coal handling(power plant engg)

8/9/2019 Coal Handling(power plant engg)

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Thermal ( Steam ) Power plants

mainly consists of 4 circuits

Coal and ash Circuit

– Coal produced in the mining site is

transported to power plant site

– Coal handling equipment for generation of

steam

– The combustion of coal produces ash which

is collected and removed to ash storageyard through ash handling equipments


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– ! or "! fans are used for supply the air tocombustion chamber of the boiler through air#preheater

– The air preheater is placed in the path of flue gasesto preheat the air

– The flue gases produced by combustion of fuels inthe boiler furnaces after passing around boiler tubes

and super heater tubes – Pass through a dust collector or precipitator where

most of dust is removed before venting it of toatmosphere through chimney

$ir and gas circuit


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– Prime mover develops power by utili%ing steam generated in the boiler

– Then condenser is used to condense the steam coming out of prime mover

– $ pump is used to feed the condensate to the boiler

– The condensate leaving the condenser is heated in

feed heaters through e&tracted steam from lowestpressure e&traction point of the turbine

– The feed water may also be supplied from e&ternalsource to compensate any loss of steam and water '

– "n the boiler shell and tubes water circulation issetup due to density difference of water betweenlow and high temperature sections

– $ super heater is used to super heat the wet steam from boiler drum and is then supplied to prime

movers

eed water and steam circuit


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– "n the condenser quantity of cooling water

required to condense the steam is large andis ta*en either from la*e river or sea

– The cooling water is ta*en from upper side

of the river and then passed through the

condenser

– The hot water is then discharged to lower

side of the river

– The system is *nown as open system – +here water is not available in abundant

water from condenser is cooled either in

cooling pond or in cooling tower the system

is *nown as closed system

Cooling water circuit


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,ain Components of Thermal

Power Plants

-' .oiler

/' Super heater

0' 1conomi%er

4' $ir preheater 2' 3eheater

' Steam turbine

5' 6enerator

7' Condensers

8' Cooling towers

-9' Pumps

--' Coal mills

-/'! and "! ans

-0'$S: Precipitators

-4'+ater treatment plant


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1ssentials of steam power plant


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; -' $ furnace to burn the fuel'; /' Steam generator or boiler containing water' :eat generated in the furnace is utili%ed to

con# vert water in steam'; 0' ,ain power unit such as an engine or turbine to use the heat energy of steam and

perform wor*'; 4' Piping system to convey steam and water'; "n addition to the above equipment the plant requires various au&iliaries and accessories

depending upon the availability of water fuel and the service for which the plant isintended' The flow sheet of a thermal power plant consists of the following four maincircuits

(i) eed water and steam flow circuit (ii) Coal and ash circuit (iii) $ir and gas circuit (iv)Cooling water circuit' $ steam power plant using steam as wor*ing substance wor*sbasically on 3an*ine cycle' Steam is generated in a boiler e&panded in the prime moverand condensed in the condenser and fed into the boiler again'

(ii) The different types of systems and components used in steam power plant are as follows (i) :igh pressure boiler (ii) Prime mover (iii) Condensers and cooling towers (iv) Coal

handling system (v) $sh and dust handling system (vi) !raught system (vii) eed waterpurification plant (viii) Pumping system (i&) $ir preheater economi%er super heater feedheaters'


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uels used in thermal power plantCombustion of fuel is accomplished by mi&ing

with air at elevated temperatures

=&ygen in the air chemically unites with Carbon

:ydrogen of fuels and produce heat

"n thermal power plants normally steam is

produced from water by using combustion heat offuels (1&cept in 6as turbines)

6as) – "ndustrial waste gases

– Synthetic fuels or S?@A1BS


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Coal; Coal is the principal energy source for India because of its large

deposits and availability

; Coal originated from vegetable matter , which grew millions ofyears ago

; Trees and plants falling into water decayed and later producedpeat bogs

; Huge geological upheavals buried these bogs under layers ofsilt

; Subterranean heat, soil pressure and movement of earth's crustdistilled off some of the bog's moisture and hardened it to formcoal

