CA1042366A - Electrostatic scrubbers involving charged liquid sprays and oppositely charged dust particles - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An improved electrostatic scrubber to remove contaminant aerosol particles such as fly ash from the fluid discharge of for example coal fired power plants. The scrubber includes a cooling chamber followed by a mist eliminator and at least one spray tower having a contaminant aerosol particle charger, a cleaning liquid droplet charger, and liquid spray nozzles. The fluid discharge is first Passed through the spray tower where an aerosol particule electrostatic charger is arranged and located both to give the aerosol particle a sufficient residence time in the charging zone and to avoid giving any transfer of charge to the liquid droplets. Droplets of cleansing liquid are sprayed into the flowing fluid stream and are charged to a potential differing from that impressed on the aerosol particles. Dry purge air is employed to protect electrical leads and nozzle components from reverse flow of liquids. A
bubble scrubber receives the flowing fluid stream containing charged aerosol particles and charged liquid droplets from the spray tower. The bubble forming liquid in the scrubber has a charge opposite to the charge on at least one of the aerosol particles and the liquid droplets. The flowing fluid stream is passed through the bubble forming liquid to remove the particles and droplets therefrom.

Description

BACKGROUND OF INVENTION
The cleansing of industrial exhaust gases using wet electrostatic scrubbers whereTn a surface area or volume of flne water spray droplets Is used 7n place of respective collector plates of an electrostatlc prec7p7tator has been prev70usly undertaken. The 7nteractlon of a f7nely atom7zed water spray un7formly charged to a pos7t7ve polar7ty, w7th a negatlvely charged contamlnated aerosol, creates a comb7ned spray scrubber and collector apparatus. Such apparatus 7s presented by Gaylord W. Penney 7n h7s patents, namely, United States patents No. 2, 357, 354 and No. 2, 357, 355, where7n he describes h7s electr7cal dust precip7tators ut717zing 17qu7d sprays. Other persons have made 7mprovements, for example, as Floyd V. Peterson has done 7n prevent7ng flashovers 7n spray towers, us7ng the type of electr7cal 17quid spray prec7p7tator apparatus he descr7bes 7n his United States patent No. 2,949,168.
There remained, how~ver, several needs for addit70nal improvements 7n these wet electrostatic scrubbers and collectors to 7ncrease their eff7ciency mak7ng them more commerc7ally feas7ble and effect7ve. Th7s 7nvention prov7des additional improvements 7n the contam7nated aerosol charg7ng apparatus, the 17qu7d spray charging apparatus, the spray towers, and 7n the series arrange-ment of all these 7mproved components to create a more commerc7ally feas7ble 20 and effective wet electrostat7c scrubber and collector of contaminate aerosol particles to prevent the7r release 7n commercial stack gases.
SUMMARY OF INVENTION
The cleans7ng of 7ndusir7al exhaust gases conta7nlng contam7nat7ng ~ s aerosol part7cles, such as fly~sk, 7s undertaken more effect7vely, feas7bly, and eff7c7ently by an 7mproved ser7es of spray towers and assoc7ated equ7pment in which moreeeffect7ve scrubb7ng and collect70n occurs of contam7nate aerosol particles from commerc7al exhaust gases.
The improvements are:
1 ) more trouble free electrostatlc charger of the contam7nant aerosol 30 particles util7zlng a unldlrectlonal electrlc potentlal between rough surface corona dlscharge electrodes and spaced non-dlscharge electrodes, whereln a flow of dry air 7s used to purge the charglng area and control the aerosol particle flow to keep the electrodes as clean as posslble w7thout reduc7ng the res7dence t7me of the aerosol partlcles In the7r charglng zone; ~

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104Z3f~6

2) more trouble free charging of the 17quid droplets, as dry air 7s used to purge the leads to the charg7ng plates or the electrodes and the7r leads, keeping them clean for longer operatlng perTods, and as more Insulators and insulating materials are used to prevent and to retard leakage of electr7cal energy; and

