calcium. A complication arises when ashing liquefaction residues for material balances or other purposes (e.g. the ash tracer method). If a substantial amount of sulfur has been removed as a result of hydrodesulfurization of the lignite, then the amount of sulfur fixed in the ash as SO3 will not be as great as would be anticipated from ashing untreated lignite [41]. The reduced amount of sulfur fixation will affect the observed ash yield, and corrections would have to be made before using the ash in a material balance calculation. Sulfur retention arises mainly from reaction of calcium and magnesium, released from decomposition of carboxylate salts, with organic sulfur, the products of the reaction being mainly calcium and magnesium sulfates [42]. The amount of total sulfur retained in lignite ash prepared by standard ashing techniques ranges from 60 to 95% [42]. In extreme cases, sulfur retention results in a positive error by six percentage points in the amount of ash determined, but for lignites the error is more often of the order of 2 to 3 percentage points [42]. (This positive error in ash determination will therefore introduce negative errors into the calculated amounts of fixed carbon and oxygen, unless a correction is made by subtracting the SO3 from the determined ash; furthermore, the corrected ash value should be used when calculating the mineral matter content by the Parr or similar formulas.) The amount of sulfur retained in the ash is determined primarily by the total amount of sulfur in the coal, irrespective of its distribution among the sulfur forms [42]. The rule of thumb has been suggested that the SO3 retained, when calculated on a coal basis, is double the value of the total sulfur in the coal [42]. Variations of heating rate and maximum temperature (575 ~ vs. 815~

did not decrease the

amount of sulfur retention in a suite of four coals, a North Dakota and a Texas lignite and two

306 German brown coals [42]. (In the original literature [42] the former temperature is referred to as "low-temperature ashing," but of course is not representative of the current commonly accepted meaning of the term.) Variations in air flow rate though the furnace, the depth of sample in the ashing dish, and the depth of the dish itself also had little effect on the retention of sulfur [42]. A devolatilization step preliminary to ashing, effected by heating the lignite to 8150C in a covered ASTM ashing capsule and then ashing the remaining char, reduced the sulfur retained in the ash by about 50% [42]. For northern Great Plains lignites in pulverized-coal-fired combustors, an average of 25% of the sulfur does not appear as SOx in the flue gases [43]. In some cases the figure is as high as 50%. An empirical equation to determine the percentage of fuel sulfur emitted as SOx as a function of ash composition is [43] % S as SOx = 110.0 - 12.7 (CaO/A1203) - 48.1 (Na20/SiO2) Combustion of a suite of coals, including a Texas lignite, in a down-flow reactor shows that calcium initially chemically bound to the fuel is more effective for retaining sulfur under fuelrich conditions than is limestone [44]. The ion-exchanged calcium forms a highly active oxide during devolatilization of the lignite, the oxide being well distributed through the pore system. Addition of carbon dioxide and steam decrease sulfur retention by shifting the equilibria for calcium sulfide decomposition toward increased sulfur species, as well as accelerating carbon conversion. The approach to equilibrium is determined by the Ca/S ratio for reaction times of 1 s at 1400~

For low-sulfur coal, a Ca/S ratio of 2.5 is needed to insure good sulfur capture. Samples of lignite char from a gasifier showed 75-100% of the sulfur retained in the ash as

water-soluble sulfates [45]. 6.3.14 Volatilization during ashing The standard ASTM method for ashing may result in the determination of sodium being erroneously low [46]. The water-soluble sodium content of Texas lignites was greater than the amount of sodium found in the ash. The discrepancy is reduced somewhat if the lignite is ashed at 500* instead of the standard 750* C. The data are summarized in Table 6.3 [46]. The comparison is TABLE 6.3 Effect of ashing temperature on determined total sodium content of Texas lignite [46].

Test 1 2 3

H20 sol. ~,~,t~,_, Ash % ,,,,T.. ,-,,, % 9.95 0.128 39.87 0.110 21.49 0.068

Na20 Leached from Residue 0.040 0.120 0.048

Sum 0.168 0.230 0.116

Na20 Found in Ash at 500* at 750". 0.048 0.028 0.171 0.128 0.092 0.047

307 made between the sum of the sodium contents determined by leaching the lignite with water and further dissolving sodium from ash prepared from the water-leached lignite, and sodium determined by ashing the untreated lignite at 500 or 750~

The factors, other than temperature,

that contribute to the volatilization of sodium were not determined. Material balances on ash entering a slagging, fixed-bed gasifier and slag leaving it provide an estimate of the amount of ash constituents vaporized at temperatures of about 1700~

[47].

When Indian Head lignite was used as the feedstock, 80% of the sulfur, 30-35% of the sodium and potassium, and 25% of the phosphorus were vaporized. Smaller amounts of magnesium, calcium, and silicon also seemed to be lost. 6.4 T H E R M A L

PROPERTIES

OF ASHES AND SLAGS

6.4.1 Specific heat The specific heat of ash for four low-rank coals was found to be given by Cp = 0.189 + 2.084x10-4T - 6.927x10-8T2 - 9.584x10-11T3

where T is in Celsius [48]. The equation is suitable over the range 0-1000~

The equation can be

simplified to Cp = O. 189 + 1.840x lO-4T

with little loss of accuracy [49]. 6.4.2 Radiative heat transfer The radiative heat transfer of ash (e.g., an ash deposit in a boiler) is governed by the equation qr = o(otTf4 - rTa4)

[50], where qr is the net radiative heat flux, o the Stefan-Boltzmann constant, cz the total absorptivity of the ash, e its total emissivity, and T f and Ta are, respectively, the flame and ash temperatures. Normally one would expect Tf > Ta, and, since the heat flux equation contains the fourth powers of these temperatures, it then follows that absorptivity will usually be much more important than emissivity. Experimental difficulties in measuring the absorptivity have led to measuring emissivity instead, and substituting it for absorptivity. Such a substitution is acceptable if the ash obeys Kirchoff's law (see, e.g., [51]), which in essence means that the ash must behave as a grey body with emissivity independent of temperature and wavelength. However, ash deposits

308 are not in fact grey bodies [52]. Methods of determining total absorptivity involve measuring the emissivities of the ash in five wide-band regions in the near infrared [53]. The spectral emissivity is substituted for the spectral absorptivity, a valid substitution under conditions of local thermodynamic equilibrium. The total absorptivity then becomes a function of the flame and ash surface temperatures. The latter can have a significant effect on the total absorptivity [50]. The total emissivities and total absorptivities for ash deposits from four lignites are shown in Table 6.4 [50]. The ash samples were collected from the first probe bank of an experimental combustor operated by the State Electricity Commission of Victoria. No correlation was observed of absorptivity or emissivity with mass mean particle size of the ashes or with iron oxide contents. TABLE 6.4 Total absorptivities and emissivities for ash deposits [50]. Beulah 0.528 0.563

Total Absorptivity (a) Total Emissivity (b)

Decker 0.520 0.565

Texas 0.565 0.615

Velva 0.575 0.600

(a) Evaluated at radiation source temperature 1600 K and averaged over surface temperatures 1023-1173 K. (b) Averaged over 1023-1173 K.

Total absorptivity is shown as a function of radiation source temperature in Fig. 6.1 [50].

1

0.9 0.8

:~ 0.7 ~, 0.6 ,ta

0.5 0.4

r

o.3 0.2 0.1 0

''

0 0

'1

0 0

''

'1'

0 0

''

i'

0 0

''1

''

0 0

'1

0 0

'''

I'

,i

0 0

0 0

Radiation source temperature, K Figure 6.1. Total absorptivity of lignite ash deposits as a function of radiation source temperature [50]. Absorptivity values lie in a range denoted by shaded area.

309 6.5 S I N T E R I N G

The Barnhardt and Williams sintering test was developed to evaluate laboratory-generated ash which should simulate the fly ash passing through a boiler [54]. (This test cannot be applied to ash prepared in the standard ASTM ashing method.) The ash sample is ignited at 4900C to remove carbon and then crushed to -100 mesh. The -100 mesh sample is pressed into 15mm diameter, 19.1 mm long pellets at a pressure of 1.034 MPa. Starting in a cold furnace, the pellets are heated to the desired sintering temperature in 1.5 h, sintered for 15 h, and cooled in the furnace for 2.5 h. The compressive strength of the cooled pellets is measured. The strength of the ash sintered at 930~ is related to the fouling anticipated from combustion of this coal as follows: 6.89 MPa, low fouling; 6.89-34.47 MPa, medium fouling; 34.47-110.3 MPa, high fouling; and >110.3 MPa, severe fouling. Beulah lignite fly ash had a sinter point of 550~

as measured by changes in electrical

resistance as a function of temperature [55], much lower than observed for ashes of several bituminous coals tested under comparable conditions. (However, as will be discussed below, one must be cautious in interpreting sinter point measurements [56].) The strength of ash pellets heat treated at 950 to 1050~ showed no significant increase with heat-treatment time, nor did the pellets exhibit shrinkage. However, at a heat-treatment temperature of 1150~ the pellet strength increased with increasing heat-treatment time, and the pellets shrank. No maximum was observed in pellet strength as a function of heat-treatment temperature, whereas fly ash from a bituminous coal did show such a maximum. Sintering time affects sintering behavior, as do the heat-treatment temperature and the composition of the ash. The effects of ash particle size on sinter point are shown, for Beulah lignite ash, in Table 6.4 [57]. TABLE 6.4 Relationship of particle size and sinter point for Beulah lignite ash [57]. Particle size range, ~rn <38 38-53 53-75

Sinter point, ~ 448 500 550

The sinter point decreases as a function of particle size because the ratio of surface area to volume of the ash particles increases with decreasing particle size. Thus a proportionately greater fraction of the surface area of the particle is able to participate in the formation of interparticle contacts. Heated stage optical microscopy of mixtures of silica (30x60 mesh), ash from a highsodium Beulah lignite prepared at normal ASTM ashing conditions, and the low-temperature ash from Beulah lignite showed that sintering occurred in the range 880-970~

[58]. The mixture

310 containing low-temperature ash sintered at lower temperatures than mixtures containing ASTM ash. Samples of bed material withdrawn from a pilot-scale fluidized bed combustor burning Beulah lignite in a silica bed showed no evident physical changes until a temperature of 1000~ was reached, at which temperature a liquid phase developed [58]. Sintering can also be investigated using a dilatometer to measure volumetric change as a function of temperature. Samples of Beulah lignite ash prepared at 800~

in a muffle furnace

showed different sinter points (that is, the point of maximum expansion) depending on sodium content. A high-sodium ash (8.0% Na20) had a sinter temperature of 843~

whereas a low-

sodium ash (3.2% Na20) sintered at 927"C [59]. This behavior was observed only for ashes prepared in a muffle furnace; samples of the same lignite ashed in a test furnace designed to simulate the combustion characteristics of a full-scale boiler [60], or actually collected from a boiler firing this lignite did not show the sintering behavior observed for the laboratory-prepared ash [59]. The method appeared to be extremely sensitive to the bulk density of the sample being used for the test. The sinter temperatures correlate with the observed fouling tendencies of the high- and low-sodium Beulah lignites, in that the high-sodium lignite had a much higher propensity for ash deposition on boiler tubes [60]. Scanning electron microscopy of ashes prepared from four coals (including Beulah lignite) by ashing at 750~ showed that extensive sintering had occurred in these ashes [56]. This indicates that when experiments are performed using ash prepared by standard ASTM ashing methods, those experiments are using a material which is already partially sintered. To avoid sintering during ashing, it is necessary to ash Beulah lignite (the only lignite tested in this work) at 430~ [56]. The minimum sintering temperature range for Beulah lignite ash is 430-455~

this being--depending

on point of view--the lowest temperature at which the ash can be heated to initiate significant sintering, or the highest temperature at which the ash can be heated without sintering. (A temperature range was given in the original literature because the ashing and SEM experiments were not carried out at small,e.g. 10, intervals.) The determination of the minimum sintering temperature range was determined by passing the ash through a drop-tube furnace at various temperatures, with residence time of nominally 1 s [57]. This method provides results different from those obtained from the more traditional electrical resistivity method, as summarized for Beulah ash in Table 6.5 [57]. TABLE 6.5 Comparison of minimum sintering temperatures of Beulah ash by different methods [57]. Experimental method Drop-tube furnace Resistivity

Ash particle size, ~m <75 53-75 38-53 <38

Minimum s.intering temp., ~ 43O--455 550 500 448

311 For low-rank coal ashes older values of sintering temperatures obtained by the electrical resistance method may be incorrect because the existence of sinter points and the possibility of sintering occurring below the temperature at which the ash sample was prepared were not considered. A comparison of the effects of heating materials loosely packed in a muffle furnace with heating the same substances tightly packed into a crucible is shown in Table 6.6 [61]. TABLE 6.6 X-ray diffraction of model compound mixtures heated loosely and tightly packed [61]. Mixture Calci um Acetate Kaolinite Sodium Acetate

Loose Pack, 1000~ Gehleni te Nepheline

Tight Pack, 10500C Anorthite Corundum K Nepheline Nepheline

Calcium Acetate Kaolinite Sodium Sulfate

Gehlenite Hauyne Nepheline NazSi307

Camegieite Corundum Hauyne Nepheline Thenardite Calcite*

Kaolinite Sodium Acetate

Nepheline

Corundum Nepheline Quartz* Na2Si307

Kaolinite Sodium Sulfate

Camegieite Nepheline

Thenardite AI2SiO5

Calcium Acetate Kaolinite

Gehlenite Mullite Quartz

Anhydrite Anorthite Gehlenite Melilite Sodium Sulfite

*Observed in trace amounts (The original literature does not indicate the quantities of each material used in the reaction nor the reaction times.) Identification of both nepheline and its high temperature modification camegieite in the same sample suggests that equilibrium was not established. Nevertheless, if it is reasonable to assume that the reaction conditions and quantities of materials used were comparable for the two series of experiments, then it appears that for these systems heating under conditions promoting sintering gives rise to a greater diversity of phases. Formation of nepheline is favored in silicadeficient systems having abundant alkali cations. Sodium melilite undergoes a sub-solidus decomposition at 1000~ to produce nepheline and wollastonite [62]. Gehlenite is a likely reaction product of kaolinite in the presence of abundant calcium cations. Generally, heating under

312 conditions favoring sintering resulted in production of higher amounts of sulfates, aluminosilicates, and corundum. Anorthite derives from the aluminum in kaolinite and a source of calcium [29]. Mullite can become a major product when kaolinite is subjected to temperatures in excess of 1000~ [29]. The identification of sodium sulfite as a reaction product of calcium acetate and kaolinite is questionable. Strengths of sintered specimens were measured in a hydraulic laboratory press as a function of the temperature at which the samples were heat treated. (The heat treatment involved heating to temperature in 4 h, heat treating at temperature for 16 h, and then cooling to ambient temperature in the furnace.) The comparative behavior of fly ash produced by burning high- and low-sodium Beulah lignites in an operating boiler is shown in Figure 6.2 [59]. The sodium contents of these fly ashes were 7.9 and 2.8% Na20, respectively. The behavior of ash prepared in a laboratory muffle furnace is also shown in Figure 6.2 [59]. Although the behavior of the two types of ash (i.e., laboratory vs. fly ash) is different, the same qualitative trends and correlation with ash deposition behavior are evident for both. A preliminary guideline of sinter strengths above 41.2 MPa being indicative of high-fouling tendencies and strengths below 13.7 MPa being lowfouling has been developed from ashes studied under these conditions [59].

<> High sodium 140-

A

120.2 i

100 80" ~ ~

60

20" 0

.... LD 0:)

I'

'1'''

'1 ....

0",

C::) LD (7",

O Q Q

,,--,I

I .... ~ LD Q v--I

I .... C:) Q ','-, ,,,-,,4

I'''' ~ IX) ---4 ,1--4

,t::) .l--,I

Sinter temperature, ~ Figure 6.2. The effect of sintering temperature on the sinter strength of fly ash from low-sodium and high-sodium Beulah lignites [59].

Strength development in Beulah ash began at 950"C, 400 ~ above the sinter point [55]. Xray diffraction indicated significant sodium and mixed alkali sulfates, materials which have low melting points. The melting of these sulfates would give rise to liquid phase ionic conduction at

313 relatively low temperatures and could be responsible for the change in slope of the log (resistance) vs.

1/T curve used to determine the sinter point. Among a suite of four ashes, derived from coals

of various ranks, Beulah ash had the lowest sinter point and the highest content of sodium (5.5%, reported as Na20) and alkaline earth elements [57]. The sinter point measurements were made by the electrical resistivity method. The composition dependence of the sinter point among these four ashes suggests that the ashes have conductivity behavior similar to that of glasses [57]. Temperature, time, and the nature of the atmosphere affect the compressive strength of sintered ashes [63]. Compressive strength of ash sintered in a 60:40 CO:CO2 atmosphere exceeds that of the same ash sintered in air, at otherwise comparable conditions. Sintering in a reducing atmosphere generates a greater quantity of liquid phase, possibly as a result of the fluxing action of iron(II). Reactions and phase transformations, such as the conversion of nepheline to gehlenite, may cause sharp changes in strength by introducing cracks into the material. The effects of ash particle size and sintering temperature on the development of strength in Beulah lignite ash are summarized in Table 6.7 [57]. TABLE 6.7 Effects of temperature of sintering and ash particle size on strength of sintered Beulah lignite ash [571. Sintering temperature, ~ 750 850 950

Ash particle size, ~tm 53-75 38-53 <38 53-75 38-53 <38 53-75 38-53 <38

Compressive strength, MPa 9.3 11.0 13.2 15.8 25.3 27.6 26.2 35.2 40.0

For a given ash particle size (before sintering) the compressive strength of the sintered ash increases with increasing sintering temperature. At a given sintering temperature, the compressive strength of the sintered ash increases with decreasing particle size. Strength development in sintered (or partially sintered) low-rank coal ash deposits produced in a laboratory-scale drop-tube furnace can be expressed by an equation of the type S = a exp (bH) where S is the strength at some point in the deposit, in MPa; H is the height of the deposit above its base, in mm; and a and b are constants [64]. A direct relationship (based on Spearman rank correlation) exists between a and the sodium content of the ash, with an inverse relationship

314 between a and both calcium and aluminum. Just the opposite composition dependence was observed for b: a direct relationship with calcium and aluminum, and an inverse relationship with sodium. These results suggest that sodium and calcium are antagonists, rather than acting together. They also suggest that interactions between gehlenite and sodium melilite may be important in determining both the initial strength and the growth of strength in these deposits [64]. Increasing sintering temperature of Beulah ash resulted in a depletion of anhydrite and corresponding increase in the amount of calcium aluminosilicate phases [57]. The compressive strength of the sintered ash related inversely to the amount of anhydrite in the sintered material, but related directly to the amount of hauyne [57]. These observations suggest that the growth of compressive sintered strength may be due to the reactions of anhydrite with quartz, clays, or both, to produce calcium aluminosilicates, which then act as the liquid glue to promote sintering. For ashes sintered by passing through a drop-tube furnace, the onset of sintering correlates with the sodium content: the higher the sodium content, the lower the minimum sintering temperature range [56]. However, the growth of large sintered particles correlates with an increasing formation of anhydrite. A linear relationship exists between the anhydrite content and the size of the largest sintered particles. The increase in the amount of anhydrite, relative to that amount present in the ash before the onset of sintering, is an important factor in the growth of sintered ash particles [56]. In sintered samples of Beulah ash, crystalline phases predominated relative to glass [55]. In comparison, significant amounts of glass and of alkali sulfates were observed in an untreated fly ash from this lignite. Crystalline phases formed at 950"C included gehlenite, sodium melilite, and nepheline. The sodalite minerals hauyne and nosean were also identified. The X-ray diffraction peaks of the melilites increased in intensity with increasing heat treatment temperature. The glass and alkali sulfates could, in principle, provide the liquid phase necessary for sintering to occur. However, some of the liquid formed at the heat treatment temperature may have reacted quickly with aluminosilicates to form crystalline phases. The observed strength of the sintered fly ash pellets resulted primarily from the formation of newly formed crystalline phases acting as bridges between particles. Melilites and sodalites have been found in ash deposits produced during firing of highsodium lignites

(e.g., [65,66]; see also Chapter 11). The strength resulting from the presence of

these phases may make the ash deposits difficult to remove by sootblowing. (However, in comparing the laboratory results with the behavior of actual boiler deposits, it is important to remember that the latter are continuously bathed in a stream of alkali compounds and sulfur oxides, which could potentially react with the sodalites and sodium melilite in the deposit to produce new liquid phases which enhance the sintering rate and development of strength, thus making the boiler deposit stronger and harder than the laboratory test specimen.) Sintering of model compounds and minerals showed that a mixture of sodium sulfate and kaolinite, and a mixture of sodium sulfate, calcium acetate and kaolinite had sinter points of 890891 ~

as measured by mechanical displacement [61]. These sintered ashes showed an expansion

315 on further heating, attributed to the structural reordering of metakaolinite to mullite. Kaolinite itself, heated in the absence of other model compounds, shows a sinter point of 952~ and a subsequent expansion at 998~

[61].

Sintering temperatures of mixtures of model compounds and minerals are shown in Table 6.8 [67]. TABLE 6.8 Sinter points of model compound mixtures [67]. Mixture Calcium acetate + kaolinite Sodium acetate + kaolinite Sodium acetate + calcium acetate + kaolinite sodium sulfate + kaolinite sodium sulfate + calcium acetate + kaolinite

Molar Ratio 1:1 1:1 1:1:1 1:1 1:1:1

Sinter Point, ~ 725 865 785 >1050 890*

*Additional displacement observed at 99 I~ The relationship between sintering temperature and sodium content of bed material from a fluidized bed combustor is shown in Fig. 6.3 [32,58]. These data were obtained from combustion of Beulah lignite in a silica bed. 90O r) 880o

~'860 ~'

.~ 820" r.a 800780

....

0

I ....

1

I ....

2

I ....

3

I ....

4

I ....

5

I ....

6

7

Weight percent Na20 in bed material Figure 6.3. The sintering temperature of bulk bed material from fluidized-bed combustion of lignites, as a function of sodium oxide content [32].

These effects of ash composition on sintering behavior likely derive from the dependence of two important properties of the melt phase--viscosity and surface tension--on composition. An

316 increase in SiO2 causes a more significant increase in viscosity than in surface tension, resulting in reduced sintering [68]. Conversely, increased sodium, and possibly an increase in total alkalies, decreases viscosity more rapidly than it does surface tension, resulting in an increase in sintering [68]. These relationships derive from the fact that ashes that tend not to sinter or agglomerate form melt phases having relatively high viscosities but relatively low surface tensions (at a given temperature), whereas those ashes which do agglomerate have relatively low-viscosity, highsurface tension melts [68]. The density of the ash passes through a minimum and then increases again as sintering proceeds [63]. Sintering of pulverized coal ashes occurs via a three-step mechanism [63]: Closed pores form in the sintering material at temperatures below the point of minimum density. The pores shrink at temperatures above the minimum density. These two processes are accompanied by diffusion or reactive diffusion (or both) of melt phases within the sintering mass. The strength of the sintered ash is related to the variation in porosity [63]. Agglomerate formation in quartz beds proceeds through four steps [69]: First, the quartz grains are coated by ash to a thickness of about 50 tam. The coating then thickens in nodules and becomes sulfated, which sets the stage for initial agglomeration. Then the first agglomeration of particles occurs, with sulfated aluminosilicates from the ash acting as the glue or cement holding the particles together. Finally, strong bonds between particles are formed by sulfated ash which has partially melted and recrystallized. In comparison, a three-step process is proposed for agglomeration in limestone beds [69]: An initial calcining of the limestone allows for its subsequent sulfation. Then, as in the case of quartz beds, continued sulfation builds up a nodular coating which begins the agglomeration process. In the third step, extensive agglomeration occurs with the growth of large calcium sulfate crystals. 6.6 A S H

FUSION

It is important to recognize that ash formed in the laboratory under controlled conditions bears no immediate relationship, on the one hand, to the original inorganic and mineral constituents of the coal, nor, on the other hand, to ash formed in actual utilization processes. Strictly speaking, there is no ash in coal; ash is the product of complex reactions and thermally induced transformations of the minerals originally present in the coal, complicated in the case of low-rank coals by the reactions of the cations associated with the carboxyl groups. In the laboratory, ash is prepared under carefully controlled conditions with the specific intention of producing a reproducible, uniform material. In a combustor or gasifier, however, the conditions of temperature and atmosphere may be quite variable with respect to the exact position in the unit, and hence the inorganics and minerals in the coal will be exposed to a sequence of temperatures and oxidizing or reducing conditions. It is unlikely that any modem coal processing equipment operates at the exact conditions specified in the ASTM ashing test [70]. Despite these caveats, and despite the facts that the softening temperature correlates with neither the ash deposition on boiler tubes nor the slag

317 viscosity, the ash fusion test is nevertheless the most widely used approach to characterizing the high-temperature behavior of the ash. The range of fusion temperatures for lignites does not differ from that for bituminous coals. The major distinction between fusion behavior of lignites and bituminous coals is the effect of changes of composition. Compared to the ashes of most bituminous coals, lignite ashes have relatively high concentrations of sodium, calcium, and magnesium, and correspondingly low proportions of silicon and aluminum. As a result, the fusion temperature of lignite ash is reduced by additions of silicon or aluminum and is raised by additions of the alkali or alkaline earth elements. The opposite behavior is observed for bituminous coal ashes. Thus for example, the ASTM softening temperature of lignite ashes is expressed as ST (~

= 2326 - 6.9 SiO2 + 0.1 A1203 - 4.3 -

8.7 Na20 + 80 K20

-

5.1

Fe203

-

128 TiO2 + 8.5 CaO + 14.9 MgO

SO3

[20,71]. Here silicon has a large negative coefficient, while calcium, magnesium, and potassium all have large positive coefficients. The fusion temperatures for lignite ashes cover a very large range. A survey of 69 North Dakota lignites showed softening temperatures from 1093 ~ to 1427~ lignites showed softening temperatures from 1093 ~ to 1477~

[72]. A compilation of 98

[73]. This range of almost 400 ~ is

larger than the tabulated ranges of softening temperatures for any other rank of coal [73]. A notable property of the ashes of North Dakota lignites is the high calcium content, which averages 25% when reported as CaO [71]. Calcium has a refractory effect on the lignite ash, so that a moderately good correlation of softening temperature can be based on CaO alone: Ts = 1876 + 16.95(CAO) where Ts is in degrees Fahrenheit, and the least squares correlation coefficient is 0.778 [74]. An equation to include both lignite and bituminous coal ashes is based on first calculating the basic constituents ratio and the dolomite ratio from the equations BC = (Fe203 + CaO + MgO + Na20 + K20) / (SiO2 + A1203 + TiO2) and DR = (CaO + MgO) / (Fe203 + Na20 + K20) where BC is the basic constituents ratio and DR the dolomite ratio [74]. From these two parameters, the softening temperature in degrees Fahrenheit can then be calculated from

318 Ts = -3631.0(BC) + 2993.3(BC)2- 1456.5(DR) + 1022.1(DR)2 + 1558.7(BC)(DR) + 3238

with a standard error of estimate of 105.8~ and a correlation coefficient of 0.812 [71]. A minimum in softening temperature, as a function of the basic constituents ratio, occurs around 0.40 to 0.50 BC. Furthermore, it is generally true that bituminous coal ashes tend to have BC < 0.45 and lignite ashes tend to have BC > 0.40 (although exceptions can be encountered; see for example [71]). Thus it can be argued that in general a correlation developed specifically for bituminous coal ashes is likely to give poor or misleading results when applied to lignite ashes, and vice v e r s a .

Plots of softening temperature as a function of BC tend to more divergence at high

values of BC; consequently it is specifically for the lignite ashes that the dolomite ratio is important as a discriminator of ash behavior. The hemispherical temperature decreases as the proportion of basic components in the ash decreases, over the range of 75 to 40% [75]. At a given percentage of basic components in the ash, the softening temperature will increase with increasing dolomite ratio. The data on which this relationship is based show considerable scatter, but the relationship seems satisfactory for a rule-ofthumb correlation. A regression equation to predict the softening temperature (in degrees Fahrenheit) of lignite ash from its composition is given by Ts = -3.725(SIO2) + 39.067(Fe203) + 93.634(CAO) + 0.8155(SIO2)(A1203) - 0.22 14(SiO2)(CaO) - 0.0975(-SiO2)(Na20) - 0.4880(SIO2)(SO3) - 0.8674(AI203)(CaO) - 2.4234(Fe203)(CaO) + 2.0770(Fe203)(MgO) - 0.8765(CAO)2- 1.343 l(MgO)(Na20) + 0.0764(SO3)2 + 1027

where the formulae represent the weight percents of the respective components in the ash [71]. The equation was based on the analyses of 338 lignite ashes. The correlation coefficient is 0.875 and the standard error of estimate is 83.8~

No regression equation should be used outside the range

of compositions of the original data from which it was derived; the limits of ash composition for which this equation is suitable are 6.3-48.1 SiO2, 4.2-26.1 A1203, 1.2-28.5 Fe203, 0.1-0.7 TiO2, 0.0-1.6 P205, 11.7-52.0 CaO, 2.8-13.6 MgO, 0.2-27.8 Na20, 0.1-1.7 K20, and 5.2-32.0 SO3 [71]. Occasionally the composition of the lignite ash will be modified by the addition of extraneous minerals. This addition of minerals may be a result of adventitious mixing with overburden or seam partings during the mining operation, or may be the result of deliberate addition of minerals in an attempt to modify the ash behavior during processing. To take this possibility into account, a second regression equation was developed on the basis of the 338 natural ash analyses with an further 279 analyses of ashes which had been modified by the addition of various minerals. The recommended equation, in degrees Fahrenheit, is

319

Ts =-9.666(SIO2) + 0.1467(SIO2)2 + 0.2898(SIO2)(A1203) + 0.1984(SiO2)(Na20) + 0.2839(SIO2)(SO3) - 17.209(A1203) + 0.6223(A1203) 2 + 18.441(Fe203) -0.8671(FezO3)(CaO) + 6.423(TIO2)- 14.262(P205) + 0.4531(P205)2 + 35.281(CAO) -0.3142(CAO)2 + 0.233 I(CaO)(SO3) + 23.544(MGO) + 7.545(Na20) - 0.9299(MgO)(Na20) + 1654

with a correlation coefficient of 0.856 and a standard error of estimate of 86.8~ [71]. The range of compositions for which this equation may be used is 9.7-52.6 SiO2, 5.9-45.2 A1203, 3.4--43.4 Fe203, 0.0-33.6 TiO2, 0.1-38.2 I:'205, 10.9-54.6 CaO, 2.5-42.9 MgO, 0.2-43.0 Na20, 0.1-20.2 K20, and 2.2-35.5 SO3 [71]. Modification of the inorganic composition of lignites by ion-exchange has been suggested as a beneficiation procedure, particularly as a means to reduce ash deposition on boiler tubes. Data for 63 lignite samples treated by ion exchange was used to derive the equation (again in degrees Fahrenheit)

Ts = -38.742(SIO2) + 0.1129(SIO2) 2 + 1.4270(SIO2)(A1203) 0.1681(SiO2)(CaO) -47.574(A1203) + 0.4752(A1203)2+ 0.6464(AIzO3)(CaO) - 2.107(Fe203) + 0.1714(FezO3)(CaO)- 26.856(CAO) + 0.171 l(CaO)2 + 1.1542(CAO)(SO3) + 14.253(MGO) - 3.274(Na20) + 0.0390(Na20)2 - 19.945(SO3) + 3183

[71]. For this equation the correlation coefficient is 0.927 and the standard error of estimate is 86.9~

The composition range of ashes used to derive this equation is 13.2-56.4 SiO2, 9.5-46.7

A1203, 5.0-54.6 Fe203, 0.1-1.2 TiO2, 0.1-2.7 P205, 0.%51.8 CaO, 0.0-23.5 MgO, 0.1-18.4 Na20, 0.0--0.5 K20, and 0.3-22.1 SO3 [71]. Because comparatively few data points were used to establish this equation, its use should be restricted to ashes from lignites modified by ionexchange, rather than being used as a general equation for the softening temperature of lignite ashes. For a suite of seven coals of various ranks, including a Saskatchewan lignite, the secondorder regression equation

Tsoft~ = 1815 + (1.35 x 105)[TIO2] - (2.89 x 105)[SiO2][TiO2] - (7.54 x 105)[SO3] [P205] - (1.60 x 104)[Fe203][SO3] - (3.54 x 104)[Fe203][K20 ] + (1.03 x 105)[AIzO3][K20] + (6.54 x 106)[Na20][P205]

was developed for calculation of softening temperatures in a reducing atmosphere [76]. The root mean square error is 45.0~

For oxidizing atmospheres, the initial deformation temperature can be

320 calculated from IDo = 1686 + 0.392 IDR + 22,600 [A1203][SO3] + (1.04 x 106)[NazO][TiO2] - 25,300 [SO3][SIO2]. where ID is the initial deformation temperature and the subscripts O and R refer to oxidizing and reducing atmospheres, respectively. For this equation the coefficient of determination, r2, is 0.928 and the root mean square error is 41.5~ [77]. The softening temperature in oxidizing atmosphere is given by STo= 399 + 0.678 STR + 814 [SiO2] + (1.18 x 105)[Fe203] [TiO2]

for which r2 is 0.933 and rmse is 40.3~ [77]. The temperature at which the ash is prepared appears to affect the observed fusion behavior. Table 6.9 compares measurements on Beulah-Zap lignite ash prepared at two different temperatures, 700 and 1000~ [78]. TABLE 6.9 Fusion temperatures (~ [78].

Onset of agglomeration Onset of fusion Fluidity

of Beulah-Zap lignite ashes prepared at different ashing temperatures

Ash preparation temperature 700 *C 1000~ 677 877 1052 1102 1377 1367

This work may relate to observations, discussed in the section on ash sintering, that some partial fusion of low-melting phases may occur at temperatures below those at which the ash is prepared in the laboratory. 6.7 S L A G V I S C O S I T Y AND S U R F A C E T E N S I O N

6.7.1 Laboratory data on slag viscosity (i) Background. A lignite-type ash has been defined to be an ash in which the sum of calcium and magnesium oxides is greater than the "equivalent" ferric oxide [75]. Equivalent ferric oxide is the sum of all the iron--ferric, ferrous, and metallic--expressed as Fe203. The viscosity vs. temperature curves for two lignites, Baukol-Noonan (North Dakota) and Rockdale (Texas) are shown in Fig. 6.4 [75] and are generally typical of the kinds of behavior

321 Baukol-Noonan

9

1000

o Rockdale

.

100O r.d,

10-

1

'

1100

'

'

'

I

' '

1200

' '

I'

1300

' ' ' 1

' ' ' '

1400

1500

Temperature, ~ Figure 6.4. Viscosity-temperature relationships for slags from Baukol-Noonan and Rockdale lignites [79]. Measurements were made in 80:20 N2:H2 atmosphere.

observed for lignite ash slags. At high temperatures the logarithm of viscosity is linear with temperature down to the point known as the temperature of critical viscosity (Tcv), below which a definite change of slope is observed. In some cases, such as the Rockdale slag, T cv is fairly sharply defined and the viscosity curve below this temperature is almost vertical. Somewhat more common is the case in which the viscosity appears to undergo a smooth, continuous transition through Tcv and the slope below T cv, though large, does not become essentially vertical. The hysteresis observed between data taken when the slag is cooling and again when the cooled slag is being reheated is believed to be a result of a slow redissolution of crystals which may have formed when the slag was cooled through T cv, the time required for dissolution being longer than the time interval between successive temperature increases and data readings. The straight portion of the viscosity

vs.

temperature curve is the region of Newtonian flow

behavior. Tcv represents a temperature at which partial crystallization of the melt begins; this region represents plastic or non-Newtonian behavior. However, the distinction between Newtonian and non-Newtonian behavior must be judged on the basis of the shear stress vs. shear rate relationship, since it would be possible at least in principle for a slag to have Newtonian behavior below T cv. In fact many lignite ash slags are either pseudoplastic (viscosity decreasing with increasing shear rate) or thixotropic (viscosity decreasing with increasing shear rate and time) [75]. The most extensive study of the viscosity of lignite slags was undertaken by the former Bituminous Coal Research (now BCR National Laboratory) under the aegis of the U. S. Department of Energy. Measurements were performed using a rotating bob viscometer in reducing

322 (80:20 N2:H2), neutral (N2), and oxidizing (air) atmospheres, although the bulk of the work was performed in reducing atmospheres because a principal aim of the project was to relate the viscosity measurements to performance of slagging gasifiers. Full details of the experimental procedure have been published [31,75,79], along with a compilation of results [75]. (ii) Viscosity in reducing atmospheres. The viscosity vs. temperature curves (reducing atmosphere) for slags produced from two samples of Beulah lignite are shown in Fig. 6.5 [79]. The low-sodium slag contained 4.2% Na20 while the high-sodium slag contained 8.1%. The comparative viscosity results are counterintuitive, in that the alkali metal oxides should function as oxide donors or "polymer breakers" in the melt, decreasing the viscosity. That is, it would be expected that the high-sodium slag should have the lower viscosity at a given temperature. These results are, however, similar to addition of K20 to synthetic slags, which resulted in a viscosity increase [80]. A direct comparison is not possible because the SiO2 and A1203 contents of the Beulah slags were not the same as those of the synthetic slags and the oxide donor capabilities of K20 and Na20 are different. 9 Low sodium

1000.

o High sodium

100o

.

10-

<11

1

'

'

1100

'

'

I

'

1200

'

'

'

I ' ' '

1300

'

I

'

1400

'

'

'

1500

Melt temperature, ~ Figure 6.5 9Viscosity-temperature relationships for slags from Beulah lignites of different sodium contents [79]. Measurements were made in 80:20 N2:H2 atmosphere 9

Two reducing-atmosphere viscosity vs. temperature curves for Indian Head lignite are shown in Fig. 6.6 [79], representing lignite samples taken approximately two years apart, and thus reflecting the effects of variation of composition within a mine. The analyses of the two samples are given in Table 6.10 [79].

323

100

9

1983 DATA

o

1981 DATA

O

.~

10-

' "

'1

"

"

I'

"

'i

"

"

I'

"

'1

"

"

I'

"

'1

"

"

Melt temperature, ~ Figure 6.6. Viscosity-temperature relationships for slags from two samples of Indian Head lignite mined about two years apart [79].

TABLE 6.10 Analyses of Indian Head lignite slags from lignite samples taken two years apart [79]. (Elements reported as oxides, normalized to 100%.) 1981 Sample 20.0 36.7 12.5 0.6 0.0 17.0 7.1 5.4 0.1 0.0

Silicon Aluminum Iron Titanium Phosphorus Calcium Magnesium Sodium Potassium Sulfur

Shear stress

vs.

1983 Sample 31.5 21.1 9.5 0.9 0.1 26.0 6.6 3.7 0.4 0.0

shear rate measurements suggest that these slags are in the non-Newtonian region

throughout the temperature range studied. If a temperature of critical viscosity exists for these slags, it must be above 1315~ and at a viscosity lower than 2 Pa.s. Viscosities of fly ash and bottom ash from lignite from southern Saskatchewan [81], compared with American coals of similar ash composition (e.g., [75]) are substantially higher at

324 comparable temperatures. No detailed study has been made to explain this difference. (iii) Comparative effects of atmosphere. The results of viscosity tests on Choctaw (Alabama) lignite are shown in Figure 6.7 [79]. Clearly there are dramatic differences depending on the atmosphere used for the experiment. The Choctaw ash has the highest Fe203 content of any U. S. lignite. The effect of atmosphere is the conversion of Fe203 present in oxidizing atmospheres to FeO in reducing atmospheres. Fe203 can behave as an oxide acceptor or "polymer former" potentially contributing to a high viscosity. On the other hand, FeO is an oxide donor and a good flux. Since iron is the only element exhibiting multiple oxidation states which occurs in appreciable quantities in coal ash slags, the extent to which viscosity is affected by atmosphere should roughly parallel the iron content of the slag. In a reducing atmosphere a short region of Newtonian flow occurs above 1200~

whereas in both the neutral and oxidizing atmospheres the

slag appeared to be in the non-Newtonian flow regime throughout the entire range of measurement.

9 Reducing 1000

o Neutral & Oxidizing

100 o

r,d,

10

< 1

1000

. . . .

I

1100

. . . .

I

. . . .

1200

I

'

'

1300

1

1

1400

Melt temperature, ~

Figure 6.7. Viscosity-temperature relationships for Choctaw lignite slag as a function of atmosphere [79]. The reducing atmosphere was 80:20 Nx:H2; neutral, N2; and oxidizing, air.

Comparable test results for Baukol-Noonan lignite slags, for which the iron content is substantially lower than the Choctaw slags [79], show that the effect of the different atmospheres is not as pronounced for the Baukol-Noonan slag. For the inert and oxidizing atmosphere tests a transition is noted at 1388 and 1427"C, respectively. Below these temperatures the viscosity curve is again linear, suggestive of Newtonian or at least a pseudo-Newtonian behavior. The principal effect of oxidizing vs. reducing conditions is to change the temperature of

325 critical viscosity without affecting the viscosity in the Newtonian range [82]. The results obtained for lignites generally support this contention. Rockdale lignite slag heated at 1455~ overnight showed an increase in viscosity from about 3.5 to about 8.0 Pa.s on standing for 16 h [28]. Similar, but smaller, increases in viscosity have been noted for Decker slag maintained at temperature for three hours. These observations suggest that equilibration periods may be on the order of hours, if not days, for these slags. (There is also a possible, and unexplored, role for slow but steady contamination of the slag by dissolution of the crucible, thus continuously altering the slag composition and, consequently, continuously altering the viscosity.) (iv) Hysteresis in viscosity measurements. Much of the recently published data on the viscosity of lignite ash slags (e.g., [31,79]) was obtained by beginning the experiment at a high temperature, measuring viscosity as the slag cooled until a low temperature limit was reached at which the slag was too viscous to permit accurate measurement, and then reheating to take a second set of measurements as the temperature increased. For convenience these two sets of data are referred to as the cooling curve and the heating curve, respectively. Many slags exhibit a distinct hysteresis between the cooling and heating curves, with the viscosity on the heating curve being higher than the viscosity at the same temperature on the cooling curve. Hysteresis is generally observed at the low-temperature end of the viscosity curves; at higher temperatures the heating curve rejoins the cooling curve. Hysteresis seems to be associated with the temperature of critical viscosity, since glassy slags which do not exhibit a sharp break in slope of the log (viscosity) vs. temperature curve also do not exhibit hysteresis. It has not been unequivocally established what phenomenon causes the rapid increase in viscosity below T cv, but it is generally assumed that the transition is due to precipitation of a solid phase. If that assumption is correct, hysteresis likely derives from the time required to dissolve the solid back into the liquid slag. If the dissolution time is long relative to the heating rate and the times between viscosity measurements, then measurements made as the slag is heated may be measurements of a slurry of not-yet-dissolved crystals in liquid slag, whereas at the same temperature on the cooling curve the slag may have been completely fluid. Thus it is not unreasonable that the two measured viscosities would be different. The time required for the solid to dissolve back into the slag would depend in part on the size of the solid particles. This line of reasoning is tested by calculating the temperature susceptibility factor. For a suspension, the change in viscosity with temperature is related to the number and degree of dispersion of the suspended particles. The temperature susceptibility factor is given by S = 0.221 x {log[(log VI + 0.08) / (log V 2 + 0.08)]} ] log Tz/T1 where V1 and V 2 are absolute viscosities measured at absolute temperatures T1 and T2, respectively [83]. The temperature susceptibility factor reflects the fact that the dependence of viscosity on temperature is a function of (among other things) the number of suspended particles

326 and the extent to which they are dispersed. As the size of particles decreases and the number of particles increases, the smaller will be the change of viscosity with temperature. The larger the value of S, the larger and fewer are the suspended particles. Larger particles would take a longer time to redissolve and would therefore have a greater contribution to hysteresis. The value of S was calculated for slags showing various degrees of hysteresis. The larger the observed hysteresis loop, the larger the value of S [18]. For three slags having S values of 6.4, 11.9, and 20.5, the larger the value of S the wider was the hysteresis loop between the viscosity curves taken during cooling and subsequent reheating of the slag in the non-Newtonian region [84]. The larger the value of S, the larger and less numerous are the suspended particles. This observation substantiates the hypothesis that hysteresis arises from a slow redissolution of a second phase formed when the slag cools through the temperature of critical viscosity; viz., the larger the particles of second phase the longer it takes to redissolve and hence the bigger the hysteresis. (v) Iron loss from slags. Empirical relationships have been developed to describe the effect on viscosity of reduction of iron compounds in the slag to iron metal. Formation of iron metal would remove FeO from the slag system; FeO functions as an oxide donor, helping to cleave aluminosilicate polymers and reduce viscosity. Consequently loss of FeO would raise the viscosity. Reduction to iron metal is essentially nil in alumina but can be extensive in a carbon crucible. At a given temperature the viscosity measured in a carbon crucible was invariably higher than that of the same slag measured in an alumina crucible. However the difference in viscosities measured in the two crucibles varies widely, from about 0.4 Pa.s for Baukol-Noonan slag to 67 Pa.s for Kemmerer (Wyoming) subbituminous slag [18]. For slags exposed to comparable environments, loss of iron by reduction should be a function of at least two factors: the total amount of iron initially in the slag, and the relative ease of reduction of iron in a given slag. The Fe+3/Fe+2 ratio in a given slag increases with increasing concentration of oxide donors and decreases with increasing concentration of oxide acceptors [85]. To develop an empirical expression for iron reduction, a comparable relationship was assumed for the Fe+2/Fe0 ratio. An empirical expression for iron lost by reduction was developed by multiple linear regression and is given by log (Fe loss) = 3.508 - 0.171 ("Fe203") - 1.850 [(CaO + MgO + Na20 + K20)/(SiO2 + A1203 + TiO2) where the molecular formulae represent mole fractions, and "Fe203" is the mole fraction of total iron compounds [18,86]. The coefficient of determination, r2, was 0.79 for seven data sets. The iron loss is the difference between the mole fractions of "Fe203" in slags after tests in iron and carbon crucibles. The change in viscosity at 1300"C caused by reduction is given by

327 AVI = 0.14 (% Fe loss) + 2.57 (VIA) - 16.37

where nA is the viscosity (in Pa.s) of the "unreduced" slag [18,86]. For this equation the regression coefficient of determination r2 was 0.85 [ 18]. Slags for which A~I is small are low-silica slags. Low-silica slags will have a large concentration of oxide donors (CaO, Na20, etc.) and the loss of a portion of the oxide donors by reduction of FeO will have a relatively minor effect on viscosity. Slags with a large An are high silica slags that have a low concentration of oxide donors. In these slags loss of oxide donor by reduction of FeO will have proportionately a much greater effect on viscosity. (vi) Non-Newtonian viscosity. Lignite ash slags can be placed into one of four categories of non-Newtonian flow behavior, ranging from a sharp Tcv with a near-vertical logrl vs. T plot below Tcv to no transition at all to non-Newtonian flow anywhere in the measurable range. The Urbain B factor (see below), a key datum for calculation of the Newtonian portion of the viscosity curve, appears to have no relationship to the type of non-Newtonian behavior [87]. 6.7.2 Calculation of viscosity from composition The experimental measurement of slag viscosity at temperatures in excess of 10(O~ is in principle no different from measurements made on familiar liquids at or near room temperature, but in practice high temperature viscometry can be a difficult and time-consuming experiment. Furthermore, the equipment is more expensive than viscometer installations for aqueous solutions or organic liquids because of the need for a high-temperature furnace and attendant temperature shielding and insulation. Thus few laboratories are equipped to make such measurements, and it is useful to be able to calculate viscosity at a given temperature from composition. The development of appropriate equations is an exercise which has been tried repeatedly for at least a half-century. (i) Modified Urbain Equation. If one has only the ash composition to work with, the best calculation procedure now available for low-rank coal slags is the so-called modified Urbain "equation" (which in fact is actually a series of equations). The original equation was developed for metallurgical slags [88]. One of the attractive features of the original work was that the equations are based on the SiO2 - A1203 - CaO pseudoternary system, and these three species are major components of most low-rank coal slags. Modification of the Urbain equation for application to low-rank coal slags has been detailed in several publications [31,75]. The modified Urbain equation is written as

In VI = In A + In T + 103B/T - 6 In A = -(0.2693B + 11.6725) and 6=mT+b

328 where T is in kelvins, rl is in poises, and B, m and b are calculated as discussed below [31,75]. The application of the modified Urbain equation to low-rank coal slags depends first on classifying the slag as high-, medium-, or low-silica. This classification is based on the value of B, which is calculated from the following sequence of equations:

M = CaO + M g O + Na20 + K20 + FeO + 2TIO2 + 3SO3 et = M/(M + A1203) Bo = 13.8 + 39.9355~ - 44.049c~2 B1 = 3 0 . 4 8 1 - 117.1505et + 129.9978~2 B2 = --40.9429 + 234.0486a - 300.04ct2 B3 = 6 0 . 7 6 1 9 - 153.9276a + 211.1616~2 B = Bo + BI(SiO2) + B2(SiO2) 2 + B3(SiO2) 3

where the molecular formulae represent the quantities of the components in the slag expressed in

mole fraction [31,75]. Having calculated B, the slag is then classified on the basis of the criteria shown below in Table 6.11. Then for low-silica slags

F = CaO/(CaO + MgO + Na20 + K20) loom = - 5 5 . 3 6 4 9 F + 37.9186 b = -1.8244(103m) + 0.9416 For the intermediate-silica case F = B(A1203 + FeO) loom = -1.3101F + 9.9279 b = -2.0356(103m) + 1.1094

Finally, for the high-silica slags

F = SiO2/(CaO + MgO + Na20 + K20) 10Om = -1.7264F + 8.4404 b= -1.7737(loom) + 0.0509

Tests of the modified Urbain equation with viscosity data for synthetic slags [89] showed that in some instances the some ambiguity may result when a slag is classified as low-, medium-, or high-silica based on the calculated value of B and when classified on the silica content itself

329 [ 17]. The classification criteria are summarized in Table 6.11 [ 17]. TABLE 6.11 Criteria for the classification of low-rank coal ash slags for application of the modified Urbain equation [ 17]. Classification High silica Intermediate Low silica

B >28 24-28 <24

Wt. % SiO2 >46 38--46 <42

Thus two "selection rules" were developed [17] to refine the procedure for using the modified Urbain equation: When the B value and SiO2 content suggest different classifications of the slag, select the classification based on the SiO2 content. If the classification of a slag based on SiO2 content is borderline, the higher of the two classifications should be selected. A comparison of calculated viscosity with experimental results for some lignite ash slags is shown in Table 6.12 [90]. TABLE 6.12 Comparison of calculated and experimental viscosity for lignite ash slags, modified Urbain method [9O].

Slag Baukol-Noonan Baukol-Noonan*

Silica Classification Intermediate Low

Gascoyne

Low

Indian Head 1 Indian Head 2

Low Low

Temp, ~ 1200 1300 1400 1200 1300 1400 1200 1300 1300 1300 1400

Viscosity, Poises Calcd Exptl 28 25 8.3 7.1 2.8 2.3 78 84 15 21 3.4 5.9 12 15 3.8 4.3 1.8 4.8 18 82 3.5 6.4

*In air atmosphere; all others in 80:20 N2:H2. (ii) Bills method. This correlation method [84] has given good preliminary results for some low-rank coal slags. The Bills method begins with the mole fraction of alumina in a slag. Using published graphs [91] one determines the "silica equivalence of alumina," Na. The sum of Na and the mole fraction of silica is used to read viscosity at different temperatures from a second graph. This correlation method is based on phase relationships in the CaO - MgO - A1203 - SiO2

330 quaternary system; the test of this method for applicability to low-rank coal slags was restricted to those slags for which the sum of these four components exceeds 90%. With that restriction, however, good correlations were obtained. Data for Gascoyne lignite are shown below [92]. TABLE 6.13 Comparison of experimental viscosities and predictions of Bills method for Gascoyne slag [92].

Viscosity, Pa.s Temperature, *C 1200 1250 1300 1350

Experimental 18.5 11.1 5.7 3.1

Calculated 15.2 7.4 4.2 2.4

Like most viscosity calculation models, the Bills method does not predict the temperature of critical viscosity, and as a result may give very poor predictions below T cv. In the Newtonian flow range and with slags for which the sum of calcium, magnesium, aluminum and silicon oxides is above 90% the method appears to give reasonably accurate predictions. (iii) Bottinga and Weill method. This method was originally developed for calculating the viscosity of magmas [93]. Like the modified Urbain equation, the Bottinga-Weill method divides slags into classes on the basis of silica content. The Bottinga-Weill method adds the further refinement of calculating some of the slag components as aluminates rather than oxides; thus Na20 is "converted" (mathematically) to NaAIO2 and CaO to CaAI20 4. The calculation of viscosity is based on the equation

In ~1 = ~ XiDi

where X is the mole fraction and D an empirical constant for component i. Values of D i are tabulated for the slag components at various ranges of the mole fraction of silica. The calculation is then repeated for a series of temperatures to generate the viscosity vs. temperature curve. Like many other methods, the Bottinga-Weill method has the disadvantages that sometimes the composition of the lignite slag is well outside the range of tabulated Di, the method does not predict Tcv, and in some cases the mole fraction of silica in the slag falls on the border between two ranges of mole fractions. With the caveats that one restricts the method to slags for which Di are tabulated and to the Newtonian flow range, one can get reasonable predictions even when the silica content gives an ambiguous choice of Di. Again using Gascoyne lignite slag as an example, the comparison of calculated and experimental values is shown in Table 6.14 [92]. Two sets of calculated values are shown because the mole fraction of silica in this slag, 0.456, fell between the

331 TABLE 6.14 Comparison of experimental viscosities of Gascoyne slag with calculated values from the BottingaWeill method [92].

Temperature, ~ 1200 1250 1300 1350

Viscosity, Pa.s Experimental Calculated 15.5 6.4 17.5 7.4 4.7 10.7 4.4 4.1 5.7 2.4 2.3 3.2

ranges of 0.35-0.45 and 0.45-0.55 for which Di are tabulated. (iv) Petrographic Calculations. A limited study of the calculation of lignite slag viscosities indicated the potential of classifying slags in the basis of petrographic (using the term as applied to igneous rocks, not coal petrography) compositions to establish categories of slags, each having a unique set of coefficients for calculating the terms in the Watt-Fereday equation. This approach is conceptually similar to that in the modified Urbain equation, which, as mentioned above, first classifies slags as high-, medium-, or low-silica, and then uses a separate calculational procedure for each. The Watt-Fereday equation is given by log rl = [ 107 M / (T - 150)2] + C [94], where T is the temperature in degrees Celsius, rl the viscosity in poises, and C and M are empirical constants calculated from the composition. Neither the original Watt-Fereday equation nor its subsequent modification [95] appeared to give accurate predictions of lignite slag viscosities. However, when slags were classified according to the petrographic normative calculation [96,97] it was possible to develop empirical equations to calculate C and M for each petrographic class. Doing so gave excellent fits of experimental data for the few cases in which the method was tested [98,99]. For example, Indian Head lignite slag was found to be pyroxenenormative; for this class of slags C = 4.8749 - 0.8143 (Fe203) and M = 0.1135 (Fe203) - 0.5219 [98], where Fe203 represents the weight percent of iron in the slag, expressed as ferric oxide. The fit of Indian Head data is shown in Fig. 6.8 [98].

332 9 Experimental data

100

<> Calculated values

r

~9

10

O t.)

.~t

''

'I

''

'I'

''

I'

''I

''

'I

'''

I'

''

(::) (:::)

(::) ~

C:) ~

C:) '43

C:) 0:::.

0 C:)

0 C,~

(::) '~"

,...=4

....=4

....=i

..==l

,.==i

..==i

~

,...=4

Melt temperature, ~ Figure 6.8. Comparison of experimental viscosity data for Indian Head lignite slag with values calculated from a model based on petrographic normative prediction of silicate structures [98].

(v) Thermodynamic-based correlations. The original work on applying petrographic normative calculations sprang from the realization that silicates and aluminosilicates exist in a remarkable diversity of oligomeric or polymeric structural types (e.g., chains, rings, sheets, and infinite networks) and it is quite unlikely that the hydrodynamics of, say, a ring structure in a liquid phase will be similar to that of a chain structure; hence the viscosities would also be different. Thus if slags could be sorted according to structure, it would be possible to refine predictive equations to describe more accurately the viscosity behavior of a limited class of slags. An even more powerful strategy for achieving the same goal is to use thermodynamic calculations to predict the liquidphase composition and then to sort slags into classes based on the calculated composition of the liquid. Virtually all other methods for predicting viscosity from composition can be applied using a hand-held calculator or even, with great patience, the "cellulose-graphite" method. Use of thermodynamic modeling requires, as a minimum, a reasonably powerful microcomputer. Prediction of liquid-phase composition uses a computer program called SOLGASMIX, which uses minimization of free energy to predict the composition of a system (including gaseous and solid phases) as a function of temperature. The input data are, in the case of coal ash slags, the composition of laboratory-prepared ash. The program relies on the existence of a base of thermodynamic data on the phases expected to occur; it cannot create phases for which the data are

333 not in its base. Accurate prediction of liquid-phase composition depends on the supporting thermodynamic data base containing information on all of the species likely to occur in the system. Nevertheless, in the best cases very accurate predictions of liquid-phase compositions are possible, as shown for Martin Lake (Texas) lignite slag given in Table 6.14 [100]. TABLE 6.14 Comparison of slag composition predicted at 1475~ and composition observed in solidified slag for Martin Lake lignite [100] Component SiO2 A1203 Fe203 YiO2 P205 CaO MgO Na20 K20 SO3

Predicted composition, % 48.5 15.0 11.9 1.0 0.0 10.5 3.5 0.9 1.1 7.5

Observed composition, % 48.3 14.9 11.8 1.0 0.0 10.5 3.5 0.9 1.1 7.5

Furthermore, the prediction of the actual species present in the melt is also, in best cases, quite accurate, as shown in Table 6.15 [ 100]. This table compares species predicted by SOLGASMIXto occur in the Martin Lake slag and those observed by X-ray diffraction in the solidified slag. TABLE 6.15 Comparison of species predicted to occur in Martin Lake lignite slag and those observed by X-ray diffraction [ 1001. SOLGASMIX prediction (a) CaA12Si208 SiO2 MgSiO3 CaSO4 FeO Fe304

X-ray observation CaAlzSi208 (c) SiO2 (b) MgSiO3 (c) CaSt4 (b) Fe203 (c)

Notes: (a) Listed in decreasing order of abundance; (b) identified as "major" species; (c) identified as "minor" species.

The use of SOLGASMIXas a predictive tool allowed establishment of a series of empirical guidelines for selecting the predictive equation giving the smallest deviation between measured and calculated viscosities [101]. In most cases the ash compositions were determined on ash prepared at 1000~

rather than the customary 750~

calculated to an SO3-free basis. Using ash

334 composition determined in this way, for those ashes for which the lignite factor (the ratio of CaO + MgO to Fe203) is in the range 1-3, SiO2 is in the range 42-55%, and Na20 _<5%, the most suitable predictive equation is the modified Watt-Fereday [95]. For the lignite ashes meeting these criteria, the average deviations between experimentally observed viscosities and those calculated fall in the range 10-23%. For ashes for which the lignite factor is 1-3, SiO2 is 34-42%, and Na20 _<5%, the Hoy equation [102] is most suitable, and average deviations for the lignite slags in this category are in the range 5-19%. Some other lignites fall into other categories, but most lignite slags appear to be reasonably well treated by either the modified Watt-Fereday or the Hoy method, as appropriate. Examples of the application of this approach to slags from coals ranging in rank from brown through bituminous, with extensive tables of composition data, are given in the original literature [ 101]. (vi) Other Correlations. The chemistry of lignite-type ashes is dominated by the relative amounts of CaO and MgO present [74]. The dolomite ratio is defined from the formula DR = 100.(CaO + MgO) / (CaO + MgO + Fe203 + Na20 + K20) [74]. The temperature at which viscosity is 250 poises (T250) decreases with a decrease in the percent of basic components in the slag over the range 75 to 40%; at a given percentage of basic components in the slag, T250 increases as the dolomite ratio increases in the range 50 to 95. However, the data on which this relationship is based show considerable scatter. The F e O - Fe203 -SiO2 system is one in which ideal solution assumptions agree with experimental data [103]. The mole fractions of Fe203 and SiO2 were calculated for ten slags from the compositions of the solidified slags after viscosity tests. In six samples, the Fe203 value was well above the saturation value [103], and in each of these cases, discrete iron-containing phases were observed in the X-ray diffractogram of the solidified slag [ 104]. In two samples, the Fe203 of the solidified slag was at the saturation value, and in these two cases, no iron-containing phases were detected by X-ray diffraction. The remaining two samples had mole fractions of Fe203 slightly above the saturation value; here iron-containing phases were detected in one of the two samples. A series of 25 slags, including subbituminous and a few bituminous coal ash slags as well as lignites, were ranked according to the temperature at which the viscosity is 100 poises (i.e., T100) [84]. X-ray diffraction of the solidified slags shows clear trends in the phases observed as a function of T 100. Slags for which T100 ranges from 1436" to > 1538"C are all amorphous. Where 1407" > T 100 > 1327"C, the phases generally observed are plagioclase and pyroxene. For 1285" > T100 > 1182"C gehlenite and nepheline are commonly observed. The correlation is not perfect; furthermore, at least twelve other phases have been observed occasionally. Nevertheless a clear trend exists which substantiates the idea that silicate or aluminosilicate molecular structures have a strong influence on viscosity behavior.

335 6.7.3 Applications of viscosity data in lignite processing (i) Gasification. The criterion established for satisfactory slagging operation in a fixed-bed slagging gasifier was that the viscosity should be less than 10 Pa.s at 1300~

[31,105]. This

criterion was established on the basis of laboratory data obtained for slags from lignites and subbituminous coals, with comparison to operating experiences in the pilot scale fixed-bed gasifier at the University of North Dakota. Viscosity vs. temperature results for Velva (North Dakota) lignite ash slag [30] are of particular interest because of the unusually high calcium content of this slag (43.3%), which, among other things, makes it very difficult to work with in the laboratory. Despite a very high temperature of critical viscosity, about 1300~

and an extreme corrosiveness toward refractories,

the slag nevertheless gave satisfactory performance in a pilot-scale fixed-bed slagging gasifier [30]. Most slags show distinct hysteresis between data taken while cooling and subsequent data taken as the slag was reheated. For a given temperature, the viscosity was higher during reheating than during cooling; that is, to achieve a desired viscosity, it is necessary to attain a higher temperature if the slag is being heated after having been cooled. If a process upset causes the slag to be cooled to a viscosity too high for adequate slag flow, it will be necessary to heat the slag to a temperature above that from which it had cooled to restore satisfactory flow. Such behavior was in fact observed in a pilot plant [ 105]. During fixed-bed gasification, some of the iron present in the lignite as various ironcontaining minerals is reduced to iron metal. Since in a reducing atmosphere the iron in the slag would be present as Fe+2, a good oxide donor or "polymer breaker," the reduction to iron metal would deplete the concentration of Fe+2 and potentially raise the viscosity of the slag. The consequence is most marked for high silica slags, where the concentration of oxide donors is low to begin with, and any losses of oxide donors will have a proportionately large effect [ 105]. In the case of low silica slags, which contain significant amounts of sodium, calcium, and magnesium in addition to iron(II), the effects of the reduction of some of the iron to the metal will not be as severe. (ii) Combustion. The maximum viscosity at which slag can be tapped from a boiler is 25 Pa.s [ 106]. For operation of "wet bottom" furnaces the range of slag viscosities that permit troublefree tapping is 1-10 Pa.s [106]. For operation of a cyclone furnace, the temperature at which slag viscosity is 250 poises (often referred to simply as T250) should be less than 14250C [ 106]. 6.7.4 Surface tension The surface tension of coal ash slags, and its dependence on composition, temperature, and atmosphere, are much less explored than viscosity. Little has been reported in the open literature on surface tensions of lignite ash slags. The surface tension is greater in a reducing atmosphere than in an oxidizing atmosphere [68]. This is shown by the data in Table 6.16 [68].

336 TABLE 6.16 Effect of atmosphere on surface tension of lignite slags [68]. Lignite Atmosphere Gascoyne (Red pit) Reducing Oxidizing Beulah Reducing Oxidizing Gascoyne (Blue pit) Reducing Oxidizing

Temperature, ~ 1234 1248 1254 1274 1260 1310

Surface tension, dyne/cm 454 323 477 230 377 262

The dependence of surface tension on atmosphere is also shown, for Beulah lignite slag, by the data in Table 6.17 [107]. TABLE 6.17 Dependence of surface tension of Beulah lignite slag on atmosphere [107]. Temperature, ~ 1285 1275 1200

Atmosphere Air 8:92 H2:N2 60:40 C0:C02

Surface tension, dyne/cm 802 615 408

Slags having higher silica content tend to have higher surface tensions, while slags having higher sodium content tend to have lower surface tensions [68]. The dependence of surface tension on SiO2/Na20 molar ratio is illustrated in Figure 6.9 [68], which has data for six lignite slags (in some cases measured in both reducing and oxidizing atmospheres). The linear least squares fit of these data has a coefficient of determination, r2, of 0.410. The trend is substantiated by results for slags from coals of higher rank, which have higher SiO2/Na20 ratios and higher surface tensions [68] can be affected by the formation of thin layers of new liquid phases on the surface [107]. For example, the degree of polymerization of silicates or aluminosilicates in surface layers may be different from that in the bulk of the liquid, and cause changes in the apparent surface tension. Surface tension is also affected by the time at which the slag is held at temperature, as shown by the data in Table 6.18 for Beulah lignite slag [107].

337 650

550-

450-

~

350" 300

''''

0

I''''

i ' ' ' '

I''''

I''''

5 10 15 20 SiO2/Na20 mole ratio

25

Figure 6.9. Dependence of lignite slag surface tension on SiO2 / Na20 molar ratio [68].

TABLE 6.18 Effect of annealing time on surface tension of Beulah lignite slag [107]. Time, min. 0-5 30 45 60 120 180 240

Surface tension, dyne/cm 420 460 472 468 423 369 211

REFERENCES

R.N. Miller and P.H. Given, A geochemical study of the inorganic constituents in some low-rank coals, U.S. Energy Res.Devel. Agency Rept. FE-2494-TR-1, 1978. J.C. Nankervis and R.B. Furlong, Phase changes in mineral matter of North Dakota lignites caused by heating to 1200~ Fuel, 59 (1980) 425-430. R.N. Miller, R.F. Yarzab, and P.H. Given, Determination of mineral matter contents by low-temperature ashing, Fuel, 58 (1979), 4-10. F.W. Frazer and C.B. Belcher, Quantitative determination of the mineral matter in coal by a radiofrequency oxidation technique, Fuel, 52 (1973) 41-46. S.K. Falcone and H.H. Schobert, Mineral transformations during ashing of selected lowrank coals, in: K.S. Vorres (Ed.), Mineral Matter and Ash in Coal, American Chemical Society, Washington, 1984, Chapter 9. H.H. Schobert and S.K. Falcone, Ash and slag characterization, in: G.A. Wiltsee (Ed.), Low-rank Coal Research Under the UND/DOE Cooperative Agreement, U.S. Dept.

338

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Energy Rept. DOE/FE/60181-17, (1983), pp. 12-1 - 12-11. M.E. Morgan, R.G. Jenkins, and P.L. Walker Jr., Inorganic consitiuents in American lignites, U.S. Dept. of Energy Rept. FE-2030-TR21, (1980). R.C. Neavel, E.J. Hippo, S.E. Smith, and R.N. Miller, Coal characterization research: Sample selection, preparation, and analyses, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 25(3) (1980) 246-255. M.E. Morgan, R.G. Jenkins, and P.L. Walker Jr., Analysis of the inorganic constituents in American lignites, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 25(1) (1980) 219222. H.J. Gluskoter, Electronic low-temperature ashing of bituminous coal, Fuel, 44 (1965) 285-291. J.V. O'Gorman and P.L. Walker Jr., Mineral matter and trace elements in U.S. coals, U.S. Office Coal Res. Rept. 14-01-0001-390, (1972). L.R. Radovic, P.L. Walker Jr., and R.G. Jenkins, Effect of lignite pyrolysis conditions on calcium oxide dispersion and subsequent char reactivity, Fuel, 62 (1983) 209-212. W.W. Fowkes and J.J. Hoeppner, Sulfur in lignite: Form and transformations on thermal treatment, U.S. Bur. Mines Rept. Invest. 5626, (1960). H.H. Schobert and S.K. Falcone, Ash and slag characterization, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/UNDERC/QTR-85/3-4, (1986), pp. 13-1 - 13-9. S.K. Falcone and S.A. Braun, Ash and slag characterization, in: G.A. Wiltsee (Ed.), Lowrank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1642, (1984), pp. 13-1 13-7. S.K. Falcone, Oral presentation, Project Sodium semiannual sponsors' meeting, Grand Forks, N.D., November 1985. H.H. Schobert and S.K. Falcone, Ash and slag characterization, in: G.A. Wiltsee (Ed.), in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1682, (1984), pp. 13-1 - 13-11. H.H. Schobert, S.A. Benson, and S.K. Falcone, Ash and slag characterization, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1574, (1984), pp. 13-1 - 13-14. R.J. Quann and A.F. Sarofim, A scanning electron microscopy study of the transformations of organically bound metals during lignite combustion, Fuel, 65 (1986) 4046. Energy Resources Co., Low-rank coal study, Vol. 3, technology evaluation, U.S. Dept. Energy Rept. DE-AC18-79FC10066, (1980). J.B. Stone, K.L. Trachte, and S.K. Poddar, Calcium carbonate deposit formation during the liquefaction of low rank coals, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 24(2) (1979) 255-262. B.W. Farnum, C.L. Knudson, and D.A. Koch, Products of liquefaction of lignite with synthesis gas by product slurry recycle, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 24(3) (1979) 195-197. A. Kassoli-Fournaraki, A. Georgakopoulos, and A. Filippidis, Heating experiments of the Ptolemais lignite in the temperature range from 100*C to 500"C, Neues Jahrb. Mineral. Monatsh., 11 (1992) 487-493. M.E. Morgan, R.G. Jenkins, and P.L. Walker Jr., Inorganic constituents in American lignites, Fuel, 60 (1981) 189-193. P.C. Painter, M.M. Coleman, R.G. Jenkins, and P.L. Walker Jr., Fourier transform infrared study of acid-demineralized coal, Fuel, 57 (1978) 125-126. S.K. Falcone, J.P. Hurley, and H.H. Schobert, The effects of ion-exchangeable cations on coal ash mineralogy, U.S. Dept. Energy Rept. DOE/FE/60181-107, (1985). R.E. Grim, Clay Mineralogy, McGraw-Hill, New York, 1968, Chapter 9. H.H. Schobert, S.A. Benson, and S.K. Falcone, Ash and slag characterization, in: G.A. Wiltsee (Ed.), Low-rank Coal Research Under the UND/DOE Cooperative Agreement, U.S. Dept. Energy Rept. DOE/FE/60181-26, (1983), pp. 13-1 - 13-18. R.S. Mitchell and H.J. Gluskoter, Mineralogy of ash of some American coals: variations with temperature and source, Fuel, 55 (1976) 90-96. G.H. Gronhovd, A.E. Harak, M.M. Fegley, and D.E. Severson, Slagging fixed-bed

339

31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

47 48 49 50 51 52 53

gasification of North Dakota lignite at pressures to 400 psig, U.S. Bur. Mines Rept. Invest. 7408, (1970). H.H. Schobert, R.C. Streeter, and E.K.Diehl, Flow properties of low-rank coal ash slags: Implications for slagging gasification, Fuel, 64 (1985), 1611-1617. M. Bobman, B.J. Zobeck, and D.R. Hajicek, Fluidized bed combustion of low-rank coals, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1682, (1984), pp. 11-1 - 11-25. R.P. Tripathi and K.L. Shrivastava, Characterization of Merta Road lignite and decompositional study of its pyrite using Mi3ssbauer spectroscopy, Fuel, 69 (1990) 928930. (Unknown) Characteristics of gasification char from Dakota Star lignite, Unpublished manuscript, Grand Forks, ND, ca. 1950. P.G. Christman, M.P. Athans, and T.F. Edgar, The sulfur balance in pyrolysis and combustion, AIChE Symp. Ser. 78(216) (1982) 30-41. J. Dollimore, S.F. Ross, and G.M. Schelkoph, Pyrolysis and devolatilization, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1574, (1984), pp. 18-1 - 18-11. T.C. Owens and J.R. Rindt, Low-rank coal liquefaction, in: G.A. Wiltsee (Ed.), Lowrank Coal Research Under the UND/DOE Cooperative Agreement, U.S. Dept. Energy Rept. DOE/FE/60181-17, (1983), pp. 5-1 - 5-36. J. Boow, Sodium/ash interactions in the formation of fireside deposits in pulverized-fuelfired boilers, Fuel, 51 (1972) 170-173. W. Eitel, Silicate Science, Vol. 2, Academic Press, New York, 1964. E.A. Sondreal, J.L. Elder, and W.R. Kube, Characteristics and variability of lignite ash from the northern Great Plains province, in: J.L.Elder and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Bur. Mines Info. Circ. 8304, (1966), pp. 39-50. C.H. Wright, Coal rank effects in the SRC II process, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 27(3-4) (1982) 43-51. W.H. Ode and F.H. Gibson, Effect of sulfur retention on determined ash in lower-rank coals, U.S. Bur. Mines Rept. Invest. 5931, (1962). G.H. Gronhovd, P.D. Tufte, and S.J. Selle, Some studies on stack emissions from lignitefired power plants, in: G.H. Gronhovd and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Bur. Mines Info. Circ. 8650, (1974), pp. 83-102. H. Freund, R.K. Lyon, and W. Bartok, The sulfur retention of calcium-containing coal during partial oxidation, Proceedings 1981 International Conference on Coal Science, pp. 253-257. A. Magnusson, Sulfur in North Dakota lignite, Unpublished manuscript, University of North Dakota, ca. 1950 A.F. Duzy, M.P. Corriveau, R. Byrom, and R.E. Zimmerman, Pretinent qualitative evaluations prior to mining and utilization, in: G.H. Gronhovd and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Dept. of Energy Rept. GFERC/IC-77/1, (1977), pp. 13-42. D.K. Rindt, F.R. Kamer, W. Beckering, and H.H. Schobert, Current research on the inorganic constituents in North Dakota lignites and some effects on utilization, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 25(1) (1980) 210-217. M. Gomez, J.B. Gayle, and A.R. Taylor, Heat content and specific heat of coals and related products. U.S. Bur. Mines Rept. Invest. 6607, (1965). E.A. Sondreal and R.C. Ellman, Laboratory determination of factors affecting storage of North Dakota lignite, U.S. Bur. Mines Rept. Invest. 7887, (1974). R.C. Ledger, Parallel tests on United States lignites at the SECV and a U.S. laboratory: SECV results, State Electricity Commission of Victoria Report SO/85/88, (1985). F.J. Bayley, J.M. Owen, and A.B. Turner, Heat Transfer, Barnes and Noble, New York, 1972, pp. 303-305. I.R. Mikk and T.B. Tiykma, The absorptivity of slag covered radiantly heated surfaces of steam generators, Heat Trans. Sov. Res. 10 (1978) 39-44. H.B. Becker, Spectral band emissivities of ash deposits and radiative heat transfer in pulverised-coal-fired furnaces, Ph.D. Thesis, University of Newcastle, Newcastle, NSW, Australia, 1982.

340 54 55 56 57 58

D.M. Barnhart and P.C. Williams, The sintering test as an index to ash fouling tendency, Trans. ASME, 78 (1956) 1229-1236. L.G. Austin, S.A. Benson, H.H. Schobert, and M. Tangsathitkulchai, Fundamental studies of the mechanism of slag deposit formation, U.S. Dept. of Energy Rept. DOE/FE70770, (1987). B. Jung and H.H. Schobert, Viscous sintering of coal ashes. 2. Sintering behavior at short residence times in a drop-tube furnace, Energy Fuels, 6 (1992) 59-68. B. Jung and H.H. Schobert, Viscous sintering of coal ashes. 1. Relationships of sinter point and sinter strength to particle size and composition, Energy Fuels, 5 ( 1991) 555-561. D.R. Hajicek, Fluidized bed combustion of low-rank coals, in: G.A. Wiltsee (Ed.), Lowrank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1642, (1984), pp. 11-1 11-7.

59 60 61 62 63 64 65

66 67 68 69 70 71 72 73 74 75 76 77

M.L. Odenbaugh, Sintering characteristics of lignite ash, M.S. Thesis, University of North Dakota, Grand Forks, N.D., 1966. R.C. Attig and D.H. Barnhart, A laboratory method of evaluating factors affecting tube bank fouling in coal-fired boilers, in: The Mechanism of Corrosion by Fuel Impurities, Butterworths, London, 1963, pp. 173-182. D.P. McCollor, M.L. Jones, B.G. Miller, and R.A. Brown, Combustion research and ash fouling, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1682, (1984), pp. 10-1 - 10-15. R.W. Nurse and H.G. Midgley, Studies in the melilite solid solutions, J. Iron Steel Inst., 174 (1953) 121-135. J.W. Nowok, S.A. Benson, M.L. Jones, and D.P. Kalmanovitch, Sintering behavior and strength development in various coal ashes, Fuel, 69 (1990) 1020-1029. H.H. Schobert, S.A. Benson, and L.G. Austin, Relationships among fusing and sintering behavior, strength, and composition of slag deposits from U.S. lignites, Proceedings 1987 International Conference on Coal Science, pp. 897-899. D.K. Rindt, M.L. Jones, and H.H. Schobert, Investigations of the mechanisms of ash fouling in low-rank coal combustion, in: R.W. Bryers (Ed.), Fouling and Slagging Resulting from Impurities in Combustion Gases, Hemisphere, Washington, 1983, pp. 117141. M.H. Massa and J.S. Wilson, X-ray diffraction examination of coal combustion products related to boiler tube foiling and slagging, Adv. X-Ray Anal., 20 (1977) 85-93. D.P. McCollor, M.L. Jones, and W.P. Kinneman, Combustion research and ash fouling, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1642, (1984), pp. 10-1- 10-17. S.F. Miller and D.P. Kalmanovitch, Relation of slag viscosity and surface tension to sintering potential, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33(2) (1988) 42-49. D.W. Brekke and F.R. Karner, Analysis and characterization of atmospheric fluidized-bed combustion agglomeration, U.S. Dept. Energy Rept. DOE/FC/10120-T7, (1982). American Society for Testing and Materials, Standard test method for fusibility of coal and coke ash, in: 1991 Annual Book of ASTM Standards, American Society for Testing and Materials, Philadelphia, PA, 1991, Vol. 05.05. E.A. Sondreal and R.C. Ellman, Fusibility of ash from lignite and its correlation with composition, U.S. Energy Res. Devel. Admin. Rept. GFERC/RI-75-1, (1975). N.H. Snyder and S.J. Aresco, Analyses of tipple and delivered samples of coal, U.S. Bur. Mines Bull. 516, (1953). U.S. Bureau of Mines, Technology of lignitic coals, U.S. Bur. Mines Info. Circ. 7691, (1954). A.F. Duzy, Fusibility-viscosity of lignite type ash, ASME Paper 65-Wa/FU-7, (1965). R.C. Streeter, E.K. Diehl, and H.H. Schobert, Measurement and prediction of low-rank coal slag viscosity, in: H.H. Schobert (Ed.), The Chemistry of Low-Rank Coals, American Chemical Society, Washington, 1984, Chapter 12. W.G. Lloyd, J.T. Riley, M.A. Risen, S.R. Gilleland, and R.L. Tibbetts, Estimation of ash softening temperatures using cross terms and partial factor analysis, Energy Fuels, 4 (1990) 360-364. W.G. Lloyd, J.T. Riley, S. Zhou, M.A. Risen, and R.L. Tibbetts, Ash fusion temperatures under oxidizing conditions, Energy Fuels, 7 (1993) 490-494.

341 78 79 80 81 82 83 84 85 86 87 88 89 90

R. Ledesma and L.L. Isaacs, Thermal properties of coal ashes, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 36 (1991) 223-230. R.C. Streeter, Slag viscosity determination, U.S. Dept. of Energy Rept. DOE/FC/101591, (1984). H.P.R. Frederikse, T. Negas, and S.J. Schneider, Development, testing, and evaluation of MHD materials. U.S. Nat. Bur. Standards Rept. E(49-18)-1230, (1975). D.H.H. Quon, S.S.B. Wang, and T.T. Chen, Viscosity measurements of slags from western Canadian coals, Fuel, 63 (1984) 939-942. W.T. Reid, External Corrosion and Deposits, Elsevier, New York, 1971. H.H. Lowry (Ed.), Chemistry of Coal Utilization, Vol. 2, Wiley, New York, 1945, pp. 1308-1310. H.H. Schobert, Ash and slag characterization, University of North Dakota Energy Research Center monthly report, January 1984. E.G. Turkdogen and P.M. Bills, Some general considerations on the state of oxidation of iron in molten silicates, phosphates, and silicophosphates, in: G.R. St.Pierre (Ed.), Physical Chemistry of Process Metallurgy, Interscience, New York, 1961. H.H. Schobert, Ash and slag characterization, University of North Dakota Energy Research Center monthly report, March 1984. H.H. Schobert, Ash and slag characterization, University of North Dakota Energy Research Center monthly report, February 1986. G. Urbain, F. Cambier, M. Deletter, and M.R. Anseau,Viscosity of silicate melts, Trans. J. Brit. Ceram. Soc., 80 (1981), 139-141. J. Chen, S. Greenberg, and R.B. Poeppel, The viscosity of coal slags as a function of composition, temperature, and oxygen partial pressure, Argonne National Laboratory Rept. ANL/FE-83-30 (1984). H.H. Schobert and S.K. Falcone, Ash and slag characterization, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1846, (1986), pp. 13-1 13-6. P.M. Bills, Viscosities in silicate slag systems, J. Iron Steel Inst., (1963) 133-140. H.H. Schobert, An informal review of viscosity-composition correlations applied to lowrank coal ash slags, Grand Forks Energy Technology Center internal report GFETC/IR-8, (1982). Y. Bottinga and D.F. Weill, The viscosity of magmatic silicate liquids: A model for calculation. Amer. J. Sci. 272 (1972) 438-475. J.D. Watt and F. Fereday, The flow properties of slags formed from the ashes of British bituminous coals: Part 1: Viscosity of homogeneous slags in relation to slag composition, J. Inst. Fuel, 42 (1969), 99-103. D. Bomkamp, Properties of mineral matter, ash, and slag, in: Institute of Gas Technology (Eds.), Coal Conversion Systems Technical Data Book, U.S. Dept. Energy Rept. HCP/T2286-01, (1978), Section IA.40. T.F.W. Barth, Theoretical Petrology, Wiley, New York, 1962. A. Johansen, A Descriptive Petrography of Igneous Rocks, Univ. of Chicago Press, Chicago, 1939. R.C. Ellman, B.C. Johnson, H.H. Schobert, L.E. Paulson, and M.M. Fegley, Current status of studies in slagging fixed-bed gasification at the Grand Forks Energy Research Center, in: G.H. Gronhovd and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Dept. of Energy Rept. GFERC/IC-77/1, (1977), pp. 207-247. H.H. Schobert, Petrochemistry of coal ash slags. 2. Correlation of viscosity with composition and petrographic class, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 22(4) (1977) 143-146. H.H. Schobert and B. Jung, Thermodynamic and rheological modeling of coal ash behavior, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33(2) (1988) 28-33. B. Jung and H.H. Schobert, Improved prediction of coal ash slag viscosity by thermodynamic modeling of liquid-phase composition, Energy Fuels, 6 (1992) 387-398. H.R. Hoy, A.G. Roberts, and D.M. Wilkens, Some investigations with a small cyclone combustor, J.Inst. Fuel, (1958) 429-442. J. Lumsden, The thermodynamics of liquid iron silicates, in: G.R. St.Pierre (Ed.), Physical Chemistry of Process Metallurgy, Interscience, New York, 1961. -

91 92 93 94 95 96 97 98

99 100 101 102 103

342 104 105 106 107

H.H. Schobert, Ash and slag characterization, Grand Forks Energy Technology Center monthly report, December, 1982. H.H. Schobert, Ash and slag characterization, Proc, 5th Gasif. Proj. Contractors' Meeting, U.S. Dept. Energy Rept. DOE/METC-85/6024, (1985), pp. 43-47. E.C. Winegartner, Coal Fouling and Slagging Parameters, American Society of Mechanical Engineers, New York, 1974. J.W. Nowok, J.A. Bieber, S.A. Benson, and M.L. Jones, Physicochemical effects influencing the measurements of interfacial surface tension of coal ashes, Fuel, 70 (1991) 951-956.

343

Chapter 7

P H Y S I C A L P R O P E R T I E S OF LIGNITES

The strength of lignite particles and changes in strength and volume with drying or heating can offer insights into the three-dimensional macromolecular structure of the lignite and into the ways in which moisture is incorporated in the particles. The lignite structure may be analogous to reinforced concrete, in that dehydrated cellulose residues play the role of the reinforcing steel and the coalified lignin the cement [ 1]. Other species, such as trapped alkanes, are the analogs of the sand in the reinforced concrete. While this analogy provides a way of visualizing the importance of the coalified and altered cellulose and lignin in the long-range structural order of the lignite, it also suggests that there may be value in treating the measured mechanical properties of lignites in terms of multicomponent beam theory. The interrelationships between supramolecular order and mechanical or physical properties are poorly explored, but research in this area might develop new insights in both directions, that is, the role of supramolecular structure in determining or affecting mechanical properties, and the use of mechanical property measurements for inferences on structural arrangements. 7.1 BULK M E C H A N I C A L PROPERTIES

7.1.1 Volumetric shrinkage Volumetric shrinkage on drying can be determined by a simple laboratory test. A graduated cylinder is filled with 2x10 mm lignite, the particles settled by tapping, and the volume reduction after heating (and resettling by tapping) determined quickly from the volume scale of the cylinder. The changes in sample volume and sample weight for Indian Head (North Dakota) lignite are shown in Figure 7.1 [2,3]. The first portion of moisture to be removed has the greatest effect on particle shrinkage and breakage. The volumetric shrinkage of Indian Head lignite after seven hours of drying at 60~ was 12% [4]. After sixteen hours at 105~

the cumulative shrinkage was 20%

[4]. Heating to 540~ results in a total shrinkage of 35%. Since the free swelling index of a noncaking coal is 1, the marked shrinkage of lignite accompanying severe heating suggests the definition of fractional free swelling indices, or a "free shrinking index" [4]. In this system, the behavior of Indian Head lignite heated to 5400C would be equivalent to a fractional free swelling index of 0.65 [4]. The size reduction and bulk shrinkage of crushed Indian Head lignite that accompanied

3 4 4

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50 30

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I ....

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200 300 400 Temperature, ~

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Fig. 7.1. Changes in volume, weight, and particle size of Indian Head lignite with drying and devolatilization [2]. Samples were held 16 h at 40 and 120"; 4 h at 260 and 540*. drying by acetone extraction of the moisture were not significantly different from the same changes accompanying gentle thermal drying [3,5]. This suggests that the removal of water molecules from the structural framework of the lignite, rather than effects of thermal expansion, is responsible for loss of structural integrity. The volumetric shrinkage of lignite particles on drying or devolatilization indicates that the changes in size distribution on mechanical tumbling or heating are not due solely to comminution accompanying these actions, but may also reflect, in part, the shrinkage of particles. Shrinkage during drying and devolatilization will have an effect on the size distribution in addition to that caused by the fracturing of particles [4]. Drying and pyrolysis can produce a bulk shrinkage of 35% in Indian Head lignite [4]. In practical applications particle shrinkage can be an important factor in providing stress relief normal to planes of shear failure through compacted zones, affecting, for example, the settling behavior of lignite in process vessels. For practical design applications, properties of lignite beds such as shear strength and permeability are defined by the size distribution of the particles regardless of how that size distribution came to be established (i.e., regardless of the relative contributions of thermal and mechanical friabilities and volumetric shrinkage).. A rapid initial shrinkage can set up large stresses in the lignite particles, which in turn can cause macroscopic cracking [6]. Macroscopic mechanical factors determine the susceptibility of a given lignite to cracking. Neither the rate nor the extent of shrinkage is isotropic [6]. There is no specific relationship between shrinkage and orientation of a particle relative to its original bedding plane. However,

345 crack formation will occur along the bedding planes before it will proceed across bedding planes. The shrinkage accompanying drying is not completely reversible [6]. Rewetting of samples does not fully restore the original volume; about 80% of original volume is recovered, and this extent of recovery seems independent of the nature of the sample or its previous storage conditions [6]. The structural irreversibility occurs even with only partial drying. Very large voids in the structure can lose the water they hold even at storage conditions near 100% relative humidity (implying that the water in such spaces is essentially surface moisture) [6]. The irreversible structural shrinkage may involve loss of water from macropores or transition pores (or both). The drying and shrinkage are accompanied by a loss of porosity, primarily due to collapse of the transitional pores in the 1.5--40 nm size range [6]. The extent of pore collapse increases with increasing severity of drying. As the structure shrinks, new internal hydrogen bonds may form, and these bonds may be responsible for the lack of ability to regain completely the original sample volume. 7.1.2 Friability Lignites are the least friable of coals; the percentage friability is 12% for lignites, ranging up to 70% for low volatile bituminous coals, and dropping again to 33% for anthracites [7]. (i) Mechanical friability. Mechanical friability can be determined by the vertical displacement of a standard size distribution curve after a standardized tumbling test. Mechanical friability results for two samples of Indian Head lignite of different initial size distributions are shown in Figure 7.2 [2]. When standardized samples of 19x13 mm coal are used, the friability can

I

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Figure 7.2. Mechanical friability of Indian Head lignite, after tumbling for 30 minutes [2].

346 be expressed as the fraction (or percentage) reduced to-13 mm [2]. The mechanical friability of Indian Head lignite calculated in this manner is 38%. Alternatively, the data can be plotted on probability paper and be fitted by least squares techniques to an equation of the form log[log(100/X)] = a + b log(d) [8]. Here d is a particle size, X the cumulative weight percent greater than d, and a and b are empirical constants. This method of treating the data provides a convenient definition of friability as the shift in weight percent over 10 mm,

i.e.

X(10), since d = 10 mm is approximately midway

through the size range. Each lithotype of lignite has significantly different physical properties. Fusain is the most friable, and although it constitutes only 5-10% of Beulah-Zap (North Dakota) bed lignites, the high friability of fusain makes it responsible for a disproportionate amount of fines. Fusain is typically rich in inertinite macerals. It is easily degraded during mining, crushing, and transportation. Attritus is the most resistant lithotype. It typically produces cubic shaped particles larger than average. Vitrain is intermediate in friability between fusain and attritus. It typically forms cubic and rod-shaped particles, but when it is found intimately associated with fusain the particles tend to be blade-shaped [9]. Fusain is often the origin of extremely fine particles. During handling and transportation fusain is easily removed from the large particles as fines or dust. Fusain dust readily adheres to any moist surface. Although Beulah-Zap lignite contains about 5-10% fusain, the fusain is not evenly distributed throughout the seam. The upper 60-90 cm of the seam contains up to 25% fusain [9]. Since inertinites are more abundant in the upper portion of the seam, Beulah-Zap lignite is more friable near the top. The thickness of the friable layer varies but is typically 60-90 cm, corresponding with the region of high fusain content. The middle and lower sections of the seam are composed of the more competent maceral groups, the huminites and liptinites. Size reduction of Freedom (North Dakota) lignite to 19x25 mm produces particles which can be classified by shape into four general categories: disk, cubic, rod, and blade [9]. The disk-shaped particles typically originate from the upper 60-90 cm; blades and rod-shaped particles are more common in the middle two-thirds; and large cubic particles generally originate from the lower half of the seam. The disk-shaped particles are rich in inertinites and are therefore the most friable of the various kinds of particles. The mechanical friability of low-rank coals increases sharply with the amount of drying prior to tumbling [4]. For example, removing approximately half the original moisture content of Indian Head lignite increased the mechanical friability from 20% to 70% [ 10]. Significant changes in slope in a plot of mechanical friability

vs.

dryness suggest that the removal of water bound in

different ways to the organic structure has significantly different effects on the structural integrity of the lignite particles. Because of the two-phase (i.e., an aqueous gel) structure of lignites, their mechanical properties depend very significantly on the relative amounts of solid and liquid phases

347 [11]. Mechanical friability also depends on the initial moisture content of the lignite. The effect of moisture was determined by testing mine-fresh samples after drying at 60~

for successive

increments of 30 minutes. If mechanical friability is plotted as a percentage of the initial moisture content still remaining in the lignite, rather than the actual moisture content, then all of the lignites tested tend to fall roughly along the same line, albeit with some scatter, as shown in Figure 7.3 [2,4,8]. The changes in slope suggest that the first and last portions of water removed from the lignite have less disruptive effect on the mechanical integrity of the particles than the water that is being removed in the range of about 80% of the initial moisture content [4].

0- ~

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Percent of original moisture remaining

Fig. 7.3. Mechanical friability as a function of the percentage of the original moisture content still remaining in coal after partial drying [2]. The data points shown are for Indian Head, Beulah, and Elgin-Butler lignites and Spring Creek subbituminous coal. The curve is a visual fit of the data for the four coals.

An alternative approach to measuring friability involves dropping a 4.5 kg sample from a height of 2.1 m onto a concrete floor; after five repetitions the amount of lignite still retained on a 38 mm screen is measured [ 12]. The ratio of weight of dry lignite retained after five repetitions to that of as-mined lignite retained under the same experimental conditions provides an index that defines the friability of dried lignite relative to the untreated material. For tests with 5, 7.5, and 10 cm lumps, the index ranged from 0.124 to 0.294, with no evident correlation with the size of the original particles [ 13]. These indices indicate that dried lignite is substantially more friable than

348 untreated material. For example, in tests with 5 cm lumps, 83.0% of untreated lignite was still retained on a 38 mm screen after being dropped to the floor five times [ 13]. In comparison, only 10.5% of the dried 5 cm lumps were retained on the screen after the same test. Unfortunately this work was not extended to explore the effects of drying to different extents on friability. Lignite which has been steam-dried, rather than dried in air, is also more friable than untreated lignite, but is not as friable as the air-dried material. For example, the friability index for 10 cm lumps, comparing air-dried and untreated samples, is 0.219, whereas the index for 10 cm lumps of steamdried vs. untreated lignite is 0.423 [ 13]. (ii) Thermal friability. Thermal friability is a measure of the thermal fracturing resulting from differential expansion of different portions or components of lignite particles. Expanding water vapor generated as moisture is vaporized may push particles apart along planes of weakness. Additional planes of weakness may be generated as a consequence of the removal of the so-called bound water and of volatile organic materials. Thermal friability is measured by a change in size distribution after heating at standardized test conditions. The thermal friability of Indian Head lignite heated to 540~ is shown in Figure 7.4 [2]. Comparison of Figures 7.2 and 7.4 shows a significantly greater thermal than mechanical friability for this lignite. When expressed as the percentage of standard 19x13 mm samples reduced t o - 1 3 mm, the thermal friability of Indian Head lignite is 86% [2]. Using the alternative definition of the shift in X(10), the thermal friability is 29% [8].

o INITIAL

100-

+ HEATINGTO 540~ ..~~

~N~

~~

X TUMBLING30 MIN.

10o

gh

1

t"

1

X

0

0.10

2

4 6 8 Particle size, mm

10

12

Fig. 7.4. Thermal friability of Indian Head lignite [2].

349 (iii) Combined mechanical and thermal friability. When Indian Head lignite is heated to 540~

more comminution occurs than is observed by mechanically tumbling the untreated lignite

[4]. When the devolatilized char is then tumbled, the disintegration of particles is far greater than achieved by either heating or mechanical action alone [4]. The effects of heating and tumbling are measured by the combined friability test, which determines the effect of tumbling after heating. The suitability of nonagglomerating coals such as lignites as feedstocks for fixed bed gasifiers will be determined in large part by their thermal and mechanical friability behavior [4]. As an example of relative values of the different types of friability discussed here, the mechanical, thermal, and combined friabilities of Indian Head lignite, are, respectively 38.3%, 86.0%, and 97.3% [4,14]. Combined friability results for Elgin-Butler (Texas) lignite are shown in Figure 7.5, along with the thermal and mechanical friabilities [2,4]. Comparison with Figure 7.2 shows a lower mechanical friability for Elgin-Butler lignite, but the change in the shape of the size distribution curve suggests preferential generation of fine particles rather than the equally probable fracturing predicted by brittle fracture theory. These results suggest that the Elgin-Butler lignite may be less isotropic or less homogeneous than Indian Head [2,4]. Unfortunately no indication was given of the petrographic analyses of the original lignite nor of the size fractions at the end of the test. A comparison of Figures 7.4 and 7.5 shows that Elgin-Butler lignite has a much greater thermal friability than Indian Head.

100

1 INITIAL 2 TUMBLING30 MIN

~

3 HEATING540~

N

.~

",,

1

4 HEATTHEN TUMBLE

0.1 0

2

4

6 8 10 12 Particle size, mm

14

16

Fig. 7.5. Mechanical and thermal friabilities of ElginButler lignite [2]. The curves show the effects of tumbling without heating, heating to 540~ without tumbling, and the combination of tumbling following heating to 540~

350 The friabilities of six lignites are summarized in Table 7.1 [15]. All of the samples are from North Dakota with the exception of the Phillips lignite, which is from Texas. TABLE 7.1 Mechanical, thermal, and combined friabilities of lignites [ 15].

Baukol-Noonan Beulah Gascoyne Glenharold Indian Head Phillips

Relative Friabilities, % Mechanical Thermal Combined 15.1 63.2 97.4 11.0 89.0 99.9 16.9 97.5 99.9 33.9 74.7 99.9 10.7 29.3 74.4 15.9 66.5 99.8

As with thermal and mechanical friabilities, the combined friability can be determined from the fraction reduced to -13mm, or from the shift in X(10). The friabilities of the lithotypes of Beulah lignite are summarized in Table 7.2 [8,15]. The difference between lithotypes is substantial, suggesting that unless mechanical property data is obtained from large, well-mixed samples of lignite, petrographic analysis of the sample is crucial for interpreting the results of the mechanical testing data. The data shown for the unseparated sample were obtained on a sample collected from a different position in the seam and crushed separately from the lithotype samples. From this data it does not seem possible to relate the properties of "whole" coal to some average or other statistical treatment of data for the individual lithotypes. TABLE 7.2 Friabilities of lithotypes of Beulah lignite [15]. Relative Friabilities, % Mechanical Thermal Combined Vitrain 18.4 65.2 100 Attritus 36.1 76.0 99.9 Fusain 32.3 49.0 96.6 Unseparated sample 11.0 89.0 99.9

Deviation from ideal brittle fracture behavior may reflect the different mechanical properties of the coal components. The friabilities of the lithotypes of Beulah lignite differ in ways consistent with concepts of their origin. For example, attritus, believed to be of detrital origin and to have passed through an unconsolidated phase in the early stages of its formation, has a higher mechanical and thermal friability than vitrain. Fusain, which may be the product of ancient bog fires, is less subject to thermal degradation than the other two lithotypes.

351 Freedom lignite becomes very friable as the moisture is decreased; furthermore, the rate of moisture loss affects the friability. Lignite dried quickly, by heating to 260~

for 30 minutes, is

much more friable than the same lignite dried by heating at 60~ for three hours [9]. Rapid drying increased the fines content by 26% [9]. A friability index (FI) was defined as the ratio of the amount of fines after tumbling to the amount before tumbling [9]. The friability index for the samples dried quickly was 3.79, while for samples dried slowly the index was 2.55 [9]. The range of thermal and combined friabilities of these lignites shows no overlap with the few sets of data obtained for higher rank coals. A potential source of error in any friability data is the extent of drying--either deliberate or inadvertent--experienced by the sample before the friability testing, since friability can be very sensitive to moisture content [2]. The lignite with the lowest friability (Indian Head) gave the best performance in a pilot scale fixed-bed slagging gasifier which is markedly sensitive to the physical and mechanical properties of the fuel (Chapter 12). 7.1.3 Grindability The Hardgrove test uses a 50 gram sample of 16x30 mesh coal which has been air dried until weight loss is not more than 0. l%/hr. The sample is ground by eight 25 mm steel balls which revolve in a stationary ring and are driven by an upper rotating ring carrying a weight of 29 kg for 60 revolutions. The grindability index is based on the amount o f - 2 0 0 mesh material formed and is calculated from the equation H = 13 +6.93W where H is the Hardgrove index and W is the amount of -200 mesh material, determined by subtracting the amount of material retained on the 200 mesh sieve from the original sample weight (50 grams). All weight loss is assumed to be dust loss, and is therefore considered to be part of the -200 mesh material. The Hardgrove grindability as a function of rank, in air-dried samples, shows a minimum at 0.4-0.6% maximum reflectance in oil, corresponding to about 75% carbon [ 16]. Grindability data for North Dakota lignites, determined by the Hardgrove method, have been published [ 17]. When coals having high moisture content, such as lignites, are tested in the Hardgrove apparatus, some moisture loss may occur during the course of the test. By the standard method of calculating the grindability index, the lost moisture would be assumed to be part of the -200 mesh coal, thus introducing an error into the calculation. Calculation of the grindability index by directly weighing t h e - 2 0 0 mesh material leads to a term reported as "Hardgrove index, corrected." For lignites the difference can be appreciable; for example, a value of 59.5 calculated by the standard method

vs.

52.4 for the Hardgrove index, corrected [17].

The rank dependence of Hardgrove grindability results in the size of pulverized coal tending to increase with decreasing rank [18]. Typical values of grindability are 55 for bituminous coals, 43 for subbituminous coals, 48 for Texas lignite, and 35 for northern Great Plains lignite

352 [19,20]. All other factors being equal, the increase in size could result in poorer combustion and reduced carbon burnout. However, the higher reactivities of lignites relative to bituminous coals negate the anticipated poorer combustion performance. Despite the rule-of-thumb grindability of Texas lignite being 48 and northern Great Plains lignite 35 [20], it must be understood that Hardgrove indices for lignites are difficult to interpret, and difficult to use as design data. The Hardgrove index varies, sometimes significantly, with moisture content, and it is difficult to maintain conditions of constant moisture during the test. A modification of the Hardgrove test uses several samples of the lignite, each dried to different moisture levels; the grindability value is then selected as that at the moisture level expected to occur when grinding takes place in the power plant. Tipple samples of North Dakota lignites showed a wide range of variation from one lignite to another when compared at the same moisture content. At the as-mined moisture content, the standard grindability index varied from 54.7 to 87.8; the same samples when air-dried had indices ranging from 37.0 to 81.0. For a given lignite, grindability varied with moisture content. The extent of moisture dependence was itself characteristic of each lignite. The extreme variation of grindability with moisture was shown by Peerless (North Dakota) lignite, changing from 79 at the natural moisture content of 44.8% to 51 when dried to 12.0% moisture [17]. Although individual lignites show characteristic changes in grindability with moisture, the general trend is the same for most lignites. Initially grindability decreases as the moisture content decreases. A minimum in grindability occurs in the so-called mid-moisture range, followed by an increase in grindability as further drying occurs. Most lignites will show a second decrease in grindability with drying to very lowmoisture levels. This behavior is illustrated in Figure 7.6, which is a plot of the Hardgrove index as a function of moisture content for Peerless lignite [17]. Grindability is also dependent on ash content. For some lignites the grindability of the carbonaceous part of the lignite is much lower than that of the inorganic impurities. The dependence of grindability on ash is particularly important for high ash lignites, which would have a low heating value but higher grindability than comparable lignites of lower ash. An example of how the higher grindability compensates for lower heating value is shown in Table 7.3 [21 ]. The petrographic components of lignite also affect the grindability. For three lithotypes of Savage (Montana) lignite, the grindability varies from 87.2 to 112.1 [22]. (Relevant composition data on these lithotypes has been given in Table 3.18). The Hardgrove indices of these lithotypes showed a definite trend with the volume percent of fusinoids, rising from 87.2 for a fusinoid content of 9.6% to 107.9 for 11.4% fusinoids, and 112.1 Hardgrove for 21.9% fusinoids [21]. Petrographic analysis of screened fractions of Savage lignite showed a increase in proportion of the fusinoids in the finer sizes [22]. In this respect the behavior of lignite parallels that of bituminous coals. However, in bituminous coals the vitrinoids tend to concentrate in the softer (and hence finer) fractions, whereas the experience with the Savage lignite was the opposite. The vitrinoids in lignite are less brittle and tend to behave more as a hard material than do the vitrinoids in bituminous coals.

353

X

80

"~ 70

~

60

~

5o. 40

m

1~ 30

~0 0

2o 10" 0

bO

0

LO

0

LO

0

~

0

Moisture content, percent Figure 7.6. Variation of corrected Hardgrove grindability index of Peerless lignite with moisture content [ 17].

TABLE 7.3 Compensation of high grindability for low heating value in mill design [21]. System Ash content (dry basis), % Hardgrove Grindability Index Moisture (as fired), % Heating value (as fired), MJ/kg Mill capacity, t/hr Coal input, TJ/hr Coal input per mill, t/hr Mill capacity, %

A 14.0 40 15.0 22.9 72.5 7.4 64.4 88.7

B 30.0 48 13.0 19.1 87.1 7.4 77.2 88.7

The high grindability of as-mined lignite is attributed to the ability of fine particles of lignite at their natural moisture content to cling together. The association of particles in this manner may prevent their easy slipping out of the path of the balls, with consequent increased comminution. A reduction in moisture content would also reduce the tendency of the particles to associate, thereby allowing the particles move about more easily and avoid the action of the grinding race. The reduction of moisture below about 25% introduces a second mechanism. The loss of moisture at this stage weakens the physical structure of the lignite, by one or both of two processes: the introduction of stresses in particles by shrinkage during drying, or weakening the macromolecular

354 coal matrix by removal of water molecules which may have served as a contributor to the strength of the physical structure. Consequently grindability increases in this moisture range. No hypothesis has been developed to explain the final decrease in grindability that occurs below about 10% moisture. It has been argued that the high Hardgrove grindability index at high moisture content is "a fictitious number" [23], and that in pilot plant or field tests lignite becomes much easier to grind as the moisture is removed. In a pilot plant pulverizer a 75% reduction in power required was observed on drying lignite from 35% to 15% moisture [23]. The grindability of dried lignite seems only slightly dependent upon the method of drying. Preliminary results show that for an unidentified (but probably North Dakota) lignite Roto-Louvre drying and air drying to 19.0% moisture gave products having Hardgrove grindabilities of 43.6 and 39.3, respectively; lignite steam-dried to 15.2% moisture had a grindability of 43.9 compared to an air-dried sample of 15.8% moisture with a grindability of 41.4 [24]. High grindability at high moisture content may not necessarily be the most desirable for grinding in practice, as, for example, in preparation of lignite for pulverized combustion. The tendency of particles to cling together at their natural moisture content, discussed above, may cause smearing or caking in the pulverizers, which in turn causes operating problems both for the pulverizers and for the burners [25]. 7.1.4 Hardness Hardness can be defined as a measure of the damage that one material can inflict on another as a result of sliding contact or impact [8,26]. No universal standard of hardness is available that satisfies the criteria of being applicable to brittle and ductile materials and that does not in principle differentiate between the material acting as the abrasive and the material being abraded. The magnitude and units of hardness should be independent of geometry and should be measured in the same way for both the abrasive and the material being abraded. To use hardness as a property for equipment design (to measure the abrasive behavior of coal-water slurries, for example), an empirical function could be established relating the rate of surface damage to the hardness of the two materials (coal and slurry handling equipment, to continue the previous example) such as S = f(Ha/Hb) where a and b represent the two materials and H the hardness [8,26]. The applicability of such an equation requires a reciprocal relationship between the hardness measurements of the materials; that is, it should be equally valid regardless of which of the two is inflicting damage on the other. The only hardness definition which meets the requirement of reciprocity is the Mohs scale. The Mohs scale is commonly used to determine the hardness of mineral specimens; its development and use are discussed in textbooks of geology or mineralogy (e.g., [27]). The Mohs scale has been expanded to accommodate fractional hardness values [8,26].

355 North Dakota lignites, tested in conjunction with supplemental standards to obtain fractional values, gave Mohs hardness values of 2.2-2.6 for Indian Head, 1.3-1.8 for Baukol-Noonan, and 2.3-2.6 for Freedom [8,26]. The apparent hardness of lignite varies depending on whether the scratch is made along a cleavage plane or crosses the cut ends of laminae. At low hardness values, lignites tend to crumble rather than scratch. Handling of small particle sizes of coal provides a need to extend the macroscopic Mohs hardness measurement to the characterization of coal powders. Two devices have been developed to measure the Mohs hardness of granular materials [8,26]. Tests with -20 mesh Indian Head lignite gave results which were device-specific, 2.6-3.1 in one tester and 1.7-2.6 in the other [8,26]. There is also an apparent effect of particle size. Using the second device, -100 mesh Indian Head lignite gave values of 3.1-3.4 [26]. Comparison o f - 2 0 mesh granular Indian Head, Baukol-Noonan, and Freedom lignites gave Mohs hardness ranges of, respectively, 2.2-2.6, 1.2-1.8, and 2.3-2.6 [28]. There was no petrographic data for these samples. In the application of hardness data to equipment design, it is important to recognize an ambiguity in the results. Lignites contain about 10% mineral matter. Some of the minerals are much harder than the lignite itself, having Mobs hardness values as high as 7.5 [28]. Since the random motion of lignite particles against a surface will from time to time actually expose the surface to pieces of the much harder minerals, the effective hardness of lignite in long-term erosion or abrasion may be somewhat larger than the value determined by a short duration laboratory test. This concern should be incorporated into any predictive equations developed to relate erosion or abrasion of equipment in service to the hardness of the lignite determined in a laboratory test. Reichert microsclerometer for European lignites indicates hardnesses of 30-45 kg/mm2 [29]. The Vickers microhardness of Palana lignite (Rajasthan, India) is 22.6 kg/mm2 [30]. This sample had an average reflectance (Ro, in oil) of 0.38%. 7.1.5 Free sliding shear strength Shear failure cannot occur unless associated normal stresses can be relieved by displacement perpendicular to the shear plane. With a completely constrained shear test cell (with no vertical stress relief possible), increasing applied shear stress produces an increasing, induced compressive stress without ever achieving shear failure [4]. For materials smaller than about 20 mesh (0.79 mm) a standard free sliding shear strength procedure using a 75 mm diameter test cell has been developed [31]. However, the size specifications for the process equipment are often much larger than this. For example, in studies of free sliding shear strength done in support of a fixed-bed gasification pilot plant, the feed size specifications were 19x13 mm; thus a special test cell of 203 mm diameter was constructed [2,4,8]. Vertical deformation normal to the shear plane is observed when the shear strength is measured in an unconstrained device [4]. When the vertical motion was constrained, higher shear forces were required, at any compressive load, to achieve shear failure [4]. An applied shear force

356 will at first cause some reorientation of particles in the direction of shear. As the shear force is increased, failure along the shear plane occurs by particles "climbing over" adjacent particles, thus producing an apparent vertical expansion. Complete shear failure will occur when the applied shear force is sufficient either to rotate those particles resisting shear failure, or to break them. Data for two particle size ranges of Indian Head lignite are shown in Figure 7.7 [2,4,8]. The shear strength for the smaller size range is lower than that of the larger range. Data obtained for a 50% mixture of the two particle size ranges (also shown in Fig. 7.7) follows a line similar to that of the smaller size range alone, in keeping with the findings of fine particle flow theory that shear behavior of mixtures is dominated by the behavior of the smaller components.

