Ignition of Lean Methane-Based Fuel Blends at Gas Turbine Pressures

Shock-tube experiments and chemical kinetics modeling were performed to further understand the ignition and oxidation kinetics of lean methane-based fuel blends at gas turbine pressures. Such data are required because the likelihood of gas turbine engines operating on $CH4$-based fuel blends with significant $(>10%)$ amounts of hydrogen, ethane, and other hydrocarbons is very high. Ignition delay times were obtained behind reflected shock waves for fuel mixtures consisting of $CH4$⁠, $CH4∕H2$⁠, $CH4∕C2H6$⁠, and $CH4∕C3H8$ in ratios ranging from 90/10% to 60/40%. Lean fuel/air equivalence ratios $(ϕ=0.5)$ were utilized, and the test pressures ranged from 0.54 to $30.0atm$⁠. The test temperatures were from $1090K$ to $2001K$⁠. Significant reductions in ignition delay time were seen with the fuel blends relative to the $CH4$-only mixtures at all conditions. However, the temperature dependence (i.e., activation energy) of the ignition times was little affected by the additives for the range of mixtures and temperatures of this study. In general, the activation energy of ignition for all mixtures except the $CH4∕C3H8$ one was smaller at temperatures below approximately$1300K$$(∼27kcal∕mol)$ than at temperatures above this value $(∼41kcal∕mol)$⁠. A methane/hydrocarbon–oxidation chemical kinetics mechanism developed in a recent study was able to reproduce the high-pressure, fuel-lean data for the fuel/air mixtures. The results herein extend the ignition delay time database for lean methane blends to higher pressures $(30atm)$ and lower temperatures $(1100K)$ than considered previously and represent a major step toward understanding the oxidation chemistry of such mixtures at gas turbine pressures. Extrapolation of the results to gas turbine premixer conditions at temperatures less than $800K$ should be avoided however because the temperature dependence of the ignition time may change dramatically from that obtained herein.