Note: Descriptions are shown in the official language in which they were submitted.
METHOD TO INTEGRATE REGENERATIVE RANKINE CYCLE INTO
COMBINED CYCLE APPLICATIONS USING AN INTEGRATED HEAT
RECOVERY STEAM GENERATOR
Inventors: Mark J. Skowronski and Ronald F. Kincaid
References Cited
U.S. PATENT DOCUMENTS
4,829,938 .............. Motai, et at
4,961,311 .............. .James
4,976,100 .............. .Lee
5,799,481 .............. .Fetescu
5,649,416 .............. Rollins
6,363,711 .............. .Aktiengesellschaft
6,606,848 .............. Rollins
US Application 13/987,439 .. Skowronski, et at
TECHNICAL PAPERS AND PUBLICATIONS
GE Combined Cycle Product Line and Performance GER-3574G by D. L. Chase and P.
T. Kehoe
Comparison of Power Enhancement Options for Greenfield Combined Cycle Power
Plants, Thomas C. Tillman, February 2004, Rev. 2
Economic and Technical Considerations for Combined Cycle Performance
Enhancement
Options by Chuck Jones and John A. Jacobs III GE Power Systems GER-4200
Introduction to the Complementary Fired Combined Cycle Power Plant, Power-Gen
International 2006, Siemens
CROSS-REFERENCE TO RELATED APPLICATIONS
10001] This application claims the benefits of the U. S. Nonproyisional Patent
Application No. 15/530,258 entitled "Method to integrate regenerative rankine
cycle into
combined cycle applications using an integrated heat recovery steam generator"
filed on
December 15, 2016. This application is incorporated by reference herein in its
entirety.
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BACKGROUND OF THE INVENTION
[0002] Combined cycle power plants have come of age due to the advances in
combustion turbine technology and, most recently, due to the new natural gas
recovery
technology of "fracking". Fracking has significantly increased the gas
reserves of the
United States and has significantly lowered the cost of gas recovery. The
state of the
combustion turbine technology and the availability of long term and relatively
low cost
natural gas has made the combined cycle the prominent choice for both future
generation
needs to serve new loads and to replace coal generation in the near and mid-
term future.
[0003] Early applications for combustion turbines were aero derivative models
which were, essentially, modified jet engines originally designed for aircraft
and
modified for land base use. However, the design of this type of technology,
i.e.
combustion turbines, gradually became specific to the needs of the electric
utility
industry such that by the 1970's specific combustion turbines with
characteristics
specifically designed to optimize performance in combined cycle operation were
commercially available.
[0004] A combined cycle can be described in two parts; the "top" cycle which
is the combustion turbine utilizing a Brayton cycle, and the "bottom" cycle
which is the
steam Rankine cycle. Shaft power is initially generated through the use of
combustion
turbines; the turbine section of the combustion turbine used for land base
power
generation is designed such that there is no thrust as all developed power is
recovered in
the shaft; however, there is still significantly high exhaust temperature
which, in
standalone applications, is wasted. The "bottom" cycle of a combined cycle is
a Rankine
cycle which uses the waste heat from the combustion turbine. The turbines used
in
combine cycle applications have not been designed necessarily to be the most
efficient in
a standalone configuration, but rather to be the most efficient when used in
tandem with a
bottoming Rankine cycle. Typically, these types of combustion turbines
normally have a
low pressure ratio which results in a high exhaust temperature. The high
exhaust
temperature is beneficial to the Rankine cycle which, in tandem use with the
combustion
turbine, can produce combined overall efficiencies in the 60% range.
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[0005] Increasing the efficiency of a combustion turbine typically requires
higher firing temperatures at the turbine inlet and higher pressure ratio to
use the higher
thermodynamic availability resulting from the higher firing temperature.
