Plasma-assisted ignition of methane/air and ethylene/air mixtures: Efficiency at low and high pressures

The ignition of methane/air and ethylene/air mixtures by nanosecond pulsed discharges (NSPD) was numerically studied using a zero-dimensional isochoric adiabatic reactor. A combustion kinetics model was combined with a non-equilibrium plasma mechanism, which features vibrational and electronic excitation, dissociation, and ionization of neutral particles (O2 and N2) via electron impact. A time to ignition metric τ was defined, and ignition simulations encompassing a wide range of pressures (0.5-30 atm) and pulsing conditions for each fuel were executed. For each fuel, τ depended primarily on initial pressure and energy deposition rate, and scaling laws were derived. The benefit gained from plasma-assisted ignition (PAI) was quantified by comparing τ with a thermal ignition time. For both fuels, PAI resulted in a faster ignition at low pressures, while at higher pressures (p0 ≥ 5 atm), methane/air ignition became inefficient (meaning a longer ignition time for the same input energy compared to thermal ignition). Ethylene/air PAI showed only a modest deterioration. The drop in performance with pressure was due to the mean electron energy achieved during the pulse, which exhibited an inverse relationship with pressure, leading to fewer excited species and combustion radicals. The poor performance of methane/air mixture ignition at high pressure was explained by an analysis of the reaction pathways. At high pressures (p0 ~30 atm), H is consumed mostly to form hydroperoxyl (HO2), leading to a bottleneck in the formation of formyl (HCO) from formaldehyde (CH2O). Instead, for ethylene/air ignition, at both low and high pressures there exist several bypass pathways that facilitate the formation of HCO and CO directly from various intermediates, explaining the more robust performance of PAI for ethylene at pressure.SCOPUS: cp.j