9 6 x 12mm

8

7

§

12 x 19 mm

6

r

5 4 O0

9

2 r

1

0

"'

o

I'"'

I'"'

t.o...,.-.,

I'"'

~

I'"'

~

I'"'

t.o..

I'"'

~

I'"'

tO.

I .... I ....

',~"

t.o..

t.o

Compressive stress normal to shear stress, kPa

Fig. 7.7. Shear strength v s . normal compressive stress for crushed Indian Head lignite in unconstrained shear test cell [2].

Deformation measurements are shown in Figure 7.8 [4]; shear failure is indicated by the slope of the curves approaching infinity. Before shear failure, the curves are not smooth, but proceed through several discontinuous steps as the individual particles rearrange along the shear plane. The necessary normal stress relief should be at least equal to the minimum dimension of the largest particles. The relationship between free sliding shear strength and normal compressive stresses for coarsely crushed coal is similar to the established theory for finer particles [4]. The shear strength of a mixture of particles is dominated by the finer components in the mixture.

357

30 ~' 25-

l

20-

x

0.47 kPa

A

1.71 kPa

9 2.41 kPa o

2.94 kPa

o

3.56 kPa

,~

'~ 15 iv

,~ 10 ~' 5 o .1~t

~.x_ 0

' ' ' '

0

I ' ' ' ' 1 ' ' ' '

1

I ' ' ' ' 1 ' ' ' '

2 3 4 Applied shear stress, kPa

I ' ' ' '

5

Figure 7.8. Plastic deformation preceding shear failure of crushed Indian Head lignite, 6.3 x 3.3 mm, for various values of compressive stress normal to shear plane (indicated in legend) [4].

7.2 D E N S I T Y

7.2.1 Density measurements by immersion in fluids The specific gravity of lignite varies between 1.2 and 1.8, depending on the kinds and amounts of impurities in the sample [32]. The density of a North Dakota lignite measured in a pycnometer in various fluids was 1.546 g/cm3 in water, 1.556 in benzene, and 1.517 in xylene [32]. In each case the sample was boiled in the test fluid for 15 minutes before measuring the density. 7.2.2 Bulk density The bulk density of lignite varies according to the moisture content and size of lumps. An "average bulk density" of lignite has been estimated to be about 1250 kg/m3 [32]. The bulk density of Baukol-Noonan lignite, dried to 22% moisture, was 809 kg/m3; vibrating the container increased the bulk density to 852 kg/m3 [33]. 7.2.3 Mercury densities The densities of eight northern Great Plains lignites measured in mercury were very uniform, with a mean value of 1.310 g/cm3 and standard deviation of 0.046 [34]. The mercury density of a hand-picked sample of telocollinite from a lignite in the Fort Williams Formation (Wyoming) was 1.315 g/cm3 [35].

358 7.2.4 Helium densities The densities of eight northern Great Plains lignites measured in helium had a mean value of 1.538 g/cm3 and standard deviation of 0.054 [34]. The helium density of Fort Williams Formation telocollinite is 1.465 g/cm3 [35]. For the coals in the Exxon sample library, which included lignites, the helium density can be related to the elemental composition by the equation DHe = 0.023 (C) + 0.0292 (O) -0.0261 (H) + 0.0225

(Sorg) -0.765

[36] where all of the parameters are expressed on a dmmf basis. The coefficient of determination, r2, for this equation is 0.943 [36]. A somewhat different equation was developed on the basis of measurements of 39 vitrinite concentrates, ranging in rank from low volatile bituminous to lignite [351: PIle = 0.033 (C) + 0.036 (O) - 0.031 (H) § 0.009 (Sorg) - 1.577 where again the composition parameters are expressed on a weight percent dmmf basis. This equation accounts for 83.3% of the variance in densities among the 39 samples tested [35]. Mercury and helium densities for three lignites are compared in Table 7.4 [37]. Darco is a Texas lignite; the others are North Dakota lignites. TABLE 7.4 Helium and mercury densities (mineral-matter-free basis) for three lignites [37]. Lignite Helium density, g/cm3 Mercury density, g/cm3 Total open pore volume, cm3/g Open porosity, %

Darco 1.35 1.17 0.114 13.3

Zap 1.40 1.22 0.105 12.8

Harmon 1.45 1.31 0.073 9.6

7.3 T H E R M A L P R O P E R T I E S

7.3.1 Specific heat The specific heat of North Dakota lignite varies between 1.0 and 2.5 J/g, depending on the moisture content [38]. A commonly used value is 1.26 J/g [32]. The exact value of the specific heat depends on the petrographic composition and the moisture content of the lignite, and the temperature at which the measurement is made. The specific heat dependence on the moisture content [38] is expressed by the relationship

359 Cp = 0.251 + 0.893 M

at 25~

where M is the moisture content in percent and Cp is expressed in units of cal/g [32]. The

temperature dependence of the specific heat lies in the range +0.003 to +0.006~ -1 above 25~ [39,40]. The variation of specific heat with moisture content appears to be linear, for any given temperature of measurement, above about 10% moisture. Below 8% moisture, the linear relationship no longer applies. This observation indicates a marked change in the physical structure of the lignite when it is dried below 8-10% moisture [32]. 7.3.2 Thermal conductivity The thermal conductivity of solid lignite is shown as a function of moisture in Figure. 7.9 [38]. 0.3 o 0.25-"

~ o.2-:. ~ IN o.15-.

N0

J

f

J

J

o.1

''~'1'

0

~''1'''

5

~l''W'l

''''1

w~ ' ' 1

~'~

l0 15 20 25 30 Moisture content, percent

35

Figure 7.9. Variation in thermal conductivity of lignite with moisture content [38].

Thermal conductivity was determined in two ways, estimation from data on porous beds, which then allows calculation of the thermal conductivity of the solid via Russell's equation [41]:

kbed/ksolid - [1 - 130.667 (1 - ct)] / [1 - (130.667 _ E)(1 - ct)]

and calculation from published equations for determining the thermal conductivity of wood [41]. In this equation e is the porosity of a bed and (x is kgas/ksolid. The effect of moisture is expressed by

360 the equation k = 0.111 - 6.36xlO-4M + 9.92xlO-SM2

[38]. The value of 4x10-3 J/s.cm.*C [42] has been taken to be a reasonable value for lignite at room temperature [43].

7.4 S U R F A C E A R E A AND P O R O S I T Y

7.4.1 Measurements of surface area (i) Gas adsorption. Pore structure data obtained for Peerless lignite are shown in Table 7.5, summarized from [44]. These data represent the most complete characterization of a lignite ever reported. The data were corrected to a mineral-matter-free basis (mmf) by assuming a value of 2.7 g/cm3 as the density of the mineral matter [45]. The mineral matter content was established by lowtemperature ashing. The nitrogen surface area at 77 K was calculated from the Brunauer-EmmettTeller (BET) equation [46] using a cross-sectional area of 0.162 nm 2 for the nitrogen molecule. The carbon dioxide surface area at 298 K was calculated from the Dubinin-Polanyi equation [47] using a cross-sectional area of 0.253 nm 2 for carbon dioxide. The pore volumes were determined by combining density data, mercury porosimetry, and nitrogen adsorption measurements. The total open pore volume, VT is a measure of pores accessible to helium at 32.5~

V 1 is the volume in

pores greater than 30 nm, and is determined from porosimeter data. The volume of pores in the range 1.2-30 nm, V2, is obtained from the nitrogen adsorption isotherm at a relative pressure of 0.93. Pores smaller than 1.2 nm have a volume V3 determined by difference [VT - (V 1 + V2)].

TABLE 7.5 Surface area and porosity characteristics of Peerless lignite [44].

Mineral matter, % Mercury density, g/cm3 Helium density, g/cm3 Total open pore volume, VT, cm3/g Nitrogen surface area, mZ/g Carbon dioxide surface area, m2/g Vb cm3/g V2, cm3/g V3, cm3/g Vl/V T, % V2/VT, % V3/VT, %

As determined 13.72 1.41 1.55 <1 238

mmf 1.31 1.45 0.073 <1 276 0.063 0.001 0.008 87.00 1.30 11.70

361 A remarkable difference exists between surface areas determined by nitrogen and carbon dioxide adsorption. That the two methods give different results is certainly not unique to lignite, but in an extensive survey of American coals, none of the others [44] showed such a large difference. The internal structure of the lignite is almost totally inaccessible to nitrogen molecules at 77 K, possibly due to physical shrinkage of the pores. For coals of other ranks, low nitrogen surface areas were attributed to penetration being an activated diffusion process, which would be slow at 77 K. Gascoyne (North Dakota) lignite in nitrogen has a BET surface area of 8.5 m2/g; using carbon dioxide with the Dubinin, Polanyi, and Astakhov equation, the calculated surface area was 165 m2/g [48]. Gascoyne lignite outgassed under vacuum at 100~ C for 12 h showed a BET surface area in carbon dioxide at 20~

of 187 m2/g [49]. Another sample of the same lignite

outgassed at 80~ overnight showed a CO2 surface area (at -780C) of 177 m2/g by the PolanyiDubinin equation [50]. Comparison of results for CO2 and N2 measurements of Beulah-Zap lignite shows differences of two orders of magnitude, with a possible slight dependence on particle size [51]. Thus a 200x230 mesh sample showed CO2 surface area of 229+_50 m2/g, with N2 surface area of 0.5+_0.2 m2/g [51]. The corresponding values for a 325x400 mesh sample of the same lignite were 255_+62 and 1.0+-0.6 ma/g, respectively [51]. Low values for surface areas by nitrogen adsorption may be a result of irreversible alteration of the lignite gel structure during drying. Interaction of polar oxygen groups in the lignite with carbon dioxide may account for the values obtained with that gas [ 13]; in relation to this, the micropore walls in low-rank coals may be lined with carboxyl groups [52]. However, when samples are degassed after the initial carbon dioxide adsorption and then treated again with carbon dioxide, the calculated surface areas agree quite well with the initial values (e.g., 230 and 238 ma/g). This suggests that interactions of carbon dioxide with oxygen functional groups are not important in these lignites [37]. Nitrogen adsorption on an unidentified lignite showed a small increase in BET surface area with decreasing particle size [51,53]. Thus the surface area of 35 mesh lignite was 17.8 m2/g; of 200 mesh, 19.9 m2/g; and of 400 mesh, 21.1 m2/g [53]. These values [53] show the same trend, but are an order of magnitude larger than, measurements for Beulah-Zap lignite of about the same particle sizes [51]. This small increase is attributed to the increased external surface area as particle size is reduced, the internal surface area remaining constant. Gascoyne lignite outgassed at 100~ and 0.133 Pa for 12 h had a surface area of 142 m2/g (calculated by Dubinin-Polanyi method) for CO2 adsorption at -78~

[54]. Similar experiments

with propane and butane as the adsorbing gases gave values of 120 m2/g [54]. BET surface areas for a Texas lignite were 1.12 m2/g for nitrogen, 192.2 ma/g for carbon dioxide, and 355.4 m2/g for water [55]. The heat of immersion (discussed below) was 108 J/g [55]. The surface areas of eight northern Great Plains lignites, measured in carbon dioxide, had a mean value of 231 m2/g with a standard deviation of 58 [36]. Independent measurements made in other laboratories show remarkable agreement with this observation. Thus, for example, some

362 CO2 surface areas are reported as follows: Hagel (North Dakota), 238 [35] and 225 m2/g [56]; Beulah-Zap, 229+50 m2/g [51]; and Zap, 274 m2/g [57]. The CO2 surface area of hand-picked telocollinite from the Fort Williams Formation was 303 m2/g, calculated by the Polanyi-Dubinin equation [35]. Surface areas determined in nitrogen and carbon dioxide for five lignites are shown in Table 7.6 [37]. TABLE 7.6 Comparative nitrogen and carbon dioxide surface areas (m2/g) for five lignites [37].

Sample Darco Darco Lower Tongue River Zap Harmon

Nitrogen 2.2 2.3 2.0 <1.0 <1.0

Carbon Dioxide 225 250 264 268 238

The Lower Tongue River sample is a Montana lignite. Carbon dioxide also swells coals. The expansion of Hagel lignite in 0.1 MPa CO2, as measured by dilatometry, was 0.099% [56,58]. At higher pressures the swelling becomes larger, ranging from 1.03% expansion in 0.68 MPa CO2, to 3.79% in 4.8 MPa CO2 [58]. The value of a surface area measurement will be affected by several effects, including imbibition of the sorbate into the coal structure, penetration to the closed pores via swelling, and activated diffusion into small micropores [58]. Adsorption of eight species, ranging from water to toluene in molecular size, on Newvale (New Zealand) lignite showed adsorption rates decreasing as the size and minimum cross-sectional area of the molecules increased [59]. No molecular sieving effect (as would be found, for example, with zeolite crystals) was found. With methanol and ethanol adsorption, a time dependence of results is caused by swelling of the lignite. (Methanol can be a good swelling solvent for lignites; an expansion of 12.9% occurs for a lignite from Richland County, Montana [58].) The rate and extent of adsorption correlated with the solubility parameters of the sorbed species. The surface areas of Zap lignite measured by adsorption of ethane, cyclopropane, cyclobutane, cyclopentane, and cyclohexane were all of essentially the same value, 8+_3 m2/g [57]. (The value measured using c 0 2 for this sample was 274 m2/g [57].) These results suggest that this lignite does not have a rigid, interconnected pore network, but rather has isolated pores that could be reached only by diffusion through the solid lignite [57]. Carbon dioxide is soluble in coals, and hence will rapidly diffuse and quickly reach all of the available pores. Thus CO2 will report greater surface areas than would non-polar species (such as ethane or cyclopropane) that do not interact with the coal substance and hence do not rapidly diffuse. However, a counter-argument suggests that the reporting of different surface areas by molecules of similar molecular dimensions is not

363 necessarily an indication that access to the microporosity is only a result of diffusion through the solid coal [60]. That is, the relative contributions of diffusion through the solid coal and diffusion through an interconnected pore network depend on the molecule doing the diffusing. (ii) Heat of immersion calorimetry. The surface area of Gascoyne lignite was determined as 166 m2/g using methanol as the wetting agent [61]. The conversion factor used for obtaining surface area from heat of immersion was 2.56 m2/J (10.7 m2/cal), taken from the Bond and Spencer calculation for carbons [62]. The heat output (65.74 J/g) was nearly instantaneous [61]. No variation of heat output with particle size was observed for mesh sizes of 16, 30, 60, and -60, indicating that the internal pore size is dominant compared to the external surface area. This same lignite in tetralin gave an indicated surface ares of 5.2 m2/g, presumably showing that the tetralin wets only the external surface and possibly the very largest of pores. Heats of immersion in methanol gave surface areas, for North Dakota lignites, of Falkirk, 154 m2/g; Velva, 169 m2/g [63]; Center, 181 m2/g [64]; Beulah lignite with high sodium content, 165 m2/g [65]. For Martin Lake (Texas) lignite, the surface area measured by this method was 160 m2/g [66]. Three Indian lignites immersed in methanol showed the highest heat of wetting from the sample having the highest hydroxyl and carboxyl content [13]. The experimental results are summarized in Table 7.7 [ 13]. TABLE 7.7 Heats of wetting of Indian lignites in methanola [13]. Sample Kashmir Palana South Arcot

Hydroxylb 12.14 9.99 10.11

Carboxylb_ Heat of wettingc_ 2.33 162 2.05 120 2.20 113

aAll data is on a dry ash free basis, bParts per 100 grams, cj/g.

Since the lignite having the lowest amounts of these polar groups had the intermediate value of heat of wetting, it is not clear whether the high result obtained for the Kashmir lignite reflects a hydrogen bonding interaction of the methanol and the polar groups in the lignite. For methanol adsorption on a suite of coals of ranks lignite through bituminous, the moles of sorption sites per gram of coal, A, is given by A =0.6670, in which O represents the moles of oxygen per gram of coal [67]. Sorption was presumed to occur on --OH groups, which might be phenolic or the --OH of a carboxylic group.

364 Liquids having molecular volumes larger than methanol should not be able to penetrate the pore structure as well as methanol. In particular, large molecules should not be able to penetrate the micropore region, which can account for a large proportion of the total surface area. The much smaller apparent surface area determined from heat of wetting of Gascoyne lignite in tetralin, 5.2 m2/g, compared to 166 m 2/g for the same lignite in methanol [68], was mentioned above. Heats of immersion for Gascoyne and Indian Head lignite in benzene were found to be, respectively, 51.0 and 56.9 J/g [48]. Comparable data for heat of immersion in methanol were 64.4 and 74.9 J/g, respectively. Air-dried Indian Head lignite showed a heat of immersion of 74.5 J/g in methanol, 46.9 J/g in chloroform, and 15.6 J/g in cyclohexane [69]. Assuming that the conversion 2.56 m2/J is valid for all these liquids (probably a rather questionable assumption), the corresponding surface areas accessible to the liquids are 190 m2/g in methanol, 120 m2/g in chloroform, and 40 m 2/g in cyclohexane. The heats of immersion of Beulah-Zap lignite in twelve bases, measured at 75~

are

shown in Table 7.8 [70]. TABLE 7.8 Heat of immersion at 75~ for Beulah-Zap lignite in liquid bases [62]. Liquid acetonitrile n-butylamine cyclohexanone 2,6-dimethylpyridine dimethyl sulfoxide ethylenediamine n-hexylamine 4-methylpyridine propylene carbonate pyridine toluene 2,4,6-trimethylpyridine

AH, J/~ - 62.15 -394.8 + 2.64 - 5.44 - 187.4 -433.5 - 67.53 - 22.93 - 4.10 -104.6 - 8.58 + 1.72

A rough relationship exists between heat of immersion in pyridine, dimethyl sulfoxide, and ethylenediamine for five coals and the oxygen content of the coals. For Beulah-Zap lignite, the relative importance of interactions to the observed heat of immersion are 50.3% from hydrogen bonding, 31.6% from dispersion forces, and 18.1% from interactions with Bronsted acids. The heat of immersion can be described by a regression equation indicating the relative importance of the pK of the conjugate acid form of the base, pKBH+, the hydrogen bonding solvatochromic parameter 13and the polarizability parameter a~: All = 154.244 + 0.024

pKBH + -

229.218 a~ + 39.080 [3

with a regression coefficient of 0.976 [70].

365 (iii) Small angle X-ray scattering. The surface areas of Czechoslovakian brown coals and lignites determined by small angle X-ray scattering were in good agreement with results of methanol adsorption at 20~ [71]. Studies of the specific surface of lignites by small angle X-ray scattering are summarized in Table 7.9 [72]. (Useful reviews of the theory and concepts of this technique have been published,

e.g., [4].) Here S is the specific surface in ma/g, 1 the characteristic pore length in A, and V the specific pore volumes in 10-3 cm3/g; the subscripts T, ma, t, and mi represent, respectively, total, macropore, transition pore, and micropore. TABLE 7.9 Small angle X-ray scattering results for North Dakota lignites [72].

Lignite

Radiation

Beulah Beulah Gascoyne Indian Head*

Cu Mo Cu Mo

~ 3.94 8.92 3.57 38.2

S..Sma_ 0.556 0.605 3.55

...SSt_ .._SSmi It 0.834 2.54 2.90 6.02 0.640 2.32 6.00 28.66

149 169 146 93.5

l..[mi." 12.2 8.6 12.0 10.2

Wt

__VVmi_

3.10 12.3 2.33 14.02

0.78 1.3 0.70 7.27

*This sample was dried at 110~ These results show a dependence of the calculated results on the type of X-radiation used for the experiment. Furthermore, the results for Beulah lignite with copper radiation differ from earlier values obtained by the same technique using this lignite [73]. The differences are outside the range of even a liberal estimate of experimental error; the reason for these differences is not known. Small angle X-ray scattering results for Gascoyne lignite showed a surface area of 3.57 m2/g, with 0.605 m2/g in macropore surface, 0.64 m2/g in transition pore, and 2.32 ma/g in micropore surfaces [54,74]. 7.4.2 Porosity and pore volume The porosity of lignite is important in determining its behavior in heterogeneous reactions. The size of the pores and extent of internal void spaces may determine the penetration of solvents or reagents, the removal of products, and the internal surface area available for reaction. The porosity of low-rank coals is generally in the range of 0.1 cm3/g. Values of 0.073-0.114 cm3/g for three lignite samples [37] and 0.123 cm3/g for a fourth lignite have been reported [75]. Such values, however, depend on the substance used for the measurement. Thus for a hand-picked telocollinite sample from the Fort Williams Formation, pore volumes were 0.0780 measured in helium, 0.1351 in methanol, 0.0761 in water, and 0.0774 in carbon dioxide, all values having units of cm3/g on a dmmf basis [35]. The micropore volume of Hagel lignite, measured with carbon dioxide, agrees well, being 0.077 cm3/g [76]. (An independent measurement for Hagel lignite, though not the same actual sample, indicates a micropore volume of 0.063 cm3/g in CO2

366 [56].) Although the porosity of low-rank coals is not significantly different from data for some bituminous coals, in low-rank coals macropores account for about 60-90% of the porosity, whereas in many bituminous coals only 20-30% of the porosity was accounted for by these large diameter pores [37]. The microporosity of Peerless lignite, 11.70%, was the lowest of the ten samples (ranging in rank from lignite to anthracite) examined, and the macroporosity of 87% was the highest [44]. In general, for coals with carbon contents below 75%, porosity is due mainly to macropores [77]. Eight northern Great Plains lignites had a mean value of the porosity of 14.6%, with a standard deviation of 2.4 [34]. The mean total pore volume--the porosity expressed as cm3/g--of these samples was 0.112 cm3/g with a standard deviation of 0.020 [34]. About half of the total porosity of these samples was contained in the micropores. When coal is saturated at a relative pressure of 0.97, water will condense only in pores with radii less than 30 nm, but when the relative pressure is 1.0, water will condense in most submicron pores, as well as in surface roughnesses and interparticle contacts [78]. For Zap lignite, the upper size limit for the pore size distribution calculated for 0.97 relative pressure is much less than that for relative pressure of 1.00. Both pore size distributions have a maximum in the range 5-10 nm, but a second maximum around 200 nm is observed when the sample is saturated at the higher relative pressure. This lignite has a wide distribution of pore sizes, with a maximum of about 500 nm. The fraction of total filled pore volume in pores less than 0.5 nm is about 0.39 for P/Po of 0.97; this number drops to 0.05 for P/Po of 1, because of the addition of fluid to larger pores. Pore v o l u m e distributions for three lignites, calculated from adsorption isotherm and mercury porosimetry data, are shown in Table 7.10 [37]. TABLE 7.10 Pore volume distributions in lignites [37]. Lignites Pore volumes, cm3/g Total Macropores >30 nm 1.2 n m < Transition pores <30 nm Micropores <1.2 nm Contribution to total, % Macropores Transition pores Micropores

Darco

Zap

Harmon

0.114 0.088 0.004 0.022

0.105 0.062 0.000 0.043

0.073 0.064 0.000 0.009

19.3 3.5 77.2

40.9 -59.1

12.3 -87.7

Information on the pore radii can be obtained from heat of immersion measurements using liquids of various molecular sizes. Studies on Saran chars have shown that the minimum

367 dimension of the adsorbate molecule, rather than the average dimension as inferred from molecular volume, determines the amount of surface area contacted by the liquid [79]. Heats of immersion for Gascoyne and Indian Head lignites in seven liquids are shown in Table 7.11 [80]. The samples were prepared by outgassing at 100~ for 1000 minutes. TABLE 7.11 Heats of immersion of Gascoyne and Indian Head lignites in liquids of various sizes [80].

Benzene Chloroform Cyclopentanol Methanol Pentane Tetralin Water

Heat of Immersion,J/g Gascoyne Indian Head 116 126 81.6 54.8 32.2 34.3 155 159 97.9 87.4 14.6 13.4 153 160

For these liquids the heat of immersion is not invariably higher in one or the other of the two lignites; for example, Gascoyne has the higher heat of immersion in chloroform but has the lower value in benzene. At the same time, though, the values of the heat of immersion are quite similar for both lignites in all the liquids except chloroform. These observations suggest that while the total surface available to the various liquids is of about the same order of magnitude in the two lignites, each lignite has subtle differences in the distribution of pore radii. Comparative heat of wetting data obtained with cyclopentane and cyclohexane provides some insights into the shapes of pore openings. For an unidentified air-dried lignite, the heat of wetting in cyclopentane was 85.8 J/g, while with cyclohexane the value was 0.96 J/g [81]. Although the cyclopentane ring is non-planar, it is not as greatly non-planar as is the chair form of cyclohexane [82]. The significant difference in heat of wetting results suggests that cyclohexane molecules are unable to penetrate the pore structure, while the slightly smaller cyclopentane molecules do so much more readily. The shapes of pores in lignite is also a complicated issue. Gas release and changes in porosity and surface area during ball milling of Savage lignite suggest that the closed pores are relatively short but of large diameter [83]. The diameter of particles below which all closed pores have been opened by the grinding process is less than 38 ~tm [83]. The number of cylindrical pores, measured in a suite of five coals of different ranks, increases with decreasing rank, and with increasing oxygen content (which is also an indicator of decreasing rank) [84]. Cylindrical pores were also observed by impregnating Darco lignite with a low-melting point alloy [85], as discussed further in the subsection on fractal surfaces. Spherically shaped pores increase with decreasing rank [86]. Swelling Beulah-Zap lignite in toluene decreases the number of such pores, and

368 swelling in pyridine at 333 K essentially removes all of the spherical pores [87]. Mild swelling with nitrobenzene appears to increase the number of spherical pores; such pores become elongated as swelling severity increases [86]. 7.4.3 Hydraulic conductivity The hydraulic conductivities of some North Dakota lignites were determined from piezometers in single well response tests; values have been tabulated as a function of depth [88]. In the Falkirk area the average hydraulic conductivity is 2.5 x 10-4 cm/s, ranging from 4.6 x 10-7 to 9.7 x 10-3 cm/s [88]. In contrast, lignites in the Dunn Center area have a somewhat comparable mean value, 6 x 10-4 cm/s, but a much different range, 1 x 10-6 to 6 x 10-1 cm/s [88,89]. Two measurements of the hydraulic conductivity of the Beulah-Zap bed yielded results of 1 x 10-3 and 2 x 10-4 cm/s [88]. A relationship between hydraulic conductivity and depth below ground surface has been determined to be log K = - 4 . 4 7 5 9 - 0.0636 d where K is the hydraulic conductivity in cm/s and d is the depth in meters [88]. The correlation coefficient is 0.487. If it is assumed that the water flows through the lignite in horizontal fractures, the size of the fracture openings, a factor which determines the horizontal conductivity, should be a function of the pressure arising from the weight of the sediments above the lignite. Lignite aquifers in the Underwood area have an average hydraulic conductivity of 2.5 x 10-7 m/s [90]. 7.4.4 Effects of heating on surface area and porosity Lignite samples vacuum dried overnight at 100~ showed the results given in Table 7.12 [54,91]. The apparent surface areas were calculated assuming a conversion factor of 2.56 m 2/j for all the liquids; the validity of this assumption has not been checked. TABLE 7.12 Heat of immersion data for vacuum-dried lignites [54,91].

Liquid Benzene Methanol Water

Gascoyne Heat of Apparent Immersion Surface Area (J/g) (m2/g)._ 116 297 155 396 153 390

Indian Head Heat of Apparent Immersion Surface Area (J/g) (m2/_g)_ 126 322 159 406 160 408

Comparative data for samples which had been air dried for 48 hours are shown in Table 7.13 [54,91 ].

369 TABLE 7.13 Heats of immersion data for air-dried lignites [54,91]. Gascoyne Heat of Apparent Immersion Surface Area

Liquid Benzene Methanol

Indian Head Heat of Apparent Immersion Surface Area

(J/l~)

(m2]g)._

I'J/l~)

(rnZ]_g)._

51_0 60.2

131 154

56.-9 74.5

146 191

Vacuum drying roughly doubles the heat of immersion, for a given lignite-liquid pair, compared to air drying. The fact that polar liquids give higher heat of immersion values than the non-polar benzene adds credence to the idea that observed heats of immersion are affected by hydrogen bonding between the liquid and the lignite. The data in Tables 7.12 and 7.13 show an average ratio of heat of immersion in methanol to heat of immersion in benzene of 1.27 with a standard deviation of 0.07 regardless of the lignite or the drying method used [92]. Vacuum drying at ambient temperature (24 h at 0.67 Pa) increases the total pore surface area by about 55--60% [93]. This result is in qualitative agreement with comparable studies on the removal of moisture from the lithotypes of Victorian brown coal [94,95]. The evaluation of thermal effects on the pore structure of the Beulah lignite was done in terms of macropores, transition pores, and micropores using established small angle X-ray scattering procedures [96]. Small angle X-ray scattering from Beulah lignite heated to five different heat treatment temperatures in argon show (based on an eight-parameter fit of the scattering data [4]) a steady increase in specific surface area associated with the different classes of pores as the heat treatment temperature is increased [73]. However, the sizes of the smallest classes of pores undergo a sharp drop between 250 and 313~

and thereafter increase slowly with increasing heat

treatment temperature. These results suggest that the effect of heating in inert atmosphere is to create a new micropore structure different from the one originally present in the lignite. Possibly the smallest pores originally in the lignite remain reasonably intact, but a new, and smaller, set of micropores comes into existence and so dominates the scattering behavior that scattering from the original micropore structure is masked by the scattering from the newer pore system. Comparison of the scattering behavior at the various temperatures shows no sudden shifts, which indicates that the changes in the structure occur gradually, without a major structural reorganization (as, for example, would occur with the onset of plasticity in a bituminous coal). Heating Beulah lignite in air at 2400C for 4 h approximately doubles the surface in both the macropores and the transition pores, and increases the micropore surface area by a factor of eight [93]. At the same time, the carboxyl content increased from 2.97 to 4.15 meq/g on a dmmf basis [93], as measured by the standard barium exchange method [97]. The increase of 40% in carboxyl groups was corroborated by a 40% increase in the C=O signal in the X-ray photoelectron

370 spectrum. In comparison, heating in argon for 4 h at 225~ results in very little change in the structure detected by small angle X-ray scattering. The temperature of 225~ is below the region where extensive thermal decarboxylation would be expected to begin, so the principal effect of heating should be dehydration. In fact the carboxyl content showed only a small change, dropping from 2.97 to 2.54 meq/g [93]. The small change in scattering behavior upon drying indicates that the pores were not filled with water in the unheated sample. Thermal treatment in argon at higher temperatures does cause changes in the small angle Xray scattering behavior, which derive primarily from changes in the micropore structure. These changes arise from loss of carboxyl groups by thermal decarboxylation. After heating to 550~ in argon for 4 h, the carboxyl content had decreased to 1.07 meq/g, a substantial reduction from the original value of 2.97. The increase in pore volume after heating to 550 ~ was 11.6 x 10-8 m3/g; the volume increase predicted by calculating the volume which would have been occupied by the carboxyl groups removed by the heating was 8.7 x 10-8 [93]. Furthermore, the rate of change of micropore surface area with temperature was a maximum around 470~

an excellent agreement

with independent measurements of the maximum rate of CO2 evolution occurring at 475* for slow heating rate pyrolysis of North Dakota lignite [98]. The principal effect of the heat treatment at 550~ for 4 h is to enhance the X-ray scattering at the largest angles, that is, to create very small pores following the escape of volatiles during heating [3]. Thermal treatment in water (so-called hot water drying [99]) affects mainly transition pores. Small angle X-ray scattering from hot-water dried Indian Head lignite shows that the specific surface in transition pores increases from 2.86 to 6.00 m2/g and the specific volume of transition pores increases from 8.27 to 14.02 x 10-3 cm3/g, when compared with a sample of the same lignite dried conventionally at 110~ [72]. The surface areas of untreated and hot-water-dried Indian Head lignite were 32.0 and 38.2 m2/g, respectively, when measured by small angle scattering with molybdenum radiation [54]. Hot water drying increased the macropore surface from 2.60 to 3.55 m2/g; the transition pore surface from 2.86 to 6.00 m2/g; and the micropore surface from 26.5 to 28.7 m2/g [54]. The largest change occurs in the transition pore region [69]. Hot-water-dried Indian Head slurry was treated further by gentle air drying and by vacuum drying. The surface areas of the two dried materials were 102 m2/g and 180 m 2/g respectively, as measured by heat of immersion calorimetry in methanol [66,68]. The same method showed the surface area of the untreated lignite to be 192 m2/g. These results imply that the hot water drying process altered the pore structure in some way (i.e., restricting access to the pores) that reduces the apparent surface area by almost half. Since access of the wetting liquid to the pores was restored nearly completely by the vacuum drying, hot water drying may block the pore openings (possibly with waxes, derived from the original plant material, mobilized by the high temperatures of the process) such that the pressure differential established between the sealed pore and the surrounding vacuum during vacuum drying is strong enough to break open the seals. Hot-water-dried Mississippi lignites show a linear relationship between the temperature of

371 drying and the heat of immersion [69]. The comparison is shown in Table 7.14 [69]. TABLE 7.14 Effects of hot-water drying temperature on apparent surface area of Mississippi lignite [69]. Temp, ~ (feedstock) 144 240 270 300 340

Apparent surface area, m2_/_g 131 74 51 42 26 12

The linear relationship between drying temperature and surface area has a least squares correlation coefficient r2 of 0.902 [69]. The effect on apparent surface area of grinding in steam at various temperatures is shown for a Mississippi lignite in Table 7.15 [65]. Measurements were made in methanol. TABLE 7.15 Effects of steam grinding temperature on apparent surface area of Mississippi lignite [65]. Temp.,*C 121 163 204 238

Apparent surface area, m2_/_g 88 85 111 112

Similar experimental results on samples of steam-ground Martin Lake lignite are shown in Table 7.16 [54]. TABLE 7.16 Effects of steam grinding temperature on apparent surface area of Martin Lake lignite [54].

Temp., *C (feedstock) 47 107 154 177 204 253

Apparent surface area, m2_/_g 117 122 129 208 230 232 279

372 7.4.5 The fractal structure of lignite The pore boundaries in many porous solids are rough, rather than smooth, surfaces and consequently may be fractal surfaces [100,101]. Suppose that one wishes to cover a surface with a monolayer of spheres having a radius R. The number, n, of such spheres will be given by the equation n = cR-D

where c and D are characteristics of the particular surface and D is the fractal dimension [ 100]. Neither c nor D is a function of R. If D = 2, the surface will be smooth

(e.g., a planar surface) and

if D = 3 the surface will fill some three-dimensional region of space. For fractal surfaces 2 < D < 3. Scattering of X-rays at small angles from a substance can be analyzed using the term q, which is a function of the scattering angle and is given by the expression

q = 4 n ~-1 sin(O/2)

where Z, is the wavelength of the X-rays and O is the scattering angle [ 102]. The structure from which the scattering occurs will have a characteristic length, 1. The determination of 1 from scattering experiments can be done for 0.1 < ql < 10 [103]. There are at least two different pore distribution models which can explain observed small angle X-ray scattering data [ 104-106]. One relates to a distribution of spheres of various radii, the radii being distributed such that the number

Nr of spheres of radius r greater than rl is proportional

to n-B for some exponent B [ 105]. In this model the spheres are positioned randomly but with the constraint that they may not overlap. The second model assumes the boundaries of the pores to be fractals [ 104]. The pores are uniformly distributed in space, but, as the case with the first model, the pores are assumed not to intersect. It is questionable whether the non-overlapping of pores is reasonable. The description of lignite as a fractal structure was developed as a result of small angle Xray scattering studies of porosity [107]. The fractal model considers lignite to be a threedimensional porous solid with uniform electron density 8 and empty pores having maximum dimension R. Assuming that the pores scatter small angle x-rays independently, so that the total scattering is the sum of the scattering from each of the pores, the scattered intensity is given by

I(q) = 4 n Ie b2 V fr2 g(r) [ sin(qr)/qr] dr

where V is the total volume of the pores, Ie is the scattered intensity from a single electron, and g(r) the correlation function (the average probability that if a point lies in a pore, a second point a distance r from the first point will be in the same pore) obtained by an average over all the pores.

373 The integral is evaluated from 0 to R, where R is the maximum dimension of any of the pores [104]. For small values of r, g(r) is 1 at all points in a pore except those located within distance r of a pore surface. The number of cubes of edge r needed to provide a layer coveting the pore surface is

n =

No

r -D

where No is a constant characteristic of the surface and D is the fractal or Hausdorf dimension of the surface [104]. For a smooth pore surface D = 2. When D = 2, No = S, where S is the total surface area between the two phases lignite and air, that is, the surface area of the pores. If M is the mass of the sample, then N o/M is analogous to S/M, the specific surface for pores with smooth surfaces. No/M can be evaluated from the scattering data. The scattering curve for Beulah lignite, Figure 7.10 [107], shows that for a wide interval of q the intensity is proportional to q-3.44. The

l x l O 11

lxlO lolxlO 9l x 1 0 8-

lxlO 7-

lxlO 6" o

i~

lxlO 5 lx10 4

lxlO 3

' '"'"~

''""'!

' """I

,,m,,

(~)

O

(~)

(Z)

C)

M

M

M

M

N

Scattering angle (radians)

Figure 7.10. Small angle X-ray scattering curve for Beulah lignite [ 104].

374 scattering data can be described by I(q) = • No ~)2 Ie { F(5-D) sin [n (D-l) / 2} / q6-D }

Thus the fractal dimension of the pore surfaces is 6 - 3.44, that is, 2.56. Therefore for this lignite the value of No/M is 20+2 m 2.5/g. Because the -3.44 power dependence on q holds to the smallest angles at which data were obtained, R must be at least 100 nm. Scattering curves similar to Figure 7.10 have been obtained for all lignites studied [107]. Therefore it is likely that most lignites must have fractal dimensions of pore surfaces of about 2.5. Small angle x-ray scattering curves for both Beulah and Gascoyne lignites exhibit fractal properties [108]. For Beulah lignite, D = 2.56 [93,103]. When ql >> 1, small angle scattering from a fractal surface in a porous solid is given by I(q) = n c 82 Ie F(5-D)sin[~t(D- 1)/2]q-(6-D)

[93]. In this equation 8 is the difference in electron densities between the two phases (e.g., the lignite substance itself and the air in the pores), Ie is the scattering intensity per electron, and F(5D) is a gamma function (see, e.g., [109]). Again provided that ql >> 1, the scattering intensity I(q) will be proportional to q-(6-D), so that plotting log[I(q)} as a function of log(q) should given a straight line from which D can be calculated readily. In this model the pore positions and radii are random variables with certain distributions. An example of a randomly generated computer representations of a shape function is shown in Figure 7.11 [80]. The set which results from removing from a uniform block of material the intersection with a set of cavity-generating spheres is called a trema set [ 110]. Small angle X-ray scattering data indicate that over a scale ranging from about 1 nm to microns the pore sizes follow a Korcak distribution N r ( R > r l ) = CrI-B

with the pores distributed uniformly in space, the distribution being

Nr (R >r I) = C / (E CE vIE)

[110] where E is a dimension (in normal Euclidian space E = 3) and C E is the volume of the Edimensional unit sphere. The constant C is given by C = E - D, where D is the fractal dimension which controls the scaling over the range of rl. The range of rl has two cutoffs, an outer cutoff f~, which is limited by the size of the sample particles themselves, and an inner cutoff ~ which is limited by the molecular (lattice) spacing in the solid. If V = 1, then f2 = 1 also, and the pore

375

Figure 7.11. Computer representation of a particular fractal pore structure in lignite. In this structure D is 1.9 and the inner cutoff is =0.01 [80].

distribution will then be defined by two parameters, e and D. Within the range of rl the pore distribution has perfect translation and dilation symmetry. The size of the pores and the tendency of the pores to percolate, i.e., to form channels to the edges of the sample, are not independent of D. (In the language of fractal geometry, the size of the pores and the tendency to percolate are known as lacunarity and succolarity, respectively.) Presumably every physical property of the lignite that is related to the pore distribution would be governed by D and e, which would be characteristics of the individual lignite. The scaling of individual measurements of physical properties could be correlated by various functions of D and e. To approach more realistically the anisotropic pores in lignites, the spherical pore cavities could be replaced by ellipsoidal cavities, but this extension of the model has not yet been done. Mercury porosimetry data for a Canadian lignite have been interpreted to follow a power law distribution having a fractal dimension of 2.84 [ 111]. This interpretation is valid for mercury pressures below 20 MPa. The drying of lignite and mass transfer of reactants in or products out are governed by the extent to which the pores inside the lignite particles connect to the surface. Water, reactants, and products move in and out of the lignite across an interface made up of the combined surface area of all the pores that communicate with the external environment. An extension of the application of

376 fractal geometry to lignite should examine how porosity, the fraction of pores connecting to the surface, and the effective surface area vary with e. Visual confirmation of the fractal structure of lignites was obtained by observation of dried slurries of hot-water-dried Indian Head lignite using transmission electron microscopy [ 112]. The degree of complexity of the edge structure of lignite particles did not appear to vary over several orders of magnitude. The invariance with scale of the structure is an important property of fractal structures. The transmission electron microscopy results present independent corroboration of the small angle X-ray scattering results. An alternative approach to visual examination of pores, and confirmation of a fractal model, is obtained by filling the pores with Wood's metal, followed by microscopy of the metalimpregnated sample [85]. (Wood's metal is an alloy of bismuth, tin, lead, and cadmium melting at 66"C 113].) Darco lignite showed a distinctly different texture from five samples of higher rank coals. The macropores are elliptical in cross-section, with rough boundaries between the pores and lignite matrix. (An elliptical-shaped cross-section observed in a "slice" through the sample is consistent with cylindrical macropores.) The macropore population was highest near the edge of the lignite particles, and non-existent near the centers. This shows that the alloy wetted a rough, and texturally complex, surface rather than penetrating an interconnected network of cylindrical pores. The surfaces of the Darco lignite appear very rough at all levels of magnification. These observations support the concept of a fractal nature for lignite particle surfaces [85]. A unified picture of lignite porosity and surface area and the relationship of these properties to other measurable properties, such as moisture content, behavior on drying, and mechanical properties would be a significant step toward a sound fundamental understanding of relationships between physical structure and properties. A difficulty in developing such a picture is that many measurement techniques used to study physical properties, such as gas adsorption, heat of immersion calorimetry, and friability, often lead to inferences or conclusions that contradict those from a different measurement. An understanding of pore sizes and their distribution is central to understanding equilibrium moisture, behavior on drying, mass transfer of reagents and reaction products, and the dependence of mechanical properties on physical structure. A start toward developing such a model is the approach via fractal geometry, based on observations of the fractal nature of lignite [80,104,114]. A trema fractal ("trema" referring to hole or cavity [110]) can be defined by assuming a distribution of spherical pores with the radii of the pores distributed as discussed above, where Nr is proportional to vl-B for some exponent B but in this model the spheres are allowed to overlap. The trema set is outside the union of the overlapping spheres. B = 3 is chosen to fulfill the fractal requirement that the pore size distribution appear the same on all length scales. The fractal model of lignite derives from its shape function F(r), which is one for the lignite substance and 0 for the pore cavities. A cube of lignite of volume V and side L is assumed to contain spherical pores of random positions and random radii. The function O(rl - r) has the value 0 when its argument is negative and the value 1 when its argument is positive. In the simplest case,

377 a lignite containing one pore of radius 1"11and position rl,

F(r) = 1 - O

[1"11 -

abs(r- rl)]

[80]. Then for a lignite with two pores an analogous equation would be

F(r) = 1 - 0

[1"]1 -

abs(r-rl)]

- 0[/]2-

abs(r-r2)]

[80]. However, this equation does not take pore overlap into account and thus would count the overlapping region of two pores twice. If the general equation for the k-th pore is written as

fk(r) = 0 [rl- abs(r-rk)]

then the correct form of the shape function for lignite having N pores would be

N

F(r) - ~ (

1 - fk(r) )

[80]. The small angle X-ray scattering intensity is determined by , where F(k) is the Fourier transform of F(r) and the use of angular brackets demotes the average over the pore distribution. Averaged products of this kind relate not only to the X-ray

scattering

but also relate

via percolation theory to other physical properties. A simple model of the mechanical properties can be developed by considering the special case of fracture in a plane, where the fracture stress is dependent on the relative cross sectional area of the pores to the total cross sectional area. In the extreme case of e = 0, the pores would cover the entire cross section. In realistic cases, where e has some finite value, the pores will account for some fraction of the total cross section. For a given value of D the fraction of the plane covered by pores will vary with ~. The trema fractal model, which depends only on D and e offers the prospect of a unified conceptual approach to the systematics of pore structure. Drying, fracture, combustion, and gasification all depend in some way on the pore structure. Thus gaining a general understanding of the pore structure characteristics will eventually lead to a more unified theoretical basis for many aspects of lignite processing.

378 R E F E R E N C E S

10 11 12 13 14 15 16 17 18

19 20 21

22 23 24

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382 100 101 102 103 104 105 106 107

108 109 110 111 112 113 114

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383

Chapter 8

M O I S T U R E IN LIGNITE

8.1 I N T R O D U C T I O N Unquestionably the most difficult and perplexing problem in the characterization of lignites is determination of the moisture content, and developing an understanding of the ways in which moisture is adsorbed into, retained in, and desorbed from lignites. Decades of research have been devoted to trying, first, to measure accurately the total moisture content of a sample and, second, to determine the ways in which water might be incorporated in lignite and how the total moisture content is apportioned among them. The problem remains unsolved. At least five categories of water have been defined to occur in coals: 1) surface adsorbed water; 2) capillary condensed water; 3) water of hydration of the inorganic species; 4) so-called combined water, released by decomposition of the organic components; and 5) dissolved water [ 1]. Considering the current state of knowledge of the general topic of moisture in lignite, it is safest to regard any moisture determination simply as an operational definition. That is, "moisture" determined by, for example, measuring weight loss during heating to 105~ certainly does not measure the total amount of moisture in the lignite. However, this, or some comparable analytical technique, may be taken as a standardized, reproducible method of comparison of various lignite samples. When samples of two lignites that are otherwise reasonably similar in composition and properties are treated under identical conditions at 105~

the difference in the results reasonably

indicates the amount of loosely held moisture in each [2]. Often this comparison provides adequate information for the user of the lignite. Customarily the moisture content of coals is considered as the water driven off at 105~ in a specified period of time, or, when the xylene moisture determination is used, the amount of water driven off at the boiling point of the xylenes, 138-144~

Accepting either of these methods as the

measurement of moisture content ignores water more tightly bound to the lignite structure, hydrogen bonded to functional groups or present as water of hydration and the so-called water of decomposition, (water formed during thermal dehydration of the lignite by, for example, loss of a hydroxyl group and a neighboring hydrogen). Most approaches to the determination of water represent so-called indirect methods, because they do not use a reagent that reacts specifically with water. (Direct methods have occasionally been suggested, an example being the reaction of lignite with calcium carbide and

384 measurement of the acetylene generated. Such methods are generally rather cumbersome and in some cases may involve expensive reagents or apparatus.) Indirect methods are not undesirable per se; often in analytical chemistry their use is convenient and relatively easy. The difficulty arises

when an indirect method is applied to a material as complex and heterogeneous as coal, which furthermore easily undergoes competing reactions, such as oxidation, that complicate analytical measurements. Specific problems have been summarized [2] as follows: 1) the moisture content is sensitive to prevailing atmospheric conditions, especially with fine particle sizes; 2) the possibility of volatizing substances other than water (e.g., carbon dioxide, methane, or nitrogen); and 3) reactions with oxygen, including adsorption of oxygen, its reaction with carbon to form carbon dioxide, and reaction with hydrogen to form water. The methods for determining moisture in coal originally developed by the U. S. Bureau of Mines and later standardized by the American Society for Testing and Materials were developed for coals in the eastern United States, that is, bituminous coals and anthracites. The problems mentioned in the preceding paragraph for the application of the indirect moisture determination to coal are especially severe in regard to the weight loss during air drying of lignites. For lignites, the loss in sample weight does not necessarily mean that an equivalent weight of water has been lost, and even if it were, the loss in weight does not mean that all the water has been removed. Ideally, a method for the determination of moisture in lignite should meet the following criteria: 1) it should be able to determine the total water content; 2) it should not cause oxidation of the sample; 3) it should be practical, that is, reasonably easily carried out in the laboratory; and 4) it must be accurate [3]. The inherent moisture in low-rank coals should be determined [4] by ASTM procedure D 1412 (as indicated in procedure D388 [5]). Research continues unabated on the development of new methods for determining moisture, often ones which rely on sophisticated analytical instrumentation. Several examples are cited here. Moisture can be determined by measuring the phase shift in a guided microwave train, the phase shift being related to the amount of moisture. For a suite of five Palana (India) lignites, the difference between the microwave method and oven drying ranged from +0.50 (i.e., oven drying indicating 18.40% moisture; microwave, 17.90%) to -0.40 (16.80% by oven drying vs. 17.20% by the microwave method) [6]. Water can be determined by measuring the free induction decay of a single rd2 pulse in the 1H nuclear magnetic resonance (NMR) spectrum. For a suite of nine German brown coals, agreement with oven drying ranged from + 1.9 (11.5% by oven drying, 9.5% by NMR) t o - 1 . 5 (9.8% by oven drying, 11.3% by NMR) [7]. Extraction of water from coal into pyridine, reaction of the water with organophosphorus compounds and subsequent determination by 31p NMR provides a relative error, compared to the ASTM method, of +__2%[8].

385 8.2 EQUILIBRIUM MOISTURE The equilibrium moisture content can be used as an indicator of the anticipated as-mined moisture content [9]. The equilibrium moisture is also used to determine the bed moisture. The bed moisture and estimated surface moisture provide an estimate of the total moisture in the lignite. Equilibrium moisture varies inversely with the ash content (expressed on a dry basis), because the inorganic components may have much lower moisture holding capacities than the lignite substance itself. For example, at 40% (dry basis) ash, a lignite had an equilibrium moisture of 28%, but had an equilibrium moisture of 36% at an ash content of 8% [9]. For lignites, the equilibrium moisture is invariably lower--sometimes significantly so--than the so-called inherent moisture as determined by the ASTM method [10]. Samples of lignite that appeared to be dry visually but which probably still contained all of the bed moisture showed, on comparison with samples of the same lignite deliberately wetted by shaking in water, that both eventually attained the same equilibrium moisture, but that the wetted sample generally required an additional 24 h to come to equilibrium. For the samples that appeared dry, a equilibrium value of 32.5% was attained in 48 h over saturated potassium sulfate solution; 72 h was required to achieve this value with the wetted samples [ 11]. For lignites the equilibrium condition preferably should be approached from the "wet state," that is, from moisture contents above the equilibrium moisture [11]. Recommended analytical procedures have been published [11]. The relationship between moisture on an as-received basis and oxygen content of the coal is shown in Figure 8.1 [12]. Moisture shows a positive correlation with oxygen content [13]. This is attributed to the formation of hydrogen bonds between water molecules and oxygen functional groups on the lignite surface. The principal factor governing the equilibrium moisture content of low-rank coals is the content of carboxyl and phenolic hydroxyl groups [14]. At 52% relative humidity and 20~

the equilibrium moisture content can be expressed by the regression equation

% Equilibrium moisture = 2.181(COOH) + 0.599(OH) + 4.90 for which the correlation coefficient is 0.957. Here the terms (COOH) and (OH) represent the carboxylic and phenolic hydroxyl contents expressed as meq/g of dry acid-form coal. (The socalled acid form coal is coal treated with hydrochloric acid to remove ion-exchangeable cations and convert all the carboxylic and hydroxyl groups to their protonated forms.) The strong correlation shows that the oxygen functional groups have a very powerful influence on the adsorption of water. The coefficients in this equation suggest that the carboxyl group has roughly three times as much water associated with it as does the phenolic group. If the phenolic contribution is ignored, then the simpler equation % Equilibrium moisture = 2.239(COOH) + 6.60

386 0

70!

60

504o~ 30: ~ 20: 100

''''I

0

''''I

5

''''I'

'''I''

''I'''

'I''''

10 15 20 25 30 Percent oxygen, Parr basis

35

Figure 8.1. Relationship between moisture and oxygen content. Data for 64 samples of Canadian lignites, American peats, German brown coals, and Upper Silesian bituminous coals lie in the band [12].

can be derived [14]. These two equations were derived for coals having carboxyl contents of 0.944.01 meq/g and phenolic hydroxyl contents of 2.51-3.95 meq/g. Extrapolation outside these ranges would be of questionable validity; for example, there are bituminous coals for which the carboxyl content is essentially zero but which certainly have measurable equilibrium moisture contents. Coals in the acid form can be converted to a salt form either by reaction with an aqueous solution of the desired cation or by ion-exchange reactions after first converting the coal to its sodium form. The equilibrium moisture content of various salt form coals varies in the order of the hydration of the cations in aqueous solution. Furthermore, the moisture content increases linearly with cation content [14]. Thus for as-mined low-rank coals the carboxyl content essentially determines the equilibrium moisture content, along with the amount of cations incorporated (that is, the amounts of carboxyl in the acid and salt forms) and the kinds of cations and relative amounts of each. The importance of knowing the kinds of cations derives from the strength of the hydration. For example, water is weakly associated with sodium or potassium cations and is probably evolved on drying at temperatures around 110~

other cations, such as magnesium, have much

stronger bonds to the water of hydration and not all of the water will be released on mild thermal drying. In such a case, the moisture content determined as weight loss on heating will be low and

387 subsequent corrections of ultimate analysis data to d ~ - or moisture-free bases will give high values of organic hydrogen and oxygen. Correlations of moisture contents of North American lignites with petrographic composition have not been developed. South Australian lignites show a correlation of moisture with desmocollinite and telovitrinite, in that an increased moisture content is observed as the abundance of these macerals increases [15]. The effects of air drying on the restoration of the bed moisture for Beulah (North Dakota), Sandow (Texas), and Alnador County (California) lignites is shown in Table 8.1 [ 11]. Regardless of whether the lignite is wetted again before the equilibrium moisture determination, air drying destroys some of the moisture capacity of the lignite.

T A B L E 8.1 Effect of different degrees of air-drying on moisture-holding capacity [11].

Moisture, percent Before equilibration Sample

As-received

Air-dried

North Dakota

34.7 35.8 37.7 -

25.1 19.8 14.6 25.5 19.4 15.2 23.9 19.0 14.3

Texas

California

Equilibrated at 30 ~ 97% RH* Unwetted 33.9 30.5 28.7 26.7 35.0 30.3 27.1 25.2 35.0 28.0 25.5 22.8

Wetted 34.3 33.4 33.1 32.1 34.8 33.5 32.2 30.6 36.4 33.1 32.6 31.0

*Relative humidity

8.3

MEASUREMENT

OF

MOISTURE

BY

E L E V AT E D - T E M P E R A T U R E

METHODS

One of the most straightforward techniques, at least in principle, for the determination of moisture is heating in a suitable inert atmosphere and measuring the resulting weight loss. In practice, however, the situation is not as simple as would appear at first sight. The potential processes that might affect the amount of moisture determined by a thermal method are: 1) water would actually be given off; 2) various gases such as methane or the carbon oxides may also be given off; 3) other gases--oxygen or nitrogen--may be adsorbed; 4) the coal may be oxidized and lose weight due to the loss of carbon dioxide or water (the water in this case being the product of

388 the oxidation reaction); and 5) some water may be retained in the coal. In most laboratories the detennination of moisture is done simply by measuring the weight loss after heating in a prescribed time-temperature regime. Equating the observed weight loss to the moisture content presumes that the only process occurring during the moisture determination is the first. That none of the others occur seems a very risky presumption. Exposing a dried sample of lignite to the laboratory air for 10, 60, and 120 seconds indicated moisture contents of 18.80, 18.41, and 18.27%, respectively [16]. On the basis of these results, and similar tests with bituminous coals, exposure of samples to the air during transfer from the oven to the desiccator should be for the minimum practical time [ 16]. Oxidation of low-rank coals at 105-110~

may introduce errors into the moisture

determination. The Stansfield-Gilbart method minimizes such errors [17]. In this procedure, 5 g samples o f - 1 4 mesh coal are placed in a desiccator over saturated potassium sulfate solution, which is then evacuated and placed in a constant temperature oven at 30 + 0.05~ for 48 h. Moisture is then determined by heating in a vacuum oven for 3 h at 105-110~ in a stream of methane at subambient pressure. The effects of passing dry nitrogen at 10 kPa pressure or of dry air at ambient pressure while drying -16 mesh lignite at 105-110~

have been compared [11]. For 1.5 h in air, the

moisture content as determined was affected by the thickness of the layer of lignite in the sample container. For example, using a 43 mm crucible (resulting in a 9 mm thick layer of lignite), the moisture content was determined to be 29.2%, whereas the same sample treated in a 70 mm bottle (2 mm thick layer) gave the result of 30.8% moisture. Variations in the thickness of the layer of the lignite had no evident effect when heating in nitrogen at reduced pressure. Longer heating times, up to 4.5 h, had a limited effect, except for samples in 43 mm crucibles in air. Using a 70 mm bottle, the moisture content was determined to be 30.8% with 1.5 h heating in air and 31.0% with 4.5 h heating. For equal heating times, the moisture content as determined in reduced pressure of nitrogen averaged 0.5 percent higher than that determined in air. Under other conditions, heating in nitrogen and in air show only very small changes between the two atmospheres. At 1 h in a flow rate equivalent to 90 volume changes of the oven per hour, the moisture contents of an unidentified lignite determined in nitrogen and air were 18.21 and 18.24%, respectively [16]. When the heating time was extended to 2 h and the flow rate increased to 180 volume changes per hour, a small but discernable difference was observed: 18.63% determined in nitrogen and 18.46% in air [16]. Thermogravimetric analysis of Beulah-Zap lignite, coupled with Fourier transform infrared determination of evolved products, showed reasonable agreement with the ASTM moisture determination. The TG-FFIR method indicated 30.6% moisture, compared with 32.2% determined by the ASTM method [18]. Thermogravimetric analysis of Gascoyne (North Dakota) lignite shows a steady moisture loss at temperatures to about 90~ [19]. A slow weight loss is still recorded in the temperature range 100-260~

This latter weight loss might be ascribed to tightly bound

moisture. The relative proportions of loosely bound moisture, which comes off at or below 90 ~

389 and the tightly bound moisture are 80:20, which agrees with studies of the same lignite conducted by dielectric relaxation spectroscopy (Section 8.4). Water continued to be released from Kincaid (North Dakota) lignite for heating times as long as 150 h at temperatures as high as 175 ~ [20,21]. The results are shown in Figure 8.2. The moisture release depends upon heating time, temperature, and gas atmosphere. The nature of the gas affecting water release indicates a diffusion-controlled mechanism. If the release of water were governed by the thermal decomposition of species within the lignite (e.g., hydrates) the atmosphere should have no effect, since then the vapor pressure of the water would eventually be sufficient to force its release. A similar argument can be lodged against the hypothesis that the rate of water release is controlled by temperature. The smooth curves, with no evident changes of slope, indicate that there is not likely a sudden switch from one mechanism to another at some point in the drying process, nor that there is a change in the way the released water had been bound to the lignite. Australian brown coal has shown similar behavior; when the coal was dried in a stream of inert gas, measurable moisture evolution persisted for several days [22]. Other studies, however, show a transition from one rate of moisture loss to another [23,24]. For Beulah-Zap lignite, a plot of the logarithm of water remaining as a function of time has a constant slope for about 85% of the moisture loss from the lignite. A transition to a slower rate, considered to be indicative of a structural rearrangement, then occurs.

1 He, 175"

4 3.5-

2 N2, 150"

3L-

1 2.5-

17t

~I

2-

u.,

~

2

1.51 "

5-~ 0"0"1

!

0

i

I I

20

I

I

'1

40

'''

I'

60

''

I1

I '

I

I I

80 100 Time, hours

I I

I I I

120

I I

140

,

I

160

Figure 8.2. Water recovery as a function of drying time, gas atmosphere, and temperature for Kincaid lignite [20]. The water recovery is actual water recovered during the drying minus the ASTM moisture, expressed as a percentage of sample weight.

390 Changes in the surface area or depth of the bed of sample being dried had no effect on the rate of water release from Kincaid lignite, indicating that diffusion of the water through the bed of lignite particles is not a limiting factor in removal of water. This, together with the process being diffusion-controlled within the particles, suggests that removal of moisture from lignite is similar to the drying of porous solids [21]. However, this conclusion must be contrasted with a result obtained as a by-product of a study of the effects of heating on the pore structure of Beulah lignite [25]. The small angle X-ray scattering behavior of a sample heated to 225~ in argon for 4 h was very similar to that of the unheated sample. Since the basis of the small angle X-ray technique is that the intensity of the scattering depends on the square of the density difference between two phases, if the pores had been filled with water in the untreated lignite and with argon in the dried lignite, the difference between the coal/water and coal/argon systems should produce a change by a factor of seven in the X-ray results for the two systems. That very little change was observed suggests that the moisture in the untreated sample was not in the pores [25]. The average latent heat of vaporization of water from lignite, calculated from the ClausiusClapeyron equation, was 2.55 kJ/g in the range 20-40 ~ The latent heat of evaporation of water from a plane surface, calculated by the same equation, is 2.43 kJ/g [26]. Both the vapor pressure of water and the calculated heat of vaporization vary with moisture content, as shown in Table 8.2 [26]. Measurement of the heat of evaporation of water from Romanian lignite has shown that the percentage increase in heat of evaporation, relative to the heat of evaporation of pure water, is greatest when the last gram of water is being evaporated from the lignite [27]. TABLE 8.2 Calculated heat of vaporization of moisture from lignite [26]. Moisture per 100 grams dry lignite 40 35 30 25 20 15 10

Vapor pressure, mm Hg Heat of vaporization, 20"C 90"C calories per gram 16.53 53.55 592.9 15.61 50.51 593.8 14.33 46.46 594.8 12.54 40.99 598.9 10.24 34.40 612.8 7.37 25.45 626.7 3.95 14.11 643.8 Average ....................... 609.1

Comparison of heating in inert atmosphere with the xylene moisture determination (discussed below) shows that, for the same lignite, moisture evolution ceases after 1.5-2.5 h in xylene. The xylene penetrates the pore system of the lignite and either reduces subsequent moisture loss to a rate limited by liquid phase diffusion or blocks water release entirely [21]. The utility of the xylene distillation method for moisture determination in lignites and its

391 superiority to the conventional air drying methods have been established by meticulous research [3]. using six coal samples that included two lignites--Dakota Star (North Dakota) and Sandow. The essence of the method is distilling xylene in the presence of the coal sample; the water in the coal and the xylene form an azeotrope that distills at about 140~ into a graduated receiver from which the volume of water can be read. Assuming a density of 1 g/cm3 immediately gives the weight of water, from which calculation of the moisture content is straightforward. For the most precise work, corrections can be introduced for the density of water at the ambient temperature and for the solubility of water in xylene. The xylene method has been compared with standard air drying, drying in a vacuum oven, and Schoch's oil dehydration technique [28]. The xylene distillation and oil dehydration gave comparable precision, but because the latter is very cumbersome (it involves heating a 250 g sample of coal in the presence of 500 mL kerosene to 225 ~ the xylene method is preferable. For coals having 30-40% moisture the ASTM oven drying method gave 95-96% of the moisture reported by the xylene method. The vacuum oven gave lower values than either the ASTM or xylene methods. The xylene method accounts for all the moisture in a coal except for the combined moisture [3]. When large particle sizes are used, the xylene method is not satisfactory. Eight samples of 6x 13 mm Wyodak (Wyoming) subbituminous coal gave results ranging from 26.7% to 28.6%, with a standard deviation of 0.62, but eleven samples of 6x0 mm Dakota Star lignite ranged from 39.2% to 39.8% with a standard deviation of 0.17 [3].

8.4 E V I D E N C E F O R B I M O D A L I N C O R P O R A T I O N OF W A T E R

The incorporation of moisture in low-rank coals involves at least two fundamentally different mechanisms. The first "type" of moisture behaves as if it were free, the vapor pressure vs.

temperature behavior being that of pure water. Such moisture may be present in macroscopic

amounts in cracks or large pores. The second type of moisture is bound more tightly, having a lower vapor pressure than pure water at the same temperature. Such water may be hydrogenbonded to oxygen containing functional groups in the coal [ 13,29] or it may be present as water of hydration with various cations. Although various lines of evidence suggest that there may be several ways in which water is incorporated into lignites, it is important to note that the quantitative differentiation among these types or forms of moisture depends on the experimental technique used to make the measurements [30]. Dielectric relaxation is useful for studying bulk matter containing specific polar molecules. The graphs produced in a dielectric relaxation experiment are often called spectra, and the technique is sometimes called dielectric relaxation spectroscopy. It is not, however, spectroscopy in the quantum mechanical sense of resonant absorption of electromagnetic energy corresponding to the energy differences between two discrete quantum states. It is rather a classical phenomenon in

392

which energy in the dielectric medium is dissipated when the cycle time of an applied oscillating electric field is of the same order of magnitude as the relaxation time of the polar molecules in the medium. Useful background discussions of dielectric relaxation are provided in [31-33]. A parallel plate capacitor with a vacuum between its electrodes ceases to behave as a pure capacitance when a condensed matter dielectric is inserted between the electrodes [34]. If an alternating voltage is then applied, the current and voltage will no longer be precisely rd2 radians out of phase (rd2 being the value for a capacitor with a vacuum between its electrodes). The degree of departure from rd2 is expressed by the angle 8 and depends on the "lossyness" of the dielectric. If e represents the dielectric constant of the medium filling the capacitor, in complex notation

E* = e ' - i e"

The phase relationship between current and voltage is n / 2 - 6 . The detailed derivation of the relationship of 8 to E' and E" has been published [35,36]; condensing somewhat, tan 8 = E"/e'

and further,

t a n 8 = (E R - eLl) / (ER + 032 1;2 EU)

where ER is the low frequency limit (relaxed) dielectric constant,Eu is the high frequency limit (unrelaxed) dielectric constant, to the angular frequency, and x the extrinsic relaxation time. The quantitative interpretation of dielectric measurements requires that, ideally, the material being studied should consist of a single phase, or, if the system is multiphase, the arrangement of the phases in three dimensions be known. Obviously, neither condition exists for studies of water in coal. Thus calculated values of ER, EU, and x may not be accurate, and experimental studies have focused on calculation of tan 8. By measuring tan 8 vs. temperature at constant frequency, a quantity proportional to x can be determined. The large ionic double layer capacitance in coal containing water would seriously impede measurements made above 0~

Experimental results have been published for Gascoyne

lignite [35,36] and for San Miguel (Texas) lignite [36]. Gascoyne lignite is of particular interest because it can contain in excess of 40% moisture as mined, the highest moisture content of any commercially mined North American lignite. The sample is initially cooled to about-190"C. As it warms, the frequency F, capacitance C, conductance G and the temperature are measured [34]. The phase shift, tan 8, is obtained as a function of frequency from the equation

393 tan 6 = G/2nFC The tan 6 values plotted as a function of temperature yield the dielectric spectrum. The dielectric constant of brown coal has been measured; the dielectric constant as a function of water content showed a maximum around 50% moisture [37]. Nazarovo (Russian) brown coal displayed a non-linear tangent of the loss angle as a function of moisture content [38]. The dielectric spectrum of Gascoyne lignite shows two sets of peaks, as displayed in Figure 8.3 [34]. Depending on the frequency of measurement, one peak appears a t - 1 0 2 , - 9 2 , or -80~ the second appears at - 6 0 , - 5 0 , or -36~

The latter is attributed to the presence of macroscopic

crystals of ice [39]. After air drying to a constant weight (over several days) the high temperature peak disappeared from the spectrum. Upon vacuum drying of the air-dried sample, virtually all structure disappeared from the spectrum (Figure 8.4). Reconstituting the sample with water after freeze drying restored the spectrum to that of the original untreated lignite. The experiments suggest that this lignite contains about 80% of its moisture in a "loosely bound" form which freezes to ice below 0~

and the remaining 20% in a tightly bound form, possibly water of

hydration, which does not freeze to solid ice [40].

.

c3 4" X

~3 v

r

1O"

''

' I v-i i

''

'1'

''

~ ~ ~

i

I' (:2, O0 i

''1

''

'1

,t::) '4:) i

~ "~ i

'''

I' (:D 0,,,,I i

'' ,D

Temperature, ~

Figure 8.3. Dielectric relaxation spectrum of water-saturated Gascoyne lignite [35]. Data collected at 0.1 kHz.

The moisture distribution in Beulah-Zap lignite was measured by extracting the moisture into pyridine, followed by reaction of the water (now in the pyridine) with diphenylphosphinic

394

.

o4: .

~3 v

~2 .

1" ~

0 -200

-150

-100 -50 Temperature, *C

0

50

Figure 8.4. Dielectric relaxation spectrum of Gascoyne lignite after vacuum drying [35]. Virtually all the signal has been lost.

chloride and subsequent quantitation by 3]p nuclear magnetic resonance [41]. This work showed that 89% of the total moisture was "very loosely bound," and is probably surface moisture; 9% is "loosely bound," and 2% is "tightly bound." Unfortunately, the same lignite sample has not been examined by both techniques, and, further, one must use great caution in interpreting qualitative descriptive terms such as "loosely" and "tightly." However, the dielectric spectroscopy work suggests an 80:20 split between two forms of moisture [40]; 31p NMR suggests an 89:11 split [41], and drying measurements on the same lignite as used for the NMR work suggests an 85:15 differentiation [23,24]. San Miguel lignite, of about 30% moisture, showed only a single set of low-temperature dielectric relaxation peaks (Figure 8.5). These peaks actually appear as shoulders on a continuously rising ionic peak; the true, unperturbed maxima are 10 to 15~ lower, thus falling in the temperature region observed for other coals [36]. It was concluded that San Miguel lignite does contain chemically bound water [36]. Both Falkirk (North Dakota) and Martin Lake (Texas) lignites have large proportions of their moisture content in the tightly bound mode [42]. From the expression

"l;max = 1;o e x p (Ea/RTmax)

where Tmaxis the temperature (in kelvins) of the maximum in tan 6, it follows that

395 35 3025,,,==i

20"~ 15-

1o" 5

0 ' -140

'

'

I

'

'

'

I

'

'

'

I

'

'

'

-60

-120 -100 -80 Temperature, ~

Figure 8.5. Dielectric relaxation spectrum of San Miguel lignite at 0.1 kHz [36]. Only the low-temperature peaks are present.

log f = C2 - (Ea/2.3 RTmax) [40]. The activation energy for motion of the water molecules can be obtained from measurements at several frequencies by plotting log f

vs.

reciprocal temperature. For air-dried Gascoyne lignite, a

value of 67 kJ/mol was derived from the low-temperature peaks in the dielectric spectrum [40]. The activation energy for pure ice is 55.3 kJ/mol [33]. Since the low-temperature peaks in the dielectric relaxation spectra of lignites differ both in peak position and in activation energy for those in pure ice, it is unlikely that the moisture responsible for the low-temperature peak is present as ice. That the peaks occur at lower temperature than for pure ice suggests that the water molecules in lignite are in a less restricted environment than those in ice crystals. The less restricted environment, compared to that in ice crystals, enables significant molecular motion to occur at a lower temperature. These less restricted water molecules may be present as water of hydration of the cations present as counterions for the carboxyl groups [35,36]. From an estimated of 84 J/g for the heat of wetting of dried coal in water, and a molecular weight of a hypothesized structural unit in lignite of 450 [43], the energy associated with retention of water in lignite would be about 38 kJ/mol [44]. This value is in the range of bond energies normally associated with hydrogen bond formation [45]. This calculation does not prove that socalled bound water is held in the lignite via hydrogen bonding, but is not inconsistent with that idea. Oxygen functional groups have been suggested to be responsible for hydrogen bonding of

396 water in lignites [ 13,29,46]. That the organic oxygen functional groups play a role of some sort in the moisture content of lignite is also suggested by the correlation of the as-received moisture content with the humic acid content of peat and low-rank coals [26]. Hysteresis in the adsorption and desorption of moisture has also been attributed to the availability of hydroxyl groups for interaction with the water molecules--the availability of these groups must in some way be different during the desorption and adsorption processes [26]. Computer-aided molecular design also indicates differences in drying and rehydration. The shape of the Beulah-Zap lignite "molecule" configuration changes from an extended shape when fully hydrated to a more contracted shape as drying proceeds [47]. As drying proceeds, the number of intramolecular hydrogen bonds (i.e., hydrogen bonds entirely within the lignite structure, and not lignite-water bonds) increases from 4 to 10. As water is readsorbed (in the computer simulation) the number of intramolecular hydrogen bonds stays at about 10. This result suggests that the conformational change in lignite structure on drying is irreversible [47]. The heat of desorption of water from lignite, measured by differential scanning calorimetry, indicates that the interaction of lignite with readsorbed water is stronger than the interaction with the water molecules originally present [47].

8.5 A D S O R P T I O N

A N D D E S O R P T I O N OF M O I S T U R E

Three North Dakota lignites (Velva, Garrison, and Columbus) stored in desiccators over pure sulfuric acid required about 40 days to attain constant weight [48,49]. The three lignites showed very similar behavior, in that the dehydration could be divided into three distinct stages: first, a gradual decrease in relative vapor pressure from 100% to 80%; second, a rapid drop from 80% to 15%; and finally another gradual decrease from 15% to 0%. This behavior of lignite is very similar to the desorption of moisture from wood and peat [48,50,51]. The corresponding time for equilibration with the atmosphere in a sealed desiccator at 50~ is about 8 days [49]. The difference in equilibration time at the two temperatures reflects the increased diffusion rate of the moisture at the higher temperature. Hysteresis between the adsorption and desorption isotherms (discussed in more detail below) is also less at 50~ suggesting that a change in the physical structure of the lignite has already occurred even at this comparatively low temperature. The practical importance of these observations is that, upon exposure to the atmosphere, lignite will dry to a point at which it is in equilibrium with the prevailing humidity. For a typical relative humidity of 60%, no practical benefit would accrue from drying the lignite below about 16% total moisture, because the dried lignite would be hygroscopic and absorb water to reattain this moisture level, unless the drying process also effects some fundamental change in the lignite structure to prevent such absorption. Desorption isotherms of four North Dakota lignites are shown in Figure 8.6 [52]. The zero value of the ordinate represents a condition obtained by keeping the sample at 30~ and relative

397 50

1 Freedom 2 Beulah 3 Glenharold

o 'O ~0

ca ca

4 Gascoyne

35 30"

~ --m

.

ff

25

~o

2o

~

15-

.~1 o

10~

52 T''''I ''''I'''' I''''I''''I''''I''''I''''I''''I'''' .

.

C~

.

C~

.

.

C~

.

C~

.

C~

.

.

C)

C~

C~

P/Po, Relative humidity Figure 8.6. Moisture adsorption isotherms for four North Dakota lignites at 30~ [52].

humidity of 0 until an apparent equilibrium was attained. This condition is not the same as attained by thermal drying in nitrogen at 100~ for an hour. Drying these lignites under the latter conditions always causes a small additional mass loss--presumed to be moisture--compared to the former condition. Comparable behavior is observed with Australian brown coals. That different mass losses are observed when drying at 30 ~ and 0 relative humidity for many days

vs.

drying at 100~

in nitrogen for an hour suggests that there are different sites in the lignite responsible for retaining moisture, and the strength of retention is different for the different sites. Moisture contents below about 50% relative humidity are governed by some process, such as adsorption, other than simple capillary filling of pores [52]. That is, up to half the moisture content of lignites may be governed by adsorption behavior, again similar to that of Australian brown coals. Micron-sized porosity is not able to hold more than about 25% of the lignite moisture [52]. One micron pores are filled only at relative humidities above 99.9% [52]. Sorption of water on Texas lignite in the range of 1.2-2.3 kPa partial pressure water showed that Fickian diffusion combined with adsorption was the best theoretical model for describing the process [53], although the best correlation of the data was obtained via straightforward mass action kinetics. The desorption of water from lignite (as well as from peat and from birch wood) shows the same behavior as when water is reduced below its saturation value [50]. The vapor pressure

398 gradually decreases in the range 100-85%, followed by a rapid decrease from 85-15%, and then another gradual drying in the range 15--0%. Moisture absorption follows an S-shaped curve similar in shape to the absorption isotherm of a swelling gel. The first traces of moisture are adsorbed on the surface, and the vapor pressure increases slowly. A point will be reached at which continued absorption of water begins filling the smaller pores. With continued moisture absorption beyond this point, the vapor pressure increases rapidly, as a result of the changes in the radius of curvature of the water meniscus in the pores. When the smaller pores have been filled, the last stage of water absorption consists of filling the largest pores until the sample is saturated with water. On the other hand, inferences from heat of immersion experiments suggest that the initial reabsorption of moisture into dried lignite occurs in the largest diameter pores [ 19,54]. The heat of immersion of vacuum-dried lignite in cyclohexane was 3.9 J/g. However, after a comparable sample of vacuumdried lignite was allowed to stand in the laboratory for a short time, the heat of immersion became essentially zero. This observation suggests that the large pores, amenable to penetration by cyclohexane, quickly filled with water. In principle, water vapor can be taken into a solid adsorbent in three ways: by pure adsorption on active surfaces, by capillary adsorption and condensation to a liquid, or by adsorption or other penetration into the bulk interior of the solid. Analysis of the absorption isotherms indicates that lignite adsorbs water by two mechanisms: adsorption on active surfaces, and absorption in capillaries with subsequent condensation to liquid water [26,51]. This observation shows further analogy between moisture retention behaviors of lignite and silica gel. The physical structure of lignite can be regarded as a colloidal, sponge-like mass with "ultramicroscopic" capillaries of various radii [48]. The higher than normal vapor pressure of water may be due to water being present in a sponge-like structure of extremely small capillaries that retain the liquid with a convex meniscus (i.e., concave toward the vapor side) [51]. Hysteresis results from capillary collapse due to shrinkage during drying and from the difficulty of displacing gas molecules which had become adsorbed on the water-free surface [26]. The adsorption of moisture into the dried lignite in essence reverses the desorption process. There is first a gradual adsorption, then a very rapid increase in adsorption, followed by another gradual adsorption as the lignite approaches saturation. This behavior is typical of the adsorption isotherms of swelling gels [50]. The hydrogel or colloidal gel structural model of lignite seems generally agreed upon by investigators of moisture in lignite [55-57]. The first small amounts of moisture are adsorbed on surfaces. Further adsorption results in filling small capillaries. As more moisture is adsorbed larger capillaries will fill; this effect changes the radius of curvature of the water meniscus and hence increases the vapor pressure. In the very final stages the largest capillaries become filled until the lignite is fully saturated. The cumulative amount of moisture held within pores of various radii is shown in Table 8.3 [50]. The adsorption isotherm for brown coal, which shows the same general shape as that for lignite, shows monolayer adsorption from 0 to 20% relative vapor pressure, multi-layer adsorption in the range 20-50%, and then adsorption of water into the capillary structure above 50% [14].

399 TABLE 8.3 Moisture held in various pore radii [50] Radius interval, 10-7 cm

Cumulative moisture, %

>56.73 10.31-56.73 6.20-10.31 4.65--6.20 3.93-4.65 2.60-3.93 1.65--2.60 1.35-1.65 0.97-1.35 0.67--0.97 0.57-0.67 0.45-0.57 0.32-0.45 <0.32

14.8 38.5 44.3 47.7 49.4 54.0 61.8 66.6 75.9 81.8 85.9 89.6 92.5 100.0

If lignite did have a colloidal or gel structure, it would be expected that water retention would be governed by the sorption laws for gels. For example, the heat of vaporization of water from the gel should be close to that for vaporization of water from a plane surface. The heat of vaporization of water from freshly mined samples of five North Dakota and one Saskatchewan lignite agreed to within 7% of the value for vaporization from a surface [31]. The destruction of the gel structure should also bring with it a change in heat of vaporization. This too was verified experimentally. Thermal treatment of the same six lignites to 370~ in a rotary drum retort showed an average heat of vaporization of water from the heat-treated samples of 3.85 MJ/kg, compared with 2.44 MJ/kg for evaporation from a plane surface [31 ]. In the swollen gel structure of the lignite, movement of water through the lignite may involve some cooperative interaction between the water molecules and the lignite structure. In the swollen structure entanglements that would restrict the diffusion of the water molecules may be easier, energetically, to distort. At low moisture contents the adsorption and desorption curves are reversible, but above about 5% moisture a pronounced hysteresis effect is observed. The average results for Velva lignite are shown in Figure 8.7 [48]. The region of hysteresis in adsorption and desorption of water is greatest for lignite, and least for birch wood, with peat showing intermediate behavior [50]. (These data pertain only to these three materials.) The desorption ---, adsorption isotherms are reversible at moisture contents of 0-2.5% [50], indicating that adsorption is a true surface phenomenon in this moisture range. In North Dakota lignite, 50% of the bound water

400 100

2

~

8O

~

70 1

~

50"

'

1

40

1 Dehydration

30

2

2 Rehydration

~, 2o a~

lO 0

"'' 0

I .... I .... I'''' 10 20 30

I .... I .... 40 50 60

Water, g/100g dry lignite

Figure 8.7. Dehydration and rehydration isotherms for Velva lignite [48].

is held in pores less than 3.7 ~tm radius [50]. Hysteresis results from two effects of drying: shrinkage of the lignite with attendant collapse of capillaries, and the difficulty of rewetting the capillary surfaces because, in the dried lignite, those surfaces become covered with films of adsorbed gases [49]. If the capillaries that had collapsed during drying were part of the colloidal structure of the lignite, rehydrating the dried material will not restore fully the original moisture content. The radii of pores containing 60% of the bound water varies with temperature as <1.75 ~tm at 20~

<1.71 ~tm at 40~

<2.45 ~tm at

50~ and <1.40 for steam-dried lignite held at 20~ [49]. The sorption hysteresis in Balakhovskii (Russian) brown coal suggests a decrease in the total surface area accompanying the contraction of the capillaries as a result of drying [58]. The effect of temperature on hysteresis was studied by measuring the area between the adsorption and desorption curves. At 20~ the so-called hysteresis area was 34.31 cm 2 (of course, both the magnitude of the number and the units depend on the physical size of the plots, and, out of context, are meaningless; however, differences between the numbers are still instructive). The corresponding values at 40 ~ and 50 ~ were 20.12 and 14.25 cm2, respectively [26]. The basic characteristics of the sorption curves at 50~ are similar to data obtained at lower temperatures, but the region of hysteresis at 50~ is decidedly smaller than at lower temperatures. This observation indicates that the 50 ~ drying temperature has in some way altered the physical structure of the lignite [49].

401 Air drying of Australian lignite introduces in-eversible changes in luster and hardness [59]. Prior to air drying, the samples were soft and dull, but upon air drying became hard and brittle with a bright luster. The samples were believed to be at a state of coalification in which structural micelles were just touching, that is, had very small contact areas. Removal of water during air drying deformed the micelles, with a corresponding reduction of the spaces between them. Thus the contact areas, and hence the cohesive forces, between micelles were increased, increasing the hardness of the specimen. If the internal pressure generated by moisture upon its readsorption were less than the cohesive forces between deformed micelles (as was evidently the case) the structural changes accompanying drying would be reversible. Heating of lignite collapses the pores in all size ranges [49]. In the temperature range 2040~

the effect is minor. The proportion of pores of various sizes which do collapse is a direct

relationship to the distribution of pore sizes in the original lignite. At 50~ a disproportionately large number of the small pores suffer collapse, as shown by the data mentioned above: the radius of pores in which 60% of the moisture is held has increased considerably at 50~ compared to the virtually identical results at 20 and 40~

[49]. On the other hand, steam drying affects mainly the

larger pores. Steam drying causes the gel structure of the lignite to "set", meaning in essence that no further shrinkage occurs on heating beyond 50~ heated to 50~

[49]. By contrast, if freshly mined lignite is

the structure is not "set" and continued dehydration results in continued pore

collapse. The shrinkage behavior as a function of continuing dehydration is a significant factor in hysteresis. Steam-dried lignite had a hysteresis area of 0 at 20~

indicating destruction of the colloidal

nature of the solid by the steam drying process [26]. Although lignite dried with saturated steam at elevated pressures retains its lump form (e.g., see the discussion of the Fleissner process, Chapter 10), steam drying changes the colloidal structure of the lignite. In that regard, steam-dried lignite can be subsequently air-dried to a much lower moisture content than a freshly mined sample of lignite air-dried at similar conditions [49]. Steam-dried lignite shows essentially no hysteresis. The effect of the steam pressure used for the drying is essentially nil in the pressure range from 0.7-1.5 MPa, as indicated by the fact that the adsorption curves for lignites dried at various pressures in this range are virtually identical. Dielectric relaxation spectroscopy of hot-water-dried (at 340 ~ C) and untreated samples of Indian Head (North Dakota) lignite indicated a total loss of the lower temperature peak in the hotwater-dried sample [60]. When both samples had been brought to equilibrium moisture conditions, the lower temperature peak was still not observed in the hot-water-dried sample spectrum. This observation indicates that the chemically bound water contained in the untreated lignite sample was removed by the hot water drying process, and that reconstituting the sample to its equilibrium moisture condition did not replace the chemically bound water. It was not demonstrated whether the inability to resorb chemically bound moisture was a result of the destruction of functional groups to which the water might attach. Among four North Dakota lignites, the samples having the highest pore volume accessible

402 to nitrogen are those dried in a helium carrier gas atmosphere only to the extent that the moisture remaining in the lignite would not interfere with the detection of nitrogen desorption. Results of surface areas determined in nitrogen (BET method) as a function of drying method are summarized in Table 8.4 [52].Comparison of the adsorption and desorption branches of the nitrogen isotherms suggests little evidence for ink bottle pores.

TABLE 8.4 Nitrogen surface areas as a function of drying method, m2/g [52].

Beulah Freedom Gascoyne Glenharold

He Atmos. 4.61 3.71 4.51 3.93

0% RH*, 25~ 2.52 3.15 3.29 3.80

0% RH, 100~ 2.21 2.80 2.68 3.19

*Relative humidity

As the severity of the drying procedure increases, the nitrogen surface area decreases. Drying at 25~

for several months causes further slow pore collapse. The rate of pore collapse is

increased substantially at 100~

even for heating for only 30 minutes; but more extensive heating

(e.g., 200~ for 1 hour) causes no further pore collapse until pyrolysis begins around 350~

at

which point surface area increases. The effect on carbon dioxide surface area of heating Beulah lignite at 108~ is that after 12 h the surface area was 139 m2/g, which dropped to 109 m2/g after 36 h [52]. Dehydration of lignite at 105~ and subsequent rehydration shows a gradual decrease of the rehydrated weight after successive cycles [61]. The dehydrated weight does not change. These results show that the gradual decline in the ability of lignite to adsorb water is not a consequence of loss of mass, but rather a result either of a change in surface chemical properties or an alteration of the physical structure which makes water penetration more difficult. However, a surface chemical change would have to involve no significant loss or gain of mass. A further argument against the idea that a surface chemical change is involved is that heating dried lignite in liquid water at 100~ for 24 h restored its ability to readsorb water. Moisture reabsorption in steam and hot water dried lignite as a function of time shows that drying temperatures above 270~ are required to alter the lignite structure to prevent substantial moisture reabsorption during extended storage [62]. This temperature is of interest in comparison with a reordering of the micropore structure observed to occur between 225 and 313~

[63] and the

onset of thermal decarboxylation of the lignite structure, which begins around 30(0350 ~ [64]. Thus the reabsorption of moisture may be prevented by loss of a major oxygen functional group capable of hydrogen bonding to water molecules, or by changes in the lignite pore structure, or both.

403 The shrinkage that accompanies drying is not equal in rate or extent in different directions, nor is there a clear relation between the shrinkage and the orientation relative to the bedding plane. For example, Gascoyne lignite equilibrated at 33% relative humidity showed shrinkage parallel to the bedding plane to be greater than perpendicular to the bedding plane; but at 52% relative humidity shrinkage perpendicular to the bedding plane is greater than that parallel to the plane [52]. Cracking accompanying shrinkage generally began in directions parallel to the bedding plane before proceeding across bedding planes. The volumetric shrinkage upon drying and the volume of moisture lost correlate well [52]. Thus the pore structure collapses as water is removed, rather than creating substantial amounts of open porosity. In this respect the lignites differ from the behavior of Australian brown coals. For a range of coal ranks, the volumetric shrinkage on drying can be related to the moisture loss by the equation % (volumetric shrinkage) = 0.94 (wt. % moisture loss) - 0.6 [65]. Lignites swell by about 30% in water, and can double in volume in pyridine [65]. By comparing the swelling behavior of wet and dry Texas lignites in pyridine, the rate of swelling of the wet lignite is considerably higher than that of the dry lignite. (However, if the equilibrium swelling ratio--not swelling rate--of the wet lignite is corrected to account for the fact that the wet lignite is already partially swollen by the water, the swelling ratio achieved with wet or dry lignite is the same.) Similar results have been achieved for swelling in tetrahydrofuran and in experiments with Beulah lignite. The more swollen a coal is to begin with, the less time is required to attain a given amount of additional swelling. The effects of relative humidity, time, and temperature on the shrinkage of Beulah lignite are illustrated in Figure 8.8 [52]. Both the extent and rate of shrinkage are greater at higher temperatures. At very low relative humidity the equilibrium shrinkage values are approached more quickly than at high relative humidities, indicating that shrinkage rates are not linear functions of relative humidity. Because the water in lignite is an integral part of the physical structure, removal of moisture from lumps of lignite will eventually result in destruction of the lumps. A hypothesis explaining this behavior as a consequence of coalification has been advanced [49]. Coalification involves destruction of the tracheids and middle lamellae of the wood, as well as destruction of some of the cementing material in the cell walls. As a result, in lignite the orderly arrangement of cells in the wood has been lost and has been replaced by an arrangement that is essentially chaotic. Consequently, the loss of moisture will necessarily result in uneven shrinkage, which in turn sets up unequal forces that eventually are strong enough to shatter the lumps. In this concept, the drying of lignite in such a way as to retain the physical structure can be done only when moisture is removed in some fashion which prevents the establishment of unequal forces, or when the drying

404 16

1 1 day

14 1

.~

12

~

2 2days ~

3 5 days

.~1

4 8 days ,,

ys

420 0

10

20

30

40 50 60 70 Relative humidity, percent

80

90

100

Figure 8.8. The shrinkage of Beulah lignite at 40"C as functions of relative humidity and time [52].

process modifies the structure so as to prevent uneven shrinkage in any given direction. The best approach for accomplishing this would be controlled humidity drying at high temperature. The successful drying of lump lignite, without significant size degradation, would require fulfilling two criteria: 1) achieving a very gradual removal of moisture by slowing the rate of evaporation from the surface, and 2) increasing the rate of transport of moisture from the interior of the particle to the surface [49]. Although moisture accelerates the rate of swelling in pyridine, the activation energy for swelling is also increased in pyridine [66]. Heating lignites in the range 100-300~

causes

irreversible pore collapse and crosslinking [65,67]. Increasing the severity of this thermal treatment increases the crosslinking, which in turn decreases the activation energy for diffusion and swelling. For example, in the swelling of Beulah lignite by pyridine, drying at 100~ before swelling produces an equilibrium swelling ratio of 1.86, the activation energy for this process being 74.1 kJ/mol [66]. However, if the drying temperature is increased to 300~

the swelling

ratio decreases to 1.70 and the activation energy decreases to 58.2 kJ/mol [66]. Thus the increase in activation energy seen for wet lignites arises because the water functions to decrease the glassy nature (i.e., raise the glass transition temperature) of the macromolecular lignite structure. With highly crosslinked, rigid (network) macromolecular structures, a greater proportion of the activation energy is concentrated in a smaller number of degrees of freedom. An increase in the activation energy accompanying a greater amount of water being already present in the structure

(i.e., indicating, in effect, a greater degree of "pre-swelling") reflects the fact that the portions of

405 the macromolecular structure in the vicinity of the diffusing solvent molecules are more and more mobile the more the structure is swollen, and the greater the mobility, the greater the amount of thermal energy will be required to attain the less probable macromolecular configurations that are necessary to allow further swelling to take place [66]. Since water swells lignites, and since the presence of water in lignite allows faster swelling by organic solvents, it is crucial to report the moisture content of the lignite sample actually used for any swelling experiments [66].

8.6 H E A T OF W E T T I N G IN WATER

The applications of heat of wetting for exploring the pore structure of lignite have been discussed in Chapter 7. The role of heat of wetting of stockpiled lignite in autogenous heating or combustion is discussed in Chapter 10. Studies of well-characterized carbons have led to the general relationship that the heat of wetting is 2.56 m2/J [68]. This relationship assumes that the surface is free of oxidation. Immersion of an oxidized surface in water could result in a substantially higher heat effect because of the added heat liberated by hydrogen bond formation between water molecules and the oxygen functional groups on the surface. Furthermore, a lignite, even if it has not been oxidized unduly, will nevertheless have substantial numbers of oxygen functional groups on its surface because of its naturally high oxygen content. Consequently the heat of wetting for lignite in water is probably closer to 1.2 m2/J [69]. The heat of wetting, as a function of moisture content, of an unidentified lignite decreases from 109 J/g from lignite of supposedly 0.0% moisture to 2.3 J/g for lignite of 21.3% moisture [26]. The variation of heat of wetting with moisture content is shown in Figure 8.9 using data for North Dakota lignites from several laboratories [26,69-71]. The heat of wetting is independent of particle size below 20 mesh [69]. The heat of wetting for completely dry lignite is about 105 J/g [26,69]. Australian and North American low-rank coals show a correlation between the amount of carboxyl groups and the wettability [72].Comparison of several ranks of coal and a peat sample shows that lignite had the highest heat of wetting of any of the samples when compared at the same initial moisture content [26].

406 30.

20 Ix0

15"t~ 10b-i O

5-

.

z 0

. . . .

0

I

5

. . . .

I

10

. . . .

I

15

. . . .

!

20

. . . .

25

Lignite moisture content, percent

Figure 8.9. Heat of wetting as a function of moisture content for a North Dakota lignite [26, 69-71].

REFERENCES

10 11 12 13

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408 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

O.P. Mahajan, Coal porosity, in: R.A. Meyers (Ed.), Coal Structure, Academic Press, New York, 1982, Chapter 3. W.B. Hauserman and R.M. Neumann, Physical properties and moisture, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1531, (1984), pp. 16-1- 16-30. A.E. Wroblewski and J.G. Verkade, Moisture release from Argonne premium coal samples. A quantitative 31p NMR spectroscopic study. Energy Fuels, 6 (1992) 331-335. C. Tye and H.D. Bale, Physical properties and moisture, University of North Dakota Energy Research Center monthly report, December 1984. E.A. Sondreal, W.G. Willson, and V.I. Stenberg, Mechanisms leading to process improvements in lignite liquefaction using CO and H2S, Fuel, 61 (1982) 925-938. H.H. Schobert, Characteristics of low-rank coals important in their utilization, Short Course on Coal Characterization, The Pennsylvania State University, November 1982. L. Pauling, The Nature of the Chemical Bond, Cornell, Ithaca NY, 1960, pp. 449ff. G. Urry, Elementary Equilibrium Chemistry of Carbon, Wiley, New York, 1989. H. Kugami, K. Nakamoto, M. Sasaki, Y. Yoneda, and Y. Sonada, Computer simulation of the conformation of lignite molecule with water, Proceedings 1993 International Conference on Coal Science, Vol. I, pp. 136-139. I. Lavine and A.W. Gauger, Studies in the development of Dakota lignite. I. Aqueous tension of the moisture in lignite, Ind. Eng. Chem.~ 22 (1930) 1226-1231. M. Gordon, I. Lavine, and L.C. Harrington, Studies in the development of Dakota lignite. VII. Effect of temperature and pressure on sorption of water vapor by lignite, Ind. Eng. Chem., 24 (1932) 928-932. M. Larian, I. Lavine, C.A. Mann, and A.W. Gauger. Sorption of water vapor by lignite, peat, and wood, Ind. Eng. Chem. 22 (1930) 1231-1234. I. Lavine, Progress in low-rank coals, Ind. Eng. Chem., 26 (1934) 154-164. S.C. Deevi and E.M. Suuberg, Changes in the physical characteristics of lignites as a result of drying and heating, in: M.L. Jones (Ed.), Technology and Utilization of Low-rank Coal, U.S. Dept. Energy Rept. DOE/METC-86/6036(Vol.2), (1986), pp. 543-558. T.B. Irwin, A.M. Grosset, and J.S. Sood, Modeling of gas sorption kinetics in coal, U.S. Dept. of Energy Rept. ORNL/MIT-305, ( 1981). C. Tye and H.D. Bale, Physical properties and moisture, University of North Dakota Energy Research Center monthly report, February 1985. X. Young, A.R. Garcia, J.W. Larsen, and B.G. Silbernagel, Moisture determination and structure investigation of native and dried Argonne premium coals. A 1H solid-state NMR relaxation study, Energy Fuels, 6 (1992) 651-655. E.M. Suuberg, Y. Otake, Y. Yun, and S.C. Deevi, Role of moisture in coal structure and the effects of drying upon the accessibility of the coal structure, Energy Fuels, 7 (1993) 384-392. E.M. Suuberg, Y. Otake, S.C. Deevi, and Y. Yun, The role of moisture in the macromolecular structure of coals, Proceedings 1991 International Conference on Coal Science, pp. 36-39. A.A. Agroskin, V.I. Lyashchenko, and I.E. Svyatets, Sorption capacity of brown coals, Solid Fuel Chem., 11(5) (1977) 18-22. J.A. Dulhunty, Some effects of compression on the physical properties of low-rank coal, J. Proc. Roy. Soc. New South Wales, 83 (1949) 265-271. C. Tye, Physical properties and moisture, University of North Dakota Energy Research Center monthly report, April 1985. C.G. Pope, Lignite porosity study by kinetics of gas adsorption, Fuel, 63 (1984) 16811686. G.G. Baker, D.J. Maas, T.A. Potas, and R.E. Sears, Coal-water slurry preparation, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/601811531, (1984), pp. 4-1 - 4-22. M.L. Carlson, Small angle x-ray scattering from lignite coal, M.S. Thesis, University of North Dakota, Grand Forks, ND, 1983. J.B. Goodman, M. Gomez, V.F. Parry, and W.S. Landers, Low-temperature carbonization assay of coal in a precision laboratory apparatus, U.S. Bur. Mines Bull. 530, (1953).

409 65 66 67 68 69 70 71 72

E.M. Suuberg, Y. Otake, and S.C. Deevi, The macromolecular structure of coal--its relationship to diffusion and reaction processes in coals, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33 (1988) 387-394. Y. Otake and E.M. Suuberg, The effect of moisture on the diffusion of organic molecules in coal, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33 (1988) 898-905. S.C. Deevi and E.M. Suuberg, Physical changes accompanying drying of western U.S. lignites, Fuel, 66 (1987) 454-460. I.G.C. Dryden, Chemical constitution and reactions of coal, in: H.H. Lowry (Ed.), Chemistry of Coal Utilization, Supplementary Volume, Wiley, New York, 1963, pp. 272282. E.A. Sondreal and R.C. Ellman, Laboratory determination of factors affecting storage of North Dakota lignite, U.S. Bur. Mines Rept. Invest. 7887, (1974). H.C. Porter and O.C. Ralston, Some properties of water in coal, U.S. Bur. Mines Tech. Paper 113, (1916). H.M. Scholberg, The heat of hydrolysis of methyl and ethyl acetate and the heat of wetting of lignite, M.S. Thesis, University of North Dakota, Grand Forks, N.D., 1933. T. Murata, The effect of carboxyl oxygen in coal on the pelletizability of pulverized coal by oil agglomeration (VII): a study on the wetting of coal, Nippon Kogyo Kaishi, 99 (1983) 911-914.

410

Chapter 9

MINING, T R A N S P O R T A T I O N , AND STORAGE

9.1 L I G N I T E MINING 9.1.1 Historical development of lignite mining Lignite was used in Texas as early as 1850 [1]. In the late nineteenth and early twentieth centuries, lignite was a major contributor to the energy economy of Texas. In a quarter century lignite production increased from 13,600 tonnes per year to a high of 1.1 Mt per year in 1913 [1]. Production then remained reasonably constant, approximately a million tonnes per year, until the 1930's. Continued development of cheap natural gas and petroleum eventually resulted in a precipitous decline of lignite-fired steam generating units from the early 1900% through 1950. A mine was opened at Darco in 1931 to produce lignite for activated carbon production [2]. The first mine of the modern era was opened in 1954 by Alcoa to provide lignite for power generation for aluminum production. By 1976 production of lignite had risen nearly a thousand-fold from its low point in 1950, to 12.9 Mt [1]. In 1950 there was a single lignite mine in Texas, producing 16,483 tonnes of lignite from seams averaging 3.7 m thickness [3]. The average overburden thickness was 7.6 m, requiring the removal of 2.6 m3 per tonne of lignite mined. Thirty years later there were seven active mines with an annual production of 27 Mt [4]. Lignite mining began in the northern Great Plains in the late 1800's. Commercial production of lignite began in the 1870's, with the first recorded production occurring in 1884, although mining by individuals for domestic use had no doubt been practiced for years before commercial mining began [5]. The first commercial mining of North Dakota lignite, in 1884, was from a mine operated by the Northern Pacific railroad [6]. The output of lignite in that year was 32,000 tonnes [6]. In 1888 mines were opened at New Salem and Dickinson, and by 1894 there were eight mines in operation [6]. Production was stimulated by legislation in 1896 requiring State institutions to obtain their fuel supply from North Dakota mines. Production reached 90,000 tonnes in 1899 [6]. An annual production of 0.9 Mt was achieved in 1920 and 1.8 Mt by 1940 [5,7]. After another quarter century the production reached 2.7 Mt per year (i.e., by the mid 60's), but only ten years later the annual production reached 9.3 Mt. Lignite mining in North Dakota was started by the early settlers, who obtained lignite from outcrops for domestic use. The Native Americans may have used lignite on a very small scale for centuries before the settling of North Dakota, and it may have been the Native Americans who

411 showed the settlers the locations of the lignite outcrops [5]. Lignite outcrops in many parts of the state, and in many of those places would be the only large-scale fuel supply. A century ago, surface mining was performed mainly by hand, or using horse-drawn equipment, and excavations were very limited in depth. Consequently, much early commercial mining was done underground, usually by following seams originally exposed at the surface. A seam was followed until it pinched out or became uneconomical to mine. Once steam- or electric-powered mining equipment became available, surface mining became much more efficient than underground mining, and in a relatively short time surface mining came to dominate lignite mining in North Dakota. In the ten-year span from 1923 to 1933, the percentage of lignite mined by stripping rose from 24% to 61% [5]. Mining began in exposures of lignite along the banks of coulees. A drift could be driven into the exposed seam and the seam then worked in a direction away from the stream. As true of any early underground coal mining, the size of the mine was limited by two factors: the availability of timber for roof supports, and the mechanical strength of the roof rocks. Timber is in exceptionally short supply in North Dakota, and, to make matters worse, both the roof and floor rocks are generally quite soft and unconsolidated. Miners often had to leave a considerable amount of lignite as the roof and floor of the mine (in some cases, 60 cm of lignite for both roof and floor [5]) as well as large pillars. Recovery was severely reduced under such circumstances. Despite that, primitive early surface mining techniques (by human or horse power) were so inefficient that underground mining was generally preferred. Development of surface mining technology led to rapid increases in its importance in the North Dakota lignite industry. Indeed, North Dakota was a pioneer in the widespread application of surface mining. In 1927, 40% of the lignite mined in North Dakota was obtained by surface mining, whereas only 2% of the total coal production throughout the United States was from surface mines [5]. By 1950, the comparable figures were 87% in North Dakota but only 22% nationwide. In 1950, 37 surface mines were producing 2.6 Mt of lignite per year. The seams being stripped had an average thickness of 2.9 m. In addition, seven underground mines contributed 0.4 Mt per year from seams averaging 3.1 m thick [3]. In the surface mines the average overburden thickness was 11.6 m, requiring the excavation of 4.7 m3 of overburden per tonne of lignite mined

[31. Much of the initial commercial production was for domestic use, with the eventual development of lignite as fuel for power generation as towns grew and spread with the increasing settlement of the area. A period of stagnation in lignite mining occurred from 1950 into the mid1960's [5], in concert with the general situation in the coal industry in the United States. In North Dakota, the stagnation in the lignite industry was due to three factors: development of hydroelectric power from the Missouri River, continued penetration of fuel markets by natural gas, and conversion of railroads from steam to diesel-electric locomotives. The late 1960's marked the emergence of large, lignite-fired power plants, the application which now dominates the lignite industry in North Dakota. Production passed 3.6 Mt/y in 1967 and surpassed 6.4 Mt/y only six years later [5]. If a major lignite gasification industry should ever develop in North Dakota, the

412 impact on the mining industry would be enormous. The plant originally proposed by American Natural Gas, and which today exists in a much more modest version (Chapter 12), would, according to the initial design, have consumed lignite equivalent to the annual production of the state, and four plants of similar size would have an annual consumption equivalent to one-fourth of the total of all the lignite ever mined in North Dakota since 1884 [5]. Other uses of lignite in North Dakota include the manufacture of "charcoal" cooking briquettes, process heat

(e.g.,

supplying

evaporative heating in sugar refineries), and occasional domestic use. In the 1960's some North Dakota lignite was mined for its uranium content. The first observations of lignite in South Dakota are attributed a geologist attached to an expedition into the Black Hills led by Colonel George A. Custer in 1874 [6]. Lignite has been mined on a small scale for fuel for ranches and small communities ever since the settlement of the state. Production has always been very small, and the amount of coal used in South Dakota but brought in from other states has always exceeded the amount of South Dakota lignite mined. 9.1.2 Overview of surface mining The first stage of locating and exploiting a new lignite deposit is an appraisal of the geologic features of the region of interest. That would be followed by detailed field investigations of the sections containing lignite seams, with reconnaissance drilling to determine the seam thickness and the stratigraphy of the strata in which the seam is found. Usually at this stage proximate and ultimate analyses and calorific value determinations from outcrop samples or drill cuttings will be made. If the results of these reconnaissance activities are favorable, a surface investigation of the mining area will be conducted for detailed mapping of the outcrops and stream sediments. The final stage of activity will involve a detailed three-dimensional survey of the proposed mine area. This stage includes drilling and logging, with down-hole geophysical surveying as appropriate, analyses of core samples or drill cuttings, and the opening of a test pit to obtain bulk samples for larger scale evaluation of the behavior of the lignite in its intended use. This information can be used to compute the reserves and make a valuation of the deposit. Before proceeding with the opening of the mine, local water problems will be evaluated to determine the availability of water supplies or their possible disruption by mining (if, for example, the lignite seam is an aquifer). The time elapsed from initial detailed planning to full production usually amounts to three to five years, depending on size [8]. In some cases, up to seven years may be required. Another factor affecting the time needed for development is the procurement and erection times for major equipment items. For example, dragline erection may require a year. The cost per tonne of lignite can be affected by inflation occurring during the long equipment delivery and erection times, as well as by delays occasioned by the regulatory and permitting process. Although the underground mining of lignite has been practiced in the past, it is now considered that lignite can be mined only by surface techniques [9]. A useful compilation of information on surface mining in the Northern Great Plains has been published by the U.S.

413 Environmental Protection Agency [ 10]. This publication tabulates, for 21 mines, such information as the stratigraphic data on the coal, average coal analyses, and an inventory of the major operating equipment, supplemented by color aerial photographs of the mining operations. The types of mining practice employed are contour strip mining and area stripping. As the name implies, contour strip mining follows the contour of the local terrain. In area stripping, the pit advances down dip. As a result, the overburden tends to become thicker as mining advances. The advantages of this method are that different types of mining equipment (presuming they are available) can be used simultaneously in different locations along the cut, and that the spoil from one advance can be piled in the mined-out section of the previous advance [ 11,12]. Area mining is the major mining method in Texas. Mining is done by dragline sidecasting or by removing overburden with scrapers. The average stripping ratios are less than 7 m3 per tonne of lignite, the maximum being about 13 m3 [13]. Area mining removes the topsoil and overburden in box cuts, which typically are strips about 1.6 km long and 30 m wide[14]. Area mining is best applied to situations of shallow seams and fairly fiat terrain. The mining of deeper seams or multiple seams involves open pit mining, which begins with the excavation of larger size strips up to 300 m wide [14]. In open pit mining the overburden is removed in benches. Blasting may be employed to loosen the overburden, lignite, or both. In virtually all mining operations the topsoil and overburden will be kept separate, with exceptions where local soil and rainfall conditions allow revegetation of the reclaimed mined land without separate handling of the topsoil. Reclamation involves first replacing and grading the overburden to achieve a topography similar to that before mining, then replacing the topsoil and revegetating the area. Except for regions of favorable rainfall and soil conditions in Texas, topsoil must normally be separated from the overburden and stored separately for later reclamation. Topsoil is usually removed by bulldozers. Several kinds of equipment can be used for removal of overburden; the choice depends on the geological type of overburden and its thickness; the size, shape and thickness of the lignite seam and the production rate; and requirements for reclamation. Draglines have a great deal of flexibility and range in handling overburden and moving it around the mining area. Compared to power shovels, draglines can handle much deeper overburden, and can dig a much deeper cut. Furthermore, draglines often have a lower initial cost per unit bucket capacity, and have better operating times and lower maintenance costs than shovels. In most operations the dragline will operate ahead of the mine cut rather than in the pit. This characteristic allows greater flexibility of siting the dragline for mining operations. In comparison, power shovels operate sitting on the lignite itself. This factor can reduce costs for development of roadways in the mine. Shovels require less preparation of the surface and highwall than when draglines are used, and correspondingly lower costs for drilling and blasting. The costs of both power and rope per unit bucket capacity will be lower for shovels than for draglines. Shovels can usually handle larger single pieces of material than draglines. Bucket wheel excavators provide essentially continuous excavation of the highwall and can move the spoils a much greater distance than either a shovel or

414 dragline. This ability in placing the spoils may help reduce reclamation costs. However, the effective use of bucket wheel excavators requires relatively unconsolidated highwall material. The use of highly mobile crawler tractors and front end loaders allows moving the overburden any desired distance, which provides the opportunity of doing reclamation simultaneously with mining. These small units, being diesel powered, do not require the power distribution system necessary for the large draglines, shovels, or bucket wheel excavators. Although the crawlers and front end loaders do offer some advantages of flexibility, their use as primary mining equipment seems to be restricted to smaller mines having low annual production. Larger mines rely on draglines, shovels, or a combination of the two. Bucket capacities of these units have reached 170 m3 for draglines and 140 m3 for shovels [14]. In favorable cases, bucket wheel excavators can be used in conjunction with other equipment to remove the unconsolidated upper portion of the overburden. Some of the major U. S. lignite mines are summarized in Table 9.1 [14,15]. This table is intended to be illustrative rather than an all-inclusive listing; up-to-date information on mines and their production is available in standard reference sources [ 15]. TABLE 9.1 Selected production data for major lignite mines in the United States.