However, while
increasing the firing temperature without a commensurate increase in the
pressure ratio
may minimally increase the efficiency of the turbine, the higher exhaust
temperature
resulting from the higher firing temperature can significantly increase the
efficiency of
the Rankine cycle. In accordance with the second law of thermodynamics, the
efficiency
of any heat cycle can be expressed as:
Efficiency = 1 ¨ (TOT)
Where TL is the low temperature of the working fluid, i.e. the low temperature
of the
steam in the cycle, where heat is exhausted to the heat sink. TH is the high
temperature of
the working fluid, in our case, steam, and is the point where expansion of the
working
fluid is used to produce work.
[0006] Consequently, it is always thermodynamically preferable to have the
working fluid to be expanded at the highest possible temperature. In order to
achieve a
high steam temperature, typically around 1050 F, a high exhaust temperature is
required;
this exhaust temperature must be higher than the operating steam temperature
in order to
affect heat transfer. It is also noted that for a high Rankine cycle
efficiency, the steam
must be expanded to the lowest possible temperature and pressure. Typically
the
temperature is around 115 F at about 1.5 psia or so. However, herein lays a
problem for
an efficient combined cycle.
[0007] By expanding and condensing the steam to a low temperature, a
regenerative Rankine cycle is not possible for a conventional combined cycle
configuration. In order to achieve a low stack gas temperature, low feedwater
temperature must be supplied to the waste heat boiler. For example, if the
feedwater is
heated through regeneration to a temperature of, say, 500 F then it is
impossible for the
stack gas temperature to be lower than 500 F and, in fact, since a temperature
difference
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must be maintained in order to achieve heat transfer (usually a minimum of 50
F or so),
then the stack temperature must exit at around 550 F and this hot gas
represents an
enthalpy loss to the overall cycle. Therefore, feedwater heating, if any, can
only be used
sparingly in order to maintain a sufficiently low feedwater temperature in
order to ensure
there is no unreasonable stack loss.
[0008] To date, turbine manufacturers have concentrated on increasing firing
temperatures of the combustion turbines for increased efficiencies; but high
firing
temperature requires enormous research and development costs as well as costly
material
and blade cooling methods. The novelty proposed herein goes back to the basics
and
proposes an alternative that increases the efficiency of the Rankine cycle not
through
higher operating working fluid temperatures but employing regenerative heating
to
increase the Rankine cycle efficiency.
[0009] Overall, the energy consumption in the United States has declined
slightly over the last 5 years and much of this decline can be attributed to
the overall
economic decline of the past several years. However, domestic production has
still
increased by about 3% per year due to a decrease in the importation of
electricity from
Mexico and Canada. Overall, in the next ten years, electric consumption in the
United
States is expected to grow incrementally at about 1 to 1.5% per year. Even
though this is
a small number, the total installed capacity in the United States in 2010 was
about 1,140
GW's. Therefore, even a 1 % increase would require construction of about
twenty 500
MW power plants every year. And this does not include the replacement capacity
due to
aging plants, and, in particular, aging coal plants.
[0010] There is a significant market driven by the aging coal plants in this
country. Over the next 10 ¨ 15 years, dozens of coal units will be replaced
with gas-
fueled combined cycle units. It is unlikely that the power plant operators
will walk away
from an existing power plant site which has high value infrastructure
including
transmission and water rights as well as a certain ease of permitting since
development
would occur on an already despoiled plant site. There is significant
difficulty in
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developing a new coal plant since coal has increased in price and natural gas
has
decreased. In addition, the combined cycle is about 40-45% more efficient than
a coal
plant and the capital cost is about 1/3 the cost of a coal plant. And this
price differential
does not include the cost of greenhouse gas (CO2) clean up which would add
considerably to the cost of coal generation.
[0011] Greenhouse gases will be a significant driver not only for renewable
energy resources but also for combined cycle plants as well. Combined cycle
plants emit
less than 50% greenhouse gas than a similar size coal plant operating at the
same capacity
factor. Green house gas reduction is a significant driver for the construction
of combined
cycle power plants. Consequently, a low cost and highly efficient Regenerative
cycle
integrated in a combined cycle novelty will be received favorably in the
commercial
markets.
