IEEE Transactions on Dielectrics and Electrical Insulation Vol. 16, No. 4; August 2009

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Large Area Pulsed Corona Discharge in Water for Disinfection and Pollution Control

Werner Hartmann, Michael Roemheld, Klaus-Dieter Rohde and Franz-Josef Spiess

Siemens AG Corporate Technology Guenther-Scharowsky-Str. 1 91052 Erlangen, Germany

ABSTRACT

To investigate the efficiency of submerged pulse corona (SPC) discharges in water we built a laboratory scale, parallel-plate reactor that is part of a closed loop water circulation system. A pulsed voltage is applied across the electrodes. One of the electrodes is coated with a porous ceramic layer to create local field enhancements to initiate corona discharges. For energization of the SPC reactor a pulse generator was developed which is based on a capacitor discharge initiated by a semiconductor switch. A pulse transformer, followed by two magnetic pulse compression stages, produces voltage pulses with amplitudes of up to 30 kV at a pulse width of 0.3 µs. Simulation of the circuit behavior leads to good agreement with voltage and current measurements. Details of the pulse generator and first experimental results concerning the efficiency of radical production are presented. Depending on the conductivity of the water to be treated, pulse currents of > 600 A at a voltage of 20 kV to > 30 kV are obtained for electrode sizes of around 50 cm². The efficiency of the radical production is measured in terms of the hydrogen peroxide (H2O2) concentration, which is formed by recombination of hydroxyl radicals (OH.) at sufficiently high concentrations downstream of the plasma reactor. At pulse repetition rates of 20 to 100 Hz, H2O2 concentrations of several mg/l are produced, at efficiencies in the range of up to ≈1 g/kWh.

Index Terms — Pollution control, water pollution, corona, pulse generation.

1 INTRODUCTION

AFFORDABLE supply of drinking water as well as water for industrial processes, irrigation, food production and cooling processes will be one of the major challenges in the following decades. In particular, a sufficient supply of drinking water is of major concern not only for developing and third world countries, but in the future will also be of paramount concern for well developed countries in a world of rapidly changing economy, increasing water usage, and climate change. While standard water treatment processes like filtration are able to remove most contaminants, there is still a necessity for processes which are able to remove bacterial and viral contamination as well as trace chemical pollutants such as phenol, dyes, endocrine and endocrine-like substances, and the like in drinking water. Although a large part of today’s drinking water supply is already treated with ozone, the most common water treatment method for sterilization still is chlorination due to its low cost. Chlorination has some disadvantages, however, which makes it only second choice for drinking water treatment:

- Chlorine easily reacts with natural organic compounds and rather harmless man-made hydrocarbons, which may then be turned into carcinogenic chlorinated hydrocarbons.

- Removal of chemical pollutants through reaction with chlorine is close to impossible due to the low oxidation potential of chlorine.

Ozonisation, on the other hand, is considerably more suited to break down chemical impurities found in water owing to the higher oxidation potential of ozone as compared to chlorine, c. Table 1. Additionally, it is also possible to generate hydroxyl radicals (OH). by UV irradiation of dissolved ozone in water. OH. radicals have an even higher oxidation potential of about 2.8 V and thus are suited to remove essentially all pathogenic and organic pollutants from drinking water at sufficiently high concentration. This makes combined ozone-UV treatment increasingly attractive for many applications. Common commercial large-scale ozonizers are based on silent discharges (dielectric barrier discharges, DBD) and have efficiencies of the order of 80 to 100 g/kWh using pure oxygen as working gas, which is 20% to 25% of the theoretical limit [1]. The common cryogenic production of oxygen, however, has a comparable energy consumption. Manuscript received on 10 July 2008, in final form 22 December 2008.

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Therefore, the overall energy efficiency of large commercial ozonizers is on the order of 40 g of ozone per kilowatt-hour of electrical energy. Although experimental reactors may reach higher production rates ([2] and references therein), most of these methods are not suitable for extrapolation to high production rates as they most often utilize nanosecond pulses to increase the efficiency, which makes up-scaling very difficult.

Table 1. Oxidation potential of common oxidants.

