Mechanistic insight into the effect of CO on NH₃, HCN, and HNCO release reaction during coal pyrolysis

The release mechanisms of NH₃, HCN, and HNCO during coal pyrolysis remain unclear, especially the lack of attention to HNCO. Their release characteristics and mechanisms influenced by CO were studied deeply by TG-MS experiments and DFT calculations, respectively. The experiments indicated that NH₃ primarily originates from the direct hydrogenation of coal-N rather than the hydrogenation of HCN during coal pyrolysis. A CO-enhanced HNCO release and a CO-reduced release of NH₃ and HCN are simultaneously proven. The calculations demonstrated that the adsorption of CO on coal surface increases the number of O-containing functional groups and modifies coal-N release reaction pathways and interatomic interaction forces. Consequently, CO elevates the rate-determining step energy barriers of NH₃ and HCN release reactions from 180.8kJ·mol⁻¹ to 240.7 kJ·mol⁻¹ and from 212.8 kJ·mol⁻¹ to 254.6 kJ·mol⁻¹, respectively, but provides a more competitive release pathway for HNCO with a low-energy barrier of 213.3 kJ·mol⁻¹. More N₂O emissions will be found due to the reaction between the enhanced HNCO and NO in CFB, while the regulation of free radicals in CFB, especially H radicals, has the potential to reduce N₂O emissions caused by CO. In this study, the release mechanisms of HNCO, NH₃, and HCN influenced by CO during coal pyrolysis were clarified for the first time, which is a key supplement for the theory of nitrogen migration and transformation during coal pyrolysis.