3) more effective 7nteraction flows of charged cleansing liquld droplets and contaminate aerosol part7cles are undertaken 7n the precip7tators, 7. e. spray towers, by effectlve arrangement of the spray nozzle ex7ts, ut717za-t70n of smaller nozzle open7ngcs7zes, bubble producing devices, employment of 10 non-metallic materials, and creating magnetic fields to effect the flow path of the charged cleansing liquid droplets and to some extent the flow path of the charged contaminate aerosol particles.
It is one object of the invent70n to prov7de an electrostat7c wet scrubber and collector assembly to remove contaminate aerosol part7cles from exhaust gases compris7ng a receiving duct to direct contaminate exhaust gases to a cooling chamber, where the temperature of contam7nate exhaust gases 7s reduced. An electrical charg7ng subassembly rece7ves the cooled contam7nate exhaust gases and dur7ng a brief res7dence t7me 7n th7s subassembly electr7cally charges the contam7nate aerosol particles. A transm7tt7ng duct dlrects the 20 contaminate exhaust gases, with charged aerosol partlcles, to a s7de entry near the top of a spray tower 7n wh7ch scrubb7ng, precip7tat70n and collect70n of the aerosol particles occurs. A 17qu7d droplet charg7ng assembly rece7ves cleans7ng liqu7d and discharges the 17qu7d 7n droplets and/or bubbles and dur7ng a brief residence time in th7s subassembly electr7cally charges them w7th a charge opposite to the charge rece7ved by the contaminate aerosol part7cles and then dieects the charged 17quid droplets and/or bubbles into the top of a spray tower.
The spray tower receives the charged contaminate aerosol particles and the oppositely charged cleansing 17qu7d droplets and/or bubbles and provldes suffi-c7ent residence for the t7mely attract70n of many contamlnate aerosol partTcles 30 to the cleanslng llquld droplets and/or bubbles. A draln to remove collected droplets and the7r attracted contamlnate aerosol partlcles Is provlded. A
second electrlcal charglng subassembly Is provlded to recelve contamlnated exhaust gases, contaminate aerosol part7cles, and entralned 17quld droplets from the lower level of the spray tower and dur7ng a brlef residence tlme In this iO4;~1t;6 Second subassembly to again electrically charge the contaminate aerosol particles. A second transmitting duct, liquid droplet charging assembly, spray tower, drain, and a third electrical charging subassembly to aga7n charge the contaminate aerosol particles are provided, along wlth a third transmitt7ng duct, a third liquid droplet charging subassembly, a third spray tower and a third like drain, along with a mist eliminator to receive the exhaust gases from the third spray tower and the entrained liquid, and an exhaust fan, to complete the discharge of the exhaust gases, now clean, is also provided.
A principal object is to provide a method of removing aerosol particles from a gas stream comprising the steps of: imposing a first electro-static charge upon the particles contained in said flowing gas stream; dispersing a cleansing liquid into droplets to contact the flowing gas stream, the droplets having a second electrostatic charge thereon; flowing the gases containing at least a portion of said charged aerosol particles and said charged cleansing liquid droplets through an electrostaticblly charged bubble-forming apparatus, said apparatus having a cleansing bubble-forming liquid thereon, said bubble-forming liquid having a third electrostatic charge of a polarity opposite to at least one of said first and second charge whereby the charged contaminant aerosol particles are attracted to and collected by at least one of the charged liquid droplets and the charged bubble-forming liquid.
DESCRIPTION OF DRAWINGS
Figure 1 is a schematic elevational view of an electrostatic scrubber and collector involving charged liquid spray droplets and/or bubbles and oppositely charged contaminative aerosol particles, wherein spray towers, etc., are utilized imseries;
Figure 2 is a schematic cross section of one spray tower and its associated support apparatus, indicating the co-current directional flow of `I charged contaminative aerosol particles and the oppositely charged cleansing spray droplets;
F7gure 3 7s a schematlc cross sectlon of one spray tower and its associated support apparatus, 7nd7catlng the counter current flow of charged contam7native aerosol part7cles and the oppos7tely charged cleans7ng spray droplets;
F7gure 4 is a schemat7c elevation of a charg7ng apparatus used to 104'~;~6 charge contaminative aerosol particles by employing corona discharges;
Figure 5 is a schematic plan view of the charging apparatus of Figure

4;
Figure 6 is a schematic top view of a charging apparatus used to charge cleansing liquid spray droplets by direct voltage charges applied with7n cleansing liquid supply lines near the entries of the liquid spray nozzles;
Figure 7 is a Partial cross section taken on line 7-7 of Figure 6, of the direct voltage charging apparatus used to charge cleansing liquid spray droplets;
Figure 8 is a partial cross section taken similarly to Figure 7, showing, however, the charging of cleansing liquid spray droplets by induction charging;

