Nanosecond pulsed plasma assisted combustion of ammonia-air mixtures: Effects on ignition delays and [formula omitted] emission

Abstract

Computational investigation of plasma assisted combustion of ammonia-air mixtures in constant volume and constant pressure reactors are conducted, to determine the impact of operating conditions on ignition delays and NOx emissions. Due to the lack of a chemical kinetic mechanism for plasma discharge of ammonia (NH3)/air mixtures, a mechanism has been assembled in this work using well-validated plasma reactions of NH3 with O2 and N2, alongside plasma kinetics of air from the literature. The impact of the reduced electric field (E/N), equivalence ratio, pressure, pulse frequency and the energy density on the ignition delays and NO/NO2 emission is discussed. At lower E/N, vibrational-to-translational (VT) relaxation of the vibrational states of NH3 and N2 is observed to play a dominant role in the gas heating process on account of the higher vibrational energy contribution. The ignition event is observed to be faster for fuel-lean mixture (ϕ = 0.5) compared to stoichiometric and fuel-rich (ϕ = 1.2) conditions owing to the lower consumption of OH radicals through the reactivity-inhibiting reaction NH3+OH→NH2+H2O between plasma pulses for leaner mixtures. Nevertheless, the fuel-lean mixture is observed to exhibit higher production of NOx than stoichiometric and fuel-rich mixtures, resulting from plasma chemistry involving oxygen radical and electronic excited states of N2. At the higher pressure of 3 atm, the pressure dependent recombination reaction H+O2+M→HO2+M is found to delay the ignition by limiting the reactive radicals compared to the corresponding 1 atm case. Higher rates of collisional quenching at higher pressures during the inter-pulse gaps resulted in lesser amount of electronically excited states of N2 and O2, which resulted in lower production of air-bound NOx during the pulses. Pulse frequency and energy density per pulse are seen to exhibit an inversely proportional effect on the ignition delay times. Most importantly, a faster ignition and lower production of NOx is observed in the case of plasma discharges compared to thermal energy deposition, owing to the enhanced production of OH radicals and the reforming of NH3 to produce N2 with plasma, respectively. Interesting roles of fuel-radicals such as NH2 and NH in both producing and reducing NO at different instants have also been discussed.