6. Ash resistivity
. Particle size distribution
. Number of ESP per boiler
. Minimum No. of fields required
. Minimum specific collecting area
. Maximum gas velocity
. Minimum aspect ratio
. Maximum area connected to one
TR set
. Collecting electrode spacing
7. . Recovery of material for economic reasons
Pulp and paper Industries (sodium sulphate )
. Removal of abrasive material in the dust to
reduce wear and tear of the Fan components
. Removal of objectionable matter in the dust
-NO2 and SO2
8.
9.
10.
11.
12.
13. Specific Collecting Area
Amount of collecting area required to be provided to
collect dust in gas flow rate of 1 m3 /s.
Flue gas Velocity, m/s = Flue gas flow in
m3
ESP effective cross section m2
Aspect ratio = Effective Length of
ESP
Collecting electrode height
Treatment Time, sec =Effective Length of ESP in
m
Flue gas Velocity in m/s
14. Gas Velocity.
. Velocity is decided by the gas flow and
collection efficiency required
. Higher the gas velocity,higher the carryover of
dust particles without Collection - Re –
entertainment
. Very poor velocity alters the flow distribution
and effects settling of Dust particles
. Optimum velocity depends upon the
application will improve the Performance.
15. Aspect Ratio.
. During the rapping, the falling of dust particle
take a trajectory form
. Lower the aspect ratio, the trajectory dust
travel along with gas flow
Without falling in to hoppers – Leads to re-
entrainment loss.
. Higher the ratio, performance will be good
. Optimum aspect ratio depends on allowable
velocity, required collection
Efficiency and available space.
16. Treatment Time.
. Time available for capturing the dust particle
. More treatment time at reasonable velocity
improves the collection efficiency
. Probability of capturing the re-entrained
particles improves with time.
17. RECOVERY ELECTOSTATIC
PRECIPITATOR
The Paper mills are often located in a sensitive
environment with strict requirements of emission of
dust particles and gaseous pollutants to the
atmosphere. The dust particles are very fine and
sticky in nature. The gases are also highly corrosive.
Dedusting by means of Electrostatic Precipitators are
the preferred technology in Paper mills. Black liquor
recovery boilers are de-dusted by a multi chamber
ElectrostatiC Precipitator often with a casing made of
concrete
18. The casing of the precipitator for recovery boiler
applications are preferred to be made of
REINFORCED CEMENT CONCRETE. As the gas is
rich in moisture and highly corrosive due to the
presence of sulphur compounds ( sodium sulphate
and sodium sulphide used in the pulp digesters ), the
concrete casing is preferred to withstand corrosion.
19. For the same reason, the collecting electrode
( the thinnest part in the electrode system ) is made of
corrosion resistant steels – CORTEN - A or CORTEN –
B or equivalent. The thickness can be 1.5 mm to
provide for an enhanced life of the collecting system.
The emitting electrode shall be of austenitic
stainless steel having excellent corrosion resistant
properties (conforming to UHB 904L or AISI 316L or
equivalent)
20. The load of the collecting and emitting systems are
transferred to the casing through load bearing
members called ‘casing inserts’. These are small parts
made of steel and embedded in the concrete casing at
the time of casting the same. This is done in-site.
The hopper system for these precipitators shall be of
flat bottom. No pyramidal nor trapezoidal type of
hoppers are used for such applications. The bottom
floor of the casing itself serves as the hopper and the
dust from the collecting / emitting and the gas
distribution screens are allowed to fall on to this floor.
21. The collected dust on the floor is scrapped by means
of ‘SCRAPPER CONVEYOR’ which runs between the
inlet of the precipitator and the outlet. Structural
members are mounted at desired locations on two
end-less chains and scrap the collected dust to bring
it to the inlet end of the precipitator casing. The
conveyor is electrically driven by motors mounted on
the outside of the casing
22. In addition to the scrapper conveyor, a CHAIN
CONVEYOR is also employed to transfer the dust to a
ROTARY FEEDER mounted external to the
precipitator casing. The chain conveyor runs across
the precipitator at the inlet end of the casing and is
located inside the precipitator casing. The chain
conveyor is also electric driven by a motor mounted
external to the precipitator casing.
