Preface Coal continues to be an important energy source in many parts of the world. The increasing world population and the continued improvement in living standards have demanded that coal scientists and technologists develop new technologies for cheaper energy with less environmental impacts, particularly reduced emissions of greenhouse gases and other air pollutants. The development of clean coal utilisation technologies has required deeper understanding of coal structure and properties than ever before. The past 15 years have seen major progress in coal science and technology with some new coal utilisation technologies now moving towards demonstration and commercialisation. Victorian brown coal has many unique physical and chemical properties when compared with internationally traded black coals. The efficient utilisation of Victorian brown coal must consider its special properties. The book "The Science of Victorian Brown Coal: Structure, Properties and Consequences for UtiHsation" (edited by Dr R.A. Durie), as a comprehensive treatise of Victorian brown coal, was published 13 years ago. The book covered almost every topic on Victorian brown coal, ranging from its geology to its utilisation as an energy source and a chemical feedstock. During the past 13 years, significant progress has been made in the understanding of structural features and properties of Victorian brown coal as well as in the development of new technologies for efficient and environmentally friendly utilisation. The present book was inspired by this scientific and technological progress and aims to update our knowledge about the science and technologies of Victorian brown coal. This book is divided into 8 chapters. Chapter 1 provides an overview of the importance of Victorian brown coal and its properties. The use of Victorian brown coal as an important energy source and a potential chemical feedstock as well as other non-traditional use of the brown coal is then briefly introduced. Chapter 2 begins with a brief introduction of the petrography of Victorian brown coal. This is then followed by a detailed description of its physical and chemical structural features and properties. Many physical and chemical methods have been used to characterise the structural features and properties of Victorian brown coal, including its abundant pore structure, its diverse functional groups, ion exchange properties and its macromolecular structural features. One of the most important features of Victorian brown coal is its high moisture content, affecting every aspect of its utilisation. While some pertinent description of the water in brown coal and its drying may be found in other chapters in this book. Chapter 3 is specially devoted to a detailed description of water in the brown coal and various drying technologies. The reactions taking place during the pyrolysis of Victorian brown coal, involving its organic and inorganic constituents, are discussed in detail in Chapter 4. The chapter
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begins with a summary of the main experimental techniques for studying coal pyrolysis and ends with a description of the kinetic modelling of coal pyrolysis. The potential to produce chemicals from the pyrolysis of (modified) Victorian brown coal is also described in this chapter. The fundamental aspects of brown coal gasification and combustion are given in Chapter 5. Again, reactions involving both organic and inorganic constituents in Victorian brown coal during gasification and combustion, including the catalytic effects of metallic species inherently present in the coal or externally loaded into the coal, are described in detail. In parallel to Chapters 4 and 5, the transformation of nitrogen and sulphur in brown coal during pyrolysis, gasification and combustion is described in Chapter 6. The chapter also provides a general overview of the recent progress in the characterisation of coal-N and coal-S using modem analytical methods. A brief summary of the literature on the pyrolysis of various N-containing model compounds, ranging from simple pyrrole/ pyridine to polymeric nylons and proteins, is provided before the detailed discussion of coal-N conversion. Various possible power generation technologies using Victorian brown coal are described in Chapter 7 with a particular focus on the gasification-based power generation technologies using either gas turbines or fuel cells. The efficiencies of these cycles are analysed based on the thermodynamic simulation of the cycles. Chapter 8 is devoted to the direct liquefaction of brown coal including the fundamental understanding of brown coal liquefaction and the development of brown coal liquefaction technologies, with a particular focus on the liquefaction process originally developed for Victorian brown coal. The book will be useful to those engaged in the research and development of technologies for the clean utihsation of coal especially low rank coals. It will be particularly useful for postgraduate students and researchers entering this field. Victorian brown coal has many similarities to biomass. It is certainly much closer in properties to biomass than to high rank coals. For example, both Victorian brown coal and biomass have high contents of moisture and volatile metallic species such as sodium, potassium, magnesium and calcium. Therefore, this book should also be useful to researchers working on biomass utilisation. I am honoured and privileged to be the editor of this book. The completion of this book is truly a team effort by all authors. Despite their other heavy commitments, they have given their time and expertise freely and often worked during weekends and night time to complete their manuscripts. I would like to express my sincere gratitude to all authors for their great efforts and co-operation during the course of this project. Many colleagues have provided generous support and help for this project and special thanks to Alan N Buckley and Zhiheng Wu must be recorded for reviewing some chapters. It should be recorded with sincere appreciation and thanks that the majority of the research work on Victorian brown coal described in this book was carried out by Australian and Japanese researchers with funding from Australian and Japanese governments and industries, which together made this book possible.
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vii
I would like to thank the staff in Elsevier, especially Victoria Thame, Salma Azmeh and Sharon Brown for their support and help at all stages of this project. The authors wish to thank Elsevier for kindly granting permission to use materials from many of its journals and books in this book. The full reference details of each figure or table from an Elsevier journal or book are given in the figure caption or table heading where the figure or table is reproduced in this book. Permission was also kindly granted by other organisations to use their copyrighted materials as acknowledged in the book. Finally, I would like to thank my wife Fan and my daughter Merry for their support and encouragement throughout this project.
Chun-Zhu Li Melbourne, Australia June 2004
Foreword Victoria has one of the largest deposits of high quahty brown coal in the world and this is an important resource for the State. This coal has long been a low cost and reliable energy source for the social and economic development of the State. Victorian brown coal will continue to be an important energy source for the foreseeable future. It will also play an important role in providing affordable energy. The continued use of Victorian brown coal will depend on the development of technologies that minimise environmental impacts and reduce the emission of greenhouse gases. The Victorian and the Commonwealth Governments, together with industry, have actively promoted and supported the development of clean coal technologies. This book outlines the recent advances in the science of Victorian brown coal and the development of technologies for its clean and efficient utiUsation. Written by Australia and Japanese experts in the field, this book reflects the productive alliance between the two countries in their efforts to address the energy and environmental challenges facing the world today. This book is an important milestone in the science of Victorian brown coal and will help to facilitate the demonstration and adoption of clean coal technologies in Victoria. I congratulate the editor and the authors for the completion of this important project.
Theo Theophanous MP Minister for Energy Industries and Resources June 2004
Advances in the Science of Victorian Brown Coal Edited by Chun-Zhu Li © 2004 published by Elsevier Ltd.
Chapter 1 Introduction Chun-Zhu Li CRCfor Clean Power from Lignite, Department of Chemical Engineering PO Box 36, Monash University, Victoria 3800, Australia
1.1. IMPORTANCE OF VICTORIAN BROWN COAL AS AN ENERGY SOURCE Brown coal is found in three major Tertiary basins in the State of Victoria in Australia [1]: the Murray Basin, the Otway Basin and the Gippsland Basin. Around 19,600 Mt of brown coal is deposited in the Murray Basin (mainly Kerang deposits) [1] and around 15,500 Mt in the Otway Basin (Bacchus Marsh-Altona and Angelsea) [1]. The Gippsland Basin is much bigger than the Murray Basin and the Otway Basin. It is one of the major coal and petroleum bearing basins in the world. Only one-fifth of the basin is onshore with the reminder extending beneath the Bass Strait [1]. The Gippsland Basin includes the Latrobe Valley Depression, Moe-Yarragon, Stradbroke, Won WronBoodyam, Alberton and Gelliondale. The total brown coal resource in the onshore Gippsland Basin is estimated as 395,000 Mt [1,2]. About one third of this coal is classified as demonstrated and the reminder as inferred. As was reviewed recently by Holdgate and co-workers [3] in detail, the Traralgon Formation contains by far the largest brown coal resources in the Gippsland Basin. The latest published reserve estimates for the Gippsland Basin are 98,000 Mt, with some 33,000 Mt excluded for town reserves and so on, giving a balance of 65,000 Mt of available brown coal [2]. These brown coal reserves usually have very thin overburdens and thick coal seams, ideal for large-scale open cut operation. A comparison of Victorian brown coal with other brown coal or lignite resources in other countries indicates [4] that the brown coal in the Latrobe Valley is one of the largest high quality brown coal/lignite reserves in the world. A good summary of the history of the use of brown coal in the Latrobe Valley has been given by Harvey [5], who described the history of the power industry fuelled with the brown coal in Victoria using the Yalloum Power Station as an example. The first open cut development in the Latrobe Valley took place at Yalloum North in 1879 and ceased operation in 1963 after about 18 Mt of brown coal had been excavated [2]. The Yalloum Open Cut was the first major open cut operated by the State Electricity Commission of Victoria. The use of brown coal in Victoria has grown steadily to meet the energy demand in the State of Victoria. Brown coal production by the State Electricity Commission of Victoria between 1923 and June 1989 from all Latrobe
2
Chapter 1
Valley open cuts amounted to approximately 1000 Mt [2]: mainly from the Yalloum Open Cut (580 Mt), the Morwell Open Cut (332 Mt) and the Loy Yang Open Cut (54 Mt) [2]. The annual brown coal production in Victoria was 49 Mt in 1988-89 [4] and over 65 Mt in 2000 and 2001 [6,7]. The main use of the brown coal has been the generation of electricity through the direct combustion of the brown coal, although minor amounts of the brown coal have also been used for other purposes (see below). It is only fair to say that the Victorian brown coal in the Latrobe Valley has been the main energy source for the steady economic development in Victoria. It is expected that the brown coal resources will continue to serve as the cheap energy source for the Victorian economy in the foreseeable future. The future use of Victorian brown coal is likely to be beyond its direct combustion and expand into other areas of brown coal utilisation (see below). While a true sustainable use of coal is not possible, even with the vast reserves of brown coal in Victoria, the environmentally friendly use of the Victorian brown coal will play an important and indispensable role in our transition to a true sustainable development. The full potential of the Victorian brown coal can only be realised through a good scientific understanding of the composition and behaviour of the brown coal. Since the publication of The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilisation [8] in 1991, significant progresses have been made in this area. The purpose of this book is to critically review these scientific progresses.
1.2. AN INTRODUCTION TO THE TYPICAL PROPERTIES OF VICTORIAN BROWN COAL The brown coal in the Latrobe Valley is of low rank and Tertiary age. The greater part of the brown coal is classified as soft brown coal with the reminder as hard brown coal. The US ASTM classification system would consider all Victorian brown coal in the Latrobe Valley as lignite B [2]. Victorian brown coal has many unique physical and chemical properties and structural features. This will be discussed in detail in Chapter 2. Barton and co-workers [2] presented typical properties of Victorian brown coal from the operating open cuts in the Latrobe Valley, as is shown in Table 1.1. It should be emphasised that the data in Table 1.1 should be taken as average values because the properties of brown coal, as a heterogeneous natural resource, vary from one location to another even with the same seam. One of the most important features of the brown coals in the Latrobe Valley is their high moisture contents, ranging from 50 to 66 wt% in the as-mined brown coal, as is shown in Table 1.1. This has been a major challenge in the efficient use of the brown coals. The recent progresses in the understanding of water in the brown coals and the development of dewatering/drying technologies will be described in Chapter 3. Dewatering/drying has become an important part of any advanced brown coal utilisation technologies, as will be discussed in detail in Chapters 3, 4, 5, 7 and 8.
Introduction
3
The brown coals in the Latrobe Valley usually have very low ash yields, often < 2 wt% (db). The discrete minerals in the brown coals are in far less concentrations and thus of far less importance than those in most bituminous coals. A major class of the ash-forming species in the Victorian brown coal are alkali and alkaline earth metallic (AAEM) species present either as ion-exchangeable carboxylates/phenolates or as salts (e.g. NaCl) dissolved in the coal-bed moisture. Some other elements such as iron can also exist as ion-exchangeable cations in the brown coal. The presence of these AAEM species is a major structural feature of the Victorian brown coal and an important consideration in all aspects of brown coal utilisation. For example, in the advanced power generation processes using the brown coal, the volatilisation of AAEM species during gasification would cause the corrosion/erosion of the downstream turbine components and thus presents a challenging problem for the hot gas cleaning. However, these AAEM species retained in the char would act as excellent catalysts for the gasification of char [9]. Throughout the book, the structures involving these AAEM
Table 1.1 Typical properties of Latrobe brown coals from operating open cuts [2]. Properties
Yalloum SeamY
Morwell Seam M1
Coal properties Moisture, % (ar) Ash, %(db) Volatile Matter, % (db) Carbon, % (db) Hydrogen, % (db) Sulphur, % (db)
65.5 1.7 51.1 66.7 4.7 0.3
60.1 3.3 48.2 67.8 4.8 0.4
Specific energy Gross dry (MJ/kg) Net wet (MJ/kg)
25.9 7.1
Ash analysis Si02, % AI2O3, % Fe203, % Ti02, % CaO, % MgO, % Na20, % K2O, % SO3
26.9 8.6 20.0 0.5 6.0 14.3 6.5 0.3 17.1
Open Cut Yalloum N Extension Seam M2
Ley Yang SeamMlB
Seam M2
51.7 4.4 48.8 66.7 4.7 0.5
62.5 1.5 51.3 68.3 4.8 0.4
61.0 1.7 50.5 69.2 4.9 0.4
26.5 8.8
26.2 11.0
27.0 8.1
27.6 8.8
16.4 3.4 9.3 0.3 24.7 14.2 4.9 0.3 26.6
8.6 5.0 19.8 0.6 25.1 8.6 3.5 0.2 28.6
17.2 12.4 11.5
45.5 8.5 17.4
-
-
3.0 11.6 17.4
4.8 6.6 4.6
-
-
26.9
12.7
All percentages are by weight. Nitrogen contents of Victorian brown coals are normally between 0.5 and 0.7 wt% (db), see Chapter 6.
4
Chapter 1
species (and other ion-exchangeable species such as iron) in coal and the fates and roles of these species during all processes of brown coal utilisation will be discussed. Due to the relatively low degrees of coalification, the brown coals in the Latrobe Valley feature low carbon contents (often < 70 wt%) and high oxygen contents (often > 25 wt%). Oxygen exists in the brown coals in a wide variety of functional groups, playing important roles in the structure of the brown coals and thus the physical and chemical properties of the brown coals. This will be discussed in detail throughout the book and a few typical examples are given here. For example, the carboxyl and phenol groups are responsible for the acidic nature of the brown coals (see Chapters 2 and 3). The oxygen-containing groups are involved in/responsible for the cross-linking reactions during the pyrolysis of the brown coals (see Chapter 4). These functional groups are also important consideration in the liquefaction of the brown coals (see Chapter 8). As is shown in Table 1.1, the sulphur contents are generally low for the brown coals from the current open cut operations. However, the viability of the Victorian brown coals as a future energy source will largely depend on the minimisation of the environmental impacts from the use of the brown coals, for example, by developing zero-emission technologies. In addition to CO2, the emissions of nitrogen and sulphur oxides (and heavy metals) must be minimised. Moreover, high-sulphur Victorian brown coals do exist (see Chapter 6). The conversion of coal-N and coal-S during the combustion, gasification and liquefaction of Victorian brown coal will be discussed in detail in Chapters 5, 6 and 8.
1.3. AN INTRODUCTION TO THE MAJOR UTILISATION OF VICTORIAN BROWN COAL Victorian brown coal has found wide application, which has been reviewed by Allardice and Newell [10] and Allardice and Young [6]. In what follows, only the main utilisation processes of Victorian brown coal will be introduced briefly. In particular, for utilisation processes that will be discussed in detail in later chapters, only a brief introduction will be given here and the readers will be referred to the relevant chapters in this book. 1.3.1. Pulverised Fuel Combustion The pulverised fuel (pf) combustion of Victorian brown coal in thermal power stations for the generation of electricity has so far been the main use of the brown coal. Many books are available for the general discussion of pf combustion technologies. The pf combustion technology for the brown coal includes an integrated mill/drying system where a large portion of the hot exit flue gas is recycled via large off-take ducts to dry the as-mined high moisture brown coal before combustion [6]. Because of the high inert gas loading, furnace gas temperatures and flame temperatures are several hundred degrees lower than comparable black coal units [6]. Furthermore, the power plants
Introduction
5
burning brown coal are much larger in size than the plants burning black coal with a similar generating capacity. Further discussion is given in Chapters 5 and 7. Currently, 6220 MW of brown coal fired generating capacity is installed in the Latrobe Valley [6]. 31 % of the electricity generated in Australia in 2000-01 was from brown coal [11], the majority of which was from the Victorian brown coals. There are a number of R&D initiatives (e.g. oxyfuel and supercritical boiler technologies) in the direct combustion of brown coal, aiming at the improved efficiency and the production of CO2 that can be sequestrated easily. The use of advanced drying technologies (see Chapter 3) will also bring about increases in the power generation efficiency. Further discussion will be given in Chapter 7. 1.3.2. Gasification Gasification refers to the conversion of solid coal into gaseous fuels. There has been a long history of brown coal gasification. The gasification of brown coal (briquettes) in the Latrobe Valley was a source of town gas before the availability of Bass Strait natural gas in the late 1960s. The current interests are in the retrofitting of power plants and the development of advanced gasification-based power generation technologies. The aims of these advanced gasification-based technologies are to convert the solid brown coal into gaseous fuels (particularly H2), which can then be combusted in a gas turbine system or used in a fuel cell system to generate electricity. As an endothermic process, the gasification of brown coal allows for the recuperation of thermal energy into chemical energy and, coupled with the high efficiencies associated with the gas turbines and fuel cells, result in an increased overall efficiency. The gasification of brown coal for power generation has the potential of producing CO2 in high concentrations to facilitate the sequestration of CO2. The gasification-based power generation technologies using brown coal also have the potential of greatly reducing the emissions of other air pollutants such as SOx, NOx and particulates to meet the increasingly stringent future environmental standards. The thermodynamic analysis of these advanced processes will be presented in Chapter 7. Fundamental reactions taking place during the gasification of Victorian brown coal will be discussed in Chapters 4, 5 and 6. 1.3.3. Liquefaction Liquefaction aims to convert solid coal into liquid fuels. While the high moisture contents and great propensity of spontaneous combustion do not make the Victorian brown coal an exportable commodity, the liquefaction of the brown coal may convert the brown coal into liquid fuels suitable for long distance transportation and make it an internationally traded commodity. The brown coal may be liquefied indirectly through the gasification of the brown coal: the syngas (CO+H2) produced from gasification can be used to synthesise liquid fuels and chemicals, e.g. dimethyl ether [12], that can be transported conveniently and economically.
6
Chapter 1
The Victorian brown coal can also be liquefied directly through the hydrogenation of the brown coal in the presence of catalysts. The direct liquefaction of Victorian brown coal has been extensively studied and demonstrated at a large pilot plant scale. While the current price of crude oil may still make the brown coal liquefaction unviable economically, the future increases in oil price may well bring the brown coal liquefaction back to the international agenda. A detailed description of the Brown Coal Liquefaction process developed for the Victorian brown coal will be given in Chapter 8. The technology can also be applied to other low rank coals e.g. Indonesian low rank coals. Some efforts have been made to investigate the solubilisation of Victorian brown coal by micro-organisms [e.g. 13-15]; some further discussion will be given in Chapter 2. 1.3.4. Briquetting, Carbonisation and Production of Carbon Materials Briquette production has been a major use of Victorian brown coal [6,10]. Some further discussion may be found in Chapter 3. Guy and Perry [16] reviewed the production of carbon materials (semicoke and activated carbon) from Victorian brown coal. The carbonisation (up to 800°C) of brown coal briquettes has been used to produce a lump char [6,16] with high purity (~3 % ash yield), high reactivity and good absorbent characteristics. The char can be used for a number of applications including ferroalloy production, recarburiseing of steel, organic syntheses, barbecue fuel and preparation of activated carbons [16]. The highly porous nature of the brown coal (see Chapter 2) makes it a good starting material for making carbon materials [6]. Surface areas exceeding 1000 m^ g"^ can be achieved [16]. Based on the carbon deposition according to the Boudouard reaction of 2C0 = C + CO2, the surface area of an activated carbon sample from Yalloum brown coal could be increased from 900 to 1400 m^g"^ [17]. The preparation of a mesoporous carbon was attempted by a steam activation of a mixture of coal and metal acetylacetonate at 900°C [18]. The main problem with the activated carbon produced directly from the brown coal has been the lack of mechanical strength. A combination of physical and chemical digestion involving potassium hydroxide followed by carbonisation appears to be a promising route towards the production of premium-grade activated carbons from Victorian brown coal [16]. Carbon materials from the Victorian brown coal may be used as cheap adsorbents for the water treatment to remove volatile or low molecular mass organic compounds [1923]. Many other less common applications of the carbon materials from the Victorian brown coal are investigated, for example, for methane storage [24] and as an anode for the lithium-ion battery [25]. Victorian brown coal was also considered as a raw material for the preparation of fullerenes [26] and the separation of fullerenes by chromatography on coal [27]. Activated carbons from the Victorian brown coal, due to their large surface areas and other favourable porous structural features, may also act as cheap supports for
Introduction
7
(disposable) catalysts [28] or as cheap supports for adsorbents in the hot-gas cleaning of gas products from gasification [29]. This is clearly important for applications requiring a large amount of catalyst or the recovery of catalyst is difficult. Brown coal derived chars were also investigated for flue gas desulphurisation and denitrogenation [30]. 1.3.5. Production of Chemicals Victorian brown coal has been considered as a feedstock for the production of chemicals. Pyrolysis [31,32], liquid-phase oxidation [32,33], extraction [34,35] or a combination of these processes have been proposed as effective ways to produce chemicals from Victorian brown coal. For example, when Morwell brown coal was oxidized for 24 h at 60°C using 30%-H2O2 aqueous solution, the yield of water-soluble organics was as large as 0.60 kg/kg-coal, 0.28 kg/kg-coal of which were chemicals such as oxalic acid and acetic acid. The decomposition of the water-soluble organics in a subcritical water produced 0.12 kg/kg-coal of benzene or 0.236 kg/kg-coal of methanol, depending on the conditions employed for preparing the water-soluble organics [33]. The effects of pretreatment of the Victorian brown coals on their pyrolysis behaviour will be discussed in Chapter 4. Microwave plasma technology has been applied to the direct conversion of Yalloum brown coal under the successive supply of methane [36]. In addition to oily products, CO and H2 accompanied by a molar fraction of <16% acetylene were mainly produced. 1.3.6. Use of Victorian Brown Coal as an Adsorbent The presence of abundant carboxyl (-COOH) and phenol (-0H) groups in the Victorian brown coal has stimulated research into the use of the brown coal as a cheap ion-exchange medium for the recovery of precious metals or the removal of heavy metals from aqueous systems [e.g. 37-39]. A detailed discussion the acidity and ionexchanging ability of these groups will be given in Chapter 2. 1.3.7. Agricultural Use Victorian brown coal has been used for a variety of agricultural and horticultural purposes. For example, sized brown coal can be used directly as a soil conditioner or in potting mixes [6]. Humates etc derived from the brown coal have been used as fertilisers [6]. Sorption of ammonia on conditioned Yalloum brown coal was studied from the point of view of increasing its nitrogen content to produce a nitrogenous fertilizer [40]. Nitrohumic acids and their ammonium salts can be made from the oxidation of Victorian brown coal [41,42]. The addition of brown coal can also have influence on the soil composition and root grow1;h in acid soil from wheatbelt of Western Australia [43]. The use of Ca-modified, brown-coal-derived humates and fulvates was also investigated for the treatment of soil acidity [44].
8
Chapter 1
1.3.8. Ash Utilisation Black and co-workers [45] reviewed the utilisation of ash from the combustion of Victorian brown coal. The ash from the Latrobe Valley brown coals is unique and has a wide variety of potential applications including soil conditioning [6], concrete and cement production [46-48], fillers in non-metallic materials and components of building materials. The relatively high magnesium contents in the Latrobe Valley brown coals also means the possibility of recovering magnesium from the fly ash [45]. MgO in the fly ash can be leached as MgCl2, which is then sent for electrolysis [49]. In concluding this Introduction, it is fair to state that the use of Victorian brown coal can be much more than its current combustion for electricity generation. Its more efficient and environmentally friendly utilisation, relying a better understanding of its composition and behaviour, will make this vast resource a cheap and reliable energy source for the generations to come.
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[email protected]/0/lDD46A713657BA33CA256E00 0075736B?Open [8] Durie RA (editor). The Science of Victorian Brown Coal, ButterworthHeinemann, Oxford, 1991. [9] Li C-Z, Sathe C, Kershaw JR, Pang Y. Fuel 2000;79:427. [10] Allardice DJ, Newell BS. Chapter 12 in The Science of Victorian Brown Coal (Ed: R.A. Durie), Butterworth-Heinemann, Oxford, 1991. [11] Australian Bureau of Statistics, Year Book Australia, Energy, Energy production, 27 February 2004, HTTP://www.abs.gov.au/Ausstats/
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9
Adachi Y, Komoto M, Watanabe I, Ohno Y, Fujimoto K. Fuel 2000;79:229. Catcheside DEA, Mallett KJ. Energy & Fuels 1991;5:141. Ralph JP, Catcheside DEA. Fuel 1993;72:1679. Ralph JP, Catcheside DEA. Applied Microbiology and Biotechnology 1994;42:536. Guy PJ, Perry GJ. Fuel 1992;71:1083. Meguro T, Tanaka H, Komeya K, Tatami J. Processing and Fabrication of Advanced Materials VIII, Proceedings of a Symposium, 8th, Singapore, 8 - 1 0 September 8-10, 1999, pp.611-617. Yoshizawa N, Yamada Y, Furuta T, Shiraishi M, Kojima S, Tamai H, Yasuda H. Energy & Fuels 1997; 11:327. Hobday MD, Li PHY, Crewdson DM, Bhargava SK. Fuel 1994;73:1848. Li PHY, Roddick FA, Hobday MD. Journal of Chemical Technology & Biotechnology 1998;73:405. Othman MZ, Roddick FA, Hobday MD. Water Research 2000;34:4351. Othman MZ, Roddick FA, Snow R. Water Research 2001 ;35:2943. McCallum R, Roddick F, Hobday M. Water Science & Technology: Water Supply 2002;2:265. Chaffee AL, Pandolfo AG, Verheyen TV. Carbon'Ol, An International Conference on Carbon, Lexington, KY, United States, July 14-19, 2001 2001;1391. Chang YC, Sohn HJ, Korai Y, Mochida I. Carbon 1998;36:1653. Pang LSK. Fuel Processing Technology 1993;34:147. Inukai Y, Attalla MI, Pang LSK, Wilson MA. Fuel 1995;74:83. Nakamura I, Fujimoto K. Sekiyu Gakkaishi 1996;39:245. Ikenaga N-o, Ohgaito Y, Matsushima H, Suzuki T. Fuel 2004;83:661. Fujitsu H, Mochida I, Verheyen TV, Perry GJ, Allardice DJ. Fuel 1993;72:109. Miura K. International Journal of the Society of Materials Engineering for Resources 1999;7:222. Miura K. Fuel Processing Technology 2000;62:119. Mae K, Shindo H, Miura K. Energy & Fuels 2001; 15:611. Miura K, Shimada M, Mae K, Sock HY. Fuel 2001 ;80:1573. Miura K, Mae K, Shindo H, Ashida R, Ihara T. Journal of Chemical Engineering of Japan 2003;36:742. Kamei O, Onoe K, Marushima W, Yamaguchi T. Fuel 1998;77:1503. Lafferty C, Hobday M. Fuel 1990;69:78. Lafferty C, Hobday M. Fuel 1990;69:84. Bums CA, Cass PJ, Harding IH, Crawford RJ. Colloids and Surfaces, A: Physicochemical and Engineering Aspects 1999;155:63. Oussa AR, Evans DG. Fuel 1989;68:1363. Patti AF, Verheyen TV, Douglas L, Wang X. Science of the Total Environment 1992;113:49. Wang XJ, Douglas LA, Patti AF. Agrochimica 1995;39:73. Yazawa Y, Wong MTF, Gilkes RJ, Yamaguchi T. Communications in Soil
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Chapter 1
Science and Plant Analysis 2000;31:743. [44] Peiris D, Patti AF, Jackson WR, Marshall M, Smith CJ. Australian Journal of Soil Research 2002;40:1171. [45] Black C, Brockway D, Hodges S, Milner A. Publications of the Australasian Institute of Mining and Metallurgy 1992;3/92:149. [46] Macphee DE, Black CJ, Taylor AH. Cement and Concrete Research 1993;23:507. [47] Jo YM, Guang D, Raper J A. Advances in Cement Research 1996;8:21. [48] Cashion JD, Brown LJ, Hannaford CS, Hall JS. Hyperfme Interactions 2002; 139/140:417. [49] Hill S. Materials World 2002; 10:10.
Advances in the Science of Victorian Brown Coal Edited by Chun-Zhu Li © 2004 Elsevier Ltd. All rights reserved.
Chapter 2 Structure and Properties of Victorian Brown Coal Jun-ichiro Hayashi^ and Chun-Zhu Li^ Centre for Advanced Research of Energy Technology Hokkaido University, N13-W8, Kita-ku, Sapporo 060-8628, Japan ^CRCfor Clean Power from Lignite, Department of Chemical Engineering PO Box 36, Monash University, Victoria 3800, Australia
2.1. INTRODUCTION Victorian brown coal is a family of low-rank coals with unique physical and chemical properties. Almost every aspect of its properties is unique compared with those of other solid fuels such as biomass, peat, bituminous coal and anthracite. The efficient and environmentally friendly utilisation of the Victorian brown coal resource must consider its special structural features and properties. Therefore, the understanding of the unique structural features, physical and chemical properties of Victorian brown coal has been the subject of many studies. The contributions that these studies have made form the subject of this chapter.
2.2. PETROGRAPHY OF VICTORIAN BROWN COAL Coal is a fossilised plant material. Studies of Victorian brown coal in Latrobe Valley seams indicate that these coal seams are mainly autochthonous in origin [1], i.e. the plant material that was transformed into coal had grown in situ. For example, large tree stumps still in upright or growing position occur in the Yalloum seam [1]. Examining at a microscopic scale, plant debris (e.g. cell wall structures) can be observed in the coal. Therefore, like any other coals, Victorian brown coal is not a homogeneous substance but an organic rock whose composition varies within a coal seam or even within a small piece of coal. The structure and composition of this organic rock reflects the types of coal-forming plants, the depositional environment when the coal was formed and the thermochemical reactions that took place during coalification under geological conditions. This section deals briefly with the petrography of Victorian brown coal, i.e. its structure and composition at the macroscopic and microscopic levels when it is examined as an organic rock using optical microscopy at up to 1,000 magnifications. A detailed account of the petrological studies of Victorian brown coal carried out before 1990 has been given by George and Mackay [1]. This section is not meant to be a
12
Chapter 2
comprehensive description of the geology of Victorian brown coal seams. Interested readers should consult relevant recent reviews on the geology of Victorian brown coal available elsewhere [2-10]. 2.2.1. Lithotypes Victorian brown coal in the Latrobe Valley with their full natural moisture contents (i.e. bed moist brown coal with its moisture content in the range of 50 - 70 %) has a colour ranging from red brown to dark brown [1]. On loosing moisture, the colour changes significantly. When the brown coal is air-dried so that the moisture in coal (about 1 0 - 1 5 % ) is in equilibrium with the moisture in the ambient air, its colour may range from nearly black to brown yellow depending on coal type and rank. At an open cut face where the coal at the surface has been at least partially dried (even if not completely air-dried), bands of different colours are easily observed. These macroscopic ingredients, appearing visually as bands in coal, are the rock types occurring in coal, referred to as "lithotypes". In other words, the study of coal lithotypes considers coal as an organic rock. Coal lithotypes are also very important to the study of coal geology. Identification and classification of coal lithotypes is the examination of coal structure and composition at a macroscopic scale. For black coals, the lithotype nomenclature proposed by Marie C Stopes in 1919 has been widely used [11], which describes the lithotypes in humic coals as vitrain, clarain, durain and fusain. For sapropelites, the terminologies of "boghead coal" and "cannel coal" have been used. Victorian brown coals, particularly those currently being used, are generally not buried in great depth and thus suitable for open cut. They are of relatively low rank, mostly considered as soft or hard brown coals in rank. They are generally at an early stage of coalification. In addition to groundmass matrix consisting of compacted fragments of plant remains that are too small to be identified at a macroscopic scale, larger wood or plant tissue structures are still recognisable in coal. Lithotypes in Victorian brown coal may be sharply bounded or gradational and vary in thickness from several centimetres to a few metres [8]. Individual lithotypes can extend over 1 to 2 km within a seam while lithotype zones where one or two lithotypes predominate may be traced up to 5 km [1], reflecting the changes in the depositional environment when coal was formed [5,12-15]. Gelification has also taken place to different extents in the woody tissues and in the groundmass. The classification of lithotypes in Victorian brown coal has used a system different from that for higher rank humic coals. The classification of lithotypes in Victorian brown coal is mainly based on the work by George [16] on coals in the Yalloum and Morwell Formations in the Latrobe Valley. Five lithotypes have been identified and classified using colour and texture as the primary parameters and degree of gelification, weathering pattern and some other physical properties as the supplementary parameters. Table 2.1 describes the main characteristics of these lithotypes. Due to the extremely low contents of minerals (normally < 5%) in these coals and the lack of correlations between the properties of minerals and those of organic mass, no special consideration has been given to the classification of inorganic rock types (minerals) in these coals.
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Chapter 2
The first parameter in the identification and classification of lithotypes in Victorian brown coal, shown in Table 2.1, is the colour of lithotype when it is fully air-dried. The colour of "groundmass" may differ from those of "plant tissues" or "wood" remains within the same lithotype. The colour in Table 2.1 refers mainly to the colour of the groundmass and small plant material remains [1,16]. As is obvious in Table 2.1, the names of the lithotypes in Victorian brown coal owe to their colour, being differentiated in a colour scalefi-omdark/black brown to pale/yellow brown. The second parameter in Table 2.1 is the "texture", which considers the relative proportion of large pieces of "wood" or tissue remains to the fine groundmass in the coal. Whilst these structures or "particles" may be clearly distinguished in appearance from the groundmass both macroscopically and microscopically, their separation from the groundmass using simple physical or chemical methods is not always possible [1,16], although sometimes well preserved woody fragments may be obtained by breaking down the coal mass. The separation becomes increasingly difficult with increasing rank, as the tissue remains tend to be digested into the groundmass of coal, making the boundaries between groundmass and tissue remains very gradual or even indistinguishable. The content of "wood" remains decreases as the colour of the lithotype lightens from the dark to pale lithotypes. The third parameter to differentiate lithotypes, as is shown in Table 2.1, is the gelification of "wood" and groundmass, the formation of a gel-like substance with little or no structure macroscopically. This humic gel becomes relatively hard on loosing moisture and tends to develop cracks. Gelification is more likely with "woody" structures than with the groundmass. As is shown in Table 2.1, while gelification is common with the more "woody" dark lithotype, gelification is rare with the less "woody" pale lithotype. The fourth parameter in Table 2.1 is weathering that refers to processes taking place when coal is exposed at an open cut surface, mainly the loss of coal moisture. At an
Table 2.2 Relative abundance and average bed thickness for lithotypes in the Traralgon Formation coal seams (after Holdgate and co-workers [8]). rv Bore no.
Depocentre Location
Oark ^^'''
——'— '^^^^^"^ Medium dark light ^'^^^
Average bed thickness, m ^^'^
Traralgon 2 seam W200 STR64
Western edge Centre
Traralgon 1 seam W200 West centre STR64 Main centre STR - Stradbroke; W - Willung
2 2
37 27
48 57
12 14
1 0
0.83 1.85
2 2
39 51
54 44
4 2
1 1
0.85 1.49
15
structure and Properties
open cut face, the coal at the outer surface looses more moisture than that inside. The shrinkage of coal on losing moisture (see Section 2.3 and Chapter 3) means that the outer layer of coal shrinks more than the inner layer, causing cracks to develop. Thus, the weathering pattern is a reflection of such coal properties as porosity and permeability. The crack patterns vary considerably between lithotypes. While a pale lithotype would have little or no extensive cracks on drying, cracks of 5 - 30 mm wide and 300 - 450 mm deep may develop with a dark lithotype. As is shown in Table 2.1, lithotypes differ considerably in hardness and density. Associated with the extensive gelification and high "wood" content, a dark lithotype is relatively strong and dense. On the contrary, a pale lithotype of relatively low density is soft and crumbles easily. Relative lithotype concentrations in Victorian brown coals change from seam to seam. Table 2.2 shows the percentages of each lithotype for the Traralgon Formation coal seams in two boreholes as well as average lithotype bed thickness [8]. The medium-dark (M-d) and medium-light (M-1) lithotypes are the dominant lithotypes. The relative thin lithotype layers are believed to be due to the rapid oscillations in the peat depositional environment near seam edges [8].
Table 2.3 Typical chemical analysis of lithotypes in Victorian brown coal (after George [16], with permissionfromthe publisher of Australian Coal Geology). Lithotype
•' ^
^O/,AU\
wt% (db)
Volatile matter
C
H
0
Specific energy MJ/kg (gross dry)
wt^y^ (Ar.f\
/. Yallourn open cut (26 samples) 50.6 Dk 0.9 50.4 M-d 0.9 51.3 M-1 0.8 56.6 Lt 1.2 63.4 Pa 1.1
68.0 68.3 68.0 69.3 70.7
4.7 4.7 4.8 5.5 6.5
26.4 26.1 26.3 24.2 21.9
26.36 26.48 26.27 27.78 29.26
2. Morwell open cut (35 samples) 48.1 Dk 3.1 48.6 M-d 3.2 51.0 M-1 3.7 54.4 Lt 3.8 57.4 Pa 4.4
69.3 68.6 69.8 70.5 70.9
4.8 4.7 5.1 5.4 6.0
25.0 25.8 24.2 23.2 22.2
26.89 26.50 27.45 28.08 29.03
3. Loy Yang (bore LY1280, M-d 1.9 M-1 0.6 Lt 1.0 Pa 1.0
66.9 68.9 69.5 70.7
4.5 4.8 5.3. 5.8
27.4 25.5 24.5 22.7
25.50 26.52 27.45 28.54
26 samples) 49.3 52.6 '55.0 58.5
16
Chapter 2
Lithotypes differ not only in their appearance but also in their physical and chemical properties. A detailed description of the differences in the physical and chemical properties among the lithotypes has been given previously by George and Mackay [1]. Table 2.3 shows the typical chemical analyses of lithotypes from three typical Victorian brown coals [16]. No typical dark lithotypes was present in the sequence analysed from bore LY1280. Volatile matter yields generally tend to increase from dark lithotypes to light and pale lithotypes, accompanied by the increases in hydrogen content and the decreases in oxygen contents. Characterisation of lithotypes with various analytical methods has been an important part of study on coal formation [12,15,17-29]. These studies showed that lignin contributed much more to the coal formation than cellulose (if at all) while some resins and tannins were also retained in coal. Dark lithotypes have a higher aromatic carbon content and more lignin structure than lighter lithotypes [27]. Lithotypes also differ in their behaviour during utilisation; more details may be found in later chapters in this book. 2.2.2. Macerals The heterogeneity of coal is also obvious even when coal is examined under a microscope at a magnification up to 1000. At microscopic levels, coal may be examined as thin coal sections with transmitted light Alternatively, coal may also be examined as a polished block (often coal particles are mounted and set in resins) with reflected light. Examination using reflected light is now widely used. In analogy to minerals in inorganic rocks, the microscopic components of coal, differing in their form, texture, structure and so on when examined under a microscope, are called macerals. Macerals also differ in their physical/chemical properties. Some macerals such as vitrinite/huminite, fusinite and semi-fusinite have their origin in woody and cortical tissues [11]. For example, fusinite always shows a cellular structure, i.e. carbonised cell walls and hollow lumens, although the cell walls may occasionally appear as being crushed. It is sometimes believed that forest fires, for example caused by lightening, which "carbonised" the plant tissues, might have been a cause of fusinite. Some macerals such as sporinite, cutinite and resinite have their origin in plant materials other than woody tissues [11]. For example, sporinite represents the fossilised remains of spore exines. Resinite is the fossilised remains of plant resins. The origin of some macerals such as micrinite and macrinite has not been traced with certainty to a specific plant tissue [11]. There has been a lack of unified nomenclatures for coal macerals. van Krevelen [30] compared the nomenclatures for lignite/brown coal and hard/black coals recommended by the International Committee for Coal and Organic Petrology (ICCOP, formerly known as the International Committee for Coal Petrology, ICCP). While the liptinite and inertinite macerals have the same names in lignites and brown coals as those in hard/black coals, the vitrinite groups in hard coals corresponds broadly to the huminite group in lignites and brown coals.
structure and Properties
17
The Australian standard [31] states that "huminite is a synonym of vitrinite in lower rank coals" and therefore is not specially defined in the standard. The Standard considers that some macerals (textinite, texto-ulminite, eu-ulminite, attrinite, densinite, porigelinite) commonly or only occur in lower rank coals. Experimentally, the quantification of macerals in a coal sample is carried out using a polished particulate block or a polished block. In preparing a polished particulate block, pulverised coal particles are mixed with a binding agent, poured into moulds and formed into grain mounts. The grain mounts are ground to form a very flat surface for the exposure of an adequate number of sample grains (particles). The surface is then finely polished so that it is virtually free of scratches. Lump coal sections may also be mounted and polished. Standard methods for preparing coal sample for this purpose are available, such as Australia Standard AS 2856.1 - 2000 [32]. The block thus prepared is examined using a reflected light microscope and the macerals are identified under an immersion medium by their relative reflectance, colour, morphology and fluorescence characteristics [31]. The proportion (on volume basis) of each maceral or mineral is determined by a point-count procedure and more than 500 points are normally counted. In brown coals, the reflectance decreases going from inertinite to vitrinite/huminite and to liptinite macerals. A plot of reflectance versus maceral population is often made as a part of petrographic study of coal. When excited with lights of short wavelengths such as UV or blue lights, liptinites often show distinct fluorescence. George and Mackay [1] summarised the characteristic maceral composition of lithotypes. The light and pale lithotypes are characterised by high concentrations of detrovitrinite and liptinite and low concentrations of telovitrinite and gelovitrinite. Densinite, being a maceral from the gelified groundmass and thus an indication of gelification degree, is mainly confined to dark and medium-dark lithotypes. Kershaw and co-workers [13] analysed the petrographic data (based on the ICCP system) obtained by the former State Electricity Commission of Victoria (SECV), showing that, statistically, lithotypes differ in their petrographic composition. All humotelinite macerals as well as telogelinite, corpohuminite, the larger particles of liptinite macerals (suberinite, cutinite, and resinite) and inertinite macerals (fusinite and semi-fusinite) were referred to as "coarse tissue". There is an increase in coarse tissue from Pale to Dark and a concomitant decrease in liptinite content (dominated by sporinite and liptodetrinite). There is also a general increase in variation between lithotypes with decreasing coal age (coal age decreases in the order of Traralgon 2, Traralgon 1, Morwell 2, Morwell IB, Morwell 1A and Yallourn). While the liptinite contents vary little (around 5%) in lithotypes in the oldest Traralgon 2, the liptinite contents in lithotypes in the youngest Yallourn coal change from - 20% (Pale) to < 5% (Dark).
2.3. PHYSICAL STRUCTURE AND PROPERTIES OF VICTORIAN BROWN COAL From the petrographic description of Victorian brown coal given above, it is clear that the brown coal has not undergone extensive coalification that could only take place
18
Chapter 2
at elevated temperature and pressure for prolonged time under geological conditions. As a result, the coal seam contains large amounts of water, amounting up to 66% of its mass. The transformation of coal at this stage has conferred special physical structure and properties to the Victorian brown coal. For example, in its natural status in the coal seam, the brown coal behaves like a colloid and has its special rheological properties. On loosing its moisture, the brown coal becomes a porous solid, possessing all properties of a porous material. This section summaries briefly the special physical structure and properties of Victorian brown coal. A more detailed treatment of the literature on this topic before 1990 has been presented elsewhere [33]. 2.3.1. Victorian Brown Coal as a Colloid The plant remains in Victorian brown coal have undergone varying extents of transformation. Various entities, ranging in size from small molecules (e.g. fulvic acids), humic acids and small plant residues (e.g. spores) to large pieces of semi-transformed "woody" plant fragments, are dispersed in water in coal seam. Therefore, by the definition of a colloid being the dispersion of small particles of one substance in another, Victorian brown coal is a colloid. The stability of this colloid, i.e. as dispersion of particles in water instead of forming compact solid mass, is mainly due to the presence of large amounts of oxygen in the organic matter of brown coal. As will be discussed later in this chapter, carboxyl/carboxylate and acidic phenol groups are abundant in the brown coal. Surrounded by water, these (acidic) groups tend to dissociate at least partially to reach equilibria, making the coal seam mainly acidic, ranging in pH between 3.5 (Rosedale field of lower rank) to 6.5 (Bacchus Marsh field of higher rank) [33]. As in the case of a metal electrode immersed in a solution containing its ions, the dissociation of the acidic groups in the brown coal leads to the formation of an electric double layer [33]. The dissociation of the acidic groups makes the surface of the coal organic matter negatively charged. There will be an immediate layer of cations, the inner shell of the electric double layer, that tightly adhere to the coal organic matter surface. The cations in this layer include H^ from the dissociation of acidic functional groups as well as other cations such as Na^ that forms a part of carboxylates in the coal. This gives rise to a net charge for the coal matter and its tightly attached ion layer (i.e. the inner shell), termed as zeta potential. Away from the coal matter surface and beyond this firmly attached ion layer, there is a diffusion layer, the outer ionic atmosphere, in which cations tend to distribute more densely than the anions to eventually make the system overall neutral electrically. Unlike the cations in the firmly attached layer (the inner shell), the cations in the diffusion layer are not firmly "attached" to the coal organic matter; rather these cations move relatively freely due to the thermal motion. The inner shell of charge and the outer ionic atmosphere make up the electric double layer. The main role of the electric double layer is to confer kinetic stability. Colliding colloidal particles break up due to the double electric layers and coalesce only if the collision is sufficiently energetic to disrupt the layers of ions (and their solvating water
Structure and Properties
19
molecules). The particle-particle interactions in brown coal are governed by the opposing effects of the van der Waals attractive forces and the electrostatic repulsion forces between the particles [33]. The extent of acid dissociation in brown coal largely depends on the strength of the acidic groups, determining the/?// of the coal seam water (solution) in its natural status. The extent of acid dissociation can also be changed externally, shifting the acid dissociation equilibria. As the solution becomes increasingly acidic, less and less amounts of acidic functional groups would be dissociated. The zeta potential, reflecting the extent of charging for the system of coal matter plus the inner shell of ions, would also decrease. At a low enough pH value, very few acidic functional groups would be dissociated and thus the coal molecules would tend to be neutral. This/?// value is called the isoelectric point. For most of Victorian brown coals, the isoelectric point occurs at a pH value around 2 [33]. This rather low/?// value for the isoelectric point is a reflection of the strength of the carboxylic acids in the brown coals. The acidities of functional groups in Victorian brown coal will be further discussed in next section. 2.3.2. Alkaline Digestion and Humic Acids Victorian brown coal as a colloid can be peptised (partially solubilised) using alkali as a peptising agent. In other words, the solubility of brown coal in aqueous solution increases significantly with increasing pH value of the solution because increasing pH increases the extent of dissociation of acidic functional groups. Because of the varying strengths of acidic functional groups in brown coal (see Section 2.4.2), the exact amount of materials extracted with alkali solution would obviously depend on the pH value of the solution. At relatively low pH values, only carboxyl groups dissociate. At relatively high/?// values, both carboxyl and phenol groups would dissociate. However, increasing NaOH concentration above 0.5 M does not seem to extract much more materials from the brown coal [34]. Experimentally, the extraction of brown coal with alkaline solution separates the brown coal into three fractions: humin (insoluble in alkaline solution, i.e. the solid residue), humic acids (soluble in alkali solution and insoluble in acidic solution) and fulvic acids (soluble in both alkaline and acidic solutions). Fulvic acids are often treated as a part of humic acids. Fulvic acids are normally obtained by acidifying the alkaline solution after the extraction of brown coal, during which the humic acids would precipitate. Thus, fulvic acids are often of small molar masses and amount only to a negligible portion of Victorian brown coal when extraction is carried out at close to ambient temperature. However, heating brown coal to elevated temperatures (e.g. 250°C [34]) can produce additional amounts of fulvic acids, probably as a result of the breakage of weak bonds in the brown coal or slight oxidation by trace amounts of oxygen in the solution. Under conditions relevant to the extraction of humic acids, progress of chemical degradation should also be taken into account. A treatment of the brown coal in an alkaline medium, if performed while letting the suspension be exposed to air, would involve oxidation of the coal. Hayashi and co-workers [35,36] treated an acid-washed
20
Chapter 2
Yalloum brown coal in an aqueous solution of 0.5 M sodium carbonate at 20-85°C, into which atmospheric O2 was continuously bubbled. The mass fraction of solventextractable material in the treated coal increased with chemical O2 consumption. The increase in the extractability resulted from degradation of macromolecular network that was caused by oxidative decomposition of aromatic rings. For example, the treatment at 85°C for 12 h increased the solvent extractability of the coal from 14% up to 97% with an O2 consumption of 5.8 mmol-02/g-coal. Even in the absence of O2, the brown coal can also undergo chemical degradation. Kashimura and co-workers [37] investigated the extractability of Loy Yang brown coal in alkaline solutions in the absence of dissolved O2. Using a solvent-flowing reactor (a type of semi-batch reactor), it was found that the brown coal is completely dissolved into 0.01 M NaOH solution at 200°C. Kashimura and co-workers [38,39] also showed the progress of hydrolysis of functional groups such as ethers into hydroxyls during the heat treatment of Loy Yang coal in sub-critical water. The degradation through the hydrolysis as well as a solvent power of the alkaline solution was essential for the complete solubilisation of the coal. These results [34-39] suggest that it is difficult to define humic acid as an inherent alkali-solution-extractable material in the brown coal unless the above-described chemical degradation reactions are inhibited during the extraction. In addition to reactions taking place with the organic matter during the extraction of Victorian brown coal with an alkaline solution, reactions may also take place with the inorganic matter in coal. Pang and co-workers [40] found that Fe (likely in the form of HFe02") could be oxidised to produce H2 according to the following reaction: HFe02' + H2O + OH- = Fe04^- + 2H2
(R2-1)
2.3.3. Water as a Molecular Probe for the Physical Structure and Property of Brown Coal Victorian brown coal contains abundant oxygen-containing functional groups, making the coal hydrophilic. This is the primary reason for its water contents as much as around 65 wt%) on a wet basis. Therefore, water is an integral part of the brown coal in its natural status. Allardice and Evans [41], in their pioneer work on the drying of the Victorian brown coal, constructed isotherms relating the equilibrium water content and the water vapour pressure for Yalloum brown coal, up to the saturated water vapour pressure. They observed a "sigmoid" desorption isotherm that is typical of desorption of condensable vapours from porous adsorbents. Based on the generally accepted interpretation, they classified the water in Yalloum coal into four types of water: bulk water, capillary water, multilayer water and monolayer water. In general, desorption and/or adsorption isotherms for rigid porous solid material are analysed for measuring porosity and pore size distribution of the material. However, it is difficult to determine such properties for brown coal by analysing desorption/adsorption
structure and Properties -^
200
o o
I50h
21
100 h Q.
E 3
50 h a
o c o 0
50 wo 150 Volume of water lost ( cm^ /100 g dry coal)
I 200
\ 1 I tSO 100 50 Moisture content of coal (g/IOOg dry coal)
I 0
Figure 2.1 Volume losses as a function of volume of water evaporatively removed from Yalloum brown coal [42]. •—•, loss of lump volume, differential shrinkage. Region a, monolayer water; region b, multilayer water; region c, capillary water and region d, bulk water.
isotherms, because the coal shrinks and also swells in response to desorption and adsorption of w^ater (or polar solvent vapour), respectively. Evans [42] examined the change in volume of a cylindrical lump of bed moist Yalloum coal with the extent of drying. On drying of the Victorian brown coal, the reduction in its bulk volume is not identical with or proportional to the volume of water removed. As is shown in Figure 2.1, at the initial stage of drying, the bulk water is removed and the shrinkage is less extensive than expected from the volume of water removed. The bulk water is believed to have little bonding (e.g. hydrogen bond) with the coal organic matter and its removal leaves air-filled macropores. However, the shrinkage of coal accelerates with further removal of water (the capillary water) and the differential shrinkage, i.e. shrinkage in volume (cm^) per unit volume (cm^) of water removed, ranges from 0.7 to 1.9 cm^/cm"^. Differential shrinkages greater than 1 cm /cm indicate not only collapse of pores that held the capillary water but also loss of
22
Chapter 2
air-filled macropores that was created during the removal of the bulk water. In the multilayer region, the differential shrinkage shows a maximum (beyond 4 cm /cm ) and this is attributed to shrinkage or collapse of mesopores as well as further loss of macropores. As the region of the monolayer water is approached, the differential shrinkage drops to zero and may possibly be negative, i.e., swelling may occur in the monolayer region rather than collapse of micropores. It is thus evident that formation, shrinkage and loss of pores occur simultaneously or in sequence during drying. In recent years, the porous structure of moist (raw) coal and the interaction of water with the coal matrix have been studied extensively by employing the water itself as the molecular probe. Information on the porous structure of moist brown coal and its drying-induced change will be discussed here, based on the results of recent studies on the drying of the brown coal at temperatures lower than 373 K. More detailed discussion on drying, particularly at higher temperatures may be found in Chapter 3.
Pi
E
100
200
300
Temperature [K]
Figure 2.2 DSC thermograms of the coal samples and pure water. YL, Yalloum; LY, Ley Yang; MW, Morwell; SB, South Banko; BZ, Beulah Zap; WY, Wyodak; IL, Illinois #6 and BL, Blind Canyon. Reprinted with permission from Ref 43. Copyright 1998 American Chemical Society.
structure and Properties
23
In general, water retained in pores of solid materials with diameters smaller than 10 nm has properties that differ from those of bulk water in its normal thermodynamic states. In other words, water retained in such pores freezes at temperatures lower than 273 K, releasing latent heat smaller than 334 J/g. Figure 2.2 illustrates differential scanning calorimetry (DSC) thermograms for pure water and coals (including 3 Victorian brown coal samples) at a cooling rate of 8 K min'^ [43]. The peaks seen in the thermograms reflect exothermic processes, i.e., the congelation as the transition of water sorbed in the coals into ice. For each coal, peaks are seen around 258 K and 226 K. The peak around 258 K is due to the congelation of water having a property nearly identical to that of bulk water, while those around 226 K are attributed to the congelation of water condensed in pores with diameters less than several micrometers [44]. No exothermic peaks were detected at temperatures lower than 213 K. Figure 2.3 illustrates the relationship between the quantity of heat evolved, AH, and the water content for Yalloum coal [43]. The partially dried samples were prepared from the raw coal by drying at ambient temperature in a nitrogen atmosphere. AH decreases linearly with decreasing water content in the range from 1.3 to 0.6 g/g-daf-coal (Figure 2.3a), where the exothermic peak around 258 K diminishes with the extent of water removal. The slope, 333 J/g-water, is in good agreement with the congelation heat (heat of fusion) of bulk water, 334 J/g. Thus, water desorbed in this range is ascribed to the bulk water having no specific interaction with coal. For water contents ranging from 0.65 to 0.30 g/g-daf-coal, the peak around 226 K diminishes while AH decreases with a slope of 188 J/g. This peak is arisen from the congelation of water condensed in pores that is generally called "bound water". It is also noted that the sum of these two types of water accounts for 78% of the total water contents. This indicates the presence of another type of water that does not undergo the congelation as a primary phase transition; hence this type of water is referred to as "non-freezable water". The DSC analysis also revealed that desorption of the bulk, bound and non-freezable waters occurred successively on drying at ambient temperature.
300 (a) 250 200
I
• YL-Bound o YL-Free
150 100 50
333 [J/g watern
0.00.2 0.40,6 0.81.01.21.4 Water content [g/g-m.f.coal]
0.0 0.1 0.2 0.3 0.4 0.5 0.6 Water content [g/g-m.f.coal]
Figure 2.3 Quantity of heat generated by the congelation as a function of water content: (a) Yalloum (YL); (b) Beulah Zap (BZ), South Banko (SB) and Illinois #6 (IL). Reprinted with permissionfromRef 43. Copyright 1998 American Chemical Society.
24
Chapter 2 Nonfreezable CD O U
^
• /
15
•<»)!
E
S 10 • o E "
1 Bound^l
1
Tr
/
Free
„
j]
Oo <3
1
1
H
5 I....i...ii
) 10 20 30 40 50 60 70 80
(
iv[mol/k g-m.f. coal]
i [ 1 1 1 11» 11 n
1II1
CO
o o
E o E X
..i
0
" ' M ' " ' ! •1 1 1
15
' • • 7 1 • • • ' 1 • ' " J " i " 1 » • " > • • ' » 1 ' " ' T ' " !J
»—I 2 0
i
: (b)
^
10
cA/ 5
y
^AQX
o ^; A B2 1 a WY \
r J..LU. I l l
0
5
1 1 i i 1 t 1 tiA.A.1 i t l I I i l l
10
15
20
>• 1
25
30
iy[mol/kg-m.f. coal]
Figure 2.4 Changes in the content of mobile hydrogen (C^//) along with water removal, w is the residual water content, (a), Yalloum (YL); (b), Beulah Zap (BZ) and Wyodak (WY). Reprinted with permission from Ref. 47. Copyright 1998 American Chemical Society.
For determining the contents of three different forms of water by DSC, samples with different water contents must be prepared by partially drying the original sample. The drying will cause shrinkage and/or collapse of pores, thereby inducing the transition of one type of water to another. Norinaga and co-workers [43] examined the validity of DSC-based water classification by using another method, a ^H-NMR relaxation technique [45], that allowed the analysis of water in the brown coal without drying. The congelation of bound water as well as that of fi^ee water can be directly observed as the conversion of mobile water proton undergoing slow and exponential magnetisation decay into immobile or rigid proton experiencing Gaussian decay in the transverse spinspin relaxation. In the analysis of each original coal sample, proton transverse relaxation was measured using a solid-echo pulse sequence during cooling from 293 to 213 K. It was found that the amount of mobile proton of water decreased rapidly at 273 - 263 K while slowly at 263 - 213 K for every brown coal sample examined. The decrements in the former and latter temperature ranges on the basis of the mass of water agreed well with the amounts of the free and bound waters, respectively. The amount of proton mobile even at 213 K was in good agreement with that of the non-freezable water. The total content of the bound and non-freezable waters in the raw Yalloum coal, 0.65 g/gdaf-coal [43], was in good agreement with that of the capillary, multilayer, and monolayer waters in the same coal (0.72 g/g-daf-coal) reported by Allardice [46]. This agreement strongly suggests that the inherent pores filled with the water other than the bulk water in the raw coal are preserved during the loss of the bulk water. The amount of non-freezable water retained in the brown coal is in an order of 1 mol per mol of oxygen involved in the organic matrix of the coals [43], suggesting the direct interaction between this type of water and oxygen-containing functional groups via specific bonds such as hydrogen bonds. Norinaga and co-workers [47] examined changes in molecular mobility of coal macromolecules by means of ' H - N M R using a
structure and Properties
25
solid echo pulse sequence. They found that a portion of coal hydrogen (not including water hydrogen), about 15 mol/kg-daf-coal, was in mobile states at 298 K for Yallourn and Morwell brown coals. Figure 2.4 shows the content of mobile coal hydrogen, CMH» as a function of the water content (w) for Yallourn and other low rank coals. CMH decreased linearly with a decrease in the content of the non-freezable water at a rate of about 0.5 mol-H/mol-H20 while it remained nearly unchanged with the loss of the bulk and bound waters. This result indicates that a portion of coal hydrogen is mobilised in the NMR sense due to solvation of polar hydrogen-containing functional groups by nonfreezable water [48] and the desorption of the non-freezable water leaves the hydrogen immobile probably due to the formation of specific interactions between hydrogencontaining functional groups. Then, expecting that hydroxylic hydrogen is immobilised due to the loss of non-freezable water, Norinaga and co-workers [47] deuterated this type of hydrogen selectively and completely by exposing the coal to D2O vapour using a technique similar to those reported by Blom and co-workers [49] and Qian and coworkers [50]. They treated Yallourn coal in a sequence of complete drying, deuteration and swelling in liquid D2O. The drying reduced CMH from 15 to 1 mol-H/kg-daf-coal. The subsequent deuteration replaced all of the hydroxylic hydrogen (8 mol) with deuterium and reduced CMH to zero. However, CMH increased to 7 mol following the swelling in D2O due to the mobilisation of non-hydroxylic hydrogen. It was also found that CMH regained to 15 mol after the cycle of complete drying and swelling in H2O without deuteration. The above results reported by Norinaga and co-workers [47] demonstrate the followings: all of hydroxylic hydrogen (8 mol-H/kg-daf-coal) and some of other types of hydrogen are mobile in the presence of the non-freezable water. Methoxyl hydrogen is abundant in the brown coal [51] and they would be the most important nonhydroxylic ones that are mobilised by water. Mobilisation of methoxyl groups by water is in fact predicted from a molecular simulation of lignin-water systems [52]. Nearly all of this type of hydrogen is immobilised by removing the non-freezable water and remobilised by the subsequent swelling in water. Norinaga and co-workers [47] also reported that non-freezable water in matrices of a lignite and sub-bituminous coals mobilised hydroxylic hydrogen exclusively [47]. The total volume of pore-condensed water (bound and non-freezable waters) in asreceived Yallourn brown coal is 0.65 cmVg-daf-coal. This volume corresponds to half of the volume of the water-swollen coal. When partially or completely dried brown coal is exposed to water, it swells but often does not regain its volume prior to drying [33]. Such irreversible change in volume in the cycle of water removal and swelling was examined by Norinaga and co-workers [53]. Raw Yallourn coal was partially or completely dried at 303 K and then immersed in water at the same temperature. The effect of the predrying extent on the content of pore water after the re-swelling was investigated by means of ^H-NMR. Figure 2.5 shows the volumes of the non-freezable water (Fnf), bound water (Kf) and their total (Fp) after the re-swelling as a function of the water content after the predrying, w. Vp decreases with the extent of loss of nonfreezable in the predrying, which can be explained by an irreversible reduction of the bound water content. Fp is steady unless the non-freezable water is released. The
26
Chapter 2
predrying induced no irreversible changes in CMH and the density of the coal that was measured by a water pycnometry. The irreversible reduction of Vf may result from the decrease in the size of pores confining the pore water. It should also be stated that no irreversibility has been evidenced for the content of the non-freezable water that interacts directly with hydroxyl groups. As is predicted by the Gibbs-Thompson equation [54], the freezing point temperature, />, of water condensed in a micro space is a function of the space size. Hence, 7} of bound water could be converted to the size of pores where the water resides. For water confined in micro- and meso-pores of solid materials, TV is related to the pore dimension (diameter for cylindrical/spherical pores and width for slit-like pores) [55,56] as D=-
a - + 2fi 273.15-r
(2-1)
/
where cir is a constant depending on the type of porous material and can be determined analytically from the contents of non-freezable and bound waters. J3 \s the thickness of layer of water molecules that acts as a shield between pore surface and core of ice from bound water. Based on the above equation, a pore model was proposed with the following assumptions [53]: (1). Pores are cylindrical or slit-like in shape and they are filled with the core of ice from bound water with a diameter Dpc or thickness Dps and non-freezable water surrounding or sandwiching the core with a thickness of ^ as the interface shield between the core and the solid pore wall.
[Nonffreezable
Free
Bound
0.8 ^P CO
0.6 h
>^ 0.4
•" *
r^___^. A
p^ ^
>f^0.2
•
^
•
•
*
%
Vf
0.5 1 w [g/g-mf coal]
•
1.5
Figure 2.5 Volumes of non-freezable water {Vj,f\ bound water (V^) and their total (^'p) after a cycle of drying and re-swelling in water at 30 °C as a function of residual water content in Yalloum brown coal after drying. Reprinted with permission from Ref 53. Copyright 1999 American Chemical Society.
structure and Properties
27
(2). Since the molar ratio of non-freezable water to hydroxylic groups is about 2, the thickness of the non-freezable water layer is 0.6 nm corresponding to that of two water molecules. (3). Non-freezable water resides exclusively at the interface between the ice core and pore wall when the bound water is frozen. The average pore dimensions calculated by the model for Yalloum brown coal containing 0.64 g water per g daf coal were as follows: Dpc - 4.4 nm (cylindrical) and Dp, - 2.6 nm (sht-like) [53]. Regardless of the pore shape, the model can explain the irreversible decrease in the volume of pore water by that in the pore dimension, i.e., the shrinkage of pores. Norinaga and co-workers [53] estimated the distribution of the pore size for samples after drying/re-swelling cycle based on the observed distribution of 7} andEq.(2-l). Among the assumptions made in the pore model [53], the first and second ones were not experimentally verified, although they have been generally employed in the analysis of meso- and macro-porous ceramics sorbing water. Porous structures of solid materials sorbing water can be analysed by means of ^H-NMR employing a special pulse sequence, Carr-Purcell-Meiboom-Gill (CPMG) one [57,58]. The CPMG method can realise transverse relaxation of water proton with relatively long relaxation times by removing the effect of magnetic field inhomogeneity upon linewidth and reducing the diffusion term that is manifest in the spin echo sequence. The pore size distributions for porous ceramic materials have been estimated by analysing the relaxation characteristics of proton of water sorbed in the materials based on a theoretical relationship among the longitudinal or transverse relaxation time constant (T] or T2) for bulk water, that for pore water, and the dimension of pores [59]. The NMR analysis is thus expected to provide the size of pores in moistened coals and also allows the examination of the above-described pore model. A particular advantage of this technique is that the pore water can be characterised without letting the bound water as well as the bulk water freeze. Hayashi and co-workers [60] investigated the molecular mobility of water in the raw and partially dried Yalloum brown coal samples. The samples were subjected to HNMR analysis at 303 K taking a CPMG pulse sequence. They analysed the transverse relaxation signals from the samples and found the amounts of bulk water, pore water (bound water plus non-freezable water) and mobile coal hydrogen quantitatively. It was also found that the non-freezable water is non-separable from the bound water unless the latter freezes, supporting the first assumption of the model proposed by Norinaga and co-workers [53]. Hayashi and co-workers [60] further analysed the relaxation characteristics on the basis of a theory that relates the dimension of pore to the relaxation time of pore liquid [59]: ' +^^\^ ^2P
^2B
(2-2)
28
Chapter 2
0.4
0.8
1.2
1.6
w, ko-HzQ/kg-mf ooal
0.0
01
0.2
0.3
0.4
0.5
0.6
w, kg-HjO^tg-mf coal
Figure 2.6 Estimated pore dimensions d^c (under the assumption of cylindrical pores) and d^^ (slit-like pores) as a function of residual water content in Yalloum brown (YL) and Beulah Zap (BZ) coals. Reprinted with permission fi-om Ref 60. Copyright 2001 American Chemical Society.
V
d„.
S 2 —= y
for cylindrical pores
(2-3)
for slit-like pores
(2-4)
dps
where T2P and T2B are the relaxation times for the pore liquid and the bulk liquid,
29
structure and Properties
mobile hydroxyls
non-freezable water
bound water
o
Figure 2.7 A model of slit-like pores of moistened Yalloum brown coal. All of the hydroxylic protons are mobile unless the drying is so extensive as to remove non-freezable water. All of the hydroxylic groups are solvated by pore water at pore surface. Bound water is not distinguished from non-freezable water unless the bound water freezes. Reprinted with permission from Ref 60. Copyright 2001 American Chemical Society.
respectively, and T2S denotes that for liquid molecules being closest to the pore surface with a distance A thus interacting with the surface. For water molecules, A is usually taken as 0.3 nm. S and V are the volume and surface area of the pore filled with the liquid, respectively. Among the relaxation times, Tjs can be represented by T2 for the mobile coal hydrogen while T^p by T2 of the pore water. As reported by Norinaga and co-workers [47], the mobile coal hydrogen detected in the coal sorbing water is identical with hydroxylic protons that are solvated by water molecules probably with a distance of A via hydrogen bonding and therefore mobilised in the NMR sense. It is hence reasonably assumed that the hydroxylic protons undergo relaxation at the same rate as the neighbouring water molecules. As a proof of this, the mobile coal proton and non-freezable water proton could not be distinguished from each other when the mobile coal hydrogen became as abundant as the remaining pore water proton after a deep drying. Figure 2.6 exhibits the estimated changes in the pore dimensions, dpc (in the case of cylindrical pores) and dp,, (slit-like pores), upon drying for Yalloum and Beulah Zap coals, dpc - 5.6 nm and dps - 2.8 nm for Yalloum with a water content of 0.64 kg/kg-dafcoal may be compared with Dpc = 4.4 nm and Dp, ~ 2.6 nm reported by Norinaga and co-workers [53]. The pore dimensions estimated from the different methods are in good agreement by assuming the pores are slit-like rather than cylindrical or spherical in shape.
30
Chapter 2
The findings described in this section can be summarised in a structural model of moist coal (Figure 2.7) [60], in which hydrophobic and water-impervious phase and pore water phase are separated in a nanometer-scale. The micro-phase separation is not a special feature of the brown coal sorbing water. It has been demonstrated that Yalloum coal swollen in organic polar solvents such as pyridine consists of solvent-rich and solvent-impervious phases with sizes up to several nanometers [61]. 2.3.4. Pore Volume and Density It is clear from the aforementioned that, like any other coals, Victorian brown coal is a porous material. Therefore, the density of coal needs to consider the volume of pores inside the coal. "Apparent density" refers to the density of coal including its pore volume and "true density" of coal refers to the density of coal excluding its pore volume. Various displacement fluids may be used to determine the density of coal pycnometrically. Mercury and helium are often used. Mercury at low pressures, due to rather large contact angle (130 to 140°) between coal and mercury, is not able to penetrate into small pores. The radius {r, \im) of pore into which mercury can penetrate is calculated using the Washburn equation: 2/ . 62350 ,^ ^. r = —^cos^ = (2-5) P P where/? is the pressure (Pa), y is the surface tension of mercury (0.485 N/m) and 6 is the contact angle between coal and mercury, which is about 130°. At the normal pressure of 101 kPa, the radius r is about 6.2 ^m. The density determined using mercury at low pressure (e.g. 1 atm), for example, by following the Australian Standard AS 2434.4— 2002 [62], is often taken as the apparent density. Clearly, with increasing pressure, the radius of pores into which mercury can penetrate would decrease. This means that experiments at different pressures can give a plot of pore volume versus pore radius. In practice, the experimental data at very high pressures need to be corrected by considering the compressibility of coal [64]. On the other hand, helium, being the smallest and inert molecule, thus having negligible adsorption, can penetrate into pores even smaller than 0.3 nm [64]. The density determined using helium as the displacement fluid is generally considered as the true density. The pore volume and porosity of the brown coal can be calculated from its apparent and true densities: Pore volume (cm'^/g) =
(2-6)
Porosity (%) = f l . ^ l x l O O
(2-7)
structure and Properties
31
where dy{^ and d^e are the densities determined using mercury and helium as displacement fluids, respectively. The main problem with the determination of brown coal density using mercury or helium as displacement fluids is that the coal normally needs to be dried prior to measurement. As was discussed above, the complete removal of water would cause changes in the porous structure of brown coal. This means that the densities thus measured pycnometrically do not always represent those for the coals containing water. Although the errors introduced due to the shrinkage associated with the removal of some residual moisture are normally small for most applications, sometimes they are too big to be ignored. Yost and Creasy [63] pointed out that, due to the shrinkage of coal on drying, the density from helium displacement could not be used to calculate the volumetric concentration of suspensions of brown coal in water. Instead, the true coal density should be determined from liquid pycnometry tests using the relevant fluid and coal pre-treatment. A similar approach has also been outlined by Woskoboenko [33] where the true density of coal is calculated from the coal-water slurry density and the water content of the slurry. 2.3.5. Surface Area and Gas Adsorption Victorian brown coal has abundant pore structures with a wide range of sizes including macropores (>50 nm in diameter), mesopores (2-50 nm), micropores (0.4-2 nm) and submicropores (<0.4 nm). While the larger pores contribute significantly to its pore volume and porosity, the smaller pores are often more responsible for the large surface areas of Victorian brown coal, often exceeding 200 m^ g"'. There have been several alternative methods to determine the surface area of brown coal, based on the capacity of brown coal to adsorb gases or vapours. The surface area is calculated based on certain assumptions about the way these molecules adsorb on coal surface. When a liquid like methanol is adsorbed onto the coal, heat will be released. This is referred to as heat of wetting, which reflects the interactions between the coal surface and the vapour. The earlier studies have used the heat of wetting to determine the surface area of coal. Methanol was often used for this purpose because its heat of wetting could be released rapidly. To quantify the amount of molecules adsorbed on the pore surface of coal, a necessary assumption is that the heat of wetting is a constant. Extensive survey of the early literatures by Krevelen [64] and Woskoboenko and coworkers [33] has indicated that this assumption is wrong. There is a significant interaction between coal and the polar adsorbate methanol. In addition to the van der Waals forces, the strong hydrogen bonding between methanol and the oxygen functional groups in brown coal contributes significantly to the heat of wetting. In fact, the heat of adsorption has been used as an important way to probe the surface structure of Victorian brown coal. Wang and co-workers [65,66] recently studied the adsorption of methanol on Yalloum brown coal in comparison with that on a Japanese bituminous coal (Akabira) using flow microcalorimetry operated at both continuous and pulsed modes. For continuous flow microcalorimetric measurement, methanol
32
Chapter 2
concentration in n-hexane was from 2 to 20 g L'' to minimise the swelling of coal by methanol. They found that a methanol concentration of 2 g L'^ was enough for methanol to occupy all the sites in coal and increasing methanol concentration did not lead to any significant increases in methanol adsorption. While the Yalloum brown coal and the bituminous coal had similar adsorption amounts of methanol per unit weight of coal (about 55 mmol g'^), the heat of equilibrium adsorption of methanol on a unit weight of Yalloum brown coal (210 J g'), measured with continuous flow method, was much higher than that (70 J g'^) of the bituminous coal. This is clearly because of the polar nature of the brown coal surface. It was concluded that the adsorption of methanol on Yalloum coal included both strong chemisorption (including hydrogen bonding) and weak physisorption. The heat of adsorption of methanol on Yalloum coal was then measured using pulse injection method, as is shown in Figure 2.8 (left). It changes with the coverage of methanol on coal, indicating the heterogeneity of sites for methanol adsorption. The differentiation of the data gives the distribution of molar heat of adsorption, as is also
Surface Coverage of Methanol (-) OS
1.0
1.5
2.0
2.5
1.0 ^'allourn Brown Coal
0.9 O.K 0.7 0.6 0..^ 0.4 0.3 0.2 0.0
0.5
1.0
1.5
2.0
Amount of methanol adsorptkm (mmol/g-coal)
1.0
1.5
,n.n,n,r
1.0
Surface Coverage of Methanol (-) 0,5
0.1 0.0
0.9
2.0
-
.\kubira Bituintoous Coal
0.8 t».7
0.6
-
o..*; 0.4 03 0.2 0.1
0.0
0.5
1.0
1.5
Amount of methanol adsorptiftn (mmol/g-coal)
2.0
J IJ l.n.n.^. Mi iZ
.—1
34 36 . ^ 40 42 44 46 4 8 5 0 5 2 54 56 58 60 Molar heat of .stronn adsorption ( b j / n i o l )
Figure 2.8 Molar heat of strong adsorption of methanol on Yalloum brown coal in comparison with that on Akabira bituminous coal. Reprinted with permission from Ref 66. Copyright 1997 American Chemical Society.
33
structure and Properties
0.24
0.28
Atomic 0/C ratio
Figure 2.9 Heat of wetting by water of raw and thermally modified Victorian brown coals (Morwell, Morwell preheated in nitrogen to 320°C for 1 h, Yalloum, Loy Yang and Yalloum North) with varying atomic 0/C ratios [67].
shown in Figure 2.8 (right). Clearly, even for the strong adsorption (mainly chemisorption), the heat of adsorption of methanol on Yalloum coal varies over a wide range, showing great difference from the bituminous coal. Water is another polar molecule used as a probe to investigate the surface structure of coal. Christie and Mainwaring [67] measured the heat of wetting of water for a series of unheated and preheated Victorian brown coal samples. They found that the heat of wetting was closely related to the presence of oxygen in coal, as is shown in Figure 2.9. Following a multiple linear regression of their data, they found the following correlation between the heat of wetting and the contents (meq g"^) of carboxyl, phenol and ionexchangeable cations Af^ in the coal samples: •AH^=35.O{M'''
+0.74[COOH] + 0.40[PhOH])+constant
(2-8)
where the constant represents the contribution of other components not included in this idealised equation. The heat generated by the cations is at the same order of magnitude as that by other polar functionalities in coal. A better way to determine the surface area is to directly quantify the amount of gas adsorbed onto the pores in coal. Among various gases/vapours tried, N2 and CO2 have been investigated most for this purpose. Nitrogen adsorption has been studied at low temperatures e.g. 77 K. The adsorption of nitrogen is described using the well-known Brunauer-Emmett-Teller (BET) model, assuming the multiplayer physisorption of nitrogen onto pore surface. On the contrary, CO2 adsorption has been studied at room temperature. The adsorption of CO2 is described using the Dubinin-Polanyi model. The surface area measured using nitrogen adsorption at 77 K is often lower than that measured using CO2 adsorption at 25°C. The possibility of coal contraction at low
34
Chapter 2
temperature [Allardice and Stacy, see Ref. 33], thus the reduction in pore size, cannot be entirely ruled out as a reason for the low surface area by nitrogen adsorption at 77 K. Equally, the swelling of coal by CO2 cannot be completely ignored. However, it appears, based on the calculation presented by van Krevelen [64], that the slow diffusion of nitrogen into the micropores at low temperature, i.e. non-attainment of true equilibrium of adsorption that would require too long to be achieved in practice, is the main reason that the surface area from nitrogen adsorption at 77 K is unrealistically low. The diffusion into the micropores is an activated process and requires the gas molecules to have certain energy to be able to penetrate into the micropores. "Low pressure adsorption of CO2 at room temperature combined with DubininPolanyi analysis is now the generally accepted method for the study of coal surface area" [64]. However, it must also be pointed out that the surface area measured by CO2 adsorption should only be treated as a better estimate of the true surface area (if such a definition of surface area exists, see below) of brown coal, for example, the assumptions in the Dubinin-Polanyi analysis must not be forgotten. A recent study by Unal and co-workers [68] has confirmed the preferential adsorption of water over those of nitrogen and CO2 by Loy Yang brown coal. The measurement of surface area (e.g. CO2 adsorption) or pore volume requires the drying of coal sample, which can cause irreversible changes to coal structure. Therefore, there is highly a need to study the porous structure of Victorian brown coal without the complete removal of water. Indeed, significant progress has been made in using water as a probe for the structure of brown coal (see Section 2.3.3). 2.3.6. Information from Small-Angle Scattering and Electron Microscopy From the discussion given above, brown coal is a structurally heterogeneous solid containing capillaries, cracks, open and closed pores. The use of small angle scattering [69-75] and transmission electron microscopy [75] has provided some valuable information about the nature and structure of the porosity. The length scales over which these techniques may be used are typically from the meso- to micro-porous regimes. The pore structure of coal has traditionally been described by Euclidean models that consider the pore structure of coal as a series of discrete pores in a continuous medium [75]. While these Euclidean models have been used to interpret measurements using small-angle X-ray scattering (SAXS), BET gas adsorption and mercury intrusion, they often become overly complex. This is particularly the case when considering that the pores inside coal are not necessarily regular in shape. Avnir and co-workers [76] have pointed out that the pore structure of coal, like many other solids, may not be adequately described by simple Euclidean models. Efforts have been made to use fractal concepts to describe the pore structure of coal. There are two important concepts used: self-similarity and fractal dimension. A structure is self-similar if it "looks the same" on all length scales, e.g. under all magnifications when observed under a microscope [72]. Normal Euclidean geometric measures of structure such as surface area, S, or mass density, p, will scale according to the length of the probe used to measure the structure and thus will not be constants. The exponents in
structure and Properties
35
these scaling relationships are termed as fractal dimension [72]. For a surface fractal, S = Ny-''^
(2-9)
and for a mass or volume fractal, (2-10)
P^POR""'"-'
where r is the radius of the yardstick used to measure the surface area and R is the radius of the sphere over which the density is sampled. D, is the surface fractal dimension and D^ is the mass fractal dimension. The following limits apply: 2
and
0
(2-11)
Physically D^ is a measure of the surface roughness and Dm is a measure of the nonuniformity of mass distribution. NQ and p are the true non-Euclidean probe length independent measures of the surface extent and mass distribution. Hence, fractal geometry enables an irregular surface to be described in terms of two well-defined parameters, NQ and D, (or D^). D,—^ 2 for an open smooth pore surface. A increases with increasing surface roughness. D,-^ 3 for a volume-filling pore structure with little open porosity. Such fractal analysis of small-angle X-ray scattering data involves fitting the data with a function of the form l{q)o^Aq-''
(2-12)
where I(q) is the intensity of scattered X-rays when the sample is illuminated with an ideal point source, g represents the modulus of the momentum transfer vector (nm"), A is related to the characteristics of the fractal, and a is related to the geometry of the material and for porous solid, a = 6-D, (surface fractals)
2
cc = D^ (volume/mass fractals)
(2-13) (2-14)
For small angle scattering, the following relationship holds [70]: /(^) = /o^-^
(2-15)
Ds {Dm) can be obtained from the log-log plot oflvsq (scattering angle). Reich and co-workers [70] showed the log-log plots of scattering intensity versus scattering angle to be linear over a wide range of scattering angles for both wet and dry
36
Chapter 2
Table 2.4 Comparison of D^ determined for a suite of samples prepared fi*om the thermal treatment of Loy Yang brown coal using ultra-small-angle neutron scattering (USANS) and small-angle X-ray scattering (SAXS) (from Refs. 7 1 , 74 and 75). Treatment USANS q range used to temperature (°C) fmdD,(nm-')'
from USANS'
from SAXS ^
200
in CO/H2O atmosphere for raw coal 0.00617-0.0234 2.63 ±0.21
2.28 ± 0.07
300
0.00617-0.0234
2.13 ±0.09
2.33 ±0.2
325
-
-
2.25 ± 0.07
350
0.00982 - 0.0224
2.31 ±0.23
2.34 ±0.11
375
0.00982 - 0.0224
2.47 ± 0.22
2.39 ±0.09
400
-
-
2.39 ± 0.09
425
-
-
2.46 ± 0.04
300
in H7 for Cu[ impregnated coal 2.28 ±0.2 0.0065 - 0.0677
2.35 ± 0.08
325
0.0065 - 0.0677
2.21 ±0.2
2.37 ±0.06
350
0.01234-0.0361
2.61 ±0.25
2.79 ±0.03
375
0.01234-0.0361
2.89 ±0.23
2.76 ± 0.03
400
0.01234-0.0361
2.74 ±0.22
2.76 ± 0.04
425
-
-
2.86 ±0.01
300
in CO/H2O atmosphere for acid-washed coal 0.00651-0.051 2.15 ±0.21
340
0.00651-0.051
2.04 ± 0.22
1.96 ±0.11
375
0.00651-0.051
1.94 ± 0.2
1.95±0.12
425
-
-
2.03 ±0.10
1.88±0.12
in N2 for raw coal
2.47 ± 0.07
25^^
205
0.00708 - 0.06498
2.40 ±0.21
2.37 ±0.07
300
0.00708-0.06498
2.32 ±0.21
2.24 ± 0.07
350
0.00708 - 0.06498
2.24 ±0.2
2.21 ±0.07
375
0.00708 - 0.06498
2.32 ±0.2
2.21 ±0.08
400
0.00708 - 0.06498
2.21 ±0.21
2.27 ± 0.08
2.32 ± 0.06 425 2.25 ±0.22 0.00708 - 0.06498 a. The USANS results are from Ref 75. b. The SAXS results are from Ref 71 that thj used a q range of 0.0203 - 0.367 nm"'. c. Untreated Loy Yang coal, from Ref 74.
structure and Properties
37
Victorian brown coal, indicating that their pore structures could be described using fractal concepts. For wet bed-moist brown coal, the SAXS data are consistent with a mass fractal ofD^ - 2.5. This observation is in agreement with the fact that the pores in coal are filled with water whose density is different from that of coal mass. On drying in air, the SAXS data are consistent with a surface fractal of Ds - 2.6. This observation indicates that the pores in the dried coal are filled with gas on the removal of water. Johnston and co-workers [71 ] investigated the changes in pore structure of Loy Yang brown coal when thermally treated, as is shown in Table 2.4. While the pyrolysis of Loy Yang raw coal in N2 at low temperatures (< 425°C) does not lead to significant changes in D„ thermal treatment of Loy Yang raw coal in hydrogen-rich environment leads to increases in D/. the pore surface roughened and the extent decreased. However, the thermal treatment of acid-washed Loy Yang coal even in hydrogen-rich environment does not lead to changes in A , apparently signalling the importance of metallic species in the development of pore structure during the processing of Loy Yang coal. As the roughening of pore surface may be due to the release of gases, the changes in D^ may be a reflection of the influence of metallic species on the thermal decomposition of coal. In that study, the absolute value of/ was not measured and only relative values of TVo could be obtained. These conclusions have largely been confirmed by ultra-small-angle neutron scattering (USANS) [75], as is shown in Table 2.4. The high-resolution transmission electron microscopy (TEM) [75] observation supports the results fi*om USANS and SAXS measurements. The rapid improvement of the X-ray and neutron experimental facilities is likely to further shed light on the pore structure of Victorian brown coal and its products.
2.4. CHEMICAL STRUCTURE AND PROPERTIES OF VICTORIAN BROWN COAL 2.4.1. Inorganic Matter in Victorian Brown Coal A detailed account of inorganics in Victorian brown coal has been presented by Brockway, Ottrey and Higgins [77]. The inorganic matter in Victorian brown coal includes minerals and non-mineral inorganics. Minerals exist in the brown coal at very low concentrations, except near overburdens and interseam sediment layers [77]. The main minerals reported in Victorian brown coal are quartz and a variety of clays [77]. Under the acidic conditions in coal seam, there are no or little carbonate minerals present [77]. In fact, Ohtsuka and Asami [78] found that kneading Loy Yang brown coal and CaCOs in water was sufficient to break down CaCo3 to ion exchange Ca into the brown coal at room temperature. The non-mineral inorganics are present in Victorian brown coal as carboxylates and salts dissolved in the bed moisture. Strictly speaking, the inorganics such Na^, Ca^^ and Mg^^ bonded to the -COG' groups are an integral part of carboxylates forming part of coal organic matter. The presence of these carboxylates confers special properties to the
38
Chapter 2
Victorian brown coal and is a major consideration in the utilisation of Victorian brown coal. This will be discussed further in more detail in Section 2.4.2 NaCl is always present in Victorian brown coal. The chlorine K-edge X-ray absorption near-edge structure (XANES) spectra of Loy Yang brown coal (> 60 wt% moisture) showed great similarities to that of NaCl solution, indicating that NaCl is dissolved in bed moisture [79]. On losing moisture, NaCl would be expected to interact with the coal organic matter strongly. One distinct feature of non-mineral inorganics in Victorian brown coal is their ability to migrate through the coal bed when coal is being dried [77]. The relative rates of ion movement due to the evaporation of water from the coal surface are in the order of: Cr > Na^ > Mg^^ > Ca^^ > Al'^ > Fe'^ Decreasing pH tends to speed up the ion movement. The detailed reasons appear to be unclear, but the repeated ion-exchanging processes (see Section 2.4.2) with the coal organic matter acting as exchange media are believed to be important [77]. This is somewhat similar to the processes taking place inside an ion chromatographic column. Interactions with other functional groups or other physical forces may also be important. The movement of ions also appears to be consistent with the distribution profiles of these ions in the coal seam, particularly their enrichment at the top and/or close to interseam sediments. These may have resulted from the upward movement of water through coal, driven by the evaporation of water at surface from the overburdens [77]. The presence of inorganics, particularly non-mineral inorganics, in Victorian brown coal is responsible for the notorious fouling/slagging problems during its combustion. The quantification of non-mineral inorganics has traditionally been carried out by leaching the coal with acid followed by the analysis of the leachate, for example, by following the Australian Standard AS 2434.9-2000 [80]. Efforts have been made [81] to develop a new technique for the elemental analysis of coal, suitable for the on-site analysis of coal. Known as a laser plasma spectrometer (LPS), the technique uses laser induced breakdown spectroscopy (LIBS) to undertake simultaneous elemental analysis of many elements including low atomic number elements (hydrogen to sodium) that are hard to analyse using other instrumental techniques such as X-ray fluorescence spectroscopy. 2.4.2. Functional Groups and Ion-Exchange Properties Compared with higher rank coals, Victorian brown coal features relatively low content of carbon and high content of oxygen. This can be seen clearly in Table 2.3. The nitrogen and sulphur contents (normally < 1 wt% for the coal seams currently being mined) are very low (see Chapter 6). While the carbon content of Victorian brown coal rarely exceeds 71 wt% of the organic matter, its oxygen content often exceeds 25 wt%. As is shown in Table 2.3, even the pale lithotypes contain around 22 wt% oxygen in their organic matter. Oxygen exists in Victorian brown coal in a wide variety of functional groups and the high oxygen
39
structure and Properties
content has conferred very special properties to Victorian brown coal. As was discussed in Section 2.3.1, the colloidal properties of Victorian brown coal are largely because of their carboxylic groups, in acidic or carboxylate forms. The high oxygen content is also the fundamental reason for its high moisture content (also see Chapter 3). The high oxygen content and the presence of a wide range of oxygen-containing functional groups are always a major consideration in the utilisation of Victorian brown coal. 2.4.2.1. Oxygen-Containing Functional Groups and Their Quantification A large number of analytical methods have been used to quantify the oxygencontaining functional groups in Victorian brown coal. These methods can be briefly classified into two categories: chemical and physical methods. The former relies on
900
eooh
700H
> E
600 UJ
500h
iiOOh
2
3 ^ 5 Tiirant vol. ( mi )
6
Figure 2.10 Non-aqueous titration curves for low-rank coals from various localities [84].
40
Chapter 2
chemical reactions involving these functional groups while the latter uses modem physical instrumental techniques. A comprehensive review of the literature in this area prior to 1990 has been given by Schafer [82]. In what follows, the main analytical methods used to identify and quantify each functional group in Victorian brown coal will be discussed. A.
Wet Chemical Methods for the Quantification of Acidic Functional Groups
The main acidic functional groups in Victorian brown coal are carboxylic groups and phenolic groups. Carboxylate groups are also important. Non-Aqueous Titration. One of the earlier methods to quantify the carboxyl groups in Victorian brown coal has been non-aqueous potentiometric titration [83,84]. In a strongly basic solvent, carboxylic acids behave as strong acids with phenols somewhat weaker. The titration is normally carried out in a C02/H20-free environment, e.g. in a dry box flushed with N2 although a trace amount of H2O can be tolerated in the solvent. The coal sample suspended in anhydrous ethylenediamine (EDA, H2NCH2CH2NH2) is magnetically stirred; the difference in potential between two antimony electrodes (one acting as the reference electrode) is measured [83] until the system approaches equilibrium. Titrant (sodium aminoethoxide) is then added in small increments slowly while the potential is recorded: Ar,R-COOH + NaOCH2CH2NH2 -^ Ar,R-COONa + HOCH2CH2NH2
(R2-2)
Ar-OH + NaOCH2CH2NH2 -^ Ar-ONa + HOCH2CH2NH2
(R2-3)
where Ar and R represent aromatic and non-aromatic groups respectively. Other titrants such as n-Bu40H in pyridine [85] may also be used. A titration curve of potential versus titrant volume can then be constructed. Typical examples are shown in Figure 2.10. Inflection points on the titration curve (or peaks in a derivative curve) are considered as endpoints. Normally, two endpoints are observed corresponding to strong (-COOH) and week (phenolic -OH) acidic groups. Thus, the method may be used to determine the total acidity (-COOH and phenolic -OH) of coal. However, it should be borne in mind that the inflection points only indicate the relative strengths of the acidic groups. Some coals show 3 inflection points (Figure 2.10). Moreover, structures containing multiple phenolic groups (e.g. ortho-diphenols [86]) may show acidities stronger than some carboxylic groups, depending on their positions relative to each other as well as relative to other functional groups. Even the non-acidic oxygen functional groups can decrease tht pKa value of phenolic groups to the range of 1 - 5 [87]. In a polyelectrolyte (such as low rank coal [87]), large changes in pKa can occur because of the effects of neighbouring acidic groups [88]. Thus, the distinction of -COOH and -OH groups based on the inflection points does not have a sound theoretical ground. A major advantage of this method is that the titration is carried out in an organic solvent in which coal swells and the accessibility of the functional groups is improved.
41
structure and Properties
This advantage is however quite limited for brown coals due to their limited swelling in the solvent. This method has been widely used to quantify the acidic groups in coal and coalderived products. However, a recent study by Clemow and Jackson [89] has signalled the danger of using this method to quantify the acidic groups in coal. They found the method to give unacceptable scatters when applied to a suite of Victorian brown coal samples, although the method gave satisfactory results for simple model compounds. Firstly, the persistence of a solid phase (coal) throughout the titration process means a slow exchange process between the solid coal and the liquid phase, leading to extremely slow achievement of equilibrium. Secondly, the presence of solid coal also means that irreversible and erratic adsorption of ions on the solid coal may perturb the potential and distort the titration curve [89]. As Clemow and Jackson [89] pointed out, this method works well only if the analyte is dissolved in the solvent, which is in fact a fundamental prerequisite for any good non-aqueous titration. Ion-Exchange Method. Another method to quantify the -COOH groups in brown coal is through ion exchange, taking advantages of the reactions: (R2-4)
n Ar,R-COOH + M"^ -^ M(COO-Ar,R)n + n H^
y •X >'X
7h
^^\o^
7 8 9 Equilibrium pH
13
Figure 2.11 Extents of ion exchange as a function of equilibrium/?// [91]. 0, acid-group content calculated from Ba content; x^ acid-group content calculated from decrease in Ba(0H)2 concentration (total [Ba^^] constant at IN).
42
Chapter 2
Various ion-exchanging procedures differing in the metallic ion (M" ) used, the ionexchanging conditions or the species quantified (e.g. the formed acid or the cation M" uptake) have been used. Schafer [82] has discussed the historical development in this area. The method developed by Schafer [90-92] has been widely accepted. Briefly, to determine the total carboxyl content of a coal, the coal sample is ion-exchanged with barium in a buffer of pH 8.3 consisting of barium chloride, triethanolamine and hydrochloric acid. The carboxyl content of coal is then calculated from the amount of acid required to fully reconvert the coal to its acid form according to the following reversible reaction: Ba(Ar,R-C00)2 + H"^ -> 2 Ar,R-COOH + Ba'2+
(R2-5)
As some carboxyl groups may be exchanged with triethanolamine rather than barium [90,92], quantifying the acid used to reconvert the coal into acidic form gives better results than quantifying the barium being exchanged into the coal in the above reactions. A detailed description of the experimental procedure has been given by Schafer [92]. A slightly different method uses barium hydroxide [91] or barium acetate [92] as the ion-exchange agent. The choice of a pH of 8.3 to carry out the ion-exchange process has been based on a study on the extent of exchange (cation uptake) as a function of pH. As is shown in Figure 2.11 for Ba uptake by coal in BaCl2/Ba(OH)2 solution, an inflection point in the /?//range of 8 to 8.5 was taken to indicate the completion of the exchange of carboxyl groups and the start of the exchange of phenolic groups. A pH around 12.6 to 13 corresponded to the full exchange of the phenolic groups. Again, this distinction between phenolic and carboxylic groups is based on the acidity and there is no clear evidence that the phenolic groups in coal always have lower acidities than the carboxylic groups.
~ 250 mg coal db
Stir 30 min
Filter
50/60 ml Barium Solutbn in
Rinse Fitter Cake
250 ml Flask
Add10ml0.2MHCI
Stir 30 min
Fitter Titrate a 20ml alkjuot using 0.005M NaOH The endpoint is at pH 5.00
Rinse Filter Cake with 3 X 10ml Distilled H2O Wash filtrate into 100ml Volumetric Flask
Figure 2.12 Flow diagram of newly modified Schafer acid determination procedure [87].
structure and Properties
43
This ion-exchange method can be appHed to both raw coal and acid-washed coal. The content of carboxylates can be determined separately by measuring the acid required to convert the carboxylates into the acidic forms. A study by Schafer and Womat [93] suggested that acid-washing the coal prior ion-exchanging with barium would greatly improve the accessibility of carboxyl groups in coal. This ion-exchange method for quantifying the carboxyl content in coal has recently been improved by Allardice and co-workers [87]. Figure 2.12 shows a schematic flow diagram for the modified Schafer's method. Briefly, about 250 mg coal is ionexchanged with either 60 ml of BaCl2/triethanolamine/HCl buffer (for carboxylic acid determination) or 50 ml of 0.4 M BaCb/O.l M Ba(0H)2 buffer (total acidity determination) under vacuum for 30 min. The mixture is then filtered and the filter cake rinsed with 3 x 5 ml of either distilled water (carboxylic acid determination) or BaCl2/NaOH solution (total acidity determination), before being collected and stirred under vacuum with 10 ml, 0.2 M HCl (rather than HCIO4 in Schafer's original method). The acid mixture is then filtered and rinsed with 3 x 10 ml distilled water and the filtrate made up to 100 ml. The resulting solution is finally titrated with 0.05 M NaOH. This modified method may be quicker, cheaper and safer than the original one [87]. A trace amount of methanol (<0.3 vol%) may be added as a swelling and wetting agent [87]. Direct Titration with NaOH. Murakami and co-workers [94] quantified the carboxyl content of Loy Yang brown coal based on titration with NaOH. Briefly, about 250 mg of coal sample is immersed in 50 ml of 0.1 M NaOH solution to convert all acidic functional groups into sodium salts. The solution with coal is then back titrated with 0.5 M HCl. A blank experiment without coal is also carried out. The difference in HCl titre at each pH between titration curves with and without the added coal specimen is attributed to the amount of alkali bonded to the acid sites. The difference at pH 8.3 is assumed to be the amount of total carboxyl groups because it is assumed that all carboxyl groups would be exchanged with Na at/?// 8.3 as discussed above. This NaOH titration method is similar to the traditional ion-exchange and titration method, for example using NaHCOs [51,95]. The main improvement is that the solution containing ion-exchanged coal is titrated directly without separation. The titration of solution after filtering out the ion-exchanged coal could cause potential errors due to the solubilisation of coal in the alkali solution. Clearly, the distinction between -COOH and -OH in both cases is again based on the traditionally perceived acidities of-COOH and -OH without considering the possible acidity enhancement of -OH groups by other neighbouring functional groups. This NaOH titrafion method gives -COOH contents similar to the barium exchange method [94] for Loy Yang coal samples treated at < 100°C. However, it gives higher carboxyl contents than the barium exchange method for Loy Yang coal treated at higher temperatures (< 300°C) [94]. This may be due to the difference in the size between Na^ and Ba^^ and/or their accessibility to the acidic sites. Borohydride Method, Based on experimental studies on coal swelling, Aida and coworkers [85,96-98] developed a new method to quantify the carboxyl and phenolic
44
Chapter 2
groups in coals of varying rank. The method is based on the reactions of these functional groups in coal with various borohydrides in pyridine to form H2 that can be quantified relatively easily, e.g. with a gas burette: Ar,R-OH + MBH4 — Ar,R-OM + BH3 + H2
(R2-6)
Ar,R-COOH + MBH4 -^ Ar,R-COOM + BH3 + H2
(R2-7)
where M represents Li, Na, K, n-Bu4N, etc. The distinction among the various functional groups is based on their reactivities (kinetics) towards various borohydrides. Within a limited reaction time (e.g. < 60 min), LiBH4 can be used to determine the total acidic groups (carboxyl and phenolic groups) while n-Bu4NBH4 can be used to determine the carboxyl groups [98]. The reactions of H2O and alcohol with the borohydrides are very slow. The method has some potential merits. It is relatively simple: the reaction with a borohydride proceeds quantitatively in a single step to produce H2. The reaction in pyridine is irreversible and gives better accessibility to the functional groups inside the swollen macromolecular network in coal although brown coal may swell only to a limited extent. When the method was applied to a wide range of coals ranging in rank from brown coal to low-volatile bituminous coal [98], it gave results comparable to those from ^ CNMR. However, while the result for brown coal based on this method is as expected, the results for bituminous coals based on this method are several times those from the traditional methods (e.g. the ion-exchange method). The development of this method might have posed serious questions about our understanding of the chemistry of high rank coals [98]. Quantification of-OH Groups. Phenolic -OH groups are often quantified from the difference between total acidity and carboxyl content, as was discussed above. Direct -OH quantification can also be carried out by the traditional acetylationhydrolysis method proposed by Blom [99]. Following the esterification by refluxing with acetic anhydride in pyridine Ar,R-OH + V2 (CH3CO)20 -^ CH3C00Ar,R + '^ H2O
(R2-8)
and the evaporation of solvent, the acetylated sample is saponified with baryta: CH3COOAr,R + V2 Ba(0H)2 -^ Ar,R-OH + V2 Ba(OOCCH3)2
(R2-9)
Acidification with phosphoric acid produces acetic acid that can be distilled and quantified: Vi Ba(OOCCH3)2 + V3 H3PO4 -^ 1/6 Ba3(P04)2 + CH3COOH
(R2-10)
structure and Properties
45
Instead of baryta, Ca(0H)2 can also be used and the acetate can be quantified directly with ion chromatography [51]. B. Pyrolysis as a Method for Quantification ofAcidic Functional Groups Schafer [82] found a near 1:1 relationship between the total acid group content measured using the barium exchange method and the total amount of CO and CO2 evolved during pyrolysis at 900''C. Schafer believed that carboxyl groups produced CO2 and that phenolic groups produced CO. Clearly, this is an over simplification of the pyrolysis chemistry (see Chapter 4). It should not be considered as a reliable estimate of oxygen-containing functional groups in Victorian brown coal. Pyrolysis in combination with other analytical techniques such as pyrolysis-gas chromatography-mass spectrometry with in situ methylation [100] have been used to gain some information about the oxygen-containing structures in Victorian brown coal. C Spectroscopic Methods for Acidic Functional Groups NMR Spectroscopy. Nuclear magnetic resonance (NMR) spectroscopy has long been used in the study of Victorian brown coal structure [27], including the efforts made to quantify the acidic functional groups. Pang and co-workers [34] recorded CP/MAS '^C-NMR spectra of three Victorian brown coals, their humic acids and humins. Each spectrum is grouped into three major regions for the integration purpose: 220 - 170, 170 - 77 and 7 7 - 0 ppm, corresponding to carboxyl/carbonyl, aromatic and aliphatic carbons respectively. Absorption by ketone/aldehyde structures at -210 ppm is generally small. They found that humic acids generally contained higher concentrations of carboxyl groups than the corresponding raw coals and humins, as is shown in Table 2.5. Attempts were also made [101] to use ^^C-13 CP/MAS and H-1 combined rotation and multiple pulse nuclear magnetic resonance spectroscopy (CRAMPS) to identify carboxylic acids. Murata and co-workers [51] deconvoluted their SPE/MAS ^^C-NMR spectra into 16 peaks, as is shown Table 2.6. However, they considered that their quantification of COOH and C=0 groups were not very accurate due to low signal-to-noise ratios. Combining their chemical analysis and ^^C-NMR, they reported the oxygen distribution for Yalloum coal: 1.9 R-OH, 9.1 Ar-OH, 2.9 Ar,R-COOH, 8.6 - O - and 5.7 C=0 groups per 100 carbon atoms in coal. Yoshida and co-workers [102] used 24 peaks (also shown in Table 2.6) to deconvolute their CP/MAS '^C-NMR spectra, which gave very good curve fits to the spectra of 3 Yalloum lithotypes. Fluorination of coal (e.g. with diethylaminosulphur trifluoride, (Et)2NSF3, [103]) can convert primary, secondary and tertiary alcohols into alkyl monofluorides, aldehydes and ketones into geminal difluorides and free carboxylic acids into acyl fluorides. The fluorine-containing structure can then be analysed using '^F-NMR. The methods may provide detailed information about the 0-containing functional groups in coal such as the detection of aromatic/aliphatic carboxylic acids and primary and secondary alcohols.
46
Chapter 2
There remains much to be done for the quantification of acidic groups by NMR spectroscopy. In addition to improving signal-to-noise ratios, it is still not clear if all carbons would have equal sensitivity. It is well known that -OH of phenols, carboxyls and alcohols in coal can be converted into -OD selectively and quantitatively only by exposing coal to D2O vapour at a reduced pressure and at a temperature lower than 100°C. ' H - N M R can detect all of the protons regardless of their mobility (in terms of the proton magnetisation decay) while it cannot detect deuterium at all. Therefore, by comparing the signal intensity arisen from the protons between the original coal sample and the deuterated sample, the fraction of OH proton to the total proton can be quantified. Norinaga and co-workers [47] analysed original and deuterated brown coal samples by means of ^H-NMR, and determined the total content of-OH groups in Morwell and Yalloum coals as 6.8 and 8.1 mol-OH/kg-daf-coal, respectively. The amount for Yalloum coal is in good agreement with that determined by Murata and co-workers [51], 7.7 mol-OH/kg-daf coal. FT-IR Spectroscopy. Fourier transform infrared (FT-IR) spectroscopy is another technique used often to characterise the structural features of Victorian brown coal. Supaluknari and co-workers [104] used FT-IR to quantify the oxygen-containing functional groups in 28 Australian coals and their liquefaction products. FT-IR spectra were obtained using KBr pellets (1 vv^% of coal). The hydroxyl groups of both phenolic and carboxylic structures contribute to the peak at 3200 cmV A good correlation between FT-IR spectral intensity at 3200 cm'' and the total acidic oxygen determined by the non-aqueous titration method was observed.
