Tài liệu Coal gasification and its applications

COAL GASIFICATION AND ITS APPLICATIONS DAVID A BELL BRIAN F TOWLER MAOHONG FAN Amsterdam • Boston • Heidelberg • London • New York • Oxford Paris • San Diego • San Francisco • Singapore • Sydney • Tokyo William Andrew is an imprint of Elsevier William Andrew is an imprint of Elsevier The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA First edition 2011 Copyright Ó 2011 Elsevier Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: permissions@elsevier. com. 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Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-8155-2049-8 For information on all William Andrew publications visit our web site at books.elsevier.com Printed and bound in Great Britain 11 12 13 14 15 10 9 8 7 6 5 4 3 2 1 INTRODUCTION This is a book about coal gasification and its related technologies. The relationship between these technologies is shown in Figure 0.1. The gasification process begins with a viable feedstock. In this book, we focus on one of those feedstocks that must go through the gasification process, coal. The nature of coal, including its properties and availability, are described in Chapter 1. Petcoke, petroleum coke, a solid, high-carbon byproduct of petroleum refining, can also be gasified. Gasifiers designed for coal, especially high temperature, entrained flow gasifiers, are used for this application. Biomass gasification has a great deal in common with coal gasification, but biomass gasifiers are optimized for biomass feedstock. The product of gasification is syngas, which is primarily a mixture of carbon monoxide and hydrogen. Most syngas, however, is not currently made by gasification, but rather by the steam reforming of natural gas. In this process, steam and natural gas are fed to catalyst-packed tubes, which are held inside a furnace to provide the endothermic heat of reaction. Figure 0.1 also shows other gases, which can be blended with syngas for further processing. One such gas under consideration is hydrogen, which can be produced by electrolyzing water using off-peak power from a nuclear power plant. In a few cases, carbon dioxide from an external source may supplement the carbon monoxide in syngas. Just as coal is not the only feedstock for gasification, gasification is not the only use of coal. Most coal is burned to produce electric power. Chapter 2 describes a few of the non-gasification uses of coal. Gasification is described in Chapters 3, 4, and 5. Chapter 3 describes gasification as a chemical reaction system. Although this chapter may look complex, our knowledge of the chemistry of gasification is far from complete. Chapter 4 covers several gasifier designs. These designs were selected because they are now in commercial use or development, or because they illustrate interesting concepts. One gasification approach is sufficiently different that it deserves its own chapter, underground coal gasification, covered in Chapter 5. Instead of mining coal and transporting it to a gasifier, the coal is left in place underground, and the reactant gases are brought to the coal. Deeply buried coal seams, which are uneconomic to mine, may be exploited by underground coal gasification. Syngas leaving the gasifier contains numerous impurities. The inorganic fraction of the feedstock leaves as solid ash or molten slag. Ash or slag removal is usually an integral part of the gasifier design. If the gasification occurs at relatively low temperatures, then tar will be produced. Tar removal is also an integral part of gasifier design. Highertemperature gasifiers do not produce significant tar. The syngas also contains sulfur in the ix x Introduction hydrogen, electric power ammonia, nitrogen fertilizers methanol, dimethyl ether, hydrocarbons substitute natural gas, Fischer-Tropsch hydrocarbons Products sequestration steam Syngas processing water gas shift impurity removal steam