Coupling thermal deactivation with oxidation for predicting the combustion of a solid fuel

Abstract

A thermal annealing model has been coupled with a detailed single particle model for char oxidation to predict the burning of solid fuels in combustion facilities. The combined model is tested against reported laminar flow reactor burning profiles of several coal chars at the intermediate stages of combustion and for a single coal char also at the later stages of combustion. The model shows for all coals a transition from combustion controlled by oxygen bulk diffusion at low char conversions to kinetically controlled combustion at high conversions. The impact of annealing on char reactivity is strongest at the early stages of combustion, and when the simultaneous annealing of the char is not accounted for, shorter burning times are obtained. In many cases, the experimental burning profiles of the coal chars can only be explained by taking into account the presence of fragments being released at the periphery of the particle. This type of fragmentation appears to be the prime means of particle shrinking and is most accentuated at higher oxygen concentrations in the reactor. The model was able to predict reasonably well the unburned carbon fraction of a single coal char at the latest stages of combustion (up to ∼98%). The model relies on reasonable 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 important for determining burnout at the final stages of combustion. This can 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.