FIELD OF THE INVENTION
The present invention relates to a method and a system for the removal of
carbon dioxide (C02) from a flue gas stream with a solvent-based system. In
particular, the present invention relates to a method and a system for the
regeneration of solvents and removal of carbon dioxide (C02) from carbon
dioxide (C02) rich solvent streams.
BACKGROUND OF THE INVENTION
Flue gases from power plants and other industrial activities include pollutants,
for example greenhouse gases. One such greenhouse gas is C02. Emissions
of C02 to the atmosphere from industrial activities are of increasing concern to
society and are therefore becoming increasingly regulated.
To reduce the amount of C02 released into the atmosphere, C02 capture
technology can be applied. The selective capture of C02 allows C02 to be reused
or geographically sequestered.
CN107970743 A discloses a carbon dioxide separation method that uses a
two-tower multi-stage absorption and desorption method. C N 1 0797 43 A
discloses the use of low-grade heat to flash regenerate a semi-lean solvent.
However, the use of low-grade heat as disclosed in CN1079743 A is
insufficient to achieve the level of liquid solvent regeneration of the invention
presented herein.
The C02 capture method of the present invention is directed to C02 capture
from flue gases and industrial gases, e.g. emissions from plants that burn
hydrocarbon fuel. The C02 capture methods of the present invention are also
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applicable to C02 capture from coal, gas and oil fired boilers, combined cycle
power plants, coal gasification, hydrogen plants, biogas plants and waste to
energy plants.
Known C02 capture technology can be divided into physical adsorbents and
chemical absorbents (commonly referred to as carbon capture solvents).
The C02 capture methods of the present invention use a solvent (i.e. carbon
capture solvents). The solvent removes C02 from one or more gas streams.
The C02 in the gas streams selectively react with components in the solvent,
resulting in C02 being removed from the gas phase and absorbed by the
solvent to form a C02 rich solvent. The C02 rich solvent is then heated, C02 is
released back into the gas phase and the C02 rich solvent is depleted of its
C02 content, forming a C02 lean solvent. The C02 lean solvent is recycled
within the system to capture additional C02.
Figure 1 illustrates a block diagram 1 00 of a conventional method and system
for capturing C02 from flue gases.
In the conventional method and system for capturing C02 from flue gases,
C02 is separated from a mixture of gases using a solvent (initially a C02 lean
solvent), which selectively reacts with the C02 (to form a C02 rich solvent).
After the C02 has reacted with the solvent (C02 lean solvent), the solvent
(C02 rich solvent) can be regenerated (to C02 lean solvent) using heat to
release the C02 and regenerate the solvent for further C02 processing.
As shown in Figure 1 (indicating a prior art method and system), a flue gas
101 containing C02 enters the system. The temperature of the flue gas 101
when entering the system is typically greater than 1 00°C. The flue gas 1 01
optionally passes through a booster fan 1 02. The booster fan 1 02 increases
the pressure of flue gas 1 01 to compensate for the pressure drop through the
system, thereby ensuring that the pressure of the resultant C02 lean flue gas
(flue gas 1 07) is at the same pressure as flue gas 1 01.
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The flue gas 101 passes through a direct contact cooler 103. In the direct
contact cooler, the flue gas 1 01 is contacted with a recirculating loop of cool
water 104 in a counter-current configuration. Through this contact, the flue
gas 101 is cooled to a temperature of typically 40°C, forming flue gas 101 a.
The flue gas 101 a enters an absorber column 105, where the flue gas 101 a is
counter-currently contacted with a liquid solvent 106 (cool, C02 lean solvent).
The flue gas 101 a rises through the absorber column 105. The liquid solvent
106 (cool, C02 lean solvent) enters the absorber column 105 via a liquid
distributor (not shown in Figure 1) positioned at the top of the absorber
column 105, and cascades down through the absorber column 105. The
absorber column 105 contains packing to maximise the surface area to
volume ratio. The active components in the liquid solvent 106 (cool, C02 lean
solvent) react with the C02 in the flue gas 1 01 a.
When the liquid solvent 106 (cool, C02 lean solvent) reaches the bottom of the
absorber column 105, it is rich in C02 and forms liquid solvent 108 (cool, C02
rich solvent).
