Energy & Fuels 1991,5,317-322
317
Influence of Structural Parameters of Coal Char on K- and Ca-Catalyzed Steam Gasifications Tetsuya Haga,* Masayuki Sato, and Yoshiyuki Nishiyama Chemical Research Institute of Non- Aqueous Solutions, Tohoku University, 2-1 - 1 Katahira, Sendai 980, Japan
Pradeep K. Agarwal and John B. Agnew Chemical Engineering Department, The University of Adelaide, North Terrace, Adelaide, S.A. 5000, Australia Received June 25, 1990. Revised Manuscript Received November 27, 1990 Various coal chars with well-characterized surfaces were subjected to K- and Ca-catalyzed steam gasifications at 800 or 850 "C in atmospheric flow of equimolar H20-He or H20-H2 mixture, to elucidate the relation between the reactivity and structural properties of char. The catalytic effect of Ca was sensitive to the preparation temperature of char and the kind of parent coal, while the activity of K was essentially independent of those. The catalytic effect of Ca was well correlated to the structural parameters of char, which represent surface area, surface hydrophilicity, and crystallinity. The result is discussed in comparison with our previous studies on Ni-catalyzed gasification of coal chars.
Introduction The structure and reactivity of coal is a fundamental subject of coal science and important for coal technology such as gasification and activated carbon preparations. Coal and char gasifications with 02,H20, and C02 are well-known to be catalyzed by alkali, alkaline earth, or iron-group metal compounds.' It has been a subject of intensive studies to establish relationships between the structure of char and its reactivity for noncatalytic gasif i c a t i ~ nbut , ~ ~there ~ are only a few works reporting the influence of char structural properties on catalytic gasification." We have previously shown that the reactivity of coal char for Ni-catalyzed steam gasification is sensitive to the coal type and char preparation temperature in a different way from the case of noncatalytic reaction and is well correlated to the surface area and crystallinity parameters of char, which are obtained from gas adsorption and X-ray diffraction measurements, respe~tively.~~' Especially interesting was the positive effect of carbon crystallinity on the Ni-catalyzed reaction. More workers have recently revealed that the major role of nickel in carbon gasification above 700 OC is to activate a bulk carbon by dissolving the carbon.*'O The crystallinity of carbon is suspected to influence such a Ni-C interaction dominating the catalytic activity. If so, the nickel catalyst may be different in dependence of activity on char structures from the potassium and calcium catalysts,which are generally believed to increase only the number of active (1) Pullen, J. R. Catalytic Coal Gasification; I.E.A. Coal Research London, 19&1; ICTIS/TR26. (2) Radovic. L. R.: Walker, P. L... Jr.:. Jenkins. R. G. Fuel 1983. 62.
846868. (3) Haga, T.; Nishiyama, Y. Fuel 1988,67,743-747. (4) McKee. D. W... Sdro. _ .C. L... Kcmkv. - . P. G.: Lambv. - . E. J. Fuel 1983, 62,'217-220. . (5) Takarada, T.; Tamai, Y.; Tomita,A. Fuel 1986,65,679-683. (6) Haga, T.; Nishivama, Y. Fuel 1988, 67. 748-752. (7) Haga, T.; Nbhiyama, Y. Energy Fuels, in preaa. (8) Figueiredo, J. L.; Bernard, C. A.; Chludzinski, J. J., Jr.; Baker, R. T. K. J. Catal. 1988,110,127-138. (9) Y q ,R. T.; coethel, P. J.; Schwartz, J. M.; Lund, C. R. F.J. Catal. 1990,122,206-210. (10) Goethel, P. J.; Yang,R. T. J. Catal. 1987,108, 356-363.
sites which activates a gasifying agent without changing the activation energy of gasification." This paper reports the influence of char structures on steam gasification catalyzed by potassium and calcium. The net catalytic effects of K and Ca on gasification are evaluated based on the kinetic features of gasification and compared with the structural parameters of char, which have been obtained by our previous studies??' The result is discussed in comparison with the result on the Ni catalyst:
Experimental Section Char Preparation. Char specimens used in this study were prepared from six coals: Indian Ridge (C 88.1 wt %, daf), Big Ben (85.4), Blair Athol (81.9), Millmerran (78.3),Taiheiyo (75.0), and Yallourn (66.1). Char specimens were prepared by heating the coals in nitrogen for 1 h, at 750 OC for all the coals and at several temperatures of 240-900 OC only for the BA coal. The heat treatment temperature for char preparation is designated as HTT thereafter. The six coals were also demineralized by the treatment with a mixed HCl and HF acid at 70 OC for 20 h prior to char preparation. Char Characterizations. Chemical and X-ray diffraction analyses were employed to estimate the bulk structural properties of char.3 In the X-ray diffraction examinations, a parameter CR = A/(A[graphite](A+B)),which was obtained from the integrated intensities of the (002) reflection peak (A) and background (E) in a range of 28 = 15-33', was defined assuming to represent the state (crystallinity) of carbon in the char. The procedure of CR determination is a simplified version of those frequently used to estimate the crystallinity of organic polymer.lZ The surfaces of chars were characterized by means of gas adsorption and HzO-immersional calorimetric measurements.' The amounts of COz (0 'C) and CHSOH (25 "C) adsorbed were measured at vapor pressures of
