Plasma reforming for enhanced ammonia-air ignition: A numerical study

Abstract

Using an in-house 0D plasma chemical solver, this paper investigates the species involved in plasma-assisted reforming of both pure ammonia and stoichiometric ammonia-air mixtures. A nanosecond repetitively pulsed plasma is simulated for dielectric barrier discharge conditions, with reduced electric fields of 180 and 360 Td, energies per pulse of 0.5 and 1 mJ/cm³, and pulse repetition frequencies up to 500 kHz. To show the effect of reforming on combustion performance, the reformates are fed into a neutral species combustion chemistry solver to calculate the ignition delay time at gas-turbine relevant conditions. For a reformed stoichiometric mixture, it is possible to achieve a reduction of two orders of magnitude in ignition delay time. This reduction, however, comes at the cost of lost enthalpy, as ammonia reacts with oxygen to create water. Path flux and sensitivity analyses were performed, and it as found that the two most crucial species in the reformate were H₂ and NH₂. The presence of NH₂ in high concentration also resulted in lower concentrations of NO after ignition, compared to the unreformed mixture. When reforming pure ammonia, the same number of pulses and energy as in the stoichiometric case reduce ignition by one order of magnitude. A higher reduction is possible with more pulses, unlike the stoichiometric reforming case in which ignition is reached during the reforming process, and with no loss of enthalpy due to oxidation. At 200 kHz, a reduction of two orders of magnitude is possible after 1500 pulses. These results support the feasibility of plasma-assisted reforming for the improvement of ammonia combustion characteristics at relevant conditions.