electro static precipitator

38
ELECTRO STATIC PRECIPITATOR INTRODUCTION: The need for rapid power development, in our country has led to more and more power plants being planned and built using fuels even with high ash contents. it has hence become necessary for thermal engineers to exercise sufficient checks to see that the plant emissions are kept down to the minimum without causing environmental pollution. Two types of air pollution control equipment namely ash precipitators and electro precipitators are available. Among these, selection has to be made taking into account a) Volume of flue gas. b) Temperature of flue gas. c) Size of ash particles d) Concentration of ash particles e) Efficiency of the equipment f) Space required to install the equipment g) Initial as well as operational and maintenance cost. It is established that sustained collection efficiency of 98.5% and above can be achieved by employing EP alone. Also for the better performance of EP, particles at the inlet to it

Upload: saravan1891

Post on 06-Nov-2015

27 views

Category:

Documents


3 download

DESCRIPTION

ESP

TRANSCRIPT

ELECTRO STATIC PRECIPITATOR

ELECTRO STATIC PRECIPITATOR

INTRODUCTION:

The need for rapid power development, in our country has led to more and more power plants being planned and built using fuels even with high ash contents. it has hence become necessary for thermal engineers to exercise sufficient checks to see that the plant emissions are kept down to the minimum without causing environmental pollution.

Two types of air pollution control equipment namely ash precipitators and electro precipitators are available. Among these, selection has to be made taking into account

a) Volume of flue gas.

b) Temperature of flue gas.

c) Size of ash particles

d) Concentration of ash particles

e) Efficiency of the equipment

f) Space required to install the equipment

g) Initial as well as operational and maintenance cost.

It is established that sustained collection efficiency of 98.5% and above can be achieved by employing EP alone. Also for the better performance of EP, particles at the inlet to it should be heterogeneous in nature (i.e.) flue gas should contain particles of various sizes ranging from coarser to finer.

Electro static precipitators are widely employed in modern steam generators as they posses following advantages over mechanical precipitators.

a) Very high efficiency in the order of 99.99%

b) Possible to recover ultra fine particles also.

c) Low pressure drop across EP

d) Low operating and maintenance cost.

e) Continuous and reliable service

f) Less temperature drop across EP.

In tune with modern trends, steam generators of Neyveli Thermal Power station II are provided with electrostatic precipitators.

WORKING PRINCIPLE OF ELECTRO STATIC PRECIPITATOR

The EP consists of two sets of electrodes, one known as Emitting electrodes and the other the collecting electrodes. A high D.C voltage is applied between these electrodes. The dust-laden gas is made to pass through the electrodes. The electro static precipitation is based on the principle that the forces acting on electrically charged particles in the presence of an electric field can be utilized to effect the separation of the solid or liquid aerosols from a gas stream. In the process adopted, the dust suspended in the gas is electrically charged and passed through an electric field where electrical forces cause the particles to migrate towards the collecting electrodes. Dust gets retained in the collecting electrode s and subsequently removed.

The charging of dust particles is accomplished by means of a high voltage direct current corona. Corona is a phenomenon associated with the ionization of gas molecules by electron collision in regions of high electric field strength. A non-uniform electric field is required for generation of corona and this is achieved by the use of a small diameter wire as emitting electrodes and a plate or cylinder as the collecting electrode. High voltage is applied between these electrodes configuration sets up a high electric field near the wire and this electric field decreases inversely with the radius from the surface of the wire.

Due to natural radiation or other sources (thermionic emission, secondary emission, photo ionization) electrons are presenting the region near the wire and when high electric field is set up corona process gets initiated. The electron is accelerated to high velocities and has sufficient energy. Due to their impact with the gas molecules I the region, they release orbital electrons from the gas molecules. The additional free electrons are also accelerated and get involved in the ionization process which continues until the electric field decrease to a point when the electrons released do not acquire enough energy for ionization.

