Modeling of Plasma-Driven Electrified Chemical Conversion Processes by Non-Equilibrium Kinetics

Electrified chemical conversion has emerged as a pivotal technology for advancing renewable energy and achieving carbon neutrality. Among sustainable solutions, plasma-based nitrogen fixation (NF) shows exceptional promise. By harnessing renewable electricity, this method enables carbon-free nitrogen fertilizer production. However, a critical challenge persists: although plasma-based NF is theoretically more efficient than the Haber-Bosch process, its experimental energy requirements remain prohibitively high, undermining economic feasibility. Further research is essential to unravel the mechanisms of NO formation and enhance energy efficiency. Due to the inherent challenges of plasma diagnostics—such as limited accessibility, small spatiotemporal scales, or the inability to directly measure key quantities—numerical simulation has become indispensable. This thesis presents the development of several fully native codes for studying microwave plasma-based NF applications. Firstly, based on the Fridman–Macheret model, an improved method for calculating vibrationally enhanced reaction rate constants is introduced as vibrational temperature is different from gas temperature. The method is firstly applied to CO2 dissociation, as CO2 molecules possess a more complex structure than N2, resulting in fewer available quasi-classical trajectory (QCT) results. The generalized Fridman–Macheret (GFM) approach achieves accuracy comparable to QCT calculations while improving computational efficiency dramatically, enabling more feasible simulations of complex non-equilibrium systems. The zero-dimensional two-temperature model demonstrates that non-thermal vibrational excitation significantly enhances the dissociation efficiency, aligning more closely with experimental observations than thermal models, validating the accuracy of the GFM method. Secondly, leveraging the GFM method, a one-dimensional multi-temperature quenching model is presented, that analyses NOx formation mechanisms in the afterglow of atmospheric-pressure microwave air plasmas. The model accounts for all necessary vibrational-vibrational (V-V) and vibrational-translational (V-T) relaxation processes, allowing for a comprehensive description of vibrational energy transfer pathways. The plasma region is assumed to be in thermal equilibrium, consistent with experimental observations at high pressure, with electron-kinetic effects neglected for computational simplicity. By varying the plasma gas temperature and cooling rates, the study elucidates the reaction pathways for NOx synthesis. The results reveal that, regardless of cooling rate, the energy cost of NOx formation remains constrained by thermodynamic limits. Subsequently, a quasi-1.5D model is introduced where radial transport and electron kinetics are supplemented to the former model, investigating NOx production in both discharge and afterglow regions of microwave air plasmas at intermediate pressure. In the discharge zone, the non-thermal behavior induced by electron energy transferring to N2 vibrations enhances the NOx production. However, the non-thermal behavior diminishes rapidly in the afterglow. Turbulent effects facilitate radial NO diffusion into cooler regions while simultaneously enhancing thermal conductivity. These findings highlight the potential to improve NOx synthesis efficiency by optimizing turbulence and maintaining non-thermal conditions. Lastly, a generalized quasi-1.5D multi-temperature model where more reactors are used to capture the temperature gradient in the radial, is used to examine NO synthesis in N2-O2 mixtures at 80 mbar under varying gas flow rates and compositions. As the N2 fraction increases, more electron energy channels into N2 vibrational excitation rather than other inelastic processes. Nearly all the NO formation occurs in the plasma core, and then a part of NO diffuses to the periphery in the afterglow. The results highlight that N2 vibrational activation provides a more energy-efficient pathway for NOx production compared to thermal processes. The algorithms and codes developed in this thesis offer critical insights into the interplay of electron kinetics, vibrational excitation, chemical kinetics, and transport phenomena in microwave plasma-based NF. These advancements pave the way for optimizing plasma reactors to achieve energy-efficient NF relevant for industrial applications.