Dry ESPs

Most electrostatic precipitators are operated dry and use rappers to remove the collected particulate matter. The term dry is used because particles are charged and collected in a dry state and are removed by rapping as opposed to water washing which is used with wet ESPs. The major portion of this course covers dry ESPs that are used for collecting dust from many industries including steel furnaces, cement kilns and fossil-fuel-fired boilers

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Wet ESPs

Any of the previously described ESPs can be operated with a wet spray to remove collected particles. Wet ESPs are used for industrial applications where the potential for explosion is high (such as collecting dust from a closed-hood Basic Oxygen Furnace in the steel industry), or when dust is very sticky, corrosive, or has very high resistivity.

The water flow may be applied continuously or intermittently to wash the collected particles from the collection electrodes into a sump (a basin used to collect liquid). The advantage of using a wet ESP is that it does not have problems with rapping reentrainment or with back corona. Figures 1 and 2 show two different wet ESPs. The casing of wet ESPs is made of steel or fiberglass and the discharge electrodes are made of carbon steel or special alloys, depending on the corrosiveness of the flue gas stream. In a circular-plate wet ESP, shown in Figure 1-15, the circular collection plates are sprayed with liquid continuously. The liquid provides the electrical ground for attracting the particles and for removing them from the plates. These units can handle gas flow rates of 30,000 to 100,000 cfm. Preconditioning sprays located at the inlet remove some particulate matter prior to the charging stage. The operating pressure drop across these units is typically 1 to 3 inches of water.

Rectangular flat-plate wet ESPs, shown in Figure 2, operate similarly to circular plate wet ESPs. Water sprays precondition the gas stream and provide some particle removal. Because the water sprays are located over the top of the electrical fields, the collection plates are continuously irrigated. The collected particulate matter flows downward into a trough that is sloped to a drain

                                          figure 1

                                                           Figure 2

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Hot Side

Hot-side ESPs (Figure 1-14) are placed in locations where the flue gas temperature is relatively high. Their collection electrodes can be either tubular or plate. Hot-side ESPs are used in high-temperature applications, such as in the collection of cement kiln dust or utility and industrial boiler fly ash. A hot-side precipitator is located before the combustion air preheater in a boiler. The flue gas temperature for hot-side precipitators is in the range of 320 to 420°C (608 to 790°F).

The use of hot-side precipitators helps reduce corrosion and hopper plugging. However, these units (mainly used on coal-fired boilers) have some disadvantages.

Because the temperature of the flue gas is higher, the gas volume treated in the ESP is larger. Consequently, the overall size of the precipitator is larger making it more costly. Other major disadvantages include structural and mechanical problems that occur in the precipitator shell and support structure as a result of differences in thermal expansion.

For years, cold-side ESPs were used successfully on boilers burning high-sulfur coal. However, during the 1970s when utilities switched to burning low-sulfur coal, cold side ESPs was no longer effective at collecting the fly ash. Fly ash produced from low sulfur coal-fired boilers has high resistivity (discussed in more detail later in the course), making it difficult to collect. As you will learn later, high temperatures can lower resistivity. Consequently, hot-side ESPs became very popular during the 1970s for removing ash from coal-fired boilers burning low sulfur coal. However, many of these units did not operate reliably, and therefore, since the 1980s; operators have generally decided to use cold-side ESPs along with conditioning agents when burning low sulfur coal.

Hot-side ESPs are also used in industrial applications such as cement kilns and steel refining furnaces. In these cases, combustion air pre-heaters are generally not used and hot side just refers to the high flue gas temperature prior to entering the ESP.

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Cold Side

Cold-side ESPs (Figure 1-13) have been used for over 50 years with industrial and utility boilers, where the flue gas temperature is relatively low (less than 204°C or 400°F). Cold-side ESPs generally use plates to collect charged particles. Because these ESPs are operated at lower temperatures than hot-side ESPs, the volume of flue gas that is handled is less. Therefore, the overall size of the unit is smaller, making it less costly. Cold-side ESPs can be used to remove fly ash from boilers that burn high sulfur coal. As explained in later lessons, cold-side ESPs can effectively remove fly ash from boilers burning low-sulfur coal with the addition of conditioning agents.

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Cold-side and Hot-side ESPs

Electrostatic precipitators are also grouped according to the temperature of the flue gas that enters the ESP: cold-side ESPs are used for flue gas having temperatures of approximately 204°C (400°F) or less; hot-side ESPs are used for flue gas having temperatures greater than 300°C (572°F). In describing ESPs installed on industrial and utility boilers, or municipal waste combustors using heat recovery equipment, cold side and hot side also refer to the placement of the ESP in relation to the combustion air preheater. A cold-side ESP is located behind the air preheater, whereas a hot-side ESP is located in front of the air preheater. The air preheater is a tube section that preheats the combustion air used for burning fuel in a boiler. When hot flue gas from an industrial process passes through an air preheater, a heat exchange process occurs whereby heat from the flue gas is transferred to the combustion air stream. The flue gas is therefore "cooled" as it passes through the combustion air preheater. The warmed combustion air is sent to burners, where it is used to burn gas, oil, coal, or other fuel including garbage. APTI Course SI:428A Introduction to Boiler Operation describes boilers and heat recovery equipment in greater detail.

