Fuel gas production system, fuel gas production method, and computer program

Abstract

To provide a technique for improving the utilization rate of carbon dioxide in a fuel gas production system.SOLUTION: The fuel gas production system includes a storage device including a recovery device configured to recover carbon dioxide, a recovery unit configured to recover carbon dioxide separately from the recovery device, and a storage unit configured to store the carbon dioxide recovered by the recovery unit, a fuelization device configured to supply hydrogen and at least one of the carbon dioxide recovered by the recovery device and the carbon dioxide stored by the storage device to a reactor to generate fuel gas, and a control unit configured to control the storage device to supply the carbon dioxide recovered by the recovery unit to the storage unit when a differential pressure becomes smaller than a differential pressure threshold.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a fuel gas production system, a fuel gas production method, and a computer program. [Background technology]

[0002] BACKGROUND ART Conventionally, a fuel gas production system has been known that produces fuel gas such as methane gas from carbon dioxide recovered from a gas containing carbon dioxide such as combustion exhaust gas and hydrogen (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-142806 Summary of the Invention [Problem to be solved by the invention]

[0004] However, even with prior art such as that disclosed in Patent Document 1, there is still room for improvement in the technology for improving the utilization rate of carbon dioxide in a fuel gas production system.

[0005] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a technology for improving the utilization rate of carbon dioxide in a fuel gas production system. [Means for solving the problem]

[0006] The present invention has been made to solve at least part of the above-mentioned problems, and can be realized in the following aspects.

[0007] (1) According to one aspect of the present invention, there is provided a fuel gas production system comprising: a capture device that captures carbon dioxide from a gas containing carbon dioxide supplied from a carbon dioxide supply source; a storage device having a capture unit that captures carbon dioxide contained in the gas separately from the capture device; and a storage unit that stores the carbon dioxide captured by the capture unit; a fuel production system having a reactor connected to each of the capture device and the storage device, wherein the fuel production system supplies at least one of the carbon dioxide captured by the capture device and the carbon dioxide stored in the storage device, and hydrogen supplied from a hydrogen supply source, to the reactor to produce fuel gas; a first pressure detection unit that detects a fuel production reaction pressure in the reactor; a second pressure detection unit that detects the pressure of the carbon dioxide stored in the storage unit; and a control unit that controls the storage device so that the carbon dioxide captured in the capture unit is supplied to the storage unit when a differential pressure, which is a value obtained by subtracting the pressure detected by the first pressure detection unit from the pressure detected by the second pressure detection unit, becomes smaller than a predetermined differential pressure threshold.

[0008] According to this configuration, in the fuel gas production system, the storage device that supplies carbon dioxide to the reactor of the fuel production device includes a storage unit that stores carbon dioxide recovered by a recovery unit that recovers carbon dioxide contained in the gas, separate from the recovery device. This allows the amount of carbon dioxide that the storage device can supply to the fuel production device per unit time to be leveled. Therefore, even if the amount of carbon dioxide that the recovery unit can supply to the storage unit is relatively small, the carbon dioxide recovered in the storage device can be used to generate fuel gas in the reactor. Furthermore, the storage device supplies the carbon dioxide recovered in the recovery unit to the storage unit when the differential pressure, which is the value obtained by subtracting the fuel production reaction pressure in the reactor from the pressure of the carbon dioxide stored in the storage unit, becomes smaller than a predetermined differential pressure threshold. Here, the "fuel production reaction pressure" refers to the pressure of the fuel production reaction field in the reactor, which changes depending on the production amount of fuel gas produced using carbon dioxide and hydrogen supplied to the reactor as raw materials. This prevents the pressure of the carbon dioxide stored in the storage unit from decreasing to a pressure that makes it impossible to supply the carbon dioxide to the reactor. In this way, fuel gas can be generated using the small amount of carbon dioxide remaining in the recovery unit, thereby improving the utilization rate of the carbon dioxide contained in the gas.

