By Weihao Xu and Aran Jian

By use of static electricity, particles are attracted in the same way that static electricity in clothing picks up small bits of dust and lint. Electrostatic precipitators, 98 to 99 percent effective, are used instead of baghouses when the particles are suspended in very hot gases, such as in emissions from power plants, steel and paper mills, smelters, and cement plants.Static electricity is used in pollution control by applying a static charge to dirt particles in the air and then collecting those charged particles on a plate or collector of the opposite electrical charge. These devices are commonly known as “electrostatic precipitators”.

Smokestacks

 An electrostatic precipitator is a large, industrial emission-control unit. It is designed to trap and remove dust particles from the exhaust gas stream of an industrial process. Precipitators are used in these industries:

•Power/Electric •Cement •Chemicals •Metals •Paper

Electrostatic Precipitator

In many industrial plants, particulate matter created in the industrial process is carried as dust in the hot exhaust gases. These dust-laden gases pass through an electrostatic precipitator that collects most of the dust. Cleaned gas then passes out of the precipitator and through a stack to the atmosphere. Precipitators typically collect 99.9% or more of the dust from the gas stream.Precipitators function by electrostatically charging the dust particles in the gas stream. The charged particles are then attracted to and deposited on plates or other collection devices. When enough dust has accumulated, the collectors are shaken to dislodge the dust, causing it to fall with the force of gravity to hoppers below. The dust is then removed by a conveyor system for disposal or recycling.Depending upon dust characteristics and the gas volume to be treated, there are many different sizes, types and designs of electrostatic precipitators. Very large power plants may actually have multiple precipitators for each unit.

Gas characteristics and particle properties define how well a precipitator will work in a given application. The main process variables to consider are:

Gas flow rateThe gas flow rate in a power plant is defined by coal quality, boiler load, excess air rate and boiler design. Where there is no combustion, the gas flow rate will have process-specific determinants.

Particle size and size distributionThe size distribution in a power plant is defined by coal quality, the coal mill settings and burner design. Particle size for non-combustion processes will have similar determinants.

Particle resistivityThe resistivity of fly ash or other particles is influenced by the chemical composition and the gas temperature.

Gas temperature

Discharge electrodes emit charging current and provide voltage that generates an electrical field between the discharge electrodes and the collecting plates. The electrical field forces dust particles in the gas stream to migrate toward the collecting plates. The particles then precipitate onto the collecting plates. Common types of discharge electrodes include:

•Straight round wires •Twisted wire pairs •Barbed discharge wires •Rigid masts •Rigid frames •Rigid spiked pipes •Spiral wires

Collecting plates are designed to receive and retain the precipitated particles until they are intentionally removed into the hopper. Collecting plates are also part of the electrical power circuit of the precipitator. These collecting plate functions are incorporated into the precipitator design. Plate baffles shield the precipitated particles from the gas flow while smooth surfaces provide for high operating voltage.Collecting plates are suspended from the precipitator casing and form the gas passages within the precipitator. While the design of the collecting plates varies by manufacturer, there are two common designs:

•Plates supported from anvil beams at either end The anvil beam is also the point of impact for the collecting rapper •Plates supported with hooks directly from the precipitator casing Two or more collecting plates are connected at or near the center by rapper beams, which then serve as impact points for the rapping system

Top, center, or bottom spacer bars may be used to maintain collecting plate alignment and sustain electrical clearances to the discharge system.

Static electricity can also be used to reduce air pollution. Smoke is made from very small solid particles. If charged plates are placed inside of a chimney, the smoke particles will be attracted. The smoke therefore does not end up in the atmosphere. The plates can then be removed from time to time for cleaning.