thermal power plant.doc

8/11/2019 thermal power plant.doc

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THERMAL POWER PLANT

General Layout of Thermal Power Plant:

The general layout of thermal power plant consists of mainly four circuits as shown in [1].

The four circuits are

1. Coal and Ash circuit

2. Air and Gas circuit

3. Feed ater and !team circuit

". Cooling ater circuit

Coal and Ash Ciruit:

#n this circuit$ the coal from the storage is fed to the %oiler through coal handling

e&uipment for the generation of steam. Ash produced due to com%ustion of coal is

remo'ed to ash storage through ash(handling system.

Air and Gas Ciruit:

Air is supplied to the com%ustion cham%er of the %oiler either through forced

draught or induced draught fan or %y using %oth. The dust from the air is remo'ed %efore

supplying to the com%ustion cham%er. The e)haust gases carrying sufficient &uantity of

heat and ash are passed through the air(heater where the e)haust heat of the gases is gi'en

to the air and then it is passed through the dust collectors where most of the dust is

remo'ed %efore e)hausting the gases to the atmosphere.

!eed Water and "team Ciruit:

The steam generated in the %oiler is fed to the steam prime mo'er to de'elop the power.

The steam coming out of the prime mo'er is condensed in the condenser and then fed to

the %oiler with the help of pump. The condensate is heated in the feed(heaters using the


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steam tapped from different points of the tur%ine. The feed heaters may %e of mi)ed type

or indirect heating type. !ome of the steam and water are lost passing through different

components of the system$ therefore$ feed water is supplied from e)ternal source to

compensate this loss. The feed water supplied from e)ternal source to compensate the

loss. The feed water supplied from e)ternal source is passed through the purifying plant to

reduce to reduce dissol'e salts to an accepta%le le'el. This purification is necessary to

a'oid the scaling of the %oiler tu%es.

Coolin# Water Ciruit:

The &uantity of cooling water re&uired to condense the steam is considera%ly high

and it is ta*en from a la*e$ ri'er or sea. At the Colum%ia thermal power plant it is ta*en

from an artificial la*e created near the plant. The water is pumped in %y means of pumps

and the hot water after condensing the steam is cooled %efore sending %ac* into the pond

%y means of cooling towers. This is done when there is not ade&uate natural water

a'aila%le close to the power plant. This is a closed system where the water goes to the

pond and is re circulated %ac* into the power plant. Generally open systems li*e ri'ers are

more economical than closed systems.

Wor$in# of the Thermal Power Plant:

!team is generated in the %oiler of the thermal power plant using heat of the fuel

%urnt in the com%ustion cham%er. The steam generated is passed through steam tur%ine

where part of its thermal energy is con'erted into mechanical energy which is further used

for generating electric power. The steam coming out of the steam tur%ine is condensed in

the condenser and the condensate is supplied %ac* to the %oiler with the help of the feed

pump and the cycle is repeated. The function of the +oiler is to generate steam. The

function of the condenser is to condense the steam coming out of the low pressure tur%ine.


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The function of the steam tur%ine is to con'ert heat energy into mechanical energy. The

function of the condenser is to increase the pressure of the condensate from the condenser

pressure to the %oiler pressure. The other components li*e economi,er$ super heater$ air

heater and feed water heaters are used in the primary circuit to increase the o'erall

efficiency of the plant.

"ite "eletion of a Thermal Power Plant:

The important aspect to %e %orne in mind during site selection for a thermal power

plant are a'aila%ility of coal$ ash disposal facility$ space re&uirement$ nature of land$

a'aila%ility of water$ transport facility$ a'aila%ility of la%or$ pu%lic pro%lems$ si,e of the

plant.

Power Plant En#ineerin# Rele%ant to Colum&ia Thermal Power Plant

Coal:

At the Colum%ia power plant the coal is shipped in %y means of rail. The coal is

o%tained primarily from yoming. This is done in most cases$ %ut coal may also %e

shipped %y truc*s or %y pipelines. #nside the power plant there is an unloading doc* where

the carts of the rail are heated to melt the snow on the rail carts %efore unloading them.

