Modern Engineering Thermodynamics

ðn i /n fuel Þðc pi Þ E system
Summary 643
The often complex formulae of chemical thermodynamics provide an excellent topic for personal computer software
development. In this chapter, simple Excel spreadsheet programs are developed for calculating heats of
combustion and dissociation equilibrium conditions. The reader is encouraged to utilize and modify this material,
to take full advantage of the computer’s ability to remove the tedium of calculation and expand the scope
of the analysis.
Some of the more important equations introduced in this chapter follow. Do not attempt to use them blindly
without understanding their limitations. Please refer to the text material where they were introduced to gain an
understanding of their use and limitations.
1. Air/fuel ratio. For a chemical reaction combustion equation of the form
αðFuelÞ + βðO 2 + 3:76 N 2 Þ!Products
the molar and mass air/fuel ratios are
αð1 + 3:76Þ
ðA/FÞ molar
= = αð4:76Þ
β
β
ðA/FÞ mass = αð4:76ÞðM airÞ
= αð4:76Þð28:97Þ = αð137:9Þ
βðM fuel Þ βðM fuel Þ βðM fuel Þ
where α and β are the stoichiometric coefficients in the reaction equation, and M fuel is the molecular mass of
the fuel.
2. Stoichiometric reaction equation for the combustion of a hydrocarbon of the form C n H m in 100%
theoretical air:
C n H m + n + m
½O 2 + 3:76ðN 2 ÞŠ6nðCO 2 Þ + m
ðH 2 OÞ + 3:76 n + m
ðN 2 Þ
4
2
4
3. The temperature at which water condenses out of the products of combustion:
T condense = T sat ðp H2 OÞ
where p H 2O = (χ H 2O/χ total )p total is the partial pressure of the water vapor in the products.
4. The standard reference state (SRS):
Temperature: T SRS = T° =25:0°C = 298 K = 77:0°F = 537 R
Pressure: p SRS = p° =0:100 MPa = 14:5 psia ≈ 1 atmosphere
5. Molar specific heat of formation of a hydrocarbon C n H m in MJ/kgmole:
ðq f °Þ CnH m
= − ½nð393:5Þ + ðm/2Þð285:5Þ + HHV Cn H m
Š
6. Molar specific enthalpy of formation of a compound at the standard reference state is the same as its molar
specific heat of formation:
ðh f
°Þ compound = ðq f °Þ compound
7. Molar heat of reaction:
q r = ∑ðn i /n fuel Þh i −∑ðn i /n fuel Þh i
P
R
8. Higher and lower heating values of a fuel:
HHV = q r ðSRS and H 2 O in the liquid stateÞ
LHV = q r ðSRS and H 2 O in the vapor stateÞ
9. Open and closed system adiabatic flame temperature:
ðT A Þ open
= T°+
ðh f
°Þ fuel
−∑ðn i /n fuel Þðh f
°Þ i
P
∑
P