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THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS
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Process Optimization of an Integrated Combined Cycle The Impact & Benefit of Sequential Combustion
Dr. Walter Jury
ABB Power Generation Ltd.
Baden, Switzerland
1111111111 E1111111111
David E. Searles
ABB Power Generation Inc.
Richmond, Virginia USA
Abstract
Introduction
Advanced gas turbine designs require revisiting the optimization
process to provide maximum competitiveness of new
generating installations. This counts specifically for those
designs created for combined cycle applications. Gas turbine
The onset of deregulation will change fundamental business
practice within the electric power industry. This change requires
electric power producers to find increasingly innovative ways to
provide competitive power. These ways must provide an edge
performance and its associated exhaust temperature has been
increasing at a rapid pace over recent years. The conventional
method of selecting a GT based upon price and performance,
over the competition and improve profit levels at the same time.
Previously, a common solution was for the gas turbine
manufacturer to increase engine firing temperatures. This
and then designing a complex bottoming cycle does not provide
sufficient solutions for power generation in an open access
marketplace. The optimal solution takes into account the
interrelation between the CT and WS cycle, leading to a more
efficient, simplified and flexible power plant. This analysis
shows how different levels of GT exhaust energy lead to
different optimum cycle solutions. It shows, as postulated
above, that considering the WS cycle demands in gas turbine
design leads to a simpler cycle with inherent advantages in
efficiency, reliability and flexibility.
increase provided for both simple cycle and combined cycle
improvements. However, the ability to finance and insure these
large machines has recently become an issue, especially as a
deregulated marketplace eliminates the financial security of a
long term power purchase agreement.
Nomenclature
3PRH Triple Pressure Reheat
2PRH Dual Pressure Reheat
CC
Combined Cycle
FOR
Forced outage rate
EFOR Equivalent Forced Outage Rate
GT
Gas Turbine
HP
High Pressure
LP
Low Pressure
NPV
Net present value
RH
Relative humidity
SP
Superheater
ST
Steam Turbine
WS
Water Steam
VP
Evaporator
LO
Loss of Output
Specific exergy
Specific enthalpy
Specific entropy
amb
ambient
Efficiency
Traditionally, gas turbines provided emergency peaking power.
With the growing importance of energy efficiency, combined
cycle concepts based on such gas turbines, designed primarily
for simple cycle operation, became economical. However, to
utilize the full potential of the combined cycle power plant it is
not enough to optimize either the gas turbine or the bottoming
water steam (WS) cycle independently. The most economical
and hence most competitive solution must consider a plant
approach that integrates the CT and WS cycle in a manner
most beneficial to the owner of the plant. The CT exhaust
energy is the link between CT and WS cycle. Therefore, the
design and operating concept of the CT must consider this link
and provide exhaust energy at a level leading to a simple,
reliable and efficient WS cycle concept. To ensure high
reliability and financeability, the plants design criteria should be
based upon proven concepts, rather than new technology that
consistently 'pushes the envelope'.
Gas turbine exhaust energy - link between
topping and bottoming cycle
Over the last 15 years, gas turbine exhaust temperature has
risen from below 480°C (900 °F) to almost 650°C (1200 °F) [1,
2]. Due to this fact, CC performance was marginal in the early
years (<50% gross thermal efficiency). With CT exhaust
temperatures of 480 °C, the efficiency of the bottoming steam
cycle was much below that of conventional rankine cycle plants.
Presented at the International Gas Turbine Sr Aeroengine Congress & Exhibition
Orlando, Florida
— JuneTerms
2—June
5,1997
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on 12/29/2014
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Significant improvements (ft 56% gross thermal efficiency)
began only a few years ago when GT exhaust temperatures
rose to 540°C (1000 °F) and beyond. This allowed live steam
temperatures of CT based combined cycles to match that of
steam boiler plants (540°C - 565°C (1000 °F - 1050°F)),
increasing bottoming cycle efficiency and improving overall CC
plant performance.
