Combustion Equipment and Methods

Unit-4: Combustion Equipments
Prepared by:
Ankur Sachdeva
Assistant Professor, ME

Analysis of Combustion
• To have proper control on combustion process,
an idea about complete combustion of fuel is
made by the analysis of flue gas
• If complete combustion of fuel takes place then
Carbon Dioxide is released.
• If incomplete combustion of fuel takes place then
Carbon Monoxide is released.
Ankur Sachdeva, Assistant Professor, ME

Composition of Flue Gases
• If the flue gases contain considerable amount
of carbon monoxide, it indicates
– incomplete combustion is occurring (i.e.
considerable wastage of fuel is taking)
– short supply of oxygen for combustion
• If the flue gases contain a considerable
amount of oxygen, it indicates
– the oxygen supply is in excess, though the
combustion may be complete.

Orsat Apparatus:
Measurement of Flue Gases

Construction of Orsat Apparatus
• Consists of a water-jacketed measuring burette, connected
in series to a set of three absorption bulbs, each through a
stop-cock.
• The other end is provided with a three-way stop-cock, the
free end of which is further connected to a U-tube packed
with glass wool (for avoiding the incoming of any smoke
particles, etc.)
• The graduated burette is surrounded by a water-jacket to
keep the temperature of the gas constant during the
experiment.
• The lower end of the burette is connected to a water
reservoir by means of a long rubber tubing.
• The absorption bulbs are usually filled with glass tubes, so
that the surface area of contact between the gas and the
solution is increased.

Construction of Orsat Apparatus
• The absorption bulbs have solutions for the absorption of CO2, O2
and CO respectively.
• First bulb has ‘potassium hydroxide’ solution (250g KOH in 500mL of
boiled distilled water), and it absorbs only CO2.
• Second bulb has a solution of ‘alkaline pyrogallic acid’ (25g
pyrogallic acid+200g KOH in 500 mL of distilled water) and it can
absorb CO2 and O2.
• Third bulb contains ‘ammonical cuprous chloride’ (100g cuprous
chloride + 125 mL liquor ammonia+375 mL of water) and it can
absorb CO2, O2 and CO.
• Hence, it is necessary that the flue gas is passed first through
potassium hydroxide bulb, where CO2 is absorbed, then through
alkaline pyrogallic acid bulb, when only O2 will be absorbed
(because CO2 has already been removed) and finally through
ammonical cuprous chloride bulb, where only CO will be absorbed.

Working Principle of
Orsat Apparatus
• Flue gas is passed through fused Calcium Chloride
which absorbs the water vapor present due to its
hygroscopic properties.
• Three way stopcock is opened and the flue gas is
filled in the graduated burette.
• The whole set up is water jacketed to maintain a
constant temperature.
• The stopcock of the KOH reservoir is opened and
the water reservoir is moved up.
• Water inflows in the graduated burette and
pushes the gas to flow in the KOH reservoir.

Working Principle of
Orsat Apparatus
• In that reservoir carbon dioxide is absorbed. The whole CO2
in the sample is absorbed.
• The water reservoir is brought down so that the air again
can rush to the burette.
• Again the stopcock of the alkaline pyrogallic acid reservoir
is opened and the water reservoir is moved up.
• Water inflows in the graduated burette and pushes the gas
to flow in the alkaline pyrogallic acid reservoir where the
absorption of Oxygen takes place.
• The same process is repeated with the ammonical cuprous
chloride reservoir.
• The Volume increase of all the reservoir is measured and
the amount of Carbon Dioxide, Carbon Monoxide and
Oxygen are determined.

Combustion Equipment
• Combustion equipment are those appliances
that are used for burning fuels for heating.
• These includes heaters, ovens, stoves,
furnaces, fireplaces, dryers, burners, stokers,
and many more.
• Combustion equipments can be used for
solids, liquids, and gaseous fuels.
• These allow the proper combustion of fuels

Requirements of Combustion
Equipment
• Fresh charge of fuel should be freely ignited as it
enters the burning zone.
• Steady combustion is the basis for obtaining the
desired amount of heat release.
• Adequate combustion space should be provided for
driving the process.
• Sufficient temperature of the combustion gases should
be maintained.
• Quantity of air supply is important in achieving proper
combustion.
• The method of air supply is another vital factor of
efficient combustion.

