Combustion kinetics of high-ash coals in fluidized beds

Abstract

Burnout times of high-ash coals in a fluidized bed were determined for coal particle sizes ranging from 0.125 to 4 mm, over a temperature range of 973–1173 K and fluidization velocities varying from 3.7 U mf to 6.2 U mf. The results show that the mode with which anthracite particles burn resembles that of a shrinking core of unchanged initial particle diameter. With these anthracites it was observed that diffusion through an ash layer becomes more important as the burning core diameter decreases with time. It can also be concluded that the combustion of the anthracites appears to be most influenced by the bed temperature and particle size. The combustion of particles at 1173 K is primarily controlled by diffusion of oxygen to the surface of the particles and through the ash layer. The influence of chemical kinetics on the combustion process increases considerably with decreasing bed temperature and is the dominant resistance below 1073 K for all the particle sizes tested.