ESP Operation

7/29/2019 ESP Operation

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INTRODUCTION

As you may know, particulate matter

(particles) is one of the industrial air

pollution problems that must becontrolled. Of the major particulate

collection devices used today,

electrostatic precipitators (ESPs)

are one of the more frequently used.


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They can handle large gas volumes with a

wide range of inlet temperatures, pressures,

dust volumes, and acid gas conditions. They

can collect a wide range of particle sizes, and

they can collect particles in dry and wet

states. For many industries, the collection

efficiency can go as high as 99%. ESPs aren't

always the appropriate collection device, but

they work because of electrostatic attraction(like charges repel; unlike charges attract).


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ELECTRO STATIC PARECIPITATOR


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ELECTRO STATIC PRECIPITATOR


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ELECTRO STATIC PRECIPITATOR


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PRECIPITATION

Every particle either has or can be given achargepositive or negative. Let's suppose weimpart a negative charge to all the particles in agas stream. Then suppose we set up a groundedplate having a positive charge. What wouldhappen? The negatively charged particle wouldmigrate to the grounded collection plate and becaptured. The particles would quickly collect onthe plate, creating a dust layer. The dust layer

would accumulate until we removed it, which wecould do by rapping the plate or by spraying itwith a liquid. Charging, collecting, andremovingthat's the basic idea of an ESP


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ELECTROSTATIC PRECIPITATORWORKING PRINCIPLE

The precipitation process involves 4

main functions

Corona generation

Particle charging

Particle collection Removal of particles


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CORONA DISCHARGE

The applied voltage is increased until it produces a corona discharge, which can

be seen as a luminous blue glow around the discharge electrode. The free

electrons created by the corona are rapidly fleeing the negative electric field, which

repulses them. They move faster and faster away from the discharge electrode. Thisacceleration causes them to literally crash into gas molecules, bumping off

electrons in the molecules. As a result of losing an electron (which is negative), the

gas molecules become positively charged, that is, they become positive ions . So,

this is the first thing that happensgas molecules are ionized, and electrons are

liberated. All this activity occurs very close to the discharge electrode. This process

continues, creating more and more free electrons and more positive ions. The name

for all this electron generation activity is avalanche multiplication .


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CORONA GENERATION


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AVALANCHE MULTIPLICATION OF GAS MOLECULES


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PARTICLE CHARGING

ESP has thin wires called discharge electrodes, whichare evenly spaced between large plates called collectionelectrodes, which are grounded. Think of an electrodeas something that can conduct or transmit electricity. A

negative, high-voltage, pulsating, direct current isapplied to the discharge electrode creating a negativeelectric field. You can mentally divide this field intothree regions The field is strongest right next to thedischarge electrode, weaker in the areas between the

discharge and collection electrodes called the inter-electrode region, and weakest near the collectionelectrode. The region around the discharge electrode iswhere the particle charging process begins.


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ESP ELECTRIC FIELD


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RIGID FRAME DISCHARGE ELECTRODE


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RIGID DISCHARGE ELECTRODE


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IONIZATION OF GAS MOLECULES

As the electrons leave the strong electrical field area around thedischarge electrode, they start slowing down. Now they're in theinter-electrode area where they are still repulsed by the dischargeelectrode but to a lesser extent. There are also gas molecules in theinter-electrode region, but instead of violently colliding with them,

the electrons kind of bump up to them and are. This imparts anegative charge to the gas molecules, creating negative gas ions.This time, because the ions are negative, they too want to move inthe direction opposite the strong negative field. Now we haveionization of gas molecules happening near the discharge electrodeand in the inter-electrode area, but with a big difference. The ions

near the discharge electrode are positive and remain in that area. Theions in the middle area are negative and move away, along the pathof invisible electric field lines, toward the collection electrode.


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REPRESENTATION OF GAS FLOW IN A

PRECIPITATOR


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GAS FLOW THROUGH A PLATE

PRECIPITATOR


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NEGATIVE GAS IONS FORMED IN THE INTER ELECTRODE

REGION


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Charging of Particles: These negative gas ions play a key

role in capturing dust particles. Before the dust particles can be

captured, they must first acquire a negative charge. This is

when and where it happens. The particles are traveling alongin the gas stream and encounter negative ions moving across

their path. Actually, what really happens is that the particles

get in the way of the negatively charged gas ions. The gas ions

stick to the particles, imparting a negative charge to them. Atfirst the charge is fairly insignificant as most particles are huge

compared to a gas molecule. But many gas ions can fit on a

particle, and they do. Small particles (less than 1 m diameter)

can absorb tens of ions. Large particles (greater than 10 m)

can absorb "tens of thousands" of ions.


