Shock tube ignition delay times and methane time-histories measurements during excess CO2 diluted oxy-methane combustion

The combustion of methane in air results in large amounts of CO2 and NOX emissions. In order to reduce the NOX emissions, one possible solution is the oxy-methane combustion with large CO2 dilution so that the combustion products can be reduced mainly to CO2 and H2O. However, there are very few studies on the chemical kinetics of oxy-methane combustion in a CO2 diluted environment. In this study, methane time-histories, CH* emission profiles, and pressure time-histories measurements were conducted behind reflected shock waves to gain insight into the effects of CO2 dilution of the gas mixtures on the ignition of methane. The measurements were carried out for mixtures of CH4, CO2 and O2 in argon bath gas at temperatures of 1577-2144 K, pressures of 0.53-4.4 atm, equivalence ratios (Φ) of 0.5, 1, and 2, and CO2 mole fractions (XCO2) of 0, 30, and 60%. The laser absorption measurements were conducted using a continuous wave distributed feedback interband cascade laser (DFB ICL) centered at 3403.4 nm. The results showed the decrease of activation energy and the increase of ignition delay time as the amount of CO2 dilution was increased. However, the changes were minor and within the experimental uncertainties of the measurements. Also, the results were compared to the predictions of two different natural gas mechanisms: GRI 3.0 and AramcoMech 1.3 mechanisms. In general the predictions were reasonable when compared to the experimental data; however, there were discrepancies at some conditions. Three different influences of CO2 addition to the argon bath gas in regards to chemistry, collision efficiencies, and heat capacities were examined. In addition, the present study included experimentally obtained correlations for absorption cross sections of methane for its P(8) line in the v3 band in argon bath gas with and without carbon-dioxide dilutions at temperatures between 1200 < T < 2000 K and pressures between 0.7 < P < 1.2 atm.