Coal and Ash Handling

COAL AND ASH
HANDLING
--VANITA THAKKARVANITA THAKKAR
ASSOCIATEASSOCIATE PROFESSORPROFESSOR
MECHANICAL ENGINEERING DEPARTMENT,MECHANICAL ENGINEERING DEPARTMENT,
BABARIA INSTITUTEBABARIA INSTITUTE OFOF TECHNOLOGY, VARNAMA,TECHNOLOGY, VARNAMA,
VADODARA.VADODARA.

Coal is the main fuel used in Thermal Power
Plants across India.
TYPES OF COAL:
 Peat
 Lignite
 Semi bituminous
 Bituminous
 Anthracite
INTRODUCTION
2VANITA THAKKAR BIT, VARNAMA

COAL ANALYSIS
Ultimate Analysis : to
determine the
constituent of coal, but
rather in a form of its
basic chemical
elements.
Constituents %
Moisture 3-30
Volatile Matter 3-50
Ash 2-30
Fixed Carbon 16-92
Constituents %
Carbon 50-95
Hydrogen 1-5.5
Oxygen 2-40
Sulpher 0.5-3
Nitrogen 0.5-7
Ash 2-30
Proximate Analysis : to
determine the amount of
fixed carbon (FC), volatile
matters (VM), moisture, and
ash within the coal sample.

RECALL :
GCV : Gross calorific value : The amount of heat released during the
combustion of a specified amount of fuel, determined by bringing
all the products of combustion back to the original pre-combustion
temperature, and in particular condensing any vapor produced.
NCV : Net calorific value : The amount of heat released during the
combustion of a specified amount of fuel, determined by
subtracting the heat of vaporization of the water vapor from the
higher heating value (GCV).
Volatile Matter: Volatile matter present in coal may be combustible
gases like CH4, H2, CO and other hydrocarbon or
incombustible gases like CO2, N2 etc. Presence of incombustible
gases is always undesirable because it reduces the GCV of the coal.

SOME IMPORTANT DEFINITIONS
Clinker : When Temperature developed in coal burning is higher than the
fusion temperature of ash, the ash melts. The melting ash engulfs the
unburnt coal particles and forms a solidified mass which is known as
clinker. It is formed while burning coal containing ash which has low
fusion temperature.
Weathering of coal: All coals tend to combine with oxygen and slow
oxidation takes place. This process of oxidation without burning is
called weathering of coal.
Primary Air: The amount of air which is used to carry the coal and to
dry it before entering into the combustion chamber is known as
‘Primary Air’
Secondary Air: The amount of air which is supplied separately for
completing the combustion is known as ‘Secondary Air’.

OUT-PLANT HANDLING OF COAL
Out plant handling means transfer of coal from
mine or port to storage of coal at plant site
(dead storage).
1. Transportation by Rail: 4000 TO 13000 TONS PER TRIP.
2. Transportation by Ropeways
3. Transportation by Transportation by Sea or River
4. Road
5. Transportation of coal by Pipeline

TRANSPORTATION OF COAL BY PIPELINES

UNLOADING OF COAL AT SITE
 Unloading from ship :
 Unloading equipment.
 Self unloading ship.
 Unloading from Rail car.
 Unloading from truck (lifting trucks).

Rail car unloading
featuring bottom dump
by unloading bridge :

Rail car Unloading
by rotary dump :

COAL STORAGE
Storage of coal is undesirable,
because it costs more as there
is
 risk of spontaneous
combustion,
 weathering,
 possibility of loss and
deterioration during storage,
 interest on capital cost of coal
lying dormant,
 cost of protecting the stored
coal from deterioration
and so on. 11VANITA THAKKAR BIT, VARNAMA

TYPES OF COAL STORAGE
 Dead Storage :
 Supplies the coal where there is a shortage of
coal in plant due to failure of normal supply
of coal.
 This is long term storage and 10% of annual
consumption, so it requires protection
against weathering and spontaneous
combustion. 12VANITA THAKKAR BIT, VARNAMA

TYPES OF COAL STORAGE
 Live storage :
 Supplies coal to plant for day to day usage.
Capacity of live storage is less then that of dead
storage.
 Usually stored in vertical cylindrical bunkers or
coal bins or silo, from where coal is transferred to
boiler grate.
 Bunkers are normally diamond-shaped cross-
section storage area, made up of steel or
reinforced concrete. 13VANITA THAKKAR BIT, VARNAMA

PURPOSE OF DEAD STORAGE OF
COAL
 To prevent shutdown of power plant in case
of failure of normal supplies of coal due to
coal strike, failure of the transport system etc.
 Storage permits choice of the date of purchase
allowing management to take advantage of
seasonal market conditions.

