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Energy & Fuels 2005, 19, 2114-2120
High-Yield Hydrogen Production by Steam Gasification of HyperCoal (Ash-Free Coal Extract) with Potassium Carbonate: Comparison with Raw Coal Jie Wang,* Kinya Sakanishi,* and Ikuo Saito Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8569, Japan
Takayuki Takarada and Kayoko Morishita Department of Biological and Chemical Engineering, Gunma Univeristy, Kiryu 376, Gunma, Japan Received November 1, 2004. Revised Manuscript Received May 16, 2005
Steam gasification of the HyperCoals (ash-free coal extracts) with the physical addition of 5.8%6.0% K2CO3 was conducted at 1023 K on a thermogravimetric apparatus that was equipped with an on-line quadrupole mass spectrometer. The catalytic gasification of the HyperCoals demonstrated a much higher gasification rate than the catalytic gasification of the raw coals. Interactions of K2CO3 with mineral matter in the raw coal formed water-insoluble potassium compounds, such as potassium aluminosilicates, and reduced the catalytic activity, whereas no such negative reactions occurred for the HyperCoals. The steam gasification of the HyperCoals with K2CO3 was also determined to be favorable for the high-yield production of hydrogen. From these experimental results, the catalytic steam gasification of HyperCoal would potentially be a more efficient process for the production of hydrogen in the future.
Introduction Hydrogen is considered to be a potential energy carrier. Despite many obstacles to the development of the hydrogen energy system for the production, transportation, and storage of hydrogen, breakthroughs in fuel cell technology for mobile and stationary applications, together with the growing public impatience with the environmental pollution caused by burning fossil fuels, may promote options for hydrogen as a fuel. Steam reforming of natural gas currently provides the cheapest source of industrial hydrogen;1,2 however, it may not become a commercial process for producing hydrogen fuel, because of less-salient benefits from the conversion of this high-quality fuel. Compared to natural gas, the world’s coal reserve is relatively abundant. In the interim, it may be more viable to use coal as a source of hydrogen. Also, coal is concentrated in carbon that is associated with a variety of harmful impurities and it is more unclean than natural gas. Therefore, the conversion of inferior coal to hydrogen, accompanied by cost-effective CO2 sequestration, is environmentally attractive. Coal gasification with steam and oxygen is a mature commercialized process for syngas (CO + H2) produc* Authors to whom correspondence should be addressed. Telephone: +81-29-8618437. Fax: +81-29-8618408. E-mail address:
[email protected] (for Dr. Wang). Telephone: +81-823-721947. Fax: +81-823-721947. E-mail address:
[email protected] (for Dr. Sakanishi). (1) Dunn, S. Int. J. Hydrogen Energy 2002, 27, 235. (2) Dicks, A. L. J. Power Sources 1996, 61, 113.
tion. Purified syngas can technologically be used as a fuel for high-temperature fuel cells; however, the oxidation of CO on the anode reduces the power generation efficiency. For low-temperature fuel cells, CO is generally required at low levels. To comply with a future fuel cell application, it is desired to develop new coal gasification technologies for the more-efficient production of hydrogen. Some research and development activities have focused on hydrogen production from coal via the extraction of hydrogen with a membrane reactor3 and via the catalytic gasification of coal with steam,4 subcritical water,5 or supercritical water.6,7 Presently, however, no gasification plant in the world is configured to optimize hydrogen production. Catalytic coal gasification is a very old technology. A rich array of publications has reported the enhanced gasification reactivity with various alkali salts.8-13 The (3) Doong, S. L.; Ong, E.; Lau, F.; Bose, A. C.; Carty, R. Direct Extraction of Hydrogen from Coal Using a Membrane Reactor Within a Gasifier. In Proceedings of the 21st Annual International Pittsburgh Coal Conference, Osaka, Japan, 2004. (CD-ROM.) (4) Timpe, R. C.; Kulas, R. W.; Huserrman, W. B.; Sharma, R. K.; Olson, E. S.; Willson, W. G. Int. J. Hydrogen Energy 1997, 22, 487. (5) Kuramoto, K.; Furuya, T.; Suzuki, Y.; Hatano, H.; Kumabe, K.; Yoshiie, R.; Moritomi, H.; Lin, S. Y. Fuel Process. Technol. 2003, 82, 61. (6) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Energy Fuels 2001, 15, 339. (7) Wang, J.; Takarada, T. Energy Fuels 2001, 15, 356. (8) Wen, W. Y. Catal. Rev. 1980, 22, 1. (9) Figueiredo, J. L.; Moulijn, J. A. Carbon and Coal Gasification; Martinus Nijhoff: Dordrecht, Boston, Lancaster, 1986; pp 1-655. (10) McKee, D. W.; Spiro, C. L.; Kosky, P. G.; Lamby, E. J. Fuel 1983, 62, 217. (11) Huttinger K. J.; Minges, R. Fuel 1985, 64, 486.
10.1021/ef040089k CCC: $30.25 © 2005 American Chemical Society Published on Web 07/02/2005
Steam Gasification of HyperCoal with K2CO3
catalytic coal gasification is well-known to have the advantage that a high reaction rate at lower temperature can reduce the capital investments per throughput and assist in realizing a pure steam gasification process by supplying reaction heat from another energy.14 Catalytic gasification also inhibits the agglomeration of caking coal, so that it can utilize a wider range of coal resources for gasification. Potassium carbonate is regarded as a preferable catalyst, because of its high activity and fair abundance for availability. In 1970s, a catalytic coal gasification process using K2CO3 was developed at Exxon Research and Engineering Co.;15 however, the process has not yet achieved commercial success. A fatal demerit of the catalytic coal gasification is the additional cost of the catalyst. K2CO3 interacts with mineral matter in coal, which makes recycled use of the catalyst and disposal of the ash difficult.16,17 In this paper, our renascent study on the catalytic gasification of coal is motivated by the HyperCoal production research and development program, which is supported financially by NEDO and CCUJ in Japan. The so-called HyperCoal is an extremely low-ash or ashfree coal that is produced by the thermal extraction of coal with an appropriate solvent. Currrently, a 0.1 ton/ day scale test plant is under construction at Kobe Steel, Ltd., in Japan. It has been claimed that the cost of HyperCoal would be competitive to a common coal price if the ash disposal cost in Japan is included.18 The principal incentive to develop the HyperCoal production process is to manifest its potential applications for various advanced coal-fired power generation technologies instead of raw coal. Here, we report an experimental study on the catalytic gasification of HyperCoal and raw coal with steam. Catalytic steam gasification of HyperCoal not only demonstrates a higher gasification rate and much lower loss of catalyst, but also has selectivity for generating more hydrogen. Experimental Section Coal and HyperCoal Samples. Guregory (GG) coal and Oaky Creek (OC) coal, from Australia, were used in this study. The HyperCoals produced from these two coals were supplied by Kobe Steel, Ltd., in Japan. Detailed information in regard to the HyperCoal production has been described in detail elsewhere.18 Briefly, the HyperCoal was obtained by extraction of coal with 1-methylnaphthalene at 360 °C, and finally by separation of solvent from the extract. The extraction yields were 67% and 69% for the GG coal and OC coal, respectively. The particle sizes of the raw coal and HyperCoal samples were
