Experimental study on pollutant emissions and fly ash characteristics in ammonia-bituminous coal co-firing under different ammonia injection modes in a swirl burner

This study investigates ammonia-bituminous coal co-firing using a 200 kW down-fired combustion system, examining three ammonia co-firing methods—premixed (PM) and staged modes (SM(A)/SM(B))—at 0–30 % ratios. The research systematically analyzes their effects on combustion characteristics, flue gas emissions, and fly ash properties within a swirl burner configuration. The results demonstrate that the premixed ammonia injection method exhibits minimal impact on flame temperature distribution, while staged ammonia injection shifts the peak temperature location downward within the flame structure. CO/CO₂ decreased after ammonia co-firing due to the carbon-free property of ammonia. NO_x generation exhibited mode-dependent behavior: under PM mode, it followed an initial increase then a decreasing trend, recovering to pure coal levels at 30 % ammonia blending ratio. SM(A) mode achieved minimum NO_x at a 20 % co-firing ratio, while SM(B) mode showed a monotonic rise with increasing ammonia fraction. Under PM mode operation at 10 % ammonia co-firing, the NO_x was reduced by 63.3 % when the OFA ratio was increased from 10 % to 30 %, which verified the effectiveness of the hierarchically optimized ammonia-coal co-firing process for NO_x control. After ammonia co-firing, the unburned carbon content of fly ash was reduced, the average particle size Dv₅₀ decreased, the proportion of 2–10 μm fine particles increased, ammonia doping promoted the generation of spherical Al/Si/Fe oxides, and the porous flocculent decreased. This study provides a key design basis for the efficient and clean combustion of ammonia-coal in swirl burners.