JP7443723B2 - Carbon dioxide capture device, hydrocarbon generation device, carbon circulation system, and carbon dioxide recovery method - Google Patents

Description

The present invention relates to a carbon dioxide recovery device, a hydrocarbon generation device, a carbon circulation system, and a carbon dioxide recovery method.

2. Description of the Related Art Conventionally, carbon dioxide recovery apparatuses have been known that recover carbon dioxide from a mixed gas containing a plurality of types of gases. Carbon dioxide recovery devices generally include an adsorbent that adsorbs carbon dioxide. For example, Patent Document 1 describes a technology in which carbon dioxide contained in combustion exhaust gas generated in a combustion furnace is adsorbed by an adsorbent, and the carbon dioxide is desorbed from the adsorbent by heating the adsorbent that has adsorbed carbon dioxide. Disclosed.

Japanese Patent Application Publication No. 2019-142806

However, in the carbon dioxide recovery device described in Patent Document 1, when carbon dioxide is adsorbed by the adsorbent, nitrogen oxides and moisture contained in the combustion exhaust gas are also adsorbed by the adsorbent. Therefore, the amount of carbon dioxide that can be adsorbed by the adsorbent decreases. In particular, nitrogen oxides are irreversibly adsorbed by the adsorbent, so if the adsorbent repeatedly adsorbs carbon dioxide from combustion exhaust gas and desorbs the adsorbed carbon dioxide, the nitrogen oxides adsorbed by the adsorbent As the amount of oxides accumulated increases, the capacity to adsorb carbon dioxide decreases. Therefore, the carbon dioxide recovery performance deteriorates.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a technique for suppressing a decrease in carbon dioxide recovery performance in a carbon dioxide recovery device.

The present invention has been made to solve at least part of the above-mentioned problems, and can be realized as the following forms.

(1) According to one embodiment of the present invention, a carbon dioxide recovery device is provided. The carbon dioxide recovery device has a CO ₂ adsorbent that adsorbs carbon dioxide, is supplied with combustion exhaust gas discharged from the combustor, and has a CO ₂ recovery unit that recovers carbon dioxide from the supplied combustion exhaust gas. , a NOx removal unit that removes nitrogen oxides from the combustion exhaust gas supplied from the combustor to the CO ₂ recovery unit; and a NOx removal unit that removes moisture from the combustion exhaust gas supplied from the combustor to the CO ₂ recovery unit. A dehumidifying section.

According to this configuration, nitrogen oxides and moisture contained in the combustion exhaust gas are removed from the combustion exhaust gas by the NOx removal unit and the dehumidification unit, respectively, before carbon dioxide is recovered from the combustion exhaust gas in the CO ₂ recovery unit. removed from As a result, when carbon dioxide is recovered from the combustion exhaust gas in the CO ₂ recovery section, the amount of nitrogen oxides and moisture adsorbed to the CO ₂ adsorbent _along with carbon dioxide is reduced, so that the amount of carbon dioxide can suppress the adsorption performance from decreasing. Therefore, deterioration in carbon dioxide recovery performance in the carbon dioxide recovery device can be suppressed.

(2) In the carbon dioxide recovery device of the above embodiment, the NOx removal section has a NOx adsorbent that adsorbs nitrogen oxides, and the dehumidification section is configured to dispose of the combustion exhaust gas on the upstream side of the NOx removal section. Water may be removed from the According to this configuration, before the nitrogen oxides contained in the combustion exhaust gas are adsorbed by the NOx adsorbent in the NOx removal section, the moisture contained in the combustion exhaust gas is removed from the combustion exhaust gas in the dehumidification section. Thereby, it is possible to suppress a decrease in the adsorption performance of nitrogen oxides in the NOx adsorbent due to moisture being adsorbed by the NOx adsorbent. Therefore, in the CO ₂ recovery section, the amount of nitrogen oxides adsorbed by the CO ₂ adsorbent can be further reduced, so that deterioration in the carbon dioxide recovery performance in the carbon dioxide recovery device can be further suppressed.

(3) The carbon dioxide recovery device of the above embodiment further includes an upstream detection section that is arranged upstream of the NOx removal section and that detects the concentration of nitrogen oxides contained in the combustion exhaust gas, and an upstream detection section that detects the concentration of nitrogen oxides contained in the combustion exhaust gas. switching between supplying the combustion exhaust gas supplied from the above to the CO 2 recovery unit via the NOx removal unit, or supplying it to the CO ₂ recovery unit without passing through _the NOx removal unit. an upstream switching valve; and a control unit that controls switching by the upstream switching valve, and the control unit controls when the concentration of nitrogen oxides detected by the upstream detection unit is greater than a first predetermined value. , the combustion exhaust gas is supplied to the CO ₂ recovery unit via the NOx removal unit, and when the concentration of nitrogen oxides detected by the upstream detection unit is equal to or lower than the first predetermined value, the combustion exhaust gas is may be supplied to the CO ₂ recovery unit without passing through the NOx removal unit. According to this configuration, the upstream switching valve switches the route through which the combustion exhaust gas flows according to the concentration of nitrogen oxides in the combustion exhaust gas detected by the upstream detection section. Specifically, when the concentration of nitrogen oxides in the combustion exhaust gas upstream of the NOx removal section is higher than the first predetermined value, the combustion exhaust gas is supplied to the CO ₂ recovery section via the NOx removal section. Thereby, the nitrogen oxides contained in the combustion exhaust gas are removed in the NOx removal section, so that the amount of nitrogen oxides adsorbed by the CO ₂ adsorbent in the CO ₂ recovery section can be reduced. Further, when the concentration of nitrogen oxides in the combustion exhaust gas is equal to or lower than the first predetermined value, the combustion exhaust gas is supplied to the CO ₂ recovery unit without passing through the NOx removal unit. As a result, nitrogen oxides are not removed in the NOx removing section, so if the NOx removing section includes a NOx adsorbent, the amount of NOx adsorbent used can be reduced. Therefore, it is possible to reduce the number of times maintenance is performed to replace the NOx adsorbent whose adsorption performance has deteriorated in the NOx removal section. In this way, it is possible to reduce the number of times the NOx removal section needs to be maintained while suppressing deterioration in the carbon dioxide recovery performance of the carbon dioxide recovery device.

