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, it can be reduced to some extent by taking steps such as:

• Use of modern techniques for efficient reduction of emissions and effluents.

• Use of appropriate raw materials,

• Selection of proper sites for industrial units.

• Create laws/legislation to force the reduction of air pollution.

Control of CO

Some measures that can be adopted for control of CO pollution are:

• Automobiles can be fitted with anti-pollution exhaust catalyst to treat exhaust before discharging it into the atmosphere. When exhaust gases are passed over catalysts with copper supported on ceria or palladium/ platinum supported on alumina, it brings about oxidation of CO to CO₂.

• Modification and improvements in internal combustion engine: Strict standards of emission levels enforced in India under law, have forced car manufacturers to modify engines to comply with combustion specifications conforming to Euro-II pollution standards.

• Use of alternate fuels such as CNG (Compressed natural gas), LPG (Liquefied petroleum gas) and LHG (Liquefied hydrogen gas) in vehicles. These fuels are almost pollution free.

• Use of alternative power sources such as solar energy, fuel cells, etc.

Control of CO

₂

• The CO₂ level can be maintained in industrial units by encouraging better air quality standards and guidelines for achieving them and/ or enforcing severe CO₂ pollution emission standards.

• Controlling vehicular exhaust, can be done by more efficient engines or using different fuels etc. Hydrogen is being looked upon as pollution less future fuel.

• By pollution legislation.

• Another practical way is to increase the forest cover. The larger the number of trees, the greater would be the extent of photosynthesis that uses up CO₂ from the atmosphere.

• At an individual level people can try to use less electricity (reducing the load on thermal power plants), more public transportation, bicycling or walking and support environmental organizations.

Control of NO

_x Pollution

• Power plants and industrial units emit large amounts of NO_x. Such emission can be reduced by almost 90% with the help of a two step combustion process as given below:

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.

Second step

The un-burnt 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.

• Industrially, at factories and power plants, some pollutants can be removed using scrubbers in smoke stacks. Large amounts of nitrogen oxides emitted from power plants and industrial units can also be removed by scrubbing the flue gases with sulphuric acid. SO_x and NO_x present in the flue gases first react to give NO and H₂SO₄·

NO₂ + SO₂ + H₂O

H₂SO₄ + NO

NO and NO₂ then react to form N₂O₃ which is scrubbed by sulphuric acid to form H(NO)SO₄:

NO + NO₂ N₂O₃

N₂O₃ + 2H₂SO₄ 2H(NO)SO₄ + H₂O

This results in the removal of NO₂ as well as SO₂ pollutants. The cleaned flue gases are then released into the atmosphere. The compound H(NO)SO₄ is decomposed in a separate chamber by heating where the following reaction takes place :

2H(NO)SO₄ + ½O₂ + H₂O 2H₂SO₄ + 2NO₂

The liberated H₂SO₄ is recycled into the scrubbing chamber.

• NO_x is a vehicular exhaust and is converted to N₂ and trace amount of NH₃, with the help of anti-pollution exhaust catalyst such as finely divided platinum supported on ceria or alumina.

Control of SO

₂ pollution

Slurry of CaCO₃ (lime stone) is used as chemical scrubber for removing sulphur dioxide from flue gases. Removal of SO₂ is known as flue gas desulphurization (FGD). In this process SO₂ is absorbed forming CaSO₃.

CaCO₃ + SO₂ CaSO₃ + CO₂

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

CaCO₃ + SO₃ CaSO₄ + CO₂

The above method is quite economical, but involves the disposal of huge amounts of CaSO₃ and CaSO₄ formed during scrubbing.

Control of particulates

To control the outflow of particulates at their industrial source of origin is the most effective and efficient method of control. This is achieved by the use of electrostatic precipitator. It is based on the principle that all particles acquire an electrical charge when exposed to a high potential electric field.

Fuel gases are passed through a tall chamber fitted with two electrodes.

When a high voltage potential difference is applied through the electrodes the air inside the chamber gets ionized into positively charged ions and free electrons. Free electrons get attached to the particles present in flue gases, which then become negatively charged. These particles are attracted towards positive electrode and settle down there. Giving a vibratory motion to the electrodes dislodges the negatively charged particulates and they get collected in a reservoir. In this way about 99% of particulate matter is removed from flue gases.