Wet Electrostatic Precipitators
Wayne T. Hartshorn
Hart Environmental, Inc., Lehigh ton, Pennsylvania
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 flushing 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/ rectifier 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.
Typical Applications and Uses
WESPs are frequently used to collect submicron particulate that arises from combustion, 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 flushed. They often replace fiberbed filters or similar coalescing devices where solid particulate is present that could plug the fiberbed design.
WESP components. (Entoleter, Inc.)
Wet precipitators are increasingly being used as final cleanup devices behind and in combination with other air pollution control devices. Applications include chemical and hazardous waste incinerators; hog fuel boilers; acid mists; steel mill applications; vapor-condensed organics; non- ferrous metal oxide fumes from calciners, roasters, and reverb furnaces; phosphate rock; veneer dryers; sludge incinerators; 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 fine or submicron particulate control, a final cleanup of mist elimination.
Another common application is on particle board dryers. These emissions can contain a combination of large particulate fines 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
WESP on veneer dryer. (Geoenergy International Corp.)
picture, and a droplet eliminator and fan is to the left of center. The gas flow is downward, thereby flushing 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-fired boiler burning wood waste as the
WESP on combination fuel-fired boiler, (photo courtesy of McGill AirClean)
primary fuel. The other fuels may at any time be fuel oil, coal, natural gas, or noncondensable gases. The emissions arrive at about 450°F; therefore, a quench-type gas inlet is used. In this specific project, a gas flow of about 337,000 acfm was split between the two WESPs. The system controls particulate emissions to meet a strict opacity limitation. When gas volumes are high, the low-pressure drop of a WESP can be attractive in that the low-flow resistance saves energy at the prime mover (in this case, fans).
Primary Mechanisms Used
Electrostatic forces as well as diffusional forces are used to accomplish the separation. On some designs wherein the collecting tubes or surfaces are air or liquid cooled, thermophoretic forces are also used. In general, a series of zones are created wherein electrostatic forces sweep the particulate from the gas stream toward the contact (collecting) surface, which is periodically flushed with water to prevent the buildup of a resistive layer. One such application is shown in Figure 20.4. The WESP is in use on a particle board dryer.
To a minor extent, the WESP is also a gas absorber. The flushing system can also provide some mass transfer of contaminant gases into the liquid.
Particle board dryer WESP. (Geoenergy International Corp.)
WESPs consist of emitting electrodes mounted inside collecting tubes. A high voltage is introduced to the emitting electrode and a corona (charged field) is produced between the emitting electrode and the collecting electrode. Pollutant particles (sometimes solids, sometimes aerosols, often a mixture of both) pass through this corona and are moved toward the collecting electrode where they momentarily attach. Periodically, a flush of liquid (usually water) flushes the particulate away.
Many manufacturers have extended and extrapolated methods of sizing electrostatic precipitators. However, there has not been significant change in the state-of-the-art of electrostatic precipitation. Concentration has been centered on hardware improvements for reliability (Figure 20.5); voltage, and spark controls to maintain maximum stable electrical fields (Figure 20.6); increasing sizes to secure compliance with new and more stringent regulations; and attention to new and improved materials of construction for longer life and more resistance to corrosive gases (Figure 20.7). Further development work has resulted in more effective arrangements and configurations of collection and charging zones in the devices (Figure 20.8). Some of this work has provided for higher particle charging or more intense ionization
Electrode support of WESP. (Hart Environmental, Inc.)
Modern WESP high-voltage controls. (Hart Environmental, Inc. Installation/NWL Control Corp.)
Picture of sonic development WESP designed and serviced by Wayne T. Hartshorn.
All-alloy WESP electrode bank. (Hart Environmental, Inc.)
Multiple discs on electrode. (Hart Environmental, Inc.)
(Figure 20.9). This has definitely added improvements to the state-of-the-art of fine particle collection.
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 controls are used that incorporate feedback-type logic. They bring the voltage up to the sparking potential then back off slightly, automatically, although the conditions in the WESP may vary.
The vertical tubular arrangement of the collecting tubes is shown in Figure 20.7. These tubes may be round or multisided, depending on the vendor.
To keep the discharge electrode masts centered, some firms use frames top and bottom. Modern designs use specially designed swivels that allow alignment of the electrodes and then lock them in place. These swivels are shown in Figure 20.8 just below the cross members. Because a WESP often handles corrosive gases, the vessel can be made from corrosion-resistant alloys or even nonmetallic fiberglass (if the surface is suitably prepared with a conducting surface).
To produce high efficiency, some vendors use multiple emitting discs on the discharge electrodes. These discs are shown in Figure 20.9 as they extend down into the collecting tube.
Discs are used instead of wire so that a series of intense corona fields can be produced. This can best be seen diagrammatically in Figure 20.10. The use
Disc versus wire corona formation comparison. (TurboSonic Technologies, Inc.)
of modern sparking controls has allowed the use of multiple discs and therefore multiple corona zones to be produced. A strong corona field can be produced between the edge of the disc and the collecting tube, much like the electrode to ground on an automotive spark plug. The controls of the WESP, however, allow a corona to be formed before the spark jumps the gap. This combination produces the greatest particulate control efficiency.
