EPA-452/F-03-030
Air Pollution Control Technology
Fact Sheet
Name of Technology: Wet Electrostatic Precipitator (ESP)- Wire-Plate Type
Type of Technology: Control Device - Capture/Disposal
Applicable Pollutants:
Particulate Matter (PM), including particulate matter less than or equal to 10 micrometers (|j,m) in aerodynamic
diameter (PM10), particulate matter less than or equal to 2.5 |j,m in aerodynamic diameter (PM25), and
hazardous air pollutants (HAPs) that are in particulate form, such as most metals (mercury is the notable
exception, as a significant portion of emissions are in the form of elemental vapor). Wet ESPs are often used
to control acid mists and can provide incidental control of volatile organic compounds.
Achievable Emission Limits/Reductions:
Typical new equipment design efficiencies are between 99 and 99.9%. Older existing equipment have a
range of actual operating efficiencies of 90 to 99.9%. While several factors determine ESP collection
efficiency, ESP size is most important. Size determines treatment time; the longer a particle spends in the
ESP, the greater its chance of being collected. Maximizing electric field strength will maximize ESP collection
efficiency (STAPPA/ALAPCO, 1996). Collection efficiency is also affected to some extent by dust resistivity,
gas temperature, chemical composition (of the dust and the gas), and particle size distribution. Cumulative
collection efficiencies of PM, PM10,
presented in Table 1.
and PM25 for actual operating ESPs in various types of applications are
Table 1. Cumulative PM, PM10, and PM25 Collection Efficiencies for Wet ESPs
(EPA, 1998; EPA, 1997)
Application
Primary Copper Production
Multiple hearth roaster
Reverbatory smelter
Iron and Steel Production
Open hearth furnace
Sinter oven
Collection
Total PM
(EPA,
1997)
99.0
99.0
99.2
98.0
Efficiency (
PM10
(EPA,
1998)
99.0
97.1
99.2
94.0
%)
PM25
(EPA,
1998)
99.1
97.4
99.2
90.0
Applicable Source Type: Point
EPA-CICA Fact Sheet
Wet Electrostatic Precipitator (ESP)
Wire-Plate Type
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Typical Industrial Applications:
Wet ESPs are used in situations for which dry are not suited, such as when the material to be collected is wet,
sticky, flammable, explosive, or has a high resistivity. Also, as higher collection efficiencies have become
more desirable, wet ESP applications have been increasing (EPA, 1998). Wet ESPs are commonly used by
the wood products, metalurgical, and sulfuric acid manufacturing industries, though other ESP types are also
employed. Common applications of wet wire-plate ESPs are presented in Table 2.
Table 2. Typical Industrial Applications of Wet Wire-Plate ESPs (EPA, 1998)
Application Source Category Code
(SCC)
Chemical Manufacture 3-01-001...999
Non-Ferrous Metals Processing (Primary and
Secondary):
Copper 3-03-005
3-04-002
Lead 3-03-010
3-04-004
Zinc 3-03-030
3-04-008
Aluminum 3-03-000...002
3-04-001
Other metals production 3-03-011...014
3-04-005...006
3-04-010...022
Ferrous Metals Processing:
Iron and Steel Production 3-03-008...009
Steel Foundries 3-04-007,-009
Mineral Products:
Stone Quarrying and Processing 3-05-020
Other 3-05-003...999
Wood, Pulp, and Paper 3-07-001
Emission Stream Characteristics:
a. Air Flow: Typical gas flow rates for wet wire-plate ESPs are 50 to 250 standard cubic
meters per second (sm3/sec) (100,000 to 500,000 standard cubic feet per minute (scfm)).
Most smaller plate-type ESPs (50 sm3/sec to 100 sm3/sec, or 100,000 to 200,000 scfm) use
flat plates instead of wires for the high-voltage electrodes (AWMA, 1992).
b. Temperature: Wet wire-plate ESPs are limited to operating at temperatures lower than
approximately 80 to 90-C (170 to 190-F) (EPA, 1998; Flynn, 1999).
c. Pollutant Loading: Typical inlet concentrations to a wire-plate ESP are 2 to 110 g/m3 (1
to 50 gr/scf). It is common to pretreat a waste stream, usually with a water spray or scrubber,
to bring the temperature and pollutant concentration into a manageable range. Highly toxic
flows with concentrations below 1 g/m3 (0.5 gr/scf) are also sometimes controlled with ESPs
(Bradburn, 1999; Boyer, 1999; Brown, 1999).
EPA-CICA Fact Sheet Wet Electrostatic Precipitator (ESP)
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d. Other Considerations: Dust resistivity is not a factor for wet ESPs, because of the high
humidity atmosphere which lowers the resistivity of most materials. Particle size is much less
of a factor for wet ESPs, compared to dry ESPs. Much smaller particles can be efficiently
collected by wet ESPs due to the lack of resistivity concerns and the reduced reentrainment
(Flynn, 1999).