State Montana North Dakota

Texas

Company Knife River BNI Coteau Fal kirk Knife River Knife River San Miguel Texas Utilities Texas Utilities Texas Utilities

Mine Name Savage Center Freedom Falkirk Beulah Gascoyne San Miguel Big Brown Martin Lake Monticello

Production, tonnes 1978 [14] 1991-92 [151 272,161 246,453 3,084,480 3,906,421 12,409,902 129,102 6,291,469 1,712,129 1,947,372 2,605,332 2,098,252 2,790,485 4,806,604 4,867,128 5,417,572 11,640,283 6,319,980 9,655,330

The mining cost per tonne of lignite for surface mining is generally low. The productivity per man day is about 23-27 t [14], which is high compared to underground mining. With reasonable planning in advance of mining and careful handling of overburden and topsoil during mining, the costs of reclamation are not exorbitant. These advantages of surface mining relative to underground mining are offset by the very high initial capital costs needed for the surface mining equipment. These costs keep small operators out of the business, and corresponding economies of scale are achieved by using larger and faster equipment in larger mines. In the late 1970's, 22 of the 25 largest coal mines in the United States were lignite or subbituminous surface mines in the west [ 14].

415 In comparison with the mining of bituminous coal in the eastern United States, the surface mining of lignite offers a number of economic advantages. The Coal Mine Health and Safety Act of 1969 was estimated to have increased the costs of underground mining by $2.30 to $2.70 per tonne, but had a negligible impact on surface mining in the west [16]. As a rule, surface mining is capital intensive rather than being labor intensive. In underground mining of bituminous coal 50-60% of the cost of the coal is labor costs, while the comparable figure for surface mining of lignite is 20-35% [16]. As a result, increases in wages and benefits for the work force have roughly half the impact on the coal cost for a surface mine as for an underground mine. The productivity of an underground bituminous coal mine may be 7-14 tonnes/man-day, whereas a lignite strip mine may have a productivity of 90-118 tonnes/man-day [16]. At a time when lignite was mined both by surface and underground mines in North Dakota (1950), the comparative productivities were 24.9 tonnes/man-day for surface mines and 9.1 tonnes/man day for underground mines [3]. 9.1.3 Preliminary steps in opening a mine One of the difficulties in opening a mine is successfully locating the first strip pit to obtain commercial quality lignite. Lignite at the outcrop has often degraded and may be of very high moisture and ash content, and consequently low calorific value. This low quality lignite can persist for up to a half mile from the outcrop toward the center of the deposit [14]. Delineating the point at which mining should begin is accomplished by drilling test holes from the outcrop toward the center of the bed (the limits of the bed itself having first been determined by drilling), with analyses of the drill cuttings. The point at which the analyses indicate lignite of satisfactory quality becomes the boundary of the commercial lignite. The first major operation at a mine is the removal of soil. Conventional earth-moving equipment such as tractors and scrapers is used. When the topsoil is to be saved for reclamation efforts, it is removed and segregated using scrapers or front-end loaders. Otherwise, vegetation and topsoil may be removed by a dozer to provide access to the overburden, or in some cases the vegetation and topsoil will actually be removed by the dragline. If the mine is to be worked with a dragline, a flat bench must be created as a platform of sorts on which the dragline can operate. The bench will follow some route, not necessarily straight, which may be determined by any of several factors including a constant depth of overburden, the quality of lignite to be mined, necessary haul routes, or ownership of the land [ 10]. Diversion dams and ditches may be created to help prevent runoff water from reaching the pit. Pre-stripping operations rely on bulldozers or scrapers to prepare the ground for the walking draglines. Stripping will start with a box cut along the outcrop of the lignite below any oxidized material (for example, in Texas generally this depth is 4-8 m) [8,17]. Subsequent pits are parallel to the box cut and proceed down dip into deeper overburden. (Typically Texas lignite dips at 1-2 ~ [8].) The most common method of mining is dragline sidecasting [18]. The spoil is placed in the previous cut where the lignite had been removed. Then successive strips are made down dip

416 into deeper cover. Initially the pits may be 50-55 m wide, narrowing to 3 6 m at the maximum depth of overburden [17]. The width is a function of the depth, but 36 m is about the minimum which can accommodate the movement of equipment in and out of the pit. The dragline can usually remove overburden without drilling and blasting. The pit lengths are 1-3.2 km. The highwall slope is 60-70 ~. 9.1.4 Overburden removal The stripping ratio (expressed as cubic meters of overburden removed per tonne of lignite) increases as mining progresses until some maximum depth is reached. The maximum depth depends both on the types of mining equipment being used as well as the local mining conditions. Normally the maximum depth is 46 m, although multiple seam mines may reach 76 m [8]. The average stripping ratios range from 3.8 to 10 m3/t. The overburden removal cost per tonne of lignite is directly related to the stripping ratio. The higher the stripping ratio, the higher the cost per tonne, because overburden removal represents a significant percentage of the direct mining costs. The primary concerns for removing and handling the overburden are the need to minimize or avoid rehandling, having a system that can cope with changing overburden conditions, and simplifying the reshaping of contours for reclamation. Overburden removal and handling is simple when a single seam is being mined. In the case of lignite with a parting, or a mine with multiple seams, the operations are necessarily more complex. In mines in which more than one seam is being worked an around-the-pit conveyor system is used. This system allows the spoil to be reclaimed in the same sequence as it was removed without the necessity of rehandling it. The overburden thickness hasno evident relationship to the quality of the lignite beneath. Sometimes a thin bed of highly weathered, and often high ash, lignite will be found in the overburden. This material is called "slack" and is discarded. In places the lignite may also be overlain by leonardite, a soft, brown, earthy material completely soluble in aqueous sodium hydroxide. Unfortunately, the terms leonardite and slack are not used unambiguously in the literature; leonardite having occasionally been taken to refer to the alkali-soluble portion of slack or to be synonymous with slack. (Leonardite has been mined from the Harmon bed, North Dakota; its commercial applications are discussed briefly in Chapter 12.) (i) Draglines. A dragline is often used for removal of the overburden. In operation, the dragline pulls a bucket that fills as it is dragged across the overburden. (This operation is in contrast to a power shovel, which must fill its bucket by undercutting; i.e., digging first out and then up.) The draglines are usually characterized by two parameters: the capacity of the bucket and the radius within which the dragline can excavate and dump, the so-called throwing radius. The throwing radius is a function of the length of the boom; since the boom must provide support to the bucket, the length is related to the bucket capacity. The length of the boom will also determine the depth to which the dragline can operate. A longer boom also may permit a more selective placement of the spoils, and hence allow segregation of various types of soil for later use in reclamation. The characteristics of the overburden also influence dragline design, since a hard overburden requires a

417 reduction in boom length [10]. Draglines have the flexibility to handle materials of various characteristics, including the hard soil or rock found in the overburden. A second advantage of draglines is that usually the operating costs, including maintenance, are low, although the capital cost can be much higher than the more common trucks and power shovels. Draglines also provide some capability for segregating materials to achieve a degree of selective mining. A dragline is necessarily limited in the distance to which it is able to move either the overburden or the lignite. Walking draglines have been the most successful machines for overburden removal [ 17]. Overburden is removed by appropriate equipment, which may involve bulldozers, scrapers, power shovels, draglines, or power excavators. Generally if the overburden is less than 12 m thick power shovels or draglines are used. An example is the Big Brown mine at Fairfield, Texas. Here the overburden is typically 12-15 m, and is removed by two draglines which have 54 m3 buckets [11,12]. When the overburden is deeper, a combination of several types of equipment might be used. Soft or fragmented overburden, combined with the gentle topography of the Gulf Coast lignite deposits, contributed to the widespread use of draglines in Texas lignite mining operations [19]. Some of the draglines exceeded 150 m3 bucket capacity and 90 m boom lengths [19]. The advantages of draglines, relative to shovel-and-truck operations, are that draglines have a deep excavation capacity, the ability to dump over the area to be reclaimed, ability for selective digging and dumping [ 19]. Draglines are effective to an average overburden depth of about 24 m [19]. However, some mining operations eventually extend beyond the reach of draglines, even with the longest boom lengths. The overburden in North Dakota mines is generally less than 36 m thick. Any topsoil suitable for plant growth (in North Dakota, this amounts to a maximum of 1.5 m [7]) is removed using scrapers and stockpiled for reclamation. The overburden is removed by a dragline positioned atop a level bench, and is placed downhill from the cut. When the first strip has been finished, the dragline is then moved to begin a second strip parallel to the first. The overburden from the second strip is placed in the previous strip, with an analogous process used for the third and further strips. On removal, the soil expands about 20%. The appearance of the operation is that of a series of peaked hills which mark the filled strips. Eventually the peaks are smoothed by bulldozers. Usually the overburden is removed without blasting. The draglines have bucket sizes exceeding 23 m3 [201. Most North Dakota operations use draglines to remove the overburden and power shovels to load the lignite. Prior to the enactment of the first mined-land reclamation law in 1970, the practice was to deposit the overburden in spoil ridges or spoil piles such that the top layers (including the topsoil) were put into the bottom of an adjacent pit, with the deeper overburden subsequently covering the topsoil. This practice generated a sequence of alternating ridges and valleys, with the valuable topsoil usually being buried beneath the ridges. The ridges and valleys were usually parallel, occasionally arranged concentrically around a box-cut pit, which sometimes

418 wound up containing a pond. Direct casting is generally preferred when practical, particularly dragline sidecasting in area mining. Although some overburden of Gulf Coast lignites is amenable to removal by bucket wheel excavators, draglines have the advantage of lower capital and operating costs. Furthermore, the costs for a dragline may be also be favorable in comparison with smaller units of equipment (such as scrapers) which may have to be replaced more frequently during the lifetime of a mine. The limitations on the dragline are the thickness of the overburden and the fact that draglines are difficult to adapt to changing mining conditions. Given the reach of the dragline and the width of the cut, the face height (or mine depth) that can be obtained depends on the highwall stability and on the spoil angle. If the highwall is not stable, the dragline operator must keep back some distance from the edge; as the spoil angle becomes flatter, the height of the spoil pile must be reduced to avoid reburying the lignite. Both considerations may limit the effective reach of the boom, and the depth of the mine may in turn be less than would be forecast solely from consideration of the economic stripping ratio. A secondary consideration may be the compressive and shear strengths of the overburden, since draglines have sole pressures on the order of 0.10-0.12 MPa [13]. However, this consideration has not limited the use of draglines in Texas.The largest draglines used in the Gulf Coast have a 110 m boom [13]. These units are limited to a maximum mining depth of 38 m if the overburden is not to be reworked [13]. Benching and rehandling the overburden could allow these draglines to mine to 49 m [13]. Rehandling the overburden will necessarily increase mining costs. The dragline is not efficient in mining multiple seams. In such cases, draglines work best in mining only two seams (or in some cases three seams if the lower two are close to each other but are separated from the upper seam by at least 10 m of interburden) [13]. Multiseam mining can be accomplished with draglines working in tandem. The dragline on the highwall side (usually this will be the larger of the two draglines) removes overburden to expose the first seam. The interburden is removed from the spoil side to expose the second (deeper) seam. Draglines provide the lowest mining cost per cubic meter of overburden to be excavated and piled as spoils. The lowest mining costs are achieved by using a dragline to remove as much of the overburden as possible and to minimize rehandling the overburden. (ii) Bucket wheel excavators. A bucket wheel excavator consists of a series of buckets arranged around the circumference of a large diameter wheel. The bucket wheel digs into unconsolidated material and dumps the excavated material onto a conveyor system for transfer or loading. Bucket wheel excavators do not work well in hard material or in material containing boulders. Attempts to use a bucket wheel excavator in the Glenharold mine (North Dakota) were unsuccessful [10]. Bucket wheel excavators have been used successfully in lignite mining in Turkey [21] and in India [22]. They were introduced in the Big Brown mine to handle greater thicknesses of overburden than could be removed with walking draglines and to accommodate recovery of lignite from multiple seams [ 17]. New developments in bucket wheel excavator technology make these machines

419 increasingly attractive compared to earlier versions. Despite a long-standing interest in continuous excavation, early bucket wheel excavators suffered the disadvantages of their difficulties in handling particularly hard coal (or rock), high maintenance costs, and the general lack of flexibility due to their enormous weight and size. However, a unit designed for use in Texas lignite mining has a bucket wheel 7.7 m in diameter, comparable to the once-standard 8.5 m, yet weighs only onetenth as much as the earlier machines [23]. The unit, with its supporting conveyors and transporters, is planned to be able to strip 66,200 m3/day [23]. In comparison to draglines, bucket wheel excavators are better limited to relatively soft overburden and thick seams lignite and less severe mining conditions. The complicated machinery of a bucket wheel excavator means that it will have a very high capital cost and, likely, high maintenance, including preventative maintenance. In operation a bucket wheel excavator can provide a high throughput because it usually operates continuously. Making optimum use of a bucket wheel excavator would require more detailed mine exploration and planning of mining operations; however, the bucket wheel excavator also serves well for selective mining if appropriate data on the variation of lignite quality with location are available. The first bucket wheel excavators had capacities of about 1000 m3/day [24], and would seem toy-like in comparison with today's machines. The development of these machines is chronicled in the German brown coal mining literature [25-27], with capacities now reaching 240,000 m3/day [28] and bucket wheels 17.5 m in diameter [29]. Bucket wheel excavators require an unconsolidated overburden that is free of hard nodules, concretions, or layers. Hard layers must be ripped apart by a bulldozer and then loaded by a frontend loader or backhoe for removal. In the early 70's an attempt was made to use a bucket wheel excavator for removing overburden in a mine in North Dakota, but the attempt was abandoned because the excavator could not handle the large, well-cemented sand lenses encountered in that mine. Provided the overburden characteristics allow the use of bucket wheel excavators, these machines can rapidly remove overburden in excess of 38 m thick. They are also very well suited to multiple seam mining, since they can selectively mine seams less than 60 cm thick or seam partings 30 cm thick [13]. The efficiency decreases if the seams are of variable thickness. Prebenching provides an approach to mining deeper seams. In this operation, scrapers remove excess overburden before the dragline is moved in. However, the most efficient solution to the problem of mining deeper is the bucket wheel excavator [19]. A combination approach using a dragline and bucket wheel excavator is able to mine at depths greater than 46 m [19]. The bucket wheel excavator removes about 6 m of overburden ahead of the dragline. With multiple seam mining, the bucket wheel excavators may work in combination above one seam, while the dragline removes the interburden over the lower seam. The use of small and relatively mobile bucket wheel excavators is facilitated by a moveable conveyor system. In cases where a single seam is being mined, the spoil is transferred to large cross-pit conveyors that transport the spoil directly over the lignite which has been exposed by the dragline. This conveyor system can dump the spoil directly on the area to be reclaimed.

420 (iii) Other eqtdpment. The final stripping of the last 15 to 30 cm of overburden above the lignite may be done with bulldozers or scrapers to a~;oid excessive dilution of the mined lignite with overburden. In most mines scrapers are used to strip off and segregate the various horizons of topsoil before the rock overburden is removed by the dragline. In cases where the stripping ratio is less than 4.2 m3/t, scrapers can be used to remove the entire overburden. Where mining operations involve multiple seams, scrapers can be used to remove the parting. Bulldozers are widely used in virtually all operations where dirt or rock is being moved. Overburden may also be removed by large power shovels. Because of the characteristic "up and out" motion of shovels [10], they must work from below the material actually being excavated. Thus the shovel may be sitting on the top of the lignite, and therefore requires mechanical strength of the lignite sufficient to support the weight of the shovel. Normally, a shovel will work in conjunction with trucks which actually haul away the excavated material. The use of small electric or diesel powered shovels for overburden removal with haulage by rear dump trucks provides some advantages compared to larger equipment. First, the delivery and erection times of large mining equipment can be very long (for example, field erection of a dragline can require a year [10]), the use of more readily available, smaller equipment provides a shorter start-up time. Second, the truck and shovel operation provides greater flexibility in mining, since the equipment is much more mobile than huge draglines or shovels. Third, the ready ability of the small equipment to selectively segregate and dump soil materials can facilitate subsequent reclamation operations. In some situations the overburden can be removed by equipment used commonly for earth moving operations, such as scrapers and dozers. In particular, if the overburden is soft enough not to require blasting prior to removal, or if the blasted overburden is fairly fine, scrapers could handle the majority of overburden removal. In cases where direct cast mining is not applicable, the major alternative is a system which relies entirely on small equipment: scrapers, bulldozers, power shovels, front-end loaders, backhoes, and trucks. At high stripping ratios these systems are less efficient than the dragline or bucket wheel excavator. However, the advantages include flexibility for handling multiple seam operations and competitive operating costs with draglines or bucket wheel excavators in situations where extensive rehandling of the overburden is necessary. Often overburden removal and reclamation can be carried out simultaneously with the same equipment. Scrapers or bulldozers are applied most efficaciously when the overburden is shallow and production of lignite is small. Scrapers can adapt easily to irregular, lenticular deposits or multiple seams, but have high labor and maintenance costs per volume of overburden removal. Power shovels are proven technology and have the flexibility to adapt readily to most changing mining conditions, thin seams, and multiple seams. In some operations with shovels, production rates of up to 23 Mt/year can be achieved [13]. Mining can be done to depths of 200 m [131. In some situations a combination of dragline or bucket wheel excavator with the smaller

421 equipment can be advantageous, especially as the depth of the mine increases. As depth increases, the dominant cost factor becomes the removal of the overburden. Capital costs for this operation may exceed 50% of the mine investment [13]. A combination of, say, a bucket wheel excavator with some smaller equipment can ease the removal of the thicker overburden, assist reclamation, and minimize overall costs. 9.1.5 Lignite removal In Gulf Coast mines, lignite loading and hauling is normally done with a power shovel and trucks. The shovels have 6 to 15 m3 buckets [8,17]. The shovel will load lignite down to the parting, the parting is then removed with a bulldozer, and the lignite below the parting is loaded on a second pass of the shovel. Partings as thin as 8 cm can be removed [17]; thinner partings are loaded with the mined lignite. Front end loaders having 7.5 m3 capacity are used to supplement the shovels for loading, parting removal, or cleanup operations [17]. The trucks typically have 75 to 110 t capacity [8,17]. The trucks leave the area of active mining via ramps cut into the spoil area; scrapers are used to advance the ramps as needed. The advantage of front-end loaders for removing lignite is that their greater mobility allows more flexibility for changing mining conditions. When partings thicker than 8 crn are encountered, they are removed using a bulldozer to shove the partings against the spoil pile. Then a second pass of the lignite excavation equipment is needed to remove the lignite that was beneath the parting. Thinner partings are simply loaded with the lignite and consequently cause some dilution of the product. Mine haulage is sometimes accomplished by tractors with dump-bottom trailers [8]. The Sandow mine includes a 3 km conveyor belt. A combination of truck and unit train haulage is used at the Monticello and Martin Lake mines. Easi-Miners were introduced at the San Miguel mine to allow for selective recovery of thin seams. Basically, the Easi-Miner is a crawler excavator with a cutting head the height of which is adjustable to within 3 mm [19]. The Easi-Miner allows precise removal of lignite and spoils. This ability makes it possible to mine very thin or interbedded lignite seams. An Easi-Miner can load a 110 t truck in three minutes [19]. However, in order to minimize time wasted turning the EasiMiner around, it is necessary that the pits be fairly long. The Easi-Miner has proven to be especially useful for mining the Jackson and Yegua lignites. The lignite is normally exposed in well in advance of the mining and loading operations. As a result, a layer of weathered or slacked lignite may have formed by the time the lignite is to be loaded. This slack and any remaining overburden are removed by a bulldozer. In addition, the top 15 cm of a lignite seam is often of very high ash value [14]. This top layer can be removed by cleaning the bed with bulldozers or scrapers after it is exposed. The surface is usually cleaned again immediately before the daily loading operation. In the 1970's the use of mechanical brooms to sweep the surface before drilling, blasting, and loading was begun. The lignite may require blasting, after which it can be loaded by power shovels or front-end loaders. The lignite shovels and front-end loaders load trucks that haul the lignite to a central collection and transfer point. If the

422 lignite or underburden is not strong enough to support a shovel, the lignite could also be removed using a small dragline. The lignite is taken from the pit, using off-road trucks, to a mine-mouth power plant or to a rail loading facility. The limit for economical truck haulage of lignite is about 13-16 km [10]. If the lignite is to be shipped by rail, the haul trucks will take it from the pit to crushing equipment, where it will be sized to -5 cm [ 10] and then transferred to temporary covered storage, such as silos. The silo capacity may be 9,0(0)-11,000 t, which is sufficient to load a train of one hundred 90 t cars [10]. If more than one bed of lignite is being mined, the parting will be removed by a dragline if it is thick enough, whereas thinner partings will be removed by scrapers or front-end loaders. In the case of thin partings that are quite hard, dozers may be used to remove it for loading into trucks. The maximum recovery attainable is about 90%, though in some instances only 60% is recovered [30]. A block of lignite is left at the end of the spoil pile to prevent contamination of the lignite. A thin layer of lignite is left at the bottom of the pit to prevent contamination by the material underlying the lignite. A wall of lignite (sometimes called the berm) is left standing to prevent the spoil piles from contaminating the lignite. Some lignite may be left as a floor in the mine, particularly when the lignite overlies clays which could become softened by groundwater. A small amount of lignite may be lost with the last bit of overburden as it is removed before loading the lignite. In North Dakota, the average yield of lignite is 3.4 Mt/km2 (34,000 t/ha) [31]. The Surface Mining Control and Reclamation Act of 1977 requires control of the hydrologic effects of mining [14]. Factors that must be controlled include the depth to the groundwater, the location of drainage channels for surface water, and groundwater recharge capacity. The acid mine drainage frequently associated with coal mining in the eastern United States is not usually a problem in lignite mining, in part because of the low sulfur content of the lignite. However, alkaline mine drainage can be a problem when the lignite, overburden, or both are characterized by high concentrations of sodium, calcium, magnesium, carbonate, bicarbonate, sulfate, or chloride ions [ 14]. In North Dakota much of the stripping follows the land contour, with shallow overburden at the outcrop and elevation increasing toward the center of the deposit. Drainage will flow from the interior elevation toward pits on the perimeter of the bed, and drainage water will collect in the open cut. Wherever possible the surface runoff must be channeled around the mining operation. The sources of water in the mine are surface water runoff, rain water runoff, or water from shallow groundwater aquifers [14]. In most cases the first two factors are negligible or can easily be rectified, as, for example, by surface water diversion. However, disruption of aquifers can be a problem, particularly when it is the lignite itself that is serving as the aquifer. The usual practice for dealing with water problems in the mine is to prepare a sump in the floor, from which water can be pumped to the surface and, if necessary, treated before discharge. Restoration of the lignite seam aquifers after mining is difficult, and useful solutions do not seem to be well developed. One approach is to segregate mined material of permeability similar to the original lignite, and to redeposit this in place of the lignite seam, to create in effect an artificial aquifer [ 14].

423 9.1.6 Mined-land reclamation In the United States coal mine reclamation is governed by federal legislation. Much of this legislation was designed to address reclamation of bituminous coal mines in the eastern United States. However, reclamation problems and issues for bituminous coal mines can be significantly different from those at low-rank coal mines in the western United States [31]. Generally no top soiling is needed for reclamation in the Gulf Coast region because the overturned material will support pasture growth about as well as the original topsoil. (Topsoil is also referred to as "surfacesoil" in some legislation.) The spoil is graded to about the same contour as existed prior to mining, and then seeded with Crimson clover during cool weather or Coastal Bermudagrass during the summer. In North Dakota, after the lignite has been recovered, bulldozers or small draglines are used to reshape the spoil piles. The stockpiled topsoil is then replaced. The area is tilled, fertilized, and seeded to begin the reclamation process. Usually the spoil piles are levelled and the original contours are reestablished within 6-8 months after mining [8]. The overburden generally swells during stripping, so the reclamation elevations may be slightly higher than the elevations before mining. After the spoil areas have been levelled, they are fertilized, disked, and either sprigged with Coastal Bermudagrass (in the spring) or seeded with Crimson clover (in the fall). A rapid revegetation is necessary to inhibit erosion and siltation. Experience with Spanish lignite mines suggests that humification and pedogenesis are fairly rapid, and that considerable evolution of the soil characteristics can occur within three years [331. In the northern Great Plains, the topsoil is removed and stockpiled separately. Large, temporary mounds are constructed for the topsoil, which is later spread on top of the spoils after they have been regraded. Regrading is done to approximate the original contour of the land. However, a swelling factor of about 25% is associated with excavation, so the overall elevation of the reclaimed lands might be higher than the surrounding, undisturbed areas [5]. Regrading is usually accomplished with crawler dozers, and the spreading of the topsoil over the regraded spoils is done with pan scrapers. In both North Dakota and Montana the spoils are characterized by greater than 50% clay content (usually montmorillonite) and high concentrations of sodium, calcium, and magnesium, which make the spoils highly alkaline or saline [34]. In Wyoming, the overburden typically consists of shales, sandstones, and clay. These spoils have low structural stability and a low permeability. The soil moisture is generally low, and any moisture that is present may have a high salt load, which places considerable stress on the vegetation. This stress on the vegetation is the major mined land reclamation problem in the northern Great Plains [34]. The water stress problem is made worse by the impermeability of the subsurface spoil materials, which results from sodium - clay interactions. Greenhouse experiments have demonstrated the potential for toxic levels of aluminum, manganese, and zinc to accumulate in rescue and clover tissues when the lignite mine spoils have high potential acidity [35]. In some cases, the rescue showed signs of apical necrosis (i.e., the death of cells near the tips). Spoils dumped by the dragline are graded to the desired configuration to be achieved after

424 the mining operation. Topsoil is spread over the graded spoils to facilitate revegetation. In addition, fertilizers and soil conditioners such as gypsum may be added after grading. Seeding is done using conventional agricultural techniques. Mulching, hydromulching with wood fibers, or planting of annuals to provide stubble have been tried as ways of improving soil stability and seedling emergence. In Germany, one of the techniques used to condition soils for mined land reclamation is to mix low-sulfur lignite ash with the soil [36]. Similarly, in Denmark the mixing of fly ash with the fairly sandy soil in amounts of 250-500 t/ha improved grass growth and both the growth and survival of shrubs and trees [37]. Application of the ash increased both soil pH and the waterretaining capacity. Higher dosages of fly ash, 1000 t/ha, were less successful in stimulating plant growth, possibly because of an increase in boron concentrations in the soil [37]. Evaluation of the 1977 Surface Mining Control and Reclamation Act and its impact on lignite mining in the Gulf region identified several major environmental issues [38]. These include land disturbance, changes in land use, deterioration of air quality, impacts on surface and ground water, and impacts on endangered or threatened species. Additionally, concern must be given for the fact that some ecosystems can never be reclaimed once disturbed, and that some lands may be declared unsuitable for some types of lignite mining. Assessment of groundwater conditions in the pre-mine overburden and resaturated mine spoil in 25 year old mined land indicated that the long term impact of mining is minimal [39]. Surface mining of lignite should not cause a significant deterioration of groundwater quality beyond the mine boundaries. Given an adequate supply of water and nutrient, mixtures of soil horizon materials showed the same potential for forage grass production as did topsoil from the same mine site [40]. Thus any of the soil horizons, or any combination of them, should be able to be used successfully for revegetation over mine spoil. A comparison of the productivity of reclaimed mined lands with undisturbed lands showed no substantial differences, suggesting that reclamation was effective [41]. The parameters evaluated included production of vegetation, the composition and diversity of the vegetation, and the extent of plant and canopy cover. 9.1.7 Specific examples of mining practices (i) Gulf Coast. Most of the lignite mined in Texas comes from the Wilcox Group, and the remainder from the Jackson Group, as summarized in Table 9.2 [19]. With the exception of the Darco mine, all lignite mines in Texas are minemouth operations supplying fuel to electric power stations. The fuel demands of these stations require mines of large area. Fortunately, the local topology is generally flat and the overburden fairly unconsolidated, both features being advantageous for area mining. Both the overburden and the lignite can be removed without blasting. Furthermore it is usually not necessary to handle the topsoil separately for reclamation. Thus the mining operations can be restricted to overburden removal and removal and haulage of the lignite. Pre-stripping operations are accomplished with bulldozers or scrapers. Typically a pit will be 36 m wide and 1 to 3 km long [14]. In a multiseam mine the maximum depth may reach 76 m [ 14]. The overburden and topsoil are mixed during the mining operation. After

425 TABLE 9.2 1985 Lignite production (tonnes) in Texas, by mine [19].

Geologic Unit Wilcox Group

Jackson Group

Mine Production % of Total Martin Lake 12,299,469 South Hallsville 2,750,592 Darco 247,721 Sandow 6,227,660 Big Brown 4,980,121 Jewett 790,000 Powell Bend 287,756 Monticello Winfield 9,504,566 Thenno 2,485,788 (subtotal) ............. 39,573,673 ............ 88 Gibbons Creek 2,883,158 San Miguel 2,528,557 (subtotal) ................ 5,411,715 ............ 12 T O T A L ................ 44,985,388

regrading, the spoil areas are fertilized and revegetated with grass or clover. The Big Brown mine contains two seams, each about 2.5 m thick, separated by about 12 m of interburden [13]. A dragline on the highwall side of the pit removes the overburden from the upper seam. The dragline then moves to the bench formed by mining the upper seam and removes the interburden covering the lower seam. A bucketwheel excavator combined with a cross-pit conveyor system (referred to as the BWE-XPS system) used at the Big Brown mine provides a 300 m lateral transfer of overburden much faster and cheaper than more conventional mining methods [42]. The prcxtuction with this system is equal to that of a 53.2 m dragline, yet the BWEXPS system can cast material three times as far as the dragline. The Thermo mine (Hopkins County, Texas) is worked using a system of scrapers and bulldozers. The mine contains four or five lenticular seams, ranging from 0.9 to 1.5 m in thickness, separated by 3-6 m intervals of interburden. Mining depths range from 6 to 41 m with an average stripping ratio of 3.8 m3/t [13]. The overburden and interburden are removed by scrapers. The lignite is ripped by bulldozers before loading, and is loaded by backhoes and front end loaders. Despite the reliance on small equipment, the annual production is about 1.8 Mt with a recovery factor of 90-95% [ 13]. The San Miguel mine uses an Easi-Miner, which employs continuous cutting to remove overburden, remove and load lignite, remove seam partings, and assist in reclamation. The unit can break out and load 2500 t/h with a single operator [13]. The mine has a lignite interval 3 to 4.5 m thick with 4 to 6 partings, each 15 to 30 cm thick [13]. The pit is oriented along the strike. The overburden is removed by two draglines, one having a 49 m3 bucket, and the other a 24 m 3 bucket [ 18]. The lignite is transported to the adjacent mine mouth power plant using 90 t haulers.

426 The Sandow mine is located in southern Milam County, Texas. Operations began in 1954, long before the opening of most of the large lignite mines in Texas. The annual production from the mine is about 5.5 Mt [43]. The lignite is used in a mine-mouth power plant from which the output is primarily used for smelting of alumina by Alcoa. Some of the electricity is also fed into the distribution network owned by Texas Utilities Company. Overburden removal is accomplished by two electric powered draglines, each having 73 m3 buckets. Lignite is loaded by means of front end loaders, backhoes, and electric powered shovels. Both trucks and conveyor belts are used to transport the lignite to the crusher. The Beckville mine (Texas Utilities Mining Company) in eastern Texas uses three draglines to produce about 7 Mt/y of lignite [44]. A dragline maintenance program at this mine schedules biweekly maintenance periods of 8-12 h per dragline. The program is successful at keeping dragline availability in the 80-90% range. Three draglines are also used at the Jewett mine (Northwestern Resources); production is about 7.2 Mt/y [44]. As in the case of the Beckville mine, a scheduled preventive maintenance program (12 h maintenance every 10 days) provides an 86% availability of the draglines. The Dolet Hills Mining Venture (Louisiana) uses a dragline with a 98 m and a 59 m3 bucket for overburden removal [45]. The dragline can move over 1700 m3 of overburden per hour [45]. The lignite is removed by a hydraulic backhoe which has an 11 m3 bucket. The machine operates on top of the seam. The backhoe loads lignite into 77 t capacity bottom dump trucks, which transport the lignite about 3 km to a truck dump. From the hopper at the truck dump, the lignite is crushed to 15x0 cm by a feeder breaker and then transported 12 km to an adjacent power plant via conveyor belt. The belt system operates at 244 m per minute with a capacity of 900 t/h. After mining, the land is backfilled and graded and then quickly revegetated. The rapid revegetation is a tactic to control erosion. A variety of land uses are projected for the reclaimed mined lands, including forest, pastureland, water, and fish and wildlife habitats. (ii) Northern Great Plains. Lignite is mined in Mercer, Bowman, Oliver, Burke, Ward, and Stark counties in North Dakota. Mining in Montana is conducted in the southeastern comer of the state, in Rosebud and Bighorn counties. In Wyoming, strip mining occurs in Sheridan, Johnson, Campbell, and Converse counties [34]. All of the lignite reserves accessible to commercial scale mining in North Dakota and Montana are found in the Tongue River and Sentinel Butte Members of the Fort Union Formation. Typically the overburden contains loosely consolidated clays and shales. In some locations the lignite may have burned at the outcrop, leaving a material called clinker or scotia. The northeastern half of the lignite area is covered by glacial till. A deposit containing about 4.5 Mt is considered to be the smallest capable of sustaining commercial lignite production [34]. Current practice provides a 90% recovery factor of reserves in place [34]. The strippable reserves, identified by deposit, are summarized in Table 9.3 [20]. The beds range from 1 to 7.6 m in thickness [45], the weighted average calculated as 2.9 m [46]. The overburden depth ranges from 3.7 to 21 m [3]. The maximum stripping ratio (overburden thickness to bed thickness) considered acceptable for economic mining is 10.0 [46].

427 TABLE 9.3 Strippable lignite reserves in North Dakota [19].

Deposit Avoca Beach Beulah-Zap Bowman- Gascoyne Center Dickinson Dunn Center Hazen M&M New Leipzig Niobe Noonan-Kincaid Renner's Cove Stanton Washburn Wilton Velva

Strippable Reserves Mt 345 408 345 1245 230 724 1360 64 91 95 132 14 71 19 27 14 5

Maximum Overburden Thickness m 23 37 37 37 15 30 30 15 36 33 30 15 15 15 15 15 15

The stripping ratios for the Fort Union lignites are among the most favorable in the United States. The operations are generally categorized as area striping, contour stripping, or hilltop removal. Area stripping is used if the lignite bed has little or no dip and occurs in an area of fairly consistent topography. If the overburden thickness increases to the economically limiting value in a relatively short distance, contour stripping is necessary. In this case the mining operations follow the topographic contour. Although contour mining is not currently used in the northern Great Plains, future recovery of lignite deposits in southern Montana and northern Wyoming will likely require contour mining [47]. Hilltop removal is required when the lignite outcrop surrounds some broad, high feature in the topography. Generally, combinations of equipment are used in the operations. The most commonly used combination is the truck-shovel-dragline operation. Overburden is removed by draglines, lignite is mined by shovels, and trucks transport the lignite out of the pit. The dragline operates on the highwall side of the pit and casts the overburden across the pit to a spoil area. The size of the dragline required is determined by the desired lignite production and the depth of the overburden. Usually the bottom of the pit is the base of the lignite bed. Shovels operate in the bottom of the pit and are used to load the trucks. Front-end loaders are sometimes used in place of shovels when the trucks are loaded on the pit floor. When the total depth of the pit becomes very large, the truckshovel combination is used. In these cases the pit width is usually too great for a dragline to cast overburden across the pit. Thus both the overburden and the lignite are loaded by the shovels and transported by the trucks.

428 At the Falkirk mine overburden had been stripped using a walking draglines with a 80 m3 bucket [49,50]. With the overburden removed, the lignite seam is cleaned by sweepers before drilling blast holes. (At this mine, blasting is required only one or two days per year [50].) The fractured lignite is loaded by a tracked loading shovel which has a 14.5 m3 scoop. The loading shovel transfers the lignite to bottom-dump coal trucks. The interseam parting, which is up to 9 m thick [50], is removed by a dragline having a 13 m3 bucket. The lower seam is dislodged by bulldozers and loaded using front-end loaders. The overburden at the Falkirk mine is glacial till, a sandy, unconsolidated material containing abundant boulders. The nature of the overburden requires special care to avoid slumping the highwalls and spoil piles, especially during wet weather. Very large draglines are needed for overburden removal in order to achieve the required fairly flat angles of repose for the highwall and spoil piles. The draglines have 107 m boom lengths (providing an operating radius of 100 m) [51]. The dragline buckets have been lined with sheets of ultra-high molecular weight polyethylene, which has improved productivities by 20% [51]. Wear studs are used instead of wear plates on the dragline buckets. The average overburden depth is 30 m, with a range of 6--46 m [51]. The presence of the boulders ruled out application of bucket wheel excavators. Haulage of overburden is done in 145 t trucks. Mining of partings less than 60 cm thick is done using tractorscrapers. Because the quality of the lignite varies both between the seams and within a given seam, an effort is made to blend the various grades of lignite in the mine site stockpile to achieve an average daily quality within the limits specified in sales agreements. The facilities are capable of processing 3600 t/h of lignite [46], and are supplied using six bottom-dump haulers [50]. The facility includes two 630 t capacity hoppers and a 14,400 t capacity silo [50]. The 3.8x0 cm lignite is conveyed to the adjacent Coal Creek electrical generating station via a 2300 t/h conveyor belt. The Beulah mine is located in Mercer County. It is owned and operated by the Knife River Coal Mining Company. All of the lignite produced from this mine is used for power generation, being burned in a number of power plants. Removal and stockpiling of topsoil and subsoil, as well as road construction and reclamation operations, are done using a fleet of scrapers. Three draglines, of 57, 14, and 9 m3 capacity, remove overburden [52]. Overburden thickness is 6-12 m [52]. Three seams are mined: the top seam is about 2 m thick; the second is 2.4-3 m thick and lies at depths of about 21 m; the third is about 1 m thick and is separated from the second by a 1.2 m parting [52]. Production is 2-3.2 Mt/y [52]. The Center mine, in Oliver County, is owned and operated by Baukol-Noonan Inc. The lignite is used in the adjacent Milton Young power plant of the Minnkota Power Cooperative. The lignite is mined from the Hagel bed of the Tongue River Formation; the bed averages about 13.4 m in thickness at the mine [5]. The Glenharold mine is owned and operated by Consolidation Coal Company. The mine is located in Mercer County, and produces lignite from the Stanton bed of the Tongue River Formation. The seam is about 2.5 m thick at the Glenharold mine [5]. The lignite is shipped to the nearby Leland Olds power plant of the Basin Electric Power Cooperative. The

429 Indian Head mine, owned and operated by North American Coal Corporation, is located in Mercer County. The lignite is mined from the Beulah-Zap bed of the Sentinel Butte Formation, the seam being 3-3.6 m thick [5]. Most of the Indian Head lignite is burned in the Stanton power plant of the United Power Association. Some is used for process heat, or for building heat by state institutions. At both the Indian Head and Center mines, overburden is removed by walking draglines; the spoils are recontoured by pan scrapers [53]. A problem frequently encountered in North Dakota is the flow of ground water into the mines. In most cases the water must be removed by pumping. (iii) Future directions. The greatest potential for increasing return on investment may derive from increasing the economically recoverable reserve base at existing mines [54]. Techniques for extending the recoverable reserve base augering or otherwise extending the recovery beyond the final highwall. Further development of crosspit conveying systems may allow mining of deeper lignite.

9.2 T R A N S P O R T A T I O N Because of the relatively low calorific value of lignite compared to higher rank coals, transportation is affected by a high volume-to-value (and weight-to-value) ratio. The cost of transportation can be a large fraction of the total delivered cost of the lignite. Thus, for example, transportation is considered to be one of the largest obstacles in shipping North American low-rank coals to Pacific Rim nations [54]. Therefore means must be sought to obtain economies of scale or at least economies deriving from optimum design and utilization of transportation, storage, and handling facilities. Unit trains now provide the economies of scale with efficiencies derived from specialized loading and unloading practices. Slurry pipelines may provide advantages in the future. Currently unit trains provide the most efficient long-distance transportation mode for lignite. Depending on the needs of the user, some lignite is still shipped in single rail cars (less than 90 t), multiple car units (e.g., fifteen 90 t cars) or by the trainload (usually fifty or more cars) [ 14]. The distinction between a single trainload shipment to a user and the unit train concept is that the locomotives and cars of the unit train are dedicated to continuous cycles between the mine and unloading point (an electric power station). The loading and unloading facilities servicing the unit train are automated and specifically designed to accommodate the type of cars being used in the train. High productivity can be obtained by combining the use of dedicated equipment with quick turnaround during loading and unloading, and the elimination of the need for switching cars en route. Over the course of a year a typical car in a unit train will carry five to six times as much coal as a car of comparable size not assigned to a unit train [ 14]. (In part this difference derived from time lost as ordinary cars sit in switch yards or are loaded and unloaded slowly.) A typical unit train will carry 9000 t of lignite in 103 cars of 90 t capacity [10,14]. Because of the higher moisture content of lignite, and lower calorific value, relative to

430 bituminous coal, larger quantities of lignite have to be shipped to provide an equivalent amount of delivered energy. Despite a lower cost per tonne of lignite, when the transportation costs are calculated as a percentage of delivered energy, the cost per unit of delivered energy can be significantly higher for lignite. Lignite is shipped by unit train from the Gascoyne mine in the southwestern corner of North Dakota to the Big Stone power plant in Big Stone City, located in the northeastern comer of South Dakota. The trains are scheduled with a transit time of 20 h, allowing four hours for unloading at the plant and four hours for loading again at the mine [56]. A loop spur at the mine is long enough to hold the entire train while it is being loaded. The cars are loaded from an over-track tipple which allows them to be loaded without stopping the train. At the plant a rotary dumping mechanism clamps each car to a track platform and then rotates the car through nearly 180 ~ The emptied car is returned to its normal position and an automatic car positioner then moves the train forward by one car length so that the dumping process can be started with the next car. Each car is equipped with swiveling couplers so that it is not necessary to uncouple individual cars for unloading. The cycle time is about two minutes per car. The coal cars for this train were specially designed for this application by the Bechtel Corporation. Each car is a 90 t capacity flat-bottomed gondola with hinged roof covers. The hinged roof is the key to rapid loading and unloading of the cars. The roof shields the lignite from rain or snow in transit, retaining heat and reducing freezing of lignite in the cars. Similarly, it prevents snow from accumulating in the empty cars en route back to the mine, which helps to prevent buildup of frozen lignite on the walls or floor of the car. The roof also prevents windblown loss of lignite in transit. Each of the roof segments is equipped with a roller mast which is engaged by a scroll rail at the mine. As the car moves forward under the tipple, the sliding of the roller mast along the scroll rail raises the roof segment to an open position. As the car passes the tipple, the roof segments are closed, again by the roller masts sliding along the scroll rail. The lids are latched down in transit, the latches being released at the mine or plant by contact with a bar which applies lateral pressure to the spring-loaded unlatching mechanism. A pressure bar to release the roof segment latches is used at both mine and power plant. No scroll rail is needed at the plant, because the roof segments will fall open under their own weight as the car is tipped upside down. The special nature of unit trains means that, as new coal-hauling business is added to a railroad, this new demand cannot simply be met by adding new cars onto existing trains. Rather, each new unit train requirement brings with it an associated requirement for its own dedicated locomotives, personnel, cars, and trackage [57]. The use of western low-rank coals by utilities in the eastern United States to achieve compliance with air pollution regulations was forecast to have a potential of 23-113 Mt/y of new business [57]. This amount represents the need for 2200-8700 new loaded trains per year; and since consistent uninterrupted operation would likely require an empty train travelling back to the mine for each loaded train headed to the consumer, the total requirement could be 4400-17,400 new trains per year [57]. The transportation of lignites in the northern Great Plains is complicated by a tendency for

431 the lignite to agglomerate by freezing. Both dried and undried lignites are susceptible to this problern. In the case of dried lignites, a continuous release of water vapor by the relatively hot lignite causes condensation on the walls of the rail cars and on cooled particles of lignite. Cooling dried lignite before loading or spraying with oil to reduce the reactivity (and thus to reduce temperature increases during transportation) can help prevent freezing [14]. As-mined coal can freeze because the vapor pressure of water over the lignite particles is higher than it is in the void spaces between the particles. Thus water vapor is released from the lignite to establish an equilibrium, and, in freezing weather, the released vapor will condense as a frost layer which can continue to grow until the particles have become agglomerated. The extent of agglomeration can be reduced by drying a portion of the lignite and selectively placing it at the car walls, with as-mined lignite in the center of the car. The efficacy of this method may derive from the ability of the dried lignite to absorb the moisture released from the as-mined lignite, preventing the condensation of moisture as frost [ 14,58]. The calorific value of lignite is an important consideration for long-distance shipment. The lower the calorific value, the greater the cost of shipping per unit of energy. Since the calorific value decreases with increasing moisture content, the higher the moisture content of the lignite the higher is the cost of shipping each unit of energy. A secondary problem associated with the high moisture content of the lignite is the freezing of the lignite in the cars during cold weather shipment. A freezeproofing method involves drying lignite equivalent to about 15% of a normal carload to a nominal 20% moisture and then mixing with untreated lignite at the walls and bottoms of the cars [59]. Shipping this lignite presented no difficulties. A carload (44 t) of 6x0 mm lignite was dried from 35 to 22.5% moisture and shipped from Noonan to Grand Forks, North Dakota in a standard open-top hopper car [59]. The shipment was in transit for five days. The average temperature of the lignite on departure from the mine was 47~ had risen to 68~

On arrival, the average temperature

with a maximum of 86 ~ No hot spots or fires were noticed. A second test

involved 39 t of lignite dried to 21.3% moisture. The average temperature in the car was 62~ After a transit time of seven days, the lignite had heated to an average temperature of 76 ~ with a maximum of 109 ~. The lignite will of course be at an elevated temperature, and will likely be fairly reactive, when loaded directly from the dryer. Thus heating in transit is not unexpected. Gascoyne lignite was dried in a rotating drum dryer in Pekin, Illinois from 39.0% moisture to 22.0% [60]. The dried coal was shipped by rail in open hopper cars from Pekin to Grand Forks, North Dakota, a distance of approximately 1300 km and a transit time of three days. Seven cars were loaded with dried lignite. In six of the cars, oil was sprayed on the lignite as a means of controlling dust. The oil application rates were varied systematically in the range of 4.2 to 7.9 L/t. The seventh car received no oil treatment. The moisture content upon arrival in Grand Forks was 21.9% [60]; no precipitation had been encountered during transit. Within an hour of loading the cars the oxygen content in the void spaces between lignite particles decreased from 21 to 1% [60]. The bulk density of the dried lignite when loaded ranged from 760 to 774 kg/m3. Upon arrival the

432 bulk density was found to have increased to 798 to 838 kg/m3 [60]. The amount of dust collected from the dried lignite not sprayed with oil was about six times that of the oil-treated dried lignite [60]. Furthermore, the dust collected from the untreated dried lignite was about three times as much as collected during shipment of the as-mined lignite from Gascoyne to Pekin, despite the particle size of the as-mined lignite being larger than the dried lignite and the distance of shipment being longer. Ignition of the dried lignite during shipment was successfully prevented by filling the hopper door openings on the cars with fiber insulation and covering them with plastic sheeting before loading the cars. These precautions had not been employed in comparable tests with dried subbituminous coal, and ignition around the hopper openings had occurred in that case [60].

9.3 H A N D L I N G

AND STORAGE

9.3.1 Handling of lignite

Problems encountered in handling lignite include dust formation by the breaking apart of the friable material, accumulation of surface water on conveyors, and the sticking of particles via the interaction of moisture with the clays [60]. Dusting is a particular problem in periods of hot, dry weather, while during times of heavy rainfall problems associated with moisture accumulation and sticking can become severe. In addition, problems are encountered with the lignite freezing during the extreme cold encountered in the Dakotas, Montana, and Saskatchewan. Freezing of the lignite itself causes problems with discharge from rail cars and storage bins. In addition, the extreme cold takes a toll on the operation of the mechanical equipment in the lignite handling system. The tendency of lignites to slack can worsen problems of size degradation and dust emissions during handling. Dust emissions from long-term storage piles can be reduced by placing a drift fence perpendicular to the wind direction [14,61]. In live storage, where the lignite may be moved about more often than in long-term storage, dust emissions can be reduced by spraying with No. 6 fuel oil [14,59]. Slacking may also exacerbate autogenous heating (discussed below) because of the creation of fresh surface area when the lignite particles degrade. If oxygen is readily available to the newly exposed surface, oxidation will certainly occur. When as-mined lignite becomes wet, lignite fines and clay minerals become sticky and adhere to surfaces. Experience at the Gibbons Creek power station in Texas has shown that silos work well to provide storage for lignite to maintain low surface moisture during rainy weather when wet, as-mined lignite would otherwise have to be handled [62]. Moving wet lignite through the conveying system results in pluggage at transfer points. The flow of wet lignite can be improved by several design modifications: increasing cross sectional area for flow, decreasing the number of turns in the system, and increasing the angle of slope. The modifications, however, also increase the impact loading on the receiving end of the conveyor. Pluggage at boiler bin outlets was alleviated by eliminating inverted vibrating cones originally in the bins. Pluggage problems in

433 downcomers to gravimetric feeders were eliminated by increasing the slope of the transition section at the bin outlet. The experience at the Gibbons Creek station has suggested that future units should include provisions for short term storage with protection from the weather (e.g., silos). Conveyors must be designed for high impact on loading, with minimum number of transfer points, and with the transfer points as open and accessible as possible. 9.3.2 Stockpiling Lignites have a reputation for being particularly prone to "spontaneous" combustion during storage. (Although such terms as spontaneous heating and spontaneous combustion are in widespread use, these phenomena are not literally spontaneous. The term "autogenous" [e.g., 65} seems more appropriate.) The reputation is not wholly merited because in fact virtually all coals, possibly excepting anthracites, can experience spontaneous combustion in improper storage conditions. Furthermore, lignites can be stored entirely successfully if appropriate guidelines for establishing and maintaining a stockpile are established. The area selected for the stockpile should have good drainage. The lignite selected for stockpiling should have a minimum of size segregation [64]. A wide range of particle sizes, when further exacerbated by size segregation, will very likely pile with regions of tightly packed small particles and loosely packed large particles. This type of segregated packing increases the tendency to autogenous heating and combustion [47]. The lignite should be emplaced in benches approximately 30 cm high, and then compacted with heavy equipment such as a roller or bulldozer. Compaction limits air flow through the pile and thus limits oxidation. In addition, compaction, by increasing the number of particles per volume in the pile, can also increase the rate at which oxygen is scavenged from the air flowing through the bed, so that air penetrating deeply into the bed will likely have a low oxygen content [66]. Further benches can be added in roughly 30 cm increments until the desired height of the pile is attained. Time delays between spreading and compacting the added layers allow some of the loose coal to become oxidized and thus less susceptible to further reaction in the pile. A period of 1 to 2 weeks is recommended when constructing a stockpile during the summer [64]. The sides of the pile should be compacted and sloped at approximately a 20 ~ angle. Temperature monitoring is desirable to detect the onset of any hot spots in the pile. Sealing the pile, e.g. by spraying with asphalt, is not necessary [471. The extent of compaction needed depends on the moisture content. Long-term stockpiling of as-mined lignite at the Garrison Dam (North Dakota) has shown that compaction to a bulk density of 1120 kg/m3 is adequate [62]. This compaction is equivalent to a 12% interparticle void fraction [64]. Lignite dried to a 20% moisture content would require compaction to 1070 kg/m3, equivalent to an interparticle void fraction of 9% [64]. Protection of the interior of the pile derives in part from the normal slacking process. Lignite in the outer layers of the pile will lose moisture from the roughly 35% as-mined to an equilibrium value in the range of 15-20% (depending upon prevailing humidity). As the lignite

434 dries, water evaporates from the outer portions of a particle faster than it is replaced by diffusion from the interior. Shrinkage of the outer surface sets up stresses which cause the particle to crack and disintegrate. During periods of rain the process reverses, in that moisture is absorbed by the outer portions of the particle faster than it can penetrate to the interior, and stresses are again set up, leading to further cracking or disintegration. Cyclic drying and wetting eventually reduce the lignite on the surface of the pile to fines which themselves serve as the protective coating, limiting or preventing the access of air to the bulk of the pile. Stockpiling large sizes of screened lignite can be facilitated by allowing air to circulate through an uncompacted pile at a rate adequate for removal of any heat generated in the interior. The success of this method depends on the use of large sizes and the absence of fines; oxidation rate is a function of particle size, and is generally slow for large particles with comparatively small surface areas. If a hot spot should develop, the remedial action depends on the temperature in the heated area. Below 70-80"C, the problem will eventually correct itself, because oxidation of lignite will reduce its reactivity toward further oxidation [64]. However, temperatures may not decrease for some time because piled lignite is a good insulator. Above this temperature range, moisture is being evaporated from the lignite, and it is primarily the residual moisture content that prevents the lignite from heating to its ignition temperature. The temperature will remain at about 93"C until the moisture has evaporated [64]. If an area of the pile becomes this hot, the available strategies are to remove the hot lignite from the pile or to flood the region with a large excess of water. The former is the preferred strategy. First, water must be applied cautiously because wetting of lignite already thoroughly dried will release substantial quantities of heat as heat of wetting. If large quantities of water are sprayed on the pile, some of the compacted particles forming a restriction to air may be washed away, allowing more air to enter the pile, possibly exacerbating the heating situation. If heating is widespread through the pile and not confined to a hot spot, it is best to try to use the lignite as soon as possible. The only other alternative to having fire break out would be to demolish the pile, spread the lignite to allow it to cool, and the prepare a new, better compacted pile. Dried Baukol-Noonan lignite (44 t) was stockpiled in a 1.5 m pile having a 8xl 1 m base and 2.4x4.9 m top [59]. Immediately after stockpiling the average temperature of the pile was 51~

as measured at 41 locations. Approximately three months after stockpiling, small areas of the

pile twice ignited after periods of unusually high winds. The average temperature of the pile was still 38*. During the following three months the pile slowly cooled to 14~ (in January) and for the next two years the temperature followed the seasonal variations. The changes in lignite quality after 27 months are shown in Table 9.4 [59]. The change in composition of lignite during eight years of stockpiled storage in Pittsburgh is shown in Table 9.5 [67]. A 18 t pile of lignite was established on the ground and covered with a 10 cm layer of soil. The lignite was analyzed after being uncovered. On a moisture-and-ash-free basis the lignite shows, after eight years' storage, a remarkable similarity to the fresh lignite. Dried lignite remains reactive even after lengthy periods of stockpiling. Even with extensive

435 TABLE 9.4 Changes in lignite quality after 27 months' stockpiling [58].

Proximate (% as-received) Moisture Volatile matter Fixed carbon Ash Calorific value, MJ/kg, maf

July 1970

October 1972

22.5 31.2 37.1 9.2 28.3

22.5 30.8 35.7 11.0 27.1

TABLE 9.5 Changes in lignite quality after eight years' stockpiling [67].

Proximate (% as-received) Moisture Volatile matter Fixed carbon Ash Ultimate (% maf) Hydrogen Carbon Nitrogen Oxygen Sulfur Calorific value, MJ/kg, maf

May 1909

August 1917

40.2 24.9 28.0 6.8

34.8 26.6 29.7 8.9

4.8 70.8 0.8 22.4 1.2 27.4

5.0 72.5 1.0 20.6 0.8 27.2

aging the lignite can still undergo quite rapid spontaneous heating if conditions become favorable. A problem in stockpiling dried lignite is that the dried particles are more likely to slide or flow than are particles of as-mined lignite; consequently it is difficult to obtain good compaction of the dried lignite. A low degree of compaction will allow greater air infiltration, which in turn may promote heating. Furthermore, lesser compaction means that the pile is more readily penetrated by rain or snowmelt. Air penetration can be reduced by building the pile with gently sloping sides (thus reducing wind pressure), but gentle slopes also reduce runoff of rain or snowmelt. The piles with low compaction and gentle slopes are less susceptible to autogenous heating but more likely to experience wide variations in moisture content [59]. Spraying the stockpile with heavy protective material has been recommended to reduce losses of calorific value of brown coals in storage [68]. Inhibition of oxidation of Russian brown coals has been achieved by the addition of sulfur compounds such as mercaptans, disulfides, and sulfenamides [69], although the environmental consequences of this treatment do not seem to have

436 been considered. 9.3.3 Low-temperature air oxidation The behavior of lignite on exposure to air or oxygen at near-ambient temperatures plays an important role in determining any subsequent autogenous heating [70]. The reactions involved in the self-heating include the exothermic formation of hydroperoxides or peroxides, with subsequent decomposition into aldehydes and carboxylic acids [70]. Low-temperature oxidation follows a three-step mechanism: a) surface adsorption of molecular oxygen; b) formation of reactive surface groups (e.g., peroxides); and c) formation of less-reactive oxygen functional groups, such as aldehydes and carboxylic acids, on the surface [71]. Below 700C chemisorption of oxygen dominates the process; above 70 ~ decomposition of peroxides and formation of new functional groups dominate [71]. The oxygen complexes produced at lower temperatures decompose exothermally between 70-90"C to form the new, lessreactive functional groups and gaseous products CO, CO2, and H20 [72]. Oxidation of Anderson (Wyoming) lignite in dry air (80 mL/min at 80~ for up to 10 days) resulted in a significant reduction of dihydroxybenzenes, and some reduction of phenols, among the products of subsequent pyrolysis [73]. The greater relative decrease of the dihydroxybenzenes

(i.e., relative to the phenols) suggests that one or both of the hydroxyl groups in the dihydroxybenzenes may derive from hydroxyl functional groups in the lignite that are directly affected by the low-temperature oxidation reactions, rather than from ether or ester groups. During weathering in air some of the hydroxyl groups may undergo condensation reactions via ether formation [73]. Oxidation of Beypazari (Turkey) lignite at 50-150~

for up to 120 h formed

anhydrides, aryl esters, and carboxylates as reaction products [74], based on identification by infrared spectroscopy. Aliphatic sulfides are converted into oxidized forms, while aromatic sulfur-containing compounds are unreactive [75]. The oxidation products are generally sulfones and sulfoxides; Rasa (Yugoslavia) lignite, which has a remarkably high organic sulfur content (=10%), mainly produced sulfonic acids [75]. About 20% of the total sulfur on the surface of Rasa lignite was oxidized after treatment at 105~ for 111.7 days [76]. The same amount of oxidation could be achieved after 130 days at ambient temperature. The extent of oxidation of sulfur levelled off after 60 days at 105~ [76]. 9.3.4 Autogenous heating and combustion A wide variety of terms has been employed to describe aspects of the autogenous heating and combustion of coals. The self-ignition temperature is that at which a powder or dust begins to heat "spontaneously", as measured by comparing the increase in temperature of the test specimen with a control material not susceptible to autogenous heating (e.g., graphite) [77]. The ignition temperature is the lowest temperature at which a dust-air mixture will burn with a definite flame [77]. The smoldeling temperature is the temperature of a hot surface on which a layer of powder 5

437 mm thick will begin to smoulder within two hours [77]. Lignite has a kindling temperature of 150~

[78]. For coals of the same rank, the volatiles content relates directly to the ignition

temperature [79]. The ignition temperature is the temperature at which ignition followed by selfsustained combustion occurs; whereas the liability to autogenous combustion is the property of a given material to heat "spontaneously" to the ignition temperature [80]. A critical oxidation temperature for autogenous combustion is defined by heating a coal sample at a constant rate while passing oxygen through the sample [81]. (This temperature has also been called the relative ignition temperature [82].) The temperatures of the sample and the furnace are monitored. Initially the sample temperature will lag the furnace temperature. However, as the sample begins to heat, a rapid increase in sample temperature will occur. Eventually the sample temperature will become equal to the furnace temperature. This point is called the critical oxidation temperature (COT). As a rule, critical oxidation temperature increases with rank. Ten North Dakota lignites showed no significant difference in critical oxidation temperature, the value being 183~ [47]. A suite of ten additional North Dakota lignites also showed no significant variation in COT, but the average value was somewhat different, 226~

[83]. The COT decreases with decreasing

moisture content and decreasing particle size. The decrease with moisture content occurs regardless of whether the lignite is dried in air or is dried by equilibrating it over solutions capable of maintaining specific humidities [83]. The cfitic~ oxidation temperature test can be augmented by measuring at the same time the so-called carbon dioxide index. In this experiment the effluent gas from the furnace is passed through aqueous ammonia. The conductivity of the ammonia solution is monitored, a distinct increase being attributed to the absorption of the first carbon dioxide issuing from the furnace. The temperature corresponding to the point of increase of conductivity of the ammonia solution (and thus to the point of first production of carbon dioxide) is called the carbon dioxide index. For a suite of ten North Dakota lignites the carbon dioxide index ranged from 71 to 80~ (average 76.6 ~ with no evident correlation with the COT [83]. Like the COT, the carbon dioxide index also decreases with decreasing moisture content [83]. The critical oxidation temperature test is sensitive to experimental conditions. Changing the oxygen flow rate changes the COT. Therefore the test should be used as a guide for comparison of coals tested under identical conditions and not as an absolute determination of a uniquely defined oxidation temperature. The dependence of both the COT and the carbon dioxide index on the moisture content suggests that the tendency to autogenous combustion increases as the lignite dries, or that dried lignite is more prone to autogenous combustion than moist lignite. The COT of Kincaid (North Dakota) lignite is strongly affected by moisture [84]. A sample of about 31% moisture showed an average COT (for duplicate determinations) of 228.5~

The COT decreased

steadily with decreasing moisture content to an average of 190~ at 6.5% moisture. When the lignite was dried to essentially zero moisture (by storage at 0% relative vapor pressure), the COT showed a sharp drop to 104~

Comparable data for Dakota lignite show a decrease in COT from

228 ~ to 192 ~ with a decrease in moisture from 34.9% to 8.0% [85]. Heating in wet oxygen (of

438 unspecified moisture content) resulted in a small increase in the COT, about 7* for the lignite of nominally 0% moisture and 10" for lignite of 18.4% moisture [84]. The COT decreases with decreasing particle size, from 215~ for both 20x40 mesh and 60x80 mesh to 185" for -200 mesh lignite [84]. The COT decreased with increasing oxygen flow rate; and eight-fold increase in oxygen flow decreased the COT from 202~ to 179~ [84]. Lignites, which have high inherent moisture contents, are more likely to undergo autogenous heating and combustion than are coals of higher ranks. The heat released by oxidation in essentially dry conditions is much less than the heat released by oxidation in moist air [61]. The heat of wetting of dried lignite can be enough to raise the temperature of the lignite to levels close to those that would initiate combustion. The significance of the heat of wetting is demonstrated by passing moist nitrogen over lignite, in which case temperature rises of the order of 25-500C are observed [61]. The moisture content of the stockpiled lignite has some role in autogenous heating. In the range 25-95~

the maximum oxidation rates occur at the moisture contents near the

equilibrium moisture value (about 20% moisture) [ 14]. The oxidation rates decreased by about half when the moisture content was reduced to 5% or increased to 36% [14]. The additional moisture needed to increase the moisture content of lignite by 3 to 4 percentage points can liberate enough heat of condensation to increase the temperature by about 17~ [ 14,64]. The reactivity of dried lignite to oxidation by the atmosphere increases with decreasing moisture content [86,87]. However, at moisture contents below about 25% the trend appears to reverse when reactivity is measured as the rate of oxygen absorption. Thus a lignite containing 5% moisture has about the same reactivity as does the undried sample having 33% moisture [86]. The presence of water in the lignite may be necessary for autogenous heating, the argument based on the temperature being too low for atmospheric oxygen to react with carbon [88]. For lignites stored under static conditions (i.e., conditions in which the atmosphere is not replenished), the rate of oxidation is directly proportional to the oxygen concentration [86]. In actual stockpiles or other storage conditions, the likelihood of autogenous heating is reduced as the oxygen concentration in the gas surrounding the lignite particles is also reduced. At some of the power plants of the northern Great Plains, lignite might be stored for up to seven days as a buffer against interruptions of supply [61]. Air can easily penetrate unconsolidated stockpiles, so that careful monitoring is needed to detect self-heating. The concentration of oxygen has a linear effect on oxidation rate at constant pressure. The oxygen concentration depends ultimately on the balance between consumption by reaction with the lignite and replenishment by admission and circulation of air to the pile. Air circulation depends on the void space between particles, on the pressure gradient, and, at high temperatures in the pile, on convection. Oxygen depletion can reduce the rate of autogenous heating. For 10x20 mesh coals in a closed vessel exposed to dry air at 25~

oxygen reduction is not governed by first-order kinetics

over the entire period of the experiment [89]. Oxygen concentration decreases exponentially with time for about 24 h and then changes to a lower rate. This effect is illustrated in Figure 9.1 [89]. In a closed vessel the total pressure will vary; because of this, the initial oxygen absorption rates are

439 100

l ca,

10

ca,

x

o 1

0

1

2

., . . . . , . . . . , . . . . , . . . . 3 4 5 6 7 Time, days

Figure 9.1. Variation of oxygen partial pressure with time for 10x20 mesh Gascoyne lignite exposed to air at 25~ in a closed container [87].

considered to be the most meaningful measure of relative reactivity of a suite of coals. The initial rate of oxygen absorption is a function of the oxygen content of the coal and is given by the equation dO2/dt = 0.062[0] - 0.055

[89] where dO2/dt is in cm3/hr and [O] is in weight percent, maf basis. Dry, finely divided coals having the greatest susceptibility to spontaneous combustion have values of dO2/dt exceeding 0.016 cm3/h [89]. Oxygen uptake by lignite increases with increasing temperature [90]. The effect of temperature follows the Arrhenius equation. Reported values of the average activation energy show a considerable range, 52.5 k.l/mol [86] and 39 k.l/mol [87] for North American lignites, and 102 kJ/mol for three Thai and one Australian lignite [91]. Oxidation using 1802 (studied by secondary ion mass spectrometry) had two activation energies, 47.1 kJ/mol in the range 23-70~ kJ/mol at 70-90~

[71]. The lower activation energy, below 70~

and 82.0

suggests that the process is

dominated by chemisorption of 1802, resulting in the formation of reactive oxygen species peroxides) [71]. The higher value, above 70~

(e.g.,

typifies processes of peroxide decomposition and

formation of oxygen functional groups on the coal surface [71]. The relationship between particle size and oxidation rate is roughly proportional to the cube root of the external surface area of the particles [87]. This relationship holds quite well for particles in the size range -12 mm through +70 mesh, in which the particle size appears to be the least

440 important of the various factors affecting reaction rate [86]. A situation in which the effect of particle size may become of concern is when a large accumulation of very fine sizes occurs. Ten North Dakota lignites showed critical oxidation temperatures in the range 215-232~ (for an average of two determinations on each of the lignites) [84]. No correlation of these values was sought with other physical, chemical or petrographic properties of the lignites. Repeating the critical oxidation temperature test on the same sample of lignite (i.e., after it had once been heated to the COT) showed that the COT was reduced in the second determination. For example, the COT of an unidentified lignite was measured at 183 ~ C; but a second determination on the same sample showed a COT of 155~ [84]. This difference might be a result of alteration of the pore structure of the lignite or alteration of its reactivity to oxygen as a result of the first heating [84]. The reactivity of lignite is reduced as oxidation proceeds [65,86]. This "ageing effect" moderates the amount of self-heating that can occur [65]. The reduction in reaction rate results from blocking of active sites by accumulated oxygen. The reactivity depends inversely on the amount of oxygen absorbed, or, alternatively, directly on the amount of carbon dioxide given off [86]. The time for deactivation depends upon the rate constant for the reaction, the oxygen concentration in the gas around the particles, and the decreasing capacity of the lignite to absorb oxygen. At 25~ in air (i.e., 21% oxygen) the time required for 95% deactivation of lignite is 14.4 days [86]. The mechanism of deactivation or reduction in rate by blocking active sites by absorbed oxygen, or perhaps by carbon dioxide still retained on the surface, indicates that desorption of carbon dioxide should reactivate the lignite. Heating is necessary to force carbon dioxide desorption; therefore the reactivation is temperature-dependent. Below 70~ reactivation increases only gradually with increasing temperature, but above 70 ~ the reactivation increases rapidly with temperature [86]. In the lignite industry there has long been the idea that 150~ (i.e., 66~ represents a critical temperature in storage, above which autogenous heating becomes accelerated. Since the rate of oxidation after reactivation of lignite accelerates rapidly above 70~

it appears that

the critical storage temperature results from the reactivation occurring with thermally induced desorption of carbon dioxide [84]. An approach to understanding the chemical mechanism leading to autogenous heating focuses on the possible role of the metal carboxylate salts, or coordination complexes of metals in lignite, or both. No work of this sort seems to have been done in North America, but the ideas should be applicable to North American lignites. Exchanging barium onto the acidic groups in brown coal promotes oxidation during the air-drying of the coal [9~

Oxidative decomposition of

metal carboxylate salts has been suggested to be one of the main processes responsible for heating in Russian brown coals [93-95], and particularly iron carboxylates [95]. Other work has implicated coordination compounds of metals [95-97], with a possible catalytic role ascribed to these complexes [96]. Mathematical models have been developed to determine the temperature rise in a stockpile given a specified set of initial and boundary conditions [64,65]. The important processes include transport of air (oxygen) into the pile; the reactions between oxygen and the coal; and heat transfer

441 within, and out of, the pile. Any model must take these factors into account [98]. The sources of heat in stockpiled lignites, and the agents causing temperature changes, are the heat of oxidation, heat of wetting, heat of evaporation or condensation, and any externally generated heat effects [64]. If oxidation is occurring because of accumulation of oxygen on the lignite surface, the rate will decrease with time. However, the evolution of the oxidation product carbon dioxide at temperatures of 70-80~ will result in an increase in oxidation rate. The effect of the temperature itself on rate is expressed through a rate equation which has an activation energy of 50 kJ/mol 02 [64]. Air can be transported into a stockpile by a variety of processes. These include convection induced by self-heating (sometimes called the chimney effect), flow caused by wind pressure, molecular diffusion arising from concentration gradients, and "breathing" of the pile resulting either from daily temperature variations or from daily variations in barometric pressure [65]. The last effect is unimportant. Air fluxes due to barometric breathing can not sustain combustion, while thermal breathing arising from temperature variations occurs so close to the surface of the pile that any heat produced is lost [65]. Natural convection is the main source of oxygen giving rise to autogenous combustion [65]. Pulverized lignite contained in a cube 100 mm on an edge and heated at 10~ increments in the temperature range I(D-140~ showed that, at 110~ and above, a temperature of 2000C was reached inside the lignite in four to twelve hours [77]. Samples of pulverized lignite of 2.5 L volume were heated to 115, 140, and 1720C, then blown with air heated to the same temperature. The formation of smouldering pockets was observed at various times ranging from 6 h for the 1150C case to about 1 h at 1720C [77]. Samples of identical volume but held in wire netting baskets of various shapes showed self-ignition temperatures ranging from 110~ for a cone to 165~ for a flat cylinder [77]. Collectively, these observations show that the self-ignition temperature of pulverized lignite depends on the quantity of fuel involved, the geometric shape of the stored mass of fuel, and the thermal conductivity of the mass [77]. The lowest self-ignition temperature observed in any experiment was 110~ C; thus, to allow an adequate margin of safety, the maximum temperature to which the lignite should be exposed during handling, transportation, or storage should be 800C [77]. In the absence of air circulation (and hence heat transfer) by convection, the temperature rise caused by a localized hot spot spreads very slowly through a well-compacted pile because of the low thermal conductivity of the lignite. A time period measured in years would be necessary to achieve complete thermal equilibrium throughout the pile. Although the low thermal conductivity may be advantageous in this respect, if heat generation in a hot spot ceases, this same low thermal conductivity means that months could be required for the hot spot to cool to a temperature at which the lignite would not longer be susceptible to heating once again if oxygen were readmitted. An uncorrected hot spot at temperatures of 930C will eventually result in the lignite in that area becoming completely dried and thus heating quickly to its ignition temperature. If a fire were to start, it could spread rapidly through the pile.

442 Stockpile size is important in determining whether spontaneous ignition will occur [98]. A small pile may not experience combustion because it loses heat rapidly. In a large pile the rate of heat loss will be small, but little oxygen will be available because it will have reacted near the surface of the pile. This analysis suggests the existence of a "worst depth" for a pile at which the balance between heat loss to the surroundings and heat generation by reaction may be just sufficient to support ignition [98]. Probably the most common warning sign of autogenous combustion, or its imminence, is temperature rise. A utogenous heating is an exothermic reaction, which may or may not be sustained depending upon the extent of heat losses. The rate of self-heating of a stockpile is a balance between the heat generated by reaction, which presumably follows an Arrhenius relationship, and the heat loss. If the heat loss mechanism is primarily by conduction, then the self heating rate, q, is simply q = q l +q2 where ql is the rate of heat generation via the chemical reactions and q 2 is the conductive heat loss. This equation can be written in greater detail as 9c (aT / at) = pQZ exp(-E / RT) + ),.•T where p is the density; c, the specific heat; T, the temperature; t, time; Q, heat of reaction, Z, the rate constant; E, the activation energy; R, the gas constant; Z, the thermal conductivity, and V2 is the Laplacian differential operator [89]. In principle, at steady state ql = q2 so that one could calculate the temperature if values of Z and E were known. Calculation of Z and E at adiabatic conditions can be done using rates of autogenous heating at various temperatures [89]. However, in practice additional heat losses may occur via convection and moisture evaporation and depletion of oxygen may reduce the reaction rates. The temperature increase may sometimes be small. Two reasons can account for this [66]. First, all the oxygen entering the pile may react near the surface. In this case, any heat generated can be lost to the surroundings. This type of stockpile is called oxygen-limited, and is characteristic of reactive coals of fine particle size. Second, the reaction rate may be sufficiently low that any heat generated due to reaction can be lost at near-ambient conditions. This type of stockpile is heattransfer limited and is characteristic of unreactive coals of large particle size. Which of these is the limiting factor is important in considering the design of a stockpile or choices for remedial action. Adding fine coal to an oxygen-limited pile will make it safer by reducing the void volume and increasing the rate of oxygen depletion [66]. Removing fine material (or not incorporating it in the first place) will help make heat-transfer-limited stockpiles safer by lowering the reaction rate [66]. The characteristics of coals most susceptible to autogenous heating are high pyritic sulfur,

443 high moisture, and high oxygen contents [89]. Lignites are generally low in pyritic sulfur, but have high moisture contents and relatively large amounts of oxygen functional groups. Carbon monoxide formation is particularly a function of moisture and oxygen contents, as shown in Figure 9.2 [89] for lOx20 mesh samples of coals of various ranks exposed to air at 25~

0 0.5

5 1

Oxygen, percent maf 10 15 20 25 30 I I i I i

0.45-

35

,.,,.. ,J

0.4-

/,,

0.35-

,a:,

.,,,,.

~, "~

//

0.3-

0 00!t O.

0

l

5

l

I

l

I

10 15 20 25 Moisture, percent

I

30

35

Figure 9.2. Variation of CO formation as a function of moisture (dashed line) and oxygen content (solid line) of lOx20 mesh bituminous, subbituminous, and lignitic coals on exposure to air at 25~ [87].

The tendency to autogenous heating of a stockpile can vary as functions of porosity of the pile, moisture content, rate of air flow through the pile, and humidity of the flowing air. The effects of coal moisture or air humidity are greatest for low-rank coals [89]. Since the experimental conditions are adiabatic and the air flow through the coal sample is negligible (most of the 50 mL3/min of air flows over the sample, not through it), the self-heating rate can be assumed to be given by the rate of heat evolution from the chemical reaction. Then dT/dt = (QZ/c) exp(-E/RT) [89], from which equation E and Z can be evaluated. However, under actual conditions of stockpiling the pile is not likely to be completely adiabatic, and self-heating temperatures are lower under adiabatic conditions than otherwise.

444 9.3.5 Storage of dried lignite Regardless of the drying process used, the temperature of dried lignite will initially be above ambient. Some cooling is necessary before the dried lignite can be stored. In fact the high temperature of the dried lignite as it leaves the drier is considered to be a serious problem [64]. At 20% moisture, lignite will absorb oxygen from the air twice as rapidly as it would at either 5% or 35% moisture [63]. Since 20% is about the equilibrium moisture value of lignite, dried lignite which is then allowed to equilibrate will unfortunately reach a moisture level at which it is rapidly absorbing oxygen. However, it is also likely that many drying processes will cause some oxidation of the lignite along with the drying itself. This oxidation during drying may be sufficient to reduce the rate of further oxidation enough to prevent problems during storage or transportation. A computer simulation of the effects of the temperature of the lignite leaving the drier and the moisture content has been published [64]. Recommendations for safe and stable storage of dried lignite include, first, a thorough cooling of the dried product before storage. Dried lignite has a very low thennal conductivity. Thus it is not sufficient to presume that the dried lignite will cool adequately without specific action being taken. Cooling the dried lignite can be accomplished by spreading it in thin layers, using cooling air at the drier exit, or by forced ventilation. A second procedure is screening. Screened lignite has been found to be more stable in storage than either fines or a mixture of fines and lumps. Limitation of exposure of the dried lignite to oxygen is important. During transportation this can be effected by use of closed rail cars. In storage in bins, the container should be kept as full as possible and the lignite fed with an inert gas purge. Thorough compaction is required for long-term storage in stockpiles. Layers of stored lignite should be about 30 cm thick [99]. Results of various small-scale tests on storing dried lignite have been compiled [47]. In one of the more promising experiments, steam-dried Dakota Star (North Dakota) lignite was stored outdoors in wire mesh containers for 27 months. A sample screened to 38x19 cm increased in moisture from 12.4% to 15.8% in this time, and showed a slight decrease in maf calorific value from 28 to 27.3 MJ/kg. A second sample, screened to 12x6 mm, increased in moisture from 10.0% to 16.3% and experienced a very small increase in calorific value, from 27.2 to 27.5 MJ/kg (maf) [47]. When lignite originally steam dried to a moisture content of 15% was stored with protection from prevailing weather conditions, the moisture content actually fell to 11-12% [99]. However, when the lignite was stored outdoors where it could be affected by rain- and snowfall, the moisture content fluctuated from 15% to 22%, averaging 18% over the course of a year [99]. Gascoyne lignite, sized 7.5x0 cm, was dried from 39% to 22% moisture and sprayed with Bunker C oil at application rates of 4.2-7.9 L/t [100]. The oil treatment helps control dust formation during the handling of the dried lignite and reduce wind losses during transportation [60]. Drying increased the calorific value of the lignite from 14.9 to 19.3 MJ/kg; oil spraying resulted in a further slight increase, to 19.6 MJ/kg [60]. The dried lignite was shipped from Pekin, Illinois to Grand Forks, North Dakota, where it was stockpiled with 1040 kg/m3 compaction. The

445 temperature of the dried lignite decreased from 49~ in the car to 10 ~ in the constructed pile. The temperature rose to 16 ~ after 40 days of storage, but thereafter the pile temperature tended to fluctuate with seasonal variations in the ambient temperature. No problems with heating or fires occurred in the nearly four-year period from November 1974 to June 1978. The initial moisture content of the stockpiled lignite was 22%. The moisture content of the lignite on the surface will increase with rainfall and decrease by subsequent drying. The moisture content of the lignite below the pile surface did not change appreciably over the nearly four year period of monitoring the pile [100,101]. The calorific value of the dried lignite when first stockpiled was 26.7 MJ/kg on a moisture-free basis. After 1150 days of storage, the calorific value at the surface was 25.1 MJ/kg; 25.6 MJ/kg fifteen cm into the pile, and also 25.6 MJ/kg thirty cm into the pile [100]. The percentage reductions in calorific value were, respectively, 6, 4, and 4%. The effect of storage on the lignite quality is summarized in Table 9.6 [101]. TABLE 9.6 Change of Gascoyne lignite quality after four years' stockpiling [ 101].

Analysis Proximate (% as-received) Moisture Volatile matter Fixed carbon Ash Ultimate (% moisture-free) Carbon Hydrogen Nitrogen Sulfur Oxygen Ash Calorific value,MJ/kg, mf

As stockpiled December 1974

Core samples October 1978

21.7 33.4 33.9 11.0

22.3 33.3 34.4 10.0

63.0 4.2 0.7 1.7 16.2 14.2 26.6

63.1 4.5 1.0 2.4 16.1 12.9 25.1

Recommendations have been published for the storage of dried, pulverized lignite in bins [102]. Similar recommendations are made for the storage of brown coal [68]. Bins should not be installed near process equipment that emits heat nor near pipes carrying hot fluids. Indeed, such operations as welding or brazing should not be allowed near the bin. If the lignite begins to smoulder, any feeding of lignite to the bin that might be in progress should be discontinued and the bin flooded with an inert gas. A dosage of 1.5 kg CO2/ln3 is recommended [102], but in any case enough inert gas should be added to reduce the oxygen content in the bin below 8%. Sealing the bin will then extinguish the smoldering because of the lack of oxygen. The detection of smoldering in bins of dried, pulverized lignite can be accomplished by monitoring the CO content of the air in the bin [77]. After two hours' storage of dried, pulverized

446 lignite at 40~ in a gas-tight, 15 m3 bin, the oxygen content dropped to 8.8% and carbon monoxide increased to 2000 ppm. After 10.5 hours the comparable figures were, respectively, 0.8% and 3500 ppm. At an initial storage temperature of 63~

the oxygen content was 3.2% after two hours

and 0.4 after 10.5 h, while the carbon monoxide contents at the respective times were 3000 and 3800 ppm [77]. A single, specific value of carbon monoxide concentration that is an invariable indicator of the onset of smouldering can not be defined, because that value will depend on such factors as the level of filling of the bin, its geometric shape, and whether the bin is used continuously or intermittantly [77]. Tests of the possibility of using some gas other than CO as an indicator of the onset of smouldering show that both hydrogen and methane are detected when the smoldering temperature is reached but not below it. (By contrast, some carbon dioxide and monoxide are invariably present and detectable in the bin atmosphere, even if the lignite is not smoldering.) Thus the detection of either of these gases and a subsequent rapid rise in their concentrations is an approach to the early detection of smoldering. The use of methane as the "indicator gas" was recommended on the basis of easier detection and measurement in practical applications [77]. 9.3.6 Dust Explosions Very little is known about lignite dust explosions. It has been speculated that there may be chemical or physical factors peculiar to low-rank coals that could be exploited for prevention of dust explosions [ 103].

REFERENCES

10

W.R. Kaiser, Electric power generation from Texas lignite, in: G.H. Gronhovd and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Dept. Energy Rept. No. GFERC/IC77/1, (1978), pp. 328-358. E.A. Elevatorski, Strip-Mineable Coals Guidebook, Minobras, Dana Point, CA, 1980. W.H. Young and R.L. Anderson, Thickness of bituminous coal and lignite seams at all mines and thickness of overburden at all strip mines in the United States in 1950. U.S. Bur. Mines Inf. Circ. No. 7642, (1952). W.R. Kaiser, W.B. Ayers, Jr., and L.W. LaBrie, Lignite resources in Texas, Texas Bur. Econ. Geol. Rept. Invest. 104, (1980). J.R. LaFevers, D.O. Johnson and A.J. Dvorak, Extraction of North Dakota lignite: environmental and reclamation issues, U.S. Dept. Energy Rept. No. ANL/AA-7, (1977). A.C. Fieldner, Analyses of Michigan, North Dakota, South Dakota, and Texas coals, U.S. Bur. Mines Tech. Paper 700, (1948). R.E. Reis, F.M. Sandoval and J.F. Power, Reclamation of disturbed lands in the lignite area of the northern plains, in: G.H. Gronhovd and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Dept. Energy Rept. No. GFERC/IC-77/1, (1978), pp. 309-327. J.E. Russell, Surface mining technology, practices, and plans for Gulf Coast lignite, in: W.R. Kube and G.H. Gronhovd (Eds.), Technology and Use of Lignite, U.S. Dept. Energy Rept. No. GFETC/IC-79/1, (1979), pp. 388-425. Energy Information Administration, Demonstrated reserve base of coal in the United States on January 1, 1979, U.S. Dept. Energy Rept. No. DOE/EIA-0280(79), (1981). C.H. Wayman and J.A. Green, Surface coal mining in the northern Great Plains of the

447 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

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448 37 38 39 40 41 42 43

44 45 46 47 48 49 50 51 52 53

54 55 56 57 58 59 60

J. Waagepetersen and J. Kofod, Research with coal fly ash as soil amendment in Denmark, Proc. 9th Intl. Ash Use Symp., Elect. Power Res. Inst. Rept. No. GS-7162, Vol. 3, pp. 58/1-58/14, (1991). R.J. Haynes, R.M. Cushman, J.F. McBrayer and R.D. Roop, Determining environmental impacts of future lignite mining in the South, U.S. Dept. Energy Rept. CONF-791009, (1979). C.R. Pollock, Long-term impacts of surface mining on groundwater in Texas delta plain lignite mines, Bull. Assoc. Eng. Geol., 20 (1983) 1-4. F.W. Chichester, Premining evaluation of forage grass growth on mine soil materials from an east-central Texas lignite site: 2. Soil profile horizons, Soil Sci., 135 (1983) 236-244. L. Hofmann, R.E. Ries and R.J. Lorenz, Livestock and vegetative performance on reclaimed and nonmined rangeland in North Dakota, J. Soil Water Conserv., 36 (1981) 4144. F.J. Kay and D. Bartsch, Spreader and bucketwheels prestrip mine, World Min. Equip., 8 (1984) 52-53. W.B. Ayers, Jr. Sedimentologic controls on lignite quality and mining: Sandow lignite mine, lower Calvert Bluff formation, east-central Texas, in: R.B. Finkelman and D.J. Casagrande (Eds.), Geology of Gulf Coast Lignites, Environmental and Coal Associates, Houston, 1986, pp. 40-53. A.P. Sanda, Texas lignite mining, Coal, 97(7) (1992) 36-39. D.R. Williamson, Lignites of northwest Louisiana and the Dolet Hills lignite mine, in: R.B. Finkelman and D.J. Casagrande (Eds.), Geology of Gulf Coast Lignites, Environmental and Coal Associates, Houston, 1986, pp. 13-28. Ford, Bacon and Davis Inc., The synthetic liquid fuels potential of North Dakota and South Dakota, U.S. Bur. Mines Special Rept., 1951. U.S. Bureau of Mines, Technology of lignitic coals, U.S. Bur. Mines Inf. Circ. No 7691, (1954). R.C. Rice, R.J. Pederson, G.W. Parry and K.L. Williams, Activities in low-rank coal in the northern Great Plains, in: W.R. Kube, E.A. Sondreal and D.M. White, Technology and Use of Lignite, U.S. Dept. Energy Rept. No. GFETC/IC-82/1, (1982), pp. 54-91. W.W. Hickok, D.A. Taylor, R.W. Dutton and R.C. Keirstead, Coal Creek Station steam generators, in: G.H. Gronhovd and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Dept. Energy Rept. No. GFERC/IC-77/1, (1978), pp. 43-73. Anonymous, Falkirk finds its new ideas work, Coal, 95(4) (1990) 38-39. R.E. Murray: The Falkirk Mining Company: From concept to major lignite producer, in: W.R. Kube, E.A. Sondreal and C.D. Rao (Eds.), Technology and Utilization of LowRank Coals, U.S. Dept. Energy Rept. No. DOE/METC/84-13Vol. 1, (1984), pp. 390-398. Anonymous, Knife River hones an edge, Coal, 94(9) (1989) 63-64. G.H. Groenewold, R.D. Koob, G.J. McCarthy, B.W. Rehm and W.M. Peterson, Geological and geochemical controls on the surface evolution of subsurface water in undisturbed and surface-mined landscapes in western North Dakota, N.D. Geol. Surv. Rept. Invest. No. 79, (1983). A.K. Neill, Western mining--future challenges, in: W.R. Kube, E.A. Sondreal and C.D. Rao (Eds.), Technology and Utilization of Low-Rank Coals, U.S. Dept. Energy Rept. No. DOE/METC/84-13Vol. 1, (1984), pp. 361-363. Anonymous, Pacific rim coal market shows potential for western coal exports, Coal, 95(5) (1990), 9. O.B. Johnson and R.F. Middleton, Big Stone plant: design features and fuel handling, in: G.H. Gronhovd and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Bur. Mines Inf. Circ. No. 8650, (1973), pp. 34-48. H.L. Arms, Deliverability--2 years later, Proc. Low-rank Coal Upgrade Technol. Workshop, EPRI Rept. No. TR-102700, (1993), pp. 10-1 - 10-25. R.C. Ellman, J.W. Belter and L. Dockter, Freezeproofing lignite, U.S. Bur. Mines Rept. Invest. No. 6677, (1965). L.E. Paulson, S.A. Cooley and R.C. Ellman, Shipment, storage, and handling characteristics of dried low-rank coals, in: G.H. Gronhovd and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Bur. Mines Inf. Circ. No. 8650, (1973), pp.49-75. R.C. Ellman, L.E. Paulson and S.A. Cooley, Commercial-scale drying of low-rank

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77 78 79 80 81 82 83 84 85 86

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450 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101

102 103

J.L. Elder, L.D. Schmidt, W.A. Steiner and J.D. Davis, Relative spontaneous heating tendencies of coal, U.S. Bur. Mines Tech. Paper No. 681, (1945). Energy Resources Co. Inc., Low-rank coal study. Vol. 5. RD&D program evaluation, U.S. Dept. of Energy Rept. No. DOE/FC/10066-I (Vol. 5), (1980). .I.M. Kuchta, V.R. Rowe and D.S. Burgess, Spontaneous combustion susceptibility of U.S. coals, U.S. Bur. Mines Rept. Invest. No. 8474, (1979). Z.G. Yalcin, S. Piskin, S. Unal, S. Dincer, and N. Kadirgan, Determination of oxidation in air and ignition tendency of some Turkish lignites, Kim. Kim. Muhendisligi Semp., 4 (1992) 485-487. .I.C. Jones and M. Vais, Factors influencing the spontaneous heating of low-rank coals, J. Hazard. Mater., 26 (1991) 203-212. H.N.S. Schafer, Aerial oxidation of brown coals follwoing exchange of acidic groups, Fuel, 57 (1978) 686-692. V.A. Sukhov, V.B. Zamyslov, O.I. Egorova, Z.A., Davydova, and A.F. Lukovnikov, Initiation of the oxidation of coal of the Irsha-Borodino deposit by molecular oxygen, Solid Fuel Chem., 11(1) (1977) 38-41. Z.A. Davydova, V.A. Sukhov, Y.N. Nedoshivin and A.F. Lukovnikov, Formation of paramagnetic centers in the oxidation of brown coal, Solid Fuel Chem., 12(1) (1978) 4753. V.A. Sukhov, V.B. Zamyslov, Z.A. Davydova and A.S. Lukovnikov, Question of initiation in the oxidation of brown coal by oxygen, Solid Fuel Chem., 11(4) (1977) 9899. I.V. Aleksandrov and A.I. Kamneva, Derivatographic investigation of the organomineral compounds of brown coals, Solid Fuel Chem., 10(2) (1976) 76-79. N.K. Larina, O.K. Miesserova, Z.S. Smutkina and G.S. Kovalenko, Investigation of the nature of the organomineral compounds in brown coals, Solid Fuel Chem., 10(3) (1977) 35-40. K. Brooks, N. Svanas, and D. Glasser, Evaluating the risk of spontaneous combustion in coal stockpiles, Fuel, 67 (1988) 651-656. O.C. Ongstad, Storage of dried lignite, Unpublished manuscript, Grand Forks, ND, c a . 1950. S.A.Cooley, L.E. Paulson and R.C. Ellman, Observations on test stockpiles of dried lignite and subbituminous coals, U.S. Dept. Energy Rept. GFETC/RI-80/5 (1981) L.E. Paulson, D.N. Baria and W.R. Kube, Preparation activities with low-rank coals at the Grand Forks Energy Technology Center, in: D.M. White, E.A. Sondreal and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Dept. Energy Rept. No. GFETC/IC-82/1, (1982), pp. 756-774. D. B0cker and H. Kreusing, Pulverized lignite--production and use, ZKG International Jahrgang 34(5) (1981) 221-226. R.G. Anthony, Reactivity of low-rank coals, in: H.H. Schobert (Ed.) Low-rank Coal Basic Coal Science Workshop, U.S. Dept. Energy Rept. CONF-811268, (1982), pp. 227242.

451

Chapter 10

B E N E F I C I A T I O N OF L I G N I T E

This Chapter discusses processes for the beneficiation of lignites. Since the as-mined moisture content of lignites may exceed 40%, and since the moisture content affects many aspects of transportation, storage, and use of lignites, the most important process is drying. Combustion of lignites for power generation dominates the commercial application of lignites, and in this application the most significant technical problem is the deposition of ash and slag on heat exchange surfaces in boilers. Although reduction of inorganic components, and particularly the removal or reduction of ion-exchangeable sodium, is not presently a commercial practice, this topic has received considerable attention at the bench and pilot plant scales, and may attain cornmercial importance in the future. For most North American lignites the sulfur content is not high enough to warrant processes especially designed for desulfurization, although research proceeds vigorously on desulfurizing lignites from other parts of the world.

10.1 DRYING 10.1.1 Rotary drying Full understanding of all the forms of moisture in lignite and processes of moisture removal or release does not yet exist. Moisture is incorporated in lignite as water absorbed on the surface, water condensed in the pores, water of hydration of inorganic components such as clay minerals, and water released via the thermal decomposition of organic oxygen functional groups. Lignites contain more natural bed moisture than coals of higher rank, reflecting the dependence of pore volume on rank, which shows a progressive decrease from peat through the bituminous coals. Freshly mined lignite undergoes rapid moisture loss on exposure to air. The vapor pressure of water lost during this phase of drying is that of ordinary water, suggesting that the first water to be lost is that absorbed as surface or "loose" moisture. This type of moisture may also correspond to the so-called "freezable" water [1]. With continued drying, the vapor pressure decreases, suggesting that water is being lost from fine pores, where forces of capillary attraction must be overcome to achieve evaporation. (This moisture may be the "non-freezable" water [1].) As the lignite dries, shrinkage collapses some of the pores, with the effect that dried lignite cannot reabsorb as much water as originally held. The pore collapse thus introduces a hysteresis

452 effect during cycles of moisture loss and reabsorption. A very rapid initial shrinkage characterizes the drying of four North Dakota lignites (Beulah, Freedom, Gascoyne, and Glenharold) [2]. Pore collapse occurs at the expense of large voids that lose their water at nearly 100% relative humidity [2]. These large voids likely contain only surface moisture [2]. Irreversible collapse occurs during loss of water from transitional pores, macropores, or both [2]. For any drying process, the relationship between moisture removal and the moisture content of the dried product is shown in Figure 10.1 [3]. For thermal drying, the thermal requirements, expressed either as per tonne of lignite or per tonne of moisture removed, are shown in Figures 10.2 and 10.3, respectively [3].

9 35% moisture

~500 "~I ~ ' 0 400 --~

9 40% moisture ~-~

9

~ 2oo

~ loo o

0

. . . .

0

i

10

. . . .

i

20

. . . .

i

30

. . . .

i

40

. . . .

50

Moisture content of dried lignite, percent Figure 10.1. Moisture removal as a function of the moisture content of dried product [3].

Lignites dried in air show a pronounced tendency toward slacking. The initial shrinkage accompanying loss of water sets up large stresses in the lignite which can then result in macroscopic cracking [2]. Consequently, handling operations involving dried lignites can generate dust. Dust formation is related to the extent of drying, the initial particle size, and the petrographic composition of the lignite [4,5]. There is conflicting evidence whether the air-drying of lignites increases their reactivity. The reactivity of freshly dried lignites toward oxygen does not vary with moisture content [5]. Rather, the temperature of the dried lignite appears to be more significant. Cooling the product from the dryer before exposing it to air reduces the reactivity toward oxygen. Reactivity in liquefaction is greatly reduced by the collapse of the pore structure during air drying [5]. (This problem led to investigations of drying coals in a heated recycle solvent stream to avoid air-drying.) Steam-dried lignite does not have the flaky appearance of air-dried lignite. In fact, the

453 1.2

....I I

9

120~ exit gas

9 175~ exit gas as

.~0

1-

0

0

N o.8 .~

-

~ 0.6-

0.4

....

0

I ' ' ' ' 1

5

....

10

I ....

15

I ....

20

I ....

25

30

Moisture content of product, percent Figure 10.2. Thermal requirements for drying lignite initially of 37% moisture content, for different dryer exit gas temperatures [3].

exterior surfaces of steam-dried lignite are quite tough, showing less size degradation on subsequent handling than does air-dried lignite. Steam-dried lignite does not reabsorb moisture from the atmosphere to the extent that air-dried lignite does. On exposure to saturated air, dried lignites regain only a portion of the moisture removed [5]. The lower the moisture content of the dried lignite, the lower will be the moisture content after reabsorption. Reabsorption is a slow process, requiring three to four days to complete [5,6]. (In this context, reabsorption reaching completion means only that reabsorption ceases; it is not meant to imply that the dried lignite fully regains the moisture content it had before drying.) The Roto-Louvre dryer exemplifies the general class of through-circulation dryers, which operate by passing hot gases through a permeable bed of the wet solids. Because of the large bed area exposed to the hot gases and the short distance for moisture to travel between the solids and the gas stream, through-circulation dryers can provide relatively high drying rates.The RotoLouvre dryer operates by passing heated gas through the bed of lignite without the lignite having to fall through a gas stream, as in conventional rotary dryers. As the dryer rotates, the louvres impart a lifting and then cascading motion to the bed and at the same time help move material to the discharge end. This design of the Roto-Louvre equipment should, in principle, make it less likely

454

120~ exit gas 4.5

9 175~ exit gas §

O

230~ exit gas

4

~" 3.5

3 v

4 2.5 1 . . . . i . . . . i . . . . i . . . . i . . . . i . . . . 0 5 10 15 20 25 30 Moisture content of product, percent Figure 10.3. Thermal requirements for drying lignite of 37% moisture content, based on amount of water removed [3].

to cause excessive size reduction than would be experienced in other rotary dryers. The amount of product, from an unspecified lignite of 34% moisture sized to 38x13 mm, passing a 4 mesh screen ranged from 29% of feed for a final moisture content of 19.4% to 59% of feed for a final moisture content of 15.1%. Drying to an 18.6% moisture content required 4.4 MJ/kg of water evaporated [7]. The Roto-Louvre dryer has low capacity and heat efficiency, and the size degradation experienced by the product far exceeds that typically associated with steam drying. These disadvantages outweigh the advantages of continuous operation and easy control of the RotoLouvre system. Rotary drying of Gascoyne (North Dakota) lignite, with feed size of 75x0 mm, decreased the amount of + 19 mm material by 40%. The amount of 9.5 mm material dropped from 60% in the feed to 36% in the product [4]. The moisture was reduced from 39% to 22%. The effect of this drying operation on the size consist is shown by the data in Table 10.1 [4]. The moisture content of the dried product as a function of particle size is shown in Figure 10.4 [4]. The +38 mm particles had no significant moisture loss, but the 28x0 mesh product had dried to 12% moisture [4]. For this lignite in this test all 9.5x0 mm material had dried to less than the average moisture content, while larger particles had moisture contents greater than the average. The existence of the

455 40

35 " ~I 30

.25 "

~ 2o 0

~ 15 1 10 .~a O 5 0

!

i

i

i c:) v

Particle size of product, mm Figure 10.4. The moisture content of dried Gascoyne lignite as a function of particle size [4]. The moisture content of the feed was 39.0%. The average moisture content of the product was 22.0%.

TABLE 10.1 Change in size distribution of lignite during rotary drying [4]. Size +38 mm 38x 19 mm 19x9.5 mm 9.5 mm x 4 mesh 4x8 mesh 8x14 mesh 14x28 mesh 28x50 mesh 50x 100 mesh lOOx200 mesh -200 mesh

% in Feed 22.9 19.3 17.7 13.5 10.8 7.9 4.4 2.2 1.1 0.4 O. 1

% in Product 9.0 15.1 12.4 10.5 11.6 13.7 13.3 8.4 4.0 1.5 0.4

+9.5 mm particles in the product may derive from the fact that they had not experienced significant moisture reduction [4]. The dust loss during this drying operation was 7.7% of the feed (mf basis) [4]. The dust was 40% -200 mesh and had a moisture content of 7.5% [4]. The heat requirement was 6.3 MJ/kg water for drying Gascoyne lignite from an average 39.0% moisture to 22.0% in a rotating drum dryer [4]. This work was conducted in a unit having a

456 nominal 13 t capacity. The lignite, sized 75x0 mm, was dried in seven lots, at a rate of 14 t/h. Gas inlet temperatures were 346-468~ 6-10~

and exit temperatures, 64-690C. Lignite temperatures were

entering the dryer and 60-68"C exiting. Considerable size degradation occurred during

drying. The amount of +9.5 mm material decreased from 60% in the feed to 36% in the product [4]. The variation of moisture as a function of particle size in the dried lignite is shown in the histogram in Figure 10.5 [4]. All 9.5x0 mm particles had moisture contents less than the average 22.0% of the dried product, while larger particles contained more-than,average moisture. The +38 mm particles had lost virtually no moisture. The dust produced amounted to 7.7% (mf basis) of the feed; 40% was -200 mesh. The moisture content of the dust was 7.5%. Bulk density was determined first on an uncompacted basis using a quantity weighed in a 2000 mL graduated cylinder; a compacted bulk density was then determined after extended shaking of the cylinder and contents. The values were 647 and 811 kg/m3, respectively. The dried lignite was shipped from Pekin, Illinois to Grand Forks, North Dakota in open rail cars (the shipment was discussed in Chapter 9).

300

9

Untreated 5gnite

9

HWD, 270~

<> HWD, 300~ +

HWD 330~

100-

r

l0

....

0

i ....

l0

I ....

20

I ....

30

I ....

40

I ....

50

I''''1

60

....

70

I ....

80

I ....

90

100

Cumulative weight percent oversize

Figure 10.5. The particle size distribution of 225 ~tm Indian Head lignite after hot-water drying at various temperatures [ 14]. The particle size distribution of the untreated lignite was obtained by wet sieving.

Lignites dried in a Roto-Louvre dryer to about 8% moisture will reabsorb moisture to about

457 the 22-25% range. Most of this gain of moisture occurs in the first two to three days after exposure to air [6]. Russian brown coals show a decrease in porosity when dried in the temperature range 20-80~

followed by an increase in porosity in the range 80-140 ~ and then

another decrease with drying temperatures above 1400C [8]. The dried coal is less hygroscopic with increased drying temperature. 10.1.2 Entrained and fluidized drying Size degradation on thermal drying of lignite in heated gases can be turned to advantage when it is desired to dry fine particles. Processes using entrained flow or fluidized beds are therefore potentially useful for drying lignite in fine particles. The Raymond flash dryer mixes hot combustion gases with pulverized coal in a so-called drying column [7]. The dried product is recovered in a cyclone separator. Particles up to 9.5 mm can be dried in this equipment. The Raymond system is limited to material having a maximum moisture content of about 25% [7]. Feeds of higher moisture content would have to be blended with a recycle stream of a portion of the dried product. Finely pulverized, thoroughly dried lignite will likely undergo autogenous combustion. This problem affects all entrained or fluidized drying systems. Unless the dried product will be used immediately, special precautions must be taken for its safe storage or shipment. Furthermore, lignite dried in this way readily regains moisture upon exposure to the atmosphere. Design data for entrained-flow drying, including heating time as a function of gas temperature for various heat-transfer coefficients, have been published [3]. 10.1.3 Hot-Water Drying Hot-water drying involves heating a lignite-water slurry under pressure to remove water from the high-moisture lignite [9]. Some of the carboxylic acid groups might decompose to carbon dioxide. Formation of carbon dioxide enhances dewatering by forcing liquid out of the pores; furthermore, the loss of hydrophilic carboxyl groups from the surface reduces the tendency to reabsorb water. (The loss of carboxyl also involves a loss of some of the sodium originally bound to these groups, providing an advantage when the product is to be used as a boiler fuel.) Since the water removed from the lignite is taken off as liquid rather than vapor, energy requirements are more favorable than for drying processes which involve evaporating the water. With high solids loadings, the water removed in the drying process can be used as the vehicle for slurrying the solids, thus reducing water consumption for the process. Hot-water drying of Victorian brown coal has achieved moisture reductions of 75-80% [ 10,11]. Hot-water drying removes both moisture and carbon dioxide from lignite, increasing the calorific value of the product to about 27.9 to 32.6 MJ/kg [12]. Drying North Dakota lignite in a water slurry for 1 h at 320~

increased the calorific value from 21.8 to 26.5 MJ/kg [13]. This

process was accompanied by a mass loss of 21.5% [13]. The ash content decreased by 16% and the total sulfur content by about 10% [13]. Water and carbon dioxide are the principal products.

458 Water comes from both the moisture in the lignite and the thermal destruction of oxygen-containing functional groups. Carbon dioxide arises from cleavage of the carboxyl groups, which may account for up to 25% (by weight) of the oxygen in lignite. Carbon dioxide may derive in part from carbonyl groups as well [ 13]. The hot-water drying of Indian Head (North Dakota) lignite (48 ~tm mass mean diameter particle size) at temperatures of 275, 300, and 335"C decreases the equilibrium moisture of the product as drying temperature increases [14]. The equilibrium moisture of the untreated lignite was 33.0%; the equilibrium moisture values and respective drying temperatures were 275"C, 18.4%; 300"C, 14.6%; and 335~

11.2%. The principal gaseous products are carbon oxides. As drying

temperature increases, the percentage of carbon dioxide in the gaseous product increases from 86 to 92%. Carbon monoxide decreases from 7 to 2%. Most of the other gases--ethane, propane, hydrogen, methane, hydrogen sulfide, and ammonia--show little change. Comparable tests with 225 ~tm particle size Indian Head lignite showed very similar reductions in equilibrium moisture as a function of drying temperature, and increases in carbon dioxide concentration in the product gases. The most notable difference was a higher carbon dioxide concentration in the gas. For example, drying 225 l~m lignite at 279~ produced 91% carbon dioxide in the gas, compared with 86% for 48 0rn lignite at 275~ Dielectric relaxation spectroscopy of hot-water-dried Indian Head lignite showed that all types of moisture detectable with that technique were removed, i.e., not only loosely bound water but also the tightly bound water of hydration [15,16]. In that respect, hot-water drying has the same effect as freeze drying. Reconstituting hot-water-dried and vacuum-dried samples to equilibrium moisture conditions failed to restore the dielectric peak assigned to chemically bound water in the hot-water-dried sample [17]. Thus not only does the hot-water drying remove the chemically bound water, the drying process alters the lignite in such a way as to prevent its reabsorption. Dehydration accompanying drying above 230"C is not completely reversible [18]. Moisture reabsorption by hot-water-dried lignite is very low, even with drying temperatures as low as 270~

For example, hot-water-dried (300~

Indian Head lignite had an equilibrium moisture

content of 14.4%; after exposure of the dried lignite to ambient air for 30 days the moisture content was 15.1% [14]. Soaking the dried lignite in water for 60 days raised the moisture content to this same value [14]. The main process variables that affect hot-water drying and the characteristics of the product slurry are the specific coal used, the temperature, residence time, and the nature of the drying medium. Temperature is of paramount importance, with changes in the structure of the coal occurring above 230~

[18]. In pilot-scale tests with Indian Head lignite, the dehydration,

decarboxylation, pyrolysis, extraction, and carbonization all increased linearly with temperature in the range 230-348~

[18]. In addition, the calorific value increased in this temperature range,

consistent with a drop in oxygen content resulting from decarboxylation. For example, the oxygen content decreased from 17.3 to 9.2% and the calorific value increased from 23.5 to 27.0 MJ/kg

459 (dry basis) as the drying temperature increased over this range. An increase of temperature decreases the equilibrium moisture content of the product [ 19]. An increase of drying temperature increases decarboxylation, dehydration, and pyrolysis of lignite [14]. Drying at 350~

increased the yields of methane, ethane, hydrogen sulfide, and

ammonia, as well as phenol and alcohol compounds. A linear increase in (carbon dioxide + water) yield occurs with increasing drying temperature [20]. A consequence of the loss of carbon via decarboxylation and pyrolysis is a decrease in yield of dried lignite as drying temperature increases. In comparing drying at 300* and 350~

the recovery of original lignite as dried product

dropped from 91% to 80% [14]. Similarly, the net yield of dried lignite drops from 99% (maf basis) at 270"C to 87% at 3300C [20]. Particle size of the feed has only a slight effect on these results. Most of the "lost" material appears as carbon dioxide. A second consequence of decarboxylation is that loss of carbon dioxide increases the calorific value of the dried product. The calorific values of untreated lignite and lignite hot-water dried at 350~ were 23.5 and 28.1 MJ/kg (dry basis), respectively [14]. Comparable work with the same lignite showed an increase in calorific value, from 24.4 MJ/kg (dry basis) for the product dried at 270 ~ to 25.6 MJ/kg for drying at 3300C [20]. Up to about 95% of the energy content of the untreated lignite can be retained in the dried product. Evidence for a more severe pyrolysis at the higher drying temperatures is the observed formation of tars and light oils during drying at 350 ~. Some of the sulfur in the lignite is lost as hydrogen sulfide, and some of the exchangeable inorganic ions (i.e., sodium, calcium, and potassium) were extracted into the surrounding aqueous drying medium. Results for hot-water drying of Indian Head lignite are summarized in Table 10.2 [ 19]. Table 10.2 Comparison of results for hot-water drying Indian Head lignite at various temperatures [ 19]. Temperature, ~ 231 Pressure, MPa 2.8 Net yields, wt% maf basis Decarboxylationa 1.6 Pyrolysis gasesb 0.0 Pyrolysis liquidsc 0.0 Residual lignite 98.1 Product lignite characteristics Cal. value, MJ/kg 24.1 Equil. moist., wt% 20.9 Lignite slurry properties Max. solids conc., % 51.0 Cal. value, MJ/kg 12.3

272 6.2

302 9.7

333 15.9

348 17.2

9.8 0.0 0.2 92.2

13.8 0.2 0.4 87.7

14.8 0.2 0.4 85.9

17.5 0.1 0.8 83.1

25.0 18.0

26.0 14.5

26.4 13.5

26.9 7.4

50.9 12.8

54.2 14.1

55.3 14.6

55.6 15.0

Notes: a(CO + CO2 + H20), b(H2 + C1-C4 hydrocarbons + H 2S + NH3), c(phenols, methanol, acetone, and methyl ethyl ketone)

The extent of decarboxylation depends strongly on the drying temperature. The greatest

460 decrease in carboxylic acid groups occurs in the range 260-340~

[9]. Increased drying rates are

observed in this same temperature range. Residence time is not an important factor in hot-water drying, when the equilibrium moisture content of the product or the characteristics of the resulting slurry are used as criteria, presumably because the rate of heat transfer through the coal particles is very rapid. For -60 mesh particles most of the dewatering occurs in the first five minutes of processing; only minor reductions in equilibrium moisture of the product are obtained with longer residence times [18]. After 15 min at a nominal 300~ drying temperature, the equilibrium moisture of the product was 15.2%; increasing the residence time by a factor of 16, to 240 min, reduced the equilibrium moisture of the product only to 13.3% [18]. However, both decarboxylation and attendant sodium reduction increase with residence time. In these same experiments, the yield of the carbon oxides and water increased from 7.0 to 17.7% (maf basis) and the percentage of original sodium removed increased from 46% to 67% as residence time increased from 15 to 240 min [18]. Drying of 9.5x6 mm particles Indian Head lignite and particles from three levels of grinding, having mass mean diameters ranging from 250 ~tm (95% -720 ~tm) to 25 0m (95% -210 ~tm), showed the maximum solids content for the product slurry (on a bone-dry solids basis, for the maximum loading still providing a free-flowing slurry without additives) to be independent of the original particle size, being about 56% [18]. However, the viscosity of the slurry produced from the finest grind lignite was nearly double that of the slurries of the other tests. The equilibrium moisture content of the finest grind material was slightly lower (9.5% and the calorific value slightly higher (26.3

vs.

vs.

