WO2005120684A1

WO2005120684A1 - Non-thermal plasma reactor for low pressure drop and low specific energy density

Info

Publication number: WO2005120684A1
Application number: PCT/KR2005/001715
Authority: WO
Inventors: Min-Suk Cha; Young-Hoon Song; Jae-Ok Lee; Kwan-Tae Kim; Seock-Joon Kim; Hong-Sik Kim
Original assignee: Korea Institute Of Machinery And Materials
Priority date: 2004-06-08
Filing date: 2005-06-08
Publication date: 2005-12-22
Also published as: KR100471107B1

Abstract

The present invention is related to a cold plasma reactor for low pressure drop and low energy density using a dielectric barrier discharge and a method for fabricating the same, the reactor being used for (1) treating various noxious gases such as a nitrogen oxide(NOx), a volatile organic compounds(VOCs) , and a stinking material, (2) generating ozone and ozone water, and (3) generating a chemically reactive species having high chemical reactivity in order to treate a surface of a metal, a ceramic and a polymer material. The cold plasma reactor comprises a cylindrical electrode rod, the cylindricalelectrode rod comprising a dielectric cylinder with a hole formed in the center thereof and a metal rod inserted into the hole, or a metal paste applied on the inner surface of the hole, and a both side structure formed by installing numbers of the cylindrical electrode rods spaced apart with a regular distance in parallel. In the both side structure, metal leads of neighboring electrode rods are positioned opposite to each other to prevent arc discharge from being generated and to generate stable plasma. The cold plasma reactor for low pressure drop and low energy density according to the present invention is applied to a system which (1) minimizes pressure drop of a flow occurred in a conventional art, (2) consumes low plasma generating power for handling mass flow and prevents oxidation and corrosion of the metal electrode, and (3) is not influenced by a direction of a flow.

Description

Description NON-THERMAL PLASMA REACTOR FOR LOW PRESSURE DROP AND LOW SPECIFIC ENERGY DENSITY Technical Field

[1] The present invention relates to a non-thermal plasma generating apparatus using a dielectric barrier discharge technology for low pressure drop and low energy density, and more particular, to a plasma generating apparatus adapted for a system which minimizes pressure drop of a flow occurred in a conventional art, consumes low plasma generating power for handling mass flow, and is not influenced by a direction of a flow. Background Art

[2] A cold plasma generating apparatuses using a stacked planar dielectric electrode is disclosed in Korean patent applications No. 10-2001-0062501 and No. 10-2003-0077053. As the ratio of input electric power to its volume is very high, the plasma reactor is suitable for an application requiring low handling flow/high energy consumption. In an application according to the present invention requiring high handling flow/high energy consumption, there is a problem that the prior art plasma reactors for high energy density can be hardly applied to a high flow system, because a volume of plasma reactor is too smallfor mass treating flow as well as pressure drop with which the flow suffers for passing through a gas floating layer having a height of several mm and a length of several mm is also increased.

[3] A cold plasma generating apparatusaccording to a prior art is depicted in Fig. 1. The cold plasma generating apparatus is a cylinder stacked type in which cylinder type electrode rods are spaced apart with each other in the vertical direction with a regular space to form an electrode structure, and such structured electrode structures are installed with a specific spacing in the horizontal direction. In the apparatus, AC electric power with different polarity is applied to neighboring electrode structures with respective, and a gas flow intended for treating is supplied in the gravity direction.

