DE2756524A1 - ELECTROSTATIC SEPARATOR - Google Patents

Description

The invention relates to an electrostatic precipitator for the removal of particulate material in contaminated Gas streams is entrained. In particular, the invention relates to a resistance coating for a passive anode, to prevent secondary corona formation and arcing.

The permissible standard values for the emission of particulate matter in flue gases emitted from the chimneys of coal-fired electrical power plants, are becoming increasingly strict. The quality regulations for flow air requires that more than 99% of the Fly ash generated by the burning coal is removed from the chimney before the combustion gases are discharged Need to become. Thus, the effectiveness for collecting particulate matter must increase as the Ash content of coal increases. In an effort to reduce the emission of certain gaseous pollutants, with what especially the sulfur oxides are meant, it became additionally more and more necessary in the power stations for electric Power generation using coal grades of low sulfur content.

The electrostatic precipitator is the most common device for the removal of particulate matter produced by coal-fired power plants. In a two-stage electrostatic separator, the gas loaded with particles flows one after the other through separate charging and collecting levels. In the charging stage, the gases flow through a corona discharge, so that the particulate material that the Charger leaves, has a positive or negative charge. The charged particles then flow through an electrical one Low intensity field with corona formation in the collection stage, causing the particles to hit a collection electrode to wander where they pile up and one by one

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can be removed and collected using various techniques. The particles flow in between in a single-stage separator a pair of electrodes having an electrostatic field extending between the electrodes and a corona flow generated; there they are charged first; then they migrate to one of the electrodes, where they accumulate and then removed. Thus, in a single-stage separator, both the charging stage and the collecting stage are one single unit connected. The effectiveness of an electrostatic precipitator is largely determined by the level of charge, which is allocated to the particulate material by the charging stage. The charging height can be increased by increasing the intensity the electrostatic field that generates the corona discharge is increased. The maximum intensity of the electrostatic Field is limited to a value at which flashover and secondary coronary formation occur when the Particulate matter precipitates on that electrode which does not emit a corona and is called a passive electrode. Even though Side corona effects can be reduced to some extent by certain techniques, among which, for example, a reduction the thickness of the particle layer on the passive electrode generates the achievable electrostatic field intensities nevertheless only a limited particle charge to a certain extent. So that the collection efficiency must be improved be that the residence time of the particulate material in the electric field during the collection is thereby increased is that either the speed at which the particle-laden gas flows through the collecting stage is reduced or by increasing the length of the pick-up stage. However, a decrease in the throughput speed due to the collecting

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grade their capacity; an increase in the size of the collection electrodes increases the capital expenditure for such a system. The intensity of the electrostatic Field, in which the charger can work without secondary corona and without sparkover, is for a particulate material lower, which shows higher resistance. Because the resistance of fly ash is inversely related to the proportion of combustible There is sulfur in the coal, an increasing use of coal grades with low Sulfur content the cost of having a high collection effectiveness to achieve, since secondary corona formation and arcing are increasingly problematic.

Attempts have already been made to reduce the occurrence made by secondary corona and arcing to reduce the intensity of the electrostatic field in the ionizers to increase; a number of techniques have been used to achieve this, none of which are entirely satisfactory is. The earliest attempts, described for example by H.J. White, Industrial Electrostatic Precipitation at 328 Addison-Wesley 1963, focused on the treatment of particulate matter before it was incorporated into the ionizer enters. Particulate matter showing high resistance has been subjected to moisture and acid conditioning treated. Other techniques have attempted the deposition of a layer of particulate material on the passive one Electrode for example by preventing moving tape electrodes, rotating brushes and various other mechanical devices were used. These latter techniques generally failed because even one thin film of particulate material can produce considerable side corona effects if the resistance of the particulate matter + ■ prials is sufficiently high. However, the accumulation could vcr. Particulate matter prevented to some extent thereby

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be that the passive electrode with a film of water continuously was rinsed. Attempts were also made to adjust the temperature of the electrodes up and down in order to move the temperature of the particulate material to lower resistance values. However, this technique consumes generally a lot of power to generate the necessary temperature shifts.

In order to adjust the electrical characteristics of the passive electrode for a reduction of the secondary corona and the sparkover, a current limiting resistor of non-critical value was generally connected in series with the discharge electrode in previous attempts. The effect of the resistor is that at the points in time at which abnormal transition states occur when arcing occurs, the possible current flow is limited and the intensity of the electrostatic field at the electrode gap is simultaneously reduced. This attempt at a solution is not believed to be feasible because an effective application of this technique requires a great deal of power. One embodiment of a current limiting resistor called "stepped resistor" is described by HJ White in "Resistivity Problems in Electrostatic Precipitation", Journal of the Air Pollution Control Association, 24,336-37 (1974). According to this technique, a thick, flat, semiconducting plate electrode made of concrete reinforced with steel was used. This experiment never gave definitive results η relating to the volume resistivity at and near the anode surface, which would enable this technique to be used with a variety of ionizing devices.

