Electrostatic Precipitators

Bob Taylor

BHA Group, Inc., Kansas City, Missouri

Device Type

Electrostatic precipitators are used for the purpose of removing dry particulate matter from gas streams. They basically apply an electrostatic charge to the particulate and provide sufficient 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.

Typical Applications and Uses

Dry electrostatic precipitators are used to remove particulate matter from flue gas streams exiting cement kilns, utility and industrial power boilers, catalytic crackers, 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 particulate removal device offering exceptionally high particulate removal efficiency.

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 find the offending word.

Air splitter switch: An air splitter switch is mounted at the high- voltage bushing contained on the transformer rectifier. The purpose of the switch is to isolate one of the two electrical sections served by the transformer rectifier while the other operates.

Antisneak baffle: A deflector or baffle that prevents gas from bypassing the treatment zone of the precipitator.

FIGURE 5.1

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

Arc: Arcs occur within the high-voltage system because of uncontrolled sparking. If measurable current flow is detected, damage will occur to internal components.

Aspect ratio: The treatment length is divided by treatment height. A higher number is more favorable for collection efficiency.

Back corona: Occurs in high-resistivity dust applications. As a result of the dust resistivity, a voltage drop occurs across the layer of dust on the collecting plates. The application of current to the field builds the charge on the surface of the dust layer until the breakdown voltage of the dust is achieved. At this point, a surge 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 carrying a charge opposite of the electrons and gaseous ions. This causes collection efficiency 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 discharge electrodes are housed.

Chamber: Common mechanical field divided in the direction of gas flow by a partition. 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 flow of electrons and gaseous ions from the discharge electrode toward the collecting plates. Corona discharge occurs after the discharge electrode has achieved high enough secondary voltages.

Current-limiting reactor: This device provides a fixed amount of inductance into the transformer rectifier 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 purpose of charging dust particles.

Disconnect switch: A switch mounted in the high-voltage guard or transformer rectifier that allows the electrical field to be disconnected from the transformer rectifier.

EGR: Electromagnetic impact gravity return rapper used for cleaning discharge electrodes, collecting plates, and gas distribution devices. An electromagnetic coil when energized raises a steel plunger, which can free fall onto the rapper shaft after the coil is deenergized.

Electrical bus: The electrical bus transmits power from the transformer rectifier to each electrical field. Generally fabricated from piping or tubing.

Electrical field: An electrical field comprises one or more electrical sections energized by single transformer rectifier. A single voltage control serves the electrical field.

Gas distribution device: A gas distribution device is any component installed in the gas flow for the purpose of modifying flow characteristics.

Gas passage: The space defined between adjacent collecting plates.

Gas passage width: The distance between adjacent collecting plates. Consistent within a mechanical field but can vary between fields contained in a common casing.

Gas velocity: Gas velocity within a precipitator is determined by dividing total gas volume by the cross-sectional area of the precipitator.

Ground switch: A device mounted in the high-voltage guard or the transformer rectifier for the purpose of grounding the high-voltage bus. This does not disconnect the field from the transformer rectifier.

High-voltage guard: High-voltage guard surrounds the electrical bus. Generally fabricated from round sections that provide adequate electrical clearances for the applied voltages.

High-voltage support insulator: The ceramic device fabricated from porcelain, alumina, or quartz that isolates the high-voltage system from the casing. Typically, a cylindrical or conical configuration, but some manufacturers use a post-type insulator.

Hopper: A casing component where material cleaned from the discharge electrodes and collecting plates is collected for removal from the system. Can be pyramidal, trough, or flat bottom.

Hot roof: Comprises the top gas-tight portion of the casing.

Insulator compartment: An enclosure for a specific quantity of high- voltage support insulators. Typically contains one insulator but may contain several. The insulator compartment does not cover the entire roof section.

Key interlock: A key interlock system provides an orderly shutdown and startup of a precipitator electrical system. A series of key exchanges connected to de-energizing equipment eventually provides access to the internals of the precipitator.

Lower frame stabilizer: A lower frame stabilizer controls electrical clearances of the stabilizer frame relative to the mechanical field. This device typically contains an insulator referenced to the hopper, casing, or collecting plate and attached on the other end to the stabilizer frame.

Mechanical field: This is the smallest mechanical section that comprises the entire treatment length of a collecting plate assembly and extends the width of one chamber.

Migration velocity: The velocity at which the particulate moves toward the collecting plate. Measured in either feet per second or centimeters per second.

Normal volume: This is the normalized condition when using metric measurements.

