Page 88 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

62 3 Basics of Gas Combustion

Example 3.2: Stoichiometric combustion
Determine the overall chemical equation for the stoichiometric combustion of
propane (C 3 H 8 ) with oxygen as oxidant.

Solution
Start with a general chemical reaction equation as

C 3 H 8 þaO 2 ! bCO 2 þcH 2 O

with a, b, and c to be determined.
According to the conservation of mass, the amount of atoms cannot be created or
destroyed. Therefore, the coefficients of a, b and c in the above reaction formula can
be computed from the atom balances of carbon, hydrogen, and oxygen:

Carbon balance : b ¼ 3
Hydrogen balance : 2c ¼ 8
Oxygenbalance : 2b þ c ¼ 2a

Solving the above three equations gives a =5, b = 3 and c = 4. Thus the above
stoichiometry is
C 3 H 8 þ 5O 2 ! 3CO 2 +4H 2 O

3.2.1 Stoichiometric Combustion with Dry
Air at Low Temperature

In most engineering applications, air instead of pure oxygen is used as an oxidizer
for fossil fuel combustion. In other words, oxygen for combustion is obtained from
the air for the sake of low cost. Our analysis herein further simplifies the ambient air
as a mixture of 21 % oxygen and 79 % nitrogen molecules by volume. In this case,
air is defined as

1Air ¼ 0:21O 2 þ 0:79N 2 ð3:6Þ
This simplified definition is reasonable in that in one mole of air there is about
0.21 mol of oxygen and 0.79 mol of nitrogen. And the corresponding density of air
3
is 1.21 kg/m and the molar weight of air is 28.82 g/mol under standard condition.
All these values are close to the measured ones.