[4] In such cylinder stacked type cold plasma generating apparatus, as the position of the cylinder type electrode rod is at the vertex of a square or a lozenge, it is possible to make a reactor having big volume comparing with the planar type cold plasma generating apparatus. The reactor can lead, however, a large pressure drop in an actual application, because treating gases collide and rub with a downstream part of the cylinder type electrode rod in the course of passing from the upper stream to the downstream through the stacked cylinder type electrodes. Further, there is a shortcomings that the cold plasma generating apparatus has not flexibility to a direction for installation as the treating gas should be supplied in the gravity direction in order to a contamination of the plasma reactor. [5] Further, in the prior art, as there is not a detailed description about the method for fabricating a metal electrode and a ceramic or quartz tube enclosing the metal electrode, if the metal electrode and the ceramic or quartz tube are applied in the present invention, a severe problem such as lack of the electrode s durability can be occurred due to oxidation of erosion of the metal electrode. Disclosure of Invention Technical Solution [6] Therefore, an object of the present invention is to solve the problems involved in the prior art, and to provide a plasma generating apparatusfor low pressure drop and low energy density, which is adapted for a system which minimizes pressure drop of a flow occurred in a conventional art, consumes low plasma generating power for handling mass flow, and is not influenced by a direction of a flow. [7] Another object of the present invention is to provide a fabricating method for ensuring durability for a plasma generating apparatus having low pressure drop and low energy density, and to provide a process for forming a cylindrical electrode by using a metal paste and a dielectric column. [8] Brief Description of the Drawings [9] The above objects, other features and advantages of the present invention will become more apparent by describing the preferred embodiment thereof with reference to the accompanying drawings, in which: [10] Fig. 1 is a perspective view of a cold plasma generating apparatus according to a prior art; [11] Fig. 2 is a perspective view of a cod plasma generating apparatusaccording to the present invention; [12] Fig. 3 is a cross-sectional view of a discrete electrode rod according to the embodiment 1 in the Fig. 2; [13] Fig. 4 is a schematic view of a position relation between a fixing structure for electrode rod and a metal electrode boundary in the electrode rod for forming the cold plasma reactor in the Fig. 2; [14] Fig. 5 is a cross-sectional view of a connection between an electrode rod and a fixing structure for electrode rod for forming a stacked cylinder type cold plasma reactor in the Fig. 2; [15] Fig. 6 is a side view of a connection between an electrode rod and a fixing structure for electrode rod for forming a stacked cylinder type cold plasma reactor in the Fig. 2; [16] Fig. 7 is a schematic view of an electrode rod according to the second embodiment of the present invention; and

[17] Fig. 8 is a schematic view of an electrode rod according to the third embodiment of the present invention. Best Mode for Carrying Out the Invention

[18] Reference will now be made in detail to preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

[19] The present invention is devised for providinga stacked cylinder type cold plasma reactor using a dielectric barrier discharge and a method for fabricating the same, the reactor being used for (1) treating various noxious gases such as a nitrogen oxide(NOx), a volatile organic compounds(VOCs) , and a stinking material, (2) generating ozone and ozone water, and (3) generating a chemically reactive species having high chemical reactivity in order to treat a surface of a metal, a ceramic and a polymer material. The stacked cylinder type cold plasma reactor is characterized by a cylindrical electrode rod, the cylindrical electrode rod comprising a dielectric cylinder with a hole formed in the center thereof and a metal rod inserted into the hole, or a metal paste applied on the inner surface of the hole, and a both side structure formed by installing numbers of the cylindrical electrode rods spaced apart with a regular distance in parallel. In the both side structure, metal leads of neighboring electrode rods are positioned opposite to each other to prevent arc discharge from being generated and to generate stable plasma.

[20] The cold plasma generating apparatusfor low pressure drop and low energy density according to the present invention is devised to adapt to system which minimizes pressure drop of a flow occurred in a conventional art, consumes low plasma generating power for handling mass flow, and is not influenced by a direction of a flow.

[21] Next, the structure of the stacked cylinder type cold plasma reactor using the dielectric barrier discharge according to the present invention will be described in detail with referring to Figs. 2 4.

[22] Fig. 2 is a perspective view of a cold plasma reactor according to the present invention, and shows a cold plasma reactor having numbers of stacked cylindrical type electrode rods with parallel.Fig. 3 is a cross-sectional view of a discrete electrode rod according to the first embodiment shown in Fig. 2. Fig. 4 is a schematic view of a position relation between the edge of a metal electrode in the cylindrical electrode rod and the edge of a structure for fixing the cylindrical electrode rods to form a whole reactor. In the reactor, numbers of cylindrical electrode rods are spaced apart with each other. [23] The cold plasma generating apparatus for low pressure drop and low energy density according to the present invention has numbers of ceramic electrode rods 10 on the same axis to form numbers of ceramic electrode rod layers. Passages with a regular spacing are formed among the ceramic electrode rods. The apparatus also has an electrode rod fixing structure 15 for fixing the ceramic electrode rod stably.

[24] The ceramic electrode rod 10 is a hollow ceramic tube 11 with a constant thickness, a metal electrode 12 applied on the inner circumferenceof the hollow ceramic tube and a glass layer 13 coated on the metal electrode in order to prevent the metal electrode from a damage such as a oxidation caused by contact of the metal electrode with air.