The dust discharged from the chain conveyor
into the rotary feeder is further conveyed to the
mixing chamber where it is mixed with the spent
liquor and recycled
23. The drives of the scrap per conveyor, chain conveyor and
the rotary feeder are to be interlocked in a particular
sequence by monitoring their operation through speed
monitoring devices mounted on the drive shafts
of these conveyors. This is essential to avoid
overloading of the conveyors / their drives. The
operation of the scrapper conveyor shall be
interlocked with the Transformer – Rectifier set
so that the fields are de-energized automatically
when the scrapper conveyor is NOT in
operation.
As the dust is sticky in nature due to the
high moisture content, the gas distributor
screens at the inlet of the precipitator will be
rapped at the same frequency as that of the
24. As the flue gas is highly corrosive and rich in
moisture content, special care has to be taken to
ensure that the flue gas temperature at the inlet of
the precipitator is sufficiently above the acid /
moisture dew point to avoid any condensation on
the precipitator surfaces and cause corrosion.
Temperature monitors are required to be installed
at the inlet duct. Some customers may prefer to
have a bye-pass duct when the gas temperature is
NOT sufficiently above the dew points. In such
cases, diverter dampers may be required at the
inlet and outlet of the precipitator casing to
prevent gas flow through the precipitator. This
will add to the cost of the precipitator system.
25. . Gas tight dampers are required to be installed at
the inlet and outlet of the precipitator casing for
purposes of maintenance.
The ingress / leakage of atmospheric air into the
precipitator casing has to be completely avoided
from the point of eliminating the possibility of any
local corrosion. The inspection doors on the casing
have to be therefore of double construction. One
inspection door located very close ( on the concrete
casing ) and the other one mounted over the inner
door.
The concrete casing also requires thermal
insulation on the outside. Light Resin Bonded (LRB)
mattresses of adequate thickness can be used.
26. CONSTRUCTION OF ELECTROSTATIC
PRECIPITATOR
This consists of Supporting structure and support
Bearing , these are the rigid structure supporting the
entire load of the ESP. The bearings are provided
between the casing colume and supporting structure to
act freely for thermal expansion.
CASING.
The casing is known as IB casing, the side walls are
made of horizontal panels, it is a leak proof arrangement
with roof beams of Longitudinal and Transverse to
support the internals of Collecting and Emitting
systems.
HOPPER.
Pyramidal and flat bottom hoppers are provided under
the casing to collect the ashes. It should not be treated
as storage bunker.
27. EMITTING SYSTEM.
Emitting system consists of rigid emitting frame suspended
from four points on the top. The four suspension points are
supported on support Insulators to give electrical
insulation to the emitting frame.
EMITTING ELECTRODES.
The Discharge electrodes consist of hard drawn spiral
wires. The coil spring form emitting electrodes are self
tensioning, this stabilized
positioning permits the highest possible operating voltage.
The self tensioning spiral discharge electrodes allow for
better transmission of the rapping force. The spiral wire
electrode provides a uniform current distribution and the
corona discharge occurs around the entire surface of the
wire.
28. Rapping mechanism for Discharge electrode.
A Traction of the dust will be collected on the
discharge electrode and the corona will be suppressed
as the dust layer grows. Frequent rapping is required
to keep the electrode clean always.
COLLECTING SYSTEM.
The collecting system is of dimensional stability. The
upper edges of the collecting plate are hung on hooks
provided on the roof and the bottom is fixed with the
shock bar. The collecting electrodes are made of cold
rolled carbon steel or corton steel material of the
order of 1.5mm thickness with G profile at the end
29. RAPPING MECHANISM
FOR GOLLECTING
ELECTRODE.
The system employs tumbling hammers which are mounted on
a horizontal shaft in a staggered fashion with one hammer for
each shock bar. The shock bar transmits the blow
simultaneously to all of the collecting plates in one row because
of their direct contact with the shock bar.
ELECTRICS .
