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1 저작자표시 2.0 대한민국 이용자는아래의조건을따르는경우에한하여자유롭게 이저작물을복제, 배포, 전송, 전시, 공연및방송할수있습니다. 이차적저작물을작성할수있습니다. 이저작물을영리목적으로이용할수있습니다. 다음과같은조건을따라야합니다 : 저작자표시. 귀하는원저작자를표시하여야합니다. 귀하는, 이저작물의재이용이나배포의경우, 이저작물에적용된이용허락조건을명확하게나타내어야합니다. 저작권자로부터별도의허가를받으면이러한조건들은적용되지않습니다. 저작권법에따른이용자의권리는위의내용에의하여영향을받지않습니다. 이것은이용허락규약 (Legal Code) 을이해하기쉽게요약한것입니다. Disclaimer

2 工學碩士學位論文 글라이딩아크플라즈마를이용한유해가스처리 Decomposition of Hazardous Gases using Gliding Arc Plasma 2009 年 2 月 仁荷大學校大學院 化學工學科 ( 化學工學專攻 ) 李種文

3 工學碩士學位論文 글라이딩아크플라즈마를이용한유해가스처리 Decomposition of Hazardous Gases using Gliding Arc Plasma 2009 年 2 月 指導敎授朴東化 이論文을碩士學位論文으로提出함 仁荷大學校大學院 化學工學科 ( 化學工學專攻 ) 李種文

4 이論文을李種文의碩士學位論文으로認定함 2009 年 2 月 主審 印 副審 印 委員 印

5 요약 본연구에서는글라이딩아크플라즈마를이용한벤젠과일련의 PFCs (CF 4, NF 3, SF 6 ) 분해실험을수행하였다. 글라이딩아크플라즈마는에너지소모가상대적으로적으면서도단위시간당많은양의대기오염물질을처리할수있으며, 저온플라즈마로구분되지만방전과정이열플라즈마의성격을지니고있기때문에높은에너지효율과선택성을제공한다. 반응기는 Pyrex관과두개의 Bakelite판으로이루어져있다. 아크는두개의칼날모양전극의최단간격에서발생하며희석된벤젠과 PFC 가스는아크밑에설치된노즐을통해반응기로도입된다. 도입가스의흐름에의해아크가전극을미끄러져올라가면서플라즈마영역이형성되고벤젠과 PFC 가스가분해되었다. 아크발생을위해 1 kw급정류 DC power generator와 2 kw 급 short-pulsed AC power generator를사용하였다. 기상생성물의정량, 정성분석은 FT-IR과 GC를통해이루어졌으며, 벤젠및 CF 4, NF 3, SF 6 를각각 90.0%, 82.7%, 98.8%, 98.9% 의분해율로처리하였다. Digital oscilloscope을이용한전압, 전류의측정을통해글라이딩아크방전의특성을관찰하였다. PFC 분해과정에서반응기내벽에고체분말이생성되었는데이는 FeF 3 로확인되었고, 분해반응에서 Fe의촉매적인역할에대해서도고려하였다. i

6 Abstract This study examined the decomposition of Benzene and typical PFCs (CF 4, NF 3, SF 6 ) using gliding arc plasma. Gliding arc plasma can treat air pollutants at high flow rates with relatively low energy consumption. Although gliding plasma is classified as non-thermal plasma, some characteristics of thermal plasma are existed during its evolution. Therefore, it offers high energy efficiency and selectivity for chemical reactions. The reactor consisted of a Pyrex tube and two Bakelite plates. The arc was first generated at the shortest gap between the two knife-shaped electrodes, and the diluted benzene and PFC gas were introduced though a nozzle placed underneath the arc. As the arc slid up the electrodes by the flow of input gas, a plasma zone was formed decomposing the benzene and PFC gases. A rectified DC power generator with a maximum power of 1 kw and a short-pulsed AC power generator with a maximum power of 2 kw were used to generate the arc. Quantitative and qualitative analyses of the gaseous products were carried out by FT-IR and GC. Up to 82.7 %, 98.8 % and 98.9 % of the CF 4, NF 3, SF 6 gases were decomposed by this process, respectively. To examine the discharge characteristics of gliding arc plasma, voltage and current analyses were also carried out by digital oscilloscope. In addition, the solid powder deposited inside the Pyrex tube during the PFC decomposition was identified as FeF 3. The catalytic role of Fe in the decomposition reaction of PFC gas was considered. ii

