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ELECTROSTATIC PRECIPITATOR
Dust extractions from industrial gases become a necessity for environmental
reasons. Most of the plants in India use coal as fuel for generating steam. The
exhaust gases contain large amount of smoke and dust, which are being emitted
into atmosphere. This poses a real threat to the mankind as a health hazards.
Hence it has become necessary to free the exhaust gases from smoke and dust.
Need For Installation Of New Electrostatic Precipitator at
GNDTP Units: -
The electrostatic precipitators installed at GNDTP units are designed to give an
emission level of 789 mg/NM3
for a coal having an ash content of not more than
30%. However on actual testing it has been found that emission level from
ESPs was about 3.0 mg/M3
. The high level of emission is due to the fact that
coals burnt in the boiler have much higher ash content than what boilers are
designed for. The pollution control board of Punjab Govt. has specified an
emission level of 380 mg/M3
from chimney. In order to achieve this new
emission level additional ESPs have been installed at GNDTP Bathinda.
Working Principle: -
The Electrostatic precipitator utilizes electrostatic forces to separate the dust
particle form the gas to be cleaned. The gas is conducted to a chamber
containing Curtains of vertical steel plates. These curtains divide the chamber
into a number of parallel gas passages. The frames are linked to each other to
form a rigid framework.
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The entire framework is held in place by four supports insulators, which
insulates it electrically from all parts, which are grounded.
A high voltage DC is applied between the framework and the ground
thereby creating a strong electrical field between the wires in the framework and
the steel curtains. The electrical field becomes strongest near the surface of the
wire, so strong that an electrical discharges. The Corona discharge is
developed along the wires. The gas is ionized in the corona discharge and large
quantities of positive and negative ions are formed. The positive wires are
immediately attracted towards the negative wires by strength of the fieldinduced. The negative ions however have to travel the entire space between the
electrodes to reach the positive curtains. On routes towards the steel curtains the
ions collide with each other and get charged and also this charge is transferred
to the particles in the gas. The particles thereby become electrically charged and
also begin to travel in the same direction as the ions towards the steel curtains.
The electrical force on each particle becomes much greater than gravitational
force. The speed of migration towards the steel curtains is therefore much
greater than the speed of sedimentation in free fall.
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General Description: -
There various parts of the precipitators are divided into two groups: -
a. Mechanical system comprising of casing, hoppers, gas distributionsystem, collecting and emitting systems, rapping mechanism, stairway
and galleries.
b. Electrical system comprising of transformer rectifier units with ElectronicController, Auxiliary Control Panels, Safety Interlocks and Field
Equipment Devices.
1) Precipitator Casing: -
The precipitator casing is an all welded pre-fabricated wall and roof panels.
The casing is provided with inspection doors for entry into the chamber at
each field. The doors are of heavy construction with machined surface to
ensure a gas tight seal.
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The roof carries the precipitators internals, insulator housings,
transformers etc. The casing rests on roller supports which allows for free
thermal expansion of the casing during operating conditions. Galleries and
stairway are provided on the sides of the casing in easy access to rapping
motors, inspection doors, transformers etc. walkways are provided inside EP
between fields for inspection and maintenance. The dust is collected in large
quantities on the curtains, the collected electrodes. Due to periodic rapping,
the dust falls into the hopper.
2)
Hoppers: -
The hoppers are sized to hold the ash for 8 hrs. collection. Buffer plates
provided in each hopper to avoid gas leakage. Inspection door is provided on
the one side of hoper wall. Thermostatically controlled heating elements are
arranged at the bottom portion of the hopper to ensure free flow of ash.
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3) Gas Distribution System: -
The good performance of the precipitators depends on the event distribution
of gas over the entire cross-section of the field. As the gas expands ten-fold
while entering the precipitator, guide vanes, splitters and screens are
provided in the inlet funnel to distribute the flue gas evenly over the entire
cross section of the EP.
4) Collecting Electrode system: -
The collecting plates are made of 1.6 mm cold rolled mild steel plate and
shaped in piece by roll forming. The collecting plates and shaped in one
piece by roll forming. The collecting electrode has unique profile with a
special configuration on its longitudinal edges. This profile is designed to
give rigidity and to contain the dust in quiescent zone free from re-
entertainment; collecting plates are provided with hooks at their top edge for
suspension. The hooks engage in slot of the supporting angle. All the
collecting plates in arrow are held in position by a shock bar at the bottom.
The shock bars are spaced by guides.
5) Emitting Electrode system: -
The most essential part of precipitators is emitting electrode system. Four
insulators support this, the frames for holding the emitting electrodes are
located centrally between collecting electrodes curtains. The entire discharge
frames are welded to form a rigid box like structure. The emitting electrodesare kept between the frames.
