Abstract: Ammonia (NH3), a carbon-free and hydrogen-rich fuel, can be co-fired with pulverized coal in conventional coal-fired units to reduce carbon emissions at the flame source. In this study, a 300 MW tangentially fired boiler is taken as the reference case. A three-dimensional CFD model coupling NH3/coal co-combustion, particle dynamics, and NOx formation pathways is developed to investigate how the co-firing elevation and co-firing ratio affect combustion and NOx emission characteristics. The model employs an improved realizable k–ε turbulence closure, the discrete ordinates (DO) radiation model, and the weighted-sum-of-gray-gases model (WSGGM) for gas radiative properties, and consistently accounts for coal devolatilization, char oxidation, and ammonia pyrolysis/oxidation. The modeling framework is validated against one-dimensional drop-tube furnace experiments. Subsequently, full-scale boiler simulations are performed. The results indicate that NH3 co-firing slightly lowers the flue-gas temperature near the furnace exit. Moreover, as the co-firing location is shifted to lower elevations, both NH3 slip and NO emissions decrease significantly. Under the same NH3 co-firing ratio, the B+C dual-layer configuration achieves more effective NO control and reduces NH3 slip compared with the C+E arrangement. These findings provide theoretical guidance for operating-condition optimization and low-NOx combustion control in coal-fired units co-firing ammonia.