On Flame Morphologies and Stabilities of NH3/Air Premixed Flames in a Dual-Swirl Gas Turbine Model Combustor: A Comparative Study of NH3 and CH4 as Pilot Fuel

Substituting hydrocarbon fuels such as methane (CH4) with ammonia (NH3) reduces CO2 emissions in gas turbines, but ammonia’s low reactivity challenges flame stability. Dual-swirl staged combustors using a low thermal power (Ppilot) pilot flame can stabilise the main flame. This work compares the morphologies and stabilities of NH3/air premixed swirl flames using ammonia and methane pilot flames (APF and MPF). Flame imaging and simultaneous OH-NH planar laser-induced fluorescence (PLIF) are employed to analyse flame morphology. Main flame stability is assessed by measuring the lean blow-off equivalence ratio (ϕb,main). The results show that MPF significantly outperforms APF in main flame stabilisation. At Ppilot = 1.2–1.8 kW (14.2–21.3% of Pmain), the dual-swirl flames exhibit a stratified structure, with OH concentrated in the pilot stage. Flames with MPF exhibit considerably lower ϕb,main than those with APF. For example, at Ppilot = 1.6 kW, ϕb,main is reduced to 0.42 with MPF, compared to 0.56 with APF, demonstrating MPF’s superior stabilisation capability. MPF can reduce CO2 emissions by 82.4–87.6% compared to a CH4 flame of equivalent thermal power. Two stabilisation modes are identified, namely primary recirculation zone-dominated and pilot-dominated modes. These findings demonstrate that a low-power MPF provides an effective strategy for enhancing ammonia flame stability and reducing CO2 emissions in gas turbines.