Impact of Power Plants on the Environment

Environment is a concern for the modern civilization. Conversion of energy from one form to another has undesired effects and the pollutants produced in the process have to be disposed off. Pollution has become a nightmarish problem and strong national and international pressure groups have sprung up and are having a definite impact on the development of energy resources.

Government awareness ahs created increasing legislation at national and international levels. The power engineers have to be fully conversant with these.

Emphasis is being laid on energy conservation issues, curtailment of transmission losses, theft, subsidized power supplies and above all on sustainable development with appropriate technology wherever feasible. In development of power plants (thermal, hydro and nuclear), we will have to assume that no irreversible damage is caused to the environment which would affect the living  conditions of the future generations.

Irreversible damages, such as, ozone layer holes and global warming caused due to increase in carbon dioxide in the atmosphere are already going up.

Air Pollution:

The life zone of earth is known as “biosphere” or “ecosphere” which includes lithosphere (solid), atmosphere (air) and the entire hydrosphere (the world’s total water resources—rivers, lakes, oceans etc.). For the economical advancement, man has been restoring to careless and reckless exploitation of the various energy sources which is gradually affecting the ecological balance of nature and making the environment more and more unfit for supporting life. Such a deterioration of environment is termed as “pollution”.

Thus, pollution may be defined as contamination of soil, air and water with undesirable amounts of material and/or heat. The substances (including heat) causing pollution are called the pollutants. Pollution has always been around, but it has become acute during the last few decades owing to rapid technological advancement being made by man.

The major contribution of electric power generation is from thermal power plants. The thermal power plants definitely contribute to the economic growth, but they bring with it like air and water pollution, health hazards, displacement of population, resettlement, and rehabilitation of project affected people and changes in land use patterns.

The air pollution by thermal plants is caused due to emission of flue gases into air. The air pollution may be divided into two groups. Stack emission and cooling tower emissions.

I. Stack Emission:

Thermal power plants using fossil fuels emit large amount of harmful gases such as sulphur oxides, nitrogen oxides (NO₂), CO, CO₂, certain hydrocarbons and particulates. These pollutants are emitted from power plant stacks along with flue gases. The plume contents get dispersed owing to atmospheric turbulence and spreads over wide region, down-wind from power plant. Such pollutants cause deterioration of human health, damage to vegetation and structural finishes and poor visibility.

Effects can be classified based upon duration in which they occur such as acute (caused owing to short exposures to high concentration of pollutants), chronic (caused due to exposure or organisms to low concentration of pollutants for extended period of time and long term (i.e., abnormal changes occurring in environment over a period of decades).

Air Pollution Performance Standards:

Air pollution standards can be categorised as primary and secondary. Primary standards are those which protect public health and secondary standards are those which protect public welfare. The air pollution standards provide the maximum values of pollutants that are permissible.

A brief account of various pollutants, their likely impact and methods of abatements are given below:

1. Oxides of Sulphur (SO₂):

Most of sulphur present in the fossil fuel is converted into SO₂ in the combustion chamber before being emitted by the chimney. Particularly injurious to plants and animal life is the pollution of air by SO₂. The latter cause’s respiratory ailment when present in amounts of 20 mg/m³ and endanger to life in amounts of 400 to 500 mg/m³. Vegetable and plant life are most sensitive to the content of SO₂, gas in the atmosphere.

Owing to toxic effect of SO₂, the surfaces of leaves or needles get deteriorated due to destruction of their chlorophyll. SO₂ gas is gradually oxides to sulphur trioxide (SO₃) under the influence of sunlight. This in turn interacts with atmospheric moisture and forms sulphuric acid (H₂SO₄), which produces a strong acidic mist (toxic mist). This corrodes metals and destroys living tissues.

Also, it gradually destroys building materials such as marble. The droplets of H₂SO₄ in the acidic mist are so fine that easily penetrate the tissue and damage it. Effect of toxic atmospheric pollutants is associated with chronic non-specific diseases most significant is an atherosclerosis and the related coronary and degenerative cardiac diseases, chronic bronchitis, emphysema, bronchi oil asthma etc. Hydrogen sulphide (H₂S) is another air pollutant containing sulphur. It is more poisonous than even CO. The sources of H₂S in air are organic matter undergoing decomposition, sewage or some industrial operation. H₂S in air blackens lead paints.

