Refined kinetic mechanism for modeling ammonia combustion in air assisted by nanosecond discharged plasma

Abstract

This study explores the effects of Nanosecond Pulsed Discharge Plasma (NSPD) on the ignition and flame propagation characteristics of ammonia (NH3)/air mixtures at low and intermediate temperatures under atmospheric pressure. A newly developed and validated plasma-assisted kinetic mechanism is proposed to evaluate both Ignition Delay Time (IDT) and Laminar Flame Speed (LFS) across a range of temperatures and equivalence ratios. Results show that plasma significantly reduces IDT and enhances LFS by generating excited species and radicals, such as H, O, OH, NH2, and O(1D), that accelerate reaction pathways and enable earlier chain-branching. The effect is most pronounced at low temperatures (T < 950 K), where thermal chemistry is limited, and plasma-induced kinetics play a dominant role. Sensitivity analyses reveal that reactions involving NH2 and H atoms are the most impactful in reducing IDT, with NH2 + NO ⇌ NNH + OH emerging as the key pathway, especially under plasma conditions. The role of H atoms also becomes up to three times more significant in the presence of plasma. For LFS, the chain-branching reaction H + O2 ⇌ OH + O is consistently the most influential, with plasma further amplifying its contribution. The maximum LFS is observed at Φ ≈ 1.1, for both plasma and non-plasma cases, however, plasma-induced enhancements are more evident at lean equivalence ratios (Φ = 0.8), where the additional radicals generated by NSPD have the greatest relative impact. At stoichiometric and rich conditions (Φ ≥ 1.0), thermal activation prevails and the plasma effect becomes marginal. Overall, the study demonstrates that NSPD is a promising strategy to enable and control low-temperature ammonia combustion by actively modulating ignition chemistry and flame dynamics.