U.S. patent application number 14/777177 was filed with the patent office on 2016-02-04 for power generation system and method to operate.
This patent application is currently assigned to SIEMENS AKTIENGESELLSCHAFT. The applicant listed for this patent is SIEMENS AKTIENGESELLSCHAFT. Invention is credited to Anders Stuxberg.
Application Number | 20160033128 14/777177 |
Document ID | / |
Family ID | 47997092 |
Filed Date | 2016-02-04 |
United States Patent
Application |
20160033128 |
Kind Code |
A1 |
Stuxberg; Anders |
February 4, 2016 |
POWER GENERATION SYSTEM AND METHOD TO OPERATE
Abstract
A method to operate a power generation system and the device
itself includes an oxy-fuel burner, a first heat exchanger
assembly, and a rankine-cycle. The oxy-fuel burner generates an
exhaust fluid submitted to an exhaust fluid line and the
rankine-cycle is operated with the working media which is
circulating separately from the exhaust fluid. The exhaust fluid
line is provided with a recirculation line downstream the first
heat exchanger assembly and upstream the working media heat
exchanger extracting exhaust fluid from the exhaust fluid line,
conducting extracted exhaust fluid to a compression unit to
increase pressure and injecting downstream the extracted exhaust
fluid into the oxy-fuel burner.
Inventors: |
Stuxberg; Anders; (Finspong,
SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SIEMENS AKTIENGESELLSCHAFT |
Munchen |
|
DE |
|
|
Assignee: |
SIEMENS AKTIENGESELLSCHAFT
Munich
DE
|
Family ID: |
47997092 |
Appl. No.: |
14/777177 |
Filed: |
March 21, 2013 |
PCT Filed: |
March 21, 2013 |
PCT NO: |
PCT/EP2014/055758 |
371 Date: |
September 15, 2015 |
Current U.S.
Class: |
60/645 ; 110/204;
60/670; 60/679; 60/691; 60/692 |
Current CPC
Class: |
F23L 7/007 20130101;
Y02E 20/344 20130101; F01K 7/34 20130101; F01K 7/22 20130101; F01K
23/10 20130101; Y02E 20/32 20130101; F23C 9/08 20130101; Y02E 20/34
20130101; Y02E 20/326 20130101; F01K 9/02 20130101 |
International
Class: |
F23L 7/00 20060101
F23L007/00; F01K 9/02 20060101 F01K009/02; F01K 7/22 20060101
F01K007/22; F23C 9/08 20060101 F23C009/08; F01K 7/34 20060101
F01K007/34 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 21, 2013 |
EP |
13160405.0 |
Claims
1. A power generation system (PGS) comprising an oxy-fuel burner
(OXB), a first heat exchanger assembly (HEA1), a rankine-cycle
(RC), wherein said rankine-cycle (RC) comprises at least one
turbine (ST) for expansion of a working media (PF), downstream said
turbine (ST) at least one condenser (CON) for condensing of said
working media (PF), wherein said rankine-cycle (RC) comprises
downstream said condenser (CON) at least one first working media
pump (FWP1) delivering said working media (PF) to a higher pressure
level, wherein said rankine-cycle (RC) comprises downstream said
first working media pump (FWP1) at least one first working media
pre-heater (PH1) heating said working media (PF) by extracted
working media (XPF2) from said turbine (ST), wherein downstream
said first working media pre-heater (PH1) said working media (PF)
passes said first heat exchanger assembly (HEA1) to be boiled and
superheated, wherein said oxy-fuel burner (OXB) generates an
exhaust fluid (EXH) by combustion of fuel (F) and oxygen enriched
gas (O2), wherein a first part (EXH1) of said exhaust fluid (EXH)
is provided to an exhaust fluid line branch (EXLB) and a second
part (EXH2) of said exhaust fluid (EXH) is provided for
recirculation to said oxy-fuel burner (OXB), wherein said
rankine-cycle (RC) is operated with said working media (PF) which
is circulating separately from said exhaust fluid (EXH), wherein
said first part (EXH1) of said exhaust fluid (EXH) is provided to
at least one working media heat exchanger (FWE) that is provided to
heat up said working media (PF) of said rankine-cycle (RC)
downstream said first working media pump (FWP1) and upstream said
first working media pre-heater (PH1) by said first part (EXH1) of
said exhaust fluid (EXH), wherein said second part (EXH2) of said
exhaust fluid (EXH) is provided downstream of heat exchangers of
said first heat exchanger assembly (HEA1) to a compression unit
(PU) to increase pressure of said second part (EXH2) in order to
re-inject said second part (EXH2) into said oxy-fuel burner
(OXB).
2. The power generation system (PGS) according to claim 1, wherein
said power generation system (PGS) is operated at a pressure level
for said exhaust fluid (EXH) of several bar above atmospheric.
3. The power generation system (PGS) according to claim 1, further
comprising a catalyst unit for cleaning of said exhaust fluid (EXH)
from residual content of oxygen by addition of further fuel and/or
other combustible media.
4. The power generation system (PGS) according to claim 1, wherein
said turbine (ST) is a combination of at least a high pressure
turbine (HPST) and a low pressure turbine (LPST), wherein between
said high pressure turbine (HPST) and said low pressure turbine
(LPST) said working media (PF) is led through a reheater (AH1),
wherein said reheater (AH1) is part of said first heat ex-changer
assembly (HEA1), so that said working media (PF) is reheated by
said exhaust fluid (EXH) downstream said high pressure turbine
(HPST) and upstream said low pressure turbine (LPST).
5. The power generation system (PGS) according to claim 1, further
comprising at least one adjustable valve (CV) or a capacity control
of said compression unit (PU) to control the flow of said second
part (EXH2) of said exhaust fluid (EXH).
6. The power generation system (PGS) according to claim 1, further
comprising in respect of a fluid flow of a mixed working media
(PFM), upstream of said at least one first working media pre-heater
(PH1), a mixing pre-heater (MP) for mixing a third extracted
working media (XPF3) from said turbine (ST) with said working media
(PF) downstream said condenser (CON) to result in said mixed
working media (PFM).
7. The power generation system (PGS) according to claim 1, further
comprising upstream said oxy-fuel burner (OXB), an air separation
unit (ASU) as part of said power generation system (PGS) to purify
ambient air to generate said oxygen enriched gas (O2).
