Review pubs.acs.org/CR
Advances in Coal Gasification, Hydrogenation, and Gas Treating for the Production of Chemicals and Fuels Christopher Higman*,† and Samuel Tam‡ †
Higman Consulting GmbH, 65824 Schwalbach, Germany Advanced Energy Systems Division, Office of Fossil Energy, U.S. Department of Energy, Washington, D.C. 20585, United States
‡
3.7.2. Hydrogasification 3.7.3. Chemical Looping 3.7.4. Other 4. Gas Treating 4.1. Desulfurization 4.1.1. Zinc 4.1.2. Iron 4.1.3. Calcium 4.1.4. Other Sorbents 4.1.5. COS Hydrolysis and Removal 4.2. Trace Element Removal 4.2.1. Mercury 4.2.2. Arsenic and Selenium 4.3. Water Gas Shift (WGS) 4.3.1. Alternative Catalysts 4.3.2. Reactors 4.4. Hydrogen−Carbon Dioxide Separation 4.4.1. CO2 Sorbents 4.4.2. Hydrogen Membranes 5. Chemicals from Syngas 5.1. Ammonia 5.2. Hydrogen 5.3. Methanol and Derivatives 5.3.1. Methanol Derivatives 5.4. Ethanol 5.5. Oxo Alcohols 5.6. Monoethylene Glycol (MEG) 5.7. Substitute Natural Gas (SNG) 5.8. Liquefied Petroleum Gas (LPG) 5.9. Fischer−Tropsch Synthesis 5.10. Direct Reduced Iron (DRI) 6. Chemicals from Pyrolysis Byproducts of Gasification 7. Direct Hydrogenation to Liquids 7.1. Process Description 7.2. Commercial Plant 7.3. Research and Development (R&D) Activities 8. Conclusions Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations and Acronyms
CONTENTS 1. Introduction 2. Background to Gasification 2.1. Chemistry and Thermodynamics 2.2. Process Realization 2.2.1. Operating Temperature 2.2.2. Bed Type 3. Gasification Research and Development 3.1. Coal Properties Relevant to Gasification 3.1.1. Reactivity of Coal and Other Chars 3.1.2. Behavior of Mineral Matter 3.2. Coal Preparation and Feeding 3.2.1. Fines Reduction in Crushing Facilities 3.2.2. Slurry Feeding Systems 3.2.3. Dry Feeding Systems 3.2.4. Dry Solids Pumps 3.2.5. Cogasification of Alternative Feedstocks 3.3. Gasification Reactors 3.3.1. Fixed Bed Gasifiers 3.3.2. Fluid Bed Gasifiers 3.3.3. Entrained Flow Gasifiers 3.3.4. Reactor Containment 3.3.5. Two-Stage Gasification 3.3.6. Other Issues 3.4. Contaminant Species in Raw Syngas 3.4.1. Nitrogen Species 3.4.2. Alkali Metals 3.4.3. Trace Elements 3.5. Syngas Coolers 3.5.1. Radiant Coolers 3.5.2. Quench Systems 3.6. Primary Gas Cleaning 3.6.1. Solids Removal from Raw Syngas 3.7. Alternative Configurations 3.7.1. Catalytic Gasification
© XXXX American Chemical Society
B B B C C D D D E E G G G H I I J J K K L M M N N O O P P P Q Q Q Q
R R R R S T T T T T U U U U V V V V V V W W W W X X X X Y Y Y Y Y Z Z Z AA AA AA AA AA AB AB
Special Issue: 2014 Chemicals from Coal, Alkynes, and Biofuels Received: April 6, 2013
A
dx.doi.org/10.1021/cr400202m | Chem. Rev. XXXX, XXX, XXX−XXX
Chemical Reviews Nomenclature References
Review
particularly those incorporating carbon capture. Schoff et al. have reviewed the research and development needs for IGCC, and the reader is referred to that source for further detail.6 Although many of the techniques used for coal gasification have their analogues in biomass gasification, the differences, both in the combustible material and in the associated mineral matter, call in many cases for different solutions in detail. Therefore, while this review will occasionally mention biomass gasification as a starting point for chemicals manufacture, e.g., under the heading of cogasification, there is no attempt to cover the topic in a comprehensive manner. As referred to above, the coals-to-chemicals industry is an established and mature industry. It will therefore be an aim of this review to look at potential advances in this light, examining the status quo of industrial practice and identifying what improvements are needed or could be useful and relating work performed in the past five years to those needs.