11-12%)

24.9-25.6 MJ/kg, mf basis). The calorific values

of the slurries from the four samples were in the range 13.7-14.6 MJ/kg [18]. Slurry concentration does not affect moisture concentration, for drying North Dakota lignite at solid/liquid ratios of 1-3, temperatures of 200-285~

and particle size 100x140 mesh [21].

Moisture reduction varied linearly with temperature. Sodium content of the lignite decreased. The equilibrium moisture content of the dried lignite decreased with increased drying temperature. Untreated Indian Head lignite had a surface area of 153 m2/g (by heat of immersion calorimetry), whereas the lignite hot-water dried at 340~ had a surface area of 98 m2/g [22]. In comparison, the lignite dried in nitrogen at 330~ had a surface area of 263 m2/g. Nuclear magnetic resonance spectroscopy of untreated and hot-water-dried Indian Head lignite shows a marked loss of aliphatic waxes and long-chain alkanes on drying [19]. However, the tars appear to be of greater relevance, with tar condensation on pore surfaces and in pore openings decreasing the surface area by 30-40% as measured by gas adsorption [19]. Tars or waxes mobilized during the hot-water drying process apparently act as plugs or caps, simply sealing over the mouths of the pores. This hypothesis is substantiated by the observation that vacuum drying of a hot-water-dried sample increases the surface area, in this case from 98 to 171 m2/g [22]. This increase may arise from the mechanical effect of the reduced external pressure providing sufficient pressure differential to dislodge the "plug" or "cap" in the pore mouth. (The near doubling of surface area during evaporative drying was attributed to the driving off of tars and waxes; other effects known to

461 increase surface area, such as loss of carboxyl groups [23], were apparently not considered.) Fluorescence microscopy (blue light excitation with a mercury light source) showed that the liptinitic macerals experienced a distinct decrease in the intensity of fluorescence and change in color after both hot-water and nitrogen drying processes. Resinites showed no strong evidence of deformation due to heating [24]. Hot-water-dried lignite had a total surface area of 38.2 m2/g, which was distributed among the macro-, transitional, and micropores as 3.55, 6.00, and 28.7 m2/g, respectively (by small angle X-ray scattering, using molybdenum radiation) [25]. In comparison, the untreated sample had a total surface area of 32.0 m2/g, which included 2.60 m2/g in macropores, 2.86 m2/g in transitional pores, and 26.5 m2/g in micropores. Surface areas of Indian Head lignite dried at essentially the same temperature (specifically, 340~ for hot water drying, and 3300C for steam and nitrogen drying) by three different methods are shown in Table 10.3 [25]. The data derive from heat of immersion calorimetry in methanol; a conversion of 2.56 m2/J was assumed to calculate the apparent surface area. TABLE 10.3 Apparent surface areas of Indian Head lignite dried at 330-3400C [25].

Drying Method None Hot Water Dried Steam Dried Dried in Hot Nitrogen

Heat of Immersion (cal/g) 59.8 38.5 14.6 102.5

Apparent Surface Area (m2/g) 153 98.0 37.8 263

Proximate and ultimate analyses of untreated and hot-water-dried Indian Head lignite are shown in Table 10.4 [14]. These data are for 48~tm mass mean diameter lignite dried at 330~ The principal cause of the changes observed is decarboxylation. Other pyrolytic reactions are relatively minor, even at 3480C. At this temperature the combined yields of phenols, alcohols, and hydrocarbons amounted to less than 1% (maf basis) of the lignite, while the yields of carbon oxides and water were 18% [18]. Loss of carboxyl groups destroys sites for incorporation of exchangeable cations of alkali and alkaline earth elements; consequently the amounts of these elements in the lignite are reduced by the drying process. For Indian Head lignite, 20% of the sodium was removed at 230~ and 60% at 348~ [ 18]. Incorporating a washing step with filtration after drying can effect sodium reductions of 90% for lignite dried at 348 ~. The percentage of the ash, determined on the untreated lignite, lost in hot-water drying of 9.5x6 mm Indian Head lignite ranges from 17.8% at 301 ~ to 37.9% at 3500C [14]. Water can extract soluble inorganic species. Decarboxylation destroys some of the exchange sites responsible

462 TABLE 10.4 Analyses of untreated 48 lxm MMD Indian Head lignite and material hot-water dried at 3300C [14]. Untreated

Dried

33.9 31.3 24.5 10.3

43.1" 23.3 25.1 8.5

61.0 3.7 1.3 1.0 17.4 23.5

64.4 4.2 0.8 0.8 14.8 25.6

Proximate (as-received) Moisture Volatile Matter Fixed Carbon Ash U1timate (moisture-free) Carbon Hydrogen Nitrogen Sulfur Oxygen Calorific Value, MJ/kg (mf)

*Moisture content of the recovered filter cake.

for holding alkali and alkaline earth cations. Only about 20% of the sodium in the untreated lignite remained in the dried product. Conversion of some of the sulfur to hydrogen sulfide and its subsequent loss removes sulfur which might otherwise have been captured during the ashing process (Chapter 6), forming sulfates that contribute to the weight of the ash. Some inorganic material may be liberated by physical washing and then removed when the process water is drained. Lignite particle size decreases during hot-water drying. Furthermore, the lignite particles become increasingly finer as drying temperature increases [14]. This behavior is illustrated in Figure 10.6 [ 14]. The ability to form stable slurries in water increases as the drying temperature increases, and represents an improvement over the ability to slurry the raw coal. For example, for Indian Head lignite the bone-dry solids loading of a slurry of untreated lignite is 40%, while the comparable value for a slurry of lignite dried at 230~ is 51% [18]. "Slurrability" also increases with increasing residence time [19].

463 100

a~

80-

,1)

ga~ o -.o

60 40

o

20 0

'

'

'

2130

'

I

'

250

'

'

'

I

'

300

'

'

'

350

Drying temperature, ~ Figure 10.6. Moisture retention during steam drying as a function of temperature (adapted from [26]). The curve is a fit of data for three lignites and two subbituminous coals with particle sizes in the range 1.3-3.8 cm.

10.1.4 Steam and hot-water drying The comparative performances of steam and hot-water drying were evaluated in batch autoclave tests of a suite of low-rank coals including Velva, Baukol-Noonan (both North Dakota) and Fairfield (Texas) lignites. Steam drying is effected by charging the autoclave with water and suspending the sample in a stainless steel basket above the water, at autoclave temperatures in the range 230-300~

In hot-water drying, the autoclave is charged with a slurry of lignite and water

and then heated to the desired drying temperature [26]. Moisture removal is a function of residence time for times less than 15 minutes, but reached a constant value at times greater than 15 minutes [26] for both steam and hot-water drying. Moisture removal does not depend on particle size in the size ranges tested. For hot-water drying the mean particle diameters ranged from 0.07 to 4.70 mm; for steam drying, from 1.27 to 3.81 cm. In steam drying, moisture reduction depends on drying temperature. This effect is illustrated in Figure 10.7 [26]. The maximum moisture reduction, about 85%, occurs above 300~

after which

further increases in temperature do not cause additional significant removal of moisture. The change of slope at 250~

corresponds to the onset of decarboxylation, and the carbon dioxide

formed assists in pushing water out of the pores [27]. The effect of drying temperature on carboxyl groups in Fairfield lignite is shown in Figure 10.8 [26]. Considerably more scatter occurs in the relationship between moisture reduction and drying temperature for hot-water drying, but, very roughly, the same trend is seen [26]. The equilibrium moisture content of the dried lignites is substantially below that of the

464 2.5 2

1.5 " O o

~

1-

M

~O 0.5 0 0

'

200

'

'

'

I

'

'

'

'

I

'

'

'

'

350

250 300 Drying temperature, ~

Figure 10.7. The carboxyl group content of hot-water-dried Fairfield lignite as a function of drying temperature [26].

~i 9

7-

.~

6-

"~4 ~ O

5

4 o

3

~

2

,,,,i,

0

'''1'''

'1''''1

''''1''

''1''''

1 2 3 4 5 6 pH of aqueous sulfuric acid

Figure 10.8. Sodium removal as a function of solution pH for the ion exchange of Beulah lignite with aqueous sulfuric acid [26].

untreated coals. For example, the moisture content during storage at 100% humidity of Fairfield lignite is about 36%" after steam drying at 322"C the moisture content is about 8% even after storage at 100% humidity for 32 days [26]. Similarly, the moisture content of Baukol-Noonan

465 lignite stored at 100% humidity is about 33.5%; this lignite hot-water dried at 284~ has an initial moisture content of 8%, which rises to 15.5% after 24 days' storage at 100% humidity. Even though the hot-water-dried lignites increase in moisture during storage, the moisture content eventually attained still does not approach that of the untreated lignite. About 80% of the total sodium content was removed from Fairfield lignite by steam drying at 320~

and roughly 65% by hot-water drying. Sodium removal during hot-water drying was not

affected by temperature in the range 260-320~

[26]. Magnesium and calcium are reduced by

10--40% on steam drying and 0-10% on hot-water drying [26]. Results for potassium vary greatly and depend on the specific lignite being dried. During steam drying, potassium removal ranged from 10% to 90%, depending on the lignite. With hot-water drying, however, the maximum potassium reductions were on the order of 15% [26]. The increased calorific value varies linearly with the drying temperature. For hot-waterdried Fairfield lignite, the calorific value of the material dried at 340~ increased by 12%, being 25.0 MJ/kg (dry basis) for the dried product and 22.3 MJ/kg for the untreated lignite [26]. Even better improvements in calorific value occur in steam or hot-water drying of Usibelli and Beluga (Alaskan) low-rank coals. For these coals, calorific value increases from = 18.5 to =27.8 MJ/kg on drying at 275-325~

[28].

10.1.5 Steam drying Saturated steam at high pressure allows the drying of lignites with a minimum of physical disintegration [29]. So-called woody lignites give the best performance in this process, being subjected to the least decrepitation [30]. Dehydration is accompanied by a collapse of some of the residual plant cell structures in the coalified woody tissue [31 ]. Steam drying uses saturated steam under pressure to remove moisture. In this process the lignite shrinks but the particles retain their lump form. The mechanisms of water removal are evaporation by heating, physical removal of liquid water from the pores, and flash evaporation during depressurization [32]. At the same time, carbon dioxide is lost by thermal decarboxylation. The surface of the steam-dried lignite is chemically altered to reduce the moisture retaining capacity. A variety of mechanisms have been advanced to explain the physical removal of liquid water (i.e., without evaporation) [33]. These include the contraction of pores during particle shrinkage squeezing water out, the difference in thermal expansion of the water and lignite forcing water from the pores, chemical modification of the lignite surface making it less able to retain water, the reduction of viscosity of the water with increasing temperature allowing it to run out of the pores, and the evolved carbon dioxide pushing the water out ahead of it. The observation that increasing amounts of moisture can be removed from low-rank coals in saturated steam by raising the pressure suggests the presence of up to three types of moisture [34]. Free moisture in large pores can be removed easily. Water held tightly in micropores requires increases in pressure (and, consequently, increases in temperature) for removal. Finally, at still higher temperatures and pressures water is released by the decomposition of oxygen-containing functional groups or by

466 liberation of water of hydration contained in minerals. Hysteresis in moisture absorption and desorption has been discussed previously (Chapter 8). Hysteresis occurs for lignite dried over sulfuric acid. Steam-dried lignite, however, does not show comparable hysteresis, nor does steam-dried lignite reabsorb as much moisture as air-dried lignite. These observations support the idea that the steam drying modifies the lignite surface, changing its moisture-absorbing or-retaining properties. At low moisture contents, the partial pressure of water over steam-dried lignite is 60% greater than that over freshly mined, air-dried lignite of the same moisture content [32]. Moisture in the fleshly mined lignite not contributing to the vapor pressure may be strongly bonded to the phenolic --OH groups [32]. This suggestion agrees with observations of drying western Canadian lignite in steam, air, or nitrogen. For this lignite, the content of phenolic --OH had a much greater role in determining equilibrium moisture than either carboxyl or carbonyl groups [35]. Process variables that affect steam drying include steam temperature, the particle size, the type of coal being used, and the drainage time. Pressure does not influence water removal. Water removal increases with increasing temperature at least until the point at which thermal decarboxylation ends. The mechanisms for removal of liquid water, listed above, will also be more effective as temperature increases. Steam drying is heat-transfer controlled, on the basis of kinetic measurements for the drying of Australian brown coal [36]. Improved drainage of water from smaller particles generally increases water removal with decreasing particle size. Four North Dakota lignites dried at 230~ showed increases of 3 to 5.5 percentage units (e.g., from 84% to 87%) for a reduction of particle size from 30x100 mm to 25x38 mm [37]. In principle, water lost during depressurization should be independent of particle size; however, smaller particles will have greater surface area per unit mass and therefore water removal by aeration will be greater for smaller particles [32]. The optimum pressure and dewatering time depend on particle size of the lignite [34]. The degree of drying attained depends on the steam pressure and temperature, the particle size of the moist lignite and the moisture content of the lignite. Because it is not necessary to supply the latent heat of vaporization of the water being removed, the energy requirements for steam drying are less than those for drying with hot gases. Shrinkage of the lignite particles on moisture removal increases the apparent density of the lignite by 15--20% during steam drying [34]. Optimum conditions for dewatering of Austrian and Czechoslovakian brown coals are 211 ~ with a saturated steam pressure of 2.0 MPa [38]. With German brown coal having a 60% moisture content as-mined, dehydration to 20-30% moisture was achieved by treating 10-60 mm particles for 30 min at 230~ with 2.5 MPa saturated steam [39]. For temperatures up to 900C a moisture increase, due to steam absorption, occurs [40]. Thermal pressure dewatering starts at 900C. Dewatering varies linearly with temperature to 200~

due to water draining from the pores.

Above 2300C, decarboxylation accelerates water removal. The prol:x~rtion of the total water lost that is removed by the first step increases with temperature, temperatures above 2400C being recommended [40].

467 Steam drying of lignite also accomplishes sulfur removal [34]. Reaction conditions that cause the breakdown of oxygen functional groups may also be sufficiently severe to decompose sulfur functional groups; hence the prospect exists for a reduction of organic sulfur during the steam drying process. No good predictor of suitability of a given coal for steam drying exists, despite the fact that steam drying characteristics vary from coal to coal. The quantity of carboxyl groups may be a predictor of process performance [32]. Indeed, steam drying may be dominated by destruction of carboxyl groups [32]. Decarboxylation increases rapidly above 180"C in the heating of Australian brown coals [27]. Carbon dioxide is first noted in the vent gases from the Fleissner process when the autoclave temperature reaches 180~

[41]. A normally hydrophilic coal surface becomes

hydrophobic when heated above 250~ [32]. The Koppelman process is a continuous process (as opposed to the batch operation of the Fleissner process discussed below) that will produce lignite having a calorific value of 28 MJ/kg and no tendency to reabsorb moisture [5,42]. The Koppelman process has been developed through the pilot-plant scale. A lignite/water slurry is pumped into a tubular reactor at 10 MPa; pyrolysis occurs in steam at 540~

[5,42]. The dried lignite discharges through a lock hopper system to be

cooled. The pyrolysis produces gases having a calorific value of 14.9-18.6 MJ/m3; these are collected and burned for power production [5]. The product gases from the Koppelman reactor resemble, in composition, those produced during pyrolysis of lignite in a dry, inert atmosphere. As an example of process performance, lignite having a calorific value of about 16.3 MJ/kg can be treated at 7-10 MPa and 40(O675~ to produce a treated product with a calorific value in the range 27.9-30.2 MJ/kg. A thermal efficiency of 90-92% has been claimed [42], equivalent to a recovery of 80% of dry lignite [32]. Pyrolysis of North Dakota lignite in the absence of steam yields 65% dry solids [10], and extrapolation of early data on Fleissner drying of Kincaid (North Dakota) lignite [43] suggests that a 55% recovery of dry solids is more likely [32]. Pulverizing Mississippi lignite in a fluid energy mill with steam at 240~

reduced the

moisture content from 40.9 to 14.1% [44]. When compared to particles pulverized with air at 120~

the steam-dried lignite seemed to be smoother and less porous.

10.1.6 Fleissner process Drying of lignite, whether by natural weathering or by deliberate thermal drying procedures, inevitably causes size degradation from the unequal stresses set up in the lignite as shrinkage occurs during water removal. To minimize the decrepetation or disintegration accompanying drying, the entire lump of lignite should be dried at the same rate, that is, moisture removed from anywhere in the lump should be removed at the same rate as moisture being lost from any other site. Drying of lump lignite involves two processes, the transfer of moisture from the interior to the surface, and evaporation from the surface. The observed rate of drying will be governed by whichever of these two processes is the slower. For lignite, migration of moisture

468 from the interior to the surface is the slower step [29]. Thus the available strategies for drying lump lignite with minimal size degradation are to retard the evaporation rate from the surface or to enhance the rate of migration from the interior to the surface. Size degradation has been greatly reduced by heating lignite in the presence of saturated steam, followed by pressure let-down and then moisture evaporation. The Fleissner process represents an approach to the steam drying of lignite on a commercial scale. Steam drying of lignite by the Fleissner process has been operated in Europe since 1927 [5]. Commercial plants based on the Fleissner process operate in Eastern Europe and Turkey. The Fleissner process heats lignite with saturated steam; the high temperatures and pressures of the steam drive moisture from the lignite without significant disintegration of the particles. Changes in the physical--and possibly chemical--structure of the lignite reduce slacking during storage after the drying. The Fleissner process involves charging lignite to an autoclave, introducing high-pressure saturated steam until the lignite particles have reached the steam temperature, venting the steam, and then removing or discharging the dried lignite. These actions are sometimes referred to (e.g., [29]) as periods, viz.: the preheating period, in which the steam pressure is raised to the maximum; the heating period, in which the maximum pressure is held for the desired residence time; the release period of pressure let-down; and the aeration period, in which air is blown over the dried lignite. Because the steam is saturated, the actual drying does not occur until the venting cycle. The heat needed to vaporize the moisture in the lignite derives from the heat in the particles themselves. An advantage of this approach to drying is much less particle breakage than occurs in other drying processes. Consequently the process may be particularly attractive when a lump fuel is the desired product. The combined effects of heat and pressure allow some of the moisture to be expelled as liquid water. Thus a second advantage of the process is that heat requirements are lower than other methods because some moisture is removed without being vaporized. The major disadvantage is that the process is a batch operation; a second disadvantage is the capital cost required for the autoclaves, which is higher than the investment for other types of dryers. The process was introduced in 1927 by Hans Fleissner, a professor of mining in Leoben, Austria. Fleissner hypothesized that controlled removal of the moisture could prevent the uneven shrinkage of the lignite particles which leads to size degradation in conventional thermal drying. The atmosphere of saturated steam prevents the evaporation of water until the lignite particle has been thoroughly heated; then, controlled loss of moisture can be effected by gradual reduction of the steam pressure. Subsequent investigations showed that a portion of the moisture was expelled as liquid water while the lignite was being heated by the saturated steam. This is due to a change in the colloidal properties of the lignite, and a simultaneous reduction in the viscosity of the water. As the temperature increases, some of the pores or cells in the lignite may collapse, and since the viscosity of liquid water is dropping at the same time, conditions are favorable for the expulsion of at least part of the moisture as liquid. (In pilot-scale tests large variations in the release of water were observed from different lignites, this behavior being attributed to differences in the

469 temperature dependence of capillary shrinkage or colloidal properties of different lignites.) In a conventional thermal drying process using hot gases as the drying medium, moisture is removed from the surface of the lignite as vapor at a temperature corresponding to the boiling point of water at the prevailing pressure. Therefore the interior of the lignite particle will not get hotter than this temperature until the moisture has been completely removed, and any structural changes in the lignite occur after removal of the moisture. The first commercial plant for Fleissner drying was installed at the Austrian-Alpine Coal Mining Company facilities in Koflach, Austria in 1927. This plant was increased in capacity several times in the late 20's. In the early 1930's commercial Fleissner plants were installed in Czechoslovakia and Hungary. Observations based on plant operating experience and collateral research investigations established a number of facts regarding the drying process. The so-called woody lignites are the best feedstocks. (The actual amount of water removed as liquid from a given lignite relates to the extent of the colloidal structure of the lignite; the water removed as liquid ranges from 10 to 40% of the total moisture [37].) The degree of moisture reduction obtained depends on the moisture content of the lignite, its particle size, the steam temperature and pressure, and the physical and chemical structure of the lignite. Exposure of a lignite particle to high-pressure saturated steam heats the particle and causes loss of some of the moisture as liquid water as the particle shrinks. The shrinkage reduces the apparent density by about 20%. Reducing the pressure results in evaporation of additional moisture by the sensible heat stored in the particle; further reductions in pressure continue removal of moisture, cooling the particle. No latent heat is required for removing that portion of the water expelled as liquid; consequently, the thermal requirements for steam drying are less than those for drying with hot gases. An increase of about 25% in thermal efficiency is realized [45]. (Removal of some of the moisture as liquid water during drying of lignite with saturated steam is a general phenomenon not unique to the Fleissner process; for example, a similar effect has been reported in autoclave drying of lignites for the Glover-West gasification process [46].) Most of the steam required is actually consumed during the preheating of the lignite particles. Indeed, this accounts for about 75% of the total steam consumption [37]. Size degradation is not as severe as in thermal drying processes. The principal factors that govern the application of the Fleissner process to North American lignites are 1) the amount of water removed depends on the temperature to which the lignite is heated; 2) the time required to heat the lignite is a function of the particle size and is given by t=7D2 where t is the time in minutes and D the particle diameter in inches (the time is apparently not a function of final temperature); and 3) size degradation is not a function of operating pressure, but the larger the size of particles being dried, the greater is the percentage size reduction [7]. The time required to dry 32 mm lignite with steam at 2.8 MPa is 20 minutes [46]. Freshly mined lignite dries better than lignite which has been in storage long enough to slack [37].

470 Although the process performance varies somewhat with the actual lignite being dried and the specific process conditions, typical performance data with a North Dakota lignite of 37% moisture dried at 2.8 MPa show that the weight of the untreated lignite to dried lignite is 1.48, and the ratio of the calorific values of dried to untreated lignite is also 1.48 [37]. The calorific value of the dried lignite increases in direct proportion to the amount of water removed. Roughly 0.75 kg of steam are required to remove 1 kg of water from the lignite [37]. The calorific value of the dried product will be about 23 MJ/kg [37]. The use of different lignites in the Fleissner process can result in wide variations in the release of water from the lignite. These variations have been attributed to differences in the shrinkage behavior of the pore system from one lignite to another, or to differences in "the temperature-dependent colloidal properties," or both [47]. No systematic study of the effects of the chemical and physical structure of lignites on their performance in the Fleissner process seems to have been undertaken. Pilot-scale drying tests with Kincaid lignite have shown the possibility of drying lignite to virtually zero moisture content (typically about 0.6%) by the Fleissner process with saturated steam at 313"C [43]. However, with increasing steam temperatures above 229~

the product became

increasingly hygroscopic and regained moisture during handling. (Since saturated steam serves as the drying medium, specifying a pressure also uniquely fixes the temperature, and vice versa.) Some decomposition of the lignite occurs, the extent increasing with increasing operating pressure. The dried lignite will likely regain moisture to a level of 8-9%, depending on prevailing humidity. Formation of gas ranged from 0.12 m3 to 1.4 m3 per 100 kg of lignite as pressure increased from 2.8 to 10 MPa. Regardless of pressure, 96 to 98% of the gas was carbon dioxide. The CO2 production has been reported to be 1 L/kg of lignite [29], accompanied by small quantities of nitrogen. Evolution of these gases results in an improvement, albeit small, in the calorific value of the lignite. Fleissner drying (at 230~

and 2.5 MPa) of Onakawana (Ontario) lignite reduced the

moisture content from 50% as-received to 23%, with an attendant increase in calorific value from 12.8 to 20 MJ/kg, corresponding to an improvement ratio of 1.56 [48] (the ratio of the calorific value of the dried lignite to that of the raw lignite). The charge was principally +5 cm material; after drying 33.7% remained as +5 cm, the remainder occurring in smaller size fractions to 13.6% in the -6 mm fraction [48]. During the preheating period the lignite absorbs some additional moisture, about 0.21 kg per kg of untreated lignite [37]. (This observation does not agree with results from European laboratories [33].) The improvement ratio increases with steam pressure. In the range of 1.1 to 1.5 MPa the improvement ratio increases from 1.19 to 1.33 [37]. Some gas formation occurs, the amount of noncondensable gas produced being about 0.8 m3/t of untreated lignite at a drying pressure of 1.3 MPa [37]. There does not appear to be a strong correlation between steam pressure and size degradation during drying, although degradation is slightly less at the higher drying pressures. As a rule, the drying of +50 mm lignite will result in conversion of about 30% of the material to-25 mm [37].

471 The time required for the drying cycle relates the size of the particles to be dried. The customary equations governing the conduction of heat through a homogeneous body should not apply to lignite, since lignite is not homogeneous in structure and experiences changes in physical--and probably chemical--structure during heating. Nevertheless, analysis of the heat conduction through lignite has been performed [49] using the law of squares, which relates the time required to heat a body to a specific temperature or temperature distribution to the square of some linear dimension of that body. For spherical particles, the appropriate linear dimension is the radius or the diameter of the sphere. At any steam pressure, the limit of drying in a practical sense occurs when the temperature at the center of a lignite particle is 95% of the surface temperature [37]. With this criterion, a graphical solution to the heat conduction problem for spherical particles shows that T = 0.37RZ/K where T is the heating time in minutes, R the particle radius, and K the thermal diffusivity of the lignite [49]. Thermal diffusivity is the ratio of the thermal conductivity to the product of density and specific heat: K = k/rcp

For lignite the values of these properties were assumed to be a specific heat of 0.58, a density of 1250 kg/m3 (1.25 g/cm3), and thermal conductivity of 0.28 J/h.cm2 [37]. Thus the thermal diffusivity is 5.13 cm2/h, so that T = 0.072R2 or, in terms of particle diameter D, T = 0.018D2 [37]. The heating time for lignite particles ranging in diameter from 0.62 to 17.8 cm will therefore range from 0.44 to 340 minutes [37]. In terms of practical operation of a Fleissner plant, it is likely that for particles smaller than 3.8 cm the total cycle time will be governed by the time needed to open and close the autoclaves and to operate the valves, rather than by heat transfer to the particles. The steam required per kilogram of moisture removed decreases as operating pressure increases [37]. This relationship reflects an increase in the amount of water expelled from the lignite as liquid with increasing pressure. In turn, since increased pressure accompanies increased temperatures, the increase in water expulsion presumably is a consequence of the decrease in viscosity of liquid water with increasing temperatures. In the range of 0.7 to 2.8 MPa, the steam

472 requirements per kilogram of moisture removed decrease from 1.25 to 0.54 kg [37]. For practical application of the Fleissner process, a steam pressure of 2.8 MPa appears optimum [43]. This pressure will result in a product of about 15% moisture which will be stable (with respect to gain or loss of moisture) in storage. If the lignite were to be used immediately upon drying, then drying to lower moisture levels would be repaid by the weight reduction for the coal handling equipment and higher efficiencies during combustion. Drying Dakota Star (North Dakota) lignite at 2.5 MPa consumes 0.65 kg steam per kg water removed [7]. Operation above 2.8 MPa provides no advantage in decreased steam consumption, because of the reduced steam enthalpies and the reduced moisture content of the dried product. A major effect of the Fleissner process is an increase in calorific value of the dried lignite to 22.1-26.0 MJ/kg on an as-received basis, compared to 15.8-17.4 MJ/kg for the untreated lignite. The improvement ratio generally equals the ratio of weights of untreated to dried lignite [37]. During drying a slight amount of inorganic material is lost from the lignite, and a small amount of fixed gases evolve. However, both of these changes have little effect on the weight ratio. That being the case, the improvement ratio can also be used to calculate the change in proximate analysis (on a moisture-free basis) resulting from drying. Particle size, bulk density, and size stability (as measured by the drop shatter test [50]) all decrease When comparing the dried to the untreated lignite. The extent to which any of these properties decrease depends upon the processing severity as expressed by the temperature or pressure of the saturated steam. The stability of the dried product on exposure to the atmosphere was very good. Lumps of lignite dried at 10 MPa retained their form for over a year [43]. The relationship between vapor pressure and moisture content observed for untreated lignites (e.g., [51]) is not identical to that observed for lignites dried in the Fleissner process [29]. At comparable temperatures and moisture contents, the vapor pressure of water over the treated lignite exceeds considerably that of untreated lignite [29]. Furthermore, the difference in vapor pressure vs. moisture behavior increases as the pressure at which the drying was carried out increases [29]. These observations suggest the existence of a colloidal structure of lignite. During drying, the capillary-sized pores are collapsed by the combined effects of temperature and pressure; the extent of pore collapse being directly proportional to the actual temperature and pressure used. A lignite treated by the Fleissner process will air-dry to a lower moisture content than achieved for an untreated lignite at identical conditions of air drying [29], manifesting the higher vapor pressure of the former. Moisture held in pores of large radii will have a hi gher vapor pressure than that held in small radius pores; consequently, drying can proceed to a greater extent. Size degradation on drying is related to the petrographic composition of lignite, though unfortunately much of the relevant work was done a half-century ago and is couched in obsolete terminology. As a rule, woody lignites are preferred for Fleissner drying. So-called peaty lignites break into irregular masses, along various fracture planes. Earthy lignites experience extensive disintegration. Any particular as-mined lignite will be composed of various proportions of these components; consequently, size stability during processing varies from one lignite to another. Size

473 degradation also relates to the size of the feed material. Large lumps show an average size reduction of 35%, whereas the average reduction for 57x6 mm feed is 10.5% [37]. Steam pressure does not correlate strongly with size degradation. Lignite shows some plasticity at the conditions of the Fleissner process. A smoothing of particle surfaces occurs for lignite dried at 7 MPa, while at 10 MPa the dried product retains impressions of the metal basket used to suspend the charge in the autoclave [43]. Unfortunately the original literature contains no petrographic data for the lignite used in the tests. Alaskan low-rank coals also appear to pass through a fluid stage on steam or hot-water drying [28]. The economics of the process require that most of the steam produced during the release period be utilized in the drying. This steam utilization can be effected by using a second autoclave which receives the steam coming from the first. The practical application of the two-autoclave system to North Dakota lignites has been demonstrated [29]. 10.1.7 Drying in hot oil During low-rank coal liquefaction, moisture functions as a diluent of the gas phase, requiring larger processing vessels and operation at higher pressures, with adverse effects on process economics. Though there drying the coal before liquefaction may provide some advantages, conventional drying seriously reduces the reactivity of the dried coal, because of surface oxidation, collapse of the pore structure, or both. High liquefaction conversion depends on the ability of the donor solvent to penetrate the pore structure of the coal, so that a source of donatable hydrogen for capping free radicals is available within a very short distance (on an atomic scale) of sites in the macromolecular structure that rupture during thermal decomposition~ Conventional thermal drying of lignites collapses the pore structure and at the same time exposes sites that are highly reactive toward oxidation. If the moisture could be removed in an inert liquid or gaseous medium, reactive sites created by drying possibly could be preserved for subsequent reaction with the liquefaction medium. A drying process developed in conjunction with the Exxon Donor Solvent liquefaction process (Chapter 12) treats crushed coal and hot recycle solvent in a stirred tank at 120-175~ and atmospheric pressure [5]. The evolved moisture bubbles up through the solvent, while at the same time the solvent prevents air from attacking the coal. (The heating of coal in a hot bath of oil in this manner is sometimes colloquially referred to as "French frying" the coal.) With adequate residence time, the moisture content can be reduced below 4% [5]. Subsequent liquefaction of the dried coal recycle solvent slurry suggests that reactivity of the coal does not decrease in this drying process. In contrast to the drying in a hot recycle solvent, attempts to dry North Dakota lignite in hot nitrogen from 35-38% moisture to 21-23% moisture for the Solvent Refined Coal process resulted in poor conversions of the dried lignite [5].

474 10.2 R E M O V A L OF M I N E R A L M A T T E R

10.2.1 Introduction Currently little incentive exists to clean lignites. Since all lignites are produced from surface mines, they have fewer extraneous mineral impurities than bituminous coals from underground mines. Extraneous impurities may constitute much of the material removed in primary cleaning steps applied to bituminous coals. In lignites, a significant fraction of the total inorganic constituents is incorporated in the organic structure, and many of the discrete minerals are finely dispersed. Thus many of the common coal washing operations provide little or no benefit when applied to lignites. The high moisture content of lignites militates against using wet cleaning processes, which would increase the amount of surface moisture on the lignite particles and further reduce the calorific value. Only a third to half the sulfur content--usually low to begin with--occurs as pyritic sulfur, the form of sulfur most readily removed in traditional coal washing operations. As a rule, most North American lignites have sufficiently low ash yields and sulfur contents that coal cleaning operations are not warranted. Furthermore, the generally low cost of lignites militates against cleaning, because such processing steps would inevitably add to the cost of the final product. Nevertheless, general considerations have been developed relevant to the cleaning of lignites [52]. Because lignites may slack on prolonged exposure to air, lignite should be treated in the first wet cleaning operation as soon as possible after mining. Any increase in the moisture content of the lignite because of the wet cleaning operations should be minimized. Dewatering or thermal drying represent significant cost considerations, not only in terms of the operations themselves, but also for cost penalties associated with transportation of wet lignite or derating boilers because of the high moisture content of the lignite being fired. Inevitably, some fines (i.e., 6x0 mm) will be created during the cleaning operations. Handling and cleaning operations should be run to minimize fluctuations in the amount of fines produced. Design and operation of a fine-coal cleaning circuit will be important for overall successful operation of a cleaning plant. Separate cleaning and storage for the fine and coarse lignite may be desirable. The relatively low commercial value of lignites dictates that cleaning costs be kept to a minimum. This consideration limits the type and amount of cleaning that can be performed. The high moisture content, and its possible increase during cleaning, may dictate thermal drying of the cleaned lignite. The special sensitivity of thermally dried lignite to autogenous combustion indicates that special storage, handling, and transportation procedures may have to be instituted. The production of fines offers the potential of significant energy losses, which would add to the cost of cleaning unless a fine-coal circuit could be designed to clean and recover the fines. Cleaning large amounts of near-gravity material generally rules out the use of jigs. In such a case, heavy medium vessels are superior for cleaning coarse lignites, though lignites having high contents of clay minerals or generating large amounts of fines during cleaning may cause problems with the recovery of the magnetite. Hydroclones may be useful for pyrite removal or rejection of

475 free, heavy impurities in a pre-cleaning operation. Successful application of flotation may require further development of additives to obtain improved selectivity. Results obtained with Spanish lignite suggest that a differential sedimentation technique may be superior to selective flocculation for removal of sulfide minerals [53]. The inorganic constituents present as cations associated with the carboxylic acid groups in the lignite structure cannot be removed by relatively simple operations such as jigging or flotation. Some prospect exists for their removal or reduction by ion-exchange operations, as discussed in Section 10.3 below. 10.2.2 Gulf Coast lignites The high moisture content of lignites complicates the interpretation of washability data. If the inherent moisture content of the lignite is not preserved throughout the sequence of tests, erroneous results for ash value, yield, and specific gravity can be obtained [54]. Washability characteristics of seven Texas and four Arkansas lignites were surveyed to determine whether these lignites could be upgraded to meet the U.S. Environmental Protection Agency (EPA) New Source Performance Standard of 1.2 lb SO2/106 Btu (0.52 g/MJ). An extensive compilation of the test results, together with much supporting analytical data on the eleven samples, has been published [55]. Only two of the Arkansas lignites, and none of the Texas lignites, could be upgraded to meet the EPA standard. Although all of these lignites would generally be considered low sulfur (total sulfur contents were 0.65-1.77%), the problem lay in the fact that much of the sulfur was organic sulfur. The organic sulfur contents were 0.56-1.30%. Expressed as a percentage of total sulfur, the median value of organic sulfur of the eleven samples was 86%. Regardless of particle size or the specific gravity of the separation fluid, it would be impossible to effect much sulfur reduction via conventional coal cleaning processes. Despite the inability to meet the EPA standard for nine of the eleven lignites, washing resulted in substantial reductions in ash value, in some cases exceeding 40% reduction. As an example, a sample from the Lower Calvert Formation (Wilcox Group, Milam County, Texas) showed 19.8% ash, 0.15% pyritic sulfur, and 1.31% total sulfur, with a calorific value of 23.1 MJ/kg. Crushing to 9.5 mm top size and removing the 1.40 specific gravity (sp. gr.) sink fraction gave a product of 11.5% ash, corresponding to a 42% reduction of ash level [55]. Recovery of calorific value was 90%. Washability data on wet-screened samples of Texas lignites are shown in Table 10.5 [56]. Cleaning Texas lignite reduced the ratio of discrete mineral species to exchangeable or complexed organics from 2 to less than 1, but did not affect the relative proportion of the various minerals [57]. Quartz dominated among the minerals in both the untreated and cleaned samples. The cleaning process affected all minerals in this lignite similarly. Some float-sink data for Texas lignites have been complied as part of a broad study to evaluate the cleaning of western U.S. coals and to establish a data base for such coals [58]. Floatsink separation of Martin Lake (Texas) lignite at 1.4 sp. gr. provided 62% recovery of the float

476 TABLE 10.5 Analytical data on wet-screened samples of Texas lignites [56]. Lignite San Pedro (Webb County)

Bull Creek (Coleman County)

Mesh Size Wt.% Retained 65 66.9 100 23.4 200 3.4 325 3.4 -325 2.8

Moisture, % 2.4 3.0 3.2 3.3 3.0

Ash, % 11.8 15.0 16.4 16.4 19.6

Total S, % 1.61 1.95 2.20 2.19 2.27

65 100 200 325 -325

71.3 25.6 1.1 0.8 1.1

4.3 4.6 3.3 3.4 3.0

21.9 22.3

2.95 3.03

26.2 34.5

3.63 2.54

65 100 200 325

44.6 48.5 3.3 1.9

2.9 2.8 2.7 2.5

22.2 24.5 24.3 27.1

3.89 4.66 4.92 5.50

- 325

1.7

2.3

32.3

5.06

Titus County Lignite Field

65 100 200 325 -325

38.3 51.0 5.4 1.9 3.3

10.5 9.7 8.3 7.8 7.7

20.0 26.8 31.0 35.6 41.6

Titus County Lignite Bed

65 100 200 325 -325

46.2 42.1 4.7 2.1 4.9

12.0 11.6 10.4 9.7 9.2

14.4 19.3 23.2 25.2 33.7

Thurber (Erath County)

fraction, which had a 6.8% ash level and 10% inertinite [59]. Treating coarse Martin Lake lignite with aqueous sulfur dioxide reduced the ash level of the float fractions by about 50% [59]. Much of the ash reduction derives from partial removal of alkali and alkaline earth cations [60]. Float-sink separation (1.40 sp. gr.) of the SO2-treated coarse lignite gave a recovery of 90% float fraction with 3.3% ash. The lignite retains about 1-2% sulfur dioxide [60]. Martin Lake lignite is readily cleaned by oil agglomeration, with ash rejections exceeding 50% in favorable cases [61]. Using No.6 fuel oil as the agglomerating agent provided ash rejection of about 46%. This lignite is very difficult to agglomerate; for example, it did not agglomerate with No.2 diesel oil, nor with three coal-derived liquefaction oils. Martin Lake lignite is more difficult to agglomerate than various subbituminous coals [62]. Cresylic acid (i.e., a mixture of the isomers of cresol) serves as an effective promoter for lignite agglomeration when a lignite-derived liquefaction oil is used as the agglomerating agent [61,62], though cresylic acid did not improve ash rejection

477 when No.6 fuel oil was used [61]. Cresylic acid substantially enhances the kinetics of agglomeration (e.g., from a 40 min induction period without cresylic acid to only 6 min [62]) and appears to aid dissolution of some of the mineral matter [61]. 10.2.3 Northern Great Plains lignites Float-sink testing of a variety of western U.S. coals of various ranks showed the poorest 1.4 sp. gr. float yields with Hagel and Beulah-Zap (North Dakota) lignites, a Wilcox (Texas) lignite, and a Washington subbituminous coal [63]. The distributions of ash, sulfur, and calorific value with particle size are shown in Table 10.6 for wet sieve data obtained for four North Dakota lignites [64]. TABLE 10.6 Variation of ash, sulfur, and calorific value with size fractions of North Dakota lignites [64].

Lignite Center (Kinneman Creek

bed)

MeshSize (Bulk) 65 100 200 325 - 325

Ash, % 9.70 7.06 8.56 9.76 14.92 19.88

Total Sulfur, % 1.17 0.93 1.06 1.12 1.34 1.28

Pyritic Calorific Sulfur, % Value(MJ/kg) 0.18 22.4 0.01 24.1 0.19 23.6 0.20 23.4 0.40 22.3 0.08 20.6

Center (Hagel bed)

(Bulk) 65 100 200 325 - 325

12.04 6.57 8.19 9.82 11.22 21.69

1.18 0.65 0.91 1.08 1.46 1.20

0.36 0.03 0.37 0.50 0.50 0.57

21.8 23.6 24.4 24.2 23.6 20.6

Beulah

(Bulk) 65 100 200 325 -325

9.54 5.88 6.70 7.42 8.52 13.82

1.32 0.84 0.92 0.97 1.02 1.12

0.06 0.03 0.06 0.09 0.16

21.6 23.3 24.0 24.1 23.3 22.2

(Bulk) 65 100 200 325 -325

11.29 7.13 8.50 9.52 10.91 15.67

1.28 1.00 1.02 1.14 1.20 1.13

0.08 0.01 0.08 0.19 0.52 0.60

21.7 23.7 24.2 24.2 23.9 22.7

Gascoyne

Washability data obtained from float-sink testing of Center (North Dakota) lignite are shown in Table 10.7 [65]. The lignite was sized 200x0 mesh.

478 TABLE 10.7 Washability data for Center lignite [65].

Sp. Gr. F l o a t - 1.30 1.30- 1.40 1.40- 1.60 1.60 - Sink

Cumulative Recovery_, % Weight Cal. Value Cal. Value, MJ/kg 0.0 . . . . 12.6 14.0 24.8 93.4 96.7 23.0 100.0 100.0 22.2

Ash 5.2% 7.4 10.4

Total S 0.6% 0.6 1.0

A more extensive compilation of washability data for Center, Beulah, and Gascoyne lignites is provided in the original literature [64]. Data for the 1.60 sp. gr. float-sink products of 65x0 mesh lignites are shown in Table 10.8 [63]. TABLE 10.8 Float-sink results at 1.60 sp. gr. for Hagel and Beulah-Zap lignites [63]. Hagel Weight, % Moisture, % Ash, % Total sulfur, % Pyritic sulfur, % Calorific value, MJ/kg

Float 95.66 8.20 6.80 0.62 0.01 22.2

Sink 4.34 8.32 53.46 4.64 0.47 9.80

Beulah-Zap Float 87.53 7.53 9.60 0.92 0.12 22.7

Sink 12.47 3.32 62.16 4.59 3.33 7.52

The effects of changing the grind of the lignite to 200x0 mesh appear to be relatively minor. For example, the ash yield of the floats becomes 7.47% for the Hagel (North Dakota) lignite and 9.50% for the Beulah-Zap; the pyritic sulfur in the floats becomes 0.02% and 0.05% for Hagel and Beulah-Zap, respectively [63]. Changing the specific gravity of the separation from 1.60 to 1.40 has a marked effect on the characteristics of the products. Table 10.9 provides data on the 1.40 sp.gr, floats of these same two lignites. Froth flotation of Indian Head lignite produced no beneficiation [65]. A thin, unstable froth formed, showing no evidence of preferential adsorption of lignite particles. Burleigh (North Dakota) lignite produced negligible amounts of 1.50 sp. gr. sink material [5,66]. The 1.50 sp. gr. sinks are usually comprised of the extraneous mineral matter. If 50% of this lignite were rejected as

479 TABLE 10.9 Nature of the 1.40 sp. gr. float products from Hagel and Beulah-Zap lignites [63].

Weight, % Moisture, % Ash, % Total sulfur, % Pyritic sulfur, % Calorific value, MJ/kg

Hagel 26.15 8.10 4.89 0.54 0.01 24.2

Beulah-Zap 31.45 8.02 6.74 0.76 0.02 24.2

refuse, the ash reduction in the cleaned lignite would amount only to 1%. Low-rank coals containing finely disseminated mineral matter are difficult or impossible to beneficiate by gravity separation. An alternative approach is the use of selective oil agglomeration. [67]. The oil preferentially wets the carbonaceous portion of the coal, so that the carbon-rich portions are agglomerated in the oil phase, while mineral-rich particles, not wetted by the oil, remain in the aqueous phase and are rejected. Oil agglomeration is also difficult for lignites because the surfaces of lignites are hydrophilic. Selective oil agglomeration may be an appropriate beneficiation method for the northern Great Plains lignites. This technology could be attractive as a "front-end" beneficiation step in a coal-petroleum coprocessing plant [67,68]. Combustibles recovery from oil agglomeration of Willowbranch (Saskatchewan) lignite with vacuum bottoms increases from 50 to 93% as the vacuum bottoms concentration increases from 20 to 40% [67,68]. If the amount of vacuum bottoms is too small for effective agglomeration, many of the lignite particles will not agglomerate and will be lost with the aqueous suspension of the tailings. As the amount of vacuum bottoms used increases, agglomeration and combustibles recovery improve. Ash rejection shows a monotonic increase with increasing vacuum bottoms concentration. Given processing times of 5-10 min, and temperatures of 110-140~

combustibles recoveries of 97.6-98.1% can be

obtained from this lignite, with ash rejections of 52.5-53.1% [68]. Using three types of vacuum bottoms at processing temperatures of 175~ provided 95-96% recovery of combustibles with 53-58% ash rejection [67]. The vacuum bottoms usage ranged from 40 to 51% on a moisture-free lignite basis. The adsorption of vacuum bottoms was facilitated by preconditioning the lignite with sulfuric acid, and adding cresylic acid as a surfactant. At high levels of combustibles recovery the rejection of mineral matter is not effective because of the minerals finely dispersed through the lignite. Furthermore, some mineral particles can be inadvertently trapped in the vacuum bottoms-lignite agglomerate. Further marginal increases in combustibles recovery can result in a decrease in the amount of mineral matter rejection. Indian Head lignite can be agglomerated with Mandan decant oil only after long periods of standing, but mixing 50% of the decant oil with p-xylene or naphtha from the Rectisol process

480 provides agglomeration within several minutes [69]. Ash rejection from 325 mesh as-received lignite using the decant oil mixed with p-xylene was 17%. Higher ash rejections derive from use of higher mixing speeds and longer mixing times [69]. Combining a high-shear coal cleaning step with a low-shear agglomeration step offers advantages [70]. Ion-exchangeable cations are leached from the lignite in the first step. Hot-water drying of the lignite enhances its agglomerating properties [69]. Treatment of Estevan (Saskatchewan) lignite with sulfur dioxide (by bubbling the SO2 through a 12% slurry of lignite in water in a bubble column reactor) provided removal of about half the alkaline earth cations at pH 3.5 [71]. This loss of alkaline earths corresponds to a reduction of ash level by about one-third. This process provides an opportunity for simultaneous flue gas desulfurization and lignite deashing. Bioelectric deashing provides a novel approach to the cleaning of lignites. Powered lignite is placed at the anode of an electric circuit, the electrolyte being pH 2.5 sulfuric acid, and exposed to a suspension of

Thiobacillusferrooxidans. The electric current produced definite reductions of

ash. Thus Pust (Montana) lignite, treated for 41 h at 15 V and 5.0 mA, showed a reduction of ash level from 6.90 to 3.20% [72]. Results with Gascoyne lignite treated for 66 h at 10 V and 2.5 mA showed a reduction of ash from 8.89 to 6.69% [72]. A Spanish lignite (66 h, 10 V, 2 mA) was cleaned from 12.48 to 9.46% ash [72]. Roughly comparable results are obtained regardless of whether bacteria are present, presumably because these three lignites have low pyrite contents.

10.3 R E M O V A L OF INORGANIC CONSTITUENTS BY ION E X C H A N G E 10.3.1 Introduction Interest in ion-exchange processes for beneficiation of lignites derives mainly from the role of sodium, which occurs in lignites primarily as an exchangeable cation, as a principal causative agent for deposition of ash on heat-exchange surfaces in boilers. This problem, generally referred to as ash fouling, represents the most serious problem in current lignite utilization technology. (Ash fouling is discussed extensively in Chapter 11.) Pilot-scale tests clearly confirm that reduction of sodium by ion exchange leads to substantially reduced ash deposition. 10.3.2 Fundamentals of ion exchange with lignites The ion-exchange process consists of five steps [52,73]: 1) diffusion of the exchanging ion from the bulk of the liquid through a boundary layer to the lignite surface; 2) diffusion of the exchanging ion through the pores of the lignite to the exchange site; 3) the actual exchange of the exchanging ion for the cation originally bound to the lignite; 4) diffusion of the formerly bound cation outward through the lignite pores; and finally 5) diffusion of the formerly bound cation outward from the lignite surface through the boundary layer into the bulk of the liquid phase. Lignite acts as a weakly acidic ion exchanger. Cationic charge and ionic size determine the

481 order in which ions can be replaced. In terms of ease of replacement (from easiest to most difficult) the order of ions is Na+ > K+ > Mg+2 > Ca+2 > AI +3 > Fe+3 > H+ [52,73-75]. The seemingly anomalous position of the proton derives from the fact that the acid form of the carboxyl group is only partially ionized; that is, the proton is bound to the carboxyl partially covalent bonding stronger than the completely ionic bonding experienced by the metal cations [76]. Generally, sodium removal depends on particle size and moisture content of the lignite, ionic strength of the exchange solution, contact time, lignite-to-solution ratio, and equilibrium between the ions in the solid and in solution [77]. Aqueous sulfuric acid effectively removes sodium, and essentially all the H+ is utilized in the process. The equilibrium ion-exchange behavior of metal cations for H+ depends directly on solution pH, regardless of the concentration of cation in solution [78]. The available H+ therefore determines the extent of exchange. Liquid-film diffusion governs the transport of the exchanging ion (e.g., a proton if aqueous acid were used as the exchange reagent) through the boundary layer around the lignite particle; particle diffusion governs subsequent transport through the pores to reach the exchange site. The rapid exchange itself provides no resistance to mass transfer [77]. The exchanged ion (e.g., sodium) then follows a reverse process of particle diffusion first and film diffusion second. Any one of the four diffusion steps could be rate-controlling. Liquid-film diffusion controls when a stagnant boundary layer exists, whereas particle diffusion controls for thin liquid films. Particle diffusion could be the rate-determining step during vigorous agitation of the system or for particles of relatively large dimensions [77]. In batch tests, particle diffusion controlled ion exchange of Beulah lignite with aqueous sulfuric acid, while surface-film diffusion was the controlling step in a continuous countercurrent unit [79]. Agitation of the batch system reduced the thickness of the surface film. The fraction of sodium removed from the lignite is given by [77] R = U(-~) X where U(x) = {1 - exp[p2(fi(c~)'t + fz(a)'t2 + I3(c0-t3)]}0.5 fl(a) = - 1 / (0.570 + 0.430 c~0.775) f2(o0 = - 1 / (0.260 + 0.782 or) f3(ot) = - 1 / (0.165 + 0.177 or) -t = (I:Na) t / ro 2

= DNa / DH

The value of ot was detennined from the empirical equation

482 c~ = exp[-0.6991 In(2 C s ) - 1.8329]

and DNa was found from iteration to be 6.5x10-7 cm2/s [79]. In these equations, R is the fractional removal of sodium from the lignite, U the fractional attainment of equilibrium Ibr sodium in the lignite, X the fractional removal of sodium at equilibrium, DNa and DH the self-diffusion coefficients for sodium and hydrogen ions inside the particle (in cm 2/s), t the contact time, ro the particle radius in cm, and Cs the total sulfate and bisulfate ion concentration in geq/L. These equations allow prediction of the sodium content of the treated lignite product from batch exchange in the ranges ro 0.0082-0.202, solid/liquid ratio 0.5-1.0, t of 0.5-50 min, and sulfuric acid concentrations of 0.005-0.535 mol/L [52,79]. The model yields accurate predictions only when sufficient protons are present initially to exchange all the sodium [77]. In a stirred batch process sodium removal does not depend on stimng speeds greater than 2 revolutions per second, indicating that exchange is controlled by particle diffusion [77]. Sodium removal does not depend on liquid-to-solid ratio, provided that sufficient protons are available to remove all the exchangeable sodium from the lignite. Removal rate increases with decreasing particle size, indicating that removal is proportional to the length of the diffusion path within the lignite particle. In principle, the concentration in the bulk of the liquid should not affect exchange in situations in which particle diffusion is controlling. The Donnan potential, which measures the exclusion of the anions attempting to enter the pores, should maintain a high concentration of protons and low concentrations of sulfate or bisulfate inside the lignite particles. However, the Donnan potential can be overcome in three situations, all of which apply during the treatment of lignite with aqueous sulfuric acid: relatively high acid concentration in the bulk of the liquid, multivalent anions (e.g., sulfate), or a weakly acidic exchanger (e.g., lignite). Although the theory of particle-diffusion control indicates that the removal rate should be independent of the H+ concentration in the bulk of the solution, sodium removal depends directly on sulfuric acid concentration [77]. The Donnan potential apparently becomes insignificant for high concentrations of sulfuric acid in combination with the weakly acidic ion-exchanger lignite [80]. For countercurrent extraction the equations shown below were developed [77,79]: R = U(x)X U('t) - 1 - exp[(-1.964x10-5 C 1:) / ro 8] 6 = 0.2 ro/(1 + 70 ro V) where 6 is the film thickness in cm, V the liquid linear velocity in cm/s, and C the initial sulfuric acid concentration in the bulk liquid in geq/L. This equation predicts the sodium content of 10x48 mesh lignite for solid residence times of 10-100 min, sulfuric acid concentrations of 0.0205-0.4 mol/L, and liquid flow rates of 8---40 mL/min [52,79]. The equation is film-diffusion dependent.

483 The variables affecting sodium reduction are, in order of decreasing significance, initial acid concentration, solid residence time, particle size, and solid/liquid ratio [52,79]. With minimal agitation, the liquid film surrounding the lignite particle adds significant resistance to the ionic transport. Film diffusion becomes rate-controlling. In these situations, sodium removal increases with lignite residence time up to an equilibrium value below which no further sodium removal occurs [77]. High liquid-to-solid ratios increased sodium removal; above a ratio of 3, sodium removal becarne independent of the value of the ratio. Liquid film thickness will be a function of the liquid-to-solid ratio. Increasing the liquid velocity decreases the film thickness, providing increased sodium removal by reducing the resistance to mass transfer. Higher acid concentrations and smaller lignite particle sizes increased sodium removal. The material balance equation for the distribution of sodium and other ions between the solid and liquid phases is -S/L = (xl

- x2) /

(Yl - Y2)

where S/L is the ratio of the weight of lignite (maf basis) to the volume of solution, x is the concentration in the solution, in ~g/mL, of the cation being exchanged, y the concentration of the same cation in the lignite in ~g/g (maf basis), and the subscripts 1 and 2 refer to the initial and final states, respectively [74]. This equation applies even to some non-equilibrium situations. It also applies for any aqueous exchange medium so along as the cation whose concentration is used in the equation is not also the exchange cation. That is, the equation could not be used to determine the distribution of calcium for lignite being treated with a solution of calcium ions. The equilibrium sodium ion concentration in lignite is independent of whether sulfuric acid or calcium chloride solution is used as the exchange reagent. The equilibrium sodium ion concentration can be determined from y = -(3250 / 2.303) In N - 3352 where N is the initial solution concentration in geq/L and y the sodium ion concentration in the lignite [74]. Thus both calcium chloride and sulfuric acid solutions of identical normality yield treated lignites of identical sodium contents. This equation is valid for a liquid/solid mass ratio of 10 and initial solution concentrations of 0. 001-0. 06N. 10.3.3 Bench-scale tests Gulf Coast lignites showed reductions of 37-91% of sodium content (expressed as Na20 in the ash) when treated with aqueous calcium chloride [5,55]. Calcium chloride was used in amounts equivalent to 5 kg/t of dry lignite. Conditions were not optimized for a particular lignite; calcium concentration, particle size, and contact time were uniform for each of the lignites. The potential thus exists for achieving even higher sodium reduction by finding the optimum treatment

484 conditions for the specific lignite of interest. Lignite from the Yegua Formation (Angelina County, Texas) gave the best results: sodium reduction from 1.27% Na20 in the ash of untreated lignite to 0.12% Na20 in the ash after treatment. Results for tests of eleven Gulf Coast ligi]ites are in the literature [55]. Sodium removal increases with increasing contact time between lignite and 0.1N sulfuric acid [73]. Sodium concentration eventually drops to an equilibrium value which represents the maximum amount of removable sodium [80]. Other ions were also removed, in the order K+ > Mg+2 > Ca§

The amount of sodium removed increases with decreasing particle size [73],

consistent with the mechanism of diffusion of the exchanging ion through the pore structure of the lignite, and suggests that this step is rate controlling. However, for particles of Beulah lignite smaller than 250 ~tm or larger than 2 mm the exchange rate was reduced significantly [81[, ascribed to the inability of the solution to wet the very small particles and to diffusion limitations in the large particles. For otherwise constant experimental conditions, the percentage of sodium removed dropped from 80% for 40x0 mesh Beulah lignite to 63% for 8x0 mesh, and to 15% for 6x0 mm [821. The rate and extent of exchange between lignite and aqueous calcium chloride decreased with decreasing moisture content of the lignite [73]. This suggests that the drying process may have resulted in particle shrinkage and pore collapse. Reducing the moisture content of Beulah lignite from 34% to 28% had little effect on ion-exchange rates, but below 28% moisture the rate decreased considerably [82]. Again, this effect likely derives from pore collapse on drying. Sodium removal does not depend on stimng speeds for speeds greater than 100 rpm [80], but does so at lower speeds. At high stirring speeds (i.e., >100 rpm) the liquid film is very thin and rate is determined by particle diffusion, but at slow speeds the lignite particles are surrounded by a stagnant liquid boundary layer, in which case liquid-film diffusion is the rate-determining step. Sodium removal also does not depend on liquid-to-solid ratio, provided that sufficient exchange ion (e.g., H+) is present to effect the exchange [80]. A direct relationship exists between the concentration of calcium in the exchange solution and the amount of sodium removal from Beulah lignite [82]. Further, the calcium content in the lignite increases in proportion to the amount of sodium removed. Increasing the system temperature from 21" to 49~

increased the sodium removal from 63 to 72%. No further

improvements in sodium removal occurred above 49~

Increasing the liquid-to-solid ratio

increased the amount of sodium removed. Sodium removal increased with increasing contact time up to 30 min, but longer contact times provided no further significant increase. Although ion-exchange beneficiation of lignite generally focuses on sodium removal or reduction, other ions are removed as well. For example, treatment of Beulah lignite (0.0502 cm particle radius, 0.0535 mol/L sulfuric acid, liquid/solid ratio 10) resulted in 89% sodium removal after 20 min, accompanied by removal of 74% of the potassium, 65% of the magnesium, and 54% of the calcium [77]. The fraction of cations replaced by protons when treating lignite with acid is a

485 logarithmic function of the equivalent ionic fraction of protons in solution, This factor, represented by XH, is given by

xH = (ZHmH) / (zimi) where z is the charge and m the molality, the subscripts H denoting the proton and i the metal cations. An equilibrium pH <2.7 was required for complete removal of calcium [81]. In the exchange of protons for cations on the lignite, the cation of higher valence and smaller hydrated equivalent volume is preferred. A separation factor (~, given by

ct = (ma/mb)sorbent (mb/ma)solution where m i is the molality of species i, expresses this preference. The separation factor for replacement of calcium ions by protons was 0.5, and for sodium ions by protons, 0.005 [81]. The rate of cation removal by protons and the rate of replacement of protons by cations is about eight times faster for sodium than for calcium [81]. Treatment of lignite with dilute solutions of strong acids or bases affords an opportunity for combining ion-exchange processes with outright dissolution of some of the mineral components. Treatment of Indian Head lignite with 0.11N hydrochloric or sulfuric acid, or 0.05N nitric acid (liquid/solid ratio 2) all resulted in extraction of approximately 45% of the sodium initially in the lignite, equivalent to 1200 mg/L sodium taken into solution [83]. Increasing the nitric acid concentration to 0.1 IN increased the sodium dissolution to 1750 mg/L, equivalent to 74.3% of the sodium. Nitric acid treatment removed about 30% of the sulfur, regardless of concentration of the nitric acid. Treatment with 0.11N potassium hydroxide removed a higher proportion of aluminum, iron, and silicon, and a lower proportion of calcium, magnesium, and sodium, than did treatment with acid [83]. The use of potassium or sodium hydroxide has the disadvantage of partially solubilizing the lignite, making subsequent separation of the lignite and the process solution difficult. Treatment with sulfuric and hydrochloric acids and potassium hydroxide all increased the equilibrium moisture of the lignite [83]. This effect did not occur with nitric acid. A neglected, but potentially very significant, consideration is the extent to which the ionexchange process somehow redistributes the sodium still remaining in the lignite. After fluidizedbed combustion of Beulah lignite treated with sulfuric acid to reduce sodium from 7.4 to 6.4% (as Na20 in the ash), the sodium content of the bed material was 9.4% Na20, well above levels expected to result in severe agglomeration [84]. Nevertheless, no significant agglomeration occurred. This unexpected behavior may be due not only to removal of a portion of the sodium from the lignite, but also to a redistribution of the remaining sodium into forms less likely to react with the bed material to cause agglomeration [84]. Testing of this hypothesis does not seem to have been pursued.

486 10.3.4 Pilot-scale tests During treatment of lignite with aqueous calcium chloride in a continuous countercurrent exchanger, the removal of sodium increased with increasing concentration of the exchange solution [73]. Using 0.1N sulfuric acid as the exchange solution showed that removal of cations increased with increasing liquid-to-solid ratio up to a value of 2; above this value no additional increase in cation removal occurred [73]. Sodium removal in countercurrent exchange increases with lignite residence time until an equilibrium value is reached [80]. For an initial sulfuric acid concentration of 0.056M, liquid/solid ratio 2, and mean particle radius 0.68 mm, the equilibrium value occurs at a lignite residence time of about 38 min, with maximum sodium removal of 95% [80]. No additional sodium could be removed, even at residence times exceeding 100 min. Sodium removal increases with increasing liquid-to-solid mass flow ratios up to a value of about 3 [80]. Sodium removal increases with initial sulfuric acid concentration to about 0.13M, with only small increases thereafter, even up to acid concentrations of 0.4M. Increasing particle size decreases sodium removal, beginning to decrease as reciprocal particle radium falls below 1.7 cm-1, and dropping off sharply below 1 cm-1 [80]. Sodium content of the lignite decreases with decreasing pH of the exchange solution [85]. A pH of 2 provides the most significant reduction, reducing the initial sodium content of 7.4% (reported as Na20 in the ash) to 2.4%. Corroborating the bench-scale results described in the previous subsection, sodium removal with aqueous calcium chloride depends on moisture content, particle size, temperature, contact time [82], and calcium content of the exchange solution [82,85]. Greatest exchange occurred with lignite of >28% moisture and 80x0 mesh particle size. Slight increases in exchange rate with temperature occur up to 49~

These results are consistent with the liquid-film diffusion

model described above. The ion migration rate within the lignite particle determines the particle-size dependence of sodium removal from Beulah lignite by treatment with calcium-containing solutions [85]. In commercial practice the process would be facilitated by using fine particles of lignite. Sulfuric acid and aqueous calcium chloride provide the most effective sodium reduction among a suite of prospective calcium sources, including calcium oxide, calcium sulfate, and cement. (Sulfuric acid removed calcium and magnesium as well as sodium.) Calcium oxide (supplying 475 ppm calcium, reported as CaO, to the exchange solution) and cement (supplying 77 ppm calcium as CaO) were ineffective in removing sodium. Other work shows that calcium concentrations below 60 ppm (as CaO) had little effect on sodium reduction; only when the calcium concentration exceeded 243 ppm did noticeable sodium reduction occur. The most extensive reduction took place with an exchange solution having a calcium content of 2630 ppm (again, reported as CaO). Liquid residence time did not significantly affect the residual sodium content in the lignite during these experiments. The highest calcium ion utilization, as determined by removal of calcium from the exchange solution, occurred in the range 60-780 ppm CaO, suggesting that commercial operation might have to accept reduced calcium utilization to achieve a treated lignite product with acceptably low sodium content.

487 The residence time of solids has an appreciable effect on the sodium content of the product [85]. For a given ratio of volumetric solution rate to weight of lignite in the bed, sodium content of the treated lignite depends inversely on residence time. Furthermore, as the ratio of solution rate to lignite weight increases, the sodium content of the treated lignite decreases for any given residence time [85]. Residence times in excess of 2 h do not materially enhance sodium reduction. Presuming a feed material with 8.5% Na20 in the ash, the respective costs for reduction to 4, 2, and 1% Na20 were $4.59, $6.42, and $9.81 per tonne, in 1982 U.S. dollars [77]. These estimates are considerably higher than the estimated replacement cost for electric power due to boiler outages caused by ash deposition, which is $2.65-3.42 per tonne of coal burned [86].

10.4

DESULFURIZATION

As a rule the sulfur content of North American lignites is low enough that desulfurization of the lignite prior to use is not considered necessary. However, some exploratory research has been done on procedures for chemical desulfurization of lignites. In addition, lignites outside North America have, in many cases, much higher sulfur contents--and particularly organic sulfur--than those of North American lignites. Such lignites sometimes have sulfur contents exceeding 10% (dry basis); much useful research on desulfurization of high-sulfur lignites is underway in several countries. No sulfur removal occurred during treatment of three Texas lignites--Freestone, Gibbon Creek, and Martin Lake--with vapors of nitrous oxide in moist air, trimethylamine in moist nitrogen, or dimethyl ether in moist nitrogen [87]. The Freestone and Martin Lake lignites did show some enhancement of calorific value as a result of this treatment. Reaction of an unidentified North Dakota lignite with carbon monoxide (initial pressure 6.9 MF'a at ambient temperature) at 372~ for 30 min removes 23.5% of the sulfur [88]. The reaction is carried out in the presence of a coal-derived solvent containing, in this instance, 25% cresol and 25% tetramethylammonium hydroxide. The JPL chlorinolysis process involves reaction of coal with chlorine in trichloroethane, which is used as a solvent. Zap (North Dakota) lignite provided the data shown in Table 10.10 [5]. TABLE 10.10 Desulfurization of lignite by the JPL chlorinolysis process [5]. Chlorination Time (Min.) (none) 30 60

Residual Sulfur (Wt. %) Org. Pyr. Sulf. Total 0.63 0.52 0.03 1.22 0.35 0.23 0.17 0.75 0.32 0.35 0.06 0.73

Sulfur Removal, % Org. l:'yr. Total 44 50

59 37

39 39

488 Treatment of San Miguel (Texas) lignite with Pseudomonas putida in a pipeline bioreactor removes 74% of the pyritic sulfur and 37% of the organic sulfur [89]. Anaerobic microorganisms remove organic sulfur as hydrogen sulfide [90]. Microorganisms capable of attacking and reducing the dibenzyl disulfide linkage may prove useful for treatment of North Dakota lignites [90]. Desulfurization of Turkish lignites can be effected by treatment with 20% aqueous sodium hydroxide solution [91]. About 12.5% of the organic sulfur content (0.9%, daf basis, in the untreated lignite) could be removed in this way. The treatment also removes some of the mineral matter. Pakistani lignites treated in similar manner showed 15-18% reductions of total sulfur, though accompanied by 3-18% increases in ash level [92]. Molten sodium hydroxide treatment also removes sulfur from Turkish lignites and significantly reduces mineral matter, in one case dropping the ash level from 24.4% to 2.0% [91]. Other work with Turkish lignites indicates 53% removal of sulfur by treatment with alkali [93]. Treatment of Turkish lignite with aqueous copper(II) chloride solution (50 g CuC12 per 100 mL solution) reduces total sulfur by about 53% during reaction at 150~ for 210 min [94]. This level of sulfur reduction derives from removal of virtually all the pyritic and sulfatic sulfur, and 30% of the organic sulfur. Pyrolysis represents a straightforward way of reducing sulfur content. In Turkish lignites, pyrolytic decomposition of pyrite to iron sulfide and sulfur begins at 300~

increases sharply

between 400 ~ and 5000C, with complete disappearance of pyrite occurring around 6000C [95]. Organic sulfur begins to decrease above 7000C, with a significant increase in organic sulfur removal occurring between 800 ~ and 9000C. Carbonization of Turkish lignite in a stream of ammonia provides desulfurization of up to 71%, observed for reaction at 9000C [93]. The extent of desulfurization depends on temperature, dropping, for example, to 64% for reaction at 700~

The reaction removes all pyritic and sulfatic

sulfur, and some organic sulfur. Hydrodesulfurization of lignite can effectively reduce the amount of organic sulfur [96101]. About 70% of the sulfur can be removed from Spanish lignite by hydroliquefaction at 3500C in the absence of solvent [102]. Treatment of lignite with a soluble salt of a transition metal, such as ammonium tetrathiomolybdate or nickel sulfate, and subsequent reaction with 7 MPa H2 at temperatures of 275--325~ can, in favorable cases, effect removal of about 75% of the total sulfur in the lignite. The transition metal salts appear to decompose to active sulfided catalysts, such as MoS2 or NiS, at the reaction temperatures. Most of the work published in the recent literature has focused on the lignites of Spain and Turkey, which have exceptionally high organic sulfur contents

(e.g., in excess of 10% in some Spanish lignites); however, some data are also reported for Hagel lignite, which has organic sulfur <1%. A drawback to this process, from the standpoint of hydrodesulfurization of the lignite to a low-sulfur char, is that the reaction seems inevitably accompanied by the formation of significant amounts of liquid, not unexpectedly, since both MoS 2 and the reaction product H2S are good liquefaction catalysts.The reaction proceeds by a sequential mechanism, involving first the breakdown of the lignite structure to liquid organosulfur

489 compounds, followed by hydrotreatment of the liquid sulfur compounds to produce hydrogen sulfide [100]. A change, however, appears to occur around 325~

Below 325~

most of the

sulfur removed from Spanish lignite is removed as organosulfur compounds in the liquid products; above 325~

most of the sulfur comes off as H2S [96]. Tungsten sulfide, formed in situ from

decomposition of ammonium tetrathiotungstate, appears to provide the best selectivity to HzS (i.e., rather than to liquid organosulfur compounds) during hydrodesulfurization [96]. A Turkish lignite showed 42% reduction of total sulfur, for reaction at 325 ~ in the presence of WS2, with 72% selectivity to HzS [100]. (The comparable results in absence of WS2 were 25% sulfur removal with 26% selectivity to HzS [100].) Depolymerization of Spanish lignite in phenol at 200~

catalyzed by boron trifluoride,

results in a 75% decrease of sulfur content [ 102].

10.5 SIZE P R E P A R A T I O N

10.5.1 Size reduction at the mine In the mine, lignite usually cleaves easily in directions parallel to the bedding plane, but show little tendency to cleave vertically. As a result, it often breaks out of the seam during mining in large slabs that may be longer than 1 m [7]. Depending on distance between bedding planes, the slabs of lignite may thus be several meters thick and several meters across. Freshly mined lignite has a tough, resilient structure, perhaps manifesting its woody character. This characteristic of the lignite may result in the crushing equipment having about half the capacity as it would with bituminous coal [7]. The properties of low-rank coals that affect comminution are the moisture content, slacking behavior, reactivity, and grindability [5]. A high moisture content requires higher throughput requirements to obtain given rates of energy production. As mined, lignite particles are fibrous, resilient, and tough. However, as the moisture evaporates, the surface of the particles becomes brittle and can spall off without application of external force. The process is generally referred to as slacking, considered undesirable in the handling and storage of lignite. During pulverization, weakening of the particles by drying the lignite before grinding or in the grinding operation itself can be a benefit. By the time a lignite has been ground and partially dried for feed to a pulverized coal burner, the particles are highly reactive, resulting in ignition characteristics superior to those of bituminous coals. Therefore lignites are ground to 5.5-65% -200 mesh rather than 70% -200 mesh for bituminous coals [5]. (In this connection, however, particle size effects on combustion and ash deposition, described in Chapter 11,must be taken into account.) The initial size reduction is usually performed with toothed, double-roll crushers. The ability of the teeth to engage the lump of lignite and then tear or shred it as it passes through the rollers helps to break down the resilient lignite structure. Double-roll crushers with cone-shaped teeth are used for secondary size reduction. A size reduction ratio of 2 is desired in each roll

490 crushing stage [7]. Swing-hammer mills provide a greater size reduction ratio

(e.g. reduction of

125x200 mm lump to stoker size, nominally 38x9.5 mm [7]) but have the disadvantage of producing more fines. Size reduction may occur in several stages between the mine and the end use of the lignite. Primary breaking usually takes place at the mine (or near it) and involves a reduction of top size to 10-20 cm [5]. Secondary crushing usually occurs at the power station, and involves a reduction of top size to 38 mm. If the lignite is not being used in pulverized-coal-fired boilers, screening the products of secondary crushing produces the various commercial sizes (e.g., 25x9 mm feed for stokers) [5]. The Hardgrove grindability of lignites has been discussed in Chapter 7. To recap briefly, the Hardgrove grindability index varies with moisture content, so that lignites are relatively easy to grind at 35--40% moisture, become more difficult to grind at 20-25% moisture, and then become more grindable again at lower moisture contents [5]. The Hardgrove grindability index is not easy to correlate, for lignites, with performance of pilot or commercial scale pulverizers. Specifically, the pulverizer performance suggests that grindability is lowest at the natural moisture content of the lignite and increases steadily as moisture is removed. As the moisture is removed, the fineness of the product and capacity of the mill increase and the power requirements decrease. These relationships are illustrated in Figures 10.9 and 10.10, respectively [5].

~

20-

~

1816-

"~

~I

14-

~

12-

.

lo-

"~ p,. V

8" -

N

6

....

0

22 02 0

" 10

20

30

I 40

" 50

Moisture content of pulverized lignite, percent Figure 10.9. The amount of pulverized lignite produced per megajoule as a function of moisture content of the lignite [5]. The shaded region includes results for hammer mill pulverization of two lignites, with in-mill drying.

491 300 250[::I 200

~9

15010050-

0

' "

0

'I

5

"

"

I'

"

'I

"

"

I'

"

'I

"

"

I'

"

'I

"

10 15 20 25 30 35 Moisture content, percent

"

40

Figure 10.10. The effect of drying on the capacity of pilot-plant pulverizers (adapted from [5]). The line is the best fit of data for hammer and ball mills with both pre-dried feed and in-mill drying. 100% of capacity is at 35% moisture.

10.5.2 Pulverization for combustion One of the first large-scale installations with a pulverized-coal fired-boiler was the W. J. Neal plant in Voltaire, North Dakota. This plant used Raymond bowl mill pulverizers which had a rated capacity of 9 t/h o f - 1 9 mm lignite. The energy requirements were estimated to be 52 MJ/t. Notwithstanding the generally lower Hardgrove grindabilities of lignites compared to bituminous coals, this energy requirement falls midway in the 36-67 MJ/t range for pulverizing bituminous coal [7]. The pulverizer requirements for feeding a nominal 600 MW unit (requiring 5.6 TJ/h) are shown in Table 10.11 [5]. TABLE 10.11 Pulverizer requirements for nominal 600 MW unit [5].

Hardgrove Index Energy Content, m,mmf, MJ/kg Number of mills req'd. Mill capacity, t/h Primary air temp., ~

Texas Lignite 48 14-20 6 83 400

Great Plains Lignite 35 14-20 7 91 400

492 For comparison, a eastem U. S. bituminous coal having a Hardgrove grindability of 55 and energy content on a moist, mineral-matter-free basis of 24.4-32.6 MJ/kg would require 6 mills of 45 t/h nominal capacity, and a primary air temperature of 274~ for drying [5]. Fracture of lignite during ball milling proceeds first through the weakest part of the physical structure. The weakest materials are characterized by pores up to 5 ~m [ 103]. Thus comminution rates are initially large. However, as comminution proceeds, stronger materials (i.e., having smaller pores) must now be fractured, and therefore the rate decreases. After ball milling for an hour, comminution proceeds through material having closed pores with diameters of about 21 nm (assuming cylindrical pores). Grinding will open pores which were originally closed in the lignite particles prior to grinding. The opening of previously closed pores results in increases in density and surface area; ball milling of Savage (Montana) lignite for 7 h results in an increase of surface area as measured by nitrogen adsorption by a factor of about 2.5 [103]. A critical diameter of particles can be defined as that particle size at which all closed pores have been opened. For the Savage lignite the critical diameter is below 38 ~

[103].

10.5.3 Micropulverization The simultaneous grinding and drying of Mississippi lignite using steam or air in a fluid energy mill can reduce the average particle size to seven microns and at the same time effect a reduction in the equilibrium moisture content [ 104]. As the temperature of the grinding medium increases, the equilibrium moisture content of the product drops from about 37% with a grinding temperature of 38~

to about 14% for grinding at 1630C [104]. Above that temperature, the

equilibrium moisture of the product appears to be independent of temperature. This observation is consistent with reductions in apparent surface area, measured by heat of immersion calorimetry, with increasing temperature of the grinding medium. In the temperature range 107-253~

the

power required for grinding and drying increases linearly with temperature of the grinding medium [105]. Particle size distributions are shown in Fig. 10.11 [104] for pulverizing Indian Head lignite in a Mikro-pul hammer mill at hammer speeds of 3300 and 6800 rpm. Different lignites give different grinding behaviors; the particle size distribution obtained by grinding a Texas lignite at 6800 rpm with the 3 mm screen is not the same as that obtained with the Indian Head lignite. Indian Head lignite had to be ground in two stages at 6800 rpm, once with a 3 mm screen and then with a 0.8 mm screen, to match the particle size distribution obtained with the Texas lignite at 6800 rpm with the 3 mm screen. Unfortunately no analytical or petrographic characterization of these lignites was reported with the grinding tests.

493 300 100

t~

3300 rpm

10-

9

''''

I''''

20

6800 rpm

I''''

40

I''''

60

I''''

80

100

Cumulative weight percent oversize Figure 10.11. Particle size distributions for Indian Head lignite pulverized in a hammer mill, as a function of hammer speeds [ 104]. Product was screened on a 3.2 mm round perforated screen.

10.6 BRIQUETTING 10.6.1 Laboratory_ and pilot scale studies Interest in the briquetting of lignite dates at least from the early years of the twentieth century. Lignites can be briquetted by the application of pressure alone, without the use of binders such as tar or pitch, provided that the lignite is at an optimum moisture content [106]. Moisture content and particle size are critical parameters in binderless briquetting [107]. The strength of lignite briquettes is related to moisture content and is not affected by variations in mineral matter content over the range of 8.25 to 56% (dry basis) [ 106]. Adhesion forces greatly affect the strength of briquettes. The optimum moisture content for satisfactory briquetting is 16-17% at low briquetting pressures, and decreases with an increase in pressure [106]. Briquette strength increases with decreasing particle size of the starting lignite. Briquette strength arises largely from hydrogen bonding between polar oxygen-containing functional groups in the lignite. The hydroxyl and carboxyl groups of lignites can form hydrogen bonds with each other, or indirectly with each other via a water molecule. In addition, the possibility exists for hydrogen bonding between two hydroxyl or two carboxyl groups, again either directly or indirectly through the intermediary of a water molecule. Acetylation o f - O H groups or barium acetate exchange of the carboxyl groups reduces briquette strength" using both treatments on the lignite reduces briquette strength by up to 90% [106]. This observation suggests that the important bonding mechanism for strength development in briquettes is a hydroxyl hydrogen-bonded to a carboxyl. At low briquetting

494 pressures this hydroxyl-carboxyl linkage is probably effected through a water molecule, since the optimum moisture content decreases with increasing briquette strength. That is, as pressure increases, these reactive oxygen groups tend to form a higher proportion of hydrogen bonds directly between them without the intervention of a water molecule. For briquetting at 45 t pressure, maxima in compressibility and plastic deformation and minima in elastic deformation and bulk modulus occurred at 15% moisture [106]. Thus at this particular pressure a moisture content of 15% would be optimum for briquetting. At the optimum moisture capacity the polar functional groups are almost completely converted to a hydrogenbonded state. Without coverage of the polar groups with hydrogen-bonded water molecules, repulsive interactions between the polar groups can hinder the movement of portions of the organic structure relative to each other. But at the optimum moisture content the repulsive interactions are removed (because of coverage by water molecules) and translational or rotational motion of the organic structure becomes easier. The easier movement of portions of the structure contributes to the maximum in compressibility and plasticity and to the minimum in bulk modulus. Hungarian brown coal briquettes with 16--17 MPa strength could be produced using a briquetting pressure of 120-140 MPa, temperature of 60-80~

particle size of 2x0 mm, and a coal

moisture content of 17-20% [108]. The ash level should not exceed 20% on a dry coal basis. These briquettes could be stored for at least three months if kept out of contact with water. Briquetting without a binder is restricted to soft, unconsolidated lignites having particle size of 100% -4 mm and 60--65% -1 mm and 8-12% moisture [109]. The briquetting process operates at 38-66~

with pressures of 69-103 MPa [109]. High amounts of xylite in the coal are

detrimental to stability of briquettes from German brown coal [110]. Low-density biomass materials such as bagasse increase the volume of the mix fed to briquetting machinery but reduce the weight of material passing between gravity-fed rolls. Production of briquettes stable to handling requires a screw feed with 2.5:1 compression ratio and a briquetting pressure of 14 MPa. Carbonizing the lignite first and then briquetting the char with a binder of 5-8% coal tar pitch and 1-2% flour provided good quality briquettes, of 27.2 MJ/kg calorific value [111]. An emulsion of 25% asphalt, 4% starch, and 71% water provides a satisfactory binder [5]. A mixture of 90-95% lignite or lignite char is blended with 5-10% of this emulsified binder, and, after crushing, grinding, and mixing, is pelletized in a bailing disc at 40-42% moisture content [5]. A satisfactory size consist for the grind is -10 mesh, with 50% 10x50 mesh and 20% -200 mesh [5]. The resulting balls, about 25 mm in diameter, are dried at 110~ until the moisture is in the range 10-16% [5]. Slow drying produces stronger pellets than rapid drying. Holding the moisture content at the maximum permissible level enhances the mechanical properties of the pellets. In contrast to many pelletizing operations, maximum strength does not occur at "zero" moisture. With pellets at 10% moisture the compression strength is significantly higher than at zero moisture. Gilsonite does not make as effective a binder as asphalt, possibly because of the higher softening temperature of gilsonite, or the greater difficulty in getting it to penetrate the lignite particles. Tars produced from pyrolyzing lignite at 370-540~ do not make good binders either. The mechanical

495 properties of pellets from lignite char are comparable with those from unreacted lignite. However, pellets from char have greater weathering resistance and better potential as a smokeless fuel. Laboratory scale briquetting of Maissade (Haiti) lignite char in a hydraulic press produced satisfactory briquettes by blending 10-25% bagasse, 15% or less of a 2:1 mixture of molasses and calcium hydroxide, 0.2% borax, and 2% sodium nitrate [112]. (The data are shown as percentages of dry weight of char.) The char was produced by pyrolysis at 800~ temperatures

pyrolysis at lower

(e.g., 550 ~) does not sufficiently drive out all of the tar and organic sulfur that would

contribute to excessive smoke or odors if the briquettes were used for domestic cooking. Borax serves as a mold release agent. Sodium nitrate improves the ignition behavior. Bagasse improves briquette strength, ignitability, and heating value. Molasses serves as a binder; however, the efficacy of the molasses depends in some unknown way on source, native Haitian molasses being superior in this application to U. S. molasses sold as a grocery item. (Agricultural binders such as corn stalks and wood waste have been used in the briquetting of German brown coals [113].) The lime serves as a sulfur-capture agent to trap sulfur oxides released during combustion. Combustion tests with various forms of the briquettes established that overall shape is not important, provided that the highest practical surface-to-volume ratio is obtained. Pelletizing lignites with 5 to 10% of a 25% aqueous emulsion of asphalt, followed by drying to 10% moisture content, results in agglomerates with good crushing and impact strengths, and 22.1 MJ/kg calorific value [114]. The pellets most resistant to degradation, water reabsorption, spontaneous ignition, and dust Ibrmation were made with 5% or more asphalt binder [115]. Bitumen emulsion also serves as a useful binder for bfiquetting German brown coals [113]. Laboratory-scale briquetting of Elgin-Butler (Texas) lignite in a hydraulic press with a 50 mm diameter die at pressures of 97, 156, and 195 MPa indicated an optimum briquetting pressure of 156 MPa, since the strength of the resulting briquettes was highest with this compaction pressure, although in fact there was little variation in briquette strength from the weakest to strongest (52-57 kg/cm2) [109]. At the optimum briquetting pressure, the strength of briquettes increased from 57 to 65.7 kg/cm2 with a decrease in moisture content from 18.4 to 8% [109]. At a compaction pressure of 156 MF'a and moisture content of 11.4%, briquette strength increased significantly as the particle size of the lignite was decreased. For lignite of 100% -1.6 mm, the briquette strength was 64.0 kg/cm2; but the strength increased to 90.2 kg/cm2 Ibr particle size 100% -0.5mm [109]. The high ash content of the Elgin-Butler lignite (26%, dry basis), and the fact that 63% of the ash is silica and alumina, suggest that significant abrasion of the ram presses might occur during briquetting [109]. (Briquetting practice in Greece limits the ash content in the lignite to 9% maximum [116].) Binderless briquetting of this lignite would probably best be done using ring roller presses rather than extrusion presses. Elgin-Butler lignite was also tested in the Papakube system, designed to compact biomass into rhomboids 3.2 x 3.2 x 5 cm. In this process the feed is sized t o - 3 . 8 cm, dried to 20-25% moisture, and compacted at 55-69 MPa [109]. Briquettes prepared in this way showed considerable degradation on handling, and possibly could not withstand normal handling

496 operations. After heating in inert atmosphere to 815~ for 30 minutes, the Papakube briquettes showed a failure stress of 21 kPa, whereas those produced in the extrusion tests showed a failure stress of 66 kPa [109]. For feed to a fixed-bed gasifier, the Papakube briquettes would probably fail under the loading imposed by the weight of the bed, whereas the extruded briquettes should work satisfactorily. However, breaking the extruded briquettes exposes a granular surface that has a tendency to shed fines, excessive fines production presenting operating problems in fixed-bed gasifiers. Both types of briquettes disintegrated after exposure to water for 1 to 3 min [109]. (Products having a low degree of water readsorption by slurrying a low-rank coal in ammonium, potassium, sodium, or calcium hydroxide at pH >10 and treating the slurry at 80-3600C [117].) Sulfite liquors from a pulp and paper mill have been used as a binder in briquetting Turkish lignites [ 118]. Compressive strength of the briquettes is affected by particle size, moisture content, binder concentration, and temperature and pressure of briquetting. Desirable characteristics include 12-16% moisture content in the lignite, <3 mm particle size, and 8-10% binder. Compressive strength increases as a function of temperature up to 1400C, though rate of increase diminishes above 80-100~

Pressing at 100 MPa provides maximum compressive strength in the briquettes.

The reactivity of lignite char (produced by pyrolysis in a fluid bed reactor) is so great that special handling precautions are needed for subsequent curing and coking in the FMC briquetting process [5]. The briquettes formed from the calcine and the expelled tar (as binder) are heat-treated in an oxidizing atmosphere at 230~ in a travelling grate unit to produce a carbon from the binder having about the same chemical reactivity as the calcined coal. After this heat-treating or curing step the briquettes are devolatilized in a shaft kiln at 870~ to produce the finished coke. 10.6.2 Commercial briquetting The only commercial briquetting operation in the United States using lignite is the Husky operation in Dickinson, North Dakota. (In fact, there is only one other commercial briquetting operation in the U.S. using low-rank coals of any kind, the FMC Corporation operation in Kemmerer, Wyoming producing briquettes for metallurgical use from subbituminous coal char. During the early 50's the Parry process was operated experimentally by the Texas Power and Lignite Company at the Alcoa facility in Rockdale, Texas.) The Husky process uses Lurgi-Spulgas carbonizers (described below) to produce char. Pyrite is separated from the char and the char is then ground, mixed, and briquetted. Currently the output of the plant is entirely devoted to briquettes ("charcoal") for barbecues. Those briquettes are made by mixing the char with a starch binder and water. Prior to 1964 the Husky plant also manufactured fuel briquettes during the winter months. Fuel briquettes were produced using pitch and asphalt as the binder [7]. The char was reduced to 3 mm top size. A binder consisting of a 1:2 mixture of lignite pitch (from the carbonization) and petroleum-derived asphalt was used. The binder requirement was 7-9%. Two plants for commercial briquetting of lignite char were erected in 1928, one in Dickinson, North Dakota and the other in Bienfait, Saskatchewan, to manufacture briquetted fuels having a high calorific value and reasonably smokeless combustion. Both plants used the Lurgi-

497 Spulgas process. In the Lurgi-Spulgas process as applied to lignite, the feed is sized to 100x6 mm. Drying occurs in the upper section of the retort; the consequent decrepitation of the lignite particles reduces the maximum size to 19 mm entering the lower carbonization section. Hot gas produced by burning the carbonization gas enters the retort at 700~ and heats the lignite to about 600~ product char is cooled by recycled combustion gas. Char is discharged at about 90~

The

The binder

needed for briquetting derives from distilling the tar, producing a distillate useful as a wood preservative and a pitch. The pitch, blended with petroleum-derived asphalt, serves as binder [ 119]. The tar-oil yield was about 20 L/t on an as-mined basis [ 119].

10.7 L I G N I T E / W A T E R

SLURRIES

10.7.1 Pilot-scale preparation Hot-water drying of lignites provides an approach to the production of stable lignite/water slurries. These slurries could be used for transport of lignite and potentially could be burned without being dewatered. As discussed previously, some reduction in sodium content is obtained in the hot-water-drying process. Therefore this technology represents a new approach to beneficiation, transportation, and combustion of lignites. The changes to the lignite resulting from hydrothermal treatment are destruction of carboxyl groups, reducing the hydrophilic sites on the lignite surface; extraction of sodium and calcium cations into the process water, removing species deleterious to slurry concentration; and devolatilization, forming tars or waxes which could resolidify to plug pore entrances, thus reducing surface area. Apparent viscosities of lignite/water slurries prepared from hydrothermally treated lignite (5 minutes at 330~

are compared in Figure 10.12 with slurries prepared from untreated lignite

[120]. The lignites for which these data were obtained are Indian Head and South Hallsville (Texas). The hydrothermally treated coals produced slurries which flowed at viscosities above 8 Pa.s. The slurries showed pseudoplastic flow behavior with little or no yield stress. Stability varies considerably; a slurry of hydrothermally treated Indian Head lignite was stable for over a year with no evidence of sedimentation even in the absence of slurry-stabilizing additives, but in comparison, the South Hallsville lignite slurry settled to a hard pack solid in 5-10 hours. The effects of hydrothermal treatment on the properties of Indian Head and South Hallsville lignites are summarized in Table 10.12 [121]. Except for a significant decrease in equilibrium moisture and a slight reduction (about 9 and 18% in the two lignites) oxygen with concomitant increase in carbon, the properties of the lignite change little as a result of the hydrothermal treatment. The most significant change in ash composition is a reduction in sodium content, from 4.3 to 2.9% (as Na20) in Indian Head and 0.9% essentially to zero in the South Hallsville lignite [121].

498 TABLE 10.12 Comparative feedstock and product analyses for Indian Head and South Hallsville lignites hydrothermally treated at 330~ [121].

Equil. Moisture Proximate (dry basis) Ash Volatile matter Fixed carbon Ultimate (dry basis) Carbon Hydrogen Nitrogen Sulfur Oxygen Calorific Value, MJ/kg mf maf

Indian Head Feed Product 33.0 20.7

South Hallsville Feed Product 28.5 23.2

15.0 38.9 46.1

14.3 38.0 47.7

13.3 42.2 44.5

15.5 40.0 44.5

60.5 4.0

63.2 4.0

59.6 4.1

61.5 3.9

1.4 1.1

1.3

1.4

1.2

18.0

0.9 16.3

1.3 20.3

1.3 16.6

23.6 27.8

24.8 29.0

24.5 28.2

23.8 28.2

2000 ?

1000-

O

Untreated Indian Head

"~ 100

9 UntreatedSouthHallsville + HWD Indian Head

<

0

HWD South HaUsville m

10

''''

35

I''''

I''''

I''''

I''''

I''''

I

40 45 50 55 60 65 Bone-dry solids loading, weight percent

Figure 10.12. Apparent viscosity at 100 s-] and 28~ as a function of solids loading for various untreated and hot-water-dried lignite/water slurries [ 120].

499 10.7.2 Rheology of Lignite-Water Slurries Systems containing over 20% lignite behave as pseudoplastic fluids [122]. Viscosity increases with lignite concentration and decreases with increasing temperature. All slurries produced from Indian Head lignite of 48 ~tm mass mean diameter particle size at five conditions--as-received, thermally dried in a rotary drum dryer to 14% moisture, and hotwater dried at 270, 300, and 330~

pseudoplastic with significant thixotropy at high shear

[ 14]. Beulah lignite slurry also shows pseudoplastic behavior, which becomes increasingly evident at high shear rates [123]. Measurements with otherwise comparable slurries produced from 225 ~tm particle size Indian Head lignite did not show such consistent behavior. Slurries from asreceived and thermally dried lignites were Bingham plastics. However, the slurries prepared from hot-water-dried lignite were pseudoplastic as were those with the smaller particle size lignite. Generally slurries of smaller average particle size show more pronounced pseudoplasticity than those from lignites of larger average particle size [ 123]. Increasing the temperature reduces the apparent viscosity of slurries produced from thermally dried lignite. For a slurry from 48 ~tm particle size Indian Head lignite, the apparent viscosity at a shear rate of 410 cm-1 decreased from 2 Pa.s at 28 ~ to 0.92 Pa.s at 790C [14]. Lignites of average particle size 10-27 ~tm show increased pseudoplasticity at shear rates of 103-105 s-1 relative to that observed below 450 s-1 [123]. 10.7.3 Solids Loadings of Lignite-Water Slurries The maximum bone-dry solids loadings for pourable slurries of Indian Head lignite are shown in Table 10.13 [14]. TABLE 10.13 Maximum bone-dry solids loadings for pourable slurries of Indian Head lignite [ 14].

Particle size, mass mean diameter As-received lignite Thermally dried Hot water dried at 270 ~ Hot water dried at 300" Hot water dried at 330"

Solids Loading, % 48 am 225 ~m 40.4 39.8 42.6 48.5 53.3 52.1 56.0 58.0 58.3

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500

10 11 12 13 14 15 16 17 18

19 20 21 22

23 24 25 26

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53

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502 54

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flocculation for sulfur removal from Teruel lignites, Fuel, 69 (1990) 166-171. A.F. Duzy, M.P. Corriveau, R. Byrom and R.E. Zimmerman, Western coal deposits-pertinent qualitative evaluations prior to mining and utilization, in: G.H. Gronhovd and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Dept. Energy Rept. GFERC/IC77/1, (1978), pp. 13-42. J.A. Cavaliaro and A.F. Baker, Washability characteristics of Arkansas and Texas lignite: report of investigations, U.S. Environ. Protect. Agency Rept. EPA-600/7-79-149, (1979). M.J. Mitchell, Fine coal cleaning, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1531, (1984), pp. 11-1 - 11-12. F.E. Huggins, G.P. Huffman and A.A. Levasseur, Comparison of combustion deposits from bituminous coals and a lignite, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33(2) (1988) 73-80. M.J. Mitchell, Fine coal cleaning, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1642, (1984), pp. 9-1 - 9-22. D.C. Cronauer, J.T. Joseph, A. Davis, J.C. Quick, and P.T. Luckie, The beneficiation of Martin Lake Texas lignite, Fuel, 71 (1992) 65-73. D.C. Cronauer and A.J. Swanson, Coal beneficiation: process development for liquefaction, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 36 (1991) 67-74. G.A. Robbins, R.A. Winschel, C.L. Ames, and F.P. Burke, Agglomeration of low-rank coal as a pretreatment for direct liquefaction, Fuel, 71 (1992) 1039-1046. G.A. Robbins, R.A. Winschel, and C.L. Ames, Agglomeration of low-rank coal as a pretreatment for direct coal liquefaction, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 36 (1991) 58-66. D.J. Brown, Fine coal cleaning, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181 - 1846, (1986), pp. 9-1 - 9-11. D.J. Brown, Fine coal cleaning, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/UNDERC/QTR-85/2, (1986), pp. 9-1 -9-6. D.J. Brown, Fine coal cleaning, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/UNDERC/QTR-85/3-4, (1986), pp. 9-1 -9-4. T. Fraser, W.L. Crevetz, and O.T. Barrett, Preparation characteristics of some coals available for the synthetic liquid fuels industry, U.S. Bur. Mines Bull. 495, (1950). N. Ikura, J.F. Kelly and C.E. Capes, Selective oil agglomeration of lignite using vacuum bottoms only as an integral part of coprocessing, Energy Fuels, 3 (1989) 132-136. N. Ikura, J.F. Kelly, and C.E. Capes, Beneficiation of lignite by oil agglomeration as an intergral part of coprocessing, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33(1) (1988) 50-57. R.C. Timpe, C.L. Knudson and P. Mack, Beneficiation of a bituminous coal and a lignite coal by agglomeration using novel binding oils, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33(2) (1988) 352-358. M.A. Musich, R.A. DeWall and R.C. Timpe, Preparation of dewatered and deashed lowrank coals by oil agglomeration, U.S. Dept. Energy Rept. DE92 002606, (1991), pp. 5468. D.S. Scott and A.J. Royce, A new method for flue gas desulfurization / coal deashing for low-rank coals, Energy Fuels, 5 (1991) 612-613. N. Lazaroff, J.E. Wey and P.R. Dugan, Bacterioelectric deashing of coals, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 35 (1990) 926-942. L.E. Paulson, D.N. Baria and W.R. Kube, Inorganic constituents of northern Great Plains lignite and their modification, in: M. Fuerstenau and B. Palmer (Eds.), Gold, Silver, Uranium, and Coal--Geology, Mining, Extraction, and Environment, American Institute of Mining, Metallurgical, and Petroleum Engineers, New York, 1983, Chapter 25. B.T. Reski, Ion exchange equilibrium in North Dakota lignite, B.S. (Hon.) Thesis, University of North Dakota, Grand Forks, ND, 1981. L.E. Paulson, D.N. Baria and W.R. Kube, Design, cost and economic benefits of ion exchange process to remove sodium from lignite, in: Energy Resources Co. Inc. (Eds.), Proceedings of the low-rank coal technology development workshop, U.S. Dept. Energy Rept., (1982), pp. 1-35- 1-46. R. Kunin and R.E. Berry, Carboxylic weak acid type cation exchange resin, Ind. Eng. Chem., 41 (1949) 1269-1272.

503 77 78 79 80 81 82 83 84 85 86 87 88

89

90 91 92 93 94 95 96 97 98 99 100

D.N. Baria, W.R. Kube and L.E. Paulson, Reduction in ash fouling potential of lignite by ion exchange, Amer. Inst. Chem. Eng. Symp. Set., 78(219) (1982) 54-63. M.H. Bobman, T.C. Golden, and R.G. Jenkins, Ion exchanges in selected low-rank coals. Part I. Equilibrium, Solvent Extr. Ion Exch., 1 (1983) 791-831. L.E. Paulson, D.N. Baria and W.R. Kube, A possible solution for reducing cost of ash fouling in burning low-rank coals, Amer. Soc. Mech. Eng. Paper 81-WA/Fu-1, (1981). D.N. Baria, W.R. Kube, and L.E. Paulson, Conceptual design of a commercial plant for sodium removal, Amer. Inst. Min. Met. Petrol. Eng. Paper A81-15 (1981). C.L. Wagoner, G. Haider, J.W. Berthold and R.A. Wessel, Measurement of fundamental properties characterizing coal minerals and fire-side deposits, U.S. Dept. Energy Rept. DOE/PC/40266-3, (1984). L.E. Paulson and J.R. Futch, Removal of sodium from lignite by ion exchanging with calcium chloride solutions, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 25(1) (1980) 224-227. G.G. Baker, R.C. Ellman, D.J. Maas, T.A. Potas and R.E. Sears, Coal-water slurry preparation, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181 - 1846, (1986), pp. 4-1 - 4-27. D.R. Hajicek, Fluidized-bed combustion of low-rank coals, in: G.A. Wiltsee (Ed.), Lowrank Coal Research Under the UND/DOE Cooperative Agreement, U.S. Dept. Energy Rept. DOE/FE/60181-17, (1983), pp. 10-1 - 10-9. L.E. Paulson and R.C. Ellman, Reduction of sodium in lignite by ion exchange: a pilot plant study, U.S. Dept. Energy Rept. GFETC/RI-79/1, (1979). F.R. Burkhardt and M.M. Persinger, Economic evaluation of losses to electrical power utilities caused by ash fouling, U.S. Dept. Energy Rept. DOE/GFETC/0059-1, (1980). S.D. Merritt, J.P. Wagner, T.G. Rozginyi and J.H. Zoeller, Jr., Hot vapor treatment of Gulf province lignites, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 37 (1992) 10061011. M.D. Baldwin, C.A. Smelker and H.F. Silver, Comparison of proposed clean coal process reduction mechanisms, in: R. Markuszewski and T.D. Wheelock (Eds.), Processing and Utilization of High-Sulfur Coals III, Elsevier, Amsterdam, 1990, pp. 725734. C. Rai and S. Mohaghigh, Microbial desulfurization of Illinois coals and texas lignite by microorganisms of the genus Pseudomonas, in: R. Markuszewski and T.D. Wheelock (Eds.), Processing and Utilization of High-Sulfur Coals III, Elsevier, Amsterdam, 1990, pp. 459-468. K.W. Miller, Evaluation of sulfur-reducing microorganisms for organic desulfurization, U.S. Dept. of Energy Rept. DOE/PC/90176-T52, (1991). H. Kara and R. Ceylan, Removal of sulfur from four Central Anatolian lignites by NaOH, Fuel, 67 (1988) 170-172. S.A.H. Zaidi, Ultrasonically enhanced coal desulfurization, Fuel Proc. Technol., 33 (1993) 95-100. S. Ktiqtikbayrak and E. Kadioglu, Comparison of some desulfurization methods for Turkish lignites, in: R. Markuszewski and T.D. Wheelock (Eds.), Processing and Utilization of High-Sulfur Coals III, Elsevier, Amsterdam, 1990, pp. 353-360. M. Oguz and A. Olcay, Desulfurization of Bolu-G~3yntik lignite using cupric chloride, Fuel, 71 (1992) 199-202. S. Ktiqtikbayrak and E. Kadioglu, Desulfurization of some Turkish lignites by pyrolysis, Fuel, 67 (1988) 867-870. A.B. Garcia and H.H. Schobert, Hydrodesulfurization of a Spanish lignite, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33(1) (1988) 241-246. A.B. Garcia and H.H. Schobert, Comparative performance of impregnated molybdenumsulfur catalysts in hydrogenation of Spanish lignite, Fuel, 68 (1989) 1613-1616. A.B. Garcia and H.H. Schobert, Hydrodesulfurization of Spanish lignite with high organic sulfur content, Coal Prep., 7 (1989) 47-54. A.B. Garcia and H.H. Schobert, Hydrodesulfurization of a high organic sulfur Spanish lignite with impregnated nickel sulfate, Coal Prep., 9 (1991) 185-197. A.B. Garcia and H.H. Schobert, Catalytic hydrodesulfurization of a high organic sulfur Turkish lignite: amount, form, and mechanism of sulfur removal, Fuel Proc. Technol. 26

504 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123

(1990) 99-109. A.B. Garcia and H.H. Schobert, Liquefaction behavior of high-sulfur lignites, in: H.H. Schobert, K.D. Bartle and L.J. Lynch (Eds.), Coal Science II, American Chemical Society, Washington, 1991, Chapter 16. A.M. Mastral and B. Rubia, Sulfur evolution in mild conversion of a Spanish low-rank coal, Fuel, 68 (1989) 80-83. J.M. Lytle, J.L. Daniel and G.L. Tingey, Effect of grinding on porosity and surface area of coal, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 27(3-4) (1982) 307-311. W.G. Steele, C.W. Bouchillon and R.B. Ross, Beneficiation processes for ultrafinely ground low-rank coals, in: M.L. Jones (Ed.), Technology and Utilization of Low-rank Coal, U.S. Dept. Energy Rept. DOE/METC-86/6036(Vol.2), (1986), pp. 483-495. C.W. Bouchillon, W.G. Steele and J.D. Burnett, Power requirements for ultrafine grinding and drying of low-rank coals in a fluid-energy mill, Proc. Intl. Symp. Adv. Fine Part. Process., (1990) 19-30. M.S. Iyengar, D.N. Sibal and A. Lahiri, Role of hydrogen bonds in the briquetting of lignite, Fuel, 36 (1957) 76-84. R.K. Komarek, Binderless briquetting of peat, lignite, subbituminous and bituminous coals in roll presses, Proc. 1991 Bienn. Conf. Inst. Briquet. Agglom., pp. 233-242. W. Bossan, J. Budde, E. Kuenstner, M. Schmidt and M. Suess, Laboratory briquetting tests of soft brown coal (lignite) from the Gyoengyoes-Visonta deposit (Hungary), Neue Bergbautech., 13 (1983) 396-400. R.E. Maurer and M. Colaluca, Briquetting Texas lignite, in: W.R. Kube, E.A. Sondreal and D.M. White (Eds.), Technology and Use of Lignite, U.S. Dept. Energy Rept. GFETC/IC-82/1, (1982), pp. 775-792. R. Rammler, Improvement of the efficiency of the Koppers synthesis gas producer, Freiberg. Forschungsh., A99 (1958) 5-79. I. Lavine, Progress in low-rank coals, Ind. Eng. Chem. 26 (1934) 154-164. W.B. Hauserman and M.D. Johnson, Production of cooking briquettes from Maissade (Haiti) lignite: feasibility study and preliminary plant design, U.S. Dept. Energy Rept. DOE/FE/60181-198, (1986). B. Bodi, L. Laposa, D. Pal, G. Ludanyi, L. Zsori, I. Szilagyi, A. Baal, I. Czabula and O. Lazar, Briquetting of lignite with resin binder for air pollution prevention, German Democratic Republic Patent No. DD 297,442 (1992). A.J. Grant and R.E. McKeever, Pelletizing and drying of lignite, Coal Technol. 2 (1978) 941-957. A.F. Baker, R.E. McKeever and A.W. Deurbrouck, Upgrading of lignite to enhance fuel value, American Mining Congress International Coal Show, Paper No. 2, Set No. 4, 1980. E. Kullik and D. Thormann, Ptolemais: Greece's largest lignite mine trebles production, World Mining, 11 (1971) 62-65. D.E. Mainwaring, G.B. Christie and D.V. Boger, Treating low-rank coal, Australian Patent No. 601,983 (1990). M. Saglam, M. Ytiksel, J. Yanik, M. Tutas, M. Karaduman and G. Llsttin, Production of water-resistant briquettes from Turkish lignites using sulfite liquor binders, Fuel, 69 (1990) 60-64. U.S. Bureau of Mines Staff, Technology of lignitic coals, Part 2. U.S. Bur. Mines Inf. Circ. 7692, (1954). T.A. Potas, G.G. Baker and D.J. Maas, Pilot-scale preparation of low-rank coal/water fuels, J. Coal Qual., 6 (1987) 53-57. G.G. Baker, W.B. Hauserman, R.C. Ellman, D.J. Maas, T.A. Potas and R.E. Sears, Coal/water slurry preparation, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/UNDERC/QTR-85/2, (1986), pp. 4-1-4-29. R.N. Paul, S.K. Basu and M. Chakrabarty, Studies on the rheology of the suspension of lignite in medium viscous fuel oil, Fuel Sci. Technol., 10 (1991)89-90. S.F. Ahmed and R. Hasan, Rheology of low-rank coal-water slurries at both high and low shear rates, Fuel, 72 (1993) 763-769.

