The decarbonization of gas turbine technology requires combustion systems capable of operating with hydrogen and hydrogen–natural gas blends while maintaining low NOx emissions. Hydrogen combustion increases flame temperature and reactivity, which might increase thermal NO formation through the Zeldovich mechanism and increase the flashback risk. Steam dilution is a promising strategy to mitigate NOx and improve flashback resistance without major hardware modifications, yet its performance in partially premixed, swirl-stabilized flames with different fuel composition, steam delivery strategy, and operating conditions is not yet fully understood.

This thesis investigates the effect of steam injection on NO emissions in lean premixed CH4/H2 flames using a swirl-stabilized vertical combustor operating at 7–11 kW. Four experimental series were conducted to isolate the influence of (i) thermal load, (ii) H2 blend fraction, (iii) steam injection amount, and (iv) injection location (side vs. axial). Emissions were measured using an ABB AO2000 gas analyzer, and the results are reported as raw, 15% O2 normalized and mass- & energy-based emission indices (EINO). Flame structure was characterized using DSLR imaging and OH∗ chemiluminescence. A simplified analytical estimate based on thermal NO scaling was developed to contextualize the measurements.


Steam injection consistently reduced NO under all operating conditions. Measured reductions ranged from 30–40% at moderate steam loadings (17.7 g/min) to 45–55% under higher steam addition (24.4 g/min), in close agreement with the simplified thermal prediction. NO reductions exceeded 80% in high methane fraction blends in lean conditions but approached local quench limits, as indicated by elevated CO and unburned CH4 measured at the exhaust. High hydrogen blend fraction flames produced higher absolute NO but tolerated steam injection without evident oxidation penalties. Axial steam injection provided more uniform premixing than side injection and maintained stable operation across all H2 blends, achieving comparable or improved NO reduction without CO rise. Flame Imaging revealed that steam addition cools and weakens the flame core, producing annular reaction zones consistent with reduced peak temperatures and suppressed radical pools.


The results demonstrate that steam injection is an effective NO control strategy for hydrogen partially- premixed combustion. Its effectiveness depends on injection strategy, blend ratio, and operating power. A more uniform steam–air partial-premixing have a more favorable effect on NO reduction and stability. The findings provide design guidance for implementing steam dilution in practical low-NOx hydrogen combustion systems and highlight directions for future work, including high-pressure testing and resolved diagnostics of radical fields and local temperature