Coal and ash handling systems

Coal handling storage of coal
• The main purpose of storage of coal is
• To store the coal for a period of 30 to 90 days,
therefore the plant is not required to be shut
down due to failure of normal supply of coal.
• To permit the choice of the date of purchase
allowing the management to take advantage
of seasonal market conditions in prices of
coal.

Dead storage or outdoor storage
• In this storage the coal received at the power
plant is stored in dead storage in the form of
piles laid directly on the ground. the coal
stored has the tendency to combine with
oxygen of air and during this process coal loss
some of its heating value and ignition quality.
• Due to oxidation the coal may ignite
spontaneously. This can be avoided by storing
coal in following ways.

• Stocking the coal in helps or piles:
• Generally concrete floored area is used to
prevent the flow of air from the bottom for
stocking the coal.
• Coal is stored in the form of heaps or piles up
to a height of 10 to 12 m directly on concrete
ground, the coal should be compacted in layer
of 15 to30 cm in thickness.

• This effectively prevents the air circulation in
the interior of the pile. The pile top should be
given a gentle slope in which the rain may be
drained off.
• Second method for removing the heat of
oxidation is the air is, allowed to move
through the layers evenly so that air may
remove the heat of reaction and avoid
burning.

• Sealing of stored pile is desirable in order to
avoid the oxidation of coal after packing an air
tight layer of coal. Asphalt, fine coal dust, and
bituminous coating are the materials
commonly used for this purpose.

• Under water storage:
• The possibility of slow oxidation and
spontaneous combustion can be completely
eliminated by storing the coal under water.

Live storage or active storage
• Coal from a live coal storage pile is usually
supplied to combustion equipment without the
use of mobile equipment.
• The coal is usually stored in vertical cylinder
bunkers or coal bins or silo. Coal from silos is
transferred to the boiler grate.
• Live coal storage bunkers are normally
constructed with a diamond-shaped cross section
storage area.
• The storage bunkers are made of steel or
reinforced concrete to store the live coal.

Requirement of good coal handling
plants at site
• It should have minimum maintenance.
• It should be simple
• It should be reliable
• It should supply coal continuously as per the
demand of power plant.
• It should have minimum wear in running the
equipment.

Burning systems
• Fuel is burnt in a confined space known as
furnace. The furnace provides supports and
enclosures for burning equipment.
• Solid fuels such as coal, coke,wood are burnt
by means of stockers whereas burners are
used to burn pulverized coal and liquid fuels.
• Solid fuels require a grate in the furnace to
hold the bed of fuel.

Selection of fuel firing methods
• The characteristics of the available coal
• Capacity of the plant
• Efficiency and reliability of the combustion
equipments.
• The power plant load factor
• Nature of load fluctuation.

Solid fuel firing
• Hand firing system: it is simple and suitable for
small power plant. Since it is difficult to achieve
the uniform combustion and it gives low
combustion efficiency.
• Stoker firing: stoker is a power operated fuel
feeding mechanism and grate. With stoker firing ,
it allows to burn large quantities of fuel, with
easy in controlling the combustion with higher
efficiency, less labour of handling ash and are self
cleaning.

Advantages of stoker firing
• Easy of control of combustion
• High combustion efficiency
• Cheaper grade of fuel can be used
• Smokeless combustion
• Less labour required
• System is realiable

Disadvantages of stoker firing
• Capital cost is high
• Loss of coal through the grates
• Steam demand due to load fluctuations on
plant can not be met efficiently
• Complicated construction and arrangement
• Excessive wear of parts.

Types of stoker their working
• A grate is used at furnace bottom to hold a
bed of fuel. There are two ways of feeding
coal on to the grate
• Over feeding
• Under feeding
• Therefore automatic stockers may be
classified as overfeed and under feed stockers.

Principles of overfeed stokers

• It receives the coal on its top surface and is
characterized by the following five zones from top to
the bottom.
• A layer of fresh or green coal-fresh coal zone
• A layer of coal losing moisture-drying zone
• A cooking layer of coal losing its volatile content-
distillation zone.
• A layer of incandescent coke where the fixed carbon is
consumed-combustion zone
• A layer of ash progressively getting cooler-ash zone.