Types of construction could be used. Furthermore , the article gives no indication of the permissible ranges for such critical parameters as material resistance, material thickness or the / electrical strength or dielectric strength.

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Recently, high intensity ionizers have become available in which a single electrode geometry generates a stable, high-intensity corona discharge which the particle-laden gas stream runs. These high intensity ionizers charge the particulate matter to a much higher degree than would be achievable with conventional ionization devices, the Diaht cylinder or wire-plate geometries, for example use. Although the collection effectiveness of two-stage electrostatic precipitators is largely due Use of this single high-intensity ionization device can be improved as the charging level, secondary corona and arcing still occur and particularly with very high resistance particulate matter when the particulate matter settles precipitates on a passive metal electrode.

The object of the invention is to create secondary corona formation and spark flashover in an electrostatic precipitator thereby preventing the anode from being coated with a layer of resistive material.

Furthermore, according to the invention, the permissible electrical Properties of the anode resistor coating such as the resistance of the material, the dielectric strength and the thickness of the coating are determined, to prevent secondary corona and arcing.

In addition, resistance materials to be provided, whose electrical properties, such as recordable, the resistance and the dielectric strength within

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half of a predetermined area for use as a resistive coating for a passive electrode.

The object of the invention has been achieved in that the anode of an electrostatic device such as the charging or collecting stage of a two-stage electrostatic precipitator or a single-stage electrostatic precipitator coated with a resistive material to contribute to the intensity of the electrostatic field of the device which the electrostatic device can operate without secondary corona and arcing. About secondary corona formation and to reduce arcing, the material has a resistance that is preferably greater than 10 ohm-cm. Of the Resistance of the material is less than the ratio of the dielectric strength of the material to the current flowing through it the material flows, thus preventing the field from being inside of the material exceeds the dielectric strength of the material, which is preferably greater than 80 kV / cm. The thickness of the outer coating should exceed 0.01 cm at atmospheric pressure and 149 ° C (300 ° F) for strikethrough of the material and the resulting secondary corona formation and arcing. The maximum thickness of the material is chosen so that the voltage drop across the material is less than 15% and preferably between 5% and 10% of the applied voltage. The resistor material resists degradation in a corona environment and is special where the particulate material showing abrasive properties is charged becomes, abrasion-resistant.

The invention is illustrated in the following, for example, with reference to FIG Drawing described; in this shows:

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1 is a schematic side view of a multi-stage separator equipped with a charging ionization device is equipped, which has the resistor anode according to the invention;

FIG. 2 shows an enlarged side view of an ionization stage of the system according to FIG. 1; there were some Components are partially omitted to show the ionization arrangement;

Figure 3 is a side view of the ionization stage according to FIG Fig. 2; the inlet has been partially cut away to show the ionization arrangement;

FIG. 4 is an enlarged partial sectional view of a single ionization venturi device, the Electrode arrangement is shown;

FIG. 5 shows a further enlarged sectional partial view of the electrodes according to FIG. 4, the electrode structure is reproduced in more detail;

6 is a schematic layout plan showing the control elements for an ionization stage;

7 shows a schematic drawing of the mode of operation of an ionization device which comprises a layer of resistance material, that coats the anode;

8 is an enlarged partial sectional view of an anode region according to Fig. 7;

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9-11 alternative embodiments according to the invention;

Figure 12 is a cut away isometric view of a Ionization device of the wire-cylinder geometry with an anode covered with resistant material is coated;

13 is a cut away isometric view of a Ionization device with wire-plate geometry, the anode with resistant material is coated; and

Figure 14 is an isometric view; another embodiment of a high intensity ionization device is shown with an anode that is coated with resistant material.

A detailed description of the preferred embodiment now follows.

Fig. 1 shows schematically a side view of a two-stage electrostatic precipitator system equipped with the subject matter of the invention. From this figure it can be seen that the separator system has a gas inlet 11, into which the gases that are to be cleaned are directed as indicated by arrow 12 is reproduced, has a gas outlet / away from which the suitable downstream devices for purified gases, for example an atmospheric discharge channel, as indicated by arrow 14, and a cascaded pair of ionizing devices s separator units contains, in general

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are denoted by the reference numerals 15, 15 '. Every Ionizing separator unit 15, 15 'includes an ionizing stage 16 (16') and a pair of conventional electrostatic ones Separators 17, 18, (17 ', 18'). Each ionization stage 16 ,, 16 ' and separator stage 17, 17 ', 18, 10' is provided with a cable entry connector 19 for high voltage, which is connected to a suitable high voltage source, as described more precisely in the Description of Fig. 6 is explained, is connected, and equipped with a collecting container area 2o, the collects the particulate matter which is separated from the gas as the gas flows through the units 15, 15 '.

