CA1101789A - Resistive anode for electrostatic precipitation - Google Patents
Resistive anode for electrostatic precipitationInfo
- Publication number
- CA1101789A CA1101789A CA291,388A CA291388A CA1101789A CA 1101789 A CA1101789 A CA 1101789A CA 291388 A CA291388 A CA 291388A CA 1101789 A CA1101789 A CA 1101789A
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- Canada
- Prior art keywords
- electrode
- anode
- resistive
- corona
- ohm
- Prior art date
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
- B03C3/34—Constructional details or accessories or operation thereof
- B03C3/40—Electrode constructions
- B03C3/41—Ionising-electrodes
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
- B03C3/34—Constructional details or accessories or operation thereof
- B03C3/38—Particle charging or ionising stations, e.g. using electric discharge, radioactive radiation or flames
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- Electrostatic Separation (AREA)
- Elimination Of Static Electricity (AREA)
Abstract
TITLE
RESISTIVE ANODE FOR ELECTROSTATIC PRECIPITATION
ABSTRACT OF THE DISCLOSURE
A resistive anode for electrostatic precipitation uti-lized either in the charging stage or collecting stage of a two-stage electrostatic precipitator, or in a single stage electro-static precipitator. The resistive anode suppresses hack corona and prevents sparkover being produced by dielectric breakdown of particle layers which normally build up on the anode. The resis-tive anode is formed by a conductive electrode coated with a layer of resistive material having a high dielectric strength and a predetermined thickness and resistivity A resistive anode of this construction may be employed in a variety of electrode designs including conventional wire-plate and wire-cylinder configuration, as well as in high intensity ionizers utilizing a planar discharge electrode concentrically mounted in a tubular anode.
RESISTIVE ANODE FOR ELECTROSTATIC PRECIPITATION
ABSTRACT OF THE DISCLOSURE
A resistive anode for electrostatic precipitation uti-lized either in the charging stage or collecting stage of a two-stage electrostatic precipitator, or in a single stage electro-static precipitator. The resistive anode suppresses hack corona and prevents sparkover being produced by dielectric breakdown of particle layers which normally build up on the anode. The resis-tive anode is formed by a conductive electrode coated with a layer of resistive material having a high dielectric strength and a predetermined thickness and resistivity A resistive anode of this construction may be employed in a variety of electrode designs including conventional wire-plate and wire-cylinder configuration, as well as in high intensity ionizers utilizing a planar discharge electrode concentrically mounted in a tubular anode.
Description
7~,9 This invention relates to electrostatic precipitation for removing particulate matter entrained in contaminated gas streams. ~lore specifically, the invention is directed to a re-sistive coating for a passive anode to prevent back corona and sparkover.
Standards for ~missions of particulate in flue gases issuing from coal fired electrical power station stacks are be-coming increasingly rtore stringent. Current air quality stand-ards require that more than 99% of the fly ash produced by burn-ing coal be removed prior to discharge of the combustion gases from the stack. Thus, the ef~iciency of particulate collection must increase in proportion to the ash content of the coal. In addition, in an effort to reduce the emissions of certain gaseous pollutants, particularly the sulphur oxides, it has become increas-i~gly necessary to use low sulphur coal in electrical power gener-ating plants.
The electrostatic precipitator is the most cor~only used device for the removal of particulate matter produced by coal fired power plants. In a two-stage electrostatic precipita-tor the particulate-laden gas sequentially passes through separate charging and collecting stages. In ~he chargin~ stage the gases pass through a corona discharge so that the particulate matter leaving the charger has a positive or negative charye. The charged particles then pass through a low intensity corona electric field in the collecting stage which causes the particles to mi~rate toward a collecting electrode where they agglor~lerate an~ are sub- ~
sequently removed and collected by various techniques. In a single-stage precipitator particies flowing between a pair of electrodes ~, .
--1-- .
` 11~17X9 haviny a corona current proclucing electrostatic field e~tending therebetween are first charged and then migrate toward one of the electrodes where they a~glomerate and are subsequentl~ removed.
Thus, in a single-sta~e precipitator both the charging stage and the collectin9 stage are combined into a single unit. The efficien-cy of an electrostatic precipitator is deterrnined to a large extent by the maynitude of the charge placed on the particulate matter by the charging staye. The charge maynitude may be increased by increasing the intensity of the electrostatic field producing !O tlle corona discharge- The r~a~-imum intensity of the electrostatic field is limited to a value at which sparkover and back corona occurs as the particulate matter builds up on the passive or non-corona emitting electrode. Althouyh back corona effects can be reduced to some extent by such techniques as limiting the thick-ness of the particulate layer on the passive electrode, achievable electrostatic field intensities nevertheless provide somewhat limited particle charging. Thereafter, the collection efficiency must be improved by increasing the residence time of the partic-ulate-matter in the electric field during collection either by O reducing the speed at which the particulate-laden yases pass through the collection stage, or by increasing the length of the collection stage. However, a decrease in transit speed through the collection stage reduces the capacity of the collection stage, and increasing the size of the collecting electrodes increases the capital cost of such equipment. The intensity of the elec-trostatic field at which the charger can operate without bac~
corona and sparkover is lower for higher resistivity particulate matter. Since fly ash resistivity is inversely related to the level of combustible sulphur in coal, the increasing use of low O sulphur coals increases the cost of achieving a hiyh collection efficiency since back corona and sparkover probler.~s are increased.
Attempts have been made to reduce the incidence of back .7139 corona and spar~over in order to increase the intensity of elec-trostatic fields in ionizers throuyh a number of techniques none of which are entirely satisfactory. Earliest attempts, as de-scribed by ~. J. ~hite, Industrial ~lectrostatic Precipitation at 32~, Addison-i~esley 1963, were directed to treating the partic-ulate matter before entering the ionizer. ~igh resistivity par-ticulate matter was generally treated ~y moisture and acid con-ditioning. Other techniques atternpted to prevent the buildup of a layer of particulate material on the passive electrode such as by employing moving belt electrodes, rotating brushes and various other mechanical devices. These later tecnniques generally failed since even thin films of particulate matter can produce severe back corona effects if the resistivity of the particulate matter is sufficiently high. However, particulate matter buildup has been successfully prevented to some extent by continuously flushing the passive electrode with a water film. Still another approach attempts to adjust the temperature of the ~lectrodes upwardly and downwardly in order to shift the temperature of the particulate matter toward a lower resistivity value. However, this technique generally requires a large amount of power to produce the required temperature shifts.
Previous attempts to adjust the electrical characteristics of the passive electrode in order to reduce back corona and spark-over have generally inserted a non-critical value of current lim-iting resistance in series with the discharge electrode. The resistance has the effect of simultaneously limiting the current that can flow and lowering the intensity of the electrostatic field across the electrode gap in times of abnormal transient conditions occurring at the onset of sparkover. This approach is not considered feasible since effective use of this technique requires a great deal of power. A form of current limiting resis-1~17~9 tance called a "graded resistance" is described in H.
J. White, Resistivity Problems in Electrostatic Precipitation, Journal of the Air Pollution Control Association, 23, 336-37 (1974). In accordance with this technique a thick, flat semi-conducting plate electrode made of steel reinforced concrete was used. The results of this approach were never definitive as to the specific volume resistivity at and close to the anode surface which would allow the technique to be utilized with a variety of ionizer designs. Further-more, the article does not indicate the allowable range for such critical parameters as material resistivity, material thickness or dielectric strength.
Recently, high intensity ionizers have been developed in which a unique electrode geometry produces a -stable, high intensity corona discharge through which the particulate-laden gas passes. These high intensity ionizers charge the particulate matter to a much higher level than is achievable with conventional ionizers utilizing, for example, wire-cylinder of wire-plate geometries. Although the collection efficiency of two-stage electrostatic precipita-tors can be greatly improved by employing this unique highintensity ionizer was a charging stage, back corona and sparkover has nevertheless been a problem, particularly with very high resistivity particulate matter, as the particulate matter builds upon on a metal passive electrode.
This invention seeks to prevent back corona and sparkover in an electrostatic precipitator by coating the anode with a layer of resistive material. More specifically ; the invention is an apparatus having a discharge electrode, a passive electrode spaced apart from said discharge elec-7~3~
trode by an electrode gap, power supply means connectedbetween said discharge and passive electrodes for applying a voltage therebetween, said applied voltage being of suffi-cient magnitude to effect a corona current producing electro-static field between said discharge and passive electrodes having a current flux greater than 10 9 A/cm2 at said passive electrode, a resistive material for preventing back corona and sparkover at said passive electrode comprising a layer of material on said passive electrode between said discharge electrode and said passive electrode, said material having a volume resistivity in the range from about 106 ohm-cm to about 103 ohm-cm under the operating temperature and electric field intensity in the resistive material.
The passive electrode may be, for example, the anode of an electrostatic device such as the charging or collecting stage of a two-stage electrostatic precipitator, or single-stage electrostatic precipitator. The electrode is coated with the resistive material in order to increase the intensity of the device's electrostatic field at which the electrostatic device can operate without back corona and sparkover. The material preferably has a resistivity less than the ratio of the dielectric strength of the material to the current flux passing through the material in order to prevent the field within the material from exceeding its dielectric strength which is preferably greater than 80 kv/cm. The thickness of the anode coating should exceed .01 cm. at a~mospheric pressure and 300F. in order to prevent ~-puncturing of the material and resultant back corona and sparkover. The maximum thickness of the material is selected , .:
so that the voltage drop across the material is less than 15%, and preferably between 5% and 10%, of the applied voltage. The resistive material resists deterioration in a corona environment, and is resistant to abrasion especially where abrasive particulate matter is being charged.
