# Patent

## Title
CO2 recovery system

## Abstract

Provided is a CO2 recovery system including: a high-pressure absorption tower; a high-pressure regeneration tower that partially regenerates a CO2 absorption solution from the absorption tower through a first liquid feed line; a second liquid feed line that extracts a semi-lean solution having a heat resistance temperature thereof or lower from the high-pressure regeneration tower and introduces a portion of the semi-lean solution into a middle stage of the high-pressure absorption tower; a branch line that introduces a rest of the semi-lean solution into a flash drum; a third liquid feed line that introduces a lean solution after adding pressure thereto into a top of the high-pressure absorption tower; a high-pressure CO2 compression device where high-pressure CO2 gas from the high-pressure regeneration tower is introduced; and a low-pressure CO2 compression device where low-pressure CO2 from the flash drum is introduced.

## Background
TECHNICAL FIELD
The present invention relates to a CO2 recovery system that absorbs CO2 from gas and reduces CO2 compression power.
BACKGROUND ART
As one of the causes of global warming, the greenhouse effect by CO2 has been identified, and a countermeasure thereof is urgently required internationally from the viewpoint of protecting the global environment. Sources of CO2 emission spread over various fields of human activity where fossil fuel is burned, and demand on emission reduction of CO2 tends to increase. As a result, for power generation facilities such as a thermal power plant where a large amount of fossil fuel is used, a CO2 recovery method has been actively studied in which flue gas of a boiler is brought into contact with an amine-based absorption solution such as an aqueous amine compound solution so as to remove and recover CO2 from the flue gas.
In the related art, PTL 1 discloses an amine recovery process in which an amine compound accompanying decarbonated flue gas, from which CO2 has been absorbed and removed by gas-liquid contact with an absorption solution, is sequentially recovered in plural stages of water-washing sections that are provided for bringing the decarbonated flue gas into gas-liquid contact with a washing solution to recover the amine compound.
In addition, in the related art, PTL 2 discloses a configuration including: a cooling unit that cools decarbonated flue gas from which CO2 has been absorbed and removed by gas-liquid contact with an absorption solution; and a contact unit that brings condensate, which has been condensed in the cooling unit, into counterflow contact with the decarbonated flue gas. Further, PTL 2 discloses a configuration including a water-washing section in which an amine compound accompanying decarbonated flue gas is recovered. In this configuration, condensate, which has been condensed in a cooling tower in which flue gas is cooled before CO2 is recovered therefrom, is used as washing water.
CITATION LIST
Patent Literature


    [PTL 1] Japanese Unexamined Patent Application Publication No. 2002-126439
    [PTL 2] Japanese Unexamined Patent Application Publication No. 8-80421


