Application of microwave plasma technology to convert CO₂ into high value products

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.