Nanosecond pulsed plasma assisted combustion of ammonia-air mixtures: Effects on ignition delays and NO_x emission

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 NO_x 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 NO_x 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 NO_x 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 NO_x 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.