59071 - Nox Emission Modelling for Lean Premixed Industrial Combustors With a Diffusion Pilot Burner 

One of the main development goals for gas turbines is the reduction of pollutant emissions across the full load range. Nitrogen Oxides (NOx) are very harmful combustion pollutants that are restricted strongly by legislation all over the world. Industrial gas turbine combustors must operate at very high temperatures, using premixed technology to provide ultra-low NOx emissions without further catalysts and fulfill the restrictions. This study provides a methodology to effectively model these NOx emissions for industrial gas turbine combustion chambers, and shows its application to an industrial combustor operated at different cycle conditions. It combines one-dimensional flame simulations using detailed chemistry with a probabilistic approach for equivalence ratio fluctuations to account for the effect of fuel-air unmixedness. This split allows for computationally fast variations of the gas inlet conditions and the consideration of different shares of pilot gas. 

The generation of emissions is split into a share of prompt formation at the flame front and a slower formation mechanism, occurring within the combustion products in the post flame region. The influence of unmixedness of the fuel-air mixture on both effects is taken into consideration by means of probability density functions (PDFs) of the equivalence ratio. These are modeled on the basis of sampled values from Large Eddy Simulations (LES) at the flame front and further downstream.  The model is calibrated for a specific industrial combustor through burner-specific parameters, originating from geometric estimations and emission measurements obtained during experimental tests at atmospheric conditions. Full load single can high pressure tests and full engine tests are used for cross-validation of the model. The influence of pressure and temperature on the NOx emissions are analyzed and compared to findings from the literature. To exploit the full potential of this method, a detailed study on the influence of the pilot gas ratio on the shape of the PDFs is conducted and compared to corresponding measurement results. Finally, potential measures for emission reduction are discussed. The method is able to asses emission values for a broad range of operating conditions. The presented method allows the prediction of NOx emissions at engine conditions based on atmospheric testing. This possibility makes it a suitable tool for future burner design.