In operation, the gas containing particulate material enters the system of FIG. 1 through inlet 11 and flows through the first ionization stage 16, in which the particles in the gas are electrostatically charged. The gas that the carries electrostatically charged particles with it, flows next into successive separator stages 17,18; In each of these separator stages, the charged particles are under the influence of an electric field across the Flow path is established, diverted from the flow path of the gas; the particles are in the container areas 2o the separator stages 17,18 deposited. The gas emerging from the separator 18 is passed through the ionization stage 16 ' and passed through the separator stages 17 ', 18' for additional cleaning of the gas; the purified gases that come out the separator stage 18 'emerge via the gas outlet 13 to suitable devices located downstream directed.

FIGS. 2 and 3 show the gas inlet 11 and the first ionization stage 16 with more details. From these figures it can be seen that the gas inlet 11 is a hollow pipe from

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trapezoidal or other suitable geometric shape which is connected to a gas distribution area 22 on the downstream side. The distribution area 22 is coupled to an entry chamber 23, which within the housing of the ionization unit 16 of the Side and bottom walls of the housing and a vertically arranged intermediate wall 24 is formed. The partition 24 and a second vertically arranged partition 25 define with the side, top and bottom walls of the ionization stage 16, a pressure distribution line 26, the purpose of which is still is described.

A large number of Venturi diffusers 27 and 27 are assigned in a regular arrangement within the ionization stage 16 Center electrode support members 28 each positioned in the upstream end of the associated venturi diffuser 27 extends into it and is substantially coaxial with the associated venturi diffuser 27 is arranged. Each element 28 is connected to a busbar line network with the general reference number 29 connected, which consists of three vertically arranged parallel busbars, which at their upper ends by a common busbar element 31 connected to one another are; the element 31 is connected to a single Santnelschiene element 32 which extends from the inside of the ionization stage to an external conventional high voltage socket 3 3 extends to the high voltage via a high voltage connector 34 from a suitable power supply (not shown) is fed. The downstream end or outlet of each venturi diffuser 27 is connected to an exit chamber connected to his-

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on the other hand with the inlet of the electrostatic separator stage

17 is connected.

The storage container 20 is equipped with a removable door 40 which is provided for inspection and cleaning tasks is, and has a vibrator holder 41 with which it is possible to use any conventional vibrator to use as a support for the particulate matter collected in container 20 to settle on the lower edge of the container 20 to discontinue. The lower edge 42 is provided with suitable openings (not shown) so that the particulate material can be removed from container 20 in a conventional manner. The containers 20 of the remaining system elements 16 ', 17, 17',

18 and 18 'are designed in a substantially identical manner.

Each venturi element 27 and each associated coaxial element 28 generally comprises a pair of electrodes for generation a high-intensity electrostatic field which runs transversely to the path of the gas flow through the ionization stage 16. For this purpose, an electrode, described below, is carried by each element 28 and is relative to a source high negative voltage connected via the busbar line network 29, while each Venturi line 27 via the The framework of the structure is connected to earth potential. Thus, each Venturi line 27 serves as an anode, while each element 28 as Cathode support is used.

During operation, a high voltage is applied between the cathode and anode; the particles that are held in suspension in the gas flowing through the ionization stage 16 are at Pass through the neck of the \ enturi line 27 electro-

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statically charged. To ensure that essentially all of the charged particles in the gas stream are kept in suspension for so long until they arrive at the downstream separator 17 or 18 and not at the one at ground potential Adhere to the anode surface, the electrode shape as shown in Figs. 4 and 5 can be applied.

In Fig. 4 it can be seen that each Venturi element 27 with an inwardly tapering, conical inlet section 45, having a generally cylindrical central portion or neck 46 and having an outwardly flaring one conical outlet region 47 is formed. The cathode has a planar electrode such as a disk 5o, which may have a curved peripheral edge protruding outwardly from the outer surface of member 28. The disc 5o is substantially coaxial in the throat of the venturi 27 and, when a high voltage is applied, causes a highly concentrated high intensity electrostatic field in the form of a corona discharge between the curved peripheral area of the disc 5o and the surrounding anode surface 52.

It can best be seen in FIG. 5 that the anode surface 52 is composed of a series of flanged conical wings 53 which, by their construction connected in a nested arrangement to a fastener 54 via spacers 54a; along the The axis of the Venturi element 27 are the blades 53 by means of spacers 54a closely spaced so that air passages 55 are thereby formed between adjacent blades. In terms of their effect, the wings 53 form a cylindrical anode wall with a slightly inclined, interrupted upper

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area. The inner surfaces of the wings 53 are provided with a resistive coating, as will be discussed later will be described with FIG. 11. The anode surface 52 is surrounded by a storage space 26, the clean Compressed air is supplied by a pump from an external source as described in connection with FIG will.

