Shock-induced ignition and pyrolysis of high-pressure methane and natural gas mixtures

A high-pressure shock tube was used to study ignition delay times (IDT) of CH₄/O₂/Ar and natural gas/O₂/Ar mixtures behind reflected shock waves. Reaction progress was monitored using sidewall pressure and direct laser absorption diagnostics of CH₄ near 3.175 µm and ethylene near 10.532 µm. Stoichiometric, fuel-rich and fuel-lean mixtures of CH₄/O₂, highly dilute in argon, were studied over a temperature range from 1450 to 1850 K and pressures between 10 and 55 atm. Of note are the experiments conducted with fuel-rich mixtures, as there is a lack of literature data in this regime. In the current study, the methane absorption diagnostic provided a unique tool enabling both speciation of methane and a clear definition of ignition delay time. In addition to methane oxidation, we have measured ignition delay times of commercial natural gas blends over a temperature range of 1408–1541 K, at pressures near 12 atm, and at an equivalence ratio of 1. To understand the effects of minor constituents (such as ethane and propane) in commercial natural gas blends, ethylene concentration during pyrolysis experiments was monitored using a two-wavelength scheme (10.532 µm and 10.674 µm) using a CO₂ gas laser. The deficiency of existing kinetic models towards predicting the high-temperature kinetics of natural gas blends was highlighted through our measurements. Therefore, this study also provides data critical for refining these models. Extensive sensitivity analysis emphasizes the importance of the reaction CH₃+C₂H₆→CH₄+C₂H₅ during natural gas pyrolysis, and the accuracy of the chemical kinetic models is significantly improved by using a revised reaction rate constant (Shao et al. 2019) for this reaction. These measurements extend the test conditions of earlier studies of methane and commercial natural gas.