[25] A spacing between the ceramic electroderod 10 and a neighboring ceramic electrode rod 10 can be varied from several centimeters to several hundred micrometers in accordance with the kind and density of a gas treated by the cold plasma reactor according to the present invention. Therefore, the spacing is selected by taking into account pressure drop and applied energy density.

[26] An electrode lead 14 is connected to the metal lead 12 applied to each of the ceramic tubes 11 of the electrode rods 10 to be connected the outside. The portion with the electrode lead 14 of the electrode rods 10 is arranged opposite to and cross to the part with the electrode lead of neighboring electrode rod 10.

[27] The electrode leads 14 in the ceramic electrode rods which are arranged on the same axis are connected to a terminal of a high voltage generator. When an high alternating voltage(a high frequency alternating electricity or a bidirectional pulse electricity) is applied to the passage of the reactor which is constructed by numbers of electrode rod layers, a streamer shaped cold plasma region is created by a micro discharge.

[28] Fig. 4 is a schematic view of a position relation between a fixing structure for electrode rod and a metal electrode boundary in the electroderod for forming the cold plasma reactor. As will be seen from the figure, the fixing structure 15 for fixing the electrode rod is provided to both ends of each of the ceramic electrode rods 10 to fix the electrode rod. The fixing structure is positioned beyond the boundary of the metal electrode 12 inthe electrode rod 10 to prevent the streamer discharge from being occurred in the vicinity of the fixing structure 15.

[29] As shown in Fig. 2, when the ceramic electrode rods 10 are positioned by the fixing structure 15 with a regular spacing, a discrete space, having a rectangular section, for a gas flow is formed as shown in Fig. 4. At this time, the streamer discharge is generated only in the spacein which the metal electrodes in the ceramic electrode rod 10 on the left side and the rod 10 in the right side are faced each other horizontally.

[30] That is, the boundary of the fixing structure 15 in the reactor is positioned at several mm beyond the outside of the tip end of the metal electrode in the electrode rod to prevent the streamer discharge from being occurred around the boundary of the fixing structure. [31] As will be seen from Fig. 5, in connecting the ceramic electrode rod with the fixing structure 15 according to the present invention, the ceramic electrode rod and the fixing structure are connected with each other by a bearing. [32] Fig. 6 is a side view of a connection between the electrode rod and the fixing structure for electrode rod for forming a stacked cylinder type cold plasma reactor in the Fig. 2. Gears 17 are attached to an end of each of the ceramic electrode rods connected to the fixing structure 15 through the bearing. Arotatable adjusting knob 20 having a gear attached thereto is installed on an edge of the fixing structure. [33] Such arranged gears 17 are connected with each other through a timing belt 16 or a chain 16. The ceramic electrode rod is rotated by adjusting the knob 20 attached to the edge of the fixing structure. If an electric motor 19 is installed on the knob, the ceramic electrode rods 10 can be automatically rotated, respectively, in accordance with a program. [34] As shown in Fig. 6, a guide roller 18 is installed between the neighboring ceramic electrode rods 10 in the fixing structure 15 in order to ensure the engagement of the gears 17, attached to the ceramic electrode rods 10 arranged in parallel, and the timing belt 16 or the chain 16. Through this arrangement, an appropriate tension is occurred to push the timing belt 16 or the chain to the each of the gears. [35] For fabricating the ceramic electrode rods 10, a process for applying the metal electrode 12 to the inside of the ceramic tube 11 and coating the glass layer 13 to the metal electrode is performed. [36] The process is performed through a printing. In first step, a metal paste with several micrometers to several hundred micrometers is applied on a place for making a metal electrode. [37] Next, the applied metal paste is heated at 100-200°C for 0.1 1 hour to remove the organic solvent included in the metal paste. [38] Next, the applied metal paste is further heated at 400-400 °C for 0.5- 1 hour to remove the organic polymer included in the metal paste. [39] Then, a side of the place applied withthe metal paste is welded to a lead of an electrode for electrical connection to the outside. [40] In next step, a region somewhat larger than the place applied with the metal paste is coated with a glass paste with a thickness of several micrometers to several hundred micrometers. [41] Next, the applied metal paste is heated at 100-200°C for 0.1 1 hour to remove the organic solvent included in the metal paste. [42] Next, the applied metal paste is further heated at 400-400 °C for 0.5- 1 hour to remove the organic polymer included in the metal paste.

