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Effective ozone generation utilizing a meshed-plate electrode in a dielectric-barrier discharge type ozone generator
ARTICLE IN PRESS
Journal of Electrostatics 64 (2006) 275–282 www.elsevier.com/locate/elstat
Effective ozone generation utilizing a meshed-plate electrode in a dielectric-barrier discharge type ozone generator Seung-Lok Parka, Jae-Duk Moonb,, Seug-Hoon Leeb, Soo-Yeon Shinc a
Extra HV Group, Electric Power Research Lab, LG Cable, 190 Gongdan-Dong, Gumi, Gyeongbuk 730-708, Republic of Korea School of Electronic and Electrical Engineering, Kyungpook National University, 1370 Sankyuk-Dong, Buk-Gu, Daegu 702-701, Republic of Korea c Digital Appliance Research Laboratory, LG Electronics Co. Ltd., 391-2, Gaeumjeong-Dong, Changwon, Gyeongnam 641-711, Republic of Korea
b
Received 23 July 2004; received in revised form 16 February 2005; accepted 22 June 2005 Available online 19 July 2005
Abstract A meshed-plate electrode, an alternative to the conventional plate electrode of the dielectric-barrier discharge (DBD) ozone generator, has been studied in order to show the effectiveness of the meshed-plate electrode in improving ozone generation. A DBD ozone generator with mesh electrode is shown to have the following two advantages: First, such a device produces corona at its thin, sharp edges, thereby decreasing corona onset voltage for a given gap spacing. Second, by utilizing the air-voids of the meshed-plate electrode, the dense ozone produced in the small, high temperature gap region of the DBD can escape rapidly to the low temperature region. As a result, decomposition of the ozone can be reduced. In this study, surface discharge, meshed-plate electrode, and conventional DBD ozone generators are studied and compared experimentally. The maximum ozone generation concentration and ozone generation efficiency are obtained with the meshed-plate type electrode. r 2005 Elsevier B.V. All rights reserved. Keywords: Ozone generator; Dielectric-barrier discharge (DBD); Meshed-plate electrode; Air-void; Gap spacing
1. Introduction Ozone (O3), a strong oxidizing agent made from stable molecular oxygen (O2), is replacing chlorinated compounds in a variety of applications including wastewater treatment, polluted air treatment, the ashing and oxidation of semiconductors, and as a disinfectant [1–3]. Ozone is unstable and decomposes slowly (in minutes) at ambient temperatures and rapidly (o1 s) at higher temperatures [4]. Current technology in ozone production utilizes several techniques. One method involved electrochemical reactions to produce ozone [5–7]. These systems suffer from high electrical consumption, and the cells produce chemicals that can be toxic or difficult to dispose [2]. The second technique involves the use of high-energy methods such as UV light, beta rays, and Corresponding author.
E-mail address: [email protected] (J.-D. Moon). 0304-3886/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2005.06.007
lasers to convert oxygen to ozone. Such methods have found minimum large-scale commercial applications [8–9]. The third general approach utilizes electrical discharges [10–17]. In this approach, pure oxygen or air is passed through an electric field generated an AC high voltage applied across plate and ground electrodes. The ground electrode is typically covered by a dielectricbarrier. These systems typically suffer from high electrical consumption and a relatively low efficiency (about 1–15%) for converting oxygen to ozone. The aim of the study presented in this paper is to show the effectiveness of using a meshed-plate electrode in an ozone generator as a way to improve the ozone generation efficiency. In a system of this type, the electric field will concentrated at the thin, sharp edges of the mesh electrode, thereby improving corona generation [12,18]. No such corona occurs at the edges of a plate electrode of the same configuration in a dielectricbarrier discharge (DBD) type ozone generator [17–19].
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Thus, by using a meshed-plate electrode, corona onset voltage can be decreased significantly. Because of the air-voids in a meshed-plate electrode, the dense ozone produced in the small, high-temperature gap spacing can escape rapidly to the low temperature region that resides above [15–17], thus greatly reducing decomposition of the ozone. As a result, ozone generation and efficiency can be further increased.