Solid Fuel Firing
• Fuel bed combustion (Coarse Particles)
– Hand Firing
– Mechanical Stokers
• Pulverized Fuel Firing (Fine Particles)
– Unit System
– Central or Bin System
• Fluidized bed combstion (Crushed Small particles)
– Atmospheric Fluidized Bed Combustion
– Pressurized Fluidized Bed Combustion

Solid Fuel Combustion
• Coal may be burnt in a
grate by hand firing or by
using mechanical stokers
• Hand Firing :-
• The grates are usually
made of iron bars with 6 to
10 mm gaps between
them.
– Hand firing can be done
either by spreading or
coking method

Types of Hand Firing
• In Spreading method,
– a small quantity of coal is supplied at a time by spreading it
over a part of the fuel bed.
– In this method, care is to be taken to maintain uniform bed
thickness.
• In Coking method,
– Considerable amount of coal is fed onto a plate. The heap
of fresh coal is slowly carbonized by heat of the glowing
bed.
– The volatile products pass over the bed and get burnt in
the air rising through the grate. Then formation of coke
takes place.
– The coke is then distributed over the bed.

Disadvantages of Hand Firing
• Though hand firing is simple and cheaper it is
not generally used, because of the following
reasons:
– It has low combustion efficiency.
– Slow response to the load fluctuations.
– Combustion control is difficult.
– Suitable only for small power plants.

Mechanical Stokers
• It functions on the principle of continuous coal
feeding.
• The evolution of volatile matter is thus uniform
and it becomes easier to control the air required
for combustion.
• The mechanical means used are, depending on
design, combinations of the screw feed, the
conveyor belt, the bucket chain, the paddle and
the ram.
• There are 3 types, the over-feed, the under-feed
and the cross - feed.

Principle 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.

• A fully built up overfeed stoker will have the beds of green coal (raw coal),
incandescent coke and ash over the grate.
• In this the primary air enters the grate from the bottom, which cools the grate
while moving up and gets heated as it passes through hot ash bed.
• The hot air then passes through the bed of incandescent coke, where oxygen reacts
with the carbon in the coke to form carbon dioxide, carbon monoxide and
hydrogen.
• Part of carbon dioxide formed reacts with carbon in the fuel to form carbon
monoxide.
• The gases leaving the bed of incandescent coke consist of nitrogen, carbon dioxide,
carbon monoxide, hydrogen and water.

• To these gases, then an additional air termed the secondary air is
supplied from the sides to burn the combustible gases like the carbon
monoxide, hydrogen and other volatile matters.
• The burnt hot gases entering the boiler consist of carbon dioxide,
nitrogen, oxygen, and water.
• It may also contain carbon monoxide, if the combustion is incomplete.
• The primary and secondary air to the stoker is supplied under pressure
with the help of blowers.

Travelling Grate Stoker
• A chain grate stoker consists
of an endless chain which
forms the support for the
fuel bed.
• The chain is made of cast
iron links connected by pins.
• The chain is held over two
sprockets as shown figure,
and travels from one end of
the furnace to the other
end.
• The sprocket at the front
end is driven by an electric
motor.
• The coal is fed at the front
end through a hopper
which is carried by the
chain to the other end,
hence into the furnace.

Travelling Grate Stoker
• The air necessary for the
combustion of the fuel is
supplied through the air inlets
below the traveling grate.
• The secondary air is supplied
through the openings in the
top roof.
• The rate of fuel supplied to the
grate and hence the heat to
the boiler can be controlled by
two means.
• The first means is to control
the depth of the coal bed on
the grate by controlling the
feed to the hopper.
• In the second method, the
speed of the chain grate can
be adjusted to meet the boiler
operation requirements.

Spreader Stoker
• In this type stoker, coal
from the hopper is fed on
to a rotating feeder which
in turn feeds the coal to a
spreader or sprinkler, and
feed according to the
requirements.
• Feeder is a rotating drum
fitted with blades on its
periphery.

