Homogeneous and heterogeneous contributions of CO2 and recycled NO to NO emission difference between air and oxy-coal combustion

High concentration of CO2 and recycled NO-containing flue gas are known to affect the total NO emission during oxy-coal combustion through both homogeneous and heterogeneous reactions. To evaluate each contribution, this work investigated the NO emission behavior in air and oxy-coal combustion atmosphere (O2/CO2 and O2/recycled flue gas) with three different oxygen concentrations (21%, 27% and 32%). Three typical Chinese coals and their derived chars were employed and all of the experiments were conducted in a drop tube furnace at a temperature of 1373 K. The results show that the conversion ratio of coal nitrogen (coal-N) to NO increased with decreasing coal rank in all atmospheres. The NO emission rate in oxy-coal was lower than that in air combustion. The heterogeneous effect of CO2 mitigated NO emission in oxy-coal combustion, whereas homogeneous contribution of CO2 may have increased NO formation due to a great amount of coal-N was released in the form of volatile nitrogen (volatile-N) at a high concentration of CO2. The reduction of recycled NO was the major reason for the lower NO emission in oxy-coal combustion, but the main removal route differed. The reduction of recycled NO through homogeneous reactions was more important for both the lignite and the high volatile content bituminous coal, whereas the heterogeneous reactions were more important for the anthracite NO removal.