An Intrinsic Kinetics-Based Model for Pulverized Coal-Char Combustion

摘要

Experimental studies have shown that both char particle diameter and apparent density vary asrnburning progresses in combustion environments typical of pulverized coal combustors. Powerlawrnexpressions have been used to correlate D/Do and r/ro with m/mo but these approaches fail tornaccount for functional variations in mass, size, and density as burning progresses. To overcomernthis limitation, an intrinsic kinetics-based, particle population balance model has been developed.rnIn this model, particle size and apparent density variations are dependent on the instantaneousrnphysical and chemical characteristics of the char. The model allows for char reactivity to bernbased on a char oxidation mechanism that takes into account (ⅰ) oxygen chemisorption at activernand deactivated carbon sites, (ⅱ) the transition of active sites to deactivated sites due to thermalrnannealing during the combustion process, and (ⅲ) desorption of CO and CO_2 as heterogeneousrnreaction products. Kinetic parameters are adjusted to fit available experimental data.rnIn this paper, the model is described and calculated profiles are discussed. Mass loss andrnapparent density profiles are compared with measurements for the char particles of Pittsburgh #8rncoal burning in 6 and 12 mole-% oxygen environments at nominally 1600 K. The model isrnshown to predict many characteristics experimentally observed during the combustion ofrnpulverized coal-chars including reductions in both apparent density and diameter with mass loss,rnvariations in the temperatures of particles nominally the same size, and variations in the sizes ofrnash particles produced during the course of burning. The model is also capable of characterizingrnreductions in char reactivity owing to thermal annealing while burning.