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Investigations on Electrostatic Precipitator : A Case Study
Avinash Chandra
Centre for Energy Studies
Indian Institute of Technology
New Delhi-110 016, India
Abstract - Some investigations have been carried oo
an old ESP unit to determine dust collection
efficiencies and effective migration velocities after
incorporating microprocessor based intermittent
charging and rapping systems. The collection
efficiencies and migration velocities can be maximized
for optimum value of intermittent charging ratios,
rapping rates and proper choice of charging currents.
The level of emissions however depend on the inlet
dust loading and flue gas flow rates and may not
meet stipulated standards it ESP if operated under
highly variant conditions.
L
INTRODUCTION
The increased awareness on the harmful effects of
paniculate .emission from power plants and other
industries has resulted in demand on suitable measures
to reduce the emissions from chimneys by adopting
additional equipments/controls. The electrostatic
precipitator (ESP) are widely used for removal of solid
and liquid particles from Industrial gases with collection
efficiencies exceeding 99 percent for wide range of
particle size (l-100)/an . They can handle large volume
of gases (25-1000) m3/s, have low pressure drop and can
operate continuously with little maintenance [1-2]. The
performance of ESFs is found to be affected with me
passage of time in power plants due to (i) change in the
characteristics of coal feed and hence that of fly ash
(ii)the quality and quantity of flue gases coming out
from the boiler (iii) charging of fly ash which depends
on current supplied by microprocessor controlled
Transformer/Rectifier sets, electrical field developed on
the collection electrodes, electrode spacing, configuration
etc. (iv) Collection and dust removal, the particles are
collected at anode which are grounded, dust is removed
through rapping and is collected in hoppers stationed
below in electrodes in ESPs.
IL
ESP AND CONTROL UNITS
The aim of experiments was to maximize the
collection efficiencies of an ESP unit for a given power
plant by choosing appropriate operating parameters e.g.
(a)Electrical like current limits, charging ratio, peak
voltages etc. (b) Rapping rates once the flue gases are
1947
passing through EP System. The temperature pressure,
moisture contents of flue gases are decided by boiler
conditions. The parameters of ESP units are slow in
table A.
With the power unit (- 110MW) there arc two
precipitators connected with a boiler. The flue gases
coming out of the boiler is divided into two paths A
and B. The gas is further divided into two subpaths. In
each path there are four fields in series. The dust is
being collected at 16 points into rite hoppers from which
it is discharged in to ash ponds with the help of water
stream.
a) Dust Charging Unit
The transformer rectifier (rating 70kv 1000 mA)
supplies the power for charging and collection, the
electronic control [EC] is connected whose function is to
feed the precipitator with maximum power input under
constant current regulation. In case of any flash over
between collecting and emitting electrodes, the EC sense
it and quickly react by bring the input voltage to zero
and blocking it for specified period. After ionized gases
are cleared and dielectric strength restored roe EC
control quickly brings back the power to a preset value
and raises it to the original non sparking level. The
transformer rectifier is mounted on the roof of the
precipitator unit while EC is located in the control unit
A careful study of voltage-current characteristics in
different fields in conventional mode (one cycle on
another off) shows the development of back corona near
the collecting electrodes even at very low current
densities 10"A/cnr at all fields. The back corona
develops due the deposition of high resistivity (10"
-1013) Q-cm dust (fly ash) on the collecting electrode.
The development of back corona results in (i) Spark
over and hence lower potential development between the
collecting and emitting electrodes and hence lowering of
migration velocity (ii) loss of dust charging power and
as a result of all these factors lowering in collecting
efficiency of ESP.
The intermittent energizing/charging [4-6] system have
been developed to over come these problems. The
system simply energizes EP for a specified number of
cycles and suppresses the EP energization for specified
number of cycles by not gating thyristors. The system
works because of time dependance of the formation of
back corona in a resistive dust layer which can be
considered equivalent to an electrical circuit having
capacitor and resistor in parallel, these capacitor must be
charged to break-down voltage of the dust layer before
back corona can form. With normal continuous operation
with high resistivity paniculate this condition is met
continuously. However wim intermittent charging the
voltage is never allowed to reach the critical break down
level, the time it takes for the voltage to break down
level depends on a number factors such as resistivity,
dielectric constant, break down strength, current density
etc. Intermittent charging is the periodic gating (on) and
suppression of gating (off) of the thyristor. The duty
cycle or the charge ratio is defined as the ratio of the
number of on cycles to the sum of the on and off cycle.
