Removal of adhesive dusts from flue gas using corona discharges with spraying water

Journal nf Erarwroremerul riences vol. 15. No. 4. PP. 554--561, 2003Article I:10010742(2003)04055408CL C number: 0646; X513 Document code: ARemoval of adhesive dusts from flue gas using corona dischargeswith spraying waterXU De-xuan, ZHAO Jian-wei, DING Yun-zheng, GE Wei-liDeparment of Environmental Science and Engineering. Nortveasl Nornal University, Changchun 130024. China. E-mail: jwzhan2 (4 )163Abstract: Effective removal of adhesive and fine dusts from flue gas is very difficull. A uew inethod of electrostatic precipitation of the coronadischarges with spraying water( CDSW)was introduced. A new electrode configuration and the circulation spraying of water were employed inthe methed. The efficient eiertostatie preeipitation for adhesive and fine dusts ran be accomplished without any drain water durinG aoperating period. The fundamental structure, discharge characteristics, Mechanism of spraying andion principle of the electrostaticnuperalure and clean of the electrodes wrre rewarrhrd in serirs experimentS. Thr treuting rffects of circulating spraying using the nce ofusing CIw were described and anAlyzed. The v-I characteristics, spraying state, supplying water quantity, influeplasma at the same time of electrostatic precipitation were investigated. The fundamental theories and experimental data were propped,in urderfectively remove the adhesive dusts from flue gas using Nsw in praeticeKeywords: teruel with adhesive dusts: corona discharges with spraying water; electrostate precipitationIntroductionIn the varied contamination sources of atmosphere there are some kinds of exhaust gases. A largenumber of adhesive and fine suspending paricles with the diameters of micrometers or submicrometer occurin the gases, for example, the flue gases of asphalt or tar as well the cooking fume in Chinese cateringIrade. It is very difficult to remove the adhesive dusts or droplets from flue gas using the: traditionalrecipilatinn methods. Although the filtrating method is able to remove the fine dusis, the adhesive dustsare easy to slop up the filtrating holes and can not be cleaned. Therefore the: frequent changes of filtratingmaterials are necessary and the increases of both equipment maintenance and operating cost areunavoidable. The traditional wet electrostatic precipitation is able to remove the adhesive and fine dusts aswell to keep the plate electrode cleaning, but the discharge electrodes have to be covered by the adhesivedusts, even though the intermitted scrubbing with water is employed. Therefore the corona dis: harges arequenched and the precipitation effecis of the equipment losta kind of charged drmplel scrubbing was developed to form the sprays of high voltage jets according tohe principle of electrohydrodynamic( EHD)spraying( Lear, 1975 ). The high-voltage insulation of thesupplyingelectrostatic precipitator with discharge electrode spraying was able to keep both discharge and plateelectrodes cleaning( Xu, 2001), the safely and stable operation during a long period was difficult, becauselage insulation of the supplying water sy Hlem was accomplished by mechanic sprayersAn eletRostatic precipitation method using the corona discharges with spraying water( CDSW )wasintroduced in this paper. The method employed new electrode configurations and avoided the difficulty ofhigh-voltage insulation in the supplying waler system. The CDSW was formed on the discharge electrodesaccording to the EHD spray and corona discharge mechanisms. The discharges not only keep the cleaning ofboth the discharge and collecting electrodes using a small amount of water, but also can treat the sprayingdischarge electrodes. There fore the efficieelectrostatic precipitation for a long period is realized and a large amount of drain water is avoided in someapplicationsIn this paper the experimental researches of the CDSw were introduced in detail. The discharFoundation itrm: The National Natural Science Foundation of China( No. 50277044)中国煤化工CNMHGRemoval of adhesive dusts from flue gas using corona discharges with spraying water555nechanism and prec ipilalion principle were analyzed. The treating process of discharges on circulalingsprays wasI Experimental apparatusThe experimental apparatus for the electrostatic precipitation of Cisw is shown in Fig. IIn the contrast with the traditional electrostatic precipitation, in thelectrostatic precipitation of CDSW the plate electrodes 2 were connectedto the llv power supply through cable 7. The discharge electrode I wasgrounded through the tube dividing water 12, soft tube: 3. valve 4 andwater container 5. All the processes of precipitation, the scrubbing forclectrodes and the plasma treatment for circulating water were conductedhel ween the discharge electrode and the plate electrodes. Thedroplets between the electrodes flowed down