Steam Boiler
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STEAM BOILERSSTEAM BOILERS
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DefinitionDefinition
A closed vessel in which steam is produced from A closed vessel in which steam is produced from water by combustion of fuelwater by combustion of fuel
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PurposePurpose of boilersof boilers For generating power in steam engines or For generating power in steam engines or
steam turbinessteam turbines
In textile industries for sizing and bleachingIn textile industries for sizing and bleaching
For heating the buildings in cold weather and For heating the buildings in cold weather and for producing hot water for hot water supplyfor producing hot water for hot water supply
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Primary requirements of a boilerPrimary requirements of a boiler
The water must be contained safelyThe water must be contained safely
The steam must be safely delivered in desired The steam must be safely delivered in desired condition (as regard its pressure, temperature, condition (as regard its pressure, temperature, quality and required rate)quality and required rate)
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Boiler termsBoiler terms ShellShell: Consists of one or more steel plates bent : Consists of one or more steel plates bent
into a cylindrical form and riveted or welded into a cylindrical form and riveted or welded together. The shell ends are closed with end platestogether. The shell ends are closed with end plates
SettingSetting: The primary function of setting is to : The primary function of setting is to confine heat to the boiler and form a passage for confine heat to the boiler and form a passage for gases. It is made of brick work and may form the gases. It is made of brick work and may form the wall of the furnace and combustion chamberwall of the furnace and combustion chamber
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GrateGrate: it is a platform in the furnace upon : it is a platform in the furnace upon which fuel is burntwhich fuel is burnt
FurnaceFurnace: it is the chamber formed by the : it is the chamber formed by the space above the grate and below the boiler space above the grate and below the boiler shell, in which combustion takes place. shell, in which combustion takes place.
Water space and steam spaceWater space and steam space: the volume of : the volume of the shell that is occupied by the water is the shell that is occupied by the water is termed as water space while the entire shell termed as water space while the entire shell volume less the water and tubes is called steam volume less the water and tubes is called steam spacespace
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MountingsMountings: The items which are used for : The items which are used for safety of boiler are called mountingssafety of boiler are called mountings
AccessoriesAccessories: The items which are used for : The items which are used for increasing the boiler efficiency are called increasing the boiler efficiency are called accessoriesaccessories
Water levelWater level: The level at which water stands : The level at which water stands in the boiler is called water levelin the boiler is called water level
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RefractoryRefractory: insulation material used for lining : insulation material used for lining combustion chambercombustion chamber
FoamingFoaming: Formation of steam bubbles on the : Formation of steam bubbles on the surface of boiler water due to high surface surface of boiler water due to high surface tension of watertension of water
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ScaleScale: A deposit of medium due to extreme : A deposit of medium due to extreme hardness occurring on the water heating hardness occurring on the water heating surfaces of boiler because of an undesirable surfaces of boiler because of an undesirable condition in the boiler watercondition in the boiler water
Blowing offBlowing off: The removal of mud and other : The removal of mud and other impurities of water from the lowest part of the impurities of water from the lowest part of the boiler. Accomplished with the help of blow off boiler. Accomplished with the help of blow off cock or valvecock or valve
LaggingLagging: Insulation wrapped on the outside of : Insulation wrapped on the outside of the boiler shell or steam pipingthe boiler shell or steam piping
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Boiler accessoriesBoiler accessories Feed pumpsFeed pumps: Used to deliver feed water to the : Used to deliver feed water to the
boiler. It is desirable that the quantity of water boiler. It is desirable that the quantity of water supplied should be at least equal to that supplied should be at least equal to that evaporated and supplied to the engineevaporated and supplied to the engine
Two types of which are commonly used as Two types of which are commonly used as feed pumps are (1) reciprocating pump (2) feed pumps are (1) reciprocating pump (2) rotary pumprotary pump
