combustion of fuel

COMBUSTION OF FUEL
The burning of fuel in presence of air is
known as combustion. It is a chemical
reaction taking place between fuel and
oxygen at temperature above ignition
temperature.
Heat is released during combustion
process.
12:57:42
By: Prof K. M.Joshi,
Assi. Professor, MED,
SSAS Institute of Technology, Surat.
Introduction
The substances taking part in combustion are
called reactants and the substances produced,
during combustion are called products of
combustion.
 fuel + air = products of combustion + heat
 If the heat is released during the chemical process, it
is called an exothermic reaction. When heal is
absorbed from the surroundings during the chemical
reaction, it is called an endothermic reaction.

By: Prof. K. M. Joshi
12:57:43
Combustion Equations


When combustion of fuel takes place, the constituents of
fuel react with oxygen. The molecular mass of various
constituents of fuel are given below :
Constituent






C
02
H2
S
N2
Molecular mass
12
32
2
32
28
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The relation between kg mass and kg mole of a substance is
given by
mass
Number of moles =
-----------------molecular mass
Carbon :
C + O2 = CO2
1 mole of C + 1 mole of 02 = 1 mole of C02
12 kg of C + 32 kg of 02 = 44 kg of C02
1 kg C + 8/3 kg 02 = 11/3 kg C02
Hydrogen :
2 H2 + 02 = 2 H20
2 moles of H2 + 1 mole of 02 = 2 moles of H20
4 kg H2 + 32 kg 02 = 36 kg H20
......... (1)
......... (2)
By: Prof. K. M. Joshi
12:57:43
Sulphur :
S + O2 = So2
1 mole of S + 1 mole of 02 = 1 mole of S02
32 kg S + 32 kg 02 = 64 kg S02
1 kg S + 1 kg 02 = 2 kg S02
Methane :
CH4 + 202 = C02 + 2H20
1 mole CH4 + 2 moles 02 = 1 mole C02 + 2 moles H20
16 kg CH4 + 64 kg 02 = 44 kg C02 + 36 kg H20
1kg CH4 + 4 kg 02 = 11/4 kg C02 + 9/4 kg H20
......... (3)
...... (4)
By: Prof. K. M. Joshi
12:57:43



The composition of air is approximately as under :
02
N2
Total air
by volume
21%
79%
100%
1 mole 3.76 mole
4.76 mole
by mass
23%
1 kg
77%
3.35 kg
100%
4.35 kg
12:57:43
By: Prof. K. M. Joshi

