Electrostatic Precipitators or ESPs have been used in industry for over 60 years. They can collect particles sized 0.1 to 10 microns very efficiently. They are generally more efficient at collecting fine particles than scrubbers or cyclones.
Electrostatic Precipitators are used for collection of fine ash particles from flue gases of boilers. Static Precipitator is a technique used to decrease the pollution content in the flue gasses. It is generally used in Thermal power plants to control the pollution caused by flue gasses.
Flue gas is cooled to as low a temperature as possible by its heat having been recovered for super heating steam, heating feed water and heating combustion air being fed to boiler furnace. This serves two purposes namely, firstly heat recovery results in increase of efficiency and secondly reduction of temperature due to heat recovery makes ash particles less electrically resistive so that they respond better to electrostatic collection process.
Basic Principle of ESP
Supplying high voltage between the Collecting Electrode and Discharge Electrode generates a corona discharge that produces minus ions. The electrically charged dusts are attracted towards the Collecting Electrodes by an electrostatic force. The accumulated dusts, due to the impact strength of hammer rapping to the Collecting Electrodes, are dropped and collected in the hopper. The dust is discharged in the hopper by flushing using a spray washing system. The dusts inside the hopper are discharged and transported by ash handling system.
In high- voltage electrostatic field, affected by the electric field force, gas ionization takes place. There are tremendous amount of electrons and ions existing in the ionized gas. After the dust particles are combined with these electrons and ions, they will be polarized; most of them are negatively polarized. Under the action of the field force, negatively charged particles migrates towards the positive electrode and in turn release electrons and attach to the positive electrode.
When the particles agglomerate and the layers reaches a certain thickness on the plate, rapping system will start to work and the particles will be dislodged from the collecting plate by vibration and falling into the hopper. That ends the collection process.
ESP size is often measured in terms of Specific Collection Area (SCA). The Specific Collection Area (SCA) has units, of square feet per 1,000 actual cubic feet per minute (acfm) of flue gas flow. The SCA for the required performance can be determined by using the Deutsch-Anderson equation, which relates the collection efficiency (E) to the unit gas flow rate, the particulate’s effective migration velocity and the collection surface area:
E = 1 - e (-wA/V)
or
A = [ ln (1/(1-E)]. V/w
Where,
E = ESP removal efficiency, %
= 100 [(Inlet dust loading - Outlet dust loading)/(Inlet dust loading)]
w = effective migration velocity, m/s
A = Collection surface area, m2
V = gas flow, m3/s
Because of the assumption about an effective migration velocity to make use of this sizing equation, the empirical nature of ESP design is obvious.
Factors that influence precipitator sizing are:
•gas volume
•precipitator inlet loading
•precipitator outlet loading
•outlet opacity
•particulate resistivity
•particle size
Particles in the medium resistivity range are the most acceptable for electrostatic precipitators. Particles in the low range are easily charged; however upon contact with the collecting electrodes, they rapidly lose their negative charge and are re-entrained back into the gas stream to either escape or to be recharged by the corona field. Particles in the high resistivity category may cause back corona which is a localized discharge at the collecting electrode due to the surface being coated by a layer of non-conductive material.
Resistivity levels are generally broken down into three categories:
•low; under 1x108 ohm-cm,
•medium; 1x108 to 2x1011 ohm-cm
•and high; above 2x1011 ohm-cm.
ESPs can handle large volumes of hot exhaust gases - beneficial for high-temperature exhausts found at Cement plants, steel industry furnaces and industrial and utility boilers.
Advantages:
Low operating cost
Ability to remove dry as well as wet particles
Temperature flexibility in design
Very high efficiency, even for smaller particles
Ability to handle very large gas flow rates with low pressure losses
Disadvantages:
High capital cost
Failure to operate on particles with high electrical resistivity
Taking a lot of space
Not flexible once installed