LIGNITES OF NORTH AMERICA
COAL SCIENCE AND TECHNOLOGY Series Editor:
Larry L. Anderson Department of Fuels Engineeri...
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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.
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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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109 110 1.11
112 113 114 115 116 117
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135 118 119 120 121 122 123 124
125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141
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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
M.L. Gorbaty, G.N. George, S.R. Keleman, and M. Sansone, Direct determination and quantitation of sulfur forms in coals from the Argonne premium sample bank, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 35 (1990) 779-783. H. Marsh, P.M.A. Sherwood, and D. Augustyn, XPS study of binding energies of sulfur in carbon, Fuel, 55 (1976) 97-98. G.H. Gronhovd, Personal communication, Grand Forks, ND, 1976. F.E. Huggins, G.P. Huffman, F.W. Lytle, and R.B. Gregor, The form-of-occurrence of chlorine in U.S. coals: an XAFS investigation, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 34 (1989) 551-558. I. Lavine, Progress in low-rank coals, Ind. Eng. Chem. 26 (1934) 154-164. L.M. Lucht, J.M. Larson, and N.A. Peppas, Macromolecular structure of coals. 9. Molecular structure and glass transition temperature, Energy Fuels, 1 (1987) 56-58. H.L.C. Meuzelaar, Y. Yun, N. Simmleit, and H.R. Schulten, The mobile phase in coal viewed from a mass spectrometric perspective, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 34 (1989) 693-699. B.M. Benjamin, E.C. Douglas, P.M. Hershberger, and J.W. Gohdes, New chemical structural features of coal, Fuel 64 (1985) 1340-1348. L.M. Lucht and N.A. Peppas, Macromolecular structure of coals. 2. Molecular weight between crosslinks from pyridine swelling experiments, Fuel, 66 (1987) 803-809. J.R. Nelson, Determination of molecular weight between crosslinks of coals from solventswelling studies, Fuel, 62 (1983) 112-116. G.D. Cody Jr., J.W. Larsen, and M. Siskin, Anisotropic solvent swelling of coals, Energy Fuels, 2 (1988) 340-344. I.E. Svyatets and A.A. Agroskin, Supramolecular structure of lignite material, Khim. Tverd. Topl. (5) (1983) 16-22. D.C. Doetschman and D.W. Dwyer, A pulsed electron paramagnetic resonance study of eight North American premium coals, Energy Fuels, 6 (1992) 783-792. J.Dollimore and M.L. Swanson, Supercritical solvent extraction, University of North Dakota Energy Reseach Center monthly report, December 1984. J. Dollimore and M.L. Swanson, Supercritical solvent extraction, University of North Dakota Energy Research Center monthly report, February 1985. M.L. Swanson, Supercritical solvent extraction, University of North Dakota Energy Research Center monthly report, October 1985. J.R. Rindt, Low-rank coal liquefaction, in: G.A. Wiltsee (Ed.), Low-rank Coal Research Under the UND/DOE Cooperative Agreement, U.S. Dept. Energy Rept. DOE/FE/6018126, (1983), pp. 6-1 - 6-31. J.R. Maxwell, C.T. Pillinger, and G. Eglinton, Organic geochemistry, Quart. Rev., 25 (1971) 571-629. P. Albrecht and G. Ourisson, Impurities in organic geochemistry, Angew. Chem., 10 (1971) 209-228. P.H. Neill, Y.J. Xia, and R.E. Winans, Identification of the heteroatom containing compounds in the benzene/methanol extracts of the Argonne premium coal samples, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 34 (1989) 745-751. M. Nishioka and J.W. Larsen, Mild pyrolytic production of low molecular weight compounds from high molecular weight coal extracts, Energy Fuels, 2 (1988) 351-355. R. Hayatsu, R.E. Winans, R.G. Scott, L.P. Moore, and M.H. Studier, Characterization of organic acids trapped in coals, Nature, 275 (1978) 116-118. B. Kybett, J. Potter, M. Etter, and M. Krahe, The effect of solvent extraction on the reflectance of coal and coal-oil mixtures, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 32(1) (1987) 9-11. E.C. Jeffrey, The origin and organization of coal, Mem. Amer. Acad. Arts Sci. XV(1) (1924) 1-52. L.A. Her6dy, A.E. Kostyo, and M.B. Neuworth, Studies on the structure of coals of different rank, Fuel, 44 (1965) 125-133. J.A. Franz, J.R. Morrey, J.A. Campbell, G.L. Tingey, R.J. Pugmire, and D.M. Grant, Inferences on the structure of coal: 13C NMR and IR spectroscopy, Amer. Chem. Soc. Div. Fuel Chem. Preprints, 20(3) (1975) 12-15.
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 am