The invention is illustrated, by example only, in the drawings, in which:
Figure 1 is a schematic side elevational view illustrating a multi-stage precipitator employing a charging ionizer having a resistive anode of the present invention;
Figure 2 is an enlarged side view of one ionizer stage of the apparatus of Fig. 1 partially broken away to show the ionizer array;
Figure 3 is an end elevational view of the ionizer stage of Figure 2 with the inlet partially broken away to show the ionizer array;
Figure 4 on the first sheet of the drawings, is an enlarged partial sectional view of a single ionizer venturi illustrating the electrode arrangement;
Figure 5 on the first sheet of the drawings, is a further enlarged partial sectional view of the electrodes of Figure 4 showing the electrode construction in greater detail;
Figure 6 is a schematic system diagram showing the control elements for an ionizer stage;
Figure 7 is a schematic diagram illustrating the operation of an ionizer having a layer of resistive material coating the anode;
Figure 8 is an enlarged partial sectional view of a portion of the anode of Figure 7;
7~39 Figures 9-11 illustrate alternative embodiments of the invention;
Figure 12 is a broken isometric view illustrating an ionizer of the wire-cylinder geometry having a resistive coated anode;
Figure 13 is a broken isometric view illustrating an ionizer of the wire-plate geometry having a resistive coated anode; and Figure 14 is an isometric view showing another embodiment of a high intensity ionizer having a resistive coated anode.
In the drawings, Figure 1 shows, in schematic side elevational view, a two-stage electrostatic precipitator system incorporating the invention. As seen in this Figure, the precipitator system includes a gas inlet 11 into which gases to be cleaned are directed as indicated by arrow 12, a gas outlet 13 -6a-7t39 from which cleaned gases are supplied to appropriate downstream apparatus, e.g. an atmospheric dischar~e duct, as indicated by arrow 14, and a cascaded pair of ionizer-precipitator units yen-erall~ designated by reference numerals 15,15'. ~ach ionizer-precipitator unit 15,15' includes an ionizer stage 16 (16') and a pair of conventional electrostatic precipitators 17, 18, (17', . Iach ionizer stage 16, 16' and precipitator stage 17, 17', 18, 1~' is provided with a high voltage input cable connector 19 coupled to a suitable source of high voltage as described more fully below with reference to Fig. 6, and a collecting bin portion 20 for collecting particulate matter precipitated from the gas as the latter flows through units 15, lr~ I .
In operation, gases containing particulate matter enter the Fig. 1 apparatus via inlet 11 and pass through the first ionizer stage 16 in which the particles in the gas are electro-statically charged. The gas bearing the electrostatically charged particles next flows into successive precipitator stages 17, 18 in each of which the charged particles are deflected out of the flow path of the gas under the influence of an electrical field established across the flow path, the particles being deposited in the bin portions 20 of the precipitator stages 17, 18. The gas exiting from precipitator 18 is passed through ionizer stage 16', and precipitator stages 17', 18', to provide additional clean-ing therefor, and the cleansed gases emerging from precipitator stage 18' are conducted via gas outlet 13 to appropriate down-stream apparatus.
Figs. 2 and 3 illustrate the gas inlet 11 and the first ionizer stage 16 with more particularity. ~s seen in these Figs., gas inlet 11 comprises a hollow conduit of trapezoidal or other ~0 suitable geometric configuration which is coupled at the downstream side to a gas distributor portion 22. ~istributor portion 22 is coupled to an entry chan~er 23 formed within the housing of Y~
ioni~ing unit 16 by the side and bottom walls tnereof and a v~rti-cally arrang~d bulk}lead 2~. Bul~}-~ead 24 an~ a second vertically arrange~ bulkhead 25 define with the side, top and bottom walls of ionizer stage 1~ a pressure manifold ~6 for a purpose to be described.
Positioned within ionizer stage 16 in a regular array are a plurality of venturi diffusers 27 and associated central electrode support members 28 each projecting into the upstream end of the associated venturi 27 and substan'ially coaxial there-with. Each rnember 28 is coupled to a bus bar network generally designated by reference numeral 29 and consisting of three ver-tically arranged parallel bus bars interconnected at the upper ends thereof by a common bus bar element 31, the element 31 being connected to a single bus bar element 32 extending from the inter-ior of ionizer stage 16 to an external conventional high voltage connector shroud 33 to which a high voltage is supplied from a suitable power source ~not shown) via high voltage connector 34.
The downstrean end or outlet of each venturi 27 is coupled to an exit chamber 36 which is-in turn coupled to the inlet of elec-trostatic precipitator stage 17.
Storage bin 20 is provided with a removable door 40 for purposes of inspection and cleaning, and a vibrator bracket 41 for permitting the use of an optional conventional vibrator to assist in settling any particulate matter collecting in bin 20 towards the bottom edye 42 thereof. Bottom edge 42 is provided with suitable apertures (not shown) for enabling the particulate matter to be removed from the bin 20 in a conventional manner.
Bins 20 of the remaining system eler~lents lb', 17, 17', 18, and 18' are configured in a substantially i~entical nlanner.
Each venturi element 27 and associated cozY~ial member 28 generally comprises an electrode pair for generating a high intensity electrostatic field across the path of gas flow through -, l~q~78~
the ionizer staye 1~. For this purpose, an electrode (described below) is carried by each member 28 and is coupled to a source of relatively high negative potential, via bus bar network 29 while each venturi conduit 27 is coupled via the framework of the struc-ture to ground potential. Thus each venturi 27 serves an anode and eac~l me~ber 2~ serves as a catho~e support.
In operation, with the high voltage applied between the cathode and anode, particles suspended in any gas flowing through the ioni~er stage 16 are electrostatically charged when passing through the throat of venturi 27. In order to ensure that sub-stantially all charged particles remain suspended in the flowing yas until arriving at the downstream precipitator 17 or 18, and do not adhere to the ground potential anode surface, the electrode configuration shown in Figs. 4 and 5 may be employed.
With reference to Fig. 4, each venturi element 27 is formed with an inwardly tapering conical inlet section 45, a gen-erally cylindrical central section or throat 46 and an outwardly tapering conical ou~let portion 47. The cathode includes a planar electrode such as a disc 50 which may have a curved peripheral edge which projects outwardly from the outer surface of member 28. ~isc 50 is mounted substantially coaxially in the throat of venturi 27 and provides a highly constricted high intensity elec-trostatic field in the form of a corona discharge between the curved periphery of disc 50 and the surroundiny anode surface 52 when a high potential is applied.
As best shown in Fig. 5, anode surface 52 comprises a series of flanged conical vanes 53 structurally connected in a nested arrange~ent to a mounting mer.~er 54 by spacers 54a and closely spaced along the a~is of venturi 27 by spacers 54a to cefine air passages 5~ between adjacent vanes. Vanes 53 effectively form a cylindrical anode wall with a slightly sloped interrupted surface~ The inner surfaces of the vanes 53 are provided with a 17~9 resistive coating as described llereinafter in ronnection with Fig. 11. Anode surface 52 is surrounded by plenum chamber 26 to which clean air under pressure is supplied from an external sourcs by a pump as ~escribed ~elow in connection with Fig. ~.
In operation, clean air is injected into venturi throat 46 via air passages 55 which effectively form a plurality of annu-lar nozzles and which are oriented to direct circum~erential jets of clean yas along the inner anode surface of venturi 27 in essentially the same direction as the main stream of contaminated gaS passing through venturi 27. The clean gas injected via pass-ages 55 flows along the ano~e surface in a substantially laminar film and provides an effective fluid barrier which also functions to entrain and aid the flow of the main gas stream. This has -' been found to significantly reduce the deposition of charged particulate matter on the anode surfaces as compared with known prior art devices. In addition, the orientation of the clean gas injection nozzles 55 reduces the pressure loss normally associated with the passage of gases through a venturi diffuser not provided with such noz~les. Also, as mentioned above, ~ack corona prob-lems encountered with prior art venturi ionizers can be substan-tially reduced by carefully contouring the edges of vanes 53.
~ig. 6 sch~natically illustrates the electrical~power connections and clean gas injection control system of ionizer ~-stage 16. Hiyh voltaye is supplied to catho~e bus network 29 via hi~h ~oltage cable 3~ from a transformer rectifier set 70 coupled to a control unit 71, both latter ele~ents being of conventional design. ~lean gas is supplied to manifold chamber 26 from a blower 73 via a ~ea~er 74,conduit 75, a controlled,damper 76 and , ' a conduit 77. ~eater 74 is connected to a temperature controller unit 78 for maintaLning ~he temp~rature of the clean gas supplied to manifold chamber 26 within a desired temperàture range. A
aifferential pressure sensor 79 having a pair'of'pressure trans--7~39 ducers ~0, ~1 provide a feedbac}; signal to controlled ~amper 76 in order to provide pressure regulation for the clean air within manifold chamber 26. Elements 73-~1 are all conventional units, the structure of which is well within the ordinary skill of the art.