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InjectorInjector Function of injector is to feed water into the boilerFunction of injector is to feed water into the boiler
It is commonly employed for vertical and It is commonly employed for vertical and locomotive boilers and does not find its locomotive boilers and does not find its application in large capacity high pressure boilersapplication in large capacity high pressure boilers
Also used where the space is not available for the Also used where the space is not available for the installation of feed pumpinstallation of feed pump
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EconomizerEconomizer Is a device in which the waste heat of the flue Is a device in which the waste heat of the flue
gases is utilized for heating the feed watergases is utilized for heating the feed water
Economizers are of two types Economizers are of two types Independent type Independent type Integral typeIntegral type
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Air Pre-heaterAir Pre-heater The function of the air pre-heater is to increase the The function of the air pre-heater is to increase the
temperature of air before it enters the furnace.temperature of air before it enters the furnace. It is placed after the economizer.It is placed after the economizer. Flue gases pass through the economizer and then to Flue gases pass through the economizer and then to
the air preheaterthe air preheater Degree of preheating depends on Degree of preheating depends on
Type of fuel Type of fuel Type of fuel burning equipment, andType of fuel burning equipment, and Rating at which the boiler and furnace are Rating at which the boiler and furnace are
operatedoperated
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Types of air preheatersTypes of air preheaters
I.I. Tubular type Tubular type
II.II. Plate typePlate type
III.III. Storage typeStorage type
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Super heaterSuper heater The function of a super heater is to increase The function of a super heater is to increase
the temperature of the steam above its the temperature of the steam above its saturation point saturation point
The super heater is very important accessory The super heater is very important accessory of a boiler and can be used both on fire tube of a boiler and can be used both on fire tube and water – tube boilers.and water – tube boilers.
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Advantages of super heated steamAdvantages of super heated steam
Steam consumption of the engine or turbine is Steam consumption of the engine or turbine is reduced reduced
Erosion of turbine blade is eliminatedErosion of turbine blade is eliminated
Efficiency of the steam plant is increasedEfficiency of the steam plant is increased
Losses due to condensation in the cylinders and the Losses due to condensation in the cylinders and the steam pipes are reduced.steam pipes are reduced.
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Steam separatorSteam separator The function of a steam separator is to remove The function of a steam separator is to remove
the entrained water particles from the steam the entrained water particles from the steam conveyed to the steam engine or turbine.conveyed to the steam engine or turbine.
It is installed as close to the steam engine as It is installed as close to the steam engine as possible on the main steam pipe from the possible on the main steam pipe from the boiler boiler
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According to principle of operation the steam According to principle of operation the steam separators are classified as followsseparators are classified as follows
Impact or baffle typeImpact or baffle type
Reverse current typeReverse current type
Centrifugal typeCentrifugal type
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Boiler mountings Pressure gauge Fusible plug Steam stop valve Feed check valve Blow off cock Mud and man holes
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Pressure gauge
To record the steam pressure at which steam is generated in the boiler
A bourdon pressure gauge in its simplest form consists of a simple elastic tube
One end of the tube is fixed and connected to the steam space in the boiler
Other end is connected to a sector through a link
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Pressure gauge
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Fusible plug To extinguish fire in the event of water level in
the boiler shell falling below a certain specified limit
It is installed below boiler’s water level
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Working of Fusible plug When the water level in the shell falls below the top
of the plug the steam cannot keep it cool and the fusible metal melts due to over heating.
thus the copper plug drops down and is held with in the gun metal body by the ribs.
Thus the steam space gets communicated to fire box and extinguishes the fire.