When the flue gas 101 a reaches the top of absorber column 105, it is
depleted of C02 and forms flue gas 107 (C02 lean). The flue gas 107 (C02
lean) is released from the top of the absorber column 105.
The liquid solvent 108 (cool, C02 rich solvent) is regenerated in regenerator
109 with high-grade heat, to reform liquid solvent 1 06 (cool, C02 lean solvent).
The liquid solvent 108 (cool, C02 rich solvent) enters the regenerator 1 09
(high-grade heat) via a cross-over heat exchanger 11 0. In the cross-over heat
exchanger 11 0, the liquid solvent 1 08 (cool, C02 rich solvent) is heated by a
liquid solvent 111 (hot, C02 lean solvent) to form liquid solvent 112 (hot, C02
rich solvent).
The liquid solvent 112 (hot, C02 rich solvent) enters the top of the regenerator
1 09 (high-grade heat) and cascades down the regenerator 1 09 (high-grade
heat). Inside the regenerator (high-grade heat), the liquid solvent 112 (hot,
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C02 rich solvent) is heated through contact with a vapour 114 (high-grade
heat). Typically, the vapour 114 (high-grade heat) flows upwards through the
regenerator 109 (high-grade heat), counter-current to the liquid solvent 112
(hot, C02 rich solvent). Upon heating, the reaction between the active
components of the liquid solvent and C02 reverses, releasing C02 gas 115
and forming a liquid solvent 111 (hot, C02 lean solvent).
Gaseous C02 115 leaves the top of the regenerator 109 (high-grade heat).
Gaseous C02 115 can be used in downstream processes.
The liquid solvent 111 (hot, C02 lean solvent) is fed into a reboiler 113 (highgrade
heat). Within the reboiler 113 (high-grade heat), the liquid solvent 111
(hot, C02 lean solvent) is boiled resulting in formation of the vapour 114 (highgrade
heat). The vapour 114 (high-grade heat) is used in the regenerator 1 09
(high-grade heat).
The liquid solvent 111 (hot, C02 lean solvent) passes into the cross-over heat
exchanger 11 0 and is cooled through contact with the liquid solvent 1 08 (cool,
C02 rich solvent) to form liquid solvent 106 (cool, C02 lean solvent). The
freshly formed liquid solvent 106 (cool, C02 lean solvent) is now ready to
repeat the absorption process again.
The liquid solvent 106 (cool, C02 lean solvent) may pass through an additional
cooler (not shown) before entering the absorber column 105.
In typical C02 capture methods that use chemical absorbents, regeneration of
the chemical absorbent requires a high amount of energy. Regeneration of the
chemical absorbent is therefore one of the largest operating costs for
capturing C02.
There is a need for a lower cost method of regenerating the absorbent (i.e. the
liquid solvent) after the absorbent has become a C02 rich chemical absorbent.
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SUMMARY OF THE INVENTION
The ability to generate the necessary quantity and quality of the heat required
to regenerate the chemical absorbent is important. In general, the higher the
temperature of the heat generated, the more valuable the heat is. In typical
C02 capture processes, the heat required to heat the C02 rich chemical
absorbent (i.e. the C02 rich liquid solvent) is supplied in the form of any
heating fluid such as a condensing steam, hot gases, hot water or thermal oil.
In typical C02 capture processes that use chemical absorbents, regeneration
of the chemical absorbent requires a temperature of equal to or greater than
120°C (high-grade heat). It is desirable to use lower-value, low-grade heat
sources to the greatest extent possible to remove C02 from a C02 rich
chemical absorbent, so that the regeneration method is as cost effective as
possible.
The present invention provides a method and a system of removing C02 from
a solvent (e.g. a method of forming a C02 lean chemical absorbent from a
C02 rich chemical absorbent).
The present invention provides a method and a system of removing C02 from
a solvent, wherein lower temperature heat sources (i.e. low-grade heat) are
used to partially or wholly regenerate the lean chemical absorbent.
The present invention provides a method and a system of removing C02 from
a solvent, wherein the high-grade heat (equal to or greater than 120°C) is
partially replaced with low-grade heat in the range of from 60 to less than
120°C. This advantageously reduces the high-grade heat required by from 30
to 50%, typically 50% (plus or minus 1 0%), and decreases the overall
operating cost.
