ELECTROSTATIC PRECIPITATOR (ESP)

Presented byRajeev saini

ELECTROSTATIC PRECIPITATOR (ESP) IS THE INDUSTRIAL EQUIPMENT THAT REMOVES THE DUST PARTICLE AND SMOKE THROUGH THE EFFECTIVE FILTRATION PROCESS.

What is Electrostatic Precipitator?

Control PanelVFD room

Rectifier

95 KV HVDC

BASIC DIAGRAM OF AN ELECTROSTATIC PRECITATOR

Supply 440V AC, 50Hz

Dust gas

Clean gas

ESP Hooper (Fly Ash)

Emitting electrode

CollectingPlate

440V /95KV

Transformer

95 KV DC

ESP-AESP-

A

ESP-B

M

M M

M

M

M

M

M

M

MM

M

MM

MM

ESP-C

ESP-D

FUR

NA

CE

DPSH

PSH

RH

LTSH

EC

ON

OM

ISE

R

STA

CK

APH-A

APH-B

ID FAN-A

ID FAN-B

Flue gas path

COMPONENTS USED IN ELECTROSTATIC PRECIPITATOR

Emitting electrodes (9900 Nos.)

Collecting plates (11400 Nos)

440V 50HZ 3φ AC supply

High voltage transformer(80 Nos)

Rectifier (80 Nos.)

Insulators (80 Nos)

ESP Hooper (160 Nos.)

GDRM (08 Nos.)

EERM (80 Nos.)

CERM (80 Nos.)

Hopper heater(160 Nos)

Gas flow rate 968.6(m3/s),641.8 Nm3/s

Gas temperature at ESP inlet (ºc) 148

Dust concentration at inlet (gm/Nm³) (Gm/Nm3) 82.39

Efficiency 99.939%

Pressure drop across the precipitator (in mmwc) 25

Velocity of gas in ESP 0.72 (m/s)

total number of collecting plates per boiler/Pass/Field 11400/2850/305

Nominal height of collecting plates in meters 15

Type of emitting electrodes Spiral with hook

Size (diameter in mm) 2.7

Total number of Emitting electrodes per boiler/Pass/Field 9900/2475

No. of hoppers/boiler 160

Capacity of hopper 8 hour storage

Coal Ash Content 42%

Concentration/Emission 50 mg/NM3

Spark Rate 5/Minute

Technical specifications

Aspect Ratio

The aspect ratio, which relates the length of an ESP to its height, is an important factor inreducing rapping loss (dust reentrainment). When particles are rapped from the electrodes, the gas flow carries the collected dust forward through the ESP until the dust reaches the hopper. Although the amount of time it takes for rapped particles to settle in the hoppers is short (a matter of seconds), a large amount of "collected dust" can be reentrained in the gas flow and carried out of the ESP if the total effective length of the plates in the ESP is small compared to their effective height.

For example, the time required for dust to fall from the top of a 9.1-m plate (30-ft plate) is several seconds. Effective plate lengths must be at least 10.7 to 12.2 m (35 to 40 ft) to prevent a large amount of "collected dust" from being carried out of the ESP before reaching the hopper.

Impacts of Flue Gas temperatures: - The operation of precipitator at gas temperature below

a. acid due point ( < 120°C) results in : b. Failure of emitting electrodes due to stress corrosion cracking.c. Corrosion of terminals.d. Collection of wet ash on the electrodes leads to formation of hard- to- rap layers of ash.e. Difficulty in removal of wet ash from the hoppers.

Specific Collection Area

The specific collection area (SCA) is defined as the ratio of collection surface area to thegas flow rate into the collector. It is an important determinant of collection efficiency.Charge RatioThe duty cycle or the charge ratio is defined as the ratio of thenumber of on cycle to the sum of the on and off cycle

The charge ratio can be varies in the range 1:1 to 1:159, which is necessitated to tackle the highresistivity of fly ash encountered in precipitators in India.At higher charge ratios the basevoltage reduces to very low values. The provision of base charging is made during some ofthe skipped half cycles there by avoiding the effect of low voltage. A facility is provided forthe measurement of peak and valley voltages of the charging signal. A high peak voltageincreases the effective migration velocity and thus increases the collection efficiencies.