(4) In the carbon dioxide recovery device of the above embodiment, the NOx removal section includes a first remover that removes nitrogen oxides from the combustion exhaust gas, and a first remover that removes nitrogen oxides from the combustion exhaust gas separately from the first remover. and a second remover for removing nitrogen oxides, the carbon dioxide recovery device is further disposed downstream of the NOx removal section, and detects the concentration of nitrogen oxides contained in the combustion exhaust gas. The combustion exhaust gas supplied to the NOx removal section by switching between the downstream detection section and the upstream switching valve is supplied to the CO ₂ recovery section via the first remover, or a downstream switching valve that switches between supplying the CO 2 to the CO ₂ recovery unit via the CO 2 remover; If it is larger than a predetermined value, the combustion exhaust gas that is being supplied to the CO ₂ recovery unit via one of the first remover and the second remover is removed from the first remover or the second remover. The downstream switching valve may be controlled to supply the CO ₂ to the recovery unit via the other one of the second removers. According to this configuration, the downstream switching valve switches the route through which the combustion exhaust gas flows according to the concentration of nitrogen oxides in the combustion exhaust gas detected by the downstream detection section. When the flue gas is being supplied to the NOx removal section by switching the upstream switching valve, the concentration of nitrogen oxides in the flue gas on the downstream side of either the first remover or the second remover is set to a second predetermined value. If the value is greater than the value, the flue gas is passed to either the first remover or the second remover. The concentration of nitrogen oxides detected by the downstream detection unit located downstream of the NOx removal unit indicates the nitrogen oxide removal performance of the remover through which the combustion exhaust gas is flowing, of the two removers. Therefore, when the concentration of nitrogen oxides detected by the downstream detection section becomes higher than the second predetermined value, it means that the nitrogen oxide removal performance of the remover is degraded. Therefore, by switching between the two removers using the concentration of nitrogen oxides detected by the downstream detection unit, the amount of combustion exhaust gas can be continuously reduced compared to the case where nitrogen oxides are removed using one remover. Nitrogen oxides can be removed.

(5) According to another aspect of the present invention, a hydrocarbon generation device is provided. This hydrocarbon generation device includes the carbon dioxide recovery device described above, and a generator that generates hydrocarbon compounds using hydrogen and carbon dioxide recovered by the CO ₂ recovery section. According to this configuration, a hydrocarbon compound is generated using hydrogen and carbon dioxide recovered by the above-mentioned carbon dioxide recovery device. In the carbon dioxide recovery device described above, the carbon dioxide recovery performance is less likely to deteriorate, so that the number of maintenance operations required to return the carbon dioxide recovery performance to its original state can be reduced. Therefore, hydrocarbon compounds can be stably produced.

(6) According to yet another embodiment of the present invention, a carbon circulation system is provided. This carbon circulation system includes the above-described hydrocarbon generation device and the combustor that combusts the hydrocarbon compound generated by the generator and discharges the combustion exhaust gas. According to this configuration, a hydrocarbon compound generated using hydrogen and carbon dioxide recovered by the carbon dioxide recovery device is combusted to recover thermal energy. Carbon dioxide produced by combustion of hydrocarbon compounds is recovered by a CO ₂ recovery device and becomes a raw material for hydrocarbon compounds again. In this way, since carbon can be recycled and used, the amount of carbon dioxide discharged outside the system can be reduced.

(7) According to yet another aspect of the present invention, a carbon dioxide recovery method is provided. This carbon dioxide recovery method includes a dehumidification step of removing moisture from the combustion exhaust gas, a NOx removal step of removing nitrogen oxides from the combustion exhaust gas from which moisture has been removed in the dehumidification step, and a CO ₂ adsorbent. and a CO ₂ recovery step of recovering carbon dioxide from the combustion exhaust gas from which nitrogen oxides have been removed in the NOx removal step. According to this configuration, moisture is first removed from the combustion exhaust gas in the dehumidification process. Nitrogen oxides are removed from the combustion exhaust gas from which moisture has been removed in the NOx removal step. As a result, when recovering carbon dioxide in the CO ₂ recovery process, the amount of nitrogen oxides and moisture that are adsorbed to the CO ₂ adsorbent along with carbon dioxide is reduced, which improves the carbon dioxide adsorption performance of the CO ₂ adsorbent. It is possible to suppress the decline. Therefore, deterioration in carbon dioxide recovery performance can be suppressed.

Note that the present invention can be realized in various aspects, including, for example, a system including a carbon dioxide recovery device, a control method for these devices and systems, and a computer program that causes these devices and systems to execute a carbon dioxide recovery method. , a server device for distributing the computer program, a non-temporary storage medium storing the computer program, etc.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows the schematic structure of the carbon circulation system of 1st Embodiment. It is a flow chart showing carbon dioxide recovery processing of a 1st embodiment. It is a schematic diagram showing a schematic structure of a carbon circulation system of a 2nd embodiment. It is a flow chart which shows carbon dioxide collection processing of a 2nd embodiment.

<First embodiment>
FIG. 1 is a schematic diagram showing a schematic configuration of a carbon circulation system 1 according to a first embodiment. The carbon circulation system 1 of the first embodiment generates thermal energy by burning methane and recovers carbon dioxide (CO ₂ ) generated by burning methane. It also produces methane from the recovered carbon dioxide. In this way, in the carbon circulation system 1, by repeating the combustion of methane containing carbon, the collection of carbon dioxide, and the generation of methane, carbon is circulated and thermal energy is generated. The carbon circulation system 1 includes a combustor 5, a methane generator 6, and a control section 55. The methane generator 6 includes a carbon dioxide recovery device 10, a methanation reactor 50, and a generated gas dehumidifier 51. The carbon circulation system 1 of the present embodiment is also applicable to carbon circulation systems using hydrocarbon compounds other than methane, such as compounds composed of carbon and hydrogen such as ethane and propane, and It is also applicable to a carbon circulation system using a hydrocarbon compound composed of carbon and hydrogen. The methane generator 6 corresponds to a "hydrocarbon generator" in the claims.

The combustor 5 is connected to a produced gas dehumidifier 51 (described later) via a methane gas flow path 91, and to a carbon dioxide recovery device 10 via an exhaust gas flow path 92. The combustor 5 uses air supplied from the outside to burn methane supplied from a methanation reactor 50, which will be described later, to generate thermal energy. The generated thermal energy is used outside the carbon circulation system 1. The combustor 5 also discharges gas generated by combustion of methane as combustion exhaust gas. The combustion exhaust gas contains carbon dioxide, nitrogen oxides (NOx), and moisture in addition to nitrogen and oxygen. The combustion exhaust gas is supplied to the carbon dioxide recovery device 10 through the exhaust gas passage 92 .