8. The power generation system (PGS) according to claim 1, wherein
said power generation system (PGS) is set up such that temperature
level at design working conditions for said second part (EXH2) of
said exhaust fluid (EXH) is at least 2/3 of saturation temperature
measured in Celsius of boiling occurring in said first heat
exchanger assembly (HEA1).
9. The power generation system (PGS) according to claim 1, wherein
said feed water heat exchanger (FWE) comprises an output port to
release gaseous carbon dioxide (CO2) and other output port to
release water (H2O), said carbon dioxide (CO2) and said water (H2O)
separated from said first part (EXH1) of said exhaust fluid (EXH)
within said feed water heat exchanger (FWE).
10. A method to operate a power generation system (PGS) comprising:
providing an oxy-fuel burner (OXB), a first heat exchanger assembly
(HEA1), a rankine-cycle (RC), generating an exhaust fluid (EXH) by
said oxy-fuel burner (OXB) by burning oxygen enriched gas (O2) and
fuel (F), wherein a first part (EXH1) of said exhaust fluid (EXH)
is provided to an exhaust fluid line branch (EXLB) and a second
part (EXH2) of said exhaust fluid (EXH) is provided for
recirculation to said oxy-fuel burner (OXB), expanding a working
media (PF) in said rankine-cycle (RC) comprising at least one
turbine (ST) of said rankine-cycle (RC), condensing said working
media (PF) downstream said turbine (ST) by at least one condenser
(CON) of said rankine-cycle (RC), delivering said working media
(PF) to a higher pressure level downstream said condenser (CON) by
at least one first working media pump (FWP1) of said rankine-cycle
(RC), heating said working media (PF) by extracted working media
(XPF2) from said turbine (ST) downstream said first working media
pump (FWP1) by at least one first working media pre-heater (PH1) of
said rankine-cycle (RC), boiling and superheating said working
media (PF) downstream said first working media pre-heater (PH1) by
said first heat exchanger assembly (HEA1) of said rankine-cycle
(RC), operating said rankine-cycle (RC) with said working media
(PF) which is circulating separately from said exhaust fluid (EXH),
providing at least one working media heat exchanger (FWE) for
heating up said working media (PF) of said rankine-cycle (RC)
downstream said first working media pump (FWP1) and upstream said
first working media pre-heater (PH1) by said first part (EXH1) of
said exhaust fluid (EXH), routing said second part (EXH2) to a
compression unit (PU) to increase pressure of said second part
(EXH2) in order to inject said second part (EXH2) into said
oxy-fuel burner (OXB).
11. The method according to claim 10, further comprising: providing
said turbine (ST) as a combination of at least a high pressure
turbine (HPST) and a low pressure turbine (LPST), conducting said
working media (PF) is through a reheater (AH1) located downstream
of said high pressure turbine (HPST) and upstream of said low
pressure turbine (LPST), wherein said reheater (AH1) is part of
said first heat ex-changer assembly (HEA1), so that said working
media (PF) is reheated by said exhaust fluid (EXH) downstream said
high pressure turbine (HPST) and upstream said low pressure turbine
(LPST).
12. The method according to claim 10, further comprising:
controlling the flow through said recirculation line (RCL) by at
least one adjustable valve (CV) or by speed control of said
compression unit (PU).
13. The method according to claim 10, further comprising: mixing a
third extracted working media (XPF3) from said turbine (ST) with
said working media (PF) downstream said condenser (CON) to a mixed
working media (PFM) by a mixing pre-heater (MP), and providing the
mixed working media (PFM) to said at least one first working media
pre-heater (PH1).
14. The method according to claim 10, further comprising: providing
an air separation unit (ASU) as part of said power generation
system (PGS) to purify ambient air to generate said oxygen enriched
gas (O2).
15. The power generation system (PGS) according to claim 1, wherein
said power generation system (PGS) is operated at a pressure level
for said exhaust fluid (EXH) of more than 5 bar above
atmospheric.
16. The power generation system (PGS) according to claim 1, further
comprising a catalyst unit for cleaning of said exhaust fluid (EXH)
from residual content of oxygen by addition of further fuel and/or
other combustible media, connected such that heat of a catalyst
process running in the catalyst unit is recovered for use in said
rankine-cycle (RC) and/or for preheating of fuel (F) or said oxygen
enriched gas (O2).
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is the US National Stage of International
Application No. PCT/EP2014/055758 filed Mar. 21, 2014, and claims
the benefit thereof. The International Application claims the
benefit of European Application No. EP13160405 filed Mar. 21, 2013.
All of the applications are incorporated by reference herein in
their entirety.
FIELD OF INVENTION
[0002] The invention relates to a power generation system
comprising an oxy-fuel burner, a first heat exchanger assembly, and
a rankine-cycle, wherein said rankine-cycle comprises at least one
turbine expanding a working media or process fluid, downstream said
turbine at least one condenser condensing said process fluid,
downstream said condenser at least one first working media pump (or
feed water pump) delivering said process fluid to a higher pressure
level, downstream said working media pump at least one first
working media pre-heater (or feed water pre-heater) heating said
process fluid by extracted process fluid or extracted working media
from said turbine, and downstream said working media pre-heater
said process fluid passes said first heat exchanger assembly to be
boiled and superheated. The oxy-fuel burner will be provided with
recirculated and compressed exhaust fluid from said oxy-fuel
burner.
BACKGROUND OF INVENTION
[0003] Power generation systems and respective methods to operate
such systems are known for a long time since mechanical power or
electrical power is generated especially by burning a fuel with an
oxygen containing gas. Recently concerns came up about
carbon-dioxide content in air increasing up to an amount where a so
called green-house effect might occur. Since such awareness is
rising several projects are initiated to reduce the emission of
carbon-dioxide. One of those projects is burning a fuel with an
oxygen containing gas other than air to avoid the generation of NOx
(nitrogen oxides) and to avoid the mixing of essential inert
components with the carbon-dioxide generated during combustion to
more easily enable the separation of carbon-dioxide from the
exhaust gas generated. This easy separation simplifies storage of
pure carbon-dioxide in a final storage capacity. Essentially pure
carbon-dioxide can further better be used for subsequent chemical
processes.
