Unit-1 EE Updated

ENERGY ENGINEERING UNIT-1

1 Joseph Sajan, Asst. Professor, CMRIT, Bangalore

UNIT-1

STEAM POWER PLANT

In a steam power plant, the thermal energy is used to raise steam that runs a steam

turbine, which produces mechanical energy. This mechanical energy is then converted

to electrical energy in a generator. Steam power plants are suitable for large scale

production of electrical power.

LAYOUT OF STEAM POWER PLANT

(Either use the diagram above or the diagram explained in the class)

The layout of steam power plant can be understood with certain circuits namely:

a) Coal and ash Circuit

b) Air and gas circuit

c) Feed water and steam circuit

d) Cooling water circuit



COAL AND ASH CIRCUIT

The coal which is stored is sent to the boiler

through a coal handling plant.

The coal is burnt inside the boiler through suitable

mechanisms and produces ash.

The ash is collected and stored through ash

handling plant.

AIR AND GAS CIRCUIT

Air is the component which contributes for

combustion in steam power plant.

Air is drawn with the help of a Forced draft fan

or Induced draft fan.

The air passes through a pre-heater in order to

make use of the heat of the flue gases.

After burning, the flue gases passes around the

boiler tubes, super heater, economizer and the

air pre-heater and escapes through the chimney.

FEED WATER AND STEAM

CIRCUIT

The feed water enters the

economizer where its

temperature is raised, which

then enters the boiler for

phase change process from

water to steam.

The steam formed then enters the super heater, where its temperature is

raised above the saturation level.

The super-heated steam enters the turbine, where it is expanded and thus

creates mechanical energy.

The exhaust steam enters the condenser, where it is condensed back to the

liquid state.



The condensate then enters a set of heaters and pumps and enters the

economizer as feed water.

COOLING WATER CIRCUIT

The purpose of the cooling water is to

change the phase of steam to liquid.

The cooling water is obtained from a river or

canal.

After extracting heat from the steam, the

cooling water is sent to cooling tower, where

it is cooled back and then sent back to the river or canal.

BURNING OF COAL

BURNING OF COAL

HAND FIRED

STROKER FIRING

OVERFEED STROKER

UNDERFEED STROKER

PULVERISED FIRING

UNIT SYSTEM

BIN SYSTEM



SELECTION CRITERIA FOR COMBUSTION EQUIPMENT FOR COAL

While selecting the combustion equipment, the following criteria should be satisfied

Initial Cost

Sufficient combustion space

Area of grate

Operating cost

Minimum smoke

Flexibility of operation

Arrangements for thorough mixing of air and fuel

STOKER FIRING A stoker is a power operated fuel feeding mechanism and grate.

CLASSIFICATION OF STOKER FIRING

Over feed Stoker

Under feed Stoker

OVER FEED STOKER

Principle:

The air along with water vapour enters from the bottom of the grate.

In flowing through the grate opening, the air is heated as it cools the grate.



The warm air then passes through the layer of hot ashes and picks up the

heat.

The immediate region above the ash is incandescent coke.

As it passes through this region, O2 in air reacts with carbon and forms CO2.

The water vapour (H2O) reacts with coke and forms CO, CO2 and H2.

On further travel along the coke region, CO2 reacts to form CO.

Hence no more O2 will be present in the gases leaving the coke region.

The region above the coke is the fresh coal, where due to immense heat the

volatile matter is distilled and forms coke, which settles below.

Secondary air is provided to burn the combustible gases. It is supplied at high

speeds to create turbulence.

The combustible gases then burn completely in the furnace.

TYPES OF OVERFEED STOKER

TRAVELLING GRATE STOKER

Construction

The stoker consists of flexible endless chain which forms the support for the

fuel bed.

The chain travels over two sprocket wheels one at front end and other at the

rear of the furnace.

The speed of stoker is around 15cm to 50 cm per minute.



Working

The coal is supplied onto the grate with the help of a hopper as shown in the

figure.

Since the grate should be protected from overheating, the coal should have

certain ash content, which forms a layer on the grate.