Table 2.5 Estimates of proportions of different carbon types in brown coal humic acids and humins (alkali digestion at 25°C with 0.5 M NaOH) by '"^C CP/MAS NMR [34]. Q ^ Bacchus Mash Raw coal Humic acid Humin Loy Yang Raw coal Humic acid Humin Yalloum Raw coal Humic acid Humin
12.7
14.9
13.1
'
Carboxyl
Aromatic
Aliphatic
12.8
57.8
29.4
15.8 11.4
44.6 52.1
39.7 36.4
10.5 15.5 9.7
56.1 62.6 43.9
33.3 21.9
13.9 15.1
53.5 65.8
32.7 19.2
9.3
50.1
40.7
46.5
47
structure and Properties
The spectra in the carbonyl stretching region (1850 - 1500 cm"^) were curve-fitted with 7 bands at 1760 cm'^ (R-COOAr), 1735 cm"^ (R-COOR', Ar-COOAr' ArCOOR), 1700 cm"' (-COOH, Ar-COOR), 1670 cm"' (ketonic structure), 1620 cm'\ 1570 cm"' and 1500 cm"' [104]. The latter three bands are associated with the aromatic ring stretching vibrations. The integrated area of each component band has been used as a measure of the amount of the relevant functional group, assuming that the absorptivities
Table 2.6 Peaks for the deconvolution of '^C NMR spectra of coal. Yoshida and co-workers [102] Peak position, Half width, ppm
Peak assignment
Murata and co-workers [51] Peak position, Half width, ppm
L-30~H-10 11.3 18.8
<30 <12 9
CH3
13 20
10-12 10-12
L24 -^ H26 L30 ~ H35 38.9 49
7 <10 15 7
CH2
31 40
11-13 11-13
L54 ~ H56 62.5 L72 ~ H75 80 84 89
<10 <10 <10 9 7 4
R-0
56 70 93
16-18 16-18 16-18
100 105 L114~H116 124
<10 5 <13 <12
Ar-H
Ar-C or Ar-H
101 113 126 140
16-18 16-18 16-18 16-18
131 L140~H141
12 <12
Ar-C
L145~H146 L150~H153 160
<12 <14 <14
Ar-0
153 167
15-16 15-16
175 180
15 15
COOH
178 187
12-15 12-15
200 L250 ~ H300
15 <20
C=0
202
12-15
OCH3
R-OH R-O-R
48
Chapter 2
(extinction coefficients) for the absoq^tion band are uniform across the samples studied. This is rather approximate and thus the results should only be considered as semiquantitative. Murakami and co-workers [94] analysed heat-treated Loy Yang coal samples by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. They deconvoluted DRIFT spectra at wavenumber range fi-om 1500 - 1800 cm' into 7 peaks. The absorption at 1710 cm"' was used as a measure of the amount of carboxyl groups. For thermally (< 300°C) treated Loy Yang brown coal, the DRIFT spectra gave similar response to the NaOH titration method, while much higher than the barium exchange method. Acidic oxygen-containing functional groups show specific FT-IR absorption arisen from O-H bond stretching at 2400 to 3600 cm'. IR absorption within this band should therefore be a direct measure of OH content. Moreover, the O-H bond stretching absorption band provides information of hydrogen bonds occurring between O-H groups. However, it seems difficult to obtain quantitative FT-IR spectral information about O-H bond stretching by using conventional KBr pellet methods or even DRIFT techniques with KBr as the diluent of coal sample. Firstly, the main difficulty comes from a requirement of complete removal of moisture from not only coal particles but also KBr pellets (particles) and the gas phase [105]. Secondly, it is impossible to prepare two pellets (or thin fixed-beds) with and without coal particles under identical conditions and therefore to obtain the difference spectrum with a proper baseline. Thirdly, it is essential for both techniques to grind coal and KBr particles to sizes less than wavelength of infrared light. The second and third difficulties can occur in any IR absorption bands. Miura and co-workers [106,107] presented a DRIFT spectroscopy technique by which most of the problems associated with KBr pellet methods and conventional DRIFT were resolved. They claimed that not only the content of OH groups but also distribution of hydrogen bonds between them can be quantified by using this technique. Further discussion will be given later in this chapter. XPS Spectroscopy. X-ray photoelectron spectroscopy may also be used to get some information about the oxygen-containing functional groups on coal surface [108], although little has been done to apply this technique to Victorian brown coal. D. Other Oxygen-Containing Functional Groups While earlier studies (as summarised by Schafer [82]) did not find strong evidence for the presence of alcohols in Victorian brown coal, some recent study [51] found the presence of alcoholic groups in low concentrations (1.9 R-OH groups per 100 carbon atoms) in a Yalloum coal sample. The total oxygen present as -COOH and -OH groups accounts for less than 70% of the oxygen in Victorian brown coal [51]. Other functional groups such as ethers (e.g. furans, diaryl, alkylaryl and dialkyl ethers) and carbonyls also exist in Victorian brown coal, although they are much less well studied than -COOH and -OH groups. Murata
structure and Properties
49
and co-workers attempt the evaluation of ethers "reactive" towards sodium iodide and silicon tetrachloride: Ar,R-0-Ar',R + ^ S i - 1 -> ^ S i - 0 - A r ' , R ' + Ar,R-l
(R2-11)
(hydrolysis) -^Ar,R-OH + A r \ R - O H The degree of ether bond cleavage is evaluated by the increases in Ar-OH and R-OH that can be analysed e.g. by wet chemical analysis. For a Yalloum coal sample studied, they found about 1.8 reactive ethers per 100 carbon atoms in the coal [51]. Carbonyl groups can be estimated using wet chemical methods (conversion to oxime through reaction with hydroxylamine or to phenylhydrazone through reaction with phenylhydrazine) or spectroscopically [102,104] with FT-IR or NMR spectroscopies (see Table 2.6). As is shown in Table 2.6, methyl (-CH3) and methylene (~CH2-) groups can be relatively easily quantified from the NMR spectra of coal. Analysis of coal-derived products (e.g. pyrolysis tars) is an important way to understand the functional groups in coal. For example, Nelson [109] found the presence of long chain (up to C30) aliphatic moieties in Yallourn brown coal by analysing its tar. 2.4.2.2, Other Functional Groups An array of modem analytical techniques, particularly XPS, NMR and XANES, have been used to study the nitrogen functionalities in coal. Nitrogen exists in brown coal in a wide variety of forms, including pyrrolic nitrogen, pyridinic nitrogen, pyridonic nitrogen, "quaternary nitrogen" and arguably a trace amount of amino nitrogen. A detailed discussion of coal-N will be given in Chapter 6. Both wet chemical methods, thermal methods and modem instrumental methods have been used to study the sulphur functionalities in coal. Thiophenes, sulphides and disulphides are the dominant forms of sulphur in Victorian brown coal. A detailed discussion of coal-S will be given in Chapter 6. 2.4.2.3. Ion-Exchange Properties One of the most prominent features of Victorian brown coal is the presence of significant amounts of alkali and alkaline earth metals (AAEM) associated with the carboxylic and phenolic functionalities in the coal structure [77,110]. The metallic species such as Na, Mg and Ca in the coals can be easily removed by washing with acids. Individual cations (e.g. Na^ or Ca^^) can then be ion-exchanged onto the coal stmcture. As was discussed above, the ion-exchange property of Victorian brown coal has been used as a method to quantify its acidic (-COOH and -OH) functional groups. Although the AAEM species generally account for less than 1 wt% of the raw coal, they play very important roles in the utilisation of the brown coals. For example, they strongly affect the pyrolysis, gasification, combustion and liquefaction behaviour of
50
Chapter 2
Victorian brown coal (see Chapters 4, 5, 6 and 8). In the future power generation processes based on the gasification of Victorian brown coal, the volatilisation of the AAEM species [e.g. 111-115] will probably cause severe problems for the operation of gas turbines due to the corrosion/erosion of the turbine blades. On the other hand, if these metallic species are retained in the char after pyrolysis, they can act as good catalysts for the subsequent gasification/combustion of the char [e.g. see Refs. 116 and 117]. The presence of ion-exchangeable cations also affects the less-common use of Victorian brown coal such as the production of novel carbon materials (fullerenes or nanotubes [118,119]). The ion-exchanging capacity is clearly an important consideration if Victorian brown coal is to be used as an adsorbent [120-122], for example, for the removal of heavy metals (also see Chapter 1). It is then not surprising that significant efforts have been made to understand the ionexchange properties of Victorian brown coal. This has formed an important part of the understanding of Victorian brown coal structure. A,
Dissociation Equilibria ofAcidic Functional Groups The ion exchange process is based on the following dissociation equilibria: Ar,R-COOH -> Ar,R-COO- + H^
(R2-12)
Ar-OH -> Ar-O" + H^
(R2-13)
At low pH values, ion exchange mainly takes place with the carboxyl groups to form carboxylates. At high pH values, phenolic groups will also be involved in the ion exchange process due to its acidic nature, particularly considering coal as a solid polyelectrolyte (see Section 2.4.2.1). The dissociation of carboxyl groups can be described as [123,124]
[Ar,R-COO-H [Ar,R-COOH] where K^ - acid dissociation constant(s) of carboxyl groups; [Ar,R-COO] = "molar concentration" of dissociated carboxyl groups per 1 g coal (mmol g"' coal); [Ar,RCOOH] = "molar concentration" of nondissociated carboxyl groups per 1 g coal (mmol g"^ coal); [H^] - molar concentration of H^ per 1 g of coal (mmol g'' coal). However, it should be pointed out that, because coal is a solid or gel, the above equation is only an approximation of the acid dissociation equilibria for the carboxyl groups. The mass balance on the carboxyl groups gives [Ar,R-COOH}r =[Ar,R-COOH]+[Ar,R-COO"]
(2-l'7)
51
structure and Properties
where [Ar,R-COOH]T = "molar concentration" of total carboxyl groups per 1 g of coal (mmol g'^ coal). The "concentration" of ion-exchanged carboxylates can then be described as (2-18) Ka+[H^] If all carboxyl groups in coal had the same acid dissociation equilibrium constant (e.g. pKa - 6.0), calculation would indicate that the amount of dissociated (i.e. exchanged) carboxyl groups would increase sharply at pH - 6.0, as is shown by the dotted line in Figure 2.13 (left). Clearly, the dotted line does not fit the experimental data for Loy Yang brown coal dried at 50°C under vacuum (LY50H) or Loy Yang brown coal heated at 200 and 250°C for 2 hr in N2 (LY200H and LY250H). The discrepancy reflects the fact that the carboxyl groups have different acidity in brown coal due to the difference in their chemical environment in the coal molecules. Hayashi and co-workers [125] also pointed out the need to consider the distribution of acidity of carboxyl groups in Victorian brown coal. Murakami and co-workers [123,124] attempted to describe the experimental data by assuming the distribution of acidity (pKa) to follow the Gaussian distribution with a mean/?A^^ of ju and half-width of w:
—I
1
r
o LY50H • LY200H A LY250H
1
o
'
1
—
'•"
1 "
•
'
0.8
H
LY50H 0.6
^
>^LY200H ^ N J
CO cH
o o
0.4 ' y/y;>^
LY250H
'^ J
0.2 2
4 6 Equilibrium pH
n
_L
.
4
1
.
6 pKa
Figure 2.13 Left, the extents of exchange of Na into Loy Yang brown coal samples as a function of/?// as determined by NaOH titration [123]; right, calculated distribution of the acidity ipK^i) in the samples [123]. A dotted line in the left panel indicates the result of calculation based on pK^ 6.0 and sohd lines represent the calculation based on SipKa of 5.8 and half-width of 4.2, 4.6 and 4.9 for LY50H (Loy Yang heated to 50°C), LY200H (heated to 200°C) and LY250H (heated to 250°C) respectively
52
Chapter 2
Table 2.7 Calculated acidity distribution for heat-treated Loy Yang brown coal sample (based on Murakami and co-workers [126]). Heat treat temperature N2, °C 50
Group A
Group C
Group B
III
0.85
l o i a i /V/
P^a.i
M
P^a,i
Ni
P^aJ
3.81
0.92
5.19
0.72
6.83
2.49 1.83
250
0.67
3.79
0.71
5.40
0.45
6.25
300
0.40
3.45
0.58
4.94
0.24
6.29
1.22
325
0.34
3.42
0.32
5.33
0
-
0.66
350
0.23
3.44
0.15
5.10
0
-
0.38
400
0.20
3.37
0
-
0
-
0.20
[Ar,R-COOH]. r
1
'"
[Ar,R-COO-J= | -
V mi^
xexp -41n2
10'+10
f-ju
xlO"' -dt
-pH
(2-19)
The total carboxyl groups in that particular set of coal samples heated at 50, 200 and 250°C were determined to be 4.0, 3.4 and 3.1 mmol g ' coal respectively [94]. Curvefitting the data in Figure 2.13 (left) to the above equation gave a constant value of mean pKa as 5.8 and half-widths as 4.2, 4.6 and 4.9 for treatment temperatures of 50, 200 and 250°C respectively. The solid lines in Figure 2.13 represent the model prediction, appearing to agree reasonably well with the experimental data. They concluded [123,124] that, while the total amount of carboxyl groups decreased with increasing temperature, the mean pK^ value remained unchanged. Their calculated distribution of acidity (pKJ is also shown in Figure 2.13 (right). More recently, Murakami and co-workers [126] re-examined the acidity distribution in a set of heat-treated Loy Yang brown coal samples. Rather than assuming the acidity (pKa) in a brown coal to follow a Gaussian distribution as discussed above, they used several Gaussian distributions to describe the acidity distribution [126]. The acidity distribution function is expressed as the sum of several Gaussian distributions: -0.5
(pH-pK,j
(2-20)
where A^, is the number of carboxyl groups, a, is the standard deviation. Starting with 10 Gaussian functions, the deconvoluting procedure found only 4 Gaussian functions to be sufficient to describe the experimental data: average pKa values of 3.8 (Group A), 5.2 (Group B), 6.8 (Group C), and 8.3 (Group D). Groups A, B, and C are assigned to carboxyl groups while the assignment of Group D is yet unclear. Table 2.7 summarise
53
structure and Properties
the results for the heat-treated brown coals. It is seen that the heat treatment appears to decompose the weaker acidic groups to larger extents than the stronger acidic groups. B. Extents of Cation Exchange Effects of pH. Clearly, pH is one of the most important parameters influencing the extent of cation exchange onto brown coal. Increasing pH will lead to enhanced dissociation of the carboxyl (and phenolic) groups and therefore enhanced extent of cation exchange. This has been confirmed experimentally by many researchers [94,121,122,] although various cations may not follow the same trend [94]. Lafferty and Hobday [121] warned that buffer solutions themselves, while giving a stable pH environment, often provide a source of cations that might be involved in the ion exchange process: the buffer cations increase the ionic strength of the solution and may also compete for the exchange sites in coal, distorting the overall cation exchange characteristics of the coal 121]. Therefore the use of a buffer should be avoided when investigating the extent of cation exchange as a function oipH. Effects of Cation Charge/Size Ratio and the Stability of Cation-Carboxyl Association. The charge/size ratio (charge density) of the metal cations affects the extent of cation exchange. Lafferty and Hobday [121] demonstrated this by exchanging three Victorian brown coals with Na^, Ni^^ and Al^^ as is shown in Table 2.8. Although the solution/?// with A1(N03)3 was lower than with Na2S04 and NiCl2, the extents of ion exchange with Al^^ were higher than with Na^ and Ni^^. In fact, H^ is one of the cations having strong affinity to the carboxyl groups in Victorian brown coal: at [H^] concentrations higher than 10""^ to 10"^ M {pH < 3), H^ is the dominant form of cation exchanged onto the brown coal (acid-washing being effective in removing all other cations from brown coal). A more detailed study [122] showed that the selectivity of ion exchange followed the Irving-Williams order of Mn^^ < Co^"^ < Ni^^ < Cu^"^ > Zx\^\ Stuart [120] found that the selectivity of cation exchange was enhanced by washing the Morwell coal with acid prior to ion exchange.
Table 2.8 Extent of ion exchange as a function of cation charge/size ratio [121]. Loy Yang
Yalloum
Yalloum North Extension
pH
Extent of ion exchange, meq g"^
pH
Extent of ion exchange, meq g'^
pH
Extent of ion exchange, meq g"^
Na2S04
3.39
0.02
3.74
0.07
5.97
0.06
NiCl2
2.02
0.14
2.19
0.21
5.41
0.53
A1(N03)3
1.70
0.32
1.78
0.45
2.60
1.44
54
Chapter 2
.J
£ B
'
JL.
•
'
""T
"»" " I ' J
Exdiangeable amount of M^* 1
______
Mi
1 o
1
1.
J
o
"
B
o
o
A
A
oJ
O
Aj
5
A
A T3
1
^)
O A
Co'" Mg^*
•
Co2"+Mg^*|J
IJ IJ
P"
0.2 0.4 0.6 ' Initial Mg concentration (mol/1)
Figure 2.14 Influence of the initial concentration of Mg^^ on the extents of ion exchange of Mg^^ and Co^^ onto Loy Yang brown coal (dried at SOX under vacuum) [127]. The horizontal broken line indicates the maximum exchangeable amounts of divalent metallic ions at pH 6. The initial concentration of Co"^ is constant at 0.085 mol L"'.
In a more recent study, Murakami and co-workers [127] studied the relative selectivity of cation exchange between alkaline earth metal ions (Mg'^^ and Ca^^) and transition metal ions (Ni'^^ and Co^^). They found that the ability of cation exchange of Co^^ and Ni^^ was much higher than that of Mg^^ and Ca^"^, as is shown in Figure 2.14 for the Co'^VMg'^^ system at a pH of 6. At an equal concentration of 0.085 mol L'\ the extent of Co*^^ exchange was about 8 times that of Mg'^^. With increasing Mg^^ concentration, the amount of Mg^^ exchanged increased and that of Co^^ decreased. Based on the stabilities of the complexes between these cations and other simple chelating agents (ethylenediaminetetraacetic acid and trans-cyclohexane-1,2diaminetetraacetic acid), they attributed the high selectivities of Ni^^ and Co^^ to the high stabilities of the complexes formed between them and carboxyl groups in coal. Selectivity of Co^^ (or Ni^^) decreases with decreasing/?// [127]. This is because the coal structure becomes more "rigid" with decreasing pH as a result of enhanced hydrogen bonding. Transition metals (Co'^^ and Ni^^) have bond directivity compared with alkaline earth metals (Mg'^ and CsT^) [127]. The number of carboxyl groups accessible by Ni'^^ and Co^^ for ion exchange is reduced in the "rigid" structural configuration. An extended X-ray absorption fine structure (EXAFS) and FT-IR spectroscopic study [128] showed that Ni exchanged onto Loy Yang brown coal was likely to be octahedrally surrounded by six oxygen atoms. In addition to oxygen in carboxylates, oxygen as H2O, carbonyl and OH' were also coordinated with the nickel atom. Further studies [129-131] indicated that the exact configuration of Ni in Loy Yang brown coal changed with the Ni loading level. At a low Ni loading level (0.77 wt% Ni), nickel is in
55
structure and Properties :
• 1
I
t—I'-^1—,—1—J—,—p-
""-]
. m
H
|L 2h
o E U
[
in
J
-
AL
H
I
p I h|L h
4 6 Equilibrium pH
(a), exchange solution containing no ethanol
2
JrF
J
/vjf
J
^LYISOH^V/ ^^^^^-^--^.,7/
-^
^^^^Vjir
J
LY50H and LYIOOH
^#^r . 4
H
__!_._ I
6
f
pH
(b), exchange solution containing about 50 vol% ethanol.
Figure 2.15 The p//dependence of extents of cation exchange (ECE) in 10,000 ppm Ni^^of Loy Yang brown coal samples heated in N2 for 2 hr at 50°C (LY50H, drying), 100°C (LYIOOH), 150°C (LY150H) and 200°C (LY200H) [94].
a bidentate structure: two of the six oxygen atoms are derived from a carboxylate group and the other oxygen atoms from carbonyl groups, H2O and OH'. At a high loading level (6.4 wt% Ni) beyond the number of carboxylate groups, it becomes a bridge-type structure: one of the six oxygen atoms is derived from a carboxylate group and other oxygen atoms from carbonyl groups, H2O and OH'. Bocquet and co-workers [132] showed Mossbauer spectroscopic evidence indicating that Fe^ may be possibly octahedrally coordinated to a range of oxygen-containing ligands in coal, including O ', OH', water and carboxylates. Effects of Heat Treatment and Swelling by Organic Solvent. Heat treatment of brown coal would lead to thermal decomposition of carboxyl groups and cross-linking reactions. The removal of water also leads to changes in the physical configuration of the macromolecular structure. Murakami and co-workers [94] found that the extents of cation exchange with Ni^^ decreased with increasing heat treatment temperature, as is shown in Figure 2.15a. For each sample, there existed a characteristic pH value above which the extent of cation exchange increased rather steeply. These characteristic pH values shifted to higher values with increasing heat treatment temperature, partly due to the thermal decomposition of carboxyl groups and partly (more importantly) due to the changes in the accessibility of the carboxyl groups by cations. When ethanol was added into the ion exchange solution, the extents of cation exchange increased, particularly for the samples treated at higher temperatures (Figure 2.15b). Apparently, ethanol caused
56
Chapter 2
the heat-treated coal to swell for the carboxyl sites to be accessible by cations for ion exchange. The extents of cation exchange of the hydrothermally treated (in the presence of water) Loy Yang coal are higher than the samples treated in N2 [94]. The differences are at least partly due to the fact that the hydrothermally treated coal has high wettability by water than the coal thermally treated in N2: hydrothermal treatment might have led to hydrolysis [133]. The differences in the extents of cation exchange between the two types of coal sample reduced when the ion exchange was carried out in the presence of ethanol [94]. A more detailed study [123] showed the effects of ethanol on the extent of cation exchange to change with the nature of cation being exchanged onto Loy Yang brown coal: 1) For Li^, Na^ and K^, the extents of cation exchange in ethanol-containing solution were smaller than those in aqueous solution. This appears to be an artefact. The presence of ethanol in the exchange solution made the solution darker due to increased solubility of coal. When the residue was analysed for its Na (or Li, or K) content to determine the extents of cation exchange, low values were obtained for ion exchange in ethanol-containing solution. 2) For Mg^^, Ca'^^ and Ba^^, the extents of cation exchange did not change with the addition of ethanol. Ethanol did not significantly enhance the solubility of coal in alkaline earth solutions. 3) For Co^^ and Ni^^ of large hydrous ionic radii, the extents of cation exchange increased with the addition of ethanol, presumably due to the swelling of coal and improved accessibility of the carboxyl sites by the large cations. Effects of Pre-existing Cations. Murakami and co-workers [94] found that the extent of ion exchange with Fe^^ was similar for the acid-washed (H-form) and Na-exchanged (Na-form) Loy Yang coal samples under similar ion exchange conditions. However, when the Loy Yang raw coal (dried at 50°C under vacuum) and its Ca-exchanged (Caform) coal samples were exchanged with Co^^ and Mg^^, the Ca-form sample gave much higher extents of ion exchange [134], as is shown in Figure 2.16. Murakami and co-workers [135] also observed the extent of cation exchange with iron to increase linearly with the amount of Ca pre-exchanged into Yalloum brown coal. C Effects of Ion Exchange on Coal Structure Ion exchange is a chemical reaction. It brings about significant changes to coal structure. Replacement of H^ with a metal cation (M"^) results in cleavage of hydrogen bonds that are very important for the physical configuration of the macromolecular network in coal. The macromolecular structure is clearly affected by ion exchange. When multivalent ions are exchanged into the brown coal, a single cation can be bonded to two carboxyl groups (or other acidic functional groups). This requires the two carboxyl (or other acidic) groups to be (often through re-arrangement of the configuration of the
structure and Properties
57
w 0
Equilibrium pH Figure 2.16 Effects of pH on the extents of ion exchange of (a) Co^^ (1 wt% Co^^ solution) and (b) Mg^^ (1 wt% Mg^^ solution) for the Loy Yang raw coal (o) and Ca-form coal (•) samples [134]. The broken line indicates the precipitation for 1 wt% Co^^ alone.
macromolecular network) at certain configuration to suit the steric requirement of the cation. In addition to the above-mentioned experimental evidence from the measurement of the extents of cation exchange, many other experiments show evidence for the changes in brown coal structure upon ion exchange and are discussed below. Ion exchange with H^, Na^ and Ca^^ is often considered as a "reversible" process: brown coal can be acid-washed or ion-exchanged with H^ and metallic cations (e.g. Na^ and Ca^^) reversibly. However, the ion-exchange processes may bring about irreversible changes to coal structure. A Loy Yang brown coal sample was acid-washed (to give Hform-1 sample), ion-exchanged with Na^ (Na-form sample) and Ca^^ (Ca-form sample) and re-acid-washed (H-form-2 and H-form-3) to prepare a series of coal samples [111,136]. When these samples were pyrolysed in a wire-mesh reactor, the three H-form coal samples did not give the same tar and total volatile yields, indicating that they do not have the same structure [111,136]. The authors discussed the possible structural changes responsible for the changes in the pyrolysis yields. Firstly, during the ion exchange processes (particularly with Na^), a significant proportion of humic acid is dissolved and lost during filtration. Secondly, the ion-exchange process with Na^ has caused significant changes in the physical structure: some humic materials have been mobilised out of their original configuration in the macromolecular network. Following filtering and drying, the Na-exchanged coal could become a cake rather than individual particles. Clearly, the pore structure of coal has changed that affects the pyrolysis
58
Chapter 2
behaviour of coal. Thirdly, the ion-exchanged Ca^^ has become a cross-linking point (one Ca ^ is bonded with two carboxylate groups) and brought the macromolecular structure tighter. Much less humic materials were lost during the preparation of Ca-form coal sample. Even when Ca is replaced with H \ the coal could not recover its original physical configuration. The authors [111,136] concluded that the effects of ionexchanged Na on pyrolysis behaviour should be better assessed by comparing the pyrolysis behaviour of Na-form coal with that of H-form-2 coal (derived from the reacid-washing of the Na-form coal). Similarly, the effects of ion-exchanged Ca should be the differences in pyrolysis behaviour between the Ca-form and H-form-3 coal samples. Proton magnetic resonance thermal analysis measurement [137] showed the coal matrix densities for ion-exchanged Yalloum coal samples to follow the order: H-form < N-form < Ba-form = Ca-form. As was discussed above, ion-exchanged cations may not be just bonded to the carboxylate groups. They could also be co-ordinated with other oxygen-containing species (carbonyl, H2O, OH' etc). Therefore, there is a possibility that additional oxygen is brought into the coal together with the ion-exchanged cations. How these oxygencontaining species could behave during the utilisation of brown coal is yet unknown. 2.4.3. Macromolecular Structural Features 2.4.3. /. Solvent Extraction and Swelling It has long been recognised that a portion of coal can be extracted with solvent. This is attributed to the nature of coal that consists of solvent-insoluble macromolecular network and low molecular mass material, and also the primary basis of "host-guest" models of coal [138-142]. It is believed that occurrence of extraction, i.e., dissolution of low molecular mass material into solvent, requires penetration of solvent molecules into the matrix of coal, which accompanies swelling of the matrix. The solvent molecules have non-specific and/or specific interactions with coal molecules in the coal matrix. Non-specific interactions are represented by van der Waals forces. The solvation of coal molecules needs their similarity in "cohesive energy" to the solvent molecules. This similarity is essential for mixing of solvent molecules and coal molecules. It is well known that the Hildebrand solubility parameter [143], hereafter denoted by SP, is a quantitative measure for the cohesive energy, and the parameter has therefore been used for explanation or prediction of solubility and swellability of polymers in solvents on the basis of the regular solution theory [144]. SP is defined as the square root of cohesive energy density on volume basis: ^A//v./''
SP =
^m
(2-21)
J
where IS^H^ap and F^ are heat of vaporisation and molar volume of solvent, respectively. Table 2.9 shows SP values of typical solvents. It has been widely accepted that
Structure
and
59
Properties
Table 2.9 Solubility parameter and donor number of typical solvents. Solvent
SP ( c a p cm-'') ^
n-hexane cyclohexane benzene carbon disulfide diethylether dioxane acetone methanol ethanol acetonitrile tetrahydrofuran dimethylformamide dimethylsulfoxide pyridine water N-methylpyrrolidinone ethyl enediamine
7.0 7.0 8.9 10.3 7.4 9.9 9.9 14.5 12.8 11.8 9.1 12.1 12.0 10.7 23.1 11.3 12.4
DN [148] 0 0 0.1 19.2 14.8 17.0 19.0 20.0 14.1 20.0 26.6 29.8 33.1 33.0 27.8 55.0
a, calculated using the solvent properties found in Ref 145.
hydrogen bonds are the most important specific interaction that can be formed between coal and solvent molecules, while other non-covalent bonding interactions such as n-n (aromatic-aromatic) interactions [146] and charge-transfer [147] would also be important particularly for higher rank coals. A more detailed discussion on FT-IRderived information on hydrogen bonding in Victorian brown coal is given later. Electron donating capability or Lewis basicity is essential for a solvent to form hydrogen bonds with acidic OH groups. The electron donating capability is represented by Gutmann's donor number (DN) [148]. It is known that the electron donating capability is very well correlated with heat of mixing with phenols that have acidic OH groups [149] and hence with the Lewis basicity. DN is correlated with the heat of mixing with/7-fluorophenol (AHQ,JP) [150] as follows: A//^^^^^ =0.6835 + 0.3063(DiV)-0.0002775(Z)A^f
(2-22)
Thus, a stronger electron donor solvent can potentially form stronger hydrogen bonds with acidic OH groups in coal, breaking original intra-/inter-molecular hydrogen bonds. The solvation of coal molecules by solvent molecules through non-specific and/or specific interactions directly leads to the swelling of the macromolecular network of coal. The swelling will create diffusion paths for "solvated" relatively small coal molecules toward the external solvent phase. In this sense, the extent of swelling, if
60
Chapter 2
corresponding to that of expansion of the diffusion paths, would be a factor determining the yield and rate of extraction. The discussion below focuses on recent studies on the solvent extraction and swelling of brown coal. Solvent properties relevant to the specific and non-specific interactions mentioned above are believed to be main factors influencing the extraction yield and the extent of swelling. Characteristics of solvent swelling of bituminous coals are also referred for examining the particular features of brown coal as macromolecular solids. A, Solvent Extraction In general, less than 20 wt% of Victorian brown coal can be extracted by organic solvents unless the extraction temperature is above 150 °C or the brown coal has been chemically depolymerised [125]. Some increases in the extraction yield by acidwashing pretreatments were reported due to the breakdown of linkages between acidic functional groups via divalent cations such as Ca^^ and Mg^^. However, the increases are at most several wt%-daf [151]. It is therefore difficult to attribute the low extractability of brown coals mainly to the presence of ionic bonds (cross-links). Many attempts have so far been made for solubilising coal without breaking covalent bonds between aromatic ring clusters (chemical structural units), i.e., without chemical depolymerisation. O-alkylation [152], which replaces protons of acidic OH groups by alkyl groups, increases solubility of higher rank bituminous coals (C = 85 - 89 wt%) in solvents such as pyridine up to 50 - 70 wt% daf [153]. The increase in the solubility is believed to be due to the elimination of hydrogen bonds between OH groups and also steric effects of the introduced alkyl groups, the latter of which suppresses the polarisation force between aromatic rings causing n-n interactions. Much more hydrogen bonds occur in brown coals than in bituminous coals. Nevertheless, the Oalkylation seems to be much less effective on the solubilisation of brown coals compared with that of bituminous coals [153]. Thus, a most plausible explanation of the ineffectiveness of the O-alkylation is that covalently cross-linked macromolecular network is a major constituent of the organic portion of brown coals. Here are considered main factors determining the extraction yield from brown coal based on the results from the studies in which coals were exhaustively extracted at ambient temperature with sufficient amounts of solvents. Figure 2.17 compares the extraction yields from Loy Yang coal [154] with various solvents with those from a bituminous coal [155]. In Figure 2.17a [155] are shown the extraction yields from a bituminous coal with various binary mixtures, each of which is an equivolume mixture of carbon disulfide (CS2) and an electron donor solvent (EDS; DN> 17). The figure also shows the extraction yield with single EDS. SP of a mixture of solvents A (mole fraction/^/4) and B (mole fraction/^z^) is given by SPAB = {/VASP} + fvBSPl T
(2-23)
61
Structure and Properties
where SP^, SPB and SPAB are the solubility parameters of solvent A, solvent B and their mixture, respectively. It appears for the CS2-EDS mixed solvents that the extraction yield from the bituminous coal is correlated well with SP. The extraction yield seems to be maximised at iSP = 10.5 - 11 caf ^cm'^^. As stated above, SP is based on a regular
100
1
(a)
cr (0
'
• CS2-EDS O CS2-NMP a Single EDS
80
?;5
^^ •0
60
.9>
>* c
0 40
^ ^ V. *rf
UJ
20
n ^a^ °^. 10
12
14
-o—^
16
SP [caP-5cm-''-5] 25
— I
1
1
1
,
1
1
I (b) (0
"9
20
A Mixed solvent of NMP-EDS orNMP-non EDS O Mixed solvent of Methanol-NMP • Single EDS
10
12
14
16
SP [ca|0-5cm-''-5] Figure 2.17 Solvent extraction yields from (a) a Japanese bituminous coal (C: 86.7 wt%-daf) (based on the data in Ref 155) and (b) Loy Yang brown coal (based on the data in Ref. 154) at ambient temperature as a function of the solubility parameter of single or binary solvents used for the extraction.
62
Chapter 2
• 1MN/M«0H ATHF/M«OH oPh/MaOH /!^m-X/MeOH V2MN/III«0H IMW/EtOH
& §20 •^ 0
8 10 12 14 16 Solvent parameter, 5 ((cal/cm^)^'-^]
(a) Morwell coal oxidised in a H2O2 aqueous solution at 60''C for 2 h [156]
9
10
11
12
13
14
15
Hildebniiid Solubility Parameter [cal^'^cm'i-^]
(b) Acid washed Morwell coal before and after oxidation with O2 in a Na2C03 aqueous solution at 85°C for 12 h [36]
Figure 2.18 Extraction yieldsfromchemically depolymerised Morwell brown coal as a function of the solubility parameter of single or binary solvents used. Solvents for Figure 2.18b: (1) THF, (2) THF-methanol (9:1 v/v), (3) pyridine, (4) THF-methanol (8:2), (5) THF-methanol (7:3), (6) DMF, (7) THF-methanol (6:4), (8) THF-methanol (5.5), (9) THF-methanol (3:7), (10) methanol. Reprinted with permission from Refs. 156 and 36. Copyright 1997 and 1999 American Chemical Society.
solution theory by which interactions between solvent(s) and macromolecules in coal are considered to be entirely non-specific. Although not shown in the figure, the extraction yields for any solvents having no or little electron donating abilities were negligibly low. The presence of EDS is thus essential for significant extraction with CS2-EDS mixed solvents. Taken together with this fact, the apparent correlation seen in Figure 2.17a may be partly explained by the dissociation of inherent hydrogen bonds in the coal by EDS, which can be followed by the dissociation of non-specific interactions if the solubility parameter of the CS2-EDS mixed solvents matches that of the coal molecules or coal molecules carrying EDS molecules as adducts. However, the presence of EDS and the matching of SP are not sufficient for enhanced extraction. It is noted in Figure 2.17a that single EDS can extract much less material from the coal than CS2-EDS. Furthermore, lino and co-workers [155] reported that the use of solvents such as dioxane and nitrobenzene instead of CS2 resulted in much lower extraction yields in spite of the fact that these solvents have very similar SP to that of CS2. lino and co-workers [155] explained the essentiality of the use of CS2 by a significant reduction of the viscosity of NMP by adding CS2, resulting in enhanced diffusion of "extractable" molecules toward the external solvent phase. Probably only very small (short or thin) molecules such as CS2 are allowed to enter and penetrate EDS-solvated but tightly assembled macromolecules to breakdown the assembly.
Structure and Properties
63
Figure 2.17b shows three different sets of extraction yields from Loy Yang brown coal [154]: those with mixtures of methanol and NMP, those with mixtures of NMP and an EDS or non-EDS, and those with single EDS. It is seen that the extraction yield is correlated with the solubility parameter regardless of the presence of CS2, although there is an exception (DMF: SP = 12.1 caf^cm"^^). Thus, the use of an EDS and appropriate SP of mixed or single solvents may be sufficient conditions to enhance or maximise the extraction yield, and there seems to be no necessity of using CS2 as the co-solvent of EDS. Takanohashi and co-workers [154] analysed the extracts and residues of Loy Yang brown coal by FT-IR and found no significant difference between them. Mae and co-workers [156] depolymerised Morwell brown coal in an acid solution of an oxidant, H2O2, with a carbon loss and oxygen uptake as small as several wt%-daf They found that more than 80 wt% of the depolymerised coal was extracted with a mixture of methanol and 1-methylnaphthalene. This extraction yield was equivalent to that attained by using a strong electron donor solvent, dimethylformamide (DMF), 90 wt%. They further investigated the extractability of the depolymerised coal by using various mixtures of EDS (methanol, ethanol or THF) and non-polar aromatic solvents and found a good correlation between the extraction yield and SP of the mixed solvents. As can seen in Figure 2.18a, the maximum extraction yields were obtained with solvents having SP around 11.5 caf ^cm'^ \ Hayashi and co-workers [35,36] depolymerised Morwell and Yalloum brown coals by means of oxidation using O2 in an alkaline medium and investigated the extractability of the depolymerised samples with different extents of the depolymerisation. They found that the extraction yield was given as a function of SP of mixed or single solvents used when both solvents had DN > 20. Their results are shown in Figure 2.18b. They reported that the maximum extraction yield was reached at SP around 12 cal^^cm"^^ for any depolymerised samples while the maximum yield depended on the extent of the oxidation. This trend is in good agreement with that reported by Mae and co-workers [156]. It is noted that the extraction yield using pyridine (DN - 33.1) is clearly lower than those with mixtures of methanol and tetrahydrofuran (THF) possessing much lower DN (19 and 20 respectively). This indicates that excess electron donicity is not necessarily needed to maximise the extraction yield. In summary, the extractabilities of both brown coals and depolymerised brown coals are determined mainly by the solubility parameter of the solvent if it involves sufficient amount or fraction of EDS that has sufficient electron donicity for breaking original hydrogen bonds in the coals. B, Solvent Swelling Solvent swelling is deemed to be an essential step for extraction since the extraction needs creation of pores inside coal particle large or wide enough for "extractable" coal molecules to diffuse towards the external solvent phase. Pores of Yalloum coal are in fact expanded to a significant degree in the course of swelling [157]. In general, the extent of swelling is defined by the swelling ratio.
64
Chapter 2
One of the most typical methods for measuring the swelling ratio of coal was proposed by Green and co-workers [158]. Briefly, a fixed bed of powered coal is formed in a glass tube and the bed height, /?/, is measured after centrifugal compression of the bed. A sufficient volume of solvent is poured into the glass tube, and then the solvent and the coal sample are mixed well so that voids and bubbles of air are removed from the mixture completely. The height of the fixed bed is again measured after centrifugation. A cycle of stirring of the solvent/coal mixture (or shaking the glass tube) and measuring the height of the fixed bed (after centrifugation) is repeated until an equilibrium height of the fixed bed, /z?, is attained. The equilibrium swelling ratio, Q, (hereafter referred simply as "swelling ratio") is given by (2-24)
Q=-
Theoretically, Q is equal to the rate of increase in the total volume of coal particles only when the void fraction in the fixed bed before the swelling is the same as that after the swelling. Although not referred here, gravimetric measurement is more general than volumetric one in studying the swelling with solvent vapour. One of the events essential for causing swelling is breakage of hydrogen bonds between macromolecules. Moist brown coal is a typical example. As was described in Section 2.3, about a half of the volume of raw Yalloum brown coal is occupied by pore water, which solvates nearly all of coal OH groups [53]. However, the solvation of coal OH groups may not necessarily lead to the enhanced extraction, since little extraction
'
I
•
O MeOH -tetralin • MeOH-benzene
.S to O)
£
10
12
14
16
SPIcalO^cm-iS]
Figure 2.19 Swelling ratios of Yalloum brown coal in binary solvents of methanol-benzene or methanol-tetralin at 30 °C as a function of the solubility parameter (based on the data in Ref 160).
structure and Properties
65
0»
18 20 22 24 26 28 30 Solubility parameter [J^*^cm"l'^] Figure 2.20 Plots of swelling ratio versus the solubility parameter for Yalloum coal in binary solvents: methanol-benzene, pyridine-benzene and DMSO-benzene. Reprinted with permission from Ref 162. Copyright 2000 American Chemical Society.
occurs from any coal with water. Jones and co-workers [159] demonstrated that the swelling ratio of a German brown coal in water was even higher than those in acetone and methanol. Similarity in cohesive energy between the coal and solvent molecules, as indicated by the solubility parameter, seems to be another important factor for swelling as well as for extraction. Kawano and co-workers [160] studied the swelling of Yallourn brown coal in binary solvent systems. They found positive synergistic effects of mixing methanol with a non-polar aromatic solvent, benzene or tetralin, on the swelling ratio. As can be seen in Figure 2.19, the maximum swelling ratio is attained at SP of the mixed solvent around 13 caf^ cm"' ^ It is also noted that methanol-tetralin gave higher swelling ratio than methanol-benzene at the same SP of the mixtures. This implies particular interactions of coal macromolecules with non-polar aromatic solvents, which will be discussed later. Amemiya and co-workers [161] found synergisms, similar to those reported by Kawano and co-workers [160], for the swelling of Yalloum coal in mixtures of methanol (or ethanol) and tetrahydroquinoline (THQ). Their results are qualitatively explained by considering that SP of Yalloum coal is greater than those of benzene, tetralin and THQ but smaller than those of methanol and ethanol. Norinaga and co-workers [162] investigated the swelling of Yalloum brown coal in binary solvents (methanol-benzene, DMSO-benzene and pyridine-benzene) at a solventto-coal ratio of about 5 ml-solvent/g-coal. Figure 2.20 shows the swelling ratio for each system as a function of SP [162]. The largest value ofSP for each system corresponds to that of methanol (14.6 caFcm"'^), DM SO (12.8) or pyridine (10.7). The smallest value
66
Chapter 2
of SP corresponds to that of benzene (8.9 cal^'cm''^). The swelling ratio in methanolbenzene is maximised at SP of 12-13 caf "cm"^^, where the swelling ratio is much lower than that in DMSO-benzene. This clearly indicates that not only SP but also another solvent property are influencing the swelling ratio. A most plausible explanation for this result is that the breakage of original hydrogen bonds in the coal by methanol (DN - 20) is much less extensive than those in DMSO {DN - 29.8) and pyridine (DN =33). Norinaga and co-workers [162] also investigated the solvent-induced mobilisation of hydroxylic proton and other types of proton. They applied a ^H-NMR technique to distinguish "mobile" protons from "immobile" protons. Since solvent swelling causes self-associated molecular segments to gain significant configurational freedom, the protons involved in the segments are detected as mobile protons through ' H - N M R transverse relaxation measurements. In the course of the swelling of Yalloum coal in deuterated EDS (EDS-d; DMSO-d^, pyridine-ds) or mixed solvents of benzene-d^ and EDS-d, the transformation of protons
DMSO-de/benzene-de Hydroxylic proton CO
o u
i o c o
o a
Non-hydroxylic proton
•o 0)
eg !Q
Pyridine-d^benzene-dQ
o E c
Hydroxylic proton
3 O
E
<
Non-hydroxylic proton
15 Ms [mol/kg-coal]
Figure 2.21 Changes in the amount of mobile hydrogen in solvent swollen Yalloum coal in binary solvents (benzene-DMSO or benzene-pyridine) as a function of molar amount of EDS (DMSO or pyridine) per unit mass of coal (based on the data in Ref 162). Solvent to coal ratio: 5 ml/g-coal.
structure and Properties
67
from immobile to mobile states occurred due to the solvation of macromolecules, i.e. the replacement of inherent inter-/intra-molecular interactions by interactions between solvent and coal molecules. They applied a special technique of using pretreated Yalloum coal in which OH groups (8.1 mol-OH/kg-daf-coal) had been deuterated completely. This enabled the "masking" of the hydroxyls and the observation of the transformation of non-hydroxylic proton selectively. Figure 2.21 illustrates changes in the amount of mobile hydroxylic and nonhydroxylic protons as a function of the amount (Ms) of DMSO-d^ or pyridine-d6 [162]. These EDS-d were added to the benzene-coal mixture. No proton became mobile without adding EDS-d. By adding EDS-d to benzene-ds, both hydroxylic and nonhydroxylic protons were mobilised, while the formers were selectively mobilised when Ms was in the range from 3 - 5 mol/kg-coal, which was no more than the amount of OH groups in the coal (8.1 mol/kg-daf-coal). The selective mobilisation of hydroxylic protons may have resulted from the selective hydrogen bonding of EDS to hydroxylic protons. Addition of more than 5 mol/kg-coal EDS caused the mobilisation of nonhydroxylic protons much more significantly than that of hydroxylic protons. It should also be pointed out that 25% and 5% of hydroxylic protons remained immobile even in pure pyridine-ds and DMSO-d^, respectively. Norinaga and co-workers used 60 - 120 mol of solvent per kg of coal for its swelling in pure EDS, which was an order of magnitude more than the amount of OH groups in Yalloum coal, 8.1 mol/kg-coal. On the basis of the results reported by Norinaga and co-workers [162], it is clear that the swelling of brown coal seems to be contributed largely by the hydrogen-bonding capability of solvent even if it has DN> 20. The capability may be better represented by equilibrium constant (K^) for the formation of complexes between EDS and OH groups such as phenolic ones [163,164] than DN of EDS. At 298 K, K^ of DMSO with phenol is 124 and is greater than that of pyridine (29), while DN of DMSO (29.8) is smaller than that of pyridine (33). This may explain why the amounts of mobilised hydroxylic protons in DMSO-benzene were more than in pyridine-benzene at the same amounts of EDS. Norinaga and co-workers [165] proposed a simple model to relate the swelling ratio with the fraction of OH groups remaining immobile semi-quantitatively. The model was originally developed by Painter and co-workers [166,167]. They demonstrated that the extent of breakage of hydrogen bonds between coal macromolecules was an essential factor governing the swelling based on thermodynamic consideration. The synergistic effects of mixing EDS (e.g. methanol) with non-EDS (e.g. benzene) on the swelling ratio of brown coal are considered further below. Based on the reports by Kawano and co-workers [160] and Norinaga and co-workers [162], the addition of benzene to a methanol-swollen brown coal caused further swelling until SP of the methanol/benzene mixture reached a value of 12-13 cm^^cm"'^. Meanwhile, the addition of benzene resulted in the dilution of methanol, i.e. the reduction in the activity of methanol. This may result in decrease in DN of the mixed solvents and also the extent of solvation of coal OH groups by methanol. On the other hand, as described previously, tetralin-methanol mixture gave a higher swelling ratio of Yalloum brown coal than did benzene-methanol mixture at equivalent SP. This suggests that aromatic
68
Chapter 2
(«) j
i o n*AMqrlb«nitn« J A MdiiylDsimfMs m PAHS 1 o BnMton#d MiyiDMtnnM
?
5
Molecular volume [ nm^]
?
Figure 2.22 Relationship between capacity ratio (k) at 30 °C and molecular volume of aromatic probe compound, (a), Pocahontas coal; (b), Illinois No. 6 coal; (c), Morwell brown coal. Solvent: acetonitrile. Reprinted with permission from Ref. 168. Copyright 1993 American Chemical Society.
solvents play roles not only as diluents of EDS but also agents interacting with coal molecules. Hayashi and co-workers [168,169] investigated interfacial properties of solventswollen Morwell brown coal by means of an inverse liquid chromatography technique, which was first developed by Winans and co-workers [170]. By using this technique, the adsorption of probe molecules onto the coal (as the packing material) swollen in a carrier solvent can be characterised based on properties such as the capacity ratio (equivalent to adsorption equilibrium constant) and the net enthalpy change due to adsorption: H„=H
CP
H PS
•H
sc
(2-25)
where Hcp, Hps and Hsc are affinity energies between coal and probe, between probe and solvent and between solvent and coal, respectively.
structure and Properties
69
Figure 2.22 illustrates the relationship between the capacity ratio (K) and the molecular volumes of n-alkylbenzenes (NAB), methylbenzenes and non-substituted polyaromatic hydrocarbons (PAH) for the carrier solvent of acetonitrile {DN= 14.8; SP = 11.8 cal^^cm"^^) [168]. The figure also shows the relationship for the case of acetonitrile and a bituminous coal. For each set of PAH and NAB (except the case of combination of Morwell coal and NAB with shorter alkyl chains), logA' increases linearly with the molecular volume. The slope of the line for PAH (S^r) is greater than that for NAB (Sai), indicating that the surface of the swollen coal interacts with aromatic rings selectively to alkyl chains. The degree of the selectivity is represented by Sa/ Sa/. Sa/Sai of Morwell coal (8.6 in acetonitrile; 14.6 in methanol at 30°C) was much higher than those of the bituminous coals (2.6 - 4.2) and even graphite (4.9) examined. Hayashi and co-workers [168,169] attributed such highly selective interaction of Morwell coal with aromatic rings to a kind of specific interaction, OH-TT hydrogen bonding, while those such as n-K interactions would be important for the bituminous coal. They reported that S^r and S^/ S^i considerably decreased after elimination of OH groups by 0-alkylation [152] or "capping" of OH with pyridine. After analysing H„ for the individual aromatic probes, they concluded that the adsorption of PAH and NAB such as toluene, xylene and tetralin was accompanied by an endothermic process, probably, breakage of original hydrogen bonds in Morwell coal. Such an endothermic process was not detected for NAB with longer alkyl chains that could not interact with OH groups due to steric hindrance. The specific interactions between aromatic rings (in particular, larger ones) and OH groups seem to be supported by the report of Mae and co-workers [156], who investigated the extractability of oxidatively depolymerised Morwell brown coal. They examined the variation in extraction yield with binary solvents of methanol and nonpolar aromatic solvent. As seen in Figure 2.18a, the extraction yield with xylenemethanol was much lower than that with 1-methylnaphalene-methanol while these binary solvents gave maximum yields at very similar SP values. C Problems in the Theoretical Analysis of Solvent Swelling Solvent swelling is one of the most convenient techniques that can potentially provide structural information such as the molecular mass between cross-link points in the macromolecular network of coal [138]. This is the primary reason why researchers have been investigating solvent swelling of coal for long time. For the theoretical analysis of the solvent swelling of coal, the Flory-Rehner theory [144] has frequently been employed to relate the macromolecular network parameters with the extent of swelling [171-173]. It has also been recognised that use of the Flory-Rehner theory is faced with serious problems in analysing the solvent swelling of coal. The Flory-Rehner theory considers non-specific interactions (dispersive forces) as the only intermolecular ones. It is evident that hydrogen bonds (see below for further discussion) occurring between OH groups in coal significantly influence its swellability and also extractability. A number of researchers [150,166,167,171-174] have therefore modified the Flory-Rehner theory and other similar ones by taking into account the
70
Chapter 2
contribution of hydrogen bonds to the thermodynamics of the swelling. However, no theoretical approaches have been established for explaining solvent swelling of coal quantitatively. Main difficulty in relating the swelling characteristics and macromolecular network parameters seems to be arisen from the microheterogeneity of solvent-swollen coal. The Flory-Rehner theory also assumes that the macromolecular network undergoes 'affme' deformation, namely, the segments of the network are deformed in the same way as the macroscopic deformation of the rubbery solid that is usually detected as swelling. The models so far proposed for analysing the swelling of coal take the idea of the affine deformation, although stiffness/rigidity of segments and network present in coal are considered well in some models [166,167]. Based on the ^H-NMR transverse relaxation characteristics of pyridine-swollen bituminous coals, these consisted of liquid and solid components with clearly different relaxation characteristics [175-179]. Barton and co-workers [175] established that pyridine-swollen coal has a phaseseparated structure comprising of a solvent-rich phase and an apparently solvent impervious phase. Such phase separation has been predicted by a model of Painter and co-workers [166,167]. Norinaga and co-workers [61] investigated the phase-separated structure of the pyridine-ds-swoUen Yalloum brown coal by means o f ' H - N M R . From the observed characteristics of spin diffusion, i.e., the transfer of proton magnetisation from the solvent-rich phase to the solvent-impervious one, they estimated the scales of the phase separation as 5 and 8 nm based on a two-dimensional and a three-dimensional spin diffusion models, respectively. 2.4.3,2, Hydrogen Bonds Associated with its high contents of 0-containing functional groups, the abundance of hydrogen bonds is a very important feature of Victorian brown coal. The hydrogen
Table 2.10 Peak assignment for the deconvolution of DRIFT spectrum of coal in the region of 2400 to 3750 cm"' (based on Miura and co-workers [106]). Peak position, cm' 3640 3530 3400 3280 3150 2940 2640 3050 2993 2920
Hydrogen bond name shown in Figure 2.23 HBl HB2 HB3 HB4 HB5 HB6
Assignments free OH groups 0H-7I hydrogen bonds self-associated /7-mers (n > 3) OH-ether 0 hydrogen bonds tightly bound cyclic OH tetramers OH-N (acid^ase structures) COOH dimers aromatic hydrogens aliphatic hydrogens aliphatic hydrogens
structure
so
and
Properties
71
100 ISO 200 250 " 50 100 ISO 200 250 Temperature [X] Temperature rC] Figure 2.23 Changes in (noH)j (see Table 2.10), (nonXotai and (-AH)totai with increasing temperature for Morwell (MW), Beulah-Zap (ND), Illinois #6 (IL), Blind Canyon (UT), Pittsburgh No. 8 (PITT), Upper Freeport (UF) and Pocahontas coals. Reprinted with permission from Ref. 106. Copyright 2001 American Chemical Society.
72
Chapter 2
bonds play an important role in the macromolecular network of Victorian brown coal. The hydrogen bonds affect many aspects of brown coal processing and utilisation such as drying, solvent extraction/swelling, pyrolysis and liquefaction; detailed discussion may be found in relevant sections in this book. Miura and co-workers [106] presented a brief review of the past studies on hydrogen bonds in coal. Various methods have been used to gain information about hydrogen bonds in coal, including solvent swelling, DSC, NMR and FT-IR. Miura and co-workers [106] investigated the distribution of hydrogen bonds in terms of the bond strength during the thermal treatment (< 300°C) of Morwell brown coal (and Argonne premium coals) using in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Neat (undiluted) coal samples were used, avoiding the potential problems associated with the use of KBr. This technique also overcomes many other common problems encountered in the quantitative analysis of coal by means of FT-IR spectroscopy, including the correction of absorptivity. The absorption in the range of 2400 to 3750 cm"' was deconvolved into 10 bands (Table 2.10). They also used the following relationship to determine the absorptivity: «^oH = «^oH.o (1 + 0.0147AVoH )
(2-26)
where OOH is the absorptivity of the hydrogen-bonded OH and aoH,o is the absorptivity of the free OH. AVQH is the frequency shift. Figure 2.23 compare the distribution of hydrogen bonds in Morwell brown coal with those in Argonne premium coals [106]. The total enthalpy (-AH),otai for the formation of all OH associated hydrogen bonds at each temperature is also shown in the figure. The amounts of weaker hydrogen bonds (e.g. HBl to HB4) decreased significantly with increasing temperature to 150°C, due to the desorption of adsorbed water [106]. The decreases in the total number of hydrogen bonds starting at around 200°C for the low rank coals (e.g. Morwell) may be due to the thermal decomposition of carboxylic groups [106]. The average strength of hydrogen bonds for Morwell brown coal, BeulahZap, Wyodak, Illinois #6 and Blind Canyon coals was about 20 - 24 kJ mof' and remained almost unchanged during heating [106]. Miura and co-workers [180] have applied this in situ DRIFT method to study the hydrogen bonding between coal and solvent. Tracing the hydrogen bonded OH groups used this technique, Mae and co-workers [181] also studied the cross-linking reactions during the pyrolysis of Morwell and oxidised Morwell brown coal samples. Li and co-workers [182] also found the SH-N bond at about 2514 cm'\ particularly in higher rank coals. 2.4.3,3. Chemical Degradation and Biodegradation Significant efforts have been made to study the biodegradation of Victorian brown coal [183-192]. A variety of micro-organisms were found to solubilise the brown coal [184]. Micro-organisms are able to use untreated Morwell brown coal and the fraction insoluble in THF as the sole carbon and energy sources [185]. The solubilised coal
Structure and Properties
73
matter is still of high-molecular-mass in nature: the modal molecular mass of solubilised coal could range between 355,000 and 33,000 as determined with size exclusion chromatography (SEC) [190], although care must be exercised that metal ions in the growth media could cause an apparent increase in molecular mass and must be reduced prior to testing. Chemical degradation of Victorian brown coal [36,39,181,193-198] has been investigated as a tool to understand the structural features of the brown coal as well as a process to convert the brown coal into valuable chemicals e.g. through subsequent pyrolysis or extraction. The effects of chemical modification of brown coal on its extraction/swelling characteristics were discussed above in Section 2.4.3.1. Trifluoromethanesulphonic (TFMS) acid could solubilise Yalloum brown coal at 150°C in the presence of isopentane [193], producing 63 wt% pyridine solubles. Coal depolymerisation with H^ from TFMS was believed to be an important reaction for the solubilisation of Yalloum brown coal [193]. Acid-catalysed solubilisation of Yalloum brown coal was further studied using HP and superacid HF/BF3 [194]. The high acidity of HF/BF3 would promote the generation of carbenium ions as fragments from the cleavage of ether and methylene bridges in coal [194]. As a strong base with the presence of abundant heteroatoms, Yallourn brown coal would weaken the acidity of HF alone through the formation of complexes. However, the strong acidity of HF/BF3 would be sufficient to depolymerise and deoxygenate Yalloum brown coal, leading to its almost complete solubilisation [194]. Oxidation of brown coal in the presence of O2 bubbling through a 0.5 N Na2C03 aqueous solution at 85°C provided important information about the macromolecular structure of brown coal [36]. The results indicated that the oxidation occurred exclusively in aromatic ring clusters, converting aromatic carbon into peripheral carboxyl groups. The average number of bridges per eliminated clusters ranged from 2.9 to 1.3 during the early and later stages of oxidation [36]. This gives some insights into the cross-links in the brown coal. Oxidation of brown coal breaks down its macromolecular network, increasing its solubility during extraction (also see Section 2.4.3.1). The oxidation of brown coal was also investigated as a way of pretreatment for the production of chemicals e.g. from the pyrolysis and extraction of oxidised brown coal [e.g. 195-197]. Murata and co-workers [198] studied ruthenium ion catalysed oxidation of Yalloum brown coal (and other coals), confirming the brown coals to contain smaller aromatic clusters with larger numbers of alkyl side chains or bridges. Their results also suggested that brown coals contain longer alkyl side chains (up to 38 carbons) on aromatic rings that can be converted into acids. 2.4.3.4. Aromatic Features NMR has been widely used to study the aromatic stmctures in Victorian brown coal [51,102,199-204]. Table 2.6 hsted the position of peaks often used to deconvolute the ^^C NMR spectra of brown coal; the use of NMR to study the 0-containing functional groups was discussed in Section 2.4.2. Supaluknari and co-workers [199] reported
74
Chapter 2
carbon aromaticity (based on TOSS ^"^C-NMR spectra) of Victorian brown coal to range from 0.29 (Bacchus Marsh Pale) to 0.63 (a Loy Yang bore sample) depending on the lithotype composition, with pale lithotype being the most aliphatic. Yoshida and coworkers [102] also confirmed the great difference in the CP/MAS ^''C-NMR spectra among the lithotypes of Yalloum brown coal. Following the procedure developed by Solum and co-workers [205], Mae and coworkers [195] estimated the average structural parameters for Morwell and oxidised Morwell samples. They estimated the average molecular mass of the aromatic ring cluster (excluding the functional groups and chains attached) in Morwell brown coal to be 156 with an aromaticity of 0.63 [195]. The average amount of oxygen functional groups per monomer was 2.68. Li and co-workers [206] pointed out the limitation of this NMR technique to derive information about the size of aromatic ring systems in coal. There has been a general lack of information about the size and relative abundance of aromatic ring systems in Victorian brown coal or indeed in any coal [206]. This is partly because solid coal is not suitable for analysis by many analytical techniques such as UV absorption and fluorescence spectroscopies that can give direct information about the size and abundance of aromatic systems. Analysis of products from coal has been used to infer information about the aromatic ring systems in coal. Chaffee and Johns [207] and Chaffee and co-workers [208] found aromatic compounds consisting of anthracene and chrysene nuclei in a chloroform-methanol
c £f o (Q
c
(f! r o ()
.C (0
u. .c U)
^F o •D o c E (H O c CO
3
S
•e m
c')
"CD
E o o V c 9 m JQ h o o o fc E < "o _ j Q.
350
400
450
600
550
600
Wavelength, nm
Figure 2.24 UV/VIS absorption spectra of solubilised Loy Yang brown coal and lignin [37].
Structure and Properties
75
extract of Loy Yang brown coal. Although not indicated in their reports, the yields of those compounds would be very low. Ouchi and Brooks [209] isolated naphthalene and phenanthrene homologues from an acid-catalysed depolymerisation product from Yalloum brown coal. The total yield of those homologues was less than 1 wt% of the coal. As was discussed above, Murata and co-workers [198] applied the ruthenium ion catalysed oxidation for the decomposition of Yalloum brown coal. They found benzene poly-carboxylic acids that could be derived from larger aromatic ring systems with three and/or more condensed rings, but with a total yield of less than 1 % on a coal carbon basis. UV absorption and/or fluorescence spectroscopies have also shown the presence of large (especially larger than 3 fused rings) aromatic ring systems in tars from the pyrolysis of Victorian brown coal [111,210-213]. In particular, a series of cyclopentafiised aromatic ring systems were identified in the tars from the pyrolysis of Yallourn brown coal in a fluidised-bed reactor [211]. However, the source of these aromatic ring systems, i.e. whether present in coal or formed during pyrolysis, remains unclear. Clark and co-workers [214] recorded the fluorescence spectra of pyridine extracts of Yalloum brown coal. Emissions at wavelengths longer than 560 nm with excitation at 500 nm were observed, indicating the presence of very large aromatic ring systems in the extract. The fluorescence lifetime measurement confirmed that the extract behaved as an aromatic polymer. The near total dissolution of Victorian brown coal in (alkaline) aqueous solution provides new opportunities for the investigation of aromatic ring systems in the brown coal [37]. The solubilisation of Victorian brown coal in alkaline aqueous solution at temperatures lower than 200°C in the absence of oxygen in a flowing-solvent reactor is unlikely to lead to any changes to the aromatic ring systems [37]. The UV absorption and fluorescence spectra of the solubilisation product can therefore be expected to give information about the aromatic ring systems in the original coal. Figure 2.24 shows the UV/VIS absorption spectra of solubilised Loy Yang brown coal and lignin. While the spectrum of lignin may be explained by considering substituted mono-aromatic structures such as vanillyl alcohol, acetovanillone, vanillic acid/ion, trans-coniferyl aldehyde, the spectrum of the solubilised Loy Yang brown coal clearly indicates the presence of larger aromatic ring systems: its absorption decreased with increasing wavelength much more slowly than that of lignin and extended to wavelengths longer than 600 nm. Figure 2.25 shows the emission spectra of solubilised Loy Yang brown coal and those of a tar from the pyrolysis of Loy Yang raw coal at 1000 K s"^ to 600°C in a wiremesh reactor. The "apparent quantum yields" in the figure represent the fluorescence intensities per unit absorbance at the excitation wavelength. The emission spectra of the solubilised Loy Yang brown coal clearly indicates the presence of larger (more than 3 fused rings) aromatic ring systems in the Loy Yang brown coal at non-negligible concentrations: even excitation at 410 nm (or even at 490 nm, not shown) resulted in strong emissions. The tar was seen to have much higher apparent quantum yields than the solubilised Loy Yang brown coal, i.e. the coal substrate from which tar was prepared. This was
76
Chapter 2 800
1
1
1
1
290 nm
1
1
,
, 1 (A)J
700 d
600
/VK
-J
H
500 330 nm \
E n c
40n
cr c
300
CD D
/
370nnrv
(D
J^^^HJJJ^
/yy
410nm^Nj^/
/
^JT
j |
/
J- -
Q. Q.
<
200 100 1
^^
I
\
250
300
350
400
450
1
1
500
550
^1 600
Emissbn wavelength, nm
16000
n
k
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12000 ^^"^250 nm 10000
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410 nm J
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300
1 350
\ 400
1 450
Ir 500
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600
Emission wavelength, nm
Figure 2.25 Apparent quantum yields (fluorescence intensities per unit absorbance at the excitation wavelength) for different excitation wavelengths of 250 to 410 nm [37]. (a), solubilised Loy Yang brown coal; (b) tar from the pyrolysis of Loy Yang brown coal at heating rate and peak temperature of 1000 K s"^ and 600°C respectively in a wire-mesh reactor.
Structure and Properties
77
ascribed to the more intensive energy transfer process in the solubilised coal than in the tar when excited with UV/VIS Hght, suggesting that the coal contained much higher concentrations of larger aromatic ring systems than the corresponding tar. Not only 3 to 6 fused aromatic ring systems but also even larger ones are present in the Loy Yang brown coal and the smaller aromatic ring systems are preferentially released from the coal during pyrolysis.
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Advances in the Science of Victorian Brown Coal Edited by Chun-Zhu Li © 2004 Elsevier Ltd. All rights reserved.
Chapter 3 Water in Brown Coal and Its Removal David J. Allardice\ Alan L. Chaffee^'\ W. Roy Jackson^ "* and Marc Marshall^'"* ^Allardice Consulting, PO Box 88, Vermont, Victoria 3133, Australia ^School of Chemistry, P.O. Box 23, Monash University, Victoria 3800, Australia ^CRCfor Clean Power from Lignite, 677 Springvale Road, Mulgrave, Victoria 3170, Australia "^Centrefor Green Chemistry, P.O. Box 23, Monash University, Victoria 3800, Australia
3.1. INTRODUCTION The brown coals of Victoria represent an important state and national resource, as they did when the first critical review 'the Science of Victorian Brown Coal' was published in 1991 [1]. Chapter 3, The Water in Brown Coal [2], reminds readers that the high residual moisture content (in the 45 - 70 % range) critically impacts on virtually every facet of brown coal utilisation. This statement is even more pertinent today in view of the energy involved in drying the coal with its consequent increase in CO2 emissions. There are several policy drivers to facilitate reduced CO2 emissions from brown coal utilisation, which have contributed to a major shift in brown coal research emphasis both locally and internationally from the situation in 1991. These drivers have stimulated research into more efficient methods of drying brown coals and this in turn has revitalised fundamental studies of brown coal-water interactions. These fundamental studies have mainly been carried out in Victoria but there have also been important contributions from other Australian States, notably South Australia, and from overseas, particularly Japan. In contrast, many countries that bum low rank coals are active in developing new drying technologies. This chapter is divided into three parts. The first part deals with the physics, chemistry and analytical aspects of water in brown coal, the second with the drying and dewatering technologies and the third with the binderless briquetting technology for brown coal where the water in the coal plays a critical role. Emphasis will be placed on advances that have been made since the publication of Allardice's chapter in 1991 [2] but there will be some repetition where it is necessary for ease of comprehension.
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3.2. PHYSICS, CHEMISTRY AND ANALYTICAL ASPECTS OF WATER IN BROWN COAL 3.2.1. Isotherms and Hysteresis The simplest way of obtaining information on coal-water interactions is still by measuring the equilibrium moisture content of coals at different water vapour pressures at a fixed temperature [2]. Recent measurements have used both classical desiccator experiments [3-8] and an automated microbalance [4,7,8]. Victorian brown coals show sigmoid isotherms, typical for physical adsorption of condensable vapours on porous substances [9]. Figure 3.1 is an example based on results from a microbalance [10]. It shows the typical irreversible loss of moisture in the initial desorption isotherm from the bed moist state, which has been attributed to irreversible collapse of the pore structure of the coal during initial drying [3,10,11]. The subsequent adsorption cycle shows significant hysteresis, with the re-adsorption curve following a lower trajectory over the full relative pressure range. This has been attributed to swelling and shrinkage effects, which alter the structure of a colloidal gel such as low rank coal [11]. In contrast, the hysteresis observed in porous solids due to capillary filling is usually confined to relative vapour pressures above 0.5 p/po.
RAW COAL MOfSrUf^e 1 CONTfNJ 7'Q kg/kg T
a-e
^
0'? RtLATiVt
Oi VAPOR PfftSSURE
p/p^
Figure 3.1 Water sorption isotherms on Yalloum brown coal at 30°C [10].
Water in Brown Coal and Its Removal
87
0.16
^ 0.12 i
0.08
0.04
L
• Isotherm 1 :ads • Isotherm 2:des A Isotherm 3:acls
1 .
0
0.2
X Isotherm 4:des
,,,,,,,, 0.4
0.6
0.8
1
Relative vapour pressure P/PO
Figure 3.2 The effect of adsorption-desorption cycling on the isotherm shape for a Loy Yang coal sample previously dried in vacuo at 30°C to constant mass. Isotherms were obtained at 30°C in an automated microbalance [12].
Clemow and Chaffee [12] have constructed isotherms using an automated microbalance (the Hiden Intelligent Gravimetric Analyser). Commencing with vacuum dried coal, the irreversible portion of the isotherm is no longer present in Figure 3.2. The isotherm still exhibits the typical hysteresis loop, but the multiple adsorption/desorption cycles are nearly coincident, illustrating that, after the initial desorption, the process appears to be reversible. An extensive study of the relationship between several coal analytical parameters and sorption isotherms was carried out by Hall and co-workers [5] for four Victorian coals, Morwell, Loy Yang, Yalloum and Yallourn North Extension. This detailed report, commissioned by the Coal Corporation of Victoria, is available from the library of HRL Pty Ltd. The authors found that the Morwell, Loy Yang and Yalloum samples showed almost identical equilibrium water values at 9.2 and 25.3 % relative humidities and 21°C, irrespective of their total acidity, carboxylic group content and surface area. Yalloum North Extension showed a significantly lower sorption value. It must be noted that the variations in coal parameters were very small e.g. total carboxylic acid group content between 2.97 and 3.25 mmol/g db. Therefore it is not surprising that correlations of the type noted by Schafer [13] for a wide range of coals with a wide range of functional group parameters, e.g. total carboxylic acid group content from ca. 1.0 to 4.0 mmol/g db, were not observed.
88
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Only small effects of acid washing were observed, probably because of the low inorganic content of the coal samples. Use of ultrasonic mixing in leaching experiments generally led to increased sorption values. Addition of cations to the acid-washed coals increased the equilibrium moisture content over a range of relative humidity (9 - 75 %) [5,6], in agreement with previous observations made by Schafer [13] at a single relative humidity of 52%. The authors also examined four lithotypes (light, medium light, medium dark and dark) all from the same Yalloum seam [5]. There was clearly a trend for the darker lithotypes to hold more water in both desorption and adsorption experiments. The Yalloum lithotypes showed a greater variation in functional group content than the runof-mine (ROM) coals discussed previously (e.g. total carboxylic acid group content from 2.6 to 3.2 mmol g'^ db) and the adsorbed and desorbed water values showed some increase with acidity. Oxidised samples of the same lithotype showed increased sorption in all cases. The isosteric heat of sorption obtained from desorption plots at three temperatures for Yalloum and Morwell ROM coals were similar to values reported by Allardice and Evans [10] for a Yalloum coal. Both sets of values showed a similar decrease with increasing sorbed water content. The electrophoretic mobility of the ROM coals fell into two classes at lower pH with Yalloum and the Yalloum North Extension coals showing less negative values than the Morwell and Loy Yang coals. The coals with the less negative curves had lower equilibrium moisture contents [5]. The small number of coals used in the above work made it difficult to draw general conclusions. Allardice and co-workers [4] and Clemow and co-workers [14], as a prelude to a study of freezing and non-freezing water in Victorian brown coals, carried out desiccator experiments on 15 raw brown coals, 4 acid-washed coals and 7 hydrothermally dewatered (HTD) products from the coals. Statistically significant correlations were noted between the water contents at 30°C for a given coal or coal product at 15 % and 52 % relative humidity. Similar correlations were found between the 30''C water contents at 15 % and 92 % relative humidity. However, the water content of the 15 raw coals at 96 % relative humidity or 100 % relative humidity (bed moisture) did not correlate with the water content at lower humidities. This contrast suggests that the interactions that control water content at very high relative humidities (> 92 %) are different from those that operate at lower values. These interactions are further discussed below (see Section 3.2.4). These results also confirm the earlier conclusion that the bed moisture does not equal the moisture holding capacity, for raw coals with a moisture holding capacity greater than ca. 40 wt % (wet basis) i.e. 60 g water/ 100 g dry coal [2]. A compilation of the results from the two studies [2,4] is illustrated in Figure 3.3. Acid washing the coals to remove soluble inorganic salts and inorganic cations led to a consistent but variable reduction of equilibrium moisture content at 15, 52 and 92 % relative humidities but not at 96 % [4]. Schafer [13] had previously established this reduction in equilibrium moisture content on acid washing, but only at 52 % relative humidity.
89
Water in Brown Coal and Its Removal
Previous work has demonstrated that acidic functional groups in the coal are the major hydrophilic groups responsible for strong binding of water to the coal surface [2]. Clemow and co-workers [14] determined the non-acidic oxygen, phenolic (plus phenolate) and carboxylic (plus carboxylate) contents and correlated them with the equilibrium moisture contents of the coals at 15 % relative humidity, which approximate to the monolayer water as determined from the BET equation [9]. The only significant linear correlation was with the carboxylate plus carboxylic acid content, in general agreement with the results of Schafer [13] for measurements at 52 % relative humidity and those of Boger and co-workers [15] for heats of wetting. The authors made no attempt to differentiate between the effects of carboxylic acids and carboxylate groups but a regression coefficient of 1.2 ± 0.7 (90 % confidence limits) suggested that each of these functionalities was strongly associated with one water molecule. The correlation between Yalloum lithotypes and equilibrium moisture content noted by Hall and co-workers [5] was confirmed. However, the correlation was not found for lithotypes from other seams [4]. 3.2.2. Heats of Sorption from Isotherm Data Thermodynamic information on the water in the coal, such as heats of desorption and adsorption, can be obtained from moisture isotherms [10]. However, to get sensible
280
240 i i"220 o o
200
B 180 .io 160 I 140 CO
120 100
30
50
70
90
110
130
150
170
190
210
Moisture Holding Capacity (g/IOOg coal db) • This Study • Allardice
Figure 3.3 Relationship between bed moisture and moisture holding capacity for brown coals, including data (•) from Ref [2] and (•)fromRef [4].
Chapter 3
90
thermodynamic data, the isotherms must be constructed with a zero moisture datum point which is also isothermally determined i.e. by drying or evacuation at the isotherm temperature and not by subsequently determining the moisture content of the samples in an oven at a higher temperature such as 105°C. From a series of isotherms determined in this way, the Clausius-Clapeyron equation, modified for the equilibrium between a vapour and its adsorbed state on a solid, can be used to determine the heat of condensation or evaporation at different degrees of adsorption, i.e. the isosteric heat of sorption [2]. ainp_ q,t dT RT^
(3-1)
where qst is the isosteric heat of sorption (or the latent heat of vaporisation at constant moisture content V), p is the water vapour pressure, T is the absolute temperature and R the universal gas constant, qst can be determined from the slope of a plot of log p against 1/T for the selected moisture contents. Figure 3.4 shows that the isosteric heat of desorption of water from brown coal, from the saturated or bed moist state and through the capillary region of the isotherms, is
J-S
g
B-C
%
¥
I I
IB lATUn
2H>
RAW COAL MOISTURE
HEAT Of
-L 0^2
0-4 MOiSTUdE
CONTENT
I C'6
1
o-e
kgJHg DRY COAL
Figure 3.4 Isosteric heat of desorption of water from Yalloum brown coal as a function of moisture content [10].
Water in Brown Cool and Its Removal
91
essentially identical to the latent heat of vaporisation of liquid water. At lower moisture contents, there is a gradual increase in the heat of desorption through the multilayer and monolayer regions. 3.2.3. ^H NMR Experiments Early work by Lynch and his colleagues using ^H NMR techniques showed that most of the water associated with brown coal is mobile at temperatures above 0°C and not bonded to the surface but that water below 0.25 kg/kg dry coal is bonded through a continuous distribution of states [16]. More recent work by Lynch and co-workers [17] not only used ^H NMR to determine water binding capacities of coals but also to provide data on the extent to which the coal structure is changed by its exposure to water. The results showed that the fraction of mobile hydrogen increased with a decrease in coal rank as an increasing portion of the structure was 'plasticised' by interaction with water. Norinaga and coworkers [18] later reported similar results. Further ' H NMR work by Norinaga and coworkers [19] confirmed that the amount of mobile hydrogen decreased with the nonfreezable water content of both Victorian brown coals and two American lignites. Quantification of the hydroxyl group content of the coals by infrared measurements on D2O exchanged samples suggested that the amount of mobile coal hydrogen in asreceived Yalloum and Morwell coals exceeded the hydroxylic hydrogen content. Thus a significant proportion of hydrogen atoms that are not exchanged by D2O treatment appear to be mobilised by interaction with water. The authors do not speculate as to the chemical structure of these hydrogen atoms. ^H NMR studies of Yalloum, Beulah Zap and Illinois coals that had been partially dried and re-wetted were used by Norinaga and co-workers [20] to estimate the amounts of non-freezable pore water, freezable pore water and bulk water contents. The amounts of non-freezable and freezable pore water for the low rank coals were found to be in excellent agreement with values determined by DSC measurements (see below). It should be noted that the boundary between bulk water and freezable pore water was set at 260 K, somewhat lower than the 270 K used by other workers [14]. The freezing point distributions of water in the pores were used to calculate the pore size distribution following the Gibbs-Thompson equation (also see Chapter 2). The results suggested that removal of non-freezable water (but not freezable or bulk water) led to an irreversible decrease in the average pore diameter that was attributed to pore collapse. This observation can be linked with the irreversible change noted in the adsorption isotherm (see Section 3.2.1). Further work by Hayashi and his co-workers [21] using a different pulse sequence showed that the initial amplitude of the signal corresponded quantitatively to the amount of'mobile' proton. The signal was interpreted in terms of three components with different T2 values that were attributed to free water, pore water (freezable bound water and non-freezable water) and mobile coal hydroxyls. Analysis using a theory which relates pore dimension to pore liquid relaxation time concluded that the water-filled pores were slit-like rather than cylindrical, initially ca. 3
92
Chapter 3
nm in dimension but decreasing in dimension with loss of pore water in a roughly linear fashion. The rate of loss of water from two Victorian brown coals has been measured using ^H NMR relaxation time measurements [22]. A waxy, high atomic H/C ratio (1.13) Yalloum coal was shown to lose water at ca. 3 times the rate of lower H/C ratio (0.81) Loy Yang coal. 3.2.4. Differential Scanning Calorimetry The first attempt to characterise freezable water in Victorian low rank coals using differential scanning calorimetry (DSC) was published by Norinaga and co-workers [18]. The DSC traces showed two exothermic peaks centred at 226 and 258 K during cooling for Yalloum, Loy Yang and Morwell coals, whereas only a single broad endothermic peak centred at 273 K was observed during heating. The enthalpies of freezing associated with the 226 and 258 K peaks were determined by measuring the changes in heat generated due to congelation as a function of water content. Values of 333 J g"' water for the 258 K peak and 188 J g ' for the 226 K peak were obtained implying that the 258 K peak was due to water whose properties were very similar to those of bulk water. The amounts of bound (226 K) and bulk (258 K) water could be calculated and the amount of non-freezable water obtained by difference. Two other recent studies have reported on the distribution of freezing (freezable) and non-fi-eezing water in Victorian brown coals [4,14,23]. The non-freezing water contents of three brown coals, Loy Yang Low Ash (LYLA), two Yalloum Township coals, a
Table 3.1 Non-freezing water contents as measured by proton nuclear magnetic resonance spectroscopy ('H NMR) and differential scanning calorimetry (DSC) [4]. /->
1
1
Coal 01 product
LYLA HTD250 HTD290 HTDHI HTDLO YTP YTD
a
Non-freezing water (g/1 OOg coal db) DSC
'HNMR 56.2 25.5 17.9 10.3 9.5 50.0 77.8
56.2 ND^ ND 13.7 ND 60.9 89.2
a, LYLA, Loy Yang Low Ash coal, medium dark lithotype; HTD250, LYLA hydrothermally treated at 250°C to give a high-porosity product; HTD290, LYLA hydrothermally treated at 290°C to give a high-porosity product; HTDHI, LYLA hydrothermally treated at 320X to give a high-porosity product; HTDLO, LYLA hydrothermally treated at 320°C to give a low porosity product; YTP, Yalloum Township pale lithotype coal; YTD, Yalloum Township dark lithotype coal. References to coal analyses are given in [4]. b, ND = not determined.
Water in Brown Coal and Its Removal
93
pale lithotype (YTP) and a dark lithotype (YTD), and a high porosity hydrothermally dewatered LYLA coal product were determined by the DSC method [4,14]. These results together with ^H NMR determinations of non-freezing water are summarised in Table 3.1. Results for other hydrothermally dewatered products from LYLA coal are also included in the Table. The DSC results for non-freezing water were on average 12 % greater than those from ^H NMR measurements. Barton and Lynch [24] reported a similar difference for some bituminous coals, which was attributed to differences in sample preparation for the two techniques. Identical sample preparations were used in this later study and a probable explanation is that there is an underestimation of freezing water by the DSC method because of the assumption that the latent heat of fusion of all freezing water present in the coal is equal to that of bulk water. Other DSC studies [25] have shown that the heat of fusion of water adsorbed as thin films (>10 nm) on solid surfaces is lower than that of bulk water leading to an underestimate of freezing water. Norinaga and co-workers [18] also noted a component of adsorbed water with a low heat of fusion. Comparison of the non-freezing water (^H NMR values) with the equilibrium moisture content as determined by desiccators indicated that for the coals the nonfreezing water corresponded to the equilibrium moisture content at ca. 93 % relative humidity in all cases [4,14]. This is lower than the relative humidity of 96 - 100 % found for two bituminous coals by Barton and Lynch [24]. Correlation of the non-freezing water (^H NMR values) with the functional group contents indicated a dependence almost entirely on the carboxylic acid plus carboxylate content. The plot showed a linear regression coefficient of 10.9 suggesting that about 10 water molecules were associated with each acidic site sufficiently strongly to be inhibited from freezing. Combining this result with the correlation between monolayer water and carboxylic acid plus carboxylate content [14] (see Section 3.2.1) suggests a picture at equilibrium wherein each strong acid site has one very strongly bound water molecule attached with another 10 or so molecules still recognisably bound to the water molecule - strong acid complex. These differences between freezing and non-freezing water explain the change in the interactions that control the equilibrium moisture below and above 92 % relative humidity (see Section 3.2.1). The water that is lost at relative humidities greater than ca. 93 % is similar to bulk water and is not meaningfully associated with functional groups in the coal. Experiments carried out by these workers [4,14] involved heating the sample in the DSC and thus no peak at 230 K was observed as predicted by Norinaga and co-workers [18]. A further DSC study [23] of water in six brown coals in which the DSC samples were cooled as by Norinaga and co-workers [18] has reported one or two peaks at around 230 K. A similar peak was observed at the same temperature for solutions of sodium chloride, as previously reported by Hvidt and Borch [26]. However, the intensity of the 230 K peak for the coals did not correlate with the sodium ion concentration of the coal and indeed persisted with no change in coal samples which had been acid washed to remove soluble salts. Solutions of MgCl2, Mg(0Ac)2, CaCli, AICI3, FeCl2, succinic acid and benzene-1,2,4-tricarboxylic acid showed no peak other
94
Chapter 3
than the bulk water peak [27]. A mixture of the benzene tricarboxylic acid and NaCl also showed only the bulk water peak but a mixture of succinic acid and NaCl showed a significant peak at 230 K. The specific phenomenon, which causes the appearance of the 230 K peak in the coals, is thus uncertain. Norinaga and co-workers [18] attributed the 230 K peak to water whose properties were modified by enclosure in pores (also see Chapter 2). The DSC curves for the Victorian brown coal sample appear similar in the two publications [17,23] and the proportion of this pore water is calculated to be ca. 26 % of the total water by Norinaga and co-workers [18]. Miura and co-workers [28], using FTIR and DSC, attempted to estimate the strength of the brown coal-water interaction and reported that the enthalpy decreased as desorption progressed. 3.2.5. Molecular Modelling of Brown - Coal Water Interactions In a bid to develop a better molecular level understanding of brown coal-water interactions, two groups have applied molecular simulation approaches to the evaluation of model coal structures. Kumagai and co-workers [29] modelled the structure of Yalloum brown coal using two oligomers, namely a tetramer (MW 1540) and a pentamer (MW 1924) based on a monomer of composition C21H20O7, as illustrated in Figure 3.5. The unit structure was constructed on the basis of combined data from elemental analysis (C: 65.6, H: 5.2, O: 29.2 wt %) and ^^C-NMR spectroscopy. Initially, the two oligomers were combined with 360 water molecules, corresponding to 65.3 % moisture content (wet basis). After the minimum energy configuration was identified, the potential energy and volume occupation were calculated. The process was repeated in a step-wise fashion for analogous systems with progressively fewer water molecules, down to 0 (simulating dry coal). A monotonic decrease in the volume occupied by the brown coal-water model was observed, with the completely dried product occupying approximately one half of the volume of the original wet coal model (Figure 3.6). The volume occupied by the brown coal model itself (i.e. excluding the water from consideration) also decreased as water was removed, as a result of changes in molecular configuration of the oligomers. This change in molecular configuration was found to be irreversible; in other words, the reintroduction of water did not cause the configuration of the brown coal oligomers to revert to their original arrangement and the original volume of the brown coal-water model was not recovered. These model results were observed to correlate closely with the results of concurrent experimental measurements in which both the % moisture and volume of Yalloum brown coal were determined after equilibration at relative humidities over the range 0-85 %. Rather than attempting to model 'whole' coal, Vu and co-workers [30,31] used fossil wood as the basis of their modelling strategy to study brown coal-water interactions. In many parts of the Victorian brown coal deposits, morphologically distinct macroscopic fossil wood can be readily handpicked from the surrounding coal matrix. Chemical and spectroscopic data from such a sample were used to construct a model (C]ooH8o028) of
Water in Brown Cool and Its Removal
95
what is essentially degraded lignin (Figure 3.7). A 3-dimensional packing arrangement of three of these model units together with 470 water molecules (62 % by weight) into a periodic unit cell with dimensions 27.3 x 27.3 x 27.3 A is depicted in Figure 3.8. Vu and co-workers [31] used a dynamic modelling approach to investigate the interactions between water and structural subunits of degraded lignin and, also, three idealised, non-degraded lignin systems. Their work indicates that the diffusion of water molecules in the vicinity of lignin is substantially reduced compared to pure water, largely as a result of hydrogen bonding interactions (Table 3.2). The significantly reduced mobility of water molecules in proximity to carbonyl groups is attributed to
H3CO Figure 3.5 Monomer structure used as the basis for the Yalloum brown coal molecular model by Kumagai and co-workers [29], corresponding to C:65.6; H:5.2; 0:29.2 wt % (MW=384.4).
'""" >"' r k A
«' A
'^
• •
i '"""f a ik
t'" '»"" 1 '""* 4 ^ ^
1 -1
T
m
1 m\
ml
• j#OCX>MiWAraEl 1
1 1. »
m
t
m
i
1
m
J * s J
1
1
m
limn
1
tm
MiiLflure rentov^ wi% Figure 3.6 Monotonic reduction in volume occupied by the wet Yalloum coal model structure as it is dried from 65.3 % down to 0 % moisture (•). Upon rehydration, the volume returns to only ca. 80 % of its original value (o). The volume occupied by the coal itself (A) does not change until more than 80 % of the original water present has been removed. Reprinted from Ref 29 with permission from the authors.
96
Chapter 3
hydrogen bond formation. Conversely, the relatively enhanced diffusion of water molecules in proximity to methoxy groups is attributed to a hydrophobic effect. 3.2.6. Other Brown Coal - Water Surface Chemistry Studies Crawford and co-workers [32] measured the advancing and receding contact angles for light gas oil in water against the surfaces of a range of coals including a Loy Yang sample. Such measurements give information regarding the surface hydrophobicity, which is important in several coal beneficiation methods, e.g. aggregation, flotation.
• • • • • • a .
^ • ^ . . . • ^
Figure 3.7 Model of degraded lignin (C100H80O28) used to simulate brown coal-water interactions. The (degraded) lignin monomer units are circled.
Water in Brown Cool and Its
97
Removal
Figure 3.8 3-dimensional unit cell constructed from 3 degraded lignin structures (refer Figure 3.7) and water and used as the basis for molecular dynamics simulation.
Table 3.2 Time-averaged diffusion constants for water molecules in proximity to specific functional groups corresponding to the model depicted in Figure 3.8. Diffusion constants were determined fi-om molecular dynamics runs of 100 ps duration at a simulation temperature of 298 K. Local Water Group Bulk water*
Diffusion constant, cm^ s"^
Rate relative to pure water
1.94x10-'
0.49
< 2.85 A from the H of OH groups
1.84x10-'
0.47
< 4.65A from the O of all 0CH3 groups
2.88x10-'
0.73
<2.85AfromtheOofcarbonyl (C=0) groups
1.45x10-'
0.37
2.14x10-'
0.54
< 2.85A from the H of carboxylic acid groups * > 6 A from lignin surface.
98
Chapters
The Loy Yang coal showed a smaller contact angle hysteresis (i.e. the difference between advancing and receding contact angles) and a larger contact angle in the mineral free limit than higher rank coals. The larger concentration of oxygen-containing functional groups in the Loy Yang coal makes the organic component of the surface more hydrophilic than in higher rank coals, which will increase the contact angle, and closer in hydrophilicity to the mineral component of the surface, which reduces the hysteresis. Information concerning the structure of pores in brown coal that are open to fluid exchange has been obtained from small angle neutron scattering [33]. The influence of micro- and macro-pores on liquid exchange after four weeks of incubation with H2O/D2O mixtures was observed. It was suggested that this method could be used to measure liquid penetration into the coal matrix. 3.2.7. Factors Causing Variations in Bed Moisture Content Five factors that influence the bed moisture content of brown coal have been extensively discussed by Allardice [2] (depth of burial, compression by folding, petrographic, weathering and thermal effects). A phenomenon not noted previously which appears to influence compression and hence the coal moisture is sea level changes over the period of deposition. Evidence from marine microfossils and sand layers indicates periods when sea levels were high and flooding of the peat swamps occurred [34]. This flooding and sand deposition led to compression of the organic matter directly below and this has resulted in a decrease in moisture content and increase in rank for coal just below the marine boundary [35]. 3.2.8. Determination of Moisture Content 3.2.8.1. Definition of Moisture Content and Standard Methods of Determination Allardice [2] stated that the widely accepted definition of moisture content of brown coal is the amount of water that is released from the coal at 105 - 11 O^'C excluding water that is derived from the decomposition of functional groups. Allardice also pointed out that this definition is not an operational one, since the standard methods of moisture determination do not distinguish water originally present in the coal from that produced by decomposition up to 105-110°C. It is therefore, strictly speaking, necessary to quote the method used with any value given. The Australian Standard Methods for Moisture Determination in Lower Rank Coals have been changed. The method AS2434.5 [36] involving azeotropic distillation with toluene has been retained but only for determinations of moisture in chars from lower rank coals. A new standard AS2434.1 [37] was developed and incorporates three methods. The first is a two-stage procedure for bulk samples {ca. 500 g) in which equilibration on a tray at 38°C for 5 h and then at ambient temperature until constant weight is attained (stage 1) is followed by heating at 105 - 110°C for 3 h under N2 flowing at a fixed rate (stage 2) and final weighing. The second procedure for samples
Water in Brown Cool and Its Removal
99
ca. 10 g involves drying at 105-110°C under a flow of N2 at a fixed rate for 3 h and final weighing. The third method is the well-established azeotropic distillation with toluene. It should be noted that sampling procedure has also been changed as of 1996. The current edition of AS2434.1 has removed the options of gas heating and the use of chromic acid to clean the glassware in the azeotropic distillation method. There is also a separate standard A2434.7 [38] for an analysis sample of coal (1 g, <212|Lim particle size, equilibrated in air), which involves heating the coal at 105110°C in a stream of nitrogen as above. The moisture is determined by weighing the sample or by absorption in magnesium perchlorate. 3.2.8,2. Rapid Methods for Moisture Determination There continues to be interest in developing rapid, on-line methods for moisture analysis for power generators, where a variation in moisture content from 64 to 68 % requires a 19 % increase in the coal feed rate to provide the same net heat input to the furnace [39]. This sensitivity has led to a target accuracy within 0.5 wt % for the moisture content of as-received coal. On-line methods of moisture determination will be even more critical for operators of advanced power generation plants. A large number of techniques have been evaluated and aspects of recent work emphasising applications to Victorian brown coals are summarised below. It should be noted that these methods all require calibration using samples analysed by the standard methods described above and are not a substitute for these methods. Normal-incidence-geometry X-ray diffraction has been used to measure moisture content of coals from several seams in the Latrobe Valley [40] but an accuracy of only 5 % was claimed. FTIR [41] in conjunction with a CIRCOM factor analysis package gave a standard error of 9 wt %, was slow and could only use very small samples. A different infrared technique, using the combination vibrations of water in the 1800 - 2100 cm"' range, has been patented [42], but its success in practice has not been reported. Fast neutron and gamma ray transmission (FANGAT) was evaluated using Yalloum and Loy Yang samples [43]. Accuracies of 0.8 to 1.8 wt % were obtained but were still considered unacceptable for raw brown coal moisture analysis. Prompt gamma-ray neutron activation analysis (PGNAA) can be used to simultaneously determine moisture, ash and the major components of Latrobe Valley brown coal (e.g. C and H values) [44,45]. It was hoped to develop an on-line estimation of a fouling index for brown coals based on this technique. Moisture determination was achieved with a root mean square (RMS) error of 0.6 to 0.8 wt % depending on the type of gauge used. Microwave transmission was considered by Allardice [2] to be a promising technique. A gauge developed for black coal determinations operating in the frequency range 2 - 4 GHz was shown to be too inaccurate for brown coal moisture determinations [41] with RMS errors of 0.7 to 1.7 wt % depending on the coal. More recent work, operating at a lower frequency of 1 GHz, gave determinations of brown coal moisture with a standard error of 0.67 wt % for bed depths of up to 350 mm [46].
100
Chapters
3.2.8,3. Instrumental Methods of Moisture Determination Methods utilising nuclear magnetic resonance (see Section 3.2.3) or electron spin resonance can be faster than the standard methods described in Section 3.2.8.1. They may be able to distinguish between water present as such in the coal and that formed by chemical decomposition, but procedures that have been applied to brown coal to date require calibration standards like the on-line methods in Section 3.2.8.2. Barker and Smith [47], using a small commercial NMR spectrometer, found a good correlation between the water content of twelve Victorian low rank coal samples (55 70 wt % moisture) and the amplitude of the slowly-relaxing component of the NMR signal. A similar method was patented by an East German group [48]. Electron paramagnetic resonance determination of moisture content of low rank coal was patented by another East German group [49]. Thermogravimetric determination of moisture content automated e.g. in a LECO system, which has been used for many years as a rapid substitute for the standard determinations [2], could be developed as a standard technique to rank alongside them, but this has not yet been done.
3.3. DRYING OF LOW RANK COALS 3.3.1. General The high moisture content of brown coals makes drying an essential component of any upgrading or utilisation process. Water comprises a substantial proportion of the asmined coal (60 - 70 % in the case of Latrobe Valley brown coals) and the cost of evaporating or otherwise removing this moisture from low rank coals is frequently overlooked or under-estimated by developers. When more detailed evaluations are performed, the cost of drying and associated problems is often the major barrier to the development of competitive new brown coal technologies. The high moisture content of low rank coals has led to a plethora of drying processes being developed, but a major breakthrough on drying cost is still awaited. Water removal processes for low rank coals can generally be divided into evaporative drying or non-evaporative dewatering, where the water is removed from the coal in liquid form. Fohl and co-workers [50,51 ] reviewed both types of water removal from brown coal. The status of several processes in each category will be reviewed or updated below. In considering the drying requirements of low rank coals, it is informative to express the moisture content on a dry coal basis to indicate the quantity of water to be removed per unit mass of dried coal and hence the energy which has to be provided. Table 3.3 indicates the moisture contents of some low rank coal fields in Victoria and elsewhere. This shows that the moisture contents (per kg of dry coal) of Latrobe Valley brown coals can be more than 3 times that in a typical USA lignite.
101
Water in Brown Coal and Its Removal Table 3.3 Moisture contents of low rank coals, as received and dry basis. Field Yalloum, Victoria Loy Yang, Victoria Morwell, Victoria Anglesea, Victoria Leigh Creek, South Australia Lower Rhine, Germany Fort Union, Nth Dakota, USA
Typical moisture contents %, as - received
kg H20/kg dry coal
66.7 62.6 60.9 46.6 31 56 37.2
2.03 1.67 1.56 0.88 0.45 1.27 0.59
3.3.2. Evaporative Drying In evaporative drying, the water removal is achieved by applying heat to the coal, either directly or indirectly, comparable to the latent heat of vaporisation of the water, and removing the water in vapour form. One of the disadvantages of evaporative drying processes is the high energy requirement to evaporate the water, in some cases up to 25 % of the energy in the coal is required to dry off the moisture before any useful energy is obtained. This leads to correspondingly higher emissions of CO2 per unit of useful energy. However, in processes where a relatively pure steam effluent is achieved, vapour recompression and condensation can recover much of the evaporative energy. In evaporative drying processes, the moisture is removed predominantly as water vapour (and small amounts of CO2 and CO) from the coal. The ash forming elements in the coal are not removed, as they are not volatile at drying temperatures. Ash fouling in boilers may in fact be worse because of the higher temperatures encountered in dried brown coal flames. The dried product is normally used on-line because of difficulties in storage and handling due to safety risks from spontaneous combustion and dust explosions. If bulk storage and/or long distance transport is required, the dried coal needs to be agglomerated in some way, for example by briquetting, to facilitate safe transport, storage and handling. 3.3.2. /. Rate of Evaporative Drying Brown coal behaves as a porous solid with respect to drying kinetics. If the coal is dried slowly in a controlled humidity environment used to obtain moisture isotherms, there will be uniform moisture content and zero gradient through the coal. During air-drying under ambient conditions, the bulk moisture can diffuse to the surface of the coal particle via the pore structure and evaporate with minimal moisture gradient through the particle. The slow drying, which occurs in this phase, follows the
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classic constant rate drying period where evaporation occurs from the particle surface [52]. Following the removal of the bulk water in the constant rate period, the rate of drying will generally fall monotonically as the water is increasingly strongly bound to the coal. For higher rates of drying in thermal evaporative systems, particularly at elevated gas temperatures, the rate of evaporation exceeds the rate of diffusion to the particle surface. Under these conditions, the surface dries and evaporation occurs within the coal particle. This drying mechanism can be modelled as a dry shell/receding wet core with the drying rate controlled by conductive heat transfer through the dry shell, as established by Mcintosh [53,54]. The drying of brown coal is accompanied by substantial shrinkage (up to 50 % by volume for Latrobe Valley coals), particularly as removal of capillary water occurs at moisture contents below about 40 %. This shrinkage generates significant stress within larger particles during thermal drying under the (shrinking) dry shell/receding wet core regime, and explains the loss of lump strength, friability and breakage which frequently occurs as in association with the thermal drying of raw brown coal lumps or agglomerates. 3,3.2,2. Direct Evaporative Drying Processes 3.3.2.2. L Mill Drying Mill drying, also known as Flash Mill Drying, is the process currently used in conventional brown coal power stations where the coal is milled and dried simultaneously while entrained in a flow of hot gas recycled via the furnace gas off-take (Figure 3.9). Modelling studies by Mcintosh [53-56] demonstrated that, in the integrated mill drying systems in Latrobe Valley power stations, the bulk of the drying occurs after the feed coal has been milled to fine particle sizes, facilitating heat transfer from the hot recycle gas. This means that the 'drying shaft', where the coal is mixed with the hot gas before the mill, is in fact a misnomer, as most of the drying occurs in the mill and the subsequent burner feed ducts. A stand-alone version of the mill drying process is feasible using hot gas generated by burning some of the product and separating the dried coal product from the transport gas with cyclones and dust filtration devices. This was the basis of the Drikol development piloted by APM [57] at the Maddingley Brown Coal Mine in Victoria. 3.3.2.2.2. Hot Gas Rotary Drum or Fluidised Bed In both hot gas rotary drum and fluidised bed drying systems for brown coal, problems have been experienced with fires and explosions, with air contacting the hot coal particularly during start up and shut down [58,59]. It is interesting to note that the former SECV's first pilot plant venture in 1925 was a flue gas drum dryer at Newport, under the direction of Dr H Herman [60]. This project was abandoned after about 12
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months over concerns on operating safety. A hot gas fluid bed dryer demonstration project by AM AX at its Belle Ayr mine in Wyoming was eventually closed over concerns with fires and explosions in the drying plant and spontaneous combustion in transport and storage of the product [59]. 3.3.2.2.3. Steam Fluidised Bed Drying Steam Fluidised Bed Drying (SFBD) was invented by Prof Potter at Monash University in the 1970's [61,62]. The initial concept was for a cascading multistage fluidised bed dryer with heat recovery between successive beds. The concept was further developed as a single stage process with an 8 t h'^ (evaporation) pilot plant at Boma in the former East Germany [63]. German brown coal technologists recognised the safety advantages of the process where the drying occurs in an inert steam atmosphere, eliminating the principal risk of fires and explosions and enabling energy recovery to be achieved by vapour recompression.
GAS OFF TAKE
-20-25V^RF.(fines}and70^0 gas to 'inects burner
DE SWfRi VANES 75-BO'A ofPFand30Vnofgas to main burner
Classifier Volute - Coarse RF. return to mill Swirl Vanes in mill outlet
Figure 3.9 Typical mill drying and separation firing system developed for power stations burning Victorian brown coal. Reproduced with permission from D Clark, AusIMM Monograph Series 11, Victoria's Brown Coal, pp. 127-154. Copyright 1984 AusIMM.
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The single stage process involved drying the coal in a superheated steam fluidised bed with heat recovery through recompression of the product vapour. Some of the compressed vapour provides the fluidising steam while the bulk of the steam is passed through a heat exchanger submerged in the bed and condenses. As the evaporated moisture is recovered in liquid form, the process offers major efficiency advantages over conventional evaporative drying systems. Rheinbraun and Lurgi designed a commercial demonstration steam fluidised bed drying plant with an output of 20 t h"^ of dried brown coal. Two of these plants, with nominal annual capacities of 150,000 t yr"^ (dry coal), were built by Lurgi [64,65]. The first of these plants to come on-line was at Loy Yang in 1992 and the second at
Table 3.4 Typical properties of a steam fluidised bed dryer (based on Ref 64). Bed conditions Fluidising steam Heating steam Coal feed size Product size Product moisture Coal residence time
1 -lOkPag, 106-120°C 15-25 kPag, slightly superheated 400 - 500 kPag, saturated 0 - 6 mm 0 - 4 mm 10-20% about 60 minutes
(b)
(a) Raw brown coal
Steam
Condensate
Dry brown coal
Raw brown coal
Condensed vapours
Dry brown coal
Figure 3.10 Alternative configurations of 20 t hr'^ steam fluidised bed dryers (modified from Ref 65): (a), with external steam supply; (b), with integral energy recovery.
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Rheinbraun's Wachtberg briquette factory at Frechen near Cologne. The principal difference was that the Loy Yang plant was supplied with live steam from the Loy Yang A power station while the Wachtberg plant included energy recovery with vapour recompression. In the latter case, the only external energy supply is from the electric motors for the steam compression. Table 3.4 shows typical properties of a steam-fluidised bed dryer for brown coal, while Figure 3.10 shows the flow diagrams for the two variants. The product from the 150,000 t yr'^ (dry coal) steam fluidised bed drying plant at Loy Yang was milled to 75 % < 90 |im and pneumatically transported through a pipe to Edison Mission's Loy Yang B power plant for use as a start up and auxiliary fuel. The drying plant operated from 1992 to 2003 but Edison Mission has recently converted from dried coal to natural gas for this purpose. The Loy Yang drying plant is therefore closed at the time of writing (April 2004) and, unless another major customer can be located, seems unlikely to reopen. The performance of this plant was reviewed by Schmalfeld and Twigger [65]. The Wachtberg plant operated from 1993 to 1999 supplying dried coal into the briquette factory system, which supplied the briquette presses, and granulated coal for fluidised bed combustion and high temperature Winkler gasification. It is significant to note that basic laboratory measurements of the equilibrium moisture content of brown coal at elevated temperatures and pressures demonstrated significant differences between the Rhenish and Loy Yang brown coals, requiring modifications to the German design for use on the Loy Yang coal. While a typical product moisture content of 12 % is achieved with Loy Yang coal at a steam temperature of 107°C, the Rhenish coal, with lower initial moisture content, would require 112°C to achieve the same degree of drying [65]. The stable performance, reliability and flexibility of these two 20 th'^ plants exceeded expectations and demonstrated the commercial potential of the technology. As a result, the wider use of the technology in power generation was proposed in Germany and a 66 th'^ (dry coal) demonstration plant was built at RWE's Niederaussem brown coal power station in the Rhine area in 2000 but closed in 2002 [66]. As recently reported by Ewing and co-workers [66], further development of SFBD at the Wachtberg facility has demonstrated that even better performance and substantial cost reductions could be achieved by moving to a fine grain version of SFBD. Reducing the feed coal size from -6 mm to -2 mm results in an 80 % improvement in heat transfer due to the smaller particle size. As a result, the volume of the fluid bed can be reduced by 70 %, allowing investment costs for a commercial plant to be cut by an estimated 60 %. This improved version of the process is being piloted by Rheinbraun (now RE Engineering) at 17 t h'' dry coal scale at Wachtberg. The development is being fast tracked for inclusion as the pre-drying step for the brown coal feed to the next generation of German supercritical brown coal boilers, designated BoA Plus [66]. A demonstration plant integrated with a power station is scheduled for 2007 with full implementation in new plants from 2011.
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3.3.2.2.4. Pressurised Hot Gas Entrained Flow Drying This technology has arisen from the IDGCC project [67] where entrained flow drying occurs by injecting the moist coal via lock-hoppers into the high pressure hot gas stream exiting the gasifier. This technology provides a low cost pre-drying technique for incorporation into other high pressure processes for low rank coals such as gasification and high pressure fluidised bed combustion. There is a substantial cost reduction due to the simplicity of the drying plant (essentially a high pressure tube), relative to the cost of stand alone dryers. This drying technology has also been proposed as an option to retrofit to existing brown coal boilers. 3.3.2.2.5. NBCL/UBC Process An integral part of the Brown Coal Liquefaction technology (see Chapter 8), this process dries the moisture from crushed coal prior to hydrogenation by mixing the raw coal into an oil slurry and pumping under pressure through a heat exchanger to supply the evaporative energy [68,69]. As in the SFBD process, most of this energy can be recovered by recompression and condensation of the high pressure water vapour evolved. This concept has been further developed by Deguchi and co-workers [70] as a standalone upgrading process to produce ^upgraded brown coal' (UBC). If waste or low grade oil is used as the slurry heat transfer medium, the process can also upgrade the quality of the oil recovered from the process. The heavy oil components can be preferentially absorbed into the porous coal structure, leaving the lighter fractions available for recovery. The dry coal product is reportedly less reactive to spontaneous combustion because the residual heavy oil components block access to the pore structure. A 5 t d"' demonstration plant is reportedly under construction in Indonesia [71]. 3.3.2.2.6. Carbon Dry Process The Carbon Dry process was developed and piloted in North Dakota. Coal is heated in hot oil (Trench fried') to evaporate the water, with residual heavy oil fractions reportedly adsorbed into the coal, blocking the pores and thus reducing the spontaneous combustion risk. The economics of the process are strongly influenced by the degree of oil recovery, which is in turn affected by the porosity of the coal. Less than 5 % oil retained in the coal can more than double the cost of the dried product. As yet, we are not aware of a commercial plant to demonstrate this technology. 3.3.2.2.7. Microwave Drying Attempts to develop a viable microwave drying process for low rank coals [72] have not been successful, although it can be a useful technique for rapidly drying of laboratory samples. While microwaves can undoubtedly provide the energy to dry the
Water in Brown Coal and Its Removal
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UtiE
'^y\^-^^,.?^^' €041
ri
Figure 3.11 Solar dried brown coal plant concept. Reprinted from Ref. 2 with permission from SECV.
coal, there is a substantial fire risk in the event of over drying, which led to the closure of a commercial microwave drying plant for peat in southern NSW. The intense energy transfer to the coal by microwaves can result in problems due to entrainment of fine particles out of the bed by the rapid evolution of water vapour. 3.3.2 J.8. Solar Drying Where the coal is left exposed to direct sunlight and unsaturated air, it will dry down to an equilibrium moisture content, about 15 % for Victorian brown coals. In the 1980s, the SECV operated a 2,200 t yr"^ Solar Dried Brown Coal pilot plant at Hazelwood in the Latrobe Valley. The process requires wet milling of coal to a pumpable slurry, which is dried in open-air ponds to provide a dense lump product [73,74]. Figure 3.11 presents a schematic illustration of the Solar Dried brown coal plant concept. The process requires large land areas and the production is seasonally variable depending on climatic conditions. It does not benefit from economies of scale to the same extent as other drying technologies because the major cost in civil works is effectively proportional to the production. However, the technology could be well suited to low rank coal deposits in arid areas with low labour costs.
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Chapter 3 40 n
-;, 3oH
•
§ 20
a 10 E o o PH
Figure 3.12 Compressive strength of densified brown coal (10mm pellets) as a function of the pH of densified coal slurries [76]. •, Loy Yang coal; o, Morwell No 1 seam, H1317 bore coal; • , Maddingley coal.
3.3.2.2.9. Densified Brown Coal and Pellet Drying Another process was developed at Melbourne University by Johns and co-workers [75,76,77] to convert Victorian brown coal-water slurries into a material known as Densified Brown Coal (DBC). This process requires no addition of water, since the slurry is formed using only the inherent moisture. Attritioning in, for example, a kneader, sigma (or Z-arm) blender reduces the particle size to 5-10 [im and is said to 'release' the water from its matrix through physical disruption and collapse of the coal pore structure. A plastic mass is formed which can be readily extruded into pellets or blocks of convenient dimensions. The 'green' product is then air dried at ambient conditions, with or without the assistance of a draught, to form a product, which possesses a net wet specific energy similar to that of bituminous coal. During the drying step, the pellets/blocks shrink by an amount that is approximately equal to the volume of water that evaporates. The material behaves as a carbonaceous gel [78] in which, as the water evaporates, strong inter- and intra-particle attraction forces develop to produce a hard, dense product. The crush strength of the dried products was shown to be dependent on the pH of the initial coal slurry (Figure 3.12) and is attributed in part to the presence of ionic association between coal particles to give a stronger three-dimensional network. Wilson and Duane [79] reported further development of this concept as the Coldry Process, which proposed accelerating the air-drying of the raw pellets using hot gas and waste heat. However, they reported that the accelerated drying reduced the strength of the pellets although attempts are still being made to overcome this problem.
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Raw coal pellets can also be made in a drum pelletiser [80] but attempts to accelerate the drying of the wet spherical pellets using hot gas fluid bed drying by Swinburne University of Technology and the Coal Corporation of Victoria encountered similar problems of loss of strength, breakage and fires. The problem with accelerating the drying of raw coal pellets arises from the shrinking core drying mechanism that applies to drying of such pellets. The outer shell of the pellet dries first and shrinks, while the wet core does not change volume, thus weakening the structure of the pellet and leading to breakage and degradation. 3.3.2J. Indirect Drying Processes Indirect or contact drying occurs when the moist solid contacts a hot surface heated on the other side by a heat transfer medium such as steam, oil or hot gas. This contrasts with direct drying processes where the solid is contacted directly by the heat transfer fluid. The most significant indirect drying process is the steam tube rotary dryers used m brown coal briquette factories in Victoria and Germany, illustrated in Figure 3.13. The performance of these dryers has been reviewed in detail by Herman [81] and Kurtz [82]. These plants dry the bed moist brown coal from up to 66 % (in the case of Yalloum coal) to 15 % prior to briquetting.
#
-
*
^-^-CL^^-^-^J^^^-^-CL-:-^-^-^^^^
Figure 3.13 Steam tube dryer used in brown coal briquetting. Reprinted from Ref 81 with permission from SECV.
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Steam is passed through the shell of the rotary drum to heat the tubes through which crushed coal flows in a relatively inert atmosphere created by the evaporated water vapour. Individual briquette factory dryers can have heat transfer surfaces of up to 4,000 m^ in the tubes. The tubes are fitted with retarder 'rods' to slow the passage of larger particles through the dryer and provide a more uniformly dried product. This arrangement is the inverse of the usual shell and tube heat exchanger, where the steam would pass through the tubes and the material to be heated is in the larger volume of the drum. This latter arrangement has been used for brown coal drying but is perceived to have a higher risk of fires and explosions with the partially dried coal being tumbled around in the larger gas volume of the shell. The briquette factory dryers use back pressure steam from the steam turbines of an integrated power station. The briquette factory is thus a cogeneration plant, providing steam for power generation and coal drying, with a higher combined efficiency than a stand-alone power station. The driers effectively behave as the condensers for the power station steam cycle, while the power station provides the energy for drying the coal, at least in part from the 'waste' heat in the steam cycle which would otherwise finish in a cooling water circuit. There are also a number of other contact dryers, such as shell and tube dryers and heat screw conveyors which function by indirect heating. They have had limited application to low rank coal drying and in each case the safety aspects require careful consideration before committing to an installation. 3.3.3. Non-evaporative Dewatering Non-evaporative dewatering processes involve the removal of water from the coal in liquid form, thus saving the latent heat of vaporisation and hence reducing greenhouse gas emissions. Another benefit is that some of the 'dissolved' inorganics in the coal (particularly sodium) are removed in the liquid water, thus reducing the ash fouling propensity of the coal. However, this can generate effluent water treatment problems, particularly in thermal dewatering processes, which can contain organic as well as inorganic contaminants in the expressed water. The cost of treating the effluent water to acceptable standards has been a major impediment to the commercialisation of thermal dewatering processes. i. i. 3. L Thermal Dewatering Processes Thermal dewatering is a generic term to describe processes where the application of heat enables the removal of moisture from the coal in liquid form rather than as water vapour by evaporation. These processes in effect accelerate the coalification process and produce a harder product with lower moisture, oxygen and porosity, and higher carbon and heating values. The effect is attributed to thermal decomposition of the hydrophilic oxygen containing functional groups in low rank coals, accompanied by the release of CO2, shrinkage of the pore structure and expansion of the water in the pores. The processes typically involve heating the coal to temperatures in the 200 - 350°C range at
Water in Brown Coal and Its Removal
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pressures above the saturated steam pressure, i.e. high enough to prevent evaporative drying of the coal. Under these conditions, water can be exuded from the coal and separated physically in liquid form. Allardice [2] provided a detailed review of the research into the kinetics and mechanisms of thermal dewatering processes on brown coal. This section will therefore focus on the subsequent research and process development. 3.3.3.1.1. Fleissner Drying The earliest thermal dewatering process was developed in the 1920's in Austria and is known as Fleissner drying, after the inventor [83]. This involves batch autoclave treatment of coarse lumps of low rank coal in steam at 180 - 240°C to produce an upgraded hard lump fuel. The water expressed in the steam treatment is drained from the autoclave and the sensible heat remaining in the coal is used to evaporate more water on depressurising. The technology was licensed by Voest Alpine and several plants have operated on harder brown coals in central European countries [50,51], with one commissioned in the late 1980's. A continuous version of the process was developed in the 1980's but to our knowledge not commercialised. The process was piloted in Victoria [84] and South Australia and was close to commercial application to supply the South Australian Railways before the introduction of diesel locomotives. The difficulty in applying the process to the soft Victorian brown coals arose from the need for the feed to be in lump form and maintain its integrity through the process. Assessments indicated that less than 50 % of a Loy Yang feedstock would have sufficient lump size to facilitate drainage of the expressed water from the coal. A Japanese version of the Fleissner process (the DK process) with improved heat recovery was piloted in the 1980's by Kamei and co-workers [85]. A significant difference between the earlier Fleissner process and the subsequent developments was in the process temperatures. The Fleissner process focussed on temperatures around 200°C where water exclusion due to shrinkage is maximised but organic contamination of the water, which increases with temperature, is confined to acceptable levels. 3.3.3.1.2. Evans-Siemon Process To improve the heat transfer to the coal, a process was developed by Evans and Siemon at University of Melbourne in 1970. Their idea [86] was to heat the coal in water or oil rather than steam in a continuous flow reaction system under sufficient pressure to prevent evaporation. This concept could be applied to lump, granular or slurried coal systems. A pilot plant for lump coal operating at up to 280°C was designed [87] but the project lapsed because of the lack of incentive at that time to improve the efficiency of brown coal use unless it also reduced costs.