reforming gasification Gasification Feedstocks CO2 removal coal petcoke biomass natural gas other gas Figure 0.1 Gasification and related technologies. form of H2S, with lesser quantities of COS. Sulfur must be removed from syngas either to prevent emission of SO2 when syngas is burned, or to prevent catalyst poisoning in downstream reactors. Sulfur removal is described in Chapter 6. Carbon dioxide removal can occur either as a part of impurity removal, or after water gas shift, as shown in Figure 0.1. The traditional carbon dioxide removal techniques are closely related to sulfur removal, and are described in Chapter 6. The ability to remove carbon from syngas and sequester it in a geological formation is one of the major attractions of coal gasification. This allows coal to be used while minimizing greenhouse gas emissions. A major objection to this approach is that carbon capture and sequestration are expensive. This prompted a great deal of research into new carbon dioxide separation technologies, and which is described in Chapter 10. Syngas contains a number of minor impurities, and one of the more significant is mercury, a neurotoxin. Removal of mercury is discussed in Chapter 9. For some applications, a nearly pure hydrogen stream is desired. In others, such as methanol synthesis, a specific ratio of carbon monoxide to hydrogen is required. In either Introduction case, the gasifier usually produces a higher ratio of carbon monoxide to hydrogen than desired. This ratio needs to be shifted towards a greater hydrogen content. The usual way to do this is through the water gas shift reaction in which carbon monoxide reacts with steam to form hydrogen and carbon dioxide, as described in Chapter 7. Hydrogen can then be burned in a turbine to generate electric power, an application known as integrated gasification combined cycle. This is a means of producing electric power from coal with minimal greenhouse gas emissions. Hydrogen is also a potential transportation fuel. The usual approach is to produce electric power from hydrogen in a fuel cell, and then use that power in an electric motor. One of the main technical obstacles is a practical means of storing hydrogen in a vehicle. Chapter 9 explores hydrogen storage for this application. Nearly all synthetic nitrogen chemicals start as ammonia, synthesized from hydrogen and nitrogen gas. Nitrogen fertilizers are, by far, the largest volume synthetic nitrogen chemicals. Chapter 11 describes ammonia synthesis and some of the more common nitrogen fertilizer compounds. Methanol is a major commodity chemical made from syngas, as described in Chapter 12. Methanol is an intermediate used to make a wide range of products. One of these, dimethyl ether (DME), is especially interesting. DME can be used as a fuel or converted to hydrocarbons, including gasoline and olefins for polymer production. Chapter 13 describes the direct conversion of syngas to hydrocarbons, including substitute natural gas (methane) and Fischer-Tropsch liquid, a synthetic crude oil. The Fischer-Tropsch liquid is then refined to meet petroleum product specifications. Coal is an inexpensive feedstock, but gasification-based plants tend to have very high capital construction costs. In concept, one could build a single plant that would incorporate all of the elements shown in Figure 0.1, but such a complex plant would be extraordinarily expensive to build. Instead, gasification-based plants have a more limited set of features dictated by economics and the regulatory environment. There are two major trends that prompt current interest in coal gasification. The first is the widely held belief that conventional petroleum supplies are declining, while demand for transportation fuels continues to rise. This has led to heightened interest in alternative energy supplies, including coal. The second major trend is concern about global warming. Gasification offers a relatively