Electrostatic Precipitation.pdf

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of proteins and other macromolecules. Continuedadvances in membrane science and manufacture andengineering improvements to modules and systemswill allow a greater penetration of this technology ina variety of industries in the future.

See also: II /Membrane Separations: Filtration; Micro-

filtration.

Further Reading

Bhave RR (ed.) (1991) Inorganic Membranes. Synthesis,Characteristics and Applications. New York: Van Nos-trand Reinhold.

Cheryan M (1998) UltraTltration and MicroTltrationHandbook. Lancaster, PA: Technomic.

Cheryan M and Alvarez J (1995) Membranes in food pro-cessing. In: Noble RD and Stern SA (eds) Membrane

Separations. Technology, Principles and Applications,p. 415. Amsterdam: Elsevier.

Cheryan M and Rajagopalan N (1998) Membranetreatment of oily streams. Wastewater treatment andwaste reduction. Journal of Membrane Science 151:15}38.

Ho WSW and Sirkar KK (eds) (1992) Membrane Hand-

book. New York: Chapman and Hall.Hsieh HP (1996)Inorganic Membranes for Separation andReaction. Amsterdam: Elsevier.

Lloyd DR (ed.) (1985) Materials Science of Synthetic Mem-branes. Washington, DC: American Chemical Society.

Matsuura T (1994) Synthetic Membranes and MembraneSeparation Processes. Boca Raton, FL: CRC Press.

Mulder M (1991) Basic Principles of Membrane Technol-ogy. Norwell, MA: Kluwer Academic Publishers.

Singh N and Cheryan M (1998) Membrane technology incorn reRning and bio-products processing. Starch/Sta(rke50: 16}23.

PARTICLE SIZE SEPARATION:Electric Fields in Field Flow Fractionation

See II / PARTICLE SIZE SEPARATION/ Field Flow Fractionation: Electric Fields

PARTICLE SIZE SEPARATION

Electrostatic Precipitation

J. J. Harwood, Tennessee Technological University,

Cookeville, TN, USA

Copyright^ 2000 Academic Press

Introduction

Electrostatic precipitators (ESPs) are the most com-monly used devices for the removal ofRne particles inexhaust from industrial and utility processes. Wire-plate ESPs consist of three or more sections of arraysof large (e.g., 15 m5 m), grounded metal collectorplates between which are situated wire or other nar-row, high voltage electrodes (Figure 1). Less com-monly, a wire-cylinder electrode conRguration isused. Particles entering the Rrst section are quicklycharged by ions generated by the plasma coronas

around the wires. (Current does pass between the

electrodes, hence the term electrostatic is not reallyaccurate, but indicates the small current-to-electrodearea.) The charged particles are drawn toward anddeposit upon the collector plates, which are period-ically cleaned by mechanical rapping. This methodis very ef Rcient in removing particles in the1}'10 m range. The most common use of ESPs isin control of exhaust from coal combustion utilities.

Precipitators are also used in the cement, ore smelt-ing, steel production, pulp and paper manufacturing,and chemical processing industries, and in wastecombustion utilities. Small units are used in cleaningdomestic and workplace air, and have been con-sidered for use in animal production facilities.

M. HolReldRrst demonstrated the removal of par-ticles by electrostatic charging in 1820. HolReldshowed that tobacco smoke can be cleared in a bottleby applying a spark-producing voltage to a pointedelectrode inserted in the bottle. In 1850, C. F. Guitardobserved that a steady corona discharge is effective in

dissipating smoke. Sir Oliver Lodge Rrst attempted

1802 II/PARTICLE SIZE SEPARATION/Electrostatic Precipitation


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Figure 1 Rigid frame electrostatic precipitator. (Reproduced with permission. Copyright Wheelabrator Air Pollution Control Inc.)

the commercial application of this phenomenon byconstructing an electrostatic precipitator to controlfumes at a lead smelter. Unfortunately, lead oxide istoo resistive to allow sufRcient particle charging witha constant DC discharge, and the application failed.The Rrst successful commercial application of elec-trostatic precipitation was by F. G. Cottrell, whodesigned an ESP to remove acid mist from sulfuricacid plants. Cottrell facilitated the establishment ofthe technique by developing the Rrst suitable high-voltage power supplies. ESP particle control becamewidely used between 1910 and 1930.

