Combustion and NOx Emission Characteristics in Premixed Swirling Flames of Partially Cracked Ammonia

Partially cracking ammonia into hydrogen offers a practical route to overcome its inherently low reactivity while leveraging existing ammonia infrastructure for zero-carbon combustion. In this study, we designed and tested a ∼10 kW premixed swirl burner to systematically investigate the influence of swirler geometry, ammonia cracking ratio, and excess air ratio on flame stability and NOx emissions. Complementary numerical simulations were conducted to elucidate the principal elementary reactions and pathways governing nitrogen oxides (NOx) formation. The results indicate that under lean premixed conditions, nitrous oxide (N 2 O) concentrations remain at single-digit parts per million (ppm) levels, while residual ammonia and nitric oxide (NO) concentrations in the exhaust exhibit a cracking-ratio-dependent trade-off: stronger swirl over-stretches the flame and elevates NH 3 slip at low cracking ratios, whereas at higher cracking ratios it promotes complete oxidation at the cost of increased NOx. In contrast, weaker swirl suppresses NO but leaves elevated NH 3 slip ratios due to incomplete burnout. Increasing the cracking ratio shortens the visible flame, shifts the flame morphology from V-shaped to straight, and promotes NOx via enhanced radical chemistry. A higher excess air ratio further accelerates NO production due to intensified OH-driven pathways. Overall, this study provides direct experimental and numerical evidence that swirl design and controlled cracking are critical levers for optimizing ammonia-hydrogen premixed burners. The insights contribute to the rational design of ammonia-hydrogen burners for high-temperature industrial applications.