[0004] The oxygen containing gas is basically pure oxygen with
minor impurities generated by for example an air separation unit,
which can be of conventional membrane type. In the context of this
invention an oxy-fuel burner is characterized by burning basically
a fuel with an oxygen containing gas wherein said oxygen containing
gas has significant higher oxygen content than ambient air and
wherein oxygen is its main component and wherein said oxygen
containing gas is preferably pure oxygen with some impurities. This
oxygen containing gas may contain some further additives but its
main component is preferably oxygen. In other words, the oxygen
containing gas is a gas with an elevated oxygen content compared to
ambient air.
[0005] One known power generation system is disclosed in U.S. Pat.
No. 7,021,063 B2, which deals with an oxy-fuel burner respectively
gas generator comprising a recuperative heat exchanger for
reheating of steam that has passed a first expansion machine stage,
which heat exchanger is heated by outlet steam respectively exhaust
from said gas generator.
[0006] The total efficiency of a conventional power generation
system with an oxy-fuel burner is significantly below the
efficiency of an ordinary power generation system if the energy
consumption of the air separation unit is considered. The
efficiency is therefore to be improved to make this technology
economically feasible and to have a positive effect on the
environment.
SUMMARY OF INVENTION
[0007] It is one object of the invention to improve the efficiency
of the known power generation system comprising an oxy-fuel
burner.
[0008] The object of enhancing the efficiency of the incipiently
defined power generation system is achieved by a power generation
system according the claims. Further the object is achieved by a
method according to the claims. Embodiments can be found in the
dependent claims.
[0009] One essential aspect of the proposed improvement of the
power generation system respectively the method according to the
invention is the combination of oxy-fuel combustion principle with
a boiler design separating the carbon dioxide-steam cycle from the
steam-water cycle. This unique feature enables the operation of the
exhaust fluid--i.e. a heat carrying media--at elevated pressure
above atmospheric pressure. Further high efficiency is achieved by
taking the recirculation of exhaust fluid from upstream economizers
in the boiler such that as little heat as possible is moved from
high temperature to a low temperature parts of the cycle.
[0010] Said oxy-fuel burner according to the invention is basically
a gas generator generating an exhaust gas respectively exhaust
fluid from a fuel burned with essentially pure oxygen. This exhaust
gas is referred to as exhaust-fluid since it might contain liquid
components or parts of the fluid might condense to a liquid.
[0011] A further beneficial efficiency improvement of the process
according to the invention or an embodiment thereof is obtained by
providing said turbine as a combination of at least a high pressure
turbine and a low pressure turbine, wherein between these two
turbines the working media or process fluid--both terms will be
used to identify the closed loop fluid or medium of the steam
cycle--is led through a reheater, wherein said reheater is part of
said first heat exchanger assembly, so that said process fluid is
reheated by said exhaust fluid downstream said high pressure
turbine and upstream said low pressure turbine.
[0012] Another beneficial improvement of the invention is given by
providing at least one adjustable valve and/or one adjustable
pump--which can be a multiphase pump or might as well be a
compressor--to control the flow through said recirculation line.
When gases are recirculated--which may be a particular embodiment
--, then the pump may be replaced by a compressor or fan. This
control feature allows maintaining the desired exhaust-fluid
temperature downstream said oxy-fuel burner respectively before
said heat exchanger assembly. Advantageously a control unit
controls the position of said adjustable valve or pump in the
recirculation line according to a temperature measurement located
advantageously upstream said heat exchanger assembly. This control
unit is designed such that it receives the measurement results from
temperature measurement and submits control signals to said control
valve. The control method in particular is designed such that the
valve opens further when exceeding a temperature limit is
recognized. Further the valve control can be designed such that
upper limits of temperature increases respectively steep
temperature transients in a turbine of the power generation system
are avoided.
[0013] Another embodiment is given by a mixing pre-heater is
provided upstream of said at least one first working media
pre-heater (or first feed water pre-heater). Said mixing pre-heater
mixes a third extracted process fluid (or extracted working media)
from said turbine with said process fluid downstream said
condenser.
[0014] Another embodiment of the invention provides an air
separation unit upstream of said oxy-fuel burner to advantageously
separate oxygen from ambient air to be burned with a fuel in said
oxy-fuel burner. This air separation unit can be of a membrane
type.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The above mentioned attributes and other features and
advantageous of this invention and the manner of attaining them
will become more apparent and the invention itself will be
understood by reference to the following description of the
currently known best mode of carrying out the invention taken in
conjunction with the accompanying drawings, wherein
[0016] FIG. 1 shows a schematic flow diagram of an oxy fuel power
plant comprising the arrangement according to the invention and
depicting the method according to the invention;
[0017] FIG. 2 shows a schematic flow diagram of an oxy fuel power
plant comprising the arrangement according to a second embodiment
of the invention and depicting the method according to the
invention.
DETAILED DESCRIPTION OF INVENTION
[0018] FIG. 1--and also FIG. 2 later on--is a schematic depiction
of a simplified flow diagram showing a power generation system and
illustrating a method according to the invention.
[0019] According to FIG. 1, fuel F and oxygen O.sub.2 from an air
separation unit ASU are both elevated to a higher pressure level by
compressors C1, C2, which compressors C1, C2 might be provided with
not shown intercoolers before both fluids (F and O.sub.2) are
injected in an oxy-fuel burner OXB at a pressure of around 20 bar.
In said oxy-fuel burner OXB--which can also be considered as a gas
generator--combustion takes place of said fuel F with said oxygen
O.sub.2 generating exhaust gas hereinafter referred to as
exhaust-fluid EXH.
[0020] It has to be noted when the term said oxygen O.sub.2 or
"pure oxygen" is used, a gas with an elevated content of oxygen is
meant, e.g. of 95% oxygen content.
[0021] The exhaust fluid EXH--or more generally called heat
carrying media--exits said oxy-fuel burner OXB and enters a first
heat exchanger assembly HEA1.
[0022] Downstream said first heat exchanger assembly HEA1 said
exhaust fluid EXH is divided at a division point DP into
recirculated exhaust fluid EXE stream and the remaining exhaust
fluid (referred to as first part EXH1 which it is diminished by
recirculated exhaust fluid stream EXE, which is also called second
part EXH2) being conducted through a continued exhaust fluid line
EXLB. Alternatively, the division of fluids can be performed even
inside the first heat exchanger assembly HEA1.