Basic components of a WESP. (TurboSonic Technologies, Inc.)
There are two types of electrostatic precipitator technologies. There is the dry electrostatic precipitator, which is cleaned of collected material by means of rapping and/or vibrating mechanisms. The wet precipitator is cleaned of collected material by means of irrigated collecting surfaces (Figure 20.11).
Until recently, the wet precipitators comprised a small share of the market for electrostatic precipitators. Originally, the leading application for wet precipitators was the collection of sulfuric acid. A typical unit was self- irrigating, tube type, and lead-lined fabrication. Reinforced thermosetting plastic has gained increased acceptance as well.
Types of Wet Precipitators
The design of WESPs can be characterized by configuration, arrangement, irrigating method, and materials of construction. Alloy construction of the entire WESP or just the emitting electrodes and tube bundle is common.
There are two basic precipitator configurations: plate and tube. The plate type consists of parallel plates with discharge elements assembled between each plate. The tube type consists of an array of tubes, round or multisided, with a discharge electrode located in the center of each.
Gas flow can be arranged in parallel or series and horizontally or vertically. This feature also distinguishes a wet from a dry precipitator—because particles are removed from the latter through rapping, it is always arranged horizontally.
This has a greater impact on the operation of a wet precipitator than any other factor. There are many irrigation methods.
In self-irrigation, the most common method, captured liquid droplets wet the collecting surface. This method works only when the particles are mostly liquid. In a specialized variation, condensation from the gas stream wets the collecting surfaces. A cold fluid, usually air, is circulated on the outside of the collecting tube to promote condensation. As with mist collectors, irrigation by condensation works best with a gas stream high in moisture content and low in particle concentration. For this reason and others, the WESP is often used as a very high-efficiency mist eliminator after other gas cleaning devices such as fluidized beds and Venturi scrubbers. As shown in Figure 20.12, it can also be used after gas absorber/coolers such as packed towers wherein gases are cooled and then subcooled to condense water vapor onto water droplets (flux force condensation).
In spray irrigation, spray nozzles continuously irrigate the collecting surfaces. The spray droplets and the particles form the irrigating film. In intermittently flushed irrigation, the precipitator operates cyclically. During collection, it operates as a dry precipitator without rapping. It is periodically
Flux force condensation type system with WESP. (TurboSonic Technologies, Inc.)
flushed by overhead spray nozzles. This method works well only if the particles are easily removed.
In film irrigation, a continuous liquid film flushes the collecting surface. Because the film also acts as the collecting surface, the plate or tube does nothing more than support the film. Therefore, the electrical conductivity of the irrigating fluid becomes an important factor. Nonconductive irrigants will not work. Also important are the physical properties of the film and the liquid-distribution network. The film must be smooth and well distributed to avoid high-voltage arcing, which can damage the unit and result in poor performance. Additionally, the distribution piping, plenums, and weirs must be designed to avoid dead zones that promote settling or plugging.
Electrostatic precipitation is made possible by the corona discharge. Through an effect known as the avalanche process, the corona discharge provides a simple and stable means of generating the ions to electrically charge 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 20.13). Free electrons in this region are then repulsed toward the positive, or grounded, surface and finally collide with gas molecules to form negative ions.
Electrostatic basics. (Wayne T. Hartshorn)
These ions, being of lower mobility, form a space-charge cloud of the same polarity as the emitting surface. By restricting further emission of high-speed electrons, the space charge tends to stabilize the corona. With a corona established, dust particles or mists in the area become charged by the ions present and are driven to the positive electrode by the electric field. Of course, for the foregoing to be successful, the proper electrode geometry, gas composition, and voltage must be present.
Particle charging is only the first step in the precipitation process. Once charged, the particles must be collected. As explained, this happens as a matter of course because the same forces that cause a particle to acquire a charge also drive the like-polarity particle to the grounded surface.
The next step is particle removal. In a wet precipitator, the material is rinsed from the collecting surface with an irrigating liquid.
Selecting a Wet Electrostatic Precipitator
The Deutsch equation describes precipitator efficiency under conditions of turbulent flow:
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 x cross-sectional area of the passage)
The derivation of the equation depends on simplifying assumptions, the most important being that 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.
To account for the numerous variables, a modified Deutsch equation is used, in which the term W (particle migration velocity) is replaced by another known 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 in the modified form are considerably lower than true particle velocities calculated or measured in the laboratory.
Most WESPs do not suffer from the nonidealities encountered by the dry- type devices. Also, because the wet-type precipitator is frequently configured for vertical gas flow, sneakby is avoided. Therefore, EMV values for wet precipitators are usually higher than those for dry precipitators. This means that, for a specific application, a wet device can be smaller than an equivalent dry device. This is additionally true because a wet precipitator operates on a cooled, lower volume gas stream.