Property
ST exhaust mass flow
GT exhaust temperature
ST cooling energy'
Value/ Range of Values
540 Ir4/s (4,285,786 Ibihr)
Variable (550°C / 600"C / 650°C)
(1022° F/1112° F/1202°F)
19 MW (64.8 MMlittuihr)
70 ban (1015 psia)
Minimum live steam generating pressure
The heat recovery curves associated with CT exhaust
temperatures between 550°C and 600°C (1000°F - 1100°F)
(Figure 1) favor the introduction of multi-pressure level cycles
for improved cycle efficiency, leading to today's 'industry
standard' of a triple pressure reheat cycle (31 Figure 1 also
shows the reason for these multi-pressure cycles: the heat
recovery steam generator (HRSG) stack temperature for a
single pressure level is very high and leaves room for
economical intermediate and low pressure steam production.
Reheat pressure
1/5 of live steam pressure
Steam turbine backpressure
42°C (110°F)
Minimum HRSG stack temperature
80°C (175°F)
Drum approach temperature
Minimum superheater approach
gig
Maximum steam temperature
lailM --tingmtm
.
e
10 K (18W)
5 K (9°F)
251< (45°F)
565"C (1050°F)
Slagle p eeeeee tint
• ■••
■•
Tab. 1.: Parameters used for cycle investigations. Exhaust
mass flow is approximately equal to the exhaust mass
flow of the largest industrial gas turbines available
today. Live steam pressure values and maximum live
steam temperature cover the well proven experience
range of combined cycle power plants and
conventional steam plants (most conventional steam
plants commissioned in the 1980s run with live steam
pressures of 170 bar (2470psia) and five steam
temperatures of 540°C to 565°C and use single or
double reheat at 35 bar (500psia) 540°C to 565°C (2)).
Reheat pressures at 1/5 of live steam pressure ensure
acceptable temperature differences at the steam
turbine (cold reheat/ hot reheat) and in the HRSG
(parallel superheater and reheater). steam turbine
backpressure of 0.065 bar is economically achievable
with a wet cooling tower at 15 °C (59°F) ambient at
60% RH. FIRSG feedwater at 42°C is appropriate for
combined cycles firing natural gas with low sulfur
content (the pipe metal temperature of the feedwater
preheating economizer remains above the water
dewpoint). Values for pinch points and approach
temperatures follow the recommendations of the
international standard ISO 3977 Annex F.
IME
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:
'414
C
•. ■■■■■■
0.065 bara (7 Hg)
HRSG feedwater temperature
Evaporator pinch point
C
IMO;
190 bare (2755 psia)
Maximum live steam generating pressure
.• ■.1
••
•
■ • ■■
•■ • ■
•.■,
••••
LDS
MSS
Heat transferred
Fig. 1.: Heat recovery profile of a triple pressure reheat cycle at
CT exhaust temperatures of 550°C (1020 °F). The
dashed line shows the heat recovery profile of a single
pressure reheat cycle. The high stack temperature
leaves room for economical production of low- and
intermediate pressure steam
The following section investigates the effect of increasing CT
exhaust temperatures on heat recovery cycle design. It answers
the question if and how optimum cycle design will change with
increasing gas turbine exhaust temperatures. Table 1 shows the
parameters used in this investigation.
Heat Recovery & Exergy Transfer
This section compares the suitability of three cycle designs for
given levels of gas turbine exhaust energy: the single pressure
reheat, dual pressure reheat and triple pressure reheat cycles.
Figure 2 shows flow diagrams of the HRSGs of the compared
cycles.
'Most high efficiency gas turbines require some amount of external cooling
of high temperature parts, either by direct steam cooling or external cooling
of compressor air. This investigation assumes that 20% of compressor
massflow are used as cooling air and cooled down by 200°C. This heat is
Introduced into the cycle as high pressure steam.
2
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Figures 4 and 5 show block diagrams of the HRSGs used for
the reliability calculations.
