pollution & control

Pollution & control

Man has been using the natural energy resources without care of environment for his economical advancement of power.

This is gradually affecting the ecological balance of nature and making the environment more and more until for supporting life.

Pollution & control

Pollution can be defined as contamination of soil, air and water with undesirable amount of matter and or heat. The matter and or heat causing pollution are called pollutants.

Pollutants from thermal power plant

The pollutants from thermal power plant includes unwanted gas, ash, dust, heat and contaminated waste waters. The gases coming from thermal power plants through stack causes air pollution in large measure.

This is due to combustion of conventional hot condenser water and water discharged into the rivers carrying the ash of the plant.


The pollutants or emission from thermal power plants can be classified as follows:

Gaseous emission: SO2, Nox, CO2, CO, H2S, and many other.

Particulate emission: smoke, dust, fumes, fly-ash, cinders

Solid waste emission: ash

Thermal pollution: waste heat.


During stack emission, SO2 and Nox are realeasedwhich subsequently get oxidised to sulphate(S04) and nitrate(NO3). In the presence of water vapoursin the atmosphere these are changed to sulphuricacid and nitric acid.

Control of air pollution due to thermal power plant

Control of sulphur oxide

Control of nitrogen oxide

Control of particulates.

Control of pollutants(SO2)

Control of SO2:Methods used to control SO2 is either by removing or reducing the sulphur content from fossil fuel or to remove SO2 from flues gases.

Desulphurisation of fuels: desulphurisation of oil can be done by chemical process using hydrogen gas. In this tratment hydrogen gas combines with sulpher of fuel to form hydrogen sulphide (H2S) which is then isolated by absorption in suitable chemicals.

Froth floatation process is used to reduce sulpher in coal. In this process coal is suspended in water through which air is bubbled. These air bubbles tend to attack coal particles and other mineral waste including sulphur falls at the bottom and discharged.

Control of pollutants(SO2)

In other method, crushed coal is passed through a magnetic field which removes pyrite (FeS2) from coal. This method of sulphur removal is called magnetic separation.

Removal of SO2 from flue gases:

The SO2 from flue gases can be removed by using wet scrubbers and electrostatic precipitators.

Another method used for removal of SO2 is by catalysticoxidation. It is used to produce H2SO4 from dilute SO2 in flue gases. Sulphuric acid is seperated from flue gas before discharged through the chimney.

Controls of SPM

The particulate matter from fuel may be in the form of dust (up to 1 micron), ash (less than 100 microns) or cinders (more than 100 microns).

Methods of removing dust and ash from the flue gases can be done by fabric filters, hydraulic and pneumatic ash handling systems, wet and electrostatic scrubbers for dust collection.

Control of Nox emissions

Nox formed increase with the increase in combustion temperatures above 1000° C. thus to reduce NOxemissions either the fuel used should have very low nitrogen or the combustion temperatures need to be controlled to avoid the fixation of nitrogen of atmospheric air. The methods used for reducing Nox are:

Reduction of temperature in combustion zone: it is achieved by decreasing the temperature of combustion zone by injection of steam in gas turbine power plants or by recirculation of furanace gas into the combustion zone in thermal power plants.

Control of Nox emissions

Reduction in residence time in high temperature combustion zone: it is promising method since it reduces not only the formation of Nox but produces less amount of CO2, SO2 and hydrocarbons.

Increase in equivalence ratio of 1.6 to 1.8 in combustion zone so it will decrease local max temp as well as increase the time of burning of the fuel.

Using fluidized bed combustion system for coal based thermal power plants:

The combustion takes place in the range of 850° C- 900°C due to which formation of Nox and SO2 (due to dolomite) is considerably reduced.

Effect of different pollutants on human health

Carbon monoxide(CO): CO is produced by incomplete combustion of fuel, generally caused by insufficient oxygen or by dissociation of gases at high temperature.

CO causes headache, nausa and breathing problems. This gas has strong affinity to combine with haemoglobin in the blood and reduces its oxygen carrying capacity.


Hydrocarbons(HC): HC is raw/unburnt fuel escaping to atmosphere. Several types of hydrocarbon are contained in oil fuel.

Major sources of HC emissions are I.C engine, gas turbine, oil refineries etc. these emissions are minimum with chemically correct mixtures and are more pronounced with rich and lean mixtures.

HC cause irritation in respiratory system.


Oxides of nitrogen (Nox): Nox are more pronounced when N2 and O2 combine at high temperatures of combustion ranging 1000° C upwards. These are formed due to complete combustion of the fuel. Their permissible limit is 0.05 to 0.1 ppm.

Nox emissions irritates the eyes, nose and throat causing coughing, headache and damage to lungs. NO2 in presence of HC and sunlight causes smog(reduced visibility).


Soot: soots are solid particles of pure carbon. They get suspended in air and are breathed by humans and animals. Their size is few microns. Soot produces lung cancer.

SO2 and H2S: toxic gas SO2 and bad smell of gas H2S are formed on burning of sulpher present in petroliumproducts and coal.

SO2 causes suffocation, irritation of throat and eyes, asthama, lung cancer in people.