[0023] The temperature of said first part EXH1 of the exhaust fluid
stream EXH is adjusted by controlling said flow of recirculated
exhaust fluid EXE (as said also called the second part EXH2 after
the branch off) to the oxy-fuel burner OXB to be mixed with the
fuel F and oxygen containing gas OCG and thus cool the exhaust
fluid EXH (i.e. the second part EXH2) to the right temperature to
subsequently enter said heat first exchanger assembly HEA1. This
control is done by a control unit CU controlling a compression unit
PU and/or a control valve CV. Optionally only one of the
compression unit PU or the valve CV can be provided. The
compression unit PU can be a pump can as well be a multiphase pump
or a compressor or fan depending on the phase of the recirculated
exhaust fluid EXE. The pump or multiphase pump will be used for
liquid content, the compressor or fan for gaseous content. In a
further embodiment, gases will be guided through the recirculating
line RCL, so a compressor or fan will be used for the compression
unit PU.
[0024] Past the compression unit PU the fluid in the recirculating
line RCL may be called pressurized recirculating fluid PEXE which
then is delivered to the oxy-fuel burner OXB.
[0025] Downstream said division point DP said exhaust fluid EXH
passes a first cooler COL1 before it enters a feed water heat
exchanger FWE (or working media heat exchanger) transferring
thermal energy to said process fluid PF (also called working media)
of said rankine cycle RC. This additional sub-cooling effect
further separates carbon dioxide CO.sub.2 from water H.sub.2O of
the exhaust fluid EXH. Said feed water heat exchanger FWE provides
further the feature of separating the gaseous phase from the liquid
phase so that said carbon dioxide CO.sub.2 is divided from the
water H.sub.2O to be stored or to be recycled separately.
[0026] The stream of carbon dioxide CO.sub.2 and water H.sub.2O are
respectively compressed and cooled by a respective intercooled
compressor assembly (which will be called CCCO2, CCH2O).
[0027] Said first exchanger assembly HEA1 comprises several single
heat exchangers designed for different temperature levels of heat
exchange. FIG. 1 shows three of these heat exchangers: a first
assembly heat exchanger AH1 (or first reheater), a second assembly
heat exchanger AH2 (or second reheater) and a third assembly heat
exchanger AH3 (or third reheater).
[0028] The first heat exchanger AH1 is also identified as reheater
for the process fluid between two turbine stages.
[0029] A fourth reheater (not shown) may optionally be present
within the oxy-fuel burner OXB to pre-heat the working media PF to
be provided to the third assembly heat exchanger AH3.
[0030] Said rankine cycle RC comprises a high pressure turbine HPST
and a low pressure turbine LPST, which are basically designed as
steam turbines, wherein said turbines respectively said rankine
cycle are/is operated using in particular water as a process fluid
PF.
[0031] Said first exchanger assembly HEA1 works as the boiler of
said rankine cycle RC boiling the water and superheating the steam
generated to be expanded first in said high pressure turbine HPST
starting from an entrance pressure level of around 150 bar.
[0032] Upstream of said high pressure turbine HPST a full capacity
first bypass station BST1 is provided to allow full operation
flexibility especially during start-up and shut-down. Said high
pressure turbine receives its steam respectively process fluid PF
not from the most upstream first assembly heat exchanger AH1 but
from said second assembly heat exchanger AH2 and is therefore not
using the highest temperature level available from the exhaust
fluid line EXL. After said process fluid PF as passed the high
pressure turbine HPST it is conducted to the first assembly heat
exchanger AH1 for being reheated to further downstream pass a
second full capacity bypass station BST2 and to further downstream
enter a low pressure turbine LPST to be expanded from 40 bar down
to around 0.045 bar.
[0033] Both turbines HPST, LPST are driving a generator GEN but can
as well be used to drive a different consumer.
[0034] During this expansion a first extracted process fluid stream
XPF1 (also called first extracted working media stream), a second
extracted process fluid stream XPF2 (also called second extracted
working media stream), a third extracted process fluid stream XPF3
(also called third extracted working media stream) and a fourth
extracted process fluid stream XPF4 (also called fourth extracted
working media stream) are separated from the process fluid PF to
provide thermal energy to downstream process steps of the rankine
cycle RC. The process fluid exiting said low pressure turbine LPST
enters a condenser CON, where it is condensed to liquid together
with said fourth extracted process fluid stream XPF4, which is
recycled into the main process fluid PF.
[0035] Water or steam may be a advantageous process fluid PF.
Therefore in the following the term "feed water" is also used, also
in combination with devices like "feed water pump" or "feed water
pre-heater" or "feed water heat exchanger". Nevertheless also
different media can be used in the cycle, not only water. Therefore
the general term instead of "feed water" would be "working media"
and therefore the devices "feed water pump" or "feed water
pre-heater" or "feed water heat exchanger" or the like may be
called more generally "working media pump" or "working media
pre-heater" or "working media heat exchanger". Thus, even though
the embodiment will use feed water as an example, this should not
be considered limiting in respect of the used working media.
[0036] The term "working media pump" stands for example for a feed
water pump but also for a condensate pump.
[0037] Downstream said condenser CON said process fluid PF enters a
first feed water pump FWP1 (or first working media pump) before
receiving thermal energy from said fourth extracted process fluid
stream XPF4 in an intermediated heat exchanger THE. Further
downstream said process fluid PF enters a mixing pre-heater MP and
is mixed with said third extracted process fluid stream XPF3
directly coming from the extraction point of said low pressure
turbine LPST. Said first and second extracted process fluid streams
XPF1, XPF2 and said fifth extracted process fluid stream XPF5 (also
called fifth extracted working media stream) are injected into said
mixing pre-heater MP, too, after they respectively were used to
preheat said process fluid PF. Downstream said mixing pre-heater MP
said process fluid PF enters a second feed water pump FWP2 (or
second working media pump) increasing the pressure well above 150
bar before said process fluid enters downstream said feed water
heat exchanger FWE. Subsequently said process fluid PF enters a
preheating assembly PAS comprising a sequence of three feed water
pre-heaters (or working media pre-heaters), a first feed water
pre-heater PH1 (or first working media pre-heater), a second feed
water pre-heater PH2 (or second working media pre-heater), a third
feed water pre-heater PH3 (or third working media pre-heater).