The carbon dioxide recovery device 10 includes a dehumidifying section 20, a NOx adsorption device 30, and a CO ₂ recovery device 40. The carbon dioxide recovery device 10 is connected to the combustor 5 via an exhaust gas flow path 92, recovers carbon dioxide contained in the combustion exhaust gas, and supplies it to the methanation reactor 50.

The dehumidifying section 20 is arranged upstream of the NOx adsorber 30 and the CO ₂ recovery device 40 in the flow of combustion exhaust gas discharged by the combustor 5. The dehumidifier 20 includes an exhaust gas dehumidifier 21 and a dryer 22. The exhaust gas dehumidifier 21 is arranged upstream of the dryer 22 in the flow of combustion exhaust gas. The exhaust gas dehumidifier 21 removes moisture contained in the combustion exhaust gas by cooling the combustion exhaust gas to about room temperature, for example. Note that "removal" herein includes not only removing the entire object but also removing a part of the object to be removed.

A NOx concentration sensor 11 is arranged in an exhaust gas flow path 92 between the combustor 5 and the exhaust gas dehumidifier 21. The NOx concentration sensor 11 is electrically connected to a control section 55, which will be described later. The NOx concentration sensor 11 detects the concentration of nitrogen oxides in the combustion exhaust gas flowing through the exhaust gas flow path 92 between the combustor 5 and the exhaust gas dehumidifier 21 (hereinafter referred to as “NOx concentration”), and outputs the detected concentration to the control unit 55. do. The NOx concentration sensor 11 corresponds to the "upstream detection section" in the claims.

The dryer 22 is arranged downstream of the exhaust gas dehumidifier 21 in the flow of combustion exhaust gas, and in this embodiment is connected to the downstream end of the exhaust gas passage 92. The dryer 22 cools the combustion exhaust gas to, for example, about -20°C. Thereby, moisture that was not removed by the dehumidifying section 20 is further removed from the combustion exhaust gas.

An exhaust gas flow path 93 and a detour flow path 94 are arranged between the dryer 22 and the CO ₂ recovery device 40. The exhaust gas flow path 93 is connected at one end to the dryer 22 and directly connects the dryer 22 and the CO ₂ recovery device 40 . A switching valve 12 electrically connected to the control unit 55 is arranged in the exhaust gas flow path 93 . The switching valve 12 has a connection port 12a connected to the dryer 22 via the exhaust gas flow path 93, a connection port 12b connected to the CO ₂ recovery device 40 via the exhaust gas flow path 93, and one end of the detour flow path 94. A connection port 12c is formed to which the connection port 12c is connected. The other end of the detour flow path 94 is connected to the exhaust gas flow path 93 at a confluence point (represented by reference numeral P1 shown in FIG. 1) located downstream of the switching valve 12 in the exhaust gas flow path 93. A NOx adsorber 30 is arranged in the detour passage 94 . The switching valve 12 selects whether to supply the combustion exhaust gas from the dryer 22 flowing through the exhaust gas flow path 93 to the CO ₂ recovery device 40 or to the detour flow path 94, in accordance with a command from the control unit 55. Switch. Details of the operation of the switching valve 12 will be described later. The switching valve 12 corresponds to an "upstream switching valve" in the claims.

The NOx adsorber 30 has a NOx adsorbent (not shown) that adsorbs nitrogen oxides. The NOx adsorber 30 collects nitrogen contained in the combustion exhaust gas flowing through the detour passage 94a that connects the switching valve 12 of the dehumidification passage 94 and the NOx adsorption unit 30, that is, the combustion exhaust gas from which moisture has been removed by the dehumidifier 20. Adsorbs oxides. This removes nitrogen oxides from the combustion exhaust gas. The NOx adsorbent included in the NOx adsorbent 30 is, for example, zeolite with a Si/Al ratio of 2.7 or more and a carbon dioxide adsorption amount of 0.5 mmol/g or less at a carbon dioxide partial pressure of 10 kPa. Note that the NOx adsorbent is not limited to this. Activated carbon, an alkali metal salt, an alkaline earth metal salt, a basic organic substance, or a metal oxide may be supported on a carbon porous body or a metal oxide porous body. The NOx adsorption device 30 corresponds to a "NOx adsorption section" in the claims.

The NOx concentration sensor 13 is disposed in a detour passage 94b of the detour passage 94 that connects the NOx adsorber 30 and the confluence P1 of the exhaust gas passage 93. The NOx concentration sensor 13 is electrically connected to the control unit 55, detects the NOx concentration of the combustion exhaust gas discharged by the NOx adsorber 30, and outputs the detected NOx concentration to the control unit 55.

In the detour flow path 94b, a switching valve 14 is arranged downstream of the NOx concentration sensor 13. The switching valve 14 is electrically connected to the control section 55. The switching valve 14 is connected to a connection port 14a that connects to the NOx adsorption device 30 via a detour flow path 94b, a connection port 14b that connects to the exhaust gas flow path 93 via a detour flow path 94b, and an exhaust port 95. A connection port 14c is formed. The switching valve 14 determines whether to supply the combustion exhaust gas discharged from the NOx adsorber 30 to the CO ₂ recovery device 40 or to discharge it to the outside of the system via the discharge port 95, according to a command from the control unit 55. , switch. Details of the operation of the switching valve 14 will be described later.

The CO ₂ recovery device 40 is connected to the other end of the exhaust gas flow path 93 and includes a CO ₂ adsorbent that adsorbs carbon dioxide. The CO ₂ adsorbent included in the CO ₂ recovery device 40 is, for example, zeolite or activated carbon. The CO ₂ recovery device 40 uses a CO ₂ adsorbent to remove CO 2 contained in the combustion exhaust gas flowing through the dryer 22 via the exhaust gas flow path 93 or in the combustion exhaust gas flowing from the NOx adsorption device 30 via the detour flow path 94. Carbon dioxide is separated from these combustion exhaust gases by adsorbing it. Carbon dioxide adsorbed by the CO ₂ adsorbent is desorbed from the CO ₂ adsorbent and recovered by pressure swings and temperature swings. The recovered carbon dioxide is sent to the methanation reactor 50 via the CO ₂ supply channel 96 . The CO ₂ recovery device 40 corresponds to a "CO ₂ recovery unit" in the claims.

Methanation reactor 50 is connected to CO ₂ supply channel 96 . The methanation reactor 50 houses a catalyst made of a metal having methanation catalytic performance. Examples of metals having methanation catalytic performance include ruthenium and nickel. The methanation reactor 50 generates methane through a methanation reaction between carbon dioxide supplied from the CO _{2 recovery device 40 via the CO 2 supply} channel 96 and hydrogen supplied from the outside. The product gas generated in the methanation reactor 50 is sent to the product gas dehumidifier 51 via a methane gas flow path 91 connected to the combustor 5. The methanation reactor 50 corresponds to a "generator" in the claims.

