unit i thermal power plants

7/28/2019 Unit i Thermal Power Plants

1/37

1

Unit I Course material

N.KARTHIKEYANUNIT I

EE1252-POWER PLANT ENGINEERING

UNIT I THERMAL POWER PLANTS

Basic thermodynamic cycles -Various components of steam power plant-LayoutPulverized coal burners- Fluidized bed combustion-Coal handling systems-Ash handling

systems-Forced draft and induced draft fans-Boilers-Feed pumps- Super heater- Regenerator- Condenser-Dearearators-Cooling tower.

A thermodynamic cycle consists of a series of thermodynamic processes transferring heat and

work, while varying pressure, temperature, and other state variables, eventually returning

a system to its initial state

The Zeroth Law of Thermodynamics

This law states that if object A is in thermal equilibrium with object B, and object B is

in thermal equilibrium with object C, then object C is also in thermal equilibrium with

object A. This law allows us to build thermometers. For example the length of a mercury

column (object B) may be used as a measure to compare the temperatures of the two otherobjects.

The first law of thermodynamics

The first law of thermodynamics says that energy cant be destroyed or created. Whenone energy form is converted into another, the total amount of energy remains constant. An

example of this law is a gasoline engine. The chemical energy in the fuel is converted intovarious forms including kinetic energy of motion, potential energy, chemical energy in the

carbon dioxide, and water of the exhaust gas.

The second law of thermodynamicsThe second law of thermodynamics is the entropy law, which says that all physical

processes proceed in such a way that the availability of the energy involved decreases. This

means that no transformation of energy resource can ever be 100% efficient.


2/37

2

The Third Law of Thermodynamics

The Third Law of Thermodynamics refers to a state known as "absolute zero." This is

the bottom point on the Kelvin temperature scale. The Kelvin scale is absolute, meaning 0

Kelvin is mathematically the lowest possible temperature in the universe. This corresponds to

about -273.15 Celsius, or -459.7 Fahrenheit.

Power plants cycle generally divided in to the following groups,

(1) Vapour Power Cycle

(Carnot cycle, Rankine cycle, Regenerative cycle, Reheat cycle, Binary vapour cycle)

(2) Gas Power Cycles

(Otto cycle, Diesel cycle, Dual combustion cycle, Gas turbine cycle.)

CARNOT CYCLEThis cycle is of great value to heat power theory although it has not been possible to construct

a Practical plant on this cycle. It has high thermodynamics efficiency.

It is a standard of comparison for all other cycles. The thermal efficiency () of Carnot cycle

is as follows:

The most efficient heat engine cycle is the Carnot cycle, consisting of two isothermal

processes and two adiabatic processes. The Carnot cycle can be thought of as the most

efficient heat engine cycle allowed by physical laws. When the second law of

thermodynamics states that not all the supplied heat in a heat engine can be used to do work,

the Carnot efficiency sets the limiting value on the fraction of the heat which can be so used.

= (T1 T2)/T1

Where, T1 = Temperature of heat source

T2 = Temperature of receiver

In order to approach the Carnot efficiency, the processes involved in the heat engine

cycle must be reversible and involve no change in entropy. This means that the Carnot cycle

is an idealization, since no real engine processes are reversible and all real physical processes

involve some increase in entropy.

RANKINE CYCLE

The Rankine cycle is a thermodynamic cycle which converts heat into work. The heat

is supplied externally to a closed loop, which usually uses water as the working fluid. This

cycle generates about 80% of all electric power used throughout the world, including


3/37

3

virtually all solar thermal, biomass, coal and nuclear power plants. It is named after WilliamJohn Macquorn Rankine, a Scottish polymath.

Steam engine and steam turbines in which steam is used as working medium follow

Rankine cycle. This cycle can be carried out in four pieces of equipment joint by pipes for

conveying working medium as shown in Fig. 1.1. The cycle is represented on PressureVolume P-V and S-T diagram as shown in Figs. 1.2 and 1.3 respectively.

Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid

at this stage the pump requires little input energy.

Process 2-3: The high pressure liquid enters a boiler where it is heated at constantpressure by an external heat source to become a dry saturated vapour.

Process 3-4: The dry saturated vapour expands through a turbine, generating power. This

decreases the temperature and pressure of the vapour, and some condensation

may occur.

Process 4-1: The wet vapour then enters a condenser where it is condensed at a constant


4/37

4

pressure and temperature to become a saturated liquid. The pressure andtemperature of the condenser is fixed by the temperature of the cooling coils as

the fluid is undergoing a phase-change.

Brayton cycle

The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine

engine, basis of the jet engine and others. It is named after George Brayton (18301892),

Actual Brayton cycle:

adiabatic process - Compression.

isobaric process - Heat addition.

adiabatic process - Expansion.

isobaric process - Heat rejection.

The efficiency of the ideal Brayton cycle is ,

Where is the heat capacity ratio.


5/37

5

THERMAL POWER PLANTS LAYOUT

1. A furnace to burn the fuel.2. Steam generator or boiler containing water. Heat generated in the furnace is utilized to

convert water in steam.

3. Main power unit such as an engine or turbine to use the heat energy of steam andperform work.

4. Piping system to convey steam and water.

In addition to the above equipment the plant requires various auxiliaries and accessories

depending upon the availability of water, fuel and the service for which the plant is intended.