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HP
HP
13P
HP
-
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m P4
HP
-
GP
HP
Er
LP
VP
LP
DIA
LP
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I
tea
CP
LP
1
650
tea • 600
X
xmoo
VV
CT
CITM
CP
LP
tea • SSO
MO
x
Fig. 4.: Block diagram for reliability analysis of the dual
pressure reheat HRSG. EC ...economizer, VP...evaporator, SP...superheater, CP...circulation pump,
VV. valves, CT...controls, OTH...other
MOO
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Evaporation pressure [bare]
••■■•■
EC
VP
HP
Fig 5
SP
EC
IP
VP
IP
DM
IP
EC
LP
VP
LP
DM
LP
I.
Fig. 6.: Heat exchange surface difference between dual
pressure reheat and triple pressure reheat cycles as a
function of live steam pressure with gas turbine
exhaust temperature as parameter. This picture shows
that the narrowing gap between dual pressure reheat
and triple pressure reheat cycle is not 'bought' with a
more elaborate cycle design. This improvement is - as
also shown in the previous section - a result of the
better 'fit' of the dual pressure reheat cycle for higher
gas turbine exhaust temperatures.
VV - CT OTM
CP
LP
•■■• •••■
Block diagram for reliability analys's of the triple
pressure reheat HRSG. Symbols a e explained in
figure 4 above.
The results presented in the previous two sections lead to the
following conclusions:
Table 5 below shows the reliability comparison of dual and triple
pressure reheat HRSG. The comparison shows an advantage of
about 0.1% points in equivalent forced outage rate for the dual
pressure reheat HRSG.
System
Dual Pressure
FOR %
High pressure
Intermediate
• ressure
Low pressure
General (Valves,
control etc.
Total
0.22
Tnple Pressure
LO % EFOR % FOR %
100
0.22
LO %
EFOR %
0.21
100
0.21
0.19
100
0.19
0.20
100
0.20
0.13
70
0.09
0.09
100
0.09
0.10
100
0.10
0.51
0.63
0.51
(1) The thermal advantage of a triple- over a dual pressure
reheat cycle sharply diminishes with increasing OT exhaust
temperatures.
(2) With increasing GT exhaust temperatures, higher HP steam
pressures show an increasing output benefit for the dual
pressure reheat cycle.
(3) The simpler design of the dual pressure reheat HRSG
results in an advantage of about 0.1% points in equivalent
forced outage rate (EFOR).
(4) The dual pressure reheat cycle requires consistently less
HRSG surface than the triple pressure reheat cycle
irrespective of GT exhaust temperature.
0.59
Combining these facts and carrying out the net present value
(NPV) calculation with the parameters from Table 4 results in
Figure 7 below. Figure 7 shows the difference in NPV between
dual pressure reheat and triple pressure reheat cycles with gas
turbine exhaust temperature as parameter. The figure shows
how the facts work in combination to result in an increasing
NPV advantage of the dual pressure reheat cycle over the triple
pressure reheat cycle with increasing GT exhaust temperature.
Tab. 5.: Reliabi ity values for dual pressure reheat and triple
pressu e reheat HRSGs [4, 51. FOR ... forced outage
rate; LO ... loss of output; EFOR ... equivalent forced
outage rate
Cycle investment cost: The primary difference between the two
cycles compared is the design of the HRSG. The dual pressure
reheat cycle is a simpler design requiring less heat exchange
surface. Figure 6 shows the heat exchange surface difference
between dual and triple pressure reheat cycles with gas turbine
exhaust temperature as parameter. The figure shows how the
dual pressure reheat cycle maintains an almost constant HRSG
surface advantage over the triple pressure reheat cycle
regardless of OT exhaust temperature.
5
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One important conclusion is that an increase in gas turbine
exhaust temperature significantly increases the effectiveness
and efficiency of heat recovery irrespective of the cycle
configuration employed. For an optimum bottoming cycle, a gas
turbine should, therefore, deliver the highest exhaust
temperature possible with the chosen gas turbine cycle concept
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...dr.