Removal of small particles from gas streams isimportant for process, health, and environmental rea-sons. Centrifugal separators are used to remove par-ticles larger than 10 m; fabric Rlter baghouses areused to removeRner particles and constitute a viablealternative to ESPs. Capital costs are similar for bag-

houses and ESPs; however, maintenance, includingenergy cost, is lower with ESP units.The increased control of sulfur and nitrogen

oxides in the 1970s, and recent calls for the controlof very Rne particles, challenge scientists and de-signers concerned with the ESP technique. Use of lowsulfur coal reduces Sy ash resistivity below that ap-propriate for conventional ESP operation. Treatmentby the addition of limestone to coal increases theparticle load on precipitators. Conventional precipi-tators do not efRciently remove submicron particles.As will be discussed in this chapter, researchers are

developing means of adapting ESP design and opera-

tion to meet the challenges of improved environ-mental control.

Characteristics of Fly Ash

Hart et al. recently performed a comprehensive in-vestigation of the composition and mineralogy ofSyash from three utility boilers. Using instrumental neu-tron activation analysis and X-ray Suorescence spec-troscopy, they found Si, Al, Fe and Ca to account formore than 90% of major elements from all threeboilers. Scanning electron microscopy with energydispersive spectroscopy revealed successive ESP ashesto be composed mainly of spherical particles whichdecrease in average diameter with increasing distancefrom the boiler. Concentrations of As, Co, Cr, Ni,Mo and Sb increased from bottom ash through thesequence of ESP ashes. These trace elements are vola-

tilized and transported to cooler regions, where theycondense or are adsorbed onto Sy ash particles. ThemajorSy ash mineral phase found by these and otherresearchers is quartz (SiO2) with magnetite (Fe3O4),anhydrite (CaSO4), and mullite (Al6Si2O13) amongother minerals commonly present.

Resistivity () is the most important property ofmaterial to be collected by an ESP. The optimumrange of resistivity is 104}1011 -cm. On collection,low resistivity particles (4103 -cm) release chargeto the collector plate and may be re-entrained. Par-ticles with'1011 -cm insulate the collector plate,

ultimately producing a sufRciently large electricReld

II/PARTICLE SIZE SEPARATION/Electrostatic Precipitation 1803


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Figure 2 Effect of surface film resistivity on flue-dust resistivity. (Reproduced with permission from Busby HGT and Darby K (1963)

Journal of the Institute of Fuel36(268): 184. Copyright The Institute of Fuel).

within the dust layer to cause a counterproductiveback corona.

Two types of resistivity may be important in par-ticle collection in an ESP. Ions collected at the surfaceof particles control surface resistivity, which domin-ates at temperatures below 2503C. As indicated inFigure 2, particle resistivity Rrst increases with tem-

perature, then decreases. Removal of the surfaceRlm(adsorbed water) by heating in vacuo at 3603Celiminates this initial increase in resistivity. Above2003C, removal of adsorbed material no longer af-fects resistivity, and at higher temperatures resistivityis attributed to ions in the bulk of the particles,

volume resistivity. Both types of resistivity are prim-arily functions of Na# and Li# ion concentrations,and in some cases K# and Iions.

Two aspects of the }T relationship affect ESPcollection efRciency. First, the maximum resistivityofSy ash occurs within the range of temperature atwhich ESPs are commonly operated, 130}1803C.

Second, SO3, produced from sulfur in coal,adsorbs onto the Sy ash particles and has traditionallybeen responsible for lowering the resistivity of theparticles to the optimum range for collection. Presentuse of low sulfur coals ((1% S) leads to inadequatecollection.



