U.S. patent application number 15/530258 was filed with the patent office on 2018-06-21 for method to integrate regenerative rankine cycle into combined cycle applications using an integrated heat recovery steam generator.
The applicant listed for this patent is Ronald Farris Kincaid, Mark Joseph Skowronski. Invention is credited to Ronald Farris Kincaid, Mark Joseph Skowronski.
Application Number | 20180171827 15/530258 |
Document ID | / |
Family ID | 62557305 |
Filed Date | 2018-06-21 |
United States Patent
Application |
20180171827 |
Kind Code |
A1 |
Skowronski; Mark Joseph ; et
al. |
June 21, 2018 |
Method to integrate regenerative rankine cycle into combined cycle
applications using an integrated heat recovery steam generator
Abstract
A system is disclosed that incorporates a regenerative Rankine
cycle integrated with a conventional combined cycle. This novelty
requires minimal changes to a conventionally designed Heat Recovery
Steam Generator and uses an added duct firing array(s) to boost the
enthalpy of combustion turbine exhaust. The higher enthalpy in said
exhaust is then extracted with the co-shared heating elements of
the conventionally designed combined cycle to produce high pressure
main and reheat steam. In practice, the condensate stream from the
condenser is bifurcated such that a separate and dedicated
feedwater flow, used for regeneration, is directed to feedwater
heaters and then converted to steam with the provided additional
enthalpy at the same pressure and temperature as the main steam in
the conventional combined cycle. The fractional amount of
condensate that is not sent through the feedwater heaters is
directed to the HRSG to be heated in conventional fashion.
Inventors: |
Skowronski; Mark Joseph;
(Irvine, CA) ; Kincaid; Ronald Farris; (Los
Alamitos, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Skowronski; Mark Joseph
Kincaid; Ronald Farris |
Irvine
Los Alamitos |
CA
CA |
US
US |
|
|
Family ID: |
62557305 |
Appl. No.: |
15/530258 |
Filed: |
December 15, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F01K 7/38 20130101; F01K
7/40 20130101; F01K 11/02 20130101; F01K 23/10 20130101; F01K 3/24
20130101; Y02E 20/16 20130101 |
International
Class: |
F01K 3/24 20060101
F01K003/24; F01K 11/02 20060101 F01K011/02 |
Claims
1. A method for generating electric power that incorporates the use
of a regenerative Rankine cycle with a combined cycle, the method
comprising the steps of: Bifurcating the condensate from a
condenser into two or more separate condensate feed streams whereby
the condensate in at least one condensate feed stream is
pressurized to feedwater and sent directly to a heat recovery steam
generator and the condensate in at least one condensate feed stream
is pressurized to feedwater and sent to one or more common heating
elements that is co-shared with the first stream first being
preheated by a one or more feedwater heaters utilizing extraction
steam from an extraction turbine; generating steam in a parallel
cycle using a regenerative Rankine cycle and co-mixing said steam
with the steam produced in a traditional non-regenerative combined
cycle and transferring the steam to an extraction steam turbine
having one or more extraction ports; converting the steam into
electricity through the use of an extraction steam turbine and
generator and extracting some of the steam for heating
feedwater.
2. The method of claim 1, wherein additional heat enthalpy is
supplied to the common heating elements and used to boost the
temperature and enthalpy of the combustion turbine exhaust flow
such that there is additional enthalpy in said combustion turbine
exhaust flow to generate steam for use in a regenerative Rankine
cycle.
3. The method of claim 1, wherein the separately fired duct burner
is placed inside the combustion turbine exhaust ducting and before
the heat recovery steam generator.
4. The method of claim 1, wherein the commonly fired heating
elements of the heat recovery steam generator may be configured in
a "once through" or drum design.
5. The method of claim 1, wherein the method may be utilized in
conjunction with a single pressure or multiple pressure heat
recovery steam generator.
6. The method of claim 2, wherein some or all of the additional
heat enthalpy supplied to the co-shared heating element is
generated from combusting fuel in at least one duct burner.
7. The method of claim 2, wherein the additional heat enthalpy
supplied to the co-shared heating element may be generated from
fossil fuel or non-fossil fuel or a combination of both.
8. The method of claim 2, wherein substantially all of the
additional heat enthalpy supplied to the co-shared heating element
is utilized to generate steam.
9. The method of claim 2, wherein some or all of the additional
heat enthalpy supplied to the co-shared heating element is supplied
through the use of one or more duct burners placed in the
combustion turbine exhaust ducting and before the commonly fired
heating element.