• Pressurized air--- FD Fan--- enters--- bottom of
the grate----- heated by absorbing heat from ash
and grate-----ash and grate are cooled------ hot
air----- incandescent coke----- O2 reacts with C-----
-CO2-----hot gases----- distillation zone-----volatile
matter added-------drying zone-----moisture is
picked up-----finally come out from the bed
containing N2,CO2,CO,H2-----secondary air
supplied-----high speed----turbulence---for
complete combustion

Types of overfeed stoker
• It may be classified as
• Travelling grate stoker
• Spreader stoker

Travelling grate stoker
• It may be chain grate type or bar grate type.
These two are different only in the details of
grate construction.
• The travelling grate stoker is also known as
conveyer grate stoker.
• In the chain grate stoker the grate is made of a
series of cast iron links connected by bars or
pins to form an endless chain.

• In the bar grate stoker the grate surface consists
of series of cast iron bars joined together by links
to form an endless belt running over two sets of
sprocket wheels with a wide surface as per need.
• The chain grate stoker consists of an endless
chain which forms a support for the fuel bed. The
chain travels over two sprocket wheels one at the
front and other at the rear of furnace. The front
sprocket is connected to a variable speed drive
mechanism.

• The travelling chain receives coal by gravity at
its front end through a hopper and carries it
into the furnace. The depth of the coal on the
grate is regulated by adjustable gate. The
speed of the grate varies at the rate at which
the coal is fed to the furnace. The ash is tipped
from the rear end of chain. The air required
for combustion enters through the air inlets
situated below the grate.

• The secondary air is supplied through the
openings provided in the furnace wall above
the grate. The combination of primary air and
secondary air provide turbulence required for
rapid combustion.
• These grate are suitable for low grade coal
because the fuel must be burnt before it
reaches the rear end of the furnace.

• Advantages:
• Simple in construction
• Maintenance cost is low
• Initial cost is low.
• Self cleaning stoker
• Heat release rate can be controlled just by
controlling the speed of chain.

• Disadvantages:
• Always some loss of coal in the form of fine
particles carried with the ashes.
• Temperature of preheated air is limited to
180°C.
• Not suitable for high capacity boiler
• Clinker trouble are very common.

Spreader stoker
• In this stoker the coal from the hooper is fed
on to a feeder which measures the coal in
accordance to the requirement. Feeder is a
rotating drum fitted with blades. Feeder can
be reciprocating rams, endless belts, spiral
worms etc.

Spreader stoker
• From the feeder the coal drops on to spreader
distributor which spread the coal over the
furnace. The spreader system projects the coal
particles in a continuous stream on to the
grate holding an ignited fuel bed. the fine
particles of coal burn in suspension where
corse particles burn on the grate.the
secondary air is admitted above the fuel bad
to promote the turbulence and to complete
combustion.

Spreader stoker
• The grate made of CI bars is connected to a
lever through links underneath the grate.
• In order to allow the ash to fall below the ash
pit, the lever is moved back and forth which
makes the bar rock about the pivot. Spreader
stoker is applied to a wide range of boiler sizes
155 MW to 265 MW.

Spreader stoker
• Advantages:
• A wide variety of coal can be burnt easily
• A thin fuel bed on the grate is helpful in
meeting the fluctuating loads.
• Operation cost is low
• Use of high temperature preheated air is
possible.
• Clinkering difficulties are reduced.

Spreader stoker
• Disadvantages:
• Spreader does not work satisfactory with
varying size of coal.
• Possibility of some fuel loss,
• Fly ass is much more due to suspension
burning of fine fuel particles.
• Clinker problem can not be avoided.

Principle of underfeed stokers

Principle of underfeed stokers
• In this type of stoker the fuel and air move in the
same direction. The coal is fed from below the
grate by a screw-conveyer or ram.
• Air----mixes----formed volatile matter-----passes---
---ignition zone------enters ----region of
incandescent coke.
• When gas stream----passes-----incandescent coke
region-----volatile matter breaks up and readily
burns with secondary air fed at the top.

Single retort stoker
• The coal from large hopper is fed by
reciprocating ram or screw conveyer into the
bottom of the horizontal trough. the air is
supplied through the tuyeres provided along
the upper edge of the grate shown in figure.
the ash from the grate falls and it is collected
in the ash pit. Some of the unburned coal may
also pass through the grate. The coal feeding
capacity of single retort stoker varies from 100
to 2000 kg per hour.

Multi retort stoker
• It is generally used for increasing the burning
capacity of the stoker. It consists of a series of
alternate retorts and tuyere boxes for supply
of air. Each retort is fitted with a reciprocating
ram for feeding and pusher plates for the
uniform distribution of coal.

