A Study on the Arrangement of Outer Steam Cooler for 1000MW
Double Reheat Ultra-Supercritical Unit
Weijun Mi
1
, Haiwen Chen
2
and Wenfeng Fu
3
1
Shaanxi Xin Yuan Clean Energy Co.Ltd. ,
Yulin 719400, Shaanxi Province, China
2
School of Energy, Power and Mechanical Engineering,
North China Electric Power University,Baoding 071003, Hebei Province, China
3
North China Electric Power University, Key Laboratory of Condition Monitoring and Control for Power Plant Equipment,
Baoding 071003, Hebei Province, China
Keywords: Double reheat, fuel specific consumption, outer steam coolers.
Abstract: Taking the 1000MW double reheat ultra-supercritical unit as an example, based on analysis of the specific fuel
consumption, the effects of following arrangement modes of outer steam cooler (OSC) on the energy
consumption of unit are studied, such as the single series arrangement, double series arrangement and double
parallel arrangement, etc., and subsequently the optimum arrangement mode of OSCs is obtained, in which case
the variation law of specific fuel consumption is analyzed for each part of the thermal system and for the whole
unit. Results show that by adopting the OSCs, the feed water temperature is raised, the irreversible loss of boiler
is reduced, thus lowering the specific fuel consumption of unit. In the single arrangement mode, the specific
fuel consumption can be reduced by 0.632g/kWh at most when the OSC is arranged at No.2 high pressure (HP)
regenerative heater (RH);whereas in the double arrangement mode, the specific fuel consumption can be reduced
by 1.122g/kWh at most when the coolers are arranged in series at RH2 and RH4.
1 INTRODUCTION
Technical upgrading of coal-fired power plants to
achieve ultra-low emissions is an important measure
to make the utilization of fossil fuels clean, improve
the air quality and ease the resource constraints.
In recent years, the research on the double reheat
technology has drawn more and more attention.
Bugge et al. (
2006) pointed out that, compared with
single reheat unit, the extraction steam of the double
heat unit has a higher degree of superheat, resulting
in a high exergy loss of RH, which suppresses the unit
efficiency further improved. In an effort to undermine
the adverse effect of steam superheat at the heater
inlet on the unit, Xu et al. (2015) devised a scheme to
set up a back pressure extraction steam turbine in the
system and calculated the improved results. Kan et al.
(2014) pointed out that the use of OSC is another
simple and effective measures, while having a good
load adaptability. Niu et al. (2011) and Xia et al.
(2015), respectively, for single reheat unit and double
reheat unit, analyzed the effect of OSC on reducing
fuel consumption.
The existing research results show that the OSC
can play a significant role in reducing the superheat
of the extraction steam at the inlet of the double reheat
unit and increasing the efficiency of the unit.
However, there are few reports on the influence of
different arrangement of OSCs on the efficiency of
double reheat unit. Therefore, with a 1000 MW
double reheat ultra-supercritical unit as the research
object, we proposed a variety of schemes to deploy
OSCs. And the efficiency of the various schemes is
compared, the better arrangement is obtained.
Mi, W., Chen, H. and Fu, W.
A Study on the Arrangement of Outer Steam Cooler for 1000MW Double Reheat Ultra-Supercritical Unit.
In 3rd International Conference on Electromechanical Control Technology and Transportation (ICECTT 2018), pages 59-62
ISBN: 978-989-758-312-4
Copyright © 2018 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
59
2 METHODOLOGY
Figure 1: Thermodynamic system of the reference double
reheat power plant.
Table 1: Extraction steam parameters at various stages of
the system.
Extraction Pressure
(MPa)
Temperature
()
Superheat
()
1 8.932 415.624 112.824
2 6.009 525.462 249.778
3 3.334 433.645 193.865
4 1.855 529.304 320.691
5 1.038 442.431 260.921
6 0.718 389.589 123.609
7 0.392 309.989 167.109
8 0.127 189.002 82.572
9 0.059 118.761 33.391
10 0.022 61.984 0
Taking a 1000MW double reheat ultra-
supercritical unit as the Base Case, its thermal system
is shown in Fig.1. The initial parameters of the unit
are 31MPa / 600 / 610 / 610 , the second
and fourth extraction steam are the first extraction
after the first reheat and the second reheat,
respectively. The regenerative system has four HP
RHs, five low pressure (LP) RHs and a deaerator.
Table 1 shows the steam parameters of the
regenerative system. The OSCs are respectively
arranged in single series mode on the extraction pipes
of four HP RHs of the unit, all the feed water passes
through the steam cooler and then enters the boiler.
3 RESULTS AND DISCUSSION
Table 2: Comparison of thermal efficiency among
different arrangements of single OSC.
Position of
OSC
Feed water
temperature
()
Superheat
reduction ( )
Thermal
efficiency
of the unit
(%)
Specific
fuel
consumpti-
on of the
unit
(g
/kWh
)
Base
Case
- 304.503 - 50.979 266.826
Schem
e1
RH1 307.992 81.486 51.014 266.659
Schem
e2
RH2 312.452 208.126 51.099 266.207
Schem
e3
RH3 307.783 127.157 51.036 266.592
Schem
e4
RH4 308.562 227.481 51.085 266.361
Based on Ebsilon platform, the thermal system
with integrated OSC is simulated and the system node
parameters are obtained. The thermal efficiency
parameters of the four schemes and the Base Case are
calculated as shown in Table 2 by using the analysis
of specific fuel consumption. When taking scheme 2,
the OSC is arranged in front of the RH2, the best
energy-saving effect can be obtained. Compared with
the Base Case, the scheme can reduce the specific
fuel consumption of the unit by 0.619 g/kWh and the
thermal efficiency of the unit by 0.12%. The
distribution of additional specific fuel consumption is
shown in Table 3. Compared with the Base Case, the
additional specific fuel consumption of each
equipment in scheme 2 is reduced, except for the feed
pump system.
