Numerical predictions of pollutant emissions of novel natural gas low nox burners for heavy duty gas turbine

Abstract

In the present work, a numerical analysis of the pollutant emissions of a novel dry low-NOx burner for industrial gas turbine applications is presented. Experimental results available from a recent campaign, performed by BHGE in cooperation with the University of Florence, served as validation tool for the simulation setup and the employed models. A RANS approach and the FGM combustion model have been used to reproduce the global flame behavior and to provide an accurate description of temperature and major species distributions. However, FGM is generally not able to predict the flame emissions, such as NOx, due to a strong separation between the time scales related to the turbulent mixing and to the nitrogen oxidation. To address this issue, specific NOx models are normally used to interrogate the flow field provided by FGM and to recover a more detailed estimation of the burner emissions. Moreover, when operating in part-load conditions, also CO emissions are not likely to be correctly predicted. In fact, as the load decreases, the flame gets closer and closer to its extinction limit and shows a superequilibrium CO concentration in the exhaust gases, which can not be properly described by flamelet-based combustion models. Therefore, based on the original idea of the CO burn-out model proposed by Klarmann, a correction to decouple the effective rate of production of CO from the one provided by FGM has been introduced in ANSYS Fluent. The implemented modification represents a robust and cost-efficient numerical setup for the prediction of CO emissions in heavy-duty gas turbine applications. Moreover, the obtained results provide useful information for the design process, allowing a more detailed description of NOx and CO production, in the perspective of future improvements on nozzle geometry and combustor operability.