Introduction
The notes below relate to the combustion process. Combustion is a rapid reaction between a fuel
and oxygen that produces heat (the chemical energy content of a fuel is converted to
heat energy).
Composition of Dry Air
Oxygen is involved in the majority of combustion reactions and this is present in
the air.
The composition of dry air as mole fractions is 0,7809 N2,
0,2095 O2, 0,0093Ar, and 0,003 CO2.
For combustion equations
it is convenient and practical to treat Ar and CO2 as Nitrogen. The composition
of molar air can then be taken as (approx.) 0,79kmol N2 and 0,21 kmol O2 per kmol of Air.
Note: I am using kmols (The molar volume is 22,414 m�/kmol at 0 �C and 101.325 kPa absolute pressure )
The equivalent mass fractions of air are 0,768N2 and 0,232 O2 per unit of air. These
values are conveniently represented as
1 kmol O2 + 3,76 kmol N2 = 4,76 kmol Air
1 kg O2 + 3,31 kg N2 = 4.31 kg Air
Combustion Notes
When considering the combustion of fuels with air equations are used to determine
the proportions of the various chemicals involved. The fuels generally are
composed of carbon, hydrogen and sulphur with other substances including oxygen and ash.
The carbon, hydrogen and sulphur combine with oxygen in the air and the nitrogen and other
gases in air are assumed to take no part in the combustion process.....
The proportion of air for complete combustion is called the stoichiometric air/fuel ratio. Normally
an excess of air is available and the mixture is weak or lean. When insufficient air
is available for complete combustion the mixture is rich...
Some of the terms used in combustion are identified below.....
- Air/Fuel Ratio R = (Amount on air) /(Amount of fuel)
- Stoichiometric air/fuel Ratio for complete combustion = R s
- Percentage excess Air E = [(R - R s) /R s]100%
>
- Mixture Strength M s = ( R s /R )100 %
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Weak mixture M s less than 100%
Rich mixture M s greater than 100%
It is important to note that the total combustion of a fuel requires ideal conditions.
The fuel must be intimately mixed with the oxygen, the temperature must be appropriate, the ignition
cannot start without a source of activation energy ( a spark, or flame, or local high temperature).
Once the ignition has commence the combustion will generally spread spontaneously.
For an car engine it may be desireable to have a rich mixture to allow for maximum power or at
start up. A boiler requires an excess or air to ensure complete combustion of the fuel
for efficient operation....
Combustion Equations
The following typical equations are used to determine the combustion process
- Carbon ...C + O 2 -> CO 2
- Carbon ...2C + O 2 -> 2CO...Incomplete Combustion
- Hydrogen ...2H 2C + O 2 -> 2H 2O
- Sulphur ...S + O 2 -> SO 2
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Reaction equations are generally based on volumes for gases and masses for liquids and solids.
An reaction equation can include the enthalpy change Δ H at the end of the equation eg
C H4 + 2 O 2 -> CO 2 + 2 H2 - Δ H
The enthalpy term is negative if the reaction is exothermic and positive if the reaction is endothermic.
Consider the combustion of Propane gas (C3 H8 )as an example...
Writing down the basic reaction equation without identifying the quantities
C 3 H 8 +x O 2 -> y C O 2 + z H 2 O..x, y z being unknown
There are three carbon atoms on the LHS and therefore y = 3. There are 8 Hydrogen atoms on the left hand side ( H 8) and therefore
z = 4 (4.H2) . The equation resulting is therefore
C 3 H 8 +x O 2 -> 3 C O 2 + 4 H 2 O..x being unknown
To determine x it is easily calculated that there are now 10 Oxygen atoms on the RHS (3 O2 + 4 O) x is therefore 5.
The reaction equation is therefore .
C 3 H 8 +5 O 2 -> 3 C O 2 + 4 H 2 O..
In terms of kmoles
1 kmol C 3 H 8 + 5 kmol O 2 = -> 3 kmol C O 2 + 4 kmol H 2 O
Transfer into mass units ( 1 - kmol has a mass in kg = Molecular Weight)
[1 kmol of C 3 H 8 = 44kg + 5 kmol of O 2 = 160 kg ]
->
[3 kmol of C O 2 = 132 kg + 4 kmol of H2 O = 72 kg ]
Now considering the equations with respect to Air. For each volume of O2
there are 4,76 volumes of Air. Therefore the stoichiometric ratio of for combustion
of propane is 1 to 5.4,76 = 1:23.8 based on volume..
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Table showing various substances involved in combustion with their Molecular Weights
| Substance |
Formula |
Approx.
molecular
Weight |
| Benzene |
C 6H 6 |
78 |
| Butane |
C 4H 10 |
- |
| Carbon |
C |
12 |
| Carbon Monoxide |
CO |
28 |
| Carbon Dioxide |
CO 2 |
44 |
| Ethane |
C 2H 6 |
30 |
| Ethanol |
C 2H 5OH |
46 |
| Ethene |
C 2H 4 |
28 |
| Hydrogen |
H 2 |
2 |
| Methane |
CH 4 |
16 |
| Nitrogen |
N 2 |
28 |
| Octane |
C 8H 18 |
114 |
| Oxygen |
O 2 |
32 |
| Pentane |
C 5H 12 |
72 |
| Propane |
C 3H 8 |
44 |
| Propene |
C 3H 6 |
42 |
| Sulphur |
S |
32 |
| Sulphur Monoxide |
SO |
48 |
| Sulphur Dioxide |
SO 2 |
64 |
| Water |
H 2O |
18 |
For combustion Notes Combustion Notes
....More notes to follow....
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