These equations are helpful for calculating the amount of
oxygen required to burn the fuel. Instead of supplying only
oxygen, the fuel is generally burnt in presence of air which
contains mainly O2 and N2 with small amount of CO2. argon
etc.
N2 and other gases in air' do not take part in
chemical reaction.
Mass fraction
The ratio of mass of a constituent of a mixture or a compound to the total
mass of a mixture or compound is called mass fraction.
e.g. consider a gas CH4,
Molecular mass of CH4 = 12 + 4 x 1 = 16 kg,
in which C is 12 kg and H2 is 4 kg.
By: Prof. K. M. Joshi
12:57:43
Mole fraction
The ratio of volume of gas in a mixture to the total volume of mixture
is called the mole fraction. It is also equal to the ratio of no. of moles
of gas in a mixture to the total no. of moles of mixture.
e.g. for octane gas C8H18, no. of moles of C is 8 and that of H2 is 9.
Total no. of moles = 8 + 9 = 17
12:57:43
Minimum Air Requirement
The no. of moles of 02 required for complete combustion of 1
mole of fuel is known as the theoretical amount of 02 (molar
basis). OR
The mass of 02 required for complete combustion of 1 kg of
fuel is known as the theoretical amount of 02 (mass basis).
The mass of 02 and N2 together (i.e. the air) required for
complete combustion of 1 kg of fuel, is known as minimum air
requirement. It is also called the theoretical or stochiometric
amount of air.
A mixture of theoretical air and fuel for complete
combustion of fuel is called stochiometric
mixture or
chemically correct mixture.
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Excess air
In practice, the combustion of fuel is never complete
with the theoretical amount of air because of imperfect
mixing of fuel and air. Mixture of fuel and air is never
homogeneous.
So, to ensure the complete combustion of fuel- usually
the actual air supplied is more than the theoretical air
required for complete combustion of fuel.
By: Prof. K. M. Joshi
The difference of actual air supplied and the theoretical
or stochiometric air required for complete combustion of
fuel is called excess air.
Air Fuel Ratio
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 If actual AF ratio is more than stochiomelric AF ratio, mixture of air
and fuel is said to be weak mixture or lean mixture. If actual AF ratio is
less than stochlometric AF ratio, mixture is said to be a rich mixture.
 Mixture strength or equivalence ratio is defined as the ratio of
stochlometric AF ratio to actual AF ratio.
If mixture strength is more than 100%, it represents a rich mixture.
If mixture strength is less than 100%. it represents a weak mixture.
12:57:43
Calculation for minimum air
requirement per kg of fuel
 Consider 1 kg of fuel. It contains x1 kg of carbon. x2 kg of
hydrogen, x3 kg of sulphur and x kg of oxygen.
 The amount of oxygen required for complete combustion can be
calculated using eq. (1), (2) and (3).
 1 kg carbon requires 8/3 kg of O2
x1 kg carbon requires ( 8/3 X x1 ) kg of O2
 Similarly
x2 kg of hydrogen requires 8 x2 kg of O2
x3 kg of sulphur requires x3 kg of O2
 Total oxygen required = 8/3 x1 + 8 x2 + x3 kg
 x kg oxygen is already presents in the fuel.
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x1
x1
x2
x3
x
x2
x1
x
x3
x2
x
x3
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Conversion of volumetric analysis into mass
analysis or gravimetric analysis
Multiply the percentage volume of each constituent by
their
respective
molecular
masses.
It
gives
the
proportionate mass of each constituent in the flue gas.
Add these proportionate masses of all constituents to
get total mass.
Divide the mass of each constituent by total mass and
express it as percentage. This gives the mass analysis of
flue gas.
12:57:43
Calorific Value Of Fuel
The amount of heat energy produced by combustion of unit
mass or volume of fuel is called its calorific value (CV).
For the solid and liquid fuels, unit mass is considered and
the unit of CV will be kJ/kg. For gaseous fuel, unit volume is
considered under NTP and the unit of CV will be kJ/m3.
Most of the fuels contain hydrogen. During combustion
process. H2 combines with 02 and forms steam (water vapour).
If this water vapour is condensed at constant temperature,
large amount of heat is released.
12:57:43
On account of this, two types of calorific values are defined.
(1) Higher Calorific Value (HCV) :
The higher calorific value is defined as the total heat liberated by
combustion of unit mass of fuel when the water vapour formed by
combustion is completely condensed at constant temperature
releasing its latent heal.
(2) Lower Calorific Value (LCV) :
The lower calorific value of fuel is defined as the net heat liberated
by combustion of unit mass of fuel when the water vapour formed
by combustion exists completely in vapour phase.
12:57:43
Determination of calorific value of fuel by bomb
calorimeter
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Calorific value of solid and liquid fuels can be determined with the help of
bomb calorimeter.
 The basic principle used for determination of calorific value of fuel is that
the known quantity of fuel is burnt and the heat energy liberated is
transferred to a medium of known mass and sp. heat and rise in
temperature of that medium is measured.
Construction :
The calorimeter C consists of a thick
walled bomb B made of stainless steel. It
has a capacity of about 650 c.c. and it is
designed to with stand high pressure upto
200 atmosphere.
The non return oxygen valve V and a
release valve U are connected to a bomb
at the top.
The crucible made of silicon or quartz is carried on support ring R
which can slide on insulated pillars P. The fuse wire W is passed though
the slots kept in pillars. The fuse wire is connected to electrical circuit.
The sensitive thermometer T is used to measure the temperature of
water filled in the calorimeter. Stirrer is provided in the water to stir the
water.
Working :
A pillet of solid fuel whose calorific
value is to be determined is prepared
and 1 gm of this pillet is kept into the
crucible.
A
known
quantity
of
water
(approximately 2500 c.c.) is filled
around
the
bomb
inside
the
calorimeter. The crucible and the
calorimeter are weighed before starting
the experiment.