The region where ionization takes place is the corona glow discharge region where there are free electrons and positive ions resulting from the ionization impact. Depending upon the polarity of the electrodes, these charged particles behave i.e. if the discharge electrode is negative the corona is called negative corona and if it is positive, the corona is called positive corona. Negative corona is popularly adopted.

In negative corona, positive ions generated in the corona region are attracted towards the negative wire electrode and electros toward the positive electrode. The electric field diminishes beyond the corona glow region and if electric negative gases are present here, the gas molecules on impact will capture electrons.

The negative ions generated at this stage will be moving towards the collecting electrode. These negative ions colliding with dust particles charge the dust and cause them to get deposited over the positive collecting electrodes.

GENERAL ARRANGEMENT OF EP

M/S. Bharat heavy electrical limited supplies Electro static precipitators; Ranipet and they are manufactured in collaboration with m/s Svenska Flakt Faluiken of Sweden. This of type2xFAA-6x36-2x90-125-2 and the nomenclature is as follows.

2- two independent streams

F- Name of equipment i.e. EP.

A-Gas flow in horizontal direction

A-Steel casing

6- six fields per stream

36- width of one field in decimeters

2- two bus sections per field

90- length of one bus section in decimeters

125- height of collecting electrode in decimeters

2- pyramidal type hopper

EP is provided in between regenerative air pre heaters and induced draught fans. Each stream located in between a regenerative air pre heater and I.D fan is capable of handling 60% flue gases at VWO (valve wide open) condition. The vertical flue gas duct from each RAPH takes a 90 bend divides into two and join each section of EP. The cross section of ducts is increased before EP to give a diffuser effect (i.e.) to educe velocity of gas in EP. In order to increase the effectiveness of ash separation in EP the retention time of gas is increased further by providing 2 numbers of vertical perforated plate type gas distribution screens placed in series at the inlet of EP. These distribute the flue gas uniformly into EP, there by ensuring effective removal of dust from the gases.

The electro static precipitator has six fields per stream along the flow direction. In each field there are 61 rows of collecting electrode curtains across the gas path. Each curtain consists of 9 corten steel plates of height 12.5 m and width 387 mm. The collecting electrode plates are pressed to a special G profile and are hung from the top with a pitch of 400mm , between the two electrodes. Thus each field width is 3.6 m.

In between the collecting electrode curtains, the helically wound emitting electrode wires of 2.7 mm diameter are arranged. The emitting electrodes are arranged in three tiers each of 4 m height and each tier has 18 emitting electrodes along the length of the gas flow. They are hooked to the specially constructed frames placed in between the collecting electrodes curtains.

Each emitting electrode row (consisting of 18x3=54 electrodes)flanked by collecting electrode curtains on either side is called as a box and there are 60 such box sections in each field.

The emitting electrodes are connected to the negative polarity of high voltage D.C potential of 70 KV.peak and the collecting electrodes to the positive side and grounded. Two 6.6 KV/0.433KV transformers feed A.C power at 433v to 24 transformer rectifier units arranged on the top of EP.Every one of the six fields of stream is divided into two bus sections and each bus section (consisting of 30 sets of electrodes) is connected to a transformer rectifier unit. the entire emitting system is insulated from the other parts of EP and is suspended from the top.

Periodic rapping is done for collecting electrodes, emitter frames and gas distribution screens(at inlet of EP only) by tumbling hammers. These are driven by motors which are placed outside the EP. There are 4 ash collecting hoppers below a field in each stream. Totally there are 48 ash hoppers in EP.Each hopper has an ash mixing vessel below it and the collected ash is disposed off through hydro ash disposal system.

After EP, the cross section of the two gas ducts from each stream gets reduced and the two ducts form a single duct and join the suction of the I.D fan.

DETAILED DESCRIPTION OF EP.

The different sections of EP can be classified under 2 major groups(i) mechanical system and (ii) electrical system . the mechanical system consists of casing, gas distribution system collecting and emitting systems, rapping mechanisms, hoppers, stair way and galleries. The electrical system comprises of transformer rectifier units with electronic controllers, auxiliary panels, safety interlocks and field equipments/devices.

