Chapter : 4
FIRST LAW APPLICATION TO CHEMICALLY REACTING SYSTEMS
STOICHIOMETRY
The stoichiometric quantity of oxidizer is just that amount needed to completely burn
a quantity of fuel. That is combustion of all the carbon, hydrogen and sulphur present
in the fuel.
It is also called as chemically correct or theoretical amount of air.
If more than a stoichiometric quantity of oxidizer is supplied, the mixture is said to be
fuel lean, or just lean.
While supplying less than the stoichiometric oxidizer results in a fuel-rich, or rich
mixture
AMOUNT OF AIR REQUIRED FOR COMBUSTION
Minimum Air Required for complete combustion of Solid Fuels:
For 1 kg of fuel consisting of carbon, hydrogen and sulphur, the minimum amount of
O2 required can be obtained with the help of basic combustion Equations.
1 kg of carbon requires 8/3 kg of O2 for complete combustion.
� + �2 → ��2
On molar mass basis
12�� � + 32�� �2 → 44�� ��2
C kg of carbon requires 8/3C kg of O2
Similarly,
1 kg of sulphur requires 1 kg of O2.
1 kg of hydrogen requires 8 kg of O2.
Total oxygen required = (8/3C +8H+S) kg
All the oxygen supplied with air must be consumed by the fuel elements to form the
completely oxidised products. No free oxygen should appear in the products.
, If (O) kg of oxygen is already present in the fuel, then minimum mass of oxygen
required for complete combustion is given by
= 8/3C + 8H + S – O
= 8/3C + 8(H – O/8) + S
Minimum mass of air required becomes,
��� � �
��� = �+� �− +�
�� � �
Since Air contains 23% of Oxygen by mass.
Similarly,
The minimum Air required for combustion of Gaseous fuel is given by
���
��� = �. ��� + �. ��� + ���� + ��� �� − �� �3
��
Composition of Dry Air:
Instead of supplying only oxygen for the chemical reactions, atmospheric air is supplied.
Atmospheric air is the mixture of mainly O2 and N2 with small traces of argon, carbon
dioxide, water vapour, etc. The composition of dry air on molar basis is given by 78.09% of
N2, 20.95% of O2, 0.93% of argon and 0.03% of CO2. The molecular weight of this mixture is
28.97 kg/kmol. For all calculations, argon and carbon dioxide are considered as an additional
nitrogen, because they are also inert gases like N2 and appear in very small quantities.
Accordingly, air is considered to consist of 79% of nitrogen and 21% of oxygen by volume or
on a molar basis. With this assumption, the molar (volume) ratio of nitrogen to oxygen is
0.79
= 3.76, that is air supplied for combustion contains 3.76 mole of nitrogen with each
0.21
mole of oxygen.
Molar basis:
0.21 mole O2 + 0.79 mole N2 = 1 Mole of Air
1 mole of O2 + 3.76 mole of N2 = 4.76 mole of air
Mass basis:
0.23 kg of O2 + 0.77 kg of N2 = 1 kg of air
1 kg of O2 + 3.347 kg of N2 = 4.347 kg of air
Molecular weight of Air = 28.97 kg/kmol
EXCESS AIR:
In actual combustion process, the amount of air supplied is either greater or less than
the theoretical amount.
FIRST LAW APPLICATION TO CHEMICALLY REACTING SYSTEMS
STOICHIOMETRY
The stoichiometric quantity of oxidizer is just that amount needed to completely burn
a quantity of fuel. That is combustion of all the carbon, hydrogen and sulphur present
in the fuel.
It is also called as chemically correct or theoretical amount of air.
If more than a stoichiometric quantity of oxidizer is supplied, the mixture is said to be
fuel lean, or just lean.
While supplying less than the stoichiometric oxidizer results in a fuel-rich, or rich
mixture
AMOUNT OF AIR REQUIRED FOR COMBUSTION
Minimum Air Required for complete combustion of Solid Fuels:
For 1 kg of fuel consisting of carbon, hydrogen and sulphur, the minimum amount of
O2 required can be obtained with the help of basic combustion Equations.
1 kg of carbon requires 8/3 kg of O2 for complete combustion.
� + �2 → ��2
On molar mass basis
12�� � + 32�� �2 → 44�� ��2
C kg of carbon requires 8/3C kg of O2
Similarly,
1 kg of sulphur requires 1 kg of O2.
1 kg of hydrogen requires 8 kg of O2.
Total oxygen required = (8/3C +8H+S) kg
All the oxygen supplied with air must be consumed by the fuel elements to form the
completely oxidised products. No free oxygen should appear in the products.
, If (O) kg of oxygen is already present in the fuel, then minimum mass of oxygen
required for complete combustion is given by
= 8/3C + 8H + S – O
= 8/3C + 8(H – O/8) + S
Minimum mass of air required becomes,
��� � �
��� = �+� �− +�
�� � �
Since Air contains 23% of Oxygen by mass.
Similarly,
The minimum Air required for combustion of Gaseous fuel is given by
���
��� = �. ��� + �. ��� + ���� + ��� �� − �� �3
��
Composition of Dry Air:
Instead of supplying only oxygen for the chemical reactions, atmospheric air is supplied.
Atmospheric air is the mixture of mainly O2 and N2 with small traces of argon, carbon
dioxide, water vapour, etc. The composition of dry air on molar basis is given by 78.09% of
N2, 20.95% of O2, 0.93% of argon and 0.03% of CO2. The molecular weight of this mixture is
28.97 kg/kmol. For all calculations, argon and carbon dioxide are considered as an additional
nitrogen, because they are also inert gases like N2 and appear in very small quantities.
Accordingly, air is considered to consist of 79% of nitrogen and 21% of oxygen by volume or
on a molar basis. With this assumption, the molar (volume) ratio of nitrogen to oxygen is
0.79
= 3.76, that is air supplied for combustion contains 3.76 mole of nitrogen with each
0.21
mole of oxygen.
Molar basis:
0.21 mole O2 + 0.79 mole N2 = 1 Mole of Air
1 mole of O2 + 3.76 mole of N2 = 4.76 mole of air
Mass basis:
0.23 kg of O2 + 0.77 kg of N2 = 1 kg of air
1 kg of O2 + 3.347 kg of N2 = 4.347 kg of air
Molecular weight of Air = 28.97 kg/kmol
EXCESS AIR:
In actual combustion process, the amount of air supplied is either greater or less than
the theoretical amount.
