EP2062649B1 - Electrostatic separator with particulate rejection means, heating system and method for operation - Google Patents

Description

The invention relates to an electrostatic precipitator, in particular for an exhaust pipe of an exhaust gas purification system, according to the preamble of claim 1. Furthermore, the invention relates to a heating system for generating energy by burning an energy source with an electrostatic precipitator according to the preamble of claim 5. In addition concerns The invention relates to a method for reducing deposits of particles of an exhaust gas stream on an electric field-generating electrode of an electrostatic precipitator of a heating system according to the preamble of patent claim 6.

The GB 2 045 647 A describes an electrode of an ionization device for an electrostatic air filter. The electrode comprises a current-conducting wire which is clamped in a frame in a plurality of mutually parallel, electrically connected series sections. The electrode also includes glands, sleeves and springs. The wire is hung in the anchor bolt, passed freely through the space formed by the frame, threaded through the fitting, the hose-like protective sleeve and the screw, again guided through the free space, etc. Finally, the wire ends in the compression fitting with which the wire can be stretched.

The JP 2007 263 754 A describes a cleaning device for an electrode of an emission spectrometer. The cleaning device comprises a brush with a metal wire brush attachment. The brush is functionally and constructively separated from the electrode and is used outside of the meter operating times. In a pause between two measuring operations, the cleaning device brushes over the electrode and removes scraping residues of condensed metal vapor.

Due to emissions from heating systems and global efforts to reduce such emissions - see, for example, the Kyoto Protocol - heating systems use appropriate emission control systems. These are in particular to filter out the harmful substances and particles from exhaust gases, so that the remaining, purified exhaust gas can be safely discharged to the environment. In particular, such emission control systems are used in biomass heating systems, where in addition to otherwise economic and environmental benefits increased emissions of pollutants in the exhaust gases can occur. Especially the relatively high emission of particulate matter as a pollutant component is a problem in biomass heating systems.

From the EP 1 193 445 A2 An emission control system is known, which is used for biomass heating systems to reduce particulate matter emission. The device described therein can be installed in a flue gas channel and for this purpose has a lid which can be placed gas-tight on an associated opening on a flue gas channel. On the inside of the lid, a spray electrode, for example in the form of a tensioned rod, is held over an insulating holder. A high-voltage transformer with rectifier function allows the construction of a high DC voltage between the wire and the lid, which is electrically connected to the furnace tube, so that it acts as a collector electrode.

Such an electrostatic filter with a spray electrode and a collector electrode is also known as an electrostatic precipitator. This is used for exhaust gas purification in an exhaust pipe of a heating system. In this case, a capacitor is formed by the spray, which runs approximately centrally through the exhaust pipe and therefore also referred to as the center electrode, and a peripheral surface of the exhaust pipe, which is also referred to as a cylindrical capacitor in a cylindrical tube-shaped design of the exhaust pipe. The spray or center electrode generally has a circular cross section in the flow direction of the exhaust gas, wherein the diameter of the cross section or the radius of curvature is generally formed relatively small (for example, less than 0.4 mm). In order to deposit the pollutants, more precisely the particles not to be emitted to the environment, of the exhaust gas from the exhaust gas flow, a field extending transversely to the flow direction is formed by the center electrode and the collector electrode formed by the lateral surface with field lines from the center electrode to the collector electrode. For this purpose, a high voltage is applied to the center electrode, for example in the range of 15 kV. As a result, a corona discharge is formed, through which the particles flowing through the field in the exhaust gas are charged in a unipolar manner. Due to this charge, most of the particles migrate through the electrostatic Coulomb forces to the inner wall of the exhaust pipe, which serves as a collector electrode.

As mentioned above, the particles are electrostatically charged by the corona discharge which forms along the surface of the electrode. This is done at the molecular level by the following process: If the electrode is e.g. relative to the exhaust pipe to negative high voltage, so a large number of gas molecules is negatively charged. They move in the electric field applied by the electrode and the exhaust pipe in the direction of the exhaust pipe. If these meet on their way through the exhaust pipe to electrically neutral particles, they stick to these and charge the previously neutral particles also negative. The charged particles flow driven by electrostatic deflection forces to the inner wall of the exhaust pipe. Here the particles stick, lose their charge and are safely removed from the exhaust stream. This is the core process of an electrostatic precipitator and, depending on the geometry, height of the corona current, electrode shape, etc., leads to deposition rates of up to more than 90%. This core process can be disturbed by the following effects:

Burning produces bipolar charged particles. By means of Boltzmann distribution, the proportion of single or multiply charged particles can be estimated. The distribution is symmetrical, ie there are the same number of positive and negatively charged particles. For conditions such as those present in the exhaust gas of biomass heating systems, between 15 and 20% of the particles carry an elementary electric charge. Although the number of charged particles is reduced by approx. 10% per second due to coagulation, there are still more than 10% charged particles at the electrostatic precipitator (corresponding to about one to two seconds of particle flying time from the place of combustion). Now get the charged particles in the vicinity of the lying on negative high voltage electrode of the charger (unit exhaust pipe, electrode), the negative particles will flow away from the electrode towards the exhaust pipe inside. The positive particles, on the other hand, flow towards the electrode. Of this, a part is neutralized or negatively charged while flowing through the charger, but the rest of the particles reaches the electrode and deposits there. Over the service life it comes therefore to function restrictions of the electrostatic deflector. Because the fine dust deposited on the electrode locally prevents the formation of the corona. As a result, the electrical charge of the particles deteriorates. The deposition efficiency of the system is degraded. In addition, in the immediate vicinity of the corona (within a radius of a few millimeters around the electrode) there is a bipolar charge area. Electrically neutral particles which flow through this area can also be positive by a negative electrode to be charged. They then flow to the electrode. One part is neutralized or negatively charged by the corona, but a small remainder reaches the electrode and also deposits there.

A disadvantage of the electrostatic precipitators according to the prior art is that it comes after a longer period of operation to a continuous degradation of the corona current at a constant high voltage. As a result, the charging efficiency of the electrode decreases, which in turn reduces the separation efficiency of the entire system.

The JP 04301117 discloses a particle separator having a center electrode and a peripheral electrode, in which the particles are separated by centrifugal forces. In this case, the center electrode comprises an electrode region in the flow direction and a downstream electrode region.

The invention has for its object to provide an electrostatic precipitator, which overcomes this disadvantage and in particular prevents or reduces the deposition of particles on the electrode in order to increase the service life of the electrostatic precipitator.

Further, the invention has for its object to provide a heating system with a separator according to the invention, which guarantees reliable exhaust gas purification.

In addition, the invention has for its object to provide a method according to which the permanent deposition of particles on the electrode is prevented or at least reduced.

This is achieved by the objects with the features of claim 1 and of claim 5 and by a method having the features according to claim 6. Advantageous developments can be found in the dependent claims.

The inventive electrostatic precipitator, in particular for an exhaust pipe of an exhaust gas purification system, with a flow channel having a channel wall and a channel inside, through which a particle-containing exhaust gas flows in a flow direction, and in the channel interior substantially in Flow direction extending electrode, for forming an electric field between the electrode and the channel wall, and an electrode feed to feed at least the electrode, wherein the electrode feed is at least partially encased with an insulator, further comprising a particle repelling agent, which prevents particles of the exhaust gas deposit on the electrode. The particle repelling agent effectively prevents or reduces deposition of particles on the electrode, and includes at least one pre-electrode unit disposed upstream of the electrode in the flow direction of the exhaust gas to apply an electric field in the flow direction in front of the electrode when voltage is applied to the corresponding portion of the channel wall form. Due to this field, particles which pass along their flow the electric field of the Vorelektrodeneinheit, upstream of the electrode, at least for the most part, effectively removed from the exhaust gas flow. Particles that would reach the electrode without Vorelektrodeneinheit are thus effectively filtered out in front of the electrode. The pre-electrode unit has a pre-electrode which has the same polarity as the electrode. In this way, it is ensured that particles which would reach the electrode without a pre-electrode unit reach the pre-electrode at the upstream pre-electrode unit. The pre-electrode thus acts as a kind of "sacrificial electrode". The electrostatic precipitator according to the invention is characterized in that at least one of the electrodes is at least partially formed as a tuft-like arrangement of wires, which are fixed at one end and freely arranged at its other end and align themselves upon application of a voltage along the field lines formed thereby. By moving the wires, adhering particles can easily be shaken off. For this purpose, a voltage can be applied several times in succession at short intervals, so that the wires move accordingly and shake off particles.

