| description abstract | Abstract. Staged combustion systems like the rich–relaxation–quench–lean (RRQL) offer the potential for low nitrogen oxides (NOx) emissions while burning ammonia (NH3). This process involves rich premixed NH3–air combustion in a primary zone, allowing time for NOx relaxation, followed by a lean secondary combustion zone via air injection. However, improper design of the secondary stage can lead to high NOx and nitrous oxide (N2O) emissions, offsetting the climate benefits of carbon-free fuels. This study investigates the effects of primary zone length and secondary stage geometry on the stability and emissions of a lab-scale RRQL system. Experiments were conducted at atmospheric pressure using a modular axial swirl burner (swirl number = 1.1, 16 vanes), with primary equivalence ratios (ϕprimary) of 1.13, 1.15, and 1.18. Two quartz lengths (76 and 178 mm) were tested using a five holes (2.03 mm) secondary injection design. Strong flame interaction and elevated NOx were observed for the 76 mm liner. A longer chamber allowed better NOx relaxation and NH3 cracking. Additional tests with 5-, 10-, and 16-holes configurations showed that fewer holes, implying higher momentum flux ratios, yielded lower NOx–N2O emissions, especially at ϕprimary = 1.13. These effects diminished as ϕprimary decreased from 1.18 down to the optimum 1.13. Diffusion-like combustion was seen for 0.90 ≤ ϕglobal ≤ 1.10, leading to inefficient combustion marked by excess O2 values compared with equilibrium. Optimal performance was achieved with ϕprimary = 1.13 and 0.70 ≤ ϕglobal ≤ 0.90, with estimated burner outlet temperatures between 1720 and 1970 K. | |