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Eventually, there are so many ions stuck to theparticles, the particles emit their own negativeelectrical field. When this happens, the negative

field around the particle repulses the negative gasions and no additional ions are acquired. This iscalled the saturation charge.

Now the negatively-charged particles are feeling

the inescapable pull of electrostatic attraction.Bigger particles have a higher saturation charge(more molecules fit) and consequently are pulledmore strongly to the collection plate. In otherwords, they move faster than smaller particles.Regardless of size, the particles encounter theplate and stick, because of adhesive and cohesiveforces.


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CHARGING OF PARTICLE


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CHARGING OF PARTICLE


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Particles are charged by negative gas ions movingtoward the collection plate by one of these twomechanisms:

FIELD CHARGING orDIFFUSION CHARGING In field charging, as particles enter the electric field,

they cause a local dislocation of the field. Negative gasions traveling along the electric field lines collide withthe suspended particles and impart a charge to them.The ions will continue to bombard a particle until thecharge on that particle is sufficient to divert the electriclines away from it. This prevents new ions fromcolliding with the charged dust particle. When a particle

no longer receives an ion charge, it is said to besaturated. Saturated charged particles then migrate tothe collection electrode and are collected.


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FIELD CHARGING


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Diffusion charging is associated with the randomBrownian motion of the negative gas ions. The

random motion is related to the velocity of the gasions due to thermal effects: the higher thetemperature, the more movement. Negative gasions collide with the particles because of theirrandom thermal motion and impart a charge on

the particles. Because the particles are very small(sub micrometer), they do not cause the electricfield to be dislocated, as in field charging. Thus,diffusion charging is the only mechanism by

which these very small particles become charged.The charged particles then migrate to thecollection electrode.


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ELECTRIC FILED STRENGTH

In the inter-electrode region, negative gas ions migrate toward the

grounded collection electrode. A space charge, which is a stable

concentration of negative gas ions, forms in the inter-electrode region

because of the high electric field applied to the ESP. Increasing the applied

voltage to the discharge electrode will increase the field strength and ion

formation until spark over occurs. Spark over refers to internal sparking

between the discharge and collection electrodes. It is a sudden rush oflocalized electric current through the gas layer between the two electrodes.

Sparking causes an immediate short-term collapse of the electric field

(Figure 1-8.) For optimum efficiency, the electric field strength should be

as high as possible. More specifically, ESPs should be operated at voltages

high enough to cause some sparking, but not so high that sparking and thecollapse of the electric field occur too frequently. The average spark over

rate for optimum precipitator operation is between 50 and 100 sparks per

minute. At this spark rate, the gain in efficiency associated with increased

voltage compensates for decreased gas ionization due to collapse of the

electric field.


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SPARK GENERATION PROFILE


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PARTICLE COLLECTION

When a charged particle reaches the grounded collectionelectrode, the charge on the particle is only partiallydischarged. The charge is slowly leaked to the groundedcollection plate. A portion of the charge is retained andcontributes to the inter-molecular adhesive and cohesive

forces that hold the particles onto the plates. Adhesiveforces cause the particles to physically hold on to each other

because of their dissimilar surfaces. Newly arrived particlesare held to the collected particles by cohesive forces;

particles are attracted and held to each other molecularly.

The dust layer is allowed to build up on the plate to adesired thickness and then the particle removal cycle isinitiated.


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PARTICLE COLLECTION AT COLLECTION ELECTRODE


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COLLECTION PLATES


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PARTICLE REMOVAL

Dust that has accumulated to a certain thicknesson the collection electrode is removed by one oftwo processes, depending on the type of

collection electrode. As described in greater detailin the next section, collection electrodes inprecipitators can be either plates or tubes, withplates being more common. Tubes are usually

cleaned by water sprays, while plates can becleaned either by water sprays or a process calledrapping.