SUMMARY OF INVENTION
Technical Problem
However, recently, from the viewpoint of protecting the environment, when a CO2 recovery device is installed to process flue gas in, for example, a thermal power plant where the flow rate of gas to be processed is high, the amount of CO2 to be removed is large. Therefore, for example, when the CO2 recovery device is buried in the ground, it is desired to reduce power relating to CO2 compression in a CO2 recovery plant.
The present invention has been made in order to solve the above-described problems, and an object thereof is to provide a CO2 recovery system that absorbs CO2 from gas and reduces CO2 compression power.
Solution to Problem
According to a first aspect of the present invention for solving the above-described problems, there is provided a CO2 recovery system including: a high-pressure absorption tower that brings high-pressure gas containing CO2 into contact with a CO2 absorption solution to remove CO2 from the high-pressure gas; a high-pressure regeneration tower that liberates a portion of CO2 from a CO2-absorbed high-pressure rich solution, which has been introduced from the high-pressure absorption tower through a first liquid feed line, and partially regenerates the CO2 absorption solution to obtain a semi-lean solution; and a branch line that branches the semi-lean solution extracted from a tower bottom portion of the high-pressure regeneration tower, in which the partially regenerated CO2 absorption solution is extracted from the bottom portion of the high-pressure regeneration tower in a semi-lean solution state having a heat resistance temperature of the CO2 absorption solution or lower, a portion of the extracted semi-lean solution is introduced into a middle stage of the high-pressure absorption tower through a second liquid feed line to be reused as the absorption solution, the rest of the extracted semi-lean solution is introduced into a flash drum through the branch line to obtain a normal-pressure lean solution, and the lean solution is pressurized and is introduced to the top of the high-pressure absorption tower through a third liquid feed line to be reused as the absorption solution, high-pressure CO2 gas separated in the high-pressure regeneration tower is introduced into a CO2 compression device having a predetermined compression pressure, and low-pressure CO2 gas separated in the flash drum is introduced into a CO2 compression device having a predetermined compression pressure.
According to a second aspect of the invention, the CO2 recovery system according to the first aspect may further include: an inert gas separator that separates inert gas between the high-pressure absorption tower and the high-pressure regeneration tower; and an inert gas introduction line that introduces the separated inert gas into the high-pressure absorption tower.
Advantageous Effects of Invention
According to the present invention, in the high-pressure regeneration tower, the high-pressure rich solution is introduced at a predetermined compression pressure and is emitted from the tower bottom portion at a heat resistance temperature of the absorption solution or lower as the semi-lean solution obtained by liberating a portion of carbon dioxide (CO2) from the high-pressure rich solution and partially regenerating the high-pressure rich solution. The separated high-pressure CO2 gas having the desired pressure is introduced into a compressor of a CO2 compression device that is provided on a gas flow downstream side of the high-pressure regeneration tower to compress CO2. As a result, deterioration of an absorption solution is suppressed, compression power for compressing CO2 gas, which is emitted from the high-pressure regeneration tower, can be significantly saved unlike the related art, and the size of compression facilities can be reduced.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic diagram illustrating a CO2 recovery system according to Embodiment 1.
FIG. 2 is a schematic diagram illustrating the CO2 recovery system according to Embodiment 1.
FIG. 3 is a schematic diagram illustrating a CO2 recovery system according to Embodiment 2.



