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

Engineering Application of Exergy Analysis: Gas
Recovery System in Steel Industry
Sergio Usón a , Javier Uche a , Juan José Arribas b , Rocío Llera c and Alicia Valero a
a CIRCE Institute Universidad de Zaragoza, Zaragoza, Spain
b Arcelor-Mittal España, Avilés, Spain
c Project Engineering Area of University of Oviedo, Oviedo, Spain
Abstract: The Basic Oxygen Furnace (BOF) for steel making produces a stream of waste gas at high
temperature which contains carbon monoxide. Accordingly, its proper recovery entails important
energy savings in the steelmaking process. Exergy analysis is applied in this paper to analyze a
system designed to use both thermal (in order to produce steam) and chemical energy of this gas. The
system is characterized by transient operation and storing of both gas and steam. Actual plant data is
used to calculate time evolution of exergy flows and irreversibilities. Besides, integral indicators are
defined for characterizing the whole recovery cycle. The analysis provides important conclusions for
improving plant operation, in order to maximize recovered exergy.
Keywords: exergy analysis, gas recovery, basic oxygen furnace
1. Introduction
According to the Best Available Techniques
Reference Document (BREF) on Iron and Steel
[1], ‘Steel sector consume huge amounts of
energy: specific consumption of 23 GJ/t of liquid
steel in EU plants in 1980 has been reduced to 18
GJ/t in modern plants in 2004, but still strong
reductions are required’. In this framework, steel
companies are making a big effort to reduce
energy consumption in order to decrease
environmental impact and to improve their
competitiveness.
Besides the development of innovative
steelmaking processes and the improvement of
existing ones, other ways of obtaining energy
savings can be considered. Examples of them are
the recovery of waste streams (both energy and
materials) and the development and application of
advanced analysis techniques, such as the concept
of exergy [2, 3].
Exergy is a convenient magnitude for the analysis
of systems and processes within steel industry for
two main reasons. First, it is able to measure in
homogeneous units not only different forms of
energy but also different kinds of materials.
Second, it takes into account not only losses
through the system boundary but also
irreversibilities within the processes. For this
reasons, exergy analysis has been applied in steel
industry for accounting energy and materials flows
[4] and for improving energy efficiency [5]. This
analysis has been applied not only to global
analyses, but also to specific issues. For example,
Corresponding Author: Sergio Usón, Email: suson@unizar.es
it has been used for analyzing an electric arc
furnace [6], for proposing innovative methods for
improving hot gas stoves efficiency in blast
furnaces [7], or for analyzing methods for recover
gas streams [8]. Not so related to steel works but
also with steel is the application of exergy for the
life cycle analysis of this material [9].
In this paper, exergy is applied to the analysis of
the operation of a system for the recovery of gas
from Basic Oxygen Furnace (BOF gas), in order to
detect possibilities for improving its efficiency.
The gas recovery system not only recovers the gas
but also produces steam, which is used in steel
treatments and can also be exported to the
steelworks steam net [10]. Furthermore, both
steam and gas can be stored.
Proper operation of such system is a complex
issue, and the analysis developed here can shed
light to improve it. The approach developed
consists of two complimentary parts. First, actual
operation data is used to obtain the time evolution
of exergy-based parameters, which allows a
detailed analysis of each recovery cycle. Second,
integral cycle indicators are defined for
characterizing the whole cycle, in order to
compare it with other ones.
2. Development of the model.
2.1. Description of the system
A scheme of the gas recovery system is depicted in
Fig. 1. Numbers 1 to 10 are used for gas flows,
while numbers from 101 on refer to water or
steam.
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2
EXTERNAL AIR
103
M
101
DRUM
HRSG
3
1
M
104
P
102
COND.
1
COND.
2
ACC.