BHEL's Advanced Precipitator Controllers (BAPCON)
have been used for intermittent charging and controlling
the current to the ESP. The charge ratio can be varies in
the range of 1:1 to 1*159, which is necessitated to tackle
the high resistivity of fly ash encountered in Indian
precipitators. At higher charge ratios the base voltage
reduces to a very low value. The provision of base
charging associated with the unit allows small current
during some of the skipped half cycles thereby avoiding
the effect of low voltage. A provision is provided for the
measurement of peak and valley voltages of the charging
signal. A high peak voltage increased the effective
migration velocity and thus increasing the collection
efficiencies of ESPs [4].
b)
Rapping system
The collected dust from the electrodes is removed by
rapping which imparts acceleration to dislodge the dust
in to the hoppers for subsequent removal. Successful
rapping depends upon accumulation of sufficient
thickness of material on the plates so that it falls in large
agglomerates into the hoppers. The acceleration required
to remove the collected dust vary wim the properties of
dust and gas stream. Forces of cohesion and adhesion
consisting of molecular (Vanderwalis), electrical and
mechanical forces the dust. Electrical forces, which
are related to current density and dust resistivity are
significant in holding the collected dust on the
electrodes. A larger force is, therefore, required to
dislodge the dust in presence of current conduction
through E.S.P. [1,4]. As the current flowing and
collection rates are different in the four fields (16
hoppers), the rapping rates and force required to
dislodge the dust will be different in each field. The
object of the rapping is to shear the dust/ash and have it
1948
slide down from the collection electrodes into hoppers
without expanding in to many small particles that could
easily be reentrained back into the gas stream. A thicker
ash cake developed may reduce the passage of the
current and hence the charging current through the ESP.
Thus a optimum thickness of ash cake be allowed to be
deposited on plates. This can be controlled by rapping.
The rapping timings depend on many factors like
location of ESP field, Boiler load, collection efficiency,
field availability etc. For controlling the rapping rates
rapper controller (RAPCON) supplied by BHEL, India
has been used. The RAPCON is a dedicated
microprocessor based device for controlling the rapping
motors. There are 16 rapping motors. The start time,
Run time and Repeat time for all the motors can be set
either in local or remote mode. The controller has error
check provisions. It has a time factor feature which
adjusts the repeat time of rapping motors.
IIL
INVESTIGATIONS
Investigations have been carried out on ESP units at
a Thermal power station in New Delhi, the parameters
of ESP units are given in table 1. The investigations can
be divided in two parts (a) The fly characterization and
(b) Experiments conducted on ESP unit at Thermal
Power Station to evaluate and optimize its performance.
a)
Fly Ash Charterisation
The fly ash samples were collected from the inlet and
outlet of ESP unit and from hoppers placed below the
electrodes in four fields. These samples have been used
to determine (i) particle size distribution after each of
collecting field (ii) Chemical composition and (iii)
Electrical resistivity. The sab-sieve particle size analysis
has been conducted by using Laser Size analyzer. The
chemical composition has been determined by using
inductively coupled plasma atomic emission
spectroscopy. The electrical resistivity of the pallets of
fly ash is measured by means of well shielded
(electrically) resistivity bridge at different pallet
temperatures. The coal feed varies from time to time to
the power station. The average proximate analysis of the
coal presently fed to the power station is given as
follows :
Total Moisture
Volatile Modter
Fixed Carbon
Ash
Calorific Value
8.96%
17.3%
31.8%
42.0%
4000kcal/kg
A detailed description of the experimental set up is
given elsewhere f3].
