Investigation on nitrogen oxidation characteristics and volatile-char interaction during co-combustion of semi-coke and bituminous coal

Blending bituminous coal (BC) in semi-coke (SC) is a promising approach to improve the combustion behaviors of SC, while the nitrogen conversion characteristics during the co-combustion are still unclear. This paper deals with the oxidation characteristics of volatile-N and char-N and their contributions to total NO during the co-combustion of SC and BC using a volatile-char separated combustion system. By comparing the volatile-char separated combustion with the entrained flow combustion, the interaction between volatile-N and char-N has also been elucidated. Results indicated volatile-N was the main contribution to total NO formation for both SC and BC at temperatures above 1000 degrees C. The volatile-NO and char-NO conversion ratios presented distinct variations along with the temperature, and the temperature had different impact on the two combustion modes. The char-NO was more sensitive to the variation of oxygen concentration, and the nitrogen conversion of SC was more susceptible to oxygen compared with that of BC. The blending of BC inhibited the volatile-NO and char-NO formation of SC, and the inhibition effect enhanced in pace with the blending ratio of BC. The influence of volatile-char interaction on nitrogen conversion was temperature-dependent and fuel-varying: for SC, it was more significant in pyrolysis stage in the temperature range of 900 degrees C to 1400 degrees C, and sufficient pyrolysis measures can effectively reduce the NO formation; however, the identical result occurred when the temperature exceeded 1100 degrees C for the blend.