저온 플라즈마-촉매 하이브리드 공정을 이용한 휘발성 유기화합물과 CO 동시 제거 특성 연구

Abstract

This study dealt with simultaneous removal of volatile organic compounds and carbon monoxide by nonthermal plasma-catalytic hybrid system. Volatile organic compounds and carbon monoxide emitted from many artificial sources (e.g., combustion engines, power plants, incinerators, boilers, etc.) can cause various adverse effects on both environment and human health. In this work, n-heptane was chosen as the target volatile organic compound. Different catalysts including γ-Al2O3, Ag2O/γ-Al2O3, MnO/γ-Al2O3, RuO2/γ-Al2O3 and PdO/γ-Al2O3 were prepared to be combined with nonthermal plasma for the simultaneous removal of n-heptane and carbon monoxide. In order to effectively use the heat generated during the plasma discharge for catalytic reactions, the plasma reactor was insulated outside with a glass wool jacket. The increase in the reactor temperature could improve the CO2 selectivity and extend the lifetime of the catalysts. Nonthermal plasma used in this work was generated by dielectric barrier discharge, which was observed to proceed radially from the high-voltage electrode by gradually increasing the applied voltage. The effects of temperature, specific energy input, initial concentrations of n-heptane and CO, humidity, concentration of oxygen, and PdO loading on the decomposition efficiency and formation of byproducts were examined. The results from separate removals of n-heptane and carbon monoxide showed that n-heptane was mainly removedby plasma with a minor influence of the catalysts. Meanwhile, the oxidation of carbon monoxide showed a significant difference and strongly depended on the type of catalysts. By plasma alone, the removal efficiency of CO was found to be less than 20 %. As plasma-catalytic system was used, the CO removal efficiency was greatly enhanced and reached 100 % with PdO/γ-Al2O3 catalyst at a specific energy input of 400 J L-1. The oxidation of carbon monoxide was described as follows: 1) Absorption of oxygen on the surface of the catalyst, 2) Dissociation and activation of oxygen, 3) Reaction of the activated oxygen and carbon monoxide, 4) Generation and desorption of carbon dioxide. Similarly, for simultaneous removal of n-heptane and carbon monoxide, the change in the n-heptane removal efficiency with different catalysts was insignificant, which was less than 10 %. However, the oxidation of carbon monoxide was found to strongly affected by the catalysts whose catalytic activities followed the order: MnO/γ-Al2O3 < Ag2O/γ-Al2O3 < RuO2/γ-Al2O3 < PdO/γ-Al2O3. By loading PdO on γ-Al2O3 with 0.005 wt% Pd, about 50 % of carbon monoxide was plasma-catalytically removed. In the hybrid reactor, the mixture of n-heptane and carbon monoxide was mainly converted into carbon dioxide. The removal efficiency of the carbon monoxide was found to decrease with increasing the concentration of the n-heptane. However, the performance for removal of n-heptane was independent on the concentration of carbon monoxide. In order to confirm the effect of nonthermal plasma, simultaneous removal of n-heptane and carbon monoxide in the plasma-catalytic system and thermal catalytic system were performed and compared. It was showed that the removal efficiency of n-heptane was higher when using the plasma- catalytic system at the same reactor temperature. For removal of carbon monoxide, similar behavior was observed at the temperature below 190oC. Nevertheless, at more than 190oC, the removal efficiency of the carbon monoxide was higher in the thermal catalytic system. The reason is that n-heptane was effectively decomposed in the nonthermal plasma-catalytic system at low temperature to generate water, which inhibited the oxidation of carbon monoxide. However, the simultaneous removal of n-heptane and carbon monoxide using the nonthermal plasma-catalytic system can be carried out at a much lower temperature than that of the thermal catalytic processes.