[0038] Said first feed water pre-heater PH1 includes a first
sub-cooler SC1 and a first main heat exchanger MH1.
[0039] Said second feed water pre-heater PH2 includes a second
sub-cooler SC2 and a second main heat exchanger MH2, wherein said
first feed water pre-heater PH1 receives said second extracted
process fluid stream XPF2 and said second feed water pre-heater PH2
receives said first extracted process fluid stream XPF1. The
respective sub-coolers are located upstream of the main heat
exchangers with regard to said process fluid PF stream.
[0040] Said third feed water pre-heater PH3 is heated by a fifth
extracted process fluid stream XPF5 extracted from said high
pressure turbine HPST, wherein said process fluid PF first passes a
third heat exchanger HEX3 of said third feed water pre-heater PH3
before it enters said third feed water pre-heater PH3 and
downstream enters said second heat exchanger HEX2 also heated by
said fifth extracted process fluid stream XPF5. Downstream said
second heat exchanger HEX2 said process fluid PF enters said third
assembly heat exchanger AH3. Downstream said second assembly heat
exchanger AH2 said process fluid PF passes said first bypass
station BST1 and further downstream enters said high pressure
turbine HPST.
[0041] One feature to mention one more time is that the
rankine-cycle RC is operated with the working media (the process
fluid PF) and that this working media is circulating separately
from the exhaust fluid EXH. "Separately" means in this respect that
the two media do not mix with each other. The rankine cycle is a
closed cycle without input or output during normal operation.
Particularly no exhaust fluid EXH is transferred into the rankine
cycle RC as or to mix with working media of the rankine cycle RC.
The exhaust fluid EXH and the working media PF are kept separate or
unmixed. The exhaust fluid EXH is also rerouted back to the
oxy-fuel burner but a part of the fluid may be extracted, but not
to enter the rankine cycle RC.
[0042] Different to what is shown in FIG. 1, there may not be a
common output port of the first heat exchanger assembly HEA1 for
outputting the exhaust fluid EXH with a later element to branch off
into a first part EXH1 and a second part EXH2. Possibly the first
heat exchanger assembly HEA1 may have two output ports included
into the first heat exchanger assembly HEA1. As a further
alternative the complete recirculation via recirculation line RCL
and the compression unit PU may also be incorporated into the first
heat exchanger assembly HEA1.
[0043] The system according to FIG. 1 can also be explained in
slightly different terminology:
[0044] A power generation system (PGS) is shown comprising an
oxy-fuel burner (OXB), a first heat exchanger assembly (HEA1), and
a rankine-cycle (RC),--wherein said rankine-cycle (RC) comprises at
least one turbine (ST) expanding a process fluid (PF), downstream
said turbine (ST) at least one condenser (CON) condensing said
process fluid (PF),--wherein said rankine-cycle (RC) comprises
downstream said condenser (CON) at least one first feed water pump
(FWP) delivering said process fluid (PF) to a higher pressure
level,--wherein said rankine-cycle (RC) comprises downstream said
feed water pump (FWP) at least one first feed water pre-heater (PH)
heating said process fluid (PF) by extracted process fluid (XPF1,
XPF2) from said turbine (ST),--wherein downstream said feed water
pre-heater (PH1, PH2, PH3) said process fluid (PF) passes said
first heat exchanger assembly (HEA1) to be boiled and superheated,
wherein,--said oxy-fuel burner (OXB) generates an exhaust fluid
(EXH) submitted to an exhaust fluid line (EXL),--said rankine-cycle
(RC) is operated with said process fluid (PF) which is circulating
separately from said exhaust fluid (EXH),--wherein downstream said
first heat exchanger assembly (HEA1) at least one feed water heat
exchanger (FWE) is provided to heat up said process fluid (PF) of
said rankine-cycle (RC) downstream said feed water pump (FWP) and
upstream said feed water preheater (PH) by said exhaust fluid
(EXH), --wherein said exhaust fluid line (EXL) is provided with a
recirculation line (RCL) downstream said first heat exchanger
assembly (HEA1) and upstream said feed water heat exchanger (FWE)
extracting exhaust fluid (EXH) from said exhaust fluid line (EXL),
conducting extracted exhaust fluid (EXE) to a pump (PU) to increase
pressure and injecting downstream said extracted exhaust fluid
(EXE) into said oxy-fuel burner (OXB).
[0045] By such a system a following method can be performed:
[0046] Method to operate a power generation system (PGS) comprising
the following steps: --providing an oxy-fuel burner (OXB), a first
heat exchanger assembly (HEA1), a rankine-cycle (RC),--generating
an exhaust fluid (EXH) by said oxy-fuel burner (OXB) submitted to
an exhaust fluid line (EXL) by burning oxygen O2 and fuel
F,--expanding a process fluid (PF) in said rankine-cycle (RC)
comprising at least one turbine (ST) of said rankine-cycle (RC),
--condensing said process fluid (PF) downstream said turbine (ST)
by at least one condenser (CON) of said rankine-cycle
(RC),--delivering said process fluid (PF) to a higher pressure
level downstream said condenser (CON) by least one first feed water
pump (FWP) of said rankine-cycle (RC),--heating said process fluid
(PF) by extracted process fluid (XPF1, XPF2) from said turbine (ST)
downstream said feed water pump (FWP) by at least one first feed
water pre-heater (PH) of said rankine-cycle (RC),--boiling and
superheating said process fluid (PF) downstream said feed water
pre-heater (PH1, PH2, PH3) by said first heat exchanger assembly
(HEA1) of said rankine-cycle (RC),--operating said rankine-cycle
(RC) with said process fluid (PF) which is circulating separately
from said exhaust fluid (EXH),--providing at least one feed water
heat exchanger (FWE) downstream said first heat exchanger assembly
(HEA1) and heating up said process fluid (PF) of said rankine-cycle
(RC) downstream said feed water pump (FWP) and upstream said feed
water preheater (PH) by said exhaust fluid (EXH),--providing said
exhaust fluid line (EXL) comprising a recirculation line (RCL)
downstream said first heat exchanger assembly (HEA1) and upstream
said feed water heat exchanger (FWE) extracting exhaust fluid (EXH)
from said exhaust fluid line (EXL), --conducting said extracted
exhaust fluid (EXE) to a pump (PU) to increase pressure and
--injecting downstream said extracted exhaust fluid (EXE) into said
oxy-fuel burner (OXB).
