Summary of typical ESP applications (by industry)

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Other Industries

ESPs are used in many other large and small industrial processes including glass melting, sulfuric
acid production, food processing, and chemical manufacturing. Glass melting furnaces
usually use hot-side ESPs because the flue gas temperature in this process is approximately
230 to 260°C (450 to 500°F). Sulfuric acid plants usually use plate or tube-type ESPs to collect
sulfuric acid mists. Collected mists are removed from collection electrodes by water
sprays. Some smaller industries that produce coatings, resins, asphalt, rubber, textiles, plastics,
vinyl, and carpet frequently use a small two-stage precipitator to control particles and smoke.
The two-stage ESPs use liquid sprays to remove collected particles, smokes, and oils from the
collection plates.

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Lead, Zinc, and Copper Smelters

Plate ESPs are used to reduce particulate emissions from a number of processes in the smelting
of lead, zinc, and copper metals. Since lead, zinc, and copper are found in sulfide ore
deposits, the release of sulfur compounds is a problem during the smelting process. Before
being smelted, ore concentrates are often treated, or prepared, by two processes called sintering
and roasting. Sintering changes the physical form of a material, usually by taking an ore
mixture of large and fine pieces and fusing them into strong, porous products that can be used
in the smelting processes. ESPs are commonly used to reduce emissions from lead and zinc
sinter plants. ESPs are also effective in reducing emissions from zinc and copper roasters.
Roasters prepare zinc and copper ores by removing unwanted materials such as sulfur. The
roasted ore is then sent to other refining processes to produce zinc and copper metals.

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Kraft Pulp and Paper Mills

Plate or tube-type ESPs are used in the kraft pulp and paper industry to reduce particulate
emissions from the recovery furnace. In the kraft process of making pulp and paper, chemicals
are recovered by using evaporators, recovery furnaces, and reaction tanks. As part of the pulping
process, a waste product, black-liquor, is produced. After it is concentrated, the blackliquor
concentrate is burned in the recovery furnace to provide heat and steam to various processes
in the plant. The recovery furnace is essentially a boiler designed to effectively burn the
black-liquor concentrate. The resulting flue gas contains particulate matter that is usually
removed by an ESP before it is exhausted into the atmosphere. Dust can be removed from collection
electrodes by rapping or by using water sprays.

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Hazardous Waste Incinerators

ESPs are used in combination with a number of other air pollution control (APC) devices
including wet scrubbers and dry scrubbers (also called spray dryer absorbers) to clean the flue
gas generated by burning hazardous wastes. Some facilities have been designed to use spray
dryers to remove the acid gases including HCl, HF, and SO2 followed by the ESP to remove
the acid gas reaction salts, any unused sorbent, and particulate matter. Other facilities have
been designed with an APC system consisting of a spray dryer, baghouse, wet scrubber, and a
wet ESP (Figure). The spray dryer cools the flue gas and reduces some of the acid gas
components. The baghouse collects the particulate matter (including metals) and the wet
scrubber removes HCl (> 99%) and other acid gases. The wet ESP collects any particulate
matter not removed by the baghouse. The wet scrubbing system is a closed loop. The effluent
produced in the scrubbers is ultimately sent to the spray dryer to evaporate the liquid, therefore eliminating the need for a waste water treatment system. A number of facilities using this APC
system configuration are permitted to burn PCBs and other Toxic Substance Control Act
(TSCA) and Resource Conservation and Recovery Act (RCRA) wastes.

APC system for a hazardous waste incinerator consisting of a

spray dryer, baghouse, wet scrubbers, and wet ESPs

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Municipal Waste Incinerators

