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Energy & Fuels 2008, 22, 1720–1730
Synthesis Gas Production with an Adjustable H2/CO Ratio through the Coal Gasification Process: Effects of Coal Ranks And Methane Addition Yan Cao,*,† Zhengyang Gao,†,‡ Jing Jin,†,§ Hongchang Zhou,†,| Marten Cohron,† Houying Zhao,† Hongying Liu,†,| and Weiping Pan†,⊥ Institute for Combustion Science and EnVironmental Technology (ICSET), Western Kentucky UniVersity (WKU), 2413 NashVille Road, Bowling Green, Kentucky 42101, North China Electric Power UniVersity, BaoDing 071003, HeBei, P.R. China, College of Power Engineering, UniVersity of Shanghai for Science & Technology, Shanghai 200093, P.R. China, Department of EnVironmental Science and Engineering, Nanjing UniVersity of Information Science & Technology, Nanjing, Jiangsu 210044, P.R. China, and Center for Mineral & Waste Material Processing, Chinese UniVersity of Mining and Technology, Beijing 100083, P.R. China ReceiVed September 22, 2007. ReVised Manuscript ReceiVed January 25, 2008
With the decline of oil reserves and production, the gas-to-liquids (GTL) part of Fischer–Tropsch (F-T) synthesis technology has become increasing important. Synthesis gas (H2 + CO) with a stoichiometric ratio (H2/CO) at 2 or ranging from 1 to 2 is generally used in major synthesis-gas-based chemicals production. There are growing interests in the development of an alternative technology, other than the expensive naturalgas-based catalytic process, for cost-effective production of synthesis gas with a flexible hydrogen/carbon monoxide (H2/CO) ratio. Direct production of synthesis gas using coal as a cheap feedstock is attractive but challenging due to its low H2/CO ratio of generated synthesis gas. Three typical U.S. coals of different ranks were tested in a 2.5 in. coal gasifier to investigate their gasification reactivity and adjustability on H2/CO ratio of generated synthesis gas with or without the addition of methane. Tests indicated that lower-rank coals (lignite and sub-bituminous) have higher gasification reactivity than bituminous coals. The coal gasification reactivity is correlated to its synthesis-gas yield and the total percentage of H2 and CO in the synthesis gas, but not to the H2/CO ratio. The H2/CO ratio of coal gasification was found to be correlated to the rank of coals, especially the H/C ratio of coals. Methane addition into the dense phase of the pyrolysis and gasification zone of the cogasification reactor could make the best use of methane in adjusting the H2/CO ratio of the generated synthesis gas. The maximum methane conversion efficiency, which was likely correlated to its gasification reactivity, could be achieved by 70% on average for all tested coals. The actual catalytic effect of generated coal chars on methane conversion seemed coal-dependent. The coal-gasification process benefits from methane addition and subsequent conversion on the adjustment of the H2/CO ratio of synthesis gas. The methane conversion process benefits from the use of coal chars due to their catalytic effects. This implies that there were likely synergistic effects on both.
1. Introduction Gas-to-liquid (GTL) conversion is a well developed and proven technology, which is capable of converting gas to clean liquid hydrocarbons and oxygenates, such as methanol, fuel additives (DME), and many other chemicals. The GTL process of Fischer–Tropsch (F-T) synthesis is one such technology, through which synthesis gas (CO + H2) can be converted into liquid hydrocarbons.1,2 With the decline of oil resources (liquid hydrocarbon mixtures), the GTL of F-T synthesis technology * Corresponding author. E-mail address:
[email protected]. Phone: 2707790202. Fax: 270-7452221. † Western Kentucky University (WKU). ‡ North China Electric Power University. § University of Shanghai for Science & Technology. | Nanjing University of Information Science & Technology. ⊥ Chinese University of Mining and Technology. (1) Overett, M. J.; Hill, R. O.; Moss, J. R. Organometallic chemistry and surface science: mechanistic models for the Fischer–Tropsch synthesis. Coord. Chem. ReV. 2000, 206–207, 581–605. (2) Wang, T.; Wang, J.; Jin, Y. Slurry Reactors for Gas-to-Liquid Processes: A Review. Ind. Eng. Chem. Res. 2007, 46 (18), 5824–5847.
has become increasingly important and received much attention from both academic and industrial interests.3–5 The Sasol Company in South Africa is one such successful example of achieving the F-T synthesis process for the production of liquid fuels and chemicals using synthesis gas.6 The synthesis gas can be produced through reforming or the partial oxidation of methane.7,8 The methane source can be natural gas or coal-bed methane (CMB). However, economically viable processes for (3) Dry, M. E. High Quality Diesel via the Fischer–Tropsch Process -a Review. J. Chem. Technol. Biotechnol. 2001, 77, 43–50. (4) Tijm, P. J. A.; Waller, F. J.; Brown, D. M. Methanol Technology Development for the New Millennium. Appl. Catal. A: Gen. 2001, 221, 275–282. (5) Fleisch, T. H.; Sills, R. A.; Briscoe, M. D. 2002-Emergency of the Gas-to-Liquids Industry: a Review of Global GTL Developments. J. Nat. Gas Chem. 2002, 11, 1–14. (6) Wilhelm, D. J.; Simbeck, D. R.; Karp, A. D.; Dickenson, R. L. Syngas production for gas-to-liquids applications: technologies, issues and outlook. Fuel Process. Technol. 2001, 71, 139–148. (7) Rostrup-Nielsen, J. R.; Sehested, J.; Nørskov, J. K. Hydrogen and synthesis gas by steam- and C02 reforming. AdV. Catal. 2002, 47, 65–139. (8) Ross, J. R. H. Natural gas reforming and CO2 mitigation. Catal. Today 2005, 100 (1–2), 151–158.