Sulphur emission can be controlled by:

(i) Using fuel with less than 1% sulphur; generally not a feasible solution;

(ii) Using chemical reaction for removal of sulphur in the form of H₂SO₄ from combustion products by limestone scrubbers or fluidized bed combustion; and

(iii) Removing sulphur from the coal by gasification or flotation processes.

It has been noticed that the byproduct sulphur could off­set, the cost of sulphur recovery plant.

2. Oxides of Nitrogen (NO₂):

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 NO and NO₂. These are toxic and produce a sharp irritating effect, especially on the mucous, membrane of the eye. Nitrogen oxides are poorly soluble in liquids and therefore, can penetrate deep into lungs and cause injury to the alveolar epithelium and bronchi.

People living in NO₂ contaminated areas suffer from reduced respiratory function, have a higher percentage of respiratory disease and exhibit certain changes in the peripheric blood. In large concentration, NO₂ may lead to serious lung congestion and even death. NO₂ had also got adverse effects on plant life. Nitrogen oxides absorb the natural radiation both in the ultraviolet and visible region of spectrum and thus reduce the transparency of the atmosphere and provoke the formation of photochemical mist or smoke (smoke + fog).

NO in air is converted into NO₂. At the stack exit, NO₂ constitutes only 10-15% of all nitrogen oxides present in the flue gases of thermal power plants, the remaining 85-90% being mainly NO. As the smoke plume moves in the atmosphere, the concentration of NO₂ in the flue gases increases due to the photochemical actions up to 60-70% in the zone of maximum concentration. This circumstance is very important, it results in a stronger harmful effect of combustion products on nature and living organisms, since NO₂ is roughly 3-3.5 times more toxic than NO.

Emission of NO₂ can be controlled by fitting advanced technology burners which can assure more complete combustion, thereby reducing these oxides from being emitted by the stack. These can also be removed from the combustion products by absorption process by certain solvents going on to the stack.

3. Oxides of Carbon (CO, CO₂):

Carbon monoxide is very toxic pollutant but it gets converted into carbon dioxide in the open atmosphere (if available) surrounding the power station. On the other hand, CO₂ has been identified as a major cause of global warming. It is not yet a serious problem in developing countries.

4. Hydro Carbons:

During oxidation process in combustion chamber certain light weight hydrocarbons may be formed. The compounds are major source of photochemical reaction that adds to depletion of ozone layer.

5. Air Borne Particulates:

Flue gases emitted from stacks of coal-based thermal power plants contains a substantial quantity of particulates. The amount of particulates depends upon ash content in the coal. Ash content is particularly high in the Indian coal. Particulates come out of the stack in the form of fly ash. Particulates pose a significant threat to human health. When flue gases contain both particulates and SO₂ the damage becomes significant. The hazards include bronchitis, respiratory diseases and excessive death rate. In addition particulates cause poor visibility.

Concentration of pollutants can be reduced by the dispersal over a wider area by using high stacks. Precipitation can be employed for removing particles as the flue gases rise up the stack. Precipitators have a high efficiency, up to 99% for large particles, but poor performance for particles of size less than 0.1 µm in diameter.

The efficiency of precipitators is high with reasonable sulphur content in flue gases but drops for the low sulphur content coals; 99% for 3% sulphur and 83% for 0.5% sulphur. Fabric filters in the form of bag houses also have been used and are located before the flue gases enter the stack.

II. Cooling Tower Impacts:

Steam from low-pressure turbine has to be liquefied in a condenser and reduced to lowest possible temperature to maximize the thermodynamic efficiency. The best efficiency of steam cycle practically achievable is about 40%. It means that 60% of the energy in steam at the end of cycle is discharged into the environment in the form of waste heat. Fossil fuel plants discharge about 15% of the waste heat directly to atmosphere (along with flue gases) and the remaining waste heat to cooling water. A nuclear power plant discharges all the waste heat to cooling water.

