In 1905, Dr. F. G. Cottrell, Professor of Physical Chemistry at the University of California, conducted a series of laboratory experiments that results in the development of the first commercial electrostatic precipitator. It was an immediate success and the precipitator soon came to be widely used in power plants, smelters, steel plants, paper mills and many other industries.

The principal components of an electrostatic precipitator (ESP) are two sets of electrodes insulted from each other. The first set is composed of rows of electrically grounded vertical parallel plates, called the collection electrodes, between which the dust-laden gas flows. The second set of electrodes consists of wires, called the discharge or emitting electrodes that are centrally located between each pair of parallel plates (Fig. 6.60). The wires carry a unidirectional negatively charged high-voltage (between 20 & 100 kV) current from an external dc source. The applied high voltage generates a unidirectional, non-uniform electrical field whose magnitude is greatest near the discharge electrodes. When that voltage is high enough, a blue luminous glow, called a corona, is produced around them. Electrical forces in the corona accelerate the free electrons present in the gas so that they ionize the gas molecules, thus forming more electrons and positive gas ions. The new electrons create again more free electrons and ions, which result in a chain reaction.

The positive ions travel to the negatively charged wire electrodes. The electrons follow the electrical field toward the grounded electrodes, but their velocity decreases as they move away from the corona region around the wire electrodes toward the grounded plates. Gas molecules capture the low velocity electrons and become negative ions. As these ions move to the collecting electrode, they collide with the fly ash particles in the gas stream and give them negative charge. The negatively charged fly ash particles are driven negative charge. The negatively charged fly ash particles are driven to the collecting plate by the force which is proportional to the product of this charge and the strength of the electric field.

Vertical electrodes and grounded plates in an ESP showing four basic operations

When the particles collect on the grounded plates, they lose their charge on the ground. The electrical resistivity of the particles, however, cause only partial discharging, and the retained charge tends to hold the particles to the plates.

High resistivity causes retension of most of the charge, which increases the forces holding the particles to the plates and makes removal more difficult. This can be rectified either by operating at high gas temperatures (before APH) or by superimposing a high voltage pulse on the base voltage to enhance ESP performance during operation under high-resistivity conditions. Collected particulate matter must be removed from the collecting plates on a regular schedule to ensure efficient collector operation. Removal is usually accomplished by a mechanical hammer scrapping system. The vibration knocks the particulate matter off the collecting plates and into a hopper at the bottom of the precipitator.

Electrostatic Precipitators: 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 ionizes 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 characteristic of an ash collector is the degree of collection.

η=Degree of collection

=$\frac{G_1-G_2}{G_1}$

=$\frac{C_1-C_2 }{C_1}$

where

Gl = 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 physio-chemical properties 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 color varies from light tan to grey to black. Tan color 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.