The flow sheet of a thermal power plant consists of the following four main circuits:

1. Feed water and steam flow circuit2. Coal and ash circuit3. Air and gas circuit4. Cooling water circuit.

A steam power plant using steam as working substance works basically on Rankine cycle.Steam is generated in a boiler, expanded in the prime mover and condensed in the condenser

and fed into the boiler again.

The different types of systems and components used in steam power plant are as follows:

1. High pressure boiler2. Prime mover3. Condensers and cooling towers4. Coal handling system5. Ash and dust handling system6. Draught system

7. Feed water purification plant


6/37

6

8. Pumping system9. Air pre-heater, economizer, super heater, feed heaters.

Fig. shows a schematic arrangement of equipment of a steam power station. 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 boilerdrum into steam at suitable pressure and temperature. The steam generated is passed through

the super heater. Superheated steam then flows through the turbine. After doing work in the

turbine pressure of steam is reduced. Steam leaving the turbine passes through the condenser

which maintains the low pressure of steam at the exhaust of turbine.

Steam pressure in the condenser depends upon flow rate and temperature of cooling water

and on effectiveness of air removal equipment. 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. Bled steam taken from the turbine at suitable

extraction points is sent to low pressure and high pressure water heaters.

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 super heater tubes, flow through the dust collector and then through economiser,air pre-heater and finally they are exhausted to the atmosphere through the chimney.

Steam condensing system consists of the following:

1. Condenser2. Cooling water3. Cooling tower4. Hot well5. Condenser cooling water pump.

CHARACTERISTICS OF STEAM POWER PLANT

The desirable characteristic for a steam power plant are as follows:

1. Higher efficiency.2. Lower cost.3. Ability to burn coal especially of high ash content.4. Reduced environmental impact in terms of air pollution.5. Reduced water requirement.6. Higher reliability and availability.


7/37

7

Various components of steam power plant

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 steamturbine

20. Forced draught (draft) fan

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

4. Step-up transformer (3-

phase)13. Feed water 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

turbine18. Bottom ash hopper 27. Flue gas stack


8/37

8

Main components of steam power plant1. Boiler2. Turbine

3. Deaerator4. Heat Exchangers5. Super Heater6. Economizers7. Condenser8. Feed water heater9. Electrical generator.

1. Boiler

A boiler is an enclosed vessel that provides a means for combustion heat to be transferred

into water until it becomes heated water or steam. The hot water or steam under pressure is

then usable for transferring the heat to a process. Water is a useful and cheap medium for

transferring heat to a process. When water is boiled into steam its volume increases about

1,600 times, producing a force that is almost as explosive as gunpowder. This causes the

boiler to be extremely dangerous equipment that must be treated with maximum care.

Transfer of heat

Heat is transferred from one body to another by means of

(1) Radiation, which is the transfer of heat from a hot body to a cold body without aconveying medium,

(2) Convection, the transfer of heat by a conveying medium, such as air or water.

(3) Conduction, transfer of heat by actual physical contact, molecule to molecule.

TurbineTurbine that convert the energy from moving stream to mechanical energy. The basic

element in a turbine is a wheel or rotor with paddles, propellers, blades, or buckets arranged

on its circumference in such a fashion that the moving fluid exerts a tangential force that

turns the wheel and imparts energy to it. This mechanical energy is then transferred through a

drive shaft to operate a machine, compressor, electric generator, or propeller. Turbines areclassified as hydraulic, or water, turbines, steam turbines, or gas turbines. Today turbine-

powered generators produce most of the world's electrical energy. Windmills that generate

electricity are known as wind turbines.

Deaerator

A deaerator is a device that is widely used for the removal of air and other dissolved gasesfrom the feed water to steam generating boilers. In particular, dissolved oxygen in boiler feed

waters will cause serious corrosion damage in steam systems by attaching to the walls of


9/37

9

metal piping and other metallic equipment and forming oxides (rust). It also combines withany dissolved carbon dioxide to form carbonic acid that causes further corrosion.

Heat Exchangers

Heat exchangers are equipment that transfers heat from one medium to another. The properdesign, operation and maintenance of heat exchangers will make the process energy efficient

and minimize energy losses. Heat exchanger performance can weaken with time, off design

operations and other interferences such as fouling, scaling etc. It is necessary to assess

periodically the heat exchanger performance in order to maintain them at a high efficiency

level.

Heat exchangers may be classified according to their flow arrangement. In parallel-flow heat

exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one

another to the other side. In counter flow heat exchangers the fluids enter the exchanger from

opposite ends. The counter current design is most efficient, in that it can transfer the most

heat. See counter current exchange. In a cross-flow heat exchanger, the fluids travel roughly

perpendicular to one another through the exchanger.

For efficiency, heat exchangers are designed to maximize the surface area of the wall

between the two fluids, while minimizing resistance to fluid flow through the exchanger. The

exchanger's performance can also be affected by the addition of fins or corrugations in one orboth directions, which increase surface area and may channel fluid flow or induce turbulence.

Super Heater

A super heater is a device in a steam engine that heats the steam generated by the boileragain, increasing its thermal energy and decreasing the possibility that it will condense inside

the engine. A super heater is a device used to convert saturated steam or wet steam

into dry steam used for power generation or processes. Super heaters increase the efficiency

of the steam engine, and were widely adopted

1. A radiant super heater is placed directly in the combustion chamber.

2. A convection super heater is located in the path of the hot gases.

3. A separately fired super heater, as its name implies, is totally separated from the

boiler

CondenserA condenser is a device or unit used to condense a substance from its gaseous to

its liquid state, typically by cooling it. The surface condenser is a shell and tube heat

exchanger in which cooling water is circulated through the tubes. The exhaust steam from the

low pressure turbine enters the shell where it is cooled and converted to condensate (water)

by flowing over the tubes as shown in the adjacent diagram.