[0047] It has to be mentioned that in the power generation system
it is fired externally to the power cycle in a boiler where the
exhaust gas side may be pressurized. Exhaust gas may be
recirculated to reduce firing temperature. This is advantageous to
increase plant reliability by decreased complexity and avoidance of
risks inherent in known cycles where exhaust from a gas generator
is led through next burner platelet narrow channels and through
blade cooling. The solution provides a better performance. Further,
it is less sensitive to combustion disturbances or fuel quality
variations and also less sensitive to corrosion as the exhaust
gases are kept outside the power cycle.
[0048] In general, FIG. 1 is directed to a process for production
of power via combustion by combining a fuel gas stream and an
oxygen rich stream in burner(s) in a steam generating boiler. The
resulting exhaust gas, mainly comprising CO.sub.2 and steam is
cooled to a desired temperature by adding a recirculated exhaust
flow. The process is based on steam turbine technology and the
configuration is similar to a reheat steam cycle seen in known
applications. The steam generating boiler according to FIG. 1 is
different to a conventional boiler in that it is closed at the
exhaust gas side to deliver carbon dioxide CO.sub.2 produced to a
compressor and that it has a large recirculation of exhaust gas.
Water formed in the combustion is condensed before the remaining
carbon dioxide CO.sub.2 rich exhaust is led to a compressor and
clean-up for delivery to a carbon dioxide consumer or injection in
ground.
[0049] The process separates the exhaust gas from the power cycle
working media (water/steam circuit) such that turbo-machinery of
known existing type can be selected and will run in an environment
as designed for. By separating the exhaust from the water/steam
circuit the process will also be relatively insensitive to soot or
emissions resulting from the fuel or combustion.
[0050] The boiler can be operated at an exhaust gas pressure of
about atmospheric pressure but can also be designed to be operated
at an exhaust gas pressure elevated above atmospheric pressure in
order to save boiler size, reduce size of burners, reduce size of
recirculation duct and fan and to reduce size of final carbon
dioxide (CO.sub.2) compressor. The elevated pressure may also lead
to marginally better cycle performance due to less expansion of
delivered fuel gas and oxygen and less compression work for
delivered carbon dioxide CO.sub.2, however depending on specific
project pre-requisites. Boiler elevated exhaust pressure may also
enable modularization of boiler supply to enable road transport of
boiler proper where an atmospheric boiler would be constructed at
site or be divided in a number of modules. Boiler type may be a
traditional drum type boiler or a once-through (Benson) type. Steam
cycle may be with or without reheat and high pressure main steam
pressure below of above critical.
[0051] A special feature of the process is the combination of
oxy-fuel combustion principle with a boiler design separating the
CO.sub.2/steam from the steam/water cycle (and optionally by
operation of the exhaust gas at elevated pressure above
atmospheric). High efficiency is achieved by taking the
recirculation of exhaust from upstream economizers in the boiler
such that as little heat as possible is moved from high temperature
to low temperature parts of the cycle.
[0052] Fuel gas and oxygen are supplied to a boiler where
combustion takes place in burners(s) at atmospheric pressure or at
increased pressure (in order to reduce size and cost). A
recirculation of exhaust gas is applied to reduce the firing
temperature.
[0053] The exhaust passes a superheater, a reheat superheater and
evaporator coils. Finally the exhaust is split in two streams, one
recirculation back to the burner section and one exit stream which
is led through a heat recovery section. The pressure resistant
boiler casing is protected from hot gases by water cooling either
combined in the shell design or by internal separate water cooled
lining. Internal insulation may be applied instead of water cooling
or as a complement.
[0054] The steam generation side of the boiler may be a
conventional single pressure drum type or a once through (e.g.
Benson) type. The heat recovery section downstream the main boiler
cools the exhaust stream which is not re-circulated back to the
burner section. This heat recovery is anticipated to comprise heat
exchange utilized for preheating of oxygen and fuel to the burners
and a condenser transferring the heat to the feed water preheat
section in the closed steam cycle. The condenser condenses only the
steam in the exhaust gas resulting from the combustion of the
hydrogen in the fuel, i.e. a rather small heat load. The rest of
the exhaust gas, comprising mainly carbon dioxide CO.sub.2,
extracted from the condenser is compressed and cleaned. The
pressure in the condenser and thus also the CO.sub.2-extraction is
as in the boiler minus some pressure drop, i.e. atmospheric or
increased depending on design.
[0055] High pressure (HP) steam generated and superheated in the
boiler is admitted to the high pressure steam turbine and is
expanded to an intermediate pressure, producing power. The steam is
then returned to the boiler to pass a second superheat (reheat) and
then forwarded to a low pressure (or intermediate pressure) steam
turbine where it is further expanded to the near vacuum condition
in the condenser producing more power.
[0056] The condenser is in particular to be water cooled. However
an air cooled condenser may be used as well. Cooling water may come
from an open source or a cooling tower. Condensate collected in the
condenser is pumped through a sequence of pre-heaters to the
deaerator/feed water tank (i.e. the mixing pre-heater MP). Feed
water is taken from the deaerator (mixing pre-heater MP) and is
pumped through further pre-heaters before being introduced in the
boiler again. The pre-heaters are supplied by steam taken from
steam turbine extractions or other source. Some heat for the feed
water preheating is also recovered from cooling and condensing
water from the exhaust stream exiting the boiler.
[0057] In case of using drum type boiler, water chemistry is
managed by reject of about 1% of the flow rate as boiler blow down.
In case of a Benson type a smaller bleed rate is applied but then
there is a requirement of inline water polish. In case an
alternative burner should be used the oxygen may be blended into
re-circulated exhaust prior to introduction to the burner.
[0058] To allow full flexibility the cycle may be equipped with
turbine full capacity bypasses and startup vents both at high
pressure and low pressure. By running steam in bypass mode the
CO.sub.2 production can be maintained even if one or both turbines
are stopped. Since the boiler provides a thermal storage capacity
turbine trips can be managed without disturbing the O.sub.2
production.
