THE EFFECT OF FORCED CONVECTION ON MASS AND HEAT TRANSFER DURING SINGLE COAL PARTICLE COMBUSTION

MILD coal combustion is one of promising technologies for clean coal utilization due to uniform heat flux and low NOx emission, while the effect of turbulent mixing on single coal particle combustion under high temperature and low oxygen concentration remains to be studied for micron level particles. In this paper, a 1-D transient coal combustion model was applied to describe mass and heat transfer around a single particle, and the effect of forced convection was modeled to represent turbulent mixing according to similarity analysis. Based on that, effect of particle Reynolds number (Re-p) on single coal particle combustion was investigated at various temperature (T-a), oxygen concentration (x(O2)) and particle diameter (d(0)). As Re-p increases, ignition time (t(i)) decreases quickly at first and then decreases slowly. t(i) of larger particle is more sensitive to Re-p. As Re-p increases, flame temperature (T-f) for 40 mu m coal particle decreases, while T-f for 80 mu m coal particle barely changes, and that for 160 mu m coal particle increases a little. The recommended d(0) for MILD coal combustion is smaller than 80 mu m. As x(O2) decreases from 21% to 5%, t(i) apparently increases and T-f decreases. t(i) at lower T-a is more sensitive to Re-p. T-f decreases with increasing Re-p when T-a < 1200 K. But it appears the opposite trend at T-a = 1600 K. The recommended T-a. for MILD coal combustion is lower than 1400 K, while it cannot be too low considering the burnout of char particle.