The blow-off and transient characteristics of co-firing ammonia/methane fuels in a swirl combustor

Recent studies have demonstrated that ammonia could be one of the most promising hydrogen carrier candidates which can be used in large-scale power plants. However, it is challenging to burn ammonia in gas turbines due to its narrow flame stabilization limits. In this study, the blow-off characteristics and flame macrostructure transition behavior of ammonia/air flame (NH₃ flame) and ammonia/methane/air flame (50%NH₃ flame) in a swirl combustor were investigated. Flame structures and flow fields near the blow-off limit were analyzed. Methane/air flame (CH₄ flame) was also examined for comparative purposes. The flow field and instantaneous OH profile were measured with PIV and OH-PLIF technique, respectively. Large eddy simulation (LES) was conducted to extend the understanding of the experimental findings. Results revealed that the NH₃ flame possesses a poor lean flame stability limit which can be largely extended by adding CH₄ in the fuel. Changing swirl number (S) had no apparent effect on the lean blow-off limit (Φ_b) for the NH₃ flame. On the contrary, a clear extension on Φ_b was found for the 50%NH₃ flame when increasing S. Four flame macrostructure modes were identified when decreasing equivalence ratio (Φ). The transition from flame II to flame III ((Φ_tdescribes the transition equivalence ratio) can be considered as the early warning of blow-off for a swirl stabilized flame. For the NH₃ flame, there was no clear flame macrostructure transition at small inlet velocities (U < 3.8 m/sec), i.e., Φ_b ≈ Φt. The difference between Φ_b and (Φ_twas observed as the inlet velocity increased. However, for the 50%NH₃ and CH₄ flames, a clear flame macrostructure transition from flame II to flame III was observed even for a lower inlet velocity. The LES results demonstrated that the NH₃ flame has a faster blow-off process compared to the CH₄ flame, which is mainly attributed to the excessive stretch causing local extinction during the blow-off process.