Abstract: Ammonia-coal co-firing involves complex chemical reaction processes, and it is highly concerned that whether co-firing exacerbates the generation of nitrogen oxides. Using the method of chemical reaction kinetics calculation, the reaction pathways of NO generation and reduction in the gas-phase reaction of ammonia-coal co-firing were studied, as well as the effects of different factors on NO generation. Results show that compared to pure coal combustion, the NO volume fraction at the outlet of the reactor can be reduced by 96.5% with the ammonia co-firing ratio of 0.3 at temperature (T) of 1 300 ℃ and excess air coefficient (α) of 0.84. Since ammonia decomposition generates a large number of free radicals such as NH₂ and NH, which leads to a faster reduction reaction rate and the conversion of N-containing elements into N₂ under co-firing working condition. At T=1 300 ℃, an ammonia co-firing ratio of 0.3 is suitable, since the volume fractions of NO and NH₃ at the reactor outlet are both low. Small ammonia co-firing ratio will increase the rate of NO generation and NH₃ decomposition, leading to an increase in NO volume fractions at the reactor outlet. The higher the temperature is, the more obvious this phenomenon is. Reducing atmosphere (α<1) and a lower temperature (T≤ 1 300 ℃) can effectively reduce NO emissions.