MECHANICAL SYSTEM:

CASING:

It is designed to with stand the pressure and temperature of gas and fabricated with 6 mm mild steel plates with suitable stiffeners outside and necessary inspection doors which are of heavy construction with machined surfaces to ensure gas tight seal. These doors not only access for maintenance and inspection but also help in increasing the cooling rate of EP after shut down.

Designed gas flow through each stream

: 275 m3/sec.

Actual gas flow through each stream

: 228.2 m3/sec.

Flue gas velocity at the inlet of EP

: 14.4 m/sec.

Dust loading at inlet of EP with 12%ash in lignite : 13.2 gm/m3

Flue gas temperature at inlet : 148 c

HOPPERS:

Forty eight pyramidal hoppers are provided under the casing to collect ash. Each hopper has 8 hours storage capacity, bulk density of fly ash adopted for its design being 1200 kg/m3.in order to prevent untreated flue gas from passing through the hopper , suitable gas deflector plates are installed in the hopper.

The hopper has the following provisions.

1. two poke tubes in the bottom portion for manual poking in case of choke.

2. twelve electric heating elements with thermo static control for wall heating to avoid condensation of moisture. The total power consumption for the heater is 6kw.

3. one number of ash level indicator in the top portion to give alarm signals when the ash level in the hopper becomes high.

4. an inspection door.

GAS DISTRIBUTION SYSTEM

The velocity of the gas in the inlet duct before EP is 14.4 /sec and this velocity is to be reduced to a value between 1.0 and 1.5 m/sec(1.225m/sec) in order to (i0 ensure that as laden gas remains in the EP field for sufficiently long period(17.56 sec.) to achieve maximum collection (2) prevent erosion of electrodes due to high velocity(3) prevent possible carry over of ash without depositing over electrode, affecting efficiency of the precipitator.

In order to reduce the gas velocity through the EP as above and to ensure uniform distribution over the entire cross sectional area of the precipitator, two specially perforated vertical screen plates are provided in each stream. (primary and secondary).

The screen sheets are provided with hooks from coming off the slots in he frame. As the dust concentration is high at inlet of EP, the ash particles may deposit over the screens and are removed periodically with a rapping mechanism arrangement with 18 hammers each positioned at 210 angular displacement with the adjacent hammer is provided for primary and secondary screen plates.

COLLECTING SYSTEM:

There are 61 rows of collecting electrodes (across gas flow)per field and each row consists of 9 electrodes (along gas flow) per field. All the nine collecting electrodes put together is called as a curtain. Each collecting electrode is formed in a G profile.

DATA ON COLLECTING ELECTRODE SYSTEM

1. Number of collecting electrodes per curtain

: 9

2. Total number of curtains per field

: 61

3. Collecting electrodes per field(61x9) : 549

4. Collecting electrodes for six fields(6x549) : 3294

5. Collecting electrodes for two streams (3249x2) : 6588

The electrode curtain is divided into 9 strips of smaller width plates in order to facilitate production, transport, erection, and alignment. Also the vibration produced during rapping can easily be controlled.

ADVANTAGES OF G PROFILED ELECTRODES:

1. I t ensures adequate mechanical rigidity.

2. It imbibes favorable rapping characteristics

3. It acts as a shield to minimize the re- entertainment of precipitated dust.

Aspect ratio is a term defined as the ratio of achieve treatment length stream to the height of the collecting electrodes. It determines the collection area and hence performance of the precipitator.

Height of the collecting electrode

: 12.5 m

Length of the each field(9x400)

: 3.6 m

When all the six fields are kept charged

Then aspect ratio

: 6x3.6/12.5= 1.728

Normally it is enough if five fields are kept charged

Aspect ratio

:5x3.6/12.5=1.44

EMITTING SYSTEM

The emitting electrodes are of spiral form since this form possesses the following advantages

1. It imbibes good corona characteristics.

2. It results in good self tensioning and stabilized positioning of the wire , which permits application of highest possible voltages.

