Plasma reactors have the potential to enable CO₂ utilization technologies and so there is need to investigate their performance from a chemical or process engineering perspective. Multiphysics models are excellent tools to carry out this analysis; however, practical engineering models of plasma reactors are limited. Herein a two-step modelling approach for plasma reactors is presented. In the first step, a 2D plasma reactor model with a simple chemistry is used to characterize the discharge. The result of this step is used in the second step to develop a global (volume averaged) model of the reactor with the actual chemistry. The approach is applied in the case of CO₂ dissociation in a non-thermal surface wave microwave plasma reactor. Preliminary calculations reveal the need to include the vibrationally enhanced dissociation of CO₂ in the chemistry of the model. Reduced vibrational kinetics are employed for this purpose by introducing the fictitious species . The model predictions are compared to experimental results to validate the model and obtain insight into the performance of the reactor. In comparison to the experimental results the conversions obtained with the model are underestimated between 11% and 25%. The dominant dissociation paths in the plasma reactor are also identified. Further calculations are performed to show the importance of an approximate description of the power deposition. Limitations of the approach are discussed as well, especially those with major contribution to the discrepancies between experimental and modelling results.

Non-thermal microwave plasma reactors can efficiently split the CO₂ molecule. However, big challenges remain before this technology can become a feasible industrial technology. Computer modelling can be very useful to tackle such challenges. Detailed kinetic modelling is commonly used to gain insights into the complex vibrational kinetics of CO₂, as vibrational excitation is strongly related to the energy efficiency in the dissociation process. The vibrational-to-translational temperature ratio has been identified as a key variable to achieve high energy efficiencies. This ratio has also been used to simplify detailed CO₂ vibrational kinetics, notably reducing the number of species and reactions required to model the non-thermal plasma. In this paper we use an isothermal reaction kinetics model to study the vibrational kinetics of CO₂ under the typical conditions used in non-thermal microwave plasma experiments. The importance of the different collisional processes is evaluated with respect to the different conditions and timescales at which CO₂ dissociation takes place. The long timescale behavior of the vibrational-to-translational temperature ratio under different conditions is discussed in detail. It is shown that the behavior at increasing gas temperatures can be fitted to an expression that incorporates the Landau-Teller temperature dependence. This is confirmed by the average adjusted R-square values higher than 0.99 and the average root mean square error values smaller than 0.22 at low gas temperatures. The limitations of the fitting expression are also discussed, especially the conditions and timescales at which it yields better results.

In the context of converting electricity into value-added chemicals, the reduction of carbon dioxide (CO₂) with hydrogen (H₂) in a surface-wave-induced microwave plasma discharge, so-called surfatron, was investigated. The effect of different input variables such as gas flow rate, feed gas composition ratio (H₂:CO₂) and specific energy input (SEI) on the reactor performance, i.e. the CO₂ conversion and energy efficiency, was assessed. A maximum CO₂ conversion of 85% is obtained when the feed gas mixture ratio (H₂:CO₂) was equal to 3. Moreover, a trade-off between CO₂ conversion and energy efficiency was clearly noticed when varying the supplied microwave power. High SEI resulted in high conversions and low energy efficiencies and vice-versa. Furthermore, the saturation of the carbon monoxide (CO) production was found at high SEI. These results were rationalized by means of a simplified reaction scheme and by optical emission spectroscopy analysis, which showed that the formation of hydrogen (H) and oxygen (O) atoms in the plasma are the dominant channels driving the reaction pathway. We also observed higher electron densities and temperatures at higher H₂ content, which may explain the high conversions achieved in the plasma reactor at high H₂:CO₂ ratios. H₂ is then not only capable of acting as a “catalyst” for CO₂ decomposition but also modifies the plasma properties, which seems to greatly enhance the potential of chemical reactions and thus the dissociation rates.