505

Chapter 11

C O M B U S T I O N OF LIGNITES

The combustion of lignite to raise steam for electric power generation is by far the most import commercial application of North American lignites, and will almost certainly remain so for the next several decades. Smaller scale combustion applications are important for space heating and raising steam for industrial processing. This chapter discusses some of the fundamental aspects of lignite combustion, small-scale combustion applications, burning lignites in electric power stations and the attendant ash fouling problem, and an important emerging technology, fluidized bed combustion.

11.1 FUNDAMENTALS OF LIGNITE COMBUSTION 11.1.1 Calorific values of lignites The average calorific value for U.S. lignites, on an as-received basis, is 16.3 MJ/kg, with a range of 15.3-18.2 MJ/kg [1]. For North Dakota lignites the average is 16.2 MJ/kg with a range of 14.9-17.6 MJ/kg; for Texas lignites the respective values are 18.3 and 17.7-18.8 MJ/kg. A reasonable linear relationship exists between the total moisture expressed on an ash-free basis and the gross calorific value expressed on a moist, ash-free basis [2]. This relationship is illustrated in Figure 11.1 [2]. If the calorific value of coals of various ranks is expressed per weight of maf carbon (rather than per weight of coal), and plotted as a function of the aromaticity, a broad maximum of about 40.7 MJ/kg is found in the range of fa of 0.7-0.8 [3]. If only the contribution of carbon and hydrogen were considered, the calorific value should decrease monotonically with fa, reflecting the greater stability of the aromatic hydrocarbons. However, the low-rank coals, which have the lowest fa values, also contain appreciable oxygen, and the presence of the oxygen in the molecular structure effectively lowers the calorific value. Thus in the low ranks, calorific value increases with increasing fa, an artifact of the concomitant loss of oxygen accompanying increased coalification. In the high ranks calorific value decreases with further increases in fa, reflecting the stability conferred by the increasing stabilization of the large aromatic structures [3]. For calculating the calorific values of coals, an equation that gives satisfactory results for the coals in the Penn State Coal Sample Bank and Data Base, a collection which contains coals of

506

25000 -~ 9 20000 15000r.)

=

,._q 1 ~ =

gt

O

5000 0

''''I'

0

'''I'''

10

20

'I''''I

30

''''I''

40

50

''I''''

60

70

Total moisture, % (ash-free) Figure 11.1. The relationship of total moisture and calorific value for coals with gross calorific value less than 24 MJ/kg [2].

all ranks, is Q (kJ/kg) = 328.4 C + 1422 H + 92.7 S - 138.0 O + 636 [4]. In this equation the symbols represent the percentages of the elements expressed on a dry, mineral-matter-free basis. For the total data base, which includes all ranks of coals, 86% of the samples have a difference between calculated and observed calorific values lying between +700 kJ/kg. Based only on 44 lignite samples from the Penn State Coal Sample Bank (but including both Gulf Coast and Northern Great Plains lignites), the equation giving the best fit of calorific value is Q (kJ/kg) - 327.8 C + 1419 H + 92.6 S - 135 O + 132 [4]. Much of the oxygen in low-rank coals is released as carbon dioxide during carbonization, whereas for high-rank coals much of the oxygen is released as water and carbon monoxide [5]. Non-linear regression analysis led to an equation for low-rank coals containing a quadratic term in oxygen:

507 Q (Btu/lb) = 144.5 C + 610.2 H + 40.5 S - 6 5 . 9 O - 0 . 3 1 0 02

[5]. Regression analysis of data for 775 samples of coals of all ranks, including lignites, led to the equation: Q (Btu/lb) = 146.58 C + 568.78 H + 29.4 S - 6 . 5 8 A - 51.53 (O + N) [6], where Q is the calorific value on a dry basis, A is the ash content in weight percent on a dry basis, and the other symbols are the percentages of the various elements, also on a dry basis. If the oxygen plus nitrogen term is obtained by summing the other constituents and subtracting from 100, one can use an equivalent expression: Q (Btu/lb) = 198.11 C + 620.31H + 80.93 S + 44.95 A - 5153 [6]. The bias in these equations when applied to the calculation of the calorific values of lignites is -45 Btu/lb [6]. An alternative approach for calculation of the higher heating value is the equation HHV (Btu/lb) = 141.05 C + 610.74 H + 39.78 S - P. O + A where the coefficient P and the term A depend on rank [7]. For lignites, P = 58.3 Btu/lb, and A = 158.4 Btu/lb [7]. Pressure differential scanning calorimetry (heating at 20 ~ C/min in 3.5MPa oxygen) of coals generally shows a thermogram consisting of two distinct peaks [8]. For samples ranging in rank from peat to anthracite, an excellent linear relationship exists between the integrated area of the high-temperature peak and the fixed carbon content of the sample [8]. Linear least squares analysis yields the equation FC = 4.7(Area) + 15.8 for 18 samples, with a coefficient of determination of 0.965. Integration of the total thermogram provides the calorific value of the sample. For two coals having fixed carbon contents C 1 and C2 and corresponding peak areas A1 and A2, the incremental increase in calorific value calculated from A2-A1 is roughly equal to the increase in calorific value calculated directly from the increase in fixed carbon, C2-C1. 11.1.2 Mechanisms of combustion Combustion of a lignite particle proceeds in four stages: removal of surface moisture at low

508 temperatures, evolution of water from organic functional groups or minerals, evolution and combustion of the volatiles, and finally burnout of the char [9]. In comparison to bituminous coals, where tar is the major volatile material, significant quantities of water, carbon monoxide, and carbon dioxide are evolved from lignites [10]. Lignites are commonly said to be more reactive fuels than bituminous coals; however, for practical design purposes the reactivity advantage of lignites may be lost via the coarser fuel grind obtained and the lower flame temperature resulting from the high moisture content [9]. The Flammability Index is determined by heating 0.2 grams of 200x0 mesh coal in oxygen in a preheated furnace; the temperature of the furnace is increased in increments until a temperature is reached at which the sample ignites. When a variety of coal samples is tested, the Flammability Index provides a relative measure of ignition temperatures. The Flammability Index of Monticello (Texas) lignite was 480~

the lowest of four ranks of coals tested [11]. A Flammability Index in

this range indicates that the fuel ignites easily and does not present flame stability problems in pulverized-coal-fired boilers. Ignition Index values of eight northern Great Plains lignites averaged 6.1 with a standard deviation of 1.7 [ 12]. The Ignition Index measures ignition reactivity of brown coals in conditions simulating those in a pulverized-coal-fired boiler. Single particles of coal, 1 mm in diameter and air dried to approximately 15% moisture, are injected into a stream of an equivolumnar mixture of air and nitrogen (i.e., 10.5% oxygen). The gas temperature is 5800C and its velocity is 3.50 m/s. The time elapsed until the particle ignites is measured electronically, based on the output of a photocell. The Ignition Index is calculated from the mean of ten replicate tests; the values have units of seconds. The Ignition Index varies inversely with ignition performance; that is, as the Index increases the performance deteriorates. The mean of 6.1 for the northern Great Plains lignites compares with a mean Index of 2.8 for brown coals from the Latrobe Valley [12]. A higher Index--or lower ignition reactivity--correlates with porosity: the lower the porosity the lower the ignition reacti vi ty. The ignition delay is the time from the initial mixing of the fuel with the oxidant until the beginning of the heat release. For vitrains from Beulah (North Dakota-) lignite, there is no effect of moisture on the ignition delay, the combustion rate, or the CO2/CO ratio [ 13]. Increasing the gas temperature decreased the ignition delay [13,14]. Kinetic parameters for this process are an activation energy of 84 kJ/mol and a pre-exponential factor of 200 g/(cm2 s) [13]. Lithotype composition has an effect on ignition behavior (i.e., of lignites not separated into lithotypes) [ 13]. Fusain shows the shortest ignition delay [ 13,14]. The reactivity of lignite char is similar to that of the most reactive lithotype (fusain) and the combustion rates are reasonably similar, although that of the lignite is slightly lower than that of the char. CO and CO2 production is similar for lignite and its char. These similarities suggest that lignite burns via heterogeneous combustion, that is, without releasing a cloud of volatiles which ignite first and burn homogeneously, followed by a separate, heterogeneous char burnout step [13]. The mechanism of ignition depends on oxygen concentration [14]. Higher oxygen

509 concentrations favor heterogeneous ignition. Essentially, heterogeneous ignition competes with volatiles release; the higher the oxygen concentration, the greater the reaction rate at the particle surface and the greater the likelihood of heterogeneous ignition occurring before devolatilization. At low oxygen concentrations, devolatilization occurs before ignition [14]. At 650~ devolatilization of Beulah lignite precedes heterogeneous ignition only when the oxygen concentration is less than 11% [14]. At 877~ essentially no combustion of pulverized lignite occurs unless the oxygen mole fraction is greater than about 0.15 [15]. In the size ranges 140x170 and 200x230 mesh at 877+20~

no combustion can take place unless the mole fraction of oxygen in the gas phase is at

least in the range 0.15-4).2 [ 15]. Wet Beulah lignite showed a jump of particle temperature to 1327~ (measured by Fourier transform infrared spectroscopy) upon ignition in an entrained-flow reactor [16]. The maximum temperature reached was 14270C. Moisture has little effect on ignition [ 14]. German brown coal showed maximum temperatures of 1450~ for 90-360 ~tm particles, but smaller particles could not attain temperatures higher than the maximum gas temperature of about 1200~ [ 17]. The high internal porosity of lignites contributes to their high reactivity, and is the factor providing uniform release of volatiles over the whole lignite surface, as opposed to the release of volatiles from individual pores of bituminous coals [18]. The most abundant material emitted during devolatilization is carbon dioxide. Tar and oil yields are low, as are yields of combustible gases such as carbon monoxide, methane, and hydrogen. The yield of combustible volatiles is much lower than that from a bituminous coal; while the combustible volatiles help stabilize ignition of bituminous coal, for lignites the char reactivity is more important in determining ignition characteristics [ 18]. The high moisture content affects the initial heating of the lignite particles prior to ignition. Ignition can also be affected by the ion-exchangeable cations normally present in lignites [ 18]. In particular, calcium seems to be a key promoter of lignite oxidation [ 14]. Volatiles yield is important in determining flame stabilization. In an industrial swirl burner a stable flame could not be achieved with coals of less than about 30% volatile matter if the furnace had water walls, although flame stabilization could be achieved with refractory walls [ 19]. Coals that provide a high volatile yield allow use of a smaller burner to achieve the same throughput when compared to coals of lower volatile yields. Furthermore, the high volatile yield coals allow greater flexibility in burner design and operating conditions. For low-rank coals, the volatile yield is not well predicted by the H/C ratio, presumably because of the high oxygen content of these coals. A better correlation is obtained from the percent carbon expressed as a function of the atomic (H+O)/C ratio [19]. The high oxygen content of lignites also results in the volatiles being highly oxygenated, and, as a consequence, the calorific value of the volatile matter is lower than that of the undecomposed lignite. For Beulah-Zap lignite, the ratio of calorific value of the volatile matter to calorific value of the lignite is about 0.93 [ 19]. This is not the case for bituminous coals, where volatile matter/coal calorific value ratios of up to 1.15 were observed [ 19].

510 Beulah-Zap and Darco (Texas) lignites, as well as eleven other coals of higher rank, showed higher yields of volatiles at a higher devolatilization temperature [ 19]. With rapid heating (105-106 *C/s) to temperatures in the range 1227-2227~

the volatile yields exceed those

predicted by the ASTM volatile matter test. The relationship between the mass yield of volatiles and the percentage of carbon converted to volatiles obtained in this investigation [19] agrees with earlier work [20]. For Darco lignite at 1327~

for example, the mass yield of volatiles was about 64%,

with a 62% conversion of carbon [ 19]. Under these conditions the volatile yields correlate directly with the carbon content of the coal. The conversion of carbon to volatiles correlated reasonably well with the mean maximum vitrinite reflectance through the range of ranks tested, although little comparison can be made among lignites because the reflectance values are so similar, in the range of 0.3-0.4% [19]. For several ranks of coal, the yield of tar aerosols (precursors to soot) varies as hvb > mvb > lignite > anthracite [21]. Aerosol yields vary with rank in the same order as tar yields. Below 1127"C the aerosol produced from a Montana lignite was gold to brown in color, suggestive of condensed tar. Significant conversion of tar to soot occurs above 1227~

Particle sizes of soot

produced in a laboratory scale laminar flow furnace increase with residence time, and with coal feed rate. For Montana lignite, the log normal average particle size of the soot is 14 nm for a coal feed rate of 2.5 g/h; increasing the feed rate to 8.4 g/h resulted in an increased particle size of 34 nm. An increase in the concentration of soot particles and soot precursors increases particle growth by coagulation and surface condensation. At higher temperatures condensed ash also contributes to the aerosol yield, the ash comprising up to 12% of the aerosol yield. Ash vaporized from the char recondenses in the aerosol. Above 1227~ most of the ash recondenses on the surface of the soot, because the high surface area of the soot facilitates heterogeneous condensation. If the soot were not present, then ash recondensation would proceed by homogeneous nucleation. In a laminar flow reactor, Hagel (North Dakota) lignite, which has a low tar yield during devolatilization, has a very low yield of agglomerated soot [22]. A brightly radiating region around individual lignite particles characterizes the earliest post-ignition phase. The radiation was attributed to soot formed during diffusion-limited burning of ejected volatiles. When lignite reacts in air, the presence of metal cations results in a decreased weight loss compared to a lignite that had been treated with acid. After 0.05 seconds acid-washed lignite will show an 80% weight loss, whereas untreated lignite will have a weight loss of about 30% [23]. The difference is related to the decrease in the rate of volatile evolution caused by the metal cations. The flame temperatures in lignite combustion are relatively low; hence the formation of thermal NOx is also low [18]. Fuel nitrogen is released initially as HCN or is retained in char [24]. In a reducing atmosphere HCN will be converted to N2; consequently, a proper staging of the combustion air may allow control of NO formation from this fraction of the fuel nitrogen. Since an oxidizing atmosphere is required for char combustion, formation of NO from the fraction of fuel nitrogen initially retained in the char would be very difficult to control. In the first stage of fuelrich, pulverized-coal staged combustion (up to 100 ms) the temporal profiles of nitrogen species

511 depend on coal composition as well as on the stoichiometric ratio and temperature [25]. Coal composition controls the environment and temperature history through the combustor. Under fuelrich conditions in the post-flame (1-2 s after ignition) a substantial portion of nitrogen in the coal is converted to N2 [25]. At the end of this phase of combustion, the yield of ammonia from lignite exceeds that of hydrogen cyanide, whereas the reverse is true for bituminous coals. The relative amounts of HCN and NH3 are a property of the coal composition, but both increase with decreasing temperature. Texas lignite char is the most easily ignited and burns the most rapidly of chars produced from a variety of ranks of coal [26]. (These tests were conducted in a thermogravimetric analyzer.) Indeed, low-rank coal chars are generally considered more reactive than chars from higher rank coals [7,27]. Since the chemical structures of chars, produced from five coals of ranks through low-volatile bituminous, appear to be similar, this rank dependence of char reactivity must reflect differences in physical structure such as internal surface area or porosity [27]. Pore structure is very important in char combustion. As a rule, the higher the total open porosity the greater will be the char reactivity [28]. In a flat-flame burner, chars produced from Beulah-Zap lignite show a three- to four-fold increase in total porosity [29]. Most of this increase occurs in the macropore range. For example, the nitrogen surface areas, indicating pores in the 1.5-20 nm radius range, increased by factors of 20-200 relative to the unreacted lignite, while carbon dioxide surface areas (indicating micropores of <1.5 nm radius) increased only two- or three-fold [29]. (The great porosity increase reduces the char particle densities by up to 50% [29].) The evolution of the macropore structure of Wilcox (Texas) lignite is shown in Table 11.1 [30]. TABLE 11.1 Effect of pyrolysis conditions on macropore structure of Wilcox lignite char [30]. Pyrolysis heating rate, *C/s

Sv (a), cm2/cm2

Macroporosity

Sg (b), cm2/cm3

0.1

0.14

1010

1180

10 1000

0.15 0.19

1105 1375

1300 1705

Average particle radius, tam 128 115 99

Notes: (a) Macropore surface area density, cm2 of pore surface per cm2 of char particle; (b) specific macropore surface area, cm2 of pore surface per cm3 of microporous char.

Effective pore diffusion coefficients for char from a Lower Wilcox (Texas) lignite are 2-3x10-9 m2/s in the temperature range 60-227~

[31 ].

The burning profile is the rate of weight change experienced when a coal sample is oxidized at a constant rate of temperature increase. An empirical comparison can be made of a test coal with a coal having known combustion characteristics in a large scale boiler; similar burning profiles would imply similar large scale combustion performance. The burning profile can be interpreted to

512 determine the ignitability, false ignition, and the rate of combustion and char burnout. Burning profiles for a lignite and a high volatile bituminous coal are shown in Figure 11.2, adapted from [32]. The lignite begins oxidation at a lower temperature, suggesting that lignites will ignite and burn out sooner than bituminous coals [32].

9 Lignite

20 18" "~

A High volatile bituminous

16 14

, ta

,..q Z~

12

10"

.~0

8 ~ 0

6"

~

4

2

.

o

"

o

'

'

'

200

~

~

I

'

;

;

I

400 600 800 Furnace temperature, ~

'

'

'

I

1000

1200

Figure 11.2. Comparison of burning profiles for a lignite and a high volatile bituminous coal [32].

For combustion of lignite char particles, the burning rate can be calculated from particle temperature and size if it is assumed that the particle is in thermal equilibrium with the surroundings. The overall burning rate 0 is given by the equation p AH = (2Z. / d) ( T p - Tg) + ~ o (Tp 4 - T w4)

[33] where AH is the heat of reaction, ~. the gas thermal conductivity, d the particle diameter, e the particle emissivity, o the Stefan-Boltzmann constant, and Tp, T g, and Tw are respectively the particle, gas, and wall temperatures. Assuming that the primary reaction occurring at the surface of the lignite char particles is the oxidation of carbon to carbon monoxide, then AH has the value 9600 J/g. The overall burning rate is a linear function of the oxygen partial pressure for Beulah lignite chars in the 63-75 van and 75-90 ~tm size ranges [33]. The particle temperature also increases with increasing oxygen partial pressure. Combustion rates of 69 ~tm Beulah lignite char prepared by rapid pyrolysis in nitrogen or

513 oxygen:nitrogen mixtures are shown in Figure 11.3 [34]. The burning rate is a linear function of the oxygen partial pressure in the range of 5-16 kPa. The burning rate is also a function of particle size; for example, the burning rates for 58 and 82.5 ~tm particles are 0.0193 and 0.0126 g/cm2 s [34]. A measure of char reactivity is provided by the ratio of the overall burning rate to the rate calculated assuming a diffusion limited case. For 69 ~tm chars the value of this ratio, z, ranges from 1.31 at 5 kPa oxygen partial pressure to 0.72 at 20 kPa. Any values of Z above 0.5 indicate that the burning rate is mainly controlled by mass transfer of oxygen to the particle surface, rather than by limitations of chemical kinetics or pore diffusion. When the burning rate becomes so high that oxygen is completely depleted at the particle surface mass transfer control occurs. A situation of complete mass transfer control corresponds to ;~ becoming 1.

0.03 0.025 0.02 I:~

-

~" 0.015 "

~

~9

-

0.01 0.005 ' "

I'

"

I'

"

I'

"

I'

"

I'

"

I'

"

I'

"

I'

"

I'

"

Oxygen partial pressure, Pa Figure 11.3. Dependence of the overall burning rate on oxygen partial pressure for 69 vtm Beulah lignite char at 742"C [34]. The data include chars prepared in N2, 5:95 N2:O2 and 10:90 N2:O2 atmospheres.

In principle it should be impossible to have values of z above 1. Observed values of ;~ greater than this limit may reflect the contributions of carbon dioxide, rather than carbon monoxide, as the primary product of combustion, since formation of carbon dioxide releases over three times as much heat as the formation of carbon monoxide. However, there is little information in the literature on the relative amounts of the carbon oxides formed during lignite combustion. Since the diffusion rate of oxygen in helium is nearly four times faster than in oxygen, combustion of lignite char in oxygen-helium mixtures should reduce the dependence on mass transfer observed in the oxygen-nitrogen mixtures. Chemical and physical control (i.e., z < 0.5)

514 was observed for combustion of 69 0m Beulah lignite char in O2:He at 25 kPa 02 pressure [34]. The ratio of the overall combustion rate to the rate limited by external film diffusion, ;~, shows, for all ;~ > 1, a linear dependence between particle temperature and oxygen concentration [35]. This result indicates film-diffusion control. At high oxygen levels, a smooth transition to kinetically controlled conversion to carbon monoxide occurs. Lignite char shows a vigorous reactivity even at low levels of oxygen. Metal cations, particularly of those which catalyze the carbon-oxygen reaction, increase the reactivity of lignite char in air [36]. The initial rate of weight loss is enhanced during combustion in an entrained flow reactor by alkaline earth cations, in the order Ba+2 > Ca+2 > Mg+2. The increased weight loss may be due to catalysis of the heterogeneous char combustion reaction. Regardless of the cation present, reactivity decreases as weight loss increased. For Texas lignite subjected to various treatments, pyrolysis dominated the early stages of reaction for an acid-washed sample, with ignition and heterogeneous combustion occurring after a 60% weight loss. For a sample backexchanged with calcium, heterogeneous combustion began after a weight loss of 30% Char prepared from acid-washed Beulah lignite showed lower particle temperatures and lower ;~ factors than char from untreated lignite combusted at the same conditions [37]. Char from acid-washed lignite had a very swollen structure. Below 6% oxygen, chars from acid-washed lignite failed to ignite. At higher levels of oxygen concentration such chars did ignite, but ;~ was less than 1 [37]. Similar behavior occurred if the lignite were treated with ammonium acetate prior to char preparation. The char burning rates exceed the theoretical maximum for conversion to carbon monoxide. Inorganic constituents promote conversion to carbon dioxide at low temperatures. The very high reactivity shown by chars from untreated lignite did not occur in the chars from acid-washed lignite. Analysis based on diffusion-limited conversion to carbon monoxide and carbon dioxide fits the combustion data for these chars. The combustion efficiency of Texas lignite char in a drop-tube furnace depends strongly on time and temperature [28]. Using the bulk gas temperatures to determine a first-order Arrhenius equation gives an apparent activation energy of 88.1 kJ/mol and a pre-exponential factor of 57 g/cm2.s.atm. If calculated particle surface temperatures are used to derive the kinetic factors, then the apparent activation energy is found to be 85.2 and the pre-exponential factor 36, in the same units. Thermogravimetric measurements show activation energies of 11-32 kJ/mol in the diffusion-controlled region and 110-140 kJ/mol in the chemical-reaction-controlled region [38]. The transition from chemical to diffusion control occurs around 425~ [38]. The pre-exponential is 7.9x107 mg/mg.s.MPa. The reaction order varies in the range 0.4-1.1, with an average value of 0.8. (The value should be 1 in the diffusion-controlled region.) No evidence of pore-diffusion control could be seen in the Arrhenius plots. In the lower temperature range (in this case, below 407~

quite similar activation energies (28.2 k.l/mol) were observed for Xundian (China) lignite,

also using thermal analysis methods [39]. However, above 407~ the activation energy of 73.2 kJ/mol for the Chinese lignite [39] is lower than this thermal analysis data (averaging 120 k.J/mol)

515 [38], and is more in agreement with drop-tube furnace results (88.1 kJ/mol [28]) and ignition delay measurements (84 k.l/mol [ 13]). Maximum flame temperatures appear to be nearly the same for coals at both ends of the rank range [40]. The coals used to establish this observation include Zap (North Dakota) lignite. 11.1.3 Inorganic transformations during combustion During combustion the inorganic components can experience one of several possible fates: volatilization, followed by condensation on boiler surfaces or on fly ash particles; melting to a liquid which may stick to boiler surfaces; or remaining in the solid state and passing through the boiler as fly ash [9]. For most lignites, all three kinds of behavior are likely to occur for at least some of the inorganic species present. Once the inorganics have been released, in whatever guise, their release is likely to be followed by a sequence of chemical reactions among them, reactions with the flue gas, and reactions with materials already deposited on boiler surfaces [41]. Those inorganic components that vaporize may be important in the formation of a sticky layer to which the solid ash particles can become attached and initiate fouling deposition. Sodium species vaporize from char particles without reacting with inherent minerals [4244], and rapidly enter the vapor phase as the hydroxide or sulfate [45]. The extent to which any sodium would be captured by inherent aluminosilicate minerals depends on the mass transfer of sodium through the char matrix [43]. Sodium hydroxide dominates the sodium species in the gas phase at thermodynamic equilibrium [42]. During combustion of Beulah lignite in a laboratoryscale laminar flame burner, with flame gases are sampled and identified by molecular beam mass spectrometry,the mass peak 40 (NaOH) appears in the gas phase as the peak of mass 23 (Na+) disappears [46]. The NaOH may have formed by reaction of sodium with water vapor in the gas stream. The particulates show a large increase in both sodium and sulfur with increasing height above the burner grid [46], indicating progressive condensation of sodium-sulfur species. Most of the increase in the concentration of these elements in the particulate occurs at heights greater than 2.5 cm above the burner. In this region the concentration of Na+ in the gas phase had already dropped to a low level suggesting that Na+ disappears from the flame by conversion to NaOH rather than by condensation [46]. Particles sampled 3 mm above the burner show the highest concentration of sulfur at the particle surface [46], suggesting that alkali and alkaline earth oxides absorb sulfur oxides from the gas. The sulfur content was roughly equivalent to the calcium content, and a calcium sulfate peak was observed in the X-ray photoelectron spectrum (XPS). Sodium had a role in sulfur capture by forming a surface coating of molten sodium hydroxide on a calcium-rich particle, the sodium hydroxide capturing SO3 from the gas phase and transferring it to the calcium with the formation of CaSO4 [46]. XPS shows 10% (atomic basis) sodium on particles collected 3 mm above the grid [47]. The sodium content of particles collected 7.5 cm above the grid reached 35% [47]. Sodium increases in the first 2.0-3.5 nm below the particle surface but does not increase below that depth [47]. Scanning electron microscopy indicated the presence of sodium sulfate, hypothesized to have been deposited as very fine particles, since no sodium sulfate

516 peak was observed in the gaseous products [47]. Volatile sodium hydroxide reacts with sulfur oxides [44]. The only sodium-containing vapors detected were sodium and sodium hydroxide. Particles sampled 10 cm from the burner (flame temperature 800~

showed sodium concentration

profiles with depth (determined by XPS) comparable to those for particles sampled 3 mm above the burner, where the flame temperature was 1500~C. The sulfur content, however, was about four times larger than the amount required for conversion of the available calcium in the particle to calcium sulfate [46], suggesting that as the particles move away from the burner sodium hydroxide continues to condense on the surface, but becomes increasingly converted to sodium sulfate at the lowered temperatures. Presumably the sodium hydroxide could become converted entirely to sodium sulfate, given enough time. Although some sodium attached to carboxyl groups will be vaporized as Na, Na20, or NaOH [48], the initial form from which the sodium is vaporized may be unimportant, as indicated by tests in which sodium added to lignite as the acetate, hydroxide, chloride, carbonate, or sulfate had essentially the same effect on fouling behavior regardless of the specific compound added [48]. As the temperature drops, these very reactive species probably form Na2CO3 or NazSO4 [48]. The reaction of volatilized sodium with kaolin particles can form sodium aluminosilicates which melt in the range of 900-1100~

These compounds can react with molten sulfates in the

accumulating ash deposit to form complex melilites of the general formula (Na, Ca, K)2[(Mg, Fe§

Fe§

AI, Si)307] [49]. That portion of the sodium not volatized will be retained in the ash,

where it will act to lower the melting point. Volatilization of sodium was studied by treating a subbituminous coal (Decker, Montana) with 24Na§ after first removing the naturally occurring exchangeable sodium. The doped coal was burned in a drop-tube furnace. About 80% of the total activity was found on the constrictor which accelerates the gas stream before it impinges on a substrate used to simulate a boiler wall. No macroscopically observable ash particles had accumulated on the constrictor, suggesting that the sodium, in whatever form it leaves the combustion zone, is very finely dispersed in the gas phase [50]. Partitioning of sodium among gas, liquid, and solid phases is important in establishing the ash deposition behavior. The variation with temperature of sodium-containing species in the gas and liquid states was calculated by free energy minimization [51]. The results for Gascoyne (North Dakota) White pit lignite are shown in Figures 11.4 and 11.5 [51], in general agreement with previous work [52]. The inorganic composition of the lignite affects the results of such predictions. For example, Center (North Dakota) lignite contains more alkali and alkaline earth elements and less silicon that Gascoyne White pit lignite. A comparable thermodynamic calculation for Center lignite predicts high concentrations of sodium sulfate up to 1700 K, while the mole fractions of NaAISi308, NaAISizO6, and Na2Si205 are less than 10-5 [51]. The influence of sodium was modelled in calculations based on Gascoyne Yellow pit lignite, in which the value of Na20 in the ash was increased in increments of 1.5%. The liquid species predicted at 1600 K are

517

9 Na2SO4 0.0001 -~

0O

O

O

9 NaOH

m

0.00001 -..

0.~1

/

-~

0.oooo001 .

0 . ~ 1

'"'

1000

'-

I

'

1200

'

,v

I

'

'

'

1400

I

1600

'

'

'

1800

Temperature, K Figure 11.4. Sodium-containing gaseous species predicted to form during the combustion of Gascoyne lignite at 20% excess air [51 ].

0.1

.~i

0.001

O

N

Na2SO4

9

Na2Si205

A

NaOH

o

Na2SiO3

o.ol

0 r

9

o.oool 0.00001 1000

1200

1400

1600

1800

Temperature, K

Figure 11.5. Distribution of sodium-containing liquid species predicted to form during the combustion of Gascoyne lignite at 20% excess air [51].

518 shown as a function of sodium content in Figure 11.6 [51]. In the solid, gehlenite increases with increasing sodium content and akermanite remains constant. Sodium melilite was not included in the thermodynamic data base used for the calculations, and it is not clear what effect this omission may have had on the predictions.

9 NaOH 9 CaAI2Si208

0.1

A

NaAISiO4

o

NaA1Si308

4- NaA1Si206 o

Na2Si205

O

*

Na2SiO3

O

t

Na2SO4

0.01

O O 'm

0.001

O

0.0001

0.00001 0

2

4

6

8

10

12

Percent sodium oxide in ash

Figure 11.6. Influence of sodium content on the distribution of liquid species predicted to form on combustion of Gascoyne lignite [51].

The fume produced during combustion of lignites is mainly magnesium and calcium oxides [53]. Most of the calcium in lignites is dispersed at the molecular level, because it is associated with carboxylate groups [44]. During combustion, calcium rapidly agglomerates into C a - O clusters and CaO [44]. The CaO may be emitted from the char as a CaO fume, or it may react with

519 mineral species in the char. The organically bound inorganics have significantly higher volatilization rates than the same elements incorporated in discrete mineral phases. By the time the coal particles have completely burned, significant interaction between calcium and the silicate minerals has taken place, even though these species were physically separated in the original coal particle. The mixing of the acidic components, such as the silicates, and the basic components, such as the calcium, lowers the thermodynamic activities of slag components and hence also substantially reduces vapor pressure. Fly ash beads may form at the surface of the lignite char particle as the char is undergoing burnout. The beads may grow individually or coalesce to form larger particles. In turbulent flames some mixing of the fly ash constituents may occur via continual coalescence and redispersal. Spherical calcium-rich ash droplets formed as the carbonaceous material was consumed during combustion of Zap lignite in an entrained flow reactor [54]. This process occurs preferentially at the outer surface of the char. Growth of the ash particles increases with residence time in the reactor. Organically bound calcium reacted at the outer surfaces of the burning char particles to form spherical calcium-rich droplets that grow in size during continued combustion. Removing calcium and magnesium prior to combustion had no effect on the vaporization rate of silica [53]. Volatilization of silica occurs inside the char particle, where a locally reducing atmosphere effects formation of volatile silicon monoxide. Little agglomeration of ash occurs inside the char particle, because the chemical interaction of the acidic and basic components of the ash is brought about by the external surface of the char receding during combustion. Organically bound calcium reacts extensively with quartz and clay minerals to give both amorphous and crystalline calcium silicates and aluminosilicates [55]. This reaction occurs quickly within char particles [45]. The products of reaction have lowered melting points; hence their formation may favor enhanced deposit formation. In Beulah lignite the CaO fume reacts with clays to form a calcium aluminosilicate glass [56]. San Miguel (Texas) lignite provides a slightly different case, because this lignite contains zeolites. In this lignite the resulting calcium aluminosilicates are richer in silicon and aluminum than in, e.g., the Beulah lignite case, because a dominant reaction is the decomposition of the zeolite and subsequent reaction with calcium [56]. Calcium sulfate forms as a result of reaction between calcium oxide fume and sulfur oxides [56]. Sodium species also experience sulfation in the vapor phase to form sodium sulfate. Rapid cooling of the vapor results in formation of a calcium-sodium-sulfate solid solution [56]. Bench-scale studies of volatilization of Red and Blue pits Gascoyne lignite [57], heated at 5~

from ambient to 1000~ and 2~

from 1000 to 1500~

showed deposits formed on a

water-cooled disk 7.5 cm above the crucible to be distributed in discrete clusters having different structures. With Red pit lignite, the deposits included mixed sodium, potassium, and calcium sulfates; sodium and potassium aluminosilicates; and vesicular spheres predominantly of silica with small amounts of aluminum oxide. Deposits from the Blue pit lignite included calcium sulfate, calcium aluminosilicate and calcium sulfate, and mixed sodium, calcium, and potassium silicates and sulfates.

520 Clay minerals are converted to alkali and alkaline earth silicates in the temperature range 1000-1300~

[58], during which process a small portion of the alkalis are vaporized. The

vaporization process might be retarded by using heavy metal additives, with a reduction in vaporization of alkali translating to a reduction in deposit formation. Aluminum may transport as the reduced species AIO or AIS [58]. The A1203/SIO2 ratio in lignite fly ash particles clusters around 0.8 [59]. This ratio varies from 0.67 to 1.00 for the clay minerals in Beulah lignite. The ratio in fly ash being about in the middle of the range for the clay minerals suggests that the aluminosilicate framework of the minerals in the lignite remains intact through combustion. The range of composition of the common clays in Beulah lignite is 25--45% A1203 and 35-50% SiO 2. The composition of most fly ash particles is below these values, suggesting that the aluminosilicates in the lignite either combined with other elements or became coated. Fly ash analyses show negative correlations of SiO2 or A1203

vs.

SO3 [59]. Expressing SO3

vs.

SiO2 graphically shows that most data tend to

cluster around a tie line corresponding to the connection of clay ( i . e . , 0% SO3) with sodium sulfate (0% SiO2) [59]. This result suggests that the clay particles become coated with sodium sulfate. Xray photoelectron spectroscopy shows the sulfates of Na, Ca, and Ba; sputter etching confirms that the sulfur is concentrated on the surface of the particles. Sodium and sulfur contents increase as fly ash particle size decreases, also consistent with the formation of sodium sulfate coatings, presumably by vaporization-condensation mechanisms. Pyrite fragmentation produces iron-containing particles with diameters about 1.8 times smaller than in the original lignite [60]. In those lignites for which pyrite constitutes a major portion of the extraneous mineral matter, the pyrite fragmentation can dominate the processes establishing the particle size of the ash, resulting an ash particle size distribution finer than that of the mineral matter originally in the lignite [60,61]. Rapid fragmentation of large extraneous pyrite particles forms smaller iron oxide particles [62]. Finely disseminated pyrite particles may not share this fate. Fine pyrite particles undergo heating in locally reducing conditions. Pyrite decomposes to FeS and forms a partial melt phase [60]. FeS may react with other inorganics or decompose to iron. The reduced, molten iron-containing phases are highly mobile and readily react with small clay particles within the char. Fuel particle size does not solely determine the particle size distributions of ash [63]. Char and large mineral particles undergo shedding and fragmentation during combustion [64]. Char particles may fragment before spheres of molten or partially molten ash have a chance to coalesce. The char particle surface recedes during combustion, contributing to the release of molten inorganic particles prior to their coalescence [62]. Fragmentation of individual large mineral particles produces several smaller ash particles. Thermal shock causes fragmentation of large quartz particles when they enter the hot combustion environment [62]. These processes together result in smaller ash particle sizes. The ds0 (the volumetric diameter below which 50% of the volume of the particles lie) of mineral matter in Beulah pulverized lignite is reduces by 62% during pilot-scale combustion [62].

521 11.2 S M A L L - S C A L E

C O M B U S T I O N SYSTEMS

11.2.1 Historical development Use of lignite for domestic heating began in about 1855, in trading posts and forts built in the Great Plains [65]. Commercial utilization for power generation began about 1925. Use of lignite continued to increase thereafter, but, because of the high moisture content and low calorific value, there has been no substantial use of lignite outside the area in which it is mined. Despite these seeming disadvantages, lignite is a satisfactory fuel for power generation, provided that it is fired in equipment designed for its use. The lower calorific value compared to bituminous coals and anthracites makes it necessary to burn a greater amount of lignite to raise comparable amounts of steam. Over the years lignite has been burned in a variety of equipment, including spreader stokers, travelling grate stokers, underfeed stokers, cyclone burners, and pulverized coal burners. The application of traveling grate stokers was the first major step forward in development of combustion equipment for what, at the time, was large-scale lignite firing. The travelling grate stokers were made obsolete by the development of spreader stokers. In recent years pulverized coal firing has become the standard for what are now considered large scale plants, that is, on the order of 400 MW. Early (i.e., in the 1920's) production of electric power from lignite in North Dakota relied on stoker-fired furnaces in small plants near the larger towns. Combustion of lignite on a grate led to high fuel losses in the ash, caused by disintegration of lignite lumps during combustion and the resulting smaller particles of lignite dropping through the grate [66]. This suggested that the most economical approach to large scale lignite combustion would be suspension firing. For suspension firing, pre-drying the lignite to about 10-15% moisture was found necessary to maintain full mill capacity, although it was also found that grinding to 70% -100 mesh, 40% -200 mesh would produce as good combustion with lignite as the finer grinding of bituminous coal [66]. The engineer responsible for most of this work, G. B. Wharen, was remarkably prescient: "Undoubtedly, only a few years ahead, perhaps as soon as the year 1950, there will be established a vast network of electric distribution systems running out from central generating stations, located at the base of lignite and water supplies, and furnishing to the people of the state the means for engaging in industrial enterprises, and making available to all the comforts and happiness of modem civilization" [66]. Actually, the first pulverized-lignite-fired boilers were installed in the lignite fields of western North Dakota in 1958-1960, and today the area along the Missouri River is sometimes facetiously referred to as "tx~wer plant alley." 11.2.2 Domestic use of lignite In what is surely one of the more remarkable documents in the lignite literature, the potential use of lignite in domestic stoves was evaluated by the actual preparation of meals for one week using a kitchen range with draft modified to provide temperatures of up to 315~ with lignite

522 [67]. The range was kept in continuous use for seven days, being banked overnight. The authors concluded that it is entirely possible to use lignite for cooking, although it was felt that, in addition to the draft, the oven insulation would need to be modified as well. The lignite was found to be cleaner to handle and to burn more completely than "soft coal" (presumably bituminous coal). Cooking time for a variety of dishes was no greater than that using a bituminous coal heated range. In the late 1930's a survey of the fuel situation in North Dakota indicated a consensus that domestic burning of lignite "was not a very pleasant operation" [68]. The primary difficulty was that most of the commercially available combustion equipment for domestic use had been developed for burning bituminous coals. The equipment could not adequately cope with slacking of lignite on the grate, and consequent loss of unburned lignite through the grate, nor with the requirement for different amounts of secondary air when burning lignite instead of bituminous coals. An extensive investigation of domestic scale heating equipment for lignite firing compared the performance of a Booker magazine-type space heater, Worsham furnace, and Harrington ramtype stoker [69]. Although this work is now about forty years old, it is unlikely that there have been substantial technical developments in the interim, due to the very small market for home heating uses of lignite. In the Booker heater, burnoff rotes of 200% of the rated capacity could be maintained for at least several hours with no evident damage to the unit. Efficiencies approached 75%. The Worsham unit provided poor control of air admission, resulting in excess air levels sometimes exceeding 350% with consequent low efficiency. The wide spacing of grate bars resulted in substantial fuel loss with the ash. The Harrington stoker provided efficiencies up to 70% during continuous operation, but efficiencies were necessarily lower during intermittent or holdfire operation. Useful design details are provided in the original literature [69]. In the past thirty years very little effort has been devoted to the improvement of small stoker furnaces or to the development of new designs [70]. 11.2.3 Stoker firing Lignite burns well in mass-burning stokers, despite its high moisture content. The maximum particle size for lignite should be 32 mm [48]. Normally any fines produced in the crushing process should be left in the fuel. However, when the moisture content of the lignite is in the range of 36--40% the particle size should the controlled at 19x32 mm [48]. North Dakota lignite is difficult to ignite on grates [71]. The lignite breaks into small flakes, which pack densely on the grate, thus requiring a high draft to force air through the bed. This behavior also contributes to the problem of the fire tending to burn through the bed in spots. The cracking of the lignite is due to the sequential loss of moisture and volatiles from the surface layers as the particle is heated. As the heat penetrates further into the particle, the cracks become more extensive until eventually thc cracks are of sufficient size to allow spalling of small pieces. The original large particle can be reduced to small chips in this way. Cracking and crumbling is probably impossible to prevent, but may be mitigated somewhat by using firing methods that cause

523 little physical or mechanical disturbance to the lignite on the grate. Partially burned flakes of lignite produced by thermally induced decrepitation may sift through the grate, contributing to high losses of combustible fuel with the ash. The cracking and crumbling is in contrast to bituminous coals fired under comparable conditions, since the bituminous coals would likely fuse when heated. Heating a 35% moisture lignite to its ignition temperature requires more than twice the amount of heat needed for a 10% moisture bituminous coal [71]. It is almost impossible to supply all of this heat from a firebrick ignition arch, since the radiative heat transfer rate is a function of the temperature (i.e., the arch would need to be at a much hotter temperature for the lignite than for the bituminous coal). Unfortunately in a lignite furnace it is not possible to get the arch as hot as (let alone much hotter than) a bituminous-coal-fired furnace. Thus for good ignition in a lignite furnace, it is also important to take advantage of convective heat transfer from the hot gases. Lignite char reacts readily with both oxygen and carbon dioxide. The first 7.5 cm of the lignite bed above the grate are a region in which the lignite burns to carbon dioxide. Oxygen is entirely consumed within 3.5-5 cm of the grate [72]. Above 7.5 cm, the carbon dioxide reacts with carbon to carbon monoxide. A consequence is that the surface of the lignite bed is nearly black, and this condition contributes to the difficult ignition. The relationship between rate of combustion and pressure drop through the bed is shown in Figure 11.7 [71]. The curve for the 10 cm bed of char shows that this bed offers much higher resistance to air flow than a bed of lignite three times as deep. The high pressure drop required to attain a given rate of combustion in the char bed is due to a high concentration of fines in the char bed, the fines reducing the bed permeability [71]. Combustion of a 15 cm bed of Leigh (North Dakota) lignite at various rates shows rapid disappearance of oxygen, followed by nearly complete reduction of the carbon dioxide to carbon monoxide [71]. Data for 15 and 290 kg/mZ.h are shown in Figure 11.8 [71]. Volatiles emission occurs uniformly throughout the firing period. The combustible volatiles are primarily light gases that burn easily without production of smoke. Comparable experiments using char from this lignite show essentially complete disappearance of the oxygen about 4 cm above the grate. At the surface of the bed no free oxygen or carbon dioxide could be detected. The rapid reaction of both oxygen and carbon dioxide may be due to the favorable gas/lignite contact, caused by the development of numerous fine passages for air through the bed, the passages themselves being the result of the disintegration of the lignite particles as the lignite gets to within 5-7.5 cm of the grate. The NO emission for stoker firing lignite is given by the equation NO = -70 + 350 (fuel %N) - 109 (fractional char N retention) where the fractional char nitrogen retention is the percentage of nitrogen originally in the coal retained by the char after isothermal pyrolysis in argon at 1097~

[73]. This equation was

developed for coals ranging in rank from lignite to high volatile A bituminous. For seven coals the coefficient of determination (r2) was 0.93. The specific performance of a North Dakota lignite

524

40 r %

35

Total combustibles (a)

9 Carbondioxide (a)

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o- .o _<;' ~ f

o

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* Oxygen (a)

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O Total combustibles (b) A Carbondioxide (b)

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Distance from grate, cm

Figure 11.7. Composition of gases in a 15 cm fuel bed of natural lignite at combustion rates of 14.6 (case a, solid line) and 293 (case b, dashed line) kg/m2, h [71 ].

(1.1% daf; 27.7% residual char nitrogen) fired with a bed region stoichiometric ratio of 1.44 was 173 ng NOz/J [73].

At combustion rates higher than 49 kg/m2 h (using Leigh lignite) the ash showed a tendency to melt to a solid and virtually impervious clinker which covered the grate but did not adhere to it [71]. Clinker formation is a problem Ibr lignite combustion on grates. The clinker is difficult to remove without disrupting the fuel bed and possibly accidentally dislodging large portions of the fuel bed into the ash pit. It is particularly exacerbated by the tendency of the lignite to decrepitate in the fuel bed, because the loss of bed permeability due to the generation of fines requires high drafts to sustain even moderate rates of combustion. Thus combustion is intense in the region where the air enters the grate, and the likelihood of the ash melting is increased. The foregoing can be summed up by listing the negative features of burning lignite on grates as difficult ignition; disintegration of the lignite particles in the fuel bed, which may lead to increased resistance to air flow and losses of fuel with the ash; and clinkering (also possibly indirectly related to the disintegration of the fuel particles). These considerations suggest that a

525 1.2 "

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,~

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Residue, 15 cm bed

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Rate of combustion, kg/m2.h Figure 11.8. Relationship between the rate of combustion and pressure drop through the fuel bed for lignite and lignite combustion residues [71].

small furnace for successfully firing lignite should provide for rapid ignition, an ample air supply to give reasonably high rates of combustion, a grate designed to prevent sifting of partially burned fuel particles into the ash and at the same time designed for removal of clinkers with minimal disturbance to the fuel bed. A furnace designed to accommodate these provisions features an inclined step grate with an arch extending nearly the entire length of the grate [71]. The arch deflects the flames and hot gas back over the incoming fuel, to facilitate ignition. Air admitted at the bridgewall is also defected by the arch over the fuel bed, thereby increasing the rate of combustion. For horizontal grates a firing rate of 195 kg/m2 h is recommended [71]. The spreader stoker throws coal into the furnace. The fines dry and burn while still in suspension, even before reaching the grate. Larger particles will dry partially in suspension and then burn completely on the grate. Because of its high moisture content, lignite will shatter when suddenly exposed to high temperatures which evaporate almost all of the moisture at once. (This behavior is colloquially referred to as the "popcorn effect.") The consequence is that the percentage of lignite which will burn in suspension is higher than that of bituminous coals. However, a higher percentage of fly ash will be generated fi-om the lignite as well. Furthermore, the greater volume of gases generated as a result of the evaporation of the moisture in the lignite means that less superheater surface per weight of fuel burned is needed for lignite. Operation is very sensitive to the size distribution in the feed, but a given spreader stoker will burn almost any kind of coal provided that the appropriate size distribution is used. This makes the spreader stoker an attractive

526 provided that the appropriate size distribution is used. This makes the spreader stoker an attractive option for small installations that might be using coal from several sources. Some of the fine lignite particles may bum in suspension above the bed. The bed itself is thin and relatively fast burning, which means that any variations in load can be accommodated fairly easily by appropriate adjustment of the feed rate. The most efficient operation involves firing a carefully sized mixture of coarse and fine lignite particles. Carbon losses in stoker firing amount to 4-8% [9]. The capacity of a boiler fired by a spreader stoker is limited by the distance over which the spreader can maintain a good distribution of the fuel. This distance is about 5.8 m [65] and thus sets a design upper limit for economic boiler operation of 31 kg/s steam [74]. In practice the largest spreader stoker unit firing North Dakota lignite was 20 kg/s steam [65]. Spreader stokers using stationary or dumping grates have been used successfully in units up to 31 kg/s steam [75]. The heat release rates per grate area or per furnace volume are slightly lower than for high-rank coals. Emissions of various organic compounds for lignite combustion in a spreader stoker are formaldehyde, 3.1 mg/MJ; fluorene, 14.2 ~tg/MJ; fluoranthene, 47.3 l~g/MJ; pyrene, 14.2 lxg/MJ; benzofluorene, 21.1 lxg/MJ; and benzo[a]pyrene, 28.0 ~tg/MJ [7]. A distinction can be made between chain grate stokers and travelling grate stokers in some features of their mechanical design, but both operate on the same principles. Since the fuel bed is not agitated in these stokers they are better suited for the combustion of lignite than is an underfeed stoker. In the quarter-century from the early 1920's to the end of World War II, travelling grate stokers were installed in a number of lignite-fired power plants in the United States and Canada. The evolution of the design of the travelling grate stoker has led to the following specifications or features for optimum performance: air preheated to 2000C; a water-cooled rear arch to force hot combustion gases across the top of the bed; and overfire air jets [65]. High-moisture North Dakota lignite has been burned successfully on forced draft travelling grate stokes which have nonsifting grates and preheated air [75]. Travelling grate stokers have been used for firing boilers as large as 12.5 kg/s steam [65]. The underfeed stoker has never been popular or widely used for firing lignite. In this design coal is introduced beneath the fuel bed. In principle, a thick bed of coal is maintained so that volatiles are evolved from the green coal and burn off as the coal or char moves upward through the hot bed. Lignite, however, does not agglomerate and tends to burn in a thin layer of independent particles. The air draft, supplied through tuyeres in the side of the unit, or mechanical agitators, used to keep a thick bed of bituminous coal moving, are likely to cause the lignite to accumulate in drifts with the consequence that burning is uneven across the area of the stoker. Vibrating-grate stokers operate with water-cooled walls to prevent slag formation adjacent to the feed point. A rear arch, extending over about a third of the furnace length, serves to direct hot combustion gases forward to mix with volatiles being released from the fuel. Turbulent mixing and combustion are also promoted by high pressure air jets in the front arch. The maximum heat release rate is recommended to be 4500 MJ/m2 h to keep carbon carryover to a minimum [48].

527 11.3 C U R R E N T

COMBUSTION PRACTICE

This section discusses the current practice for firing lignite in large boilers to raise steam for electric power generation. This application is the major use of North American lignites, and will likely remain so for the next several decades. There seems little doubt that the principal use of lignite at least through the year 2010 will be for commercial electric power generation [76]. The operating efficiency of a pulverized-coal unit firing lignite is about 2% higher than a comparable installation using a spreader stoker and maintaining the same exit gas temperature [77]. Pulverized-coal-fired units also offer a quick response to load changes and the lack of moving parts in high-temperature locations which would normally be inaccessible for maintenance. As a rule of thumb, the current commercial plant conve~ts about a third of the energy in the coal into electricity [78]. The physical limitations of stokers, particularly the burning rate per grate area, restricted spreader stoker units to approximately 25 kg/s steam maximum capacity [79]. At larger sizes, to 640 kg/s steam (750 MW electrical equivalent), pulverized-coal firing is preeminent [80]. The design change from spreader stoker to pulverized-coal units resulted in a great increase in the amount of ash entrained in the flue gas, from about 25% in a stoker unit to 83% in a pulverizedcoal unit [80]. This change also resulted in the deposition of ash on the steam tubes, a problem that remains the most serious technical problem in current lignite utilization practice. This aspect of ash behavior, slagging and fouling on heat exchange surfaces, is discussed separately in Section 11.4. 11.3.1 Historical development of pulverized firing of lignite The first pulverized-coal-fired (pc) units burning lignite in the United States were installed at the Trinidad plant of Texas Power and Light in 1926 [81]. The plant used four boilers each rated at 23 kg/s steam. Five years later, two much larger boilers were added to the plant; these were rated at 41 kg/s steam. It was not until 1945 that a pulverized-coal-fired unit burning North Dakota lignite was installed, at the Crookston, Minnesota plant of Otter Tail Power Company [82]. This unit had a capacity of 9 kg/s steam at 2.9 MPa and 440~ [65]. Use of lignite as a boiler fuel became common in the northern Great Plains in the late 1930's [78]. The boilers typically were of 10-20 MW equivalent and were used for industrial and institutional heating, utility steam, and electricity generation. Although fouling of the boiler tubes was a problem, the units were cleaned frequently by hand, so ash deposition did not appear unmanageable. The first three pulverized-lignite-fired boilers, each of about 50 MW capacity, were installed in 1958, 1959, and 1960 [78]. Two 200 MW capacity pc-fired units were installed in 1966, and the first cyclone-fired unit (235 MW) in 1970 [78]. The growth of the electric power industry in the upper midwest was very rapid in the 1960's and 1970's. Prior to 1960, the total installed capacity was 237 MW, including 10 stoker units which had an average capacity of only 12 MW per unit. By 1970, the capacity had increased to 788 MW, and by 1980, to 3,031 MW. In the years from 1980 to 1983 an additional 1,840 MW were installed [78]. However, the increase has been even more dramatic in Texas. Texas pioneered the use of lignite, the 40 MW plant at

528 Trinidad being the first major pulverized-lignite-fired power plant in the world. This plant operated during the 1920's and 1930's but was closed by 1940 because it could not compete with cheap natural gas [78]. Only one lignite-fired unit existed in Texas as recently as 1970 [78], and in 1975 90% of the electricity generated in Texas was still produced in natural gas fired boilers. In the years 1970--1984 fifteen new lignite-fired boilers with a total capacity of 9,685 MW were installed, together with ten subbituminous coal fired units adding another 5,030 MW [78]. 11.3.2 Evaluation of lignites as potential fuels for electric power plants Useful procedures for evaluating a lignite deposit have been published [83,84]. The discussion in this subsection is based on these two sources, unless noted otherwise in the text. The first activity is a compilation of existing data, including geological data, results of any previous drilling, and information on coal quality in nearby mines. The first active investigation, a "Phase I" drilling program, provides cores for determination of equilibrium and total moisture, ultimate analysis (including chlorine), sulfur forms, calorific value, and water-soluble sodium. The Phase I drilling density is one to two holes per km 2, with complete cores taken on 1.6 km centers. Portions of the samples are saved for grindability testing. If the ratio of silica to alumina is high, the relative quartz value will also be determined. Samples of the roof and floor material are analyzed for ash, sulfur, ash composition, water-soluble sodium, and, if these samples are carbonaceous, equilibrium moisture and calorific value. If the sulfur content is low and appears to remain reasonably uniform as data continues to be accumulated in Phase I, the sulfur forms analysis can be discontinued. Composite samples accumulated during Phase I are also subjected to proximate analysis, ash fusion, and grindability testing (at several levels of moisture content). Data accumulated in Phase I testing allow calculation allows the calculation of ranges and weighted averages for each seam. Evaluation of the data may then suggest the desirability of further testing, which could include the grindability as a function of ash content, the grindability of the impurities

(e.g., seam partings), size distribution on crushing, washability of the various size fractions, drying, pilot-scale tests of fouling or slagging and ash sintering strengths, and large-scale tests of coal feeding, grinding, and abrasion. If the results of Phase I are favorable, a Phase II drilling program is initiated, with a density of four to eight holes per km2, and complete cores on 1 km centers. Phase II provides data that allow determination of the desirability of opening a test pit to obtain bulk samples of lignite. A test pit can provide information on the nature of the overburden, stability of the highwall, ground water analysis, and prospects for reclamation. Alternatively, bulk samples could be obtained by largediameter auger drilling or driving an adit. Analyses of composite samples from the Phase II program allows determination of the ranges and weighted averages for each seam, as well as the overall field-weighted averages. These data can then be adjusted to a diluted basis, which includes consideration of loss of lignite in the pit and dilution of the lignite by in-seam partings and floor material. Area composite samples are analyzed for trace elements; these data being useful to assess potential trace element emissions.

529 Low-rank coals are sold on the basis of coal quality expressed on an as-received basis. The as-received values of coal parameters obviously depend on the moisture content. The high moisture content of lignites, and the dependence of the calculated values of other lignite properties on moisture, make the determination of moisture crucial. Air drying can be performed on 50x5 mm material. After air drying, the sample is crushed to --60 mesh (<0.25 mm) to determine the residual moisture. The as-received moisture is then the sum of the moisture lost on air drying and the residual moisture. The moisture content affects boiler efficiency in that the higher the moisture content, the lower the boiler efficiency. A decrease of 2% in boiler efficiency can increase power costs by 1% over the life of the boiler [84]. Flue gas volume will also increase with increasing moisture content. As a rough rule of thumb, combustion of North Dakota lignite or Jackson (Texas) lignite will increase the flue gas volume by about 25% compared to bituminous coal; Wilcox lignite will increase the volume by about 15% [84]. The design, and thus the costs, of ducting, air heaters, and particulate and SOx control systems are related to volumetric gas flow rates. The high moisture content of lignites requires a high primary air temperature in the mills for pulverization. Depending on the source of the high temperature air, the throughput of the pulverizer might be limited by this requirement. High surface moisture on lignite can impede the flow in conveyors, bins, and feeders, and, in severe weather, can cause freezing of the lignite in handling and storage systems. Volatile matter and fixed carbon contents are useful only for classification purposes. Carbonate minerals in lignite may contribute to erroneously high volatile matter determinations. Another possible source of error in the volatile matter determination is the tendency of high moisture content lignites to "pop" or "spark" during the analysis, ejecting some material from the sample crucible and again contributing to an erroneously high value. The problems associated with the determination of volatile matter may cause errors of up to 20% in combustor design calculations relating volatile matter to theoretical air. Carbon, hydrogen, sulfur, and oxygen data obtained from the ultimate analysis are used for determination of theoretical air for combustion. Oxygen also provides an estimate of the degree of oxidation of the lignite, whether in-seam oxidation or oxidation that has occurred after mining. The sulfur content is also important in determining SOx formation. Similarly, the nitrogen content is an indicator of NOx. The chlorine content may signal potential problems from wall slagging, fouling of the convection tubes, and gas-side corrosion of boiler internals. The sulfur forms analysis is particularly important for determining the pyritic sulfur. The amount of pyrite is important in washability or other beneficiation studies. The pyrite content can also indicate problems to be expected in pulverizer wear or in furnace slagging. The Hardgrove grindability index is used to determine mill capacities, the power requirements for milling, and the primary air fan requirements. The Hardgrove grindability index should be determined as functions of moisture content and of ash value. The dependence of grindability on moisture content means that to obtain adequate design data the grindability should be tested at three moisture levels at least, from as-received moisture to a nearly dry condition. Until

530 the dependence of physical properties on moisture content is much better understood than at present, there will be no substitute for actual test data. The situation is further complicated by the complex behavior of lignites as a function of moisture content, some becoming harder, others becoming more friable, and still others going through an intermediate maximum or minimum as moisture content decreases. The size of mills is also affected by the low grindability of lignite. At the Leland Olds station, slagging problems were attributed to the low grindability of the lignite

[85]. The ash value will determine the quantities of ash to be handled and have effects on precipitator costs and on the costs of ash disposal [84]. The ash value is also used to determine relationships with the Hardgrove grindability index, fouling and slagging, and slag viscosity [83]. Lignites which contain large quantities of carbonate minerals may give erroneously low values of the ash value, because of the decomposition of carbonates during ashing. Carbonate decomposition would lead to a loss of weight, causing the ash value to be low and the volatile matter to be high. Ash fusibility data must be used with caution because of a number of potential sources of error. Basic ashes from lignite could react with the ash cone support, which is predominantly composed of silica and alumina. Gas evolution and swelling could occur during heating. These factors could affect the apparent height of the ash cone, and, since the softening and hemispherical temperatures are determined on the basis of height-to-base ratios, could thereby affect the apparent values of these fusion temperatures. Ash composition--especially the sodium content--is of crucial importance in determining the fouling and slagging behavior. The high sodium content of many lignites is notorious for its role in ash deposition. In severe cases of deposition, derating the boiler seems to be the only effective cure. With increasing fouling potential, the boiler furnace size must be increased to control the flue gas temperature at the furnace exit below 1040-1090~ to minimize superheater fouling. High-sodium lignites will produce a fraction of the fine particulate rich in sodium sulfate; this problem can be a concern in design of particulate collection systems. Since most salts of sodium are water-soluble, leaching of sodium from disposed ash by groundwater or meteoric water can be a concern in waste disposal. A high sodium content is undesirable if the ash is to be used as an additive in cement manufacture [48,86]. On the other hand, high sodium content in the ash reduces the ash resistivity, making it easier to collect in electrostatic precipitators. The general effect of an ash with relatively high concentrations of alkaline components (calcium, magnesium, sodium, and potassium) is that a partial removal of SO2 from the flue gas is effected by retention on the fly ash. The sodium content of the ash seems to have the greatest effect on sulfur retention. Ash composition gives some indication of problems that might be expected in grinding. A high ratio of silica to alumina indicates the probable presence of quartz, which can cause excessive wear in the mills, coal-air pipes, and burners. The lignites which are high in silica (a characteristic more commonly found in Gulf Coast lignites rather than Northern Great Plains lignites) can cause erosion of coal feeding systems and burners. High-silica ash can cause rapid deterioration of fabric filters in baghouses. A high silica content in the ash, when combined with high sodium levels, can

531 result in severe ash deposition problems. Unusually high (e.g., about 4%) contents of titania can indicate the presence of rutile or brookite, which are very abrasive minerals. The higher heating value dictates the tonnes/hour or tonnes/year requirements for the boiler, once the boiler efficiency and the unit size have been established. These lignite requirements then determine the size and cost of the lignite handling system, and the transportation costs. The low calorific value of lignites increases the amount of fuel to be pulverized and burned to achieve a given steam or electric power production. Compared to a power station burning bituminous coals, a lignite-fired plant requires more and larger mills and a larger furnace to achieve the same energy output. Consequently the auxiliary equipment such as for ash handling and disposal must also be larger in a lignite-fired plant. Lignites contain relatively high concentrations of oxygen, much in thermally labile functional groups such as carboxyl. During initial heating in the very early stage of combustion, thermolysis of these labile groups can create reactive sites in the char for subsequent combustion [48]. The greater reactivity of lignite chars means that it is not necessary to achieve the same degree of fineness to obtain complete combustion as with bituminous coals. Thus the nominal size for pulverized-lignite firing is less than 2% >300 ~Lm(>50 mesh) and 65-70% <74 ~tm [87]. 11.3.3 Commercial firing for power generation The practical upper limit for stoker firing is 12.5 kg/s steam [88]. Cyclone furnaces are designed to operate at temperatures above the ash fusion temperature. Molten ash forms a liquid slag on the walls of the furnace; at steady state the outer layer of slag is sufficiently fluid to flow to the slag tap, where it drains continuously. In principle up to 70% of the ash is removed as slag, while the remainder leaves the furnace as entrained ash particles. The very high operating temperatures lead to high NOx production, and current utility preference is for pulverized-lignite firing in dry-bottom furnaces. The very high temperatures and great turbulence in the cyclone should promote complete combustion before furnace gases enter the convection section. Cyclones can fire any coals, including lignites, which have slag viscosities below 25 Pa.s at 1425~

except

for high-iron (or high-pyrite) ashes [48]. The advantages perceived for cyclones are a reduction of the fly ash in the flue gas, reduction in furnace size, and cost savings in fuel preparation (since crushing, rather than fine grinding, is required). Larger installations tend to rely on pulverized-coal (pc) firing, although a 82 kg/s (steam) spreader stoker is used at the power station of Montana-Dakota Utilities in Bismarck, North Dakota [88]. Surveys of commercial power plants firing lignite have been published [48,89,90]. The plants currently operating or in an advanced stage of construction genera;tiy fall into three size categories. The predominant plant size is in the range of 400 to 450 MW. Piants of this capacity typically produce 390 to 445 kg/s steam (i.e., over 375 kg/s) generated at pressures of 16.5-18 MPa and 540~

A second general category is the 500-600 MW range. Steam generation is

necessarily higher, 470-520 kg/s at 5400C. A few plants operate at hig~er-0.han-typica] pressures, 26 MPa. A third category is plants in the capacity range of 750 MW, which may generate in excess

532 of 640 kg/s steam. These plants are summarized in Table 11.2, using data abstracted from [68]. Some specific design considerations and operation features of selected plants are discussed below. TABLE 11.2 Ranges of design parameters for lignite-fired electric power station boilers [89]. Parameter

Capacity, net MW Steam load, kg / s * Steam pressure, MPa Steam temperature, *C Average calorific value of lignite, kJ / kg Average sulfur content of lignite, % Predominant particulate removal process Predominant flue gas desulfurization process

Range of values 400-750 380-720 16.5-26.5 538-543 10,40(O16,200 0.3-1.67 Electrostatic precipitators Lime or limestone scrubbers

*Increases with net generating capacity

The first cyclone-fired unit to be installed in North Dakota was the at the Milton R. Young Station (Center, North Dakota) of the Minnkota Power Cooperative. The unit had a 212 MW boiler firing low-sodium lignite. Fouling was insignificant, and the unit availability was very high. The experience at the Milton R. Young Station led to the installation of three additional 450 MW units. However, two of those plants have experienced severe fouling. The fouling problem encountered with cyclones is attributed to the fact that the fly ash passing through the boiler is enriched in sodium, even though the total ash loading is lower in a cyclone than in a pc-fired unit [88]. The beneficial effects that would be expected from the reduced ash loading are negated by the increased sodium level in the ash. Cyclone firing also produces higher NOx emissions than pc-firing [88]. The U.S. Environmental Protection Agency established an emission level of 0.8 lb NOx per million Btu (344 ng/J) for cyclone boilers firing North Dakota lignite as part of the New Source Performance Standard [88]. Other boilers are restricted to 258 ng/J NOx. A cyclone-fired boiler has a higher horsepower requirement than a comparable pulverized-fired unit. However, this requirement is compensated by a greater net plant capacity achieved through auxiliary turbine drives on forced draft fans and boiler feed pumps. Furthermore, a pulverized-coal-fired unit would require a larger building. The burning profile test shows that lignites are easy to ignite and burn, despite the high moisture content. Because of their low calorific value, greater amounts of lignites have to be burned to obtain the same fixed heat input to a boiler as would be obtained from firing bituminous coals. Because of the high moisture content, the furnace temperature should be maintained as high as possible and the lignite should be retained in the combustion zone longer than would bituminous coal to insure complete combustion. Although lignites have good ignition characteristics, other properties make it necessary to give careful consideration to the design of boilers for firing lignite.

533 These properties include the higher moisture content, lower heating values, and poorer pulverization characteristics of lignites compared to bituminous coals. In addition, the ash value of lignites may be variable within a lignite seam, and the ash may be quite troublesome in regard to fouling behavior. The principal disadvantage of pulverized-coal firing with lignite is the high auxiliary power required for the pulverizers. Moist ligniteparticles tend to agglomerate during grinding. To prevent this, primary air preheated to 300--425~

is introduced to the mills. This

practice reduces the moisture content of the lignite by about one-third [65]. The large size of boilers for lignite firing (i.e., compared to bituminous-coal-fired boilers) derives mainly from two factors: the lower heating value of lignite, and the potential for slagging and fouling. One strategy for decreasing the chance of fouling is to keep flue gas temperatures at the inlet to the secondary superheater fairly low. The maximum design temperature is 1283 ~

and

the temperature at the San Miguel plant is held to 1010~ [91]. The temperature limit therefore increases the size of the boiler needed to maintain good burner efficiency and fuel/air ratio. Further steps to decrease the possibility of fouling are to increase the tube spacing in the secondary superheater and reheater and to increase the number of wall and sc~t blowers. Because the slag or ash deposits insulate the wall or tube surfaces which they cover, the heat transfer patterns in the boiler are affected by extensive deposition. A consequence of this shift in heat transfer is inadvertent overheating of other surfaces in the boiler. Prolonged overheating can result in a general deterioration of boiler components over time, as well as catastrophic failures by warpage and cracking [92]. Removal of deposits can be effective in preventing physical damage to the boiler. Because of the relatively low cost of lignite in Texas, the lignite burning power plants are operated as base-load plants at the maximum possible load. Reliable operation at 90-110% of design load has been reported, but slagging or fouling can decrease the maximum capacity by 10% [91]. Adequate cleaning of furnace walls and superheater screen tubes is very important for firing lignites with high or severe slagging potentials. If a slag coating forms on the furnace walls, heat transfer through the walls will be reduced and gas temperatures will therefore increase. If the gas temperatures in the superheater region increase, fouling becomes more severe. Thus in a sense the way to combat fouling in the superheaters is to control slagging in the furnace, although of course keeping the furnace clean does not guarantee that no fouling will occur in the superheaters. However, as long as gas temperatures are not excessively high, air or steam soot blowers should keep the superheater clean. Generally the deposits are fairly soft and easy to remove by soot blowing when they are first formed. Many lignite-fired plants in Texas have experienced problems with running slag. The problem appears to be related to low fusion temperatures of the ash and to the level of excess air. Specifically, decreasing the excess air from the design level of 3% O2 to 2.5% O2 can lead to running slag [91 ]. Increases in the slagging potential require a decrease in the mounting centers of soot blowers in order to maintain an effective peak impact pressure (PIP). Since fouling will increase with gas temperature, an increase in gas temperature will result in a decrease in the distance from

534 the soot blower nozzle to the surface to be cleaned (this distance is termed the range). The energy delivered by the stream issuing from the soot blower is inversely proportional to the range. Firing a lignite with a high fouling potential (i.e., high Na20 in the ash) and a severe slagging potential

(i.e., low fusion temperatures) showed the PIP requirement of a steam soot blower to be 14.6 MPa [32]. Even then the performance was marginal, the soot blower being able to remove dry deposits, but if the deposit were liquid or partially liquid (plastic) the removal was difficult to impossible. With this high-fouling, severe-slagging lignite, an enormous amount of effort and energy were required to keep the unit operating. Design criteria to achieve successful pulverized firing of lignites without using additives have been summarized: volume at least 21m3/MW; gas temperature entering the superheater convection pass no higher than 1065"C; at least 20 wall blowers and 23 retractable sootblowers per 100 MW; and minimum tube spacings of 60 cm in the secondary superheater, 23 cm in the reheater, 11.5 cm in the primary superheater, and 10 cm in the economizer [93]. 11.3.4 Survey of Current Commercial Practice This section presents design and operating information for selected lignite-fired electric power stations, to indicate some of the approaches taken to designing boilers for successful burning of lignites and some of the strategies to maintain good operability once the plant is on line. It is not intended to be a complete catalog of such installations. An excellent compilation exists in the primary literature [90]. An overview of lignite-fired electric power stations in the United States is provided in Table 11.3 [48]. TABLE 11.3 Summary of U.S. lignite-fired electric power stations [48]. Location: Furnace Pulverized coal Stoker Cyclone Wet scrubber Limestone Ash alkali Particulate removal Precipitator Baghouse Mechanical

Fort Union Number MW Capacity

Gulf Coast Number MW Capacity

7 8 4

1171 212 1538

9

3 3

720 1220

4

3000

14 1 5

2329 550 210

11 2

5660 1150

(i) Milton R. Young station. This plant is located in Center, North Dakota and is jointly owned by the Minnkota Power Cooperative and the Square Butte Electric Cooperative. The plant has two units, both Babcock and Wilcox cyclone-fired boilers. Unit 1 is 230 MW and Unit 2, 438

535 MW. Furnace exit gas temperature is 1095~

and the steam temperature is 540~

The plant fires

lignite from the Baukol-Noonan Center mine, Hagel seam. The seam in this location contains a clay parting dividing a top seam of low-sodium, high-ash lignite and a bottom seam of low-ash, high-sodium lignite. The two lignites are blended to produce a feed of no more than 4% sodium oxide in the ash. Unit 1 was designed for steam conditions of 12 MPa, 5 3 7 C , and 537~ reheat [94]. Fuel consumption for Unit 1 is about 160 t/h [94]. The normal maximum continuous steaming capacity of Unit 1 is 208 kg/s, with 13 MF'a and 543~

at the superheater outlet and

543~ at the reheater outlet [94,95]. The boiler is balanced draft, normal circulation. Wide tube spacing is used in the convection section, due to the fouling characteristics of the ash. The secondary superheater tubes are spaced at 60 and 30 cm., and reheater pendants at 23 and 15 cm [94]. The furnace design is also conservative, with heat release rate not exceeding 766 MJ/m2. h and a maximum exit gas temperature of 1065~

The cyclone burner is designed to fire 6x0 mm

lignite, thus eliminating the need for pulverizing. A hammer mill crusher is used to prepare the lignite. The lignite is predried by mixing with 400~ primary air as it is carried through the hammer mill. This treatment removes 10-12% of the total moisture (the as-received moisture content being 38--41%) [94]. The cyclone temperature is about 1925~ [94]. Seven cyclones are mounted on the front wall, with four in a bottom row and three in a top row. Each cyclone has a crushing and drying system. The seven cyclones firing together will carry 125% of normal maximum capacity; ordinarily, six will provide the desired 208 kg/s steam. Unit 2 is a balanced-draft pump-assisted circulation boiler (Carolina type). Commercial operation began in May, 1977. The design capacity is 403 kg/s with 18 MPa mean steam pressure (a 5% overpressure condition) [96]. The main steam reheater steam temperatures are controlled at 540~ by two approaches: gas recirculation and spray attemperation. The capacity of the turbinegenerator unit is 438 MW [96]. Unit 2 is equipped with 213 sootblowers [96]. In the furnace itself, these are concentrated between the lower furnace slope and the gas tempering ports. The convection pass has 84 sootblowers, concentrated in the secondary superheater and reheater sections. As ash accumulates on bare tube surfaces, an increasing consumption in sootblower steam is observed. In addition, increased sootblowing is required as ash deposition increases as a consequence of firing lignite of higher ash value. Prolonged operation at 400-440 MW can be achieved by combined use of gas recirculation, gas tempering, excess air, and sootblowing, along with taking advantage of opportunities to shed slag [96]. Slag shedding through periodic load reductions is desirable when the furnace exit gas temperature exceeds 1095~

or the gas

temperature at the reheater inlet exceeds 815~ [96]. (ii) Hoot Lake station. The Hoot Lake plant of Otter Tail Power Company is located in Fergus Falls, Minnesota. The plant has three units: Unit 1 is a 7.5 MW travelling grate spreader stoker manufactured by Detroit Stoker. Steam temperature is 440~

Unit 2 is a 58 MW

Combustion Engineering boiler, with 16 pulverized-coal-fired burners firing tangentially. Furnace exit temperature is 1055~ with a steam temperature of 535~

Unit 3 is a Babcock and Wilcox

536 pulverized-coal boiler with eight front wall burners. Furnace exit temperature is 1145~ and steam temperature, 510~

The Hoot Lake plant is a swing load station, so the various units are rarely

required to operate at full capacity. Fouling in the two pc units has been minimized by addition of calcium carbonate. The additive is shot-dosed at a rate of 90 kg every two hours, introduced through the mills. On the stoker-fired unit the calcium carbonate is aspirated with the overfire air. The Hoot Lake plant fires Beulah lignite. (iii) Big Stone station. This plant, in Big Stone City, South Dakota, is owned by the Otter Tail Power Company. The plant is a single 420 MW Babcock and Wilcox radiant-reheat cyclonefired boiler. Commercial operation began on May 1, 1975. The plant is fired with Gascoyne lignite, which can cause severe slagging and fouling problems. Lignites from three pits--Blue, Red, and White--are blended to a feed containing no greater than 4.25% sodium oxide in the ash. The lignite design criteria, on an as-received basis, are 14.5 MJ/kg calorific value, 41.3% moisture, 25.24% volatile matter, 27.0% fixed carbon, 6.46% ash, and softening temperatures in the range 1127-1410"C [97]. The nameplate rating of the boiler is 414.6 MW at 17.2 MPa with 538~ steam temperature and 538~ reheat [98]. At 5% overpressure and a maximum steam flow of 410 kg/s the net output of the unit is 440 MW. This steam flow is achieved with a firing rate of 330 t/h. The steam rate at Big Stone is 410 kg/s at 18 MPa and 540~

[92,97-99]. Heat release is 885 MJ/m2.h and 450

MJ/m3.h [99]. The gas temperature entering the first convection pass is 1065~

Temperature

control is achieved by gas recirculation. The tube spacings are 60 cm in the secondary superheater, 30 cm in the finishing superheater, 23 cm in the reheater, and 11.5 cm in the primary superheater [97]. The major problem encountered at Big Stone has been fouling. Fouling is most severe in the second bank of the superheater and the first bank of the reheater; however, the specific region in which fouling is most severe can be shifted toward the front or back of the convection section depending on the gas temperatures. In addition to tempering by gas recirculation, the gas temperatures in the superheater and reheater also depend on the cleanliness of the furnace wall, primary superheater, and economizer. The first material deposited presents a very sticky surface; continued capture and accumulation of fly ash by this sticky surface produces a soft, rubbery deposit which is difficult to remove because it is not brittle but yet is tightly adherent to the tube surface. (In this connection it may be noted that gasification of North Dakota lignites in a slagging fixed-bed gasifier occasionally resulted in the formation of stalactites of slag which adhered strongly to the refractory hearth and which were sufficiently rubbery that a reciprocating mechanical slag breaker would rebound from the surface [100]. Insofar as is known, the mechanical properties of lignite slag and ash deposits at high temperatures represents an almost totally unexplored area.) The density and hardness of the deposit increase with prolonged heating, although the suggestion [92] that the deposit chemically transforms to sodium sulfate is almost certainly erroneous. Superheater deposits were cemented together with a mixture of sodium and calcium sulfates [101]. The amount of the sodium and calcium sulfate mixture depends on the

537 amount of sodium in the lignite. Increasing the mole percentage of calcium sulfate about 40% increases the melting point of the mixture, which in turn makes the superheater deposits more friable and easier to remove. To prevent fouling, the boiler is operated using two strategies to lower the furnace exit gas temperatures. If the furnace exit gas temperature is allowed to increase, the unit will foul more severely. The strategies used to lower the furnace exit gas temperatures are to allow no wall slag to form and to limit the extent of use of gas recirculation [97]. Of course, low-temperature operation means low steam temperatures, which in turn translate into increased turbine heat rate and a lower overall plant efficiency. The boiler is equipped with 139 short-wall sootblowers and 114 long, retractable blowers [99]. Steam sootblowing seems to have little effect on removing deposits [97]. Operation of the blowers appeared to change the color of the deposits removing them. Rodding the deposits was also of little use, since the rods often poked holes in the deposits rather than dislodging them from the tubes. Water sootblowing has been found effective for removing slag and ash deposits. Direct impact of the water stream on the deposit provides kinetic energy for dislodging the deposit. In addition, the rapid chilling of the deposit by the comparatively cold water both cracks the deposit by thermal shocking and permeates the pores of the deposit. Liquid water penetrating the pores of a deposit will flash to steam with near explosive violence, further cracking the deposit and causing pieces of the deposit to fly off. The popping of a kernel of popcorn has been proposed as an analogy [92]. (iv) Coyote station. The Coyote station is owned by Montana-Dakota Utilities and is located in Beulah, North Dakota. It is a 415 MW Babcock and Wilcox cyclone-fired boiler. This station burns lignite from the Beulah mine. The sodium content of this lignite is quite variable, averaging about 8% with a range of 4% to 12% (as sodium oxide in the ash). The severity of slagging and fouling has been reduced by careful mining practices, vigorous soot blowing, and reduction of load by 20% for cleaning the dry scrubber and baghouse. Load reduction results in some shedding of slag from the furnace walls. (v) Coal Creek station. The Coal Creek Station is a two-unit, 1100 MW mine-mouth plant located near Underwood, North Dakota [ 102]. The plant uses General Electric tandem compound four-flow reheat turbines that generate 550 MW at 17.3 MPa steam at 538~ with reheat to 538~ [102,103]. Combustion Engineering pulverized-fired controlled circulation boilers are used, each rated at 1695 t/h at 17.3 MPa and 540~ with 540~ reheat. The boilers are balanced-draft, dividedfurnace units of fairly conventional design with furnace deflection arch and radiant reheater wall. Each is divided into two cells by a center wall. The combustion rate is 283 MJ/m3.h, which gives a net heat release rate of 539 MJ/m2 to the exit of the reheater platen. The maximum exit gas temperature is 1038~ Feedwater temperature is 255~

Inlet conditions for the multistage reheater

are 336~ and 4.2 MPa. Each unit requires 360 t/h lignite. Pulverizing is accomplished in Combustion Engineering bowl mills having 2.8 m diameter bowls. At the time these units were placed in service, they were

538 the largest coal pulverizers being used at any power plant. The fuel grind is 65% -200 mesh. Tangential firing causes adjacent fuel-air streams to impinge on each other to produce a vortex having a single flame. Since in essence the entire furnace acts as one gigantic burner, the precise control of fuel-air ratio at any given point is not necessary, since locally fuel-rich or air-rich situations are mixed in passing through the furnace. Internal recirculation in the furnace vortex and a long residence time for burning result in low NOx emissions and essentially no CO or hydrocarbon emissions. NOx reduction is also achieved by using overfire air. The overfire air reduces both the peak flame temperature and the bulk flame temperature by extending the size of the combustion zone and the time lbr fuel burnout. The 550 MW boilers at the Coal Creek and the 450 MW boiler at the Leland Olds station (discussed below) are designed to burn high-fouling lignite. To accommodate the fouling characteristics of the lignites, the boilers were designed with low volume heat release rates of 270 MJ/m3-h for pulverized coal firing and 451 MJ/m 3.h for cyclone firing, and with heights of about 62 m [104]. Other features of these boilers specifically intended to control fouling are wide tube spacing with shallow tube bank depths in the convection section, large numbers of soot blowers, and steeply sloped fltx~rs under the pendant surfaces. The Leland Olds boiler was designed with a flue gas temperature of 1065~

entering the secondary superheater [104], since fouling is

significantly affected by gas temperature as the gas the gas temperature approaches the ash fusion temperatures. (vi) Leland Olds station. The Leland Olds station, Stanton, North Dakota, is owned by Basin Electric Power Cooperative. There are two units at Leland Olds. Unit 1 is a 220 MW Babcock and Wilcox pulverized-coal boiler with opposed burners. Furnace exit temperature is 1150"C with a steam temperature of 540"C [99]. The steam rate is 198 kg/s at 17 MPa and 540~ [97,99]. Heat release is 908 MJ/m2.h [99]. Soot blowing is provided by 43 short wall blowers and 57 long, retractable blowers [99]. The element spacings are 45 cm at the superheater outlet, 23 cm at the superheater inlet, and 15 cm at both the inlet and outlet of the reheater [97]. The Leland Olds station burns lignite from the Glenharold mine.The lignite design criteria, on an as-received basis, are a calorific value of 15.2-15.7 MJ/kg, 36.4-39.6% moisture, 27.4-28.8% volatile matter, 26.5% fixed carbon, and 6.5-6.8% ash with softening temperatures in the range 1065-13150C [97]. Leland Olds Unit 2 is a 460 MW Babcock and Wilcox cyclone-fired boiler. Furnace exit temperature is 1065~ and the steam temperature is 540~

The steam rate is 378 kg/s at 18 MPa

and 540*C [97,99]. The heat release is 885 MJ/m2.h on an areal basis and 451 MJ/m3.h on a volumetric basis [99]. Unit 2 is equipped with 124 short wall sootblowers and 157 long, retractable types [99]. The element spacings are 60 cm in the secondary superheater, 30 cm in the finishing superheater, 23 cm in the reheater, and 11.5 cm in the primary superheater [97]. Limestone is periodically injected into Unit 1. Limestone injection into Unit 2 did not improve conditions, and in fact the increased calcium content raised the fusion temperature and viscosity of the slag. Injection of vermiculite through the furnace sidewalls ahead of the convection pass has

539 substantially improved availability of Unit 2. In a survey of the economic problems of losses to utilities caused by ash fouling, Unit 2 at Leland Olds was the most severely curtailed of any boiler included in the study [97]. The average daily load loss was 1494 MWh, corresponding to a loss in capacity of 62 MW, or 15% of the unit's capacity [97]. Including the hours of outages for deslagging, the average loss becomes 2068 MWh, which corresponds to 21% of the net generation capacity [97]. (vii) Big Brown station. The Big Brown station, which has two 575 MW units, was the first large-scale (i.e., by present-day standards) lignite-fired power plant in Texas. Unit 2 came on line in November 1972 [105]. The boilers are Combustion Engineering tangential-fired units each having a rated capacity of 507 kg/s of steam, at a pressure of 26.3 MPa, with steam and reheat temperatures both 540~

[97,99,105]. Each boiler has a furnace center division wall. The

pulverizer air inlet temperature is 315-370~

and air exit is 49-54~

[106]. The volumetric heat

release is 413 MJ/m3.h and the area heat release is 624 MJ/m2.h [99,105]. The flue gas temperature at the secondary superheater is 1065~ [ 105]. The element spacings are 56 cm in the platen superheater, 44 cm in the finishing reheater, 17 cm in the finishing superheater, and 11 cm in the primary reheater [97]. The lignite design criteria are, on an as-received basis, a calorific value of 17.6 MJ/kg, 31.74% moisture, 10.44% ash with an ash fusion temperature of 1215~ [97]. Four Combustion Engineering bowl mills are located on each side of the unit to pulverize the lignite. Each mill supplies pulverized lignite to a single level of burners, a level consisting of eight fuel nozzles (one at each corner). The lignite is pulverized to 65% -200 mesh [91]. Sootblowing capability is provided by 192 short wall blowers and 34 long, retractable sootblowers [99]. T h e two main turbine generator units are Westinghouse, designed for operating at 24 MPa, 5380C throttle, and 5380C reheat steam conditions [ 105]. Concerns for the relatively high ash content of the lignite and its rate of deposition in the boiler led to a very conservative design for the Big Brown units. A comparably sized unit firing subbituminous coal would generate about 750 MW [91]. The Monticello Units 1 and 2 (discussed below) are generally similar in design to Big Brown. The pulverizers at Monticello are larger because the lignite fired at this station is of somewhat poorer grade than that burned at Big Brown. Subsequent units (Monticello Unit 3, Forest Grove, and Martin Lake) were designed with lower volumetric heat release rates, in the range of 302-351 MJ/h.m3 [91]. (viii) Monticello station. The Monticello plant, located in Mount Pleasant, Texas, has two 575 MW pc tangential-fired units, and a third, 750 MW pc wall-fired boiler [99]. Units 1 and 2 have a steam rate of 507 kg/s at 26.3 MPa and a reheat temperature of 5400C. They are designed to be identical to Units 1 and 2 at Big Brown [97]. Element spacings are the same [97]. The heat release is 624 MJ/m2.h on an areal basis and 413 MJ/m3.h on a volumetric basis, with an exit gas temperature of 1010~

These units have 192 short wall sootblowers, and 34 long retractable

sootblowers. The lignite design criteria, on an as-received basis, are 15.7 MJ/kg calorific value, 31.27% moisture, 31.40% volatile matter, 23.02% fixed carbon, and 14.31% ash with a fusion

540 temperature of 1215~ and hemispherical temperature of 1276~ [97]. In comparison, the larger wall-fired Unit 3 provides steam at 696 kg/s at 26.4 MPa and 543~ [97,99]. The comparable area and volumetric heat release rates are 913 MJ/m2.h and 326 MJ/m3.h, respectively. The element spacings are 60 cm in the secondary superheater, 23 cm in the reheater, and 10 cm in the economizer [97]. Monticello Unit 3 has a 33 MW/h input per burner, a burner zone release of 4.5 GJ/m2.h, furnace cross section input of 17.5 G.l/m2.h, and a maximum convection pass velocity of 18 m/s [91]. (ix) San Miguel station. The San Miguel plant burns very high fouling Jackson lignite, which is the lowest quality lignite used in any commercial power plant in the United States. The range of calorific values of this lignite is 9.8-14.8 MJ/kg [107]. The very low grade of Texas lignite has 28% ash and 5.0% alkali metals in the ash [104]. The plant is very conservatively designed, with a low gas exit temperature (1010~

and a very low volumetric heat release of only

280 MJ/m3 "h. Extra furnace height, a wider tube spacing, and ample steam soot blowers were incorporated in the design. The very conservative design is also reflected in the burner inputs of 30 MW/h, burner zone release of 4.1 GJ/m2.h, furnace input of 16.6 GJ/m2.h and maximum convection pass velocity of 14 m/s i91]. These data should be compared with those given previously for the Monticello Unit 3, which fires Wilcox lignite. (Although the lignite being used at San Miguel is the lowest quality lignite ever used in a large power station in the United States, there are coals of even poorer quality being burned elsewhere. Some European brown coals of 8.4 MJ/kg heating value and 30-65% moisture, with 10-50% ash containing up to 8% Na20, are being used successfully [91 ].) The boiler is a natural circulation, balanced draft, single reheat drum unit. The maximum continuous steaming capacity is 385 kg/s; the superheater pressure is 18 MPa; and the terminal temperatures of the superheater and reheater are 5400C [107]. At low load, the reheat steam temperature is controlled by recirculating gas from the economizer outlet to the furnace hopper. At full load, the furnace exit gas temperature is controlled by gas tempering in the upper furnace region below the furnace arch. The furnace is a dry bottom unit 20 m wide, 14 m deep, and 67 m high [107]. The challenges in designing this boiler arose from the anticipated severe slagging nature of the lignite ash, and the desire to operate a 16.5 MPa drum boiler with this low-quality fuel. A major consideration was the necessity of minimizing slagging and controlling slag formation. To fulfill this consideration, the following design parameters were adopted: a maximum heat input per furnace cross sectional area of 17 GJ/m2.hr; a maximum burner heat input of 132 GJ/h" a maximum furnace exit gas temperature of 1050~

and a maximum furnace volume heat

release rate of 372 MJ/mJ.h [ 107]. In fact, the actual volume heat release rate was 279 MJ/mJ.h for a burner input of 128.2 GJ/h [107]. The furnace exit gas temperature was chosen to be below the initial deformation temperature of the ash, because preliminary tests suggested that the ash could be quite difficult to handle, and because the ash has a wide plastic range. However, the use of this fuel in a drum

541 boiler with desired steam conditions of 16.5 MPa/540/540*C required some careful choices in the design of the boiler. In a drum boiler, steam generation occurs in the furnace wall. If the exit gas temperature is restricted to 1050~ from consideration of the slagging behavior of the ash, and if no superheater or reheater surfaces are in the furnace itself, there is not sufficient heat in the flue gas to achieve the desired steam conditions. Since other considerations ruled out the use of superheater platens in the furnace, gas tempering was selected as the strategy for reaching these design steam conditions. Gas tempering allows reducing the furnace exit gas temperature without increasing the size of the furnace itself. For the San Miguel plant, gas tempering permits the furnace to be 23 m shorter than would otherwise have been necessary to achieve the desired exit gas temperature [ 107]. To help control slagging and fouling, the San Miguel unit was originally designed with 76 furnace wall sootblowers, 56 long-travel soot blowers, and 24 extended lance blowers, with an additional 140 wall boxes provided for relocation of sootblowers if necessary [ 107]. The furnace wall blowers are on 3 m vertical and 2.7 m horizontal centers [107]. Sootblowers in the convection section are spaced on 1.8 to 2.1 m centers [107]. To help the sootblowers function most effectively, the tube bank depths are limited to 1.5 m for pendant surfaces and 1.3 m for horizontal surfaces [107]. Uniformity of gas temperatures entering the convection section is helped by opposed wall firing in the furnace, which provides a uniform heat input and flat temperature profile across the furnace exit. In the 1035-925~ range the secondary superheater sections are spaced on 60 cm side centers; the spacing was reduced to 30 cm for the 925-835~ pendant reheater, in the temperature range 835-620~

region [107]. The

has sections on 23 cm side centers [ 107].

Wide spacings reduce the likelihood of ash deposits forming bridges from one tube to another. In all cases an excessive accumulation of ash deposits can lead to increased gas temperatures, accelerated deposition, and reduced effectiveness of heat transfer surfaces. However, the concern is heightened for the San Miguel lignite ash, which has sintering strengths above 137 MPa at temperatures as low as 870~

[107]. The pendant convection pass floor has a 35 ~ slope from the

furnace arch to the rear boiler screen; this steep slope is intended to reduce ash carry-over into the horizontal convection section. (x) The Antelope Valley Station. The Antelope Valley Station, owned by Basin Electric Power Cooperative, is on a site adjacent to the nation's only commercial-scale coal gasification plant (Chapter 12). The steam generator is a Combustion Engineering tangentially fired design to provide 394 kg/s steam with a superheater outlet pressure of 18 MPa, superheater outlet temperature of 5400C and reheater outlet temperature of 5400C [ 108,109]. The temperature of the steam is controlled by burner tilt and by main and reheat steam desuperheaters. A spray-type desuperheater is provided between the horizontal superheater and the vertical platen superheater, This unit is supplied with condensate from the boiler feed pumps discharge. The maximum spray rate of this desuperheater is 25 kg/s [ 108]. The reheat steam temperature is also controlled by spray type desuperheaters which have a maximum spray of 4.7 kg/s [108]. The steam pressure at the reheater outlet is 3.4 MPa [109]. The unit is designed for balanced draft furnace operation firing

542 high-sodium lignite. The steam generator is one of the tallest boilers in the world, standing 106 m to the top of the boiler steel. The furnace is 20 m wide and 16.6 m deep, with a heat release rate of 874 M.l/m2.h and a furnace exit gas temperature of 1065~

[108,109]. The furnace has 36 fuel

injection nozzles in nine elevations. Each nozzle is rated at a maximum heat input of 190 GJ/h, providing a heat release of 2.9 GJ/m2 [ 108]. Adiabatic flame temperatures are about 1650~

The

nozzles fire tangentially. Overfire air ports are used to help control NOx formation. The superheating system consists of the furnace roof, side walls, back pass roof and walls, vertical platen superheater, a low-temperature horizontal superheater, and the finishing superheater. The vertical pendant superheater has 6.4 cm outside diameter tubes spaced 62 cm; the finishing pendant and horizontal low temperature superheaters have 5 cm tubes spaced at 18 and 10 cm, respectively. The gas velocity in the vertical pendant is 8.5 m/s and is 17 m/s in both the finishing pendant and the horizontal low temperature superheaters. The fuel and air nozzles can be tilted to help control the heat absorption in the furnace and the exit gas temperature. Combustion Engineering model RP-1003 Raymond-type pulverizers are used. Each of the nine pulverizers has a base design capacity of 68,000 kg/h of lignite with a Hardgrove grindability index of 49. The pulverizer product is 98.5% -50 mesh and 65% -200 mesh [108]. For grinding lignite with a 38% moisture content having a Hardgrove grindability index of 25, the pulverizers are derated to 45,600 kg/h. The steam generator is equipped with 455 steam sootblowers provided by Diamond Power Specialty Corp.[ 109]. These sootblowers include 136 retractable units for cleaning the convection surfaces in the superheater, reheater, and economizer; 311 wall blowers for cleaning boiler walls, 2 retractable blowers in the furnace ash hopper throat, and 6 retractable blowers for cleaning the regenerative air heaters. A unique feature of the Antelope Valley steam generator is a provision to produce 295,000 kg/h of steam for use as process steam at the adjacent coal gasification plant. The steam pressure delivered to the gasification plant is 10 MPa. This steam requirement is in addition to the normal demands on the boiler for auxiliary steam. (xi) Boundary Dam Generating Station. This plant is located near Estevan, Saskatchewan. It is operated by Saskatchewan Power Corporation. Unit 3 is a 150 MW balanced draft, tangentially fired, radiant reheat steam generator produced by Combustion Engineering - Canada [110]. The boiler fires the locally available Estevan lignite, which is generally high fouling. (xii) Gibbons Creek station. At the Gibbons Creek station (408 MW) concerns for the large quantities of ash to be handled (45 tonnes of ash are removed from the walls per hour ) and the high potential for slagging and fouling led to a design with a plan-area heat release rate of 16 GJ/m2.h, volumetric heat release rate of 287 MJ/m3.h, and a furnace exit gas temperature of 1025~ [111]. These design parameters have been effective in preventing accumulation of tightly bonded ash deposits. Furthermore the low exit-gas temperature, combined with a wide tube spacing in the convective pass, have eliminated problems in the superheater pendant and the convective section. The boiler is equipped with 162 wall blowers and 56 steam soot blowers

543 [ 111]. The wall blowers are operated in pairs on a continuous basis. 11.3.5 Emission control technology (i) Particulates. Electrostatic precipitators are commonly used for removal of particulates from stack gases. However, some lignites have ashes which show high resistivities at the typical operating conditions of a precipitator. High resistivity upsets the electrical conditions inside the unit and reduces the efficiency of collection. Collection efficiency of an electrostatic precipitator depends on the residence time of the ash particles in the electric field, the strength of the field, the resistivity of the ash and its composition, and the temperature and composition of the flue gas [112]. A high sulfur content in the flue gas lowers resistivity. More likely in the case of lignites, the resistivity is also lowered by high moisture content in the flue gases. Even with the advantage provided by the high moisture in the flue gas, the resistivity of some lignite ashes may still be too high for efficient collection under normal operating conditions. In that case, alternatives are to reduce the flue gas temperature prior to its entering the precipitator, add sodium compounds or ammonia to the flue gas to reduce resistivity, or forego the use of precipitators for baghouses [9]. Because of the high resistivity of some lignite ashes, collection efficiencies can be drastically reduced, compared to behavior with bituminous coal ashes. Strategies for coping with this problem include sizing the precipitators with larger specific collecting areas, locating the precipitator on the "hot side" of the air preheater, or using additives to change the ash resistivity [48]. An advantage of baghouses for high resistivity lignite ashes is that resistivity has no effect on their operation. Baghouses are more effective at collecting submicron particles than are precipitators. However, the performance of baghouses can be adversely affected by surges of flue gas, particularly if the gas temperature exceeds the normal operating range of 120-180~

[9]. The

behavior of fly ash in a baghouse is not easily predicted, and baghouse design must often rely on previous experience with similar types of ashes. Baghouses possess an economic advantage relative to precipitators in that future tightening of emission standards for particulate matter will require larger (and hence more expensive) precipitators, while present baghouse performance is already superior to that required by existing emission standards. (ii) Sulfilr oxides. Emission regulations which require removal of 70% of the sulfur force the use of some type of emission control devices [48]. However, the low sulfur content of lignites, combined with the alkaline nature of the ash, provides the potential for SO2 removal without added sorbents such as limestone. A spray dryer system using sodium carbonate as the active reagent is used for flue gas desulfurization [9]. The products of sulfur capture, sodium sulfate and sulfite, are collected and disposed with the fly ash. Wet scrubbers generally use limestone as the active sorbent, the limestone being converted to gypsum. Direct injection of a sulfur-capture sorbent into the furnace is practiced at the Shand power station in Saskatchewan, the plant having two 300 MW units. The ash alkali wet scrubbing process essentially leaches some of the alkaline components of the fly ash for use in sorbing SO2. In favorable cases, the addition of lime or limestone is not necessary. Wet scrubbers using the alkaline ash, or alkaline ash supplemented with lime or

544 supplemented with lime or limestone, have been installed on about 2600 MW of generating capacity [ 113]. Using an aqueous slurry of fly ash from a cyclone boiler, it is possible to exceed SO2 emission requirements for a 1000 ppm concentration of SO2 in flue gas (dry basis) with a liquid/gas ratio in the scrubber of 30 [114]. Increasing the level of sodium in the solution, from 1.5% to about 4%, increased the sulfur removal efficiency by about 10 to 15% [114]. This increase in sodium also eliminated scale formation in the scrubber. The possibility of adding a sulfur capture agent to lignite before combustion provides a potential route to the reduction of sulfur emissions. Treatment of lignite with calcium nitrate solution followed by sodium hydroxide (to precipitate calcium hydroxide) and subsequent combustion of the impregnated sample resulted in a sulfur emission equivalent to a hypothetical sulfur content in the coal of 0.004% [115]. Unfortunately, the effects of this treatment on the concomitant ash deposition behavior were apparently not investigated. (iii) Nitrogen oxides. The NOx level in flue gases can be reduced by a small amount by reducing excess air to the boiler [48]. However, this procedure can significantly increase ash deposition and slag formation. Thus reducing NOx by controlling excess air is undesirable in pc fired boilers. NOx formation can also be changed by changing the burner tilt angle. Lignite chars prepared by fluidized-bed pyrolysis provide good catalytic activity for reaction of nitric oxide with ammonia [116]. Lignite chars are superior to chars from subbituminous or bituminous coals. High surface area and a large amount of oxygen functional groups contribute to catalytic activity. Since oxygen functional groups decrease with increasing pyrolysis temperature, but surface area increases with temperature, the maximum catalytic activity is obtained for chars produced at intermediate pyrolysis temperatures (600-700~

[ 116].

11.4 ASH DEPOSITION, SLAGGING AND C O R R O S I O N

11.4.1 Introduction A 1937 survey of lignite utilization [68] indicated that, "no important technical problems hold back the proper use of lignite as an industrial fuel." Events twenty years later were to prove this forecast wrong, and would in fact unveil what is almost certainly the most serious technological problem facing current lignite utilization practice. Ash fouling, the accumulation of deposits on heat-exchange surfaces in utility boilers, is a serious operating problem in many power plants fired with lignites. Ash fouling is a complex function of the boiler design, the method and conditions of operating the boiler, and the properties of the coal. In extreme cases it is necessary either to schedule frequent shutdowns to remove the deposits or to derate the boiler. An early instance of the ash deposition problem occurred with the 54 MW Hoot Lake plant of the Otter Tail Power Company. The plant was installed in 1959 and experienced severe fouling problems during its first years of operation. The deposition problems were neither consistent nor predictable, but coordination between Otter Tail and the Knife River Coal Mining Company

545 established a relationship between severity of fouling in the boiler and the location in the pit from which the lignite had been mined (in the Beulah mine). It was then possible to outline areas of "troublesome" lignite, which were not mined during winter, when boiler loads were highest. A survey established that the Beulah lignite ash was highly variable in composition [117]. Subsequent tripartite cooperative work involving Otter Tail, Knife River and the U. S. Bureau of Mines testing low- and high-sodium Beulah lignite at Hoot Lake clearly established a relationship between sodium content and severity of fouling [ 118,119]. A 1980 survey of six power plants estimated that the total costs of curtailments due to ashrelated problems were $20.6 million over a six-month period [99]. The cost of shutting down a 750 MW plant to remove deposits is as high as $300,000 per day [ 120]. Losses due to ash fouling ranged from zero at the Big Brown and Monticello stations in Texas to 426,354 MWh (16.1% of capacity) at the Leland Olds station in North Dakota. The highest losses for any unit at a station were 321,917 MWh (18% of capacity), also at the Leland Olds station [121]. Using data from the Leland Olds and Big Stone (South Dakota) stations, the costs of outages or curtailments due to ashrelated problems could reach $8.4 million per year for a 500 MW plant firing a high-fouling North Dakota lignite. This cost is the equivalent of an extra $2.50/t of lignite, and gives an indication of the cost limits of remedial measures (e.g., removal of sodium by ion exchange) that could hope to be cost effective [ 121]. An indication of the severity the ash slagging and fouling problem is provided by a sixmonth survey of power stations burning U.S. low-rank coals [99]. During this six-month period two plants firing North Dakota lignite, with a total generating capacity of 1007 MW in three units, experienced 104,295 MWh of outages or curtailments due to slagging, representing 2.37% of available boiler capacity. During the same period, 671,559 MWh were lost due to outages or curtailments attributable to fouling; this lost production represents 15.27% of boiler capacity. Substantially better performance was observed in plants burning Texas lignite. Two plants, having a total of five units with an aggregate 3090 MW capacity, experienced only 10,676 MWh of outages due to slagging (0.08% of boiler capacity), and no fouling losses. An outcome of this survey was a correlation of the total ash-related losses with the weight of deposit produced under standardized test conditions in a pilot-scale combustor. This relationship is illustrated in Figure 11.9 [99]. More recent work has verified that the mechanisms affecting the fate of sodium, as well as iron and calcium, are consistent among all scales of combustion experiments [122]. Those lignites which are particularly prone to cause ash-related problems can, in extreme cases, cause repeated, unscheduled shutdowns. During operation at high fouling, the boiler capacity may be reduced by as much as 20%, the thermal efficiency reduced by 10%, and 1% of the total steam may be required to operate sootblowers [ 123]. The number of operating days can be reduced by 10% [ 123]. In economic terms, the revenue lost due to a forced outage in a 500 MW unit can exceed $100,000 per day [123].

546 20 j~.

18

~

16

~

14

o

12

~~ 1~ ~" 81

o

o

4 0~ .... o

I

1O0

200

300

400

Deposit weight, g Figure 11.9. Ash fouling losses (as percent of annual capacity) in electric power stations as a function of ash deposit weight in standardized pilot-scale combustion tests [99].

11.4.2 Factors affecting fouling and slagging (i)

Summa~..Ash deposition is principally, but not entirely, a factor of the properties of the

ash. The instantaneous deposition rate is influenced by the cleanliness of the boiler, after cleanup from the most recent shutdown. Given a constant boiler load and quality of fuel, fouling typically follows a slowly accelerating rate. Although the accumulating deposits may be removed by sootblowers, the removal is not usually complete, with the consequence that the net accumulation of deposits shows a slow but steady increase. The temperatures in the boiler will be increased to maintain heat transfer across the increasing thicknesses of thermally insulating deposits. The fouling rate increases with temperature, so that deposition is accelerated and eventually moves out of the furnace itself into the convection section. In some cases temperatures in the furnace may become high enough to cause melting of the deposits and the formation of running slag on the furnace walls. The observed severity of deposition is dependent not only on the specific lignite being fired but also on the specific design of the boiler, its method of firing, its load factor, the tube metal temperature, and soot blowing practices [91,124,125]. That high percentages of sodium oxide in the ash are strongly associated with heavy deposition has been established for about a quartercentury [124]. The key factors that affect deposition are summarized in Figure 11.10 [9,91,121]. Factors such as the specific combustion conditions, the cleanliness of the boiler, and the flow patterns of flue gases can "all play a role in affecting fouling behavior [9]. If one begins with a clean boiler and assumes a constant load and fuel specifications, fouling normally proceeds with a

547 MAJOR FACTOR SODIUM (%, dry coal basis) <0.1

[

<1

[

0.1-0.5

]

>0.5

]

>5

(% Na20 in ash)

1-5 Low

<4%

Low

4 - 8%

Low

Low-medium

Medium-high

Z

> 8%

LOw

Medium-high

HIGH

ga

< 870 ~ C

Low

O

O

0

Ash, dry' coal basis

Flue gas temperature 870 - 1040 ~ C

0

> 1040 ~

h)w

bw

Low Low-medium Medium

Low-medium

Low Medium HIGH

Figure 11.10. Matrix of the key variables affecting fouling potential [91]. Effect of :Ash content is based on constant 1040"C flue gas temperature; effect of flue gas tem~rature is based on constant 8% ash in coal (dry basis).

slowly accelerating rate. Removal of deposits by sootblowing seldom achieves a complete removal, with the result of a slow, net accumulation of deposits. As the deposits slowly accumulate, it will be necessary to increase the boiler temperature in order to achieve a constant heat-transfer rate. However, the rate of fouling also increases with boiler temperature, thus initiating a spiraling escalation of temperatures and fouling rates. In severe instances, the wall deposits will melt to a running slag. (ii) Sodiunr The association of sodium with fouling for large lignite-fired units was first established in cooperative work between Otter Tail Power Company and what was then the Lignite Research Laboratory of the Bureau of Mines (now the Energy and Environmental Research Center of the University of North Dakota). When a lignite having 9% Na20 in the ash was burned, only three days' operation could be achieved before it was necessary to shut down for cleaning the tubes. In contrast, fouling was not excessive when low-sodium-oxide lignite of the same total ash content was fired with similar boiler load. Pilot-scale tests established that lignite containing 8-10% Na20 in the ash results in a much higher deposition rate than a lignite with less than 2% Na20 [ 118], later confirmed in full-scale tests [125]. A general rule, derived from tests on 31 coals, including 25 lignites, is that coals containing less than 1% Na20 in the ash are low fouling [ 126]. Coals in the range of 1% to about 5% sodium

548 less than 1% Na20 in the ash are low fouling [126]. Coals in the range of 1% to about 5% sodium oxide are medium fouling. Above 5% sodium oxide in the ash, the lignite would be considered to be high fouling. These criteria are not absolute guidelines because factors other than sodium also affect deposition. For example, Rockdale (Texas) lignite has less than 1% sodium oxide in the ash and might therefore be expected to be low fouling; however, it also has about 39% silica in the ash, a relatively high value for lignites, and actually shows medium fouling behavior. Velva (North Dakota) lignite is not as severe a fouling lignite as would be predicted solely on the basis of sodium oxide content, because it also contains over 40% calcium oxide in the ash, an unusually high value. A coal in which the sodium content exceeds 5%, expressed as Na20 in ash, and the ash value exceeds 5% (dry basis) can be expected to result in high to severe fouling [127]. The single exception among the lignites tested was the medium-fouling Rockdale lignite with 39% SiO2 in the ash. Again with one exception, none of the coals with greater than 3% Na20 in the ash were low fouling. In this case the exception was Velva lignite. For coals having a lignitic type ash (i.e., CaO + MgO > Fe203) the effect of sodium on fouling is summarized by the data in Table 11.4 [126,128]. TABLE 11.4 Relationship between sodium oxide in ash and fouling [126,128]. % Na~O in Ash < 2.0 2-6 6-8 >8

Fouling Low Medium High Severe

Fouling is not solely a function of sodium content; for example, at a given level of sodium, increasing calcium content decreases fouling while increasing silicon increases fouling. The relationship between sodium content and rate of deposition is shown for Beulah lignite in Figure 11.11 [ 126]. This lignite typically contains about 20% CaO in the ash, which is close to the average value for North Dakota lignites. The fouling rate becomes essentially independent of sodium content above about 10% Na20; a similar effect was noted for other lignites. To fully understand fouling, however, it is not enough to have information on the oxide content of the ash, because only certain forms of sodium, before conversion to the ash, are the troublemakers; and other components besides sodium may have a significant role in fouling [ 124]. All other factors being equal, deposition rates in the superheater and reheater areas were a linear function of the sodium content up to 6% sodium oxide in the ash. Above that level, the deposition rate was essentially independent of higher sodium content. (see, e.g., Figure 11.12.) Pilot-scale combustion studies of Beulah lignite of various sodium contents established a so-called saturation

549 .

