Summary:
Electrified chemical conversion has emerged as a key enabler of renewable energy utilization and carbon neutrality. Among the available sustainable pathways, plasma-based nitrogen fixation (NF) stands out as a particularly promising route, enabling the carbon-free synthesis of nitrogen fertilizers by directly harnessing renewable electricity. Despite its theoretical advantage over the energy-intensive Haber–Bosch process, experimental implementations of plasma-based NF still face prohibitively high energy requirements, posing a critical barrier to industrial viability. Understanding and improving the underlying mechanisms of NO formation and energy transfer are therefore essential. However, direct plasma diagnostics are often limited by restricted accessibility, nanosecond-scale dynamics, and difficulties in measuring key species, making numerical modeling indispensable. This thesis presents a comprehensive suite of in-house computational models and algorithms designed to elucidate plasma-assisted NOx synthesis mechanisms in microwave-driven nitrogen fixation.
First, an enhanced approach for calculating vibrationally promoted reaction rate constants is proposed based on the Fridman–Macheret framework, which distinguishes between gas and vibrational temperatures. The generalized Fridman–Macheret (GFM) method is first validated on CO₂ dissociation, a system with complex molecular structure and limited quasi-classical trajectory (QCT) data. The GFM method reproduces QCT-level accuracy while dramatically improving computational efficiency, making it highly suitable for large-scale plasma simulations. A zero-dimensional two-temperature model demonstrates that non-thermal vibrational excitation substantially enhances molecular dissociation, aligning more closely with experimental trends than conventional thermal models, thus validating the predictive power of the GFM approach.
Building upon this foundation, a one-dimensional multi-temperature quenching model is developed to investigate NOx formation in the afterglow of atmospheric-pressure microwave air plasmas. Incorporating all relevant vibrational–vibrational (V–V) and vibrational–translational (V–T) relaxation processes, the model captures detailed vibrational energy transfer dynamics. Assuming local thermal equilibrium in the plasma region—consistent with experimental conditions at high pressure—the study explores the influence of plasma temperature and cooling rate on NOx yields. The findings reveal that, regardless of quenching conditions, the overall energy cost for NOx synthesis remains constrained by thermodynamic limits.
To further resolve spatial and kinetic effects, a quasi-one-and-a-half-dimensional model is introduced, integrating radial transport and electron kinetics to study both discharge and afterglow phases at intermediate pressure. The simulations show that electron-driven vibrational excitation of N₂ enhances NOx formation in the discharge region, though this non-thermal effect rapidly decays in the afterglow. Turbulent diffusion facilitates NO transport toward cooler regions while simultaneously enhancing heat transfer, suggesting that optimized turbulence and sustained non-thermal conditions can improve synthesis efficiency.
Finally, a generalized quasi-1.5D multi-temperature model is employed to study NO formation in N₂–O₂ mixtures at 80 mbar under various flow rates and compositions. With increasing N₂ fraction, electron energy preferentially channels into vibrational excitation rather than inelastic collisions. The results indicate that most NO originates within the plasma core and subsequently diffuses outward. Overall, vibrational activation of N₂ offers a more energy-efficient route for NOx production compared to purely thermal mechanisms.
Collectively, the numerical frameworks and codes developed in this work provide fundamental insights into the coupled roles of electron kinetics, vibrational excitation, chemical kinetics, and transport phenomena in microwave plasma-assisted NF, paving the way toward the rational design of energy-efficient plasma reactors for industrial nitrogen fixation.
First promoter: Richard van de Sanden
Second promoter: Vasco Guerra (Instituto Superior Técnico).
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