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Two Stage

The two-stage precipitator differs from the single-stage precipitator in both design and amount of voltage applied. The two-stage ESP has separate particle charging and collection stages (Figure). The ionizing stage consists of a series of small, positively charged wires equally spaced 2.5 to 5.1 cm (1 to 2 in.) from parallel grounded tubes or rods. A corona discharge between each wire and a corresponding tube charges the particles suspended in the air flow as they pass through the ionizer. The direct-current potential applied to the wires is approximately 12 to 13 kV.

The second stage consists of parallel metal plates less than 2.5 cm (1 in.) apart. The particles receive a positive charge in the ionizer stage and are collected at the negative plates in the second stage. Collected smoke or liquids drain by gravity to a pan located below the plates, or are sprayed with water mists or solvents that remove the particles and cause them to fall into the bottom pan. Two-stage precipitators were originally designed for air purification in conjunction with air conditioning systems. (They are also referred to as electronic air filters). Two stage

ESPs are used primarily for the control of finely divided liquid particles. Controlling solid or sticky materials is usually difficult, and the collector becomes ineffective for dust loadings greater than 7.35 x 10-3g/m3 (0.4 gr/dscf). Therefore, two-stage precipitators have limited use for particulate-emission control. They are used almost exclusively to collect liquid aerosols discharged from sources such as meat smokehouses, pipe-coating machines, asphalt paper saturators, high speed grinding machines, welding machines, and metal-coating operations.

            Representation of gas flow in a two-stage precipitator

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Single Stage

A single-stage precipitator uses high voltage to charge the particles, which are then collected within the same chamber on collection surfaces of opposite charge. Most ESPs that reduce particulate emissions from boilers and other industrial processes are single-stage ESPs (these units will be emphasized in this course). Single stage

ESPs use very high voltage (50 to 70 kV) to charge particles. After being charged, particles move in a direction perpendicular to the gas flow through the ESP, and migrate to an oppositely charged collection surface, usually a plate or tube. Particle charging and collection occurs in the same stage, or field; thus, the precipitators are called single-stage ESPs. The term field is used interchangeably with the term stage and is described in more detail later in this course. Figure 1 shows a single stage tubular precipitator. A single-stage plate precipitator is shown in Figure 2

                                                                 Figure 1

                                               Figure 2

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Plate

Plate electrostatic precipitators primarily collect dry particles and are used more often than tubular precipitators. Plate ESPs can have wire, rigid-frame, or occasionally, plate discharge electrodes. Figure 1 shows a plate ESP with wire discharge electrodes. Dirty gas flows into a chamber consisting of a series of discharge electrodes that are equally spaced along the center line between adjacent collection plates. Charged particles are collected on the plates as dust, which is periodically removed by rapping or water sprays. Discharge wire electrodes are approximately 0.13 to 0.38 cm (0.05 to 0.15 in.) in diameter. Collection plates are usually between 6 and 12 m (20 and 40 ft) high. For ESPs with wire discharge electrodes, the plates are usually spaced from 15 to 30 cm (6 to 12 in.) apart. For ESPs with rigid-frame or plate discharge electrodes, plates are typically spaced 30 to 38 cm(12 to 15 in.) apart and 8 to 12 m (30 to 40 ft) in height. Plate ESPs are typically used for collecting fly ash from industrial and utility boilers as well as in many other industries including cement kilns, glass plants and pulp and paper mills

                                   Gas flow through a plate precipitator

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Tubular

Tubular precipitators consist of cylindrical collection electrodes (tubes) with discharge electrodes (wires) located in the center of the cylinder (Figure 1). Dirty gas flows into the tubes, where the particles are charged. The charged particles are then collected on the inside walls of the tubes. Collected dust and/or liquid are removed by washing the tubes with water sprays located directly above the tubes. The tubes may be formed as a circular, square, or hexagonal honeycomb with gas flowing upward or downward. A tubular ESP is tightly sealed to minimize leaks of collected material. Tube diameters typically vary from 0.15 to 0.31 m (0.5 to 1 ft), with lengths usually varying from 1.85 to 4.0m (6 to 15 ft).

Tubular precipitators are generally used for collecting mists or fogs, and are most commonly used when collecting particles that are wet or sticky. Tubular ESPs have been used to control particulate emissions from sulfuric acid plants, coke oven byproduct gas cleaning (tar removal), and iron and steel sinter plants.

                                    

                                                    Gas flow through a tubular precipitator

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Types of Electrostatic Precipitators

ESPs can be grouped, or classified, according to a number of distinguishing features in their design. These features include the following:

• The structural design and operation of the discharge electrodes (rigid-frame, wires or plate) and collection electrodes (tubular or plate)

• The method of charging (single-stage or two-stage)

• The temperature of operation (cold-side or hot-side)

• The method of particle removal from collection surfaces (wet or dry)

These categories are not mutually exclusive. For example, an ESP can be a rigid-frame, single- stage, cold-side, plate-type ESP as described below.

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