Power Plant Unit 1

8/2/2019 Power Plant Unit 1

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POWER PLANT ENGINEERING

Introduction

Large amount of power is generated using prime movers in a site called power plants.Power plant is a unit built for production and delivery of an electrical energy.

Sources of energy

The basic sources of energy for power generation are coal, oil, nuclear fuels, and gas.

These sources are known as conventional sources of energy. These sources will one beused up and are exhaustible.

The most reassuring and promising energy which is abundant in supply and is

inexhaustible is non conventional sources of energy: such as solar, wind, tidal, geo-

thermal etc.

Primary energy sources and secondary energy sources

At the power station electricity is made from energy released by burning coal or oil or

by using nuclear fuel or water power. Each of these four sources of energy is called a

primary source. Electricity is a secondary source, because it is produced from a primarysource.

Non-renewable sources

Most of the energy sources like coal, oil, natural gas and nuclear fuels are called

Non-renewable energy sources because once they have been used up, they cannot be


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replaced. One day all the coal (within 120 years), oil & natural gas and nuclear fuels (less

than 50 years) on earth will be used up and these may no more be available.

Renewable energy sources

Sources of energy that can be used over and over again are called renewable sources. Energy from the sun (Heat and light energy)

Energy from the wind (Kinetic energy)

Energy from the waves and tides (Kinetic energy)

Energy from the Earths core (Heat energy)

Sources used for producing energy

1. Fossil fuels

Fossil fuels may be classified as solid, liquid and gaseous. Fossil fuels mat also

be classified as natural fuel and prepared fuel (artificial fuel)

Solid fuels: Naturally occurring solid fuels include wood, varieties of coals such as

bituminous coal, anthracite coal and lignite. While prepared fuels are charcoal, coke,briquetted (compressed) coal, and pulverized (powdered) coal.

Liquid fuels: Liquid fuels are the derivatives from oil extracted by drilling from theground/off-shore. Liquid fuels include petrol, diesel, paraffin, and crude oil.

Gaseous fuels: These fuels are natural gases which are present under the ground/off

shore, these gases are brought to the earths surface by drilling gas wells and petroleum


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wells. Examples of prepared gaseous fuels are producer gas, blast furnace gas, coke

oven gas, blue water gas, carbureted water gas, illuminating gas, etc.

2. Nuclear energy or Atomic energy

It is the most powerful kind of energy known. The atoms of a few metals can bechanged or split, to make new, different atoms. The nucleus of an atom of metal called

uranium can be split into two. This is called nuclear fission. The word fission means

breaking apart. In a nuclear power station this heat energy is used to produceelectricity. The complete fission of 1 kg uranium U235 releases as much energy as the

burning of 4500 tonnes of coal or 2000 tonnes of oil.

3. Hydro-electric power or Hydel power

In a hydro electric power station, electricity is produced without burning, coal or

oil and without using nuclear fuel.

A hydro-electric power station can turn the potential energy of water at higher levelbehind a dam into kinetic energy and then convert this energy into electrical energy by


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using a generator. This kind of power station is called hydro-electric or hydel power

station.

The energy stored in water is inexhaustible source of energy. It is the renewable and

cheapest source of energy.

STEAM POWER PLANT (THERMAL POWER PLANT)

It is the plant where the chemical energy from the coal is converted into heat

energy of a steam to kinetic energy of a turbine and then converted into electrical

energy from the generator.

Energy transfer diagram

Working principle:


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Power stations that burn coal or oil have three main parts the boiler, the turbine

and the generator. The power station burns coal or oil to produce heat in its boiler. The

boiler is lined with pipes carrying water, which boils and turns to steam. The steam isthen passed through pipes to machines called turbines. Turbines have huge wheels

fitted with hundreds of steel fan like blades.

The steam rushes through the turbines at high speeds causing both the wheels and the

turbine shaft to spin. The spinning shaft turns the rotor of the electric generator creating

magnetic field, thereby producing electricity. In a large power station steam passesthrough several wheels until almost all of its energy has been used up. The used steam

then cools in the cooling towers and changes back into water. This changing process is

called condensation. The water returns to the boiler and is heated up again.

LAYOUT OF STEAM POWER PLANT

(i) Coal and ash circuit: In this circuit, the coal from the storage area called stackis taken to the boiler by means of coal handling equipments such as belt

conveyors, bucket elevators etc. A thermal power plant of 400 MW capacity

requires 5000 to 6000 tonnes of coal per day. So highly reliable and efficientdevices should be used.

After the pulverized coal is burnt at 1500C - 2000C by combustion it gets

collected in the ash pit. It is removed from the ash pit by ash handling systemlike belt conveyors, screw conveyors etc. In a power station producing 400

MW capacity of electricity 10 hectares area is required per year if ash is

dumped to a height of 6.5meters. So sufficient space should be provided forash storage.

