Applied Thermal Engineering 120 (2017) 402–415
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Applied Thermal Engineering
journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Experiment and optimization of a new kind once-through heat recovery
steam generator (HRSG) based on analysis of exergy and economy
Jinbo Li, Kunyu Wang, Lin Cheng ⇑
Center of Thermal Science and Technology, Shandong University, Ji’nan 250061, Shandong, China
h i g h l i g h t s
A novel U-typed once-through HRSG is presented for the low temperature heat source.
Heat utilization and exergy of HRSG are analyzed by experiments in a cement plant.
HRSG-flash system is modeled and validated to find the optimal system solution.
Exergy and economic research are conducted and contrasted for different system.
a r t i c l e
i n f o
Article history:
Received 14 January 2016
Revised 11 January 2017
Accepted 6 April 2017
Available online 8 April 2017
Keywords:
Heat recovery steam generator (HRSG)
Exergy efficiency
Flash system
Operating optimization
Thermal equilibrium analysis
a b s t r a c t
In this work, we proposed a novel once-through heat recovery steam generator (HRSG) which could be
used for low temperature heat resource recovery. Experiments have been done in a cement plant under
different conditions to study its thermal performance. Exergetic and economic analyses of the HRSG have
been performed. We also built a mathematical model based on the energy and mass balance equations.
Moreover, a flash tank is implemented in this study and optimized researches are carried out to find the
best exergy efficiency of the HRSG. Multiple sets of optimized results are compared and experiments have
been done to show the rationality of the model. Results show that the HRSG is highly efficient in recovering energy from low temperature heat source. Higher feed water flow rate is more suitable for the
working conditions than the lower one. When the flow rate is around 16 m3/h and the diversion water
temperature is around 413 K, the highest exergy efficiency of the generating system and the HRSG can
be up to 44.43% and 54.61%, respectively. The economic analysis result shows that annual income can
be 121,242 € and the construction cost can be recovered in 1.1 years.
Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
Energy shortage is a serious problem in social development.
Energy recycling has become the focus of the energy industry. Heat
recovery steam generator (HRSG) is one of the most important
parts in recovering waste heat. Analysis and optimization of HRSG
is a significant subject due to the increasing fuel prices and
decreasing fossil fuel resources [1,2].
Increasing thermal efficiency and steam quality of HRSG has
interested numerous researchers. The optimum application of
energy and the energy consumption management methods are
also very important. Hence, the thermodynamic [3], exergoeconomic [4], exergetic [5,6] and exergoenvironmental [7] analysis
and optimizations have been used widely in thermal systems. Con-
⇑ Corresponding author.
E-mail address: cheng@sdu.edu.cn (L. Cheng).
http://dx.doi.org/10.1016/j.applthermaleng.2017.04.025
1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
sidering minimizing the generating cost or maximizing the annual
cash flow of the plant, the structure of heating surface [1,8] is also
analyzed and optimized. Soft computing methods can be used to
optimize model parameters over a full range of input and output.
For design and operation of HRSG and other heat exchangers,
genetic algorithm (GA) [4], particle swarm algorithm (PSO) [9]
are the most widely used as optimization methods.
For the optimization of the heating surface and the specific
components of the HRSG, some scholars have put forward some
new models or methods. Feng et al. [10] have presented a new
algorithm model of multi-pressure HRSG. They analyzed the model
characteristics by changing different feed water/steam flowing
route and optimizing heat exchangers layout. Hanafizadeh et al.
[11] have presented part elimination lattice search (PELS) method
to optimize the HRSG inlet duct geometry. They also applied this
method to improve a 5 MW HRSG inlet duct and enhanced the flow
uniformity and efficiency. In and Sang [12] presented a new
J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
403
Nomenclature
c
C
CE
_
Ex
h
K
_
M
P
s
T
vp
_
W
gex
gex, HRSG
gpump
specific heat capacity
cost (€)
unit price of electrical energy
exergy flow rate (kW)
specific enthalpy (kJ/kg)
average investment cost of HRSG equipment
mass flow rate (kg/s)
pressure (MPa)
specific entropy
temperature (K)
specific volume (m3/kg)
net power (kW)
exergy efficiency of the generating system (%)
exergy efficiency of HRSG
efficiency of the pump
approach to find the optimum design parameter of a single pressure HRSG. It could maximize the exergy efficiency by minimizing
the unavailable exergy. Sharma and Singh [13] have done the
exergy analysis of the dual pressure HRSG for varying experimental
steam conditions and dead states. They concluded the HP evaporator was major source of irreversibilities, which were useful to find
the thermodynamic states and reduce the exergy destruction.
Mansouri et al. [14] performed an economic and exergetic study
for a double pressure and two triple pressure HRSGs (with and
without reheat) in a gas-steam combined cycle power plant. They
estimated the exergy loss of each component of the HRSGs. The
result showed that an increase in the number of pressure levels
caused a tangible increase in exergy efficiency of the system.
Dumont and Heyen [15] built a mathematical model of an
advanced once-through HRSG. They used this model to study the
thermodynamic characteristics under high temperature and pressure conditions. Rovira et al. [16] described a one-dimensional
model to simulated the performance of an once-through HRSG at
supercritical pressure. The model toke account the strong variation
of some thermal and transport properties of fluid and discussed
their influence. Vandani et al. [9] added a flash tank to recover
the energy content of blowdown water and made optimization
for extraction pressure. The results indicated that using blowdown
recovery technique, the net generated power and exergy efficiency
increased. Baig et al. [17] built a once-through multi-stage flash
distillation system. They have analyzed the effect of various design
and operating conditions on the performance ratio.
Most of the research and optimization of the HRSG were based
on theoretical studies and model calculation. Because of the fluctuation of the working conditions in the actual production, the input
parameters are difficult to match the optimized ones, and it is
almost impossible to achieve the optimal results. Hence, analysis
and optimization should be based on experiments and operating
conditions. Hanafizadeh et al. [18] have done experiments to optimize the HRSG inlet duct design. They also performed numerical
simulation to validate the experimental study and presented the
optimized inlet duct angle and movable plate length.