; Basically classification of coal is based on Physical andchemical composition

– Peat

– ignite and brown coal

– Bituminous coal

– !nthracite


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Coal analysis; Two types of coal analysis

–Pro"imate !nalysis 6ives .ehavior of coal when heated

#C $ %& $ & $ ! ())* by mass

– +ltimate !nalysis

6ives chemical elements along with ash and moistureC $ H $ ) $ $ S $ & $ ! ())* by mass

based on

(a) as#received basis (useful for combustion calculations)

(b) dry or moisture free basis

(c) dry mineral#matter#free or combustible basis


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Coal properties

; There are certain properties of coal whichare important in power plant applications

; They are – sulphur content

– heating value

– ash softening temperature

– swelling inde& – grind ability

– weather ability


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!esirable properties of coal

– :igh calorific value

– Small sulphur content ( les than -)

– 6ood burning characteristics for complete

combustion

– :igh grind#ability inde&

– :igh weather#ability

6rading of coal done on the basis

– :eating value

– Si%e

– $sh content

– Sulphur content

– $sh softening temperature


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Peat

; Bow grade coal

; irst stage coal formation

; Contains 89 moisture

; Small amount of volatile matter ; @ot suitable for power plants

; Ased in domestic purposes

; 3equires -#/ months for drying in sunlight; Peat (/9 water !ried) has C< of - ,D E *g


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Bignite (.rown coal)

; "ntermediate stage of coal development

; :igh amount of moisture 09#49

; .rown in color

; :igh heating value and carbon compared to

peat

; Should be stored to avoid spontaneous

combustion

; Ased in pulveri%ed form

; Can be air dried easily

; Suitable for local use instead of transporting


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.ituminous coal; Containing 4#7 of fi&ed carbon and /9#49 of


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$nthracite Coals; Contains more than 7 fi&ed carbon >


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Biquid fuels

; They are easy to handle store and to burn; They have nearly constant heating values; They are primarily a mi&ture of hydrocarbon compounds which

may also contain nitrogen o&ygen and sulphur

; The bul* of the hydrocarbons belong to the paraffin series li*emethane (C:4) ethane (C/:) propane (C0:7) and butane(C4:-9) which are gaseous and pentane (C2:-/) he&ane(C:-4) and octane (C7:-7) which are liquid at STP

; "n addition there can be isoparaffins cycloparaffins and

aromatic compounds


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; Carbon 70#75 :ydrogen --#-

; =&ygen G @itrogen 9#5 Sulphur 9#4

; There can also be some moisture and sediment; Crude oil distilled into a number of fractions

gasoline aviation fuel *erosene light diesel oil

heavy diesel oil lubrication oil ; The heavier fractions are used for boiler fuels

and chemical production

; The required physical properties of fuel oil are – specific gravity

– viscosity pour point

– flash point and heating value


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6aseous fuels; Transportation of natural gas is made through

pipelines; @atural gas is the cleanest of all fossil fuels; "t is free from ash and mi&es well with air to undergo

complete combustion producing very little smo*e

; "t consists of a mi&ture of the most volatile paraffins#methane to pentane; "t has high hydrogen content and produces a

considerable amount of water vapour when burned; The heat of combustion varies from 00'2 to 49 ,DEm0; Since the maHor constituent of all natural gases is

methane; Biquid natural gas (B@6) is transported by special

tan*ers and stored in spherical pressure vessels to beused when needed particularly during pea* load'

