Model predictions of the extent of heterogeneous coal combustion during rapid evolution of volatiles

Two relatively simple, single-particle coal combustion models are developed and used to predict the extent of heterogeneous combustion during rapid devolatilization. The first model uses the flame sheet approximation while the second employs finite gas-phase reaction rates. Both models predict that small particles (< 10 μm diameter) burn heterogeneously and large particles (> 100 μm diameter) burn homogeneously, but heterogeneous mass loss is severely underestimated by the simpler flame-sheet model in comparison to the more realistic finite-rate-chemistry model for intermediate sized particles typical of a utility grind. Small particle size, high oxygen concentration and low devolatilization rate, in that order of importance, yield more extensive heterogeneous combustion, while temperature, volatiles composition, gas-phase kinetics, and surface reaction rates have negligible impact.