The cement industry is an industry that consumes a considerable quantity of resources and energy and has a very large influence on the efficient use of global resources and energy [19]. In
typical cement production process, the waste heat for the HRSG
comes from the grate cooler. It is the air mixed with cement clinker
particles after clinker cooling. The heat source can be divided into
three sections: high temperature air, intermediate temperature air
and low temperature air. For high temperature air, the temperature is about 500–900 °C, and it is mainly used as the secondary
Subscripts
0
environmental conditions
C
condensate removal pump
dp
diversion point
flash
flash system
HRSG
HRSG system
in
inlet
out
outlet
r
rated parameter
s
steam
w
water
air and tertiary air respectively into the rotary kiln and calciner
[20,21]. For intermediate temperature air, the temperature is about
300–500 °C, and it is mainly used in cement plant double-pressure
and multi-pressure HRSG for waste heat power generation [22].
For the low temperature air in our experiment cement plant, in
addition to low temperature (less than 300 °C), it also has the characteristics of small flow rate (less than 30,000 N m3/h) and high
dust content. Mixing it with intermediate temperature air will
reduce the quality of waste gas. Designing a single pressure HRSG
individually comes with high cost, poor economy and low efficiency. Hence, this exhaust air in cement companies is directly discharged into the atmosphere which results in low temperature
heat loss.
It is found from the literature survey that researchers focused
on the analysis and modeling of multi-pressure HRSG and supercritical once-through HRSG. Once-through HRSG which is suitable
for low temperature conditions is in the blank stage. The present
work has been carried out to fill this gap by designing and building
a novel once-through HRSG-flash system. In the previous paper,
the author has analyzed the advantage of U-type HRSG heating
surface in inhibiting the particle deposition [23]. In this article,
we will analyze its experimental performance in terms of economy
and exergy and do operating optimization under low temperature
conditions. The following are the specific contributions of this
paper in the subject matter area:
To present a U-typed once-through HRSG for the low temperature heat source.
To verify its feasibility and analyze the utilization of waste heat
and heat exchange on different heating surfaces by experimental researches in a cement plant.
To construct HRSG-flash system model and optimize system
configurations to find the optimal cogeneration system
solution.
To conduct the exergy and economic aspects of the research and
contrast the results of different constructions.
2. Energy analysis and exergy analysis
The temperature profile of the HRSG and the energy consumption of each heating element are estimated to study the HRSG performance. For one heating surface, the energy balance equation of
exhaust gas and feed water are given in Eq. (1) [6]:
_ w ðhw;out hw;in Þ
_ g cg ðT g;in T g;out Þ ¼ M
M
ð1Þ
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J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
_ T and h are the mass flow rate, absolute temperature and
where M,
specific heat, respectively. Subscript (w) and (g) refer to water and
gas. It must be mentioned that the exhaust gas for the HRSG is the
hot air from grate cooler. Hence, the specific heat at constant pressure, cg is considered as temperature-related variable as follows [24]:
3:8371
9:4537 2
cg ðTÞ ¼ 1:04841 T
þ
T
107
104
5:49031 3
7:9298 4
þ
T
T
1010
1014
ð2Þ
Exergy is defined as the maximum theoretical useful work,
which means the work obtained by heat source is completely
cooled to ambient temperature. It is defined as follows [6]:
_ ¼M
_ ½ðh h0 Þ T 0 ðs s0 Þ
Ex
ð3Þ
where s is the specific entropy and subscript (0) refers to environmental conditions.
In reversible systems, it is also the theoretical work done by the
temperature of waste heat completely cooled to the ambient temperature T0, which is obtained from infinite number of small Kanocycle cumulative, as Eq. (4):
Z
Z
_ g T T 0 dT
cg M
T
T0
T0
_ g T g;in T 0 T 0 ln T g;in
¼ cg M
T0
_ ¼
Ex
T g;in
dEx ¼
T g;in
ð4Þ
Exergy efficiency of the generating system is given by:
X
gex ¼
n
_n
W
ð5Þ
_
Ex
P _
n W n is the net power, as the following:
X
X
_n¼
_ n ðhn hc Þ
W
M
n
ð6Þ
n
To calculate the exergy efficiency of HRSG, the following Eq. (7)
is given by:
_
Ex
_
Ex
gex;HRSG ¼ _ s;out _ w;in
Exg;in Exg;out
ð7Þ
where subscript (s) refers to steam.
3. Performance experiment of the U-type once-through HRSG
3.1. Introduction for experiment
According to the basic characteristics of the cement plant waste
heat, our research group has designed and installed a novel oncethrough HRSG with U-type structure. It is consist of 8 heating surfaces, named as Evaporator (EVA) 1–3 and Economizer (ECO) 4–8.
The experimental apparatus picture and the schematic diagram of
the heat-recovering system are shown in Fig. 1. The size of a single
heating surface is 2.5 m 2.5 m 2.5 m. It consists of 48 serpentine tube screens (£38 mm 3.5 mm each) and 2 headers.
The exhaust gas is extracted by an induced draft fan. During the
experiment, the gas flows into HRSG from the flue entrance near
Evaporator 1 and continues going downward vertically until reaching the ash bucket at the bottom. After that, the movement direction of gas is changed to upward. Then it flows through Economizer
4–8 and dust collector. At last, it is discharged into the atmosphere.
Feed water is supplied by two centrifugal pumps from a 32 m3
cylindrical tank and the rated flow is 32 m3/h. The flow direction
of water is opposite to that of exhaust gas to ensure contraflow
heat transfer. Therefore, the feed water flows from Economizer 8
to Evaporator 1. After heated by 8 heating surfaces, it flows into
the steam-water separator. Then saturated steam is used to perform work in low-pressure cylinder of steam turbine, while
remaining water returns to tank for a new cycle of the experiment.
