Fundamental and applied studies of non-thermal plasma

Abstract

This thesis reports a pure and applied study on non-thermal plasmas (NTPs) produced using Dielectric Barrier Discharge (DBD) generators of various forms. The main aim of the pure aspect of the study was to obtain a better understanding of the chemistry taking place within the NTP in an air-fed DBD and to what extent the plasma output varies from the glow to the downstream regions under various operational conditions. Thus, a DBD plasma jet generator was designed and employed for the investigations of these regions. The analyses of the plasma glow region and the downstream exhaust were carried out using Fourier Transform InfraRed (FTIR) and UV–Vis absorption spectroscopies. The applied studies focussed on the development of a novel, dielectric barrier discharge-packed bed reactor (DBD-PBR) for effective ozone generation from oxygen or air, and its application to the remediation of Cu(II)-EDTA and Fe(III)-EDTA containing water in combination with an oscillatory baffled reactor (OBR) and a UV irradiation reactor (UVR). In-situ analysis of the plasma glow region of the plasma jet identified: O3, N2O5, N2O, NO2 HNO3, CO2, CO and, for the first time, a vibrationally excited form of CO2 (i.e. CO2*(v)), while O3, N2O5, HNO3 and N2O were detected in the downstream exhaust. The behaviour of these species was monitored as a function of a range of experimental conditions including: input power, gas flow rate, relative humidity, gas temperature and feed gas composition, and mechanisms postulated based on literature precedent. It is clear from this work that the feed gas composition, input power, gas temperature and relative humidity have a significant effect upon the NTP chemistry in the glow and post glow regions. Unexpectedly, the spectroscopic analyses of O3, NO2 and N2O in the plasma glow region and in the downstream exhaust suggested the occurrence of chemical reactions in the afterglow region rather than simple diffusion. This behaviour rules out the general assumption that reactive chemistry is confined to the glow region. The DBD-PBR was designed, fabricated, characterized and optimized for ozone generation from oxygen and air. The effects of reactor arrangement, feed gas flow rate, coolant temperature, input power and dielectric material on ozone generation were investigated. The highest ozone concentration of 152 g m-3 was obtained using 2.0 mm glass beads and an oxygen feed at 5 oC, and 0.06 dm3 min-1, while the highest ozone yield efficiency was 210 g kW-1 h-1 at an oxygen feed rate of 15 dm3 min-1 this compares to 173 g kW-1h-1 reported in the literature. The highest ozone concentration produced from air was 15.5 g m-3 at flow rate Preface v of 0.06 dm3 min-1 with Al2O3 beads as the dielectric. It was found that the dielectric employed in the DBD-PBR had a significant effect upon the selectivity towards ozone at low feed gas flow rates. Different NOx by-products were formed along with ozone when the DBD-PBR was fed with air depending on the coolant temperature and the dielectric material. The efficiency of ozone generation via DBR-PBRs was significantly enhanced reducing the discharge current during the generation of NTP by decreasing the capacitance of the dielectric and by effective heat removal. Finally, a MnO2-based catalyst (CARULIT 200) tested for DBD-PBR exhaust control, and was found to be effective for simultaneous ozone and NOx removal at room temperature. The DBD-PBR was coupled to an OBR to intensify ozone-to-water mass transfer. The OBR was operated as a semi-batch and as a co-current, up-flow continuous reactor. The effect of input ozone concentration, input gas & water flow rates, and oscillation amplitude and frequency on gas hold up, volumetric mass transfer coefficient and mass transfer efficiency were determined. The same reactor was operated as a bubble column (i.e. without baffles or oscillation) and as a baffled column (without oscillation) to assess the effect of the reactor arrangement on the mass transfer. The results show that the OBR was 5 and 3 times more efficient for ozone-water mass transfer than the baffled and bubble columns, respectively. The enhancement obtained with the OBR over the baffled column reactor was found to decrease with gas flow rate due to changes in bubble flow pattern from homogenous to heterogeneous. Under continuous flow conditions, the performance of the baffled reactor and the OBR were found to be twice as efficient for ozone-water mass transfer than when operating under semi-batch conditions. The mass transfer efficiency (MTE, %) was found to increase from 57 % using the baffled reactor to 92 % with OBR under continuous flow at water and gas superficial velocities of 0.3 and 3.4 cm s-1, respectively. From these results it is clear that the OBR and baffled reactor are promising approaches for enhancing ozone-water mass transfer and its application in water treatment. One of the targeted fields of the DBD-PBR/OBR/UVR system is in water treatment, and hence it was important to evaluate its performance in such application. Therefore, the system was employed for the treatment of water samples contaminated with Cu(II)-EDTA and Fe(III)-EDTA. These compounds were chosen because they are difficult to remove from water using conventional methods. The effects of reactor arrangement, ozonation time and ozonation plus UV irradiation on remediation of the complexes were investigated under Preface vi continuous flow conditions. The results suggest that Cu(II)-EDTA was decomposed completely by ozonation within 17 minutes using the OBR, with no significant enhancement by UV irradiation. However, the Fe(III)-EDTA was converted to other stable complexes (i.e. Fe(III)-ED3A and Fe(III)-IDA) by ozonation, and hence following the ozonation by UV irradiation was essential to ensure complete degradation. The total organic carbon (TOC) of the Fe(III)-EDTA and Cu(II)-EDTA solutions was reduced by 50% after 17 minutes of O3/UV treatment using the OBR. Some of the final products were identified using Ion Chromatography and included: oxalic acid, formic acid, acetic acid, glycolic acid, nitrate and nitrite ions. From these results, it is clear that the enhancement in ozone-water mass transfer using the OBR or the baffled reactor was essential for reducing the treatment time and ozone dosage required for the remediation of Cu(II)-EDTA and Fe(III)-EDTA over conventional bubble column reactors.