The coal is stored in huge heaps or piles of a half a mile in diameter. The reason for

stoc*ing this much of coal is %ecause the loss due to loss of generation due to lac* of coal

is 'ery high. The upper layer of the coal heap has to %e compacted to ma*e it into a

airtight surface to pre'ent loss of coal due to o)idation. -ther methods of pre'enting

o)idation are %y *eeping it under water or %y spraying chemicals on it.

Coal handlin#:

#n plant coal handling is a 'ery important aspect of power plant safety. #n 'aria%ly

the coal is not e)posed as it can pollute the air and release poisonous gases li*e car%on


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mono)ide. The coal from the heaps is mo'ed into the plant %y means of long con'eyors

that are electrically operated. There are many different types of con'eyors and coal(

handling de'ices li*e screwing con'eyors$ %uc*et ele'ators$ gra%%ing %uc*et con'eyors

etc.

Coal Crusher:

+efore the coal is sent to the plant it has to %e ensured that the coal is of uniform si,e$ and

so it is passed through coal crushers. Also power plants using pul'eri,ed coal specify a

ma)imum coal si,e that can %e fed into the pul'eri,er and so the coal has to %e crushed to

the specified si,e using the coal crusher. otary crushers are 'ery commonly used for this

purpose as they can pro'ide a continuous flow of coal to the pul'eri,er.

Pul%eri'er:

/ost commonly used pul'eri,er is the +oul /ill. The arrangement consists of 2 stationary

rollers and a power dri'en %aul in which pul'eri,ation ta*es place as the coal passes

through the sides of the rollers and the %aul. A primary air induced draught fan draws a

stream of heated air through the mill carrying the pul'eri,ed coal into a stationary

classifier at the top of the pul'eri,er. The classifier separates the pul'eri,ed coal from the

unpul'eri,ed coal.

Advantages of pulverized coal:

0ul'eri,ed coal is used for large capacity plants.

#t is easier to adapt to fluctuating load as there are no limitations on the

com%ustion capacity.

Coal with higher ash percentage cannot %e used with out pul'eri,ing %ecause

of the pro%lem of large amount ash deposition after com%ustion.

#ncreased thermal efficiency is o%tained through pul'eri,ation.


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The use of secondary air in the com%ustion cham%er along with the powered

coal helps in creating tur%ulence and therefore uniform mi)ing of the coal and the air

during com%ustion.

Greater surface area of coal per unit mass of coal allows faster com%ustion as

more coal is e)posed to heat and com%ustion.

The com%ustion process is almost free from clin*er and slag formation.

The %oiler can %e easily started from cold condition incase of emergency.

0ractically no ash handling pro%lem.

The furnace 'olume re&uired is less as the tur%ulence caused aids in complete

com%ustion of the coal with minimum tra'el of the particles.

The pul'eri,ed coal is passed from the pul'eri,er to the %oiler %y means of the primary air

that is used not only to dry the coal %ut also to heat is as it goes into the %oiler. The

secondary air is used to pro'ide the necessary air re&uired for complete com%ustion. The

primary air may 'ary anywhere from 1 to the entire air depending on the design of the

%oiler. The coal is sent into the %oiler through %urners. A 'ery important and widely used

type of %urner arrangement is the Tangential Firing arrangement.

Tangential Burners:

The tangential %urners are arranged such that they discharge the fuel air mi)ture

tangentially to an imaginary circle in the center of the furnace. The swirling action

produces sufficient tur%ulence in the furnace to complete the com%ustion in a short period


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of time and a'oid the necessity of producing high tur%ulence at the %urner itself. igh heat

release rates are possi%le with this method of firing.

The %urners are placed at the four corners of the furnace. At the Colum%ia 0ower

0lant si) sets of such %urners are placed one a%o'e the other to form si) firing ,ones.

These %urners are constructed with tips that can %e angled through a small 'ertical arc. +y

ad4usting the angle of the %urners the position of the fire %all can %e ad4usted so as to raise

or lower the position of the tur%ulent com%ustion region. hen the %urners are tilted

downward the furnace gets filled completely with the flame and the furnace e)it gas

temperature gets reduced. hen the %urners are tiled upward the furnace e)it gas

temperature increases. A difference of 1 degrees can %e achie'ed %y tilting the %urners.

