Power Plant

Engineering

EEE-4707

Prepared By:

Dr. Sikder Sunbeam Islam

Associate Professor,

Dept. of EEE, IIUC

Steam

Power

Plant

2

Introduction

A generating station which converts heat energy of coal

combustion into electrical energy is known as steam

power plant.

This type of power station is suitable where coal and water are

available in abundance and a large amount of electric power is

to be generated.

• A steam power station basically works on the Rankine cycle.

• Steam is produced in the boiler by utilizing the heat of coal

combustion.

• The steam is then directed to the prime mover (i.e., steam

turbine) and is condensed in a condenser to be fed into the

boiler again.

• The steam turbine drives the alternator which converts

mechanical energy of the turbine into electrical energy.

Basic Principle

Advantages and Disadvantages

3

Advantages:

• The fuel (i.e., coal) used is quite cheap.

• Less initial cost as compared to other generating

stations.

• It requires less space as compared to the hydroelectric

power station. The cost of generation is lesser than

that of the diesel power station.

Disadvantages:

• It pollutes the atmosphere due to the

production of large amount of smoke and

fumes.

• It is costlier in running cost as compared to

hydroelectric plant.

4

Schematic Arrangement of Steam Power

Station

The whole arrangement can be divided into

the following stages for the sake of simplicity :

1. Coal and ash handling arrangement

2. Steam generating plant

3. Steam turbine

4. Alternator

5. Feed water

6. Cooling arrangement

5

Fig.1. Steam Power Plant.

Schematic Arrangement

of Steam Power Station

Operating Procedure 6

• Coal received in coal storage yard of power station is

transferred in the furnace by coal handling unit.

• Heat produced due to burning of coal is utilized in

converting water contained in boiler drum into steam at

suitable pressure and temperature.

• The steam generated is passed through the superheater.

• Superheated steam then flows through the turbine. After

doing work in the turbine die pressure of steam is reduced.

• The steam turbine is coupled to an alternator. The

alternator converts mechanical energy of turbine into

electrical energy.

• Steam leaving the turbine passes through the condenser

which maintain the low pressure of steam at the exhaust of

turbine.

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• Water circulating through the condenser may be taken from

the various sources such as river, lake or sea.

• If sufficient quantity of water is not available the hot water

coming out of the condenser may be cooled in cooling towers

and circulated again through the condenser.

• Air taken from the atmosphere is first passed through the air

pre-heater, where it is heated by flue gases.

• The hot air then passes through the furnace. The flue gases

after passing over boiler and superheater tubes, flow through

the dust collector and then through economizer, air pre-

heater and finally they are exhausted to the atmosphere

through the chimney.

Operating Procedure (continues)

8

Choice of Site for Steam Power Stations

In order to achieve overall economy, the following points should be

considered while selecting a site for a steam power station :

(i) Supply of fuel. The steam power station should be located near the

coal mines so that transportation cost of fuel is minimum.

(ii) Availability of water. As huge amount of water is required for the

condenser, therefore, such a plant should be located at the bank of a river

or near a canal to ensure the continuous supply of water.

(iii) Transportation facilities. A modern steam power station often

requires the transportation of material and machinery. Therefore,

adequate transportation facilities must exist.

(iv) Cost and type of land. The steam power station should be located

at a place where land is cheap and further extension, if necessary, is

possible. Moreover, the bearing capacity of the ground should be adequate

so that heavy equipment could be installed.

(v) Nearness to load centers. In order to reduce the transmission cost,

the plant should be located near the center of the load.

(vi) Distance from populated area. To avoid the pollution arises form

smoke and fume of coal combustion, the plant should be located at a

considerable distance from the populated areas.

Coal and Ash Handling Plant

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The coal is transported to the power station by road or rail and is

stored in the coal storage plant.

From the coal storage plant, coal is delivered to the coal handling

plant where it is pulverized (i.e., crushed into small pieces) in order

to increase its surface exposure, thus promoting rapid combustion

without using large quantity of excess air.

The pulverized coal is fed to the boiler by belt conveyors [see Fig.2].

The coal is burnt in the boiler and the ash produced after the

complete combustion of coal is removed to the ash handling plant .

Then delivered to the ash storage plant for disposal.

The removal of the ash from the boiler furnace is necessary for proper

burning of coal.

Fig.2. Pulverized coal firing

Advantages and Disadvantages of Pulverized Coal

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Advantages

• Low grade coal can be burnt easily.

• Powdered coal has more surface area. They permits rapid and high

rates of combustions.

• High temperature can be produced in furnace.

• Using pulverized coal rate of combustion can be adjusted to meet

varying load.

• Free from clinker (stony residue) troubles.

Disadvantages

• It requires additional equipment to pulverized the coal.

• Pulverized coal firing produces fly ash (fine dust) which requires

separate fly ash removal equipment.