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104;~366 Flgure 9 Is a partlal cross sectlon taken slmllarly to Flgures 7 and 8~ indlcating, however, the charging of cleansing liqu7d spray droplets by corona ionization;
Figure I0 ls a bottom view of the corona lonlzatlon structures shown in FTgure 9, surrounding the passlng cleanslng 17quid spray droplets;
Figure I I is a schematic cross sectlon of one spray tower and some of its support apparatus, 7nd7cat7ng the use of a magnet7c f7eld to control the flow path of the charged cleans7ng llquid droplets dur7ng the7r res7dency, keeping them clear of the 7nter70r surfaces of the spray tower;
I0 F7gure 12 7s a schemat7c top cross sect70nal v7ew of one spray tower, 7nd7cat7ng the use of multiple magnetic f7elds about the spray tower to selectively control the flow path of the charged cleans7ng 11qu7d droplets during their res7dency, keeping them clear of the inter70r surfaces of the spray tower;
Flgures 13 and 14, respectlvely, Illustrate In schematlc top and cross sectional v7ews, the centralized location of cleans7ng 17quid sprays to start the charged 17qu7d droplets on the7r way well clear of the 7nter70r surfaces of the spray tower;
F7gures 15 and 16, respect7vely, 711ustrate 7n schematlc top and cross sect70nal v7ews, the complete c7rcumferent7al dlspersed rad7al entr7es of 20 contaminated aerosol gases 7nto the spray tower utilizing an outer aerosol gas chamber and an inner scrubbing chamber;
Figure I 7 7s a schemat7c elevat70n of two spray towers and support7ng apparatus, one spray tower operation 7nvolv7ng the co-current downwardly flow of both charged aerosol part7cles and oppos7tely charged cleans7ng 17qu7d droplets, and the other involving the co-current upwardly flow of both charged aerosol particles and oppositely charged cleansing 17quid droplets and bubbles and comblnations thereof;
Figures 18, I9, 20 and 21, are enlarged schematic cross sections of respective 7nd7vidual comblnatlons of droplets wlthln bubbles showlng respec-30 t7ve obtalnable dlspersements of the respectlvely charged and centered cleanslngllquld droplets, the surroundlng charged aerosol partlcles, and the all encom-passlng bubble structures wlth thelr respectlve charges; and Flgures 22 and 23, showlng graphlcally the collect70n efflclencles at respect7ve operat7ng cond7t70ns of one model of thls electrostatlc wet 104Z;~66 scrubber and collector~ when operated to remove aerosol partlcles from the smoke stack gases or emissions leav7ng a coal fired power bo71er.
DESCRIPTION OF PREFERRED EMBODIMENTS
Wet Electrostatic Scrubber Uslng Three SpraY Towers In Serles The improvements in electrostatlc scrubbers, coupled with their inherent contamlnant aerosol particle collector functlon, are 711ustrated and described in reference to several embodlments, the components of several embodiments being optionally used and 7nterchangeably used in other embodiments.
All the improvements involve the objectives of improving the electrostat7c I0 charges and increasing the residency times~ during whlch periods the well charged cleansing liquid droplets and/or bubbles have the opportunity of attracting opposTtely well charged contaminant aerosol particles, such as the dust carried in stack gases leaving a coal fired power boiler. ~oreover, In obtaining these objectives, there Is an improvement in obtalng longer effective operational`periods wh1ch are undertaken at a lower overall cost. For example, clean dry air is employed as a directional flow purging medlum to keep electrical leads and connectors and other components free of non-soluble cloggTng deposits and other hindrances.
In Figure 1, an overall wet electrostalc scrubber is illustrated 20 schematically. During initial operations~ even before Improvements and adjust-ments were undertaken, this scrubber operated at htgh eff7ciencles as 7ndicated In the graphs of Flgures 22 and 23. Leglons are included In these illustrations to qu7ckly identify the arrangement of various components and the operatlons of thls wet electrostatlc scrubber whlch ut71Tzes the three spray towers arranged t in serles.
The contamlnated aerosol gases emltted from a coal fired power boiler are Initlally cooled to brlng the temperatures down low enough to avoid heat damage to any of the components, many of whlch are preferably made of t~ ~
~ non-metalllc, 7.e., non-conductlve materlals, such as luclte and flberglass.
30 Also, the temperature of thls contamlnated aerosol gas Is lowered, because thls entlre wet electrostatlc scrubber must be operated at lower temperatures to be efflclently effectlve. At the cooler temperatures~ the relatlve humldlty Increases and the water or llquld evaporatlon rate decreases, thereby maklng the generation and dlspersement of the cleanslng liquld spray droplets and/or bubbles more _5--/c ~ c~r ~

