ELECTROSTATIC PRECIPITATOR

Electrostatic precipitators: This electrical equipment was
first introduced by Dr.F.G. Cottel in 1906 and was first economically
used in
1937 for removal of dust and ash particles with the exhaust gases of
thermal
power plants

PRINCIPLES OF ELECTROSTATIC PRECIPITATOR
Electrostatic precipitation is a method of dust collection that uses
electrostatic forces, and consists of discharge wires and collecting
plates.
A high voltage is applied to the discharge wires to form an electrical field
between the wires and the collecting plates, and also ionizes the gas
around the discharge wires to supply ions.
When gas that contains an aerosol (dust, mist) flows between the
collecting plates and the discharge wires, the aerosol particles in the
gas are charged by the ions. The Coulomb force caused by the electric
field causes the charged particles to be collected on the collecting
plates, and the gas is purified.
This is the principle of electrostatic precipitation, and Electrostatic
precipitator apply this principle on an industrial scale.

ESP OPERATION
Electrostatic precipitators use electrostatic charges to
separate particles from a dirty gas stream.
High voltage, direct current electrodes are used to
establish a strong electric field.
This field (known as a corona) delivers a (usually)
negative charge to particles as they pass through the
device.
This charge forces the particles onto the walls of
collection plates or tubes.
These collection surfaces (or collection electrodes) are
then rapped, vibrated, or washed with water to
dislodge the particles, which fall into a hopper.

IN SUMMARY, THE MAJOR COMPONENTS OF A PRECIPITATOR
INCLUDE:
Collection electrodes
Discharge electrodes
High voltage power supply
Precipitator controls
Rapping or spray washing systems
Purge air systems
Efficiency losses occur in ESPs due to reentrainment, sneakage, and
back corona. Reentrainment occurs due to rapping, causing a small
percentage (10-15 percent) to be projected back into the gas
stream. Sneakage is the small part of gas flow that moves around
the charging zones untreated due to practical design constraints.
Back corona occurs when an electric field becomes large enough
to cause an electrical breakdown which reduces the charge on
particles.

ESP DESIGN
To begin, a manufacturer will ask for a number
of process variables which describe the conditions
and requirements of the system. These variables
include:
Gas flow rate - how fast the gas is moving through the
system. At higher flow rates, particle reentrainment
increases rapidly, but insufficient flow will result in
poor gas distribution or particle dropout.
Particle size and size distribution - the average size
and size distribution of PM in the gas flow. Larger
particles pickup charge more easily, while an
abundance of small particles may suppress the
generation of the corona.

A typical ESP has thin wires called discharge
electrodes (DE), which are evenly spaced between
large plates called collection electrodes (CE).

Design factors that determine an ESP's performance include :
Particle resistivity - measure of the particles' resistance to
electrical conductance. Particles with high resistivity have
difficulty acquiring charge, while particles with low resistivity
may lose their charge too easily and not stick to the collection
plate. Resistivity is influenced by the particulate's chemistry
and the gas temperature.
 Gas temperature - the temperature of the gas flow in the system.
 Particle chemistry - the chemical makeup of the particulate matter in
the gas flow.
Precipitator size - the size of the precipitator affects its collection
efficiency, footprint, and gas flow capacity.
Power input - the power supplied to the system to induce the
electric field. Increasing power input improves collection
efficiency under normal conditions.

Types of ESPs :
ESPs are classified based on a number of different factors,
including the collector design, the number of stages,
and whether the process is dry or wet.
Plate or Tubular
The functional design of an ESP incorporates either plate or
tubular collection surf Plate Precipitators
Plate ESPs primarily collect dry particles and are used more often
than tubular precipitators. They can have wire-plate or flat-plate
electrodes.
Plate-Wire Precipitators
In a plate-wire ESP, gas flows between parallel plates of sheet
metal and high-voltage long metal wires. It allows many flow
lanes to operate in parallel, making it suitable for handling large
volumes of gas.