Because the collecting surfaces in a wet precipitator are cleaned by a liquid, 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 that are normally of concern in the design of dry precipitators, such as electrical resistivity, surface adhesion, and flammability. A possible exception is the dielectric constant of the particles. It has a weak effect on the maximum charge that can be achieved, according to the theoretical relationship for predicting particle saturation charge.
N = saturation charge К = dielectric constant E0 = charging field A = particle diameter E = electron charge
The effect of dielectric constant on performance is not normally considered in the design of precipitators because the dielectric constant of most particles is high and has little effect on the charge. However, the constant may be important in oil mist collection by a wet precipitator. Some oils tend to have very low constants, which can markedly lower collection efficiencies.
Nevertheless, there are many applications for which a wet precipitator should be carefully considered and even some for which wet precipitation should be the only technology of choice (Figure 20.14). Some such conditions occur when the gas stream has already been treated in a wet scrubber, the temperature of the gas stream is low and its moisture content is high, gas and particles must be simultaneously removed, the loading of submicron particles 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 precipitators strongly depends on the particular design. In some cases, certain wet precipitator designs may not be suitable for certain applications. For instance, a precipitator for gas streams containing adherent particles must be continuously, not intermittently, irrigated.
The second most important factor in design after the type and configuration have been decided is materials of construction. Wet precipitators operate at, or below, the adiabatic saturation temperature of the irrigating fluid (usually water), and corrosion is a constant concern.
Application comparison chart. (Wayne T. Hartshorn)
Wet precipitators are rarely made of carbon steel, at least the surfaces that are in contact with the gases to be treated. Carbon steel construction may be feasible only when the gas stream is high in pH and low in oxygen. Ordinarily, wet precipitators are constructed of one or more corrosion-resistant materials. These materials can include simple stainless steels, exotic high-nickel alloys, reinforced thermo-setting materials, and thermoplastics.
From a materials standpoint, the casing, or housing, is the least critical element. The outside of the shell housing not in contact with the gases need not even be corrosion resistant, only capable of withstanding ambient conditions. The collecting surfaces should afford the maximum resistance to chemical attack. Also, fabrication points subject to corrosion should be minimized because failures in the collecting surfaces can disturb the electric field and cause arcing and lowering performance. Because the discharge electrodes are usually not irrigated, there is a concentrating effect on their surfaces that does not occur on wetted areas. For example, if the gas stream contains 200-500 ppm S02, 10-20 ppm HC1, and 0-5 ppm HF, the pH on the moist surface of the discharge electrodes will be about 1.0, even if the irrigant is kept at a pH of 3.0 or higher. The galvanic effect of operation in the range of 40,000-V direct current compounds the corrosion potential of the concentrating effect. For these reasons, the discharge electrodes should always be fabricated of a material of significantly greater corrosion resistance than that of any other part of the wet precipitator.
Wet precipitators capture fine or submicron particles without high-energy consumption (Figure 20.15). Their capture efficiency of submicron particles is greater than that of the highest energy wet scrubber. The size of the wet precipitator strongly affects its performance in collecting fine particles.
Relative energy consumption. (Hart Environmental, Inc.)
Wet precipitators are particularly effective in capturing large particles. However, most gas cleaners do a good job of this; 30%-40% of the emissions from a dry precipitator consist of large particles, mainly because of emissions due to rapping and reentrainment. Similarly, a considerable portion of the emissions from a wet scrubber is caused by mist carryover (another form of large particles).
Wet precipitators are relatively insensitive to the chemical and physical characteristics of the gas stream or the particles. Gas streams at almost any temperature or of any composition can ultimately be treated with the proper design. With added quenching and conditioning, wet precipitators can handle flue gases at over 2000°F because the adiabatic saturation temperature will always be less than approximately 180°F. Because wet precipitators can be constructed from a wide variety of materials, they can treat the most aggressive gas streams.
The factors that most influence the cost of wet precipitators are collection efficiency requirements, materials of construction due to the corrosive nature of the gas stream, and physical size due to the gas volume to be treated.
The actual cost of a wet precipitator in most cases will be site specific. A cost and systems analysis should be performed to determine the configuration, materials of construction, and size. Typically, a wet precipitator system to treat corrosive gases can run from $100 to $300 per square foot of collecting surface area; for noncorrosive applications, the price may be in the $50-$90 range.
Wet precipitator operating costs are among the lowest for gas cleaning equipment. They operate at lower pressure drops than scrubbers or fabric filters and generally have less collecting area and require less high-voltage power than dry precipitators. For estimating purposes, high-voltage power consumption will usually range between 0.1 and 0.5 W/actual ft3/min gas volume, depending on collection efficiency requirements. Auxiliary equipment, such as purge air blowers, heaters, and pumps, are highly site specific, so estimates of their power consumption should be done on a case-by-case basis.
Regarding installation orientation, it is suggested that the high-voltage supply be mounted in the serviceable area as close as practical to the WESR This keeps the high-voltage runs minimal in length and therefore less expensive to install and maintain.
The WESP is a very effective device for use in the collection of submicron particulate and mists where those contaminants can be water flushed from the collecting surfaces.