Importance of geometric effects in scaling up energy-efficient plasma-based nitrogen fixation

Abstract

Despite the recent promising potential of plasma-based nitrogen fixation, the technology faces significant challenges in efficient upscaling. To tackle this challenge, we investigate two reactors, i.e., a small one, operating in a flow rate range of 5–20 ln min⁻¹ and current range of 200–500 mA, and a larger one, operating at higher flow rate (100–300 ln min⁻¹) and current (400–1000 mA). Both reactors operate in a pin-to-pin configuration and are powered by direct current (DC) from the same power supply unit, to allow easy comparison and evaluate the effect of upscaling. In the small reactor, we achieve the lowest energy cost (EC) of 2.8 MJ mol⁻¹, for a NO_x concentration of 1.72%, at a flow rate of 20 ln min⁻¹, yielding a production rate (PR) of 33 g h⁻¹. These values are obtained in air; in oxygen-enriched air, the results are typically better, at the cost of producing oxygen-enriched air. In the large reactor, the higher flow rates reduce the NO_x concentration due to lower SEI, while maintaining a similar EC. This stresses the important effect of the geometrical configuration of the arc, which is typically concentrated in the center of the reactor, resulting in limited coverage of the reacting gas flow, and this is identified as the limiting factor for upscaling. However, our experiments reveal that by changing the reactor configuration, and thus the plasma geometry and power deposition mechanisms, the amount of gas treated by the plasma can be enhanced, leading to successful upscaling. To obtain more insights in our experiments, we performed thermodynamic equilibrium calculations. First of all, they show that our measured lowest EC closely aligns with the calculated minimum thermodynamic equilibrium at atmospheric pressure. In addition, they reveal that the limited NO_x production in the large reactor results from the contracted nature of the plasma. To solve this limitation, we let the large reactor operate in so-called torch configuration. Indeed, the latter enhances the NO_x concentrations compared to the pin-to-pin configuration, yielding a PR of 80 g h⁻¹ at an EC of 2.9 MJ mol⁻¹ and NO_x concentration of 0.31%. This illustrates the importance of reactor design in upscaling.