Table 3: Distribution of specific fuel consumption in
reference system and scheme 2.
Specific fuel consumption
Base
Case
Scheme 2
Reduction
of Specific
fuel
consumpti-
on
Additional specific
fuel consumption
(g/kWh)
Boiler 119.473 118.632 0.841
Turbine 6.738 6.733 0.005
Condenser 11.860 11.815 0.045
RHs 2.901 2.887 0.014
Pump 1.058 1.069 -0.011
Specific fuel consumption of the
uni
t
(g/kWh)
266.826 266.216 0.610
Utilization of the steam superheat at the inlet of
the heater reduces the heat exchange temperature
difference of RH2 by 208.126K, reducing the
additional specific fuel consumption of the RH by
0.014 g/kWh. At the same time, the temperature of
feed water is increased by 7.949K, which reduces the
additional specific fuel consumption of the boiler by
0.841g/kWh. Due to the increase of feed water mass
ICECTT 2018 - 3rd International Conference on Electromechanical Control Technology and Transportation
60
flow and flow resistance, the additional specific fuel
consumption of pump is increased by 0.011g/KWh,
but the value is far less than the overall reduction of
additional specific fuel consumption of other
equipment. In conclusion, the contribution of the
OSC to the thermal efficiency of the unit is mainly
reflected in reducing the additional specific fuel
consumption of the boiler and less affecting the
reduction of the irreversible loss of the RH itself. The
reason is that the utilization of the superheat of the
steam at the heater inlet greatly increases the
temperature of feed water to bring it closer to the
optimal value of the system.
It can be seen from Table 1 that the superheat of
the second extraction and the fourth extraction has the
most potential for utilization. Therefore, when using
double steam coolers, it should be arranged at RH2
and RH4, respectively, with series and parallel
arrangement,
recorded as scheme 5 and scheme 6.
Table 4: Thermal efficiency comparison of installing
double OSCs.
Temperature
()
Thermal
efficiency of
the unit
(%)
Specific fuel
consumption
of the unit
(g/kWh)
Base Case
304.503 50.979 266.826
Scheme 5 317.124 51.192 265.513
Scheme 6 312.857 51.158 265.742
In scheme 5, the main feed water passes through
RH1 and then enters into the double OSCs
respectively and then enters the boiler. In order to
reduce the thermal deviation at the outlet flow mixing
of double OSCs, the feed water mass flow into the
RH2 steam cooler is set to be 70% of the total flow.
In scheme 6, the inlet water supply of the double
OSCs comes from the outlet of their corresponding
RHs, respectively. According to the method of Ref.
(
XU Chuanpu, 1990), the feed water splitting
coefficient is chosen. It is calculated that when the
mass flow of RH2 steam cooler and RH4 cooler is
respectively 5.5% and 3% of the feed water mass
flow before shunting, the thermal efficiency of the
unit is the best. The comparison of thermal efficiency
between the two schemes and the Base Case is shown
in Table 4.Compared with the Base Case, the
temperature of feed water is increased by 12.621K,
the specific fuel consumption of the unit is reduced
by 1.313g/kWh, and the thermal efficiency of the unit
is improved by 0.213%. Because of the smaller mass
flow in the steam coolers in scheme 6, the
temperature of the feed water is less increased and the
reduction effect is lower than that of scheme 5.
Table 5 shows comparison of specific fuel
consumption of between the best single OSC
arrangement (scheme 2), the best double arrangement
(scheme 5) and the Base Case. Compared with the
single arrangement, when the double arrangement is
adopted, the superheat of the steam at the inlet of the
heater can be utilized to a greater extent. The
additional specific fuel consumption of the boiler,
turbine, condenser and RHs in the system is reduced.
And there is not much difference between the
additional specific fuel consumption of the pump
system. Therefore, the use of double series
arrangement is significantly better than single
arrangement.
Table 5: Distribution of fuel specific consumption in
different systems.
Specific fuel consumption
Base
Case
Scheme
2
Scheme
5
Additional specific
fuel
consumption(g/kWh)
Boiler 119.473 118.632 117.934
Turbine 6.738 6.733 6.861
Condenser 11.860 11.815 11.768
RHs 2.901 2.887 2.647
Pum
p
1.058 1.069 1.081
Specific fuel consumption of
the unit (g/kWh)
266.826 266.216 265.483
4 CONCLUSION
When using a single steam cooler, the best effect can
be obtained by arranging it at the RH2, which can
reduce specific fuel consumption by 0.619g/kWh.
When using double OSCs arrangement, arranged in
the RH2 and RH4 in series of the best way, can reduce
specific fuel consumption 1.313g/kWh.
The OSC enhances the thermal efficiency of the
unit mainly by reducing the additional specific fuel
consumption of the boiler, while contributing little to
reduce the additional specific fuel consumption of the
RH itself.
ACKNOWLEDGMENTS
This research is supported by the Special Funds of
the National Natural Science Foundation of China
(No. 51606066).
A Study on the Arrangement of Outer Steam Cooler for 1000MW Double Reheat Ultra-Supercritical Unit
61
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