2. Experimental method and setup Fig. 1 shows a schematic diagram of the experimental setup consisting of the ozone generator, a high-voltage pulsed-power supply, a cooling water system (chiller) for the ozone-generating space, an oxygen gas cylinder with flow meter and valve, and an ozone monitor to measure ozone output. Fig. 2 shows the details of the ozone generator, which consists of two parallel sets of meshedplate/gap/dielectric-barrier/induction-plate systems. The corona emitter in this case is an etched, stainless steel, rectangular electrode of dimensions 60 mm 88 mm 0.20 mm. As shown in Fig. 2(a), the dimension VM of the square-shaped air-voids of the meshed-plate electrode, produced by a wet chemical etching method, vary in size between 0.50 and 1.50 mm. The total width WM of the meshed-plate electrode is fixed at 0.30 mm. The grounded, aluminum induction electrode (180 mm 250 mm 10 mm) has a water cooling path that is 5 mm deep, 30 mm wide, and 830 mm long. This pathway also has a concaved rear edge. A 92% alumina ceramic plate of dimensions 84 mm 112 mm 0.8 mm is used as the dielectric-barrier. A 2 mm 3 mm 0.80 mm Teflon spacer sets the gap spacing between the dielectric-barrier and the meshed-plate electrode. The gap spacing S was adjusted by the thickness (S ¼ 0:0020:80 mm) of the Teflon spacer. Note that a discharge-free region exists between the upper and lower meshed-electrodes as shown in Fig. 2, where a no discharge occurs and the temperature (of ions and ambient gases) is lower than
3. Experimental results and discussion
chiller cooling water flow meter O2 in valve Oxygen gas
the discharging gap spacing between the meshedelectrodes and the dielectric-barrier. Fig. 3 shows the high-voltage pulsed-power system consisting of a diode rectifier to produce dc voltage, a semiconductor inverter (IGBT Module, Mitsubishi, CM100BU-12 H) with gate drive, a pulse-control circuit for shaping the high-voltage pulse, and a high-voltage transformer that raises the pulse voltage. As indicated in Fig. 3, when a square-wave voltage is applied as an input to the transformer, the output becomes a triangle-like voltage wave. This occurs because the time-varying portion of the input waveform is altered by the inductance and capacitance of the transformer. Fig. 4 shows the measured voltage and current waveforms applied to the ozone generator. The triangular waveform has a peak voltage of about 6 kV and a peak current of about 0.8 A. The frequency and duty cycle of the applied high voltage pulse are known to be important factors that influence ozone production [20]. Although our system is capable of producing frequencies over the range 0.1–10 kHz and duty cycles between 0.05 and 0.50, the values 4.5 kHz and 0.10 were chosen for all experiments reported in this paper. The various voltages and the corona current were measured, respectively, using a Tektronix P6015A probe, and a Tektronix A-6312 Rogowski coil current probe with AM 503B current probe amplifier. Waveforms were recorded using a Tektronix 100 MHz TDS360A digital storage oscilloscope. The output ozone concentration was measured using the conventional method described in Refs. [11–17,20]. Specifically, output gas was monitored with an ultraviolet ozone monitor (Dasibi, Model DY-1500). Industrial-grade oxygen of 99% purity was fed to the generator at a fixed rate of 2.0 l/min. Chilled tap water at 5 1C was supplied to the cooling path of the induction electrode (Accurate Gas Control System Inc. Model AG-T). All experiments were carried out in an electromagnetically shielded booth at a constant ambient temperature of 20 1C and standard atmospheric pressure.
Ozone Generator
O2+O3 out Ozone moniter
HV power GND
Fig. 1. Schematic diagram of experimental setup.
Fig. 5 shows the current–voltage (I– V) characteristics of the generator at different values of gap spacing S for the case V M ¼ 0:80 mm and W M ¼ 0:30 mm. This figure shows that gap spacing significantly influences the corona discharge, as one would expect. The onset of corona occurs at 1.5 kV, where a weak glow discharge occurs at the edges of the meshed-plate electrode. This is evident in Fig. 7. The output ozone can be detected at an applied voltage of 1.5 kV, which thus reveals that a weak glow discharge exists and ozone production begins. The corona current then increases abruptly for higher
ARTICLE IN PRESS S.-L. Park et al. / Journal of Electrostatics 64 (2006) 275–282
induction electrode
meshedelectrode
dielectricbarrier
277
gap spacing, S
O2 in
O2+O3 out
HV power
WM
cooling waterpath
VM
meshed-electrode (closeup)
GND
WM : width (net) of meshed-plate VM : air-void of meshed-plate
(a)
Proposed Ozone Generator
O2 in
O2+O3 out
meshed-plate
dielectric barrier
thin and sharp edge
electric flux
gapspacing, S
(b)
induction electrode
Fig. 2. Configuration of mesh-electrode ozone generator (a) Configuration of proposed ozone generator with closeup of meshed-plate (side view); (b) Sketch of electrode configuration and electric flux (closeup view).
voltage over the range 3–5 kV, where an additional intense DBD is generated in the gap. Afterwards, a strong streamer corona discharge is generated, and, it saturates at an applied voltage in the range 5–6 kV. From the DBD mechanism, the corona discharge cannot proceed further to an arc discharge in the gap between the discharge electrode and the dielectricbarrier [1,21]. Figs. 6 and 7 shows the corona I– V characteristics and ozone generation, respectively, as functions of gap spacing S for the fixed values V M ¼ 0:80 mm and W M ¼ 0:30 mm. Fig. 6 shows that the gap spacing affects the corona current, and that there is an optimal value of gap spacing at about 0.7 mm [22]. Fig. 7 shows
that ozone is generated just above the 1.5 kV corona onset voltage. For the case of no gap spacing, which was realized by omitting the Taflon spacer between the meshed-electrode and the dielectric-barrier, however, the corona onset voltage is about 2.5 kV, which is higher than the onset values for larger gap spacing. Fig. 8 shows the ozone concentration of the output as a function of gap spacing. The maximum ozone concentration of about 2.96 vol% is obtained at S ¼ 0:65 mm. This value is about 1.32 times higher than the 2.25 vol% value obtained with no gap spacing. This difference occurs because, with no gap spacing, ozone is generated only from the edges of the meshed-plate electrode, causing it to operate as a surface discharge (Masuda
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Diode rectifier
HV transformer 300V/10kV
IGBT inverter
Vac 220V
C
Ozone Generator 20V DC
DC source
Output Current Sensing
Gate Signal Generating
Gate driving circuit Output Voltage Control
±15V DC
Protecting Circuit
Control circuit
Fig. 3. Circuit diagram of high-voltage pulse power.
Peak Corona Current, I [A]
1.0
0.8
0.6 4KV 5KV
0.4
6KV
0.0
0.2
0.4
0.6
0.8
1.0
Gap Spacing, S [mm] Fig. 4. Oscillograms of input pulse voltage and corona current waveforms.