7 Contents 국문요약 ⅰ Abstract ⅱ Contents ⅲ List of tables ⅴ List of figures ⅵ 1. Introduction What is plasma? Types of plasma Applications of plasma Gliding arc plasma Necessity of study 7 2. Experimental Experimental apparatus Experimental methods Results and discussion Thermodynamic analysis Voltage-current characteristics of gliding arc discharge Effect of current and flow rate on the conversion of benzene Effect of total gas flow rate on the conversion of PFCs (CF 4, NF 3, SF 6 ) FT-IR analyses of exhaust gases after PFCs decomposition By-product analyses Future works Treatment or controlling of by-products Minimizing dead volume in the reactor 37 iii

8 4. Conclusions 39 References 40 iv

9 List of tables Table 1. Industrial/commercial applications of plasmas Table 2. Atmospheric lifetimes and global warming potentials (GWPs) of some greenhouse gases Table 3. Experimental conditions for benzene decomposition Table 4. Experimental conditions for PFCs decomposition v

10 List of figures Figure 1. Plasma state. Figure 2. Temperature and pressure domain for equilibrium and non-equilibrium plasmas. Figure 3. Plasma processing in industry. Figure 4. A cycle of gliding arc discharge. Figure 5. Thermal and non-thermal regimes of gliding arc discharge. Figure 6. VOCs effects on environment. Figure 7. Schematic diagram of the gliding arc plasma system. Figure 8. Three types of electrods. Figure 9. Thermodynamic equilibrium diagrams of hazardous gases with air at 1 atm: (a) CF 4, (b) NF 3, (c) SF 6 and (d) benzene. Figure 10. Voltage waveform of gliding arc discharge using rectified DC power supply. Figure 11. Voltage and current waveform of gliding arc discharge using pulsed AC power supply controlling voltage. Figure 12. Voltage and current waveform of gliding arc discharge using pulsed AC power supply controlling current. Figure 13. Effect of current on the conversion of benzene gas. Figure 14. Effect of flow rate on the conversion of benzene gas. Figure 15. Effect of total gas flow rate on the decomposition rate. Figure 16. Qualitative analyses of exhaust gases from the reactor by FT-IR spectra: (a) CF 4, (b) NF 3, (c) SF 6. Figure 17. XRD patterns of the powder (FeF 3 ) deposited inside the reactor tube. Figure 18. Thermodynamic equilibrium diagrams of PFCs with air and Fe at 1 atm: (a) CF 4, (b) NF 3, (c) SF 6. Figure 19. SEM image of FeF 3. Figure 20. Water treatment of exhaust gas. Figure 21. Expected effects of additive gases on NF 3 decomposition. Figure 22. Development of electrode type for minimizing dead volume in the reactor. vi

11 1. Introduction 1.1 What is plasma? A plasma is a gas containing charged and neutral species, including some or all of the following: electrons, positive ions, negative ions, atoms, and molecules. On average a plasma is electrically neutral, because any charge imbalance would result in electric fields that would tend to move the charges in such a way as to eliminate the imbalance. As a result, the density of electrons plus the density of negative ions will be equal to the density of positively charged ions [1, 2]. Plasma is often called the fourth state of matter. A solid substance in thermal equilibrium generally passes into a liquid state as the temperature is increased at a fixed pressure. The liquid passes into a gas as the temperature is further increased. At a sufficiently high temperature, the molecules in the gas decompose to form a gas of atoms that move freely in random directions, except for infrequent collisions between atoms. If the temperature is further increased, then the atoms decompose into freely moving charged particles (electrons and positive ions), and the substance enters the plasma state (Figure 1). This state is characterized by a common charged particle density n e n i n particles/m 3 and, in equilibrium, a temperature T e = T i = T. The temperature required to form plasmas from pure substances in thermal equilibrium range from roughly 4000 K for easy-to-ionize elements like cesium to 20,000 K for hard-to-ionize elements like helium. The fractional ionization of a plasma is x iz i (1) n g n n i where n g is the neutral gas density, and n i is the ionized gas density. x iz is near unity for fully ionized plasmas, and x iz 1 for weakly ionized plasmas [3-5]. 1

