The global energy challenges along with global warming are regarded as the most important issues faced by humankind in the 21st century. A fossil fuels-based energy economy cannot support the rapidly increasing world energy demand in a sustainable manner. Hence, the development and implementation of alternative solutions to the use of fossil fuels have become a top priority for goverments, industries and academia. In this regard, a collaborative project (ALTEREGO) – funded by the European Union with the involvement of four industrial partners and four academic institutions, was carried out to develop novel forms of energy for intensified chemical manufacturing. In this thesis, the application of microwave plasma technology to convert carbon dioxide (CO2) into added-value products was studied with a twofold purpose: the storage of electricity into chemicals and the chemical recycling of CO2. This thesis is divided into four different sections where fundamental and engineering aspects of microwave plasma and its application to CO2 transformation are investigated. The first section tries to determine whether microwave plasma reactors can outperform conventional thermal chemical reactors, particularly when CO2 is part of the feedstock. The second section explores further optimization of microwave plasma reactors by combining experimental and modelling work. The third section tackles the problem of implementation of complex kinetic models, exemplified for CO2 dissociation, into multidimensional multiphysics simulations. The last section discusses scale up of microwave plasma technology, potential applications in the chemical industry and the milestones on the way to implementation of the technology to commercial scale. In this doctoral work, a bench-scale microwave plasma reactor was built to investigate two key chemistries: the reduction of CO2 with hydrogen (H2) and the splitting of pure CO2. In Chapter 2, we prove that microwave plasma can outperfom conventional thermal reactors; a chemical CO2 conversion as high as ~80% was attained under microwave plasma conditions, compared to ~60% via thermal processes. High microwave power input, high H2 content in the feed and low operating pressure favoured the attaitment of high CO2 conversions. Chapter 3 shows that two-dimensional multiphysics models with simple chemistries (e.g. argon) allow to study different reactor configurations in order to find the optimum performance. Thus, modelling results were used to develop a modified downstream section of the microwave plasma reactor that led to the improvement of chemical CO2 conversion (from 40 to 60%) at low H2 content in the feed, which is beneficial given the current limited scalability of the microwave plasma technology. In Chapter 4, a new simplification approach of state-to-state kinetic models in microwave plasma conditions is presented for the CO2 molecule. By means of chemical lumping, significant reduction in the number of species and reactions, 13 and 44 respectively, was achieved as opposed to its benchmark state-to-state kinetic model that required about 100 species and 10000 reactions. Lastly, Chapter 5 summarizes the current state-of-the-art applications of the microwave plasma technology, along with the existing possibilities for scale up. Additionally, a detailed description of the scientific and engineering challenges towards the commercialization of this technology is given. In the last chapter (Chapter 6), the major conclusions of the project are summarized and recommendations for continuation of the research are provided. @en

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

Microwave plasma (MWP) technology is currently being used in application fields such as semiconductor and material processing, diamond film deposition and waste remediation. Specific advantages of the technology include the enablement of a high energy density source and a highly reactive medium, operational flexibility, fast response time to inlet variations and low maintenance costs. These aspects make MWP a promising alternative technology to conventional thermal chemical reactors provided that certain technical and operational challenges related to scalability are overcome. Herein, an overview of state-of-the-art applications of MWP in chemical processing is presented (e.g. stripping of photo resist, UV-disinfection, waste gas treatment, plasma reforming, methane coupling to olefins, coal/biomass/waste pyrolysis/gasification and CO₂ conversion). In addition, two potential approaches to tackle scalability limitations are described, namely the development of a single unit microwave generator with high output power (>100 kW), and the coupling of multiple microwave generators with a single reactor chamber. Finally, the fundamental and engineering challenges to enable profitable implementation of the MWP technology at large scale are discussed.

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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