| dc.description.abstract | In recent years, non-thermal plasma technology is becoming popular for chemical applications involving conversion/abatement/decomposition of compounds of interest. The specialty of non-thermal plasmas is to activate chemical processes at atmospheric conditions, which are thermodynamically limited. However, plasma reactors for chemical conversion are still on the laboratory scale (flow rates of the order of a few mL·min-1) and pose scale-up challenges. The challenges mainly arise due to the strong interaction between the plasma and gas, whose interaction behavior is not fully understood. The rotating gliding arc (RGA) plasma reactor is well adapted to scale up for high flow rate applications. The reactor has a tangential entry of the treatment gas, creating a swirl flow that rotates the arc formed between two diverging electrodes. The complex behavior of the rotating gliding arc poses challenges in quantifying the plasma parameters governing the chemistry, such as reduced electric field, gas temperature, electron temperature, and discharge size. Nevertheless, to meet the growing scale of the demand for sustainable energy and chemistry, it is necessary to investigate the performance and behavior of plasma reactors (characterization) with large throughputs. This dissertation: First part proposes a new electrode configuration for the rotating gliding arc plasma reactor that facilitates scaling up of plasma volume without the need to scale up the reactor size. The flow regime (laminar, transitional, and turbulent) in the electrode region is characterized for flow rates between 5 SL·min-1 and 50 SL·min-1 for multiple tangential entries, using the Reynolds number defined based on the tangential velocity. The defined Reynolds number is found to have a linear relationship with the arc’s rotation; the arc rotation and gas rotation are comparable. The second part validates the applicability of a method to estimate the reduced electric field, using a multi–diagnostic approach and collisional-radiative modeling. Image processing techniques are developed to estimate the length and diameter of the rotating arc. In the third part, the reactor’s electrical, optical, morphological, and chemical characteristics are investigated at transitional (5 SL·min-1) and highly turbulent (50 SL·min-1) flow regimes, using nitrogen as a plasma-forming gas. When changed from transitional to turbulent flow, operation mode transitioned from glow to spark due to frequent reignition events; the average reduced electric field and electron temperature raises (38→92 Td, 0.84→2.2 eV); plasma’s gas temperature is slightly cooled (2973→2807 K). The G–factor (molecules generated per 100 eV of energy input) of the chemically active singlet and triplet metastable states of N2 increases by a factor of 20 and 65, respectively—indicating increased energy efficiency, a promising feature for chemical applications. This result is further confirmed by a dilute toluene stream (112±10 ppmV) conversion experiment, which shows a three-fold increase in the energy efficiency achieving 3.0±0.2 g·kWh-1 at a highly turbulent flow. The final part investigates the decomposition of dilute methane (1% by volume) in a nitrogen stream. When changed from transitional (5 SL·min-1) to turbulent (50 SL·min-1) flow, the operation mode changes from glow to spark type; the average electric field, plasma’s gas temperature, and electron temperature raises (106→156 V·mm-1 , 3681→3911 K, and 1.62→2.12 eV). The energy efficiency in decomposing CH4 increases by a factor of 3.9 (16.1→61.9 g·kWh-1). Chemical kinetics simulation shows that the reactions induced by H, CH, and CH3 (key–species) are the first three dominants in CH4 consumption yet differ by their contribution values for both the flow regimes. The rate of the dominant CH4 consuming reactions increases by 80–148% involving electron and singlet state of N2 and decreases by 34–93% involving CH, CH3, and triplet state of N2. The electron–impact processes generate at least 50% more key–species and metastables for every 100 eV of input energy, explaining the increased energy efficiency at turbulent flow. These observations clearly show that the flow regime does influence plasma chemistry. This work highlights the significance of the flow regime, which is often overlooked by the plasma community. From the application point of view, based on this work, it is demonstrated that the developed RGA reactor is suitable to decompose dilute hydrocarbons with energy-efficient operation at high flow rates—a promising feature for upscaling. The understandings evolved in this work will help optimize the reactor for improved conversion/decomposition efficiency. | en_US |
