INCREASING OPERATIONAL STABILITY IN LOW NOx GT COMBUSTOR BY A PILOT FLAME

The need for NOx reduction in gas turbine (GT) stimulates research for new combustion methods. Lean combustion is a method in which combustion takes place under low equivalence ratio and relatively low combustion temperatures. As such, it has the potential to lower the effect of the relatively high activation energy nitrogen-oxygen reactions which are responsible for substantial NOx formation during combustion processes. Moreover, lowering temperature reduces the reaction rate of the hydrocarbon oxygen reactions and deteriorates combustion stability. The objective of the present study is to reduce the lower equivalence ratio limit of the stable combustion operational boundary in lean GT combustors. A lean premixed gaseous combustor was equipped with a surrounding concentric pilot flame operating under rich conditions, thus generating a significant amount of reactive radicals. The main combustor's mixture composition was varied from stoichiometric to lean mixtures. The pilot's mixture composition varied by changing the air flow rate, within a limited reach mixtures range. The pilot gas flow rate was always lower than five percent of the total gas supply at the specific stage of the experiments. The experiments and simulation showed that despite the high temperatures obtained in the vicinity of the pilot ring, the radicals' injection by the pilot combustion has the potential to lower the limit of the global equivalence ratio (and temperatures) while maintaining stable combustion. Therefore the amount of generated NOx is expected to be significantly reduced as compared to a similar combustor of identical inlet and exit temperatures. In order to study the relevant mechanisms responsible for combustion stabilization, CFD and CHEMKIN simulations were performed to reveal the detailed flow characteristics and their spatial distribution within the combustor. Based on the CFD results, the CHEMKIN model was developed. The CHEMKIN simulations for atmospheric pressure showed satisfactory agreement with experimental results. Further simulation confirmed the advantageous of the technique also at elevated pressures. It is therefore important to understand the relevant mechanisms responsible for combustion stabilization and their spatial distribution within the combustor. The present work discusses an experimental- CFD-CHEMKIN combined approach aimed at studying the influence of radicals generated in the pilot ring combustion on the processes taking place in the main combustor.