DEVELOPMENT OF LARGE VOLUME AIR PLASMA FOR PRACTICAL APPLICATION TO GAS CONTAMINANT REMOVAL

Abstract

Non-thermal plasma catalyst systems have been used for environmental control, including abatement of VOC and NOx removal at atmospheric pressure because of their hazardous impact on the environment and human health. This dissertation investigates non-thermal corona plasma generated in a honeycomb monolith catalyst for VOC decomposition and rotational gliding arc plasma produced for NOx reduction from the diesel exhaust gas. The general aim of the research is developing non-thermal air plasma catalyst systems to improve the removal efficiency of VOC and NOx via investigating the effects of several input parameters, processes, and experimental conditions. In the case of VOC decomposition, the effective removal of acetaldehyde by humidified air plasma was investigated with a high throughput of contaminated gas in a sandwiched honeycomb catalyst reactor at surrounding ambient temperature. Here, acetaldehyde at the level of a few ppm was successfully oxidized by the honeycomb plasma discharge despite the harsh condition of large water content in the feed gas. The conversion rate of acetaldehyde increased significantly with the presence of catalysts coating on the surface channels. The increased conversion rate was also obtained with a high specific energy input (SEI) and total flow rate. Interestingly, the conversion changed negligibly under the acetaldehyde concentration range from 5 to 20 ppm. However, the conversion rate decreased toward increased water amount in the feed gas. Notably, about 60% of acetaldehyde in the feed was oxidized under SEI of 40 J/L at water amounts ≤ 2.5%, approximately 0.5 g/kWh for acetaldehyde removal. Also, the plasma-catalyst reaction was superior to the thermal reactive catalyst for acetaldehyde removal in airborne pollutants. In comparison with other plasma-catalyst sources for acetaldehyde removal, the energy efficiency under the condition is comparable. Moreover, the honeycomb plasma discharge features high throughput, avoiding pressure drop, and straightforward reactor configuration, suggesting potential practical applications. The removal of NOx over Ag/γ-Al2O3 catalyst coupled with gliding arc plasma at low temperatures is demonstrated. Specifically, n-heptane (the reducing agent) was pretreated by exposure to gliding arc plasma (the outlet gas temperature of 73.4 °C) before injecting into the simulated diesel exhaust gas and passing it through the catalyst zone. As a result of the plasma treatment, the feed gas consisted of oxygenated hydrocarbons (OHCs), which serve as reducing agents, instead of only n-heptane without plasma treatment. Consequently, the NOx removal efficiency increased substantially by approximately 10% at temperatures of [165-225 °C], owing to the presence of the OHCs. The dependence of the NOx removal efficiency on typical reducing agents was examined; these results agreed with our hypothesis that aldehyde derivatives were more effective than the parent compound (n-heptane) for NOx removal at low temperatures. However, enhancement of the NOx removal efficiency after plasma pretreatment was not observed at high plasma discharge power. This is because NOx is formed from the air and a significant amount of n-heptane is completely oxidized to CO2 when the gliding arc plasma is operated at high power. Besides, the plasma treatment of n-heptane did not improve the NOx removal under high operating temperature conditions at which the catalyst itself exhibits high catalytic activity. This led us to surmise that boosting the effectiveness of the OHCs generated during plasma pretreatment would require the ratio of the exhaust gas flow rate to the reducing agent flow rate to be high, which is challenging to realize in laboratory-scale experiments. This method would lower the energy consumption of the plasma stage.