-`" 1042;~66 effectlve.
~ 0/~0~1 There is a coolant entry and ~ for the coollng chamber and there are cleansTng liquTd entrles, contaminated liquld dralns, and electrlcal power leads illustrated in conjunction wlth other components throughout Figure 1, ind7cating their need in performing their respective funct10ns. Also sampling ports are used wlth instrumentatlon to continuously or perlodically monltor the overall continuing effectiveness of the operation of the wet electrostatic scrubber.
After belng cooled, the contamlnated aerosol gases from the 10 industrial-lib~ source, such as the coal fired power bo71er, are directed through electrostatic chargers, wherein the contaminant aerosol particles, often referred to as dust, are given preferably a negative electrostatic charge.
Thereafter, the contaminated aerosol gases are dellvered to the f7rst spray tower near its top, as illustrated in Flgure 1.
From another location in the spray tower, well separated and often above the aerosol particle charger, cleansing llqu7d enters another electro-stat7c charger, resultTng in the creation of droplets of cleansing liquld and/or ubbles, often usTng spray nozzles, wh7ch leave the charger with a pos7tive electrostatic charge. The electrical leads and equlpment, and nozzles are 20 protected from deposits of non-soluble coattngs by the directional flow of clean dry aTr which purges the surrounding volume, generally established by insulator structures spaced about the electrical leads and equipment, and nozzles or other dlscharge units.
From above, the charged droplets and bubbles of cleans7ng liquld travel below jolnlng the contamlnated gases w7th oppos7tely charged dust, 7. e.
aerosol partlcles, creatlng an overall comb7ned gas stream. The res7dency time of the charged droplets and bubbles of cleans7ng ITquid, in theTr act7ve posltlons, away from the interlor surfaces of the spray tower, T. e., precTpTtator, ls ma7ntaTned at a maxTmum to Tnsure thelr collectlon eMlclency. Such resldency 30 tlme Ts dependent on many conslderatlons such as relatlve veloclty rates of the charged cleanslng llquld droplets and/or bubbles and the contamlnant charged aerosol particles, thelr respectTve flow paths, thelr respectlve surface areas, thelr respectlve electrostatTc charges, theTr respectTve welghts~ etc.
Although the collectTon of aerosol partTcles Is consldered successfully ~ iO4Z~66 undert~ken in the flrst spray tower, second and thlrd spray towers are used, as Illustrated In Ftgure 1~ to thoroughly remove substantlally all the aerosol particles down to and below practtcal removal llmlts. Those aerosol partlcles not charged or not recelvlng a sufflclent charge to saturate them go on to the second and posslbly thlrd aerosol partlcle chargers and thereafter Into the respectlve second and thlrd spray towers.
At the dralns for liguld recovery at the bottoms of the respective spray towers and at the bottom of the mlst ellmlnator, the charged aerosol partlcles captured by the oppositely charged cleansing water droplets and/or 10 bubbles are directed to one or more liqu7d recovery systems. 80th these ~ liquid recovery systems and the mlst elTmlnator are essentlally arranged us7ng ; ~ ~ selected conventlonal components.
At the exlt of this overall system of three spray towers, arranged ln serles as illustrated In Flgure 1, there Is another sampl7ng port used ln conJunctlon wlth perlodlc and/or contlnuous monTtor7ng equ7pment. Also an exhaust fan Is operated as necessary to control the overall speed of the gas flow t hroughout tbls wet electrostatlc scrubber.
Use of DrY Clean A7r as Purge Fluld. Insulated Inter70r Structures of SDrav Towers.` SPray Trees. and MonltorTn~ EqulDment ~20 ~ In Flgure 2, components of a wet electrostatlc scrubber 30 are schematlcolly shown In an elevatlonal cross sectlon of a spray tower 32 and lts assoclated components. Contamlnated gases~ such as those em7tted from a coal fi d power boller, enter at gas Inlet 34. Shortly thereafter, these gases pass through an aerosol partTcle charger 36 whTch utlllzes a h7gh voltage power supply 38 creatlng dtrect current used In a selected electrostatlc charger >nf7guration, such as the corona d7scharge embodlment shown 7n Flgures 2, 4 and 5. The dust, contamlnants, or aerosol partlcles, 40 are negatlvely charged and then dTrected Into the spray tower 32.
Located at a well separated hlgher posltlon Is another selected 30 electrostatlc charger referred to as the cleanslng llquld droplet charger 42~
whlch Is shown In Flgures 2, 6 and 7 In a selected dlrect charglng conflguratlon.
The cleanslng llquld enters through plpes 44 and contlnues on to the spray nozzles 46. Inserted Into the plpes 44 are Insulated hlgh voltage leads 4~, from a power supply 38, whTch are uncovered at thelr ends In the plpes 44 Just above the spray - ~04Z~66 nozzles 46. Surroundlng the non-conductlve p7plng 44, contaln7ng 7n turn the electrjcal leads 48, and supporting in turn the spray nozzles 46, are insulator structures 50 secured to the spray tower. Through the 7nterlor of the 7nsulator structures 50, clean dry a7r 7s d7rected along the p7pes 44 to purge the surround-7ng volumes thereby protectlng all these 7mportant operat70nal components from all hazards such as the otherwise occurrlng deposlts of insoluble coat7ngs.
The lower portions of the spray tower 32 have their interior surface structures electrically insulated and monltored and corrected, as necessary, as shown by the potential control un7ts 52 illustrated in Figure 2, and/or the entire 10 constructlon materials may be non-metallic, using, for example, luclte and fTberglass. Such operational control units 52 and/or non-metallTc materl~als provide the Insurance to keep the charged droplets from migrating to the 7nterior surfaces of the spray tower before they have been effect1ve in perform-7ng their role in the scrubbing function by attractlng and capturing oppositely charged contaminant aerosol particles.
The spray tower 32 is equipped downstream with a spray tree unit 54 to inject additional pos7tively electrostatically charged droplets of cleansing liquid. The flow rates of these droplets and those from above are mon7tored by subassembly 56 of the collectlng funnel 58, liquid flowmeter 60, electrometer 20 62, and recorder 64.
Also the efficiency of the scrubber is monitored as gas samples are taken by the subassembly 66 of the sampl7ng port or duct 68, electrometer 62, filter 70 and vacuum pump 72.
Electrostaticallv Charqlna the Aerosol Partlcles and the DroDlets of Cleansina Liquid In describing the three spray tower wet electrostatlc scrubber 711ustrated in Figure 1, the corona dlscharge aerosol particle charger 36 was referred to. In the schematic views of Flgures 4~and 5, the arrangement of the corona wlee frame 74 of dlscharge electrodes 76 Is shown In Its relatlve positlon 30 to the ground plates 78 servlng as the non-dlscharge electrodes. The corona dlscharge electrodes 76 are formed wlth screw-llke exterlor surfaces to enhance the effectlveness of the corona dlscharglng functlons. The flow dlrectlon of the contaminated gases by the uprlght corona dlscharge electrodes 76 Is Illustrated by the flow arrows In F1gure 5. Around the entry of the hlgh voltage leads 48 ~04Z3~6 tnto the partlcle charger 36 clean dry alr enters through duct 80 servlng the purglng functlon to keep the electrlcal energy supply free from coatlng and plugging substances.