500

Beulah lignite (--20% CaO) 9 Velvalignite (--36% CaO)

400.

f

300

o

200lOO o

'

0

'

'

'

I

'

'

'

'

5

I

10

'

'

'

'

I

'

' '

15

'

20

Percent sodium oxide in ash

Figure 11.11. Comparison of fouling in a pilot-scale combustor for high- and normal-calcium content lignites as a function of sodium content [126].

effect above which deposition rate was independent of sodium content. The saturation effect occurred in the range of 8-10% sodium oxide in the ash. The fact that the saturation effect does not occur at a fixed, invariant level of sodium oxide in the ash for all conditions reflects the fact that other properties of the lignite and the boiler operating conditions also affect deposition rates. The threshold concentration of sodium necessary to produce the low-melting, sticky aluminosilicates which form the deposit matrix is 4--5% Na20 in the ash [129]. These sodium aluminosilicates are sticky enough to retain fly ash particles impacting the growing deposit. An early rule-of-thumb was developed which indicated that 0.4 to 0.5% sodium oxide in the coal (not ash) was the limit above which alkali-bonded deposits could occur. The chemical form of sodium was important, suggesting a rough distinction between "volatile" and "stable" sodium. In lignites of North America, the sodium is mainly incorporated as relatively mobile cations associated with carboxylic acid functional groups. The reason that empirical correlations which relate ash deposition to sodium oxide content of the ash provide a reasonable predictor of deposition without making a distinction between volatile and stable sodium is that for most lignites most of the sodium occurs in the cationic form. Sodium associated with the organic portion of the lignite is worse, from a fouling perspective, than sodium contained in minerals, because the organically associated sodium is both highly volatile and in a highly reactive form in the furnace. Support for the idea is derived from the

550 200

9

180 160~a0

~0 .),,4

to o

9

9

1014~ : 100-

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9

80 60 40 20 [""l ,=,

,~.

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r

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,~.

r,.-

,0q.

Sodium oxide in coal, % dry basis

Figure 11.12. Deposit weight in pilot-scale combustion of North Dakota lignites as a function of sodium content [130].

observation that the sodium concentration in deposits taken from convection surfaces in boilers is often higher than in the ash of the coal being burned [88]. The importance of organically bound sodium was demonstrated by first removing the sodium by ion exchange and then backexchanging sodium acetate. The fouling behavior of the lignite doped with sodium acetate in amount equal to the original sodium content was essentially identical with that of the untreated lignite. Pilot-scale data for Beulah lignites with sodium removed and back-exchanged with sodium bicarbonate are shown in Table 11.5 [ 130]. TABLE 11.5 Effects of changing sodium content on ash deposition behavior of Beulah lignite [130]. Condition natural, low-sodium natural, high-sodium ion-exchanged high-sodium ditto, back-exchanged with NaHCO3

% NagO in Ash 1.1 6.3 1.0 6.7

Deposit weight, g 118 399 127 438

The relationship between sodium, expressed as percent Na20 in the ash, and the percentage of annual output lost due to ash-related problems is shown in Figure 11.13 [ 130] for seven power stations. For six of the plants a remarkably linear relationship exists, highlighting the profound

551 25 o

20

0

N

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15

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,_q

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Percent sodium oxide in ash Figure 11.13. Total ash-related losses (as a percent of annual output) in electric power stations as a function of sodium content in ash [121]. The outlier is a coal of unusually low ash yield (5.7%, dry basis).

effect of sodium on ash behavior. The single point lying well off the line is data for a power station burning a subbituminous coal with ash content (5.7%) well below that of the coals burned in the other stations (all >8%). A survey of fouling losses at several power plants burning lignites or subbituminous coals confirmed the general relationship of sodium in ash to fouling [97]. Stations burning low-sodium Texas lignite had no fouling losses. A low-sodium subbituminous coal produced low fouling losses at the Jim Bridger station, and a subbituminous coal of somewhat higher sodium content had higher fouling losses (at the Four Comers power plant). The highest fouling losses in the study [97] were incurred at Big Stone and Leland Olds, which burn high-sodium lignites, the worst case being the Leland Olds Unit 2, which burns lignite of higher sodium content than is used at Big Stone. (iii) Flue gas temperature. The gas temperature is an important operational parameter affecting deposition. Indeed, the gas temperature at the point of deposition is second only to ash composition (particularly sodium) in importance in affecting fouling [ 131]. For lignite-fired units a rule of thumb is that there is a possibility of deposits forming on convection surfaces in all areas where the gas temperature is above 815~

As a rule, the amount of deposit formed in a given time

increases with increased flue gas temperature [126,127]. This effect is illustrated in Figure 11.14 [127], which presents data from a 34 kg/h pc-fired test combustor. The temperature of the gas entering the first bank of test probes (designed to simulate secondary superheater tubes in the convection section of a boiler) was varied from 980 ~ to 1150 ~

a threefold increase in deposit rate

552 700 600~0 5009r-I

400-

~9 3000

121 200" 100

o 900

'

'

I

'

'

'

1000

I

'

1100

'

'

1200

Flue gas temperature, ~

Figure 11.14. Ash deposit formation in a pilot-scale combustor as a function of the flue gas temperature, with deposition rates corrected to constant coal consumption rates [127].

occurred over this temperature range [ 126]. The effect of increasing the gas temperature is ascribed to the gas temperature approaching the ash softening temperature [126], although there appears from other studies to be no direct relationship between ASTM fusion data obtained on the laboratory ash and the formation of ash deposits. Rapid fouling certainly occurs when the temperature of the gases reaching the convection banks is well above design temperature. For example, in a unit designed for a flue gas temperature of 1137"C entering the secondary superheater, unusually heavy buildup of wall slag resulted in the gas temperature rising to 1315~

This condition resulted in rapid fouling of the superheater. Any

combination of high values of sodium (>5% Na20 in the ash), high ash levels (>8% ash on a dry coal basis) and flue gas temperatures above 10370C is a recipe for serious trouble. Comparable observations are made with Siberian lignites having 40% CaO in the ash; for example, exit gas temperatures of 1020-1050~ are recommended [132]. When the condensation of sodium sulfate, and its subsequent participation in formation of a sticky surface layer that accumulates particles, governs fouling, a temperature "window" may exist in which deposit accumulation is particularly severe. In this model, at gas temperatures above 9770C the ash particles will not have a molten surface layer, and any particles that have accumulated on the tube surface will also be dry, because the tube metal temperature is below the melting temperature of the sulfate [133]. Any sodium sulfate that condenses on the tube is removed by erosion; a deposit would not be expected to form. At gas temperatures of 977~ or

553 below, sodium sulfate will begin to condense on suspended particles; eventually enough sodium sulfate will accumulate to make a sticky surface layer on some impacting particles, the sticky layer being sufficiently thick that the rate of deposition and accumulation exceeds the rate of erosion [133]. Deposition should cease when the gas temperature drops to 884~

or below, this

temperature being the assumed melting temperature of the sulfate [ 133]. Because of the role of gas temperature in affecting fouling, it can be a serious mistake to increase firing rate to regain boiler output once the boiler has begun to foul [ 134]. Doing so will increase the outlet gas temperature, which will in turn increase the slag buildup even further. Outlet gas temperature can be reduced by increasing windbox pressure, reducing primary air temperatures, or lowering the burner tilts. Furnace exit gas temperatures are very important when burning high-fouling lignites [97]. If gas temperatures become too high, fouling will be a problem. Water blowers on furnace walls are the most cost-effective measure for controlling furnace exit temperatures [97]. When fouling becomes severe enough to affect the load, usually virtually all of the tubes in the convection section are heavily fouled. As the fouled area suffers reduced heat-transfer ability, the temperature will increase through the convection section. Consequently the zone in which gas temperatures are high enough to promote fouling steadily moves back through the convection section until eventually the fouling becomes so severe that draft losses or the temperature at the back of the convection section force a curtailment [97]. However, the accumulation of fouling throughout the convection section is slow, and often remedial measures such as sootblowing or hand lancing can be applied during periods of load reduction without requiring a curtailment. In cyclone furnaces, low slag viscosities, which are otherwise desirable for good slag tapping, could increase the carry-through of coal particles burned in suspension (as opposed to those trapped and retained by a sticky, high viscosity slag) again raising temperatures at the furnace outlet and exacerbating deposition. (iv) Tube metal temperature. The temperature of the tube surface has a moderate effect on ash deposition. As the temperature of the metal tubes increases, the total weight of deposit accumulated in a specified time also increases [126,127,131]. However, the weight of the inner white layer decreases with increasing metal temperatures. The effects of tube metal temperature in the temperature range 425-650~

are shown in Figure 11.15 [127]. In the test furnace used to

acquire these data, the deposit forms on metal tubes simulating the steam tubes in the convective pass of a utility boiler. The suggested explanation is that at lower temperatures whatever deposit is already on the tube solidifies faster and is less sticky, therefore being less able to trap or retain additional material. When particles solidify, at temperatures below the melting temperature of the sulfates, deposition should cease [ 133]. (v) Calcium.

A high calcium content in the ash reduces deposition [48,131]. Large

amounts of calcium may prevent formation of large amounts of glassy ash particles of high iron content, the calcium serving as a nucleating agent [135]. (High magnesium levels may have the same effect [48].) Large grains of caicite could "dilute" deposit strength [45]. Calcium could also

554 600

6

500

-5

~4oo-

-4

.~0 .

~1 9 3000

-3

"

K

,~ 200 -

~,4

-

1000

--1

. . . .

400

i

500

. . . .

I

. . . .

600

~

0

700

Tube metal temperature, ~

Figure 11.15. Deposit formation in pilot-scale combustion as a function of tube metal temperature [127].

increase the viscosity of the liquid phase in a deposit, decreasing sintering and hence reducing deposit strength [45]. However, with Nazarovo (Russian) brown coal, sulfation of calcium oxide is a causative factor for the formation of strongly bonded deposits on the convective heating surfaces [ 136]. Calcium sulfate has been implicated as the "glue" in some superheater deposits of Texas lignite ashes [ 137]. Sulfation of calcium occurs after the deposition of calcium-rich particles [138]. The propensity toward sulfation appears to depend on the nature of the association of the calcium with other elements in the deposit [45]. Calcium silicate particles are stickier at high temperatures than are other ash particles, and are more likely to stick to boiler surfaces [45]. The reaction between calcium-rich particles and aluminosilicates leads to a deposit matrix rich in calcium, aluminum, and silicon [137]. Fine calcite grains may react with silicates to depress liquid viscosities, thereby enhancing ash stickiness and deposit formation [45]. Velva lignite has an unusually high calcium content, about 40% CaO in the ash, double the average for North Dakota lignites. The deposition rote for this lignite is about half that of a lignite with the more typical 20% CaO [126]. Velva lignite has a lower ash value than some other North Dakota lignites, but even when deposition rates are corrected to an equal ash input basis the highcalcium lignite has a lower rate of fouling. Similarly, deposition is greater for a lignite of lowerthan-normal calcium con~,ent when compared with a lignite having normal calcium and comparable sodium and ash levels, The behavior of calcium relative to sodium may at first seem counterintuitive. Except 7or the obvious difference in ionic charge, the two elements have much in common. They form ionic compounds, their cationic radii are virtually identical, and their oxides

555 are strong bases. Increasing the amount of calcium in ash increases the ash softening temperature for lignitic ashes, but there does not seem to be a correlation between ash fusion temperature and fouling. Calcium affects the melting temperatures of the matrix material in the outer sinter layer of deposits. Sodium melilite has a melting point in the range 1120-1200~

In high-calcium systems,

there is a likelihood of forming gehlenite, Ca2AI2SiO7, which has a much higher melting point, 1590"C. A system high in calcium and magnesium could form akermanite, which also has a melting point much higher than sodium melilite, 1438~

The high melting temperatures of

gehlenite and akemlanite suggest that the role of the high calcium content in reducing fouling may result from increasing the melting temperature of the matrix. Reactions of kaolinite with sodium and calcium acetates, discussed in Chapter 6, have shown that kaolinite and sodium acetate produce nepheline, whereas kaolinite and calcium acetate produce gehlenite, suggesting that different mineral assemblages may fornl in the early stages of combustion of high-sodium and highcalcium lignites. Since the subsequent behavior of ash in the combustion system is dependent on composition and mineralogy of the ash particles, the different compounds formed early in combustion could have a profound effect on deposition behavior when the ash reaches the boiler tubes. (vi) Ash value. The ash value per se is not a predictor of fouling behavior. Some coals with high ash levels show pronounced fouling whereas others do not. However, when the total ash value is considered in conjunction with ash composition, an effect of ash value can be observed. At a given level of ash value, the fouling rate is a direct function of the sodium content. For any given level of sodium content, the fouling rate increases exponentially with ash value. Thus ash value can be an important predictor of fouling. The most severe fouling conditions occur when both the sodium and ash levels are high, as shown in Figure 11.16. However, a contrary view is that lignite having high sodium but low ash value is more troublesome than a high sodium lignite of high ash value. The role of the sodium as a flux or in promoting sintering may be diluted when larger quantities of ash are present [96]. In a survey of the economic effects of fouling, a plant burning a subbituminous coal having the highest sodium content of any low-rank coal included in the survey experienced only moderate fouling, and no fouling losses during the six month period of study. This uncharacteristic behavior of a high sodium coal was attributed to the very low ash levels in the coal, which were 4.1-5.1% on an as-received basis [97]. Blended coals show fouling strongly influenced by the sodium and ash values of the blend. However, these two parameters do not always vary linearly, because the fouling behavior of the blend was not necessarily that predicted from the weighted average of the sodium contents or ash values of the unblended coals. Thus fouling is a function of the characteristics of the blend itself regardless of the characteristics of the s~arting coals. (vii) Other factors of lignite composition. Moisture, sulfur content, and ash fusion temperatures have minor roles in affecting deposition behavior [ 126,131 ]. None have the same

556 800 700-

~,"

600-

9 l%Na2Oash

500-

9 5% Na20 in ash

.w-i

400-

o

10% Na20 in ash

oga~ 300 200 100 0 0

2

4

6

8

10

12

Ash content, % dry basis Figure 11.16. Relationship of deposit weight in pilotscale combustion of subbituminous coals as a function of the ash yield, for three levels of sodium in ash [130].

degree of importance as sodium content. Fouling from a coal with its normal 30% moisture content and the same coal dried to 15-20% moisture, show no significant differences.The silica content of the ash tends to increase fouling, but is generally has a minor role compared to sodium and total ash value. Ash deposits obtained after shutdown of the boiler usually contain a greater amount of sulfatic sulfur than would be indicated by the sulfur-to-ash ratio of the lignite. This suggests some interaction between the components of the ash and sulfur oxides. Further confirmation was provided by measurements of flue gas SO2 and SO3 when high- and low-sodium coals of comparable sulfur content were fired. The SO2 content was higher by about a factor of two for the flue gas from the low-sodium coal. For the high-sodium coals about 60% of the input sulfur was found as sulfur gases, whereas in the low-sodium case virtually 100% of the sulfur was in the flue gas. These observations suggest a minor role for sulfur in deposit formation, but large differences in sulfur content are not nearly as important a factor as sodium content in determining fouling behavior. (viii) Factors of boiler operation. Excess air level during firing appears to have a minor role in affecting deposition behavior [126,~t31].The relationship of excess air to fouling is not clearly understood. In some tests no effect of excess air levels in the range of 5-50% was observed. Tests in a drop-tube furnace showed no effects of oxygen concentration on the composition of adhering particles in the deposit [ 139]. Other work has shown an increase in deposition rate at low excess air levels. However, in these cases wall slagging also had increased, and thus the effect of the low excess air may have been to trigger first the increased wall slagging, which itself leads to higher

557 gas temperatures, and the higher gas temperatures then become responsible for increased deposition in the convective region. An increase in excess air level has been claimed as a method for controlling slagging problems [9]. When control of NOx emissions is crucial, it may not be feasible to use excess air adjustments as a control method for slagging. Change in damper positions from normal to overfire air conditions does not increase ash deposition [48]. Changes in burner tilt angle modify asia deposition to some extent, as well as affecting the NOx concentration in the flue gases. Boiler load factor can influence deposition. Lignite which causes severe fouling under high load conditions produces much less fouling at lower loads. However, the load factor may also be an indicator of changes in llue gas temperature, which affects deposition significantly. Variations in soot-blowing practice have been claimed to affect deposition; however, this topic does not appear to have been the subject of systematic research. At constant load, deposit rate can be a function of burner position. The deposit rate is more a function of the burner position than of the overfire air condition. Changing damper positions from normal to overfire air conditions does not appear to increase fouling. Deposit rate thus does not appear to be a function of air distribution. The effects of operating parameters in full-scale boilers are generally related to the gas temperature in the secondary superheater region. The effect of the boiler parameters is illustrated in Figure 11.17 [91].

BOILER PARAMETERS

Major operating factors affecting fouling.

Possibly affecting fouling.

Effect on fouling potential as parameter decreases.

Flue gas temp.

Decreases significantly as temperature drops from 1150 ~ to 870 ~ C.

Plant load

Decreases significantly from 100% to 70% of load.

Excess air

Increases - slightly to significantly - as 02 drops below design level.

Pulverized coal size

Decreases as size changes from <40% to >70% passing 200 mesh screen (74 0.m).

Volumetric heat release

Decreases as volumetric heat release is reduced from normal to 60% of normal for non-fouling coal.

Figure 11.17. Boiler operating factors related to ash fouling for a plant burning 5% Na20 in ash coal, 70% -200 mesh grind, 426 MJ/m3.h volumetric heat release rate [91].

Any variable affecting burning rate of the lignite will also affect fouling. A coarse grind from the pulverizers will mean that larger lignite particles may continue burning as they are carried upward through the furnace, thus increasing I:x)th the temperature in the convection section and, as a consequence, deposition. Changes of burner tilt, excess air, and the air and fuel distribution could have similar effects. Canadian lignites show a deposition rate greater with coarse

558 pulverization and with increased gas velocity [140]. (ix) Sintering behavior o f ash. Measurements of the sintering behavior of the ash (as determined from inflections in a resistance vs. reciprocal absolute temperature plot) have shown that ashes from high-fouling North Dakota lignites have lower sinter points (625-800~

and

generally lower resistances (6x 103 to 105 ohms) [ 141 ]. Ashes from low-fouling Alabama lignites had higher sinter points (about 950~

and higher resistances (about 2 x 106ohms).

The sintering behavior of ashes from five North Dakota lignites--Blue and Red pit Gascoyne, Velva, Center, and Indian Head--as well as Choctaw (Alabama) lignite and Navajo (Arizona) subbituminous coal were measured for comparison to observed deposition behavior [142]. Measurements were made by two techniques--observation of changes in shrinkage or electrical resistance with temperature--on ashes prepared by the standard ASTM technique from -60 mesh coals. The results are shown in Table 11.6. By either method the sinter point is determined as a change in slope of the shrinkage (or resistance) vs. temperature curve. The deposition behavior is a qualitative observation of fouling tendency based on tests in the 34 kg/h pilot-scale combustor.

TABLE 11.6 Sintering temperatures correlated with ash deposition behavior for low-rank coal ashes [142].

Lignite Center Choctaw Gascoyne (Blue pit) Gascoyne (Red pit) Indian Head Navajo Velva

Sinter Point,*C Shrinkage Resistance 890 825 900 * 760 975 840 * 890 850 900 945 910 960

Ash Fouling Tendency High Low High Low High Low Low/Moderate

*No distinct break was observed in the resistance vs. temperature curve.

In neither method does it appear that any useful correlation exists between the observed sinter point and the fouling tendency; compare for example the shrinkage data for Navajo and Center coals. However, the shrinkage observed above the sinter point correlates reasonably well with fouling. At 950"C the ashes from both the high-fouling Gascoyne Blue pit, Center and Indian Head lignites showed a shrinkage of 25-27% from those observed at the sinter point. In comparison, at the same temperature the ashes from the low-fouling Velva, Choctaw, and Navajo coals showed shrinkages of only 0.5-2% of the sinter point shrinkage, and the value for Gascoyne Red pit lignite was about 7.5% [142]. It may be noteworthy that the sinter points measured by the shrinkage test tend to cluster around the melting point of sodium sulfate (884"C), the single exception being the Blue pit

559 Gascoyne lignite. In a reducing atmosphere, viscosities of slags from seven lignites and three coals of higher rank decrease, while surface tensions increase [143]. This results in an increased sintering potential of ash in a reducing environment relative to an oxidizing environment. Increased silica content increases the viscosity at a greater rate than it does the surface tension, thus reducing sintering [143]. On the other hand, increased sodium oxide content decreases viscosity at a greater rate than surface tension, increasing sintering behavior [143]. The observation of the effect of sodium may extend to total alkali content. In general, ashes that tend not to sinter or agglomerate form a melt phase having high viscosity relative to surface tension at a given temperature, while ashes that tend to agglomerate have lower viscosities relative to surface tension. (x) Prediction from pilot-scale testiug. The Predicted Ash Collection Efficiency (PACE) Index was developed originally for Australian brown coals. A regression analysis of total ash collected in a test furnace as a function of composition led to the equation PACE INDEX = 0.67 + (0.13 Na + 0.08 Ca) 9(100/Ash) [12] where the Na and Ca are expressed as percentage of the element (not oxide) on a dry coal basis. Generally, low-fouling coals have a PACE Index less than 2 while high-fouling coals have an Index greater than 3 [12]. In applying the Index to lignites, it should be remembered that the lower moisture of the lignites, relative to brown coals, could result in higher flame temperatures which might make the fouling experienced by lignites worse than might be anticipated from the calculated Index. In a 34 kg/h pulverized-coal-fired furnace, the relative fouling potential is assessed on the basis of the weight of deposit collected on the first probe bank, with a deposit weight below 150 grams indicating a low fouling potential, 150 to 300 g medium, and weights greater than 300 g a high fouling potential [91]. A regression equation calculated from results of 44 ash deposition tests is log (FDW) = 1.21 + 0.45 log(Ti) + 1.46 log(S) + 0.38 log(ash) + 1.14 log(Ca/S) + 0.63 log(alk) [144] where FDW is the fouling deposit weight expressed in grams per million Btu fired, Ti is the percentage of titanium dioxide in the sulfate-free ash, S is the percentage of sulfur in the dry coal, ash is the percentage of ash in the dry coal, Ca/S is the ratio of the percentages of calcium and sulfur in the dry coal, and alk is the alkali ratio, which is calculated from alk = [(ash) 9(Na20 + 0.659 K20)] / (CaO + MgO)

where again the ash is expressed as percentage of the dry coal and the molecular formulas represent the percentages of each compound in the sulfate-free ash. The error associated with the predicted FDW is 20-30% [144]. The largest errors occur for coals giving the highest deposit weights; the

560 magnitude of the error for such coals is unimportant since in any case a severe fouling problem will exist. This work indicates that coal having a low ash value will have a low fouling behavior, while coals of high ash value could exhibit severe lbuling, provided that there is sufficient adhesive material to agglomerate the del:x)sit [ 144]. The term including titanium is an indicator of the amount of detrital contribution to the ash [ 144]. 11.4.3 The mechanism of deposition (i) The sulfate layer. Ash deposition is initiated by formation of a layer rich in sodium sulfate on the boiler tubes. This layer is called the inner white layer. The layer forms completely around the tube. Its formation appears to proceed via a diffusional process [36]. Thermal decomposition of sodium carboxylates in the coal starts a sequence of reactions which lead ultimately to the formation of sodium sulfate in the flame or flue gas. Convective mass transfer diffusion of sodium-containing species through a boundary layer around the tube deposits sodium sulfate on the tube surface [36,125,145]. Deposition of the white layer occurs when the metal temperature was raised (using a test probe of which the temperature of the metal surface could be varied) from 870 ~ to 980~ [125]. The inner white layer contains most of the elements present in the coal ash but with sodium and sulfur predominating. X-ray diffraction has shown that the material is crystalline with the major phase being ct-NazSO4. Calcium sulfate and silica have been identified in the inner white layer. Generally the concentrations of calcium and magnesium are higher than in the original coal ash, while those of silicon and sodium are lower. The amount of water-soluble material in the inner white layer increases with increasing sodium content in the coal, ranging from about 7% for a lignite with 2% sodium oxide in the ash to 54% for a lignite with 9% sodium oxide in the ash. In some deposits the total amount of sulfate is greater than can be balanced by the alkali and alkaline earth elements, suggesting the presence of iron or aluminum sulfates. Condensed sodium sulfate tends to be most concentrated in small particles with low impaction efficiencies. Thus the concentration of sCx:lium in a deposit is not useful in indicating the importance of sodium in the deposition process [ 133]. Combustion of Beulah lignite in a drop-tube furnace provided no unequivocal evidence for "gluing" of the deposit by sulfate or sodiumcontaining species [ 139]. Particles adhering to a boiler steel substrate contained significant quantities of calcium and sulfur (probably as calcium sulfate) [51]. These particles were produced in the combustion of Center lignite in a laboratory-scale drop-tube furnace. Adhesive strengths were 3.0-3.7MPa [51]. Center lignite that had been ion-exchanged to remove the cations produced adherent calcium-sulfur particles in greatly reduced numbers compared to the untreated lignite. The particles having greatest adhesive strength had high concentrations of iron and silicon, probably as fayalite. (Fayalite has a relatively low melting point of 1200~

With the ion-exchanged lignite, the iron-silicon particles

made up a greater proportion of the adherent particles, but the total number of adherent particles of all compositions was less. In similar tests with San Miguel and Martin Lake (Texas) lignites, most

561 of the adherent particles of high adhesive strength were rich in iron and silicon [51]. The adhesive strengths of the iron-silicon particles were in the range 7.1-76 MPa. In comparison, iron-sulfur particles showed adhesive strengths in the range 2.2-4 MPa, and the calcium-sulfur particles, 3.0-3.7 MPa. Strengths in excess of 50 MPa are comparable to those of enamel coatings on metals, and 40 MPa is comparable to a strong bond between epoxy resin and steel [146]. An adhesion strength of 0.1 kPa is adequate to support a 10 mm lightly sintered ash deposit of 1000 kg/m3 density [147]. The adhesive layer formed from combustion of a Texas lignite was very calcium-rich with abundant sulfur and some iron-rich particles [137]. The presence of a melt phase of calcium-magnesium-sodium sulfates on deposits is consistent with role for calcium sulfate in fouling at temperatures near 927~ [ 133]. X-ray diffraction analyses of white layers of deposits produced from combustion of various lignites in a pilot-scale combustor are shown in Table 11.7 [148]. TABLE 11.7 X-ray diffraction analyses of white layers from lignite ash deposits [148]. Lignite Baukol-Noonan Beulah Gascoyne

Major Phases Minor Ph~tses Trace Phases CaSO4 MgO Fe304 CaSO4 Fe203 SiO,,_ CaSO4

Iron and aluminum may assist in fomling a low-melting material that would allows sticking of ash particles. Thus ash deposits from Beulah lignite (produced in a drop-tube furnace) contained 1-3 ~m particles of calcium aluminosilicates enriched in iron, as well as sodium and magnesium [139]. Iron or aluminum sulfate can lower the melting point of alkali sulfate mixtures by several hundred degrees. As the inner white layer builds up, it acts as a porous mat into which fly ash particles striking the tube can sink. As the relatively hot fly ash particles sink into the inner white layer, they may partially melt the sodium sulfate or mixed metal sulfates, thereby improving bonding or sticking. Bench-scale studies of ash volatilization from the Red, Blue and White pits of the Gascoyne mine and a low-sodium Beulah lignite were conducted at heating rates of 25~ [149]. A water-cooled platinum disk 7.5 cm above the crucible was used to collect the vaporized material. The White and Blue pit Gascoyne lignites produced substantial deposits of potassium and sodium sulfates having the appearance of condensed droplets. These droplets were absent from the immediate vicinity of aluminosilicate particles, suggesting that the sulfates had been incorporated into the aluminosilicates. Reports in the literature differ considerably about the adhesion of the inner white layer to the tube surface, the layer being variously described as "tightly attached" and "loosely attached and

562 ... easily removed with a toothbrush." The gas and tube temperatures would determine whether the sodium sulfate condenses as a liquid or solid. Thickness is dependent on temperature. On a test probe for which the metal temperature varied over the range 425 ~ to 650~

the inner white layer

reduced in thickness with increasing temperature. In other work, the white layer "disappeared" (which evidently means that it did not form in the first place) when the tube temperature was held at 815~

Once the bulk of the deposit forms, the inner white layer cannot be identified on the

upstream side of the tube under the deposit. Formation of the inner white layer is important for the initiation of fouling but that it subsequently has little effect on the build-up of the deposit. During combustion, sodium present as carboxylates could react to form a number of sodium-containing compounds, depending on the temperature and other species available in the diffusion environment [52]. Above 1400 K vapor-phase sodium will occur either as sodium metal or sodium hydroxide [150]. Vapor-phase sodium that remains in the flue gas below 1400 K will condense on ash or boiler surfaces and can then react with sulfur species to form sodium sulfate [150]. Calculations based on a pilot-scale combustor with a gas temperature of 1093 ~ entering the convection section indicate a diffusivity of sodium sulfate of 9.55 x 10-5 m2/sec, a Sherwood number of 22.1 (assuming a Schmidt number of 1), and a mass transfer coefficient of 5.0 x 10-2 m/sec [ 151]. These values are very close to data reported for the deposition of material in oil-fired boilers [152]. Thus it was assumed that the formation of the inner white layer proceeded via a similar physical process, namely convective mass transfer diffusion of sodium-containing species through a boundary layer on the probe. (ii) Tile inner sinter layer. The inner sinter layer is characterized by discrete ash particles bonded to each other or to the tube via fairly weak bonds. There is no distinguishable melt phase. Deposition of the sulfate layer is followed by accumulation of a layer of ash particles which builds up via inertial impaction [151]. Some particles may be molten or partially molten when they strike the tube [145]. A significant increase in adhesion efficiency occurs as temperatures in the combustion system are increased [ 153]. If these particles do not freeze immediately upon impact, they provide a sticky surface for the subsequent trapping of additional particles, regardless of whether the newly arrived particles are molten themselves. Surface stickiness of the ash particles provides enough particle-to-particle adhesion to allow the inner sinter layer to form and grow. The inner sinter layer acts as an insulator, so that it takes longer and longer for the growing upstream surface to cool by conduction to the tube; eventually the inner sinter layer becomes a good enough insulator so that its upstream surface stays hot enough for a melt phase to form. Accumulation of particles gradually thickens the inner sinter layer until a point is reached at which heat loss to the tube is sufficiently slow to allow reactions with gas-phase sodium-containing species to occur. At that point a glassy matrix begins to form. This matrix material, and the ash particles subsequently trapped in it, constitutes the outer sinter layer, the bulk of the deposit. Several mechanisms have been suggested for transport of the particles to the surface, including thermal diffusion caused by the large temperature gradient between the tube and the bulk of the flue gas, and Brownian particle motion in the tx~undary layer. Larger-than-average or denser-

563 than-average particles may preferentially impact the surface. The smaller-than-expected deposits produced during combustion of micronized lignite have been attributed in part to the ash particles being much smaller than normal and thus better able to follow the flow streamlines around the tube. Particle viscosity dominates the factors affecting inertial impaction [154]. A critical viscosity for adhesion is in the range 105-108 Pa.s; below the critical viscosity, particles will adhere to a substrate, but will not do so above the critical viscosity [154]. The calculated critical viscosity for deposition of San Miguel lignite is 107 Pa.s [154]. A simplified physical picture of the events lk:~llowingparticle impaction is that the particles deposit in sublayers in a fashion in which the first particles to arrive cover the available surface of the probe, forming a sublayer which is then itself covered by a second sublayer of particles, and so on. It is likely that the particles in the first sublayer will cool very quickly because they are in contact with the relatively cold tube. Particles in the second sublayer will not cool quite as rapidly, because the conductive heat transfer to the probe must now proceed through the intervening first sublayer. With each successive accumulated sublayer, the particles in that sublayer will cool less and less rapidly because of the increasing insulating effect of the previously deposited sublayers. At some point the particles in the "n-th" sublayer will cool so slowly that time is available for crystallization or reactions with gas-phase sodium species to begin the fomaation of matrix-- i.e., to begin the formation of the outer sinter layer. If it is assumed that the thermal conductivity of the ash particles is independent of composition (or, in essence, is not coal-specific) and that the physical packing of particles in the various sublayers is also independent of the specific coal being burned, then the pathways for conductive heat transfer from the accumulating ash particles to the tube will be the same for all coals. This deduction leads further to the notion that the inner sinter layer would therefore be the same thickness regardless of the coal being burned (for a constant configuration of the combustor and temperatures at the inlet to the convection pass). Indeed six samples of deposits from a pilot-scale combustor showed very similar thicknesses of the inner sinter layer [151]. Measurements by two independent observers showed average thicknesses of 0.94 and 1.07 mm with relative standard deviations of 5.83 and 13.5%, respectively. Little work has been done on characterizing the inner sinter layer. Individual particles of silica, iron oxide, aluminum silicate, magnesium aluminum silicates, and calcium aluminum silicates have been identified. Virtually all particles have coatings of stx:lium sulfate; some particles have coatings of calcium sulfate. Detailed examination of the inner sinter layer of a deposit produced from Beulah lignite showed the following constituents, listed in approximate order of abundance: 1) calcium-sodium-l:x~tassium aluminosilicate glass spheres containing minor amounts of unfused clay minerals, quartz and other minerals; 2) quartz groins of various sizes; 3) anhydrite occurring both as a matrix and as a coating of spheres; 4) spheres of hematite and iron oxide glass; 5) sulfates of magnesiurn, aluminum and sodium; and 6) calcium- and magnesium-rich spheres of sintered carbonates [ 155]. X-ray diffraction analyses of the sintered layer from deposits produced from three lignites in a pilot-scale combustor are shown in Table 11.8 [ 148].

564 TABLE 11.8 X-ray diffraction analyses of sintered layers from lignite ash deposits [ 148]. Lignite Baukol-Noonan Beulah

Gascoyne

Major Phases Minor Phases CazAI2SiO7 Si02 CaSO4 Ca2AI2SiO7 SiO2 CaSO4 Fe203 (Na,Ca)8(Si,AI) 12024(SO4)2 Fe304 CaMgSi206 SiO2 Ca2AlzSiO7

Trace Phases

CaSO4

(iii) The outer sinter layer. For low-sodium, low-fouling lignites the entire deposit consists essentially of the inner sinter layer and the inner white layer. However, for the large, troublesome deposits by far the bulk of the deposit consists of an outer sinter layer. The outer sinter layer is a coarse, brownish, strongly integrated deposit. It consists of semicontinuous fused skeletal material (the melt phase, often referred to as matrix material) with seemingly unreacted ash particles embedded in and bonded to the matrix. In contrast to the sulfate layer, which forms all around the tube, the sinter layers form only on the side of the tube facing the gas stream. The outer sinter layer sometimes has a glassy appearance, from which it was inferred that the matrix material was probably amorphous. However, X-ray diffraction has shown that the matrix is largely crystalline. The crystallinity likely contributes to the strength of the deposit. Virtually all of the elements in ash are also found in the outer sinter layer. The dominant elements affecting formation of the outer sinter layer matrix are calcium, sulfur, iron, and sodium [58]. For example, decomposition of calcium sulfate (e.g., as gypsum in the lignite) occurs around 1200~ the CaO and SO 3 are able to recombine at the growing surface of the deposit. Magnesium, aluminum, iron, and silicon are usually enriched compared to their concentrations in the original coal ash, while sodium and sulfur are depleted. Pilot-scale tests of Bienfait (Saskatchewan) lignite showed a selective deposition of ash components to produce a eutectic composition enriched in silicon, iron and sulfur, and depleted in calcium and magnesium relative to the lignite ash [ 156]. These deposits also were rich in water-soluble alkali sulfates. The ranking of elemental compositions is Ca >> A1,Si > Mg,Fe > Na,S. Calcium, magnesium and silicon are evenly distributed throughout the deposit, as are sodium and sulfur. Iron is present in discrete particles, sometimes seen as small balls of iron oxide. In some deposits these balls are surrounded by a ring in which sodium is at a much higher concentration than in the bulk of the deposit. Examination of fragments of outer sinter layer material shows that there are high concentrations of sodium and sulfur on the surface, with calcium, aluminum, and silicon concentrations higher in the core. Calcium sulfate is also occasionally seen as a crystalline coating, possibly as a result of CaO and

565 SO3 (initially produced by thermal decomposition of gypsum in the lignite) recombination at the growing surface of the deposit. If vapor-phase sodium encounters a molten silicate or aluminosilicate grain, it will most likely react with the grain to form a variety of mineral species or glasses upon cooling [ 157]. If these particles collide with the ash deposit they may become part of the silicate-aluminosilicate matrix that binds the deposit. Sodium bound in clay or tectosilicate minerals will mainly be retained in the particle, although new minerals may be fomaed in reactions with the flue gas or by structural reordering Ul:X-mcooling from a molten state [ 158]. Several mechanisms transfer ash constituents from the vapor phase to the tube surface, including vapor phase diffusion, thermal diffusion, electrostatic interaction, and inertial impaction [48]. Inertial impaction is the principal mechanism, as suggested by the observation that deposits build up on the upstream face of the tube. Accumulation of the outer sinter layer will be affected by the rate of particle impaction and the retention of particles in the sticky layer. The efficiency of particle retention is influenced by the size, geometry, and melting behavior of the particles [48]. The inertial forces favoring impaction of an ash particle into the deposit are counteracted to some extent by fluid dynamic forces which keep ash particles moving with the flow streamlines around the tube. Of five coals of various ranks, a North Dakota lignite had the highest ash content and the highest sticking coefficient, but there was no general correlation of rank with sticking coefficient [159]. For self-regulated capture of super-micron ash to be enhanced by liquids, the inertial impaction rate depends on the steady-state sticking coefficient, s, but s in turn depends on the amount and physical properties of the liquid phase "glue" available to each particle in the surface layer of the deposit [ 159]. Values of s for steady-state deposition and of the particle deposition rate must be obtained from a set of coupled nonlinear equations. Iron, silicon, calcium, and especially aluminum all act to suppress "glue" formation. Predicted sticking values can be obtained by calculating the fraction of liquid phase by thermodynamic analysis and using the amount of glue as an input to the coupled nonlinear equations R-~rthe self-regulated capture model. The growth rates of ash deposits for North Dakota lignites in a drop-tube furnace are shown in Figures 11.18 through 11.21 [51]. The distinction between the Indian Head and Beulah on the one hand and Center and Velva on the other may arise from the higher concentration of sodium in the first pair. The sodium may form small particles more likely to follow the flow streamlines and thus less likely to be captured by the deposit. The accelerated growth rate of the White pit Gascoyne lignite is a result of the formation of abundant liquid at the top of the growing deposit. This liquid would create a captive surface that would retain most of the ash particles impacting it. The dependence of sticking fraction on time is illustrated in Figure 11.22 for San Miguel and Martin Lake lignites [51]. The San Miguel lignite has the higher sticking fraction. Zeolite minerals in San Miguel lignite contain significant amounts of sodium [160], which can reduce the melting temperature and enhance formation of a very sticky captive surface. In the Martin Lake lignite virtually 'all of the alkali and alkaline earth elements are associated with the carboxyl groups

566

.i,-I

9 Beulah

[~ 0.1-

9 Velva

or

A Indian Head A// 0.01 0

o

''''l''''l''''l''''l''''l''''l 5

10

15

20

25

Center

30

Time, minutes

Figure 11.18. Deposit weight in bench-scale drop-tube furnace combustion of North Dakota lignites as a function of time [51].

1

9 Beulah

0.9-

9 Velva

0.8a~

.r

0.7-

A Indian Head

0.6o ~

0.5

g

0.4

I~ 9

o.3

o

Center

.

i

0.2 0.1 0 0

''''1''''1''''1''''1''''1'''' 5

10

15

20

25

30

Time, minutes Figure 11.19. Sticking coefficients as a function of time for bench-scale drop-tube furnace combustion of North Dakota lignites [51]. Sticking coefficient is a ratio of deposition rate to rate of firing of ASTM ash.

567

,

Red pit 9

0.1-

White pit

A Blue pit

o

o

0.01

....

0

I''''1

5

....

10

I ....

15

I ....

20

Yellow pit

I ....

25

30

Time, minutes Figure 11.20. Deposit weight as a function of time in bench-scale drop-tube furnace combustion of Gascoyne lignites from different pits in the same mine [51].

0.9 ,

0.8 ~

Red pit 9 White pit

0.7

.r,,l

~

0.6"

o

0.5

A Blue pit o

.~ 0.4 ~9

~'1

Yellow pit

0.3" -

0.2 0.1

"

o

''''

0

I''''

5

I''''

I''''

lO 15 Time, minutes

I''''

20

25

Figure 11.21. Sticking coefficient as a function of time for bench-scale drop-tube furnace combustion of Gascoyne lignite from different pits [51].

568 1

.

0.9" a~

0.8

9 Martin Lake

0.7

9 San Miguel

..=.1

0.6 o

0.5"

.~ 0.4 ~9

o.3

0.2 II,'"

0.1-

A

0

'

0

' '

I"'

2

-

''~'

4

v

''

I'

6

''

I'

8

''

I'''

10

Time, minutes Figure 11.22. Sticking coefficient as a function of time for bench-scale combustion of Texas lignites [51].

rather than with aluminosilicate minerals. The factors that may affect the cohesion in the deposit include van der Waals interactions, liquid film effects, retention in a liquid matrix, or even trapping of particles by geometric effects, such as might be hypothesized for an ash particle being trapped in a mat of whiskery particles [48]. The factors which will determine whether a partial melt phase will form in the deposit include the fusion temperatures of the ash particles, the flue gas temperature, the boiler tube temperature, and the thickness of whatever ash layer has already been deposited. Once a melt phase has been formed, liquid-phase diffusion may also affect deposit growth [48,161]. The outer sinter layer is characterized by the presence of matrix material, which forms an effective, strong bond holding this layer together. Formation of the outer sinter layer depends on the temperature of the deposit surface becoming high enough either to allow the molten material striking the surface to CD'stallize slowly or to allow ash particles to react with some fluxing material in the gas phase (possibly sodium compounds) to form a new low-melting phase which can act as the matrix. The surface temperature rises because of the insulating effect of the inner sinter layer and interior regions of the outer sinter layer. The matrix consists of complex aluminosilicates, of which sodium melilite is a particularly important constituent [162]. Sodium melilite is a lowmelting material which could stay fluid long enough to react with other silicates, aluminosilicates, or sulfates and which could crystallize into a hard, strong deposit. The reaction anhydrite + quartz + AI,Fe,Na aluminosilicate glass --, melilite + Ca,Na,Mg,AI sulfate

569 has been postulated as a mechanism for the formation of melilite in the deposits [ 155]. Reaction of volatilized sodium with kaolinite particles can form sodium aluminosilicates melting in the range 90(O1100~

These compounds can react with molten sulfates in the accumulating ash deposit to

form complex melilites of the general formula (Na,Ca,K)2[Mg,Fe+2,Fe+3,A1,Si)307] [49]. The important role of sodium in these instances suggests a rough correlation: the more sodium in the coal, the more melilite which could form; the more melilite, the more matrix; and the more matrix, the worse the fouling problem. This is the chemical basis for the rule-of-thumb correlation between the amount of sodium in the coal and the severity of the fouling. Fly ash from Beulah lignite was heated to 90(O1000~ for 10 minutes, and then cooled to 5000C over a period of two hours [151] to approximate exposure of ash particles to the gas temperature entering the convection section of a pilot-scale combustor with subsequent cooling to the tube temperature. A weight loss of 1-2% was observed, in good agreement with the calculated weight of a 0.3 ~tm layer of sodium sulfate coating a 50 tan ash particle. The fly ash before heating was essentially amorphous, although traces of magnetite, magnesioferrite, and (Na0.8 Ca0.1)SO4 were observed. After heating, X-ray diffraction indicated the presence of melilite (gehlenite or sodium melilite), monticellite, and (Na0.8 Ca0.1)SO4. In igneous rock petrology, both melilite and monticellite are known to form in situations where high temperatures prevailed for a short time only, and cooling was relatively rapid [163]. Other crystalline phases which have been identified are pyroxene and plagioclase [164]. Crystallization of melilite, pyroxene, and plagioclase is most pronounced when the ash is rich in sodium and calcium. Crystallization is accompanied by formation of a residual liquid phase which may be the agent responsible for growth of the deposit and enhancement of deposit strength. If the residual liquid eventually solidifies into a glassy phase, then the relative amounts of crystalline and glassy material that form in a deposit may be important in determining deposit growth and strength [164]. Deposits collected on a test probe inserted into a boiler at the Hoot Lake station showed compositions of the matrix consistent with a mixture of sodium melilite, quartz, and a sodium aluminosilicate hypothesized to be NaAISi 206 [ 151]. Sulfur species were virtually non-existent in the matrix of these samples. The absence of sulfur species may be a result of the concentration being below detection limits or, in the case of X-ray diffraction, of the material being noncrystalline. X-ray diffraction is capable of identifying both sodium and calcium sulfate in a mixture of 2% of each of these salts with fly ash [151]. Crystalline melilite is the major strength component of the deposit [ 151]. The relative amount of melilite increases with increasing distance from the tube. Increased distance from the tube provides a longer time for cooling and therefore increased time for crystallization and for reactions of sodium with ash material to form melilite. Quartz particles can be quite large in lignite ash deposits [ 137]. Assimilation of quartz into a melt phase might be retarded because of slow reaction rates and, possibly, liquid-phase immiscibility. The slow assimilation of quartz results in the melt phase being more basic than would be predicted from bulk composition; in turn, the basicity of the melt enables crystallization

570 of phases such as melilite [ 137]. The gradual assimilation of quartz into the melt eventually allows the composition of the melt to shift toward a more acidic composition, and the major phase to be crystallizing becomes anorthite [ 137]. Quartz and clays are easily able to form glass at high temperatures. In the glass structures the SiO4-4 tetrahedra are joined by linkage of three corners. However, alkali or alkaline earth oxide addition provides a source of oxide ions that modify the glass structure because of the increased O/Si ratio. Disruption of the extensively linked glass structure by the added oxide ions (referred to in this context as "network modifiers") leads to crystallization of some of the components [165]. This argument is analogous to the role of oxide ions as "polymer breakers" in affecting the viscosity of slags. In the early 1970's the theory was advanced that organically bound cations react with finely disseminated clays or silica to form a type of ash particle called "matrix parent." The matrix parent particles deposited on the tube can then be fluxed with sodium to soften, flow, and collect and cement other ash particles to form the matrix. The flux was postulated to be sodium sulfate depositing from the vapor. Successful formation of matrix requires high temperature and long times to allow reactions to occur. According to this theory, the key variables affecting ash deposition would be the amount of sodium, the amount of matrix parent, and the melting properties of the reaction products. In recent years several pieces of evidence have been collected which relate closely to the original matrix parent theory. First, the plateau in the fouling

vs.

sodium content

curves can be shown to arise from straightforward stoichiometry. Above about 8% sodium oxide in the ash all of the available aluminum and silicon have been converted to sodium melilite, representing therefore the maximum amount of sodium melilite which can form. Since the relative amounts of sodium, calcium, aluminum, and silicon vary from coal to coal the point at which the curve levels off will vary also, in the range of about 6 to 10% sodium oxide. Second, studies with mixtures of kaolinite and sodium and calcium acetates (Chapter 6) suggest that an important reaction early in the combustion process is the reaction of organically bound alkali and alkaline earth elements with clay to form new phases. Analogous prcxzesses in lignite would give rise to the matrix parent. Third, tests with micronized coal have shown much less ash deposition than might be predicted for the same coal at a standard utility grind. A possible explanation is that the micronizing has removed more of the silica and clay particles from juxtaposition with the organically bound species, and thus there is less opportunity for the formation of matrix parent. X-ray diffraction indicated gehlenite, sodium melilite, and akermanite as principal phases constituting the matrix material. The amount of sodium melilite increases with increasing distance from the tube. X-ray diffraction of a deposit produced from Beulah lignite in a pilot-scale combustor showed a decrease or disappearance of quartz, anhydrite, and hematite in comparing the inner sinter layer with inner and outer zones of the outer sintcr layer [ 155]. (The inner zone of the outer sinter layer is transitional between the inner sinter layer and the outer layer of the outer sinter layer and contains a mixture of the constituents of those two layers.) In this same transition, from inner sinter layer to inner and then outer zones of the outer sinter layer, magnetite and melilite

$71 increased. In terms of bulk composition, silicon, aluminum, and iron show decreases in composition whereas calcium, magnesium, sodium and sulfur increase in an outward direction through the deposit [1551]. An iron-rich zone has been observed as the outer transition from the inner to outer zones of the outer sinter layer; the iron-rich zone shows a change in the oxidation state of iron [155]. Electron microprobe analysis of deposits from six North Dakota lignites showed that high concentrations of iron may be necessary for the formation of strong deposits, unless the sodium content is also extremely high [ 145]. Deposits produced from Beulah, Decker (Montana), Velva, and a Texas lignite in a pilot scale combustor had, as dominant phases,wollastonite, gehlenite, and anorthite [166]. Occasional regions of high calcium and sulfur (presumably anhydrite) were observed, as were particles of quartz and particles having high iron contents. The Texas lignite deposit showed substantial numbers of ash "bubbles" in the deposit, which could have arisen either from cenosphere formation in the fly ash or fusion of clusters of particles forming a pocket of entrapped gas. Cenospheres were also observed in the deposit from the Velva lignite. Electron microscopy of deposits from an unidentified North Dakota lignite showed the primary components to be calcitnn aluminosilicates (55~3%), along with small amounts of alkali sulfates mixed with calcium sulfates [ 167]. Other components identified included calcium- and ironrich aluminosilicates, silica, hematite, a calciunl iron femte, and calcium magnesium sulfate [167]. Further analysis by potassium K-edge X-ray absorption spectroscopy revealed characteristic X-ray absorption near-edge structures (XANES) useful for identifying the sulfate species. XANES of a secondary superheater detx~sit from a boiler was very similar to that of potassium hydrogen sulfate [167]. A deposit formed at 1090~ in a test combustor had a XANES spectrum typical of a 60:40 mixture of potassium sulfate and a potassium alurninosilicate glass [ 167]. Deposits formed from a Texas lignite showed a matrix rich in calcium aluminosilicates which appeared to have formed as a result of reactions of calcium with aluminosilicate minerals [137]. The most common crystalline phases in the matrix are melilite, Ca2(AI,Mg,Fe,Si)2SiO7, and anorthite, CaAI2Si2C~. The melilite occurs in the less agglomerated areas of the matrix, while the anorthite occurs in the areas of greater agglomeration. As indicated above, quartz particle assimilation into a melt phase may be slowed by unfavorable reaction kinetics or by liquid-liquid immiscibility [ 137]. Deposits at the entrance or front of the convective pass are enriched in iron [125,161]. Ash deposited further away from the furnace enclosure had a diminished iron content, and ash collected at 430~

had less than half the iron content of the original coal ash. This segregation may be an

effect of particle density. Ash particles containing iron are among the most dense particles, and are more likely to impinge on furnace walls or tubes rather than to follow the flow streamlines. Furthermore, iron-rich ash particles are among the most adhesive and are thus more likely than alkali aluminosilicates to adhere to surfaces once they do impinge.

572 11.4.4 Growth of strength in deposits The strength of the deposits is variable and is a function of the composition, temperature, and age of the deposit [51]. The strength of sintcred ash specimens increases with corresponding increases in sintering time at any given temperature. Coals with high sodium levels and low ash values may produce small deposits but ones which are strong, while conversely low-sodium, highash coals may produce large but weak deposits which are easily removed. Time, composition, and temperature can all influence the crystallinity of the deposit; the crystallinity may in turn contribute to strength. The sintering strength of lignite ash is proportional to its sodium content [32]. This relationship is illustrated in Figure 11.23 [32]. There is also a direct relationship between the sinter strength of ash and the temperature [32]. For short periods of time (i.e., under 25 hours) the sinter strength does not change with time. However, lignite ashes increase in strength with longer periods of heating. Consequently the older (and thus nearer the tube) portions of a deposit on a superheater tube may be very hard while the leading edge is soft. 20

it,

16 14

d ~

12 1!

5-1

2 0

~ ....

I'"'l'"'i

....

I'"'l'"'l'"'

% Sodium oxide in ash Figure 11.23. Effect of sodium oxide content on the sintering strength of a North Dakota lignite ash [32].

Deposits from high-sodium lignites have a continuous melt phase that provides the material for a strongly bonded network in the deposit. In comparison, deposits from low-sodium lignites have no continuous melt phase, and seem to be held together only by particle-to-particle interactions. Deposit strength, expressed in units of MPa/mm, is shown as a function of sodium content (as Na20 in the ash) in Figure 11.24 [51]. The curve appears to level off around 8.8%

573 10

I

4,-'

0.1

. . . .

0

I

5

. . . .

I

10

'

'

'

'

I

....

15

20

Sodium oxide in ash, %

Figure 11.24. Ratio of deposit strength to height as a function of sodium oxide content for bench-scale droptube furnace combustion of various lignites [51].

Na20. When ash deposition ( e.g., weight of ash collected on probes in a pilot scale combustor) is plotted against sodium, that curve also tends to level off in the region of 8.0-8.5% Na20 [161]. The similarity suggests a correlation between the growth of strength in a deposit and the ability of the deposit to form a large mass on a boiler tube. For ash deposits produced in a laboratory-scale drop tube furnace, strength varies exponentially with the height at which the strength was measured [168]. This relationship had the form S = a exp(bH) where S is the strength and H the height. The values of a and b differ for different lignites, but for an 3, given lignite, the coefficient of determination, r2, for the exponential fit varied from 0.94 to 0.99. For the coefficient a, a negative Spearman rank correlation was found with calcium and aluminum and a positive correlation with sodium. For coefficient b, the same elements contributed to significant Spcarman rank correlations, but in the opposite sense, i.e., positive correlation with calciuln and aluminum and a negative correlation with sodium. The results confirm observations that calcium and sodiurn act as antagonists rather than acting together, and indicate that a relationship bctwecn gehlenite, CazAIzSiO7, and sodium melilite, NaCaAISi207, may be important in determining both the initial strength and the growth of strength of the deposits. An empirical characteristic of deposits, the relative rcfractory nature R, was defined as the mole fraction of SiO2 in the deposit to the mole fraction of SIC)-,_in the original ash of the lignite [168]. For deposits from differcnt lignites, as R increases the deposit is increasingly refractory relative to the ash from which it formed. For ash deposits formed in a drop-tube furnace, an

574 increase in R accompanies visual evidence that the top of the deposit has experienced less fusion, evidence for sintering in the lower portion of the deposit occurs further and further toward the base, and the base layer becomes thinner. In addition, when the strengths of a series of deposits are measured at a comparable height from the base, the strength decreases as R increases. For example, deposits produced from Indian Head and Velva lignites for which R = 1.00 and 1.67, respectively, showed that the strength measured 13 mm above the base was 7 MPa for the deposit from Indian Head but only 4.6 MPa for the Velva deposit [168]. In drop-tube furnace tests, the strongest deposits contained massive fused regions near the top, with abundant crystal growth [51]. Weak deposits are characterized by high porosity, suggestive of less extensive sintering or fusing of the deposit. With increased sintering, crystallization of aluminosilicates such as pyroxenes, melilites, and plagioclase from the melt begins. These aluminosilicates appeared to nucleate from regions of high iron content. Sodium was concentrated in the glass adjacent to crystals, suggesting that the development of deposit strength is due to viscous flow sintering and not, as suggested in earlier work [ 161] due to the formation of crystalline sodium melilite. Development of deposit strength being due to viscous flow sintering is consistent with the action of availability of alkali or alkaline earth elements to act as fluxes for aluminosilicates, and with the importance of gas temperature in the region of deposition as the primary factors affecting formation of strong deposits [51]. The degree of interaction between the consitiuents in a deposit is expressed to some extent by the formation of crystalline material. In drop-tube furnace tests some of the strongest deposits contained abundant pyroxene and melilite crystals [51]. In the strong deposits, sodium accumulated in the glass phase. The glass is primarily responsible for viscous flow sintering and thus for the development of deposit strength. The presence of sodium in the glass reduces its viscosity, improving the ability of the liquid to increase deposit strength via sintering. When strength development is due to viscous flow sintering, the sintering rate can be determined by Sr = kl(Rg/c)(1/V) [169] where Rg/c is the ratio of glass to crystalline material and V is the viscosity of the glass. Deposit strength is shown as a function of glass viscosity in Figure 11.25 [51]. Here the composition of the glass was determined by X-ray microprobe analysis and the viscosity then calculated from composition via the modified Urbain equation [170]. The inverse relationship of deposit strength and glass viscosity is consistent with viscous flow sintering theory. Sintering or melting of a deposit that contains abundant alkali or alkaline earth elements can produce highly crystalline deposits [164] containing melilite, plagioclase, and pyroxene. Crystallization is most pronounced for sodium- and calcium-rich deposits, but is accompanied by partitioning between the glassy and crystalline phases. For example, with deposits produced from Gascoyne lignite, sodium tends to concentrate in the glass phase [ 164]. As crystallization occurs, the residual liquid is responsible for the growth of size and of strength in the deposit.

575

7-

\

.

6-

4-

~9

3

o

2

'

'

'

I

200

'

'

'

I

'

'

'

400

I

600

'

'

'

800

Viscosity, Pa.s Figure 11.25. Strength of deposits produced in bench-scale drop-tube furnace combustion of low-rank coals as a function of the viscosity of the glass phase at 1300~ [51].

The quantity of liquid expected to exist at a given temperature can be predicted from thermodynamic calculations that minimize the free energy of the systern (e.g., [171]). Deposit strength plotted as a function of the predicted quantity of liquid is shown in Figure 11.26 [51]. As presumed from viscous flow sintering theory, deposit strength increases with increasing quantity of liquid.The formation of crystals could raise the viscosity of the residual liquid and retard the sintering rate. Additives that promote crystallization thus reduce the strength of the deposit.. The physical strength of the deposits increases as a result of continued exposure to sulfur oxides at high temperatures, a process known as sulfating [49]. The amount of anhydrite present was greater than the amount of melilites in sulfated deposits obtained from the Hoot Lake station [49]. 11.4.5 Remedial measures for combatting fouling and slagging The basic approaches for controlling fouling are conservative design of furnace height and area, to allow ample time for burnout thereby to minimize the furnace exit temperature; installation of an adequate number of sootblowers at spots likely to be troublesorne (and in fact to allow the ability for repositioning some of the sootblowers if necessary); to use fuel additives containing calcium or magnesium to reduce the fluxing ability of any molten ash phases, or containing aluminum to form high-melting point materials; and to limit the sodium content of the lignite by selective mining, blending, or ion-exchange [91. (i) Boiler design. The methods suggested for control of fouling include boiler design, sootblower placement, restricting the sodium level in the lignite, and using additives. For new

576

7-.

6"

9

3-

o =

1 ''''

0

0

I''''

0.1

I''''

0.2

I''''

0.3

I''''

0.4

0.5

Liquid-to-solid ratio

Figure 11.26. Strength of deposits produced in benchscale drop-tube furnace combustion of low-rank coals as a function of the predicted quantity of liquid in deposits [51].

construction, the most economically attractive option to combat fouling is modification of the boiler design to accommodate the high-fouling fuel [ 123]. The objective of conservative design is to limit the volumetric heat release rate to 250 MJ/h m3, about half the value which can be tolerated for a low-fouling fuel [9]. Further design aspects include ample spacing between the burners and between the tubes in the convection section, steeply sloping floors at the base of the convection section (to handle dislodged slag or ash deposits falling through the furnace), and large numbers of sootblowers. An example of conservative design practices is provided by the Antelope Valley station, which includes a furnace volume exceeding 2 lm3/MW; furnace exit gas temperature of 575~

minimum of 20 wall blowers and 23 soot blowers per 100 MW capacity; and tube spacings

of 60 cm in the secondary superheater, 23 cm in the reheater, 11 cm in the primary superheater, and 10 cm in the economizer [9]. Successful firing of high-fouling lignites eventually derives from conservative boiler design, particularly in the heat release rate [97]. The strategy is to produce very conservative designs which, in a sense, derate the boiler before it is built. A large height and large furnace volume provides a low volumetric heat release rate to give ample time for burnout of suspended particles and to have a low furnace exit gas temperature. For burning a high-fouling lignite, a conservative value of the volumetric heat release rate would be 268 MJ/m3 h [48]. This value will give a furnace exit gas temperature of about 1035~

In addition, wide spacing of tubes in the

convection section, steeply sloping floors under the superheaters (to help shed deposits into the main furnace) and ample provision for sootblowers can be incorporated into the boiler design.

577 While these design procedures can increase reliability and operability for combustion of high fouling lignites, they add about 15% to the capital cost of the boiler [48]. For new construction, boiler modification is economically attractive, since it provides cost savings in purchased replacement power during outages, and in operations and maintenance costs associated with additive injection systems or sootblowers. Recommended design practice for a new boiler burning a high-sodium North Dakota lignite would be the following: heat release at full load of 850 MJ/m2; plan heat release in the burner zone of 14.8-15.9 GJ/m2; gas velocities of 9 m/s in the platen superheater and 18 m/s in the convection reheater; 60 cm spacing in the superheater platen and 30 cm spacing elsewhere; and a division wall and upper furnace division panels in the furnace [123]. For high-fouling lignites, pulverized-coal firing appears preferable to cyclone firing. A good comparison is provided by Units 1 and 2 at the Leland Olds station. With both units burning the identical lignite, Unit 1 (pulverized-coal fired), has a smaller percentage loss and uses only steam sootblowing, whereas the cyclone-fired Unit 2 requires water sootblowers [97]. In addition, Leland Olds Unit 1 experiences less fouling than Big Stone, even though the Big Stone plant burns a lignite of lower sodium content [97]. (ii) Boiler load. Boiler load has a large effect on fouling. The most common practice to reduce problems associated with ash fouling is reduction of load on the boiler [ 123]. In severe cases, the unit must be derated or even shut down completely. It is preferable to reduce the nameplate rating of the boiler by a predetermined amount to assure continued reliable operation than to endure unplanned outages resulting from severe fouling. During an unscheduled outage, the cost of replacement power is at a premium, whereas with scheduled curtailments the replacement power can be negotiated in advance at a less-than-premium cost. A conservative estimate for deration to insure boiler availability and freedom from unscheduled outages is 23% [ 123]. Load reduction is a costly alternative to combatting fouling. However, since load reduction is preferable to paying a premium for replacement power during unscheduled outages, load reduction is the recommended alternative unless more sophisticated approaches can be taken. (iii) Fuel additives. Additive injection is also an economically attractive approach to combatting fouling [ 123]. For existing power plants, a combination of sootblowing and additive injection is the most economically competitive approach. New plants should have appropriately designed (i.e., conservative) furnaces with sootblowers installed, and should reasonably be expected to give satisfactory performance with high fouling lignite. Should any other actions be necessary, additive injection would be the most attractive economically. X-ray diffraction the matrix has shown high concentrations of sodium melilite, NaCaA1Si207 [ 172]. This material melts in the range of 1120-1200~

The prospect of forming

higher-melting phases and thus possibly altering the melting characteristics of the matrix is a basis to evaluate additives to reduce fouling [ 172]. Replacing the sodium with calcium should produce gehlenite, Ca2AlzSiO7, which has a melting point of 1600~

The use of calcium and magnesium

S78 to replace sodium would produce akermanite, Ca2MgSi207, which melts at about 14400C. Therefore addition of calcium or magnesium could reduce fouling by increasing the melting temperature of the matrix material [172]. The most promising fuel additives seem to be calcium or magnesium compounds or minerals containing these elements. Presumably the additive acts by converting sodium melilite either to gehlenite or to akermanite [88]. In pilot-scale combustion tests of a high-fouling lignite, pulsed injection of additives did not significantly affect the rate of deposition, but did reduce the deposit strength, comparable to that obtained by lowering the flue gas temperature from 1095~ to 980~ without use of additives. A survey of about twenty fuel additives showed that monticellite, Niobrara shale, calcium carbonate, and magnesium oxide appeared to have the most potential to reduce deposit strength [99], based on measurements of the friability (ASTM tumbler test D-44145) of ash deposits produced in a pilot scale combustor firing Beulah lignite. To meet the criteria of having low deposit weight and strength with high friability, the best additive was calcium carbonate, injected at a rate of 2.6 kg/t of lignite [172]. A deposit produced while injecting magnesium oxide at a dosage of 3.8 kg/t had a friability of 80.6%, the weakest deposit produced in the test series. Pilot-scale tests with magnesium oxide using an intermittent injection equivalent to 50 kg additive/t for 30 second intervals every six minutes reduced ash deposit weights by 35% [99]. Dosage of 50 kg/t at this interval is equivalent to a steady injection dosage of 2.5--5 kg/t. In full-scale tests, addition oi 270 kg calcium carbonate, in the form of Number 6 poultry grit, every four hours to a 216 MW wall-fired pulverized-lignite unit (Leland Olds Unit 1) proved to be very successful in maintaining a clean boiler [ 172]. Tests in a 53 MW tangential-fired and a 79 MW wall-fired pc units produced some improvement in operation and facilitated cleanup after a shutdown. Excellent results were achieved with the addition of lime or limestone at 23 kg every two hours (1.2 kg limestone/t lignite) in a 15 MW spreader-stoker unit [172]. Results are not as promising in cyclone-fired units. In the Leland Olds Unit 2, a 440 MW cyclone firing the identical coal as in Unit 1, addition of pulverized limestone at 540 kg every two hours was not particularly successful, an experience attributed to the enrichment of sodium in the ash deposits in cyclone boilers [172]. Similarly, there was no apparent effect on operability or cleanability of the Big Stone plant, which is also a 440 MW cyclone [172]. Limestone has been used successfully to reduce fouling in full-scale boilers fired with high-sodium Saskatchewan lignite. The success of the limestone additive is to decre~tse the compressive strength of the deposits, making them more easily removed by soot blowing [173]. The first commercial-scale experience which contradicted the then-prevailing conventional wisdom that additive injection v,,ould be costly and troublesome was obtained by Saskatchewan Power Corporation in 1977, demonstrating positive results of periodic shot-dosing calcium carbonate during the firing of a high-sodium Estevan (Saskatchewan) lignite [88,134]. Increasing the calcium or magnesium content (and, to a lesser extent, the aluminum content) increased the ash softening temperature [134]. A successful method was developed for shot-dosing calcium carbonate grit into the pulverizer inlet to prevent buildup of slag or fouling deposits and to soften

579 existing deposits. A boiler rated at 66 MW but reduced in load to 60 MW because of fouling deposits was restored in only 8 h to operation at 65 MW by lime addition. Routine addition of calcium carbonate reduced both the length and the severity of boiler cleaning during overhaul. With calcium carbonate addition, superheater and economizer deposits are more easily removed. Double dosing with calcium carbonate at the onset of observed deposition lowered the gas temperature and increased steam output. These results were substantiated by experience at Basin Electric Cooperative and Otter Tail Power Company. The remedial action of lime- or limestone-based additives appears to be limited to pulverized-lignite-fired units, and does not extend to cyclones. The range of conditions for best results with calcium-based additive injection appears to be 0.5-2 kg of additive per tonne of lignite, 1-4 hour intervals, and injection times of 15 s to 5 min. [88]. Injection of "LiquiMag" (a suspension of magnesium oxide in fuel oil) at a rate of 0.15 L/t substantially reduced ash deposition in plants operated by Winnipeg Hydro [174]. The plant in Winnipeg, Manitoba has two Babcock and Wilcox pulverized-lignite-fired boilers rated at 21 and 38 kg/s steam, and burn high-sodium lignite from Saskatchewan. Pilot-scale tests with Saskatchewan and James Bay (Ontario) lignites showed that injection of either magnesium oxide or dolomite ahead of the screen tubes could sufficiently alter the deposit structure to improve the prospects for removal by sootblowing [156]. Vermiculite has been used successfully as a fireside additive to reduce fouling in Unit 2 of the Leland Olds Station of Basin Electric Power Cooperative [ 175]. It is injected into the furnace on the front wall at the same elevation as the arch. As the vermiculite is heated by the gas stream, the particles expand and are swept into the superheater pendants by the gas. The expanded particles become incorporated into the deposits, increasing the friability to increase the susceptibility of the deposits to removal by sootblowing. Without vermiculite addition, the deposits in this boiler have been described as being "rock-like" [ 175]. Vermiculite addition rates of up to 2 kg/t of lignite have been used, the necessary feed rate varying with boiler load [175]. High temperatures attendant on sustained operation at high load can overwhelm any possible beneficial effect of vermiculite injection. Injection of vermiculite is not of itself adequate to eliminate deposition [9,175]. Other tactics include limiting the gas temperature to 1065~ using gas recirculation fans to temper the gas stream at high load; and selective sootblowing to concentrate the cleaning on problem areas in the superheater region. After the vermiculite addition program was adopted, only one outage for deslagging was necessary in 27 months, whereas before vermiculite addition 31 deslagging outages had occurred in 45 months [175]. Other benefits include improved heat transfer from the superheater; reduced sootblowing, rodding, and shotgunning to remove deposits; reduced shutdown, startup and (overtime) labor costs; and improved safety [ 175]. Perlite has also been tested in analogous fashion, also in the Leland Olds Unit 2. Perlite provides some benefits, but it is not as effective as vermiculite [ 175]. A small portion of alkalis vaporizes from clays as they convert to alkali and alkaline earth silicates in the temperature range 10(0)-1300~ [58]. The vaporization process might be retarded by using heavy metal additives, the reduction in vaporization of alkali translating to a reduction in

580 deposit formation [58]. (iv) Sootblowing. Economic analysis of potential solutions has shown, for existing plants, the most economical option is a combination of sootblowing with additive addition [123]. Sootblowers physically remove accumulated ash and slag deposits. In some cases it may be difficult to predict the exact locations in the boiler where fouling or slagging problems may occur; because of this, the necessary sootblowers may be mounted in the wrong locations or may even be lacking completely. In those cases, a retrofit relocation (or addition) of sootblowers is necessary. The need for sootblowing may be determined by monitoring the steam temperature; if the steam temperature is too low, this may indicate that the superheater tubes are fouled and are in need of cleaning. (An ash deposit 3 mm thick can reduce heat transfer to boiler tubes by 50% [123].) Alternatively, the low steam temperature may indicate that the gas temperatures are too low. The increase in the size of the boilers, as well as an increase in the number of sootblowers installed per boiler, has in some cases placed constraints on the number of sootblowing cycles per day due to limitations in the air or steam supply system for the sootblowers. An increase in the number of cycles per day can be attained by increasing the travel time of the sootblowers. For long retractable sootblowers, cleaning ability does not deteriorate even when the travel speed is doubled [123]. This finding negates the previously held concept that cleaning was proportional to nozzle dwell time. By establishing an appropriate sootblowing cleaning pattern on the furnace walls and superheater screens, it is possible to burn high-fouling lignites successfully. For existing plants, sootblowing can be an economically attractive option to combatting fouling [ 123]. The amount of water needed to produce sufficient thermal shock to break a deposit off a tube can be estimated from the viscosity-temperature behavior of the ash. Many lignite ashes have non-Newtonian (plastic) viscosities over a fairly narrow range of temperature, generally a range of 40-85"C. The non-Newtonian range also occurs at low temperatures, often below 1200~

[32].

These deposits will require much less water to cool to a point where they can be broken off the tube with a water lance or water blower than would the deposit from an ash having a plastic viscosity over a large temperature range. The mechanical or physical properties of ash at high temperatures seems to be a neglected area of research. More information is needed in the susceptibility of different kinds of ash deposits to sootblowing. Most units would eventually plug without some degree of sootblowing. However, at the Big Stone and Leland Olds plants a major problem with fouling is not so much the rate of deposition as the tenacity with which the deposit adheres to the tubes [97]. (v) Fine gri, ding ("micro, izi,g"). Comparison of deposition using a standard utility grind

(i.e., 80% _-<74lam) and micronized lignite (100% _<15 ~tm) showed that the micronized lignite produced smaller and weaker deposits than the conventional grind [ 141,176] in combustion tests of a high-sodium Beulah lignite in a pilot-scale combustor. The center of the deposit from the micronized lignite was depressed, extending nearly to the inner white layer. The strength of the deposit from the micronized lignite was less than that from the standard grind lignite, the former being easy to remove from the tube and to crumble into small (but hard) fractions.The total deposit

581 weight from the micronized lignite was about half that from the standard grind (204 vs. 401 g), but the inner white layer of the deposit from the micronized lignite was nearly double the weight of that from the standard grind (28 vs. 16 g). Other work shows an inner white layer from micronized lignite to be 3.5 times as heavy as that from the standard grind lignite [176]. The compositions of the inner white layers were nearly identical. However, the outer sinter layer from the micronized lignite had more silica and iron oxide, and less alumina and calium oxide, than the outer sinter layer of the deposit from the standard grind. Increased calciumcontaining phases in the deposit from the micronized lignite evidently formed at the expense of sodium-containing phases. In particular, the outer sintered layer of the micronized lignite deposit contained no sodium sulfate (identifiable by diffraction) and less sodium melilite than the outer sintered layer of the deposit from the standard grind. The reduced amount of sodium melilite may relate to the substantially reduced strength. Analyses of the inner white layers and outer sinter layers of the deposits are shown in Table 11.9 [ 176]. TABLE 11.9 Comparative analyses of deposits from micronized and standard grind Beulah lignite [ 176].

SiO2 A1203 Fe203 YiO2 1:'205 CaO MgO Na20 K20

Inner White Layer Micronized Standard 22.7 28.8 13.7 14.1 9.7 12.9 1.1 1.1 1.1 1.0 25.9 21.3 6.1 5.7 19.2 14.6 0.5 0.5

Outer Sinter Layer Micronized Standard 25.1 34.7 14.5 11.1 10.5 17.3 1.1 0.8 0.8 0.6 30.8 19.3 8.0 4.8 9.0 11.2 0.1 0.2

If the lignite feed is a relatively coarse grind, the burning time will increase, and particles will be carried further up the furnace, which raises the temperature higher in the furnace and hence increases the fouling rate [9]. An inadvertent cause of the coarseness of grind is wear of the grinding surfaces in the mills. (vi) Co-firing with other fuels. Co-firing lignite with fuel oil or natural gas produces deposits which contain primarily silicon, iron, and aluminum, and less than 0.8% sodium [177]. Next to ash composition (and particularly sodium) the flue gas temperature at the point of deposition is the second most important factor affecting fouling [131]. In the co-firing of lignite with natural gas or fuel oil, low heat release rates (which produce low combustion gas temperatures) do not produce hard, fused deposits [177]. Deposition is minimized by a short, compact flame, as is obtained by co-firing Number 6 fuel oil and lignite.

582 11.4.6 Ash-related corrosion. Wastage or thinning of waterwall tubes arises from two mechanisms [178]. In the first case, alkali sulfate deposits on tubcs react with sulfur trioxide and the iron(Ill) oxide normally on the tube surface: 3M2SO4 + Fe203 + 3SO3 ~ 2M3Fe(SO4)3

where M is the symbol for an alkali metal. Evidence for this mechanism is the existence of deposits of mixed alkali sulfates and alkali iron(III) sulfates on corroded tubes. The reaction of the alkali iron(Ill) sulfate with the metal surface is

4M3Fe(SO4)3 + 12Fe ---, 3FeS + 31Ze304 + Fe203 + 3M2SO4 + 3SO2

More severe corrosion proceeds via a second mechanism, involving formation of pyrosulfates [179]. An alkali pyrosulfate forms from the reaction of sulfur trioxide with an alkali sulfate:

M2SO4 + $03 ~

M28207

Mixed alkali pyrosulfates can be liquid at remarkably low temperatures, depending on composition. For example, with a 3:1 ratio of K:Na, the pyrosulfate K1.sNa.sS207 melts at 279~ in the presence of 0.0007% SO3. The addition of 1% lithium depresses the melting point to 232~ in 0.0002% SO3. Rapid deterioration of the metal is facilitated by the formation of a liquid corrodant [180]. The formation of the liquid pyrosulfate means that the attack on the iron(III) oxide

3M2S207 + Fe203 ~ 2M3Fe(SO4)3 can proceed much more rapidly than the solid-state reaction of the alkali sulfate and iron(Ill) oxide. A similar mechanism has been postulated for liquid phase ash corrosion of superheater and reheater surfaces. "Acid-soluble" alkali correlates better with corrosion than the total alkali content of the coal [ 178]. The acid-soluble alkali was present in a "simpler" form than insoluble alkali and therefore could be more readily available Ibr participation in reactions leading to corrosive species. For lignites, the alkali elements soluble in acid would primarily be those associated with carboxyl groups as ion-exchangeable cations. Alkali elements in this form readily participate in a variety of reactions when they are liberated by thcrmal decomposition of the carboxyl groups. Alkaline earth elements are beneficial. Their participation in the formation of alkali - alkaline earth double sulfates

(e.g., K2Ca2(SO4)3) in effect ties up some of the alkali sulfate and thus reduces the potential for formation of the corrosive alkali iron(Ill) sulfate. A nomograph has been published to predict the

583 corrosiveness of bituminous coal ashes on the basis of calcium, magnesium, and iron contents of the ash and acid-soluble sodium and potassium [178]. The extension to lignite ashes is unfortunately limited because many lignites have compositions outside the range for which the nomograph was developed. Deposits consisting of iron(II) sulfide are also observed. In that case, the problem originated with deposition of pyrite on the tubes, the pyrite adhering to the alkali sulfate layer. A slow oxidation of pyrite produces iron(II) sulfide and magnetite, forming enough sulfur trioxide to participate in the formation of the alkali iron(Ill) sulfate.

11.5 F L U I D I Z E D - B E D

COMBUSTION

11.5.1 Introduction For lignites the potential advantages of atmospheric fluidized-bed combustion, relative to pulverized-lignite-fired combustion, include reduced sensitivity to the quality of the lignite and the variability of the lignite quality, elimination of slagging and fouling, and higher sulfur capture by the ash, with attendant elimination of the need for flue gas desulfurization [181]. The higher reactivity of low-rank coals relative to high-rank coals results in combustion efficiencies in excess of 99% for low-rank coals, without the need for ash recycle or a carbon burnup cell [ 182]. Lower combustion temperatures may offer the advantage of substantial reductions in thermal NOx formation [9]. Operation on lignites of very high ash content is also an advantage [9]. The solids contact and radiant heat transfer from the bed to in-bed tubes provide heat transfer rates greater by up to a factor of five relative to the convection section in a pulverized-lignite-fired boiler, thus reducing the overall combustor size [ 182]. A tolerance for larger and less uniform fuel particle sizes reduces fuel preparation costs; and the dry, solid waste is more acceptable from an environmental standpoint than the wet sludge from a scrubber [182]. Furthermore, units with capacities smaller than 6 kg/s steam are amenable to easy modular construction. Some lignites are more difficult to pulverize in conventional grinding equipment than are bituminous coals. The Hardgrove grindability varies widely with moisture, so grindability data given in the literature without specification of the moisture level at which the test was conducted must be used with great caution. Because fluidized-bed combustors offer the possibility for burning crushed, rather than pulverized, lignite, these combustors can handle a great variety of lignites without as much concern for grindability or moisture content [183]. These advantages are tempered by some problems in adapting fluidized-bed units to lignite firing, particularly agglomeration problems when using sand as the bed material, and a potential disposal problem if the spent bed material contains significant quantities of water-soluble sulfates [9]. Since the lignites of North America are generally low in sulfur content, the ability to accommodate high-sulfur fuels and attendant operating and disposal problems are not a major consideration in the fluidized-bed combustion of lignites. (In comparison, some of the lignites

584 elsewhere in the world, such as Spain and Turkey, are notorious for extremely high sulfur contents--in excess of 10% in some of the Spanish lignites--and with such fuels fluidized-bed combustion might offer significant advantages.) The high calcium and magnesium contents of many lignite ashes provide inherent sulfur capture ability at low bed temperatures. If all of the calcium and magnesium could be'utilized in sulfur capture, the ash itself would be an adequate inert bed material without the necessity for the addition of limestone. The necessity for relatively low bed temperatures derives from the thermal stability of calcium sulfate, the principal product of the sulfur capturing reactions. Calcium sulfate begins to decompose around 980"C [ 183]. The principal design parameters affecting the performance of a fluidized bed combustor are the in-bed heat extraction system, the combustor area, the feed system, the type (if any) of solids recycle, and the heat extraction from drained solids [182]. In addition, the combustor area, the feed system, and the solids recycle can affect the environmental performance of the combustor. At velocities in the range 2-4 m/s, combustion efficiencies will be in the range 92-96% [48]. Higher efficiency at the higher velocity derives from the concomitant increase in bed temperature from 700 to 840"C [48]. Generally the combustion efficiencies of lignites are higher than those of bituminous coals, reflecting the higher reactivities of lignite char. If ash recycle is incorporated in the system, virtually complete carbon utilization can be obtained. Normally fluidized bed combustors operate with a low inventory of combustibles in the bed. Thus the design is well suited to burning high ash fuels. This fact could become advantageous in the future if necessity dictates buming lignites with ash contents above 30%. In conventional pulverized-lignite-fired equipment, the high ash value could re,suit in a large ash burden in the flue gas, with resultant problems of ash deposition on heat exchange surfaces and the ash settling out of the gas stream [ 183]. Such problems would not be likely in a fluidized-bed combustor operating with low gas velocity. In-bed heat extraction is effected by horizontal tubes in the upper regions of the bed or by water wall tubes. The horizontal tubes are located high in the bed to minimize corrosion in localized regions of reducing atmosphere or regions in which the atmosphere fluctuates between oxidizing and reducing. Normally the tubes are widely spaced; improper spacing can exacerbate erosion in localized areas of high gas velocity. The tube spacing can also affect pressure drop in the bed and bubble formation. Vertical tubes could help channel gas bubbles, and have a second disadvantage of having lower heat transfer coefficients than horizontal tubes. The combustor area, along with the bed depth and heat removal requirement, defines the bed zone. To achieve complete combustion, the bed zone must be within the established limits of heat release rate and gas residence time. If the area is too small, the throughput will be too high; if the depth is too low, the gas residence time will be too short. In either case, incomplete combustion will occur, resulting in emissions of char, carbon monoxide, and hydrocarbon gases from the bed. Furthermore, if the gas residence time is too short, SOx emissions may become unacceptably high. Without proper dispersion of the lignite via properly designed feed systems, some

585 combustion can occur above the bed. The loss of combustible materials such as carbon monoxide or hydrocarbons can result in reduced combustion efficiency. SOx emissions could be increased. If significant heat release occurs above the bed, the exhaust gas temperature may be higher than the optimum for the existing convection section, causing a reduced thermal efficiency. 11.5.2 Combustion behavior During fluidized-bed combustion, lignite chars burn by a constant-density, shrinkingsphere process [ 184]. Combustion efficiency increases with increasing bed temperature and excess air [185]. Results obtained from atmospheric fluidized-bed combustion of six Spanish lignites relate an apparent kinetic constant kc to external surface area of the particle by kc = ko T0.5 exp(-Ea/RT)

where T is the temperature of the burning particle [186]. Values of Ea are in the range 43.5--68.6 kJ/mol; and of ko, 2.8-57.1 m/s.K0.5 [186].

11.5.3 In-bed Sulfur Capture Fluidized-bed combustion of North Dakota lignite with limestone addition to the bed generally results in SOx and NOx emissions below the emission standards [ 187,188]. Combustion of some low-rank coals in a sand bed requires the addition of supplemental alkali to ensure compliance with sulfur emission standards [188]. Limestone was the least effective of three additives tested at an alkali/sulfur ratio of 1.6, with a sulfur retention of 80% [48]. Nahcolite and trona produced sulfur retentions of 96% and 98%, respectively [48]. The additives were used in amounts equivalent to an added alkali/sulfur ratio of 1.0, so that the total alkali/sulfur ratio would be 2.6 [48]. The alkalinity of lignite ash effects significant sulfur capture even without additives. For example, during combustion of Seyitomer (Turkey) lignite at 850~

about 65% of the sulfur

was captured by the lignite inorganics [ 189]. The most important factor affecting sulfur capture and retention in the bed is the total alkalito-sulfur molar ratio [48,88,190-192]. The composition of both the lignite and any added sorbent must be considered in determining this ratio. Total alkali includes calcium, magnesium, and sodium. Sulfur retention as a function of alkali-to-sulfur ratio is shown in Figure 11.27 [ 190]. Sulfur retention can be predicted from the ratio of total alkali to sulfur. Generally retention by the inherent alkali or by added limestone can be predicted to within _+10% [193]. The effect of temperature on sulfur retention varies with the coal being used and with other operating conditions of the combustor. With an emission standard of 1.2 lb SO2/106 Btu (516 ng/J), many lignites could be burned in a fluidized-bed combustor without the need for added sorbent or downstream flue gas desulfurization [ 194]. However, with tighter emission standards, addition of supplemental sorbent would be needed to achieve compliance. The reinjection of fly ash can increase sulfur

586 100

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Alkali-to-sulfur ratio Figure 11.27. Sulfur retention in fluidized-bed combustion of low-rank coals as a function of total alkali-to-sulfur ratio [190]. Points include data for inherent alkali and addition of limestone, trona and nahcolite.

capture, but this strategy is not of itself sufficient to meet stricter sulfur emission standards [ 194]. Addition of sufficient limestone to achieve an alkali/S mole ratio of 1.5 reduces SO2 emissions to 172 ng/J [193]. NOx emissions from Beulah lignite are below the New Source Performance Standard of 0.5 lb/106 Btu (215 ng/J) [193]. Combustion efficiencies up to 99% can be achieved under these conditions [ 193]. Alkaline components of the lignite ash provide high sulfur capture in the bed. The natural alkalinity of the ash reduces, and, in favorable cases eliminates, the need for added sorbent to meet New Source Performance Standards for SO2 emissions. Sulfur retentions exceeding 90% have been observed in pilot-scale testing [ 182]. With a Ca/S ratio of 3, SO2 retentions of 92% and 96% have been achieved for Tavsanli and Orhaneli (Turkey) lignites, respectively [191]. In other work with Turkish lignites, SO2 reductions of 90% were attained using dolomite with a Ca/S ratio of 4 [195]. Sulfur retentions greater than 90% can also be achieved for Spanish lignites if the Ca/S molar ratio is greater than 2.0 [ 192]. Sulfur retention can be determined from the empirical equation R = 12.7 (CaO / A1203) + 48.1 (Na20 / SiO2) - 10.1 [ 196] where the molecular formulae represent the weight percentages of the compounds in the coal ash. The presence of silica and alumina in the denominators of the terms of this equation reflects

587 the fact that some of the sodium and calcium which would otherwise react with sulfur become unavailable for sulfur capture by reaction with the silica and alumina. The reaction of sodium and calcium with alumina and silica may be facilitated by higher temperatures and thus accounts for the fact that the peak temperature for sulfur retention in lignite is about 55~ lower than that for bituminous coal combustion. Electron microprobe analyses of ash particles from combustion of Northern Ireland lignite indicate that CaO may react preferentially with the acidic silica and alumina to form calcium aluminosilicates [ 197]. Thus the determination of the amount of lime or limestone to be added to achieve acceptable sulfur capture must take into account possible reactions of acidic ash components with the additive. Among the operating variables that affect sulfur retention are the average bed temperature, superficial gas velocity, and air-to-fuel ratio [48,88,190,198]. Retention of sulfur by the ash appears to reach a maximum at 760~ without ash reinjection and at 815~ if ash is reinjected into the bed [48,88,190,196]. Thus combustion of Beulah lignite in a 15 cm diameter fluidized-bed combustor shows maximum sulfur retention (51%) at an average bed temperature of 760~

[ 196].

The conditions were 30% excess air and superficial gas velocity of 2.2 m/s. For comparison, the sulfur retention in pulverized-lignite-combustion of this lignite is estimated to be about 30%. Other work has indicated optimum sulfur capture in the temperature range 800-850~ 848~

[195], e.g. at

[199]. The lower temperature without ash reinjection may reflect the fact that local

temperatures in the region of burning coal particles (and presumably where sulfur capture might occur) may be higher than the average bed temperature. Sulfur retention at a given alkali-to-sulfur ratio is decreased as the superficial gas velocity is increased [190]. Increasing air-to-fuel ratio generally increases the sulfur retention [48,190]. The effect of excess oxygen could be different depending on the oxidation states of the materials in the bed. Furthermore, oxidation of sulfite to sulfate might be catalyzed by some trace species in the ash (such as vanadium). Sulfur retention increases with increasing excess air. For example, increasing the excess air from 30 to 61% increased the sulfur retention from 51 to 58%. The additional oxygen supplied by increasing the excess air may drive reactions of calcium oxide, sulfur dioxide, and oxygen further to completion: CaO + 0.5 02 + S02 ~ CaS04

The level of secondary air injection also affects sulfur capture [ 198]. Both the lignite and sorbent particle sizes also have a role in determining the effectiveness of sulfur capture. With Turkish lignites, SO2 concentration in the flue gas decreased with increasing lignite particle size [191]. The effectiveness of sorbent action with increasing lignite particle size reflects the slower rate of release of SO2 from the larger particles [ 195].The slower release provides a longer time period for reaction of SO2 with the sorbent.

Sulfur capture

increased with decreasing sorbent particle size [191]. Particles larger than 1.2 mm show a low degree of sulfation [ 192]. Eliminating larger limestone particles from the sorbent feed will result in

588 increased sulfation and improved sorbent utilization [ 192]. SO2 emissions are shown in Figure 11.28 for various low-rank coals [ 190]. Different coals burned under similar combustion conditions can result in significant differences in sulfur retention. These differences are attributed, in unspecified ways, to chemical and physical differences among the ashes [88].

700

600 ~

400

-

300 2oo

~

100 0 0

....

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. . I. . . . 1 4 6

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Alkali-to-sulfur ratio Figure 11.28. Sulfur dioxide emissions as a function of alkali-to-sulfur ratio for fluidized-bed combustion of low-rank coals [190]. Bed temperature 760"C, with 40% excess air.

11.5.4 NO,, Formation NOx emissions are a strong function of the fuel nitrogen content [ 182]. This relationship is particularly important for fluidized bed combustion, since the fixation of atmospheric nitrogen is generally very low as a result of the low bed temperatures. However, lignite-fired fluidized bed combustors seem able to meet the 1979 New Source Performance Standards for NOx emissions. The percentage of fuel nitrogen converted to NOx increases with average bed temperature [196] and with increasing air/fuel ratio. The fluidized combustion of Beulah lignite at 760~ and 60% excess air converts about 65% of the fuel nitrogen to NOx [196]. NOx emissions increase with excess air. For five coals from lignite through bituminous in rank, the NOx emission increases from 172 to 387 ng/J for an increase in excess air from 10 to 90% [190]. NOx emissions correlate with average bed temperature. With Beulah lignite ( 1% nitrogen, maf basis), the NOx emission doubled from 172 to 344 ng/J as the bed temperature increased from 665 to 815~

[48]. In these tests the excess air was held at 30%. At a bed temperature of 760~

589 the NOx emissions increased from 215 to 387 ng/J as excess air increased from 10% to 90% [48]. The 1979 New Source Performance Standards specify NOx emissions of 0.6 Ib/106Btu (258 ng/J) for lignite in pc-fired boilers and 0.8 lb/106 Btu (344 ng/J) for cyclones. It appears that lignites can meet the NSPS requirements for bed temperatures of 760~ and 30% excess air [48]. When fluidized-bed combustors are operated at low temperatures to enhance sulfur capture and reduce formation of the higher oxides of nitrogen, the formation of nitrous oxide can increase [200]. Effects of bed temperature on formation of N20 and the higher oxides of nitrogen are inversely related [200]. In circulating fluidized-bed combustion, nitrogen bound in the char reacts via heterogeneous oxidation processes that are only minor contributors to N20 formation. Lowrank coals should produce lower N20 emissions than bituminous coals [201]. Below 850~ bituminous coals produce the highest N20 emissions [200]. This behavior is due to the high reactivities of low-rank coal chars for destruction of N20. For example, char produced from Center lignite begins reacting with N20 at 290~ [201]. Decreasing the amount of excess air and increasing the amount of limestone both reduce N20 emissions [200]. 11.5.5 Bed agglomeration The sodium content of the ash is the single most important factor in determining the severity of agglomeration [88]. When the sodium content of the ash is greater than 3%, and a sand bed is being used, the bed has a tendency to agglomerate. The agglomerates may be suspended in the bed or stick to internal surfaces in a manner roughly analogous to ash deposition in pc-fired combustion. Agglomeration appears to be initiated by the formation of low-melting sodium aluminosilicates from the ash and sand. The addition of limestone or alumina will depress the silica concentration in the bed and may help to suppress agglomeration. The tendency for bed agglomeration to occur is determined by bed temperature (increasing temperature increases the likelihood of agglomeration), sodium and calcium contents of the lignite (increasing sodium increases the tendency of the bed to agglomerate, whereas increasing calcium content delays the onset of agglomeration); and operating methods such as ash recycle, position of coal feed, and performance of distributor plate [202]. These observations for North American lignites are also seen in fluidized-bed combustion of Turkish lignites, in which, for example, there is a direct relationship between sodium and potassium content of lignites and agglomeration tendencies [195]. The agglomeration temperature increases with increasing particle size, because the surface area decreases, and with increasing air velocity, because of improved bed mixing [ 195]. Agglomeration decreases the fluidization quality of the bed, resulting in poor bed mixing, increased temperature gradients, and poor combustion efficiency. As agglomeration proceeds, the size of bed material increases, as do the concentrations of sodium and iron in the bed. The heat transfer coefficient drops. Agglomeration problems can be expected as the sodium concentration in the bed approaches 4% [88]. The bed temperature and the use of ash recycle also affect

590 agglomeration. Additives such as limestone may help control agglomeration. Agglomeration will occur if a lignite having 4-5% (or higher) Na20 in the ash is burned in a silica bed with little or no limestone addition [88]. Moderate agglomeration can cause poor fluidization and reduce the heat transfer rate. Severe agglomeration can defluidize the bed, making it necessary to shut down the unit. Agglomeration is manifested in three ways: adherence of bed material to surfaces in the bed, actual agglomeration of the bed particles, and growth of size of the particles [203]. Surface agglomeration occurs only on surfaces at or near the bed temperature (760-980~ tube surfaces are cold relative to the bed (<175~

The in-bed

and do not experience surface agglomeration.

The growth of particles appears to be an increase in size of individual particles and not, in general, and agglomeration of several smaller particles followed by coalescence. Agglomerate formation among quartz particles follows a four-stage process [202,204]. An initial ash coating of about 50 ~m in thickness forms on the particle. The coatings contain some coarse ash particles, and show some penetration of the quartz particles. Second, thicker ash coatings, of about 10(O300 ~m, form with nodular outer surfaces caused by the incorporation of larger ash particles. As the coatings thicken, they become sulfated. The sulfated aluminosilicate ash acts as the cement to exacerbate agglomeration. This results in the quartz grains being loosely held together by the sulfated aluminosilicate ash. In the final stage additional bonding between particles results from recrystallization of the partially melted sulfated ash. Hot ash and quartz grains react, with the bonding proceeding via a calcium-rich glass. With limestone bed materials, agglomeration follows a three-step mechanism [202,204]. Initial calcining of the bed material allows for the sulfation of the particles. The limestone grains may incorporate sulfur, iron, or sodium in concentric alteration zones. The accumulation of thickened nodular ash coatings provides opportunities for continued sulfation. The third stage is marked by the formation of a weakly bonded agglomerate of masses of sulfated ash and altered limestone. This in turn sets the stage for agglomeration and growth of large, coarse calcium sulfate crystals. In either case agglomeration may be initiated by temperature excursions in the bed. The initial layer to which ash particles subsequently adhere during agglomerate growth is calcium sulfate and pseudowollastonite (CaSiO3) [205]. Mechanical adhesion of this layer to bed particles is very strong in the presence of SO-,,. Later, sintering begins to join the particles via the surface layers, forming "layer-particle" aggregates. Some volatilization of sodium and sulfur may occur at temperatures as low as 750~

[206]. Stable species such as gehlenite and akermanite

appear to form in the bed at temperatures well below their melting points. (These melilite minerals were identified in agglomerates formed in the bed of a pilot-scale combustor.) The vaporization of sodium at relatively low temperatures may contribute to reactions which form these melilite species. During fluidized-bed combustion of high-sulfur low-rank coals, molten ash material can form on the char surface [207]. In circulating fluidized-bed combustors, a portion of this melt phase is capable of transferring to the surface of bed particles, on which the melt phase has an interfacial tension lower than on char particles. This "transferred" melt phase on the bed particles

591 then renders them capable of sintering and agglomerating. The formation of the melt phase on the char surface is facilitated by reactions of the organically bound inorganics. In pressurized (0.9 MPa, 8N)-875~

fluidized-bed combustion of Beulah lignite in a dolomite bed, only 0.24% of the

total sodium is released as vapor species [208]. Presumably the remainder could be available for reactions with other inorganic components of the lignite. Pilot-scale tests have shown some relationship between the extent of agglomeration in silica beds and the extent of sulfur capture, in that agglomeration occurs more readily when sulfur capture is effective [209]. Average bed temperature is also an important parameter affecting agglomeration; operation in the region 843-900~ proceeds without agglomeration of silica beds [209]. Particle agglomeration is more prevalent when SO2 is present in the gas, based on data obtained in a 5 cm reactor [205]. Agglomeration was more severe at 788 ~ than at 843~

bed

temperatures. Limestone aggregates more severely than silica under these conditions. With a silica bed, particle diameter increased due to ash accumulation on the surface of the silica particles, but severe agglomeration did not occur in the absence of SO2. Bed material from pilot-scale combustion of a high-sodium Beulah lignite in which agglomeration occurred showed a low-temperature sinter point in the region 900--940*C and a second sinter point around 1025~ [209]. These sinter points were not observed for bed material from tests in which agglomeration did not occur. Agglomeration with the high-sodium lignite is least severe in the range of bed temperatures of 840-900~

below the region of the onset of

sintering of the bed [209]. The alkaline ash has a beneficial effect on in-bed sulfur capture, but at the same time contributed to bed agglomeration by participating in the formation of compounds with relatively low melting points [203]. Texas lignites showed essentially no agglomeration, in contrast to the experience with the Beulah lignite [203]. The Texas lignites ',all had lower concentrations of alkalies in the ash than did the Beulah lignite.

REFERENCES

G.J. Flynn, Average heating values of American coals by rank and by states, U.S. Bur. Mines Inf. Circ. 7538, (1949). W.H. Ode and F.H. Gibson, International system for classifying brown coals and lignites and its application to American coals, U.S. Bur. Mines Rept. Invest. 5695, (1960). F.P. Miknis, G.E. Maciel, and V.J. Bartuska, Carbon-13 NMR studies of coals by cross polarization and magic-angle spinning, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 24(2) (1979) 327-333. P.H. Given, D. Weldon, and J.H. Zoeller, Calculation of calorific values of coals from ultimate analyses: theoretical basis and geochemical implications, Fuel, 65 (1986) 849-854. R.A. Mott and C.E. Spooner, Calorific value of carbon in coal--Furlong relation, Fuel, 19 (1940) 226-242. D.M. Mason and K. Gandhi, Formulas for calculating the heating value of coal and coal char: development, tests and uses, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 25(3) (1980) 235-244.

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(1970), pp. 103-112. D.P. McCollor, R.E. Conn, and M.L. Jones, Combustion research and ash fouling, in: G.A. Wiltsee (Ed.)~ Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/601811531, (1984), pp. 9-1 -9-12. D.P. McCollor, R.E. Conn, and M.L. Jones, Combustion research and ash fouling, in: G.A. Wiltsee (Ed.) Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/601811574, (1984), pp. 10-1 - 10-11. S.F. Miller and D.P. Kalmanovitch, Relation of slag viscosity and surface tension to sintering potential, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33(2) (1988) 42-49. J.D. Yeakel and R.B. Finkelman, A new model for predicting the fouling deposit weight of coal, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33(2) (1988) 91-102. T.D. Wilhelm, Sample preparation and electron microprobe analysis of lignite ash deposits, M.S. Thesis, University of North Dakota, Grand Forks, ND, 1968. B.W. King, H.P. Tripp, and W.H. Duckworth, Adherence of porcelain enamels to metals, J. Amer. Ceram. Soc., 42 (1959) 504-525. E. Raask, Mineral Impurities in Coal Combustion, Hemisphere, Washington, 1983. B.G. Miller, Slides presented at Project Sodium review meeting, Grand Forks, ND, April, 1986. S.K. Falcone and M.M. Fegley, Ash and slag characterization, Unpublished monthly report, Univ. North Dakota Energy Res. Center, February 1986. J.P. Hurley, Sodium in low-rank coal, 5th Intl. Coal Testing Conf., Lexington, KY, 1986. D.K. Rindt, M.L. Jones, and H.H. Schobert, Investigations of the mechanism of ash fouling in low-rank coal combustion, in R.W. Bryers (Ed.), Fouling and Slagging Resulting from Impurities in Combustion Gases, Hemisphere, Washington, 1983, pp. 117141. P.J. Jackson, Deposition of inorganic material in oil-fired boilers, in: R.W. Bryers (Ed.), Ash Deposits and Corrosion Due to Impurities in Combustion Gases, Hemisphere, Washington, 1978, pp. 147-161. J.J. Helble, S. Srinivasachar, and A.A. Boni, A fundamental study of ash particle adhesion, Proc. 7th Intl. Pittsburgh Coal Conf., (1990) 52-61. S. Srinivasachar, J.J. Helble, and A.A. Boni, An experimental study of the inertial impaction of ash under coal combustion conditions, Proc. 23rd Intl. Symp. Combust., (1991) 1305-1312. F.R. Karner, Preliminary study of inorganic constituents of Beulah standard lignite and derived fly ash probe deposits, Unpublished report, Grand Forks, ND, 1977. F.D. Friedrich, G.K. Lee, and E.R. Mitchell, Combustion and fouling characteristics of two Canadian lignites, J. Eng. Power, 19 (1972) 127-132. S.K. Falcone, H.H. Schobert, D.K. Rindt, and S.A. Braun, Mineral transformations during ashing and slagging of selected low-rank coals, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 29(4) (1984) 76-83. M. Neville and A.F. Sarofim, The fate of sodium during pulverized coal combustion, Fuel, 64 (1985) 384-390. J.S. Ross, R.J. Anderson, and R. Nagarajan, Effect of sodium on deposition in a simulated combustion gas turbine environment, Energy Fuels, 2 (1988) 282-289. S.K. Falcone and H.H. Schobert, Mineral transformations during ashing of selected lowrank coals, in: K.S. Vorres (Ed.), Mineral Matter and Ash in Coal, American Chemical Society, Washington, 1986, Chapter 9. E.A. Sondreal, P.H. Tufte, and W. Beckering, Ash fouling in the combustion of low rank western U.S. coals, Combust. Sci. Technol., 16 (1977) 95-110. E.A. Sondreal, G.H. Gronhovd, P.H. Tufte, and W. Beckering, Ash fouling studies of low-rank western U.S. coals, in: R.W. Bryers (Ed.), Ash Deposits and Corrosion Due to Impurities in Combustion Gases, Hemisphere Publishing Corp., Washington, DC, 1978, p. 135-152. T.F.W. Barth, Theoretical Petrology, Wiley, New York. S.A. Benson and L.G. Austin, Crystallization in coal ash slags and its effect on slag deposit strength, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 33(2) (1988) 50-63. S.A. Benson, Formation of alkali and alkaline earth aluminosilicates during combustion of

599

166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181

182

183 184 185 186

western low-rank coals, Unpublished review paper, The Pennsylvania State University, University Park, PA, 1985. R.C. Ledger, Parallel tests on United States lignites at the SECV and a US laboratory: SECV results, State Elect. Comm. Victoria Rept. SO/85/88, (1985). G.P. Huffman and F.E. Huggins, Reactions and transformations of coal mineral matter at elevated temperatures, in: K.S. Vorres (Ed.), Mineral Matter and Ash in Coal, American Chemical Society, Washington, 1986, Chapter 8. H.H. Schobert, S.A. Benson, and L.G. Austin, Relationships among fusing and sintering behavior, strength, and composition of slag deposits from U.S. lignites, Proc. 4th Intl. Conf. Coal Sci., (1987) 897-899. E. Raask, Flame vitrification and sintering characteristics of silicate ash, in: K.S. Vorres (Ed.), Mineral Matter and Ash in Coal, American Chemical Society, Washington, 1986, Chapter 11. R.C. Streeter, E.K. Diehl, and H.H. Schobert, Measurement and prediction of low-rank coal slag viscosity, in: H.H. Schobert (Ed.), The Chemistry of Low-rank Coals, American Chemical Society, Washington, 1984, Chapter 12. G. Eriksson, Thermodynamic studies of high-temperature equilibriums. XII. SOLGASMIX, a computer program for calculation of equilibrium compositions in multiphase systems, Chemica Scripta, 8 (1975) 100-103. F.I. Honea, D.K. Rindt, R. Middleton, and D. Rothe, The use of additives to reduce ash fouling problems in lignite-fired boilers, Amer. Soc. Mech. Eng. Paper 80-JPGC/Fu-3, 1980. W.J. Marner, Gas-side fouling, Mech. Eng., (3) (1986) 70-77. E.A. Sondreal, Control of ash fouling, in: G.H. Gronhovd (Ed.), Quarterly Technical Progress Report, July-September, 1975, U.S. Energy Res. Devel. Admin. Rept. GFERC/ QTR-76/1, (1975), pp. 19-22. J.P. Rudolf and L.D. Huff, Use of vermiculite as a fireside additive to reduce superheater fouling, in: M.L. Jones (Ed.), Technology and Utilization of Low-rank Coal, U.S. Dept. Energy Rept. DOE/METC-86/6036 (Vol.1), (1986), pp. 58-64. M.L. Jones, D.P. McCollor, and B.G. Miller, Combustion research and ash fouling, in: G.A. Wiltsee (Ed.), Low-rank Coal Research Under the UND/DOE Cooperative Agreement, U.S. Dept. Energy Rept. DOE/FE/60181-26, (1983), pp. 10-1 - 10-12. V.A. Cundy, D. Maples, and N. Estep, Ash deposition and composition resulting from the combustion of lignite coals combined with fuel oils and natural gas, Coal Technol., 6 (1983) 215-234. A.L. Plumley, Predicting and assessing tube metal wastage in boilers fired with low-rank coal, CE Power Systems Rept. TIS-6948, (1981). H.E. Crossley, External fouling and corrosion of boiler plant: a commentary, J. Inst. Fuel, 40 (1967) 342-346. W. Nelson and C. Cain, Corrosion of superheaters and reheaters of pulverized-coal-fired boilers, J. Eng. Power, 82 (1960), 194-198. C.R. McGowin, C. Aulisio, and S. Ehrlich, Technical and economic aspects of a lignitefired AFBC boiler, in: Energy Resources Co. (Eds.), Proceedings of the Low-rank Coal Technology Development Workshop, U.S. Dept. Energy Rept. DE-AC01-80ET17086 (1981), pp. 4-25-4-44. M.D. Mann, D.R. Hajicek, B.J. Zobeck, and B.G. Miller, FBC: an environmentally and economically acceptable alternative for burning low-rank coal, in: M.L. Jones (Ed.), Technology and Utilization of Low-rank Coal, U.S. Dept. Energy Rept. DOE/METC-86/ 6036 (Vol.1), (1986), pp. 342-356. K.D. Kiang, H. Nack, J.H. Oxley, and W.T. Reid, Fluidized-bed combustion of coals, in: W.R. Kube and G.H. Gronhovd (Eds.), Technology and Use of Lignite, U.S. Energy Res. Devel. Admin. Rept. GFERC/IC-75/2, (1975), pp. 36-64. S. Halder and R.K. Saha, Combustion kinetics of lignite char in a fluidized bed, J. Inst. Energy, 64 (1991) 55-61. N. Selcuk and U. Kirmizigul, Characteristics of a fluidized-bed combustor burning lowquality lignite, J. Inst. Energy, 64 (1991) 151-156. J. Ad~inez, L.F. de Diego, and J.C. Ab~inades, Determination of coal combustion reactivities by a combined method. Proc. 7th Intl. Conf. Coal Sci., 2 (1993) 125-128.

