Modeling CO2 dissociation in microwave plasma reactors through quasi-steady state non-equilibrium vibrational kinetics

Abstract

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.