The produced gas dehumidifier 51 is arranged between the methanation reactor 50 and the combustor 5 in the methane gas flow path 91 . The produced gas dehumidifier 51 removes moisture contained in the produced gas produced in the methanation reactor 50 from the produced gas, for example, by cooling the produced gas. The generated gas from which moisture has been removed by the generated gas dehumidifier 51 is sent to the combustor 5.

The control unit 55 is a computer including a ROM, a RAM, and a CPU. The control unit 55 controls the entire carbon circulation system 1, such as switching the flow paths with the switching valves 12 and 14 in the carbon dioxide recovery process to be described later, and desorbing carbon dioxide with the CO ₂ recovery device 40. . Details of the control content of the control unit 55 will be described later.

FIG. 2 is a flowchart of the carbon dioxide recovery process of this embodiment. Next, carbon dioxide recovery processing in the carbon circulation system 1 will be explained. The carbon dioxide recovery method shown in FIG. 2 is constantly repeated in the carbon circulation system 1 while methane gas is being burned in the combustor 5.

Before the flowchart shown in FIG. 2 is started, the switching valve 12 is in a state where the connection port 12a and the connection port 12b are connected. Further, the switching valve 14 is in a state where the connection port 14a and the connection port 14b are connected. At this time, when the methane gas is burned in the combustor 5, nitrogen oxides and moisture are generated in addition to carbon dioxide inside the combustor 5. The generated carbon dioxide, nitrogen oxides, and moisture are discharged from the combustor 5 as combustion exhaust gas along with nitrogen and the like. The combustion exhaust gas is sent to the CO ₂ recovery device 40 through exhaust gas channels 92 and 93. When the combustion exhaust gas flows through the exhaust gas passage 92, moisture is removed by the exhaust gas dehumidifier 21 and the dryer 22. In this embodiment, the amount of moisture contained in the combustion exhaust gas is reduced to the saturated amount of water vapor at room temperature in the exhaust gas dehumidifier 21, and becomes 1 ppm or less in the dryer 22. The combustion exhaust gas from which moisture has been removed flows through the exhaust gas flow path 93.

In the carbon dioxide recovery process of this embodiment, first, it is determined whether the NOx concentration upstream of the carbon dioxide recovery device 10 is 200 ppm or less (step S11). Specifically, the NOx concentration sensor 11 detects the NOx concentration of the combustion exhaust gas flowing through the exhaust gas passage 92 upstream of the exhaust gas dehumidifier 21. The NOx concentration sensor 11 outputs the NOx concentration detection result to the control unit 55. When it is determined that the NOx concentration detected by the NOx concentration sensor 11 is 200 ppm or less (step S11: YES), the control unit 55 shifts the process to step S12. When it is determined that the NOx concentration detected by the NOx concentration sensor 11 is greater than 200 ppm (step S11: NO), the control unit 55 shifts the process to step S13. Note that the NOx concentration, which is the criterion in step S11, is not limited to 200 ppm. For example, it may be set lower than 200 ppm in order to suppress the amount of nitrogen oxides adsorbed in the CO ₂ recovery device 40 at the subsequent stage. 200 ppm in step S11 corresponds to the "first predetermined value" in the claims.

In step S12, the control unit 55 determines not to switch the switching valve 12. Thereby, the combustion exhaust gas flowing through the exhaust gas flow path 93 is directly supplied from the dryer 22 to the CO ₂ recovery device 40 . The CO ₂ recovery device 40 adsorbs carbon dioxide contained in the combustion exhaust gas supplied from the dryer 22 and separates carbon dioxide from the combustion exhaust gas. The main components of the combustion exhaust gas from which carbon dioxide has been separated are nitrogen and oxygen. The combustion exhaust gas discharged from the CO ₂ recovery device 40 is discharged outside the carbon circulation system 1 . In the CO ₂ recovery device 40 , carbon dioxide separated from the combustion exhaust gas is desorbed from the CO ₂ adsorbent and supplied to the methanation reactor 50 .

In step S13, the control unit 55 performs control to switch the switching valve 12. Specifically, the control unit 55 controls the switching valve 12 so that the connection port 12a and the connection port 12c of the switching valve 12 are connected. Thereby, the combustion exhaust gas flowing through the exhaust gas flow path 93 flows into the detour flow path 94a. The combustion exhaust gas flowing through the detour flow path 94a is supplied to the NOx adsorption device 30. In the NOx adsorber 30, nitrogen oxides contained in the combustion exhaust gas are adsorbed using a NOx adsorbent. The combustion exhaust gas on which nitrogen oxides have been adsorbed by the NOx adsorber 30 flows through the detour passage 94b of the NOx adsorber 30.

Next, it is determined whether the NOx concentration downstream of the NOx adsorber 30 is 200 ppm or less (step S14). Specifically, the NOx concentration sensor 13 detects the NOx concentration of the combustion exhaust gas flowing through the detour flow path 94b. The NOx concentration sensor 13 outputs the NOx concentration detection result to the control unit 55. When it is determined that the NOx concentration detected by the NOx concentration sensor 13 is 200 ppm or less (step S14: YES), the control unit 55 ends the current carbon dioxide recovery process. At this time, the combustion exhaust gas flowing through the detour flow path 94b is supplied to the CO ₂ recovery device 40. When it is determined that the NOx concentration detected by the NOx concentration sensor 13 is greater than 200 ppm (step S14: NO), the control unit 55 shifts the process to step S15. Note that the NOx concentration, which is the criterion in step S14, is not limited to 200 ppm. For example, it may be set lower than 200 ppm in order to suppress the amount of nitrogen oxides adsorbed in the CO ₂ recovery device 40 at the subsequent stage.

In step S15, the control unit 55 performs control to switch the switching valve 14. Specifically, the control unit 55 controls the switching valve 14 so that the connection port 14a and the connection port 14c of the switching valve 14 are connected. Thereby, the combustion exhaust gas discharged by the NOx adsorber 30 flows into the exhaust port 95 and is discharged outside the carbon circulation system 1 . This completes the current carbon dioxide recovery process.

