Air Pollution Control Equipment Selection Guide, 2nd Edition

31

5

Electrostatic Precipitators

Bob Taylor

BHA Group, Inc., Kansas City, Missouri

5.1 DEVICE TYPE

Electrostatic precipitators are used for the purpose of removing dry particulate mat-

ter from gas streams. They basically apply an electrostatic charge to the particulate

and provide sufcient surface area for that particulate to migrate to the collecting

plate and be captured. The collecting plates are rapped periodically to disengage the

collected particulate into a receiving hopper.

5.2 TYPICAL APPLICATIONS AND USES

Dry electrostatic precipitators are used to remove particulate matter from ue gas

streams exiting cement kilns, utility and industrial power boilers, catalytic crack-

ers, paper mills, metals processing, glass furnaces, and a wide variety of industrial

applications (see Figure5.1).

An electrostatic precipitator is a constant pressure drop, variable emission par-

ticulate removal device offering exceptionally high particulate removal efciency.

There is a unique jargon involving electrostatic precipitators. If you contemplate

purchasing or studying the use of one, perhaps the following buzzword list will prove

helpful. It is in alphabetical order, so if you see a word that you do not understand,

just jump down the list to nd the offending word.

Air splitter switch: An air splitter switch is mounted at the high-voltage bushing

contained on the transformer rectier. The purpose of the switch is to iso-

late one of the two electrical sections served by the transformer rectier

while the other operates.

Antisneak bafe: A deector or bafe that prevents gas from bypassing the treat-

ment zone of the precipitator.

Arc: Arcs occur within the high-voltage system as a result of uncontrolled spark-

ing. If measurable current ow is detected, damage will occur to internal

components.

Aspect ratio: The treatment length divided by treatment height. A higher number is

more favorable for collection efciency.

Back corona: Occurs in high-resistivity dust applications. As a result of the dust resis-

tivity, a voltage drop occurs across the layer of dust on the collecting plates. The

application of current to the eld builds the charge on the surface of the dust

layer until the breakdown voltage of the dust is achieved. At this point a surge

32 Air Pollution Control Equipment Selection Guide

of current occurs from the surface of the dust to the collecting plate, causing

localized heating of the dust. The dust explodes back into the gas stream carry-

ing a charge opposite of the electrons and gaseous ions. This causes collection

efciency to degrade and dust reentrainment to increase.

Bus section: Smallest isolatable electrical section in the precipitator.

Casing: Gas-tight enclosure within which the precipitator collecting plates and dis-

charge electrodes are housed.

Chamber: Common mechanical eld divided in the direction of gas ow by a parti-

tion. The partition is either a gas-tight wall or open structural section.

Cold roof: This is the walking surface immediately above the hot roof section.

Collecting surface: Component on which particulate is collected. Also known as

collecting plate or panel.

Corona discharge: The ow of electrons and gaseous ions from the discharge elec-

trode toward the collecting plates. Corona discharge occurs after the dis-

charge electrode has achieved high enough secondary voltages.

Current-limiting reactor: This device provides a xed amount of inductance into

the transformer rectier circuit. Some current-limiting reactors have taps

that allow the amount of inductance to be varied manually when the circuit

is not energized.

Direct rapping: Rapping force applied directly to the top support tadpole or lower

shock bar of a collecting plate.

Discharge electrode: The component that develops high-voltage corona for the pur-

pose of charging dust particles.

Disconnect switch: A switch mounted in the high-voltage guard or transformer rec-

tier that allows the electrical eld to be disconnected from the transformer

rectier.

FIGURE5.1 Typical electrostatic precipitator in operation (BHA Group, Inc.).