Experimental investigation of the characteristics of NOx emissions with multiple deep air-staged combustion of lean coal

Abstract

Deep air-staged combustion is the most promising low-NOx combustion technology to reduce NOx emissions. Both the single and multiple air-staged combustion were adopted to understand the NOx evolution behaviors and to explore the efficient NOx reduction measures for lean coal. The air-staged combustion experiments of lean coal were carried out in one-dimensional self-sustained pulverized coal combustion experimental furnace with the detailed measurements of flue gas species along the furnace height. And the effects of the excess air coefficient in the primary combustion zone (αM) and the burnout air port position on the NOx emissions and burnout ratio were evaluated. The results indicate that the NOx reduction performance under deep air-staged combustion (αM ≤ 0.85) is much better than that under shallow air-staged combustion (αM = 1.05). And an optimal burnout air port position of Ms = 0.4 exists for the lower NOx emissions under single deep air-staged combustion. The burnout air streams are supplied step by step under multiple air-staged combustion to form a low-oxygen reducing atmosphere in the burnout zone, which is conducive to the heterogeneous reduction between char and NOx on the char surface. Therefore, the NOx abatement under multiple air-staged combustion is greater than that under single air-staged combustion. Triple air-staged combustion presents the best NOx reduction efficiency in the experiments and its burnout ratio has little variation compared with single air-staged combustion. These results will provide useful fundamentals for the optimal control of NOx emissions during lean coal combustion process in the utility boilers.