Page 181 - Entrophy Analysis in Thermal Engineering Systems
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176 Entropy Analysis in Thermal Engineering Systems
Table 11.2 The enthalpy and entropy values of water, oxygen,
and carbon dioxide at 298.15K, 1bar.
Substance h (kJ/mol) s (J/molK)
Water (liquid) 285.8 69.95
Water (vapor) 241.8 188.8
Oxygen – 205.15
Carbon dioxide 393.5 213.79
On the other hand, the higher heating value may be represented as
y
HHV ¼ h f ,0 xh CO 2 ,0 h H 2 O lðÞ,0
2
y
¼ h f ,0 + x 393:5Þ + ð 285:8Þ kJ=molÞ (11.26)
ð
ð
2
Substituting Eqs. (11.25) and (11.26) into Eq. (11.23) and grouping the sim-
ilar terms allows one to obtain the following expression for the chemical
exergy.
ð
ð
ψ ¼ x 396:08ð Þ + y 138:04Þ + z 30:58Þ + h f ,0 T 0 s f ,0 (11.27)
ch
where ψ has units of kJ/mol, h f ,0 is the formation enthalpy of the fuel in
ch
kJ/mol, s f ,0 isthe standard entropy ofthefuel in kJ/molK,andT 0 ¼298.15K.
Dividing Eq. (11.27) by the molecular weight of the fuel leads to an alter-
native relation for the chemical exergy of a hydrocarbon fuel in units of kJ/g.
ch (11.28)
ψ ¼ 31:1C½ + 136:1H½ +1:91 + h f ,0 T 0 s f ,0
where [C] and [H] denote the carbon and hydrogen contents per unit mass
of fuel, respectively.
Another useful correlation can be obtained for the chemical exergy as a
function of the heating value of fuel using the data of Table 11.1. Hence,
ψ ¼ 0:967HHV 38:0 (11.29)
ch
Note that Eq. (11.29) is obtained using a limited number of data points.
However, Eqs. (11.27) and (11.28) are derived from basic thermodynamic
principles and by using the empirical data of NIST [3]. So, they can be
applied for estimating the chemical exergy of any fuel with a chemical for-
mula of C x H y O z .