b) Site Experiments
Investigations were carried out to choose the optimum
operating conditions namely, input current, charge ratio,
rapping rate, voltage developed, in all the 16 fields of
the unit just before the break down. The rapping rates
were controlled and measured by microprocessor based
RAPCON units. Advanced precipitator
controllers
(BAPCON) have been used for intermittent charging and
measuring Peak and valley voltages, spark ratio etc. in
the fields. The magnitude of charging current to different
fields is decided by the maximum allowable [5] spark
rate of 5 per minute. A high spark rate (>5) indicates the
heavy back corona conditions to be avoided for the
efficient collection of fly ash. Thus conditions were
established for getting maximum peak voltage just
before high spark area conditions (>5), which depends
on charge ratio, current supplied to the field, rapping
rates at a given temperature pressure and volume of flue
gases passing through ESP unit The migration velocity
of charged particles is directly proportional to the peak
voltage obtained during the working of ESP. The charge
ratios were optimized on the basis of ma«innim peak
voltage obtained, these value were(15,19,25,31) in first,
second, third and fourth fields respectively.
Dust leading tests were conducted at the inlet and out
let of the ESP unit. For conducting the dust loading tests
the services of Messes SIMA Lab Pvt. Ltd., New Delh,
an agency recognized by pollution Board New Delhi and
BTPS, Badarpur, were obtained. Besides measuring the
dust loading (mg.NM3) the other measured parameters
are flue gas emission rate (NM3), stock velocity (m/s)
and inlet gas temperatures at each available points at the
inlet and outlet. It was observed during the investigations
that all the parameters like dust loading, flue gas flow
rate temperatures and pressures, stock velocities vary
widely during the investigations, which were conducted
for number of days during roe period 5.6.% to 26.6.96.
As the flow rate of flue gases effects the coDection
efficiency quite significantly. It was thought proper to
compared the migration velocities in different conditions
of operations. Table 1 shows the collection efficiencies,
and migration velocities obtained during the
investigations. During these investigations the charge
ratio was fixed at IS, 19,25, 31 at first, second, third and
fourth field respectively. The rapping rates and limiting
currents in the four fields have also been shown, the
limiting currents as described in section (d) earlier have
been decided on the basis of spark rates limited upto 5
per minute. The collection efficiency (n) and migration
1949
velocities (w) have been determined by using following
expressions.
.
Inlet dust loading Outlet dust loading
(I)
Q In (In)
A
(2)
W=B
(3)
Which is Deutch Anderson equation for migration
velocity
A - Collection area of ESP
Q - Flue gas flow rate
Eo - Charging field,
Ep - Collecting field
a ■ Diameter of particle, K - Coulomb's constant /u
- Gas Viscosity , g- Acceleration due to gravity Bconstant depending on the system, configuration and
other non idealities For Ideal condition B-l
IV. RESULTS AND DISCUSSIONS
The following are the some major results found during
investigations.
a) The particle size distribution varies at each field
and lie in the range (0.3-120)^m. At the inlet of the unit
the sample is dominated by large particles. (s30/im)
having an average diameter 40/an. On the other hand at
the outlet of the unit before join to stack small size
particles dominate (0.3-20)/^ having an average
diameter (~9)/jn. For rest of the fields the average
particle size lie in between. A majority of dust is
collected in the first field (~70%) which in the last field
the percentage of collected dust is small ("3.5%)
although the area of collecting surfaces in all the four
fields is same
b)A number of fly ash samples were collected and
chemical analysis was carried out The fly is dominated
by Silica SiO, (58-62%), alumina (28-32%). The
electrically conducting ion species like K2OJMa2O are in
small percentage (s l%). Other notable compounds arc
Iron Oxide (FeO), Magnesium Oxide, Calcium Oxides
etc. Some percentage of fuel remain unburnt (COl)
"4.0%.
c)As expected from chemical analysis the electrical
resistivity of fly ash is very high 110"-10u)Onm-cm. It
varies with its temperature reaching a maximum value in
the range (100-120)°C. This variation of electrical
resistivity is shown in Pig.l.
d)Table 1 shows the results obtained under different
conditions of operation. Column 7 shows the
recommended conditions of operations provided by the
suppliers namely Bharat Electrical limited, India (5J.
The ESP is expected to work in pulsed mode having
charge ratio 1 with the application of intermittent
charging the charge ratio can be varied from 1 to 159.
