Coupling thermal deactivation with oxidation for predicting the combustionof a solid fuel

A thermal annealing model has been coupled with a detailed single particle model fur char oxidation to predict the burning of solid fuels in combustio n facilities. The combined model is tested against reported laminar flow re actor burning profiles of several coal chars at the intermediate stages of combustion and for a single coal char also at the later stages of combustio n. The model shows for all coals a transition from combustion controlled by oxygen bulk diffusion at low char conversions to kinetically controlled co mbustion at high conversions. The impact of annealing on char reactivity is strongest at the early stages of combustion, and when the simultaneous ann ealing of the char is not accounted for, shorter burning times are obtained . In many cases, the experimental burning profiles of the coal chars can on ly be explained by taking into account the presence of fragments being rele ased at the periphery of the particle. This type of fragmentation appears t o be the prime means of particle shrinking and is most accentuated at highe r oxygen concentrations in the reactor. The model was able to predict reaso nably well the unburned carbon fraction of a single coal char at the latest stages of combustion (up to similar to 98%). The model relies on reasonabl e estimates of the temperature and oxygen profiles in the reactor, as well as the char density, particle size, porosity and initial reactivity (e.g., at room temperature) of the non-annealed fuel. The latter is particularly i mportant for determining burnout at the final stages of combustion. This ca n be determined by simply combining the char's reactivity (extracted from a single TGA experiment of the fuel involving pyrolysis at high temperature and subsequent char oxidation), with the proposed annealing model. (C) 2001 by The Combustion Institute.