NO Formation in counterflow diffusion flames of partially cracked ammonia: Effects of flame structure, cracking ratio and strain rate

Partial cracking improves NH 3 flame reactivity, but its effect on NO formation is not universal and may depend on flame organization. Premixed and technically premixed flames of partially cracked NH 3 often show large NO increases at intermediate cracking ratios, whereas the response in non-premixed flames remains underexplored because spatially resolved quantitative NO profiles are scarce. In this study, NO formation in canonical counterflow diffusion flames of partially cracked NH 3 was investigated using microprobe sampling coupled with mid-infrared laser absorption spectroscopy and kinetic simulations. The results identify radical spatial coupling as a key factor controlling NO formation in partially cracked NH 3 counterflow diffusion flames, leading to two distinct responses. First, increasing the ammonia cracking ratio in counterflow diffusion flames raises the peak flame temperature but monotonically decreases the peak NO mole fraction. Kinetic analysis shows that this decrease results mainly from reduced NO formation through the HNO-mediated fuel-NO pathway, rather than from enhanced NO-consuming reactions. As the cracking ratio increases, the diffusion flame structure increases the spatial segregation between fuel side NH i radicals and oxidizer side O/OH radicals, thereby weakening the NH i → HNO → NO sequence. Second, the effect of strain rate depends on whether fuel-bound nitrogen remains in the inlet stream. In fully cracked H 2 /N 2 flames, increasing strain rate decreases NO through lower temperature and shorter residence time. In partially cracked flames, however, increasing strain rate increases NO despite the lower temperature, because flame stretch enhances NH i /O/OH interaction and strengthens the HNO pathway. These results shows radical spatial coupling is a critical factor in NO formation in partially cracked NH 3 diffusion flames and provide spatially resolved data for validating kinetic mechanisms for non-premixed partially cracked NH 3 flames. Novelty and significance statement: Partially cracked NH3 is often expected to increase NO because H2 enrichment raises flame temperature and radical production, as commonly observed in premixed flames. This study shows that this expectation does not apply directly to non-premixed counterflow diffusion flames. Spatially resolved NO measurements and kinetic simulations demonstrate that increasing the cracking ratio monotonically decreases NO despite increasing peak temperature. In contrast, increasing effective strain rate increases NO in partially cracked flames, while decreasing NO in fully cracked H2/N2 flames. Both trends can be explained by changes in the spatial coupling between fuel-derived NHi radicals and oxidizing O/OH radicals, which controls the HNO-mediated fuel-NO pathway. These results provide benchmark data and a mechanistic basis for low-NO non-premixed cracked NH3 combustion.