Development of Subair Technique for Combustibility Enhancement and NOx Reduction in a Pulverized Coal-Fired Boiler

In this work, subair injection was proposed to improve the combustibility and NOx emission in a 500 MW tangentially fired coal boiler. The location of injection ports was determined based on the coal particle trajectory and its effect was investigated numerically. The flow rate of subair was set to 0, 5, and 10% of the total combustion air. The secondary air flow rate was decreased appropriately to ensure that the total quantity of combustion air remained constant. The over-fire air was not adjusted to retain the effect of an air-staged combustion. The simulation results showed that the subair improved the combustibility of coal particles originating from burners A and B in the lower part of the furnace. Particles from other burners were not affected significantly. In addition, this method achieved reduction of NOx by 6.3 and 13.2% when the subair accounted for 5 and 10% of the combustion air, respectively. This reduction was attributed to the decrease in the peak temperature as a result of a wider combustion region. The proposed subair technique improved the coal combustibility and reduced the NOx emissions successfully in the furnace.