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Rapping is a process whereby deposited, dry particles aredislodged from the collection plates by sending mechanicalimpulses, or vibrations, to the plates. Precipitator plates are

rapped periodically while maintaining the continuous flue-gas cleaning process. In other words, the plates are rappedwhile the ESP is on-line; the gas flow continues through the

precipitator and the applied voltage remains constant. Platesare rapped when the accumulated dust layer is relativelythick (0.08 to 1.27 cm or 0.03 to 0.5 in.). This allows thedust layer to fall off the plates as large aggregate sheets andhelps eliminate dust re-entrainment. Most precipitators haveadjustable rappers so that rapper intensity and frequencycan be changed according to the dust concentration in theflue gas. Installations where the dust concentration is heavy

require more frequent rapping.


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Dislodged dust falls from the plates into the

hopper. The hopper is a single collection bin

with sides sloping approximately 50 to 70 to

allow dust to flow freely from the top of the

hopper to the discharge opening. Dust should

be removed as soon as possible to avoid (dust)packing. Packed dust is very difficult to

remove. Most hoppers are emptied by some

type of discharge device and then transportedby a conveyor.


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REMOVAL OF DEPOSITED DUST


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HOPPER


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HAMMER/ANVIL RAPPERS FOR

COLLECTION PLATES


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MAGNETIC IMPULSE RAPPERS FOR

COLLECTION PLATES


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HIGH VOLTAGE EQUIPMENT

High-voltage equipment determines and controls

the strength of the electric field generated

between the discharge and collection electrodes.

This is accomplished by using power supply setsconsisting of three components:

1. step-up transformer

2. high-voltage rectifier3. control metering and protection circuitry

(automatic circuitry).


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The Transformer-rectifier (T-R) set steps up thevoltage from 400 volts to approximately 50,000volts. This high voltage ionizes gas molecules thatcharge particles in the flue gas. The rectifierconverts alternating current to direct current.Direct (or unidirectional current) is required for

electrical precipitation. Most modern precipitatorsuse solid-state silicon rectifiers and oil-filled,high-voltage transformers. The control circuitry ina modern precipitator is usually a Silicon-

controlled Rectifier (SCR) automatic voltagecontroller with a linear reactor in the primary sideof the transformer.


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HIGH VOLTAGE SYSTEM


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CIRCUITRY ASSOCIATED WITH PRECIPITATORS


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METERS INSTALLED ON CONTROL CABINET FOR EACH


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METERS INSTALLED ON CONTROL CABINET FOR EACH

PRECIPITATOR FILED


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Primary voltmeter. This meter measures theinput voltage, in a.c. volts, coming into the

transformer. The input voltage ranges from 220to 480 volts; however, most modernprecipitators use 400 to 480 volts. The meter islocated across the primary winding of the

transformer. Primary ammeter. This meter measures the

current drawn across the transformer in

amperes. The primary ammeter is locatedacross the primary winding (wires wound inthe coil) of the transformer.


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Secondary voltmeter. This meter measures, ind.c. volts, the operating voltage delivered to

the discharge electrodes. The meter is locatedbetween the output side of the rectifier and thedischarge electrodes.

Secondary ammeter. This meter measures the

current supplied to the discharge electrodes inmill amperes. The secondary ammeter islocated between the rectifier output and the

automatic control module. The combination ofthe secondary voltage and current readingsgives the power input to the dischargeelectrodes.


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Spark meter. This meter measures the number

of sparks per minute in the precipitator section.

Sparks are surges of localized electric currentbetween the discharge electrodes and the

collection plate.


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PROTECTIONS

1. AC current high = Primary AC current high trip2. DC under voltage = Sec. DC voltage low trip

3. DC over voltage = Sec. DC voltage high trip

4. H/W SCR firing = Single SCR firing trip

5. Synchronization fail = Syn. signal fail trip

6. Top float = Top float alarm

7. Bottom float = Bottom float trip

8. Oil temp. high = Oil temp. high alarm

9. Oil temp. high = Oil temp. high trip

10. SCR temp high = SCR temp high trip11. Over load = Over load trip


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E-1 = Safety line (hardware interlock)

E-2 = Program controller is faulty

E-3 = Internal mains faulty

E-4 = Controller communication faulty


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DESCRIPTION OF THE

INSTALLATIONS

INSPECTION DOORS: Inspection doors are

provided at different locations of the

electrostatic precipitator. They are to be used

for checks and inspections during standstillperiods only E.g: supporting insulators,

heating elements, discharge electrodes, rapping

devices, collecting electordes.