There are 6 platinum resistance thermometers configured in the
upper and lower parts of each heating surface to measure temperature of gas, water and steam. In order to ensure the accuracy of
temperature measurement, the average value is taken as the
experimental temperature. A turbine flow meter is installed at
the feed water entrance of boiler and a vortex street flow meter
is installed at the outlet of steam-water separator to measure the
feed water and steam production, respectively. Pressure sensors
are installed on each entrance of evaporative heating surfaces to
measure the pressure of saturated water. Detailed parameters of
the measurement instruments are summarized in Table 1.
3.2. Analysis of HRSG pitch point and approach point
The pinch point temperature is the difference between the saturation temperature and the gas temperature at the economizer
inlet. We have chosen two experimental conditions to reflect the
pitch point temperature. The temperature of U-type HRSG heating
surfaces is shown in Table 2 and Fig. 2(a) and (b).
It can be seen from the figures that the pinch point temperature
of the U-type once-through HRSG is 26.5 °C. The once-through
HRSG is different from the drum HRSG. The feed water gradually
completes the phase change in different heating surfaces instead
of drum. When the feed water starts to change from simple fluid
to steam/water mixtures, the transition curve is smooth without
obvious inflection point. It is shown as the curves between the
fourth and fifth red point in Fig. 2. When the pinch point temperature decreases, the outlet gas temperature of the HRSG will decline.
Then, the recovery of the waste heat from the exhaust gas will
increase, as well as the output of power of the steam turbine and
the HRSG efficiency. However, it will increase the heat exchange
area of the HRSG and reduce the economic performance. Hence,
the pinch point temperature is not allowed to be equal to 0 °C.
Otherwise, the heat exchange area of the HRSG will be infinite.
Approach point temperature refers to the difference between
the water temperature at the economizer outlet and the saturated
water temperature. If the approach point temperature increases,
the total heat transfer area of the HRSG will increase, and it will
lead to increase of the investment cost. If the temperature is too
small, under the low load condition or during the start-up period,
the feed water may vaporize in the economizer, which is not
allowed for the single pressure or multi-pressure HRSG. Hence,
the selection of the approach point temperature is a difficult task,
because it can lead to economic or operational problems. The drum
HRSG is not suitable for low load conditions.
For our working conditions in the cement plant, the exhaust gas
temperature is 200–300 °C, and mass flow rate is 20,000–
35,000 N m3/h, which are much smaller than the input parameters
of the HRSG in combined cycle power plant. It is a typical low load
conditions. Therefore, our research group designed a once-through
HRSG, which could avoid the choice of approach point temperature. For the once-through HRSG, the feed water is gradually changed into steam through the heating of the exhaust gas in the
heating surface. There is no obvious boundary between the economizer and the evaporator. Hence, there is no obvious approach
point temperature.
3.3. Experimental data and working process of the U-type oncethrough HRSG
In this article, thermal performance of the HRSG is studied
under different working conditions shown in Table 3.
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J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
P
F
Steam
turbine
T
T
T
P
Steam-water
separator
P
EVA 1
T
ECO 8
EVA 1
EVA 2
T
ECO 7
EVA 3
T
ECO 6
P
Condenser
Condensate
pump
P
Water tank
ECO 5
ECO 4
Steam flow
Water flow
Gas flow
F
P
Pressure transmitter
T
Temperature thermometer
F
P
T
Feed water pump
Flow meter
Fig. 1. Experimental apparatus picture and schematic diagram of the waste-heat-recovering system.
Table 1
Model, range and precision of experiment instrument.
Equipment
Model
Range
Unit
Precision
Thermal resistance
Turbine flow meter
Vortex street flow meter
Pressure sensor
Pt100
LWGY-80
DY100
511
0–450
0–100
0–8.3
0–1.6
°C
m3/h
t/h
MPa
±0.1 °C
±0.5%
±0.75%
±0.3%
Table 2
Inlet parameters of different operating conditions.
Experimental
condition
in
Exhaust gas inlet temperature, Tg,
(°C)
_g
Exhaust gas mass flow rate, M
(N m3/h)
Feed water inlet temperature, Tw,
in (°C)
_w
Feed water mass flow rate, M
(t/h)
A
B
286.50
333.15
23,800
23,900
96.6
74.3
4.56
5.17
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J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
300
350
280
exhaust gas
feed water
260
Temperature T (°C)
Temperature T (°C)
240
exhaust gas
feed water
300
220
200
180
160
250
200
150
140
100
120
100
50
0
200
400
600
800
1000
1200
1400
0
500
1000
1500
2000
Total heat transfer rate (kW)
Total heat transfer rate (kW)
(a)
(b)
Fig. 2. Temperature of feed water and exhaust gas on different heating surfaces: (a) condition A; and (b) condition B.
On the other, through the study in experiment, the measure
curves of temperature and steam production are obtained in
Figs. 3–5.
Fig. 3 shows that exhaust gas temperature of different heating
surfaces varies over time under test conditions. It can be seen that
temperature fluctuation of exhaust gas is obvious, especially for
the gas inlet temperature (EVA 1 Inlet). The reason is that the
exhaust gas is from the grate cooler. Temperature of the cement
clinker changes large, and exhaust gas temperature has the same
trend.
Fig. 4 shows the temperature of water/steam varies over time.
The comparison among feed water temperature turns out that
the stable temperature decreases from Evaporator 3 to Evaporator
1, while flue gas temperature and the quantity of heat exchange
have the opposite change. The reason is that in these three heating
surface, steam/water mixtures are the final products, whose temperature is determined by steam pressure. For single-phase heating surface (ECO 4–8), the pressure loss can be ignored. However,
flow resistance of steam is much greater than that of feed water.
In evaporative heating surfaces, the pressure drop is obvious,
which leads to the temperature decrease rather than increase, even
though the feed water has absorbed more heat and gas temperature is higher than that in economizers. Steam production
increases in spite of the gradual lowering of the water-vapor temperature from Evaporator 3–1. For the two operating conditions 1
and 2, the feed water pressure is 0.65 MPa, but there is a big gap
between the temperature and pressure of the final steam. The reason is that when the flow rate is around 4 m3/h, the feed water
reaches saturated state in Economizer 5 and then the HRSG begins
to produce steam. With the gradual increase of the steam/water
mixture, the pressure drop caused by the steam is gradually
increased (0–0.23 MPa). When the flow rate is around 16 m3/h,
steam/water will not be generated until in Evaporator 3, the pres-
Table 3
Average flow rate of exhaust gas, feed water and steam of typical working conditions.