SUMMARY OF THE INVENTION
[0012] Using the concept of a combined cycle, this novelty creates a separate
and designated stream of preheated feedwater mass flow rate by bifurcating the
flow
from the condenser hotwell (condensate/feedwater) to allow harvesting of
additional
enthalpy resulting from duct firing. One stream is the traditional low
temperature
condensate/feedwater (at condenser saturation pressure) that is fed directly
to the Heat
Recovery Steam Generator (HRSG) and the other is a separate preheated
condensate/feedwater that is fed into common heating elements of the HRSG.
This
preheated condensate/feedwater is generated through steam extractions thereby
creating a
separate regenerated Rankine cycle within the combined cycle. This is
differentiated
from the traditional method of duct firing whereby the increased low
temperature
feedwater flow is added to the existing feedwater flow from the condenser and
fed
directly to the HRSG without regeneration. Consequently, in a traditional
arrangement of
feedwater flow to the HRSG, the feedwater flow must be kept at low temperature
prior to
entering the HRSG in order to ensure that the stack gas temperature does not
rise.
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[0013] By having the additional and differentiated feedwater mass flow rate in
the cycle, which has been preheated through regeneration and delivered by a
separate
feed, the preheated feedwater can be further heated through common heating
elements
and the addition of one or more duct firing arrays in a modified design of a
conventional
Heat Recovery Steam Generator (HRSG). In this application, the term "common
heating
elements" is defined as those pipes, headers, drums and other associated
heating elements
and components which are co-shared with the flows of a traditional combined
cycle in a
non-regenerative Rankine cycle and those flows resulting from a separately
generated
regenerative Rankine cycle. It is important to note and differentiate the
primary
difference between this novelty and the previously submitted concept is that
this novelty
co-shares heating elements, tubes, headers and drums that are common to a
traditional
Heat Recovery Steam Generator (HRSG). In other words, the heated feedwater
flow that
has been pre-heated through regeneration is continued to be heated in tubes,
heating
elements and flows through headers and drums that are also utilized in the
production of
steam common to a non-regenerated steam cycle. In this manner, less exotic
tube
material that is less expensive is used and the design is also simplified. In
the previous
design, which incorporated direct duct firing on separate heating elements,
high
temperature dictated a costly design and expensive tube material.
[0014] In addition, another strong advantage of this novelty's design over the
predecessor is the reduction of cooling flow required. In this novelty's
design, where
common flow is shared in the same tubes, headers, and heating elements,
cooling is
reduced to near zero. The previous design required cooling flows during those
periods
when no pre-heated feedwater, heated by extraction steam, was available. Since
the
previous design had separate and flow dedicated heating elements in the
exhaust gas flow
prior to the HRSG, these forward located heating elements, located immediately
downstream from the duct firing arrays, had to be cooled. Consequently, the
cooling
requirement resulted in a heat penalty attributed due to the overall system
heat rate.
[0015] The additional heat added to the separate preheated feedwater mass flow
results in the production of additional main and reheat steam flow produced by
the
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common heating elements used in combination with one or more duct firing
arrays
integrated with the conventional combined cycle production of main and reheat
steam for
generation of energy in a Rankine non-regenerative system. The main and reheat
steam
produced by the preheated regenerative feedwater mass flow rate is
thermodynamically
compatible with the main and reheat steam produced by the conventional
combined cycle
and these steam flows are combined prior to steam turbine entry.
