Non-Thermal Plasma Based Decomposition of Volatile Organic Compounds in Industrial Exhaust Gases

Int. J. Environ. Sci. Technol. (2015) 12:3745–3754 DOI 10.1007/s13762-015-0814-1 ORIGINAL PAPER Non-thermal plasma based decomposition of volatile organic compounds in industrial exhaust gases 1 2 3 1 M. Schmidt • I. Jo˜gi • M. Hołub • R. Brandenburg Received: 9 December 2014 / Revised: 9 March 2015 / Accepted: 19 April 2015 / Published online: 5 May 2015 Ó Islamic Azad University (IAU) 2015 Abstract The decomposition of various volatile organic Keywords Non-thermal plasma Á Dielectric barrier compounds (VOCs), namely butyl acetate, styrene, and discharge Á Exhaust gas Á VOC Á Decomposition Á methanol, by means of non-thermal plasma technology was Pollution control Á Industrial scale investigated in different industrial exhaust gases. Although a great deal of effort on the investigation of these VOCs can be found in literature, data regarding the application in real exhaust gases are missing and provided in this con- Introduction tribution. Measurements were performed in an oil shale processing plant in Estonia and in a yacht hall production Many industrial processes cause emissions of gaseous site in Poland. Up to 71 % of butyl acetate at a specific pollutants endangering public health and the environment. input energy (SIE) of about 220 J/L and more than 74 % of These are e.g., oxides of nitrogen (NOx), oxides of sulfur styrene and methanol at SIE [ 300 J/L were decomposed. (SOx), and volatile organic compounds (VOCs). Estab- The energy efficiency of the decomposition processes was lished technologies for gas cleaning are, e.g., catalysis and analyzed as well as the products formed by the plasma thermal oxidation, dry and wet scrubbing, and biofiltra- process. Energy constants of kE \ 9.85 L/kJ for butyl ac- tion (Wang et al. 2004). These methods are efficient with etate and kE \ 2.75 L/kJ for styrene and methanol were respect to the level of removal of pollutants and energy obtained. Most of the VOCs were partly oxidized to carbon consumption. Catalytic processes and thermal oxidation monoxide and, to lesser extent, totally oxidized to carbon require high temperatures (*1000 °C). Beside the large- dioxide. scale installations of most of the technologies, they suffer from poor applicability to processes varying fast in con- centration of pollutants and gas flow volumes. A prospective means to overcome at least the latter is the use of non-thermal plasma (NTP) technology. The term NTP refers to an ionized gas containing, additional to the neutral bulk gas molecules and atoms, also ions, electrons, and excited species. The species are not in thermal equilibrium. A common way to generate an NTP is to & M. Schmidt applyanelectricfieldtoaneutral gas. If the breakdown [email protected] field strength is exceeded, a gas discharge is forming the 1 Leibniz-Institute for Plasma Science and Technology, plasma. The electric field can be applied by using two Felix-Hausdorff-Str. 2, 17489 Greifswald, Germany electrodes, one connected to ground and the other one to a 2 Institute of Physics, University of Tartu, Ravila 14c, high voltage supply. Dielectric barrier discharges (DBDs, 50411 Tartu, Estonia sometimes also referred to as silent discharges) are a 3 West Pomeranian University of Technology, al. Piasto´w 17, prevalent type of NTP generated at atmospheric pressure 70-310 Szczecin, Poland and are used in exhaust treatment, for ozone generation, 123 3746 Int. J. Environ. Sci. Technol. (2015) 12:3745–3754 surface treatment, and many other applications (Wagner This work focuses on the removal of butyl acetate, et al. 2003; Kogelschatz 2003; Brandenburg et al. 2011). styrene, and methanol in the exhausts of a shale oil pro- In DBDs, at least one of the electrodes is covered with a duction plant in Estonia and a yacht hull production site in dielectric material (e.g., glass or alumina). Typical con- Poland, respectively. Butyl acetate (C6H12O2) is widely figurations are a metal wire in a glass tube wrapped with a used as a solvent in the production of coatings, finishes, metal foil or metal plates separated by an alumina plate. inks, and cosmetics (MCgroup 2014a, butyl acetate) with a The use of NTP allows the initiation of various chemical global production of 528,000 tons in 1998 (WHO 2005). processes without a significant increase in the gas tem- Vapor and liquid of butyl acetate are inflammable and form perature. The chain of reactions starts among others with explosive mixtures. Butyl acetate induces drowsiness; the ionization, excitation, dissociation, or dissociative ioniza- workplace exposure limit is 62 ppm (BGRCI 2014a, BA, tion of bulk gas molecules (nitrogen, oxygen, and water, if all legislative limitations given are valid for Germany). present), by direct collisions of these molecules with free Styrene (C8H8) is used in the production of rubber, plastics, electrons. Besides direct electronic processes, there are, for adhesives, and inks with a global production of more than example, indirect