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Operation at non-optimal temperature can beavoided by lowering the temperature, but this re-quires energy input to a cooling device, and also canlead to difRculties with corrosion due to condensa-tion. The temperature of the exhaust is normallycooled by heat exchange in an air pre-heater prior toinjection into the precipitator, hence the name cold-

side precipitator. Hot-side precipitators operate attemperatures as high as 3703C; resistivity is oftenreduced to a desirable level of 21010 -cm at above3153C. However, difRculties are encountered withthe greater volume of the hot gas, and these unitsrequire more careful construction.

More commonly, resistivity of high-resistance ashis lowered by chemical conditioning.

Theoretical and Practical Aspects

of Electrostatic Precipitation

Charging of Particles

The corona surrounding a discharge electrode of anESP is a gas plasma of electrons and cations. Thecations are drawn into the electrode, which is biasednegative. The electrons are repelled into the interelec-trode space where, within one centimetre travel, theyionize gas particles. These gas particles are drawntoward the grounded collector electrodes. On theway, many of them collide with and attach to theparticles to be collected, charging them. The charged

particles are then drawn towards the collector elec-trodes. This particle charging is efRcient. Typicalcharging time is 5 ms; typical residence time in an ESPis 2}5 s.

Charging processes are of two types: Reld charging,where ions moving with the electric Reld charge theparticles, and diffusion charging, where charging isdue to collision with ions moving with random ther-mal motion. The types are distinguished to facilitatemathematical description. Field charging is dominantin charging particles with radius (a) greater than0.5 m; diffusion charging is dominant with

(0.2 m particles. Both mechanisms are importantin the intermediate size range.

The charge on an ion at timet(q(t)) resulting fromReld charging, derived by Pauthenier and Moreau-Hanot is:

q(t)"12

#20a

2E0t

t#

where is the relative dielectric constant; 0 is thepermittivity of free space (C2 N1 m1); a is the par-

ticle radius (m);E0is the electricReld (V m

1

);tis the

time (s) and:

"40/N0eb

where N0 is the number density of particles (m3);e is

the electronic charge (C); b is the ion mobility(m2 s1 V1). (This and other formulae are in the

form presented in Oglesby and Nichols (see FurtherReading.) The corresponding expression for diffusioncharging presented by White is:

q(t)"akT

e ln1#

avN0e2t)

kT wherek is the Boltzmanns constant (J K1); v is therms thermal velocity of ions (m s1);Tis the absolutetemperature (K).

Smith and McDonald derived an expression forcombined diffusion and particle charging. Theydivided the surface of a particle being charged intothree regions: region I, where the electric Reld of theparticle and that within the ESP duct intersect, sweep-ing ions onto the particle surface; region II, where theradial component of the electric Reld of the particledominates; and region III, where the electric Reld ofthe particle and duct are in the same direction, sweep-ing ions away from the particle. Giving the electricReld about the particle, Er, as:

Er"E0cos1#2!1

#2

a3

r3!ne

40r2

where Er is the radial component of electric Reld(V m1); E0 is the external Reld (V m

1); r is theradial distance to point of interest (m); is the azi-muth angle measured from the z axis (toward thedischarge electrode) (rad); ne is the particle charge(C); these researchers produced a combined chargingrate equation:

dq

dt"

dq

dtI#

dq

dtII#

dq

dtIII

"(N0Anse2/40)

1!nns

2

#ea2vN0

2

/2

0

exp!ne2(r0!a)

40kTar0

#[3ar20!r

30(#2)#a

3(!1)]eE0cos

kTr20(#2) sind

#ea2vN0

2

exp(!ne2/40akT)



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Figure 3 Modes of particle charging. }}, field charging; }}, diffusion charging; }}, total charge; }}, Boltzman charge AC.(Data from Kanazawa et al. (1993) Journal of Electrostatics29: 193. Copyright: Elsevier Science.)

where A is the surface area of the particle onwhich ions may impinge (m2); ns is the saturationcharge; 0 is the greatest angle of region from z axis(rad).

This expression agrees well with measured charg-ing rates.