[0016] In this manner, the steam turbine serves as the primary mover for both
the steam extraction regenerative Rankine cycle resulting from the additional
heat that is
added and the non-regenerative or straight through non-regenerative Rankine
cycle
resulting from the traditional combined cycle. This additional heating of the
regenerated
preheated feedwater is through an integrated design of the HRSG which allows
heating of
both the non-preheated condensate/feedwater and the separately fed preheated
condensate/feedwater. The co-sharing of flows, one flow generated by
extraction steam
and the other flow generated by the once through cycle of the HRSG, allows for
a simple
and cost effective design of the HRSG. The additional heating required due to
the
additional flow produced by the heated feedwater, is performed with one more
duct firing
arrays within the HRSG such that there is no or minimal increase in the stack
temperature. By firing the added duct burner array, there is additional
enthalpy provided
to the feedwater, main steam and reheat steam that is the result of the added
regenerative
cycle.
[0017] The integrated design of the HRSG that is capable of heating both the
preheated and non-preheated condensate will require larger piping diameters in
order to
optimize the overall heat absorption in the HRSG. The co-sharing of the tubes
is only
necessary for the evaporator, superheating and reheating portion of the HRSG.
However,
some economizer heating, feedwater preheating and low pressure steam
generation may
be required since additional enthalpy supplied by the duct firing must also be
absorbed in
the low temperature end of the HRSG. The amount of enthalpy that must be
absorbed to
preclude a rise in stack temperature is dependent on the overall HRSG design
and at what
temperature the pre-heated feedwater is brought to the HRSG. The additional
enthalpy in
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the back end of the HRSG can be used for low pressure steam generation and
also used
for steam dearating purposes of the overall cycle.
[0018] Typically, the mass ratio of the total steam extraction flows to the
main
throttle steam flow is in the order of .35 or so to fully utilize regeneration
and to pre-heat
the feedwater as much as possible. This ratio assumes that the amount of main
steam
throttle flow is essentially the same as the condensate/feedwater flow rate as
there are
practical considerations regarding the amount of heat that can be transferred
from the
steam to the condensate/feedwater. However, in this case, since the feedwater
heating
only applies to that amount of additional flow attributed to duct firing, this
dedicated flow
of preheated feedwater could be raised close to or even to the saturation
temperature of
the operating pressure of the waste heat boiler. Limitations would ensue based
on
amount of total flow of main steam throttle flow to the dedicated feedwater
flow for duct
firing. In traditional regenerative cycles, regeneration is normally limited
by the amount
of heat that can be transferred from the main throttle steam flow; in this
invention, the
limitation can be the amount of heat absorbed by the feedwater stream. In any
case, the
heating of the independent preheated feedwater flow in an integrated designed
MSG
will result in a significant gain in thermodynamic efficiency.
[0019] By switching "off' this novelty's concept, the HRSG can still be
operational in a "normal mode" though a small incremental amount of duct
firing may be
required during normal mode operation; additional duct firing may be necessary
in order
to keep the regenerative portion of the heating elements "hot". This flow
would be a
nominal few percent of maximum flow to eliminate possible thermal shock and to
ensure
that water is not transported back to the turbine via the extraction piping.
In this mode of
operation, the overall capacity would be reduced since there is minimal or no
additional
preheated feedwater being delivered to the HRSG; however, the overall
efficiency would
be improved since a combined cycle operation will normally have a higher
efficiency
when compared to a steam regenerative Rankine cycle. Alternately, when high
capacity
is preferred, the plant can be operated in the enhanced regenerative mode as
described
herein; however, the overall efficiency may be slightly lower when the steam
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regenerative Rankine cycle is averaged with the combined cycle efficiency.
When higher
capacity is required the switch can be made to this novelty of increasing
capacity through
the addition of regenerative Rankine generation. Depending on need and design,
the
added amount of generating capacity resulting from the added regenerative
cycle is
significantly higher than can be achieved when using traditional duct firing
and merely
increasing the low temperature feedwater flow into the traditional combined
cycle.
[0020] The reheating elements for the production of intermediate pressure
steam
or hot reheat required for the regenerative steam cycle within the HRSG would
also be
integrated and combined with the reheating of the combined cycle steam cycle.