Also In descrlbing the wet electrostatlc scrubber Illustrated 7n Flgure I, a droplet charger 42 was referred to. In Flgures 6, 7, 8, 9 and 10, three embodiments of such cleanslng liquid droplet chargers 42 are Illustrated.
Figure 6 indicates their overall cross sectlonal posltlons of their nozzles 46 in the spray tower 32 with their respectlve Insulators 50, liqu7d supply p7pes 44, purge a7r ducts 80, and hlgh voltage leads 48. In Figure 7, taken on line I0 7 - 7 of Figure 6, part of the components of the d7rect wire or direct voltage droplet charger 42 are illustrated in a partial cross sectlon. The cleanlng liquid is directed downwardly Into pipes 44, then charged at uncovered end of hlgh voltage leads 48, and thereafter emltted through the spray nozzles 46 as electrostatical positively charged droplets 82 ready to start thelr scrubbing funct70n. It is to be noted that non-conductlve piplng 44 for the liquid is pre-ferred to reduce the loss of electrical energy.
In Flgure 8, simllar to the setting of Figure 7, another droplet charger 84 is illustrated, wherein an Induction charglng plate 86 Is utilized having openlngs 88 which are spaced around the spray of droplets of cleansing 20 llquld leaving nozzles 46. Conductlve piplng 44 for the llquld is used. Also the metallic spray nozzles 46 are supported from this piping to complete the Induction charglng circult. Hlgh voltage leads 48, from a high voltage power supply 38, brlng electrical energy to the Induction charglng plate 86. The volume around tbe high voltage leads 48 Is protected by the Insulator structure 50, through which clean dry air enters from duct 80 to serve Tn purglng this sur-roundlng volume, thereby protectlng these Important operational components from coatlngs and other interference, for example, to be otherwise caused by deposlts of Insoluble materlals.
In Flgures 9 and 10, Flgure 9 belng slmllar In settlng to Flgures 7 30 and 8, another droplet charger 90 Is Illustrated, whereln corona wlres are used In corona lonlzatlon methods of electrostatlcally charglng the droplets of cleanslng llquld, shortly after thelr dlscharge fr-m spray nozzles 46. In Flgure I0, the circular conflguratlon of the corona wlre 92 and Its radlally spaced supports 94 Is 711ustrated. Electrlcal leads 48 from a hlgh voltage power _g_ "` l()~Z366 supply 38 leadlng down to the corona wires 92 are protected by employing the insulator structures 50 and the dry clean purging a7r dellvered through ducts 80. The liquid supply pipes 44 and spray nozzles 46 are made of conductive mater~als to complete the corona lonlzatlon circutt.
Control of Flow Paths of Electrostatlcally Charged Cleansing Llquld DroPlets During Thelr ResTdenc~ in the SpraY Tower by Utll7zlng Magnet7c Flelds In Flgure 11, a spray tower 32 ts schemattcally illustrated with some of tts assocTated equipment which have been pre~usly described, however, there Is added the magnetic coil subassembly 96 and Its electrical energy source 10 98. By selective use of the operat1On effects of introducing a magnetic field, the flow paths of the charged droplets 82 are controlled with one of the principle objectives of keepTng the charged droplets away from the tnterior surfaces of the spray tower 32.
In Figure 12, several smaller magnetlc coil subassemblles 100 are c7rcumferentially spaced about a spray tower to obtain a more selectlve control over the flow paths of the cleanslng liquld charged droplets. They are kept In selected motion paths during their res7dency in the spray tower to insure their maxTmum opportunlty to successfully collect many aerosol oppositely charged particles; i.e. dust, thereby increasing the overall efflciency of the 20 wet electrostatic scrubber.
The Central Posittonlng of the Cleanstng Ltquid Spray Nozzles Coupled w7th the Annular Flow of Contamlnated Gas Flow In Flgures 13 and 14, the spray nozzles 46 are arranged In a central cluster 102 and the contamlnated glas flow 7s thereby dTrected somewhat annu-larly around th7s central cluster 102 of nozzles 46. By start7ng the em7sslon of the charged droplets 82 nearer the center of the spray tower 32, thelr tendency to migrate to the interior surfaces of the spray tower Is reduced and often avoided, even In the pres~oe of an unfavourable space charge. The radlal ;~ supports 104 of these central nozzle clusters 102 are kept clean by uslng Insulator - 30 structures 50 and dry clean purglng atr. The cleanslng liquid supply llne 44 Is non-conductlve.
The EntrY of Contamlnated Gas Flrst Throuqh Surroundln~ Volume and Then Into an Inner Volume of a SPraY Tower In Figures 15 and 16, the entry of contamlnated gas Into a spray tower --10_ 16~4Z366 106 occurs flrst into a surrounding volume 108. Then the contaminated gas is redirected radlally into an inner volume 110 of this spray tower 106 The majority of the volume of the contaminated gas enters through a smaller~ hlgher supply duct 112 and the minority volume enters through a lower, larger supply duct 114 at a lower pressure. By uslng th7s dual vdiL~me spray tower conflgura-tion the cleansing liquid droplets 82 are kept from migrating to the inner surfaces of the spray tower 106 and the respectTve flow paths of the charged aerosol particles 40 and the oppositely charged droplets 82 are more conductlve to the attraction and capture of the aerosol particles 40. Also the droplets are pre-ferably formed and released by a central cluster 102 of nozzles 46.
Capture of Contaminant Aerosol Particles bV Combined Units of Cleansinq LiquTd Droplets Pos7tioned Within and Outslde of Cleansing Liquid Bubbles The embodiment of this wet electrostatic scrubber, schematicallyillustrated in Figure 17, involves the effectlve use of both electrostatically charged liquid droplets and electrostatically charged 17guid bubbles to collect oppositely charged aerosol partlcles, for example, from contaminated aerosol gases enroute to and/or up an exhaust stack. The charged liquid droplets may be of either equal or opposite polarity from the bubbles of cleansing liquids.
The corona wlres are connected to a high voltage power supply and 20 ionlze the gases and 7ncoming aerosol particles. The non-dTscharge electrodes are connected to ground. The cleans7ng llquld droplets are generated by dls-charging liquids under pressure from spray nozzles, or by discharging liquids, not under pressure, but in the presence of flowlng a1r under pressure, as occurs when a pneumatic nozzle is used.
The liquid electrostatic charge is imparted by a conductor whlch extends from a high voltage power supply. The spray tower chamber walls are constructed of either conduct7ve or non-conductive materials, or a combination of them. The Itquid supply lines are made of non-conductlve materials, and they are connected to the chamber walls by uslng flttlngs that provlde for the entry 30 and dlscharge of dry clean alr whlch purges the volume around the non-conductlve materlal serving to keep the non-conductlve supply llne materlals and some other mater7als dry and therefore substantlally free of Insoluble deposlts.

