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Transcript of coalhandling
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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'
<arious equipments
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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)
<arious fuels were used in thermal power plants
– ossil fuels ( Coal =il > 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 millionsof years ago
; Trees and plants falling into water decayed and laterproduced peat bogs
; Huge geological upheavals buried these bogs under layers
of silt; Subterranean heat, soil pressure and movement of earth's
crust distilled off some of the bog's moisture and hardenedit to form coal
; 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
moisture
C $ H $ ) $ $ S $ & $ ! ())* by mass
based on
(a) as#received basis (useful for combustion calcu)
(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
– 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
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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 <,
; $sh content may vary #-/ ; :igh percentage of volatile matter C< of 0/ ,D E Fg
; $vailable in two forms C$F"@6 and @=@ Ca*ing
; Sub#.ituminous is having less moisture ash thanlignite and no C$F"@6 power used in briquette or
pulveri%ed form
; Semi#.ituminous coal high carbon and heating value
contain less moisture ash sulphur ><, :astendency to brea* into pieces
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$nthracite Coals; Contains more than 7 fi&ed carbon ><,
around 7; "gnites slowly has high C< of 0 ,D E Fg
; Bow ash %ero C$F"@6 power
; !ifficult to pulveri%e $nthracite coal
Bi id f l
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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 l h dli
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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 bellow the wagons are hydraulic controlled; .y opening the gates the coal flows downwards due to gravitational force
and is moved to the bellow 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 crus 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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=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
; $dHustable plate and the screen determinethe ma&imum si%e of the discharged coal
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=P13$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
; .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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.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
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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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luidi%ed bed
; $ir passed through the coal particles
; $s the air velocity increase it create the
pressure drop equal to the weight of coal
particles'
; Particles are buoyed up'
; :igh degree of particle mi&ing'
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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
; 3eceives fresh coal from top surface has
following distinct %ones
– resh or green coal
– Coal losing moisture (!rying %one)
– Co*ing layer (loosing of <,) !istillation %one
– "ncandescent co*e ( i&ed carbon is
consumed) Combustion %one – $sh layer progressively cooled
=perations
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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(-/99 C) 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 <,
is added
; "n distillation %one moisture is added
; now stream contains N,CO2 ,CO,H 2 ,VM&moisture
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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 <, which will be
crac*ed to form free carbon at high
temperature suspended in gas stream
; "f .urner is not designed properly or
operated properly leads to unburnt carbon
particles this appears as blac* smo*e onchimney top
A d f di
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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 <, is added
; "n incandescent %one <, brea*s readily burnswith secondary air present above it where it is
fed from top; <, burning is somewhat cooler need longer
time to ignite and burn
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,echanical Sto*ers
=verfeeding
-' Traveling grate sto*er
/' Chain grate sto*er
0' Spreader sto*er
4' <ibrating grate sto*er
Anderfeed sto*er
-' Single retort
/' ,ultiretort
Traveling 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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$dvantages > !isadvantages
; Simple and "nitial cost is low
; 3eliable in service and maintenance is low
; 6ives high rate of heat release per volume of
the furnaceBimited coal can be carried on grate
Clin*er problems are common
"gnition arches are requiredThere is always some loss in the form of
particles
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Spreader sto*er
; Coal from hopper is fed by rotating feeder ; eeder normally will have .lades fitted on the
drum; ine particles burn in suspension; Speed of the feeder varies with the steam output
of boiler ; 6rate is made up of C" bars Bin*s underneath
the grate are connected to a lever
; uels used may be .ituminous lignite woodwaste baggase
; Coal si%e used in #0 cm
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$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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+hy pulverisersJ
; Si%e reduction
(mm to micron)
=ther things
; !rying (primary
airLappro& 099
degree)
; eeding
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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
Pulverised coal firing
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Pulverised coal firingPulverised 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'
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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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Crushed coal from the raw coal buner is fed by gravity to a dryer where hot air is
!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.
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.
.in or central system
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.in or central system
Pulverised fuel firing system
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Pulverised fuel firing system
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Pulveri%ed coal handling system
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.all and tube mill
. ll d t b ill
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.all and tube mill
. ll d t 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 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'
" tE: ,ill
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"mpactE:ammer ,ill
" tE: ,ill
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"mpactE:ammer ,ill
" tE: ,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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.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'
. ll d ill
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.all and race mill
. ll d ill
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.all and race mill
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
$sh handling system
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$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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Hydraulic !sh Handling System ow %elocity System
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ow %elocity System
Hydraulic !sh Handling System High %elocity
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Pneumatic !sh Handling System
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Pneumatic !sh Handling 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.
6as borne !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 <+#+++ volts. "he dust !articles are removed from the collecting electrode byra!!ing the electrode !eriodically. "he electrostatic !reci!itator is costly but has low
maintenance cost and is freuently em!loyed with !ulverised coal fired !ower stations for its
effectiveness on very fine ash !articles and is su!erior to that of any other ty!e.
1SP
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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.