Fig. 6. Corona current as a function of gap spacing for different values of applied voltage.
4.0 S = 0.00mm S = 0.40mm S = 0.65mm S = 0.80mm
0.8
Ozone Concentration, O3 [Vol%]
Peak Corona Current, I [A]
1.0
0.6
0.4
0.2
0.0
S = 0.00mm S = 0.40mm S = 0.65mm S = 0.80mm
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
1
2
3
4
5
6
7
8
Applied Pulse Voltage, V [kV] Fig. 5. Voltage–current (I– V) characteristics of mesh-electrode ozone generator for different gap spacings.
0
1
2
3
4
5
6
0
8
Applied Pulse Voltage, V [kV]
Fig. 7. Ozone generation as a function of applied pulse voltage for different gap spacings.
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0.9 Peak Corona Current, I [A]
type) ozone generator only [20]. With gap spacing, two kinds of corona can occur, one from the sharp edges, and the other from the dielectric-barrier. This the ozone output increases with gap spacing between the meshedelectrode and the dielectric-barrier. Fig. 9 shows the I– V characteristics of the meshelectrode ozone generator for different values of VM for fixed values S ¼ 0:65 mm and W M ¼ 0:30 mm. As suggested Fig. 2(b), as VM become larger, more electric flux concentrates at the mesh edges, and a stronger discharge occurs [22]. The maximum corona current added by this additional discharge was about 0.1 A, or about 14% of the peak corona current, for a constant applied pulse voltage of 6 kV. Fig. 10 shows the corona current as a function of VM for various values of applied voltage for the case S ¼ 0:65 mm and WM ¼ 0.30 mm. Maximum current occurs at all voltages for a VM of about 0.8–1.0 mm.
279
0.8
0.7
4KV 5KV 6KV
0.6
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Air-Void Size of Mesh Electrode, VM [mm] Fig. 10. Corona current as a function of air-void size for different values of applied voltage.
4.0 Ozone Concentration, O3 [Vol%]
Maximum Ozone Concentration, O3 [Vol%]
3.5
3.0
2.5
2.0
1.5 0.2
0.4
0.6
0.8
1.0
1.2
Gap Spacing, S [mm]
2.5 2.0 1.5 1.0 0.5
0.8
0.6 VM = 0.5mm VM = 0.8mm VM = 1.0mm VM = 1.5mm
0.2
0.0 1
2
3
4
5
1
2
3
4
5
6
7
8
Fig. 11. Ozone generation as a function of applied voltage for different air-void sizes of the meshed-plate electrode.
1.0
0.4
0
Applied Pulse Voltage, V [kV]
Fig. 8. Ozone output concentration as a function of gap spacing.
Peak Corona Current, I [A]
3.0
0.0 0.0
0
VM = 0.5mm VM = 0.8mm VM = 1.0mm VM = 1.5mm
3.5
6
7
8
Applied Pulse Voltage, V [kV] Fig. 9. I– V characteristics of mesh-electrode ozone generator for different air-void sizes.
Fig. 11 shows the ozone output as a function of applied voltage for different values of VM, again with S ¼ 0:65 mm and W M ¼ 0:30 mm. Ozone output begins at the onset voltage of 1.5 kV and increases steeply thereafter. Above about 5 kV, ozone output begins to fall. We hypothesize that this decrease occurs because the temperature in the air gap rises with stronger corona discharge. Thus ozone concentration falls due to rapid decomposition at the elevated temperature. Fig. 12 shows the peak ozone output as a function of VM for S ¼ 0:65 mm; W M ¼ 0:30 mm and V ¼ 5:6 kV. As this plot shows, the peak ozone concentration of 2.96 vol% occurs for V M 0:8 mm. These results suggest that, while more ozone can escape from the high temperature region for higher values of VM, less is produced, leading to the optimum value somewhere in the middle of the VM range shown.
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1.6 A: S = 0.00mm, VM = 0.80mm B: S = 0.65mm, VM = 0.80mm C: Ceramic to Ceramic type (none mesh electrode)
1.4 Peak Corana Current,I [A]
Maximum Ozone Concentration, O3 [Vol%]
3.5
3.0
2.5
1.2
C
1.0
B
0.8 A
0.6 0.4 0.2
2.0 0.0
0.0
0.4
0.8
1.2
1.6
2.0
0
1
Fig. 12. Ozone output concentration as a function of air-void size.
4
5
6
7
8
4.0 A: S =0.00mm, VM = 0.80mm B: S =0.65mm, VM = 0.80mm C: Ceramic to Ceramic type (none mesh electrode)
C
Specifications
type
Vlow
Vm
Vhigh
type
Vc [kV]
2.5
---
1.5
---
4.0
Vp [k V]
5.5
5.0
5.3
5.6
6.0
Ip [A]
0.69
0.80
0.81
0.83
0.90
O3m [vol%]
2.25
2.88
2.92
2.96
2.66
Wp=VpIp [k W]
3.80
4.00
4.29
4.65
5.40
EF=O3m / Wp [vol%/kW]
0.59
0.72
0.68
0.64
0.49
Ozone Concentration, O3 [Vol%]
3.5
B type
3
Fig. 13. Comparison of I– V characteristics of three types of ozone generators.