In operation, clean air is blown into the venturi throat 46 through the air passages 55 which, by their effect, have a Forming a plurality of annular nozzles and oriented so that they are along the inner anode surface of the venturi element 27 circumferential jet streams of clean gas in the essentially steer the same direction that the main stream of contaminated gas flowing through the venturi 27 has. That clean gas injected through the passages 55 flows along the anode surface in a substantial manner laminar film and creates an effective fluid barrier, the function of which is also to prevent the flow of the main gas flow to be carried away and to support. It has been found that this process causes the deposition of charged particulate matter on the Anode surfaces considerably reduced if this is done with known, prior art devices are compared. In addition, the orientation of the Injection nozzles 55 for the clean gas reduce the pressure drop normally associated with the gas flow through a venturi diffuser, the is not equipped with such nozzles is assigned. As mentioned earlier, the secondary corona problems, the prior art venturi ionizers occur, are substantially reduced by carefully contouring the edges of the wings 53 to be edited.

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In Fig. 6 the lines for the electrical power and the control system of the ionization stage are shown schematically 16 for the injection of clean gas. High voltage is supplied via a high voltage cable 34 from a transformer-rectifier system 7o, which is connected to a control unit 71, is fed to the cathode bus network 29; the transformer-rectifier system 7o and the control unit 71 are of conventional construction. The clean gas is controlled by a fan 73 via a heater 74, a line 75, a throttle and supplied via a line 77 of the United distribution chamber 26. The heater 74 is connected to a temperature control unit to control the temperature of the clean gas that the Ver distribution chamber 26 is supplied to keep within a desired temperature range. A differential pressure sensor 79, which has a pair of transformers 8o, 81, provides a feedback signal for the controlled throttle 76, in order to regulate the pressure for the clean air within the distribution chamber 26. The components 73-81 are of conventional design; their structure is known to the person skilled in the art.

As noted, there are problems associated with such electrostatic devices. This is particularly true when the electrostatic devices are used to charge particles of high resistance. Such high resistance particulate material can be, for example, the fly ash from boilers that are made using a type of coal with a low sulfur content can be used as fuel. A major problem with such Electrostatic devices have been the advent of arcing and sub-coronation in general represents the factor that increases the intensity of the

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limited electrostatic field. Secondary corona and sparkover occur when the intensity of the electrostatic field within the particulate material at the passive, d. H. no current-emitting electrode exceeds the dielectric strength of the particulate material. For example, the dielectric strength of fly ash is that of burning coal that has a low sulfur content is generated, generally between 1 kV / cm and 1o kV / cm. When the dielectric strength is exceeded a small hole or crater will form. As the corona current tries to take the path of lowest resistance to follow, the corona current focuses on the site of dielectric breakdown, creating a localized zone extremely high current flow is generated. When this phenomenon occurs, causes the great concentration a strongly negative field on negative ions, which accelerates free electrons to the breakdown. the Free electrons collide with gas molecules at a relatively high velocity or effective velocity together, knock off electrons from the molecules according to the "avalanche-like process" and thereby turn the molecules into positive ions. The positive ions are then accelerated towards the cathode and generate adjacent to the Anode has a positive corona effect. Those electrons that are released by the gas molecules during the avalanche-like process removed are accelerated towards the anode, thereby increasing the negative field and increasing production Electrons and positive ions. The result is then a positive feedback effect that lasts as long as until the voltage between anode and cathode is greatly reduced. During this process, the electron diffusion tries to to increase the diameter of the discharge while the diameter created by the flow of electrons in the discharge is circular

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Magnetic field in turn tries to compress the diameter of the discharge. As the gas pressure increases, the total corona flow increases since the increased concentration of electrons is the cause that the electrons with greater Collide frequency, reducing the mobility of the electrons. However, the increased magnified Electron concentration also one's own magnetic field; this effect prevails so that the local current density or the local current flow is increased with increasing gas pressure.

The field that prevails through the particulate material layer is represented by the formula:

= J

P (Formula 1)

where J is the current flow or current density through the material and P is the specific volume resistivity of the material. The current flow J for ionization devices is generally

—8 2 —6 2 mean on the order of 1o A / cm and 2 3d ο Α / cm. It follows that for a particulate material with a dielectric strength of 10 kV / cm, secondary corona formation and flashovers are not a problem until the volume contradicts:
exceed.

9 12

Volume resistances fourth between 5x1 o ohm-cm and 1o ohm-cm

The secondary corona and the spark flashover represent a disturbance for the operation of the ionization device, since the strong negative field adjacent to the anode greatly reduces the field intensity in the area between the electrodes; In addition, the positive ions discharge the negatively charged particulate material, whereby the object of Aifladestuf e to-

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not done.