Species Oxidation potential in V

fluorine F2 3.03 hydroxyl radical OH. 2.79 oxygen radical O. 2.42 ozone O3 2.07 hydrogen peroxide H2O2 1.79 chlorine Cl2 1.36

Even systems with high efficiency at moderate pulsed power requirements [3] suffer from the necessity of dissolving the ozone – which is produced in the gas phase, at concentrations of hundreds to thousands of ppm (parts per million) – in the water. This mixing itself is energy intensive, and may lead to only partial usage of the ozone, which further reduces the overall efficiency of gaseous ozone generators for water treatment. Hence, it is highly desirable to have additional methods for the generation of highly reactive radicals such as ozone, hydroxyls, oxygen atoms, peroxy hydroxyl radicals HO2

. and the like directly in water. A large amount of work has been done in this area (see e.g.

references [4-10]) although some of this work [4-6] still uses hybrid reactors – i.e., a combination of gaseous and water corona – where most of the active species such as ozone are produced in the gas phase and therefore still require proper mixing of the gas and liquid phases after radical production by gas discharges. Some of the published work is particularly interesting because the methods used do not require gas phase – liquid phase mixing but rather provide in-situ generation of oxidants such as radicals by using submerged pulse corona (SPC) discharges. This leads to an intense, large-contact area mixing of the radicals and the liquid. Specifically, these methods are needle – plane and wire – plane pulse corona discharges [7-9], and, most interestingly, a special kind of wire – plane configuration where the wire has a porous ceramic layer to reduce the onset voltage of streamer discharges by roughly an order of magnitude as compared to similar configurations [10]. It is the latter method that was explored in detail in this work. In particular, a more suitable pulsed power supply was developed to drive this kind of SPC plasma reactor which is described in detail in section 2.1. The discharge principle as described in ref. [10] was further developed in this work towards a planar design which is thought to be more suitable for upgrading to large mass flows. The experimental setup is described in detail in section 2.2, while the experimental results concerning pulse generator performance and reactor electrical-to-radicals efficiency are presented in section 3.

2 EXPERIMENTAL SETUP The work presented hereafter is divided into two parts:

- a power modulator suitable to drive a transient load such as the SPC reactor described thereafter,

- development and characterization of a laboratory-size SPC plasma reactor suitable for the in-situ production of highly reactive radicals in liquids such as water.

.

2.1 PULSED POWER SUPPLY The pulsed power generator used in this work was

developed specifically for this application, and is based on a capacitor discharge followed by a step-up transformer and a two-stage magnetic pulse compression unit similar to the pulse generator shown in [3]. The circuit is shown schematically in Figure 1.

Figure 1. Schematic of the pulse power generator used in this work. C1, C2: pulse capacitors with 10 nF each.

The pulse generator consists of a 1 µF discharge capacitor Cp at the primary of a 1:10 pulse transformer T1. The capacitor is charged up to 3.3 kV by a capacitor charging power supply, and discharged into the transformer primary by a high-power semiconductor switch (IGCT, isolated gate-commuted thyristor). At the transformer secondary, a 10 nF pulse capacitor C1 is charged to voltages up to 30 kV within 2.1 µs. A saturable core magnetic switch MS I isolates C1 from a second stage pulse capacitor C2 of 10 nF capacitance. When MS I saturates, C1 is discharged into C2 within less than 400 ns; MS II is chosen such as to saturate when the voltage at C2 peaks at around 32 kV. At saturation of MS II, C2 discharges into the load Rload at a risetime of around 80 to 100 ns. Figure 2 shows a SPICE simulation of the voltages at C1, C2 and the load as a function of time. Simulated and measured voltage waveforms show a reasonable agreement. The main differences result from accurately determining the correct inductances and saturation levels of the saturable magnetic switches MS I and MS II, respectively. While pure theoretical estimates of the MS characteristics were used in the simulation shown in Figure 2, it was found that experimentally determined core parameters lead to a far better match of simulation and experiment than that shown in Figure 2.

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Figure 2. Simulated voltage waveforms of the pulse generator of Figure 1, for a load resistance of 10 ohms. Trace 1: voltage at C1; trace 2: voltage at C2; trace 3: voltage at the load.

Figure 3. Measured voltage waveforms of the pulse generator of Figure 1, supplied into the SPC cell at a conductivity of ca. 46 µS/cm.

The SPC treatment cell used shows a widely varying resistance depending on the experimental parameters: water conductivity and electrode distance. Therefore, the voltage at

the load also varies widely, between 10 kV and 30 kV, dependent on the impedance matching between the final pulse compression stage C2 - MS II and the load resistance. Figure 3 shows experimental waveforms of the pulse generator supplied into the cuvette at an electrolyte conductivity of only

46 µS/cm; the load impedance is mainly resistive (ca. 150 Ω

in this case), with a parallel capacitance of the order of 1 nF.

2.2 SPC TREATMENT CELL Based on the work of [10], a substantial decrease in the onset voltage of submerged pulse corona discharges of the order of one order of magnitude can be achieved by using a porous ceramic coating on the cathode. It is caused by the local increase of the electrical field strength at the triple points of fluid – electrode – dielectric interface. Using alumina coatings with an effective dielectric constant of εr ≈ 8 in an aqueous environment (εr ≈ 81), the local electric field strength increases by roughly a factor of

εwater / εalumina ≈ 10.