cost-effective means of using coal while minimizing greenhouse gas emissions. xi CHAPTER 1 The Nature of Coal Contents The Geologic Origin of Coal Coal Analysis and Classification Coal Rank Ash Thermal Properties Coal as a Porous Material Spontaneous Combustion Reserves, Resources, and Production References 1 2 4 5 9 10 11 15 THE GEOLOGIC ORIGIN OF COAL Coal is fossilized peat. A peat bog is a marsh with lush vegetation. Plant matter dies and falls into the water, where partial decomposition occurs. Aerobic bacteria deplete the water of oxygen, and bacterial metabolic products inhibit further decomposition by anaerobic bacteria. Plant matter accumulates on the marsh bottom faster than it decomposes, and, over a period of many years, a layer of peat forms. The peat that became today’s coal was laid down millions of years ago. Buried peat is converted to coal when high pressure and elevated temperature is applied to the buried layer. This process is known as coalification. The physical and chemical structure of the coal changes over time. As shown in Figure 1.1, the youngest (least converted) coal is known as lignite, which can be further converted to sub-bituminous coal, bituminous coal, and finally anthracite. These coal types strongly influence the properties and use of coal, and will be discussed further. Peat Lignite Increasing age, conversion Sub-bituminous Bituminous Anthracite Figure 1.1 Coalification. Coal Gasification and Its Applications. ISBN B978-0-8155-2049-8.10001-4, doi:10.1016/B978-0-8155-2049-8.10001-4 Ó 2011 Elsevier Inc. All rights reserved. 1 2 The Nature of Coal Petrography is the visual inspection of a rock sample to determine the mineral types in the sample. When applied to coal, the different coal types are known as macerals. Table 1.1 lists coal macerals, and shows how they are derived from plant material. COAL ANALYSIS AND CLASSIFICATION Coal is used primarily as a fuel, so its most important property is its heat of combustion. Gross calorific value, also known as higher heating value (HHV), is determined by measuring the heat released when coal is burned in a constant-volume calorimeter, with an intitial oxygen pressure of 2 to 4 MPA, and when the combustion products are cooled to a final temperature between 20 and 35
C (ASTM D 5865-04). The tests mentioned in this book are primarily based on the American Society for Testing and Materials (ASTM) specifications.1 Coal is a variable, widely distributed and widely used material so a wide range of standard tests have been developed by a variety of individuals and organizations. Coal is a porous medium, and these pores, especially in low rank coals, can contain substantial quantities of water even though the coal appears to be dry. The water is either adsorbed onto hydrophilic surface sites or held in pores by capillary forces. When this moist coal is burned or gasified, a substantial fraction of the combustion heat is required Table 1.1 Coal macerals, based on ASTM D121-05 and ASTM D 2799-05a.1 Maceral group Maceral Origin Comments Vitrinite Vitrinite Liptinite Alginite Cutinite Intertite Resinite Sporinite Fusinite Inertodentrinite Macranite Micranite Funginite Secretinite Semifusinite Woody tissue of plants (cellulose, lignin) Botryoccus algae Waxy coating (cuticle) of leaves, roots and stems Plant resins Spores and pollen grains Some structures of plant cell wall still visible Fragments incorporated within other macerals. No plant cell wall structure, larger than 10 mm No plant cell wall structure, less than 10 mm, and typically 1 to 5 mm Fungi No obvious plant structure, sometimes containing fractures, slits or notch. Like fusinite, but with less distinct evidence of cellular structure. Most common maceral Waxy, resinous materials Derived from strongly altered and degraded peat The Nature of Coal to vaporize water. Since the final temperature in the gross calorific value test is 20 to 35