into a water basin 9were pumped by the pump 10 into the water container 5 throughlube: 6. Therefore the circulating water was forned. The: supplying watersystem connecting with the discharge electrode was grounded and thedifFicult HV insulation of supplying water system in Iraditional methodS:hematic: nf experimentalwas avoided by this way. The discharge electrode with smaller curvature apparatus for electrostatic precipitator ofradius can form non-uniform electric field and produce stable corona romna discharges with water sprayingdischarges under the induction effecls of hv plate electrodes. The 1.discharge electrode;2. IIv platedischarge electrode was made fron the stainless steel wire with 1 mm electrode: 3.sufi tube: 4valve: 5.waterdiameter and the length of the electrode was 700 mm. The waler on the container; 6, water tube: 7. Hv ahle:discharge electrode flowed down from a hale of the tube dividing water pump;ll.overflow tube:12.tubeand there are some approac hes for avoiding blockage in the hole. The dividing waterHV plate electrodes of stainless steel were 500 mm in width and 900 mmin height. The HV electrodes were suspended and fixed on the insulating frame:. The spacing D betweenwo plate elecTrodes was adjustable as 300 mm or 400 mmIn most electrostatic precipitators negative corona disc harges areaveThichel discharge model( Zhang, 1990). Therefore in the electrostatic preeipitation of CDSW the: positiveHV DC power supply was employed, in order to generate negative corona discharges on the dischargeelectrode. The HV value was measured by the HV probe from the plate electrodes. Between watercontainer and grounded wire an amperemeter was inserted in order to determine: the electric current valuesin the practical applicalwas reading the current value in theexperiments, the pumping water should slop, in order to insure all the discharge current to flow through theamperemeterexperiments were carried out in room temperature. When we: measured the influenees of watertermperature on the discharges, the heated water was put in the water container and the water temperatureswere respectively measured before and afler applying HV under the water hole2 Experimental results and discussions2.1 Spraying process of discharge electrodesWhen posilive HV is applied on the plate electrodes opposite to the water- wire electrode, the non-uniform elee Iric field is formed between wire and plate electrodes. The very strong electric field occurs nearthe of the water-wire electrode, because the curvature radius of the electrode is much smaller thanthat of plate elecinMdes. The electric field produces an electrostatic force, which is outward andpendicular to the water surface. The electrostatic force is proportional to the electric field strengt中国煤化工CNMHGXL De- xuan et alVol, I5Therefore the instability of water surface increases and the Taylor Cone is formed. At firsl the water surfaceis uneven, after that the protrusions are: foreed by both the electrostatic force and the gravity then extendoutward to form filament flows. When electric field is strong enough, the numbers of filament flows increaseand at their ends Ehd spray phenomena are produced( Hara. 1981: Jaworek, 1999; Borra, 1999).Thespraying droplets with diameters bigger than 20 um were observed in the similar condition( Huneiti, 199Peter, 2000).When vollage is higher than the onset voltage of corona discharges, besies the mec haof EHD spray, the water sputtering is produced too( Moon, 1998). At this time, the positive ions wereproduced by corona dis: harges and bombard the water surface of water-wire electrodeThe spraying droplets of CDSw were analyzed by microscope, when the discharge electrode wasgrounded. The: spacing het ween discharge electrode and plate electrode was 50 mm. When HV was12.7kv, the stable corona discharges occurred and pe Hfecl multi-dispersion spray was formed. A lol ofdroplets with a diameler about 100 pm existed in the aerosol. But there are only a few droplets with adiameter above I mm in it. The mass of the droplets with a diameler above 80 um was above 99.6total droplet mass. Therefore the droplets above 80 um prolueedl by EHD spraying play a very importantrole in the electrostatic precipitation of CDSw2.2 Discharge characteristics and capture for particlesAfter the water-wire electrode produced EHD spraying, we continually increased the voltages of platelectrode to the onset corona voltage. At this moment, the electric field near the wire electrode was highenough to accelerate the free electrons, which occurred in this region. The electrons bombarded gasmoleculae and ionized a few of them at first. Though treated nue gases usually were negative electric onesand water moleculae had high electron attachment coefficient, eletron avalanche was easily produced ancorona discharges were formed near the wire electrodeThe critical electric field( Ec)for the