EXPERIMENT NO. 2

AIM: To study coal based thermal power plant including (a). Site selection (b). Classification

(c). Merits and demerits (d). Environmental impacts (e). Basic layout (f). Various parts

(g).Working

THEORY:

WHAT IS THERMAL POWER STATION?

A thermal power station is a power plant in which the prime mover is steam driven. Water is

heated, turns into steam and spins a steam turbine which drives an electrical generator. After it

passes through the turbine, the steam is condensed in a condenser and recycled to where it was

heated; this is known as a Rankine cycle. The greatest variation in the design of thermal power

stations is due to the different fuel sources. Some prefer to use the term energy center because such

facilities convert forms of heat energy into electricity. Some thermal power plants also deliver heat

energy for industrial purposes, for district heating, or for desalination of water as well as deliver ing

electrical power.

Installed thermal power capacity

The installed capacity of Thermal Power in India, as of October 31, 2012, was 140206.18 MW

which is 66.99%of total installed capacity.

Current installed base of Coal Based Thermal Power is 120,103.38 MW which comes to

57.38% of total installed base.

Current installed base of Gas Based Thermal Power is 18,903.05 MW which is 9.03% of

total installed capacity.

Current installed base of Oil Based Thermal Power is 1,199.75 MW which is 0.57% of total

installed capacity.

The state of Maharashtra is the largest producer of thermal power in the country.

In thermal power stations, mechanical power is produced by a heat engine that transforms thermal

energy, often from combustion of a fuel, into rotational energy. Most thermal power stations

produce steam, and these are sometimes called steam power stations. Not all thermal energy can

be transformed into mechanical power, according to the second law of thermodynamics. Therefore,

there is always heat lost to the environment. If this loss is employed as useful heat, for industr ia l

processes or district heating, the power plant is referred to as a cogeneration power plant or CHP

(combined heat-and-power) plant. In countries where district heating is common, there are

dedicated heat plants called heat-only boiler stations. An important class of power stations in the

Middle East uses by-product heat for the desalination of water.

CLASSIFICATION OF THERMAL POWER PLANT

By fuel

• Fossil-fuel power stations may also use a steam turbine generator or in the case of natural

gas-fired plants may use a combustion turbine. A coal-fired power station produces electricity by

burning coal to generate steam, and has the side-effect of producing large amounts of sulfur dioxide

which pollutes air and water and carbon dioxide, which contributes to global warming. About 50%

of electric generation in the USA is produced by coal-fired power plants

• Nuclear power plants use a nuclear reactor's heat to operate a steam turbine generator.

About 20% of electric generation in the USA is produced by nuclear power plants.

• Geothermal power plants use steam extracted from hot underground rocks.

• Biomass-fuelled power plants may be fuelled by waste from sugar cane, municipal solid

waste, landfill methane, or other forms of biomass.

• In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energy-

density, fuel.

• Waste heat from industrial processes is occasionally concentrated enough to use for power

generation, usually in a steam boiler and turbine.

• Solar thermal electric plants use sunlight to boil water and produce steam which turns the

generator.

By prime mover

• Steam turbine plants use the dynamic pressure generated by expanding steam to turn the

blades of a turbine. Almost all large non-hydro plants use this system. About 90% of all electric

power produced in the world is by use of steam turbines.

• Gas turbine plants use the dynamic pressure from flowing gases (air and combustion

products) to directly operate the turbine. Natural-gas fuelled (and oil fueled) combustion turbine

plants can start rapidly and so are used to supply "peak" energy during periods of high demand,

though at higher cost than base-loaded plants. These may be comparatively small units, and

sometimes completely unmanned, being remotely operated. This type was pioneered by the UK,

Prince town being the world's first, commissioned in 1959.

• Combined cycle plants have both a gas turbine fired by natural gas, and a steam boiler and

steam turbine which use the hot exhaust gas from the gas turbine to produce electricity. This greatly

increases the overall efficiency of the plant, and many new base load power plants are combined

cycle plants fired by natural gas.

• Internal combustion reciprocating engines are used to provide power for isolated

communities and are frequently used for small cogeneration plants. Hospitals, office buildings,

industrial plants, and other critical facilities also use them to provide backup power in case of a

power outage. These are usually fuelled by diesel oil, heavy oil, natural gas, and landfill gas.