Representative saturated particle charge as a func-

tion of particle radius is presented in Figure 3.

Chemical Conditioning

Conditioning can enhance collection efRciency by re-ducing resistivity to prevent back corona, increasingcohesivity of the collected layer, enhancing the elec-tricReld by increasing the space charge contribution(see below), and increasing agglomeration of smallparticles. Some exhaust gas conditioning methodswhich have been used to enhance ESP performanceare cooling by in-leakage of cold air, heat exchange to

a waste heat boiler, evaporative spray, and chemical

conditioning. The latter has proven most popular inutility applications.

Most commonly, sulfur is combusted and the SO2produced is catalytically converted to SO3. On com-bination with the combustion exhaust, SO3 hydro-lyses to produce H2SO4, sulfuric acid. The acid de-posits efRciently on Sy ash particles, decreasing the

particle surface resistivity. SO3 is added at levels of1}10 ppm; up to 30 ppm can be added without signif-icant increase in sulfur emission from the ESP. SigniR-cant SO2 is produced on combustion of even lowsulfur coal, but only about 1% is converted to theuseful SO3. Knowledge of the importance of SO3wasinitially gained through experience with conditioningof smelter dusts by sulRde ores.

Ammonium salts were found to be effective whenH. J. White was consulted on the problem of de-creased ESP efRciency which resulted from a changein petroleum cracker catalyst. He found that replac-

ing ammonia, released by the initial catalyst,



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Figure 4 Electric field on the collector electrode just under the

corona wire as voltage is applied; d, discharge electrode dia-

meter. 2D"20 cm; , d"0.5 mm; , d"0.71 mm; ,d"1.09 mm. (Reproduced with permission from Ohkubo et al.

1985.)

Figure 5 Pilot ESP V}I curves before and after development ofsevere back corona., 1 hour after startup;, 8 days after startup.(Reproduced with permission from DuBard and Nichols 1990.)

recovered ESP performance. As NH3increases collec-tion efRciency with both low and high resisitivityboiler ash, and may or may not decrease resistivity, itis thought that this conditioner functions by furnish-ing salt particles which increase the space chargecontribution to the electric Reld and, by increasingdust cohesiveness, reduce rapping losses.

Characteristics of the Corona Discharge

As voltage applied to a wire discharge electrode isincreased, visible discharge points, tufts, begin toappear dispersed along the wire. Eventually the tuftpattern stabilizes, appearing asRngers which elongateand retract at 1}10 Hz and remain at somewhat Rxedlocations along the wire. This stabilization of thecorona is indicated by a distinct increase in theslope of the electric Reld vs. applied voltage curve(Figure 4). Peek determined a semi-empirical expres-

sion for the corona onset voltage (Vc) in air:

Vc"3106 am(1#0.03/a)ln(b/a)

where Vc is the corona onset voltage (V); a is theradius of discharge electrode (m); m is the wire rough-ness factor (0.5}0.9); is the relative air density;P isthe air pressure (Pa).

This discharge produces a current density (withrespect to the area of the collector electrode) inthe order of 150 nAcm2. While the ionic composi-tion of the interelectrode space has not been wellstudied, ionization properties indicate that this cur-rent is carried by oxygen, sulfur dioxide, and water,which are negatively charged by the electron current

near the corona, and by particles ionized by theseprimary ions.

The Electric Field

Current vs. voltage curves (Figure 5) are commonlyused in monitoring the operation of ESP. For bestcollection efRciency, ESP units are operated at the

maximum voltage which produces a stable current.A discussion of operation monitoring of ESPs is givenby DuBard and Nichols.

Much effort has been given to the task of determin-ing the electric Reld within an ESP. Besides the burdenof inability to solve known expressions analyticallyfor theReld in a wire-plate electrode system, this taskis made difRcult by uncharacterized changes in thecollector electrode as the dust layer collects on it. Theadvent of high-speed computing has allowed satisfy-ing solution of theReld equations for a clean system.

