U.S. patent application number 11/959885 was filed with the patent office on 2008-06-05 for carbon dioxide capture systems and methods.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Michael Adam Bartlett, Matthias Finkenrath, Stephanie Marie-Noelle Hoffmann, James Anthony Ruud.
Application Number | 20080127632 11/959885 |
Document ID | / |
Family ID | 39474177 |
Filed Date | 2008-06-05 |
United States Patent
Application |
20080127632 |
Kind Code |
A1 |
Finkenrath; Matthias ; et
al. |
June 5, 2008 |
CARBON DIOXIDE CAPTURE SYSTEMS AND METHODS
Abstract
A carbon dioxide separation system includes a compressor for
receiving an exhaust gas comprising CO.sub.2 and generate a
compressed exhaust gas and a separator configured to receive the
compressed exhaust gas and generate a CO.sub.2 lean stream. The
separator includes a first flow path for receiving the compressed
exhaust gas, a second flow path for directing a sweep fluid
therethrough, and a material with selective permeability of carbon
dioxide for separating the first and the second flow paths and for
promoting carbon dioxide transport therebetween. The system further
includes an expander coupled to the compressor for receiving and
expanding the CO.sub.2 lean stream to generate power and an
expanded CO.sub.2 lean stream.
Inventors: |
Finkenrath; Matthias;
(Munich, DE) ; Bartlett; Michael Adam; (Munich,
DE) ; Hoffmann; Stephanie Marie-Noelle; (Munich,
DE) ; Ruud; James Anthony; (Delmar, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
SCHENECTADY
NY
|
Family ID: |
39474177 |
Appl. No.: |
11/959885 |
Filed: |
December 19, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11564912 |
Nov 30, 2006 |
|
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11959885 |
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Current U.S.
Class: |
60/274 |
Current CPC
Class: |
F23J 2215/50 20130101;
Y02C 10/10 20130101; B01D 53/22 20130101; Y02E 20/326 20130101;
F23J 15/02 20130101; Y02A 50/2342 20180101; Y02C 20/40 20200801;
F02C 1/04 20130101; Y02E 20/32 20130101; B01D 2257/504 20130101;
Y02C 10/04 20130101; Y02A 50/20 20180101; B01D 53/62 20130101; F02C
1/02 20130101 |
Class at
Publication: |
60/274 |
International
Class: |
F01N 3/00 20060101
F01N003/00 |
Claims
1. A carbon dioxide separation system comprising: a compressor for
receiving an exhaust gas comprising CO.sub.2 and generate a
compressed exhaust gas; a separator configured to receive said
compressed exhaust gas and generate a CO.sub.2 lean stream, said
separator comprising; a first flow path for receiving said
compressed exhaust gas; a second flow path for directing a sweep
fluid therethrough; and a material with selective permeability of
carbon dioxide for separating said first and said second flow paths
and for promoting carbon dioxide transport therebetween; and an
expander coupled to said compressor for receiving and expanding
said CO.sub.2 lean stream to generate power and an expanded
CO.sub.2 lean stream.
2. The carbon dioxide separation system according to claim 1,
wherein said sweep fluid is at a sub-atmospheric pressure.
3. The carbon dioxide separation system according to claim 1,
wherein said exhaust gas is generated from one or more of a
coal-fired power plant or a natural gas combined cycle power plant
or an industrial process generating a CO.sub.2 rich flue gas.
4. The carbon dioxide separation system according to claim 3,
wherein a portion of said exhaust gas is recycled back into said
coal-fired power plant or natural gas combined cycle power
plant.
5. The carbon dioxide separation system according to claim 1
further comprising a heat exchanger configured to receive said
compressed exhaust gas and CO.sub.2 lean stream to generate a cold
compressed exhaust gas.
6. The carbon dioxide separation system according to claim 1
further comprising a pre-cooler to cool down said compressed
exhaust gas.
7. The carbon dioxide separation system according to claim 1,
wherein said exhaust gas comprises at about 3% to about 15%
CO.sub.2 by volume.
8. The carbon dioxide separation system according to claim 1
further comprising a particle removal unit configured to remove
particles from said exhaust gas.
9. The carbon dioxide separation system according to claim 1,
wherein said separator is a membrane separator configured to
generate a permeate stream comprising CO.sub.2.