In this embodiment, if it is determined in step S14 that the NOx concentration detected by the NOx concentration sensor 13 is greater than 200 ppm, it is expected that the adsorption performance of the NOx adsorbent of the NOx adsorber 30 has decreased. . Therefore, if it is determined in step S14 that the NOx concentration detected by the NOx concentration sensor 13 is greater than 200 ppm, the NOx adsorbent included in the NOx adsorption device 30 is replaced.

According to the carbon dioxide recovery device 10 of the present embodiment described above, each of the nitrogen oxides and moisture contained in the combustion exhaust gas is converted into NOx before carbon dioxide is recovered from the combustion exhaust gas in the CO ₂ recovery device 40. It is removed from the combustion exhaust gas by the adsorber 30 and the dehumidifier 20, respectively. Thereby, when carbon dioxide is recovered from the combustion exhaust gas in the CO ₂ recovery device 40, the amount of nitrogen oxides and moisture adsorbed by the CO ₂ adsorbent along with the carbon dioxide is reduced. In particular, by removing nitrogen oxides that are irreversibly adsorbed on the CO ₂ adsorbent from the combustion exhaust gas supplied from the combustor 5 to the CO ₂ recovery device 40, the nitrogen oxides adsorbed on the CO ₂ adsorbent are removed. The accumulated amount of can be reduced. Thereby, it is possible to suppress a decrease in the carbon dioxide adsorption performance of the CO ₂ adsorbent, and thus it is possible to suppress a decrease in the carbon dioxide recovery performance of the carbon dioxide recovery device 10.

Furthermore, according to the carbon dioxide recovery device 10 of the present embodiment, it is possible to suppress a decrease in the carbon dioxide recovery performance of the CO ₂ recovery device 40, and thus the performance life of the CO ₂ recovery device 40 can be extended. Thereby, in the CO ₂ recovery device 40, it is possible to reduce the number of maintenance operations such as replacement and regeneration of the CO ₂ adsorbent whose carbon dioxide adsorption performance has decreased.

Further, according to the carbon dioxide recovery device 10 of the present embodiment, before the nitrogen oxides contained in the combustion exhaust gas are adsorbed by the NOx adsorbent in the NOx absorber 30, the moisture contained in the combustion exhaust gas is is removed from the flue gas. Thereby, it is possible to suppress a decrease in the adsorption performance of nitrogen oxides in the NOx adsorbent due to moisture being adsorbed by the NOx adsorbent. Therefore, in the CO ₂ recovery device 40, the amount of nitrogen oxides adsorbed by the CO ₂ adsorbent can be further reduced, so that the deterioration of the carbon dioxide recovery performance in the carbon dioxide recovery device 10 can be further suppressed. can.

Further, according to the carbon dioxide recovery device 10 of the present embodiment, the switching valve 12 switches the route through which the combustion exhaust gas flows according to the NOx concentration of the combustion exhaust gas detected by the NOx concentration sensor 11. Specifically, when the NOx concentration of the combustion exhaust gas on the upstream side of the NOx adsorption device 30 is higher than 200 ppm, the combustion exhaust gas is supplied to the CO ₂ recovery device 40 via the NOx adsorption device 30. Thereby, the nitrogen oxides contained in the combustion exhaust gas are removed in the NOx adsorber 30, so that the amount of nitrogen oxides adsorbed by the _CO2 adsorbent in the _CO2 recovery device 40 can be reduced. Further, when the NOx concentration of the combustion exhaust gas is 200 ppm or less, the combustion exhaust gas is supplied to the CO ₂ recovery device 40 without passing through the NOx adsorption device 30. Thereby, since nitrogen oxides are not removed in the NOx adsorber 30, the amount of NOx adsorbent used can be reduced. Therefore, in the NOx adsorber 30, it is possible to reduce the number of times maintenance is performed to replace the NOx adsorbent whose adsorption performance has deteriorated. In this way, it is possible to reduce the number of times the NOx adsorber 30 needs to be maintained while suppressing deterioration in the carbon dioxide recovery performance of the carbon dioxide recovery device 10.

Further, according to the carbon dioxide recovery device 10 of the present embodiment, for example, when high concentration nitrogen oxides are generated due to abnormal combustion in the combustor 5, the route through which the combustion exhaust gas flows is switched by the switching valve 12, and the combustion Nitrogen oxides can be removed from exhaust gas. Thereby, it is possible to suppress a sudden increase in the amount of nitrogen oxides adsorbed by the CO ₂ adsorbent, and therefore it is possible to further suppress a decrease in the carbon dioxide recovery performance of the carbon dioxide recovery apparatus 10.

Moreover, according to the methane generation device 6 of this embodiment, methane is generated using carbon dioxide recovered by the carbon dioxide recovery device 10 and hydrogen. In the carbon dioxide recovery device 10, nitrogen oxides and moisture contained in the combustion exhaust gas are removed by the NOx adsorber 30 and the dehumidifier 20, respectively, so that the carbon dioxide recovery performance is unlikely to deteriorate. This makes it possible to reduce the number of maintenance operations required to restore carbon dioxide recovery performance to its original state. Therefore, in the carbon circulation system 1, hydrocarbon compounds can be stably produced.

Furthermore, according to the carbon circulation system 1 of the present embodiment, methane generated using carbon dioxide recovered by the carbon dioxide recovery device 10 and hydrogen is combusted to recover thermal energy. Carbon dioxide produced by the combustion of methane is recovered by the CO ₂ recovery device 40 and becomes a raw material for methane again. In this way, since carbon can be recycled and used, the amount of carbon dioxide discharged outside the system can be reduced.

<Second embodiment>
FIG. 3 is a schematic diagram showing a schematic configuration of a carbon circulation system 2 according to the second embodiment. The carbon circulation system 2 of the second embodiment differs from the carbon circulation system 1 of the first embodiment (FIG. 1) in that two NOx adsorbers are provided in parallel.

The carbon circulation system 2 of this embodiment includes a combustor 5, a methane generator 7, and a control section 55. The methane generator 7 includes a carbon dioxide recovery device 60, a methanation reactor 50, and a generated gas dehumidifier 51. The carbon dioxide recovery device 60 includes a dehumidifying section 20, a first NOx adsorption device 61, a second NOx adsorption device 62, and a CO ₂ recovery device 40. The carbon dioxide recovery device 60 is connected to the combustor 5 via an exhaust gas flow path 92, recovers carbon dioxide contained in the combustion exhaust gas discharged by the combustor 5, and supplies it to the methanation reactor 50. The methane generator 7 corresponds to a "hydrocarbon generator" in the claims.

