Engineering Application of Exergy Analysis - circe - Universidad de ...

where xi,j is the molar fraction of component j in
flow i, bch,j is the specific exergy of component j
and R is the constant of gases. The second part of
the equation refers to the difference between the
exergy of the different components of a mixture
isolated, and the exergy of the components all
mixed together. Szargut reference state [2] has
been used for the calculation of chemical exergy.
After calculating the exergy of all flows, it is
possible to obtain the irreversibility of each
component (Ik). It should be noted that, in this
paper, irreversibility will also include losses
(external irreversibilities).
I = B − B
, (4)
∑ ∑
k i i
inputs outputs
In order to put in perspective the values of exergy
and irreversibility obtained, two non-dimensional
parameters have been defined. First, several
efficiencies calculated by dividing the exergy of
the outputs of the system into the main input
(exergy of gases leaving the converter):
Bi
η i =
B
, (5)
1
Besides, irreversibility of components is made
non-dimensional by dividing it also into the exergy
of flow 1:
I
k ϕ k = , (6)
B
1
2.4. Parameters for characterizing the
whole cycle.
Exergy flows, irreversibility and non-dimensional
parameters defined in the previous section vary
with time, and thus they are suitable for a detailed
study of a given recovery cycle. However, in order
to summarize the results of a cycle and to compare
it with others, integral indicators characterizing the
whole cycle are needed.
Accordingly, the exergy of a flow i over a cycle
(kJ) is defined as:
Bcycle, i = ∫ Bi() t ⋅dt
, (7)
cycle
Besides, the irreversibility in a component k over a
cycle (kJ) is defined as:
Icycle, k = ∫ Ik() t ⋅dt
, (8)
cycle
Finally, the non-dimensional parameters η and φ
can also be calculated for the whole cycle:
η
ϕ
cycle, i
cycle, k
cycle,1
1
cycle
()
B i t dt
B ∫ ⋅
cycle, i cycle
= =
, (9)
B B t ⋅ dt
∫
∫
cycle,1
1
cycle
()
()
Ik t ⋅ dt
Icycle,
k cycle
= =
, (10)
B B t ⋅ dt
3. Results
∫
()
The model described above is being applied for
the study of the operation of the gas and steam
recovery system by using information (detailed in
Fig. 1 and at the beginning of section 2.2), which
is stored by the plant information system every 16
seconds. In this section, the most important results
corresponding to an example of a gas recovery
cycle are presented. First, evolution of both exergy
and irreversibility is presented, and then the cycle
is summarized by the integral cycle indicators.
3.1. Exergy and irreversibility versus
time.
Figure 2 shows the evolution of the exergy of flue
gases leaving the BOF during a cycle. It can be
seen how this amount increases during the first
half of the cycle and decreases later, with smooth
variations according to the blowing pattern of
oxygen inside the converter.
B[1] (kW)
350000
300000
250000
200000
150000
100000
50000
0
0 200 400 600 800 1000 1200
Time (s)
Fig. 2. Exergy of gases leaving the converter.
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