Emission Stream Pretreatment Requirements:
When the pollutant loading is exceptionally high or consists of relatively large (> 2 • m) particles, venturi
scrubbers or spray chambers may be used to reduce the load on the ESP. Much larger particles (> 10 • m),
are controlled with mechanical collectors such as cyclones. Gas conditioning equipment to reduce both inlet
concentration and gas temperature is occasionally used as part of the original design of a wet ESP (AWMA,
1992; Flynn, 1999).
Cost Information:
The following are cost ranges (expressed in 2002 dollars) for wire-plate ESPs of conventional design under
typical operating conditions, developed using EPA cost-estimating spreadsheets for dry wire-plate ESPs with
adjustments made to reflect wet wire-plate ESPs (EPA, 1996). Costs can be substantially higher than in the
ranges shown for pollutants which require an unusually high level of control, or which require the ESP to be
constructed of special materials such as titanium. Capital and operating costs are generally higher due to
noncorrosive materials requirements, increased water usage, and treatment and disposal of wet effluent. In
most cases, smaller units controlling a low concentration waste stream will not be as cost effective as a large
unit cleaning a high pollutant load flow (EPA, 1998).
a. Capital Cost: $42,000 to $85,000 per snf/sec ($20 to $40 per scfm)
b. O&MCost: $11,000 to $85,000 per snf/sec ($5 to $40 per scfm), annually
c. Annualized Cost: $19,000 to $100,000 per snf/sec ($9 to $47 per scfm), annually
d. Cost Effectiveness: $53 to $570 per metric ton ($48 to $520 per short ton)
Theory of Operation:
An ESP is a particulate control device that uses electrical forces to move particles entrained within an exhaust
stream onto collector plates. The entrained particles are given an electrical charge when they pass through
a corona, a region where gaseous ions flow. Electrodes in the center of the flow lane are maintained at high
voltage and generate the electrical field that forces the particles to the collector walls. In wet ESPs, the
collectors are either intermittently or continuously washed by a spray of liquid, usually water. The collection
hoppers used by dry ESPs are replaced with a drainage system. The wet effluent is collected, and often
treated on-site (EPA, 1998).
In the wire-plate ESP, the exhaust gas flows horizontally and parallel to vertical plates of sheet metal. Plate
spacing is typically between 19 to 38 cm (9 to 18 inches (in.)) (AWMA, 1992). The high voltage electrodes
are long wires that are weighted and hang between the plates. Some later designs use rigid electrodes
(hollow pipes approximately 25 mm to 40 mm in diameter) in place of wire (Cooper and Alley, 1994). Within
each flow path, gas flow must pass each wire in sequence as it flows through the unit. The flow areas
between the plates are called ducts. Duct heights are typically 6 to 14 meters (m) (20 to 45 feet) (EPA, 1998).
EPA-CICA Fact Sheet Wet Electrostatic Precipitator (ESP)
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The power supplies for the ESP convert the industrial AC voltage (220 to 480 volts) to pulsating DC voltage
in the range of 20,000 to 100,000 volts as needed. The voltage applied to the electrodes causes the gas
between the electrodes to breakdown electrically, an action known as a "corona." The electrodes are usually
given a negative polarity because a negative corona supports a higher voltage than does a positive corona
before sparking occurs. The ions generated in the corona follow electric field lines from the wires to the
collecting plates. Therefore, each wire establishes a charging zone through which the particles must pass.
As larger particles (>10 • m diameter) absorb many times more ions than small particles (>1 • m diameter),
the electrical forces are much stronger on the large particles (EPA, 1996).
Due to necessary clearances needed for nonelectrified internal components at the top of the ESP, part of the
gas may flow around the charging zones. This is called "sneakage" and places an upper limit on the collection
efficiency. Anti-sneakage baffles are placed to force the sneakage flow to mix with the main gas stream for
collection in later sections (EPA, 1998).
Wet ESPs require a source of wash waterto be injected or sprayed nearthe top of the collector plates either
continuously or at timed intervals. This wash system replaces the rapping mechanism usually used by dry
ESPs. The water flows with the collected particles into a sump from which the fluid is pumped or drained.
A portion of the fluid may be recycled to reduce the total amount of water required. The remainder is pumped
into a settling pond or passed through a dewatering stage, with subsequent disposal of the sludge (AWMA,
1992).
Unlike dry ESPs, resistivity of the collected material is generally not a major factor in performance. Because
of the high humidity in a wet ESP, the resistivity of particles is lowered, eliminating the "back corona" condition.
The frequent washing of the plates also limits particle buildup on the collectors (EPA, 1998).
Advantages:
Wet wire-plate ESPs and other ESPs in general, because they act only on the particulate to be removed, and
only minimally hinder flue gas flow, have very low pressure drops (typically less than 13 mm (0.5 in.) water
column). As a result, energy requirements and operating costs tend to be low. They are capable of very high
efficiencies, even for very small particles. Operating costs are relatively low. ESPs are capable of operating
under high pressure (to 1,030 kPa (150 psi)) or vacuum conditions, and relatively large gas flow rates can be
effectively handled (AWMA, 1992).
