Effect of air and N2 dilution on combustion stability and emission in a swirl stabilized premixed combustor operating on H2 and mixtures of H2 and CH4

• Investigated recirculated flue gas for H 2 -enriched methane premixed combustors. • Analyzed combustion dynamics and pollutant emissions in H 2 –CH 4 flames. • Scaled combustion data using extinction strain rate for predictive capability. • Air and N 2 dilution yield similar flame topology and dynamics; higher H 2 shifts transitions to lower ϕ. • Dilution lowered NO x but increased CO, showing a trade-off with stability. A hydrogen (H 2 ) or hydrogen-enriched methane (CH 4 )-fired gas turbine could play a critical role in significant decarbonization. However, the combustion of these fuel mixtures at lean premixed conditions is highly prone to flashback and combustion instabilities and may even lead to high NO x emissions. This study investigates the use of recirculated flue gas to mitigate combustion instability and emissions in H 2 and H 2 -enriched methane flames in a premixed swirl-stabilized combustor. Flue gas effects were modelled via air and nitrogen blending, and dilution flow rates were adjusted to match the baseline methane flame. The results suggest that the flame topology transitioned from a “V” to “M” shape with increasing equivalence ratio or decreasing dilution, with the transition occurring at lower equivalence ratios for higher hydrogen content. Spectral and modal analysis revealed low-frequency “ORZ flickering” (∼11 Hz) for pure methane and 40% H 2 flames, whereas thermo-acoustic instabilities dominated at 210–220 Hz for higher H 2 fractions. Pure hydrogen flames exhibited a second harmonic longitudinal mode at approximately 440 Hz, in addition to thermo-acoustic instability at frequencies ranging from 240 to 280 Hz. OH-PLIF imaging revealed increased flame wrinkling with hydrogen enrichment, primarily driven by preferential fuel diffusion and the effective Lewis number. Scaling combustion data with extinction strain rate successfully collapsed flame dynamics up to 80% H 2 , but failed for pure hydrogen. Pollutant measurements indicated a decrease in NO x and an increase in CO with dilution; excessive dilution for high H 2 fractions led to high CO emissions, highlighting a trade-off between flame stability and pollutant reduction.