10. The carbon dioxide separation system according to claim 9,
wherein said membrane separator is selected from a group consisting
of mixed matrix membranes, facilitated transport membranes, hollow
fiber membranes, spiral wound membranes, ionic liquid membranes and
polymerized ionic liquid membranes.
11. A carbon dioxide separation system in accordance with claim 1,
wherein said exhaust gas is in the temperature range between about
150.degree. C. to about 700.degree. C.
12. A carbon dioxide separation system in accordance with claim 1,
wherein said sweep fluid is a condensable fluid.
13. A carbon dioxide separation system in accordance with claim 1,
wherein said sweep fluid is steam.
14. A carbon dioxide separation system in accordance with claim 1,
wherein said sweep fluid is an organic compound.
15. The carbon dioxide separation system in accordance with claim
1, wherein said sweep fluid is selected from the group consisting
of refrigerants; alcohols; fluorinated and non-fluorinated
hydrocarbons, ketones, esters, and ethers; siloxanes and
combinations thereof.
16. The carbon dioxide separation system in accordance with claim
1, wherein said sweep fluid is at a pressure of about 0.1 to about
0.3 bar.
17. The carbon dioxide separation system in accordance with claim 1
further comprising a post purification system and a compressing
system.
18. The carbon dioxide separation system in accordance with claim
17, wherein said compressing system comprises a compressor with at
least one stage to compressed said permeate stream to produce
CO.sub.2 rich stream at high pressure.
19. The carbon dioxide separation system in accordance with claim
1, wherein said exhaust gas is produced from at least one of a gas
turbine, a furnace, a thermal oxidizer, metal processing systems,
or an industrial process.
20. A carbon dioxide separation system comprising: a compressor for
receiving an exhaust gas comprising CO.sub.2 and generate a
compressed exhaust gas; a membrane separator configured to receive
said compressed exhaust gas and generate a CO.sub.2 lean stream,
said membrane separator comprising; a first flow path for receiving
said compressed exhaust gas; a second flow path for directing a
sweep fluid therethrough wherein said sweep fluid is at a
sub-atmospheric pressure; and a material with selective
permeability of carbon dioxide for separating said first and said
second flow paths and for promoting carbon dioxide transport
therebetween; and an expander coupled to said compressor for
receiving and expanding said CO.sub.2 lean stream to generate power
and an expanded CO.sub.2 lean stream.
21. A carbon dioxide separation system in accordance with claim 20,
wherein said sweep fluid is steam.
22. A carbon dioxide separation system comprising: a compressor for
receiving an exhaust gas comprising CO.sub.2 and generate a
compressed exhaust gas; a facilitated transport membrane separator
configured to receive said compressed exhaust gas and generate a
CO.sub.2 lean stream, said facilitated transport membrane separator
comprising; a first flow path for receiving said compressed exhaust
gas; a second flow path for directing a sweep fluid therethrough,
wherein said sweep fluid is at a sub-atmospheric pressure; and a
material with selective permeability of carbon dioxide for
separating said first and said second flow paths and for promoting
carbon dioxide transport therebetween and an expander coupled to
said compressor for receiving and expanding said CO.sub.2 lean
stream to generate power and an expanded CO.sub.2 lean stream.
23. An exhaust gas treatment system comprising: a compressor for
receiving an exhaust gas comprising CO.sub.2 and generate a
compressed exhaust gas, wherein said exhaust gas is generated from
a coal gasification plant or a natural gas combined cycle power
plant; a membrane separator configured to receive said compressed
exhaust gas and generate a CO.sub.2 lean stream, said membrane
separator comprising; a first flow path for receiving said
compressed exhaust gas; a second flow path for directing a sweep
fluid therethrough, wherein said sweep fluid is at a
sub-atmospheric pressure; and a material with selective
permeability of carbon dioxide for separating said first and said
second flow paths and for promoting carbon dioxide transport
therebetween; an expander coupled to said compressor for receiving
and expanding said CO.sub.2 lean stream to generate power and an
expanded CO.sub.2 lean stream; and a post purification system and a
compressing system to generate a high pressure CO.sub.2 rich
stream.