[43] Lastly, the applied metal paste is further heat-treated in a furnace at 700-900 °C for 1-2 hours to be fused.

[44] The material for the metal electrode can be selected from conductive metal pastes such as copper, silver, gold and tungsten.

[45] In addition to the above process, there are lots of processes for fabricating the ceramic electrode rod 10, and some of the processes will be described in detail.

[46]

[47] (Embodiment 2)

[48] Fig. 7 is a schematic view of an electrode rod according to the second embodiment of the present invention. The embodiment comprises steps of: forming a recess in the outer circumference of the ceramic tube having a constant thickness; applying the metal paste 12 into the recess; heating the applied metal paste 12 at 100-200 °Cfor 0.1-1 hour to remove an organic solvent therein; further heating the applied metal paste 12 at 400-500 °C for 0.5-1 hour to remove an organic polymer therein; welding an end of the applied metal paste to a lead of an electrode for electrical connection to the outside after heating of the metal paste 12; applying the glass paste 13 on the metal paste 12, the glass paste being extended beyond the boundary of the metal paste; heating the applied glass paste 13 at 100-200 °C for 0.1-1 hour to remove an organic solvent therein; further heating the applied glass paste 13 at 400-500 °C for 0.5-1 hour to remove an organic polymer therein; and firing the glass paste 13 in a furnace at 700-900 °C for 1-2 hours.

[49] In the electrode rod fabricated by the above steps, the finally processed glass part acts as a dielectric in case of dielectric discharge. This electrode rod has an advantage that the treatment of the outer surface of the ceramic rod is easy, and it is possible to vary a pattern of the metal electrode.

[50]

[51 ] (Embodiment 3)

[52] Fig. 8 is a schematic view of an electrode rod according to the third embodiment of the present invention. A process for positioning a metal plate between ceramicsis accompanied. The process will be described in detail below.

[53] In forming a ceramicelectrode rod for using in the stacked cylinder type cold plasma reactor using dielectric barrier discharge, a ceramic rod 21 is positioned in the inside of the metal electrode. The method for forming the electrode rod for using in the cold plasma generating apparatus according to the third embodiment is characterized by steps of: printing a metal paste 12 on a green sheet 22, which is a ceramic material prior to a firing step, in order to make a metal electrode of a desired pattern; heating the metal paste at 100-200 °C for 0.1-1 hour to remove an organic solvent therein; further heating the metal paste at 400-500 °Cfor 0.5-1 hour to remove an organic polymer therein; surrounding the green sheet printed 22 with not hardened metal electrode around the ceramic tube or rod 21 such that the surface with the printed metal electrode is contacted with the ceramic rod; and firing the ceramic tube or rod 21 surrounded by the green sheet printed with the metal electrode 12 to merge the metal electrode into the ceramic.

[54] According to the present invention, the flow pressure drop in the cold plasma generating apparatus using dielectric barrier discharge is significantly improved. Damages such as breakdown of the dielectric and oxidation of the metal electrode caused by discharge between the ceramic and the metal electrode is also radically prevented, as the ceramic and the metal electrode are tightly connected without an air gap. As the ceramic electrode rod can be rotated with a regular time interval, overall durability of the apparatus is improved. In addition, as there is an advantage that the present invention is free from flow direction of a treating gas, one of problems in prior art, the present invention can flexibly cope with a gas treating application.

[55] In the conventional cold plasma generating apparatus using numbers of dielectric electrode rods, the electrode rods arranged in parallel are considered as a unit electrode structure, numbers of unit electrode structures are stacked in neighboring with each other, and the direction of flow is in gravity direction in order to minimize the effect of moisture or particles for the generation of plasma. In such construction, as each of the electrode rods is at the vertex of a square or a lozenge, spatial efficiency is in- creasedwith respect to the generation of plasma, while the volume of treating gas is also increased. Thus, in the case of a duct to be installed in the cold plasma reactor relatively large or a treating gas requiring low plasma energy for the volume for treating, the loss of pressure of a flow is too large to apply the cold plasma generating apparatus in actual. Further, as a solid metal rod is used as a metal electrode to be positioned in an inside of a dielectric tube such as a ceramic, the metal electrode and the dielectric tube are not tightly connected to generate an electric discharge therebetween and the breakdown of the dielectric and oxidation of the metal electrode are occurred.