Multi retort stoker
• Coal---falling from hopper-----push forward------
inward stroke of stoker------distribution
ram(pusher)---push entire coal----down the
length of stoker.
• Slope of stoker----help in moving the fuel bed and
fuel bed movement keeps it slightly agited to
break up clinker formation.
• Primary air supplied to fuel bed from wind box
situated below stoker. The partly burnt coal
moves on the extension grate. The quality of air
supplied is regulated by an air damper.

Multi retort stoker
• Advantages:
• Higher efficiency
• High part load efficiency
• Grate is self cleaning
• Smokeless operation
• Combustion rate is high
• Grate bars, tuyeres, retorts are not subjected to high
temperature
• Substantial amount of coal always remains on the grate so
that boiler may remain in service in the event of temporary
breakdown
• Different varieties of coal can be used.

Multi retort stoker
• disadvantages:
• Initial cost high
• Space requirement high
• Clinker formation
• High rate of wear and tear
• Low grade fuels with high ash content can not
be burnt economically.

Pulverized fuel handling systems
• In pulverized fuel firing coal is reduced to a fine powder in
grinding mills or pulverizers and then projected into the
combustion chambers by means of current of hot air.
• The coal is pulverized in order to increase its surface area,
thus promotes the rapid combustion without need of
supplying much of excess air, result into high thermal
efficiency.
• The amount of air which is used to dry the coal and convey
the powdered fuel to the furnace is known as primary air.
• Amount of air which is blown in separately to complete the
combustion is known as secondary air.

Process of coal pulverization
• Stage 1: raw and lump coal is crushed to a
particle size not more than 15-25 mm in the
crusher.
• Stage 2: crushed coal is delivered into raw
bunkers and from here it is transferred to
grinding mills that grind the feed into the final
particles of 200-300 mesh size. During
grinding hot air is blown through the fuel to
dry it to impart good fluidity of the coal dust.

Advantages of pulverized coal firing
• Large surface area per unit mass allows faster and efficient combustion.
• Less excess air is required for complete combustion
• Any type of coal can be used
• Better response to variation in load
• Large amount of heat release makes it suitable for super thermal power
station
• Clinkering and slagging problems are low
• Carryover of unburnt fuel to ash is nill
• Low ash handling problem
• Operate successfully with gas and oil fired system
• Less furnace volume required
• Smokeless operation
• Use of highely preheated air helps for rapid flame propagation
• No moving parts in furnace so life of the system is more.

Disadvantages of pulverized coal firing
• Capital cost is high
• Operation cost is high
• Lost of fly ash in the exhaust which makes the
removing of fine dust uneconomical.
• Since coal will burn so it wiil be danger of explosions
• Special equipment is required for starting
• Maintenance of furnace is costly
• Skilled operators are required
• High air pollution due to emission of fine particles of
dirt and grit.

Pulverized fuel handling system
• It is a family of equipment in which coal is
ground, dried and fed to the burners of a
boiler furnace.
• There are two methods used to feed the
pulverized fuel to the combustion chamber of
the boiler
• Unit system
• Central or bin system

Unit system
• Crushed coal from raw coal bunker is fed to
the pulverizer through feeder at a variable
rate governed by the combustion
requirements of boiler furnace and steam
generation rate.
• Hot air or flue gases are passed through
feeder to dry the coal before feeding to the
pulverizer.

Unit system
• Pulverized coal is carried from the mill with
the help of induced draught fan via a
separator and blown into the furnace along
with the hot air. In the separator the big
particles of coal are separated from the fine
dust and these again fall down into the mill.
the secondary air is supplied to the burner
before entering the fuel into the combustion
chamber.

Unit system
• Advantages:
• Simple in layout, design, operation
• Initial cost is less
• Allows direct combustion control from the
pulverizer
• Lower maintenance charge
• Less space required
• No drying unit required.

Unit system
• Disadvantages:
• If one pulverizing unit goes out then its corresponding
boiler unit has to be shutdown.
• Pulverizing unit operates at variable load as per the
load on power plant which results in poor performance
at part load
• Lesser degree of flexibility
• Greater wear off an blades as these handle both air
and abrasive coal particles.
• Exhaust fan is subjected to excessive wear since it
handles coal dust particles also.

Central system
• In this system coal is pulverized on a central
basis, the pulverized fuel is stored in a central
bin wherefrom it is distributed through
pipelines between the boilers.
• The coal from raw coal bunkers is fed to the
drier. The coal drying is achieved by using hot
gases, preheated air or bled steam. Feeder is
used to feed dried coal to the pulverizer.