In this
manner, an integrated design of the HRSG heating elements serves both the
needs of the
combined cycle steam production, both main steam and reheat steam, and the
regenerative Rankine steam production, both main steam and reheat steam.
Consequently, the integrated HRSG design produces main and reheat steam at the
same
pressure since this steam is produced by co-sharing the same tubes, headers,
drums and
overall heating elements as the steam produced by the once through steam cycle
typical
with standard combined cycle operation. Additional duct firing would be
required to
provide the necessary enthalpy to create steam from the preheated condensate
and to
increase the reheat steam temperature. This technique allows for reheating
back to the
original main steam temperature without impacting the stack gas temperature.
[0021] It is noted that this novelty can be applied to new installation or to
existing regenerative Rankine cycle installations. In particular, coal plants
that are near
end of life operation could be repowered utilizing the existing steam turbine
generator,
feedwater train and associated piping, and equipment as well as the indigenous
infrastructure such as site and transmission. In this embodiment, at least one
combustion
turbine with at least one HRSG could be used to incorporate the existing coal
plant's
equipment.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Drawing 1 is a sketch that diagrammatically shows the proposed
concept. The drawing shows an inner feedwater loop, shown in dotted lines,
employing
feedwater heaters supplying additional feedwater flow in a designated flow
path such that
common heating elements and added duct firing results in a separate
regenerative
Rankine cycle. The duct firing shown is an added array and is not to be
confused with
conventional duct firing used to increase the steaming capacity of the HRSG.
The
novelty's proposed additional duct firing does not increase the feedwater flow
rate from
the condenser directly to the HRSG to produce more steam, this novelty
proposes a
separate loop method allows the feedwater to be preheated in a separate loop
using
extraction flows from the steam turbine with additional enthalpy added for
steam
production using a dedicated duct burner array. The novelty's added duct
firing
precludes the installation of a conventional duct firing array but does not
impact or
impede the operation of using the new array for conventional duct firing and
can be used
in tandem with the proposed novelty.
[0023] Drawing 2 is similar to Drawing 1 but shows the additional embodiment
of reheating that would be available, if deployed, under this novelty. In this
scheme, the
cold reheat steam is bifurcated with the majority of steam flowing to the
common and co-
shared heating elements and the remaining steam flow used for regenerative
heating in
the first point heater.
DETAILED DESCIPTION OF THE INVENTION
[0024] The numbers and data shown are general approximations only in order
to more fully delineate the principles of the proposed novelty and the overall
flow
schematic should not be construed as a final thermodynamic analysis. Referring
to
Drawing 1, if we assume a closed operating Rankine cycle, condenser 1
condenses the
CA 2988273 2017-12-08
steam flow 18 from the low pressure steam turbine 12. This novelty separates
that
amount of condensate into two streams 2 and 3 where stream 2 is the additional
mass
flow rate used for regeneration and absorbs the heat from steam extractions
from
appropriate ports in the extraction turbine. In practice, the fraction
dedicated to the
regenerative portion of the condensate flow 2 from the condenser is,
typically, about 40-
45 % of total condensate flow. However, these values can be adjusted for cycle
optimization. The pre-heated feedwater 7 is shown in Drawing 1 as a dedicated
feed to
the co-shared heating elements 8. The amount of condensate 3 used for non-
regenerative
cycle operation is fed directly to the HRSG 9 for feedwater heating,
evaporating and
superheating and then directed to the High Pressure (BP) steam turbine 11.
Condensate 2
flows through the regenerative heater #3 4, then through heater #2 5 and then
completes
its pre-heating through heater #1 6. Typically, in traditional Rankine
regenerative reheat
cycles that are non-critical, the first point heater (heater #1) 6 receives
steam extraction
from the cold reheat line; this embodiment of reheat is described further in
Drawing 2.
The herein embodiment description assumes that the first point heater 6
receives its
extraction flow from the cold reheat line from the HP turbine 11. For
simplicity, boiler
feed pumps and other associated flow lines, such as feedwater drip lines, have
not been
shown.