Ash Handlin#:

The e'er increasing capacities of %oiler units together with their a%ility to use low

grade high ash content coal ha'e %een responsi%le for the de'elopment of modern day ash

handling systems. The widely used ash handling systems are

1. /echanical andling !ystem

2. ydraulic !ystem

3. 0neumatic !ystem

". !team 5et !ystem

The ydraulic Ash handling system is used at the Colum%ia 0ower 0lant.

Hydraulic Ash Handling System:

The hydraulic system carried the ash with the flow of water with high 'elocity

through a channel and finally dumps into a sump. The hydraulic system is di'ided into a

low 'elocity and high 'elocity system. #n the low 'elocity system the ash from the %oilers

fall into a stream of water flowing into the sump. The ash is carried along with the water


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and they are separated at the sump. #n the high 'elocity system a 4et of water is sprayed to

&uench the hot ash. Two other 4ets force the ash into a trough in which they are washed

away %y the water into the sump$ where they are separated. The molten slag formed in the

pul'eri,ed fuel system can also %e &uenched and washed %y using the high 'elocity system.

The ad'antages of this system are that its clean$ large ash handling capacity$ considera%le

distance can %e tra'ersed$ a%sence of wor*ing parts in contact with ash.

Hi#h Pressure (oiler:

#t is a common practice to use high pressure and temperature %oilers to increase

the efficiency of the plant and to decrease the cost of electricity production. The %oiler at

the power plant is a water tu%e %oiler$ which means that water that is con'erted to steam

is passed through the tu%es inside the %oiler. The tu%es are %ent %ac* and forth many times

to ensure that all the water is con'erted to steam. et steam is not desira%le when it goes

to the tur%ine as it may cause corrosion on the tur%ine %lades. A high pressure %oiler is not

a simple assem%le of certain components li*e %urners$ super heaters$ air heaters and

others. The function of the components is interrelated. The location of the heat transfer

surfaces is 'ery important and it depends on the re&uired duty of the %oiler and the &uality

of the coal used. The most commonly used furnace layout or pul'eri,ed is shown in the

figure. #n ,one 1$ heat transfer is primarily %y radiation. As the gases mo'e upward and

secondary air is added$ the effect of radiation is reduced and con'ection %ecomes

predominant. The heat transfer in 6one 2 and 6one 3 ta*es place mainly %y con'ection.

6ones 2 %eing a high temperature ,one and 6one 3 %eing the low temperature ,one. The

e'aporators are placed in the 6one 1 as it is desira%le to ha'e lowest possi%le tu%e metal

surface temperature %ecause of AFT 7ash fusion temperature8 issues. !ince the 6one 2 has

high temperatures$ slagging is an important concern in this ,one. The e)cess heat is


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remo'ed %y using panels or platens$ which may either %e super heaters or e'aporators. The

6one 3 %ecause of comparati'ely low temperatures is ideally suited for heat reco'ery

e&uipment li*e economi,ers and air pre heaters.

(oiler Aessories:

A large amount of fuel is used in thermal power plant and 'ery large amount of

heat is generated and carried %y waste gases. The loss would %e 'ery high if the waste

gases carry all the heat away. The loss can he hal'ed %y installing an economi,er and a pre(

heater in the path of the waste gases. The economi,er transfers the heat from the waste

gases to the incoming feed water. This reduces the heat re&uired to con'ert the feed water

to steam. The air pre heater increases the heat of the air supplied into the %oiler for

com%ustion. This increases the efficiency of the %oiler.

Eonomi'er:

The economi,er is a feed water heater$ deri'ing heat from the flue gases. The

4ustifia%le cost of the economi,er depends on the total gain in efficiency. #n turn this

depends on the flue gas temperature lea'ing the %oiler and the feed water inlet

temperature. A typical return %end type economi,er is shown in the figure.

Ty)es of eonomi'er:

Plain Tube Economizer:

These are generally used in case of %oilers with natural draught. The tu%es are

made of cast iron and their ends are pressed into top and %ottom headers. The economi,er

is placed in the main flue gas path %etween the %oiler and the chimney. The waste flue

gases flow outside the tu%es and heat is transferred to the water flowing inside. igh

efficiency can %e achie'ed %y maintaining the water walls soot free.