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3.3.3.1.3. Hydrothermal Dewatering or Hot Water Drying The slurry version of Evans-Siemon was developed further at University of North Dakota EERC [88] and SECV [89,90]. This version is generally referred to as Hydrothermal Dewatering (HTD) or less frequently as Hot Water Drying. In the process (Figure 3.14), a slurry of brown coal is heat-treated to around SOO^'C under sufficient pressure to prevent evaporation. After cooling and depressurising, excess water can be separated from the product slurry. The decomposition of the coal structure, which occurs under these conditions, could be regarded as analogous to accelerated coalification. Applied to high moisture brown coals, the product is an upgraded coal slurry with an energy content greater than the solid as-mined coal. Allardice and co-workers [90] reported the results of the hydrothermal dewatering (HTD) studies by the SECV/HRL in a 1 m^ hr' HTD slurry pilot plant. These studies confirmed that: • HTD treatment at 275 - 325°C resulted in non-evaporative moisture reduction and produced a pumpable coal water slurry which, for Latrobe Valley brown coals, had a lower moisture content and higher net wet specific energy than the starting coal. • The process also removed some inorganic constituents from the coal, which reduced ash deposit formation problems in combustion and liquefaction systems. Acid washing or the addition of multivalent exchangeable cations to the feed slurry further reduced the concentration of troublesome ion exchangeable elements such as sodium. • The overall energy recovery was about 97 %. • The average dry solids recovery was over 90 % including the losses due to soluble organic material and to water and gas released from the breakdown of coal functional groups during the HTD process. For Morwell and Loy Yang coals, the soluble organics emitted within the water was about 1 % (on dry coal basis) and the CO2 yield varied from 5.0 to 6.5 %. • Several treatment processes were investigated at laboratory and/or pilot scale using effluent water from the HTD pilot plant although with limited success. The processes tried included: - flocculation/filtration and membrane filtration; - reverse osmosis and electro dialysis reversal for inorganic removal, both of which suffered from membrane fouling by the organic wastes; - aerobic and anaerobic biological treatment for removal of the organic wastes; and - absorption onto feed coal or char. • Tests on a suite of international low rank coals demonstrated the wide applicability of the process for upgrading low rank coals, although the greatest beneficiation was achieved for coals with greater than 50 % raw coal moisture and 15 % oxygen. The HTD product was found to be technically suitable for use in coal fired gas turbine power generation, industrial boilers, pressurised fluidised bed combustion.
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Water in Brown Coal and Its Removal WASTE GAS£S
^/'^^4^^^^^^''''^''''^^^^-'QM
SEPmAim
yATER
miEQ
COAL,/.;
CENfRIFUOE
yA"E«
SlUHRt crSl nOlSTURPi
PPODUCT SMiRRr CSO% MOISTURE)
Figure 3.14 Hydrotheraial dewatering process concept. Courtesy of SECV.
gasification and liquefaction applications. Unfortunately, the most prospective short term application of the technology, power generation in a coal slurry fired gas turbine [89], was abandoned by the SECV, in favour of other options such as IDGCC, for future power generation. The CRC for Clean Power from Lignite has also conducted further studies on the process [Chaffee and co-workers [91]), but its emphasis has shifted to MTE for predrying brown coal for power generation (Mcintosh, [92]). A Japanese HTD variant piloted by Hashimoto and Tokuda [93] reportedly produced higher solids density slurry products (> 60 % dry coal) from Loy Yang coal than achieved in Victoria. The use of surfactants contributed to this improvement, but differences in viscosity measurement techniques and the definition of 'pumpable' slurries may also have contributed to the difference. The Japanese have also studied the process's potential with respect to upgrading Indonesian brown coals but were discouraged by the cost and difficulty in cleaning up the effluent water. Companies such as Shell, Bechtel and Koppelman (K-Fuel) have patented other variants of the thermal dewatering process for low rank coals and the process has also been investigated for lignite upgrading in India and China. Recently Nakagawa and co-workers [94] reported that the organic contaminants in the recovered water from HTD of brown coal increased with increasing process temperature, reaching 1.5 % of the coal carbon at 300**C. They demonstrated that the organic carbon contaminants could be completely removed by pressurised hydrothermal gasification at temperatures as low as 350*'C using a novel Ni/carbon catalyst.
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Most of the studies on the development of pumpable brown coal slurries have involved adding water and, usually, surfactants. The addition of water is detrimental since it further reduces the already low heating value of the fuel or increases the quantity of water to be removed in the slurry upgrading process. Recent work by the Institute of Applied Energy, Japan, has demonstrated that a pumpable brown coal slurry can be produced from Latrobe Valley brown coals without the addition of more water as is normal in HTD studies. Katayama and Onozaki [95] achieved this by kneading the coal at elevated temperature (150 - 200°C) under saturated steam pressure to produce a stable slurry. This could make an improved feedstock for HTD treatment at higher temperatures. When the product is pumped into a hot reactor, the water in the slurry rapidly evaporates and the coal particles are 'atomised', so that the slurry is suitable as a direct feed to a coal gasifier. The authors claim that the slurry can be stored and transported and/or used as feed for briquette production. We suggest that the HTD process still has unrealised potential to convert brown coals into a clean, safe, exportable form. Trends in international coal markets are likely to make the technology more attractive as a means of accessing under-utilised low rank coal resources in the longer term, as black coals become scarcer and more expensive. The major concerns with generic thermal dewatering processes are the high capital cost of the plant, particularly the heat exchangers, the problems of treating the contaminated effluent water, and the low slurry densities achievable relative to black coal water slurries (> 70 %). 3.3,3.2, Mechanical Thermal Expression Initial Victorian studies of "press dewatering" at ambient temperatures by Banks and Burton [96,97] and later by Guo and co-workers [98] found that such processes could remove up to 80 % of the water in the coal. However, further development was abandoned as impractical because of the high pressures, which had to be applied to the coal for residence times of 20 minutes or more. Mechanical thermal expression (MTE) is a promising improvement initiated by Strauss and co-workers at the University of Dortmund, Germany [99-102]. The MTE process (Figure 3.15) combines the mechanical press dewatering concept with the use of elevated temperatures in the range 150 - 220°C. Such temperatures are high enough to 'soften' brown coal and enable dewatering at substantially lower mechanical pressures (e.g. 2 - 1 2 MPa) and residence times. The temperature is also low enough to avoid any appreciable chemical change, for example in elemental composition, during dewatering. The CRC for Clean Power from Lignite has recognised the potential of applying MTE dewatering to Victorian brown coal. Extensive studies on a laboratory scale have been carried out in Australia [98, 103-106] and in Germany [107,108]. Hulston and coworkers [106] have shown that the percentage water removal increases approximately linearly as a function of temperature within the range examined (Figure 3.16). The effectiveness of water removal is remarkably improved by doubling the applied mechanical pressure from 2.5 to 5.1 MPa. However, a further (approx) doubling of the applied mechanical pressure from 5.1 to 12.7 MPa does not have so marked an effect.
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Thus, the higher temperature used in MTE decreases the pressure and residence time required to manageable levels and enables significant moisture reduction in a realistic time frame and for a minimal expenditure of energy. Some energy can be recovered from the expressed water. MTE processing of brown coal produces strong, dense compacts with variable amounts of residual water, dependent upon processing conditions. Hulston and coworkers [106] found that, as the samples equihbrate with the atmosphere, they lose both
Steam
MTE
waste water 'cold'
Figure 3.15 Mechanical thermal expression of water from brown coal. Reprrinted with permission from Ref 102. Copyright 1999 VGB. MTE Product: Loy Yang Coal 100 90 80 70 60 50 40 30 20 10 100 120
140 160 180 200 220 240 260 Temperature (C)
Figure 3.16 Water removed from Loy Yang Coal by MTE as a ftinction of process conditions (after Hulston and co-worker [106]). Initial Moisture = 59.7 % (1.48 g/g db).
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"o
>
2.5 MPa
5.1 MPa
12.7 MPa
D Raw lignite to wet MTE product volume reduction D Wet to dry MTE volume reduction • Dry MTE product volume Figure 3.17 Volume reduction during MTE processing and drying [106].
moisture and volume. Figure 3.17 illustrates the volume losses that occur during and after MTE processing. The CRC for Clean Power from Lignite has also piloted a version of the process at 1 t h"' scale and plans for 15 t h ' and 100 t h'^ demonstration units have been developed with support of the local brown coal power generators and the State Government, as reported by Brockway and Jackson [110]. Few details of the Australian process configuration are publicly available, but it is targeted at a more continuous operation. The CRC is promoting the development of MTE as a practical concept to retrofit to existing boilers or to pre-dry the feed coal for an IGCC plant. As reported by Mcintosh [92], the CRC has concluded that MTE is less expensive and provides greater efficiency improvements in these applications than HTD or steam fluidised bed drying. Bergins and co-workers [109] calculated that potential power generation efficiency improvements of between 13 and 21 % could be achieved for a brown coal dewatered from an initial moisture content of 57.5 % and 65.5 % respectively to a final moisture content of 25 % as fired.
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3.3.3.3. Solvent Dewatering Miura and co-workers [111] recently introduced a new concept of non-evaporative dewatering coal via solvent extraction. Using a fixed bed arrangement for contacting the coal, they investigated a range of solvents at temperatures up to 200°C. The extent of dewatering was governed by solvent polarity, temperature and solid-liquid contact time, but water contents as low as 2 % could be achieved with Morwell coal under appropriate conditions. Since the solvent is recycled, a practical system must employ a solvent that can solubilise substantial proportions of water at high temperature (150 - 200°C), but which is also substantially immiscible with water at low temperature. This facilitates separation of the organic and aqueous phases by decantation at ambient conditions. Polar solvents (e.g. methanol and ethyl acetate) provided good water removal, but the separation of water from the liquid mixture was difficult. With non-polar solvents (e.g. tetralin), good liquid separation efficiency was achieved, but large quantities of solvent were required to extract all the water. Using tetralin as the solvent, Miura and coworkers [111] claimed that this process had an energy requirement of < 1 MJ per kg of removed water. A recent report by Kanda and co-workers [112] indicates that this concept can be extended to the use of dimethyl ether (DME) as a solvent. Water is highly miscible in DME, which can be used to 'extract' the water from coal at ambient temperature (30°C) and modest pressures {ca. 0.8 MPa). The use of pressure is required to maintain DME in the liquid phase; since, at ambient pressure, it boils at -25°C. In its current stage of development, the process involves passing liquid phase DME through a fixed bed of brown coal. The liquid phase is then de-pressurised to evaporate DME, leaving the separated water as the liquid phase product. DME is recovered by compression and a series of heat exchangers are incorporated to facilitate recovery of most of the heat that would otherwise be lost during DME expansion. Kanda and co-workers [112] reported that, using this method, the moisture content of a Loy Yang brown coal sample could be reduced from 54 % to only 4 % in laboratory studies. The authors claim that the process consumes 948 kJ kg'^ of removed water, similar to Miura's estimate [111] and lower than the values reported by most other drying processes. The DME process has several other potential positive features. Because the process operates at ambient temperatures, the extracted water is unlikely to suffer from the organic contamination and clean up problems associated with thermal dewatering processes. DME is non-toxic and will become more widely available, because it is increasingly seen as a possible substitute fuel for diesel and LPG. The cost is also expected to fall as production grows. 3.3.3.4. Effluent Waterfi'omNon-Evaporative Dewatering Processes Unlike conventional evaporative processes, non-evaporative dewatering processes, such as Fleissner, HTD and MTE, produce a liquid by-product stream that must be
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Chcq)ter3
managed and disposed of or re-used in an environmentally responsible way. The liquid water stream contains both organic and inorganic components. An understanding of the composition and concentration of species contained in this effluent water is clearly essential to a full evaluation of water quality issues associated with the various dewatering technologies. 3.3.3.4. J. Inorganic Components Inorganic components within the natural brown coal may be present in at least three forms (also see Chapter 2) as shown schematically in Figure 3.18. Some of the inorganic components are present as discrete mineral inclusions (e.g. pyrite and clays) and are largely unaffected by the dewatering processes. A second type of inorganic matter is present in the form of water-soluble salts. These are contained within the interstitial pore water and are removed with the dewatering product water. Favas and co-workers [104] found that this soluble inorganic matter is almost entirely removed in the product water stream in the HTD process. In the MTE process, Kealy and coworkers [103] reported that it was only removed in proportion to the reduction in total moisture. The situation is also depicted in Figure 3.18. For Latrobe Valley brown coals, the water-soluble material is mostly sodium chloride; but soluble sulphate salts may also be present [113]. A third type of inorganic matter consists of inorganic cations that are ion exchanged as cations at the site of carboxylate functional groups on the brown coal surface. Some of these cations can also be removed by MTE when acid is added [104], although it seems unlikely that this would be practicable on an industrial scale. Reduction in exchangeable cations also occurs with acid addition in HTD processing [90] and was found in earlier Evans-Siemon studies. In non-evaporative dewatering processes that operate at higher temperatures, more of these ion exchanged cations are released to the product water stream as a result of the decomposition of carboxylates [2]. Hence, the concentration of residual inorganics in HTD dewatered products (produced at higher temperatures) is generally lower than for MTE products [105]. 3.3.3.4.2. Organic Components Organic matter is also released into the product water during the dewatering processes. A comparison of the organic carbon levels in product water from a variety of non-evaporative dewatering studies has been recently assembled (Table 3.5). Where possible, the organic carbon data are compiled both in terms of product water concentration (g L"^) and in terms of organic carbon released from the raw coal (g kg" dry basis). The latter measure generally provides a better basis for comparison, as they are less influenced by experimental procedures (for example, different ratios of water to coal charged to experimental reaction vessels by different researchers). Table 3.5 demonstrates that higher yields and higher concentrations of organic carbon are produced at higher treatment temperatures. This same point can also be concluded from the specific reports cited in this table (see also Qi and Chaffee [114]
Water in Brown Cool and Its Removal
119
and Favas and co-workers [104]). In fact, it is reasonable to conclude that the final treatment temperature is more critical to the degree of organic contamination in the effluent water than the particular thermal dewatering process used. Detailed analytical work has shown that the most abundant organic compounds that can be identified by gas chromatography-mass spectrometry (GC-MS) are low molecular weight organic acids and phenols. The detailed identification of individual
1. Mineral Inclusions
Surface Hydroxyl Group
^ ;) Multilayer Water
3. Dissolved Salts
"^ 2.Ton Exchanged
MTE
^ Surface Hydroxyl Group
Dissolved salts are expressed in proportion to expressed water Figure 3.18 Physico-chemical model of natural brown coal illustrating three different types of inorganic matter. Dissolved salts are removed in proportion to the volume of expressed.
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compounds has been reported for water from pressurised steam drying [117], HTD [115] and MTE [113]. However, it appears that only a fraction of the organic components present are usually accounted fr)r. For effluent water from the MTE process, the development of improved analytical techniques has shown that, of the identifiable components, aliphatic and aromatic acids are 1 - 2 orders of magnitude more abundant than phenolic components (Table 3.6). From a comparison of the GC-MS results with those from field flow fractionation (FFF) analysis, it has been estimated that the GC-MS identifiable components account for only ca. 25 % of the total organic carbon for a variety of Latrobe Valley coals [113].
Table 3.5 Comparison of organic carbon levels reported in the product water from nonevaporative dewatering Victorian brown coalfromQi [113]. K/iTr: n 1 -51 D ^ u UTT^ n i ^i ^TD pilot plant MTE[n3] BatchHTD[115] f"^^^^^ Temperature (°C) Brown coal TOC (g/L) OC (g/kg dry coal)
120-200 Loy Yang 0.08 - 0.4 0.4-2.2
Pressurised steam drying**[in] 182-222 Loy Yang NA 0.1*-2.3
300 Loy Yang L32 NA
250-350 Loy Yang 0.3*-7 2*-50*
* estimated from charts presented in the papers. ** this procedure is similar to a Fleissner steam autoclave procedure. NA - not available. Table 3.6 Quantities of major organic groups in MTE water produced under a range of conditions,fromQi [113]. 150°C 25MPa
Loy Yang A 200^C 200^C 25MPa 6MPa
200X 6 MPa
Morwell 200"C 6 MPa
200°C 25 MPa
Total mono- & dihydroxy phenols (mg/kg, db)
0.31
3.0
13
18
22
52
Total tri-hydroxy phenols (mg/kg, db)
0.038
0.26-0.85
0.32-1.1
0.89-3.0
1.0-3.4
5.6-19
Total aliphatic acids (g/kg, db)
0.091
0.43
0.65
0.83
0.74
1.3
Total aromatic acids (g/kg, db)
0.070
0.19
0.38
0.47
0.54
0.67
Total of compounds identified by GC-MS (g/kg, db)
0.16
0.62
1.0
1.3
1.3
2.0
Total organic carbon (gC/kg,db)
0.71
1.3
2.2
2.5
2.9
3.6
Water in Brown Coal and Its Removal
121
3.3.3.4.3. Effluent Water Utilisation The quantity of water that would be released by any of the non-evaporative drying processes is large. For example, under modest MTE conditions (180°C and 6 MPa), it is expected that 60 - 70 % of the original water would be removed. For a typical Latrobe Valley brown coal (with 60 % moisture), if MTE is applied to the output from a new mine producing say 30 million tonnes per annum of brown coal (comparable in scale to the Loy Yang development), it can be estimated that the MTE plant would produce ca. 13 Gigalitres (GL) of effluent water per year. This is a substantial portion of the total water usage in the Latrobe River Basin (currently ca. 130 GL) and it is clear that a plan would be needed to manage this by-product water in an appropriate manner. Butler and co-workers [118] have considered the potential uses for MTE water and identified industrial cooling water, agricultural irrigation in the region and environmental recharge (re-injection to the underground water table) as possibilities. Since the concentrations of both organic and inorganic components exceed guideline levels for these applications, it is clear that some remediation will be required [118]. However, the extent of remediation for MTE water will not be as great as required for the effluent water derived from dewatering processes that function at higher temperatures such as HTD. The cost and difficulty in treating HTD effluent water has been a major obstacle to its commercial development, compared for example with the lower temperature Fleissner process effluent. MTE by-product water has now been reasonably well characterised by classical procedures. However, there is a further range of industrially relevant physical characterisation parameters (e.g. colour, turbidity, conductivity, biochemical oxygen demand, total dissolved solids) for which systematic data must still be gathered to enable comparison with guidelines/limits for the envisaged applications. These parameters form the basis for specification of public water quality classifications. It is understood that the water produced in the demonstration scale MTE trials at Frechen and Niederaussem was treated using a microbiological fixed bed process. This approach was adapted from a process to treat wastewater from coke manufacture. Only limited details are available from work by Reich-Welber and Felgener [119]. The pressurised hydrothermal gasification process recently reported by Nakagawa and co-workers [94] for treatment of HTD effluent offers a novel treatment alternative for MTE water. In this process, the organic components in the effluent are gasified under water at temperatures as low as 350''C and pressures above saturation to prevent evaporation of the water.
3.4. MOISTURE AND BINDERLESS BRIQUETTING OF BROWN COAL 3.4.1. Background to the Briquette Industry Brown coal briquettes have been produced in Europe for centuries, initially by kneading and moulding brown coal and water to form 'mud bricks' which were dried in
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the sun and stockpiled for later use. 'Modem' binderless briquetting of dried brown coal was developed in central Germany around 1850 and is now the second largest use for brown coal around the world (after power generation). Brown coal briquettes led the industrial development of Germany as the production increased from 750,000 tonnes in 1885 to 60 Mtpa during World War II [81 ]. In recent years, world briquette production has been substantially reduced by the decimation of the industry in Germany, where production fell from over 50 Mtpa in 1990 to less than 4 Mtpa in 1998. This resulted from the restructuring of the German economy following unification, the environmentally unacceptable quality of much of the coal available for briquetting in eastern Germany and the expansion of the natural gas grid through areas previously reliant on brown coal for industrial and domestic fuel. Large-scale briquetting of Victorian brown coal commenced at Yalloum in 1924 using the German technology. This followed abortive attempts at Yalloum North in the 1890s. The original Yalloum plant closed in 1970. The Morwell briquette and power plant commenced operation in 1959, with a nominal production capacity of 1.2 Mtpa of briquettes and 170 MW of electricity. It operated until December 2003 when a major fire in the dry coal handling sections closed the briquetting plant. The Morwell briquette and power complex was one of the largest cogeneration plants in the Southern Hemisphere, but 4 months after the fire, a decision is still awaited on whether to partially rebuild the briquetting capacity. This is a salutary lesson on the risks associated with the production, storage and handling of dried brown coal, even for an experienced operator. Briquette production in Victoria peaked at 1.9 Mtpa in 1966 prior to the introduction of natural gas. Before the fire, production had fallen to below 400 ktpa, primarily for
CRUSHING 3jfc AND SREENING
Figure 3.19 A schematic diagram of brown coal briquetting. Courtesy of SECV.
Water in Brown Coal and Its Removal
123
industrial and commercial heat applications, char production and occasional niche market export opportunities because of the high quality of the product. Briquettes, as a cheap source of energy, underpinned the industrial development of Victoria and its manufacturing industries after the first and second world wars. 3.4.2. Briquetting Process In the briquetting process, shown schematically in Figure 3.19, the raw brown coal is crushed to - 8 mm, dried in rotary steam tube driers from 66 % to about 15 % moisture. The dry coal is then cooled to about 40°C and briquetted without a binder, using an Exter reciprocating extrusion press at 1200 kg cm'^ pressure to form hard compacts with an energy content (21 MJ kg'^ net wet basis) comparable to many higher rank coals. The briquette factory dryers use low grade 'waste' energy (backpressure steam from the integrated power station) to dry the coal. This cogeneration process therefore produces lower greenhouse gas emissions than stand alone brown coal briquetting processes, which use high grade heat to dry the coal. Herman in his classic 1952 book 'Brown Coal' [81] devotes 279 pages to a detailed description of the binderless briquetting process. More recent reviews of the technology have been published by Kurtz [120-123] and others [2,124]. Because of the poor strength and weathering characteristics of briquettes made from Morwell brown coal, production at the Morwell briquette factory from the outset used Yalloum coal transported to Morwell. However, because of changes in the quality of Yalloum coal with the development of Eastfield and the competition between the privatised coal mines in the 1990's, Loy Yang coal has recently provided 90 % of the briquetting coal, with Yalloum Eastfield providing the balance. With appropriate selection of coal from the Loy Yang mine, an improvement in briquette quality, with ash yields generally below 1.5 % dry basis, has been achieved. A number of alternative agglomeration technologies for brown coal have also been proposed. These could supplement or eventually replace the current technology. Processes such as double roll pressing, pellet milling and drum pelletising could offer improved economics for a new installation, although questions remain on product quality and self-heating characteristics of these products. To date none of these alternative briquetting processes have replicated at a pilot scale the strength and weather resistance of the commercially produced binderless extruded briquettes. Another approach has been to use binders to briquette the dried brown coal. Unfortunately, the high cost of suitable binders and the relatively high concentrations required because of the porosity of the dried coal make this an unattractive route to develop a low cost bulk commodity fuel. 3.4.3. Impact of Moisture on Briquettes Moisture is a critical factor in the binderless briquetting or pelletising of dried low rank coals. The bonding mechanism is generally accepted as hydrogen bonding between
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Chapters
coal particles via the oxygen containing functional groups on the coal surfaces and the moisture remaining in the coal after drying down to the multilayer moisture region [125-128]. Control of the moisture content is critical to optimising the briquette strength and minimising the shrinkage (and breakage) on equilibration with the atmospheric humidity in storage and handling. For Victorian brown coals, the optimum moisture content for briquetting is around 15 %. The inferior weathering behaviour found with briquettes made from Morwell coal, relative to briquettes from Yalloum or Loy Yang coal, is also associated with the coal moisture. The inferior performance of Morwell in this regard is attributed to rapid swelling of the briquettes on wetting, due to the different exchangeable cation composition of this coal producing greater cation hydration, higher swelling and loss of strength. Morwell coal contains much higher levels of exchangeable cations, particularly calcium and magnesium, than Yalloum or Loy Yang. Moisture can also be a contributing factor in the spontaneous combustion of briquettes (and dried brown coal) in storage. Heat (of adsorption) is generated when moisture is re-adsorbed onto briquettes. This can occur if there is an increase in humidity or even light rain, if storage under low humidity conditions has partially dried the briquettes. The resulting increase in briquette temperature can accelerate oxidation by air to the point where spontaneous combustion occurs. As a rule of thumb, the rate of oxidation approximately doubles for each 10°C rise in temperature. The industry has developed safe operating practices to minimise and manage the risk of spontaneous combustion in storage and transport, which can be a serious concern for new operators with unproven products. The subject of spontaneous combustion in brown coal based materials was reviewed in detail by Mulcahy and co-workers [129] and case studies on the shipping of brown coal briquettes have been reported by Cunningham and co-workers [130]. Procedures for the safe shipping of brown coal briquettes have been incorporated into the International Maritime Organisation, Code of Safe Practice for Shipping Bulk Cargos [131].
3.5. CONCLUSION In a Royal Commission on Coal in 1890, the Victorian Government Analyst Cosmo Newberry commented that "the brown coal in the Latrobe Valley would be an excellent fuel if it could be rid of its water'' [129]. Over 100 years later, this is still the major impediment to the economic development of Victorian brown coal and other low rank coal resources. A huge market exists for a technology, which can convert low rank coals to high-energy transportable fuels at a comparable cost to exported black coals. Despite the development of many innovative drying technologies, none have yet managed to clear this hurdle. It is important to note that any dewatering or drying process that can remove the water from brown coal in liquid form, i.e. without the need to supply the evaporative
Water in Brown Coal and Its Removal
125
energy to dry the coal, has the potential to reduce the greenhouse gas emissions from use of these coals by up to 25 %. The current emphasis on reducing greenhouse gas emissions has added an extra factor for consideration in the selection of brown coal drying processes for further development. The two attracting the most interest at present for application to Victorian brown coal are mechanical thermal expression at the CRC for Clean Power from Lignite in Victoria [92] and the fine grain version of steam fluidised bed drying in Germany [66]. In addition, two emerging Japanese developments, the upgraded brown coal pilot plant in Indonesia [71] and the concept for solvent dewatering with DME [112], also warrant further evaluation.
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Advances in the Science of Victorian Brown Coal Edited by Chun-Zhu Li © 2004 Elsevier Ltd. All rights reserved.
Chapter 4 Pyrolysis of Victorian Brown Coal Jun-ichiro Hayashi^ and Kouichi Miura^ ^ Centre for Advanced Research of Energy Technology Hokkaido University, N13-W8, Kita-ku, Sapporo 060-8628, Japan "Department of Chemical Engineering, Kyoto University, Kyoto-Daigaku Katsura, Nishikyo-ku, Kyoto 6J5-85JO, Japan
4.1. INTRODUCTION When coal is heated up to elevated temperature, it will undergo pyrolysis. The thermochemical reactions during pyrolysis include both bond-breaking and bondforming reactions, resulting in the formation of volatile products and non-volatile solid residue termed char. Pyrolysis or the thermal decomposition of coal can be considered as the initial steps of all thermochemical utilisation processes of coal such as combustion, gasification, carbonisation and liquefaction. Therefore, understanding the thermal decomposition behaviour of the Victorian brown coal is central to all thermochemical utilisation processes of the brown coal. Due to its great importance, great efforts have been made to understand the pyrolysis behaviour of the Victorian brown coal under a wide range of conditions using a variety of experimental techniques. Since Jones [1] reviewed the previous studies on the pyrolysis of the Victorian brown coal in 1991, there have been undoubtedly significant progresses in this area. These progresses, from both experimental and modelling studies, form the topics of this chapter.
4.2. EFFECTS OF OPERATING VARIABLES ON THE PYROLYSIS OF BROWN COAL 4.2.L General Feature of Coal Pyrolysis When a coal particle is heated up, the initial intra-particle thermochemical reactions that result in the formation of volatiles and char are in general summarised as the primary pyrolysis. The primary volatiles consist of light gases (such as H2, CO, CO2, H2O, CH4 and other low hydrocarbons) and heavier products termed tar. The tar is usually defined as the products heavier than C6 compounds such as benzene, or otherwise as the products that are not evaporated at ambient temperature. The tar from the primary pyrolysis, hereafter referred to as the primary tar, is a major component of
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the primary volatiles. The volatiles released from the particle (the primary volatiles) further undergo thermal cracking in the extra-particle gas phase at elevated temperatures, which is called the secondary pyrolysis. During the secondary pyrolysis, the primary volatiles are converted into light gases and carbonaceous solid termed soot or coke. Similar secondary thermal cracking reactions also take place inside the solid char particles. As was described in Chapter 2, coal macromolecules can be represented by two kinds of structural units: aromatic clusters (aromatic ring systems) each of which consists of a mono or polyaromatic ring with peripheral functional groups, and inter-cluster linkages connecting two or more clusters. It is believed that aromatic clusters are the main sources of tar while non-aromatic light gases are derived from the peripheral groups and inter-cluster linkages. The volatiles are mainly evolved from a coal particle through the formation of low-molecular-mass fragments from the thermal degradation of macromolecules followed by their intraparticle transport out of the particle, which involves diffusion and/or convection. Small molecules also exist in Victorian brown coal although they make much smaller contributions to the formation of volatiles (especially tar) than the macromolecules. It is clear that the evolution of volatiles and the formation of char is a very complicated process, involving both chemical and physical processes. In addition to the physico-chemical properties of coal, many operating variables can also affect the yields and chemical composition of the primary volatiles and char. The pyrolysis of the Victorian brown coal has been studied under a variety of conditions that are defined by the coal particle time-temperature history and the temperature and chemical nature of the gas atmosphere surrounding the particles. The findings from these studies are summarised below. 4.2.2. Reactors Available for Investigating Pyrolysis Characteristics A number of reactors and reactor systems have so far been proposed and used to provide a wide range of experimental conditions to understand the pyrolysis behaviour of Victorian brown coal. The features of several types of reactors employed in the studies introduced in this chapter are briefly described below. 4,2.2,7. Fixed-Bed Reactors Several types of fixed-bed reactors have been used for studying the pyrolysis of brown coal. Thermogravimetric reactors (TGR), also called thermogravimetric analysers or thermobalance reactors, are employed for the in situ measurement of the mass of coal sample upon heating. In general, a fixed-bed of a small amount (1 - 100 mg) of coal is placed in a cell (cup or pan) made of inert material such as platinum. The cell is heated at a constant heating rate lower than 1 °C s'^ in a flow of inert gas. Because no flow of inert gas is forced through the fixed bed (or little flow when a basket made of mesh is used), the primary volatiles have to diffuse through the fixed bed toward the carrier gas phase.
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Pyrolysis with slow heating rates is also performed in a fixed-bed with forced flow of carrier gas through the bed. The forced inert gas flow can sweep the primary volatiles away from the fixed bed. Increasing the carrier gas flow rate and/or reducing the bed height can shorten the residence time of the volatiles and therefore suppress or minimise their secondary pyrolysis within the bed. This is particularly important if the secondary pyrolysis of the volatiles is significant over the surface of pyrolysing coal particles. However, for the pyrolysis under atmospheric pressure with heating rates lower than 1 °C s', the presence or absence of forced inert gas flow through the fixed bed is not necessarily a determining factor for the total volatile yield [2]. 4.2.2.2. Wire-Mesh Reactors This type of reactor, hereafter referred to as WMR, has particular features effective for investigating the primary pyrolysis over a wide range of heating rates up to even 5000 °C s'^ [3-7]. A mono-layered coal sample (amount < 10 mg; particle size < 200 ^m) is sandwiched between two wire-mesh sheets made of stainless steel or platinum. The wire-mesh serves as the resistance heater that is stretched between two electrodes and heated with an alternating current using a computer-based temperature control system. The temperature of the wire-mesh, that is believed to be nearly equal to that of the coal/char particles, can be raised at a constant rate even when it is of the order of 10^ °C s'\ While the coal particles are heated, the carrier gas continuously passes through the mesh at a velocity sufficiently high to sweep the primary volatiles away from the vicinity of coal/char particles and cool them to ambient or cryogenic temperatures immediately. In other words, the secondary pyrolysis of the volatiles can be minimised in a WMR regardless of the heating rate. 4.2.2.3. Curie-Point Reactors Curie-point reactors, hereafter referred to as CPRs, enable the pyrolysis of coal at a heating rate in the order of 10^ °C s"' with minimised extents of the secondary pyrolysis of the primary volatiles [8,9]. In a usual pyrolysis run, a small amount of coal particles ( 1 - 3 mg) with sizes smaller than 200 jam is wrapped tightly in a ferromagnetic foil made of iron or iron-based alloy. After placed in a quartz tube, the foil is heated inductively up to its characteristic Curie-point temperature (e.g. 764 °C for pure iron) within a period as short as 0.2 s and the temperature is kept until the inductive heating is stopped. Due to the accuracy of the peak temperature that is defined as the Curie-point temperature, there is no need of measuring it. The vapour of the primary volatiles is evolved fi-om particles into the inner space of the folded foil and then forced out of the foil by the convective flow of the vapour itself. The volatiles are then cooled down to a temperature low enough to avoid the secondary pyrolysis because only the ferromagnetic foil is heated inductively. The secondary pyrolysis, only to a small extent in a CPR, may occur inside the foil. As mentioned in this chapter, it seems to be difficult to minimise the secondary pyrolysis when the primary volatiles are extremely unstable. It is also possible that very
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rapid evolution of the primary volatiles causes some build up of pressure inside the foil. Another disadvantage of using a CPR compared with a WMR is the difficulty to vary the heating rate. However, a CPR has a definite advantage over a WMR in terms of analysing the volatile products, particularly, lighter ones. In general, the flow rate of the carrier gas used for the pyrolysis in a CPR is only 50 - 100 ml min'' (corresponding to a gas velocity of ca 0.1 m s'^ through the quartz tube) and this is much lower than that for WMR, at least 3000 ml min"\ Such a low carrier gas flow rate makes it possible to subject the whole gaseous products to analysis using, for example, gas chromatography without any special techniques (such as a trap-and-purge technique). The direct analysis of dilute volatile products from a WMR is a very difficult task. 4.2.2.4. Drop-Tube Reactors Drop-tube reactors have been employed for the pyrolysis of pulverised coal particles by heating them at rates in the order of 10^ - 10^ °C s'\ In general, coal particles are continuously introduced at a fixed rate together with a downward flow of carrier gas into a vertical tubular reactor. Reactors of this type are often called entrained-flow reactors if the linear gas velocity is much higher than the terminal velocity of dropping particles, while termed fi-ee-fall reactors if the terminal velocity is comparable or even higher than the gas velocity. The heating rate of particles is affected by operating variables such as temperature distribution along the reactor axis, particle size/density and gas velocity. It is therefore difficult to exactly determine or control the timetemperature history of the particles such as the residence time and heating rates of the particles. However, drop-tube reactors can provide environments where the primary pyrolysis and secondary pyrolysis take place concurrently as in practical reactors for coal gasification and combustion. Drop-tube/fixed-bed reactors [10,11,12], that have features of drop-tube and fixedbed reactors, have been developed and used for particular purposes. For one type of such reactor [10], pulverised coal sample is fed into a quartz-made drop tube at a constant feeding rate together with the carrier gas. The bottom of the tube is equipped with a quartz-made frit. Coal/char particles are dropped onto the frit and a fixed-bed of char particles is formed. The volatiles formed are swept away from the drop tube by the carrier gas that is forced to pass through the fixed-bed of char particles and then the frit. Thus, this type of reactor is suitable to investigate chemical interactions between nascent char and volatiles formed from the rapid pyrolysis. For another type of such reactor [12], a pulse of pulverised coal (less than 10 mg) is injected into a drop tube and then a monolayer of char particles is formed on the frit. The volatiles formed in the drop tube are swept away from the vicinity of nascent char particles by the carrier gas flowing through the tube at a very high velocity. The interactions between the volatiles and char particles are effectively minimised. Moreover, the nascent char can further be heated for a desired period.
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4.2.2.5. Fluidised-Bed Reactors Fluidised-bed reactors have been employed by many researchers for the pyrolysis of coal with the main purposes of investigating the effects of operating variables such as temperature and pressure under conditions where both the primary pyrolysis and the secondary pyrolysis reactions take place. Coal particles are fed into a bed of hot fluidised particles together with the carrier gas: the rapid heat transfer from the fluidised particles to the coal particles realises heating rates as high as 10^ - 10^ °C s"\ Among the nascent products from the primary pyrolysis, volatiles undergo secondary reactions while they pass through the fluidised bed (dense phase) and the free board above the bed. The residence time of the char particles depends on their sizes and bulk densities; the time distributes over a certain range even when well-sized particles are used. Very coarse particles reside in the fluidised bed without being elutriated out of the bed. A tendency of some brown coal particles to melt upon heating means that these char particle may stick to the fluidised particles until the conclusion of the experiment. Silica or zircon sand particles that have nearly inert surfaces are used for investigating the vapour phase secondary pyrolysis of volatiles in the fluidised-bed (dense bed) and/or in the free board. Particles having catalytic activities can be employed instead of inert particles to study the heterogeneous catalytic cracking of the volatiles in the fluidised bed. Some of the fluidised-bed reactors so far reported have particular features. Temperatures of the fluidised bed and free board can be controlled independently. Hayashi and co-workers [13] carried out the pyrolysis of a brown coal where the temperature of the fluidised bed was fixed at 600 °C while the free board temperature was varied in a range from 600 to 900 °C. With this method, the volatiles formed under the same pyrolysis conditions were subjected to secondary pyrolysis at different temperatures. The concept of independent control of the fluidised bed and freeboard temperatures was also realised by Li and co-workers [14] who developed a two-stage fluidised-bed/tubular reactor. A quartz frit was equipped between the two reaction zones in order to avoid the introduction of char particles into the tubular reactor. Li and coworkers [15] also developed another fluidised-bed reactor that was equipped with a quartz frit above the fluidised bed. Nascent char particles were elutriated out of the fluidised bed but kept underneath the quartz frit to form a fixed-bed of char particles. The char particles in the fixed bed were continuously exposed to volatiles coming up from the fluidised bed together with additional char particles. 4.2.3. Effects of Heating Rate on the Primary Pyrolysis under Atmospheric Pressure Effects of heating rate on the tar and total volatile yields from the Victorian brown coal was first reported by Tyler [16], who found that these yields from the pyrolysis in a fluidised-bed reactor were much higher than those from the pyrolysis in a fixed-bed reactor. It is clear that the primary pyrolysis rather than the secondary one is responsible for such effects.
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139
Pyrolysis of the Victorian brown coals using a WMR was first reported by Brockway and Stacy [7,17]. They pyrolysed Yalloum coal at heating rates of about 10^ and lO"^ °C s'^ and found that about a half of the total volatiles consisted of Hght gases containing oxygen, namely, CO, CO2 and H2O. They also reported that the total volatile yield was more than 60 wt%-daf when the peak temperature was higher than 900 °C. Xu and Tomita [9,18] investigated the effects of temperature and coal type on the distribution of products from the pyrolysis in a CPR. The yields of volatile products from Yalloum coal and Morwell coal are summarised in Table 4.1. The tar yield from Morwell coal was constant at temperatures of 590 - 920 °C while those of the other products increased with temperature. The same trends in the tar yield were reported by Miura and co-workers [19] and Hayashi and co-workers [20]. Sathe and co-workers [21] pyrolysed Loy Yang coal in a WMR to investigate the effects of peak temperature (300 to 1000 °C) and heating rate (1 to 2000 °C s'^) on the yields of total volatiles and tar from the primary pyrolysis. The tar was defined as the material nonvolatile at atmospheric pressure and 35 °C. Figure 4.1 shows the total volatile and tar yields as a function of peak temperature for a heating rate of 1 or 1000 °C s'^ and a holding time of 10 s [21]. The total volatile yield increased rapidly from 300 to 600 °C and slowly at higher temperatures. It is noted that, at 600 to 1000 °C, the heating at 1000 °C s'' resulted in a total volatile yield by several wt%-daf higher than the heating at 1 °C s'\ Matsuo and co-workers [2] compared the total volatile yield from the pyrolysis of Morwell coal in a CPR at a heating rate of about 3000 °C s"^ with those in a TGR and a fixed-bed reactor at a heating rate of 0.167 °C s''); they found a heating rate effect very similar to that shown above. Figure 4.1 also shows an appreciable effect of heating rate on the tar yield. While the yield seemed to reach an asymptotic one around 600 °C for both heating rates, the asymptotic yield for 1000 °C s'\ around 20 wt%-daf, was more than three times of that for 1 °C s'^ As shown in Figure 4.2, the asymptotic tar yield from Loy Yang coal increased in a linear manner with logarithm of the heating rate [21]. Increasing heating rate thus caused the tar yield to increase much more significantly than the total volatile yield. This in turn meant a reduction in the yields of light gases by increasing heating rate. Matsuo and co-workers [2] reported that the rapid pyrolysis in the CPR gave lower yields of CO, CO2 and H2O as a function of the total volatile yield (on a coal mass basis) than did the slow pyrolysis in the fixed-bed reactor. Such a heating rate effect is clearly different from that found for the pyrolysis of bituminous coal. For example, Gibbins and Kandiyoti [22] reported that higher heating rates gave higher tar yields from bituminous coals without changing light gas yields significantly. Sathe and co-workers [21] also investigated the effects of holding time on the tar and total volatile yields and found that the tar yield was nearly independent of the holding time at temperatures higher than 600 °C while the total volatile yield increased with time. This result indicated that the evolution of the tar completed before the particle temperature reached 600 °C during heating, even at a rate of 10^ °C s'\ The same trend was confirmed by Jamil and co-workers [23]. However, this trend cannot be generalised over a wide range of external gas pressure, as described later.
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Table 4.1 Product yields (wt%, daf) from the pyrolysis of Morwell and Yallourn coals in a CPR [18]. Coal Morwell
Temperature, °C 445 500 590 650 764 920 764
Yallourn
HCG 0.3 0.8 1.3 2.2 2.8 5.2 2.4
lOG 11.3 14.6 18.7 21.7 25.6 29.7 27.2
TVM 28 38 48 52 56 63 51
HCL 0.5 0.9 1.3 1.6 1.6 2.5 1.5
Tar 15 22 26 26 26 26 20
1. TVM: total volatiles; lOG: inorganic gases (H2, CO, CO2 and H2O); HCG: hydrocarbon gases (CpCs); HCL: hydrocarbon liquids (benzene, toluene, xylene, phenol and cresol). 2. Total pyrolysis time including that required for heating up is 4.0 s. Tar yield was determined by the difference between the total volatile yield and the combine yields of the products measured by gas chromatography.
15 80 o u
60
5
A A • o
Total volatile (1 000 **C s-i) Total volatile (1 »C s-i) Tar( Tar(
40
Z 20 o
o 01— 200
400
600
800
1000
Peak temperature, ''C
Figure 4.1 Changes in total volatile and tar yields from the pyrolysis of Lx)y Yang coal in the wire-mesh reactor with peak temperature (based on the data in Ref 21). Conditions: pressure, 0.1 MPa; holding time at peak temperature, 10 s; heating rate, 1 or 1000 °C s'^
4.2.4. Physical and Chemical Processes Responsible for Heating Rate Effects on the Product Distribution Kershaw and co-workers [24] analysed the tars from the pyrolysis of Loy Yang brown coal in the same WMR as used by Sathe and co-workers [21] by means of UV-
141
Pyrolysis
1
10
1000
100
Heating rate, K s
-1
Figure 4.2 Eifects of heating rate on tar yield from the pyrolysis of Loy Yang coal in the wiremesh reactor. Conditions: pressure, 0.1 MPa; peak temperature, 600 °C; holding time at peak temperature, 10 s. Reprinted with permission from Ref 21. Copyright 1999 American Chemical Society.
VIS absorption and UV-VIS fluorescence spectroscopies to investigate the aromatic features of the tars from the pyrolysis at different heating rates. They detected aromatic clusters containing three or more condensed fused rings in the tar and found that increasing heating rate enhanced the release of such larger aromatic clusters. It was also evidenced that increasing heating rate increased the average number of aromatic clusters per tar molecule, i.e. the molecular mass (corresponding to degree of polymerisation for polymers) of the tar. Kershaw and co-workers [24] also analysed the tars by means of ' H - N M R and found that long-chain methylene (-CH2-) protons were much less abundant in the tar from the pyrolysis at 1 °C s'' than at 1000 °C s'\ These results suggest that thermally induced fragments of coal macromolecules as volatile precursors undergo thermal cracking inside the particle and the extent of cracking depends on the heating rate. Figure 4.1 also indicates that the temperature range for the tar evolution, approximately 300 - 600 °C, does not depend on the heating rate so significantly. This is indicative of the fact that the rate of tar evolution under heating at 1000 - 2000 °C s" is orders of magnitude higher than that during heating at 1 °C s'\ Compared with the heating at 1 °C s'\ heating at 1000 - 2000 °C s"' should cause much more rapid generation of volatile precursors. This results in the formation of a much larger pressure or concentration gradient across the matrix of the pyrolysing coal particle, which is the driving force of intra-particle transport of the volatile precursors by diffusion or convection, respectively. Regardless of the predominance of either convection or
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diffusion, high heating rate can thus shorten the residence time of the fragments within the particle. The volatile precursors may experience intra-particle thermal cracking to form lighter species while their polymerisation leads to char formation. The shortened residence time would therefore suppress both the thermal cracking and polymerisation of fragments inside the particle. This can be a qualitative explanation of higher yield and molecular mass of the tar, lower light gas yield and higher total volatile yield by the pyrolysis at a higher heating rate. Chemical process governing the net degradation of the macromolecular network of coal consists mainly of breakage of inter-aromatic-cluster bridges and cross-linking. The bridge breaking involves the formation of radicals by homogeneous cleavage of labile C-C and C-0 bonds and the stabilisation of the radicals by donatable hydrogen, both of which are required for depolymerisation. On the other hand, the cross-linking produces bridges causing polymerisation. The rate of bridge breaking relative to that of crosslinking is therefore a definite factor determining the extent of the net depolymerisation: a larger relative rate of bridge breaking would result in a greater yield of tar, i.e., aromatic clusters that have escaped from the macromolecular network. Competing bridge breaking and cross-linking are considered in a recent model of coal pyrolysis, the FG-DVC model (Functional Groups - Decomposition, Vaporisation and Cross-linking model) [25,26]. This model describes the chemical structure of coal as a mixture of networked macromolecules and oligomers. The major sources of donatable hydrogen, as a stabiliser of fragment radicals, are hydroaromatic rings and ethylene-type bridges [25,26]. Supply of donatable hydrogen from these structures is inevitably accompanied by the aliphatic-to-aromatic carbon conversion; the amount of released hydrogen consumed will correspond to that of aliphatic carbon converted into aromatic carbon. Thus, the loss of bridges due to the bridge breaking may be estimated from the aliphatic-to-aromatic carbon conversion. The FG-DVC model [25,26] also assumes that cross-linking is brought about by the thermal decomposition of specific functional groups in coal into specified gases. Efibrts have been made to define the cross-linking experimentally [27-30]. It is believed that the formation of cross-links is closely related to that of gases such as CO2 and H2O from oxygen-containing groups. These inorganic gases are evolved from brown coal even at temperatures from 150 to 300 °C where tar formation is negligible [31]. This suggests smaller activation energies for the formation of CO2 and H2O than those for tar evolution as taken in the FG-DVC model. The Victorian brown coals contain quite a number of carboxyl and hydroxyl groups, most of which are associated via hydrogen bonds. Hydrogen bonds can destabilise 0-H bonds and induce condensation between the groups, which would form cross-links such as ethers, anhydrides and esters [30]. Mae and Miura [30] quantified changes in the amounts of these cross-links and the yields of inorganic gases and concluded that the molar amount of cross-links formed was equal to that of H2O evolved during the pyrolysis of raw and chemically modified Morwell coal samples. Hayashi and co-workers [20] investigated the pyrolysis of Yalloum brown coal in order to experimentally establish the variations with heating rate of the extents of bridge breaking and cross-linking, which were evaluated from the aliphatic-to-aromatic carbon
143
Pyrolysis
•t I I I I I I I ' ' '
I I I I i-r-r
t(b)
(c)
O Yco/cp •
^CO/FB
O yC02/DT
j
•
J
i
^
0
10
20
30 0
10
20
30 0
10
^COl/FB
t
I
1
1 i 1 1
20
30 0
(d)
O YQHC/CP •
. .
^
10
yCHC/FB
. 1 . 1
20
30
ACai [mol-C/100 mol-C in YL] Figure 4.3 Yields of H2O, CO, CO2 or H2O from the primary pyrolysis of Yalloum coal as a function of aliphatic-to-aromatic carbon conversion (ACai). •, atmospheric pyrolysis in the fixedbed reactor (heating rate; 0.17 °C s''); o, atmospheric pyrolysis in the drop-tube reactor or CPR (heating rate; 2 - 3^10^ °C s''). Reprinted with permission from Ref 20. Copyright 2000 American Chemical Society.
conversion and the yields of oxygen-containing inorganic gases, respectively. The pyrolysis was performed with a slow heating rate of 0.17 °C s"' in a fixed-bed reactor (with a forced flow of inert gas through the bed) and with a fast heating of 2x10 3x10^ °C s'^ in a CPR or a drop-tube reactor. The asymptotic tar yield from the fast pyrolysis was 26% on a coal carbon basis. The secondary pyrolysis of the primary tar inevitably occurred in the drop-tube reactor and produced CO and hydrocarbon gases (HCG) by decomposing phenolic groups and alkyl groups of the tar, respectively. Increased yields of CO and HCG were then quantified from the difference in the yields of these gases between the CPR and drop-tube reactor. The asymptotic tar yield from the slow pyrolysis was 15%, which was clearly lower than that from the fast pyrolysis. Figure 4.3 shows the molar yields of CO, CO2, H2O and HCG as a function of the aliphatic to aromatic carbon conversion [20]. The conversion, indicated by ACai, was determined as the difference in the content of aliphatic carbon (mol-C/100 mol-C in the coal) between the coal and the primary pyrolysis products (char, tar plus HCG). The figure clearly indicates that the fast heating gives the H2O yield lower than the slow heating at every ACai. Unlike the H2O yield, those of the other light gases hardly depend on the heating rate at equivalent ACai. Thus, the fast heating, in comparison with the slow heating, enhances bridge breaking that requires donatable hydrogens from aliphatic groups and/or suppresses cross-linking that forms H2O. It is also seen in Figure 4.3 that the formation of CO, CO2 and HCG is not responsible for the heating rate effect on the asymptotic tar yield from the coal. As suggested above, activation energies of cross-linking reactions forming H2O are lower than those of the bridge breaking reactions. Increasing heating rate would then increase the rate of bridge breaking relative to that of cross-linking (forming H2O),
144
Chapter 4
leading to enhanced net depolymerisation of coal macromolecules and therefore tar evolution. Based on the statistics of macromolecular network [32-35], the enhanced intensive decomposition of the network would lead to the formation of fragments having a large number of aromatic units (higher molecular mass). This is consistent with the report by Kershaw and co-workers [24] on the effects of heating rate on the number of aromatic clusters per tar molecule. The strong positive effects of heating rate on the asymptotic tar yield can be explained based on either of the two different concepts: shortened residence time of fragments of coal macromolecules (as precursors of tar) inside the particle or enhancement of bridge breaking relative to cross-linking. Although each concept does not necessarily need the other for explaining the effects of heating rate on the tar yield, there seems to be no inconsistency between the concepts. For example, suppressed H2O yield may be partly contributed by shorter intraparticle residence time of fragments carrying hydroxyl groups. On the other hand, enhancement of bridge breaking relative to cross-linking would make the formation of fragments more rapid than expected only from the more rapid heating itself 4.2.5. Selective Release of Smaller Aromatic Clusters as Tar The carbon aliphaticity of tar from the primary pyrolysis of Yalloum coal was investigated by Hayashi and co-workers [20] and estimated to be around 0.4 regardless of the tar yield and the heating rate over a range from 0.17 to 3000 °C s"'. The estimated aliphaticity is similar to that of the parent coal [20,36]. However, this does not necessarily mean either non-selective release of aromatic clusters in the coal as tar or chemical homogeneity of the coal in terms of the structure of aromatic clusters. Kashimura and co-workers [37] made an attempt to obtain UV/VIS absorption and fluorescence spectra of Loy Yang coal itself Because the absorption and fluorescence spectra of the solid coal cannot be recorded easily, they depolymerised the coal through hydrolysis in an aqueous alkali solution at 200 °C to convert the coal completely into alkali soluble products. These spectra allowed them to investigate the abundance of aromatic clusters with poly-condensed aromatic rings in the coal. The results of the spectroscopic analyses of solutions of the depolymerised coal demonstrated that aromatic clusters with three and more condensed rings were too abundant in the coal to be considered as trace components in the coal. The fluorescence spectra of the solubilised coal also gave an indication of the presence of even larger clusters. Spectra were also obtained from a tar sample prepared by the pyrolysis of the parent coal in a WMR (heating rate, 1000 °C s"': peak temperature, 600 °C). The comparison of the spectra of the solubilised coal with those of the tar revealed that the size distribution of aromatic clusters in the tar was biased toward smaller ones than that in the solubilised coal. Smaller aromatic clusters were thus released from the coal preferentially to larger ones during pyrolysis. The selective release of smaller aromatic clusters to that of larger ones is qualitatively explained as follows. Smaller clusters may have smaller 'coordination numbers'. In other words, they have connections to fewer clusters and/or substitution
Pyrolysis
145
groups than do larger ones on average. Smaller clusters would therefore be released from the macromolecular network during the bond breaking and cross-linking more easily than the larger ones [38]. In fact, Griffiths and Mainhood [39] reported that aromatic hydrocarbons with more fused rings underwent 'charring reactions' in or over carbonaceous matrices more extensively than those with fewer fused rings. 4.2.6. Effects of External Gas Pressure on the Evolution of the Primary Volatiles A wealth of fundamental information exists in the literature [40-45] about the effects of pressure on the pyrolysis behaviour of bituminous coals. Some common effects of increasing pressure have been reported: suppressed evolution of tar and resulting decrease in the total volatile yield with pressure. However, such information on the pyrolysis of bituminous coals is unlikely applicable to that of the Victorian brown coal. This is because, unlike the bituminous coals that melt to some extent during the pyrolysis, the Victorian brown coal melts to an extremely limited extent during pyrolysis [1,46]. Therefore, during the pyrolysis of the brown coal, the transportation of volatile precursors through the abundant pore system is expected to play an important role, or even become the rate-limiting step. This was in fact predicted by Gavalas [47]. Sathe and co-workers [48] studied the pyrolysis of Loy Yang coal in a WMR at a heating rate of 1000 °C s"^ and pressure over a wide range from 0.1 to 6.1 MPa. They found that the pressure influenced the tar yield in a complicated manner. As seen in Figure 4.4, the asymptotic tar yield decreased from 20 down to about 10 wt%-daf with an increase in the pressure from 1 to 1.1 MPa, then increased up to 16 wt% from 1.1 to 2.0 MPa, and again decreased gradually to 12 wt% with further increases in pressure from 2.0 to 6.1 MPa. It is believed that the main mechanism for the intraparticle transport of volatile precursors is either bulk diffusion flow driven by concentration gradient or forced convection driven by pressure gradient [47,48]. Increases in the pressure of gas surrounding the particle would reduce diffusivities of the volatile precursors. This would slow down the diffusion of the volatile precursors through the pore system toward the external gas phase. The increase in the external pressure can also reduce the pressure gradient. Thus, the slow down of the intraparticle mass transport would extend the residence time of the volatile precursors within the particle. The extended residence time could lead to enhancement of the thermal cracking and/or charring reactions of the volatile precursors. This would result in a generation of large amounts of light gaseous species. The combined effects of continuous generation and accumulation of the light gaseous species result in a rapid pressure build-up inside the particle. If the pressure build-up is large enough to overcome the reduction of pressure gradient by increasing external pressure, the volatile precursors could come out of the particle with a forced convection flow [47]. This process must be much more rapid than the diffusion and may allow the volatile precursors to come out of the particle without undergoing further thermal cracking. The contribution of the bulk diffusion and that of the forced convection flow to the overall mass transport rate would vary depending on the external pressure as well as the
146
Chapter 4
Pressure, MPa
Figure 4.4 Effects of pressure on the asymptotic tar yieldfromthe pyrolysis of Loy Yang coal in the wire-mesh reactor (based on the data in Ref 48). Conditions: heating rate, 1 or 1000 °C s'; peak temperature, 900 °C; holding time, 10 s.
rapidity of the intra-particle conversion of the volatile precursors. According to the mass transport mechanisms outlined above, the decrease in the tar yield with increasing pressure from 0.1 to 1.1 MPa can be explained by the suppressed diffusion of volatile precursors. In the temperature range of 600 - 700 °C, holding times of several to even longer than 10 s were required for the tar evolution to complete [48]. The increase in the tar yield from 1.1 to 2.0 MPa may be arisen from the transition of the main mass transport mechanism from the diffusion flow to the forced convection flow. The decrease in the tar yield at the higher pressure is due to reduced pressure gradient. Sathe and co-workers [48] reported that the char yield was much less sensitive to changes in the pressure than the tar yield. They suggested that a major part of the decrease in the tar yield was compensated by an increase in the light gas yield due to the cracking of volatile precursors, rather than their conversion into char. However, insignificance of charring reaction could not necessarily be concluded. Bazardorj and co-workers [49], who pyrolysed Loy Yang coal at a heating rate higher than 10^ °C s'^ in a drop-tube/fixed-bed reactor, reported that the char yield, not on the basis of coal mass but on the basis of carbon, from the pyrolysis at 1.2 MPa was 6 % higher than that at 0.1 MPa. The pressure-insensitivity of the char yield on the mass basis could be a net
Pyrolysis
147
Table 4.2 Typical contents (wt%) of AAEM species in the Victorian brown coals. Coal Loy Yang^ Yalloum^ Morwelf
Na 0.13 0.12 0.34
K 0.012 n.d.^ 0.03
Ca 0.034 0.29 0.81
Mg 0.058 0.70 0.52
Refs 65 20 66
a. The coal sample was ashed in O2 with slow heating up to 600 °C. The resulting ash was digested in a hot mixture of HF:HN03 solution for at least 16 h. After the evaporation of the acid solution, the solid was re-dissolved in an aqueous solution of CH3SO3H and analysed by ion chromatography. b. The contents of acid-soluble AAEM species are indicated. The coal sample was treated in an aqueous solution of 5N HCl at 60 °C for 72 h. The solution was analysed by inductively coupled plasma emission spectrometry (ICP-ES). c. The coal sample was subjected to wet oxidation with HF, HNO3 and HCIO4 followed by atomic absorption spectrometric analysis of AAEM speciers. d. Not determined.
result of a higher char yield on the carbon basis and a lower char yield on the oxygen basis. In fact, the carbon/oxygen ratios of chars from Yalloum coal are greatly influenced by heating rate [2]. Sathe and co-workers [48] also investigated the effects of pressure on the yields of tar and total volatiles from the pyrolysis of Loy Yang coal at a slow heating rate of 1 °C s"\ An increase in the pressure from 0.1 to 2.0 MPa caused the tar yield to not decrease but increase from 6 w^%-daf up to 10 wt%-daf. However, such pressure effects were not seen in the absence of the inherent alkali and alkaline earth metallic (AAEM) species in the coal. The combined effects of pressure, heating rate and the presence of AAEM species in coal will be discussed below. 4.2.7. Effects of Inherent and Extraneous Metallic Species during Primary Pyrolysis 4.2,7. /. Roles of Inherent AAEM Species during Primary Pyrolysis As was discussed in Chapter 2, the presence of alkali and alkaline earth metallic (AAEM) species is a particular feature of the Victorian brown coal. Some AAEM species exist as ion-exchangeable cations of carboxylates in coal and some as chlorides dissolved in the moisture in the pore systems [50-53]. Table 4.2 shows contents of inherent AAEM species in Victorian brown coals as examples. Roles and behaviours of AAEM species during the pyrolysis of the Victorian brown coal have been investigated in two different ways [7,16,17,20-24,38,54-65]. One way is to compare the pyrolysis characteristics between the raw coal and an AAEM free-coal from which the inherent AAEM has been removed by acid washing. The other way is to load AAEM species into coal by means of ion-exchanging or physical impregnation and then compare the pyrolysis characteristics of the AAEM-loaded coal with those of the
148
Chapter 4
raw coal, or otherwise, with those of the AAEM-free coal. The effects of inherent and externally loaded (extraneous) AAEM species on the primary pyrolysis of the brown coal have been investigated mainly by using WMRs, CPRs and fixed-bed reactors. Studies have been carried out to clarify the effects of the removal of AAEM species fi-om Loy Yang coal on its atmospheric pyrolysis behaviour when heated up at 1 and 1000 °C s'^ in a WMR [21,23,64]. The asymptotic tar yields from the acid-washed coal were 37 and 20 wt%-daf at the heating rates of 1000 and 1 °C s'\ respectively. These yields were nearly twice of those from the raw coal (20 and 9 wt%-daf at 1000 and 1 °C s'\ respectively). Sathe and co-workers [21,64] observed the evolution of tar in an eruptive manner during the pyrolysis of the acid-washed coal at 1000 °C s'\ Such eruptive evolution of the tar was never detected during the pyrolysis of the raw coal. Sathe and co-workers [64] believed that the eruptive evolution made the residence time of volatile precursors extremely short and thereby minimised the extent of their intraparticle thermal cracking. Hayashi and co-workers [63] pyrolysed raw and chemically modified Morwell brown coal samples in a CPR (pressure: 0.1 MPa; heating rate: ca 3000 °C s'^). They reported that the removal of the inherent AAEM species from Morwell coal increased the asymptotic tar yield fi-om 17 to 23 wt%-daf. This increase was accompanied by the corresponding decreases in the char yield (3 wt%-daf) and inorganic gas yields (CO2 and H2O; 3 wt%-daf). It was also reported that the removal of AAEM species from Yalloum coal increased its tar yield from 26% to 32% (on the coal carbon basis) when pyrolysed in a CPR [20]. Although there has been no study comparing the tar yields from the pyrolysis of the same coal sample in a WMR and in a CPR, both reactors give very similar tar yields fi-om the raw coals (Loy Yang, Yalloum and Morwell) [9,18,30,21]. This suggests that the extraparticle secondary pyrolysis of the primary volatiles, if any, occurs to a minimal extent in both types of reactors. Different from this trend, much more tar seems to have been recovered from the fast pyrolysis of the acid-washed coal in a MWR than that in a CPR. The primary tar fi-om the fast pyrolysis of the AAEM-free brown coal would be so thermally unstable that it could have undergone extraparticle reactions even before escaping fi-om the ferromagnetic foil heated at its Curie-point temperature in a CPR. The divalent cations (Ca^ and Mg^^) bonded to carboxylic groups serve as virtual cross-link points in the macromolecular network of coal [69] and increase the matrix density of the coal [70]. Regardless of their valence, AAEM species can suppress the vapour pressure of fragments of the macromolecular network containing the AAEM species and affect the vaporisation of these macromolecular fragments. The roles of AAEM species also have their origin in their chemical transformation during pyrolysis. It is well known that carboxylates in the brown coal undergo thermal decomposition even at temperatures lower than 300 °C [67,68]. With the release of CO2, AAEM species originally associated with -COO' groups may be re-bonded to the pyrolysing coal/char matrix (-CM) through reactions as presented below [21]. (-COO-Ca-OOC-) + (-CM) = (-COO-Ca-CM) + CO2
(R4-1)
149
Pyrolysis (-COO-Ca-CM) + (-CM) = (CM-Ca-CM) + CO2
(R4-2)
(-COO-Na) + (-CM) = (CM-Na) + CO2
(R4-3)
As will be described in Section 4.4, the formation of CM-bonded cations occurs even from the decomposition of chlorides. (R4-4)
NaCl + (CM-H) = (CM-Na) + HCl
Thus, AAEM species can serve as virtual cross-linking points continuously, making the formation and release of fragments (as tar precursors) more difficult. Hayashi and co-workers [20] investigated the fast pyrolysis of raw and acid-washed Yalloum coal samples in a CPR and in a drop-tube reactor that gave equivalent char yields from each coal sample. They estimated the relative rates of cross-linking and bridge breaking by means of the same analytical techniques as mentioned above. It was found that the presence of the inherent AAEM species did not influence the progress of aliphatic-to-aromatic carbon conversion or the evolution rate of inorganic gases significantly. This indicates that the AAEM species suppress the tar evolution mainly by serving as cross-linking points before and during pyrolysis. The effects of the external gas pressure were investigated on the tar yield from the pyrolysis of an acid-washed Loy Yang coal in a WMR [64]. The asymptotic tar yields
40 1000**Cs-i
1 X s-^
^Raw
• O A A
O Acid-washed A Na-exchanged A Ca-exchanged -co
Raw Acid-washed Na-exchanged Ca-exchanged
30
20
10
1
2 Pressure, MPa
3
4 0
0.5
1.0
1.5
2.0
Pressure, IMPa
Figure 4.5 Combined effects of ion-exchange, heating rate and pressure on the asymptotic tar yields from the pyrolysis of Loy Yang coal in the wire-mesh reactor (based on the data in Ref 64).
150
Chapter 4
under different heating rates or pressures are shown in Figure 4.5. For the pyrolysis at 1000 °C s'\ the intra-particle transport of volatile precursors is mainly governed by the forced convection flow regardless of the pressure, rather than the diffusion flow, since the tar evolution in an eruptive manner has been confirmed [21,64]. Increases in the pressure thus decreased the driving force of the convection flow, namely, the pressure gradient across the pore system of the particle. This would result in extended residence time of the volatile precursors. In fact, the tar yield decreased from 37 to 13 wt%-daf with the pressure increasing from 0.1 to 3.6 MPa. For the pyrolysis of the acid-washed Loy Yang coal at 1 °C s'^ [64], the main mechanism governing the intraparticle mass transport should be the diffusion flow that is suppressed by increasing the external gas pressure. The tar yield decreased from 19 to 11 wt%-daf as the pressure increased from 0.1 to 1.1 MPa. It should be noted that the tar yield at a pressure of 1.1 MPa was little influenced by the heating rate. This shows how the intra-particle transport of volatile precursors is an important process to determine the primary tar yield from the pyrolysis at elevated pressures. In terms of only relative rates of bridge breaking and cross-linking, an increase in the heating rate can provide a condition more favourable for enhanced tar evolution. However, at elevated pressures, suppressed intra-particle transport of volatile precursors can cause a great negative effect on the tar yield from the acid-washed coal so as to offset the positive effect brought about by increasing the heating rate. Figure 4.5 also compares variations with pressure of asymptotic tar yields from the raw and acid-washed Loy Yang coals from the pyrolysis in a WMR with a heating rate of 1000 °C s"'. The data demonstrate that "enhancement of tar evolution by removal of inherent AAEM species" is valid only at lower pressures. Comparable tar yields from the raw and acid-washed coals at higher pressures show a significant negative effect of pressure on the tar yield from the acid-washed coal and, moreover, suggest even a positive effect of the presence of AAEM species on the tar yield, which appears at higher pressures. Roles of AAEM species causing such an effect are discussed below. 4,2.7.2, Effects of Loading AAEM Species onto Brown Coal on Its Pyrolysis Characteristics Sathe and co-workers [21,64] and Jamil and co-workers [23] ion-exchanged Na^ (2.8 wt% on a dry basis) or Ca^^ (3.27 wt%) onto carboxyl groups in Loy Yang coal and then pyrolysed the ion-exchanged coal samples in a WMR. Figure 4.5 summarises the combined effects of heating rate and pressure on the asymptotic tar yields from the pyrolysis of the Na- and Ca-exchanged coals heated at 1 and 1000 °C s"' [21,23,64]. From the comparison of the tar yields from the raw and acid-washed coals, it is clear that the introduction of ion-exchangeable cations reduces the tar yield. It should also be noted that the tar yield from the Na-exchanged coal increased with an increase in pressure while that from the Ca-exchanged coal decreased for both heating rates. It is believed that the intraparticle transport of volatile precursors during the pyrolysis of Na- and Ca-exchanged coals is governed by their diffusion at lower pressures [48]. Increasing pressure would suppress the diffusion and extend the residence time of
Pyrolysis
151
volatile precursors. Murakami and co-workers [71] pyrolysed Na- and Ca-exchanged coal samples prepared from an acid-washed Loy Yang coal at a heating rate of 0.093 °C s'\ They found that the surface area of the resulting char, measured by means of N2 adsorption at -196 °C, decreased from about 300 m^ g'' to less than 100 m^ g ' with increasing contents of the metallic species. This suggests that Na and Ca both suppressed the development of mesopores and macropores during pyrolysis. The effects of increasing pressure on the rate of transport of volatile precursors would therefore be even more significant during the pyrolysis of Na- and Ca-exchanged coals than those during the pyrolysis of the acid-washed and raw coals. It is also known that both ionexchanged Na and Ca greatly influence the formation of inorganic light gases such as CO, CO2 and H2O [16,69-73]. This is due to the catalytic decomposition of oxygen functional groups such as phenol [16], ether and carbonyl [73] groups that would otherwise be decomposed mainly into CO in the absence of these AAEM species. At the same contents, Na changes the yields of CO and CO2 more significantly than Ca, indicating a higher catalytic activity of Na than Ca [71]. The decrease in the tar yield from the Ca-exchanged coal with increasing pressure (Figure 4.5) is explainable from the extended residence time of volatile precursors within the particle to enhance the thermal cracking of the precursors. However, this explanation cannot be applied to that for the tar evolution from the Na-exchanged coal. The Na-catalysed decomposition of volatile precursors would bring about enrichment of species such as hydrogen radicals as stabilisers of radical fragments (as tar precursors). Fynes and co-workers [74] evidenced the roles of H2 (or hydrogen radicals) as a stabiliser of volatile precursors even at atmospheric pressure. Inorganic light gases such as CO2 and H2O are the most abundant gases in the pore systems of the pyrolysing Naexchanged coal particles as is in the cases of the other types of brown coals. Studies on the pyrolysis of Victorian brown coals in a drop-tube reactor showed that inherent AAEM species greatly catalysed the steam reforming of the nascent tar from the coals [12]. Thus, it is plausible that the AAEM species, particularly Na, catalyses the reactions of volatile precursors with H2O and/or CO2 as well as thermal cracking and thereby the formation of intermediate products such as hydrogen radicals inside the pyrolysing Na-exchanged coal. As shown previously, the asymptotic tar yield from the pyrolysis of raw Loy Yang coal at a heating rate of 1 °C s'^ increased from 5 to 10 wt%daf as the pressure increased from 0.1 to 2.0 MPa. It is difficult to explain this fact without considering the roles of Na contained in the raw coal (about 0.1 wt%). For the fast pyrolysis at 1000 °C s'\ the rapid formation of lighter species from the Na-catalysed decomposition of volatile precursors might have caused rapid pressure build up inside the particles, which could led to the formation of forced convective flow of volatile precursors. This could be an authentic explanation for the occurrence of the forced convection flow of volatile precursors during the pyrolysis of the raw coal [48] at pressures higher than 1.0 MPa. Again, it is strongly suggested that the Na contained in the raw coal, although at a much lower concentration than that in the Na-exchanged coal, is a key species for the increase in the tar yield with increasing pressure.
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4.2.7.3 Effects ofAAEM species on the Formation of Light Gases during Pyrolysis The effects ofAAEM species on the evolution of light gases have been investigated for the slow pyrolysis in fixed-bed reactors under atmospheric pressure [16,69-73]. In these reactors, it is believed that the secondary pyrolysis of the primary volatiles w^as not so significant as to influence the yields of light gases [73]. Schafer [69,70] was the first researcher who made systematic investigations on the effects of inherent and extraneous AAEM species on the pyrolysis of the Victorian brown coal. He prepared an AAEM-free brown coal sample by means of acid-washing and then prepared metal-ionexchanged coal samples by exchanging protons (H^) of the carboxyl groups
-I
1
I
1
I
*
1 —
Acid-washed
2 h CO2
CO
o 3
" 3
CO2
200
400
Na-exchanged (3.2 wt%)
600
800
Temperature, °C
Figure 4.6 Characteristics of evolution of CO and CO2 from the pyrolysis of Yalloum coal in the fixed-bed reactor (based on the data in Ref 71). Condition: heating rate, 0.083 °C s"^; pressure, 0.1 MPa.
Pyrolysis
153
quantitatively with AAEM cations. In the absence of AAEM species, most of the carboxylic groups in Yalloum coal were decomposed into CO2. Its formation was associated with that of H2O: the H2O yield was about 3 times of the CO2 yield on a molar basis for a range of temperature up to 600 °C. The exchange with Ca enhanced the formation of CO2 but hardly changed that of H2O. At a temperature of 600 °C, the yield of CO2 (= 21 wt%) was 2/3 of the H2O yield on the molar basis. It was also found that other AAEM species (Na, K and Mg) promoted the formation of CO2 as significantly as Ca. Thus, AAEM-exchanged brown coals evolve more CO2 than can be accounted for by carboxylic groups alone. Murakami and co-workers [71] investigated the pyrolysis of Ca- and Na-exchanged Loy Yang coal samples. They measured the evolution rates of CO, CO2 and CH4 upon heating at a rate of 0.083 °C s'\ Figure 4.6 shows that ion-exchanged Ca and Na both enhance the formation of CO2 and suppress that of CO in a temperature range from 300 to 600 °C where the tar was also evolved. At higher temperatures, i.e., after the completion of tar evolution, the chars from the Na- and Ca-exchanged coals evolved more CO than that from the acid-washed coal. Mechanisms for the CO evolution above 600 °C will be discussed in Section 4.4 in connection with chemical transformation of AAEM species during pyrolysis. 4.2.8. Primary Pyrolysis in Reactive Atmosphere As was described in Section 4.2.3, the evolution of tar from the brown coal is completed before the particle temperature reaches around 600 °C even for a heating rate in the order of 10^ °C s'\ unless the pyrolysis is performed under pressure within a particular range [48]. Due to such rapidity of the tar evolution under rapid heating, it is believed that the reactive atmosphere around the particles is minimally involved in the tar evolution. In fact, it was reported that changing the extra-particle gas phase from He to H2 had no significant effects on the tar yield from the rapid pyrolysis of coals even at pressure as high as 5 - 7 MPa [75-77]. Miura and co-workers [77] found that the rapid pyrolysis of Morwell coal in pressurised H2 in a CPR gave higher yields of light hydrocarbons such as CH4 and BTX than in He of the same pressure. This is explained by the progress of reactions of the nascent char with H2 (or H2-derived hydrogen radicals), termed hydrocracking or hydrogasification, mainly after the completion of tar evolution. However, loading of an active catalyst or its precursor can change the distribution of products from the rapid pyrolysis under pressurized H2 [78,79]. Matsui and co-workers [78] prepared Ni^^loaded (Ni content: 7.3 wt%-db) and Fe^^-loaded one (Fe content: 8.5 wt%) Loy Yang coal samples by means of an ion-exchange technique. They pyrolysed the coal samples in H2 of 7 MPa in a WMR with a heating rate around 500 °C s"' and a holding time of 6 s. They found that the loaded Ni and Fe both increased the tar yield (by 5 to 10 % on a coal carbon basis) as well as the BTX and CH4 yields. Thus, enhanced supply of active hydrogen with the aid of an excellent catalyst enabled the participation of pressurised H2 gas in the rapid primary pyrolysis.
154
Chapter 4
Clearly different from the pyrolysis under rapid heating, gaseous H2 seems to be involved in the primary pyrolysis under slow heating. Fynes and co-workers [74] pyrolysed an Australian brown coal in a fixed-bed reactor at a heating rate of 5 °C s"'. They found that the tar yield from the pyrolysis in H2 atmosphere was increased from 22 to 32 wt%-daf as the H2 pressure increased from 0.1 to 3.0 MPa. It was also found that the tar yield from the pyrolysis in H2 of only 0.1 MPa, 22 wt%-daf, was higher than that in N2 of the same pressure, 16 wt%-daf Thus, H2 can participate in intra-particle reactions of volatile precursors by playing roles of a stabiliser of radicals and/or an inhibitor of their recombination even at ambient pressure. Jamil and co-workers [23] investigated the pyrolysis of raw, acid-washed, Naexchanged and Ca-exchanged Loy Yang coal samples in atmospheric He or CO2 in a WMR. They found no significant effect of changing the atmosphere on the asymptotic tar yields from the individual coal samples over a range of heating rate from 1 to 1000 °C s'\ while they observed the extremely rapid CO2 gasification of nascent char that occurred simultaneously with the thermal cracking of the char after completion of the tar evolution. 4.2.9. Secondary Reactions of the Primary Volatiles 4.2.9.1. Rapid Pyrolysis in Fluidised-Bed Reactors Early studies on the rapid pyrolysis of brown coal in fluidised-bed reactors [16,54,80] showed that the tar yield was maximised around 600 °C. This may be explained by considering the intensified formation of tar with increasing temperature up to 600 °C and the intensified the secondary pyrolysis of volatiles above 600 °C. The maximum tar yields from the fluidised-bed pyrolysis of raw brown coals at 600 °C were around 20 wt%-daf [16,54,80], nearly equivalent to those from the pyrolysis in WMRs and CPRs [9,18-21]. This suggests that the progress of the secondary pyrolysis in terms of the tar yield is insignificant at 600 °C within the range of residence times of the volatiles through the fluidised bed and freeboard above the bed, normally shorter than 1 s. The yields of light gases increase with temperature and this is attributed to the formation of light hydrocarbons (mainly Cj - C3) and CO from the thermal cracking of tar as well as char [16,54,58,80]. Figure 4.7 exhibits the changes in the product yields from the pyrolysis of Yalloum coal in a fluidised-bed reactor [58]. Womat and Nelson [81] pyrolysed raw and Ca-exchanged Loy Yang coal samples in a fluidised-bed reactor similar to that of Tyler [16]. They analysed the dichloromethane (DCM)-soluble portions of tars by means of FTIR. The pyrolysis of the Ca-exchanged coal gave DCM-soluble tar yields lower than those from the raw coal at temperatures lower than 900 °C, but the DCM tar yields from those coals were nearly equivalent with each other at 900 - 1000 °C. As described previously, Ca-exchange does decrease the primary tar yield by more than a half [21,23]. Thus, after extensive thermal cracking in the gas phase and/or over surfaces of the bed material and char particles, the Ca exchange may no longer be a factor reducing the tar yield. It was also found from the tar analysis that the average chemical structure of the DCM-soluble tar from the Ca-
155
Pyrolysis
exchanged coal was different from the counterpart from the raw coal in terms of the contents of aromatic hydrogen, aliphatic hydrogen, unsaturated hydrocarbon substituents to aromatic rings and oxygen-containing functional groups (on the DCM tar basis).
1100
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1100 Temperature, **C
Figure 4.7 Effects of bed temperature on product yields from the pyrolysis of Yalloum coal in the fluidised-bed reactor (based on the data in Ref 58).
156
Chapter 4
Caution should be exercised in evaluating the yields of CO and CO2. The use of metal components in an experimental reactor system could cause some uncertainties in the yields of CO and CO2: some metal oxides (e.g. formed when the reactor is cleaned by the combustion of soot and residual char/tar) may be reduced by volatiles at high temperatures to give COx. Li and Nelson [82] studied the pyrolysis of Yalloum coal and other two brown coals in a fluidised-bed reactor, focusing on the effects of pyrolysis temperature on the yields of aromatic rings in the DCM-soluble tar. They observed maximum DCM soluble tar yields from the coals at 600 °C or even lower temperature, but also found that the yields of aromatic rings, that were estimated semi-quantitatively from UV absorption and UV fluorescence spectroscopies combined with size exclusion chromatography, reached maxima around 700 °C. They claimed that a maximum tar yield (on the mass basis) in the fluidised-bed pyrolysis does not necessarily mean a maximum yield of aromatic rings. Such separation of the two maxima could be attributed to incompleteness of the tar evolution at 600 °C within a very short residence time of coal/char particles under rapid heating at a rate over 10"* °C s'\ which is an order of magnitude higher than those generally used for the pyrolysis in WMRs and CPRs. Another possibility is the dependency of heating rate on the peak temperature that is inevitable for fluidised-bed reactors and drop-tube reactors. As shown in Figure 4.2, the asymptotic tar yield from the primary pyrolysis of Loy Yang coal increases with increasing heating rate. Hayashi and co-workers [83] investigated the secondary pyrolysis of tar from Yalloum coal in a particular type of fluidised bed reactor [13]. They controlled the temperature of the fluidised-bed (dense phase) as the zone for the primary pyrolysis and that of the freeboard as the zone for the vapour phase secondary pyrolysis of the tar formed in the fluidised-bed. They fixed the temperature of the fluidised-bed at 600 °C while varied that of the free board in a range from 600 to 900 °C. Thus, the primary tar formed under the same conditions underwent the secondary pyrolysis at different temperatures. At the fixed temperature for the primary pyrolysis (600 °C), the tar yield decreased with increasing temperature for the secondary pyrolysis. On the other hand, increasing the temperature for the secondary pyrolysis increased the yields of monoand di-aromatics (BTX and naphthalene derivatives), most of which were produced by the secondary pyrolysis of the primary tar. Hayashi and co-workers [83] also pyrolysed the same Yalloum coal as above at 600 900 °C in a drop-tube reactor, in which an identical temperature was provided for both the primary and secondary pyrolysis. At the same secondary pyrolysis temperature in a range from 800 - 900 °C, the drop-tube reactor gave higher yields of the mono- and diaromatics than did the fluidised-bed reactor. This result suggests that the temperature for the primary pyrolysis, even above 600 °C, can influence the nature of the primary tar as the precursor of light aromatics. The result reported by Hayashi and co-workers [83] could be explained in a way similar to that for the above-described results reported by Li and Nelson [82]. However, Hayashi and co-workers [83] found no significant differences in the yields of mono- and di-aromatics between the fluidised-bed and drop-tube reactors for the pyrolysis of a subbituminous coal. Taken together with the above-described results, this result implies a
Pyrolysis
157
particular feature of the secondary reactions of tar from the brown coal, while no experimental proof is available. 4.2,9.2, Rapid Pyrolysis in Entrained-Flow or Drop- Tube Reactors Miura and co-workers [84] studied the pyrolysis of Morwell coal in a type of entrained-flow reactor that was made of a coil of stainless tube with a length of 3.3 m. The residence time of coal/char particles in the reactor was about 1.3 s. For the pyrolysis with N2 as the carrier gas, the tar yield reached a maximum of about 20 wt%daf at 650 °C. They examined the effects of adding vapour of an organic solvent, ethylbenzene, 2-methyl-l-propanol, tetralin or methanol, to the carrier gas. It was found that the decomposition of each solvent vapour formed radicals to promote particular types of reactions, the rates of which matched with those of formation of radicals from the solvent. Adding 2-methyl-l-propanol vapour greatly enhanced the formation of BTX from tar, increasing the BTX yield by about 5 times at 750 °C, while BTX was hardly produced from the solvent alone. The addition of ethylbenzene vapour at 650 °C resulted in not only an increase in the tar yield but also a decrease in the char yield. Donation of radicals from decomposing ethylbenzene to the pyrolysing coal was so fast that the radicals participated even in the primary pyrolysis. Hydrogen radicals from the vapours of tetralin and methanol were transferred to tar, causing significant increases in the yields of light hydrocarbon gases at 850 °C. Thus, the results of Miura and coworkers [84] demonstrate the importance of the transfer of radicals among the volatiles in the gas phase during the secondary pyrolysis of tar. Hayashi and co-workers [85] pyrolysed Yalloum coal in a drop-tube reactor focusing on the mechanisms of the secondary pyrolysis of the primary volatiles. They evaluated the progress of the secondary pyrolysis by comparing the product yields from the pyrolysis in the drop-tube reactor with those from the pyrolysis in a CPR under conditions where both reactors gave equivalent char yields at the same temperature. Over a range of temperature from 600 to 800 °C, the difference in the tar yield (on a coal carbon basis) between the two reactors was in good agreement with the difference in the total yields of carbon oxides (CO and CO2) and HCG (mainly CH4 and C2H4). The secondary pyrolysis decreased the tar yield mainly by thermal decomposition of oxygen-containing functional groups and dealkylation to form CO and HCQ respectively. Figure 4.8 [85] shows the tar yields from the pyrolysis in the drop-tube reactor as a function of temperature. In the figure are also shown the yields of CO/CO2 and HCG formed by the secondary pyrolysis. The sum of the yields of tar, CO/CO2 and HCG is therefore equivalent with the tar yield from the primary pyrolysis in the CPR. The tar from the pyrolysis in the drop-tube reactor at 600 °C was analysed by ^^C-NMR to quantify the alkyl groups and oxygen-functionalities and then estimate the maximum yields of carbon oxides and HCG potentially formed by the secondary pyrolysis of the tar. In Figure 4.8 is indicated a minimum tar yield that is given by assuming complete deoxygenation to form CO and dealkylation to form HCG. It is noted that the tar yield at 900 °C is even much lower than the estimated minimum tar yield, while the CO/CO2
158
Chapter 4
yield is much higher than the estimated maximum yield. At 900 °C, a considerable portion of H2O formed by the primary pyrolysis (12 moles per 100 moles carbon in the coal) was consumed by the secondary pyrolysis. Based on these results, it was concluded that the tar underwent a rapid steam reforming with pyrolysis-derived H2O within a gas residence time as short as 2 s. Matsuo and co-workers [2] reported the consumption of pyrolysis-derived H2O and the concurrent reduction in the tar yield for the pyrolysis of Fe-exchanged Yallourn coal in a drop-tube reactor at 800 °C. Hayashi and co-workers [12] extended their study by using another drop-tube reactor in which a gas residence time as long as 10 s could be realised. They pyrolysed raw and acid-washed coals (Yallourn and Loy Yang) with or without adding steam (40 kPa) to the carrier N2 gas. The product distributions from the pyrolysis under different conditions are summarised in Figure 4.9. For the pyrolysis of every coal without the addition of steam, the total yield of the volatile products and soot deposited onto the wall of the quartz-made reactor tube (soot -R) was lower than the total volatile yield from the primary pyrolysis (Xp, broken line). This is due to the direct or indirect deposition of soot onto the surface of char particles: the yield of such soot is given by the difference between X^ and the total yield of the volatiles and soot-R. The total yield of soot from the acid-washed coals is in a range of 15 - 20 % on the coal carbon basis and appreciably higher than those from the raw coals. Such high yields accounted for 30 - 50% of the primary tar on the carbon basis. As is seen in Figure 4.9, the addition of steam greatly suppresses the soot formation from the tar for the pyrolysis of the raw coals and also promoted the decomposition of
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Figure 4.8 Cumulative yields of tar and gaseous products (hydrocarbon gases and carbon oxides) formed by the secondary pyrolysis of the primary tar from the pyrolysis of Yallourn coal in the drop-tube reactor (based on the data in Ref 85). K(tar), tar yield; AK(HCG), yield of hydrocarbon gases formed by the secondary pyrolysis of tar; Ay(COx), yield of carbon oxides formed by the secondary pyrolysis of tar; Ceo, fraction of oxygen-bonded carbon in tar at 600 °C; Cal, fraction of aliphatic carbon in tar at 600 °C.
159
Pyrolysis
the tar mainly into carbon oxides. In particular, the tar yield from the raw Yalloum coal was as low as 0.3%. This indicates that the primary tar from the coal (yield: 25%) was converted into gases nearly completely through steam reforming as well as thermal cracking. Such extensive conversion of the tar hardly occurred for the pyrolysis of the acid-washed coals that contained no AAEM species. Thus, the AAEM species can
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160
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enhance the steam reforming of the tar and inhibit the soot formation. Figure 4.10 presents the fates of the primary tar from the raw and acid-washed Yalloum coals under the same conditions as in Figure 4.9. It is also seen in Figure 4.9 that the total yield of the volatiles and soot-R exceeded Xp. This demonstrates the rapid progress of steam gasification of nascent char within its residence time of 4 - 5 s.
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Pyrolysis
161
4,2.9.3. Effects of Pressure on the Pyrolysis in Drop-Tube Reactors and FluidisedBed Reactors Yeasmin and co-workers [86] performed the rapid pyrolysis of Yalloum coal in a drop-tube reactor at pressures from 0.1 to 1.0 MPa and temperatures from 600 to 1000 °C within a range of particle residence time from 0.02 up to 3 s. The heating rate of particles was estimated in the order of 10"^ °C s"\ The rate of volatile release decreased as the pressure increased. As a result of this, increasing pressure from 0.1 to 1.0 MPa decreases the total volatile yield by 10 to even 20 wt%-daf at a residence time of about 1.5 s for every temperature examined. It was also reported that the total volatile yield reached an extremely high yield of about 80 wt%-daf at 0.1 MPa and 1000 °C. This yield is much higher than that from the pyrolysis of Victorian brown coals in WMRs and CPRs at similar temperatures and much longer holding times [8,9,20,21,85]. In addition, no significant pressure effect was found on the total volatile yield from the rapid pyrolysis in a WMR [48]. Factors causing these disagreements are not clear. However, it should be pointed out that Yeasmin and co-workers [86] used ash as a tracer to determine the weight loss (total volatile yields). The volatilisation of AAEM species that are important ash-forming species in the brown coal may have caused some uncertainties in their total volatile yields. Shibaoka and co-workers [87] studied the effects of Ca-exchange on the distribution of pyrolysis products from the pyrolysis of Yalloum coal in a fluidised-bed reactor at a pressure of 1.1 MPa. The total volatile yield at 600 °C was reduced by the Ca-exchange, which was mainly explained by a decrease in the tar yield. The secondary pyrolysis of the primary volatiles was not significant at 600 °C and therefore the effects of the Caexchange on the observed product distribution occurred mainly during the primary pyrolysis. It appears that the ion-exchanged Ca may have slowed the formation of volatile precursors during pyrolysis and led to the reduction in volatile yields when the particle residence was short. 4.2.10. Other Studies on Rapid Pyrolysis in Drop-Tube Reactors or Fluidised-Bed Reactors Drop-tube reactors and fluidised-bed reactors were also employed for investigating the effects of chemical or physical pretreatment on the evolution of tar and the effects of operating variables on the release of minor species such as AAEM species and nitrogenor sulphur-containing gases during the pyrolysis of brown coals. Details of those studies are described in Section 4.4 and in Chapters 5 and 6.
4.3. FLASH PYROLYSIS OF BROWN COAL FOR PRODUCING CHEMICALS Although the use of coal as energy (fuel) may continue and will be indispensable, the "non-fuel" use of coal is another aspect of coal utilisation. Now, the main "non-fuel" uses of coal are the production of metallurgical coke and coal tars formed as by-
162
Chapter 4
products. Coal tars are still an important source of aromatic chemicals: they account for about 15 - 25% of benzene, toluene and xylene production, and 95% of the larger aromatics [88]. Furthermore, the pitch fraction of tar is an important raw material of carbon materials such as graphite, carbon fibre, activated carbon fibre, etc. The aromatic structures in coal are utilised in these usages and naphthalene derivatives obtained from coals would be an important raw material for the next generation of polymers such as engineering plastics and new carbon materials [89]. All of the aromatic carbons from coal are produced from the metallurgical coke production by-products, tars, as stated above. Another method is the liquefaction of coal, which mainly produces benzene derivatives. Since the tar yield from the metallurgical coke production is very small (less than 7 to 8%) and the coal liquefaction has not been commercially realised for economical reasons, both processes would not supply enough aromatic carbons required in the future. Then it would be very important to search another economical conversion routes to produce aromatic carbons from coal [89]. Economical conversion routes will surely be realised using rather mild conversion conditions as compared with the conditions utilised for conventional coal conversion processes. In this section, special attentions will be paid to new pyrolysis methods performed to increase the yields of aromatic compounds and tar from brown coal. Following the brief review of the recent progress in this field, attempts made by Miura and co-workers will be introduced. 4.3.1. Flash Pyrolysis of Brown Coal Performed for Increasing Liquid Yields Pyrolysis is the first step of coal conversion processes such as gasification, liquefaction and combustion. Since pyrolysis proceeds under rather mild reaction conditions (low temperature and low pressure), attentions have been paid to recover liquid products in high yields by utilising pyrolysis: it has been called "mild gasification of coal" or "skimming of coal" [90]. Pyrolysis for this purpose is performed under rather high heating rates over 1000 °C s ' and is called "flash pyrolysis". Figure 4.11 shows a schematic how flash pyrolysis of coal proceeds [91]. Flash pyrolysis consists of two sets of reactions: primary devolatilisation reactions and subsequent secondary gas phase reactions. The formers are very rapid reactions that consist of radical formation reactions, polymerisation-condensation reactions, radical recombination reactions, hydrogen addition reactions, etc., and the latter reactions are decomposition reactions of the volatile products produced through the primary reactions. Flash pyrolysis has been paid attention as a method to produce liquid products because it is known to increase the total volatiles (TVM) over the volatile matter of the proximate analysis (PVM). Figure 4.12 shows the ratio of TVM to PVM against pyrolysis temperature [18], which shows the ratio increases up to 1.05 to 1.45 above the pyrolysis temperature of 800 °C. Although the mechanism by which TVM is increased is not clear, it is clear that the primary reactions must be controlled to further increase TVM. The secondary gas phase reactions solely change the distributions of TVM that are important to increase the yields of some specialized products.
163
Pyrolysis
Many attempts have been performed to increase the yields of tar and aromatic compounds such as benzene, toluene and xylene (BTX) through controlling either the primary reactions or the secondary gas phase reactions. Most commonly employed method is the so-called flash hydropyrolysis under the presence of high pressure of hydrogen over 20 bar at high temperatures over 900°C. This method is actually effective to increase both TVM and the BTX yield significantly [92,93], but it requires expensive hydrogen and employs rather severe reaction conditions. Much milder reaction conditions must be employed to meet with the demand for energy saving and economical coal conversion processes.
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Figure 4.12 Ratio of TVM to PVM as a function of pyrolysis temperature [18]. Circles and bars, data of the authors: (solid bar) Menster; (oblong with dots) Solomon; (big broken bars) Tyler, (oblong with diagonal lines) Scott; (tiny broken lines) Teo.
Table 4.3 summarises some pyrolysis methods that were performed to increase TVM and/or the BTX yield under rather mild experimental conditions. They are grouped into the following three categories: /. Pyrolysis in reactive gas atmospheres. These methods intended to supply CH3 and/or OH radicals to coal fi-agments in addition to H radicals by performing pyrolysis in methane [94,95], toluene [96] or methanol [97] atmosphere. However, these methods could not realise a significant increase in TVM probably because the rate of radical supply from the gases did not match the formation rates of coal fragments. 2. Pyrolysis of pretreated coal. Several methods pyrolysing coals pretreated with various gases such as H2, He, CO2, H2O [98] and hydrogen donor [99] were proposed, but the effectiveness of the pretreatment has not been elucidated. For example, Graff and co-workers [10,100] proposed that pyrolysing the coal pretreated with steam at 50 bar and 320 - 360 °C increased both TVM and the liquid yield significantly. The increases were presumed to be realised through the breakage of ether linkages in coal during the pretreatment. However, such a significant effect has not always been detected in spite of the re-examination of the method by many investigators. The effect is believed to be significantly coal rank dependent [101]. The trial of Ofosu-Asante [102] is unique: they alkylated coal by the so-called Oalkylation method to replace OH groups by alkyl groups. By pyrolysing the pretreated
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166
Chapter 4
coal, they realised significant increases in TVM. It is also reported that TVM can be increased slightly when ion exchangeable Ca was removed before pyrolysis for low rank coals [103]. The success of these methods in increasing TVM is believed to be realised by suppressing the cross-linking reactions between OH groups during the pyrolysis. 3, Catalytic pyrolysis of coal. Several trials have been reported on the catalytic flash pyrolysis of coal, but catalysts are effective in general only to the secondary gas phase reactions. However, catalysts are of course effective to slow pyrolysis performed at rather low heating rates in the presence of high pressure of hydrogen [109]. Of the pyrolysis methods given in Table 4.3, only the methods proposed by OfosuAsante [102] and Franklin and co-workers [104], except for the methods proposed recently by the authors [19,103,105-108], realised significant increases in TVM. Methods that intended to supply radicals from other sources to coal fragments during pyrolysis were in general not successful as shortly reviewed above. This is because the rate of radical supply did not match the formation rate of coal fragments. To match both rates, control of the primary reactions based on the understanding of coal structure seems to be indispensable. 4.3.2. New Pyrolysis Methods Recently the macromolecular and the chemical structure of coal have been almost completely elucidated through various analytical methods and advanced spectroscopic instruments (also see Chapter 2). Non-covalent interactions such as hydrogen bonds, electron donor-acceptor interaction, van der Waals force, etc. are believed to play an important role to keep the macromolecular structure of coal [111-115]. Then the noncovalent interactions would affect the pyrolysis behaviour of coal. The success of Ofosu-Asante and co-workers in increasing TVM is judged to be realised by removing hydrogen bonded OH groups before pyrolysis. Based on the understanding of these noncovalent interactions the authors have recently proposed the following pyrolysis methods. 4.3.2.L Pyrolysis of Coal Swollen with Solvent /19J05J It is essential to control the primary devolatilisation reactions for increasing TVM. To do so, the cross-linking reaction between hydrogen-bonded oxygen functional groups must be suppressed as stated above. It is well known that brown coal is swollen to a great extent in polar solvents such as methanol, pyridine, etc. This is because the coal hydrogen bonds between oxygen functional groups are replaced by the hydrogen bonds between the coal oxygen functional groups and the solvents. Miura and co-workers intended to utilise this coal swelling phenomenon as a means to control the primary devolatilisation reactions of brown coal. Several coals were treated by hydrogen donor
167
Pyrolysis
solvents such as tetralin at 70 to 200°C under 1 MPa of nitrogen pressure. Through this treatment coal hydrogen bonds are released and solvent molecules penetrated coal particles to swell them by 30 % or so in volume as shown in Figure 4.13. The solvent molecules are believed to be occluded between the coal oxygen functional groups. The solvent swollen coals thus prepared were pyrolysed in a flash mode by use of a Curie point pyrolyser and/or a free fall reactor. The TVM (total volatiles), tar yield and H2O
Hydrogen bondmg
Intimate contact between tetralin Microporesx and functional ^Ngrougs^^ _^
Release of hydrogen bondings
! 1^4 Raw coal
Tetralin swollen coal
Figure 4.13 Image of the tetralin swelling of coal.
590
670 764 920 Pyrolysis temp. [*C]
Figure 4.14 Comparison of the yields during the pyrolysis between the raw coal and the tetralin treated coal (TTC) for Morwell coal.
168
Chapter 4
yield obtained by pyrolysing Morwell coal are shown in Figure 4.14. The TVM of the tetralin swollen coal (TTC) was larger than that of the raw coal at all the temperatures, and larger by 12 % at 920 °C. The tar yield of the TTC was also significantly larger than that of the raw coal and surprisingly it reached 42 % at 920 °C, which was twice as large as that of the raw coal. This significant increase in the tar yield was compensated by the decreases in the char and the water yields as shown in Figure 4.14. The yield of hydrogen of the TTC was also significantly smaller than that of the raw coal. These results indicated that the increases in the TVM and the tar yield of the TTC were brought about by the suppression of H2O forming cross-linking reactions, which is realised by taking apart the hydrogen-bonded coal oxygen functional groups spatially, and by effective hydrogen transfer ft-om tetralin to radical fragments of coal. In other words, the solvent swelling of coal increased the coal conversion and the tar yield through physical and chemical effects. The point of this method lies in fixing the hydrogen donor in the vicinity of the coal oxygen functional groups just when radical fragments are being formed from coal. In this sense Miura and co-workers called this method a "flash liquefaction in a microspace of coal". 4.3.2.2. Pyrolysis of Coal in a Flow of Solvent Vapour [84/ Next, flash pyrolysis of coal in a solvent vapour was performed to increase the selectivity of chemicals such as BTX. This method intended to control the secondary gas phase reactions by effectively supplying reactive radicals from solvent to the primary pyrolysis products. When Morwell coal was pyrolysed at 750 °C, the tar yield increased by 0.1 kg/kg-coal in a vapour of ethylbenzene, and the BTX yield reached 0.064 kg/kg-coal in a vapour of 2-methyl-l-propanol, which was 10 times larger than the BTX yield obtained in an inert atmosphere. This method presented the possibility to control the product distribution by controlling the secondary gas phase reactions. 4.3.2.3. Pyrolysis of Coal-Solvent Slurry /106J It was then intended to develop a method combining the two methods introduced above to simultaneously control the primary devolatilisation reactions and the secondary gas phase reactions. We tried to pyrolyse the tetralin-swollen coal in a vapour of 2methyl-1-propanol [116]. Figure 4.15 compares the yields obtained by four different pyrolysis methods for Morwell coal at 750°C. The bars represent the yields for (1) pyrolysis of the raw coal in an inert atmosphere (white bars), (2) pyrolysis of the tetralin-swollen coal in an inert atmosphere (cross-hatched bars), (3) pyrolysis of the raw coal in the vapour of 2-methyl-1-propanol (hatched bars), and (4) pyrolysis of the tetralin-swollen coal in the vapour of 2-methyl-l-propanol (black bars)). The yields obtained by pyrolysing tetralin and 2-methyl-l-propanol are also shown. These yields were utilised to estimate the actual effect of the proposed methods. The char yields of methods (1) and (3) and those of methods (2) and (4) almost exactly coincided, which means that only the tetralin-swelling is effective to control the primary devolatilisation reactions. The BTX yields of methods (3) and (4) are large and almost coincided, which
Pyrolysis
169
Char
Figure 4.15 methods.
Comparison of the product distributions obtained by four different pyrolysis
means that 2-methyl-l-propanol in the gas phase mainly controlled the secondary gas phase reactions. Thus method (4) gave the sum yields of methods (2) and (3), and is effective to increase both the TVM and the BTX yield. Although the concept of method (4) seemed attractive, it would be almost impossible to realise method (4) as it is in practical applications. Then, to realise the concept in practical processes, the pyrolysis of coal-solvent slurry was proposed, where coal particles swollen by solvent was directly served to the flash pyrolysis with the solvent. We have examined the validity of this method using a coal-methanol slurry as an example. When the methanol slurry prepared from Morwell coal was pyrolysed, the TVM, the tar yield and the BTX yield increased significantly as compared with the pyrolysis of the raw coal in an inert atmosphere as shown in Figure 4.16. Thus, it was found that the pyrolysis of coal-solvent slurry was effective to increase both the TVM and the BTX yield. This method has many additional merits: it does not use expensive hydrogen, the pyrolysis condition is rather mild, it is very easy to add hydrogen donors and/or catalysts if necessary, and the handling of coal can be greatly facilitated. 4.3.2.4. Pyrolysis of Solvent Solubilised Coal 1118J19] The method pyrolysing coal-solvent slurry was extended to use the coal pre-oxidised by H2O2 [117]. The pre-oxidation was performed to increase the swelling sites, carboxylic groups, in coal. The extended method further increased the total volatiles and the tar yield. Miura and co-workers have also found that the oxidised coal (oxidised for 2 h at 60 C in 30% H2O2 aq) can be solubilised up to 85 % in methanol-based binary
170
Chapter 4 :?60
650
750 850 Pyrolysis temp. pC]
Figure 4.16 Comparison of total volatiles, tar yield, and BTX yield between the pyrolysis of coal-methanol slurry and the pyrolysis of raw coal in an inert atmosphere for Morwell coal.
solvents such as methanol/1-methyl naphthalene as shown in Figure 4.17 [118,119]. This presented us the possibility to develop more effective coal utilisation processes. Then Miura and co-workers presented a new coal pyrolysis method pyrolysing the solubilised coal [120]. When pyrolysing Morwell coal oxidized and then solubilised in a mixed solvent of methanol/tetralin, the TVM reached surprisingly 0.87 on solubilised coal basis at the pyrolysis temperature of 750 °C. The overall balance for this method was established as follows: the yield through the pre-oxidation is 0.79, the yield of the solubilised coal is 0.66 on coal basis, and the volatile matters obtained through this oxidation, solubilisation, and pyrolysis is 0.56 kg/kg-coal. In addition, 0.17 kg/kg-coal of water soluble organics were produced during the pre-oxidation, and pyrolysing the residue obtained during the solubilisation produced 0.08 kg/kg-coal of volatile matters. Then 0.81 kg/kg-coal of volatile matters can be obtained by the proposed method. It is noteworthy that TVM obtained by pyrolysing the solubilised coal as precipitated solid was only 0.54 under the same pyrolysis conditions. This clarified that pyrolysing the highly dispersed coal molecules in solvent realised the high TVM. On the other hand,
Pyrolysis 100
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Figure 4.17 Extraction yield of the oxidized coal (HPO60.2) prepared from Morwell coal at 25^C in various methanol-based binary solvents. Reprinted with permission from Ref. 119. Copyright 1997 American Chemical Society.
coal molecules form an aggregate in the precipitated solid, which could not suppress cross-linking reactions among coal molecules during the pyrolysis. Thus, the pyrolysis of solvent solubilised coal was found to be a promising method to recover volatile matters from coal in high yield. As reviewed in this section, special attentions have been paid to new pyrolysis methods that enable to recovery raw materials for valuable chemicals, clean fuels and carbon materials from brown coal at high yields under rather mild operating conditions. After reviewing the trials made to obtain aromatic compounds and tar from coal in high yields through each method several attempts made by Miura and co-workers were introduced. (1) Flash pyrolysis of solvent swollen coal, (2) pyrolysis of coal in a solvent vapour, (3) flash pyrolysis of coal-solvent slurry, and (4) flash pyrolysis of solvent solubilised coal were introduced as new coal pyrolysis methods. These methods were developed to control the primary devolatilisation reactions and/or the secondary gas phase reactions. When pyrolysing Morwell coal after partial oxidation by H2O2 in liquid phase and solubilisation in a mixed solvent of methanol/tetralin, pyrolysis method (4), it was found 0.81 kg/kg-coal of volatile matters in total could be obtained. Methods introduced here were developed based on the recent progress in the understanding of coal structure. The authors expect that further progress in coal science will surely bring forth new and efficient brown coal utilisation methods.
172
Chapter 4
4.4. VOLATILISATION OF ALKALI AND ALKALINE EARTH METALLIC SPECIES Although the total contents of alkali and alkaline earth metallic (AAEM) species are generally less than 1 wt% of coal on a dry basis, they are clearly responsible for the characteristics of the pyrolysis of the brown coal, as was described in Section 4.2. AAEM species are also responsible for problems encountered in combustion processes such as fouling, slagging and defluidisation [121]. AAEM species undergo volatilisation during pyrolysis as well as subsequent combustion or gasification. This will cause erosion/corrosion of turbine blades for power generation from the brown coal by advanced fluidised-bed combustion or gasification. On the other hand, AAEM species retained in the char from the primary pyrolysis can serve as catalysts of char combustion and gasification while volatilised species might catalyse reactions of volatiles in the gas phase. Hence, future successful development of advanced technologies converting the brown coal relies on the understanding of the behaviour of AAEM species during conversion. In this section, studies on the volatilisation of AAEM species during the pyrolysis of brown coals, mainly Victorian brown coals, are reviewed and then the volatilisation mechanisms are discussed. 4.4.1. Volatilisation of AAEM Species during Primary Pyrolysis Manzoori and Agarwal [61] performed the rapid pyrolysis of single particles of an Australian brown coal (Lochiel coal; particle size in the range of 5.5 to 9.0 mm) at 700 - 830 °C in a convective gas flow. The coal had relatively high contents of AAEM species (Na: 0.93 wt%-db; Ca: 1.36; Mg: 0.94), sulphur (3.4 wt%-db) and chlorine (0.6 wt%-db). Figure 4.18 illustrates the volatilisation (loss) of Na and CI during pyrolysis as a function of temperature and time. It is clearly seen that the volatilisation of Na is much less extensive than that of CI. It was estimated that most of CI and a part of Na (about 42%) in the coal were present as NaCl. Then, even by assuming that all of the volatilised Na was initially present as NaCl, only a part of the volatilisation of CI could be explained by that as NaCl. This is an indication of disproportionate release of Na and CI and also decomposition of NaCl into organically bound Na (char-bonded Na) and a gaseous Cl-containing gas, probably HCl. Furthermore, it is suggested that the majority of Na in the coal transformed into char-bonded Na regardless of its initial chemical form (as carboxylates or NaCl). The volatilisation of Na separately from that of CI was also claimed by Srinivasachar and co-workers [62], who investigated the volatilisation of Na from chars prepared by low temperature pyrolysis of Loy Yang coal and Beulah Zap lignite. Clearly different from Na, neither Ca nor Mg underwent volatilisation within the range of conditions employed by Manzoori and Agarwal [61]. Most of Ca and Mg in the raw coal were insoluble in water but soluble in an acidic solution (0.1 N HCl aq.). These species became acid-insoluble during pyrolysis. However, the formation of acidinsoluble compounds containing Ca or Mg was neither evidenced from XRD analysis of chars nor predicted from equilibrium thermodynamic calculations. Acid-insoluble
173
Pyrolysis
40
60
80
120
Time ( s )
Figure 4.18 Volatilisation of Na and CI from a single particle of Lochiel coal during pyrolysis [61]. Conditions: temperature; 700, 770 or 830 °C, pressure; 0.1 MPa. Solid lines (~), Na; dashed lines (—), CI.
compounds were formed from Na to a very limited degree and its solubility in water was maintained over the ranges of pyrolysis temperature and holding time. This may be explained by considering that most of the char bonded Na was present in forms of organic salts, or otherwise, as water-soluble inorganic salts such as Na2C03. Takarada and co-workers [122] impregnated Na2C03 or mixed NaCl with raw and acid-washed Yalloum coals at a Na loading of 5 wt%-db, and they pyrolysed the Naloaded coal samples in a fixed-bed reactor at a heating rate of about 13 °C s' . Volatilisation of Na took place above 600 °C and the degree of volatilisation reached about 90% and 70% at 1000 °C for the NaCl-mixed and NasCOs-impregnated samples, respectively. Li and co-workers [65] investigated the volatilisation of AAEM species during the pyrolysis of Loy Yang coal (Na: 0.13 wt%-db; Mg: 0.06 v^%-db) in a WMR at atmospheric pressure. The extent of volatilisation was influenced by the peak temperature much more significantly than by the heating rate. Figure 4.19 shows the variations in the retentions of Na and Mg with temperature. The retentions of Na and Mg decreased to about 30% and 50% as the temperature increased up to 1200 °C. The volatilisation of Ca occurred in a manner similar to that of Mg.
174
Chapter 4 100
80
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• O
1000 "Cs-l 1 X s-^
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1> 80 60
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20
400
600
800
1000
1200
Peak temperature, °C
Figure 4.19 Effects of heating rate and holding time on the volatihsation of Na and MgfromLoy Yang coal for the pyrolysis in the wire-mesh reactor (based on the data in Ref 65). Conditions: pressure; 0.1 MPa, holding time at the peak temperature; 10 s.