600 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207

R.L. Rice, J.Y. Shang, and W.J. Ayers, Fluidized-bed combustion of North Dakota (USA) lignite, Proc. 6th Intl. Conf. Fluid. Bed Combust., (1980) 863-871. R.D. Talty, D.R. Hajicek, M.L. Jones, and D.R. Sears, Performance characteristics of low-rank coals in atmospheric fluidized-bed combustion, Proc. 16th Intersoc. Energy Conversion Eng. Conf., 2 (1981) 1141-1148. T. Eskikaya, Fluidized-bed combustion of a low grade lignite and emission control, J. Mines Met. Fuels, 38 (1990) 126-130. G.M. Goblirsch and E.A. Sondreal, Low-rank coal atmospheric fluidized bed combustion technology, in: W.R. Kube and G.H. Gronhovd, Technology and Use of Lignite, U.S. Dept. Energy Rept. GFETC/IC-79/1, (1979), pp. 75-107. M. Ozcan and H.A. Heperhan, Sulfur retention during fluidized-bed combustion of some Turkish lignites, in: R, Markuszewski and T.D. Wheelock (Eds.), Processing and Utilization of High-sulfur Coals III, Elsevier, Amsterdam, 1990, pp. 585-595. E. Men6ndez and L. Mateo, PFBC experiences in Escatron, in: T.J. Boyd and J.W. Wheeldon (Eds.), Proc. Conf. Appl. Fluid. Bed Combust. Power Gen., Elect. Power Res. Inst. Rept. TR-101816, (1993), pp. 20-1 - 20-15. G.M. Goblirsch, R.H. VanderMolen, K. Wilson, and D.R. Hajicek, Atmospheric fluidized bed combustion testing of North Dakota (USA) lignite, Proc. 6th Intl. Conf. Fluid. Bed Combust., (1980) 850-862. G.M. Goblirsch, R.W. Fehr, and E.A. Sondreal, Effects of coal composition and ash reinjection on sulfur retention burning lignite and western subbituminous coals, Proc. 5th Int. Conf. Fluid. Bed Combust. (1977) 729-743. E. Ekinci, Fluidized-bed combustion studies of Turkish lignites, Energeia, 4(4) (1993) 16. G.M. Goblirsch and E.A. Sondreal, Fluidized combustion of North Dakota lignite, in: G.H. Gronhovd and W.R. Kube (Eds.), Technology and Use of Lignite, U.S. Dept. of Energy Rept. GFERC/IC-77/1, (1978), pp. 82-99. M.E. Brady, M.G. Burnett, and A.K. Galwey, Aspects of the efficiency of calcium salts for the retention of sulfur in coal ash during combustion at 1200 K, J. Chem. Technol. Biotechnol. 5 (1993) 313-320. K. Riley, Texas-New Mexico Power Company: two 160 megawatt CFBs three unit-years of operation, in: T.J. Boyd and J.W. Wheeldon (Eds.), Proc. Conf. Appl. Fluid. Bed Combust. Power Gen., Elect. Power Res. Inst. Rept. TR-101816, (1993), pp. 3-1 - 3-23. A.K. Henderson, T.A. Moe, D.R. Hajicek, and M.D. Mann, Design and operation of the EERC pilot-scale circulating fluidized-bed combustor, Proc. Biennial Low-rank Fuels Symp., U.S. Dept. Energy Rept. CONF-910571-DE92002606, pp. 407-420. M.E. Collings, M.D. Mann, and B.C. Young, Effect of coal rank and circulating fluidizedbed operating parameters on nitrous oxide emissions, Energy Fuels, 7 (1993) 554-558. B.C. Young, M.E. Collings, and M.D. Mann, Formation and destruction of N20 in fluidized-bed combustors, Proceedings 1993 International Conference on Coal Science, II pp. 205-208. S.A. Benson, F.R. Karner, G.M. Goblirsch, and D.W. Brekke, Bed agglomerates formed by atmospheric fluidized bed combustion of a North Dakota lignite, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 27(1) (1982) 174-181. R. Vander Molen, Fluidized bed combustion bed properties with low-rank coal, in: Energy Resources Co. (Eds.), Proceedings of the Low-rank Coal Technology Development Workshop, U.S. Dept. Energy Rept. DE-AC01-80ET17086 (1981), pp. 4-45-4-66. D.W. Brekke and F.R. Karner, Analysis and characterization of atmospheric fluidized-bed combustion agglomeration, U.S. Dept. of Energy Rept. DOE/FC/10120-T7 (1983). M.D. Mann, M.H. Bobman, and D.R. Hajicek, Fluidized bed combustion, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/UNDERC/QTR85/3-4, (1986), pp. 11-1 - 11-19. D.R. Hajicek, Fluidized-bed combustion of low-rank coals, in: G.A. Wiltsee (Ed.), Lowrank Coal Research Under the UND/DOE Cooperative Agreement, U.S. Dept. Energy Rept. DOE/FE/60181-26, (1983), pp. 11-1 - 11-11. A.R. Manzoori, E.R. Lindner, and P.K. Agarwal, Inorganic transformations during the circulating fluid bed combustion of low-rank coals with high content of sodium and sulfur, in: S.A. Benson (Ed.), Inorganic Transformations and Ash Deposition in Combustion,

601 208 209

American Society of Mechanical Engineers: New York, 1992, pp. 735-762. S.H.D. Lee and E.L. Carlo, Measurement of sodium and potassium vapors in pressurized fluidized-bed combustion of Beulah lignite, J. Inst. Energy, 63 (1990) 203-210. M.D. Mann, M.H. Bobman, and D.R. Hajicek, Fluidized bed combustion, in: G.A. Wiltsee (Ed.), Low-rank Coal Research, U.S. Dept. Energy Rept. DOE/FE/60181-1846, (1986), pp. 11-1 - 11-9.

602

C h a p t e r 12

A L T E R N A T I V E USES OF LIGNITES

Nearly all of the lignite used commercially in North America is consumed in combustion applications, mostly for steam generation in electric power stations, and to a lesser extent for industrial steam or process heat. This chapter surveys alternative uses of lignites, both current commercial practice and some additional technologies that have been used in the past, or that offer potential future markets. A very small proportion of total lignite production is currently used commercially for gasification, extraction of montan wax, or production of "charcoal" briquettes. In the past, lignites have been used as a source of uranium, and leonardite, a weathering product of lignite, has been used as a source of dyes and stains. Potential future applications of lignites include liquefaction to synthetic fuels and conversion to chemical feedstocks.

12.1 G A S I F I C A T I O N OF LIGNITE The only gasification process in current commercial use in North America is the Dakota Gasification plant in Beulah, North Dakota. This plant relies on Lurgi fixed-bed gasification technology. This section will first treat fixed-bed gasification, at the fundamental and at the practical plant level. Then other gasification technologies that have been used commercially for lignites, though not in North America, will be discussed. Finally, a few gasification technologies that have been tested with lignites at the pilot scale will be surveyed. 12.1.1 Fundamentals of gasification (i) Reactivities and effects of lignite properties. Reactions of steam with carbonaceous material that generate synthesis gas are

C + H20 ~

CO + I-I2

CO + H20 ---" CO2 + H2

The first is strongly endothermic while the second is exothermic. The net overall process is endothermic and therefore requires a steady heat source. The available options are to supply external heat through the walls of the gasification vessel, or to provide another, exothermic reaction

603 inside the gasifier. Gasification of lignite in externally heated retorts was successfully demonstrated at the pilot scale (e.g., [1]). Externally heated retorts offer some process advantages, such as requiring no oxygen and the provision of thermal efficiencies as high as 80% [ 1]. To obtain high rates of heat transfer, metal walls are used for the retort, but the temperature to which the metal can be heated for long-lived continuous operation is limited. Consequently, the capacity of externally heated retorts is limited, and all process designs involve the use of steam-oxygen or steam-air mixtures so that the heat necessary to drive the endothermic steam-carbon reactions is supplied internally by simultaneous combustion reactions. The advantages provided by using an oxygen-steam mixture in a fixed-bed gasifier are that the countercurrent flow of ascending product gases and descending fuel provides efficient heat exchange, with the result that gas temperatures at the outlet are low; a high thermal efficiency is obtained; and good oxygen utilization occurs. Carbonization of the fuel takes place in the upper region of the fuel bed; to obtain continuous free flow of char to the reaction zone, operation is restricted to noncaking coals unless special provisions are made for decreasing the caking tendency before gasification or stirring the caking mass inside the gasifier. The exothermic combustion reactions supply the heat to drive the endothermic gasification reactions in the gasification/combustion zone of a fixed-bed gasifier. The reactions occurring in this zone, shown with heat of reaction, aH298 in kJ/mol, are

C + Oz---" CO2

-394

C + 0.5 O 2 ~ CO

-111

C + H20 ~ CO + H2

+ 131

C + CO2 ~ 2CO

+ 172

[2]. For a slagging fixed-bed gasifier, selection of the appropriate oxygen/steam ratio makes it possible to maintain a temperature high enough to consume the char completely and to melt the ash. The temperatures in the gasification/combustion zone of a fixed-bed slagging gasifier are in the range 1540-1700~

[2]. At these temperatures the reaction of carbon to CO2 is about a thousand

times as fast as the endothermic water gas and Boudouard reactions. Nevertheless, these latter reactions do occur when sufficient gas residence time is provided in the gasification/combustion zone. The apparent reaction order for reaction of lignite char in carbon dioxide is 1 (with respect to carbon dioxide); for Coronach (Saskatchewan) char the activation energy is 79.0 kJ/mol [3]. The reaction proceeds by a shrinking core process. Activation energies for gasification in air of Canadian lignite chars lie in the range 120-140 kJ/mol [4]. The shrinking core model with chemical reaction as the rate-controlling step also describes the steam gasification [5]. Hydrogen, carbon monoxide, and methane concentrations in the product gas from steam gasification of

604 Mississippi lignite increase with temperature [6]. Kinetic constants for a first-order model of steam gasification are given in Table 12.1 [6]. TABLE 12.1 Kinetic constants for first-order steam gasification of various lignites at 870~ [6]. Lignite "/'exas Mississippi Montana North Dakota

k, min-1 0.091 0.13 0.162 0.194

E/R, K-1 m 12280 8480 12985

The effect of the oxygen content in the oxygen-steam mixture on the product gas composition for gasification of lignite char is shown in Figure 12.1 [7]. This Figure derives from thermodynamic calculations [8]; since kinetics are not taken into account, percentages of the various products in the exit gas actually obtained in gasification will differ somewhat from these theoretical predictions. Oxygen-steam ratios below about 0.7 would be used in dry-ash operation. Ratios in the range 0.7-1.2 produce temperatures high enough to cause melting of the ash, suitable for slagging gasification. Above a value of 1.2, the process would still operate in the slagging mode, but temperatures might become so high that significant volatilization of the ash components could occur. The methane yields and the oxygen and heat requirements for various types of gasification processes are shown in Figures 12.2 and 12.3, respectively [9]. For a lignite having 70% carbon and 5% hydrogen on an maf basis, the atomic H/C ratio is 0.85 and the C/H weight ratio is 14. Increasing the operating pressure (1.4

vs.

2.8 MPa) during pilot-scale slagging gasification of

Indian Head (North Dakota) lignite resulted in the following effects: 1) an increase in the total amount of hydrocarbon gases (5.52 to 6.03%), 2) an increase in the ratio of alkanes to alkenes, and 3) a decrease in the amount of alkenes [10]. The methane content of the gas increased from 4.98 to 5.60% [ 10]. The increased amount of hydrocarbon gases at the higher pressure correlated with an increased cracking of tar. At 2.8 MPa, the yield of tar per tonne of as-received lignite gasified decreased by 4.5 kg compared to tests at 1.4 MPa. However, an increase of 5.6 m3 of gaseous hydrocarbons per tonne represents 4.5 kg. of gases, based on a weighted density [10]. The increase in methane agrees with postulates [11] that methane formation in the gasification/combustion zone is due to the reaction

605 80 +

CO

709

d~

60-

J

~'50

H2

A H20

O

~ej

o D

30"

D

20

-....

-

O" 0

'

'

'

'

I

'

'

'

'

I

'

'

'

'

I

'

0.5 1 1.5 Oxygen / Steam Molar Ratio

'

'

'

~

CO2

*

CH4

]"

2

Figure 12.1. Wet gas composition from the gasification of lignite char at 1017"C, 3MPa [7].

2CO + 2H2 ---, CH4

+ CO 2

[2]. That being the case, higher pressure favors methane formation by Le Chatelier's principle~ Furthermore, the higher pressure increases residence time of the gases, thus allowing more time for the reaction to occur. Lignites yield the largest increase in carbon conversion, relative to subbituminous or bituminous coals, as operating pressure is increased, at least for operation at moderate pressures (0.5-1.0 MPa) [12]. A major parameter affecting the rate of conversion in a given gasifier is the reactivity of the coal. Low-rank coal chars can show reactivities over 100 times as great as chars from high-rank coals [13]. As examples, Velva (North Dakota) lignite produces a more reactive semicoke than a bituminous coal tested under the same conditions [ 14]. Similarly, a North Dakota lignite provided higher carbon conversions than subbituminous or bituminous coals tested under the same conditions [12]. Thus it should be possible to process low-rank coals at higher space velocities than achievable for high-rank coals. In turn, either smaller gasifiers or a lower total number of gasifiers of a given size would be needed for a plant operating on low-rank coal, thus reducing capital costs of a gasification plants. In dry ash, fixed-bed gasifiers the temperatures in the gasification/combustion zone are maintained at low levels to prevent fusion of the ash. The superior reactivity of low-rank coal chars is an important advantage in such applications. In slagging gasifiers this difference is much less noticeable because of the higher temperatures used in

606 + Process 1 0.9~ 9 Process 2 0.8

A Process 3 --"~ 0.7

J

d0.6 ~,

"

r ~

* Process 4 ~"

* Process 5 x Process 6

~ 0.4 "

~

/

~ 0.30.2-. 0.1

0

0.5

1 1.5 2 2.5 3 3.5 Atomic H/C Ratio in Fuel

4

Figure 12.2. Maximum methane yields as a function of atomic H/C ratio in the fuel for various processes. Process 1, pyrolysis; process 2, complete hydrogenation with hydrogen produced by catalytic reforming of the product methane; process 3, complete hydrogenation with hydrogen produced by partial combustion of the product methane; process 4, complete hydrogenation with hydrogen produced from gasification of the fuel; process 5, complete hydrogenation with hydrogen produced from gasification of the carbon residue: and process 6, complete gasification and methanation [9].

the unit. The oxygen chemisorption capacity of demineralized lignite chars at 102~ in 0.1 MPa air provides a relative indication of the concentration of carbon active sites in the chars, and thereby provides an indication of the gasification reactivity of the char [ 15]. The decrease in reactivity of char with increasing severity of pyrolysis is a result of the decrease in carbon active surface area. Char reactivity also depends on the amount of surface oxygen complexes, and their formation appears in turn to be related to the amount and type of mineral matter in the coal [ 16]. Mineral matter clearly has an effect on lignite reactivity during gasification. Indeed, catalysis by mineral matter may be the most important factor affecting reactivity of chars from lowrank coals [4,16]. Highly dispersed alkali and alkaline earth elements--sodium, potassium, magnesium, and calcium--can act as effective gasification catalysts [17]. A good dispersion of these elements in lignite chars likely arises from their being dispersed throughout the parent lignite on ion-exchange sites. Chars produced under mild conditions provide a highly dispersed form of calcium oxide, which effectively catalyzed the air gasification of Highvale (Alberta) lignite [18]. Catalysis is not so effective if severe pyrolysis causes coalescence or crystallization of the

607 18-

-1.4

16

"1.2

--.

14

i- 1

f...~"~

~" + Process 1 ~

9 Process 2 A Process 3

10

0.8

8

.0.6

ro ~ Process 4

-

6.. 4 2.

Process 5

-0.4 0.2

-

"

o-

.~ =, ,, i~1

;.o

0

0.5

1

1.5

2

2.5

3

3.5

4

o

Atomic H/C Ratio in Fuel Figure 12.3. Oxygen and fuel requirements for methane production as a function of atomic H/C ratio in fuel for various processes. Process 1, complete gasification and methanation; process 2, complete hydrogenation with hydrogen from partial combustion of methane; process 3, complete hydrogenation with hydrogen from complete gasification of fuel; process 4, partial hydrogenation, with hydrogen from complete gasification of carbon residue; process 5, complete hydrogenation with hydrogen from catalytic reforming of the product methane [9].

dispersed calcium oxide particles. The presence of the mineral matter increased gasification reactivity of Spanish lignites in carbon dioxide at 900-1000~

the effect being temperature-

dependent [19]. Calcium and sodium are the key inorganic components affecting gasification reactivity of central German lignites [20]. Demineralization, as for example in the steam gasification of Velva lignite, reduces reactivity [21]. The demonstrated effects of the inorganic components of lignites enhancing the gasification reactivity naturally lead to the question of further affecting reactivity by added catalysts. Alkali carbonates substantially enhance the reactivity of Velva char [21]. The reactivity in steam gasification at 700 and 7500C increased by factors of 3 to 10 [21]. In atmospheric-pressure carbon dioxide or a 12:68:17 CO2:CO:H2 mixture, potassium carbonate provides rate enhancements by factors of 30 to 40 [22]. Potassium carbonate provides effective gasification catalysis for lignites, at least in part because of its strong interaction with the lignite surface during catalyst loading [23]. Potassium carbonate is more effective than other potassium salts, specifically, the acetate, chloride, nitrate, and sulfate [23]. The reaction order with respect to carbon was 0 for potassium-carbonatecatalyzed steam gasification [5]. The reaction rate increases linearly with potassium carbonate loading, while the activation energy decreases with increased loading. With identical reaction conditions and catalyst loadings (3%), the relative catalytic activities of various carbonates for

608 steam gasification of Australian lignite char are sodium > mixed sodium-potassium > potassium > lithium [5]. Nickel, added from an aqueous solution of the nitrate, is an effective catalyst for steam gasification at relatively low temperatures (400 and 500~

as demonstrated for Indian lignites

[241. Although the highly dispersed alkali and alkaline earth elements may serve asuseful catalysts, a potential negative impact also exists. These elements can volatize in high-temperature gasifiers and subsequently condense into a very fine alkali fume carried over into downstream equipment [25]. This behavior would be a special concern in combustion turbines or other advanced power applications because of possible deposition on, and corrosion of, turbine blades. Thus some form of hot-gas clean-up would be required. Submicron fumes might even pass through the hot-gas clean-up equipment. At the very high temperatures expected in entrained-flow gasifiers (1260-1650~

the alkali chlorides and hydroxides have vapor pressures exceeding 1 kPa

[25], requiting alkali gettering for effective control. The high moisture content of lignite may present problems in handling crushed coal, and will reduce the temperature of the product gas exiting the gasifier [26]. In addition, some low-rank coals have low ash fusion temperatures. The use of these coals in dry-ash gasifiers such as the Lurgi or Wellman-Galusha may require additional steam injection to control the bed temperature below the ash fusion temperature. In the case of slagging gasifiers, high levels of moisture can cause problems by quenching the gasification reaction or reducing temperatures enough to cause operational difficulties with slag flow. The heat produced in the gasification/combustion region of the gasifier may not be enough to evaporate completely the moisture coming in with the feed, and the consequent drop in temperature might prevent ash fusion or stop slag flow. This problem was observed in the pilot-scale gasification of Gascoyne (North Dakota) lignite, which has a moisture content in excess of 40% [27]. In principle, the moisture in the lignite would act as a diluent in the process (i.e., on an as-received basis, less carbon is available per kilogram of fuel than is obtained with a coal of lower moisture content). Thermal drying before gasification requires large amounts of heat and can generate fines as a result of size degradation of the lignite particles during drying. Significant quantities of fines in the feed are detrimental to the performance of a fixed-bed gasifier (thought they might be acceptable in a fluid bed unit and are desirable for entrained flow gasification). When lignites of high moisture content are gasified in a fixed-bed unit, large volumes of wastewater may be generated, due to the evaporation of the moisture in the lignite and the subsequent dissolution of polar organic compounds emitted as the lignite is devolatilized. Thus large gas liquor separation units may be needed, as well as wastewater treatment facilities. Fine particles are a problem in the operation of fixed-bed gasifiers. Maintaining a proper gas flow requires a well-controlled particle size distribution, with the particles normally having to be +6 mm [28]. Fines introduced at the top of the gasifier may be entrained out of the bed. In fluid bed gasifiers gas velocities may also be high enough to cause carryover of the fines. Loss of fines will reduce thermal efficiency (because of the carbon lost in the fines) and may contribute to erosion, fouling, or plugging of downstream equipment.

609 (ii) Properties of lignite beds. Uniform and predictable operation of a fixed-bed gasifier requires that the upward gas flow be distributed reasonably uniformly over the vessel's cross section [29]. If the bed permeability is such as to cause pressure drops of the same magnitude as the bed weight minus the localized shear strength, settling will be impeded. If a bridge or bubble in the fuel bed becomes self-supporting, a large void space may form beneath it, with consequent reduction of the gas-solid contact volume, disruption of the vertical temperature profile, and stoppage of the settling of the bed. If the bridge or bubble is not self-supporting, the upward pressure may be relieved by following a random channel of low resistance to gas flow. In this instance, much of the gas stream may actually bypass the fuel bed, resulting in reduced contact time and incomplete reactions. Channelling may be caused by excessively fine material in the fuel bed, while the formation of stable bridges may be a consequence of excessive gas flow rates. The permeability of a bed of lignite particles to the flow of gases will be determined by the pressure drop across the bed, the properties of the gas or gas mixture flowing through the bed, and the characteristics of the void space available for gas flow. The last factor will in turn be determined by the size, shape, exterior roughness, and orientation of the lignite particles. For lignites sized 5x0 cm in 15 cm deep beds and pressure drops on the order of 10 Pa/cm, the percentage of fine particles had a minor effect on the flow of air through the bed [30]. (In this study, fine particles were defined as 6x0 mm.) The effect is illustrated in Figure 12.4 [30]. For a given degree of bed compaction, the air velocity through the bed is given by log/k_P = k log V - log Kp

where AP is the pressure gradient, V the air flow, and

Kpthe permeability constant

[30]. For this

work, the viscosity of the air was treated as a constant. For moisture contents in the range 1.2-35.4% and bulk densities of 840-1180 kg/m3, k is in the range 1.0-1.5 [30]. As the bulk density (i.e., extent of compaction) increases, k approaches 1, which indicates that the air flow is purely viscous and the flow is governed by Darcy's equation [31] for fluid flow through porous media AP = (P1 - P2)/L = -V/Kp

[30]. The dependence of air flow on bulk density, compaction pressure, and moisture content is illustrated in Figure 12.5 [30]. Flow as a function of bed porosity is given by the equation log V = [log AP + 0.135 e - 7.53 - log ~] / [0.806 + 0.0248 e] [30]. The relationship of k log V to bed porosity for the special case of a pressure drop of 83 Pa/cm is shown in Figure 12.6.

610 10

'

+

1% 6x0 mm

9 5% 6x0 mm gt

1

+

A

15% 6x0 mm

,

*

25% 6x0 mm

+~

* 35% 6x0 mm

9 +

+A Ix)

,-"

0.1-

o~ 0 . 0 1 -

r

-.

I~

0.001

....

i .... 1000

900

i .... 1100

J .... 1200

1300

Compaction (Bulk Density), kg/m3 Figure 12.4. The effect of the percentage of fine (50 x 0 mm) lignite on air flow as a function of compaction (expressed as bulk density) [30].

10 +

A

1.2% Moisture

9 26.8% Moisture

t~

It

A 37.2% Moisture

1

r d~

.a pl r

0.1-

pl

~.

.

0.01

\ ''''

800

I''''

900

I''''

I''''

I''''

1000

1100

1200

1300

Compaction (Bulk Density), kg/m3 Figure 12.5. Air flow through a bed of crushed (50 x 0 mm) lignite as functions of bed compaction and lignite moisture content [30].

611 3

4

s

+

2

1.2% Moisture

9 10.8% Moisture A 20.7% Moisture ~ 26.8% Moisture ~o

0

* 35.4% Moisture x 37.2% Moisture -X--

,q -2

0

''''i'

'~176

5

10

'1''''1

15

''''1''

20

25

''1''''

30

35

Porosity (Percentage Voids) Figure 12.6. The air flow through a bed of lignite particles as a function of bed porosity [30]. Flow is expressed as k log V*, where k is the exponent in the flow equation a P - (~t/Kp) Vk.

Sieve analyses of samples taken from the bed of a pilot-scale slagging fixed-bed gasifier show the greatest amount of degradation occurs in the upper portion, during drying and devolatilization [2]. Dry char then undergoes only minor changes in mean particle diameter until it is consumed by gasification and combustion reactions. The packing density of the fuel bed in an operating gasifier depends on the continual rearrangement of the settling fuel particles, the extent of reduction of particle density due to drying and devolatilization, and shrinkage and fracture of lignite particles [29]. Cold flow tests with lignite beds indicate that fluidization begins as the pressure drop exceeds an average column density of 480 kg/m3 [29]. The permeability of the bed will be dominated by the finest components present in any significant quantity [29]. The permeability of a bed containing a broad range of particle sizes will be roughly approximated by the permeability behavior expected for the mass median size [29]. For example, at a gas flow rate of 28 m3/h the addition of 25% of 10x20 mesh lignite to 12x19 mm material increases the pressure drop by up to a factor of 8 [29]. A 50% addition of the 10x20 mesh material would cause a pressure drop exceeding the weight of the bed. Hence the specified size range for the feed to fixed-bed gasifiers is + 12 mm [29]. For example, when a pilot-scale fixedbed gasifier operating normally on lignite screened to 60% + 12 mm, 2% -10 mesh was switched to the same lignite ground but not screened to meet a size specification, a forced shutdown occurred due to channelling that allowed the hot gases from the reaction zone to bypass most of the fuel bed [29 ].

612 For a granular solid flowing downward through an enclosed vessel, the downward motion requires that the particles overcome a supporting shear force that is a function of force components acting on a vertical shear plane. Uniform plug flow occurs when the shear strength of the material in sliding flow against other particles of the same material is greater than the shear strength of the material sliding against the walls of the vessel. In plug flow of solids, relative motion among the flowing particles is insignificant compared to the superficial velocity of the flowing mass of particles relative to the walls. Funnel flow occurs when the shear resistance at the walls is greater than the shear strength of the material against itself. In funnel flow, the material flOWS downward through a central channel while the material contacting the walls remains relatively motionless. In this case, the material first fed into the vessel is generally the last to exit. In a fixed-bed gasifier the distinction between plug and funnel flow is crucial [32]. If funnel flow occurs, lignite fed to the reactor may pass through much more quickly than would have been presumed on the basis of a plug-flow design, while the walls of vessel accumulate an essentially stagnant mass of coal or char that may undergo unwanted secondary reactions. The ability of a given coal to sustain bridges during flow through a fixed-bed gasifier is of major importance to its acceptability as a gasifier feedstock. Since lignite does not agglomerate during gasification, problems of lignite bridging in a gasifier depend on the amount of dust present, the gasifier diameter, the flow rates of lignite and product gases, and the bulk shear strength of the lignite bed. The measurement of thermal and mechanical friability (Chapter 7) can predict how much dust can be expected from a given lignite. In a pilot-scale slagging fixed-bed gasifier, bridging was promoted and maintained more by reduced bed permeability (i.e., a higher zff') than by the effects of the dust on the shear strength of the feed material [33]. Lignite briquettes fed to this gasifier gave unsatisfactory behavior [34]. The briquettes disintegrated during their descent through the shaft, one consequence being the loss of fine char by being blown out of the gasifier. It is likely that some of the fines remained in the bed and adversely affected bed permeability, but that problem was not assessed. If the gasifier of diameter D having rough walls (in which the irregularities on the walls are of comparable size to the larger particle diameters) contains a bridged mass or plug of fuel bed, of height h, the effective weight W of the plug will be the difference between the actual weight and the partial support provided by the pressure drop through the plug. The effective weight W is supported by the shear forces between the plug and the vessel walls. In the case of a lignite with an average bulk density of 640 kg/m3, the bed weight is 64 Pa/cm [33]. Thus an average AP of 64 Pa/cm will more than support the bed, requiring the wall shear forces to keep it from rising or fluidizing. Methods are available (e.g., [35]) to translate free-sliding shear stress data into bed settling behavior. Constrained samples of lignite have sliding shear stresses less than those of the unconstrained samples (Figure 12.7 [33]). A shear stress is applied to a plane through a bed of particles induces a compressive stress normal to the shear plane. This compressive stress must be relieved for shear failure to occur. The greater the particle size, the greater the dimension required

613 10

9

+

.

.

.

++

+

7-

+ +

y. |

1"

"* 5-" i~ ~

9

9 9

ill

+:

4"

+ Shear stress

oo

4=.:. 2

-

+-I:

.be

9

9 Normal compressive stress

i

1" 0 0

' ' ' ' 1 ' ' ' ' 1

1

....

2

I ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1

3

4

5

....

6

I ' ' ' ' 1 ' ' ' '

7

8

9

Normal Compressive Stress, kI~ Figure 12.7. Comparison of free sliding and constrained bulk shear stress relations for 12x20 mm particles of Indian Head lignite [33].

for this stress relief to occur. Thus for impacted masses to resume downward motion through the bed, some mechanism of bed motion is required to allow this stress relief. As the fuel bed is consumed by the gasification and combustion reactions, creation of a void by reaction of the fuel bed insures that any impacted mass collapses inward. The simplest model of collapse is that particles will fall from the "roof" of the void into the reacting cavity, resulting in the void migrating upward through the bed. If a localized concentration of dust causes a higher than normal pressure drop (i.e., a pressure drop approaching the weight of the bed), particles of the void "roof" become essentially weightless. Restraint of the free downward fall of fuel particles may also be caused by the condensation of tars into or onto localized concentrations of fine particles. If the sloughing off of an unconstrained void "roof" becomes critical for stress relief, a slight stickiness of the particles caused by the condensation of a thin film of tar may significantly alter the bed behavior. In a packed column of lignite, the shear planes will be vertical and the compressive forces radial. As the ratio of column diameter to particle size increases, the required radial compression for stress relief will become smaller and more easily achieved. To achieve uniform slug flow, the lignite must be in an incomplete, but uniform, state of consolidation so that any necessary stress relief can be accomplished with a rearrangement of particles across the vessel. When impacted zones form in the bed, stress relief is possible only at the bottom of the zone, as the ceiling of the zone drops off. If the "bubble" is not a cavity but a volume filled with lignite at lower density, disintegration of the ceiling can be impeded, and therefore the impacted zone can be maintained

614 [29]. During gasification, drying and devolatilization in the upper portion of the bed result in shrinkage, and the bed will be consumed entirely by the gasification and combustion reactions occurring below. The location in the bed at which each of these processes occurs depends on the durability of temporary domes, in that, if the dome can collapse, it will do so to supply more fuel to the steam and oxygen, or if the dome resists collapse, the steam and oxygen reactions will shift upward in the bed to consume the dome. The latter case can have potentially serious consequences for process stability. Thus it is desirable to provide stress relief for impacted zones in the fuel bed. Since a stirrer operating in a bed on non-caking coal will generate excess fines by attrition of the friable lignite, some new design concepts for providing stress relief without a traditional rotating stirrer would be useful. The mechanical behavior of packed columns of lignite depends so strongly on changes in the structural properties of the lignite that models based on the assumption of ideal slug flow and isotropic bulk properties are useless [36]. Accepted feed size specifications for operation of fixedbed gasifiers are conservative. Fixed-bed gasifiers can be operated with finer-than-specified feeds if the thermal and mechanical friabilities and the volumetric shrinkage of the feed can be predicted [36]. The finer particles dominate pressure drop through a bed with a broad range of particle sizes, to an extent beyond that predicted by the weight fraction of the fine particles [36]. Both conduction and convection contribute to heat transfer in lignite fuel beds. At high compactions and low temperatures, conduction predominates. However, with severe heating, characterized by high temperatures and the evaporation and recondensation of moisture, or at low compactions, convection will predominate. The thermal conductivity of a porous bed is given by Russell's equation

kbed ] ksolid =

[1

- i;0.67

(1 - ct)] / [1 -

(1~0.67 -

e)(1 - ct)]

[37]. (Thermal conductivity of solid lignite has been discussed in Chapter 7.) The effects of compaction and moisture content on the thermal conductivity of beds of lignite particles are shown in Figures 12.8 and 12.9, respectively. The thermal conductivity of crushed lignite lies in the range of 0.31 to 1.56 kJ/h.m.~ in conditions where heat transfer by conduction predominates [30]. In a fixed-bed gasifier the incoming lignite is heated by ascending product gases to the temperature at which the moisture in the lignite can be vaporized. This process largely determines the offtake temperature of the product gas, since much of the sensible heat in the ascending gas is needed to vaporize the water. With lignites, reducing the moisture content of the feed from 35% to 3% could increase the offtake temperature from under 150~to 540~ [2]. 12.1.2. Commercial gasification in North America The first two subsections provide the background and description of the Dakota Gasification plant in Beulah, North Dakota, which is the only large-scale gasification plant producing substitute natural gas in North America. The third subsection is a brief description of the

615 0.4

~

ro

0

+ 5% Moisture 9 20% Moisture

~0.3

A 30% Moisture

~0.2-

~

O

36% Moisture

o

0.1

0 7( ~)

'

'

'

'

I

i

900

'

'

'

'

I

1100

'

'

!

'

1300

Compaction (Bulk Density), kg/m3 Figure 12.8. The effect of compaction on the thermal conductivity of a bed of crushed (50x0 mm) lignite at 21-100"C, for various lignite moisture contents [30].

0.4 + 800 kg/m3

r.)

9 960 kg/m3

0.3~

A 1120 kg/m3 ~ 1200 kg/m3

0.2

o

~ 0.1-~ -

0

'

0

'

'

'

I

'

' '

'

I

' '

' '

I

'

'

10 20 30 Lignite Moisture Content, %

'

'

40

Figure 12.9. The effect of lignite moisture content on the thermal conductivity of a bed of crushed (50xO mm) lignite at 21-100~ for different degrees of compaction (as expressed by bulk density) [30].

616 Elgin-Butler gasifier, which provides an example of what can be accomplished in the small-scale commercial gasification of lignite for industrial use. (i) Background. In the late 1960's American Natural Gas Service Company (Detroit) began active consideration of alternatives to natural gas. Importation of liquefied natural gas was undesirable considering the geopolitical uncertainties of the time [38]. Production of gas from liquid petroleum feedstocks was of questionable economic viability given the rapid changes in petroleum prices then occurring [38]. Consequently, attention was given to producing substitute natural gas from coal. Coals in the western United States were attractive because they are won by strip mining, with productivity in tonnes/man-day higher by up to a factor of 10 than underground mines producing bituminous coal in the east [38]. Thus the cost per tonne of strip-mined coal was less likely to be affected by rapid increases in labor costs. Furthermore, a strip mine requires smaller capital investment per tonne of production than does an underground mine. North Dakota lignite was attractive because of the large reserves which were not already dedicated to other consumers. Large, privately held reserves were available at low cost. In the early 70's the projected requirement for substitute natural gas involved four plants each producing 7

Mm3/dayof

gas. Coal options began to be acquired in 1972 [39]. That year an agreement was reached with North American Coal Corporation to lease much of North American's holdings of North Dakota lignite to American Natural Resources [40]. Coteau Properties Corporation was formed as a subsidiary of North American to acquire and maintain the coal leases and to mine the lignite for American Natural Resources. The gasification technology selected for the proposed plants was the Lurgi process. The decision was based largely on the fact that Lurgi gasification is commercially proven technology and on the presumption that the so-called second generation gasification processes would not be commercially developed by the late 1980's [38]. The Lurgi process is a fixed-bed gasification process capable of operation at elevated pressures [7]. Originally developed in Germany prior to World War II, by the 1950's it had been used commercially in at least six other countries [41-43]. Commercial Lurgi gasifiers operate on the dry-ash principle; that is, the ash is removed as a solid through a rotating grate. Successful operation requires that the temperature be controlled carefully to prevent formation of clinkers on the grate. To insure that temperatures are kept below the softening temperature of the ash, the oxygen supplied is generally limited to 15-20% (by volume) of the oxygen-steam mixture [7]. Doing so results in the amount of steam supplied to the gasifier being well in excess of the amount needed for gasification; a consequence during operation is that the water vapor content of the exit gas is high. In July 1974 11 kt of lignite was shipped to the SASOL plant in South Africa for full-scale trials in a Lurgi gasifier. The oxygen-blown Lurgi gasifiers have by now shown quite clearly their suitability for operation on lignite. Several positive features have been identified [44,45]: the non-caking nature of lignites makes their transit through the gasifier easier than for caking coals; and the high reactivity of lignites allows for a high throughput rate with virtually complete carbon conversion. In addition, the steam generated in the water jacket of the gasifier can be used in the process, thus

617 ultimately capturing 95% of the energy potential of the lignite in the product gas; and low gas temperatures (230-315~

at the gasifier exit eliminates carryover of alkalis [44].

Although the original concept was based on four plants each producing 7 Mm3/day of substitute natural gas (with the total 28 Mm3/day (one billion scf/day) representing 40% of American Natural Resources's requirements), by 1974 the rapid escalation in the cost of the plant reduced the project to a single plant to supply 10% of the company's needs [40]. In 1976 a decision was made to build the plant in two phases of 3.5 Mm3/day each [40]. In 1975 the cost of gas in the first year was estimated to be $0.11/km3 ($3.00/Mcf) and, over the estimated 25 year life of the plant, then projected to be 1981-2006, the average gas cost was projected to be $0.07/ km3 ($2.00/Mcf) [38]. The original schedule presumed, quite optimistically as events subsequently proved, that all of the necessary bureaucratic approvals would be obtained within a year, and that site work would start in the fall of 1976. The start-up of the first gasification train was projected for January 1, 1981, with the complete plant being on stream by October 1, 1981 [38]. A history of the financial, bureaucratic, and legal travail associated with the project is outside the scope of this book, but is well chronicled in the literature (e.g.,[36,40,46]). In 1978 the Great Plains Gasification Associates partnership was formed among ANR Gasification Properties Company (a subsidiary of American Natural), The Peoples Energy Corporation, Columbia Gas System, Tenneco, and Transco Companies. The project administrator was ANG Coal Gasification Company, another subsidiary of American Natural. Ultimately, forty-two permits and approvals were required to construct the plant, and nine additional permits for the coal mine [40]. Together with the continual reduction in the scope of the project, these administrative changes resulted in a significant redirection of the project from an entity first planned to be a major source of substitute natural gas for a single company had become a commercial-sized demonstration plant for the natural gas industry [40]. By 1980 it was estimated that gas would be produced by 1984. The cost estimate, in current dollar costs, was $9.35/GJ ($8.87/million Btu), substantially above the predicted cost of new wellhead natural gas ($4.19) and imported crude oil ($8.21). However, projections indicated that gas from the plant would become less expensive than the new wellhead natural gas by 1990 and would cost less, in dollars per gigajoule, than imported crude oil by 1986 [36]. Ground-breaking for the plant at Beulah, North Dakota was held on July 25, 1980 [47]. The plant was then expected to come on line in late 1984 with a planned throughput of 12.7 kt/day of lignite and output of 3.9 Mm3/day of substitute natural gas. Full-scale construction began in August, 1981 and took about three years [39]. Commissioning of the utilities began in August, 1983 with the plant fully commissioned and operating in the fall of 1984 [39]. The energy efficiency was projected to be 66% [47]. The U.S. government took over the plant in August, 1985, after the five partners in GPGA defaulted on $1.5 billion in government loans and abandoned the facility. A mechanism was sought to return the plant to the private sector, while at the same time keeping the plant operating and recovering some or all of the government's investment. Nine offers were received for the

618 plant, and in August, 1988 the offer from Basin Electric Power Cooperative Association of Bismarck, North Dakota was accepted. On August 5, 1988, then-Secretary of Energy John Herrington announced the sale of the Great Plains plant to Basin Electric Power Cooperative [48]. Basin Electric owns the adjacent power station, the two facilities sharing lignite and water supplies. Through a complex series of arrangements (the sale of the plant to Basin Electric is said to have involved the signing of 70 documents [49]). Basin Electric has received the title and fights to the gasification plant and the adjacent lignite mine. It was intended that the plant should be kept running through 2009 provided that the revenues exceed expenses. Basin Electric took over operation from the federal government on October 31, 1988 [50]. Dakota Gasification Co., the subsidiary of Basin Electric, reported a profit of $35 million in the first year of operation [50]. In that year the plant averaged gas production of 4.2 Mma/day, occasionally exceeding 4.5 Mm3/day, well in excess of the design capacity of 3.9 Mm3/day [50]. This was accomplished while holding actual production costs 12% below anticipated costs [50]. Basin Electric and its Dakota Gasification subsidiary are not required to share profits from byproducts with the government [50]. The profitable by-products potentially include phenols, for production of adhesive resins for plywood and chipboard manufacture; krypton and xenon (from the air separation units) for various types of lights; and carbon dioxide, which could be used for enhanced oil recovery in the Williston Basin. Unfortunately, the "by-product bonanza" [51] has not fully materialized because of high costs for adding some of the necessary capital equipment and reduced demand for some of the by-products (e.g., argon, naphtha, and creosote). Nevertheless, the plant has operated profitably at least through 1992, producing 1.6 Gma/y of gas from 5.4 Mt of lignite [51]. (ii) Process and plant description. Lignite is delivered to the plant by truck from the nearby Freedom Mine (Coteau Properties Company). The lignite is crushed to 20 cm particles in a primary crusher [39,52]. The particle size is reduced to 5 cm in secondary crushers. The 5-cm sized lignite is transferred to live storage, a building having a capacity of 110 kt. Lignite from the live storage is screened. Approximately 12.7 kt of crushed, 50x6 mm sized lignite per day are used for gasifier feed [52,53]. The -6 mm fines, amounting to about 7.2 kt/day, are sent to the adjacent electric power plant owned by Basin Electric Power Cooperative. The Lurgi Mark IV gasifiers, which accept lignite feed screened to 50x6 mm [47], operate at 3 MPa [28,47,52,54]. Each gasifier has a nominal output of 52 km3/h [28,47]. The plant has fourteen gasifiers, of which twelve are normally in operation, with two spares. The gasifiers operate with countercurrent flow of coal, fed through the top, and a steam/oxygen mixture injected in the bottom. The maximum temperature attained in the gasifier is 1200~ [53]. The raw gas exiting the gasifiers has a calorific value of 11.5 MJ/m3 [28]. Initially problems existed with excessive fines being fed to the gasifiers as a result of friability of the lignite and size separation [45]. Improved screening and handling methods reduced these problems. The reactivity of the lignite proved to be higher than anticipated, requiting adjustments in the steam temperature and flow rate to achieve satisfactory control [45].

619 One-third of the gas stream is shifted over sulfur-resistant cobalt molybdenum catalysts. The shift reaction adjusts the H2/CO ratio to slightly over 3 [28]. The mixture of shifted gas and the remaining two-thirds of the raw product gas are cooled to remove tar, oils, ammonia, and phenols. Cooling to condense tars, oils, and ammonia occurs in four stages. The first cooling stage produces 225,000 kg/h of steam at 800 kPa [39]. This steam is used elsewhere in the plant. The next two cooling stages produce 520 and 275 kPa steam [39]. The last stage removes heat from the gas with cooling water. A low-temperature (-73~

methanol wash (the Rectisol process)

is used to remove carbon dioxide, naphtha, and sulfur compounds. Four additional stages of cooling reduce the gas temperature to --40~ before it enters the Rectisol units. The total sulfur content of the gas exiting the Rectisol units is less than 100 parts per billion [39]. The recovered naphtha, along with the tar and oil condensate, are used as boiler fuel. The sulfur compounds removed by the Rectisol process were then treated to produce elemental sulfur in the Stretford process. The off-gases from the Stretford unit were incinerated in the boilers. The original Stretford process has now been replaced by the Sulfolin process for sulfur control [45]. Methanation of the sulfur-free synthesis gas is performed in Lurgi methanators in the presence of nickel catalysts. Methanation is exothermic; cooling the product gas produces 363,000 kg/h of 9 MPa steam [39]. The steam is superheated to match the quality of the high-pressure steam used for the gasifiers; as a result, about 50% of the total energy of the plant is obtained from waste heat recovery from the methanators [39]. Any residual carbon monoxide is removed in a clean-up methanator. The product gas has a calorific value of 36.4 MJ/m3 [28]. The final handling of the gas involves cooling, drying, and compression in two stages to the pipeline pressure of 10 MPa [28]. The plant ties into existing pipeline networks through a 50 cm, 588 km-long pipeline from the plant to connect with the Great Lakes Gas Transmission Company system at Thief River Falls, Minnesota. Gas liquor is condensed during the cooling of the raw gas leaving the gasifiers. The gas liquor, produced at a rate of 450,000 kg/h [39], is essentially an aqueous emulsion of ammonia, phenols and other water-soluble organic compounds, oils, and tars. The tars may contain some lignite fines. Gravity separation of the gas liquor removes a "dusty tar" which is reinjected into the gasifiers [39,45]. Oils are separated and used as boiler fuel. The aqueous stream is extracted with diisopropyl ether in the Lurgi Phenolsolvan process. This extraction removes phenols; subsequent distillation of the ether stream recovers the ether for reuse and separates phenols. The dephenolized water is then treated in the USX Corporation (formerly U.S. Steel) Phosam process. This process heats the water to the bubble point and the vapors are stripped. The vapors are washed with a solution of diammonium phosphate to recover ammonia. Stripping the ammonium phosphate solution allows recovery of the ammonia, which is eventually marketed as 99.99% pure anhydrous ammonia [39]. The water remaining after phenol and ammonia recovery serves as cooling tower makeup. The use of treated wastewaters in process cooling towers reduces the net water demand from aquifers [25]. The production of phenols and cresols amounts to about 80 t/day [53]. These compounds can be used with the naphtha, tars, and oils as boiler fuel. In the long term, these

620 materials may be marketable by-products, since phenols and cresols have a wide range of uses in the chemical industry. The yields of aromatic compounds, on a moisture-and-ash-free lignite basis, are 0.47% benzene, 0.23% toluene, 0.10% xylene, 0.66% phenol, 0.54% cresols, 0.32% xylenols, and 3.50% creosote [55]. Pure nitrogen is produced as a by-product of the oxygen plant, but it is not clear whether there would be a market for this commodity. Other potential by-products from the oxygen plant include krypton (236 L/h) and xenon (22 L/h), which may eventually find use in the lamp industry [53]. The plant produces about 5.7 Mm3 of by-product carbon dioxide per day [53]. The primary use for the carbon dioxide is injection into oil wells for enhanced oil recovery. By-product sulfur, amounting to 70-90 t/day, was projected to be used in manufacture of sulfuric acid for eventual use in the Minnesota and Wisconsin paper industries [53]. The daily production of anhydrous, fertilizer-grade ammonia is 84--113 t [53]. The ammonia has two potential markets, one as a fertilizer in the northern Great Plains during the spring and fall planting seasons, and the second as a commodity in the paper industry. The ash from the gasifiers amounts to 800 t/day [52]. In principle, an enormous number of products could be made from the liquid by-product streams. However, refined fuel products do not seem likely to be competitive with petroleumderived products in the near future [56]. High-volume petroleum refining enjoys economies of scale which make it unlikely that a small by-products operation would be cost effective [56]. Only one refinery in the northern Great Plains (in Regina, Saskatchewan) appears to have the capability to accept the tar oil stream as a feedstock. The tar oils have higher phenol concentrations and a high aromaticity, and products taking advantage of this unique composition may offer greater potential for profitable operation than refining the tar oil stream to liquid fuels. An economic analysis suggested that the optimum product slate would include military jet fuel produced from the tar oil stream, along with benzene from the naphtha and phenols from the crude phenol streams; however, the production of jet fuel would require financial support from the government [56]. Indeed, a fairly extensive study was made of the feasibility of producing military jet fuels from the tar oil stream, and of the properties of such fuels

[e.g., 56-59]. It appears, though, that this

program has been supplanted by one aimed at making jet fuels of high thermal stability by direct liquefaction of bituminous coals

[e.g., 60].

The most important result of building and operating the Dakota Gasification plant has been the demonstration that it is indeed possible to construct a large synthetic fuels plant on schedule and within budget, a feat all the more noteworthy because of the remoteness of the plant site (Beulah, North Dakota) and the extreme climatic conditions that prevail there [45]. The experience has demonstrated a number of other factors [45]. Operating problems with the gasifiers and variations in gas production relate to the quality of the lignite feed, specifically, to reactivity, composition, and ash fusion behavior. Thus the variations of lignite quality within the mine should be known in advance of start-up. Also, it is indeed feasible to design for zero liquid discharge, using gas liquor (after removal of phenols, oils, and ammonia) as makeup to the cooling towers. (iii) The Elgin-Butler

gasifier. The gasifier system installed by the Elgin-Butler Brick Co.

621 (near Austin, Texas) provides an example of the small-scale gasification of lignite for industrial purposes. The Elgin-Butler company strip mines clay for brick making. The clay is overlain by lignite, which for many years had simply been stockpiled. Brick manufacturing requires large quantities of a clean burning and inexpensive fuel. A typical tunnel kiln for firing bricks at 120ffC requires 13.2 GJ/h of thermal energy from gas [61]. Around 1980 a decision was made to use the stockpiled lignite as a source of gas for the brick kilns.The circumstances peculiar to the ElginButler company dictated the requirements for the gasification system. The specific concerns were a) the ability to use stockpiled lignite without beneficiation, b) the ability to produce 32 GJ/h of gas of 5.1 MJ/m3 calorific value, c) provision for removal of ash and sulfur prior to firing the gas in the kilns (to avoid fouling the brick materials), and d) low capital and operating costs. The unit selected to meet these requirements is a travelling grate sawdust gasifier. The system is illustrated in Figure 12.10 [61]. The perceived process advantages of the travelling grate sawdust gasifier are its ability to accept-10 cm lignite (including fines) at up to 30% moisture, sulfur retention in the ash, and cracking the tars to gas.

AIR

I LIGNITE

HExEAT FIRST STAGE

~AIR

v

CHANGER)

VOLATILES

CHAR and ASH HOT GAS and STEAM THIRD STAGE

AsH I

r

IAIR v

GAS

GAS to KILN GRIT

Figure 12.10. Block flow diagram of Elgin-Butler travelling grate gasification plant [61].

In operation, lignite is fed through the rotary air lock into the drying tube. The drying tube, forming the top of the pyrolysis chamber, exposes the lignite to gas temperatures of about 870~ The dried lignite, and the vaporized steam, are discharged by auger into a pyrolysis chamber. Here the lignite temperature is raised to 3200C. Injection of air into the travelling bed of hot, dry lignite

622 provides enough combustion to supply the heat needed to drive the endothermic drying, pyrolysis, and steam-carbon reactions. The second stage of the process occurs in a thermal cracking unit. The feed to this unit consists of gases and fines from the pyrolysis chamber. Here further air injection maintains the temperature at 9800C. Cracking of tars and the steam-carbon reaction occur in this unit. Char from the pyrolysis chamber is fed via a rotary lock feeder to a travelling bed gasification chamber where, as in the thermal cracking unit, more air is injected to maintain the temperature at 980~ and generate sufficient heat to drive the gasification reactions. The total carbon conversion efficiency of the system, on an energy basis, is 70--76% [35]. Ash is discharged via an auger. The gas and entrained particulates are passed through a series of cyclone separators and through a heat exchanger which reduces the gas temperature to 200"C. The gas, which is eventually discharged into the kiln piping system, is typically 55% nitrogen, 21% carbon monoxide, 14% hydrogen, and 10% carbon dioxide, with traces of steam and hydrogen sulfide. The calorific value is 5.1 MJ/m3 at standard conditions [61]. 12.1.3 Commercial gasification outside North America This section briefly discusses gasification processes that have been used successfully outside North America, and which therefore should be easily adapted to North American lignites. (i) Winkler gasification. The Winkler gasification process is based on fluidized-bed reactors. Outside the United States the Winkler process has been used commercially in a wide range of applications, which include production of gas of both low and medium calorific value, production of synthesis gas for the Fischer-Tropsch and other processes, and production of hydrogen. Successful commercial operation has been obtained on low-rank coals in the 9x0 mm size range with high sulfur contents and up to 50% ash [62]. Although these gasifiers are not in commercial use in North America for gasification of lignite, there appears to be no technical reason forestalling their immediate application to North American lignites on a commercial scale. The Winkler process is, in principle, useful for any carbonaceous fuel from wood to graphite; but wood, peat, and low-rank coals are preferable to the high-rank coals [62]. Only minimal fuel preparation is needed--crushing to 10x0 mm and drying sufficiently to allow good flow of the solid fuel. A screw conveyor charges lignite from a feed hopper into the lower portion of the gasifier. A mixture of preheated air or oxygen and steam is used as the fluidization and gasification medium. The mixture is also injected into the freeboard above the bed to insure complete gasification of any lignite particles entrained by the product gases. No tars or liquids are produced, apparently as a result of reforming reactions in the bed or the freeboard. With oxygen blowing, a Winkler gasifier will produce a mixture of about 82% 1:1 CO:HE and 14% CO2 [62]. Small amounts of methane are produced, but no higher hydrocarbon gases. An extensive list of commercial Winkler operations has been published [62]. Of these, plants in Germany, the former Soviet Union, Bulgaria, Yugoslavia, Turkey and India have successfully operated on lignite. Some of the plants, which have as many as seven gasifiers, have been in operation since 1950. A commercial fertilizer plant in Kutahya, Turkey has successfully

623 used Winkler gasifiers for over 20 years. The design capacity is 24,000 m3/h, though the plant has operated at 130% of design capacity with 73.7% carbon Turkish lignites containing up to 50% ash [62]. Operating data for an oxygen-blown Winkler gasifying an unidentified lignite are presented in Table 12.2 [28]. TABLE 12.2 Gasification of lignite in a Winkler gasifier [28]. Gas composition (volume %) Hydrogen Carbon Monoxide Methane Nitrogen Carbon Dioxide Higher heating value, MJ/m3 Feed requirements, kg/kg lignite Oxygen Steam Operating conditions Gas outlet temp., ~ Pressure Lignite residence time, min. Turndown capability Thermal efficiency, % Hot mw gas Cold clean gas

35 48 2 1 14 10.8 0.5 0.4-0.7 620 atmospheric 20-30 4:1 81.8 74.9

Gasification of low-rank coals in a Winkler gasifier is subject to a number of considerations, most of which are advantages for lignites compared to other coals. Coals having a high, or fluctuating, ash content can be used without upsetting performance of the unit. The upper limit on coal moisture for satisfactory operation appears to be about 18%, which would require some drying for lignites [28]. Coals of high reactivity are preferred, because a higher reaction rate reduces carbon carryover with the product gas. Non-caking behavior is preferred as an aid to gassolid contacting and maintaining proper particle size distribution in the bed. The preferred feed size is -9 mm. The ability to tolerate fines allows the gasification of friable coals. The temperature in the bed must be maintained 20-28~ below the ash softening temperature [28]. The high-temperature Winkler, or HTW, process operates at temperatures to 1090"C and pressures to 1 MPa [45]. These temperatures provide some risk of ash fusion in the bed. Limestone or dolomite additions raise the fusion point of the ash sufficiently to provide satisfactory control. The HTW process has several advantages relative to the conventional Winkler process. These advantages result mainly from the increased temperature and pressure, and include more effective utilization of the lignite, improved gasifier capacity, and avoidance of by-product

624 formation [63]. In the HTW process, cyclones remove dust from the hot raw product gas. The dust recycling from the hot cyclones provides carbon conversions up to 98% [63]. Lignites from Canada, the United States, and Greece have given satisfactory performance in small-scale tests of the HTW process [63]. (ii) Koppers-Totzek gasification. There are no commercial scale Koppers-Totzek gasifiers operating in North America, but lignite-fed commercial units have been installed in Spain, Portugal, Greece, Thailand, and Turkey [28]. The Koppers-Totzek gasifier is horizontal, with two (or, in some designs, four) "heads" in the shape of truncated cones on the ends. A waste-heat boiler is mounted vertically over the gasifier. Each head contains two adjacent burners through which coal, steam, and oxygen are fired. The feed coal is pulverized t o - 2 0 0 mesh and dried to 2-8% [28]. The pulverized coal contacts the premixed oxygen-steam at the injection nozzles of the burners. The reaction temperature inside the gasifier is about 1900"C; reactions are complete in 1 s [28]. The endothermic reactions and heat losses reduce the temperature of the gasifier effluent to about 1500~

At these extreme temperatures the inorganic components of the coal melt. About half

the slag drains into a slag quench tank. The remainder is entrained in the effluent gas. Water sprays at the gasifier exit cool the gas and solidify the slag. Coals of very high moisture contents, such as lignites, are at an economic disadvantage because of the costs and difficulties in drying the coal before gasification; however, it may be possible to tolerate coals of moderately high moisture content by reducing the steam injected with the feed to the gasifier. Although steam consumption is low, the oxygen consumption is quite high for a gasifier, being 0.9 kg/kg lignite [28]. Since no tars, oils, phenols, or naphthas are produced, necessary wastewater treatment is minimal. Carbon conversion reaches nearly 100% for lignites [28]. This is again an indication of the higher gasification reactivities of low-rank coals and the potentially higher process efficiencies attainable with such feedstocks. If gasification of low-rank coals could be carried out at temperatures lower than the typical 19000C, the loss of efficiency caused by the water quench step immediately after gasification could be reduced. Despite a low concentration of coal in the process stream, entrained-flow gasifiers are capable of providing high mass throughputs and high energy outputs per unit volume. The high reaction rates achieved by the use of finely pulverized coal make this possible. However, the conversion efficiency in entrained-flow gasification is sensitive to coal rank. In the KoppersTotzek gasifier, the highest efficiencies are obtained with lignites (>98%), with efficiencies of about 85% for subbituminous coals and about 60% for anthracites [28]. The lower reactivity of high-rank coal chars, combined with the very short residence times of entrained flow gasifiers, are the factors responsible for the superior performance of lignites. 12.1.4 Pilot-scale gasification of lignites This section discusses some of the processes that have been tested at the pilot-plant scale, generally in continuous flow reactors, for the gasification of lignites. (i) The C02 Acceptor Process. The CO2 Acceptor Process is a fluidized-bed gasification

625 system designed to convert lignite or subbituminous coal into a substitute natural gas having a calorific value of 33.5-35.4 MJ/m3 [64]. Development of the CO2 Acceptor Process by the Consolidation Coal Company began in the early 1960's. The novelty of the process is that it uses the exothermic carbonation reaction of calcined limestone or dolomite with carbon dioxide to provide the heat necessary for the gasification process. The limestone or dolomite is re-calcined in a separate fluidized-bed reactor heated with char. The C O 2 Acceptor Process produces a substitute natural gas without using oxygen. Carbon utilization is over 99% [28,45]. The cold gas thermal efficiency is 77% [45]. Hydrocarbons other than methane are not present in the substitute natural gas, and concentrations of carbon dioxide and hydrogen sulfide are minimal. In addition, the H2/CO ratio in the product gas is greater than 3, eliminating the need for shift conversion prior to methanation. Since the steam-carbon reaction is endothermic, any gasification process must have a strategy for supplying enough heat to overcome the endothermicity of the steam-carbon reaction. The unique feature of the CO2 Acceptor Process was the use of the reaction between calcined dolomite or limestone (literally, the "CO2 acceptor") and carbon dioxide to serve as the exothermic heat source. At the same time, the capture, or acceptance, of carbon dioxide removes much of this gas and thus reduces the burden on downstream gas purification operations. The CO2 Acceptor Process used two fluidized bed reactors. In the first, the devolatizer/gasifier, lignite that has been ground (pre-screened to 8x100 mesh [45]), dried, and preheated was reacted with steam in the presence of hot, calcined dolomite. Operating conditions were 815~ and 1-2 MPa [64,65]. (This reactor temperature limits operation to low-rank coals, since feedstocks must have high reactivity at this temperature [45].) The lignite enters a bed of lignite char. The acceptor is then "showered" downward through the bed, and collects in the lower section of the gasifier, called the "boot." The heat needed to overcome the endothermic steamcarbon reaction was derived from the heat of recarbonization of the calcined acceptor and partly from the sensible heat it supplied. In the second reactor, the regenerator, char remaining from the partial gasification of the lignite was burned at about 1040~

[65] to supply the heat necessary to

calcine the spent acceptor. Since the gasifier uses no air (other than at start-up) the product gas has essentially no nitrogen. A flowsheet of the CO2 Acceptor Process is shown in Figure 12.11 [66]. The lignite is reduced to 8x100 mesh and dried to 5% moisture [64]. Lignite is fed into the bottom of the gasifier, where it experiences rapid hydrodevolatilization, followed by steam gasification of the char. The purpose of feeding the lignite deep into the bed is to provide the maximum retention time for volatiles, to allow them to crack to methane, hydrogen, and the carbon oxides in the bed. Steam is injected at the bottom of the gasifier. The char bed depth is about 7.6 m [64]. Heat for the endothermic gasification reaction comes from reaction of the acceptor (either calcined limestone or dolomite) with carbon dioxide. The reaction of calcium oxide with carbon dioxide liberates 177 kJ/mol at 250C [66]. The gasification rate is 0.0040 kg C gasified/kg C in the bed/min at 840~

626

"- FLUE GAS

I~ ASH

9

~

GAS

1-,

D \

~ ,

LIGNITE

~

GAS

MAKE-UP ACCEPTOR;,.d" "-I

MgO-CaC03

STEAM and AIR

L

LII~ GAS

S

ik REIECT ACCEPTOR

Figure 12.11. Block flow diagram of the CO2 Acceptor process [66].

[66].The purpose of the regenerator is to remove carbon dioxide from the spent acceptor. The regenerator operates at 1010~

heat is supplied by burning the residual char from the gasifier. The

acceptor gradually loses reactivity after multiple passes through the system. Some of the acceptor inventory is withdrawn from the gasifier and replaced by fresh makeup. The makeup is added as the acceptor is being returned to the regenerator. There is little evidence for ash fusion in the regenerator. Some problems were encountered as a result of partial conversion of calcium sulfide, present in char ash, to calcium sulfate, by reaction with either oxygen or carbon dioxide (or both). Above 925"C a mixture of calcium sulfide and calcium sulfate melts, the melt then trapping char ash and acceptor fines [64]. The sulfide and sulfate can react with each other to produce calcium oxide and sulfur dioxide; the calcium oxide resolidifies as a deposit on the wall. Devolatilization and gasification reactions in the gasifier consume about 63% of the carbon in the lignite [66].Total consumption of carbon in the process amounted to 95.6%, with 0.7% in the ash leaving the regenerator and 3.7% lost through the gasifier cyclone [66]. The product gas has a H2/CO ratio of 3.6. Since a value of 3 is needed for methanation, the excess hydrogen could be used to methanate some of the carbon dioxide. The only hydrocarbon in the product gas was

627 methane, obviating the need for removal of tars and oils. The only gas clean-up needed is removal of ammonia, hydrogen sulfide, and a portion of the carbon dioxide. The gasifier product gas flows through a steam-generating heat exchanger, a quench tower, a scrubber, acid gas removal system, and then into a packed-bed methanator. The raw product gas typically has very high hydrogen content and relatively high methane. The waste products from the CO2 Acceptor Process are the ash, spent limestone or dolomite acceptor, acid gases, and some sour water. Operating data obtained with an unspecified lignite feedstock are summarized in Table 12.3 [281. TABLE 12.3 Operating data for CO2 Acceptor Process [28] Gas Composition, mole % Carbon monoxide Carbon dioxide Hydrogen Methane Hydrogen sulfide + carbonyl sulfide Nitrogen + argon Raw Gas Heating Value, MJ/m3 Feed Requirements, kg/kg lignite Air (regenerator) Steam (gasifier) Operating Conditions Pressure, MPa Temperatures, ~ Regenerator Gasifier Gas velocities, m/s Cold Gas Thermal Efficiency, %

15.5 9.1 58.8 13.7 0.0 2.9 14.3 2.3 1.1 1 1010 815 0.55- 0.73 77

Among the advantages claimed for the C O 2 Acceptor Process are the ability to use fines in the gasifier, elimination of the need for oxygen, a lower water requirement than the Lurgi dry-ash gasifier, elimination of production of tars and oils due to the high operating temperatures, and minimal contamination by carbon dioxide, hydrogen sulfide, and higher hydrocarbons [28]. Water gas shift is not necessary if the product is to be methanated for pipeline gas production. The capture of significant amounts of CO2, along with H 2S, in the devolatilizer means that only small amounts of these gases need to be removed during a final gas purification. A disadvantage of the process is the need to dry the lignite to about 5% moisture, this being an expensive and difficult operation. Other disadvantages recognized for the process were 1) very low methane concentrations in the product gas require a large separate methanation operation; 2) the relatively low pressures of operation require compression of the gas for introduction to a pipeline; and 3) running two fluidized bed reactors in synchronization is a complex process and may have potential

628 problems in controlling the operation. The CO2 Acceptor Process was tested in a 36 t/day pilot plant in Rapid City, South Dakota. By 1975 the plant had been operated in a test in which the recycling acceptor supplied the entire heat requirements of the gasifier for 171 hours and an overall process efficiency of 77% was obtained [66]. The feedstock for this test was Velva lignite. (ii) The Wellman-Galusha gasifier. The Wellman-Galusha gasifier is an atmospheric pressure reactor that can be operated with either steam/air or steam/oxygen mixtures as the gasifying medium. A typical Wellman-Galusha gasifier is a cylindrical vessel 0.5-3 m in diameter [28]. The gasifier is refractory lined or cooled by a water jacket. Coal is fed through vertical feed pipes; the gasification medium is introduced through the bottom of the bed. The product gas exits the top of the gasifier at temperatures in the range of 480-650~ [28], though some temperatures as low as 200~ have been reported [67]. After removal of entrained particulate, the gas can be used directly as a fuel gas. The high moisture level of low-rank coals can cause problems in handling the crushed coal, and will lead to increased production of tar liquor. On the other hand, the high moisture level in the feed coal can reduce the temperature of the exit gas. Low-rank coals which have ashes of low softening temperature will certainly require careful temperature control. They may also require injection of additional steam to keep temperatures in the reaction zone below the fusion temperatures. If additional steam injection is required, gasification efficiency will be reduced and the production of tar liquor will be increased. The Wellman-Galusha gasifier is a countercurrent-flow, fixed-bed unit. The gasifier can be operated at atmospheric or elevated pressure and with air or oxygen blowing. Lignite is fed from a lock hopper into the gasifier, where, as typical of fixed-bed gasification processes, the lignite is successively dried, devolatilized, gasified, and combusted as it descends through the vessel. The feedstock size range for optimum performance is 50x6 mm [67]. If the amount of -6 mm material in the feed is large, some problems can be encountered during operation. Ash is removed through a grate. For an air-blown, atmospheric-pressure unit, air saturated with water vapor is injected through the grate; by varying the saturation temperature (i.e., the steam/air ratio) it is possible to maintain the temperature of the combustion zone below the point at which ash fusion would begin. For any given feedstock the product gas composition is determined by the steam/air ratio of the blast, As this ratio increases, the proportions of hydrogen and carbon dioxide in the product gas increase, and the carbon monoxide decreases. This behavior reflects attainment of equilibrium in the water gas shift reaction inside the gasifier. The proportion of carbon dioxide in the product gas increases as the rank of the feedstock decreases. As rank decreases, more gaseous sulfurcontaining products appear as mercaptans rather than as hydrogen sulfide. With lignites, up to 20% of the sulfur can be appear in the products as mercaptans [67]. With air blowing at atmospheric pressure the ash is generally low in carbon. Usually less than 1% of the carbon in the feed remains in the ash [67]. The unconsumed carbon in the ash increases as the steam/air ratio increases. Since a greater amount of steam entering the combustion

629 zone will lower the temperature there, at high steam/air ratios the carbon conversion may be kinetically controlled. The alkaline nature of lignite ashes provides some sulfur-capturing ability, but even with Ca/S molar ratios of 2.0 the amount of feedstock sulfur appearing in the ash never exceeded 25% [67]. Pilot-scale tests of lignites have been conducted in an air-blown atmospheric pressure unit [67]. Indian Head lignite gasified well in the Wellman-Galusha unit, and provided very high throughputs. However, the friability of this lignite resulted in some size degradation during handling. An unidentified Texas lignite provided the same problems with friability, and, in addition, caused problems in controlling ash removal because of a highly variable ash content. Moderate throughputs were achieved, although the quality of the product gas was very high. Benton (Arkansas) lignite also gasified well with very high throughputs. With this lignite the throughput was limited by the ability to supply steam to the gasifier. The friability of this lignite seemed markedly dependent on moisture, the lignite being very hard while wet, but very friable when dry. Some performance data obtained with Benton lignite are shown in Table 12.4 [67]. TABLE 12.4 Process behavior of Benton lignite in air-blown Wellman-Galusha gasifier [67]. Dry gas higher heating value, MJ/m3 Water in gas, kg/m3 Tar in gas, kg/m3 Tar yield, % of coal weight Hot, raw gas efficiency, % Maximum throughput, kg/h. m2

6.5 0.62 0.1(3" 11" 97 913

*Estimated

Of seven fuels tested, including four subbituminous coals and two peats, Benton lignite showed the highest maximum throughput rate. The limitation to the throughput rate was the capacity to deliver steam to the gasifier. Indian Head lignite has also been gasified in a Wellman-Galusha unit modified to operate at pressures to 2 MPa with oxygen blowing [68]. The results of these tests, which provide a comparison of air and oxygen blowing, and shown in Tables 12.5 and 12.6 [68]. (iii)

Slagging fixed-bed gasification. The

slagging, fixed-bed gasification of lignite was

examined in a pilot-scale reactor at the Bureau of Mines and Department of Energy laboratory in Grand Forks, North Dakota (now the Energy and Environmental Research Center of the University of North Dakota). The pilot-scale gasifier was the only one of its type in North America. Although the conventional Lurgi fixed-bed gasifier is successful on a commercial scale, a disadvantage of the conventional ("dry ash") Lurgi is that considerable amounts of excess steam must be fed to the unit to maintain the hearth temperature below the ash fusion temperature of the

630 TABLE 12.5 Process behavior of Indian Head lignite in pressurized Wellman-Galusha gasifier [68]. Oxygen-blown 182 Exit gas temperature, ~ 1.0 Gasifier pressure, MPa Steam/lignite, kg/kg 0.73 0.29 Air or oxygen/lignite, kg/kg 2.50 Steam/air or oxygen, kg/kg 46 Ash, kg/t lignite 20.7 Dust, kg/t lignite 2.22 Raw gas, Mm3/t lignite 12.2 Tars and oils, kg/t lignite 60 Illuminants, kg/t lignite Raw gas, Mm3/h 3.56 43 Water in raw gas, volume % 6.59 Raw gas calorific value, M.l/m3 10.8 Product gas calorific value, MJ/m3* 78 Raw gas efficiency, %

Air-blown 300 1.7 0.29 1.44 0.20 41 20.2 2.72 16.7 62 4.52 24 5.36 6.18 88

*After cleaning

TABLE 12.6 Average raw gas composition (vol. percent) from Wellman-Galusha gasification of Indian Head lignite [68].

Hydrogen Carbon dioxide Carbon monoxide Nitrogen Methane Hydrocarbon gases Tars and oils Water Hydrogen sulfide

Oxygen-blown 22.5 16.5 13.5 0.5 2.5 0.8 0.1 43 0.21

Air- blown 16.5 10.5 11.5 35.5 2 0.8 0.1 24 0.17

coal. Most of this excess steam serves only as a diluent to control hearth temperatures. It passes through the gasifier without reacting, and on condensation serves to dilute the gas liquor produced as by-product. Alternatively, the oxygen concentration in the steam-oxygen mixture can be increased to a point at which hearth temperatures are high enough to melt the ash, thus resulting in slagging rather than dry ash operation. Slagging operation significantly reduces the amount of steam that must be generated for operation. In turn, this reduces the volume of gas liquor and increases the concentration of dissolved by-products in this stream. Slagging operation also offers significant improvements in the rate of gas production per unit cross-section area of the gasifier.

631 The unit was able to operate at oxygen/steam molar ratios of 0.7 to 1.2 [34]. The limiting factors were too low a hearth temperature (and hence a slag viscosity too high for good operability in a continuous tapping mode) at 0.7 oxygen/steam ratio; and vaporization of ash components, causing fuel bed bridging, at 1.2 oxygen/steam ratio [34]. The gas production rate and the load on the gasifier depend entirely on the rate of oxygen supply [69]. For lignites, the maximum gasification rate at 2.8 MPa was determined to be 9,800 m3/h.m2 [69]. Operations were conducted at various operating pressures in the range 0.6--2.8 MPa. The pressure dependence of oxygen and fuel requirements per unit gas production are shown in Figure 12.12 [69]. Pressure had little effect on gas production rate and steam, oxygen, and lignite consumption [70]. The product gas composition is shown as a function of pressure in Figure 12.13 [69]. Generally, increasing operating pressures increased the carbon dioxide and methane contents of the gas and decreased carbon monoxide [70]. Operating pressure did not affect wastewater production appreciably, the main parameter affecting water production being the lignite moisture content [70]. 600 t"o

AA

~500

A

A

+ Oxygen

A

9 Steam

~" 4 0 0 -

A Fuel (maf)

.~ 300-

+ +

+

t#

2009

9

;

100 O" ' ' ' ' 1 .... 0 0.5

I ....

1

I''''1

1.5

....

2

i ....

2.5

3

Pressure, MPa Figure 12.12. Material requirements per unit gas production as a function of pressure in slagging fixed-bed gasification of lignite [69].

Operation with Indian Head lignite indicated that fuel rate, product gas rate, and oxygen and steam consumption were not significantly affected by pressure when comparing tests at 1.4 and 2.8 MPa [71]. The oxygen and steam requirements per amount of (CO + H2) produced were higher at the higher pressure. Cold gas efficiency and operational efficiency also increased with pressure. The cold gas efficiency is the ratio of potential heat in the product gas to potential heat of the fuel, usually expressed as a percentage [71]. The operational efficiency is the ratio of the sum of potential heats of product gas and tar to that of the fuel, usually expressed as a percentage [71].

632 80

+ CO

70"

9 1+

o

+

!

4

A CO2

50-

~ CH4

~ 40o

H2

30

9 9

9

9

O 2010, , , , i , , , , i , , , , i , , , , i , , , , i , , , ,

0

0

0.5

1

1.5

2

2.5

3

Pressure, MPa Figure 12.13. Product gas composition as a function of pressure in slagging fixed-bed gasification of lignite [69].

A comparison of the results from this pilot-scale unit with three other gasifiers is provided in Table 12.7 [69]. The lignite gasified in the Grand Forks unit was not identified in the original literature, but was very likely either Baukol-Noonan (North Dakota) or Velva. The major differences between slagging and non-slagging operation shown in Table 12.7 can be attributed to the lower steam consumption of the former, and the consequent higher capacity per unit hearth area. Fuel consumption and gas production rates are higher by factors of 3 to 6 for slagging gasifiers relative to non-slagging units. Operating data for four different North Dakota lignites are summarized in Table 12.8 [27,69]. Summary results on three of these lignites have also been published [70]. In comparing these data it should be noted that the data for Velva lignite [69] were obtained at a different pressure and oxygen rate (as well as being obtained about 15 years earlier) than the data for the other three lignites [27]. The effects of pressure on operation with a single lignite (Indian Head) are illustrated by the data in Table 12.9 [27]. The moisture content of the lignite feed is considered to be a limiting factor in operation. Operations with Gascoyne lignite, of 40% moisture, experienced significant difficulties attributed to the heat produced in gasification and combustion reactions deep within the fuel bed being insufficient to vaporize all the moisture in the incoming coal [27]. The effect is a steady reduction of temperatures in the gasifier hearth with eventual cessation of slag flow. However, entirely successful tests were performed with Indian Head lignite at 38.6% moisture [27]. A much more serious problem is bridging of the lignite particles in the upper portion of the fuel bed. The bridging causes the combustion and gasfication reaction zone to migrate upward, depriving the

633 TABLE 12.7. Comparison of slagging fixed-bed gasification of lignite with other fixed-bed gasification processes [69].

I_xxzation Grand Forks Type of operation Slagging O2/steam molar ratio 0.9 Operating pressure, MPa 2.8 Hearth area, m2 0.14 Fuel Rank Lignite Size, mm 19x10 Ash, % 6.9 Moisture, % 22.7 Calorific value, MJ/kg 20.2 Gas composition (N2 free) CO2 5.9 Illuminants 0.5 O2 0.4 H2 29.0 CO 58.7 C2H6 0.3 CH4 5.2 Gas calorific value, MJ/m3 13.1 Fuel rate, t (maf)/m2 hearth area. h 4.3 Gas rate, m3/m2 hearth area. h 7960 Oxygen consumption m3/Mm3 gas 192 m3/GJ of product gas 14.6 m3/Mm3 (CO + H2) 223 m3/kg maf fuel 0.30 Steam consumption kg/Mm3 gas 193 kg/GJ of product gas 12.3 kg/Mm3 (CO + H2) 224 kg/kg maf fuel 0.30 Fuel consumption, maf kg/Mm3 gas 640 kg/GJ gas produced 41.0 kg/Mm3 (CO+H2) 743 MJ fuel/Mm3 gas produced 1.56 MJ fuel/MJ gas produced 1.18 Cold gas efficiency, % 84.4 Operational efficiency, % 94.2

Morwella Dry ash 0.11 2.8 4.9 Brown coal 60x55x36d 1.7 15.3 22.2 34.4 0.3 0.2 36.6 14.6 0.6 13.3 11.6 1.2 5280

Westfieldb Dry ash 0.20 2.4 5.9

Solihullc Slagging

hvB 32x10 14.6 15.6 22.4

hvB 38x25 4.9 13.8 25.4

2.1 0.66

24.9 1.1 0 40.3 24.9 e 8.8 11.7 1.2 2760

2.7 0.5 0.1 28.1 60.9 e 7.7 14.0 7.0 13000

120 10.3 235 0.19

155 13.2 238 0.29

205 14.7 230 0.32

966 69.7 1890 1.51

697 49.9 1070 1.31

187 11.3 210 0.29

642 46.3 1255 1.44 1.24 80.7 86.9

530 38.1 813 1.45 1.24 80.5 88.0

642 38.6 722 1.38 1.21 82.7 90.0

Notes: aVictoria, Australia. bScotland, cEngland, dThis fuel was in briquette form. eNot available.

634 TABLE 12.8 Comparative performance of four North Dakota lignites in slagging fixed-bed gasification [27,69] Lignite Moisture content, % Operating pressure, MPa Oxygen rate, m3/h Oxygen/steam mol ratio Test duration, h Fuel rate, kg/h Fuel rate kg (maf)/h Product gas rate, m3/h Slag rate, kg/h Oxygen consumption m3/Mm3 gas m3/Mm3 (CO + H2) m3/kg (maf) fuel Steam consumption kg/Mm3 gas kg/Mm3 (CO + H2) kg/kg (maf) fuel Product gas composition, % CO2 H2 CO C2-C4 hydrocarbons CH4

Baukol-Noonan 30.3 1.4 95 1.0 11.7 535 340 518 43

Gascoyne 40.5 1.4 95 1.0 3.6 619 322 544 30

Indian Head 34.7 1.4 95 1.0 8.9 573 339 525 24

Velva 37.2 2.8 119 0.9 4.7 661 393 600 22

185 212 0.28

175 202 0.30

182 212 0.28

198 234 0.30

168 191 0.25

158 182 0.14

164 192 0.25

193 235 0.30

6.4 30.5 57.3 0.7 5.1

7.7 31.3 56.1 0.6 4.3

8.4 32.5 53.6 0.6 4.9

7.4 28.8 56.4 0.5 6.9

hearth of heat and resulting in slag freezeing [72,73]. This problem is very likely a result of the unusually narrow (41 cm) diameter of the gasifier vessel in the configuration used for several of the pilot plant campaigns, and not a generic problem of slagging gasification of lignites [72]. Subsequent enlargement of the interior diameter to 56 cm substantially increased reliability of operation [73]. There appears at present to be no interest in construction of a commercial-scale slagging gasifier for lignites. A useful summary of design considerations to be taken into account should the process be scaled up is available [73]. (iv) The Riley gasifier. The Riley gasifier is also a fixed-bed unit, but has the novel feature that the entire fuel bed is caused to rotate slowly, as a means of insuring an even distribution of fuel in the bed [74]. The Riley gasifier features a very thin fuel bed, at most 1.4 m, designed mainly as a means of varying the coal particle heating rate to cope with gasification of caking coals. Some pilot-scale data have been obtained for gasification of a northern Great Plains lignite [74]. The lignite was sized 50x12 mm and fuel bed depth was 1.2 m. The air requirement was 2.44 kg air/kg lignite (daf basis), with a steam/air mass ratio of 0.18. These conditions provide a gas offtake temperature of 270"C. The gas yield was 3.5 m3/kg lignite (daf basis), with a calorific value of 6.2 MJ/m3. The gas contained 44.5 N2, 28.1 CO, 17.3 H2, 6.1 CO2, 1.5 CH4, with the balance

635 TABLE 12.9. Effects of operating pressure on slagging fixed-bed gasification of Indian Head lignite [27]. All data were obtained using an oxygen rate of 113 m3/h and oxygen/steam molar ratio of 1.0. Pressure, MPa Moisture in lignite, % Fuel rate, kg/h Fuel rate, kg (maf)/h Product gas rate m3/h Slag rate, kg/h Oxygen consumption m3/Mm3 gas m3/Mm3 (CO + H2) m3/kg (maf fuel) Steam consumption kg/Mm3 gas kg/Mm3 (CO + H2) kg/kg (maf fuel) Product gas consumption, % CO2 H2 CO C2-C4 hydrocarbons CH4

0.7 22.9 476 331 569 33

0.88 36.4 559 319 588 25

1.4 35.7 571 331 623 30

2.8 38.6 588 330 596 23

199 224 0.29

193 219 0.30

182 212 0.29

190 230 0.29

174 196 0.26

169 191 0.27

159 186 0.26

166 202 0.27

5.0 29.1 59.6 1.0 4.3

6.6 31.3 57.0 0.6 4.5

8.4 31.5 54.2 0.6 5.3

10.5 28.5 54.4 0.5 6.1

being higher hydrocarbons, COS and H2S, and inert gases. By-product tar formed in amount equivalent to 0.02 kg/kg lignite. (v)

Entrained-flow gasification.

Shell conducted a four-year pilot-scale program on

entrained-flow gasification at a facility in Deer Park, Texas. This campaign included operation for a total of 647 h on Texas lignite from the Alcoa mine at Sandow (Milam County), the most reactive of all coals tested in this unit [75]. Carbon conversion of greater than 99.0% was achieved, with cold gas efficiencies up to 80% [75,76]. Methane is the only organic compound surviving the gasification reaction; tars, phenols, or other hydrocarbons do not occur in the product streams. The acid gas system removed up to 99.8% of the sulfur species in the gas [75]. The feed, gasification, and solids removal systems of the Shell process are shown in Figure 12.14, modified from [75]. Details of the coal receiving and handling system and the gas and water treatment systems have also been published [75]. Some of the key results, obtained for gasification of two different batches of Sandow lignite (designated as high-ash and low-ash), are summarized in Table 12.10 [75,76].

636 Lignite

t

Steam v

..~smam

Syngas cooler

r

Slag lock hoppers Lignite feed vessel

I J~rnite v

---

~lag dewatering

Flyslag

~ Slag

Figure 12.14. Coal feed, gasification, and solids removal systems for pilot-plant Shell entrained-flow gasification (adapted from [75]).

TABLE 12.10 Operating results for Shell gasification of Sandow lignite [75,76].

Lignite fed to plant, t/d Oxygen/maf lignite ratio Offgas composition, dry, vol% Carbon monoxide Hydrogen Carbon dioxide H2S + COS N2 + Ar + CH4 Sweet syngas production Mass basis, kg/h HHV energy basis, GJ/h Sulfur removal from syngas, % Carbon conversion, % Cold gas efficiency, %HHV Slag production, kg/h Flyslag production, kg/h Acid gas, kg/h

High-ash lignite 334 0.877 60.59 28.20 5.38 0.71 5.08 12,060 155 99.1 99.7 78.8 2,210 410 1,690

Low-ash lignite 248 0.865 61.82 28.21 4.47 0.80 4.83 10,380 131 99.8 99.4 80.3

637

The relatively high carbon dioxide level in the product gas relates to the high oxygen content of the lignite. The high exit gas temperatures (1300-1400"C) facilitate high carbon conversion [44]. Problems at the mill hampered early operations with the Sandow lignite [75]. The mill feed chute plugged with wet fines. As a result, mill outages limited feed rates to the gasifer to 60% of design. These problems were subsequently overcome.The drying gas entering the mill was preheated to 370-4000C because of the high moisture content of the lignite. Despite these temperatures, the hydrocarbon concentration in the vent from the drying loop was less than 100 ppb [75]. The Shell process has been selected for a plan for a nominal 500 MW integrated gasification combined cycle plant planned for a Houston Power and Lighting site in Malakoff, Texas [77]. The plan is based on use of lignite with 15.1 MJ/kg calorific value from the Trinity mine. Various design cases have been considered, including two gasification trains with no coproduct, two gasification trains with methanol as a co-product, and three trains with ammonia and urea as co-products. In contrast to the Shell system, the Texaco and Dow entrained-flow gasifiers are fed with a coal/water slurry. Using high-moisture lignite efficiently in these gasifiers depends on hydrothermal dewatering

(e.g., hot-water drying, Chapter 10) to produce a dense slurry of high

energy content [44,45]. Otherwise, excessive moisture in the lignite, in addition to the water used as the slurry vehicle, limits gas yield and efficiencies [45]. For the Texaco gasifier, a minimum of 50% dry solids loading in the slurry is needed to achieve low oxygen demand and efficient operation [45]. The Riser Cracking process was developed as an entrained-flow hydropyrolysis that produced both gas and liquid products from a helical tube reactor [78]. The hydropyrolysis occurred at 760-816~ and 10.3-13.8 MPa, with a residence time of 2--4 s. Depending on reaction severity, the liquids could be used as aromatic feedstocks for chemicals production or blending stock for high-octane gasoline. Low-severity operation favors the latter. Reaction of lignite at 14 MPa system pressure (hydrogen partial pressure of 11.2 MPa) and coil outlet temperature of 760~ provided a carbon conversion of 47.7% [78]. This was achieved with a residence time of 3.1 s and hydrogen feed of 35.3% by weight of the maf lignite. Gaseous products, expressed as weight percent of the maf lignite, were 10.5 CH4, 5.2 C2H6, 1 C3H8, 0.3 other light hydrocarbon gases, and 12.0 carbon oxides. Hydrocarbon liquid yield amounted to 11% of maf lignite; of this amount of liquid, 53% could be considered 2050C endpoint gasoline. The major components of the liquid included benzene, toluene, naphthalene, and phenol; other components included xylenes, ethylbenzene, indenes, cresols, biphenyl, and quinoline. 12.1.5 Underground gasification of lignite During the so-called synfuels boom of the late 1970's much of the interest, and experimental effort, in underground gasification of coal was directed to the subbituminous coal

638 deposits of the western United States, especially in Wyoming. (A fine overview discussion of underground gasification has been published [79].) The deep-basin Texas lignites were also thought to have considerable promise for underground gasification [80]. In Texas, deep-basin seams 61-610 m below the surface having thickness greater than 1.5 m contain 31.2 Gt of lignite, representing three times as much energy as the proven oil and natural gas reserves in the state [80]. Three potential uses of gas were envisioned: a) low calorific value gas for mine-mouth generation of electricity; b) medium calorific value gas transported off-site for process heating and steam raising; and c) conversion of the gas, either on- or off-site, to chemicals, such as methanol. Carbon dioxide could be a by-product for use in enhanced oil recovery in Texas and Louisiana. The lower calorific value of lignite, relative to the subbituminous coals, results in a higher gasification cost, assuming other factors are equivalent [80]. Furthermore, the candidate lignite seams in Texas are thinner than the subbituminous seams in Wyoming. These two factors combine to require a greater number of injection and production wells to achieve the same gas production as from a thicker subbituminous seam. The drilling costs represent a significant fraction of the total cost for underground lignite gasification. The higher moisture content of the lignite is also a disadvantage, in part because the moisture participates in the gasification reaction, thus giving a higher steam/oxygen ratio than had been desired. This effect results in poor oxygen utilization and poor gas quality [81 ]. Underground lignite gasification conducted near Fairfield, Bryan, and Alcoa, Texas in 1976-79 showed that air blowing produced gas of 3.2-3.7 MJ/m3 calorific value [80]. Oxygen blowing produced gas with calorific value of 8.5 MJ/rrd at a thermal efficiency of 60-70% [80]. 12.1.6 Biological gasification of lignites Bacterial enzymes break down the macromolecular structures of lignites to lower molecular weight products. Soluble material extracted from Vermont lignite with 1M sodium hydroxide, and subsequently precipitated at pH 5.5, had an apparent molecular weight (by gel permeation chromatography) of 167,000-174,000 daltons [82]. Treatment with aerobic, Gram-positive Bacillus or any of three strains of aerobic Gram-negative bacteria for 1-4 h produced cell-free

culture filtrates with a sharp chromatogram peak at 113,000 daltons [82]. The bacterial enzymes cleave ether or ester linkages. The breakdown of the structure may facilitate further conversion of the lignite to desired gaseous or liquid products. Microbial solubilization of lignite depends on the extent of oxidation of the lignite [83]. Increased solubilization correlates with increases in the oxygen content of the lignite, as well as with increases in the pH of the culture broth. Bioconversion of lignite to methane could occur in a two-stage process. The first stage would involve an aerobic alkaline oxidation and solubilization of the lignite, with the second stage involving anaerobic conversion to methane of both the products and the cell mass from the first stage [83,84]. Lignite solubilization is generally necessary prior to biogasification [85]. The formation of methane from pretreated Texas lignite using anaerobic bacteria can occur in concentrated brines [86]. In this process, the optimal salt concentration is

639 8--12% and the optimal temperature is 35--42"C [86]. Thus it is possible that the process could be carried out in underground salt caverns. Texas lignites are the most reactive substrates for bacterial consortia derived from termites [87]. Maximum methane production was 7195 ~mol/g lignite. Lignite particle size in the range 44-650 ~tm did not affect biogasification, but micronized lignite (=10 ~tm particles) enhanced methane production by 26.6% [87]. Biogasification in this system is a multistep process in which the lignite structure is degraded to compounds of 2-3 tings, which are suitable substrates for acetogenic organisms. This initial degradation is the rate-determining step. The acetogenic organisms convert the 2-3 ring compounds to volatile aliphatic carboxylic acids and short-chain aliphatic alcohols. Methanogenic bacteria then convert the acids, acetic acid in particular, along with carbon dioxide and hydrogen, to methane. Texas lignite in the 28-325 mesh size range produced methane at rates of 50(0700 ~tmol/day.g lignite [87].

12.2 DIRECT L I Q U E F A C T I O N OF LIGNITES

Direct liquefaction is the conversion of coal to a liquid product by hydrogenation of the coal, usually slurried in a solvent medium, with no intervening process steps between the coal and liquid (this of course does not rule out down-stream treatment of the liquid product). Indirect liquefaction is the formation of liquids from synthesis gas produced by coal gasification. Usually indirect liquefaction would employ the Fischer-Tropsch synthesis or some variant of the FischerTropsch as the liquefaction step. Neither the direct nor indirect liquefaction of lignites is current commercial practice in North America, and it does not seem likely that liquefaction will become commercial in the near future. This section provides information on some pilot-scale processes for direct liquefaction of lignites. Fundamental aspects of the chemical reactions of lignite relevant to liquefaction processing are treated in Chapter 4. 12.2.1 The Exxon Donor Solvent Process In both the Exxon Donor Solvent process and the Solvent Refined Coal II process (SRC II) lignites process more readily than subbituminous coals and produces higher yields on an maf basis [88]. With addition of pyrite, Texas lignite can be processed to >60% oil yields in the SRC II process. Lower yields of oil were obtained if no additional pyrite was added. Lignites have the potential of being excellent feedstocks for direct liquefaction. The Exxon Donor Solvent process, hereafter referred to as EDS, is based on the reaction of coal with hydrogen in a hydrogenated recycle solvent. Components of the solvent serve as hydrogen donors to the coal; the process name derives from this behavior. As originally conceived, a slurry of coal in solvent was fed to the liquefaction reactor with hydrogen. Typical operating conditions were 14 MPa total pressure, 450~

and upward plug flow in the reactor [89]. The

material leaving the reactor consists of a depleted recycle solvent, gaseous hydrocarbons, C4-

640 540"C distillate, and vacuum bottoms that contain 540*+ liquids, unconverted coal and mineral residues. The configuration of the EDS process is shown in Figure 12.15 [90]. The dried, crushed coal is fed to the liquefaction reactor with hydrogenated recycle solvent and hydrogen. Distillation of the reactor products provides a depleted recycle donor solvent stream, as well as light gases, C4-540"C liquids, heavy vacuum bottoms (540"C+) and the unconverted coal and mineral matter. Hydrogenation of the recycle solvent is performed in a conventional fixed bed catalytic reactor. The vacuum bottoms are fed to a Flexicoking unit (a fluidized bed process combining coking and gasification) which produces some liquid products along with fuel gas for process furnaces. The necessary hydrogen is obtained by steam reforming the light hydrocarbon gases. Several options also exist for the 540*+ vacuum bottoms: partial recycle to the liquefaction reactor, flexicoking to liquids and fuel gas; partial oxidation to produce hydrogen or fuel gas, or simply feeding directly to a furnace for process heat [91]. The vacuum gas oil, which boils in the range 425-540"C, can also be recycled to the liquefaction reactor.

HYDROGEN

I HYDROGEN .._1

GAS

SOLVENT HYDROGENATION

STEAM REFORMING

l LIQUEFACTION

HYDIOGEN

..._1

w,.---

|

~STEAM

DISTILLATION

..--I v

~0ITOMS FLEXICOKING

-.- FUELGAS

,,,...-

ASH Figure 12.15. Block flow diagram of Exxon Donor Solvent process, operating with bottoms treatment by Flexicoking [90].

641 Pilot-scale testing in the late 1970's suggested that lignites would be more difficult to process in EDS than bituminous coals [89]. Big Brown (Texas) lignite produced total liquid yields of about 37%, 28% from the liquefaction reactor and 9% from the Flexicoker. By comparison, three bituminous coals, Monterey, Burning Star (both Illinois), and Ireland (West Virginia), had total liquid yields of 44--46%, with yields from the liquefaction reactor in the range of 29-35% [89]. The EDS results for various ranks of coal show quite clearly that for each coal there is an optimum combination of operating parameters, such as residence time, for maximizing the yields of liquids. For example, the maximum liquid yield from Big Brown lignite occurs at 25--40 min. residence time. The total process yield (that is, from both liquefaction and flexicoking) to C4+ products is around 40% (daf basis) for Big Brown and Indian Head lignites. Of the total C4+ product from these lignites, roughly 15-18% is derived from Flexicoking, and the balance from the liquefaction process itself. Product yields from EDS as a function of rank show some correlations with the expected rank dependence of composition. For example, the yields of water and the carbon oxides from low-rank coals are higher than from bituminous coal, reflecting the higher oxygen content of the lower rank coals [90]. Coals that are higher in sulfur content yield a greater amount of hydrogen sulfide in the product gases. The C 1-C3 gas yield is also slightly higher for low-rank coals. With more of the carbon reporting to the carbon oxides and light hydrocarbons, the yield of

C4-540~ and 540~

liquids from low-rank coals is somewhat lower than from bituminous

coals. The effect of reactor conditions and mode of operation on the yields of gas, naphtha, and oil obtained from Big Brown lignite is shown in Figure 12.16 [91]. Using solvent, coal, and vacuum bottoms in proportion 1.6:1:1 increases the yield of C3-540~

liquids by about a third

compared to once-through operation. The naphtha yield also increases by about the same proportion, although the selectivity to naphtha is not improved. Changes in process variables allow optimizing the selectivity to naphtha. For example, by changing to a solvent:coal:bottoms mixture in proportion 3:1:1, the naphtha cut becomes 85% of the total liquid product [91]. Bottoms recycle can significantly improve yields of C3-5400C liquids compared to once-through operation. On a daf coal basis, the yield of C3-540~

liquids from Big Brown lignite was 49% with bottoms

recycle, but only 35% for once-through operation [91]. For Big Brown lignite processed at 450~ and 10.5 MPa, the total conversion to 540~ products increases with increasing residence time up to about 40 min [90]. Higher residence times, up to 110 minutes, have essentially no effect on lignite conversion to 540~ - products. However, the conversion to C4-540~ liquids drops steadily as residence time is increased, from 33% at 20 min to 27% at 110 minutes. At constant temperature, the yield of the C4-540~ liquids increases to a maximum and then decreases as a function of residence time. The time at which the maximum yield occurs is coal-specific; for Big Brown lignite processed at 450~ the maximum yield occurs in 20 min [28,91]. In comparison, the maximum for Illinois bituminous coal is reached in about 40 min. The residence time corresponding to the maximum yield changes as temperature changes. As

642

[~

50_

,-1 ~

4o_

GAS

~] NAPHTHA

30_ r I/-I I.-4 >'

20_

t..r

O' ~

xx

10..

O b ,

o

0

A

B

Figure 12.16. Comparative effects of bottoms recycle and once-through operation for liquefaction of Big Brown lignite [91]. Reaction conditions are defined as follows: A, 448"C. 40 min. residence time, 10.3 MPa, 1.6:1:0 solvent:coal:bottoms ratio; B, 427"C, 100 min. residence time, 10.3 MPa, same solvent:coal:bottoms ratio; C, 448~ 40 min. residence time, 14 MPa, 1.6:1:1 solvent:coal:bottoms; D, 448~ 40 min. residence time, 14 MPa, 1.3:1:0.5 solvent:coal:bottoms; and E, 448~ 40 min. residence time, 14 MPa, 3:1:1 solvent:coal:bottoms.

residence time increases at 450"C the viscosity of the heavy oils (bottoms) decreases sharply. Bottoms viscosity also drops with increasing temperature. Operation of the EDS process on low-rank coals has shown that problems arise from formation of less liquid product and correspondingly more carbon dioxide and water (from the higher oxygen content of low-rank coals), a higher bottoms viscosity for low-rank coals, and the formation of free flowing agglomerates and reactor wall scale of calcium carbonate [28,90]. (The calcium carbonate problem is discussed in Chapter 6.) The extent of calcium carbonate deposition relates to the amount of ion-exchangeable calcium in the coal. Hence these deposits were believed to originate from decomposition of calcium carboxylates in the lignite. The approaches to controlling the problem include removal of some of the free-flowing agglomerated particles during operation and acid washing of the wall scale during reactor downtime. Deposition can be alleviated by bottoms recycle operation. The higher fluid density and viscosity enabled more solids to be swept out of the reactor, and a moderation in reaction rate due to the dilution of the reactor contents with some of the recycled bottoms [91]. Indeed, recycling the vacuum bottoms, including the ash, appears to be a useful remedy for the precipitation of calcium salts [45]. Contacting the lignite with

643 appears to be a useful remedy for the precipitation of calcium salts [45]. Contacting the lignite with liquid sulfur dioxide at temperatures of 0-90~

and pressures of 70--350 kPa for 0.1 to 4 h

suppresses formation of calcium carbonate scale [92]. Ash from the residue of untreated lignite liquefied at 4500C in tetralin for 40 min contained 40% calcium carbonate. Treatment with liquid sulfur dioxide, followed by liquefaction at the same conditions, resulted in 2.9% calcium carbonate in the residue ash. Gaseous sulfur dioxide treatment also effectively suppresses calcium carbonate formation, but not as well as the liquid sulfur dioxide. Lignite treated with gaseous sulfur dioxide produced a residue containing 8% calcium carbonate in the ash. .The viscosity of the bottoms is higher for low-rank coals than for bituminous coals [90]. A comparison of bottoms viscosity for Big Brown and Monterey coals is shown in Figure 12.17 [89], for aperation at 4500C and 10 MPa H2. These viscosities represent a mechanical operability of the process since they relate directly to pumping the bottoms from a vacuum tower to a coking or gasification operation. The criterion of pumpability was a viscosity below 5 Pa.s [89]. The high viscosity is undesirable for good operation of the vacuum tower and for operation of the liquids feed system for bottoms recycle. For low-rank coals, the bottoms viscosity decreases with increasing residence time. For example, the viscosity of bottoms diluted with 10% 540~ - material decreased from about 16 Pa.s for material produced at a residence time of 20 min to about 1 Pa.s for material produced with a residence time of 100 min [90]. (The viscosity data were obtained at 290~

Bottoms dilution is also a strategy for decreasing bottoms viscosity; increasing the

concentration of the 5400C - from 10 to 15% decreases the viscosity by a factor of about four for material produced with a residence time of 20 min. Any 540~ - liquids used for bottoms dilution can be recovered in the Flexicoking operation. Bottoms recycle during the processing of Big Brown lignite reduces the proportion of preasphaltenes (pyridine-soluble, benzene-insoluble) in favor of increasing the proportion of asphaltenes (540~

benzene-soluble). Vacuum bottoms from

bottoms recycle operation show improved thermal stability on storage. For example, the relative viscosity of vacuum bottoms from Big Brown bottoms recycle operation increased only from 1 to 2 units in an accelerated aging test at 315~ for 28 h in nitrogen [91]. 12.2.2 Project Lignite The original concept, dating from the 1960's, that motivated work on Project Lignite, was the prospect of developing a "lignite refinery" that would produce a slate of useful products from lignite. The products envisioned were solvent-refined coal (or, more specifically, a solvent-refined lignite); distillate liquids from hydrogenation of the solvent-refined lignite; products from lignite carbonization, including solid char, liquid products, and gases; and further liquids from direct hydrogenation of the lignite [93]. Thus the products leaving the plant gate would include synthetic crude oil, fuel gases, a low-ash char, a solvent-refined lignite, and possibly even feedstocks for the chemical industry. As the project eventually evolved, however, the dominant focus was on the production of solvent-refined lignite at the pilot scale, with some supporting bench-scale work on direct hydrogenation of lignite. The pilot-scale operations processed 0.5 tonne/day of lignite.

644 20 + Big Brown 9 Wyodak

15

A Burning Star ~100

"

5

0

' ' ~ '

0

I

' ~ ' '

I ~ w ~

I ~ ' ' '

I ' ' ' '

20 40 60 80 Residence Time, min.

100

Figure 12.17. The effect of liquefaction residence time (at 450~ 10 MPa Hz) on residual bottoms viscosity at 10 s-1 shear rate [91]. Operability problems are encountered at viscosities above 5 Pa.s. Wyodak is a Wyoming subbituminous coal; Burning Star, an Illinois high volatile C bituminous coal.

Broadly, the lignite feedstock was slurried, without drying, with a donor solvent. The slurry is then pressurized, preheated, and reacted in a synthesis gas atmosphere at temperatures to 510" and pressures to 18 MPa. At a process feed rate of 23 kg/h, the production of solvent-refined lignite was about 7 kg/h [93]. Temperatures in excess of 480 ~ in the dissolver caused excessive coking in the unit [93]. The effect of reaction temperature, for operation at 17.5 MPa of 1:1 H2/CO, is shown in Figure 12.18. The temperature data used to illustrate the effect are the maximum temperatures observed in the dissolver unit. The yield of oil and solvent-refined lignite passes through a maximum at 440~ At this condition, the amount of pyridine-insoluble unreacted lignite remaining is about 8% (ashfree basis). The change in slope represents the combination of two opposing effects. On the one hand, the conversion of lignite to solvent-refined lignite and oil increases as a function of temperature. Offsetting this, however, is the fact that the rates of retrogressive recombination reactions, or coking of the solvent, or both, also increase with temperature. Above =440~

the

recombination or coking reactions dominate and consequently the net conversion decreases. In Figure 12.18 the effects of temperature on the yields of solvent-refined lignite and oil are shown. The solvent-refined lignite yield decreases monotonically as a function of temperature. The oil yield increases as a function of temperature to about 440~

and appears to be independent of

temperature at higher temperatures. Distillation of the oil into a cut boiling below 2800C (so-called

645 70 ~60

t" x ~ - ~ . - - . . ~ _ ~+, +

+ +,

+ Product

+

++ ~ . , . . , . ~ +

-

9 Gas

+

,..150

+

-

9

A Unconverted

9

30 A

2o

A

z lO-

A

A

A

A AA AAA~x A

0

. . . .

430

I

440

. . . .

I

450

. . . .

460

Maximum Reactor Temperature, ~ Figure 12.18. Yields of product (oil plus solvent-refined lignite), gas, and unconverted lignite as a function of maximum reactor temperature [93].

light oil) and solvent (boiling between 280 ~ and the solvent-refined lignite) allows separation of the oil yield as shown in Figure 12.19. While production of light oil increases as a monotonic function of temperature, the solvent yield seems to pass through a maximum in the vicinity of 440~

It

should be noted that over much of the temperature range the solvent yield, when expressed on an maf coal basis, is actually negative, and it is only in the fairly narrow temperature range around 439-443~

that positive solvent yields are obtained.

The solvent-to-coal ratio could be varied in the range 1.4-3.0 [93]. At lower ratios, e.g., 1.33, problems occur with mixing and pumping the slurry. An increase in the solvent-to-coal ratio from 1.3 to 2.3 increases the gas make by about 5 percentage points, as a result of increased gas production from the solvent itself when the amount of solvent present in the system is increased. For successful long-term operation of a process such as this, establishing and maintaining "solvent balance" is critical [93,94]. That is, the process must produce enough solvent to maintain its own operability. If quantity of solvent is the only criterion, then in some cases it is possible to process lignite with solvent balances in excess of 100%; that is, more solvent is produced than is required for recycle to keep the process operating. However, combined with the issue of solvent balance is the question of solvent quality. In continuous runs in which all available liquid products were recycled, conversion of lignite decreased as a function of time, suggesting that the solvent quality was also decreasing. In comparison, if only liquids boiling above 2000C are recycled, then the solvent appears to remain effective. (Scouting studies in batch autoclaves of 200 g maf lignite capacity verified that light, low-boiling solvents produce lower solvent recoveries and reduced yields of light oils, relative to heavier solvents [93].) Under these circumstances, solvent balance

646 70

+ Oil 609

.~0 50-

SRL

4o~. 30+

- 20"

+-!-+

+ ..,_....,_,..,..,~

10" +

4--"

2;

O-

+

-10

'

+ '

'

'

I

'

'

'

'

I

'

'

'

'

440 450 460 4130 Maximum Reactor Temperature, ~ Figure 12.19. Yields of oil and solvent-refined lignite as a function of maximum reactor temperature [93].

was obtained only for reactions at 17.5 MPa. Successful operation of the process was achieved at pressures of 10.5, 14, and 17.5 MPa [93]. The effects of reactor pressure are illustrated in Figure 12.20, and the yields of solvent, light oil, and solvent-refined lignite are shown in Figure 12.21. The yield of solvent-refined lignite decreases slightly as pressure is increased above 14 MPa. On the other hand, the yield of light oil increases significantly at the high pressures. At low pressures solvent is actually lost; although the solvent loss decreases with pressure, for the conditions in which these data were generated (440~ 1.8-2.2 solvent-to-coal ratio, and 71-74% H2 in the gas feed) the system does not achieve solvent balance at any pressure. The best hope for achieving solvent balance would occur at higher pressures. Changes in the gas composition have only minor effects on the product slate, that is, on oil plus solvent-refined lignite, gases, water, and unconverted lignite [93]. However, among the desirable product fraction, the oil plus solvent-refined lignite, the yield of solvent-refined lignite decreases as the mole fraction of H2 in the gas increases. This effect is most pronounced as the H 2 increases from 25% to 50%. In comparison, the light oils increase with increasing H2. For reaction at 17.5 MPa, 4400C, 1.97 solvent-to-coal ratio, there is a very slight positive yield of solvent at H2 mole fractions <_50%, but a loss of solvent is observed when the H2 mole fraction is 75%. Storage of lignite in air for 70 weeks has a slight negative effect on the process, based on bench-scale studies in batch autoclaves [94]. Compared to testing of fresh lignite at otherwise comparable conditions, the total liquid yield drops from 66% to 58%, the solvent-refined lignite yield drops from 45% to 37%, and the amount of unconverted lignite increases from 9% to 12%

647

30

~

o =

+ o

Solvent

o

20"

o Light Oil

,--1

4o

10" o

44-

o"

gs

o 444-

,1-.' -10 " Z -

-t-4-

"

4-

-20

!

|

|

430

!

"|

|

440

|

+ |

|

I

!

|

450

!

!

460

Maximum Reactor Temperature, *C

Figure 12.20. Yields of light oil (IBP to 280"C) and solvent (280"C to SRL) as a function of maximum reactor temperature [54].

70 +

Oil + SRL

9 Gas

~1761

~9 50

A Unconverted

~40"

o Water

~. 30

~. 20 ~9 lO2 z

0 -10

''''

10

I''''

I''''

I''''

I''''

12 14 16 18 Reactor Pressure, MPa

20

Figure 12.21. Yields of major products as a function of reactor pressure, for reaction at 441--444"C, 1.3-1.4 linear hour space velocity, 1.8-2.2 solvent/lignite ratio, 0.7--0.8 mS gas/kg lignite, and 71-74% I-t2 in gas charged to reactor [93].

