Combustion reactions involve the release or absorption of energy as heat during chemical reactions where products form from reactants. A combustion reaction between fuel and oxygen releases heat. Stoichiometric coefficients in chemical equations represent equal amounts of each chemical element on both sides of the equation. The air-fuel ratio refers to the ratio of air to fuel, either by mass or moles. The theoretical air is the minimum amount of air needed for complete combustion of carbon, hydrogen, and sulfur in the fuel. Analysis of exhaust and flue gases can be done through wet analysis including steam or using an Orsat apparatus. Enthalpy of formation is the change in enthalpy when a compound forms from its elements in their standard states.

1. Combustion Reactions
S.Gunabalan
Associate Professor
Mechanical Engineering Department
Bharathiyar College of Engineering & Technology
Karaikal - 609 609.
e-Mail : gunabalans@yahoo.com
Part - 1

2. Combustion Reactions
The entropy of substances at the absolute zero of temperature,
called absolute entropy (third law of thermodynamics).
A combustion reaction is a chemical reaction in which
products are formed from reactants with the release
or absorption of energy as heat is transferred to as
from the surroundings

3. The numerical coefficients in the equation,
which precede the chemical symbols to give
equal amounts of each chemical element on
both sides of the equation, are called
stoichiometric coefficients.
the total mass of reactants must equal the total mass of products
Since 1 kmol of H2 equals 2 kg, 1 2 kmol of O2 equals 16 kg, and
1 kmol of H2O equals 18 kg,

4. The air–fuel ratio is simply the ratio of the
amount of air in a reaction to the amount of
fuel. The ratio can be written on a molar basis
(moles of air divided by moles of fuel)
or on a mass basis (mass of air divided by mass
of fuel).

5. Theoretical Air
The minimum amount of air that supplies
sufficient oxygen for the complete combustion
of all the carbon, hydrogen, and sulfur present
in the fuel is called the theoretical amount of air
Gas Molar Weight
( M)Kg/Kmol
Air 28.97
Nitrogen 28.01
Oxygen 32
Hydrogen 2.016
Helium 4.004
Carbon dioxide 44.01
Steam 18.02

7. Stoichiometric (or chemically correct) mixture of
air and fuel is one that contains just sufficient
oxygen for complete combustion of the fuel.
A weak mixture is one which has an excess of air.
A rich mixture is one which has a deficiency of air
The ratios are expressed as follows :
For gaseous fuels :By volume
For solid and liquid fuels : By mass

8. AIR-FUEL RATIO
• air-fuel ratio can be calculated by the
following methods :
1. Fuel composition known
(i) Carbon balance method
(ii) Hydrogen balance method
(iii) Carbon-hydrogen balance method.
2. Fuel composition unknown
(i) Carbon-hydrogen balance method.

9. Carbon balance method : combustion takes place
with excess air and when free (solid) carbon is not
present in the combustion products.
Hydrogen balance method is used when solid
carbon is present in the combustion product.
Carbon-hydrogen balance method: This method
may be employed when there is some uncertainty
about the nitrogen percentage reported by the
Orsat analysis.

10. ANALYSIS OF EXHAUST AND FLUE GAS
Wet analysis : An analysis which includes the
steam in the exhaust.
Orsat apparatus : is used for the analysis of
combustion products.

12. ENTHALPY OF FORMATION
The enthalpy of formation (∆Hf) is the increase in
enthalpy when a compound is formed from its
constituent elements in their natural form and in a
standard state. The standard state is 25°C, and 1 atm.
∆ 0 = ∆ 0
− ∆ 0
∆ 0 = . ∆ 0
− . ∆ 0
Where
n – number of moles
Also Enthalpy of formation of elements in its elemental form is
zero. For example H2,O2 ext

15. Enthalpy of formation of elements in its elemental form
is zero. For example H2,O2 ext

18. http://en.wikipedia.org/wiki/Standard_enthalpy_of_formation

19. Reference
• Moran, M. J. 2011. Fundamentals of engineering thermodynamics. Wiley,
[Hoboken, N.J.?].
• Rajput, R. K. 2010. Engineering thermodynamics. Jones and Bartlett
Publishers, Sudbury, Mass.
• Nag, P. K. 2002. Basic and applied thermodynamics. Tata McGraw-Hill, New
Delhi.
• http://www.weebly.com/uploads/3/1/9/7/3197205/heats_of_formation_
worksheet.pdf