Toward the Development of an NH3-RRQL System—Part II: Effects of the Primary Combustion Zone Length and Secondary Stage Number of Holes on Stability and Emissions

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.