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COLLECTING ELECTRODE SYSTEM: The

collecting electrodes are made of cold rolled

profiled plates. The collecting electrodes aremovably hung in rows on suspension bars.

They are guided at the bottom end by the

rapping bars without hampering their thermalexpansion. Two collecting electrodes of each

row are connected to the rapping bar by

supporting pins.


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DISCHARGE ELECTRODE SYSTEM:

Which are virtually unbreakable. They are

fixed at the upper discharge frame so that theyare exactly in the centre of two collecting

electrodes.

GAS DISTRIBUTION: The gas distributionconsists of perforated sheets in the inlet

transition and the baffle plates in the outlet

transition. Every row of perforated sheets in

the inlet is connected with the next row by

connection bars.

COLLECTING ELECTRODES RAPPING DEVICE


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COLLECTING ELECTRODES RAPPING DEVICE:

The collecting plates rapping device consists ofthe rapping hammers. Which are fixed in spiralson the rapping shaft, the shaft itself and thegeared motor. Every electrical field has its owncollecting electrodes rapping device.

DISCHARGE ELECTRODES RAPPING DEVICE:The discharge rapping device consists of therapping hammers, which are fixed in spirals onthe rapping shaft, the shaft itself, the pinwheel,

the ratchets, the lifting rod, the rapping insulatorand the geared motor. Every electrical field hasits own discharge electrodes rapping device.

INSULATOR HEATING: The insulator heatings


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INSULATOR HEATING: The insulator heatingsaround the supporting insulators hold thetemperature inside the insulator compartments

above the dew point temperature of the gas to becleaned. This prevents that the surface of theinsulator itself becomes humid. This would leadto flash over's on the surface under high tension

and consequently destroy the insulator. Theheating elements are constantly turned ON. Theyare switched OFF only during longer standstillperiods, for example maintenance stops. The

insulator heating have to be started at least 4hours before starting, respectively gas flowsthrough the electrostatic precipitator.


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COLLECTING ELECTRODES RAPPING DEVICE:

The collecting electrodes are held in rows by

rapping bars, which are equipped with anvils.Flail hammers, which are mounted in spirals

on the rapping shaft, hit the anvils of the

rapping bars. The impact of the blow is

transferred to the collecting electrodes, shaking

off the dust layer. The hammers are hitting the

anvils individually in a certain sequence in

order to prevent a temporary overload of thedust conveying system.


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The rapping shaft is driven by a geared motor,

which is connected by a flexible coupling.

The default rapping cycle setting is asfurnished below.

This rapping cycle time is to ensure adequate

rapping to the CE system for dislodging the

dust.

1st FIELD 2nd FIELD 3rd FIELD 4th FIELD

ON TIME 6 sec. 6 sec. 6 sec. 6 sec.

OFF TIME 10 min. 45 min. 100 min. 120 min.

S CO


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BUS COUPLER:

Bus couplers provided in the ESP- one in

between 1st

and 2nd

field and second inbetween 3rd and 4th field. The bus coupler will

take care of 2 fields through a single

transformer.

EARTHING SWITCH:

Every high tension set is equipped with a single

pole earthing switch. All parts of the electrostatic

precipitator, which are on high tension, canimmediately be earthed by the earthing switches.


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prior to entering into an electrostatic precipitator, all

parts which are on high tension must be manually

earthed at the inspection door itself. This is a very

important personal safety protection against switching on

the high voltage by any possible mistake.

MECHANICAL INTERLOCKING SYSTEM:The inspection doors of the electrostatic arelocked by a mechanical interlocking systemagainst unauthorized opening. They can onlybe opened after the high voltage is turned OFFand the high tension sets are earthed. In the

reverse case the high voltage cant be turnedON as long as some inspection doors areopened and the high tension sets are earthed.

PARAMETERS AFFECTING THE


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PARAMETERS AFFECTING THE

PERFORMANCE OF ESP

Temperature of gas

Gas volume

Moisture content

Gas velocity within the ESP

Inlet dust concentration

Size of the dust particle

Dust resistivity

Dust composition


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The electrical properties detoriate with increase

in temperature. Flashover limit decreasesthereby operating voltage have to be broughtdown to avoid back corona. This weakens fieldstrength and consequent force on the particles.

So migration velocity decreases and thus ESPperformance deteriorates.