(ii) Air and flue gas circuit: The air from the atmosphere gets heated in the air preheater. The air receives its heat from the hot flue gas passing to the


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chimney. The hot air enters the boiler and helps in combustion of fuel in the

boiler.

The flue gases after combustion in the boiler furnace pass around the boilertubes, heating the water present in the tubes. The fuel gases then pass through a

dust collector which removes any dust or solid particles. The filtered flue gas

passes through the economizer and preheater and is forced out through thechimney by a draught fan.

(iii) Feed water and steam circuit: The superheated steam from the boiler enters

the steam turbine; the superheated steam temperature is about 600C at a

pressure of 30 MPa. The steam expands in the turbine causing the turbine

blades to rotate. After doing mechanical work on the blades and loosing itsenergy, the steam becomes wet and the pressure of steam becomes less.

The wet steam passes through a condenser where it is completely becomes

water. The condensed water that has a temperature of 30C to 40C is collectedin a tank called hot well. The water from the hot well is fed into the boiler

through economizer. The economizer preheats the water before entering it. Theeconomizer receives its heat from the flue gases leaving the boiler. Preheating

the feed water in the economizer increases the boiler efficiency and helps

quicker production of steam.

(iv) Cooling water circuit: In this circuit water is circulated around the condenser

to condense the steam coming out of the turbine. Cooling water enters thecondenser at 10C to 15C and leaves at 20C to 25C.

Plenty of water is required for condensing the steam in the condenser. Thewater may be taken from sources such as river or lake.


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HYDRO ELECTRIC POWER PLANT

It is the plant in which the hydraulic energy of water is converted intomechanical energy of a turbine and then converted into electrical energy from the

electric generator.

Working principle:

Potential energy is the energy which a substance has due to its position or state.The water behind the dam has potential energy because of its position. The water can

fall from this position and exert a force over a distance and therefore do work. In hydro

electric power plant the force is used to drive a turbine, which in turn drives the

electric generator. Because gravity provides the force which makes water fall, theenergy stored in the water is called gravitational potential energy.

Layout of hydro-electric power plant

(i) Water reservoir: In a reservoir the water collected from the catchment area is

stored behind a dam. Catchment area gets its water from rain and streams.Continuous availability of water is a basic necessity for a hydro electric plant.

The level of water surface in the reservoir is called head water level.

(ii) Dam: The purpose of the dam is top store the water and to regulate the out

going flow of water. The dam helps to store all the incoming water. It also

helps to increase the head of the water. In order to generate a required quantity

of power it is necessary that a sufficient head is available.

(iii) Spillway: Excess accumulation of water endangers the stability of dam

construction. Also in order to avoid the over flow of water out of the dam


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especially during rainy seasons spillways are provided, this prevents the rise of

water level in the dam. Spillways are passages which allow the excess water to

flow to different storage area away from the dam.

(iv) Gate: A gate is used to regulate or control the flow of water from the dam.

(v) Pressure tunnel: It is a passage that carries water from the reservoir to the

surge tank.

(vi) Surge tank: A surge tank is a small reservoir or tank in which the water level

rises or falls due to sudden changes in pressure.

A surge tank serves the following purposes:

(a) To reduce the distance between the free water surface in the dam and the

turbine, thereby reducing the water hammer effect (The water hammer is

defined as the change in pressure rapidly above or below normal pressure

caused by sudden change in the rate of water flow through the pipe,according to the demand of prime mover i.e. turbine) on penstock and also

protect the upstream tunnel from high pressure rise.

(b) To serve as a supply tank to the turbine when the water in the pipe is

accelerated during increased load conditions and as a storage tank when thewater is decelerating during reduced load conditions.

(vii) Penstock: It is a closed pipe of steel or concrete for supplying water under

pressure to the turbine.

(viii) Inlet valve: Water from the penstock flows to the turbine through the inlet

valve. The valve may be partially closed or open thereby regulating thepressure of water flowing to the turbine.

(ix) Hydraulic turbine (prime mover): The hydraulic turbine converts the energyof water into mechanical energy (rotation) available on the turbine shaft of an

electric generator and electricity is produced.

The water after performing the work on turbine blades is discharged through

the draft tube. The prime movers which are in common use are pelton wheel,Kaplan turbine, Francis turbine.

(x) Draft tube: It is connected to the outlet of the turbine. It allows the turbine tobe placed above the tail water level.

(xi) Tail water level or tail race: Tail water level is the water level after dischargefrom the turbine. The discharged water is sent to the river, thus the level of the

river is the tail water level.


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(xii) Electric generator, step-up transformer and pylon: As the water rushes

through the turbine, it spins the turbine shaft, which is coupled to the electric

generator. The generator has a rotating electromagnet called a rotor and astationary part called a stator. The rotor creates a magnetic field that produces

an electric charge in the stator. The charge is transmitted as electricity. The

step-up transformer increases the voltage of the current coming from the stator.The electricity is distributed through power lines also called as pylon.

