Releases of NO and its precursors from coal combustion in a fixed bed

Experiments have been carried out to investigate the emissions of nitrogen species including NO and its precursors during temperature-programmed coal combustion by TG/EGA method. Experimental results show that the conversion ratio of fuel nitrogen to NO is the highest, followed by that of fuel nitrogen to HCN and the conversion ratio to NH₃ is negligibly small. Nitrogen is retained in the char and released mainly as NO at the later stages of coal combustion. HCN and NO are both primary products from coal char oxidation. Coal rank, heating rate, indigenous minerals and external additives are the major influential factors of the nitrogen species release. Higher rank coals with higher fuel ratio have higher NO releases. HCN release decreases as fuel ratio increases for most coals. The fuel nitrogen conversion to NO increases and the fuel nitrogen conversion to HCN decreases with the increase of heating rate, which may imply that the char nitrogen prefers to react with oxygen to form NO instead of HCN while coal char is combusted at higher temperatures. Different metallic additives show different effects on nitrogen species emission and the effects of indigenous minerals on nitrogen release can be qualitatively estimated by ash analyses.