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

Ammonia combustion poses challenges due to low reactivity and high NOx emissions, requiring optimization of combustor designs and fueling strategies. This study examines NO emissions, flame stability, and structure in cofired 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 (xNH3: 0 to 1) and equivalence ratios (Phi in: 0.4 to 1.4), while the outer stream contains CH4/air mixtures with Phi out ranging from 0.5 to 0.8 and Reynolds numbers (Reout) of 4350, 5250, and 6000. NO emissions varied significantly with Reout, Phi in, and Phi out, prompting further investigation of flame structure using OH-NO PLIF and PIV diagnostics for three flame sets: FA (Phi in=0.4), FB (Phi in=0.8), and FC (Phi 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 Reout facilitated V-OH/NO formation through enhanced mixing, increased recirculation, and more effective ammonia cracking in rich mixtures. At Reout=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 NHi 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 Reout, increase Phi out, and avoid premixing NH3/CH4 in the inner stream. At high Reout, limiting rich Phi in to 1.2 or leaning it out, combined with increasing Phi out, was the most effective strategy for reducing NO emissions.