12 Figure 1. Plasma state. 1.2 Types of plasma In general, plasma can be divided into two categories: equilibrium and nonequilibrium plasmas (Figure 2). The equilibrium plasma, known also as thermal plasma, is in local thermodynamic equilibrium and has almost equal temperatures of electrons and ions (T e T i ). In a non-equilibrium plasma, the electrons are hotter than the ions (T e T i ) but the gas pressure is lower [4, 6, 7]. Equilibrium (thermal) plasma has a powerful energy to destruct stable chemicals and the chemical reaction is occurred in very short time. On the other hand, although it is not as strong as equilibrium plasma, non-equilibrium (non-thermal) plasma is convenient to control the chemical reaction [8, 9]. 2

13 Figure 2. Temperature and pressure domain for equilibrium and non-equilibrium plasmas. 1.3 Applications of plasma Plasma is conductive and respond to electric and magnetic fields and can be efficient sources of radiation, they are usable in numerous applications where such control is needed or when special sources of energy or radiation are required such as plasma processing of semiconductors, sterilization of some medical products, lamps, lasers, diamond coated films, high power microwave sources, and pulsed power switches. In addition, plasma technology is environmentally cleaner than similar processes used in existing chemical industries. Therefore, plasma technology also has important potential applications such as the generation of electrical energy from fusion and pollution control and removal of hazardous chemicals. The applications of plasma in industry are shown briefly in Figure 3 and Table 1 [1-7]. 3

14 Figure 3. Plasma processing in industry. 4

15 Table 1. Industrial/commercial applications of plasmas Processing Flat-Panel Displays Volume Processing Radiation Processing Chemical Synthesis Switches Light Sources Energy Converters Surface Treatment Medicine Etc. Surface Processing Non-equilibrium (low pressure) Thermal (high pressure) Field-emitter arrays Plasma displays Flue gas treatment Metal recovery Waste treatment Water purification Plant growth Plasma spraying Diamond film deposition Ceramic powders Electric power Pulsed power High intensity discharge lamps Low pressure lamps Specialty sources MHD converters Thermionic energy converters Ion implantation Hardening Welding Cutting Drilling Surface treatment Instrument sterilization Propulsion Isotope Separation Lasers Material Analysis 5

16 1.4 Gliding arc plasma Gliding arc plasma can be described as a flame between more than two diverging electrodes. A short equilibrium plasma column is formed at the shortest gap between the electrodes and the gas flow drags up the column. As the length of the column increases, the surrounding flow cools the arc and the power supply compensates the heat losses until the maximum power is reached. And after that, the discharge disappears and a new cycle starts (Figure 4). In principle, gliding arc region could be divided into three phases. The equilibrium stage takes place after formation of initiated arc in the breakdown channel. It produces a stable plasma region with the length of flowing arc. In the mean time, equilibrium state of the arc is transferred into non-equilibrium state. Because of comprising both equilibrium and non-equilibrium plasma conditions (Figure 5), gliding arc plasma offers high energy efficiency and selectivity for chemical reactions [10-12]. Figure 4. A cycle of gliding arc discharge. 6