3. It offers good mechanical strength to resist fatigue failures and spark erosion.

When the emitting electrodes are exposed to atmosphere for a longer time, they are susceptible to stress corrosion, due to which frequent snapping of electrodes takes place. Stress crack corrosion is due to chlorides and sulphides. Chlorides are present in atmosphere. Hence it is advisable to erect the emitting electrode just before commissioning.

RAPPING MECHANISM:

Rapping is a means adopted for dislodging of ash deposits over the electrodes during the process of precipitation by vibrating the electrodes. Both the collecting electrodes and emitting electrodes are rapped to get good performance. Otherwise accumulation of ash deposits over the collecting electrodes may lead to BACK CORONA EFFECT, thus reducing the collection rate. If the emitting electrode is not rapped then the process of corona discharge itself will get affected. The rapping frequency will have to be carefully chosen such that at the time of rapping there will be sufficient accumulation dense enough to fall as an agglomerate, but not so dense to make dislodging difficult. The rapping mechanism employed is of the tumbling hammer type.

RAPPING OF COLLECTING ELECTRODES

For the rapping system of he collecting electrodes, the tumbling hammers are mounted on the shaft in a staggered manner at an angular interval of 210 when the angle is reckoned in an anticlockwise direction.

For the first field the rapping mechanisms are in the front and for all other fields in the rear of the field. The rapping system should be switched on before switching on the supply to emitting system. The frequency of rapping is controlled by timers and synchronous programmers provided in the auxiliary panel. Since ash colleted in the first field will be high and lesser in the subsequent fields., the frequency of rapping is reduced in the latter in steps as under.

Field numberRapping frequency per hourAsh collected hopper in kg per rapping

110255

2693

3330

4212

5116

6110

RAPPING OF EMITTING ELECTRODES

The system is similar to that of collecting electrodes except that the drive is located at the top of the electro static precipitator. The angle between the hammers will be 240 when the angle is reckoned in a clockwise direction, except in the cases of angles between hammers 4&5 and 11&12, which are 210.

Since the shaft of rapping mechanism of emitting system will be at high potential while rapping, it is provided with shaft insulator to ensure electrical insulation of the drive shaft. Space heaters are also provided to avoid any condensation on shaft insulator.

INTERLOCKS:

Each precipitator stream is provided with an interlocking system to ensure safety. The inter locking is made such that access to the EP internals through inspection door, interior of the insulator housing and disconnecting switch is possible only when all the transformer rectifier units of one stream are tripped. De- interlocking to be sequentially done for gaining access to the internals of the EP parts and interlocking operation will also to be done sequentially for re start of the transformer rectifier units after a shut down for maintenance or inspection of EP streams.ELECTRO STATIC PRECIPITATOR

DATA SHEET FOR ELECTRO STATIC PRECIPITATROR

1) 6.6 KV /433V Transformer

Number of transformer : 2

Make : Kirloskar; oil cooled

Rating

: 1600KVA, 3 Phase

Input A.C Voltage / Current : 6.6 KV/140A

Winding input / Output : Delta / Star

Out put A.C Voltage /Current : 433V/ 2133.4A

2) 415V(AC) SINGLE PHASE /70 KV(DC) TRANSFORMER RECTIFIER UNIT :

Number of transformers : 24

Rating

: 75KVA,/59KW

Input A.C Voltage / Current : 415V/181A

Out put A.C Voltage /Current : 70KV/ 1A

3)RAPPING MOTORS

SL.NODESCRIPTIONCOLLECTING ELECTRODE SYSTEMEMITTING ELECTRODE SYETEMGAS DISTRIBUTION SCREEN BEFORE ESP

1Number of rapping motor 24242

2LocationAt 8 Metre levelAt 20 Metre level atAt 10 Metre level

3Motor Rating0.25KW,415V 900RPM,1.1A0.25KW,415V 900RPM,0.8A0.25KW,415V 900RPM,1.1A