As noted above, a major problem encountered with elec-trostatic devices, particularly when employed to charge particu-late matter of high resistivity such as fly ash from coal fired boilers using low sulphur coal as a fuel, has been the incidence of sparkover and ~ack corona which is generally the limiting factor for increasing the intensity of the electrostatic field.
Bac~ corona and sparkover occurs when the intensity of the elec-trostatic field within the particulate matter on tl-e passive or noncurrent emitting electrode exceeds the dielectric strength of the particulate material. For example, the dielectric strength of fly ash produced by burning low sulphur coal is generally be-tween 1 ~v/cm. and 10 kv/cm. When the dielectric strength is exceeded a small hole or crater is formed. Since the corona current tends to follow the path of least resistance the corona current concentrates at the point of dielectric breakdown pro-ducing a localized area of extremely high current flux. When this occurs the high ne~ative ion concentration produces a strong negative rield which accelerates free electrons toward the punc-ture. The free electrons collide with gas molecules at a relatively ; high rate of speed knoc~ing off electrons from the molecule accord-ing to the "avalanche process" there~y transforming the molecules into positive ions. The positive ions arF then accelerated toward the cathode to produce positive corona adjacent the anode.
Electrons removed from the gas molecules during the avalanche process are accelerated toward the anode thereby increasing the negative field and increasing the production of eIectrons and positive ions. The result is a positive feedback effect which ' .
7~9 continues until the voltage bet~een the anode an~ cathode is greatly reduced. ~uring this process, electron difusion tends to increase the ~iameter of the discharge, while the circular mag-net~c field produced ~y the electron flow i~ tlle ~ischarge tend3 to compress the ~i~leter of the discharge. As the pressure of the gas increases the total corona current decreases since the increased concentration of electrons causes them to strike each other with greater frequency thereby reducing electron mobility.
However, the increased electron concentration also increases the self magnetic field, and this effect predominates so that the local current density or current flux is increased with increas-ing gas pressure.
The fields through the layer of particulate matter is given by the formula:
~ = 3~ OR~IULA 1) where J is the current flux or current density through ~he material, and _,~ is the volume resistivity of the material.
The current flux J for ionizers is generally on the order of 10 8 A~cm and 2 X 10 6 A/cm. Consequently, for particulate matter hav-ing a dielectric strength of 10 kv/cm back corona an~ sparking arenot a problem until volume resistivities exceed between 5 X 109 ohm-cm and 10 ohm-cr.l.
Back corona and spar~over interfere with the operation of the ionizer since the strong negative field adjacent the anode greatly reduces the intensity of the field in the interelectrode region and the positive ions discharge th~ negatively charged particulates thereby defeating the purpose of the charging stage.
In accordance with the present invention, the passive electrode of an electrostatic device is coated with a resistive material having a high dielectric strength. The term~"electro-stati~ device" as used herein refers to either the charging stage or the collecfing stage of a two-stage electrostatic precipitator, or a single-stage electrostatic precipitator employing a unitary ~ -12-char~in~ and collccting stage. The passive electro~e is gen~rally an ano~e since the effects of bac~ corona an~ sparl;over are more serious with negative corona in which the cathode is the corona emitting electrode. In the n~gative corona, most of the current is carried by negative ions which originate from electrons lib-erated from the cathode or discharge electrode surface by pos-itive ion bombardment. The positive ions in turn are generated in the high field region near the cathode by electron ionization of the gas molecules. In the positive corona, on tlle other hand, the current is carried primarily by positive ions which oriyinate from electron ionization of the gas în the high field region near the anode or discharge electrode.The electrons in ~is case are produced by photoelectric ionization of the ~as molecules in the region between the high field zone and the ground electrodes.
These differences in ionization processes have a large influence on the spark brea~down voltages of the negative and positive coronas. As mentioned above, spark breakdown for corona is aue to the formation of self-propagating strearders or current flow wh~ch originate from the anode. For the positive corona, high electric fields exist near the discharge electrode even a$ rel-atively low voltages, so that the spark breakdown streamer trig-gered ~y the high fields forms at lower voltages also. This ~eans that the operating voltages of electrostatic devices using pos-itive corona are limited to relatively low values, even for low resistivity particulate matter. However, for negative corona the electric field is relatively low near the anode so that much higher voltages can be applied before the fields build up to the point where spark breakdown occurs. In practicey spar~over voltages for negative corona are found o be about double those for pos-itive corona. ~he use of positive corona to improve performanceunder high resistivity conditions is of no advantage~ however, because of the inherently low sparkover voltages of positive . :
corona. These differences between negative and positive corona lead to different e~fects for back corona in the two cases. The main difference is that bac}; cGrona has little effect on spark-over voltage for the positive corona since in this case the back corona does not trigger a spark as it does for neyative corona.
Thus, back corona, which has such a disruptive effect on the negative corona, has only a small effect on positive corona.
Consequently, the resistive material coating the passive elec-trode is generally more useful for negative corona than for positive corona. Thus the term "anode"is used herein synonymously with "passive electrode" and is intended to include the cathode - of a positive corona electrostatic device.
The resistive layer on the anode reduces back corona and sparkover by two distinct processes. First, current flowing through a resistive material will follow the path of least resis-tance. This means that a current flowing in the resistive mater-ial which would otherwise be in a relatively narrow path, and hence have a large local current flux, is unable to concentrate in this manner so that the current flux passing through the re-sistive material and the particulate matter is kept relativelylow. r~he result is that the current flowing between the discharge electrode and the passive electrode cannot concentrate to allow the discharge to convert from a relatively low current density, high electric field form, to the high current density, relatively low electric field form associated with sparkover. Since the current flux passing through the particulate matter is maintained at a relatively low value, the intensity of the field within the particulate matter is I~intained below its dielectric strength in accordance with FO~lUL~ 1 above. The second process by which the electrostatic field is stabilized by the resistive anode is that ; the resistive material reduces the voltage between the cathode ,, . - , ~, '. - ~
and the sur~ace of the resistive coating as current fluY. incre~s~s~
Tllat is, for an electrostatic device having an applied voltage of 75 kv and an average current flux of 10 6 A/cm, a 1 cm thick coating of resistive material having a volume resistivity of 10 ohm-cm has a voltage difference ~ V across the resistive coating of 10 ~v in accordance with the following formula:
V = J t (Fo~lULA 2) where t is the thic~ness of the resistive coating.
Clearly the maximum current flux which can flow through the resis-tive material is 7.5 Y 10 A/cm since at tnat current flux the voltage across the resistive coating would be 75 kv. ~ven at this maximum current flux of 7.5 X 10 6 A/cm, the intensity of the electrostatic field through a particulate matter having a resistivity of 101 ohm-cm is only i.5 kv/cm - a field intensity less than the 10 kv/cm dielectric strength of most high resistivity ~articulate matter. Of course, there would be a substantial vol-tage drop within the electrode gap in order for the corona dis-, charge to continue so that the intensits~ of the electrostatic I field in the particulate matter would be somewhat less.
The electrical properties of the resistive material are i deterr.lined as a function of the construction of the electrostatic device and the electrical properties of the particulate matter.
The volume resistivity of the resistive material is initially determined. It has been experimentally determined that the resis-tivity for a high intensity charger or ionizer having a current ~lux greater than 10 6 A/cm must be greater than 108 ohm-cm in order to prevent bac~ corona and sparl;over. The maximum allowable reslstivity is selected so that the dielectric strength of the material is not exceeded at the current flux of the ionizer.
Thus, for example, for a rclatively high intensity ionizer having a current flux of 2 X 10 A/cm2 and a breakdown voltage of 100 kv the maximum r~sistivity is calculated by formula 1 as 5 X 101 .
.
ohm-cm. After the resistivity of the resistive material is selected at some value between the maximum and miniMu~ values, the thick-ness of the coating is calculated to maintain a voltage drop across the resistive coatiny of less than 1577 ~ and preferably about 5%, of the applied voltage. Thus, in the above example if 2 X 101 ohm-cm is selected as the resistivity of the resistive coating and 75 ~v is applied between the anode and cathode of the ionizer, the thickness of the resistive coating is calculated by formula 2 as about .09 cm. It should be noted, howPver, that the resistivity of the coating is inversely proportional to temperature so that temperature fluctuations r~ust be accounted for when selecting a resistivity value. It has also been found e~perimentally that a resistive coating thickness of about .025 cm is sufficient to prevent corona, and theoretical analysis suggests that a thickness of 0.01 cm should be acceptable. The theory predicts the minirnum thickness of the resistive material is about 4 times the diame~er of the ~ischarye terminating on the resistive surface when it has not had an op~ortunity to concentrate.
The above described technique is also utilized for rela-tively low intensity electrostatic devices. For example, a device having a current flux of 10 7 A/cm, and an interelectrode voltage of 50 ~v would require a resistive coating between 106 and 1012 ohm-cm. The thickn~ss of the coating is then calculated to be 5 X 10 2 cm. It is thus seen that the resistive material coating the ano~e of a lower intensity electrostatic device may have a higher resistivity. For high intensity devices, the resistivity is generally between 10 ohm-cm and 5 X 10 ohm-cm, and the coatings less than 0.5 cm. A relatively low intensity device generally utilizes a resistivity between 10 and 10 ohm-cm and a relatively thicker coating of resistive l~terial. A lower intensity device having a current flux bet~een 10 and 10 9 A/cm may utilize a material having a resistivity as low as 10 ohm-cm.
