Juliusz Kruszelnicki

Propagation of negative electrical discharges through 2-dimensional packed bed reactors

Plasma-based pollutant remediation and value-added gas production have recently gained increased attention as possible alternatives to the currently-deployed chemical reactor systems. Electrical discharges in packed bed reactors (PBRs) are of interest, due to their ability to synergistically combine catalytic and plasma chemical processes. In principle, these systems could be tuned to produce specific products, based on their application by combinations of power formats, materials, geometries and working gases. Negative voltage, atmospheric-pressure plasma discharges sustained in humid air in a PBR-like geometry were experimentally characterized using ICCD imaging and simulated in 2-dimensions (2D) to provide insights into possible routes to this tunability. Surface ionization waves (SIWs) and positive restrikes through the lattice of dielectric rods were shown to be the principal means of producing reactive species. The number and intensity of SIWs and restrikes are sensitive functions of the alignment of the lattice of dielectric beads (or rods in 2D) with respect to the applied electric field. Decreased spacing between the dielectric elements leads to an increased electric field enhancement in the gas, and therefore locally higher plasma densities, but does not necessarily impact the types of discharges that occur through the lattice.

Time-resolved evolution of micro-discharges, surface ionization waves and plasma propagation in a two-dimensional packed bed reactor

Plasma packed bed reactors (PBRs) are being investigated for applications ranging from pollution remediation to chemical synthesis, including plasma catalysis. Plasma PBRs typically operate as dielectric barrier discharges where the plasma propagating through the PBR strongly interacts with the dielectric packing media and gas in the interstitial spaces. The nature of plasma propagation through this macroscopically porous-like medium is not well understood. Plasma formation in PBRs is a function of many parameters, including dielectric media composition and surface morphology, dielectric constant, packing fraction, pressure, and the applied voltage waveform. Imaging the plasma propagation through the complex three-dimensional geometry of the packing media and interstitial spaces that make up the PBR is difficult to experimentally execute. In this regard, a two-dimensional PBR composed of dielectric disks was developed to enable optical imaging of plasma formation and propagation. The mode of plasma propagation and the sensitivity of discharge formation to material dielectric constant, applied voltage, and pressure were experimentally and computationally investigated. We found that higher dielectric constants of the packing material produced more intense, localized filamentary micro-discharges between disks. In general, plasma propagation through the PBR at 1 atm is initiated by localized micro-discharges between adjacent dielectric disks, which in turn give rise to surface ionization waves (SIWs) that propagate along the dielectric surface. At pressures below 1 atm, the discharge was more diffuse regardless of the dielectric media, filling the interstitial space instead of forming SIWs.

Properties of atmospheric pressure plasmas in packed bed reactors

Summary form only given. Atmospheric pressure dielectric barrier discharges (DBDs) sustained in packed bed reactors (PBRs) are being investigated for remediation of toxic gases, CO2 removal and conversion of waste gases into higher value compounds. Though investigated extensively in experiments, few computational studies of PBRs have been performed to date [1]. For applications involving chemical reprocessing which require a high degree of reactant selectivity, the ability to control plasma properties is particularly important. In this paper, we report on the results of a computational investigation of PBR-DBD properties using the multi-fluid plasma hydrodynamics simulator nonPDPSIM [2].PBRs were simulated in 2-dimensions using coplanar electrodes with dielectric beads (rods in 2D) having different configurations. The base case gas is humid air at ambient temperature and atmospheric pressure. The reaction mechanism includes 33 species and 147 reactions. Electrons were seeded near the cathode, and a negative pulse was applied, forming an anode-seeking streamer. The influence of packing geometry, filler gas and bead material on discharge properties was investigated. We found that increasing the rod dielectric constant or decreasing inter-rod separation increases regional electric field enhancement, and therefore electron densities, temperature, and discharge propagation velocity. Propagation mechanisms were also examined. It was found that photoionization plays an important role in discharge propagation around the beads, as it serves to seed charges across regions of low electric field, and in areas of high field enhancement. In plasmas where ionizing radiation has a short penetration distance, discharge propagation around rods is significantly reduced. Upon breakdown in regions of high electric field, electron impact (as opposed to photoionization) was the primary ionization source. Formation of inter-rod, cathode-seeking restrikes, reminiscent of the experimentally-observed filamentary microdischarges [3], was seen. Positive and negative surface ionization waves were also observed. Preliminary results indicate that radical generation takes place near rod surfaces and is largely influenced by these ionization waves. This is likely due to characteristically high electric fields and electron energies in the ionization front, which lead to increased reaction rates.