The most relevant fact for further investigations, however, is
that the advantage of a triple pressure reheat cycle sharply
diminishes with increasing gas turbine exhaust temperatures.
The next section will, therefore, answer the question whether the
triple pressure reheat cycle remains the economical cycle at
higher gas turbine exhaust temperatures?
Dual Pressure Reheat versus Triple Pressure
Reheat Cycle: Economic Comparison
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Evaporation pressure (bare]
This section compares the dual pressure reheat and triple
pressure reheat cycles in respect of economical properties over
the range of exhaust temperatures and live steam pressures
specified in Table 1. The economical comparison uses the
parameters listed in Table 4
Property
Value
Financial assessment period
15 years
Discount Factor for NPV Analysis
Plant operating hours
Electricity price
10%
7900 hours/year
3.5 centsAd/Vh
Escalation Electricity Revenue
Fuel Price (LHV)
1.5%
Fixed O&M Costs
Cycle availability: Dual / and triple pressure systems differ
through the additional components required for intermediate
pressure steam generation by the triple pressure reheat cycle.
This comparison considers only the HRSG components. The
comparison assumes that failures of the high pressure part
(dual and triple pressure cycle) or of the intermediate pressure
part (triple pressure cycle) of the HRSG result in a plant
shutdown. Failure of the low pressure part results in a plant
shutdown for the dual pressure cycle and in a load reduction to
70% for the triple pressure cycle. Scheduled HRSG outages
(maintenance) coincide with the gas turbine scheduled outages,
differences in availability between the cycle concepts are,
therefore, equal to differences in reliability. 3
2.25 USS/MMBTU
Escalation Fuel Cost
Variable O&M Costs
Fig. 3. Steam turbine output of the dual pressure reheat cycle
as a percentage of the steam turbine output of the
triple pressure reheat cycle with gas turbine exhaust
temperature as parameter. This picture confirms the
conclusion of the previous section that with moderate
exhaust temperatures between 550 °C and 600°C the
triple pressure reheat cycle with a five steam pressure
of 110 bar (1550psia) was a good choice. With higher
exhaust temperatures it gets significantly more
interesting to increase pressure and switch to dual
pressure reheat.
2%
0.4 cents/IcWh
3.8 MM US$/year
Escalation O&M
2%
Tab. 4.: Parameters used for economical comparison
Cycle characteristics governing the economical comparison are:
• cycle efficiency (steam turbine output differential)
• cycle availability
• cycle investment cost
Cycle efficiency: Figure 3 shows the steam turbine output of the
dual pressure reheat cycle as a percentage of the steam turbine
output of the triple pressure reheat cycle with gas turbine
exhaust temperature as parameter. The shown percentages
reflect the trend explained in the previous section. The relative
advantage of the triple pressure reheat cycle decreases
significantly with increasing GT exhaust temperature. The figure
also shows another interesting trend, the benefit of higher
pressures for the dual pressure reheat cycle with increasing
exhaust temperatures.
In combined cycle power plants scheduled outages (maintenance) are
coordinated with the gas turbine as lead machine. As the duration of HRSG
maintenance is significantly shorter than gas turbine maintenance
irrespective of cycle concept, differences in HRSG maintenance duration do
not influence overall availability.
3
4
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Figures 4 and 5 show block diagrams of the HRSGs used for
the reliability calculations.
vr
"Tr
HP
HP
13P
HP
-
E
m P4
HP
-
GP
HP
Er
LP
VP
LP
DIA
LP
"Tr
I
tea
CP
LP
1
650
tea • 600
X
xmoo
VV
CT
CITM
CP
LP
tea • SSO
MO
x
Fig. 4.: Block diagram for reliability analysis of the dual
pressure reheat HRSG. EC ...economizer, VP...evaporator, SP...superheater, CP...circulation pump,
VV. valves, CT...controls, OTH...other
MOO
Met
SO
I.
