CONTROL OF PARTICULATE POLLUTANTS

The equipments used are:

• Gravity settling chambers

• Centrifugal collector including cyclones

• Wet Scrubbers

• Electrostatic precipitators

• Fabric filters

Gravity Settling Chamber

• It consists of a large expansion chamber inwhich dust is separated from the gas byreducing the velocity of the gas. Due to this,the heavy dust particles settle down undergravity.

• In order to reduce the size of the chamber,the gas velocity is kept between 0.5 m/s (toobtain good results) to 3 m/s (to prevent re-entrainment of settled particles).

• Hence such chambers are capable ofremoving only the large size particlesbetween 40 to 50 u.

• Usual n is about 50%.

• Settling chambers are used widely for theremoval of large solid particulate from foodand metallurgical industries, furnaces andkilns etc.

Centrifugal Separator

• Cyclones use centrifugal force to separateparticulate out of the airstream. There are nomoving parts inside a cyclone body and nofilter media.

• Air and particulate enter the inlet tangentiallywith a high velocity along the inner wall at thetop of the cylindrical cyclone body.

• The gas spirals downwards to the bottom ofthe cone.

• Due to their inertia, the dust particles tend toconcentrate on the surface of cyclone wallfrom where they are led to the receiver. Thisclean air is exhausted through an outletchamber that starts in the center of thecyclone and exits out the top.

• Cyclones are cheaper in cost and are bestsuited to dry dust particles of size 10 – 40 u.

Wet Scrubbers

• A scrubber is a device which utilizes aliquid to assist in the removal ofparticulates from the carrier gasstream.

• The scrubber transfers suspendedparticulate matter in the gas to thescrubbing liquid which can be readilyremoved at the bottom.

• The advantage offered are that (i) thehot gases can be cooled of, and (ii) thecorrosive gases can be removed.

• Efficiency of the wet scrubbers are inthe range of 80-99%.

ELECTROSTATIC PRECIPITATORS

• In an electrostatic precipitator the flue gas is made to pass through ahighly ionized zone, where the particles get electrically charged andare separated out from the gas with the help of powerful electricfield.

• 99% n is obtained and even small particles can be removed.

• They are widely used in thermal power plants, pulp and paper mills,mining and metallurgical industries, iron and steel plants andchemical industries.

FABRIC FILTER

• In this system flue gas is allowed topass through a woven or feltedfabric, which filters out theparticulate matter and allows thegas to pass.

• Larger particles are retained on thefabric initially and as the collectionbecomes more even smallerparticles are removed.

• The fabric is shaken intermittentlyto collect the dust at the bottom.

CONTROL OF GASEOUS POLLUTANTS

Emission of gaseous pollutants can be controlled by :

•Absorption

•Adsorption

•Combustion

Absorption :

• This involves the transfer of the pollutants from the gas phase to theliquid phase by dissolution.

• The effectiveness of the device depends upon the solvent through whichgas is passed. When water is used the removal is restricted to ammonia,chlorine and sulphur dioxide

CONTROL OF GASEOUS POLLUTANTS (Contd.)

Adsorption :

• This involves passage of dirty gases through bed of such adsorbentmaterials where the pollutants are effectively caught and removed.

• Activated carbon beds can very effectively catch hydrocarbons, H2Sand SO2.

Combustion or Incineration :

• This is used when pollutants in carrier gas are oxidizable to an inertgas. Pollutants like hydrocarbons and carbon monoxide can be easilyburnt, and removed in such equipments.

CONTROL OF POLLUTION FROM AUTOMOBILES• Automobiles chiefly emit CO, HC and Nox.

• The contents of these pollutants usually vary as:

• Carbon monoxide – 0.5-6.4%. CO is a poison for any air-breathing animal.

• HYDROCORBONS – 300-1000 ppm. HC produce smog.

• Nitrogen oxides – 500- 3000 ppm. Nox lead to smog and acid rain

• Depending upon the type and condition of engine of the automobile.

CONTROL OF POLLUTION FROM AUTOMOBILES (Contd.)

• A catalytic converter is a device that uses a catalyst to convert threeharmful compounds in car exhaust (CO, HC, Nox) into harmlesscompounds.

• In a catalytic converter, the catalyst (in the form of platinum andpalladium) is coated onto a ceramic honeycomb or ceramic beadsthat are housed in a muffler-like package attached to the exhaustpipe.

• The catalyst helps to convert carbon monoxide into carbon dioxide. Itconverts the hydrocarbons into carbon dioxide and water. It alsoconverts the nitrogen oxides back into nitrogen and oxygen