ESP - Basic principles of Electrostatic Precipitator

Electrostatic Precipitator Principles

Electro Static Precipitator or "ESP" is a globally used system for effective filtration/control of particulate emission. It is a filter that removes minute dust particles from factory exhausts. ESPs can collect dust particles of size 0.1 to 10 micron effectively. They are efficient than scrubbers and cyclones for collecting particulate suspended matters from air or gaseous content.

     1.PRINCIPLE OF OPERATION

Electrodes at high voltage create a corona effect (Ionized atmosphere around them)

This charges the passing particles. Once charged the particles are subjected to a transverse electrostatic force that pulls them towards the collecting plates.

Plates are periodically hammered, it causes mechanical vibrations and makes the collected dust particles to fall into collecting hoppers.

     2.BACK CORONA

High resistivity dust is stubborn. Let us try to understand back corona with the following schematics.

Normally, dust in the range of 104-1011 Ω ･ cm can be collected easily.
Having been easily charged by the negative ions from the discharge electrode.

The dust is attracted to the collecting plate by Coulomb force.

Collected dust is easily separated from the collecting plate by rapping.







High resistivity dust (1011 Ω ･ cm or greater) is hard to remove because

Having been easily charged by the negative ions from the discharge electrode.

the dust is attracted to the collecting plate. Up to this point, it is pliant and controllable.Due to its strong adhesion force, the high resistivity dust is not easily separated by the shock of rapping.

Moreover, this dust layer starts releasing positive ions that cancel the negative ions from the discharge electrode, making the charging process unstable (back corona). This results in the deterioration of dust collection efficiency.

In case of high resistive dust, the dust layer creates insulation between the positively charged collecting plates and negatively charged dust particles.

In such condition spark/arc within the layer of dust particles is formed with the increase of kV(DC). This phenomenon is known as back corona.

To avoid back corona, the field voltage kV has to be reduced sufficiently, such measurer finally reduce the collection efficiency of the field.

     3.FIELD SHORT

Dry and high resistivity dust causes back corona while with wet and low resistive dust, field shorting occurs. The wet dust layer gets positively charged easily.In such condition whenever the gap between positively charged dust particles and negatively charged electrodes gets reduced due to accumulation of dust layer, spark gets emitted from emitting electrodes to collecting plates.This may cause the failure of the high voltage winding if transformer is not switched off immediately after field short.

Volt-Ampere characteristics

Let us graphically try to visualize the actual voltage-ampere condition within the esp equipment in case of normal running condition, back corona and field short.

a)Normal Operating zone(Linear operation) :- This operation mode is shown in green colour in the graph.With the increase in field voltage(kV dc) the field current increases linearly (mA).
b)Back corona zone:- Sparking due to reduced resistivity of the dust , causing decrease in field voltage and increase in field current.
c)Field short point:- The point where the voltage in the field reduces to zero short circuiting the field and the current raises to a dangerous value in the secondary of the high voltage transformer.

     4.PARAMETERS THAT AFFECT THE PERFORMANCE OF THE ESP

a)Gas temperature: Normally ESP  is designed to operate in the temperature range of 200-300 degree centigrade. At high temperature the quality of insulation deteriorates and flash over voltage limit decreases. In such condition operating voltage has to be brought down to avoid back corona that results in lower dust collection efficiency. At temperatures below the acid dew point , deposition of acid in the structure leads to faster corrosion.

b)Moisture content: moisture content has a large influence on the performance of esp. moisture content increase the ionization tendency and decreases the resistivity of the dust particles. As an effect of these factors dust collection efficiency increases with reduced back corona tendency.

c)Dust Particle size: The collecting efficiency increases with increase in particle size, since larger particle receive charge more quickly and attain migration velocity. Migration velocity is directly proportional to diameter. Hence collection efficiency decreases with the increase in fineness of the dust particle.

d)Dust resistivity: Dust resistivity increase with the increase in dryness of dust and quality of fuel. At higher dust resistivity, internal spark over between two layers of dust takes place as a result of potential difference created by the resistance of the dust. This phenomenon is called back corona. Once back corona starts, field intensity start reducing with reducing of field current. This reduced the collecting efficiency of the ESP.

e)Rapping frequency: Whenever the electrode surface is subjected to hammering or rapid shock, the re-entertainment of particles take place in the main flow path and carried away by the gas causing increase in emission level.
 

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