Spreader Stoker
• The feeder continuously supplies the coal on to the spreader,
a fast moving drum with blades, which in turn distributes and
feeds the coal on to the grate.
• The fuel feed rate and the supplied to the boiler can be
controlled by controlling the feed to the hopper are by
controlling the spreader speed.

Principle of Underfeed Stokers
• In this the coal is charged from the bottom, and the primary air under
pressure also moves from the bottom through the holes in the grate.
• This stoker has the layers of ash, incandescent coke bed and raw coal,
in the reverse direction as compared to that of the overfeed stoker.
• In operation the primary air entering from the bottom through the
grate holes comes in contact with the coal and then passes through
the bed of incandescent coke.

Principle of Underfeed Stokers
• In operation the primary air entering from the bottom through the
grate holes comes in contact with the green coal and then passes
through the bed of incandescent coke.
• Initially, air reacts with carbon in the coke to form carbon dioxide, and
the moisture in the air reacts to release carbon dioxide, carbon
monoxide and hydrogen.
• While these gases pass over the ash bed, secondary air is supplied for
their complete combustion.
• This method is most suitable for semi-bituminous and bituminous coals
which have high volatile-matter

Advantages of Mechanical Stokers
• Even though it is costlier, generally they are used
to feed the solid fuels in small and medium size
power plants, because of the following reasons:
– Combustion is more efficient.
– Fuel handling is automatic and combustion control is
easier.
– Faster response to load fluctuations.
– Low quality fuels can be successfully burnt.
– Suitable for small to high capacity plants

Coal handling Plant

Combustion Equipment for Coal
• Coal burning methods are classified into two types:
– Stoker firing – used for solid coal
– Pulverized fuel firing – used for pulverized coal
• Selection of one of the above methods depends upon
1. Characteristics of the coal available
2. Capacity of the plant
3. Load fluctuations
4. Efficiency / Reliability of combustion equipments.
• The boiler furnaces that burn coal can be classified as follows:
– Fuel bed furnaces (coarse particles)
– Pulverized coal furnaces (fine particles)
– Cyclone furnaces (crushed particles)
– Fluidized bed furnaces (crushed small particles)

Pulverized Fuel Firing
• In pulverized fuel firing system, the coal is powdered
and then charged into the combustion chamber with
the help of hot air current.
• The main purpose of pulverizing coal is to increase the
surface area of exposure to the combustion process,
which results in faster and efficient combustion.
• In burning the pulverized coal, the secondary air
required for the complete combustion of fuel is
supplied separately to the combustion chamber.
• The resulting turbulence in the combustion chamber
helps for uniform mixing of fuel and air.

• The air required to carry the pulverized coal and dry it
before entering the combustion chamber is termed
the Primary Air, and the air supplied separately for
complete combustion is termed the Secondary Air.
• Pulverized coal firing systems are universally adopted
for large scale power plants.
• The choice of pulverized fuel firing system depends
upon the size of the boiler unit, type of coal available,
cost of coal, type of load (i.e., fluctuating or constant),
the load factor and availability of trained personnel.
• Generally such systems are not economical for small
capacity thermal power plants

Advantages of
1. A wide variety of low grade fuels (coal) can be used and burnt easily.
2. Greater surface area is exposed for combustion and hence
combustion is faster and efficient.
3. The system is free from clinker and slagging troubles.
4. Combustion control is easy, and hence the system gives fast
response to load changes.
5. Preheated secondary air (up to 350°C) can be used, resulting in rapid
flame propagation and faster heat supply to the boiler.
6. The pulverizing system can be maintained or repaired without
affecting the combustion process.
7. It has a very high rate of heat release.
8. Banking losses (unburnt fuel with ash) are lower, as compared to
stoker firing.
9. The boilers can be started from cold very rapidly.
10. Usually combustion will be smokeless

Disadvantages of
1. The capital investment of the system is high as it requires
additional equipments (for pulverizing, and handling).
2. Its operation and maintenance costs are very high.
3. It produces fly-ash/fine dust and needs costly fly-ash
removal equipments like electrostatic precipitators.
4. The chances of explosion are high as coal burns like a gas.
5. The storage of powdered coal requires special attention as
it has possibilities of fire hazards.
6. Skilled workers are required for safe-operation and
maintenance.
7. Air pollution takes place by the emission of fine particles
of grit and dirt.
8. The removal of liquid slag formed from low fusion
temperature ash requires special handling equipments.