10. The method of claim 2, wherein the low temperature enthalpy
supplied by the duct burner is used for steam dearation or low
pressure steam for power production.
11. The method of claim 2 where additional enthalpy is added by
conventional duct firing and there is a proportional increase in
the low temperature feedwater flow to provide additional capacity
and energy through a straight through non-regenerative Rankine
cycle in the conventional manner of duct firing.
12. The method of claim 2 where a combination of increase of the
low temperature feedwater flow in conjunction with an increase of
the high temperature regenerative feedwater is used in parallel to
optimize both capacity and energy output.
13. A method to generate reheated steam utilizing a co-shared
heating element in a regenerative Rankine cycle used in conjunction
with a combined cycle, the method comprising of: Partially expanded
steam from the high pressure turbine exhaust is sent to a co-shared
heating element to boost said steam to a temperature that is
compatible with the hot reheat steam produced by the heat recovery
steam generator for mixing with total mix directed to the
intermediate pressure turbine inlet; a duct burner to provide for
the necessary enthalpy into the separately fired heating element to
reheat the steam from the high pressure turbine exhaust is located
upstream of the heat recovery steam generator.
14. A method for generating electric power that incorporates the
use of a regenerative Rankine cycle with a combined cycle, the
method comprising the steps of: Bifurcating the condensate from a
condenser into two or more separate condensate feed streams whereby
the condensate in at least one condensate feed stream is
pressurized to feedwater and sent directly to a heat recovery steam
generator and the condensate in at least one condensate feed stream
is pressurized to feedwater and sent to at least one co-shared
heating element first being preheated by a one or more feedwater
heaters utilizing cold reheat steam from a non-extraction turbine;
generating steam in at least one co-shared heating element and
transferring the steam to a non-extraction steam turbine;
converting the steam into electricity through the use of a
non-extraction steam turbine and generator.
Description
REFERENCES CITED
U.S. Patent Documents
[0001] U.S. Pat. No. 4,829,938 . . . Motai, et al [0002] U.S. Pat.
No. 4,961,311 . . . James [0003] U.S. Pat. No. 4,976,100 . . . Lee
[0004] U.S. Pat. No. 5,799,481 . . . Fetescu [0005] U.S. Pat. No.
5,649,416 . . . Rollins [0006] U.S. Pat. No. 6,363,711 . . .
Aktiengesellschaft [0007] U.S. Pat. No. 6,606,848 . . . Rollins
[0008] U.S. application Ser. No. 13/987,439 . . . Skowronski, et
al
Technical Papers and Publications
[0008] [0009] GE Combined Cycle Product Line and Performance
GER-3574G by D. L. Chase and P. T. Kehoe [0010] Comparison of Power
Enhancement Options for Greenfield Combined Cycle Power Plants,
Thomas C. Tillman, February 2004, Rev. 2 [0011] Economic and
Technical Considerations for Combined Cycle Performance Enhancement
Options by Chuck Jones and John A. Jacobs III GE Power Systems
GER-4200 [0012] Introduction to the Complementary Fired Combined
Cycle Power Plant, Power-Gen International 2006, Siemens
CROSS-REFERENCE TO RELATED APPLICATIONS
[0013] This application claims the benefits of the U.S. Provisional
Patent Application No. 62/386,908 entitled "Method to integrate
regenerative Rankine cycle into combined cycle applications using
an integrated heat recovery steam generator" filed on Dec. 16,
2015. This provisional application is incorporated by reference
herein in its entirety.
BACKGROUND OF THE INVENTION
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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-(T.sub.L/T.sub.H)
Where T.sub.L 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. T.sub.H 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.
[0018] 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.
[0019] 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 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.
[0020] 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.
[0021] 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.
[0022] 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 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 (CO.sub.2) clean up which would add
considerably to the cost of coal generation.
[0023] 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
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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 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.
[0028] 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.
[0029] 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 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.
[0030] Typically, the mass ratio of the total steam extraction
flows to the main throttle steam flow is in the order of 0.35 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 HRSG
will result in a significant gain in thermodynamic efficiency.
[0031] 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
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.
[0032] 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.
[0033] 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.
[0034] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] 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.
[0036] 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 DESCRIPTION OF THE INVENTION
[0037] 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 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 (HP) 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.
[0038] 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.
[0039] 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 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 500 F 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
[0040] 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 HP 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.
[0041] 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 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.
* * * * *