Electrostatic Precipitator Electrostatic Charging of Dust Particles Cutaway

Electrostatic Precipitator Electrostatic Charging of Dust Particles Cutaway of Electrostatic Precipitator

Electrostatic Precipitator Principle · The particles in a polluted gas stream are charged by passing them through an electric field. · The charged particles are led through collector plates · The collector plates carry charges opposite to that on the particles · The particles are attracted to these collector plates and are thus removed from the gas steam Construction and Operation of Electrostatic Precipitator · Charging Electrodes in the form of thin wires are placed in the path of the influent gas. · The charging electrodes generate a strong electric field, which charges the particles as they flow through it. · The collector plates get deposited with the particles are occasionally removed either by rapping or by washing the collector plates.

Six major components • • • A source of high voltage Discharge electrodes and collecting electrodes Inlet and outlet for gas A hopper for disposal of collected material An electronic cleaning system An outer casing to form an enclosure around electrodes

Electrodes : Based on DC current flow terminals elctrodes can be divided as below: - Discharge electrode : Electrodes wire which carries negatively charged high voltage (between 20 to 80 KV) act as discharge or emitting electrodes. Collector electrode : Electrode wire which carries positively charged high voltage act as Collecting electrodes. Collector electrodes Discharge electrode

WORKING OF ELECTROSTATIC PRECIPITATOR Stage - 1 Several things happen very rapidly (in a matter of a millisecond) in the small area around the discharge electrode. Electric field is emerged due to dc terminal arrangement. The applied (-) voltage in discharge electrode is increased until it produces a corona discharge, which can be seen as a luminous blue glow around the discharge Electrode. Due to the formation of corona discharge, free electrons are emitted with high velocity from discharge electrode. This fast moving free electrons strikes the gas molecule thus emission of free electron from gas molecules takes place. The positive ion molecule move towards discharge electrode by electrostatic attraction As a result using gas molecule more free electrons are emitted near the discharge electrode.

Stage - 2 As the electrons leave the strong electrical field area around the discharge electrode, they start slowing down. This free electron again strikes the gas molecule but this time they are captured by gas molecule and became negatively charged ion. As the gas molecule are negatively ionized they move towards the (+) electrode (i. e. , collector electrode). This negative gas ion fills the space of Dust particle and becoming negatively charged particle. This particle are captured by collector electrode using electrostatic attraction.

CORONA FORMATION Corona discharge The flow of electrons and gaseous ions from the discharge electrode toward the collecting plates. Corona discharge occurs after the discharge electrode has achieved high enough secondary voltages.

Principles Gas stream passed two electrodes. High potential difference is maintained. Out of two electrodes, one is discharging other collecting. Potentials of 100 kv are used. Ionization creates active glow ( )ﺗﺨﻠیﻪ zone called “corona”. Gas ionization is dissociation of gas molecules into free ions. As particulates pass through field, they get charged and migrate to oppositely charged electrode. • Particles deposited on collecting electrodes, lose charge and removed mechanically by rapping. , vibration or washing to a hopper. • •

Single stage and two stage precipitators • Single stage gas ionization and particulate collection in a single stage. • Two stage, particle ionized in first chamber and collected in second chamber. • Two stage used for lightly loaded gases. • Single stage for more heavily loaded gas streams.

Wet electrostatic precipitator • A wet electrostatic precipitator (WESP or wet ESP) operates with saturated air streams (100% relative humidity). • WESPs are commonly used to remove liquid droplets such as sulfuric acid mist from industrial process gas streams. • The WESP is also commonly used where the gases are high in moisture content, contain combustible particulate, have particles that are sticky in nature. • The preferred and most modern type of WESP is a downflow tubular design. This design allows the collected moisture and particulate to form a slurry that helps to keep the collection surfaces clean. • Plate style and upflow design WESPs are very unreliable and should not be used in applications where particulate is sticky in nature.

Design of Electrostatic Precipitators • The efficiency of removal of particles by an Electrostatic Precipitator is given by η = fractional collection efficiency w = drift ( )ﺭﺍﻧﺶ velocity, m/min. A = available collection area, m 2 Q = volumetric flow rate m 3/min

Migration velocity Where, q = charge (Columbus) Ep = collection field intensity (volts/m) r = particle radius (m) μ = dynamic viscosity of gas (Pa-S) c = Cunningham correction factor

• Cunningham correction factor where, T = absolute temperature (°k) dp = diameter of particle (μm)

Problem • An ESP is designed to treat 50, 000 m 3/min with 97 % efficiency. Assuming an effective drift velocity of 2. 5 m/min, calculate the required plate area and the number of plates. The plate size is 10 m by 5 m (height by length).

Solution • Step 1: Efficiency of an Electrostatic Precipitator is given by A =-[ (Q/w)*ln(1 - η)] A = 70, 000 m 2 • Step 2: Number of plates = total area/plate area = 1400

Efficiency • General collection efficiency is high, nearly 100% • Acid mist and catalyst recovery efficiencies in excess of 99%. • Carbon black, because of agglomeration tendency collection efficiency less than 35%. • The efficiency is usually at a minimum in the range of 0. 1 to 0. 5 micrometers.

Design parameter • • Volumetric flow rate Composition Temperature Dew point Dust particle conc. Size of particle Bulk density Tendency of allgomoration

Resistivity of dust 1. When dust resistivity drops below this range, the dust releases its charge readily to the collecting surface. As a result, the dust migrates to the collecting plates where it immediately loses its charge. The charge in conjunction with the cohesive nature of the dust keeps the dust on the collecting plates. If the charge is lost, the dust is likely to be re-entrained back into the gas stream. • high resistivity dust retains charge for extended periods. the high resistivity dust deposits on the collecting plates, charge does not dissipate. As a result, high resistivity dust is very difficult to remove from the collecting plates. It is not uncommon for high resistivity dust applications to require periodic manual cleaning to restore precipitator performance.

Advantages of Electrostatic Precipitators · Electrostatic precipitators are capable very high efficiency, generally of the order of 99. 5 -99. 9%. · Since the electrostatic precipitators act on the particles and not on the air, they can handle higher loads with lower pressure drops. · They can operate at higher temperatures. · The operating costs are generally low. · High collection efficiency. · Particles may be collected dry or wet. · Can be operated at high temp. (300 -450˚c). · Maintenance is normal. · Few moving parts.

Disadvantages of Electrostatic Precipitators · The initial capital costs are high. · Although they can be designed for a variety of operating conditions, they are not very flexible to changes in the operating conditions, once installed. · Particulate with high resistivity may go uncollected. · Require high voltage. · Collection efficiency reduce with time. · Space requirement is more. · Possible of explosion during collection of combustible gases or particulates.

Application • • Cement factories Pulp and paper mills Steel plants Non- ferrous metal industry Chemical industry Petroleum industry Carbon black industry Electric power industry