An investigation of ammonia primary flame combustor concepts for emissions reduction with OH*, NH2* and NH* chemiluminescence at elevated conditions

With developing interest in NH3 as a prospective energy carrier, combustor designs and fuelling concepts require optimisation to reduce NOx emissions. Through the introduction of staged combustor concepts, pathways have previously been identified that limit NOx production whilst improving combustor efficiency and reducing unburned NH3. However, the efficacy of secondary air staging is sensitive to the primary flame behaviour, and whilst low NOx emissions can be achieved at rich conditions, high unburned NH3 leads to greater global NOx concentrations from downstream production. Here, time-resolved OH*, NH2* and NH* chemiluminescence were employed together for the first time for NH3-air and NH3 H2-air flames to investigate a primary flame configuration that produced the lowest combined emissions concentration. A generic, fuel-flexible burner was developed to enable partial and full premixing, together with operation of a swirl-stabilised non-premixed flame. Initially, NH3 H2-air flames were employed in a range of configurations and produced markedly different chemiluminescence and emissions results as functions of global equivalence ratio. The performance of a pure NH3-air flame was subsequently investigated and compared to the blended fuel results. Optical trends complemented changes in sampled exhaust emissions, enabling analysis of intermediate chemistry. Burner inlet temperature and pressure were then increased proportionally to maintain equivalent bulk nozzle exit velocities. Contrasting trends were identified as functions of fuel composition and equivalence ratio, with a comprehensive database of optical and analytical results generated. Results obtained for NH3 H2-air suggest the most favourable configuration resulted from a partially premixed flame employing H2 as a pilot, operating under rich conditions (Φ=1.2). However, at higher temperatures and pressures, the trends observed for non-premixed NH3-air flames will lead to superior performance, particularly with a small increase in equivalence ratio.