modern thermal power plant

Power Plant Engineering

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Modern Thermal Power

PlantGuided byAsst. Prof. Piyush MandaliyaDepartment of Mechanical EngineeringAITS- Rajkot

Prepared ByTaral G SoliyaMechanical DepartmentBatch ~ FYEnrollment no. 130030119109

Introduction A steam power plant / thermal power plant is using steam as working

fluid. A thermal power station is a power plant in which the prime mover is

steam driven. Steam is produced in a boiler using coal as fuel and is used to drive

the prime mover, namely the steam turbine. Water is heated, turns into steam & spins a steam turbine which

drives an electrical generator. After steam passes through the turbine, it’s condensed in a condenser &

recycled to where it was heated; known as a Rankine cycle. In the steam turbine, heat energy is converted into mechanical

energy which is used for generating electric power. Generator is an electro-magnetic device which makes the power

available in the form of electrical energy.2

General Layout

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General Layout A schematic arrangement of equipment of a modern coal fired steam

power plant may be divided into four main circuit as

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Coal – Ash circuit Air – Flue Gases circuit

Feed Water – Steam circuit Cooling Water circuit

General Layout

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Coal – Ash Circuit The mixture (pulverized coal + preheated air) is taken into boiler,

then burnt in combustion zone. On ignition of fuel a fireball is formed at the center of the boiler and large amount of heat energy is radiated from it.

The flue gases from the boiler make their way through superheater, economizer, air preheater and finally get exhausted to the atmosphere from the chimney.

Coal Handling : Coal from the storage yard is transferred to the boiler furnace by means of coal handling equipment like belt conveyor, bucket elevator, etc.,

Ash Disposal : Indian coal contains 30% to 40% ash. A power plant of 100MW 20 to 25 tons/hour of ash.

Ash collects at the back of the boiler and is removed to the ash storage yard through the ash handling equipment.

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Air – Flue Gases Circuit Air is taken from the atmosphere to the air preheater. Air is heated in the air preheater by the heat of flue gas which is

passing to the chimney. The hot air is supplied to the furnace of the boiler.

The flue gases after combustion in the furnace, pass around the boiler tubes.

The flue gases then passes through a dust collector, economizer and pre-heater before being exhausted to the atmosphere through the chimney.

By this method the heat of the flue gases which would have been wasted otherwise is used effectively.

Thus the overall efficiency of the plant is improved.

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Feed Water – Steam Circuit The steam generated in the boiler passes through super heater and is

supplied to the steam turbine. Work is done by the expansion of steam in the turbine and the

pressure of steam is reduced. The expanded steam passes to the condenser, where it’s condensed. The condensate leaving the condenser is first heated in a water

heater using the low pressure extraction point steam of the turbine. High pressure extraction point steam of turbine is used for heating

the feed water in the high pressure water heater. The hot feed water is passing through the economizer, where it is

further heated by means of flue gases. The feed water which is sufficiently heated by the feed water heaters

and economizer is then fed into the boiler.

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Cooling Water Circuit Abundant quantity of water is required for condensing the steam in the

condenser. Water circulating through the condenser may be taken from various

sources such as river or lake, provided adequate water supply is available from the river or lake throughout the year.

If adequate quantity of water is not available at the plant site, the hot water from the condenser is cooled in the cooling tower or cooling ponds and circulated again.

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Principle Components The principle components of the modern thermal power plant

are listed below Furnace : To burn fuel. Steam Generator (Boiler) : In which heat generated in the

furnace is utilized to convert water into steam. Prime Mover (Steam Turbine) : To use the heat energy of

steam & converts into mechanical work. Piping System : To convey steam & water. Electric Generator or Alternator : To convert mechanical

power into electricity.

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Brief About Components Boiler : The heat energy is utilized to convert the water into steam at

high temperature and pressure. Steel tubes run along the boiler walls in which water is converted in steam. Superheater : The saturated steam in the boiler tubes is superheated to

540 °C at high pressure.

Economizer : Essentially a feed water heater which heats the water before supplying to the boiler.

Air pre-heater : The primary air fan takes air from the atmosphere & it’s then warmed in the air pre-heater. Pre-heated air is injected with coal in the boiler. Pre-heating the air improves the coal combustion.

Steam Turbine : High pressure super-heated steam is fed to the steam turbine. The pressure and temperature of the steam falls to a lower value and it expands in volume as it passes through the turbine. The expanded low pressure steam is exhausted in the condenser.

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Brief About Components

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Boiler Economizer

Generator : To convert mechanical power of turbine shaft into electric energy. The generator shaft is coupled to the turbine shaft.

Alternator : The steam turbine is coupled to an alternator. When the turbine rotates the alternator, electrical energy is generated.

Condenser : Exhausted steam is condensed in the condenser by means of cold water circulation. Condensing is essential compressing a gases requires huge amount of energy with respect to energy required in compressing liquid. Condensing increases efficiency of the cycle.

Cooling Tower : To cool the hot water coming from the condenser with help of the atmospheric air.

Feed Water Pump : The condensed water is again fed to the boiler by a feed water pump. Some water may be lost during the cycle, supplied from an external water source.

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Circulating Water Pump : To circulate cooling water in the condenser & cooling tower closed circuit.

Coal Handling Plant : Wagon Tripler : To unload the coal when transported to the plant site.

Crusher House : To crush coal to a size of 10mm.

Coal mill : To pulverize the raw coal into a fine powder.

Ash Precipitators : To remove flyash & dust particles from the flue gases before emitting into atmosphere to avoid air pollution.

Chimney : It’s Ferro-concrete structure lined with bricks used to escape flue gases atmosphere at 30m above the ground level.

Water Treatment Plant : To remove impurities of water before water supply to the boiler.

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Draft system : The circulation of air is caused by a difference in pressure, known as Draft. There is a differential pressure b/w atmosphere & inside the boiler. It is necessary to cause the flow of gases through boiler setting Natural draft

Mechanical draft :

Forced draught fan (FD)

Induced draught fan (ID)

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Thermal Efficiency : Thermal efficiency of modern thermal power stations is about 30%.

Overall Efficiency : The overall efficiency of a thermal plant is about 29% (slightly less than the thermal efficiency).

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Efficiency of Thermal Power Station

Advantages Initial cost is low compared to other generating stations. It requires less land as compared to hydro power plant. The fuel (i.e. coal) is cheaper. Cost of generation is lesser than that of diesel power plants. The power plant can be located near load center, so the

transmission losses are considerably reduced. The generation of power is not dependent on the nature’s

mercy like hydro plant. The construction and commissioning of thermal plant requires

less period of time than a hydro plant.

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Disadvantages Air pollution : The pollution of the surrounding atmosphere is

caused by the emitting large amount of objective gases & dust through the chimney.

The air pollution and smoke cause nuisance to people surrounding the planet.

This is one of the causes of global warming. The overall efficiency of a thermal power station is low (less

than 30%). Major part of heat is lost in the condenser. That is why the

efficiency of thermal plants is quite low.

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