10.1021/ef7005707 CCC: $40.75 2008 American Chemical Society Published on Web 03/25/2008
Syn Gas Production through Coal Gasification
synthesis-gas production based on methane only typically produce a gas that is too rich in hydrogen (H2/CO ) 3) to meet the required stoichiometric ratio (H2/CO ) 2) in synthesis gas. Some other value-added chemicals may require the H2/CO ratio of the synthesis gas to be varied between 1 and 2.9–12 Consequently, a downstream facility to adjust the H2/CO ratio is generally necessary prior to the chemical synthesis. The synthesis gas manufacturing systems based on natural gas are also capital intensive due to the presence of an expensive catalyst and higher energy consumption. Hence, there is growing interest in the development of an alternative technology for cost-effective production of synthesis gas. Coal can be a cheap feedstock to produce synthesis gas though the gasification process. However, coal-derived synthesis gas production is generally not the dominant process on an industrial scale when natural gas is available. There is a technical challenge in decreasing the capital investment and the operating cost to direct production of synthesis gas with a flexible H2/CO ratio using the coal gasification process. The first technical challenge is that coal gasification provides synthesis gas with a H2/CO ratio close to 1. To adjust the H2/CO ratio of coal-derived synthesis gas, the water-gas-shift reaction is required after coal gasification. The water-gas-shift reaction is a feasible process, but its lower kinetics requires catalyst promotion, which increases capital and operational costs. Second, the process economics of coal-derived synthesis gas production is largely dependent on the gasification reactivity of coal. Coal reactivity determines the coal-carbon conversion efficiency and the synthesis gas yield and, consequently, the capital cost of a coal gasifier. Thus, coal reactivity and the H2/CO ratio are two important parameters used to determine the economics of the synthesis gas process. Coal gasification reactivity is likely correlated to coal ranks. According to the definition of coal ranks, there are three main coals of different ranks in the U.S. In order of increasing coal rank, they are the following: lignite, sub-bituminous, and bituminous coal. The Department of Energy (DOE) Energy Information Administration (EIA) estimates that over 50% of the coal reserve base in the United States (U.S.) is bituminous coal, about 30% is sub-bituminous, and 9% is lignite.13 Methane could be a H2 source for synthesis gas production, while coal-bed methane (CBM) seems a good selection as a methane source since it is cheap and available during coal production.14 In the United States, conservative estimates suggest that more than 700 trillion cubic feet (TCF) of coal-bed methane exists in place, with 100 TCF economically recoverable with existing technologysequivalent to about a 5-year supply at (9) Wender, I. Reactions of synthesis gas. Fuel Process. Technol. 1996, 48, 189–297. (10) Song, X. P.; Guo, Z. C. Technologies for direct production of flexible H2/CO synthesis gas. Energy ConVers. Manage. 2006, 47 (5), 560– 569. (11) Verkerk, K. A. N.; Jaeger, B.; Finkeldei, C. H.; Wilhelm, Keim. Recent developments in isobutanol synthesis from synthesis gas. Appl. Catal. A: Gen. 1999, 186 (1–2), 407–431. (12) Ancillotti, F.; Fattore, V. Oxygenate fuels: Market expansion and catalytic aspect of synthesis. Fuel Process. Technol. 1998, 57 (3), 163– 194. (13) U.S. Department of Energy, Energy Information Administration. U.S. Coal ReserVes: 1997 Update; DOE/EIA-0529(97), Office of Coal, Nuclear, Electric and Alternate Fuels, Office of Integrated Analysis and Forecasting, Washington, DC. February 1999; available online at http:// www.eia.doe.gov/cneaf/coal/reserves/front-1.html. (14) Policy Facts, Coalbed Natural Gas; U.S. Department of Energy, Office of Energy, National Energy Technology Laboratory, February 2005, available online at http://www.netl.doe.gov/publications/factsheets/policy/ Policy019.pdf.