Cristina et al. present a modern method of deter-

mining the electric Reld. These researchers use thefollowing expressions for charge and electricpotential:

) ()"!Q

) (Q)"0

where is the permittivity of air (C2 N1 m1); Q isthe ionic charge density (C m2); is the electricalpotential (V).

These equations have a unique solution for DC

ion Sow Relds. An iterative numerical procedure



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is used to balance the electricReld and charge densityof elements in an array of triangular elements super-imposed on the inter-electrode space. These elementsare each speciRed to have a constant ionic chargedensity. The contribution of space charge by ionizedparticulate matter is disregarded in this treatment. Inthe more dust-laden regions of an ESP, particulate

matter is found to alter electric Reld slightly.Equipotential Reld maps developed by this treat-

ment show a fairly linear decrease in potential fromthe discharge to the collector electrode, with circularRelds to about one-third distance across the channel(from 57.5 kV to 20 kV), then equipotential Reldlines parallel to the collector. Near the collector elec-trode, at about Rve-sixths the channel width, thepotential has decreased to 5 kV. Electric Relds ofadjacent discharge electrodes interact, producingbell-shaped electricReld patterns radiating from eachwire to the adjacent collector plates.

Back Corona

In working ESPs, a portion of the applied potentialis dropped across the resistive layer of the collecteddust. As the dust layer thickens the resistance,hence voltage drop, across the layer increases. Ifthe dust resistivity is high ('1010-cm), eventuallythe Reld becomes great enough to cause electricalbreakdown of gas within the dust layer, generatinga back corona. Electrons are ripped from the gas, andthe resulting positive ionsSow into the inter-electrodespace, where they attach to negatively charged par-ticles. The diminished net charge of the particulatesleads to greatly diminished collection efRciency; re-entrainment also occurs. A back corona occurs withan electricalReld strength across the dust layer (resist-ance of layer times current) of 8}12 kV cm1.

A back corona can be visually observed as a glowon the collection surface. It is also characterized by anincrease in corona onset voltage followed by an in-crease in current for a given applied voltage as com-pared with a correctly functioning system (Figure 5).

The dust layer acts as a resistive-capacitative elementin having a lag time before discharge. Both intermit-tent and pulse energization take advantage of this lagtime to allow the application of high voltage withoutthe occurrence of back corona.

Current Density

The electric Reld at ESP collection surfaces is notuniform due to both the electrode}collector geometryand to irregular resistance resulting from uneven dustlayer thickness. The parameter determined in study-

ing the situation at the collection surface is current

density, j (nAcm2). With poor distribution of cur-rent over the dust layer, collection efRciency is dimin-ished. Re-entrainment can occur in regions of lowcurrent density, and high local currents can lead toback corona even with a low average current. Land-ham et al. measured the current density distributionin a pilot scale ESP by inserting a segmented copper

electrode sensor board into a collector electrode.Conventional, intermittent, and pulse energizationwith both wire and barbed-strip discharge electrodeswere investigated. (Barbed-strip electrodes are usedby some manufacturers to force a more uniform cur-rent distribution at the collector.)

Intermittent energization (IE) is achieved by inter-rupting the rectiRer circuit output for one to twentyhalf cycles of the supply power line. A baseline volt-age is maintained, with relatively broad pulses super-imposed. In pulse energization (PE), pulses of 1 s to1 ms width and up to !75 kV are superimposed ona DC voltage set just below the spark limit. Bothtechniques have been found to save energy and toreduce the incidence of back corona. PE is moreeffective in countering problems with low resistivitydust. Dubard and Nichols set IE to one full AC cycleon and four off (duty cycle 0.2), and PE base voltage!20 kV with 50 Hz, &125 s width pulses of!25 kV. Conventional full-wave rectiRed energiz-ation (CE) was set at about!35 kV.