By duty

Power plants that can be dispatched (scheduled) to provide energy to a system include:

• Base load power plants run nearly continually to provide that component of system load

that doesn't vary during a day or week. Base load plants can be highly optimized for low fuel cost,

but may not start or stop quickly during changes in system load. Examples of base-load plants

would include large modern coal-fired and nuclear generating stations, or hydro plants with a

predictable supply of water.

• Peaking power plants meet the daily peak load, which may only be for a one or two hours

each day. While their incremental operating cost is always higher than base load plants, they are

required to ensure security of the system during load peaks. Peaking plants include simple cycle

gas turbines and sometimes reciprocating internal combustion engines, which can be started up

rapidly when system peaks are predicted. Hydroelectric plants may also be designed for peaking

use.

• Load following power plants can economically follow the variations in the daily and

weekly load, at lower cost than peaking plants and with more flexibility than base load plants.

ADVANTAGES OF THERMAL POWER

1. The fuel used is quite cheap.

2. Less initial cost as compared to other generating plants.

3. It can be installed at any place irrespective of the existence of coal. The coal can be

transported to the site of the plant by rail or road.

4. It requires less space as compared to Hydro power plants.

5. Cost of generation is less than that of diesel power plants.

6. They can be located very conveniently near the load centers.

7. Does not require shielding like required in nuclear power plant

8. Unlike nuclear power plants whose power production method is difficult, for thermal

power plants it is easy.

9. Transmission costs are reduced as they can be set up near the industry.

10. The portion of steam generated can be used as process steam in different industries.

11. Steam engines and turbines can work under 25% of overload capacity.

12. Able to respond changing base loads without difficulty.

DISADVANTAGES OF THERMAL POWER

1. It pollutes the atmosphere due to production of large amount of smoke and fumes.

2. Large amounts of water are required.

3. Takes long time to be erected and put into action.

4. Maintenance and operating costs are high.

5. With increase in pressure and temperature, the cost of plant increases.

6. Troubles from smoke and heat from the plant, disposal of ash.

7. The following list corers most of the factors that should be studied and considered in

selection of proper sites for power plant construction:

a) Transportation network: Easy and enough access to transportation network is

required in both power plant construction and operation periods.

b) Gas pipe network: Vicinity to the gas pipes reduces the required expenses.

c) Power transmission network: To transfer the generated electricity to the

consumers, the plant should be connected to electrical transmission system.

Therefore the nearness to the electric network can play a roll.

d) Geology and soil type: The power plant should be built in an area with soil and

rock layers that could stand the weight and vibrations of the power plant.

e) Earthquake and geological faults: Even weak and small earthquakes can damage

many parts of a power plant intensively. Therefore the site should be away enough

from the faults and previous earthquake areas.

f) Topography: It is proved that high elevation has a negative effect on production

efficiency of gas turbines. In addition, changing of a sloping area into a flat site for

the construction of the power plant needs extra budget. Therefore, the parameters

of elevation and slope should be considered.

g) Rivers and floodways: Obviously, the power plant should have a reasonable

distance from permanent and seasonal rivers and floodways.

h) Water resources: For the construction and operating of power plant different

volumes of water are required. This could be supplied from either rivers or

underground water resources. Therefore having enough water supplies in defined

vicinity can be a factor in the selection of the site.

i) Environmental resources: Operation of a power plant has important impacts on

environment. Therefore, priority will be given to the locations that are far enough

from national parks, wildlife, protected areas, etc.

j) Population centers: For the same reasons as above, the site should have an enough

distance from population centers.

SCHEMATIC DIAGRAM OF THERMAL POWER PLANT

Fig. Typical diagram of a coal-fired thermal power station

1. Cooling tower 10. Steam Control valve 19. Superheater

2. Cooling water pump 11. High pressure steam turbine 20. Forced draught (draft) fan

3. transmission line (3-phase) 12. Deaerator 21. Reheater

4. Step-up transformer (3-phase) 13. Feedwater heater 22. Combustion air intake

5. Electrical generator (3-phase) 14. Coal conveyor 23. Economiser

6. Low pressure steam turbine 15. Coal hopper 24. Air preheater

7. Condensate pump 16. Coal pulverizer 25. Precipitator

8. Surface condenser 17. Boiler steam drum 26. Induced draught (draft) fan

9. Intermediate pressure steam

turbine 18. Bottom ash hopper 27. Flue gas stack

MAIN AND AUXILIARY EQUIPMENTS-

Coal handling plant

• The function of coal handling plant is automatic feeding of coal to the boiler furnace.

