Effect of the Coal Blending Ratio on NOx Formation under Multiple Deep Air-Staged Combustion

The air-staged combustion experiments of lean coal, bituminous coal, and their blends were performed in a one-dimensional coal combustion experimental furnace. The influences of excess air coefficient in the primary combustion zone, multiple air-staged combustion, and coal blending ratio (CBR) of bituminous coal in coal blends on the burnout ratio and NOx emissions were evaluated. The coal blend pyrolysis under both Ar and NO/Ar environments was performed in a fixed bed to investigate the reduction mechanism between pyrolysis products and NO. The results show that, under single air-staged combustion, the burnout ratio of coal blends is lower than that of linear calculation result when the CBR increases up to 50% and then shows the opposite trend with further increasing CBR. This reflects the competition between the improvement and inhibition effects of bituminous coal combustion on the combustibility of lean coal. The CBR value of the critical point reduces to 25% under multiple air-staged combustion in comparison to single air-staged combustion. The emission index of NOx of coal blends decreases when the CBR increases up to 50% and then changes a little with further increasing CBR under different air-staged combustion conditions. The combination of triple air-staged combustion with the CBR of 50% is optimal to achieve the lower NOx emissions. Moreover, the experiments in the fixed bed indicate that CO released from coal blend pyrolysis can effectively reduce NOx which can be used to explain the NOx reduction mechanism under coal blending combustion.