Plate-wire precipitators are among the most common
types of ESPs. In industry, they are used in cement
kilns, incinerators, boilers, cracking units, sinter
plants, furnaces, coke oven batteries, and a variety
of other applications.
Flat Plate Precipitators: Smaller precipitators use flat
plates instead of wires for high-voltage electrodes.
The flat plates increase the average electric field
used to collect particles and provide additional
surface area for particle collection. They are less
susceptible to back corona than conventional
plate-wire precipitators but also have higher
rapping losses.
Flat plate ESPs can be used in applications with
high-resistivity particles with small (1 to 2 µm)
diameters. Fly ash can be captured using flat plate
ESPs, but typically requires low velocities to
prevent significant rapping losses.

Tubular Precipitators
Tubular ESPs consist of parallel arrangements of tubes with
high-voltage electrodes running on their axis. The tubes
may be arranged as a circular, square, or hexagonal
honeycomb with gas flowing upwards or downwards.
They are designed as one-stage units in which all the gas
passes through the tube, eliminating sneakage. They are
still susceptible to inefficiencies from corona non-
uniformities.
Tubular precipitators are less common than plate types.
They are used in applications involving wet or sticky
particulate, and are typically cleaned with water for lower
reentrainment losses than typical ESPs. They also can be
tightly sealed to prevent leakage of material, an important
consideration for valuable or hazardous substances.

Single or Two Stage
ESPs can be designed as either single or
two stage configurations.
Single-Stage Precipitators
Most industrial scale ESPs are single stage.
They use very high voltages to charge
particles and incorporate charging and
collection together in the same stage.
Sets of electrodes and collector surfaces
(plates or tubes) operate in parallel to
each other.

Two -Stage Precipitators
Two-stage ESPs operate in series rather than parallel
configuration. Instead of using a side by
side design, they incorporate separate particle
charging and collection stages. This allows more
time for particle charging, less susceptibility to back
corona, and economical construction for smaller
sizes.
Two-stage precipitators are separate and distinct from
other ESPs, originally designed for air purification
in conjunction with air conditioning systems. They
are typically used for smaller, lower-volume
applications. They are usually applied to submicron
sources emitting oil mists, smokes, fumes,
or other liquid aerosols. Many are sold as pre-
engineered, package systems.

DRY OR WET
ESPs can also be classified based on whether
they operate using a dry or wet process.
Dry ESPs
Dry electrostatic precipitators are used to capture
particles in dry product streams. They use periodic
rapping to separate the accumulated dust from the
collector plates and discharge electrodes. The dust
layer (released by rapping) is collected in a hopper
and then removed by an ash handling system.
Typically, rapping will also project some of these
particles (around 10-15 percent) back into the gas
stream (known as reentrainment).
Dry electrostatic precipitators are often not suitable
for submicron particulate applications because of
particle size, resistivity, and other issues

WET ESPS (WESPS)
Wet electrostatic precipitators are used to strip wet
(saturated) gas streams of particles. They use water
sprays to condition/trap particles for collection and also
to clean the particles off collection surfaces. WESPs
collect particulate matter not suitable for dry ESPs,
including sticky, moist, flammable, explosive, or high
resistivity solids. WESPs can also remove very fine
(submicron) particulate that dry ESPs cannot capture
effectively. The use of water also gives these devices gas
scrubbing capabilities. Most wet precipitators are tubular
designs.
However, WESPs are more costly than dry ESPs. Because
they incorporate water and corrosive gases, they must be
designed from more expensive corrosion-resistant
materials. Another disadvantage of WESPs is that the PM
is collected as a slurry instead of a dry solid. This form is
unsuitable for high value or recyclable materials and is
more expensive to handle and dispose. If the water is
being recycled and reused, the system also must
incorporate a water purification step.

