Coal-particle size effects on NO reduction and burnout characteristics with air-staged combustion in a pulverized coal-fired furnace

The influence of coal-particle size on nitrogen oxide (NOx) emission and burnout characteristics was experimentally investigated in a pulverized coal-fired furnace. This study was carried out for a range of particle sizes, namely, mean sizes of 52, 73, 102, and 107 μm. Detailed in-furnace measurements of gas temperature and gas species concentrations (O2, CO2, CO, and NO) were performed for two particle sizes (52 and 107 μm) in both unstaged and air-staged flames. The results show that the overall temperature of the flames with a mean particle size of 52 μm is higher than that of the flames with a mean particle size of 107 μm, because burning of fine coal particles improves the heating rate of other larger pulverized coal particles. The value of the NO emission measured at the furnace outlet depends on the pulverized coal-particle size and decreases by 20% with an increase in the mean particle size from 46 μm to 118 μm. Two trends are observed in the NO emission with a mean particle size. For the unstaged combustion, a linear relationship exists between the NO emission and mean particle size, whereas the NO emission is constant at the air-staged combustion. The burnout performance increases with an increase in the level of coal fineness. The effectiveness of air staging on the NO reduction and burnout performance is significant in the flames with fine pulverized coal particles. The NO-reduction efficiency for flames with mean particle sizes of 52 and 73 μm is almost twice that for flames with mean particle sizes of 102 and 107 μm. For the burnout performance, the deteriorating effect of air staging is more profound at the flames with high level of coal fineness. The reduction rate in the burnout performance is 1.7% for flames with fine particles (52 and 73 μm) and 0.7% for flames with coarse particles (102 and 107 μm).