Mechanistic insights into NO reduction during ammonia-coal co-combustion: A DFT study on char surface functional groups

Nitrogen oxides (NO_x) in flue gas generated by coal combustion can cause severe environmental pollution, and ammonia is regarded as a promising zero-carbon energy source. However, ammonia also has limitations, including low flammability and NO_x emissions. Ammonia-coal co-combustion technology is a mainstream choice. Relevant research indicates that the presence of oxygen can assist in the reduction of NO by creating new active sites. Previous studies on ammonia-coal co-combustion technology combined with air-staged combustion were mainly based on experimental research, lacking exploration of the reaction mechanism. Therefore, we used density functional theory (DFT) to explore the effects of dangling bonds, C=O, and −OH groups on the reactivity of char, NO reduction pathways, and reaction kinetics. The conclusion indicates that the Zig@OH model has the strongest NO adsorption capacity. Both –OH and C=O groups are beneficial to shortening the reaction pathway and promote the formation of N-N structure. Therefore, O₂ promotes the co-combustion of ammonia and coal, which is consistent with the experimental results. The Zig@O model has the lowest rate-determining step energy barrier. The rate-determining step energy barriers of the three models are not significantly different overall. The kinetic calculation illustrated that within the temperature range of 800–1600 K, three char models with ammonia can effectively promote the reduction of NO. This study aims to provide new insights into the microscopic mechanism of the NO/O₂/NH₃-char reaction in ammonia-coal co-combustion technology.