4Speed After Gear Box1.1RPM2.5RPM1.1RPM

5Out put Torque215Kg.m97Kg.m215Kg.m

4) POWER INPUT

a) Total power input to ESP for six fields

: 1190 KW

b) Total power input to ESP for Five fields

: 990 KW

c) Power density / Unit area of collecting electrode: 96 Watts/M

d) Corona Power Available

: 16.3 Watts /M

e) Input rapping power to rapping system : 2 KW / Stream

5) WEIGHT OF ESP STRUCTURE

a) Total Weight of ESP

: 2328 Tonnes

b) Weight of ESP Including Ash

: 5124.48 Tonnes

OPERATION OF ELECTRO STATIC PRECIPITATOR

PreparationsThe following preparatory checks are to be carried out before starting the ESP.

1. The precipitator internals are to be checked and any tools or foreign materials inside should be cleared off.

2. Temporary ear thing connection, if any, on the emitting system should be removed.

3. All the insulators are to be checked.

4. The oil level in the rapping system gear boxes should be checked and oil is to be topped up if necessary.

5. The rapping motors are to be inspected and run to ascertain their normal functioning.

6. The heating element are to be checked for proper functioning.

7. The di-electric strength of the transformer oil is to be checked. The oil may be centrifuged if necessary.

8. To ensure the healthy condition of the transformers an open circuit test may be conducted.

9. All the inspection doors and covers are to be closed and interlocking system to be checked.

10. All the fields are to be meggered and tested on air load.

START UP PROCEDURE

1. The heating elements are to be switched on at least 24 hours before the start up of the boiler.

2. The rapping mechanism are to be switched on as soon as the boiler lighted up.

3. The transformer should be energized only when gas temperature is above 100C.Even though the precipitator design allows start up from a clod condition, it is likely that considerable moisture from the gases may condense on to the cold collector System, when a precipitator is charged in cold condition. This will result in dust adhering to the collecting electrodes forming a wet layer on the electrodes. As the precipitator dries out .this layer will get backed / on to the collecting electrodes and it will be extremely difficult to remove it by normal rapping .Wet dust falling on to the hopper may also stick on the hopper walls and even the hopper opening. Hence the transformers are to be energized only when the gas temperature reaches 100C and is increasing at a sustained rate rather then dwelling at a moderate temperature slightly below 100C.

4. The optimum amperage setting depends upon various factors such as flue gas condition, dust concentration etc. Excess amperage setting may cause frequent snapping of emitting electrodes due to spark erosion and bring down the useful corona power. The optimum amperage for each fields will be just below the spark overload .

OPERATION CHECKS

I .The following equipments should be periodically checked to ensure normal functioning of ESP

a) Transformers

b) Electronic controllers

c) Rapping Mechanisms

d) Heating Elements

e) Ash Handling System

f) Auxiliary Control Panels

II. The following observation should be recorded at least once in an hour in the logbook

a) Boiler Load

b) Flue gas temperature at the inlet of the precipitator

c) All the reading in the electronic controller and Auxiliary control panel.

III. Any failure of components, the outage ,the probable cause and the corrective action taken are all to be recorded

IV. Operation of ESP under following condition may cause excessive dust deposits on the internals

a) Prolonged operation of ESP without rapping mechanisms being operated

b) Energisation operation of ESP with wet internals

c) Operation of ESP below Acid dew point

d) The flue gas entering the ESP being misty due to incomplete flue combustion particularly at the time of initial boiler operation

Accumulation of such dust deposited not only causes reduction of precipitator efficiency, but also result in unstable operation rapid corrosion of electrodes and failure of insulators