10/37

10

Such condensers use steam ejectors or rotary motor-driven exhausters for continuous removal

of air and gases from the steam side to maintain vacuum.For best efficiency, the temperature in the condenser must be kept as low as practical in order

to achieve the lowest possible pressure in the condensing steam. Since the condensertemperature can almost always be kept significantly below 100 oC where the vapor pressure

of water is much less than atmospheric pressure, the condenser generally works under

vacuum.

EconomizersA boiler economizer is a heat exchanger device that captures the "lost or waste heat"

from the boiler's hot stack gas. The economizer typically transfers this waste heat to the

boiler's feed-water, but it can also be used to heat domestic water or other process fluids.

Capturing this normally lost heat reduces the overall fuel requirements for the boiler. Less

fuel compare to money saved as well as less emission - since the boiler now operates at a

higher efficiency. This is possible because the boiler feed-water or return water is pre-heated

by the economizer therefore the boilers main heating circuit does not need to provide as muchheat to produce a given output quantity of steam or hot water. Again fuel savings are the

result. Boiler economizers improve a boiler's efficiency by extracting heat from the flue gases

discharged.

Feed water heaterIn the case of a conventional steam-electric power plant utilizing a drum boiler, the surface

condenser removes the latent heat of vaporization from the steam as it changes states from

vapour to liquid. The condensate pump then pumps the condensate water through a feed

water heater. The feed water heating equipment then raises the temperature of the water by

utilizing extraction steam from various stages of the turbine.


11/37

11

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 isintroduced back into the steam cycle.

Electrical generatorIn electricity generation, an electrical generator is a device that converts mechanical

energy to electrical energy, generally using electromagnetic induction. The source ofmechanical energy may be a reciprocating or turbine steam engine, water falling through a

turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, the sun

or solar energy, compressed air or any other source of mechanical energy.

PULVERIZED COALCoal is pulverized (powdered) to increase its surface exposure thus permitting rapid

combustion. Efficient use of coal depends greatly on the combustion process employed. For

large scale generation of energy the efficient method of burning coal is restricted still to

pulverize coal combustion. The pulverized coal is obtained by grinding the raw coal in

pulverising mills.

The various pulverising mills used are as follows:1. Ball mill2. Hammer mill3. Ball and race mill4. Bowl mill.

The essential functions of pulverising mills are as follows:1. Drying of the coal2. Grinding

Proper drying of raw coal which may contain moisture is necessary for effective grinding.

BALL MILL

A line diagram of ball mill using two classifiers is shown in Fig. It consists of a

slowly rotating drum which is partly filled with steel balls. Raw coal from feeders is supplied

to the classifiers from where it moves to the drum by means of a screw conveyor.


12/37

12

As the drum rotates the coal gets pulverized due to the combined impact between coal andsteel balls. Hot air is introduced into the drum. The powdered coal is picked up by the air and

the coal air mixture enters the classifiers, where sharp changes in the direction of the mixturethrow out the oversized coal particles. The over-sized particles are returned to the drum. The

coal air mixture from the classifier moves to the exhauster fan and then it is supplied to the

burners.

BALL MILL

BALL AND RACE MILL

In this mill the coal passes between the rotating elements again and again until it has

been pulverized to desired degree of fineness. The coal is crushed between two movingsurfaces namely balls and races. The upper stationary race and lower rotating race driven by a

worm and gear hold the balls between them. The raw coal supplied falls on the inner side of

the races. The moving balls and races catch coal between them to crush it to a powder. The

necessary force needed for crushing is applied with the help of springs. The hot air supplied

picks up the coal dust as it flows between the balls and races, and then enters the classifier.

Where oversized coal particles are returned for further grinding, where as the coal particles of

required size are discharged from the top of classifier.

BALL AND RACE MILL


13/37

13

PULVERIZED COAL BURNERS

Burners are used to burn the pulverised coal. The main difference between the various

burners lies in the rapidity of air-coal mixing i.e., turbulence. For bituminous coals the

turbulent type of burner is used whereas for low volatile coals the burners with long flame

should be used. A pulverised coal burner should satisfy the following requirements:

1. It should mix the coal and primary air thoroughly and should bring this mixture beforeit enters the furnace in contact with additional air known as secondary air to create

sufficient turbulence.2. It should deliver and air to the furnace in right proportions and should maintain stable

ignition of coal air mixture and control flame shape and travel in the furnace.

3. The flame shape is controlled by the secondary air vanes and other control

adjustments incorporated into the burner.

4. Secondary air if supplied in too much quantity may cool the mixture and prevent its

heating to ignition temperature.

Pulverized coal burners

A Pulverised Coal Burner System.


14/37

14

1. The concept of burning coal that has been pulverized into a fine powder stems fromthe belief that if the coal is made fine enough, it will burn almost as easily and

efficiently as a gas.

2. The feeding rate of coal according to the boiler demand and the amount of airavailable for drying and transporting the pulverized coal fuel is controlled by

computers.3. Pieces of coal are crushed between balls or cylindrical rollers that move between two

tracks or "races." The raw coal is then fed into the pulveriser along with air heated to

about 650 degrees F from the boiler.4. As the coal gets crushed by the rolling action, the hot air dries it and blows the usable

fine coal powder out to be used as fuel.