Opacity: An indication of the amount of light that can be transmitted through the gas stream. Measured as a percentage of total obscuration.

Partition ivall: Divides adjacent chambers in a multiple chamber precipitator. Can be gas tight, but also can be a row of supporting columns.

Penthouse: An enclosure that houses the high-voltage support insulators. Typically covers the entire roof section of the precipitator casing. This is a gas-tight enclosure that cannot be entered when the precipitator is operating.

Perforated plate: A perforated steel plate, typically 10 gauge, that is placed perpendicular to gas flow for the purpose of redistributing the velocity pattern measured within the precipitator. The perforation pattern is typically not uniform across the panels providing specific flow patterns.

Primary current: The current provided at the input of a transformer rectifier. It will be measured in alternating current (AC) amps.

Primary voltage: The voltage provided at the input of a transformer rectifier. It will be measured in AC volts.

Purge heater system: Intended to provide heated, pressurized, and filtered air into the insulator compartments or penthouse. An electric heater element or sometimes steam coil heats air that has been drawn through a filter by a blower. The conditioned air is then distributed into the support insulators.

Rapper: A device responsible for imparting force into a collecting plate or discharge electrode for the purpose of dislodging dust.

Rapper insulator shaft: An insulator shaft that isolates the high-voltage rapping system from the casing. Can be fabricated from any material with high dielectric, but typically porcelain, alumina, or fiberglass-reinforced plastic.

Rigid discharge electrode: A discharge electrode that is self-stabilizing from the high-voltage frame down to the stabilizer frame. Typically constructed from tubular or roll-formed material. Individual emitter pins or other corona generators are affixed to the surface for the purpose of generating high-voltage corona.

Rigid frame: Rigid frames are associated with tumbling hammer-type precipitators. A rigid frame that encompasses the entire gas passage area is provided for the purpose of support of individual discharge electrodes.

Saturable core reactor: Sometimes also called an SCR, this is an antiquated method of providing inductance into the transformer rectifier circuit. The saturable core does vary impedance but is extremely slow to react and introduces distortion into the waveform. Replaced by the current-limiting reactor.

Secondary current: Current measured at the output side of a transformer rectifier. It will be measured in direct current (DC) milliamps.

Secondary voltage: Voltage measured at the transformer rectifier output. It is measured in DC kilovolts.

Silicon control rectifiers: Silicon control rectifiers are the switches that control power input to the electrical field. The voltage control turns the silicon control rectifier on and off based on the sparking occurring within the field.

Spark: A spark within a precipitator occurs between the high-voltage system and the grounded surfaces. There is a minimum of current flow during a spark, and as a result internal components are not damaged. Sparking is the method by which voltage controls determine the maximum usable secondary voltage that can be applied to an electrical field.

Specific collecting area: Specific collecting area (SCA) is the total amount of collecting plate area contained in a precipitator divided by the gas volume treated. When referenced to a common gas passage width, values for SCA can be compared to define relative capability of precipitators.

Transformer rectifier: A device to rectify the AC input to DC and step up the voltage to the required level. A single voltage control serves each transformer rectifier.

Treatment length: Total length of all mechanical fields in the direction of gas flow.

Treatment time: Treatment time or retention time is calculated by dividing the treatment by the gas velocity.

Tumbling hammer rapping: A rapping system utilizing a series of hammers mounted on a shaft common to a mechanical field. When the shaft rotates or drops, the hammers strike an anvil connected to the collecting plates or high-voltage frames.

Turning vane: Turning vanes are installed within ductwork or precipitator inlet and outlet transitions to direct flow to a specified position.

Voltage control: A voltage control serves a single transformer rectifier for the purpose of maximizing power input to the electrical field that it serves.

Weather enclosure: This is a weatherproof enclosure over the top of a precipitator for the purpose of facilitating maintenance during adverse weather. It is not for the purpose of isolating high-voltage electrical sections.

Weighted wire: A discharge electrode fabricated from wire that is tensioned by a cast iron weight.

To make sense of these terms, the following illustrations indicate some of the terms for standard configuration electrostatic precipitator components: Figure 5.2 shows a complete electrostatic precipitator. The cutouts show specifics that will become clearer. The details shown will become more obvious

FIGURE 5.3

Exploded detail of single field (BHA Group, Inc.).

as we look more deeply at selected components. Figure 5.3 shows better detail of a single field. Note the detail of the rapper tranes. The rappers that clean the collecting plates are configured differently from those for the high- voltage system. The collecting rapping system is shown in Figure 5.4, and the high-voltage rapping system is shown in Figure 5.5.

 
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