The inventive resistive coating has been described as a technique for preventing back corona and sparkover, but it also allows the intensity of the electrostatic field in a given electrostatic device to be increased without producing back corona and sparkover. As illustrated in Fig. 7, the problems of sparkover and back corona are reduced according to the invention by providing a layer of resistive material 85 on the inner surface of the anode 27 in the region adjacent the planar electrode 50 in which the electrostatic field therebetween represented by dotted lines 87 is concen-trated. The physical and electrical properties of theresistive material 85 are calculated in accordance with the above described procedure.
The simple annular band shown in Fig. 7 for resistive layer 85 is only one of several possible configura-tions envisioned. For example, with reference to Fig. 9 ananode 90 is shown which comprises inlet and outlet wall sections 91, 92 fabricated from an electrically conductive material, a plurality of conductive anode segments 93 also fabricated from a good electrically conductive material and electrically insulative spacers 94 interposed between adjacent conductive elements 91-93 for providing electrical isolation therebetween. A layer of resistive material 85 is provided on the inner surface of each of the anode segments 93. Each of the conductive segments 93 can be also provided with a suitable terminal adapted to be coupled to indepen-dent high voltage supplies (not shown) in order to permit electrical field shaping by regulation of the individual voltage supplies.
3~ Fig. 10 shows an alternative embodiment of the .
3L~L5~:~7~9 invention in which anode segments 93 are mutually spaced to provide air passages therebetween for a similar purpose to that described above with reference to Figs. 4 and 5, with each anode segment 93 being provided with a layer of resis- :
tive material 85 on the inner surface thereof.
Fig. 11 illustrates still another embodiment of the -17a-7~9 invention ta~cn after Fi~. 5 in which the indiviclual conical seg-mental vanes 53 are each urovided with a layer of resistive mater-ial ~5 along the inner surface thereof.
l'he collection efficiency of two-stage electrostatic precipitators employing other ty~es of particle charging ionizers as well as single-stage precipitators may also be improved in accordance with this invention. With reference to Fig~ 12, a conventional, relatively low intensity electrostatic device of the wire-cylinder geometry includPs a wire discharge electrode 100 suspended from a feed-through insulator 102 secured to a precipitator shell 104. The discharge electrode 100 is concen-trically mounted with a tubular passive electrode 106 which also forms a duct for the particle-laden gases. A weight 108 is sus-pended from the discharge electrode 100 to maintain the position of the electrode 100 constant as gases flow through the passive electrode 106. ~ transformer rectifier set of conventional variety 110 is connected between the discharge electrode 100 and the passive electrode 106. In operation the particle-laden gas enters the passive electrode 106 through an inlet duct 112 and exits through an outlet duct 114 after passing through the full length of the electrostatic field extending between the discharge electrode 100 and passive electrode 106. The electrostatic device may be used as either the charging stage or the collecting stage of a two-stage electrostatic precipitator depending upon such physical and electrical desiyn parameters as electrode size, field intensity and gas flow rate. The device may also be used as a single-stage electrostatic precipitator. The voltage between the discharge electrode 100 and the passive electrode 106 may be increased without causing back corona and sparkover beyond a value heretofore possible by coating the inside surface of the passive electrode 106 with a resistive material calculated in accordance with the above descri~ed technique. Consequently, the capacity and/or charging efficiency of electrostatic precipitators employin~
7æs ire-cylinder devices as illustrated in ~iy. 12 can be vastly improved in accordance with this invention.
A conve~tional electrostatic of the wire-plate geometry is illustrated in Fig. 13. These conventional wire-plate devices utilize several spaced apart, wire discharge electrodes 120 sus-pended fron a conductive bus bar 122 and supporting respective stabilizing weights 124. The discharge electrodes 120 are pos-itioned between parallel plates 126 generally having deflector members 128 extending along the plates 126 transverse to the direction of gas flow through the ionizer. A relatively high voltage is maintained between the discharge electrodes 120 and plates 126 by a conventional transforr,ler rectifier set (not shown).
As with the conventional wire-cylinder device of Fig. 12, the collection efficiency and/or ca~acity of electrostatic precipitators employing conventional wire-plate devices may be greatly increased by coating the plates 126 with a layer of resistive material hav-ing electrical and physical properties calculated in accordance with the above described procedure.
A high intensity ionizer somewhat silnilar to the ionizer ; 20 illustrated in Fig. 7 and having a resistive anode is illustrated in Fig. 14. The ionizer utilizes a planar discharge electrode 130 mounted at the end of a support ~e~ber 28 which places the dischar~e electrode 130 coa~.ial with a glass tube 134. The outer ; surface of the glass tube 134 adjacent the discharge electrode 130 is coated with a conductive material 136. A relatively high voltage is then ~laced between the discharge electrode 130 and metal coating 136 by a conventional transformer rectifier 138 which is connected to the disch~rge electrode 130 through a con-ductor 132 in the support 23. The metal layer 136 forms the anode of the ionizer, and the physical and electrical properties of the glass tube 134 are selected so that the tube 134 forms a resis-tive coating ,or the metal layer 136.
:
A variety of resistive materials may be used to fabricate resistive anodes in accordance with this inven-tion. The resistive material may comprise an epoxy resin having the required volume resistivity and dielectric strength.
Aluminum-oxide provided with a suitable dopent oxide and/or metal may be required to obtain specific resistivities.
However, presently known epoxy resins deteriorate in a corona environment, and they may not be sufficiently resis-tive to abrasive wear to be advantageously employed. These materials include:
I. ORGANIC MATERIALS
a. SYTCAST 2762FF, a trade mark for an epoxy sold by Emerson Cumings Stycast-resistivities are suitable for low intensity b. STYCAST 2762, a trade mark for an epoxy sold by Emerson Cumings Stycast- -resistivities are suitable for low intensity ionizers. Can be molded in place on anodes.
c. Type C-26, a trade mark for an epoxy sold by Emerson Cumings - resistivities are suitable for both high and low intensity ionizers. May be applied to anodes in thin coats by spraying or painting.
II. INORGANIC MATERIALS
a. Type LA-2-500 aluminum oxide coating sold by Union Carbide - resistivities are suitable for low intensity ionizers or high intensity ionizers above 550F. The volume resistivity is 10 2 ohm-cm at 300F. and 101 ohm/cm at 550F. The coating is applied ~: -- ' ' ~ ' with a specially developed plasma gun. Since the material was developed as an anti-wear coating its resistance to abrasion is excellent.
b. Porcelainized steel sold by Erie Ceramic, Inc. volume resistivity ranges between lO 2 ohm-cm and 2 X 1011 ohm-cm at 300F. Thicknesses range between 0.02 cm to 0.05 cm.
c. Pyrex (trade mark) pipe 7740 sold by the Corning Glass Company. Resistivities are suitable for either high or low intensity ionizers since the resistivity is about 101 at 300F. Available in 1/4 to 1/8 inch thick tubes having inside diameters between 6 and 18 inches. This material can be -advantageously used in the embodiment illustrated in Figure 13.
d. Pyrocexam (trade mark) sold by Corning Glass Company:
l. Type 9606-resistivities are proper for low intensity ionizers or high ', ~ intensity ionizers at temperatures above 500E'. Resistivity is 5 X 10l ohm-cm at 550F. and about 5 X lOll ohm-cm at 300F.
Standards for ~missions of particulate in flue gases issuing from coal fired electrical power station stacks are be-coming increasingly rtore stringent. Current air quality stand-ards require that more than 99% of the fly ash produced by burn-ing coal be removed prior to discharge of the combustion gases from the stack. Thus, the ef~iciency of particulate collection must increase in proportion to the ash content of the coal. In addition, in an effort to reduce the emissions of certain gaseous pollutants, particularly the sulphur oxides, it has become increas-i~gly necessary to use low sulphur coal in electrical power gener-ating plants.
The electrostatic precipitator is the most cor~only used device for the removal of particulate matter produced by coal fired power plants. In a two-stage electrostatic precipita-tor the particulate-laden gas sequentially passes through separate charging and collecting stages. In ~he chargin~ stage the gases pass through a corona discharge so that the particulate matter leaving the charger has a positive or negative charye. The charged particles then pass through a low intensity corona electric field in the collecting stage which causes the particles to mi~rate toward a collecting electrode where they agglor~lerate an~ are sub- ~
sequently removed and collected by various techniques. In a single-stage precipitator particies flowing between a pair of electrodes ~, .
--1-- .
` 11~17X9 haviny a corona current proclucing electrostatic field e~tending therebetween are first charged and then migrate toward one of the electrodes where they a~glomerate and are subsequentl~ removed.