The charged aerosol partlcles and the opposltely charged cleanslng llquld droplets flow through performed plates, whlch Is made from non-conductlng _l 1--1~4;~3~6 material. Th7s perforated plate contalns circular holes through which the gases,aerosol particles, and liquld droplets flo~w The gas, particle and droplet mlxture, then bubble through a layer of liquid and foam malnta7ned at a glven level on top of the perforated plate. Thls ever charglng llquid may contaln a foaming agent to asslst in the generatlon and stablllzation of the bubbles. The charged aerosol particles are collected upon the surfaces charged to the opposite polarity, whlch are the surfaces on either or both of the liquld droplets or thebubble. Aerosol particle collect70n wlll occur upon both the inside and outside surfaces of the respective bubbles. The liqu7d and foam maintained at a given level upon the perfoeated plate is electrostatically charged by a high voltage power supply to a polarity either the same or opposlte from the liquid droplet, and to a polarity e7ther the same or opposite from the aerosol particles. The bubbles and droplets and gases then pass out the exit and on to either another scrubber chamber or to a mist droplet and bubble collector.
in Figure 18, the bubble is electrostatically charged positively, the particle Is electrostatlcally charged negatively, and the liquid droplet is electrostatically charged positively. The negatively charged partlcles are each respectively attracted to and collected upon the positively charged inside surface of the bubble, the outside surface of the bubble, or the outsTde surfaceof the droplet.
In Figure 19, the bubble is charged posltively, the particle is charged negatively, and the droplet is charged negat7vely. The negatively charged particles are repelled by the negatively charged droplets, which are located both inside and outside of the bubbles, and are attracted to and collected upon the positively charged inside and outside surfaces of the bubble.
In Figure 20, the bubble is charged negatively, the droplets are charged positively, and the particles are charged negatively. The negatlvely charged partlcles are repelled by the negatlvely charged bubble surfaces and are attracted to and collected upon the posltlvely charged droplets.
In Flgure 21, the bubble Is electr~ statlcally charged negatlvely, the droplets are charged posltlvely, and the partlcles are charged posltlvely. The positlvely charged particles are repelled by the posltlvely charged droplets andare attracted to the negatively charged Inside and outslde surfaces of the bubble.