=th f l


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=ther fuels; Industrial -astes . Byproducts

blast furnace gas co*e oven gas and refinerygas sugar factory refuse (bagasse)I saw mill

wood dust rice hus*

; Synthetic fuels6aseous and liquid fuels from coal

economically and environment friendly manner

i/uid fuels using mi"tures of fine coal in oilhave been 0nown as colloidal fuel, coal1in1

oil and more recently, coal1oil mi"tures

2C3&4

C


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Coal :andling


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Coal handling system

"n Case of 3ail !elivery,ost Commonly Ased are

; +agon Tippler :opper; :ydraulic 6ates =pening

6 =


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:ydraulic 6ates =pening

=peration; The gates below the wagons are hydraulic controlled; .y opening the gates the coal flows downwards due to gravitational force

and is moved to the below pit

!emerits

; !ue to the dust the :ydraulic will not wor* smoothly; 3equire a lot of maintenance

+ Ti l :


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+agon Tippler :opper

P ti


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Crushers and brea*ers used to crush the big si%e coal into small si%e

coal

!riers used to dry the coal if it contains lot of moisture it is also further

done in the pulveri%es

,agnetic sprayers used to separate the iron parts that come along withthe coal from the mines these may damage the parts of pulveri%es or

may have a bad effect on the boiler

Preparation


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Coal Crushers

+hyJ3educe si%e from /99mm to /9mm

Types of crushers

-'3ing Type Coal Crusher

/':ammer ,ill Coal Crusher

0'.rad ord .rea*er


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3ing Type Coal Crusher


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=peration

; The coal is fed at the top left corner

; "s crushed in between the plane loose

rings and the adHustable plates'

; The loose rings with the Haws used to ta*e

the crushed coal down words

; $dHustable plate and the screen determinethe ma&imum si%e of the discharged coal


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:ammer ,ill Coal Crusher


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=P13$T"=@

; The coal is fed at the top

; "s crushed by the action of rings that pivot

off center on a rotor or by swinging

hammers attached to it

; $dHustable screen bars determine the

ma&imum si%e of the discharged coal


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.rad ord .rea*er


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.rad ord .rea*er

; .radford brea*er which is used for largecapacity wor*

; "t consists of a large cylinder made up of

perforated steel plates to which liftingshelves are attached on the inside

; The cylinder rotating slowly at about /9

rpm receives coal at one end; The shelves lift the coal up and then the

coal drops down by gravity


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Transfer

.elt Conveyor


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.elt conveyor


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.elt conveyor


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Screw conveyor


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.uc*et > 6rab buc*et elevator


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S*ip :oist and flight conveyor


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Sto*er and 3e#claimer

Sto*er terms for storing the coal intothe stoc* yard

3e#claimer means to ta*e the coal

bac* from the stoc* yard

"t moves on the rail trac* along withadHustable length of belt conveyor


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Combustion 1quipments

for Bumped coal burning

-' 6rate#fired furnaces

/' Chamber#type or flame

furnaces


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6rate ired urnace

Ch b t


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Chamber type


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Combustion 5/uipment #or

Burning Coal

; uel bed furnaces (coarse particles)

; Pulveri%ed coal furnaces (fine particles)

; Cyclone furnaces (crushed particles); luidi%ed bed furnaces (crushed small

particles)


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uel bed furnaces (coarse particles)

There are two ways of feeding coal on to the

grate

-' =verfeeding

/' Anderfeeding

= f d t *


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=verfeed sto*er

Anderfeed sto*er


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=verfeeding


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=verfeeding

; 3eceives fresh coal from top surface has

following distinct %ones

– resh or green coal

– Coal losing moisture (!rying %one)

– Co*ing layer (loosing of


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=perations

; Primary air gets warmed up as it flows through the ash

layer

; $s it passes through the incandescent co*e

layer(-/99oC) Carbon converted to Carbon dio&ide

releasing heat continues till o&ygen is consumed iflayer is thic* C=/ is converted to C= reducing layer

temperature

; The stream while passing through distillation %one


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or combustion of this stream

