Abstract: Co-firing ammonia in coal-fired boiler is significantly important for carbon dioxide emission reduction, and China has proposed a development target of co-firing ammonia in coal-fired boiler with a co-firing ratio of 10%. While, co-firing with ammonia is probable to cause some serious problems, like severer NO emission and larger challenge in NO emission control. Accordingly, this numerical work presents a deep insight into the effects of co-firing strategy, ratio and air supply on ammonia conversion and NO emission in the co-firing ratio range of 0–20%, based on experiments in a 45 kW down-fired furnace. Results show that ammonia conversion and NO emission are fairly dependent on the specific ammonia co-firing strategy. For ammonia injection in the burner, applying separated ammonia channel is more beneficial for NO emission control, compared with blending it into the O₂-contained primary air; For ammonia injection in the furnace, NO emission is quite sensitive to its specific position, and increasing the distance between it and the coal burner is beneficial for NO emission reduction. In addition, increases of ammonia co-firing ratio and burnout air ratio cause more and less NO emission, respectively. Besides, there is a remarkable difference of ammonia conversion paths between the strategies of ammonia injection in the burner and the furnace, which are dominated by the competition between NO formation and reduction and the path of ammonia decomposition, respectively. Accordingly, a new co-firing mode named the double-position mode is introduced, where ammonia injections in the burner and the furnace are combined. Numerical results show the NO emission concentration in the new mode is lower than that in the other two strategies with only one injection position, and the lowest NO emission concentration reaches 76 mg/m³. Based on the further mechanism analysis, it is found that the variation nature in the ammonia conversion process is significant for NO emission control. Low NO emission can be achieved through enhancing the advantage of burner-blending in ammonia consumption and that of furnace-injection in NO reduction.