Percolation Model for Coal Char Particle Combustion and Fragmentation

Abstract

A char combustion model based on percolation theory is developed. The concept of percolation is used to derive the time and position dependent transport and structural properties of the char particle during combustion. By adopting existing kinetic parameters for the surface reaction of the char particle, porosity and oxygen concentration profiles within the particle are calculated for progressive carbon conversion values. Assuming an initially uniform porosity of the char particle, and a Bethe lattice to describe the pore structure, a critical porosity is calculated at which the particle surface layer breaks into fragments. The effects of particle diameter, combustion temperature, pore structure, and fragmentation porosity upon particle reactivity and burnout times are evaluated to determine the sensitivities of the results to the assumption of initial and operating conditions. The impact of fragmentation on the fly ash particle size distribution during pulverized coal combustion is discussed.