APPLICATIONS
ESPs may be specifically designed to meet the needs of certain
industries or applications. Some applications and media
types include:
Abrasives - baghouse fabrics are designed to withstand and
capture abrasive particles.
Coolant and oil mists - unit is capable of filtering coolant smoke
and mist from metal finishing and forming processes, and
machining oil mists.
Explosive media - unit is capable of filtering explosive dusts, mists,
and/or fumes.
Fine powders - unit is capable of filtering fine powders such as
carbon black, talc, pigments, oxides, and plastic compounding
dusts.
Metalworking chips & fluids - unit can capture aerosols and fumes
emitted by metalworking fluids, including oils, lubricants, and
coolants.
Toxic media - unit is capable of filtering toxic materials such as
dust, mist, fume, or smoke from the air.
Welding fumes - unit is designed specifically for the collection of
welding fumes or dust; these may include flux recovery

Depending on the design, ESPs are capable
of handling large gas volumes across a
wide range of temperatures, pressures,
dust volumes, and acid gas conditions.
In the case of an ESP, a negative, high-voltage,
pulsating, direct current is applied to the DEs in
order to create a negative electric field and induce
ionization of the passing PM.
. For the sake of understanding the charging process,
the negative electric field can be mentally divided
into three regions. The first region is right next to
the discharge electrode, where the field is the
strongest. The second region includes space
between the DE and CE called the inter-electrode
region, and is weaker than the first. The third region
is located near the collection electrode and has the
weakest field strength of all.

RESISTIVITY OF THE PARTICLES
•
Particulate resistivity is probably the most important basic variable
influencing the precipitator and therefore is an important design
consideration.A too high level of electrical resistivity or too low level
causes collection difficulty. A high resistivity dust, such as sulphur,
does not readily give up its negative charge and assumes a positive
charge. This causes the particulate to be repelled back into the gas
stream of negatively charged particles. A low resistivity dust can be
collected and repelled in this manner many times before finally being
emitted to the atmosphere. Therefore, the presence of large quantities
of carbon in the ash can adversely affect the collection efficiency of a
precipitator. One thumb rule followed by designer is to downgrade the
efficiency of the unit by 1% for every 1% of carbon in the gas over 15%.
Therefore, one always wishes a medium resistivity for good collection
efficiency. In coal fired boilers, sulphur in the form of SO2 affects
resistivity. Resistivity has two components, one related to the bulk of
the material and another is related to the surface of the particle,
absorbed layer of gas. As the temperature increases, the absorbed
surface contaminants evaporate and surface resistivity increases. And
with all insulating materials, the volume resistivity increases with
decreasing temperature.

Six steps typically take place:
Ionization – Charging of particles
Migration – Transporting the charged particles to the collecting surfaces
Collection – Precipitation of the charged particles onto the collecting surfaces
Charge Dissipation – Neutralizing the charged particles on the collecting surfaces
Particle Dislodging – Removing the particles from the collecting surface to the hopper
Particle Removal – Conveying the particles from the hopper to a disposal point

The major precipitator components that accomplish these activities are as
follows:
Discharge Electrodes
Power Components
Precipitator Controls
Rapping Systems
Purge Air Systems
Flue Gas Conditioning / Sorbent Injection Systems

CORONA DISCHARGE:
FREE ELECTRON GENERATION
The first region is where the particle charging process begins, and in this small area immediately surrounding the
discharge electrode several things happen very rapidly (in a matter of a millisecond). As voltage applied to the DE is
increased, it eventually reaches a point when the electric field around the conductor is high enough to form a
conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects. This
phenomenon is commonly referred to as corona discharge and can be seen by the human eye as a luminous blue
glow surrounding the DE. As free electrons created by the corona discharge are repulsed by the negative electric
field, they move faster and faster away from the DE. This acceleration causes the electrons to literally crash into
passing gas molecules and occasionally knock off some of their electrons. As these gas molecules lose electrons
that are negatively charged, they become positively charged ions. So, this is the first thing that happens – gas
molecules are ionized, and electrons are liberated. All this activity occurs very close to the discharge electrode, and
as the process continues, it creates more and more free electrons and positive ions. The name for all of this
electron generation is avalanche multiplication.

IONIZATION OF GAS MOLECULES
As electrons leave the strong electrical field region surrounding the discharge electrode, they enter
the inter-electrode region where they begin to lose energy and slow down. Though there are still
gas molecules in the inter-electrode region, the electrons kind of bump up to them and get
captured instead of violently colliding with them, creating negative gas ions. Now we have
ionization of gas molecules happening near the discharge electrode as well as in the inter-
electrode area, but with a big difference. The ions created near the discharge electrode are
positive and remain in that area. However, because the ions created in the inter-electrode area
are negative, they want to move with the electrons in the direction opposite the strong negative
field.

