Effects of CO2 Dilution on Methane Ignition in Moderate or Intense Low-oxygen Dilution (MILD) Combustion: A Numerical Study

Homogeneous mixtures of CH4/air under moderate or intense low-oxygen dilution (MILD) combustion conditions were numerically studied to clarify the fundamental effects of exhaust gas recirculation (EGR), especially CO2 in EGR gases, on ignition characteristics. Specifically, effects of CO2 addition on autoignition delay time were emphasized at temperature between 1200 K and 1600 K for a wide range of the lean-to-rich equivalence ratio (0.2 similar to 2). The results showed that the ignition delay time increased with equivalence ratio or CO2 dilution ratio. Furthermore, ignition delay time was seen to be exponentially related with the reciprocal of initial temperature. Special concern was given to the chemical effects of CO2 on the ignition delay time. The enhancement of ignition delay time with CO2 addition can be mainly ascribed to the decrease of H, 0 and OH radicals. The predictions of temperature profiles and mole fractions of CO and CO2 were strongly related to the chemical effects of CO2. A single ignition time correlation was obtained in form of Arrhenius-type for the entire range of conditions as a function of temperature, CH4 mole fraction and O-2 mole fraction. This correlation could successfully capture the complex behaviors of ignition of CH4/air/CO2 mixture. The results can be applied to MILD combustion as "reference time", for example, to predict ignition delay time in turbulent reacting flow.