MEANS OF COAL STORAGE
Storage in Coal Heaps :
It is required to :
 Keep coal at low temperature (>70ºC)
 Prevention of air circulation from bottom of coal piles
 Proper drainage of rainy water to prevent weathering – drainage
should not be rapid to prevent washing away of coal.
Hence, ground used for stocking should be dry and leveled for
proper drainage and concrete-floored to prevent flow of air
from bottom.
Coal is piled at a height of about 10 to 12 m and compacted in
layers of 15 to 30 cm.
In dead storage, coal pile is sealed by asphalt, fine coal dust,
bituminous coating or other coating material.

MEANS OF COAL STORAGE (contd.)
Underwater Storage :
Possibility of slow oxidation and
spontaneous combustion can be
completely eliminated by storing coal
under water.

COAL STORAGE AT SITE : DEAD STORAGE

SITE SELECTION FOR COAL DEAD
STORAGE
POINTS TO REMEMBER :
1. The storage area should be free from standing water.
2. If well drainage is not available artificial drainage should be
provided.
3. It should be free from all foreign materials like wood, paper
rags, waste oil or material having low ignition temperature.
4. Handling cost should be minimum.
5. Pile should build up in successive layer and compact.
6. Pile should dressed to prevent entry of rainy water.
7. Alternative drying and wetting should avoided.

SITE SELECTION FOR COAL DEAD
STORAGE (contd.)
8. Stoker size coal should be oil treated to prevent absorption of
water, O2, and compaction which is not needed.
9. Side of pile should not be steep.
10. Air may circulate freely through pile for proper ventilation to
keep temperatures low.
11. Hot surfaces or boiler blow down or hot water or steam pipes
and tanks should far from coal storage.
12. Hot bright days are to be avoided.
13. There should be provision for temperature measurement at
different points.
14. Conical piling should be avoided.
15. Fire fighting equipment should be easily available.

INPLANT COAL HANDLING
Coal Delivery
Road Rail & Ship
Unloading
Preparation
Transfer
Inplant Handling
Weighting
Furnace Fire
Car shakers, Rotary car dampers, unloading bridges, Self
unloading ship, lifts trucks cranes and buckets.
Crushers, Sizers, driers
Belt conveyers, Screw conveyers, Bucket elevator, skip
hoist, flight conveyers.
Bulldozers, scrapers, tramways, cranes and conveyers.
Bins, bunkers, indicators, gates, and valves.
Scales, coal meters and samplers.
Covered storage
Outdoor Storage
InPlantHandling
Transfer coal from coal storage at
plant site to point of utilization.

DESIGN OF INPLANT COAL HANDLING
SYSTEM
POINTS TO REMEMBER :
1. Simple and sound, requiring minimum operations and
transportation.
2. No double handling of coal in plant.
3. Handling unit should be centralized to facilitate inspection
and maintenance.
4. Electric motors can be used as driver of mechanism.
5. Working parts should be enclosed to avoid abrasion and
corrosion.
6. System should be able to supply required quantity of coal as
per demand.

Inplant handling of Coal

COAL PREPARATION PLANT

CRUSHING OF COAL
Coal is crushed in two stages. In the first stage the
300mm size is reduced to 100 to 75mm and in the
second stage from 75 to 100mm to below 6mm.
In figure double roller crusher, single roller
crusher and rotary breaker is shown.

VIBRATING SEPARATOR
Sizer or vibrating separator is fitted after crusher to screen the coal
crushed below set size. Over size coal is returned to crusher for
further proper crushing. Generally it separates out oversize coal of
size > 6mm.