SHUT DOWN

1. When the gas temperature drops to the acid dew point of the gas ,the transformer should be switched off

2. The rapping motor, should be run for at lest 30 minutes after switching off the power to the transformers

3. The fly ash evacuation system (Ash mixing Vessels) should be in service until all the hopper are completely emptied

4. Only when the precipitator internals have cooled down to a safe level , the access doors should be opened duly following the interlock sequence .hopper access door must be opened with caution as hot ash may fall out

5. During shut down all the internals should be inspected for

a)Snapped or loose wires

b)Excessive ash accumulation

c)Displacement of shock bars guides and collecting electrodes

d)Any other abnormality

6. During short duration shutdown ,when no work is expected inside ESP,ESP inlet and outlet damper should be kept closed the casing doors should not be opened .This helps in retaining as much heat as possible and thus reduced the precipitator start up time .The heaters need not be switched off during such conditions.

7. If an inspection of ESP is required during short duration shutdown, the door are to be closed as soon as the internals checks are completed. In this case heaters may have to be switched off for carrying out inspection.

WARNING

On no account must access to ESP internals attempted until HV system are switched off, earthling rods are applied, interlock sequence is adhere to and inside temperature is dropped to a safe level

8.Normally the precipitator does not required any periodical manual cleaning , since the rapping system ensure adequate cleaning of the internals, during the operation. However during mini shut down, if excessive ash deposit is noticed, the internals are to be cleaned. Removal of sticky dust may be done with brushes, scrappers, compressed air or water. Use of water is most effective method. The cause of such excessive deposits will have to be identified and rectified.

MAINTAINENCEGENERALTo ensure required performance of ESP, periodical maintenance may have to be carried out during shut down. Every available shut down may be utilized to clear the excessive ash build up, if any. During routine checkup for maintenance (a) All the internals, casing, hoppers and roof shall be inspected for corrosion, erosion, wear and tear and leakages. (b) The asbestos ropes and gaskets shall be replaced whenever required. (c) The position of guide vanes , spillters, deflectors plates on the gas distribution screen should not be disturbed during inspection or maintenance.

MECHANICAL SYSTEM

COLLECTING ELECTRODE SYSTEM

a) The collecting electrodes, shock bars and guides are to be checked for alignment

b) If the shock pad is worn out excessively, it is to be replaced

c) If dust building is excessive, the rapping mechanism is to be checked

EMITTING ELECTRODES

a) The emitting electrodes, should be checked for excessive erosion due to sparking and for snapping .Such electrodes are to be replaced .care should be taken to ensure that the healthy electrodes are not disturbed , while replacing the snapped or eroded electrodes

b) When ever access is difficult, the intervening electrodes may be unhooked for easy approach and re hooked while returning after replacement

c) While stretching an electrode for hooking the stretching tool must be used always. If an electrode is over stretched, the same should be discarded.

d) The support insulator and screen tube are to be inspected for excessive dust buildup. If dust build up on frame is excessive, the rapping mechanism is to be checked.

RAPPING SYSTEM

a) The plain bearing in the rapping shaft is to be checked for wear. When the wear is more then 8mm,it is to be replaced. During such replacement, it is preferable to replace the complete set of plain bearing ,since retaining some old bearing will result in excessive wear on the new ones. When replacing the bearings, alignment should be maintained.

b) The worn out shaft could be replaced along with the bearing after the replacement, the rapping system should be test run to establish satisfactory performance and the bolt as and nuts must be locked by tack welding

c) The hammer and arm assembly shall be inspected for excessive wear on hammer /pins worn-out assemblies are to be replaced .the inner arm of the should be locked by tack welding the bolts and nuts .the hammer should be so fixed that they hit at the center of the shock pad beam

d) The SKF bearing assembly in the rapping drive system normally requires no maintenances. However if the bearing is disturbed for replacement of any components, the following procedure should be adopted for assembly.