processes like Penning ionization and 26.4 million tons in 2012 (MCgroup 2014c, styrene). Va- many others. Thus, various intermediate and reactive spe- por and liquid of styrene are inflammable and health cies are formed. The most important species utilized for the damaging when inhaled and lethal if swallowed. The removal of gaseous pollutants in water containing air are workplace exposure limit is 20 ppm (BGRCI 2014c, sty- radicals (e.g., O, OH, HO2) and ozone (O3) with different rol). Methanol (CH4O) is used as a feedstock in chemical oxidation potentials. A detailed discussion of plasma che- industry and fuel for combustion engines. The global pro- mical processes can be found in (Fridman 2008) and, duction of methanol reached almost 63 million tons in 2012 especially for pollutants decomposition, in (Kim 2004). (MCgroup 2014b, methanol). Vapor and liquid of methanol The decomposition of VOCs with NTPs is widely stud- are easy inflammable. Methanol is toxic when swallowed, ied for laboratory conditions with and without the use of inhaled or at skin contact. The workplace exposure limit is catalysts (e.g., Huang et al. 2011; Vandenbroucke et al. 200 ppm (BGRCI 2014b, methanol). 2011; Aerts et al. 2013). One of the most intensively studied To the authors’ present knowledge, there are only few VOCs is toluene (C7H8) consisting of a benzene ring with a data available for the abatement of butyl acetate, styrene, methyl group (Van Durme et al. 2007; Vandenbroucke et al. and methanol, which were found to be the most important 2012). It was found that the decomposition process starts pollutants in the field tests, by means of NTP (Demidiouk with the hydrogen abstraction from the methyl group by et al. 2003; Zhang et al. 2009). Thus, the plasma reactor, its electron impact or reaction with hydroxyl radical. The fol- application, and the removal efficiencies for butyl acetate, lowing reaction proceeds to the formation of ring-contain- styrene, and methanol as well as the formation of reaction ing compounds (e.g., benzaldehyde or benzoic acid) or ring- products and by-products are presented in this contribution. opening products (e.g., formic acid). There are also studies about the removal of other VOCs available in literature (see, e.g., Kim 2004; Fridman et al. 2005). Materials and methods However, most of the studies in the reviews and con- tributions mentioned above are performed in the laboratory The experiments took place in an oil shale processing plant and thus under well-defined conditions. These studies deal in Estonia and in a company producing hulls for boats and with a general understanding of the mechanisms of plasma yachts in Poland. The experiments were performed in the chemistry and discharge development which is very valu- bypass of the emission duct during the industrial process. able and desired for the development of new processes. On This process was not interfered by the experiments. In the other site, data regarding the application in real exhaust order to study the general plasma chemistry and to evaluate gases are only few, and most of them are concerning SOx the plasma effect, sensitive laboratory equipment was ap- and NOx treatment (Kim 2004; Hammer 2014). One ex- plied to industrial environments under rough conditions ample of the treatment of VOCs, namely methanol, ace- (dusty air, heat, moisture, and vibrations of installations tone, dimethyl sulfide, and a-pinene in the exhaust of pulp and devices). Although the conditions of the process gases mills and wood product plants is described in (Fridman to be treated were varying (fluctuation of concentrations of et al. 2005). Mizuno is presenting the removal of ac- the pollutants, moisture, and gas flow), it was possible to etaldehyde (ethanal, C2H4O) in an indoor air cleaning obtain sufficient and reliable results after careful selection process (Mizuno 2007). of the data to be used. 123 Int. J. Environ. Sci. Technol. (2015) 12:3745–3754 3747 In general, the experimental setup was almost identical and ground. Thus, the plasma was formed by a stacked for both experiments in Estonia and Poland. It consisted of volume DBD reactor configuration. This discharge con- the plasma source, a power supply to energize the plasma, figuration was mounted and sealed in a stainless steel and a set of diagnostics to analyze the gas composition and housing also containing the electrical connectors and gas the electrical parameters of the plasma source. The gas in- and outlet connectors (100 mm diameter). sample was extracted through the bypass and directly The plasma source was energized with sinusoidal high treated by the plasma in all experiments. A more detailed voltage with two different power supplies. For the ex- description follows together with the differences of the periments in Estonia, a programmable high-voltage (HV) experimental apparatus. power supply (Chroma 61604) with a matched high- The treatment of the gas samples was performed with a voltage transformer was used.

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