Parameters
Unit
Condition 1
Condition 2
_g
M
_w
M
N m /h
32,222
29,000
m3/h
15.87
4.02
_s
M
t/h
2.44
1.82
3
sure drop is less than that under condition 2. The final steam temperature is decided by the pressure.
Fig. 5 shows the flow rate changing of feed water and steam
over time. It shows that the volume of feed water and steam is negatively correlated. This is because the flow rate and the temperature of the waste heat are low and certain. The more feed water,
the more waste heat can be utilized and the thermal efficiency
can be higher. But the more feed water, the less steam can be generated. When the flow rate is around 16 m3/h, the mass flow of
steam is around 2.5 t/h. When the flow rate is around 4 m3/h,
the mass flow of steam is around 1.8 t/h. For the two operating
conditions 1 and 2, the flow rate and inlet temperature of exhaust
gas are almost the same, but the final steam production have a
great difference. This is because less flow rate and lower velocity
under condition 2 reduce the convective heat transfer coefficient,
which makes the waste heat in the exhaust gas poorly utilized.
It is obvious that temperature and flow rate tend to be stable
after 325 min. Averaging all data during 325–375 min and the
result is given in Tables 4 and 5.
3.4. Experimental results and analysis
Figs. 6 and 7 show released heat of exhaust gas and exergy
destruction in different heating surfaces. For both condition 1
and 2, these two parameters of the heating surfaces show the similar trend. They both decrease gradually from Evaporator 1 to Economizer 8. The reason is that in the process of heat transfer, the
temperature difference between exhaust gas and feed water
decreases gradually, which is shown in Table 4. But for Economizer
8 under condition 2, the heat transfer and exergy destruction are
higher than those of Economizer 7. The water flow rate is around
4 t/h, and inlet water temperature is 90.61 °C. The inlet temperature difference between exhaust gas and feed water of Economizer
8 is 37.89 °C which is higher than that under condition 1 (9.75 °C).
Hence, the parameters are higher. From Fig. 6, it can be seen that
the highest heat exchange rate appears in Evaporator 1. From
Fig. 7, exergy loss in evaporator 1–3 is significantly higher than
that in the economizers. This is because feed water is in saturated
state in evaporators. The temperature difference between exhaust
gas and feed water is large, so as the irreversibility of the heat
exchange surface.
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J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
350
350
300
300
250
250
200
200
150
150
100
EVA 1 Inlet
EVA 2 Inlet
EVA 3 Inlet
ECO 4 Inlet
ECO 5 Inlet
HRSG Outlet
50
0
0
50
100
150
200
250
300
350
400
450
500
100
EVA 1 Inlet
EVA 2 Inlet
EVA 3 Inlet
ECO 4 Inlet
ECO 5 Inlet
HRSG Outlet
50
0
0
50
100
150
Time (min)
200
250
300
350
400
450
400
450
Time (min)
(a)
(b)
Fig. 3. Temperature curves of heat source under typical operating conditions: (a) condition 1; and (b) condition 2.
180
180
160
160
140
140
120
120
100
100
EVA 1 Outlet
EVA 2 Outlet
EVA 3 Outlet
ECO 4 Outlet
ECO 5 Outlet
HRSG Inlet
80
60
EVA 1 Outlet
EVA 2 Outlet
EVA 3 Outlet
ECO 4 Outlet
ECO 5 Outlet
HRSG Inlet
80
60
40
40
0
50
100
150
200
250
300
350
400
450
500
Time (min)
(a)
0
50
100
150
200
250
300
350
Time (min)
(b)
Fig. 4. Temperature curves of water and steam: (a) condition 1; and (b) condition 2.
Table 6 shows the exhaust parameters of steam turbine which
are measured in the cement plant. Available steam power can be
given according to the result calculated in Eq. (8). Exergy efficiency
of generating system and HRSG are defined as the following Eqs.
(9)–(11). The result is shown in Table 7.
From the experimental results, low temperature waste heat can
be recovered by the U-type once-through HRSG. Under the experimental condition, the power generated by the steam cycle is
327.35 kW, and the exergy efficiency of generating system can be
31.05%. When the feed water flow rate is low (condition 2), steam
production and available steam power are both low. Hence, utilization of waste heat is poor.
4. U-type once-through HRSG-flash system model equation and
optimization
Through the analysis of pitch point and approach point temperature, it is concluded that for the condition of low mass flow rate
and low temperature exhaust gas, the once-through HRSG can be
a good choice. For the once-through HRSG, the feed water is gradually changed into steam through the heating of the exhaust gas in
the heating surface. During the low water flow experiment, steam/
water mixture can be produced in Economizer 5 or 6. When the
mixture flows through Economizer 4 and Evaporator 3–1 in turn,
it will have large pressure loss, which leads to low vapor pressure
and poor steam quality. Besides, the low water flow rate can also
lead to low convective heat transfer coefficient and low waste heat
utilization efficiency. Hence, in the experiment, we choose high
feed water flow rate. In the Economizer 4–8, the feed water can
fully utilize waste heat and steam/water mixture will be generated
in evaporating heating surfaces (Evaporator 1–3). In this way, large
vapor pressure loss is avoid and steam quality is improved, while
high temperature water contains much heat which is not fully
utilized.
In order to utilize this part of the waste heat and improve
exergy efficiency, a HRSG-flash system is designed. In this paper,
a bypass valve is installed and different water flow can be dispensed into the heat recovery steam generator and the flash evaporator respectively with energy balance discipline. The water flow
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J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
50
3.0
7
45
2.5
6
2.0
5
2.5
1.5
Feed water
Steam
30
25
1.0
20
0.5
2.0
1.5
4
3
1.0
Feed water
Steam
2
Steam flow (t/h)
35
Steam flow (t/h)
Feed water flow (m3/h)
Feed water flow (m3/h)
40
0.5
1
15
0.0
10
0
50
100
150
200
250
300
350
400
450
500
0.0
0
0
50
100
150
Time (min)
200
250
300
350
400
450
Time (min)
(b)
(a)
Fig. 5. Flow curves of water and steam: (a) condition 1; and (b) condition 2.