The oxygen is admitted until a pressure of about 25 atmosphere.
The stirring of water is continued throughout the experiment and
the temperature readings are taken every minute.
After five minutes when the equilibrium sets, the fuel is ignited.
The temperature of water rises quickly due to heat energy released
by the combustion of fuel.
 After the temperature has reached its maximum value, it again
starts falling due to heat transfer losses to the surroundings.
 At the end of experiment, the release valve is opened so that the
pressure inside the bomb reduces to atmospheric pressure.
12:57:47
Let,
mf
CV
mc
CC
mw
Cw
δt
=
=
=
=
=
=
=
mass of fuel
calorific value of fuel
mass of calorimeter
sp. heat of calorimeter
mass of water
sp. heat of water
rise in temperature of water and calorimeter
Heat released from combustion of fuel =
heat gained by calorimeter and water
mf x CV
=
mc CC δt
+
mw Cw δt
When other quantities are known, CV of fuel can be calculated.
By: Prof. K. M. Joshi
12:57:47
Junkers gas calorimeter
By: Prof. K. M. Joshi
12:57:47
Construction :
Junkers gas calorimeter consists of a combustion chamber
surrounded by water jacket.
A gas pipe line is connected with a burner kept in combustion
chamber.
A gas flow meter and
pressure
regulator
are
provided in a gas pipe line.
Thermometers are used to
measure the temperature of
water at inlet and outlet.
Condensate from the gases
is collected in condensate pot.
12:57:47
Working :
The gas whose calorific value is to be measured is supplied
through a pipeline to the gas burner where it is burnt.
The flow rate of gas is measured by a flow meter. The pressure
of gas is measured by a manometer attached to the pressure
regulator.
 The heat produced by combustion of gas is absorbed by the
cold water flowing through water jacket. The gases are cooled
upto room temperature as far as possible, so that entire heat
released from the combustion may be absorbed by circulating
water.
 The temperature of cooling water at inlet and outlet and exit
gas temperature are measured. Mass flow rate of cooling water
is also measured. Volume flow rate of gas is converted to STP
condition.
12:57:48
By: Prof. K. M. Joshi
12:57:48
Enthalpy Of Reaction
During the constant pressure process, the heat energy released by
combustion of fuel at STP is called enthalpy of reaction.
 It is also known as heating value or heat of reaction or calorific value of fuel
at constant STP.
It is denoted by Qp or H0.
Applying 1st law of thermodynamics to combustion process at constant
pressure,
12:57:48
By: Prof. K. M. Joshi
12:57:48
Enthalpy Of Formation
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It is assumed that the reactants C
and O2 and the product CO2 both
enter and leave the combustion
chamber at STP.
The chemical energy of fuel carbon is 393520 kJ/kg mole. Above reaction
is exothermic, so the amount of heat energy transferred from system to
surroundings is equal to 393520 kJ/kg mole which is equal to chemical
energy of carbon.
Enthalpy datum for all naturally occuring stable elements is assigned as
zero enthalpy, e.g. the stable form of gases oxygen, hydrogen and nitrogen
are respectively O2, H2 and N2 while the natural form of carbon is.
graphite. All these elements are assigned zero enthalpy of formation.
So. the change in enthalpy of above chemically reactive system at STP
is [0 - 393520 ] = - 393520 kJ/kg mole
12:57:49
By: Prof. K. M. Joshi
Negative value indicates that heat energy is released in forming the
compound from its stable elements. A positive value will Indicate that the
heat energy is absorbed in formation of compound from its stable
elements.
Adiabatic Flame Temperature
Adiabatic flame temperature is defined as the theoretical
temperature attained by the products of combustion in an
adiabatic process assuming complete combustion.
If the combustion chamber is completely insulated, the
chemical energy released during combustion will be utilized to
raise the temperature of products of combustion alone and no
heat will be transferred to the surroundings.
 In this situation, the combustion process is said to be
adiabatic.
12:57:49
In actual practice, the actual flame temperature is less than
adiabatic flame temperature due to following reasons :
(1) Heat transfer to the surroundings since the system can not
be made perfectly insulated.
(2) Combustion is never complete.
(3)At high temperatures, the gases may not be stable, they
may dissociate and absorb energy internally from the
system, thus reducing the flame temperature.
Factors affecting adiabatic flame temperature;
(1)
Heat losses from combustion chamber
(2)
Incomplete combustion of fuel
(3)
Dissociation of gases
(4)
Excess or deficient air
12:57:49
As adiabalic flame temperature is the maximum temperature in
combustion chamber, it helps in selecting the material for
combustion chamber and design of combustion chamber.
Adiabatic flame temperature can be reduced by increasing the
air fuel ratio.
It is lower if excess air is supplied, since the total heat of
reaction of fuel is utilized by more no. of moles of products of
combustion.
The adiabalic flame temperature is also lower with deficient air
as heat of reaction is not fully released since 02 available is not
sufficient to burn the entire fuel.
12:57:50
By: Prof. K. M. Joshi
Adiabatic flame temperature is maximum with stochiometric air.
Equation for adiabatic flame temperature
Consider a perfectly insulated combustion chamber. The reactants are
supplied at P0, T1 and the products leave at P0 , T2.
According to definition, T2 is adiabaltic flame temperature. From steady
energy equation, with negligible changes in KE and PE.
Where Hp = enthalpy of products and HR = enthalpy of reactants
As combustion chamber is completely insulated, Q = 0
No work is done. So W = 0
12:57:50
As the products and reactants are respectively at temperatures T2 and T1,
their enthalpy is more than enthalpy of formation by the amount equal to
difference between enthalpy at that temperature and enthalpy at STP.
12:57:50
By: Prof. K. M. Joshi
Using this equation, adiabatic flame temperature T2 can be calculated
with the help of chemical reaction equation, enthalpy of formation
table and enthalpy of gas table.