Page 179 - Entrophy Analysis in Thermal Engineering Systems

174 Entropy Analysis in Thermal Engineering Systems

where SEG is the specific entropy generation, and we may denote ψ de as the
specific exergy destruction (SED). It was shown in Chapter 8 that the min-
imization of SEG is identical to the maximization of thermal efficiency in
combustion power plants.
The chemical exergy of fuel is traditionally determined with the entro-
pies of the combustion gases evaluated at the fixed atmospheric concentra-
tions. It is further assumed that combustion takes place in pure oxygen, water
content of the products mixture remains in gas phase, and the combustion
products behave like ideal gases. It should be noted that the entropy gener-
ation term in Eq. (11.17) accounts for the irreversibility within the system
and that due to the rejection of the waste heat to the atmosphere. Also, the
entropy of the combustion products at the exit of the system, S e , and at the
p
ambient temperature and pressure, S 0 , should be determined based on the
p
concentrations of the gaseous species in the combustion products. Further-
more, the possibility of condensation of water vapor needs to be accounted
for; see Chapter 8.
Important to note is that heat engine is a device that converts heat into
mechanical work. It is not designed to produce work through isothermal
mixing of the flue gases with the atmospheric air, which would occur outside
the boundary of the system. The chemical exergy of a fuel should in accor-
dance with Eq. (11.18) represent the maximum possible work extractable
from the thermal energy liberated through burning the fuel. On the other
hand, as discussed in Chapter 4, the isothermal mixing of ideal gases is in
principle equivalent to the expansion of individual gases from an initial pres-
sure to a final pressure; a process which according to the first law, involves an
amount of heat identical to the expansion work of the gases. If one desires to
account for the mixing of the flue gases with the atmospheric air as an addi-
tional source of entropy generation, it would also be necessary to take the
heat effect of the mixing process (at the ambient temperature) into consid-
eration. To summarize, it is irrelevant to calculate the fuel chemical exergy
based on the atmospheric concentrations of the individual gases released
from combustion of fuel.
The maximum theoretical work, or chemical exergy, extractable from a
unit mole of fuel in a combustion-driven heat engine (see Table 8.1) is com-
pared with the traditional chemical exergy in Table 11.1 for several fuels.
The chemical exergy, ψ , is a few percentages less than the traditionally
ch
defined chemical exergy, ξ ch . The specific work production of a power plant
driven by combustion of a fuel should indeed be compared to ψ in order to
ch
have an accurate measure of performance from a second law perspective.