CHARGING & MIGRATION
Before PM can be captured, it must first acquire a negative charge and the negative gas ions created in the inter-electrode
region play a crucial role in this process. When PM and negative gas ions cross paths, the gas ions stick to the
particles and impart a negative charge to them. At first the charge is fairly insignificant, as most particles are huge
compared to a gas molecule, but many gas ions can fit on a single particle, and they do. Small particles (less than 1
μm diameter) can absorb “tens” of ions, while large particles (greater than 10 μm) can absorb “tens of thousands”
of ions. Eventually, there are so many ions sticking to the particles, that the particles begin to emit their own
negative electric field. When this happens, the negative fields surrounding the saturated particles start to repulse
negative gas ions and no additional ions are acquired. This is called the saturation charge and is responsible for
inducing the PMs inescapable pull of electrostatic attraction, or migration. Bigger particles have a higher saturation
charge and are consequently pulled more strongly to the collection plate than smaller particles that have a smaller
saturation charge. Regardless of size, the particles eventually encounter the CE and stick due to adhesive and
cohesive forces.

THEORY OF OPERATIONS: REMOVAL
Particulate matter that has accumulated to a certain thickness on the CE is
removed by one of two processes, depending on the type of CE used.
Collection electrodes in ESPs can be either plates or tubes, with plates
being the more common of the two. Tubes-type ESPs are usually
cleaned by water sprays, while plates can be cleaned by either water
sprays or a process called rapping. We will focus on the latter.

RAPPING: DISLODGING PARTICULATE MATTER
Rapping is a process whereby deposited, dry particles are dislodged from the CE by sending mechanical
impulses, or vibrations, to the plates. Precipitator plates are rapped periodically while maintaining the
continuous flue-gas cleaning process. In other words, the plates are rapped while the ESP is on-line; the
gas flow continues through the ESP and the applied voltage remains constant. Plates are rapped when
the accumulated dust layer is relatively thick (0.08 to 1.27 cm) so that the dust layer is coaxed to fall off
the plates as large aggregate sheets instead of small sections, helping to minimize re-entrainment. Most
precipitators have adjustable rappers that allow rapper intensity and frequency to be changed according
to the dust concentration in the flue gas. Precipitator fields with heavy dust concentrations require more
frequent rapping than fields with light dust concentrations.

RECYCLE OR DISPOSAL
As PM is dislodged from CEs, is falls into the hopper. A hopper is a dedicated collection bin with sides sloping
approximately 50 to 70o so that PM is allowed to flow freely from the top of the hopper to the discharge
opening in the bottom. Particulate matter collected in hoppers should be removed as soon as possible in
order to avoid packing that is very difficult to remove. Most hoppers are emptied by some type of
discharge device and then a conveyor transports the collected PM to its final destination for recycling or
disposal.
In an ESP using liquid sprays to remove accumulated PM, the sludge collects in a holding basin at the bottom
of the vessel. The sludge is then sent to settling ponds of lined landfills for proper ultimate disposal.
Spraying can occur while the ESP is on-line and is typically intermittent. While water is generally used as
the spraying liquid in wet ESPS, other liquids could be used if absorption of gaseous pollutants is also a
goal.

Advantages of electrostatic precipitator
•
This is more effective to remove very small particles like smoke, mist
and fly ash. Its range of dust removal is sufficiently large (0.01 micron
to 1.00 micron). The small dust particles below 10 microns cannot be
removed with the help of mechanical separators and wet scrubbers
cannot be used if sufficient water is now available. Under these
circumstances, this type is very effective.
• This is also most effective for high dust loaded gas (as high as 100
grams per cu. meter)
• The draught loss of this system is the least of all forms(1 cm of water)
• It provides ease of operation.
• The dust is collected in dry form and can be removed either dry or wet.