It is noted in Figure 4.19 that the retentions of Na and Mg were already 80 - 90% at 300 °C where the tar yield was negligibly low [21]. It is impossible for AAEM species to be released as inorganic salts (carbonate, chloride or hydroxide) or metals at such low temperatures [123]. Quyn and co-workers [15] pyrolysed Na-exchanged, Ca-exchanged and NaCl-impregnated samples from Loy Yang coal, which contained 2.80 wt%-db of Na, 3.27 wt%-db of Ca and 0.89 wt%-db of Na, respectively. They found that about 10% of the loaded Na was volatilised from the Na-exchanged and NaCl-impregnated coals at 300 - 400 °C while a similar fraction of Ca was volatilised from the Caexchanged coal. They also observed the evolution of light carboxylates such as formates, acetates and benzoates with sufficient amounts to carry Na or Ca that were volatilised at 300 °C. Based on this result, they attributed the volatilisation of Na and Ca at lower temperatures to their release as light carboxylates. The volatilisation of AAEM species as carboxylates may be caused by reactions such as
Pyrolysis
175
CM-CH2-COO-M + H =:: CM +CH3COO-M
(R4-5)
CM-CH2-COO-M-OOC-CH2-CM' + 2H = CM + CM' + (CH3COO)2-M
(R4-6)
M denotes mono or divalent cations. CM (CM') and H represent coal/char matrix and donatable hydrogen, respectively. It is seen in Figure 4.19 that the further volatilisation of Na and Mg was insignificant at temperatures from 300 to 600 °C where the tar was evolved. Rather, the main part of volatilisation took place above 600 °C, in other words, after completion of the tar evolution [21]. Li and co-workers [65] and Quyn and co-workers [15] proposed mechanisms for the intraparticle transformation and volatilisation of AAEM species as follows. AAEM species associated with carboxylic groups first undergo transformation to char-bonded species with the decomposition of the carboxylates to form CO2 [15,65], unless the species are released as light carboxylates according to Reactions (R4-5) and (R4-6). CM-COO-M-OOC-CM' = CM-COO-M-CM' + CO2
(R4-7)
CM-COO-M-CM' = CM-M-CM' + CO2
(R4-8)
CM-COO-M = CM-M + CO2
(R4-9)
CM-M and CM-M-CM' may further experience decomposition (breaking of CM-M bonds) forming carbon-containing gases such as CO and CO2 and new bonds between M and CM would be formed [64]. Huggins and co-workers [124] reported that bulk CaO was virtually absent in chars from the rapid pyrolysis of a lignite that contained Ca as carboxylates. Yamashita and co-workers [125] pyrolysed Ca-loaded Yalloum coal samples and analysed the resulting chars by means of an X-ray absorption fine structure (XAFS) spectroscopy. They detected no Ca-Ca bonds assigned to crystalline CaCOs or CaO even at 800 °C unless the initial Ca content was well above 2 wt%-db. Instead, it was found that the Ca species were dispersed with CaO^ moiety. Thus, Ca in the char was highly dispersed in the char matrix being bonded to the matrix via oxygen atoms without forming phases (particles) of carbonates or oxides. More extensive volatilisation of CI than that of Na was also reported by Quyn and co-workers [126]. They pyrolysed raw and NaCl-impregnated Loy Yang coals (Na contents: 0.13 and 0.89 wt%-db respectively) in a fluidised-bed/fixed-bed reactor. They found that 60 - 80 % of CI was released during the pyrolysis up to 500 °C where only about 10% of Na was volatilised. This result indicates that NaCl in the coal was decomposed into gases (probably HCl) and char-bonded Na (CM-Na) at low temperatures. Thus, the transformation of NaCl may be represented by reactions such as NaCl + CM-H = CM-Na + HCl
(R4-10)
176
Chapter 4
4.4.2. Volatilisation of Char-Matrix-Bonded AAEM Species The volatilisation of char-bonded AAEM species requires the breakage of bonds between the species and the char matrix (CM). Dissociation of CM-M bonds (in the case of monovalent M such as Na) can possibly be caused by the following reactions [15,65,127]. CM-M + R = CM-R + M
(R4-n)
CM-M + R = CM + R-M
(R4-12)
CM-M = CM + M
(R4-13)
Among these types of reactions, Reaction (R4-13) is implausible because homogeneous cleavage of CM-M bonds requires extremely high level of thermal energy. Instead of Reaction (R4-13), Reactions (R4-11) and (R4-12) may be responsible for the release of M from the char matrix. Li and co-workers [127] pyrolysed Yalloum coal in a fluidised-bed reactor, and they found that significant volatilisation of not only Na but also Ca occurred although the exact extents of volatilisation were not certain [127]. Quyn and co-workers [11,126] pyrolysed raw, Na-exchanged and NaCl-impregnated Loy Yang coal samples in a
1
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Figure 4.20 Volatilisation of Na and Ca as a function of temperature during the pyrolysis of NaCl-impregnated (NaCl loading; 0.89 wt% on a dry basis), Na-exchanged and Ca-exchanged Loy Yang coal samples in the fluidised-bed/fixed-bed reactor [15].
111
Pyrolysis
fluidised-bed/fixed-bed reactor. In the reactor, the coal sample was rapidly pyrolysed in the fluidised-bed and then the char formed was retained in the freeboard forming a fixed-bed underneath a frit. Thus, the fixed-bed was exposed to the flow of the vapour of nascent volatiles coming up from the fluidised bed together with the nascent char. Typical results are shown in Figure 4.20. The pyrolysis in the fluidised-bed/fixed-bed reactor caused nearly all of Na to be volatilised from the raw and NaCl-impregnated coals at 800 - 900 °C. Such extensive volatilisation of Na never occurs during the pyrolysis at equivalent temperatures in the WMR (see Figure 4.19) in which contact between the char and volatiles are minimised. The results shown in Figure 4.20 would be reasonably explained if considering that the volatiles supply reactive species such as free radicals that play roles of R for inducing reactions (R4-11) and (R4-12). Wu and co-workers [128] further examined the enhanced volatilisation of Na from the char by its exposure to volatiles. They developed and used a two-stage fluidisedbed/fixed-bed reactor as presented schematically in Figure 4.21. A Na-exchanged or
Fiuidising gas H4orm coat Loading
Figure 4.21 A schematic diagram of a two-stage fluidised-bed/fixed-bed reactor [128].
178
Chapter 4
101
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Figure 4.22 Retentions of AAEM species in the char from the pyrolysis of a NaCl-impregnated Loy Yang coal (NaCl loading: 2.09 wt%-db) as a function of time at 900 °C for exposure of the char to nascent volatiles from the fast pyrolysis of acid-washed Loy Yang coal at 900 °C in the two-stage fluidised-bed/fixed-bed reactor (based on the data in Ref 128). Pressure: 0.1 MPa.
NaCl-impregnated Loy Yang brown coal was preloaded in the upper section of the reactor and heated up to 900 °C at a rate of 0.083 °C s"^ with a holding time long enough to complete the volatilisation of Na at the temperature. This slow pyrolysis caused 50 - 70 % of the initial Na to be volatilised. The char formed in such a way was in situ exposed to the vapour of the volatiles from an AAEM-free Loy Yang coal that was continuously pyrolysed at 900 °C in the bottom section, i.e., the fluidised-bed. Figure 4.22 shows the changes in the retentions of Na, Ca and Mg with increasing exposure time, giving a direct evidence of the volatilisation of Na induced by the interactions between char and volatiles. During the exposure, neither soot deposition onto the char nor its gasification occurred to a detectable degree. The diffusion of tar vapour through the pore system of the char was thus unlikely, and it was therefore concluded that small radicals such as hydrogen radicals migrated into the char and participated in reaction (R4-11) serving as R. Clearly different from the volatilisation of Na, that of Ca and Mg was hardly induced by the volatile-char interactions and this is consistent with the data in Figure 4.20. Wu and co-workers [128] ascribed the difficulty of volatilisation of Ca and Mg to their divalent natures. Namely, simultaneous breakage of two bonds is needed for the release of Ca or Mg from the char matrix. CM-M-CM' + 2H = CM-H + CM'-H + M
(R4-14)
It should be reminded that the volatilisation of the char-bonded AAEM species occurs mainly after completion of tar evolution even in the absence of the interaction
179
Pyrolysis 100 |(a) (0 O
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200
400
600
800
1000
1200
Temperature, "C Figure 4.23 Effects of peak temperature on the char yield, tar yield and retention of Ca or Na during the pyrolysis of (a) Ca-exchanged and (b) Na-exchanged Loy Yang coal in the wire-mesh reactor (based on the data in Ref 65). •, char yield at heating rate of 1000 °C s'*; A, tar yield at 1000 °C s-1. D, retention of Ca or Na at 1000 ""C s ; o, retention of Ca at C s"'. Conditions: pressure, 0. MPa; holding time at the peak temperature, 10 s.
between volatiles and char as during the pyrolysis in the WMR [65]. It is believed that char undergoes thermal cracking forming light gases such as CO, H2 and CH4 after completion of the tar evolution. This in turn means the presence of radicals such as hydrogen radicals in the char matrix. Hydrogen radicals formed from the thermal cracking of the char can be involved in Reaction (R4-11). Li and co-workers [65] observed significant volatilisation of Na and Ca during the pyrolysis of Na-exchanged and Ca-exchanged Loy Yang coals in a WMR at 1000 1200 °C. As is seen in Figure 4.23, the releases of Na and Ca are both associated with those of tars. This result suggests that the AAEM species can be released as M-R or RM-R' formed from reactions such as (R4-12); alternatively, severe thermal cracking causes lack of oxygen in the char matrix as sites to bond AAEM species. Figure 4.24 compares the volatilisation between Na and CI during the rapid pyrolysis of raw and NaCl-impregnated Loy Yang coals in a fluidised-bed/fixed-bed reactor
180
Chapter 4 ->
1
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100 (A) Raw coal (fast heating) 80 CI Na
60 40 O O
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40
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ok. 100 o
80
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60
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Raw coal (slow heating) CI Na
40 20 0 200
400
600
800
1000
Temperature, 'C
Figure 4.24 Volatilisation of Na and CI during the pyrolysis of raw, NaCl-loaded (NaCl loading: 0.89 wt%-db) Loy Yang coal samples as a function of temperature in the fluidised-bed/fixed-bed reactor at fast and slow (ca 10 °C min"') heating rates [126]. Conditions: holding time at peak temperature, 10 min; pressure, 0.1 MPa.
[126]. For the pyrolysis above 500 °C, the fraction of volatilised CI decreased and then increased with temperature, while that of Na increased monotonically up to nearly 100% due to the volatile-char interaction as explained above. The complex behaviour of CI may result from recombination of Cl-containing gases (e.g., HCl) back into the char forming char-bonded CI. The formation of NaCl in/on the char is unlikely because the volatilisation of Na is much more extensive above 600 or 700 °C. Although exact nature of bonds between CI and the char matrix is unknown, they may not be so stable thermally and therefore decompose releasing CI into the gas phase at higher temperature.
181
Pyrolysis 4.4.3, Effects of Pressure on the Volatilisation of AAEM Species
Sathe and co-workers [129] investigated the volatilisation of AAEM species during the pyrolysis of Loy Yang coal in a WMR at a heating rate of 1000 °C s'^ in ranges of pressure and temperature from 0,1 to 6.1 MPa and from 600 to 900 °C, respectively. Figure 4.25 illustrates the effects of pressure on the retention of Na in the char at different temperatures with a holding time of 10 s. It is seen that pressure influenced the volatilisation of Na in a very complex manner.
1
1
1
1
i
J^
90 75
-
j J
60 600X 45 90
1 1
1 1
1 1
1 1
1 1
\r^\ A-^---*
75 60
1 1
•
.V
700X
45
1 1
1 1
1 1
1 1
1 1
1 1
90 75
/ ^^^
1 J
60 800X 45 1 1
1 1
1 1
1 1
90
1 1
1 1
900X
-\
75 60 45
i
• \ 1
] 1
1
1
1
1
J
Pressure, MPa
Figure 4.25 Combined effects of peak temperature and pressure on the volatilisation of Na from Loy Yang coal during the pyrolysis in the wire-mesh reactor (based on the data in Ref 129). Condition: heating rate, 1000 °C s"^; holding time at peak temperature, 10 s.
182
Chapter 4
For explaining the complex effects of pressure, it is essential to consider not only the mechanisms for the AAEM volatilisation as outlined above but also the pressure effects on the evolution of tar and other volatiles. As described in Section 4.2.6, pressure affects the main mechanism of the intraparticle transport of volatile precursors, thereby influencing the yields of volatiles especially tar. Figure 4.26 [48] shows the tar yields at 600 and 700 °C for a holding time of 10 s. Decreases in the tar yield with increases in pressure from 0.1 to 0.6 or 1.1 MPa were due to the suppressed intraparticle diffusion of volatile precursors that extended their residence time and enhanced their thermal cracking inside the char particle. On the other hand, increasing pressure from 2.0 to 6.1 MPa slowed down the forced convective flow of volatile precursors by decreasing the pressure gradient across the pore system of the char. Changes in the tar yield at 0.6 or 1.1 to 2.0 MPa were results from the transition of the main mechanism from the diffusion to the convection flow. As is seen in Figure 4.25, the Na retention at 600 °C increased from 78 to 98% with increasing pressure. At temperatures up to 600 °C, Na was released mainly as light carboxylates before the tar evolution [15]. Increasing pressure slowed down the intraparticle diffusion of light carboxylates that are believed to be relatively labile [15], thus resulting in their decomposition into CO2 and char-matrix-bonded Na. Light carboxylates are in fact unstable. For the pyrolysis performed in a TGR [126], the volatilisation of Na occurred only to a negligible extent, because light carboxylates, even if released from a particle, were trapped by another particle and then decomposed before escaping from the fixed bed by diffusion. The Na retention at 700 °C went
1
r
\
1
F
T—1
20
C^
75 0
it 16
H
CO •0
0
1 2
R
0)
0
^
4
- 1/ •
A 0 600 °C
8
• 0
I
I
L
1
1
2
3
4
700 X
_,
-\
Pressure, MPa
Figure 4.26 Effects of pressure on the tar yield from the pyrolysis of Loy Vang coal in the wiremesh reactor [48]. Conditions: heating rate, 1000 °C s"'; peak temperature, 600 or 700 °C; holding time, 10 s.
Pywlysis
183
through a minimum at 1.1 MPa, where the tar yield was minimised due to extended residence of volatile precursors inside the particle. At 1.1 MPa, it took several seconds for the tar evolution to be completed [129]. This trend of the Na retention at 700 °C can be explained based on the importance of Reactions (R4-11) and/or (R4-12). In the diffusion controlled regime (0.1 to 1.1 MPa), the intensive thermal cracking inside the particle produced a large amount of radicals that might have served as agents for breaking CM-Na bonds to form free Na or Na-R. Sathe and co-workers [129] investigated the changes in the Na retention and tar yield with holding time at 700 °C and 1.1 MPa and confirmed that the progress and termination of Na release corresponded well with those of the tar evolution. Thus, increasing residence time of volatile precursors intensified the volatilisation of Na at 700 °C due to Reactions (R4ll)and/or(R4-12). For the pressures in the range from 2.0 to 6.1 MPa, the decreases in the Na retention at 600 - 900 °C were due to the Na volatilisation not during the tar evolution but afterward, because the rapid tar evolution by the forced convective flow was completed before the temperature reached 600 °C [21]. It is seen in Figure 4.25 that the progress of Na volatilisation at 600 - 900 °C became increasingly rapid or extensive as the pressure increased. This shows that the release of Na was not controlled by its intraparticle diffusion but by a chemical process. Char undergoes thermal cracking generating hydrogen and other radicals even after completion of the tar evolution. Their diffusion through the pore system of the char would be suppressed by increasing pressure. This would result in an increase in their concentrations, promoting Reactions (R4-11) and (R4-12). The progress of Na volatilisation at 2.0 - 6.1 MPa and 700 - 900 °C was much more significant than that at lower pressures. As was described above, the effects of pressure on the volatilisation of Na during the pyrolysis in the WMR are reasonably explained by mechanisms for the transformation of Na, i.e.. Reactions (R4-5) - (R4-12) and those for the intraparticle transport of volatiles. Sathe and co-workers [129] also reported that the pressure influenced the volatilisation of Mg and Ca in a manner similar to that of Na, although Mg and Ca were less volatile than Na. 4.4.4. Effects of Volatilisation of AAEM Species on Char Gasification There have been a number of studies focusing on the effects of inherent and externally added AAEM species on the gasification of char after pyrolysis. In many of those studies, apparent reactivities of chars were correlated with the initial contents of AAEM species in the substrate coals or in the chars. However, without considering the volatilisation of AAEM species (as catalysts) before gasification, either during pyrolysis or during the heat up of cooled char for reactivity measurement, it seems difficult to understand the characteristics of gasification, in particular, the effects of catalyst concentration on the observed char reactivity. Wu and co-workers [127,130] prepared char samples by pyrolysing Loy Yang coal samples with different loadings of NaCl in a range from 0.29 to 2.01 vv^%-db. They observed the 'saturation' of the char reactivity with O2 at a certain NaCl loading level and claimed that the saturation was not due to
184
Chapter 4
the poor dispersion of NaCl-derived Na at higher loading levels but the saturation of Na retention that was determined mainly by the coal substrate and pyrolysis conditions e.g. temperature. In addition to the volatilisation of AAEM species, that of CI should also be taken into consideration. As is shown in Figure 4.24, the retention of CI relative to that of Na is considerably influenced by the pyrolysis conditions. This causes broad variations in the char reactivity with the condition of pyrolysis for the preparation of char [131]. A more detailed discussion on the gasification reactivity of char may be found in Chapters.
4.5. MODELS FOR COAL PYROLYSIS 4.5.L Chemical and Macromolecular Structure of Coal and Pyrolysis As stated in previous chapters, great progress has been made in recent years to understand the chemical and macromolecular structure of coal (see Chapter 2). It is therefore very important to consider the structure when we analyse the coal pyrolysis. Figure 4.27 shows a schematic macromolecular structure of coal that was proposed by Larsen and co-workers [132], where aromatic clusters represented by the pentagons are connected by covalent bonds (alkyl, etheric, oxygen and sulphur bridges; shown by the solid lines) and non-covalent bonds (van der Waals interactions and hydrogen bonds; shown by s). The pentagons circled represent cross-linking points of covalent bonds. The non-covalent bonds also form weak apparent cross-links. This model shows that the number of non-covalent cross-linking points is much larger than that of covalent crosslinking points, which means that the non-covalent bonds play significant roles to retain the macromolecular structure of coal. It is also noted that small molecules, the so-called guest molecules, are occluded within the macrostructure in Figure 4.27. Based on such information on coal structure, the chemical and physical processes of coal pyrolysis are discussed. Solomon and co-workers, for example, depicted the changes of a bituminous coal during the pyrolysis as shown in Figure 4.28 [133]. They further examined the coal pyrolysis process in detail and divided it into the following nine steps: Step 1: Rupture of hydrogen bonds Step 2: Diffusion and devolatilisation of "guest molecules" Step 3: Formation of cross-links at low temperatures. This is significant for brown coal and is closely related to the formation of H2O and CO2 Step 4: Change of macromolecules to radical fragments due to the breakage of weak covalent bonds Step 5: Stabilisation of the radical fragments produced in step 4 through hydrogen transfers Step 6: Diffusion and devolatilisation of small molecules formed in step 5 Step 7: Recombination of the radical fragments produced in step 5 to form large molecules
Pyrolysis
185
Figure 4.27 A schematic model for macromolecular structure of coal by Larsen and co-workers [132].
Step 8: Decomposition of functional groups forming light gases Step 9: Formation and growth of poly-aromatic structures accompanied by H2 formation at high temperature In this section, recent advances in the kinetic modelling of coal pyrolysis are introduced, starting from a simple global model to sophisticated models incorporating the progress of the recent understanding of coal structure. Some problems in relation to the interpretation of global model are also discussed. A new simple analysis method is also presented for estimating the distributions of activation energy and frequency factor in the so-called distributed activation energy model. Since the kinetic analysis is not restricted to brown coal, no distinction is made between brown coal and high rank coals in this section. 4.5.2. Single Reaction Model This model represents the complex coal pyrolysis reactions by a single reaction based on a simple reaction stoichiometry as
186
Chapter 4
a)
H2
»Z CH3
CH3
b)
I
C«4
CH3
COAl
CH3
0-
Tgr
N
CM3
C,K 2"5
*
PRIMARY PYROLYSIS STAGE II •<2 CH3
"•^"%,
^^
'""^^O^cl,
xcoo
Hi
^H,'
\
c) Hj . ^ ^
SECONDARY PYROLYSIS STAGE i n
Figure 4.28 Hypothetical coal molecule during stages of pyrolysis. Reprinted with permission from Ref. 144. Copyright 1987 American Chemical Society.
187
Pyrolysis
(R4-15)
Coal-» Volatile + Char
Global or apparent reaction rate parameters are determined based on several analysis methods proposed in the field of thermal analysis. Here several analysis methods for the single reaction are briefly reviewed, and the applicability of the methods to coal pyrolysis is discussed. Using the conversion of solid reactant, X, the pyrolysis rate is defined by the conversion rate, dX/dt, written as dX = kQe -E/RT f(X) dt
(4-1)
where Tk the temperature, ko is the frequency factor and E is the activation energy. The function/(1\5 represents the change in the solid reactant with the progress of reaction. It is, for example, given as f{x)=] - X when the reaction rate is directly proportional to the unreacted fraction of the solid. This may simply be called the first-order reaction in
1.0
"1—I—\—1—I—\—r
1/r 1
1
'
-T"" —1
a = 83 ? ~ ^2 1
\ V \j J j T
^
\
*
1
(d) J
^'JJA—\^_\ ^ ^^ % % _\^ * ^^{«^
\
^ !..._ 1
1
\ a = a,\ J -^
X= X
1
1
1/r Figure 4.29 Schematic representation of X vs. T relationships and Arrhenius plots for estimating activation energy, (a), (b): single reaction; (c), (d): parallel reactions.
188
Chapter 4
the field of thermal analysis. The target of the thermal analysis is the determination of the functional form of J(X) and the estimation of the values of ko and E from experimentally obtained X vs. / relationships. Pyrolysis reaction is highly temperature dependent in general, which means that the pyrolysis reaction is not completed at a low temperature but is completed instantaneously at a high temperature. This makes it very difficult to obtain reliable X vs. / relationships from pyrolysis experiments performed at constant temperatures. To overcome this difficulty, pyrolysis experiments are performed in general by heating the solid reactant from a low temperature TQ, at which reaction rate is appreciably zero, to a higher temperature at a constant rate a. Then the temperature of the solid reactant T is related to time / by T--To-^at
(4-2)
Inserting this relationship to Eq. (4-1) gives ^ = ^e-''''f(X) dT a
(4-3)
When X vs. T relationships are measured at three different a values, they shift to higher temperature region with increasing a value {a\
(4-4)
This equation enables to estimate E without knowing the functional form off^X). When \x\{dXldt) or \n{adXldT) estimated at selected X values of X], Xi, and X^ for different a values are plotted against \IT at each given X value, parallel linear relationships can be obtained as shown by the solid lines in Figure 4.29(b), because \T\{k(f{X)} is same for the different a values at the same X value. The E value can be obtained from the slope of the linear relationships and \x\{kof{X)} values can be obtained from the intercepts. If linear parallel relationships are not obtained when making the plot of ln(dA7d/) vs. MT, the reaction can not be represented by a single reaction. If it is a case, this analysis method must be applied with care as will be stated later in this section.
Pyrolysis
189
4.5.2.2. Integral Method Integrating Eq. (4-3) gives ^(^) - (-77^^ 4) f{X)
= - {^-'"'dT ah
(4-5)
where the initial temperature TQ was set equal to 0, because TQ can be chosen so that the reaction rate dX T - TQ can be regarded as zero. Rearranging Eq. (4-5) with the substitution of w = E/RT gives ,(;,) = M ( £ : l . r £ : ! ^ , ) = M , ( „ ) aR u Ju u aR
(4-6)
where p{u) is called the "/>-function" in the field of thermal analysis. Although p{u) cannot be integrated analytically, the following approximate equations hold with high accuracy ioxp{u) [135] p{u) = exp(-2.315 - 0.4567w),
p(u) = —r^ u
(20 < w < 60)
(20 < w < 50)
(4-7)
(4-8)
Inserting Eq. (4-7) into Eq. (4-6) gives lna= I n ^ - Ing(X)-2.315-0.4567—/? RT
(4-9)
On the other hand, inserting Eq. (4.8) into Eq. (4.6) gives ln(^)= lnM_,ng(X)-41 T^ E RT
(4-10)
Equations (4-9) and (4-10) show that the Arrhenius type plots. In a vs. 1/7 for Eq. (4-9) and ln(a/7^) vs. \IT for Eq. (4-10), give straight lines for a same X value of X vs. T relationships which are obtained at different heating rates of a\, a^, a^ and so on, because the corresponding g(X) values are the same at the same X level. The activation energy can be obtained from the slope of the linear line. The plot utilising Eq. (4-9) may be called Ozawa's method [136] and that utilising Eq. (4-10) may be called Akahira's method [137], respectively. These methods allow us to estimate E values without knowing the functional form of/A). What must be stressed here is that these methods can be applicable only when parallel linear lines are obtained for different X values, because Eqs. (4-9) and (4-10) are derived for a single reaction, in other words, by
190
Chapter 4
assuming a constant E value. Nonparallel plots for different X values indicate that the assumption is not valid. In this case neither Eq. (4-9) nor Eq. (4-10) can be applied. When/A) is given SLS J{X) - {\'Xf , Eq. (4-6) can be reduced to the following equations with Eq. (4-8) for/?(t/): -ln(i-;^) = M Z _ ^ - ^ ' / ? ^ , (^=1) aE
(4-11)
l_(,_X)'-"_M'"".^-£/«r \-n aE
(4.,2)
^„^,)
Taking the logarithm of the above equations give a ^ , k^R E1 ln(-^)= l n ^ - l n { - ] n ( l - ; ^ ) } - ^ ^ , ( « = 1 ) T^ E R T
(4-13)
.n)i^^l::^^)=.nM_£l,(„„)
(4-14)
T^{]-n)
aE RT
When utilising Eq. (4-14), n is determined so that the plot between the left hand side and 1/rmay give a linear line. The activation energy, E, is obtained from the slope of the linear relation. This analysis method is called Coat and Redfem's method [138]. 4,5.2.3. Validity of the Single Reaction Model for the Analysis of Coal Pyrolysis As is given above, it is possible to determine both k^ and E from a single X vs. T relationship when/A) = {\-Xf by using Eqs. (4-13) or (4-14). Even if the functional form o^j{X) is not given, it is possible to estimate E from X vs. T relationships obtained at different heating rates by using Eq. (4-4), (4-9) or (4-10). However, these methods must be applied with particular care for the analysis of coal pyrolysis. In the field of coal pyrolysis the degree of the reaction is represented by the amount of volatile matter formed V, The V value is converted to a kind of conversion using the ultimate amount of volatile matter formed V* dis X - V/V*. The values of V and V are conventionally estimated from weight loss curve during pyrolysis. Since the first order reaction model may be the first one that will be tested in many reaction systems, it has also been applied for the analysis of coal pyrolysis by many investigators. Figure 4.30 shows the first order rate constants estimated for various coals, where the first order rate constant k is defined as A; = {dV/dt)/Y* -v). This figure was constructed by Anthony and co-workers by collecting data from the literature [139]. It is seen that the k values are significantly dependent on the coal type and/or the experimental conditions. They are different by 4 orders of magnitude for similar coals. Furthermore,
191
Pyrolysis
1800
iZm
800
iOD 500
400
iO%
10
o DC
ro •2
to
lO'
-4
10
0,4
Figure 4.30 Diagram prepared by Anthony and Howard for comparing simple first-order coal pyrolysis rate constants from different investigators. Reproduced with permission from Ref. 139. Copyright © 1976 AIChE. All rights reserved.
the activation energies range from 8 to 200 kJ mol"^ It is said that the uncertainty and the difficulty of measuring coal temperature during fast pyrolysis caused these significant difference in the rate constants [27]. On the other hand, Miura and Mae have pointed out that special care must be taken when the first order reaction model is applied to the reaction system that contains many parallel reactions [140]. This can be exemplified by analysing some experimental data. Figure 4.31 shows X vs. T and dX/dt vs. T relationships measured at different heating rates by Van Krevelen and co-workers in 1956 [141]. The first-order reaction rate constants were calculated by using the relation k = [dV/dt)/\y* -Vj at different
Chapter 4
192
conversion levels at each heating rate and their Arrhenius plots are shown by the solid lines in Figure 4.32. They are dependent on the heating rates and they even show maxima. The k values estimated at higher heating rates tend to be larger, which apparently means that the rate is dependent on the heating rate. It is clear that neither true activation energies nor reaction rates are estimated from such Arrhenius plots as shown by the solid lines in Figure 4.32. This strange result at a first sight arises from the mal-application of the first order reaction model. It is essential to use X vs, T data obtained at least three different heating rates simultaneously for estimating activation energy and reaction rate. More detailed discussion will be given later. 4.5.3. Parallel First Order Reactions of Finite Number This model assumes that each pyrolysis product is formed by a unique first order reaction. Then the formation rate of i-th component dV/dt is given by (4.15) d/ where V*, koi, and E, are the values for the i-th component; they are dependent on the coal type and the component. Even for the same component, different sets of values must be given when the component is formed through different reactions. The values of Vi*, itoi, and E^ of main components have been estimated for several coals [142,143]. This model requires to determine many parameters beforehand to calculate yields and
400
450
500
550 600 Temptr^tun (•C)
400
450
500
550
600
Figure 4.31 Weight loss and weight loss rate curves for a coal at different heating rates obtained by Van Krevelen and co-workers [141].
Pyrolysis
193
formation rates of the main components. However, Serio and co-workers have proposed that the values of koi and E, are almost independent of coal types [144]. The values reported by them are listed in Table 4.4. If this is the case, only the values of V* are necessary for using this model. 4.5.4. Distributed Activation Energy Model (DAEM) 4.5.4.L Basic Equations This model is an extension of the parallel reactions of finite number and is generally called the distributed activation energy model (DAEM). Originally developed by Vand [145], this model has been widely utilised to analyse complex reactions including coal pyrolysis [146-152]. When the model is applied to the analysis of coal pyrolysis, the
10*^
10-^ b
(0
c E 10"* b
10-^ 1.30 1.35 irr [K"^]
1.50x10"'
Figure 4.32 Arrhenius plots of first-order rate constants estimated from Fig.4.31. Solid lines: at constant heating rates; broken lines: at same conversion levels. The slopes of the broken lines give activation energies at the conversion levels. Reprinted with permission from Ref 154. Copyright 1995 American Chemical Society.
194
Chapter 4
change in total volatiles, V, against the time, /, is given by
(4-16)
where J{E) is a nomnalized distribution curve of the activation energy defined so as to satisfy
Table 4.4 Kinetic rate coefficients and species composition parameters for FG model. Reprinted with permission from Ref 26. Copyright 1988 American Chemical Society. Pittsburgh North primary No.8 Dakota ftinctional Zap composition bituminous group coal lignit^ source rate eq.^ params gas C 0.821 0.665 H 0.056 0.048 N 0.017 0.011 0.024 0.011 S(org) 0.082 0.265 0 1.000
1.000
carboxyl ki - • 0.81 E+13 exp(-(22500±1500)/T)
0.000
0.065
carboxyl k^ -= 0.65E+17 exp(-(33850±1500)/T) k3 == 0.11 E-Hl 6 exp(-(38315±2000)/T) hydroxyl A.v = 0.22E+19 exp(-(30000±1500)/T) hydroxyl ks -= 0.17E+14 exp(-(32700±l 500)/T) k,--= 0.14E+19 exp(-(40000±6000)/T) ether 0 kj -= 0.15E+16 exp(-(40500±l 500)/T) h--:: 0.17E+14 exp{-(30000±l 500)/T) k,--= 0.69E+13 exp(-(42500±4750)/T) ^10 = 0.12E+13 exp(-(27300±3000)/T) H(al) k n = 0.84E+15 exp(-(30000±1500)/T) methoxy ki2 = 0.84E+15 exp(-(30000±1500)/T)
007 005 012 012 050 021 009 023 000 207 000
0.030 0.005 0.062 0.033 0.060 0.038 0.007 0.013 0.001 0.102 0.000
kjs = 0.75E+14 exp(-(30000±2000)/T) kj4 = 0.34E+12 exp(-(30000±2000)/T) k„ = 0.1OE+15 exp{-(40500±6000)/T)
020 015 013 000 020 562 024 000
0.017 0.009 0.017 0.000 0.090 0.440 0.011 1.000
total Y," Y,"
Yf Y: Y." Y,"
Yf
r/' Y," Y ^ '10
Y ^^ Y ^^
'11
'12
Y ^ 113
Yif Yi^' Y ^ '16
Y ^ Y ^ ^ IH
Y ^
'19
total
)^
CO2 extra loose CO2 loose CO2 tight H2O loose H2O tight CO ether loose CO ether tight HCN loose HCN tight NH3 CHx, aliphatic methane extra loose methane loose methane tight H aromatic methanol CO extra tight C nonvolatile S organic
methyl methyl H(ar)
ki6
ether 0 kj-/ = 0.20E+14 exp(-(45500±1500)/T) C(ar) kj, = 0
k^ ^ i^^= 0.86E-H5 exp(-(27700± 1500)/T) tar ' The rate equation is of the form k„ = k„ exp(-(E/R±G/R)/T, with ko in s', E/R in K, and a/R in K. G designates the spread in activation energies in a Gaussian distribution. The notation for k„ is defined as follows: 0.81E+13 is equivalent to 0.81 x i o ' ' ' etc.
Pyrolysts
|?(^)ci^ = l
195
(4-17)
and ko is the frequency factor corresponding to the E value. Knowing y(£') and ko, we can calculate the change in V/V for any heating profiles with the aid of Eq. (4-16). When using Eq. (4-16), most of researchers including Vand assumed a constant ko value for all reactions to simplify the analysis. However, it is well known that ko and J{E) are interrelated [147,152153]. The work of Lakshmanan and co-workers [153], for example, clearly showed that a number of sets of ko and J{E) describe the experimental data within the limits of experimental accuracy. This means that ko and J(E) may be just adjustable parameters. To overcome this weakness, Miura has presented a simple method to estimate both/^") and ko(E) from three sets of experimental data obtained at different heating rates without assuming any functional forms for J{E) and ko(E) [154,155]. The concept of the method is explained by referring to Figures 4.29c and 4.29d. 4,5.4.2. Methods to Estimate V/V^ vs. Efrom Experimental Data When a finite number (e.g. five) of first-order irreversible reactions that have different rate parameters occur in parallel, the X {-V/V*) vs. T relationships obtained at different heating rates of «], ai and a^ are schematically represented by the broken lines in Figure 4.29c. At a given heating rate, reactions having larger E values tend to occur at higher temperature and all reactions tend to shift to higher temperatures at higher heating rate as stated in a previous section. By analogy with the analysis method for the single reaction model presented above (Figs.4.29a and 4.29b), the activation energy of each reaction is obtained by performing the Arrhenius plots of d(K/F*)/d/ obtained from the three curves at same conversion levels as shown by the solid linear lines in Figure 4.29d. On the other hand, the Arrhenius plots of d{VIV*)ldt at same heating rates are given by the dotted waved lines in Figure 4.29d. It is impossible to use these Arrhenius plots to estimate E. Furthermore, we can easily imagine that the X vs. T relationships will be schematically represented by the solid lines in Figure 4.29c when infinite number of first-order reactions occur in parallel. Even if the number of reactions is infinite, the Arrhenius plot of d{VIV*)ldt obtained from the three curves at a given conversion level will surely give the activation energy at the conversion. By performing the Arrhenius plots at different conversion levels, we can obtain the change of £" values against X, namely the relationship between VIV* vs. E. The broken lines in 4.32, for example, show the Arrhenius plots at same conversion levels. They are well represented by linear lines and their slopes tend to increase with increasing conversion level. This result supports the validity of the above discussion. The above discussion can be expressed mathematically as follows. The discussion is based on an approximation: only the j-th reaction having rate parameters of ko and E occurs at specified a and T. This approximation enables us to replace the overall pyrolysis rate dVldt by the rate of the j-th reaction dVJdt as
196 ^ , ^
Chapter 4 =,,e---(K;-Kp
(4-18)
where we approximated an actual reaction system consisting of infinite number of reactions by a finite number of reactions to facilitate the discussion. Eq. (4-18) can be rewritten as
The K/Kj values can be regarded as same for different a values at the same V/V value at which only the j-th reaction occurs. Then the plot of ln{d(K/K )/d/} v.v. 1/7 at the same V/V value for different heating rates gives the E value corresponding to the V/V value. This plot, shown by the broken line in Figure 4.32., is the plot proposed for estimating E by the differential method for the single reaction model. By repeating this procedure we can obtain the relationship between V/V* vs. E. Then we will call this method Differential DAEM method here. Since k(, and E in Eq. (4-18) are the values for the j-th reaction, they are constants. Then Eq. (4-18) can be integrated as follows: l - ^ = exp(-Ao fe-^"'^d/) = e x p ( - M I _ e ^ ' « ^ ) VJ) aE
(4-20)
By taking the logarithm, Eq. (4-20) is rewritten as
This equation can also be used to estimate ko and E for the j-th reaction. Namely, the plot of ln(<^/r^) vs. ]/Tat the same V/V* value gives the E value at the V/V value. This is the plot proposed for estimating E by the integral method for the single reaction model. By repeating this procedure we can obtain the relationship between V/V* vs. E. We will call this method Integral DAEM method here. The discussion in this section clarified that V/V vs. E relationship could be estimated from the X{-V/V ) vs. Trelationships obtained at different heating rates of ai, a2, and a^ by resorting either the Differential DAEM method or the Integral DAEM method. The Arrhenius plots performed to estimate £, ln{d(K/K )/d/} v^. 1/rplot for the Differential method and \n{a/T^) vs. \/T plot for the Integral method based on respectively Eqs. (4-19) and (4-21), are apparently the same as those proposed for the analysis of single reaction, Eqs. (4-4) and (4-10), but it must be stressed that the concept of Eqs. (4-19) and (4-21) is significantly different from that of Eqs. (4-4) and (4-10) as the discussion developing Eqs. (4-19) and (4-21) clarified.
197
Pyrolysis 4.5.4.3. Method to Estimate f(E) and kg
Here we will show how f(E) and ko are estimated from the Differential DAEM method and the Integral DAEM method. When Eq. (4-16) is applied to the case when coal is heated at a constant heating rate a{T-TQ+ at).
150
200
250
300
350
Activation energy, E [kJ/mol]
Figure 4.33 Typical changes in (E,T) and ^(E,T)f{E) against E at several temperatures. The hatched area in the middle graph gives the \-V/V* value at 7=750 K. The horizontally hatched area in the lower graph gives the \-V/V* value at T =750 K when ^(E,T) is approximated by a step function at E = E^ Reprinted with permission from Ref 154. Copyright 1995 American Chemical Society.
198
Chapter 4
-v/v* = jo p>(£,7)/(£)d£
(4-22)
is obtained. Here, the function 0(£,7) is set equal to 0 ( £ , r ) = e x p ( - ^ (c-^'^^dT) a i)
= exp(-MZle-^/^^) aE
(4-23)
This function represents the unreacted fraction of the reaction having ko and E as can be deduced from Eq. (4-11) and hence changes between 0 and 1. First, we will show how Eqs. (4-22) and (4-23) relate VIV* and T The upper graph in Figure 4.33 schematically shows how 0(£',7) changes against E at several temperature levels for « = 10 K/s and arbitrarily assigned k(, values. If we see the curve for T- 750 K, the ^{E,T) value changes from 0 to 1 rather steeply when E increases from 230 to 265 kJ/mol. This means that at 750 K the reactions having E values smaller than 230 kJ/mol are completed and that the reactions having E values larger than 265 kJ/mol are not initiated yet. Only reactions having E - 230 to 265 kJ/mol are occurring at 750 K. The middle graph in Figure 4.33 schematically shows an/^") curve by the solid line and shows Q>{E,T)f{E) values by the dotted lines. The area below the dotted line gives \-VIV at a given temperature T as Eq. (4-22) indicates. The hatched area, for example, gives the \-VIV* value at T-150 K. Thus the relationship between \-VIV* and Tis calculated when ko diX\dJ{E) are given. To estimate the J{E) curve from experimental data of V/V* vs. T relationships, the possibility of approximate representation for Eq. (4-22) was examined. Since the (E,T) function changes rather steeply with £" at a given temperature, it seemed to be allowable to assume 0(£',7) by a step function U at an activation energy £ = £§ as a>(£,7)-^£-£,(7)]
(4-24)
This approximation, which corresponds to assume that only the reaction having E^ is occurring at the specified Tand a, is exactly the same as the approximation employed to obtain the V/V* vs. E relationship from experimental data in the previous section. Inserting Eq. (4-24) into Eq. (4-22) gives ]-V/V* = ff(E)dE
(4-25)
The area represented by this equation is shown by the horizontally hatched area in the bottom graph in Figure 4.33. If the hatched area in the middle graph and the horizontally hatched area in the bottom graph are close in their magnitudes, the approximation of Eq. (4-25) is judged to be valid. This condition is found to hold approximately when E^ was chosen so as to satisfy 0(£'s,7) = 0.58 for many combinations of/:o and/£). Then E^ is related to a, 7 and ^o using Eq. (4-23) as given by
Pyrolysis OM5aEs/koRf == Qxp(-EJRT)
199 (4-26)
This relationship is utilised to estimate ko as follows. The K/F* v^-. E relationship can be obtained experimentally as stated above. This means that we can obtain the relationship between E and T for a selected a value. Then by applying Eq. (4-26) at each set ofE, T and a values we can estimate ko for the set. Then we can obtain the ko vs. E relationship for the selected a value by repeating this procedure for different sets of E and T. If the analysis method is sound, the ^o v.v. E relationship is independent of the a value. The distribution curve /(£") can be obtained by using Eq. (4-25). First, Eq. (4-25) is rearranged with the aid of Eq. (4-17) as follows:
v/v *=]-
^f(E)dE = ^f(E)dE
(4-27)
This equation shows that /£") is obtained by simply differentiating the V/V vs. E relationship obtained experimentally by E. Thus, we can estimate both f(E) and ko from the V/V vs. E relationship obtained experimentally by resorting to either the Differential DAEM method or the Integral DAEM method. If we compare Eq. (4-20) and Eq. (4-26) carefully, we find that the approximation of Eq. (4-26) is equivalent to setting 1 - Vj/V* = 0.58. Then Eq. (4-21) is reduced to ln(-^)= i n M + 0 . 6 0 7 5 - - T^ E RT
(4-28)
This equation allows us to estimate both E and ko from only the plot of \n(a/T^) vs. \/T at the same V/V* value. This equation can also be directly derived by simply taking logarithm and rearranging Eq. (4-26). The term 0.6075 in Eq. (4-28) may be set equal to 0 for simplicity, which simplifies Eq. (4-28) to \n(^)= T^
inM-J^l E RT
(4-29)
This corresponds to assume that 1 - V^/V*- Q>(EJ) = e'^ in Eq. (4-20) and is equivalent to replace 0.545 by 1 in Eq. (4-26). This is the approximation introduced by Vand for obtaining j{E) using a completely different procedure with an assumed ko value [145]. Equations (4-28) and (4-29) are utilised as the simple forms of the Integral DAEM method. The Integral DAEM method does not require tedious differentiation procedure to calculate d{V/V )ldt, thereby simplifying and making more accurate the procedure to determine/£) and ko. The procedure to estimate/£") and ko using the integral method is summarised as follows: 1. Measure V/V* vs. T relationships at three different heating rates at least.
200
Chapter 4
2. Calculate the values of a/f at selected V/V* values from the V/V* vs. T relationships obtained at different heating rates. 3. Perform the plots of ln(a/7^) vs. 1/rat the selected V/V* values and determine E and ko values from the Arrhenius plots at different levels of V/V* by utilising the relationship of either Eq. (4-28) or (4-29). From the slope and the intercept of each plot, both E and kp values corresponding to the V/V* value can be obtained. 4. Plot the V/V* value against the activation energy E obtained above, and differentiate the V/V* vs. E relationship to obtain/£). 4.5.4.4. Analysis of Pyrolysis Reactions of Argonne Premium Coals and Victorian Brown Coals by the New DABM Method The V/V* vs. rdata obtained experimentally at three heating rates oi a- 5, 10, 20 K min" for the Argonne premium coals are shown in Figure 4.34. These data have been
I.U
1
1
1
0.8 - a = 5 K/min UT^ 0.6
I
0.4 0.2 n Hi
WY
Jj
W '
1
f7'yC poc
-
Sc PITT^^
-
F/ /
~J^^0^
ST
I
I 600
700
800
900
1001
Temperature [K] Figure 4.34 V/V* vs. T relationships measured at three different heating rates for the Argonne premium coals. ND: Beulah-Zap lignite; WY: Wyodak; IL: Illinois No. 6; UT: Blind Canyon; ST: Levinston-Stockton; PITT: Pittsburgh No. 8; UP: Upper Freeport; POC: Pocahontas.
201
Pyrolysis
analysed by the Differential DAEM method and the Integral DAEM method to estimate J{E) and k^E) for the coals. Figure 4.35 shows the plots of ln{d(PyF*)/d/} vs. 1/7 and \n{a/f) vs. 1/rat selected V/V* values for Pittsburgh #8 coal (PITT) as an example. It is clearly seen that the slope of the plots increases with the increase of V/V value. The E value was obtained at each V/V value from the slope of the plot and the V/V value was plotted against E to obtain the V/V* vs. E relationship. It was graphically differentiated to obtain J(E). The ko value corresponding to each E value was estimated from the intercept of the plot for the Integral DAEM method and by applying Eq. (4-26) for the Differential DAEM method. ThQj(E) curves and the ko vs. E relationships estimated by the both methods are compared in Figure 4.36 and Figure 4.37, respectively. ThQj{E) curves obtained by the integral method were a little steeper than those obtained by the differential method, but the peak E values estimated by both methods are almost the same as shown in Figure 4.36. The shape and peak E value of J(E) were highly dependent on coal type, and the peak E value tended to increase with increasing coal
—I
4
Differential method
o • D
10-^ k
5 K/min 10 K/min 20 K/min
4h
S ^
0.6 0.5 0.3^ Q 10''t
VAr=OA
ep 2|
10"" t
Integral method
(0
10
2"-
1.1
o • D
5 K/min 10 K/min 20 K/min
1.2
0.8
oyo.e„.»n.3 0.2>*^=»-^:
1.3 1/7
1.4
1.5
1.6x10
[K'^l
Figure 4.35 Arrhenius plots for estimating activation energies at different conversion levels for PITT coal. Upper: Differential DAEM method; Lower: Integral DAEM method.
202
Chapter 4
rank. On the other hand, the ko value changed significantly with the increase of E and was approximately represented by the relation Ao' = C6e^^ {a, P'. positive constants)
(4-30)
This relation is well known as the "compensation effect" of ko and E in the field of catalytic reaction. This clearly shows that ko cannot be represented by a single value as was done by many investigators. Furthermore, the ko vs. E relationships are almost independent of coal type, which means that the reactions involved in pyrolysis are the same for different coals. In other words, this implies that the rate parameters of CO2 formation reaction, for example, are the same for all coals. This is judged to be reasonable and hence circumstantially supports the validity of the proposed analysis method. The Integral DAEM method was judged to be more accurate and easy to apply than the Differential DAEM method. Therefore, we recommend the use of the integral method, although both methods are the same in principle. The DAEM method has been applied to the analysis of the pyrolysis and gasification of Victorian brown coal [156]. Figure 4.38 shows the/E) curves for the pyrolysis of
1.5x10''
n
200
1 \ Differential method
250 300 350 Activation Energy [kj/mol]
400
Figure 4.36 The distribution curves f(E) for the Argonne premium coals: estimated by Differential method (upper) and Integral method (lower) Reprinted with permission from Ref. 155. Copyright 1998 American Chemical Society.
203
Pyrolysis
1
""I'--
J
' Integral method 10^*
/a//'
10^
/A y 1
10^
-
10^^
1
y^
~«-...
j^/fy
10^^
ND WY IL UT - o - ST PITT UF POC
TSaf^'' 10^* • y^j^t^
10^^
150
400
200 250 300 350 Activation energy, E [l(J/mol]
1 200 ....
,.J J 300 350 Activation Energy, E [l<J/moI]
inio 150
,
n J 1 A 1 J 1
„ 1
250
400
Figure 4.37 The k^ vs. E relationships for the Argonne premium coals: estimated by Differential method (left) and Integral method (right). Reprinted with permission from Ref 155. Copyright 1998 American Chemical Society.
0012
OOD
0008
UU
0006
0.004
1
T"
, pyrolysis 1 by Int Method 1 —•—dried coal • HT300 V ^ HT350 —•—SC400 [ • SC450 N-+-'SC500 L
•
r
/\
0.002 h
1 0.000 tX3
•T
- '
]
:\ '
+f ..••
.- • ' • ' • • 1
/ • V ••'
-J
1
\
J
t •
• ^ A
A
«
.jk
A
•
\
1
300
•
#
-•-
• • • ' 200
^—1
1—
400
.i 500
-1
J GOO
E(kJ/mol) Figure 4.38 The distribution curvesX^) for raw and treated Loy Yang brown coal [156]. HT300 and HT350 represent the Loy Yang coal treated in hot water at 300 and 350°C respectively; SC400, SC450 and SC500 represent the Loy Yang coal treated in supercritical water at 400, 450 and 500°C respectively.
204
Chapter 4
raw Loy Yang brown coal and pretreated Loy Yang brown coals where the Loy Yang brown coal was pretreated in hot water (hydrothermal treatment at 300 or 350 °C) or supercritical water (400, 450 and 500°C). After pretreatment, J{E) shifted rightward fi-om ca. 200 to 300 kJ mol'; the values of E at which maximum off{E) occurs were nearly the same ft)r pretreated coals in spite of the fact that the volatile matter of these coals apparently decreased with thermal pretreatment. 4.5.5. Models Based on New Understanding of Coal Structure With the recent development of analytical methods, great progress has been made in the structural characterisation of coal (see Chapter 2). Based on such new understanding of coal structure, new coal pyrolysis models have bee proposed. Gavalas and coworkers [157,158] depicted coal as a collection of 14 functional groups, including aromatic rings and aliphatic chains, and proposed a model taking into account more than 40 elementary reactions of the functional groups. In the late 1980s' three simulation models for coal pyrolysis were presented by taking into account the change in coal structure during pyrolysis. They are called the FG-DVC, FLASHCHAIN, and CPD models, and were presented by Solomon and co-workers [26,144], Niksa and coworkers [159,160], and the group of Grant, Fletcher and co-workers [35,161,162], respectively. The models assume that coal is a three dimensionally cross-linked polymer of small monomers involving aromatic nuclei and peripheral groups. The coal structure was concretely depicted by taking into account the accumulated works on coal structure such as '''C NMR studies, coal extraction studies, etc. Reactions breaking bonds connecting monomers, decomposition reactions of peripheral groups, recombination reactions of decomposed fragments, and evaporation of small-molecular-mass compounds were taken into consideration to formulate the rate process of the pyrolysis. Utilising these models, the yields of gases, tar, char and metaplast can be estimated using only the information on the raw coal. Development of these simulation models has been truly epoch making in the field of simulation of coal pyrolysis. A comprehensive review of these models was made by Smith and co-workers [163]. Here we will introduce the concept of the FG-DVC model and FLASHCHAIN model briefly, and show the concept of the CPD model, which we think is a very smart model, a little bit in detail. 4.5.5.1. FG'DVC Model Solomon and co-workers visualised the progress of coal pyrolysis as shown in Figure 4.28. This process was modelled by two models. The first one is the model called the DVC model that represents depolymerisation, vaporisation and cross-linking during the pyrolysis as shown in Figure 4.39. Figures (a), (b), and (c) in this figure, respectively, correspond to figures (a), (b), and (c) in Figure 4.28. The circles represent monomers consisting of aromatic clusters and functional groups. The numbers on the circles represent the molecular mass of the monomers. The monomers are connected by two kinds of bridges: one is a breakable bridge that produces donatable hydrogen when it is
205
Pyrolysis
broken (shown by —) and the other is an unbreakable bridge (shown by =). Coal macromolecule is modelled by assigning the bridges and the sizes of monomers at random using the Monte-Carlo routine as given in Figure 4.39a, which constitutes the distribution of molecule sizes as the histograms on the right of the figure show. Small molecular mass components correspond to the so-called "guest molecules" and large molecular mass components correspond to the "host molecules". On heating the
a. Starting Molecule Guest Molecule Pyridine Insoluble
20-1 Pyridine Soluble
-j
^
i •
f
'
4">n
I
w
Molecular Weight (AMU)
"^^^
b. D u r i n g Tar Formation
Pyridine ]Insoluble
20-1 Tar
50
Pyridine Soluble
Molecular Weight (AMU)
"^^^
c. C h a r F o r m e d 201 Tar
. -
50 Tar
^
Char
PJS.
Pyridine Soluble
1 -^
.
r—r-
Pyridine p Insoluble
,
Molecular Weight (AMU)
^ ^
Char
Figure 4.39 Representation of coal molecule in the DVC simulation and corresponding molecular weight distribution. Reprinted with permission from Ref. 26. Copyright 1988 American Chemical Society.
206
Chapter 4
modelled coal, the breakable bridges are decomposed at different rates to form new molecular size distribution, which are grouped into tar, pyridine soluble fraction (PS) and pyridine insoluble fraction (PI) in the char. The PS fraction in the char corresponds to the molecules that cannot escape from coal due to their large sizes. The decomposition of functional groups is represented using another model called Functional Group (FG) model. Table 4.4 summarises the functional groups taken into account, the first order rate constants of their decomposition, k^, and ultimate yields of the components produced by the decomposition, Yio, for a Pittsburgh coal and a BeulahZap lignite. Although the K/^ values are dependent on coal type, the ^i values are regarded to be independent of coal type. In a later work, Solomon and co-workers correlated the Y^Q values with the coal analysis data [164]. This model is reported to represent pyrolysis behaviour well under various conditions. This was also extended to express the softening and melting of coal during the coking process [165]. 4.5.5.2. FASHCHAIN Model Niksa and co-workers [159,160] depicted that coal consists of units (monomers) containing aromatic nucleus. The monomers are connected either a char link consisting of aromatics or a link called labile bridge as shown in Figure 4.40. The labile bridge is assumed not to contain aromatics and is breakable on pyrolysis. Then j-mer is a molecule which consists of j monomers connected by j-1 char links and labile bridges and two peripheral groups attached on the monomers of both ends. The size of the peripheral group is assumed to be a half of the labile bridge. When a labile bridge in the j-mer is broken, two molecules are formed. The reaction that converts the labile bridge to the char link and a gaseous molecule was also taken into account. Gaseous products are also formed by the decomposition of the peripheral groups. It is assumed that j-mers ranging j = 1 to oo exist in coal, and the j-mers are grouped into three components depending on their sizes as follows: 1 <j<J* J*+l < j < 2J* 2J*+1 < j < 00
: Metaplast(MP) : Intermediate fragments (IF) : Reactant fragments (RS)
The parameter J* is the maximum degree of polymerisation of metaplast. These three components change by following the paths shown in Figure 4.41. Each j-mer gradually loses the labile bridges with the progress of pyrolysis and the pyrolysis is completed when all labile bridges are consumed. Then IF and RS fragments remain as char in the coal particles. The component MP is assumed to exist as condensed phase (liquid) and it has a vapour in equilibrium with the condensed phase. A part of the MP components in the vapour phase are carried away by gaseous components produced by the pyrolysis to the outside of coal particles. The components correspond to the so-called tar. The saturated vapour pressures of the MP components are represented as a function of molecular mass and temperature by an empirical equation and the Raoult's law is assumed to hold as the liquid-gas equilibrium.
Pyrolysis
207
^CH2
CH3
Figure 4.40. Molecular model employed in the FLASHCHAIN. Reprinted in part with permission from Ref. 160. Copyright 1991 American Chemical Society.
208
Chapter 4
FLASHCHAIN
^
Gas from spontBKOus condensation of bridges iniodii char links
Gufnm condensation of bridles inco ch«r links n Tar Vapor
pliase equilibrium
. ^
l<j<J* Gas uOflB spontaneous ^r condensation of Md^s toco char links convected away in stream of gas
2 meiaplast fragmentt leconibine witfi a new char link, and expel gas
imennediate Fragnems J^+l<j<2J*
Figure 4.41. A schematic illustration of the reaction paths in the FLASHCHAIN. Reprinted with permission from Ref. 160. Copyright 1991 American Chemical Society.
This model predicts that the tar formation rate is proportional to the formation rate o f gaseous components and the vapour pressure o f the MP component within coal particles, which means that it is not necessary to take into account o f mass transfer effects to represent the pressure effect on the tar formation rate. This model has been improved to predict the product yields during the pyrolysis from only the information o f H/C and O/C ratios o f coal by utilising regression analysis o f many data [ 1 6 6 ] .
4.5.5.3.
CPD Model
Grant and co-workers [35] presented a coal pyrolysis model called CPD model (Chemical Percolation Model for Devolatilisation) by applying the lattice statistics proposed by Fisher and Essam [167] to coal pyrolysis. The basic concept of this model is rather close to that of the FLASHCHAIN model, but this model is treating sophisticatedly the change in cluster size during pyrolysis and needs less number of parameters to calculate product yields. Then this model will be introduced in more detail below. Consider a square two-dimensional lattice of coordination number 4 (four bridges attached to each lattice site; a + l = 4 ) shown in Figure 4.42. There exists site problem and bond problem in lattice statistics. In the simplest case, consider the problem in which particles occupy at random the sites of the lattice. Each site can accommodate only one particle and is occupied with a constant probability p. A group of particles which are linked by nearest neighbour bonds from one occupied lattice site to an adjacent occupied site are called a cluster. The size of the cluster (the number of sites in the
209
Pyrolysis
cluster) is represented by s. The task to evaluate the distribution of clusters as the function ofp is called the site problem. This problem has been applied in various physical contexts such as the formation of cross-linked polymer gels and the clustering of impurities and defects in crystal. Another version of the problem is to consider the
B
rT¥TriTnn
IMS hUiM
5SZ29U
szuz
'uuSl LZ|
m
rf^yjffl f 1
[SBSltS i l i i i i i b H * ! ' WH
•
• 1 •
•Kir •
•
1
r
m
\m\..
1 J
••:.
1
Figure 4.42. Monte Carlo simulation of the square lattice with coordination number of 4 and for various bridge populations: (a)p = 0.1, (h)p = 0.8, and (c)p- 0.55,finiteclusters only, and (d)p - 0.55, infinite cluster only. Reprinted with permissionfi-omRef 35. Copyright 1989 American Chemical Society.
210
Chapter 4
occupation of the bonds of a lattice. Each bond can be occupied with probability/?, and occupied bonds which meet at a lattice site are considered as linked together in a cluster. The size s cluster is one that involves ^-1 bonds in the cluster. This problem, closely related to the so-called percolation process, is called the bond problem. Figure 4.42 is an example of the bond problem, showing how the cluster size distribution changes with the p value. For a relatively low bond population of/? = 0.1 (a), the cluster sizes are only 1 to 4. Conversely, for/? = 0.8 (b), only three independent sites are found and the remaining sites are connected each other through one or more bonds. The size of this cluster is regarded as infinite. Figure 4.42c shows the clusters of finite sizes for/? = 0.55, and Figure 4.42d shows the cluster of infinite size. The cluster of infinite size is formed when p reached a threshold value of/?c that is called a critical point. The value of/?c is dependent on the coordination number. The abrupt change that occurs atp-pc is related to various physical changes such as phase transition, sol-gel transition, etc. Lattices are grouped into real lattices and pseudo lattices: real lattices such as square lattice, honeycomb lattice, and diamond lattice are ones that have loops and pseudo lattices such as Bethe lattice are ones that have no loops as shown in Figure 4.43. Using Bethe lattice, the cluster size distribution can be expressed analytically as the function of p. By virtue of this merit, Grant and co-workers [35] used Bethe lattice to represent the network structure of coal.
HONEYCOMB LATTICE
DIAMOND LATTICE
TRIGONAL BETHE LATTICE
TETRAGONAL BETHE LATTICE
Figure 4.43. Representative real lattices (honeycomb and diamond) and Bethe pseudolattices (trigonal and tetragonal) for coordination numbers 3 and 4 Reprinted with permission from Ref. 35. Copyright 1989 American Chemical Society.
211
Pyrolysis 4,5.5.4, Coal Pyrolysis and Bethe Lattice
The structure of coal and its change through the pyrolysis were modelled as shown in Figure 4.44. The monomers correspond to the sites and the bridges correspond to bonds of the Bethe lattices. The possible bridge number per monomer is equal to the coordination number of the Bethe lattice, a+1. The ratio of the actual number of bridges to the possible bridge number is represented by/?. The bridges were divided into breakable bridges (labile bridges, Q and non-breakable bridges (char links, C). A portion of the
Under pyrolysis
Coal
O
Labile iinl(
Monomer
p (t)
Functional group
Char link
Figure 4.44. Coal molecular structure assumed in the CPD model and its change with the progress of pyrolysis.
25
c S: Labile bridge c: Char link
kg
+
2gi
2g2
S *: Reactive bridge intermediate
5 : Side chain
g2* gas2
gi: gasl
Figure 4.45. Reaction sequence assumed in the CPD model (based on Refs. 35 and 161).
212
Chapter 4
monomers was attached by functional groups called peripheral groups (S). Calculating the changes in the numbers of labile bridges and char links by a suitable reaction model gives the change in the number of bridges, namely the change in p, from which the change in the distributions of cluster size can be calculated. By employing the Bethe lattice of coordination number C7+ 1 to represent the coal structure, the number of the cluster involving n monomers («-mer), Qnip), is represented by [167] Qnip)^nb,p"-'{\-py
(4-30)
where ris the possible number of bridges per monomer, and nbj^ is the possible number of n-mers to be formed, including a specified bridge. They are respectively given by the following equations: r = (A7-lXc7-l)+cT + l=«(c7-l)+2 nb„ =
a +\ na-¥\
n-\
(4-31) (4-32)
To formulate the rate equations for the pyrolysis, the five reaction routes given in Figure 4.45 were taken into account. By applying the steady state approximation to the intermediate ^ , the differential equations representing the changes in ^, C, S, and gases gi and g2 are given by (4-33)
AC_
( h ^
d/
p+\
dt
p+\
\C-k,5
(4-34)
(4-35)
= k„S
(4-36)
dg2 J 2^A 1^ At p+\
(4-37)
dt
where
p=— k..
Pyrolysis
213
To solve Eqs. (4.33) to (4.37), the numbers of labile bridges ^, char links C, and side chains S at time 0 were set equal to satisfy ^o "•" Q + ^/2 = 1. Then the weight of a monomer mi(0) at /=0 is given by m,(0) = m „ + m , ( l - C o ) { ^ ^ |
(4-38)
where m^ is the weight of aromatic cluster and m^ is the weight of a functional group (side chain). The accumulated amount of non-condensable gases produced m^as, expressed on a per site basis, is given by
{g:^g2)[ <^ + li
(4_39)
The weight of the size n cluster mtar,n? also given on a per site basis, is
P
2
f(i-p) Wip)
(4-40)
By assuming that all the finite size clusters are tar, the accumulated amount of tar produced Wtar, also given on a per site basis, is given by summing up mtar,n over all finite clusters. From these equations the mass fractions of gas, tar, and char at time t are represented as follows: Non-condensable gases: /gas = — 7 ^
(4-41)
Tar:
/.ar = ^
(4-42)
Char:
/char = 1 -/gas -/mr
(4-43)
Table 4.5. Rate parameters and coal structural parameters used for the simulation of the pyrolysis of the Beulah-Zap lignite [35]. Rate parameters: ^ob = 2.6x1 O^-^' s^
£ob = 55.4 kcal/mol
C7b = 1.8 kcal/mol
^og = 3.0 X 10^-' s"^
£og = 69.0 kcal/mol
cjg = 8.1 kcal/mol
Coal structural parameters: cr+1 = 4.5 po = 0.61
Co = 0.30
r^m^m^^
0.82
214
Chapter 4
All the equations required for calculating the yields of gas, tar and char are derived as shown above. The parameters required for the calculation are Rate parameters: k^y, k^, p Coal structure parameters: Co, ^o, ^ "•" U ^ - Wg/^b It is noted that only the ratio of Wg to m^ (- r) is necessary to calculate the mass fractions as Eqs. (4.38) to (4.43) show. The activation energies in ^b. k^, and pare distributed by using the DAEM model with constant RQ values, which means that three parameters, k^ and mean activation energy EQ and its standard deviation a, are required to represent the rate parameters. Even so, the total number of parameters required is still only 13. Table 4.5 lists the parameters employed to simulate a set of pyrolysis data for a North Dakota lignite and Figure 4.46 shows the comparison between the simulation by the CPD model and the experimental data. A fairly good agreement between the simulation and the experimental data suggests the validity of the CPD model. The CPD model was developed based on a sophisticated mathematical treatment and can simulate complex coal pyrolysis reaction fairly well using only 13 parameters. In this sense the CPD model is judged to be a very smart model. Conversely, however, the original CPD model introduced here had several drawbacks. Tar components stay in coal particles, cross-linking reactions are not taken into account and gas species are not identified. Later Genetti and Fletcher [168] have tried to improve the model so as to predict product yields from only the analyses of raw coal. 4.5.6. Summary of the Progress in Kinetic Modelling Recent advance in kinetic modelling of coal pyrolysis was introduced, starting from a simple global model to sophisticated models incorporating the progress on the recent understanding of coal structure. It was clarified that the global model must be applied with care: at least three weight loss curves measured at different heating rates are necessary to estimate the rate parameters rationally. It was shown that the distributions of activation energy and fi-equency factor in the distributed activation energy model (DAEM) could also be obtained by a simple procedure from three weight loss curves measured at different heating rates. These results will be surely useful to estimate the kinetic parameters of coal pyrolysis reaction. Finally, three simulation models that take into account the change in coal structure during pyrolysis, the FG-DVC, FLASHCHAIN and CPD models, were briefly introduced. These models assume that coal is a three dimensionally cross-linked polymer of small monomers involving aromatic nuclei and peripheral groups. The coal structure was concretely depicted by taking into account the accumulated works on coal structure such as '^C NMR studies, coal extraction studies, etc. Reactions breaking bonds connecting monomers, decomposition reactions of peripheral groups, recombination reactions of decomposed fragments and evaporation of small-molecular-mass compounds were taken into consideration to formulate the rate process of the pyrolysis. Utilising these models, the yields of gases, tar, char and metaplast can be estimated using only the information on
215
Pyrolysis
the raw coal. Development of these simulation models has been truly epoch making in the field of simulation of coal pyrolysis.
O D A
Char Tir Gat
0.8
I
0.4-1 ....-ft.
^
0.2
"° ->
0
o-
( I r I 1 j 1 r i t ] I I I " T-] I I I
-r^ 0.8
— —
aridgat Char Sid« Chains Oflil
C
s a
£
Ga«2
0.6 H
'"'""Jii"»»*^;
0.4
I
CQ
0.2 H
0.0-H
•^•1 I I
20
30
40
I I
60
60
70
Time (ms) Figure 4.46. Top: simulations of devolatilisation yields of char, tar, and light gases for BeulahZap lignite. Experimental data from Solomon and co-workers [144]. Bottom: changes in bridges with time on a per site basis. Reprinted with permission from Ref 35. Copyright 1989 American Chemical Society.
216
Chapter 4
4.6. CONCLUDING REMARKS In this chapter were reviewed experimental studies on the pyrolysis of Victorian brown coal and also studies on kinetic modelling of coal pyrolysis which were reported for the last one and half decades. Knowledge of the primary pyrolysis has been accumulated through systematic experimental investigations using particular types of reactors over the wide ranges of the operating variables such as the heating rate, peak temperature, holding time, external gas pressure and also those of physical/chemical properties of the brown coal. The knowledge encourages quantitative elucidation of the secondary reactions of the nascent volatiles and char that take place concurrently with the primary pyrolysis in practical reactors. Efforts have also been made on evaluating depolymerisation (bond-breaking) and cross-linking (bond-forming) reactions during the primary pyrolysis, the extents of which are factors determining the yields and composition of the volatiles. For example, understanding of the mechanism of low temperature cross-linking between hydrogenbonded oxygen-containing functional groups seems to have reached a molecular level. Methods of flash pyrolysis, as reviewed in Section 4.3, are good examples associated with a concept of controlling the cross-linking reactions by breaking or weakening hydrogen bonds prior to the pyrolysis. The presence of AAEM species is a particular feature of the Victorian brown coal. Results from extensive studies on the dynamic behaviours of AAEM species allow us to draw mechanisms of chemical transformation and volatilisation of AAEM species during the primary pyrolysis and subsequent interactions between the volatiles and char. Significant catalysis of AAEM species on reactions with steam of not only nascent char but also volatiles has also been demonstrated. This may lead to a concept of rapid gasification at low temperatures applicable to the brown coal. Advance in modelling the pyrolysis kinetics has been significant. Effectiveness of analysing the kinetics of coal pyrolysis based on DAEM has been demonstrated. The model can represent the kinetics of devolatilisation by the distributions of activation energy {E) and frequency factor {UQ) that are derived purely from experiments. The relationship between E and ^o is nearly independent of coal nature, while the distribution depends on it. FG-DVC, FLASHCHAIN and CPD models simulate bondbreaking and bond-forming reactions in the matrix of coal as a three dimensionally cross-linked polymer and vaporisation of low-mass components as the volatiles, and thereby describe the kinetics of formation of light gases, tar, metaplast and char. These models predict the kinetics of pyrolysis based on chemical properties of coal (before pyrolysis) given by spectroscopic and other analyses. Thus, it is beyond doubt that there has been considerable advance in understanding the pyrolysis of brown coal over the last one and half decades. However, it should also be said that many problems remain to be solved for quantitative and comprehensive understanding of mechanisms of the pyrolysis and its application to the development of advanced technologies for converting the brown coal.
Pyrolysis
217
There have been arguments about factors causing significant effects of the heating rate on the yields of tar and lighter gases, which can be distinguished from those for the pyrolysis of bituminous coals. Only simultaneous progress in experimental and modelling studies on the low-temperature cross-linking and intraparticle decomposition of tar precursors will lead to clear explanation of the factors. Another example of the problems is a gap between experimental facts and model predictions. Increasing external gas pressure does not necessarily cause a decrease in the tar yield from the primary pyrolysis of brown coal, but even brings about increase in a certain range of the pressure. Such an influence is unusual for the pyrolysis of coals of higher ranks and also not predicted by any of the pyrolysis models so far proposed. As was stated in Section 4.2, AAEM species are responsible for such complex pressure effects. Further experimental investigation is necessary aiming at the quantitative evaluation of the behaviours of AAEM species and their roles in chemical and physical processes in the matrix of pyrolysing coal, and achieved knowledge, together with that of intraparticle mass transport, is to be implemented in the kinetic models.
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Chapter 4
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Advances in the Science of Victorian Brown Coal Edited by Chun-Zhu Li © 2004 Elsevier Ltd. All rights reserved.
Chapter 5 Gasification and Combustion of Brown Coal Akita Tomita and Yasuo Ohtsuka Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan
5.1. GASIFICATION OF BROWN COAL 5.1.1. Introduction This section addresses several subjects relevant to the gasification of Victorian brown coal. This section focuses on the data published since this topic was reviewed by Mulcahy and co-workers in 1991 [1]. The gasification characteristics of Victorian brown coal will also be compared briefly with those of other low rank coals and bituminous coals. The terms brown coal and lignite are essentially synonymous. Australian and most European systems use brown coal, while lignite is the US ASTM rank terminology. In Chapter 5, these two terms are used interchangeably. Coal gasification refers to the reactions of coal with air, oxygen, steam, carbon dioxide, hydrogen or a mixture of these gases to yield a gaseous product, which can be used either as a source of energy or as a raw material for the synthesis of chemicals, liquid fuels or other gaseous fuels. In other words, the gasification converts solid coal into gaseous products. Since coal combustion is defined as the reactions of coal with air or oxygen, it can be regarded as a special case of coal gasification. There are great similarities between coal gasification and combustion in terms of chemical and physical processes, for example, many reactions take place during both gasification and combustion. However, the combustion essentially aims at utilising the reaction heat as a source of thermal energy e.g. for the direct generation of electric power. Power generation technologies based on direct coal combustion or via gasification will be further discussed in Chapter 7. Since coal gasification or combustion proceeds in the reducing or oxidizing atmosphere, respectively, the processes of pollutant formation are different between the two. The sulphur or the nitrogen in coal is converted mostly to H2S or NH3 during gasification, respectively, whereas these heteroatoms in coal are emitted mainly as SOx and NOx during combustion. Further details are provided in Chapter 6. When as-received Victorian brown coal is heated up under gasification conditions, it firstly undergoes dewatering and pyrolysis. Unlike bituminous coals and many other low rank coals, Victorian brown coals have large contents of moisture (60 - 65 %). The dewatering process is therefore particularly important from a practical point of view
224
Chapter 5
Table 5.1 Basic chemical reactions in char gasification. AH298 K
Type of reaction
Reaction
Combustion
C + O2 = CO2 C+1/202 = CO
(R5-2)
-111
Steam gasification
C + H2O = CO + H2
(R5-3)
+130
Boudouard reaction
C + CO2 = 2C0
(R5-4)
+171
Partial oxidation
No. (R5-1)
(kJ mor^) -394
Hydrogasifi cation
C + 2H2 = CH4
(R5-5)
-75
Water-gas shift reaction
CO + H2O = CO2 + H2
(R5-6)
-40
Methanation
CO + 3H2 = CH4 + H2O
(R5-7)
-206
(Chapter 3). In the pyrolysis step (sometimes referred to as devolatilisation), inorganic and hydrocarbon gases as well as tarry vapours evolve as volatile matters to leave a solid residue as char. Then, the relatively slow reaction of the devolatilised char with a gasifying agent takes place predominantly. The gasification of the char can be summarised as simple basic chemical reactions in Table 5.1. Coal combustion is expressed as Reaction (R5-1) and highly exothermic. Most of the oxygen (either pure O2 or air) injected into the gasifier is used for this reaction as well as the combustion of volatiles, which provides the thermal energy required to dry the coal, break chemical bonds and heat up the products to reaction temperature. In the gasification zone, partial oxidation [Reaction (R5-2)], steam gasification (sometimes referred to as water-gas reaction) [Reaction (R5-3)] and Boudouard reaction [Reaction (R5-4)] proceed predominantly to yield mainly a mixture of CO and H2, so-called syngas. Reactions (R5-3) and (R5-4) are endothermic and thus favourable at high temperature. The hydrogasification reaction [Reaction (R5-5)] is very slow even under pressure. Unlike these solid-gas reactions. Reactions (R5-6) and (R5-7) are secondary homogeneous reactions in the gas phase, affecting CO/H2 ratio in the product gas. Full details concerning the mechanisms and kinetics of these reactions may be found elsewhere [2-5]. 5.1.2. Gasification Technologies and Techniques 5. /. 2. L Gasification Processes It is appropriate to first review advanced gasification technologies for understanding experimental techniques for the study of brown coal gasification. Gasification processes have been reviewed in more detail by several workers [6-8, also see Chapter 7] and are thus summarised briefly in this section. There are three main types of gasifiers: moving bed (sometimes referred to as fixed bed) gasifiers, fluidised bed gasifiers and entrained flow gasifiers. These processes are different in respect of the type and rank of coal that
Gasification and Combustion
225
can be used, the particle size distribution, the solid residence time and the gasification temperature and pressure. A moving bed gasifier operates with a countercurrent flow of coal and gasifying agent (usually air/oxygen and steam) at pressures of 3 - 10 MPa, using lump coal of 5 80 mm in size. The residence time is normally in the range of 0.5 - 1 h. The moving bed gasifiers can be further classified into two categories with respect to ash removal: nonslagging (or dry ash) and slagging. The highest temperature of a non-slagging or slagging gasifier normally ranges from 1473 to 1573 K or from 1773 to 2073 K, respectively. In a fluidised bed gasifier (abbreviated as FBG), coal particles of less than 5 - 6 mm in size are used and fluidised together with supporting material such as sand, limestone (CaCOs) or dolomite (MgCOs-CaCOs). The latter two materials act as in situ sulphur removal agents. Highly caking bituminous coals are not recommended, since these particles agglomerate in the bed and thus cause fluidisation problems. Low rank coals are thus suitable for this system. This type of gasifier is operated usually at low temperatures (lower than 1273 K) to maintain the bed below ash fusion temperature and consequently to avoid slag formation. Since the feed coal is heated up rapidly to reaction temperature in this gasifier system, any volatile compounds released are readily cracked so that the product gas contains less tars or hydrocarbons than that from a moving bed gasifier. The solid residence time in the FBG is typically in the range of minutes. In the FBQ unlike the moving bed gasifiers, the perfect mixing of coal particles and gasifying agent takes place, which leads to uniform temperature distribution in the bed. The FBG system may be divided roughly into two categories: with or without recycling fines. In an entrained flow gasifier, pulverised coal (< 75 ^m in size) is entrained with a cocurrent flow of gasifying agent. All types of coals can be used, because the swelling and caking properties of coal do not affect the operation of this process. The solid residence time is as short as few seconds, in contrast to the long time in the moving bed gasifier. The entrained system is operated under high pressures of 3 - 8 MPa and at high temperatures of 1773 K or above to ensure high carbon conversion and enhance slag formation. Almost no tars and heavy hydrocarbons are produced and the product gas is composed mainly of syngas. Entrained flow gasifiers may be further classified into two categories according to the method of coal feed, i.e., water slurry or dry feed. Entrained flow gasifiers may be the most common systems for power generation application, such as the Integrated Gasification Combined Cycle (IGCC) and the Integrated Gasification Fuel Cell (IGFC). Although slag removal is a common issue in the entrained flow gasifier, this technology is relatively well proven compared with moving bed and fluidised bed gasification. Although brown coals show high reactivity and thus provide high gasification rates at moderate temperatures compared with high rank coals, brown coals have high moisture contents and low calorific values, making gasification-based power generation processes (e.g. IGCC) directly using these low value coals uncompetitive. Drying must therefore be an integral part of advanced gasification-based power generation technologies using the brown coals, see Chapters 3 and 7 for more detailed discussion.
226
Chapter 5 COAL OTf!!-.:AND GAS CCOIING
NITROGEN ALTERNATOR
COAL ^ESSIJRISAT 0^^
, CONDENSER
lOCKHOPPER WATER
SUFFER/W£;GHING HOPPER
EXHAUST GASES
ASH- CHAR
o HOT GAS
GAS TURBINE DRYER
GASIFIER
PmP
xv
STEAtVl O TURBINE HEAT RECOVERY
CLEANING
Figure 5.1 Flow sheet of the IDGCC process. Reprinted with permission from Ref. 9. Copyright 2001 Pittsburgh Coal Conference.
Figure 5.1 shows a flow sheet of the Integrated Drying Gasification Combined Cycle (IDGCC) process being developed by HRL in Australia to make IGCC technology suitable for Victorian brown coals with high moisture contents [6,9]. It combines an entrained flow dryer with an air-blown fluidised bed gasifier. The key feature of the IDGCC process is that the feed coal is dried under pressure by direct contact with the hot product gas from the fluidised bed gasifier without the need for expensive heat exchangers. The plant at 10 t h"' was operated under 2.5 MPa at 1173 K that is below fusion temperature of the coal ash. HRL estimates that, by scaling up this technology, a 125 MWe (2230 t d ' ) plant could achieve a net efficiency of 43 % (LH V) [6,9]. 5.7.2.2. Experimental Techniques for the Study of Brown Coal Gasification Most of reactors for the study of brown coal pyrolysis (see Chapter 4) are also used for the experimental study of brown coal gasification. Laboratory scale gasification experiments are usually carried out with fixed bed, fluidised bed, drop tube reactors using small amounts of sample in the mg or g order. These reactors diflbr from each other with respect to the sample amount, the heating rate and the gasification temperature and pressure. The advantages and disadvantages of several gasification techniques will be discussed below. In the fixed bed gasification, a thermobalance is convenient to determine efficiently not only the reactivity of several types of brown coals but also the effectiveness of different kinds of catalysts loaded into each brown coal [10-12]. About 10 - 20 mg of the sample is held onto quartz wool in a quartz or ceramic cell mounted on a thermocouple device and the weight change during gasification is on-line monitored.
227
Gasification and Cornbustion
Ring
(Q)
being dropped
(b) fixed position
Figure 5.2 Thermobalance designed to heat coal particles rapidly [13].
The use of an infrared image furnace makes it possible to heat the sample at a relatively high heating rate of 300 - 1000 K min"^ The presence of small holes in the cell enhances the contact between the sample and the gasifying agent. A fixed bed reactor equipped with a conventional thermobalance was specially designed to heat coal particles rapidly without using the infrared furnace [13], as is shown in Figure 5.2. In this reactor coal particles could be heated at 1600 K min'^ [13]. A thermobalance installed in a pressurised vessel can be used to examine gasification reactivity of brown coals under pressure. Since the hydrogasification [Reaction (R5-5) in Table 5.1] is favourable under pressure and the rate is thus measurable, the pressurised thermobalance may be suitable for this purpose [14]. A wire-mesh reactor was originally designed for studying the hydrogasification of coal under pressure [15] and frequently used for studying the rapid pyrolysis of coal [16]. The development of computer-based temperature control systems has provided well-controlled heating rates in the wide range of 0.1 - 5000 Ks'^ for coal pyrolysis [17]. In the reactor, a small amount (of the order of 5 to 10 mg) of coal particles
228
Chapter 5
(typically around 100 ^m) is sandwiched between two layers of a folded wire mesh, which is heated directly by an electric current. This technique has recently been applied to the study of pyrolysis and CO2 gasification of a Victorian brown coal [18,19]. Since the sample amounts used in the gasification with a thermobalance and wiremesh reactor are in the order of 10 mg, it is essential to make the sample homogeneous and check the reproducibility in repeated runs in order to ensure the data reliability. These reactors are not necessarily suitable for the analysis of product gas due to the use of small amounts of coal sample. A fluidised bed reactor is suitable for gasifying a relatively large amount of coal sample. Since the perfect mixing of coal particles and gasifying agent occurs in the bed, the uniform temperature distribution is achieved, whereas secondary reactions of tar and gas evolved cannot be avoided. Tomita and co-workers [20] used a quartz-made fluidised bed reactor to study the Ni-catalysed gasification of Yalloum coal in steam. A pressurised fluidised bed gasification system was also designed to examine the performance of several catalysts in the direct production of CH4 from Victorian brown coal and steam under high pressure [21,22]. A flow sheet of the pressurised gasification system is shown in Figure 5.3 [22]. The reactor was made of Incoloy, 5 cm in inner diameter, and the bed height was 7.5 or 15 cm. Coal particles were continuously injected to the electrically heated reactor at a rate of usually 100 g h"'. The product gas after water washing, such as CO, CO2 and CH4, was analysed on-line by IR analysers. The gasification tests revealed that significant agglomeration of non-caking brown coal took place under pressure, possibly through bridging by formation of tarry materials.
Water tank
Fasd pump
Figure 5.3 Schematic diagram of a pressurized fluidised bed gasification system. Reprinted from Ref 22 with permissionfromthe Japan Institute of Energy.
Gasification and CornbusUon
229
inner tyb#
ciiar quartz WQQI^ ^^
sintared qwartz filter
I V" i^ ^-J
outer fy foe HC gas & volatiles
Figure 5.4 Drop tube reactor equipped with a fixed bed [24].
However, the use of Ni- or Ca-loaded brown coals caused no feeding problems, since the formation of tarry materials did not take place significantly. The detailed results are shown later. A drop tube reactor achieves high heating rates of coal particles in the order of 10^ 10"^ K s"' and short solid residence time in the order of seconds [23-25]. Hayashi and coworkers used this type of reactor to examine secondary reactions of tar and char evolved during the rapid pyrolysis of Victorian brown coals [23,24]. In this case, the pulverised coal was dropped into a vertical stainless steel reactor at a rate of 50 mg min"^ and the residence time was estimated to be a few seconds. When there is some concern about the influence of the reactor wall, a graphite-made drop tube reactor may be recommended [26]. A fixed bed equipped with a drop tube reactor or a fluidised bed reactor has recently been developed to study the pyrolysis and gasification of Victorian brown coals [24,27]. The details of the former system are shown in Figure 5.4 [24]. This type of reactor may be suitable for determining the gasification reactivity of nascent char [28,29]. The latter system that has some features of both the fluidised and fixed beds has been mainly used in the study of the release of alkali and alkaline earth metallic species during pyrolysis [27, also see Chapter 4]. 5.1.3. Gasification of Volatiles and Char Figure 5.5 shows a simplified scheme of coal gasification. When the coal is injected
230
Chapter 5 Volatiles H2/COx/H20/hydrocarbon gas/heteroatom ga^ Hydrocarbon/tar vapor Inorganic species (alkali metals)
I Brown coal
Char Fixed carbon (C, H, N, S, O) Inorganic matters (MgO, CaO, Fe...) Pyrolysis
Gasification
Figure 5.5 Simplified scheme of brown coal gasification.
into a gasifier, the pyrolysis (i.e. coal devolatilisation) takes place first to evolve volatiles with the residue remaining as char. The major components of volatiles are gas and hydrocarbon/tar vapour as well as trace amounts of inorganic species. Large amounts of oxygen functional groups present in Victorian brown coals [30, also see Chapter 2], such as carboxyl and phenol groups, are decomposed to produce H2O, CO and CO2. In addition, H2 and light hydrocarbon gases (mainly CH4) are formed. Significant amounts of heteroatom-containing gases, such as H2S, HCN and NH3, are also released (Chapter 6). The remaining char comprises fixed carbon (the main element being carbon. Figure 5.5) and inorganic matters. The relative proportions of volatiles and char depend strongly on coal type, reactor type and heating conditions such as heating rate, temperature and pressure. In the gasification step, as is shown in Figure 5.5, reactions of volatiles and char with gasifying agents proceed predominantly and will be highlighted below. 5. /. 3, L Secondary Reactions of Volatiles The extent of secondary reactions of volatiles is affected by the type of reactor, the kind of gas atmosphere, the reaction conditions, the composition of inherent minerals and the addition of catalyst components. Table 5.2 shows the effects of gas atmosphere on product gas composition from the pyrolysis and gasification of Yalloum brown coal in a pressurised fluidised bed reactor without externally added bed materials (Figure 5.3) [22]. When pure N2 was replaced with steam, weight losses at 870 - 980 K remained almost unchanged at 54 - 59 wl % (daf), which were nearly equal to the volatile matter yield of 56 vv^% (daf) determined by the proximate analysis, i.e. the coexistence of steam did not cause significant additional weight loss. Nevertheless, the sum of H2, CO, CO2 and CH4 yields was higher in steam than in pure N2 and the increment was larger at 973 K. The gas
Gasification and Combustion
231
Table 5.2 Effects of gas atmosphere on product gas composition during the pyrolysis/gasification of Yalloum coal at 1.1 MPa in a pressurised fluidised bed reactor (reproduced from Ref 22 with permissionfromthe Japan Institute of Energy). Temperature K 773 879 872 978 973 975 1075
Steam/coal kg/kg 0 0 2.5 0 2.5 5.0 2.5
Weightless % (daf) 47 54 55 n.d.
57 59 73
Total 4.0 10 16 13 38 42 72
H2 CO CO2 mol/kg-coal(daf) 0.4 1.6 1.3 1.9 3.1 3.5 4.7 2.7 6.5 3.1 4.3 3.3 13 3.2 16 15 19 3.0 22 34 8.9
CH4 0.7 1.9 2.5 2.5 5.3 4.6 7.2
composition also changed considerably at a higher temperature: H2 and CO2 increased predominantly. The quantitative analysis of the changes in the gas composition may show the occurrence of the following reactions as well as the water-gas shift reaction [Reaction (R5-6) in Table 5.1]: (CKHnO)„ + 15«H20 -^ 4](n/2)U2 + 8«C02
(R5-8)
(C8HnO)„ ^
(R5-9)
7nC + nCO + 11«/2H2
Reactions (R5-8) and (R5-9) mean the steam reforming and secondary decomposition of tar, respectively. The apparent chemical formula of the tar can be estimated to be (CsHnO)^, where the value ofn is 2 - 3, on the basis of the elemental analysis [20]. The CO formed in Reaction (R5-9) may further be converted to H2 and CO2 through Reaction (R5-6). It is of interest to clarify the roles of catalytic components in the secondary reactions of volatiles. Table 5.3 shows the effects of Ca catalyst on weight loss and product gas composition from pyrolysis and gasification of Yalloum coal [22]. Ca was ionexchanged onto Yalloum coal from a saturated solution of Ca(0H)2. Weight loss of the 4.4 vs^% Ca-loaded coal at 880 K in N2 was low compared with that of the raw coal (Table 5.2). The tar decomposition on the Ca catalyst may have taken place [Reaction (R5-9)], since the carbon deposited would apparently decrease the weight loss. The presence of steam mainly increased H2 and CO2. Since the weight loss at a steam/coal ratio of 2.5 was higher for the Ca-loaded coal than for the raw coal, the gasification of the devolatilising nascent char as well as the steam reforming of tar also proceeded at about 870 K. When the Ca-loaded coal was fluidised at 970 - 975 K in steam, weight loss reached 80 wt% (daf) (Table 5.3), indicating a larger extent of the Ca-catalysed gasification of char. It should be noted that the yields of not only H2 but also CO2 are higher at a larger steam/coal ratio, in spite of the almost same weight loss, whereas the yields of CO and CH4 were unchanged. The increment ratio of AH2/ACO2 observed by
232
Chapter 5
Table 5.3 Eifects of Ca catalyst on weight loss and product gas composition during the pyrolysis and gasification of Yalloum coal at 1.1 MPa (reproduced from Ref 22 with permission from the Japan Institute of Energy). Temperature
steam/coal
Weight loss
Total
K 880 869 871 873 982 970 975
kg/kg 0 1.0 2.5 5.0 0 2.5 5.0
% (daf) 43 51 63 65 51 81 79
10 27 37 48 20 73 103
CO. CO mol/kg-coal(daf) 1.8 3.9 2.8 11 2.0 10 14 1.5 18 1.5 18 25 5.5 4.3 7.2 5.9 23 38 5.6 35 58
H.
CH4
1.6 3.7 3.5 3.6 2.6 5.6 5.3
Table 5.4 Effects of Ni catalyst on weight loss and product gas composition during the pyrolysis and gasification of Yalloum coal at 1.1 MPa (reproduced from Ref 22 with permission from the Japan Institute of Energy). CH4
11 42
H2 CO CO2 mol/kg-coal(daf) 6.4 2.6 1.1 12 1.7 15
18 69 74 82 90 122
11 27 23 35 46 70
1.8 16 14 12 12 9.8
Temperature K 773 788
Steam/coal kg/kg 0 1.2
Weight loss % (daf) 26 57
Total
873 875 868 871 868 853
0 1.2 1.4 2.0 2.5 4.4
36 75 75 76 nd 79
4.0 7.7 5.9 6.6 5.5 4.1
1.4 19 21 28 27 38
0.9 14
increasing steam/coal ratio was estimated to be 1.7, which was close to the stoichiometric ratio (2.6) of H2/CO2 calculated based on Reaction (R5-8). It is likely that Ca remarkably promotes the gasification of the tar evolved to produce H2O and CO2 at high steam/coal ratios. As is summarised in Table 5.4, the Ni catalyst, which was added to Yalloum coal at a loading of 12 wt% and highly dispersed on the char [20], exhibited the effects distinct from those of the Ca catalyst [20,22]. In pure N2, weight losses at 773 - 873 K of the Ni-loaded coal were much lower than those of the raw coal and the Ca-loaded coal. Since soot-like carbon was formed, Ni catalysed the tar decomposition considerably [Reaction (R5-9)]. The carbon deposition on the Ni resulted in an apparent decrease in weight loss. On the other hand, the coexistence of steam more than doubled the weight loss at 788 K and increased CO2 and CH4 yields drastically (Table 5.4). The increment ratio of ACO2/ACH4 was almost unity and the sum of ACO2 and ACH4 corresponded to
233
Gasification and Combustion
the increase in weight loss. It is evident that Ni promotes the tar decomposition to carbon and subsequent steam gasification of the carbon to produce CO2 and CH4 according to Reactions (R5-9) and (R5-10). It may be quite reasonable to believe that, in place of the deposited carbon (C), nascent carbon precursors (CH^c) formed by the tar decomposition must have reacted with steam according to Reaction (R5-11), which may be evidenced by a significant increase in the H2 yield in the presence of steam (Table 5.4). 2C + 2H2O - ^ CH4 + CO2
(R5-10)
2CU, + 2H2O -^ CH4 + CO2 + XH2
(R5-11)
At about 870 K, as is shown in Table 5.4, the weight loss increased dramatically from 36 % in pure N2 to 75 % at a steam/coal ratio of 1.2, which means that the steam gasification of char as well as the secondary reactions of tar proceeded to a large extent. Although the weight loss in steam was almost unchanged for steam/coal ratios in the range of 1.2 - 2.5, the H2 and CO2 yields were larger at higher steam/coal ratios while the corresponding CO and CH4 yields were smaller. It is suggested that the increases in the yields of H2 and CO2 arise mostly from the decreases in the yields of CO and CH4 through both the water-gas shift reaction of CO [Reaction (R5-6)] and the steam reforming of CH4 [Reaction (R5-12)].
873
973 1073 Temperalyre, K
1173
Figure 5.6 Distribution of the hydrogen in Yalloum coal during its pyrolysis in a drop tube reactor [23].
Chapter 5
234
(R5-12)
CH4 + 2H2O -^ CO2 + 4H2
Fine particles of metallic Fe formed from FeOOH precipitated onto Loy Yang coal catalysed the deposition of carbon from the tar and the subsequent hydrogenation to produce CH4 according to Reaction (R5-5) at 700 - 800 K in pressurised H2 [14]. It has recently been reported that the steam evolved during the pyrolysis of Yalloum (YL) coal can also react with light hydrocarbons and tarry materials to produce H2 [23]. The temperature dependency of the fate of the hydrogen in YL coal is illustrated in Figure 5.6, where GHC denotes gaseous hydrocarbons. Excellent hydrogen balances were observed. The amount of H2O evolved was unchanged between 973 and 1073 K. When the temperature was raised to 1173 K, however, the H2O yield decreased drastically and the amounts of H in the GHC and tar decreased, whereas the yield of H2 increased remarkably. Considerable increases in the yields of CO and CO2 were also observed. These results show that the reforming of GHC and tar with the H2O evolved proceeds to produce H2, CO and CO2 even under the conditions of rapid heating in the order of 10"^ K s ' and short residence times of volatile matters [23]. It is reasonable to imagine that such reactions also take place under the fluidised bed conditions.
y^^^^
1073
LY-PY
1123
S^
1173
77^ »V/A
1073
LY-RE
k s % • \ s X v I .' / / 1 \/ / .* / J' / A/ / / A
1123
f •
?^^l^
3
1173
a E
1073
VWT)^
1123
'^ .-V//A
a>
n—'—!—^— LYA - PY
ZA
1173 y ••' ^Y y /
1073
A
LYA - RE
1123 1173 -J
I
10
V f i f •'• T i' i «
20
30
I
40
50
60
70
Yield [moi-C /100 mol-C in coal] I
I CO
E3C02
E l GHC Q T a r
^Soot-R
Figure 5.7 Carbon distribution during the rapid pyrolysis/gasification of Loy Yang coal in a drop tube reactor [24].
235
Gasification and Combustion
The influence of acid washing on carbon distribution during the rapid pyrolysis and gasification/reforming of Loy Yang (LY) coal in a drop tube reactor is illustrated in Figure 5.7, where PY or RE denotes the pyrolysis in pure N2 or gasification in 40 vol % H2O/N2, respectively, and LYA denotes the acid-washed (demineralised) LY coal [24]. The contents of Na, Mg, Ca and Fe were determined to be 0.11, 0.06, 0.08 and 0.03 wt% (dry) in the original LY, respectively. As is seen in Figure 5.7, acid-washing did not affect carbon distribution in pure N2 significantly, but it changed the yields of gaseous carbon-containing species in the presence of steam. Compared with the results for the acid-washed coal, it is evident that the inherent metallic species promote not only the secondary reactions of volatiles, such as steam reforming of the nascent tar and the water-gas shift reaction, but also the gasification of the nascent char with steam to increase the yields of CO2 and H2 predominantly. Figure 5.8 shows the relationship between (CO + 2CO2) yield and H2O yield upon pyrolysis [24]. A negative slope of 45 degrees observed indicated both a one-to-one stoichiometry between H2O consumed and CO/CO2 formed and the larger extent of H2O consumption in the presence of the inherent metallic species, irrespective of the gas atmosphere. The results mentioned here point to the conclusion that ion-exchangeable metal cations naturally present in Victorian brown coals play crucial roles in the secondary reactions of volatiles.
r-, "re o o
.E o
-60
0 h
•5
£
o o
-20 h o E
•u
-40
*>. O CM
X
.50
I
0
,
i
20
,
1
40
1
60
80
100
Yco + 2yco2 [ mol-C /100 mol-C in coal ]
Figure 5.8 Relationship between (CO + CO2) yield and H2O yield during the rapid pyrolysis of Loy Yang coal in a drop tube reactor [24].
236
Chapter 5 Temperature, 'C 500
700
1 50
900
.
'
1 :
I
- t.
(0
•a
-
(0
o «
\
40
\ 1
•- ^ O
-
CM
5: O
14 30
^--4fc__
1V
o '^
^^'^^^
20
y
^--A Q
O 0) - JD
10
-
-« /' 1
n
. . «-..K::-„..1_._
-0.4
-0.2
0
_.!......_
20
i.. -...„..i......
40
, 60
Holding time at 900 'C, s
Figure 5.9 Difference in char yield in He and CO2 during the rapid heating of Victorian brown coal with a wire mesh reactor [18]: • in He; A in CO2; • the difference in CO2 and He.
5.7.3.2. Gasification of Char As was mentioned in Table 5.1, Reactions (R5-2), (R5-3) and (R5-4) are the predominant reactions in char gasification. Since the latter two reactions with H2O and CO2 are endothermic and slow, most of the research about brown coal gasification focuses on these reactions and the results from these recent studies form the topic of the discussion given below. Victorian brown coals include significant amounts of alkali and alkaline earth metal cations, which are initially ion-exchanged with oxygen-functional groups or present as salts (NaCl) dissolved in coal moisture [30, also see Chapter 2] and then remain in char in highly dispersed forms upon pyrolysis, though some of alkali metallic species are volatilised (Figure 5.5, also see Chapter 4). Since finely dispersed Na, K and Ca species remarkably catalyse the gasification with H2O and CO2, their catalytic effects on the gasification reactivity of char should be taken into account even in the absence of
Gasification and Combustion
237
• rapid in-situ O slow in-situ m slowex-s/fti JL 1.0
2.0
3.0
time, ks Figure 5.10 Steam gasification of Loy Yang coal at 1173 K in a drop tube reactor equipped with a fixed-bed (ReproducedfromRef. [28] with permission).
externally added catalysts. It is well known that Morwell coal has higher reactivity than Yalloum coal because of the larger amounts of Na and Ca ions naturally present in Morwell coal than in Yalloum coal [10]. It has been reported that nascent coal chars prepared by rapid pyrolysis at a heating rate of more than 1000 K s'' are highly reactive in subsequent gasification [31,32]. Hayashi, Li and co-workers have been working extensively on the reactivity of nascent chars formed from rapidly heated Victorian brown coals [18,28,29,33]. The difference in char yield in He and CO2 at 1173 K is shown in Figure 5.9, where about 5 mg of the brown coal with the particle size of 100 - 150 ^m was heated up at 1000 K s'^ in a wire mesh reactor [18]. They estimated from the yield difference observed that 20 wt % of the char per second could be gasified with CO2 and suggested that such a rapid reaction rate was affected by the concentration of the radicals formed during the thermal cracking of the nascent char. Acid-washing (demineralisation) had almost no effect on the initial rate of CO2 gasification, while ion-exchanged Na increased it [18]. Hayashi and co-workers also examined the effects of pyrolysis and gasification conditions on the reactivity of Loy Yang coal char in steam in a drop tube reactor equipped with a fixed bed (Figure 5.4) [28,29,33]. The results at 1173 K are shown in Figure 5.10 [28], where about 10 mg of the coal was firstly pyrolysed at a rapid or slow heating rate and then subjected to the gasification in the "in situ" or "ex situ" mode, which denotes the nascent char or the char cooled after pyrolysis, respectively. They demonstrated that the steam gasification of char proceeds in two separate manners, that is, rapidly or slowly at the initial or latter stage of reaction, respectively, irrespective of
238
Chapter 5
Table 5.5 Coals gasified in the IDGCC process. Coal
Country
Loy Yang Zhaotong Mibrag Branko Barat Mae Moh Elbistan Texas lignite
Australia China Germany Indonesia Thailand Turkey USA
Moisture % (as received) 62 62 51 26 30 52 41
Ash % (dry) 0.9 16 9.0 8.3 36 25 21
LHV MJ/kg 8.4 6.6 12 19 11 4.6 11
Sulphur % (dry) 0.3 0.3 3.3 0.3 3.0 3.0 3.4
the thermal history of the char. Char thus consists of two components with different reactivity. It is also suggested that the rate of the rapid reaction depends on the amount of catalytically active species present in the char [33]. When the steam gasification of Yalloum coal was carried out continuously under pressure in a fluidised bed reactor (Figure 5.3), conversion of carbon in the coal at 1073 K was 75 % [22] and the product gas mainly comprised H2 and CO2 (Table 5.2). In the IDGCC pilot plant (Figure 5.1), a wide rang of low rank coals from different countries shown in Table 5.5 have been gasified [6,34]. Victorian brown coals are characterised by very high moisture contents, extremely low ash yields and low sulphur contents, compared with other low rank coals [35]. As this plant is based on a fluidised bed gasification system, the coals with low reactivity and with low ash melting points are not recommended, as for most fluidised gasification technologies. To process the coals with high levels of sulphur in the IDGCC process, sorbents for in-bed sulphur removal, such as limestone or dolomite, can be used as supporting materials [6,34], as mentioned in Section 5.1.2. 5.1.4. Changes in the Structures of Organic/Carbon and Inorganic Matters during Gasification 5.L4.L Changes in the Properties and Structures of Organic Matters Unlike caking coals used for metallurgical coke production, Victorian brown coals have essentially no caking properties and do not usually become plastic upon heating. Thus, the transformation of amorphous carbon in the devolatilised char into more ordered/crystallised carbon is not so easy. On the other hand, the crystallisation of caking coals proceeds readily through plastic-phase reactions to form the welldeveloped crystalline structures. As expected, no formation of plastic particles was observed during the pyrolysis of Yalloum coal at a slow heating rate [36]. When this coal was rapidly heated up in a fluidised bed reactor, however, plastic particles were formed. This behaviour was
Gastfication and Combustion
239
Figure 5.11 Photomicrographs of Yalloum coal chars partially gasified in a pressurised fluidised bed reactor [37].
'.^^... Figure 5.12 TEM pictures of Loy Yang brown coal chars after rapid pyrolysis and subsequent steam gasification (ReprintedfromRef [38]).
strongly dependent on the type of maceral: phlobaphinite, attrinite and sclerotinite were not plasticized, whereas relatively large particles of eu-ulminite became plastic [36]. The proportion of plastic particles increased at an elevated pressure while it decreased by Ca addition. The formation of plastic particles also took place during the steam gasification of Yalloum coal in a pressurised fluidised bed reactor (Figure 5.3). Figure 5.11 shows two pictures observed using a light microscope of char particles partially gasified at 1075 K to a coal conversion level of 75 %(daf) [37]. Most of the particles consisted of plastic and non-plastic parts. The presence of Ca inhibited the development of plasticity, as was
240
Chapter 5
observed in the fluidised bed pyrolysis [36]. Since fine particles of CaO derived fi*om the exchanged Ca promoted the steam reforming [Reaction (R5-8)] of tar evolved during gasification [22], the plastic behaviour observed may be related to the formation of tarry materials. As was seen in Figure 5.10, the gasification rate of Loy Yang coal char lowered considerably at the latter stage of reaction. This phenomenon can usually be observed in the gasification of not only brown coals but also high rank coals. Such a slow rate may be caused by the enrichment of crystallised and thus less reactive carbon. As is shown in Figure 5.12, the TEM observations of the brown coal char quenched after about 90 % gasification conversion at 1173 K revealed the formation of graphitic carbon beads with the size of 20 - 30 nm [38]. The magnified picture in Figure 5.12 showed that each bead was composed of a shell with 5 - 1 5 layers of graphene structures. When the brown coal was firstly demineralised by acid washing and then gasified under the same conditions, however, the graphitic structures were much less developed. Some inorganic matter in the brown coal may enhance the graphitisation of the char at the latter stage of gasification, which results in the remarkable lowering in the gasification rate. It has been well established that several metals and their oxides can promote crystallisation and graphitisation reactions of amorphous carbons prepared from polymers and organic compounds [39]. In fact, carbon crystallisation took place during pyrolysis around fine particles of Fe and CaO produced fi*om inherent Fe and Ca ions exchanged onto low rank coals [40,41]. The relationship between crystallised carbon
40 RB^
^ 30 h o
V
ZN •
o a o
AD •
/
/
10
L£/ 0
IP 1
0
1
1
1
1
1
1
1
•
0.2 0.4 0.6 0.8 1.0 Inherent calcium content, wt%(db)
Figure 5.13 Relationship between extent of carbon crystallisation and inherent calcium in coal. Reprinted with permissionfromRef. 42. Copyright 2003 American Chemical Society.
Gasification and Combustion
241
formed at 1623 K and Ca content naturally present in coal is shown in Figure 5.13 [42], where T-carbon, determined by the X-ray diffraction analysis, means crystallised carbon with turbostratic structures [39], and LY and RB denote Loy Yang and Rhein Braun (from Germany) coal, respectively. The proportion of T-carbon tended to increase with increasing Ca content: it was much larger with RB char than with LY char. A higher content of Fe ions in RB coal should also be responsible for the development of the crystallised structures. 5.1,4.2. Release and Retention of Inorganic Matters In an advanced gasification-based combined cycle for power generation such as IGCC and IGFC, alkali compounds emitted from a gasifier cause serious problems on their materials and performances. The release of alkali and alkaline earth metals added to Victorian brown coals during pyrolysis has recently been studied extensively by Li and co-workers [19,27,43-45]. Na species was readily volatilised while almost no volatilisation of Mg and Ca species occurred up to 1173 K during pyrolysis [44]. The detailed results are described in Chapter 4. Figure 5.14 shows the reactivity of Na2C03-impregnated Yalloum coal during CO2 gasification at 1173 K and the volatilisation of Na at 1023 - 1173 K [46]. As expected, the Na catalyst greatly enhanced the reaction rate. On the other hand, the extent of Na volatilisation increased almost linearly with gasification conversion. Since the volatilisation was not so significant upon pyrolysis, the gasification atmosphere enhanced the release of Na species. As is seen in Figure 5.14, the addition of kaolinite deactivated the Na catalyst almost completely and thus suppressed the Na volatilisation remarkably, because kaolinite reacted with Na to transform it into inactive and thermally stable species. Mineral matters including silica components may work effectively as the sorbents for in situ removal of alkali species during coal gasification.
^100
1
1
1
r
Na2CX)3 inqjieft. char
g 80 'S3
^ 60
add - washed coal char J
.„v.
40 60 Time [ min 1
100
20 40 60 80 Fixed-carbon conversion \%]
Figure 5.14 Reactivity of Na-loaded Yalloum coal and volatilisation of Na species during CO2 gasification (reprintedfromRef 46 with permissionfromthe authors).
242
Chapter 5 lOOi
^
Figure 5.15 Retention of Na, Mg, and Ca species during the gasification of Loy Yang coal in steam at 1173 K under total pressures of (a) 0.10 MPa and (b)1.2 MPa. Reprinted from Ref 29 with permission from the Australian Institute of Energy (12th ICCS).
It is of interest to examine how alkali and alkaline earth metallic species naturally present in Victorian brown coals behave during gasification. The retention of Na, Mg and Ca in Loy Yang coal char during steam gasification is shown in Figure 5.15 [29], where the nascent char was prepared by rapid heating and then partially gasified in a drop tube reactor equipped with a fixed bed (Figure 5.4). As the reaction proceeded, the retention of Na, Mg and Ca all decreased although the retention of Na was the lowest. In other words, the volatilisation of catalytically active Na, Mg and Ca species took place and consequently lowered the gasification rate of the char [29]. It is noteworthy that Mg and Ca species are volatilised significantly in the gasification process of the brown coal char in spite that the carbon is sufficiently remained, since these species have been believed to exist in thermally stable forms such as MgO, CaCOs and/or CaO under the reaction conditions. The surface of these bulk species might be transformed into the chemically unstable peroxides and carbides [42] in the working state. To make clear the volatilisation mechanism should be the subject of future work. 5.1.5. Catalysis of Char Gasification by Inherently Present Inorganic Constituents As observed by many workers, the gasification reactivity of char is dependent on carbon content (C %) of the parent coal. Miura and co-workers analysed a large amount of gasification data for 95 chars prepared from 68 coals by 4 groups of Japanese researchers and plotted them as a function of C % [47]. The results for gasification in
Gasification and Cornbustion
'
~o-] Ah
'
\
T= 800X,
243
r~
PH20= 0-5 Qtm
'in
X o
2
ho
^ O
oo o AAA
P
/)
Q o.^ o
ot
70
90
80 C
C'/. 3
(d.Q.f.)
Figure 5.16 Reaction rate against carbon content of coal during the gasification of different coals in steam [47].
3
RB
IH
•
4
VL
•
-*.
SB
£ QC
NK
3
•
MW TH • WD •
-
X
X3
c
>. >
•
•
2
+ j
u o
4> Q C
1 "• YL
SY
g^l^KN • M P OHG NV
•^•^ L 0.5 Ca + Na (mmol g'^char)
Figure 5.17 Correlation between (Ca + Na) content leachedfromcoals and reactivity in steam at 1023 K [10].