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104~3~;6 Performance Informat70n Re: One Model of the Electrostat7c Scrubber The portable electrostatic scrubber pllot plant was cannected to the duct exhausting from the number 3 coal-flred power bo71er at the Unlverslty of Washington. PartTcle collection efflclency eests were conducted on March 25 and 26, 1974. These were the first tests conducted with our electrostatlc scrubber us7ng an emTssion from an actual operating source. All previous tests were conducted with an artificlally generated aerosol of either dibutyl phthalate, di-cotyl phthalate, or wax. During these tests the gases sampled from the number 3 power boiler exhaust were passed through a water spray IO coolTng chamber to cool the gases to about 140-150F. The cooling was per-formed in order to protect the plexlglass ducting and flberglass reTnforced plaseic scrubber chambers from being damaged by excesslve temperatures.
Simultaneous measurements of the partTcle mass concentration and particle size distribution were conducted at the inlet, downstream of the cooling chamber as shown 7n Figure I, and at the outlet down~eam of the mist ellmlnator a~id upstréam of the fan as shown tn Figure 1, using Mark 3 University of Washington Source Test Cascade Impactors.
The first test conducted on March 25, 1974 at an average gas flow rate of 585 acfm and an Tnlet particle concentrat70n of 0.165 gralns/sdcf w7th 20 part7cle charging voltage of 37 ~cilovolts, negat7ve, water chargTng voltage of 2 k~lbvolts, pos7tive, water flow of i~19 gallons per minute, water to gas flow rate rat70 of 6.7 gallons/1000 actual cub7c feet of gas showed an overall particle collect70n eff7ciency of 98.7% by we7ght and a particle collectlon efflc7ency as a function of part7cle s7ze as shown 7n F7gure 22.
The second test was conducted on March 26, 1974 at an average gas flow rate of 877 acfm and an 7nlet part7cle concentration of 0.153 grains/sdcf w7th part7cle charg7ng voltage of 30 k710volts, negat7ve, water charg7ng voltage of 2 k710volts, pos7tive, water flow of 2.2 gallons per mlnute, water to gas flow rate ratio of 2.5 gallons/IOOO actual cublc feet of gas, showed an overall 30 particle collectTon efflclency of 98.2% by welght and a partlcle collectlon efflclency as a functlon of partlcle slze as shown In Flgure 23.
These first fleld tests demonstrated the effectlveness of thls electrostatic scrubber for controll7ng the em7sslons of flne partlcles from coal-fired power bollers. Slnce these tests were conducted the electrostatic _13_ -" 104Z366 scrubber has been modifled. The part7cle charglng sections of the p71ot plant now received voltages of about 60 kilovolts, and the water charging sections now receive voltages of about 12 kilovolts. With these 7mproved particle charging and water charging sectlons, even hlgh collectlon efflclencles are obtainable for particles emltted from coal-fTred bollers.
SUMMARYOF SOMEOF THEADVANTAGES
-When the operation of these descrlbed and Illustrated wet electro-static scrubbers 7n the7r various embodiments, Is compared with the operation of wet scrubbers which are operated w7thout electrostatically charging the 10 aerosol particle collection efficiencies, especially in the range of aerosol particle sizes of 0.01 to 10 microns in diameter, are significantly higher.
These increases 7n efficlency are realized because: there Is a lower gas pressure drop wh7ch reduces the operat7ng energy requ7rements; there is a lower usage of liquid, such as water; there Is a lower liquid pressure drop which also reduces the operating energy requirements; and in initial design considerat70ns, a des7gner real7zes there 7s a reduced requirement for larger overall s7zes of equ7pment~ thereby reduc7ng cap7tol costs, as smaller s7zed units of equ7pment are operated to meet the operat70nal requirements; and in operatTng smaller sizes of equipment there are fewer surfaces and parts to 20 become coated or plugged w7th 7nsoluble solids, thereby reducing comparative maintenance costs.
Also when the operation of these described and Illustrated wet electrostatlc scrubbers, in their various embodiments, is compared w7th the operat70n of previous wet electrostatic scrubbers, there Is an overall realiza-tion of the attainment of substant7ally hlgher aerosol part7cle collect70n efficiencies, especially over susta7ned per70ds of operat70n.