••
126
III
1111
Evaporation pressure [bare]
••■■•■
EC
VP
HP
Fig 5
SP
EC
IP
VP
IP
DM
IP
EC
LP
VP
LP
DM
LP
I.
Fig. 6.: Heat exchange surface difference between dual
pressure reheat and triple pressure reheat cycles as a
function of live steam pressure with gas turbine
exhaust temperature as parameter. This picture shows
that the narrowing gap between dual pressure reheat
and triple pressure reheat cycle is not 'bought' with a
more elaborate cycle design. This improvement is - as
also shown in the previous section - a result of the
better 'fit' of the dual pressure reheat cycle for higher
gas turbine exhaust temperatures.
VV - CT OTM
CP
LP
•■■• •••■
Block diagram for reliability analys's of the triple
pressure reheat HRSG. Symbols a e explained in
figure 4 above.
The results presented in the previous two sections lead to the
following conclusions:
Table 5 below shows the reliability comparison of dual and triple
pressure reheat HRSG. The comparison shows an advantage of
about 0.1% points in equivalent forced outage rate for the dual
pressure reheat HRSG.
System
Dual Pressure
FOR %
High pressure
Intermediate
• ressure
Low pressure
General (Valves,
control etc.
Total
0.22
Tnple Pressure
LO % EFOR % FOR %
100
0.22
LO %
EFOR %
0.21
100
0.21
0.19
100
0.19
0.20
100
0.20
0.13
70
0.09
0.09
100
0.09
0.10
100
0.10
0.51
0.63
0.51
(1) The thermal advantage of a triple- over a dual pressure
reheat cycle sharply diminishes with increasing OT exhaust
temperatures.
(2) With increasing GT exhaust temperatures, higher HP steam
pressures show an increasing output benefit for the dual
pressure reheat cycle.
(3) The simpler design of the dual pressure reheat HRSG
results in an advantage of about 0.1% points in equivalent
forced outage rate (EFOR).
(4) The dual pressure reheat cycle requires consistently less
HRSG surface than the triple pressure reheat cycle
irrespective of GT exhaust temperature.
0.59
Combining these facts and carrying out the net present value
(NPV) calculation with the parameters from Table 4 results in
Figure 7 below. Figure 7 shows the difference in NPV between
dual pressure reheat and triple pressure reheat cycles with gas
turbine exhaust temperature as parameter. The figure shows
how the facts work in combination to result in an increasing
NPV advantage of the dual pressure reheat cycle over the triple
pressure reheat cycle with increasing GT exhaust temperature.
Tab. 5.: Reliabi ity values for dual pressure reheat and triple
pressu e reheat HRSGs [4, 51. FOR ... forced outage
rate; LO ... loss of output; EFOR ... equivalent forced
outage rate
Cycle investment cost: The primary difference between the two
cycles compared is the design of the HRSG. The dual pressure
reheat cycle is a simpler design requiring less heat exchange
surface. Figure 6 shows the heat exchange surface difference
between dual and triple pressure reheat cycles with gas turbine
exhaust temperature as parameter. The figure shows how the
dual pressure reheat cycle maintains an almost constant HRSG
surface advantage over the triple pressure reheat cycle
regardless of OT exhaust temperature.
5
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increased bottoming cycle efficiency as these elevated
pressures define the top of the ST expansion curve and an
overall increase in the power density of the cycle (power per unit
mass flow of working fluid).
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X
x
x
In considering higher live steam pressures, the next logical step
is evaluating the use of a high speed, high efficiency geared HP
turbine section. With higher live steam pressures - which are
favored by sequential combustion and a dual pressure reheat
cycle, as shown in the previous sections - the value (kW/kg) of
live steam increases significantly. Hence higher HP steam
turbine efficiency pays off disproportionately. The geared ST
technology, with many years of proven experience, allows for
complete optimization of the steam flow profile for maximum
efficiency resulting in increasing benefits with increasing steam
pressures (Figure 8). Through their unique design, high speed
turbines are more efficient that 3600 rpm turbines. The reason
is that the circumference (blade-hub-blade) of the stages is
smaller than.a comparable 3600 rpm turbine. This in turn yields
longer, more efficient blades since although the circumference
is smaller, the steam path area is constant (the tip speeds of
both turbine types are identical). Since tip losses and root
losses are basically constant, longer blades (high speed) are
always more efficient than shorter blades. Therefore, the
resulting HP turbine section can achieve higher efficiency with
fewer stages than a comparable direct drive unit.