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Thus damage to the fire box which could burn up is avoided
By removing the gun metal plug and copper plug the Fusible plug can be put in position again by inserting the fusible metal usually lead or metal alloy
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Steam stop valve A valve is a device that regulates the flow of a fluid
(gases , fluidized solids slurries or liquids) by opening or closing or partially obstructing various passageways
Function : to shut off or regulate the flow of steam from the boiler to the steam pipe or steam from the steam pipe to the engine
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Steam stop valve
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Feed check valve
To allow the feed water to pass in to the boiler
To prevent the back flow of water from the boiler in the event of the failure of the feed pump
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Blow off cock To drain out water from
the boiler for internal cleaning inspection or other purposes
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Mud and man holes To allow men to enter in to the boiler for
inspection and repair
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Classification of boilersClassification of boilers Horizontal, vertical or inclinedHorizontal, vertical or inclined Fire tube and water tubeFire tube and water tube Externally fired and internally firedExternally fired and internally fired Forced circulation and natural circulationForced circulation and natural circulation High pressure and low pressureHigh pressure and low pressure Stationary and portableStationary and portable Single tube and multi tubeSingle tube and multi tube
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HorizontalHorizontal, , verticalvertical oror inclinedinclined
If the axis of the boiler is horizontal, vertical If the axis of the boiler is horizontal, vertical or inclined then it is called horizontal, vertical or inclined then it is called horizontal, vertical or inclined boiler respectivelyor inclined boiler respectively
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Fire tube and water tubeFire tube and water tube If hot gases are inside the tube and water is If hot gases are inside the tube and water is
outside the tube, it is called fire-tube boiler. outside the tube, it is called fire-tube boiler. Examples: Cochran, Lancashire and Examples: Cochran, Lancashire and
locomotive boilerslocomotive boilers If water is inside the tube and hot gases are If water is inside the tube and hot gases are
outside the tube, it is called fire-tube boiler. outside the tube, it is called fire-tube boiler.
Examples: Babcock and Wilcox, Stirling, Examples: Babcock and Wilcox, Stirling, Yarrow boiler etcYarrow boiler etc
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Externally fired and internally firedExternally fired and internally fired The boiler is known as externally fired if the The boiler is known as externally fired if the
fire is outside the shell. fire is outside the shell. Examples: Babcock and Wilcox, StirlingExamples: Babcock and Wilcox, Stirling
The boiler is known as internally fired if the The boiler is known as internally fired if the furnace is located inside the boiler shell.furnace is located inside the boiler shell.
Examples: Cochran, Lancashire Examples: Cochran, Lancashire
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Forced circulation and natural Forced circulation and natural circulationcirculation
In forced circulation type of boilers, the In forced circulation type of boilers, the circulation of water is done by a forced pumpcirculation of water is done by a forced pump
Examples: Velox, Lamont, Benson boilerExamples: Velox, Lamont, Benson boiler In natural circulation type of boilers, circulation In natural circulation type of boilers, circulation
of water in the boiler takes place due to natural of water in the boiler takes place due to natural convection currents produced by the application convection currents produced by the application of heatof heat
Examples: Lancashire, Babcock and WilcoxExamples: Lancashire, Babcock and Wilcox
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High pressure and low pressureHigh pressure and low pressure The boilers which produce steam at pressures The boilers which produce steam at pressures
of 80 bar and above are called high pressure of 80 bar and above are called high pressure boilersboilers
Examples: Babcock and Wilcox, Velox, Examples: Babcock and Wilcox, Velox, Lamont, Benson boilersLamont, Benson boilers
The boilers which produce steam at pressure The boilers which produce steam at pressure below 80 bar are called low pressure boilersbelow 80 bar are called low pressure boilers
Examples: Cochran, Cornish, Lancashire Examples: Cochran, Cornish, Lancashire and locomotive boilersand locomotive boilers
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Stationary and portableStationary and portable
Stationary boilers are used for power plant-Stationary boilers are used for power plant-steam, for central station utility power plants, steam, for central station utility power plants, for plant process steam etcfor plant process steam etc