Classification of Hydro-electric plants

Hydro-electric plants are usually classified according to the available head of water.

(i) High head power plants: In this the head of water is more than 100 meters.The turbine used in such plants is pelton wheel.

(ii) Medium head power plants: In this the head of water ranges from 30 to 500

meters. The turbine used in such plants is Francis turbine.

(iii) Low head power plants: In this the head of water ranges from 25 to 80 meters.The turbine used in such plants is Kaplan turbine and Francis turbine.

NUCLEAR POWER PLANT

It is the plant in which the heat energy from the nuclear fission is converted into electricalenergy.

Working principle:


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Nuclear energy, also called atomic energy is the most powerful kind of energy known.

A nuclear power plant works in a similar way as a thermal power plant. The difference

between the two is in the fuel they use to heat the water in the boiler (steam generator).Inside a nuclear power station, energy is released by nuclear fission in the core of the

reactor. The energy heats water in the steam generator. The water boils and produces

steam. This steam turns the huge turbine wheels and the turbines drive the generator thatproduces the electricity.

It is estimated that 1 kg of Uranium U235 can produce as much energy as the burning of4500 tonnes of high grade variety of coal or 2000 tonnes of oil.

Nuclear fission

The fuel inside the reactor is metal called uranium. Uranium exists as an isotope in the

form of U235 splits into two fragments of approximately equal size. About 2.5 neutrons are

released and a large amount of energy (200 million electron volts, Mev) is produced. The

neutrons produced move at a very high velocity 1.510

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m/s and fission other nucleus ofU235. Thus fission process and release of neutrons take place continuously throughout the

remaining material.

Controlled chain reaction in a Nuclear reactor

It may be observed in the figure that 2.5 neutrons are released in fission of each nucleus of

U235, out of these one neutron is used to sustain the chain reaction, 0.9 neutrons is absorbed

by U238 and becomes Pu239. The remaining 0.6 neutrons escape from the reactor. Moderators

are provided to slow down the neutrons from the high velocities but not to absorb them.

Control rods limit the number of fuel atoms that can split. They are made up of a materialthat absorbs neutrons. To turn on the reactor some rods pulled out. The rods are made of

boron or cadmium.

Nuclear power plant


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Pressurized water reactor plant (PWR)

(i) Nuclear reactor: A nuclear reactor is similar to a furnace in a steam power

plant. Heat is produced in the reactor due to the nuclear fission and there is a

chain reaction. The heat generated in the reactor is carried away by the coolantcirculated through the core. Water or heavy water is cheaply available in plenty

and is used as coolant.

The purpose of the pressure equalizer is to maintain a constant pressure of14MN/m2. This enables water to carry more heat from the reactor. The purpose

of the coolant pump is to pump coolant water under pressure into the reactor

core.

(ii) Steam generator: The steam generator is a heat exchange where the heat from

the coolant is transferred on to the water that circulates through the steam

generator. As the water passes through the steam generator it gets converted

into steam.

(iii) Turbine: The steam produced in the steam generator is sent to the turbine. Theturbine blades rotate. The turbine shaft is coupled to a generator and electricity

is produced.

(iv) Condenser: After the steam performing the work on the turbine blades by

expansion, it comes out of the turbine as wet steam. This is converted back into

water by circulating cold water around the condenser tubes.

(v) Feed pump: The feed pump pumps back the condensed water into the steam

generator.

DIESEL POWER PLANT

It is the plant in which the heat energy from the diesel engine in converted into electricalenergy from the electric generator.


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This is a fossil fuel plant since diesel is a fossil fuel. Diesel engine power plants are

installed where supply of coal and water is not available in sufficient quantity.

(i) Diesel Engine: A diesel engine is required for a diesel power plant. A diesel

engine is a compression ignition engine.

A diesel engine may be four stroke or two stroke cycle engine. The two stroke

cycle engine is more favoured for diesel power plants. Air is admitted into thecylinder of the engine and is compressed; fuel (diesel) is injected into the

cylinder through fuel injectors. Due to the high temperature and pressure

obtained during compression, it ignites. The fuel burns and the burnt gasesexpand to do work on the moving part inside the cylinder called piston. This

movement of the piston rotates a flywheel. The engine is directly coupled to the

electric generator. The gases after expansion inside the cylinder are exhaustedinto the atmosphere and passes through a silencer in order to reduce the noise.

The fuel for the diesel engine is drawn through a filter from the all day tank.

The air required for the diesel engine is drawn through the air filter from theatmosphere.

(ii) Starting system: diesel engine used in diesel power plants is not self starting.

The engine is started from cold condition with the help of an air compressor.

