COMBUSTION OF SINGLE COAL PARTICLES IN CONVECTIVE FLOW

An experimental method has been developed to investigate the combustion rate of single mm-sized coal particles in a convective flow. Experiments were conducted schematically at free stream temperatures of 800-1200 K, flow rates of 2-8 scfm, and free stream oxygen concentrations of 4.5%-21%. Four coal types, HVCB, HVBB, SUBB, LIGA, were tested at various sizes of 4.0 mm-12 mm. Experimental data of transient mass, particle size change, characteristic combustion times, particle surface temperature, and gaseous products during combustion were established. This information was not available before and it will prove useful for the study of coal combustion systems.; Experimental results indicate that free stream temperature and flow rate are the major factors in the devolatilization stage. Increasing free stream temperature and flow rate will heat the coal particle to a higher temperature in less time. Since the effect of free stream oxygen concentration was found to be negligible, it is concluded that chemical kinetics is the controlling mechanism. Similarly, small particles are heated faster and more uniformly and thus are more reactive. In addition, lower rank coal particles have a higher reactivity.; During char combustion the lower rank coal is also more reactive but the kinetic factors have little influence. Judging from the measurement of particle size, density and reactivity at various particle sizes, free stream oxygen concentrations, and flow rates, the external diffusion and intra-particle diffusion both dominate the char burn period. However the free stream temperature will change the initial conditions of char and thus has an indirect effect on char combustion.; A numerical model for predicting char combustion was developed. The model indicates that the Thiele modulus is on the order of 10('2) and that the intra-particle mass transfer is the major factor on char combustion. The char model is basically in agreement with the experimental results. It correctly predicts the effect of the free stream temperature, free stream oxygen concentration, particle size, and flow rate on transient mass for HVCB coal char.