Abstract: Ammonia (NH₃) is a carbon-free and hydrogen-rich fuel. NH₃ co-firing in coal-fired boiler is an effective approach to reduce carbon emissions from the combustion source. However, it can lead to problems such as high NO_x emissions and poor coal burnout. To address the issues above, this paper investigated the Fuel-N conversion and NO_x emissions during co-firing 20%NH₃ in gasifier (G-20% NH₃) and in combustor (C-20%NH₃) using a self-built mild gasification-combustion experimental rig. The research found that the co-firing NH₃ in the gasifier can decrease the gasifier temperature. However, it has little effect on the coal gasification reactions in gasifier. Compared with a pure coal condition, the conversion ratio of coal-nitrogen was only reduced by 3.64%, and the conversion ratio of volatile matter and fixed carbon decreased by only 0.70% and 2.54%, respectively. Besides, the conversion of NH₃ in the gasifier can reach 69.55% during the condition of G-20%NH₃. The 68.88% of total fuel-N was conversed to N₂ in the gasifier, with NH₃-N converting to N₂ was 67.73% in G-20%NH₃. In addition, the NH₃ co-firing in the gasifier promoted the transformation of coal-N and NH₃-N to HCN, with the value of 0.36%. Moreover, the coal pyrolysis in the gasifier promoted the NH₃ pyrolysis. Compared with pure coal, the NH₃ co-firing in the gasifier increased the H₂ concentration at the outlet of the gasifier by 69.23% than that of pure coal. The NH₃ co-firing also decreased the temperature during the preliminary stage of combustion. Compared with pure coal, the NH₃ co-firing in the gasifier and combustor decreased the temperature peak by 46 ℃ and 62 ℃, respectively. Simultaneously, the NH₃ co-firing delayed the occurrence of temperature peaks. Additionally, the NH₃ co-firing delayed the gasified char combustion in the combustor. The average temperature in the burnout zone increased by 135.8 ℃ and 72.8 ℃ during G-20%NH₃ and C-20%NH₃, respectively, compared to PC. It was more conducive to the burnout of pulverized coal. The NH₃-cofiring in combustor had more significant promoting effects on the burnout of pulverized coal, in which the unburned carbon in fly ash decreased by 1.8% compared PC. However, the NH₃ co-firing in gasifier was more effective in reducing NO_x emissions. During the G-20%NH₃, NO emissions were decreased by 23.51% compared to C-20%NH₃. The study offers some innovative ideas and data reference for the development of low NO_x emission technology for NH₃ co-firing in coal-fired boiler.