Thus, in a single-sta~e precipitator both the charging stage and the collectin9 stage are combined into a single unit. The efficien-cy of an electrostatic precipitator is deterrnined to a large extent by the maynitude of the charge placed on the particulate matter by the charging staye. The charge maynitude may be increased by increasing the intensity of the electrostatic field producing !O tlle corona discharge- The r~a~-imum intensity of the electrostatic field is limited to a value at which sparkover and back corona occurs as the particulate matter builds up on the passive or non-corona emitting electrode. Althouyh back corona effects can be reduced to some extent by such techniques as limiting the thick-ness of the particulate layer on the passive electrode, achievable electrostatic field intensities nevertheless provide somewhat limited particle charging. Thereafter, the collection efficiency must be improved by increasing the residence time of the partic-ulate-matter in the electric field during collection either by O reducing the speed at which the particulate-laden yases pass through the collection stage, or by increasing the length of the collection stage. However, a decrease in transit speed through the collection stage reduces the capacity of the collection stage, and increasing the size of the collecting electrodes increases the capital cost of such equipment. The intensity of the elec-trostatic field at which the charger can operate without bac~
corona and sparkover is lower for higher resistivity particulate matter. Since fly ash resistivity is inversely related to the level of combustible sulphur in coal, the increasing use of low O sulphur coals increases the cost of achieving a hiyh collection efficiency since back corona and sparkover probler.~s are increased.
Attempts have been made to reduce the incidence of back .7139 corona and spar~over in order to increase the intensity of elec-trostatic fields in ionizers throuyh a number of techniques none of which are entirely satisfactory. Earliest attempts, as de-scribed by ~. J. ~hite, Industrial ~lectrostatic Precipitation at 32~, Addison-i~esley 1963, were directed to treating the partic-ulate matter before entering the ionizer. ~igh resistivity par-ticulate matter was generally treated ~y moisture and acid con-ditioning. Other techniques atternpted to prevent the buildup of a layer of particulate material on the passive electrode such as by employing moving belt electrodes, rotating brushes and various other mechanical devices. These later tecnniques generally failed since even thin films of particulate matter can produce severe back corona effects if the resistivity of the particulate matter is sufficiently high. However, particulate matter buildup has been successfully prevented to some extent by continuously flushing the passive electrode with a water film. Still another approach attempts to adjust the temperature of the ~lectrodes upwardly and downwardly in order to shift the temperature of the particulate matter toward a lower resistivity value. However, this technique generally requires a large amount of power to produce the required temperature shifts.
Previous attempts to adjust the electrical characteristics of the passive electrode in order to reduce back corona and spark-over have generally inserted a non-critical value of current lim-iting resistance in series with the discharge electrode. The resistance has the effect of simultaneously limiting the current that can flow and lowering the intensity of the electrostatic field across the electrode gap in times of abnormal transient conditions occurring at the onset of sparkover. This approach is not considered feasible since effective use of this technique requires a great deal of power. A form of current limiting resis-1~17~9 tance called a "graded resistance" is described in H.
J. White, Resistivity Problems in Electrostatic Precipitation, Journal of the Air Pollution Control Association, 23, 336-37 (1974). In accordance with this technique a thick, flat semi-conducting plate electrode made of steel reinforced concrete was used. The results of this approach were never definitive as to the specific volume resistivity at and close to the anode surface which would allow the technique to be utilized with a variety of ionizer designs. Further-more, the article does not indicate the allowable range for such critical parameters as material resistivity, material thickness or dielectric strength.
Recently, high intensity ionizers have been developed in which a unique electrode geometry produces a -stable, high intensity corona discharge through which the particulate-laden gas passes. These high intensity ionizers charge the particulate matter to a much higher level than is achievable with conventional ionizers utilizing, for example, wire-cylinder of wire-plate geometries. Although the collection efficiency of two-stage electrostatic precipita-tors can be greatly improved by employing this unique highintensity ionizer was a charging stage, back corona and sparkover has nevertheless been a problem, particularly with very high resistivity particulate matter, as the particulate matter builds upon on a metal passive electrode.
This invention seeks to prevent back corona and sparkover in an electrostatic precipitator by coating the anode with a layer of resistive material. More specifically ; the invention is an apparatus having a discharge electrode, a passive electrode spaced apart from said discharge elec-7~3~
trode by an electrode gap, power supply means connectedbetween said discharge and passive electrodes for applying a voltage therebetween, said applied voltage being of suffi-cient magnitude to effect a corona current producing electro-static field between said discharge and passive electrodes having a current flux greater than 10 9 A/cm2 at said passive electrode, a resistive material for preventing back corona and sparkover at said passive electrode comprising a layer of material on said passive electrode between said discharge electrode and said passive electrode, said material having a volume resistivity in the range from about 106 ohm-cm to about 103 ohm-cm under the operating temperature and electric field intensity in the resistive material.
The passive electrode may be, for example, the anode of an electrostatic device such as the charging or collecting stage of a two-stage electrostatic precipitator, or single-stage electrostatic precipitator. The electrode is coated with the resistive material in order to increase the intensity of the device's electrostatic field at which the electrostatic device can operate without back corona and sparkover. The material preferably has a resistivity less than the ratio of the dielectric strength of the material to the current flux passing through the material in order to prevent the field within the material from exceeding its dielectric strength which is preferably greater than 80 kv/cm. The thickness of the anode coating should exceed .01 cm. at a~mospheric pressure and 300F. in order to prevent ~-puncturing of the material and resultant back corona and sparkover. The maximum thickness of the material is selected , .:
so that the voltage drop across the material is less than 15%, and preferably between 5% and 10%, of the applied voltage. The resistive material resists deterioration in a corona environment, and is resistant to abrasion especially where abrasive particulate matter is being charged.
The invention is illustrated, by example only, in the drawings, in which:
Figure 1 is a schematic side elevational view illustrating a multi-stage precipitator employing a charging ionizer having a resistive anode of the present invention;
Figure 2 is an enlarged side view of one ionizer stage of the apparatus of Fig. 1 partially broken away to show the ionizer array;
Figure 3 is an end elevational view of the ionizer stage of Figure 2 with the inlet partially broken away to show the ionizer array;
Figure 4 on the first sheet of the drawings, is an enlarged partial sectional view of a single ionizer venturi illustrating the electrode arrangement;
Figure 5 on the first sheet of the drawings, is a further enlarged partial sectional view of the electrodes of Figure 4 showing the electrode construction in greater detail;
Figure 6 is a schematic system diagram showing the control elements for an ionizer stage;
Figure 7 is a schematic diagram illustrating the operation of an ionizer having a layer of resistive material coating the anode;
Figure 8 is an enlarged partial sectional view of a portion of the anode of Figure 7;
7~39 Figures 9-11 illustrate alternative embodiments of the invention;
Figure 12 is a broken isometric view illustrating an ionizer of the wire-cylinder geometry having a resistive coated anode;
Figure 13 is a broken isometric view illustrating an ionizer of the wire-plate geometry having a resistive coated anode; and Figure 14 is an isometric view showing another embodiment of a high intensity ionizer having a resistive coated anode.
In the drawings, Figure 1 shows, in schematic side elevational view, a two-stage electrostatic precipitator system incorporating the invention. As seen in this Figure, the precipitator system includes a gas inlet 11 into which gases to be cleaned are directed as indicated by arrow 12, a gas outlet 13 -6a-7t39 from which cleaned gases are supplied to appropriate downstream apparatus, e.g. an atmospheric dischar~e duct, as indicated by arrow 14, and a cascaded pair of ionizer-precipitator units yen-erall~ designated by reference numerals 15,15'. ~ach ionizer-precipitator unit 15,15' includes an ionizer stage 16 (16') and a pair of conventional electrostatic precipitators 17, 18, (17', . Iach ionizer stage 16, 16' and precipitator stage 17, 17', 18, 1~' is provided with a high voltage input cable connector 19 coupled to a suitable source of high voltage as described more fully below with reference to Fig. 6, and a collecting bin portion 20 for collecting particulate matter precipitated from the gas as the latter flows through units 15, lr~ I .
In operation, gases containing particulate matter enter the Fig. 1 apparatus via inlet 11 and pass through the first ionizer stage 16 in which the particles in the gas are electro-statically charged. The gas bearing the electrostatically charged particles next flows into successive precipitator stages 17, 18 in each of which the charged particles are deflected out of the flow path of the gas under the influence of an electrical field established across the flow path, the particles being deposited in the bin portions 20 of the precipitator stages 17, 18. The gas exiting from precipitator 18 is passed through ionizer stage 16', and precipitator stages 17', 18', to provide additional clean-ing therefor, and the cleansed gases emerging from precipitator stage 18' are conducted via gas outlet 13 to appropriate down-stream apparatus.
Figs. 2 and 3 illustrate the gas inlet 11 and the first ionizer stage 16 with more particularity. ~s seen in these Figs., gas inlet 11 comprises a hollow conduit of trapezoidal or other ~0 suitable geometric configuration which is coupled at the downstream side to a gas distributor portion 22. ~istributor portion 22 is coupled to an entry chan~er 23 formed within the housing of Y~
ioni~ing unit 16 by the side and bottom walls tnereof and a v~rti-cally arrang~d bulk}lead 2~. Bul~}-~ead 24 an~ a second vertically arrange~ bulkhead 25 define with the side, top and bottom walls of ionizer stage 1~ a pressure manifold ~6 for a purpose to be described.
Positioned within ionizer stage 16 in a regular array are a plurality of venturi diffusers 27 and associated central electrode support members 28 each projecting into the upstream end of the associated venturi 27 and substan'ially coaxial there-with. Each rnember 28 is coupled to a bus bar network generally designated by reference numeral 29 and consisting of three ver-tically arranged parallel bus bars interconnected at the upper ends thereof by a common bus bar element 31, the element 31 being connected to a single bus bar element 32 extending from the inter-ior of ionizer stage 16 to an external conventional high voltage connector shroud 33 to which a high voltage is supplied from a suitable power source ~not shown) via high voltage connector 34.