1
Gas leaving the converter (flow 1) reacts with
external air entering through the hole between the
converter and the heat recovery generator (flow 2)
to form flow 3, which is cooled in the heat
recovery steam generator (HRSG) down to point
4. It should be noted that part of the burning
reactions take place inside the HRSG, but here
reaction and heat transfer have been separated for
simplicity. Besides, the operator has two ways of
controlling the amount of external air (under
certain limits). First, the hood that directs gases
towards the heat recovery steam generator can be
moved upwards and downwards (and thus the gap
between the converter and the hood can be
modified). Second, position of venturi located
downstream can also be modified.
Gas leaving the HRSG (flow 4) is washed and
cooled in two washing venturi. Afterwards, a third
venturi is present to measure the gas flow, and a
blower establishes the draft needed to impulse the
gases. Finally, a three-way valve is located in
ACC.
2
4 5 6 7 8 10
WASHING WASHING VENTURI
BLOWER GASHOLDER
VENTURI 1 VENTURI 2
GAS
T X
T T
M
T
GAS FROM
CONVERTER
TO STEEL
TREATMENT
131
105
106
109 110
M M
111 112
107
108 114
113
121
131
117
122
M
VENT
132
133 134
115 116
118 119
123 124
ACC.
3
120
125
126
129
ACC.
4
128
130
FLARE
Fig. 1. Flow scheme of the gas recovery system.
127
9
136
DEAERATOR
138
STEAM TO NET
135
137
order to choose whether the gas is flared (flow 9)
or stored in the gasholder (flow 10). In this choice,
quality requirements to store gas are considered.
Heat released during gas cooling within the HRSG
is used to transform saturated water from the drum
(flow 101) into a mixture of liquid and steam
going back to that component (flow 102). Part of
the steam produced in the drum (flow 103) is used
for steel treatments (flow 104). Another part flows
through a pressure regulator and then can flow
towards the general steam network of the
steelworks (flow 135) or be used by the deaerator
(flow 136). It should be noted that it is possible to
import medium pressure from the steam network;
accordingly, flow 135 can have two senses.
Steam generated in the drum can also be stored in
four accumulators for later use (flows 114 to 120).
Besides, if there is excess of steam, it is possible to
condense part of it in two condensers (flows 109
and 110). For safety reasons, if pressure increases,
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P
M
P
M
M

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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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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The exergy of gas which is recovered in the
gasholder is shown in Fig. 3. In this figure and in
the following ones, two families of points are plot:
the absolute value (left axis) and the relative value
(according to 5 or 6) in the right axis. Due to
quality requirements of gas to be stored (e. g.
minimum amount of CO), gas recovery only takes
place during part of the cycle. It should be noted
that the exergy of this gas represents between 50
and more than 80% of exergy of flow 1.
B[10] (kW)
300000
250000
200000
150000
100000
50000
0
B[10] (kW) eta[10] (‐)
0 200 400 600 800 1000 1200
Time (s)
Fig. 3. Exergy of recovered gas.
B[9] (kW)
250000
200000
150000
100000
50000
0
B[9] (kW) phi_flare (‐)
0 200 400 600 800 1000 1200
Fig. 4. Exergy of flared gas.
Time (s)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
The part of the gas which is flared is represented
in Fig. 4. This amount represents more than 50%
of flow 1 before the recovery but falls at the end of
the cycle. This result indicates that it is very
interesting to extend the duration of the recovery
as much as possible and to flare gas only when this
is the only possibility.
As explained previously, the system not only
recovers gas, but also produces steam. Figure 5
shows the evolution of exergy of steam used in
several steel treatments (flow 104). This amounts
eta [10] (‐)
phi_flare (‐)
around 12 MW, which corresponds to less than
10% of the exergy of converter gas.
B[104] (kW)
14000
12000
10000
8000
6000
4000
2000
0
B[104] (kW) eta[104] (‐)
0 200 400 600 800 1000 1200
Time (s)
Fig. 5. Exergy of steam to steel treatments.