1
X
texh•550C
1601090
Evaporation pressure [bare]
Fig. 7.: Difference in NPV between dual pressure reheat and
triple pressure reheat cycle as a function of live steam
pressure with gas turbine exhaust temperature as
parameter. This picture shows that for CT exhaust
temperatures above approximately 630 °C (1150°F) the
dual pressure reheat cycle is the most economical
solution regardless of live steam pressure. For a CT
exhaust temperature of 650 °C (1200°F) the picture also
shows an increasing advantage of the dual pressure
reheat cycle with increasing evaporation pressure.
This NPV comparison shows that the NPV advantage of the
dual pressure reheat cycle increases with increasing live steam
pressures. Figure 7 compares dual pressure reheat and, triple
pressure reheat cycles. Therefore, it does not give any
information as to where the optimum live steam pressure for the
dual pressure reheat cycle lies. The following section answers
that question and draws some conclusions as to which
components shall be applied to make this cycle work most
effectively.
Figure 8 shows the power output difference between a geared
ST at 170 bar (2465psia) evaporation pressure and a nongeared ST at 110 bar (1600psia) evaporation pressure as a
function of CT exhaust temperature with the cycle type as
parameter. The figure shows the increasing rewards of using
higher steam pressures and a geared steam turbine with
increasing CT exhaust temperatures. With increasing exhaust
temperatures the geared steam turbine becomes ever more
favorable for the dual pressure reheat cycle.
Specification of the dual pressure reheat
cycle
t it
2
0
This section optimizes the live steam pressure of the dual
pressure reheat cycle based on the economical parameters
shown in Table 4. It also shows which components the
optimized cycle requires to work most effectively.
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F 2.6
2 2
o.
< .2 21
0
The previous section has shown that above CT exhaust
temperatures of approximately 630 °C the dual pressure reheat
cycle is the most economical cycle regardless of live steam
pressure (Figure 7). However, it has also shown that increasing
live steam pressures yield increasing benefits for the dual
pressure reheat cycle with increasing CT exhaust temperatures
(Figure 3). Conventional plants, where heat input at high
temperature levels has always been available, have used live
steam parameters of 170 bar (2465psia) and 565°C (1050psia)
for more than two decades. This was not possible with
combined cycle plants until recently when high GT exhaust
temperatures have become available.
2, to
uti
"0
III
In
710
OT exhaust temperature [C]
Fig. 8.: Difference in power output between a geared ST at 170
bar evaporation pressure and a non-geared ST at 110
bar evaporation pressure
The CT exhaust energy determines the amount of live steam
that can be produced at a given pressure level. Increasing live
steam pressures produce a smaller mass- and volume flows of
higher energy steam. In principle, this provides two benefits:
Evaluating the dual pressure reheat cycle at an elevated gas
turbine exhaust temperature with a geared steam turbine results
in the curve shown in Figure 9 below. Figure 9 shows the NPV
6
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of the dual pressure reheat cycle as a function of evaporation
pressure with the cycle at 70 bar (1000psia) as reference.
Figure 9 shows that the economical evaporation pressure lies
between 150 and 170 bar (2200 and 2500psia) for an optimized
dual pressure reheat cycle with a geared steam turbine at a ST
exhaust temperature of 650 °C.
•
a decreasing output difference between dual pressure reheat
and triple pressure reheat cycles and an increasing benefit
of higher live steam pressures for the dual pressure reheat
cycle with increasing ST exhaust temperatures.
