1. Electrostatic Precipitator
• It is a device to precipitate suspended fly-
ash and dust particles from the flue gas by
ionizing the particles in an electric field and
collecting them subsequently on oppositely
charged electric plates (or rods). It was first
introduced by Dr. F. G. Cottrell in 1907.
3. Working of E.S.P.
• The dust laden flue gas enters the ESP
through the perforated gas distributor to the
parallel arrays of discharge electrodes
(Emitter; negatively charged) spaced
alternately with collecting electrodes
(Collector; positively charged).
4. Working of E.S.P.(contd.)
• The high intensity of Electric Field
energized at 45-75 kV causes the
particulates in the gas stream to acquire
negative charges transferred from ionized
gas molecules of the same polarity.
• These charged dust particles accelerate
towards the collection electrodes where
their charges get neutralized.
5. Working of E.S.P.(contd.)
• Due to rapping action the neutralized dust
fall by gravity into collection hopper below
the unit.Rapping is the periodic agitation or
vibration of discharge wires and collection
electrodes with hammers called rappers.
• The high voltage discharge system is
supported by insulators that also isolate it
from grounded components.
6. Mechanism of Dust Collection
• Ionization of Gas Molecules to form a train
of Plasma-- a stream of positively charged
ions.
• Corona generation which is a negatively
charged space cloud.
• Charging and collecting dust particles.
• Particle removal through Rapping.
8. Advantages of E.S.P.
• It is the most effective method to remove
very fine particulates, as fine as 0.01 ,
which escape by mechanical separators.
• Useful for high dust loaded gas(100g/Nm3).
• High dust removal efficiency(99 to 99.5%).
• Minimum draught loss.
• Least maintenance cost.
9. Disadvantages of E.S.P.
• High capital cost of equipment.
• Power requirement is considerably high.
• Removal efficiency drops with the increase
of gas velocity.
• A good amount of floor space is occupied.
10. Factors Affecting Dust Removal
• Particle size
• Particle Resistivity
• Field Strength
• Corona Characeristics
• Flue Gas Velocity
• Area of Collecting Surface
• Rapping
11. Influence of particle size
• Collection efficiency increases with the
increase of migration velocity. As migration
velocity is directly proportional to the
particle size, collection efficiency will
increase with the increase of particle size
and decreases with the fineness of the
particle.
12. Effect of Flue Gas Velocity
• Collection Efficiency increases with the
decrease of gas velocity and vice-versa.The
ionized particulates follow the path of the
resultant of two mutually perpendicular
forces, that is, electrostatic force(E.F.) and
gas flow(G.F.) If E.F.>G.F., the particle
will follow the path AB and get deposited
on the collector plate., otherwise it will
escape precipitation.
13. Effect of Gas Velocity(contd)
• The linear flue gas velocity should be low in
order to avoid turbulence in the inter-
electrode space and in the hoppers.
Turbulence or any other eddies in these
regions will lead to re-entrainment of dust
particles. Also, limitation to higher gas
velocity is imposed by the residence time
required for particle charging and collection.
14. Effect of Electric Field Strength
• Electric field strength is directly related to
applied voltage.The average velocity with
which the particles migrate towards the
collecting electrode for eventual deposition
increase roughly as the square of the voltage
since an increasing voltage between the
electrodes increases the corona discharge and
thus the number of free charge carriers.
15. Effect of Corona Characteristics
• In the normal case , the corona character-
istics shows the increase of precipitator
voltage with the increase in precipitator
current up to the optimum point.Beyond this
point, there is a decreases in voltage with the
increasing current and this region is called
‘Back Corona’ region. This reduces the
efficiency of the precipitator and consumes
unnecessary increased power.
16. Effect of Particle Resistivity
• Particle Resistivity is a measure of the
ability of a particle to accept a charge, that
is, to become ionized. Higher resistivity of
a particle means greater is its resistance to
become ionized and so it offers greater
resistance to the passage of electric current.
Hence lower resistivity means it is
conductive.
17. Parts of Precipitator
• High Voltage Transformer/Rectifier Set:
It is an assembly of a single phase high
voltage transformer rated 110 kV peak and
the rectifier set connected in a bridge circuit
whose negative terminal are brought out for
connection with emitter.This set is designed
for heavy duty operation, that is 24 hrs duty,
with frequent sparking in the precipitator
load, occasionally 200 sparking per min.
18. Parts of Precipitator(contd.)
• Emitter:It is in the form of vertical rod
having spikes for corona discharge. It
receives the negative high tension produced
by HT sets.
