Particulate material, found in ambient air, originates from both stationary and mobile sources. An estimate of particulate emissions into US atmosphere for 1969 has been broken down as follows:
To exercise control at source or to reduce the effects of particulate pollution, the following techniques can be used:
1. Gas-cleaning devices for removal of particulates
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2. Source relocation
3. Fuel substitution
4. Process changes
5. Good operating practices
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6. Source shutdown
7. Dispersion
Criteria for Selection:
The selection of a specific type of collector or cleaning device involves the following considerations:
(i) Particulate characteristics,
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(ii) Process factors,
(iii) Economic factors and
(iv)Collection efficiency.
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Types of Gas Changing Devices:
Terminology and classification of these devices are: mechanical devices, wet collectors, filter systems, electrostatic precipitators, and after burners.
Mechanical Devices:
These are settling chambers and cyclones.
Wet Collectors:
These can be classified according to the method of particle collection, as liquid carriage or particle conditioning. Liquid carriage consists of a moving body of liquid carrying trapped dust particles to a location outside the collector.
Separation of particles from the gas stream occurs when the particles are made to strike a liquid surface within the collector. The liquid serves to prevent particle detrainment and to carry the particles to some place of ultimate disposal.
Particle conditioning consists of increasing the effective size of small particles so that collection can be better accomplished by other collection mechanisms. The increase in size may be accomplished by condensation of water upon particles by causing the water temperature to pass through its dew point.
Filter Systems:
They consist of a woven or filter fabric through which dust laden gases are forced. The particles are collected on the fabric fiber.
Electrostatic Precipitators:
These utilize an extremely high voltage electric current to separate dust, fumes or mist from a gas stream. Four basic steps are involved: electrically charging the particles by ionization, transporting the charged particles to a collecting surface by the force exerted upon them in the electric field, neutralizing the electrically charged particles precipitated on the collecting surface, and remaining the precipitated particles from the collecting surface. Removal of particles is performed by rapping or by washing.
After Burners:
(Direct flame or catalytic) are considered mostly as a control device for gaseous pollutants.
Advantages and disadvantages of particulate collection devices are selection of these devices depends upon the degree of control designed and the economics involved.
Techniques for air pollution control are briefly compared and different processes available for sulphur dioxide removal are both the tables are considered useful for selection of pollution control devices.