An intrinsic kinetics-based, particle-population balance model for char oxidation during pulverized coal combustion

Abstract

Experimental studies have shown that both char particle diameter and apparent density vary as burning progresses in combustion environments typical of pulverized coal combustors. Power-law expressions have been used to correlate D/D0 and p/p0 with m/m0, but these approaches fail to account for functional variations in mass, size, and apparent density as burning progresses. To overcome this limitation, an intrinsic kinetics-based particle population balance model has been developed. The model represents an improvement over apparent kinetics-based models that employ the power-law-controlled mode of burning, in that particle size and apparent density variations are dependent on the instantaneous physical and chemical characteristics of the char. The model also allows for char reactivity to be based on a char oxidation mechanism that takes into account (1) oxygen chemisorption at active and deactivated carbon sites, (2) the transition of active sites to deactivated sites due to thermal annealing during the combustion process, and (3) desorption of CO and CO2 as heterogeneous reaction products. In this paper, the intrinsic kinetics-based char oxidation model is described, and a particle population balance model for pulverized coal char combustion is presented. The model is shown to predict many characteristics experimentally observed during the combustion of pulverized coal chars, including variations in the mode of burning and reductions in char reactivity owing to thermal annealing while burning. Calculations indicate that char particles formed during devolatilization that have relatively high apparent densities contribute significantly to unburned carbon in ash.