Theoretical analysis of reaction regimes in low-temperature oxidation of coal

This paper examines the low-temperature oxidation of coal using pore model resembling ordinary tree structures, where the trunk of each effective pore, reaches the exterior of the coal particle. Theoretical analysis shows that, at low temperatures and atmospheric pressure, the mean diffusivity of oxygen in a coal particle is related to the porosity and particle size, and varies between 10(-8) and 10(-6) m(2)/s. When the particle size is large (more than 1 mm in diameter), the coal oxidation is controlled by continuum diffusion, while for a very fine particle the reaction regime switches to Knudsen-diffusion controlled (for active coal) or kinetically controlled (for less active coal). With increasing porosity of fine coal particles, the trend for the reaction regime to be kinetically controlled becomes more significant. For the less active coal with high porosity and particle size of several tens of microns, the reaction regime is almost entirely kinetically controlled. The rate of oxygen consumption of coal usually shows a dependence on particle size, but in the case of the less active coal and a particle size of a few tens of microns, the rate of oxygen consumption is virtually independent of the particle size. The independence of the rate of oxygen consumption of the particle size is also observed for larger particles (even around 500 mu m in radius), when the coal reactivity is sufficiently low. The predictions from the present model are in agreement with published experimental findings, and have application to the modelling of spontaneous combustion of coal. (C) 1999 Elsevier Science Ltd. All rights reserved.