Investigation of the pure ammonia flames in a novel two-stage burner

Ammonia's slow chemical kinetics and fuel-bound NOx emissions present challenges for combustion applications. However, its carbon-free nature, easy storage and transport, and high hydrogen content have attracted growing research interest. To improve the burning efficiency of pure ammonia flames, this study introduces a novel burner design with two reactor sets, aimed at achieving stable ammonia-air flames with reduced NOx zero NH₃ slip. Experiments are conducted and successfully stabilised the pure ammonia flames under the wide range of global equvalence ratios (0.3-1.3) and thermal intensities (∼1.5 MW/m³ to ∼9.2 MW/m³). Emissions of NH₃, NO, and NO₂, along with temperatures at various combustor levels, are measured. Computational simulations using Large Eddy Simulation (LES) are conducted to study flame dynamics and mixing in pure ammonia flames. The results indicated that the new burner design enhanced flame stability (0.3-1.3), improved mixing, achieved nearly zero NH₃ slip, and reduced NOx levels in non-premixed ammonia-air flames. Both experimental and predicted data revealed that higher thermal intensities are key to reducing NH₃ and NOx emissions across all equivalence ratios. At lower thermal outputs (10 kW and 20 kW), minimal NH₃ emissions were noted at rich conditions (1.3), while higher thermal outputs completely eliminated NH₃ emissions. The burner's air staging and recuperative design resulted in lower NO emissions compared to previous studies, with the lowest NO levels (420, 302, 390, and 299 ppm) at 10, 20, 40, and 60 kW, respectively, without NH₃ emissions. Rich conditions produced well-distributed flames at 40 kW and 60 kW. A chemical reaction network (CRN) analysis showed the influence of O₂ availability and thermal intensities on NO emissions, confirming that uniform mixing from tangential air inlets effectively controlled ammonia consumption.