ELECTROSTATIC PRECIPITATOR

INTRODUCTION In our country more and more plant being planned and built using the fuel even at high ash content. So the air pollution control equipments used to control the emission of the plant. There are the two types of air pollution control equipments are available. A) Ash precipitator B) Electrostatic precipitator

Selection is of the equipment is based on 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.

WORKING PRINCIPLE OF ESP ESP works 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. High voltage is applied between the collecting electrode & emitting electrode configuration sets up a high electric field near the wire.

Due to natural radiation electrons are present in the region near the wire and when high electric field is set up corona process gets initiated. Corona is a phenomenon associated with the ionization of gas molecules by electron collision in regions of high electric field strength. 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.

GENERAL ARRANGEMENT ESP This is of type2xFAA-6x36-2x90-125-2. 2 – two independent streams F – 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

DETAILED DESCRIPTION OF ESP ESP has the two different section. MECHANICAL 1. Casing. 2. Hoppers. 3. Gas distribution system. 4. Collecting system. 5. Emitting system. 6. Rapping mechanism. 7. Rapping mechanism of collecting electrode. 8. Rapping mechanism of emitting electrode. 9. Rapping of gas distribution system.

INTERLOCKS Each precipitator stream is provided with an interlocking system to ensure safety. The inter locking is made such that access to the ESP 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 ESP 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 ESP streams

OPERATION OF ELECTRO STATIC PRECIPITATOR PREPARATIONS: 1. The precipitator internals are to be checked and any tools or foreign materials inside should be cleared off. 2. Temporary earthing 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 gearboxes 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 elements 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

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 mechanisms are to be switched on as soon as the boiler is lighted up. 3. The transformer should be energized only when gas temperature is above 100 C. 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.

OPERATION CHECKS Equipment check : a) Transformers b) Electronic controllers c) Rapping Mechanisms d) Heating Elements e) Ash Handling System f) Auxiliary Control Panels

observation record: conditions : Boiler Load b) Flue gas temperature at the inlet of the c) All the reading in the electronic controller and Auxiliary control panel. conditions : 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

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 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 Snapped or loose wires Excessive ash accumulation Displacement of shock bars guides and collecting electrodes Any other abnormality 6. During short duration shutdown, when no work is expected inside ESP, ESP inlet and outlet dampers should be kept closed and the casing doors should not be opened. This helps in retaining as much heat as possible and thus reduce 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 doors 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.

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) should be checked for excessive erosion due to sparking and for snapping. b) Whenever access is difficult, the intervening electrodes may be unhooked for easy approach and rehooked 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 insulators and screen tubes 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: The plain bearing in the rapping shaft is to be checked for wear. When the wear is more than 8 mm, it is to be replaced. 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. The hammer and arm assembly shall be inspected for excessive wear on hammer.

ELECTRICAL SYSTEM TRANSFORMERS AUX.CONTROL PANELS: Proper up keep and maintenance of the transformer is essential for achieving the desired performance level. Master controllers and timers 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: 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 require 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 terminal box covers should also be kept watertight.

FACTORS AFFECTING ESP PERFORMANCE The performance of the precipitator is influenced by a number of factors, many of which are controllable RESISTIVITY CLEANLINESS OF ELECTRODES GAS TEMPERATURE SPARK RATE RAPPING FREQUENCY OIL COMBUSTION DUST CONCENTRATION IN FLUE GASES HOPPER EVACUATION AIR CONDITIONING OF CONTROL CABINS