AB AB
1. INTRODUCTION World ammonia production in 2010 is reported to be 159 million tons per annum,1 of which Chinese coal-based production is estimated to be 39 million tons.2 This amounts to about 25% of total worldwide production. A similar situation obtains for methanol. Global capacity is reported as 69 million tons per annum,3 of which coal-based capacity, again mostly in China, is estimated as 27 million tons or about 39% of the world total. While much of this methanol is further processed to conventional methanol derivatives such as formaldehyde, solvents, methyl tert-butyl ether (MTBE), and acetyls, other markets have opened up in recent years. Newer methanol derivatives include dimethyl ether (DME), gasoline via the ExxonMobil MTG (methanol to gasoline) and similar processes, and olefins. Other chemicals being manufactured on an industrial scale from coal-derived synthesis gas (H2 + CO or syngas) include oxo alcohols, monoethylene glycol (MEG), substitute natural gas (SNG), and Fischer−Tropsch liquids. It is almost exactly 100 years since the start-up of Carl Bosch’s first industrial synthetic ammonia plant in Oppau, Germany, on Sept 9, 1913.4 The hydrogen for this plant was produced from coal using the water gas process. The capacity was 30 metric tons per day (t/d), modest compared with the 3300 t/d units being offered today. However, this was not the beginning of chemicals from coal gasification. In 1910 Kietaible5 discussed the manufacture of both hydrogen and formic acid from “generator gas” or water gas. At the time, the development of airships was seen as a mass market for coalbased hydrogen production. Formic acid was used as an intermediate for synthetic dyestuffs and oxalic acid. Coal tar based chemistry is even older, but it is not the subject of this review. Chemicals production from syngas, whether generated by coal gasification or steam reforming of natural gas, is a major and largely mature industrial activity with a broad range of commercial licensors. Much research and development takes place in the commercial laboratories of such companies and is therefore not accessible in the same manner as academic research. Nonetheless the results of this work become visible through the patent literature and with each further improvement in the industrial processes. Coal gasification is but one means of generating synthesis gas. The synthesis of chemicals from syngas is largely independent of the source of the syngas, so that most developments applicable to syngas from steam reforming of natural gas are applicable to syngas generated from coal gasification. This review will therefore focus on advances in gasification and associated gas treatment technology with only brief mention being made of the chemicals that can be manufactured from the treated syngas. Synthetic fuels have been included in the definition of chemicals, so brief mention is also made of Fischer−Tropsch synthesis. Direct hydrogenation of coal is however discussed in more detail. Coal gasification is used in the power industry in the integrated gasification combined cycle (IGCC) configuration, but this cannot be considered as part of the chemical industry and is therefore not discussed. The demands placed particularly on gas treatment are different, but many of the techniques described here will find their way into advanced IGCCs,
2. BACKGROUND TO GASIFICATION Gasification can be described as the “conversion of any carbonaceous feedstock into a gaseous product with a useful chemical heating value.”7 Initially the focus was on devolatilization and pyrolysis since the 19th century town gas market was mainly for lighting, where a high hydrocarbon content was of benefit. For modern chemical synthesis applications a heavy hydrocarbon free gas mainly consisting of hydrogen and carbon monoxide is desired, so the emphasis is on the partial oxidation reactions. 2.1. Chemistry and Thermodynamics
The principle reactions which take place during the gasification of pure carbon are those involving carbon, oxygen, and hydrogen and in particular their compounds carbon monoxide, carbon dioxide, water (or steam), and methane. Simplifying coal to pure carbon, the most important reactions are partial oxidation: C + 1 2 O2 → CO
0 ΔH298K = −111 MJ/kmol
(1)
CO oxidation: CO + 1 2 O2 → CO2
0 ΔH298K = −283 MJ/kmol
(2)
water gas reaction: C + H 2O(g) ⇄ CO + H 2 0 ΔH298K = +131 MJ/kmol
(3)
and Boudouard reaction: C + CO2 ⇄ 2CO
0 ΔH298K = + 172 MJ/kmol
(4)
In gasification processes the reactions involving free oxygen are essentially complete. The carbon conversion is usually 95% or higher, whereby the failure to reach 100% conversion is due to nonthermodynamic effects. At the high temperatures at which most processes operate, the reactions reach close to equilibrium and the final gas composition is determined by the CO shift reaction: CO + H 2O(g) ⇄ CO2 + H 2 0 ΔH298K = −41 MJ/kmol
B
(5)
dx.doi.org/10.1021/cr400202m | Chem. Rev. XXXX, XXX, XXX−XXX
Chemical Reviews
Review
Figure 1. Sequence of reactions in coal gasification. [Adapted with permission from Reimert, R. and Schaub, G. Gas Production. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH: Weinheim, Germany, 1989; Vol. A12, p 215. Copyright 1989 Wiley-VCH.]
Table 1. Characteristics of Different Categories of Gasification Process [Adapted with Permission from ref 7. Copyright 2003 Elsevier.] category ash conditions typical processes feed characteristics size acceptability of fines acceptability of caking coal preferred coal rank operating characteristics outlet gas temperature oxidant demand steam demand other characteristics
fluid bed
moving bed
entrained flow
dry ash Lurgi, SEDIN
slagging BGL
dry ash Winkler, HTW, CFB, TRIG
agglomerating KRW, U-Gas, AFB
slagging Shell, GEE, E-Gas, Siemens, KT, and others; see Table2
1/4−2 in. limited
1/4−2 in. better than dry ash
1/4−1/2 in. good
1/4−1/2 in. better
