electro-static precipitator
TRANSCRIPT
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