5. The powdered coal from the pulveriser is directly blown to a burner in the boiler. Theburner mixes the powdered coal in the air suspension with additional pre-heated

combustion air and forces it out of a nozzle similar in action to fuel being atomized by

a fuel injector in modern cars.6. Under operating conditions, there is enough heat in the combustion zone to ignite all

the incoming fuel.

The various types of burners are as follows:

1. Long Flame Burner (U-Flame Burner). In this burner air and coal mixture travels a

considerable distance thus providing sufficient time for complete combustion [Fig. (a)].

2. Short Flame Burner (Turbulent Burner). It is shown in Fig. (b). The burner is fitted inthe furnace will and the flame enters the furnace horizontally.

3. Tangential Burner. A tangential burner is shown in Fig. (c). In this system one burner isfitted attach corner of the furnace. The inclination of the burner is so made that the flame

produced are tangential to an imaginary circle at the centre.

4. Cyclone Burner. It is shown in Fig. (d). This burner uses crushed coal intend of pulverised

coal. Its advantages are as follows:


15/37

15

FLUIDISED BED COMBUSTION (FBC)

Need for FBCBurning of pulverised coal has some problems such as particle size of coal used inpulverised firing is limited to 70-100 microns, the pulverised fuel fired furnaces designed to

burn a particular cannot be used other type of coal with same efficiency, the generation of

high temp. about (1650 C) in the furnace creates number of problems like slag formation on

super heater, evaporation of alkali metals in ash and its deposition on heat transfer surfaces,

formation of SO2 and NOX in large amount.

Working Fluidised Bed Combustion


16/37

16

1. Fluidised Bed combustion system can burn any fuel including low grade coals (evencontaining 70% ash), oil, gas or municipal waste.

2. Fluidized bed combustion (FBC) is a combustion technology used in power plants.Fluidized beds suspend solid fuels on upward-blowing jets of air during the

combustion process.

3. The result is a turbulent mixing of gas and solids. The tumbling action, much like abubbling fluid, provides more effective chemical reactions and heat transfer.

4. FBC plants are more flexible than conventional plants in that they can be fired oncoal and biomass, among other fuels.

TypesFBC systems fit into essentially two major groups, atmospheric systems (FBC) and

pressurized systems (PFBC), and two minor subgroups, bubbling (BFB) and circulating

fluidized bed (CFB).

FBC

Atmospheric fluidized beds use limestone or dolomite to capture sulfur released by thecombustion of coal. Jets of air suspend the mixture of sorbent and burning coal during

combustion, converting the mixture into a suspension of red-hot particles that flow like a

fluid. These boilers operate at atmospheric pressure.

PFBCThe first-generation PFBC system also uses a sorbent and jets of air to suspend the mixture of

sorbent and burning coal during combustion. However, these systems operate at elevated

pressures and produce a high-pressure gas stream at temperatures that can drive a gas turbine.

Steam generated from the heat in the fluidized bed is sent to a steam turbine, creating a highly

efficient combined cycle system.


17/37

17

Circulating Fluidized Bed combustion

Fine particles of partly burned coal, ash and bed material are carried along with the flue gases

to the upper areas of the furnace and then into a cyclone. In the cyclone the heavier particles

separate from the gas and falls to the hopper of the cyclone. This returns to the furnace for

recirculation. Hence the name Circulating Fluidized Bed combustion. The hot gases from thecyclone pass to the heat transfer surfaces and go out of the boiler.

COAL HANDLING SYSTEMS

Coal delivery equipment is one of the major components of plant cost. The various steps involved

in coal handling are as follows

1. Coal delivery2. Unloading3. Preparation4. Transfer5. Outdoor storage6. Covered storage7. In plant handling8. Weighing and measuring9. Feeding the coal into furnace.

Coal Delivery. The coal from supply points is delivered by ships or boats to power stations

situated near to sea or river whereas coal is supplied by rail or trucks to the power stationswhich are situated away from sea or river. The transportation of coal by trucks is used if the

railway facilities are not available.

Unloading. The type of equipment to be used for unloading the coal received at the power

station depends on how coal is received at the power station. If coal is delivered by trucks,

there is no need of unloading device as the trucks may dump the coal to the outdoor storage.Coal is easily handled if the lift trucks with scoop are used. In case the coal is brought by

railway wagons, ships or boats, the unloading may be done by car shakes, rotary car dumpers,

cranes, grab buckets and coal accelerators. Rotary car dumpers although costly are quite

efficient for unloading closed wagons.


18/37

18

Preparation. When the coal delivered is in the form of big lumps and it is not of proper size,

the preparation (sizing) of coal can be achieved by crushers, breakers, sizers driers andmagnetic separators.

Transfer. After preparation coal is transferred to the dead storage by means of the following

systems:

1. Belt conveyors.2. Screw conveyors.3. Bucket elevators.4. Grab bucket elevators.

3. Bucket elevators.

1. Belt conveyors.

2. Screw conveyors.

4. Grab bucket elevators.

1. Belt conveyor. Fig. shows a belt conveyor. It consists of an endless belt. Moving over a

pair of end drums (rollers). At some distance a supporting roller is provided at the middle.

The belt is made, up of rubber or canvas. Belt conveyor is suitable for the transfer of coal

over long distances. It is used in medium and large power plants. The initial cost of the

system is not high and power consumption is also low. The inclination at which coal can be

successfully elevated by belt conveyor is about 20. Average speed of belt conveyors varies

between 200-300 r.p.m. This conveyor is preferred than other types.


