NATIONAL INSTITUTE OF TECHNOLOGY, DURGAPUR

ELECTROSTATIC PRECIPITATOR (ESP)

PRESENTED BY:FATEH RANA 13/ME/22

DEPARTMENT OF MECHANICAL ENGINEERING

INTRODUCTIONAn electrostatic precipitator is a large, industrial emission- control unit. It is designed to trap and remove dust particles from the exhaust gas stream (flue gases) of an industrial plant.ESPs are used in Chemical, Paper, Power sector units, etc But most exclusively it is used in coal based thermal power plants.

Purpose of using ESP:

Removal of objectionable matter in the dust/flue gas , SO2 and NOx and PM2.5 .

NOx causes depletion of ground layer ozone and SO2 causes acid rain

Recovery of material for economic reason.

Removal of abrasive material in the dust to reduce wear and tear of the fan components.

Construction:(i) Discharge Electrodes:Electrode wires which carries negatively charged high voltage DC (between 20 to 80 KV) act as discharge or emitting electrodes.

The cross section of the wire may be round , square, or further complicated to increase the local electrical field and thus the corona discharge efficiency.

(ii) Collector Electrode Electrode plates which carries positively charged high voltage DC act as collector plate.

Collector plates are earthed , so they have zero potential.

The collector plates are separated by a distance of 1cm to 18 cm depending upon the working condition.

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(iii) Casing :The side walls are made of horizontal panels ,it is a leak proof arrangement with roof beams of longitudinal & transverse to support the internals of collecting and emitting electrodes.

Casings are generally made of Reinforced cement concrete, to withstand corrosion due to the presence of moisture and sulphur Compounds. Pyramidal and flat bottom hoppers are provided under the casing to collect the ashes.It should not be treated as storage bunker.

(iv ) Hopper :

Working Principle:

The precipitation process involves 4 main functions:

(i) Corona generation

(ii) Particle charging

(iii) Particle collection

(iv) Removal of particles

Corona generation

When high voltage is applied in the charging electrodes ,a blue luminous glow called “Corona “ is generated around the electrode.And it causes ionisation of gas molecules , due to which +ve ions, -ve ions , and free electrons are generated.Particle charging:When –ve ions travel towards +ve electrodes (Collecting Plate) ,the –ve charges get attached to the dust particles, and thus the dust particles are electrically charged.

Particle collection:

The –vely charged particles get attracted towards the +vely charged collecting electrodes and form a layer on the surface of the electrode.Similarly the +vely charged particles are deposited on the discharging electrode.But this is merely 10% of the total deposited material.Removal of particles:The deposited particles forms a layer on the electrodes, and after certain time interval they are removed from the electrodes by mechanisms depending upon the type of ESP.Ash particles are collected in the hopper , and from there they are transported to somewhere else.

Types of ESP(i) Dry-type ESP

A collecting plate rapping device is used to remove the dust collected on the plate , when the dust has accumulated up to a certain thickness. Due to rapping re-entrainment of particles takes place which reduces efficiency. In addition to this , dry ESPs are not suitable to collect particles that are sticky, corossive,or have high dust cake resistivity. Dry ESPs exhibits reduced collection efficiency for fine particles in the 0.1-1 micrometer range.

(ii) Wet-type ESP Wet ESPs are designed and

developed to eliminate the problems of Dry ESPs ,by using periodic or continuous scrubbing water to remove deposited particles on the collection electrodes. Wet ESPs exhibits good control on fine particles in industrial applications.

Performance of ESPThe basic formula describing ESP performance is Deutsch-Anderson equation , giving the outlet dust concentration (Cout), as function of inlet concentration(Cin) , gas flow volumetric rate (Q),and precipitator collecting area (A)Cout =Cin.exp(-wA/Q)

Where, w=effective migration velocity of particle.It’s a function of several variables ,including particle size distribution ,ash composition,flue gas temperature etc

In practicle it is an over-simplified equation.

………continued.

ESP has very high mass based collection efficiency which exceeds 99%

Efficiency=(mass of all particles retained by collector )/(mass of all particles entering collector) This high collecting effiency of ESP is of great

importance , because flue gases contain N2 , CO2, NOx ,SO2, H2O etc.

SO2 and NOx are the major pre-cursor of acid rains which has acidified soils, lakes, accelerated corrosion of buildings and reduced visibility.

The dust particles present in flue gas creates respiratory and cardio-vascular illness.

Advantages of ESP:(i) This is more effective to remove small particles like smoke ,mist, fly ash. Unlike Cyclone separator which is not efficient in removal of fine particles (particles smaller than 5 micron).It’s range of dust removal is sufficiently large (0.01 micron to 1.0 micron).(ii) ESPs are durable, cost effective and easy to operate.(iii) ESPs have very high efficiency i.e more than 99%(iv) Generally ESPs operate at a temperature range of 363K-573K. But in case of ultra-fine particles it can be operated at a higher temperature range of 623K-673K or higher.

Disadvantages of ESP:(i) DC is not available with modern plants , therefore considerable electrical equipment is necessary to convert low voltage (400V) AC to high voltage (60KV) DC ,this increases the capital required for the set-up.(ii) The running charges are also high as the amount of power required for charging is considerably large.(iii) The equipment is complex, requires equipment transport and maintenance of high level is required.(iv) Because of 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 ionising chamber.

Conclusion:As one of the principle source of energy ,Coal is important because of its stability of supply and relatively low cost .However, any greenhouse gases emitted during the combustion of coal must be reduced in order that coal can continue to be used in CO2 constrained future.The problem of air pollution from particulate emission has recently shifted from micro meter to nano meter range .As particle size decreases the potential for chemical and catalytic effects on human health dramatically increases.

So , despite of its high capital investment ESP is the necessity of present and the hope for the future.

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