[0025] The amount of reheating, and the number of feedwater heaters, is an
economic evaluation whereby the cost of preheating is evaluated against the
gain in
efficiency; typically large coal plants use 7 or 8 heaters; if a new facility
is used, an
economic evaluation will determine the number of feedwater heaters used.
Drawing 1
shows only three for simplicity. While this novelty permits heating close to
the
saturation point, it is assumed here for illustrative purposes that the pre-
heated feedwater
7 is heated to approximately 500 F. Heating elements 8 provide sensible
heating,
evaporation and superheating required for production of main steam.
100261 While the exhaust of the combustion turbine 13 is shown as 1160 F, the
additional duct firing 14 adds heat such that the overall gas temperature is
now 1540 F.
The amount of heat required to evaporate and superheat the main steam and to
reheat the
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steam from the feedwater 7 would then bring down the combustion turbine's
exhaust gas
temperature as the gas flow travels from the high temperature heating elements
to the
lower heating elements (feedwater heating and economizers). Since there would
be
excess heat in the lower temperature end of the HRSG due to the duct firing
and heating
the 500F preheated feedwater, excess enthalpy is used for lower steam pressure
generation and to preheat the steam used for dearation. In this manner, any
increase in
the stack temperature, as compared to the stack temperature when no
regenerative steam
is being produced and there is no duct firing, can be held to a minimum
[0027] Referring again to Drawing 1, the feedwater 7 is heated in the co-
shared
heating elements used for production of steam and reheat steam in the non-
regenerative
combined cycle, the feedwater stream which is now superheated steam 10 is
directed to
the inlet of the HIP steam turbine 11 where it is mixed with the main steam
produced by
the CT exhaust flow in the HRSG 9 at the same pressure and enthalpy for
expansion in
the HP turbine 11. A separate line 10, as shown in Drawing 1 may be necessary
depending on the design of the existing turbine; otherwise, the steam is fed
to the turbine
in a common header. It is noted that this example depicts a three pressure
combined
cycle and that the low pressure steam 15, and the intermediate pressure steam
16 are
directed to the IP/LP steam turbine 12 as appropriate. The main steam 17, the
intermediate pressure steam 16 and the low pressure steam 15 have all been
generated
with minimal changes to the HRSG 9. The primary design parameter proposed in
this
novelty is that the heating of the separated and designated regenerative
feedwater 7 is
performed by integrating with the heating elements required for the combined
cycle
although larger carrying capacity is required. These co-shared heating
elements 8 and the
added duct firing 14 in the duct upstream of the conventionally designed HRSG
9 where
the said HRSG design is, essentially, unaltered and the stack temperature 23
remains,
essentially, unchanged.
[0028] Referring to Drawing 2, the addition of a reheat section is shown in
conjunction with the production of main steam produced by the previously
described
regenerative Rankine cycle in Drawing 1. The cold reheat working fluid 24 is a
separate
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loop used to reheat that portion of the main steam that has been generated
through a
regenerative Rankine cycle. It is noted that the main steam produced by the
HRSG using
solely the waste heat of the CT 13 is reheated through the HRSG operation
only.
Drawing 2 is the same as Drawing 1 except for the addition of the specific
equipment and
lines required for reheating of the main steam. In Drawing 2, we follow the
assumption
that most non-critical Rankine cycles take the first point heater steam
extraction 22 from
the cold reheat line 19. The remaining fraction of the cold reheat 24 is then
directed to a
co-shared reheater 21 used by the traditionally designed combined cycle. The
reheated
steam 20 is directed to the intermediate steam line 16 and mixed with the
combined
cycle's production of intermediate steam and directed to the IP/LP steam
turbine 12.
Although a separate line is shown, the delivery of the hot reheat may also use
a co-shared
header, drum and other heating elements. The reheating process does not impact
the
stack temperature 23.
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