19/37

19

Advantages of belt conveyor1. Its operation is smooth and clean.2. It requires less power as compared to other types of systems.3. Large quantities of coal can be discharged quickly and continuously.4. Material can be transported on moderates inclines.

2. Screw conveyor. It consists of an endless helicoids screw fitted to a shaft. The screw while

rotating in a trough transfers the coal from feeding end to the discharge end. This system is

suitable, where coal is to be transferred over shorter distance and space limitations exist. The

initial cost of the system is low. It suffers from the drawbacks that the power consumption is

high and there is considerable wear of screw. Rotation of screw varies between 75-125 r.p.m.

3. Bucket elevator. It consists of buckets fixed to a chain Fig. The chain moves over two

wheels. The coal is carried by the buckets from bottom and discharged at the top.

4. Grab bucket elevator. It lifts and transfers coal on a single rail or track from one point to

the other. The coal lifted by grab buckets is transferred to overhead bunker or storage. Thissystem requires less power for operation and requires minimum maintenance. The grab

bucket conveyor can be used with crane or tower as shown in Fig. Although the initial cost ofthis system is high but operating cost is less.

DEWATERING OF COAL

Excessive surface moisture of coal reduces and heating value of coal and creates handling

problems. The coal should therefore be dewatered to produce clean coal. Cleaning of coal has

the following advantages:

1. Improved heating value.

2. Easier crushing and pulverising3. Improved boiler performance4. Less ash to handle.5. Easier handling.6. Reduced transportation cost.

Ash Handling Systems

ASH DISPOSAL

A large quantity of ash is, produced in steam power plants using coal. Ash produced

in about 10 to 20% of the total coal burnt in the furnace. Handling of ash is a problem

because ash coming out of the furnace is too hot, it is dusty and irritating to handle and isaccompanied by some poisonous gases. It is desirable to quench the ash before handling due

to following reasons:

1. Quenching reduces the temperature of ash.2. It reduces the corrosive action of ash.3. Ash forms clinkers by fusing in large lumps and by quenching clinkers will

disintegrate.

4. Quenching reduces the dust accompanying the ash.

Handling of ash includes its removal from the furnace, loading on the conveyors and

delivered to the fill from where it can be disposed off.


20/37

20

Types of Ash:

1. Fly Ash2. Bottom Ash3. Mill Reject

Ash Handling Systems

1. Hydraulic system2. pneumatic system3. Mechanical system.

The commonly used ash discharge equipment is as follows:1. Rail road cars2. Motor truck3. Barge.


21/37

21

Mechanical ash handling system. Water jet system.

Hydraulic System.

1. In this system, ash from the furnace grate falls into a system of water possessing highvelocity and is carried to the sumps. It is generally used in large power plants.

2. Hydraulic system is of two types namely low pressure hydraulic system used forcontinuous removal of ash and high pressure system which is used for intermittent ash

disposal.

3. In this method water at sufficient pressure is used to take away the ash to sump. Wherewater and ash are separated. The ash is then transferred to the dump site in wagons, railcars or trucks.

4. The loading of ash may be through a belt conveyor, grab buckets. If there is an ash

basement with ash hopper the ash can falls, directly in ash car or conveying system.

Mechanical ash handling system.

Fig. shows a mechanical ash handling system. In this system ash cooled by water seal

falls on the belt conveyor and is carried out continuously to the bunker. The ash is then

removed to the dumping site from the ash bunker with the help of trucks.

Pneumatic system.

1. In this system ash from the boiler furnace outlet falls into a crusher where larger ashparticles are crushed to small sizes.

2. The ash is then carried by a high velocity air or steam to the point of delivery. Air leavingthe ash separator is passed through filter to remove dust etc. so that the exhauster handles

clean air which will protect the blades of the exhauster.

Water Jetting.

Water jetting of ash is shown in Fig. In this method a low pressure jet of water

coming out of the quenching nozzle is used to cool the ash. The ash falls into a trough and is

then removed.


22/37

22

DRAFT FANS

The draft system is one of the most essential systems of thermal power plant. The

purpose of draught is to supply required quantity of air for combustion and remove the burnt

products from the system.

To move the air through the fuel bed and to produce a flow of hot gas through theboiler, economiser, preheater and chimney.

Types of draft (draught) system1. Natural draft system.2. Artificial draft system.

1. Forced draft system.2. Induced draft system.3. Balanced draft system.

1. Natural draft system.

Natural draft is obtained with the help of tall chimney. A chimney is a vertical tubular

structure of building material; brick, steel or reinforced concrete. This is designed once at the

initial stage only future expansion is not possible. The air circulation is due to pressure

difference and also temperature difference. Normally the pressure in the chimney at top and

at bottom is different due to its variation of atmospheric air level.Advantage

a. It does not require any external power for producing the draught.b. Chimney keeps the flue gases at a high place in the atmosphere which prevents the

contamination of atmosphere in a crowded locality and maintains the cleanliness.

c. It has long life.Imitationsa. It has been seen that draught produced by chimney is affected by the atmospheric

conditions.

b. It has no flexibility, poor efficiency and tall chimney is required.

2. Artificial draft system.

1. Forced draft system.

In a forced draught system, a blower is installed near the base of the boiler and

air is forced to pass through the furnace, flues, economiser, air-pre heater and to the


23/37

23

stack. This draught system is known as positive draught system or forced draughtsystem because the pressure of air throughout the system is above atmospheric

pressure and air is forced to flow through the system.