• A thermal power plant burns enormous amounts of coal.

• A 200MW plant may require around 2000 tons of coal daily

Pulverizing plant

In modern thermal power plant, coal is pulverised i.e. ground to dust like size and carried to the

furnace in a stream of hot air. Pulverizing is a means of exposing a large surface area to the

action of oxygen and consequently helping combustion.

Pulverizing mills are further classified as:

1. Contact mill

2. Ball mill

3. Impact mill

Draft system

• The circulation of air is caused by a difference in pressure, known as Draft.

• Draft is a differential pressure b/w atmosphere and inside the boiler.

• It is necessary to cause the flow of gases through boiler setting

• It may be –

1. Natural draft

2. Mechanical draft

Boiler

• A boiler or steam generator is a closed vessel in which water under pressure, is converted

into steam.

• It is one of the major components of a thermal power plant

• Always designed to absorb maximum amount of heat released in the process of

combustion

Boilers are of two types-

1. Fire tube boiler

2. Water tube boiler

Superheater and reheater

Superheater:

Superheater is a component of a steam-generating unit in which steam, after it has left the boiler

drum, is heated above its saturation temperature. The amount of superheat added to the steam is

influenced by the location, arrangement, and amount of super heater surface installed, as well as

the rating of the boiler. The super heater may consist of one or more stages of tube banks

arranged to effectively transfer heat from the products of combustion. Super heaters are

classified as convection, radiant or combination of these.

Reheater: Some of the heat of superheated steam is used to rotate the turbine where it loses some

of its energy. Reheater is also steam boiler component in which heat is added to this

intermediate-pressure steam, which has given up some of its energy in expansion through the

high-pressure turbine. The steam after reheating is used to rotate the second steam turbine where

the heat is converted to mechanical energy. This mechanical energy is used to run the alternator,

which is coupled to turbine, there by generating electrical energy.

Steam turbine

A steam turbine converts heat energy of steam into mechanical energy and drives the generator.

It uses the principle that steam when issuing from a small opening attains a high velocity. This

velocity attained during expansion depends on the initial and final heat content of the steam. This

difference b/w initial and final heat content represents the heat energy converted into kinetic

energy.

These are of two types:-

Impulse turbine

Reaction turbine

Ash handling plant

The percentage of ash in coal varies from 5% in good quality coal to about 40% in

poor quality coal

Power plants generally use poor quality of coal , thus amount of ash produced by it is

pretty large

A modern 2000MW plant produces about 5000 tons of ash daily

The stations use some conveyor arrangement to carry ash to dump sites directly or for

carrying and loading it to trucks and wagons which transport it to the site of disposal

Condenser

Steam after rotating steam turbine comes to condenser. Condenser refers here to the shell and

tube heat exchanger (or surface condenser) installed at the outlet of every steam turbine in

Thermal power stations of utility companies generally.

These condensers are heat exchangers which convert steam from its gaseous to its liquid

state, also known as phase transition.

In so doing, the latent heat of steam is given out inside the condenser. Where water is in

short supply an air cooled condenser is often used.

An air cooled condenser is however significantly more expensive and cannot achieve as

low a steam turbine backpressure (and therefore less efficient) as a surface condenser.

The purpose is to condense the outlet (or exhaust) steam from steam turbine to obtain maximum

efficiency and also to get the condensed steam in the form of pure water, otherwise known as

condensate, back to steam generator or (boiler) as boiler feed water.

Cooling towers and ponds

A condenser needs huge quantity of water to condense the steam.

Typically a 2000MW plant needs about 1500MGallon of water.

Most plants use a closed cooling system where warm water coming from condenser is

cooled and reused

Small plants use spray ponds and medium and large plants use cooling towers.

Cooling tower is a steel or concrete hyperbolic structure having a reservoir at the base

for storage of cooled water

Height of the cooling tower may be 150 m or so and diameter at the base is 150 m

Feed water heater

Advantages of heating water before feeding back to the boiler:-

a) Feed water heating improves overall plant efficiency.

b) The dissolved oxygen and carbon dioxide which would otherwise cause boiler corrosion

are removed in feed water heater

c) Thermal stresses due to cold water entering the boiler drum are avoided.

d) Quantity of steam produced by the boiler is increased.

