Feature Summary

Types of Designs

Single- or two-stage

Applications

Single-stage units are used in coal-fired power plants to remove fly ash, cement kiln dust, lead smelter fumes, tar, and pulp and paper alkali salts. Two-stage units are used for air conditioning applications.

Flue Gas Limitations

Flow up to 4 X 106 acfm, temperature up to 800°C, gas velocity up to 10 ft/sec, pressure drop is under 1 in w.g.

Treatment Time

2-10 sec

Power Requirement

Dust Particle Size Range

Collection Efficiency 99.5 to 99.99%

Partial List of Suppliers

Air Cleaning Specialists, Inc.; Air Pol Inc.; Air Quality Engineering, Inc.; ASEA Brown Boveri Inc.; Babcock and Wilcox, Power Generation Group; Belco Technologies Corp.; Beltran Associates, Inc.; Dresser Industries Inc.; Ducon Environmental Systems; GE Company; Joy Environmental Technologies, Inc.; North American Pollution Control Systems; Research-Cottrell Companies; Scientific Technologies, Inc.; United Air Specialists, Inc.; Universal Air Precipitator Corp.; Wheelabrator Air Pollution Control.

Electrostatic precipitation uses the forces of an electric field on electrically charged particles to separate solid or liquid aerosols from a gas stream. The aerosol is deliberately charged and passed through an electric field causing the particles to migrate toward an oppositely charged electrode which acts as a collection surface. Gravity or rapping the collector electrode removes the particles from the precipi-tator. Various physical configurations are used in the charging, collection, and removal processes.

ESPs are characterized by high efficiencies, even for small particles. They can handle large gas volumes with low pressure drops and can be designed for a range of temperatures. On the other hand, they involve high capital costs, take up a lot of space, and are not flexible after installation to changes in operating conditions. They may not work on particles with high electrical resistivity.

Commercial ESPs accomplish charging using a highvoltage, direct-current corona surrounding a highly charged electrode, such as a wire. The large potential gra dient near the electrode causes a corona discharge comprising electrons. The gas molecules become ionized with charges of the same sign as the wire electrode. These ions then collide with and attach to the aerosol particles, thereby charging the particles. Two electrodes charge the particles, and two electrodes collect the particles, with an electric field between each pair.

When the same set of electrodes is used for both charging and collecting, the precipitator is called a single-stage precipitator. Rapping cleans the collecting electrodes; thus, these precipitators have the advantage of continuous operation. The discharge electrode consists of a wire suspended from an insulator and held in position by a weight at the bottom. A power source supplies a large direct current (DC) voltage (~50 kV) which can be either steady or pulsed.

Parallel plate ESPs are more widely used in industry. Here, gas flows between two vertical parallel plates with several vertical wires suspended between them. The wires are held in place by weights attached at the bottom. These wires constitute the charging electrodes, and the plates are the collecting electrodes. Rapping the plate removes the collected dust; the dust gathers in a dust hopper at the bottom. Figure 5.17.1 shows a single-stage, parallel plate pre-cipitator with accessories such as hoppers, rappers, wire weights, and distribution baffles for the gas.

If different sets of electrodes are used for charging and collecting, the precipitator is called a two-stage precipitator (see Figure 5.17.2). Cleaning involves removing the collecting plates and washing them or washing the collecting plates in place. In a two-stage ESP, the charging section is short, providing a short residence time, and the collection section is five or more times longer to provide sufficient time for collection (Crawford 1976). Two-stage ESPs are used in air conditioning applications, while single-stage ESPs are used in industrial applications where dust loadings are higher and space is available.

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