These researchers found that CE and PE give Gaus-sian current density proRles across the sensor platesurface, while IE gives a more even, but lower magni-tude distribution. Under good operating conditions(clean collector surface), PE had less collector areaabove dust layer breakdown current than CE or IE,with 96% useful area as compared to 83% and 86%,respectively. Under severe back corona conditions, PEmaintained 84% useful area, CE 12%, and IE 54%.Similar results were obtained with both barbed-stripand wire electrodes. The results indicate a linear rela-tionship between the increase in collector area receiv-ing useful values of current density and collectionefRciency.

Among alternative discharge electrode designs,barbed plate discharge electrodes appear moreeffective in producing uniform current distribu-tions (see McKinney and Davidson). This designmay also have some advantage with respect to Sowturbulence.

Flow

Besides electricReld and current density distribution,gasSow is important in ESP performance. ESP Sowvelocities vary between 0.2 and 2.0 m s1. Schwab

and Johnson suggest an optimum velocity of between



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1.22 and 1.52 m s1. Lower velocities diminish theturbulent mixing which brings small particles into thelow Sow region near the collector electrode; highervelocities overwhelm electrostatic attraction ofparticles to the collector electrode, and can lead tore-entrainment.

Flow within an ESP is inherently turbulent due to

high gas Sow velocity, which typically results ina Reynolds number of about 10 000 }Rve times thatat which turbulent Sow replaces laminar Sow. Turbu-lence in ESPs is complicated by Sow obstructions}discharge electrodes and collector stiffening bafSesand connectors, and by a non-uniformSow proRle atthe ESP inlet. At low velocities (0.5 m s1) turbu-lence resulting from the ion current between dis-charge and collector electrodes (ionic wind) contrib-utes signiRcantly. TheSow regime in an ESP is thusseen to be quite complicated. BafSes within the ESPduct and porous plates at the inlet and outlet areemployed to create uniform Sow.

Schwab and Johnson have produced a computermodel based on Navier}Stokes Suid Sow equations asan alternative to traditional reduced scale physicalmodels for Sow design. The model can be used todetermine inlet plate perforation patterns which pro-duce uniform Sow without high Sow near the ductwalls.

Collection Ef\ciency

Most important in ESP operation is the particulatecollection efRciency, , the fraction of particulatesremoved. Around 1920, Deutsch and Anderson inde-pendently derived an expression for , now com-monly referred to as the Deutsch equation:

"1!exp!A

Qe

whereA is the collector electrode area (m2); Q is thevolumetric gas Sow rate (m3 s1); e is the

effective particle migration velocity (m s1

).EfRciency is also expressed as fractional particlepenetration,P"1!.A/Qis the speciRc collectionarea (SCA), the parameter of importance in sizing anESP. Assumptions of the Deutsch equation are thatparticles are instantaneously fully charged and accel-erated, turbulent and diffusive forces cause particlesto be distributed uniformly in any cross section of theprecipitator, the velocity of the gas stream does notaffect the migration velocity of the particles, particlemotion is governed by viscous drag where StokesLaw applies, the effect of collision between ions and

neutral gas molecules can be neglected, and there are

no disturbing effects such as erosion, re-entrainment,uneven gas Sow distribution, sneakage, or back co-rona. A number of researchers have adjusted theDeutsch equation to account for neglected factors.Especially questionable is the assumption of completeturbulent mixing (for instance, see Zhibin andGuoquan). The Deutsch model represents one mixing

extreme, with the other extreme being laminarSow (noturbulence). The Deutsch model tends to underestimatefor particles with diameter less than about 1 m, andoverestimates the collection of larger particles.

The Matt}OG hnfeldt modiRcation attempts to ac-count for variation in particle size distribution:

"1!exp!A

Qk

m

where k is the particle migration velocity (m s1);

m is an exponent which depends on particle sizedistribution.

An m of 0.5 is commonly used with Sy ash orcement dust; increasing values can be used withRnersize distributions.

In comparing ESP performance under varyingconditions, the migration velocity is more useful than, being independent of ESP collection area and gasSow rate. An important consideration in using values is that migration velocity of large particlesdecreases at a much lower rate with increasing gasSow (or decreased plate size) than that of small par-ticles, hence though small particles will be less efR-ciently collected, measured can increase with gasSow rate.

