Environmental Engineering Reference
In-Depth Information
Figure 2.2.2
Exergy components including the mechanical exergy (ezergy).
0) has the traditional physical
exergy
b
, expressed by formula (2.2.15), equal to the work which could be done by
the gas during the equalizing of its parameters,
T
and
p
, with respective environment
parameters
T
0
and
p
0
.
Exergy definition postulates the exergy to be the maximum possible work. There-
fore, the larger work of the two,
b
b
+
On the other hand, the gas at the actual altitude (
x
=
b
H
or
b
, is the true exergy called the mechanical
exergy.
b
m
=
max[(
b
b
+
b
H
),
b
]
(2.2.18)
Including the mechanical exergy (eZergy) into consideration, the scheme of the all
components of the exergy of substance is now shown in Figure 2.2.2.
The eZergy is applied only for the substance (e.g. not for heat or radiation) and it
replaces the two traditional exergy components: physical (
B
ph
) and potential (
B
p
). To
better distinguish exergy of the substance from the eZergy of the substance, different
symbols could be used:
B
for eXergy and
Z
for eZergy, (e.g. in paragraph 2.4.3:
B
m
≡
Z
=
f
(
B
ph
,
B
p
).
2.2.3.3 Chemical exergy of substance
In a chemical process, in contrast to a physical process, substances change and only
the chemical elements remain unchanged. Therefore, to calculate the chemical energy
(or exergy) of substances the reference substances have to be assumed appropriately.
The most common methods for determination of the chemical energy of substances
are
enthalpy formation
and
devaluation enthalpy
, which differ mainly by the definition
of reference substances. In the enthalpy formation method, the reference substances
are the chemical elements at standard temperature and pressure.
In the devaluation reaction method the number of reference substances is the same;
however, they are not the chemical elements but the devaluated substances (compounds
or chemical elements most commonly appearing in the environment). For example, the
reference substance of C is gaseous CO
2
, for H it is gaseous H
2
O, and for O it is just
O
2
. Therefore, in any particular case, when a substance is composed only of C, O, H,
N, and S, then the devaluation enthalpy of the substance is equal to its calorific value.
Contrary to the devaluation enthalpies, the values of the enthalpy of formation
are a little illogical. For example, the enthalpy of formation for valuable pure carbon
C is zero and the enthalpy of formation of not valuable CO
2
is significantly different
from zero (
−
394 MJ/kmol). However, for comparison, the devaluation enthalpy of C
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