Two-stage air injection for NOx reduction in ammonia-hydrogen flameless combustion

Ammonia-hydrogen blends are emerging as promising carbon-free fuels for gas turbine and industrial combustion systems. However, lean combustion typically produces high NO x emissions due to fuel-bound nitrogen in ammonia and elevated concentrations of reactive O/H radicals, while rich combustion leads to excessive unburned ammonia and hydrogen emissions. This study addresses these challenges using a novel two-stage air-injection strategy integrated with flameless combustion to achieve low emissions over a wide equivalence ratio range (0.4 ≤ ϕ ≤ 1.3). Numerical simulations using a chemical reactor network (CRN) coupled with CFD analysis demonstrate that two-stage combustion significantly suppresses NO x formation across lean, near-stoichiometric, and rich regimes. The staged configuration substantially lowers unburnt NH 3 and H 2 emissions in rich mixtures. CFD results indicate improved dilution characteristics in the two-stage mode (R dil ≈ 4) compared to single-stage combustion (R dil ≈ 1), contributing to highly recirculated and distributed reaction zones. Experimental validation in a 30 kW conical combustor confirms the numerical predictions. For a 70/30 NH 3 /H 2 blend, NO x emissions as low as 83–207 ppm were achieved under lean conditions (ϕ = 0.4–0.6), with optimal combined NO x and NH 3 slip values of 200 ppm and 115 ppm at ϕ = 1.1. Peak NO x emissions are reduced by 20–30% compared to single-stage operation, while lean-condition reductions reach up to 70–80%. Optical diagnostics further revealed a more uniform intensity distribution in the two-stage configuration, confirming enhanced mixing and flameless characteristics. Overall, the proposed strategy demonstrates a viable pathway for low-emission, carbon-free combustion systems for industrial applications.