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Energy & Fuels 1988,2, 18-26
Intrinsic Reaction Kinetics of Microporous Carbons. 1. Noncatalyzed Chars J. K. Floess,* J. P. Longwell, and A. F. Sarofim Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received June 24, 1987. Revised Manuscript Received October 1, 1987
This paper reports the results of kinetic studies conducted with two microporous carbons over a wide range of conditions. The purpose of this study was to establish an understanding of the reaction behavior from which to test and improve currently available structural models. Kinetic data were obtained with a thermogravimetric analyzer, a r d structural characterization of the chars was made by analysis of gas adsorption measurements and by application rf the random pore model. The results of this study showed the following: (1)The determination of kinetic data for the carbon-oxygen reaction from gravimetric data at conversions below 20% requires correcting for the oxygen content of the char. This correction changes the shape of the rate v8 conversion curve from the usual concave shape to a monotonically decreasing rate with conversion. Substantial amounts of oxygen, approaching monolayer coverage, are adsorbed on the char during this reaction. Rate vs conversion data could be scaled to a single characteristic curve for each char for a broad range of temperatures and reactant concentrations. (2) At conditions believed to correspond to intrinsic kinetics (based on the magnitude of the activation energy and a straight Arrhenius line spanning over 2 decades of reaction rate), the reactivity of one char, which lacked a significant macropore network, was found to be a function of particle size for particles between 50 and 200 bm in diameter. The other char, which has a significant macropore network, did not display such an effect. It is believed that this particle size effect may be a characteristic of large microporous domains in chars.
Introduction Chars formed by pyrolysis of coal and other polymeric organic material typically have porosities of 2040% and surface areas of 200-1000 m2/g. Typically, much of the surface area is found in micropores (