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6) Rapping System: -
Rapping mechanism is provided for collecting and emitting electrodes.
Geared motors drive the rapping mechanism. The rapping system employs
tumbling hammers, which are mounted on a horizontal shaft. As the shaft
rotates slowly the hammers which are mounted on a horizontal shaft. As the
shaft rotates slowly the hammers tumble on the shock bar/shock, which
transmits blow to the electrodes. One complete revolution of the rapping
shaft will clean the entire field. The rapper programmer decided the
frequency of rapping. The tumbling hammers disposition and the periodicity
of the rapping are selected in such a way that less than 2% of the collecting
area is rapped et one time. This avoids re-entertainment of dust and puffing
at the stock outlet.
The rapping shaft of emitting electrodes system is electrical isolated from
the geared motor driven by a shaft insulator. The space around the shaft
insulator is continuously heated to avoid condensation.
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Following Are The Modules For The Outgoing Feeders: -
Hopper heater for each field Support insulator heaters. Shaft insulator heaters. Collecting electrode-rapping motor for each field. Emitting electrode rapping motor for each filed.
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ELECTRICAL SYSTEM
1. High Voltage Transformer Rectifier (HVR) with ElectronicControlled (EC): -
The rectifier supplies the power for as particle charging and collection. The
basic function of the EC is to feed the precipitator with maximum power
input under constant current regulation should there be any flash between
collecting and emitting electrodes, the EC will sense the flash and quickly
react by bringing the input period voltage to zero and blocking it for a
specific period. After the ionized gases are cleaned and the dielectric
strength restored, the control will quickly bring back the power to a present
value and raise it to the original non-sparking level. Thus the EC ensure the
electrical disturbance within precipitator. Regulated AC power from EC is
fed to the primary of the transformer, which is stepped up and rectified togive a full wave power output. The transformer is mounted on roof of the
precipitator while the EC is located in an air conditional room.
2. Auxiliary Control Panel (ACP): -
The ACP houses the power and circuits required for energizing rapping
motor and heating elements of the precipitator. ACP controls each gas path.
The complete ACP is of modular type with individual module for each
feeder. Each module houses the power and control circuit with meters. Push
buttons, witches and indicating lamps are mounted on the door of the
compartments.
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Flue Gas Velocity (Flow): -
If the flue gas velocity is more than desired, the treatment time in the fields will
reduce. It will cause poor performance of EPs. Percentage oxygen on higher
side is an indication of excess flow of the flue gases. Efforts should be made to
bring percentage oxygen near to 6% at boiler outlet. Proper flue gas flow can be
achieved by plugging air leakages into the boiler. The ducts and the EPs and
also by regulating primary air and secondary air required for proper combustion
in the furnace.
Maximizing The Performance OF ESP: -
The performance of the ESP is influenced by a number of factors many of
which may be controllable. It should be the aim of every operator to maximizethe performance by judiciously adjusting the controllable variables.
Cleaning Of Electrodes: -
The performance of the ESP depends on the amount of electrical power
absorbed by the system. The highest collection efficiency is achieved when
maximum possible electric power for a given set of operating conditions is
utilized on the fields. Too thick a dust layer on the collecting plates will lead to
drop in the effective voltage, which consequently reduces the collection
efficiency. It also leads to unstable to unstable operating conditions. Therefore
the rapping system of collecting and emitting electrodes should be kept in
perfectly working condition. All the rapping motors have been programmed to
achieve the optimum efficiency.
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Spark Rate: -
The operating voltage and current keep changing with operating conditions. The
secondary current of HVRs have been set just below the spark level, so that
only few sparks occur during an hour. Spark rate between 5 to 10 sparks per
minute is the most favorable limit, as per the practical experience. Too high
flash over will not only result in reduction in useful power and interruption of
precipitation process but will cause snapping of emitting electrodes due to
electrical erosion.
How To Control The Spark Rate: -
One number s-pot and one number t-pot have been provided on the front of
each electronic controller. The s-pot controls the drop rate of rise of field
current after the spark is over. The operator can control the rate of spark by
adjusting these two pots manually. Both the pots if turned anticlockwise will
cause increase in spark rate.
Ash Hopper Evacuation: -
Improper/incomplete hopper evacuation is a major cause for the precipitator
malfunction. If the hopper are not emptied regularly, the dust will build up to
the high tension emitting system causing shot circuiting. Also the dust can push
the internals up causing misalignment of the electrodes. Though the hoppers
have been designed for a storage capacity of 8 hours, under MCR conditions,
this provision should be used in case of emergency. Normally, the hopper
should not be regarded as storage as storage as storage space for the collected
ash.