Central System
• The crushed raw coal is dried using hot air or flue gases and
fed to the pulverize.
• The pulverized coal from the pulverizing mill is passed to
the cyclone separator where over-sized particles are
separated and fed back to the mill.
• The pulverized coal is then transferred from the separator
to the central bunker (bin) through a conveyer system.
• The pressurized air from the forced draft fan, supplies the
stored coal to the burner.
• This air not only carries the fuel, but also acts as the
primary air for the combustion of the fuel.
• Secondary air is supplied to the burner separately to assist
in the complete combustion.

Merits and Demerits of
Central System
Advantages
1. Central system is highly
flexible and hence can meet
any quick changes in the
demand
2. Burner operation is
independent of coal
pulverization.
3. The pulverizing mill can be
stopped when there is a good
stock of pulverized fuel in the
bin.
4. The fan wear is less as it
handles only natural air.
5. Coal size can be controlled
efficiently.
Disadvantages
1. Central system is expensive, and
occupies more space.
2. It requires complicated coal
handling systems.
3. Power consumption in
auxiliaries is high.
4. Chances of fire hazards are more
since the pulverized fuel is stored.
5. Operation and maintenance
costs are high.

Unit System
• In this system, each burner and a pulveriser constitute a unit.
• It consists of a raw coal bunker, a feeder, pulverizing mill,
separator, and the burner.
• In operation, the raw coal is supplied to the bunker, where it
is crushed to the required sizes, the crushed coal is then fed
to the pulverizing mill through the feeder at the required
rate, depending upon the combustion requirements.
• Hot gases are passed through the feeder to dry the coal.
• The dried coal is pulverised in the mill and it is carried to the
burner.
• An induced draft fan is used at the pulverizer to carry the
powdered coal to the burner.
• A separator is provided to separate the grains of bigger size
from the powder and returned to the pulverize for further
crushing

Merits and Demerits of
Unit System
Advantages
1. It is simple in operation
and economical than the
central system.
2. Combustion is controlled
directly after pulverize.
3. Maintenance cost is low.
4. Fuel supply to the burner
can be controlled easily
Disadvantages
1. The performance of the
pulverizing mill is poor as
the system operates at
variable loads.
2. The total capacity of mills
must be higher than the
central system.
3. The unit system of fuel
burning is less flexible.
4. Whenever any of the
auxiliaries fails the burner
has to be put-off

Introduction to FBC
• The major portion of the coal available in India is of low quality,
high ash content and low calorific value.
• The traditional grate fuel firing systems have got limitations and are
techno-economically unviable to meet the challenges of future.
• Fluidized bed combustion has emerged as a viable alternative and
has significant advantages over conventional firing system and
offers multiple benefits – compact boiler design, fuel flexibility,
higher combustion efficiency and reduced emission of noxious
pollutants such as SOx and NOx.
• The fuels burnt in these boilers include coal, washery rejects, rice
husk, bagasse and other agricultural wastes. The fluidized bed
boilers have a wide capacity range- 0.5 T/hr to over 100 T/hr.

Mechanism of FBC
• When an evenly distributed air or gas is passed upward through a
finely divided bed of solid particles such as sand supported on a
fine mesh, the particles are undisturbed at low velocity.
• As air velocity is gradually increased, a stage is reached when the
individual particles are suspended in the air stream – the bed is
called “fluidised”.
• With further increase in air velocity, there is bubble formation,
vigorous turbulence, rapid mixing and formation of dense defined
bed surface.
• The bed of solid particles exhibits the properties of a boiling liquid
and assumes the appearance of a fluid – “bubbling fluidized bed”.
At higher velocities, bubbles disappear, and particles are blown out
of the bed.
• Therefore, some amounts of particles have to be recirculated to
maintain a stable system - "circulating fluidised bed"

Types of FBC

Types of FBCs
• There are three basic types of fluidised bed
combustion boilers:
1. Atmospheric classic Fluidised Bed Combustion
System (AFBC)
2. Atmospheric circulating (fast) Fluidised Bed
Combustion system(CFBC)
3. Pressurized Fluidised Bed Combustion System
(PFBC).

