AIR POLLUTION CONTROL EQUIPMENT SELECTION GUIDE - CHAPTER 20 (end) ppsx

of either a low level shown or high level gas inlet, collecting tubes, masttype electrodes mounted on a grid or frame, a high voltage insulator section,an air-purged insulator compartmen

of either a low level (shown) or high level gas inlet, collecting tubes, masttype electrodes mounted on a grid or frame, a high voltage insulator section,

an air-purged insulator compartment to prevent particulate from coating thehigh voltage insulator section, a high voltage power supply (trans-former/rectifier set), and a gas outlet

The designs also include various types of cleaning or irrigation systemsthat are used to purge the tubes of captured particulate These purge systemsmay include fog nozzles, spray nozzles, or weir type irrigation systems

Typical applications and uses

WESPs are frequently used to collect submicron particulate that arises fromcombustion, drying operations, process chemical production, and similarsources They are also used as polishing devices to reduce particulate load-ings to extremely low levels They are generally used where the inlet loading

of particulate is under 0.5 grs/dscf and where corrosive gases may bepresent They also excel where the particulate is sticky but can be waterflushed They often replace fiberbed filters or similar coalescing deviceswhere solid particulate is present that could plug the fiberbed design.Wet precipitators are increasingly being used as final cleanup devicesbehind and in combination with other air pollution control devices

* This chapter is contributed by Wayne T Hartshorn, Hart Environmental, Inc., Lehighton, Pennsylvania.

Applications include chemical and hazardous waste incinerators; hog fuelboilers; acid mists; steel mill applications; vapor-condensed organics;nonferrous metal oxide fumes from calciners, roasters, and reverb fur-naces; phosphate rock; veneer dryers; sludge incinerators; and blue haze

emis-is a good candidate for its control On themis-is unit, the WESP emis-is in the center

of the picture and a droplet eliminator and fan is to the left of center Thegas flow is downward, thereby flushing solids toward the sump, assisted bygravity The bypass stack for the dryer can be seen in the background

Primary mechanisms used

Electrostatic forces as well as diffusional forces are used to accomplish theseparation On some designs wherein the collecting tubes or surfaces are air

or liquid cooled, thermophoretic forces are also used In general, a series ofzones are created wherein electrostatic forces sweep the particulate from thegas stream toward the contact (collecting) surface, which is periodicallyflushed with water to prevent the buildup of a resistive layer

Figure 20.1 WESP components (Entoleter, Inc.).

CLEAN GAS OUT

PURGED INSULATOR COMPARTMENT

DIRTY GAS IN

COLLECTING TUBE

HIGH VOLTAGE INSULATOR ELECTRODE SUPPORT BEAM

To a minor extent, the WESP is also a gas absorber The flushing systemcan also provide some mass transfer of contaminant gases into the liquid.

Figure 20.2 WESP on veneer dryer (Geoenergy International, Corp.).

Figure 20.3 Particle board dryer WESP (Geoenergy International, Corp.).

of both) pass through this corona and are moved toward the collectingelectrode where they momentarily attach Periodically, a flush of liquid (usu-ally water) flushes the particulate away.

Many manufacturers have extended and extrapolated methods of sizingelectrostatic precipitators However, there has not been significant change inthe state-of-the-art of electrostatic precipitation Concentration has been cen-tered about hardware improvements for reliability (Figure 20.4), voltage, andspark controls to maintain maximum stable electrical fields (Figure 20.5),increasing sizes to secure compliance with new and more stringent regula-tions (Figure 20.6), and attention to new and improved materials of construc-

Further development work has resulted in more effective arrangements andconfigurations of collection and charging zones in the devices (Figure 20.8).Some of this work has provided for higher particle charging or more intense

Figure 20.4 Electrode support of WESP (Hart Environmental, Inc.).

Figure 20.5 Modern WESP high voltage controls (Hart Environmental, Inc tion/NWL Control Corp.).

Installa-Figure 20.6 Picture of sonic development WESP designed and serviced by Wayne T Hartshorn.

Figure 20.7 All alloy WESP electrode bank (Hart Environmental, Inc.).

Figure 20.8 Multiple discs on trode (Hart Environmental, Inc.).

elec-ionization (Figure 20.9) This has definitely added improvements to the of-the-art of fine particle collection.

state-Notice the insulators on either side of the discharge electrode mast(center), which passes through to the electrode frame located below

To control the WESP and reduce sparking, modern solid state controlsare used that incorporate feedback type logic They bring the voltage up tothe sparking potential then back off slightly, automatically, although theconditions in the WESP may vary

The vertical tubular arrangement of the collecting tubes is shown in

Figure 20.6 These tubes may be round or multisided, depending on thevendor

To keep the discharge electrode masts centered, some firms use framestop and bottom Modern designs use specially designed swivels that allowalignment of the electrodes, then lock them in place These swivels are shown

in Figure 20.7 just below the cross members Because a WESP often handlescorrosive gases, the vessel can be made from corrosion resistant alloys oreven nonmetallic fiberglass (if the surface is suitably prepared with a con-ducting surface)

To produce high efficiency, some vendors use multiple emitting discs onthe discharge electrodes These discs are shown in Figure 20.8 as they extenddown into the collecting tube

Discs are used instead of wire so that a series of intense corona fields

Figure 20.9 Disc vs wire corona formation comparison (TurboSonic Technologies Inc.).

