Plasma-assisted CH₄/NH₃/H₂ reaction varying thermal NOₓ formation and critical safety boundary under different swirling intensity

For the sake of controlling carbon emissions imposed by the United Nations Framework Convention on Climate Change (UNFCCC) and China Regulation (3060 scope), the international community is accelerating for implementing green energy, e.g. NH₃ and H₂. NH₃/H₂ dual fuel is well studied, however CH₄/NH₃/H₂ ternary fuel as a significant alternative fuel remains rarely researched, particularly regarding its thermal NOₓ formation mechanisms under dielectric barrier discharge (DBD) plasma coupled with swirling effectiveness. This study conducts emission experiments across φ = 0.5–1.1(with boundary tests extending to φ ≈ 0.2–0.4), swirling angle 20°-80°, and DBD voltage 0–16 kV in combination with OH-PLIF, flue gas analyzer, and DBD high-frequency power supply techniques. The major findings are obtained with fundamental mechanisms illustration: 1) Under non-DBD combustion, reducing the swirling angle from 80° to 20° at φ = 0.7 decreases NOₓ by 49%. Lower swirling promotes flame laminarization, reduces reaction residence time and limits thermal OH generation, suppressing NOₓ formation. 2) Under DBD combustion (as φ = 1.1 and voltage exceeds 8 kV), NOₓ maximally drops by 27% reduction because of electron-driven NH₃ dissociation and excited species (O( 1 D), N( 2 D)) production that induce thermal NH₂/NH, thus causing NO reduction. 3) DBD significantly widens the critical safety boundary, as critical ignition equivalence ratio decreases from φ = 0.34 to φ = 0.24 (a 29.4% widening of the lean ignition limit). This is attributed to direct dissociation of O₂ and fuel by high-energy electrons, bypassing thermal pyrolysis and reducing the ignitability barrier. The findings demonstrate that DBD plasma offers regime-dependent functionality, providing safety benefits under lean conditions and emissions benefits under rich conditions, thereby enabling flexible control of CH₄/NH₃/H₂ ternary fuel combustion.