(iii) Fuel supply system: Fuel from the storage tank is pumped through a filter into

a smaller tank called all day tank. This tank supplies the daily requirements ofthe diesel engine. The all day tank is placed high so that the fuel flows to the

engine under gravity and sufficient pressure.

(iv) Air intake system: The air required for the combustion of fuel inside the diesel

engine cylinder is drawn through the air filter. The purpose of the filter is to


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remove dust from the incoming air. The dry filter used may be made of felt,

wood or cloth. In wet filter, oil bath is used; in this the air passes over a bath of

oil where the dust particles get coated on the oil.

(v) Exhaust system: The exhaust gases coming out of the engine is very noisy. In

order to reduce the noise a silencer (muffler) is used..

(vi) Cooling system: The temperature of the burning fuel inside the engine cylinder

is in the order of 1500C to 2000C. In order to lower this temperature water iscirculated around the engine. The water envelops (water jacket) the engine, the

heat from the cylinder, piston, combustion chamber etc, is carried by the

circulating water. The hot water leaving the jacket is passed through the heat

exchanger. The heat from the heat exchanger is carried away by the watercirculated through the heat exchanger and is cooled in the cooling tower.

(vii) Lubricating system: This circuit includes lubricating oil tank, oil pump and oil

cooler. The purpose of the lubrication system is to reduce the wear of the enginemoving parts. Part of the cylinder such as piston, shafts, and valves must be

lubricated. Lubrication also helps to cool the engine.In the lubrication system the oil is pumped from the lubricating oil tank through

the oil cooler where the oil is cooled by the cold water entering the engine. The

hot oil after cooling the moving parts return to the lubricating oil tank.

GAS TURBINE POWER PLANT


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Elements of a gas turbine power plant

(i) Starting motor: Gas turbines are not self starting. They require a starting motor

to first bring the turbine to the minimum speed called coming in speed, for this

purpose a starting motor is required.

(ii) Low pressure compressors (LPC): The purpose of the compressor is tocompress the air. Air from the atmosphere is drawn into the LPC and is

compressed.

(iii) Intercooler: The air after compression in the LPC is hot. It is cooled by the

intercooler. The intercooler is circulated with cooling water.

(iv) High pressure compressor (HPC): The air from the intercooler enters the

HPC where it is further compressed to a high pressure. The compressed airpasses through a regenerator.

(v) Regenerator (Heat Exchanger): The air entering the combustion chamber forthe combustion must be hot. The exhaust gases leaving the turbine are passedthrough the regenerator. The heat from the exhaust gases is picked up by the

compressed air entering the combustion chamber.

(vi) Combustion chamber: As the name suggests, it is a chamber where

combustion takes place. For any combustion, fuel and air is required. The fuel

used in the gas power plants may be natural gas, pulverized (powdered) coal,


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kerosene (or) gasoline. The fuel is injected into the combustion chamber. The

fuel along with the high pressure air is ignited sometimes with a spark plug.

(vii) High pressure turbine (HPT): In the beginning the starting motor runs the

compressor shaft. The burnt gases expand through the high pressure turbine.

When the turbine shaft rotates it in fact drives the compressor shaft which iscoupled to it. Now the HPT runs the compressor and starting motor is stopped.

About 66% of the power developed by the turbine is used to run the

compressor, and only 34% of the power developed is used to generate electricpower.

(viii) Low pressure turbine (LPT): The purpose of the LPT is to produce electric

power. The shaft of the LPT is coupled with the generator. The burnt fuel(gases) after leaving the HPT is again sent to a combustion chamber where it

further undergoes combustion.

Even if there is any left out unburnt fuel from the previous turbine it gets fullyburnt in the combustion chamber. The burnt gases run the low pressure turbine

(LPT). The shaft of the turbine is directly coupled with the generator forproducing electricity.

The exhaust hot gases after leaving the LPT passes through the regeneratorbefore being exhausted through the chimney into the atmosphere. The heat from

the hot gases is used to preheat the air leaving the HPC before it enters the

combustion chamber. This preheating of the air improves the efficiency of the

combustion chamber.

ADVANTAGES AND DISADVANTAGES OF POWER PLANTS

THERMAL POWER PLANT

ADVANTAGES DISADVANTAGES

1. Cost of construction of the plant is

low compared to hydro-electricpower plant.

2. The power plant may be located nearthe load centre, so that the cost of

transmission and the losses due to

transmission are considerablyreduced.

3. This power plant does not depend on

1. The fuel (coal or oil) used in thermal

power plant will one day getexhausted. It is a non-renewable

source of energy.

2. It cannot be used as a peak load

plant.

3. The transportation of fuel is a major

problem for power plants located

away from coal fields.


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nature like rain and rivers.

4. The construction of thermal power

plant takes less time.

4. The cost of power generation isconsiderably high compared to

hydro electric power plant.

5. Air pollution.

Power Plant Unit 1

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