; $ hot ignition point ( "n the range of -999#-099oC

– !one by providing a fire bric* lined arch which stores

up the heat and remains at high temperature

; Sufficient fresh air – Secondary air or over#fire air

; Turbulence

– Providing secondary air at right angles to up#flowing

gas stream emerging out of fuel bed


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3esults

; Carbon is in C= C=/ both are color less

; Carbon with hydrogen in


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Anderfeeding


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Anderfeeding

; "n underfeeding coal is fed from below

; Primary air passing through holes diffusesthrough spaces in the raw green coal pic*ing up

moisture; "n distillation %one to stream


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,echanical Sto*ers

=verfeeding

-' Traveling grate sto*er

/' Chain grate sto*er

0' Spreader sto*er

4'


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Travelling grate sto*er


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Traveling grate sto*er

; 6rate surface is made up of Cast "ronbars Hoined together by lin*s to formendless belt

; .elt wound around two sproc*ets; $ coal gate regulates the depth of fuel bed

; Simultaneous adHustment of uel bed

thic*ness primary air flow controls theburning rate so that at the end of its rearash only remains


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$dvantages > !isadvantages

; Simple and "nitial cost is low

; 3eliable in service and maintenance is low

; 6ives high rate of heat release per volume of

the furnaceBimited coal can be carried on grate

Clin*er problems are common

"gnition arches are requiredThere is always some loss in the form of

particles


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Spreader sto*er


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Spreader sto*er

; Coal from hopper is fed by rotating feeder ; eeder normally will have .lades fitted on the

drum; ine particles burn in suspension; Speed of the feeder varies with the steam output

of boiler ; 6rate is made up of C" bars Bin*s underneath

the grate are connected to a lever

; uels used may be .ituminous lignite woodwaste baggase

; Coal si%e used in #0 cm


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$dvantages and !isadvantages

; $ wide variety of fuels can be used

; Clin*er formation is reduced

; :igh temperature preheated air can be used

; Kuic* response to load variation; 6ives equal pressure drop and proper air

distribution

; =peration cost is low

!ifficult to operate variable si%ed coal particles

ly ash and entrapped carbon particles


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Anderfeed sto*ers


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,ultiple retort


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Pulverised coal


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Types of Pulverisers

; .all and tube mill (low speed M 52 rpm)

; .all and tube race mill (medium speed 52#

//2 rpm)

; .owl mill (medium speed 52#//2 rpm)

; "mpact or hammer mill (N //2 rpm)

L'depending upon the nature of coal


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.all and tube mill


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.all and tube mill


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.all and tube mill

. ll d t b ill


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.all and tube mill

. ll d b ill


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.all and tube mill

; ,ills can be designed and manufactured for very largecapacity' $lso each mill can supply coal to two sets ofburner' .oiler can be designed with lesser number ofmills'

; :igher power consumption per tons of coal and highstarting torque ma*es drive motor large'

; Suitable for base load stations only where load variation isless'

; 3esponse to load change is very fast' !uring operationpresence of relatively large quantity of pulveri%ed coal ingrinding %one acts as a storage reservoir' "n case of

sudden increase in fuel demand fuel flow can beincreased almost instantaneously'

; Start up and shutdown time is longer'; Product fineness is very good'


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.owl ,ill

.owl ,ill


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.owl ,ill

.owl mill


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.owl mill

. l ill


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.owl mill

; 3eplacement of grinding elements ta*es lesser time'; Capacity and product fineness deteriorate with wearing

out of grinding elements' 3ate of erosion of bowl andrings are very high'

; requent shutdown of mills are required for adHustment of

loading springs which becomes loose for wearing out ofroller and bowl'; "ngress of non crushable material may damage mill

internals'; 3eHect rate is very high' 1laborate mill reHect system

becomes essential for larger units' !isposal of mill reHectis another problem to be tac*led by power station'


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"mpactE:ammer ,ill


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"mpactE:ammer ,ill


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"mpactE:ammer ,ill

; $ simple compact and low cost mill'

; ,ill can be designed for very high temperature' Thisfacility ma*es this mill suitable for use with high moisture

lignite fuel'

; +ear of grinding elements reduces mill capacity'

; Product fineness is low'