Table 1 Detailed parameters for three types of ozone generators
A
2
Applied Pulse Voltage, V [kV]
Air-Void Size of Mesh Electrode, VM [mm]
3.0
B
2.5 2.0
C A
1.5 1.0 0.5 0.0 0
1
2
3
4
5
6
7
8
Applied Pulse Voltage, V [kV]
To further evaluate the effectiveness of the meshedplate electrode ozone generator, three types of generators—a surface discharge type [20], the meshed-plate electrode type, and a DBD type, labeled A, B, and C, respectively in the Figures to follow, were compared experimentally. The various parameters of these devices are shown in Table 1. The results of these tests are shown in Figs. 13–15. Figs. 13 and 14 show the I– V characteristics and ozone output of the three types of ozone generators, A, B and C, as a function of applied voltage. For each generator, corona discharge begins at its respective onset voltage and increases continuously with increasing voltage. Each generator also displays a displacement current, indicated in each case by dotted lines in Fig. 13. These latter currents, which are due to the capacitive loads of the ozone generators, should not affect ozone generation. As shown in Fig. 14, ozone outputs for curves A, B and C begin at each corona onset voltage. The latter are equal to about V ¼ 2:5, 1.5 and 4.0 kV, respectively. At
Fig. 14. Comparison of ozone output of three types of ozone generators.
these voltages, the corona discharges also occur and start to produce ozone. Ozone output for curves A and C increases slowly in each case until V ¼ 4:5 kV, then increases more rapidly for higher voltages above 5 kV. For curve B, ozone output increases rapidly from the onset voltage of V ¼ 1:5 kV, then begins to decrease above V ¼ 5:6 kV. The details of the various parameters of these three devices are shown in Table 1. Fig. 15 and Table 1 provide comparisons of the maximum ozone concentration and ozone generation efficiency of the three types of ozone generators. As Fig. 15 and Table 1 show, the mesh-electrode ozone generator has the highest values of both ozone concentration and efficiency. These data clearly reveal the effectiveness of the meshed-plate electrode compared to both the conventional plate-electrode and dielectricbarrier ozone generators.
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1.0
4
Maximum Ozone Concentraion, O3[Vol%]
ozone generation efficiency 0.8 3
0.6 2 0.4
1 0.2
0
Ozone Generation Efficiency, EF [Vol%/KW]
maximum ozone concentration
0.0 A
B Type of Ozone Generator
C
Fig. 15. Maximum ozone concentrations and efficiencies for three types of ozone generators. (A) Surface discharge type S ¼ 0.00 mm, VM ¼ 0.80 mm, (B) Proposed meshed type S ¼ 0.65 mm, VM ¼ 0.80 mm (C) DBD type S ¼ 0.65 mm, ceramic-to-ceramic.
4. Conclusions
References
A meshed-plate electrode, having thin and sharp edges and air-voids, is proposed as an alternative to conventional plate-electrode and dielectric-barrier ozone generators. It was found that the meshed-plate electrode improves the level of ozone produced. The meshed-plate electrode contributes to this increase in two ways. First, by utilizing the thin and sharp edges of meshed, corona onset voltage can be reduced, and starting corona power can be decreased also. Second, by utilizing the air-voids of the meshed-plate electrode, the ozone produced in the high-temperature gap spacing between the dielectricbarrier and the plate electrode can escape to regions of lower temperature, thus reducing ozone decomposition. As a result, ozone generation and efficiency can be increased. A parametric study yields optimum conditions for the geometry and spacing of the mesh-electrode device. A maximum ozone concentration of 2.96 vol% was obtained at an applied voltage of 5.6 kV, air-void size of 0.80 mm, a gap spacing of 0.65 mm, electrode width of 30 mm, and an oxygen flow rate of 2.0 l/min at room temperature and atmospheric pressure.
[1] J.-S. Chang, A.J. Kelly, J.M. Crowley, Handbook of Electrostatic Processes, vol. 26, Ozone Generation and Applications, Marcell Dekker, Inc., 1995, pp. 581–605. [2] T.J. Manning, Production of ozone in an electrical discharge using inert gases as catalysts, Ozone Sci. Eng. 22 (1999) 53–64. [3] K. Omiya, Y. Kataoka, Effect of addition on ozone ashing, J. Electrochem. Soc. 145 (12) (1998) 4323–4327. [4] J.-D. Moon, G.-T. Lee, S.-T. Geum, Discharge and NOx removal characteristics of non-thermal plasma reactor with a heated corona wire, J. Electrostat. 50 (2000) 1–15. [5] J.-D. Moon, United States Patent No. 5,154,895, Ozone Generator in Liquids, 1992. [6] P. Foller, W. Tobias, United States Patent No. 4,316,782, Electrolytic Process for the Production of Ozone, February 23, 1982. [7] S.G. Chang, United States Patent No. 5,332,563, Yellow Phosphorous Process to Convert Toxic Chemicals to Non-Toxic Products, July 26, 1994. [8] C.J. Arthurson, United States Patent No. 5,223,105, Ozone Generator, June 29, 1993. [9] D.E. Pelster, United States Patent No. 5,387,400, Apparatus and Method for Water Purification Using Ozone Generated by UV radiation with a Continuous Filament Bulb, February 7, 1995. [10] U. Kogelschatz, Ozone generation and dust collection, in: E.M. van Veldhuizen (Ed.), Electrical Discharge for Environmental Purposes: Fundamentals and Applications, Nova Science Publishers, Inc., New York, 2000. [11] J.-D. Moon, S.-T. Geum, Discharge and ozone generation of a ferroelectric-ball/mica-sheet barrier, IEEE Trans. IA 34 (6) (1998) 1206–1211. [12] U. Kogelschatz, B. Eliasson, M. Hirth, Ozone generation from oxygen and air: discharge physics and reaction mechanisms, Ozone Sci. Eng. 9 (1987) 367–377.