C, most of the water is condensed, thereby recovering the heat of vaporization. Water in the HHV test is primarily a non-combustible diluent. For example, a Wyoming Powder River Basin coal typically has an HHV of 19.8 MJ/kg (8500 Btu/lb) and a 28% moisture level. One can then calculate an HHV value for the coal if it is dried:   19:8 MJ=kg MJ Btu ¼ 27:5 11; 800 Eqn. 1.1 HHV ; dry ¼ 1  0:28 kg lb If coal is burned or gasified near atmospheric pressure, then the heat of condensation for the water may not be recovered. For example, in a coal-fired power plant, the water contained in the coal may go up the stack as steam. In other situations, the heat of condensation is recovered, but the value of this heat is relatively low because of its temperature. In these cases, a better estimate of coal heat of combustion is the net calorific value, also known as Lower Heating Value (LHV), which assumes that vaporized water remains as steam and that the heat of condensation is not recovered. Water in the coal reduces its heating value by its heat of vaporization, 2.395 MJ/kg water (1055 Btu/lb water). Again, for a typical PRB coal: 19:8 MJ MJ kg water  2:395  0:28 kg coal kg water kg coal   MJ Btu 8; 200 ¼ 19:1 kg coal lb coal LHV ; moist ¼ Eqn. 1.2 Proximate Analysis (ASTM D 3172-89) involves a series of tests that heat and burn coal. Moisture is measured (ASTM D 3173-03) by determining the weight loss after coal is dried at 104 to 110
C. Volatiles are then measured (ASTM D 3175-02) by determining additional weight loss when coal is pyrolyzed at 950
C. Ash is determined (ASTM D 3174-04) by the weight of inorganic materials remaining after coal is burned. Fixed carbon is the fraction of coal that is not moisture, volatiles, or ash. Fixed carbon, which is mostly carbon but can contain other elements represents the combustible portion of the coal char that remains after the volatiles have been removed. Proximate analysis results are sometimes reported on a dry mineral matter-free basis. Mineral matter is calculated using the following equation: Mm ¼ 1:08A þ 0:55S Where: Mm ¼ percent mineral matter A ¼ percent ash S ¼ percent sulfur (ASTM D 3177 or D 4239) Eqn. 1.3 3 4 The Nature of Coal The 1.08 factor presumes that minerals in the coal are hydrated. This water of hydration is lost when the coal is burned. The 0.55 factor assumes that sulfur is present as pyrites, which in many areas are converted to the corresponding oxides during combustion. Ultimate analysis (ASTM D 3176) describes coal in terms of its elemental composition. For a dried coal, weight percentages of carbon, hydrogen, nitrogen, sulfur, and ash are measured. The remainder of the coal sample is assumed to be oxygen. COAL RANK In the coalification process, the coal rank increases from lignite to anthracite, as shown in Figure 1.1. Coal rank is useful in the market, because it is a quick and convenient way to describe coal without a detailed analysis sheet. A more detailed description of coal rank is shown in Tables 1.2 and 1.3. Bituminous and sub-bitumous coals are the primary commercial coals. A relatively small amount of anthracite is available. In the USA, anthracites are produced only in northeastern Pennsylvania. Lignites are abundant. But the economics of hauling a low-grade fuel long distances are unfavorable; so most lignite is consumed close to where it is mined. Peat is also mined and generally used close to where it is mined. Peat may be either considered old biomass or very young coal. In nations that regulate greenhouse gas emissions, the difference between the two is more than mere semantics. Carbon dioxide emissions from biomass combustion are not considered a contributor to global warming, because these emissions are offset by carbon dioxide uptake by growing biomass. On the other hand, the same emissions from fossil fuels, are restricted. Emissions from peat combustion are a regulatory gray area. Some coal, particularly