According to the invention, the passive electrode of an electrostatic device is coated with a resistance material which has a high dielectric strength. As used herein, the term "electrostatic device" refers to either the charging stage or the collecting stage of a two stage electrostatic precipitator or a single stage electrostatic precipitator using a unitary charging and collecting stage. The passive electrode is generally designed as an anode, since the effects of secondary corona formation and arcing are much more severe with a negative corona, where the cathode emits the corona discharge. In the negative corona effect, most of the current is carried by negative ions; these negative ions originate from electrons that are released from the cathode or discharge electrode surface by being shot with positive ions. The positive ions in turn are generated in the zone with high field strength near the cathode by electron ionization of gas molecules. In the positive corona, on the other hand, the current is primarily carried by positive ions which originate from the electron ionization of the gas in the zone with high field strength near the anode or discharge electrode. In this case, the electrons generated by photoelectric ionization of the gas molecules in the area between the high field zone and the earth electrodes. These differences in the ionization process have a great influence on the spark breakdown voltages of the negative and positive corona formations. It has already been mentioned that a spark breakdown for a corona is associated with the formation of self-propagating streamers or with the current flow originating from the anode. For the posi-

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tive corona even exist near the discharge electrode at relatively low voltages high electric fields, so that the triggered or triggered by the high field strengths Spark breakdown streamers even at lower Stress is formed. This means that the operating voltages of electrostatic devices which use the positive corona effect, also for particulate material with low resistance to relatively low values are limited. With the negative corona effect, the electric field near the anode is relatively low, so that it is much higher Voltages can be applied before the fields build up to the point at which a spark breakdown occurs he follows. In practice it turns out that the arcing voltages for negative corona formations are about twice as large as for the positive corona effect. The usage however, there is a positive corona effect to increase performance under high resistance conditions not an advantage, since the sparkover voltages of the positive corona effect are inherently low. Those differences between the negative and the positive corona effect lead to different cases for these two cases Effects for a secondary corona. The main difference is that a minor corona has little effect on the arcing voltage for the positive corona shows, as in this case the secondary corona as it does for a negative Corona is the case, does not trigger a spark. The secondary corona thus has a breakdown effect on the negative one Corona shows only a slight effect on positive corona formation. It follows that a coating of the passive electrode with resistive material is generally much more suitable for a negative corona than for a positive one Corona is. The term "anode" is used here synonymously with

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"passive electrode" used; this is intended to enclose the cathode of a positive corona electrostatic device be.

The resistive layer on the anode reduces secondary corona formation and arcing from two different processes. First, it runs through a Resistance material flows current along the path with the least resistance. This means that one in the resistor material flowing current that would otherwise run in a relatively narrow path and thus a large local current flow unable to concentrate that way; thus the current flow through the resistor material and the particulate material runs, kept relatively low. The result of this is that between the discharge electrode and current flowing to the passive electrode cannot concentrate in such a way that the discharge moves out of a relative state low current density and high electric field strength in the state of high current density associated with a sparkover and can convert relatively low electric field strength. Since the current flow that flows through the particulate material, on is kept at a relatively low value, the strength of the field within the particulate material will be in accordance with FORMULA 1 given above is kept below the dielectric strength of the particulate material. Of the The second process by which the electrostatic field is stabilized by means of the resistance anode is that the Resistance material the voltage between the cathode and the surface of the resistance coating with increasing current flow reduced. That means that for an electrostatic

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direction with an applied voltage of 75 kV and a

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average current flow of 10 A / cm a 1 cm-thick coating from resistance material with a volume resistance of 10 ohm-cm, a voltage difference of 4V at the resistance coating of 1o kV in accordance with the following formula:

Δ V = J t (FORMULA2) where t is the thickness of the resistive coating .

Obviously, the level of the maximum current flow that can flow through the resistor material is 7.5 χ 10 A / cm, since with this current flow the voltage across the resistor coating would be 75 kV. Even with this maximum current flow of 7.5 10 A / cm, the electrostatic field intensity through a particulate material with a resistance of 10 ohm-cm is only 7.5 kV / cm, which corresponds to a field intensity that is less than the dielectric strength of 10 kV / cm of most particulate materials is high resistance. Of course there would be a substantial voltage drop within the electrode gap to maintain the corona discharge so that the intensity of the electrostatic field in the particulate material would be somewhat less.

The electrical properties of the resistive material are determined as a function of the design of the electrostatic device and the electrical properties of the particulate material. The volume resistivity of the resistor material is initially determined. It was found experimentally that the resistance for a high-intensity charging or

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Ionization stage with a current flow greater than 1o A / cm would have to be greater than 1o Ohm-cm to avoid arcing and

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To prevent secondary corona formation. The maximum permissible resistance is selected so that the dielectric Strength of the material is not exceeded when current flows through the ionization device. Thus, for example for a relatively high-intensity ionizing device