[0059] If an increased efficiency is desired the steam conditions
may be increased. As an example, high pressure steam at
supercritical condition 250 bars and 600 degrees would provide an
increased net efficiency compared to examples given in figure
above. However a third steam turbine module would then be
installed. If double reheat is introduced in a supercritical cycle
even a further improvement would be possible. By pressurizing the
exhaust gas side of the boiler a significant reduction in size and
hopefully in cost could be gained. This is a difference of this
cycle compared to existing boilers where pressurization would
require a turbine for pressure recovery when letting the exhaust to
a stack. In this case there is no stack but instead a requirement
to further compress. The largest cost and space saving would be
gained by a pressure of about 5 to 8 bars. Up to 20 bars may be
advantageous but above that the benefit can in some cases be
overruled by pressure vessel issues which may need to be
considered. To save in design efforts scalability may best be
handled by designing a pressurized boiler as a standard module to
be put in parallel for scalability but with a common drum, e.g. 100
MW fired per module. Road transport limits would probably dictate
the sizing.
[0060] The previous configuration can be operated with exhaust
fluids with atmospheric pressures, but possibly also with elevated
pressure up to 20 bar.
[0061] In the following a configuration is explained in which the
exhaust fluid is operated with elevated pressure, for example above
5 bar or above 10 bar, particularly between 10 to 40 bar,
advantageously between 15 to 30 bar. A further configuration is
operated with 20 bar (plus/minus 10%). Thus, the pressure is
significantly above atmospheric pressure. This is further explained
in relation to FIG. 2.
[0062] Basically FIG. 2 shows almost all components as already
explained in conjunction with FIG. 1. Therefore only the
differences will be discussed. All previously said still applies
also for FIG. 2.
[0063] What is different to FIG. 1 is that FIG. 2 is designed so
that the exhaust fluid is operated on elevated pressure level. A
further control unit or the existing control unit CU is arranged to
control a pressure level for said pressurized recirculating fluid
(PEXE), particularly by balancing supplied fluids and extracted
fluids such that the wanted pressure level is reached. Said
supplied fluids comprise the fluids that are provided to said
oxy-fuel burner (OXB) and/or to said recirculation line (RCL). Said
extracted fluids comprise the fluids that are separated or
extracted from said recirculation line (RCL).
[0064] So the pressure level within the recirculation line can be
set-up by controlling an amount of fuel (F) provided to said
oxy-fuel burner (OXB) and/or by controlling an amount of oxygen
(O2) provided to said oxy-fuel burner (OXB) and/or by controlling a
ratio between said first part (EXH1) of said exhaust fluid (EXH)
and said second part (EXH2) and/or by controlling the outtake of
CO2. The pressure level specifically can be set by controlling
valves--e.g. said control valve CV--and/or compressors and/or
fans--e.g. said compression unit PU and/or an additional
compression unit CU2 and/or compressors C1, C2.
[0065] The pressure for the exhaust fluid of the oxy-fuel burner
(OXB) elevated above atmospheric pressure (typical magnitude of 10
to 40 bar) increases exhaust fluid water gaseous phase partial
pressure and thereby enable recovery of latent heat of condensation
of said water, particularly at temperature level for efficient use
within the rankine cycle RC.
[0066] To use this effect, some modifications in the system may be
beneficial. Particularly it may to provide the working media PF
when first feed water pump FWP1 is guided to the feed water heat
exchanger FWE to be able to use the heat of the first part EXH1 of
the exhaust fluid EXH and to use also its latent heat. Past the
feed water heat exchanger FEW the working media PF now has an
increased temperature and will be provided to the mixing pre-heater
MP, which has--as before--a further source from the first
sub-cooler SC1 and is also provided with working media via the
third extracted process fluid stream XPF3 from the low pressure
turbine LPST. The mixing pre-heater MP can provide working media
via the second feed water pump FWP2 directly to the first
sub-cooler SC1 to be further guided to the first feed water
pre-heater PH1.
[0067] Compared to FIG. 1, an extraction of the fourth extracted
process fluid stream XPF4 from the low pressure turbine LPST may
become superfluous and can be omitted. This again improves the
effectiveness of the cycle.
[0068] Downstream the division point DP said exhaust fluid EXH
passes the first cooler COL1 before it enters the feed water heat
exchanger FWE. The first cooler COL1 may be upstream or downstream
of the feed water heat exchanger FWE, or may even be incorporated
into a common vessel with the feed water heat exchanger FWE. The
first cooler COL1 can be connected to a heat exchanger in a branch
that is providing the fuel F. Alternatively the first cooler COL1
can be connected to a heat exchanger in a branch that is providing
the oxygen O.sub.2 to preheat the oxygen O.sub.2. As a further
alternative, the feed water heat exchanger FWE can provide heat to
the working media stream in the rankine cycle RC in various
positions.
[0069] Fuel F provided by a pipeline may already be provided at an
elevated pressure level, often of about 30 bar. In this case the
compressor C2 may be superfluous. So the system can be simplified.
Even more, possibly the pressure of the fuel F from the pipeline
way need to be reduced--e.g. if the pipeline is operated by for
example 85 bar. The reduction would then in particular take place
in an expander replacing the compressor C2.
[0070] If natural gas is provided a fuel F, it has to be noted that
natural gas typically anyhow is provided pressurized so it is
advantageous if also the exhaust fluid is pressurised.
[0071] The oxygen O.sub.2 may even be provided in liquid form, e.g.
when provided as liquid oxygen if no air separation unit ASU is
used on site. A then needed regasification of the liquid oxygen can
be integrated in the system such that the heat reaction can be
utilized in the system, particularly via a further heat exchanger
unit.
[0072] If preheating of fuel F and oxygen O.sub.2 is wanted, then
the preheating operation can utilize heat from the rankine cycle
RC.
[0073] As an output to the exhaust stream carbon dioxide CO.sub.2
and water H.sub.2O is extracted. With the pressurization of the
exhaust stream a further compressor CU2 may be superfluous as the
CO.sub.2 is already delivered at a wanted pressure level.
Alternatively the further compressor CU2 may be present to elevate
CO.sub.2 to a wanted pressure level.
[0074] As an option, a catalyst unit for cleaning of said exhaust
fluid EXH from residual content of oxygen may be provided--e.g.
removing oxygen content that was not burned or removing unburned
hydrocarbons --, particularly in the exhaust fluid line branch
EXLB. The catalyst unit is operated by adding of further fuel
and/or other combustible media to the first part EXH1 exhaust fluid
EXH. The catalyst unit may particularly be connected in such way
that heat of a catalyst process running in the catalyst unit is
recovered for use in said rankine cycle RC and/or for preheating of
fuel F or said oxygen enriched gas O.sub.2. This allows to
facilitate efficient use of generated heat in the cleanup process
by the catalyst unit.