COLLECTION EFFICIENCY: The weight of dust collected per unit time
divided by the weight of dust entering the precipitator during the same
unit time expressed in percentage. The computation is as follows:
(Dust in) – (Dust out)
Efficiency = (Dust in) X 100

DISADVANTAGES OF A ELECTROSTATIC PRECIPITATOR
•
The direct current is not available with the modern plants, therefore considerable
electrical equipment is necessary to convert low voltage (400 V) A.C to high
voltage (60000 V) D.C. This increases the capital cost of the equipment as high as
40 to 60 cents per 1000 kg of rated installed steam generating capacity.
• The running charges are also considerably high as the amount of power required
for charging is considerably large.
• The space required is larger than the wet system.
• The efficiency of the collector is not maintained if the gas velocity exceeds that for
which the plant is designed. The dust carried with the gases increases with an
increase of gas velocity.
• Because of closeness of the charged plates and high potential used, it is
necessary to protect the entire collector from sparking by providing a fine mesh
before the ionizing chamber. This is necessary because even a smallest piece of
paper might cause sparking when it would be carried across adjacent plates or
wires

Factors affecting the performance of E.S.P.:
The present trend in adopting the gas cleaning device is to discharge the clean gas without containing SO2 to the atmosphere. One
solution to this problem is to burn fuels containing less sulphur, but unfortunately low sulphur fuels are costly to use. However, in
most cases burning low sulphur fuel increases the electrical resistivity of fly ash, particularly at low temperatures. This higher and
unpredictable resistivity at low temperatures coupled with high collection efficiencies demand can spell trouble for low temperature
precipitators. That's why pollution engineers are leaning towards precipitators operating at about 345 degrees where resistivity is not
dependent on sulphur level in the flue gases.
The principle of electrostatic precipitator is described in 3 stages as charging of the suspended particles, collecting of particulates
under the influence of electrostatic field and removal of the precipitate from the collector plate.
Many factors influence these three fundamental steps but they are critical to the reliability and performance of high temperature
precipitators which are
listed below:

CORONA CHARACTERISTICS:
• Initiation of corona depends upon free electrons by random sources such as natural radioactivity. Under the influence of an
electrical field, these electrons are accelerated to a terminal velocity. The rapidly moving electrons produce additional free
electrons y colliding with the orbital electrons of gas molecules and by ionization. At higher temperatures, flue-gas density is
reduced, resulting in a reduced starting potential. Thus, at higher temperatures, lower voltages initiate the corona to start the
precipitation process, resulting in more collection for a given voltage than at lower temperatures.
Electrostatic precipitators operated at maximum power input have steep corona characteristics; that is, the rate of change of
corona current is much greater than the concurrent charge in precipitator-circuit voltage. The steeply rising corona current is
further enhanced by increasing temperature of the stack gases. The net effect is to maximize power levels to achieve high
efficiency.

RAPPING BEHAVIOUR
This is perhaps the most complex among the three performance steps. Non electrical adhesive forces which play a
significant role in plate rapping, vary inversely with particle diameter, but depend generally on the chemical and
physical nature of the particle. Moisture can increase adhesion at lower temperatures. Particle resistivity has a
critical effect on the electrical force causing particles to slick to the collection plates: the more resistive the particle,
the greater the force. Operation at low temperatures and high resistivity requires considerably more rapping
acceleration on the collection plates than it does under normal resistivity, and higher temperatures.
Conventional practice limits maximum average gas velocity in high resistivity and low temperature operation to
approximately 1.2 m/s. this limit avoids losses due to re-entrainment of particles which can occur when the dust
layer is dislodged violently. In contrast, precipitators run at 1.7 m/s gas velocity at higher temperature.

GAS VELOCITY
• There are two forces acting on a particle having direct right angles to each
other. First is due to the flow of gas and second is produced by the electric
force on the ionized particle perpendicular to the motion of the gas. The path
followed by the particle will take direction which is resultant of the two forces
mentioned above. Therefore the efficiency of the collector decreases with an
increase in velocity which can be compensated by increasing the voltage
supplied to the plates.

BPA Quality Air Solutions LLC -
Electrostatic Precipitation for Dust
Collection
EPA - Electrostatic Precipitator Operation
(pdf)
Infohouse - Electrostatic Precipitators (pdf)
Neundorfer - Electrostatic Precipitator
KnowledgeBase

http://www.neundorfer.com/knowledgebase-posts/introduction-to-esp/

ELECTROSTATIC PRECIPITATOR

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