Rectifier Transformers are provided on top of ESP, the control
panels are located in ESP control room situated in the ground.
Auxiliary control panels are housed in the ESP control room to
control the auxiliary equipments of ESP like Heaters, Rapping
motors, conveyers etc.LT distribution board also housed in
control room.
30. OPERATION AND MAINTENANCE OF
RECOVERY ESP.
ESP’s are constant efficient equipment, if the input
parameters are maintained to the design value then the out
put efficiency (emission) will be maintained, Provided the
ESP fields should be healthy.
We have to ensure the healthiness of each and every
equipment independently.
The HVR and EC controllers should be tuned to the
optimum level
Depending upon the load in the steam unit.
Monitor the optimum operation of the boiler by periodical
check of O2 levels in different point in the flue gas circuit.
The controllers should be kept at just below spark level
Always monitor the conveyors to work smooth to avoid any
Jamming.
31. MAINTENANCE.
Check all the Heaters are in service with
thermostat control in operation.
Check all the rapping motors are working as per
the program set.
Check the conveyors are running smooth
Check the current and voltage are to the set level
in the controllers.
Check the boiler is operated with optimum design
condition without any excess flue gas
Any pluggage problem in the entire flue gas path
from boiler outlet to chimney Inlet.
To monitor the maximum solid content in the
liquor to be fired.
32. Maintain history of firing proportion to emission
with parameter recording
The gas distribution to be studied for better
correction.
Optimization of rapping to avoid offset in the system.
Repair and replacement of rapping mechanism by
suitably replacing the worn-out components.
Field alignment to be checked perfectly to attain
max. current and voltage
Corona quenching problems to be studied and
attended.
The ESP rapping system should impart as high
acceleration to the precipitator internals as possible
to increase the intensity of rapping by increasing the
size.
33. Poor ESP power input
Dust build-ups
Gas flow issues
Over load
. Poor ESP power input : Due to mechanical alignment
deficiency, that reduces the gap between +ve and –ve
electrodes, sparks controls the current build up and
reduce the collection efficiency.
Dust build- up : The formation of accumulation is due to
the reaction between solid sodium sulfate and gaseous
So2, which results in the formation of acid sulfate,NaHso4
and thus corona quenching.
34. Gas Flow : Gas distribution, if it is not even then
current distribution will be uneven, In leakage of air
increases the flow rate , sneakage of gases flowing in
untreated levels carry the dust.
Over load : Due to higher production in the mill, Poor
boiler operation with high amount of excess air,
leakages in the flue gas path.
35. Leakages in the flue gas path to be controlled
Internal alignment to be checked and corrected,
Gas distribution to be checked for uniform
distribution,
Gas sneakage points to be arrested for efficiency
improvement,
Rapping mechanism to be checked for effective
dislodging of dust particles,
Cleaning of internals either by air lashing or water
washing.
Power supply sources to the ESP to be checked.
36.
37. Electrical migration
Electrical mobility
Corona discharge
ESP theory
Charging mechanisms
Ash resistivity
Flue gas conditioning
Power consumption
Reading: Chap. 5
Positive Negative
Republican Democrat
Love Hate
Ying Yang
Man Woman
Hell Heaven
Cation Anion
War Peace
Attraction Repel
38. Coulomb’s law
Statcoulomb (stC): the charge that causes a repulsive force of 1
dyne when 2 equal charges are separated by 1 cm (3.33×10-10
C)
Unit charge: 4.8 ×10-10
stC (1.6×10-19
C)
2
21
r
qq
KF EE = E
F
q
E
= (q=ne)
Electric Field
40. Terminal velocity in an electrical field
(electrical migration velocity/drift velocity)
c
TEp
C
Vd
qE
πµ3
=
( ) qEB
d
qEC
wV
p
c
TE ===⇒
πµ3
qB
d
qC
E
V
Z
p
cTE
===⇒
πµ3
(force balance)
DE FF =
(for Re < 1)
Q: What is the physical meaning of electrical mobility?
Q: When does a particle have a higher mobility?
May the force be with the
particles!