In this embodiment, an exhaust gas flow path 93 and a detour flow path 97 are arranged between the dryer 22 and the CO ₂ recovery device 40, as shown in FIG. The detour passage 97 includes a connection passage 97a connected to the connection port 12c of the switching valve 12, two branch passages 97b and 97c, and a confluence point located downstream of the switching valve 12 in the exhaust gas passage 93 (see FIG. A connecting flow path 97d connected to the exhaust gas flow path 93 is formed at reference numeral P2) shown in FIG. The connecting flow path 97a and the two branch flow paths 97b and 97c are connected by a switching valve 15. The switching valve 15 is formed with a connection port 15a that connects to the connection flow path 97a, a connection port 15b that connects to the branch flow path 97b, and a connection port 15c that connects to the branch flow path 97c. The switching valve 15 supplies the combustion exhaust gas flowing through the connection flow path 97a to the CO ₂ recovery device 40 via the first NOx adsorption device 61 in accordance with a command from the electrically connected control section 55, or Alternatively, it is switched whether to supply the CO ₂ to the CO 2 recovery device 40 via the second NOx adsorption device 62 or not. The details of the operation of the switching valve 15 will be described later. The switching valve 15 corresponds to a "downstream switching valve" described in the claims.

The first NOx adsorber 61 is disposed in the branch flow path 97b, and has a NOx adsorbent (not shown) that adsorbs NOx. The first NOx adsorber 61 adsorbs NOx contained in the combustion exhaust gas flowing through the branch flow path 97b. The second NOx adsorption device 62 is disposed in the branch flow path 97c, and has a NOx adsorbent (not shown) that adsorbs NOx. The second NOx adsorber 62 adsorbs NOx contained in the combustion exhaust gas flowing through the branch flow path 97c. The NOx adsorbent included in the first NOx adsorber 61 and the second NOx adsorber 62 has a Si/Al ratio of 2.7 or more, and a carbon dioxide adsorption amount of 0.5 mmol/g or less at a carbon dioxide partial pressure of 10 kPa. It is zeolite. The first NOx adsorber 61 corresponds to a "first remover" in the claims. The second NOx adsorber 62 corresponds to a "second remover" in the claims.

A NOx concentration sensor 16 that detects the NOx concentration of the combustion exhaust gas flowing through the connection flow path 97d is arranged in the connection flow path 97d. The NOx concentration sensor 16 is electrically connected to the control section 55. The NOx concentration sensor 16 detects the NOx concentration of the combustion exhaust gas discharged by the first NOx adsorption device 61 or the second NOx adsorption device 62, and outputs it to the control unit 55. The NOx concentration sensor 16 corresponds to the "downstream detection section" in the claims.

FIG. 4 is a flowchart of the carbon dioxide recovery process of this embodiment. Next, carbon dioxide recovery processing in the carbon circulation system 2 will be explained. The carbon dioxide recovery process shown in FIG. 4 is constantly performed in the carbon circulation system 1 while methane gas is being burned in the combustor 5. Before the flowchart shown in FIG. 4 is started, the switching valve 12 is in a state where the connection port 12a and the connection port 12b are connected. Further, the switching valve 15 is in a state where the connection port 15a and the connection port 15b are connected.

In the carbon dioxide recovery process of this embodiment, first, similarly to step S11 of the first embodiment, it is determined whether the NOx concentration upstream of the carbon dioxide recovery device 60 is 200 ppm or less (step S21). . When it is determined that the NOx concentration detected by the NOx concentration sensor 11 is 200 ppm or less (step S21: YES), the control unit 55 shifts the process to step S22. When it is determined that the NOx concentration detected by the NOx concentration sensor 11 is greater than 200 ppm (step S21: NO), the control unit 55 shifts the process to step S23.

In step S22, similarly to step S12 of the first embodiment, the control unit 55 determines not to switch the switching valve 12. Thereby, the combustion exhaust gas flowing through the exhaust gas flow path 93 is directly supplied from the dryer 22 to the CO ₂ recovery device 40 .

In step S23, similarly to step S13 of the first embodiment, the control unit 55 performs control to switch the switching valve 12. Specifically, the control unit 55 controls the switching valve 12 so that the connection port 12a and the connection port 12c of the switching valve 12 are connected. Thereby, the combustion exhaust gas flowing through the exhaust gas flow path 93 flows into the connection flow path 97a of the detour flow path 97. The combustion exhaust gas flowing through the connection passage 97a is supplied to the first NOx adsorber 61 via the switching valve 15 and the branch passage 97b, in which the connection port 15a and the connection port 15b are connected. In the first NOx adsorber 61, nitrogen oxides contained in the combustion exhaust gas are adsorbed by the NOx adsorbent. The combustion exhaust gas in which nitrogen oxides have been adsorbed by the first NOx adsorber 61 flows through the connecting flow path 97d.

Next, it is determined whether the NOx concentration of the combustion exhaust gas flowing downstream of the first NOx adsorber 61 is 200 ppm or less (step S24). Specifically, the NOx concentration sensor 16 detects the NOx concentration of the combustion exhaust gas flowing through the detour flow path 94b. The combustion exhaust gas at this time is the combustion exhaust gas that has passed through the first NOx adsorber 61. The NOx concentration sensor 16 outputs the detection result of NOx concentration to the control section 55. When it is determined that the NOx concentration detected by the NOx concentration sensor 16 is 200 ppm or less (step S24: YES), the control unit 55 ends the current carbon dioxide recovery process. When it is determined that the NOx concentration detected by the NOx concentration sensor 16 is greater than 200 ppm (step S24: NO), the control unit 55 shifts the process to step S25. Note that the NOx concentration, which is the criterion in step S24, is not limited to 200 ppm. It may be set lower than 200 ppm in order to suppress the amount of nitrogen oxides adsorbed in the CO ₂ recovery device 40 at the subsequent stage. The 200 ppm in step S24 corresponds to the "second predetermined value" in the claims.

In step S25, the control unit 55 performs control to switch the switching valve 15. Specifically, the control unit 55 controls the switching valve 15 so that the connection port 15a and the connection port 15c of the switching valve 15 are connected. Thereby, the combustion exhaust gas flowing through the connecting flow path 97a is caused to flow into a branch flow path different from the branch flow path 97b, that is, the branch flow path 97c. The combustion exhaust gas flowing through the branch flow path 97c is supplied to the second NOx adsorber 62. In the second NOx adsorber 62, nitrogen oxides contained in the combustion exhaust gas are adsorbed by the NOx adsorbent. The combustion exhaust gas in which nitrogen oxides have been adsorbed by the second NOx adsorber 62 flows through the connecting flow path 97d. In this embodiment, in step S25, after switching the switching valve 15 so that the connection port 15a and the connection port 15c are connected, the NOx adsorbent of the first NOx adsorption device 61 is replaced.