This accounts for the observed decrease of the threshold electric field strength for corona onset in [10]. Once the

threshold voltage is exceeded, a localized, microscopic gas discharge (SPC) forms, in which water is vaporized and dissociated to form many strongly oxidizing radicals such as OH., O., HO2

., which are rapidly dissolved in the surrounding liquid.

In contrast to the coaxial wire-cylinder system used in [10], however, in this work a parallel-plate SPC reactor was chosen in order to increase the contact area and allow higher treatment rates.

Figure 4. Top view (upper drawing) and cross-sectional view (lower drawing) of the parallel-plate, flow-through SPC reactor. The size of the stainless steel electrodes 4, 4’ is 50x100 mm. 1, 1’: water inlet / outlet; 2, 3: high voltage feedthroughs; 5: acrylic housing.

The SPC reactor is shown schematically in Figure 4. It consists of a pair of plane-parallel, stainless steel electrodes of 50x100 mm2 size each, mounted in a flow-through acrylic cuvette and contacted through the cuvette walls to the high-voltage pulse power supply described in section 2.1. Either one or both electrodes are coated with a porous alumina ceramic having a thickness of several tens of microns.

Figure 5. Photograph of the large area, multiple SPC discharge on the cathode (top), and the SPC reactor (bottom).

The water is circulated in a closed loop of ca. 1.5 l total volume by a rotary pump at a rate of about 0.5 l/min. At pulse repetition rates above a few pulses per second (pps), the water is cooled by a heat exchanger in order to avoid heating of the water. The conductivity is adjusted to values between 50 μS/cm and 1 mS/cm by adding sodium chloride, while the pH is adjusted to a pH ≈ 6-7 by adding a Na2HPO4/KH2PO4 buffer solution. Conductivity, temperature, and oxygen and ozone concentrations are continuously monitored using Wallace & Tiernan (W&T) analyzing equipment. The H2O2 concentration formed in the SPC discharge is measured every 5 minutes in a W&T P42 i-cal analyzer.

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3 EXPERIMENTAL RESULTS The short risetime and width (80 ns and 300 ns, respectively) of the high voltage pulse at the electrodes leads to a large number of simultaneous SPC discharges on the cathode if the voltage amplitude is sufficiently above the threshold voltage for SPC initiation. Figure 5 shows such a broad-area SPC with many simultaneous microscopic discharges evenly distributed over the cathode. The OH. radicals produced in this discharges are rapidly dissolved in the liquid and recombine quickly to form H2O2 due to the high local concentration of these radicals. The H2O2 concentration was measured every 5 minutes, and is shown in Figure 6 as a function of time for the conditions indicated in the caption of Figure 5.

Figure 6. Hydrogen peroxide concentration (in mg/l) as a function of time, for different electrode distances. Pulse repetition rate is 20 pps, conductivity = 50 μS/cm; the cathode is coated, the anode is pure stainless steel.

Based on these measurements, the hydrogen peroxide production efficiency (in grams of H2O2 per kilowatt-hour, kWh) is calculated as a function of the electrode distance, and is shown in Figure 7.

Figure 7. Hydrogen peroxide production efficiency of the planar SPC discharge as a function of the electrode distance, for parameters shown in Figure 6.

Obviously, at distances > 8 - 10 mm the electric field strength is insufficient to ignite SPC discharges, hence the H2O2 production efficiency rapidly approaches zero. For smaller gaps, the efficiency quickly rises since there is considerably less energy deposited in the conducting liquid layer in comparison to the energy consumed in the pulse

corona. The efficiency calculation shown in Figure 7 is based on the energy input into the SPC cell from measurements shown in Figure 3. Additional losses occur in the pulse generator itself, as a result the wall plug efficiency is accordingly lower. Most of the energy is dissipated as Joule heat in the water (electrolyte) layer between the electrodes, while only a small amount is dissipated in the corona discharge itself.

4 CONCLUSIONS

We demonstrated that submerged pulse corona discharges in water with porous ceramic-coated electrodes can be scaled to large areas and pulse repetition rates without sacrificing the radical production efficiency. The maximum production efficiency achieved in our experiments is around 1 g/kWh, which compares well with results published by different authors for other, spatially more inhomogeneous discharge geometries (point – plane; wire – cylinder; etc.). More detailed investigations show that the efficiency is not very sensitive to the conductivity as long as the pulse amplitude is sufficiently large in order to warrant ignition of multiple SPC discharges simultaneously. Furthermore, the efficiency does not depend on the flow rate or pulse repetition rate up to 200 pps if the water temperature is kept constant. Theoretical estimates of the intrinsic efficiency of SPC discharges hint towards values as high as 17 g (H2O2)/kWh, which explains the steep rise of efficiency towards smaller electrodes distances.

