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52.5 Control of Air Pollution

Air pollution can be controlled either at source or at the dispersal stage of various pollutants. Although, the problem of controlling air pollution at source is not easy, yet it can be reduced to some extent by taking steps such as:

  • use of appropriate raw materials,
  • selection of proper sites for industrial units, and
  • use of modem techniques for efficient reduction of emissions and effluents.

 

Different methods for the control of various air pollutants are being described as follows:

 

  1. 1.      Control of CO. Some of the measures which can be adopted for control of CO pollution are:

a)      Use of anti-pollution exhaust catalyst in automobiles.

b)      Modification and improvements in internal combustion engine.

c)      Use of other fuels such as CNG (Compressed natural gas), LPG (Liquefied petroleum gas) and LHG (Liquefied hydrogen gas) which are almost pollution free.

d)     Alternative power sources such as solar energy, fuel cells, etc.

 

  1. 2.      Control of CO2 CO2 level can be maintained by reducing its release from industrial units and controlling vehicular exhaust, etc. Hydrogen is being looked upon as
    pollution less future fuel.

 

  1. 3.      Control of NOx Pollution.
    1.                                 i.            Power plants and industrial units emit large amounts of NOx. Such emission can be reduced by almost 90% with the help of two step combustion process as given below:

 

  • In the first step, the fuel is burnt at high temperature in a limited supply of air. Under these conditions only a limited ‘quantity of NO is formed.
  • In the second step, the unburnt fuel is burnt at lower temperature in excess of air. Since the formation of NO requires higher temperature, only a small quantity
    of NO is formed at the lower temperature in the second step.
  1.                               ii.            Acidic scrubbing of flue gases. The large amounts of nitrogen oxides emitted from power plants and industrial units can also be removed by scrubbing the flue gases with sulphuric acid.

 

  1.                             iii.            NOx in vehicular exhaust is converted in N2 and trace amount of NH3 with the help of anti-pollution exhaust catalyst (finely divided platinum).

 

Pollutants

Major Sources

Maximum permissible exposure in ppm during an average
period of
8 hours per day

CO Incomplete combustion of carbonaceous matter in automobile engines and defective furnaces, incomplete combustion of agricultural and slash matter, volcanic eruptions, forest fires. 

50

SO2 Combustion of sulphur-bearing fuels such as coal and oil, volcanic eruptions.

5

NO, NO2 Combustion of fuel, interaction of N2 and O2 of the atmosphere at high temperature.

5

H2S Biological decay of organic matter, oil refineries

20

Cl2 Industrial units producing. paper, plastics, chlorinated hydrocarbons, dyes, chlorochernicals, etc. 

 

1

Hydrocarbons  Combustion of fuel in automobiles, refineries, anaerobic bacterial decomposition of organic matter, natural gas. 

1000

Chlorofluoro-
carbons 
Industrial units manufacturing refrigerants, fire fighting agents, their use as solvents. 

-

Pesticides  Application of pesticides and their volatilization from soil, water, treated surfaces
and industrial units producing pesticides. 

-

Particulates  Volcanic eruptions, fly ash, smelting and mining operations, smoke from incomplete combustion, dust from crushers and grinders. 

-

PbCl2 PbBr2  Addition of (C2H5)4Pb + C2H4Cl2 + C2H4Br2 to gasoline to improve its antiknock
property. 

-

CO2  Combustion of fossil fuels, bacterial decay of plants/animals. 

5000

 

  1. 4.      Control of SO2 Pollution. The process of removal of
    SO2 is known as flue gas desulphurisation (FGD). Sulphur
    dioxide from flue gases is commonly removed by using
    chemical scrubbers consisting of a slurry of CaCO3 (lime
    stone) when S02 is absorbed forming CaSO3.

CaCO3 + SO2 → CaSO3 + CO2

If the flue gases happen to contain SO3 CaSO4 is formed:

CaCO3 + SO3 → CaSO4 + CO2

 

  1. 5.      Control of Particulates. The most effective and efficient method of control of particulates is based on the use of electrostatic precipitator. It is based on the principle that particles of all sizes acquire electrical charge when exposed to a high potential electric field. The flue gases are passed through a tall chamber fitted with two electrodes. A high voltage potential difference is applied through the electrodes. The particulates settle down and get collected in a reservoir. In this way about 99% of particulate matter gets removed from flue gases.