Modelling of NOx Formation of a Premixed DLE Gas Turbine Combustor

Abstract

To comply with the stringent emission regulations industrial gas turbines operate under lean premixed conditions. To be able to predict emissions in an early design phase, advanced premixed combustion models are required. The rate of NOx formation is sensitive to temperature and some radical concentrations, therefore the flame predictions have to be accurate and detailed. It is well known that there are several routes (thermal, prompt, N2O mechanism) by which NOx can be formed. As lean premixed gas turbines operate at relative low flame temperatures, the contribution of the prompt NO and N2O mechanisms has to be accounted for. The modelling of NOx formation is done in a post-processor, as the influence of nitrogen species on the main combustion characteristics is negligible. This post-processor is based on flame calculations using a Flame Generated Manifold method. This is a reduced reaction mechanism, so that only a limited number of variables have to be solved during the CFD computations. A post-processor method has been developed to compute NO formation. Differential equations are solved for NO and N2O, other species including nitrogen are solved using a steady-state equation. The flame and post-processor models are applied to 2-D and 3-D models of an industrial series staged gas turbine. Parameter studies for the fuel split between the two stages and inlet fuel/air ratio variations have been carried out and the data has been compared with generic engine data.