17 Figure 5. Thermal and non-thermal regimes of gliding arc discharge. 1.5 Necessity of study The emission of hazardous gases, including VOCs (volatile organic compounds) and PFCs (perfluorocompounds), has become an important issue with the increasing concern about global air pollution. VOCs are compounds that have a high vapor pressure and low water solubility. Many VOCs are human-made chemicals that are used and produced in the manufacture of paints, pharmaceuticals, and refrigerants, etc. VOCs not only cause air pollution like photochemical smog, creating ozone (Figure 4), and global warming but can also contaminate soil and ground-water. Furthermore, the aromatic compounds, such as benzene, toluene and xylene are suspected carcinogens. The existing methods for the VOCs decomposition are thermal oxidation, catalytic oxidation, bio-membrane filtration, UV oxidation, adsorption, absorption, etc. However, non-thermal plasma process for the decomposition has been variously investigated recently [13-19]. 7

18 Figure 6. VOCs effects on environment. PFCs, which are used mainly in the semiconductor and LCD industries, are considered to be extremely potent greenhouse gases on account their chemical inertness and strong absorption in the infrared spectrum with long lifetimes in the atmosphere [20-24]. Therefore, they have relatively larger global warming potentials (GWPs) than other greenhouse gases. In the case of SF 6 (atmospheric life time = 3,200 year, GWPs = 23,900), the emission of 1 kg of SF 6 is equivalent to the emission of 23.9 metric tons of CO 2 [24, 25]. GWPs of some PFCs are listed in Table 2. 8

19 Table 2. Atmospheric Lifetimes and Global Warming Potentials (GWPs) of Some Greenhouse Gases Gas Atmospheric lifetime (year) GWPs CO 2 CF 4 C 2 F 6 SF 6 C 3 F 8 CHF 3 C 4 F 8 CH 4 N 2 O NF ,000 10,000 3,200 2,600 7, , ,500 9,200 23,900 7,000 11,700 8, ,000 Thus far, there have been many studies on the decomposition of PFCs using a variety of technologies [21, 24, 26], with microwave plasma appearing to be the most efficient [20, 22-24]. However, the gliding arc plasma system is more economical than microwave plasma devices [10]. Although gliding arc plasma is classified as nonthermal plasma, a thermal plasma condition can also be produced. An equilibrium plasma column is formed at the shortest distance between more than two diverging electrodes. The discharge length increases due to the flow of input gas. The power also increases until the power supply reaches a maximum value in order to maintain the equilibrium state by compensating for the energy loss caused by thermal conduction. Subsequently, a transition to a non-equilibrium state occurs and up to 80 % of the total energy that is dissipated during this period contributes to the generation of chemically active species [10-12]. Therefore, this type of plasma can provide a high level of selectivity and energy efficiency for various chemical reactions such as the abatement of air pollutants [10, 11, 27-30], the production of synthesis gas [31-34], surface treatment [35, 36], etc. This paper reports the decomposition of benzene and three types of PFCs; CF 4, NF 3, SF 6 using a gliding arc plasma system at atmospheric pressure and ambient 9

20 temperature. The composition of the exhaust gas was analyzed by FT-IR, and the effects of the total gas flow rate and stainless steel electrodes were examined. 10

21 2. Experimental 2.1 Experimental apparatus The decomposition process was operated using a gliding arc plasma reactor consisting of a Pyrex tube, two Bakelite plates, and two stainless steel electrodes at atmospheric pressure and ambient temperature. Figure 7 shows a schematic diagram of the gliding arc plasma system and Table 3 and 4 summarize each experimental conditions. The arc was generated at the shortest gap between the stainless steel electrodes and was spread by gliding up the electrodes in the direction of the gas flow. Benzene and PFC gases diluted with air were introduced to the reactor through a nozzle that was placed at the bottom of the reactor. A mixing chamber was then placed before the reactor to enhance the mixing of gases. Rectified DC (direct current) power supply (Plasmatech, Korea) and pulsed AC (alternating current) power supply (IHP-1002, EN Technologies, Korea) were used to generate the arc. Three types of electrodes with different D, distance between the electrodes edge (Figure 8) were used for benzene decomposition and only electrode A was used for PFCs decomposition. 11

