Flammability enhancement of swirling ammonia/air combustion using AC powered gliding arc discharges

Aiming at the potential development of ammonia as a carbon-free renewable fuel, this work investigates the flammability and NOx emission of swirling ammonia/air flames and utilizes plasma to enhance ammonia combustion. First, the well-designed rapidly-mixed swirl burner can anchor compact ammonia/air flames under a wide range of flow conditions, with the current maximum heat release of approximately 4.7 kW. The flame is progressively detached from the quartz confinement tube and goes blow-off when the equivalence ratio drops below approximately 0.7–0.8. Then, to alleviate the problem of low flammability, gliding arc discharges driven by a 12.5 kHz alternating current (AC) power supply are facilitated to extend the lean blow-off margin to approximately 0.3–0.4. The localized flame kernels surrounding the discharge column are detected by high-speed photography. The planar laser-induced fluorescence (PLIF) imaging of OH radicals, optical emission spectroscopy, and NH2* chemiluminescence measurement are performed to interpret the intermediate chemistry. Finally, the NOx emission of the swirling ammonia/air flame is measured by a flue gas analyzer. Results show that although the AC-powered gliding arc exhibits weak global effects on the NOx reduction of burner-stabilized ammonia/air flames prevailing at higher equivalence ratios (e.g., φ > 0.75), leaner flames stabilized by discharges can achieve NOx emissions below 100 ppm due to the thermal DeNOx mechanism.