Modeling the fragmentation of non-uniform porous char particles during pulverized coal combustion

Abstract

Char fragmentation during pulverized coal combustion was studied using an Australian bituminous coal. The coal was combusted with air in a drop tube furnace at a gas temperature of 1300°C. The char samples were collected at different levels of char burnout, and their structure was examined using scanning electron microscopy. Approximately 40% of the char particles formed after pyrolysis were cenospheres with a highly non-uniform porous structure and a large central void. A large number of fine particles were also observed in the char samples with burnout levels between 30 and 50 vol%, which suggests that significant fragmentation occurs during the early combustion stage. A mathematical model was developed relating the fragmentation of cenospherical char particles with the macropores in the particle shell. The formation of these macropores partially results because of the carbon removal from the surface of the thin shell due to surface oxidation. A percolation model was used to simulate the char structural changes during combustion in regime III, and the predicted particle size distributions qualitatively agreed with the experimental measurements.