Acknowledgement This work was financially supported by MOCIE through the EIRC program, Korea.
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[13] I. Chalmers, L. Zanella, S.J. MacGregor, Ozone synthesis in oxygen in a dielectric barrier free configuration, 10th IEEE International Pulsed Power Conference, Albuquerque (1995) 1249–1254. [14] I. Chalmers, L. Zanella, S.J. MacGregor, I.A. Wray, Ozone generation by pulsed corona discharge in a wire cylinder arrangement, IEE Colloquium Digest 229 (1994) 1–4. [15] J. Kitayama, M. Kuzumoto, Theoretical and experimental study on ozone generation characteristics of an oxygen-fed ozone generator in silent discharge, J. Phys. D: Appl. Phys. 30 (1997) 2453–2461. [16] J. Kitayama, M. Kuzumoto, Analysis of ozone generation from air in silent discharge, J. Appl. Phys. D: Appl. Phys. 32 (1999) 3032–3040. [17] Y. Nomoto, T. Ohkubo, S. Kanazawa, T. Adachi, Improvement of ozone yield by a silent-surface hybrid discharge ozonizer, IEEE Trans. IA 31 (6) (1995) 1458–1462.
[18] NATO, Advanced research workshop on non-thermal plasma techniques for pollution control, Cambridge University, England UK 1–8 (1992) 21–25. [19] B. Eliasson, U. Kogelschatz, M. Hirth, Ozone synthesis from oxygen in dielectric barrier and discharges, J. Appl. Phys. 20 (1987) 1421–1437. [20] S. Masuda, A. Kensuke, M. Kuroda, Y. Awatsu, Y. Shibuya, A ceramic-based ozonizer using high-frequency discharge, IEEE Trans. IA 24 (2) (1988) 223–231. [21] E.E. Kunhardt, L.H. Luessen, Electrical Breakdown and Discharges in Gases, Fundamental Processes and Breakdown, NATO ASI Series, Series B: Physics, vol. 89a, 1981. [22] J. Harry, White, Industrial Electrostatic Precipitation, Addison-Wesley Publishing Co., INC., Reading, MA, 1963, pp. 90–100.
Journal of Electrostatics 64 (2006) 275–282 www.elsevier.com/locate/elstat
Effective ozone generation utilizing a meshed-plate electrode in a dielectric-barrier discharge type ozone generator Seung-Lok Parka, Jae-Duk Moonb,, Seug-Hoon Leeb, Soo-Yeon Shinc a
Extra HV Group, Electric Power Research Lab, LG Cable, 190 Gongdan-Dong, Gumi, Gyeongbuk 730-708, Republic of Korea School of Electronic and Electrical Engineering, Kyungpook National University, 1370 Sankyuk-Dong, Buk-Gu, Daegu 702-701, Republic of Korea c Digital Appliance Research Laboratory, LG Electronics Co. Ltd., 391-2, Gaeumjeong-Dong, Changwon, Gyeongnam 641-711, Republic of Korea
b
Received 23 July 2004; received in revised form 16 February 2005; accepted 22 June 2005 Available online 19 July 2005
Abstract A meshed-plate electrode, an alternative to the conventional plate electrode of the dielectric-barrier discharge (DBD) ozone generator, has been studied in order to show the effectiveness of the meshed-plate electrode in improving ozone generation. A DBD ozone generator with mesh electrode is shown to have the following two advantages: First, such a device produces corona at its thin, sharp edges, thereby decreasing corona onset voltage for a given gap spacing. Second, by utilizing the air-voids of the meshed-plate electrode, the dense ozone produced in the small, high temperature gap region of the DBD can escape rapidly to the low temperature region. As a result, decomposition of the ozone can be reduced. In this study, surface discharge, meshed-plate electrode, and conventional DBD ozone generators are studied and compared experimentally. The maximum ozone generation concentration and ozone generation efficiency are obtained with the meshed-plate type electrode. r 2005 Elsevier B.V. All rights reserved. Keywords: Ozone generator; Dielectric-barrier discharge (DBD); Meshed-plate electrode; Air-void; Gap spacing
1. Introduction Ozone (O3), a strong oxidizing agent made from stable molecular oxygen (O2), is replacing chlorinated compounds in a variety of applications including wastewater treatment, polluted air treatment, the ashing and oxidation of semiconductors, and as a disinfectant [1–3]. Ozone is unstable and decomposes slowly (in minutes) at ambient temperatures and rapidly (o1 s) at higher temperatures [4]. Current technology in ozone production utilizes several techniques. One method involved electrochemical reactions to produce ozone [5–7]. These systems suffer from high electrical consumption, and the cells produce chemicals that can be toxic or difficult to dispose [2]. The second technique involves the use of high-energy methods such as UV light, beta rays, and Corresponding author.
E-mail address: [email protected] (J.-D. Moon). 0304-3886/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2005.06.007
lasers to convert oxygen to ozone. Such methods have found minimum large-scale commercial applications [8–9]. The third general approach utilizes electrical discharges [10–17]. In this approach, pure oxygen or air is passed through an electric field generated an AC high voltage applied across plate and ground electrodes. The ground electrode is typically covered by a dielectricbarrier. These systems typically suffer from high electrical consumption and a relatively low efficiency (about 1–15%) for converting oxygen to ozone. The aim of the study presented in this paper is to show the effectiveness of using a meshed-plate electrode in an ozone generator as a way to improve the ozone generation efficiency. In a system of this type, the electric field will concentrated at the thin, sharp edges of the mesh electrode, thereby improving corona generation [12,18]. No such corona occurs at the edges of a plate electrode of the same configuration in a dielectricbarrier discharge (DBD) type ozone generator [17–19].