Electrostatic precipitators have been successfully used for many years to reduce particulate
emissions from municipal waste incinerators. Municipal incinerators, also commonly called
municipal waste combustors (MWCs) are used to reduce the volume of many different solid and liquid wastes. Generally, municipal wastes are composed of combustible materials (e.g.
paper, wood, rags, food, yard clippings, and plastic and rubber materials) and noncombustible
materials (e.g. rocks, metal, and glass). MWCs burn waste and produce ash residue that is disposed
of in landfills.
Both dry and wet plate ESPs are commonly used on municipal incinerators. Collected dust can
be removed from collection plates by rapping or by using water sprays. Plate ESPs having
rigid frame discharge electrodes are currently being used on MWCs (installed after 1982). The
designed collection efficiency is usually in the range of 96 to 99.6%. Dust resistivity can be a
problem, particularly if the refuse contains a large quantity of paper products. The dust in the
flue gas in this case usually has low resistivity. Resistivity can be adjusted by carefully controlling
the temperature and the amount of moisture in the flue gas.
Since the mid-1980s a number of large MWCs (plants having a capacity of 250 to 3000 tons
per day) with heat recovery devices have been built. More recent installations have been built
with acid gas control systems along with an ESP or baghouse. The ESP (or baghouse) collects
acid gas reaction products (mainly calcium chloride and calcium sulfate), unused sorbent
material, and fly ash. ESPs are typically designed with 3 to 5 fields and are capable of meeting
particulate emission limits of 0.015 gr/dscf and occasionally can achieve limits as low as 0.01
gr/dscf. These units have successfully reduced SO2 by 80% (24 hr avg) and HCl by 90 to 95%.
The acid gas is removed by using dry sorbent injection or spray dryer absorbers. In dry injection
systems sorbent is injected (usually hydrated lime) into the furnace or into the ducting
prior to the flue gas entering the ESP. Acid gas removal efficiencies of 50% for SO2 and 75%
for HCl are routinely achieved (Beachler 1992).
A more commonly used acid gas control system is a spray dryer absorber placed ahead of the
ESP. These systems have been able to achieve 80% removal (24 hr avg) for SO2 and 90%
removal for HCl. A wet calcium hydroxide slurry is injected into a spray dryer by a rotary
atomizer or two-fluid nozzle. The slurry is made by slaking pebble lime (CaO) with water in a
paste or detention slaker. The heat of the flue gas evaporates the liquid slurry in the spray dryer
and the dry acid gas reaction products along with the particulate matter are collected in the
ESP. Background information and data prepared as part of the promulgated NSPS and Emission
Guidelines (U.S. EPA 1991) shows very good acid gas removal and particulate emission
control for these systems.

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Petroleum Refineries

ESPs are used in petroleum refineries to control particulate emissions from fluid-catalytic
cracking units and boilers. In a refinery, heavy crude is broken down into lighter components
by various distilling, cracking, and reforming processes. One common process is to "crack"
the high-molecular-weight, high-boiling-point compounds (heavy fuel oils) into smaller, lowmolecular-
weight, low-boiling-point compounds (gasoline). This is usually done in a fluidcatalytic
cracking (FCC) unit.
In an FCC unit, the feed stream (heavy gas oils) is heated and then mixed with a hot catalyst
that causes the gas oils to vaporize and crack into smaller hydrocarbon-chain compounds.
During this process, the catalyst becomes coated with coke. The coke deposits are eventually
removed from the catalyst by a catalyst-regeneration step.
In the regenerator, a controlled amount of air is added to burn the coke deposits off the catalyst
without destroying it. The gases in the regenerator pass through cyclones to separate large catalyst
particles. The gases can sometimes go to a waste heat boiler to burn any carbon monoxide
and organic emissions present in the gas stream. The boiler's exhaust gas still has a high
concentration of fine catalyst particles. This flue gas is usually sent to an electrostatic precipitator
to remove the very fine catalyst particles.
ESPs can also reduce particulate emissions from boiler exhausts. Oil-fired and, occasionally,
coal-fired boilers generate steam that is used in many processes in the refinery. The flue gas
from boilers is frequently sent to ESPs to remove particulate matter before the gas is exhausted
into the atmosphere. ESPs designed similarly to those used on industrial and utility boilers are
used on FCC units and petroleum refinery boilers.

