Air Pollution Control Equipment Selection Guide, 2nd Edition

183

20

Wet Electrostatic

Precipitators

*

Wayne T. Hartshorn

Hart Environmental, Inc., Lehighton, Pennsylvania

20.1 DEVICE TYPE

The wet electrostatic precipitator (WESP) is a mechanical device that uses primarily

electrostatic forces to separate particulate from gas streams. The collecting surfaces

are periodically cleaned using water or other suitable conductive ushing liquid, thus

the name wet electrostatic precipitator.

The basic components of a WESP are shown in Figure20.1. They consist of either

a low-level (shown) or high-level gas inlet, collecting tubes, mast type electrodes

mounted on a grid or frame, a high-voltage insulator section, an air-purged insulator

compartment to prevent particulate from coating the high-voltage insulator section,

a high-voltage power supply (transformer/rectier set), and a gas outlet.

The designs also include various types of cleaning or irrigation systems that are

used to purge the tubes of captured particulate. These purge systems may include fog

nozzles, spray nozzles, or weir type irrigation systems.

20.2 TYPICAL APPLICATIONS AND USES

WESPs are frequently used to collect submicron particulate that arises from com-

bustion, drying operations, process chemical production, and similar sources. They

are also used as polishing devices to reduce particulate loadings to extremely low

levels. They are generally used where the inlet loading of particulate is under 0.5 grs/

dscf (grains per dry standard cubic foot) and where corrosive gases may be present.

They also excel where the particulate is sticky but can be water ushed. They often

replace berbed lters or similar coalescing devices where solid particulate is pres-

ent that could plug the berbed design.

Wet precipitators are increasingly being used as nal cleanup devices behind and

in combination with other air pollution control devices. Applications include chemi-

cal and hazardous waste incinerators; hog fuel boilers; acid mists; steel mill appli-

cations; vapor-condensed organics; nonferrous metal oxide fumes from calciners,

roasters, and reverb furnaces; phosphate rock; veneer dryers; sludge incinerators;

*

Additional information provided by Jerry Childress of McGill AirClean and David Meier of Bionomic

Industries.

184 Air Pollution Control Equipment Selection Guide

and blue haze and fume control. Figure20.2 shows a WESP on a popular application,

a veneer dryer.

The WESP can provide, in addition to ne or submicron particulate control, a

nal cleanup of mist elimination.

Another common application is on particle board dryers. These emissions can con-

tain a combination of large particulate nes plus condensable aerosols. These products

tend to be sticky, so the WESP, properly designed, is a good candidate for its control.

On this unit, the WESP is in the center of the picture, and a droplet eliminator and fan

is to the left of center. The gas ow is downward, thereby ushing solids toward the

sump, assisted by gravity. The bypass stack for the dryer can be seen in the background.

Some of these applications require versatility. Figure20.3 shows a WESP as applied

to a combination fuel-red boiler burning wood waste as the primary fuel. The other

fuels may at any time be fuel oil, coal, natural gas, or noncondensable gases (NCGs). The

emissions arrive at about 450°F; therefore, a quench-type gas inlet is used. In this specic

project, a gas ow of about 337,000 actual cubic feet per minute (acfm) was split between

the two WESPs. The system controls particulate emissions to meet a strict opacity limita-

tion. When gas volumes are high, the low-pressure drop of a WESP can be attractive in

that the low-ow resistance saves energy at the prime mover (in this case, fans).

20.3 PRIMARY MECHANISMS USED

Electrostatic forces as well as diffusional forces are used to accomplish the sepa-

ration. On some designs wherein the collecting tubes or surfaces are air or liquid

Clean Gas Out

Purged

Insulator

Compartment

Dirty Gas In

Collecting

Tube

High Voltage

Insulator

Electrode

Support Beam

FIGURE20.1 WESP components (Entoleter, Inc.).