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Thus, by using a meshed-plate electrode, corona onset voltage can be decreased significantly. Because of the air-voids in a meshed-plate electrode, the dense ozone produced in the small, high-temperature gap spacing can escape rapidly to the low temperature region that resides above [15–17], thus greatly reducing decomposition of the ozone. As a result, ozone generation and efficiency can be further increased.
2. Experimental method and setup Fig. 1 shows a schematic diagram of the experimental setup consisting of the ozone generator, a high-voltage pulsed-power supply, a cooling water system (chiller) for the ozone-generating space, an oxygen gas cylinder with flow meter and valve, and an ozone monitor to measure ozone output. Fig. 2 shows the details of the ozone generator, which consists of two parallel sets of meshedplate/gap/dielectric-barrier/induction-plate systems. The corona emitter in this case is an etched, stainless steel, rectangular electrode of dimensions 60 mm 88 mm 0.20 mm. As shown in Fig. 2(a), the dimension VM of the square-shaped air-voids of the meshed-plate electrode, produced by a wet chemical etching method, vary in size between 0.50 and 1.50 mm. The total width WM of the meshed-plate electrode is fixed at 0.30 mm. The grounded, aluminum induction electrode (180 mm 250 mm 10 mm) has a water cooling path that is 5 mm deep, 30 mm wide, and 830 mm long. This pathway also has a concaved rear edge. A 92% alumina ceramic plate of dimensions 84 mm 112 mm 0.8 mm is used as the dielectric-barrier. A 2 mm 3 mm 0.80 mm Teflon spacer sets the gap spacing between the dielectric-barrier and the meshed-plate electrode. The gap spacing S was adjusted by the thickness (S ¼ 0:0020:80 mm) of the Teflon spacer. Note that a discharge-free region exists between the upper and lower meshed-electrodes as shown in Fig. 2, where a no discharge occurs and the temperature (of ions and ambient gases) is lower than
3. Experimental results and discussion
chiller cooling water flow meter O2 in valve Oxygen gas
the discharging gap spacing between the meshedelectrodes and the dielectric-barrier. Fig. 3 shows the high-voltage pulsed-power system consisting of a diode rectifier to produce dc voltage, a semiconductor inverter (IGBT Module, Mitsubishi, CM100BU-12 H) with gate drive, a pulse-control circuit for shaping the high-voltage pulse, and a high-voltage transformer that raises the pulse voltage. As indicated in Fig. 3, when a square-wave voltage is applied as an input to the transformer, the output becomes a triangle-like voltage wave. This occurs because the time-varying portion of the input waveform is altered by the inductance and capacitance of the transformer. Fig. 4 shows the measured voltage and current waveforms applied to the ozone generator. The triangular waveform has a peak voltage of about 6 kV and a peak current of about 0.8 A. The frequency and duty cycle of the applied high voltage pulse are known to be important factors that influence ozone production [20]. Although our system is capable of producing frequencies over the range 0.1–10 kHz and duty cycles between 0.05 and 0.50, the values 4.5 kHz and 0.10 were chosen for all experiments reported in this paper. The various voltages and the corona current were measured, respectively, using a Tektronix P6015A probe, and a Tektronix A-6312 Rogowski coil current probe with AM 503B current probe amplifier. Waveforms were recorded using a Tektronix 100 MHz TDS360A digital storage oscilloscope. The output ozone concentration was measured using the conventional method described in Refs. [11–17,20]. Specifically, output gas was monitored with an ultraviolet ozone monitor (Dasibi, Model DY-1500). Industrial-grade oxygen of 99% purity was fed to the generator at a fixed rate of 2.0 l/min. Chilled tap water at 5 1C was supplied to the cooling path of the induction electrode (Accurate Gas Control System Inc. Model AG-T). All experiments were carried out in an electromagnetically shielded booth at a constant ambient temperature of 20 1C and standard atmospheric pressure.
Ozone Generator
O2+O3 out Ozone moniter
HV power GND
Fig. 1. Schematic diagram of experimental setup.
Fig. 5 shows the current–voltage (I– V) characteristics of the generator at different values of gap spacing S for the case V M ¼ 0:80 mm and W M ¼ 0:30 mm. This figure shows that gap spacing significantly influences the corona discharge, as one would expect. The onset of corona occurs at 1.5 kV, where a weak glow discharge occurs at the edges of the meshed-plate electrode. This is evident in Fig. 7. The output ozone can be detected at an applied voltage of 1.5 kV, which thus reveals that a weak glow discharge exists and ozone production begins. The corona current then increases abruptly for higher
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induction electrode
meshedelectrode
dielectricbarrier
277
gap spacing, S
O2 in
O2+O3 out
HV power
WM
cooling waterpath
VM
meshed-electrode (closeup)
GND
WM : width (net) of meshed-plate VM : air-void of meshed-plate
(a)
Proposed Ozone Generator
O2 in
O2+O3 out
meshed-plate
dielectric barrier
thin and sharp edge
electric flux
gapspacing, S
(b)
induction electrode
Fig. 2. Configuration of mesh-electrode ozone generator (a) Configuration of proposed ozone generator with closeup of meshed-plate (side view); (b) Sketch of electrode configuration and electric flux (closeup view).