Central system
• the pulverized coal is carried from the
pulverizer mill with the help of air and it is
separated in the cyclone separator. The
separated pulverized coal is transferred to the
central bin with the help of conveyer. Forced
draught fan is used to supply primary air to
feeder. The mixture of pulverized coal and air
is then supplied to the burners.

Central system
• Advantages:
• Greater flexibility and better response.
• Less power consumption
• Operation of burner is independent
• The fan handles only air therefore no problem of
excessive wear of fan blades.
• Pulverizer can be shut down where there is enough
reserve of pulverized coal.
• Offers good control over the fineness of coal
• Less labour
• Pulverized capacity is low

Central system
• Disadvantages:
• Initial cost is high
• More floor space.
• Possibility of fire hazard is more
• Dryer is essesntial
• Operation and maintenance cost are high
• Coal transportation system becomes more
complex.

Pulverized mill
• Pulverized mill is also known as pulverizer. It is
used to convert raw crushed coal into powder
form in order to increase surface area. A
pulverizer is the most important part of a
pilverized coal system.
• Impact mills (a) ball mill (b) hammer mill
• Attrition mills (a) bowl mill (b) ball and race
mill.

Ball mill
• Raw coal from feeders is supplied to the classifiers
from where it moves to the rotating drum by means of
a screw conveyer. As the drum rotates the coal gets
pulverized due to combined impact between coal and
steel balls. Hot air is introduced into the drum. The
powdered coal is picked up by the air and coal air
mixture enters the classifiers, where sharp changes in
the direction of the mixture throw out the over sized
coal particles. The oversized particles are returned to
the drum. The coal air mixture from the classifier
moves to the exhaust fan and then it is supplied to the
burners.

Ball mill
• Advantages:
• System is simple & low initial cost
• Grinding elements are not seriously affected
by metal scrap and other foreign materials
• Suitable for a wide range of fuels
• Low maintenance cost.

Ball mill
• Disadvantages:
• Larger and heavier in construction
• Consumes more power
• Due to poor air circulation works less
efficiently
• Operation cost is high.

Ball & race mill
• A ball and race mill also called contact mill .
• It consists of two elements which have a
rolling action with respect to each other. The
rolling elements may be balls or ring shaped
rolls that roll between two races, in the
manner of a ball bearing. The balls are
between a top stationary race or ring and a
rotating bottom Ring, which is driven by the
vertical shaft of the pulverizer.

Ball & race mill
• The coal is supplied through the rotating table
feeder at the upper right to fall on the inner
side of the races. The moving balls and races
catch coal between them to crush it to a
powder. Spring hold

Ball and race mill
• Spring hold down the upper stationary race and adjust
the force needed for crushing.
• Hot air is supplied to the mill through the annular
space surrounding the races by a forced draft fan. The
powdered coal is is picked up by the air and the coal air
mixture enters the classifiers, where sharp changes in
the direction of the mixture throw out the over sixzed
coal particles. The over sized particles are returned for
further grinding in the mill. The coal air mixture from
the classifier moves to the exhuster fan and then it is
supplied to the burners.

Ball and race mill
• Advantages:
• Lower power consumption
• Lower in weight
• Lower in operation cost
• Suitable for variable load conditions
• Can handle coal containing as much as
20%moisture.
• Disadvantages:
• Greater wear
• Leakage of fine coal through the mill

Bowl mill
• It consists of stationary rollers mounted on an
electrically driven rotating bowl. Coal fed through
the hopper gets pulverized by attrition as it
passes between the sides of the rollers and bowl.
• Hot primary air introduced into the pulverizer
through the bottom of the bowl carries of coal
dust into the centrally located classifier fitted at
the top. Coarse particles drop back into the bowl
through the centre cone of the classifier while the
fine coal dust-air mixture is led away to the
burner.

Bowl mill
• Advantages:
• Lower energy consumption
• Less over all dimensions
• Leakage of coal from the mill casing is practically
nil
• Produce less noise.
• Disadvantages:
• It is sensitive to metallic objects enter along with
coal.
• Uneven wear of the grinding parts

Hammer mill
• It is known as impact mill as pulverization takes place
due to impact. Rotating hammers with a
circumferential speed of 50 to 60 m/s strike the fuel
lumps and crush them into smaller pieces which get
pulverized by the abrasion in gap between the
hammers and casing.
• The primary air fans reduce air through the pulverizer,
a flow of air that lifts the coal dust. the air borne fuel
dust is subjected to a centrifugal dust separator to
throw the oversize particles back into the grinding
section while the finely divided fuel particles
suspended primary air are discharged through a
centrally located dust discharging duct.