Next, it is determined whether the NOx concentration of the combustion exhaust gas flowing downstream of the second NOx adsorber 62 is 200 ppm or less (step S26). Specifically, similarly to step S24, the NOx concentration sensor 16 detects the NOx concentration of the combustion exhaust gas flowing through the connection flow path 97d. The combustion exhaust gas at this time is the combustion exhaust gas that has passed through the second NOx adsorber 62. The NOx concentration sensor 16 outputs the detection result of NOx concentration to the control section 55. When it is determined that the NOx concentration detected by the NOx concentration sensor 16 is 200 ppm or less (step S26: YES), the control unit 55 ends the current carbon dioxide recovery process. When it is determined that the NOx concentration detected by the NOx concentration sensor 16 is greater than 200 ppm (step S26: NO), the control unit 55 shifts the process to step S27.

In step S27, similarly to step S25, the control unit 55 performs control to switch the switching valve 15. Specifically, the control unit 55 controls the switching valve 15 so that the connection port 15a and the connection port 15b of the switching valve 15 are connected. Thereby, the combustion exhaust gas flowing through the connecting flow path 97a is caused to flow into the branch flow path 97b. The combustion exhaust gas flowing through the branch flow path 97b is supplied to the first NOx adsorber 61, and nitrogen oxides are adsorbed by the NOx adsorbent. The combustion exhaust gas in which nitrogen oxides have been adsorbed by the first NOx adsorber 61 flows through the connecting flow path 97d. In this embodiment, in step S27, after switching the switching valve 15 so that the connection port 15a and the connection port 15b are connected, the NOx adsorbent of the second NOx adsorption device 62 is replaced. After that, the current carbon dioxide recovery process ends.

In the carbon dioxide recovery process of the present embodiment, the switching valve 15 directs the combustion exhaust gas flowing through the connection flow path 97a to the first NOx adsorbent according to the NOx concentration contained in the combustion exhaust gas flowing through the connection flow path 97d. 61 or _the second _NOx adsorber 62. Furthermore, when the switching valve 15 is switched, the NOx adsorbent of the NOx adsorber through which the combustion exhaust gas no longer flows is replaced.

According to the carbon dioxide recovery device 60 of the present embodiment described above, the switching valve 15 switches the route through which the combustion exhaust gas flows according to the NOx concentration of the combustion exhaust gas detected by the NOx concentration sensor 16. When the combustion exhaust gas is flowing through the connection flow path 97a by switching the switching valve 12, for example, if the NOx concentration of the combustion exhaust gas on the downstream side of the first NOx adsorption device 61 is higher than 200 ppm, the combustion exhaust gas is transferred to the second NOx adsorption device 62. Flow. Further, when the NOx concentration of the combustion exhaust gas on the downstream side of the second NOx adsorption device 62 is higher than 200 ppm, the combustion exhaust gas is caused to flow to the first NOx adsorption device 61 . The NOx concentration detected by the NOx concentration sensor 16 indicates the nitrogen oxide removal performance of the adsorbent through which the combustion exhaust gas flows, of the two adsorbers, and the NOx concentration detected by the NOx concentration sensor 16 is lower than 200 ppm. A larger value means that the nitrogen oxide removal performance is lower. Therefore, by switching between the two removers using the NOx concentration detected by the NOx concentration sensor 16, nitrogen oxides in the combustion exhaust gas can be continuously removed compared to the case where one remover is used to remove nitrogen oxides. can be removed.

Further, according to the carbon dioxide recovery device 60 of the present embodiment, when the NOx concentration becomes greater than 200 ppm, the combustion exhaust gas discharged from the combustor 5 is oxidized into nitrogen by the first NOx adsorption device 61 or the second NOx adsorption device 62. Objects are removed. As a result, even if the nitrogen oxide adsorption performance of one of the two NOx adsorbers deteriorates, maintenance can be performed to replace one NOx adsorbent while removing nitrogen oxides from the other. Therefore, by switching between the two NOx adsorbers to remove nitrogen oxides, it is possible to discharge the combustion exhaust gas from which nitrogen oxides could not be removed to the outside of the carbon circulation system 2, as in the first embodiment. Therefore, thermal energy can be efficiently generated from carbon.

<Modification of this embodiment>
The present invention is not limited to the above-described embodiments, and can be implemented in various forms without departing from the spirit thereof. For example, the following modifications are also possible.

[Modification 1]
In the above-described embodiment, the methane production device as a "hydrocarbon production device" produces CH ₄ as a "hydrocarbon compound." However, the hydrocarbon compounds produced by hydrocarbon production equipment include not only CH ₄ but also compounds composed of carbon and hydrogen, such as ethane and propane, and compounds composed mainly of carbon and hydrogen. May include.

[Modification 2]
In the embodiment described above, the NOx adsorber serving as the "NOx removal section" has a NOx adsorbent that adsorbs NOx, and removes nitrogen oxides from the combustion exhaust gas. However, the method by which the NOx removal section removes nitrogen oxides from the combustion exhaust gas is not limited to this. Nitrogen oxides may also be removed from the combustion exhaust gas by a reduction method using a reducing agent such as urea or a reduction method using a catalyst. Alternatively, NO may be oxidized to NO ₂ by disposing an NO oxidation catalyst in the former stage, and adsorbed by a NOx adsorbent disposed in the latter stage.

[Modification 3]
In the embodiment described above, the dehumidifying section 20 removes moisture contained in the combustion exhaust gas by cooling the combustion exhaust gas. However, the method of removing moisture by the dehumidifier 20 is not limited to this.

[Modification 4]
In the embodiment described above, after moisture is removed from the combustion exhaust gas by the dehumidifying section 20, nitrogen oxides are removed by the NOx absorber. However, the relationship between the timing of removing moisture from the combustion exhaust gas and the timing of removing nitrogen oxides is not limited to this. Moisture may be removed after nitrogen oxides are removed from the combustion exhaust gas. For example, when nitrogen oxides are removed using a reducing agent such as urea, the moisture contained in the combustion exhaust gas may increase, so it is desirable to remove the nitrogen oxides before removing the moisture.