Mobile or portable boilers include locomotive Mobile or portable boilers include locomotive type, and other small unit for temporary use at type, and other small unit for temporary use at sitessites
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Single tube and multi tubeSingle tube and multi tube The fire tube boilers are classified as single The fire tube boilers are classified as single
tube or multi-tube boilers, depending upon tube or multi-tube boilers, depending upon whether the fire tube is one or more than onewhether the fire tube is one or more than one
Examples of single tube boilers are Cornish Examples of single tube boilers are Cornish and simple vertical boilerand simple vertical boiler
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Comparison of fire tube and water Comparison of fire tube and water tube boilerstube boilers
Particulars Particulars Fire-tube boilersFire-tube boilers Water-tube boilersWater-tube boilers
Position of water and hot Position of water and hot gasesgases
Hot gases inside the tubes Hot gases inside the tubes and water outside the and water outside the tubestubes
Water inside the tubes Water inside the tubes and hot gases outside the and hot gases outside the tubestubes
Mode of firingMode of firing Generally internally firedGenerally internally fired Externally firedExternally fired
Operation pressureOperation pressure Limited to 16 barLimited to 16 bar Can go up to 100 barCan go up to 100 bar
Rate of steam productionRate of steam production LowerLower HigherHigher
SuitabilitySuitability Not suitable for large Not suitable for large power plantspower plants
Suitable for large power Suitable for large power plantsplants
Risk on bursting Risk on bursting Involves lesser risk of Involves lesser risk of explosion due to lower explosion due to lower pressurepressure
More risk on bursting More risk on bursting due to high pressuredue to high pressure
Floor areaFloor area For a given power it For a given power it occupies more floor areaoccupies more floor area
For a given power it For a given power it occupies less floor areaoccupies less floor area
ConstructionConstruction DifficultDifficult Simple Simple
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Cont…Cont…ParticularsParticulars Fire-tube boilersFire-tube boilers Water-tube boilersWater-tube boilers
TransportationTransportation Difficult Difficult SimpleSimple
Shell diameterShell diameter Large for same powerLarge for same power Small for same powerSmall for same power
Chances of explosionChances of explosion Less Less MoreMore
Treatment of waterTreatment of water Not so necessaryNot so necessary More necessaryMore necessary
Accessibility of various Accessibility of various partsparts
Various parts not so easily Various parts not so easily accessible for cleaning, accessible for cleaning, repair and inspectionrepair and inspection
More accessibleMore accessible
Requirement of skillRequirement of skill Require less skill for Require less skill for efficient and economic efficient and economic workingworking
Require more skill and Require more skill and careful attentioncareful attention
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Lancashire boilerLancashire boiler Reliable, has simplicity of design, ease of Reliable, has simplicity of design, ease of
operation and less operating and maintenance operation and less operating and maintenance costscosts
Commonly used in sugar-mills and textile Commonly used in sugar-mills and textile industries where along with the power steam industries where along with the power steam and steam for the process work is also neededand steam for the process work is also needed
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Cont…Cont… Consists of cylindrical shell inside which two large Consists of cylindrical shell inside which two large
tubes are spacedtubes are spaced
Shell is constructed with several rings of cylindrical Shell is constructed with several rings of cylindrical from it is placed horizontally over a brick work from it is placed horizontally over a brick work which forms several channels for the flow of hot which forms several channels for the flow of hot gases gases
The furnace is placed at the front end of each tubeThe furnace is placed at the front end of each tube
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Locomotive boilerLocomotive boiler
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Consists of cylindrical barrel with rectangular Consists of cylindrical barrel with rectangular fire box at one end and smoke box at another fire box at one end and smoke box at another endend
Hot gases generated due to burning of coal are Hot gases generated due to burning of coal are deflected by an arch of fire bricks, so that deflected by an arch of fire bricks, so that walls of the fire box may be heated properlywalls of the fire box may be heated properly
The heat of the hot gases is transmitted into The heat of the hot gases is transmitted into water through the heating surfaces of fire tubeswater through the heating surfaces of fire tubes
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Babcock and Wilcox boilerBabcock and Wilcox boiler