[0009] (2) The fuel gas production system of the above aspect may further include a flow rate detection unit that detects the flow rate of carbon dioxide supplied from the capture unit to the storage unit, and the control unit may increase the differential pressure threshold when the flow rate of carbon dioxide detected by the flow rate detection unit is equal to or less than a preset flow rate threshold. According to this configuration, when the amount of carbon dioxide supplied from the capture unit to the storage unit is relatively small, the control unit increases the differential pressure threshold to advance the timing of supplying the carbon dioxide captured in the capture unit to the storage unit. This extends the time for supplying the carbon dioxide captured in the capture unit to the storage unit, so that the pressure of the carbon dioxide stored in the storage unit can be increased even when, for example, the amount of carbon dioxide that the capture unit can supply to the storage unit is relatively small. Therefore, even a small amount of carbon dioxide remaining in the capture unit can be used to generate fuel gas, thereby further improving the utilization rate of carbon dioxide.

[0010] (3) The fuel gas production system of the above aspect may further include a fuel pressure control unit that controls the fuel pressure in the reactor, and a flow rate detection unit that detects the flow rate of carbon dioxide supplied from the recovery unit to the storage unit. The control unit may control the fuel pressure control unit to reduce the fuel reaction pressure in the reactor when the flow rate of carbon dioxide detected by the flow rate detection unit is equal to or lower than a preset flow rate threshold. According to this configuration, when the flow rate of carbon dioxide supplied from the recovery unit to the storage unit is relatively low, the control unit reduces the fuel reaction pressure in the reactor and reduces the amount of carbon dioxide that needs to be supplied from the storage unit to the reactor. This allows the carbon dioxide recovered in the storage device to be used to generate fuel gas in the reactor, even when the amount of carbon dioxide that the recovery unit can supply to the storage unit is relatively low. Therefore, the carbon dioxide utilization rate can be further improved.

[0011] (4) In the fuel gas production system of the above aspect, the recovery unit may have a plurality of adsorption towers containing adsorbents capable of adsorbing carbon dioxide, and the control unit may control the storage device to desorb carbon dioxide adsorbed by a first adsorption tower contained in a first of the plurality of adsorption towers, while causing a second adsorption tower contained in a second of the plurality of adsorption towers to adsorb carbon dioxide contained in the gas. According to this configuration, the recovery unit has a plurality of adsorption towers containing adsorbents capable of adsorbing carbon dioxide. The control unit adsorbs carbon dioxide in a first of the plurality of adsorption towers, while causing the first adsorption tower to desorb carbon dioxide adsorbed in a separate second adsorption tower. This allows the recovery unit to continuously supply carbon dioxide to the storage unit. This further equalizes the amount of carbon dioxide that the storage device can supply to the fuel production device per unit time.

[0012] (5) In the fuel gas production system of the above aspect, the control unit may control the storage device so that, when the differential pressure becomes equal to or less than a preset lower threshold, the first adsorbent adsorbs carbon dioxide contained in the gas while the second adsorbent desorbs carbon dioxide adsorbed therein. According to this configuration, when the differential pressure becomes equal to or less than a preset lower threshold, the control unit switches the process so that the second adsorption tower that adsorbed the carbon dioxide contained in the gas desorbs carbon dioxide, and the first adsorption tower that desorbed the adsorbed carbon dioxide adsorbs carbon dioxide contained in the gas. This prevents the storage unit from being unable to supply carbon dioxide at a sufficient pressure to the reactor, for example, by switching the process if the carbon dioxide adsorbed in the adsorbent is desorbed but the pressure of the carbon dioxide in the storage unit does not reach a pressure that allows it to be supplied to the reactor. Therefore, fuel gas can be continuously produced.

