Abstract: Ammonia cofiring in coal-fired boilers is one of the promising technical routes for the decarbonization of coal-fired power plants. However, ammonia cofiring could potentially result in drastic increase of NO_x emissions due to its high nitrogen content. Effective control of NO_x emissions is thus one of the key factors that affect the technical feasibility of ammonia cofiring in coal-fired boilers. Therefore, the divergent trends of NO_x emissions with respect to NH₃ cofiring ratio （R_\mathrmNH_3 ） observed in experimental studies were systematically reviewed. A unified mechanism underlying these divergent trends was proposed — the net NO formation is determined by the competition between the NO formation and reduction reactions of NH₃ in the varying O₂ environment of the furnace. In a boiler environment, NO_x emissions are jointly determined by the NO formation during the initial stage of combustion in the main combustion zone, NO reduction by NH₃ in the reduction zone, and NO formation by the oxidation reaction of residual NH₃ with staging air in the burnout zone. The NO formation-reduction-formation processes can add up to generate a variety of NO_x emission trends. Therefore, the divergent NO_x trends observed in the experiments should not be simply attributed to the effects of NH₃ cofiring mode or ratio but should comprehensively take into consideration the resultant changes of NH₃ combustion environment brought about those different NH₃ cofiring conditions. Based on the above NO formation mechanism of NH₃-coal cofiring, the key factors that should be considered in engineering NO_x prediction model of NH₃ cofiring were further elucidated, with particular emphasis on the necessity of converting the key boiler design and operating parameters, which directly affect the furnace flow and O₂ distributions, to the boundary conditions of the model. By simulating NH₃ cofiring in a 40 MW boiler and a 600 MW boiler, respectively, the results by different NO models were compared and validated. Results indicated that the modified Østberg mechanism showed good qualitative and quantitative agreement with the testing results. Furthermore, the results revealed a distinctive NO_x formation characteristic of NH₃ cofiring. Although NH₃ combustion may generate a large amount of NO, due to the rapid combustion consumption of O₂ by NH₃, an O₂-deficient NO reduction zone is formed adjacent to the high NO formation zone in which the initially formed NO is going to be immediately reduced by the residual NH₃. This characteristic contributes to a substantial reduction in the net NO production of NH₃ cofiring.