What is Combustion ?
A chemical reaction during which a fuel is oxidized and a large quantity
of energy/ heat is released is called combustion .
Necessary Constituents of Combustion:
i. Fuel
ii. Supporter of combustion(Air/oxidizer)
iii. Ignition / kindling temperature
Ignition temperature for different fuels
Fuel Ignition Temperature (oC)
Gasoline / Petrol 260
Carbon 400
Hydrogen 580
Carbon monoxide 610
Methane 630
Principle of Combustion
The oxidizer most often used in combustion processes is air, which is free
and readily available.
Pure oxygen O2 is used as an oxidizer only in some specialized
applications, such as cutting and welding.
Oxygen is the key to combustion
Composition of Air
Types of combustion Process
1. Complete combustion
2. Incomplete combustion
1. Complete Combustion Process
A combustion process is complete if all the carbon & hydrogen in
the fuel burns to form, CO2,& H2O.
All the sulfur (if any) burns to SO2.
It means, all the combustible components of a fuel are burned to
completion during a complete combustion process.
2. Incomplete Combustion Process
The combustion process is incomplete if the combustion products
contain any unburned fuel or components such as, C, H2, CO, or OH.
Insufficient oxygen is an obvious reason for incomplete combustion,
but it is not the only one.
Incomplete combustion occurs even when more oxygen is present in
the combustion chamber than is needed for complete combustion.
Another cause of incomplete combustion is dissociation, which
becomes important at high temperatures.
(During adiabatic combustion max. Temp. Lower than the calculated
one)
Chemical Equation for Combustion
Chemical equations are balanced on the basis of the conservation of mass
principle (or the mass balance),
Stated as follows: The total mass of each element is conserved during a
chemical reaction.
That is, the total mass of each element on the right-hand side of the
reaction equation (the products) must be equal to the total mass of that
element on the left-hand side (the reactants).
Even though the elements exist in different chemical compounds
in the reactants and products.
Example:
2H2 + O2 → 2H2O
Reactants Product
Theoretical and Actual Combustion Processes
Theoretical / Stoichiometric Combustion Process
The ideal combustion process during which a fuel is burned
completely with theoretical air(stoichiometric air) is called the
stoichiometric or theoretical combustion process.
For example,
the theoretical combustion of methane is notice that the
products of the theoretical combustion contain no unburned
methane and no C, H2, CO, OH, or free O2.
Stoichiometric air:
Minimum mass / volume of air required for completed
combustion of 1kg of fuel is known as Stoichiometric air.
Actual combustion process
In actual combustion processes, it is common practice to use
more air than the stoichiometric amount to increase the chances
of complete combustion or to control the temperature of the
combustion chamber.
Excess Air
The amount of air in excess of the stoichiometric amount is called
excess air.
The amount of excess air is usually expressed in terms of the
stoichiometric air as percent excess air or percent theoretical air.
For example,
50 percent excess air is equivalent to 150 percent theoretical air,
200 percent excess air is equivalent to 300 percent theoretical air.
Deficiency of Air
Amounts of air less than the stoichiometric amount are called
deficiency of air and are often expressed as percent deficiency of
air.
For example,
90 percent theoretical air is equivalent to 10 percent deficiency of
air.
Air-Fuel Ratio
A frequently used quantity in the analysis of combustion processes
to quantify the amounts of fuel and air is the air–fuel ratio (AF).
AF represents the amount of air used per unit mass of fuel during a
combustion process
It is usually expressed on a mass basis and is defined as,
The mass m of a substance is related to the number of moles N
through the relation m= NM, where M is the molar mass.
Example:
The air–fuel ratio can also be expressed on a mole basis as the
ratio
of the mole numbers of air to the mole numbers of fuel. But we will
use the former definition.
The reciprocal of air–fuel ratio is called the fuel–air ratio.
Example:
One k mol of octane (C8H18) is burned with air that contains 20 kmol of O2.
Assuming the products contain only CO2, H2O, O2, and N2, determine the
mole number of each gas in the products and the air–fuel ratio for this
combustion process. Assume the molar mass of air is 29.0 kg/kmol.
The chemical equation for this combustion process,
X, y,z and w represent the unknown mole numbers of the product, these
unknowns are determined by applying the mass balance to each of the
elements.
For Carbon(C): 8=x x=8
For Hydrogen (H): 18 =2y y=9
For Oxygen (O) 20x2=2x+ y+2z z =7.5
For Nitrogen (N2): (20)(3.76) =w w = 75.2
Now, substitute the values and yields
Note that the coefficient 20 in the balanced equation above represents the
Number of moles of Oxygen, not the number of moles of air.
The latter is obtained by
20x3.76 =75.2 moles of nitrogen to the 20 moles of oxygen.
Therefore, total moles of air is 95.2
The air-fuel ratio (AF) is determine by
AF = 24.2 kg air / kg fuel
It mean 24.2 kg of air is required to burn each kilogram of fuel during this
Combustion process.