244
Chapter 5
steam are shown in Figure 5.16. The reaction rate (R) is defined as 0.5/TO.5 where T0.5 denotes the time required to reach a char conversion of 50 %. The rate is normalised at 1073 K under a partial pressure (P) of H2O of 50 kPa by assuming that R is proportional to p"e"E^T where E = 45 kcal mol' and n - 0.5 [47]. As is seen in Figure 5.16, the relationship between R and C % changed drastically at about 80 % C (daf). Almost the same trend was also observed with CO2 gasification [47]. The large variation in R observed at < 80 % C originated from the catalytic effects of some inorganic matter naturally present in low rank coals, since demineralisation lowered the R remarkably by removing catalytically active species [47], as indicated by many workers. Alkali and alkaline earth metallic cations in low rank coals are usually present in ionexchangeable forms [30] and can be leached with an aqueous solution of ammonium acetate [48]. The relationship between the sum of Ca and Na ions leached and the reactivity index R at 1023 K is shown in Figure 5.17 where each sample is denoted by a code: YL or MW denotes the char prepared from Yalloum or Morwell coal, respectively [10]. Most of the chars used were from low rank coals with C % < 80. It is evident that R is larger at higher (Ca + Na), though some data were scattered. The R for MW was about four times that for YL. When the reactivities in steam and CO2 of Yalloum coal and a South Australian low rank coal were compared, larger reaction rates were observed with the latter coal of higher (Ca + Na) content [49]. The correlation observed in Figure 5.17 is reasonable, since it has been well accepted that Ca and Na ions incorporated into brown coals by the ion-exchange method are finely dispersed in the chars and very active during gasification [50,51]. The presence of other exchanged metallic species may be responsible for some scattering observed in Figure 5.17. The char from RB (Rhein Braun) coal showed the largest R, which may partly be ascribed to a significant amount of Fe ions [40]. Fe in
[Cayc
0.15 (Ca-^3FeyC
Figure 5.18 Relationship between (Ca + Fe) content in low rank coals and the average rate of CO2 gasification at 1073 K under different partial pressures of CO2 [55].
Gasification and Combustion
245
brown coals also exists in the ion-exchangeable forms [52,53] and shows large catalytic effects on the gasification of Loy Yang coal with steam [11,54]. Figure 5.18 shows the relationship between Ca or (Ca + Fe) content and mean gasification rate (denoted as r*) when five brown coals and three bituminous coals are gasified with CO2 at different partial pressures [55]. The rate was much higher with the brown coals and could be correlated with the value of (Ca + 3Fe)/C, strongly suggesting the larger catalytic effects of the inherent Fe. It can be concluded from these observations that some metal cations in low rank coals, in particular, ion-exchanged Ca, Na, K and Fe ions, catalyse char gasification and control the reactivity. 5.1.6. Catalysis of Char Gasification by Externally-Added Inorganic Compounds 5. /. 6,1. Catalysis by Alkali and Alkaline Earth Metal Compounds It has been well accepted that inexpensiveness and abundance are indispensable requirements for raw materials of gasification catalysts from a practical point of view.
0.20
0.00 0.0
0.2
0.4
0.6
0.8
1.0
Char conversion
Figure 5.19 Reactivity in air of the char prepared by the slow pyrolysis of NaCl-loaded Loy Yang coal [56].
246
Chapters 100 YL(K)
u c o
> ou
/^L(K)*LD
C
50
c
•>-''YL(K)*LD(caic.) /
%
j
v^
/ */ 11 */'/
<0
u
1 tV
il °0
1
1
1
2
Time ChD Figure 5.20 CO2 gasification at 973 K of a bituminous coal with ion-exchanged K catalyst on Yalloum coal [58].
Alkali metal chlorides such as NaCl and KCl are readily available as sea water and rock salts and thus attractive as catalyst precursors, but their activities are generally low compared with the corresponding carbonates. When Cl-free Na^ and K^ ions alone were ion-exchanged with the protons in COOH and/or phenol groups in Yalloum coal from aqueous solutions of NaCl and KCl using NH3 or Ca(0H)2 as a pH-adjusting agent, however, the Na^ and K^ cations show almost the same catalyst effectiveness as that of alkali metal carbonates [50]. Although NaCl, when simply impregnated into coal, is usually ineffective in gasification due probably to strong interactions between Na^ and CI" ions, Li and coworkers have recently observed that the CI in NaCl impregnated into Loy Yang coal is more readily released than the corresponding Na during slow pyrolysis [27] and that Na species remaining in the char is catalytically active [56,57]. Figure 5.19 shows the reactivity in air of the char after slow pyrolysis at 1173 K [56]. Na even at low loading of 1 wt% promoted the gasification. When the NaCl-loaded coal was rapidly pyrolysed in a fluidised bed reactor, on the other hand, the reactivity of the resulting char was as low as that of the raw coal, because Na was volatilised at a larger rate than CI. As expected, the ion-exchange method mentioned above is not effective for high rank coals with low contents of oxygen-functional groups. Takarada and co-workers developed a novel method of gasifying high rank coals using the K-exchanged Yalloum coal [58,59]. The results are shown in Figure 5.20, where the exchanged Yalloum coal, a bituminous coal and a physical mixture of both coal samples are denoted as YL (K), LD and YL (K) + LD respectively [58]. When the mixture YL (K) + LD was gasified with steam, fixed carbon conversion, i.e. conversion of char after devolatilisation, was much larger than the value calculated based on the conversion observed with YL (K) or LD
247
Gasification and Cornbustion
alone. The SEM-EPMA observation revealed that the K species on the YL char migrated to the LD char and consequently catalysed the gasification of the LD char. The drawbacks of alkali catalysts are their volatilisation at elevated temperatures and reaction with mineral matter such as quartz and kaolin to lose their catalytic activity. Thus, inexpensive Ca compounds such as Ca(0H)2 and CaCOa may be rather promising
100
f KGy^
SH
\^rff/ ?- 751r
-SB"
Win ^cw,-
»-- '*
/
I 501 Ui
»
> c
o o ^ O
Ml
^,-'SH
*
CW* 1
*
--'"
25
5wt%Ca
^>
-
I
I
30
None ...L-,
60
90
1
120
Reaction time, min
Figure 5.21 Profiles for steam gasification at 973 K of low rank coals without and with Ca catalyst [60].
Crystalline size of CaO, nm Q
30
60 O Inherent Ca
25
• Size of CaO 5
20
15
oo
A
o 1
O 1 2
Q
3
Inherent Ca in char, wt%
Figure 5.22 Relationship between rate enhancement by Ca addition and inherent Ca in char as well as crystalline size of CaO [60].
248
Chapter 5
Raw coal 4000
J_ 2800
1600
400
Wave number, cm'
Figure 5.23 FT-IR spectra of Loy Yang coal without and with CaCO^. Reprinted with permission from Ref. 61. Copyright 1996 American Chemical Society.
catalysts. Figure 5.21 shows steam gasification profiles at 973 K for low rank coals [including Yalloum (YL) coal] without and with 5 % Ca added from an aqueous slurry of Ca(OH)2 (so-called lime water) [12,60]. The Ca addition increased the reaction rate independently of the coal type, but the catalytic effects differed widely among these coals. In order to examine this difference, the relationship between the content of inherent Ca in char after devolatilisation and a rate enhancement index is shown in Figure 5.22 [60], where the index is defined as (/?ca - /?none)//?none, and R means the initial rate. The index decreased with increasing inherent Ca. This observation points out that the externally added Ca can work more efficiently in the gasification of low rank coals with smaller contents of naturally present Ca ions, for example, in Yalloum coal. The X-ray diffraction measurements after devolatilisation showed the transformation of the added Ca to CaO. Figure 5.22 also reveals, as expected, that CaO with a smaller crystalline size provides a larger rate enhancement. Since Ca(0H)2 is manufactured by the calcination of limestone (CaCOa) and subsequent hydration of CaO formed, the utilisation of CaC03 in place of Ca(0H)2 as a raw material can lead to a simplified catalytic gasification process. Although it is well understandable that the Ca^^ ions in a saturated solution of Ca(0H)2 can readily undergo ion exchange reactions with the protons in COOH and phenol groups in brown coals due to its strong basicity, it is not immediately clear that the Ca ions in CaCOs can also be exchanged. Figure 5.23 shows the FT-IR spectra of Loy Yang coal before and after
Gasification and Combustion
249
lOOi
80h % c o
'«
60h
k.
o > c o o
40
Cat S t e a m / c o a l 1 2.5 6
"5
o O
None A Ni Na Ca
20h
700
J_ 800
_L 900
_L 1000
9 A • d
0 ^ O
_L 1100
Temperature, K
Figure 5.24 Temperature dependency of coal conversion during the catalytic gasification of Yalloum coal in steam in a pressurised fluidised bed reactor. Reprinted from Ref 22 with permissionfromthe Japan Institute of Energy.
treatment with an aqueous slurry of CaCOj [61]. The intensity of the IR peak at 1700 cm attributable to carboxylic C=0 stretch band decreased drastically after treatment. Ion exchange reactions between the Ca ions in CaCOs, which is sparingly soluble in water, and LY coal may proceed according to Reaction (R5-13): CaCOs + 2(-COOH) = .(C00)2Ca + CO2 + H2O
(R5-13)
The formation of CO2 was confirmed. The detailed scheme for the ion-exchange reaction has been proposed elsewhere [61]. The exchanged Ca exhibited high catalytic activity comparable to that of the Ca prepared from Ca(OH)2. The effects of Ca catalyst on the steam gasification of Yalloum coal at 1.1 MPa in a pressurised fluidised bed reactor (Figure 5.3) are shown in Figure 5.24, where Ca was added from an aqueous slurry of Ca(0H)2 and the data with Ni and Na catalysts are provided for reference, metal loading being 4.4, 12 and 4.0 wt % for Ca, Ni and Na, respectively [22]. Ni was the most effective catalyst at temperatures lower than 973 K, whereas the catalytic effects of Ca were larger at a higher temperature. The lower activity of Na may be ascribed to the readily volatilisation during gasification, as was observed in Figure 5.14. Figure 5.25 shows the H2/CO ratio in the product gas [22]. At a steam/coal ratio of 2.5, the H2/CO ratio was larger with Ca than without the catalyst or with Ni catalyst. The H2/CO ratio in the presence of Ca increased with increasing steam/coal ratio. The reason for the preferential formation of H2 with the Ca catalyst is that Ca not only promotes the water-gas shift reaction [Reaction (R5-6)] but also catalyses the steam reforming of tarry materials [Reaction (R5-8)], as was described
250
Chapter 5 20
00 CO
O O
15h
lOh r
CM
X
\ca
\Ca ^ \ Na Ni ^5<—None
6h "^^ 800
NI
NonV*-*
1
1
1
900
1000
1100
Temperature, K
Figure 5.25 H2/CO ratio in product gas evolved during the catalytic gasification of Yalloum coal in steam at 1.1 MPa (Keys as in Figure 5.24). Reprinted from Ref. 22 with permission from the Japan Institute of Energy.
earlier in Section 5.1.3. These observations may point to the conclusion that the Ca catalyst is suitable for producing H2-rich gas from Victorian brown coals. 5.7.6.2. Catalysis by Transition Metal Compounds Although inexpensive Fe is the most promising catalyst for brown coal gasification among transition metals, the activity of Fe catalyst depended strongly on the kind of the precursor compound [62]. While Fe(N03)3 and (NH4)3Fe(C204) are quite effective for the gasification of Yalloum coal at a low temperature of 973 K with steam as well as H2, FeCl3 and Fe2(S04)3 were almost ineffective. Since Fe chloride and sulphates are readily available as acid wastes from steel pickling and Ti02 production plants, these compounds are considered as catalyst precursors. When brown coal is impregnated with Fe chloride and sulphates and then heated for gasification, HCl and H2S are inevitably released, causing corrosion problems and/or increasing costs for gas clean up. Cl-free or S-ft-ee Fe cations should thus be incorporated into brown coal in the step of catalyst addition. The performances of Cl-free Fe catalysts prepared from an aqueous solution of FeCl3 during the gasification of Loy Yang coal in steam are shown in Figure 5.26, where Fe(amm), Fe(ure) and Fe(cal) are precipitated from the FeCl3 solution by using ammonia, urea and Ca(0H)2 respectively [11,54,60]. The CI retention of three Fe catalysts was as low as 0.02 - 0.06 wt %, which was almost the same level as that of the raw coal. As can be seen in Figure 5.26, all of the Fe catalysts promoted the steam gasification with the Fe(cal) catalyst exhibiting the highest catalytic performance. Since
251
Gasification and Combustion
some Ca remained in the Fe(cal), there may be synergistic effects between Fe and Ca on the gasification [54]. As is summarised in Table 5.6 [54,60], the results of catalyst characterisation showed that all of these Fe catalysts were initially present as nanoscale particles of FeOOH, which were transformed to Fe304 with average crystalline size of 15 - 23 nm upon devolatilisation preceding the gasification stage. Although Fe304 particles agglomerated due to the loss of char as the reaction proceeded, the slowest rate of the agglomeration with the Fe(cal) can probably account for its highest catalytic
100
§
75!
100|
4.6 % F8(amm) 1.0 % Fe(amm) 0.95%Fe(hyd) None
V/' 5-
75
\ J! /
«
50
/ / /
•'
i
973 W
\
m
I
r 30
60
*
*
/// / 8 25 Wii •'' .' o \jf/y
90
Reaction time, min
.1 30
4.7 % Fe(cal) 1.0%F©(cai) 3.4%F0(ure)
\ 60
1
90
Reaction time, min
Figure 5.26 Profiles for steam gasification of Loy Yang coal with precipitated iron catalysts at different temperatures [60].
Table 5.6 Characterisation of Fe catalysts precipitated into Loy Yang coal from FeCls solution (Reproduced from Ref 60). Catalyst'
Fe 2p 3/2 (eV)
Fe(amm) Fe(ure) Fe(cal)
711.0 711.2 711.3 711.0
b
Mossbauer parameter H IS QS (kOe) (mm/s) (mm/s) 0 0.39 0.97 0 0.39 0.79 360 0.57 0.40
a-FeOOH^ Precipitated Fe using ammonia, urea or calcium hydroxide. IS, isomer shift; QS, quadrupole splitting; / / , hyperfine field. 'After 60 - 75 % gasification. Commercial bulk compound.
Size of Fe30/ (nm) 95 60 55 -
252
Chapter 5
activity [54]. The changes in local structures of Fe species dispersed on Loy Yang coal during pyrolysis and steam gasification were studied extensively by the transmission Mossbauer spectroscopy and extended X-ray absorption fine structure spectroscopy [6365]. It is likely that the Fe-catalysed gasification proceeds through the oxygen-transfer mechanism involving a redox cycle of Fe oxides [60]. The Fe catalyst precipitated on Loy Yang coal was also quite active in the pressurised hydrogasification at low temperatures of 700 - 800 K to provide high yields of CH4 [14]. In the direct production of CH4 fi-om Victorian brown coal and steam, Ni catalysts worked quite efficiently at low temperatures around 800 K under pressure [1,21,22], as described earlier in Section 5.1.3. The performance of Ni catalysts in the hydrogasification of Loy Yang coal and the local fine structures were reported [66,67]. It has very recently been shown that a carbon-supported Ni catalyst promotes the gasification of organic compounds soluble in the wastewater recovered after hydrothermal dewatering of Morwell coal [68]. This observation is interesting with respect of handling large amounts of wastewater in the dewatering process of Victorian brown coals.
5.2. COMBUSTION 5.2.1. Introduction According to the EI A statistics, the world lignite (brown coal) production in 2001 totalled 861 million tons, which is more or less similar to the production rates in the past two decades [69]. Over 90% of the brown coal mined worldwide is combusted in power stations close to the mine [35]. In Europe, even with increasing pressure from Kyoto Protocol, brown coal (lignite) still fuels 14% of Europe's power stations. The continent's largest brown coal user is Germany, which converts 175 million tons of brown coal into 28% of its electricity in 2001 [70]. Some other countries are more dependent on brown coal (lignite) for power: for example Greece (76%), Yugoslavia (67%) and the Czech Republic (51%). In Australia, about 85% of electricity is derived from coal. Especially, Victoria uses its abundant brown coal as the main fuel: its brown coal production was 66 million tons in 2001. USA produced 76 million tons of lignite in 2001 mainly in North Dakota and Texas. The major advantages of firing brown coal (lignite) are that it is relatively abundant, relatively low in cost, highly reactive in combustion and usually low in sulphur content. The disadvantage is that it contains a large amount of moisture. Therefore, more fuel and more capital-intensive facilities are necessary to generate a unit of power with brown coal than with bituminous coal. The moisture content in Victorian brown coals is typically 60 - 70 wt%). Many other brown coals, for example those in North Dakota, have lower moisture contents, in the 30 - 50 wt% range, but this advantage is often offset by higher ash yields. The combination of the high moisture and high reactivity necessitates the use of brown coals at the mine site. In conventional pulverised fuel combustion, a high-moisture brown coal is dried on-line in an integrated mill/drying
GasificatLon and Combustion
253
system. In this technology, a large proportion (about 50%) of the hot exit flue gas is recycled to dry the as-mined coal. To handle the high moisture content of Yalloum coal (66 wt%), a fuel-rich stream (up to 80 wt% of the feed coal) is fed to the main burners to achieve a stable flame and the remaining finer coal is carried in the bulk of the recycled flue gas and moisture vapour and over-fired through the upper burners. Morwell coal, w^hich contains less moisture (60 wt%) and has a high reactivity, does not require such device [35]. Some Texas utilities use entrained flow evaporative dryers to reduce its moisture content, but generally the drier American lignites are combusted without pre-drying. In any case, because of the high inert gas loading, furnace gas temperatures (~1200°C) and flame temperatures are several hundred degrees lower than comparable black coal units. Hence a larger radiant heat-transfer surface is required to cool the furnace exit gases and minimise superheater fouling. This results in a very large and capital-intensive plant compared with black coal units of similar capacity. The relative size for brown coal boiler to bituminous coal boiler is schematically illustrated in Figure 5.27 for the cases of circulating fluidised bed combustor (CFBC) and pulverised coal combustor (PPC). The quantity of fuel is 2 - 4 times bituminous coal boiler and the quantity of flue gas is 2 - 3 times [6,71,72]. For the foreseeable future, the dominant use of brown coal will continue to be for power generation. A trend of higher prices, lower quality and reduced availability of internationally traded black coals may facilitate the increased use of low rank coals. On the other hand, the high capital cost of conventional thermal brown coal power stations relative to black coal, coupled with their higher CO2 emissions per MWh, makes it difficult to build further units of a similar design. It is necessary to develop more efficient, clean and economical utilisation technologies. Efficiency in the future system should be high enough to markedly reduce CO2 emission and fuel consumption. In addition to SOx, NOx, soot and particulate matters, we must pay more attention to
1.3 W2 1.2W1
W1
|.3[|2 D1
D1
T
.1 HI \ 1.1
H1
i (a)
I 1.5 H2
1 (b)
CFBC boiler
(b) PPC boiler
Figure 5.27 Comparative size for CFBC and PPC boilers, (a) Medium-volatile bituminous coal, (b) high-sodium lignite.
254
Chapter 5
hazardous trace elements, volatile organic compounds and even radioactive species released during combustion. More cost reduction will be necessary in future to compensate the environmental damage caused by the use of brown coal. Probably, a sort of carbon tax will be levied to the use of hydrocarbon resources. Future more efficient brown coal power technology should have new pre-drying processes [6,72-76]. New power generation technologies (also see Chapter 7), such as Integrated Drying and Gasification Combined Cycle (IDGCC) technology for power generation from brown coal [76], Mechanical Thermal Dewatering (MTE) process [77], Upgraded Brown Coal (UBC) process [78] and others, are expected to become solutions for such requirements. A recent development in Germany is also a good example towards this direction. A 1000 MW brown coal-fired unit started up at Niederaussem on August 2002 [79]. This plant bums Rhine brown coal with a moisture content of 53 wt%. The plant thermal efficiency is about 38% (HHV), much higher than that of the existing 600 MW Niederaussem units. This modem unit, called "BoA", has a high efficiency and high availability while simultaneously considering economics and environmental protection. This unit is equipped with advanced supercritical steam cycles, more efficient turbines and many other innovations. A further 3 - 5% improvement in thermal efficiency is projected for a new BoA system, where brown coal will be dried with waste heat in a fluidised-bed fine-grain drying plant. In order to design such advanced coal combustion systems, the understanding of the reactivity of brown coal, behaviour of ash during combustion and the emission of environmentally hazardous materials would be important fundamental research areas. Thus, in this Section the focus is put on these topics and especially the difference between brown coal and bituminous coal will be discussed in detail. A comprehensive review book "The Science of Victorian Brown Coal" was published in 1991 [80]. Combustion reactivities of brown coals and kinetic parameters determined before 1990 have already been reviewed by Mulcahy and co-workers in this book [1]. The present Section mainly describes the progress made after the publication of this book. 5.2.2. Reaction Kinetics of Brown Coal Combustion 5.2.2.1. Reactivity Reactivities of brown coals have been determined using various techniques. Patel and co-workers reported the combustion reactivity of low sodium North Dakota lignite using thermobalance [81]. They found that the activation energy in the chemical reaction controlled zone was about 120 kJ mol'^ which is comparable with that for Texas lignites and French lignite of about 117 kJ/mol, while significantly lower than that of Yalloum brown coal. Generally, the reactivity of low-rank coal is controlled by catalysis of mineral matter; the content of such catalytically active species in Yalloum coal is rather low when compared with other brown coals [10]. This is likely the reason for the above difference. Ma and co-workers compared the reactivity of three brown coals and the order of reactivity was Morwell > Gelliondale > Loy Yang. Pale lithotype was found to be more reactive than dark lithotype [82]. Combustion rate of Loy Yang
255
Gasification and Combustion
10^
2000 1600
10r l H
|New Zealand (n=1) •
10
-2 |_
10-
Anthracite — . MiHmen-an ( " ^ l ) 700 & SOO'C • {n=0.5) Millmerran 585&610°C" (n=0 5) Petroleum coke (n=0.5) I I L 6 8
10
12
104/Tp(K-'')
Figure 5.28 Arrhenius plots for coal combustion at an O2 partial pressure of 10.1 kPa. Brown coals are shown in bold face. Temperature indicated with the coal name is the char preparation temperature, pc: reaction rate; n: order of reaction in O2; Tp: particle temperature. Reprinted by permission of Elsevier SciencefromRef 83. Copyright 1989 by The Combustion Institute.
brown coal char was determined over the temperature range of 940 - 1420 K and it was observed that the particles burned with a reduction both in size and in density [83]. The combined effects of pore diffusion and chemical reaction controlled the burning rate. Chemical reactions tend to dominate at the lower temperatures. Figure 5.28 illustrates Arrhenius-type plots for the combustion of Loy Yang and Yalloum coals. The data for other low rank coals, bituminous coals and petrocoke are also included in this figure. The reaction order in oxygen is around 0.5 and the activation energy is 68 kJ mol'^ 5.2.2.2. Reactivity at High Temperatures and High Pressures The reactivity of brown coal under high-temperature, high-heating rate and highpressure conditions has attracted much attention recently, because many practical coal conversion processes are operated under such conditions. However, few efforts have been made to understand the reaction at temperatures of >1700 K, partly because experimentation is not easy in this region. In 1970's, Hamor and co-workers determined the combustion reaction rate of Yalloum coal char in the temperature range between 900 and 2200 K using an entrain-type reactor [84]. At >900 K, the chemical reaction rate was independent of particle size and the activation energy was 65 kJ mor\ which is about half the value found in the lower temperature range. These are characteristic of regime II conditions [2]. The ratio of the observed to the diffusional reaction-rate
256
Chapter 5
Table 5.7 Effect of calcium on the gasification of Yalloum coal with air and CO2 at 1173 and 1773 K [85]. ka^lO's-
Sample
1
kco2/10's-'
1173
1773
1173
1773
Demineralised Yalloum
5.5
11
0.012
4
Raw Yalloum
7.5
15
0.08
5.5
2.5wt% Ca-Yalloum
6.8
19
0.77
5.1
coefficient reached unity at temperatures > 1800 K for the particle with a diameter of 89 \im. Kyotani and co-workers [85] determined the reactivity of five coals including Yalloum coal in a wide temperature range between 773 and 1773 K using a specially designed apparatus that can rapidly heat up coal samples to the reaction temperature. They found that the chemistry still played some role even in the high temperature region. The reactivity for the CO2 gasification depended on the coal rank and Yalloum coal was more reactive than other higher rank coals. As is seen in Table 5.7, some catalytic effect by mineral matter was observed. The reactivity of Yalloum coal was 1.4 times larger than the demineralised Yalloum coal at 1773 K. This ratio is much smaller than the ratio of 6.7 observed at 1173 K, but the presence of catalytic effect is unambiguous. Many coal conversion processes, like pressurised fluidised bed combustion (PFBC), are operated at elevated pressures, because it has a potential to decrease the cost of electricity and to allow for easier control of emissions. However, kinetic data on char oxidation at elevated pressures are very limited in contrast with extensive data at atmospheric pressure. Bateman and co-workers [86] measured mass loss and bumout time of large char particles at pressures of 101 - 760 kPa. In all tests, a linear decrease in the cube root of char mass with time was observed during oxidation until near the end of bumout. Increasing pressure from 101 to 507 kPa reduced oxidation time. The results with Blind Canyon bituminous coal and Beulah-Zap lignite were compared. The bumout time is somewhat shorter for the lignite and more mass loss occurs during the devolatilisation stage for the lignite than for the bituminous coal. However, the difference in the oxidation time between two coals is not significant, particularly for the larger samples. Joutsenoja and co-workers determined the effect of pressure and oxygen concentration on the combustion of different coals in a pressurised entrained-flow reactor [87]. The pressure ranged from 0.2 to 1.0 MPa. The strongest increase in combustion rate at increased pressure occurred for the least reactive anthracite. The mass loss rate and particle temperatures increased with pressure. For lignite, the pressure had almost no effect on the combustion rate. This phenomenon is related to the high reactivity of lignite and suggests that the diffusion process may control the reaction. In fact, the particle temperatures remained constant with pressure. The time required to reach 90% bum-off was also independent of pressure for lignite, whereas it
257
Gastfication and Combustion
decreased with pressure for other coals. Lin and co-workers compared the combustion kinetics of a lignite char and a bituminous coal char in a temperature range of 559 1273 K and a pressure range of 0.1 - 1.6 MPa [88]. Arrhenius plots for the combustion rates of lignite char in the low temperature range indicate that the reaction is chemically controlled and the activation energy is 172 kJmol"^ (see Figure 5.29). The plots at a higher temperature range indicate internal diffusion control regime and external diffusion control regime. Experimental initial rates agreed well with the predicted ones according to the 0.5th-order rate equation for lignite char and the first-order equation for bituminous coal char. Hecker and co-workers [89] attempted to assemble a complete set of intrinsic kinetic data for char oxidation over a wide range of experimental conditions. They applied a simple intrinsic rate expression to determine the oxidation rate for North Dakota lignite char and Pittsburgh No. 8 hvA bituminous coal char at both low and high pressures. Experiments were conducted over a pressure range of 0.1 to 3.2 MPa, oxygen partial pressures from 0.003 to 1.28 MPa, temperature from 598 to 823 K and burnout range of 20 to 60%. Intrinsic char oxidation rate is independent of char burnout level and kinetic parameters are little affected by changes in total pressure. The n-th order kinetic model fits both atmospheric and elevated-pressure char oxidation data very well, with n = 0.7 for both chars studied. Wall and co-workers have reviewed the effects of pressure on coal reactions during pulverised coal combustion and gasification, however, no emphasis was placed on lignite in this paper [90]. For the purpose of obtaining data at high pressure, high temperature and high heating rate, the most suitable equipment is a pressurised drop-tube type reactor. Some data have been collected in several laboratories [91-93]. For example, in the Japanese BRAIN-C project, several groups measured reaction rates in H2O or CO2 at a pressure
Temperature (°C)
0.001
0.8
0.9
1.1
1.2
1.3
1.5
lO-'/TCI/K)
Figure 5.29 Arrhenius plots for initial char-02 reaction rates for Wyoming lignite char in internal and external diffusion control temperature range. Particle size: 0.25 - 0.5 mm; O2 concentration: 21%. Reprinted from Ref 88 with permission from the Society of Chemical Engineers, Japan.
258
Chapter 5
Table 5.8 Selected values of activation energy and reaction order. Researcher Ref
Conditions
Lignite
Activation Order of energy (kJ/mol) reaction
Patel et al.
81
TG; Chem control
North Dakota
120
0.4-1.1
Patel et al.
81
TG; Chem./diff control
North Dakota
18
na
Intrinsic reactivity
Brown coal char
123-171
0.66
Yalloum
140
0.5
Young et al. 83
Hamor et al. 84 Entrain bed; Chem/diff control Lin et al.
88
Fixed bed; Chem control
Wyoming
170
0.5
Lin et al.
88
Fixed bed; Diff control
Wyoming
7
na
Intrinsic rate
North Dakota
na
0.7
Chem./diff control
Texas, French
120
0.7
Hecker et al. 89 Tseng et al.
94
of 1 MPa between 1400 and 1600 K; the reaction rate was formulated with Langmuirtype pressure dependence [93]. Table 5.8 summarises the results of activation energy and reaction order for the combustion of variety of coals under different conditions. It is difficult to draw some definite conclusion on the difference between brown coals and bituminous coals. The scatterings of the data between samples, researchers and equipments are larger than the difference in coal type. 5.2,2,3, Simulation Simulation studies on brown coal combustion are relatively limited. The simulation for bituminous coal combustion can be applied to brown coal by modifying several parameters like moisture content. It is essential to understand the reaction mechanism for carrying out a reliable simulation. Hayhurst made an important contribution through careful experiments on fluidised bed combustion (FBC) [95]. He considered four carbon oxidation reactions during combustion: C-O2 reaction forming CO, C-O2 reaction forming CO2, C-CO2 reaction and the gas-phase oxidation of CO. As test reactions, the combustion of C3HS, CH4 or CO with air in the bed of silica sand was investigated. Visual and high-speed camera observation together with the noise measurements during combustion revealed that combustion occurred either above the bed or in the ascending bubbles, but not in the interstices between the sand particles. Noise is emitted when bubbles are igniting. From these observations, he proposed a model of coal combustion. Oxidation of coal char in a fluidised bed takes place inside the pores of char particle. The resulting CO transfers to a bubble and is oxidised to CO2. Some of CO does bum on top of the bed. His suggestion for more precise modelling is to have correct values of
Gasification and Combustion
259
Nusselt and Sherwood numbers and the intrinsic rate constant for the initial char oxidation. Because of the increasing pressure for installing more environmentally compatible combustion systems, particularly making use of low-quality fuels, new combustion techniques must be developed. This trend has made a number of traditional design tools on the basis of the experience from older technologies obsolete and new approaches are thought necessary. For this purpose, a computational fluid dynamics (CFD)-based combustion modelling is commonly used. Only one case is shown here among many examples. At the Vanaja plant in Finland, the old plant was converted to a bubbling fluidised bed boiler where the primary fuel was changed from hard coal to peat. Coal, wood waste and natural gas were used as the secondary fuels. The change of fuel was analysed with a CFD-based modelling package called ARDEMUS [96]. The structure of ARDEMUS is shown in Figure 5.30. It is based on the use of commercial CFD programs like Phoenics or Fluent in which models for the most essential phenomena prevailing in furnaces are implemented. A turbulent flow field of the gas phase and trajectories and the behaviour of fuel particles were included. The heterogeneous gasparticle surface reactions including NO-chemistry were modelled; special attention has been placed on the turbulence-chemistry interaction and the radiative heat transfer. There were several submodels of different levels of complexity for the same physical phenomenon. In the retrofit project, the change of the combustion technology was analysed with this model. A good generality and qualitative accuracy have been achieved for the prediction of gas velocity, temperature and oxygen concentration distribution. However, a high quantitative accuracy in the prediction of complex chemical phenomena like soot, NOx and slagging information cannot be expected. It was recommended that the final design should be complemented by experimental knowledge. Generally, the prediction of NOx formation is particularly difficult. The development of modelling and prediction methods for NOx formation has been
EMISSIONS
NEAR BURNER ZONE l/AdUanced\ (tocaHy denseiN 1 NOx J \^ grid ^ ^ COMBUSTION TURBULENCE Hc-c modeP
Figure 5.30 The structure of ARDEMUS program.
260
Chapter 5
reviewed by Moreea-Taha [97]. Current approaches include the use of CFD, kinetic model, artificial intelligence methods and others. 5.2,2.4, Ignition Because of the complexity of the process, much debate has been made over the mechanism of ignition of pulverised fiiel. In homogeneous ignition, which is one of two main mechanisms of ignition, the ignition of the volatile matter takes place and oxygen is consumed in the flame and does not react with char during this step. In heterogeneous ignition, on the other hand, the direct attack of oxygen on the coal occurs at low particle temperatures. This ignition will be extinguished when the volatile matter starts to evolve at higher temperatures. These mechanisms are schematically illustrated in Figure 5.31 [98]. The effects of coal rank and particle size were examined for a high-volatile bituminous coal, a low-volatile bituminous coal and a brown coal. For brown coal, only homogeneous ignition and flame of the volatile remote from the particle surface were seen (Figure 5.31a). For bituminous coals, the visual flash was not greater than the particle size, indicating heterogeneous ignition (Figure 5.31b). It was then followed by the homogeneous ignition of the evolved volatile matter. For heat-treated char as well as petroleum coke, only the visual flash on the particle surface occurred and the glowing on the surface increased and the particle became a bright burning particle without any volatile flame (Figure 5.31c). The method to determine the ignition temperature depends on the type of experimental technique. In the captive particle technique and in the free-floating experiment, ignition is detected as a flash or light. The thermogravimetric analysis uses
^ ^
- -
•
^""^
smoking
'^•••=WM ignition
4
i r ' « " ' « ° " Char burning
Char ignition (C) Char ignition
• - • —•
Char burning
o
Figure 5J1 Three ignition mechanisms, (a) Homogenous ignition (brown coal), (b) Heterogeneous-homogenous ignition (high volatile bituminous coal and low volatile bituminous coal), (c) Heterogeneous ignition (petroleum coke and bituminous coal char). Reprinted by permission of Elsevier SciencefromRef 98. Copyright 1992 by The Combustion Institute.
261
Gasification and Combustion
the deviation of mass loss measurement as the indication of ignition. A review by Essenhigh and co-workers [99] described the state of the art in earlier days. Wall and co-workers carefully examined the previous results and found some common trends [100]. The effects of particle size and volatile matter on the ignition temperature are qualitatively the same with all experiments. Increasing volatile matter and oxygen concentration decreases the ignition temperature. However, the effect of particle size appears to depend on the experimental technique. For captive particle technique or pulse type reactor, the ignition temperature decreases with particle size, whereas cloud ignition technique and continuous flow experiments show a trend that the ignition temperature increases with particle size. Gupta and co-workers performed pulse ignition and continuous ignition experiments, using one lignite (Beular), two brown coals (Leigh Creek and Yalloum) and one subbituminous coal (Blair Athol) [101]. The minimum ignition temperature, Tg, determined in a pulse reactor decreases with an increase in volatile mater content, particle diameter and oxygen concentration. Low rank coals ignite at the lowest temperatures. In an entrained flow reactor, several characteristic
"ST
650 600
^
-
s Blair Athoi
550 bUU
X 1
1
O
o
550 500
o. E B •a an
a
-
8 Leigh Creek
•.
450 h-.....^;
X
1
600
e
d
^ Beular
500
X
400
50
100
150
200
Particle size (mm)
Figure 5.32 Characteristic ignition temperatures in continuous experiments with three coals, (x) Temperature for the first detected flash; (0) Temperature for flame streaks; (o) Temperature for rapid changes in O2 and NO concentration; (•) Minimum gas temperature for ignitionfrompulse experiments. Reprinted by permission of Elsevier SciencefromRef 101. Copyright 1990 by The Combustion Institute.
262
Chapters
temperatures were determined: the first flash temperature, Tf, the flame formation temperature, Tfs, and the rapid O2 consumption and NOx evolution temperature, T02. Figure 5.32 compares these characteristic temperatures obtained using two techniques. The minimum gas temperature for ignition, Tg, estimated from the pulse experiments agrees reasonably with the first flash temperatures, Tf, in continuous experiments. However, the temperature for the rapid increase in O2 consumption and NOx generation is much higher than Tf. The onset of flashing occurred some 200 °C below T02. Influence of volatile matter is evident. Although many experimental results have been interpreted on a heterogeneous ignition mechanism, Gupta and co-workers found some evidence thaf contradicts these arguments. They also suggested that Tfs and T02 appeared to be better indicators of the ignitability of pulverised coals in practical combustion equipment than Tg. The latter temperature corresponds to the ignition of only a few particles among 10"* -10^ particles, while the former temperature reflects the ignition behaviour of majority of particles. The same group also examined the effect of other parameters on the combustion of brown coal [102]. At low oxygen concentration, the contribution of volatile matter combustion is significant and the ignition mechanism seems to be homogeneous. With an increase in oxygen concentration, the ignition mechanism appears to shift from a homogeneous mechanism to more heterogeneous one. Phuoc and co-workers used laser-induced technique to investigate the ignition behaviour of Indian lignite and North Dakota lignite, Wyoming subbituminous coal and Pittsburgh bituminous coal [103]. For laser intensity of >800 W cm'^, two ignition mechanisms were observed; the surface ignition followed by the gas-phase ignition when Wyoming subbituminous, Indian lignite and North Dakota lignite were burnt. However, for Pittsburgh bituminous coal only the gas-phase ignition was observed at the same range of the laser intensifies. Chen and co-workers examined the effect of coal type on the ignifion mechanism using TG-DTA technique [104]. The presence of two exothermic peaks is the evidence of the homogeneous mechanism. The combustion mechanism for Loy Yang brown coal, Kaipin bituminous coal and Hongay anthracite was assigned to homogeneous, hetero-homogeneous and heterogeneous, respectively. Only for Kaipin coal, they observed the effect of particle size. With increasing particle size, the type of ignition changes from hetero-homogeneous to homogeneous. 5.2.2.5. Explosion Dust explosion can occur in any process that handles fine combustible particulates. Although few serious accidents have occurred in brown coal industries, it is without saying that the understanding of the explosibility of coal dust is quite important. Woskoboenko attempted to collect comprehensive data on the brown coal using the 1.2 L Hartmann bomb and the 20 L spherical bomb [105]. At its equilibrium moisture content, Morwell coal is explosive between 0.16 and 7.0 kg m'^ and the explosion severity peaks at 0.50 kg m"^. Explosibility increases with decreasing moisture content and particle size as well as with increasing volatile matter content. He also determined the effects of bomb type, coal rank, lithotype of brown coal and others. He found that
265
Gasification and Combustion
the Hartmann bomb seriously underestimates the explosibility of brown coal dust and empirical relations reported in the literature between Hartmann bomb and spherical bomb results are not valid for brown coal. For example, the Hartmann bomb results suggest that the explosibility of Victorian brown coal is slightly higher than that of Pittsburgh coal, whereas the spherical bomb results indicate that the explosibility of the former coal is six times higher than the latter. It was also found that the explosibility of brown coal increased from dark lithotype to run-of-mine coal and to pale lithotype. This trend is the same for the combustion of Victorian brown coal in thermobalance [82]. 5.2.3. Mine/al Matter and Ash Brown coals contain variable amounts of ash-forming species. The management and control of ash deposition during combustion of brown coal is one of the most important considerations in the design and operation of utility boilers. The volatile mineral matter constituents are thought to play an important role in fouling. The principal volatile constituents are Na species in USA and Canadian lignites, whereas it is Ca species in Australian brown coals. 5.2.3,1. Methods to Characterise Mineral Matter, Ash and Slag Much progress on the analytical methods for inorganic species has been made over the years. Among them, computer-controlled scanning electron microscopy (CCSEM) and x-ray absorption fine structure (XAFS) spectroscopy have been recognised as powerful techniques to analyse these species. For example, reaction products of calcium, sodium and potassium during the combustion of lignite were studied extensively using these techniques. The reaction of calcium with clay minerals, SO2 and volatile sodium was clarified. It was found that the composition and particle size distributions of Ca-aluminosilicate ash particles reflect the forms of occurrence of calcium in the coals [106]. Ion-exchanged metals are quite important from various aspects, but it is not easy to
n Total Ca content POC
•
Caicite
^ Other Ca compounds [^ Ion-exchanged Ca
0.5
1
1.5
2
Weight fraction (wt% In coal)
Figure 5.33 Quantification of ion-exchanged calcium using CCSEM technique [107]. Coal: POC, Pocahontas LV bituminous coal; UT, Utah HV bituminous coal; ND, North Dakota lignite.
264
Chapter 5
quantitatively determine the contents. The chemical leaching with ammonium acetate solution has been widely used for this purpose. However, this method was found to be inadequate for the quantitative analysis of organically associated calcium because a substantial amount of CaO is also soluble in this solvent. A new technique for quantifying ion-exclianged metals has been proposed, as is shown in Figure 5.33 [107]. In the CCSEM analysis, only discrete mineral matter is detected but ion-exchanged metals and finely dispersed particles in the organic phase cannot be detected. The amount of these species can be estimated by subtracting the amount of calciumcontaining mineral matter determined using CCSEM method from the total amount of calcium, which can be determined by X-ray fluorescence analysis, inductively coupled plasma spectroscopic analysis, etc. Further progress in the accuracy of CCSEM analysis may be necessary to improve the quantitativeness of this method. In order to assist the development and optimisation of clean coal technologies, online techniques for the determination of Na species concentrations is strongly desired. Some advances in this field can be seen in the report by Chadwick and co-workers [108]. They developed two techniques. The first one is a laser-induced photofragment fluorescence technique, which is extremely sensitive for dilute alkali concentrations (the detection limit is <0.1 ppb of NaCl and 1 ppb of NaOH) and can be used in situ. Limitations of this technique include interference from other chemical species like coal volatiles that are also excited by the UV light. Figure 5.34 shows the Na evolution from Loy Yang coal on heating in N2 detected by this technique. The gas-phase concentration for NaCl and NaOH was measured with 589 nm and 819 nm fluorescence, respectively. The amount released as NaCl was 10 times as large as that of NaOH. The peak temperature for NaCl evolution was more than 200 °C higher than that of NaOH.
15
-
10
-
1 589 nm fluorescence
"E"
" ^ -—^
Q.
3 6 c 0 0
^ Z
5
0 0 Q.
a. 6 0 c
3
!%M
2
0
X 0
CD
z
819 nm fluorescence
h
0 B
*» 0
*» 00
1 .BoA
0'— 500
1
1
600
700
-•"'•^^^^J 800
9c
Temperature (°C)
Figure 5.34 The gas-phase concentration of NaCl and NaOH determined by photofragment technique [108]. Sample: Loy Yang coal; atmosphere: N2; heating rate: 10 K min"^
265
Gasification and Combustion
Because other chemical species are also excited, the technique is most applicable in dilute post-flame conditions. The second method is a free-jet microwave spectroscopy designed for use in high-temperature environment. The detection limit is < 100 ppb and the sensitivity is not good for NaOH. This technique can differentiate many alkali species but cannot be used in situ. Later they used the third harmonic of a NdiYAG laser, 355 nm radiation, instead of the 193 nm excimer laser source [109]. This source has several benefits: for example, interference from other species is much less and the restriction in selecting materials is less; standard optical fibers and window materials can be used. The measurement of the strength of sintered ash is essential to assess the behaviour of coal ashes under FBC conditions. A simple laboratory test method was proposed, which is based on the compression strength measurement of sintered cylindrical pellet [110]. Temperatures at which the sintering was initiated varied between 500 and 900 °C, depending on the type of ash. The relationship between compression strength and heat treatment temperature is shown in Figure 5.35 for three brown coals, one bituminous coal and one anthracite. The strength at the horizontal part on the left end of each curve is almost equal to that for the pellet of untreated ash. With increasing heat-treatment temperature, the strength of each pellet starts to increase. The temperature indicated by an arrow for each curve corresponds to the initial sintering temperature. For the Na-rich brown coal, the sintering temperature was 500 °C, whereas that for the anthracite was 900 °C. The sintering tendency of the various coals in full-scale and pilot-scale CFBC boilers was reasonably predicted from the sintering temperature determined by this method. This method was applied to estimate the agglomeration propensity of four low-rank coal ashes from the northern part of Thailand [111]. The compressive strength of sintered ash pellets was determined over the temperature range of FBC. Main results were as follows:
200
400
600 800 Temperature (°C)
1000
Figure 5.35 Compression strength of ash pellets heat-treated at various temperatures for 4 h in air. (a) Brown coal with high Na content, (b) brown coal with high Si content, (c) brown coal with low ash yield but high Ca content, (d) bituminous coal, (e) anthracite. Reprinted with permission from Ref 110. Copyright 1992 American Chemical Society.
266
Chapter 5
(1) At 950 °C, Mae Moh ash showed the highest sintered strength. This observation is because of the relatively high content of clays and anhydrite, both of which can produce low-melting eutectics. (2) At the higher temperature range (>950 °C), the trend of sintered strength of pure ashes is as follows: Banpu > Chiengmuan > Mae Moh > Lanna. This trend is in accord with the amount of clays because the ash having higher clay content develops higher strength. Chemical reactions among the ash components also affect the sintering behaviour. For Mae Moh ash, CaS04 disappeared at this temperature range and anorthite was formed as a major crystalline phase. The new high-melting crystalline solid phases retarded the sintering rate. (3) The pellet strength of Lanna ash was very low because it contained the lowest amounts of clays compared to the other ashes. Addition of gibbsite decreased sintered strength of Banpu ash and Mae Moh ash. Gibbsite, after being transformed to amorphous alumina, removed molten ash constituents. The role of gibbsite in the strength reduction involves the combined effect of inert dilution and adsorption of molten ash in the pores. 5.2.3.2. Transformation of Mineral Matter and Ash Many ash formation models have been proposed. Figure 5.36 shows one example during coal gasification and combustion [112]. Zygarlicke and co-workers used CCSEM and chemical fractionation techniques to investigate the mechanism of coal ash formation [113]. Texas lignite was combusted in a drop-tube furnace at 1500 °C and the resulted fly ash was carefully analysed. It was found that relatively large quantities of Ca, Mg and Fe in raw coal were organically bound. When the lignite was burnt at 1500 °C, Ca was released from organic binding sites and formed tiny, CaO-rich,
Vaporous ^...;::, inorganics
Adhesion/ accumulation
Vaporization
Agglomeration Collision
Flowing slag ^Raw coaly Before reaction
>K-
•>H
Adhesion
Figure 5.36 Mineral matter transformation to ash during coal gasification and combustion. Reprinted with permissionfi-omRef 112. Copyright 2003 NEDO.
Gasification and Combustion
267
inorganic ash droplets on the receding surface of the burning char particles. The highly reactive CaO-rich phase then combined with molten silica on the surface of quartz grains. Kaolinite was fragmented to high surface area aluminosilicates. CaO combined into the structure of these aluminosilicate components in the amorphous fraction of fly ash. At a later stage, these were converted to larger fly ash particles. Huffman and co-workers also put an emphasis on the behaviour of basic elements during coal combustion [114]. The reactions of Ca, Fe and alkalis in combustion systems have been studied using XAFS, Mossbauer spectroscopy and CCSEM. Principal conclusions were as follows: (1) Ca may either transform to CaO fume that reacts with SO2 to form CaS04 or may react with clays, quartz and other minerals to form slag droplets or fly ash. (2) Pyrite may be partially devolatilised and oxidised to form molten or partially molten iron sulphide-iron oxide mixtures, or may react with other minerals to become part of the slag. (3) Alkalis in lignites (principally Na) volatise and may react with either SO2 to form sulphates or with clay minerals (principally kaolinite) to form aluminosilicate slag droplets. Potassium in illite may melt and then be incorporated into the slag phase. Vuthaluru and co-workers studied the behaviour of inorganic constituents during FBC of Victorian brown coals (Loy Yang and Morwell) and South Australian brown coals (Lochiel and Bowmans) [115]. Coals with high contents of Na and S have produced low-melting point compounds that coat the surface of bed particles. In contrast, for coals with low Na and S contents, the combustion can be operated for longer periods without agglomeration. In a series of studies on the pilot-scale combustion of pulverised lignite and lignitewater slurry fuels. Miller and Schobert examined the relationship of ash formation behaviour with lignite particle size distribution (PSD), mineral matter PSD and the occurrence and composition of iron compounds, silicate, aluminosilicates, alkalis, alkaline earth elements and sulphur [116-120]. In these studies, they attempted to clarify the difference in behaviours between pulverised lignite (PF) and lignite water slurry (CWSF). Fuel particle size alone did not determine the PSDs of the ashes. The PSD of CWSF ash was coarser than that of the PF ash even though both fuels originally had similar PSDs. The CWSF ash showed extensive agglomeration and coalescence, whereas the PF ash exhibited fragmentation. In the case of pyrite, PSD and occurrence were significant in determining the dominant mechanism of ash formation. In the CWSF combustion, coalescence and agglomeration of crushed pyrite dominated the ash formation behaviour. On the other hand, in the PF combustion, pyrite fragmentation is the major process for the formation of sub-micron iron oxide particles. The essence of their findings on Fe-containing minerals is summarised in Figure 5.37. The behaviours of silicates and aluminosilicates were also different between two fuels. The dominant mechanism for determining ash PSD during PF combustion was fragmentation of quartz, however, during CWSF combustion coalescence and agglomeration of inherent silicates and aluminosilicates were dominant. As for alkali and alkaline earth elements, no appreciable difference was observed between two fuels. In both cases, organically
268
Chapter 5 O r i g i n a l coaT^
X
L extraneous pyrite S extraneous pyrite S inherent pyrite
<Ep j; L extraneous pyrite I S inherent pyrite I I s extraneous pyrite — . _ ^ — I ^ Local reducing Oxidizing environment environment
I
I Particle fragmentation; Interact with other Little Interaction with particles; Coalescence other particles via melt phases
A
Fine iron oxide
S extraneous pyrite S inherent pyrite
T
Incorporation of ' ext. pyrite within agglomerate
1
Coarser complex iron aluminosilicates
Figure 5.37 Fate of iron compounds during the combustion of pulverised coal and coal-water slurry. Particle size, L: large, S: small. Reprinted with permission from Ref 118. Copyright 1993 American Chemical Society.
bound Na was important to form small Na2S04 particles and coatings on large silicates and aluminosilicates particles. Sulphur was fixed by alkaline elements during combustion. On the other hand, organically bound calcium was not associated with sulphur, but it participated in the formation of mixed aluminosilicates during char burnout. 5.2.3.3, Ash Troubles during Coal Combustion The inorganic species present in coal transform to various forms of ash during combustion in complicated manners (see Figure 5.36). Fouling is one of the notorious troubles among many. When the adhesion of fly ash to superheater tubes in a boiler takes place continuously, it results in the build-up of sintered ash deposits. The following preventive measures are generally considered to be effective [72]: (1) increase of the furnace height and area allowing ample time for burnout; (2) installation of a number of soot blowers; (3) reserve of space for repositioning; (4) the lowering of the Na content; (5) the use of additives. The characteristics of ash deposits on heat exchanging tubes and the influence of ash deposits on the heat transfer through walls of boiler tubes are important for smooth operation. The character of fireside ash deposits depends on the processes by which
Gasification and Combustion
269
deposits are formed and subsequent reactions within the deposit and with furnace gases. This character influences furnace heat transfer, absorptivity for radiative transfer and thermal conductivity for conductive transfer [121]. Ash deposit leads to corrosion of boiler tubes. Sakashita and co-workers investigated high temperature corrosion of tubes in combustion of pulverised Loy Yang brown coal [122]. The results were compared with those with Warkworth bituminous coal. The former contained much less ash but much more Na than the latter. The corrosion of tube was more extensive with Loy Yang brown coal. The deposit on tube contained more alkali sulphate and chloride, showing high melting behaviour. In the presence of Na2S04, the effect of CI ion on the tube corrosion was remarkable. Another type of ash problem is slagging, which results from the formation of molten ash. Liquid slag causes problems if it becomes plastic and viscous. To reduce slagging, it is desirable to have: (1) a low heat input per furnace cross-sectional area; (2) a low heat release rate at burner zone; (3) ample clearance between burners and furnace walls; (4) a low furnace exit gas temperature. The naturally occurring Na contributes to ash fouling and slagging through combination with the coal's other inorganic constituents. Some of Na may enter the gas phase and be deposited later on heat exchanger where it often aids additional fouling. In the FBC, sintering of bed material and fuel ash often causes problems. In the lower furnace of CFBC, the deposits disturb the air distribution in the bed, disturbing the fluidisation. In other parts of the furnace, the deposits affect the circulation of the gas/bed material in a negative way. In some serious cases, sintering can result in heavy agglomerate formation that completely inhibits the fluidisation. The use of standard ash melting point data for the prediction of ash sintering tendency is usually not successful. Significant sintering usually starts far below the melting temperature of ash [123]. Sintering of ash is influenced by various factors: (1) partial melting, (2) viscous flow and (3) gas-solid chemical reactions. Therefore, sintering phenomenon depends on coal type. Partial melting of ash is the main cause for brown coals, whereas viscous flow is the dominant sintering mechanism for bituminous coal ash and anthracite ash [124]. The thermal conductivity of ash deposit on boiler tube surface is a quite important parameter that determines process efficiency. Rezaei and co-workers [125] used a onedimensional heat transfer method at temperatures up to 900 "^C. The effects of temperature, sintering time and porosity of ash samples were experimentally investigated. It was found that the thermal conductivity generally increased with increasing temperature. Chemical composition has little effect, except affecting the extent of sintering. Prediction of the thermal conductivity was attempted by assuming the presence of spherical pores distributed in a continuous slag phase. A reasonable agreement with experimental data was achieved. Robinson and co-workers developed unique equipment for measuring the time-resolved thermal conductivity of ash deposit in a pilot-scale combustor [126,127]. The measurement conditions closely replicate those found in a commercial boiler. Thermal conductivity is strongly dependent on deposit microstructure and this technique is designed to minimise the disturbance of the
270
Chapter 5
deposit microstructure. The average thermal conductivity of unsintered loose deposits from a mixed fuel of bituminous coal and biomass was 0.14 ± 0.03 W m-^ K"^ This value was midway between highly porous deposit (0.06 W m'^ K'^) and well-sintered solid deposit (3 W m"' K"'). The initial stages of sintering and densification were accompanied by an increase in deposit thermal conductivity. SEM analysis revealed that the deposit consisted of two layers and that a relatively unsintered innermost layer seemed to determine the overall thermal conductivity of the deposit. 5,2.3,4. Measures to Avoid Ash Troubles Several attempts have been made to prevent fouling in pulverised fuel boilers. The use of additives like kaolin or aluminium-containing solution is a common technique. These additives can capture sodium, which is a key element in fouling. Addition of kaolin (10-20 ^m, 2-3 wt% of feed) to Victorian brown coal effectively reduces fouHng [128]. Similarly, the treatment of Loy Yang brown coal with aluminium lactate reduces the amount of Na-rich fine ash compared with the untreated coal. Both methods are effective, but the dry method would be less expensive. The effectiveness of various control methodologies in alleviating ash-related problems during FBC of South Australian low-rank coals was studied in a laboratory scale spouted-bed combustor [129]. The technologies studied are (1) the use of bauxite and calcined sillimanite as alternative bed materials and (2) Al-pretreatment, water washing and acid washing. Experiments using bauxite and calcined sillimanite showed no agglomeration for longer periods than with sand runs. Wet treatment method was also effective. Al enrichment in ash coating on bed particles has been identified as the key mechanism for prevention of agglomeration. Water washing is effective because of the reduction of Na levels in coal. Kry and Chadwick investigated the capacity of twelve minerals for the chemisorption of NaCl, NaOH and Na2S04. Their conclusion was that, on a cost basis, kaolin-type minerals and overburdens appeared to be the best candidates for use to prevent fouling in the combustion of Victorian brown coal [130]. The possibility of blending coals to alleviate particle agglomeration and bed defluidisation was exploited in the fluidised bed combustion of several low-rank coals [131]. Coal blends pf lignites with a sub-bituminous coal at ratios of 50:50 and 90:10 were combusted at 800 °C. Experiments showed significant improvements in FBC operatability with the coal blends compared to the raw lignites. Chemical analyses indicated that the formation of low temperature eutectics was suppressed by calcium aluminosilicate phases derived fi'om the sub-bituminous coal, rendering the surface of ash-coated particles dry and less sticky. Sakashita and co-workers also found a similar effect [122]. The 1:1 mixture of brown coal and bituminous coal resulted in less corrosion compared with brown coal only. This result is because of the reaction of alkali sulphate and chloride in the brown coal with the aluminosilicate type ash in the bituminous coal.
Gasification and Combustion
271
5.2.3.5. Ash Utilisation Past research on pulverised fuel combustion has mainly focused on the combustion of coal, release of toxic substances, characterisation of ash, etc. Recently, however, in order to minimise the amount of ash disposal, the utilisation of residual ash as resources has attracted attention in many fields. A series of biennial symposia covering all aspects of coal combustion by-product utilisation has been organised by Center for Applied Energy Research, University of Kentucky [132]. The most important application of ash after coal combustion is for cement production [133]. The characterisation of ash is very important from this viewpoint. The properties of the carbon remained in ash, including reactivity, surface area and adsorptivity toward surfactant, have been investigated [134]. The adsorptivity is the most important property for ash quality in concrete application. If the remaining carbon adsorbs surfactant that is used to impart freeze/thaw resistivity in the concrete, the ash quality deteriorates to a large extent. It was found that the char adsorptivity was well correlated with the surface area of char in pore diameter of >2 nm. It was also found that the oxidation of the char surface resulted in the surfactant adsorptivity. The effect of coal type was examined and it was found that the char from subbituminous coal and lignite had rather high surfactant adsorptivity. Value-added applications of fly ash have also been attempted. Iyer and Scott [135] reviewed utilisation of ash in such areas as novel materials, waste management, metal recovery and agriculture. Kikuchi [136] reported the use of coal ash for producing zeolite, potassium silicate fertiliser and flue gas desulphurisation agent. Mouhtaris and co-workers tried to use calcium-rich fly ash from Greek lignite in the production of gobinsite-NaPl zeolite [137]. Chareonpanich used fly ash from Mae Moh lignite for the production of high silica zeolite, ZSM-5 [138]. 5.2.4. Environmental Clearinghouse for Inventories & Emission Factors, US/EPA, collects air pollutant emission factors [139]. One chapter describes how lignite combustion in various modes can produce environmentally hazardous materials: particulate matter, SOx, NOx, CO, total organic compounds, greenhouse gases, trace elements, etc. Emission factors for these pollutants are presented in tabular forms. Emission factors were collected for pulverised coal combustion (dry bottom, tangential/dry bottom, wall fired), cyclone combustor, spreader stoker, travelling grate overfeed stoker and atmospheric FBC. Generally speaking, the use of low rank coals does not pose any special emission problems. Some lignites, however, form dust particles, which are difficult to trap in electrostatic precipitators (ESP) and therefore a bag filter system is required. 5.2.4.1. SOx
During the past decade relatively few studies have been performed, because extensive measures to suppress SOx emission have already been installed in many
272
Chapter 5
pulverised fuel power plants. Some recent important progresses on the NOx and SOx issues can be seen in a book edited by Tomita [140]. Notwithstanding, there still remain serious problems in some area. Electricity Generating Authority of Thailand (EGAT) operates the Mae Moh lignite-fired power plant, which is the biggest power plant in Thailand. Because of the high content of sulphur of about 3 wt%, the health problem has been quite serious. As a short term measure, EGAT shifted a part of fuel to low sulphur lignite and diesel oil. FGD systems have been installed for Mae Moh Units 8-13 since 1995, together with retrofitting of the FGD system for Units 4-13 since 2000. The number of hours in which the SO2 concentration exceeded the guideline of 1300 ]ig/m^ was 82 times in FY 1993, while it was 15 and 0 times in FY 1998 and 1999, respectively [141]. In spite of this improvement, some more time seems to be necessary for the resolution of health problems [142]. Coal-fired FBC has been originally used for steam generation and then scaled up for power generation. Inherent emissions control and fuel flexibility are the advantages of FBC over conventional boilers. Several commercial PFBC plants have started operation recently. The desulphurisation in FBC of lignite has been studied intensively. Westby and co-workers [143] reported the effects of various operating variables on carbon conversion and sulphur dioxide emission during the combustion of Texas lignite. Variables examined were average bed temperature, excess air ratio, gas velocity and carbon residence time. It was found that the combustion of lignite char in a pilot-scale unit was primarily controlled by diffusion and that the combustion of elutriated fines was limited by chemical process. The mechanism of sulphur retention by lignite ash is similar to that by limestone sorbent, but the effect of pore-plugging is less significant because of the better dispersion of calcium in the ash particles. The mechanism of desulphurisation in PFBC differs from that in atmospheric pressure system (AFBC). In contrast with AFBC, the partial pressure of CO2 is so high that CaC03 is not converted to CaO (see Figure 5.38). Thus, following the absorption of SO2 by the limestone particle, the subsequent CO2 emission opens up new pores in the particle. Therefore, a
a. o o
Q.
800
900
1000
1100
T e m p e r a t u r e ("C)
Figure 5.38 CaC03-CaO equilibrium diagram and typical operating conditions for AFBC and PFBC. Reprinted from Ref 144 with permission from lEACoal Research.
Gasification and Combustion
273
higher sulphur removal efficiency at a lower Ca/S ratio can be achieved in the PFBC system. 5.2.4.2. NOx Many reports on the NOx formation mechanism have already been published [72,140,144-146, also see Chapter 6]. In what follows, special attention is paid to the behaviour of low rank coals as contrasted with bituminous coal. High water content of the fuel results in low combustion temperatures. Furthermore, the fuel has relatively low nitrogen contents. These factors are both beneficial in terms of NOx emission. The effect of coal rank can be summarised as follows [147]. More HCN than NH3 was formed from bituminous coals compared with low rank coals, but more NH3 was formed from subbituminous coals and lignites. This observation is consistent with the understanding that the size of aromatic rings in coal increases with coal rank and the number of aliphatic and naphthenic structure decreases. However, NH3 is not generally detected during coal devolatilisation in either heated-grid or entrained-flow experiments. HCN appearance in the gas phase from devolatilisation precedes NH3 even for low-rank coals. It is considered that NH3 is formed from HCN under oxidative pyrolysis conditions. Phong-Anant and co-workers reported a good example that elucidates the effect of coal type on NOx formation during combustion [148]. Experiments were performed in a crucible, a thermobalance and a drop tube furnace. The coals examined were Liddell subbituminous coal and Morwell brown coal. They observed some difference between the two coals. During simulated pulverised coal combustion in the drop tube furnace, the combustion of the brown coal in air showed significantly higher NOx formation than that in an 02-Ar mixture. This difference cannot be attributed to thermal NO alone and they speculated the formation of some prompt NO, which is derived from the interaction between volatile matter and molecular N2. Another remarkable difference between the two coals is seen in the conversion of char nitrogen to NO. For subbituminous coal, the formation of charderived NO increased in proportion to char bum-off and combustion temperature had no effect on the NO yield. On the other hand, with brown coal, the conversion rate of charN to NO increased with combustion temperature and the formation of char-derived NO was not observed until about 35% bum-off These results were explained by the higher concentration of volatile matter in Morwell coal, which assisted the reduction of NO to N2 at lower temperatures and at lower bum-off region. Unfortunately, the number of coal examined in this study was only two and it is premature to draw some conclusion on the effect of coal type only from this observation. Shimizu and co-workers combusted nine coals, including three brown coals, using a laboratory-scale fiuidised bed [149]. The conversion of char-N to NOx was large for chars from high rank coals. A good correlation was found between the conversion to NOx and the char combustion rate. The conversion to NO decreased with increasing combustion rate. Reactivity of the char to NO per unit intemal surface area was almost the same for all the chars. In a model proposed, they considered the reduction of NO with CO catalysed by the intemal surface of char.
274
Chester 5
Pels and co-workers investigated the effect of coal type on N2O emissions in a smallscale reactor [150]. Six coals ranging from lignite to anthracite were tested. Fractional conversion of coal-N to N2O was found to increase with coal rank. Another clear dependency on coal rank was found with nitrogen functionalities in coal. The relative amount of pyrrolic-N decreased with increasing coal rank, while the relative amount of pyridinic forms increased. However, they concluded that no clear relationship was observed between the nitrogen functionality in the coals and the emissions of N2O and NO during combustion. This conclusion is reasonable because the nitrogen functionality in the char prepared at high temperature is quite different from that in the parent coal. Collings and co-workers examined the effect of coal type on the N2O emission in a pilot-scale CFBC using seven coals including North Dakota lignite and Asian lignite [151]. Among the seven coals, the fuel-N conversion to N2O is greater for the case of the bituminous coals (see Table 5.9). N2O formed at 1120 K and 25% excess air for the two lignites was 2.2 and 3.5% of total ftiel nitrogen, whereas those for the bituminous coals ranged from 11 to 14%. On the other hand, the conversion to NOx is favoured for the low-rank coals. Burch and co-workers examined the effect of coal type in a simulated rebuming system [152]. A flue gas composition of 16.8% CO2, 1.95% O2 and 0.1% NO, with He gas as a balance, was chosen to represent a coal primary flame operating at a stoichiometric ratio of 1.1. Isotopically labelled '^NO was used in order to examine the interaction of fuel nitrogen with NO in detail. Pittsburgh #8 bituminous coal and North Dakota lignite were used as rebuming fuels; either of them was fed continuously to the heated gas stream. The isotope ratio of each major nitrogen-containing species was determined. A remarkable difference between the two coals was seen in the product gas under a lean bum condition. HCN was a dominant species from bituminous coal while NH3 was dominant from lignite. The isotope distribution for N2 depended to some
Table 5.9 Nitrogen balance in combustion tests performed at 1120 K and 25% excess air using a 5 kg/h top-down flow burner furnace. Reproduced with permissionfi-omRef 151. Copyright 1993 American Chemical Society. Coal
% Conversion of fuel-nitrogen Rank
N2O
NOx
N2
HVA bit.
11
3
86
HVA bit.
n
6
83
HVC bit.
14
4
82
Black Thunder
sub. C
5
11
84
Powder River
sub. C
3
8
88
North Dakota
lignite
2
10
88
Asian
lignite
4
4
93
New Mexico Blacksville Salt Creek
lis
Gasification and Combustion
extent on the stoichiometric ratio, but it was independent of coal type. For both coals, the distribution was roughly 50% '^N^^N, 40% '"^Ns and 10% ^^Ns. To explain the N2 formation behaviour, they took several mechanisms into consideration. However, they did not consider any heterogeneous reaction. The formation of ^"^N^^N can be accounted for by the reaction between ^"^N in char and ^^NO gas as proposed by Chambrion and coworkers [153], who examined the carbon-NO reaction in detail, using phenolic resin char. This mechanism was also confirmed to be valid in the combustion of a coal char [154]. The Upgraded Brown Coal (UBC) process developed by Sugita and co-workers [78] converts low rank coal to high heating value fuels. Low rank coal is treated with light oil containing a small amount of heavy oil, and then the coal is dewatered and heavy materials are adsorbed on the pore wall of the coal, making the coal waterproof and reducing the propensity of spontaneous combustion. The treated coal tended to give somewhat higher values for NOx and unbumt carbon, but it is because of its short residence time compared to actual furnaces. Figure 5.39 shows a semi-quantitative comparison with a Chinese bituminous coal. The NOx conversion ratio and the amount of unbumt carbon for the treated coal were considerably lower than the bituminous coal. It should be noted that the unbumt carbon was little even under low NOx combustion conditions (i.e., at high overfire air factors). 5.2.4.3, Trace Elements Coal contains many toxic trace elements and it is necessary to pay attention to their emission behaviour during combustion. Among many trace elements, mercury has attracted much attention in the last decade especially in USA because of the recent EPA regulation on mercury emission under the Clean Air Act [155]. The content of Hg
10
^
10 15 Overfire air factor (-)
Figure 5.39 NOx and unbumt carbon formation as a function of overfire air factor in a pilot scale combustion test. o,«: Chinese bituminous coal; n,B: upgraded Indonesian brown coal. Reprinted with permission from Ref 78. Copyright 2003 Pittsburgh Coal Conference.
276
Chapter 5
Great Plain Lignite|%-'••
^....c. '-'I'h.>
, ^''D
^.ir.>.^-x.,Ak:".y..-....<....
Gulf Lignite [v!.p^i r'^%^^>^K^i^^^^}fm¥:,fi
^wwyf^m^^"fW¥^ ^,-^^
3
Rocky Mt. Subbituminous f.f'.'^^M' ;ai-.^^^'f„ .|,„| * / | | ^ ^ j J Great Plain Subbituminous pV^3 hm^A^"^'
\z ^' .
V \-^
J
,, 1
Great Plain Bituminous| Rocky Mt. Bituminous
\^4^^Y''"^^'i'''^^''"''
vi^i
' ^'^"f K
'.\
East Bituminous
J
Interior East Bituminous!f^'^^
\
.^\w'.w^ 1
0
0.02
I
^^^^^^^^r^Y^ 1
0.04
.f t?t
1
0.06
a 1
1
0.08
0.1
0.12
0.14
Average mercury content (ppm)
Figure 5.40 Average mercury content in US coalsfromdifferent regions.
differs from region to region. Figure 5.40 illustrates average Hg content in coals from various regions in USA [156]. The Hg content in Texas lignite is more than that in North Dakota lignite, which in turn is more than that in Wyoming subbituminous coal. Much progress has been made on the understanding about trace elements in coal, but some uncertainties still exist concerning the mode of occurrence, the transformation behaviour, the effect on human health, etc. With respect to the relationship between coal type and trace element emission, it is generally agreed that the contents of most trace elements show a positive correlation with the ash yields of the coals. Many coals are washed at mine-site and therefore some of the trace elements are already removed with the mineral matter rejects. Pyrite-associated elements. As, Se and, to lesser extents, Cd, Co, Cr, Cu, Mn, Ni, Sb, V and Zn, may partially be removed during coal cleaning process. The emission behaviour may be to some extent related to coal rank. Differences in enrichment factors during coal combustion as a function of coal type would come from differences in the volatility and differences in the predominant forms of the elements in the parent coals [157]. Trace elements partitioning was examined at a pulverised-lignite fired power plant [158]. Typical results on enrichment factors are shown in Figure 5.41. Volatile elements like As, Zn and W were enriched in the last row of ESPs (EP6), which collects finest ash particles. The results are fairly comparable with those obtained in other bituminous coal-fired plants, although the elemental modes of occurrence are somewhat different between bituminous coal and lignite. This apparent inconsistency suggests that the mode of occurrence is not an influential factor on enrichment. During combustion process, Hg in coals or lignites is converted to various forms of Hg species in different concentrations. Very little Hg is retained in the bottom ash of a coal-fired boiler and emissions are generally near 100% in most US plants. As a total.
Gasification and Combustion
«*5
277
2
Figure 5.41 Average enrichment factors of trace elements for the samples collected at different locations of a power stationfiringTexas lignite.
about 48 tons of Hg emitted from coal-fired units in USA in 1999 [159]. Japanese data also suggest that only 0.5-1.9 wt% of the Hg is retained in the coarse ash. The behaviour of Hg during combustion is not entirely understood, but substantial knowledge has been accumulated over the years [156]. Hg release starts at around 150 °C and the release is usually complete at 500-600 °C. Above this temperatures, elemental Hg is the thermodynamically stable form in flue gas. The CI content of the coal is the most important factor with respect to Hg oxidation. Under oxidising conditions and in the presence of HCl and/or CI2, elemental Hg is oxidised to HgCU at 300-400 °C. Hg may also be oxidised by NO2 to HgO, which in turn will be converted to HgCl2 at temperatures of <400 °C. CaO also influences Hg behaviour, because CaO promotes the conversion of HgCl2 to elemental Hg. Hg emissions from western US coals appear to be difficult to control with conventional technologies because they are largely elemental. Western subbituminous coals contain significantly lower levels of Hg than eastern Appalachian or interior bituminous coals (see Table 5.10). However, the content of CI in Western coals and lignites is relatively low while the content of Ca is high. Both of them are unfavourable for forming oxidised Hg, which is easiest to capture. Bituminous coals tend to produce more Hg in the oxidised form, whereas low rank coals produce mainly elemental Hg. Plants firing low rank coals will be more difficult to control Hg emission. Some of general agreements on Hg capturing during coal combustion are as follows [160,161]: (1) Hg particulate is easily captured by PM control facility like ESPs and fabric filters (FFs); the efficiency is higher for FF than ESP. Capture rate using existing
278
Chapter 5
equipment ranges from 0 to > 90%. (2) Both dry and wet FGD scrubbers capture Hg effectively. Hg^^ compounds are relatively soluble and can be captured in scrubbers, however Hg"^ is insoluble and must be either adsorbed onto solids or converted to Hg^^ for capture by scrubbing. (3) Typical Hg^VHg° ratio in flue gas is in the order of bituminous coal > subbituminous coal > lignite. In the case of low rank coal combustion, it is necessary to capture gaseous Hg by solid sorbents, where activated carbon is usually used for this purpose. The capturing efficiency is controlled by many factors such as mass transfer effects (mercury-sorbent contact), sorbent concentration, sorbent capacity to hold Hg, sorbent characteristics, temperature, mercury concentration, concentrations of sulphur trioxide (SO3) and other contaminants and the type of particle control device (FF vs. ESP). Large-scale tests in bench, pilot and full-scale facilities were performed to examine Hg control technologies for electric utilities burning lignite [161]. Activated carbons prepared from several coals and chars were tested as sorbents under different conditions. The principal results were: (1) Flue gas contained Hg° (85%), Hg'^ (15%) and Hg particulate (<1%). (2) Increasing injection rates and decreasing gas temperatures significantly improved Hg removal. (3) Increasing flue gas temperatures from 150 to 200 °C generally required 10-20% more activated carbon. (4) Some fabric filter media provide better flue gas-to-sorbent contact and showed better Hg removal relative to the ESP. (5) The cost of reducing Hg from lignite-burning plants is expected to be high when compared to plants burning bituminous coals. Figure 5.42 illustrates some examples of pilot-scale and field test results on Hg removal efficiencies, where activated carbon was injected upstream of an ESP only and ESP/pulse-jet FF system [161,162]. For a lignite and a subbituminous coal, the Hg
Table 5.10 Mercury in utility coals in USA. Reprinted from Ref 159 with permission from lEA Coal Research. ^ . , , Reg,on/rank
Hg(ppni)
^, , CI (ppm)
Emission factor ^^^^^^^
Hg in utility ^^^, ^^^
Apparachian bituminous
0.126
948
4.07
38.2
Interior bituminous
0.086
1348
2.99
4.9
Western bituminous
0.049
215
0.19
3.2
Western subbituminous
0.068
124
2.74
16.7
Fort Union lignite
0.088
139
3.56
1.2
Gulf lignite
0.119
221
5.37
4.1
Gasification and Combustion 100
279
Eastern bit./ A^ Eastern ESP/FP ^ ^ - - A bitVESP* .ignite/ ^
^^
Lignite/ESP*^
JL. -L 100 200 300 400 Injection concentration (kg/10^ m^)
Figure 5.42 Mercury removal by activated carbon injection. Coal: eastern bituminous coal, western subbituminous coal and lignite. Test scale: *DOE field test, ** EERC pilot test. Dust collector: ESP and FF.
removal efficiency was never greater than 70%, regardless of the activated carbon injection rate, if only ESP was used. In the case of bituminous coal combustion, however, the Hg removal rate increased monotonously with the feed rate of activated carbon. The difficulty of removal from low-rank coal flue gas is probably caused by the low amount of acidic flue gas constituents, such as chlorides, that promote the adsorption of Hg on activated carbon. When pulse-jet FF was combined with ESP, a high Hg removal rate was attained with a small amount of sorbent at full-scale utility boilers even for low-rank coal flue gas. The results from a long-term test were summarised as follows [156]: (1) Hg removal efficiency was 40 - 50, 50 - 60 and 60 - 70% at powdered activated carbon (PAC) injection rate of 15, 50 and 150 mg-PAC m'\ respectively. (2) PAC injection reduced both Hg^ and Hg^^ concentrations. (3) The presence of PAC rendered fly ash unusable for concrete manufacturing. Thus, great progress has been made in understanding the Hg behaviour during transformation and in controlling the Hg emission. In future, it would be required to establish more economical technology to prevent the Hg emission. Furthermore, it would be required to control the emission of other toxic elements. 5.2.5. Concluding Remarks Obviously, power generation is the most important area for the utilisation of brown coal. The problems are emissions of CO2 and environmentally hazardous materials. Of course, the emission of CO2 is inevitable if brown coal is combusted and it also holds true even if it is converted to cleaner gas and liquid forms [163]. The situation is more or less similar to other fossil fuels. However, it is most likely that the human beings cannot survive without fossil fuels at least until the end of the 21st century. Thus, it is absolutely necessary to develop new technologies to meet both energy and
280
Chapter 5
environmental requirements. Many developments have been made already as was seen in this Section, but further improvements are absolutely necessary.
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Advances in the Science of Victorian Brown Coal Edited by Chun-Zhu Li © 2004 published by Elsevier Ltd.
Chapter 6 Conversion of Coal-N and Coal-S during Pyrolysis, Gasification and Combustion Chun-Zhu Li CRCfor Clean Power from Lignite, Department of Chemical Engineering PO Box 36, Monash University, Victoria 3800, Australia
The continued use of coal as a reliable and cheap energy source in the future will depend on the minimisation of environmental impacts of coal utilisation. Power generation using coal has two major aspects of environmental impacts. Firstly, the emissions of greenhouse gases, especially CO2, are a major concern. The reduction in the emissions of greenhouse gases will most likely be achieved by developing new power generation technologies that will have much higher efficiencies than the existing pulverised fuel combustion technology, and/or that will capture CO2 from the flue gas for sequestration/storage. Various new power generation technologies will be discussed in Chapter 7. Secondly, the emissions of other air pollutants such as oxides of nitrogen (NO, NO2 and N2O) and oxides of sulphur (SOx) have been and will continue to be another major environmental concern. NOx (NO and NO2) and SOx (SO2 and SO3) are among the most important air pollutants from power generation using coal. NOx and SOx contribute to the formation of acid rain and photochemical smog, in addition to their direct effects on human health. N2O is a potent greenhouse gas. With its extremely long lifetime (more than 150 years) in the troposphere, N2O can also diffuse into the stratosphere where it is oxidised into NOx to act as catalysts for the destruction of stratospheric ozone. In addition to reduced CO2 emission, the newly developed power generation technologies must comply with the increasingly stringent future environmental standards, which are likely to call for "zero-emissions" of these pollutants from power generation using coal. Coal gasification is likely to play an increasingly important role in the future both as an integral part of new power generation technologies (see Chapter 7) using gas turbines or fiiel cells and as a source of synthesis gas. Nitrogen-containing compounds found in the gasifier product gas include ammonia, cyanides, thiocyanates, nitrogen oxides and various heteroaromatic organic compounds in addition to molecular nitrogen [1]. Significant amounts of sulphur-containing compounds, particularly hydrogen sulphide and carbonyl sulphide, also exist in the gasifier product gas. When the gasifier product gas is burned in a gas turbine, the nitrogen-containing species such as NH3 and HCN and the sulphur-containing species such as H2S and COS are NOx and SOx precursors as they may be converted into NOx and SOx during combustion. When the gasifier product gas is used in a fuel cell for electricity generation or used as a synthesis gas for the
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production of value-added chemicals or liquid fuel (e.g. methanol, dimethyl ether and diesel), the nitrogen-containing or sulphur-containing species may interfere with the functioning of the fuel cell system or poison the catalysts in the downstream reactors. Removal of these nitrogen-containing and sulphur-containing species as well as tar from the gasifier product gas is a major task. There are two approaches to the cleaning up of the gasifier product gas: low temperature cleaning and hot gas cleaning. In a low temperature gas cleaning system, most of these nitrogen- and sulphur-containing compounds end up in the wastewater or other liquid streams [1]. In addition to the difficulties associated with the handling and disposal of these liquid streams, the low temperature cleaning processes bring a significant reduction to the overall efficiency of the power generation system as the gasifier product gas is cooled down (see Chapter 7). Hot gas cleaning can greatly alleviate the problems of waste liquid stream generation and process efficiency reduction, but it, like the low temperature cleaning processes, still increases the process complexity and thus the capital and operating costs. Minimisation of the formation of these NOx and SOx precursors in the gasifier thus becomes an important aspect of the design and operation of a gasifier. The detailed information about the formation and destruction of NOx, SOx and their precursors is required for the optimum strategies for the reduction of NOx and SOx emissions from a future gasification-based power generation plant using coal. Recent years have seen intensive research and development efforts to understand the exceedingly complex reaction systems involved in the conversion of coal-N and coal-S during pyrolysis, gasification and combustion. The results from these intensive research efforts form the topic of this chapter. While the presentation of materials in this chapter will be focused on Victorian brown coal, studies on coal-N/coal-S model compounds and on other solid fuels, where relevant, will also be reviewed. However, the mention of literature on coals other than Victorian brown coal is not meant to be exhaustive.
6.1. NITROGEN IN VICTORIAN BROWN COAL Nitrogen constitutes only a small fraction of organic matter in coal, normally accounting for less than 1 wt% daf of Victorian brown coal. For 251 coal samples tested from the Gippsland Basin, their nitrogen contents ranged from 0.36 to 0.74 wt%, tending to be low in woody coal [2]. Little evidence exists for the presence of inorganic nitrogen in Victorian brown coal. The low nitrogen contents in Victorian brown coal, or indeed in any coal (normally <2.5 wt%, [3]), mean that the accurate quantification of the nitrogen content is not always a trivial task. Two types of analytical methods are commonly used [3]. The first one is referred to as the Kjeldahl method. It is considered to be suitable for determining the nitrogen contents in brown coal [4] and is adopted as an Australian Standard [5] as a part of the suite of methods for the ultimate analyses of low rank coals. The method is based on the digestion of a coal sample in concentrated H2SO4 and K2SO4 solution in the presence of a catalyst [e.g. selenium or mercury (II) sulphate] to convert coal-N into NH4SO4. The solution is then made alkaline and the ammonia is distilled into a
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sulphuric acid solution. The excess acid is titrated with sodium hydroxide solution for the quantification of the NH3 produced and thus the nitrogen content in coal. The second method for the quantification of nitrogen content in coal is referred to as the Dumas method, in which coals are burned at high temperature in the presence of a catalyst. The evolved nitrogen oxides are then catalytically reduced to nitrogen that is then quantified to calculate the nitrogen content in coal [3]. Micro elemental methods are usually based on the combustion of coal and the reduction of nitrogen oxides generated, followed by the quantification of N2 produced. Rigby and Batts [6] found that the Dumas method tended to give higher nitrogen contents than the Kjeldahl method for the 16 Australian, 1 New Zealand and 1 Antarctic coals studied, although the Dumas and Kjeldahl methods showed little difference for the 13 Australian oil shale samples studied. The difference in the nitrogen contents of coals determined using the two methods increased almost proportionally with increasing N content of the samples: a difference of about 0.3 wt% was found for coals of around 2.5 wt% nitrogen contents. The difference is possibly due to the loss of N during sample preparation using the Kjeldahl method [6] or incomplete digestion of the coal matter. The accuracy of these methods is not always satisfactory for studying the distribution of coal-N during conversion. The Australian Standard [5] based on the Kjeldahl method states that the method gives a reproducibility of 0.08 wt%. It is not too difficult to find the reported nitrogen contents for Victorian brown coal samples even from the same coalfield to vary significantly. For example, a wide range of values, from 0.55 to 0.7 wt% (daf), may be found in the literature [7-13] for the nitrogen contents in Loy Yang samples. Whilst the nitrogen contents of the Loy Yang samples used by different research groups may differ, the errors of the analytical methods used may have also contributed to some extent to the variation in the reported nitrogen contents. The difficulties in the accurate quantification of nitrogen content in coal are a major obstacle in the understanding of coal-N distribution during pyrolysis, gasification and combustion. The yields of various nitrogen-containing products during coal conversion are often expressed as the ratios (or percentages) of the nitrogen in these products to the nitrogen in coal (i.e. "% of coal-N"). Any error in the nitrogen content of coal (e.g. 0.08 wt% as the reproducibility for the Australian Standard [5]) for a coal of only 0.6 wt% coal-N would mean a big error in the yields of nitrogen-containing products from the coal-N, even if assuming that the quantification of nitrogen-containing products involves minimal errors, which in fact also has relatively large errors (see Section 6.3). This points to the special need for caution when comparing the yields of NOx and NOx precursors from different research groups. The errors in the quantification of coal-N become particularly important when the yields of some nitrogen-containing species (e.g. N2) are calculated as the difference between the nitrogen in coal and the nitrogen found in other nitrogen-containing products quantified. This will be further discussed in Section 6.3. Nitrogen exists in coal in a wide variety of structures. Traditionally, the information on nitrogen functionalities in coal has been inferred from the nitrogen-containing compounds identified in the products from the destructive conversion of coal, e.g. pyrolysis tar or liquefaction products. A brief account of the literature was presented by
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Davidson in 1994 [3]. The main problem is that, in a heterogeneous organic solid mixture such as coal, various reactions, including rearrangement/isomerisation, cracking, hydrogenation and condensation/polymerisation, could take place during the destructive conversion processes in ways that are very complicated and not well understood (also see Sections 6.3 and 6.4). What are observed in the products (e.g. pyrolysis tars or liquefaction products) are not always necessarily what would be present in the coal substrate. Non-destructive analytical techniques are required to characterise coal as a whole in its solid state in order to understand the exact structure of nitrogen in coal. With the recent advances in the development of X-ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), this non-destructive technique has been widely used to study the nitrogen functionalities in coal. Briefly, XPS determines the binding energies of both core and valence electrons by measuring the kinetic energies of electrons photoemitted from a solid when irradiated in vacuo with near-monochromatic X-radiation [14]. Since the core electron binding energies are characteristic of the element, XPS provides an elemental analysis for all elements except H and He. Changes in oxidation state and/or functional group where the element is present cause small shifts in the characteristic binding energy, which can be measured in XPS to give information about the chemical environment of the element, i.e. functional groups. An XPS spectrum of an element can often be deconvoluted mathematically into peaks (i.e. fitting) characteristic of functional groups of the element present in the sample. XPS has the advantage of constant sensitivity to an element irrespective of the functional groups in which nitrogen is present [15]. XPS has been used to determine the elemental composition of coal. For example. Perry and Grint [14] used XPS to determine the contents of oxygen, sulphur, silicon, aluminium, nitrogen, chlorine and iron in a set of 18 coals, including a Victorian brown coal. The nitrogen content (an N/C atomic ratio of 0.7%) reported for the Victorian brown coal was broadly within the range expected. However, XPS is essentially a surface technique due to the short (often 2 nm) inelastic mean free path of the photoemitted electrons, with the detected signal mainly ting from within the outmost 3 nm of the exposed surface. Questions arise if the XPSderived elemental composition of coal would represent the bulk composition of the coal, e.g. as determined through classical ultimate analyses of coal. Whilst many studies have shown, within experimental errors, that the XPS-derived surface composition (e.g. N/C atomic ratio) of coal/char largely resembles that from bulk conventional chemical analysis [14-21], evidence [14,15,17,19,22] has been presented for the enrichment/depletion of some elements on the surface for some coal samples. Most XPS studies of nitrogen in coal have proved or assumed that the XPS-derived surface composition represents the bulk composition. Perry and Grint [14] observed that the nitrogen XPS spectra of coal were dominated by pyrrolic nitrogen although "subtle changes in peak shape between different coals" were also observed probably "as a result of different distributions of nitrogen in aminotype groups" following the (wrong) suspection by Jones and co-workers in an earlier study [23].
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Since the late 1980s, there has been a considerable advance in the understanding of nitrogen functionalities in coal (and char, see Section 6.3) using XPS. The N Is XPS spectra were deconvoluted (or termed as "peak synthesis") into peaks representing different forms/functionalities of nitrogen in coal. Whilst earlier XPS studies [14-17] considered only pyridinic nitrogen ("N-6", i.e. nitrogen in six-membered rings such as pyridine and acridine) and pyrrolic nitrogen ("N-5", i.e. nitrogen in five-membered rings such as pyrrole and carbazole), with the binding energies of about 398.7 and 400.3 eV respectively, to curve-fit the N Is XPS spectra of coals, later studies [18,24-26] found that an additional peak at a higher binding energy of about 401.5 eV, also noted previously [15,17] as a small residue in curve-fitting, was necessary to obtain an acceptable fit of the N Is XPS spectra of coals. This additional peak has been termed as "quatemary-N" ("N-Q") based on its binding energy being close to (not necessarily equal to) that of quaternary nitrogen (see below), although the term has not been used to mean a true quaternary structure such as ammonium ion (also see discussion below). In an XPS study to determine the nitrogen functionalities in 20 coals from Australia, Canada, China, Japan and USA, covering a whole rank spectrum from brown coal to semi-anthracite, Kambara and co-workers [18] reported the nitrogen in a Yalloum brown coal sample to consist of 14 % quaternary nitrogen, 51 % pyrrolic nitrogen and 35 % pyridinic nitrogen. The nitrogen functionality of Yalloum brown coal is broadly within the expectation based on its rank, as is shown in Figure 6.1, in which the XPSderived nitrogen functionalities of other coals from several studies [18,19,25,27-30] are plotted as a function of coal rank (carbon content). There are a number of observations to be made from the data in Figure 6.1. In broad agreement with the study [31] on a rank-ordered series of coals from the Mahakam delta in Indonesia and the study [16] on a rank-ordered series of UK coals, the data in Figure 6.1a show a weak maximum of nitrogen content at around 85 wt% carbon content, although there are only a few datum points for coals of >85 wt% C in Figure 6.1a. Pyrrolic nitrogen (N-5) is the dominant form of nitrogen, representing around 60% of the nitrogen in all coals shown in Figure 6.1b. Many other XPS studies on nitrogen in coal [14-17,32-37] support this conclusion. X-ray absorption near-edge structure (XANES) spectroscopy confirmed the presence of pyrrolic nitrogen as the dominant form of nitrogen in coal [37-39]. Most XPS studies have indicated that pyridinic nitrogen (N-6) is another main form of nitrogen in coal, accounting for about 20 - 40 % of coal-N in most coals. In fact, the early study by Brooks and Smith [40] on the reactions of Victorian brown coal with perchloric acid indicated that 60-70% of coal-N in a Morwell and a Yalloum coal was basic. A portion of this "basic" nitrogen would be pyridinic nitrogen. However, a limited number of studies have reported that N-6 constitutes only a very small fraction of the nitrogen in coal, particularly in low rank coal samples (Figure 6.1c). Aho and coworkers [29] found no pyridinic nitrogen in some peat samples and only 9 % of coal-N as pyridinic nitrogen in a German lignite (brown coal, C = 65.8 v^% daf) using XPS. In an early study where N Is XPS spectra were deconvoluted into N-5 and N-6 peaks. Pels and co-workers [32] showed that N-6 represented less than 20% of coal-N and that a German lignite (C = 65.9 wt% daf) contained a negligible amount of N-6.
291
Coal-N and Coal-S
60
65
70
75 80 Carbon content, wt% (daf)
-I
(B), N-5
•
1
1
1
95
1
r
(D), N-Q
(C), N-6
80
90
85
4-
Yalloum
•5i.
(0
o o
60
Yalloum
C
.2 40 o c
•
+
3 0)
o S 20
'••:\t.
ik.\
60
70
80
90
70
80
90
70
80
90
Carbon content, wt% (daf)
Symbols A
•
References Kambara and co-workers [18] Buckley and co-workers [25,27,28] Kelemen and co-workers [19]. Aho and co-workers [29] Friebel and Kopsel [30
Figure 6.1 Nitrogen content and XPS-derived coal nitrogen functionalities as functions of coal rank (carbon content).
292
Chapter 6
The studies using '^N NMR and N-XANES have caused further debate in the hterature on the presence of pyridinic nitrogen in coal. N-XANES spectroscopy of coal (with errors of about ±12% [39]) appears to agree with the XPS spectroscopy of coal on the presence of pyridinic nitrogen in coal [37-39]. However, in a solid-state ^^N NMR spectroscopic study of three coals (C = 75.8, 83.6 and 84.0 wt% daf), Knicker and coworkers [41] failed to observe significant signals arising from the pyridinic nitrogen in coal. It should be noted that the signal-to-noise ratios of the ^^N NMR spectra of coal are usually relatively low due to the low natural abundance of ^^N (0.36%), its negative gyromagnetic ratio and low nitrogen content of coal [41]. A subsequent publication from the same authors [42] on the '^N NMR spectroscopy of un-degraded plant material, plant composts, sediments and coal samples also showed that pyridinic-N did not appear to be a major constituent of coal-N for the coals studied. However, Solum and co-workers [43] showed that treatment of coal with strong acid could protonate the pyridinic type nitrogen and make it observable in '^N NMR, suggesting that the pyridinic nitrogen does exist in coal but is not easy to observe with NMR spectroscopy under some NMR experimental conditions. Saito and co-workers [44] observed changes in the '^N NMR spectra with contact time (0.1 to 5 ms) to show the presence of pyridinic nitrogen in the Witbank coal studied under specific NMR conditions. The differences between the results from NMR and other X-ray techniques (e.g. XPS and XANES) are yet to be resolved [45]. It seems that further work, e.g. to develop methodologies and optimise experimental conditions to improve the sensitivity and quantitativeness of '^N NMR, is required before '^N NMR can be successfully applied to the study of nitrogen in coal. The balance of evidence indicates that amines (amino groups) account for a very limited proportion, if any at all, of the nitrogen in coal, including low rank coal, despite the fact that Brooks and Smith [40] had claimed that a substantial concentration of amines was present in Victorian brown coal. The accurate quantification of amines (amino groups) in coal by XPS has been difficult due to the closeness of binding energies for pyrrolic, pyridinic and amino nitrogen: 398.7 eV for N-6 (e.g. phenanthridine), 399.4 eV for primary amines (e.g. 1 -aminopyrene) and 400.3 eV for N5 (e.g. carbazole) [15]. Furthermore, the amino peak, if present, would lie in between the two large peaks of N-5 and N-6 in the XPS spectra. Bartle and co-workers [15] stated that their XPS spectra could be deconvoluted without the need to include a component for primary aromatic amines. Furthermore, Wallace and co-authors [17] reported that the inclusion of such an amino peak in fact resulted in a poorer fit to their spectra than without the peak. No amino groups were considered in the deconvolution of N Is XPS spectrum of Yalloum coal [18]. Aho and co-workers [29] included an amino peak (398.5 eV for N-6, 398.8 eV for amino groups and 400.2 eV for N-5) in the deconvolution of their XPS spectra but found negligible amounts of amino group even in low rank coals (peat and lignite). Kelemen and co-workers [19] carried out a sensitivity analysis for the need of including amine in their XPS spectral deconvolution and concluded that the concentration of amine in the coals studied, including a lignite, would be less than 5 % of coal-N. Kelemen and co-workers [36] also showed that XPS could detect small amounts (6-11%) of fiiel-N) of amino nitrogen in kerogen samples
Coal-N and Cocd-S
293
whilst the same experimental procedures showed the amino nitrogen to account for less than 5% of coal-N, even in a low rank coal (Beulah-Zap lignite). While the initial NXANES of coal [38] did not reveal the presence of aromatic amines, a more recent NXANES study [39] showed some evidence of the presence of aromatic amines at very low concentrations (6-10 % of coal-N in the Argonne Premium Coal samples) but no saturated amines. No amines were observed in the Argonne Premium Coal samples with high-resolution mass spectrometry (HRMS) [46]. While the difference between the findings from XPS and XANES is yet to be resolved [37], the balance of evidence indicates that little (if any at all) amine is present in coal, including low rank coal. XPS studies [22,47,48] also showed the presence of ammonium nitrogen, at 402-403 eV or 403-404.5 eV and away from 401.5 eV for N-Q [47]), associated with (clay) minerals in some bituminous coals and semi-anthracites, accounting for 4-17 % of coalN [22] or higher [48], although it was also noted that ammonium nitrogen was often not uniformly distributed in coal [47,48]. The presence of ammonium nitrogen has not been reported for Victorian brown coal. N-XANES study also revealed the presence of pyridone structure, N-6(0), in coal, particularly low rank coal samples [37,39]. Friebel and Kopsel [30] believed that pyridone structure existed in coal. However, the direct quantification of pyridone structure by XPS is hard because its binding energy is close to that of pyrrole. Friebel and Kopsel [30] used CH3I to quatemarise a Lusatian (German) lignite and its humic acid samples. The quatemarised samples showed increases in N-Q (about 10%) while N-6 remained almost unchanged. Friebel and Kopsel [30] concluded that the increases in N-Q were mainly due to the reactions between pyridone/hydroxyl pyridinic structures and CH3I. If so, the actual concentration of N-6 in the lignite is higher than what is observed by simple deconvolution of an XPS spectrum into N-5, N-6 and N-Q peaks. Perhaps the biggest controversy in the XPS study of coal-N is the nature of N-Q seen in the N Is XPS spectra of coal and char. Buckley [27] presented a detailed discussion about the nature of N-Q and some of the possibilities are briefly summarised below: • N-Q as an excited final state satellite. The satellite lines in the electron spectra of aromatic ring systems reflect the re-organisation of the valence electrons upon core electron ionisation. The position of N-Q (being closer to the main N Is spectra) and the lack of simultaneous change in N-Q with other N-5 and N-6 intensities in a number of samples examined led him to exclude this possibility. • Amines associated with other fijnctional groups such as -COOH. This possibility was ruled out due to the very low concentrations of amines in coal. In fact, quaternary salts are likely to be unstable under the X-ray radiation [15]. • Ammonium ion (NH/). No evidence for the presence of enough N H / was found to account for the N-Q. In fact, later studies [22,47,48] found that ammonium would not appear at the same position as N-Q in the N Is XPS spectra of coal. • Protonation or oxidation of pyridinic nitrogen. It was believed that N-Q could be N-6 associated with oxygen in coal. Kelemen and co-workers [19] believed that N-Q is a pyridinic or basic nitrogen associated with nearby or adjacent hydroxyl groups from carboxylic acids or phenols, in broad agreement with Buckley's suggestion mentioned above [27]. The decreases in the
294
Chapter 6
proportion of N-Q in coal-N (Figure 6.1b) with increasing rank (and thus decreasing contents of oxygen in general and acidic oxygen-containing groups in particular) would also agree with the proposals about the nature of N-Q by these groups. If so, N-Q could be considered as a variation of N-6. The actual pyridinic nitrogen in coal would need to include both pyridone and quaternary nitrogen structures and thus would be of much higher concentration than "N-6" as directly obtained from spectral deconvolution of N 1 s XPS spectra. Li and co-workers [28] more recently challenged the explanation about the nature of N-Q outlined above, at least for tars. The XPS of tar samples (discussed in more details in Section 6.3) produced at 600°C did not show any N-Q in tar although the tar would be expected to contain some -OH groups. Other forms of electron withdrawl from the nitrogen, e.g. through ionic and/or charge-transfer interactions (known to decrease with increasing rank) should also be considered to explain the nature of N-Q [28]. For highly condensed/graphitised carbonaceous materials such as char. Pels and coworkers [49] and Stariczyk and co-workers [50] proposed that the quaternary nitrogen observed in XPS spectra may represent the nitrogen incorporated into the interior ("valley-N" and "centre-N" [49]) of a large aromatic ring system or a graphene layer [51,52]. Inagaki and co-workers [53] and Nakahashi and co-workers [54] also observed a high energy (about 401 eV) peak in the carbon films. Inagaki and co-workers [53] did semi-empirical molecular orbital calculation and concluded that nitrogen could be incorporated into a carbon layer, with the localisation of electron spins around nitrogen and neighbouring carbon atoms. Casanovas and co-workers [55] also carried out calculations and concluded that high binding energies (401.0 to 403.0 eV) in the N Is XPS spectra of carbonaceous material could originate from the N atoms at "inner positions" ("graphitic" nitrogen). The XPS spectroscopic study of char will be further discussed in Section 6.3. It should be pointed out that this explanation of N-Q clearly does not apply to the nitrogen in coal [28,50]. It may now be useful to re-examine the compilation of XPS data in Figure 6.1b. Closer examination of the data would reveal that smaller scatters exist for the changes in nitrogen functionalities with rank from an individual group than for the whole compilation of the data from different groups. The overall scatters in the compilation of data become even more significant if data from more research groups are included in the plot in Figure 6.1. The scatters in the XPS data in Figure 6.1 may not be explained simply by the differences in the properties of coals used in these studies. The XPS data of several low rank coal samples are listed in Table 6.1. Even for the samples apparently from the same coalfield (e.g. Rhenish German lignite in Table 6.1), very different nitrogen functionalities have been reported by the two groups. Differences in the detailed XPS experimental procedures, peak shape (e.g. Gaussian-Lorentzian [30] versus Gaussian [29]), peak width, peak position and so on would account for at least partially the differences among the results from various research groups, in addition to the intrinsic differences among the actual samples used by individual research groups. Clearly, the XPS technique needs to be further improved for it to become a routine analytical technique. Nevertheless, the data in Table 6.1 appear to indicate that the Yalloum brown
Coal-N and Cool-S
295
Table 6.1 Nitrogen functionalities (% of coal-N) in several low rank coal samples as determined by XPS. C, wt% daf
N-5
N-6
N-Q
References
Yalloum (Victoria, Australia)
65.4
51
35
14
18
Sample Unknown name (USA)
69.2
53
33
14
18
Beulah Zap (USA)
72.9
59
25
16
19
Rhenish lignite (Germany)
65.8
59'
9
32''
29 30
Rhenish lignite (Germany)
68.1
44
13
43
Central Germany lignite
70.2
53
27
20
30
Lusatia (Germany)
67.0
68
11
21
30
a, including 1 % as amino-N at 398.8 eV. b, including 1% as oxidised nitrogen at 404.1 eV.
coal investigated might be closer to the American lignite (Beulah-Zap) than to the German lignites as far as the nitrogen functionalities are concerned. In summary, the balance of evidence from the survey of literature given above indicates that pyrrolic nitrogen (N-5) is the dominant form of nitrogen in coal. Significant amounts of nitrogen also exists in coal in six-membered rings either as simple pyridinic nitrogen (N-6) or associated with oxygen (N-Q), including pyridone structure, N-6(0). Amine (amino) structures, if at all present in coal, account for very small proportion (e.g. < 5%) of coal-N, even in brown coal. Disagreement still exists in the literature regarding the nitrogen functionalities in coal among the different spectroscopic methods and among different research groups. Further research (particularly new analytical techniques) is required to resolve the disagreement for a better understanding of nitrogen functionalities in coal.
6.2. PYROLYSIS OF N-CONTAINING MODEL COMPOUNDS Pyrolysis of N-containing compounds has long been a subject of extensive research for such purposes as elucidation of compound structure, understanding of (radical) reaction mechanisms involved in various combustion and explosion processes at elevated temperatures, formation of toxic species in a fire (including bush fire) and formation of hazardous species in smoking (cigarettes). A large number of N-containing compounds containing a wide variety of nitrogen functional groups or heteroaromatic ring systems have been studied. The purpose of this section is to review and analyse the information in literature about the pyrolysis of relatively simple N-containing compounds, which might serve as models of the nitrogen functionalities possibly present in coal (see Section 6.1 above). The information presented in this section will be helpful for understanding the conversion of coal-N during the pyrolysis, gasification and combustion of Victorian brown coal to be discussed in Sections 6.3 and 6.4.
296
Chapter 6
Table 6.2 briefly lists findings from some studies reported in the open literature [5694] about the pyrolysis of N-containing model compounds. The list is not at all meant to be exhaustive but only to provide some examples for discussion. The discussion here will be focused on the distribution of N-containing products from the pyrolysis of these N-containing model compounds as a function of pyrolysis conditions and structural features of the model compounds. A detailed discussion on the reaction mechanisms and kinetic modelling involving tens or even hundreds of elementary reactions [e.g. 95100], while important for the thorough understanding of the reactions involved in pyrolysis and for developing predictive models, is beyond the scope of this book. 6.2.1. Pyrolysis of N-Containing Model Compounds in Gas Phase Detailed studies have been carried out on the pyrolysis of relatively light Ncontaining model compounds in the gas phase, especially the studies carried out by Lifshitz and co-workers [63-71] and Mackie and co-workers [72-78] using shock tube reactors over the past several years. The inherent nature of a shock tube reactor means a very high heating rate and a very short reaction time, providing an ideal facility to study the reactions taking place in the gas phase. With the very short reaction time of a few milliseconds or even sub-milliseconds, the pyrolysis reactions in the gas phase do not normally take place to extents large enough to cause non-negligible soot formation during pyrolysis, particularly when the concentrations of model compounds are very low (see Table 6.2). These studies provide information about the reactions taking place in the initial phase of pyrolysis. The pyrolysis of the relatively simple N-containing model compounds in the gas-phase involves a large number, close to or more than 100, of elementary reactions initiated and propagated by radicals. For the pyrolysis of pyridine in a shock-tube reactor, Mackie and co-workers [72] proposed that the thermal decomposition of pyridine was a chain process initiated principally by C-H bond fission to form o-pyridyl. Many types of radicals such as H, CH3 and other hydrocarbon radicals are involved in the chain reactions. A large number of (intermediate) products, both with and without nitrogen, were detected in the pyrolysate, as is shown in Figure 6.2 for the yields of significant products [72]. Under less severe conditions (e.g. at relatively low temperatures around 1500 - 1600 K with a very short reaction time of 450 - 600 jxs in Figure 6.2), various nitriles such as HCCCN and CH3CN were observed as important N-containing pyrolysis products together with hydrocarbons such as C2H4 and CH4, representing the "debris" from the breakdown of pyridine. However, under more severe conditions (e.g. increasing temperature to 1800 K in Figure 6.2), HCN would become the most important N-containing product together with C2H2 and H2 as the most important non-nitrogenous products. A 58-step reaction model was used to model the observed major product species. Clearly, various nitriles were precursors of HCN, as is evidenced from the pyrolysis of nitriles such as acetonitrile [57]. In fact, HCN and nitriles have almost always been found as the main N-containing products from the pyrolysis of simple model compounds in the gas-phase (Table 6.2), for example, using a shock tube (or similar) reactor [63-79] of short residence time.
297
Coal-N and Coal-S Table 6.2 A brief survey of pyrolysis of N-containing model compounds. Compounds
Experimental conditions
Comments
Refs
Pyrrole
Pyrrole was injected into a Vycor tube packed with Berl Saddles heated at 850°C in a furnace.
Gaseous products of HCN, CH4, C2H4, C2H2 and NH3 together with heavy aromatic and heteroaromatic compounds (tar).
56
Acetonitrile
Acetonitrile was injected into a stirred-flow Vycor reactor at 880 to 960°C.
HCN and CH4 as the main products together with other products including brown non-volatile material soluble in acetone.
57
Amino acids
Samples were introduced into a Vycor tube heated at 700 to 1000°C
Cyclisation to form HCN favoured at 58 high temperatures. Formation of NH3 and HCN dependent on the structure of amino acids.
a-amino acids
Pyrrole was injected into a Vycor tube packed with Berl Saddles heated at 850°C in a furnace.
HCN and NH3 as the main Ncontaining products, dependent on the structure of amino acids. HNCO may be formed. HCN formation via N-heterocycles.
Pyridine, quinoline, pyrrole and benzonitrile
About 3 mole% of model compound vapour in helium was injected into a quartz capillary reactor heated at 850 tollOO°C.
HCN as the main N-containing 60 product but with trace amounts of NH3 formation (for pyridine). "Carbonaceous material" coated on the reactor wall to catalyse pyrolysis reactions for some model compounds.
Pyridine
About 10 mole% of pyridine NH3 as the main N-containing vapour in helium was fed into a product. Formation of soot (about half of the carbon in the feed could stainless steel tubular flow reactor heated at 600 to 800°C. not be detected in the gas after reaction, particularly for quinoline).
Pyridine
0.25 to 2.0 mole % pyridine was fed into a stirred-flow reactor heated at 900-1000°C.
HCN as the main N-containing product but accounting for about 50% of N in pyridine even at 1000°C. Tarry residue found at reactor exit.
62
Pyrrolidine, acrylonitrile, pyrrole, 5methylisoxazole, N-methylpyrrole, benzonitrile, indole, (iso)quinoline and acetonitrile
About 3x10"-molcm"^ of model compounds was pyrolysed at 580 to 1680°C behind reflected shocks.
HCN and nitriles as the main Ncontaining products.
63-71
59
61
298
Chapter 6
Table 6.2 A brief survey of pyrolysis of N-containing model compounds (cont'd). Compounds
Experimental conditions
Comments
Pyridine, pyrrole, 2picoline, butenentrile, pyrimidine and acetonitrile
0.06 to 0.5% of model compounds in inert gas was pyrolysed at 930 to 1580°C in shock-tube reactors.
HCN and nitriles as the main N72-78 containing products with little NH3. Little or no soot formed.
Pyridine
0.4 to 0.5 % pyridine in argon was pyrolysed in a shock tube reactor at 1310to2060X.
HCN and nitriles as the main Ncontaining products. Toluene decreased CH3CN yield.
Pyridine
Pyrolysis at 700 to 900°C using Bipyridine and cyanomethylene as the major products matrix-isolation IR.
Benzonitrile
Benzonitrile vapour in N2 was passed through a tubular quartz plug-flow reactor heated at 550 to 800X.
Refs
79
80
HCN, benzene, monocyanodiphenyl etc as the main products together with soot on the reactor wall.
81
Both HCN and NH3 as the main N14 model Powers (<60 |im) fed into an compounds of entrained flow reactor at 800°C. containing products together with N-6, N-5, Nother compounds. 6(0), and amino functionalities and other oxygen functionalities
82
Pyrrole, pyridine The model compounds were and quinoline deposited onto hydroprocessing catalysts (C0M0/AI2O3, NiMo/Al203) before pyrolysis in a fixed-bed reactor at a slow heating rate.
NH3 and HCN formation observed 83 over a wide range of temperature. N2 also possible at high temperatures. Tar and char formation observed.
Pyridine and 2-picoline
Model compounds (2-3.5% in Ar) were pyrolysed in a completely stirred fused silica reactor at 780 to 970°C.
Formation of high molecular mass materials. HCN as the main Ncontaining product from pyridine.
84
Quinoline and hydrogen ated quinolines
Pyrolysis was carried out in a curie-point pyrolyser.
Mainly HCN and trace amounts of NH3 observed with both to increase with hydrogenation degree.
85
2-azetidinone
Sample vapour was introduced into a flow system (quartz tubing) at 550 to 950°C.
HNCO and HCN reported among other products.
86
HCN, HNCO and NH3 as the main N-containing products together with traces of NO and tar. Selectivities changed with temperature.
87
2-pyridone and Particles were fed into a fluidised-bed reactor for 2,5diketopiperazine pyrolysis at 700 to 1100°C.
299
Coal-N and Coal-S Table 6.2 A brief survey of pyrolysis of N-containing model compounds (cont'd). Compounds
Experimental conditions
Comments
Refs
Methylamine
Pyrolysis in a shock tube and computational study considering previous results.
HCN and NH3 as the main Ncontaining products together with other species.
88,89
Poly(4-v The polymer v^as pyrolysed in a inylpyridine) and fixed-bed reactor (10 K s"4o its HCl salt 1000°C with 10 s holding time (quaternary and in a curie-point pyrolyser nitrogen) (2000-3300 K s"' to 1040X with 3 s holding time).
Both HCN and NH3 observed from 90 the neutral form of the polymer but at low level due to the formation of tar (containing pyridine and oligomers). Formation of NH3 from the salt form is not certain.
Polyacrylonitrile Pyrolysis in fixed-bed reactor at HCN, NH3 and N2 from PAN. NH3, 91 (PAN), and lOKmin"'. small amounts of HCN and traces of nylon 6,6; nylon N2 from nylons. 6,12 and nylon 12) Polyamide (nylon 6,6)
Polyamide was "pushed" (heating rate of about 100 K s"') into a horizontal tubular reactor heated at 800 to 1000°C.
