Modeling of effects of volatile matter cloud on heterogeneous ignition of single coal particles

A simple model has been developed to predict the heterogeneous ignition temperature for single coal particles under conditions of negligible, natural and forced convections. These conditions cause different sizes of the volatile matter cloud surrounding the coal particle. The model is based on the ignition of a single coal particle heated by irradiation from spot heaters in cold (ambient) surroundings. Energy from the particle surface oxidation, volatile matter combustion and physical heating from the spot heaters are all included in the particle energy balance. General ignition criterion is used to determine the ignition point. The model results agree quite well with experimental findings obtained earlier as far as the effects of particle size and volatile matter are concerned. Gas temperature emerged as the most effective model parameter influencing the ignition temperature. In the absence of volatiles in the particle vicinity, as for the forced convection case, the gas temperature remains almost constant and so does the ignition temperature. For cases where volatiles surround the particle, high gas temperatures as well as ignition temperatures are obtained. This is mainly attributed to the combustion of the volatiles contributing to the gas temperature rise, and possibly raising the particle temperature.