Introduction - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 23

1.2.4 T HERMODYNAMICS OF H IGH T EMPERATURE A IR C OMBUSTION

It is quite reasonable to use exergy loss in precise evaluation of the usefulness of a
thermal system aiming at the transformation of heat energy into power as well as
to define its theoretical limit for thermal efficiency. However, when we utilize heat
in heating furnaces, we usually use the specific units of fuel consumption instead
of thermal efficiency units, because there are always nonequilibrium processes
represented by, for example, heat conduction. Nevertheless, a thermodynamic anal-
ysis can be applied if we assume a quasi-equilibrium change for the heating process,
and such thermodynamic discussion can be useful in evaluating the ability of energy
saving of the process.
The available energy of fuel E av is the difference between Gibb's energy of
reactant (fuel and air) at ambient condition and that of burned product at the same
condition:

E av ≡ Ex I = G I - G E

Exergy, Ex , and Gibb's free energy, G , are expressed by using temperature, T ,
enthalpy, H , and entropy, S :

Ex = H - H 0 - T 0 ( S - S 0 ) (1.1)

G = H - TS (1.2)

where subscript 0 denotes the ambient condition. Of course, there must be an
exergy change of mixing between fuel and air, because the initial value of exergy
was estimated from the separated fuel and air. However, because it varies depend-
ing on the equivalence ratio and because the exergy change due to mixing is
relatively small as compared with that of combustion, it will be ignored here to
simplify the explanation.
Thermodynamic analysis is conveniently made using H - T diagram shown in
Figure 1.12 , where the ordinate is enthalpy of unit mole (specific enthalpy) and the
abscissa is temperature. The Hg and Hp curves are the relation of enthalpy of
unburned mixture and its preheat temperature and the relation of enthalpy of chem-
ically equilibrated burned product and its temperature, respectively.
Ordinary combustion of non-preheated mixture is ideally considered to be an
isenthalpic change, and it is exemplified by the change from the initial point I on
the Hg -curve to the theoretical burned point F . In general cases associated with a
heat loss, the final point F c is somewhat lower than point F . If the mixture is preheated
to the point H , the burned point will be F r the temperature that increases with
preheating. However, the Hp -curve increases exponentially in the high temperature
range because of the increase of apparent specific heat caused by thermal dissociation
in product, which makes the heat of combustion less. Since it becomes zero at the
crosspoint of the two curves, L , which corresponds to the adiabatic limit temperature,