In one embodiment, it is provided that the pre-electrode unit is designed such that it uses a voltage level that is the same as the electrode. On the Vorelektrodeneinheit but also a different voltage level to the electrode may be applied, which is higher or lower than that of the electrode. This realizes an effective filter function.

An embodiment provides that the pre-electrode is formed as conductively connected to the electrode. This ensures that the electrode and the pre-electrode have the same polarity.

Yet another embodiment provides that the pre-electrode is formed differently with respect to the geometry, the material, the manufacturing method and / or the surface structure. The pre-electrode is formed differently from the electrode, in particular with regard to the parameters required for generating an electric field. The pre-electrode is formed in one preferred embodiment in one piece with the electrode. Preferably, the pre-electrode is formed with respect to the electrode as a thickening.

The heating system for generating energy by burning from an energy source such as biomass comprises a particulate matter emitting heating system, such as a biomass heating system, for burning the energy source, wherein particulate exhaust gases are formed, an electrostatic precipitator in an exhaust pipe, comprising a flow channel with a channel wall and a Channel interior through which the particle-containing exhaust gas flows in a flow direction, an electrode extending in the channel interior substantially in the flow direction and an electrode feed to feed the electrode, wherein the electrode feed is at least partially encased with an insulator. Furthermore, a particle-repelling agent, which prevents particles of the exhaust gas (P) from depositing on the electrode (6), wherein the particle-repelling means comprises at least one pre-electrode unit (9), which in the flow direction of the exhaust gas (P) in front of the electrode (6). is arranged to form an electric field in the flow direction (P) in front of the electrode (6) upon application of a voltage to the corresponding portion of the channel wall 4, and wherein the pre-electrode unit (9) has a pre-electrode (10) having the same polarity The heating system according to the invention is characterized in that the electrostatic precipitator is designed in accordance with the electrostatic precipitator according to the invention, wherein at least one of the electrodes (6, 10) is at least partially formed as a tuft-type arrangement of wires (11). which are fixed at one end and freely arranged at the other end and when donning a r Align voltage along the field lines established by it.

The inventive method for reducing deposits of particles of an exhaust gas stream on an electric field generating electrode of an electrostatic precipitator of a heating system, wherein the electrostatic precipitator is formed according to the electrostatic precipitator according to the invention comprises the steps of feeding at least the electrode by means of an electrode feed, generating a electric field between a channel wall and the electrode to move the particles flowing along the electrode along the field lines from the exhaust gas flow, and further generating an electric ramp to upstream particles moving in the electric field to the electrode upstream of field lines of the electric apron to move to a pre-electrode for generating the electric apron. The method further comprises the steps of forming at least one of the electrodes at least in part as a tuft-like arrangement of wires which are fixed at one end and free at the other end and align themselves with voltage applied along the field lines established thereby, and move at least one these electrodes by applying a voltage to shake off particles adhering to the corresponding electrode. The movement can be rotational, translatory (eg vibrating) or as a combination thereof. Preferably, the movement takes place jerkily.

• Eine Vermeidung bzw. Reduzierung von Feinstaubablagerungen auf der Elektrode wird realisiert. Damit die positiv geladenen Partikel im Abgasstrom sich nicht auf der Elektrode ablagern können, müssen sie aus der Abgasströmung entfernt werden. Dies geschieht vorliegend durch ein zweites elektrisches (Vor-) Feld stromaufwärts vor der Elektrode. Dieses kann durch eine getrennte Hochspannungszuführung auf einem anderen Spannungsniveau wie jenes der (Mittel-) Elektrode realisiert werden. Die Polarität der (Mittel-)Vorelektrode des Vorfeldes wird dabei vorzugsweise identisch wie die der nachgeschalteten Mittel- oder Corona-Elektrode gewählt. Abhängig von der Strömungsgeschwindigkeit bzw. der Geometrie der Einheit Elektrode-Abgasrohr, welche auch als Aufladungseinheit bezeichnet wird, existiert ein optimaler Abstand zwischen Corona- und Vorelektrode bzw. deren optimale Geometrie (Länge, Breite, Querschnitt etc.). Stromaufwärts zur Aufladungseinheit ist ein metallischer Körper, der zum Beispiel als Verdickung der Elektrode ausgebildet sein kann, elektrisch leitend mit der Elektrode verbunden. Der Körper liegt daher auf demselben Spannungsniveau wie die Elektrode. Durch eine entsprechende Dimensionierung des Körpers bzw. der Elektroden kann erreicht werden, dass an dem Körper oder der Vorelektrode zuverlässig alle aus dem Verbrennungsprozess stammenden positiv geladenen Partikel abgeschieden werden. Der Körper oder die Vorelektrode, welche somit als "Opfer-Elektrode" fungiert, bietet bevorzugt eine viel größere Ablagerungsfläche als die Mittelelektrode der Aufladungseinheit. Dadurch kann die maximale Betriebszeit des elektrostatischen Abscheiders bis zu Funktionseinschränkungen und somit einem nächsten Wartungsabschnitt verlängert werden.

