Combination of plasma with catalysis and adsorption towards enhancing the decomposition of volatile organic compounds

Abstract

In this thesis, non-thermal plasma (NTP) has been combined with catalysts and dual functional adsorbent/catalysts for abatement of volatile organic compounds (VOCs). The main aims of this work are to optimize the VOC decomposition and energy efficiencies as well as to understand the effect of the scaling up a NTP reactor on the reactor performance. The work is divided into six studies dealing with plasma-catalytic decomposition of acetone, diethyl ether (DEE) and ethylene. For the purpose of optimizing the VOC decomposition efficiency, various tandem catalyst arrangements of supported transition metal oxides have been investigated for NTP continuous treatment of acetone and DEE (Chapter 3, Sections 3.1 and 3.2). Despite the synergistic effect of NTP and catalysis, the energy efficiency of the process is still poor, especially at low VOC concentration. The cyclic adsorption/plasma oxidation of VOCs is therefore intensively studied in the next studies (Sections 3.3-3.5) in order to further enhance the energy efficiency. Finally, for the sake of practical application, effects of scalability on the NTP reactor performance are under investigation and presented in Chapter 4. From the first study, the experimental results show that more than 90 % of acetone has been decomposed with a catalyst arrangement of in-plasma MnO2 (0.1wt% Mn) followed by post-plasma MnO2 (5.0 wt% Mn), showing a great performance enhancement compared to the tandem bare supports and ZnO (0.1 wt% Zn)-MnO2 (5.0 wt% Mn) arrangement. The use of MnO2 either in or post plasma region substantially promotes the acetone decomposition, obviously due to the dissociation of ozone into far more reactive oxygen atoms available for oxidizing acetone. However, as dealing with DEE, an ozone-reactive compound, the presence of Mn-based catalysts in plasma does not positively affect the DEE decomposition compared to the bare and Fe2O3 coated cordierites. Even worse, Mn-Fe mixed oxide, the best catalyst for decomposing ozone among prepared catalysts, lowers the DEE removal efficiency because of a large amount of ozone catalytically decomposed in plasma to molecular oxygen. The presenceof a catalyst with a high catalytic activity for zone decomposition in plasma is therefore not beneficial for abatement of the ozone-reactive VOCs. However, as Mn-Fe/cordierite is used in the post-plasma region of the Mn/cordierite one-stage reactor, the removal efficiency has been greatly enhanced by more than 10 %. Choosing appropriate catalysts to couple with NTP is therefore crucial and the nature of treated VOCs should be considered. The abatement of acetone by cyclic treatment is performed in the third study using silver coated -zeolite. Dilute acetone (300 ppm) is completely removed from the gas stream by adsorption on zeolite for 100 min and subsequently oxidized in oxygen plasma within 15 min. The acetone abatement by the cyclic operation has largely improved the energy efficiency with about 6.5 times higher than the continuous treatment at the same operating conditions. Different from acetone, VOCs without polar groups within the molecule will be highly volatile and have low ability of adsorption. In such a case, modification of available adsorbents is needed to improve the adsorption capacity. In the fourth study, cyclic adsorption and oxidation of ethylene on 13X modified with Ag and Ag-MxOy (M: Co, Cu, Mn, and Fe) are investigated (Chapter 3, Section 3.4). The incorporation of Ag into zeolite affords a marked enhancement in ethylene adsorption capacity due to the Ag-C2H4 complex formation. Among additional metal oxides, FexOy with high oxidation catalytic activity is able to reduce the ozone emission while keeping a high effectiveness for oxidative removal of adsorbed ethylene. In the next study (Section 3.5), it is found that the zeolite modification method (i.e., ion exchange and impregnation) strongly affects the ethylene adsorption capacity, by which Ag exchanged 13X (Ag-EX/13X) is superior over Ag impregnated 13X because of the higher dispersion of exchanged Ag+ active cites. The adsorption and decomposition of ethylene are then performed on Ag-EX/13X with different reactor configurations including one-stage, two-stage and the combination of the two (hybrid). The use of hybrid reactor results in a moreeffective generation of ozone and other reactive species, thereby shortening the oxidation time and therefore achieving higher energy efficiency, which is evaluated to be ca. 2.4 g (kWh)-1. In the final study, the specific input energy and VOC decomposition efficiency are found to be independent on the reactor size and the way reactor scaled up (i.e., in series or parallel). Based on the finding, the required energy and reactor size can be predicted for treatment of a specific gas stream with known VOC concentration and gas flow rate.