Hammer mill
• Advantages:
• Simple and low in capital cost
• Requires minimum floor area
• Operation speed is high
• Wet coal can be pulverized
• Disadvantages:
• Power consumption is high
• Its capacity is limited
• Care is required to prevent metal scrap entering
to the pulverizer mill.

Pulverized coal burners
• The function of the coal burners is to fire the
pulverized coal along with primary air into the
furnace. The secondary air is admitted
seperately below the burner, around the
burner or elsewhere in the furnace. A good
coal burner shoul be able to produce a
uniform and stable flame with almost
complete combustion of fuel. A pulverized
coal burners should satisfy the following
requirements.

• It should mix the coal and primary air
thoroughly and bring this mixture in contact
with secondary air to create sufficient
turbulance.
• Control the flame shape and its travel in the
furnace.
• It should ensure adequate protection against
overheating and excesssive abrasive wear.
• It should ensure complete combustion.

• Following factors affect the performance
• Characteristics of the fuel
• Particles size
• Mixing place of the fuel and air
• Proportion of primary air and secondary air
• Volatile matter content
• Furnace design

Cyclone burner
• Fuel particles are subjected to a great
turbulence by the combustion air supplied
and they burn of more quickly.
• The cyclone burner is essentially a water
cooled horizontal cylinder located outside the
main boiler furnace in which the crushed coal
is fed and fired with high rate of heat release.
• Combustion of coal is completed before the
resulting hot gases enter the boiler furnace.

Cyclone burner
• The primary air enters the burner tangentially ,
thus imparting a centrifugal motion of the coal.
• The secondary air is also admitted tangentially at
the top of the cyclone at high speed, imparting
further centrifugal motion.a small quantity of air
called tertiary air is admitted at the center. A high
temperature is developed in this furnace, with
the effect that slags produced are in a molten
state and liquid slag is discharged off. Hence such
furnace are also called slagging type.

Cyclone burner
• Advantages:
• Only crushed coal is used no pulverization
equipment is needed.
• It removes much of the ash.
• It reduce erosion and fouling of steam generator
surface due to low ash content in the flue gases.
• Furnace gives best result with low grade fuel like
indian coal.
• Can be operated with less excess air.
• High furnace temperature are obtained.

Cyclone burner
• Disadvantages:
• Higher forced draft fan pressure is required
• Formation of relatively more Nox
• Not suitable for high sulphur content coal.

Oil burners
• The function of the oil burner are to mix the
fuel and air in the proper proportion and to
prepare the fuel for combustion.
• Generally the oil burners are used for small
capacity boilers for industrial process heating.
• The mixing of the oil with air is done in such a
way that the oil could be burnt with minimum
excess air for achieving the maximum
temperatures.

Oil burners
• (1) vapouring burners: in this oil may be
vapourised or gasified by heating within the
burner. Used in blow torch, gasoline stoves.
Etc. not used in boiler for steam generation.
• (2) atomising burners: in this oil may be
atomised by the nozzle of burner and its
vapourisation occurs in combustion space.
Used for oil fired furnaces and boiler furnaces.

• Injection under high pressure of 25 to 35 bar.
The oil leaves the nozzle in atomised form.
• In case of rotating wheel burner the oil leaves
in the form of hollow cone of fine fuel
particles due to centrifugal action.
• The modern trend is towards mechanical
atomisation since the auxiliary consumption is
less and less excess air is required.

Necessity of ash disposal
• Ash is the incombustible material that remains
when coal is burned. The ash came into the body
of coal from the original vegetable matter from
which coal was formed.
• Ash content in coal may vary from 8 to 10 % to
ash high as 20 to 50 % depending upon the grade
of the coal. Better the quality of the coal lower
the ash content, hence for inferior grade coal, the
ash content is higher than the anthracite or semi-
anthracite coal.

Necessity of ash disposal
• Ashes have to be discharged and dumped at a
site sufficiently far away from the thermal
power plant for the following reasons.
• The ash is dusty and irritating
• Generates toxic gases, corrosive acids
• Sufficiently hot when it comes out of the
boiler furnace.

Coal and ash handling systems

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Coal and ash handling systems