24. A method for separating carbon dioxide comprising: compressing
an exhaust gas comprising CO.sub.2 and generating a compressed
exhaust gas; receiving said compressed exhaust gas in a separator
and generating a CO.sub.2 lean stream, said separator comprising; a
first flow path for receiving said compressed exhaust gas; a second
flow path for directing a sweep fluid therethrough wherein said
sweep fluid is at a sub-atmospheric pressure; and a material with
selective permeability of carbon dioxide for separating said first
and said second flow paths and for promoting carbon dioxide
transport therebetween; and expanding said CO.sub.2 lean stream to
generate power and an expanded CO.sub.2 lean stream.
25. The method of claim 25, wherein said separator is a membrane
separator.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation-In-Part of U.S. patent
application Ser. No. 11/564,912, Docket Number 207795-1, entitled
"CARBON DIOXIDE CAPTURE SYSTEMS AND METHODS," filed Nov. 30, 2006,
which application is herein incorporated by reference.
BACKGROUND
[0002] This invention relates generally to carbon capture and more
specifically to methods and systems for capturing carbon
dioxide.
[0003] Before carbon dioxide (CO.sub.2) gas can be sequestered from
power plants and other point sources, it must be captured in a
relatively pure form. On a mass basis, CO.sub.2 is the nineteenth
largest commodity chemical in the United States, and CO.sub.2 is
routinely separated and captured as a byproduct of industrial
processes such as synthetic ammonia production, hydrogen (H.sub.2)
production or limestone calcination.
[0004] Existing CO.sub.2 capture technologies, however, are not
cost-effective when considered in the context of sequestering
CO.sub.2 from power plants. Most power plants and other large point
sources use air-fired combustors, a process that exhausts CO.sub.2
diluted with nitrogen. For efficient carbon sequestration, the
CO.sub.2 in these exhaust gases must be separated and
concentrated.
[0005] CO.sub.2 is currently recovered from combustion exhaust by
using, for example, amine absorbers and cryogenic coolers. The cost
of CO.sub.2 capture using current technology, however, can be as
high as $150 per ton--much too high for carbon emissions reduction
applications. Furthermore, carbon dioxide capture is generally
estimated to represent three-fourths of the total cost of a carbon
capture, storage, transport, and sequestration system.
[0006] Accordingly, there is a need for a new CO.sub.2 separation
system and method to make CO.sub.2 separation and capture from
power plants easier and more cost effective.
BRIEF DESCRIPTION
[0007] In one aspect, a carbon dioxide separation system includes a
compressor for receiving an exhaust gas comprising CO.sub.2 and
generate a compressed exhaust gas and a separator configured to
receive the compressed exhaust gas and generate a CO.sub.2 lean
stream. The separator includes a first flow path for receiving the
compressed exhaust gas, a second flow path for directing a sweep
fluid therethrough, and a material with selective permeability of
carbon dioxide for separating the first and the second flow paths
and for promoting carbon dioxide transport therebetween. The system
further includes an expander coupled to the compressor for
receiving and expanding the CO.sub.2 lean stream to generate power
and an expanded CO.sub.2 lean stream.
[0008] In another aspect, a carbon dioxide separation system
includes a compressor for receiving an exhaust gas comprising
CO.sub.2 and generate a compressed exhaust gas and a membrane
separator configured to receive the compressed exhaust gas and
generate a CO.sub.2 lean stream. The membrane separator includes a
first flow path for receiving the compressed exhaust gas, a second
flow path for directing a sweep fluid therethrough wherein the
sweep fluid is at a sub-atmospheric pressure and a material with
selective permeability of carbon dioxide for separating the first
and the second flow paths and for promoting carbon dioxide
transport therebetween. The system further includes an expander
coupled to the compressor for receiving and expanding the CO.sub.2
lean stream to generate power and an expanded CO.sub.2 lean
stream.
[0009] In yet another aspect, a carbon dioxide separation system
includes a compressor for receiving an exhaust gas comprising
CO.sub.2 and generate a compressed exhaust gas and a facilitated
transport membrane separator configured to receive the compressed
exhaust gas and generate a CO.sub.2 lean stream The facilitated
transport membrane separator includes a first flow path for
receiving the compressed exhaust gas, a second flow path for
directing a sweep fluid therethrough, wherein the sweep fluid is at
a sub-atmospheric pressure and a material with selective
permeability of carbon dioxide for separating the first and the
second flow paths and for promoting carbon dioxide transport
therebetween. The system further includes an expander coupled to
the compressor for receiving and expanding the CO.sub.2 lean stream
to generate power and an expanded CO.sub.2 lean stream.