The downstrean end or outlet of each venturi 27 is coupled to an exit chamber 36 which is-in turn coupled to the inlet of elec-trostatic precipitator stage 17.
Storage bin 20 is provided with a removable door 40 for purposes of inspection and cleaning, and a vibrator bracket 41 for permitting the use of an optional conventional vibrator to assist in settling any particulate matter collecting in bin 20 towards the bottom edye 42 thereof. Bottom edge 42 is provided with suitable apertures (not shown) for enabling the particulate matter to be removed from the bin 20 in a conventional manner.
Bins 20 of the remaining system eler~lents lb', 17, 17', 18, and 18' are configured in a substantially i~entical nlanner.
Each venturi element 27 and associated cozY~ial member 28 generally comprises an electrode pair for generating a high intensity electrostatic field across the path of gas flow through -, l~q~78~
the ionizer staye 1~. For this purpose, an electrode (described below) is carried by each member 28 and is coupled to a source of relatively high negative potential, via bus bar network 29 while each venturi conduit 27 is coupled via the framework of the struc-ture to ground potential. Thus each venturi 27 serves an anode and eac~l me~ber 2~ serves as a catho~e support.
In operation, with the high voltage applied between the cathode and anode, particles suspended in any gas flowing through the ioni~er stage 16 are electrostatically charged when passing through the throat of venturi 27. In order to ensure that sub-stantially all charged particles remain suspended in the flowing yas until arriving at the downstream precipitator 17 or 18, and do not adhere to the ground potential anode surface, the electrode configuration shown in Figs. 4 and 5 may be employed.
With reference to Fig. 4, each venturi element 27 is formed with an inwardly tapering conical inlet section 45, a gen-erally cylindrical central section or throat 46 and an outwardly tapering conical ou~let portion 47. The cathode includes a planar electrode such as a disc 50 which may have a curved peripheral edge which projects outwardly from the outer surface of member 28. ~isc 50 is mounted substantially coaxially in the throat of venturi 27 and provides a highly constricted high intensity elec-trostatic field in the form of a corona discharge between the curved periphery of disc 50 and the surroundiny anode surface 52 when a high potential is applied.
As best shown in Fig. 5, anode surface 52 comprises a series of flanged conical vanes 53 structurally connected in a nested arrange~ent to a mounting mer.~er 54 by spacers 54a and closely spaced along the a~is of venturi 27 by spacers 54a to cefine air passages 5~ between adjacent vanes. Vanes 53 effectively form a cylindrical anode wall with a slightly sloped interrupted surface~ The inner surfaces of the vanes 53 are provided with a 17~9 resistive coating as described llereinafter in ronnection with Fig. 11. Anode surface 52 is surrounded by plenum chamber 26 to which clean air under pressure is supplied from an external sourcs by a pump as ~escribed ~elow in connection with Fig. ~.
In operation, clean air is injected into venturi throat 46 via air passages 55 which effectively form a plurality of annu-lar nozzles and which are oriented to direct circum~erential jets of clean yas along the inner anode surface of venturi 27 in essentially the same direction as the main stream of contaminated gaS passing through venturi 27. The clean gas injected via pass-ages 55 flows along the ano~e surface in a substantially laminar film and provides an effective fluid barrier which also functions to entrain and aid the flow of the main gas stream. This has -' been found to significantly reduce the deposition of charged particulate matter on the anode surfaces as compared with known prior art devices. In addition, the orientation of the clean gas injection nozzles 55 reduces the pressure loss normally associated with the passage of gases through a venturi diffuser not provided with such noz~les. Also, as mentioned above, ~ack corona prob-lems encountered with prior art venturi ionizers can be substan-tially reduced by carefully contouring the edges of vanes 53.
~ig. 6 sch~natically illustrates the electrical~power connections and clean gas injection control system of ionizer ~-stage 16. Hiyh voltaye is supplied to catho~e bus network 29 via hi~h ~oltage cable 3~ from a transformer rectifier set 70 coupled to a control unit 71, both latter ele~ents being of conventional design. ~lean gas is supplied to manifold chamber 26 from a blower 73 via a ~ea~er 74,conduit 75, a controlled,damper 76 and , ' a conduit 77. ~eater 74 is connected to a temperature controller unit 78 for maintaLning ~he temp~rature of the clean gas supplied to manifold chamber 26 within a desired temperàture range. A
aifferential pressure sensor 79 having a pair'of'pressure trans--7~39 ducers ~0, ~1 provide a feedbac}; signal to controlled ~amper 76 in order to provide pressure regulation for the clean air within manifold chamber 26. Elements 73-~1 are all conventional units, the structure of which is well within the ordinary skill of the art.
As noted above, a major problem encountered with elec-trostatic devices, particularly when employed to charge particu-late matter of high resistivity such as fly ash from coal fired boilers using low sulphur coal as a fuel, has been the incidence of sparkover and ~ack corona which is generally the limiting factor for increasing the intensity of the electrostatic field.
Bac~ corona and sparkover occurs when the intensity of the elec-trostatic field within the particulate matter on tl-e passive or noncurrent emitting electrode exceeds the dielectric strength of the particulate material. For example, the dielectric strength of fly ash produced by burning low sulphur coal is generally be-tween 1 ~v/cm. and 10 kv/cm. When the dielectric strength is exceeded a small hole or crater is formed. Since the corona current tends to follow the path of least resistance the corona current concentrates at the point of dielectric breakdown pro-ducing a localized area of extremely high current flux. When this occurs the high ne~ative ion concentration produces a strong negative rield which accelerates free electrons toward the punc-ture. The free electrons collide with gas molecules at a relatively ; high rate of speed knoc~ing off electrons from the molecule accord-ing to the "avalanche process" there~y transforming the molecules into positive ions. The positive ions arF then accelerated toward the cathode to produce positive corona adjacent the anode.
Electrons removed from the gas molecules during the avalanche process are accelerated toward the anode thereby increasing the negative field and increasing the production of eIectrons and positive ions. The result is a positive feedback effect which ' .
7~9 continues until the voltage bet~een the anode an~ cathode is greatly reduced. ~uring this process, electron difusion tends to increase the ~iameter of the discharge, while the circular mag-net~c field produced ~y the electron flow i~ tlle ~ischarge tend3 to compress the ~i~leter of the discharge. As the pressure of the gas increases the total corona current decreases since the increased concentration of electrons causes them to strike each other with greater frequency thereby reducing electron mobility.
However, the increased electron concentration also increases the self magnetic field, and this effect predominates so that the local current density or current flux is increased with increas-ing gas pressure.
The fields through the layer of particulate matter is given by the formula:
~ = 3~ OR~IULA 1) where J is the current flux or current density through ~he material, and _,~ is the volume resistivity of the material.
The current flux J for ionizers is generally on the order of 10 8 A~cm and 2 X 10 6 A/cm. Consequently, for particulate matter hav-ing a dielectric strength of 10 kv/cm back corona an~ sparking arenot a problem until volume resistivities exceed between 5 X 109 ohm-cm and 10 ohm-cr.l.
Back corona and spar~over interfere with the operation of the ionizer since the strong negative field adjacent the anode greatly reduces the intensity of the field in the interelectrode region and the positive ions discharge th~ negatively charged particulates thereby defeating the purpose of the charging stage.
In accordance with the present invention, the passive electrode of an electrostatic device is coated with a resistive material having a high dielectric strength. The term~"electro-stati~ device" as used herein refers to either the charging stage or the collecfing stage of a two-stage electrostatic precipitator, or a single-stage electrostatic precipitator employing a unitary ~ -12-char~in~ and collccting stage. The passive electro~e is gen~rally an ano~e since the effects of bac~ corona an~ sparl;over are more serious with negative corona in which the cathode is the corona emitting electrode. In the n~gative corona, most of the current is carried by negative ions which originate from electrons lib-erated from the cathode or discharge electrode surface by pos-itive ion bombardment. The positive ions in turn are generated in the high field region near the cathode by electron ionization of the gas molecules. In the positive corona, on tlle other hand, the current is carried primarily by positive ions which oriyinate from electron ionization of the gas în the high field region near the anode or discharge electrode.The electrons in ~is case are produced by photoelectric ionization of the ~as molecules in the region between the high field zone and the ground electrodes.
These differences in ionization processes have a large influence on the spark brea~down voltages of the negative and positive coronas. As mentioned above, spark breakdown for corona is aue to the formation of self-propagating strearders or current flow wh~ch originate from the anode. For the positive corona, high electric fields exist near the discharge electrode even a$ rel-atively low voltages, so that the spark breakdown streamer trig-gered ~y the high fields forms at lower voltages also. This ~eans that the operating voltages of electrostatic devices using pos-itive corona are limited to relatively low values, even for low resistivity particulate matter. However, for negative corona the electric field is relatively low near the anode so that much higher voltages can be applied before the fields build up to the point where spark breakdown occurs. In practicey spar~over voltages for negative corona are found o be about double those for pos-itive corona. ~he use of positive corona to improve performanceunder high resistivity conditions is of no advantage~ however, because of the inherently low sparkover voltages of positive . :
corona. These differences between negative and positive corona lead to different e~fects for back corona in the two cases. The main difference is that bac}; cGrona has little effect on spark-over voltage for the positive corona since in this case the back corona does not trigger a spark as it does for neyative corona.