2. Induced draft system.

In the induced draught system, the blower is located near the base of the

chimney instead of near the gate. The air is sucked in the system by reducing the

pressure through the system below the atmosphere. The induced fan sucks the burnedgases from the furnace and the pressure inside the furnace is reduced below

atmosphere and induces the atmospheric air to flow through the furnace. The action of

the induced draught is similar to the action of the chimney.

Comparison of forced and induced draught system.

1. The size and power required by the induced draft fan is more than the forceddraught because the induced draft fan handles more gases and at elevated

temperature.

2. Water cooled bearing are required foe induced draught fan to withstand the hightemperatures of the flue gases.

3. When the doors are opened for firing in case of induced draught fan, there will berush of cold air in to the furnace and this reduces the draught through the system

and reduces the heat transmission sufficiency of the surfaces.

3. Balanced draft system.


24/37

24

1. It is always preferable to use a combination of forced draught and induced draftinstead of forced or induced draft system alone.

2. If the forced draft is used alone, then the furnace cannot be opened either for firingor inspection because the high pressure air inside the furnace will try to blow out

suddenly and there is every chance of blowing out the fire completely and furnacestop.

3. If the induced draft system is used alone, then also furnace cannot be openedeither for firing or inspection because the cold air will try to rush in to the furnace

as the pressure inside the furnace is below atmospheric pressure. This reduces the

effective draft.4. To overcome both the difficulties mentioned above either using forced draught or

induced draught alone, a balanced draft system is used.

Boilers

Boiler is an apparatus to produce steam. Thermal energy released by combustion offuel is transferred to water, which vaporizes and gets converted into steam at the desired

temperature and pressure.

The steam produced is used for:1. Producing mechanical work by expanding it in steam engine or steam turbine.2. Heating the residential and industrial buildings3. Performing certain processes in the sugar mills, chemical and textile industries.

Boiler is a closed vessel in which water is converted into steam by the application of heat.

Usually boilers are coal or oil fired.

A boiler should fulfil the following requirements

1. Safety. The boiler should be safe under operating conditions.2. Accessibility. The various parts of the boiler should be accessible for repair and

maintenance.

3. Capacity. The boiler should be capable of supplying steam according to therequirements.

4. Efficiency. To permit efficient operation, the boiler should be able to absorb amaximum

5. Amount of heat produced due to burning of fuel in the furnace.6. It should be simple in construction and its maintenance cost should be low.7. Its initial cost should be low.8. The boiler should have no joints exposed to flames.9. The boiler should be capable of quick starting and loading.

The performance of a boiler may be measured in terms of its evaporative capacity also called

power of a boiler. It is defined as the amount of water evaporated or steam produced in kg per

hour. It may also be expressed in kg per kg of fuel burnt or kg/hr/m2 of heating surface.


25/37

25

According to flow of water and hot gases.

1. Water tube.2. Fire tube.

1. Water tube.

1) Water tube boilers are designed to circulate hot combustion gases around the outsideof a large number of water filled tubes.

2) The tubes extend between an upper header, called a steam drum, and one or lowerheaders or drums. In the older designs, the tubes were either straight or bent into

simple shapes.

3) Newer boilers have tubes with complex and diverse bends. Because the pressure isconfined inside the tubes, water tube boilers can be fabricated in larger sizes and usedfor higher-pressure applications.

4) Small water tube boilers, which have one and sometimes two burners, are generallyfabricated and supplied as packaged units.

5) Because of their size and weight, large water tube boilers are often fabricated inpieces and assembled in the field.

6) In water tube or water in tube boilers, the conditions are reversed with the waterpassing through the tubes and the hot gases passing outside the tubes.

7) These boilers can be of a single- or multiple-drum type. They can be built to anysteam capacity and pressures, and have higher efficiencies than fire tube boilers.


26/37

26

The features of water tube boilers are:

1) Forced, induced and balanced draft provisions help to improve combustion efficiency.2) Less tolerance for water quality calls for water treatment plant.

3) Higher thermal efficiency levels are possible.

2. Fire tube.

1) Fire tube boilers consist of a series of straight tubes that are housed inside a water-filled outer shell.

2) The tubes are arranged so that hot combustion gases flow through the tubes. As thehot gases flow through the tubes, they heat the water surrounding the tubes.

3) The water is confined by the outer shell of boiler. To avoid the need for a thick outershell fire tube boilers are used for lower pressure applications.

4) Generally, the heat input capacities for fire tube boilers are limited to 50 tons per hour

or less, but in recent years the size of fire tube boilers has increased.

5) Most modern fire tube boilers have cylindrical outer shells with a small roundcombustion chamber located inside the bottom of the shell.

6) Depending on the construction details, these boilers have tubes configured in one,

two, three, or four pass arrangements.7) Because the design of fire tube boilers is simple, they are easy to construct in a shopand can be shipped fully assembled as a package unit.

8) These boilers contain long steel tubes through which the hot gases from the furnacepass and around which the hot gases from the furnace pass and around which the

water circulates.

9) Fire tube boilers typically have a lower initial cost, are more fuel efficient and areeasier to operate, but they are limited generally to capacities of 25 tonnes per hour and

pressures of 17.5 kg per cm2


27/37

27

Feed pumps

1) Boiler feed pumps are an important part of any boiler operation. They control the

amount of water fed to the boiler and the manner in which it is fed

2) A boiler feed water pump is a specific type of pump used to pump feed water intoa steam boiler. The water may be freshly supplied or returning condensate produced

as a result of the condensation of the steam produced by the boiler.

