Abstract: The effect of high proportion ammonia co-firing on combustion and furnace heat transfer of coal-fired boilers is still unknown. This paper investigates a 600 MW tangentially-fired coal boiler using numerical simulations to explore the effects of different ammonia co-firing ratios on combustion and heat transfer characteristics. It analyzes the compatibility and matching characteristics of the boile’s combustion and heat transfer with ammonia co-firing. The results indicate that within the ammonia co-firing ratio range of 0−50%, the impact of ammonia co-firing on the furnace velocity field is minimal, and the tangentially-fired combustion pattern remains effective. Although the longitudinal gas flow velocity in the furnace increases slightly, it does not affect combustion stability. Compared with pure coal combustion, the average flue gas velocity under 50% ammonia co-firing demonstrates a 9.4% increase, with furnace outlet flue gas velocity rising by approximately 1.5 m/s. This enhancement is attributed to the 10% growth in total flue gas volume generated during 50% ammonia-coal combustion relative to pure coal combustion. After ammonia co-firing, the in-furnace flame temperature significantly decreases. This thermal behavior primarily stems from the inherent lower theoretical flame temperature of NH₃ compared to coal. Under constant total heat input conditions, the temperature decrease results from the combined effect of increased flue gas flow rate and specific heat capacity during ammonia co-firing. Furthermore, as the ammonia co-firing ratio increases, the flue gas temperature at the arch throat cross-section decreases slightly. The high-temperature region at the arch throat is primarily concentrated in the central area of the furnace. This indicates that ammonia co-firing in the range of 0−50% does not lead to excessive wall temperatures or slagging, thereby ensuring the safe and stable operation of the boiler. However, the furnace’s radiative heat transfer capacity is reduced. The molar fraction concentration of NH₃ at the furnace outlet is nearly zero, indicating complete combustion of ammonia with no leakage. After ammonia co-firing, the heat flux distribution on the furnace wall becomes more uniform compared to pure coal combustion, while the overall heat transfer in the furnace is slightly reduced. In summary, the 600 MW tangentially-fired coal boiler demonstrates good compatibility in combustion and heat transfer for ammonia co-firing ratios between 0−50%. The study’s conclusions can provide theoretical and technical guidance for the application of ammonia co-firing technology in coal-fired boilers.