The microwave plasma source invented at IGVP (Institute of Interfacial Process Engineering and Plasma Technology) at the University of Stuttgart has demonstrated the successful conversion of CO₂ into CO at atmospheric pressure, operating with high microwave power and CO₂ flow rates. This novel system ignites the CO₂ plasma without the need for additional igniters, forming a free standing and voluminous plasma. It has recently shown high energy efficiency at high microwave powers and CO₂ flows.

 

Background: CO₂ conversion in a 2.45 GHz plasma torch at IGVP

The initial development of the microwave plasma torch was designed by Leins et al. [1]. The torch consists of a cylindrical resonator with a coaxial resonator mounted below [1]. A quartz glass tube is centrally positioned within the cylindrical resonator. This arrangement allows a simple ignition and sustainable continuous operation without the need of additional igniters. Moreover, the torch is used for the decomposition of halogenated volatile organic compounds or the synthesis of NO_x [2,3]. In early investigations, Wiegers et al. achieved a maximum conversion of 8 % and an energy efficiency of 25 % at a specific energy input of 0.6 eV · molecule^-1 [4].

 

Performance Enhancements through Nozzle Integration

 

To improve conversion and energy efficiency, a nozzle was placed on the cylindrical resonator. Nozzles with different diameters were used. The diameter significantly influences the maximum conversion. The nozzle prevents the back reactions to CO₂ by an improved product gas treatment. The conversion can reach up to 22 % and energy efficiency to 35 % [5].

 

Operation at High Power and CO₂ Flow rates

 

Further experiments extended the operating plasma parameters to higher levels. The system was tested at microwave powers up to 5.7 kW and CO₂ flow rates up to 74 standard liter · minutes^-1. Under these conditions, the plasma process achieved energy efficiency up to 40 % and conversions up to 21 % [6].

The conversion decreases with increasing CO₂ flow and increases with increasing microwave power at constant CO₂ flow. The energy efficiency remains largely independent of microwave power at high CO₂ flows, highlighting the process flexibility.

 

Conclusion

These results suggest that the plasma process can be flexibly tuned. The energy efficiency can be prioritized when the renewable energy availability is limited and high conversion can be target when more energy is available.

 

Author: Katharina Wiegers

 

References

 

[1] M. Leins, L. Alberts, M. Kaiser, M. Walker, A. Schulz, U. Schumacher and U. Stroth, Plasma Processes and Polymers, 2009, 6(1), DOI: 10.1002/ppap.200930604.

[2] A. Schulz, K. Wiegers, M. Troia, M. Bresser, S. Merli, M. Walker and G. E. M. Tovar, Chem. Ing. Tech., 2025, 97(5), DOI: 10.1002/cite.202400158.

[3] M. Troia, C. Vagkidis, A. Schulz, A. Köhn-Seemann, M. Walker and G. E.M. Tovar, Chem. Ing. Tech., 2025, 97(5), DOI: 10.1002/cite.202400137.

[4] K. Wiegers, A. Schulz, M. Walker and G. E. M. Tovar, Chem. Ing. Tech., 2022, 94(3), DOI: 10.1002/cite.202100149.

[5] K. Wiegers, A. Schulz, M. Walker, F. Buck, T. Schiestel and G. E. M. Tovar, ISPC Proceedings, 2023, ISPC2025, Kyoto, Japan, www.ispc-conference.org/ispcproc/ispc25/pdf/2-P-209.pdf.

[6] M. Bresser, K.Wiegers, A. Schulz, M. Walker and G. E. M. Tovar, Chem. Ing. Tech., 2025, 97(5), DOI: 10.1002/cite.202400157.

 

Keywords

 

CO₂ Conversion, Microwave Plasma Torch, Atmospheric Pressure, High Energy Efficiency, self-igniting Plasma Source