• Collector: It is generally plane rectangular
sheets hung vertically, spaced regularly apart
and parallel to each other.It is connected to
the positive terminal of the TR Set and the
main body of ESP and finally grounded.
19. Parts of Precipitator(contd.)
• Inlet Funnel for Gas Distribution: Perforated
distribution screens are used to provide the uniform
gas flow.For achieving the best precipitation
velocities, the gas must be expanded between 5:1 and
15:1 as it passes from the duct and this is carried out
by a transition piece called the inlet funnel.
• Collector Rapper:The collecting electrodes are held
in rows by rapping bars, which are equipped with
anvils.Flail hammers, which are mounted spirally on
the rapping shaft, hit the anvils of the rapping bars.The
torque is transferred through the geared motor drive.
20. Parts of Precipitator(contd.)
• Emitter Rappers:The suspension bars for
the discharge electrodes are equipped
with anvils.Flail hammers are hitting the
anvils in a certain sequence, thus shaking
off the dust adhering to the discharge
electrodes.The drive of the rapping device
is done by a geared motor which is
electrically isolated from the parts under
HT by means of the rapper insulator.
21. Parts of Precipitator(contd.)
• Dust Conveying System: The precipitator
dust falling into the hopper is extracted by
means of the dust conveying system. For a
good function of the precipitator it is
essential that the dust conveying system is
gas-tight.
• HT Manual Isolator:It is meant for conne-
cting TR set with the field side during
oper-ation or for isolating and earthing the
field during maintenance. A moving knife is
fastened with rotary insulator column.
23. Mechanical Safety Interlock
Safety Interlock consists of the following :
• Key Exchange Panel: 1 set
• Electromechanical lock:R1,R2,R3 & R4.
• Loose locks for inspection doors: H, C & I
• Integral lock R1,R2,R3 & R4 are fitted in live
position of Isolator and E1, E2, E3 & E4 in
earth position of Isolator.
The purpose of Interlocking :
• Off load operation for Isolator w.r.t. TR set.
• To make the field earthed before opening
inspection door.
24. Operation of Interlocking
• Make the TR set OFF and release the keys R1,
R2, R3 & R4 from the TR Control panel.
• Trap these keys R1, R2, R3 & R4 on the lock
fitted on the isolator. Then isolator will be
operational.
• Turn the handle to earth position and releases
key E1, E2, E3 & E4.
• Trap key E1, E2, E3 & E4 to Exchange Box-1
and release key ‘Y’.
• Trap the key Y in Exchange Box-2 and
ultimately releases key H, C & I for operating
inspection doors.
25. Making TR set ON
• All keys are to be released from the locks fitted
on the inspection doors: Hopper door, Casing
door and Insulation roof.
• Trap these keys into the exchange box. Then
the keys E1, E2, E3 & E4 will be released from
the key exchange box.
• Release the keys R1, R2, R3, & R4 from the
isolator and trap these keys to TR Control
panel to make the TR set ON.
26. Safety Measures for ESP Inspection
• Precipitator Inspections must always be carried
out by ATLEAST TWO MEN.
• Main fuses of HT sets are to be disconnected.
• HT Switch to be switched to ‘Earth’ position.
• Before entering inside, discharge system has to
be earthed from access door and attention has
to be paid for actual temperature inside.
• Inside the precipitator, only 24 V AC/DC
Lamps are to be used with protective insulation.
27. Stopping Sequence of ESP
• The precipitator should remain in operation for
2-3 hours after stopping of the dust source.
• Rapping devices to be switched off.
• HT sets to be disconnected; fuses out.
• Dust conveying system to be disconnected.
• Insulator heaters to be switched OFF.(For
short shut-down the heaters may remain
switched ON).
• For precipitator inspection, the SAFETY
MEASURES has to be followed.
28. Start-up Sequence of ESP
• After final inspection, access doors to be
closed and locked.
• Re-connect support insulator heaters 4 hrs prior
to start-up.
• HT Switch to be switched ON - provided all
interlocking systems are closed.
• Dust conveying system to be connected.
• Rapping devices to be switched on.
• HT set to be switched ON.
29. Maint. & Insp. Of Collectors
• Check whether all collecting electrodes are
straightened are hanging vertically (max. defor-
mation to gas flow 10 mm) in correct manner.
• Check whether all collecting electrodes are
sufficiently cleaned.
• Collector rapper and timer to be checked.
• Possible adjusting of rapping sequence.
• Position of supporting pins to be checked.
• Check whether all rapping bars are correctly
fixed and freely moving during hitting.
