Use of Electrostatic Precipitators (ESP) in Thermal Power Plants

Use of  Electrostatic Precipitators:

The electrostatic precipitators (ESP) are extensively used in the thermal power plant or steam power plant for removal of fly ash from the electric utility boiler emissions. The use of electrostatic precipitators is growing rapidly because of the new strict air code and environmental laws. An electrostatic precipitators can be designed to operate at any desired efficiency for use as primary collector or a supplementary unit to cyclone collector. It is often considered worthwhile to retain an existing cyclone collector as primary collector in the case where the fly-ash collection efficiencies must be upgraded especially where there is a large amount of unburnt carbon in the fly-ash. Because the presence of large quantities of the carbon in the gas can adversely affect the efficiency of the electrostatic precipitator.

Working Principle of  Electrostatic Precipitators (ESP):

The dust laden gas is passed between the oppositely charged conductors and is becomes ionized as the voltage applied between the conductors is sufficiently large (30kV to 60kV depending upon the electrodes spacing). As the dust laden gas is passed through the highly charged electrodes, both negative and positive ions are formed (positive ions will be a high as 80%).

The ionized gas is further passed through the collecting unit which consists of set of metal plates. Alternate plates are charged and earthed. As the alternate plates are grounded, high intensity electrostatic field exerts a force on the positive charged dust particles and drives them towards the ground plate. The deposited dust particles are removed from the plates by giving the shaking motion of the plates with the help of cams driving by external means. The dust removed from the plates with the help of shaking motion is collected in the dust hoppers. Care should be taken that the dust collected in the hopper should not be entrained in the clean gas.

Some of the advantages and disadvantages of electrostatic precipitators in thermal power plants are given below:

Advantages of Electrostatic Precipitators:

• This is more effective to remove small particles like smoke, mist and flyash. Its range of dust removal is sufficienctly large. The small dust particles below 10 micron cannot be removed with the help of mechanical separators. and wet scrubbers cannot be used if sufficient water is not available. Under these circumstances,  electrostatic precipitators are very effective.
• Electrostatic Precipitators (ESP) is also most effective for high dust loaded gas (as high as 100 grams per cu meter). Its efficiency is as high as 99.5%
• The drought loss of the separator is the least of all forms
• The maintenance charges are less compared to all other separators
• Electrostatic Precipitators provides ease of operation
• The dust or fly-ash is collected in dry form and can be removed either dry or wet

Disadvantages of Electrostatic Precipitators:

• The direct current (DC) is not available with the modern thermal power plants. Hence considerable electrical equipment is required to convert from ac to dc (60kV dc). This increases the capital cost of the equipment. 
• The running charges is also high as the amount of power required for charging is considerably high
• The space required for electrostatic precipitators  is larger than wet system
• The efficiency of the  electrostatic precipitators is not maintained if the gas velocity exceeds that for which the plant is designed. The dust carried with the gases increases with an increase of gas velocity. The efficiency of electrostatic precipitators  decreases when the gas flow increases
• Because of the closeness of the charged plates and high potential used, it is necessary to protect the entire collector from sparking by providing a fine mesh before the ionizing chamber. This is necessary because even a smallest piece of conducting material can cause sparking when it would be carried across adjacent plates

However, in-spite of the cost electrostatic precipitators are widely used in pulverized coal fired thermal power stations for its effectiveness on fine ash particles compared to other methods.