Table 4
Exhaust gas and water/steam properties of different heating surface.
Heating surface
Ta,in (°C)
Ta,out (°C)
Absorbed heat (kW)
Exergy destruction rate (kW)
Tw,out (°C)
Tw,in (°C)
Pw (MPa)
Condition 1
1
2
3
4
5
6
7
8
311.61
232.12
199.96
174.76
156.66
139.63
127.43
118.11
232.12
199.96
174.76
156.66
139.63
127.43
118.11
109.95
955.42
382.34
298.11
213.40
200.25
143.16
109.21
95.52
431.79
149.18
105.08
69.33
58.56
38.19
26.97
21.99
152.58
158.73
161.82
147.19
136.52
125.93
118.85
113.35
158.73
161.82
147.19
136.52
125.93
118.85
113.35
108.36
0.51
0.6
0.65
0.65
0.65
0.65
0.65
0.65
Condition 2
1
2
3
4
5
6
7
8
292.85
239.93
219.05
193.65
179.25
166.42
152.75
142.55
239.93
219.05
193.65
179.25
166.42
152.75
142.55
128.50
536.99
212.38
257.23
146.12
130.39
138.50
103.50
142.57
273.87
85.61
96.37
50.86
42.83
42.68
29.89
38.20
145.21
152.40
154.43
157.52
161.01
156.19
132.82
114.82
152.40
154.43
157.52
161.01
156.19
132.82
114.82
90.61
0.42
0.51
0.54
0.58
0.63
0.65
0.65
0.65
Table 5
Ambient temperature and pressure parameters.
Parameters
Unit
Value
T0
P0
h0
s0
°C
MPa
kJ/kg
kJ/(kg K)
25
0.10
104.93
0.3672
to the heat recovery steam generator is completely converted to
high temperature saturated steam, while the other to be low temperature saturated steam through flash. The once-through HRSGflash optimization model is constructed taking exergy efficiency
of system as the optimization goal based on mass and energy conservation equations.
4.1. Objective function, parameters design and constraints
In this paper, a general calculation for thermodynamic performance of HRSG-flash system composed of the 8 heating surfaces
is designed based on the existing experimental data. For given
_ g Þ, water/
exhaust gas inlet temperature (Tg,in), mass flow rate ðM
steam temperature and pressure (Tw,in, Ts,out and Pw), the main
characteristics and assumptions are made as the followings:
_ r Þ of steam is certain. Steam outFor the HRSG, the rated flow ðM
_ s;HRSG Þ of the HRSG in the model is less than Dr. The pressure
put ðM
of flash steam (Ps,flash) is equal to that of supplementary steam.
For the HRSG-flash system, even though the temperature of the
feed water varies with position and condition of heating surfaces,
the water temperature of diversion point (Tw,dp) is less than the
saturated temperature (TP) corresponding with feed water pressure and more than the temperature of flash steam, which is calculated through thermodynamic properties of water by IAPWS-IF97.
The feed water is diverged at the diversion point. Part of the
water, after the shunt valve, re-enters the next heating surface of
the HRSG, and is heated to high pressure saturated steam. A certain
amount of water flows into the flash tank through the shunt valve
and nozzle, resulting in low pressure saturated steam. The remaining water after flash is conveyed to the water tank by the pump at
the bottom of the flash tank, and continues the next round of cycle.
Ignore water-side pressure drop. The steps to adjust the results are
given as:
1. Select the diversion temperature point and apply the energy
balance principle for the exhaust gas and water/steam in the
exchanger from the point to Economizer 8. Next, take the diversion
409
J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
1000
1000
900
900
800
800
Absorbed heat (kW)
1100
Absorbed heat rate (kW)
1100
700
600
500
400
300
700
600
500
400
300
200
200
100
100
0
1
2
3
4
5
6
7
0
8
1
2
3
4
5
Heat surface No.
Heat surface No.
(a)
(b)
6
7
8
Fig. 6. Absorbed heat rate in HRSG components: (a) condition 1; and (b) condition 2.
400
Exergy destruction rate (kW)
Exergy destruction rate (kW)
400
300
200
100
300
200
100
0
0
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Heat surface No.
Heat surface No.
(a)
(b)
Fig. 7. Exergy destruction rate for HRSG components: (a) condition 1; and (b) condition 2.
point of water temperature Tw,dp as a temperature reference to calculate the gas temperature Tg,dp at this point. It is shown in Eq. (8):
Table 6
Exhaust steam parameters of steam turbine.
Parameter
Value
Unit
_ w ðhw;dp hw;in Þ ¼ M
_ g cg ðT g;dp T g;out Þ
M
Steam turbine vacuum degree, Pv
Internal efficiency of steam turbine, gt
Pump efficiency, gpump
Steam turbine exhaust temperature, Tk
Steam turbine exhaust enthalpy, hks
9
85
85
49.4
2265.58
%
%
%
°C
kJ/kg
2. Eq. (9) is based on the energy balance of the heat exchanger
from Evaporator 1 to the diversion point to calculate the steam
_ s1 of HRSG.
production M
ð9Þ
Then, quantity of water to the flash system can be estimated as
follows:
Table 7
Available steam power and exergy efficiency of different conditions.
Parameter
Condition
1
Condition
2
Unit
_
Available steam power, W
Exergy efficiency of generating system,
327.35
240.38
kW
31.05
30.07
%
gex
_ s;HRSG hs;out hw;dp ¼ M
_ g cg T g;in T g;dp
M
ð8Þ
_ w;flash ¼ M
_ wM
_ s;HRSG
M
ð10Þ
3. Applying the first law of thermodynamic to the flash tank as
an adiabatic control volume, calculation formula of steam produced by flash system is shown in Eq. (11).