.all and 3ace mill


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a a d ace

. ll d ill


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.all and race mill

; Capacity and product fineness does not deteriorateappreciably with wearing out of grinding elementsbecause of self compensating mechanism'

; :ydro #pneumatic loading system maintains constantloading pressure' .ut it is costly and needs maintenance'

; 3esponse to load change is slow compared to tube mills'; Start up and shut down time is less'; "ngress of non crushable material may damage mill

internals'

; Chances of grinding %one fire are almost nil'


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.all and race mill

. ll d ill


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.all and race mill

Pulverised coal firing


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Pulverised coal firing is done by two system :

(i) Unit System or Direct System. (ii) Bin or Central System.

Unit System'; "n this system the raw coal from

the coal bun*er drops on to the

feeder' ; :ot air is passed through coal in

the feeder to dry the coal'

; The coal is then transferred tothe pulverising mill where it is

pulverised'; Primary air is supplied to the

mill by the fan'; The mi&ture of pulverised coal

and primary air then flows toburner where secondary air is

added'; The unit system is so called from

the fact that each burner or a

burner group and pulveriser

constitute a unit'

A it t


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Anit system

!dvantages

(i)The system is simple and cheaper than the central system'

(ii ) There is direct control of combustion from the pulverising mill'

(iii ) Coal transportation system is simple

Bin or Central System.

; Crushed coal from the raw coal buner is fed by gravity to a dryer where hot air is


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!assed through the coal to dry it.

; "he dryer may use waste flue gases# !reheated air or bleeder steam as drying agent.; "he dry coal is then transferred to the !ulverising mill.;

"he !ulverised coal obtained is transferred to the !ulverised coal buner (bin); "he trans!orting air is se!arated from the coal in the cyclone se!arator.; "he !rimary air is mi$ed with the coal at the feeder and the mi$ture is su!!lied to the

burner.

.in or central system


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.in or central system

Advantages

l. "he !ulverising mill grinds the coal at a steady rate irres!ective of boiler feed.

%. "here is always some coal in reserve. "hus any occasional breadown in the

coal su!!ly will

not effect the coal feed to the burner.

&. 'or a given boiler ca!acity !ulverising mill of small ca!acity will be reuired as

com!ared to

unit system.Disadvantages

. "he initial cost of the system is high.

%. Coal trans!ortation system is uite com!licated.

&. "he system reuires more s!ace.

Pulverised fuel firing system


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g y


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Pulveri%ed coal handling system


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P l i ti


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Pulveri%ation

$dvantages

; Bow e&cess air requirement; Bess fan power ; $bility to use highly preheated air reducing e&haust losses

; :igher boiler efficiency; $bility to bum a wide variety of coals; ast response to load changes; 1ase of burning alternately with or in combination with gas and

oil; $bility to release large amounts of heat enabling it to generate

about /999 tEh of steam or more in one boiler ; $bility to use fly ash for ma*ing bric*s etc'; Bess pressure losses and draught need'

!i d t


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!isadvantages

-' $dded investment in coal preparation unit/' $dded power needed for pulveri%ing coal

0' Barge volume of furnaces needed to permit desired heat

release and to withstand high gas temperature

l idi d . d C b ti


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luidi%ed .ed Combustion



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$sh handling system


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$sh handling system* large uantity of ash is# !roduced in steam !ower !lants using coal. *sh !roduced in about +

to %+, of the total coal burnt in the furnace. -andling of ash is a !roblem because ash coming

out of the furnace is too hot# it is dusty and irritating to handle and is accom!anied by some

!oisonous gases.

t is desirable to uench the ash before handling due to following reasons:

. /uenching reduces the tem!erature of ash.

%. t reduces the corrosive action of ash.

&. *sh forms cliners by fusing in large lum!s and by uenching cliners will disintegrate.

0. /uenching reduces the dust accom!anying the ash.

-andling of ash includes its removal from the furnace# loading on the conveyors and delivered

to the fill from where it can be dis!osed off.