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Steel Mills

ESPs are used in steel mills for reducing particulate emissions from blast furnaces, basic oxygen
furnaces, and sinter plants.
In a blast furnace, iron ore is reduced to molten iron, commonly called pig iron. Blast furnaces
are large, refractory-lined steel shells. Limestone, iron ore, and coke are charged into the top of
the furnace. The gases produced during the melting process contain carbon monoxide and particulate
matter. Particulate matter is removed from the blast furnace gas by wet ESPs or scrubbers,
so that the gas (CO) can be burned "cleanly" in blast furnace stoves or other processes.
Both plate and tube-type ESPs having water sprays to remove dust from collection electrodes
are commonly used for cleaning blast furnace gas.
Basic oxygen furnaces (BOFs) refine iron from the blast furnace into steel. A BOF is a pearshaped
steel vessel that is lined with refractory brick. The vessel is charged with molten iron
and steel scrap. A water-cooled oxygen lance is lowered into the vessel, where oxygen is
blown to agitate the liquid, add intense heat to the process, and oxidize any impurities still contained in the liquid metal. The hot gases generated during the oxygen blow are approximately
1090 to 1650°C (2000 to 3000°F). These are usually cooled by water sprays located in
the hood and ducting above the BOF. The cooled gases are then sent to an ESP or scrubber to
remove the particulate matter (iron oxide dust). The iron oxide dust can have high resistivity,
making the dust difficult to collect in an ESP. This problem can usually be reduced by conditioning
the flue gas with additional moisture. Plate ESPs that are rapped or sprayed with water
to remove dust from collection plates are commonly installed on BOFs.
In a sinter plant, materials such as flue dusts, iron ore fines (small particles), coke fines, mill
scale (waste that occurs from various processing steps), and small scrap are converted into a
high-quality blast furnace feed. These materials are first fed onto a traveling grate. The bed of
materials is ignited by burning gas in burners located at the inlet of the traveling grate. As the
bed moves along the traveling grate, air is pulled down through the bed to burn it, forming a
fused, porous, red-hot sinter. The resulting gases are usually sent to an electrostatic precipitator
to remove any particulate matter. If oily scrap is used as a feed material, care must be taken
to prevent ESP collection plates from being coated with tarry particulate matter. Controlling
the amount of oily mill scale and small scrap processed in the sinter plant can help alleviate
this problem. Plate ESPs are commonly used in sinter plants.

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Cement Plants

ESPs are used in cement plants to control particulate emissions from cement kilns and clinker
coolers. In a cement plant, raw materials are crushed, ground, blended, and fed into a kiln,
where they are heated. The kiln is fired with coal, oil, or gas. The material is heated to a temperature
above 1595°C (2900°F), which causes it to fuse. The fused material is called cement
clinker. The temperature of the hot, marble-sized, glass-hard clinker is cooled by the clinker
cooler. The cooled clinker is then sent to the final grinding mills.
ESPs are frequently used to control kiln emissions because of their ability to handle high-temperature
gases. These ESPs are usually hot-side ESPs with collection plates that are rapped or
sprayed with water to remove collected dust. The dust generated in the cement kiln frequently
has high resistivity. High resistivity can be reduced by conditioning the flue gas with moisture.
Many of the newer cement plants send the high temperature kiln flue gas that contains particulate
matter through a cyclone and conditioning tower (uses water to cool the gas temperature)
prior to ducting the flue gas to the ESP. The ESP is then operated at a temperature of approximately
150°C (302°F).
A special problem arises during kiln startup due to the fact that the temperature of the kiln
must be raised slowly to prevent damage to the heat-resistant (refractory) lining in the kiln.
While kilns (especially coal-fired ones) are warming up and temperatures are below those for
steady-state operating conditions, complete combustion of the fuels cannot occur, giving rise
to combustible gases in the exhaust stream leading into the ESP. Electrostatic precipitators
cannot be activated in the presence of combustibles, because the internal arcing of the precipitator
could cause a fire or explosion. Use of a cyclone preceding the precipitator helps to minimize
the excessive emissions during startup. Periods of excessive emissions during startup,
malfunction, or shutdown are specifically exempted from the federal New Source Performance
Standards for cement kilns.
ESPs can also be used on clinker coolers. However, the ESP must be carefully designed to prevent
moisture in the flue gas from condensing. Condensed moisture can combine with clinker
dust to coat the ESP internals with cement. (A case history of an ESP used on a cement kiln is
given in Szabo et al. 1981.)