voltage over the range 3–5 kV, where an additional intense DBD is generated in the gap. Afterwards, a strong streamer corona discharge is generated, and, it saturates at an applied voltage in the range 5–6 kV. From the DBD mechanism, the corona discharge cannot proceed further to an arc discharge in the gap between the discharge electrode and the dielectricbarrier [1,21]. Figs. 6 and 7 shows the corona I– V characteristics and ozone generation, respectively, as functions of gap spacing S for the fixed values V M ¼ 0:80 mm and W M ¼ 0:30 mm. Fig. 6 shows that the gap spacing affects the corona current, and that there is an optimal value of gap spacing at about 0.7 mm [22]. Fig. 7 shows
that ozone is generated just above the 1.5 kV corona onset voltage. For the case of no gap spacing, which was realized by omitting the Taflon spacer between the meshed-electrode and the dielectric-barrier, however, the corona onset voltage is about 2.5 kV, which is higher than the onset values for larger gap spacing. Fig. 8 shows the ozone concentration of the output as a function of gap spacing. The maximum ozone concentration of about 2.96 vol% is obtained at S ¼ 0:65 mm. This value is about 1.32 times higher than the 2.25 vol% value obtained with no gap spacing. This difference occurs because, with no gap spacing, ozone is generated only from the edges of the meshed-plate electrode, causing it to operate as a surface discharge (Masuda
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278
Diode rectifier
HV transformer 300V/10kV
IGBT inverter
Vac 220V
C
Ozone Generator 20V DC
DC source
Output Current Sensing
Gate Signal Generating
Gate driving circuit Output Voltage Control
±15V DC
Protecting Circuit
Control circuit
Fig. 3. Circuit diagram of high-voltage pulse power.
Peak Corona Current, I [A]
1.0
0.8
0.6 4KV 5KV
0.4
6KV
0.0
0.2
0.4
0.6
0.8
1.0
Gap Spacing, S [mm] Fig. 4. Oscillograms of input pulse voltage and corona current waveforms.
Fig. 6. Corona current as a function of gap spacing for different values of applied voltage.
4.0 S = 0.00mm S = 0.40mm S = 0.65mm S = 0.80mm
0.8
Ozone Concentration, O3 [Vol%]
Peak Corona Current, I [A]
1.0
0.6
0.4
0.2
0.0
S = 0.00mm S = 0.40mm S = 0.65mm S = 0.80mm
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
1
2
3
4
5
6
7
8
Applied Pulse Voltage, V [kV] Fig. 5. Voltage–current (I– V) characteristics of mesh-electrode ozone generator for different gap spacings.
0
1
2
3
4
5
6
0
8
Applied Pulse Voltage, V [kV]
Fig. 7. Ozone generation as a function of applied pulse voltage for different gap spacings.
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0.9 Peak Corona Current, I [A]
type) ozone generator only [20]. With gap spacing, two kinds of corona can occur, one from the sharp edges, and the other from the dielectric-barrier. This the ozone output increases with gap spacing between the meshedelectrode and the dielectric-barrier. Fig. 9 shows the I– V characteristics of the meshelectrode ozone generator for different values of VM for fixed values S ¼ 0:65 mm and W M ¼ 0:30 mm. As suggested Fig. 2(b), as VM become larger, more electric flux concentrates at the mesh edges, and a stronger discharge occurs [22]. The maximum corona current added by this additional discharge was about 0.1 A, or about 14% of the peak corona current, for a constant applied pulse voltage of 6 kV. Fig. 10 shows the corona current as a function of VM for various values of applied voltage for the case S ¼ 0:65 mm and WM ¼ 0.30 mm. Maximum current occurs at all voltages for a VM of about 0.8–1.0 mm.
279
0.8
0.7
4KV 5KV 6KV
0.6
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Air-Void Size of Mesh Electrode, VM [mm] Fig. 10. Corona current as a function of air-void size for different values of applied voltage.
4.0 Ozone Concentration, O3 [Vol%]
Maximum Ozone Concentration, O3 [Vol%]
3.5
3.0
2.5
2.0
1.5 0.2
0.4
0.6
0.8
1.0
1.2
Gap Spacing, S [mm]
2.5 2.0 1.5 1.0 0.5
0.8
0.6 VM = 0.5mm VM = 0.8mm VM = 1.0mm VM = 1.5mm
0.2
0.0 1
2
3
4
5
1
2
3
4
5
6
7
8
Fig. 11. Ozone generation as a function of applied voltage for different air-void sizes of the meshed-plate electrode.
1.0
0.4
0
Applied Pulse Voltage, V [kV]
Fig. 8. Ozone output concentration as a function of gap spacing.
Peak Corona Current, I [A]
3.0
0.0 0.0
0
VM = 0.5mm VM = 0.8mm VM = 1.0mm VM = 1.5mm
3.5
6
7
8
Applied Pulse Voltage, V [kV] Fig. 9. I– V characteristics of mesh-electrode ozone generator for different air-void sizes.
Fig. 11 shows the ozone output as a function of applied voltage for different values of VM, again with S ¼ 0:65 mm and W M ¼ 0:30 mm. Ozone output begins at the onset voltage of 1.5 kV and increases steeply thereafter. Above about 5 kV, ozone output begins to fall. We hypothesize that this decrease occurs because the temperature in the air gap rises with stronger corona discharge. Thus ozone concentration falls due to rapid decomposition at the elevated temperature. Fig. 12 shows the peak ozone output as a function of VM for S ¼ 0:65 mm; W M ¼ 0:30 mm and V ¼ 5:6 kV. As this plot shows, the peak ozone concentration of 2.96 vol% occurs for V M 0:8 mm. These results suggest that, while more ozone can escape from the high temperature region for higher values of VM, less is produced, leading to the optimum value somewhere in the middle of the VM range shown.