Some other impurities carried by the steam and condensate, due to corrosion of boiler and

condenser are precipitated outside the boiler

Economizer

Flue gases coming out of the boiler carry lot of heat. An economizer extracts a part of this heat

from flue gases and uses it for heating feed water. This use of economizer results in saving coal

consumption and higher boiler efficiency

Air preheater

After flue gases leave economizer, some further heat can be extracted from them and used to

heat incoming heat. Cooling of flue gases by 20 degree centigrade increases the plant efficiency

by 1%.

Air preheaters may be of three types

Plate type

Tubular type

Regenerative type

WORKING OF THERMAL POWER PLANT

Feed water heater

A feed water heater is a power plant component used to pre-heat water delivered to

a steam generating boiler. Preheating the feed water reduces the irreversibility involved in steam

generation and therefore improves the thermodynamic efficiency of the system. This reduces plant

operating costs and also helps to avoid thermal shock to the boiler metal when the feed water is

introduced back into the steam cycle.

Boiler

A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid

exits the boiler for use in various processes or heating applications.

Steam condensing

The condenser condenses the steam from the exhaust of the turbine into liquid to allow it to be

pumped. If the condenser can be made cooler, the pressure of the exhaust steam is reduced and

efficiency of the cycle increases.

Electrical Generator

In electricity generation, an electric generator is a device that converts mechanica l

energy to electrical energy.

Steam Turbine

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and

converts it into rotary motion.

Efficiency:

The energy efficiency of a conventional thermal power station, considered as salable energy as a

percent of the heating value of the fuel consumed, is typically 33% to 48%. This efficiency is

limited as all heat engines are governed by the laws of thermodynamics. The rest of the energy

must leave the plant in the form of heat. This waste heat can go through a condenser and be

disposed of with cooling water or in cooling towers. If the waste heat is instead utilized for

district heating, it is called co-generation.

Planning of Power Plant involves decision on two basic parameters:

1. Total power output to be installed (e.g. 1000 MW)

Installed capacity is determined from:

• Estimated Demand: - Before setting up a power plant, we need to critically analyze

demand which gives us the idea to determine capacity which needs to be installed.

The installation capacity should match the demand and hence estimation of demand

is the critical fact while setting up a power plant.

• Growth of Demand anticipated: - While determining demand, future prospects needs

to be considered so that the return on capital would be maximized and future demand

could be met easily.

• Reserve Capacity required: - Considering the various type of demand in a market

how much reserve capacity is required to be installed is determined and hence this

will help in determining installation capacity.

2. Size of generating units (e.g. 4 units of 250 MW each)

Size of the generating units will depend on:

• Variation of Load (Load Curve):- During the different hour of the day and in various

seasons the demand varies, so the load curves. Now the number of units has to be

determined to run the operations optimally and meeting the requirement daily.

• Minimum start-up and shut down periods of the units

• Maintenance programme planned

ENVIRONMENTAL IMPACTS OF COAL BASED THERMAL POWER

PLANT

Impact of Thermal Power Plant on Water Source

Water Intensive

New thermal power plant of 500 MW installed capacity requires – around 14 Million m3

of water per annum.

High impact on river & ground water

Water demand for the once-through system is 30 to 50 times that of a closed Cycle system.

Largest emitter of mercury

Typical power plant emits 90 % of its mercury into the air and 10 percent on land

On an average 65 tonnes of mercury released in the atmosphere by Indian Thermal power

plant.

Potential source of water pollution

Cooling Tower Blow Down, Boiler Blow Down, Demineralization (DM)

Plant Effluent, Coal Handling Plant Dust Suppression, Ash handling (Leachate of heavy

metal (ash pond) contaminate groundwater), Effluent from oil handling and transformer

areas, Power House and Turbine Area

Effluent, Domestic waste water

Air pollution due to thermal power plants

Air Pollution from point source:

Particulates matter, Gaseous emission - Sulphur dioxide, oxides of nitrogen, carbon monoxide,

carbon dioxide, Hydrocarbon.

Air Pollution from non-point source:

Transportation of coal, Loading/unloading of fuel, Coal storage yard, Fly ash

Handling & Transportation., Coal storage yard