H2S causes irritation, diseases of bones and breathing problems. It also causes the destruction of crops.


So2:

On vegetation: destruction of crops

On material: corrosion, H2SO4 gradually destroys building material like marbel.

H2S:

On vegetation: destruction of crops

On material: H2S in air blackens lead paints.


Suspended particulate matters (SPM): particualtes like dust,ash, solid carbon and liquids like sulphuric acid vapours are produced due to burning of fossil fuel like coal and oil. Size of SPM is generally less than 100 micron and they remain suspended in atmospheric air for long time.

SPM affects respiratory system causing cough, cold, snezzing. Fine particles less than 1 micron reach lungs and causes bronchitis and cardiac diseases. And on material corrosion and soiling.

CO2: increase in concentration of CO2 in atmosphere will lead to its ability to absorb energy and cause global warming of atmosphere known as green house effect.

Water pollution by thermal power plant and its control

The water pollution mainly caused by discharging hot condenser water, discharging water from ash handling plant, and discharging waste water from water treatment plant.

The hot water discharged from condenser is caused the thermal pollution. The hot water raises temperature of river, lakes or basins and disturbs the natural ecological balance. The addition of heat to water reduces the water’s ability to hold dissolved gases including dissolved O2 vital to aquatic life. Another important constitutes in the condenser cooling water is residual chlorine as chloride or sodium hydrochloride is used to prevent fouling of the condenser.


The water discharged from ash handling plant is contained ash. The ash gets spread over the large cultivated area along the path of the river and affects the agricultural growth very much. This is because the ash has high alkaline characteristics which are injurious for the growth of many agricultural products. The ash destroys the fertility of the land for ever.

The water coming from the water treatment plant contains large quantities of chlorides of Ca, Mg, Na and K. this water channeled out to some river to an ash pond, these salts percolate in the nearby soil and make the ground water salty.


In this ash pond the situation is worse as there is continuous accumulation of these salts and pond reaches a saturation level of these salts. These process of salt saturation in the pond is further accelerated by solar evaporation of the water. The well on the area covering a few kilometers from the ponds become salty and polluted. Water from these wells becomes harmful for human consumption as well as for irrigation purposes.

Control of salt from waste water

The waste water from water treatment plant can be treated first with lime to precipitate magnesium hydroxide(Mg(OH)2) and then with soda ash to get precipitated calcium carbonate(CaCO3) and resulting sodium chloride solution can be reused for regeneration in softners.

The recovered CaCO3 can be used in chemical , paint, paper and many other industries

Thermal pollution

Discharge of thermal energy(heat) into waters is called thermal pollution. As we know that thermal power plant discharges large amount of heat with cooling water. If this heated water from condensers is discharged into lakes or rivers, the water temperature goes up which disturbs natural ecological balance. Also, the ability of water to hold dissolved gases is reduced when the temperature increases.

If the temperature of water exceeds 35° C, the dissolved O2 content is too low to support aquatic life. But in very cold regions, the aquatic growth is usually enhanced by the warm water and the plants and fish grow at faster rate. But in our country such places are not many and hence, it is necessary that we minimize this thermal pollution.

Thermal pollution

In order to avoid effect of rejected heat on the ecological system of water basins,it is required by sanitoryspecifications that the temperature rise in water basins due to hot water discharge be not more than 5°C in winter and 3°C in summer.

By using cooling tower or cooling ponds: in this case heat rejected to cooling water by power plant is transferred to atmosphere air instead of river or any other water source.

By using separate lake for waste water: a sufficiently large water storage in the form of a lake can be built and once through cooling the condenser can be adopted.

Acid rain

Acid precipitation is the return to earth of the oxides of sulpher and nitrogen in acid form. It may be in the form of acid rain or acid snow.

acid rain is also known as acid deposition.

The deposition may be wet or dry depositions.

Wet deposition of acid occurs when any form of precipitation removes acid from the atmosphere and deliver to the earth’s surface.

In case of dry deposition it occurs in the absence of precipitation. This occur when particles and gases stick to the ground plants or other surfaces.

Acid rain

The three constitutes of the flue gases which mainly effect acidity of the rains are CO2, SO2 and Nox.

SO2 emitted by power plant combines with water to form H2SO4 and Nox combine with water to form HNO3 and during rainy season it falls in the form of acid rain.

Effect of acid rain is to increase acidic (reduction in PH value means less than 5) of rivers,lakes and well water.

Acid rain

Liquids with a PH less than 7 are acidic, and those with a PH greater than 7 are alkaline, the PH value of water is 7.

The reduction of PH value or acid rain affects life of fishes causing death(PH value less than 6.5), loss of fertility of soil and damage buildings and historic monuments, especially those made of rocks such as limestone and marble containing large amounts of calcium carbonate.

Flue gas desulfurization to remove sulfur-containing gases from their stack.

Acid rain

For a typical coal fired power station flue gas desulfurization (FGD) will remove 95 percent or more of the SO2 in the flue gas.

An example of FGD is wet scrubber.

A wet scrubber is basically a reaction tower equipped with a fan that extracts hot smoke stack gases from a power plant into the tower.

pollution & control

Engineering