Page 224 - Advanced Thermodynamics for Engineers, Second Edition

212 CHAPTER 10 THERMODYNAMICS OF COMBUSTION

nitrogen). It is normal to assume, at this level, that the nitrogen is an inert gas and takes no part in the
process. Combustion of methane with air is given by

79 79
CH 4 þ 2 O 2 þ N 2 / CO 2 þ 2H 2 O þ 2 N 2
21 21
1 kmol 9:52 kmol 1 kmol 2 kmol 7:52 kmol (10.3)
12 þ 4 2 ð32 þ 105:2Þ 12 þ 32 2 ð2 þ 16Þ 7:52 28
16 kg 274:4kg 44kg 36kg 210:67 kg

10.2.1 STOICHIOMETRY
There is a clearly defined, and fixed, ratio of the masses of air and fuel that will result in complete
combustion of the fuel. This mixture is known as a stoichiometric one and the ratio is referred to as the
stoichiometric air-fuel ratio. The stoichiometric air-fuel ratio, ε stoic , for methane can be evaluated from
the chemical equation (Eqn (10.3)). This gives
mass of air 2 ð32 þ 105:33Þ
¼ 17:17
ε stoic ¼ ¼
mass of fuel 16
This means that to obtain complete combustion of 1 kg CH 4 it is necessary to provide 17.17 kg of
air. If the quantity of air is less than 17.17 kg then complete combustion will not occur and the mixture
is known as rich. If the quantity of air is greater than that required by the stoichiometric ratio then the
mixture is weak.

10.2.2 COMBUSTION WITH WEAK MIXTURES

Aweak mixture occurs when the quantity of air available for combustion is greater than the chemically
correct quantity for complete oxidation of the fuel; this means that there is excess air available. In this
simple analysis, neglecting reaction rates and dissociation etc., this excess air passes through the
process without taking part in it. However, even though it does not react chemically, it has an effect on
the combustion process simply because it lowers the temperatures achieved due to its capacity to
absorb energy. The equation for combustion of a weak mixture is

2 1 f 7:52
CH 4 þ ðO 2 þ 3:76N 2 Þ/CO 2 þ 2H 2 O þ 2 O 2 N 2 (10.4)
f f f
where f is called the equivalence ratio, and
actual fuel air ratio
(10.5)
f ¼
stoichiometric fuel air ratio
For a weak mixture f is less than unity. Consider a weak mixture with f ¼ 0.8, then

CH 4 þ 2:5ðO 2 þ 3:76N 2 Þ/CO 2 þ 2H 2 O þ 0:50 2 9:4N 2 : (10.6)