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with a current flow of 2 χ 1o A / cm and a breakdown voltage of 100 kV, the maximum resistance according to formula 1 is calculated as 5 χ 10 ° ohm-cm. After the resistance of the resistive material is selected to be somewhere between the maximum and minimum values, the thickness of the coating is calculated to maintain a voltage drop across the resistive coating of less than 15%, and preferably about 5% of the applied voltage . If, in the example given, 2 χ 10 ohm-cm is selected as the resistance of the resistance coating and a voltage of 75 kV is applied between the anode and cathode of the ionization device, the thickness of the resistance coating can be calculated using formula 2 as approximately 0.09 cm. It should be noted, however, that the resistance of the coating is inversely proportional to the temperature, so that temperature fluctuations must also be taken into account when selecting a resistance value. It has also been found experimentally that a resistive coating thickness of about 0.025 cm is sufficient to prevent corona formation; theoretical analytical investigations suggest that a thickness of 0.1 cm should be acceptable. The theory predicts that the value of the minimum thickness of the resistor material is approximately 4 times the diameter of the discharge terminating on the resistor surface if it has not had a chance to concentrate.

The technique described above can also be used for relatively low intensity electrostatic devices .

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the. For example, for a device with a

—7 2

Current flow of 10 A / cm and an interelectrode voltage of 5o kV a resistive coating between

6 12
1o and 1o ohm-cm required. The thickness of the coating

—2
tion can then be calculated as 5x1ο cm. Thus, it can be seen that the resistive material coating the anode of a low intensity electrostatic device can be of higher resistance. For high intensity devices, the resistance is generally between 10 ohm-cm and 5 Io ohm-cm while the coatings are thinner than 0.5 cm. For a relatively low intensity device, generally

7 12

a resistance between 1o and 1o ohm-cm and a relative thicker coating of resistor material used. In a device with lower intensity,

—7 —9 2 which has a current flow between 10 and 10 A / cn, a material can be used whose resistance of 10 ohm-cm can be estimated as low.

So far, the resistive coating of the present invention has been described as a technique for preventing secondary corona and arcing; however, this technique also enables the intensity of the electrostatic field in a given electrostatic device to be increased without creating spurious corona and arcing effects. From Fig. 7 it can be seen that the problems with arcing and secondary corona formation are reduced according to the invention by applying a layer of resistance material 85 to the inner surface of the anode 27 in the area adjacent to the planar electrode 50, / aas ¹ aa prevailing, by dashed lines Lines 87 reproduced electrostatic field is concentrated

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The physical and electrical properties of the Resistance material 85 are calculated according to the procedure given above.

The simple annular band shown in FIG. 7 as resistive layer 85 represents only one of several possible configurations. For example, an anode 90 is shown in FIG. 9, the inlet and outlet wall sections 91, 92, which are made of an electrically conductive material, a plurality of conductive Has anode segments 93, which are also made of a material with good electrical conductivity, and has electrically insulating spacers 94, which between adjacent conductive elements 91-93 to form a between acting electrical resistance are placed in between. A layer of resistive material 85 is on the inner surface of each of the anode segments 93 is provided. Each of the conductive segments 93 can also be associated with a suitable one Connector that connects to independent high voltage supplies not shown can be in order to enable shaping of the electric field curve by regulating the individual voltage supplies.

Fig. 10 shows another embodiment of the invention, in which the anode segments 93 are mutually spaced so that between for a purpose similar to that in FIG In connection with FIGS. 4 and 5, air passages are formed; each anode segment 93 is provided with a layer of resistance material 85 on its inner surface.

In FLg. 11 is another embodiment of the invention

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in the sense of Fig. 5 reproduced, in which each of the individual conical segment wing 53 along its inner surface with a layer of resistance material Is provided.

According to the invention, the collection effectiveness of two-stage electrostatic precipitators that use other types of ionization to charge particles, and the collection efficiency of single-stage separators can be improved just as well.

In Fig. 12, a conventional electrostatic device of the wire / cylinder geometry of relatively low intensity is shown, which includes a discharge wire electrode 100 suspended from a bushing insulator 102 which is attached to a separator cup 1o4. The discharge electrode 1oo is arranged concentrically with a tubular passive electrode 1o6, which also forms a line for the gases loaded with particles. A weight 108 is suspended from the discharge electrode in order to keep the position of the electrode 100 constant when the gases flow through the passive electrode 106. ^{A transformer / rectifier system of conventional design r} T 1o is connected between the discharge electrode 1oo and the passive electrode 1o6. In operation, the particle-laden gas enters the passive electrode 106 through an inlet line 112 and exits through an outlet line 114 after it has fully passed through the electrostatic field existing between the discharge electrode 100 and the passive electrode 106 . Depending on such physical and electrical design variables as electrode size, field intensity and gas flow rate, the electrostatic device can either be used as the charging

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stage or as the collection stage of a two-stage electrostatic precipitator. The device
can also be used as a single-stage electrostatic precipitator. The voltage between the discharge electrode 1oo and the passive electrode 1o6 can thereby
that the inner surface of the passive electrode 1o6 with
a resistor material calculated according to the above-mentioned technique is coated, over a / value: ninaus
can be increased without going over this
Worth a. Secondary corona formation and arcing are caused. It follows that, according to the invention, the capacity and / or the charging efficiency of electrostatic precipitators with wire / cylinder devices according to FIG. 12 can be improved to a great extent.