1. The Plummer block top and bottom housing and other internals are to be cleaned

2. The end seals, adopter sleeve and the bearing are to be inserted into the shaft and should be placed on the bottom half of bearing

3. The seals are to be fixed in the corresponding grooves and the bearing must be locked by guide rings

4. The nut of the adopter sleeve is then tightened and locked by bending one tooth of the washer to the slot of the nut

5. Grease is to be applied to one-third volume of the housing and then top housing should be closed. Lithium based grease grade 3 of IS 1002 such as IOC MOBILUX 3 or SERVOGEM 3 or HP BEACON 2 can be used

ELECTRICAL SYSTEM

GENERAL

Before commencing work on any electrical equipment ear thing devices must be connected to all high voltage parts. The end of the cable on ear thing rod must always be connected to earth before the rods are applied .

TRANSFORMERS AUX.CONTROL PANELS ETC

Proper up keep and maintenance of the transformer is essential for achieving the desired performance level. Master controllers and timer are to be checked regularly for proper condition. Dust ingress should be avoided in the keyholes of the interlocking system. They can be periodically cleaned with brush or blower. The cover of the disconnecting switch should be ensured for proper closure and the gasket provided for the cover, will have to be replaced as and when required .

INSULATORS

The following checks are to be carried out for upkeep of insulator

1. The insulators are to be inspected for cleanliness and rigidity of metal parts

2. Dusty insulators should be cleaned with carbon tetrachloride and dried

3. Cracked insulators, if any should be replaced

4. Bushing insulator and pin insulator of the disconnecting switch can be easily replaced after removing the fixed screws.

5. Support insulators can be removed and replaced with the help of lifting tool and alignment jig.

HEATING ELEMENTS

Normally the heating elements required very little maintenance. Failure of the heating elements can occur either due to ingress of moisture or water into the terminal box or due to damage caused by hammering of the hopper walls. Hence hammering of hopper wall should not be restored to in any case and the thermal box covers should also be kept watertight

FACTORS AFFECTING ESP PERFORMANCE

The performance of the precipitator is influenced by a number of factors, may of which are controllable. Some of the variable factors, which influence efficiency, are narrated below. It should be the aim of the operator to obtain optimum condition by adjusting the controllable variable.

RESISTIVITY

When the ESP is in service, the negatively charged dust ions travels to the collecting electrodes and gradual buildup of dust layer may take place at certain point .the current due to corona discharge has to pass though these layers and the resistively of ash at these point result in a voltage drop across the dust layers. Consequently an electric field within the layer is setup, the field strength of which lends the dust layer a force for greater bonding at the collecting electrodes. The process of removing the dust collection would get impeded at such a situation.

Moreover, when such an ash built up cumulatively become large, the abnormally high strength of its field may cause a corona like discharge (which is aptly known as back corona) producing both positive and negative ions .the negative ions migrate quickly to the collecting electrodes .in this condition where positive and negative charges find position between electrodes, resulting in either neutralizing the other, the ash freely escape to stack .the precipitator performance will be affected in the situation.

At gas temperature above 250C, the resistively of the dust layer falls and they become conducting .in practice it is considered that the resistively value below 10 ohms are considered safe

The restively of ash dust will be low if the ESP is located before air pre heater i.e., immediately after the economizers. Such an arrangement is called as hot ESP installation. But in this case, the materials used for the ESP should be designed to withstand high temperatures. Hot ESP installation is not favored at present because of the following disadvantages

1. Higher temperature lowers the flash over voltage.

2. Due to increase in gas volume at high temperature, the collecting surface increases. Also the insulation is to be increased .the above two factors result in increase in the cost of ESP

Restively is reduced by the use of artificial conditioning agents such as SO3, Water vapor or ammonia. These agents can be injected into the gas stream. This type of conditioning called seeding is associated with corrosion problem. Chemicals used like SO3, NH3, may cause corrosion. Water sprays lower the gas temperature and consequent condensation of acidic vapor causing corrosion.

