A combustion strategy for multi-emission control via secondary ammonia injection in a swirl combustor

Ammonia has received extensive attention from the combustion community as hydrogen carrier due to its carbon-free nature. However, fuel-NOx has hindered the application of ammonia in combustion devices such as gas turbines. This study proposes a low NOx strategy by secondary ammonia injection based on the two-stage combustion. Furthermore, the low-emission effect has been experimentally and computationally validated in a generic swirl combustor. Experimentally, the emission concentrations from the two-stage combustion chamber were measured. A chemical reaction network (CRN) analysis was conducted to investigate the NO reduction/formation mechanisms under the influence of secondary ammonia. Large Eddy Simulation combined with the Flamelet-Generated Manifold (LES-FGM) method was employed to reveal the evolution of NO in the secondary combustion zone and its final emission process. Experimental results demonstrate that replacing a small volume of secondary air with NH₃ in the current combustion chamber can maintain NO emissions below 450 ppm across the entire operation range, without deteriorating other emissions. Furthermore, efficient low-NOx combustion can be achieved within the equivalence ratio range of 0.9 to 1.05 in the primary zone (Φ_pri=0.9–1.05). CRN calculation results indicate that the optimal secondary ammonia ratio (R_NH3) under current operating conditions is around 0.4%, at which NO reduction ratio can exceed 60% while effectively limiting ammonia slip. Rate of production (ROP) analysis reveals that NO reduction mainly occurs through the NH₂ and NH pathways, with N₂ as the final reduction product. LES results confirm that a small amount of ammonia introduced via secondary air enhances NOx reduction by promoting NH₂ formation and its inward transport, thereby strengthening SNCR effects. Additionally, R_NH3=0.4% is also identified as the optimal ratio for achieving effective emission control.