• There are more chances of explosion as coal burns like a gas.

• Pulverized fuel fired furnaces designed to burn a particular type of coal

can not be used to any other type of coal with same efficiency.

• Size of the coal used in pulverized coal furnace is limited to 70-100

microns.

Steam Generating Plant 11

The steam generating plant consists of a boiler for the production of steam and

other auxiliary equipment for the utilization of flue gases.

Boiler • The heat of combustion of coal in the boiler is utilized to convert water

into steam at high temperature and pressure.

• The flue gases from the boiler make their journey through superheater,

economiser, air pre-heater and are finally exhausted to atmosphere

through the chimney.

Superheater

• The steam produced in the boiler is wet and is passed through a

superheater where it is dried and superheated (i.e., steam

temperature increased above that of boiling point of water) by the flue

gases on their way to chimney [See Fig.3].

• The superheated steam from the superheater is fed to steam

turbine through the main valve.

Superheating provides two principal benefits:

• Firstly, the overall efficiency is increased.

• Secondly, too much condensation in the last stages of turbine (which

would cause blade corrosion) is avoided.

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Steam Generating Plant (Continues)

Fig.3. Superheater

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Steam Generating Plant (Continues)

Economiser

• An economiser is essentially a feed water heater

and derives heat from the flue gases for this purpose

(Fig.4a).

• The feed water is fed to the economiser before

supplying to the boiler.

Advantages:

• It reduces the losses of heat with flue gases.

• It reduces the combustion of fuel

Air preheater

• An air preheater (in Fig.4b) increases the

temperature of the air supplied for coal burning by

deriving heat from flue gases.

• Air is drawn from the atmosphere by a forced draught

fan and is passed through air preheater before

supplying to the boiler furnace.

Advantages:

• Accelerates the combustion and increases thermal

efficiency

• Increased steam capacity per square meter of boiler

surface. Fig.4b. Air preheater

Fig.4a. Economiser

Draught 14

Advantages:

• To supply required amount of air to the furnace for the combustion of

fuel

• To remove the gaseous products of combustion.

Balanced Draught System

• The balanced draught (in Fig.5) is a combination of forced and induced

draught.

• The forced draught fan forces the air through the fuel bed there fore

sufficient air is supplied to the fuel bed for proper and complete combustion.

• The induced draught fan removes the gases from the furnace maintaining

the pressure in the furnace just below atmosphere.

Fig.5.Balanced Draught

Draught is defined as difference between absolute gas pressure at any

point in a gas flow passage and ambient atmospheric pressure.

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By means of automatic combustion control it becomes easy to maintain a

constant steam pressure and uniform furnace draught (flow) and supply of

air or fuel can be regulated to meet the changes in steam demand. The boiler

operation becomes more flexible and better efficiency of combustion is

achieved. This saves manual labor also. Hagan system of automatic combustion

control is shown in Fig. 6.

Automatic Boiler Control

Fig. 6 Automatic

boiler control

16 Automatic Boiler Control(Cont.)

It has Four Sensors:

Steam flow sensor (SFS)

Steam pressure sensor (SPS)

A fuel flow sensor (FFS)

Air flow sensor (AFS)

Two Controller:

Fuel flow controller ( regulate opening-closing of fuel valve)

Air flow controller (regulate opening closing of dampers)

Fig. 4 Automatic

boiler control

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Operation • Steam pressure signal (from SPS) and flow signal (from SFS) are transmitted to a

computing relay.

• The computing relay generates the Master Signal (MS) depending on the load of the

boiler.

• The Master Signal represents the fuel requirements to the boiler.

• In the low signal selector (LSS), the MS is compared to with the air flow signal (from Air

flow sensor). The selector passes the MS only if it is lower (MS < Air flow signal).

• In the high signal selector (HSS) the MS is compared with the signal representing fuel

flow (from FFS). The selector passes the MS if it is higher (MS > FFS).

• When the steam demand is high, high signal selector passes the master signal to the Air

flow controller.

Automatic Boiler Control(Cont.)

Fig. 4 Automatic

boiler control

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• The Air flow controller increases the air flow by adjusting the damper position.

• As air flow increases, the air flow sensor transmit the signal to the low signal selector

and air flow controller.

• The MS passes to the Fuel flow controller to increase the flow of fuel by adjusting the

fuel valve position.

• If the steam demand is low, the low signal selector (LSS) permits the master signal pass

to the fuel flow controller (FFC). The FFC decreases the flow of fuel by adjusting the fuel

valve,

• As the fuel flow decreases, the FFS sensor transmit the signal to high signal selector and

to FFC controller.

• The HSS sends the MS to AFC controller to decrease the air flow by adjusting the

damper. So the whole control system prevents the incomplete composition of fuel with

changing steam demands.

Automatic Boiler Control(Cont.)