Wet ESPs can collect sticky particles and mists, as well as highly resistive or explosive dusts. The continuous
or intermittent washing with a liquid eliminates the reentrainment of particles due to rapping which dry ESPs
are subject to. The humid atmosphere that results from the washing in a wet ESP enables them to collect
high resistivity particles, absorb gases or cause pollutants to condense, and cools and conditions the gas
stream. Liquid particles or aerosols present in the gas stream are collected along with particles and provide
another means of rinsing the collection electrodes (EPA, 1998).
Disadvantages:
ESPs generally have high capital costs. The wire discharge electrodes (approximately 2.5 mm (0.01 in.) in
diameter) are high-maintenance items. Corrosion can occur near the top of the wires because of air leakage
and acid condensation. Also, long weighted wires tend to oscillate - the middle of the wire can approach the
plate, causing increased sparking and wear. Newer ESP designs are tending toward rigid electrodes (Cooper
and Alley, 1994).
ESPs in general are not suited for use in processes which are highly variable because they are very sensitive
to fluctuations in gas stream conditions (flow rates, temperatures, particulate and gas composition, and
particulate loadings). ESPs are also difficult to install in sites which have limited space since ESPs must be
EPA-CICA Fact Sheet Wet Electrostatic Precipitator (ESP)
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relatively large to obtain the low gas velocities necessary for efficient PM collection (Cooper and Alley, 1994).
Relatively sophisticated maintenance personnel are required, as well as special precautions to safeguard
personnel from the high voltage. Ozone is produced by the negatively charged electrode during gas ionization
(AWMA, 1992). Wet ESPs add the complexity of a wash system, and the fact that the resulting slurry must
be handled more carefully than a dry product, and in many cases requires treatment, especially if the dust can
be sold or recycled. Wet ESPs are limited to operating at stream temperatures under approximately 80 to
90-C (170 to 190-F), and generally must be constructed of noncorrosive materials (EPA, 1998; Flynn, 1999).
Other Considerations:
For wet ESPs, consideration must be given to handling wastewaters. For simple systems with innocuous
dusts, water with particles collected by the ESP may be discharged from the ESP system to a solids-removing
clarifier (either dedicated to the ESP or part of the plant wastewater treatment system) and then to final
disposal. More complicated systems may require skimming and sludge removal, clarification in dedicated
equipment, pH adjustment, and/or treatment to remove dissolved solids. Spray water from an ESP
preconditioner may be treated separately from the water used to wash the ESP collecting plates so that the
cleaner of the two treated water streams may be returned to the ESP. Recirculation of treated water to the
ESP may approach 100 percent (AWMA, 1992).
References:
AWMA, 1992. Air& Waste Management Association, Air Pollution Engineering Manual, Van Nostrand
Reinhold, New York.
Boyer, 1999. James Boyer, Beaumont Environmental Systems, (724) 941-1743, personal communication
with Eric Albright, January 18, 1999.
Bradburn, 1999. Keith Bradburn, ABB Environmental Systems, (800) 346-8944, personal communication
with Eric Albright, January 18, 1999.
Brown, 1999. Bob Brown, Environmental Elements Corp., (410) 368-6894, personal communication with
Eric Albright, January 18, 1999.
Cooper & Alley, 1994. C. D. Cooper and F. C. Alley, Air Pollution Control: A Design Approach, Second
Edition, Waveland Press, Inc. IL.
EPA, 1996. U.S. EPA, Office of Air Quality Planning and Standards, "OAQPS Control Cost Manual," Fifth
Edition, EPA 453/B-96-001, Research Triangle Park, NC. February.
EPA, 1997. U.S. EPA, Office of Air Quality Planning and Standards, "Compilation of Air Pollutant
Emission Factors, Volume I, Fifth Edition, Research Triangle Park, NC., October.
EPA, 1998. U.S. EPA, Office of Air Quality Planning and Standards, "Stationary Source Control
Techniques Document for Fine Particulate Matter," EPA-452/R-97-001, Research Triangle Park, NC.,
October.
Flynn, 1999. Brian Flynn, Beltran Associates, Inc., (718) 338-3311, personal communication with Eric
Albright, Februarys, 1999.
ICAC, 1999. Institute of Clean Air Companies internet web page www.icac.com, Control Technology
Information - Electrostatic Precipitator, page last updated January 11, 1999.
EPA-CICA Fact Sheet Wet Electrostatic Precipitator (ESP)
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STAPPA/ALAPCO, 1996. State and Territorial Air Pollution Program Administrators and Association of
Local Air Pollution Control Officials, "Controlling Particulate Matter Under the Clean Air Act: A Menu of
Options," July.
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