_ w;flash hw;dp ¼ M
_ s;flash hs;flash þ ðM
_ w;flash M
_ s;flash Þhw;flash
M
ð11Þ
410
J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
Steam power and exergy efficiency are calculated based on the
quantity of high pressure steam generated by HRSG and low pressure steam from flash system. And optimal water temperature of
diversion point corresponding to the highest exergy efficiency
can be given in exhaustive method.
Start
43.0
Exergy efficiency (%)
Flow diagram and the calculation results are shown in Figs. 8–
10 through FORTRAN programming calculation. It is obtained from
the following figures that: under a certain exhaust flow, different
selection of the feed water flow rate and water temperature at
diversion point cause different system exergy efficiency. There is
an optimal value. As shown in Figs. 9 and 10, the system exergy
efficiency is the highest when the flow rate is around 16 m3/h with
the diversion point water temperature about 413 K.
Fig. 9 shows the relationship between the flow rate of feed
water and system exergy efficiency. The increase of flow rate
means increase of velocity of flow and the heat transfer coefficient.
In this way, the waste heat can be utilized better and the outlet
temperature of the waste gas decreases. More saturated steam
can be made, and the system exergy efficiency increases. When
the flow rate is around 16 m3/h, the system exergy efficiency is
the highest. Then when the flow rate is more than 16 m3/h, the
influence of heat transfer coefficient is very little. But the waste
heat is not enough and saturated vapor reduces.
As shown in Fig. 10, exergy efficiency presents a linear rise followed by a downward trend, and the highest point (corresponding
water temperature of diversion point 413 K) is 43.4%. The reason is
that: when the temperature of diversion point increases to 413 K,
the feed water can be heated to a higher temperature before it is
diverged. Water flow rate and water temperature of the flash sys-
43.2
42.8
42.6
42.4
42.2
42.0
10
12
14
16
18
20
Water flow rate (t/h)
Fig. 9. Optimal exergy efficiency of HRSG-flash system on different water flow rate.
44
43
System exergy efficiency (%)
4.2. Results and analysis of model calculation
43.4
42
41
40
39
38
37
36
35
380
Energy consumption
calculation of HRSG
390
400
410
420
430
440
Temperature (K)
Fig. 10. Exergy efficiency changing with diversion point water temperature.
Diversion point water
temperature Tw,dp
tem will increase, which leads to a higher system exergy efficiency.
When the temperature of diversion water is over 413 K, with the
increase of temperature, the water flow rate of the HRSG will
reduce and main steam output will fall. Most of the feed water
flows into the flash tank and becomes low pressure steam, which
reduce the quality of steam and system exergy efficiency.
Exergy efficiency
calculation
Tw,dp >Tp
No
Changing diversion point water
temperature
Tw,dp=Tw,dp+1
Yes
Calculation result
End
Fig. 8. Flow chart of HRSG-flash system calculation.
5. U-type once-through HRSG-flash system experiment
validation
In order to validate the optimization scheme of different conditions, our group have designed and built a flash tank and connected
it with the HRSG. The high temperature and high pressure spray
flash experimental system is shown in Fig. 11. The main body is
a cylindrical cylinder with a standard ellipsoidal head. It has a
diameter of 2.5 m and a total height of 5.33 m. Its material is
Q345R, and the wall thickness is 30 mm, which can withstand
pressure up to 3 MPa. The tank and pipe are covered by insulation
material with a thickness of 100 mm. There are two water supply
pipelines of the flash tank, which are symmetrically arranged on
upper and lower of the cylinder body. Hence, it can meet the liquid
downward or upward jet requirements.
411
J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
LP cylinders
F
Flow meter
P
Pressure transmitter
T
Temperature thermometer
T
P
Electric
valve
Pump
To HRSG
F
P
T
P
T
Shunt valve
Nozzle
Flash
Evaporator
HRSG
Valve
Water tank
Fig. 11. Schematic diagram and photo of spray flash evaporation experimental system.
The specific working process is as follows: The feed water is
heated in the HRSG to the target temperature. Through the calculation of the amount of exhaust gas, water supply and temperature
parameters, we select the appropriate heating surface, and change
the opening of the valve between the heating surfaces. When the
feed water flows out from this heating surface, it does not enter
the next heating surface directly, but flows through the shunt
valve. Part of the water, after the shunt valve, re-enters the next
heating surface of the HRSG, and is heated to saturated steam. A
certain amount of water flows into the flash tank through the
shunt valve and nozzle. Then flash occurs. Flash steam is transferred through the pipeline at the top of the tank. And it is used
as supplementary steam in the low pressure cylinder of steam turbine. The remaining water is conveyed to the water tank by the
pump at the bottom of the flash tank, and continues the next round
of cycle.
The connection mode and quantity of heating surfaces in the
experiment are determined based on the theoretical calculation
results and experimental conditions. The alternative locations of
diversion point are displayed as follows:
2# experimental system: diversion point is between Evaporator
3 outlet and Evaporator 2 inlet.
3# experimental system: diversion point is between Economizer 4 outlet and Evaporator 3 inlet.
4# experimental system: diversion point is between Economizer 5 outlet and Economizer 4 inlet.
Experiments are carried out based on the three improved
schemes above. Different feed water flow rates and diversion point
temperature are selected for the experiment validation. Production
of high and low pressure saturated steam is measured. The exergy
of system and the HRSG are calculated and contrasted with each
other. Table 8 describes the different conditions of the flow rate
and diversion point temperature. Optimized architecture of
HRSG-flash system is illustrated in Fig. 12.
Fig. 13 shows comparison of system exergy efficiency between
different feed water flow rate and diversion point temperature. It is
evident that system exergy efficiency has an upward trend when
the flow rate changes from 11.70 m3/h to 15.90 m3/h; but when
the flow rate changes from 15.90 m3/h to 17.90 m3/h and 19.70
m3/h, the system exergy efficiency decreases. Furthermore, for a
Table 8
Water flow rate and diversion point temperature of different experimental system.