AFBC/Bubbling Bed
• In AFBC, coal is crushed to a size of 1 – 10 mm depending on
the rank of coal, type of fuel feed and fed into the combustion
chamber.
• The atmospheric air, which acts as both the fluidization air and
combustion air, is delivered at a pressure and flows through
the bed after being preheated by the exhaust flue gases.
• The velocity of fluidizing air is in the range of 1.2 to 3.7 m /sec.
• The rate at which air is blown through the bed determines the
amount of fuel that can be reacted.
• Almost all AFBC/ bubbling bed boilers use in-bed evaporator
tubes in the bed of limestone, sand and fuel for extracting the
heat from the bed to maintain the bed temperature.
• The bed depth is usually 0.9 m to 1.5 m deep and the pressure
drop averages about 1 inch of water per inch of bed depth.

Schematic Diagram of AFBC

AFBC/Bubbling Bed
• The combustion gases pass over the super heater sections
of the boiler, flow past the economizer, the dust collectors
and the air preheaters before being exhausted to
atmosphere.
• The main special feature of atmospheric fluidised bed
combustion is the constraint imposed by the relatively
narrow temperature range within which the bed must be
operated.
• With coal, there is risk of clinker formation in the bed if the
temperature exceeds 950°C and loss of combustion
efficiency if the temperature falls below 800°C.
• For efficient sulphur retention, the temperature should be
in the range of 800°C to 850°C.

Circulating Fluidized Bed
Combustion (CFBC)
• CFBC technology utilizes the fluidized bed principle in which
crushed (6 –12 mm size) fuel and limestone are injected into the
furnace or combustor.
• The particles are suspended in a stream of upwardly flowing air (60-
70% of the total air), which enters the bottom of the furnace
through air distribution nozzles.
• The fluidizing velocity in circulating beds ranges from 3.7 to 9
m/sec.
• The balance of combustion air is admitted above the bottom of the
furnace as secondary air.
• The combustion takes place at 840-900 °C, and the fine particles
(<450 microns) are elutriated out of the furnace with flue gas
velocity of 4–6 m/s.
• The particles are then collected by the solids separators and
circulated back into the furnace

Schematic Diagram of CFBC

Combustion (CFBC)
• There are no steam generation tubes immersed in the bed.
• The circulating bed is designed to move a lot more solids
out of the furnace area and to achieve most of the heat
transfer outside the combustion zone – convection section,
water walls, and at the exit of the riser.
• Some circulating bed units even have external heat
exchangers.
• For large units,
– Taller furnace characteristics of CFBC boiler offers better space
utilisation
– Greater fuel particle and sorbent residence time for efficient
combustion and SO2 capture
– Easier application of staged combustion techniques for NOx
control than AFBC generators.

Combustion (CFBC)

Combustion (CFBC)
• CFBC boilers are said to achieve better calcium to
sulphur utilisation – for the AFBC boilers,
although the furnace temperatures are almost
the same.
• CFBC boilers are generally claimed to be more
economical than AFBC boilers for industrial
application requiring more than 75 - 100 T/hr of
steam.
• CFBC requires huge mechanical cyclones to
capture and recycle the large amount of bed
material, which requires a tall boiler.

