Devolatilization and Combustion of Coarse-Sized Coal Particles in Oxy-Fuel Conditions: Experimental and Modeling Studies

A generalized unsteady-state kinetic model, coupled with all modes of heat transfer, was developed to describe the combined coal devolatilization and the subsequent combustion of the residual char under oxy-fuel condition in both O-2-CO2 and O-2-N-2 environments. Experiments were conducted to validate the model, which was also found to predict the experimental data published in the literature well. The effect of coal particle diameter, temperature of the reactor, and oxygen concentration on devolatilization time was investigated. Peaks in devolatilization and char combustion rates and particle center temperature were studied, and the effect of different parameters assessed. Higher reaction time was observed in an O-2-CO2 environment compared to that in an O-2-N-2 environment due to lower particle temperatures resulting from endothermic gasification reaction and the difference in thermo-physical properties. Simulation studies were carried out to generate temperature, carbon, O-2, CO, and CO2 contours to understand the char combustion reaction mechanism. The reaction started at the external surface of the particle, following unreacted shrinking core model with two zones; the solid product layer and the unreacted shrinking core, separated by a thin reaction front. Gradually, the reaction front thickness increases, leading to the shrinking reactive core model, consisting of three zones: completely reacted ash layer, partially burnt reacting char called reaction zone, and unreacted zone or slightly reacted char core. At the outset, O-2 cannot penetrate into the particle and CO produced near the surface diffuses out to the boundary layer, forming a thin flame. Subsequently, O-2 diffuses through porous ash layer into the char, and CO burns within the pores, with hardly any CO detected in the boundary layer as the particle temperature increases.