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 NO_x 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 (d₀). 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 μm coal particle decreases, while T_f for 80 μm coal particle barely changes, and that for 160 μm coal particle increases a little. The recommended d₀ for MILD coal combustion is smaller than 80 μm. 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.