bituminous coal, has the tendency to cake. With increasing temperature, coal particles simultaneously pyrolize and partially melt, causing the coal particles to stick to one another. Some gasification reactors, especially moving bed and fluidized bed gasifiers, are limited to processing coal that does not cake. Table 1.2 Classification of anthracitic and bituminous coals by rank (ASTM D 388-05).1 Volatile matter limits (dry mineral-matter-free Fixed carbon limits basis), % (dry mineral-matter-free basis), % Equal or Less Greater Equal or greater than than than less than Rank Meta-anthracite Anthracite Semi-anthracite Low volatile bituminous coal Medium volatile bituminous coal High volatile A bituminous coal 98 92 86 78 69 n/a n/a 98 92 86 78 69 n/a 2 8 14 22 31 2 8 14 22 31 n/a The Nature of Coal Table 1.3 Classification of bituminous, sub-bituminous and lignite coals by rank. (ASTM D 388-05). Note that high volatile A bituminous coal is the only rank that is listed in both Table 1.2 and Table 1.3. Gross calorific value limits (moist, mineral-matter-free basis) Btu/lb MJ/kg Equal or Less Equal or Less Rank greater than than greater than than High volatile A bituminous coal High volatile B bituminous coal High volatile C bituminous coal Sub-bituminous A coal Sub-bituminous B coal Sub-bituminous C coal Lignite A Lignite B 14 000 13 000 11 500 10 500 9 500 8 300 6 300 n/a n/a 14 000 13 000 11 500 10 500 9 500 8 300 6 300 32.557 30.232 26.743 24.418 22.09 19.30 14.65 n/a n/a 32.557 30.232 26.743 24.418 22.09 19.30 14.65 ASH THERMAL PROPERTIES The melting temperatures of coal ash impose temperature limits for coal gasification. Fluidized bed gasifiers and dry-bottom moving bed gasifiers, such as the Lurgi gasifier, require free-flowing ash. The maximum operating temperature for these gasifiers is the initial deformation temperature. When the temperature rises above the initial deformation temperature the ash becomes sticky. Fluidized bed gasifiers often run near the initial deformation temperature to maximize carbon conversion. Entrained flow gasifiers and slagging moving bed gasifiers such as the BGL gasifier require a fluid slag, so they must operate at a sufficiently high temperature to completely melt the ash. Operation at significantly higher temperatures increases oxygen consumption. Ash is a complex mixture of minerals, which will cause the coal ash to melt over a temperature range rather than at a fixed temperature. Temperatures in this range are specified by ASTM D-1857-04. A coal ash cone, 19 mm high and with an equilateral triangle base 6.4 mm on each side, is placed in an oven. Temperatures are reported for reducing or oxidizing gas environments. The initial deformation temperature (IDT) occurs when rounding of the cone tip first occurs. The softening temperature (ST) occurs when the cone has fused to produce a lump which has a height equal to its base. The hemispherical temperature (HT) occurs when the lump height is half the length of its base. The fluid temperature occurs when the fused mass has spread out in a nearly flat layer with a maximum height of 1.6 mm. A number of researchers have attempted to correlate ash thermal properties with ash composition. The most extensive effort was by Seggiani and Pannocchia,2 who correlated the behavior of 433 ash samples, based on nine elemental concentrations. 5 6 The Nature of Coal Note that mineral elemental compositions are reported as if the mineral sample were a blend of simple metal oxides. For example, the fraction of aluminum in a sample is typically reported as the equivalent weight percent of Al2O3. Seggiani and Pannocchia’s correlations are based on mole percents, rather than weight percents, on a normalized, SO3-free basis. The correlation for initial deformation temperature is given as:   IDT ; C ¼ 2; 040 exp 0:1