Thus, back corona, which has such a disruptive effect on the negative corona, has only a small effect on positive corona.
Consequently, the resistive material coating the passive elec-trode is generally more useful for negative corona than for positive corona. Thus the term "anode"is used herein synonymously with "passive electrode" and is intended to include the cathode - of a positive corona electrostatic device.
The resistive layer on the anode reduces back corona and sparkover by two distinct processes. First, current flowing through a resistive material will follow the path of least resis-tance. This means that a current flowing in the resistive mater-ial which would otherwise be in a relatively narrow path, and hence have a large local current flux, is unable to concentrate in this manner so that the current flux passing through the re-sistive material and the particulate matter is kept relativelylow. r~he result is that the current flowing between the discharge electrode and the passive electrode cannot concentrate to allow the discharge to convert from a relatively low current density, high electric field form, to the high current density, relatively low electric field form associated with sparkover. Since the current flux passing through the particulate matter is maintained at a relatively low value, the intensity of the field within the particulate matter is I~intained below its dielectric strength in accordance with FO~lUL~ 1 above. The second process by which the electrostatic field is stabilized by the resistive anode is that ; the resistive material reduces the voltage between the cathode ,, . - , ~, '. - ~
and the sur~ace of the resistive coating as current fluY. incre~s~s~
Tllat is, for an electrostatic device having an applied voltage of 75 kv and an average current flux of 10 6 A/cm, a 1 cm thick coating of resistive material having a volume resistivity of 10 ohm-cm has a voltage difference ~ V across the resistive coating of 10 ~v in accordance with the following formula:
V = J t (Fo~lULA 2) where t is the thic~ness of the resistive coating.
Clearly the maximum current flux which can flow through the resis-tive material is 7.5 Y 10 A/cm since at tnat current flux the voltage across the resistive coating would be 75 kv. ~ven at this maximum current flux of 7.5 X 10 6 A/cm, the intensity of the electrostatic field through a particulate matter having a resistivity of 101 ohm-cm is only i.5 kv/cm - a field intensity less than the 10 kv/cm dielectric strength of most high resistivity ~articulate matter. Of course, there would be a substantial vol-tage drop within the electrode gap in order for the corona dis-, charge to continue so that the intensits~ of the electrostatic I field in the particulate matter would be somewhat less.
The electrical properties of the resistive material are i deterr.lined as a function of the construction of the electrostatic device and the electrical properties of the particulate matter.
The volume resistivity of the resistive material is initially determined. It has been experimentally determined that the resis-tivity for a high intensity charger or ionizer having a current ~lux greater than 10 6 A/cm must be greater than 108 ohm-cm in order to prevent bac~ corona and sparl;over. The maximum allowable reslstivity is selected so that the dielectric strength of the material is not exceeded at the current flux of the ionizer.
Thus, for example, for a rclatively high intensity ionizer having a current flux of 2 X 10 A/cm2 and a breakdown voltage of 100 kv the maximum r~sistivity is calculated by formula 1 as 5 X 101 .
.
ohm-cm. After the resistivity of the resistive material is selected at some value between the maximum and miniMu~ values, the thick-ness of the coating is calculated to maintain a voltage drop across the resistive coatiny of less than 1577 ~ and preferably about 5%, of the applied voltage. Thus, in the above example if 2 X 101 ohm-cm is selected as the resistivity of the resistive coating and 75 ~v is applied between the anode and cathode of the ionizer, the thickness of the resistive coating is calculated by formula 2 as about .09 cm. It should be noted, howPver, that the resistivity of the coating is inversely proportional to temperature so that temperature fluctuations r~ust be accounted for when selecting a resistivity value. It has also been found e~perimentally that a resistive coating thickness of about .025 cm is sufficient to prevent corona, and theoretical analysis suggests that a thickness of 0.01 cm should be acceptable. The theory predicts the minirnum thickness of the resistive material is about 4 times the diame~er of the ~ischarye terminating on the resistive surface when it has not had an op~ortunity to concentrate.
The above described technique is also utilized for rela-tively low intensity electrostatic devices. For example, a device having a current flux of 10 7 A/cm, and an interelectrode voltage of 50 ~v would require a resistive coating between 106 and 1012 ohm-cm. The thickn~ss of the coating is then calculated to be 5 X 10 2 cm. It is thus seen that the resistive material coating the ano~e of a lower intensity electrostatic device may have a higher resistivity. For high intensity devices, the resistivity is generally between 10 ohm-cm and 5 X 10 ohm-cm, and the coatings less than 0.5 cm. A relatively low intensity device generally utilizes a resistivity between 10 and 10 ohm-cm and a relatively thicker coating of resistive l~terial. A lower intensity device having a current flux bet~een 10 and 10 9 A/cm may utilize a material having a resistivity as low as 10 ohm-cm.
The inventive resistive coating has been described as a technique for preventing back corona and sparkover, but it also allows the intensity of the electrostatic field in a given electrostatic device to be increased without producing back corona and sparkover. As illustrated in Fig. 7, the problems of sparkover and back corona are reduced according to the invention by providing a layer of resistive material 85 on the inner surface of the anode 27 in the region adjacent the planar electrode 50 in which the electrostatic field therebetween represented by dotted lines 87 is concen-trated. The physical and electrical properties of theresistive material 85 are calculated in accordance with the above described procedure.
The simple annular band shown in Fig. 7 for resistive layer 85 is only one of several possible configura-tions envisioned. For example, with reference to Fig. 9 ananode 90 is shown which comprises inlet and outlet wall sections 91, 92 fabricated from an electrically conductive material, a plurality of conductive anode segments 93 also fabricated from a good electrically conductive material and electrically insulative spacers 94 interposed between adjacent conductive elements 91-93 for providing electrical isolation therebetween. A layer of resistive material 85 is provided on the inner surface of each of the anode segments 93. Each of the conductive segments 93 can be also provided with a suitable terminal adapted to be coupled to indepen-dent high voltage supplies (not shown) in order to permit electrical field shaping by regulation of the individual voltage supplies.
3~ Fig. 10 shows an alternative embodiment of the .
3L~L5~:~7~9 invention in which anode segments 93 are mutually spaced to provide air passages therebetween for a similar purpose to that described above with reference to Figs. 4 and 5, with each anode segment 93 being provided with a layer of resis- :
tive material 85 on the inner surface thereof.
Fig. 11 illustrates still another embodiment of the -17a-7~9 invention ta~cn after Fi~. 5 in which the indiviclual conical seg-mental vanes 53 are each urovided with a layer of resistive mater-ial ~5 along the inner surface thereof.
l'he collection efficiency of two-stage electrostatic precipitators employing other ty~es of particle charging ionizers as well as single-stage precipitators may also be improved in accordance with this invention. With reference to Fig~ 12, a conventional, relatively low intensity electrostatic device of the wire-cylinder geometry includPs a wire discharge electrode 100 suspended from a feed-through insulator 102 secured to a precipitator shell 104. The discharge electrode 100 is concen-trically mounted with a tubular passive electrode 106 which also forms a duct for the particle-laden gases. A weight 108 is sus-pended from the discharge electrode 100 to maintain the position of the electrode 100 constant as gases flow through the passive electrode 106. ~ transformer rectifier set of conventional variety 110 is connected between the discharge electrode 100 and the passive electrode 106. In operation the particle-laden gas enters the passive electrode 106 through an inlet duct 112 and exits through an outlet duct 114 after passing through the full length of the electrostatic field extending between the discharge electrode 100 and passive electrode 106. The electrostatic device may be used as either the charging stage or the collecting stage of a two-stage electrostatic precipitator depending upon such physical and electrical desiyn parameters as electrode size, field intensity and gas flow rate. The device may also be used as a single-stage electrostatic precipitator. The voltage between the discharge electrode 100 and the passive electrode 106 may be increased without causing back corona and sparkover beyond a value heretofore possible by coating the inside surface of the passive electrode 106 with a resistive material calculated in accordance with the above descri~ed technique. Consequently, the capacity and/or charging efficiency of electrostatic precipitators employin~
7æs ire-cylinder devices as illustrated in ~iy. 12 can be vastly improved in accordance with this invention.
A conve~tional electrostatic of the wire-plate geometry is illustrated in Fig. 13. These conventional wire-plate devices utilize several spaced apart, wire discharge electrodes 120 sus-pended fron a conductive bus bar 122 and supporting respective stabilizing weights 124. The discharge electrodes 120 are pos-itioned between parallel plates 126 generally having deflector members 128 extending along the plates 126 transverse to the direction of gas flow through the ionizer. A relatively high voltage is maintained between the discharge electrodes 120 and plates 126 by a conventional transforr,ler rectifier set (not shown).
As with the conventional wire-cylinder device of Fig. 12, the collection efficiency and/or ca~acity of electrostatic precipitators employing conventional wire-plate devices may be greatly increased by coating the plates 126 with a layer of resistive material hav-ing electrical and physical properties calculated in accordance with the above described procedure.