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!SH H!6I7 58+IP&5T

1echanical means are reuired for the dis!osal of ash. "he handling eui!ment should

!erform the following functions:

() Ca!ital investment# o!erating and maintenance charges of the eui!ment should be low.

(%) t should be able to handle large uantities of ash.

(&) Cliners# soot# dust etc. create troubles# the eui!ment should be able to handle them

smoothly.

(0) "he eui!ment used should remove the ash from the furnace# load it to the conveyingsystem to deliver the ash to a dum!ing site or storage and finally it should have means to

dis!ose of the stored ash.

(2) "he eui!ment should be corrosion and wear resistant.

$S: :$@!B"@6 B=+ !"$63$,


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$sh handling


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$sh handling


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Hydraulic !sh Handling System ow %elocity System


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ow %elocity System

Hydraulic !sh Handling System High %elocity


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High %elocity

System


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Pneumatic !sh Handling System


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Pneumatic !sh Handling System


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!ust collection system


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"he si3e of dust !articles is designated in microns ( 4 5 +.++ mm). Dust !articles are mainly

ash !articles called fly ash intermi$ed with some uantity of carbon ash material called cinders.

6asborne !articles larger than 4 in diameter are called dust and when such !articles become

greater in si3e than ++7 they are called cinders. Smoe is !roduced due to the incom!letecombustion of fuels# smoe !articles are less than +7 in si3e.

"he dis!osal smoe to the atmos!here is not desirable due to the following reasons :

.* smoy atmos!here is less healthful than smoe free air.

%. Smoe is !roduced due to incom!lete combustion of coal. "his will create a big economic

loss due to loss of heating value of coal.

&. n a smoy atmos!here lower standards of cleanliness are !revalent. Buildings# clothings#

furniture etc. becomes dirty due to smoe. Smoe corrodes the metals and darens the !aints."o avoid smoe# the coal should be com!letely burnt in the furnace.

Types of dust collectors"he various ty!es of dust collectors are as follows :


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. 1echanical dust collectors.

%. 8lectrical dust collectors.

Mechanical dust collectors. 1echanical dust collectors are sub9divided into wet and dry ty!es.

n wet ty!e collectors also nown as scrubbers water s!rays# are used to wash dust from the air."he basic !rinci!les of mechanical dust collectors are shown in 'ig. *s shown in 'ig. by

increasing the cross9sectional area of duct through which dust laden gases are !assing# the

velocity of gases is reduced and causes heavier dust !articles to fall down. Changing the

direction of flow of flue gases causes the heavier !articles of settle out. Sometime baffles are

!rovided as to se!arate the heavier !articles. 1echanical dust collectors may be wet ty!e or dry

ty!e. et ty!e dust collectors called scrub scrubbers ma*e use of water sprays to wash the dustfrom flue gases'Dry ty!e dust collectors include gravitational# cyclone# and baffle dust collectors.

Electrostatic Precipitators. t has two sets of electrodes# insulated from each other that maintain

an electrostatic field between them at high voltage. "he flue gases are made to !ass between


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these

two sets of electrodes. "he electric field ionises the dust !article; that !ass through it attracting

them to the electrode of o!!osite charge. "he other electrode is maintained at a negative !otential

of &+#+++ to


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1SP

9:(;:( #< !SH SC=+BB5=

t is similar to a mechanical ash collector but has a flowing water film on its inner walls. Due to

this film the collected ash is removed more ra!idly from the a!!aratus to the bin "he degree of


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this film# the collected ash is removed more ra!idly from the a!!aratus to the bin. "he degree of

ash collection in scrubbers varies from +.=% to +.>+. "he dust laden gas enters through the inlet

!i!e.

Cinder Catcher. Cinder catcher is used to remove dust and cinders from the gas. n this catcher

the dust laden gas is made to strie a series of vertical baffles that change its direction and

reduce its velocity.

"he se!arated dust and cinders fall to the

-o!!er for removal. Cinder catchers

are ordinarily used with stoer firing.

coal handling(power plant engg)

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