Cooling water is supplied from a natural source of supply such as river, canal, sea or lake or cooling towers through screens to remove the matter that might choke the condenser tubes. It is circulated through the condenser for condensing the steam and finally discharged to the suitable position near the source of supply.

During the passage its temperature rises and in the case of cooling towers the heat must be extracted before the water is again pumped to the condenser. In the former case, the rise of temperature of water discharged into supply source (such as river, canal or sea), threatens sea, canal and river life around in sea and down-stream in river/canal. These are serious environmental objections and many times cannot be overruled and also there may be legislation against it.

In case of cooling towers, cool water is circulated around the condenser tube to remove heat from the exhaust steam so as to condense it. The circulating water gets hot in the process. It is pumped to the cooling towers and is sprayed through nozzles into a rising volume of air. Some of the water evaporates providing cooling. The latent heat of water is 2 × 10 J/kg and cooling can occur fast. Of course, the water evaporated must be made up in the system by adding fresh water from the source. These cooling towers are called the wet towers.

Cooling towers are huge hyperbolic structures measuring more than 100 m in height and employs air draft, which is produced due to shape and height of the tower, for cooling the water.

The harmful effects of cooling tower are:

1. Drift Deposits:

Sometimes the surrounding air cannot absorb moisture and as such small droplets of circulating water escape from the tower. These droplets, termed as drift, carry with them salts and chemicals. The approximate amount of drift is 0.002% of circulating water. When deposited, the salts and chemicals damage vegetation causes weathering and corrosion of metals and change in properties of soil.

2. Fog:

Fog is formed when cooling tower water added to the atmosphere gets condensed. It is formed at ground level. The conditions of wind may result in concentration of dense fog over a small area reducing the visibility in that area.

3. Visible Plumes:

Cooling towers may even produce visible plumes which are likely to pollute atmosphere and cause respiratory diseases.

Aquatic Impacts:

When a nearby river or estuary is available, its water can be used for cooling purposes in thermal power plants. Use of water from an open cooling system involves the withdrawal, use and discharge of water back to the source.

The impacts of these processes are hazardous as given below:

1. Thermal Effects:

Lot of heat is injected into biosphere from thermal power plants, through exhaust gases and waste water. The major problem is the effect of discharge of large quantities of heated waste water into natural water source. Discharged into natural water source, hot water raises the aquatic temperature and disturbs the natural ecological balance with an unwanted effect produced on flora and fauna.

The addition of heat to water reduces the water’s ability to hold dissolved gases, including dissolved oxygen vital to aquatic life. When the temperature of water exceeds 35°C, the dissolved oxygen content is too low to support life. At low temperatures, however, the aquatic growth is usually increased by the warm water and the plants and fishes grow at a faster rate. In fact, the warm water discharged from a power plant is usually very popular place to fish, particularly during cold weather.

Thermal power plants are sources of the following types of waste water:

(i) Waste water of water treatment plants and condensate cleaners.

(ii) Waste water contaminated with petroleum products.

(iii) Cooling water which mainly causes thermal contamina­tion of water basins or sources.

(iv) Water used for washing of external surfaces of fuel oil fired steam generators and peak load heating boilers.

(v) Used solutions after chemical cleaning and preservation of thermal equipment.

(vi) Water from hydraulic ash-disposal systems of solid-fuel- fired thermal power plants.

(vii) Used water after hydraulic cleaning of fuel conveying systems.

For minimizing thermal effects, the discharge designs should include optimal mixing and offshore, submerged heat diffusers.

2. Chemical Contamination:

Water taken from rivers and lakes contains a number of substances which cause corrosion and scaling in condenser and for removal of these impurities some chemicals are used. These chemicals are discharged along with hot water into the natural water source and produces chemical contamination. Owing to increased temperature, the metabolic rate of organism is enhanced and the chemical reactions of pollutants are speeded up. Chemical residuals are harmful to aquatic life. The other chemicals may also be toxic.

3. Entrapment:

Hydraulic forces in the stream prior to flow of streams through the screens cause entrapment. Such forces may restrict the movement of aquatic life and result in death of some species owing to starvation, exhaustion or asphyxiation. Entrapment effect can be reduced by proper design, location and capacity of intake structures.