22 Figure 7. Schematic diagram of the gliding arc plasma system. 12

23 Table 3. Experimental conditions for benzene decomposition Dilution gas Concentration of benzene Total gas flow rate Current Air 50 ~ 500 ppm 0.5 ~ 2 liter/min 50 ~ 100 ma A (D = 60 mm) Electrode type B (D = 38 mm) C (D = 23 mm) 13

24 Table 4. Experimental conditions for PFCs decomposition Dilution gas Air PFCs CF 4, NF 3, SF 6 Concentration of PFCs Total gas flow rate Voltage Frequency 5000 ppm CF 4 : 4 ~ 10 liter/min NF 3, SF 6 : 7 ~ 10 liter/min 10 kv 40 khz 14

25 (a) Electrode A (b) Electrode B (c) Electrode C Figure 8. Three types of electrodes. 15

26 2.2 Experimental methods Online analyses of the input and the output gas were carried out by Fourier transform infrared spectroscopy (FT-IR, IG-2000, OTSUKA Electronics, Japan) and gas chromatograph (GC, 6890N, Agilent Technologies, USA). Field emission scanning electron microscopy (FE-SEM, S-4300, HITACHI, Japan) and X-ray diffraction (XRD, D/MAX 2200V/PC, RIGAKU, Japan) were used to identify the powder deposited in the reactor wall during the reaction. The conversion of a target gas was calculated using the following equation: Conversion Ci Cf (%) 100 (2) C i where C i is the initial concentration of the target gas and C f is its final concentration after the plasma treatment. 16

27 3. Results and discussion 3.1 Thermodynamic analysis The composition of the exhaust gas at equilibrium was predicted through thermodynamic equilibrium calculations using the FACTSAGE program (CRCT - ThermFact Inc., Canada & GTT - technologies, Germany). Figure 9 show the thermodynamic diagrams of benzene and each PFC with air at 1 atm and temperatures ranging from 300 K to 3000 K. The diagrams show that benzene is easily decomposed, CF 4 and NF 3 are the most and least stable PFC gases, respectively. This trend is in agreement with previous knowledge on the chemical structure and bonding energy of CF 4. In addition, a large amount of fluorine gas was generated during the PFCs decomposition reaction. 17

28 Figure 9. Thermodynamic equilibrium diagrams of hazardous gases with air at 1 atm: (a) CF 4, (b) NF 3, (c) SF 6 and (d) benzene. 18

29 3.2 Voltage-current characteristics of gliding arc discharge Figure 10 shows voltage waveform of gliding arc discharge using rectified DC power supply. Because electrode A has the largest D of 60 mm, the arc length is rapidly increased and disappeared. This is because the power supply reaches its critical value to compensate heat losses from plasma column due to increase of the arc length. As shown in Figure 10 (a) and 10 (b), narrower electrodes have a longer cycle. Equilibrium and non-equilibrium region of the discharge could be found in the enlarged waveforms. Symmetrical waveforms of equilibrium state had become distorted as the transition into non-equilibrium state occurs. In the case of electrode C, Figure 10(c), the arc length was increased gradually and the arc was held at the edge of the electrodes for a while. Therefore, non-equilibrium state was sustained longer than the other electrodes during a cycle. The current and voltage waveforms of gliding arc discharge using pulsed AC power supply were also measured. Voltage was controlled in Figure 11 and current was controlled in Figure 12. In both cases, current waveforms were steady but voltage waveforms had changed moment by moment. In all cases, the arc ignites initially with high current but low power. With an increase in length, voltage on the arc increases and current drops. This results in increase in total power input to the arc, until power input reaches a maximum value. After this point, the arc still continues to elongate for considerable time in the overshooting regime [12]. 19

30 (a) Electrode A (b) Electrode B (c) Electrode C Figure 10. Voltage waveform of gliding arc discharge using rectified DC power supply. 20

