Ammonia co-firing is a promising approach for reducing the carbon intensity of existing coal-fired power plants; however, its application to large utility boilers requires simultaneous consideration of combustion stability, coal burnout, and nitrogenous emissions. In this study, three-dimensional computational fluid dynamics simulations were conducted for a full-scale 1000 MW ultra-supercritical opposed-wall-fired pulverized-coal boiler to investigate the effects of ammonia co-firing ratio, burner-level ammonia injection position, and overfire-air (OFA) ratio. The numerical model was validated against plant measurements of gas temperature, outlet O₂ concentration, unburned carbon (UBC), and NO emissions. The ammonia thermal co-firing ratio varied from 0% to 60% in 10% increments, and the effects of ammonia injection elevation and OFA ratio were further examined at a practically relevant 20% co-firing condition. Increasing the ammonia fraction progressively redistributed and fragmented the high-temperature reaction zone, while the outlet CO₂ concentration decreased from 15.23 vol.% under coal-only firing to 5.44 vol.% at 60% ammonia co-firing. In contrast, UBC and NO exhibited non-monotonic behavior, with the most pronounced deterioration occurring in the intermediate co-firing range around 30% NH₃ due to the mismatch among local oxygen availability, coal-particle conversion, and NH₃ reaction pathways. At 20% ammonia co-firing, lower-position injections provided the most favorable combined UBC and NO performance by promoting NH₃-assisted NO reduction in the oxygen-deficient lower furnace while maintaining sufficient downstream residence time for coal burnout. Upper-position injections improved coal burnout but showed a weaker NO-reduction effect, whereas middle-position injection caused the greatest interference with coal combustion. Increasing the OFA ratio shifted heat release and residual-char burnout toward the upper furnace, and the 31–36% OFA range showed the most favorable combined tendency for UBC and NO, with the lowest outlet NO obtained at 36% OFA. These results demonstrate that high-ratio ammonia co-firing in large utility-scale coal boilers should be optimized through coordinated control of the ammonia co-firing ratio, burner-level injection position, and air staging to achieve carbon reduction while maintaining acceptable combustion and emission performance.
 

Ammonia co-firing; Wall-firing boiler; Computational fluid dynamics; NO emissions; Overfire air