NH3 and HCN (and HNCO, not 92 quantified) observed as the main Ncontaining products. HCN influenced more by temperature and residence time than NH3.
Polyacrylonitrile The homopolymer was pyrolysed in DSC, hermetically sealed and open containers.
Evolution of NH3 and HCN seen at 120 to 220°C under vacuum.
93
Poly-L-leucine The protein powder was fed (1142 monomers) into a fluidised-bed reactor for and poly-Lpyrolysis at 700 and 800°C. proline (150 monomers)
HCN, HNCO and NH3 as the main N-containing products. No char formed.
94
It may be surprising to note that HCN could still be a major N-containing product from the pyrolysis of many model compounds, in which the nitrogen functionalities are not directly pyrrolic and pyridinic, such as amino acids [58,59], methylamine [89], 2azetidinone [85], 2,5-diketopiperazine [87], 2-pyridone [87] and protein [93]. Cyclisation of amino acids to form N-heterocyclic structures at high temperature is believed to be an important route for the formation of HCN during the pyrolysis of amino acids [58,59]. Therefore, substitutional groups in the amino acids that could favour or inhibit the formation of N-heterocycle intermediates would lead to increases or decreases in the pyrolysis yields of HCN/nitriles respectively. With the same reasoning, the pyrolysis of cyclic amines such as pyrrolidine [63], which could be easily dehydrogenated, yielded mainly HCN and nitriles. For the pyrolysis of 2-azetidinone, the formation of methylenimine
300
Chapter 6
0.0001 1200 1300 1400 1500 1600 1700 1800 T/K Figure 6.2 Distribution of all products of significance behind reflected shocks in the 0.7 % pyridine in argon mixture over the temperature range studied. Reprinted with permission from Ref. 72. Copyright 1990 American Chemical Society.
C3H5NO - ^ CH.NH + CH2C=0 is an important step [85,93] for HCN formation, because methylenimine is relatively unstable and would dissociate into HCN and H2. Increasing temperature often favours the formation of HCN. For example, the HCN/NH3 ratio increased from 1.7 to 2.2 when temperature was increased from 700 to 800°C during the pyrolysis of poly-L-leucine [93]. Similarly, the selectivity of HCN increased with increasing temperature for the pyrolysis of 2,5-diketopiperazine and 2pyridone [87].
Coal-N and Coal-S
301
6.2.2. Pyrolysis of Solid N-Containing Model Compounds or Pyrolysis with the Formation of Soot and/or Tarry Materials Close examination of the literature on the pyrolysis of even such simple model compounds as pyrrole and pyridine would show some apparent "contradictions" among the results obtained using different reactor systems. Li and Tan [101] believed that there appeared to be some correlation between the formation of NH3 and the presence of a condensed phase with donatable hydrogen during pyrolysis of some model compounds containing pyrrolic and pyridinic nitrogen. Whilst no NH3 was reported from the pyrolysis of pyrrole and pyridine in shock tube reactors [63,65,72,73,79], NH3 and/or N2 were reported as an important product when pyrrole and pyridine were pyrolysed in other reactor systems [56,60-62,82,83]. In the latter group of studies where NH3 was observed, the reactant concentration was often high and/or residence time was long. The formation of NH3 and/or N2 was accompanied by the formation of heavy aromatic and heteroaromatic compounds, tarry materials or even soot (see Table 6.2). In the pyrolysis of pyrrole in a Vycor tube containing Berl Saddles (as a solid surface), Patterson and co-workers [56] reported NH3 as an important product together with the identification of a large number of aromatic, cyano and heteroaromatic compounds in the products. Only 44% of pyrrole added into the reactor was recovered as pyrolysate. In reporting some trace (< 5%) amounts of NH3 and N2 from the pyrolysis of pyridine, Axworthy and co-workers [60] noted that about 49 % of nitrogen in the decomposed pyridine was found in the "carbonaceous residue". When powders of carbazole and acridine (and many other compounds) were pyrolysed in an entrained flow reactor at 800°C, Hamalainen and co-workers [82] reported that both HCN and NH3 are important N-containing products. While the channel of hydrogen abstraction (dehydrogenation) is important in the gas phase for the formation of HCN and nitriles, the reactions in the solid or on the solid surface may follow a different route. Once the reactants are adsorbed or deposited on the solid surface or incorporated into the solid matrix held in the reactor, the true residence time is necessarily much longer than that of the bulk gas stream. The carbonaceous material provides a surface for the generation and adsorption of H radicals that can hydrogenate the N-containing species on the surface for the production of NH3 [101]. The formation of NH3 through this route is clearly very limited in the gas phase, partly due to the short residence time and partly due to the low concentration of H radicals in the gas phase compared with that on the solid surface. The roles of H radicals in the formation of NH3 will be discussed in detail in Section 6.3. Sugiyama and co-workers [61] found NH3 and N2 to be the main products from the pyrolysis of pyridine in a stainless steel reactor at 600 to 800°C. It is suspected [87] that the decomposition of pyridine [61] was assisted by the catalytic surface. The catalytic surface might have catalysed the formation of NH3 by providing the H radicals necessary for the formation of NH3 [101] (see Section 6.3). The surface could also catalyse the destruction of NH3 to produce N2: Li and co-workers [102] found that many materials commonly used in the reactor systems such as quartz, zircon sand and stainless steel could lead to significant destruction of NH3 at temperatures higher than
302
Chapter 6
700 or SOO^'C. Houser and co-workers [62] also stated that the use of a metal reactor produced large amounts of N2 during the pyrolysis of pyridine. Furthermore, with an effective catalyst such as the hydroprocessing catalysts (C0M0/AI2O3 and NiMo/AliOs) used by Furimsky and co-workers [83], substantial amounts of NH3 could be formed starting even at around 200°C during the pyrolysis of pyrrole, pyridine and quinoline [83]. Apparently, H radicals must have been easily generated on the surface of hydroprocessing catalysts. The presence of substitutional groups significantly increases the ability for a model compound to form heavy compounds or even soot. For example, N-methylpyrrole seemed to form considerable quantities of tar even at 745°C [56]. 2-picoline, a nitrogen heteroaromatic analogue of toluene, has a much higher sooting propensity than pyridine [84]. The formation of a solid residue catalyses the pyrolysis process [60,84] including the formation of the solid residue itself The rate of decomposition of 2-picoline was found to be faster than predicted by a homogeneous gas-phase reaction mechanism even in a silica reactor [84]. The presence of oxygen-containing functional groups has been shown to greatly affect the pyrolysis behaviour of the N-containing model compounds. Hamalainen and co-workers [82] showed that -OH groups on the ring structure and adjacent to the N atom clearly increased the conversion to NH3 at the expense of HCN. They speculated that the reaction HCN + OH -> HNCO -^ NH. -> NH3 might become important. However, this speculation is yet to be proved or disproved, because there were no data reported on a pair of model compounds differing only by an -OH group. Nevertheless, it was believed [82] that 0-containing functional groups tended to destabilise the heterocyclic structures, affecting their pyrolysis behaviour. The discussion presented above would suggest that heterogeneous reactions involving a solid would give a different product distribution from what would be seen from the homogenous gas phase reactions, even for the same substrates of N-containing model compounds. This indicates that the kinetic models developed for gas-phase reactions could hardly be applied directly to heterogeneous reactions involving a solid such as the reaction systems inside a pyrolysing coal/char particle. Pyrolysis of polymers of known nitrogen functionalities would provide information relevant to the reactions taking place in the pyrolysing solid coal/char particles. Both HCN and NH3 were observed in the pyrolysis of poly(4-vinylpyridine) [90] although little char was finally left at lOOO'^C, presumably due to the low cross-linking density in the polymer. Quatemarising the polymer by treatment with HCl appeared to decrease the yield of NH3 below the detection limit [90]. Both NH3 and HCN were observed from the pyrolysis at >200°C of polyacrylonitrile [-CH2CH(CN)-]n in which only nitrile groups were present [85,93]. It should be bom in mind that the pyrolysis of light nitriles (e.g. acetonitrile [57] and benzonitrile [81]) did not produce appreciable amounts of NH3 although N-H bonds were noted in the polymeric residue from the pyrolysis of benzonitrile [81].
Cocd-N and Cool-S
303
Similarly, in addition to NH3 (particularly at temperatures lower than 600°C [85]), some HCN was also observed [85,85] from the pyrolysis, particularly at high temperatures (800-1000°C) [85], of polyamides (e.g. nylon 6,6 [NH(CH2)6NHCO(CH2)4CO-]n) that only contained-NH groups. The formation of HCN increases with increasing temperature and residence time, while the formation of NH3 is much less temperature dependent during the pyrolysis of polyamide at a fast heating rate [85]. The formation of HCN, sometimes as the main N-containing product (see Table 6.2), from the pyrolysis of simple amines [88,89], amino acids [58,59,82] and proteins [93], has been discussed above. These findings from the pyrolysis of polymers clearly indicate that, when the reaction system involves a solid surface or solid reactant, there is poor, if any at all, correlation between the nitrogen functionalities in the substrate and the yields of N-containing species in the pyrolysis product. Reaction conditions, especially if influencing the interactions between the intermediates, are a very important factor influencing the yields of N-containing products. Reactions on or in the solid encourage the formation of NH3. The lack of a strong correlation between nitrogen functionalities in coal and the yields of NOx and NOx precursors will be discussed in Sections 6.3 and 6.4.
6.3. REACTION PATHWAYS INVOLVING COAL-N PYROLYSIS OF VICTORIAN BROWN COAL
DURING
THE
From the information presented in Section 6.1, the nitrogen in coal mainly exists as N-containing heteroaromatic systems such as pyrrolic nitrogen (N-5), pyridinic nitrogen (N-6), "quaternary nitrogen" (N-Q) and pyridonic nitrogen [N-6(0)] . The main gaseous NOx precursors from the pyrolysis of coal are HCN, NH3 and HNCO. The conversion of the nitrogen in solid coal into these gaseous NOx precursors during pyrolysis is an exceedingly complex process, involving reactions in the gas phase, reactions in the solid phase as well as gas-solid reactions. Conceptually, the conversion of coal-N during pyrolysis can be schematically shown in Figure 6.3. Four main reaction pathways exist for the formation of simple Ncontaining products (HCN, NH3, HNCO and N2). As soon as coal particles are heated up, even at a relatively fast heating rate, the nitrogen in coal is partitioned between volatiles and char. The nitrogen in volatiles may include tar-N, HCN, HNCO, N2 and some light nitriles. Following this initial partition of coal-N, both volatile-N and char-N may be further thermally cracked into simple gaseous NOx precursors, if the volatiles and char are further exposed to elevated temperature. The thermal cracking of volatiles may also form soot (soot-N), which can be further cracked into gaseous N-containing products along with soot-N in a better-ordered (more condensed) soot structure. The conversion of coal-N through each of these major reaction pathways will be discussed below. It must be emphasised that the conceptual classification of these reaction pathways is only for the ease of presentation of literature information and that strong interactions exist among the reaction intermediates involved in these reaction pathways. It will be pointed out that the extent of the interactions strongly depends on the
304
Chapters
HCN, NH3 HNCO, N2, etc
CHAR Figure 6.3 A schematic diagram showing the main reaction pathways for the conversion of coalN during pyrolysis [7].
configuration of the reactor used in a particular study, affecting the observed yields of N-containing products. 6.3.1. Partition of Coal-N into Volatile-N and Char-N As was discussed in detail in Chapter 4, when coal particles are heated up, especially at a fast heating rate, volatiles are rapidly released. During this initial stage of pyrolysis, the nitrogen in coal is also partitioned into volatile-N and char-N. Takagi and co-workers [85] investigated the coal-N distribution during the flash pyrolysis of a Yalloum brown coal sample. The coal particles were heated at about 3000 K s"' in a curie-point pyrolyser with a total pyrolysis time of 5 s. Figure 6.4 shows the partition of coal-N into volatile-N (N2, tar-N, HCN and NH3) and char-N for Yalloum and other coal samples [85]. At about 600-700°C where the release of tar was essentially complete with a tar yield of around 22-25% and a char yield of about 5055%, volatile-N accounted for less than 30 % of coal-N. At this temperature, the yields of NH3 and HCN would be low and likely to account for around 10 % of coal-N [103]. The tar-N would account for about 20 % of coal-N: the yield of tar-N would be close to the yield of tar. Even after complicated oxidative (in H2O2) and hydrogenative (in H2) treatments (see Ref. 85 for details), the modified Yalloum coals still followed similar trends. In the pyrolysis of a Loy Yang brown coal sample in a free-fall pyrolyser, Chen and co-workers [12] also reported that tar-N reached a maximum at 600°C, accounting for 20% of coal-N, in good agreement with the study of Takagi and co-workers [85]. The results for bituminous coals (Figure 6.4) also show that tar-N was an important part of volatile-N, albeit to different extents. The study by Li and co-workers [28,104106] on the pyrolysis of a suite of Australian, German and American brown and
305
Cool-N and Coal-S
bituminous coals in a fluidised-bed reactor also showed that the main form of volatile-N at 600°C was tar-N. Therefore, all these results indicate that tar-N is a major shuttle for the release of coal-N for brown and bituminous coals. Similar conclusions were also reached in other studies [e.g. 107,108]. It should be noted that the observed yield of tarN may vary not only with coal structure/properties but also with the experimental methods to collect tar. The uncertainties in the tar nitrogen content and tar yield, together with the errors in the coal-N determination (see Section 6.1), combine to contribute to uncertainties in the yield of tar-N.
(a) Y L
(d) M l
(b) S B 100
oi 20
600 800 1000 Temperature [C]
600 800 1000 Temperature fC]
600 800 1000 Temperature ['C]
600 800 1000 Temperature [C]
600 800 1000 Temperature ['C|
600 800 1000 Temperature fC]
0 600 800 1000 Temperature ("C] I
I Yield of denltrogenation by oxidation and hydrogenation
^ 1 Ng in the gas phase (N2-N) ^
Tar-N + HCN-N -f NH3-N
Im^ Nitrogen In char determined by elemental ar^alysis (Char-N)
Figure 6.4 Nitrogen distribution from the pyrolysis of brown to bituminous coals in a curie-point pyrolyser. YL = Yalloum (C: 62.0 wt% daf); SB = South Banko (C: 68.5 wt% daf); TH = Taiheiyo (C: 78.7 wt% daf); MI = Miike (C: 79.9 wt% daf); HV = Hunter Valley (C: 83.2 wt% daf); YL-0 and YL-H are oxidised and hydrogenated Yalloum coal samples. Reprinted with permission from Ref 85. Copyright 1999 American Chemical Society.
306
Chapter 6
The formation and release of tar during pyrolysis can go to completion rather rapidly. For example, when Loy Yang brown coal was heated up in a wire-mesh reactor at 1000 K s'\ the release of tar (as seen with the yield of tar) was essentially complete as soon as the coal particles were heated to about 600°C at atmospheric pressure [109-112]. At this early stage of coal pyrolysis, N-containing structures in coal become part of tar fragments and are released as a part of tar. Therefore, tar precursors have only experienced very short time inside the hot coal particles and would not have undergone extensive decomposition, although some thermal decomposition must have happened to the N-containing structure. The "striking" similarities in the chemical composition and spectral (e.g. ^^C NMR and IR) features between coal and its tar have been the main justification for many past studies (e.g. 107) to infer the (nitrogen) structural features of coal from those of tar, especially produced at fast heating rates; such information on the (nitrogen) structure in coal is clearly of limited reliablity (see Section 6.1). The relative enrichment of nitrogen in Yalloum char and chars from other brown coals [28,104,105] at low temperature is mainly due to the release of inorganic gases such as CO2 [85] (also see Chapter 4). The data in Figure 6.4 also show that tar-N can be a much more important shuttle for the release of coal-N from some higher rank coals (e.g. MI with C = 79.9 %) than from Yalloum brown coal, in agreement with other studies [28,104,105]. As has been discussed in Chapter 4, heating rate greatly affects the yields of tar and char during pyrolysis. The heating rate was also found to affect the partition of coal-N into volatile-N and char-N during the initial pyrolysis stage. Cai and co-workers [113] also showed using a wire-mesh reactor that, for the Illinois bituminous coal they studied, the release of coal-N was broadly proportional to the tar yield; both increased with increasing heating rate. The data presented elsewhere [12,28,104,105] (and in Figure 6.4) showed that increasing temperature beyond 600°C led to decreases in the yields of char-N and tar-N and corresponding increases in the yields of gaseous N-containing products such as HCN and NH3. This is a result of thermal cracking of volatile-N (tar-N) and char-N and will be discussed in detail below. Before proceeding to the discussion of thermal cracking of tar-N and char-N, however, it should be pointed out that HCN, NH3, HNCO and nitriles may also form during the initial stage of pyrolysis (i.e. primary pyrolysis) as a result of rupture of (less stable) N-heterocyclic rings in coal, taking place concurrently with the release of tar-N. The yields of these species would likely increase with increasing temperature. However, an unambiguous distinction of the yields of HCN, NH3, HNCO and nitriles during the primary pyrolysis from their yields during the thermal cracking of tar and char would be very difficult. In particular, the nascent char could be instantly cracked in many reactors to produce these species. In this case, the experimental distinction of the yields of these species during primary pyrolysis and char cracking seems to be almost impossible. A wire-mesh reactor (see Chapter 4) may be used to minimise/control the extents of thermal cracking of tar-N and char-N, although the quantification of light species such as HCN and NH3 seems to be an almost impossible task due to the use of small quantities of coal.
Cocd-N and Coal-S
307
To absorption bottles for the collection of HCN or NH3. Tar residue can also be collected separately.
Supplementary
Fluidising gas
Figure 6.5 A schematic diagram showing a two-stage fluidised-bed/tubular reactor used for studying the thermal cracking of volatiles in isolation from the corresponding char (based on Ref 125).
6.3.2. Conversion of Volatile-N during Thermal Cracking 6.3.2.1. Formation of HCN, HNCO and NHs from Volatile-N It has long been suggested and discussed in various reviews and modelling attempts [e.g. 1,3,18,26,28,60,87,103,114-123] that the thermal cracking of volatiles is an important source of gaseous N-containing products such as HCN. However, the formation of HCN and NH3 from the thermal cracking of volatiles alone has been difficult to quantify because the observed HCN and NH3 yields often contained the contribution from the thermal cracking of char-N as well as the interactions between volatiles and char in the reactors used. Li and co-workers [105,106], Ledesma and co-workers [124] and Xie and co-workers [125] studied the formation of HCN and NH3 from the reactions of volatiles in isolation from the reactions of and with the char using a two-stage reactor. Hayashi and coworkers [126] also studied the thermal cracking of volatiles (see Chapter 4). The
308
Chapter 6 20 15
(A)
O • A
10
AC-A, Tar AC-B, Tar CC-A, Tar
5
—+—
0 15 h (B) O AC-A, HCN 10 A CC-A, HCN • CC-B, HCN 5 0 15
I (C)
10
• V
AC-B, HCN AC-C, HCN
5 0 600
700
800
900
1000
Temperature, **C
Figure 6.6 The yields of HCN and tar (CHCl3/CH30H-soluble) from the thermal cracking of volatiles as a function of temperature in the tubular reactor (Figure 6.5) [125]. The first-stage fluidised-bed/fixed-bed reactor was set at 600°C to generate volatiles in situ. The HCN yields in the figure include those fi-om the pyrolysis of the corresponding coal coals in the one-stage fluidised-bed/fixed-bed reactor (this would be about 5-6 % of coal-N for Loy Yang coal based on the experiments in a similar reactor [103]. Codes of coal samples: AC-A: Loy Yang; AC-B: an Australian coal (C= 82.2 wt% daf); AC-C: an Australian coal (C = 84.3 wt% daf); CC-A: a Chinese coal (C = 82.3 wt% daf) and CC-B (C = 90.0 wt% daf).
methodologies in these studies [105,106,124,126] are similar and the specially designed two-stage fluidised-bed/tubular reactor made of quartz used by Xie and co-workers [125] to study the formation of HCN and NH3 from the thermal cracking of volatiles for Loy Yang brown coal is shown in Figure 6.5. Briefly, a tubular quartz reactor was placed directly in tandem with a fluidised-bed reactor in which a frit was installed in the free board. While the volatiles generated in the first-stage fluidised-bed reactor passed through this frit and entered the second-stage tubular reactor, the char particles were retained by this frit. Both the first-stage fluidised-bed reactor and the separating frit between the two stages were heated with heating tapes to the same temperature of 600°C. At this temperature, the majority of volatiles would be released during the pyrolysis of coals in a fluidised-bed reactor [105,124,127]. The temperature of the second-stage tubular reactor was varied between 600 and 1000°C. Therefore, the thermal cracking reactions of volatiles in the second-stage tubular reactor took place in isolation from those of the corresponding char. There was also a port between the first-
309
Coal-N and Coal-S I
60
^
HCN
•
40 T3
•
20
c o o
T •
t1 •
•
NH,
9
•
6
1
1
4i
4 2 0
# • #
—
# i • , , i... 600
700
4
»
800
1
, 1000
a
-
•
•
ft
^
«
60
(0
D
40
^
20
E 3
f
900
Temperature (°C)
i 80 >
•
1
1
4-«
1
HNCO
9 ..<>-
Z 100
S -
T • f
6
••_;
f
3 0 8
l£.\J
-
•
0 12
o c o '
t
.
• D
•
tar HCN NHg
o
HNCO
600
700
n
-
•
• 1
800
900 1000
Temperature (°C)
Figure 6.7 Conversion of primary tar-N to HCN, NH3 and HNCO from the pyrolysis of an Australian bituminous coal (C = 83.2 wt% daf) in a two-stage fluidised-bed/tubular reactor as a function of temperature. The volatiles were generated in situ in the first-stage reactor at 600°C and the yields of HCN, NH3 and HNCO shown above do not include thosefromthe pyrolysis in the first-stage reactor. Reprinted with permission from Ref 124. Copyright 1998 American Chemical Society.
stage and the second-stage (Figure 6.5) for adding a stream of supplementary gas to adjust the residence time of the volatiles in the second-stage tubular reactor without changing the pyrolysis conditions in the first-stage fluidised-bed reactor. The supplementary gas can also be a reactive gas, allowing the study of the gasification reactions of the volatiles without changing the pyrolysis conditions in the first-stage fluidised-bed reactor and in isolation of reactions with char. Figure 6.6 shows the yields of tar (CHCl3/CH30H-soluble) and HCN during the thermal cracking of volatiles in the reactor shown in Figure 6.5. Increasing temperature for the thermal cracking of volatiles resulted in decreases in tar yield and increases in HCN yield, particularly at > 800°C [125]. These results are in very good agreement with the earlier study on an Australian bituminous coal reported by Li and co-workers [105] and Ledesma and co-workers [124], as is shown in Figure 6.7: more than 60 % of tar-N was converted into HCN at 1000°C. HCN is clearly the most important Ncontaining product from the thermal cracking of volatile-N (mainly tar-N). The pyrolysis of Loy Yang brown coal at 600°C gave much higher HCN yield (about 5 % of coal-N) than the higher rank coals (Figure 6.6) [125], indicating the high reactivity of the brown coal compared with the higher rank coals. However, the temperature dependence of HCN formation from the actual thermal cracking of volatiles of Loy Yang brown coal (Figure 6.6) [125] was very similar to those of the higher rank coals in
310
Chapter 6
Figures 6.6 and 6.7 [124,125]. The similarities in the thermal cracking of tar-N from different bituminous coals were also observed by Zhang and Fletcher [121]. Using the same reactor shown in Figure 6.5, Tian and co-workers [128] also observed the yield of HCN from the thermal cracking of sewage sludge volatiles to increase rapidly with temperature at > 800°C. Xie and co-workers [125] and Tian and co-workers [128] did not observe NH3 to be a significant N-containing product from the thermal cracking of volatiles from sewage sludge, brown coal and bituminous coals. The study by Ledesma and co-workers [105,124] showed that NH3 only accounted for less than 10% of tar-N and that the formation of NH3 might have been from the interactions of N-containing species with donatable H on the soot surface. This will be discussed later in this chapter. However, it must be pointed out that the experimental set up used in the study by Li and co-workers [105,106] and Ledesma and co-workers [124] might have also allowed fine char particles being carried to the bottom of the tubular reactor and thus being heated to temperatures higher than 600°C. This means that a portion of NH3 observed during the thermal cracking of volatiles might have actually come from the thermal cracking of these fine char particles. This problem can be avoided with the reactor set up used in the later study by Xie and co-workers (Figure 6.5) [125]. The study reported by Li and co-workers [105,106] and Ledesma and co-workers [124] also observed some HNCO (< 7% of tar-N) (Figure 6.7), a product observed in the pyrolysis of (bituminous) coals [129]. The study by Xie and co-workers [125] did not specially determine the yield of HNCO, which, even if produced, would have been seen as NH3 as the product stream passed through an acidic solution to absorb NH3 [7], particularly considering that the hydrolysis of HNCO occurs rather rapidly in acidic solution to form N H / [130]. Conversion of volatile-N (tar-N) to soot-N is an important fate of tar-N [103,116, 121,124,125,131,132]. Ledesma and co-workers [124] found that around 40 % of tar-C fed into the second stage reactor could end up in soot at 1000°C, at which more than 85% of tar-N was accounted for by HCN (the main product), NH3, HNCO and tar (Figure 6.7). Therefore, the soot at high temperatures is N-deficient. Xie and co-workers [125] found that, particularly for Loy Yang brown coal, the formation of soot at 700°C could "lock up" some volatile-N temporarily in a form slightly more stable than tar-N, reducing the yield of HCN. However, at 900°C, the formation and subsequent immediate cracking of sooty materials may be a non-negligible route of HCN formation for the thermal cracking of volatile-N. 6,3.2.2. Comparison between Pyrolysis of N-Containing Model Compounds and Thermal Cracking of Volatile-N An XPS study by Li and co-workers [28] on nitrogen functionalities in tar samples from the pyrolysis of a German bituminous coal showed that the tar produced at 600°C contained mainly pyrrolic (>60%) and pyridinic nitrogen with their relative proportions to depend upon molecular mass of the tar. A study by Kelemen and co-workers [35] also showed the dominance of pyrrolic nitrogen in tars from 8 coals followed by pyridinic
311
Coal-N and Cocd-S I.U
0.8
\
\
c q CD
M ' M •
0.6
M—
_C
"c 'cD
E
0.4
CD
0.2
n n
600
1
700
1
800
1
\V
900
^
1 1000
^
1100
Temperature, °C Figure 6.8 Thermal decomposition of pyrrole and pyridine at residence time of 0.5 s predicted by extrapolating the kinetic data obtained at much shorter residence time [73,84]. Curve 1, pyridine, kinetic datafi*omRef 84; curve 2, pyrrole, kinetic data from Ref 73. ReproducedfromRef 101.
nitrogen, although minor amounts (<10 %) of tar-N were believed to be in the forms of amino and quaternary nitrogen. The similarities among the thermal cracking characteristics of volatile-N from different coals (see discussion above) might well mean that tar-N from other coals would also be dominated by the pyrrolic and pyridinic nitrogen functionalities. Other structures such as pyridones may also be present in coal and tar [87], which may be hard to distinguish from pyrrolic and pyridinic nitrogen by XPS. Therefore, it is now informative to compare the pyrolysis of N-containing model compounds in the gas phase (see Section 6.2) with the thermal cracking of volatile-N in the gas phase discussed above. There are good qualitative agreements between the pyrolysis of model compounds and that of coal volatiles, as is summarised by Li and Tan [101] and Li and co-workers [28,106]. The first aspect of this agreement is about the onset temperature for the Ncontaining heteroaromatic rings to decompose. Figure 6.8 shows the decomposition of pyrrole and pyridine at 0.5 s residence time predicted by extrapolating the kinetic data obtained for the model compounds at much shorter residence time [73,84]. According to the data in Figure 6.8, under conditions similar to those in a fluidised-bed reactor (residence time of volatiles being around 0.5 s), the onset temperature for the decomposition of the model compounds would be around 750 - 800°C. The calculation [28,106] also showed that the curve for 2-picoline would lie in between the curves for pyrrole and pyridine in Figure 6.8. This temperature of 750-800°C was indeed seen to
312
Chapter 6
be the onset temperature for the N-containing heteroaromatic ring systems in the coal pyrolysis volatiles to decompose. Detectable amounts of HCN and HNCO were only observed around this temperature from the thermal cracking of volatiles (Figures 6.6 and 6.7). Further evidence for the onset temperature of tar-N thermal cracking may be seen from the XPS study [28] of tars produced from the pyrolysis of a bituminous coal in a fluidised-bed reactor with a volatile residence time at the magnitude of 0.5 s. The XPS results [28] showed the nitrile groups, neither present in the raw coal nor in the tars produced at temperatures lower than 700°C, began to appear in the tars when the temperature was raised to about 700 - 800°C. The appearance of the nitrile groups is an indication of the rupture of the N-containing heteroaromatic rings. This onset temperature of 700 - 800°C is in good agreement with the prediction from the Ncontaining model compounds (Figure 6.8). The second aspect of the qualitative agreement [101] between the thermal decomposition of model compounds and that of coal pyrolysis volatiles is the observation of nitriles as the intermediates. As was discussed in Section 6.2, the pyrolysis of model compounds, especially of pyrrolic and pyridinic nitrogen functionalities or involving N-heterocyclic structures as intermediates during pyrolysis (e.g. amino acids), all showed the nitriles to be the intermediates in the conversion to HCN, particularly without the formation of soot or soot precursors. The XPS studies of the tar samples [28] also showed the concentration of nitriles to increase with temperature, confirming the findings on the pyrolysis of model compounds. In agreement with the study of gas-phase pyrolysis of N-containing model compounds (see Section 6.2), no convincing evidence was found for the presence of amines in the tar samples studied [28]. The nitriles found in tar in the XPS study [28] would be in tar molecules of relatively high molecular masses as light species would have been lost in the preparation of tar sample or in the high vacuum chamber inside the XPS spectrometer. Using a high resolution gas chromatograph with nitrogen-specific (NPD) and flame ionisation (FID) detectors. Nelson and co-workers [115] found nitriles (both aromatic and aliphatic) in the light tar fractions from the pyrolysis of Yalloum brown coal (and sub-bituminous and bituminous coals) in a fluidised-bed reactor at 700 and 800°C. The yields of one-ring and two-ring nitrogen containing aromatics from the pyrolysis of Yalloum brown coal in a fluidised-bed reactor [115] are shown in Figure 6.9. Bartle and co-workers [133] also found nitriles during the GC analysis of volatiles from the pyrolysis of a UK bituminous coal, with HCN and CH3CN being particularly prominent. Clearly, significant nitriles (e.g. benzonitrile) were already present in the volatiles at 700°C. Based on the data in Figure 6.9, Nelson and co-workers [115] concluded that the reactivities of pyrrole type structures in volatiles were greater than those of pyridine type structures (e.g. pyrrole disappeared more rapidly than pyridine), in agreement with model compound studies. However, this conclusion should be treated with caution because the GC-FPD technique could not detect all N-containing structures in tar due to the difficulties for these polar and/or heavy components to pass through a GC column. In other words, not all N-containing species in tars, particularly in the tars produced at low temperatures, could be analysed with GC. Furthermore, pyrrole and pyridine
313
Coal'N and Coal-S
120r-
n
700 800 900 Temperature (^C)
1000
4UH
600
700
800
900
1000
Temperature {°C)
Figure 6.9 Yields of one-ring and two-ring N-containing heteroaromatics in CH2Cl2-soluble tar (quantified using a GC-NPD system)fromthe pyrolysis of Yalloum brown coal in a fluidised-bed reactor [115].
(different from pyrrolic and pyridinic nitrogen structures in coal and tar) do not constitute any significant fractions of coal-N. Therefore, the increases or decreases in the yields of these species as observed by the GC-NPD technique are a reflection of the net contribution from the formation and destruction of these simple compounds during pyrolysis. In fact, the XPS study [28] of a similar tar showed the absence of pyridinic nitrogen in tar once the nitriles appeared at 800°C or higher, with pyrrolic and nitrile nitrogen to be the only forms of nitrogen in tar. These results indicate that, unlike the relative thermal stability of pyrrole and pyridine that pyrrole is less stable than pyridine (see references in Section 6.2), the pyrrolic form is more stable than pyridinic form in the high temperature tar. If the pyrrolic nitrogen is retained in the tar without being decomposed, as indicated by the XPS study [28], less pyrrolic nitrogen would appear as small molecules for detection using the GC-NPD technique. Nevertheless, regardless of the relative stability of pyrrolic and pyridinic nitrogen, it is fair to conclude that nitriles are the most important intermediates during the conversion of pyrrolic and pyridinic heteroaromatic ring systems (both in the model compounds and in the coal pyrolysis volatiles) into HCN in the gas phase. Following the speculation by Ledesma and co-workers [124] that structures similar to 2-pyridone may exist in tar and the finding in recent studies (see Section 6.1) that pyridone may be an important nitrogen functionality in coal, Hansson and co-workers
314
Chapter 6
[87] studied the pyrolysis of 2-pyridone and found that HCN was the most important Ncontaining product. They also attempt to explain/model the formation of HNCO during the pyrolysis of 2-pyridone and coal volatiles. The third aspect of the qualitative agreement between the thermal decomposition of model compounds and that of coal pyrolysis volatiles is about the main final Ncontaining products. The thermal cracking of the volatiles (Figures 6.6 and 6.7), with a gas phase concentration of nitrogen-containing species at the order of the magnitude of 10'^ g-N L'\ showed that the main final product containing nitrogen in the gas phase was HCN. The formation of HCN fi-om the cracking of the volatiles started at around 700-800°C, in agreement with the prediction (Figure 6.8) from the data on the pyrolysis of model compounds. Only a very small amount of NH3, probably concurrently with the formation of soot [124] or as an artefact of fine char particle cracking (see above), was producedfi-omthe thermal cracking of volatiles. The agreement between the pyrolysis of N-containing model compounds and the thermal cracking of volatile-N should only be considered as qualitative. Ledesma and co-workers [ 124] found that the apparent activation energy for the conversion of tar-N to HCN (the main fate of tar-N) was 140 ± 15 kJ mol'^ much lower than the activation energy for the overall disappearance of model compounds, e.g. 309.7 kJ mol" for pyrrole [73]. It may be envisaged [28,106] that the concentration of radicals, e.g. H, CH3 and so on, originating fi-om the thermal decomposition of volafiles, is much higher during the thermal decomposition of volatiles than during that of the model compounds. These radicals can take part in the reactions to initiate the activation and breakdown of N-heterocyclic structures, e.g. through hydrogen abstraction reactions. The presence of oxygen-containing radicals, due to the oxygen-containing functional groups in the volatile molecules, may play an even more important role in altering the reaction schemes for the decomposition of N-containing structures. The size of a Nheteroaromatic ring system and its substitutional groups are likely to greatly affect the ring system's thermal stability [28,101,104-106]. Therefore, while studies on Ncontaining model compounds (see Section 6.2) provide vital information for the understanding of thermal decomposition of volatile-N during coal pyrolysis, caution must be taken in applying directly the results from model compounds study to the kinetics of coal-N reactions, even in the gas phase. 6.3.3. Conversion of Char-N during Thermal Cracking While tar-N is an important source of HCN, close examination of the data on the yields of HCN and NH3 would reveal that there is simply not enough tar-N to account for the observed yields of HCN and NH3 during coal pyrolysis [105], particularly at high temperatures. Thermal cracking of nascent char is an important, often more important than tar-N, source of HCN and NH3. With increasing temperature, the nitrogen retention in char always decreases [e.g. 12,104-106,113,116,134-137]. The main products from the thermal cracking of char-N would be HCN and NH3 as well as N2, as will be discussed in detail below.
315
Coal-N and Coal-S 6.3.3.1. Formation ofHCNandNH^from Char-N
Tan and Li [7] used a specially designed drop-tube/fixed-bed reactor, made of quartz, to study the importance of char cracking to the formation of HCN and NH3. It was a modified version of the previous reactor used by the same group [138] to study NH3 formation from a Loy Yang brown coal. The reactor [7,138] differed from a conventional drop-tube reactor in that a quartz frit was installed in the isothermal zone in the reactor. As coal particles entrained in a feeder were fed continuously into the reactor, the char particles were retained by the frit in the reactor while the volatiles would pass through the frit and leave the reactor as is in a normal drop-tube reactor. Therefore, the reactor has features of a drop-tube reactor (in terms of initial pyrolysis conditions such as high particle heating rates of no lower than 10^ K s"^) and features of a fixed-bed reactor (in terms of retention of char particles as a thin fixed bed in the reactor). The reactor could also be used as a fixed-bed reactor where coal particles loaded onto the frit in the reactor at room temperature could be heated up at a preset heating rate. The reactor also features strong interactions between char and volatiles when operated in the fast heating rate mode. The formation of HCN and NH3 was quantified both during feeding ("feeding" period) and after the feeding of coal had stopped ("not-feeding" periods) [7,138]. Even after the feeding of coal had stopped and thus no volatiles were generated, the formation of NH3 was observed to continue for a long time (up to 2 hours or longer), although at relatively slow rates, indicating that very significant amounts of NH3 were formed from the thermal cracking of char [7,138]. Table 6.3 shows the formation of NH3 from Loy Yang during the "feeding" and "not-feeding" periods for a wide range of temperature from 600 to 1000°C. Very significant proportions of NH3 were formed in the "not-feeding" periods. The proportion of NH3 formed in the "not-feeding" periods decreased with increasing temperature. This is because the NH3 formation rates increased with increasing temperature and thus the NH3 formation completed to larger
Table 6.3 NH3 formation observed during the pyrolysis of Loy Yang brown coal in the "feeding" and "not-feeding" periods as a function of temperature in a drop-tube/fixed-bed reactor [7]. Nominal coal feed rate: 90-110 mg min'. Total gas flow rate: 1.0 L min'* (measured under ambient conditions). NH3 observed in the "feeding" period, % of the total NH3 in the whole experiment
NH3 observed in all the "notfeeding" period, % of the total NH3 in the whole experiment
600
15
85
700
20
80
800
65
35
900
50
50
1000
75
25
Temperature, °C
316
Chapter 6
extents in the "feeding" period at higher temperatures than at lower temperatures. The formation of NH3 from the pyrolysis of a bagasse, a high volatile bituminous coal and a low-volatile bituminous coal was also observed to last for an extended period [7]. The formation of HCN from the pyrolysis of Loy Yang brown coal was also seen to last for a long time (e.g. up to an hour, depending on temperature) at very slow rates [7]. At a feeding rate of 185 mg min'^ to feed 0.5 g of coal, about 25 % of the total HCN yield observed at 800°C was formed after the feeding of coal had stopped. Therefore, the formation of HCN can go to completion much more rapidly than the formation of NH3 from the thermal cracking of char [7,101]. As Tan and Li [7] pointed out, HCN and NH3 formation observed in the "notfeeding" periods only represented "tails" of HCN and NH3 formation from the thermal cracking of char. Significant proportions of HCN and NH3 observed in the "feeding" period must have also come from the thermal cracking of nascent char. In other words, the char formed during the early stages in the "feeding" period was continuously cracked while coal particles were still being fed into the reactor at the later stage of the "feeding" period. These observations indicate that, unlike the formation and release of relatively heavy tar components, the formation of HCN and NH3 in the pyrolysing coal/char particles does not go to completion rapidly. As will be discussed later, the formation of HCN and NH3 from the thermal cracking of char is largely controlled by the slow rate of (H) radical generation in the char. The slow formation of HCN and NH3 from the thermal cracking of char means that the proportions of HCN and NH3 experimentally observed in the "feeding" and "not-feeding" periods would vary with the feeding rate of coal [7,103]. The slow formation of HCN and NH3 from char-N would also mean that the experimental values of HCN and NH3 would be related to the residence time of char and therefore related to the configuration of the reactor used in the experiment [7]. The differences among the data from various reactors will be discussed in Section 6.3.5. Tsubouchi and co-workers [139] studied the importance of holding time on the distribution of coal-N during pyrolysis using a drop-tube reactor with a graphite filter installed inside the tube to retain the char particles in the reactor (similar to the droptube/fixed-bed reactor used by Tan and Li [7]). At a rapid particle heating rate (10"^ - 10^ K s'), Tsubouchi and co-workers [139] showed that increasing holding time of char from 0 to 120 s at 1300°C resulted in increases ( 4 - 6 %) in the yield of NH3 (and decreases in the tar-N of about 2 %) from Chinese and Indonesian low rank coals (C = 68.7 wt% daf for both coals). Only small (around ±1 % of coal-N) corresponding changes in the yield of HCN yield were observed with increasing holding time [139], confirming that HCN formation is a much more rapid process than NH3 formation [7,101], particularly at this high temperature of 1300°C. The formation of HCN and NH3 from the thermal cracking of char was also observed during the pyrolysis of Loy Yang brown coal at a slow heating rate [103], as is shown in Figure 6.10. The formation of HCN and NH3 (H2S to be discussed in Section 6.5) was collected into temperature-resolved fractions as the coal particles were heated up at about 6.7 Kmin'V As was discussed in Chapter 4, the release of tar from Victorian brown coal would be complete before the coal particles were heated up to 600°C.
317
Coal-N and Coal-S c
"F 20 0) CM
X I .
o CO
X
z
z o X
D) IL (0
11 —
1—1
1 —
16 12 [ HCN 8 4 0 200 400
600
800
r-
12 10 8 6 4 2
1
8 6 4 2 J
L_
CD CD
> <
o CD O
O
CO
n 0
o
fS 10 F 1o
I
CD O
200 400 600 800 1000
c
M—
CO
1000 HA
10 8 6 : NH3 4 2 0 •
rn i-
1
T"~
1
•
1
1
1
__
50 8 40 H 30 4 20 2 10 0 0 200 400 600 800 1001D Temperature, °C
_CD -1
E 0 0
<
Figure 6.10 Formation of NH3, HCN and H2Sfromthe pyrolysis of Loy Yang brown coal as a function of temperature in a fixed-bed reactor [103]. Heating rate: 6.7 K min''.
Clearly, the thermal cracking of char, at least at > 600°C, has resulted in the formation of both HCN and NH3 after the completion of tar release. Hayashi and co-workers [140,141] studied the formation of HCN and NH3 during the pyrolysis ofYalloum brown coal at a slow heating rate (10 Kmin') together with the formation of N2. The formation of HCN from Yalloum brown coal showed two peaks at around 400 and 550°C. In comparison, the formation of HCN from Illinois bituminous coal showed one main peak at around 750°C, indicating the differences in the thermal stabilities of N-containing structures in chars from coals of different rank. Bassilakis and co-workers [142] observed that the temperature of maximum HCN and NH3 formation in a TG-FTIR increased with increasing rank [142]. Data from many other studies [e.g. 9,10,13,143] on the pyrolysis of Victorian brown coal also showed evidence for the formation of HCN and NH3 from char-N during pyrolysis at slow heating rates. 6.3.3.2. Formation of N2 from Char-N Hayashi and co-workers [140,141] detected the formation of N2. The formation of N2 started at around 600°C and occurred over a wide range of temperature up to the highest
318
Chapter 6
temperature (]000°C) investigated. Loading iron catalysts, as ferrocene [Fe(C5H5)2] or as ferric acetate [Fe(OH)(CH3COO)2], into the Yalloum coal by the solvent swelling method led to decreases in the yields of HCN and, more importantly, char-N, accompanied by drastic increases in the yield of N2 [141] from 30 % to 68 %. The iron catalysts were also effective for catalysing the N2 formation during the pyrolysis of Illinois #6 and Wyoming bituminous coals. Ohtsuka and co-workers [8-11,13,20,139,143,144-153] have carried out detailed investigation of N2 formation from the pyrolysis of Victorian brown coal and other coals over the past several years. When a Loy Yang brown coal sample was loaded with iron catalyst (precipitated into the coal as FeOOH) and pyrolysed by heating at 600 - 700 K min"^ in a fluidised-bed reactor, the iron catalyst was found to selectively favour the formation of N2 at the expenses of char-N and other forms of N-containing products (e.g. tar-N) [8,145]. Whilst the yield of N2 from the raw Loy Yang even at 900°C was only < 3 % of coal-N, loading 3 wt% of Fe into the coal resulted in drastic increases in the yield of N2, increasing with temperature and reaching nearly 60 % of coal-N at 900°C [8,145]. Increasing iron loading from 0.7 to 7.4 wt% did not make much difference in terms of N2 formation at 900°C [8,145]. The iron catalyst was found to exist as nanoparticles (20 - 50 nm, depending upon initial iron loading level) in the 900°C char [8]. Changing pyrolysis atmosphere from He to H2 did not result in any difference in the production of N2 at the highest temperature investigated (900°C). This was believed [8,145] to be due to the fact that the reducing environment during pyrolysis was sufficient to reduce the iron catalyst into active forms of a-Fe and Fe3C [8,145]. The finding by Ohtsuka and co-workers [8,145] with inorganic iron catalyst precursors precipitated into brown coal agrees well with the observation made by Hayashi and co-workers [140,141] with the organic iron catalyst precursors loaded into brown coal by the solvent swelling method. It might appear that the iron catalyst precursors in both studies were all well dispersed in coal, albeit in different forms, and might have eventually turned into the same active forms in the char. Much smaller effects of Fe on the formation of N2 (and NH3/HCN) were observed during the pyrolysis of a sub-bituminous Canadian coal [146] than for the Victorian brown coal. This demonstrates that the formation of N2 may be related to the stability of N-containing structures. Indeed, the nitrogen in the char prepared at 1000°C was harder to remove [146] than carbon during gasification in CO2. In a subsequent study using the same fiuidised-bed reactor as that in the earlier studies [8,145], Mori and co-workers [9] confirmed that the iron catalyst precipitated onto the Loy Yang brown coal increased the yield of N2, at the expense of NH3, HCN, oil-N, tar-N and char-N, as is shown in Figure 6.11. This is in general agreement with the earlier studies [8,145,146] and the study by Hayashi and co-workers [141] mentioned above. In the absence of added iron catalyst, NH3 was the dominant Ncontaining gas-phase product, in agreement with other studies (see Section 6.3.3.1). In the presence of iron catalyst, N2 became the dominant N-containing gas-phase product (Figure 6.11). Increasing temperature from 900 to 1000°C led to little changes in the distribution of coal-N, indicating that N in the char at 900°C was rather stable (Figure 6.11).
319
Coal-N and Cool-S 1000'C
900'C 100
75 n .2
50
T3 C
NH3
fedHCN
o
Oil Tar
25
^y Char 0
0.73 2,8
0
0.73
Iron loading, wt*^o
Figure 6.11 Effects of iron catalyst loading in Loy Yang brown coal on the nitrogen distribution during pyrolysis by heating at 600-700 Kmin"' in a fluidised-bed reactor. Reprinted with permissionfromRef 9. Copyright 1996 American Chemical Society.
There would not be enough volatile-N formed during primary pyrolysis (around 20 25% of coal-N, see Section 6.3.1) to account for the formation of N2 shown in Figure 6.11 in the presence of iron catalyst. Although the formation of N2 overlapped with that of HCN and NH3 during the temperature-programmed pyrolysis, N2 still formed [150] at the end of HCN, tar-N and NH3 release. Therefore, in addition to the possible (minor) conversion of N-containing gas-phase products into N2, char-N has clearly been converted into N2 [150]. The latter route was particularly important for brown coal compared with a bituminous coal [143], mainly due to the better dispersion of iron catalysts in the brown coal char (20 - 50 nm) than in the bituminous coal char (50 - 100 nm). Formation and subsequent decomposition of iron nitrides is believed to be a possible route of N2 formation in the solid phase [143]. A study [154] on the ironcatalysed formation of N2 during the carbonisation of polyacrylonitrile detected the formation of iron nitrides in the temperature range of 500 - 600°C, which decomposed to form N2. XPS study of the char samples from Loy Yang brown coal indicated [13,20] that the iron catalysed the preferential removal of pyrrolic nitrogen to that of pyridinic nitrogen. This conclusion does not agree with that of Zhu and co-workers [154] who found pyridinic nitrogen to be preferentially released as N2. The changes in the nitrogen functionalities in char during pyrolysis will be further discussed in Section 6.3.3.3. The role of iron nanoparticles in char formed during pyrolysis in the formation of N2 was further confirmed by loading iron oxide nanoparticles (3 nm in size) into the coal [9,150,143]. Studies [147,150] on other coals confirmed the possibihty of substantial formation of
320
Chapter 6
65
70 75 80 Carbon content, wt%(daf)
85
Figure 6.12 Rank dependence of coal-N conversion to HCN, NH3 and N2 for 16 coals pyrolysed in a fixed-bed reactor by heating at 400 K min"' to 1000°C. Reprinted with permissionfi-omRef 10. Copyright 1997 American Chemical Society.
N2 during the pyrolysis of low rank coals. However, for a set of 16 brown (including Loy Yang and Morwell) and bituminous coals pyrolysed in a fixed-bed reactor heated at 400 K min"\ Wu and Ohtsuka [10] did not observe a good correlation between N2 yield and coal rank, particularly for low rank coals, as is shown in Figure 6.12. While the German Rhein brown coal (C = 64.4 wt% daf) gave the highest yield of N2 (61 % of coal-N), the Victorian Loy Yang brown coal (C = 65.9 wt% daf) gave the lowest yield of N2 (13 % of coal-N). The Victorian Morwell brown coal gave a medium N2 yield (about 35 % of coal-N) (Figure 6.12). The presence of catalytic species in low rank coal may be a key factor (see below). At 400 K m i n \ more NH3 was observed than HCN for many coals. The rank dependence of N2 yield seemed to be better for higher rank coals: N2 yield decrease with increasing rank at C > 74 wt% daf, probably due to the lack in these coals of active catalytic species for N2 formation. A strong reverse correlation between char-N and N2 was observed [10,147,150,152], confirming the char-N to be the main source of N2. Alkali and alkaline earth metallic species were also found to affect the conversion of coal-N into HCN, NH3 and N2 [149]. NaOH and KOH increased the rates of HCN formation at >800°C but decreased the rate of HCN formation at abound 600°C. This was believed [149] to be due to the formation of cyanides (NaCN or KCN) at lower temperatures that subsequently decomposed at higher temperatures, although no evidence for cyanides was found from the X-ray diffraction (XRD) of the char samples, probably due to low concentrations and/or high dispersion of the cyanides in char [149].
Coal-N and Coal-S
321
The alkali and alkaline earth metallic species also enhanced the formation of NH3 and the conversion of tar-N (e.g. hydrolysis by H2O) was an important source of the additional NH3 observed [149]. While Na suppressed N2 formation, Ca promoted the N2 formation. At least a small part of N2 formation happened through the secondary reactions of tar-N [149]. Wu and co-workers [155] studied N2 formation from the pyrolysis of Yalloum brown coal by heating at 10 K min'^ to 1000°C with 30 min holding time in a fixed-bed reactor. They found [155] that the formation of N2 from Yalloum brown coal started at about 500°C and showed two maxima at 550°C and 800°C respectively. Acid-washing the coal before pyrolysis almost completely stopped the N2 formation. Loading Fe, Ca, Mg and Na into the acid-washed Yalloum coal followed by the pyrolysis under identical conditions showed that both Fe and Ca were very effective catalysts for the formation of N2 whilst Mg and especially Na had little catalytic effects on the formation of N2. Their results are shown in Figure 6.13. Fe catalyst is effective at low temperatures and Ca catalyst is effective at high temperatures (Figure 6.13). Extemally loading 3 wt% Ca (55 % N2 yield) was found to be not as effective as the inherent Fe (0.38 wt%) and Ca (0.17 wt%) in the raw coal (60 % N2 yield) in catalysing the N2 formation. They believed [155] that solid-phase reactions would be mainly responsible for the catalytic formation 0fN2.
Further studies provided more detailed information about the Ca-catalysed formation of N2. The catalytic effects of Ca on N2 formation were particularly important at high temperatures (e.g. 850 - 1000°C) for low rank coals and a char derived from polyacrylonitrile [151,155]. Clearly, this route of N2 formation mainly took place in the solid particles, due to the mobility of nano-sized CaO particles. The observed CaO particle sizes in char, 45 - 65 nm [151], 30 - 50 nm [153], 25 - 65 nm [152], appeared to increase with increasing Ca loading level. The formation of N2 (also NH3) was seen to be accompanied by the formation of well-crystallised turbostratic carbon, which could be catalysed by Ca [156]. Formation of non-stoichiometric calcium carbides (CaCx or CaCxOy) and interstitial calcium carbon nitrides (CaCxNy) was believed [152] to play important roles for the enhanced formation of N2 and NH3 as a result of ring condensation reactions to form well-ordered carbon. Calcium also catalyses the formation of N2 during the pyrolysis of biodegraded metasequoia leaf [157]. Furthermore, molybdenum has also been found to catalyse the formation of N2 from the pyrolysis of Yalloum brown coal [157]. Pyrolysis of low rank coals demineralised and subsequently loaded with Ca appeared to indicate that the enhancement of N2 formation at low temperature (820°C) for the raw coals arises from synergistic effects of Fe ions and Ca [153]. On the other hand, the enhancement of N2 formation at high temperatures (850 - 1100°C) by Ca exchanged into the demineralised coals, independent of coal type, was due to the intrinsic catalytic activity of Ca [153]. A more recent study [139] investigated the formation of N2 from the pyrolysis of two low rank coals (C - 6SJ wt% daf) from China and Indonesia under fast heating rate (10"^ - 10^ Ks'^) conditions in a drop-tube/fixed-bed reactor. It was with increasing holding time of char that the N2 formation drastically increased, from minor amounts
322
Chapter 6
(around 10 % of coal-N) at 0 s to 35 - 55 % of coal-N at 120 s of holding time at 1300°C. It also confirmed that, even under rapid heating conditions, a linear relationship existed between the formation of well-ordered carbon in char and the formation of N2 [139], concurrently catalysed by the nano-sized Ca catalyst in char. A study by Wu and co-workers [158,159] on the pyrolysis of model coals, claimed to have similar composition as Yalloum brown coal, confirmed that both calcium and iron catalysed the formation of N2, mainly via solid-phase (char) reactions, during the pyrolysis of both pyrrolic and pyridinic model coals in a fixed-bed reactor heated at 2.5 -lOKminV Wu and co-workers [160] recently compared the formation of N2 from Yalloum with that from high rank coals (C = 73.4 to 90.6 wt%). They found that the Ca and Fe catalysts had much less effects on N2 formation from high rank coals than that from Yalloum or other low rank coals. They believed [160] that the high dispersion of catalyst precursors in low rank coals (e.g. as carboxylates) would produce nm-sized and mobile catalysts for N2 formation at high temperature while the catalyst precursors loaded into the high rank coals would produce catalysts of much larger size (and presumably of lower mobility). They also believed [160] that the high yields of N2 from
Temperature. C Time, min Figure 6.13 Effects of Ca, Fe, Na and Mg loaded (3 wt%) into an acid-demineralised Yalloum brown coal (labelled as "YLD" but still containing some Fe, Ca, Mg and Na) on the formation rate of N2 during pyrolysis by heating at 10 K min'' to 1000°C with 30 min holding time in flowing helium. Total yields of N2 are 60 % of coal-N for the Yalloum raw coal, 15 % for the demineralised Yalloum coal and 55 % for the demineralised Yalloum coal loaded with Ca. Reprinted with permissionfromRef. 155. Copyright 2000 American Chemical Society.
Coal-N and Coal-S
323
low rank coals were also related to the high reactivity of their coal-N compared with that of coal-N in high rank coals. At very high temperatures [161-163] approaching pulverised-fuel combustion conditions, a very substantial proportion of coal-N could be released from coal during pyrolysis in a wire-mesh reactor [161,163]. Extended holding time at high temperatures could result in almost total release of coal-N [164]. The retention of coal-N in char increases with increasing rank [163]. More than 80% of coal-N could be released from the pyrolysis of low rank coals (C < 76 wt% daf) when the coal particles were heated at 5000 K s'^ to 1500°C with 2 s holding time in a wire-mesh reactor [163]. For Yalloum brown coal pyrolysed even at 1215°C for 5 s in a pyroprobe, 80% of coal-N was released [18]. Nitrogen released at very high temperature could be mainly in the form of HCN [162]. For example, 65 % of coal-N was converted into HCN during the pyrolysis of Yalloum brown coal at 1215°C in the pyroprobe [18]. The retention of nitrogen in char slightly decreases in the order of vitrinites > inertinites > liptinites [163]. 6.3,3,3, Nitrogen Functionalities in Char The nitrogen functionalities in coal were discussed in Section 6.1. XPS has also been used to study the nitrogen functionalities in various chars [13,18-20,22,30,33,3537,49,165-167] and carbon materials [e.g. 49,50,53-55,168-172]. The findings from these studies are briefly summarised here. Kambara and co-workers [18,165] found that quaternary (N-Q) nitrogen was less stable than pyrrolic (N-5) and pyridinic (N-6) nitrogen forms and completely disappeared from the chars prepared at 1218 K from all 20 coals investigated, covering a wide rank spectrum from brown (Yalloum) to semi-anthracite (C - 88.1 wt%) daf). Kelemen and co-workers [19,35] also observed that N-Q was less stable than N-5 and N-6 even during mild pyrolysis (400°C) and hydropyrolysis (427°C) where the net release of coal-N was minimal. In a study of changes in nitrogen functionality during the pyrolysis of a lignite. Pels and co-workers [49] confirmed the relatively high reactivity of N-Q even during mild pyrolysis (500°C). However, the exact fate of N-Q is unclear. Kambara and co-workers [18] believed that N-Q decomposed to form NH3 (also see discussion later about NH3 formation). On the other hand, Pels and co-workers reported that, under mild pyrolysis conditions (500°C) with little nitrogen release, the proportion of N-6 nitrogen increased at the expenses of N-Q and, to a lesser extent, N-5 (representing both pyrrolic plus pyridonic [49]). Friebel and Kopsel [30] also observed decreases in N-Q and increases in N-6 in chars from the pyrolysis of German brown coals under mild conditions. The decreases in N-Q intensity in the XPS of chars obtained under mild conditions are in agreement with the belief that N-Q in coal is pyridinic nitrogen somehow associated with oxygen functional groups (see Section 6.1) that disappear upon mild pyrolysis. For this reason, Friebel and Kopsel [30] believed that the char produced at low temperature (<600°C) might reflect better the tme nitrogen functionalities in coal. Somewhat opposite findings have been reported regarding the relative stability of N5 and N-6. Kambara and co-workers [18,165] reported that N-5 tended to be more stable
324
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than N-6, although the exact fractions of N-5 and N-6 decomposed varied with coal type. This finding agrees qualitatively with the report on the relative stability of N-5 and N-6 in tars during thermal cracking [28]. Kambara and co-workers [18] showed that about 60% of N-6 in Yalloum brown coal was decomposed at 580°C; correspondingly less than 20 % of N-5 in Yalloum coal was decomposed. This calculation of decomposition fraction of nitrogen forms has of course assumed that there was no interconversion among various nitrogen forms during pyrolysis, which may not always be true based on a study on the pyrolysis of polymeric N-heterocyclic compounds [171]. Kambara and co-workers [18] pointed out that the thermal stability of N-5 nitrogen might not be the same as pyrrole and that the stability of N-6 might not be the same as pyridine. This view has been shared by others [28,101,104-106]. On the other hand, Kelemen and co-workers [35,37] reported that the proportion of N-5 in chars decreased and that of N-6 in char increased with increasing pyrolysis severity (increasing temperature), indicating that N-6 was more preferred structure in char. In contrast to the report by Kambara and co-workers [18] on the disappearance of NQ at 1218 K for only 5 s, under more severe pyrolysis conditions (900°C for 1 hour [49] or heating slowly at 0.23 K s"^ [35,36]), N-Q appeared again as the main form of nitrogen in char. Friebel and Kopsel [30] found the re-appearance of N-Q in the 900°C chars to be coal-specific when 3 German brown coals were pyrolysed by heating at 0.25 K s ' to 900°C with 30 min holding time. However, the N-Q appearing in the severely heat-treated chars does not represent the same nitrogen structure as N-Q in the raw coal: it represents the nitrogen incorporated into the interior ("valley-N" or "centre-N" [49]) of a large N-heteroaromatic ring system or a "graphene" layer [51,53-55] in char or carbon materials (also see Section 6.1). Clearly, holding time at high temperature is an important factor for the nitrogen functionalities in char [22] and the formation of such N-Q could be a relatively slow process unless the temperature is extremely high. Ohtsuka and co-workers [13,20] proposed another explanation for the nature of N-Q in (brown coal) char. In a study on the pyrolysis of Loy Yang brown coal, Ohtsuka and co-workers [13,20] found that, after holding at 900°C for 10 min, both N-5 and N-6 were important nitrogen functionalities in char together with some N-Q (<20 %). Ar ion sputtering of the char samples, removing some surface and thus showing the depth profiles, was found to alter the observed char nitrogen functionalities (although care needs to be taken that the sputtering may also cause preferential release of some element). They believed [20] that inter-conversion between N-6 and N-Q took place probably through formation of surface hydroxyl groups upon char recovery and its decomposition upon Ar sputtering. They also noted that Ar ion bombardment of original coal led to changes in C 1 s binding energies, although the Ar ion bombardment resulted in no significant changes in the binding energies for the pyrolysed char. Caution must be exercised in interpreting the results from XPS studies of nitrogen functionalities in char. Li and co-workers [28] warned that the nitrogen functionalities of char as measured with XPS were likely to be complicated by the fact that XPS is a surface technique. Firstly, when char is exposed to air even at room temperature, the char surface, including its internal pore surface, is likely to be oxidised. Pyrolysing model compounds containing no oxygen in an inert atmosphere has resulted in the
325
Cocd-N and Coal-S
400
600
800
1000
Temperature, °C
Figure 6.14 Yields of NH3, HCN, H2S and H2 from the pyrolysis of Loy Yang brown coal in a drop-tube/fixed-bed reactor at a fast heating rate (> 10'^ K s'^) as a function of temperature [103]. Total gas flow rate: 1.1 Lmin"' (measured at room temperature); coal feeding rates: 90 - 135 mgmin''.
observation of forms of oxidised nitrogen in the char with XPS [49,50,168]. Secondly, if the procedures for the preparation of char samples from the pyrolysis of coal (or model compounds) cannot eliminate the re-condensation of volatiles on the char surface, the observed nitrogen functionalities of "char" would probably reflect more those of the recondensed volatiles than those of the char itself, depending on the extent of volatile recondensation. 6.3.4. Further Discussion on the Formation of HCN and NH3 during Pyrolysis Figure 6.14 shows the yields of HCN, NH3, H2 and H2S from the pyrolysis of Loy Yang brown coal in a drop-tube/fixed-bed reactor (see Figure 6.5) as a function of temperature at a fast heating rate (> 10^ Ks"^) [103]. A detailed discussion on the
326
Chapter 6
formation of H2S will be given in Section 6.5. Immediately clear from the data is that significant amounts of HCN and NH3 were formed at temperatures lower than 700°C. Pyrolysis of Loy Yang brown coal (and many other brown and bituminous coals) at a slow heating rate (see discussion in Sections 6.3.3, e.g. Figure 6.10) also showed the formation of HCN, NH3 and N2 at low temperatures. However, the pyrolysis of model compounds such as pyrrole and pyridine in the gas phase would produce mainly HCN. These results clearly indicate that the findings made with the pyrolysis of simple Ncontaining model compounds in the gas phase cannot be applied directly to the coal-N release from pyrolysing solid coal/char particles. As was discussed in detail in Section 6.2, the formation of HCN and NH3 in a solid phase, such as in a pyrolysing solid coal/char particle or in a polymeric model compound, is likely to be very different from that in the gas phase. It should also be pointed out that, as was discussed in Section 6.1, nitrogen exists in coal mainly in pyrrolic, pyridinic, pyridonic and quaternary forms. Amino nitrogen, even if representing 5% of coal-N in low rank coals [36], is likely to be released as tarN [36]. Therefore, the possible presence of amino nitrogen at low levels (hard to detect with XPS) could not be the main or only source of HCN and NH3 observed from the pyrolysis of low rank coals at T < 700°C such as those shown in Figures 6.10 and 6.14 for Loy Yang brown coal. Formation of NH3 during the pyrolysis of coal has been debated in the literature. As was stated above, Kambara and co-workers [18,165] believed that N-Q in coal was decomposed to form NH3. However, many researchers [e.g. 29,104,105,173-175] have challenged this type of direct correlation between coal-N functionality and release of coal-N during pyrolysis and combustion. Furthermore, the studies on the pyrolysis of model compounds (see a summary in Table 6.2), particularly polymeric model compounds [90-94], did not show any direct correlation between nitrogen functionality in the substrate and formation of NH3 and HCN. HCN could be formed from the pyrolysis of substrates with amino or other groups whilst NH3 could be formed from the substrates with amino, pyrrolic, pyridinic or nitrile groups. The formation of NH3 was more related to the reaction conditions (e.g. gas phase versus solid phase reactions, see Section 6.2) and the presence of other substitutional groups than to the nitrogen functionalities alone in the substrate. Treating coal with (strong) acid is likely to convert some coal-N into N-Q form [19]. However, treating coals with acids [146,152], intended for demineralisation purpose, did not seem to result in any enhanced formation of NH3 during pyrolysis. On the contrary, sometimes NH3 yields decreased with acid treatment of coal [146,152]. The observation of HNCO [129] provided another possible explanation for the formation of NH3 during coal pyrolysis. The failure to observe significant amounts of HNCO during the pyrolysis of low rank coals [129] was believed [105,129] to be due to the hydrolysis of HNCO to form NH3 since these coals have high moisture contents and/or produce considerable amounts of water upon pyrolysis. However, HNCO is mainly formed in the gas-phase cracking of volafiles during pyrolysis [105] and is not stable at high temperatures. The extent of contribution of HNCO to NH3 during the thermal cracking of char remains unclear.
Coal-N and Coal-S
327
The formation of NH3 requires the hydrogenation of coal-N. The source of hydrogen required for the formation of NH3 has long been debated and a number of hypotheses have been proposed. For example, Baumann and Moller [176] have suggested the formation of NH3 from the hydrogenation of HCN in the gas phase, mainly based on their observation that NH3 and H2 appeared simultaneously with increasing temperature in a fluidised-bed reactor. The formation of NH3 from \ow rank coals at low temperatures was attributed to the decomposition of amino groups or amides [176]. Bassilakis and co-workers [142] proposed that NH3 was mainly from the hydrogenation of HCN on the pyrolysis coal/char surface. Based mainly on this hypothesis in their FG-DVC model, they attempted to model the formation of HCN and NH3 during the pyrolysis of coals [142,177], including the HCN and NH3 formation from the pyrolysis of Yalloum brown coal [26].
0.00
0.50 1.00 1.50 2.00 Total gas flow rate, L min"''
Figure 6.15 Yields of HCN and NH3 as a function of gas flow rate from the pyrolysis of Loy Yang brown coal in a drop-tube/fixed-bed reactor at 800°C at a fast heating {>10'^ K s'^) [103]. Coal feeding rate: 90-120 mg min"\
328
Chapter 6
Schafer and Bonn [178,179] believed that the hydrolysis of HCN to NH3 [180,181] could be a potential explanation of NH3 formation during the pyrolysis of low rank coals, based on the consideration that Ca in these coals may catalyse the hydrolysis of HCN. Peck and co-workers [182], Aho and co-workers [29] and Hamalainen and coworkers [82,183] proposed that the OH radicals, related to the oxygen contents of fuel, might play important roles for the formation of NH3. These hypotheses have a common assumption that NH3 is the secondary product and that HCN is the primary product. Tan and Li [103] pyrolysed Loy Yang brown coal using their specially designed drop-tube/fixed-bed reactor (see Figure 6.5) to verify or disprove these hypotheses. Figure 6.15 shows the effects of gas flow rate on the yields of HCN and NH3 from the pyrolysis of Loy Yang brown coal [103]. Increasing gas flow rate would be expected to reduce the chance of HCN being hydrogenated on char surface, particularly when volatiles (including HCN or its precursors) had to pass through a thin char bed on the frit in the drop-tube/fixed-bed reactor. While the HCN yield did decrease with decreasing gas flow rate (increasing contact time between volatile and char), the decrease in the HCN yield was not accompanied by possible increases in the yield of NH3: the yield of NH3 remained unchanged. They concluded [103] that HCN was not converted to NH3 but HCN (as well as its precursors) was converted into soot-N or N2 during its interactions with char. Figure 6.16 shows the effects of coal feeding rate on the yields of HCN, H2, NH3 and H2S from the pyrolysis of Loy Yang brown coal in the same reactor [103]. The yields of HCN and H2 decreased with increasing coal feeding rate. However, no corresponding increases in NH3 yields were observed. Again, the decreases in the yields of HCN and H2 were not due to the hydrogenation of HCN. Tan and Li concluded [103] that HCN and/or its precursors were converted into soot-N or N2 during the interactions between volatiles and char. These results (Figures 6.15 and 6.16) did not support the above hypotheses about the formation of NH3 from HCN. Further evidence not supporting the hypotheses for NH3 formation from HCN may be found from the studies on char cracking discussed in Section 6.3.3.1. It was shown that char cracking was an important source of HCN and NH3. NH3 formation from charN could last for much longer than HCN formation. In the absence of volatiles (i.e. in the "not-feeding" periods), thermal cracking of char alone would produce negligible amounts of H2O and H2, not enough for any significant extents of HCN hydrogenation or hydrolysis. If HCN were a major source of NH3, the observation of NH3 formation lasting longer than that of HCN during char thermal cracking would have to mean that the hydrogenation or hydrolysis of HCN became intensified with the progress of char cracking. This is clearly impossible: the concentrations of H2O and H2 in the gas would only decrease with the progress of char cracking that decreases the O and H contents of char. Similarly, the production of OH radicals in the thermal cracking of char would be very limited and cannot play major roles in the direct formation of NH3 from char thermal cracking. Based on the discussion given above, it is fair to conclude that, whilst the routes of HCN hydrogenation/hydrolysis to NH3 could not be completely ruled out, they do not
Coal-N and Cocd-S
0 60
H
\
329
h-
2 CO
o C) L.
o
(n 1 CD
H2S
50 40 'M)
O
NH3
o 20 o
^ o^
10 0
40
80
120
160 200 240
Coal feeding rate, mg min'^
Figure 6.16 Yields of NH3, HCN, H2S and H2 as a function of coal feeding rate fi-om the pyrolysis of Loy Yang brown coal in a drop-tube/fixed-bed reactor at 800°C at a fast heating rate (>10' K s'^) [103]. Gas flow rate: 1.1 L min"' (measured at room temperature).
constitute major routes of NH3 formation during the pyrolysis of (at least Victorian brown) coal. In fact, there is even some contrary evidence that the presence of NH3 increased the formation of aliphatic nitriles, possible precursors of HCN (see Section 6.2) in the oil from the standard Fischer assay of Loy Yang brown coal [184]. The hydrolysis of HCN to NH3 as well as the OH radicals may play some roles in the presence of volatiles during pyrolysis or during gasification (see below). Li and Tan [101] pointed out that the formation of NH3 requires the presence of condensed phase(s) of carbonaceous materials rich in hydrogen. They proposed [101] a new theoretical framework about the formation of NH3 (and HCN) in the pyrolysing coal/char particles during the pyrolysis of coal at intermediate temperature levels (e.g. around 800°C). During coal pyrolysis, the active hydrogen required for the hydrogenation of coal-N/char-N is generated from the thermal cracking reactions taking
330
Chcq)ter 6
place inside the pyrolysing solid coal/char particles. There are several major ways for the generation of active hydrogen. For example, the bulky substitutional groups (particularly abundant in brown coals) may be broken off from the (hydro)aromatic ring systems, possibly through the formation of free radicals. Dehydrogenation of the aliphatic material or hydroaromatic ring systems would also seem to generate free radicals, i.e. the H radicals. The condensation reactions between the aromatic ring systems would also provide a source of active hydrogen. Some H may also be supplied from the volatile-char interactions (also see Chapter 4). Free radicals, particularly the H radicals, thus generated in situ (or supplied onto the char surface during volatile-char interactions) and possibly existing in the forms of adspecies on the char surface, would then attack the N-containing ring systems, resulting in the initial activation of the ring. The H radicals seem to be much more active than other radicals (e.g. CH3) in initiating the rupture of the heteroaromatic ring systems (see Refs. 28 and 106 as well as references on reaction mechanisms of decomposition of model compounds in Section 6.2). Free radicals (especially H) would probably then continue to migrate to the nitrogen-containing site, resulting finally in the full hydrogenation of the N site for the release of NH3. The formation of NH3 from the thermal cracking of nascent coal/char can be considered as a "self-gasification" process of char by the active hydrogen generated in situ within the char. In fact, hydropyrolysis of coal [185] was found to greatly favour the formation of NH3 mainly at the expenses of char-N even at 800°C. Li and Tan [101] also explained why the formation of NH3 from the reactions of Ncontaining heteroaromatic ring systems in the gas phase is diflficult. In the gas phase, under pyrolysis conditions, the chance for an N-containing radical system to come into collision with 2 or 3 H radicals together or even consecutively to form an NH3 molecule is very small. During this time, the dehydrogenation will lead to HCN formation. On the other hand, the H radicals may exist as adspecies on the char surface, essentially allowing for the "accumulation" of H to form NH3. Li and Tan [101] also believed that the initiation of the N-containing heteroaromatic rings in the solid phase by radical(s), differing from the ring activation in the gas phase, is the first step for the formation of both HCN and NH3 in the solid phase. This may be the main reason for the observation of both HCN and NH3 from the pyrolysis of coal, even at temperatures lower than 600°C (e.g. Figures 6.10 and 6.14), instead of just HCN. Clearly, the formation of NH3 requires more than one H. Li and Tan believed [101] that the selectivities of HCN and NH3 from the reactions in the solid phase depend on the relative stability of the N-containing structure. When the N-containing structures can be easily ruptured under the experimental conditions (e.g. relatively high temperature), the formation of nitriles and then HCN is favoured. For relatively stable N-containing structures that can be activated (slowly), the formation of NH3 will be favoured but as a slow process. In other words, while the thermally less stable N-containing structures are mainly responsible for the formation of HCN (as a relatively rapid process), the thermally more stable N-containing structures may be slowly hydrogenated by the H radicals to NH3. This explains why the formation of HCN can go to completion more rapidly than NH3 (see Section 6.3.3.1).
Coal-N and Coal-S
331
Figure 6.17 Effects of heating rate on the yields of HCN and NH3fi-omthe pyrolysis of a set of solid fuels in a drop-tube/fixed reactor [101]. For the fast (>10'^ K s"') heating rate experiments, the final temperature was 800°C. For slow heating rate experiments, the samples were heated at 6.7 Kmin"' to 1000°C. Carbon contents (wt% daf): 47.6 % for bagasse, 68.5 % for Loy Yang, 82.1 % for Drayton and 91.0 % for Pocahontas #3.
Li and Tan [101] explained the heating rate sensitivities of HCN and NH3 yields. Figure 6.17 shows the formation of HCN and NH3 from 4 different solid fuels. The HCN and NH3 from the fast heating rate experiments included those from the thermal cracking of char over a long period of holding time even after the feeding of coal had stopped. The yields of HCN and NH3 for coal decrease with increasing rank mainly due to the decrease in the availability of H and increased thermal stability of coal-N. The lower yields of NH3 from bagasse were due to its extremely rapid release of large amounts of volatiles (a volatile matter yield of 82.4 wt% daf), leaving little char for the formation of NH3 in the solid phase.
332
Chapter 6
The explanation of the effects of heating rate would necessarily need to consider the reactions in the gas phase and in the solid phase. At a slow heating rate, volatiles are released and swept out of reactor mostly at T <600°C and little HCN would be formed from the thermal cracking of volatiles. At a fast heating rate, volatiles had time to breakdown (formation of N-heterocyclic structures and their subsequent breakdown in the case of bagasse) to contribute to the observed HCN yields. However, the enhanced formation of NH3 (and HCN) at the fast heating rate is at least due to another two reasons (see Ref 101 for more details): match between H production and hydrogenation of char-N as well as additional sources of H (and other radicals). The formation of NH3 and HCN competes with the recombination reactions during pyrolysis. A rapid heating rate means that more H radicals are supplied rapidly, favouring the hydrogenation of N-sites to form NH3 (or HCN depending on the stability of the N-containing structure in char). A slow heating rate means that the H radicals could not be supplied rapidly, favouring the recombination reactions for the (activated) N-sites to form a more stable structure in the pyrolysing solid particle. Clearly, the sensitivity of NH3 yield to heating rate would increase with increasing availability of H radicals, explaining that the low rank coal (Loy Yang) is more sensitive to heating rate than the high rank coals. Heating coal rapidly to high temperature also favours the interactions between the volatiles and the pyrolysing coal/char particles. Some volatiles, often H-rich and containing radicals, may also react with the nascent char to yield some NH3. This is an additional source of H (or other radicals such as OH radicals) for fast heating experiments but that is not available at a slow heating rate. The volatile-char interactions, mainly involving (H) radicals, play important roles during the pyrolysis of Victorian brown coal. The interactions could lead to almost complete volatilisation of Na during the pyrolysis of Loy Yang brown coal and detailed discussion has been presented in Chapter 4. Specially designed experiments using a two-stage reactor have shown that volatile-char interactions lead to NH3 formation from char-N [186]. Limited extents of HCN hydrolysis may also happen during the volatile-char interactions, which is however unlikely to be an important source of the overall NH3 observed as discussed above. While coal rank is an important coal property affecting the availability of H radicals during pyrolysis and therefore the formation of HCN and NH3 (Figure 6.17), the petrographic composition and/or the geographical location of the coal (Northern versus Southern Hemispheres) is another important factor governing the formation of HCN and NH3. Some inertinite-rich Chinese bituminous coals were found to give higher NH3 yields than some more vitrinitic Australian coals [125]. The inertinites of poor caking properties were believed to form H radicals over a "correcf temperature range to overlap with that for the activation and hydrogenation of coal-N [125]. 6.3.5. Experimental Techniques and Observed Distribution of Coal-N Tan and Li [7] pointed out that the difference in the reactor configuration is an important factor for the differences in the HCN and NH3 yields reported in the literature.
Coal-N and Coal-S
333
For example, when a suite of rank-ordered American coals was pyrolysed in an entrained-flow reactor, very little NH3 was detected in the product gas [142]. This may be mainly because the short particle residence time in the reactor does not favour the slow formation of NH3 from char-N. However, when a similar suite of rank-ordered coals were pyrolysed in a thermogravimetric analyser (TGA) by the same group, NH3 was found to be one of the most important N-containing products from the nitrogen in coals with low yields of HCN [142]. This is mainly because the pyrolysis in a TGA gives long particle residence time for the formation of NH3. But the volatiles are easily swept out of TGA at temperatures too low for significant extents of volatile cracking to form HCN. A fluidised-bed reactor is often used in the laboratory to study coal-N distribution. When caking coals are pyrolysed in the reactor, the coal particles are likely to adhere to the sand particles and therefore have long time for the char cracking reactions to form HCN and NH3. When coals of poor caking properties are pyrolysed in the reactor, a significant proportion of char particles would be elutriated out of the reactor with little time for char cracking reactions. This type of reactor would not give a true evaluation of effects of coal rank on coal-N distribution during pyrolysis [7]. Xie and co-workers [125] designed a fluidised-bed/fixed-bed reactor to address this shortcoming of a normal fluidised-bed reactor. The extents of volatile-char interactions in a reactor are another factor contributing to the differences in the observed HCN and NH3 yields. Zhang and Fletcher [121] reported that the pyrolysis in a flat flame entrained flow reactor tended to give higher NH3 yields than in other entrained flow systems. They believed that the local environment is important for the conversion of coal-N [121]. In fact, their combustion gas from a CO/H2/O2/N2 fuel-rich flame could contain radicals normally found in the coal pyrolysis volatiles, which would interact with char-N to form NH3. Many studies [e.g. 25,26,28,105] have reported the yield of NH3 to decrease at higher temperatures (e.g. > 800°C). A typical example was shown in Figure 6.14. NH3 is known to be unstable and to decompose to form N2 and H2 at high temperature. At a residence time of about 0.7 s, less than 25 % of NH3 could be decomposed at temperatures lower than 950°C [187]. However, this non-catalytic decomposition of NH3 does not seem to account for all the decreases in NH3 yield observed during coal pyrolysis at high temperatures. It is yet to be resolved if there are changes in the intrinsic reaction pathways that are responsible for the decreases in NH3 yield at high temperatures (e.g. changes in selectivity between NH3 and HCN). However, at least a part of the NH3 yield decrease at high temperature may be due to the interactions of NH3 with materials commonly used as a part of the reactor system. Li and Nelson [102] showed that NH3 interacted with quartz, steel and zircon sand strongly at temperatures higher than 700 - 800°C, warning the need to evaluate the NH3 yields at high temperatures with great caution. Steels seem to be particularly effective in the catalytic destruction of NH3. The (catalytic) decomposition of NH3 at high temperatures poses experimental difficulties to understand the true selectivities of NH3 and N2 from the conversion of coal-N at high temperature.
334
Chapter 6
Quantification of HCN and NH3 often introduces large errors in the study of coal-N distribution. HCN and NH3 are often quantified following their absorption in aqueous solutions. Tan and Li [7] reported that placing the acid solution and the alkaline solution in series [188] could cause serious underestimates of the HCN yield due to the high solubility of HCN in the acid solution preceding the alkaline solution. Acetylene interferes with the quantification of HCN using a selective ion electrode [189]. The deterioration of CN' standards with storage time may also worth mentioning here.
6.4. FORMATION OF NOx AND NO, PRECURSORS DURING GASIFICATION AND COMBUSTION Gasification and combustion of coal can be conceptually divided into two steps: thermal decomposition (pyrolysis) of coal followed by the reactions of volatiles and char with reactive reagents. Whilst the conversion of coal-N during the pyrolysis of Victorian brown coal has been discussed in detail in Section 6.3, this section focuses on the reaction of coal-N, volatile-N and char-N, in reactive atmospheres. The formation of NO^, N2O and N2 from the reactions of simple NOx precursors, especially HCN and NH3, in the gas phase is relatively well understood. Excellent reviews (e.g. [190]) are available and many (commercial) combustion modelling codes contain detailed gas-phase N chemistry. This will not be further discussed in detail here. However, it should be pointed out that HCN and NH3 are not the only NOx precursors in the gas phase. Modelling of the NOx formation during combustion in the gas phase, particularly at relatively low temperatures, should also take into account a range of Ncontaining species [84]. Some studies have been carried out to study the oxidation of Ncontaining model compounds [89,191,192]. Ledesma and co-workers [193,194] also studied the oxidation of volatile-N in isolation from that of the char using a reactor similar to that shown in Figure 6.5. 6.4.1. Reactions with O2 Substantial efforts have been made worldwide to study the formation of NOx during coal combustion. The progress in this area has been summarised in recent reviews [e.g. 118,123,195-203]. The behaviour of coal-N in Victorian brown coal during reactions with O2 is summarised below. Cliff and Young [204,205] studied the formation of NOx from the combustion of Yalloum and Morwell brown coals in comparison with an Australian sub-bituminous coal (Millmerran). Under fuel-lean conditions at high temperature (>1270°C), the oxidation of volatile-N was always proportional to the rate of evolution of CO2 for the combustion of Yalloum brown coal in premixed hydrogen/oxygen/argon flames [204], indicating the importance of volatile-N for the formation of NO. When the brown coals were burned in a laminar-flow furnace (initiated at 1200 ± 100°C), about 64 % of coal-N could be converted into NO under fuel-lean conditions (the O2 supplied was 1.88 times of the O2 needed for complete combustion). N2
335
Coal-N and Cocd-S
accounted for around 32 % of coal-N together with 4 % of coal-N in the unbumed char. Under the fuel-rich combustion conditions (the O2 suppHed was 0.56 times of the O2 needed for complete combustion of coal), about 15 % of coal-N was converted into NO and more than 65 % of coal-N was converted into N2 together with char-N and traces of HCN and NH3 [204], as is shown in Figure 6.18. The brown coals exhibited a significantly greater potential for NO emission under the fuel-lean conditions than did the sub-bituminous coal [204,205]. However, under the fiiel-rich conditions the conversion of coal-N to NO for the sub-bituminous coal was higher than for the brown coals [204,205]. These observations appeared to agree with those by Phong-Anant and co-workers [136] who found that the formation of NOx from the volatile-N of the Morwell brown coal was more sensitive to combustion stoichiometry than that of volatile-N from a bituminous coal. Under fuel-rich conditions, char-N dominated the formation of NOx from the Morwell brown coal [136]. Jeremiejczyk and co-workers [206] also pointed out that a substantial amount of NOx could be reduced by the volatiles from brown coal. In addition to the differences in the reactivities of volatile-N, the high reactivity of brown coal char (based on its high char
to) fuel - lean •-
50
100 150 Reaction Hme Ims)
200
250
Figure 6.18 Fomiation of NO, N2, HCN and NH3 during the combustion of Morwell brown coal in a laminar furnace (heated with acetylene/oxygen diffusion flame to 1200°C) under (a) the fiiellean ((|) = O2 supplied/02 needed for complete combustion = 1.88) and (b) fuel-rich (^ - 0.56) conditions [205].
336
Chapter 6
surface area) was speculated to be an important factor for the reduction of NO [204,205]. Flue gas recirculation in the brown coal power stations was also speculated [204] to reduce the NOx emissions. In fact, flue gas recirculation has been investigated in detail [207-209] as a novel method of NO^ reduction for the fluidised-bed combustion of coal. Under the fluidised-bed conditions, the recycled NO is reduced to N2 either directly or via N2O. The combustion of volatiles is fully responsible for the NO-to-N20 conversion while that of char governs the NO-to-N2 reduction [208]. H2O enhances the heterogeneous NO-to-N2 reduction and the homogenous NO-to-N20 conversion while inhibiting the heterogeneous N20-to-N2 reduction [208]. Kambara and co-workers [165] believed that the formation of NOx during the combustion of coal (under their experimental conditions) was related to the coal-N functionalities, being inversely related to the so-called NOx index: NOx index = ([N-Q] + [ N - 6 ] + [N-5]/[C])> : [Volatile-N]x
[N-Q] ([N-6] + [N-5]/[C])
[NH3] [HCN]
where [N-Q], [N-6], [N-5] and [C] are the contents of quaternary nitrogen, pyridinic nitrogen, pyrrolic nitrogen and carbon in coal (daf) respectively. The Yalloum brown coal tested gave nearly the highest NOx index value (19.9) among 20 coals tested, corresponding to the lowest fuel-N conversion to NOx [165]. Whilst the correlation
air staging coal •
fuel staging
air
(:oal
air
>f
X = 0.95 ...0.65
reductive
A,= 1.15
A. = 1.05 air ...0.65
air X=1.15
I ash flue gas
X=1.15
I ash flue gas
Figure 6.19 Schematic diagrams showing the principles of air staging and fuel staging (rebuming) in coal combustion [210].
337
Coal-N and Coal-S
between the coal-N conversion to NOx and the NOx index (coal-N functionaHties) was very good for their experimental conditions [165], generalisation of the correlation is yet to be further verified (also see Section 6.3). Air staging and fuel staging (rebuming) [200] have been investigated as important ways to reduce NOx emissions from coal-fired power plants. Air staging divides the combustion process into a primary air-deficient zone and a second burnout zone with excess air. In fuel staging, the reducing conditions are created only subsequent to a first oxidising zone by adding a reducing fuel that is further burned in a bum-out zone with excess air [210], as is shown in Figure 6.19. Cliff pointed out [204] that the tangential coal firing practice (injecting coal and air at various levels) in the brown coal power stations effectively acted as a type of staged combustion, helping to reduce the NOx emission levels. Spliethoff and co-workers [210-212] investigated the performance of Victorian brown coal in comparison with that of German brown and bituminous coals in advanced air staging and rebuming processes. As is shown in Figure 6.20, air staging is a very effective technique to reduce the NOx emissions from coal-fired power plants. Similar to the German brown coal, the air staged combustion of Yalloum brown coal helps to
1800 1600-j
6'
reactor temperature T^pQ = 1300 *C residence time in reduction zone 3 s
1400 -•-GOttelborrk ^ _. _ , >hard coal 1000 --0-- Bayswater/ A Laubag ^^^^^^^^, 800 —V— Yalloum/ 600
1200-1
E E c
400-1 200 0,6
0,7
0,8
0,9
1,0
1.1
1.2
primary air ratio X Figure 6.20 Effects of primary air ratio on the emissions of NG^ during the air-staged combustion of Yalloum brown coal and other Australian (Bayswater) and German brown (Laubag) and bituminous (Gottelbum) coals in an electrically heated entrained-flow reactor [210].
338
Chapter 6
achieve NOx emissions far below 200 mg m"^ [210], which could not easily be achieved with rebuming. Increasing temperature appeared to favour the NOx reduction during the air-staged combustion [210]. As was discussed in Section 6.3, tar-N is an important shuttle for the release of coalN during pyrolysis. Chen and co-workers [12] estimated that, if tar could be released from Loy Yang brown coal, the emissions of NOx would be reduced by 20% for combustion at 1430°C or 40 % for combustion at 900°C. However, this estimation is yet to be proved experimentally as it may have under-estimated the roles of gas-solid interactions for the reduction of NOxOn the contrary, coal pyrolysis volatiles, including those from brown coal, were found to be a very effective rebum fuel, being more effective than methane or natural gas [210-212]. The N-containing species in volatiles (tar), especially the volatiles produced at around 800°C, seem to be responsible for the enhanced reduction of NOx [210-212]. Similar to volatile-N, char-N also plays an important role both in the formation and in the reduction of NOx during coal combustion. In a study on the conversion of char-N to NO under single particle conditions, Shimizu and co-workers [213] found that around 30 - 35 % of N in the char prepared from the pyrolysis of Yalloum brown coal in a fluidised-bed reactor at 900°C for 5 min was converted into NO. Under similar conditions, 40 - 50 % of char-N in the chars from higher rank coals (up to 80 % carbon contents) was converted into NO. The large surface area of the chars from the low rank coals was believed to contribute to the reduction of NO inside the pore surface [213]. Due to the high reactivity of chars from low rank coals, Chen and Tang [214] believed that a mixed fuel containing natural gas and lignite char can be used as a rebum fuel for the in-fumace control of NOx. The high reactivity of brown coal char led to the search to use the char as an effective reducing agent at low temperature to convert NO into N2 [215,216] or for flue gas cleaning (de-NOx and de-SOx) [217]. The C-NO reaction was remarkably promoted by the presence of O2 at around 300°C or higher [215], at least partially due to the enhanced formation of C(0) complexes on the char surface by O2. Cu, Ca and Ni were all catalysts for the C-NO reactions with Cu loaded into Loy Yang brown coal prior to pyrolysis being the most active. Chang and co-workers [218,219] investigated the release of coal-N during the oxidation of Loy Yang brown coal in 4 % O2. At a low temperature around 500°C, very significant amounts of HCN and NH3 were formed from the oxidation of Loy Yang brown coal although at very slow rates, as is shown in Figure 6.21. Formation of significant amounts of NH3 and HCN were also seen with other higher-rank coals [218,220], in general agreement with the observation made by Varey and co-workers [135], Wang and Thomas [221] and Jones and co-workers [222] of reactive Ncontaining species during the oxidation of coal and other carbonaceous materials. As the use of O2 does not introduce additional hydrogen into the system, the hydrogen required for the formation of NH3 and HCN has to come from the hydrogen originally present in the coal. Even for a high rank coal (C - 90 wt% daf), the very slow oxidation of the coal/char can generate enough H for the formation of significant amounts of NH3 [218].
339
Coal-N and Cocd-S = 10.0
E
Z
— 1 — • —
80
4.0
E
2.0
5.0
1
4.0
O
3.0
1
-
50 40
-_30 20
- y
10 1
O
'
••
J
15 8 6.0 2
-T
HCN
'
1
1
1 1
,
1
1 1
0
2^ 0)
*>»
NH3
•o 0)
-
o> 2.0 Co §
1.0
-
1
y
nn(
()
1
20
40
•
E 3
o
1
60
1
1
80
1C
Time, min
Figure 6.21 Formation of HCN and NH3fromthe oxidation of Loy Yang brown coal in 4% O2 at 500°C in a fluidised-bed/fixed-bed reactor at a fast heating rate (>10'^-10'^ K s'^). Total gas flow rate: 1.5 Lmin' (measured under ambient conditions). Average coal feeding rate: 30 mg min'^ The first steps (points) refer to the "feeding" periods and other steps (points) to "not-feeding" periods [218].
Therefore, the data such as those in Figure 6.21 serve to provide further evidence that the formation of NH3 is largely controlled by the availability of H radicals within the pyrolysing/gasifying solid [101]. The presence of O2 helps to rupture the N-containing heteroaromatic ring systems at this low temperature for the formation of NH3 and HCN. At elevated temperatures (e.g. at 500°C) in the presence of O2, some (H) radicals might have been generated through the oxidative cracking of volatiles, becoming a source of hydrogen required for the formation of NH3 and HCN during the volatile-char interactions. The oxidation of volatile-N would also be an important source of HCN based on the observation that HCN was an important product from the oxidation of pyrrole [191] and pyridine [192] (although the formation of HCN from the oxidation of pyrrole and pyridine was observed at much higher temperatures than 500°C). It is possible that the oxidation of volatiles produced much higher concentrations of radicals than in the oxidation of pyrrole and pyridine [191,192], effectively lowering the temperature for the decomposition of volatile-N. The more recent observation [223] that HCN yield changes with coal feeding rate during the oxidation of Loy Yang brown coal in the fluidised-bed/fixed-bed reactor may further signify the importance of the reactions of volatiles (including possible volatile-char interactions) for the formation of HCN. HCN seems to be a favourable product from coal-N during oxidation at low temperature (Figure 6.21) [218-220]. Firstly, this is because very limited extents of ring condensation reactions have taken place to stabilise the coal-N at temperatures lower
340
Chapter 6
than or around 500°C. Thus, the formation of HCN was still possible. Secondly, the formation of HCN may be related to the partial consumption of H radicals by O2derived species on the coal/char surface. While the presence of O2 continues to generate radicals and helps to break down the N-containing ring systems, a significant proportion of H radicals must have also been consumed by the 02-derived species on the surface. Therefore, the formation of HCN is a net result of continuous ring breakdown, generation and removal of H radicals. The consumption of H radicals by 02-derived species must have acted as a limiting factor for the formation of NH3. HCN was also observed to form during the oxidation at 600°C of chars prepared from the pyrolysis of higher-rank coals under severe conditions in other studies [167,224]. HCN was believed to be a primary product [167]. The formation of HCN was believed to be from surface -C(N) species [224], either formed from C-NO reactions [170,225,226] or inherently present in char. The presence of O2 at 600°C destabilised the -C(N) structure through the destruction of carbon network [224], in general agreement with the explanation of HCN formation from the oxidation of Loy Yang brown coal by Chang and co-workers [218-220]. NOx was also observed during the oxidation of Loy Yang brown coal at 500 to 600°C [227], including NO2. The formation of NO2 was also observed in other studies [220,228-230]. While the exact mechanisms for the formation of NO2 remain unclear, it appears that the reactions such as NO + HO2 = NO2 + OH and H + O2 +M = HO2 + M (where M is a third body) may be important. NO2 may also form on the H-rich char surface [220]. Bhattacharya and co-workers [231 ] measured NOx and N2O from the combustion of Victorian brown coal (Loy Yang, Morwell and Yalloum) in a circulating fluidised-bed combustion pilot plant, showing that the fluidised-bed combustion of Victorian brown coal would produce NOx emissions (25 - 80 ppm @ 6 % O2) lower than the pf combustion of the brown coal. 6.4.2. Reactions with CO2 CO2 affects the formation of HCN and NH3 even at low temperatures when the gasification of char by CO2 is very limited. Figure 8.22 shows the effects of CO2 on the formation of HCN and NH3 from the pyrolysis/gasification of Loy Yang brown coal in a drop-tube/fixed-bed reactor at fast heating rates [101]. In the "feeding" periods, the formation rates of HCN and NH3 were both suppressed by CO2. In the "not-feeding" periods, whilst the NH3 formation rates were enhanced, the HCN formation rates were suppressed by CO2. The effects of CO2 on the formation of HCN and NH3 were studied in more detail by the same group [218,219] for a set of rank-ordered coals. The effects of CO2 on the formation of NH3 and HCN were explained by considering the possible effects of CO2 on the concentrations of H radicals as well as their interaction with the N-sites in the pyrolysing/gasifying coal/char particles [101,218,219,227]. At around 800°C, CO2 can be chemically adsorbed strongly on the coal/char surface, even preferentially on the N-site. The adsorption of CO2 on the N-site can lead to the slow oxidation of the N-site or the blockage of the access of the N-site
341
Cocd-N and Cocd-S
by the H radicals. However, more importantly, CO2 adsorption on other nearby sites also leads to the consumption of freshly generated H radicals by the C02-derived species on the surface. All these factors combine to suppress the formation of NH3 and HCN. The evidence has been presented [112] that CO2 can react rapidly with nascent char and that the reaction is coupled with the thermal cracking of the nascent char, implying the interactions between chemisorbed CO2 and (H) radicals in the pyrolysing char. In the case of the "not-feeding" periods for the Loy Yang brown coal (Figure 6.22) and, to a much lesser extent, some "not-feeding" periods for some bituminous coals [218], the generation of H radicals from the thermal cracking of char itself is slow. The introduction of CO2 can lead to net generation of H radicals as the H-containing char structure is slowly destroyed through the slow gasification of char by CO2. This in turn means that the formation of NH3 can be slightly increased by the introduction of CO2, as is shown in Figure 6.22. For high rank coals deficient in H, NH3 formation was enhanced even during the feeding periods due to the enhanced net generation of H radicals through C02-char interactions.
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20
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Time, min
40
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• • • • Pyrolysis E^mm Gasification
Z 25
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1 1 • • 1 1 NhHs
-
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Figure 6.22 Average formation rates (left) and accumulated yields (right) of NH3 and HCN from the pyrolysis/gasification of Loy Yang brown coal in argon and CO2 at 800°C in a droptube/fixed-bed reactor operated at the fast heating rate (>103 K s-1) [101]. The first-steps (left) correspond to the periods when coal was being fed into the reactor.
Chapter 6
342
100
100
.9>
• ZN-raw O ZN-dem AHV 75|-ALY TBF VWM • TK 50 • OM
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25
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I
A
A
25 50 75 Coal conversion, wt%(daf)
100
N2 yield (% of coal-N) vs coal conversion
25
50
75
100
Char conversion, wt%(daf)
N2 yield (% of char-N) vs char conversion
Figure 6.23 Formation of N2 during the gasification of 8 coals (C: 65.9 to 81.2 wt% daf) in CO2 at 1000°C [11]. "LY" in the plots stands for Loy Yang brown coal.
Chang and co-workers [218] further concluded that the effects of CO2 on 1^3 formation was directly related to the NH3 yield during pyrolysis. Whenever NH3 formation is favoured during pyrolysis due to the presence of abundant H radicals, CO2 tends to suppress the NH3 formation. On the other hand, if H radicals are scarce during pyrolysis, CO2 would generate more H radicals through gasification to enhance MI3 formation. HCN does not seem to be a favoured N-containing product in the presence of CO2 at temperatures around or higher than 800°C (Figure 6.22 and Ref 218). Wang and coworkers [232] also observed for lower HCN yields from the temperature-programmed combustion of chars (prepared at 1000°C) than from the parent coals. The limited formation of HCN shown in Figure 6.22 and elsewhere [218] is clearly because HCN is mainly formed from relatively small N-containing heteroaromatic ring systems (see Section 6.3), which do not survive long at elevated temperatures (e.g. around 800°C or higher). Ohtsuka and Wu [11] studied the release of coal-N during gasification in CO2 at lOOO^C following the heating up of coal particles at 400 K min' in a fixed-bed reactor. Similar to the observation by Chang and co-workers [218], Ohtsuka and Wu [11] also observed significant decreases in the yield of NH3 in CO2 compared with that in helium. However, N2 was by far the most important N-containing species from gasification, as is shown in Figure 6.23. At the end of devolatilisation at lOOO'^C for 2 min, only 13 % of coal-N in the Loy Yang brown coal was converted into N2. At the end of gasification, about 50 % of coal-N in Loy Yang was converted into N2(the left panel of Figure 6.23). These N2 yields are the lowest among the 8 coals studied. When the N2 yields were plotted as % of char-N as a function of char conversion (the right panel of Figure 6.23), all coals followed the same trend, reaching 65 % of char-N. It was speculated [11] that
343
Cocd-N and Cocd-S
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100
200
300
400
500
600
700
800
1 — 1
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Reaction time, min
Figure 6.24 Accumulated NH3 yields as a function of reaction time during the gasification of Loy Yang brown coal in 15 % steam/85 % argon in a one-stage fluidised-bed/fixed-bed reactor at 800°C [233]. The first points refer to the NH3 formed in the "feeding" periods when coal was continuously fed into the reactor and all other points refer to the "not-feeding" periods, i.e. the NH3 from the gasification of char only.
N2 was formed via NO as intermediates in reactions at the char surface such as C(N) + NO = N2 + C(0) and 2Cf + 2N0 = N2 + 2C(0). The failure to detect significant amounts of NO led the authors to beheve that these reactions are very fast [11]. Further investigation is warranted to verify the above speculation on N2 formation during gasification in CO2. 6.4.3. Reactions with H2O and H2 Unlike the reactions with O2 and with CO2, the gasification of coal in H2O and H2 introduces a significant amounts of hydrogen into the reaction system. This clearly will be expected to affect the distribution of coal-N, particularly the yield of NH3. Hydropyrolysis in high pressure (up to 5 MPa) hydrogen resulted in NH3 being the dominant N-containing gaseous product, mainly at the expenses of char-N [185]. When H2O is used as a gasifying agent, hydrogen radicals (or similar species) would be produced on the char surface, as likely intermediates in the reactions between the coal/char and H2O. The H radicals, required for the formation of NH3 from char-N [101], would necessarily enhance the formation of ]S[H3. When Loy Yang brown coal was gasified in steam, NH3 was observed to be the most important N-containing gaseous product [219,227,233], as is shown m Figure 6.24. At 800°C in 15 % steam, more than 60 % of coal-N in the Loy Yang brown coal could be converted into ]S[H3.
344
Chapter 6
Similar yields of NH3 could also be observed even during gasification at 700°C although at very slow rates [227]. The majority of NH3 was observed in the not-feeding periods especially at low temperatures, indicating that NH3 mainly originates from the slow gasification of char. The gasification of other higher-rank coals in steam also resulted in NH3 being the dominant gaseous N-containing product [218]. Gasification in steam also led to increases in the yields of HCN both for Loy Yang brown coal (especially at 700°C) [233] and other higher-rank coals [218]. However, the increases in the HCN yields, while significant, were much smaller than the corresponding increases in the NH3 yields. The data shown in Figure 6.24 indicate that the ash, prepared from the combustion of the same Loy Yang brovm coal at 600°C in the same reactor prior to the gasification experiment, shows strong catalytic effects on the formation of NH3 at the expense of the formation of HCN and other N-containing species (NOx, N2O, HCNO and N2, not directly measured in the experiments) [233]. The enhanced NH3 formation mainly took place in the "not-feeding" periods, i.e. mainly during the char gasification. While the hydrolysis of HCN to form NH3, possibly catalysed by components in the ash, cannot be entirely ruled out, it was not considered to be the main or only cause for the enhanced formation of NH3 because the decreases in the yield of HCN were not enough to account for the increases in the NH3 yield. Experiments with Na-loaded Loy Yang brovm coal indicate that Na in the ash may be mainly responsible for the enhanced formation of NH3. Catal3Atic effects of ash on NH3 may be due to the catalysed formation of H radicals or catalysed interactions between H and the N-sites with the exact mechanisms remaining unclear. Nearly all studies on coal gasification have indicated that NH3 is the dominant Ncontaining gaseous product from the gasification of coal (including air-blown gasification) [e.g. Refs. 1,234-236]. The reactions of char-N with H2O, H2 or similar species appear to the main source of NH3. This may even be the case in air-blovm gasification [235,236]. In the reactions of coal with 4 % O2 in a fluidised-bed/fixed-bed reactor, the yield of NH3 would go through a minimum with increasing temperature [220]. The formation of NH3 at higher temperature is likely to be due to the reactions between coal-N/char-N with the H2O/H2 (or H radicals) formed from the oxidation/gasification of coal. Volatile-char interactions also favour the formation of NH3[186].
6.5. SULPHUR IN VICTORIAN BROWN COAL AND ITS CONVERSION DURING PVROLYSIS, GASIFICATION AND COMBUSTION 6.5.1. Sulphur in Victorian Brown Coal Sulphur contents in brown coal can be measured using standard methods, e.g. Australian Standards AS 2434.6 - 2002 [5]. With the Eschka method, coal is ignited in intimate contact with the Eschka mixture (calcined magnesium oxide and anhydrous sodium carbonate in the ratio of 2:1 by mass) in an oxidising atmosphere at 800°C to
Coal-N and Cocd-S
345
convert S to sulphate [5]. The sulphate is then quantified to calculate the sulphur content of coal. With the high temperature combustion method, coal is burned in a stream of oxygen at 1350°C. The oxides of sulphur formed, together with CI in the form of HCl, are absorbed in neutral hydrogen peroxide and then determined [5]. The combustion method also allows for the simultaneous quantification of chlorine in coal [5]. Victorian brown coal currently being used (Yalloum, Yalloum North Extension, Loy Yang and Morwell) has very low sulphur contents, often < 0.4 wt% (db) [2], thus requiring little extra sulphur removal measures for the power plants to meet the emission standards [237]. For this reason, the sulphur in Victorian brown coal has not received great attention. However, typical sulphur contents for Gelliondale and Gormandale coals are about 0.7 and 0.8 wt% respectively [2]. The sulphur content of Gippsland Basin coal increases towards the East reaching a maximum of 5 - 7 wt% in the Glencoe deposit [2]. The differences in the sulphur content among Victorian brown coal seams are beheved to reflect the different depositional environment of coal formation [238,239]. "The easterly increase in sulphur in Traralgon Formation coals appears mostly related to syndepositional effects, although in some areas the immediately overlying marine beds also post-depositionally add to or modify the sulphur distribution" [239]. The future use of the high-sulphur brown coals would require measures to reduce SOx to meet the future environmental standards. Sulphur in coal can be broadly classified into two major categories: inorganic sulphur and organic sulphur. The organic sulphur forms an integral part of coal organic matter. The inorganic sulphur includes various forms of S-containing minerals, the most important normally being FeSi (pyrite and/or marcasite). However, pyrite and/or marcasite are only infrequently present in the Victorian brown coal [237,240]. Even for the Victorian brown coals (Gippsland Basin) with sulphur contents as high as 5.7 %, their pyritic sulphur contents are still very low [241,242], often less than 0.1± 0.2 wt% db [241]. Sulphate minerals are generally absent in freshly mined Victorian brown coal [240]. Therefore, sulphur in Victorian brown coal is mainly (often almost entirely) organic sulphur. The absence of significant amounts of inorganic sulphur would greatly simplify the study on coal-S in Victorian brown coal. As in the case of coal-N, the functionalities of coal-S have been debated for long time. Many S-containing structures may be present in coal such as thiophenes (heteroaromatic), aryl sulphides (in which sulphur is connected to an aromatic ring), cyclic sulphides (in which sulphur is part of a non-aromatic ring), aliphatic sulphides and aryl/aliphatic thiols [243]. Elemental sulphur may exist in weathered coal samples. Both destructive and non-destructive methods have been used to gain information about the coal-S functionalities and comprehensive reviews have been published [e.g. 37,243-245]. Davidson gave a good account of the destructive methods up to 1994 [245]. The destructive methods for differentiating coal-S forms rely on the detection of gaseous S-containing species (mostly H2S and SO2) during the pyrolysis, hydrous pyrolysis, reduction (with solvents or H2) or oxidation of coal under chosen reaction conditions. Biodegradation of coal-S was also considered as a tool for probing the presence of elemental sulphur [246], which confirmed the absence of elemental sulphur in a high-sulphur (4.1 wt%) Victorian brown coal.
346
Chapter 6 Aliphatic Sulfides, Meicaptans, Disulfides
Aiomadc Sulfides, Mercaptans
lUoptenc Benzoduophene Dibenzoduophcnc 800
Temperature''C Figure 6.25 Pyrolysis of S-containing model compounds, 0.5 s contact time. Reprinted with permission from Ref. 248. Copyright 1992 American Chemical Society.
Calkins and co-workers [247,248] proposed that the pyrolysis of coal at a rapid (~ 10"^ Ks'^) heating rate and short gas residence time (~ 0.5 s), e.g. in a fluidised-bed reactor, could differentiate the aliphatic sulphur from the aromatic sulphur. Pyrolysis of S-containing model compounds, as is shown in Figure 6.25, was used as a "calibration" of the method. The cut-off point for the pyrolysis temperature of aliphatic S-containing structures was chosen as 700-750°C [248]. In other words, the S-containing gases collected from the pyrolysis of coal at 700-750°C were considered to have derived from labile aliphatic sulphidic structures in coal [248]. Using a temperature-programmed reduction with a "cocktail" of hydrogen donor solvents in the absence of a catalyst, Dunstan and Walker [241] determined the sulphur frinctionalities in Victorian brown coal, although the authors clearly pointed out the approximate and qualitative nature of their coal-S functionality results for the brown coals. The organic sulphur forms identified in 5 brown coal samples from the Otway and Gippsland Basins include aliphatic thiols, aromatic thiols, aliphatic sulphides, aromatic sulphides, aliphatic disulphides and aromatic disulphides. All destructive techniques for analysing sulphur forms in coal rely on the choice of suitable model compound standards. This has normally been a difficult choice as the coal-S frinctionalities are unknown. More importantly, it is yet uncertain if the model compounds would behave similarly to those S-containing structures in solid coal [249]. As was discussed in detail in Section 6.3, coal-N/char-N in the solid coal/char behave very differently from volatile-N or N-containing model compounds. Furthermore, the data on S-containing model compounds such as those shown in Figure 6.25 are kinetic data and thus the reaction time is an important parameter, which in turn depends on the
Cocd-N and Cool-S
347
reactor configuration. For example, when a normal fluidised-bed reactor is used, the residence time of solid would depend on the caking properties of coal being pyrolysed [7,249]. Whilst char particles from non-caking coals would be rapidly elutriated out of the reactor and have residence time similar to that of volatiles, the char from caking coal would agglomerate with the sand particle and thus stay a much longer time than volatiles. During the extended residence time in the reactor, the thiophenes may also contribute to the production of H2S or CS2. Volatile-char interactions, including the interactions of char with reactive species such as H2O and CO2 formed from the pyrolysis of coal, may also lead to additional formation of S-containing gaseous products. A number of non-destructive X-ray techniques have been used to investigate coal-S. Davidson [245] presented a brief account of the analysis of organic sulphur in coal using electron microscopies combined with energy dispersive X-ray (EDX) analysis, including scanning electron microscopy (SEM), electron probe microanalysis (EPM) and transmission electron microscopy (TEM). These techniques have been shown to be useful for the direct quantification of organic sulphur contents in coals [245]. However, major advances on coal-S functionalities have been made using X-ray spectroscopies [e.g. see summaries in Refs. 37 and 245], including X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) and extended Xray absorption fme structure (EXAFS) spectroscopy. Perry and Grint [14] attempted to study the sulphur in a Victorian brown coal using XPS technique although no detail was given other than noting that sulphur in most coals studied showed two peaks. Using XANES (K-edge), Huffman and co-workers [250] and Taghiei and co-workers [242] investigated the coal-S in a high-sulphur (5.73 %) Victorian brown coal (Glencoe), as is shown in Figures 6.26. The brown coal contained a negligible amount (0.02%) of pyritic sulphur. The third derivative of the XANES spectra of the brown coal clearly showed a peak representing disulphide functional groups in the brown coal, accounting for almost 30% of coal-S. All other Argonne Premium Coal samples (including Beulah Zap hgnite) did not show the presence of disulphides. Other main S functional groups in the Glencoe brown coal were found to be sulphidic (about 23 %) and thiophenic sulphur (about 43 %)). The minor peaks of oxidised sulphur functional groups are within the experimental accuracy of ± 5 - 10%. In a temperature-programmed reduction study, Dunstan and Walker [241] also reported the presence of high concentrations of disulphides in Victorian brown coal samples and some Australian black coal samples. Zhiguang and co-workers [251] studied the hydrous pyrolysis of a Loy Yang (S = 0.22 wt% db) brown coal sample and an Anglesea (S = 3.20 wt% db) brown coal sample. Up to 77 % of coal-S in the Anglesea brown coal and 50 % of coal-S in the Loy Yang brown coal were unstable during the hydrous pyrolysis at 330°C. Among many Scontaining compounds detected in the products, only long chain w-alkyl thiophenes are believed to reflect structures in the original brown coal [251]. Re-incorporation of H2S etc into the organic matter during the formation of coal is believed to be an important process for the formation of stable S-containing structure in coal [251]. Both X-ray techniques and destructive techniques (e.g. TPR) indicate that the proportion of aromatic sulphur in coal increases with increasing rank [37,252].