4. Entrainment:

Eggs and newly hatched fish are sometimes entrained into the power station along with intake water. While passing through the cooling system these organisms are subjected to thermal, mechanical and chemical stresses. These stresses may be lethal.

Miscellaneous Effects:

Coal power plants discharge water polluted by chemicals and impurities back into natural water sources, causing significant environmental damage.

Coal preparation uses large quantities of water and chemicals to separate impurities from mined coal, washing away the waste in a sludge known as slurry.

A common chemical from discharged water is copper, which can reach from water condenser piping and end up in discharge water, sometimes at toxic level. In addition, water discharged from waste treatment has been shown to have high concentrations of arsenic, cadmium, chromium, lead, selenium, sulphates and boron.

Chlorine and its by-products are present in the discharged water plume and can be toxic to aquatic life, even at low concentrations. High water temperatures can magnify the damaging impacts of chlorine.

While burning in power plants, coal releases mercury into atmosphere. This mercury falls to earth in rain, running into lakes, rivers and stream. Bacteria in the water transform this mercury into toxic methyl mercury. When fish consume this bacterium, they become contaminated. Fish that eat other contaminated fish end up with even higher levels of toxic mercury in their flesh. Humans can be contaminated with this methyl-mercury by eating contaminated fish.

Hydroelectric Plant Impacts:

Regarded usually clean and harmless, the hydroelectric power plants have following environmental impacts:

1. Since hydro power plants require huge amount of water storage, large dams have to be constructed. This leads to displacement of inhabitants of the area. As such historical and ecological nature of area gets disturbed.

2. The construction of dams for hydroelectric plants slows down the flow of rivers and thus causes the pollution of water, the growth of deleterious blue-green algae, encourages the reproduction of epidemic-carrying bacteria, checks, the flood waters with the result that water meadows cease to exist and sometimes salinization of soil occurs.

3. In addition to environmental degradation and disturbing ecological balance, the hydro plants disturb the demographic balance also. Large numbers of workers required for construction are brought into the area and they disturb the very nature of local population.

4. Large acquisition means destruction of forest cover which is harmful for environment.

Social and Economic Impacts:

Power plants create a number of social and economic impacts, which are mostly because of competition for land and water, availability of community services and cultural threats. There may be industrial growth around the power plant and this may significantly change the features of the area and population.

The social and economic impacts are summarized as below:

1. Demographic Impacts:

There may be a rapid influx of large construction force in the area. A majority of these workers tend to remain there even after the completion of construction work of the power plant. This influx may result in a change in ethnic composition, age structure, sex ratio and educational status of the population. The area may get unbanised and its population density may increase.

2. Land Use Impacts:

Land use pattern is affected due to plant construction and changing social structure. Agricultural land gets converted into residential and commercial property.

3. Housing Impacts:

There may be changes in residency duration and mobility, enhanced housing demand, increase in housing values and rents and possibility of out migration of existing residents.

4. Public Services:

Because of increase in the area population, there may be an increase in demand for water supply, waste disposal, sewerage, educational, medical and transport facilities, and police, fire protection etc.

5. Community Structure:

The traditional rural commu­nity values may be replaced by urban values and preferences. Number and type of community organisations may change. An accelerated way of life may be experienced in area.

6. Psychological Impacts:

There may be a change in type and number of crimes. There may be increase in social problems, such as, alcoholism, divorce and suicide.

7. Economic Impacts:

Because of plant construction, job opportunities and employment rate of the area will be certainly increased. There may be a change in business and industrial composition of the community. There will be an upward trend in the family incomes, retail prices, taxes and community’s gross product.

8. Recreational and Cultural Impacts:

Because of increase in population of the area there may be an increase in demand of parks, playgrounds, swimming pools, theatres and other recreational and social centres.

9. Political Impacts:

The strengths of local political parties and the extent of citizen participation in politics may change owing to additional large population of different social, professional and economic status. The appearance of trade union activities in the area may completely erode the base of the existing political parties of the area.