ELECTROSTATIC PRECIPITATOR

An ESP is an apparatus, or equipment, which utilizes an electric field to separate the suspended particles from the dust laden flue gas .

OperationTheory of precipitation:

Particle Discharge

Collection of particles

Removal of particles

Particle charging:a).Corona discharge: Free Electron Generation

b).Ionization of gas molecules:

c).Charging of particles:

d).Particle Charging Mechanisms

Diffusion charging

Field charging

Particle collection:

When a charged particle reaches the grounded collection electrode, the charge on the particle is only partially discharged. The charge is slowly leaked to the grounded collection plate.

Particle Removal:

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

Rapping: it’s a process whereby deposited, dry particles are dislodged from the collection plates by sending mechanical impulses or vibrations to the plates

Hopper: It is a single collection bin with slides sloping approximately 50-70 degrees, where dislodged dust falls from the plate into the hopper.

Precipitator Components Discharge electrodes

Collection electrodes

High voltage electrical systems

Rappers

Hoppers

Shell

Collection Efficiency (R)Precipitator performance is very sensitive to two particulate properties:

Electrical resistivity: Resistivity, which is a characteristic of particles in an electric field,

is a measure of a particle's resistance to transferring charge (both accepting and giving up charges). Resistivity is a function of a particle's chemical composition as well as flue gas operating conditions such as temperature and moisture.

Particle Size Distribution: The particle size distribution of a powder, or granular material, or particles dispersed in fluid is a list of values or a mathematical function that defines the relative amount, typically by mass, of particles present according to size.

Dust Layer ResistivityA potential electric field (voltage drop) is formed across the dust layer as negatively charged particles arrive at the dust layer surface and leak their electrical charges to the collection plate. At the metal surface of the electrically grounded collection plate, the voltage is zero. Whereas at the outer surface of the dust layer, where new particles and ions are arriving, the electrostatic voltage caused by the gas ions can be quite high. The strength of this electric field depends on the resistivity and thickness of the dust layer.

Normal Resistivity

Particles with normal resistivity do not rapidly lose their charge on arrival at the collection electrode.

These particles slowly leak their charge to grounded plates and are retained on the collection plates by intermolecular adhesive and cohesive forces.

High Resistivity

If the voltage drop across the dust layer becomes too high, several adverse effects can occur.

First, the high voltage drop reduces the voltage difference between the discharge electrode and collection electrode, and thereby reduces the electrostatic field strength used to drive the gas ion - charged particles over to the collected dust layer.

As the dust layer builds up, and the electrical charges accumulate on the surface of the dust layer, the voltage difference between the discharge and collection electrodes decreases.

The migration velocities of small particles are especially affected by the reduced electric field strength.

APPLICATIONS:

Chemical production and food processing industries

Fossil fuel fired boilers

Petroleum refineries

Municipal waste incineration plants

Kraft pulp and paper mills

Lead , Zinc , Copper smelters etc.....

ADVANTAGES:Less draught loss

Effective removal of small and even large dust particles

Ease of operation

DISADVANTAGES:

Because direct current is not available in modern plants, another electrical equipment is necessary

Power required for charging is high

Protection from sparking should be provided because of the closeness of charged plates and high potential used

ESP in Waste Incineration Plant