A conventional wire / plate geometry electrostatic device is shown in FIG. Such conventional wire / plate devices use a plurality of spaced-apart discharge wire electrodes 12o which are suspended from a conductive bus bar 122 and carry corresponding stabilizing weights 124. The discharge electrodes 12o are disposed between parallel plates 126, generally equipped with deflectors 128, which extend along the plates 126 transverse to the direction of gas flow through the ionizer. A relatively high voltage is maintained between discharge electrodes 12o and plates 126 by conventional transformer / rectifier equipment, not shown. As in the case with the conventional wire-cylinder arrangement of FIG
the collection efficiency and / or capacity of electrostatic precipitators when using conventional wire /

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Plate devices are greatly enlarged by covering plates 126 with a layer of resistive material are coated, their electrical and physical properties according to those described above Procedure.

14 is one of the ionizing devices shown in FIG somewhat similar high intensity ionization device reproduced with a resistor anode. The ionization device uses a planar discharge electrode 13o, which is arranged at the end of a support element 28, which the discharge electrode 13o coaxially positioned to a glass tube 134. The outer surface of the glass tube 134 is adjacent to the discharge electrode 13o coated with a conductive material 136. Between the discharge electrode 13o and the metal coating 136 is through a conventional transformer / rectifier 138 which is connected to the discharge electrode 13o via a Conductor 132 is connected in the support element 28,

a relatively high voltage is applied. The metal layer 136 forms the anode of the ionization device; the physical and electrical properties of the glass tube are selected so that the tube 134 has a resistive coating for the metal layer 136.

A variety of resistive materials can be used to manufacture resistive anodes in accordance with the spirit of the invention. The resistor material may comprise epoxy resin having the required volume resistivity and dielectric strength. In order to obtain appropriate resistivities , aluminum oxide may be required, mixed with a suitable doping oxide

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and / or metal is provided. However, presently known epoxy resins deteriorate in a corona environment; and they may not be sufficiently resistant to abrasive wear as that they could be used to advantage. These materials include:

I ORGANIC MATERIALS

a. STYCAST 2762 IF epoxy material used by Emerson Cumings Stycast is delivered; the resistors are for ionizers suitable for low intensity.

b. STYCAST 2762 epoxy material supplied by Emerson Cummings Stycast; the resistances are suitable for low intensity ionization devices. The material can pass through on the anodes in place a molding process can be applied.

c. Type C-26 epoxy supplied by Emerson Cumings; the resistors are both suitable for high-intensity as well as low-intensity ionization devices. The material can be thin on the anodes Coatings are applied by spraying or painting.

II. INORGANIC MATERIALS

a. a Type LA-2-500 alumina coating supplied by Union Carbide; the resistances are higher or higher for ionization devices

809843/0677 -3i-

Low intensity suitable for temperatures above 288 ° C (55o ° F). The volume resistivity is 10 ohm-cm at 149 ° C (3oo ° F) and 1o ¹ ° ohm-cm at 288 ° C (55o ° F). The coating is applied with a specially developed plasma gun. Since the material was developed as a wear-resistant coating, its resistance to abrasion is extremely good.

b. Porcelain-lined steel supplied by Erie Ceramic, Inc.; the volume resistance is in

12 11

Range between 1o ohm-cm and 2 χ Io ohm-cm 149 ° C (300 ° F). The values for the thickness are in the range between 0.03 cm and 0.05 cm.

c. Pyrex tube 11Ao supplied by Corning Glass Company; the resistors are suitable for both high and low intensity ionizers since the resistance is about 10 ° at 149 ° C (300 ° F). The material is available in 1/4 inch to 1/8 inch thick tubing with an inside diameter of between 6 inches and 18 inches. This material can advantageously be used in the embodiment shown in FIG.

d. Pyrokeram supplied by Corning Glass Company:

1. Type 96o6 - the resistors are suitable for low intensity ionizers or for high intensity ionizers at temperatures above 26o C (500 ° F). ^{The resistance value is 5 χ 1o 1} at 288 ° C (55o ° F)
Ohm - cm.

5 χ 1o ¹ ° ohm-cm and at 149 ° C (3oo ° F) about 5 χ 1o ¹¹

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2. Type 96ο8; Resistors suitable for both high intensity and low intensity ionizers as the resistance at 149 ° C (300 ° F) is 3 χ 1o ⁹ ohm-cm.

e. Soda-lime glass supplied by the Corning Glass Company. The volume resistance is at 149 ° C (3oo ° F)

2 χ Io ohm-cm.

f.VYCOR glass supplied by the Corning Glass Company. The resistors are suitable for low intensity and high intensity ionization devices at very high temperatures. At 149 ° C (300 ° F) the resistance is 1o ¹² ohm-cm.