Q: Difference between
cyclone and ESP in terms
of forces acting on the
system? What’s the
effect?
42. 1 2 3
1 2 3
(20) (12) (8)
Turbulent Flow with Lateral Mixing Model
Electrostatic Precipitator
43. Deutsch-Anderson
Equation
R
dtV
R
dtRV
N
dN TETE 22
2
−=−=
π
π
)
2
exp(
)(
0 R
tV
N
tN TE
−=⇒
−−=−=η⇒
Q
AV
P cTE
exp11 Ac/Q: Specific Collection Area (SCA)
• Turbulent flow: uniformly mixing
• Perfect Collection
•The fraction of the particles removed in
unit time = the ratio of the area traveled by
drift velocity in unit time to the total cross-
section
Q: How to increase the efficiency?
44. Q: An ESP that treats 10,000 m3
/min of air is expected to
be 98% efficient. The effective drift velocity of the
particles is 6.0 m/min. (a) What is the total collection
area? (b) Assuming the plates are 6 m high and 3 m
long, what is the number of plates required?
6 m
3 m
Internal Configuration: self-review
45. Random collisions between ions
and particles
+=
kT
tNecd
e
kTd
n
iipp
2
1ln
2
2
2
π
Q: Does q depend on time?
Does q depend on dp?
The total number of charges on a particle
(ci ~ 2.4×104
cm/s)
neq =
The total charges on a particle
Use esu, not SI units.
46. Bombardment of ions in the presence of a strong
field
eZ1
eZ
42
3
i
i
2
+
+
=
tN
tN
e
Ed
n
i
ip
π
π
ε
ε
Total number of charges by field charging
Q: Is the charging rate dependent on
particle size? On field strength? On
time? On material?
Aerosol Technology, Hinds, W. C., John Wiley & Sons, 1999.
+
=
e
Ed
n
p
s
42
3
2
ε
ε
Saturation charge (Zi ~ 450 cm2
/stV•s)
48. Typical fly ash
size distribution
Q: If the ESP is used to collect the fly ash,
how will the particle size distribution at
ESP outlet look like?
49. Impact of particles’ resistivity on ESP’s
performance:
Factors: temperature, composition
Flue gas conditioning
109
- 1010
ohm-cm is desired
Q: How does resistivity affect an ESP’s performance?
50. Effects of sulfur content and temperature on resistivity
Q: Is S in coal good or bad?
52. Effective drift velocity as a function of resistivity by
measurement
Use the same Deutsch-Anderson Equation with new we.
Q: Estimate the total collection area required for a 95% efficient fly-ash
ESP that treats 8000 m3
/min. The ash resistivity is 1.6×1010
ohm-cm.
54. High Efficiency ESP (>95%)
Matts-Ohnfeldt Equation
−−=
k
e
C
w
Q
A
exp1η
Use k = 1 for fly ash
k = 0.5 or 0.6 for
industrial category
Rule of Thumb
• Below 95%, use Deutsch-Anderson Equation
• Above 99%, use Matts-Ohnfeldt Equation
• Between them, use an average
Q: In designing a high
efficiency ESP, a smaller
drift velocity is to be
used. Why?
55. avgCC VIP =
C
C
e
A
kP
w =
Power density ~ 1-2 W/ft2
−
−=
Q
kPC
exp1η
Corona power
Drift velocity
Efficiency vs. Corona Power
k = 0.55 for Pc/Q in W/cfs up to 98.5%
60. 20/04/13 60
Types of ESPs in terms of shape
҉ Cylindrical type
҉ Plate type
Types of ESPs in terms of flow
direction
҉ Vertical gas-flow
҉ Horizontal gas-flow
Types of ESPs in terms
performance
҉ One stage or two stages
҉ Dry or wet
Plate type, horizontal gas-flow, one stage
and dry ESPs are the most common ESP
type in industrial application.
61. 20/04/13 61
Electrostatic Precipitator (ESP)
One-stage ESP Two-stage ESP
Discharge
electrodes
Collecting
electrodes
The observed
minimum is because
of Cunningham
factor in calculation
of drift velocity.
