Ignition of CH4 intensely diluted with N2 and CO2 versus hot air in a counterflow jets

The ignition temperatures of CH4 intensely diluted with N2 and CO2 have been measured in a counterflow ignition facility with mixtures of 6–25% CH4 in N2 and 9–25% CH4 in CO2. The simulation by GRI Mech 1.2, GRI Mech 3.0, USC Mech 2.0, and Aramco Mech 2.0 cannot predict well the present experimental results. A modified mechanism has been proposed by substitution of the news channels of CH3 + O and CH2 + O2 for the corresponding channels in USC Mech 2.0 and updating some reaction accordingly. The modified mechanism has been verified by present experiments and other experiments under the intensely diluted CH4 condition. The analysis by the modified mechanism shows that when CH4 is intensely diluted with N2 or CO2, the path of CO → CO2 becomes important to produce ignition, because the reaction CO + OH = CO2 + H can provide sufficient radical H for the rapid growth of the radical pool, thus, the main oxidation chain of CH3 is extended furtherly to CO2, that is, CH3 → CH2O → HCO → CO → CO2. The channels of CH3 + O becomes increasing important for the ignition with the decease of the CH4 concentration, because the reaction CH3 + O = CO + H + H2 significantly promotes the ignition of the intensively diluted CH4 due to facilitating the rapid growth of the radical pool. Consequently, the secondary CH3 oxidation path, CH3 → CO → CO2, makes additional but critical contribution to produce ignition. The channels of CH2 + O2 are CH2 + O2 = CO + H2O, CH2 + O2 = CO2 + H + H, CH2 + O2 = CH2O + O, CH2 + O2 = CO2 + H2, and CH2 + O2 = CO + OH + H. Among the channels of CH2 + O2, the variation of the effects of the reaction CH2 + O2 = CO + OH + H on the ignition is the most significant when the concentration of CH4 changes from 20% to 6%. Therefore, the third CH3 oxidation path, CH3 → CH2(s) → CH2 → CO → CO2, makes evident contribution to produce ignition under intensely diluted fuel condition due to CH2 + O2 = CO + OH + H. When diluent gas N2 is replaced by CO2 in intensely diluted CH4, the ignition temperature in the CH4/CO2 is higher than that in the CH4/N2, which is mainly attributed to the higher heat capacity of CO2 compared to N2.