With the electrostatic precipitator according to the invention, the heating system according to the invention and the method according to the invention, in particular the following advantages are realized:

• An avoidance or reduction of fine dust deposits on the electrode is realized. So that the positively charged particles in the exhaust gas stream can not be deposited on the electrode, they must be removed from the exhaust gas flow. This is done in the present case by a second electric (pre-) field upstream of the electrode. This can be realized by a separate high voltage supply at a different voltage level as that of the (middle) electrode. The polarity of the (middle) pre-electrode of the apron is preferably chosen to be identical to that of the downstream central or corona electrode. Depending on the flow velocity or the geometry of the unit electrode-exhaust pipe, which is also referred to as a charging unit, there is an optimal distance between corona and pre-electrode or their optimal geometry (length, width, cross section, etc.). Upstream of the charging unit is a metallic body, which may be formed, for example, as a thickening of the electrode, electrically conductively connected to the electrode. The body is therefore at the same voltage level as the electrode. By appropriate dimensioning of the body or of the electrodes, it is possible to reliably deposit all positively charged particles originating from the combustion process on the body or the pre-electrode. The body or the pre-electrode, which thus functions as a "sacrificial electrode", preferably provides a much larger deposit area than the center electrode of the charging unit. As a result, the maximum operating time of the electrostatic precipitator can be extended to functional restrictions and thus a next maintenance section.

Automated removal of contaminants on the high voltage electrode is realized. Despite the electric ramp, it can lead to fine dust deposits on the electrode. These can be removed automatically. It has been shown that instead of a solid wire-shaped electrode, a tuft of fine wires generates a sufficiently high corona current when a high voltage is applied. The fine wires are sufficiently mobile that they follow the electric field lines when the high voltage is applied. When the high voltage is switched off, the wires follow the gravitation again. By appropriate switching, the wires move back and forth. This movement is sufficient to shake off the loosely adhering fine dust. If the movement in the gas flow is not sufficiently pronounced, the high voltage can be switched on and off several times automatically even when the exhaust fan is at a standstill. The fine wires are naturally also subject to increased chemical and physical wear in the aggressive plasma of the corona. By appropriate geometry and material selection, the optimum is set between the formation of a sufficiently high corona current, the electrostatic mobility and chemical-physical resistance.

The electrostatic precipitator according to the invention has a minimal flow resistance in an exhaust gas system, which increases only slightly and slowly even as the loading of adhering particles increases. Electrostatic precipitators have a relatively large absorption capacity for separated particulate matter of the particle flow. At slow flow velocities of the exhaust gas stream and sufficiently long separation distances for the fine dust, electrostatic precipitators for submicron particles have a separation efficiency> 90%. For the reasons explained above, therefore, electrostatic precipitators are often used for the exhaust gas purification of heating systems such as a pellet heating system, other biomass heating systems or oil burners. Maintaining the necessary for the charging of the particles corona current even after several hours of operation is a technical difficulty in the execution of the electrostatic precipitator. The inventive keeping free of the high voltage potential center electrode prolongs the maximum operating time of the electrostatic precipitator until the next maintenance.

The electric apron of the same polarity but of an optimized voltage level, which is arranged upstream of the actual charging unit, reliably removes any fine dust particles already loaded from the combustion process, given a suitable choice of geometry. As a result, the electrode of the charging unit is reliably kept free of fine dust. In one embodiment, the pre-electrode is formed as an extension of the charging electrode to a thicker end against the flow direction of the exhaust gas. This tail serves as a "sacrificial electrode" with a much larger loading capacity for charged particulate matter than the (middle) electrode.