Disc-In-Tube

Wire-In-Plate

TUBE DISC

DISC WEAK FIELD

WEAK FIELD

SAME FIELD

TUBE

STRONG FIELD

STRONG FIELD RADIAL EXPANSION

RADIAL EXPANSION

AXIAL EXPANSION

NO AXIAL EXPANSION PLATE PLATE

The use of modern sparking controls has allowed the use of multiple discsand therefore multiple corona zones to be produced A strong corona fieldcan be produced between the edge of the disc and the collecting tube,much like the electrode to ground on an automotive spark plug Thecontrols of the WESP, however, allow a corona to be formed before thespark jumps the gap This combination produces the greatest particulatecontrol efficiency.

There are two types of electrostatic precipitator technologies There isthe dry electrostatic precipitator, which is cleaned of collected material bymeans of rapping and/or vibrating mechanisms The wet precipitator iscleaned of collected material by means of irrigated collecting surfaces(Figure 20.10)

Until recently, the wet precipitators comprised a small share of the ket for electrostatic precipitators Originally, the leading application for wetprecipitators was the collection of sulfuric acid A typical unit was self-irrigating, tube-type, and lead-lined fabrication Reinforced thermosettingplastic has gained increased acceptance as well

mar-Figure 20.10 Basic components of a WESP (TurboSonic Technologies Inc.).

INTERMITTENT FLUSHING HEADER HIGH VOLTAGE GRID HIGH VOLTAGE CONNECTION

HIGH VOLTAGE INSULATOR COMPARTMENT DRAIN

DISCHARGE DRAIN

COLLECTING ELECTRODE TUBE

PRECIPITATING ELECTRODE

DISCHARGE ELECTRODE

RIGID MAST ALIGNMENT MECHANISM

PURGE AIR CONNECTION

GAS DISTRIBUTION DEVICES

HIGH VOLTAGE SUPPORT INSULATOR

Types of wet precipitators

The design of wet electrostatic precipitators can be characterized by uration, arrangement, irrigating method, and materials of construction

config-Configuration

There are two basic precipitator configurations: plate and tube The platetype consists of parallel plates with discharge elements assembled betweeneach plate The tube type consists of an array of tubes, round or multisided,with a discharge electrode located in the center of each

Arrangement

Gas flow can be arranged in parallel or series, and horizontally or vertically.This feature also distinguishes a wet from a dry precipitator — becauseparticles are removed from the latter through rapping, it is always arrangedhorizontally

in Figure 20.11, it can also be used after gas absorber/coolers such as packedtowers wherein gases are cooled then sub-cooled to condense water vaporonto water droplets (flux force condensation)

In spray irrigation, spray nozzles continuously irrigate the collectingsurfaces The spray droplets and the particles form the irrigating film Inintermittently flushed irrigation, the precipitator operates cyclically Duringcollection, it operates as a dry precipitator without rapping It is periodicallyflushed by overhead spray nozzles This method only works well if theparticles are easily removed

In film irrigation, a continuous liquid film flushes the collecting face Because the film also acts as the collecting surface, the plate or tubedoes nothing more than support the film Therefore, the electrical conduc-tivity of the irrigating fluid becomes an important factor Nonconductiveirrigants will not work Also important are the physical properties of the

sur-film and the liquid-distribution network The sur-film must be smooth andwell distributed to avoid high voltage arcing, which can damage the unitand result in poor performance Additionally, the distribution piping,plenums and weirs must be designed to avoid dead zones that promotesettling or plugging.

Electrostatic precipitation is made possible by the corona discharge.Through an effect known as the avalanche process, the corona dischargeprovides a simple and stable means of generating the ions to electricallycharge and collect suspended particles or mists In the avalanche process,gases in the vicinity of a negatively charged surface break down to form

a plasma, or glow, region when the imposed voltage reaches a critical level(Figure 20.12) Free electrons in this region are then repulsed toward thepositive, or grounded, surface, and finally collide with gas molecules toform negative ions

These ions, being of lower mobility, form a space-charge cloud of thesame polarity as the emitting surface By restricting further emission of high-speed electrons, the space charge tends to stabilize the corona With a coronaestablished, dust particles or mists in the area become charged by the ionspresent, and are driven to the positive electrode by the electric field Of

Figure 20.11 Flux force condensation type system with WESP (TurboSonic ogies Inc.).

RECYCLE DUCT PACKED

TOWER

course, for the forgoing to be successful, the proper electrode geometry, gascomposition, and voltage must be present.

