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

摘要

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 ( NH 3 )/air mixtures, a mechanism has been assembled in this work using well-validated plasma reactions of NH 3 with O 2 and N 2 , 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/ NO 2 emission is discussed. At lower E/N, vibrational-to-translational (VT) relaxation of the vibrational states of NH 3 and N 2 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 ( 4) = 0.5) compared to stoichiometric and fuel-rich ( 4) = 1.2) conditions owing to the lower consumption of OH radicals through the reactivity-inhibiting reaction NH 3 + OH -> NH 2 + H 2 O 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 N 2 . At the higher pressure of 3 atm, the pressure dependent recombination reaction H + O 2 + M -> HO 2 + 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 N 2 and O 2 , 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 NH 3 to produce N 2 with plasma, respectively. Interesting roles of fuel-radicals such as NH 2 and NH in both producing and reducing NO at different instants have also been discussed.(c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.