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In recent years there is growing interest in the use of plasma technology for CO2 conversion. To improve this application, a good insight in the underlying mechanisms is of great importance. This can be obtained from modeling of the detailed plasma chemistry, to understand the chemical reaction pathways leading to CO 2 conversion (either in pure form or mixed with another gas). Moreover, in practice several plasma reactor types are being investigated for CO 2 conversion, so in addition it is essential to model these reactor geometries, in order to be able to improve their design, and to achieve the most energy efficient CO2 conversion. Modeling the detailed plasma chemistry of CO2 conversion in complex reactors is, however, very time-consuming. This problem can be overcome by using a combination of two different types of models. 0D chemical reaction kinetics models are very suitable for describing the detailed plasma chemistry, while the characteristic features of different reactor geometries can be studied by 2D or 3D fluid models. The latter can in first instance be developed in argon or helium, with a simple chemistry, to limit the calculation time, but the ultimate aim is to implement the more complex CO2 chemistry in these models. In the present paper, examples will be given of both 0D plasma chemistry models and 2D and 3D fluid models for the most common plasma reactors used for CO2 conversion, to emphasize the complementarity of both approaches. Furthermore, based on the modeling insights, the paper discusses the possibilities and limitations of plasma-based CO 2 conversion in the different types of plasma reactors, and what would be needed to make further progress in this field.

We present a 3D and 2D Cartesian quasi-neutral plasma model for a low current argon gliding arc discharge, including strong interactions between the gas flow and arc plasma column. The 3D model is applied only for a short time of 0.2 ms due to its huge computational cost. It mainly serves to verify the reliability of the 2D model. As the results in 2D compare well with those in 3D, they can be used for a better understanding of the gliding arc basic characteristics. More specifically, we investigate the back-breakdown phenomenon induced by an artificially controlled plasma channel, and we discuss its effect on the gliding arc characteristics. The back-breakdown phenomenon, or backward-jump motion of the arc, as observed in the experiments, results in a drop of the gas temperature, as well as in a delay of the arc velocity with respect to the gas flow velocity, allowing more gas to pass through the arc, and thus increasing the efficiency of the gliding arc for gas treatment applications.

The modelling of a gliding arc discharge (GAD) is studied by means of the quasineutral (QN) plasma modelling approach. The model is first evaluated for reliability and proper description of a gliding arc discharge at atmospheric pressure, by comparing with a more elaborate non‐quasineutral (NQN) plasma model in two different geometries – a 2D axisymmetric and a Cartesian geometry. The NQN model is considered as a reference, since it provides a continuous self‐consistent plasma description, including the near electrode regions. In general, the results of the QN model agree very well with those obtained from the NQN model. The small differences between both models are attributed to the approximations in the derivation of the QN model. The use of the QN model provides a substantial reduction of the computation time compared to the NQN model, which is crucial for the development of more complex models in three dimensions or with complicated chemistries. The latter is illustrated for (i) a reverse vortex flow (RVF) GAD in argon, and (ii) a GAD in CO2. The RVF discharge is modelled in three dimensions and the effect of the turbulent heat transport on the plasma and gas characteristics is discussed. The GAD model in CO2 is in a 1D geometry with axial symmetry and provides results for the time evolution of the electron, gas and vibrational temperature of CO2, as well as for the molar fractions of the different species.

In this study we quantitatively investigate for the first time the plasma characteristics of an argon gliding arc with a 3D model. The model is validated by comparison with available experimental data from literature and a reasonable agreement is obtained for the calculated gas temperature and electron density. A complete arc cycle is modeled from initial ignition to arc decay. We investigate how the plasma characteristics, i.e., the electron temperature, gas temperature, reduced electric field, and the densities of electrons, Ar + and Ar2 + ions and Ar(4s) excited states, vary over one complete arc cycle, including their behavior in the discharge and post-discharge. These plasma characteristics exhibit a different evolution over one arc cycle, indicating that either the active discharge stage or the postdischarge stage can be beneficial for certain applications.

A gliding arc plasma has great potential for CO 2 conversion into value-added chemicals, because of its high energy efficiency. To improve the application, a 2D/3D fluid model is needed to investigate the CO 2 conversion mechanisms in the actual discharge geometry. Therefore, the complex CO 2 chemical kinetics description must be reduced due to the huge computational cost associated with 2D/3D models. This paper presents a chemistry reduction method for CO 2 plasmas, based on the so-called directed relation graph method. Depending on the defined threshold values, some marginal species are identified. By means of a sensitivity analysis, we can further reduce the chemistry set by removing one by one the marginal species. Based on the so-called flux-sensitivity coupling, we obtain a reduced CO 2 kinetics model, consisting of 36 or 15 species (depending on whether the 21 asymmetric mode vibrational states of CO 2 are explicitly included or lumped into one group), which is applied to a gliding arc discharge. The results are compared with those predicted with the full chemistry set, and very good agreement is reached.Moreover, the range of validity of the reduced CO 2 chemistry set is checked, telling us that this reduced set is suitable for low power gliding arc discharges. Finally, the time and spatial evolution of the CO 2 plasma characteristics are presented, based on a 2D model with the reduced kinetics.

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