Water flow rate
(m3/h)
Temperature of diversion point (K)
2# experimental
system
3# experimental
system
4# experimental
system
11.70
15.90
17.90
19.70
426.25
434.97
424.43
418.46
418.96
420.34
409.05
402.95
407.08
409.67
400.22
395.19
single flow rate condition, the result shows that there is an optimal
diversion point temperature corresponding to the highest system
exergy efficiency. The experiment results are in good agreement
with the simulation ones. The result also shows that the optimal
diversion point temperature decreases when the water flow rate
increases.
By means of analyzing the experiment condition of 15.9 m3/h
feed water using different experiment schemes, results of steam
and exergy efficiency are shown in Tables 9–11, separately.
6. Result and discussion
6.1. Exergy analyses results
As shown in Tables 4, 9 and 10, the low temperature waste gas
from grate cooler which was discharged into the atmosphere
directly is efficiently utilized through the U-type once-through
heat recovery steam generator. High-temperature saturated steam
production rate is 2.44 t/h, and the generated steam power is
327.35 kW, under working condition of feed water flow 15.9 m3/
h, hot air flow 32,222 N m3/h, inlet hot air temperature
311.61 °C. The exergy efficiency of generation system and HRSG
are 31.05% and 33.64%, respectively.
Available steam power and exergy efficiency have both substantial increase with configuration optimization based on numerical model. As shown in Fig. 14, comparison is made between 3
schemes. From schemes 2 to 4, the temperature of the diversion
point decreases gradually. Available steam power of the HRSG
steam increases, while that of flash steam decreases.
Based on Tables 10 and 11 and Fig. 15, the overall quality of
steam is relatively improved (due to increased high pressure
412
J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
High pressure
HRSG steam
High pressure
HRSG steam
ECO 8
EVA 1
Low pressure
falsh steam
EVA 1
EVA 2
Low pressure
falsh steam
ECO 7
EVA 3
ECO 6
ECO 4
ECO 5
Shunt valve
EVA 1
ECO 8
EVA 1
EVA 2
ECO 7
EVA 3
ECO 6
ECO 4
ECO 5
Flash evaporator
Shunt valve
Condensated
water
Flash evaporator
Condensated
water
Water tank
Water tank
Water flow
Water flow
Steam flow
Gas flow
Steam flow
Gas flow
Feed water pump
Optimized cogeneration system 2#
Feed water pump
Optimized cogeneration system 3#
High pressure
HRSG steam
Low pressure
falsh steam
Shunt valve
Flash evaporator
Condensated
water
EVA 1
ECO 8
EVA 1
EVA 2
ECO 7
EVA 3
ECO 6
ECO 4
ECO 5
Water tank
Water flow
Steam flow
Gas flow
Feed water pump
Optimized cogeneration system 4#
Fig. 12. Optimized cement kiln waste-heat-recovering system.
steam) with the decreasing temperature of the shunt. Available
steam power, as well as exergy efficiency of system and HRSG all
increase and the optimal value is achieved in 4# experiment
schemes, being 44.43% and 54.61% respectively.
6.2. Economic analyses results
Economic analyses are important to determine whether heat
transfer equipment makes sense. As mentioned above, it is necessary to do calculation on economic aspects of generation system to
have a better evaluation about the new once-through heat recovery steam generator and its different configurations.
Cost estimation can have a major impact both on project profitability and influences of the technical solution. The total capital
investment of the once-through HRSG-flash experimental system
Ctot contains three parts: the tube bundles cost Ct, the shell cost
Cs and the insulation cost Cins. The capital investment can be
expressed as:
C tot ¼ C t þ C s þ C ins
ð12Þ
The size of a single heating surface is 2.5 m 2.5 m 2.5 m.
The tube bundles consist of 48 serpentine tube screens
(£38 mm 3.5 mm each) and 2 headers (£89 mm 6.0 mm
each). Hence, the tube bundles cost is calculated as follows:
413
J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
Table 11
Steam exergy and exergy efficiency of HRSG.
System exergy efficiency (%)
44.0
11.7 m3/h
15.9 m3/h
17.9 m3/h
19.7 m3/h
43.5
43.0
Experiment
schemes
High-temperature
saturated steam (kJ/kg)
Flash saturated
steam (kJ/kg)
HRSG exergy
efficiency (%)
1#
2#
3#
4#
302.70
258.72
340.69
396.67
0.00
201.03
138.02
94.73
33.64
51.09
53.20
54.61
42.5
550
42.0
500
450
41.5
HRSG steam
Flash steam
Total
41.0
395
400
405
410
415
420
425
430
435
Temperature (K)
Fig. 13. System exergy efficiency of different flow rate and diversion point
temperature.
Steam power (kW)
400
350
300
250
200
150
100
Table 9
Steam production of different experiment scheme.
50
Experiment
schemes
HRSG saturated steam (t/
h)
Flash saturated steam (t/
h)
2#
3#
4#
2.22
2.63
2.91
1.33
0.91
0.63
0
1
2
3
4
Experiment scheme No.
Fig. 14. Available steam power of different optimized cogeneration system.
60
HRSG
HRSG generation system
ð13Þ
where Cpm1 is the price of the tube structural material, wwt and whd
represent the tube weight per unit length of the serpentine tube
screens and headers, respectively. Nwt and Nhd are the number of
those. Ns is the heating surfaces number of the HRSG, Ns = 8.
For the shell and insulation material cost, it includes the following parts: heating surfaces, flue duct, water tank and flash tank.
Shell cost and thermal insulation material cost formulas are as
follows:
C s ¼ C pm2 qm2 ðNs Ass tss þ Afd tfd þ Afl tfl þ Atk t tk Þ
ð14Þ
C ins ¼ C pins tins ðNs Ass þ Afd þ Afl þ Atk Þ
ð15Þ
Exergy efficiency (%)
C t ¼ C pm1 ðwwt lwt Nwt þ whd lhd Nhd ÞNs
50
40
30
where Cp and q are the unit price and density of different materials,
respectively. Subscript (m2) and (ins) are the metal and insulation.