[0013] (6) According to another aspect of the present invention, there is provided a fuel gas production method using a fuel gas production system, the fuel gas production method comprising: a recovery step of recovering carbon dioxide from a gas containing carbon dioxide supplied from a carbon dioxide supply source; a storage step of recovering the carbon dioxide contained in the gas using a recovery unit and storing the carbon dioxide recovered by the recovery unit in a storage unit separately from the recovery step; a fuel production step of supplying at least one of the carbon dioxide recovered in the recovery step and the carbon dioxide stored in the storage step, and hydrogen supplied from a hydrogen supply source, to a reactor to produce fuel gas; a first pressure detection step of detecting a fuel production reaction pressure in the reactor; a second pressure detection step of detecting the pressure of the carbon dioxide stored in the storage unit; and a control step of controlling the recovery unit to supply the carbon dioxide recovered in the recovery unit to the storage unit when a differential pressure, which is a value obtained by subtracting the pressure detected in the first pressure detection step from the pressure detected in the second pressure detection step, becomes smaller than a predetermined differential pressure threshold. According to this configuration, the fuel gas production method includes a recovery step of recovering carbon dioxide from a carbon dioxide-containing gas, and a storage step of recovering the carbon dioxide contained in the gas using a recovery unit and storing the carbon dioxide recovered by the recovery unit in a storage unit. This allows the amount of carbon dioxide that the storage unit can supply to the reactor per unit time to be leveled out, so that even if the amount of carbon dioxide recovered in the storage step is relatively small, it can be used to generate fuel gas in the reactor. Furthermore, in the control step, when the differential pressure becomes smaller than a predetermined differential pressure threshold, the carbon dioxide recovered in the recovery unit is supplied to the storage unit. This prevents the pressure of the carbon dioxide stored in the storage unit from decreasing to a pressure that makes it impossible to supply it to the reactor, thereby improving the utilization rate of the carbon dioxide contained in the gas.

[0014] (7) According to yet another aspect of the present invention, there is provided a computer program for causing a computer to execute a method for producing fuel gas using a fuel gas production system. The computer program causes the computer to execute a capture function for recovering carbon dioxide from a gas containing carbon dioxide supplied from a carbon dioxide supply source, a storage function for recovering the carbon dioxide contained in the gas using a capture unit and storing the carbon dioxide recovered by the capture unit in a storage unit separately from the capture function, a fuel production function for supplying at least one of the carbon dioxide recovered by the capture function and the carbon dioxide stored by the storage unit, and hydrogen supplied from a hydrogen supply source, to a reactor to produce fuel gas, a first pressure detection function for detecting a fuel production reaction pressure in the reactor, a second pressure detection function for detecting the pressure of the carbon dioxide stored in the storage unit, and a control function for controlling the capture unit to supply the carbon dioxide recovered in the capture unit to the storage unit when a differential pressure, which is a value obtained by subtracting the pressure detected by the first pressure detection function from the pressure detected by the second pressure detection function, becomes smaller than a predetermined differential pressure threshold. According to this configuration, the computer program causes the computer to execute a capture function of recovering carbon dioxide from a carbon dioxide-containing gas, as well as a storage function of recovering the carbon dioxide contained in the gas using a capture unit and storing the carbon dioxide recovered by the capture unit in a storage unit. This allows the amount of carbon dioxide that the storage unit can supply to the reactor per unit time to be leveled, so that even if the amount of carbon dioxide recovered by the capture unit is relatively small, it can be used to generate fuel gas in the reactor. In addition, as a control function, when the differential pressure becomes smaller than a predetermined differential pressure threshold, the carbon dioxide recovered in the capture unit is supplied to the storage unit. This prevents the pressure of the carbon dioxide stored in the storage unit from decreasing to a pressure that makes it impossible to supply it to the reactor, thereby improving the utilization rate of the carbon dioxide contained in the gas.

[0015] The present invention can be realized in various forms, for example, in the form of a carbon dioxide storage device, a carbon dioxide storage method, a system including a carbon dioxide storage device, a control method for these devices and systems, a computer program that causes these devices and systems to produce fuel gas using carbon dioxide recovered in these devices and systems, a server device for distributing the computer program, a non-transitory storage medium that stores the computer program, etc. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram showing a schematic configuration of a fuel gas production system according to a first embodiment. [Figure 2] FIG. 1 is a schematic diagram showing a general configuration of a carbon dioxide storage device. [Figure 3] FIG. 1 is a diagram illustrating process switching in an adsorption tower of a carbon dioxide storage device. [Figure 4] 4 is a flowchart of a desorption step in the adsorption tower in the first embodiment. [Figure 5] FIG. 10 is a graph showing the time changes in the flow rate and pressure of carbon dioxide in the desorption step. [Figure 6] FIG. 1 is a first graph showing the change over time in the flow rate of carbon dioxide desorbed from the adsorbent. [Figure 7] FIG. 2 is a second graph showing the change over time in the flow rate of carbon dioxide desorbed from the adsorbent. [Figure 8] FIG. 10 is a schematic diagram showing a schematic configuration of a fuel gas production system according to a second embodiment. [Figure 9] 10 is a flowchart of a desorption step in an adsorption tower in a second embodiment. [Figure 10] FIG. 10 is a schematic diagram showing a schematic configuration of a modified example of the fuel gas production system of the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0017] First Embodiment FIG. 1 is a schematic diagram showing the overall configuration of a fuel gas production system according to a first embodiment. The fuel gas production system 1 of this embodiment includes a carbon dioxide capture device 10, a carbon dioxide storage device 20, a fuel production device 30, a hydrogen gas tank 40, a methane gas tank 50, and a control unit 60. The fuel gas production system 1 captures carbon dioxide contained in combustion exhaust gas generated in a combustion furnace 5 serving as a "carbon dioxide supply source," and produces methane gas as a "fuel gas" using the captured carbon dioxide and hydrogen. Note that the fuel gas produced by the fuel gas production system 1 is not limited to methane gas. It may also be a hydrocarbon compound such as methanol.