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280
1.6 A: S = 0.00mm, VM = 0.80mm B: S = 0.65mm, VM = 0.80mm C: Ceramic to Ceramic type (none mesh electrode)
1.4 Peak Corana Current,I [A]
Maximum Ozone Concentration, O3 [Vol%]
3.5
3.0
2.5
1.2
C
1.0
B
0.8 A
0.6 0.4 0.2
2.0 0.0
0.0
0.4
0.8
1.2
1.6
2.0
0
1
Fig. 12. Ozone output concentration as a function of air-void size.
4
5
6
7
8
4.0 A: S =0.00mm, VM = 0.80mm B: S =0.65mm, VM = 0.80mm C: Ceramic to Ceramic type (none mesh electrode)
C
Specifications
type
Vlow
Vm
Vhigh
type
Vc [kV]
2.5
---
1.5
---
4.0
Vp [k V]
5.5
5.0
5.3
5.6
6.0
Ip [A]
0.69
0.80
0.81
0.83
0.90
O3m [vol%]
2.25
2.88
2.92
2.96
2.66
Wp=VpIp [k W]
3.80
4.00
4.29
4.65
5.40
EF=O3m / Wp [vol%/kW]
0.59
0.72
0.68
0.64
0.49
Ozone Concentration, O3 [Vol%]
3.5
B type
3
Fig. 13. Comparison of I– V characteristics of three types of ozone generators.
Table 1 Detailed parameters for three types of ozone generators
A
2
Applied Pulse Voltage, V [kV]
Air-Void Size of Mesh Electrode, VM [mm]
3.0
B
2.5 2.0
C A
1.5 1.0 0.5 0.0 0
1
2
3
4
5
6
7
8
Applied Pulse Voltage, V [kV]
To further evaluate the effectiveness of the meshedplate electrode ozone generator, three types of generators—a surface discharge type [20], the meshed-plate electrode type, and a DBD type, labeled A, B, and C, respectively in the Figures to follow, were compared experimentally. The various parameters of these devices are shown in Table 1. The results of these tests are shown in Figs. 13–15. Figs. 13 and 14 show the I– V characteristics and ozone output of the three types of ozone generators, A, B and C, as a function of applied voltage. For each generator, corona discharge begins at its respective onset voltage and increases continuously with increasing voltage. Each generator also displays a displacement current, indicated in each case by dotted lines in Fig. 13. These latter currents, which are due to the capacitive loads of the ozone generators, should not affect ozone generation. As shown in Fig. 14, ozone outputs for curves A, B and C begin at each corona onset voltage. The latter are equal to about V ¼ 2:5, 1.5 and 4.0 kV, respectively. At
Fig. 14. Comparison of ozone output of three types of ozone generators.
these voltages, the corona discharges also occur and start to produce ozone. Ozone output for curves A and C increases slowly in each case until V ¼ 4:5 kV, then increases more rapidly for higher voltages above 5 kV. For curve B, ozone output increases rapidly from the onset voltage of V ¼ 1:5 kV, then begins to decrease above V ¼ 5:6 kV. The details of the various parameters of these three devices are shown in Table 1. Fig. 15 and Table 1 provide comparisons of the maximum ozone concentration and ozone generation efficiency of the three types of ozone generators. As Fig. 15 and Table 1 show, the mesh-electrode ozone generator has the highest values of both ozone concentration and efficiency. These data clearly reveal the effectiveness of the meshed-plate electrode compared to both the conventional plate-electrode and dielectricbarrier ozone generators.
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281
1.0
4
Maximum Ozone Concentraion, O3[Vol%]
ozone generation efficiency 0.8 3
0.6 2 0.4
1 0.2
0
Ozone Generation Efficiency, EF [Vol%/KW]
maximum ozone concentration
0.0 A
B Type of Ozone Generator
C
Fig. 15. Maximum ozone concentrations and efficiencies for three types of ozone generators. (A) Surface discharge type S ¼ 0.00 mm, VM ¼ 0.80 mm, (B) Proposed meshed type S ¼ 0.65 mm, VM ¼ 0.80 mm (C) DBD type S ¼ 0.65 mm, ceramic-to-ceramic.
4. Conclusions
References
A meshed-plate electrode, having thin and sharp edges and air-voids, is proposed as an alternative to conventional plate-electrode and dielectric-barrier ozone generators. It was found that the meshed-plate electrode improves the level of ozone produced. The meshed-plate electrode contributes to this increase in two ways. First, by utilizing the thin and sharp edges of meshed, corona onset voltage can be reduced, and starting corona power can be decreased also. Second, by utilizing the air-voids of the meshed-plate electrode, the ozone produced in the high-temperature gap spacing between the dielectricbarrier and the plate electrode can escape to regions of lower temperature, thus reducing ozone decomposition. As a result, ozone generation and efficiency can be increased. A parametric study yields optimum conditions for the geometry and spacing of the mesh-electrode device. A maximum ozone concentration of 2.96 vol% was obtained at an applied voltage of 5.6 kV, air-void size of 0.80 mm, a gap spacing of 0.65 mm, electrode width of 30 mm, and an oxygen flow rate of 2.0 l/min at room temperature and atmospheric pressure.
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Acknowledgement This work was financially supported by MOCIE through the EIRC program, Korea.
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