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Oil combustion: -
The combustion of oil used during start up or for stabilization of the flames can
have an important impact on precipitator operation. Un burnt oil, if passed into
ESP can deposit on the emitting and collecting electrodes and deteriorates the
electrical condition i.e. reduce the precipitators operating voltage due to high
electrical resistivity and consequently the ESPs performance is affected
adversely. The precipitator performance remains poor until the oil vaporizes and
the ash layer gets rapped off, which usually takes along time.
Air Conditioning Of The ESPs Control Room: -
The ESPs control room houses sophisticated electronic controller. The
operation of these controllers directly reflects on precipitator performance. In
order to ensure that the controllers are in proper working conditions, it is
essential to maintain a dust free atmosphere with controlled ambient conditions.
Therefore, the air conditioners should be kept in proper working conditions.
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GENERAL DISCRIPTION OF ELECTRONIC CONTROLLER
The EC-HVR is the High voltage DC power supply equipment for the
electrostatic precipitator used for extracting fly ash from the exhaust gases. The
equipment is supplied in two parts: -
1. The High Voltage Transformer Rectifier (HVR).
2. The Electronic Controller (EC).
The transformer rectifier unit (HVR) consists of an oil immersed step up
transformer ac reactor, high voltage, high frequency choke, measuring and
protection components.
The electronic controller (EC) contains the anti parallel-connected thyristors
pair for controlling the input voltage to the transformer rectifier unit &
necessary control circuit.
The complete equipment is designed to provide a continuously adjustable dc
output voltage up to 70 KV peak across the precipitator electrode. The controls
are arranged i.e. the unit operate as constant current source adjustable up to an
average current of 800 mA max. Occurrence of spark at the electrodes is sensed
& made to block the output voltage for a specific period & the voltage is built
up again in a specified manner to provide optimum operational efficiency of the
precipitator.
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Principle of Operation: -
Controlling the voltage on the primary of the transformer controls the output
voltage & current at high voltage DC terminals. The voltage control is achieved
by two thyristors connected in anti-parallel configuration. In normal operation,
the output of the thyristors is controlled by the gate pulse circuit, which in turn
gets its control signal from the current regular output. The output of current
regulator adjusts itself i.e. the actual current is maintained equal to set reference
value. In case of a spark detection unit detects the same. Wide ranges of
adjustment are provided for selecting blocking period & range of S & Tcontrol to make equipment suitable to different operating conditions. Persistent
low voltage at the primary of transformer or the persistent excess current on
primary side that may occur to short-circuiting initiates tripping of equipment.
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TECHNICAL DATA OF ELECTROSTATIC PRECIPITATOR
COLLECTING ELECTRODE
1. Total No. of collecting plates2. Nominal height of collecting plate3. Nominal Length of collecting plate
248012.5 m
400 mm
EMITTING ELECTRODES
1. Type2. Size3. No. of electrodes in each field4. Plate/Wire spacing.
Spiral
2.7mm
1440150 mm
Design Conditions Unit-1,3,4
1. Gas flow rate2. Temperature3. Dust concentration4. Number of precipitator5. Number of gas path per boiler6. No. of fields in series in each
gas pass
200m3/sec
1450
C
38.9 gms/Nm3
One
2
5
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RAPPERS FOR COLLECTING ELECTRODES
1.No. & type of
rapper
2.Frequency of
Rap
3.Drive
4.Location
One drop hammer per row
of collecting electrodessurface area 90 m
2
Varying from 12 raps/hr at
the inlet field to 1 rap/hr at
exist
Geared electric motor
controlled by synch.programmer
At the bottom of
collecting system
RAPPERS FOR EMMITING ELECTRODES
1.No. and type of
rappers
2. Frequency of Rap3. Driver
4. Location
Approx. one drop hammers/two
rows of electrodes
10 raps/hourGeared Electric Motor controlled by
Synch. Programmer
On the side of emitting frame
middle position
HOPPERS
1. Type
2.No of Hoppers
3. Capacity
Pyramidal
20
8 hour storage
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MOTORS
RAPPING OF EMMITING ELECTODE
1. Quantity
2. Rating
3. Location
10 Nos.
Geared Motor
0.33hp/2.5 rpm at 3-
phase 415 V 50 Hz
On the top EP
RAPPING OF COLLECTING ELECTODE
1. Quantity
2. Rating
3. Location
10 Nos.
Geared Motor, 33hp/2.5 rpm at 3
phase 415 V 50 Hz.
On the top EP
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ELECTRICAL ITEM
RECTIFIERS
1. Rectifier Rating
2. Number/Boiler
3. Type
4. Location
70 KV (peak)
800 MA (Mean)
10
Silicon Diode Full Wave,
Bridge connection
Mounted on the top of
precipitator
RECTIFIER CONTROL PANEL
1. Type of Control
2. Location
Thyristor
In the Control Room
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