The air is admitted from the underside of the grate which is divided into

several compartments, which is in turn connected to air duct.

The secondary air is passed from the parts located above the stoker.

This produces enough turbulence and also guides the gases towards the boiler.

The gases after passing through the boiler, is sent back to the atmosphere

through the exhaust.

Advantages

Simple in Construction

Low Initial Cost

Low Maintenance Cost

Self cleaning

High heat release rate per unit volume

SPREADER STOKER

Working

The coal from the hopper is fed onto the grate with the help of Feeder and

Spreader

The feeder is a rotating drum fitted with blades on its periphery.



The feeder supplies coal on to the spreader.

The spreader is a fast moving drum with blades around it.

The spreader throws the coal onto the grate. The lighter particles travel a

further distance, where as the heavier particles travel a shorter distance.

The secondary air is supplied through nozzles located directly above the

ignition zone.

The primary air is supplied from under the grate as shown in the figure.

Unburnt coal and ash are deposited on the grate which can be moved

periodically to remove ash on the furnace.

UNDER FEED STOKER

Principle:

Air entering through the holes in the grate comes in contact with the raw coal.

It then passes through the incandescent coke where reactions similar to over

feed system take place. (Please write the reactions in detail as explained in

overfeed stoker)

The difference between overfeed and underfeed is the position of layers.

(Types of underfeed stoker will be explained later

and will be updated soon)



PULVERISED FUEL FIRING In pulverized fuel firing system, the coal is reduced to fine particles with the help

of grinding mill and then projected into the combustion chamber with the help of hot

air.

The main advantage of pulverised firing is that the combustion is more effective and

the efficiency depends on the fineness of the coal.

Pulverised fuel firing can be classified into 2 types

Unit System

Bin System

UNIT SYSTEM

The unit system is so called because each burner and a pulveriser constitute a

unit.

The coal is fed into the pulveriser through a feeder. In the feeder, preheated

air is passed to dry the coal before being pulverised.

The preheated air also acts as primary air.

The dry coal is now sent to the pulveriser, where the coal is cut down to fine

particles.

The fine coal through a separator enters the exhaust fan. Only fine particles

pass out of the separator.



The job of the exhaust fan is to create suction effect, so that air along with

the coal particles travels along the circuit.

The secondary air is sent separately to the burner.

BIN or CENTRAL SYSTEM

This system employs a limited number of pulverisers at a common point for all

the burners.

The coal is pulverised in the pulverizing mill and is sent to the cyclone

separator, where the fine particles is sent to the burner and the heavier

particles are sent back to the pulveriser.

The pulverised coal is stored in a storage plant. The coal is sent from the

storage to the burner through the feeder.

PULVERISED FUEL BURNERS

LONG FLAME BURNER

It is also called U-shaped burner.

The burner is placed in such a way that it

produces a long, u-shaped flame.

The burner injects a mixture of primary air

and fuel vertically downwards in thin streams.



The secondary air is supplied at right angles, so that rapid combustion is in

place.

The tertiary air is also supplied for better mixing of fuel through turbulence.

TURBULENT BURNER

It is also called as short flame burner.

The burners are built onto the furnace

walls, so that the flame travels in the

horizontal direction.

The coal-air mixture and secondary air

pass through the burner in such a way

that there is good mixing and high

turbulence.

Due to high turbulence, the mixture burn

intensely and combustion is completed in

short distance.

TANGENTIAL BURNER

The construction is that four burners are

located in four corners of the furnace and

fired.

The four flames meet such that they form a

tangent for an imaginary circle at the centre,

hence the name tangential burner.

The swirling action produces enough

turbulence for the fuel to burn effectively.

CYCLONE BURNER

The main disadvantage of the three burners is that the fly-ash produced

requires expensive dust collectors.

This disadvantage is offset by cyclone burner.

It consists of a cylindrical drum having a diameter of 2 to 4 meters and

around 2.5 meter in length.

The coal-air mixture enters the burner from the left end. The coal is thrown

to the surface of the drum.



The secondary air enters through the tangential ports at high speeds. This

creates high turbulence.

