Quantitative comparison of the benefits of cracking and nanosecond repetitive pulsed discharges on the lean blow-off, emissions, and topology of ammonia premixed swirl flames

Abstract

In the last decade, ammonia has gained interest as an alternative fuel for the energy sector. The main advantages include its carbon neutrality and ease of storage at industrial scale. However, to consider ammonia as an alternative carbon-free fuel it is necessary to solve two main issues, namely flame stability and pollutant emission. In this work we compared two promising technologies used to enhance ammonia combustion, specifically partial cracking and plasma assisted combustion (PAC). The experiments were carried out for different blends of ammonia–hydrogen–nitrogen–air that reproduce the conditions of cracking as well as for pure ammonia assisted by nanosecond repetitive pulsed (NRP) discharges for the PAC. The cracking and PAC were compared for the same added power ratio at atmospheric conditions. Different added power ratios, from 0.1% to 4% of the flame’s total thermal power, were tested. The lean blow-off limits were measured for a range of bulk flow velocities between of 4 and 12 m/s. For the pollutant emissions, NOx, NH3, and N2O were considered. The effect of both strategies on the Laminar burning velocity (LBV) was explored by numerical simulations. For the same added power ratio, partial cracking extended the lean blow-off limits further than NRP discharges, even with a high pulse repetition frequency of 30 kHz. This result indicated that cracking was more effective in stabilizing the flame than PAC. Regarding NOx emissions, NRP discharges induced only a third of the increase observed with cracking. However, the reduction in N2O associated with cracking was twice as much as that for NRP discharges. Finally, cracking proved more efficient in reducing the flame height, which was supported by the simulations of LBV. These results suggest that PAC strategies optimized for hydrocarbon flames are less efficient on ammonia combustion, and that more work is needed to develop plasma actuators for NH3 combustion.Novelty and Significance statement This work marks the first one-to-one quantitative comparison between plasma actuation and thermal cracking on ammonia swirl flames. It serves as a pivotal exploration to assess and understand the economic aspects of these strategies, providing valuable insights into their efficiency and potential for practical implementation.