Effect of CO₂ dilution on structures of premixed syngas/air flames in a gas turbine model combustor

The impact of CO dilution on combustion of syngas (a mixture of H₂, CO, and CH₄) was investigated in a lab-scale gas turbine model combustor at atmospheric pressure conditions. Two mild dilution levels of CO₂, corresponding to 15% and 34% of CO₂ mole fraction in the syngas/CO₂ mixtures, were experimentally investigated to evaluate the effects of CO₂ dilution on the flame structures and the emissions of CO and NOx. All experiments were performed at a constant Reynolds number (Re = 10000). High-speed flame luminescence, simultaneous planar laser-induced fluorescence (PLIF) measurements of the OH radicals and particle image velocimetry (PIV) were employed for qualitative and quantitative assessment of the resulting flame and flow structures. The main findings are: (a) the operability range of the syngas flames is significantly affected by the CO₂ dilution, with both the lean blowoff (LBO) limit and the flashback limit shifting towards fuel-richer conditions as the CO₂ dilution increases; (b) syngas flames exhibit flame-pocket structures with chemical reactions taking place in isolated pockets surrounded by non-reacting fuel/air mixture; (c) the inner recirculation zone tends to move closer to the burner axis at high CO₂ dilution, and (d) the NOx emission becomes significantly lower with increasing CO₂ dilution while the CO emission exhibits the opposite trend. The flame-pocket structure is more significant with increased CO₂ dilution level. The low NOx emissions and high CO emissions are the results of the flame-pocket structures.