formation of corona discharges on the surface of water-wireelectrode is determined hy Peek formulaWhere f is the roughness coefficient of surface(f=0.6-1):8 is the relative density of flue gas withspraying droplets: k=3.08 x 10(m"): r is the radius of discharge electrode. The corona onset voltageVe in a wire-plate system with a single wire electrode can be shown as follows Moor, 1972)Ve =r Ec In(4s/r)Where s is the spacing belween wire and plate electrodesIn the CDSW, although the wire electrode is grounded, it can form discharge electrode and producestable corona discharges according to the electrostatic induction principle. Moreover the discharges keep theprincipal mechanism of corona discharges. The dis harge model and its purifying function are obviouslydifferent from traditional corona discharges, because the waler layer takes part in discharge proeessWhen the spacing D between two HV positive placharacteristics of the corona discharges with dry discharge electrode or spraying one were respectivelymeasured. Their V-I characteristic curves are shown in Fig. 2 and Fig-3Fig 2 and Fig 3 show that the onset voltages of negative coruna with spraying water were obviouslylower than thal of dry corona discharges, whenever the spacing between plate electrodes was 200 or 300mm. This phenomenon is attributed to the different radius of discharge electrode according to Eq (2).Theradius of dry corona discharge electrode was 0. 5 mm. In negative CDSW, the onset voltages weredetermined by the radius of filament flows, which were produced by EHD instability of water surface. Thefilament flows extended from discharge electrode downwards and outwards. Their radius can get smallerthan 0. 2 mm, which were much thinner than that of dry corona discharges. Therefore the onset voltage ofnegative corona with spraying water was lowed中国煤化工CNMHGNo, 4Removal of adhesive dusts from flue gas using corona discharges with spraying water557Dry electrodesectrodes◆ Dry electrodesR寻子导导的5017320252502753032535037540Voltage V, kYVoltage V.kYFig 2 T- V characteristics curves ofFIK. 3 I-V characteristicsThe curves demonstrated that current of negative CDSW was higher than that of dry negalive coronadischarges under the same voltages. In dry negative corona discharges, discharge current consisted of freeelectrons and negative ions existing between two electrodes. There were a lot of free electrons and negativeions produced by electron attachment on molecular of negative electric gases in the ionized region near thedischarge electrode. Both electrons and negative ions moved to positive plate electrodes under the electricfield. Because the electric field strength decreases steeply with the increase of distance, the free electronsout of ionized region had no enough energy to ionize gas molecular. Therefore more and more electronsattached on the gas molecules and formed negative ions. At last, almost all of the charge carriers collectedon the plate electrodes were negative ionsIn the CDSW, besides above process lots of highly charged droplets were produced. Because thewater surface on discharge electrode accumulated lots of negative charges under clectric field, the waterdroplets had to carry the charges and formed charged droplets when they left the water surface. The dropletscan be charged further in the near-ionized region of corona discharges. In this region not only the strengthof electric field was very strong, but also the concentration of both free electrons and negative ions wereluch higher. The droplets were further charged by ions according to mechanism of electric field chargingand diffusion charging. Especially a large number of free electrons existed in the near-ionized region( Xu1997).The free electrons have higher energy than gas molecule ions. Therefore they not only chargedbigger droplets effectively, but also charged smaller droplets continually, which could easily achieve theirsaturation charges in the process of ions charging(o Hara, 1989; DuBard, 1983)Under the same voltages, the current of negative CDSW was higher than that of drynegative coronadischarges. But the corond current in comma discharge of nue gas contained gaseous steam was lower thanat of dry negative corona discharges under the same voltages. The reason was that there were ions and afew free electrons in the current of dry corona discharge. In the negative CDSW, besides ions and freeelectrons there were a large amount of charged droplets in charging spacing. This was different from theexperiments of corona discharge in the flue gas contained gaseous steam, in which corona eurrent decreasedbecause of higher electron attachment coefficient of the gaseous sleamThe highly charged droplets had high charge-mass ratio. Their migration velocity was about 30 m/sunder electric: field Lear, 1975). They had efficient electrostatic and dynamic agglomeration for the dustsin flue gas( Kazimierz, 2001), which