Fig. 4 Automatic

boiler control

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Types of Boiler

According to flow of water and hot gases boiler can be classified as :Fire

tube and water tube boiler (See in Fig. 7).

The fire tube boiler passes combustion gas inside a series of tubes

surrounded by water in a vessel to produce steam, while a water tube

instead sends water through a series of tubes surrounded by combustion

gas used to transfer heat energy and produce steam.

Fig. 7

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Fire Tube boiler

Water Tube Boiler

.1. Hot flue gases flow inside the tube

and the water outside the tube.

Water flows inside the tube and the

flue gases outside the tube. 2. These boilers are generally low or

medium pressure boiler.

These boilers are generally high

pressure boiler. 3. Fire tube boiler have lower rate of

steam production compare to water

tube.

It has higher rate of steam production.

4. The maintenance of this boiler is

costly. It is required regular

inspection.

This boiler is easy to maintain.

5. It is suitable for the production

work like sugar mill, textile

industries.

It is suitable for large power plant.

6. This boiler is difficult to construct. This is simple in construction.

7. Example: Lancashire boiler Example: Babcock and Wilcox boiler

Comparison Between Water tube and Fire tube boiler

Boiler Requirements

The boiler should be safe under normal operating condition.

The various parts of the boiler should be accessible for repair and

maintenance.

The boiler should be capable of supplying steam according to the

demand.

The boiler should be able to absorb a maximum amount of heat

produced due to burning of fuel in the furnace.

It should be capable of quick starting and loading.

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Steam Turbine

• The dry and superheated steam from the superheater is fed to the steam

turbine through main valve and passed over the blades of turbine which

is converted into mechanical energy from heat energy.

• After giving heat energy to the turbine, the steam is exhausted to the

condenser which condenses the exhausted steam by means of cold water

circulation.

Furnace: Fuel is burnt in a confined space called furnace. The furnace provides supports

and enclosure for burning equipment. Solid fuels such as coal, coke, wood etc.

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Alternator.

• The steam turbine is coupled to an alternator. The alternator converts

mechanical energy of turbine into electrical energy.

• The electrical output from the alternator is delivered to the bus bars

through transformer, circuit breakers and isolators.

Feed Water.

• The condensate from the condenser is used as feed water to the boiler.

Some water may be lost in the cycle which is suitably made up from

external source.

• The feed water on its way to the boiler is heated by water heaters and

economiser. This helps in raising the overall efficiency of the plant.

Water Treatment Plant

Usually high purity feed water for boiler is required to ensure proper

operation of steam generation systems. When a boiler is used to run a

steam turbine, turbine blade erosion is reduced due to higher purity

steam generation. Therefore, the impurities present in the raw water

should be removed before it can be used in the boiler.

A basic boiler feed water treatment system typically includes some type of:

Filtration/ultrafiltration Ion exchange/softening Reverse osmosis and

Nano filtration Coagulation/Chemical precipitation

Steam Condensers 23

• In order to improve the efficiency of the plant, the steam exhausted from

the turbine is condensed by means of a condenser. Water is drawn

from a natural source of supply such as a river, canal or lake and is

circulated through the condenser.

• The circulating water takes up the heat of the exhausted steam and

itself becomes hot. This hot water coming out from the condenser is

discharged at a suitable location down the river.

• In case the availability of water from the source of supply is not assured

throughout the year, cooling towers are used. The cold water from the

cooling tower is reused in the condenser.

The steam condensers are broadly classified into two types [Fig.8]:

Surface condensers (or non-mixing type condensers): In surface

condensers, there is no direct contact between the exhaust steam and the

cooling water.

Jet condensers (or mixing type condensers): In jet condensers there is

direct contact between the exhaust steam and cooling water.

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S.NO Surface Condenser Jet or Direct Contact Condenser

1 Steam and Cooling water are

not mixed up. Cooling water and steam are mixed up

2 More suitable for high capacity

plants. Less suitable for high capacity plants.

3 Condensate is reused. Condensate is wasted

4 It requires large quantity of

circulating water.

It requires less quantity of circulating

water

5 The condensing plant is costly

and complicated.

Condensing plant is economical and

simple

6 Its maintenance cost is high. Its maintenance cost is low.

7 Less power is required for air

pump. More power is required for air pump.

Difference between surface condenser and Jet condenser

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Difference between surface condenser and Jet condenser

Fig. 8

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Problems 28

[365day ×24hrs=8760 Hrs., 1 ton [metric] = 1 000 kg.

29

Problems continues.

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Problems continues.

**Try to practice exercises in text books.**

References:

1. Principles of Power

Systems,V.K.Mehta&RohitMehta

2. Power Plant Engineering, G.R.Nagpal

3. Power Station Engineering &Economy, William A

Vopat

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End

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Fig. Relative Position of Superheater, Economiser and Air Pre-heater.