3) These pumps are normally high pressure units that take suction from a condensate

return system and can be of the centrifugal pump type or positive displacement type.

Super heater


28/37

28

A super heater is a device used to convert saturated steam or wet steam into drysteam used for power generation or processes. There are three types of super heaters namely:

radiant, convection, and separately fired. A super heater can vary in size from a few tens offeet to several hundred feet (a few meters or some hundred meters).

1) A radiant super heater is placed directly in the combustion chamber.

2) A convection super heater is located in the path of the hot gases.

3) A separately fired super heater, as its name implies, is totally separated from the

boiler

Dearearators

A deaerator is a device that is widely used for the removal of air and other

dissolved gases from the feed water to steam-generating boilers. In particular,dissolved oxygen in boiler feed waters will cause serious corrosion damage in steam systems

by attaching to the walls of metal piping and other metallic equipment and

forming oxides (rust). Water also combines with any dissolved carbon dioxide to form

carbonic that causes further corrosion. Most deaerators are designed to remove oxygen.

1. Tray-type deaerator2. Spray- type deaerator

1) Tray-type deaerator

1. The tray-type (also called the cascade-type) includes a vertical domed deaeration

section mounted on top of a horizontal cylindrical vessel which serves as the

deaerated boiler feedwater storage tank.

Tray-type deaerator


29/37

29

1. The typical horizontal tray-type deaerator in Figure has a vertical domed dearation

section mounted above a horizontal boiler feed water storage vessel.

2. Boiler feed water enters the vertical deaeration section above the

perforated trays and flows downward through the small holes.

3. Low-pressure deaeration steam enters below the perforated trays and flows

upward through the small holes.

4. The steam strips the dissolved gas from the boiler feed water and exits via the vent

at the top of the domed section.

5. The vent line usually includes a valve and just enough steam is allowed to escape

with the vented gases to provide a small and visible telltale plume of steam.

2) Spray- type deaerator

1. The spray-type consists only of a horizontal (or vertical) cylindrical vessel which serves

as both the dearation section and the boiler feed water storage tank.

2. As shown in Figure, the typical spray-type deaerator is a horizontal vessel which has a

preheating section (E) and a deaeration section (F).

3. The two sections are separated by a baffle(C). Low-pressure steam enters the vessel

through a sparger in the bottom of the vessel.

4. The boiler feed water is sprayed into section (E) where it is preheated by the rising steam

from the sparger.

5. The purpose of the feed water spray nozzle (A) and the preheat section is to heat the

boiler feed water to its saturation temperature to facilitate stripping out the dissolved

gases in the following deaeration section.


30/37

30

6. The preheated feed water then flows into the dearation section (F), where it is deaerated

by the steam rising from the sparger system.

7. The gases stripped out of the water exit via the vent at the top of the vessel. The deaerated

boiler feed water is pumped from the bottom of the vessel to the steam generating boilersystem.

Condenser

A closed vessel in which steam is condensed by abstracting the heat and where thepressure is maintained below atmospheric pressure is known as a condenser. The efficiency

of the steam plant is considerably increased by the use of a condenser. In large turbine plants,the condensate recovery becomes very important and this is also made possible by the use of

condenser.

The steam condenser is one of the essential components of all modern steam power plants.

Steam condensers are of two types:

1. Surface condenser.2. Jet condensers

1. Surface condenser.

Steam surface condensers are the most commonly used condensers in modern power

plants. The exhaust steam from the turbine flows on the shell side (under vacuum) of the

condenser, while the plants circulating water flows in the tube side. The source of the

circulating water can be either a closed-loop (i.e. cooling tower, spray pond, etc.) or once

through (i.e. from a lake, ocean, or river). The condensed steam from the turbine, called


31/37

31

condensate, is collected in the bottom of the condenser, which is called a hot well. Thecondensate is then pumped back to the steam generator to repeat the cycle.

ADVANTAGES AND DISADVANTAGES OF A SURFACE CONDENSER

The various advantages of a surface condenser are as follows:

1. The condensate can be used as boiler feed water.2. Cooling water of even poor quality can be used because the cooling water does not come

in direct contact with steam.

3. High vacuum (about 73.5 cm of Hg) can be obtained in the surface condenser. Thisincreases the thermal efficiency of the plant.

The various disadvantages of' the surface condensers are as follows:

1) The capital cost is more.2) The maintenance cost and running cost of this condenser is high.3) It is bulky and requires more space.

2. Jet condensers

In jet condensers the exhaust steam and cooling water come in direct contact with each

other. The temperature of cooling water and the condensate is same when leaving the

condensers. Elements of the jet condenser are as follows:

1) Nozzles or distributors for the condensing water.2) Steam inlet.3) Mixing chambers: They may be (a) parallel flow type (b) counter flow type depending on

whether the steam and water move in the same direction before condensation or whetherthe flows are opposite.

4) Hot well.

In jet condensers the condensing water is called injection water.


32/37

32

Cooling system1. Once through wet cooling system. (open loop cooling system)2. Recirculation wet cooling system. (closed loop cooling system)3. Dry cooling system

i. Direct dry cooling systemii. Indirect dry cooling system

1. Once through wet cooling system. (open loop cooling system)

Open cycle (once through) cooling systems may be used for plants sited beside large

water bodies such as the sea, lakes or large rivers that have the ability to dissipate the waste

heat from the steam cycle. In the open system, water pumped from intakes on one side of the

power plant passes through the condensers and is discharged at a point remote from theintake (to prevent recycling of the warm water discharge).

