ENGRD 2210
–
Thermodynamics
Cornell University
–
Fall 2009
Chapter 13 Practice Problems for the Final Exam (13.9, 13.13, 13.55, 13.61, 13.70)
Problem 13.9
Problem Statement:
A fuel mixture with the molar analysis 40% CH
3
OH, 50% C
2
H
5
OH, and 10% N
2
burns completely with 33%
excess air. Determine:
a.
the balanced reaction equation
b.
the airfuel ratio, both on a molar and mass basis
Engineering Assumptions:
1.
The textbook assumption is that air is comprised of 21% O
2
and 79% N
2
where N
2
includes
nitrogen gas along with all the other components of air, resulting in 3.76 moles of nitrogen
accompanying every mole of oxygen.
2.
Nitrogen is inert, meaning it does not react and remains at the same temperature on both sides
of the equation.
Solution:
Assume 1 mole of fuel for simplicity, and based upon the given molar ratios of the fuel components, the
reaction equation for complete combustion with the theoretical amount of air is:
where Greek notation represents the unresolved coefficients of air (alpha), carbon dioxide (beta), and
water (gamma). Note how the inert nitrogen gas terms and coefficients have simply transferred to the
right side of the equation.
Now we can solve for the coefficients:
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Now that we have our complete combustion with the
theoretical amount of air
, we want to determine
the complete combustion with the problem prescribed 33% excess air condition. We represent this with
an additional coefficient of 1.33 in front of the air term. Note that Greek notation has been replaced
with the above values and also how the added coefficients translate to either side of the equation. Also
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 '08
 TORRANCE
 Combustion, Cornell University, complete combustion

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