Numerical modeling of oxy-methane combustion in a model gas turbine combustor

There is a renewed interest in oxy-fuel combustion of natural gas for reduction of greenhouse gas emissions. This has necessitated various experimental and numerical studies of oxy-fuel combustion. In the numerical combustion study, the radiation model and combustion chemistry are critical for accurate numerical predictions of oxy-fuel combustion characteristics. In this study, three global reaction mechanisms: Westbrook-Dryer (3 equations), Jones-Lindstedt (5 equations) and Jones-Lindstedt (7 equations) for oxy-methane combustion were combined with different weighted sum of gray gas radiation models (WSGGM) available in the literature to determine the most accurate combination for oxy-methane combustion modeling and simulation. Experiments were conducted in a non-premixed swirl stabilized model gas turbine combustor at a firing rate of 4 MW/m³-bar while varying the percentage of CO₂ in the oxidizer (O₂/CO₂) mixture. Numerical model developed using ANSYS FLUENT 17 code was validated against the experimental results. The combinations of Jones-Lindstedt (5 equations) reaction mechanism and WSGGM model proposed by Bordbar gave the closest approximation of the flame temperature with an average deviation of 5.52%. The model combination also predicted the flame attachment to the fuel nozzle and flame lift-off at a high CO₂ percentage in the oxidizer mixture. The results of the parametric study on the effect of CO₂ percentage in the oxidizer mixture, combustor energy level and equivalence ratio on the combustion characteristics and CO emissions were also reported. The CO emissions monotonically increases with increasing percentage of CO₂ in the oxidizer due to decreased residence time and the reduction in the flame temperature. While the CO emission increases with the energy level in the combustor up to 3.5 MW/m³ and decreases thereafter. The optimum equivalence ratio for minimum CO emission is 0.98 with approximately 2 PPM at 40% CO₂ in the oxidizer.