Exploring optimal secondary ammonia injection strategies for efficient low-NOx emissions in a fuel-stage swirl combustor

Ammonia, as a carbon-free fuel and suitable hydrogen carrier, has attracted increasing attention for gas turbine applications. However, high NOx emissions present a significant environmental challenge. This study explores a novel fuel-staging strategy by injecting a small amount of secondary ammonia after the primary combustion zone. The effects of secondary ammonia injection ratio (R_NH3) and secondary air inlet configuration on emissions were investigated. Flame structures were visualized using Planar Laser-Induced Fluorescence (PLIF) and digital photography. Main emissions were also quantified via an exhaust gas analyzer. Mechanisms of NOx formation and reduction were further examined using Chemical Reaction Network (CRN) method with a detailed mechanism in combination with the three-dimensional numerical simulation. Results reveal a trade-off behavior between NOx reduction and unburned ammonia (NH3) slip: increasing R_NH3 improves NOx reduction but elevates NH3 emissions. The emission characteristics also strongly depend on the swirl direction of the secondary air. For the Co-Swirl (CoS) configuration, the optimal R_NH3 is 0.2 %, reducing NOx by 36.3 % with a combustion efficiency of 99.7 % and N2O emissions below 100 ppmv. For the Counter-Swirl (CnS) configuration, the optimal ratio is 0.4 %, achieving 44.2 % NOx reduction with 99.5 % efficiency and similarly low N2O. CRN analysis highlights that secondary ammonia promotes the NH2 + NO = N2 + H2O pathway, enhancing NOx consumption rates by 1–2 orders of magnitude. The simulation results indicate that the CnS configuration yields superior mixing (mixing coefficient Ω>0.9), suppressing ammonia slip and shortening the length of the SNCR reaction zone, which benefits compact combustor design.