DESCRIPTION OF EMBODIMENTS
Hereinafter, the present invention will be described with reference to the drawings. The present invention is not limited to the embodiment. In addition, in the case of plural embodiments, combinations of the respective embodiments are included in the present invention. In addition, components in the following embodiment include components which can be easily conceived by those skilled in the art, and substantially identical ones thereto.
Embodiment 1
A CO2 recovery system according to an embodiment of the present invention will be described with reference to the drawings.
FIGS. 1 and 2 are schematic diagrams illustrating the CO2 recovery system according to Embodiment 1. As illustrated in FIG. 1, a CO2 recovery system 10A according to the embodiment includes: a high-pressure absorption tower 13 that brings high-pressure gas 11 containing CO2 into contact with a CO2 absorption solution 12 to remove CO2 from the high-pressure gas 11; a high-pressure regeneration tower 15 that liberates a portion of CO2 from a CO2-absorbed high-pressure rich solution 14, which has been introduced into the high-pressure regeneration tower 15 from the high-pressure absorption tower 13 through a first liquid feed line L10, and partially regenerates the CO2 absorption solution to obtain a semi-lean solution 19; a second liquid feed line L11 that feeds the semi-lean solution 19, which has been extracted from a tower bottom portion of the high-pressure regeneration tower 15, to the high-pressure absorption tower 13; a branch line L12 that branches a portion 19a of the semi-lean solution 19 fed from the second liquid feed line L11; and a flash drum 21 that is provided in a branch line L12 and flashes the semi-lean solution 19. In the CO2 compression recovery system, the partially regenerated CO2 absorption solution is extracted from the bottom portion of the high-pressure regeneration tower 15 in the state of the semi-lean solution 19 having a heat resistance temperature of the CO2 absorption solution or lower, the extracted semi-lean solution 19 is introduced into a middle stage of the high-pressure absorption tower 13 through the second liquid feed line L11 to be reused as the absorption solution, the flash drum 21 flashes the portion 19a of the semi-lean solution branched in the branch line L12 to obtain a normal-pressure lean solution 18, the lean solution 18 is pressurized and is introduced to the top of the high-pressure absorption tower 13 to be reused as the absorption solution, high-pressure CO2 gas 22H which is a gas component separated in the high-pressure regeneration tower 15 is introduced into a high-pressure compressor 16H having a predetermined compression pressure, and low-pressure CO2 gas 22L which is a gas component separated in the flash drum 21 is introduced into a low-pressure compressor 16L having a predetermined compression pressure.
In the high-pressure absorption tower 13, the high-pressure gas 11 containing carbon dioxide (CO2) is brought into counterflow contact with the CO2 absorption solution (amine solution) 12 containing, for example, alkanolamine as a base such that the CO2 absorption solution 12 absorbs CO2 from the high-pressure gas 11 and removes CO2 from the high-pressure gas 11. The high-pressure rich solution 14 which is the CO2-absorbed CO2 absorption solution is regenerated in the semi-lean solution 19 state by a portion of CO2 being liberated in the high-pressure regeneration tower 15.
Here, in FIG. 1, reference numeral L13 represents a third liquid feed line that feeds the lean solution 18, which is separated in the flash drum 21, to the top of the high-pressure absorption tower 13, reference numeral L14 represents a gas feed line that feeds the low-pressure CO2 gas 22L from the flash drum 21 to a gas-liquid separator 23, reference numeral L15 represents a liquid return line that returns a portion 18a of the lean solution 18, which has been recovered from the gas-liquid separator 23, to the third liquid feed line L13, reference numeral L16 represents a gas feed line that feeds the high-pressure CO2 gas 22H from the top of the high-pressure regeneration tower 15 to the high-pressure compressor 16H, reference numeral L17 represents a gas feed line that feeds the low-pressure CO2 gas 22L from the gas-liquid separator 23 to the low-pressure compressor 16L, reference numeral L18 represents an introduction line that introduces compressed CO2, which has been compressed in a CO2 compression device 16, into the ground, reference numeral 31 represents purified gas from which CO2 has been removed, reference numeral 32 represents a liquid feed pump of the high-pressure rich solution 14 that is provided in the second liquid feed line L11, reference numeral 33 represents a heat exchanger that exchanges heat between the high-pressure rich solution 14 and the semi-lean solution 19, reference numeral 36 represents a cooling condenser that cools steam condensate 34 on the top portion of the high-pressure regeneration tower with cooling water 35, reference numeral 37 represents a reboiler that reheats the portion 19a of the semi-lean solution 19, reference numeral 38 represents saturated steam that is supplied to the reboiler, reference numeral 39 represents the steam condensate, reference numeral 41 represents a pressure-reducing valve that is provided in the branch line L12, reference numeral 42 represents a booster pump that pressurizes the lean solution 18, and reference numeral 43 represents cooling means for cooling the lean solution 18.
The high-pressure gas 11 is introduced into the high-pressure absorption tower 13, the CO2 absorption solution 12 absorbs CO2 contained in the high-pressure gas 11 to obtain the high-pressure rich solution 14, and the high-pressure rich solution 14 is emitted from the bottom of the high-pressure absorption tower 13. Next, the high-pressure rich solution 14 is fed to the high-pressure regeneration tower 15 by the liquid feed pump 32 and is regenerated in the semi-lean solution 19 state by a portion of CO2 being liberated in the high-pressure regeneration tower 15.
Here, when the lean solution 18 is completely regenerated, the internal temperature of the high-pressure regeneration tower 15 is a heat resistance temperature (for example, 120° C.) of the absorption solution or higher. Therefore, it is necessary that the regeneration in the high-pressure regeneration tower 15 be limited to the partial regeneration and that the CO2 absorption solution be emitted in the semi-lean solution 19 state.
As the high-pressure gas 11 containing CO2, for example, high-pressure gas for fertilizer synthesis or high-pressure natural gas (gas pressure: for example, 3,000 kPaG) may be used.
Accordingly, the high-pressure rich solution 14 that has absorbed CO2 in the high-pressure absorption tower 13 using the high-pressure gas 11 is introduced into the high-pressure regeneration tower 15 at a predetermined compression pressure and is emitted from the tower bottom portion as the semi-lean solution 19 which is partially regenerated from the high-pressure rich solution 14 by a portion of carbon dioxide (CO2) being liberated from the high-pressure rich solution 14.
