Plasma reactors emerge as a promising alternative to cope with some of the biggest challenges currently faced by humanity, the global warming, the increasing global energy demand and the need for efficient storage of electricity from renewable energy sources. Plasma reactors have the potential to enable the storage of green renewable electricity into fuels and chemicals through processes whereby CO2 can be used as a feedstock. Owing to these potential benefits there is a need to investigate this technology from a chemical and process engineering perspective. Big challenges are still hindering the development of plasma reactors into a feasible industrial technology. Despite its limitations, computer modelling is an excellent tool to tackle such challenges. It is well known that the chemistry of non-thermal plasmas is usually the most challenging and complex part of plasma modelling due to the large number of species and reactions involved, which can reach hundreds and thousands ones, respectively; hence, there is need for practical approaches to study, design and optimize plasma reactors. This thesis summarizes the research performed towards the development of engineering approaches to study and model plasma reactors by taking CO2 dissociation in a non-thermal microwave plasma reactor as the case study. The vibrational kinetics of CO2 under the typical conditions of non-thermal microwave plasma are studied through an isothermal reaction kinetics model. The importance of the different collisional processes is evaluated throughout the conditions and timescales at which the CO2 dissociation takes place. The long timescale behavior of the vibrational-to-translational temperature ratio at different conditions is discussed and it is shown that its behavior at increasing gas temperatures can be fitted to an algebraic expression. This temperature ratio has been identified as a key variable to achieve an energetically efficient dissociation. The vibrational-to-translational temperature ratio is shown to be useful for the reduction of vibrational kinetics, enabling their implementation in practical engineering models. A novel reduction methodology is developed and demonstrated for the case of CO2 by lumping relevant vibrationally excited states within a single group. Through this methodology, the dissociation and vibrational kinetics of CO2 can be captured in a reduced set of reactions, dramatically decreasing the calculation time of the model. A two-step modelling approach for plasma reactors is also developed. The approach is applied for the case of CO2 dissociation in a surface wave microwave plasma reactor. The reduction methodology is applied to incorporate the vibrationally enhanced dissociation of CO2 in the chemistry of the model. The model predictions are compared to experimental results to validate the model and obtain insight into the performance of the reactor. The reduction methodology and the modelling approach are the result of studying the CO2 dissociation in a non-thermal microwave plasma reactor. Nonetheless, these are based on general fundamentals that apply to other types of discharges and chemistries as well. The modelling approach can be used for process engineering applications involving the design, optimization and verification of plasma reactors and their performance. The reduction methodology can be implemented in the modelling approach when the vibrationally enhanced dissociation is considered relevant. @en

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.

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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.

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A novel surface-wave microwave discharge reactor configuration to generate syngas via gaseous CO2 reduction with H2 (non-catalytic Reverse Water-Gas Shift reaction) is studied in the context of power-to-chemicals concept. Improvement of CO2 conversion to maximize CO production is explored by adding an external cylindrical waveguide downstream of the plasma generation system. A 2D self-consistent argon model shows that power absorption and plasma uniformity are improved in the presence of the waveguide. We show experimentally that CO2 conversion is increased by 50% (from 40% to 60%) at the stoichiometric feed ratio H2:CO2 equal to 1 when using the waveguide. At higher H2:CO2 ratios, the effect of the waveguide on the reactor performance is nearly negligible. Optical emission spectroscopy reveals that the waveguide causes significant increase in the concentration of O atoms at a ratio H2:CO2 = 1. The effects of the operating pressure and cooling rate are also investigated. A minimum CO2 conversion is found at 75 mbar and ratio H2:CO2 = 1, which is in the transition zone where plasma evolves from diffusive to combined operation regime. The cooling rates have significant impact on CO2 conversion, which points out the importance of carefully designing the cooling system, among other components of the process, to optimize the plasma effectiveness.@en

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.

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Plasma reactor technologies have the potential to enable storage of green renewable electricity into fuels and chemicals. One of the major challenges for the implementation of these technologies is the energy efficiency. Empirical enhancement of plasma reactors' performance has proven to be insufficient in this regard. Numerical models are therefore becoming essential to get insight into the process for optimization purposes. The chemistry in non-thermal plasmas is the most challenging and complex part of the model due to the large number of species and reactions involved. The most recent reaction kinetics model for carbon dioxide (CO2) dissociation in non-thermal microwave plasma considers more than one hundred species and thousands of reactions. To enable the implementation of this model into multidimensional simulations, a new reduction methodology to simplify the state-to-state kinetic model is presented. It is based on four key elements: 1) all the asymmetric vibrational levels are lumped within a single group or fictitious species, Image ID:c6re00044d-t1.gif, 2) this group follows a non-equilibrium Treanor distribution, 3) an algebraic approximation is used to compute the vibrational temperature from the translational temperature based on the Landau–Teller formula and 4) weighted algebraic expressions are applied, instead of complex differential equations, to calculate the rates of the most influencing reactions; this decreases substantially the calculation time. Using this new approach, the dissociation and vibrational kinetics are captured in a reduced set of 44 reactions among 13 species. The predictions of the reduced kinetic model regarding the concentrations of the heavy species in the afterglow zone are in good agreement with those of the detailed model from which the former was derived. The methodology may also be applied to other state-to-state kinetic models in which interactions of vibrational levels have the largest share in the global set of reactions.@en