Organic fuel burned at thermal power stations contains harmful impurities, which are ejected into the environment as gaseous and solid components of combustion products and adversely affect the atmosphere and water and the whole biosphere. The atmosphere is also contaminated by waste gases of various industrial plants, exhaust gases of transport vehicles, abuse of agricultural chemicals and other contamination sources which are due to human activities.

In India, by the turn of the century, about 70,000 MW of thermal power will be generated using high ash content coal. The environmental pollution due to thermal power generation will increase beyond acceptable limits unless stringent measures based on Environmental Protection Act, 1986 are strictly enforced.

Causes of Thermal Pollution

The thermal power stations use thousands of tons of low quality (high ash content) coal per day. These power stations and other industries have completely changed the nature and socio­economic order of the region.

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The natural rocky hills are disappearing due to heavy quarrying. Tall chimneys and gigantic machines emit a cloud of dust with fly ash and smoke containing high level of acid forming oxides of sulphur and toxic fluorides and huge quantity of highly toxic cement particles which find easy foothold on plant leaves and human lungs.

The products of complete burning of fuel in thermal power plants mainly consist of carbon dioxide, water molecules, nitrogen, sulphur dioxide and S03 anhydride (sulphur trioxide) and ash. At high temperatures existing in the flame core of high power boilers, the nitrogen of fuel and air may partially be oxidized to form nitrogen oxides. Nitrogen dioxide dissociates in the presence of sunlight to nitric oxide and atomic oxygen. The latter combines with molecular oxygen to reform ozone (03).

The concentration of ozone in polluted atmosphere often goes up 10 to 20 times the natural ozone level (0.02-0.03 ppm).

With incomplete combustion of fuel in furnaces, carbon monoxide hydrocarbons (CH2, C2H4) etc. and some carcinogenic substances are additionally formed. Among many carcinogenic substances, of highest importance, as regards their intensity of action, are polycyclic aromatic hydrocarbons, in particular, benzapyrene. The highest quantity of benzapyrene is formed under the conditions when air is deficient and complete combustion cannot occur.

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Nitrogen oxides, even in low concentrations, can irritate respiratory organs, destroy equipme and materials, and promote the formation of smog and impair visibility. Sulphur is present iii solid fuels in three forms: as inclusions of pyrite FeS2; Sulphur in molecules of the organic ma of the fuel; and sulphate sulphur. Upon combustion of a fuel, almost all the sulphur contained in it passes to flue gases in the form of S02 and S03 oxides.

Pollutants in the effluents of thermal power stations and natural admixtures undergo complex processes of transformations and reactions. Deposited on the ground, they are washed down by atmospheric precipitates and reach the solid and water basins.

The hot fuel gases can be effectively removed in a powerful upward flow through high stacks and ejected into the atmosphere at a substantial height where they will be mixed with higher layers of the atmosphere. Before ejecting the fuel gases and ash into the atmosphere, however, the modern state of gas purifying techniques makes it possible to reduce appreciably the concentration of impurities in waste gases.

Electrostatic precipitators are employed to ensure a high degree of gas cleaning. With any method of fuel and waste gas purification, however, a certain quantity of impurities remains in the effluent gases. If harmful impurities have been dispersed in the atmosphere to concentrations not exceeding the scientifically found norms, their presence in the atmosphere has practically no effect on the living nature.

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Toxic substances both of natural and industrial (antropogenic) origin can produce deleterious effects on the whole complex of biosphere.

The biosphere comprises of the atmospheric layer near the earth’s surface and the upper layers of the soil and water basins. Though the natural sources of atmospheric pollution are sometimes more powerful than the anthropogenic ones, the latter are of great importance since they are responsible for atmospheric pollution in densely populated areas.

The main contributor to atmospheric pollution is the combustion of mineral fuels, especially the thermal power plants. The relative contribution of a particular industry to atmospheric pollution may vary with the rate of growth of the industry.