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Exergy of steam exported to the steam net of
steelworks is presented in Fig. 6. First, the system
exports 1.5 MW of steam, which are progressively
reduced and afterwards more than 2 MW are
imported (negative value). These amounts are less
than 2% of the exergy of flow 1.
B[135] (kW)
2000
1500
1000
500
0
‐500
‐1000
‐1500
‐2000
‐2500
‐3000
B[135] (kW) eta[135] (‐)
0 200 400 600 800 1000 1200
Time (s)
Fig. 6. Exergy of steam exchanged with the net.
0.06
0.04
0.02
0
‐0.02
‐0.04
‐0.06
‐0.08
‐0.1
Results presented above have shown the variation
of the exergy of several flows leaving the system.
However, a key advantage of the use of exergy is
that it allows one to consider also losses of energy
quality (irreversibilities) taking place inside the
different component.
Figure 7 represents the exergy destroyed in the
combustion process. The low value of this variable
(around 2%) can be explained by considering two
points. First, only a fraction of the CO contained in
the gas is burned (the other part contributes
strongly to the chemical exergy of gas which is
either stored or flared, as it can be seen in Fig. 3
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eta[104] (‐)
eta[135] (‐)

and 4). Second, due to the high temperature of
flow 1, the combustion takes place at a high
temperature (above 1400 ºC), which reduces
substantially irreversibility due to heat transfer.
I_combustion (kW)
6000
5000
4000
3000
2000
1000
0
I_combustion (kW) phi_combustion (‐)
0 200 400 600 800 1000 1200
Time (s)
Fig. 7. Irreversibility of combustion.
I_HRSG (kW)
40000
35000
30000
25000
20000
15000
10000
5000
0
I_HRSG (kW) phi_HRSG (‐)
0 200 400 600 800 1000 1200
Time (s)
Fig. 8. Irreversibility in the HRSG.
0.1
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Irreversibility in the heat recovery steam generator
is plotted in Fig. 8. These losses can be as high as
30 MW, or more than 15%. They might be
reduced by considering another design of the
boiler, producing steam of different levels of
pressure. It should be noted that the separation of
combustion and heat transfer has been made to
simplify the model. Accordingly, the actual
distribution of losses between these two processes
is not exactly the same as the values calculated
here.
Figure 9 represents irreversibility produced in the
first washing venturi, where gases are cooled from
more than 700 ºC down to less than 100 ºC. This
irreversibility is around 6% of the exergy of flow
1, and might be reduced by modifying the design
of the HRSG in order to use a higher part of the
thermal exergy of gases.
phi_combustion (‐)
phi_HRSG (‐)
I_venturi1 (kW)
20000
18000
16000
14000
12000
10000
8000
6000
4000
2000
0
I_venturi1 (kW) phi_venturi1 (‐)
0 200 400 600 800 1000 1200
Time(s)
Fig. 9. Irreversibility in the first washing venturi.
0.14
0.12
0.1
0.08
0.06
0.04
0.02
Finally, Fig. 10 shows how 2.5 MW of exergy are
destroyed without use in the steam condensers.
This amount may seem not too high compared to
flow 1 (between 1 and 2%), but represents around
20% of the exergy of steam used in steel
treatments (flow 104). This value only depends on
operation strategy and can be reduced substantially
by a proper management of steam storage and
steam export to plant net.
I_condensers (kW)
3000
2500
2000
1500
1000
500
0
I_condensers (kW) phi_condensers (‐)
0 200 400 600 800 1000 1200
Time (s)
Fig. 10. Irreversibility in the condensers.
3.2. Integral cycle indicators.
0
0
0.035
0.03
0.025
0.02
0.015
0.01
0.005
In this section, parameters defined in (7-10) are
used to summarize the performance of the whole
recovery cycle. The main flows appear in Table 1,
and the main components in Table 2.