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Boilers

Particulate Matter Control System
ESPs are most widely used for the control of fly ash from industrial and utility boilers and
have been used on coal-fired boilers for over 50 years. Particulate matter is generated from
boilers when fossil fuels (coal and oil) are burned to generate steam for industrial processes
or to produce electric power. Both hot-side and cold-side precipitators are used to
control particulate emissions. Other than some construction modifications to account for
the temperature difference of the flue gas handled, hot-side and cold-side ESPs are essentially
the same. Cold-side ESPs are used most often for collecting fly ash from coal-fired boilers. If the dust has high resistivity, cold-side units are used along with a conditioning agent such as sulfur trioxide

Dry Sulfur Dioxide (SO2) Control System
One technology for reducing sulfur dioxide (SO2) emissions from boilers is dry flue gas
desulfurization (FGD). In dry FGD, the flue gas containing SO2 is contacted with an
alkaline material to produce a dry waste product for disposal. This technology consists of
three different FGD methods:
• Injection of wet alkaline material (slurry) into a spray dryer with collection of dry particles
in an electrostatic precipitator or baghouse,
• Injection of dry alkaline material into the flue gas stream with collection of dry particles
in an ESP or baghouse, or
• Addition of alkaline material to the fuel prior to combustion

Spray dryers used in dry FGD are similar to those that have been used for over 40 years
in the chemical, food-processing, and mineral preparation industries. Spray dryers are vessels
where hot flue gas is contacted with a finely atomized, wet alkaline spray (see Figure). Flue gas enters the top of the spray dryer and is swirled by a fixed vane ring to cause
intimate contact with the slurry spray. Sodium carbonate solutions and lime slurries are the
most common alkaline material used. The slurry is atomized into extremely fine droplets
by rotary atomizers or two-fluid nozzles. In a rotary atomizer, slurry is broken into droplets
by centrifugal force as the atomizer wheel spins at a very high speed. In two-fluid nozzles,
slurry is mixed with compressed air, which forms the very small droplets. The high
temperature of the flue gas, 120 to 204°C (250 to 400°F), evaporates the moisture from the
wet alkaline sprays, leaving a dry, powdered product. The dry product is then collected in
an ESP or baghouse (Joseph and Beachler 1981).

Spray dryer with ESP

A number of spray dryer FGD systems have been installed on industrial and utility boilers.
They are particularly useful in meeting New Source Performance Standards (NSPS) that
require only 70% SO2 removal efficiency for utility boilers burning low-sulfur coal and as
retrofit applications for units having to meet the standards required by the 1990 Clean Air
Act Amendments (see Table).

Commercial spray dryer FGD systems using an ESP or a baghouse

Spray dryer absorbers systems can reduce SO2 emissions by 60 to 90%. They have been
used on boilers burning low-sulfur coal (usually less than 2% sulfur content) and are
attractive alternatives to wet scrubbing technology, particularly in the arid western U.S.
In dry injection systems, a dry alkaline material (sorbent) is injected pneumatically into
the gas stream by nozzles located in the ductwork prior to the flue gas entering the ESP.
Sodium-based sorbents are used more frequently than lime for industrial coal-fired boilers
but hydrated lime is prevalent for waste burning incinerators. Sodium bicarbonate is frequently
used because it is highly reactive with SO2. Sodium carbonate (soda ash),
although not as reactive as sodium bicarbonate, is also used (U.S. EPA 1980). SO2
removal efficiency for these systems is typically between 70 and 80%.
A third way to apply dry FGD is by adding alkaline material to the fuel (coal) prior to
combustion. In fluidized bed boilers, limestone or sometimes lime is added to the coal in
the fluidized burning bed. These systems are capable of removing more than 90% of SO2
from the boiler flue gas. Alkaline material can also be injected into the furnace through
ports or directly into the fuel burners. The SO2 removal is typically greater than 70% in
these systems.

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Introduction

Because ESPs can collect dry particles, sticky or tarry particles, and wet mists, they are used
by many different industries, as diverse as chemical production and food processing. This lesson
reviews the following industries that use ESPs to reduce air pollutant emissions: fossilfuel-
fired boilers, cement plants, steel mills, petroleum refineries, municipal waste incinerators,
hazardous waste incinerators, kraft pulp and paper mills, and lead, zinc, and copper
smelters.

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