Energy & Fuels 1996, 10, 225-228
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Coal Gasification Using the ZnO/Zn Redox System Masamichi Tsuji,* Yuji Wada, and Yutaka Tamaura Research Center for Carbon Recycling and Utilization, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, Japan
Aldo Steinfeld and Peter Kuhn Laboratory for Energy and Process Technology, Paul Scherrer Institute, CH-5232 Villigen, Switzerland
Robert Palumbo Mechanical Engineering Department, Valparaiso University, Valparaiso, Indiana 46383 Received June 20, 1995. Revised Manuscript Received September 25, 1995X
A two-step thermochemical process is proposed for converting coal to high-quality synthesis gas. In the first, high temperature, endothermic step, coal is reacted with zinc oxide to form metallic zinc and an H2-CO gas mixture. In the second, low temperature, exothermic step, zinc is used for splitting water and producing hydrogen and zinc oxide. The hydrogen is employed to enrich and adjust the synthesis gas mixture obtained in the first step, while the zinc oxide is recycled to the first step. Experimental studies have shown a more effective chemical conversion obtained via the proposed two-step scheme as compared to that obtained via the conventional single-step direct steam gasification. CO formation was more favorable with the coal-ZnO redox reaction than with the coal-H2O reaction in the 1173-1373 K temperature range. This highly endothermic reaction could be conducted using concentrated solar radiation as the energy source of high-temperature process heat, allowing for a combined utilization of coal and solar energy, and consequently for a significant reduction of the CO2 emissions derived from the combustion of coal.
Introduction The gasification of coal and carbonaceous materials has been widely studied and is presently practised at an industrial scale.1-4 The conversion of coal to synthesis gas (syngas, a mixture primarily of H2 and CO) provides a chemical pathway for the production of synthetic fuels and commodity organic chemicals. Coal gasification is a high-temperature energy-intensive process and coal itself is burned internally in the gasifier to supply the process heat. Off-gases usually contain substantial amounts of CO2, which, depending on their final end use, may require further energy-consuming separation systems. Catalysts can improve the kinetics and chemical conversion and reduce the operating temperature requirements,5-10 but their use is subjected to the feasibility of recovering them from the remaining Abstract published in Advance ACS Abstracts, November 1, 1995. (1) Ullmann’s Encyclopedia of Industrial Chemistry; Gerhartz, W., Ed.; VCH Verlagsgesellshaft: Weinheim, Germany, 1988; Vol. A12. (2) Wison, J. S.; Halow, J.; Ghate, M. R. CHEMTECH 1988, 123128. (3) Harig, H.-D. VDI Ber. 1992, N. 984, 169-194. (4) Keller, J. Fuel Process. Technol. 1990, 24, 247-268. (5) Schumacher, W.; Muehlen, H. J.; van Heek, K. H.; Juentgen, H. Fuel 1986, 65, 1360-1363. (6) Otto, K.; Bartosiewicz, L.; Shelef, M. Fuel 1979, 58, 85-91. (7) Morales, I. F.; Garzon, F. J. L.; Peinado, A. L.; Castilla, C. M.; Utrilla, J. R. Fuel 1985, 64, 666-673. (8) Tanaka, S.; U-emura, T.; Ishizaki, K.; Nagayoshi, K.; Ikenaga, N.; Ohme, H.; Suzuki, T.; Yamashita, H.; Suzuki, T. Energy Fuels 1995, 9, 45-52. (9) Adler, J.; Huettinger, K. J. Fuel 1984, 63, 1393-1396. (10) Greenbaum, E.; Tevault, C. V.; Ma, C. Y. Energy Fuels 1995, 9, 163-167. X
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ash. Nonetheless, environmental considerations favor converting coal into cleaner fluid fuels that have higher H/C ratios, provided the process heat required for the transformation is supplied by a clean renewable energy source, e.g., solar energy. In industry, steam and/or air is usually the oxidant in the gasification of coal. Alternatively, one could use a metal oxide as the donor of oxygen. This approach is formally equivalent to combining two processes, viz., the gasification of coal for the production of syngas and the reduction of metal oxides for the production of metals. The overall reaction, which is the first step of a twostep themochemical scheme, can be represented as
first step: CHx + (1/z)MyOz ) (x/2)H2 + CO + (y/z)M (1) where CHx represents coal, M denotes metal, and MyOz the corresponding metal oxide. In the second step, the metal M is further reacted with water to form molecular hydrogen according to
second step:
(y/z)M + H2O ) (1/z)MyOz + H2
(2)
The hydrogen produced in the second step is used to enrich the syngas mixture obtained in the first step or to adjust the H2/CO molar ratio for compatibility with the Fischer-Tropsch catalytic synthesis. The metal oxide regenerated in eq 2 is recycled to eq 1. The net stoichiometric reaction is then given by © 1996 American Chemical Society
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Energy & Fuels, Vol. 10, No. 1, 1996
CHx + H2O ) CO + [1 + (x/2)]H2
Tsuji et al.