[0018] The carbon dioxide capture device 10 includes a capture device 11 and a surge tank 12. The capture device 11 includes a plurality of adsorbers (not shown) that house adsorbents having carbon dioxide adsorption properties, and a vacuum pump (not shown). In this embodiment, the capture device 11 includes three adsorbers. The capture device 11 is connected to the combustion furnace 5 via an exhaust gas pipe 1a, and combustion exhaust gas generated in the combustion furnace 5 is sent to the capture device 11. The exhaust gas pipe 1a is provided with a dehydrator 13 that removes at least a portion of the water vapor contained in the combustion exhaust gas. The carbon dioxide captured in the capture device 11 is sent to a surge tank 12. The surge tank 12 is connected to a raw material gas pipe 1b that is connected to a methanation reactor 31. Details of the method for capturing carbon dioxide from combustion exhaust gas using the carbon dioxide capture device 10 will be described later.

[0019] The carbon dioxide storage device 20 is connected to the flue gas pipe 1a separately from the carbon dioxide capture device 10. The carbon dioxide storage device 20, separate from the carbon dioxide capture device 10, captures carbon dioxide contained in the combustion flue gas and temporarily stores the captured carbon dioxide.

[0020] 2 is a diagram showing a schematic configuration of carbon dioxide storage device 20. Carbon dioxide storage device 20 of this embodiment includes two adsorption towers 21 and 22, a vacuum pump 23, a flow rate detection unit 24, a compressor 25, a storage surge tank 26, a storage pressure detection unit 27, a flow rate control unit 28, and a plurality of pipes connecting these.

[0021] Each of the adsorption towers 21, 22 is a "recovery section" that recovers carbon dioxide from the combustion exhaust gas. Each of the adsorption towers 21, 22 is connected to a pipe 20a connected to the exhaust gas pipe 1a, a pipe 20b connected to a storage surge tank 26 (described later), and a pipe 20c for releasing gas discharged from the adsorption towers 21, 22 to the outside of the fuel gas production system 1. The pipe 20a is provided with valves 21a, 22a that control the inflow of combustion exhaust gas into the adsorption towers 21, 22. The pipe 20b is provided with valves 21b, 22b that control the outflow of gas from the adsorption towers 21, 22 to the storage surge tank 26. The pipe 20c is provided with valves 21c, 22c that control the release of gas from the adsorption towers 21, 22 to the outside of the fuel gas production system 1. Each of the valves 21a, 21b, 21c, 22a, 22b, and 22c is electrically connected to the control unit 60, and the opening degree thereof is controlled in accordance with a command signal output by the control unit 60.

[0022] The adsorption towers 21 and 22 each include an adsorbent 21d or 22d capable of adsorbing carbon dioxide. The adsorbents 21d or 22d adsorb carbon dioxide contained in the combustion exhaust gas that flows into the adsorption towers 21 or 22 through the pipe 20a. By maintaining the state in which the adsorbents 21d or 22d adsorb carbon dioxide, the adsorption towers 21 or 22 temporarily store the carbon dioxide.

[0023] The vacuum pump 23 is connected to the pipe 20b. The vacuum pump 23 is electrically connected to the control unit 60, and reduces the pressure inside the adsorption towers 21 and 22 in response to commands output by the control unit 60. When the pressure inside the adsorption towers 21 and 22 is reduced, the carbon dioxide adsorbed by the adsorbents 21d and 22d is desorbed from the adsorbents 21d and 22d. The carbon dioxide desorbed from the adsorbents 21d and 22d flows through the pipe 20b.

