Nanosecond pulsed plasma-assisted MILD combustion of ammonia

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 2 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. Novelty and Significance Statement The novelty of this research is the application of non-equilibrium plasma discharges to improve ammonia combustion in the MILD regime. It offers a new and original solution to the challenges associated with the poor ignition quality and high NOx emissions that are currently limiting its practical implementation. For the first time, a numerical analysis is provided to quantify the benefits of plasma chemistry activation on the combustion performance. Another novel aspect of this work is the use of a detailed two-temperature model to describe the plasma-combustion interactions in an integrated computational framework. The literature reports only a few references on the interaction of plasma with MILD combustion conditions. The former were modeled using a very simplified model to describe the evolution of the plasma species. Consequently, we believe that this contribution will significantly advance the state-of-the-science in field of plasma-assisted ammonia combustion.