INTRODUCTION TO AIR POLLUTION CONTROL

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INTRODUCTIONTO AIR POLLUTION CONTROL
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DIRTY AIR REMOVAL OR EMISSION CONTROL?

• The area of Eskisehir basin is about 123 km2 .
• The heavily polluted air layer is assumed to be
  610m thick on average.
• One solution to Eskisehirs problems would be to
  pump this contaminated air away.
• Suppose that we wish to pump out Eskisehir basin
  every day and that the air must be pumped 80km to
  the near suburb area.

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Map of Eskisehir
Industrial region
Population (2002) 504.724 kisi Area 123,1 km2
(EOIR 22 km2) Population density 4.998
capita.km-2 Number of districts 65 Number of
homes 175.280 Number of industrial facilities
in EOIR 198
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Calculation of the flow rate and the pipe
diameter

• Assume also that average velocity in the pipe is
  12m/s.Estimate the required pipe diameter.
• The flow rate reqired is
• QA.H/Dt(123,1km2610m)/24h(1/3600)870000m
  3/s
• and the required pipe diameter is
• D(4870000/p12m/s)1/23644m

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Acknowledgement about this solution

• This is about six times the height of the tallest
  man-made structure, and far beyond our current
  structural engineering capabilities.
• Similar calculations show that the power required
  to drive the flow exceeds the amount of
  electrical power generated in Eskisehir.

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GENERAL IDEAS IN AIR POLLUTION CONTROL

• If we have an air pollution problem there are
  three control options available
• Improve dispersion
• Tall stacks
• Intermittent control scheme
• Relocate the plant
• Reduce Emissions by Process Change, Pollution
  Prevention
• Use a Downstream Pollution Control Device

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1.1.Tall Stack

• 50 years ago
• Tall stacks to dilute the pollutants before they
  came to ground
• Dilution is the solution to pollution

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1.2.Intermittent control schemes

• Intermittent control schemes are
• predictive,
• observational
• combined predictive-observational
• To reduce emissions then, allowing emissions to
  return to normal rates at other, less critical
  times
• Short-therm emission reduction is brought about
  by
• a plant shutdown,
• fuel switching,
• production curtailment during the period
  of control.

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1.3.Relocate the plants

• It is hard to remove an existing plant, but a new
  plant can be located where it is emissions will
  have their greatest impact in non populated areas.

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2.Reduce Emissions by Process Change, Pollution
Prevention

• Water base paints for some of their oil based
  paints for some of their oil based paints and
  greatly reduce their the emissions
• Copper smelters have replaced reverberatory
  furnaces, which produce high-volume, low
  concentration SO2

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3.Use a Downstream Pollution Control Devices
(Tailpipe Control Devices)

• A contaminated gas stream and treats it remove or
  destroy enough of the contaminant to make the
  stream acceptable for discharge into the ambient
  air.

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Resource Recovery

• If the pollutant is a valuable material or a
  fuel, it may be more economical to collect and
  use it than to discard it.

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Air Pollution control strategies

• Pollution formation prevention (before)
• Pollutant formation control by the process
  (during the process)
• Pollutant reduction after the formation (after)

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The Ultimate Fate of Pollutants

• If possible, we prevent the formation of
  pollutants.
• If we can not do that, we hope to capture them
  and put to some good use

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Control Efficiency
Control Equipment
Qo Co
Q1 C1

• QoCo-Q1C1 Q1C1
• Control Efficiencyh ------------1- ------
• QoCo QoCo

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Control of Primary Particulates

• Wall Collection Devices
• Gravity Settlers
• Centrifugal (cyclone)Separators
• Electrostatic Precipitators(ESP)
• Dividing Collection Devices
• Surface filters
• Depth filters
• Scrubbers for particulate control

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Control of VOC

• Substitution
• Leakage control
• Control by adsorption
• Control by combustion
• Control by condenstaion
• Process modification

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Sulfur Oxides and Nitrogen Oxides
Similarities

• Nitrogen oxides and sulfur oxides react with
  water and oxygen in the atmosphere to form nitric
  and sulfuric acids, These two acids are the
  principal contributers to acid rain.
• Both undergo atmospheric transformations of PM10
  in urban areas.

