Plasma Catalytic Hybrid Reactor System for Air Pollution Control

Abstract

The thesis focuses on investigating plasma catalytic hybrid reactors towards controlling air pollution (e.g., NOx removal and VOC decomposition). The influence of critical process parameters on the removal process such as gas temperature, humidity, gas flow rate, hydrocarbon, energy density, and so on were investigated. The byproducts in the plasma catalytic oxidation process were also analyzed to understand the role of the plasma-activated catalytic in governing the reaction pathways according to the oxidation process. Additionally, screening of different suitable catalytic materials and suitable plasma technologies has been carried out towards practical applications. In this study, three different processes for NOx and soot removal, and VOC degradation using plasma catalytic hybrid are investigated. The first is the removal efficiency of NOx and soot at a low-temperature range below 350 ℃. The key for this study is plasma-induced activation of the catalyst at relatively low temperatures, which can efficiently remove NOx at low temperatures and fluctuating below 350 ℃. The second work has discussed the degradation of VOCs in the ambient air by using a DBD plasma reactor packed with Pd/ZSM5 catalyst that acts as both an adsorbent and catalyst. For the sake of practical application, the third work is to develop a honeycomb catalysts plasma reactor system for dealing with large amounts of air pollutions in the environment. From the first work, an evaluation of low-temperature NOx removal over Ag/ZSM5 and Cu/ZSM5 catalysts coupled with plasma is performed in a wide temperature range below 350 ℃. The experimental results show that the NOx removal rate over Ag/ZSM5 was higher than that over Cu/ZSM5. The NOx removal efficiency based on Ag/ZSM5 catalyst coupled with plasma exceeded 80% at low operating temperatures. The oxidation of NO to NO2 was approximately 100% at temperatures of 200–300 ℃. Furthermore, the effects of water vapor, hydrocarbons, and operating temperature on NOx removal efficiency were also investigated. Mechanisms for low-temperature NOx removal by plasma catalysis have also been discussed in detail. Besides, the removal of NOx and soot simultaneously by selective catalytic reduction (HC-SCR) coupled with plasma under temperature fluctuation conditions ranging from 150-350 ℃ is also investigated. The results revealed that the HC-SCR coupled with plasma maintained high NOx and soot reduction efficiency regardless of the temperature variations in the range from 150 to 350 ℃. In comparison, the average NOx removal efficiency of the HC-SCR process at various SIE was varied from 37.6 to 76.6%, depending on the specific input energy (SIE). In the second work, the styrene degradation was investigated in a DBD plasma reactor packed with Pd/ZSM5 pellets acting as both an adsorbent and a catalyst at atmospheric pressure. The effects of SIE, styrene concentration on the removal of styrene, the CO2 selectivity, and the formation of organic and inorganic byproducts were discussed. The results demonstrated that the plasma-coupled Pd/ZSM5 catalyst significantly enhanced both the styrene oxidation and the CO2 selectivity as compared with the plasma-alone case, greatly reducing the byproducts. The styrene degradation efficiency proportionally increased with increasing the specific input energy. Ozone formed by the plasma played an important role in the oxidation of styrene. The Pd/ZSM5 catalyst could be activated by the plasma, which greatly enhanced the performance of the styrene removal. In the third work, a plasma-assisted catalysis reactor system comprising different types of corona discharge and honeycomb catalysts have been investigated for degradation of ethyl-acetate (EA) in the air under various parameters such as humidity, space velocity (GHSV), and temperature. The honeycomb catalyst used in this work was successfully prepared by applying a coating of -alumina (-Al2O3) powder to the bare cordierite monolith on which to support the palladium (Pd) catalyst. An attempt has been made to successfully generate a large volume of uniform plasma at atmospheric pressure in the channels of the nearly practical-scale honeycomb catalyst. The results demonstrated DC honeycomb catalyst discharge is more stable and easier to operate at higher voltages under various atmospheric conditions. Furthermore, EA conversion in the DC honeycomb catalyst discharge (96%) is significantly higher than that at the given applied voltage. The by-products of EA degradation were also analyzed to understand the role of the plasma in governing the reaction pathways according to which the oxidation process of EA proceeds in the plasma discharge over the honeycomb catalyst. The honeycomb catalyst discharge renders the proposed system suitable for practical applications.