Chemical Reaction Kinetics Simulation Study on NO_x Generation Characteristics of Coal Combustion Coupled with Ammonia Heterogeneous Reaction

Ammonia/coal co-firing in coal-fired boilers can effectively reduce carbon emission, but it may lead to an increase in NO_x emission during the co-firing process. The influence of coal properties and ammonia distribution strategy on NO_x generation characteristics and reaction mechanism of ammonia/coal co-firing over a wide range of ammonia co-firing ratios need to be investigated further. An ammonia/coal heterogeneous reaction model was proposed, and the effects of co-firing ratio, coal properties and ammonia distribution strategy on the NO_x generation characteristics were investigated through the chemical reaction kinetics simulation method. Results show that the NO emission increases and then decreases with the increase of NH₃ ratio, and the NO content in the combustion process first increases dramatically to the peak and then decreases gradually. The conversion rate of NO increases and then decreases with the increase of coal volatile matter mass fraction at NH₃ ratio x=20%, while the conversion rate of NO decreases and then increases at x=50%. The conversion rate of NO increases with the increase of nitrogen mass fraction. By changing the ammonia distribution strategy, the emissions of both NO and NO₂ are lower than that of all NH₃ entering the main combustion area with the fuel when the proportion of NH₃ in the burnout zone ranges from 20% to 60%. Sensitivity analysis reveals that NH₃ derivatives such as NH, NH₂, N₂H₂, as well as functional groups such as OH and H, have a significant impact on the conversion of NO during ammonia co-firing. The analysis of conversion pathways shows that NH₃→NH₂→HNO/NH→NO and HCN→NCO→NO are the main pathways for NO generation. HNO and NH are key substances for NO generation, and NO reduction by NH₂ and NH can promote NO consumption, thereby reducing the generation of nitrogen oxides. The study is helpful to optimize the ammonia co-firing ratio and distribution strategy, thereby further reducing NO_x emission and providing theoretical support for the organization and parameter selection of ammonia/coal coupled combustion in actual boilers.