TRANSFER OF COAL
Equipments used in coal transfer are:
 Belt Conveyer
 Flight Conveyer
 Screw Conveyer
 Bucket Elevator
 Grab Bucket Elevator
 Skip Hoists

BELT CONVEYORS
A very suitable means of
Transporting, it can transfer
large quantity of coal over large
distance economically – low
initial cost, low power
consumption. Main parts are:
Belt (made of rubber, canvas or
balata)
End Drums or pulleys
Idlers
Return Idlers

BELT CONVEYORS (contd.)
It uses inclination upto 20º
Avg. speed : 60 to 100 m/min
Capacity : 50 to 100 tones/hr
Max distance it can transfer : 400m
Advantage:
 Economical, Low power consumption.
 Large capacity
 Rate of coal transfer rapidly change
 Low maintenance cost.
Disadvantage:
 Not suitable for shorter distance and
inclination >20º
 Not suitable for dust particles and slurry. 29VANITA THAKKAR BIT, VARNAMA

FLIGHT CONVEYER (SCRAPER)
 It is used when coal is discharged at different points in
bins situated below the conveyer.
 All parts are made of steel and iron, so it can handle hot
material.
 It is totally enclosed, so dust of coal can get transferred.
 It can transfer coal at high inclination.

FLIGHT CONVEYER (SCRAPER) (contd.)
Advantages:
 It requires small head room
 Speed and so material transfer rate
can easily change.
 It can handle hot materials also.
Disadvantages:
 High wear and tear, so short life.
 High maintenance required.
 Speed is limited up to 300 m/min due to abrasive action of
material.
 High power consumption per unit of material transfer. 31VANITA THAKKAR BIT, VARNAMA

SCREW CONVEYOR
 It is used for shorter distance.
 Totally enclosed from atmosphere.
 Coal dust can also be transferred
easily.
 It is generally used in metering of
coal.
 Driving mechanism is attached at
end of the shaft.
 Diameter : 15 cm to 50 cm.
 Speed : 70 to 120 rpm.
 Capacity : 125 tones/hr (max.).

SCREW CONVEYOR (contd.)
Advantage:
 Cheap initial cost.
 Simple and compact
 Dust tight.
 It can transfer coal at high
inclination also.
 Most suitable for short
distance.
Disadvantage:
 High power consumption.
 Length is limited up to 30 m
 High maintenance due to
high wear and tear. 33VANITA THAKKAR BIT, VARNAMA

Centrifugal Type
Continuous Type
Height: 30.5 m max.
Inclination with Vertical: 30º
Chain speed :
Centrifugal: 75 m/min
Continuous: 35 m/min
Capacity: 60 tones/hr
BUCKET ELEVATOR
 It is used for vertical
lifts.
 Buckets are fixed on
chain which moves on
two wheels or
sprockets.
 Buckets are loaded at
bottom and
discharged at top.
 Continuous Type
Bucket capacity is
more than that of
Centrifugal type.

GRAB BUCKET ELEVATOR
 It is used for lifting as well as
transfer.
 It can be used with crane or
tower.
 Initial cost is high but
operation cost is less.
 It is used when another
arrangement is not possible.
 Bucket capacity : 2 to 3 m3
 Distance : 60 m
 Capacity : 100 tones/hr.

COAL BURNING SYSTEM
 FURNACE : Confined space where fuel is
burnt. Provides supports and enclosure for
burning equipment.
 For solid fuels, like coal, coke, wood, etc. :
STOKERS to convey the coal, GRATE to hold
the fuel.
 For pulverized coal and liquid fuels :
BURNERS.

SOLID FUEL FIRING
 HAND FIRING :
 Simple method.
 No capital investment.
 Used for smaller plants.
 Not a continuous firing process.
 Only very small furnaces can be fired by this
method.
 Poor response to load fluctuations.
 Controlling draught is difficult.

SOLID FUEL FIRING (contd.) : MECHANICAL
FIRING / STOKER FIRING (Medium and large size
plants.)
ADVANTAGES :
 High combustion
efficiency.
 No coal preparation.
 Low grade coal can be
used.
 Reliable firing system.
 Less labour requirement.
 Less smoke produced.
 Minimum danger of
explosion.
DISADVANTAGES :
 High initial cost.
 Complicated construction.
 Coal loss in moving through
grates.
 High maintenance/repair cost
due to high furnace
temperatures.
 Excessive wear of moving parts
by abrasive action of coal.
 Difficult : Sudden variations in
steam demand.

