Combustion characteristics of ammonia/air flames assisted by microwave plasma in a two-staged swirl burner

As a zero-carbon fuel, ammonia (NH₃) has represented a promising candidate to replace fossil fuels for reducing greenhouse gas emissions. In this work, microwave-generated plasma torch was employed to assist two-staged swirl combustion of NH₃/air. Comprehensive experiments were conducted to elucidate the effects of plasma activation, primary equivalence ratio (Φ_p), and global equivalence ratio (Φ_g) on combustion characteristics. The plasma achieves simultaneous extension of lean blow-off (LBO) limit and reduction of both NOx and N₂O. OH planar laser-induced fluorescence (OH-PLIF) and NH₂* chemiluminescence diagnostics revealed that microwave discharge generates substantial concentrations of OH and NH₂* radicals, which markedly accelerate ammonia pyrolysis and oxidation, yielding a more compact flame and significantly widening the operable range of Φ_p. By integrating plasma assistance with staged combustion, the LBO limit of NH₃/air flames was extended from 0.55 ∼ 0.63 (without plasma) to 0.40 ∼ 0.56 within the range 2 ≤ Φ_p ≤ 6. At the same time, NO emissions were simultaneously reduced from ∼2600 ppm to ∼1000 ppm when Φ_g was fixed at 0.7. Spatial distributions of OH and NH₂*, combined with mechanistic analyses of NOx formation and consumption pathways, indicate that NOx emissions in the absence of plasma are governed primarily by OH-controlled formation processes, whereas DeNOx process dominates NO consumption under plasma activation. Meanwhile, N₂O emissions can be maintained below 50 ppm by appropriately increasing Φ_p.