Abstract: Ammonia-coal cofiring is one of the most promising technical routes to realize decarbonation of coal-fired power plants. However, due to the high nitrogen content of NH₃, ammonia-coal cofiring may lead to significant increase of boiler NO_x emission. This may become one of the key problems restricting the implementation of ammonia cofiring in coal-fired boilers. Therefore, it is imperative to study the NO_x formation characteristics of ammonia cofiring in coal-fired boilers, so as to guide the development of effective NO_x control methods. The present study numerically investigates the effects of ammonia cofiring ratio （R_\mathrmNH_3 ） on the NO_x characteristics of boiler in a 40 MW industrial scale boiler. The simulation results of boiler NO_x emission show that, under 20% overfire air flow rate, boiler NO_x emission increases first and then decreases with the increase of R_\mathrmNH_3 , reaching maximum value of 197 mg/m³ when R_\mathrmNH_3 =5%, and dropping to 95 mg/m³ when R_\mathrmNH_3 =25%, which is lower than 137 mg/m³ of pure coal combustion. The simulation results are in good agreement with the ammonia cofiring testing results of 40 MW boiler both qualitatively and quantitatively. Such trend of boiler NO_x emissions with the increase of R_\mathrmNH_3 is attributed to the competing reaction pathways between NO formation and reduction reactions in the process of NH₃ combustion. The formation of NO in the boiler is mainly composed of the initial NO formation in the high O₂ environment in the early stage of combustion and the NO reduction in the low O₂ environment in the later stage of combustion. Boiler NO_x emission is jointly determined by the formation and reduction of NO in these two stages. The formation and reduction rates of NO in the furnace both increase with the increase of R_\mathrmNH_3 . However, in the range of R_\mathrmNH_3 = 0−5%, the formation rate of NO increases faster than that of NO reduction, while in the range of R_\mathrmNH_3 = 5%−25%, the reduction rate of NO reduction increases faster than that of NO formation. As a result, the net formation of NO exhibits an increase-then-decrease trend as R_\mathrmNH_3 is increased from 0 to 25%. This is the reason why it was observed in the ammonia-coal cofiring testing of 40 MW boiler that the NO_x emission increased first and then then decreased with increase of R_\mathrmNH_3 . The simulation results reveal the key influence of the competition mechanism between the NO formation and reduction reaction pathways of NH₃ combustion on the NO_x emission of ammonia-coal cofiring, which is of great significance to realize effective NO_x control of ammonia cofiring in large scale coal-fired boilers.