•
an advantage of about 0.1% points in equivalent forced
outage rate for the dual pressure reheat HRSG
•
an almost constant HRSG surface advantage of the dual
pressure reheat over the triple pressure reheat cycle
regardless of ST exhaust temperature.
The comparison of net present value based on these facts
shows an increasing NPV advantage of the dual pressure
reheat cycle over the triple pressure reheat cycle with increasing
GT exhaust temperature. For ST exhaust temperatures above
approximately 630°C (1150°F) the dual pressure reheat cycle is
the more economical solution regardless of live steam pressure.
For a ST exhaust temperature of 650 °C (1200°F) the advantage
of the dual pressure reheat cycle continually increases with
increasing evaporation pressure.
II
HI
Also demonstrated was the increased potential of higher live
steam pressures at high gas turbine exhaust temperatures can
best be utilized with a geared high pressure steam turbine. The
geared ST technology, with many years of proven experience,
allows for complete optimization of the steam flow profile for
maximum efficiency resulting in increasing benefits with
increasing steam pressures. At a ST exhaust temperature of
650°C the resulting optimum live steam pressure for the dual
pressure reheat cycle lies between 150 and 170 bar (2200 and
2500psia). The increase in power output largely outweighs the
cost increase for the high pressure parts in HRSG and water
steam cycle.
Evaporation pressure [bare]
Fig. 9.: NPV of the dual pressure reheat cycle a function of
evaporation pressure with the cycle at 70 bar
(1000psia) as reference. As the evaluation shows the
economical evaporation pressure lies between 150 and
170 bar (2200 and 2500psia). The increase in power
output largely outweighs the cost increase for the high
pressure parts in HRSG and water steam cycle.
Summary and Conclusions
How these thermodynamic advantages of the dual pressure
reheat cycle translate into an optimum power plant concept
together with the ABB sequential combustion gas turbines 5T24
and 5126 will be the subject of a follow-up paper.
As earlier postulated, the most economical and hence most
competitive combined cycle power plant solution must consider
a plant approach that integrates the ST and WS cycle in a
manner most beneficial to the owner of the plant. As the
deregulated electric industry evolves and cost transparency is
no longer available, the owners of new generating facilities must
seek the true optimal configurations. ST exhaust energy is the
link between ST and WS cycle. Therefore, the design and the
operating concept of the ST must consider this link and provide
the exhaust energy at a level that leads to a simple, reliable and
efficient WS cycle concept.
Acknowledgements
The authors would like to acknowledge the contributions of the
following individuals to the publication of this paper Rolf
Bachmann, Yves Carels, Sep van der Linden and Melinda
Martin.
A comparision of heat recovery effectiveness (fit of a cycle with
a given level of ST exhaust energy) and heat recovery efficiency
for single pressure reheat, dual pressure reheat and triple
pressure reheat cycles showed that an increase in gas turbine
exhaust temperature significantly increases the effectiveness
and efficiency of heat recovery irrespective of the cycle
configuration employed. The comparison also showed that the
advantage of a triple pressure reheat cycle over a dual pressure
reheat cycle sharply diminishes with increasing gas turbine
exhaust temperatures.
An economic comparison of the dual pressure reheat and triple
pressure reheat cycles, considering cycle efficiency, cycle
availability and cycle investment costs, showed
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References
[1]
Jury W. and Luthi H.: 'Advanced GTs call for
advanced CC's (or the 4 S's)', ASME 94-GT-303, paper
presented at the international gas turbine and aero engine
congress and exhibition, the Hague, Netherlands - June 13-16,
1994
[2]
Chiesa P. et. al.: 'Predicting the ultimate performance
of advanced power cycles based on very high temperature gas
turbine engines', paper presented at the 38th ASME
international gas turbine and aero engine congress, Cincinnati,
May 1993
Lugand P. and Parietti C.: 'Combined cycle plants with
[3]
frame 9F gas turbines', ASME paper 90-GT-345
[4]
NERC statistics 1978-1987 and 1988-1994
VGB analysis of non-availabilities 1988-1994
(5)
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