never occurred in the traditional electrostatic precipitators. Thereforee precipitation of ( w had higher precipitation efficiencyIn the traditional electrostatic precipitators, it is difficult to form a large number of both highlyharged and flying droplets. The injected droplets from outside of the electric field can not approach the中国煤化工CNMHG58XL De-xuan et alVol, 15near-innized region. because there was strong electric wind near the discharge electrode. Generallyinjected droplets have no charges. They could not be highly charged by ions and free electrons in the2.3In CDSW, the spraying of water on the surface of discharge: clectrode was produced continuously. Sodischarge electrOde can be cleared during the operation period, which could nol be realized in thetraditional electrostatic precipitationIn order to demonstrate the cleaning effects of the plate electrodes, a thinwas smeared on the plate electrode before the operation. The spacing betweeelectrodes was 300m, water flow rate on the discharge wire was 464 ml/ min for each meter of discharge electrode, positihigh voltage was 55 kv and current was 530 HA. The results are shown in Fig. 4 after 3 minutesdischarges. The dark area on plale electrode was thoroughly cleared by waler. At the top of plate electrodethere was a small area, which was not cleaned. The area would become smaller, if the spacing betweenIn the CDSW, flying droplets bombed plate electrodes and formedwater film on their surface, And the discharge electrode was covered bydropping water along wire. Both collecting electrodes and dis : hargeadhesive liquid or solid particles in lue gas could not adhere to collectingney bumped water film of plateelectrodes and were brought away by water film immediately. In this wayoriginal contamination on plate electrodes was removed by flying dropletprecipitation of the: CDSW not only saves water, but alsog. Cleaning effects of plate has higher effects for cleaning plate electrodes2. 4 Influence of water temperature on the dischargeIn the negative CDSW, when water temperature: respectively was 22C and 64C, influence of waterlemperature on discharges is shown in Fig. 5. The discharge currents had no obvious increase under usualoperating high voltages, when water temperature ine reased from 22C to 64"CIn the usual corona discharges of heating electrode or high 60temperature flue gas, the influence of temperature on corona 550+Water 22tdischarges was researched carefully( MeDonald, 1980; Moon, sooF Water 64t2000). There was no obvious change of discharge characteristic 3450below 100C. In the CDSW, sometimes the higher lemperature Ewas applied. If the water temperature is below the boiling 300temperature of the water, the obvious increase of discharge 2505/current also can not occur. This is because of that higher walertemperature increases both the secondary electron emission0425450475500525550coefficient of flue gas. The former makes the current ine reasing, Fig. 5 Influence of water temperature m thebut the latter makes the currenl decreasing2.5 Treatment of spraying water using the dischargeThe corona discharge and EHD spray have function of decolorization and sterilization. The experimenton the decolorization of indigo solution was carried out by authors using CDSw (Wu, 2001).The initial中国煤化工CNMHGRemoval nf adhesive dusIs from Aue gas usingdischarges with spraying water559oncentration of aqueous solution of indigo carmine( Ci H N2 Na2 Oy S25.0xnol/L. Aspeclropholotneter(I cm path length) was used for measuring decolorization rate. When air gap was 30 mmand high voltage was 25 kv, the decolorization rate can gel lo 80.8% in 30 minutesThe experiment of killing bacteria in water using EHD spraying has been reported(Tee, 2001). In theexperimental apparatus, the inner diameter of nozzle was 0.2 mm and the inner diameter of ring electrodewas 20 mm. The survivability of Wscherchia coli treated only once decreased to 2,8%, when high voltagewas 5.4k\The mechanism of decolorization and sterilization using the corona discharge or/and EHD spray is notvery understandable. Many researehes have been done on the DeSO, and the DeNOx from flue gas, thedecolorization as well sterilization in water using the pulsed corona plasma( Masuda, 1990; Chang, 1989Clements, 1989), The researches demonstrated thal the: active species are dominant factors for treatinggases or water using the pulsed plasma. In the CDsw a cerlain number of energetic electrons also orcutnear disc harge electrodes, It is reasonable to deduce that the decolorization in CuSW can attribute lo theactive species, such as OH,O, H2 O2. 