Electrostatic Precipitator

Principle of operation of Electrostatic Precipitator

The gas carrying dust particles, when passed between oppositely charged conductors, gets ionized, as the voltage applied (30kv) and spacing between conductors, is adjusted that a strong electric field exists between them.

The ionized gas is further passed through the collecting unit which consists of a set of vertical metal plates.

The alternate plates are positively charged and earthed. The spacing and voltage are so adjusted that high electric field exists between the plates which exert force on the ionized particles.

The positively charged dust particles are attracted towards the grounded plates.

The deposited dusts particles are removed from the collecting plates by tumbling hammers and is collected in the dust hoppers.

It has two sets of electrodes, insulated from each other that maintain an electrostatic field between them at high voltage. The flue gases are made to pass between these two sets of electrodes. The electric field ionises the dust particle; that pass through it attracting them to the electrode of opposite charge. The other electrode is maintained at a negative potential of 30,000 to 60,000 volts. The dust particles are removed from the collecting electrode by rapping the electrode periodically. The electrostatic precipitator is costly but has low maintenance cost and is frequently employed with pulverised coal fired power stations for its effectiveness on very fine ash particles and is superior to that of any other type.

The principal characteristics of an ash collector is the degree of collection.

= Degree of collection

whereGl = Quantity of ash entering an ash collector per unit time (kg/s)G2 = Quantity of uncollected ash passing through the collector per unit time (kg/s)Cl = Concentration of ash in the gases at the inlet to the ash collector (kg/m3)C2 = Ash concentration at the exist (kg/m3).

Depending on the type of fuel and the power of bailer the ash collection in industrial boilers and thermal power stations can be effected by mechanical ash collectors, fly ash scrubbers and electrostatic precipitators. For fly ash scrubbers of large importance is the content of free lime (CaO) in the ash. With a high concentration of CaO the ash can be cemented and impair the operation of a scrubber.

The efficiency of operation of gas cleaning devices depends largely on the physico-chemicalproperties of the collected ash and of the entering waste gases.

Following are the principal characteristics of the fly ash:(i) Density (ii) Dispersity (Particle size)(iii) Electric resistance (For electrostatic precipitators) (iv) Coalescence of ash particles.

Due to increasing boiler size and low sulphur high ash content coal the problem of collecting fly ash is becoming increasingly complex. Fly ash can range from very fine to very coarse size depending on the source. Particles colour varies from light tan to grey to black. Tan colour indicates presence of ion oxide while dark shades indicate presence of unburnt carbon. Fly ash particles size varies between 1. micron (l μ) to 300 μ. Fly ash concentration in flue gases depends upon mainly the following factors :(i) Coal composition. (ii) Boiler design and capacity.

Percentage of ash in coal directly contributes to fly ash emission while boiler design and operation determine the percentage retained in the furnace as bottom ash and fly ash carried away by flue gas. Fly ash concentration widely varies around 20-90 g/mm3 depending on coal and boiler design. Fly ash particle size distribution depends primarily on the type of boiler such as pulverised coal fired boiler typically produces coarser particles then cyclone type boilers. Electrostatic precipitator (ESP) is quite commonly used for removal of fly ash from flue gases.