348
Chapter 6
RTstan
Sulf^ Q Suifonc D SuifoxKie G Daulfidic Z SvtlMic B ThM^>lHauc | Figure 6.26 Percentages of sulphur in various functional forms as a function of temperature for Australian brown coal under pyrolysis in helium atmosphere. Reprinted with permission from Ref 242. Copyright 1992 American Chemical Society.
RTstan
• Sulfone D Sulfoxide G Sulfonic Acid D Dtsulfidic D Sulfidic M Thiqihgmc | Figure 6.27 Percentages of sulphur in various functional forms as a fiinction of temperature for Australian brown coal under oxidative condition in 95% He + 5% O2. Reprinted with permission from Ref 242. Copyright 1992 American Chemical Society.
Coal-N and Coal-S
349
6.5.2. Conversion of Coal-S during the Pyrolysis, Gasification and Combustion of Victorian Brown Coal As noted above, the Victorian brown coal currently being mined has low sulphur contents and therefore the conversion of coal-S in Victorian brown coal has not been studied extensively. H2S is by far the most important S-containing gaseous product during the pyrolysis of coal. Tan and Li [103] determined the yield of H2S from the pyrolysis of Loy Yang brown coal. As is shown in Figure 6.10, heating Loy Yang brown coal at 6.7 K min'^ to lOOO^'C released about 48 % of its sulphur as H2S; the majority of this sulphur was released at temperatures lower than 600°C. On the other hand, heating the same Loy Yang at a fast heating rate (>10^ K s'^) to 800°C released about 55 % of its sulphur as H2S, as is shown in Figures 6.14 and 6.16. Heating rate had only a small effect on the H2S yield, probably as a result of volatile-char interactions (e.g. self-gasification of char by volatile species to release coal-S) in the reactor used. A small decrease in the yield of H2S was observed when the temperature was increased from 700 to lOOO^C, probably due to the re-incorporation of H2S back into the char at high temperature. Tan and Li [103] concluded that there were two types of organic sulphur in the Loy Yang coal sample. One type of the sulphur structures seems to be very thermally unstable, likely to be of aliphatic nature based on the data in Figure 6.25. Another type of the sulphur structures seems to be very thermally stable up to 1000°C (Figures 6.10, 6.14 and 6.16), likely to be of aromatic nature e.g. thiophenes. As is shown in Figure 6.26, some Victorian brown coal contains disulphides. The disulphidic structures are not stable and start to degrade at lower temperature than sulphidic structures, while the percentage of thiophenic sulphur increases as a function of temperature [242]. Taghiei and co-workers noted [242] the possible conversion of aliphatic sulphides into aromatic thiophenes during the pyrolysis of low rank coal, although care should be taken that the histogram (Figure 6.26) shows the relative percentages of the sulphur forms and not their absolute amounts. The minor oxidized forms of sulphur in coal, such as sulphoxides, sulphones and sulphates, started to disappear at 200°C (Figure 6.27). Taghiei and co-workers [242] also investigated the transformation of coal-S in the brown coal during oxidation in 5 % O2, as is shown in Figure 6.27. Qualitatively, the trends observed during oxidation were similar to those during pyrolysis with the exception of sulphonic acid, probably as a result of the oxidation of coal-S (probably disulphides). At the end of oxidation experiments shown in Figure 6.27, only about 1.5 wt% coal-S was oxidised (into SO2). The presence of metallic species, especially alkali and alkaline earth metallic species, significantly affects the conversion of coal-S during the pyrolysis of low rank coals [253-255]. Telfer and Zhang [255] showed that the presence of water-soluble inorganics in a South Australian Bowmans coal led to increased retention of sulphur during pyrolysis between 400 and 500°C. The acid-soluble (organically bound) inorganics in the coal also suppressed the release of coal-S during pyrolysis between 500 and 600°C. In a study on the catalytic effects of Ca on the gasification of coal in steam, Ohtsuka
350
Chapter 6
and Asami [256] showed that the presence of Ca helped to retain sulphur during the pyrolysis of coals (including Yalloum brown coal) at 700°C. The release of coal-S during gasification [257-261], particularly as H2S, is a major environmental concern as it can be converted into SOx during the subsequent combustion in a gas turbine. It is necessary to remove as much H2S as possible from the gasification product gas. Takarada and co-workers [259-261] have investigated the use of Ca-exchanged char from low rank coal as an absorbent for H2S. When Ca-exchanged Yalloum brown coal was pyrolysed, the char thus produced was found to be a very efficient H2S absorbent. H2S removal was not found to be a function of the particle size of the Ca-exchanged coal char [259], because ultra-fine CaO particles were produced from Ca-exchanged Yalloum coal, independent of the coal particle size. Clearly, this is an important advantage of brown coal char over other absorbents for desulphurisation, either as a means of in-situ desulphurisation in a gasifier or a down stream desulphurisation unit. CaS formed from the reactions of CaO in the char from Ca-exchanged brown coal was also shown to have extraordinarily higher reactivity than calcined limestone for its oxidation to CaS04 or CaO for safe disposal [260]. Up to 85 % of CaS was oxidised into CaS04 at 700°C. Takarada and Yamaguchi [261] further showed that CaS thus formed also had a high catalytic effect for the char gasification in steam or CO2. Char from Victorian brown coal, with its abundant porous stmcture, has also been studied for other means of gas desulphurisation [217,262]. Zinc ferrites (ZnFe204) could be prepared in the presence of Yalloum coal by impregnation and calcined at 500°C in air [262]. ZnFe204 crystallite size was about 5 - 1 3 nm. The absorbent showed ability to reduce H2S to less than a few ppm at 500°C and can be regenerated easily by oxidation inAr-02. The combustion of low-sulphur Victorian brown coal in a circulating fluidised-bed combustor gave low SOx emissions (< 150 ppm @ 6 % O2) [231]. Its low sulphur (and sodium) contents also help to reduce the ash buildup on bed material particle surfaces, thus reducing the defluidisation problems during the fluidised-bed combustion of Victorian brown coal [263], as sodium and sulphur are believed to be involved in the formation of low-mehing-point compounds (e.g. alkali sulphates) in the ash coating on the bed particle surfaces.
REFERENCES [ 1 ] Leppalahti J, Koljonen T. Fuel Processing Technology 1995;43:1. [2] Brockway, D.J. and Higgins, R.S., Chapter 5 in The Science of Victorian Brown Coal (Ed: R.A. Durie), Butterworth-Heinemann, Oxford, 1991. [3] Davidson RM. Nitrogen in coal. IEAPER/08. lEA Coal Research, 1994, London, UK. [4] Waanders J, Wall TF, Roberts J. Chemistry in Australia 1980;47:274. [5] Standards Australia, Australian Standards AS 2434.6-2002, Method for the
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(Ed: R.A. Durie), Butterworth-Heinemann, Oxford, 1991. [238] Holdgate GR, Kershaw AP, Sluiter IRK. International Journal of Coal Geology 1995;28:249. [239] Holdgate GR, Wallace MW, Gallagher SJ, Taylor D. International Journal of Coal Geology 2000;45:55. [240] Brockway DJ, Ottrey AL, Higgins RS, Chapterl 1 in The Science of Victorian Brown Coal (Ed: R.A. Durie), Butterworth-Heinemann, Oxford, 1991. [241] Dunstan BT, Walker LV. The Functionality of Organic Sulphur in Australian Coal, Final project report. School of Applied Science, Gippsland Institute of Advanced Education, Churchill, Victoria 3842, Australia, 1990. [242] Taghiei MM, Huggins FE, Shah N, Huffman GP. Energy & Fuels 1992;6:293. [243] Attar A. Fuel 1978;57:201. [244] Damste JSS, De Leeuw JW. Fuel Processing Technology 1992;30:109. [245] Davidson RM. Fuel 1994;73:988. [246] Schicho RN, Brown SH, Olson GJ, Parks EJ, Kelly RM. Fuel 1989;68:1368. [247] Calkins WH. Energy & Fuels 1987; 1:59. [248] Calkins WH, Torres-Ordonez RJ, Jung B, Gorbaty ML, George GN, Kelemen SR. Energy & Fuels 1992;6:411. [249] Lafferty CJ, Mitchell SC, Garcia R, Snape CE. Fuel 1993;72:367. [250] Huffman GP, Mitra S, Huggins FE, Shah N, Vaidya S, Lu F. Energy & Fuels 1991;5:574. [251] Zhiguang S, Batts BD, Smith JW. Organic Geochemistry 1998;29:1469. [252] Maes II, Gryglewicz G, Machnikowska H, Yperman J, Franco DV, Mullens J, Van Poucke LC. Fuel 1997;76:391. [253] Manzoori AR, Agarwal PK. Fuel 1992;71:513. [254] Telfer MA, Zhang DK. Energy & Fuels 1998; 12:1135. [255] Telfer M, Zhang DK. Fuel 2001;80:2085. [256] Ohtsuka Y, Asami K. Energy & Fuels 1995;9:1038. [257] Highsmith JR, Soelberg NR, Hedman PO, Smoot LD, Blackham AU. Fuel 1985;64:782. [258] Middleton SP, Patrick JW, Walker A. Fuel 1997;76:1195. [259] Garcia B, Takarada T. Fuel 1999;78:573. [260] Garcia B, Yamazaki Y, Takarada T. Fuel 1999;78:883. [261] Takarada T, Yamaguchi D. Proceedings of the 18th Annual International Pittsburgh Coal Conference, 3 - 7 December 2001, Newcastle, Australia, published in the form of a CD-ROM. [262] Ikenaga N-o, Ohgaito Y, Matsushima H, Suzuki T. Fuel 2004; in press. [263] Vuthaluru HB, Zhang Dk, Linjewile TM. Fuel Processing Technology 2000:67:165.
Advances in the Science of Victorian Brown Coal Edited by Chun-Zhu Li © 2004 Elsevier Ltd. All rights reserved.
Chapter 7 An Overview of Advanced Power Generation Technologies Using Brown Coal Sankar Bhattacharya^ and Atsushi Tsutsumi ^ ^Cooperative Research Centre for Clean Power from Lignite Unit 8, 677 Springvale Road, Mulgrave, Victoria 3 J 70, Australia ^Department of Chemical System Engineering, The University of Tokyo Bunkyo-ku, Tokyo, Japan
l.\.
INTRODUCTION
There are large deposits of brown coal/lignite in countries such as Australia, Germany, USA, China, Indonesia and India. Many of these low-rank coals have high moisture contents. In the case of Victorian brown coal, the moisture contents may be up to 70 % (see Chapter 3), adversely affecting the efficiency of power generation from a conventional boiler plant that leads to a concomitant increase in CO2 emission. The efficiency obtained from the modem boiler plant at Loy Yang power station is about 29 % (higher heating value, sent-out basis) for a coal moisture content of 62 %. This compares with about 36 % for a conventional boiler plant fuelled with high-rank coals. A number of advanced technologies have been or are being developed to improve thermal efficiency, to reduce NOx, SOx and CO2 emissions and to reduce the cost of electricity. These include the circulating fluidised bed combustion (CFBC), pressurised fluidised bed combustion (PFBC), integrated gasification combined cycle (IGCC), integrated gasification fuel cell (IGFC) technologies. These technologies have reached demonstration, semi-commercial or commercial scales [1-5]. Most of these studies have been carried out for high-rank coals. There are, however, specific issues to consider in the utilisation of high-moisture low-rank coals for power generation and, in particular, the need to incorporate a coal drying process. Other properties of low-rank coals such as coal reactivity, alkalinity of the ash and the difficulty of handling raw coal also strongly impact on the design of the system components. This chapter describes the analyses of various advanced power generation technologies using brown coal. The efficiencies determined here are based on the first law of thermodynamics. This is essentially an accounting of energies entering and exiting the system. Efficiencies are evaluated as ratios of energy quantities and are often used to assess the performance of a system and to compare various systems. The chapter does not include an economic evaluation of the technologies considered. It also does not delve in the liquid fuel or chemical production processes from brown coal.
Advanced Power Generation Technologies 7.2. KEY PROCESSES IN ADVANCED POWER TECHNOLOGIES - IMPACTS OF LOW-RANK COAL
361 GENERATION
Key processes in advanced power generation technologies involving low-rank coals include drying, gasification, combustion of fuel gas in gas turbines, gas cleaning, combustion of char if generated during gasification, fuel cells and their integration with the rest of the cycle. There are a number of special properties of low-rank coals that must be considered in the design of a power generation system: • the very high moisture content (up to 70 % for Victorian and South Australian coals), • the high reactivity of low-rank coals, • the generally high alkali content in ash that affects fouling. These properties impact particularly on the drying, gasification and gas cleaning systems used in advanced power generation processes. 7.2.L Drying In combustion or gasification processes, all moisture in coal has to be dried either prior to the process in a separate drier or during the process. As drying is an energy consuming process, the cycle efficiency of a power plant can be significantly improved by the optimum choice of the drying process. While a more detailed discussion on the dewatering/drying of brown coal is given in Chapter 3, three drying processes considered in the process simulations to be described in this chapter are briefly discussed below in relation to their impacts on the overall process efficiency. 7.2.7./. Drying Using Hot Gas In a conventional boiler plant, raw coal is dried with hot flue gas aspirated from the furnace by fans or beater mills. After drying, the dried pulverised coal is entrained in the cooled flue gas and directed to the furnace (via the burners). Drying through direct contact between the moist coal and the hot gas may also be accomplished in a fluidised bed [6,7]. While there has been considerable work on fluidised bed drying using hot gas at pilot plant scales, only two demonstration plants have been commissioned: one in Wyoming for drying Powder River Basin sub-bituminous coals and the other in Finland for drying peat. For hot gas drying in a fluidised bed, crushed brown coal is required in order to reduce the residence time during drying. A similar process has been proposed by HRL Technology Ltd for its Integrated Drying and Gasification Combined Cycle (IDGCC) process [8], in which the coal is dried in the hot fuel gas leaving the gasifier followed by the separation of coal from the gas in a cyclone before the dried coal is fed into the gasifier. The cooled fiiel gas, together with evaporated moisture, is directed via a filter to the gas turbine. Disadvantages of hot gas drying are the dilution of the fuel gas by the evaporated
362
Chapter 7
moisture (reducing the specific energy of the fuel gas) and the potential to trap alkali and alkaline earth metals in the gasifier/drying circuit (leading to fouling). 7.2.7.2. Drying Using Steam Steam drying can be accomplished either by direct or indirect contact of coal with steam. The most commonly used steam driers for drying brown coal in Victoria and Europe are the rotary drum steam type, which are used in the making of briquettes [9]. In these driers, coal is fed through tubes contained in a drum or shell and the heat for drying is supplied by steam condensing on the outside of the tubes. The moisture evaporated from the coal is carried through the tubes in a low velocity air stream. More recently an indirect drying process has been developed for brown coal involving a steam fluidised bed dryer (SFBD) (see Chapter 3 for details). In a SFBD, crushed coal (<4 mm) is dried in a fluidised bed using steam as the fluidising medium either at atmospheric pressure or elevated pressures. The heat for drying is supplied to the fluidised bed through tubes by steam condensing at about 5 bar (in the case of an atmospheric pressure steam fluidised bed). To enable effective heat transfer and hence drying of the coal, a temperature difference of 50°C between the heating steam and the bed is required. In one arrangement, this heating steam is extracted from the low pressure (LP) steam turbine in the steam cycle. Alternatively, the heating steam may be obtained by the recompression of the moisture evaporated from coal. The effluent from this option is the condensate formed from the water evaporated fi"om the coal. While vapour recompression does not reduce the output from the steam cycle, the vapour compressor has significant power consumption. The two SFBD options are analysed in detail in subsequent sections. One disadvantage of the extraction scheme is the lack of opportunities for using the latent heat in the saturated water vapour evaporated from the coal, particularly in the case of an atmospheric pressure SFBD. Part of the energy can be used for feed heating of the steam cycle and preheating of the incoming raw coal fed to the drier. Even then, a substantial part of the energy remains unused. Alternatively, the steam could be used to drive a separate low pressure steam turbine. Use of a vapour recompressor obviates this problem because the effluent from the process is hot water, although cost and availability of suitable vapour recompressors of adequate pressure and flow capability seems to be a major problem. In an atmospheric pressure SFBD, the capacity required from vapour compressors is around 1.1 - 1.4 x 10^ m^ hr' to meet the drying requirements of a 300 MWe power station fuelled with coal of 62 wt% moisture content. Another type of steam drier, called a bed mixing drier, is being developed by Imatran Voima Oy (IVO) of Finland. The drier uses hot bed material from a fluidised bed boiler as a heat source. A flash drier is used although in principle it can be of any type. Hot bed material, extracted from the fluidised bed, is mixed with the carrying steam just before the wet fuel is introduced. At drier exit, the dried fuel and the cooled bed material are separated from the carrying steam in a cyclone. Part of the steam is recycled from the cyclone to provide the carrying steam. The latent heat of the evaporated steam is
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recovered and used for feed heating in the steam cycle. IVO has a demonstration combined heat and power (CHP) plant that integrates a bed mixing drier with a bubbling fluidised bed boiler fired with peat and wood wastes which are dried from a initial moisture content of 50 - 55 % to a final moisture content of 10 - 15 %. 7.2.1,3. Non-evaporative Drying There are various thermal dewatering processes that have in common the ability to reduce the moisture content of coal without the evaporation of water. These processes are described in detail in Chapter 3. 7.2.2. Gasification Gasification is the reaction of a solid fuel with reactants (gasifying agents) such as oxygen, steam, carbon dioxide or a mixture of these gases to yield a fuel gas suitable for the production of power, liquid fuels, chemicals or other fuel gases. Coal gasification is conceptually a two-step process: the pyrolysis of coal to produce volatiles and char and then the gasification of the char with gasifying agents. Volatiles also react with the gasifying agents. Reactions taking place during pyrolysis and gasification have been discussed in detail in Chapters 4 and 5. In general, it is possible to get higher carbon conversion in gasifiers as oxygen feed is increased. However, this also results in higher levels of carbon dioxide and water, which are unwanted products from coal gasification. Steam acts both as a gasification agent and as a temperature moderator. Therefore, quantity of steam that is used is in excess of what is required for steam-carbon gasification. The steam must have a minimum temperature corresponding to the saturation temperature at the gasifier working pressure. This is necessary to prevent the condensation of steam in the feed lines. Three major types of generic processes exist for coal gasification depending on flow geometry: entrained flow gasifiers, moving bed (also termed fixed bed) gasifiers and fluidised bed gasifiers. Only a brief discussion about the operation characteristics of these gasifier types will be given here and a fuller discussion of these gasifiers may be found in the literature [4]. Entrained flow gasifiers operate with coal and gasifying agents in co-current flows. The coal feed is ground to/?/size (~ 100 |Lim) to promote mass transfer and transport in the gas. Residence time is of the order of a few seconds and therefore high temperatures are required to ensure good conversion. Also, it is important to ensure that the unconverted char and ash do not foul the downstream gas cooler. For these reasons, all entrained flow gasifiers traditionally run in slagging mode so that majority of the ash are collected away from the gas cooler and reliably removed from the system as slag. The high temperature requirement means the operation under oxygen rather than air. Oxygen requirement becomes higher if the feedstock has a high moisture content or a high ash yield. Fluxing agents are added to reduce the ash melting point if necessary. Moving bed gasifiers operate with coal and gasifying agents in counter-current flows. The hot fuel gas from the gasification zone is used to preheat and pyrolyse the coal that
364
Chapter 7
usually flows downward. Feed size is very large to allow slow downward movement and long contact time with the upward moving fuel gas. Excessive fines in feed or coals having propensity for caking are not preferred in a moving bed as these can block the passage of the gas. Outlet temperature of the product gas is low and the product gas does contain pyrolysis liquids. Fluidised bed gasifiers offer good mixing between coal/char and gas, facilitating heat and mass transfer. Temperature of operation is generally limited below the ash deformation temperature to maintain good fluidisation and mixing. To prevent the build up of ash and hence agglomeration, ash is removed from the bed at regular intervals. Good mixing in fluidised bed invariably means that partially reacted fuel, often with significant carbon content in it, is removed with the ash. This is one of the reasons for limiting carbon conversion in fluidised bed gasifiers. Some fluidised bed gasifier designs promote controlled agglomeration by operating into the higher temperature to improve carbon conversion. In general, lower temperature operation of fluidised bed means that these are more suitable for reactive fuels such as brown coals or biomass. The feed size in fluidised bed has to be large. Otherwise fines present in the coal or generated due to attrition and reaction elutriate away from the bed resulting in reduced carbon conversion. Majority of the fluidised bed gasifiers operate either in bubbling (~ 1 m s'^ superficial velocity) or circulating (3 - 5 m s ' superficial velocity) mode. KBR (Kellog Brown Root) transport gasifier is also being developed in the high velocity ( 1 0 - 1 8 m s'^) regime [10]. This development is intended to demonstrate higher circulafion rate, better mixing and heat transfer rates and higher carbon conversion than in fluidised beds. In general, brown coal tends to have higher reactivity than high-rank coals and thus the gasification of brown coal (lignite) may be carried out at lower temperatures than high-rank coals, resulting in higher specific energy of the fuel gas with more carbon present as carbon monoxide than as carbon dioxide. 7.2.3. Combustion of Fuel Gas Pilot plant trials have indicated that the specific energy of the fuel gas derived from the gasification of dried brown coal varies typically from 5 - 10 MJ k g ' for oxygenblown gasification and 3 - 5 MJ kg'' for air-blown gasification [11]. The major combustible components of these gases are CO, H2 and CH4; the proportioning of which is influenced by the type of gasifier, gasifying conditions and the mix of inlet gases. However, it should be noted that, even for an oxygen-blown gasifier, the specific energy of the gas at the combustor may be reduced to as low as 4 MJ kg' by admitting the nitrogen from the air separation plant into the fuel gas prior to the combustor. This has been done in the 250 MWe IGCC plant at Buggenum, Holland, for the control of NOx emissions. In general, the IGCC application requires the modification of gas turbine. Air will normally be extracted from the gas turbine compressor to supply the gasifier, although this may be optional for an oxygen-blown gasifier. It may be possible in the case of an oxygen-blown gasifier with an independent oxygen supply to accommodate the
Advanced Power Generation Technologies
365
additional mass flow into the turbine section without causing the turbine compressor to surge. The use of low specific energy gases will require the modification of the gas turbine combustor to ensure stable combustion with acceptable levels of NOx, CO and unbumt hydrocarbons. In the case of low-rank coals, water vapour is a significant diluent, whereas nitrogen is the main diluent for gas derived from high-rank coals. The different diluents may affect the minimum specific energy acceptable for use in a gas turbine, which is reported to be about 3.8 MJ kg'^ [12]. The high moisture content of low-rank coals also has a potential to cause turbine compressor surge if the flue gas mass flow that passes through the turbine section exceeds the air flow through the compressor beyond the design margin. The technologies requiring CO2 sequestration will involve separation of hydrogen from the fuel gas produced in the gasifier. This will require gas turbines to use 90 %+ hydrogen fuel [13,14]. E-class gas turbines are already in service with 65 - 95 % hydrogen in the fuel gas. While acceptable combustion under the F-class conditions has been demonstrated, it requires a diffusion combustor with diluent injection. Nitrogen diluent is preferable to steam in this regard [13]. 7.2.4. GasClean-Up For advanced technologies, the gas streams from the combustion or gasification of coal need to be cleaned to meet emission standards for particulates and gaseous pollutants and to protect the process components (furnaces, gas turbines etc.) against deposition, erosion and corrosion. Depending on the temperature of the process, the gases that need to be cleaned include H2S, HCN, NH3, NOx, SO^ and the vapour phase NaCl, NaOH and Na2S04. The limits imposed for gas turbines are approximately 20 ppbw for vapour phase alkali species and 10 - 12 ppmw for particulates [2]. Gas clean-up can be achieved either at low or high temperatures and termed as cold gas clean-up (CGCU) or hot gas clean-up (HGCU) respectively. Cold gas clean-up refers to process temperature below the dew point of the gas to be removed; typically <300 - 400°C [2]. Two major processing steps are generally employed in the cold cleaning of fuel gases: the initial wet scrubbing with or without simultaneous gas quenching followed by the acid gas treatment to reduce sulphurous gases. The first step involves the removal of particulates, tars, condensed organics, trace elements, watersoluble gas components (NH3 and HCN) and the free and reduced halogen components. The second step involves the removal of acid gases that have poor solubility in the wet scrubbing operation. This separation step may involve solvent absorption, gas adsorption, cryogenic separation or membrane separation. While cold gas cleaning is a well-established process, it does result in a loss of cycle efficiency. Gas cleaning at high temperatures offers the potential of higher cycle efficiencies from the advanced power generation technologies. However, there are practical limitations on the maximum allowable temperature for gas cleaning. Ceramic candle filters, granular bed filters and iron aluminide filters have been under development for the removal of particulates and
366
Chapter 7
alkali species. It appears that ceramic filters operating at temperatures above 600°C are not yet considered reliable; moreover, it becomes increasingly difficult to remove the alkalis at temperatures above 500°C because of the relatively high vapour pressure. Mercury emission has been of considerable concern recently for all coal-based power generation technologies. The primary forms of mercury in the post-combustion gases are elemental mercury and its oxidised forms, Hg^ and Hg^^ [15,16]. Elemental mercury is difficult to capture. There is currently no single technology that can control mercury emissions from power plant flue gas emissions [17], even though several techniques (sorbent injection - TOXECON^"^ process as an integrated system to control emissions of mercury, particulate matter, sulphur dioxide and nitrogen oxides, ultraviolet light oxidation of the elemental mercury) are being developed. However, for gasificationbased technologies, proven and economic methods of mercury removal are available and have been practiced for many years. Elemental mercury emission form gasification plants can be controlled (90 - 95 % mercury capture) by using sulphur-impregnated activated carbon bed [18]. 7.2.5. Combustion of Char As was indicated in Section 7.2.2, fluidised bed gasifiers do invariably produce char, either as fine particles (pf size) escaping to the gas filter or as coarse particles (mm sized) from the bottom of the gasifiers. These have to be further utilised in order to improve the efficiency of the process. If recycled into the gasifier, ensuring sufficient residence time for the fines for full gasification will be difficult. Rather, these fines can be further utilised through combustion in a boiler or in a fluidised bed. Combustion tests were undertaken in a drop tube furnace simulating p/boiler environment at 1100°C and 1250°C with 20% excess air and 3 - 4 seconds residence time. Carbon burnout from such tests exceeded 99 % at 1250°C for filter fines from the fluidised bed gasification of Loy Yang and Yalloum [11]. Combustion trials undertaken in a circulating fluidised bed combustion pilot plant using a char (1 - 4 mm in size) produced from Yalloum coal also showed carbon burnout in excess of 99 % [19]. 7.2.6. Electrochemical Conversion of Fuel Gas in Fuel Cells Fuel cells directly convert the chemical energy of hydrogen in a fuel gas into electrical energy by an electrochemical reaction. Fuel cell systems can reach high efficiencies of 45 - 60 % and could be a very low-emission technology. The fuel gas produced by gasification is suitable for this purpose, as the main additional treatment (apart from removing contaminants such as chlorides, sulphides, alkali, etc.) required is the water-gas shift reaction to convert CO in the gas to additional hydrogen and CO2. Various types of fuel cells [20] are being developed and Table 7.1 shows the characteristics of the major types of fuel cells currently available [21,22]. Of the fuel cells listed in Table 7.1, MCFC and SOFC are the most suitable for coal gas application due to their operability at high temperature. The most developed SOFC in terms of stack
367
Advanced Power Generation Technologies
size and accumulated operating hours is the Westinghouse tubular SOFC. However, operation of either fuel cell at high pressure is yet to be demonstrated.
7.3. ADVANCED POWER GENERATION PROCESSES Conventional power generation cycles have been based on the use of steam as the working fluid in a sequence of thermodynamic processes known as the Rankine cycle, the efficiency of which is determined by the maximum and minimum temperatures of the cycle. However, steam temperature limits (around 600°C) imposed by metallurgical considerations has meant that it has been difficult to achieve efficiencies of more than about 40% (HH V) in a Rankine cycle alone. The advanced technologies such as IGCC and PFBC use gas turbines in combined cycle to increase efficiency. The gas turbine, which utilises the Brayton cycle, has the gas turbine inlet temperature currently as high as 1350°C and rejects heat at temperatures typically in the range 500 to 600°C. This relatively high grade heat can be utilised to generate steam to drive a steam turbine operating in a Rankine cycle. For natural gas, the efficiency of a modem gas turbine in open cycle is about 33 %, which can be increased to about 60 % in a combined cycle. Figure 7.1 shows the temperatureentropy diagram of a combined cycle. The closer is the integration of the Brayton cycle (ABCDA) and the Rankine cycle (abb'c'cda), the lower is the loss from the system and the higher is the efficiency. The efficiency also increases if the sensible heat in the gas is converted via the gas turbine in Brayton cycle rather than being used to generate steam for conversion via the steam turbine in Rankine cycle. The efficiency of a combined cycle increases with increases in gas turbine inlet temperature and decreases in steam turbine exhaust temperature (line ad). Figure 7.2 shows a typical schematic diagram of a Brayton cycle - fuel cell Rankine cycle combination with fuel gas from any source including a coal gasifier. The fuel cell converts the majority of the chemical energy of the fuel gas using compressed air from the air compressor outlet, with the gas turbine utilising the residual fuel gas and the sensible heat from the fuel cell exhaust gas. In the overall cycle, the proportion of total power produced by the Rankine cycle is much smaller than the power produced the combination of the Brayton cycle and the fuel cell system. This results in a high cycle efficiency. Table 7.1 Characteristics of principal fuel cells (based on Refs. 21 and 22). Type
Electrolyte
Operating temperature
Fuel
SOFC
Yttria doped Zirconia
800-1000°C
H2,C0
Hours
MCFC
Molten Li/Na/K carbonate
650-800°C
H2,C0
Hours
PAFC
Phosphoric acid
160-200°C
H2
Hours
PEMFC Hydrated organic polymer
50-100°C
H2
Seconds-minutes
Start-up time
368
Chapter 7
7.3.1. Processes Simulated and General Assumptions This section describes the major features of the technologies that have been simulated using a methodology [23] developed using two commercially available software packages - ASPEN PLUS and GTPRO. Several variations of these technologies are possible as different combinations of the various process variables. Some of these variations are considered in these simulations to test the sensitivity of the results to various process variables. Details and limitations of these variations are described in the appropriate sections; relevant development issues are also identified and discussed in brief. Several case studies are investigated for the following power generation technologies: circulating fluidised bed combustion (CFBC), integrated gasification combined cycle (IGCC), pressurised fluidised bed combustion (PFBC), advanced pressurised fluidised bed combustion (A-PFBC), partial gasification combined with atmospheric fluidised bed combustion (PG/CFBC), integrated gasification fuel cell (IGFC) combination. Assumptions common to all simulation studies are indicated in Table 7.2. Variations from these assumptions or additional assumptions are indicated in the discussion of each technology. Properties of the coal samples considered in the simulations are given in Table 7.3. Coal A represents a typical Loy Yang brown coal (also see Chapter 2). Coal B and C are the same coal but with lower moisture contents. Both coals are assumed to be dried to a
Qin
^ 1 ^
i^
^ ^
Brayton cycle
^ ^
, D J 1^
B ^X^"^
.
ri T
Qex
by ^r b y^
V Rankine cycle T
Qcw
Figure 7.1 Temperature (T) - Entropy (S) diagram of a combined cycle.
1 d
Advanced Power Generation Technologies
Fuel gas
369
Fuel cell Anode
Figure 7.2 A schematic diagram of a typical Brayton cycle - fuel cell - Rankine cycle combination.
moisture content of 20 % whenever SFBD drying is considered. The coal might be dried to a moisture content of 15% or less (see Chapter 3). The effect of drying to a lower moisture content is also examined in the study. In cases where hot gas drying was considered, the final moisture content of coal was calculated by considering the hot gas wet bulb temperature calculated from the gas temperature and moisture content.
370
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For IGCC applications, the performance characteristics of the GE 9311 FA were used. For PFBC application, the performance is based on ABB's purpose-built GT-140P turbine that has been adapted specifically for the application to give good efficiency at low gas temperatures and to tolerate low levels of entrained fine ash. For A-PFBC and PG/CFBC applications, the existence of a gas turbine generically similar to the GE 9311 FA was assumed. The leading parameters of the GE 9311 FA turbine are listed in Table 7.4 [24]. Although a nominal size of 300 MWe is established as the standard for these simulations, this varies according to actual gas turbine outputs. 7.3.2. Circulating Fluidised Bed Combustion (CFBC) Systems Circulating fluidised bed boiler technology is essentially a developed technology, which is offered by a number of major companies. The largest unit size currently installed is 300 MWe at Jacksonville, Florida, although there are design studies to scaleup beyond 300 MWe [25]. In a CFBC, the fluidising velocity is sufficient to carry the coal and bed material through the combustion chamber into a hot cyclone where the majority of the solids are separated and returned to the combustor. The hot flue gas from the cyclone passes to a convective boiler section. A CFBC typically operates at a fluidising velocity of at least 5 m s'^ and a combustion temperature in the range 850 900°C. Steam is generated and heated both in the combustor chamber, which has a water wall construction, and in the convective boiler section. The proportion of heat transferred to the steam cycle from the convective boiler section relative to that from the fluidised bed increases with the moisture content of the coal burned [26]. A CFBC designed for a high moisture content coal will therefore have a larger convective boiler. The CFBC manufacturers offer two configurations of plant. In a design promoted by Lurgi, Foster-Wheeler, Combustion Engineering and others, an external heat exchanger (EHE) is incorporated between the cyclone and the combustion chamber (which is separately fluidised) to control the temperature in the fluidised bed. In another design, offered by Pyropower and others, additional heat from the fluidised bed section can be
Table 7.2 Common assumptions made in the simulation of power generation processes. Parameter Nominal plant capacity Carbon conversion Moisture content in the raw coal Moisture content in the dried coal Heat lossfromthe drier, gasifier, and combustor Power consumption in coal preparation Gas turbine Ambient conditions (for exergy analysis and air compressor use)
Assumed value 300 MWe 100% 62% 20% 1 MWth each 10 MWe GE 9311 FA 20°C, 1 atm
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Advanced Power Generation Technologies
transferred via a platten superheater element in the combustion chamber to control the temperature in the fluidised bed. The pressure and temperature of the steam that can be raised in a CFBC are essentially the same as in a conventional boiler, although currently CFB boilers with supercritical pressures are at the design stage. In the simulation of CFBC system to be described below, the CFBC configuration of Pyropower was used. A schematic diagram of the CFBC system is shown in Figure 7.3 for two cases: Atmospheric pressure SFBD - with extracted heating steam: Case 1 Raw coal of 62 % moisture content is dried in the atmospheric pressure SFBD to 20 Table 7.3 Properties of coal considered in the simulation (dry basis). Proximate analysis, % Fixed carbon Volatiles Ash
Ultimate analysis, % 49.3 48.5 2.2
Carbon Hydrogen Nitrogen Sulphur^ Oxygen Ash
67.56 4.6 0.59 0.35 24.7 2.2
^ all sulphur is in organic form.
Moisture content (%, wet basis) Higher heating value (MJ kg"', dry basis) Higher heating value (MJ kg"'' wet basis)
Table 7.4
Coal A
CoalB
CoalC
62 26.4 10.03
45 26.4 14.52
20 26.4 21.12
Gas turbine data (ISO conditions) [24].
Description Nominal power output, MWe Intake pressure, bar temperature, °C
GE9311 FA 231 1.013 20
Compressor airflow, t hr"' pressure ratio delivery temperature, °C
2187 14.6 400
Turbine inlet temperature, °C exhaust temperature, °C
1288 587
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% moisture content. The CFBC is supplied with dried coal at 90°C. Heating steam for the SFBD is extracted from the LP turbine at 5.2 bar and the condensate returned to the deaerator at 4.5 bar and 140°C. The moisture is assumed to be evaporated from the coal at 100°C. Part of the latent heat of this "dirty" steam is recovered for feed heating of the condensate (in the steam cycle) between the condensate extraction pump and the deaerator. Three regenerative high pressure (HP) heaters are used between the deaerator and the economises Steam is raised in the convective boiler and in the bed. Atmospheric pressure SFBD - with vapour recompression: Case 2 This system is similar to the previous one except that a vapour compressor is used to pressurise the evaporated coal moisture from 1 bar to 5.8 bar for use as heating steam. 7.3.3. Integrated Gasification Combined Cycle (IGCC) Systems Fuel gas is generated in the gasifier and purified in the gas clean-up system. Clean gas is sent to the gas turbine for combustion with compressed air to provide a stream of hot high pressure gas which drives the turbine to generate electricity. The exhaust gases from the turbine pass to a heat recovery boiler where steam is raised. This steam is passed to the steam turbine to generate additional electricity. The use of a fluidised bed gasifier is assumed for the following cases.
STEAM TURBINES
6' ASH
/*^
CONDENSATE HEATER
COAL MOISTURE HEATING STEAM
REGENERATIVE FEED HEATERS FEEDPUMP
(^~)^
*"
DRIED COAL
Figure 7. 3 A schematic diagram of the CFBC system with a seam fluidised bed drier.
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Advanced Power Generation Technologies Atmospheric SFBD - with extracted heating steam: Cases 3-8
Six different simulations were carried out under this category. In each case, raw coal is dried to 20 % moisture content. Heating steam for the SFBD is extracted from the LP turbine at 5.2 bar and the condensate returned to the deaerator at 4.8 bar. Dried coal of 20 % moisture is fed to the gasifier operated at 25 bar and 950°C. Compressed air at the outlet of the main GT compressor is at 14.6 bar and 394°C. Part of this air is extracted and cooled to 50°C, before boosting its pressure to 28 bar using a booster compressor. It is then heated to 450°C using the gas turbine exhaust for supply to the gasifier. In the process of its cooling from 394°C to 50°C, the compressed air heats part of the feed water from the IP economiser outlet, raising its temperature to saturation. This saturated steam is fed back into the steam circuit at the outlet of the IP boiler. Case 3 considers the gas entering the gas turbine combustor at 950°C without cooling. Cases 3a and 3b consider feed coal moisture contents of 45 % and 20 % respectively, with the latter not having a drying system. Cases 4 and 5 consider cooling of the gas to 650°C and 450°C respectively using part of the feed water from the HP economiser outlet. This is heated to saturation and fed back to the steam circuit at the outlet of the HP boiler. Cases 6 and 7 are similar to Case 3 except that carbon conversion in the gasifier is considered to be 98 % and 95 % respectively. Case 8 considers the use of a high gasification temperature as for an entrained flow gasifier.
GAS TURBINE
STEAM GENERATOR
GAS CLEANING STEAM TURBINE
0
COMPRESSEDAIR
FEED HEATING
ASH
ESP
CONDENSER
COAL MOISTURE HEATING STEAM
(±)^
WATER y^
FEEDPUMP
DEAERATOR
(^~")^
DRIED COAL
Figure 7.4 A schematic diagram of the IGCC system with a steam fluidised bed drier.
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Atmospheric SFBD - with vapour compression: Cases 9-11 These systems are similar to the systems in Cases 3 - 5 respectively except that a vapour compressor is used to pressurise the evaporated coal moisture from 1 to 5.8 bar for use as heating steam. Case 9 considers the gas entering the gas turbine combustor at 950°C without cooling. Cases 10 and 11 consider cooling of the gas to 650°C and 450°C respectively using part of the feed water from the HP economiser outlet. This is heated to saturation and fed back to the steam circuit at the outlet of the HP boiler. A schematic of the system for Cases 3 - 1 1 is shown in Figure 7.4. Pressurised SFBD - with extracted heating steam: Cases 12 - 14 Raw coal is assumed to be dried in a pressurised (25 bar) steam fluidised bed drier to 20 % moisture content. Heating steam is extracted from the HP turbine at 59 bar. This pressure was chosen in order to maintain a 50°C temperature difference between the saturation temperature of the heating steam and the bed temperature. The condensate from the heating steam is mixed with the feed water (to the steam cycle) before the deaerator. The moisture evaporated from the coal is at 25 bar. Three cases relate to the use of the evaporated moisture from the coal: exhausted and not used (Case 12), fed to the gasifier operated at 25 bar (Case 13) or mixed with the gasifier gas prior to cleaning (Case 14). In each case, the fuel gas is cooled to 450°C as in Case 5. Hot gas drying: Cases 15 - 21 In the current simulations, the concept of feeding raw coal or slurries of raw coal in water is investigated, as a pumped slurry or paste offers a possible approach to ameliorating the difficulties of feeding raw coal into a pressurised reactor. The system considered has the coal slurry dried in the hot product gas from the gasifier, which is diluted by the evaporated moisture. Feed water from the steam cycle is used to cool the fuel gas to its dew point temperature. The gas may also be wet scrubbed to remove condensate and entrained particulates. As the product gas after scrubbing is saturated with water vapour, the moisture content is decreased as the gas temperature is reduced. Part of the hot scrubbing water is used for slurry preparation. Seven cases are simulated in this category: six cases (15 - 20) have air blown gasification and one case (21) has oxygen blown gasification. In all cases, the gasifier is assumed to operate at 950°C and 25 bar. The gas cooling arrangements are as follows: • Case 15, raw coal (62 % moisture) with sensible cooling of the hot gas. • Case 16, raw coal (62 % moisture) with sensible cooling of the hot gas followed by water scrubbing to 150°C. • Case 17, coal slurry (66 % moisture) with sensible cooling of the hot gas followed by water scrubbing to 150°C. • Case 18, coal slurry (70 % moisture) with sensible cooling of the hot gas followed by water scrubbing to 1 OO^'C.
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• Case 19, coal slurry (70 % moisture) with sensible cooling of the hot gas followed by water scrubbing to 150°C. • Case 20, coal slurry (70 % moisture) with no cooling or scrubbing of the hot gas. • Case 21, O2 gasification of coal slurry (70 % moisture) with sensible cooling of the hot gas. A schematic of the process illustrating Cases 1 5 - 2 1 is shown in Figure 7.5. Two stage drying with char recycle and pressurised SFBD: Cases 22 - 23 In these cases, the coal is proposed to be dried in two stages in an attempt both to improve the cycle efficiency obtained from a pressurised SFBD and to provide a system conceptually more able to accept a slurry feed. In the first stage, the incoming coal is partially dried to 50 % moisture content in a steam drier operated at 25 bar using hot char recirculated from the fluidised bed gasifier (based on the bed mixing drier discussed in Section 7.1). The partially dried coal is then further dried to 20% moisture in a pressurised SFBD using extraction steam (from the HP turbine) as the heating medium. This arrangement of two stage drying requires less high pressure steam than would otherwise be required by drying in SFBD alone. The dirty steam (evaporated moisture from the coal) from the two driers is combined and added to the gasifier gas
HEAT RECOVERY BOILER
GAS TURBINE
STEAM TURBINE
GAS SCRUBBER
CONDENSER
O ...J
GAS COOLER RAW COAL
COMPRESSED AIR
V - ^
Figure 7.5 A schematic diagram of the IGCC System with a hot gas drier.
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before cleaning. Two cases have been simulated in this category assuming the incoming feed to be raw coal: dirty steam from the two driers mixed with the gasifier gas at 950°C (Case 22) or with the gasifier gas cooled to 450°C (Case 23). 7.3.4. Pressurised Fluidised Bed Combustion Systems In a PFBC system, hot pressurised combustion gas is supplied directly to the gas turbine. Combustion in PFBC systems is at a temperature (around 850°C) similar to that in atmospheric fluidised beds but at a pressure of 12 to 16 bar. The usual PFBC arrangement has the combustion chamber and the hot gas cleaning equipment enclosed in a single pressure vessel. Because the compressed fluidising air contains more oxygen per unit volume, combustion in the bed takes place at a higher intensity than in an atmospheric fluidised bed, enhancing carbon conversion. The higher combustor pressure results in a unit that is substantially smaller in size than a/?/fired boiler or an atmospheric fluidised bed boiler of equivalent output. Steam can be generated in PFBC boilers at high pressures (supercritical), whereas the steam pressure is limited to subcritical pressures in the heat recovery boiler of IGCC systems. PFBC systems may be either bubbling beds or circulating beds (PCFB). The gas velocity is about 1 m s'^ in a bubbling PFBC or 5 m s ' in a circulating PFBC. Carbon conversions in excess of 99.5 % are achieved in either type with typically 20 - 25 % and 10 % excess air respectively. The bubbling PFBC concept has reached commercial status even though its future is uncertain for new plants due to the lack of superiority in emission performance over other technologies. ABB offered bubbling PFBC in two standard unit sizes of nominally 200 MWt (P200) and 800 MWt (P800). The P200 unit incorporates a GT35P gas turbine specially modified for use in the PFBC and the P800 will use a similar but larger GT140P gas turbine. Four bubbling PFBC power plants based on the ABB P200 design have been in operation since 1990. Other companies, particularly Japanese companies, have more recently built PFBC plants similar to the ABB design. The PFBC system simulated here is based on the P800 system developed by ABB. This system employs a single gas turbine GT-140P with a nominal output of about 70 MWe under ISO conditions. Five cases were considered: • Case 24, PFBC without a drier, coal A, subcritical steam system. • Case 25, PFBC with SFBD dried coal A, extracted steam, subcritical steam system. • Case 26, PFBC with SFBD dried coal A, extracted steam, subcritical steam system. • Case 27, PFBC with SFBD dried coal A, extracted steam, supercritical steam system. • Case 28, PFBC with SFBD dried coal B, extracted steam, subcritical steam system. A schematic diagram of the system is shown in Figure 7.6. In all cases except Case 24, the coal is assumed to be dried to 20% moisture. Dried coal at 90°C (from the SFBD) is assumed to be fed directly into the pressurised combustor operated at 16 bar
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Advanced Power Generation Technologies •jGAS COOLER
-
-
EXTRACTION STEAM
t TO INTERCOOLER
FROM INTERCOOLER
CONDENSE!^
I ASH CWIN
TO ASH COOLER
CW OUT * • •
3t)
FEED HEATERS
o-
^
BFP
COMPRESSOR INTERCOOLER
-o •
Figure 7.6 A schematic diagram of the PFBC system considered in the simulation.
and 860°C. In each case, the coal flow rate was adjusted to give a constant gas turbine output of about 73 MW. The steam turbine output was allowed to vary. As in the ABB system, the air from the turbine compressor passes through the outer annulus of a coaxial duct into the pressurised combustor and the hot flue gas from the PFBC passes counter current through the inner core to the turbine. As a result, the flue gas cools from 860°C to 830°C at the turbine inlet and the compressor discharge air is heated from 300°C to about 350°C. Heat from the bed ash cooler, compressor intercooler and the gas turbine waste heat economiser is used for feed water heating prior to the deaerator. If a SFBD is used, the heating steam condensate is returned to the deaerator. The feed water at the boiler feed pump discharge is separated into two streams. One stream is heated using three high pressure regenerative type feed water heaters and the other stream is heated in the gas turbine waste heat economiser. The proportioning of the feed water between these two streams varied among the cases, depending on the heat available in the bed, the moisture content of the coal and the pressure in the steam cycle. The combined feed water streams then pass to the evaporator, superheater and reheater elements in the PFBC bed. 7.3.5. Advanced Pressurised Fluidised Bed Combustion (A-PFBC) Systems It has been intended primarily to overcome the gas turbine inlet temperature constraint of PFBC. For process reasons, the gas turbine inlet temperature in the case of
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a PFBC system is limited to around 850°C. Thus the gas turbine output from a PFBC system is limited to around 20 % of the total electrical output. By combining the PFBC with a gasifier the gas turbine inlet temperature can be raised to around 1250°C or higher. This increases the power generated from the gas turbine (Brayton cycle) to about 50 % of the total electrical output. An A-PFBC system is a hybrid system and coal reacts in two stages. The coal is firstly gasified partially in a carboniser to produce a stream of low calorific value fuel gas and char. The char is then burnt separately in a pressurised fluidised bed boiler to generate steam. The low specific energy gas from the carboniser is mixed with the combustion products of the char at a temperature of about 860°C and burnt with air in the topping combustor. Both the fuel gas and flue gas are cleaned prior to entering the topping combustor. The resulting flue gas, which is in the temperature range 1100 nOO^'C, is then expanded through the gas turbine. The air supplied to the carboniser, PFBC and topping combustor is extracted from the gas turbine compressor. A booster compressor is used only for the air supplied to the carboniser. Steam is also generated in the heat recovery boiler of the gas turbine and this, together with the steam from the PFBC, is used to drive a steam turbine. A schematic diagram of the A-PFBC system is shown in Figure 7.7. The perceived un-reliability of operation and the need for gas cleaning at high temperatures (yet unproven) has stalled development of the A-PFBC system.
FROM SFBD
BOOSTER COMPRESSOR FROM REHEATER
AIR COOLER
•.TO
*^-H
OH
e
l' ' I DEAERATOR
Figure 7.7 A schematic diagram of the A-PFBC system considered in the simulation.
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There is scope to substantially vary several of the design parameters in the simulation of an A-PFBC system. Two such parameters are the level of excess air in the char combustor and the degree of conversion of coal in the gasifier. The influence of the steam conditions, coal moisture content and the type of drying system is also investigated with the following variables: excess air flow to the char combustor (20, 200 and 325 %), yield of char in the carboniser (21.8 and 34.0 % on a dry basis), steam system conditions (subcritical or supercritical), coal moisture content change (62 to 20 % or 20 to 10 %) and drying system (SFBD with extraction heating steam or hot pressurised flue gas). In each case, a turbine inlet temperature of 1269°C was taken. The char yield in the carboniser is a function of the carbonisation temperature, residence time and the environment in which the process is carried out. Because of the limited experimental data available on the yield and composition from low-rank coal char in a carboniser environment, the sensitivity of performance to two char yields of 34.0 % and 21.8 % has been investigated at a single carboniser temperature for illustration purposes. In the simulations, the carboniser operates at 15 bar and 860°C and receives dried coal from the SFBD at 90°C. Air for the carboniser is extracted from the discharge of the gas turbine compressor and the pressure boosted from 14.6 to 16.1 bar. The steam cycle is arranged to have the exhaust gas from the gas turbine heat and partially evaporate the feed water. The wet steam from the heat recovery boiler is then directed to the PFBC where evaporation is completed and the steam is superheated and reheated. The saturated "dirty" steam, evaporated from the coal in the SFBD, is used for feed water heating. None of the systems has either low pressure (LP) or high pressure (HP) regenerative type feed water heaters. A total of eleven simulations (Cases 29 - 39) were carried out with SFBD dried coal A with different char yields in the carboniser and excess air in the char combustor: • Case 29, 21.8 % char yield and 20 % excess air, • Case 30, 21.8 % char yield and 200 % excess air, • Case 31,21.8% char yield and 325 % excess air, • Case 32, 34.0 % char yield and 20 % excess air, • Case 33, 34.0 % char yield and 200 % excess air, • Case 34, 34.0 % char yield and 325 % excess air, • Case 35, 34.0 % char yield and 200 % excess air, • Case 36, 34.0 % char yield and 325 % excess air, • Case 37, 10 % moisture, 34.0 % char yield and 200 % excess air, • Case 38, 10% moisture, 34.0 % char yield and 325 % excess air, • Case 39, 21.8 %> char yield and 325 % excess air. 7.3.6. Hybrid Partial Gasification/Atmospheric CFBC (PG/CFBC) Systems The system considered here is similar in principle to the air blown gasification (ABO) system, previously known as the British Coal topping cycle. The arrangement (Figure 7.8) has a carboniser to provide fuel gas to a gas turbine and char to an atmospheric pressure CFBC boiler. After drying in the SFBD, coal is fed
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at 90°C to the carboniser operated at 25 bar and 860°C. Air to the carboniser is extracted from the discharge of the gas turbine compressor and its pressure boosted to 28 bar in a booster compressor. The air from the turbine compressor is firstly cooled from 400 to 120°C (by heating condensate between the condenser and the deaerator) and then heated to 450°C (by cooling fuel gas from the carboniser). About 67 % of the superheated steam is generated in the HRSG of the gas turbine and the remainder in the CFBC boiler. The CFBC is used for all steam reheating. Two cases were considered for simulation: • Case 40, coal A is dried to 20 % moisture in a SFBD with extraction steam. Only about 17 % of the latent heat in the "dirty" steam from the SFBD is used for feed water heating. • Case 41, coal A is dried to 20 % moisture in a SFBD with vapour recompression. In each case, the char yield in the carboniser is 21.8 % and the gas turbine inlet temperature is 1269°C.
7.4. COMPARISON OF EFFICIENCIES OF VARIOUS PROCESSES In this section, the salient features of the results from the simulations for each
GAS T COOLER
r" •
<
>
•f COAL FROM SFBD
o, BOOSTER COMPRESSOR TO CFBC FOR REHEATING
FROM REHEATER
^ TO
t - - ^
ho
- - • . (4COMPRESSOR
....,__-.^
(^^^l''3.''J?^..m^DEmER
Figure 7.8 A schematic diagram of the hybrid partial gasification/atmospheric CFBC (PG/CFBC) system.
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technology are discussed in terms of the assumptions made and the influence of the variables studied. The different technologies are then compared in terms of their predicted performance and contrasted with predictions reported for similar technologies fuelled with high-rank coals. 7.4.1. Efficiencies of Circulating Fluidised Bed Combustion (CFBC) Systems The CFBC system is similar to the conventional sub-critical pulverised fuel (pj) fired system in that each employs the Rankine cycle and the boiler efficiency and steam cycle conditions are similar. The simulations assume the drying of coal A in a SFBD that is integrated into the CFBC cycle. Two means for sourcing the heating steam to the SFBD were evaluated: either extracting steam from the steam turbine (with return of the condensate) or recompressing the evaporated moisture from the coal. The output from the steam turbine is greater in the case of vapour recompression as steam is not extracted from the turbine. The vapour compressor, however, consumes a significant part of the increased output and improves the efficiency (HHV, sent out basis) by 0.3 % from 32.1 to 32.4%. This compares with 29 % in a conventional /?/boiler fuelled with the same coal. An integrated SFBD therefore improves the efficiency of a CFBC system by about 3 % points (10 % increase in efficiency). This improvement in efficiency may be understood by considering the details for Cases 1 and 2. For Case 1, latent heat from the extracted steam is utilised for drying, which otherwise would be transferred to the cooling water and exhausted to atmosphere. The loss of output from the steam turbine, by not expanding the extraction steam to condenser pressure, is considerably smaller than the latent heat in the steam used for drying. For Case 2, the vapour recompressor is a heat pump, which has a heat output several times greater than its power input. For Case 1, the discharge from the SFBD is the moisture evaporated from coal, which is at 100°C. About 35% of the latent heat in this steam can be used for feed heating (to increase condensate temperature from 33°C to 94°C in the simulations) in the low pressure (LP) path between the condensate extraction pump and the deaerator. Thus only high pressure regenerative feed heating is used (i.e. after the boiler feed pump) involving three feed water heaters. Conceptually, it is also possible to utilise the steam generated from drying the coal to drive an LP steam turbine. This might be done directly in a separate turbine or via a reboiler, to prevent contamination, in an existing or separate turbine. For Case 2, which uses vapour recompression, the discharge from the SFBD is "dirty" condensate (i.e. part of the heating steam condensed in the heating tubes) at about 15 P C and 5 bar. Part of the sensible heat in this stream could be used for feed heating (to heat the condensate from 33 to 62°C in the simulations) in the LP circuit between the condensate extraction pump and the deaerator. One regenerative LP and three HP feed water heaters are used to accomplish the rest of the feed heating. However, the high cost and lack of availability of vapour compressors with an adequate pressure ratio and capacity may not favour the use of vapour recompression systems.
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Use of dried coal (20 % moisture) in the CFBC boiler results in a reduction in the quantity of moisture that has to be elevated to the stack temperature, thereby reducing the amount of sensible and latent heat leaving through the stack. The efficiencies (-32 %) from the present simulations may be compared with 37 % as reported in an IE A study [1 ] for a high rank coal with a moisture content of 16 % and a HHV of 30 MJ kg'' on a dry basis. In the present simulations, the coal is dried from 62 to 20% moisture content and has a HHV of 26.4 MJ kg"' on a dry basis. The difference in efficiency is largely due to the difference in the moisture content of the as-mined fuels. 7.4.2. Efficiencies of Integrated Gasification Combined Cycle (IGCC) Systems In general the simulations show the IGCC systems to have substantially higher efficiencies than conventional boiler plant. Although the efficiency depends on the type of drying system, coal moisture content and other system variables, the efficiency determined for IGCC systems fuelled with coal A is typically about 42 % (HHV, sentout basis), which is 13 % points (45 %) higher than for conventional subcritical boiler plant. The results are discussed below according to the type of the four drying system employed. 7.4.2,1. Efficiencies of IGCC with Atmospheric SFBD Drying Effect of source of heating steam for drying: Heating steam from vapour recompression of the evaporated moisture (Cases 9 - 1 1 ) gives a higher efficiency by about 0.9 percentage points than for heating steam extracted from the steam turbine (Cases 3 - 5). Effect of fuel gas cooling (Cases 3-5 and 9-11): For the extraction steam cases, the efficiency decreased from 42.8 to 42.3 % when the product gas is cooled from 950 to 450°C. A similar decrease was also found for the cases involving vapour recompression. Maude reported [1] a fall of only 0.4 percentage points when the fuel gas was cooled from 600 to 35°C in a similar system. Effect of carbon conversion (Cases 6 - 7): Most of the simulations described here assumed 100% conversion of the coal to gas. For carbon conversions of 98 % and 95 %, the efficiency is reduced by 2 and 5 percentage points respectively. Carbon conversion has a strong influence on efficiency because all of the raw coal feed has to be dried and the energy equivalent of the unreacted carbon is 20 % greater than for dry coal. In a commercial gasifier it is anticipated that carbon conversion will be between 90 and 95 % for low rank coal. As a result, a separate char combustion cell with heat recovery in a steam circuit will be required. Effect of coal moisture: Cases 3a and 3b evaluate the use of a lower moisture content coal (moisture contents of 45 % and 20 % compared with 62 % for Case 3). The coal is dried in the SFBD to 20 % in each case. The efficiencies obtained were 42.8 %, 44.9 % and 47.3 % as drier coals were used. This corresponds to a reduction in the extraction steam to 40 % for coal B and zero for Coal C.
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Effect of a higher gasifier temperature: Case 8 shows the effect of increasing the gasifier temperature to 1500°C as might be the case for an entrained flow gasifier. Compared with Case 5, the efficiency is reduced from 42.3 to 38.6 % because more of the energy is converted via the steam turbine in Rankine cycle (rather than via the Brayton cycle) because the fuel gas is cooled by steam raising. 7.4.2,2 Efficiencies oflGCC with Pressurised SFBD Drying The efficiencies calculated for IGCC with a pressurised SFBD varied between 38.9 and 40.8 %, depending on the usage of the steam evaporated from the coal, which is at system pressure. These efficiencies are lower than those obtained for an atmospheric SFBD because of the need for higher extraction pressures (59 bar compared to 6.4 bar for atmospheric pressure operation), which decreases the output from the steam turbine. Three alternatives to use the evaporated moisture from coal drying were evaluated: Steam discharged to waste (Case 12): This arrangement gives a cycle efficiency of 38.9 %. For the purpose of comparison with other alternatives, this case gives a turbine inlet temperature of 1272°C and a fuel gas with a moisture content of 2.4 % and specific energy of 5.7 MJ kg"\ The turbine inlet temperature (TIT) for the gas turbine chosen is limited by GTPRO to be in the range 1260 - 1280°C, which represents a metallurgical limitation. GTPRO adjusts the fuel/air mixture to ensure that the allowable TIT for the gas turbine is not exceeded. If, however, the stoichiometric combustion temperature is lower than this limit, which occurs when the fuel gases have a very low specific energy, the TIT becomes the stoichiometric combustion temperature. Steam fed into the gasifier operating at 25 bar (Case 13): This arrangement improves the efficiency to 40.1 %, however, the gas specific energy falls to 3.13 MJ kg"^ and its moisture content increases to 20.2 %. The turbine inlet temperature in this case falls to 1209°C because of the lower specific energy of the gas. The reduction in specific energy results from dilution both from the additional moisture pumped into the gasifier and the need to bum additional coal to heat the water vapour to the gasifier temperature. The efficiency increase results from the increased mass flow through the turbine. Steam mixed with the fuel gas after the gasifier and prior to cleaning (Case 14): This arrangement increases the efficiency to 40.8 %. The gas has a moisture content of 28.8 % and a specific energy of 4.18 MJ kg"^ and the turbine inlet temperature is 1217°C. There are, of course, other issues to consider besides efficiency. These include the potential to reduce residence time, the size and cost of the drier and a simpler feeding system than is required for an atmospheric SFBD that requires lock hoppers. It is necessary also to consider any potential problems associated with a greater release of volatiles at the higher temperatures in the pressurised SFBD. 7.4.2.3. Efficiencies of IGCC with Hot Gas Drying The system considered has the coal slurry dried in the hot product gas from the
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gasifier (Cases 15 - 20). As the product gas is diluted by the evaporated moisture, the gas on leaving the drier can be wet scrubbed to remove condensate and entrained particulates. As the product gas after scrubbing is saturated with water vapour, the moisture content decreases as the gas temperature is reduced. Hot water from the scrubber is used to slurry the coal, increasing the temperature from 20°C to as high as 58°C. The results show that increasing the moisture content of the feed from 62 to 70 % increases the moisture content of the dried coal from 23.8 to 41.8 %. This increases the moisture content of the gas from 27.6 to 31.8 % and correspondingly decreases the specific energy from 3.97 to 2.92 MJ kg"' (LHV) at the drier exit. The cycle efficiencies decrease from 40.4 to 37.9 % (HHV). Three assumptions were made in the calculations. Firstly, the coal could be gasified in each case despite, in some cases, the coal still having a high moisture content after drying. Secondly, the gas turbine could cope with the increase in the "injected" mass flow between its compressor and turbine stages without surge of the compressor. As the net power is the difference between power developed by the turbine elements and the power consumed by the compressor, diluent injection can increase output significantly. Thirdly, the gases having such low specific energies could be burnt acceptably in a gas turbine combustor. The literature [12] suggests that the minimum acceptable specific energies (LHV) for the product gas from high-rank coals are in the range 3.8 to 4.0 MJ kg'. For the case of a slurry moisture content of 70 %, although intensifying the scrubbing can improve the specific energy of the gas from 2.92 to 4.24 MJ/kg (LHV), the cycle efficiency is shown to decrease from 37.9 to 36.6 %. The efficiency is reduced because the gas turbine output is reduced as a result of the reduced diluent flow through the turbine. The addition of water to facilitate the feeding of a high moisture coal therefore has an efficiency penalty and alternative feeding systems need to be considered. Case 21 considers the use of oxygen as the gasifying agent. In this case, the gas flow rate to the drier is much smaller than for air-blown gasification (Cases 15 ~ 20). This results in less drying and a higher moisture content of the coal leaving the drier. The estimated cycle efficiency in this case is 35.7 %, not including the power requirement for air separation plant. 7.4.2.4. Efficiencies of IGCC with Two Stage Drying - with Char Recycle and Pressurised SFBD As indicated earlier, two cases (Cases 22 and 23) were simulated under this category. In view of the results obtained from the simulations in Section 7.4.2.2, the dirty steam is mixed with the fuel gas after the gasifier. In either case, the coal (coal A) is assumed to be dried to 50 % moisture in the first drier using heat from the hot char and the final drying to 20 % moisture level is accomplished in the steam fluidised bed drier. The LHV and moisture content of the gas after mixing with the dirty steam are about 2.89 MJ k g ' and 32.8 % respectively. The simulations indicated efficiencies between 35.8 and 36.7%, which are much lower than those predicted from other simulations
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involving steam fluidised bed drier (atmospheric or pressurised) or hot gas drying. This is because the hot char, extracted from the gasifier for the first stage drying, must be reheated by burning additional coal. Any coal/char combustion over that required to gasify the coal will be reflected as a reduction in efficiency. In view of the low efficiencies and the low calorific value (< 3 MJ kg'^) and high moisture content of the fuel gas, the IGCC system integrated with two stage drying of coal as described is not a viable option. 7.4.3. Efficiencies of Pressurised Fluidised Bed Combustion (PFBC) Systems The PFBC simulations indicate that, for low-rank coals of high moisture contents, an efficiency in between those from CFBC and IGCC is obtained. Typically the efficiency obtained with an integrated SFBD is in the range of 37 to 38.4 % for coal A. The effects of the drying system, the coal moisture content and the supercritical steam conditions on the efficiency of a PFBC are discussed below. 7.4.3.7. Effect of Coal Moisture Content and Dtying A feature of fluidised bed combustion systems is the possibility to bum raw fuel because the bed is almost entirely hot inert material with coal/char constituting only a few percent of the bed. When there is no predrying of the feed coal (coal A, Case 24), the PFBC has an efficiency of 35.7 %. If the coal is dried to 20 % moisture content in a SFBD, an efficiency of 37.3% is obtained in the case of heating steam extracted from a steam turbine and 36.9 % in the case of heating steam obtained from vapour recompression. Note that, for the CFBC cases (Section 7.4.1), a SFBD with vapour recompression gave a slightly higher efficiency than a SFBD using extracted heating steam. The effect of using a coal of lower moisture content was examined in Case 28 (coal B of 45 % moisture content) for the case of a SFBD with extraction heating steam. The efficiency increases from 37.3 % for coal A to 39 % for the drier coal B, which corresponds to 60 % less extraction steam. The efficiencies for the low-rank coals given above are lower than the efficiencies from similar studies on high-rank coals. The higher efficiencies obtained from high-rank coals are consistent with the much lower moisture content of these coals. 7.4,3,2. Use of Supercritical Steam Cycle In a PFBC, steam is generated in the fluidised bed operated at a fairly constant temperature of about 850°C, which is quite suitable for supercritical steam generation. Case 27 examines a supercritical steam cycle at 228 bar/540°C/545°C which can be compared with the subcritical steam cycle at 180 bar/540°C/540°C used in Case 25. The efficiency of a PFBC operated under supercritical conditions is 1.4 percentage points greater than the subcritical cycle. This results solely from the increase in output from the steam cycle.
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1.4A. Efficiencies of Advanced Pressurised Fluidised Bed Combustion (A-PFBC) Systems In general the advanced PFBC systems give the highest efficiencies of the cycles discussed so far. Efficiencies in the range 42.4 to 44.4 % were obtained from the simulations depending on the values of the process variables chosen as discussed below. 7.4.4.1. Excess Air in Char Combustor The excess air level in the char combustor may significantly affect the performance of an A-PFBC. As excess air is increased, for a given fluidised bed temperature, more of the heat released from the combustion of the char leaves as sensible heat in the hot flue gas, leaving less heat available from the bed for raising steam. The effect is a shift towards a higher outputfi*omthe gas turbines at the expense of output fi*om the steam cycle. In the current simulations, the effect of excess air on the efficiency of A-PFBC has been investigated at three levels of 20, 200 and 325 %. The highest excess air case should be considered as an indicator of the effect of extreme excess air, as the pressure drop data for the extraction of such a large quantity of air from the compressor of a gas turbine had to be assumed. The results indicate that, for coal A, as the excess air level is increased fi*om 20 to 325 %, the ratio of gas turbine to steam turbine output increases from 1.11 to 1.51 for a char yield of 21.8% and from 0.88 to 1.34 for a char yield of 34.0 %. This increase in excess air results in cycle efficiencies increasing by 0.8% points for both char yields. The mass flow of air to be added to the topping combustor is reduced as the excess air to the char combustor is increased. Although there is a thermodynamic advantage in increasing excess air, the sizes of the ducts and plant in the air and flue gas paths must be increased. This includes the air compressor, char combustor and gas cleaning equipment. The air required for carbonisation, as a proportion of the total compressor air, varies between 11.4 % (Case 34) to a maximum of 15.1 % (Case 32). McKinsey and co-worker [27] reported 9 % for a nominal 320 MWe plant fuelled with a low moisture content (12 %) high rank coal. The optimum excess air will be a balance between operational requirements and system economics. 7.4.4.2. CharTield For the simulations, the char and gas compositions, char yield and carbonisation temperature (850°C) are specified in ASPEN PLUS, and the air/coal and gas/coal flow ratios are calculated. This creates self-consistent data, but may not reflect the actual performance of a carboniser operated under the chosen conditions. The two char yield estimates do, however, provide an opportunity to test the sensitivity of the efficiency of A-PFBC to char conversion. For a given excess air level in the combustor, an increase in the yield of char from the carboniser will result in an increase in the mass flow of gas through the turbine. This
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increases the mass flow through the turbine (relative to the air flow through compressor of the gas turbine), resulting in an increase in the overall output and efTiciency. The efficiencies obtained from the higher char yield (Cases 35 - 37) are about 1.3 % points higher than the corresponding efficiencies obtained from the lower char yield (Cases 29 -31). 7,4.43. Steam System Conditions As in the case of PFBC, the high temperature in the char combustor is appropriate for supercritical steam generation and is a feature that would almost certainly be offered in an A-PFBC. Supercritical steam conditions in Cases 35 and 36 (which have 200 % and 325 % excess air supplied to the combustor respectively) have about 0.6 % points higher efficiency than corresponding plant with subcritical steam conditions (Cases 33 and 34). 7.4.4.4. Moisture Contents of Feed Coal and Coal after Drying The effect of feeding a relatively dry coal (moisture content of 20 %) is readily modelled by taking what has been the output from the drier as the feed coal to the process. As there is no drying in this case, there is no heating steam extracted and the cycle efficiency is improved by additional output from the steam turbine. An efficiency of 46.6 % is obtained for a feed coal having a moisture content of 20 % (Case 39), which can be compared with 43.2 % for a feed coal moisture content of 62 % (Case 31). The difference in efficiency obtained by drying the coal to 10 % moisture content rather than 20 % was found to be negligible at 200 % excess air (comparing Cases 37 and 33) and 0.3 % at 325% excess air (comparing Cases 38 and 34). 7.4.4.5. Further Comments on the A- PFBC Simulations There are a number of variables, apart from the choice of drying system, such as char yield, char composition, topping combustor temperature, excess air levels in the char combustor and steam conditions that affect the efficiency of A-PFBC. There is potential, therefore, to optimise the efficiency in terms of these variables. While the yield and composition of char considered in the calculation have been based on limited experimental data, these simulations must be reviewed when the appropriate
Table 7.5 A-PFBC efficiencies reported for high-rank coals. Study
Size, MW
FW/Bechtel [27] METC[28] FW/Southem [29]
80-410 270-300 15MWth
Coal Quality M/C/Ash % 12/58/16 NA NA
Efficiency % HHV 43.0-47.3 46.5-48.2 45.0
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experimental data becomes available. The efficiencies obtained from the simulations should, therefore, be considered as indicative of the potential performance. Similar studies on high-rank coals [27-29] have shown A-PFBC to give higher efficiencies (Table 7.5) than predicted by other coal combustion and/or gasification technologies. In the high-rank coal studies, the temperatures chosen are typically 925°C in the carboniser, 870°C in the combustor, 1260''C at gas turbine inlet and 15°C ambient. In the present simulations on low-rank coals, the corresponding temperatures chosen are 860X, 860°C, 1269°C and 20°C. By varying the conditions of pyrolysis, it is possible to obtain a range of char yield and composition from the carboniser. The energy from the combustion of the char is used both to heat the flue gas and to provide superheat and reheat energy to the steam system. For a given system, it may be necessary to supplement this char flow by diverting a portion of the feed coal (to the carboniser) into the char combustor. Any need for additional coal flow to the char combustor can be determined only when experimental data on the carbonisation of dried low-rank coal becomes available. Although the simulations indicate the potential for high efficiencies from an APFBC, its development appear to have been stalled for the reasons indicated earlier. 7.4.5. Efficiencies of Hybrid Partial Gasification Atmospheric CFBC (PG/CFBC) Systems The efficiencies (~ 38.7 %) of PG/CFBC systems integrated with SFBD are shown to be 4 to 5 % less than for IGCC systems with 100 % coal conversion. SFBD with vapour recompression gives a slightly lower efficiency than SFBD with extracted heating steam. 7.4.6. Summary of Efficiencies from Different Technologies Table 7.6 provides a summary of the first law efficiencies of the processes simulated. The current simulations indicate in general the following ranking in terms of the efficiencies potentially achievable from the technologies assuming complete carbon conversion. For consistency, an integrated SFBD with heating steam extracted from the steam turbine is assumed in each case except for the IGCC-hot gas drying: CFBC < PFBC < PG/CFBC < IGCC < IGCC < A-PFBC (32.1 %) (37.0-38.4%) (38.7%) 40.4-41.0% 42.3-42.8% 42.4-44.4% The effect of the drying process is also considered as follows. In the case of coal dried in a SFBD, the output (and hence efficiency) of the waste heat steam turbine is reduced by extracting some of the steam part way through its expansion. If a drier coal is used, there will be less demand for heating steam, which will result in an increase in the steam turbine output. For the case of coal dried by hot gas, there will be no contribution to output from a separate high pressure steam generator. However, the output (and efficiency) of the gas turbine will be increased because of the increased gas
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Technologies
Table 7.6 Summary of the first law efficiencies of systems simulated (excluding gasification-fiiel cell systems). System type CFBC-Atm SFBD
IGCC - Atm. SFBD with extracted heating steam
Coal type/feed/ System description moisture content A- - dried -- 20% with extracted heating steam A- • dried •• 20% with vapour recompression A- • dried •• 20% g a s a t 9 5 0 ^ C - n o cooling B - • dried -• 20% gas at 9 5 0 T - no
Eifects studied
Case No
vapour
1
First law efficiency, % 32.1
recomp.
2
32.4
coal
3
42.8
3a
44.9
3b
47.3
moisture
cooling C - - dried -• 20% g a s a t 9 5 0 ° C - n o cooling A- • dried -• 20% gas cooled - 9 5 0 T to 650T A- • dried -- 20% gas c o o l e d - 9 5 0 T to
gas
4
42.6
cooling
5
42.3
450T A- • dried •- 2 0 % g a s a t 9 5 0 " C - 9 8 % C
carbon
6
40.7
conv. A- - dried •• 20% g a s a t 9 5 0 T - 9 5 % C
conversion
7
37.6
gasifier temp
8
38.6
conv. B - - dried -- 20% gas c o o l e d - 1 5 0 0 ° C to 450T IGCC - Atm. SFBD with
A- • dried •• 2 0 % gas at 9 5 0 T - no cooling
vapour
9
43.7
vap. recomp. IGCCPres. SFBD. with extracted heating steam
A- - dried •- 20% gas c o o l e d - 9 5 0 ° C to
recomp.
10
43.5
650^C
and
A- • dried -- 20% gas cooled - 950°C to 450^C A- • dried •• 2 0 % gas cooled - 9 5 0 T to 450^C dirty steam from drier wasted A- - dried •- 20% gas cooled - 950 to 450T dirty steam from drier fed to gasifier A- - dried •- 20% gas cooled - 950 to 450T dirty steam mixed with gas after gasifier
gas cooling
n
43.2
pressure in SFBD and
12
38.9
use of dirty
13
40.1
steam (from SFBD)
14
40.8
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Table 7.6 Cont'd. System type IGCCHot gas drying air gasification
As above, O2 gas
Coal type/feed/ System description moisture content A-• raw-62% sensible cooling only A-• raw-62% As above & gas scrubbing to 1 SOT A-• slurry-66% As above & gas scrubbing to 150°C A-• slurry-70% As above & gas scrubbing to lOOT A-• slurry-70% As above & gas scrubbing to 150°C A-• slurry-70% no cooling / scrubbing A-• slurry-70% sensible cooling only
Case No
gas drying, sensible
15 16
First law efficiency, % 41 40.4
cooling,
17
38.9
and
18
35.6
scrubbing
19
36.7
of gas.
20 21
37.9 35.7*
22
35.8
23
36.7
drying vapour
24 25
35.7 37.3
recomp.
26
38.4
super critical steam cycle
27
36.9
coal moisture
28
39
excess air
29
42.4
in char
30
42.7
combustor
31
43.2
gas turbine
32
43.3
O2
gasification
IGCC- two stage drying With char & press SFBD
A-• raw-62%
PFBC systems Atm. SFBD
A-- raw-62% without drier A-• dried - 20%) with extracted heating steam A-• dried - 20% with vap. recompression A-• dried - 20% with extracted heating steam super critical steam system B- • dried - 20%, with extracted heating steam
Advanced PFBCAtm. SFBD. with extracted heating steam
Effects studied
A-• raw-62%
gas at 950T-no cooling gas cooled - 950°C to 450°C/ dirty steam from drier mixed with gas after gasifier
A-- dried - 20%, 21.8% char yld-20% ex. air in char comb. A-• dried - 20%, As above, 200% excess air in char combustor A-- dried - 20%, As above, 325% excess air in char combustor A-- dried - 20%, 34.0% char yld-20% ex. air in char comb.
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Table 7.6 Cont'd. System type
PG/CFBC Atm. SFBD.
Coal type/feed/ Effects System description moisture studied content inlet A-dried-20% As above, 200% excess air in char combustor A-dried-20% As above, 325% excess temperature air in char combustor char yield A-dried-20% As case 33 - super critical steam cond A-dried-20% As case 34 - super critical steam cond A-dried-10% As case 33 - lower product moisture A - d r i e d - 1 0 % As case 34 - lower product moisture - 20% As case 31 - no drying, feed coal moisture 20% A - d r i e d - 2 0 % 21.8% char yld-20% ex air in char comb.with extracted heating steam A - d r i e d - 2 0 % As above, with vap. recompression
Case No
First law efficiency,
%
~'l3
43.8
34
44.1
35
44.4
36
44.8
37
43.8
38
44.4
39
46.6
40
38.7
41
38.1
vapour
recomp.
Carbon conversion is 100% unless stated otherwise; subcritical steam parameters unless stated otherwise. Power consumption of air separation plant excluded
flow through the turbine rotor, relative to the mass flow of air delivered by the turbine compressor, due to the water evaporated from the coal. CFBC systems: The CFBC systems give the lowest efficiency as it is based on the Rankine cycle alone. The efficiency can be increased by about 3 % points from the 29 % currently achievable in a sub-critical conventional boiler plant by integrating a SFBD into the system. The SFBD with extracted heating steam utilises latent heat for drying that would otherwise be rejected from the steam turbine condenser. PFBC systems'. The PFBC systems improve the performance over that of CFBC because it involves the use of a combined cycle and is able to utilise higher steam pressures (180 bar) than in a CFBC (130 bar). This technology is constrained by the allowable fluidised bed temperature, which limits the inlet temperature to the gas turbine to about 830°C. A PFBC is unable to capture the full capability of a gas turbine. As only about 20 % of the total output is derived from the gas turbine, the efficiency that can be achieved is less than from IGCC or A-PFBC. However, this technology is simpler than the gasification-based technologies and has achieved a commercial status.
392
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IGCC systems'. The IGCC systems with SFBD offer efficiencies in excess of 42 % (assuming 100 % carbon conversion in the gasifier), with 57 - 73 % of the output derived from the gas turbine depending on the type of drying and how the evaporated moisture is used. As the carbon conversion in a fluidised bed gasifier is expected to be less than 100%, it will be necessary to incorporate a separate char fired boiler. If an option is to dump the char, it has been shown that the eflTiciency is adversely affected because of the energy discarded and the need to dry all of the feed coal. This leads to the conclusion that one of the hybrid schemes (viz A-PFBC or PG/CFBC) must be adopted if a fluidised bed gasifier is to be used. If, however, a high temperature entrained flow gasifier is to be used, because of the expectation of a more complete carbon conversion, the efficiency is also reduced. Case 8 shows that gasification at 1500°C followed by cooling to 450°C (by generating high pressure steam) reduced the efficiency to 38.1 %. The IGCC technology has achieved the status where there are several major fullscale demonstration plants either operating or under construction in several countries, but all using a variety of entrained flow gasifier types. A'PFBC and PG/CFBC systems: The A-PFBC systems (with coal dried by SFBD) give the highest efficiencies of the technologies evaluated, particularly when supercritical steam conditions are used in the high pressure steam cycle. Although this technology is much less developed than IGCC, a fluidised bed gasifier will almost certainly require the use of a hybrid cycle to bum the char residue from gasification. The efficiency advantage over IGCC (assuming 100% carbon conversion) may be eroded if a gas turbine with a higher efficiency is used, or if an IGCC is to be installed with a low pressure drop topping combustor. The efficiency obtained from a hybrid system with an atmospheric pressure char combustor, assuming a char yield of 21.8 %, is 4 to 5 % points lower than that obtained from an IGCC system with 100 % carbon conversion. The lower efficiency results from the facts that a higher booster compressor power consumption is required to supply coal gas at 25 bar to the gas turbine combustor (as for the IGCC cases) and that a larger proportion of the power is generated in the high pressure steam cycle, than in an APFBC, because the exhaust gas from the boiler is not expanded through the gas turbine. General comments on gas turbine based technologies: The performance of the gas turbine based technologies is strongly influenced by the performance of the gas turbine. It is important to recognise that the efficiency of gas turbines will continue to be improved as a result of continuing developments in gas turbine technology. The efficiencies reported here correspond to an allowable maximum TIT of 1280°C considered in the simulations; however, cycle efficiency is a strong function of TIT. Implication of incomplete carbon conversion: The relative efficiencies of the various technologies have so far been compared assuming complete carbon conversion. As discussed, complete carbon conversion is unlikely to be achieved. Assuming that the char yield is 10% in the IGCC systems and 21.8 % in the two hybrid systems, the efficiencies can be compared as follows (IGCC with hot gas drying, all other cases assume the use of SFBD drying):
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CFBC < PFBC < PG/CFBC
Comparison of Efficiencies from Similar Technologies using High-Rank Coals
Table 7.7 compares published efficiencies [1,28,30,31] for high-rank coals in technologies similar to those considered in the present simulations. With the reservation that not all of the relevant details behind the published data are available, high-rank coals have an efficiency advantage over low-rank coals, although this advantage diminishes for the advanced technologies. The difference in efficiency results from the need for a drying process for the low-rank coal. In the case of IGCC, the difference between low and high rank coals is small, despite the need to incorporate a drying process for the low-rank coal. This occurs because the low-rank coal is gasified at 950°C (in a fluidised bed) and the high rank coal is gasified at say 1300 - 1600°C (in an entrained flow reactor). The lower gasification temperature results in a higher cycle efficiency, as was discussed earlier. 7.5 THERMO-CHEMICAL RECUPERATION, GASIFICATION-FUEL CELL SYSTEMS Carbon dioxide (CO2) emission control is a focus of worldwide research in the power generation industry. In conventional coal and gas-fired units, CO2 can be removed from the exhaust gas using various absorbers. The partial pressure of CO2 in the exhaust gas is low due to the near ambient pressure of the gas and also due to the substantial presence of nitrogen (N2) originating from combustion using air. The low partial pressure of CO2 makes the removal process very expensive. One way to mitigate this problem is to concentrate the CO2 in the flue gas. Advanced power generation technologies, in particular those based on oxygen-blown gasification, offer opportunities to produce flue gas with concentrated CO2. 7.5.L Concept of Thermo-chemical Recuperation Thermo-chemical recuperation is a concept originally proposed for natural gas-fired gas turbine based power generation cycles [32]. In this concept, the gas turbine exhaust heats a mixture of natural gas and steam in the presence of Ni catalyst to reform natural gas to H2, CO and CO2. Steam for reforming is usually generated from the heat recovery steam generator. Thus the natural gas/steam mixture absorbs heat thermally (as it is
394
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Table 7.7 Comparison of efficiencies (% sent out, HHV) achievablefromtechnologies using low and high rank coals. Technology CFBC
Comments subcritical cycle
High-rank coal, other studies
Lx)w-rank coal MC - 62 %
37.3
29.0 no drying SFBD dried Jl
CFBC[1]
coal moisture 6 - 16 %
37.3
32.1-32.3
PFBC[1,30]
subcritical cycle supercritical cycle
40.3-42.0 42.4
36.7-37.3
IGCC[1]
coal moisture 6 - 16 %
39.2-42.8
36.6-43.4
A-PFBC [28]
subcritical cycle supercritical cycle subcritical cycle, coal moisture not available
46.5 up to 53.4
42.4-44.4
45.0
38.1-38.7
PG/CFBC[31]
38.4
heated) and chemically (as the endothermic reaction proceeds), resulting in a thermochemical recuperation of the energy in gas turbine exhaust. This is larger than can be obtained by thermal recuperation, which recovers energy by heat alone. A somewhat similar concept can be used in coal gasification, with details provided in the next section. 7.5.2. Description of Systems Considered Two different cycle configurations (Figures 7.9 and 7.10), with and without fuel cells, were considered. The proposed cycles use an oxygen-blown fluidised bed for gasification and a shift reactor to maximise the production of hydrogen and carbon dioxide. Part of the gas turbine exhaust gas is used to heat oxygen and produce steam, both of which are used as reactants in the gasifier; steam is also used in the shift reactor. Thus thermal energy held by the exhaust gas is utilised to produce chemical energy of the reformed products, largely H2 and CO2, and to a minor extent CO and CH4. The systems consist of the following key components: • A steam-fluidised bed coal drier. As indicated later, steam for drying is obtained from the exothermic shift reaction in a shift reactor. • An oxygen-blown fluidised bed gasifier. Steam is used as the other reactant. Compared to air-blown gasification, oxygen-blown gasification has several benefits: high carbon conversion, high calorific values and low volume of the product gas, resulting in lower gas clean-up requirement. The products from gasification are predominantly CO, H2 and CO2 together with minor amounts of CH4 and H2O.
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Advanced Power Generation Technologies AIR IN (PARDN, AIR COMP
GAS TURB
TO (optional) STEAM TURBINE
—0 HRSG
STEAM
rvw^
TO EXHAU$T
SEQUESTRATION READY CO.
MEMBRANE
ASU
SEPARATOR _CW IN STEAM OUT
4
••C^ STEAM
FUELGAS
J-
f SHIFT REACTOR
TO COAL DRYER GASIFIER DRIED COAL O.
STEAM
Figure 7.9 A schematic diagram of a thermochemically recuperated gasification cycle.
Air Gas
i!^'"5-^
Combustor
Water
f
^o (optional)
Hz) ASU
Membrane Separator 4r S t e a m ^ To C o a l Dryer O2
Steam
Figure 7.10 A schematic diagram of a thermochemically recuperated gasification - fixel cell cycle.
396
Chapter 7 • A low-temperature shift reactor. Use of a low temperature shift reactor (~ 200°C) has two major benefits: high equihbrium conversion and condensation of volatile alkali species in the gas. As the shift reaction is exothermic, the heat from the reaction can be utilised to generate low-pressure (slightly) superheated steam for steam drying of coal. • A high-pressure membrane separator to separate carbon dioxide into one stream and hydrogen and other minor gases into another stream. Development of highpressure, high-temperature membranes is a significant research focus worldwide. • An air separation unit. While O2 is used for gasification and combustion of hydrogen, N2 may be used for power boosting and temperature control in the combustor. • A gas turbine capable of burning primarily hydrogen. • A heat recovery steam generator (HRSG). The GT exhaust provides thermal recuperation by producing steam, which then provides chemical recuperation when used in the gasifier as a reactant to generate the combustible constituents CO and H2. The steam is also used in the hydrogen combustor for temperature control, increased mass flow and output. • A solid oxide fuel cell (SOFC), capable of taking part of fuel gas after the membrane separator. The exhaust from the fuel cell goes directly into the gas turbine, where remaining part of the fuel gas combusts along with the exhaust from SOFC to generate the desired GT inlet temperature. The proposed cycles have several perceived advantages, e.g. use of only gas turbine, eliminating the need of a steam turbine that is a major source of inefficiency in power generation cycle, combustion of only hydrogen in the presence of oxygen, with either steam and/or nitrogen used only for temperature control and availability of compressed CO2 at low temperature amenable to further compression for easy sequestration or other uses of the compressed gas. The cycles are suitable for low-rank coals that have large amounts of inorganics that act as catalysts during gasification.
7.5.3. Process Simulation and Major Assumptions Properties of Loy Yang coal (Table 7.4) are used in this simulation. Other major assumptions include ambient temperature of 20^C, gasification temperature and pressure of 900°C and 18 bar respectively to reach complete carbon conversion in the gasifier. The simulations were carried out using ASPEN Plus. Use of a generic gas turbine is assumed with a maximum inlet temperature of 1280°C. The following five cases are simulated, with the first three based the scheme in Figure 7.9 and the last two based on the scheme in Figure 7.10. • Case 42: N2 from the air separation plant is expanded separately. Steam in excess of that required in the gasifier and water spray is used for temperature control in the gas turbine. • Case 43: Excess steam generated in the steam generator and a part of the N2 from the air separation unit are used for temperature control in the gas turbine.
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Advanced Power Generation Technologies
• Case 44: The entire amount of N2 from the air separation plant and excess steam generated in the steam generator are used for temperature control in the gas turbine. • Cases 45 and 46: different air utilisation factor (Ua) and fuel utilisation factors (Uf) in the fuel cell. The Ua and Uf values signify the proportions of fuel and oxygen in air consumed in the fuel cell [33]. The results are presented in Table 7.8. In evaluating the net efficiencies in Table 7.8, it must be remembered that the cycles produce CO2 ready for sequestration. The full reuse of the N2 (from the air separation unit) in the gas turbine combustor appears to provide an efficiency advantage compared to the cases where it is not used, or water spray is used for temperature control. When water spray is used, the mass flow through the gas turbine is reduced as water soaks up latent heat from hydrogen combustion. This results in reduction of gas turbine output and hence efficiency. The results indicate that the system incorporating fuel cells gives higher efficiency (about 45 % HHV) than the system without fuel cells (39.5 % HHV). The efficiency becomes higher when higher values of Ua and Uf are used. However, in practice there is a limit to the maximum value of Ua that can be used in the fuel cells. The results give simple and qualitative illustration of the effects of thermo-chemical recuperation on efficiency even though the cycle parameters are not necessarily fully optimised. Improvements in both systems are possible with optimised configuration of thermal and chemical recuperation of the GT exhaust energy and other operating variables.
Table 7.8 Key results of recuperated gasification systems. Parameters Case 42 Moisture in raw coal (%) 62 Moisture in dried coal (%) 20 Raw coal consumption, T/hr 210.5 Heating steam in SFBD, flow/pressure/temperature, t/hr, bar, °C Gasifier pressure and temperature, bar, °C Shift reactor temperature, °C GT inlet temperature, °C Fuel utilization factor, Uf Air utilization factor, Ua Cell voltage, V Fuel cell outlet temperature, °C Gross power output, MW Net Power output, MW Efficiency, HHV, net, %
190 19 29
Case 43 62 20 210.5
Case 44 62 20 210.5
Case 45 62 20 210.5
Case 46 62 20 210.5
135/6/200 18/900 200 Not to exceed 1280°C 0.284 0.15 0.69 475 292 250 258 29.2 25 25.8 44.9 38 39.5
0.356 0.2 0.69 505 302 30.2 46.3
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7.6. RESEARCH AND DEVELOPMENT NEEDS Gasification based power generation technologies are the key to the long-term use of low-rank coals with sequestration of carbon dioxide being a primary driver. While some of the common research and development issues in the development of advanced power generation technologies are listed elsewhere [34], there are a number of general process development issues relating to low-rank coals that must be need to be addressed: • coal feeding and handling systems for use in pressurised systems. • coal drying systems and their integration into the power system. This includes the efficient use of moisture evaporated from the coal, which can only be partly utilised in an atmospheric SFBD. Alternative drying or dewatering schemes need also to be explored. • appropriate process kinetics of coal during drying, pyrolysis, combustion and gasification and yield and composition of the char and gases as a function of pressure, time and temperature of gasification. • choice of reactors for the gasifier. Even though pressurised fluidised bed gasifiers are ideal for low-rank coal operation, no commercial sized plants have yet been demonstrated. Development of alternative gasifiers should therefore also be explored. • continued development of fuel cell cycles with lower cost materials, and capability of higher pressure operation.
References [1] Maude C. Advanced Power Generation - A Comparative Study of the Design Options for Coal. lEA Coal Research, London, 1993. [2] Thambimuthu K. Gas Cleaning for Advanced Coal based Power Generation. IE A Coal Research, London, 1993. [3] Takematsu T, Maude C. Coal Gasification for IGCC Power Generation. lEA Coal Research, London, 1991. [4] CoUot AG, Matching Gasifiers to Coals. lEA Coal Research, London, 2002. [5] Couch G. Power Generation from Lignite. lEACoal Research, London, 1989. [6] Ziesing GF, Forgaard K, LeFever J, Rhodes B, Griffiths V. Drying of Low-Rank Coal for MHD Application. 10th Biennial Lignite Symposium, Grand Forks, North Dakota, May 1979, pp. 459-495. [7] Wilver PJ, Brumbaugh CA. Thermal Drying of Low-Rank Coals Using the Fluid Bed Method. 13th Biennial Lignite Symposium, Bismarck, North Dakota, 1985, pp. 520-542. [8] Anderson B, Johnson TR, Huynh, DQ. Integrated Drying and Gasification of Low-Rank Coals for Power Generation. Proc. 6th New Zealand Coal Conference, 1995, pp. 113-122. [9] Allardice DJ. Chapter 3 in The Science of Victorian Brown Coal (Ed: R.A.
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Durie), Butterworth-Heinemann, Oxford, 1991. [10] Smith PV, Davis BM, Vimalchand P, Liu G, Longanbach J. Proc. of the Gasification Technologies Council Conf, San Francisco, October 28-30, 2002, in the form of a CD-Rom. [11] Bhattacharya SP, Beaupeurt I. Proc. 18th Low-Rank Fuels Symposium, Montana, 2003, in the form of a CD-Rom. [12] Cook CS, Corman JC, Todd DM. J. Engg. Gas Turbines and Power 1995;117:673. [13] Shilling N, Jones RM. Proc. Gasification Technologies Conference, San Francisco, 12-15 October 2003. in the form of a CD-Rom. [14] Todd DM. Proc. Gasification Technologies Conference, San Francisco, 2000. [15] Edwards JR, Srivastava RK, Kilogroe JD. Journal of the A&WMA 2001; 51:869. [16] Senior CL, Afonso RF. Proceedings of the 94th A&WMA Annual Meeting and Exhibition, Air & Waste Management Association, Pittsburgh, PA, USA, 2001. [ 17] USDOE, www.netl.doe.gov/coalpower/environment/mercury/description.html, 2004. [18] Rutkowski MD, Klett MG, Maxwell RC. Gasification Technologies Public Policy Workshop, Washington DC, October 2002, available from www.gasification.org. [19] Bhattacharya SP, Kosminski AK, Yan HM, Vuthaluru H B. Proc. of the 16th International Conference on Fluidised Bed Combustion, Nevada, 2001, in the form of a CD-Rom. [20] Scott DH, Advanced Power Generation from Fuel Cells, IE A Coal Research, London, 1993. [21] Pangalis MG, Martinez-Botas RF, Brandon NP. Proc. Inst of Mech Engrs, vol 216, part A: J Power and Energy, 2002, pp 129-144. [22] Spakovsky MR, Olsommer B. J Energy Conversion and Management 2002;43:1249. [23] Bhattacharya SP and Mcintosh MJ, Proc. 6th Japan-Australia Joint Technical Meeting on Coal, Sapporo, Japan, 4-5 June, 1996, pp 161-171. [24] GTPRO Manual, Thermoflow Inc., Wellesley, MA, USA, 1997. [25] Lafanecere L, Jestin L. Proc. Fluidised Bed Combustion, Orlando, 1995, vol. 2, pp. 971-980. [26] Lafanecere L, Basu P, Jestin L. Proc. Fluidised Bed Combustion, Orlando, 1995, vol. l,pp. 1-8. [27] Mckinsey RR, McCone AI, Wheeldon JM, Booras GS. Proc. Fluidised Bed Combustion, Orlando, 995, vol. 1, pp. 653-661. [28] Reed ME. Proc. of the Coal Fired Systems 9 4 - Advances in IGCC and PFBC review Meeting, US DOE, 1, 93-101. [29] McClung J, Quandt M, Hemmings J, Moore D, Proc. Fluidised Bed Combustion, Orlando, 1995, vol. 1, pp. 107-116.
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[30] Pillai KK and Jansson SA (1991). Utility Size PFBC Plants. Proc. Fluidised Bed Combustion, Montreal, 1, 493-502. [31] Minchiner AJ, Clark RK, Dawes SG Proc. Fluidised Bed Combustion Conference, Montreal, Canada, 1991, vol. 1, pp. 115-118. [32] Carcasi C, Facchini B, Harvey S. Energy Convers. Mgmt. 1998;39:1693. [33] Kuchonthara P, Bhattacharya S, Tsutsumi A. J Power Sources 2003;124:65. [34] Holt N. J Materials at High Temperatures 2003;20:1.
Advances in the Science of Victorian Brown Coal Edited by Chun-Zhu Li © 2004 Elsevier Ltd. All rights reserved.
Chapter 8 Liquefaction of Victorian Brown Coal Osamu Okuma^ and Kinya Sakanishi^ ^ The New Industry Research Organisation, Kobe 650-0047, Japan ^Institute for Energy Utilisation, AIST, Tsukuba West, Ibaraki 305-8569, Japan
8.1. INTRODUCTION By the early 1920's, some countries, which were not so well endowed with oil resources, had conducted research for converting coal into more convenient liquid fuels. In particular, Germany laid the foundations for most of the present day direct and indirect coal liquefaction processes with the discovery and development of the Bergius hydrogen donor process in 1913, the Pott-Broche solvent extraction process in 1927, and the Fischer-Tropsch process in 1925 for hydrocarbon synthesis with the syngas fi"om coal gasification [1]. During World War II, petroleum shortages in Germany led to the development of coal liquefaction to obtain petroleum substitute. The Bergius process was applied by using very high hydrogen pressures and inexpensive iron catalysts [1,2]. After World War II, coal liquefaction was again considered as an alternative to petroleum in USA when the rate of petroleum production could not keep up with the growing demand. A number of commercial demonstration plants were built and operated for several years. However, the discovery of massive petroleum reserves in the Middle East made coal liquefaction uneconomical. Since then, the level of the R&D efforts on coal liquefaction declined until 1973 when political unrest in the Middle East made the USA aware of its vulnerability because of the dependence on foreign countries for its major source of energy [1,2]. Although the oil crises in 1973 and 1979 had no direct connection to limited petroleum resources, oil shortages will occur early in the 21st century because of increasing demands by the developing countries with large populations. Coal liquefaction will be expected to be a major source of liquid fuel, hence its economical feasibility is very important for the commercialisation [1,2]. A route to more efficient liquefaction lies in the development of suitable catalysts to lower capital investment and operating costs by improving the selectivity to the desired products, reducing operating severity and increasing throughput rates. One of the most expensive components in direct coal liquefaction is the cost of hydrogen. Hydrogen production is estimated to represent 15 to 40 % of the capital cost of a commercial plant [3]. The investment cost escalates with increasing hydrogen requirement. For coals whose hydrogen content is the lowest of the available fossil fuel resources, it is
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necessary to add about 8 wt% hydrogen to produce gasoline and distillates [3]. The use of catalysts will help to minimise the hydrogen consumption, approaching that required by stoichiometry, contributing to the reduction of cost. However, there is the cost of the catalyst itself, which is usually cited based on the use of very expensive metals as catalysts [3]. The limited activity of catalyst and the difficulty in recovering the catalyst force a large amount of catalyst to be wasted as it is mixed with minerals and organic residues derived from coal. Recoverable catalysts offer a promising way to economise the cost of catalysts. Dow Chemical designed a process, which utilised fine powders of M0S2, and reported the possibility of catalyst recovery by hydroclone [2-4]. For catalysts that are inherently difficult to recover from residual products and thus considered disposable, the catalysts must be inexpensive. In this case, a huge amount of the fresh catalyst would have to be prepared in a commercial plant and a suitable place for catalyst disposal after extensive cleaning must be found. Recycling the catalyst for re-use and/or improving the activity of the catalyst for the drastically reduced use of the catalyst are a major goal in current studies. Sakanishi and co-workers proposed a novel type of liquefaction catalyst with much higher activity and ftmctions for recovery and repeated use. NiMo, FeMo and FeNi bimetallic sulphides supported on a particular carbon black of nano particles are catalysts providing high distillate yields with the least amounts of residual product, and they are recoverable from the solid residues by gravimetric separation for repeated use [2-6]. Solvent/coal ratio in slurry and oil yield are also important factors influencing the cost of coal-derived oil. Reduction of the solvent/coal ratio can decrease the cost of coal liquefaction because the reactor efficiency increases in inverse proportion to the ratio. Recently, the recycle solvent having a dual peaks in its distillation curve, which consists of light and heavy solvents including bottom fractions, has been proposed for coal liquefaction process [4-6]. In this chapter, the development of brown coal liquefaction processes are reviewed in terms of characteristics of Victorian brown coals, including the fundamental understanding of reactions taking place during liquefaction.
8.2. BASIC REACTIONS OF BROWN COALS IN LIQUEFACTION 8.2.1. Structure and Reactivity of Brown Coal Clearly, a wide variety of aliphatic, aromatic and heteroaromatic structural units together with abundant (oxygen-containing) functional groups are present in Victorian brown coal. The structure of Victorian brown coal, or indeed any coal, could not be represented by a single simple structure. In earlier studies, average structures [e.g. Refs. 7 and 8] representing the main features of coal were therefore commonly used to provide guidelines for understanding the chemical transformations involved during coal conversion. While some average structures were simple and only used to illustrate the changes in coal structure with rank [7,8], other structural models were more
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sophisticated. The model presented by Shinn [9] showed the presence in coal of poly aromatic rings (1-4 ring units with some naphthene rings), hetero cycles, alkyl side chains and oxygen sub units connected by methylene, ether, thioether, and aryl-alkyl linkages (sp^-sp^) to form macromolecular networks. The significance of this model is to indicate a number of relatively weak bonds such as alkyl and oxygen bridges that can account for the rapid break-up of coal into smaller and more soluble fragments during coal liquefaction [1,2,10-12]. The structure of coal is known to be dependent on rank as well as the degree of coal weathering and the distribution of macerals. The structures proposed for low rank coals emphasise their hydrogen bonding, charge-transfer interactions and ion bridges between large macromolecules [7]. The structures proposed for high rank coals emphasise their large aromatic planes [13]. The molecular size of the coal structural units decreases with increasing rank to a minimum at a bituminous coal rank of ~ 83 wt% carbon, and then the aromatic ring is enlarged to have a graphite-like structure. Thus, the highest solubility in conventional solvents is observed with bituminous coals of this rank (~ 83 wt% carbon) [14,15]. It should be pointed out that many models are representative of the active vitrinite macerals, whereas inert macerals, such as fusinite and micrinite, are believed to have large aromatic planar structures with fewer substitutional groups to show behaviours similar to those of chars [16]. Recent advances in the understanding of the structural features of Victorian brown coal were discussed in Chapter 2. In particular, studies using various instrumental methods (such as NMR, FT-IR and UV-absorption/fluorescence spectroscopies) have provided detailed information about the structural features of Victorian brown coal, including its aromatic ring systems, substitutional groups, covalent and non-covalent interactions. Knowledge about the special structural features of Victorian brown coal is important to the understanding of the chemical reactions taking place during its liquefaction and thus the design of each step of coal conversion. Attempts have been made to use structural parameters of Victorian brown coal to predict its behaviour during liquefaction. For example, continued efforts have been made to correlate the NMR-derived and/or FT-IR-derived structural parameters of Victorian brown coal with its behaviour during liquefaction under various conditions [17-21]. 8.2.2. Coal Dissolution and Depolymerisation The liquefaction of coal is the conversion of coal macromolecules into smaller hydrocarbon molecules that are distillable. During the liquefaction of brown coal, the oxygen-containing functional groups are firstly decomposed and the macromolecules then undergo thermal fission at their weak bonds, such as methylene and benzylether bonds, producing radicals [1,2]. When the radicals are capped with hydrogen from the solvent or the catalyst, they form smaller molecules that are soluble in the solvent and increase the quantity of liquid phase [2,22]. This pyrolysis continues while the reactive bonds and stabilising hydrogen are available. Hydrogen atom can hydrogenate reactive sites on the aromatic rings. When the ipsoposition of the strong aryl-aryl bond in the aromatic ring is hydrogenated, the bond
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becomes weakened and bond cleavage becomes possible via the first mechanism of depolymerisation and facile stabilisation [23-25]. Aromatic rings are very stable unless they are hydrogenated to naphthenic rings, which may thermally or catalytically crack to open the ring [26], yielding free radicals. Under the conditions where hydrogen atoms are not available or the concentration of radicals becomes excessively large, the fragments may recombine or polymerise to form high-molecular-weight compounds and coke precursors [27-29]. The primary coal-derived species is called preasphaltene, which is ftirther reduced in molecular weight to asphaltene, and then to distillable oil and hydrocarbon gases [22-24,30]. 8.2.3. Hydrogen Transfer in Liquefaction of Brown Coal Solvent is necessary in coal liquefaction because solid coal is generally transported as slurry into the reactor. However, reduction of the amount of solvent is very important in order to improve the economics of liquefaction [4]. When the solvent/coal ratio is small, the reactor volume efficiency becomes high, but the slurry viscosity also becomes high, disturbing its transportation. Therefore, the solvent/coal ratio should be optimised to balance the reactor efficiency and the slurry viscosity. Coal liquefaction reaction consists mainly of four steps: dissolution, hydrogen transfer, hydrogenation and hydrocracking. The characteristics of the solvent strongly influence the reaction rates and product selectivities in each step. As was pointed above, during the dissolution step, the coal macromolecules are thermally decomposed into fragment radicals. There are a number of pathways for the stabilisation of free radicals: 1) H-abstraction from the solvent or the hydrogen-rich portion of coal, 2) elimination of part of the radicals with associated group migration, 3) rearrangement to form another more stable structure and 4) combination reactions such as condensation or alkylation with other molecules in the solvent or coal-derived products. If H-atoms from the H-donor solvent are not available, the radicals are stabilised by condensation reactions, forming compounds of increased aromaticity and decreased reactivity [1]. The presence of catalyst and type of hydrogen donors may determine the preferred reaction path [1]. In general, the sources of hydrogen in a liquefaction process are hydrogen gas, solvent, coal, and coal-derived materials such as heavy asphaltene, preasphaltene and residue. Some catalyst (e.g. Ni-Mo/A^Os [31] and Mo(CO)6-S and Ru3(CO)i2 [32]) can effectively promote the H exchange between the gas phase and the coal. The demand for hydrogen is very high in the initial stage of liquefaction when massive thermal fragmentation of coal produces a large number of free radical species. H must be released from the H-donating solvent under conditions pertinent to those of H demand for radical stabilisation [33,34]. Nondonor species in the liquefaction solvent mixture may also play an important role (e.g. H shuttling) in the hydrogen transferring process [35]. When no hydroaromatics are present, poor solvents such as phenol derivatives can be H-donors. However, when these compounds lose hydrogen atoms, they tend to form undesirable retrogressive products of lower solubility [1].
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Coal and primary heavy products have been reported to be significant sources of hydrogen by themselves during coal liquefaction. The presence of non-donor solvent such as polycyclic aromatic hydrocarbons can effectively provide a means of hydrogen redistribution to some extent that can convert most of the reactive macerals to pyridinesoluble products in short reaction times even without hydroaromatic compounds in the solvent or gaseous hydrogen [1,36,37]. According to this mechanism, there is no net change in the overall H/C ratio, but there is a significant increase in the carbon aromaticity in the products in comparison to the case when external hydrogen is provided. This type of H-shuttling process has its limitation because of the limited amounts of labile hydrogen available within the coal and primary products for the redistribution. Consequently, if the hydrogen demand far exceeds the labile-hydrogen capacity of the coal, the coke formation reactions are not avoidable [1,37]. During coal liquefaction, the solvent quality degrades due to the conversion of hydroaromatics into aromatic compounds. If the hydroaromatic content of the solvent is not adequately replenished, unwanted side reactions such as phenol condensation, solvent dimerisation and coke formation may contribute to the lower conversion of coal. Stable solvent composition can be achieved either by converting coal to provide new solvent or by re-hydrogenating the hydrogen-depleted solvent with gaseous hydrogen. Nevertheless, both routes applied in coal liquefaction are too slow compared to the reactions of coal dissolution and depolymerisation, resulting in the fact that the yield of re-hydrogenated solvent does not become significant until over 50% of the oxygen has been removed from the coal through the decomposition of oxygen-containing functional groups [1,37]. 8.2.4. Catalytic Liquefaction of Brown Coal There are two kinds of liquefaction catalysts that are differentiated by their function. They are coal dissolution and coal liquid upgrading catalysts. Dissolution catalysts are used to promote the change of solid coal into liquid products. Normally, these catalysts are used at the concentrations around a few weight percents of coal. Dissolution catalysts are classified into the following categories: - dispersed - slurry - homogeneous - disposable - once-through The intimate interactions between catalyst and coal make it difficult to separate and recover the used catalyst from the bottom products. For this reason, once-through and disposable catalysts are preferred. Consequently, a low cost catalyst is desirable [3]. This means that the catalyst or its precursors should be cheap and be effectively utilised even at their low concentrations. The development of procedures for catalyst recovery and re-use is a target for the improvement of the catalyst cost and the variety of catalyst selection [3].