Migration of charged particles can be treatedtheoretically. Hall presents the expression:

"40kaE0EpCDf

6

where k is 3/(#2); is the dielectric constant of ash

(C N1

m1

);E0is the particle charging electricReldstrength (V m1); Epis the particle collecting electricReld strength (V m1); ais the particle radius (m); C isthe Cunningham slip correction; D is the factor toaccount for diffusion charging ofRnes;fis the factoraccounting for gas turbulence and free electron charg-ing; is the gas viscosity (Pa s); is the fractionalparticle saturation charge.

C is used to adjust diffusion of particles with sizecomparable to mean-free path of gas molecules. Thereader can understand that empirical estimation of values, from measured efRciency, is often more

productive than theoretical estimation.



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The Deutsch model is often used in determininga minimum speciRc collection area in sizing precipita-tors. Extrapolation from performance of pilot orother ESPs should be made only with similar designsand similar dusts.

Computer ModellingSeveral comprehensive models of particle collectionin wire-plate ESPs are available, including theSouthern Research Institute (SRI), Research TriangleInstitute Sectional (RTIS), Italian Electricity Board(ENEL), and Electric Power Research Institute(ESPM) models. As an example, the ESPM model willbe brieSy described (see Lawless and Altman).

In ESPM, the applied voltage for a given currentdensity is Rrst determined for a position directly be-neath the wire:

V"Vc#Ecrwz!1!lnz#1

2 whereEcis the electricReld at wire surface at coronaonset (V m1); rw is the radius of wire (m) and:

z"1#jb

0b

Vw2

whereb is the wire-plate separation (m); is the ion

mobility (m2

s

1

V

1

).Current densities at other plate locations are cal-culated as:

j()"j0cosn()

where is the angle from between wire and plate(rad);n is a function ofj, 2 (near corona onset) to 4.

Ion mobility is based on that of SO2 and SO2.Space charge resulting from the charging of particlesis treated as an equivalent ion current and used toadjust the operating voltage.

Non-dimensional parameters are used in models toadd generality to results and to simplify mathematicaltreatment. For instance, the initial particle chargingrate in an ESP model (ESPM), combining both diffu-sion and Reld charging, is:

d

d"

3w

41!

3w2

#1

where is the non-dimensional charge; is the non-dimensional exposure time;wis the non-dimensional

electric Reld.

Intermediate and saturation charging rates aremodelled by different expressions. Back coronacharging can be modelled with ESPM, though betterunderstanding of this process is needed to assureaccurate results.

Particle collection is modelled with a modiRedDeutsch expression:

P"1!A/Q

NwNw

Nw is a parameter to adjust for turbulence: a highvalue approaches Deutschian total mixing, a lowvalue approaches laminar collection (laminar whenNw"1). To correct for uneven gasSow, penetrationis calculated at several gas velocities and the resultscombined by weighted averaging. Rapping loss canbe speciRed or estimated by applying conventionaldynamics to the falling of agglomerated cake duringcollector cleaning. The model treats each ESP sectionseparately, remixing particulates exiting from eachsection.

ESPM can be used to predict V}I curves, gradeefRciency (penetration as a function of particle dia-meter), and other aspects of ESP performanceunder varying conditions.

Innovative ESP DesignsSeveral new means have been developed to facilitatecollection of submicron and highly resistive particlesby ESPs.

Pulse energization is discussed above (see Currentdensity). By superimposing a brief, high voltage pulseon a DC voltage set just below the back corona level,efRcient Rne particle charging and collection are ob-tained with highly resistive dusts.

Enhanced particle agglomeration can also facilitateRne particle collection. For instance, Kanazawaet al.

have accentuated particle agglomeration witha precharging section, in which the gas stream isdivided and particles in the two sub-streams giveneither a positive or a negative charge. On recombin-ing the sub-streams, particles agglomerate throughelectrostatic attraction. Watanabeet al. have devisedan alternative method utilizing a three sector design.Large particles are removed in the Rrst ESP sector.A modiRed quadrupole electrode system in the sec-ond sector applies an AC Reld to enhance particlecollision, hence agglomeration. The third section isagain a conventional ESP, which removes the agglom-

erated particles.



