Predicting Exhaust Gas Recirculation Impacts on Instability and Emissions of an Industrial Gas Turbine Combustor Using Large Eddy Simulations

Abstract Carbon capture at engine exhaust is a promising technology to reduce carbon footprint for industrial gas turbines. Exhaust gas recirculation (EGR) has been proposed as a potential solution to increase the exhaust CO2 concentration for reducing the cost of carbon capture. A key challenge of applying EGR to practical gas turbines is the enhanced combustion instability as the level of EGR increases. Hence, identifying the upper limit of EGR that maximizes the exhaust CO2 concentration while still allowing for stable combustion is critical to demonstrating the technology feasibility and reducing the number of expensive experimental tests. In this study, a large eddy simulation (LES) model is developed to predict the combustion instability and NOx emissions in an industrial gas turbine combustor under EGR conditions. Adaptive mesh refinement (AMR) is used to resolve complex local flow and flame structures. The finite rate chemistry based combustion model is used to predict the highly transient combustion and emissions behaviors. To accelerate the finite rate chemistry calculation, a 39-species skeletal chemical kinetics mechanism accounting for NOx chemistry is developed for methane/air mixture under EGR conditions for realistic gas turbine operations. The LES model is validated against experimental temperature profile at the combustor exit and good agreement is achieved between LES and experiment. The model is then employed to conduct a parametric study comprising of various levels of EGR and piloting. As EGR level increases, the instability amplitude, defined by the pressure fluctuation inside the combustor, is shown to increase monotonically attributed to the decreased flame speed. The flame jumps from a stable mode to an unstable mode at a threshold EGR level. Piloting is shown to promote or inhibit the instability amplitude depending on the level of EGR. Analysis of adiabatic flame temperatures for both main and pilot flames shows that piloting may not be effective in suppressing combustion instability when the corresponding pilot flame temperature is lower than the main flame temperature. Instead, the instability amplitude is found to be strongly correlated to the laminar flame speed of the main flame alone. On the other hand, when increasing EGR level, the NOx emissions first decrease in the stable regime, then increases abruptly as flame transitions from the stable to unstable mode, and finally decreases again within the unstable regime. Such complex response of NOx is found to be associated with the availability of excess O2 as well as the change of flame size and flame surface area.