A general mechanistic model of fly ash formation during pulverized coal combustion

Abstract A general mechanistic model, involving an ensemble of rate equations, is developed to predict the formation of fly ash during pulverized coal combustion. The most important pathways, including physicochemical mineral vaporizations, dynamic mineral collisions, and statistical mineral fragmentations, are addressed in the model. The coalescence between exposed minerals on the receding char surface is simulated by a three-dimensional hard sphere model, while the coagulation between ash aerosols of all sizes in the flue gas is done by a population balance model. Besides, the fragmentation of both liberated included minerals and isolated excluded minerals is assumed to follow a stochastic Poisson distribution. Numerical results fit well with the fine particulate matter (PM 10 ) data measured by the electrical low pressure impactor and the bulk ash (PM 10+ ) data measured by the Malvern particle sizer in our 25 kW one-dimensional furnace experiments. The model reproduces several features of the experimental data for coals in different ranks, including the shape of particle size distributions (PSDs) and the yield of fine particles (PM 0.26 , PM 1 , and PM 2.5 ). For instance, it indicates that the fly ash from Zhundong coal in fractions of PM 0.4 , PM 0.4-2.5 , PM 2.5-33.2 , and PM 33.2+ are mainly contributed by mineral precursors, mineral fragmentations, included minerals, and excluded minerals, respectively. With the validated model, the effects of particle coagulation, coal PSD, and ambient conditions on the fly ash formation are intensively examined.