30. Maint. & Insp. Of Collector
Rapping Device
• All hammers are to hit the anvils correctly.
• Worn out hammers and anvils are to be changed or
repaired by build up weld.
• Check all bolt connections and their locks.
• Condition of loose bearings to be checked; worn
out parts to be replaced.
• Geared motor to be checked for smooth run.
• Condition of coupling to be inspected
• Fixed bearing to be inspected regarding tight-fit
31. Maint. & Insp. Of Emitter
• Check whether all emitters are straight and
hanging vertically.
• Check the distance from discharge to collect-ing
electrodes.
• Check whether all electrodes are correctly fast-
tened and are in center-line of gas passage.
• Check whether the discharge electrodes are
sufficiently cleaned.
• Discharge electrode rapping device and timer to
be checked.
32. Maint. & Insp. Of Emitter(cont.)
• Possible adjusting of rapping sequence.
• Check the support insulators against contami-
nation and cracks;defective ones to be replaced.
• Protective tube, sealing of supporting insulator
and suspension tube also to be cleaned.
Precaution:A wrong sense of turning may cause
damages inside the precipitator. Check by a
short connection of the drive.
33. Maint. & Insp. Of Discharge
Electrode Rapping Device
• Function to be checked with operating drive ; all
hammers hit the anvils correctly.
• Worn out hammers and anvils to be replaced or
repared by build up weld.
• Pin wheel and rachets to be checked - worn out
rachets to be replaced.
• All bolt connections and their locks to be checked.
• Condition of bearing to be checked.
34. Maint. & Insp. Of Discharge
Electrode Rapping Device
• In the insulator compartment, insulator of
geared motor - smooth run, fastening.
• Bearing of eccentric to be checked.
• Cleaning with dry rag.
• Defective insulators to be replaced.
• Lifting rod to be checked.
35. Maint. & Insp. Of HT Isolator
• Check of knife against contamination.
• Adjusting of earthing knife.
• Sliding surfaces to be symmetrical and parallel
to the contact tongue.
• Knife to be in alignment.
• Contacts to be lubricated with contact grease.
• Check of working points of switch.
• Check of insulator against contamination to be
cleaned with dry rag.
• Check of function by test switching.
36. FLUE GAS PARAMETER FOR EACH BOILER
SL NO. DESCRIPTION UNIT
BMCR
DESIGN COAL
BMCR
WORST COAL
DESIGN
POINT
1 Gas flow rate to ESP m
3
403 406 505
Nm
3
/s 272 274 326
2 Gas temperature at ESP deg.C 132 132 150
3
Dust concentration at
inlet of ESP
g/Nm
3
58.44 77.25 77.25
4
Moisture content in flue
gas
%wt 7.57 7.84 7.84
5
Under pressure at ESP
inlet
mmWc -250 -250 -250
6
Dust concentration at
outlet of ESP with all
fields in service
mg/Nm
3
<30 <30 30
7
Collection efficiency with
all fields in service
% 99.948 99.961 99.961
8
Dust concentration at
outlet of ESP with one
field out of service
Mg/Nm
3
<50 <50 50
9
Collection efficiency with
one field out of service
% 99.914 99.935 99.935
37. DATA OF ELECTROSTATIC PRECIPITATOR
SL NO. SUBJECT DATA
1 Type of electrostatic precipitator offered FAA-845M-2116150-2
2 Number of precipitators/boiler TWO
3 Number of gas paths per precipitator ONE
4
Number of electrical fields (Zones) in series in the
direction of the gas flow
8
5 Total number of electrical fields per boiler 32
6
Space between the centers of collecting
electrodes across the gas path, mm
400
7 Nominal height of collecting electrode, m 15
8
Nominal length of collecting electrodes per field
in the direction of gas flow, m
4.5
Total projected collecting area provided, m
2
With all fields in service
Total projected collecting area provided, m
2
With one field out of service
9 125280
10 109620
38. DATA OF ELECTROSTATIC PRECIPITATOR
11 Treatment time,sec. (with all fields in service) 49.6
12 Gas velocity inside ESP, m/sec. 0.73
13 Specific collecting area provided, m
2
/m
3
/sec,
With allfields in service
248.08
14 Specific collecting area provided, m
2
/m
3
/sec,
With one field out of service
217.07
15
Totalnumber of HV rectifier units installed per
boiler
32
Rating of each rectifier units
.KV (peak) 95
.MA(mean) 1200
17
Predicted pressure drop across ESP (funnel
flange to flange),mmwc
25
16