TYPES OF STOKERS : based on
way of feeding coal on grate
OVERFEED STOKER :
 Coal is fed from above, air from
bottom.
 Layers from above (after
flame):
 Fresh Coal Zone.
 Drying Zone.
 Distillation Zone – losing
volatile content.
 Combustion Zone –
incandescent coke.
 Ash Zone.
UNDERFEED STOKER :
 Coal and air are fed from
bottom.
 Layers from bottom :
 Green Coal.
 Ignition Zone (VM + CO
+ CO2 + N2 + H2).
 Incandescent coke (CO2 +
O2 + N2 + H2O).
 Ash .
 Flame. 39VANITA THAKKAR BIT, VARNAMA

SINGLE RETORT UNDERFEED
STOKER • Smaller size / capicity.
• Screw feeder on a
mechanical ram forces the
coal upward along the
length.
• Typical capacity : 1500 to
15000 kg/hr.
Troughs
for
feeding
coal Openings
for air 40VANITA THAKKAR BIT, VARNAMA

MULTIPLE RETORT
UNDERFEED STOKER
Larger underfeed stokers.
Multiple retorts – can be
12.
Inclination – 25o-30o
Typical Capacity : 10000
to 250000 kg/hr.
Some over-fire air
used for complete
combustion.
Tuyeres in
between retorts
for air.
Some over-fire air
used for complete
combustion.
Steam or air operated
Water sprays used
to cool ash.

ADVANTAGES OF MULTIPLE
RETORT UNDERFEED STOKERS
 High combustion efficiency.
 High part-load efficiency.
 Self-cleaning grate.
 Smokeless operation at even very high loads.
 High combustion rate.
 Grate bars, tuyeres and retorts not subjected high
temperatures due to continuous contact with fresh coal.
 Substantial amount of coal remains on grate  boiler
remains in service during temporary breakdown.
 Different varieties of coals can be used. 42VANITA THAKKAR BIT, VARNAMA

DISADVANTAGES OF MULTIPLE
RETORT UNDERFEED STOKERS
 High initial cost.
 Larger space requirements.
 Clinker formation not eliminated.
 High rate of wear and tear due to moving parts.
 Low grade fuels with high ash contents cannot be
burnt economically.

CHAIN GRATE OVERFEED
STOKER

ADVANTAGES OF CHAIN GRATE
OVERFEED STOKERS
 Simple in construction.
 Low initial cost.
 Self-cleaning stoker.
 Heat release rates can be controlled by
controlling the speed of chains. 45VANITA THAKKAR BIT, VARNAMA

DISADVANTAGES OF CHAIN
GRATE OVERFEED STOKERS
 Always some loss of coal by fine particles carried
with ashes.
 Temperature of preheated air limited to 180oC.
 Not suitable for high capacity boilers (200 tonnes/hr
or more) as small amount of coal is carried in the
grate.
 Clinker problem – common. 46VANITA THAKKAR BIT, VARNAMA

TRAVELLING GRATE
OVERFEED STOKER

SPREADER OVERFEED
STOKER

<30 mm
PULVERISED COAL HANDLING SYSTEM

UNIT SYSTEM

ADVANTAGES OF UNIT SYSTEM
 It is simple in layout and cheaper than central system.
 The coal transportation system is simple.
 It allows direct control of combustion from the pulverize.
 The maintenance charges are less as spares required are
less.
 In the replacement of stokers, the old conveyor system can
be used without much alteration.
 Coal which requires drying for satisfactory function of the
central system is generally supplied without drying in the
unit system.
 It affords better control of fuel feed into the boiler
furnace.

DISADVANTAGES OF UNIT SYSTEM
 The mill operates at variable load as per the load on the power
plant which results in poor performance of the pulverising mill
(more power consumption per ton of coal at part load).
 The total capacity of all mills must be higher than that for the
central system with the load factors common in practice.
 The degree of flexibility is less than of central system.
 The fault in the preparation unit may put the entire steam
generator out of use because no storage of pulverize coal.
 Strict maintenance is desired as the operation of the plant directly
depends upon the pulverizing mill.
 There is excessive wear and tear of the primary fan blades as it
handles air and coal particles.

