Complete Oxidation of Volatile Organic Compounds (Vocs) Using Plasma-Driven Catalysis and Oxygen Plasma

Kim et al. 46 Complete Oxidation of Volatile Organic Compounds (VOCs) Using Plasma-Driven Catalysis and Oxygen Plasma H.H. Kim*, A. Ogata, S. Futamura National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan Abstract ⎯ The decomposition of volatile organic compounds (VOCs) was investigated using a flow-type plasma-driven catalysis (PDC) system and a cycled system. In the flow-type PDC reactor a trade-off relation was observed between the formation of nitrogen oxides and the decomposition of VOCs. Complete decomposition of VOC to CO2 was achieved with the cycled system without forming CO, aerosol, and any nitrogen oxides. The oxygen-partial pressure dependence of different catalysts on the decomposition of benzene and toluene was investigated. The influence of VOC concentration and the temperature of oxygen plasma on the equivalent energy in the cycled system will be also discussed. Keywords ⎯ Nonthermal plasma, plasma-driven catalysis (PDC), VOC, oxygen plasma, enhancement factor Ⅰ. INTRODUCTION factors to be solved before the industrial use of the NTPs for the abatement of VOCs are the reduction of energy Volatile organic compounds (VOCs) have high vapor consumption (i.e. high energy efficiency), less NOx pressure under normal conditions, so they can be easily formation) and good material balance. One of the vaporized into the atmosphere. Two important VOCs- remarkable recent trends in the NTP application for air related environmental issues in urban area are the pollution control is the combination of NTP with catalyst photochemical smog and the secondary aerosol in either configuration of single-stage [6-8] or two-stage formation. Many of VOCs are known as carcinogenic, [9-11]. The authors have been investigated the teratogenic and mutagenic. The key driving force of decomposition of VOCs using a plasma-driven catalysis environmental market is the environmental policy of (PDC) system, and demonstrated the effectiveness of the government (i.e. policy-driven market) rather than PDC in terms of energy efficiency, product selectivity technology or demand (technology/demand-driven and carbon balance [12-15]. market). Emission standards and regulations are getting Table 1 summarizes the differences between the stringent in many countries over the world. There is, conventional NTP alone processes and the PDC system. therefore, an urgent need to develop more effective and Both the plasma alone and the PDC system are not inexpensive techniques for the treatment of VOCs. influenced by gas hourly space velocity (i.e. residence In Japan, new regulation of VOC emission has been time) because the characteristic time of plasma chemical started in April 2006 to achieve 30% reduction of the reactions is usually much more shorter than the gas total VOCs emission by 2010 compared to that of 2000 residence time. One important characteristics of the PDC (1.85 million tons/ year). More accelerated movements system that we focused on in this work is the influence of towards the reduction of toxic chemicals have been oxygen partial pressure on the decomposition efficiency noticed by European Union (EU, REACH; registration, [16]. In contrast to the plasma alone processes, the evaluation and authorization of chemicals) and United decomposition efficiency of VOC linearly increased with Nations (UN, GHS; globally harmonized system of oxygen partial pressure in the PDC reactor packed with 2 classification and labeling of chemicals). The upcoming wt% Ag/TiO2 catalyst. In addition to the decomposition new regulations of REACH may bring out big change on efficiency, the selectivity of CO2 also remarkably many industrial fields (especially for chemical industry) enhanced with oxygen partial pressure. On the other over the world. Generally, the approaches toward the hand, this strong influence of oxygen partial pressure on pollution control can be divided into two major groups of the decomposition efficiency of VOC is not observed in source reduction and end-of-pipe (EOP) treatment. The the NTP alone processes regardless of the type of plasma source reduction includes the modification of process, reactors [17-19]. Based on the highly oxygen partial changing chemicals to safer ones, and recycling before pressure-dependent property of the PDC system, a cycled they are released to the open air. While the source system of adsorption and the decomposition of adsorbed reduction approach is expected to be main stream in the VOCs using oxygen plasma has been proposed. The future, significant time and investments will be required expected advantages of this cycled process include high to meet the stringent upcoming regulations through the energy efficiency, high CO2 selectivity and no formation source reduction technology alone. From a practical of nitrogen oxides. viewpoint, the EOP treatment is more reasonable way to In this work, the decomposition of benzene and toluene