[0010] In yet another aspect, an exhaust gas treatment system
includes a compressor for receiving an exhaust gas comprising
CO.sub.2 and generate a compressed exhaust gas, wherein the exhaust
gas is generated from a coal gasification plant or a natural gas
combined cycle power plant and a membrane separator configured to
receive the compressed exhaust gas and generate a CO.sub.2 lean
stream. The membrane separator includes a first flow path for
receiving the compressed exhaust gas, a second flow path for
directing a sweep fluid therethrough, wherein the sweep fluid is at
a sub-atmospheric pressure and a material with selective
permeability of carbon dioxide for separating the first and the
second flow paths and for promoting carbon dioxide transport
therebetween. The system further includes an expander coupled to
the compressor for receiving and expanding the CO.sub.2 lean stream
to generate power and an expanded CO.sub.2 lean stream and a post
purification system and a compressing system to generate a high
pressure CO.sub.2 rich stream.
[0011] In another aspect, a method for separating carbon dioxide
includes compressing an exhaust gas comprising CO.sub.2 and
generating a compressed exhaust gas and receiving the compressed
exhaust gas in a separator and generating a CO.sub.2 lean stream
The separator includes a first flow path for receiving the
compressed exhaust gas, a second flow path for directing a sweep
fluid therethrough wherein the sweep fluid is at a sub-atmospheric
pressure and a material with selective permeability of carbon
dioxide for separating the first and the second flow paths and for
promoting carbon dioxide transport therebetween. The method further
includes expanding the CO.sub.2 lean stream to generate power and
an expanded CO.sub.2 lean stream.
DRAWINGS
[0012] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0013] FIG. 1. is a schematic depiction of one embodiment of the
instant invention;
[0014] FIG. 2. is another schematic depiction of one embodiment of
the instant invention;
[0015] FIG. 3. is another schematic depiction of one embodiment of
the instant invention; and
[0016] FIG. 4. is another schematic depiction of one embodiment of
the instant invention.
DETAILED DESCRIPTION
[0017] FIG. 1 illustrates a carbon dioxide (CO.sub.2) separation
system 10 including a compressor 14 for receiving an exhaust gas 12
comprising CO.sub.2 and generating a compressed exhaust gas 20. The
separation system 10 also includes a separator 26 configured to
receive the compressed exhaust gas 20 and generate a CO.sub.2 lean
stream 32. The separator 26 includes a first flow path 28 for
receiving the compressed exhaust gas 20, a second flow path 30 for
directing a sweep fluid 36 therethrough, and a material 25 with
selective permeability of carbon dioxide for separating the first
and the second flow paths 28 and 30 and for promoting carbon
dioxide transport therebetween. The CO.sub.2 separation system 10
further includes an expander 18 optionally coupled to the
compressor 14 through a common shaft 16 for receiving and expanding
the CO.sub.2 lean stream 32 to generate power through generator 40
and an expanded CO.sub.2 lean stream 42, or reduce the overall
power requirement in the compressor-expander section to be provided
by an external source such as a motor.
[0018] As shown in FIG. 1, in operation, the compressed exhaust gas
20 from the compressor 14 is optionally sent to a heat exchanger
22. The heat exchanger 22 is configured to receive the compressed
exhaust gas 20 and the CO.sub.2 lean stream 32 from the separator
26 to generate a cooled compressed exhaust gas 24. The heat
exchanger 22 is used to utilize the heat content of the compressed
exhaust gas 20 and also to cool the compressed exhaust gas 20 to a
lower temperature for efficient separation of CO.sub.2 in the
separator 26. The cooled compressed exhaust gas 24 is introduced
into the first flow path 28 of the separator 26. In one embodiment,
the separator 26 is a membrane separation unit.
