How can static electricity prevent air pollution?

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How can static electricity prevent air pollution?
By Weihao Xu and Aran Jian
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Smokestacks
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.
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Electrostatic Precipitator

•  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

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.
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Process Variables

• 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

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About Discharge Electrodes

• 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

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About Collecting Plates

• 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.

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Eventually
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.