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Cont…Cont… It consists of a drum connected to a series of It consists of a drum connected to a series of
front end and rear end header by short riser tubesfront end and rear end header by short riser tubes
To these headers are connected a series of To these headers are connected a series of inclined (15inclined (1500 or more) water tubes or more) water tubes
A hand hole is provided in the header in front of A hand hole is provided in the header in front of each tube for cleaning and inspection of tubeseach tube for cleaning and inspection of tubes
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Cont…Cont… Feed valve is provided to fill the drum and Feed valve is provided to fill the drum and
inclined tubes with waterinclined tubes with water Through the fire door fuel is supplied to grate Through the fire door fuel is supplied to grate
where it is burntwhere it is burnt The hot gases are forced to move upwards The hot gases are forced to move upwards
between the tubes by baffle platesbetween the tubes by baffle plates The water from the drums flows through the The water from the drums flows through the
inclined tubes via down take header and goes inclined tubes via down take header and goes back into the shell in the form of water and steam back into the shell in the form of water and steam via uptake headervia uptake header
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Nestler boiler
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Nestler boiler Fire tube type of fired horizontal axis boiler The boiler shell consists of two mild steel
thick plates with large number of fire tubes fitted between two plates
A bigger diameter furnace tube extending from burner end to other end is used for carrying hot flue gases from one smoke box to other smoke box
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At the rare end smoke box chimney is provided for the rejection of exhaust gases
Hot gases passes through the furnace tube and enter into the rear end smoke box and pass through fire tubes to the front end smoke box for final discharge through chimney
Water surrounding tubes get transformed into steam and gets collected in steam space.
Oil is first heated up to 80oc by electric heater before being supplied to burner for injection into furnace tube.
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Blower is employed for atomization of furnace oil into furnace
Such a boilers are capable of generating steam up to 10-11 bar.
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Bent tube boilers
Straight tube boilers has many disadvantages like1. They had less accessibility and poorer inspection
capability, considerable time, labour and expense were required to open up or close the bolts in the headers, and to remove and replace the gaskets
2. Inadequate design and imperfect fabrication of hand hole caps (cleaning purpose) resulted in much leakage
3. Circulation was sluggish sluggish due to low head, and limited steam disengaging surface made inadequate separation of steam and water reducing steam rate
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Bent tube boilers offers many advantages over straight-tube boilers
The notable among them being greater accessibility for inspection, cleaning, and maintenance, and ability to operate at higher steaming rates and to deliver drier steam
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Four drum stirling boiler
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Five-drum form
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Water flows downwards from the mud drum to headers feeding the tubes lining the walls of the radiant surface
The low density steam-water mixture rises up to the steam drum at the upper side
The steam is separated and flows to the central drum, where it is removed
Feed water enters the drum at the left and mixes with the saturated liquid in the drum
The cooled liquid flows down to mud drum
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Cochran boiler
One of the best types of vertical multi-tube boiler
Consists of a cylindrical shell with a dome shaped top where the space is provided for steam
The furnace is one piece construction and is seamless
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Cont..
Its crown has a hemispherical shape and thus provides maximum volume of space
The fuel is burnt on the grate and ash is collected and disposed from the ash pit
The gases of combustion produced by burning the fuel enter the combustion chamber through the flue tube
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They strike against fire brick lining which directs them to pass through number of horizontal tubes, being surrounded by water
After which the gases escape to the atmosphere through the smoke box and chimney
A number of hand holes are provided around the outer shell for cleaning purposes
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It is a component of steam generatorBasic requirements :Through mixing of fuel and airOptimum fuel-air ratios leading to most
complete combustion possible maintained over full load range
Ready and accurate response of rate of fuel feed to load demand
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Contd..Contd..