A high intensity ionizer somewhat silnilar to the ionizer ; 20 illustrated in Fig. 7 and having a resistive anode is illustrated in Fig. 14. The ionizer utilizes a planar discharge electrode 130 mounted at the end of a support ~e~ber 28 which places the dischar~e electrode 130 coa~.ial with a glass tube 134. The outer ; surface of the glass tube 134 adjacent the discharge electrode 130 is coated with a conductive material 136. A relatively high voltage is then ~laced between the discharge electrode 130 and metal coating 136 by a conventional transformer rectifier 138 which is connected to the disch~rge electrode 130 through a con-ductor 132 in the support 23. The metal layer 136 forms the anode of the ionizer, and the physical and electrical properties of the glass tube 134 are selected so that the tube 134 forms a resis-tive coating ,or the metal layer 136.
:
A variety of resistive materials may be used to fabricate resistive anodes in accordance with this inven-tion. The resistive material may comprise an epoxy resin having the required volume resistivity and dielectric strength.
Aluminum-oxide provided with a suitable dopent oxide and/or metal may be required to obtain specific resistivities.
However, presently known epoxy resins deteriorate in a corona environment, and they may not be sufficiently resis-tive to abrasive wear to be advantageously employed. These materials include:
I. ORGANIC MATERIALS
a. SYTCAST 2762FF, a trade mark for an epoxy sold by Emerson Cumings Stycast-resistivities are suitable for low intensity b. STYCAST 2762, a trade mark for an epoxy sold by Emerson Cumings Stycast- -resistivities are suitable for low intensity ionizers. Can be molded in place on anodes.
c. Type C-26, a trade mark for an epoxy sold by Emerson Cumings - resistivities are suitable for both high and low intensity ionizers. May be applied to anodes in thin coats by spraying or painting.
II. INORGANIC MATERIALS
a. Type LA-2-500 aluminum oxide coating sold by Union Carbide - resistivities are suitable for low intensity ionizers or high intensity ionizers above 550F. The volume resistivity is 10 2 ohm-cm at 300F. and 101 ohm/cm at 550F. The coating is applied ~: -- ' ' ~ ' with a specially developed plasma gun. Since the material was developed as an anti-wear coating its resistance to abrasion is excellent.
b. Porcelainized steel sold by Erie Ceramic, Inc. volume resistivity ranges between lO 2 ohm-cm and 2 X 1011 ohm-cm at 300F. Thicknesses range between 0.02 cm to 0.05 cm.
c. Pyrex (trade mark) pipe 7740 sold by the Corning Glass Company. Resistivities are suitable for either high or low intensity ionizers since the resistivity is about 101 at 300F. Available in 1/4 to 1/8 inch thick tubes having inside diameters between 6 and 18 inches. This material can be -advantageously used in the embodiment illustrated in Figure 13.
d. Pyrocexam (trade mark) sold by Corning Glass Company:
l. Type 9606-resistivities are proper for low intensity ionizers or high ', ~ intensity ionizers at temperatures above 500E'. Resistivity is 5 X 10l ohm-cm at 550F. and about 5 X lOll ohm-cm at 300F.
2. Type 9608-resistivities are - proper for both high and low intensity ionizers since the resistivity is 3 X 109 ohm-cm at 300F.
' ' - -
' ' - -
3 7~9 e. Soda-Lime glass sold by Corning Glass Company. Volume resistivity is 2 X 108 ohm-cm at 300F.
f. VYCOR (trade mark) glass sold by Corning Glass Company. Resistivities are suitable for low intensity ionizers- and high intensity ionizers at very high temperatures. Resistivity is 10l2 ohm-cm at 300F.
The inventive resistive anode can thus be used in a variety of ionizers in order to improve the capacity and/or charging efficiency of two-stage electrostatic precipitators.
The resistive anode has been described herein as forming part of an electrostatic precipitator for removing fly ash from coal fired power plants. However, the resis-tive anode may also be advantageously employed in other applications including electrostatic devices used outside the power generating field as well as in electrostatic precipitators for power plants fired by such fossil fuels as ' ~ oil and mixtures of hi-sulphur and low-sulphur coal.
.:
.
,,~
.~ ~
:
.. . . . -.. ~
. - , ~ :
f. VYCOR (trade mark) glass sold by Corning Glass Company. Resistivities are suitable for low intensity ionizers- and high intensity ionizers at very high temperatures. Resistivity is 10l2 ohm-cm at 300F.
The inventive resistive anode can thus be used in a variety of ionizers in order to improve the capacity and/or charging efficiency of two-stage electrostatic precipitators.
The resistive anode has been described herein as forming part of an electrostatic precipitator for removing fly ash from coal fired power plants. However, the resis-tive anode may also be advantageously employed in other applications including electrostatic devices used outside the power generating field as well as in electrostatic precipitators for power plants fired by such fossil fuels as ' ~ oil and mixtures of hi-sulphur and low-sulphur coal.
.:
.
,,~
.~ ~
:
.. . . . -.. ~
. - , ~ :
Claims (11)
1. In an apparatus for removing particulate matter from a gas stream and having a discharge electrode, a passive electrode spaced apart from said discharge electrode by an electrode gap, power supply means connected between said discharge and passive electrodes for applying a voltage therebetween, said applied voltage being of sufficient magnitude to effect a corona current producing electrostatic field between said discharge and passive electrodes having a current flux greater than 10-9 A/cm2 at said passive electrode, a resistive material for preventing back corona and sparkover at said passive electrode comprising a layer of material on said passive electrode between said discharge electrode and said passive electrode, said material having a volume resistivity in the range from about 106 ohm-cm to about 1013 ohm-cm under the operating temperature and electric field intensity in the resistive material.
2. m e apparatus of claim 1 wherein the temperature of said particulate matter within said electrode gap is between 180°F and 750°F.
3. me apparatus of claim 2 wherein the thickness of said material is greater than 0.01 cm and less than that thickness where the voltage drop across the material is less than 15% of the applied voltage.
4. The apparatus of claim 2 wherein the thickness of said resistive material is greater than about four times the diameter of an unconcentrated discharge terminating on the surface of said material.
5. me apparatus of claim 2 wherein the volume resistivity of said material is in the range from about 108 ohm-cm to about 1010 ohm-cm for a current flux at said passive electrode greater than 10-6 A/cm2.
6. The apparatus of claim 2 wherein the volume resistivity of said material is in the range from about 106 ohm-cm to about 1013 ohm-cm for a current flux at said passive electrode in the range from about 10-9 A/cm2 to about 10-6 A/cm2.
7. The apparatus of claim 2 wherein said resistive material is an organic compound having a dielectric strength greater than 50 kv/cm.
8. The apparatus of claim 2 wherein said resistive material is an inorganic compound having a dielectric strength greater than 80 kv/cm.
9. The apparatus of claim 1 wherein said passive electrode includes a plurality of mutually spaced electrically conductive sections electrically isolated from each other, and wherein said resistive material covers the surface of each section facing toward said discharge electrode.
10. The apparatus of claim 9 further including a plurality of insulative spacers positioned between said electrically conductive sections.
11. The apparatus of claim 9 wherein said sections are spaced apart from each other to provide a plurality of interstitial fluid passages therebetween.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US78419677A | 1977-04-18 | 1977-04-18 | |
| US784,196 | 1991-10-28 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1101789A true CA1101789A (en) | 1981-05-26 |
Family
ID=25131651
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA291,388A Expired CA1101789A (en) | 1977-04-18 | 1977-11-21 | Resistive anode for electrostatic precipitation |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS53131581A (en) |
| AU (1) | AU3314278A (en) |
| CA (1) | CA1101789A (en) |
| CH (1) | CH625975A5 (en) |
| DE (1) | DE2756524A1 (en) |
| FR (1) | FR2387689A1 (en) |
| SE (1) | SE7800356L (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4239505A (en) * | 1979-09-07 | 1980-12-16 | Union Carbide Corporation | Purge gas conditioning of high intensity ionization system for particle removal |
| JP5098500B2 (en) * | 2007-01-29 | 2012-12-12 | パナソニック株式会社 | Electric dust collector |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE346235C (en) * | 1922-11-30 | Paul Kirchhoff Dipl Ing | Non-spraying electrode for electric gas cleaners | |
| US3768258A (en) * | 1971-05-13 | 1973-10-30 | Consan Pacific Inc | Polluting fume abatement apparatus |
| US4010011A (en) * | 1975-04-30 | 1977-03-01 | The United States Of America As Represented By The Secretary Of The Army | Electro-inertial air cleaner |
-
1977
- 1977-11-21 CA CA291,388A patent/CA1101789A/en not_active Expired
- 1977-12-19 DE DE19772756524 patent/DE2756524A1/en not_active Withdrawn
- 1977-12-28 CH CH1614577A patent/CH625975A5/en not_active IP Right Cessation
-
1978
- 1978-01-12 SE SE7800356A patent/SE7800356L/en unknown
- 1978-01-13 JP JP262978A patent/JPS53131581A/en active Pending
- 1978-02-09 AU AU33142/78A patent/AU3314278A/en active Pending
- 1978-03-31 FR FR7809689A patent/FR2387689A1/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| SE7800356L (en) | 1978-10-19 |
| FR2387689A1 (en) | 1978-11-17 |
| DE2756524A1 (en) | 1978-10-26 |
| AU3314278A (en) | 1979-08-16 |
| JPS53131581A (en) | 1978-11-16 |
| CH625975A5 (en) | 1981-10-30 |
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