A lower temperature also is detrimental. If itfalls below acid dew point, corrosion gives rise to

every possible problem on structural andmechanical aspect of ESP


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Increase in gas volume decreases specific

collecting area. so increase in gas volumedecreases the collection efficiency.

Specific collecting area = Total effective

collecting surface / Gas volume.


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Moisture content has a large influence on the

performance on ESP. it directly effects voltageand current characteristics thereby resistively

of dust. Increases in moisture and improves

the precipitation performance.


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The velocity of gas effects the migration

velocity. Due to high gas velocity migrationvelocity is decreased. The low gas velocity also

decreases the ESP efficiency due to gas

ratification i.e. hot gas passes through upperzone and cold gas passes through lower zone.


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Increase in inlet dust burden increases

outlet emission level even after maintainingsame efficiency


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Precipitator performance should increase with

increase in particle size because the largestparticles receive charge more quickly and

attains migration velocity.


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Worst effect of high resistivity dust is the formation ofback corona. Due to back corona spark over between

the discharge electrode and collecting electrodes alsooccurs. Energy consuming also increases. To avoid backcorona we must decrease the current density. Sooperating voltage decreases and hence consequent

loss occurs in collection efficiency Fly ash resistivity below 2X1010 ohm-cm dont affect

the performance of ESP voltage and corona current.

Above 1012 ohm-cm causes sparks and sever back

corona. Critical resistivity is 2X1010 ohm-cm.


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Performance of ESP largely influenced by the

composition and morphology of dust like

presence of SULPHUR oxides, ALKALI metals

(Sodium, Lithium, Potassium) which increasethe moisture absorbing capacity of the dust

and thus decreasing its resistivity.


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The purpose of gas distribution is to achieve

uniform gas flow and thereby distribute the

inlet dust burden evenly over the whole cross

section of the field without under utilizing or

over burdening a particular zone. Apermissible limit in the distribution is kept

within 20% to achieve the desired

performance.

START UP OF ESP


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START UP OF ESP Ensure that men and material clear from the ESP after

maintenance. Ensure that all inspection doors are locked/closed properly.

Ensure that earth switch must be open.

Ensure that all mechanical interlocks are correct and the respectivekeys should be preserved properly in place provided for smoothoperation.

Start the insulator heatings at least 4 hours before starting theplant.

Start the hopper heating at least 4 hours before starting the plant.

Start the high tension T/R sets at the in let flue gas temp 120 deg.C.

Start the dust conveying system

Start the rapping system


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SHUTDOWN OF THE ESP

Switch OFF the high tension T/R sets, by

bringing down the set values to zero.

Leave the auxiliaries e.g. hopper heaters, seal

air blower, rapping system and conveyingsystem running.

After 4 hours of Boiler shutdown keep the

auxiliaries OFF ( as per ash levels).

CHARGING OF 1ST FIELD THROUGH 2ND FIELD


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CHARGING OF 1ST FIELD THROUGH 2ND FIELD

Switch OFF 1st

field and 2nd

field T/R. Remove the keys corresponding from the panel.

Insert the key at T/R end and rotate the grounding

switch to earth position. Change the bus coupler individual position to 2nd

field position.

Change the found switch of TR2 to HT position

Remove the keys and charge the fields through

T/R-2.


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DOs

1. Maintain the process parameters within the specified limits2. Keep the EXP temp. below the max. permissible limit of

200 Deg.C.

3. Evacuate the ESP hoppers at a regular interval.

4. Adjust the TR setting to operate ESP fields with minimumsparks

5. Keep the required readings in a log book for reference

6. Put the correct GRADE of oil in rapping motor gear-box.

7. Allow proper combustion in the boiler to avoid

fire/damage to ESP.8. Avoid accumulation of un-burnt in the hoppers as it may

lead to hopper fire and damage to ESP.

DONTs


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DON Ts

1. Do not exceed the specified temp. limit. This can damagethe equipment/plant.

2. Do not bypass the interlock.3. Do not enter the ESP without cooling and gas analysis

against presence of any poisonous gases.

4. Do not stop the conveying equipments below the ESPwhen the equipments are in operation.

5. Do not by pass any safety requirement.

6. Do not keep ESP fields on if the hopper level is high.

7. Do not ignore any fault signal from any equipment.

8. Do not run the rapping motors without gear box oil.

9. Do not use the hopper as silo.10. Do not over rap the ESP.

11. Do not run the ESP with faulty controller.

ESP Operation

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