[56] In order to overcome the above problems, as a unit structure formed by ceramic electrode rods spaced apart with a regular interval is constructed as a cold plasma reactor in the present invention, the loss of pressure of a flow is about several mmAq when a desired plasma power is applied into a duct having a large section.

[57] A bearing is used when attaching the ceramic electrode rod to a fixing structure. As the electrode rod is able to be rotated by a gear installed on one end of the electrode rod and a timing belt or chain connected the gear, the surface of electrode rod to be exposed to an electric discharge is extended to entire surface of the electrode rod to maximize durability of the dielectric.

[58] In fabricating the ceramic electrode rod, an air gap between a surface of the metal electrode and a surface of the dielectric is entirelyehminated by using a metal paste and a glass paste, or a process for firing the ceramic electrode rod and a metal electrode printed on a green sheet. Therefore, an electric discharge generatable between the metal electrode and the electrode rod is essentially prevented from being occurred to maximize the durabilityof the dielectric and the metal electrode.

[59] Further, for constructing the ceramic electrode rod and the fixing structure, the region for passing a gas and the region for generating a streamer are formed on different positions. That is, the metal electrode is positioned inside the inner boundary of the fixing structure with several millimeters to prevent a streamer from being generated in the inside part of the fixing structure having a gas flow.

[60] Generally, the streamer discharge by a dielectric discharge has a characteristic that the streamer is successively generated at the discharge-originated portion. Generally, as high alternating voltage with a high frequency is usedas a driving source for the dielectric discharge, the streamer discharge is seen as if the streamer had fixed on an arbitrary position(this is, a standing streamer).

[61] As such, if the streamer is continuously generated at one point, a local heating will be increased, and the shock caused by the successive discharge will also be accumulated. If this situation is continued for long time, the part of the dielectric on which the standing streamer is generated is broken down. As a result, the streamer discharge for cold plasma is changed to an arc discharge to ruin the reactor.

[62] However, if a gas flow is present in the region in which the streamer discharge is generated, the standing streamer is spread evenly. Thus, the ceramic electrode is not damaged by the streamer as well as the reactor is not operated at the voltage exceeding the breakdown voltage. Therefore, as the application using the present invention is always accompanied with a gas flow, a drop of durability is hardly occurred. As the flow rate is 0(zero) in an inner side of a spacer according to the hydromechanical on slip conditioning, If the streamer is generated in the vicinity of the inner side of the spacer, the probabilityof changing to a standing streamer is increased to bring about shortening of life time of the reactor. With this reason, the boundary of the metal electrode and the inner boundary of the fixing structure are positioned on different places in order to prevent the streamer discharge from being generated in the inner side of the fixing structure. Through this construction, the damage of the dielectric by the streamer can be dramatically reduced. However, the space for the plasma is slightly reduced comparing with the entire region of the reactor. In actual, as a portion in the vicinity of the inner side of the spacer is a boundary layer flow having low flow rate and the volume of treating gas proportional to the reduced volume is not flowed in the portion, there is little variation for the efficiency of gas treating. [63] Any contamination by moisture and dust is not occurred in the reactor not to affect the operation of the reactor, as the cold plasma generating apparatus for low pressure drop and low energy densityis devised to be operated by high alternating voltage with a frequency of several kHz. [64] Dielectric heat is generated when the reactor is operated by high voltage with high frequency. Therefore, moisture is not condensed on a surface of the ceramic electrode rod. Preferably, if water is sprayed to the surface of the ceramic electrode rod in active, the water is immediately evaporated and the malfunction by the water is not occurred. [65] Generally, there is a phenomenon that particles such as fine dust and smoke is attached to the surface of the electrode rod due to the electrostatic precipitation. However, the surface of the electrode rod can be maintained cleanly if the electrode rod is operated by high frequency. [66] Thus, the clod plasma generating apparatus according to the present invention is devised to maintain the function of plasma generating at all times irrespective of the direction of flow.

