WELCOME

To

A Presentation

On

Electrostatic Precipitator

Why use an ESP ?

• To control atmospheric pollution caused by fly ash .

• To meet statutory requirement of pollution control (maximum permissible 150mg/Nm cube). Preferrebly 30mg/Nm cube

• To get fly ash as a by product. to use ash as filling for mines., Cement Plant as raw meterials, bricks vas etc.

Working Principle

• When passing through the high voltage electrostatic fields, dust particles will be charged by colliding with positive ions, negative ions and electrons or in the ion dispersion movement. The particles with electrons and ions on them will then move, under the influence of the electric field, toward and later accumulate on the electrodes of opposite polarity. By means of rapping, the layer of dust particles on the electrodes will be dislodged into the bottom hoppers. Practice has shown that the higher the strength of electrostatic field, the more effective as ESP will be, and that it is preferable to have an ESP operating with negative corona. Therefore, our ESPs are designed in the structure of high voltage negative corona electrodes.

Principle of operation• The ash particles

move along the stream of flue gas

• We are to change their direction of travel so that they get separated from the stream of flue gas

• How ?

Principle of operation (continued)

• We impart electrical charge to the particles.

• We have a surface with opposite electrical polarity nearby.

• The particles are electrically attracted to the surface.

+

Principle of operation (continued)

• Emitting electrode is charged at high voltage DC negative.

• Collecting Electrode (plate) is at ground potential (positive).

• Electrical field is perpendicular to flue gas flow.

+ -

Principle of operation (continued)

• Emitting electrodes sprays negative ions towards positive plate (corona discharge).

• They collide with ash particles and transfer the charge to them.

• Ash particles get negatively charged.

-+

-

-

--

- -

-

-

-

--

Principle of operation (continued)

• The positive collecting plate attracts negative ash particles.

• On reaching the collecting plate, the particles get electrically neutralized and remain there.

+

-

-

-

-

-

-

-

-

-

Principle of operation (continued)

• After a layer of ash is collected on the collecting plate,it is mechanically rapped so that the ash falls into the hopper for disposal through ash handling system .

Construction

• To achieve laminar flow at lower velocity the gas path is split in several “Pass”es.

• Perforated GD screen at the entry of each pass ensure uniform velocity distribution.

• To achieve better collection efficiency several fields are provided in series in each pass.

• Many combinations are possible depending on boiler size.

Construction(major parts)

• Collecting plates

• Emitting electrodes

• Rapping mechanism(collecting,emitting)

• Insulators

• High Voltage Rectiformers

• Heaters

• Ash Level Indicators

Construction(High Voltage Rectiformer)

• Rectiformer is a transformer and a rectifier in one unit.

• Voltage & current feedback circuits are also there.

• 70kV DC on no load.• Each field has got its

own HVR.

Controlled LT AC

High Voltage DC

Construction(ash level indicators )

• Capacitance type Ash Level indicators are provided in each hopper for sensing hopper overfilled condition.

• High level trips the particular field.

Construction(ash disposal)

• Below each hopper there is a vessel called transmitter.

• The ash from the hopper is first unloaded into the transmitter.

• The ash is then pneumatically to the silos for further disposal.

Controller

Control philosophy

• To have a voltage just below the

spark-over voltage for as long as possible.

Controller(graph)

V

t

Controller(spark sensing)

• The controller measures current and calculates dI/dt .

• When this value crosses a limit a spark is sensed.

• Voltage is made zero for few milliseconds of pause time to allow ionized air to flow away.

Controller(voltage recovery)

• Voltage again rises at a fast rate to very near the spark over value (say 90%).

• Rate of rise then slows down.

• If a spark is sensed, the process repeats itself.

• If no spark is sensed, the voltage continues to rise till voltage/current limit is reached.

Controller(back corona)

• All controllers use the above method of dynamic control of voltage.

• However use of only this method is unable to tackle the BACK CORONA problem.

• Modern controllers have micro-processor based computing facilities to take care of the back corona problem.

Controller(potential difference )

• The neutralization of charged ash particles constitute a current.

• Current has to pass through the layer of ash already collected.

• Potential difference is created across the layer. (V=IR)

+-

V = I x R

Controller(ionization of trapped air)

• Due to the high resistivity of ash the potential difference in a few mm thick layer can be several kV.

• This ionizes the air trapped between the ash layers.

+ +

+

+

-

-

-

Controller(back corona)

• The positive ions are repelled by the positive plate & they come out in a stream.

• This phenomenon is BACK CORONA.

+

++

+

+ + +

+

+

+

Controller(effects of back corona)

BACK CORONA

• Neutralize charged ash particles, so they are not collected.

• Dislodges already collected particles & allows them fly away.

• Draws extra power & reduces collection efficiency.

+-+

+

+

Controller(controlling back corona)

• To reduce Back Corona IR drop has to be reduced.

• R is a property of ash which cannot be controlled.

• To reduce I, V has to be reduced which will reduce the main corona. Hence collection itself will be reduced.

Controller(intermittent charging)

• Similar to a fluorescent tube, higher voltage is required to start a corona. Once started, the corona can be maintained by a much lesser voltage.

• The field behaves like a RC circuit. Once charged, the voltage decays only gradually through the collection current.

• It can be charged again before corona stops.

Controller(intermittent charging)

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

1:51:1

Controller(less average voltage)

• In conventional method of charging every half cycle, difference between the peak & average voltages is very small.

• In intermittent charging, firing of the thyristors is skipped for several half cycles & voltage is allowed to decay. While the peak voltage remains same, the average voltage comes down drastically.

Controller(less average current)

• Reduction of average voltage reduces average current which in turn reduces the IR drop across the ash layer. Thus back corona is controlled.

• The number of half cycles skipped is denoted by “charge ratio”. A charge ratio of 1:5 means firing takes place at every 5th half cycle.

(alarms & indications)

Alarms:• Transformer temperature high.• Buckholtz top float.• Ash level high.• Under Voltage.Indications:• Base charge ON.• Optimum reached.• Remote.

Power Flow Diagram of ESP Control Building # 2 (Unit#3)

eUnit 11KVSection ‘A’

Unit 11KVSection ‘B’

ESP ‘A’ Control Panel ESP ‘B’ Control Panel

BUS Bridge

BUS Bridge

BUS Bridge

2.5 MVA11/.433kVDry type

Transformer

2.5 MVA11/.433kVDry type

Transformer To

LT Drives

Switchgear Floor ESP (Unit#3, 18Mtr.)

Transformer Rectifying,Rapper Motor’s, collecting/Emitting system etc.

Transformer Rectifying,Rapper Motor’s, collecting/Emitting system etc.

PMCC ‘A’ PMCC ‘B’

Thank you !