The high-pressure CO2 gas 22H having the desired pressure (for example, 330 kPaG) which has been separated from the tower top portion of the high-pressure regeneration tower 15 is fed to the high-pressure compressor 16H of the CO2 compression device 16 through the gas feed line L16.
In the embodiment, the partially regenerated CO2 absorption solution is extracted from the bottom portion of the high-pressure regeneration tower 15 in the semi-lean state having a heat resistance temperature (for example, 120° C.) of the CO2 absorption solution or lower. Therefore, the absorption solution does not deteriorate.
The extracted semi-lean solution 19 is introduced into the middle stage of the high-pressure absorption tower 13 through the second liquid feed line L11 to be reused as the absorption solution.
Here, it is preferable that the semi-lean solution be introduced into the middle stage of the high-pressure absorption tower 13 for the following reason: the lean-solution 18 introduced from the tower top partially absorbs CO2 and is in the semi-rich state in the middle stage of the top, and the semi-lean-solution 19 introduced from the middle stage is the absorption solution having the same properties as the above semi-rich solution.
On the other hand, the portion 19a of the semi-lean solution 19 branched in the branch line L12 is introduced into the flash drum 21 and is flashed to perform gas-liquid separation and to obtain the normal-pressure lean solution 18.
This lean solution 18 is pressurized by the booster pump 42 and is introduced from the tower top of the high-pressure absorption tower 13 to be reused as the absorption solution.
Since the amount of the lean solution 18 introduced from the tower top portion of the high-pressure absorption tower 13 is small, the size of a tower body on the upper side of the high-pressure absorption tower 13 can be reduced, and the size of the absorption tower can be reduced.
Here, regarding the amount of the semi-lean solution branched, a molar ratio of the amount of the semi-lean solution introduced to the high-pressure absorption tower 13 to the amount of the semi-lean solution branched is about 8:2.
The portion 19a of the branched semi-lean solution is used for gas-liquid separation in the flash drum 21. The separated low-pressure CO2 gas 22L is fed to the gas-liquid separator 23 through the gas feed line L14. In this gas-liquid separator 23, the absorption solution accompanying the gas during flashing is separated as the lean solution 18, and the lean solution 18a is fed to the low-pressure compressor 16L through the gas feed line L17. The lean solution 18 which is the separated absorption solution is returned to the third liquid feed line L13 for feeding the lean solution.
FIG. 2 is a schematic diagram illustrating the CO2 recovery device to describe a temperature and a pressure of gas according to an example of the embodiment.
The high-pressure rich solution 14 is introduced into the high-pressure regeneration tower 15 and is converted into a partially regenerated absorption solution by a portion of CO2 being liberated. This partially regenerated absorption solution is emitted as the semi-lean solution 19 from the bottom portion of the high-pressure regeneration tower 15 at a heat resistance temperature (for example, 120° C.) of the absorption solution or lower.
From the tower top of the high-pressure regeneration tower 15, the liberated high-pressure CO2 gas 22H containing 93% of the total amount of CO2 is fed to the high-pressure compressor 16H at a gas pressure of 330 kPaG and 40° C. through the gas feed line L16.
In addition, the low-pressure CO2 gas 22L containing the rest of the 7% of the total amount of CO2 which has been emitted from the flash drum 21 is fed to the low-pressure compressor 16L at a gas pressure of 50 kPaG and 40° C. through the gas feed line L17.
The semi-lean solution 19 emitted from the bottom portion of the high-pressure regeneration tower 15 has a temperature of 120° C. and a pressure of 355 kPaG. Since this temperature is lower than or equal to a deterioration temperature of the amine solution constituting the absorption solution, the absorption solution does not deteriorate. The lean solution 18 emitted from the flash drum 21 has a temperature of 106° C. and a pressure of 60 kPaG.
On the other hand, as in the case of the related art, when the high-pressure rich solution is directly introduced into the high-pressure regeneration tower and the total amount thereof is regenerated, the lean solution which has been regenerated by emitting CO2 in the high-pressure regeneration tower is emitted from the bottom portion at about 150° C. or higher and a pressure of 380 kPaG. Therefore, the lean solution is in the high-temperature state, and the amine solution which is the composition of the absorption solution deteriorates.
In this way, according to the embodiment, the high-pressure rich solution 14 is introduced into the high-pressure regeneration tower 15 and is emitted as the high-pressure CO2 gas 22H from the tower top portion by a portion of CO2, absorbed in the high-pressure rich solution 14, being liberated. This emitted high-pressure CO2 gas 22H is introduced into the high-pressure compressor 16H of the CO2 compression device 16 that is provided on a gas flow downstream side of the high-pressure regeneration tower 15 to compress CO2.
As a result, compression power for compressing CO2 gas, which is emitted from the high-pressure regeneration tower 15, can be significantly saved unlike the related art, and the size of compression facilities can be reduced.
Embodiment 2
A CO2 recovery system according to an embodiment of the present invention will be described with reference to the drawings. FIG. 3 is a schematic diagram illustrating a CO2 recovery system according to Embodiment 2. The same components as in Embodiment 1 are represented by the same reference numerals, and the description thereof will not be repeated.
As illustrated in FIG. 3, the CO2 recovery system 10B according to the embodiment is the same as the CO2 recovery system 10A according to Embodiment 1 illustrated in FIG. 1, except that an inert gas separator 50 is provided between the liquid feed pump 32 and the heat exchanger 33.
When inert gas is present, the compression purity of CO2 gas cannot be improved because inert gas is non-condensable gas. Accordingly, in the embodiment, inert gas (for example, N2 or H2) present (0.1 wt % or less) in the high-pressure rich solution 14 is removed on an upstream side of the high-pressure regeneration tower 15. The removed inert gas 51 is circulated again to the high-pressure absorption tower 13 through an inert gas circulating line L20. In the inert gas circulating line L20, a cooler 52, a gas-liquid separator 53, and a compressor 54 are provided.
By providing the inert gas separator 50 to remove the inert gas 51, the purity of compressed and recovered CO2 gas is improved.
In the embodiment, gas-liquid separation is performed using the flash drum, but the invention is not limited thereto. For example, a distillation tower or a separation column may be used for gas-liquid separation.
REFERENCE SIGNS LIST