The automated cleaning concept for the corona electrode has the following effects: Numerous tests have shown that the fine dust from wood combustion, for example, adheres very loosely to the corona electrode. The movement of the electrode, or parts thereof, forced by the alignment of the wires in the electrostatic field is sufficient to shake off the particulate matter. This movement is easy to automate.

Fig. 1

schematisch einen Längsquerschnitt durch eine Ausführungsform eines erfindungsgemäßen elektrostatischen Abscheiders,

Fig. 2A-B

schematisch in einer Seitenansicht ein Ausführungsbeispiel einer büschelartigen Elektrode, einmal ohne elektrisches Feld (2A), einmal mit angelegtem elektrischen Feld (2b), und

Fig. 3A-B

schematisch in einer Draufsicht das Ausführungsbeispiel gemäß Fig. 2A und 2B.

The drawings illustrate several embodiments of the invention and show in the figures:

Fig. 1

FIG. 2 schematically shows a longitudinal cross section through an embodiment of an electrostatic precipitator according to the invention, FIG.

Fig. 2A-B

schematically in a side view an embodiment of a tuft-like electrode, once without electric field (2A), once with an applied electric field (2b), and

Fig. 3A-B

schematically in a plan view of the embodiment according to FIGS. 2A and 2B ,

Fig. 1 schematically shows a longitudinal cross-section through an embodiment of an electrostatic precipitator 1 according to the invention. The electrostatic precipitator 1 is in an exhaust pipe 2 (only partially shown) one not shown here The flow channel 3 is formed as a tubular portion of the exhaust pipe 2 and includes a channel wall 4 and a channel interior 5. Through the flow channel 3, a here represented by an arrow P, particle-containing exhaust flows into the also by the arrow P illustrated flow direction. In the flow direction P, an electrode 6, which is also referred to as a center electrode or corona electrode, extends in the interior of the flow channel 3. The flow channel 3 is preferably formed in cross-section in the flow direction P rotationally symmetrical about a central axis A. The electrode 6 extends along this central axis A. The electrode 6 is fed via an electrode feed 7, which is covered with an insulator 8. Together with the channel wall 4, the electrode 6 forms a charging unit, in which particles can be charged electrically. For this purpose, the electrode 6 forms with the channel wall 4 under application of a high voltage an electric field whose field lines are substantially radial to the electrode 6 and the channel wall 4, substantially transversely, more precisely at right angles to the flow direction P. upstream in the flow direction P. The electrode 6 is a Vorelektrodeneinheit 9 with a pre-electrode 10 is arranged. This pre-electrode unit 9 has the same polarity as the electrode 6, but at a different voltage level. In the present Fig. 1 the pre-electrode 10 is electrically conductively connected to the electrode 6, wherein in the illustrated embodiment, the pre-electrode 10 is formed in the shape different from the electrode 6, more precisely as a thickening of the electrode 6. The pre-electrode 10 forms with the corresponding portion of the channel wall 4 when applying a voltage a corresponding apron. In this apron, particles which would reach the electrode 6 without the apron are directed to the pre-electrode 10. The pre-electrode 10 has a larger capacity than the electrode 6 by their larger surface area. In this way, a longer service life of the electrostatic precipitator 1 is ensured. In FIGS. 2 and 3 shows an embodiment of an electrode 6 or 10, with which adhering particles can be removed from the electrode 6 and 10, respectively.

Fig. 2A-B show schematically in a side view an embodiment of a tuft-like electrode 6 and 10, once without electric field ( Fig. 2A ), once in an existing electric field ( Fig. 2B ). The tuft-like electrode 6, 10 has a plurality of electrically conductive wires 11. The wires 11 are clamped at one of their ends fixed in a holder 12. The other, opposite end of the wires 11 is free. In Fig. 2A There is no electric field. The wires 11 are directed according to the gravitational field. In Fig. 2B an electric field is applied. Accordingly, the electrically conductive wires 11 align according to the field lines of this generated field. By switching the electric field, or a corresponding voltage, thus the wires 11 of the electrode 6, 10 can be moved. Due to this movement, particles adhering to the wires 11 can be shaken off.

FIGS. 3A and 3B show schematically in a plan view of the embodiment according to Fig. 2A or 2B. In Fig. 3A is a state without generated electric field to see the alignment of the wires 11 can not recognize this in this illustration. In Fig. 3B on the other hand, an electric field is applied and the wires 11 are aligned in accordance with the field lines of the field extending radially from the electrode 6, 10.

