Char fragmentation during pulverized coal combustion was studied using an A
ustralian bituminous coal. The coal was combusted with air in a drop tube f
urnace at a gas temperature of 1300 degrees C. The char samples were collec
ted at different levels of char burnout, and their structure was examined u
sing scanning electron microscopy. Approximately 40% of the char particles
formed after pyrolysis were cenospheres with a highly non-uniform porous st
ructure and a large central void. A large number of fine particles were als
o observed in the char samples with burnout levels between 30 and 50 vol%,
which suggests that significant fragmentation occurs during the early combu
stion 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 r
emoval from the surface of the thin shell due to surface oxidation. A perco
lation model was used to simulate the char structural changes during combus
tion in regime III, and the predicted particle size distributions qualitati
vely agreed with the experimental measurements. (C) 2000 Elsevier Science L
td. All rights reserved.