0, and so on. The possible reactions for the generation of EtiveO+O2+M-03+M0+H2O→20H,3462OH→H2O(6)There the third body M is 02, 03, N,, O and so anAlthough the average energy of electrons in the CDSW is lower and plasma region is smaller than thosein pulsed corona discharges, the experiments have demonstrated that treating waler is very effective.Thiscan be: attributed to the special shapes of treated water. In the CDSw, all the waler has to form filamentnows and droplets, which are surrounded by comma plasma region and active species during a short timeTherefore the reaction areas bet ween plasma and treated water are significantly enlarged and the masstransfer of the active species into water is considerably improvedThe corona plasma region in CDSw is larger than that in usual corona discharges. The expandingplasma region was observed in corona discharge on waler drops dripping from a conductor under DC highroltage( Hara, 1981). In the CDSW, corona plasma is produced by filament flows, which extend outwardsand downwards from wire electrodes. Sometimes the droplets near discharge electrode are highly charged so)thal corona discharges occur on the droplets. All these expand the corona plasma region from wireelectrodes, which improved efficiency of treat ing wasterThe CDSw has the potential for removing some harmful gases from flue gas. An experiment of DCcorona discharge over waler surface demonstrated that the DC. corona with the presence of waler withprovides excellent effecis for NOx removal processes( Tomio, 2001)In the CDSW, waler quantity for cleaning electrodes was much smaller than that of traditional wetelectrostatic precipitation. So it is possible to frequently treat the water, when circulation water system ismployed. The odor cun be avoided, and the colors can be removed soorganisms can be killed inrder to prevent the equipment from corrosions and blockages. Even though the circulaling water includessome harmful resolved compounds after a long time operation, its spraying could not adld the pollution offlue gas. Most droplets have higher charges and bigger sizes of above 80 !um. They are easily removed bycorona discharge fom flue gas instead of mixing with atmosphere3 ConclusionsBased on theoretical and experimental researches, a new electrostatic precipitation method using中国煤化工CNMHGXU Dexuan et alVol. 15CDSw was proposed, in order to effectively remove the adhesive and fine dusts from flue gasIn the CDSW, both the disc harge electrodes and supplying water system were grounded according toe principle of electrostatic: induction. Therefore the difficulty of high- voltage insulation in the suppliedwater system of discharge electrodes was avoided. The stable and safe operations of the precipitators wereinsuredWhen the quantity of the supplied water was 464 ml/min for each meter of discharge electrodes,alarge number of water droplets with a diameter of about 80 um were produced from discharge electrodesaccording to the EHD spraying principle. The droplets bombarded water film of plate electrodes with thevelocity of about 30 m/s. And the discharge electrode was covered by dropping water along wireTherefore, both discharge electrodes and collecting electrodes were effectively cleanedResides the precipitation mechanism of traditional wet electrostatic precipitation, the CDSw addedsome new precipitation mechanisms on the precipitators, such as the dynamic agglomeration of fyingroplets, the electrostatic agglomeration of charged droplets and the eleclron charging of fine particleTherefore higher precipitation efficiency for fine dust was achievedThe spraying circulation water in the CDsw was treated by corona discharge plasma near the dischargeelectrodes. The color of some dye was removed and the some harmful bacterium can be killed in the waterThe water quantity employed in the CDSw was rather little. The cireulating water can be repeatedlytreated by corona discharge plasma in the precipitation process. The droplets are easy removed by coronadischarge from nue gas instead of mixing with atmosphere. All above factors providedconvenientconditions for using the circulating water during a long periodReferencesBorra J P, Tombette Y, Ehouarm P. 1999. Influence of electric field pmfile and polarity on the mode of EHDA related to electric: dischargeregimes[J|. Aerosol Sci, 30: 913-925Chang Js. 1989. The role of Hy 0 and NH, on the formation of NH, NO, acrosol particles and DeNOx under the coroma discharge trealment ofombustion fue gas[ J Aerosol Sci, 20: 1087-1090Clements IS, Sato M, Davis R H, 1987. Preliminary investigation of prebreakdown phenomena and chemical reactions using a pulsed HVdischarge in water[ JI. IEEE Tra IA, 23: 224-2Dubard I L. MeDonald J R, Spark L E, 1983. First measurement of aerosol particle charging by free elec trons--A preliminary report.JIAcrosol Sci,4:5—10.Hara M, Akazaki M. 1981. Onset mechanism and development of corona discharge on water drops dripping from a conductor under high direetvoltage[J]. J of Electrostatics,9:339-353Huneiti Z. 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J of Electrostatics, 25: 221( Received for review June 5, 2002. Accepted November 13. 2002)中国煤化工CNMHG