The current limits, set up during the experiments, use
BAPCON supplied by BHEL. These are not the actual
currents but depend on the charger ratio which can be
kept fixed during these investigation at 15,19,25,31 for
first, second, third and fourth fields. For each fields the
rap rates and current limits have been carried. As
indicated earlier at these ratio the measured peak
voltages are maximum which in rum maximize the
migration velocity with respect to charge ratios. The
following observations can be made on the basis of
investigations.
l.By getting in semi pulse mode the migration
velocities were found more as compared by designed
values, expected with pulse mode having charge ratio I.
2.For die same values of charge ratio the migration
velocities achieved depend on the raping rates and
current limits set up during the experiments.
3.For the constant current limits (columns 1,2,3. of
table) the migration velocity is changed with rapping
rate, with slower rapping the sufficiently thick layer is
deposited with can be removed more efficiently and
thereby improves the collection efficiency and effective
migration velocity. Too thick a layer hinders current
flow and deteriorations its performance.
4.For the same rapping rates the migration velocity
depends on the current limits set during the experiments.
If too high current is allowed to pass through high
resistivity dust deposited on the collecting electrodes; a
high field is developed across the dust which leads to
back corona and reduction of effective field between the
electrodes. The effective migration velocity is reduced
and so is the case with dust collection efficiency of ESP.
Columns 3,4,6 show the effect of current settings on
collection efficiencies and calculated effective migration
velocities at constant Rapping ratio and charge ratios.
1950
The migration velocity seems to improve when current
limits are reduced (quenching of corona). However,
current cannot be reduced continuously as the charging
of dust particles is not sufficient and field developed is
less thus reducing the effective migration velocities and
hence the collection efficiencies. It is, therefore, possible
to identify the rap rates, currents and charge ratios for
getting the maximum collection efficiencies. The
reduction of corona and hence the improvement in
collection efficiency can be seen by observing column
1 and 5 when collection efficiency and effective
migration velocity improve by reducing the current for
constant rapping ratios.
5.The ESP is operating under quite different
conditions for which it is designed. As a result despite
getting much better migration velocity (4.38 cm/s)
as-compared to expected attainable value (3.51 cm/s);
the actual collection efficiency achieved are however
lower (- 98.89 - 99.40) then the designed value (99.61)
[51. The treated volume flow rates are too high (208 212 Nm3/s) [5] and hence collection efficiencies are
lowered The actual emission rates are higher then the
designed one (150 mg/Nm3), because of high dust
loading at inlet of ESP (42.61 - 67.33/Nm3), compared
to design condition (38.29/Nm3).
6.The ESPs were obtained quite some times back
around twenty years or so, as per conditions at that time
since than a lot of changes have taken place like fuel
having low calorific value and higher ash contents are
being progressively used. Because of increasing demand
of energy no control on fuel quality is possible. Similar
situations exist in other thermal power plants as well. It
requires upgrading of ESPs, which may not be possible
all time for the want of space. In such cases one should
look in to possibilities of (i) either putting a mechanical
dust collector before ESPs in case the emitted particles
are dominated by large size particles or (ii) By
introducing Bag fillers if finer particles are being emitted
out after ESP units.
ACKNOWLEDGEMENT
Sincere thanks are due to (i) the management and
technical staff of Badarpur Thermal Power Station, New
Delhi for providing the site and necessary technical
assistance. (ii)Prof. S.P.Sabberwal, Dr. A.K.Mukherjee
and Mr. Ved Pal Singh for fruitful discussions and
assistance.
This work has partially been supported by Badarpur
Thermal Power Plant, New Delhi through a project.
REFERENCES
[1] S.Oglesby J., G.BJNichols, "Electrostatic
Precipitation', Marcel Dekker Inc., New York
and BNasel (1978)
[2] Report on Design and operating parameters of
Electro static Precipitator published by Member
Secretary, Central Pollution Control Board,
Delhi 110 032(1992)
[3] A Chandra, S.P.Sabherwal, A.K.Mukherjee,
"Performance Evaluation of an ESP UNit using low
grid coal", Proc. 6th International Conference on
Electrostatic Precipitation. Budapest - Hungary
(June 15-21, 1996) [4] M.G.Kumar, S.Sekar,
R.Sivasubramanian, "Environmentally Acceptable
Emission from Power Plant Through improved
Precipitator controls", Proc. 1st International
Conference on Gren Power - The need for the 21st
century P429-440 (Feb.,1997)
[5] Electrostatic Precipitator : Technical data :
Supplied by Bharat Heavy Electricals Ltd. for
Project, Badarpur TPS, Units 1,2,3, ( 1970). [6]
Robert R.Cryanck : "Improving Precipitator
Performance and Saving Energy with
intermittent Energization _ A Case study" 78th
Annual Meeting of air pollution control
Association, Detroit, Michigan (June, 1985).