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Sulfur Oxides and Nitrogen Oxides
Similarities

• Both are released into the atmosphere in large
  quantities, and both are regulated pollutants.
• In high concentrations sulfur oxides and nitrogen
  oxides are severe respiratory irritants.
• Both are released to the atmosphere chiefly by
  large combustion sources, of which coal-fired
  power plants are the largest emitters.

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Control of Sulfur Oxides and Nitrogen Oxides
Differences

• Sulfur oxides are formed by the sulfur
  contaminants in fuels or the unwanted sulfur in
  slfide ores. Removing all sulfur from the fuels
  would completely eliminate sulfur emissions from
  fuel combustion.
• The formation of nitrogen oxides in flames can be
  greatly reduced by manipulating the time,
  temperature, and oxygen content of the flames. No
  such reduction are possible with sulfur dioxide.
• Motor vehicles are a major emitter of nitrogen
  oxides, but a very minor source of sulfur oxides.
  If motor vehicle had zero-sulfur fuels, they
  would emit no sulfur oxides. If they had
  zero-nitrogen fuels, which they practically do,
  they would still be major contributors to the
  nitrogen oxides problem.

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Control of Sulfur Oxides and Nitrogen Oxides
Differences

• The ultimate fate of sulfur oxides removed in
  pollution control or fuel-cleaning processes is
  to be turned into CaSO4, which is an innocuous,
  low-solubility solid, and to be placed in
  landfills. There is no correspondingly cheap,
  innocuous, and insoluble salt of nitric acid. The
  ultimate fate of those nitrogen oxides is to be
  converted to gaseous nitrogen and oxygen, N2 and
  O2, and be returned to the atmosphere.
• It is relatively easy to remove SO2 from
  combustion gases by dissolving SO2 in water and
  reacting it with alkali. Aqueous SO2 quickly
  forms sulfurous acid, which reacts with the
  alkali and then is oxidized to sulfate.
  Collecting nitrogen oxides is not nearly assy
  this way because NO, the principle nitrogen oxide
  present in combustion gas streams, has a very low
  solubility in water.Unlike sulfur oxides, which
  quickly react with water to form acids, NO must
  undergo a two-step process to form an acid.

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Removal of SO2 from rich waste gases

• The SO2 concentrations in off-gases from the
  smelting of metal sulfide ores depend on which
  process is used and vary with time within the
  batch smelting cycle. However, they generally
  range from 2 to 12 SO2. Such gases can be
  economically treated in plants that produce
  sulfuric acid.

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Removal of SO2 from lean waste gases

• The most widely used procesure for controlling
  SO2 emissions from these sources is scrubbing
  with water containing finely ground limestone.
• Limestone wet scrubbers
• Dry systems

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Alternative control techniques for SO2

• Change to a lower sulfur content fuel
• Remove sulfur from the fuel
• Modify the combustion process

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Control of nitrogen oxide emissions

• Combustion modification
• Post-flame treatment

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• Catalytic converters in cars
• 2NO 2 CO ? N2 2CO2
• Two pollutants form two nonpollutants (if you can
  take CO2 as a nonpollutant)

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Control of nitrogen oxide emissions for non
combustion sources

• Production and utilization of nitric acid lead to
  emissions of NO and NO2, as do some other
  industrial and agricultural processes.
• The concentrations of NOx in the exhaust gases
  from these processes can be significantly larger
  than that from combustion sources, and most such
  industrial sources are under fairly strict
  control, so that their contribution to the
  overall NOx problem is generally small.

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Centrifugation for the control of particles
(CYCLONES)

• For particles less than 20 µm diameter,
  gravitational force is not strong enough to
  deposit them within a practical time. The force
  can be multiplied by passing the air through a
  cyclone
• Cyclones are cheap, reliable and straightforward
  gas-cleaning devices with a wide range of
  applications.

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Cut Diameter (D50 or Dcut)

• The diameter of a particle for which the
  efficiency curve has the value of 0.50, i.e. 50
  percent.
• It is a measure of the size of particles caught
  and the size passed for a particle collector

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A typical cyclone design
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Calculation of Dcut for a certain design
W width of entry duct Ne effective number of
turns of gas on spiral path Vg gas velocity at
entry ? particle density µ dynamic viscosity
of the gas.
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Example calculation

• Estimate the cut diameter for a cyclone with
• inlet width 25 cm
• Entry speed 30 m/s
• Number of turns 5
• Typical values that you can use in many cases for
  ? and µ are
• ? 2000 kg/m3
• µ 1.8 x 10-5 kg/m.s