BACK CORONA : 

A term that signifies that the ash layer on the collector surface has reached a level of RESISTIVITY that produces a flow of positive IONS back towards the negative high voltage discharge electrode. Most commonly observed back corona conditions result in SPARKOVER and a reduction of POWER INPUT.

COLLECTION EFFICIENCY: 

The weight of dust collected per unit time divided by the weight of dust entering the precipitator during the same unit time expressed in percentage.  The computation is as follows:                                    (ESP inlet Kg/Hr) – (Dust out let Kg/Hr) X 100            Efficiency =              (Dust in let Kg/Hr)     =99.939%   

  

Spark rate: -

The operating voltage and current keep changing with operating conditions. This takes care by the electronic controller unit. Very high flash over rateresults not only reduction of useful power and interruption of precipitation process but also cause snapping of emitting electrodes due to electrical erosion.  

Electrodes : -

Based on DC current flow terminals electrodes can be divided as below:-

Emitting electrode :-

Electrodes wire which carries negatively charged high voltage up to 95KV act as discharge or emitting electrodes.

Collector Plate:-

Electrode wire which carries positively charged high voltage act as Collecting electrodes/ Collecting plate.

Collecting Plates

Emitting electrode

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ESP – Working Principle

The precipitation process involves 4 main

functions :

Corona generation

Particle charging

Particle collection

Removal of particles

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ESP – Working PrincipleCorona generation: Due to ionization of gas molecules +ve ions, -ve ions and free electrons are generated

Particle charging:The –ve charges of ions and free electrons move towards +ve electrodes and the +ve charges of ions move towards –ve electrodes.When –ve ions travel towards +ve electrodes, the –ve charges get attached to the dust particles and thus the dust particles are electrically charged

A row of electrodes

A stack of metal plates

Gravity drops particles into the ESP hopper

The ionized particles moves to the grounded plates

Incom

ing

air

str

eam

Cle

an

ed

air

The precipitator’s operational principle uses negative voltage

to ionize particlesA negative voltage of 95kV is applied

WORKING OF ELECTROSTATIC PRECIPITATOR

Several things happen very rapidly (in a matter of a millisecond) in the small areaaround the emitting electrode. Electric field is emerged due to dc terminalarrangement. The applied (-) voltage in emitting electrode is increased until it produces a corona discharge, which can be seen as a luminous blue glow aroundThe emitting Electrode.

Due to the formation of corona discharge, free electrons are emitted with high velocity from discharge electrode.

This fast moving free electrons strikes the gas molecule thus emission of free electron from gas molecules takes place.The positive ion molecule move towards discharge electrode by electrostatic attraction

As a result using gas molecule more free electrons are emitted near the discharge electrode.

Stage - 1

Stage - 2As the electrons leave the strong electrical field area around the discharge electrode, they start slowing down. This free electron again strikes the gas molecule but this time they are captured by gas molecule and became negatively charged ion.

As the gas molecule are negatively ionized they move towards the (+) electrode (i.e., collector electrode).

This negative gas ion fills the space of Dust particle and becoming negatively charged particle.

This particle are captured by collector electrode using electrostatic attraction.

Resistivity/Conductivity

Impact of particle’s resistivity on ESP’s performance:

Factors: temperature, compositionFlue gas conditions

Q: How does resistivity affect an ESP’s performance?

Effects of sulfur content and temperature on resistivity

Q: Is “S” in coal good or bad?

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ESP – Effect of ash resistivity

ADVANTAGES OF ELECTROSTATIC PRECIPITATOR

•Removes tiny dust particle •Effective for high dust loaded gas•Dust is collected in dry form and can be disposed dry or in wet form •Low cost•Easy maintenance•Controls Air Pollution and save environment

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ESP – Problems

Dust Accumulation

Wire Breakage

Insulator cracking

Air Ingression