Claims (6)

1. Electrostatic precipitator (1), in particular for an exhaust line (2) of an emission control system, comprising a flow channel (3) with a channel wall (4) and a channel interior (5), through which a particle-containing exhaust gas (P) flows in a direction of flow, an electrode (6), extending substantially in the direction of flow (P) in the channel interior (5), for forming an electrical field between the electrode (6) and the channel wall (4), and an electrode feed (7), for feeding at least the electrode (6), the electrode feed (7) being at least partially enclosed by an insulator (8),
   also comprising a particle repelling means, which prevents particles of the exhaust gas (P) from being deposited on the electrode (6), the particle repelling means comprising at least one upstream electrode unit (9), which is arranged upstream of the electrode (6) in the direction of flow of the exhaust gas (P), in order to form an.electrical field upstream of the electrode (6) in the direction of flow (P) when a voltage is applied to the corresponding portion of the channel wall (4), and the upstream electrode unit (9) having an upstream electrode (10), which has the same polarity as the electrode (6),
   characterized in that at least one of the electrodes (6, 10) is formed at least partially as a bundle-like arrangement of wires (11), which are arranged as fixed at one end and free at their other end and, when a voltage is applied, align themselves along the resultant field lines.
2. Electrostatic precipitator (1) according to Claim 1,
   characterized in that the upstream electrode unit (9) is formed in such a way that it uses a voltage level that is the same as that of the electrode (6).
3. Electrostatic precipitator (1) according to Claim 1 or 2,
   characterized in that the upstream electrode (10) is formed as connected to the electrode (6) in a conducting manner.
4. Electrostatic precipitator (1) according to one of Claims 1 to 3,
   characterized in that the upstream electrode (10) is formed differently from the electrode (6) in terms of the geometry, the material, the production process and/or the surface structure.
5. Heating system for generating energy by means of burning an energy source such as biomass with a heating installation emitting a fine dust, such as a biomass heating installation, for burning the energy source, thereby producing particle-containing exhaust gases (P), and an electrostatic precipitator (1) in an exhaust line (2), comprising a flow channel (3) with a channel wall (4) and a channel interior (5), through which the particle-containing exhaust gas (P) flows in a direction of flow, an electrode (6), extending substantially in the direction of flow (P) in the channel interior (5), an electrode feed (7), for feeding at least the electrode (6), the electrode feed (7) being at least partially enclosed by an insulator (8), and a particle repelling means, which prevents particles of the exhaust gas (P) from being deposited on the electrode (6), the particle repelling means comprising at least one upstream electrode unit (9), which is arranged upstream of the electrode (6) in the direction of flow of the exhaust gas (P), in order to form an electrical field upstream of the electrode (6) in the direction of flow (P) when a voltage is applied to the corresponding portion of the channel wall (4), and the upstream electrode unit (9) having an upstream electrode (10), which has the same polarity as the electrode (6),
   characterized in that the electrostatic precipitator (1) is formed according to one of Claims 1 to 4, at least one of the electrodes (6, 10) being formed at least partially as a bundle-like arrangement of wires (11), which are arranged as fixed at one end and free at their other end and, when a voltage is applied, align themselves along the resultant field lines.
6. Method for reducing deposits of particles of a stream of exhaust gas (P) on an electrical-field-generating electrode (6) of an electrostatic precipitator (1) of a heating system according to one of Claims 1 to 4, comprising the steps of:

   feeding at least the electrode (6) by means of an electrode feed and

   generating an electrical field between a channel wall (4) and the electrode (6), in order to move the particles (P) flowing along the electrode (6) along the field lines out from the stream of exhaust gas,

   further comprising the step of: generating an upstream electrical field, in order to move particles (P) that are moving in the electrical field to the electrode (6) upstream thereof along field lines of the upstream electrical field to an upstream electrode (10) generating the upstream electrical field,

   characterized in that also comprised are the steps of:

   forming at least one of the electrodes (6, 10) at least partially as a bundle-like arrangement of wires (11), which are arranged as fixed at one end and free at their other end and, when a voltage is applied, align themselves along the resultant field lines,

   moving at least one of these electrodes (6, 10) by applying a voltage, in order to shake off particles attaching themselves to the corresponding electrode (6, 10).

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