CENTRALIZED SYSTEM

ADVANTAGES OF CENTRALIZED
SYSTEM
 Central system is flexible and changes can be made to
accommodate quick changes in demand.
 There is always a supply of fuel available in reserve in the boiler
bunkers.
 Since any mill can be used to supply to any boiler, the outage of
parts of the mills or even a short outage of entire pulverizing plant
will not cause a boiler plant outage.
 Pulverizer mills always runs at its rated load irrespective of the
load on the plant, therefore its power consumption per ton of coal
crushed per hour is less and capacity of mill required less
comparatively.

ADVANTAGES OF CENTRALIZED
SYSTEM (contd.)
 Pulverizer can be shutdown when sufficient reserve capacity
has been achieved. During peak load, this reserve capacity
can fulfill excess demand of coal.
 Good control over fineness of coal.
 There is greater degree of flexibility as the quantity of fuel
and air can be separately controlled.
 Burners can be operated independently of the operation of
coal preparation.
 Primary fan can handle only air so no problem of excessive
wear of blades.
 More flexibility of fitting of burners.

DISADVANTAGES OF
CENTRALIZED SYSTEM
 Higher initial cost and
 Occupies a large space due bulky storage bin
 High auxiliary power consumption. So power
consumption/ton of coal is more then unit system
 Possibility of fire hazard due to the storage of
pulverized coal.
 Complex coal transportation system.
 Driers are essential.

DISADVANTAGES OF
CENTRALIZED SYSTEM (contd.)
 Operation and maintenance cost are higher
then unit system.
Note: bin system is not adopted in large capacity
plants because of dangers from EXPLOSION of
coal air mixture and tendency of stored
pulverized fuel to cake. Also it required more
maintenance, losses of coal through vent, high
initial cost, high operating cost.
So universally Unit System is adopted for large
capacity power plants.

TYPICAL FLOW PROBLEMS IN
BINS

1.Ball mill
2.Ball and Race mill
3.Impact or hammer mill
4.Bowl mill

BALL MILL
Sharp
changes in the
direction of
the mixture
throws out
over-sized
particles
(returned to
drum)
10 tons of coal
per hour with
4% moisture
requires 28
tones of steel
balls. There is
20-25kWh
energy
consumption
per ton of coal.

ADVANTAGES OF BALL MILL
 Simple operation.
 Low initial cost.
 Grinding elements are not seriously
affected by metal scrap and other foreign
material in the coal.
 Suitable for wide range of fuels, including
anthracite and bituminous coal.

DISADVANTAGES OF BALL MILL
 Large and heavy in construction.
 Consumes more power than other mills.
 Poor air circulation, so works less
efficiently with wet coals.
 High operating cost.

BALL AND RACE MILL
Rotating.
Hold down upper race
to adjust force needed
for crushing
Very widely
used.
Principle of operation : Crushing
and attrition.

ADVANTAGES / DISADVANTAGES OF
BALL AND RACE MILL
ADVANTAGES :
 Low power consumption.
 Lower weight.
 Lower initial cost.
 Lower operating cost.
 Suitable for variable load
conditions.
 Can handle coals
containing as much as 20%
moisture.
DISADVANTAGES :
 Greater wear
compared to other
pulverizers.
 Leakage of fine
coal through the
mill.

BOWL MILL

BOWL MILL
ADVANTAGES :
 Lower energy consumption
(12-15kWh/t)
 Less overall dimensions.
 Classifier can be adjusted to
alter degree of coal fineness
while mill is on.
 Leakage of coal from mill
casing is practically nil as it
operates at negative
pressure.
 Less noisy.
DISADVANTAGES :
 Sensitive to metallic
objects that enter
with coal.
 Uneven wear of
grinding parts
presents repairing
complexities.

IMPACT OR HAMMER MILL
For brown coals, peat, oil
shales, etc.