31 10 Current (A) Voltage (kv) Time ( s) (a) Before discharge 10 Current (A) Voltage (kv) Time ( s) (b) After discharge Figure 11. Voltage and current waveform of gliding arc discharge using pulsed AC power supply controlling voltage. 21

32 6 Current (A) Voltage (kv) Time ( s) (a) Before discharge 6 Current (A) Voltage (kv) Time ( s) (b) After discharge Figure 12. Voltage and current waveform of gliding arc discharge using pulsed AC power supply controlling current. 22

33 3.3 Effect of current and flow rate on the conversion of benzene The increase of current put more energy in the reaction and conversion of benzene was also increased (Figure 13). The effect of total gas flow rate, which is related to the residence time of benzene in the reactor, was examined. Figure 14 shows the changes of benzene conversion according to the various flow rates at 100 ma. The conversion of benzene decreased with the increasing total flow rate. The increase of the flow rate reduced the residence time of benzene in the reactor, which could reduce the possibility and time of benzene to collide with electrons or other high energy species which have enough energy to destroy C-H bond. Electrode C seems to be the most appropriate for benzene decomposition due to long non-equilibrium state in which chemical active species are produced during a cycle. Sustaining non-equilibrium state might be important factor for better decomposition of hazardous materials. 23

34 Conversion (%) Electrode shape A B C 100 ppm, 500 sccm Current [ma] Figure 13. Effect of current on the conversion of benzene gas. 24

35 Conversion (%) ppm, 100 ma Electrode shape A B C Flow rate [sccm] Figure 14. Effect of flow rate on the conversion of benzene gas. 25

36 3.4 Effect of total gas flow rate on the conversion of PFCs (CF 4, NF 3, SF 6 ) Figure 15 shows the relationship between total gas flow rate (4 ~ 10 liter/min) and conversion of PFCs. The conversion is decreased with increasing gas flow rate due to the short residence time of the PFCs in the reactor. Therefore, the reaction time might not sufficient to decompose the PFCs at the higher flow rate. When the flow rate was < 4 liter/min, the arc was not dragged up the electrodes and the reactor could not tolerate high temperatures. When the flow rate was > 10 liter/min, the arc disappeared shortly after its ignition and the conversion is decreased rapidly. 26

37 Conversion (%) CF4 NF3 SF Total gas flow rate [sccm] Figure 15. Effect of total gas flow rate on the conversion. 27

38 3.5 FT-IR analyses of exhaust gases after PFCs decomposition Figure 16 shows the FT-IR absorption spectra of the total gases before and after the decomposition reaction. Some major components of the exhaust gases could be identified. Qualitative analyses of the product gases were carried out using the library of IR spectra. In all cases, NO (nitric oxide), NO 2 (nitrogen dioxide) and HF (hydrogen fluoride) were generated. CO 2 was produced in the case of CF 4, and SO 2, SO 2 F 2 were generated from the decomposition of SF 6. There was no significant difference in the intensities of the main peaks before and after the runs, as shown in Figure 16 (a) and (c). The sub peaks are sometimes used for qualitative analysis of CF 4 and SF 6, as recommended by OTSUKA Electronics according to the concentration range, because they are more accurate than the main peaks. The circled parts indicate the sub peaks of each PFC. 28