648 [93]. If the lignite is stored in a nitrogen atmosphere or under water for the same period of time, the effects on process performance are essentially nil. Drying lignite by heating to 215~

in the

presence of solvent resulted in a substantial reduction in reactivity, or, conversely, a substantial increase in yield of unconverted lignite.Using the lignite dried in this fashion, the liquid yields were in the range of 36-54%, whereas under comparable reaction conditions the liquid yields from lignite not dried prior to testing were 61--63%. Carbonization of lignite at temperatures of 260 ~, 315 ~ and 370 ~ causes a progressive decrease in liquid yield (i.e., oil plus solvent-refined lignite) with increasing carbonization temperature [93,94]. In batch autoclave tests the net liquid yields and amounts of light oils and solvent-refined lignite were comparable regardless of whether-100 mesh or 6 mm particles were used as feed [93,94]. 12.2.3 The CO-Steam Process In the presence of a suitable solvent, such as phenanthrene / 1-naphthol mixtures or isoquinoline, a carbon monoxide and water (steam) atmosphere can liquefy lignite in about 90% conversion [95-97]. The low sulfur content of North American lignites, and the possible economic advantages of using a CO-steam mixture (hence the name of the process) instead of hydrogen as the reducing gas suggested the possibility of conversion of lignite to low-sulfur oils. An additional potential economic advantage, relative to other liquefaction processes, is that high conversions can be obtained even without added catalyst [98]. Aspects of the chemistry of interaction of carbon monoxide with lignite have been discussed in Chapter 4. The CO-Steam process concept appears to have sufficient flexibility to be operated in modes similar to the Solvent Refined Coal (SRC) I, SRC II, or Exxon Donor Solvent processes [28]. Lignite is slurried with a recycle solvent and fed to the reactor with the reactive gases. Optimum conditions appear to be 460~

and 21 MPa [28]. No special added catalyst is used,

though some of the inorganic components in the lignite may catalyze some steps of the overall conversion to distillable liquids. Conversion of lignite to benzene-solubles is very rapid, even at comparatively low reaction temperatures (e.g., 380~

The effect of reaction temperature on conversion is illustrated in Figure

12.22 [99]. Oil yields after 15-20 minutes of reaction are comparable to those obtained at 2 h in a hydrogen atmosphere. Increasing the reaction temperature from 440 to 500~ increases the overall conversion, the yield of product distillable at 250"C and 133 Pa, the yield of methane and the C2--C4 hydrocarbons, and the consumption of reducing gases, based on tests with Beulah (North Dakota) lignite [ 100]. Temperature also has a significant affect on the apparent molecular weight distribution (as measured by gel permeation chromatography) of the products, the peak of the distribution shifting from 1500 to 250 daltons with a temperature increase from 404 to 500~ [100]. Conversion increases as a function of pressure, at least to 14 MPa. Figure 12.23 illustrates the results obtained for reaction of a coal-phenanthrene-water mixture (in proportion 1:1:0.5) as a function of carbon monoxide pressure [99]. The use of other solvents results in displacing the

649 80 + Solvent "~ 60-

9 Total Oil

.r,,i

'~ 4 0 -

A Light Oil

t~ 20-

o SRL

~-20-40 10

'

'

'

'

I ' ' ' '

I'

" ' '

I ' ' ' '

I

12 14 16 18 Reactor Pressure, MPa

'

'

'

'

20

Figure 12.22. Yields of oil and solvent-refined ligmte as a function of reactor pressure, for reaction at 441-444~ 1.3-1.4 linear hourly space velocity, 1.8--2.2 solvent/lignite ratio, 0.7--0.8 m3 gas/kg lignite, and 71-74% I-t2 in the gas charged to reactor [93].

100 9O 80

+ Condition 1 171

r,3

o Condition 2

401 350

....

' ....

400 Temperature, ~

I

450

Figure 12.23. The effect of temperature on lignite conversion for two reaction conditions. Condition 1, 10 minutes, 10.5 MPa, 1:1 phenanthrene: 1-naphthol solvent; condition 2, 2 hours, 7 MPa, phenanthrene solvent [99].

650 100 90-" 80-

-

70 0o

60"

2

50" 4O

Z '

0

''

I'

2

''

I'

''

I'

''

I'

4 6 8 Pressure, MPa

''

I'

10

''

12

Figure 12.24. Effect of initial CO pressure on lignite conversion, for two reaction conditions. Condition 1, 2 hours, 3800C, 1:1:1 lignite:tar:water; condition 2, 2 hours, 4O0~ 1:1:0.05 lignite:phenanthrene:water [99].

curve vertically, to higher or lower conversions at a given pressure of CO, but the slopes of the curves are nearly parallel. Tests in batch autoclaves showed that the minimum operating pressure that would provide high conversions was 21 MPa [99]. Solvent quality is of particular importance for successful operation of the CO-Steam process. Early workers achieved high conversions by using very strong solvents, such as isoquinoline [99]. High viscosity of the liquid product was a problem in early work. Substantial reductions of product viscosity can be obtained by increasing the reaction temperature to the range 450--475"C [101]. Increasing the reaction temperature from 400 to 475 ~ results in a significant decrease in the apparent molecular weight of the soluble, but non-distillable, product fraction [102]. However, operation at such temperatures in the absence of a donor solvent,

e.g.,

use of the

non-donor solvent anthracene oil, results in coke formation on reactor walls, stirrers, and sampling lines [ 102]. Thus the apparent molecular weight distribution can also be shifted by increasing the amount of tetralin in the solvent; this effect being more important at 404~ than at 460 or 500~ [ 100]. A solvent mixture such as a 90:10 blend of anthracene oil and tetralin obviates this problem. Formation of by-product methane complicates the situation. Methane production increases as a function of residence time for any temperature and solvent combination; however, the higher the temperature, the greater the slope of the methane production

vs.

residence time curves [ 102]. Thus

increased reaction temperatures need to be accompanied by reduced residence times. The combined effects of high temperature and short residence time potentially could provide rapid conversion to a low-viscosity product, thus enhancing the economic viability of the process [102]. Generally the yield of distillate product increases with increasing reaction temperature,

651 although the temperature

dependence is strongly a function of the solvent used [103].

Hydrocarbon gas yield and reducing gas consumption both increase nearly linearly with temperature [103]. In continuous operation, bottoms recycle increased distillate yield [104]. The effect of added hydrogen donor (e.g., tetralin) appears to depend on the quality of the material used as the principal solvent. At 440~

addition of tetralin to anthracene oil improves the

yield of liquids from Beulah lignite from about 42% obtained in anthracene oil without tetralin, to about 60% with tetralin addition [ 103]. At this temperature, a distillation cut from anthracene oil produced essentially the same liquid yield regardless of whether tetralin had been added. At higher reaction temperatures, tetralin addition to distilled anthracene oil gives slight improvements in liquid yield relative to the same solvent without added tetralin. However, the most dramatic effect is obtained by adding tetralin to anthracene oil. At 480~

total liquid yield from Beulah lignite in a

tetralin-anthracene oil solvent exceeds 90%, whereas the liquid yield obtained using only the anthracene oil is essentially nil [103]. Changing the coal concentration in the feed slurry has no effect on production of tetrahydrofuran-insolubles in the range 30-48% coal in slurry [ 103]. 12.2.4 Two-stage liquefaction By the early 1990's the last surviving large-scale liquefaction pilot plant was an integrated two-stage process development unit running in Wilsonville, Alabama. The plant was shut down in 1993. Although much of the effort at Wilsonville focused on subbituminous and bituminous coals, some plant trials were made with Martin Lake (Texas) lignite. The highest yields of distillate were about 50% (compared with 78% from Illinois bituminous coal) [45]. Performance with Martin Lake lignite was limited by conversion [105]. However, the residuum from the lignite was easier to convert to distillate than were residua from Illinois or Ohio bituminous coals. Optimization of two-stage performance therefore was achieved with high thermal severity (438-440~

in the first stage to maximize conversion, with lower temperatures

(380-395"C) in the second stage to enhance resid conversion [ 105]. Another advantage from the lower temperature operation in the second stage is maintaining good hydrogenation activity of the catalyst. Total lignite conversion of 90% can be achieved by combining the high thermal severity of the first stage with use of an iron oxide catalyst (and sulfiding agent, dimethyl disulfide) and high solids recycle [105]. The iron oxide catalyst was added in amounts equivalent to 0.8--2.0% of the lignite on an maf basis. Solids recycle involved 18-25% cresol-insolubles in the process solvent. The C4 + distillate yield ranged from 47 to 50%, with residuum yield of 0-7% [105]. The lower yields of distillate, compared to coals of higher rank, result from the correspondingly higher yields of carbon oxides, water, and C1-C3 hydrocarbon gases. The higher carbon oxides and water yields are consequences of the higher oxygen content of the lignite, whereas the higher hydrocarbon gas yields result from the high first-stage temperature. In the plant run providing 50% distillate yield, Martin Lake lignite was processed at a feed

652 rate of 133 kg/h (moisture-free basis), corresponding to a space velocity of 1190 kg/h.m3 per stage [105]. First-stage temperature was 450~

with hydrogen partial pressure 17.5 MPa; the

corresponding parameters for the second stage were 390"C and 18 MPa, respectively. Yield of C4§ distillate was 50.1%, corresponding to a distillate production of 58.5 kg/h, or 526 kg/h. m3 catalyst. The operation used Shell 324 catalyst, requiring a catalyst replacement rate of 0.5 kg/t lignite (maf basis). Residuum recycle was 25%. 12.2.5 Bench-scale studies for process improvement There are no commercial-scale direct liquefaction plants in operation in North America, and some question even exists as to whether a "green field" direct liquefaction plant will ever be built. Pilot-scale operations in the public sector in the United States are limited to one plant now operating near Princeton, New Jersey, and apparently no plans exist to run lignites in this plant. Nevertheless, world-wide bench-scale research continues to provide insights into lignite behavior in direct liquefaction. Generally, these studies tend to fall into the areas of effects of lignite properties, lignite pretreatment, effects of temperature (or strategies for temperature staging), effects of solvent, and use of catalysts. Organic sulfur functional groups provide a weak link for depolymerization of the lignite structure [106]. If the liquefaction catalyst also has good hydrodesulfurization ability, the hydrogen sulfide produced in the process can also catalyze further breakdown of the lignite structure [107]. The studies leading to this conclusion included Hagel (North Dakota) lignite, though the results may be more pertinent to the lignites of very high organic sulfur content found outside North America. For three lignites--Hagel, Cayhiran (Turkey), and Mequinenza (Spain)--liquid yields (at 275~

6.9 MPa cold gas, 30 min) correlated with organic sulfur content [ 106]. Both the absolute

values of liquid yield and, when ammonium tetrathiomolybdate is added, the increase in yield in a hydrogen atmosphere relative to that in nitrogen correlate with organic sulfur content of these lignites [106]. A suite of seven Turkish lignites showed a strong dependence of conversion on sulfur content (as well as on volatile matter) [ 108]. The sulfur content accounts for almost all the variance in yield of benzene-solubles [ 108]. With a larger suite of eleven Turkish lignites, liquefied in creosote oil solvent, the oil yield correlated with total sulfur, and somewhat less well with organic sulfur [ 109]. Untreated (32.2% moisture) and partially dried samples of Beulah-Zap (North Dakota) lignite provided higher yields of preasphaltenes and asphaltenes than did dried lignite, with the highest yields obtained from the partially dried sample [110]. The oil yield was highest for the dried sample and lowest for the partially dried. The total of oil, asphaltene, and preasphaltenes was almost the same for all samples. Pretreatment of Zap (North Dakota) lignite with water (350~

28

MPa, 10--1000 min.) reduced yields of toluene- and pyridine-solubles with short pretreatment times, but the yields increased with longer times [111]. However, even with long pretreatment times the yields were generally not as good as with the untreated lignite. Water pretreatment creates o-dihydroxyl functional groups that reduce liquefaction yields via retrogressive solvent

653 incorporation. Short hydrothermal treatment increases the surface concentration of the odihydroxyl groups, which then enhance solvent incorporation. These groups are destroyed at longer pretreatment times, but longer times also induce crosslinking in the lignite structure which would need to be compensated by some parallel depolymerization processes. Blends of Beypazari and Elbistan, or Beypazari and Karliova (Turkey) lignites provided higher yields of oil than would be anticipated from the arithmetical average of the oil yields from the lignites liquefied separately [112]. The conversions, however, were about the same as the average of the conversions of the individual lignites. These observations were independent of choice of solvent and catalyst. Swelling of Big Brown lignite before liquefaction improves conversion at 275~

even

without added catalyst [113]. The increased conversion is mainly a result of increased oil and gas yields [ 114]. The relative effectiveness of four solvents--methanol, pyridine, tetrahydrofuran, and 10% aqueous tetra-n-butylammonium hydroxide--for enhancing conversion was in the same general order as their effectiveness at swelling the lignite [113,114]. The combined effect of catalyst impregnation (ammonium tetrathiomolybdate) and solvent swelling doubles conversion, increasing the yields of all products (i.e., preasphaltenes, asphaltenes, oils, and gases). Pretreatment of Beulah-Zap lignite with methanol and hydrochloric acid (40 mL methanol and 0-2 mL concentrated HC1 per 5 g lignite, for 3 h) enhances liquefaction reactivity [115,116]. The increase in reactivity correlates with extent of calcium removal. Calcium catalyzes retrogressive reactions of free radicals during early stages of lignite dissolution. The hydrochloric acid destroys calcium dicarboxylate bridging structures in the lignite. Thus in the absence of this pretreatment conversion (350~

7 MPa cold HE, 30 min., dihydrophenanthrene solvent in 2:1 ratio with lignite)

was 35% tetrahydrofuran-solubles [115]. With pretreatment, conversion increased to 63%. Any simple organic solvent can be used with hydrochloric acid in the pretreatment step [ 116]. Optimum reaction temperature for liquefaction of Spanish lignite is 375-400~ Heating rate can affect conversion [ 118]. Fast heating (=80~

[117].

of Estevan (Saskatchewan)

lignite marginally improves liquefaction yield relative to heating up at =5~

[ 119]. Enhanced

yields can be obtained if the lignite-derived radicals are generated at a rate that allows efficient hydrogen capping from the surrounding hydrogen-donating medium. These observations have been verified for liquefaction of Indian Head lignite in water (with pyrrhotite catalyst and 1:2:2 H2S-CO-H2 atmosphere) [118]. Above 430~

oil yields increase with residence time [120].

Reaction provides an initial fast yield of oil, and the enhanced oil yield with increasing residence time results from increased conversion of asphaltenes. A 1:1 mixture of anthracene oil and creosote oil provided higher oil yields and conversions of Karliova lignite (at 440~

8 MPa) than did either solvent used alone [ 112]. In the solvent blend,

the oil yield reached 53%, and the conversion, 96%. Coprocessing, the simultaneous treatment of coals and heavy petroleum fractions such as residua, is an alternative to conventional direct liquefaction with a coal-derived recycle solvent. Possible advantages of coprocessing relative to conventional direct liquefaction, include

654 coprocessing occurring in a once-through operation, eliminating the solvent recycle loop, and an apparent synergy in which the oil yields from some coal/residuum combinations are greater than would have been predicted from a simple average of the oil yields from the coal and residuum processed separately. Martin Lake lignite, processed with various residua (4(K1--425"C, 8.6 MPa cold hydrogen, 30-60 min, and residuum/lignite ratio of 2), produced higher yields of hexanesolubles, by 6-20%, than did bituminous coals with any of the residua tested [121]. Also, the lignite gave a greater contribution of coal-derived material to the hexane-soluble product, and a correspondingly lower contribution to the asphaltenes, than did the bituminous coals. Using fuel oil as the liquid medium, liquefaction yields from Bergueda (Spain) lignite are higher in hydrogen than in a carbon monoxide/steam mixture (for reaction with IC141-6 cobalt molybdenum catalyst, 5 MPa cold gas pressure, 40(0-450"C) [ 122]. Hydrogen sulfide enhances conversion and generally shifts the product slate to lighter materials, as observed in liquefaction of Indian Head lignite [123]. Hydrogen sulfide catalyzes both conversion and production of distillables [124]. Production of distillable tetrahydrofuransolubles from both Big Brown and Zap lignites (at 40(0-417"C) increased with increasing hydrogen sulfide concentration in the reactor. Pretreatment of Indian Head lignite in an argon/hydrogen sulfide mixture (in tetralin, 60 min, 175~

prior to reaction in hydrogen (6.9 MPa cold, 410"C)

provided the highest conversion and most desirable product slate of all pretreatments tested [ 125]. The ion-exchange properties of lignites, important in their inorganic geochemistry (Chapter 5) and possibly for application in waste treatment (below) can also be exploited for catalyst impregnation. Exchanging Beulah and Hagel lignites with 0.05M iron(Ill) acetate at pH 2.8 (controlled with sulfuric acid) and 60~ increased the iron to 7.8% in the Beulah lignite and 5.3% in the Hagel [126]. The iron exchanges primarily with calcium. Liquefaction conversions exceeded 80%, compared to about 60% without iron ion exchange [126,127]. Preasphaltene yields decreased. Oil and asphaltene yields increased by adding dimethyl disulfide as a catalyst sulfiding agent. Smaller, but still significant, increases in yields were obtained by incorporating only 1% iron [ 128]. For Turkish lignites, a direct correlation occurs between oil yield and iron content [ 109]. Iron ores such as pyrite, hematite, and magnetite have catalytic activity in lignite liquefaction [129]. With Kansk-Atchinsk (Russia) lignite, the yield of hexane-solubles obtained with added pyrite was similar to that obtained with an industrial cobalt-molybdenum supported catalyst. Pyrrhotite and hydrogen sulfide, both produced from pyrite, assist in the liquefaction reactions. Iron carbonyl catalysts, Fe(CO)5 and (~t-S2)Fe(CO)6, also appear to be suited to low-rank coal liquefaction because of the oxygen functional groups in these coals [130]. These catalysts increased the conversion of Beulah-Zap lignite (425~

20 min, 3.5 MPa cold H2 pressure,

hexadecane solvent in 6:1 solvent-to-lignite ratio) from 24% toluene-solubles without catalyst to about 40% with added catalyst [ 130]. An Illinois bituminous coal, in comparison, showed virtually no effect of adding these catalysts. Beypazari lignite provided a 30% yield of oils at 300~ when liquefied in the presence of a

655 NiC12-KC1-LiC1 catalyst (14:36:50 mole percent) [131]. A catalyst-to-lignite ratio of 0.5 was used. At 360"C the molecular weight of the oils can be decreased to 245 daltons by increasing the catalystto-lignite ratio to 1. Sodium promotes hydrogen addition to lignite. Conversion of Indian Head lignite (at 250"C, cresylic acid solvent, carbon monoxide atmosphere) increased from 66% in the absence of sodium to 74% with sodium present [123]. Oil yields from Turkish lignites tend to increase with increasing amounts of aluminum, calcium, and magnesium in the lignite [ 109].

12.3 C H E M I C A L P R O D U C T S F R O M L I G N I T E S Almost the entirety of lignite technology today is devoted to its use as a fuel, mainly in electric power generation, and to a much lesser extent its conversion to substitute natural gas. Furthermore, results of pilot-scale testing from the synfuels "boom" of the late 1970's and early '80's suggest that lignite has significant potential as a feedstock for direct liquefaction processes. However, it is important to bear in mind that lignite--indeed all ranks of coal, and in fact all of the fossil "fuels" -- are of greater potential use simply than sources of energy from combustion processes. Natural gas, petroleum, and coals all are potential raw materials or feedstocks for processes not immediately related to fuel use. This section provides an overview of some alternative uses of lignites, some of which provide examples of non-fuel uses of lignites. Some of the technology to be discussed in this section is quite old; nevertheless it remains of potential applicability for future utilization of lignites. 12.3.1 Montan wax Montan wax belongs to a class of waxes known collectively as mineral waxes. (The term "mineral wax" implies that the waxes occur geologically, as opposed to the animal waxes such as beeswax or the vegetable waxes such as bayberry wax. Other examples of mineral waxes are ozokerite and petroleum-derived waxes.) Montan wax is hard and brittle with a high melting point. It has had numerous commercial applications in such diverse materials as leather dressings, ink, carbon paper, greases, phonograph records, electrical insulators and a variety of polishes or other kinds of protective coatings. Chemically, montan wax is primarily a mixture of esters and high molecular weight acids. The exact composition depends on the source. Minor components include assorted alcohols, ketones, hydrocarbons, resins, and asphalts [ 132]. The yield of crude wax from lignite depends on the solvent used. For solvent extraction of Antrim (Northern Ireland) lignite from the Crumlin mine in a conventional Soxhlet apparatus (lignite/solvent ratio of 0.4) the following yields were obtained: in toluene, 14.52%; hexane, 6.69%; benzene, 10.71%; methyl isobutyl ketone, 15.25%; chloroform, 13.15%, isopropanol, 12.93%; and 3:1 chloroform:hexane, 12.04% [133]. For many years montan wax was extracted commercially from brown coal in Germany.

656 The German brown coals ranged from 10 to 18% wax content. The extraction was performed using an 80:20 benzene:ethanol mixture or an 85:15 benzene:wood alcohol blend (where the wood alcohol itself was a mixture of methanol and isopropanol). The wax was sold in three grades: crude wax; so-called "deresinified" wax; obtained by reduction of the resin content by further solvent extraction; and refined wax, obtained by distillation at 5.3--6.6 kPa over the range 180-500"C [132]. Crude montan wax has also been produced in the former Czechoslovakia. Interest in the production of montan wax from American lignites was a result of the cut-off of supplies of imported wax in World War II. A plant in Malvern, Arkansas produced wax for, among other things, shoe polish. (This plant also converted Arkansas lignite into the dye Vandyke brown.) Another plant has been in operation in Buena Vista, California. The process used by the American Lignite Products Company in lone, California begins with crushing the lignite t o - 1 2 mm and then pulverizing to 80% -200 mesh. The pulverized lignite is treated in batch extractors with petroleum-derived solvents. The discharge from the extractors is filtered, and solvent removed from the extract by distillation. The wax is drained from the stills, cooled and solidified, and then crushed and packaged for shipment. About 140 kg of crude wax is obtained per tonne of lignite [ 134]. The spent lignite from the extraction is treated by steam distillation to recover any solvent retained in the filter cake. The spent lignite has been used as a filler in fertilizer, where it acts as a mulch, improves soil porosity, and contributes humic acid to the soil [ 134]. Fixed-bed retorting (heating at 4*C/min to 550"C) of lignite from the Claiborne Formation, Carlisle County, Kentucky, yielded 11.7% of a liquid pyrolyzate reported to be "very waxy" [ 135]. No work appears to have been done to recover or characterize the wax component of the liquid. 12.3.2 Activated carbon Activated carbon from lignite can be prepared to have the same adsorption behavior as activated carbon from coconut hulls, birch, or oak [ 136]. Furthermore, the activated carbon from lignite shows a high degree of hardness which makes it more easily regenerated than carbons from some other sources. Granular activated carbons made from lignite are especially suited for adsorption of dyes from textile wastes [137]. Similarly, cokes produced from German brown coals have been used as adsorbents for wastewater purification in paper, sugar, and textile industries [138], and unburned lignite chars recovered from boilers are considered suitable for water purification, deodorization, or adsorption of gases [139]. Activated carbon has been produced on a commercial scale from lignite at the Darco plant (Atlas Powder Co.) in Marshall, Texas, by a process originally patented in 1918 [140]. The lignite is ground with dolomite and pressed into cakes with a starch binder. The cakes are heated until the dolomite decomposed; carbon dioxide is also liberated within the pores of the char and is believed to be the agent that actually activates the lignite [141]. Lignite is crushed t o - 2 2 mm and is carbonized in externally heated alloy steel retorts. The retorts are heated by natural gas. The product is acid-washed, ground and classified, and then packaged for shipment [134]. Two

657 products are made. Darco activated carbon is 70-90% -300 mesh. It is used in purifying and decolorizing a variety of materials including pharamceuticals, sugars and syrups, and oils and solvents. The other product is Hydrodarco, a granular material used in water purification plants. Bench-scale steam activation of a Dickinson (North Dakota) lignite char in a fluidized-bed apparatus showed the feasibility of producing good quality carbon. The operating conditions were a temperature of 955~ and particle size of 40x60 mesh [142]. Char and steam feed rates were 300 g/h and 0.01 m3/min, respectively. At the optimum operating conditions, the activated carbon from the Dickinson char had superior service time for adsorption of carbon tetrachloride vapor from air and comparable heats of wetting in benzene to commercially available Darco carbons [142]. Reaction of Velva lignite char with steam at temperatures in excess of 950~ produced an activated carbon with benzoic acid adsorption substantially superior to Darco (42.5%

vs.

21.3%) [ 134,142].

Activation was carried out in a rabble-arm furnace. Activation of Louisiana lignites in steam at 750~ showed a slow increase in surface area, but even after four hours' reaction the surface area was still less than 300 m2/g [ 143]. However, an increase in activation temperature to 800~ resulted in reaching a surface area of 400 m2/g in about 3 h. Activation at this temperature for times greater than 3 hours resulted in only small further increases of surface area. Further increase in activation temperature, to 850~

produced a surface

area of 460 m2/g after 3 h, and a decrease in surface area from this maximum at longer times. At 9000C for 2 h the surface area reached about 400 m2/g. Although different lignites appear to show different reaction pathways (in the sense of requiring different times or temperatures to achieve a given surface area), the surface areas of different lignites measured at equal burn-off levels are quite similar. This observation suggests that the activation process may proceed at different rates for different samples, but that if the activation process is allowed to proceed to some specific extent, the products obtained from the different lignites are nearly identical. The total pore volumes in the activated carbons from Louisiana lignites were lower than those of commercial lignitederived activated carbon. The average pore size for the carbons derived from the Louisiana lignites was 2.2 nm; essentially no pores of radii >5 nm were present [ 143]. A substantial enhancement of surface area of the activated carbon can be achieved by demineralizing the parent lignite. Ptolemais (Greece) lignite activated in 3:1 N2:CO2 yielded a carbon with CO2 surface area of 200 m2/g [144]. As activation proceeded, the surface area increased to a maximum and then decreased. The initial increase resulted from opening of closed pores, widening of micropores, and formation of new pores. The decrease in surface area with longer activation times results from coalescence of adjacent pores. Pretreatment of the lignite with hydrochloric and hydrofluoric acids reduced the ash value from 22.1 to 0.14%; subsequent activation in carbon dioxide ( 100 cm3/min flow) produced a carbon with CO2 surface area of 650 m2/g [144]. Adsorption isotherms for the carbon from the acid-treated lignite are typical of materials that are exclusively microporous with a very narrow size distribution of the micropores. The activation temperature did not affect the development of surface area in either the untreated or

658 the acid-washed lignites. 12.3.3 Direct uses of lignites in waste treatment Since lignites have significant porosity and ion-exchange capabilities, the potential exists for using lignites directly as waste-treatment media, rather than taking the additional step of producing an activated carbon from the lignite. Three lignites, Beulah-Zap, Claiborne (Carlisle County, Kentucky), and a Jackson lignite (Atascosa County, Texas) show high adsorption capacities for mercury, lead, and cadmium [145]. Solutions containing 1000 ppm of these metals were treated with -50 mesh lignite at a liquid/solid ratio of 20. The extent of metal adsorption was governed primarily by the equilibrium solution pH; lignites providing the higher solution pH values showed the largest metal adsorption capacities. The metals ion-exchanged with surface carboxylic acid functional groups to form metal carboxylates on the lignite surface. The use of these lignites in multistage treatment should make it possible to meet heavy metal discharge limits at lower cost than with conventional treatment processes. Lignite has been successfully used to treat the wastewater from chromium plating [146]. The removal amounts to 80 mg Cr per gram of lignite. Wastewaters with chromium contents as high as 1000 ppm can be "de-chromed" by passage through lignite beds. An application of the absorptive properties of Canadian lignite is its application for the recovery of gold from hydrometallurgical processes [147]. Lignite effectively removes dinitrochlorobenzene, dinitrophenol, and dinitrotoluene from industrial effluents [148]. The concentration of these compounds can be reduced to less than 5 mg/L by lignite treatment. Lignites also effectively adsorb amines, e.g., dihydroxyaluminum aminoacetate (commercially known as Alamine), and alkylammonium salts, such as cetyltrimethyl ammonium bromide, from aqueous solutions [ 149]. 12.3.4 Charcoal A commercial lignite carbonization plant was opened in Dickinson, North Dakota in 1928. At first the product was a char briquette used for domestic heating. The byproduct tar was used as the binder. In the early 1960's operation was converted to make barbecue briquettes. Consumption of lignite in this operation is about 105,000 tonnes per year [150]. Husky Industries is a leading manufacturer of charcoal briquettes for the domestic barbecue market. The lignite is obtained from a nearby mine presently working the Dickinson and Leigh seams. The lignite is carbonized at 550"C [151]. The char is compacted into briquettes with a binder and additives. Lignite chars recovered from combustion chambers of boilers can be used to prepare carbon-based soil improvers for agriculture [139,152]. A suggested application is to alleviate hazards from overzealous use of agrochemicals [152]. These materials also appear to promote plant growth [139], the growth-promoting effects being evident with radishes, for example [152]. Filters made of lignite cokes provide for removal of pollutants from flue gases [ 153]. In biological wastewater treatment, lignite cokes allow improved sludge sedimentation and improved nutritional support of microorganisms [ 154].

659 12.3.5 Aoolications in extractive metallurgy Early workers have investigated the effects of inorganic salts on the carbonization of lignites [155-157]. Hydrated salts of aluminum produced a hard, compact residue resembling

(e.g., in luster) the coke produced from bituminous coals. The particles could survive a drop of about 60 cm to the floor without disintegration [141]. These materials were termed pseudocokes [ 141]. The mechanism of formation is postulated to be based on the secondary decomposition or cracking of hydrocarbons in the presence of the inorganic material with the resultant formation of graphitic layers [155]. High-temperature reactions of lignite in hydrogen atmospheres have sometimes been suggested as a way of producing a coke or coke-like product. For example, during hydrogasification of Rockdale (Texas) lignite at temperatures to 800~ and pressures to 42 MPa, the residue remaining in the reactor agglomerated [158]. Lignites pretreated with coke oven gas can be blended with coking coals to be used in metallurgical coke production [159]. Processing involves heating at 3N)--4200C for 90 min with 6 MPa cold coke oven gas (equivalent to 13-17 MPa operating pressure). The coke oven gas contained as main constituents 60.3% hydrogen, 22.3% methane, and 7.6% carbon monoxide. At 420~

the three lignites produced fused masses having mosaic textures developed to various

extents (Tunqbilek and t~an lignites (both Turkey) showed formation of small mesophase structures at temperatures as low as 3500C.) Semicokes produced at 550~ from the treated lignites showed abundant mosaic or flow domain structures; the semicokes from lignites without pretreatment were completely isotropic. A formcoke can be produced from Soma (Turkey) and Tun~bilek lignites using sulfite liquor from pulp and paper mills as a binder [160]. The liquor is concentrated to 50% solids before use. The process involves preheating --0.5 mm lignite to 120~ and preheating the liquor to 60~ The two materials are mixed (using 4-16% binder) and briquetted at 800C with up to 100 MPa pressure. Coking of the briquettes is done by heating at 4~

to 4500C and holding at that

temperature for 4 h. Injection of lignite (or indeed coals of other ranks) through blast furnace tuyeres is an alternative to production of coke. Lignite is a suitable alternative to coals of higher rank for tuyere injection [161]. Injection of lignite through tuyeres saves coke, improves productivity, raises the blast temperature, and enriches the oxygen in the blast [ 162]. Lignite has been suggested as a reducing agent in processing of taconite ore in northern Minnesota [ 150]. Taconite occurs in magnetic and non-magnetic forms, the former being more desirable. Normally magnetic separation processes are used to recover the magnetic taconite. Mild reduction of the non-magnetic taconite could convert it to the magnetic form. Since the lignite deposits of North Dakota are the nearest source of coal, it has been suggested to use lignite to carry out the reduction step. Lignite has also been suggested to convert taconite to so-called pre-reduced pellets proposed for use as blast furnace feed. Partial reduction of taconite would increase the proportion of iron in the feed, allowing blast furnaces to operate at higher capacity and with less

660 coke consumption. Unfortunately, the economic problems besetting the North American steel industry make it questionable whether a major use for lignite in the metallurgical industry will ever develop. Outside North America, both lignite and carbonized lignite have been investigated as potential replacements for coke breeze in the downdraft sintering of iron ore [ 163], and sintered iron ore pellet manufacture [ 164]. Similar efforts have been directed to the metallation of iron ore pellets in a rotary kiln [165], as well as reduction of hematite by Cheremkhovo (Russia) brown coal [166] and the production of sponge iron [167]. In the German metallurgical industry, lignite has also been used for the volatilization of zinc from waste oxides [167]. In India, pyrolyzed lignite has been tested for roasting of ilmenite sand at 865-875"C [168]. A combination of this roasting process with a subsequent leaching to remove iron oxides increases the titanium oxide content of the sand from 59% to 93%. Lignites effectively reduce manganese nodules [ 169]. The addition of sodium chloride during the roasting step provides more complete reduction at lower temperatures. Roasting the nodules at 750~ for 90 min, followed by leaching with a solution of 125 g/L ammonia and 62.5 g/L carbon dioxide provides recoveries of 93% of the nickel, 92% of the copper, and 65% of the cobalt [169]. Carbon metallurgical electrodes can be manufactured from tar derived from lowtemperature carbonization of lignite [170]. A coke was produced by thermally cracking and then coking the pitch fraction of tar. Coke production occurred at 1370~ in nitrogen The density of the coke was 1.96 g/cm3; electrical resistivity was 2.75 mQ/cm3 [170]. The electrodes were more susceptible to the Boudouard reaction than electrodes made from petroleum coke with a bituminous binder. Thus, higher anode consumption occurred in an alumina reduction cell. If care is taken to increase the compressive strength and density of the electrode, susceptibility to the Boudouard reaction decreases. Increasing the calcination temperature for formation of the coke to 1750"C improved performance in an alumina reduction cell. Higher compressive strength and lower porosity of the electrode were obtained by using binder with a hydrogen content of less than 5% [1701. Fire hardening, or induration, is used as a treatment for iron ore pellets. The current standard practice is to indurate the pellets in natural gas fired kilns. Limitations of natural gas supplies in the future could make it desirable to fire the kilns with solid fuels. Lignites and subbituminous coals have been tested in this regard [171]. The advantages of these fuels are their proximity to the iron ore mining regions and their general low sulfur content. The pellets produced in these tests had mechanical properties equal or superior to pellets produced in commercial naturalgas-fired kilns. There are two concerns in the use of any solid fuel in firing iron ore induration kilns: possible contamination of the iron ore pellets, and the and the accumulation of solid ash deposits inside the kiln. The latter problem is referred to as kiln ringing. Test firing of a pilot-plant kiln with North Dakota lignite for 120 hours produced a deposit 75 cm long and 13 cm deep [ 171]. The maximum ringing occurred about a meter beyond the lignite flame, at a region where the

661 temperature was 1160~ (the maximum temperature in the kiln was 1300~

[171]. The principal

culprit in ring formation was the lignite ash. Some fluxing action of the lignite ash with the iron ore or the bentonite binder promotes ringing at induration temperatures. For a range of coals, the best performance, in terms of minimizing kiln tinging, is obtained from coals which have ash fusion temperatures above the pellet induration temperatures. However, both the fluxing and the fusion behavior of the ash are important. Although the ash contributes to kiln tinging, it does not result in contamination of the pellets. About 80% of the ash from the lignite combustion was picked up by the pellets; however, this amounted to only 5 kg ash/t of pellets. The effect of the ash pickup on composition of the pellets is to increase the total of silica plus alumina by 0.2%, and to increase the total of lime plus magnesia by 0.15% [134]. Therefore it would seem that even if 100% of the lignite ash were picked up by the pellets, the effect on pellet composition would be quite minimal. The alkali contamination is less than the amount of alkali supplied by the bentonite clay used as a binder for the pellets. Strictly speaking, firing of brick-making kilns is not a metallurgical application, but has much in common with lignite uses in pyrometallurgy. Lignite has been evaluated as a replacement fuel for natural gas in a rotary kiln used in the manufacture of bricks and refractories [ 172]. In this application, lignite is pulverized in a hammer mill, and then air-swept into the kiln burner. The air is taken from the hood of the kiln and is at approximately 230~ [172]. This high temperature airdried both the wet lignite and the wet clay. A deflector between the air fan and the hammer mill, together with the volume of air, control the lignite particle size entering the kiln. The velocity can be increased by using an auxiliary blower. The lignite particles are blown onto the clay feed, and are ignited by means of a tangentially firing natural gas igniter burner. The burning time of the lignite is regulated by changing the particle size, the velocity of the lignite as it is blown into the kiln, the temperature of the carrying air, the amount of excess air, and the kiln draft (or some combination of these variables). The optimum operating conditions were not established, but it appears that the velocity of the lignite and the ignition mode are the most critical. It is important to create turbulence once the lignite has been ignited, to remove any ash from the outside of the burning lignite particle. The overall energy consumption of the kiln was reduced by about 20% by converting from natural gas to lignite firing, yet the production rate and the kiln temperature (104(0C) were held the same. 12.3.6 Recovery of uranium from lignites The only commercial production in the United States of uranium from organic source materials was production from the lignite of northwestern South Dakota and southwestern North Dakota. The existence of uranium in North Dakota lignite was discovered in 1948 [173]. These Sentinel Butte lignites contain up to 0.1% U308 [174]. The uraniferous lignite deposits are thin and impure. In North Dakota, the major occurrences of uraniferous lignite are in beds which are overlain or underlain by a sandstone aquifer. Uranium enters the ground water by leaching from

662 sediments of uraniferous volcanic ash. The ground water itself is presumed to be of meteoric origin, leaching uranium from the ash as it moves downward and laterally into the ground. Lignite with a uranium content high enough to be considered ore-grade was located between Belfield and Amidon in Billings County in 1955, and the first shipments of uranium ore grade lignite were made the following year [175]. The shipment of lignite for uranium processing was limited to a few hundred tonnes because the lignite could not be milled by the same processes applied to uraniferous sandstones. From 1962 to 1967 the uraniferous lignite was burned and the ash was shipped to processing plants where it could be blended with sandstones for uranium extraction. The operation was discontinued in 1967. About 270 tonnes of U308 was produced from North Dakota lignite, mainly from the lignites of the Belfield area of Billings County [176]. After the observation that the lignite could not be milled by the same equipment used for size reduction of the sandstone ores of Colorado, the lignite was first burned and the ash was shipped to uranium mills for blending with the sandstones. Heating lignite reduces the solubility of uranium in both acid and carbonate solutions [177]. Once the solubility has been reduced by heating, it can only be restored by complete destruction of the organic matter. With appropriate combustion of the lignite the reagent consumption is reduced; however, problems can arise from overheating, converting the uranium into a very refractory form. The best approach involves combustion at about 1200-1300~

Sulfur dioxide released in the combustion process can be

captured and used to supplement the acid used in the leaching step. Production of lignite for uranium extraction ended in 1967. The total production from North Dakota was 77,237 t, which yielded 268,662 kg of U308 "yellow cake" [ 150]. Based on a price of U308 of $18/kg, economically recoverable reserves of uraniferous lignite in North Dakota amount to 64,000 t, capable of producing 218,000 kg of yellow cake [150]. These amounts represent less than 1% of U. S. uranium reserves. New deposits of uraniferous lignite might be found in areas where sandstones are in contact with lignite. Even if the price of uranium or yellow cake rose sufficiently, it is unlikely that production of lignite specifically for its uranium content would ever be resumed. Although these lignites may represent a future source of uranium, environmental protection legislation enacted in the last two decades makes it unlikely that processing could be accomplished as was done in the 1960's. Their recovery by strip mining would be affected by mined-land reclamation regulations. The open-air burning of lignite for its uraniferous ash would certainly be questionable. Burning the lignite would be complicated by the presence of appreciable molybdenum and arsenic with the uranium; all three elements are health hazards in airborne smoke or dust. Thus any burning operation to recover uraniferous lignite ash would need to satisfy various air pollution regulations.Although the liberation of uranium, molybdenum, and arsenic as airborne dust or water leachates would be a serious environmental problem, an alternative would be the extraction of uranium (as well as other elements such as molybdenum, gallium, and germanium) from power plant ash.

663 12.3.7 Leonardite The material now known as leonardite was first described on the basis of analysis of a sample of material found lying in a ditch in western North Dakota [178]. The material had the appearance of a black gel, which dried to hard lumps having a coal-like appearance. When treated with water, the material dispersed to a dark brown to blackish colored suspension. The material was presumed to have derived from the interaction of oxidized lignite with alkaline groundwater, and was brought the material to the attention of the state geologist, Arthur G. Leonard, who conducted the first detailed study, and in whose honor the material was subsequently named. The term leonardite is sometimes applied in a general way to partially oxidized low-rank coals, including subbituminous coals, and carbonaceous shales. Leonardite has been defined as a "soft, earthy, medium-brown, coal-like substance" [ 179]. Leonardite is a mixture of humic acids with such minerals as gypsum, clay, and quartz [150]. It is often associated with lignite outcrops, generally overlying the lignite and grading into it, and occurs in association with virtually all outcrops of lignite in North Dakota. It has been mined commercially in Adams, Bowman, Divide, and Williams counties. A leonardite seam in Bowman County, North Dakota is about 2 m thick and lies 2.4 to 11 m below the surface [ 179]. Generally leonardite occurs in flat-lying lenses ranging from 0.3 to 2.4 m in thickness [150]. The cover is usually shallow and porous. Leonardite is generally believed to be formed from lignite by natural oxidation, either by atmospheric oxygen, or percolation of oxidizing groundwater, or possibly by both. Analysis of two samples of leonardite from North Dakota and one from Texas shows, on an maf basis, 6066% C, 3.7-4.6% H, and 28-30% O. The volatile matter ranged from 54 to 62%. The wax content, determined by Soxhlet extraction with benzene-ethanol, was 5.2-12%; humic acids, soluble in 1M sodium hydroxide, were 64--86% [ 179]. (Leonardite is easily dispersed or dissolved in aqueous sodium hydroxide at room temperature [180].) Other proximate and ultimate analysis data on Peerless (North Dakota) leonardite and Baukol-Noonan slack fall into these ranges [181]. Comparative proximate and ultimate analyses of leonardites and corresponding lignites are shown in Table 12.11 [ 150]. The higher oxygen content of leonardite relative to lignite makes leonardite undesirable as a fuel. Since leonardite is associated with most lignite outcrops, the leonardite must be removed with the overburden to mine the fuel-grade lignite. An expansion of the commercial use of leonardite could be accommodated with the addition of a single processing step--the separation of the hydrocarbon leonardite from the noncarbonaceous portions of the overburden, or alternatively, the separate mining of the overburden proper and the leonardite. However, a much-expanded commercial utilization of leonardite would also have to cope with the highly variable quality of this material. Two factors determine the quality of the leonardite: the extent of oxidation, which in essence is a measure of the extent of conversion to humic acids or humates soluble in water or alkali; and the nature of the associated inorganic materials, since leonardite with large quantities of gypsum is not as desirable for some applications [150].

664 TABLE 12.11 Comparative analyses of leonardites and lignites [150] Leonardite Bowman Divide County County Prox. (as-rec'd) Moisture V.M. F.C. Ash Ultimate (maf) Hydrogen Carbon Nitrogen Sulfur Oxygen (diff.)

53.4 23.9 15.0 7.7 4.0 63.9 1.2 2.5 28.4

Lignite Bowman Divide County County

42.6 26.4 22.6 8.4

42.6 24.5 26.0 7.0

36.2 26.4 31.0 6.4

3.7 65.7 1.3 0.9 28.4

5.1 72.9 1.0 1.5 19.5

5.1 73.8 1.2 1.0 19.9

A second type of leonardite, different from that just discussed, is believed to be the result of the leaching of lignite seams by alkaline groundwater. It has a "jelly-like" consistency and may be found some distance from the original lignite bed, whereas the first type is found overlying the lignite. Comparison of the compositions of Gascoyne leonardite, of the first type, with Bienfait (Saskatchewan) leonardite of the second type shows the principal difference to be in hydrogen content, 3.2% vs. 5.6% maf, respectively [182]. The other distinction is based on the pH of a slurry of leonardite with distilled water. The Gascoyne leonardite slurry had a pH of 4.3, compared to 9.8 for the Bienfait sample [182]. Leonardite has had variety of actual and proposed uses, including a conditioner (as a dispersant and for viscosity control) for drilling mud, a stabilizer for ion-exchange resins used for water treatment, a soil conditioner and fertilizer, and a source of brown, water-soluble wood stain. The commercial uses of leonardite, although quite varied, have nevertheless been limited, both with respect to the tonnage consumed and the geographical range in which it is used. Leonardite production has apparently never exceeded 50,000 tonnes per year [ 150]. From the 50's through the 70's the annual production generally fluctuated around 41,000 tonnes per year [ 150]. Carbonization of the leonardite at 480"C followed by activation in carbon dioxide at 955 ~ for 20 minutes yielded a product with a methylene blue number of 214 [183]. This result compares quite favorably with values of 293 and 211 for activated carbons obtained from lignite and subbituminous coal, respectively. In 1923 an operation was started near Minot, North Dakota to produce a coloring material from leonardite. The material was called "Dakalite." The production of Dakalite involved treating the leonardite with a hot alkaline solution to produce a solution of the dye. The extracted material was consideredto be a mixture of ulmic and humic acids [ 184]. Decanting the solution from the unextracted leonardite and evaporating to near dryness yields the moist Dakalite. Complete drying

665 is then effected in a rotary kiln. Various grades of oil-soluble and water-soluble Dakalite were marketed. In 1938 the American Dyewood Company began operation in El Dorado, Arkansas. The dye produced was sold as Vandyke brown. The plant also produced montan wax. In tetralin, leonardite showed a maximum solubility 74.6% [ 185], achieved at 440~

a 3:1

weight ratio of tetralin:leonardite and a reaction time of one hour, which were essentially the optimum conditions found for the reaction. Not surprisingly, conversion was found to be aided by reduction of particle size, and increase of tetralin:leonardite ratio, temperature, residence time, and reactor agitation. Leonardite has been evaluated as a soil conditioner in mined-land reclamation. In this application leonardite seems to enhance production of legumes but decreases grass production [186]. Leonardite may be useful as a soil amendment for mined-land reclamation, especially as an additive for spoils which are low in a natural organic matter [187]. Addition of 10% leonardite to a test soil produced a favorable response by legumes in controlled growth chamber studies, although grass production was decreased [187]. This difference in behavior between the legumes and grasses may reflect the effects of sorbed trace elements in the leonardite. The ideal additive would have a high humic acid content and low capacity for Fe§ and A1 +3 sorption. Na§ Ca§

and Mg§

in the leonardite could be easily lost to groundwater or meteoric water. Loss of these ions may exacerbate local salinity problems in the water. In Pakistan, a dosage of 500 g/ha of lignite-derived humic acids with half the standard dose of nitrogen fertilizer provided maximum wheat yields [188]. A trazine, a commercial herbicide, is adsorbed by humic acids extracted from North Dakota leonardites [ 189]. Adsorption results from charge transfer interactions between the electron-poor groups in the humic acids and electron-rich atoms in atrazine. Leonardite char can replace activated carbon filter materials, and provides more efficient capture of sulfur dioxide and nitric oxide from power plant flue gases than do filter materials made from high-rank coals [190]. For this application, leonardite is activated in carbon dioxide, nitrogen, or argon for 15-60 min at temperatures above 750~ 12.3.8 _l~-ospects for other chemical products from lignite It has long been known that oxidation of bituminous coals carried out in a slurry of caustic yields a variety of polycarboxylic aromatic acids [182]. However, this process has two problems as a potential source of chemicals. At mild reaction conditions there is a high yield of high molecular weight humic acids. At severe conditions there is no problem with the humic acids, but too much of the carbon gets converted to carbon dioxide. The best yield of desirable single ring aromatic acids appears to be about 30%. It appears that any aromatic cluster, regardless of the number of fused tings, yields one single ring aromatic acid and carbon dioxide. There is little ring cleavage to yield two or more single nng acids. Thus in an aromatic system of, say, three or four fused tings, only one of the tings is converted to the desired single ring acid, and the remaining carbon is lost as carbon dioxide. The advantage of lignites in this process is that they should

666 provide higher yields of the desired single ring acids and much less carbon loss to CO2 because most of the aromatic ring systems in lignites contain one or two rings. 12.3.9 Applications of lignite ashes Lignite ashes from circulating fluidized-bed combustion provide soil pH levels equivalent to those obtained from conventional lime application [191]. A better growth of plants is obtained from use of the lignite ash. Similar behavior occurs for lignite ashes from the Katek power plant in Siberia [192]. Reclamation of acidic soils with these ashes improved the quality of barley and clover crops. Concrete made with 20% fly ash from Saskatchewan lignite gives satisfactory performance, as long as its air content and strength are made comparable to concretes with no fly ash [ 193]. A high percentage (35-50%) of fly ash in concrete reduced resistance to freezing and thawing, even after curing in water for 80 days. This behavior results from migration of crystals of portlandite and ettringite to the air voids.

12.4 C O M B U S T I O N OF LIGNITE-WATER SLURRIES

In one sense, the preparation and use of lignite-water slurries can be considered a variant of liquefaction, since the slurry is a fuel in liquid form. In a different sense, slurries represent a new application of coal preparation and beneficiation. Interest in slurries likewise derives from several sources: an approach to lignite drying that gives a dried product resistant to moisture readsorption; an alternative approach to transportation (i.e., slurry pipelines); and a liquid fuel that might be used, for example, as an energy source in boilers that had originally been designed for oil firing. The 1980's witnessed commercial interest in slurry preparation and utilization--slurries of most ranks of coal, not only lignites--by several independent firms or subsidiaries of larger companies. By the mid-90's the commercial sector's participation in slurry research, production, and use has virtually vanished. The extent to which slurries will represent a viable commercial-scale technology in the future, other than in specialized niches, is questionable. 12.4.1 Slurry preparation for combustion The hot-water drying process (Chapter 10) affords a product that can be handled, atomized, and burned like fuel oil [ 194]. Utility-grind lignite slurried in water is heated to temperatures above 240"C, the exact temperature depending the lignite being processed [194,195]. Lignite moisture expelled during heating is not reabsorbed on cooling because of alterations in the lignite structure accompanying the thermal treatment. Cations of elements contributing to boiler fouling (e.g., sodium) get released from the lignite into the aqueous phase, and are removed during the dewatering [ 195]. Excess water is removed from the product slurry by centrifugation, evaporation, or filtration. The concentrated slurry has a calorific value in the range 13.8-17.6 M.l/kg [194].

667 Although the hot-water drying represents an additional processing cost for lignite utilization, the resulting slurries become a value-added product capable of competing for the same markets as heavy oil [ 195]. The slurries also offer a low-cost retrofit for oil-fired boilers. The practicability of the process has also been demonstrated for a variety of Turkish low-rank coals, with hot-water drying at 270-3300C [196,197]. 12.4.2 Combustion Performance Combustion of four low-rank coal (both lignite and subbituminous) slurries showed >99% carbon burnout at the combustor exit [198]. However, complete carbon burnout in slurry combustion may require a longer residence time, because of the high water content of the slurries. Complete combustion of the low-rank coal slurries is easier to achieve than for slurries of bituminous coals because of the non-caking nature of the low-rank coals. The lack of caking behavior eliminates agglomeration of the coal particles during the evaporation of the slurry water and devolatilization of the coal. High levels of combustion air swirl were necessary to achieve a stable flame [198]. For South Hallsville (Texas) lignite, slurries with 55-58% solids produced clearly defined, stable flames. Flue gas NOx was invariably higher for slurry combustion than in comparable tests with the same lignite burned in the pulverized coal mode. This may be attributable, at least in part, to the increased heat input needed to evaporate the slurry water and maintain furnace exit temperatures (heat inputs are higher by about 7% in the slurry tests compared with pulverized firing of the same lignite). However, slurry combustion should occur with lower flame temperature, so that it would be reasonable to expect reduced thermal NOx levels associated with slurry combustion. Comparison of bituminous coal and lignite slurries in pilot-scale combustion showed that the combustion of the bituminous coal slurry proceeded with the abundant formation of "sparklers," expanded char particles which required long times for complete burnout, whereas the combustion of a lignite slurry under the same conditions (and same burner configuration) produced a diffuse flame with only a haze of finely burning sparks [199]. These results suggest that agglomeration of char particles is not a problem with lignite slurries. In turn, this finding means that the effectiveness of slurry atomization in the combustor is not as critical for lignite slurries as for bituminous coal slurries. 12.4.3 Ash Behavior The comparative behavior of ash in the combustion of South Hallsville lignite in the pulverized coal and slurry firing modes is compared in Table 12.12 [198]. The lignite slurry was prepared by hot-water drying (Chapter 10) at 330~ The principal difference for slurry firing appears to be a much greater accumulation of ash on the wall of the combustor and in the flue; apparently because of this wall accumulation less of the ash susceptible to removal in the precipitator actually reaches the precipitator. The fact that little ash

668 TABLE 12.12 Comparative ash behavior in pulverized (PC) and slurry firing of South Hallsville lignite (adapted from [ 198]).

Ash input, gin. % of ash collected Total Combustor wall Combustor bottom Probes* Flue gas duct Precipitator

PC 21024 44.5 2.4 1.1 0.1 7.3 33.7

Slurry_ 20598 67.3 26.6 3.2 0.03 22.8 14.7

*Simulating steam tubes

was deposited on the probes in either test is a reflection of the low sodium content in the South Hallsville lignite. Pilot-scale combustion of Beulah lignite slurry showed that the droplet size is not the only factor affecting the particle size distribution of the ash [200]. Processes of agglomeration and coalescence affected the ash particle size distribution. The agglomeration and coalescence behavior is in turn influenced by the kinds of inorganic species in the lignite and their size distributions [201]. Furthermore, the amounts of various inorganics and their size distributions are not necessarily the same as those in the parent, untreated lignite. Preparation of Beulah lignite slurry by hot water drying causes a significant reduction in the particle size distribution of the pyrite and removes organically associated sodium [201]. The coalescence and agglomeration are facilitated by the fluxing action of the iron when it becomes incorporated into aluminosilicates [202]. The aluminosilicates also react with organically bound alkali and alkaline earth elements, forming new complex aluminosilicates of lower melting point than the original phases [203]. These new, lowmelting alkali and alkaline earth aluminosilicates also contribute to coalescence [203]. Several factors could contribute to the increased deposition of ash on the combustor walls. These factors include the slurry spray pattern, the intense swirl near the burner, and the longer flame (compared to a pulverized coal flame). In general, the ash deposits produced by firing low-rank coal slurries are more friable and easily removed than those produced in pulverized coal firing, even though there was little difference in the amount of ash deposited. The ash deposition rate for both South Hallsville and Indian Head lignites was lower in slurry firing than in conventional pulverized firing. The difference in physical properties of the ash deposits from pulverized and slurry firing may be related to differences in the mineral composition of the deposits produced in the two cases. As an example, Table 12.13 provides X-ray diffraction analyses of ash deposits from pulverized and slurry firing of Indian Head lignite [198].

669 TABLE 12.13 Comparative X-ray diffraction analyses of ash deposits from pulverized- and slurry-fired Indian Head lignite (Adapted from[198]) Pc-fired White Layer Major components

Quartz Anhydrite

Slurry-fired Quartz Anhydrite

Minor components

Hematite

Trace components

Lime Periclase Hematite Magnetite

Magnetite

Anhydrite Quartz Periclase

Pyroxene Melilite

Melilite

Anhydrite

Sintered Layer Major components

Minor components

Quartz Trace components

Hematite Magnetite

A general observation, based on the presence of minerals such as the pyroxenes and melilites in the ash deposits (as well as plagioclase feldspars observed in ash deposited in the flue gas duct), is that many of the phases detected in the ash from slurry firing appear to have crystallized from a melt, whereas many of the phases in ash from pulverized firing may have arisen from a simple thermal decomposition of minerals [ 198].

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679

INDEX

Abernathyite, 262 Abietic acid, 129 Absorptivity (of ash), 308 Acetic acid, 92, 103-104 Acetoguaiacone, 67 Acetone, 261 from lignite, 184, 192, 196, 200 Acetonitrile, 196, 200 Acetophenone, 148 Acetylene, 384 Activated carbon (from lignite), 4, 9, 656-657 Adipic acid, 101-102, 104 Air, reactions in, 161-163 Akermanite, 299, 301,518, 555, 570, 578, 590 Alamine, 658

Anhydrite, 240, 246, 298, 301,303-305, 314, 563,570, 575 Anisole, 94, 192 Anorthi te, 312, 571 Antelope Valley power station, 541-542, 576 Anthracene, 157 Anthracene oil, 141,170, 650-651,653 Anthracite, 66, 128, 433 Anthraquinone, 92 Anthraxylon, 52, 82, 90, 218 Anthrocyanins, 69 Antimony, 265, 268, 271-274 Antrim lignite, 655 Apatite, 246, 266 Arabinose, 120 Aragonite, 239, 245, 254

Alang iaceae, 54

Arecipites, 54

Albite, 236, 239, 249 Alcoa lignite mining, 9, 410 NMR of, 106 pyrolysis, 108, 123, 178, 192, 197 sulfur forms, 123 Alginite, 62 Alumina, 302,329, 589 Aluminum effect on pyrolysis, 184 in lignite, 230-231,268, 271-274, 277279 Aluminum chloride, 183-184 Aluminum oxide, 519 Aluminum silicate, 563 Aluminum sulfate, 563 Ammonia from lignite, 458-459, 511,619-620 for NOx control, 544 reaction with lignite, 488 Ammonium acetate, 220-223,226, 231-232, 236, 253, 255, 276, 291,293, 300, 514 Ammonium hydroxide, 496 Ammonium molybdate, 158 Ammonium tetrathiomolybdate, 488, 652-653 Ammonium tetrathiotungstate, 489

Arsenic, 265, 268, 271-273 Ash hemispherical temperature, 318 Ash softening temperature, 317-319 Atrazine, 665 Attrinite, 86-90 Attrital lignites, 82 Attritus, 16, 81-88, 90 aromaticity, 98 cellulose residues in, 120 friability, 346, 350 morganlcs in, 224 methoxyl groups, 115 reaction with hydrogen, 158 sodium in, 236 Augite, 301 Autogenous combustion, 26, 437-438, 442, 457 A utogenous heating, 433,436, 438, 440, 442 Autunite, 261

Amonoa, 54

Amorphinite, 58, 62 Analcime, 236 Anatase, 238-239, 244 Anderson lignite air oxidation, 436 Andesine, 239

Backhoes, 419-420, 425-426 Bagasse, 494-495 Baghouses, 543 Balakhovskii brown coal, 400 Barite, 239, 241,247, 254, 266, 277 Barium effect on air oxidation, 440 effect on pyrolysis, 183-184 in lignites, 253-254, 268, 271-274, 277, 279 Barium acetate, 110, 493 Barium chloride, 110 Barium hydroxide, 110 Barium sulfate, 520

680 Barnhardt-Williams sintering test, 309 Bassanite, 240-241,246-247, 292, 298, 301 Baukol-Noonan lignite aromaticity of core samples, 99 ash slag viscosity, 320, 324, 326 bulk density, 357 calcite in, 245 carbonization, 204 combustion, 535 drying, 463 microprobe analysis, 220 mine, 23, 25 minerals in, 241 Mohs hardness, 355 nacrite in, 249 pyrite in, 243 removal of cations, 221 stockpiling, 434 tetrapyrroles in, 121 Beckville lignite, 426 Benton lignite, 629 Benzaldehyde, 144, 190, 192 Benzene, 92, 192-193,261 extraction with, 128, 655 for heat of immersion, 364, 367 from lignite (see also BTX), 154, 157, 194, 203,620, 637 Benzenepentacarboxylic acid, 93-94 1,2,4,5-Benzenetetracarboxylic acid (see also pyromellitic acid), 104 1,2,3-Benzenetricarboxylic acid (see also hemimellitic acid), 106 1,2,4-Benzenetricarboxylic acid (see also trimellitic acid), 103, 106 Benzofluorene, 526 Benzofuran, 117 Benzophenone, 169 Benzo[a]pyrene, 526 Benzoxanthone, 117 Bergueda lignite, 654 Beryllium, 253,271,274 Betulaceae, 54 Betulaceae-Myricaceae, 53 Beulah lignite (also see Beulah-Zap) aluminum in, 230-231 aromaticity, 97-99 aromaticity of lithotypes, 97-98 ash deposition, laboratory, 560-561, 563,565, 570-571,578, 580581 ash formation, 519-520 ash fouling, 545, 548, 550 ash reactions, 569 ash sinter point, 309-310, 313 ash sinter strength, 312-314 ash slag surface tension, 336 ash slag viscosity, 322 barite in, 247

Beulah lignite (continued) calcite in, 245 carbonyl groups, 116 carboxyl groups, 110 carboxyl groups in lithotypes, 111 cellulose residues in, 120 cellulose in lithotypes, 120 char properties, 514 chemical fractionation, 223, 225-226, 228 clays in, 520 combustion, 515, 536-537 combustion rates, 512, 514 depolymerization, 129 dolomite in, 245 DRIFF of, 163 drying, 387 esters, 118 fluidized-bed combustion, 310, 315, 485, 586-588, 591 free radicals in, 159 friability of lithotypes, 350 FTIR of, 99, 107, 112, 116 gypsum in, 246 hematite in, 244 humic acids from, 113-114, 116 ignition, 509 ion-exchange beneficiation, 481,484, 486 iron in, 232 kaolinite in, 248 lignin residues in, 119 liquefaction, 160, 170, 297, 304, 648, 651,654 magnesium in, 233 magnetite in, 244 methoxyl groups, 115, 119 methoxyl groups in lithotypes, 115 methylation of, 103 microprobe analysis, 231,233,236-237 mine, 24, 428 minerals in, 241 montmorillonite in, 250 nacrite in, 249 nitrates in, 233 oxidation of lithotypes, 104 oxidation reactions, 70, 93, 101-103 quality, 27, 84-85 petrography, 87 polonium in, 265 potassium in, 234 pyrite in, 242-243 pyrolysis, 188-189, 191,193,205 pyrolysis of lithotypes, 184, 192 quartz in, 247 Raman spectroscopy of, 112 reaction with hydrogen, 159 SAXS of, 365, 369-370, 373-374, 390

681 Beulah lignite (continued) shrinkage, 403,452 siderite in, 246 slurry combustion, 668 sodium in, 236 solvent extraction of, 126-127 sulfur forms in, 122, 237-238 surface area, 363,402 swelling, 403-404 talc in, 250 titanium in, 238 uranium in, 262 XAFS of, 122 XANES of, 234 XPS of, 112, 369 zircon in, 244 Beulah-Zap lignite (see also Beulah and Zap

lignites) adsorption of metals antimony in, 265 arsenic in, 265 ash fusion behavior, 320 barium in, 254 beryllium in, 253 boron in, 263 bromine in, 265 cadmium in, 263 calcite in, 245 carbonyl groups, 116 carboxyl groups, 110 cerium in, 258 cesium in, 253 char properties, 511 chromium in, 255 cobalt in, 256 copper in, 257 deposit, 24-25 deposition, 60-62, 89 devolatilization, 510 dolomite in, 245 drying, 389 esters, 118 ether groups, 116 europium in, 259 FIMS of, 113, 118, 124, 128 float-sink separation, 477-478 fluorine in, 265 friability, 346 FTIR of, 112 gallium in, 263 genesis, 84 hafnium in, 257 heat of immersion, 364 hematite in, 244 hydration of, 396 hydraulic conductivity, 368 illite in, 249 inorganics in, 224

Beulah-Zap lignite (continued) iron in, 232 kaolinite in, 248 lanthanum in, 258 lead in, 264 liquefaction, 652-654 lithium in, 251 low-temperature ashing, 290 lutetium in, 259 magnesite in, 245 manganese in, 255 minerals in ash, 305 moisture, 393 molybdenum in, 258 monazite in, 246 nickel in, 256 neodymium in, 258 niobium in, 257 NMR of, 95, 100, 107 petrography, 79, 81, 83, 85-88, 90 phenolic groups in, 115 praseodymium in, 258 platinum group elements in, 259 pyrite in, 242 pyrolysis, 187, 202, 206 quartz in, 247-248 Rock-Eval analysis, 202 rubidium in, 253 rutile in, 244, 248 samarium in, 259 scandium in, 254 selenium in, 265 solvent extraction, 108-109, 113, 128 strontium in, 253 sulfur forms, 123 surface area, 361-362 swelling of, 367-368 tantalum in, 257 terbium in, 259 thermogravimetric analysis, 388 thorium in, 259 tin in, 264 tungsten in, 258 uranium in, 262 vanadium in, 255 XAFS of, 123 XANES of, 123 ytterbium in, 259 yttrium in, 257 zinc in, 263 zircon in, 244-245 zirconium in, 257 Beypazari lignite air oxidation, 436 liquefaction, 653-654 Bibenzyl, 169 Bienfait lignite ash fouling, 564

682 Bienfait lignite (continued) FTIR of, 107, 126 hydrogen bonding in, 126 NMR of, 96 BI-GAS process, 155 Big Brown lignite aromaticity, 97 carboxyl groups, 110 deposition, 61 humic acids from, 113-114 liquefaction, 141,147, 159, 170, 304, 641,643, 653-654 mining, 9, 417-418, 425 NMR of, 106 oxidation reactions, 102 petrography, 90-91 pyrolysis, 205 solvent extraction of, 126-128 solvent swelling, 125 Big Brown power station, 539, 545 Big game hunting, 249 Big Stone power station, 536, 545, 551,577578, 580 Bills method (viscosity), 329 Biotite, 239 Biphenyl, 192-193,637 Blackjack, 9 Blasting, 413,417, 420-421,428 Boehmite, 230, 244 Bombacaceae, 54 Bombacacidites claibornensis, 54 Bombapollis, 54 Bone coal, 9 Booker space heater, 522 Borax, 495 Boron, 263, 268 in soil, 424 Boron trifluoride, 109, 117, 120, 129, 489 Bottinga-Weill method (viscosity), 330 Boudouard reaction, 603,660 Boundary Dam lignite, 129 Boundary Dam power station, 542 B randon li gni te, 107 Bredigite, 299 Bromine, 265, 268, 271,273 Brookite, 238, 531 Brooms, mechanical, 421 Bryan lignite, 228 BTX (from lignite), 153, 155-157 Bucket wheel excavators, 413-414, 418-421, 425 Bulldozers, 413,415, 417, 419-421,423, 425, 433 Burleigh lignite, 478 Burning Star coal, 641 Burseraceae, 54 Butadiene, 108, 203 Butane, 202

Butanedioic acid, 103 2-Butanone, 192, 196, 200 1-Butene, 203 Butyraldehyde, 142 Butyric acid, 103 Cadalene, 127-128 Cadmium, 263,268, 271-273 sorption by lignite, 658 Cadmium oxide (see also cadoxen), 120 Cadoxen, 120 CalamuspoUenites, 54 Calamuspo Uenites-Are cipi tes, 53 Calcite (see also calcium carbonate) in ash deposition, 553-554 formation in liquefaction, 297 in lignites, 231,239-240, 245, 266, 268270, 292 manganoan, 246 reactions of, 296, 299, 301-303,305 Calcium in ash deposition, 553-555 behavior during combustion, 518-519 effect on pyrolysis, 183-184, 207 exchangeable, 223 as ignition promoter, 509 in lignite, 231,268, 271-274, 277-279 in macerals, 225 removal by ion exchange, 485-486 removal during drying, 465 in retrogressive reactions, 653 Calcium acetate, 296-297, 299, 302-303, 312, 314, 555, 570 Calcium carbide, 383 Calcium carbonate (see also calcite) effect on pyrolysis, 184 formation in liquefaction (see also oolites) 297-298, 642 suppression of ash fouling, 578-579 Calcium chloride, 483-484, 486 Calcium hydroxide, 495-496 Calcium magnesium sulfate, 571 Calcium montmorillonite, 250 Calcium nitrate, 233, 544 Calcium oxide, 298-299, 302-303,486, 518 effect on ash fusion, 317 gasification catalysis, 606 Calcium silicate, 554 Calcium sulfate (see also anhydrite, bassanite, gypsum) 299, 486, 515, 519-520, 536-537, 554, 560, 563-564, 584, 590 Calcium sulfide, 299, 306 Calcium sulfite, 299 Calvert Bluff lignite aluminum in, 230 barite in, 254 calcite in, 245

683 Calvert Bluff lignite (continued) chalcopyrite in, 257 cobalt in, 256 chromium in, 255 deposit, 9-10 gypsum in, 246 illite in, 249 iron in, 232 kaolinite in, 248 magnesium in, 233 manganese in, 256 mica in, 249 molybdenum in, 258 monazite in, 246 potassium in, 234 rubidium in, 252 scandium in, 254 siderite in, 246 silicon in, 234 strontium in, 253 titanium in, 238 rutile in, 244 vanadium in, 255 yttrium in, 257 zircon in, 245 Can lignite, 659 Canakkale-r lignite, 121 Cannel coal, 64, 105 Canneloid lignites, 82 Cannizzaro reaction, 140, 144 Carbominerite, 53, 60 Carbon County lignite, 175 Carbon dioxide from lignite, 155-156, 166, 181-183, 188, 195, 201-205, 458, 467, 470, 509, 620 reaction with lignite, 603 swelling effect, 362 Carbon dioxide index, 437 Carbon disulfide, 72 Carbon monoxide from lignite, 155, 166, 181-183, 195, 202, 445-446, 458, 509 reactions with, 139-149 Carbon tetrachloride, 239, 244-247, 249-250 Carbonyl groups in lignites, 116 loss in coalification, 68 Carboxyl groups association with cations, 219 determination, 110, 112 in lignites, 109-112 in lithotypes, 85 loss in coalification, 68 loss in pyrolysis, 180-181,188, 370 retention of moisture, 385 Carex, 53-54 Carnegieite, 296, 302-303, 311

Carya, 60, 62 Catechol, 68, 164, 182, 184-185, 190, 200 Catechols, 67, 184 Cayhiran lignite, 652 Celestite, 247, 253 Cellulose, 64-66, 84, 119-121 reactions of, 148 Center lignite ash deposition, laboratory, 560, 565 ash sintering, 558 chemical fractionation, 228 deposition, 60 float-sink separation, 477 gymnosperms in, 52 mine, 23,428 petrography, 91 pteridophytes in, 51 pyrolysis, 191, 197, 200 quality, 27 sodium vaporization, 516 surface area, 363 uranium in, 251 Central Otago lignite, 97 Cerium, 258, 268, 271-273 Cesium, 253,268, 271-273 Cesium fluorosulfonate, 94, 106 Chalcopyrite, 243,257 Charcoal (from lignite), 25, 27, 658 Cheirolepidiaceae, 52 Chemard Lake lignite deposit, 17 deposition, 56, 59, 62 palynology, 53-54, 60 quality, 18 Chemical fractionation, 220 Cheremkhovo brown coal, 660 Chlorine in lignites, 265, 268, 272-273 reaction with lignites, 487 Chlorite, 239, 241 Chlorobenzene, 125 Chloroform extraction with, 126, 655 heat of immersion in, 364, 367 Choctaw lignite ash sintering, 558 ash slag viscosity, 324 chemical fractionation, 227 Chromite, 255 Chromium, 255, 268, 271-274, 279 sorption by lignite Chrysene, 100 Cinnamaldehyde, 192 Clarain, 59 Clarite, 53, 59-60 Clarodurite, 53 Clausthalite, 243,264-265 Clinker, 426

684 CO-Steam process, 648-651 CO2 Acceptor process, 167, 624-628 Coal Creek power station, 537-538 Coastal Bermudagrass, 422 Cobalt, 256, 268, 271-273 Cobalt molybdenum catalysts, 619, 654 Coffinite, 261-262 Collinite, 89 Columbus lignite, 396 Coniferaldehyde, 192 Conveyor belts, 421,425, 432-433 Copper effect on pyrolysis, 188 in lignites, 256-257, 268, 271-274, 279 Copper(II) chloride, 488 Copper(II) oxide, 65, 94, 100, 118-119 Copper sulfate, 188 Coquimbite, 241 Coronach lignite pyrolysis, 182 reaction with carbon dioxide, 603 solvent extraction, 129 Corpohuminite, 86, 88, 90, 185 Corundum, 302, 312 Coumarins, 69 Coyote power station, 537 Crandallite, 246 Crawler tractors, 414 Creosote, 618, 620 Creosote oil, 652-653 m-Cresol, 160, 196 o-Cresol, 192 from lignite, 160, 196 p-Cresol, 160, 196 Cresols, 67, 94, 192-193 from gasification, 619-620, 637 from lignite, 184-185, 191, 197, 200 Cresylic acid, 476-477 (see also cresols) Crimson clover, 423 Cristobalite, 248, 302, 304 Critical oxidation temperature, 437-438, 440 Cupulifero idaepo llinites, 53 Custer, George A., 412 Cutinite, 40, 58, 86-87, 90-91 Cyclohexane, 364, 367, 398 Cyclopentane, 367 p-Cymene, 70 Cysteine, 71 Cystine, 71 Dakalite, 664-665 Dakota lignite, 203 Dakota Gasification plant, 614, 616-618 Dakota Star lignite drying, 472 gasification, 304 moisture, 391

Dakota Star lignite (continued) quality, 27 storage, 444 sulfur forms, 237, 304 Darco lignite activated carbon from, 4, 656 carboxyl groups, 111 density, 358 deposit, 4, 9-11 deposition, 59 devolatilization, 510 ilmenite in, 244 iron in, 232 mining, 4, 9, 410, 424 pores in, 376 pyrolysis, 174-175, 177, 181-183, 185, 187, 196, 199-200 reaction with hydrogen, 150 repeating units between crosslinks, 124 Decarboxylation in coalification, 70 in drying, 461,463,465, 467 Decker lignite ash deposition, laboratory, 571 ash slag viscosity, 325 oxidation reactions, 92, 94 solvent extraction, 127-128 Decker subbituminous coal, 516 Dehydration, 70-71 Dehydroabietic acid, 129 Dehydroxylation, 70 Desinite, 85-90 Desmocollinite, 89 Desulfovibrio, 71 Detrinite, 89 Deuterium oxide, 147, 189 Deuteroformate ion, 147 Diammonium phosphate, 619 Dibenzodioxane, 117 Dibenzofuran, 117 Dichlorodicyanoquinone, 102 Dickinson lignite, 657 Dickite, 56 1,2-Dihydronaphthalene, 143 9,10-Dihydrophenanthrene, 143, 146 as solvent, 170, 653 Dihydroxyaluminum aminoacetate, 658 Dihydroxybenzaldehyde, 190 Dihydroxybenzenes, 436 3,4-Dihydroxybenzoic acid, 118-119 1,3-Dihydroxynaphthalene, 189 2,6-Dimethoxyphenol, 190 N,N-Dimethylaniline, 169 Dimethyl disulfide, 651 Dimethyl ether, 487 2,4-Dimethylphenol, 160 Dimethyl sulfate, 70, 115 Dimethyl sulfoxide, 238, 261,364

685 Dinitrochlorobenzene, 658 Dinitrophenol, 658 Dinitrotoluene, 658 Dinoflagellates (in lignite), 61 Diphenyl disulfide, 149 1,1-Diphenylethane, 124 Diphenyl ether, 143 Diphenylmethane, 103, 161,169-170 Diphenylphosphinic chloride, 393 Diphenyl sulfide, 149, 169 Dolet Hills lignite, 426 Dolomite, 239, 245, 268, 270, 579, 623, 625, 656 Dolomite ratio, 334 Dopplerinite, 89 Dow gasification process, 637 Draglines, 412-420, 422-423,425-429 Dump-bottom trailers, 421 Durain, 184 Durite, 53 Duroclarite, 53, 60 Dyes (from lignite), 15 Dysprosium, 259, 268 Easi-Miner, 421,425 EDTA (see ethylenediaminetetraacetic acid) Elbistan lignite liquefaction, 653 pyrolysis, 195, 200 Electrodes, from lignite, 660 Electrostatic precipitators, 543 Elgin-Butler lignite briquetting, 495 friability, 349 gasification, 621-622 Emissivity (of ash), 308 Engelhardia, 53-54, 60, 62 Engelhardia dilatus, 53

Enopodios, 54

Epidote, 250 Eschka mixture, 237 Ester groups, 118 Estevan lignite anatase in, 244 aragonite in, 245 ash fouling, 578 barite in, 247 calcite in, 245 carbonyl groups, 116 carboxyl groups, 110 combustion, 542 co-processing, 171 dolomite in, 245 ethers, 116 illite in, 249 liquefaction, 145, 158, 653 mica in, 249 NMR of, 96, 116

Estevan lignite (continued) oxidation reactions, 92 petrography, 86, 89 phenolic groups, 115 pyrolysis, 181-182 reaction with sulfur dioxide, 480 siderite in, 245 uranium in, 274 Ethane from gasification, 637 from lignite, 153-157, 194, 202-203, 458-459 Ether groups, 116-117 Ethylbenzene, 637 Ethylene from hydropyrolysis, 154 from pyrolysis, 108, 194, 202-203 Ethylenediamine, 364 Ethylenediaminetetraacetic acid, 72, 230, 277 4-Ethylguaiacol, 67, 118 2-Ethylphenol, 160 4-Ethylphenol, 160 Ettringite, 666 Eudalene, 128 Eugelinite, 61 Eugenol, 66-67 Europium, 250, 268, 271-273 Eu-ulminite, 84, 86, 88-89, 120 Exinite, 64, 86, 105, 129 reaction with hydrogen, 153 Exxon catalytic gasification process, 167 Exxon Donor Solvent process, 144, 297, 473, 639-643, 648 Fairfield lignite drying, 463,465 removal of cations, 465 Falkirk lignite angiosperms in, 51-52 calcium in, 181 deposition, 60 dewatering, 234 dielectric relaxation, 394 mine, 23,428 petrography, 91 pyrolysis, 181 quality, 27 Fayalite, 560 Feldspar, 267 Fescue, 423 Flammability index, 508 FLASHCHAIN, 206 Flavones, 69 Fleissner, Hans, 468 Fleissner process, 467-472 Flexicoking, 640-641 Fluoranthene, 526 Fluorene, 526

686 9-Fluorenone, 92 Fluorine in lignite, 265 reaction with lignite, 99 Fly ash, 424 FMC process (briquetting), 496 Formaldehyde, 526 Formate ion, 140, 143-144, 148 Formcoke, 659 Formic acid, 140, 143 Freedom lignite calcite in, 245 friability, 346, 351 mine, 24 Mohs hardness, 355 petrography, 80, 86, 91 pyrite m, 243 pyrolysis, 185 quartz m, 248 selenite in, 246 shrinkage, 452 Freestone lignite, 487 Front end loaders, 414-415, 419-422, 425-427 Fundamental jelly, 64 Fusain, 81-85, 87-88, 90 aromaticity, 97-98 carboxyl groups, 111 cellulose residues in, 120 friability, 346, 350 ignition delay, 508 inorganics in, 224 methoxyl groups, 115 pyrolysis, 184 quartz in, 248 reaction with hydrogen, 158 sodium in, 236 Fusinite, 62, 85-88, 90, 129, 224-225 Fusinitization, 62 Galactose, 120 Galena, 243,264 Gallium, 263,274, 279 Garrison lignite moisture sorption, 396 stockpiling, 433 Gascoyne lignite aluminum in, 230 aromaticity, 98 aromaticity of lithotypes, 98 ash deposition, laboratory, 561,565, 574 ash formation, 300, 519 ash sintering, 558 ash slag viscosity, 330-331 barite in, 247 bio-deashing, 480 carboxyl groups, 111 chemical fractionation, 226, 228

Gascoyne lignite (continued) combustion, 536 dewatering, 234 dielectric relaxation, 392-393, 395 drying, 454-456 gasification, 608, 632 heat of immersion, 364, 367 microprobe analysis, 230 mine, 23 minerals in ash, 293 nacrite in, 249 petrography, 83, 91 polonium in, 265 potassium in, 234 pyrite m, 242 pyrolysis, 179, 200 quality, 26 reaction with EDTA, 230 removal of cations, 220-221 SAXS of, 365, 374 shrinkage, 403,452 sodium in, 236 sodium vaporization, 516 solvent swelling, 125 storage, A.A.A. surface area, 361,363-364 thermogravimetric analysis, 388 titanium in, 238 transportation, 430-431 uranium in, 262 Gasoline, 637 Gehlenite, 296-297, 299, 301-303, 311, 313314, 334, 518, 555, 569-571,573, 577-578, 590 Gelification, 61, 65 Gelinite, 61, 85-86, 88, 90, 225 Germanium, 263-264, 271,273-274 Gibbons Creek lignite, 487 Gibbons Creek power station, 542 Gibbsite, 230, 244 Gilsonite, 494 Glance coal, 38 Glenharold lignite aromaticity, 98 aromaticity of lithotypes, 98 barium in, 254 beryllium in, 253 calcite in, 245 calcium in, 231 calcium adsorption, 219 carbonization, 179 chemical fractionation, 230 chromium in, 255 combustion, 538 copper in, 256 deposition, 60 gypsum in, 246 iron in, 232

687 Glenharold lignite (continued) magnesium in, 233 manganese in, 255-256 mine, 23,418, 428 nacrite in, 249 nickel in, 256 potassium in, 234 pyrite in, 242-243 pyrolysis, 187, 198-199 quality, 27 quartz in, 248 scandium in, 254 selenite in, 246 shrinkage, 452 silicon in, 234 sodium adsorption, 219 sodium in, 236 strontium in, 253 titanium in, 237 vanadium in, 255 ytterbium in, 259 yttrium in, 257 zirconium in, 257 Glover-West process, 469 Glucose, 120 Glutaric acid, 101-102, 104, 128 Glutathione, 71 Goethite, 232, 243 Gold, 258 sorption by lignite, 658 Graphite, 143,233 Greigite, 243 Guaiacol, 66, 118, 182, 190, 192 Gun fights, 35 Gypsum decomposition, 564 in lignites, 73,223, 231,239, 242, 246-247, 254, 268-270, 292, 298, 305, 663 in scrubbers, 543 in soil, 424 Hafnium, 257, 268 Hagel lignite barite in, 247 carboxyl groups, 111 deposition, 60, 89 desulfurization, 488 float-sink separation, 477-478 methine carbons in, 124 hydropyrolysis, 152 inorgamcs in, 224 liquefaction, 159-160, 652, 654 MW between crosslinks, 125 oxidation reactions, 105 petrography, 79 pore volume, 365 pyrolysis, 177, 179

Hagel lignite (continued) reaction with hydrogen, 153-154 soot formation, 510 surface area, 362 swelling, 362 tetrapyrroles in, 121 Halloysite, 230, 248-249 Hardgrove grindability, 351-354, 490, 529530, 542, 583 Harmon lignite, 150 Harrington stoker, 522 Hat Creek lignite arsenic in, 265 boron in, 263 bromine in, 265 chlorine in, 265 chromium in, 255 cobalt in, 256 dolomite in, 245 gypsum in, 246 hafnium in, 257 illite in, 249 kaolinite in, 248 quartz in, 248 rubidium in, 252 scandium in, 254 siderite in, 246 strontium in, 253 thorium in, 259 uranium in, 262 vanadium in, 255 zinc in, 263 Hauyne, 293,296, 299-300, 303, 314 Hematite, 239, 244, 293, 301-304, 563,570571,660 Hemimellitic acid, 92 Heptanedioic acid, 103 Hercynite, 301-302 Herrington, John, 618 Hexamethylbenzene, 95 Hexane, 128, 655 Hexanedioic acid, 103 Highvale lignite, 606 Holmium, 259 Hoot Lake power station, 535-536, 544 Hornblende, 239 Hot-water drying, 457-463,465, 480, 497, 666-667 Howard, Jerome, 687 Hoy equation (viscosity), 334 Humic acids from lignites, 113, 116-117 sorption of cations, 73 Humification, 62 Huminite, 40, 45, 79, 82, 88 birefringence of, 65 reaction with hydrogen, 158 Humodetrinite, 88, 90, 224-225

688 Husky lignite charcoal from, 25 mine, 25 Hydrochloric acid, 220, 223,226, 232, 238, 246, 254-255, 277, 291-292, 304, 385, 485, 653 Hydrodarco, 657 Hydrofluoric acid, 291-292 Hydrogen from lignite, 166, 181-183, 194-195, 202-203, 446, 458, 509, 627 reactions with, 145-146, 149-155, 174 Hydrogen cyanide, 510-511 Hydrogen sulfide as catalyst, 145, 148, 169-171,488, 654 formation in coalification, 72 from lignite, 193,458-459 m-Hydroxybenzoic acid, 65 p-Hydroxybenzoic acid, 65, 118-119 Hydroxyl groups loss in coalification, 68 loss in pyrolysis, 189 3-Hydroxyl-6-methylpyridine, 233 Ignition delay, 508 Ignition index, 508 Illinois No. 6 coal, 167 Illite, 57, 230-231,234, 239-241,249, 266 Ilmenite, 238, 244, 660 Indenes, 637 Indian Head lignite aromaticity, 98 aromaticity of lithotypes, 98 ash deposition, laboratory, 565, 574 ash sintering, 558 ash slag viscosity, 322, 331 carbonization, 204 chemical fractionation, 228 dielectric relaxation, 401,458 flotation, 478 friability, 349, 351 (see also mechanical friability, thermal friability) gasification, 307, 604, 629, 631-632 heat of immersion, 364, 367 hot-water drying, 458-461,497 ion-exchange beneficiation, 485 liquefaction, 141,145, 159, 170, 641, 653-655 low-temperature ashing, 291 mechanical friability, 345-346 methoxyl groups, 114-115 mining, 429 Mohs hardness, 355 NMR of, 460 oil agglomeration, 479 petrography, 91 pulverization, 492

Indian Head lignite (continued) pyrolysis, 174, 176, 196-197, 199, 201, 205, 207, 304 quality, 27 reaction with CO, 141 reaction with hydrogen, 150, 157 reaction with steam, 167-169 SAXS of, 370, 461 shear strength, 356 shrinkage (on drying), 343-345 slurry combustion, 668-669 slurry properties, 497, 499 solvent extraction, 126 solvent swelling, 125 surface area, 460-461 TEM of, 376 thermal analysis, 172, 175-176, 179 thermal friability, 348 waxes from, 460 Inertinite, 58, 60, 79, 88, 346 Inertodetrinite, 85, 87-88, 90, 225 Ireland coal depolymerization, 129 liquefaction, 641 Iron effect on pyrolysis, 184 in lignites, 231-232, 268, 271-273,277-279 metallic, 301,326, 520 Iron(II) acetate, 232 Iron(Ill) acetate, 654 Iron carbonyl, 654 Iron(II) chloride, 232 Iron(Ill) chloride, 183-184 Iron(II) oxide, 324, 326-327 Iron(III) oxide (see also hematite), 324, 331, 334, 582 Iron(II) sulfate, 298 Iron(II) sulfide, 71,298, 520, 583 Isoeugenol, 66-67, 190 Isophthalic acid, 92 Isopropanol, 655 Isoquinoline, 146, 650 Itmann coal, 129 Jarosite, 232, 239, 243, 247 Jet fuel, 620 Jewett lignite, 426 JPL chlorinolysis process, 487 Jug landaceae , 53-54 Kansk-Atchinsk lignite, 654 Kaolin, 41,516 Kaolinite in lignites, 56-57, 230-231,234, 239241,248-249, 266, 268, 292 reactions of, 296-297, 299-303, 311312, 314-315, 555, 569-570 Karilova lignite, 653

689 Kashmir lignite, 363 Kemmerer coal, 326 Kincaid lignite carbonyl groups, 116 critical oxidation temperature, 437 drying, 389, 467, 470 methoxyl groups, 114 phenolic groups, 115 Koppelman process, 467 Koppers-Totzek gasification, 624 Lanthanum, 258, 268, 271-274, 279 Larsen lignite, 25 Lawsonite, 250 Lazurite, 301 Lead, 264, 274 sorption by lignite, 73,658 Ledbetter lignite, 97, 99 Lei gh lignite charcoal from, 27 combustion, 523-524 metamorphosed wood in, 64 quality, 27 Leland Olds power station, 538-539, 545, 551, 577-580 Leonard, Arthur G., 663 Leonardite, 23, 26, 416, 663-664 sorption of cations on, 72 Levigelinite, 87, 90 Lignin, 64-70, 94, 118-119, 129, 160, 164, 202 Lignite factor, 334 Liliacidites, 53-54 Limestone, 538, 543-544, 578, 585-587, 589591,623,625 d-Limonene, 70 Liptinite, 40, 88, 166 Liptodetrinite, 87-88, 91 Liquidambar, 54 LiquiMag, 579 Lithium, 251 Lithium aluminum hydride, 112 Loy Yang brown coal, 160 Lurgi gasification, 616, 618, 629 Lurgi methanators, 619 Lurgi-Spulgas process, 496-497 Lutetium, 259, 268 Macrinite, 90 Magnesioferrite, 560 Magnesite, 245 Magnesium effect on pyrolysis, 183-184 in lignites, 233,268, 271-274, 276277, 279 removal by ion-exchange, 486 removal during drying, 465 Magnesium chloride, 183

Magnesium oxide, 518, 578-579 Magnesium sulfate, 563 Magnetite, 244, 293,301-304, 569-570, 583 Magothy lignite, 246 Maissade lignite, 495 Malonic acid, 103-104 Manganese, 255-256, 268, 271-274, 279 Manganese nodules, 660 Marcasite, 232, 239, 243,264 Marl, 55 Martin Lake lignite ash deposition, laboratory, 560, 565 ash slag composition, 333 co-processing, 654 deposition, 59, 61 desulfurization, 487 dielectric relaxation, 394 float-sink separation, 475-476 inorganics in, 224 liquefaction, 147, 159, 651 mining, 421 oil agglomeration. 476 petrography, 89-91 pyrrhotite in, 243 reaction with steam, 167, 169 reaction with sulfur dioxide, 476 surface area, 363, 371 Mataura lignite, 150 Matrix parent, 570 Meek, F.B., 19 Megalopolis lignite, 188 Melanterite, 232, 243 Melilites, 293-294, 299-301, 314, 516, 569571,574-575, 590, 669 Mellitic acid, 93-94 Mellophanic acid, 93 Mequinenza lignite, 652 Mercury adsorption, 658 Metaautunite, 261 Metakaolinite, 302, 315 Metatorbernite, 261-262 Metauranocirite, 262 Metazeunerite, 261 Methane from biogasification, 639 from gasification, 604-605, 627, 637 from lignite, 153-157, 166, 183, 194, 201203,206, 446, 458-459, 509 from liquefaction, 650 Methanethiol, 72 Methanol adsorption on lignite, 363 m heat of immersion calorimetry, 363-364 from lignite, 184, 192, 196, 200 reaction with lignite, 653 swelling, 362, 653 Methoxyl groups, 65 decomposition, 192

690 Methoxyl groups (continued) demethylation, 67-69 determination, 114 in lignites, 114 loss in coalification, 66, 68 Methoxymethyldibenzofurancarboxylic acid, 128 2-Methyl- 1,4-butanedioic acid, 103 4-Methylguaiacol, 66, 118 6-Methylguaiacol, 118 Methyl iodide, 103, 204 Methyl isobutyl ketone, 655 2-Methyl- 1,5-pentanedioic acid, 103 4-Methylsterene, 105 Mica, 231,233-234, 248-249, 267 Micrinite, 90, 129 Microreticulatosporites, 53, 60 Mikro-pul hammer mill, 492 Millerite, 243, 256 Milton R. Young power station, 532, 534 Mohs scale, 354 Molasses, 495 Molybdenum, 257-258, 268 Molybdenum disulfide, 488 Momipites-En gelhardia, 53 Monazite, 246, 259 Montan wax, 15, 39, 655-656, 665 Monterey coal, 641,643 Monticellite, 569, 578 Monticello lignite aluminum in, 230 barium in, 254 chemical fractionation, 229 flammability index, 508 illite in, 249 mining, 421 pore volume, 186 potassium in, 234 pyrolysis, 175, 186, 198 quartz in, 247 silicon in, 234 surface area, 186 titanium in, 238 Monticello power station, 539-540, 545 Montmorillonite, 70, 230, 239-240, 250 Morton Mains lignite, 150 Morwell brown coal, 115 Moose River Basin lignite arsenic in, 265 cadmium in, 263 calcite in, 245 chromium in, 255 cobalt in, 256 copper in, 256 gibbsite in, 244 gold in, 258 gypsum in, 246 illite in, 249

Moose River Basin lignite (continued) kaolinite in, 249 lead in, 264 mica in, 249 molybdenum in, 258 nickel in, 256 platinum group elements in, 258 pyrite in, 242-243 quartz in, 247-248 rubidium in, 252 siderite in, 245 strontium in, 253-254 thorium in, 259 tungsten in, 258 uranium m, 262 zinc in, 263 zirconium in, 257 Mullite, 299, 302-303, 312, 315 Muscovite, 239 Myriceae-Betulaceae, 52 Nacrite, 249 Nahcolite as catalyst, 168 for SOx control, 585 Nakayama lignite, 100 Naphtha, 479, 618-619, 641 Naphthalene, 92 from gasification, 637 from lignite, 157 1-Naphthol, 141,146 Navajo coal, 558 Nazarovo brown coal ash fouling, 554 dielectric relaxation, 393 Neodymium, 258 Nepheline, 293,296-297, 300, 302-303, 311, 313-314, 334, 555 New Source Performance Standard, 475, 532, 586, 589 Newvale lignite, 362 Nickel, 268, 271-274, 279 catalyst, 619 Nickel nitrate, 608 Nickel sulfate, 488 Nickel sulfide, 298, 488 Nimz's procedure, 120 Niobrara shale, 578 Nipadites, 53 Nitrates, 233 Nitric acid, 94, 106, 232, 485 Nitrobenzene, 368 Nitrogen in lignites, 233 4-Nitroperbenzoic acid, 116-117 Nitrous oxide formation in combustion, 589 reaction with lignites, 487

691

NOx control, 538, 542, 544, 557, 585-586, 665 formation, 510, 531,583,588, 667

Nomapollis, 54 Nonanedioic acid, 103 Normapolle, 53 Nosean, 293,299-301, 314 Novacekite, 261 Novodmitrovskoe lignite, 108 Nuxpollenites, 54 Nymphaea, 54 Nyssa, 53-54, 59 Nyssapollenites, 54 Octanedioic acid, 103 Oil agglomeration, 476, 479 Oil shale, 106 Oligoclase, 239 Onakawana lignite drying, 470 pyrolysis, 203 reaction with air, 164 reaction with hydrogen, 158 reaction with steam, 168 Oolites, 297-298 Orhaneli lignite, 586 Orthoclase, 239 Osmunda, 52 Oxalic acid, 92 Oxaloglutaric acid, 112 Oxalomalonic acid, 112 Oxalosuccinic acid, 112 Oxygen absorption, 436, 438, 444, 606 reactions with, 164 Palana lignite moisture, 384 Vickers hardness, 355 Papakube process, 495-496 Paraffin, 99 Parry process (briquetting), 496 Patapsco lignite, 164 Patuxent lignite, 264 Peerless lignite grindability, 352 pore structure, 360 porosity, 366 Pentane, 129 Pentanedioic acid, 103 Perlite, 579 Peroxyacetic acid, 94 Peroxytrifluoroacetic acid, 94, 104, 114, 159 Phenanthrene, 97, 143 as solvent, 141,146 Phenol, 190, 192 depolymerization with, 109, 117, 129,

Phenol (continued) depolymerization (cont'd), 489 from gasification, 637 from lignites, 67, 94, 184-185, 191, 196197, 200, 459, 620 Phenolic groups in lignites, 115 retention of moisture, 385, 466 Phenols, 67, 192 from lignites, 166, 178, 436, 618-619 Phenolsolvan process, 619 1-Phenoxynaphthalene, 143 9-Phenoxyphenanthrene, 143 Phenylalanine, 68 Phenyl glycol, 148 Phillips lignite, 350 Phlobaphenite, 185 Phosam process, 619 Phosphorus, 234, 271,273,279 Photochemical oxidation, 94 Phthalic acid, 92-94, 102-104, 106 Pittsburgh coal, 124 Plagioclase, 239, 241,334, 569, 574, 669 Platinum group elements, 258 Podocarpaceae, 52

Pollienites laesius, 54 Polonium, 265 Poly(coniferyl alcohol), 119 Poly(ethylene terephthalate), 112 Poly(p-hydroxystyrene), 70 Polypodium, 54 Poly(vinyltoluene), 99 Poplar River lignite calcite in, 245 dolomite in, 245 greigite in, 243 gypsum in, 247 mica in, 249 quality, 33 siderite in, 245 Porphyrins, 121 Portlandite, 666 Potassium effect on pyrolysis, 183-184 in lignites, 234, 268, 271,273-274, 277, 279 removal during drying, 465 Potassium acetate, 037 Potassium carbonate, 146-147, 167-169, 182, 184, 037 Potassium chloride, 037 Potassium feldspar, 234 Potassium hydrogen sulfate, 571 Potassium hydroxide, 144, 485, 496 effect on pyrolysis, 191 Potassium nitrate, 607 Potassium permanganate, 92-94, 106, 115117, 164

692 Potassium sulfate, 519, 561,571,607 Poultry grit, 578 Power shovels, 413-414, 416-417, 420-422, 426-428 Praseodymium, 258 Predicted Ash Collection Efficiency Index, 559 Previtrain, 30 Project Lignite, 297, 643-648 Propane from gasification, 637 from lignite, 194, 202-203,458 1,2,3-Propanetricarboxylic acid, 103-104 Propionic acid, 103 Propionitrile, 196, 200 Propylene, 108, 194, 203 4-Propylguaiacol, 118 Pseudocoke, 659 Pseudomonas putida, 488 Pseudovitrinite, 153 Pseudowollastonite, 590 Ptolemais lignite, 657 Pust lignite aromaticity, 97 bio-deashing, 480 quality, 33 reaction in air, 162 Pyrene, 100, 526 Pyridine extraction of water with, 384, 393 heat of immersion in, 364 swelling with, 124-125, 368, 403-404, 653 Pyridinium iodide, 94, 105 Pyrite as catalyst, 145, 639, 654 decomposition, 488, 583 determination, 291 fragmentation, 520 framboidal, 58, 71,242-243 in lignites, 57, 60, 71, 73, 232, 239240, 242-243,246, 255, 264, 266, 268-269, 272, 274-275, 292, 529 molybdenum in, 257-258 nickel in, 256 nodules, 61,243 oxidation of, 293,303 reactions of, 296, 299, 304-305 zirconium in, 257 Pyrolusite, 256 Pyromellitic acid, 93 Pyroxene, 293,300, 334, 569, 574, 669 Pyrrhotite as catalyst, 653-654 in lignites, 243 Quartz agglomeration, 590

Quartz (continued) fragmentation, 520 in lignites, 56, 234, 238-241,247, 248, 266-267, 274, 292-293, 530, 563, 569, 663 reactions of, 296, 299, 301,304-305, 314, 316, 570 rutilated, 244 Quercus, 54 Quinoline, 637 Rapponmatsu lignite, 164, 166 Rasa lignite, 436 Ravenscrag lignite antimony in, 265 barium in, 254 cesium in, 251 cobalt in, 256 deposit, 35 deposition, 57-58 lithium in, 251 manganese in, 255 nickel in, 256 pyrite in, 242 rubidium in, 251-252 scandium in, 254 strontium in, 253 vanadium in, 255 Raymond bowl mills, 491 Raymond dryer, 457 Rectisol process, 479, 619 Red dog, 26 Reichert microhardness, 355 Relative refractory index, 573-574 Resinite, 58, 87, 91,461 reaction with hydrogen, 153 Resins (in lignite), 30, 38, 43, 45, 52, 64 Resorcinol, 189 Retene, 69, 97 Rhodochrosite, 246 Ribose, 120 Richland County lignite pyrolysis, 177-178, 181 reaction with hydrogen, 150 swelling, 362 Riley gasifier, 634 Riser Cracking process, 637 River King coal, 167 Rockdale lignite aromaticity, 97 ash fouling, 548 ash slag viscosity, 320-321,325 hydrogasification, 659 hydropyrolysis, 153, 155-157 NMR of, 106 oxidation reactions, 93, 103 solvent extraction, 128 Rosebud coal, 97

693 Rosenbach lignite, 218 Roto-Louvre drying, 354, 453-454, 456 Rotten coal, 9, 10 Rubidium, 251-253, 268, 273,279 Ruthenium, 271-273 Ruthenium tetroxide, 93, 101-104, 112, 124 Rutile, 238-239, 244, 248, 277, 531 RWE-K/31n lignite, 196 Sabugalite, 262 Saleeite, 262 Samarium, 259, 268, 271-273 Samarium iodide, 95 San Miguel lignite ash deposition, laboratory, 560, 563, 565 ash formation, 519 deposit, 12 deposition, 61 desulfurization, 488 dielectric relaxation, 392, 394 grindability, 12 iron in, 232 mining, 421,425 petrography, 90-91 potassium in, 234 pyritic sulfur content, 12 pyrolysis, 173, 177 XANES of, 234 zeolites in, 519, 565 San Miguel power station, 540-541 Sand, glauconitic, 55 Sandow lignite carbonization, 179, 193, 195 deposition, 61 drying, 387 gasification, 635-637 mining, 4,421,426 moisture, 391 petrography, 87

Sapindaceae, 54 Sapotaceae, 54 Sapropelinite, 61 Saran, 366 SASOL plant, 616 Savage lignite aluminum in, 230 ash formation from, 294 ball milling, 367, 492 calcite in, 245 calcium in, 231 dolomite in, 245 grindability of lithotypes, 352 magnesium in, 233 microprobe analysis, 231,233,236237 mine, 33 nacrite in, 249

Savage lignite (continued) polonium in, 265 pyrite in, 242 pyrolysis, 186-188, 198-199 quality, 33 sodium in, 236 sulfur in, 237 uranium in, 262 Scandium, 254, 268, 271-274, 279 Schizoporis parvus, 62 Schizosporis texus, 62 Schoch's method, 391 Sclerotia, 40 Sclerotinite, 90 Scoria, 426 Scranton lignite, 241 Scrapers (in mining), 413,415, 417-418, 420, 422-423,425, 428-429 Selenite, 246 Selenium, 265, 268, 271,273-274 Self-ignition temperature, 436, 441 Semicoke, 659 Semifusinite, 79, 85, 87-90, 129 Sernapollenites, 53 Seyitomer lignite, 585 Shell 324 catalyst, 652 Shell gasification process, 635-637 Sheridan lignite oxidation reactions, 64, 92-94, 105-106, 117, 119, 121 solvent extraction, 128 Shoe polish, 656 Siderite, 61,232, 239, 245, 256, 264, 266, 268 Silica, 296, 302, 329-330, 519, 530, 556, 560, 563, 571 Silicon, 234, 268, 271-272, 274, 277-279 Silicon monoxide, 296 Silver, 258, 268, 272-273 Silver oxide, 70, 94, 105 Simonellite, 128 Slack, 416, 421 Slackening (see slacking) Slacking, 26, 27, 30, 41,432-433,452, 468469, 474

SOx control, 543-544, 585-588, 665 formation, 584-585 Sodium in ash deposition, 530, 546-552, 555 in fluid bed agglomeration, 589-590 effect on pyrolysis, 183-184 in lignites, 234-236, 268-276, 278-279 in lithotypes, 236 metal, vapor, 516, 562 promotion of liquefaction, 655 removal by ion exchange, 481-484, 486487

694 Sodium (continued) removal during drying, 461-462, 465 vaporization during combustion, 515516, 591 Sodium acetate, 296-297, 302-303, 516, 555, 570 Sodium autunite, 262 Sodium bicarbonate, 302 Sodium carbonate, 233,516 as catalyst, 168-169 reactions, 302 retention of sulfur, 237 Sodium chloride, 516 Sodium dichromate, 64, 92, 94, 105, 117, 121 Sodium formate, 140, 143 Sodium hydroxide, 117, 128, 144, 166, 485, 488, 496, 515-516, 544, 562, 638 Sodium hypochlorite, 105 Sodium melilite, 311, 314, 518, 555, 568-570, 573-574, 577-578, 581 Sodium methoxide, 118 Sodium nitrate, 233,292, 495 Sodium oxide, 305 Sodium periodate, 115 Sodium silicates, 304-305 Sodium smectite, 270 Sodium sulfate in ash deposition, 552-553,560-562, 570, 581 in combustion products, 515, 519, 536, 563 in lignite, 247 reactions, 296, 302-303,305, 312, 314,516 in scrubbers, 543 Sodium sulfite, 312, 543 Sodium toxicity, 20 SOLGASMIX, 332-333 Solvent Refined Coal process SRC I, 144, 297, 473, 648 SRC II, 639, 648 Soma lignite, 659 South Hallsville lignite hot-water drying, 497 slurry combustion, 667-668 slurry properties, 497 Southland lignite, 97 Soya lignite hydropyrolysis, 156 oxidation reactions, 65, 106 Specific heat, of ash, 307 Sphagnum, 53, 62 Sphalerite, 243,263 cadmium in, 263 Spinel, 293 Spontaneous combustion (See autogenous

combustion) Sporinite, 58, 66, 86-88, 90-91

Sporinite (continued) fluorescence, 88 oxidation reactions, 93, 106, 116 solvent extraction, 128 synthetic, 66 Sporopollenin, 66, 71 Stansfield-Gilbert method, 388 Sterane, 105 Sterene, 105 Stereisporites, 53 Steriosporites antiquasporites, 52 Stretford process, 619 Strontium in lignites, 253-254, 268, 271,273-274 sorption by lignite, 73 Strontium sulfate, 254 Styrene, 192-193 Suberinite, 89 Succinic acid, 101-102, 104, 128, 159 Sulfite liquor, 496, 659 Sulfur by-product, 620 fixation during ashing, 305-306 as liquefaction promoter, 170 in lignites, 237, 268, 271-274

(elemental), 238 Sulfur balls, 242 Sulfur dioxide (see also SOx) in coalification, 72 reactions with lignites, 171,298, 476, 480, 643 Sulfuric acid, 298, 482-486 Sulfur tetrafluoride, 112 Sulfur trioxide, 292, 515, 582 (see also SOx) Sunrise lignite palynology, 53 petrography, 87 Surface Mining Control and Reclamation Act, 422, 424 Sweepers, 428 Szomolnokite, 239 Taconite, 659 Talc, 239, 250 Tantalum, 268 Tavsanli lignite, 586 Taxodiaceae, 52

Taxodiaceae pollinites, 53 Taxodiaceae-Cupressaceae, 51-52, 60 Taxodium, 53-54, 56, 62 Taylerton lignite, 119, 172 Telocollinite density, 357-3 58 pore volume, 365 Terbium, 259 Terephthalic acid, 92 Tetrabutylammonium hydroxide, 204, 653 n-Tetracontane, 97

695 Tetrahydrofuran extraction with, 128-129 swelling with, 125, 403,653 Tetralin, 161 heat of immersion calorimetry, 364 as solvent, 158, 643, 650-651,665 Texaco gasification process, 637 Texto-ulminite, 89 Thenardite, 247 Thermo lignite, 425 Thioacetic acid, 120 Thioanisole, 149 Thiobacillus ferrooxidans, 480 Thionine, 71 Thiophenol, 149 Thomsonipollis, 53, 56 Thomsonipollis magnificus, 54 Thorium in lignite, 259, 268, 271 sorption by lignite, 73 Thulium, 259 Tin, 264 Titanium, 238, 268, 271-273,277, 279, 531 Titus lignite FFIR of, 99, 107 glass transition temperature, 124 MW between crosslinks, 125 repeating units between crosslinks, 124 Toluene, 192-193 extraction with, 129, 655 from lignite, (see also BTX) 620, 637 swelling with, 367 p-Toluenesulfonic acid, 117, 129 Tourmaline, 263 Tree stumps, coalified, 27, 34, 43, 55, 82, 115, 120, 243 TriatriopoUenites arboratus, 54 Trichloroethane, 487 Tricolpopollenites hians, 54 Tricolporopollenites, 53 Tridymite, 248, 304 Triethylborane, 116 Trimellitic acid, 92, 94 Trimesic acid, 93 Trimethylamine, 487 Triplanospo ri te-De ltiospo rite, 53 Triporopollenites bituitus, 54, 60 Troilite, 303-304 Trona as catalyst, 168, 182, 184 for SOx control, 585 Tunqbilek lignite, 659 Tungsten, 258, 268 Tungsten sulfide, 489 Tyrosine, 68 Ulminite, 86-90, 224 Unit trains, 429

Uranitite, 261 Uranium from lignite, 24, 35. 661-662 in lignite, 38, 251,259-262, 268, 271272, 274, 279 sorption by lignite, 73 Uranyl sulfate, 73 Urbain equation, modified version, 327-328, 574 Vanadium, 254-255, 268, 271-274 Vandyke brown, 656, 665 Vanillic acid, 65 Vanillin, 67, 190 Vaterite, 297 Velva lignite, activated carbon from, 657 aromaticity, 98 aromaticity of lithotypes, 98 ash deposition, laboratory, 565, 571, 574 ash fouling, 548, 554 ash sintering, 558 ash slag viscosity, 335 barite in, 247 char reactivity, 605, 607 depolymerization, 129 drying, 463 gasification, 628, 632 hematite in, 244 moisture sorption, 396, 399 mine, 25 nacrite in, 249 petrography, 91 pyrolysis, 182-184, 188 quality, 27 reaction with hydrogen, 151 reaction with steam, 167-169 surface area, 363 Vermiculite, 579 Vickers microhardness, 355 4-Vinylguaiacol, 66 Vitrain, 80-85, 87-88 aromaticity, 97-98 carboxyl groups, 111 cellulose residues in, 120 friability, 346, 350 ignition delay, 508 lnorganics in, 223-224 methoxyl groups, 115 pyrite in, 242-243 pyrolysis, 184 quartz In, 248 reaction in air, 161 sodium in, 236 XPS of, 161 Vitrinite, 58, 60, 62, 89, 129 reaction with hydrogen, 153

696 Vitrinite (continued) reflectance, 88-89 Vitrite, 53, 59 Volcanic ash, 244, 249, 267 Volcanic glass, 239 Watt-Fereday equation (viscosity), 331 modified version, 334 Wax (see also montan wax), 40, 82, 113, 128, 460 Weddellite, 244, 250 Wellman-Galusha gasifier, 628-629 Wharen, G.B., 521 Whewellite, 250 Willow Branch lignite oil agglomeration, 479 petrography, 86 WinNer gasification, 622-623 Witherite, 246, 254 Wollastonite, 311,571 Wood's metal, 376 Worsham furnace, 522 Wyodak coal humic acids from, 113 liquefaction, 160 moisture, 391 Xanthone, 117 Xenotime, 246, 257 Xundian lignite, 514 Xylem, 65-66 p-Xylene, 479-480 Xylene method (moisture), 390-391 Xylenes, 192-193 from gasification (see also BTX), 620, 637 Xylenols, 94, 192, 620 Xylinite, 89 Xylinoids, 88 Xylite, 52, 60, 67-68, 494 Xyloid lignites, 8 Xylose, 120

YeguapoUis, 54 Yellow cake, 662 Ytterbium, 259, 268, 271-274, 279 Yttrium, 257, 268, 271,273-274, 279 Zap lignite (see also Beulah-Zap) ash formation, 519 desulfurization, 487 flame temperature, 515 hydrous pyrolysis, 166 liquefaction, 160, 169, 652, 654 oxidation reactions, 64, 92 porosity, 366 pyrolysis, 181-185, 205-207

Zap lignite (continued) quality, 27 reaction with hydrogen, 150 surface area, 362 Zeisel method, 114 Zeolites, 519, 565 Zinc, 263,268, 271-274, 279 Zircon, 239, 244-245, 257 hafnium in, 257 Zirconium, 257, 268, 271,273-274, 279