HAMMER / IMPACT MILL
ADVANTAGES :
 Simple construction.
 Low capital cost.
 Requires minimum
floor area.
 Operational speed is
high.
 Wet coal can be
pulverized.
DISADVANTAGES :
 High power
consumption when
fine coal is require.
 Limited capacity.
 Metal scrap cannot
be allowed to enter
the mill.

PULVERISED COAL BURNERS
REQUIREMENTS:
 Proper mixing of coal and primary air.
 Bring mixture of coal and primary air in contact
with secondary air to create sufficient
turbulence.
 Controlling of flame shape and flame travel.
 Adequate protection against overheating and
excessive abrasive wear.
 Ensure complete combustion. 69VANITA THAKKAR BIT, VARNAMA

FACTORS AFFECTING PERFORMANCE :
 Characteristics of fuel used.
 Particle size of pulverized coal.
 Mixing of air and fuel.
 Proportions of primary and secondary air.
 Volatile matter content in coal.
 Furnace design.

TYPES :
 LONG FLAME / U-FLAME / STREAMLINED
BURNER
 SHORT FLAME / TURBULENT BURNER
 TANGENTIAL BURNER
 CYCLONE BURNER

LONG FLAME/U-FLAME BURNER
 Primary, secondary and tertiary air
required for combustion.
 Hot secondary air is introduced at right
angles to the flame for necessary mixing for
better and rapid combustion.
 Flame travels longer to allow it to get
sufficient time for complete combustion.
 Used for low volatile coal.

SHORT FLAME / TURBULENT
BURNER
 Fire horizontally or at some inclinations by
adjustments.
 High turbulence is created for pulverized coal
and hot secondary air going to the burner 
mixture burns as soon as it enters 
combustion completed in a short distance.
 Generally preferred for high volatile coals.
 All modern power plants generally use it.

TANGENTIAL BURNER
 High heat release rates.
 Simple operation.
 Almost complete
combustion can be
achieved.
 Liquid, gaseous or
pulverized fuels can be
readily burnt either
separately or in
combination.
ADVANTAGES :Four burners located in four
corners of the furnace.
Fires in such a way that four
flames are tangential to an
imaginary circle formed at
the centre.

TANGENTIAL BURNER
FRONT VIEW : TOP VIEW :

CYCLONE BURNER

CYCLONE BURNER
 Water-cooled horizontal cylinder.
 Located outside main boiler furnace.
 Crushed coal is fed and fired with high rates of
heat release.
 Coal combustion is complete before the
resulting hot gases enter the furnace.
 High temperature – about 2000oC is developed.
 The liquid slag formed is discharged off.

ADVANTAGES OF CYCLONE
BURNER
 Crushed coal is used. So, no pulverizing equipment.
 Removes most of the ash, about 60% as molten slag.
Hence, only 40% of the ash leaves with flue gases (in
pulverized coal firing, 80% of ash goes with flue
gases).
 Reduces erosion and fowling of steam generator
surface due to low ash content in flue gases.
 Best results in low grade fuels like Indian Coal.
 Can be operated with less excess air.
 High furnace temperature obtained. 78VANITA THAKKAR BIT, VARNAMA

DISADVANTAGES OF CYCLONE
BURNER
 High F. D. Fan pressure required  high
power consumption.
 Formation of relatively more NOx, which
pollutes air.
 Not suitable for high Sulphur content coal.

OIL BURNERS
 Functions :
 Mixing oil and air in proper proportions.
 Prepare fuel for combustion.
 Generally used in small capacity boilers
for industrial process heating.
 It has to prepare air-oil mixture requiring
minimum excess air for achieving
maximum temperatures. 80VANITA THAKKAR BIT, VARNAMA

PRINCIPLE OF OIL FIRING

TYPES OF OIL BURNERS
 VAPOURIZING BURNERS :
 Oil is vapourized / gasified by heating within the
burner.
 Used in blow torches, gasoline stoves, etc.
 Not used in boilers for steam generation.
 ATOMIZING BURNERS :
 Oil is atomized by nozzle(s) and vapourization
occurs in combustion space.
 Used in oil-fired furnaces and boiler furnaces. 82VANITA THAKKAR BIT, VARNAMA

