Techniques to control gaseous and particulate pollution - PowerPoint PPT Presentation

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Techniques to control gaseous and particulate pollution


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Title: Techniques to control gaseous and particulate pollution

Techniques to control Gaseous
Particulate Pollution
Prepared By Shristi Soni
Our Focus-
  • Introduction
  • Methods to control gaseous pollutant
  • Control devices to control gaseous pollution
  • Control devices for Particulate Pollution
  • Conclusion

  • A substance in the air which causes harm to
    humans and environment
  • is known as pollutant.
  • Pollutants such as sulphur dioxide, hydrogen
    sulphide, and nitrogen dioxide
  • combine with moisture in the air to form acids
    that attack and damage library
  • material are known as gaseous pollutants.
  • Smoking, cooking, and off-gassing from unstable
    materials (cellulose nitrate film,
  • paint finishes, fire-retardant coatings, and
    adhesives) may also produce
  • harmful gaseous pollutants.
  • Particulate pollutants, such as soot, dirt, and
    dust abrade, soil, and disfigure
  • materials. Dust and dirt that have absorbed
    gaseous pollutants from the air
  • become sites for harmful chemical reactions
    when they settle on library
  • material. The pollution caused by this is known
    is particulate pollution.

Gaseous Substance
Gases and vapours can be categorized under
gaseous pollutants.
Cement Iron Oxide Sulphuric acid SO2 Gasoline
Coal Zinc Oxide Chromic acid NO2 Trichloroethylene
Ores Lead Oxide Oil CO Perchloroethyl
Grains ------ Grease(From cooking and smoking of metals H2S Toulene
Method to control gaseous pollutants
  • adsorption
  • absorption
  • Reduction system
  • condensation

  • Adsorption

Adsorption involves the interaction between
gaseous contaminants and its surface of a solid
adsorbent. In this phenomenon molecules from a
gas or liquid will be attached in a physical way
to a surface. The binding to the surface is
usually weak and reversible. The most common
industrial adsorbents are activated carbon,
silica gel, and alumina, because they have
enormous surface areas per unit
weight. Activated carbon is the universal
standard for purification and removal of trace
organic contaminants from liquid and vapour
Carbon adsorption uses activated carbon to
control and/or recover gaseous pollutant
emissions. In carbon adsorption, the gas is
attracted and adheres to the porous surface of
the activated carbon.

Gaseous contaminants that are soluble in aqueous
liquids can be removed in absorbers. This is one
of the main mechanisms used for the removal of
acid gas compounds (e.g., sulphur dioxide,
hydrogen chloride, and hydrogen fluoride) and
water soluble organic compounds (e.g., alcohols,
aldehydes, organic acids).The contaminant gas or
vapour is absorbed from the gas stream as it
comes into contact with the liquid. Biological
treatment systems are termed either biological
oxidation or biofilter systems. Regardless of the
term, the fundamental processes involved is the
collection of contaminants on the surface of a
media that contains viable microorganisms. The
contaminant is metabolized by the organism and
carbon oxide and water vapour is re-emitted. All
absorption processes operate best when the gas
and liquid temperatures are low. Gas and vapour
phase contaminants are most soluble under cold
  • Reduction systems

Reduction systems are used primarily for the
destruction of NOx compounds emitted from
combustion processes. These systems include
selective noncatalytic reduction systems (SNCR)
and selective catalytic reduction systems
(SCR). In both types of systems, a chemically
reduced form of nitrogen is injected into the gas
stream to react with the oxidized nitrogen
compounds, namely NO and NO2. The
reactions between the reduced and oxidized forms
of nitrogen result in molecular N2, the major
constituent of clean air SNCR and SCR systems
are believed to be effective for NO and NO2. It
is important to note that in the majority of
combustion systems, 90 to 95 of the NOX in the
gas stream is present as NO, and the remaining is
present as NO2.
  • condensation