The present invention provides a method and system that typically comprises
at least two regeneration sections. Typically, one regeneration section
comprises a regenerator for low-grade heat, and the second regeneration
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section comprises a second regenerator for high-grade heat respectively. The
regenerator (low-grade heat) produces a hot C02 semi-lean stream which is
only partially depleted of C02.The second regeneration section (high-grade
heat) produces a hot C02 lean stream, which is analogous to stream 111 in
the conventional method and system for capturing C02 from flue gases.
The present invention provides a method and a system where heat is
exchanged between liquid streams that are regenerated with both high-grade
and low-grade heat. The heat exchange advantageously allows customisation
of the system, which advantageously allows optimisation of the operating cost
of the overall energy consumption.
Representative features of the present invention are set out in the following
clauses, which stand alone or may be combined, in any combination, with one
or more features disclosed in the text and/or figures of the specification.
The present invention is now described with reference to the following
clauses:
1. A method for regenerating a solvent comprising carbon dioxide (C02),
the method comprising:
providing a solvent comprising carbon dioxide (C02);
passing the solvent comprising carbon dioxide (C02) through a lowgrade
heat regenerator to form a carbon dioxide (C02) lean solvent; and,
passing the carbon dioxide (C02) lean solvent through a low-grade
heat reboiler.

Claims
1. A method for regenerating a solvent comprising carbon dioxide (C02),
the method comprising:
providing a solvent comprising carbon dioxide (C02);
passing the solvent comprising carbon dioxide (C02) through a lowgrade
heat regenerator to form a carbon dioxide (C02) lean solvent; and,
passing the carbon dioxide (C02) lean solvent through a low-grade
heat reboiler.
2. The method of claim 1, wherein the low-grade heat regenerator
operates at a temperature in the range offrom 60 to less than 120°C.
3. The method of claim 1 or claim 2, wherein the low-grade heat
regenerator operates at a temperature in the range of: from 100 to 119°C; or,
from 100 to 115°C.
4. The method of any one of claims 1 to 3, wherein the low-grade heat
reboiler operates at a temperature in the range of from 60 to less than 120°C.
5. The method of any one of claims 1 to 4, wherein the low-grade heat
reboiler operates at a temperature in the range of: from 100 to 119°C; or, from
100 to 115°C.
6. The method of any one of claims 1 to 5, wherein the method further
comprises:
passing the solvent comprising carbon dioxide (C02) through a highgrade
heat regenerator to form a carbon dioxide (C02) lean solvent; and,
passing the carbon dioxide (C02) lean solvent through a high-grade
heat reboiler.
7. The method of claim 6, wherein the high-grade heat regenerator
operates at a temperature equal to or greater than 120°C.
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8. The method of claim 6 or claim 7, wherein the high-grade heat
regenerator operates at a temperature of from 120°C to 140°C.
9. The method of any one of claims 6 to 8, wherein the high-grade heat
reboiler operates at a temperature equal to or greater than 120°C.
10. The method of any one of claims 6 to 9, wherein the high-grade heat
reboiler operates at a temperature of from 120°C to 140°C.
11. The method of any one of claims 6 to 1 0, wherein the low-grade heat
regenerator, the low-grade heat reboiler, the high-grade heat regenerator and
the high-grade heat reboiler are in fluid communication such that solvent
comprising carbon dioxide (C02) passes between two, three or four of the
components.
12. The method of claim 11, wherein solvent comprising carbon dioxide
(C02) leaving the low-grade heat reboiler passes to the high-grade heat
regenerator; optionally, through a cross-over heat exchanger.
13. The method of any one of claims 6 to 1 0, wherein:
the low-grade heat regenerator and the low-grade heat reboiler are in
fluid communication such that solvent comprising carbon dioxide (C02)
passes between the low-grade heat regenerator and the low-grade heat
reboiler;
the high-grade heat regenerator and the high-grade heat reboiler are in
fluid communication such that solvent comprising carbon dioxide (C02)
passes between the high-grade heat regenerator and the high-grade heat
reboiler; and,
the low-grade heat regenerator and the low-grade heat reboiler are
hydraulically independent with (not in fluid communication with), and thermally
dependent with (in thermal communication with), the high-grade heat
regenerator and the high-grade heat reboiler.