Page 86 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

60 3 Basics of Gas Combustion

where n is mole amount, and the subscripts a and f stand for air and fuel,
respectively.
The air–fuel ratio of a combustion mixture determines the combustion chemistry,
and consequently the air emissions produced from the combustion process. There
are three possible outcomes of the combustion process: stoichiometric, fuel lean, or
fuel rich combustion.
• Stoichiometric combustion When the air and fuel are mixed at such a ratio that
both the fuel and oxygen in the air are consumed completely. This ratio is called
theoretical air–fuel ratio, and the combustion is considered as ideal combustion,
or stoichiometric combustion.
• Fuel rich combustion When the oxygen in the air is not sufficient to burn the
fuel completely, the combustion is referred to as fuel rich combustion. And,
there will still be fuel left in the combustion product.
• Fuel lean combustion When there is more oxygen than needed to burn the fuel
completely, the combustion is called fuel lean combustion, and there will be
oxygen left in the combustion product.
An important term that is related to air–fuel ratio is equivalence ratio (/). It is
defined as the ratio of the theoretical air–fuel ratio to the actual air–fuel ratio of the
mixture. Mathematically,

ðA=FÞ s
/ ¼ ð3:2Þ
ðA=FÞ mix
where the subscript s stands for stoichiometric. The advantage of using equivalence
ratio over air-to-fuel ratio is that the former is independent on the units being used
for the computation of the air–fuel ratio.
The equivalence ratio is more commonly used than air fuel ratio as an indicator
to show whether the mixture is stoichiometric, fuel lean, or fuel rich. When / is
greater than one there is always excess fuel in the mixture than what is needed for a
stoichiometric combustion. / less than one represents a deficiency of fuel in the
mixture.
8
/ \ 1 Fuel lean
<
/ ¼ 1 Stoichiometric ð3:3Þ
/ [ 1 Fuel rich
:
Example 3.1: Air fuel ratio and equivalence ratio
Consider a mixture of one mole of ethane (C 2 H 6 ) and 15 mol of air. Determine
(a) the air-to-fuel ratio of this mixture
(b) the equivalence ratio of this mixture if the stoichiometric air to fuel ratio based
on volume is 16.66.