Low-temperature oxidation of coal .1. A single-particle reaction-diffusion model

In this paper, the first of a three-part series, a kinetic model for low-temperature oxidation is developed. The formulation, based on a single isothermal particle, permits assessment of the relative importance of various mass transfer resistances. Through inclusion of transport resistances, the model accounts for the dependence of the observed reaction rates on particle size and specific external surface area over the full range of particle sizes, with smooth transition between different reaction regimes for dry as well as wet low-rank coal. The approach also permits development of a new model which considers the effect of bulk and surface moisture in the coal on the rate of low-temperature oxidation. This new model is expected to apply at moisture levels above similar to 0.1 kg kg(-1) coal. It is shown that when liquid water fills the porous matrix, in situ moisture in fresh coal must exert a very significant retarding influence on the rate of low-temperature oxidation at constant temperature. Further, the influence of particle size on reaction rate is pronounced even at low temperatures and small particle sizes. For dried coal on the other hand, the influence of particle size becomes important only at significantly higher temperatures or for considerably larger particle sizes. Results of the extension of the model to (1) the interpretation of rate data obtained in isothermal batch and fixed-bed reactors and (2) the modelling of spontaneous combustion in coal stockpiles are reported in parts 2 and 3 respectively.