[0019] The separation systems described herein enhance the driving
forces for CO.sub.2 removal by membranes from the cooled compressed
exhaust gas 24 comprising CO.sub.2 as the cooled compressed exhaust
gas 24 is at high pressure. The pressure difference across the
membrane, which pressure difference is the driving force for
CO.sub.2 removal is further enhanced by operating the sweep fluid
36 at a sub-atmospheric permeate pressure. In some embodiments, the
sweep fluid 36 is at a sub-atmospheric pressure of about 0.1 bar to
about 0.5 bar. In one embodiment, the sweep fluid 36 is at a
sub-atmospheric pressure of about 0.2 bar. Referring once again to
FIG. 1, in one embodiment, the compressed exhaust gas 20 containing
CO.sub.2 is directed along the first flow path 28 and the sweep
fluid 36 is directed along the second flow path 30. The separator
26 is selective to CO.sub.2 and as the sweep flow 36 has a
significantly lower CO.sub.2 partial pressure than that of the
cooled exhaust gas 24 containing CO.sub.2, the CO.sub.2 is drawn
into the sweep fluid 36 through the CO.sub.2 selective material 25.
Accordingly, the stream flowing out of first flow path 28 is the
CO.sub.2 lean stream 32, which CO.sub.2 lean stream 32 is heated in
the heat exchanger 22. The heated CO.sub.2 lean stream 34 is
introduced into the expander 18 to generate the expanded CO.sub.2
lean stream 42 and power. The independence of the
compressor-expander system makes the separation systems described
herein attractive for retrofitting into the existing power plants
with CO.sub.2 capture. In some embodiments, the power generated by
expanding the CO.sub.2 lean stream 32 may not be sufficient to run
the compressor 14, in which case external power is used to run the
compressor 14. In some embodiments, in contrast to heating the
CO.sub.2 lean stream 32 in the heat exchanger 22, the CO.sub.2 lean
stream 32 may be optionally cooled down (not shown) and expanded to
atmospheric pressure. In this case, the cooled (at very low or even
sub-zero temperatures) expanded CO.sub.2 lean stream can be used
for any cooling process in a power plant. Optionally a
dehumidification device (not shown) can be added prior to the
expansion of the CO.sub.2 lean stream 32 in the expander 42 to
avoid formation of ice or expander damage by droplets. In some
embodiments, the exhaust gas 12 is compressed to about 5 bar before
being sent to the separator 26. Since the separation system 10
described above can operate independently, it can be a retrofit
option for a cost-effective and simple CO.sub.2 separation solution
from exhaust streams in existing power plants. The expanded
CO.sub.2 lean stream 42 that is released to atmosphere is
substantially reduced in CO.sub.2 by using the technique described
above.
[0020] The membrane in the separation systems described here may
comprise any membrane material that is stable at the operating
conditions and has the required CO.sub.2 permeability and
selectivity at the operating conditions. Possible membrane
materials that are selective for CO.sub.2 include certain inorganic
and polymer materials, as well as combinations comprising at least
one of these materials. Inorganic materials include microporous
carbon, microporous silica, microporous titanosilicate, microporous
mixed oxide, and zeolite materials, as well as combinations
comprising at least one of these materials.
[0021] Polymeric materials known to be selective for CO.sub.2
include, for example, certain polymer materials, such as
polyethylene oxides, polyimides, and polyamides. While not to be
limited by a particular theory, mechanisms for CO.sub.2 selectivity
in polymeric materials include solution-diffusion and facilitated
transport. In a solution-diffusion membrane the flux of CO.sub.2 is
enhanced over the other gases in the gas stream by the virtue of
CO.sub.2 having a higher solubility in the membrane, a higher
diffusivity through the membrane or a combination of both. In a
facilitated transport membrane, functional groups with a chemical
affinity for CO.sub.2 are present within the membrane that allow a
higher flux of CO.sub.2 relative to the other gases. Examples of
facilitated transport membranes include polyethylenimine/poly(vinyl
alcohol).
[0022] In practice, the membrane often comprises a separation layer
that is disposed upon a support layer. The porous support can
comprise a material that is different from the separation layer.
Support layers for polymeric membranes can comprise polysulfone,
poly(ether sulfone), Teflon, cellulose acetate, or
polyacrylonitrile. Support materials for asymmetric inorganic
membranes include porous alumina, titania, cordierite, carbon,
silica glass (e.g., Vycor.RTM.), and metals, as well as
combinations comprising at least one of these materials. Porous
metal support layers include ferrous materials, nickel materials,
and combinations comprising at least one of these materials, such
as stainless steel, iron-based alloys, and nickel-based alloys. In
addition, polymeric membranes can be disposed on polymeric or
inorganic supports. Membranes can include polymeric materials such
as polyethers and polyether blends and hybrid membranes such as
silanized gamma-alumina membranes. Silanes, such as 2-acetoxyethyl,
2-carbomethoxyethyl and 3-aminopropyl, can be integrated with
ceramic membranes to achieve selective CO.sub.2 transport.
