Conversion characteristics of fuel-N to NO during oxy-fuel combustion of semi-coke/bituminous coal blend fuels

Given the characteristics of China’s energy structure, coal grading technology has been continuously receiving attention and developing. However, its solid by-product, semi-coke, faces issues such as poor combustion characteristics and high NO_x emissions when burned for utilization. To address these problems, co-combustion is considered as an effective way. Furthermore, in the context of the “dual carbon” goals, the application of oxy-fuel combustion technology is conducive to controlling CO₂ emission, while also influencing the generation of NO_x. Accordingly, in order to explore the mechanism of NO formation during the oxy-fuel combustion of semi-coke/bituminous coal blend fuels, the experimental research on the conversion characteristics of fuel-N to NO was conducted based on a fixed bed reactor. The research results show that the total conversion ratio of fuel-N to NO increases with the blending ratio of bituminous coal in blend fuels. Compared to the O₂/N₂ atmosphere, the presence of CO₂ in the O₂/CO₂ atmosphere can participate in the gasification reaction of char to produce more CO and then promote the NO reduction, so the conversion ratios of volatile NO and char NO in the blend fuels all decrease. As the combustion temperature increases, more fuel-N is converted to NO. The increase in the O₂ concentration in O₂/CO₂ atmosphere also increases the total conversion ratio of fuel NO. Meanwhile, the promotion on NO conversion caused by the interaction between blended fuels will increase firstly and then decrease with O₂ concentration. At the stage of volatile combustion, the increase in O₂ concentration will promote the conversion of fuel-N to NO; however, its effect on the conversion of char NO depends on the fuel type. As O₂ concentration increases, the conversion of char-N in bituminous coal is promoted. During semi-coke combustion, higher O₂ concentration will lead to the earlier release of more pyrrolic-N (N-5) from semi-coke at the devolatilization stage, resulting in the lower conversion ratios of char NO for semi-coke and its blend fuels. Additionally, smaller particle sizes are more favorable for the generation of fuel NO. The achievements of this research can provide a theoretical basis for the clean and low-carbon utilization of semi-coke.