STEAM OR HIGH PRESSURE AIR
ATOMIZING BURNERS
 VAPOURIZING BURNERS :
 Oil is vapourized / gasified by heating within the
burner.
 Used in blow torches, gasoline stoves, etc.
 Not used in boilers for steam generation.
 ATOMIZING BURNERS :
 Oil is atomized by nozzle(s) and vapourization
occurs in combustion space.
 Used in oil-fired furnaces and boiler furnaces. 83VANITA THAKKAR BIT, VARNAMA

TYPICAL OIL BURNER Preheated Oil (93oC-
120oC) delivered under
pressure through a tube.
Steam or air supplied by
annular area between oil pipe
and concentric outer tube.
Oil is broken down into small droplets
by using air or steam under pressure.

MECHANICAL BURNER
 Oil is pressurized by :
 Positive displacement pump.
 Centrifugal dispersal of oil from a rotating wheel.
 Spray nozzle is used for oil injection under high
pressure of about 25 to 35 bar.
 The oil leaves the nozzle in atomized state.
 Rotating Wheel Burner : Oil leaves orifice as hollow
cone of fine fuel particles due to centrifugal action.
 Widely used due to less auxiliary consumption and
less excess air required. 85VANITA THAKKAR BIT, VARNAMA

GUN-TYPE (MECHANICAL)
OIL BURNER

ASH HANDLING

WHY ASH HANDLING ??!!

REQUIRMENTS OF AHP

Mechanical AHS : Hot ash
coming from boiler falls on a
belt conveyor through a water
seal – gets cooled on quenching
– and is carried continuously by
conveyor to ash bunker or
dumping site.

MECHANICAL AHS :
ADVANTAGES :
 Low power
consumption.
 Continuous ash
removal.
DISADVANTAGES :
 Suitable for coal-
fired, low capacity
plants.
 Maximum capacity :
5t/h.
 Short life span : 5-
10 years only.

Low Velocity Hydraulic AHS :
Ash from furnace grate falls
into a low velocity water
channel (3 to 5 m/s) and goes to
ash sumps. There, ash and
water are separated. Water is
reused.
Capacity : 50t/h
Distance : 500m

High Velocity Hydraulic AHS :
Hoppers below boiler are fitted
with nozzles on top (to quench
ash) & sides (to drive ash).
Cooled ash goes to water
channel below, settles & is
removed. Water reused.
Capacity : 120t/h
Distance : 1km
PCBs use it.

ADVANTAGES OF HYDRAULIC
AHS :
 High capacity, so more suitable for modern
large capacity thermal power plants.
 Clean and dust-free operation.
 Ash can be disposed upto a distance of 1km
from the TPP.
 Can also handle molten ash.
 No moving parts in contact with ash. Hence
erosion problem is minimum.

ADVANTAGES OF HYDRAULIC
AHS :
 Abrasive ash forms corrosive acids
in contact with water.
 Water channel and pump must be
made of special wear resisting
material. 95VANITA THAKKAR BIT, VARNAMA

PNEUMATIC AHS :
 High velocity air steam is directed to collect ash and dust
from all discharge points.
 All collected ash – crushed on the way by crushers, if
clinkers are present – drops into air stream.
 Cyclone separators separate ash from air.
 Ash is collected in hoppers for further handling for
disposal by trucks.
 Clean air is discharged from top of secondary air
separator into atmosphere through exhauster.
 Typical Ash handling capacity : 5 to30 t/h 96VANITA THAKKAR BIT, VARNAMA

PNEUMATIC AHS :

ADVANTAGES OF PNEUMATIC
AHS :
 Dust-free operation.
 No re-handling problem.
 Dry ash handling eliminates chances of ash
freezing, picking or sticking to storage bins.
 Flexibility in operation to suit various plant
conditions.
 Installation cost per ton of ash handled is less
than that in other systems.

DISADVANTAGES OF PNEUMATIC
AHS :
 Noisy operation.
 Wearing of crusher and pipe
lines by abrasive ash inflicts high
maintenance cost.