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Fabric Rltration is the surest means of removingRne particles. EPRI has devised the Compact HybridParticulate Collector (COHPAC). This design simplyplaces a baghouse after an ESP. The ESP removesmuch of the particulates, easing the load on thebaghouse, hence reducing maintenance. The bag-house reduces pollution due to rapping loss, and is

insensitive to changes in fuel.EPRI has also developed an EPRICON process

which can replace conventional chemical condition-ing ofSy ash from low sulfur coals. In this process,a portion of the gas stream is diverted to a vanadiumoxide-based catalytic unit, which efRciently convertsSO2 to SO3. Recombination of the treated streamwith the bulk results in the necessary conditioning oftheSy ash.

See also: Particle Size Separation: Hydrocyclones for

Particle Size Separation; Sieving/Screening.

Further Reading

Busby HGT and Darby K (1963) EfRciency of electrostaticprecipitators as affected by the properties and combus-tion of coal. Journal of the Institute of Fuel36(268):184.

Dubard JL and Nichols GB (1990) Diagnosis of electricaloperation of electrostatic precipitators.Journal of Elec-trostatics25: 75.

Hart BR, Powell MA, Fyfe WS and Ratanasthien B (1995)Geochemistry and mineralogy ofSy-ash from the MaeMoh lignite deposit, Thailand.Energy Sources 17: 23.

Kanazawa S, Ohkubo T, Nomoto Y and Adachi T (1993)

Submicron particle agglomeration and precipitation byusing a bipolar charging method. Journal of Electro-statics29: 193.

Landham EC Jr., Dubard JL and Piulle W (1990) The effectof high-voltage waveforms on ESP current density distri-butions. IEEE Transactions on Industry Applications26(3): 515.

McKinney PJ, Davidson JH and Leone DM (1992) Currentdistributions for barbed plate-to-plane coronas. IEEETransactions on Industry Applications28(6): 1424.

McKean KJ (1988) Electrostatic precipitators. IEE Pro-ceedings Pt.A. 135(6): 347.

Oglesby S Jr. and Nichols GB (1978) Electrostatic Precipi-

tation. New York: Marcel Dekker Inc.Tachibana N (1989) Back discharge and intermittent ener-gization in electrostatic precipitation ofSy ash.Journalof Electrostatics 22: 257.

Watanabe T, Tochikubo F and Koizumi Y et al. (1995)Submicron particle agglomeration by an electrostaticagglomerator.Journal of Electrostatics34: 367.

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Field Flow Fractionation: Electric Fields

S. N. Semenov, Institute of Biochemical Physics,Russian Academy of Science, Moscow, Russia

Copyright^ 2000 Academic Press

Introduction

Field Sow fractionation (FFF) represents a class ofseparation techniques, which use a forceReld perpen-dicular to the direction of separation to control thelongitudinal velocity of particles injected into thesystem. It is achieved by particle redistribution in theSow with a parabolic velocity proRle due to the ac-tion of a transverse force. This transverse force maybe due to an electric Reld, a centrifugal or gravityReld, etc. In electric FFF (ElFFF), the transverse move-ment of the particles is caused by an electric Reld.

The transverse particle velocity, U, i s deRned by

the expression:

U"b ) E [1]

where b is the particle electrophoretic velocity, andEis the electricReld strength in the channel interior,

which is available both for the particles and the Sowof the carrier liquid. The particle electrophoretic mo-bility is related to the particle electrokinetic potential(zeta-potential):

b"

4f

R

[2]

whereis the dielectric constant of the carrier liquid, is the carrier liquid viscosity, is the particle elec-trokinetic potential, R is the particle diameter,

and is the Debye length characterizing the screening

II/PARTICLE SIZE SEPARATION/Field Flow Fractionation: Electric Fields 1811

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