present holding the position of Chief Scientific Officer,
which is highest cadre post, equivalent to Professor in
the same centre. He had been providing consultancy
services to Bharat Electrical Ltd. at Tiruchirappalli for
MHD Channel design. Besides MHD, his present interest
lies in air pollution control methods for power and other
industries, especially in the Electrostatic Precipitator. He
is providing consultancy services to Badarpur Thermal
Power Station Delhi, operated by National Thermal
Power Cooperation (NTPQ, India) for performance
evaluation and up gradation of ESP Unit.
He has guided around twenty students for their
M.Tech and Ph.D dissertations and published or
presented around 75 papers on above areas. Dr.
Chandra was a member of National Technical
Committee for 10* International Conference on MHD
power generation. He is also holding memberships of
several professional bodies like, International Society for
Electrostatic Precipitation, Plasma Science Society,
Indian Society for Technical Education etc.
I.
(a)
(b)
(c)
2.
3.
4.
(a)
(b)
(c)
A Chandra received the M.Sc., M.Phil, and Ph.D
degrees from Aligarh Muslim University, Aligarh India,
thereafter, be joined the Physics Department at the
Indian Institute of Technology (IIT) Delhi, India. Since
then he has set up experiments related with seeded
combustion flames, electrode and wall plasma
interactions and has made extensive studies in these
areas. With the establishment of centre for Energy
Studies at IIT Delhi he shifted to the centre and worked
on the different aspects of channel design for retrofit
MHD plants and liquid metal MHD Systems. He was
with Electrical Engineering Department of Sydney
University, Australia on a price Fellowship during
1980-81. Thereafter he joined as Assistant Professor
and at
1951
(e)
(0
5.
(b)
(c)
6.
Design conditions Gas
flow rate Temperature
320 NmVsec
Dust concentration
Table A: Parameters of ESP Units
Number of feed in series
160 C
Pressure drop across
38.2 gm/Nm3 4
precipitator for designed
conditions
Collecting electrodes No.
75 mm
of rows of collecting
We 51
electrodes per field No. of
collecting electrode plates
per field Total No. of
collecting plates per boiler
306
Nominal height of
collecting plates nominal
2448
length of collecting plates
Specific conducting area
13 5 meters
Emitting Electrode No. of
electrodes in each field
750 mm
Total length of electrodes
151.875 rtf/m'/sec
per field
Spiral with hooks
Plate/wire spacing
Electrical
2700
15147 meter
225/300 mm
: 70 KV (Peak)
: 1000 mA
(i)
Voltage (ii)
Current
Collection Efficiency & migration velocities obtained in ESP Unit
Charge ratio (15,19,25,31) Rap Rate for
discharge electrodes 10 per hour
Sample
No.
Operating Parameters Rap rate per
hour for collecting electrode current
in mA Fields I
n
III
IV
Gas Flow
rate
NM'/Sec
Dust loading
(mg/NM3)
Inlet
Outlet
Migration
Velocity
(cm/s)
Efficiency
(%)
1. Rap
Current
6
15
700 1000
2
1500
1
2000
208.75
67350
780
3.83
98.84
2. Rap.
Current
10
4
700 1000
3
1500
1
2000
212.70
47034
509
3.96
98.92
3. Rap
Current
3
8
700 1000
2
1500
1
2000
208.77
59340
432
4.23
99.27
4. Rap
Current
3
8
400 600
2
800
1
1000
208.73
43814
265
4.39
99.40
5. Rap
Current
6
15
400 600
2
800
1
1000
210.74
42617
388
4.07
99.08
6. Rap
Current
8
3
300 450
2
600
1
800
212.6
12279
129
3.98
98.95
7. Rap
15 300
150
600
4
800
160
38200
150
3.51
99.61
1952