Training Progress Report (E083053) (1)

7/31/2019 Training Progress Report (E083053) (1)

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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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Training Progress Report (E083053) (1)

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