Claims

[1] The method for fabricating a ceramic electrode rod for using in a stacked cylinder type cold plasma generating apparatususing dielectric barrier discharge comprises steps of: Applying a metal paste 12 into the inside of a ceramic tube 11 having a constant thickness; heating the metal paste 12 at the temperature of 100-200 °Cfor 0.1-1 hour to remove an organic solvent therein; further heating the metal paste at the temperature of 400-500 °Cfor 0.5-1 hour to remove an organic polymer therein; welding a lead of an electrode for electrical connection to the outside to a side of the ceramic tube applied with the metal paste 12 after the heat-treating of the metal paste 12; applying the glass paste 13 on the metal paste 12, the glass paste being slightly extended beyond the boundary of the metal electrode heating the glass paste 13 at the temperature of 100-200 °C for 0.1-1 hour to remove an organic solvent therein; further heating the glass paste 13 at 400-500 °Cfor 0.5-1 hour to remove an organic polymer therein; and firing the glass paste 13 in a furnace at the temperature of 700-900 °C for 1-2 hours. [2] The method for fabricating a ceramic electrode rod for using in a stacked cylinder type cold plasma generating apparatus using dielectric barrier discharge comprises steps of: forming a recess in the outer circumference of a ceramic tube having a constant thickness; applying the metal paste 12 into the recess; heating the metal paste 12 at the temperature of 100-200 °C for 0.1-1 hour to remove an organic solvent therein; further heating the metal paste at the temperature of 400-500 °Cfor 0.5-1 hour to remove an organic polymer therein; welding a lead of an electrode for electrical connection to the outside to a side of the ceramic tube applied with the metal paste 12 after the heat-treating of the metal paste 12; applying the glass paste 13 on the metal paste 12, the glass paste being slightly extended beyond the boundary of the recess heating the glass paste 13 at the temperature of 100-200 °Cfor 0.1-1 hour to remove an organic solvent therein; further heating the glass paste 13 at 400-500 °Cfor 0.5-1 hour to remove an organic polymer therein; and firing the glass paste 13 in a furnace at the temperature of 700-900 °C for 1-2 hours. [3] The method for fabricating a ceramic electrode rod for using in a stacked cylinder type cold plasma generating apparatus using dielectric barrier discharge comprises: a ceramic rod 21 positioned in the inside of a metal electrode; and steps of: printing a metal paste 12 on a green sheet 22 in order to make a metal electrode of a desired pattern, the green sheet being a ceramic material prior to a firing step; heating the metal paste at 100-200 °C for 0.1-1 hour to remove an organic solvent therein; further heating the metal paste at 400-500 °C for 0.5-1 hour to remove an organic polymer therein; surrounding the green sheet printed 22 with not hardened metal electrode around the ceramic tube or rod 21 such that the surface printed with the metal electrode is contacted with the ceramic rod; and firing the ceramic tube or rod 21 surrounded by the green sheet printed with the metal electrode to merge the metal electrode into the ceramic. [4] The method as claimed in any one of claims 1-3, wherein the metal paste is a conductive metal. [5] In a stacked cylinder type cold plasma generating apparatus using a dielectric barrier discharge technology for low pressure drop and low energy density, the apparatus comprising: ceramic electrode rods 10 being spaced apart with regular spacing and arranged on the same axis to form electrode rod layers passages with a regular interval being formed among the ceramic electrode rods An electrode lead 14 for connecting to the outsidebeing provided to each of the ceramic electrode rods 10, and welded to a metal electrode 12 applied to each of ceramic tubes 11 of the electrode rods 10; and the electrode leads 14 of the electrode rods 10 being arranged opposite to and cross to an electrode lead of neighboring electrode rod 10. [6] The apparatus as claimed in claim 5, further comprising a fixing structure 15 for fixing the electrode rod being provided to both ends of each of the ceramic electrode rods 10 to fix the electrode rod, and positioned beyond the boundary of the metal electrode 12 in the electrode rod 10 to prevent a streamer discharge from being occurred in the vicinity of the boundary of the fixing structure 15. [7] The apparatus as claimed in claim 5, further comprising: the fixing structure including a bearing and attached to both ends of each of the ceramic electrode rods 10; gears 17 attached to one end of each of the ceramic electrode rods 10; a timing belt 16 or a chain 16 for connecting the gears 17; and adjusting knob 20 as an adjusting means being provided the one end of each of the ceramic rods 10 having gear 17 attached thereto to rotate the ceramic electrode rods 10. [8] The apparatus as claimed in claim 7, wherein the adjusting means is an electric motor, and the motor is operated by a program to rotate each of the ceramic electrode rods 10. [9] The apparatus as claimed claims 7 or 8, further comprising a guide roller 18 provided between neighboring ceramic electrode rods 10.