Autoignition of methane-based fuel blends under gas turbine conditions

To determine gross autoignition behavior for power generation gas turbine engines over a wide range of possible blends, undiluted natural-gas-based mixtures combining CH4 with C2H6, C3H8, C4H10, C5H12, and H-2 were tested at engine-relevant conditions. The experiments were performed behind reflected shock waves at an average pressure of 20 atm, target temperatures near 800 K, and at) equivalence ratio of phi=0.5. A statistical matrix approach developed in previous papers was employed to cover as wide a range of multiple-fuel blends as possible, resulting in 21 binary and ternary mixtures with methane content in the blends as low as 50% by volume in some mixtures. Both pressure and CH*-emission histories were obtained from the endwall to determine the ignition delay time and the relative strength of the ignition process. Previous work has shown reduced ignition activation energies and similar behavior in the low-temperature, high-pressure regime for higher-order hydrocarbons such as heptane and iso-octane. Similar trends have been obtained in this study for methane-based fuel blends containing higher-order hydrocarbons up to n-pentane, implying a strong reduction in activation energy at lower temperatures and higher pressures. The mean ignition time for all mixtures around 800 K was found to be 7.9 ms with a standard deviation of 1.9 ms, or less than 25%, which is small compared to the relatively high reduction effect at higher temperatures. While most of the fuel blends exhibited strong ignition behavior, the pure methane tests exhibited weak ignition at the lower temperature. Assuming either type of ignition to be worthy of concern for autoignition in gas turbines, it was found that pure methane at low temperatures (800 K) and gas turbine pressures (20 atm) has about the same ignition delay time as other mixtures. The results from this Study can be directly used for industrial purposes, as validation of potential chemical kinetics mechanisms in this regime, and as a starting point to conduct further studies into the behaviors observed. (c) 2006 Published by Elsevier Inc. on behalf of The Combustion Institute.