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66 3 Basics of Gas Combustion

3.2.3 Fuel Rich Combustion with Dry Air at Low
Temperatures

A mixture of fuel with less than stoichiometric air is fuel rich mixture, and the
corresponding combustion is called fuel rich combustion. Since oxygen is insuffi-
cient to oxidize all the C and H in the fuel to CO 2 and H 2 O, there may be, for example
CO and H 2 in the products. The combustion reaction formula for hydrocarbon fuel
with dry air can be derived using the similar approach above.

1 b
C a H b þ aþ ð O 2 þ3:76N 2 Þ ! aCO 2 þbCO½ Šþ cH 2 O+ eH 2 ŠþdN 2 ð3:16Þ
½
/ 4
where for fuel rich mixture with insufficient air, 1// is less than one. Again from the
atom balances for carbon, hydrogen, oxygen, and nitrogen, one can get
Carbon balance: a ¼ a þ b ð1Þ

Hydrogen balance: b ¼ 2c þ 2e ð2Þ

2 b
Oxygen balance: a þ ¼ 2a þ b þ c ð3Þ
/ 4

4:76 b
Nitrogen balance: a þ 0:79 ¼ d ð4Þ
/ 4
Only d can be determined from the nitrogen balance. The other four (a, b, c
and e) cannot be determined because there are four unknowns in three equations. To
solve the problem, one more equation is needed. This equation can be derived from
chemical equilibrium (see Sect. 3.3.2). For now, we can only write the chemical
equations as follows with x and y to be determined from other known factors.

1 b
C a H b þ aþ ð O 2 þ3:76N 2 Þ
/ 4

3:76 b
½
! xCO 2 þ 1 xð½ ÞCOŠþ yH 2 O+ 1 yÞH 2 Šþ aþ N 2
ð
/ 4
ð3:17Þ
The preceding analyses were based on a common assumption that air and fuel
are perfectly mixed. However, it is very challenging in engineering practices to
achieve perfect mixing in the entire combustion device. Stoichiometric, fuel lean
and fuel rich combustion all take place at different spots in the combustion device.
As a result, the actual combustion formula is much more complicated than that in
Eq. (3.17). For instance, if we consider only oxygen as one more extra product, the
reaction becomes