Environ. Sci. Technol. 2008, 42, 4546–4550
Novel Method for Enhancing the Destruction of Environmental Pollutants by the Combination of Multiple Plasma Discharges A L I C E M . H A R L I N G , †,‡ DAVID J. GLOVER,‡ J . C H R I S T O P H E R W H I T E H E A D , * ,† A N D KUI ZHANG‡ School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K., and Plasma Clean Ltd., Broadstone Knowledge Mill, Broadstone Road, Stockport, Cheshire, SK5 7DL, U.K.
Received December 21, 2007. Revised manuscript received March 18, 2008. Accepted March 18, 2008.
A novel, multistage, dielectric, packed-bed, plasma reactor has been developed and is used to efficiently destroy environmental pollutants, such as volatile organic compounds (VOCs). A three cell plasma reactor, operated at ambient pressure and low temperatures, is found to be an effective technology for complete VOC remediation in air. The combination of plasma cells in series can significantly improve the efficiency of VOC decomposition, but the combined destruction rate is not simply an additive effect, there is a synergistic enhancement related to the effect on the plasma chemistry of sequential processing in the three cells. At the same time, the formation of byproduct such as NOx is strongly suppressed, and it is possible to remediate toluene and ethylene in air, with no detectable formation of NOx or nitric acid.
Introduction Volatile organic compounds (VOCs) are contaminants that are found across a range of market sectors from semiconductor manufacturing plants to chemical processing plants including paint, coatings, chemical manufacturing. VOCs in the air contribute significantly to photochemical smog production and certain health problems (1, 2). The remediation of dilute VOCs from waste gas streams is both a health and an environmental concern (3). This work looks specifically at the destruction of toluene and ethylene. Toluene is widely used in many applications, such as the pharmaceutical industry and it is an important feedstock in chemical processes. It is often used as a solvent because of its excellent ability to dissolve substances. Toluene is also used to make spray and wall paints, medicine, dyes, explosives, detergents, spot removers, lacquers, adhesives, rubber, and antifreeze (4). Ethylene is an unsaturated hydrocarbon; it is the most produced organic compound worldwide being widely used in the petrochemical industry. Various studies reveal that VOCs may affect the nervous system of human beings and that direct and prolonged contact with them can cause health problems (5, 6). VOCs also pollute soil, surface water, and * Corresponding author fax: +441612754598; e-mail: j.c.whitehead@ manchester.ac.uk. † The University of Manchester. ‡ Plasma Clean Ltd. 4546
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groundwater. Emission of VOCs into the atmosphere is detrimental to both human beings and the environment. It is necessary to remove VOCs from waste gas streams due to increasing legislation on emission control (6). The conventional technologies for VOC abatement from air include activated carbon adsorption, thermal incineration, and thermal catalysis (7). Each of these methods has restrictions. For adsorption methods using activated carbon, disposal costs of the spent carbon must be taken into account. Thermal incineration requires high energy to heat the large volume of air to a high temperature (700-900 °C), which introduces high costs and energy consumption because this technology demands continuous supply of external heat energy (8). Thermal catalysis often uses expensive catalysts containing precious metals, together with high energy input to obtain the necessary operating temperatures (300-500 °C). Another concern is the lifetime of the catalysts and deactivation by poisoning, coking, or sintering is a common problem. An alternative approach to these conventional VOC abatement technologies is to use nonthermal plasma generated at atmospheric or higher pressure. Atmospheric pressure nonthermal discharges, such as DC or AC corona discharge, microwave plasma, and dielectric barrier discharges (DBDs), are characterized by their high energy electrons and relatively low gas temperatures. Plasmas have now been studied for decades and are emerging as a promising technology for VOC remediation (3, 9–12). If sufficient oxygen is present, the main products of the VOC decomposition are carbon dioxide and water (13). The DBD system is commonly used for producing nonequilibrium plasmas at atmospheric pressure, and this technology has been used in industry for ozone generation for over 150 years (14, 15). The temperature of the energetic electrons ranges from 10 000 to 100 000 K, whereas the actual gas temperature remains near ambient temperature which is cool enough to provide a noncorrosive processing environment while still supporting a multitude of chemical reactions that bring about destruction of the pollutant (1, 16). Through electron-impact ionization, dissociation, and excitation of the source gases, active radicals and ionic and excited atomic and molecular species are generated, which can initiate plasma chemical reactions (9). A great advantage of DBDs over other discharges is that the average energy of the electrons can be optimized by changing either the gas pressure (or gas density) or discharge gap width, which means the reaction conditions of plasma processing can be optimized. Despite its proven ability to destroy waste gas streams, the disadvantages of plasma technology for air pollution control currently include low energy efficiency and the formation of toxic byproduct such as CO, NOx, and nitric acid; the latter when processing in air streams (17). Several groups have published work on how to restrict the formation of NOx in a plasma system (18–21). A distinct advantage of plasma technology for removing waste gas streams is the ability to combine this system with catalysts (22, 23). Plasmacatalysis has been extensively tested to improve the energy efficiency of the system and to restrict the formation of toxic byproduct in a plasma reactor (17, 24). In this paper, we report experimental results on VOC destruction in air in a novel multistage, dielectric packedbed discharge plasma reactor (MDPBD) that we have developed (25). Specifically, we summarize our results on the destruction of toluene and ethylene using a plasma reactor with three discharge cells in series operating at atmospheric pressure and ambient temperatures. The present study 10.1021/es703213p CCC: $40.75
2008 American Chemical Society
Published on Web 04/29/2008
FIGURE 1. Schematic diagram of the multicell plasma system used in the study of the destruction of toluene and ethylene. reveals that the MDPBD is an effective energy-efficient technology for complete VOC remediation in air and at the same time, the formation of byproducts such as NOx is strongly suppressed. The results reveal that the combination of plasma cells in series can significantly improve the energy efficiency of VOC decomposition. We find that the combined destruction rate is not simply an additive effect but that there is a synergistic enhancement related to the effect on the plasma chemistry of sequential processing in the three cells. The advantage of this system is that low concentration of VOCs in air can be converted into CO2 and water without producing other hazardous byproduct such as NOx (NO, NO2). These results show that it is possible to completely remediate toluene in air, with no detectable formation of NOx or nitric acid. The only other work known to us that uses multiple plasma discharges is that of Chavadej et al. for the destruction of benzene and the reforming of methane (13, 26), McAdams (27) on toluene, and a combination of plasma and catalyst steps for NOx reduction by Tonkyn et al. (28).
