Experimental investigation on the effects of gliding arc plasma on the combustion characteristics of air stratified NH3 flames

This study investigates the impact of gliding arc plasma (GAP) on emission characteristics in NH3-air flames with premixing ratios ranging from 0 % to 25 % at global equivalence ratios (ϕ) of 0.8 and 0.9. The experiments are carried out in a dual swirl burner, where a premixed NH3-air mixture is introduced through the inner nozzle and central lance for GAP interaction, and non-premixed air is supplied via the outer nozzle. This study is the first to apply gliding arc plasma (GAP) to air-stratified ammonia flames, and this approach provides a significant and novel strategy for reducing NOx emissions. The results show that GAP reduces NOx emissions by up to 40 % under fully non-premixed conditions. At ϕ of 0.8, a critical transition appears when premixed air exceeds 20 %, resulting in increased NO emissions. In contrast, for ϕ = 0.9, GAP consistently maintains stable NO reduction across all tested premixing ratios. The spectral analysis of the flame shows that the OH and NH2 radicals play a pivotal role in mediating the effect of GAP on NO formation. These radicals exhibit competing influences, with NH2 formation favoured under plasma activation. The NH2 radical primarily forms through two plasma-driven pathways: O(2D) + NH3 → OH + NH2 and N(2D) + NH3 → NH + NH2. The observed reduction of NO with plasma is primarily due to the increased production of NH2 related to OH.