This causes the temperature inside the drum to around 2000C. Thus the ash

formed is in the molten state, which is formed over the inner wall of the

furnace.

The burner is tilted at an angle. Hence the molten ash flows out of the burner.

PULVERISING MILLS or PULVERISERS

Pulverisers are the devices used for crushing the coal into finer particles

before burning them.

There are mainly four types of pulverizing mills

o Ball mill

o Bowl mill

o Ball and Race mill

o Impact mill



BOWL MILL

The bowl mill consists of stationary roller and a rotating bowl.

The stationary roller is attached to a spring which provides enough pressure

to crush the coal.

A bowl is rotated slowly by means of a bevel gear which is connected to the

motor.

The motor is also connected to the fan, which draws the air by creating

suction.

The raw coal is fed to the rotating bowl through a hopper, which falls in the

space between the rotating bowl and the roller.

Due to the pressure applied by the spring, the coal gets crushed due to

attrition.

Hot air is supplied from the bottom of the bowl, which carries the fine coal

towards the classifier.

The classifier, as the name suggests separates the fine coal from course coal.

The coarse coal is sent back to the bowl and the fine coal is sent to the burner

through the fan.



BALLL MILL

The major components of a ball mill consist of a cylinder, feeder, classifier,

and exhaust fan.

The cylinder consists of balls of varying size.

Raw coal enters initially to the classifier through the feeders.

Since there is no fine coal, all the coal particles enters the cylinder.

The cylinder is rotated slowly with respect to the horizontal axis.

The coal particles get in between the balls in the cylinder and due to attrition

the coal particles gets crushed.

Hot air is passed into the cylinder, which carries away the fine particles to the

classifier.

The coarse particles are sent back to the cylinder and fine particles are sent

to the burner or furnace through the exhaust fan.



BALLL AND RACE MILL

It is also known as contact mill, consisting of races, ball and classifier.

The crushing takes place between the races and the balls.

The pressure required for crushing is given by the spring which is connected

to the upper race.

The upper race is stationary and the lower race is rotated by means of a gear

arrangement, which also rotates the classifier

Coal is fed to the space between the races and due to the rotation of the

lower race and the moving balls, the crushing takes place.

Hot air is supplied from the side, which carries the fine coal.

Above the upper race, a classifier is placed, which is rotated by the gear.

The classifier creates a cyclone effect, which throws the lighter particles to

the discharge and heavier particles back to the race.



IMPACT MILL

This is called as impact mill since the primary stage of grinding takes place due

to impact.

The mill consists of an impact block, rotating pegs and stationary pegs.

It also consists of a shaft. The rotating pegs and the primary air fan is

connected to it.

Coal from the feeder enters the mill through the opening provided.

As the coal enters, it is first crushed into finer particles by the impact

blocks, which acts similar to a hammer.

The impact block is operated by means of an hydraulic system.

The crushed coal then enters the space between the stationary pegs and

rotating pegs.

The crushing here takes place due to attrition.

The fine coal now passes through a rejector arm, where the fine coal and

coarse coal are separated.

The fine coal enters the next region, where the fan is situated and later is

sent back to the furnace.

The Primary air fan acts an exhaust fan, which creates suction and hence the

coal travels along the system.



COAL HANDLING The various steps involved in coal handling is shown below

The coal is supplied to the power plant mainly

through railway wagons

The coal is then unloaded into underground

hoppers either manually or through some

mechanism

The coal which is in the form of big lumps is

crushed into small particles (not necessarily

pulversing)

The coal is then transferred from the

unloading point to the storage.

Coal is stored in order to ensure adequate

supply of coal irrespective of delay of

transportation

Covered storage also called as live storage is

the coal required for the days requirement

Before firing the coal, the coal is to be

measured and weighed for consistent burning

The coal is burned using certain firing

mechanisms to supply heat for the formation

of steam.

COAL DELIVERY

UNLOADING

PREPARATION

TRANSFER

OUTDOOR STORAGE

COVERED STORAGE

WEIGHING AND MEASURING

FURNACE FIRING



ASH HANDLING SYSTEMS

Unit-1 EE Updated

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