A represents the total area and t is the thickness of shell metal or
insulation layer.
The parameters of the waste-heat-recovering system structure
and materials for the economic analysis are shown in Table 12.
The total capital investment of the heat recovery system are
45346.23 € (without flash system) and 52153.10 € (with flash system), respectively.
The annual operational cost can be devised into Cpw and Cpg. Cpw
comes from pump work and it is expressed as:
1
2
3
4
Experiment scheme No.
Fig. 15. Exergy efficiency of different optimized cogeneration system.
C pw ¼
_ w DP w
M
C t
q gpump e p
ð16Þ
where q is the fluid density and DPw is the pressure drop of feed
water in the whole cycle. gpump is the pump efficiency. tp is the
operating time per year. Ce is the price of the electrical energy.
Table 10
Available steam power and exergy efficiency of system.
Experiment schemes
HRSG high-pressure saturated steam (kW)
Flash saturated steam (kW)
Total steam power (kW)
System exergy efficiency (%)
1#
2#
3#
4#
327.35
297.83
352.84
390.40
0.00
157.94
108.44
74.43
327.35
455.77
461.27
464.83
31.05
43.23
43.75
44.43
414
J. Li et al. / Applied Thermal Engineering 120 (2017) 402–415
Cpg is caused by the induced draft fans and it is expressed as:
C pg
D_ g DPg
¼
K C e tp
36; 00; 000 gf
ð17Þ
where DPg is the exhaust gas pressure drop of HRSG. g is the total
pressure efficiency. K is the motor capacity factor.
Whether only HRSG or HRSG-flash system, the final product is
saturated steam used for electricity generation in the low pressure
cylinder of steam turbine. Hence, the system profit, Cga is the economic benefits generated by waste heat power generation, which
is expressed in Eq. (18). Table 13 shows parameters about system
operational cost and profit calculation.
C ga ¼ ðW b þ W f Þ gt gg C e t p
ð18Þ
where Wb and Wf are the net power done by boiler steam and flash
steam. gt and gg are the steam turbine efficiency and generator efficiency, respectively.
The experiment measurements system is shown in Table 1. The
measurement data includes the mass flow rate and the pressure
drop of the HRSG and flash system in Tables 3–5. Taking the above
experimental analysis in Section 3 as the representative average
operating conditions of the waste heat recovery system for the year
analysis, cost recovery is calculated. Assuming that it takes n years
to start profit, and investment capital annual interest rate i is15%.
The condition for achieving profitability is shown in Eq. (19) and
the economic analysis result of different systems is shown in
Table 14 [25].
n
ðC ga C pw C pg Þ P
ið1 þ iÞ
C tot
n
ð1 þ iÞ 1
ð19Þ
Table 12
Waste-heat-recovering system structure and materials parameters.
Parameters
Value
Unit
Cpm1
Cpm2
Cins
qm1, qm2
tss, tfd
ttk
tfl
tins
lwt
lhd
Ass
Afd
Atk
Afl
0.47
0.54
53.98
7850
10
12
30
100
56.64
3.1
25
30.91
71.97
51.33
€/kg
€/kg
€/m3
kg/m3
mm
mm
mm
mm
m
m
m2
m2
m2
m2
Table 13
The annual operational cost parameters of exhaust gas and feed water.
Parameters
Value
Unit
Ce
tp
0.09
4800
85
1.1
95
€/(kW h)
h
%
–
%
gf
K
gf
As it is shown in Table 14, with this heat recovery system, 200–
300 °C low temperature waste heat of the cement industry can be
recovered well with an annual income of 85,383 €. The construction cost can be recovered in 2.7 years. After flash system optimization, the optimal results show that annual income can be 121,242 €
and only 1.1 years is needed to recover construction cost.
The U-type once through HRSG can make effective use of low
temperature waste heat from grate cooler to produce saturated
steam and to generate electricity. After incorporating flash tank
into waste heat system and configuration optimization of different
shunts, the corresponding exergy efficiency of system and HRSG
achieve the optimal values.
7. Conclusion
Based on the above-mentioned results, related conclusions are
summarized as the following:
1. A novel U-type heat recovery steam generator is proposed,
which is efficient in recovering the low temperature waste heat
originally emitted into the atmosphere. Experiments results
indicate that the heat transfer performance and exergy loss of
each heating surface in the heat recovery steam generator system go down with the decrease of heat source temperature.
Exergy loss of the remaining heat transfer surface, except Evaporator 1 is small, which indicates low irreversibility of the heat
surface. The exergy efficiency of the system and heat recovery
steam generator can be 31.05% and 33.64% respectively in typical experimental condition.
2. HRSG-flash system model is applicable and favorable to achieve
system exergy efficiency optimization based on mass and
energy balance equation. Optimized system schemes are studied by numerical simulation and experiments. Exergy efficiencies of generation system and HRSG of the optimized HRSGflash system are both higher than those when the HRSG separately operates. The corresponding optimal efficiency is
44.43% and 54.61%, respectively. It is obviously that the utilization efficiency of waste heat is enhanced.
3. The economic analysis results show that the application of the
HRSG makes an annual income of over 85,383 € and it takes
no more than 3 year to recover the cost of building the boiler.
After optimization, the annual income of the combined HRSGflash system can be 121,242 € and the construction cost can
be recovered in 1.1 years.
With the rapid development of the industrial production, the
quantity of the low temperature waste gases will increase and
the quality of the low temperature waste gases will decrease.
Hence, related researches in this paper provide a reference for
once-through HRSG in low temperature conditions, which is of
great significance and economic value in the fields of cement, steel
and other industries.
Acknowledgements
Funding for this research was provided by the National Basic
Research Program (973 Program) of China (Project No.:
2013CB228305).
References
Table 14
Economic performance and payback period of the heat recovery system.
Experimental
program
Ctot (€/yr)
Cpw (€/yr)
Cpg (€/yr)
Cga (€/yr)
n (yr)
HRSG system
HRSG-flash system
45346.23
52153.10
1434.24
1434.24
62117.28
62117.28
85383.36
121242.5
2.7
1.1
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