Particle charging is only the first step in the precipitation process Oncecharged, the particles must be collected As explained, this happens as amatter of course because the same forces that cause a particle to acquire acharge also drive the like-polarity particle to the grounded surface

The next step is particle removal In a wet precipitator the material isrinsed from the collecting surface with an irrigating liquid

Selecting a wet electrostatic precipitator

The Deutsch equation describes precipitator efficiency under conditions ofturbulent flow:

E = 1 – exp(–AW/Q)where

E = collection efficiency, 1-(outlet particle concentration/inlet particle concentration)

A = area of the collecting surface

W = velocity of particle migration to the collecting surface

Q = upward gas flow rate (gas velocity × cross-sectional area of the passage)

The derivation of equation depends on simplifying assumptions, themost important being: all particles are the same size, the gas velocity profile

is uniform, a captured particle stays captured, the electric field is uniform,and no zones are untreated

Figure 20.12 Electrostatic basics (Wayne T Hartshorn).

Plasma region Free electrons

Ions Charged particles Collected dust

Collecting electrode

Dust to removal Gas flow

Discharge electrode

High-voltage

D.C source

To account for the numerous variables, a modified Deutsch equation isused, in which the term W (particle migration velocity) is replaced by anotherknown as effective migration velocity (EMV) Empirically determined, EMV is

a characterizing parameter that accounts for all the nonidealities mentioned,

as well as for the true particle-migration velocity Values for EMV used inthe modified form are considerably lower than true particle velocities calcu-lated or measured in the laboratory

Most wet electrostatic precipitators do not suffer from the nonidealitiesencountered by the dry type devices Also, because the wet type precipitator

is frequently configured for vertical gas flow, sneakby is avoided Therefore,

precipitators This means that, for a specific application, a wet device can besmaller than an equivalent dry device This is additionally true because awet precipitator operates on a cooled, lower volume gas stream

Because the collecting surfaces in a wet precipitator are cleaned by aliquid, the wet precipitator can be used for virtually any particle emission.Generally, the physical and chemical properties of the particles are not

an important factor in the design of wet precipitators, as well as factors thatare normally of concern in the design of dry precipitators, such as electricalresistivity, surface adhesion, and flammability A possible exception is thedielectric constant of the particles It has a weak effect on the maximumcharge that can be achieved, according to the theoretical relationship forpredicting particle saturation charge

N = {1 + 2[(k – 1)/(k + 2)]}(Eo∑a2/e)where

consid-to have very low constants, which can markedly lower collection efficiencies.Nevertheless, there are many applications for which a wet precipitatorshould be carefully considered, and even some for which wet precipitationshould be the only technology of choice (Figure 20.13) Some such conditionsoccur when the gas stream has already been treated in a wet scrubber, thetemperature of the gas stream is low and its moisture content is high, gasand particles must be simultaneously removed, the loading of submicronparticles is high and removal must be very efficient, liquid particles are to

be collected, and the dust to be collected is best handled in liquid

Unlike other gas cleaning methods, the applicability of wet precipitatorsstrongly depends on the particular design In some cases, certain wet pre-cipitator designs may not be suitable for certain applications For instance,

a precipitator for gas streams containing adherent particles must be uously, not intermittently, irrigated

contin-The second most important factor in design after the type and ration has been decided is materials of construction Wet precipitators oper-ate at, or below, the adiabatic saturation temperature of the irrigating fluid(usually water), and corrosion is a constant concern

configu-Wet precipitators are rarely made of carbon steel, at least the surfacesthat are in contact with the gases to be treated Carbon steel constructionmay only be feasible when the gas stream is high in pH and low in oxygen.Ordinarily, wet precipitators are constructed of one or more corrosion-resis-tant materials These materials can include simple stainless steels, exotichigh-nickel alloys, reinforced thermo-setting materials, and thermoplastics.From a materials standpoint, the casing, or housing, is the least criticalelement The outside of the shell housing not in contact with the gases neednot even be corrosion-resistant, only capable of withstanding ambient con-ditions The collecting surfaces should afford the maximum resistance tochemical attack Also, fabrication points subject to corrosion should be min-imized, because failures in the collecting surfaces can disturb the electricfield and cause arcing, lowering performance Because the discharge elec-trodes are usually not irrigated, there is a concentrating effect on their sur-faces that does not occur on wetted areas For example, if the gas stream

on the moist surface of the discharge electrodes will be about 1.0, even if theirrigant is kept at a pH of 3.0 or higher The galvanic effects of operation inthe range of 40,000-V direct current compounds the corrosion potential ofthe concentrating effect For these reasons, the discharge electrodes shouldalways be fabricated of a material of significantly greater corrosion resistancethan that of any other part of the wet precipitator

Figure 20.13 Application comparison chart (Wayne T Hartshorn).

Fine particles

Liquid particles

Low gas temp./

high dew point

Dry ESP

Wet Precipitator

X X X

X

X X

X X X