Two-Stage Combustion of Ammonia/Hydrogen Blends: An Experimental Approach to Low-Carbon Fuel Utilisation

This study presents an experimental investigation of a laboratory-scale two-stage RQL-based combustor burning NH3/H2 fuel blends at a thermal input of 8.0–8.5 kW and atmospheric pressure using fuels containing 70–100% ammonia by volume in the first stage and 80–100% in two-stage operation. The first stage operated under fuel-rich conditions (ϕ1 = 1.2–1.4), while the second stage operated under globally lean conditions (ϕG = 0.5–0.9). Unlike conventional rich–quench–lean combustors, the secondary stage allows controlled hydrogen injection upstream of the secondary air inlet to enhance ammonia burnout. Temperature profiles and exhaust NH3, NOx and O2 concentrations were measured with and without secondary fuel injection. The burner operated stably over a wide range of fuel compositions, including pure ammonia, while maintaining reduced NOx emissions in the rich stage. Two-stage operation exhibited a clear trade-off: secondary hydrogen reduced ammonia slip, but NOx emissions increased above the 59 ppm and 24 ppm reference benchmarks under most conditions and exceeded 200 ppm in some cases. Incremental secondary hydrogen injection, tested with pure ammonia at ϕ1 = 1.3, ϕG = 0.6, identified an operating window of approximately 100–300 W, within which ammonia slip decreased to 10–20 ppm with a moderate increase in NOx above 300 W. Further reduction in ammonia slip was limited while NOx emissions continued to increase.