Question

How is a low voltage two stage electrostatic precipitator different from a high voltage single stage electrostatic precipitator? Explain the operation of a plate type electrostatic precipitator. Include a schematic view of the electrostatic precipitator and mention the size-efficiency relationship.

Answer 1

Low Voltage 2 Stage ESP

High Voltage Single Stage ESP

Two-stage ESP are designed so that the charging field and the collecting field are independent of each other. The charging electrode is located upstream of the collecting plates. In a two-stage precipitator, particles are charged by low voltage in one chamber, and then collected by oppositely charged surfaces in a second chamber. The two-stage ESP has separate particle charging and collection stages. The ionizing stage consists of a series of small, positively charged wires equally spaced 2.5 to 5.1 cm from parallel grounded tubes or rods. A corona discharge between each wire and a corresponding tube charges the particles suspended in the air flow as they pass through the ionizer. The direct-current potential applied to the wires is approximately 12 to 13 kV. Two-stage precipitators were originally designed for air purification in conjunction with air conditioning systems. Two-stage precipitators have limited use for particulate-emission control.

Single stage ESP is designed so that the same electric field is used for charging and collecting particulates. Single stage ESP are the most common types used for the control of particulate emissions. A single-stage precipitator uses high voltage to charge the particles, which are then collected within the same chamber on collection surfaces of opposite charge. Single stage ESPs use very high voltage (50 to 70 kV) to charge particles. Most ESPs that reduce particulate emissions from boilers and other industrial processes are single-stage ESPs.

Operation of Plate type ESPs

It has two sets of electrodes one is positive, and another is negative. The negative electrodes are in the form of rod or wire mesh. Positive electrodes are in the form of plates. The positive plates and negative electrodes are placed vertically in the electrostatic precipitator alternatively one after another.

The negative electrodes are connected to a negative terminal of high voltage DC source, and positive plates are connected to the positive terminal of the DC source. The positive terminal of the DC source may be grounded to get stronger negativity in the negative electrodes. The distance between each negative electrode and positive plate and the DC voltage applied across them are so adjusted that the voltage gradient between each negative electrode and adjacent positive plate becomes quite high to ionise the medium between these. The medium between the electrodes is air, and due to the high negativity of negative electrodes, there may be a corona discharge surround the negative electrode rods or wire mesh. The air molecules in the field between the electrodes become ionized, and hence there will be plenty of free electrons and ions in the space. The entire system is enclosed by a metallic container on which one side is provided with an inlet of the flue gases, and the opposite side is provided with the outlet of the filtered gases. As soon as the flue gases enter into the electrostatic precipitator, dust particles in the gases collide with the free electrons available in the medium between the electrodes and the free electrons will be attached to the dust particles. As a result, the dust particles become negatively charged. Then these negatively charged particles will be attracted due to electrostatic force of the positive plates. Consequently, the charged dust particles move towards the positive plates and deposited on positive plates. Here, the extra electron from the dust particles will be removed on positive plates, and the particles then fall due to gravitational force. We call the positive plates as collecting plates. The flue gases after travelling through the electrostatic precipitator become almost free from ash particles and ultimately get discharged to the atmosphere through the chimney. Charged particles are collected on the plates as dust, which is periodically removed by rapping or water sprays.

Size-Efficiency relationship

The efficiency of the electrostatic precipitator is dependent on the particle size of the aerosol (dust, mist) to be collected. The collection efficiency if high for more massive particles and low for small particles.

Schematic Diagram