meet the environmental goals at present stage. was investigated using a flow-type PDC reactor and a The nonthermal plasmas (NTPs) have been cycled system. The oxygen partial pressure-dependent investigated for the removal of a wide range of air characteristics of different catalysts (~ 4 wt% Ag/TiO2 pollutants during the last two decades [1-5]. Three key and 0.5 wt% Pt/γ-Al2O3) were evaluated using a new parameter of enhancement factor. The optimum range of Corresponding author: Hyun-Ha Kim plasma-catalyst process according to the gas flow rage e-mail address: [email protected] and the concentration of VOCs will be also discussed. Originally presented at ISNTPT-5, June 2006 Revised: October 29, 2006, Accepted: January 23, 2007 47 International Journal of Plasma Environmental Science & Technology Vol.1, No.1, MARCH 2007 Plasma TABLE 1. Catalyst COMPARASION OF THE NTP ALONE AND THE PDC VOCs CO2, H2O Parameter Plasma alone PDC First-order Zeroth-order Kinetics* η (%) = (-exp-{1 k ⋅SIE)} k E ⋅SIE E η (%) = ×100 (a) Flow-type PDC system C ×100 0 Adsorption mode GHSV No influence No influence Adsorbent/Catalyst O2 content No influence Large influence VOCs Clean gas Carbon balance Poor Good Aerosol Yes No IP vs DRE Highly related Unrelated * The rate constant kE is referred to as energy constant. The unit for first-order and zeroth-order are LJ-1 and Cycled operation ppm·J-1L, respectively. Regeneration mode Oxygen Ⅱ. EXPERIMENTAL CO , H O A. Reactor and Method 2 2 Figure 1 shows the conceptual diagrams of (a) the (b) Cycled system flow-type PDC reactor and (b) the cycled system for the complete destruction of VOCs. In the flow-type PDC Fig. 1. Schematic diagram of (a) the flow-type PDC reactor reactor, catalysts are packed inside the reactor and and (b) the cycled system for the total destruction of VOCs. In directly activated by a plasma application. There is a the cycled system plasma is only applied in the regeneration trade-off relation between the degree of VOCs mode, but not in the adsorption mode decomposition and the NOx formation. So the All the used catalysts were pellet type and their size were determination of the maximum applicable specific input in range of 1.6 mm – 3.2 mm. Benzene and toluene were energy (SIE) is important in the flow-type PDC reactor. used as model compounds of VOCs. Their On the other hand, nitrogen oxides are formed in the concentrations were adjusted by changing either the cycled system because the plasma is only turned on temperature of water bath or the N flow rate purging the under oxygen environment. The cycled process is based 2 bubbler. Temperature was measured near the plasma on the highly oxygen content-dependent behavior of the zone at the outlet of the reactor using a fiberoptic PDC system for the decomposition of benzene [16]. In thermometer (Anrits Co., AMOTH FX 8500). The the adsorption mode, VOCs was removed by adsorption temperature sensor was covered with insulating materials, without plasma application. When the catalyst bed which enables us to avoid interruption during the reached a saturated adsorption, then the catalyst bed temperature measurement. purged with oxygen before applying oxygen plasma to decompose the adsorbed VOCs. For the measurement B. Electrical Measurement purpose, the regeneration mode was operated with oxyge flow of 5~8 L/min. A small additional PDC reactor was Discharge power of the PDC reactor was measured set downstream of the main PDC reactor to decompose using the automated V-Q Lissajous program (Insight Co. the desorbed VOC just after the O plasma was turned on. 2 ver 1.71) [15]. A condenser (about 100 nF) was In practical case, it will be beneficial to operate the connected in series to the ground line of the plasma regeneration mode in closed system after purging the reactor. The charge Q (i.e. time-integrated current) was catalyst bed with oxygen. Since the main goal of the measured with a 10:1 voltage probe. Specific input current work is to find the dependence of various energy (P/Q ) is the ratio of discharge power (P in watt) catalysts on the oxygen partial pressure, most of the f dis to gas flow rate (Q in L/min). experiments were done with the flow-type PDC reactor f at 100 oC. The flow-type system consisted of the PDC P (J/L) SIE (J/L) dis ×= 60 (1) reactor (quartz tube of 13 mm in inner diameter and Q 200 mm in effective length), AC power supply, and f measurement system. In the case of cycled system, the In the case of the cycled operation equivalent specific adsorption mode was operated at room temperature. The input energy (SIEeq) was introduced to evaluation the regeneration mode was operatured at room temperature energy consumption. or 100 oC to see the temperature influence on the energy T )(P Avedis Oxy (2) efficiency. A Fourier transform infrared (FITR) SIEeq (J/L) ××= 60 )(Q T spectrometer (Perkin Elmer, Spectrum One) was used for Ads f Ads the gas measurements of both the reactants and products.

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