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

part of the steam can be vented (flow 132,
normally closed).
Liquid water from condensers and accumulators
(flows 111-113, 121-127) can be pumped and
returned to the drum. Besides, the drum can be fed
from external fresh water (flow 137), once the
deaerator has passed.
2.2. Thermodynamic model.
Figure 1shows plant instrumentation used for the
characterization of the thermodynamic state of the
system: flow rates (M), pressures (P), temperatures
(T) and gas composition (X). The latter refers to
concentration of CO, CO2 and O2 in dry basis.
Although they have not been represented in the
figure, levels of the four accumulators and position
of three-way valve are also available.
2.2.1. Gas flows
To characterize flows 3 to 10, measurements of
flow rate and dry gas composition are available. In
order to calculate the amount of water, it has been
considered that gas is dry in flows 3 and 4 and that
is water saturated in flows 5 and 6. In the other
flows, temperature increases slightly and no water
is added. Accordingly, it has been considered that
the flow of water is kept constant. Nitrogen
concentration is calculated by difference and, since
it is no reaction, flow of CO, CO2, O2 and N2 is
maintained in flows 3 to 8. Depending on the
position of the three way valve, flow 8 continues
either in flow 9 or in flow 10.
In order to calculate flows 1 (converter output) and
2 (external air), a balance to carbon, nitrogen and
oxygen is made. Besides, composition of 1 is
known (air) and it has been considered that gas
flow 2 does not contain significant amounts of
CO2 or O2.
There are temperature measurements in flows 4 to
8 (except in point 7, which has been supposed to
be equal to 6). Temperature of flow 2 is equal to
the environment (considering that the control
volume is far enough from the hot area around the
converter), and temperature of flow 1 is calculated
by energy balance of the HRSG.
2.2.2. Water/steam flows
First, there is no measurement of the flow of
saturated liquid leaving the drum towards the
HRSG (flow 101); accordingly, it is fixed by the
design values. Since flows 131 and 103 are
known, mass accumulated in the drum is
calculated. Due to available measurements in
flows 104 and 135 (and assuming that no steam is
vented), it is possible to calculate flows 105, 106,
107, 133 and 134. Besides, flows entering the
condensers are also measured.
In order to calculate flows corresponding to the
accumulators, rate of level variation is used. This
rate can be obtained because evolution of all
signals is available. Besides, it has been supposed
that the amount of water leaving all accumulators
is the same. Finally, measurement in 137 allows
one to calculate mass accumulation in the
deaerator.
To calculate intensive properties, three pressure
zones have been considered:
▪ Drum (high pressure): flows 103 to 133
▪ Steam network (medium pressure): flows 134
to 136
▪ Deaerator: flows 137 and 138
Besides, isentropic efficiency is imposed for
pumps. Finally, an equation is introduced relating
matter and energy accumulation in the drum in
order to calculate the quality of flow 102 (which,
in turn, allows one to calculate the temperature of
gases leaving the converter by the energy balance
of the HRSG).
2.3. Exergy analysis.
Once all flows defined in Fig. 1 have been
characterized as described in the previous sections,
exergy analysis can be performed [2,3]. Exergy of
a flow i is composed of two parts: physical and
chemical:
B = B + B
, (1)
i ph, i ch, i
Physical exergy appears because the flow has
different conditions of temperature and/or pressure
that the reference environment:
( ) ( )
B ph, i = fmi⋅⎡hih0, i T0 sis ⎤
⎣
− − ⋅ − 0, i ⎦
, (2)
where fm is the molar rate, h is specific enthalpy
and s is specific entropy. Properties are evaluated
at the conditions of flow i and at reference
conditions 0 (but for the same composition of flow
i).
Chemical exergy is due to the composition of the
flow (different than that of the environment):
n
∑
( ln )
B = fm ⋅ x b −R⋅T ⋅ x , (3)
ch, i i i, j ch, j 0 i, j
j=
1
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