(3)
A diagram of the thermochemical scheme is depicted in Figure 1. ZnO was selected as the metal oxide for conducting reactions 1 and 2. The thermodynamics of the simpler CH4/ZnO redox system serves as the reference for the present coal/ZnO redox system. The equilibrium composition of the system ZnO + CH4 has been calculated over a wide temperature range.11 The present paper describes an experimental study of the combined coal gasification and ZnO reduction (eq 1 using ZnO) and examines the advantage of using solar energy as the source of process heat to drive the reaction.
Figure 1. Two-step thermochemical scheme for the gasification of coal to high-quality synthesis gas.
Experimental Setup Figure 2 shows the experimental apparatus. It consists of an 8 mm diameter, 330 mm length, tubular quartz reactor that contains a packed bed of solid reactants. Samples were prepared by mixing ZnO powder (Wako Pure Chemical Ind. Ltd, size smaller than 400 mesh or 37 µm) and pulverized coal (Idemitsu Coal Lab.) and placed in the quartz reactor. The size of coal mostly ranges from 50 to 150 µm in diameter, and the composition is given in Table 1. The reactor was then evacuated and heated electrically to the desired temperature in 3 min. Evolved gases were determined by gas chromatography (Shimadzu GC-8A, equipped with Porapak Q or Molecular Sieve 13X column). Partial pressures of CO and CO2 were monitored by a gas analyzer (Shimadzu CGT-10-2A). The nature of the solid products after the reaction was determined by X-ray diffraction (Rigaku RINT 2000). The same experimental setup was used for the open system experiments, where a continuous flow of Ar and Ar-CO2 (50 mL/min) was passed through the packed bed.
Figure 2. Schematic illustration of the experimental setup.
Results and Discussion Figure 3 shows the volume of CO and H2 that evolved at 1173 K as a function of reaction time for different ZnO/carbon molar ratios. The system was closed; i.e., no external gas was allowed to flow through the bed. The reaction reached 80-90% of its equilibrium composition within 10 min for all the experimental conditions. Then we observed a slow evolution of CO and H2, which may be ascribed to the surface reaction between coal and ZnO. After 40 min, the reaction was complete. The product gas was rich in H2 gas. When no ZnO was added to the coal, the conversion of coal to CO gas was very low (ca. 2% of coal used) and the CO/ H2 molar ratio of the product gas was 0.15. For a ZnO/ carbon molar ratio larger than 1.4, the CO/H2 molar ratio attained a maximum of 0.72. However, the maximum value is still smaller than the C/H molar ratio of the coal used. It will be mostly due to reaction of ZnO with volatile components. The conversion of coal to CO increased monotonically with an increasing ZnO/ carbon molar ratio, as shown in Figure 4. The conversion was 10% for ZnO/carbon molar ratio of unity, and it reached a maximum of 15% for a ZnO/carbon molar ratio larger than 1.4. More ZnO than the stoichiometric amount was necessary for obtaining a constant conversion of coal. It is because a smaller, denser ZnO particle (