Energy & Fuels, Vol. 22, No. 3, 2008 1721
present rates of use.15 So far, CBM has accounted for about 7.5% of total natural gas production in the U.S. Coal-bed methane is coproduced in many coal mines, including those of lignite, sub-bituminous coal, and bituminous coal. Among them, the Powder River Basin (PRB) area of the U.S., which is also the top producer of PRB sub-bituminous coal, is one of the newest, most productive areas of CBM. Another major coalbed methane production area in the U.S. is in the Black Warrior Basin in Alabama, which is close to the geological reserve of Texas lignite.16 Methane release from coal mining accounts for approximately 10% of methane emissions in the U.S.17 Utilization of CBM also helps to preserve the nation’s environmental quality since methane is a greenhouse gas, with 21 times the global warming potential of carbon dioxide.5 Cogasification of coal and methane to produce synthesis gas has been evaluated and developed in recent years.18–21 Cao and Wu cogasified coal and methane to produce hydrogen-rich synthesis gas (H2/CO ratio greater than 1) in a fluidized-bed reactor.18,19 Cao focused on the behavior of coal’s sulfur release under a hydrogen-rich environment during cogasification. Simultaneously, tests indicated that the achievable H2/CO ratio of the produced synthesis gas was about 1.5–2 under the air blown condition.18 Wu achieved higher than 75% methane conversion efficiency in the cogasification process in an oxygenblown mode. Unfortunately, the adjustability of methane addition on the H2/CO ratio was not focused.19 It is possible the partial oxidation or burnout of methane occurred during his test. Other researchers20,21 also tested higher methane/coal ratios to achieve the adjustability of the H2/CO ratio in a fixed bed or moving bed by cogasification technologies. It seemed that the location of methane introduced into the gasifier and coal ranks have major impacts on methane conversion and the H2/CO ratio of synthesis gas. There are also some investigations on the synergistic interaction between coal gasification and methane conversion under the conventional coal gasification process.22,23 This paper focuses on the effects of coal rank and the adjustability of methane addition on the quality of synthesis gas under conditions of conventional coal gasification and cogasification. The reactivity of coals of different ranks (two bituminous coals, one sub-bituminous, and one lignite) was first evaluated in a thermogravemetric (TG) analyzer using water or (15) Coal-Bed Methane: Potential and Concerns; USGS Fact Sheet FS123-00, U.S. Geological Survey, October 2000. (16) The U.S. Geological SurVey National Coal Resource Assessment; USGS Fact Sheet FS-020-01, U.S. Geological Survey, March 2001. (17) InVentory of U.S. Greenhouse Gas Emissions and Sinks 1990–2000; U.S. Environmental Protection Agency: Washington, DC, April 2002. (18) Energy Solutions for the 21st Century- From “Miner’s Curse” to Valuable Resource; U.S. Department of Energy, Office of Energy, National Energy Technology Laboratory, Federal Energy Technology Center; available online at http://www.netl.doe.gov/publications/FETC_Focus/sept98/ 1-3.pdf. (19) Cao, Y.; Liu, K. L.; Pan, W. P.; Li, B.; Huang, J. J.; Wang, Y. Twentieth Annual International Pittsburgh Coal Conference, Pittsburgh, PA, September 15–19, 2003. (20) Wu, J. H.; Fang, Y. T.; Wang, Y. Combined Coal Gasification and Methane Reforming for Production of Synthesis Gas in A Fluidized-Bed Reactor. Energy Fuels 2005, 19, 512–516. (21) Song, X. P.; Guo, Z. C. Technologies for Direct Production of Flexible H2/CO Synthesis Gas. Energy ConVers. Manage. 2006, 47, 560– 569. (22) Bai, Z. Q.; Chen, H. K; Li, W.; Li, B. Q. Hydrogen Production by Methane Decomposition over Coal Char. Int. J. Hydrogen Energy 2006, 31, 899–905. (23) Chen, W. J.; Sheu, F. R.; Savage, R. L. Catalytic Activity of Coal Ash on Steam Methane Reforming and Waster-gas Shift Reactions. Fuel Process. Technol. 1987, 16 (3), 279–288. (24) Bai, Z. Q.; Chen, H. K.; Li, W.; Li, B. Q. Hydrogen Production by Methane Decomposition over Coal Char. Int. J. Hydrogen Energy 2006, 31, 899–905.
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Cao et al.
Table 1. Proximate and Ultimate Analyses and Major Oxides of Coal Samples (on a Dry Basis) dry basis, by weight sample name
moisture (%)
ash (%)
lignite (0.1–0.5 mm) sub-bitu (0.35–0.8 mm) bit-1 (0.5–0.85 mm) bit-2 (0.5–0.85 mm)
24.41 11.47 1.50 2.57
22.51 4.87 11.23 7.02
vol. mat sulfur btu carbon hydrogen nitrogen oxygen chloride mercury fluoride bromide (%) (%) (kJ/(K g)) (%) (%) (%) (%) (ppm) (ppm) (ppm) (ppm) 30.21 40.99 35.34 33.46
1.34 0.38 0.90 0.55
18745 24316 29717 32490
46.17 62.04 69.98 76.56
5.76 5.43 5.24 5.33
0.54 0.74 1.46 1.57
23.68 26.54 11.19 8.97
139 200 468 1637
0.25 0.06 0.10 0.07
62 98 475 204
nd nd nd nd
in percent by weight and after normalization lignite (0.1–0.5 mm) sub-bitu (0.35–0.8 mm) bit-1 (0.5–0.85 mm) bit-2 (0.5–0.85 mm)
Na2O
MgO
Al2O3
SiO2
CaO
K2 O
SO3
P2O5
BaO
SrO
Fe2O3
MnO
TiO2