Grilled Tube Economizer:


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This is the type of economi,er used in the power plant. This type of economi,er

reduced space considera%ly. ectangular grills are cast on the %are tu%e walls. 9conomi,er

tu%es may ha'e finned tu%es to increase the heat transfer rate. Thic*er fins offer greater

efficiency than thinner ones %ecause of greater surface area.

Air Pre*heater:

The flue gases coming out of the economi,er is used to preheat the air %efore

supplying it to the com%ustion cham%er. An increase in air temperature of 2 degrees can

%e achie'ed %y this method. The pre heated air is used for com%ustion and also to dry the

crushed coal %efore pul'eri,ing.

Ty)es of Air Heaters:

Tubular Air Heater:

The flue gas flows outside the tu%es in which the air flows heating it. To increase

the time of contact hori,ontal %affles are pro'ided.

Plate Type Air Heater:

#t consists of rectangular flat plates spaced 1.: to 2 cm apart lea'ing alternate air

and gas passages. This is not used e)tensi'ely as it in'ol'es high maintenance.

egenerative Air Heater:

The transfer of heat from hot gas to cold air is done in 2 stages. #n the first stage

the heat from the hot gases is passed to the pac*ing of the air heater and the temperature

of the gas is sufficiently reduced %efore letting it out in the atmosphere. This is called the

heating period. #n the second stage the heat from the pac*ing is passed to the cold air. This

is called the cooling period.


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"oot (lowers:

The fuel used in thermal power plants cause soot and this is deposited on the %oiler

tu%es$ economi,er tu%es$ air pre heaters etc. This drastically reduces the amount of heat

transfer of the heat e)changers. !oot %lowers control the formation of soot and reduce its

corrosi'e effects. The types of soot %lowers are fi)ed type$ which may %e further classified

into lane type and mass type depending upon the type of spray and no,,le used. The other

type of soot %lower is the retracta%le soot %lower. The ad'antages are that they are placed

far away from the high temperature ,one$ they concentrate the cleaning through a single

large no,,le rather than many small no,,les and there is no concern of no,,le arrangement

with respect to the %oiler tu%es.

Condenser:

The use of a condenser in a power plant is to impro'e the efficiency of the powre

plant %y decreasing the e)haust pressure of the steam %elow atmosphere. Another

ad'antage of the condenser is that the steam condensed may %e reco'ered to pro'ide a

source of good pure feed water to the %oiler an reduce the water softening capacity to a

considera%le e)tent. A condenser is one of the essential components of a power plant.

Ty)es of "team Condensers:

!i"ing or #et Type $ondenser: These type of cndensers are mainly of two types. 0arallel

flow type and Conunter flow type. The parallel flow type the steam and the water flow in

the same direction and in the counter flow type they flow in opposite directions. These

type are rarely used in high capacity modern day power plants.

%on !i"ing Type or Surface $ondensers:#n this type the cooling water and steam do not

come in contact with each other. This is used where large &uantity of inferior &uality water

is a'aila%le. #n this the cooling water flows in pipes and the steam flows in a perpendicular


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direction to the pipes. The 'elocity of water flowing is 'ery high to a%sor% the heat from

the steam. This condenser can %e classified %ased on the num%er of passes of the tu%e and

the direction of the condensate flow and tu%e arrangement$ either down flow or central

flow.

Coolin# Tower:

The importance of the cooling tower is felt when the cooling water from the

condenser has to %e cooled. The cooling water after condensing the steam %ecomes hot

and it has to %e cooled as it %elongs to a closed system. The Cooling towers do the 4o% of

decreasing the temperature of the cooling water after condensing the steam in the

condenser.

The type of cooling tower used in the Colum%ia 0ower 0lant was an #nline

#nduced ;raft Cross Flow Tower. This tower pro'ides a hori,ontal air flow as the water

falls down the tower in the form of small droplets. The fan centered at the top of units

draws air through two cells that are paired to a suction cham%er partitioned %eneath the

fan. The outstanding feature of this tower is lower air static pressure loss as there is less

resistance to air flow. The e'aporation and effecti'e cooling of air is greater when the air

outside is warmer and dryer than when it is cold and already saturated.


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