Table 1. Integral cycle indicators for the main flows
Flow Bcycle (GJ) η cycle(-)
1, gases from converter 193.31 1.0000
9, flared gas 82.92 0.4290
10, recovered gas 59.70 0.3089
104, steam to treatments 11.55 0.0597
135, steam to net -1.686 -0.0087
Accumulated steam 8.438 0.0437
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phi_venturi1 (‐)
phi_condensers (‐)

Table 2. Integral cycle indicators for the components
Component Icycle (GJ) Φcycle (-)
HRSG 24.58 0.1271
Venturi 1 12.30 0.0636
Combustion 4.03 0.0209
Condensers 2.44 0.0126
The analysis of these indicators allows one to draw
important conclusions regarding the efficiency of
the system. First, the most important part of
exergy (more than 70%) is available in gas flow
which is stored in a gasholder (30.1%) or flared
(42.9%). Accordingly, the minimization of gas
flared is a key point in increasing efficiency of the
system. The first action towards this goal is to
avoid flaring when the gas quality is suitable for
storing. The second step is to optimize the
operation of the system in order to increase the
time span when gas accomplishes the requirements
for being recovered.
11.55 GJ of steam are consumed for steel
treatments, while around 1.7 GJ are imported (this
is the meaning of the minus sign) from the general
steam net of the steelworks. In this context, the
2.44 GJ of exergy lost in the condenser represent a
big potential of steam savings. Accordingly,
operation of the steam part of the system should be
improved to reduce this amount. Importance of
these savings can be even larger, because if steam
is properly used, operation can be more easily
adapted for maximizing the gas recovered.
12.7% of exergy of entering gas is destroyed in the
heat recovery steam generator and 6.4% in the first
washing venturi. Accordingly, new developments
of gas recovery systems may include more
advanced HRSG producing steam at several
pressures. Irreversibility in the combustion process
is small (2.1%) because it takes place at high
temperatures and only affects a part of the heating
value of the gas. It should be noted that all these
values correspond to a single cycle, and they vary
according to the profile of gases leaving the BOF
as well as the operation of the recovery system.
4. Conclusions
Exergy analysis has been applied to point out the
energy savings potential of a system as complex as
an installation for BOF gas recovery, which also
produces steam and includes storage of both. The
approach comprises two parts: a detailed analysis
of the evolution of exergy and irreversibility, and
the calculation of integral parameters for
characterizing the whole recovery cycle. These
integral parameters are the key point of the paper,
because they allow to rigorously summarize the
behavior of batch processes like the one studied
here.
Results obtained in the analysis of a single cycle
point out that there is strong potential for
improvement, not only in the reduction of gas
flared but also in the operation of the steam section
(e.g. condensers). Due to the use of exergy, all
these losses can be directly compared.
Although these results correspond to an example
of converter cycle, the comparison of the integral
parameters corresponding to the operation during
several months (including different operation
strategies) will provide useful information to
improve the exergy efficiency of the system and
thus the steam and gas recovered. This comparison
may include the separation of exergy lost by flared
gas into causes (due to gas low quality or to
improper operation). Furthermore, the
development of a thermoeconomic model based on
exergy calculated here may highlight more clearly
the potential points for improving.
Nomenclature
.
B exergy, kW
Bcycle exergy in a cycle, kJ
BOF basic oxygen furnace
BREF Best Available Techniques Reference
EU
Document
European Union
fm molar rate, kmol/s
h specific enthalpy, kJ/kmol
HRSG heat recovery steam generator
I irreversibility, kW
Icycle irreversibility in a cycle, kJ
M flow rate, kg/s
n number of substances in a flow
P pressure, Pa
R constant of gases, kJ/(kmol·K)
s specific entropy, kJ/(kmol·K)
T temperature, K
X,x composition
Greek symbols
η efficiency
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φ non-dimensional irreversibility
Subscripts and superscripts
ch chemical
i flow
j substance in a flow
k component
ph physical
0 reference conditions
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