défense publique de la dissertation de doctorat de Monsieur Senne VAN ALPHEN

Titre de la dissertation: »Modelling plasma reactors for sustainable CO₂ conversion and N₂ fixation. »

Promoteurs de thèse: Monsieur Rony Snyders et Madame Annemie Bogaerts

Résumé de la dissertation: 200 years ago, humanity started the industrial revolution by discovering one of the Earth’s greatest sources of concentrated energy: fossil fuels. Thanks to fossil fuels, our civilization saw unprecedented technological advancements, facilitating work through engines and greatly improving our daily life through the production of plastics, pharmaceuticals and fertilizers.However, after 200 years of burning fossil fuels, it has become alarmingly clear that these advancements came at a great cost, as greenhouse gas emissions, associated to burning fossil fuels, threaten to cause irreversible climate changes through global warming. Given the major environmental concerns associated with fossil fuels, a short-term transition from a carbon-based energy economy to a sustainable one based on green electricity, is essential. A key step concerning this transition exists in developing electricity-driven alternatives for chemical processes that rely on fossil fuels as a raw material. A technology that is gaining increasing interest to achieve this, is plasma technology.Using plasmas to induce chemical reactions by selectively heating electrons in a gas has already delivered promising results for gas conversion applications like CO2 conversion and N2 fixation, but plasma reactors still require optimization to be considered industrially competitive to existing fossil fuel-based processes and emerging other electricity-based technologies. In this thesis I develop computational models to describe plasma reactors and identify key mechanisms in the plasma chemistry of plasma-based CO2 conversion and N2 fixation. By using modeling, I aim to answer questions that can’t be solved by experiments alone and use the combined insights to optimize the plasma process.After a general introduction in chapter 1, and a description of the developed models in chapter 2, I first use the models in chapter 3 to describe a new rotating gliding arc (GA) reactor operating in two arc modes, which, as revealed by my model, are characterized by distinct plasma chemistry pathways. By combining experiments from a fellow colleague and my modelling, we reach record values for N2 fixation into NOx in a GA reactor operating at atmospheric pressure, obtaining NOx concentrations up to 3.4%, at an energy cost of 2.4 MJ mol−1.Subsequently in chapter 4, my colleague and I even further improve the reactor’s performance by a combined computational and experimental study of an effusion nozzle added to the rotating GA reactor, reaching the best results to date for N2 fixation into NOx at atmospheric pressure, i.e., NOx concentrations up to 5.9%, at an energy cost down to 2.1 MJ/mol. My simulations reveal that the effusion nozzle acts as very efficient heat sink, causing a fast drop in gas temperature when the gas molecules leave the plasma, hence limiting the recombination of NO back into N2 and O2.In chapter 5, I investigate the possible improvement of N2 admixtures in plasma-based CO2 and CH4 conversion, as significant amounts of N2 are often found in industrial CO2 waste streams, and gas separations are financially costly. Through combining my models with the experiment from a fellow PhD student, we reveal that moderate amounts of N2 (i.e. around 20%) increase both the electron density and the gas temperature to yield an overall energy cost reduction of 21%.Finally, in chapter 6 I model quenching nozzles for plasma-based CO2 conversion in a microwave reactor, to explain the enhancements in CO2 conversion that were demonstrated in experiments. Through computational modelling I reveal that the nozzle introduces more convective cooling by mixing the gas, as well as more conductive cooling through the water-cooled walls of the nozzle. I show that gas quenching and the suppression of recombination reactions have more impact at low flow rates, where recombination is the most limiting factor in the conversion process.

Overall, I use computational models to reveal the underlying mechanisms in three different plasma reactors for three different gas conversion applications, i.e. N2 fixation, combined CO2-CH4 conversion and CO2 splitting to pinpoint the reason for the good results that were obtained in experiments. In chapter 7, I offer an overall conclusion to the previous chapters and present a future outlook for plasma-based N2 fixation and CO2 conversion.