Combustion Under Flue Gas Recirculation Conditions in a Gas Turbine Lean Premix Burner

An EV burner as installed in Alstom’s dry low NO_x gas turbines was experimentally investigated under different Flue Gas Recirculation (FGR) and engine conditions. FGR enables the reduction of the high exhaust volume flow while significantly increasing the exhaust CO₂ concentration. This may substantially improve the post-combustion capture of CO₂. However, FGR introduces consequent changes in the gas turbine combustion process mainly because of the oxygen depletion and CO₂ increase within the oxidizer. N₂ and CO₂ were mixed with air in order to obtain at the burner inlet a synthetic oxidizer mixture reproducing O₂ and CO₂ levels spanning different FGR levels of interest for engine operation. In addition, various degrees of unmixedness of the reactive mixture were investigated by varying the ratio of fuel injected at different port locations in the investigated burner set. Stable operation was achieved in all tested conditions. The lean premix flame shifts downstream when O₂ is depleted due to the decrease of the reactivity, although it always stays well within the combustion chamber. The potential for NO_x reduction when using FGR is demonstrated. Changes of the NO_x formation mechanism are described and compared to the experimental data for validation. Unmixedness appears to be less detrimental to NO_x emission when under high FGR ratio. However, CO emission is shown to increase when FGR ratio is increased. Meanwhile, with the present gas turbine combustor, the CO emission follows the equilibrium limit even at high FGR ratio. Interestingly, it is observed that when the burner inlet pressure is increased (and consequently the inlet burner temperature), the increase of CO emission due to FGR is lowered while the NO_x emission stays at a very low level. This present an argument for using a higher cycle pressure in gas turbines optimized for FGR operation.