Autoignition Delay Time Measurements for Natural Gas Fuel Components and their Mixtures

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Recent studies have indicated that small changes in concentration of higher hydrocarbons in natural gas can severely reduce ignition delay time of methane-based fuels. To increase the database of auto-ignition delay times for methane-based fuel mixtures characteristic of natural gas, experiments to measure auto-ignition delay times have been performed for a variety of gaseous fuels in an atmospheric flow reactor. Autoignition delay times were measured for pure fuels in air including methane, ethylene, ethane, and propane. The effect of higher hydrocarbon addition on methane-based fuel ignition delay was then investigated for fuel mixtures composed of methane/ethylene, methane/ethane, and methane/propane where methane composition varied from 25-95% by volume. Autoignition delay times were also measured for multi-component methane-based fuels composed of methane/ethane/propane and air. Finally the effect of CO2 addition on methane autoignition was investigated. For all experiments, equivalence ratios for the experiments ranged between 0.5 and 1.25 and temperatures ranged from 931 - 1137 K.

The most dramatic decrease (30-60%) in auto-ignition delay time of binary methane-based fuels occurred with the addition of the first 5-10% of a higher hydrocarbon. Further addition of a higher hydrocarbon continued to reduce auto-ignition delay time, but less dramatically. Similarly, addition of small concentrations of methane to higher order hydrocarbons dramatically increased auto-ignition delay time as compared with the auto-ignition delay time of the pure higher-order hydrocarbon. This trend was found for all binary mixtures of methane/ethylene, methane/ethane, and methane/propane. CO2 addition of 5-10% decreased auto-ignition delay time of fuel mixtures, but the effect was minimal. The measurements made in this study will benefit the combustion community by both providing valuable insight into the effects of additives on methane ignition as well as validation data for chemical kinetic mechanisms for predicting ignition of methane-based fuels.