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The most conventional catalytic material used in coal liquefaction is iron sulphide because of its moderate price and activity, including pyrite (FeSa), pyrrhotite, and various non-stoichiometric iron sulphides (Fei.xS) such as Fe203, Fe(0H)2 and FeOOH with sulphur. Pyrrhotite is postulated to be the active form. The catalytic activity of Fci.xS is controlled by the sulphiding extent between FeS2 and FeS through H2S evolution from FeS2 during liquefaction. Sulphur deficient sites are believed to activate the molecular hydrogen [4]. Other catalytic materials widely used are Co-Mo and Ni-Mo sulphides, which have been widely used in petroleum refineries. They are usually supported on alumina. CoMoS phase is suggested to be formed as the active species in addition to the crystallites of M0S2 and C09S8. Some acidity is induced by the mixed sulphides. Ni or Co is believed to improve the hydrogenation activity of Mo [4]. The third type of catalytic materials is chlorides of transition metals, such as ZnCb and SnCU. These catalysts work in molten state. They are primary Friedel-Crafts type catalysts of acidic function to break the C-C bonds. In addition, some transition mptal chlorides have activities for hydrogenation. However, their corrosive nature and instability may limit their practical application [4].
8.3. EFFECTS OF PRETREATMENTS ON LIQUEFACTION 8.3.1. Effects of Drying of Brown Coal on Liquefaction Water is an important integral part of Victorian brown coal and many other low-rank coals. Thermal drying of coals, which is generally applied in practice, causes irreversible changes to the structure in coals such as shrinkage, collapse of the pore structure, and changes of chemical structure in the coal [38,39]. A more detailed discussion on water in brown coal and dewatering may be found in Chapters 2 and 3. A number of studies [40-48] have been carried out to study the effects of drying/dewatering on the liquefaction of Victorian brown coal. Drying of brown coals is essential to their liquefaction, utilising the reactor volume for coal with solvent and catalyst. Water may play some important roles in the liquefaction under mild conditions around 400°C, especially under CO-steam conditions, enhancing the depolymerisation of brown coal through the hydrolytic interactions [49-61]. Co-existing alkali and alkaliearth metals may catalyse such CO-steam and hydrolytic reactions, increasing the liquefaction conversion. Iwai and co-workers [47,48] reported that drying of brown coals using supercritical CO2 with organic solvent could retain the physical pore structure without any shrinkage. 8.3.2. Effects of Acid Washing of Brown Coal on Liquefaction Alkali and alkaline earth metallic species (AAEM) species exist in Victorian brown coal either as ion-exchangeable cations in the form of carboxylates and phenolates or as
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salts (mainly NaCl) dissolved in coal bed moisture (see Chapter 2). They affect every aspect of brown coal utilisation (Chapters 3 to 7), including its liquefaction behaviour. Washing coal with aqueous acids is effective to improve the solubility, fusibility and coking or depolymerisation reactivity of coals, especially very effective for the conversion of low-rank coals such as brown and sub-bituminous coals [25,62-65]. The removal of bi-cation bridging through the oxygen-containing functional groups has been confirmed to cause such modifications by the liberation of coal macromolecular network [64,65] and therefore affect the liquefaction behaviour of Victorian brown coal [50,53,66-71]. The removal of ion-exchangeable cations was also suggested to allow the easier recovery of the used catalyst in the coal liquefaction process through reductions of organic residue and deposit on the catalyst [71-73]. However, structural changes of coals in terms of such aggregate form of macromolecules, pores and reactivity of oxygen functional groups by such pretreatment have not been fully clarified. 8.3.3. Effects of Thermal Pretreatment of Brown Coal on Liquefaction Coal consists of primary macromolecules of polyaromatic-polynuclear structure with some heteroatom groups and their secondary networks, the latter of which are derived from aromatic ring stacking, aliphatic side chain entanglement, and hydrogen bonds, cation bridges, charge transfer interactions through oxygen functional groups [74-78]. Thermal pre-treatment of Victorian brown coal is important for the pre-mixing of coal particles with catalyst with or without co-existing solvent [6]. Below 400°C, retrogressive reactions such as dehydration and decarboxylation during the pre-heating can be minimised by controlled heating rate and solvent and/or catalyst addition, especially under CO-steam conditions for the effective depolymerisation of brown coals [49-52]. 8.3.4. Solvent Swelling and Impregnation Swelling of Victorian brown coal in solvent was discussed in Chapter 2. Solvent swelling and impregnation have been reported as one of the pre-treatments for the liquefaction of brown coals [49-58,72]. Non-polar liquefaction solvents such as tetralin, 1-methylnaphthalene and their mixture may play some roles in making slurry solutions, dissolving and swelling coal particles, however, polar solvents such as pyridine, quinoline, THF, DMF, and phenol are more efficient for the intimate mixing and interactions with coal particles and liquefaction catalyst. Since brown coals are rich in oxygen functional groups such as hydroxyl and carboxyl groups, polar solvents above mentioned swell coal macromolecules very effectively. However, under the liquefaction conditions, such solvent swelling and impregnation effects are not distinguished because the catalytic liquefaction and hydrogen-transferring depolymerisation from hydrogen donor solvent such as tetralin are vigorously taking place under the liquefaction conditions (for example above 673 K and high hydrogen pressure around 15 MPa).
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8.4. DESIGN OF NEW LIQUEFACTION SOLVENT AND CATALYST The cost of coal liquefaction process must be further reduced for its commercialisation in order to meet the increasing demand of the transportation fuels in developing countries [79]. There are four approaches to the reduction of cost [80]. 1) Increasing oil yield with the least production of hydrocarbon gases 2) Increasing productivity of oil at the fixed reactor volume 3) Catalyst recycle after the separation from the residue 4) Simplified preheater and reactor configuration Mochida and co-workers [73,81] proposed a novel type of the liquefaction catalyst with much higher activity and properties that allow its recovery and repeated use. NiMo, FeMo, or FeNi bi-metallic sulphides supported on a particular carbon black composed of nanoparticles are such type of catalysts that provide high distillate yields with low yields of non-distillable product. It can be recovered from the solid residues by gravimetric separation for the repeated use [5,82]. The catalyst significantly increased the oil yield in a two-stage liquefaction process consisting of the first stage at 360°C and the second stage at 450°C [6]. A remaining problem in increasing productivity of the reactor for improved economics is to increase the coal concentration in the slurry feed to the reactor. Hulston and co-workers [83] reported the liquefaction of coal without solvent by impregnating the catalyst species directly onto the solid coal matrix, although such impregnation of the catalyst onto the coal prior to the feeding and reaction is rather costly. The solvent/coal ratio was investigated as a variable to clarify the minimum quantity of solvent (including that produced from coal itself) required in liquefaction with NiMo sulphide catalyst supported on highly dispersed carbon nanoparticles in comparison with the commercial catalysts of ordinary-size (10 - 100 fim) powders [6], revealing that nano-sized fine particles may be well dispersed in coal solvent slurry with the least amount of solvent, catalysing not only the hydrogenative stabilisation of coal fragment radicals, but also the hydrogenation of polyaromatic hydrocarbons to partially hydrogenated species which perform hydrogen donation to primary coal liquefaction products. The higher yield of lighter oil fraction is another target to make coal liquefaction process commercially viable. For this purpose, the two-stage liquefaction consisting of low temperature (around 360°C) hydrogenation in the first stage and high temperature (around 450°C) hydrocracking in the second stage is expected to be effective [6] even under the solvent-free liquefaction conditions. Based on the present approach, a highly efficient coal liquefaction process can be proposed with a recyclable carbon nanoparticle catalyst using minimal amounts of solvent in the reactor. Figures 8.1 and 8.2 illustrate the product distributions during the single- and twostage liquefaction of Yalloum coal, respectively, using NiMo supported on carbon nanoparticles (KB) as a catalyst at variable tetralin/coal ratios. The oil yield decreased significantly from 72 % to 52% when the solvent/coal ratio decreased from 1.5 to 0 in the single-stage liquefaction. Increased gas and preasphaltene plus residue yields
409
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Figure 8.1 Effects of solvent/coal ratio on the product distribution from the single-stage liquefaction of Yallourn brown coal using tetralin as a solvent. Single stage: 450°C-60 min; reaction Hj pressure: 15 MPa; coal/solvent/catalyst = 3 g/X g/0.09 g (X = 0, 1.5, 3.0, 4.5); stirring speed: 1300 rpm; heating rate: 20 Kmin"'. Reprinted with permission from Ref 6. Copyright 1998 American Chemical Society.
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Figure 8.2 Effects of solvent/coal ratio on the product distribution from the two-stage liquefaction of Yallourn brown coal using tetralin as a solvent. Two-stage: 360°C-60 min/450°C60 min; reaction H2 pressure: 15 MPa; coal/solvent/catalyst = 3 g/X g/0.09 g (X = 0, 1.5, 3.0, 4.5); stirring speed: 1300 rpm; heating rate: 20 Kmin"*. Reprinted with permission from Ref 6. Copyright 1998 American Chemical Society.
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Figure 8.4 Effects of coal type on the production distribution during the solvent-free single- and two-stage liquefaction, (a), (c) and (e): single-stage (450°C - 60 min); (b), (d) and (f): two-stage (360°C - 60 min/450°C - 60 min). (a) and (b): Yalloum coal; (c) and (d): South Banko coal; (e) and (f): Tanitoharum coal. Reaction pressure: 15 MPa. Coal/solvent/catalyst = 3 g/0 g/0.09 g. Stirring speed: 1300 rpm. Heating rate: 20 K min'\ Catalyst: NiMo/KB. Reprinted with permission from Ref 6. Copyright 1998 American Chemical Society.
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compensated for the decrease in oil yield. The decrease of oil yield was much moderate in the two-stage liquefaction, the yield being 75 % at the solvent/coal ratio of 1.5 and 64 % without solvent, respectively. The use of less solvent increased the gas yield also in the two-stage liquefaction, but did not affect the heavier products yields substantially. Figure 8.3 compares the effectiveness of NiMo/KB, commercial NiMo/A^Os and synthetic pyrite catalysts for the single-stage and two-stage liquefaction of Yallourn coal under the solvent-free conditions. It can be seen that catalytic activity of NiMo/KB was considerably higher than other catalysts, giving the oil yields of 52 and 64 % in the single and two stage liquefactions, respectively. Although the two-stage liquefaction with NiMo/alumina and pyrite catalysts increased the oil yields to 45 and 40 % from 42 and 38 %, respectively, the effects were very limited. The lower oil yields with the commercial NiMo/Al203 and FeS2 catalysts are ascribed to the much higher yields of residue which were 27 and 32 % in the single stage, and 19 and 25 % in the two stage, respectively. In contrast, the gas yield was around 25 % without solvent regardless of the catalysts used. Figure 8.4 shows the product distributions from the three coals in the solvent-free single- and two-stage liquefaction using the NiMo/KB catalyst. The oil yield was in the order of South Banko coal (SBC, C = 65.8 wt% daf) > Yallourn coal (YLC, C = 66.9 wt% daf) > Tanitoharum coal (THC, C = 71.2 wt% daf) regardless of reaction conditions. In the single-stage liquefaction, THC, which was the highest rank coal among the three coals, gave the lowest oil yield of 46 %. SBC, which is an Indonesian brown coal with high oxygen content (27.2 wt%), provided the oil yield of 60 wt%, much higher than YLC, an Australian brown coal. In the case of the two-stage liquefaction, the oil yield increased to between 60 and 68 wt% depending on the coal type with simultaneous reduction in the yields of the heavier fractions (asphaltene, preasphaltene, and residue) while the gas yield remained unchanged. Especially, the
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412
Chapter 8
two-stage liquefaction of SBC provided the highest oil yield of 68 wt% even under solvent-free conditions. With solvent-free coal liquefaction, the boiling point distribution of oil products can be easily measured by distillation GC as illustrated in Figure 8.5, because no contribution from solvent was included in the distillable products. The above-mentioned study [6] revealed that the two-stage liquefaction of brown and sub-bituminous coals catalysed by NiMo supported on carbon nanoparticles (KB) achieved a remarkable oil yield over 60 % even under the solvent-free conditions. One of the key factors in the liquefaction without solvent is that the stirring speed during the heating must be carefully controlled at as low as 500 rpm before the temperature reached 300°C for the sufficient mixing of the coal particles with the catalyst nanoparticles without loss by splashing and sticking onto the reactor wall. Above 300°C, the stirring speed can be increased up to 1300 rpm, because a considerable amount of solid coal has been converted into a liquid form and the catalyst particles are well dispersed in the viscous matrix, enhancing the catalytic hydrogenation of the primary heavy products at 360°C in the first stage reaction with minimal retrogressive reactions. The product distribution after the first stage as illustrated in Figure 8.5 confirmed that the major portion of solid coal was liquefied to act as the self-produced solvent in the liquefaction. The successive second stage reaction at 450°C effectively hydrocracked the initially hydrogenated products from the first stage, producing the relatively lighter fractions in the distillate at a very high yield over 60 %. The NiMo/KB catalyst has two advantages in the solvent-free coal liquefaction. One is the nano-scale particle size with its high surface area, hollow structure, low density, and relatively lipophilic surface nature for dispersion in the viscous matrix of the primary coal liquid. It has been reported that the higher stirring speed was very effective for achieving a remarkably high oil yield above 70 % in the liquefaction of Tanitoharum coal using NiMo/KB in tetralin solvent [84]. The nano-size particles of NiMo/KB can be well dispersed by the rapid stirring with the aid of a small amount of liquid fraction produced in the initial stage of the reaction without adding external solvent. The other advantage of the NiMo/KB catalyst is the possibility of its recovery from the residual products by the simple gravimetric separation method. The hollow carbon nanoparticles of the NiMo/KB catalyst are non-polar and floating to be dispersed well in the liquid phase, a very small amount of catalyst precipitation taking place with the large particles of the residue products. It is reported [81] that the recovered NiMo/KB catalyst together with THF insoluble residue exhibited the similar activity to the virgin catalyst for the liquefaction of Wyoming coal through the re-sulphiding treatment, suggesting that no irreversible deactivation took place during liquefaction. For the practical application, recycled use of the NiMo/KB catalyst can be combined together with the heavy distillable product and/or residue by bottoms recycle or heavy solvent recycle mode, as illustrated in Figure 8.6. In the present approach with less solvent, a certain amount of rather volatile initial solvent as a transportation mediator can be used in the coal feeding system to make slurry of low viscosity for transportation smoothly into the preheater. This type of solvent fraction can be easily vaporised by increasing the gas flow rate in the first
Liquefaction
413 Hydrocarbon Gas: 5 CO,G02:15 H 2 0 : 10
piBi,^4
Product Oil
]
J L ^ " l^00*C~400'C : 50/
First-Stage Reactor
Pressure Two-Stage ^ — ^ Reducing Reactor Q H ^ ' j x *
Heavier oil +NiMo/KB 4 0 0 X ~ ; 15 Catalvst
!>
Distiliatio
Y
Figure 8.6 Coal liquefaction process scheme using the NiMo/KB catalyst (numbers indicate approximate amounts based on the dry coal). Reprinted with permission from Ref 6. Copyright 1998 American Chemical Society.
reactor as reported by the NBCL (Nippon Brown Coal Liquefaction) group [85]. Furthermore, the two-stage process of the scheme may remove the preheater by the effective utilisation of the exothermic heat of hydrogenation in the first stage where the inlet and outlet of the first reactor may be heated to 360°C and 450°C, respectively, by controlling the additional hydrogen gas charge.
8.5. DESIGN AND DEVELOPMENT OF LIQUEFACTION PROCESS FOR BROWN COALS 8.5.1. Characteristics of Victorian Brown Coal as Feedstock for Liquefaction Victorian brown coal is porous and contains high moisture of 60-66 wt%, and is easily oxidised in air after drying, resulting in spontaneous ignition [86,87, also see Chapters 2 and 3]. Therefore, it cannot be conveniently transported and stored, and does not make an exportable commodity. However, its ash yield is very low, generally in the range of 1- 4 wt% (db) [88, also see Chapter 2]. Its reactivity for hydro-liquefaction is very high if hydrogen transfer is matched to hydrogen-demand during thermal decomposition of the coal [89, also see Section 8.2]. These are great advantages in improving the liquefied oil yield. Therefore, it is a very suitable feedstock for producing liquid fuels by direct liquefaction. Accordingly, the conversion of the coal to much more
414
Chapters
valuable transportation fuel is one of the most useful utilisation of the coal because the production of petroleum seems to decrease in the future [90]. However, the high moisture and oxygen contents (more than 25 wt% on dry basis) are crucial disadvantages for liquefaction. In addition, it also contains much ionexchangeable cations such as Ca, Mg [91,92] that forms carbonate deposits, resulting in troubles for the plant operation and deactivate the catalyst [93,94]. Consequently, the process developed must be suitable for characteristics of such low rank coals, especially Victorian brown coal [95]. 8.5.2. Concept of Process Development for Victorian Brown Coal The liquefaction process developed is required to provide high oil yield under milder conditions to reduce the cost of product oil in order to make coal liquid economically competitive. In addition, it should also be suitable for low rank coal properties such as high moisture and oxygen contents [96]. The moisture in the coal must be firstly removed before liquefaction because it raises pressure extremely at the hydroliquefaction stage and decreases the oil yield [96]. In order to increase the oil yield, the coal fragments formed at the first stage of thermal decomposition should be rapidly stabilised by hydrogen donation to prevent retrogressive reactions [63,97, also see Section 8.2,3]. This indicates that the hydrogen donor solvent and catalyst are very effective to increase the oil yield and to moderate liquefaction conditions for low-rank coal liquefaction. There are two types of catalysts used for liquefaction: disposable iron/sulphur catalyst and highly active catalysts such as Ni-Mo/Al203. For the liquefaction of low rank coal, the former is usually adopted because the deposition of the inorganic matter inherently present in coal and the heavy organic fractions tends to result in catalyst deactivation. Consequently, catalytic two-stage liquefaction processes such as Brown Coal Liquefaction (BCL) process have been proposed to optimise the conditions for the states of coals and performance of catalyst [90]. The operability and reliability of the plant are also significant factors of the process development. Therefore, the problems of scale and sediment (reactor solids) accumulation, which are caused by ion-exchangeable cations in coal, must be overcome for stable operation of the plant [98,99]. Consequently, the BCL process aimed for the liquefaction of Victorian brown coal has been developed using process development units (PDUs, 0.1 t-dry coal/d) as a twostage liquefaction process, consisting of 4 units sections: dewatering (DW), primary hydrogenation (PH), solvent de-ashing (DA) and secondary hydrogenation (SH) [44,90]. Figures 8.7 and 8.8 show the concept of development and the simplified flow diagram of BCL process. The features of BCL process are summarised as follows: 1) Milder hydro-liquefaction reaction conditions, achieved by adoption of a twostage liquefaction technology. 2) Improved oil yields, achieved by bottoms recycle (recycling the residue) through two-stage liquefaction.
415
Liquefaction
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416
Chapter 8
Naphtha)
/Recovered^
(Catalyst]
VCoat
ywatef
J
J
/Tp\
[ihJ
1
^
Solvent
("Eh
1
Primary UJNew f Brown CoaTN -*j Slurry | HydroIgenation f»|Dewalering
H^^AKtHSTngl^^
- ( Solvent/CLB]
SecoTKJary Hydroger^ation
MWdle^ OistiUatey
(Residue)
(HDAO}-
Figure 8.8 Simplified flow diagram of BCL process [44,109,131 ]
3) Improved operability, brought about by the hydrogenated de-ashed oil (HDAO) recycle process. 4) A disposable and inexpensive iron-sulphur catalyst for primary hydrogenation. 5) Better energy efficiency with the slurry de-watering method, which is highly appropriate for high moisture raw brown coal processing. 6) Improved liquefied oil yields and better reliability of process operation, achieved by the solvent de-ashing connecting a two-stage liquefaction process. 7) Improved liquefied oil yields and plant operability, attained by adoption of a newly developed catalyst for fixed bed de-ashed oil (DAO) hydrogenation. The normal operation conditions and product yields determined using a 50 ton-dry coal/day pilot plant for Victorian brown coal will be discussed later in Section 8.7. 8.5.2.1. Dewatering
(DWSection)
Various ways for dewatering have been discussed in detail in Chapter 3. Conventional drying technologies may lower the liquefaction reactivity of the coal and loss some carbon due to surface oxidation in the air [40]. A slurry dewatering process has been developed as a dewatering technology to keep the reactivity of coal and achieve high thermal efficiency. Figure 8.9 shows the simplified flow diagram of the slurry dewatering process. In this process, coal is mixed with recycled liquefaction solvent, and then dewatered in the solvent by heating [40]. The moisture content of the de-watered coal should be less than 10 wt% to suppress an excessive increase in the partial pressure of water under the liquefaction condition [96]. This dewatered coal-solvent slurry at high temperature is directly transferred to the liquefaction stage. The steam generated from the water in coal is used as a heating source for dewatering after compression, resulting in great
Liquefaction preheater
417
evaporator
slurry making
4;
1
superheater (^H
N/ STMH
^-GT HJ
'
ri^ cw })
oil water separation LIGHT Q I L ^
Figure 8.9 A simplified flow diagram of dewatering (DW) section [45].
improvement of the thermal efficiency [45]. The optimum operation conditions are determined as 418 K (150 kPa). The coal concentration in the brown coal-solvent slurry is very important to develop the process and is considered to be smaller than that of bituminous coal-solvent slurry because the brown coal absorbs much solvent due to its higher porosity [100]. The viscosity of the raw coal-solvent slurry (rjsL) can be described with the following equation [101]:
x^uiun''''\c
rjsL = 32.78^ 7;
(8-1)
where X is the volume ratio of raw brown coal to solvent (0 <X< 1), 0/C is the atomic ratio of solvent (0.002 < 0/C <0.06) and rj^oi is the solvent viscosity, rj^oi is a function of distillate and non-distillate expressed by using, respectively, 50 vol% boiling point and solvent extraction analysis [102]. The viscosity of the slurry prepared by this slurry dewatering process is higher at the same coal concentration than that of the slurry made of the coal dried by a steam dryer [40,96]. This is because the former absorbs more solvent, especially lighter fraction, than the later. However, the coal dewatered in the solvent provided higher oil yield and lower hydrogen consumption in the liquefaction compared with the slurry of the coal dried in air because the coal reactivity maintains high and the contact with the solvent is intimate [40,96].
418
Chapter 8 Makeuq^ 1. Slurry Pump 2. Preheoter 3. Reoctors 4. Sludge Withdrawing Line 5. gos-Liquid Seporarors 6. Atmospheric Distil lator 7 Vocuum
Slurry
OistillQtor 8. gos-Oil CLB To De-Ashing
Seporotor 9. gas Purifier
Figure 8.10 A simplified flow diagram of the primary hydrogenation (PH) section [44].
For design purpose, cold model test of shell and tube type heat exchanger was carried out. The overall heat transfer coefficient of the slurry flow inside a tube was obtained as follows [42]: N0.14
At preheating stage: — = •y25y';'{GzPef' k
At evaporating stage:
^SL
(8-2)
rjw k
O^ORe'-'^Pr^^^A-^^^
(8-3)
where U and Ug are the overall heat transfer coefficients (W/m^K) at preheating and evaporating stages respectively, D is the inner pipe diameter (m), k is the thermal conductivity (W/mK) of the slurry, Yp is the coal concentration (kg/kg) in the slurry, risL is the slurry viscosity (mPas), T]^ is the slurry viscosity at wall, Gz is the Graetz number, Pe is the Pe'clet number. Re is the Reynolds number, Pr is the Prandtl number, and X is the ratio (kgh''/kgh"') of gas flow rate to slurry flow rate. 8.5.2.2. Liquefaction (PHSection) In most direct coal liquefaction processes, pulverised coal is hydro-liquefied in a recycled solvent with a wide range of boiling points under high temperature (703 - 733 K) and high hydrogen pressure (total pressure: 15-20 MPa). In these processes, fine powder of iron oxide (hydroxide) with sulphur or pyrite (FeS2) is usually used as disposable catalyst, and the recycle solvent is recovered by distillation from the liquid product, resulting in equilibrated solvent [103]. In the BCL process, disposable iron/sulphur catalyst is also adopted in the primary hydrogenation (PH) section, namely hydro-liquefaction of the coal, and the recycled solvent consists of three components: distillate and bottom fraction (coal liquid bottom: CLB, b.p. > 693 K) from the PH section, and productfi-omsecondary hydrogenation (SH) section.
Liquefactio n
419
In this main PH section shown in Figure 8.10, the coal-solvent slurry with catalyst is heated up under high pressure of hydrogen through a preheater, then hydro-liquefied in the bubble column reactors in series. The liquid product is separated from vapour phase in gas/liquid separator, then fractionated by atmospheric and vacuum distillations into light, middle and heavy products. In this PH section, there are many important factors that should be clarified to develop a liquefaction process that provides higher oil yield under milder reaction conditions [90]. 8.5.2.3, Preheating for Liquefaction Upon heating, low-rank coal such as Victorian brown coal is easily decomposed to form an inert organic substance [104-106, also see Chapter 4 for more details]. This is mainly due to retrogressive reactions by the decomposition of oxygen functional groups at 420-575 K, and coupling of coal radicals with each other at above 570 K. Therefore, the conditions such as heating rate and residence time at preheating stage are very important to improve the liquefaction yield. However, the suppression of former crosslinking reaction is very difficult because it can occur in solid state before dissolution of the coal into the solvent. To avoid the latter type of retrogressive reaction, the solvent and catalyst play a very important role by supplying hydrogen to the radicals to stabilise fragments. Therefore, the hydrogen donor ability of the solvent and the activity of catalyst at lower temperature are required to increase the oil yield. In addition, temperature control to shorten the preheating time is also considered to increase the oil yield [107]. In the liquefaction plant, the slurry at the preheating stage is affected by not only temperature, but also flow patterns of the slurry and gases, because a preheater usually consists of a long pipe heated in a frirnace. Therefore, the effects of preheating conditions on the liquefaction of Victorian brown coal were investigated using a continuous liquefaction system at the standard conditions of BCL process [107]. As is shown in Table 8.1, these results show that the preheating temperature should be lower
Table 8.1 Effects of preheating conditions on brown coal liquefaction [107]. Yields (wt%, daf) Preheating temp. Solvent dafC/RS/CLB/HDAO PHT-1,2 PHT-3 CLB Dist. H2O C1-C4 CO+CO2 AH2 (wt/wt/wt/wt) (K) (K) A-1 11.6 -5.33 1.0/2.5/0/0 34.5 33.7 17.6 7.9 683 683 A-2 -5.84 13 1.0/2.5/0/0 28.5 39.7 15.4 9.3 R.T. <583 B-1 11.6 -5.11 1.0/1.5/0/1.0 29.2 38.9 17.4 8.1 683 683 B-2 14.6 -6.19 1.0/1.5/0/1.0 R.T. 16.6 47.4 16.6 11.1 <583 C-1 12.6 -5.56 8.6 1.0/2.0/0.5/0 30.8 35.6 18 683 683 C-2 -5.87 12.5 1.0/2.0/0.5/0 R.T. 21.6 36.5 13.7 9.4 <583 RS: Solvent fraction(b.p.<693 K), CLB: b.p..>693 K, HDAO: b.p.>623 K, 1.0 h. Liquefaction conditions: 723 K, 15 MPa, Cat.: 3 wt% daf-coal as Fe, S/Fe 1.2 (atomic ratio). PHT-1 and PHT-2: Di 6 mm, L 30 m, PHT-3: Di 6 mm, L 10 m. (Di: inner-diameter of pipe). Run
420
Chapter 8 iUUU
<0 OHOr O180°C A200"C
V
\ V CO
_E
100 o
\
> •
\ \
^ \ A
0.8
1
12 1.4 1.6 So Vent/Coal [Vol/Vol]
1.8
Figure 8.11 Relationship between slurry viscosity and solvent/coal ratio at the preheating stage for Morwell coal [100].
than 623 K even if the residence time is very short. It also indicates that the oil yield is further improved with more effective catalyst and hydrogen donor solvent at preheating stage. Furthermore, the viscosity change in the preheating stage was investigated to avoid a trouble of plugging or large pressure drop due to an increase in the slurry viscosity through the preheater because it increased markedly for some kind of bituminous coal [108]. Figure 8.11 shows the relationship between the slurry viscosity and the solvent/coal ratio for Morwell coal at different temperatures. Since the viscosity of Victorian brown coal-solvent slurry decreases monotonically with increasing temperature [100], the higher coal concentration of the slurry is advantageous to produce much oil in the same capacity of the plant insofar as it can be transport by pump. 8,5,2.4. Liquefaction (Hydrogenation) The crucial factors for the liquefaction stage are temperature, pressure (hydrogen partial pressure), residence time (reaction time) as well as quality and quantity of used catalyst and recycled solvent. Since the effects of these factors are very complicated, great deal of studies have been carried out in the world, and reported elsewhere. Therefore, the features of these factors are briefly described for BCL process in this section. These data were obtained by using the equilibrated solvent produced by 0.1 ton dry coal/day process development unit (PDU) and 50 ton-dry coal/day pilot plant [103, also see Section 8.7].
Liquefaction
421
O:430»c V:470*C
3
ss
u X
—O
9.8
U7
m
24.5
UO 450 460 Pressure ( HPa > Temp.Cc) Figure 8.12 Effects of liquefaction conditions on H2 consumption and the yields of C1-C4 gases and heavy fraction (CLB). Other liquefaction conditions: 1.0 h, cat. 3 wt% as Fe on daf, S/Fe (atomic ratio) (0.1 t/d PDU) [109].
i) Temperature and pressure With increasing temperature up to 733 K, the increases in coal conversion would result in increases in the oil yield. Since hydrogen gas consumption and hydrocarbon gas yield increase markedly at around 733 K and above [109], hydrogen pressure must be markedly higher to avoid the decrease in oil yield at these high temperatures. Figure 8.12 shows the effects of temperature and pressure on H2 consumption and the yields of C1-C4 gases and heavy fraction (CLB) for Victorian brown coal. Based on these results, the operating temperature and pressure of almost all liquefaction processes are fixed at 723-733 K and 15-20MPa, respectively. In the PH section of the BCL process, temperature of 723 K and pressure of 15 MPa are selected as the standard conditions with disposable iron/sulphur catalyst and hydrogen donor solvent recovered from the secondary hydrogenation [90]. Under these conditions, the amount of the catalyst and sulphur/iron (S/Fe) atomic ratio are determined to be 3.0 wt% as Fe on daf coal and more than 1.2 because the hydrogen donor solvent is effective to reduce catalyst used and hydrogen consumption [110]. ii) Coal concentration and nominal residence time of the slurry The coal concentration (solvent/coal ratio) of the feed slurry affects the liquefaction reactions in addition to its viscosity [62,100]. For the brown coal liquefaction process, the solvent/coal ratio should be small as long as the slurry can be transported by a pump
422
Chapter 8
because it improves the efficiency of processing, hydrogen consumption and distillate yield [100]. The smaller residence time in the reactors is also advantageous to increase the process efficiency of the plant. However, longer reaction time usually provides higher conversion of the coal into distillate, and the heavy liquid product derived from short residence time is unstable during distillation [111]. Consequently, a compromise is necessary to choose a suitable solvent/coal ratio and a nominal residence time. In the BCL process, the nominal residence time and solvent/coal (daf) ratio are determined to be 1.0 h and 2.5, respectively, as the standard operation conditions [44,90].
100
1
i
(a)
1000
»^3-'
V
80 h60
1
k-^-
x:-
1
J
40
•A
20 h-
H 1
1
20 TGV/STK (
1
40 ikr')
i
1
60
I
1
1
20 40 TGV/STW a k r ' )
I
60
50 100 150 200 250 300 350 Total #»T (sin) Figure 8.13 Effects of gas flow rate on vaporisation and actual residence time of reactor liquid and distillate yield [112,113]. TGV: Gas volume blown into reactors under liquefaction conditions (L/h), STW: slurry fed into reactors (kg/h), Liquefaction conditions: 723 K, 15 MPa, cat 3 wt% asFe, S/Fe 1.2.
Liquefaction
423
iii) Solvent composition and gas flow rate into reactors In the liquefaction process, H2 gas is blown into the coal-solvent slurry before the preheating stage, and H2-rich gas is recycled into the reactors to maintain the H2 pressure and to control the temperature. Therefore, in addition to temperature and pressure, the ratio of the gas flow to the slurry (solvent) flow markedly influences the vaporisation rate of the solvent. Since the recycled solvent consists of many compounds with a wide range of boiling points, the vaporisation rate of the solvent also depends on its composition. The light fraction vaporised does not contribute directly to the conversion of the coal. The vaporisation and actual residence time of liquid remaining in the reactors are estimated by the following equations obtained from the comparison of distillation results of the liquid product and the liquid sampled directly from the reactors [112,113]: W^{i)=W,
r(i)-m^R{O)\
0^^ ={l-e^ )-j
J"
^v(0 = 7;71^
"^^ W^P{i)
...
y P{i) = y R{i) = 1
(8-4)
(8-5)
where Wg(i) is the mass flow rate of component / in the vapour phase (kg h'^), W, is the total flow rate of liquid except ash, P(i) and R(i) are the contents of component / in the product and reactor liquids separated by distillation, P(0) and R(0) are the nonvaporised heavy components (ash free) in both liquids, V is the effective reactor volume (m^), p is the density of reactor liquid (kg m" ^) and Sg is the gas hold up. When the gas flow rate into reactors increases, the oil yield markedly increases because almost all of light fractions of the solvent and product oil are vaporised, resulting in the marked increases in the actual residence time, and the concentration of catalyst and heavy fraction in the remainder liquid phase in the reactors as shown in Figure 8.13 [112,113]. It is found that Victorian brown coal can be completely converted into distillate at 723 K and 18.6 MPa by increasing gas flow rate [114]. In some processes, the solvent containing heavy fraction is used as feed solvent to improve oil yield (bottom recycling) [115,116]. As is shown in Table 8.2, the bottom fraction of the product (CLB) in the solvent is not vaporised, but it also increases the oil yield because it further converts into distillate and contains the used catalyst that is still active when sulphur and/or H2S exist [117]. This bottom recycling is effective to improve the distillate yield under mild liquefaction conditions such as 723 K and 15 MPa. Therefore, the combination of increasing gas flow rate and bottom recycling realises the complete conversion of the brown coal under 723 K and 15 MPa because the operation of the plant without bottom recycling is very difficult due to drying up of the liquid phase in the reactors [118].
424
Chapter 8
Table 8.2 Effects of bottom recycling on yield structure [117]. Composition of solvent
Yield (wt% on mad-C)
(wt% on maf-C)
Run
PH-Dist
CLB'
HDAO^
CLB
Solv. Naph.
H.O
R-500
250
0
0
47.4
10.4
10.8
11.7
8.4
15.3
4.0
21.2
R-501
200
50
0
39.1
12.8
13.5
14.4
9.1
15.4
4.3
26.3
R-502
100
15.0
3.8
38.5
50
100
32.0
21.9
16.6
9.8
C,-C CO+CO2 AH2
8.5
Dist^
1. CLB contained 10.8 wt% ash. 2. HDAO contained 21.5 wt% BTM (b.p. > 420°C). 3. Distillates: solvent fraction + naphtha fraction. Reaction conditions: 450°C; 14.7 MPa; solv./maf-C (wt/wt): 2.5; Hj: 10 wt% on maf-C; cat.: 3 vA% on maf-C as Fe; S/Fe ratio: 1.2.
Table 8.3 Properties of light and middle distillates derived from Victorian brown coal [120]. PH (primary hydrogenation) Fraction
LN
Kerosen
LGO
HGO
100-200 22.8
200-240 26.2
240-360 35.2
360-420
0.993
1.074
86.8 10.1 0.60 0.04
88.4
HN
B.P. Range'^ Yield (wt%)
C5-100
S.G.(\5/4Vf
0.747
0.871
0.955
81.2 13.3 0.14 0.10
82.5 11.6 0.13 0.16
83.4 10.2 0.27 0.05
8.7
7.1
U.A.^> C H N S 0 (diff.)
8.3 1.10 0.07
3.7
4.7
6.3
2.9
2.7
H/C'^
1.95
1.68
1.46
1.39
1.12
fa^>
0.03
0.25
0.44
0.45
0.60
LN : Light naphtha, HN : Heavy naphtha, LGO : Light gas oil, HGO : Heavy gas oil 1) Boiling poin range (°C), 2) Specific gravity, 3) Ultimate analysis, 4) Atomic ratio, 5) Aromaticity Primary hydrogenation conditions : 450°C, 15 MPa.
iv) Fractionation of liquid product The liquid product is usually fractionated by atmospheric and vacuum distillations to prepare the recycle solvent and liquid products such as gasoline and diesel fractions. In a two-stage liquefaction process such as the BCL process, the conditions of these distillations are very important to prepare the feed stock for the secondary hydrogenation, because they influence the properties of heavy fractions [111,119].
425
Liquefaction
Run A Series 10
y^
/°
8
t/C--^ °
1
-a —
0
o
4
So1v/P~lr • 0 h, O 1 h Solv/S~l: « 0 h. a X h
/
• •2
?
2
4
e
I
^ 1 10
Solv/P-2: O 430'C» • So1v/$-2: a 430'C. •
2
0
4
12
8
8
10
12
460*0 ^WC
14
Press. (MPa)
C«t.,F«203/S
Solv/daf-C 2.5/1 (w/w), S/Fe 1.2, H2int. 10 MPa
Solv/daf-C 2.5/1 (w/w), cat.3 wt% as Fe, S/Fe 1.2
Figure 8.14 Effects of liquefaction conditions on hydrogen efficiency [122].
Htol JCO^'C^} | a - - C 4 | V • « i
ft t
» «
*
MAFC
HtO
15
*
« 1
—
8 .-^^11 S
n>fi»*CimFqi
BICMAFC)
10 9
Oli
6 ^Si^CM^O^
17 18 H20
iNiii&rm) 'mmmm
a~-C4
......__.«._..^gipa^j >^SS«CIIS11Mi
12 13 14
a«~oi
Mimf^mm MMmtM
Fig. 8.15. Reaction scheme on brown coal liquefaction [124]. MAFC: Moisture and ash free coal (daf coal), BI: Benzene insolubles, BS: Benzene solubles, RBTM: Recycled bottom.
426
Chapter 8
The properties of light and middle fractions derived from Victorian brown coal in the PH section are shown in Table 8.3 [120]. They contain many species containing heteroatoms, especially oxygen-containing species such as phenols. Since they are easily oxidised in air and unstable during storage [121], it is necessary to further upgrade to transportation fuels such as gasoline and diesel oil. The light fraction with high octane number is suitable as feedstock for gasoline, but the cetane number of middle fraction is low compared to that of diesel oil. 8.5.2.5. Hydrogen Consumption The efficiency of hydrogen transfer to product oil is also an important factor determining the liquefaction conditions because the hydrogen gas is very expensive [122]. As is shown in Figure 8.14, the hydrogen efficiency, which is defined as the ratio of distillate yield (DY) to transferred hydrogen [H(t): hydrogen from solvent and H2 gas], depends not only on temperature and pressure, but also on the quality and quantity of solvent and catalyst used. Therefore, it should be minimised to reduce the cost of expensive hydrogen [110,122]. The standard conditions of BCL process are selected to maximise oil yield and hydrogen efficiency under milder conditions. 8.5.2.6. Kinetic Models for Coal Liquefaction To develop a coal liquefaction process, especially to scale-up the process, it is very important to describe the process by a kinetic model based on the coal conversion mechanisms. Many kinetic models of coal liquefaction have been developed [30, 123]. They are classified into three types: lumping models, hydrogen transfer models and mechanistic numerical models. The lumping models are the most useful in scaling up and designing large scale plants. However, liquefaction reaction and product distribution depend on the coal properties such as coal-rank. Therefore, based on the liquefaction results of a 0.1 t (daf-coal)/d process development unit and a 50 t (dry coal)/d pilot plant operations, a reaction scheme shown in Figure 8.15 and a kinetic model for Victorian brown coal have been developed to estimate the effects of liquefaction conditions on product yields [124]. 8.5.3. Solid/liquid Separation (De-Ashing) All direct coal liquefaction process must include a solid/liquid separation process to remove solids from the liquid product. These solids consist of un-reacted coal, mineral matter originating from that in the coal and the used catalyst, and are concentrated in the vacuum residue (bottom). Therefore, a vacuum distillation under severe conditions is one of the solid-liquid separation processes. However, its recovery of heavy fraction is very low because it contains much non-distillable organic fractions that are polymerised under severe distillation condition [90,119].
427
Liquefaction De-Ashing Solvent
~1
3. Separator
r-^
/ Solvent
\
Recovery / 1. DIssolver
V
X —^DAO To Secondary Hydrogenatjon
v
CLB
2 2. Settler
M
3
T" Residue
Figure 8.16 A simplified flow diagram of the solvent de-ashing (DA) section [44,129,130].
Therefore, many solid/liquid separation processes, which are usually called as deashing process, have been developed on the basis of the techniques of filtration, centrifUgation, settling (sedimentation) and so on [125,126]. Each process has advantages and disadvantages. The BCL process has developed a solvent de-ashing process, which is easy to apply to a large scale pant, to enhance the recovery of heavy product and de-ashing efficiency [44,90]. In this de-ashing process, heavy product (vacuum residue) is dissolved into the solvent, and then separated by gravimetric sedimentation in a settler. The de-ashing solvent is recovered by distillation and recycled as the solvent. Figure 8.16 shows a simplified diagram of the DA section. 8.5.3.1. De-ashing Solvent The selection of a de-ashing solvent is a very important aspect of de-ashing process development. A de-ashing solvent should have high dissolving ability of vacuum residue and high settling velocity of ash (inorganic matter) to provide a high de-ashing efficiency. In addition, the sludge settled must be easy to withdraw from the settler. The effects of solvent properties on the solubility of CLB and fluidity of the sludge were investigated using cyclohexane, toluene and their mixtures. Toluene has a high solubility, dissolving more than 60% of preasphaltenes (benzene insoluble-pyridine solubles: BI-PS) in CLB, and the resultant sludge could be handled without trouble. In
428
Chapter 8
the case of cyclohexane, the sludge became sticky, plugging the underflow line and the settler bottom. This is caused by the impregnation of cyclohexane into the un-dissolved solid that has low softening point [127]. The properties of naphthas produced in the PH and SH sections are similar to those of toluene and cyclohexane, respectively. In addition, the lighter naphtha from the PH section also becomes similar to cyclohexane. Figure 8.17 shows the effects of solvent properties on the sludge properties. In order to use the naphtha produced in the PH section as a de-ashing solvent, the naphtha properties need to meet the same criteria as applied for toluene. With increasing average boiling point temperature or density, the naphtha would have higher CLB solubility and lower de-ashing efficiency expressed as the settling velocity of the undissolved solids (ash) [128]. Lighter naphtha has smaller molecules that are richer in paraffins than heaver naphtha with benzenes and phenols. The solubility of the CLB (SCLB ) measured at the de-ashing conditions using high temperature and high pressure extraction is expressed as follows [129]: ^CLB = Css + 0.6{Csi - Qv/, )[l + 6.9 X 10-^ (/? - 868)]
(8-6)
where CBS, Q / {=100- CBS) and Cash are the contents of benzene solubles, benzene insolubles and ash in CLB, respectively, p is naphtha density derived from the liquid product in the primary hydrogenation with the naphtha of p = 868 corresponding to toluene [130,131]. The properties of CLB are expressed by solvent extraction analysis and ash content. Table 8.4 shows the properties of representative CLBs and the residues obtained under the de-ashing conditions, which correspond to tetrahydrofuran insolubles (THF-I). 8.5,3.2. De-Ashing Efficiency and Effects of De-Ashing Conditions To develop a solvent de-ashing process, it is crucial to know the ash distribution in a settler. Figure 8.18 shows the vertical ash distribution in a settler of a batch de-ashing
Table 8.4 Properties of representative CLBs and residues (THF-I) [128,129].
CLB-l 723/1 SA^es 15.8 23.6
Properties o f THF-I . S/Fe H/C Density • Ash^ Fe' : Ratio) (g/cm^) HI-BS BI-PS THF-I PI (Atomic 0.6 35.8 43.9 16.2 19.4 16.3 77.8 35.6 1.1
CLB-2 723/15/No 12.5 24.9
39.6
21.8
18.0
13.7 61.7 29.5
0.8
0.7
41.9
CLB-3 723/20/Yes 9.7
44.7
15.8
13.5
12.9 78.7 34.8
1.0
0.7
48.4
Liquefaction CLB Condtions^ Ash*'
Properties of CLB Solvent Extraction^ HS
26.6
a, Temperature(K)/Pressure(MPa)/BTMR. Other liquefaction conditions: 1.0 h, cat. 3.0wt% as Fe on daf, S/Fe 1.2. b, wt%.
429
Liquefaction r-Hs-
-Hl-BS-
-BI-PS^ •Pl-\
CLB Ash
I
in Toluene a t - 250''C
^ in Cyclohexane a t ' - 2 5 0 * ^ 0
^W^ 4 ^
y////A
-B-J—C —
A
1 A dissolves in solvent, (solution) B is sticky. C is solid.
.V
Figure 8.17 Effects of solvent properties on the sludge properties [127].
system. There is an ash-free zone, a boundary and an ash-concentrated zone in the settler [130]. The undissolved solids consisting of ash (inorganic matter) and heavy preasphaltenes coagulate in the ash-concentrated zone, and the ash-boundary is settled with settling period. The settling velocity of the ash-boundary depends on ash concentration of feed, temperature and the properties of solvent and heavy product [130]. When naphtha is used as a de-ashing solvent, the settling velocity increases with the decreases in the density, average boiling point temperature and aromaticity [129]. The settling velocity of the ash boundary (F, mm/s) can be expressed by the following equations [129,130].
For toluene:
For PH naphtha:
K = 4VLB
c \ 2.5 J
V^p = A^ :LB
2.5
(8-7) l523.
l523j ,848j
(8-8)
where ACLB is the characteristic parameter of organic components of CLB (mm s"), CSA is the ash content (wt%) in feed slurry, r i s temperature (K) and/? is the naphtha density (p = 848 corresponding to toluene). However, there is a lower limit beyond which the blockage of the settler bottom would occur [127]. Therefore, a suitable naphtha must be selected within this limitation.
430
Chapter 8 Overflow
Feed
10 Underflow
Batch de-ashing Settling period : 60s (D), 120S (•) 523 K, 5 MPa, CLB/toluene 1/4 (w/w)
20 Ash Content
30
40
{wt%]
Continuous de-ashing 6h(D),9h(+), 12h(o), 17h(x) 523 K, 5 MPa, CLB/toluene 1/3 (w/w)
Figure 8.18 The ash distribution in the settler of a batch de-ashing system [129-131]
For the continuous operation of the plant, feed material moves out of the settler as the streams of overflow (OF) and underflow (UF) [129,131]. To separate the ash, the liquid up-flow velocity of the settler must be lower than the settling velocity of the ashboundary. In addition, the amount of ash in the UF withdrawn from the setter bottom has to be almost equal to the ash fed to the settler. The ash overflows into the OF stream when the rate of the OF stream is too small, and a too large UF withdrawing rate increases the loss of useful organic materials. Therefore, the ash should be as much concentrated at the settler bottom as possible to minimise the rate of the UF stream. Based on the ash concentration tests at the settler bottom using a continuous de-ashing system, the maximised ash concentration of the UF stream is expressed by the following equations [129,131]:
For toluene:
^CLB
FL 0.35
FL For PH naphtha: Z^,, = B^^^g 0.35
(8-9)
523 J
523
P_ 1,857
(8-10)
where BCLB is the characteristic parameter of organic components of CLB (mm s"'), FL is the underflow flux (kg/kg or wt%), T is temperature (K) and p is the naphtha density (293 K, p = 857 corresponding to toluene). Figure 8.19 shows the comparisons of estimated V and Z with their experimental values, showing good agreement in both cases.
431
Liquefaction 10
T—I—I—I—I—1—I—r
r=0.992
J 8 >
D»
§
D-
1
1
t
>i.
t
t
i
I
L.
1
«
1 H
,.o
1 ' -' c l 3J ^ I
r- *
#>'
! ! 12
1 2h
I
.'
S
1
T
r^ 0.979
09
4
I
1
T3 16
6
3
-—,,—, 5' 20 ^ ^ ,^
1
J
^°
n cdv ^
4
4.•Hrq.8
»
1
1
1
-
^
—
.
"1
. J
12 16 20 2 4 6 8 10 Estimated Vw and V [mm/s] Estimated Zwand Z [wt%] Figure 8.19 Comparison of estimated and experimental Fand Z values [129-131]. D, toluene; naphtha. W
00
The characteristic parameters .4^^ and BQIB are expressed as follows [129-131]:
^CLB ~ ^ • 3 |
1.7
B,CLB :3.08^^^
b_ 0.6
(8-11)
(8-12)
where a is the ratio of the content of organic THF-insolubles to that of the ash in CLB (w/w) and b is the H/C ratio of organic THF insolubles. These results indicate that the de-ashing efficiency decreases with the depth of hydrogenation of heavy product with small ACLB and BCLB values. The ash distribution in the settler of a solvent de-ashing plant can be predicted by calculating V and Z using analytical results of CLB when the slurry feed rate, OF/UF ratio and temperature are fixed. As a result, the de-ashing conditions for stable operation of the plant are represented by the following equations [129,131]: Vu\<\v\
(8-13)
yVyp > yVsA ' ^UF
(8-14)
and
where Vu is the upward linear velocity of the solution (OF) in the settler (mm s'') and V is the settling velocity (mm s"^) of the ash boundary estimated by Eq. (8-8). WSA is the flow rate of ash in the feed slurry (kg h"^). Wup and Q/F [Z = maximum Cuf estimated
432
Chapter 8 To Gos Purifier 1. Oissoiver Nap. Middle Dist.
2. Preheoter 3. Reoctors 4 QQs-Liquid Seporotof 5. Distillator
Hydrogenorion Figure 8.20 Simplified flow of secondary hydrogenation (SH) process [44].
by Eq. (8-10)] are the flow rate (kgh"^) of UF and the ash content (kg kg'') in UF respectively. These equations have been developed for the solvent de-ashing of CLB produced from Victorian brown coal in the presence of iron/sulphur disposable catalyst in the BCL process. Therefore, the constants in the equations may change for the solvent deashing of heavy product derived from other coals and processes on which the properties of heavy products depend. Since other coals usually contain much more mineral matter such as silica and clay minerals than Victorian brown coal, the equations should include a parameter that represents their effects on the settling and concentration of ash (mineral matter). 8.5.4. Hydrogenation of Liquefaction Solvent and Secondary Hydrogenation of Liquid Products Since the hydrogen donation from the liquefaction solvent to the coal fragments is very important at the early stage of the liquefaction reaction as mentioned before, in most processes, the recovered solvent fraction is re-hydrogenated over a catalyst such as Ni-Mo/Al203 to give the hydrogen donation ability before being recycled to the slurry make-up step [132]. This hydrogenation process (including the catalyst used) is similar to the upgrading process of coal-derived liquid products. Figure 8.20 shows the simplified flow diagram of secondary hydrogenation (SH) section in the BCL process [44,90]. The product in the SH section is also fractionated by distillation into naphtha (b.p. < 523 K), middle (b.p. 523 - 693 K) and heavy (b.p. > 693 K) fractions. The middle and heavy fractions (hydrogenated de-ashed oil, HDAO) are recycled into the PH section as a part of solvent to improve the liquefaction reaction and operability of the plant. In some cases, this step is combined with the secondary hydrogenation of the liquid products to increase oil yield and to improve their properties. In the SH section of the
Liquefaction
433
BCL process, the de-ashed heavy liquid product (de-ashed oil: DAO, b.p. > 693 K) is further hydrogenated with middle distillate (b.p. 453-693 K) to convert into lighter fraction and to remove the heteroatom-containing compounds [ 1 3 3 , 1 3 4 ] . The conversion rate of DAO over Ca-doped Ni-Mo/A^Os can be expressed as follows [90]: /
xO.36
ln(l-DAOconv)=-)to Y^
{LVf^{LHSV)-^^'
(8-15)
where "DAO conv" is the DAO conversion (wt%), ko = Aexp(-E/RT) is the reaction rate constant (h^^^^), P is pressure (MPa), LV is the linear velocity (m h"^) of liquid and LHSV is the liquid hourly space velocity (h'^).
8.6. PRODUCT EVALUATION AND UPGRADING Coal liquid oils (CLO) with boiling point ranges of naphtha and gas oil fractions tend to carry more heteroatoms such as nitrogen and oxygen than the corresponding petroleum products. Higher contents of such heteroatoms cause serious problems including the production of air pollutants from the use of the oil and the poor stability in storage. Especially, nitrogen-containing compounds may act as inhibitor and poisons in
1000-.
C9
C10
500-^
C15 C12 ^"^ C14 C16 C11 , C13
i
I
C17
Carbon 84.97 wt%
Sulfur 677 ppm
<
40-J
Nitrogen 0.84 wt%
c
L--J*L.JwJ,JUJJLw,,Jv-v^.^^
Dibenzofuran
150 n
Oxygen 3.74 wt%
Phenol
UJLJL^
0
5
10
15
—I— 20
Retention time [min] Figure 8.21 GC-AED chromatograms of gas oil fraction in SBCL [140].
25
—1 30
434
Chapter 8
the hydrodesulphurisation processes, where molybdenum or tungsten sulphide promoted by cobalt or nickel sulphide supported on alumina or zeolite have been usually applied as catalysts. It is very important to characterise the heteroatom-containing molecular species in the coal liquids to clarify their roles and behaviours in the storage and hydrotreatment. The molybdenum sulphide catalysts supported on the carbons of various sources have been examined in terms of coal liquefaction, hydrogenation of aromatic compounds, and hydrodesulphurisation and hydrodenitrogenation of model compounds in order to optimise the catalysts for the liquefaction and the subsequent upgrading processes [135-139]. Figure 8.21 shows the GC-AED chromatograms of gas oil fraction in the South Banko coal liquid (SBCL; boiling point: < 673 K, supplied from Kobe Steel Co. Ltd) [140]. The SBCL liquid contained paraffmic hydrocarbons up to C17. Sulphurcontaining compounds found were thiophenes and benzothiophenes. Pyridines, anilines, quinoline and indoles were found as nitrogen-containing compounds. Phenols and dibenzoftiran were found as oxygen-containing compounds. The heteroatom-containing species of SBCL before and after hydrotreatment are listed in Table 8.5. All heteroatoms contained in hydrotreated oil were reduced by catalytic hydrotreatment over a commercially available NiMo/alumina catalyst. The higher reaction temperature enhanced very much the heteroatom removal. Reaction time also enhanced the removal of heteroatoms. The use of more catalyst also enhanced the removal of heteroatoms. However, the removal efficiency on hydrotreatment over this catalyst varied with the type of heteroatom species (N, S and O). Table 8.6 summarises the effect of reaction temperature on the sulphur removal (major sulphur compounds in SBCL). Sulphur content in SBCL was reduced markedly by hydrotreatment at a relatively low temperature of 320°C. Some alkylated benzothiophene and dibenzothiophene still appeared with a total amount of about 50 ppm, while thiophene and its derivatives were almost completely removed. Increasing reaction temperature to 340°C under the same condition, most of the sulphur species were removed to around 5 ppm. Sulphurcontaining species contained in the light distillate of SBCL are highly reactive in hydrotreatment over NiMo/Al203 catalyst. As listed in Table 8.5, total heteroatom contents remaining in the hydrotreated product decrease with increasing reaction temperature. The nitrogen removal (major nitrogen compounds in SBCL) is summarised in Table 8.7. At the lowest reaction temperature of 320°C, all nitrogen species identified in the CL decreased slightly. When reaction temperature was increased to 400°C, most of pyridine and its derivatives were removed, while aniline and its derivatives still remained. It is assumed that some aniline derivatives are removed during hydrotreatment, but other types of aniline derivatives may be formed from indole or quinoline derivatives; because indole and quinoline derivatives identified in the raw coal liquid decreased, but their nitrogen-containing species remained as the intermediates of HDN reaction below 450''C. At the highest reaction temperature of 450°C, indole and quinoline derivatives were removed completely, while some aniline derivatives still survived under the severe conditions.
Liquefaction
435
Table 8.5 Removal of heteroatoms by hydrotreatment overNi M0/AI2O3 catalyst [140]. Reaction conditions Catalyst, wt%
Temperature, °C
5 5
HDS, %
HDO, %
Time, min
HDN, %
360
30
31
55
4
360
60
37
66
22
5
360
120
43
70
28
5
400
30
49
72
27
5
400
60
58
79
55
5
450
30
55
78
65
5
450
60
83
94
95
3
360
60
30
50
14
10
360
60
86
39 24
3
400
60
10
400
60
49 44 74
73 75
80
3
450
60
67
93
73
10
450
60
87
95
94
5
360
120
43
28
5
400
120
55
70 84
5
450
120
77
96
92
58
Table 8.6 Conversion of sulphur-containing compounds under various conditions [140] Feed
340°C
320°C
Sulphur composition
360°C
ppm S 4.1
3.9
0
0
Ci-thiophene
18.4
4.6
0
0
C2-thiophene
38.9
9.4
0
0
Cs-thiophene
52.4
16.9
3.4
0
C4-thiophene
114.5
0
0
0
9.9
0
0
0
Ci- Benzothiophene
131.5
8.9
0
0
C2- Benzothiophene
83.6
15.4
7
1.7
C3- Benzothiophene
94.8
25.0
2.4
1.2
C4- Benzothiophene
103.5
0
0
0
25.3
0
0
0
Thiophene
Benzothiophene
Unknown
436
Chapter 8
Table 8.7 The contents of nitrogen species before and after hydrotreatment [140]. Feed
320°C
340°C
360°C
400°C
450°C
C2-A
547 143 310 587 155 629 951
368 185 334 399 136 444 746
256 50 261 301 101 432 545
232 0 173 225 151 293 506
C3-A
2606
2014
1302
1070
Indole
322 427
244 342
C,-In
1385
1118
120 184 448 185
336 0 0 45 239 366 588 888 115 179 391 40
176 0 0 0 197 284 334 387 88 48 319 28
3587
3187
Light amine Pyridine CrPyr C2-Pyr Aniline C,-A
Quinoline
Un-known Total
884
686
141 210 506 569
8946
7016
4674
1861
Figure 8.22 illustrates the GC-AED chromatograms of oxygen species identified in the samples before and after the hydrotreatment over the NiMo/alumina catalyst. Most of phenol derivatives still remained at 400°C. However, at the highest reaction temperature of 450 °C, all of phenol derivatives were completely removed. Dibenzofuran identified as a small quantity in SBCL still remained even under this severe condition, indicating that dibenzofiiran is the most refi-actory oxygen species found in this distillate. The reactivity of heteroatom species found in SBCL on hydrotreatment over NiMo supported on alumina catalyst varied with the type/structure of the species. Generally, species appearing at longer retention time showed lower reactivity. This has been confirmed by sulphur- and oxygen-containing species. Thiophenes, benzothiophenes and dibenzothiophenes also confirmed the above trend, in agreement with the finding by Nag and co-workers [139] who reported that hydrodesulphurisation reactivities decreased with increasing numbers of the phenyl rings neighbouring to the thiophene ring. It is inferred that the difficulty of the sulphur atom in the substrate to approach the active site on the catalyst influences its reactivity due to the steric hindrance. The tendency that the lower boiling point heteroatomcontaining species exhibits the higher reactivities was also seen with nitrogencontaining species where pyridine was clearly eliminated at 400°C whilst anilines and quinoline still remained at the highest reaction temperature of 450°C. Overall, sulphur in thiophenes and benzothiophenes exhibits the higher reactivity than the nitrogen- and oxygen-containing species.
437
Liquefaction
K
= 2
2
2
«j
•-
g § «S
70 n
Feed 3.74 wt% 04 IS 70 (0 T3 C O & t^ O Lil
360 °C 3.16 wt% 0. 70
400 C 2.62 wt%
<
070
450 C 0.19 wt%
O-l 10
—r15
-1
20
1
25
Retention Time / min Figure 8.22 Oxygen GC-AED chromatograms of the gas oil fraction in SBCL [140].
8.7. DEVELOPMENT AND OPERATION OF LARGE SCALE PILOT PLANT OF VICTORIAN BROWN COAL LIQUEFACTION 8.7.1. Coal Liquefaction Processes Developed in the World Many direct coal liquefaction processes have been proposed after oil crises in 1970s to produce transportation ftiels from coal to substitute petroleum [141-143]. Table 8.8 summarises the major large scale pilot plants that have been operated in the world [142]. In addition, there are other well-known processes such as LSE process in UK [144], ST-5 process in Russia [145], ITSL and CTSL processes in USA [146], although they have not been scaled up to a pilot plan level. Most of these processes aimed to liquefy bituminous and/or sub-bituminous coals, although EDS process and ST-5 tested to liquefy Texas lignite and Kansk-Achinsk lignite, respectively. On the other hand, BCL process was develop for the liquefaction of Victorian brown coal and is now applied to the liquefaction of Indonesian brown coal [147].
438
Chapter 8
8.7.2. Brief History of Development of Brown Coal Liquefaction (BCL) Process After twice of oil crises in 1970's, the Japanese and Australian governments agreed to develop a direct coal liquefaction technology for Victorian brown coal in 1980. Commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Nippon Brown Coal Liquefaction Co. Ltd. (NBCL), comprising Kobe Steel Ltd., Mitsubishi Chemical Corp., Nissho Iwai Co., Idemitsu Kosan Co. and Cosmo Oil Co. Ltd, started to work in 1981 on the development of the BCL process, which was based on the Catalytic SRC (Solvent-Refined Coal) process developed by KOMINIC group (Kobe Steel Ltd., Mitsubishi Chemical Corp. and Nissho Iwai Co.) [90]. The Catalytic-SRC process is a hydrogenation technology to manufacture solventrefined coal in the presence of iron/sulphur catalyst from Victorian brown coal, which has been used as binder pitch to make cokes for steel making [148]. The pilot plant of 50 ton dry-coal/day was constructed in Morwell, Australia, and operated successfully from June 1985 to October 1990 [44,149]. Then the conceptual
Table 8.8 Major liquefaction processes in the world (partly based on Ref 142). Process Coal Capacity of pilot plant (t/d) Operation
lyear 1 Reactor type Catalyst
Germany USA USA USA SRC-n New-IG EDS H-Coal Subbituminous Subbituminous Subbituminous Bituminous Bituminous Bituminous Bituminous
Japan BCL Brown coal
Japan NEDOL Subbituminous' Bituminous
30
250
200-600
200
50*
150
1977-1981
1980-1982
1980-1982
1981-1987
1985-1990
1996-1998
Bubblling column None (Pyrite in coal) 733
Bubblling column
Ebullated-bed
None
Co/Mo supported
Temp.(K) 723 Press. 14 17 (MPa) Solid Vacuum Vacuum Separation distilation distilation Bottom Yes Yes Recycle Slurry 45 28 coc Oil Yield 36 55 (wt%,daf) * 150 t/d on raw brown coal
723
Bubblling column* Iron Oxide/ Fe (Red-mud) Sulphur, Pyrite 723 153 Bubblling column
Bubblling 1 column Piryte 723 17
20
30
15
Hydroclone/ Vacuum
Vacuum distilation
Solvent deashing
Yes
No
Yes
No
38
40-50
29
40-50
61
50-58
53
51-58
Vacuum 1 distilation
439
Liquefaction 1981 I 1982 Pilot Plant Construction '"Pnmery Hydrogenation (Stage I) Secondary Hydrogenation (Stage II) Pilot Plant Operation" Primary Hydrogenatbn
1983
\9eA
1987
Oct.
Dec. Mechanical ' Completion
1992
1993
Construction Ckjmmenced
Run-1
Run-2 Run-4 Run-3
IMS-1 lMS-2 Secondary Hydrogenation
mcRun-5
Run~6 Run-7 1st 2nd | Run-Si
Integrated Operation
Figure 8.23 Schedule of construction and operation of the pilot plant [90].
design of a demonstration plant and the creation of a commercial plant were carried out. NBCL continued the research and development until 1994 to solve the problems that were faced during the pilot plant operation and to improve the reliability and economics of the process [90]. Recently, based on the BCL process, Kobe Steel Ltd. and Mitsubishi Chemical Co. have developed a more practical and economic process, named the Improved BCL process, for the liquefaction of Indonesian brown coal [147]. 8.7.3. Facilities and Operation Runs of the Pilot Plant The pilot plant of 50 ton-dry-coal/day consisted of coal preparation including a pulveriser and steam dryer, primary hydrogenation (PH), solvent de-ashing (DA) and secondary hydrogenation (SH) sections. The facilities for hydrogen and steam generation were also constructed and operated. Hydrogen was produced by the steam reforming of natural gas. The de-watering (DW) plant of 6 ton-raw-coal/day was constructed and operated only to collect engineering data for a large-scale plant. The operation of the plant was carried out by the Brown Coal Liquefaction Victoria Pty Ltd (BCLV) [90,149]. Figure 8.23 shows the schedule of the construction and operation of the pilot plant. The operation runs and the specified targets of each run were as follows: Run 1: Trial operation of the PH section. Runs 2 and 3: Performance confirmation of the modified plant in the PH section and the preparation of the equilibrated solvent. Run 4: Confirmation of the data collection procedure for obtaining material balance in the PH section and trial operation with actual fluid in the DA and SH sections.
440
Chapter 8 Run 5: Confirmation of the DA and SH plant performance and the integrated operation of the PH, DA and SH sections. Run 6-1 and 6-2: Long term, continuous operation of the total system, improvement and confirmation of the liquefied oil. Run 7: Data collection for scale-up (part 1). Run 8: Data collection for scale-up (part 2). Run 9: Confirmation of the DA performance with product naphtha.
During this period, nine runs with total coal operation for more than 10,000 h were conducted and the data on the process performance and operation technology were collected. Consequently, all development objectives set for the project were achieved as follows: 1) High oil yield of more than 50 wt% daf (Run 8). 2) Continuous and stable operation of the whole process for 1,000 h (Run 6-2). 3) De-ashing performance of ash concentration less than 1,000 ppm (from Runs 6 to Run 9). 4) Long period operation of maintained secondary hydrogenation catalyst life (from Run 6 to Run 8). 5) Establishment of a new de-watering process (from Run 6 to Run 8). The normal operating conditions and yield structure (Run 8) are summarised in Table 8.9 [90,149]. 8.7.4. Process Performance 8.7.4.L New Slurry De-Watering (DW) Section Three runs of the new slurry de-watering tests provided the de-watering rates of 93% at the evaporator outlet and 97% at the super heater outlet that were better than the design values. The heat transfer performance did not decrease during the operation period and the measured overall heat transfer coefficients were 170 - 200 kcal m'^Kh for the preheater, 120-150 kcal m'^Kh for the evaporator and 220 kcal m'^Kh for the superheater. This new slurry de-watering system was confirmed that its energy consumption is 1/4-1/5 of that of a conventional steam drying system [45,150]. 8.7.4,2, Primary Hydrogenation (PH) Section The major factors affecting the coal conversion and yield structure were evaluated by the data collected during stable operations and optimised to maximise the oil yield under moderate reaction conditions. The condition providing 47 - 48 wt% daf of oil yield was determined as follows: pressure 15 MPa, temperature 723 K, the ratio of heavy product with b.p. > 693 K to coal (wt/wt daf, bottom recycle ratio) 1.0 and the ratio of solvent (including bottom) to daf coal 2.5 - 2.6.
441
Liquefaction
The fluid flow behaviour in three-phase reactors involving gas, liquid and suspended solid particles was evaluated under the hydrogenation conditions by using a neutron absorbing tracer (NAT) technique developed by the Commonwealth Science and
Table 8.9 Normal operating conditions and yield structure (Run 8) [90]. 1
Unit
Primary Hydrogenation (PH)
Solvnet De-ashing (DA)
Secondary Hydrogenation
Conditions Temperature Pressure Reaction time Catalyst
j 452°C ISMPa 1 h Pyrite: 8.3 wt% daf 2.5/1 (wt/wt)
SolventVCoal Ratio
l/l (wt/wt) 270°C 3.5 MPa 4/1 (wt/wt) Naphtha 2,000 ppm in DAO 88 wt% 380°C 15 MPa
BottomVCoal Ratio Temperature Pressure De-ashing Solvent/CLB De-ashing Solvent Ash in Settler Over-Flow CLE Extraction Yiled Temperature Pressure LHSV
1.0 h"' 1.8/1 (wt/wt) 40 wt%
(SH)
SolventVDAO Ratio HDAO Recycle a, Solvnet fraction contains bottom fraction. b, Bottom with b.p. >420°C recycled from both PH and SH sections. c, Solvent fraction with b.p.25-420°C from PH section. 1
Yield Structure (wt%daf) H2 Sulfur C0+C02 H2S C1-C4 C5-C6 Light Oil (C7-220°C) Middle Oil (220--'300°C) Heavy Oil (300^420°C) CLB (420°C+) Water Total Gas Total Oil Total Oil + CLB Coal Conv. (THF con v.)
PH -4.70 -0.95 13.19 1.20 11.05 3.34 13.04 16.82 14.85 18.35 13.81 100.00 25.43 48.06 66.41 97.95
1
SH -1.0 0.0 0.0 0.1 0.7
Total -5.70 -0.95 13.19 1.30 11.75
5.3 4.0 -5.1 -5.9 1.9 0.0
21.68 20.82 9.75 12.45 15.71 100.00
4.2
52.26
442
Chapter 8
Industrial Research Organization (CSIRO) Australia [151]. Coexistence of two kinds of flow patterns was observed: a linear flow from the reactor inlet to the outlet and another flow following the internal wall of the reactor from the top towards the bottom. These flow patterns were described using axial dispersion coefficient (£)/, cmVs) gas hold-up (Eg) as follows [90]:
D, = 2.60 U I''
(8-16)
-^-\.0{u^-^U^)=0.062
(7 +^, <0.1 ms"^ 6
^8
L^^4-^^>0.1 ms-'
-^-\.5l{u^+U^)=0.0\\ Of«
TT
{r^
C - h or^rl TT
fr^
o - l \ r^^^
(8-17)
^
(8-18)
^.
Heat of hydrogenation reaction was estimated as 850 kcal/ Nm^-H2 (470 kcal/kgdaf-coal) for Victorian brown coal and the heat flux in the slurry preheater etc were also determined [90]. 8.7.4.3. De-Ashing (DA) Section Toluene was used as a de-ashing solvent (except for Run 9). The de-ashing solvent used for Run 9 was the naphtha produced in the PH section and the density of the naphtha was controlled by distillation. This is because a suitable de-ashing solvent can be prepared from the naphtha by varying its density, boiling temperature range and aromaticity [128,129]. For most of the runs, the ash concentration in heavy product (de-ashed oil - DAO, b.p. > 693 K) recovered from the settler overflow was kept less than 3,000 ppm, which corresponded to less than 1,000 ppm on a secondary hydrogenation feed basis [90]. The content of preasphaltenes (THF insolubles) in DAO was also kept at a level enough to avoid the deactivation of the catalyst and operational troubles in the SH section [90]. 8.7.4.4. Secondary Hydrogenation (SH) Section The DAO conversion was strongly affected by the reaction temperature and the DAO properties. The conversion was about 15%, corresponding to 4 % oil yield on daf coal basis, when the PH section was operated under the high oil yield conditions. After eliminating the light fraction (naphtha) by distillation, the product oil (hydrogenated DAO - HDAO, b.p. > 523 K) was recycled to the PH section as a part of solvent. HDAO was useful to improving the oil yield and operability of the PH plant. The performance of the secondary hydrogenation catalyst (Ca-doped Ni-Mo/A^Os) was demonstrated for a total of 3,400 h of DOS (DAO + solvent fraction) operation from Runs 5 to 8. The life of this catalyst was confirmed to be more than 8,000 h by the
Liquefaction
443
Table 8.10 PropertiesJ v/A of jiwpiv/^v>ii representative oil products (Run 8) [120]. 1,14.1,1 T v/ w i i ^i\_rv^uvbO yxxvn Secondary Hydrogenation
Primary Hydrogenation Heavy ight Heavy ^^^^^^^^ Light Fraction Total' . . ^^^^] . . ^TZ Kerosene ^^ ^ Naphtha Naphtha Gas Oil Gas Oil 1
b.p.'a
^^.lArk ^ f\f\ O A A ^ 1 0 0 100-200 200-240 240-360 360-420
Yield^ ^nsity' 0.925
Light Heavy Kerosene Naphtha Naphtha ---lOO
100-200 200-240
8.7
22.8
26.2
35.2
7.1
5.1
47.2
47.7
0.7376
0.852
0.9524
0.9778
1.0874
0.7132
0.8584
0.9127
Ultimate analysis (wt%) C
84.5
81.2
82.5
83.4
86.8
88.4
84.6
85
86.6
H
10.6
13.3
11.6
10.2
10.1
8.3
15
12.8
11.6
N
0.47
0.14
0.13
0.27
0.6
1.1
0.021
0.49
0.67
S
0.085
0.1
0.16
0.05
0.04
0.07
0.01
0.01
0.01
0
4.6
3.7
4.7
6.3
2.9
2.7
U/&
1.56
1.95
1.68
1.46
1.39
1.12
2.11
1.8
1.6
Har
17.3
0.9
10.5
22.9
21.1
28
3
5.4
11.8
Ha
24.4
12.3
14
29
30.3
30.8
1.4
8.5
24.8
Hp
42.9
59.4
51.8
35.9
35.9
32.8
65.4
61.8
49.7
H,
15.4
27.4
23.7
12.2
12.7
8.4
30.2
24.4
14.1
fa
0.38
0.03
0.25
0.44
0.45
0.6
0.02
0.15
0.29
'H-NMR
FIA analysis, vol^ A
-
7.1
20.6
-
-
-
5.7
14.7
-
0
-
12.6
14.1
-
-
-
0.9
6.6
-
S
-
80.3
65.3
-
-
-
93.4
78.7
-
F-1 Octane Number
-
(82)*
(81)*
-
-
-
(72)*
(70)*
-
Smoke Point (mm)
-
-
-
9
-
-
-
-
10.5
Cetane Index
-
-
-
-
(9)**
(11)**
-
-
-
a, boiling point range, °C; b, wt% daf; c, g/cm^ (15°C); d, atomic ratio e, total distillate from the PH section. *:Run5; **:Run7
444
Chapter 8
Table 8.11 Materials recommended for main equipment in coal liquefaction plant [90]. Unit Process
Primary hydrogenation
Equipment
Materials
Preheater
SUS347
Reactor
3Cr-lMo + SUS347overley
High pressure liquid-vapor separater
3Cr-lMo + SUS347overley High temperature
Low pressure liquid-vapor separater
SUS347/321 overley or clad Low temperature Carbon steel
De-ashing
Secondary hydrogenation
Atomspheric pressure distillation column
Carbon steel+ SUS316 clad
Settler
Carbon steel + SUS304 clad
Preheater
SUS347
Reactor
3Cr-lMo + SUS347 overley
Low pressure liquid-vapor separater
SUS347/321 overley or clad
Distillation column
Carbon steel + SUS304 clad
following PDU operation, when the ash content of the fed DAO solution was always kept less than 1,000 ppm [90]. 8.7.5. Quality of Oil Products The liquefied oils produced in each run were fractionated into the same boiling point ranges as petroleum-based products and each fraction was evaluated as a fuel. Table 8.10 summarises the properties of representative oil products obtained from Run 8 operation [120]. The liquefied oils obtained from the PH section contained much heteroatom-containing species, especially oxygen-containing compounds such as phenols, because Victorian brown coal contains much oxygen-containing groups. The light and heavy naphtha fractions should be used as a gasoline blending because they have high octane numbers corresponding to much aromatic and naphtenic hydrocarbons. The kerosene and gasoline fractions have low smoke points and cetane numbers. All these fractions have poor storage stability and exhibited relatively high gum formation [121]. Therefore, they need fiirther hydro-treatment to upgrade. On the other hand, the oils from the SH section had better quality than those from the PH section, because their heteroatom contents were decreased by hydrogenation over the Ca-NiMo/AbOs catalyst [120]. However, further upgrading is still necessary to satisfy the quality of the commercial grades of gasoline and gas oil.
445
Liquefaction
8.7.6. Mechanical and Material Problems during the Operation of the Pilot Plant (Erosion and Corrosion) The major equipment troubles experienced during the pilot plant operation were caused by erosion of the materials used in the slurry handling areas; these were mainly observed in the slurry pump, pressure reducing valves and slurry piping in the PH section. These were overcome by changing the materials and/or design of the damaged parts, rearranging the pipe and improving the operational procedures. Pressure reducing valves of various materials and different designs were tested, but none of them were entirely satisfactory during the pilot plant operation [152]. Therefore,
H~-202 Run
R-201
R-203
R-202
V-241
R-204
4
NaCI CaCOa OaMg(C03)2 NaeMg2Cl2(C03)4 MgCOg Na2Mg(C03)2 Run 6-1 Fe,_,S NaCI CaCOg CaMg(C03)2 Na6Mg2Cl2(C03)4 MgC03 Na2Mg(C03)2 Run 7 NaCI CaCOa
1
CaMg(C03)2 Na8Mg2Cl2(C03)4 MgCOa Na2Mgr(C03)2 Run 8 Fe,^S NaCI CaCOa
^
CaMg(C03)2
._
Na6Mg20l2(CO3)4 MgCOa Na2Mg(C03)2
Mjuor,
- Minor.
-Trace
Figure 8.24 Major components of scale along the stream [99].
,„
~
»J
j
446
Chapter 8
new valves to reduce the pressure of the slurry were required to be developed [153]. No significant corrosion and erosion of the vessels were found throughout the operation. To select better refractory materials, many kinds of material test pieces were placed in the equipment such as the reactors [154] and three test loops were designed and installed in the slurry preheater etc [152]. Table 8.11 summarises the materials recommended for the main equipment in a coal liquefaction plant. 8.7.7. Scale Formation, Solid Accumulation and Pressure-Drop Increase in the Primary Hydrogenation (PH) Section Scale formation on the inner wall of the pipes and vessels was observed from the outlet of the slurry preheater through the reactors and high pressure gas/liquid separators in the PH section for all runs [99,155]. The composition and thickness of the scale depended on the metal species in coal and disposable catalyst and the position of the reactor system. The thickness of the scale was almost proportional to the amount of ion-exchangeable cations in the treated coal and catalyst [99]. The scale had clearly layered structures. Its major components consisted of NaCl, Fcj.xS, and Ca, Mg and Na carbonates (calcite, dolomite and northupite, etc) in this order from up-stream to downstream as shown in Figure 8.24 [99]. Morwell coal gave a larger amount of scale than Yalloum coal, because it contains much more ion-exchangeable cations (especially Ca) than Yalloum coal. The scale formed in the piping leading from the preheater outlet to the first reactor, which consisted of NaCl and Fci.xS, increased the pressure drop in the preheater during operation [99]. This problem was overcome by using iron ore and sulphur, especially pyrite (FeS2), instead of the catalyst used for the pilot plant operation, which had prepared from the dust from steel making process [156]. The solid particles (reactor solids) were accumulated in the reactors, especially the first reactor, when the slurry withdrawal system from the reactor bottom was stopped [90,99,156]. The composition of the reactor solids was similar to the wall scale though mineral matter or catalyst existed as a core of some particles. The system withdrawing the small amount of the slurry from the reactor bottom was successfully operated by adopting tact valves and prevented the solid particle accumulation in the reactors. However, the scale formation was difficult to prevent by control of the operation conditions. Therefore, the countermeasure needed to be developed for preventing the scale formation such as pretreatment of the coal [71,157,158]. Hydrothermal dewatering was developed to reduce the ion-exchangeable cations before liquefaction, preventing the deposition as the scale [157, see Chapter 3]. This pretreatment provides another advantage of increasing coal concentration of the feed slurry because the pore volume decreases due to the shrinkage of coal. The pretreatment of the coal in the liquefaction solvent was also developed [158]. In addition, acid treatment removed almost all of scale precursor and to improve the oil yield [71, also see Section 8.3.2]. These pretreatments are not only required for the prevention or suppression of the scale formation, but also advantageous to the reduction of the cost of liquefied oil.
Liquefaction
447
8.7.8. Scale-Up Design for Commercial Plant NBCL has continued the research and development efforts to improve the reliability and economics of the process after the completion of the pilot plant operation. The standard operating conditions and material balance of the whole BCL process were evaluated again in detail to design a demonstration plan of 6,000 ton-dry coal/day as one unit of a commercial plant of total 30,000 ton-dry coal/day [90]. For the commercial plant operated at the standard process conditions, the yields of liquefied oil and CLB are estimated to be 51.1 and 13.9 wt% (daf). Hydrogen consumptions are 4.6 wt% (daf) in the PH section and 1.0 wt% (daf) in the SH section, respectively. The catalyst prepared from natural pyrite (FeS2) ore was adopted as a substitute for iron oxide/sulphur Thereafter, the newly developed iron oxide (limonite)/sulphur catalyst with higher activity will be adopted for a commercial plant in future (see Section 8.8).
8.8. CURRENT AND FUTURE STATUS OF COAL LIQUEFACTION 8.8.1 Current State of the Liquefaction Process and Its Commercialisation in Future After the two oil crises in 1970's, developed countries have made great efforts to reduce the share of petroleum in the energy consumption. The present supply of petroleum is sufficient and its price is generally stable. Since the liquid fuel synthesised from coal is still expensive compared to petroleum, no necessity of coal liquefaction is recognised right now in developed countries. However, rapid economic growth in Asian countries, especially China, India and Indonesia with very large population, requires a huge amount of energy. In particular, the demand of transportation fuel will markedly increase by their motorisation and will bring about a significant gap with limited growth of petroleum supply in the near future. Coal is a major energy resource to substitute petroleum because of its very huge reserves and coal liquefaction is expected to supply the liquid fuel for transportation. Therefore, the liquefaction process must be improved to reduce the cost of liquid products to be acceptable in the market. The energy resources of fossil fuel, especially coal, are entering a crucial problem of safeguarding the global environment because their consumption emits CO2, which is a major greenhouse effect gas. Therefore, the utilisation of coal must be harmonious to environment by maximising energy efficiency and minimising emissions of harmful gases and wastes. Accordingly, many processes have been developed as 'clean coal technologies'. Coal liquefaction is an important clean coal technology that can supply high quality liquid fuels, which consists of hydrocarbons, by eliminating heteroatoms from the coal such as sulphur, nitrogen and oxygen. Since the oxygen is eliminated as H2O and condensed CO2 during processing and gas purification, CO2 can be fixed to prevent its emission.
448
Chapter 8
The liquefaction of low rank coals with low value is considered to be one of the most effective clean coal technologies that can expand energy resources in the world. It is a particularly attractive technology for the countries having large population and huge amounts of reserves of low rank coals such as China and Indonesia. Victorian brown coal is also important as a feedstock for liquefaction in Australia. 8.8.2. Improvement of the Process for Commercialisation After the completion of the pilot plant operation, NBCL continued the research and development using a bench scale unit of 0.1 ton-dry-coal/day to improve the reliability and economics of the BCL process. Based on these studies, NBCL has proposed a new technology maximising the oil yield and simplifying the process configuration [121,159]. Figure 8.25 illustrates the conceptual flow diagram of the improved BCL process, which differs fi-om the BCL process on following points: 1) Deletion of ball mills in slurry making. 2) Using highly active iron-based catalyst (limonite and sulphur). 3) Addition of in-line hydro-treating process 4) Cutback in capacity of solvent de-ashing 5) Deletion of secondary hydrogenation 6) H2S direct recycling operation The modification has been aimed at: 1) Maximising the oil yields by increasing the bottom recycle ratio and the gas flow rate in the reactors, and using the limonite catalyst of high activity. 2) Improving the quality of oil that has lower boiling point range and lower heteroatom contents by using heavier distillate fraction as a part of feed solvent, and in-line hydro-treater substituting to the secondary hydrogenation in the BCL process. 3) Reducing the cost of construction and operation of the plant by simplifying the process. The standard process conditions of the Improved BCL process are summarised in Table 8.12. This process can be applied not only to Victorian brown coal, but also to other low rank coals such as Indonesian brown coal. At the slurry making and dewatering sections, coal is crushed by a hammer mill (instead of a ball mill) and then is mixed with DAO, CLB and hydrogenated recycle solvent from the in-line hydro-treating section to make coal slurry. The moisture in the coal is removed to 5 wt% (daf base) in the slurry dewatering section. The de-watered coal slurry is sent to the liquefaction section after mixing with the pulverised natural limonite (y-FeOOH: < Ijim) of high activity in the recycle solvent. This limonite/sulphur catalyst provides very higher oil yield even at much less catalyst usage than the pyrite and hematite/sulphur catalysts [160-162], as shown in Table 8.13. The effluent from the reactors is separated into gas/vapour and liquid (slurry) streams in the gas-liquid separator. The vapour phase containing lighter
449
Liquefaction
Slurry De-wateringj Raw Brown Coal
^
C Slurry ^ (^ Making
Catalyst (Slurry) CLB Recycle Solvent Recycle
_•. CLB Run Down
DAG Recycle
Figure 8.25 Conceptual flow diagram of the improved BCL process [159].
fractions of liquefied oil and recycle solvent is further hydrogenated in the in-line hydro-treating section to produce the hydrogenated recycle solvent (hydrogen donor solvent) and upgraded product oils [121,163]. In the in-line hydro-treating section with fixed-bed hydro-treaters, the reactor vapour effluent is directly hydro-treated over the catalyst with long life at the atmosphere of high contents of H2O, CO2, NH3 and H2S. This simplifies the process and increases the thermal efficiency [147]. The in-line hydro-treatment over Ni-Mo-P/Al203 catalyst under the conditions shown in Table 8.12 markedly improved the quality of product oil such as contents of heteroatoms and storage stability [121]. A part of the liquid phase is recycled to the coal slurry making section as CLB/bottom recycle. The remaining slurry product is separated by vacuum distillation into heavy distillate and CLB. Most of CLB is sent to the solvent de-ashing section. The surplus CLB is used as a boiler fuel. 8.8.3. Applicability of the Process for Indonesian Low-Rank Coal Since 1994, the Agency for the Assessment and Application Technology (BPPT) of Indonesia has carried out research in collaboration with NEDO of Japan on coal liquefaction technology, and NBCL has been assigned by NEDO to execute the applicability study on the direct liquefaction of Indonesian low-rank coal. Accordingly, NBCL and Kobe Steel Ltd. group have proposed for the liquefaction of Banko coal with
450
Chapter 8
Table 8.12 Standard process conditions of Improved BCL processes [159]. Unit
Primary hydrogenation
Inline hydrotreating
Conditions Temperature Pressure Reaction time Catalyst Solvent*/Coal Ratio Bottom Recycle Ratio Temperature Pressure LHSV
Temperature Pressure De-ashing Solvent Ash in Settler Over-Flow CLB Extraction Ratio * Solvnet fraction contains bottom fraction Solvnet de-ashing
450°C ISMPa 65 min. Limonite: 0.6 wt% daf as Fe 2.0 wt/wt 80 wt% daf lst320°C, 2nd360°C 15MPa 0.5 h'' in total 270°C 3.5 MPa Naphtha 2000 ppm in DAO 88 wt%
Table 8.13 Comparison of activity of catalyst using a 0.1 t/d bench scale unit [147,159,160]. Run No. BK-2 BK-1 — Banko Coal -— Coal S/C ratio (wt/wt) — 2.0 Catalyst Y-FeOOH FeS2 Loading (wt% daf as Fe) 1 3 Temperature (°C) 450 450 Pressure (MPa) 15 15 CLB recycle (wt% daf) 52 55-75 3.2 Gas recycle (Nm3/kg-daf) 2.1 Oil yield (wt% daf) 71.06 57.26 CLB yield (wt% daf) 20.39 1.16 * Results of 50t/d pilot plant in Ausitralia. ** Gas recycle: 0.94 was the amount of quench gas.
YL-1 — FeS2 3 450 15 92 2.1 61.63 7.01
YL-2 PP-Run 7* - Yallourn Coa1 2.5 Y-FeOOH 1 450 15 99 2.1 66.05 2.84
2.5 NBCL-K 3 450 15 94 0.94" 40.00 26.00
the Improved BCL process, which is economical and environment-friendly process [147]. This process is also applicable for Victorian brown coal.
Liquefaction
451
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452
Chapter 8
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Index
AAEM, 3, 49, 147, 148, 149, 150, 151, 152, 153, 159, 161,172,173,174, 175, 176, 178, 179, 181, 182, 183, 184,216,217,406 volatilisation/retention of, 161, 172, 173, 174, 175, 177, 179, 180, 181, 182, 183, 184,216,241,242,235, 240, 270, 332 Acid-insoluble, 172, 349 Acid-washing, 19, 37, 43, 53, 56, 57, 60,88, 147, 148, 149, 150, 151, 152, 153, 154, 158, 159, 173,321 Activated carbons, 6, 162, 278, 279, 366 Activation energy, 142, 143, 185, 187, 189, 190, 191, 192, 193, 194, 195, 198, 200, 214, 216, 254, 255, 257, 258,314 distributed activation energy model (DAEM), 185, 193, 196, 197, 199, 200,201,202,214,216 Differential DAEM method, 188, 196, 197, 199,201,202 Integral DAEM method, 188, 196, 197,199,201,202 Adsorbent, 6, 7, 20, 50 Advanced pressurised fluidised bed combustion (A-PFBC), 368, 370, 377, 378, 379, 386, 387, 388, 391, 392, 393 Air staging, 337 Aliphaticity, 144 Alkali (also see AAEM), 3, 49, 147, 172, 229, 236, 241, 242, 244, 245, 267, 320, 349, 362, 366, 406
Alkaline earth metal (also see AAEM), 3,49,54,56,147, 172,229,236, 241, 242, 244, 267, 320, 362, 406 Alkyl, 143, 157, 164, 184, 347, 403 Aminoacids, 299, 303,312 Argonne premium coals, 72, 200, 293, 347 Aromatic ring systems (clusters), 20, 47,60,69,73,74,75,77, 135,141, 142, 144, 155, 156, 184,204,213, 273, 293, 294, 330, 345, 403,407 Aromaticity, 404, 405, 429, 442 Asphaltene, 404,411
B Bag filter, 271 BET surface area, 33, 34, 89 Bidentate structure, 55 Bimetallic sulphides, 402 Borohydride method, 43 Brayton cycle, 367, 378, 383 Breakable bridges, 204, 211 Bridge-type structure, 55 Bridge breaking reactions, 142, 143, 144, 149, 150 Briquetting, 6, 105, 109, 110, 114, 121, 122, 123, 124,362 Brown Coal Liquefaction process, 6, 106,413,414,418,419,420,421, 422, 424,426, 427, 432, 433, 437, 438, 439, 447, 448, 450 BTX, 153, 156, 157, 163, 164, 168, 169 Bum-off, 256, 273 Burnout, 256, 257, 268, 337, 366
Index
Carbonisation, 6, 134, 319, 379, 386, 388 Carbonyl, 45, 47, 48, 54, 58, 95, 151, 286 Carboxyl, 4, 7, 18, 19, 33, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 72, 73, 75, 87, 88, 89, 93, 142, 148, 150, 152, 169, 175, 230, 249, 293, 407 Carboxylate, 3, 18, 37, 39, 40, 43, 50, 51,54,58,89,93, 118, 147, 148, 172, 174, 175, 182,322,406 Catalysis, 113, 138, 151, 153, 166, 183, 202, 228, 230, 231, 232, 234, 236, 241, 244, 245, 246, 247, 248, 249, 250, 252, 256, 273,, 287, 288, 301, 318, 319, 320, 321, 322, 333, 344, 346, 349, 350, 393, 402, 403, 404, 405, 406, 407, 408, 411, 412, 414, 416, 418, 419, 420, 421, 423, 426, 432, 434, 436, 438, 440, 442, 444, 446, 447, 448 Ca-catalysed/catalyst, 231, 232, 249, 321,322 Fe-catalysed/catalyst, 234, 250, 252, 318,321,322,406,448 Na-catalysed/catalyst, 151, 241,249 Ni-catalysed/catalyst, 228, 232, 249, 252, 393 recoverable catalysts, 402 upgrading catalysts, 405 Char-N, 273, 303, 304, 306, 307, 314, 315,316,317,318,319,320,328, 329, 332, 333, 334, 335, 338, 342, 343, 344, 346 Circulating fluidised bed combustion (CFBC), 253, 265, 269, 274, 360, 366, 368, 370, 372, 379, 381, 382, 385,388,391,392,393 Clausius-Clapeyron equation, 90 Coalification, 4, 11, 12, 17, 110, 112 Coal-N
459 amino, 49, 292, 326 centre-N, 294, 324 content, 7, 273, 287, 288, 289, 290, 292, 305 Dumas method, 288 Kjeldahl method, 287, 288 N2 formation, 275, 317, 318, 319, 320,321,322,342,343 N-5, 290, 292, 293, 295, 303, 323, 324, 336 N-6, 290, 292, 293, 294, 295, 303, 323, 324, 336 N-Q, 290, 293, 294, 295,303, 323, 324, 326, 336 pyridinic, 49, 290, 292, 293, 294, 295,301,303,310,312,313,319, 323,336 pyridonic, 49, 293, 295, 303 pyrrolic, 49, 289, 290, 295, 303, 310, 313,319,336 quaternary, 49, 290, 293, 294, 303, 311,323,326,336 valley-N, 294, 324 Coal-S content, 4, 38, 238, 252, 344, 345, 347, 349, 434 disulphide, 49, 346, 347, 349 Eschka method, 344 sulphide, 49, 267, 286, 345, 346, 349, 366,402, 406,408, 434 thiophene, 49, 345, 347, 349,434, 436 Coal-solvent slurry, 169, 171, 416, 419, 420, 423 Coat and Redfern's method, 190 Coal-water interactions, 85, 86, 94 Coal liquid bottom (CLB), 418, 421, 423, 427, 428, 429, 430, 431, 432, 447, 448, 449 Cohesive energy, 58, 65 Coke, 121, 135, 161, 162, 238, 260, 404,405 Colloid, 18,19,39,86 Compensation effect, 202
460
Index
Computational fluid dynamics (CFD), 259 Contact angle, 96, 98 Convection, 135, 141, 145, 150, 151, 182 Coordination number, 144, 208, 210, 212 Corrosion, 172, 250, 269, 270, 365, 446 CPD model, 204, 208, 214, 216 Cross-linking, 4, 55, 58, 72, 142, 143, 144,145,148, 149,150,166,171, 184, 204, 209, 214, 216, 217, 302, 419 Crystallised carbon, 238, 240 Cylindrical pore, 21, 26, 27, 28, 29 Curie-point reactor (CPR), 136, 139, 143, 148, 149, 153, 154, 156, 157, 161, 167
D Dealkylation, 157 De-ashed oil, 416, 432, 433, 442 Demineralisation, 235, 237, 240, 244, 256,321,326 Denitrogenation, 7 Density apparent, 30 true, 30, 31 Deoxygenation, 157 Depolymerisation, 142, 144, 204, 216, 403,404,405, 406, 407 Desulphurisation, 7, 271, 272, 350 Dewatering/Drying, 2, 4, 5, 15, 20, 21, 22, 23, 24, 25, 27, 29, 31, 34, 37, 57, 72,85,86,90,99,100,101,102, 103, 104, 105, 106, 107, 108, 109, 110,111, 112,113, 114, 116,117, 118,119,120,121,124,223,225, 252, 254, 360, 361, 362, 363, 369, 373, 374, 375, 379, 381, 382, 383, 384, 385, 387, 388, 391, 392, 393,
394, 396, 398, 406, 413, 414,416, 417,423,440,446,448 bed mixing drier, 362, 375 Carbon Dry process, 106 Coldry Process, 108 entrained flow drying, 106 Evans-Siemon process, 111 evaporative drying, 100, 101, 104, 111,121 Fleissner process. 111, 121 hot gas rotary drum, 102 hydrothermal dewatering (HTD), 88, 93, 112, 113, 114,116, 117, 118, 120,121, 252 indirect drying, 109 mechanical thermal expression (MTE), 113, 114, 115, 116, 117, 118,120, 121, 125,254 microwave drying, 106, 129 mill drying, 102 NBCL/UBC Process, 106 non-evaporative dewatering, 100, 110, 117, 118 pellet drying, 108 press dewatering, 114 solar drying, 107 solvent dewatering, 117, 125 steam fluidised bed drying, 103, 104, 105,106, 116, 125, 129,362,369, 371, 372, 373, 374, 375, 376, 377, 379, 380, 381, 382, 383, 384, 385, 388,391,392,398 steam tube rotary dryers, 109 thermal dewatering, 110, 111, 113, 114, 117, 119, 131,363 using hot gas, 361 Diffusion, 95, 102, 135, 141, 145, 150, 178,182,183, 184,255,256,272, 365 Distillable oil, 404 Distillate, 402, 408,412, 417,418,422, 423,426,433, 434,436, 448,449 Divalent, 148, 175, 178
Index Donatable hydrogen, 142, 143, 175, 204, 301 Donor number (DN), 59 Drop-tube reactor (DTR), 137, 157, 161, 226, 229, 235, 237, 242, 273, 366 Drop-tube/fixed-bed reactor, 137 DSC, 23, 24, 72, 91, 92, 93, 94
E Electric double layer, 18 Electron donor solvent, 59, 60, 61, 62, 63, 66, 67, 437 Electron paramagnetic resonance, 100 Electron probe microanalysis (EPM), 347 Electrostatic precipitators (ESP), 271, 276, 277, 278 Equilibrium moisture content, 86, 88, 89,93, 105, 107,262 ESCA, 289 Ether, 5, 20, 48, 49, 73, 117, 142, 151, 164, 184,287,403 EXAFS, 54, 252, 347 Explosion, 262, 295 Extent of cation exchange, 42, 53, 55, 56,57 External gas pressure, 145 Extraction, 7, 19, 20, 58, 60, 61, 62, 63, 64,65,69,72,73, 117,204,214, 362, 372, 375, 379, 380, 381, 382, 383,385,386,401,417,428
Fabric filters, 277, 278 Fast neutron and gamma ray transmission (FANGAT), 99 FG-DVC model, 142, 204, 214, 216, 327 Fixed-bed reactor (FBR), 117, 121, 135, 136, 137, 138, 139, 143, 146, 148, 152, 154, 173,175,177,179,
461 182, 224, 226, 229, 237, 242, 315, 316, 320, 321, 322, 325, 328, 333, 339,340,342,344,363 416 Flash liquefaction, 168 Flash pyrolysis, 162, 171 FLASHCHAIN model, 204, 208, 214, 216 Flory-Rehner theory, 69, 70 Fluidised-bed reactor (FBR), 102, 103, 104,105,106, 112, 116, 138, 154, 156,161,172,175,176,177,179, 224, 225, 226, 228, 229, 230, 234, 238, 239, 246, 249, 253, 254, 256, 258,259,270,273,305,308,311, 312,318,327,333,336,338,339, 340, 344, 346, 347, 350, 360, 361, 362, 363, 364, 366, 368, 370, 372, 374, 375, 376, 378, 384, 385, 386, 391,392,393,394,398 freeboard, 138, 154, 156, 177, 308 Fluidised-bed/fixed-bed reactor, 175, 177,179,333,339,344 Fluidised-bed/tubular reactor, 138, 308 Fly ash, 266, 267, 268, 271, 279 Forms of water in coal bound water, 23, 24, 25, 26, 27, 91, 93 bulk, 20, 21, 23, 24, 27, 91, 92, 93, 94, 102 capillary, 20,21, 102 freezable, 88, 92, 93 monolayer, 20, 22, 24, 89, 93 multilayer, 20, 22, 24 non-freezable, 23, 24, 25, 26, 27, 29, 88,91,92,93 Fouling, 99, 101, 110, 112, 172,253, 263,268,269,270,361,362 Fractal, 34, 35, 37 Free-fall pyrolyser, 304 FT-IR, 46, 48,49, 54, 59, 63, 72, 94, 99,154,248,317,403 DRIFT, 48, 72 Fuel cell, 5, 286, 360, 361, 366, 367, 368, 394, 396, 397, 398
462
Index
Fullerene, 6, 50 Fulvate, 7 Fulvicacid, 18, 19
Gas clean-up, 365 cold, 365 hot, 287, 365, 376 Gasifiers entrained flow, 224, 225, 363 fluidised bed, 224, 363, 364, 366, 398 moving bed, 225, 363 Geology, 10, 12,78 Graphitisation, 240
H H radicals, 164, 151, 153, 164, 178, 179, 301, 330, 332, 339, 340, 341, 342, 343, 344 Hartmann bomb, 262 Heat of adsorption, 31, 32 isosteric, 88, 90 Heat treatment, 20, 53, 55, 265 Heating rate, 135, 136, 137, 138, 139, 141, 142, 143, 144, 145, 146, 147, 148, 150, 151, 153, 154, 156, 161, 162, 166, 173, 181, 189,190, 191, 195,196,197,199,200,214,216, 217, 226, 227, 229, 230, 237, 238, 255, 257, 296, 303, 304, 306, 315, 316,317,321,325,331,332,340, 346,349,407,419 Host-guest model, 58, 184, 205 Humate, 7 Humic acid, 18, 19,20,45,57,293 nitrohumic, 7 Hydrocracking, 153, 404, 408 Hydrodenitrogenation, 434 Hydrodesulphurisation, 434, 436 Hydrogasification, 153, 224, 227, 252
Hydrogen bond, 21, 24, 29, 31, 32,48, 54, 56, 59, 60, 62, 63, 64, 66, 67, 69, 70,72,95, 123, 142, 166, 184,216 Hydrogen donor, 164, 166, 169, 346, 403, 407, 401, 404, 407, 414, 419, 420,421,449 Hydrogen transfer, 168, 184, 404, 413, 426 Hydrolysis, 20, 44, 49, 56, 144, 310, 321,326,328,332,344 Hydrolysis of HCN, 328, 329, 344 Hydropyrolysis, 163, 323, 330, 343 Hydrothermal, 56, 88, 93, 112, 113, 121, 125,204,252,446 Hydrothermal gasification, 114, 122 Hydroxyl, 20, 25, 26, 27, 29, 40, 43, 44,45,46,48,49,50,66,67,91, 142, 144, 293, 294, 302, 324, 407 Hysteresis, 86, 87, 98
I Ignition, 260, 262,413 Integrated gasification combined cycle (IGCC), 116, 225, 241, 360, 364, 367, 368, 370, 372, 376, 382, 383, 384, 385, 388, 391, 392, 393, 398 IDGCC, 106, 113, 226, 238, 254, 361 Integrated gasification fuel cell (IGFC), 225,241,360,368 Inter-cluster linkages, 135 Intra-particle, 108, 134, 135, 141, 144, 145, 146, 148, 150, 154, 175, 182, 183 Ion bridges, 403 Ion exchange, 3, 7, 33, 38, 41, 42, 43, 49,50,51,56,57,58, 147, 150, 151, 152, 153, 161,231,235,236,237, 244, 245, 246, 249, 263, 406, 407, 414,446 Iron sulphides, 267, 406 Isoelectric point, 19 Isotherms, 86, 87, 89, 90, 91, 101
463
Index
Labile bridges, 206, 211,213 Laser induced breakdown spectroscopy, 38 Laser-induced photofragment fluorescence, 264 Laser plasma spectrometer, 38 Lewis basicity, 59 Lithotype, 12, 16, 88, 89, 93„ 254, 262 colour, 12, 14, 17 gelification, 12, 14, 15, 17 groundmass, 12, 14, 17 texture, 12, 14, 16 weathering, 12, 14
M Maceral, 16, 17,403,405 fluorescence, 17 Macromolecular network, 20, 24, 44, 55, 56, 57, 58, 59, 60, 62, 64, 65, 67, 69,70,72,73, 135, 141, 142, 144, 145, 148, 166, 184, 205, 403, 404, 407 Macropores, 98, 151 Mercury, 275, 278, 285, 287, 366 Mesopores, 151 Mesoporous carbon, 6 Methoxyl, 25 Micro-organisms, 6, 72 Microwave, 99, 106 microwave spectroscopy, 265 Monovalent, 176 Mossbauer spectroscopy, 55, 252, 267
NaCl-loaded, 174, 175, 176, 178, 179,246 Nano, 251, 321, 322, 402, 408, 412 Nascent char, 137, 138, 153, 154, 177, 216, 229, 231, 235, 237, 242, 306, 314,316,332,341 Nascent tar, 151, 235 Nascent volatiles, 177, 216 Nitriles, 296, 299, 301, 302, 303, 306, 312,313,326,329,330 NMR, 24, 25, 27, 29,44,45,46, 49, 66,70,72,73,74,91,92,93,94, 100,141,157,204,214,292,306, 403 ^^C-NMR,44,45,74,94, 157 ^^N NMR, 292 ^^F-NMR,45 ^H-NMR, 24, 25, 27, 46, 66, 70, 141 CP/MAS ^^C-NMR, 45, 74 CPMG pulse sequence, 27 CRAMPS, 45 deuteration, 25, 46, 66 proton magnetisation decay, 46 SPE/MAS ^^C-NMR, 45 spin-spin relaxation, 24 thermal analysis, 58 TOSS ^^C-NMR, 74 transverse relaxation, 24, 27, 66, 70 Non-breakable bridges, 205, 211 Non-covalent bonding, 59, 166, 184, 403 NOx index, 336
o Oxyfuel, 5
N N-containing model compounds, 296, 302,312,314,326,334,346 NaCl, 3, 38, 94, 149, 172, 173, 174, 175,176,178,179,183,236,246, 264, 270, 365, 407, 446
Peripheral functional groups, 135, 204, 206,212,214 Permeability, 15 Petrography, 11, 17,98,332 Petrology, 11, 16
464
Index
Phenol, 4, 7, 18, 19, 33, 40, 42, 43, 44, 45, 46, 49, 50, 53, 67, 89, 120, 143, 151, 230, 246, 248, 275, 404, 405, 407, 436 Phenolate, 3, 406 pKa, 40, 51,52 Polyamides, 303 Polymerisation, 141, 142, 162,206, 289 Porosity, 15, 20, 30, 31, 34, 35, 93, 106,110, 124,269,417 Preasphaltene, 404, 408, 411,427, 429, 442 Pressurised fluidised bed combustion, 112, 256, 272), 360, 367, 368, 370, 376, 377, 378, 379, 385, 386, 387, 388,391,392,393 Pretreatment, 161, 164, 204, 270, 407, 446 Primary pyrolysis, 134, 136, 137, 138, 139, 143, 144, 148, 153, 154, 156, 157, 158,161, 168,172,216,217, 306,319 Primary tar, 134, 143, 148, 150, 154, 156, 158, 159 Primary volatiles, 134, 135, 136, 148, 152, 157, 161 Prompt gamma-ray neutron activation analysis (PGNAA), 99 Prompt NO, 273 Pulverised fuel (pf), 4, 340, 363, 366, 376,381 Pycnometry, 26, 31
Quantification of-OH group, 44
R Radical transfer, 157 Rankine cycle, 367, 381, 383, 391 Rebuming, 274, 337 Recuperation, 393, 396, 397
Reforming, 151, 158, 159,231,233, 234, 235, 240, 249, 393, 439 Relative humidity, 87, 88, 89, 93, 94
Secondary pyrolysis, 135, 136, 137, 138,143,148, 152,154,156,157, 161 SEM, 247, 270, 347 CCSEM, 263, 264, 266, 267 EPMA, 247 Shrinkage, 15, 21, 24, 27, 31, 86, 102, 109,110, 111, 124,406,446 Shock-tube reactors, 296, 301 Size exclusion chromatography, 156 Slagging, 172, 225, 259, 269, 363 Slit-like pore, 26, 27, 28, 29, 91 Small angle scattering, 34, 35, 37, 98 Solubilisation, 6, 19, 20, 43, 60, 72, 73, 75,77, 144, 169, 171 Solubility parameter, 58, 60, 61, 62, 63, 65,67,69,220,271 Solvent/coal ratio, 402, 404, 408, 411, 420,421,422 Solvent de-ashing, 414, 416, 427, 428, 429, 431, 432, 439, 442, 448, 449 Solvent extraction, 117, 401, 417, 428 Solvent-free coal liquefaction, 408, 412 Solvent power, 20 Solvent solubilised coal, 169, 171 Soot, 135, 156, 158, 178, 232, 253, 259, 268, 296, 301, 302, 303, 310, 312,314,328 Soot-N, 303, 310,328 Specific energy, 108, 112, 362, 364, 365, 378, 383, 384 Spherical bomb, 262 Spontaneous combustion, 101, 103, 106,124,275 Spouted-bed combustor, 270 Supercritical boiler, 5 Supercritical water, 204
465
Index Surface area, 6, 29, 31, 33, 34, 35, 87, 151,267,271,273,336,338,412 Swelling, 21, 22, 25, 27, 32, 34,40,43, 44, 55, 56, 58, 59, 60, 63, 64, 65, 66, 67, 68, 69, 70, 72, 73, 86, 124, 166, 168, 169,225,318,407 Syngas, 224, 225, 401
gas turbine, 5, 50, 112, 286, 350, 361, 364, 365, 367, 370, 372, 373, 374, 376, 377, 378, 379, 380, 383, 384, 386, 387, 388, 391, 392, 393, 394, 396, 397 steam turbine, 110, 362, 367, 372, 377, 378, 381, 382, 383, 385, 386, 387,388,391,396 Turbostratic carbon, 321
T-carbon, 241 Tar yield, 139, 143, 144, 145, 146, 147, 148, 149, 150, 151, 153, 154,156, 157,159,161, 162, 167, 168, 169, 174,182,183,217,304,305,306, 309 asymptotic tar, 139, 143, 144, 145, 148, 149, 150, 151, 154, 156 Tar-N, 303, 304, 306, 309, 310, 311, 312,314,316,318,319,321,326, 338 Thermal conductivity, 269, 418 Thermal cracking, 135, 141, 142, 145, 148, 151, 154, 159, 179, 182, 183, 237, 303, 306, 307, 309, 310, 311, 312,314,315,316,324,326,328, 329,331,332,341 Thermogravimetric analyser (TGA, thermobalance or thermogravimetric reactor), 135, 139, 182, 226, 228, 254, 263, 273, 333 Titration direct with NaOH, 43 non-aqueous, 40 Total volatile yield, 136, 138, 139, 142, 145, 147, 158, 161, 162, 167, 169, 194 Trace elements, 254, 271, 275, 276, 365 Transition metal, 250, 406 Transmission electron microscopy, 34, 37, 240, 347 Turbines
u Upgraded Brown Coal (UBC) process, 254, 275 UV absorption spectroscopy, 74, 75, 144, 156,403 UV fluorescence spectroscopy, 38, 74, 75, 141, 144,156,403
van der Waals force, 19, 31, 58, 166, 184 Vapour pressure, 86, 90, 148, 206, 208, 366 Viscosity, 62, 113, 404, 412, 417, 418, 420,421 Volatile-char interactions, 178, 180, 330, 332, 333, 339, 344, 347, 349 Volatile-N, 303, 304, 306, 307, 309, 310,311,314,319,334,335,338, 339,346
W Water-gas shift reaction, 231, 233, 235, 249, 366 Wetting heat of, 31, 33 wettability, 56 Wire-mesh reactor, 57, 75, 136, 137, 139, 140, 144, 145, 148, 149, 150, 153, 154, 156, 161, 173, 177, 179, 181,183,227,228,237,306,323
466
Index
XANES, 38, 49, 290, 292, 293, 347 XPS, 48,49, 289, 290, 292, 293, 294, 310,312,313,319,323,324,326, 347 X-ray absorption fine structure (XAFS) spectroscopy, 175, 263, 267, 347 X-ray diffraction (XRD), 99, 241, 248, 320 X-ray fluorescence spectroscopy, 38
z Zeta potential, 18, 19
