Vocs Degradation with Non-Thermal Plasma and Ag-Mnox Catalysis
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VOCs degradation with non-thermal plasma and Ag-MnOx catalysis Xiujuan Tang, Meng Wang, Weiqiang Feng, Fada Feng, Keping Yan* Industrial Ecology and Environment Research Institute, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310028, P. R. China Abstract: Non-thermal plasma (NTP) is effective for degradation of aromatic molecules, achieving complete mineralization, however, is difficult. In contrast, thermal catalysis may be very effective toward achieving complete mineralization at high temperatures. Its drawbacks at a low processing temperature are undesirable intermediate byproducts production. By integrating dielectric barrier discharge with home-made Ag-MnOx catalyst, we have succeeded in achieving both VOCs complete decomposition and without toxic byproducts production. As an example, this paper reports toluene decomposition in air. The effects of the reaction temperature and the plasma specific energy density on the decomposition efficiency and byproducts formation are experimentally investigated. Very effective synergetic effect between the NTP and catalyst has been observed for VOCs decomposition. Keywords: VOCs; nonthermal plasma; ozone; Ag-MnOx catalyst 1. Introduction species toward VOC destruction and better energy efficiency [2, 4]. VOCs such as alkanes, aromatic or halogenated compounds are hazardous pollutants emitted from Although some catalysts are capable of paints, solvents, preservatives, automobile exhaust gas, decomposing ozone and removing VOCs and certain industrial processes. Some VOCs may be simultaneously at room temperature, the catalysts carcinogens or may produce respiratory diseases and would gradually deactivate because of the build-up of in addition, cause ozone layer depletion, green house organic intermediates. In the present study, a highly effect. Previous works have demonstrated that active Ag-MnOx ozone decomposition catalyst was nonthermal plasma is quite effective for the introduced into the NTP system in order to avoid such decomposition of the VOCs, but it was found that the a problem. The synergetic effect was investigated at plasma treatment alone leads to formation of atmospheric pressure and room temperature for the unwanted by-products such as ozone (O 3), NOx, oxidation of toluene as a model VOC. carbon monoxide (CO), and aerosol particles [1, 2]. In 2. Experimental order to eliminate this disadvantage, nonthermal plasma (NTP) technology for VOCs removal could A schematic diagram of the experimental system take advantage of its synergetic effect by coupling is shown in Fig. 1. The experimental apparatus with heterogeneous catalysts. consisted of a continuous-flow contaminated gas generation system, a DBD reactor driven by a 50-Hz Instead of working independently, the AC power supply, a fixed-bed catalytic reactor and an combination of NTPs and catalysts could induce extra electric and gas analysis unit. The initial toluene performance enhancement mechanisms either in a concentration in the mixed air feed was: 67 ppm. The single-stage or a two-stage configuration, in which the flow rate and toluene concentration were adjusted by catalyst is located inside and downstream from the mass flow controllers (MFC-1 and MFC-2), which nonthermal plasma reactor, respectively. And NTPs were fixed at 6 ml/min and 492 ml/min, respectively. are usually combined with an appropriate catalyst The coaxial cylindrical DBD reactor was made of such as BaTiO , Al O , TiO , MnO , and zeolites or 3 2 3 2 2 silica tube with an inner diameter of 21 mm and wall their derivatives. Among these choices of catalysts, thickness of 2 mm wrapped by the copper mesh of 10 MnO is found to be the best catalyst because it could 2 cm length as a ground electrode. The inner discharge effectively decompose ozone and generate active electrode was a stainless steel rod (17 mm in diameter) placed on the axis of the reactor and connected to the high-voltage output of the AC power supply with 50 multiple pass absorption cell. On the other hand, for Hz, where the discharge gap is 2 mm. The discharge accurate quantitative gas analysis, an on-line gas was limited in the space between the inside surface of chromatograph (Fuli 9790, Wenlin, China) equipped the silica tube covered with the copper mesh and the with a polyethylene glycol capillary column (30 m, outside surface of the stainless steel rod. The 0.25 mm), and a flame ionization detector (FID) was discharge space volume was 11.9 cm 3 and this used to measure VOCs. The experimental data taken corresponds to a gas residence time in the DBD after steady state were averaged and the values were reactor was 1.4 s. A post-plasma treatment of the used to