SiO2 SiO2 þ Fe2 O3 þ CaO þ MgO 2  þ 83:4P2 O5 þ 2:12 Al2 O3 þ 39:3TiO2 þ 0:335 ðFe2 O3 Þ2 þ 0:118 ðAl2 O3 Þ2 þ 0:135 ðCaOÞ2  0:116 ðSiO2 Þ ðFe2 O3 Þ þ 0:0768 ðSiO2 Þ ðAl2 O3 Þ   SiO2 2 þ 0:533 ðFe2 O3 Þ ðCaOÞ þ 2:42 Al2 O3  2 SiO2 þ 205 SiO2 þ Fe2 O3 þ CaO þ MgO   4 þ 780 exp 10 ðSiO2 Þ ðAl2 O3 Þ  2170 Eqn. 1.4 The correlation for softening temperature is given as:   2  SiO2 ST ;
C ¼ 5; 360 exp 0:1 þ 91:3 P2 O5 SiO2 þ Fe2 O3 þ CaO þ MgO þ 0:282 ðFe2 O3 Þ2 þ 0:178 ðCaOÞ2 þ 0:939ðMgOÞ2 þ 0:630 ðFe2 O3 Þ   SiO2 2  ðCaOÞ  1:03 ðFe2 O3 Þ ðMgOÞ þ 2:34 Al2 O3  2 SiO2  140 SiO2 þ Fe2 O3 þ CaO þ MgO   CaO þ MgO  85:9 Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 O3   þ 3; 120 exp 104 ðSiO2 Þ ðAl2 O3 Þ  7820 Eqn. 1.5 The Nature of Coal The correlation for hemispherical temperature is given as:   HT ;
C ¼ 2; 150 exp 0:1 SiO2 SiO2 þ Fe2 O3 þ CaO þ MgO 2  þ 53:1 TiO2   Fe2 O3 2 2  25:3K2 O þ 16:0 ðTiO2 Þ þ 0:0877 ðCaOÞ þ 19:3 CaO 2  SiO2 þ 0:285 Al2 O3   2  Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 O þ 910exp 0:1 1 SiO2 þ Al2 O3 þ TiO2 þ P2 O5   Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 O 2 þ 41:9 SiO2 þ Al2 O3 þ TiO2 þ P2 O5  2 Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 O þ 86:4 1 SiO2 þ Al2 O3 þ TiO2 þ P2 O5 2  SiO2 þ 216  2; 120 SiO2 þ Fe2 O3 þ CaO þ MgO Eqn 1.6 The correlation for fluid temperature is given as:   2  SiO2 þ 6:13Al2 O3 FT ; C ¼ 2; 240 exp 0:1 SiO2 þ Fe2 O3 þ CaO þ MgO
þ 58:0TiO2  13:8MgO þ 0:259 ðFe2 O3 Þ2 þ 0:278 ðAl2 O3 Þ2 þ 0:736 ðMgOÞ2 þ 0:259 ðFe2 O3 Þ ðCaOÞ  0:730 ðFe2 O3 Þ ðMgOÞ     SiO2 2 Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 O 2 þ 92:0 þ 2:03 Al2 O3 SiO2 þ Al2 O3 þ TiO2 þ P2 O5  2 SiO2 þ 231  1; 340 SiO2 þ Fe2 O3 þ CaO þ MgO Eqn. 1.7 The temperature of critical viscosity, Tcv, is not part of the ASTM D1857 test but it is important for slagging gasifiers because it marks the transition of slag from a 7 8 The Nature of Coal difficult-to-handle Bingham plastic, below Tcv, to a more easily handled Newtonian fluid, above Tcv. The correlation for temperature of critical viscosity is given as: Tcv ;
C ¼ 935P2 O5 þ 4:11Al2 O3 þ 2; 580ðP2 O5 Þ2 þ 0:254ðAl2 O3 Þ2  0:139ðNa2 OÞ2 þ 0:108 ðSiO2 Þ ðFe2 O3 Þ þ 0:0377 ðSiO2 Þ ðAl2 O3 Þ      SiO2 SiO2 2 þ 14:0 þ 0:00691 Fe2 O3 þ CaO þ MgO þ 3:05 Al2 O3 Al2 O3    Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 O þ K2 O þ Na2 O 2 þ 7:40 SiO2 þ Al2 O3 þ TiO2 þ P2 O5   Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 O 2  113 SiO2 þ Al2 O3 þ TiO2 þ P2 O5   Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 O  5:48 ðNa2 OÞ SiO2 þ Al2 O3 þ TiO2 þ P2 O5   Fe2 O3  164 Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 O   Fe2 O3 þ CaO þ MgO þ K2 O þ Na2 O 1  7:40 SiO2 þ Al2 O3 þ TiO2 þ P2 O5   þ 409 exp 104 ðSiO2 Þ ðAl2 O3 Þ þ 675 Eqn. 1.8 Seggiani and Pannocchia report standard deviations for their correlations to be 70 to 88oC. Table 1.4 compares experimental results for four American coals from Baxter3 to the temperatures predicted by these correlations. The predicted results are very close to the experimental results for the lignite and the sub-bituminous coals. The exception is the predicted temperatures are substantially higher than the experimental values for the bituminous coals. Inorganic additives have been added to coal gasifiers to modify ash thermal properties. For example; alkaline materials such as sodium, potassium and calcium compounds tend to lower ash melting temperatures. These can be added to an entrained flow gasifier to lower slag viscosity. Care must be taken with refractrory-lined gasifiers, because these compounds may attack the refractory. The opposite approach was taken by van Dyk and Waanders.4 They sought to increase the ash fusion temperature (ISO 540 and 1195E) to allow higher temperature operation in a Lurgi gasifier. Tests with Al2O3, TiO2, and SiO2 showed that Al2O3 was most effective. Addition of 6 weight % Al2O3 boosted the ash fusion temperature of a mixture of South African coals from 1,340
C to greater than 1,600
C.