[0075] The catalyst unit can be place at several positions in the
exhaust fluid stream. It may be upstream or downstream of a
condensation occurring in the feed water heat exchanger FEW. The
catalyst unit may even be incorporated in the oxy-fuel boiler
OXB.
[0076] Furthermore other refinery components may be incorporated in
the exhaust fluid line branch EXLB or in the recirculating line RCL
for gas filtration.
[0077] The system according to FIG. 2 provides improvement of
performance and reduction of component sizes for the oxyfuel boiler
process. This may improve the previously explained oxyfuel cycle,
in which the cycle configuration is based on the boiler exhaust
side operating at atmospheric pressure or slightly above
atmospheric. The condensation of water from exhaust gas is
typically not fully utilised to benefit from water condensation
latent heat.
[0078] Boiler exhaust pressure is increased to increase the
temperature level of water condensation in the exhaust gas water
separation and thereby enable usage of the latent heat in the power
cycle at beneficial temperature level. The feed water preheating
heat exchanger sequence may then be adapted to fit with the changed
recovery in the exhaust cooling, also some related changes to fuel
and oxygen preheating may be made to suit the new
configuration.
[0079] By the increased temperature level for water condensation
the latent heat in steam phase in the exhaust gas formed by
combustion is utilized in the power cycle, i.e. the power cycle
makes use of the fuel higher heating value instead of the normal
practice of only use of lower heating value (minus certain loss).
This may be seen as achieving boiler efficiency above 100% when
referring to the normal efficiency definition based on lower
heating value. A number of other positive effects are also gained
in equipment size and cost by the reduced gas volumes and increased
heat convection factors resulting from increased pressure.
[0080] By increasing gas side pressure in the boiler, the size of
downstream equipment (heat exchangers, water separation unit and
CO.sub.2 compressor, is reduced and parasitic load for the CO.sub.2
compression is reduced. Since the supplied fuel gas normally is at
a positive pressure in the order of 25 to 30 bars the boiler
exhaust pressure may be increased to about 20 bars without any
additional fuel compression.
[0081] By also changing the exhaust stream cooling section and
changing the adaption of boiler feed water preheaters the following
positive effects are gained:
[0082] 1) The increased exhaust pressure results in increased dew
point for the water phase in the exhaust gas since the water steam
partial pressure is increased to the same rate as the increase in
total pressure. As an example when firing natural gas with 95% CH4
content and operating the boiler at 20 bar the dew point is
190.degree. C. Water condensation will thus begin at 190.degree. C.
when cooling the exhaust gas.
[0083] 2) By connecting boiler feed water, to be heated, at the
secondary side of the heat exchange in the exhaust gas cooling
(element FEW in the Figures), both the convective (sensible) heat
of cooling the exhaust gas and the latent heat of condensation from
the water phase is utilised in the power cycle.
[0084] 3) By moving the mixing preheater/feed water tank to a
position downstream the feed water heat exchanger FWE and deleting
the condensate preheater--i.e. said intermediated heat exchanger
IHE--more heat is recovered from the exhaust cooling as the
original final CO.sub.2 cooler cannot utilise the added latent heat
from the exhaust stream efficiently.
[0085] 4) The feed water heat exchanger FWE may be divided in two
or more sections and the mixing preheater may be integrated between
sections.
[0086] 5) Fuel F and oxygen 02 preheating may be integrated with
the FWE as part of the heat recovery to achieve a more efficient
use of available exergy (energy related to temperature level).
[0087] 6) The higher the boiler pressure is, the higher the
temperature level is for the said recovery of latent heat.
[0088] 7) The increased heat recovery in the exhaust gas cooling is
used to reduce the steam extraction from the steam turbine. This
implies higher power production from the steam turbine as more
steam is allowed to pass through the turbine all way down to the
cold condenser. Also less number of steam extraction points and
less number of steam driven boiler feed water preheaters are
required.
[0089] Generally, the pressurization of the exhaust fluid benefits
from that a fluid under higher pressure has a higher density.
Furthermore, under pressure condensation starts at a higher
temperature level (e.g. at 190.degree. C. when operating with 20
bar instead of 90.degree. C. when operating under atmospheric
conditions). As a secondary effect, sizes of components can be
reduced due to pressurising the fluid, e.g. the size of the
oxy-fuel burner OXB or the size of the feed water heat exchanger
FWE.
[0090] It may be advantageous to set up the power generation system
PGS such that temperature level at design working conditions for
said second part EXH2 of said exhaust fluid EXH is at least two
third (2/3) of saturation temperature measured in Celsius of
boiling occurring in said first heat exchanger assembly HEAL It is
advantageous to have a temperature level being particularly above
200.degree. C.
[0091] The recirculation line RCL according to FIGS. 1 and 2 may be
implemented as shown. Alternatively the recirculated exhaust fluid
EXE may even be kept internally to the first heat exchanger
assembly HEA1. So no external piping may be required (particularly
when even the compression unit PU is integrated within the first
heat exchanger assembly HEA1) or may be limited to just exit the
first heat exchanger assembly HEA1 to provide the recirculated
exhaust fluid EXE to the compression unit PU and then provided to
the oxy-fuel burner OXB.
[0092] Valid for both embodiments of FIGS. 1 and 2, it may be
advantageous to directly drive rotating equipment (like the
compressors or pumps in the power generation system) by the steam
turbine ST. Gear boxes may be used to operate the driven rotating
equipment at a needed speed. This may increase the effectiveness as
a conversion in electrical energy and reconversion in rotational
energy can be omitted.
[0093] For startup or purging the oxy-fuel burner OXB, a further
air supply may be present to guide air into the oxy-fuel burner
OXB.
[0094] The at least one working media heat exchanger FWE may
comprise more than one element to allow a heat transfer from a hot
exhaust fluid EXH--i.e. the first part EXH1--to a cooler medium.
E.g. a first heat transfer element operates with the first part
EXH1 in gas phase and further heat transfer elements operate with
the first part EXH1 in liquid state.
* * * * *