The environmental pollution by thermal power plants using fossil fuels poses a serious health hazard to modern civilization. Air pollution by thermal plants is a contributing factor in the cause of various respiratory diseases and lung cancer and causes significant damage to the property in addition to causing annoyance to the public.

The thermal power plants burning conventional fuels (coal, oil or gas) contribute to air pollution in a large measure.

The combustion elements of the fuels are converted to gaseous products and non-combustible elements as ash. The common gaseous products of interest are sulphur dioxide, nitrogen oxide, carbon dioxide and carbon monoxide, and large quantities of particulate materials as fly ash, carbon particles, silica, alumina and iron oxide.

The energy industries are one of the largest sources of environmental pollution. A 350 MW coalfired station emits about 75 tons of SO2, 16 tons of nitrogen oxide, and 500 tons ash per day if no safeguards are adopted. All steam generating plants also discharge nearly 60% of heat produced back to the atmosphere irrespective of the fuel used.

Due to large emissions from the thermal power plants, air pollution has become an international problem.

This problem is mainly faced by 11 countries in the world, which share 80% of the world’s fossil-fired generating capacity. Emissions from their power plants have grown to point where we and all of them now must think for controlling the pollution contributing to a common atmosphere.

Many countries have unique air pollution problems. These are due to fuel characteristics, unfavorable topographical conditions, concentration of power plants in limited area and high population densities.

The production capacities of 11 countries, which share 80% world-electric generation, are given in tables 1 and 2. The major pollutants given off by fossil fuel combustion are particulates, SO2 and other gases and it will be sufficient to discuss about these pollutants.

The amount of particulates and gases emitted per year in U.S.A. are listed in tables, 3 and 4, which is sufficient to understand the seriousness of the problem.

It is clear from table 1 that U.S.A. produces 39% of the world’s electric power and emits nearly 20 million tons of SO2 per year. Therefore, the major problems with the thermal power plants are to remove particulates and SO2, which cause health hazards. The principal air pollutant from oil-fired power plants is SO2. the oil fuels used in world power plants contain the sulphur in the range of 2 to 5%. However, the high energy to weight ratio of oil reduces its net contribution of SO2 with respect to coal. On the emission basis, 2.7% sulphur coal and 4% sulphur oil are equivalent.

The sulphur dioxide is of immediate concern among the different pollutants. The SO2 is oxidized slowly in the atmosphere to sulphur trioxide and it further combines with water to form H2SO4. the H2SO4 is highly irritating to the upper respiratory tract. The H2SO4 even in minute quantities can penetrate deep into lungs and can cause damage. The acid mist can also cause extensive damage to vegetation and forestry.

Table 1

Electric and industrial power generation of 11 countries

As per the data published by U.S.A. Public Health Service, the permissible maximum concentrations of SO2 at ground level are 0.05 to 0.08 ppm for 24 hours, 0.12-0.2 ppm for 1 hour and 0.1-0.5 ppm for 5 minutes respectively. The maximum expected ground level concentration of SO2 should be such that it does not exceed the level, which might result in injury to the population and agriculture.

Table 2

Total fuel consumption for all purposes in 11 countries

The primary source of SO2 in the atmosphere is the combustion of bituminous coal and residual oil fuel. The sulphur content in the U.S. coal varies from 0.5 to 6% whereas that in Indian coals varies from 0.2 to 8%. Presently, the only effective control of SO2 emission is the use of fuels (coal and oil) containing lower percentage of sulphur. The use of fuel oil containing more than 2.5% sulphur by weight is now prohibited in some countries. For the use of present coal containing high sulphur percentage requires either removal of SO2 from exhaust gases or desulphurisation of fuel before use.

The sulphur dioxide is main pollutant from thermal power plants. 65% of total SO2 emitted into atmosphere was only from thermal plants in U.S.A. the use of coal is going to increase throughout the world in coming few decades so control of SO2 is going to be more difficult problem.

Another gas pollutant is nitric oxide (NO), which is not a very toxic gas by itself and does not play any harmful role in the atmosphere. But it oxidizes to nitrogen dioxide (NO2) as a result of series of chain reaction, which is highly irritant to the lung. Nitrogen oxide further decomposes into nitric oxide and atomic oxygen in the presence of sunlight. The formed atomic oxygen combines with the atmospheric oxygen to form O3, which is highly irritating and responsible for many health hazards.

Table 3

Total U.S.A. air pollution by source

Table 4

Emission of different pollutants in U.S.A. per year

The thermal plants contribute substantially to CO2 emissions. The concentration of CO2 could have long-range effects on the welfare of the world population. The CO2 is added in the atmosphere at a rate of 6000 million tons per year. This pollutant may modify the heat balance of the atmosphere and could cause uncontrolled change in climate.

The burning of fossil fuels is believed to be largely responsible for 10% increase in CO2 in the atmosphere in the present century. The global consequences of CO2 are not fully understood but there is concern that its accumulation in the atmosphere, as use of fossil fuels continues to grow without adequate environmental controls, may have a green-house effect, causing irreversible and dangerous changes in the world climate severe climate changes could turn fertile land into deserts or melt the polar ice cap, raising the global sea level 5 to 10 m. the implications and control of CO2 need priority study. These samples of environmental issues as yet unresolved, however, indicate the uncertainties in trying to evaluate acceptability and the future market of coal. Like many other forms of energy, it has high social costs, which cannot be measured in economic terms.

The CO is injurious to the human health as it combines with haemoglobin in the red blood corpuscles and thus interferes with their normal function of supplying oxygen to the blood tissues. The 5% of the body’s haemoglobin becomes inactive if body is exposed for one hour to a concentration of 120 ppm. This leads to dizziness, headache and lassitude to the human beings and animals.

The quantities of pollutants emitted by 400 MW plant for different fossil fuels are listed in table 5.

As mentioned earlier, the other pollutants from thermal plants using coal or powdered coal as fuel are carbon, silica, iron oxide and large quantities of fly ash. The best type 400 MW plant emits 500 tons of fly ash per day, whereas a plant using poor coal could emit 50 tons of fly ash per hour. Nearly 3 million tons of fly ash are emitted per year by 30 thermal plants in India and it is expected that this will increase to 8 million tons per year by the end of this decade.

The quantity of fly ash emitted depends upon the ash content of the coal, the type of burner and the equipment used to control the fly ash. The ash content of U.S. coal lies between 7 to 17% whereas the ash content of Indian coal varies from 3 to 42%.

The use of air cleaning equipment has reduced the particulate emission considerably from coal-fired plants. The efficiency of 95% of cyclone collector combined with electrostatic precipitator is very common nowadays. No doubt, this reduction in fly ash emission increases the cost of the plant. The cleaning equipment required for 400 MW plant costs nearly 1.5 crores of rupees and running cost of the cleaning equipment is 0.5 to 0.6 mils/kw-hr, which is 10% of the plant operating costs.