Ultrafast Kilowatt-Range Microwave Pulsing for Enhanced CO2 Conversion in Atmospheric-Pressure Plasmas

TL;DR

This study demonstrates ultrafast microwave pulsing at kilowatt power levels to improve CO2 conversion efficiency in atmospheric-pressure plasma reactors, with detailed analysis of plasma stability and temperature effects.

Contribution

It explores the scalability of microwave pulsing for CO2 conversion to kilowatt levels across different reactor configurations, highlighting stability and efficiency impacts.

Findings

Stable plasma operation at ~4 kW pulsed power with microsecond pulses.

<40% increase in CO2 conversion and <20% in energy efficiency compared to continuous wave.

No plasma reignition observed at kilowatt scale, affecting temperature modulation.

Abstract

Ultrafast microwave power pulsation is demonstrated as an effective strategy to enhance CO2 conversion in atmospheric-pressure plasma reactors. While initial experiments at several hundred watts in a compact coaxial plasma torch showed improved performance, the present study investigates the scalability of this approach to kilowatt-range microwave power. Conversion and energy efficiency were examined in two reactor configurations: a Surfaguide-based system (KIT) and a cavity-based plasma torch (IPP), and benchmarked against the compact coaxial torch. Both kilowatt-scale setups share similar microwave coupling schemes, power levels, reactor tubes, and gas injection geometries, but differ in afterglow treatment. The torch at IPP employs rapid nozzle-based quenching, whereas the Surfaguide-based reactor relies on slower cooling along an extended quartz tube. Stable plasma operation was achieved at pulsation peak powers of ~4 kW and pulse durations from sub-microseconds to microseconds, with stability limited to inter-pulse times of ~10 us (cavity-based torch) and ~12 us (Surfaguide-based reactor). In contrast to the coaxial torch, no plasma reignition regime was observed in either kilowatt-scale reactor, resulting in weaker plasma temperature modulation. Notably, the period-averaged gas temperature in the Surfaguide-based reactor exceeded that under continuous-wave operation. Under these conditions, relative enhancements of <40% in CO2 conversion and <20% in energy efficiency were measured compared with continuous-wave operation. These improvements were largely suppressed in the torch at IPP, presumably due to rapid afterglow quenching. Finally, analysis of the instantaneous reflected microwave power provided qualitative insights into electron density dynamics during the power-OFF and power-ON phases.