5 REFERENCES [1] U. Kogelschatz and B. Eliasson, in Ozone generation and applications”,

Handbook of Electrostatic Processes, J. S. Chang, A. J. Kelly, J. M. Crowley, editors, Marcel Dekker, New York, USA, p. 581, 1995

[2] M. A. Malik, A. Ghaffar, S. A. Malik, “Water purification by electrical discharges”, Plasma Sources Science & Technology Vol. 10, pp. 82-91, 2001.

[3] W. Hartmann, M. Romheld and K. Rohde, “All-Solid-State Power Modulator for Pulsed Corona Plasma Reactors”, IEEE Trans. Dielectr. Electr. Insul., Vol. 14, pp. 858-862, 2007.

[4] P. Lukes, M. Clupek, V. Babicky, V. Janda and P. Sunka, “Generation of ozone by pulsed corona discharge over water surface in hybrid gas-liquid electrical discharge reactor”, J. Phys. D: Appl. Phys. Vol. 38, pp. 409-416, 2005.

[5] P. Lukes and B. R. Locke, “Hydrogen peroxide and ozone formation in hybrid gas-liquid electrical discharge reactors”, Conf. Records, Vol. 3, IEEE 37th Industrial Applications Society (IAS) Meeting, Pittsburgh, PA, USA, pp. 1816-1821, 2002

[6] S. Ihara, T. Miichi, S. Sato, C. Yamabe and E. Sakai, “Ozone generation by a discharge in bubbled water”, Jap. J. Appl. Phys. Vol. 38, No. 7B, pp.4601-4604, 1999

[7] H Akyiama, “Streamer discharges in liquids and their applications”, IEEE Trans. Dielectr. Electr. Insul., Vol. 7, pp. 646-653, 2000.

[8] A. Abou-Ghazala, S. Katsuki, K. H. Schönbach, F. C. Dobbs, K. R. Moreira, “Bacterial decontamination of water by means of pulsed corona discharges”, IEEE 28th International Conf. Plasma Sci. and IEEE 13th Pulsed Power Conf., Nevada, USA, pp. 612-615, 2001

[9] M. Sato, T. Ohgiyama and J. S. Clemens, “Formation of chemical species and their effects on microorganisms using a pulsed high-voltage discharge in water”, IEEE Trans. Ind. Appl., Vol. 32, pp. 106-112, 1996.

[10] P. Lukes, M. Clupek, P. Sunka, V, Babicky and V. Janda, “Effect of ceramic composition on pulse discharge induced processes in water using ceramic-coated wire to cylinder electrode system”, Czech. J. Phys., Vol. 52, Suppl. D, pp. D800-806, 2000.

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Werner Hartmann (AM’87-SM’05) was born in Ansbach, Germany, in 1955. He received the M.Sc. degree in physics in 1981 and the Ph.D. degree in physics in 1986 both from the Friedrich-Alexander-Universität (FAU) Erlangen-Nürnberg, Germany. He worked as a research assistant at the FAU from 1986 to 1991, and as a research scientist at the University of Southern California, EE Dept., Los Angeles, CA, USA from 1987 to 1988. Since 1991 he works as a research scientist and project manager at the Corporate Technology of Siemens AG, Erlangen,

Germany. His main research activities comprise vacuum arc physics, pulsed corona discharges and their applications, and pulsed power technology and applications. He is a member of the German Physics Society DPG.

Michael Roemheld graduated with a M.Sc. degree in physics from the University of Freiburg, Germany, in 1972. He received a research scholarship from the German Academic Exchange Service for a fourteen-month stay at the National Research Council of Canada in Ottawa where he worked in the field of laser spectroscopy of molecules. He joined the University of Ulm, Germany, in 1974 and received the Dr. rer. nat. degree in 1979. He was awarded the dissertation prize of the University of Ulm in 1980. In 1979 he joined Siemens working first in the field of energy recovery solutions in industry before

changing to the Corporate Technology branch, where his main interests are presently pulsed power and plasma technology for environmental and industrial applications. He is a member of the Association of German Engineers VDI.

Klaus-Dieter Rohde was born in Erlangen, Germany, in 1951. He has been working at Siemens Corporate Technology as a research assistant for 35 years, mainly in the areas of plasma physics, switching arcs, and pulse power technology. He is inventor and co-inventor of numerous patents concerning switching technology, shockwave generators, and pulsed power. Photograph not available.

Franz-Josef Spiess was born in Meersburg, Germany, in 1971. He received the B.Sc. degree in chemistry in 1994 from the Universität Konstanz, Konstanz, Germany, and the Ph.D. degree in chemistry in 2003 from the University of Connecticut, Storrs, CT, USA. He worked as research assistant at the Julius-Maximilians-Universität Würzburg; Germany, from 2003 to 2004 and as a research assistant at the Universität Konstanz from 2005 to 2006. Since 2006 he has been a research scientist at the Corporate

Technology of Siemens AG, Erlangen, Germany. His main research activities comprise pulsed corona discharges and their applications, and water treatment technologies.