STEAM JET AHS :
 High steam jet is passed through a pipe to
carry off dust and ash particles, instead of air
stream.
 Ash is separated in cyclone separator and
deposited into ash hopper for further
disposal by trucks. 101VANITA THAKKAR BIT, VARNAMA

STEAM JET AHS :
ADVANTAGES :
 No auxiliary drive
required.
 Less capital cost.
 Less space required.
 Installed at inaccessible /
awkward positions also.
 Economical for ash disposal
– 200m horizontal distance,
30m vertical distance.
DISADVANTAGES :
 Noisy operation.
 Erosion in
pipelines due to
abrasive ash.
 Capacity limited
to 5t/h.

DUST COLLECTORS – WHY ?
 Flue gas exhaust contains fly ash.
 Flue gas exhaust in pulverized coal fired
boilers contains 80% of ash content of
coal.
 Fly ash is very fine – most of the
particles can pass through a 300 mesh
(40m) screen – particle size varies from
1m to 80m – basic difficulty in handling.104VANITA THAKKAR BIT, VARNAMA

DUST COLLECTORS – WHY ?
(contd.)
 Fly ash pollutes atmosphere – harms human,
animal and plant health.
 So, concentration of fly ash in flue gas must be
brought down to below 0.5 g/m3, before
discharging into atmosphere.
 High ash content in Indian coals – about 25% -
50%.
 A 200 MW plant using pulverized coal produces
about 250T of fly ash per day. 105VANITA THAKKAR BIT, VARNAMA

BASIC REQUIREMENTS OF GOOD
DUST COLLECTOR
 Ability to remove very fine fly ash particles.
 Low capital cost.
 Low operating and maintenance costs.
 Good wear resistance against erosion caused
by abrasive fly ash.
 Minimum floor space requirement.
 High efficiency at all load conditions.

TYPES OF DUST COLLECTORS
1. MECHANICAL DUST COLLECTORS :
1. DRY TYPE
1. GRAVITATIONAL
2. CYCLONE
2. WET TYPE
1. SPRAY TYPE SCRUBBERS
2. PACKED BED SCRUBBERS
3. IMPINGEMENT TYPE SEPARATORS
2. ELECTROSTATIC PRECIPITATORS 107VANITA THAKKAR BIT, VARNAMA

GRAVITATIONAL SEPARATORS :
Work on principle of gravity.
Advantages :
•Simple.
•Low capital and maintenance
costs.
Disadvantages :
•Cannot remove fine particles.
•Bulky.
•Large space requirement.

CYCLONE DUST COLLECTOR
Commonly used
for stoker as well
as pulverized fuel
firing.
Efficiency range :
60% - 90%.

CYCLONE DUST COLLECTORS
(CONTD.)
Advantages :
1. High efficiency for large size
particles.
2. Low maintenance cost.
3. Simple, cheap and easy operation.
4. Increase in efficiency with
increasing load.
Disadvantages :
1. Efficiency decreases with fineness
of particles.
2. High power requirement for
higher vortex velocity of flue gases.
3. Large space requirement.
4. High pressure loss. 110VANITA THAKKAR BIT, VARNAMA

WET TYPE MECHANICAL DUST
COLLECTOR (WET SCRUBBER)
Principle : Fly ash and sulphur
present in outgoing flue gases is
removed by water spraying – as
shown in the fig.
To minimize corrosion
problems, tank is lined with
lead, pipes are lined with rubber
and spray nozzles are made of
vitrified material.
Advantage : High efficiency
(about 90%).
Disadvantage : Large water
consumption & Corrosion.
Tangential
entry

COLLECTOR (WET SCRUBBER) (contd.)

ELECTROSTATIC PRECIPITATORS
WORKING PRINCIPLE :

WORKING PRINCIPLE (contd.):

SCHEMATIC DIAGRAM OF ESP :

THANKS !!THANKS !!
-- VANITA THAKKARVANITA THAKKAR
ASSOCIATEASSOCIATE PROFESSORPROFESSOR
MECHANICALMECHANICAL ENGINEERING DEPARTMENT,ENGINEERING DEPARTMENT,
BABARIABABARIA INSTITUTE OF TECHNOLOGY, VARNAMA,INSTITUTE OF TECHNOLOGY, VARNAMA,
VADODARA.VADODARA.
vanitaa.thakkar@gmail.comvanitaa.thakkar@gmail.com

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