Comparison Between
AFBC and CFBC
AFBC CFBC
Flue gas velocity-1.5 to 2.0 m/sec Flue gas velocity-3.7 to 4.3 m/sec
Under bed feeding Over bed feeding
No U beam U beam Technology
Bed coil No bed coil
UBC-Less than 3 to 4% UBC-Less than 2%
Efficiency low (84%) Efficiency high (88%)
Clinker formation chances low Clinker formation chances low
Tube erosion low as compared to CFBC Tube erosion high as compared to AFBC
No Ash recycle (unburned heavy particle)
system
Ash recycle (unburned heavy particle)
system
Fuel consumption is more as compared to
CFBC
Fuel consumption is less as compared to
AFBC

Pressurized Fluidized Bed Combustion
System (PFBC)
• Pressurized Fluidized Bed Combustion (PFBC) is a variation of fluid
bed technology that is meant for large-scale coal burning
applications.
• In PFBC, the bed vessel is operated at pressure upto 16 ata (16
kg/cm2).
• The off-gas from the fluidized bed combustor drives the gas turbine.
• The steam turbine is driven by steam raised in tubes immersed in
the fluidized bed.
• The condensate from the steam turbine is pre-heated using waste
heat from gas turbine exhaust and is then taken as feed water for
steam generation.
• The PFBC system can be used for cogeneration or combined cycle
power generation.
• By combining the gas and steam turbines in this way, electricity is
generated more efficiently than in conventional system.
• The overall conversion efficiency is higher by 5% to 8%.

Pressurized Fluidized Bed Combustion
System (PFBC)

Advantages of Fluidized Bed
Combustion
• High Efficiency
• Reduction in Boiler Size
• Fuel Flexibility
• Ability to Burn Low Grade Fuel
• Ability to Burn Fines
• Pollution Control
• Low Corrosion and Erosion
• Easier Ash Removal – No Clinker Formation
• Less Excess Air – Higher CO2 in Flue Gas
• High Efficiency of Power Generation
• Simple Operation, Quick Start-Up

Introduction
• For liquid fuels to burn satisfactorily, they must first be brought into
the vapour state and burners may be classified by the method
which is employed to do this.
• Different method of vaporization may be adopted, but all burners
operating on direct vaporization are referred to as vaporizing
burners.
• Light fuels such as kerosene and gas oil can be vaporized by the
direct application of heat.
• If this technique is applied to the heavier oils, the temperature
required to evaporate the fuel also produces severe cracking and
solid carbon particles are produced.
• The method adopted to overcome this difficulty is to vaporize in a
two-stage process, first by atomizing the oil, or more correctly, by
breaking down the oil into a fine spray or mist of oil particles and
then by vaporizing these fine oil particles during the actual
combustion process.

Burner and its features
• A burner is a mechanical device that.
– supplies required amount of fuel and air
– creates condition for rapid mixing of fuel and air
– produces a flame which transfers thermal energy to furnace and
charge
Features of Good Burner:
1. Stable and proper operation in the range of design
parameters
2. Low pollution
3. Low level of noise
4. Longer life time
5. Security of operation

Oil Burners
Principle of oil firing:
• The functions of an oil burner are to mix the
fuel and air in proper proportion and to
prepare the fuel for combustion.
• It is essential that the oil / air mixture is well
homogenized with as few pure droplets of fuel
oil as possible for the efficiency of the
combustion process.

Classification of Oil Burners
1. Vapourizing oil burners :
a) Wick Type Vapourizing Burners
b) Pot Type Burner
2. Atomising fuel burners :
a) Mechanical or oil pressure atomising burner
b) Rotating cup burner
c) Steam or high pressure air atomising burner

Vapourizing Burner

Pot Type Vapourizing Burner

Atomizing Type Oil Burners

Gas Burners
• Gas burners are usually classified based on their operating
gas pressure.
• They are operated both atmospheric and high-pressure
conditions.
• The gases are supplied in different ways depending on the
pressure. In low pressure burner, the gas pressure varies
from 1 to 4 kPa.
• Whereas, high pressure burners the pressure varies from 7
to 70 kPa
• Total or partial premix type, in which a part or whole of the
combustion air is mixed with the gas before it emerges out
of the nozzle.
• Nozzle-mix type, in which, the air is supplied to the burner
tip after the gas leaves the nozzle.

Gas Burners
Total or Partial Premix Type
Nozzle mix Type

Combustion Equipment and Methods

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