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.