Comparative study of four alternative models for CO oxidation around a burning coal char particle

The steady state combustion of a quiescent char particle is investigated by means of a detailed model which accounts for heterogeneous oxidation and gasification, as well as homogeneous reactions (GRI-Mech 3.0) in the particle’s boundary layer. First, the way and extent in which the mass and energy transfer are altered due to the oxidation of CO are examined by comparison with the predictions of a single-film model in the case of anthracite particles within 60–1000 μm. Then, four alternative descriptions of the gas phase (single-film, double-film, global kinetics and detailed kinetics) are evaluated for two coals of very different reactivity towards O2 and CO2, at low and high O2 concentration and in the same broad range of sizes. The overall influence of the CO conversion modeling on the particle burning rate and temperature (i.e. whether the reduction in O2 surface concentration or the heat and CO2 provided by the flame dominate over each other) is found to depend on the conditions considered. The single-film approach reasonably fits the predictions of the most complete model in all cases (and especially in the pulverized-coal size range), whereas the double-film hypothesis and the global kinetics generally overestimate the effects of the flame on the consumption rates and the particle temperatures.