evaluate the performance of the plasma- effluent was performed in a catalysis fixed-bed reactor. catalytic hybrid system. Ozone was measured by UV- A newly developed silver-cobalt composite oxides absorption at 254 nm in the late afterglow, the catalyst was used. The catalyst powder was pressed, analyzer (ZN-MODEL254) being placed outside the crushed and sieved to a size of 40–60 mesh for the reactors. catalytic evaluation. Catalyst (200 mg) was loaded in 3. Results and Discussion the quartz reactor with quartz wool held at both ends of the catalyst bed leading a weight hourly space 3.1 Decomposition of toluene by non-thermal plasma velocity (WHSV) of about 15,000 h -1. The gas effluent treatment alone was not heated before the discharge reactor or the 100 100 catalyst. 90 80 Concentration of O 80 70 60 50 60 40 3 30 (ppm) 40 20 Conversionoftoluene (%) 10 0 20 20 30 40 50 60 70 80 SED (J/L) Fi gure 2. Toluene decomposition efficiency and ozone concentration variation versus SED. Figure 1. Schematic diagram of experimental setup An air stream of 500 ml/min containing 67 ppm The high voltage applied to the plasma reactor of toluene was treated in the DBD reactor. The was measured with a 1000: 1 voltage probe (Tektronix, decomposition of C 6H8 and the formation of ozone in P6015A). The waveforms of voltage were recorded the discharge are shown in Fig. 2. It shows that the using a digital oscilloscope (Tektronix, TDS 3052). conversion of toluene steadily increases when SIE Discharge current was measured as charge by increases. Besides, huge amounts of ozone are measuring the voltage across the capacitor connected produced in the discharge, as can be seen in Fig. 2, O3 in series to the ground lines of the plasma reactors concentration at the outlet of the NTP reactor can with a 10:1 probe (Tektronix, P6109B). The V–Q reach values as high as nearly 100 ppm at a specific method was used to determine discharge power in the energy of 79.6 J/L. The high outlet ozone plasma reactors. The charge Q was determined by concentration for the treatment of volatile organic measuring the voltage across the capacitor of 100 nF, compounds in air using DBD reactor was often quite which was connected sequentially to the ground line expected, because dielectric barrier discharge (DBD) of the plasma reactors. has been widely used for ozone generation. However, The gas composition was analyzed online using a oxygen atoms that are consumed in the formation of Fourier transform infra-red absorption spectrometer ozone are not available for the elimination of (FTIR, Bruker Tensor 27), equipped with a 2 m long pollutants in the effluent. Therefore, a significant part of the energy that has been injected in the discharge to formation is generally inevitable. In previous create high energy electrons, which produce radicals investigations it has been proposed that the by dissociation of molecular oxygen, is wasted in the decomposition of ozone on transition metal oxides production of ozone. Moreover, Ozone emissions in (Co 3O4, MnO 2) leads to the formation of atomic the atmosphere are not desirable, as O 3 is responsible oxygen (probably O-) on the oxide surfaces[]. On the for respiratory diseases and is implied in the formation other hand, studies on the oxidation of CO and of smog over big cities. benzene in the presence of ozone have shown this 0.20 form to be, most probably, the oxidant. Moreover, CO it has been proved that in terms of the reactivity, an O oxygen atom is a more chemically active species than 3 0.15 CO 2 ozone [5]. Hence, it would be favorable for the 79.6 J/L oxidation reaction if ozone can be decomposed into an oxygen atom before reacting with VOC molecules or 0.10 57.2 J/L CO. In this study, we are focusing on developing a N O HCOOH 2 Absorbance highly active ozone-decomposition catalyst and C H 24.8 J/L 0.05 7 8 couple it with DBD in an arrangement of two-stage 0 J/L NTP catalysis. 0.00 400 CO 2-NTP 30002750 2250 2000 1750 1500 1250 1000 350 Wavenumber (cm -1 ) CO-NTP CO -NTP+Ag-MnOx 300 2 Figure 3. Evolution of FTIR spectra of the effluent during NTP treatment of toluene at the SED range of 0 to 79.6 J/L. 250 200 Then organic and inorganic by-products have been determined for the studied VOC. FTIR spectra of 150 the effluent treated by NTP and NTP-catalysis are 100 compared in Fig. 3. FTIR analyses show that the COx concentration (ppm) 50 toluene is only partially oxidized. Several hazardous 0 organic by-products (such as formaldehyde and formic 0 10 20 30 40 50 60 70 80 acid) and inorganic by-products (carbon monoxide SED (J/L) N2O and ozone) remain in the exhaust. No traces of Figure 4. Eeffect of Ag-MnOx catalyst on the formation of carbon nitrogen oxides [NO and NO 2] were detected among oxides during the abatement of toluene (67 ppm).