Nanosecond pulsed plasma-assisted MILD combustion of ammonia

Abstract

Ammonia is a promising clean and sustainable energy carrier, yet challenges persist in achieving stable combustion, particularly concerning poor ignition quality and elevated NOx emissions. Recent research suggests that the Moderate or Intense Low-oxygen Dilution (MILD) regime could address these challenges for ammonia combustion. This study aims to optimize the MILD regime using non-equilibrium plasma discharges, specifically nanosecond repetitive pulsed discharges (NRPD). While the beneficial effects of NRPD on ammonia chemistry have been demonstrated in traditional applications, their impact under the highly diluted conditions characteristic of the MILD regime remains unexplored. This numerical study employs a detailed two-temperature model to investigate the effects of pulsed discharges in ammonia/air mixtures, simulating conditions representative of the MILD regime. The research comprehensively explores the selection of optimal discharge settings and examines plasma effects on various parameters, including ignition delay time, flammability limit, radical production, and emissions. Equivalence ratios ranging from 0.2 to 2 and dilution levels up to 2.5% O are considered in this investigation. Results indicate that NRPD show a notable benefit by enlarging fuel-lean and fuel-rich stability limits, promising enhanced operational flexibility. Examining OH radicals and NOx emissions underscored a consistent plasma-driven mechanism, reducing emissions, also in the MILD regime.