2. Recirculation wet cooling system. (closed loop cooling system)


33/37

33

1. In closed cycle wet cooling systems, the waste energy that is rejected by the turbine istransferred to the cooling water system via the condenser.

2. The waste heat in the cooling water is then discharged to the atmosphere by the cooling

tower. In the cooling tower, heat is removed from the falling water and transferred to therising air by the evaporative cooling process.

3. Some of the warm water, typically 1 to 1.5% of the cooling water flow, is transferred tothe rising air, and this is visible in the plume of water vapour above towers in times of

high humidity.

3. Dry cooling system

Dry cooling systems are used where there is insufficient water, or where the water is

too expensive to be used in an evaporative system. Dry cooling systems are the least used

systems as they have a much higher capital cost, higher operating temperatures, and lowerefficiency than wet cooling systems.

i. Direct dry cooling system

1. In the direct dry system, the turbine exhaust steam is piped directly to the air-cooled,finned tube, condenser.

2. The finned tubes are usually arranged in the form of an 'A' frame or delta over a forceddraught fan to reduce the land area.

3. The steam trunk main has a large diameter and is as short as possible to reduce pressurelosses, so that the cooling banks are usually as close as possible to the turbine.

The direct system is the most commonly used as it has the lowest capital cost, but

significantly higher operating costs.

4. The power required to operate the fans of this system is several times that required forwet towers, being typically 4 to 5 MW for a 420 MW unit.


34/37

34

ii. Indirect dry cooling system

1. Indirect dry cooling systems have a condenser and turbine exhaust system as for wetsystems, with the circulating water being passed through finned tubes in a natural draught

cooling tower.2. The water pipe work allows the towers to be sited away from the station.

A variation on this type of indirect system is the system that uses a direct contact

condenser in place of the traditional tube type condenser.

3. In the spray condenser, the water from the cooling cycle mixes with the boiler water. Themaintenance of the water quality to suit all circuits is critical to the successful operation

of the system.

Cooling towers

A cooling tower is equipment used to reduce the temperature of a water stream by

extracting heat from water and emitting it to the atmosphere. Cooling towers make use of

evaporation whereby some of the water is evaporated into a moving air stream and subsequently

discharged into the atmosphere. As a result, the remainder of the water is cooled down

significantly

1. Evaporative or wet cooling tower.

2. Non-evaporative or dry cooling tower.i. Air-cooled condenser.ii. Air-cooled heat exchanger.

(The above two method are similar as dry cooling system)


35/37

35

1. Evaporative or wet cooling tower.

1. Cooling towers are able to lower the water temperatures more than devices that use onlyair to reject heat, like the radiator in a car, and are therefore more cost-effective and

energy efficient.

2. Wet cooling towers rely on the latent heat of water evaporation to exchange heat betweenthe process and the air passing through the cooling tower.

3. The cooling water may be an integral part of the process or may provide cooling via heatexchangers.

1. Natural draught (draft) cooling tower.2. Mechanical draught (draft) cooling tower.

1. Natural draught (draft) cooling tower.

1. Concrete natural draught towers have a large concrete shell. The heat exchange 'fill' isin a layer above the cold air inlet at the base of the shell as shown in the towersectional view.

2. The warm air rises up through the shell by the 'chimney effect', creating the naturaldraught to provide airflow and operate the tower.

3. These towers therefore do not require fans and have low operating costs. Naturaldraught towers are only economic in large sizes, which justify the cost of the large

concrete shell. Natural draught towers are the most common towers for large

generating units in Europe, South Africa and Eastern USA.

4. They are not used in the drier areas, as their performance is better suited to cooler andmore humid areas.


36/37

36

2. Mechanical draught (draft) cooling tower.

Mechanical draft towers have large fans to force or draw air through circulated water. The

waterfalls downwards over fill surfaces, which help increase the contact time between the

water and the air - this helps maximize heat transfer between the two. Cooling rates of

mechanical draft towers depend upon various parameters such as fan diameter and speed of

operation, fills for system resistance etc.

1. Forced draft cooling tower.2. Induced draft cooling tower.

1. Forced draft cooling tower.

The forced draft tower, shown in the picture, has the fan, basin, and piping located

within the tower structure. In this model, the fan is located at the base. During operation, the

fan forces air at a low velocity horizontally through the packing and then vertically against

the downward flow of the water that occurs on either side of the fan. Vibration and noise are

minimal since the rotating equipment is built on a solid foundation. Due high flow of cool air

the heat from the water is transferred to the air.


37/37

2. Induced draft cooling tower.

The induced draft tower show in the following picture has one or more fans, located at

the top of the tower, that draw air upwards against the downward flow of water passing

around the wooden decking or packing. Since the airflow is counter to the water flow, the

coolest water at the bottom is in contact with the driest air while the warmest water at the top

is in contact with the moist air, resulting in increased heat transfer efficiency.

High Pressure Boilers

The demand for the high power output from the boiler and associated plants hasincreased in the last ten years. It is a common practice to use high pressure and temperature

steam in power plants to increase the efficiency of the plant and to reduce the cost ofelectricity production. The high pressure boiler which is also known as modern boilers used

for power generation are for steam capacities 30 to 650 tons/hr and above with a pressure up

to 160 bar and maximum steam temperature of about 540C.

Types of high pressure boiler:-

1. La Mont boiler2. Benson boiler3. Loeffler boiler4. Supercharged boiler

(RefNag, P.K., book)

unit i thermal power plants

Documents