Fundamental Characteristics of Premixed Methane/air Flames in a Sequential Two-Stage Combustor of a Stationary Gas Turbine

Due to their high load flexibility and air-quality benefits, axial (sequential) stage combustion systems have become more popular among ground-based power gas turbine combustors. However, inert post combustion products passing from the first stage to the second stage affect the flame reactivity and stability in the second stage of the combustor. The present study investigates laminar flame characteristics of the second stage combustion of a sequential combustor. The constant pressure method for spherically expanding flames was employed to obtain laminar burning velocities and burned gas Markstein lengths of premixed methane/air mixtures diluted with flue gas at 3 bar and 423 K. Combustion residuals were simulated with a 9.50% CO₂+71.49% N₂+19.01% H₂O mixture by volume and the tested dilution ratios were 0%, 5%, 10%, and 15%. Experimental results showed that the laminar burning velocity was decreased by 18-23%, 36-42% and 50-52% with additions of 5%, 10% and 15% combustion products, respectively. As the equivalence and dilution ratios increased, the burned gas Markstein length increased slightly, suggesting the methane/air flames are more stable and stretched at these conditions. Numerical results were obtained from CHEMKIN using the GRI-Mech 3.0, USC Mech II, San Diego, HP-Mech, NUI Galway, and AramcoMech 1.3 mechanisms. The GRI-Mech 3.0 and HP-Mech performed best, with a maximum of 9% and 7% difference between numerical and experimental laminar burning velocities, respectively. The USC Mech II overpredicted results by up to 11%, while still maintaining the overall trend of the experimental findings. The laminar burning velocities predicted by the NUI Galway and AramcoMech 1.3 mechanisms were very close and in good agreement with experimental data, with the exception of overprediction at rich conditions. The San Diego detailed kinetic scheme poorly simulated the laminar burning velocities at ϕ≥1.0 with a maximum of 20% error. The dilution, thermal-diffusion, and chemical effects of inert post combustion gases on the laminar burning velocity were found using numerical results. The dilution effect was primarily responsible, accounting for 79-84% of the decrease in the laminar burning velocity. The chemical effect slightly outweighed the thermal-diffusion effect and was most important near stoichiometry where combustion temperatures are highest.