    
    
        10A and 10B: CO2 RECOVERY SYSTEM
        11: HIGH-PRESSURE GAS
        12: CO2 ABSORPTION SOLUTION
        13: HIGH-PRESSURE ABSORPTION TOWER
        14: HIGH-PRESSURE RICH SOLUTION
        15: HIGH-PRESSURE REGENERATION TOWER
        16: CO2 COMPRESSION DEVICE
        16H: HIGH-PRESSURE COMPRESSOR
        16L: LOW-PRESSURE COMPRESSOR
        18: LEAN SOLUTION
        21: FLASH DRUM
        22H: HIGH-PRESSURE CO2 GAS
        22L: LOW-PRESSURE CO2 GAS

## Claims

1. A CO2 recovery system comprising:
a high-pressure absorption tower that brings high-pressure gas containing CO2 into contact with a CO2 absorption solution to remove CO2 from the high-pressure gas;
a high-pressure regeneration tower that liberates a portion of CO2 from a CO2-absorbed high-pressure rich solution, which has been introduced from the high-pressure absorption tower through a first liquid feed line, and partially regenerates the CO2 absorption solution to obtain a semi-lean solution;
a second liquid feed line that extracts the semi-lean solution having a heat resistance temperature thereof or lower from a bottom portion of the high-pressure regeneration tower and introduces a portion of the semi-lean solution into a middle stage of the high-pressure absorption tower to be reused as the CO2 absorption solution;
a branch line that introduce a rest of the semi-lean solution into a flash drum to obtain a normal-pressure lean solution;
a third liquid feed line that introduces the lean solution after adding pressure thereto into a top of the high-pressure absorption tower to be used as the CO2 absorption solution;
a high-pressure CO2 compression device where high-pressure CO2 gas separated in the high-pressure regeneration tower is introduced; and
a low-pressure CO2 compression device where low-pressure CO2 gas separated in the flash drum is introduced.

2. The CO2 recovery system according to claim 1, further comprising:
an inert gas separator that separates inert gas between the high-pressure absorption tower and the high-pressure regeneration tower; and
an inert gas introduction line that introduces the separated inert gas into the high-pressure absorption tower.