The resistor anode according to the invention can thus be used in a Variety of ionization devices to improve the capacity and / or the charging efficiency of two-stage electrostatic precipitators can be used.

As the resistance anode has been described so far, it formed part of an electrostatic precipitator, to remove fly ash from coal-fired power plants. However, the resistance anode can also be used in other areas of application including electrostatic devices that are outside the power generation area, and used just as well in electrostatic precipitators in power plants that run on fossil fuels,

for example, oil and mixtures of coal types high and low sulfur content.

According to the invention, a resistance anode for electrical

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static deposition processes created either in the Charging stage or in the collecting stage of a two-stage electrostatic separator or in a single-stage electrostatic precipitator can be used. The resistance anode suppresses the formation of secondary corona and prevents the occurrence of arcing caused by triggered a dielectric breakdown of layers of particulate matter normally deposited on the anode will. The resistance anode is formed by a conductive electrode covered with a layer of resistance material high dielectric strength, predetermined thickness and predetermined resistance is coated. One Resistive anode of this type can be used in a variety of electrode types, including conventional Wire / plate and wire / cylinder configurations are included, and as well in high intensity ionization devices be used, which use a planar discharge electrode, which is concentric in a tubular anode is arranged.

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e e rs e 11 e

Claims (1)

MANITZ. FINSTERWALD & GRÄMKOW Air Pollution Systems, Inc. P / 4 / Hi-A 3159 1114 Andover Park West, Tukwila,
Washington 98188, USA Electrostatic precipitator Patent claims: 1. \ Electrostatic precipitator for particulate matter with a discharge electrode, with a passive electrode spaced from the discharge electrode by an electrode gap is, and with a power supply connected between the discharge electrode and the passive electrode for applying Voltage between the discharge electrode and the passive electrode, characterized in that the applied The voltage is high enough to build up an electrostatic field between the discharge electrode and the passive electrode, its between the discharge electrode and the passive elec- —9 2 trode prevailing current flow is greater than 10 A / cm, and that a resistor material is provided on the passive electrode to prevent secondary corona and spark flashover, which comprises a material layer arranged on the passive electrode between the discharge electrode and the passive electrode, of their volume resistance under operating temperatures and under / in Resistance material dominating% T ^ rral ^ ra ^ ke a value between 6 13 about 10 ohm-cm and about 10 ohm-cra. DR. C. MANITZ · DIfU-ING. M. FINSTERWALD DIPL.-INC, W. O R A Mt * 6 W ZtNTRALKASSI SAYEK. FOLK BANKS β MÖNCHEN 33. ROtERT-KOCH-STRASSE I 7 STUTTGART SO «« AD CANNSTATtV- MÖNCHEN. ACCOUNT NUMBER 7370 TEL. Ι0β9> 39 43 11. TELEX 05-99673 PATMF SEEL1ERCSTR.3S / 3S. TEL.I07IDS6 73 61 POSTSCHECK · MÖNCHEN 77063 - 805 OFUGINÄtSNSPEC X € D 2. Separator according to claim 1, characterized in that the temperature of the particulate material within the electrode gap is between 82 ° C (180 ⁰ F) and 399 ° C (750 ° F). 3. Separator according to claim 2, characterized in that the thickness of the material is greater than 0.01 cm and less than
15% of the ratio of applied voltage to that in the material
prevailing field strength. 4. Separator according to claim 2, characterized in that the thickness of the resistance material exceeds four times the diameter of a non-concentrated discharge which is on
the surface of the material ends. 5. A separator according to claim 2, characterized in that the volume resistance of the material changes when a current flows at the passive electrode of more than 10 A / cm in the area ο Λ Ci moved from about 10 ohm-cm to about 10 ohm-cm. 5. A separator according to claim 2, characterized in that the volume resistance of the material for a current flow to -9 2 of the passive electrode in the range of about 10 A / cm to about 10 A / cm is in the range of about 10 ohm-cm to about 10 ohm-cm. '. Separator according to Claim 2, characterized in that the resistance material is an organic compound, the electrical strength of which is greater than 50 kV / cm. ! Separator according to Claim 2, characterized in that the resistance material is an inorganic compound, the dielectric strength of which is greater than 80 kV / cm. 609843/0577 9. Separator according to claim 1, characterized in that that the passive electrode has a plurality of mutually spaced, electrically conductive, electrically from each other contains isolated sections, and that the resistance material rl the surface of each section, which is the discharge electrode facing, covered. 10. A separator according to claim 9, characterized in that a plurality of insulating spacers between the electrically conductive sections are arranged. 11. Separator according to claim 9, characterized in that that the sections are spaced from one another so that a plurality of intermediate fluid passages between the sections are present. 809843/0577