Investigating NO emissions, stability, and flame structure in co-fired premixed NH3/CH4/air swirling flames

Abstract

Ammonia combustion poses challenges due to low reactivity and high NO_x emissions, requiring optimization of combustor designs and fueling strategies. This study examines NO emissions, flame stability, and structure in co-fired premixed NH₃/CH₄/air flames using a double-swirl burner. The inner swirl stream consists of NH₃/CH₄/air mixtures with varying ammonia mole fractions (x_NH3: 0 to 1) and equivalence ratios (Φ_in: 0.4 to 1.4), while the outer stream contains CH₄/air mixtures with Φ_out ranging from 0.5 to 0.8 and Reynolds numbers (Re_out) of 4350, 5250, and 6000. NO emissions varied significantly with Re_out, Φ_in, and Φ_out, prompting further investigation of flame structure using OH-NO PLIF and PIV diagnostics for three flame sets: FA (Φ_in=0.4), FB (Φ_in=0.8), and FC (Φ_in =1.4). Far-rich (FC) and far-lean (FA) flames exhibited an early conical OH layer followed by a V-shaped OH layer, while NO dispersed across the flame, forming a thin layer at the OH boundary with a V-shaped distribution downstream. Higher Re_out facilitated V-OH/NO formation through enhanced mixing, increased recirculation, and more effective ammonia cracking in rich mixtures. At Re_out=4350, the absence of a V-OH layer in FA resulted in reduced NO emissions. Flame FB showed a broader, positively correlated NO and OH structure along the central region of the flame, indicating enhanced NH_i oxidation to NO. Overall, co-firing ammonia with methane in the outer stream was crucial for improving flame stability. To minimize NO emissions, it is important to lower Re_out, increase Φ_out, and avoid premixing NH₃/CH₄ in the inner stream. At high Re_out, limiting rich Φ_in to 1.2 or leaning it out, combined with increasing Φ_out, was the most effective strategy for reducing NO emissions.