Condensation systems are used exclusively for the
recovery of organic compounds present at moderate
to high concentrations in industrial process
effluent gas steams. The most common type of
condensers are those using cooling water in
direct contact or indirect contact vessels.
Refrigeration and cryogenic systems are used
primarily for the high efficiency recovery of
high value contaminants. Condensation systems
reduce the contaminant concentration in the gas
stream to a concentration equivalent to the vapor
pressure of the compound at the operating
temperature of the condenser. Most condensers
operate on systems with only a single contaminant
compound or a mixture of compounds that does not
usually require separation (i.e. gasoline).
Condensation process
Control devices for gaseous pollutants
  • Cyclones operate to collect relatively large size
    particulate matter from a gaseous stream through
    the use of centrifugal forces. Dust laden gas is
    made to rotate in a decreasing diameter pathway
    forcing solids to the outer edge of the gas
    stream for deposition into the bottom of the
    cyclone. Efficiencies of 90 in particle sizes of
    10 microns or greater are possible.
  • Performance Collection Efficiency
  • Linear increases with particle density, gas
    stream velocity, and rotational passes
  • Linear decrease with fluid viscosity
  • Exponential increase with particle diameter

Cyclone process
  • Incineration involves the high efficiency
    combustion of certain solid, liquid, or gaseous
    wastes. The reactions may be self sustaining
    based on the combustibility of the waste or
    require the addition of fuels.
  • They may be batch operations or continuous as
    with flares used to burn off methane from
    landfills and, they may incorporate secondary
    control methods and operate at efficiency levels
    of 99.99, as with hazardous waste incinerators.
    Combustion temperatures, contact time, and mass
    transfer are the major parameters affecting
    incineration performance.
  • It has, High destruction efficiencies possible
  • Variations in fuel content of waste
  • Transition among wastes require significant
    control changes
  • Good for gases, liquids, and solids

Incineration Process
  • Electrostatic precipitators

An electrostatic precipitator (ESP) is a particle
control device that uses electrical forces to
move the particles out of the flowing gas stream
and onto collector plates. The ESP places
electrical charges on the particles, causing them
to be attracted to oppositely charged metal
plates located in the precipitator. The
particles are removed from the plates by
"rapping" and collected in a hopper located below
the unit. The removal efficiencies for ESPs are
highly variable however, for very small
particles alone, the removal efficiency is about
99 percent.
Electrostatic Precipitator
  • Fabric Filters

Fabric filters, or baghouses, remove dust from a
gas stream by passing the stream through a porous
fabric. The fabric filter is efficient at
removing fine particles and can exceed
efficiencies of 99 percent in most applications.
The selection of the fiber material and fabric
construction is important to baghouse
performance. The fiber material from which the
fabric is made must have adequate strength
characteristics at the maximum gas temperature
expected and adequate chemical compatibility with
both the gas and the collected dust. One
disadvantage of the fabric filter is that
high-temperature gases often have to be cooled
before contacting the filter medium.
Fabric Filters
  • Venturi scrubbers

Venturi scrubbers use a liquid stream to remove
solid particles. In the venturi scrubber, gas
laden with particulate matter passes through a
short tube with flared ends and a constricted
middle. This constriction causes the gas stream
to speed up when the pressure is increased. A
water spray is directed into the gas stream
either prior to or at the constriction in the
tube. The difference in velocity and pressure
resulting from the constriction causes the
particles and water to mix and combine. The
reduced velocity at the expanded section of the
throat allows the droplets of water containing
the particles to drop out of the gas stream.
Venturi scrubbers are effective in removing small
particles, with removal efficiencies of up to 99
percent. One drawback of this device, however,
is the production of wastewater.
Venturi scrubbers
  • conclusion

The best control measure, of course, is
prevention. However, as long as there are fossil
fuel emissions from our coal-burning factories
and gas-burning automobiles, there will be air
pollution. The key to easing future impacts is
control. The sources of air pollution are many,
although most authorities identify sulphur
dioxide, nitrogen dioxide, ozone and particulate
matter as the major pollutants. Identification of
sources of pollution offers opportunities for
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