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of removing aerosol particles from a gas stream comprising the steps of:
imposing a first electrostatic charge upon the particles contained in said flowing gas stream;
dispersing a cleansing liquid into droplets to contact the flowing gas stream, the droplets having a second electrostatic charge thereon;
flowing the gases containing at least a portion of said charged aerosol particles and said charged cleansing liquid droplets through an electro-statically charged bubble-forming apparatus, said apparatus having a cleansing bubble-forming liquid thereon, said bubble-forming liquid having a third electro-static charge of a polarity opposite to at least one of said first and second charge whereby the charged contaminant aerosol particles are attracted to and collected by at least one of the charged liquid droplets and the charged bubble-forming liquid.

2. The method of claim 1 wherein said first and second charges are of the same polarity and a different potential.

3. A method as claimed in claim 1 or 2 wherein said first and second charges are of opposite polarity.

4. The method of claim 1 wherein said third charge is of a polarity opposite to both said first and second charges.

5. An electrostatic wet scrubber and collector assembly comprising:
means for imposing a first electrostatic charge upon particulate matter contained in a gas stream entering a spray chamber;
liquid spray means for injecting droplets of a cleansing liquid into said chamber;
means for imposing a second electrostatic charge on said liquid to charge said droplets;
a bubble scrubber means through which said gaseous stream is arranged to flow upwardly, said bubble scrubber means comprising at least one bubble-forming means for supporting a bubble-forming liquid thereon;
means for imposing a third electrostatic charge on said bubble-forming liquid, said third electrostatic charge being of a polarity opposite to at least one of said first and said electrostatic charges; and means for recycling said bubble-forming liquid.

6. The assembly of claim 5 wherein said first and second charges are of the same polarity and a different potential.

7. The assembly of claim 5 wherein said first and second charges are of opposite polarity.

8. The assembly of claim 5 wherein said third charge has a polarity opposite to both said first and second charges.