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64 3 Basics of Gas Combustion

The corresponding stoichiometry is

ð
C 3 H 8 þ5O 2 þ3:76N 2 Þ ! 3CO 2 +4H 2 O+18:8N 2 ð3:10Þ

3.2.2 Fuel Lean Combustion

When there is excess air in a combustion process, the combustion is called fuel lean
combustion and the corresponding mixture is called fuel lean mixture. At low
combustion temperatures the nitrogen in the air and the extra oxygen appear in the
products. For the fuel with a formula C α H β the reaction formula in general is

ð
C a H b þa O 2 þ3:76N 2 Þ ! bCO 2 þcH 2 O+ dN 2 þeO 2 ð3:11Þ
The coefficients b and c can be determined from the carbon balance and
hydrogen balance, respectively (b ¼ a; c ¼ b=2).
From the definition of the equivalence ratio above, one can get

ð A=FÞ s 4:76 b
ð A=FÞ ¼ ¼ a þ ð3:12Þ
mix / / 4
Now the fuel lean combustion reaction equation becomes,

4:76 b b
C a H b þ a þ ð 0:210 2 þ0:79N 2 Þ ! aCO 2 þ H 2 O+ dN 2 þeO 2 ð3:13Þ
/ 4 2
where / is less than one for a fuel lean mixture because of the excess air. The mole
amount of nitrogen (d) and the extra oxygen in the excess air (e) can be determined
by the atom balances of O and N:

4:76 0:79 b 3:76 b
a þ ¼ d ! d ¼ a þ
/ 4 / 4

1 b
e ¼ 1 a þ
/ 4
Therefore, the fuel-lean reaction formula for the combustion of C a H b perfectly
mixed with excess air is,

4:76 b
C a H b þ aþ ð 0:21O 2 þ0:79N 2 Þ
/ 4
ð3:14Þ

b 3:76 b 1 b

! aCO 2 þ H 2 O+ aþ N 2 þ 1 aþ O 2
2 / 4 / 4