39 Figure 16. Qualitative analyses of exhaust gases from the reactor by FT-IR spectra: (a) CF 4, (b) NF 3, (c) SF 6. 29

40 3.6 By-product analyses During the PFCs decomposition reaction, a light green powder was deposited inside the reactor tube, which was revealed by XRD (Figure 17) to be FeF 3 (iron fluoride). It was concluded that Fe participated in the reaction through the synthesis of FeF 3 and erosion of the stainless steel electrodes. From thermodynamic equilibrium calculations (Figure 18) of the PFCs with air and Fe, it was found that Fe enhanced the decomposition reaction and inhibited the generation of F 2 gas. FeF 3 is predominantly synthesized in the range of the settled temperature, and fluorine gas is reduced somewhat because each FeF 3 molecule holds three F atoms. Ceder et al. reported that metal fluorides, such as FeF 3, have a sufficiently high potential to be used as Li-ion cathodes due to the high reaction potential from the strong ionic property of the metal-fluorine bond [37]. Therefore, the byproduct of this experiment, FeF 3, might be useful in the field of electrochemistry when prepared as a nano-scale powder. However, in this experiment, the particle size of the FeF 3 synthesized was not uniform (Figure 19). In order to solve this problem, rapid plasma quenching around the reactor is required to produce ultrafine FeF 3 powders [38]. This has significant potential as this process can decompose hazardous gases and at the same time produce valuable materials. Determining the optimum conditions for the decomposition of CF 4 was the main aim of this study. Other PFCs will be decomposed easily because CF 4 is the most stable PFC and some PFCs even recombine to form CF 4 during the decomposition reaction [23]. Although HF is soluble in water, the other byproducts, such as NO, NO 2, SO 2, and SO 2 F 2, will require treatment due to their damaging effects on the environment. 30

41 Figure 17. XRD patterns of the powder (FeF 3 ) deposited inside the reactor tube. 31

42 Figure 18. Thermodynamic equilibrium diagrams of PFCs with air and Fe at 1 atm: (a) CF 4, (b) NF 3, (c) SF 6. 32

43 Figure 19. SEM image of FeF 3. 33

44 3.7 Future works Treatment or controlling of by-products When air was used as a dilution gas, NOx, SOx, HF and fluorine gas which are still toxic were generated during the reaction. To prevent this, use of additive gases has to be considered. So far, H 2 O and H 2 gases seem to be effective because hydrogen ions can capture fluorine gas and produce HF that is readily hydrolyzed in water and easy to treat (Figure 20). Although H 2 O would be preferable for the industrial application, both of them are worthwhile additive gases for the experiment. The important thing is to introduce to H 2 O the gliding arc reactor as a gas state. For NOx and SOx generation, use of nitrogen or argon gases as a dilution gas is under consideration. The expected effects of additive gases for NF 3 decomposition are shown in Figure

45 Figure 20. Water treatment of exhaust gas. 35

46 (a) Without additive gases (b) Effect of hydrogen gas (c) Effect of 1 mole of steam (d) Effect of 2 moles of steam Figure 21. Expected effects of additive gases on NF 3 decomposition. 36

47 3.7.2 Minimizing dead volume in the reactor The configurational problem of present reactor is its large dead volume in the reactor between knife-shaped electrodes and a cylinderical Pyrex tube. To maximize plasma region in the reactor, knife-shaped electrodes are expected to be changed into rod-like electrodes. Rod-like electrode is easy to twist and the arc would be rotate as sliding up the electrodes during a discharge. Therefore, two dimensional plasma region is expanded into three dimensional conical shape and then, it increases possibility of target gas to pass through the plasma region (Figure 22). 37

48 Figure 22. Development of electrode type for minimizing dead volume in the reactor. 38

49 4. Conclusions Benzene and PFCs gases were decomposed by gliding arc plasma with a high conversion. The total gas flow rate ranged from 0.5 to 2 liter/min for benzene decomposition and ranged from 4 to 10 liter/min for PFCs decomposition. The conversion decreased with increasing flow rate. The composition of the input and output gases was identified by FT-IR analysis in real time. Up to 82.7 %, 98.8 % and 98.9 % of the CF 4, NF 3 and SF 6 gases at flow rates of 5000 ppm were decomposed, respectively. During the decomposition reaction, FeF 3 was deposited as a solid powder inside the Pyrex tube. This powder might have electrochemical applications. However, the particle size will need to be more uniform before it can be used in Li-ion cathodes. Further studies will be required to determine the optimal conditions for CF 4 decomposition, quenching of the reactor and the treatment of byproducts. Therefore, reactor configuration improvement and the addition of other gases will need to be considered. 39

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