[Modification 5]
In the embodiment described above, the NOx concentration detected upstream of the carbon dioxide recovery device is used to supply the combustion exhaust gas to the CO ₂ recovery device 40 via the NOx adsorption device by the switching valve 12, or , and whether to supply the CO ₂ to the CO 2 recovery device 40 without passing through the NOx adsorption device. However, the switching valve 12 may not be provided. The entire amount of combustion exhaust gas discharged from the dehumidifying section 20 may be supplied to the CO ₂ recovery device 40 via the NOx adsorption device.

[Modification 6]
In the embodiment described above, the control of the switching valve, the control unit, and the control unit may not be necessary, and any method that can perform switching using the switching valve when a certain concentration is detected may be used.

Although the present aspect has been described above based on the embodiments and modified examples, the embodiments of the above-described aspect are for facilitating understanding of the present aspect, and do not limit the present aspect. This aspect may be modified and improved without departing from the spirit and scope of the claims, and this aspect includes equivalents thereof. Furthermore, if the technical feature is not described as essential in this specification, it can be deleted as appropriate.

1, 2... Carbon circulation system 5... Combustor 6, 7... Methane generator 10, 60... Carbon dioxide recovery device 11, 13, 16... NOx concentration sensor 12, 14, 15... Switching valve 12a, 12b, 12c, 14a , 14b, 14c, 15a, 15b, 15c,...Connection port 20...Dehumidifier 21...Exhaust gas dehumidifier 22...Dryer 30...NOx adsorber 40... _CO2 recovery device 50...Methanation reactor 51...Produced gas dehumidifier 55... Control part 61... First NOx adsorption device 62... Second NOx adsorption device 91... Methane gas channel 92, 93... Exhaust gas channel 94, 94a, 94b, 97... Detour channel 95... Discharge port 96... CO ₂ supply channel 97a, 97d... Connection flow path 97b, 97c... Branch flow path

Claims (6)

A carbon dioxide recovery device,
A CO ₂ recovery unit that has a CO ₂ adsorbent that adsorbs carbon dioxide, is supplied with combustion exhaust gas discharged from a combustor, and recovers carbon dioxide from the supplied combustion exhaust gas;
a NOx removal unit that removes nitrogen oxides from the combustion exhaust gas supplied from the combustor to the CO ₂ recovery unit;
an upstream detection section that is disposed upstream of the NOx removal section and detects the concentration of nitrogen oxides contained in the combustion exhaust gas;
Whether the combustion exhaust gas supplied from the combustor is supplied to the CO 2 recovery unit via the NOx removal unit, or to the CO ₂ recovery unit without passing through _the NOx removal unit. an upstream switching valve that switches ,
a dehumidifying section that is disposed upstream of the upstream switching valve and removes moisture from the combustion exhaust gas supplied from the combustor to the CO ₂ recovery section;
A control unit that controls switching by the upstream switching valve,
The dehumidification section is
an exhaust gas dehumidifier that removes at least a portion of moisture contained in the combustion exhaust gas by cooling the combustion exhaust gas to a first temperature;
A dryer disposed downstream of the exhaust gas dehumidifier in the flow of the combustion exhaust gas, the dryer cooling the combustion exhaust gas that has passed through the exhaust gas dehumidifier to a second temperature lower than the first temperature, a dryer that further removes moisture contained in the combustion exhaust gas,
The control unit includes:
If the concentration of nitrogen oxides detected by the upstream detection section is higher than a first predetermined value, the combustion exhaust gas is supplied to the CO ₂ recovery section via the NOx removal section;
If the concentration of nitrogen oxides detected by the upstream detection section is below the first predetermined value, the combustion exhaust gas is supplied to the CO ₂ recovery section without passing through the NOx removal section;
Carbon dioxide recovery equipment. The carbon dioxide recovery device according to claim 1,
The NOx removal section has a NOx adsorbent that adsorbs nitrogen oxides,
The dehumidification section removes moisture from the combustion exhaust gas on the upstream side of the NOx removal section.
Carbon dioxide recovery equipment. The carbon dioxide recovery device according to claim 1 or 2,
The NOx removal unit includes a first remover that removes nitrogen oxides from the combustion exhaust gas, and a second remover that removes nitrogen oxides from the combustion exhaust gas separately from the first remover. Ori,
The carbon dioxide recovery device further includes:
a downstream detection section that is disposed downstream of the NOx removal section and detects the concentration of nitrogen oxides contained in the combustion exhaust gas;
The combustion exhaust gas supplied to the NOx removal section by switching the upstream switching valve is supplied to the CO ₂ recovery section via the first remover, or via the second remover. a downstream switching valve that switches between supplying the CO 2 to the CO ₂ recovery section;
The control unit includes:
When the concentration of nitrogen oxides detected by the upstream detection section is higher than the second predetermined value, the CO ₂ recovery section passes through one of the first remover and the second remover. Controlling the downstream switching valve so as to supply the combustion exhaust gas supplied to the CO ₂ recovery unit via the other one of the first remover and the second remover. do,
Carbon dioxide recovery equipment. A hydrocarbon generating device,
The carbon dioxide recovery device according to any one of claims 1 to 3,
a generator that generates a hydrocarbon compound using hydrogen and carbon dioxide recovered in the CO ₂ recovery section;
Hydrocarbon generator. A carbon circulation system,
The hydrocarbon generating device according to claim 4,
the combustor that combusts the hydrocarbon compound produced by the generator and discharges the combustion exhaust gas,
Carbon circulation system. A carbon dioxide recovery method, comprising:
a dehumidification process that removes moisture from combustion exhaust gas;
a NOx removal step of removing nitrogen oxides from the combustion exhaust gas from which moisture has been removed in the dehumidification step;
A CO ₂ recovery step of recovering carbon dioxide from the combustion exhaust gas from which nitrogen oxides have been removed in the NOx removal step using a CO ₂ adsorbent,
In the dehumidification step, the combustion exhaust gas is cooled to a first temperature to remove at least part of the moisture contained in the combustion exhaust gas, and then the combustion exhaust gas is cooled to a second temperature lower than the first temperature. By doing so, the moisture contained in the combustion exhaust gas is further removed,
When the concentration of nitrogen oxides contained in the combustion exhaust gas detected by the upstream detection unit disposed upstream of the NOx removal unit is higher than the first predetermined value, the combustion exhaust gas is treated with nitrogen oxide in the NOx removal step. After the substances are removed, carbon dioxide is recovered in the CO ₂ recovery step,
When the concentration of nitrogen oxides contained in the combustion exhaust gas detected by the upstream detection section is equal to or lower than the first predetermined value, the combustion exhaust gas is treated in the CO ₂ recovery step without performing the NOx removal step. carbon dioxide is recovered,
Carbon dioxide capture method.

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