This work presents an experimental study on the influence of the pilot flame characteristics on the flame morphology and exhaust emissions of a turbulent swirling flame. A reduced-scale burner, inspired by that fitted in the AE-T100 micro gas turbine, was employed as the experimental platform to evaluate methane (CH₄) and an ammonia-methane fuel blend with an ammonia (NH₃) volume fraction of 0.7. The power ratio (PR) between the pilot flame and the main flame and the fuel composition of the pilot flame was investigated. The pilot power ratio was varied from 0 to 20% for both fuel compositions tested. The NH₃ volume fraction in the pilot flame ranged from pure CH₄ to pure NH₃ through various NH₃–CH₄ blends. Flame images and exhaust emissions, namely CO₂, CO, NO, and N₂O were recorded. It was found that increasing the pilot power ratio produces more stable flames and influences most of the exhaust emissions measured. The CO₂ concentration in the exhaust gases was roughly constant for CH₄-air or NH₃–CH₄–air flames. In addition, a CO₂ concentration reduction of about 45% was achieved for X_NH3 = 0.70 compared with pure CH₄, while still producing stable flames as long as PR ≥ 5%. The pilot power ratio was found to have a higher relative impact on NO emissions for CH₄ than for NH₃–CH₄, with measured exhaust NO percentage increments of about 276% and 11%, respectively. The N₂O concentration was constant for all pilot power ratios for CH₄ but it decreased when the pilot power ratio increased for NH₃–CH₄. The pilot fuel composition highly affected the NO and N₂O emissions. Pure CH₄ pilot flames and higher power ratios produced higher NO emissions. Conversely, the NO concentration was roughly constant for pure NH₃ pilot flames, regardless of the pilot power ratio. Qualitative OH-PLIF images were recorded to further investigate these trends. Results showed that the pilot power ratio and the pilot fuel composition modified the flame morphology and the OH concentration, which both influence NO emissions.

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.

Ammonia (NH3) is a carbon–free energy source and hydrogen carrier, but its fuel–bound nitrogen can lead to significant nitrogen oxides (NOx) emissions. Staged combustion strategies, such as rich–quench–lean, can achieve low NOx emissions. However, improper design may result in high NOx levels when operating with a rich head end without a sufficient post–flame relaxation time. Previous work has shown that a rich–relaxation–quench–lean (RRQL) configuration can minimize NOx emissions in the rich head end (Cole et al., 2024, “Rich Ammonia Flame Shapes and NO Relaxation: Facility Development and Characterization,” ASME Paper No. GT2024–122369). This study focuses on how different swirl geometries affect exhaust emissions and flame morphology in rich, premixed NH3–air flames, for the design of a rich relaxation head end of an RRQL combustor. Experiments were conducted using a modular swirl burner and measuring NOx, N2O, and NH3 emissions, and recording natural flame luminosity and NH2* and OH* chemiluminescence images. Swirler design affects emissions by influencing flame length (with shorter flames allowing for more postflame relaxation time) and flame–wall interactions (influencing NH3 and N2O emissions). A compact flame with minimal wall interactions allows for increased NOx relaxation, while minimal wall interactions prevent heat losses and instabilities, which minimize NH3 and N2O emissions at a richer equivalence ratio, potentially enhancing H2 production during the relaxation phase without increasing overall primary stage NOx emissions, thereby improving system efficiency. Detailed spatial evolution of NOx, N2O, and NH3 emissions further supports the RRQL as a promising combustor design for NH3 combustion, if the rich head end is designed properly.