NO and PM formation characteristics during preheating-combustion of semi-coke produced by coal pyrolysis at different temperatures

Preheating-combustion technology is widely applied in boilers for coal/coke combustion, which has great potential for synergistic reduction of PM and NO. In this study, coal was pyrolyzed at different temperatures (300 °C, 900 °C, and 1100 °C) to produce semi-coke, followed by conventional and preheating-combustion experiments conducted in a two-stage drop-tube furnace. The effects of pyrolysis temperatures and combustion modes on PM and NO formation were investigated. The results showed that preheating-combustion technology enabled synergistic source control of NO and PM emissions. Compared to conventional combustion mode, the reducing atmosphere in preheating-combustion was stronger, and the mineral gasification and the fragmentation of char were weaker. Therefore, the emissions of NO, PM₁, and PM_1-10 in raw coal decreased by 31.09 %, 13.85 %, and 5.17 % respectively. Furthermore, higher pyrolysis temperatures further decreased NO and PM emissions under conventional combustion mode. Compared to raw coal, the NO, PM₁, and PM_1-10 emissions of semi-coke pyrolyzed at 1100 °C (1100 °C-SC) were reduced by 27.40 %, 21.30 %, and 14.41 %, respectively, due to more stable nitrogen forms and increased porosity. More interestingly, compared to conventional combustion mode, the reduction rates of NO and PM showed opposite trends under preheating-combustion mode. With the increase of pyrolysis temperatures, the reduction rate of NO gradually decreased and reached a minimum of 14.85 % for 1100 °C -SC. Whereas, the reduction rates of PM₁ and PM_1-10 gradually increased and reached maximum values of 18.25 % and 10.17 %, respectively, for 1100 °C-SC. The results provided critical theoretical foundations and technical support for the large-scale application of preheating-combustion technology, highlighting its pivotal role in advancing clean coal combustion.