[0023] Hybrid membranes that incorporate inorganic particles within
a polymeric matrix can show enhanced CO.sub.2 selectivity
properties at elevated operating conditions. Mixed matrix membranes
that incorporate adsorbent inorganic particles such as zeolites or
carbon within polymeric matrices also show enhanced properties at
elevated operating conditions. This technique is not restricted to
any particular membrane material or type and encompasses any
membrane comprising any material that is capable of providing
suitable levels of permeance and selectivity. That includes, mixed
matrix membranes, facilitated transport membranes, hollow fiber
membranes, spiral wound membranes, ionic liquid membranes and
polymerized ionic liquid membranes.
[0024] In one embodiment, the separator is a facilitated transport
membrane. As an alternative to conventional polymeric membranes,
facilitated transport membranes may be used as they have the
potential of achieving both high permeability and high selectivity.
Facilitated transport membranes may selectively permeate CO.sub.2
by means of a reversible reaction of CO2 with an incorporated
complexing agent (carrier) in the membrane, whereas gases such as
H.sub.2, N.sub.2, and CH.sub.4 will permeate exclusively by the
solution-diffusion mechanism.
[0025] In one embodiment, the exhaust gas 12 is at a temperature in
the range between about 30.degree. C. to about 700.degree. C. This
system can be utilized over a wide range of systems for any exhaust
gas, for example, furnace exhaust, thermal oxidizers, metal
processing or any other industrial process.
[0026] In one embodiment, sweep fluid 36 is a condensable fluid,
like steam for example. In another embodiment, sweep fluid 18 can
be one or more of the following: refrigerants; alcohols, like
ethanol; hydrocarbons like butane; fluorinated and non-fluorinated
hydrocarbons, ketones, ethers, and ethers; and siloxanes. In
addition, while this invention is discussed in relation to CO.sub.2
capture systems, a material selective to other constituents within
an exhaust gas steam, for example, CO, nitrous oxide (NOx), or acid
gases like hydrogen sulfide (H.sub.2S), sulfuric acid
(H.sub.2SO.sub.4) or hydrochloric acid (HCl) or other pollutants or
species, may be utilized to capture the other constituents in a
similar fashion.
[0027] FIG. 2 illustrates a membrane system 50 to separate the
CO.sub.2 content in the compressed exhaust gas 12. As shown in FIG.
2, separator 26 physically separates first flow path 28 and second
flow path 30 and promotes carbon dioxide transport therebetween.
FIG. 2 also illustrates the different locations for the
purification systems. In one embodiment, a purification system 52
is provided in the flow path of the exhaust gas 12 before the
exhaust 12 is introduced into the compressor 14. In some other
embodiments, a purification system 54 is provided to purify the
compressed exhaust gas 24 before it enters the membrane 26. In
another embodiment, the sweep fluid 36 is treated in a purification
unit 56 before being sent to the membrane 26 and the CO.sub.2 rich
stream 38 from the membrane is also treated in another purification
unit 62 prior to being compressed in a compressor 60, which
compressor 60 may be a single or a multistage compressor. The
compressor 60 generates a CO.sub.2 product stream 66 at high
pressure. Optionally the compressed CO.sub.2 product 66 may also be
treated in yet another purification unit 64 after compression. The
purification units 52, 54, 56 and 64 described in this section may
include cooling, drying or particle removing systems or
combinations thereof.