Continuous and reliable ignition of fuel
Practical distillation of volatile components of coal followed by adequate action
Adequate control over point of formation and accumulation of ash, when coal is the fuel
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Solid fuels fired
Hand fired Stoker fired Pulverized fuel fired
Underfeed stockers Overfeed stockers
Unit system Central system Both
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Liquid fuel fired
Injection system
Evaporator system
Combination of both
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Gaseous fuel fired
Atmospheric pressure system
High pressure system
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Initial cost of equipment Sufficient combustion space and its liability to
withstand high flame temp Area of grate Operating cost Minimum smoke nuisance Flexibility of operation Arrangements for through mixing of air with fuel for
efficient combustion
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A stoker is a power operated fuel feeding mechanism and grate
A cheaper grade of fuel can be usedA higher efficiency can be attainedA greater flexibility of operations assuredLess smoke producedGenerally less building space is necessaryCan be used for small or large boiler unitsVery reliable , maintenance charges are reasonably low
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Practically immune for explosion
Reduction in auxiliary plant
Capital investment as compared to pulverized fuel system is less
Some reserve is gained by the large amount of coal stored on the grate in the event of coal handling plant failure
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Construction is complicated
In case of very large units the initial cost may be rather higher than with pulverized fuel
There is always a certain amount of loss of coal in the form of riddling through the gates
Sudden vibrations in the steam demand cannot be met to the same degree
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Troubles due to slagging and clinkering of combustion chamber walls are experienced
Banking and stand by losses are always present
Structural arrangements are not so simple and surrounding floors have to be designed for heavy loadings
There is excessive wear of moving parts due to abrasive action of coal
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In overfeed stokers the coal is fed into the grate above the point of air admission
The fuel bed section receives fresh coal from top surfaces
The ignition plane lies between green coal and incandescent coke
The air enters the bottom of the grate under pressure
In flowing through the grate opening the air is heated while it cools the grate
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The warm air then passes through a layer of hot ashes and picks up the heat energy
The region immediately above the ashes contains a mixture of incandescent coke and ash, coke content increasing upward direction
As the air comes in contact with incandescent coke, the O2 reacts with carbon to form CO2
Water vapor entering with the air reacts with coke to form CO2, CO and free H2
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Upon further travel through the incandescent region some of the CO2 reacts with coke to form CO
Hence no free O2 will be present in the gases leaving the incandescent region
Fresh fuel undergoing distillation of its volatile matter forms the top-most layer of the fuel bed
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Heat for distillation and eventually ignition comes from
1. By conduction from the incandescent coke below
2. From high temperature gases diffusing through the surface of the bed
3. By radiation from flames and hot gases in the furnace
4. From the hot furnace walls
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The ignition zone lies directly below the raw fuel undergoing distillation
To burn gases additional secondary air must be fed into the furnace to supply the needed oxygen
The secondary air must be injected at considerable speed to create turbulence and to penetrate to all parts of the area above the fuel bed
The combustible gases then completely burn in the furnace
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Fuel, coke and ash in the fuel bed move in the direction opposite to that of air and gases
Raw fuel continually drops on the surface of the bed
The rising air feed cools the ash until it finally rests in a plane immediately adjacent to the grate
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Types of overfeed stokers
1 Travelling grate stoker
• Chain grate type• Bar grate type
2 Spreader stoker
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A chain grate stoker consists of flexible endless chain which forms a support for the fuel bed
The chain travels over sprocket wheels one at the front and one at the rear of furnace
The front sprocket is connected to a variable speed drive mechanism
The grate should be saved from being overheated, for this, coal should have sufficient ash content which will form a layer on grate
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Simple in constructionInitial cost lowMaintenance charges lowSelf-cleaning stokerGiving high release rates per unit volume of
the furnaceHeat release rates can be controlled just by
controlling the speed of the chain
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Preheated air temperatures are limited to 1800C maximum
The clinker troubles are very commonThere is always some loss of coal in the form
of fine particles through riddlingsIgnition arches are requiredThis cannot be used for high capacity boilers
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In this type of stoker the coal is not fed into furnace by means of grate
The function of the grate is only to support a bed of ash and move it out of the furnace
From the coal hopper, coal is fed into the path of a rotor by means of a conveyor
And it is thrown into the furnace by the rotor and burnt in suspension
The air for combustion is supplied through the holes in the grate
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The secondary air to create turbulence and supply oxygen for thorough combustion of coal is supplied through nozzles located directly above the ignition arch
Unburnt coal and ash are deposited on the grate which can be moved periodically to remove ash out of the furnace
Spreader stokers can burn any type of coal
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A wide variety of coal can be burntThis stoker is simple to operate, easy to light
up and bring into commissionThe use of high temperature preheated air is
possibleOperation cost is considerably lowThe clinking difficulties are reduced even with
coals which have high clinkering tendencies
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Volatile matter is easily burntFire arches etc. Are generally not required
with this type of stokers
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It is difficult to operate spreader with varying sizes of coal with varying moisture content
Fly-ash is much moreNo remedy for clinker troublesThere is a possibility of some fuel loss in the
cinders up the stack because of the thin fuel bed and suspension burning
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Hand fired system
Manual feeding system
Very old system
Used in small scale applications.