CLEANLINESS 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 electrical power for a given set of operating condition is utilized in the precipitation process. During the operation of a precipitator the applied voltage is reduced by the potential drop across the deposited dust layer on the collecting electrodes will also lead to unstable operating condition. The dust deposited on the emitting wires results in non uniform corona

In view of the above facts, it become necessary to ensure proper working of rapping system of both collecting and emitting electrodes to get sustained high collection efficiency in ESP

GAS TEMPERATURE

The gas temperature at the ESP should be maintained sufficiently high since operation of the precipitator at gas temperature below the acid dew point results in the following

1. Failure of emitting electrodes due to stress corrosion cracking

2. Corrosion of the internals.

3. Collection of wet dust on the electrodes leading to formation of Hard to-rap layers and consequent reduction in the performance of ESP

4. Difficulty in the removal of wet dust from the hoppers

SPARK RATE

Too high a flash over rate will not only result in reduction of useful power and interruption of precipitation process, but will also cause snapping of emitting electrodes due to electrical erosion .it is recommended that for the best performance the flash over rate shall not exceed 5 spark per minute .an automatic voltage controller maintain constant flash over rate

RAPPING FREQUENCY

If the rapping frequency is insufficient dust accumulation will be there on the electrodes .Too high a rapping frequency will dislodge the dust layer before formation of agglomerates resulting in re-entrainment and puffs through the stack. Hence the time intervals between the raps for the various fields are to be optimally chosen to permit building of a sufficiently thick layer , so that when rapped ,the dust is dislodged in the form of agglomerates

OIL COMBUSTION

During initial startup, un burnt oil, if any may get carried over and form a coating on the electrodes in the ESP. This fouling of this electrodes reduce the precipitator operating voltage due to high electrical resistivity. So the precipitator performance gets affected and remain poor until the oil vaporizes and the layer gets rapped off, which usually takes a few weeks time. Also the UN burnt oil in the ESP poses the danger of fire hazard. Hence low current setting (without any flashover) are recommended during oil firing

DUST CONCENTRATION IN FLUE GASES

The dust concentration in the gases is much higher in the front part of the precipitator then in the rear. The current distributions is influenced by the dust concentration, where it is high, the current is suppressed, i.e.. Inlet fields will take less current then the outlet fields.

HOPPER EVACUATION

Incomplete hopper evacuation is a major cause for the precipitator mal-function. If the hoppers are not emptied regularly, the dust will buildup to the high tension emitting system-causing shorts. Also the dust can push the internals up causing misalignment of the electrodes. Though the hopper are designed for a storage capacity of 8 hours under MCR conditions. The hopper should not be regarded as a storage space for collected dust and ash accumulation in the hopper should not be allowed.

AIR CONDITIONING OF CONTROL CABINS

In order to ensure that the sophisticated electronic controls will be in proper working condition, it is essential to maintain a dust free atmosphere with controlled ambient condition. it is therefore necessary to keep the air conditioners in proper working conditions.

CONCLUSION

Electro static precipitator equipment has established its superiority over other method of fly ash separation.

1. It consumes only very little power say only about 0.5% of the power generation

2. Pressure drop across ESP is only one third of the widely used mechanical separation. Consequently draught loss on the gas side reduced and power consumed by the ID fans is lesser

3. A guaranteed efficiency of 99% is achieved with very little fly ash in the exit gas, the plume gas coming out of the chimneys not ever visible.

4. Since the minute ash particle of size less then 5 microns, coming out of the chimney, is also spread over a larger area due to 170 m tall chimney, air pollution is very much reduced, in TS-II dust concentration in the outgoing flue gas is 150mg/nm .98%of the particles are below 5 microns

ASH EMISSION THROUGH CHIMNEY

50 % of particles less then 2 microns

40 % of particles 2 microns to 3 microns

8 % of particles 3 microns to 5 microns

23 % of particles more then 5 microns

5. Since only very small quantity of ash particles pass through the ID fans, erosion in the ID impellers is very much reduced and maintenance is minimized.

Increasing collecting electrode height to 15 meters, broadening the pitch of the collecting electrode to 400 mm and widening the collecting electrode to 750mm are some of the improvements that are recently being implemented in the field of EP.