Experimental Section A schematic of the experimental setup is shown in Figure 1. In this process, a mixture of synthetic air (80% nitrogen and 20% oxygen) and the vapor of the VOC pass through the reactor controlled by mass flow controllers (MFC), maintained at a pressure of 1 bar, to give a total flow rate of ∼1 L min-1. When toluene is used as the pollutant to be destroyed it is introduced into the gas flow by passing a certain amount of nitrogen (controlled by a mass flow controller (MFC)) through the bubbler to give 110 ppm of toluene. In order to eliminate the influence of room temperature on the concentration of toluene in the gas flow, the bubbler is kept in a water bath at room temperature (293 K). When ethylene is used as the pollutant, it is introduced into the system directly as a gas, controlled by the mass flow controller to give an initial concentration of ethylene of 112 ppm and a total flow rate of 1 L min-1. For real applications, total removal of VOCs is needed; however, it is worth mentioning that in this setup the concentrations of VOCs were optimized to avoid complete removal so that the actual destruction of the pollutant could be quantified and compared. This allows the observation of the effects of all the process parameters and permits their optimization. An illustration of the detailed configuration for the multicell plasma reactor used is shown in Figure 2. The MDPBD reactor is made up of a plastic box (dimensions 24.81 by 4.8 by 6.5 cm, AAC Eurovent Ltd.) containing three fixed barrier discharge cells (A, B, and C). The plasma cells are based on an earlier design (29) and each is 14.8 × 6.5 × 1.6 cm and is made of clear polycarbonate (Fibercraft Plastics Limited) perforated on two faces, through which the gas can flow and contain two copper mesh electrodes (13 × 4.5 cm) made from a copper perforated sheet (RMIG Ltd.). The
thickness of the copper sheet is 0.7 mm, and the diameter of the holes is 4 mm, which gives a 58% open area. The distance between the two copper electrodes is 16 mm. Each plasma cell is filled with soda lime, glass beads (6 ( 0.3 mm in diameter) and is individually powered by a high frequency, AC high voltage power supply (either an electronic neon sign transformer or a commercial ozonizer power supply). The spacing between each plasma cell is 3 cm. The input voltage to each of the plasma power sources is controlled by a variac connected to the mains. A plug-in power meter (Prodigit Electronics, 2000MU) is used to monitor the input (or wallplug) power applied to each power supply; the power is varied by altering the input supply voltages. The output from the neon sign transformer is 10 kV, 30 mA at full supply voltage with a frequency of 21 kHz; the ozonizer power supply produces a sinusoidal output of 5-10 kV, 50-100 W at 20 kHz. The concentration of toluene, ethylene, O3, CO, CO2, and N2O are determined by FTIR spectroscopy using a long-path gas cell (4.8 m) and a Shimadzu FTIR spectrometer (8300) with a resolution of 1 cm-1. This is done through the measurement of the peak heights of the products compared to their standard reference spectra. The standard reference spectra, from QASOFT-Infrared Analysis, Inc (30). are measured at a known concentration, and calibrated in ppmmeter (concentration-length). From these measurements and the use of the Beer-Lambert Law, the end concentration can be determined. The destruction of the pollutant molecule is then determined by comparing its initial and final concentrations. In this process, neither NO nor NO2 were detected by the FTIR; their values were below the detection threshold of the FTIR spectrometer (∼1 ppm). In order to place a lower limit on the NOx levels, a gas sampling tube (Gastec 11L), with a detection limit of 0.01 ppm for NOx (NO + NO2 + HNO3) was used and it was confirmed that NOx could not be detected down to this level.
Results Ethylene Destruction. Results for the destruction of ethylene (112 ppm in air) using one plasma cell (A), two cells (A + B, A + C) and three plasma cells (A + B + C) in series with an input supply voltage of 25 V, at a flow rate of 1 L min-1, are given in Table 1. The output concentrations of ozone, N2O and nitrogen oxides (NOx) are also shown. As mentioned, NOx is not detected by the FTIR system, their quoted values represent the detection limit of the gas sampling tubes. For the purposes of comparison, a further experiment was conducted using all three cells at an input voltage of 30 V. Neon sign power supplies were used to power the cells. Toluene Destruction. The results for the destruction of toluene (100 ppm in air) using one plasma cell (A), two plasma cells (A + C), and three plasma cells (A + B + C) in series, at a flow rate of 1 L min-1, are shown in Table 2. The input voltage is kept constant but the different powers reflect the different number of cells being used, as shown in Table 2. In these experiments the ozonizer power supply was used requiring a higher input voltage. End-products of ozone, N2O, CO, and CO2 but not NOx (
