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

The impact of CO${}_2$ dilution on combustion of syngas (a mixture of H${}_2$, CO, and CH${}_4$) was investigated in a lab-scale gas turbine model combustor at atmospheric pressure conditions. Two mild dilution levels of CO${}_2$, corresponding to 15% and 34% of CO${}_2$ mole fraction in the syngas/CO${}_2$ mixtures, were experimentally investigated to evaluate the effects of CO${}_2$ dilution on the flame structures and the emissions of CO and NO$x$. 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 CO${}_2$ dilution, with both the lean blowoff (LBO) limit and the flashback limit shifting towards fuel-richer conditions as the CO${}_2$ 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 CO${}_2$ dilution, and (d) the NO$x$ emission becomes significantly lower with increasing CO${}_2$ dilution while the CO emission exhibits the opposite trend. The flame-pocket structure is more significant with increased CO${}_2$ dilution level. The low NO$x$ emissions and high CO emissions are the results of the flame-pocket structures.