Morphological Development and Intrinsic Reactivity for Combustion of Two Microporous Carbon Chars

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Abstract
In this paper, we consider the morphological development and intrinsic reactivity or two microporous carbon chars burning at conditions typical of a pulverized-fuel combustor (i.e., particle temperatures above 1300 K. and diameters below 200 μm). The model chars were chosen for their low ash and volatile matter contents to preclude the complicating effects of these components on the oxidation process. The partially oxidized samples (0 to 65% mass loss), which were collected in an entrained-flow reactor, are experimentally characterized by their total surface area, particle diameter, apparent density. pore volume distribution, and macroporous surface area. SEM photomicrographs of the external particle surface and refiected-light photomicrographs of the internal porosity of the particles are also presented. These measurements demonstrate that combustion occurs under Zone II conditions. The Thiele model of diffusion and reaction in a porous particle is used to calculate intrinsic combustion rates from the measured overall rate data for both chars. The calculations support an intrinsic activation energy of 42 ± 3kcal/mol and an intrinsic oxygen reaction order close to zero. The high temperature (1300 to 2300 K) intrinsic rate coefficients calculated here show good agreement with extrapolated intrinsic rate coefficients measured in a TGA apparatus at temperatures below 850 K. We discuss (he ramifications of this agreement on the current mechanistic understanding for the carbon-oxygen reaction.