Experimental investigation of the NO emission characteristics in swirl-stabilized technically premixed CH4/H2 flames

Hydrogen (H 2 ) is an attractive sustainable energy source, given its carbon-free combustion. However, its future availability in large quantities remains uncertain. To address this issue, designing fuel-flexible (methane - hydrogen) combustion chambers adaptable for various fuel blends is essential. One of the main challenges in these systems is achieving low NO x emissions and maintaining flashback-free operation for high hydrogen content fuels. The present paper investigates the nitric oxide (NO) emission characteristics of technically premixed, swirl-stabilized flames across fuel mixtures, ranging from 100 % methane (CH 4 ) to 100 % H 2 . To mitigate the flashback risk for high hydrogen content mixtures, a non-rotating axial air injection (AAI) jet is introduced along the centerline of the swirling flow. The NO distribution within the combustion chamber was measured using NO planar laser-induced fluorescence (PLIF) and validated against NO concentrations in the exhaust gas measured with an exhaust gas analyzer. Complementary hydroxyl (OH) PLIF measurements and particle image velocimetry (PIV) measurements provided insights into the flame location and flow field characteristics and the relationship between flame location and NO concentrations. The results showed that introducing AAI significantly impacts the flame shape and the NO concentration within the combustion chamber, ultimately leading to an increase in NO emissions. For cases without AAI, increasing the hydrogen molar fraction (up to X H 2 = 0.4) increases the NO emissions. However, for cases with a high level of AAI (and hydrogen contents up to X H 2 = 1), no clear trend was observed between the level of hydrogen content and the measured level of NO. This suggests that NO formation results from a complex interaction between local flame temperature variations, which depend on fuel-air mixing and fuel composition, and the specific mechanisms responsible for NO production.