[0028] FIG. 3 illustrates another exemplary separation system 80
wherein in operation, the exhaust gas 96 that is sent to the
compressor 14 is generated in a power generation system 82. As
shown in FIG. 3, an exhaust gas 88 can be generated from a coal
fired power plant 82. Typically a coal-fired power plant uses a
combustion or gasification process (not shown) to burn coal 84 with
air 86 to generate fuel for the turbine (not shown) or generate the
exhaust stream 88. The exhaust stream 88 from the coal-fired power
plant 82 comprises carbon dioxide CO.sub.2 in the range of about
10% to about 15%. Alternatively the exhaust stream 88 can also be
generated in a natural gas power plant. An exhaust generated from a
natural gas power plant comprises about 3% to about 8% CO.sub.2. To
achieve the high exhaust gas CO.sub.2 concentrations for natural
gas power plants, exhaust gas recirculation back to the gas turbine
may be advantageously applied, as described later. The final
exhaust 96 sent to the compressor 14 may be generated in either of
these coal-fired or natural gas power plants or a combination of
the exhausts generated from each of these plants. In some
embodiments, a portion of the exhaust gas 94 is recycled back into
the coal-fired power plant 82 to increase the concentration of
CO.sub.2 in the exhaust gas 96. Exhaust gas recirculation around
the main coal-fired power plant (or using natural gas as feed for
the combustion process) is advantageously used to increase the
CO.sub.2 concentration within the working fluid, leading to an
additional rise in CO.sub.2 partial pressure in the exhaust gas 88,
and a further increase of the driving forces for CO.sub.2
separation. In some embodiments, the exhaust gas 88 is passed
through a heat exchanger 90 to cool down the exhaust gas 88 and the
cooled exhaust stream 92 is introduced to a pre-treatment unit 52
to remove species including but not limited particles. The purified
cooled exhaust stream 96 is introduced to the compressor 14 and the
compressed exhaust stream 20 is treated in the separator 26 as
described in the earlier section to generate a CO.sub.2 rich stream
38.
[0029] FIG. 4 illustrates yet another exemplary separation system
110, wherein the CO.sub.2 rich stream 38 generated from the
separator 26 is treated in a post separation purification unit 62
to separate species like particles. In one embodiment, the
purification unit 62 may include a condenser 113 to separate the
water content in the CO.sub.2 rich stream 38. The sweep stream 36
as described in the earlier section is at a sub-atmospheric
pressure and hence the CO.sub.2 rich stream 38 at sub-atmospheric
pressure is compressed to a high pressure in a multistage
compressor 112. As shown in FIG. 4, the multistage compressor 112
comprises 5 stages 114, 116, 118, 120 and 122 with intercoolers
124, 126, 128 and 130 in between to cool the compressed gas in
between compression stages. The number of stages in the multistage
compressor 112 is determined by the pressure ratio at which the
final CO.sub.2 product 132 has to be produced at. In one
embodiment, the CO.sub.2 product 132 is generated at about 100 bar
pressure. The CO.sub.2 product 132 can be used in any industrial
application, transported and sold in merchant market or used in
enhanced oil recovery.
[0030] There are several advantages for separating CO.sub.2 from
exhaust gases using techniques described in the preceding sections.
Typically post combustion separation of CO.sub.2 from any exhaust
gas is not energy efficient due to lack of availability of elevated
pressure in the exhaust stream. In the separation systems described
herein, the exhaust gas is compressed to increase the CO.sub.2
partial pressure, which compression process allows the use of
CO.sub.2 separation technologies such as membrane technology. The
compression power required to compress the exhaust gas is partly
recovered by expanding the CO.sub.2 lean stream in an expander
coupled to the compressor. As described herein, the membrane
permeate side is operated at a sub-atmospheric conditions, e.g. by
operating a CO.sub.2 compression chain at sub-atmospheric inlet
suction pressure (e.g. at about 0.2 bar). By this a higher pressure
difference over the membrane is established at relative low
compression power, as mainly the much smaller flow of the permeate
side rich in CO.sub.2 has to be compressed in contrast to the
larger feed side. This leads to increased driving forces for
separation, and enables the use of membrane technology for CO.sub.2
capture. Due to the low permeate pressure (sub-atmospheric), steam
at a temperature lower than 100.degree. C. can be used for
sweeping, as required by a lot of polymeric membrane materials. The
separation systems described herein are easy to implement on all
existing and future power plants, as no integration with the main
power system is required. This separation system may also be used
for CO.sub.2 rich flue gases from any industrial processes.
Optionally, still heat recovery from the main power system could be
implemented, including heat recovery from the hot gas turbine
exhaust gas, or a gas turbine intercooler (if available). By
(optionally) using the exhaust gas recirculation (shown in FIGS. 3
and 4) and pressurizing CO.sub.2-rich exhaust gas, this technique
raises the partial pressure of CO.sub.2 in the power plant
exhaust-gas, thus simplifying the CO.sub.2 separation process. The
compression of the exhaust gas also decreases the volume of gas to
be treated in the CO.sub.2 separator, thus reducing the associated
capital and energy demands.
[0031] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
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