Modeling of microwave plasmas for carbon dioxide conversion.
26 March 2018
Campus Drie Eiken, R1 - Universiteitsplein 1 - 2610 Antwerpen-Wilrijk (route: UAntwerpen, Campus Drie Eiken
Organization / co-organization:
Faculty of Science
Public defence of the PhD thesis of Mr. Antonin Berthelot - Faculty of Science
There has been over the last few years a growth of the energy production by renewable energy sources. However, these energy sources are typically intermittent. Therefore, large research efforts have been recently directed towards finding solutions for energy storage. In particular, the conversion of CO2 to CO (and oxygen) by non-equilibrium plasmas, followed by conversion into hydrocarbons through the Fischer-Tropsch process, would be an interesting way to store energy via a carbon-neutral process. Microwave plasmas have been shown to be one of the most energy-efficient plasma sources.
However, the control of non-equilibrium plasmas is far from trivial, as their kinetics is complex and cannot be directly controlled by adjusting external parameters. Therefore, the energy efficiency currently achieved in plasma-based CO2 dissociation needs to be enhanced for potential industrial applications, and thus, there is a need for a more detailed understanding of the processes occurring in a CO2 plasma, notably through modeling.
First, a 2D-axisymmetric argon model was built, operating over a wide range of pressure. This model, albeit for argon as a first step into the development of a 2D model for CO2, provides good insight into the spatial properties of the plasma. This is of interest for the 0D model, subsequently developed to study more in detail the complex chemical kinetics of CO2 dissociation in a microwave plasma.
The dissociation mechanisms and energy transfers occurring in the discharge were investigated and the model reveals that the discharge tends to thermalize faster at high gas temperature and high pressure, since vibration-translation relaxation occurs faster. Optimal conditions for energy efficient CO2 dissociation are identified.
A reduction of the chemistry set and a level-lumping method designed to reduce the computational load associated with the description of the CO2 kinetics are also presented. These modeling techniques enable to model the CO2 microwave discharge in higher-dimensional models in future work. Moreover, the effect of uncertainties associated with the rate coefficients on the model results is studied and it is found that, while the error on the model results can be significant, the trends predicted by the model can be considered reliable.
This PhD thesis increases our knowledge on the kinetics of CO2 discharges and gives useful directions. There is of course still a long way to go before plasma-based CO2 conversion can be used industrially, but we believe that this work paves the way for future modeling and experimental research.