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Hand fired system
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Air entering through the holes in the grate comes in contact with the raw coal
Then it passes through the incandescent coke where reactions similar to overfeed system takes place
The gases produced then pass through a layer of ash
The secondary air is supplied to burn combustible gases
The underfeed principle is suitable for burning the semi-bituminous and bituminous coal
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High thermal efficiency as compared to chain grate stokers
Combustion rate is considerably higherThe grate is self cleaningPart load efficiency is high particularly with multi
retort typeDifferent variety of coal can be usedMuch higher steaming rates are possible with this
type of stoker
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Grate bars, tuyeres and retorts are not subjected to high temp as they remain contact with fresh coal
Overload capacity of the boiler is high as large amount of coal is carried on the grate
Smokeless operation is possible even at very light load
With use of clinker grinder, more heat can be liberated out of the fuel
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Substantial amount of coal always remains on the grate so that boiler may remain in service in the event of temporary breakdown of the coal supply system
It can be used with all refractory furnaces because of non-exposure of stoker mechanism to the furnace
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High initial costRequire large building spaceThe clinker troubles are usually presentLow grade fuels with high ash content cannot
be burnt economically
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Contd..
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Coal is reduced to a fine powder with the help of grinding mill and then projected into the combustion chamber with the help of hot air current
The amount of air (secondary air) required to complete the combustion is supplied separately to the combustion chamber
The resulting turbulence in the combustion chamber helps for uniform mixing of fuel and air
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The amount of air which is used to carry the coal and dry it before entering into the combustion chamber is known as primary air
The efficiency of the pulverized fuel firing system mostly depends upon the size of the powder
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Any grade of coal can be used since coal is powdered before use
The rate of feed of the fuel can be regulated properly resulting in the economy
Since there is almost complete combustion of the fuel there is increased rate of evaporation and higher boiler efficiency
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Greater capacity to meet peak loadsThe system is practically free from sagging and
clinkering troublesNo stand by losses due to banked firesPractically no ash handling problemsNo moving parts in the furnace is subjected to
high temperaturesThis system works successfully with or in
combination with gas or oilMuch smaller quantity of air is required as
compared to that of stoker firing
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Practically free from clinker troublesThe external heating surfaces are free from
corrosionIt is possible to use highly preheated secondary
air which helps for rapid flame propagationThe furnace volume required is considerably
less
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High capital costLot of flyash in the exhaust, which makes the
removing of fine dust uneconomicalThe possibility of explosion is more as coal burns
like a gasThe maintenance of furnace brick work is costlySpecial equipment is needed to start this system
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Skilled operators are requiredA separate coal preparation plant is necessaryHigh furnace temps cause rapid deterioration
of the refractory surfaces of the furnaceNuisance is created by the emission of very
fine particles of grit and dustFine regular grinding of fuel and proper
distribution of burners is usually difficult to achieve
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Self study topics
1. Schematic diagrams of modern steam generators
2. Cyclone furnace
3. Fluidized bed combustion
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