NO_x-formation and co-burnout in Water injected, premixed natural gas flames at typical gas turbine combustor residence times

With the transition of the power production markets towards renewable energy sources an increased demand for flexible, fossil based power production systems arises. Steep load gradients and a high range of flexibility make gas turbines a core technology in this ongoing change. In order to further increase this flexibility research on power augmentation of premixed gas turbine combustors is conducted at the Lehrstuhl für Thermodynamik, TU München. Water injection in gas turbine combustors allows for the simultaneous control of NO_x emissions as well as the increase of the power output of the engine and has therefore been transferred to a premixed combustor at lab scale. So far stable operation of the system has been obtained for water-to-fuel ratios up to 2.25 at constant adiabatic flame temperatures. This paper focuses on the effects of water injection on pollutant formation in premixed gas turbine flames. In order to guarantee for high practical relevance experimental measurements are conducted at typical preheating temperatures and common gas turbine combustor residence times of about 20 ms. Spatially resolved and global species measurements are performed in an atmospheric single burner test rig for typical adiabatic flame temperatures between 1740 and 2086K. Global measurements of NO_x and CO emissions are shown for a wide range of equivalence ratios and variable water-to-fuel ratios. Cantera calculations are used to identify non-equilibrium processes in the measured data. To get a close insight into the emission formation processes in water injected flames local concentration measurements are used to calculate distributions of the reaction progress variable. Finally, to clarify the influence of spray quality on the composition of the exhaust gas a variation of the water droplet diameters is done. For rising water content at constant adiabatic flame temperature the NO_x emissions can be held constant, whereas CO concentrations increase. On the contrary, both values decrease for measurements at constant equivalence ratio and reduced flame temperatures. Further analysis of the data shows the close dependency of CO concentration on the equivalence ratio, however, due to the water addition a shift of the CO curves can be detected. In the local measurements changes in the distribution of the reaction progress variable and an increase of the flame length were detected for water injected flames along with changes of the maximum as well as the averaged CO values. Finally, a strong influence of water droplet size on NO_x and CO formation is shown for constant operating conditions.