Investigation on primary zone length and secondary stage design effects on nitrogen oxides emissions in ammonia combustion systems, suggesting design enhancements for stability.
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 mm, 178 mm) were tested using a 5-hole (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-hole 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. Diffusion-like combustion was seen for 0.90 ≤ ϕglobal = 1.10, leading to inefficient combustion and excess O2. Optimal performance was achieved with ϕprimary = 1.13 and 0.70 = ϕglobal = 0.90, with estimated burner outlet temperatures between 1720–1970 K.
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Jimenez et al. (2025) studied this question.