Prediction and measurement of the combustion time of single coal particles

Abstract

Combustion times of single particles of four coals, ranging in rank from lignite to medium volatile bituminous, were measured thermogravimetrically. The experiments were conducted in a gas mixture containing 13 vol% oxygen in nitrogen at a flow rate of 100 cm 3 min −1 using particles ranging from 0.8 to 4.2 mm diameter, for gas temperatures from 973 to 1173 K. Gas velocity, particle size and bulk gas temperature influenced the char combustion rate, whereas the influence of coal type was relatively minor. The effects of gas velocity, particle size, and gas temperature on the oxygen diffusion rate were predicted using a char combustion model. The thickness of the mass transfer boundary layer, the fractional diffusional resistance, and the combustion times of coal particles up to 10 mm diameter were calculated for temperatures from 973 to 1373 K, and gas velocities from 0 to 200 cm s −1. The combustion times of the coal particles predicted by the model agreed with those measured experimentally and with published data in the literature for small scale fluidized bed reactors. The data indicate a mixed control (both diffusional and chemical kinetics) for conditions relevant to fluidized bed combustion.