[0024] The flow rate detection unit 24 is connected to the pipe 20b. In this embodiment, the flow rate detection unit 24 is connected downstream of the vacuum pump 23 in the flow direction of the carbon dioxide in the pipe 20b. The flow rate detection unit 24 detects the flow rate of the carbon dioxide flowing through the pipe 20b. The flow rate detection unit 24 outputs the detected flow rate of the carbon dioxide to the control unit 60.

[0025] The compressor 25 is connected to the pipe 20b. In this embodiment, the compressor 25 is connected downstream of the flow rate detection unit 24 in the flow direction of the carbon dioxide in the pipe 20b. The compressor 25 compresses the carbon dioxide flowing through the pipe 20b.

[0026] The storage surge tank 26 is a “storage unit” for storing the carbon dioxide pressurized by the compressor 25. The storage surge tank 26 is connected to the raw material gas pipe 1b, and stores the carbon dioxide pressurized by the compressor 25.

[0027] Storage pressure detection unit 27 is provided in storage surge tank 26. Storage pressure detection unit 27 detects the pressure of carbon dioxide in storage surge tank 26. Storage pressure detection unit 27 outputs the detected carbon dioxide pressure to control unit 60, which is electrically connected to storage pressure detection unit 27. Storage pressure detection unit 27 corresponds to the "second pressure detection unit" in the claims.

[0028] The flow rate control unit 28 is connected to the pipe 20b. In this embodiment, the flow rate control unit 28 is connected downstream of the storage surge tank 26 in the flow direction of the carbon dioxide in the pipe 20b. The flow rate control unit 28 controls the flow rate of carbon dioxide supplied from the storage surge tank 26 to the methanation reactor 31 in response to a command from a control unit 60 that is electrically connected to the flow rate control unit 28.

[0029] The fuel production device 30 includes a methanation reactor 31, a reaction pressure detection unit 32, and a concentration detection unit 33. The methanation reactor 31 houses a catalyst capable of producing methane gas from carbon dioxide and hydrogen. The methanation reactor 31 produces methane gas (fuel gas) using at least one of the carbon dioxide captured by the carbon dioxide capture device 10 and the carbon dioxide stored in the carbon dioxide storage device 20. The methane gas produced in the methanation reactor 31 is sent to a methane gas tank 50 via a connected methane gas pipe 1c.

[0030] The reaction pressure detection unit 32 is provided in the methanation reactor 31. The reaction pressure detection unit 32 detects the pressure (fuel conversion reaction pressure) of the methanation reaction field in the catalyst in the methanation reactor 31. The fuel conversion reaction pressure changes depending on the amount of methane gas produced using carbon dioxide and hydrogen supplied to the methanation reactor 31 as raw materials. The reaction pressure detection unit 32 outputs the detected fuel conversion reaction pressure to the control unit 60 to which it is electrically connected. The reaction pressure detection unit 32 corresponds to the "first pressure detection unit" in the claims.

[0031] The concentration detection unit 33 detects the concentration of carbon dioxide and the concentration of hydrogen in the source gas containing carbon dioxide and hydrogen flowing through the source gas pipe 1b. The concentration detection unit 33 outputs the detected concentrations of carbon dioxide and hydrogen to the control unit 60, which is electrically connected to the concentration detection unit 33.

[0032] The hydrogen gas tank 40 is connected to both the recovery device 11 and the raw gas pipe 1b provided in the carbon dioxide recovery device 10. A hydrogen flow control valve 40b that controls the flow rate of hydrogen in the hydrogen gas pipe 40a is connected to a hydrogen gas pipe 40a that connects the hydrogen gas tank 40 and the recovery device 11. A hydrogen flow control valve 40d that controls the flow rate of hydrogen in the hydrogen gas pipe 40c is connected to a hydrogen gas pipe 40c that connects the hydrogen gas tank 40 and the raw gas pipe 1b. The hydrogen flow control valves 40b and 40d control the flow rate of hydrogen supplied to the recovery device 11 or the raw gas pipe 1b in response to commands from a control unit 60 that is electrically connected to them.

[0033] The methane gas tank 50 stores the methane gas sent from the methanation reactor 31 via the methane gas pipe 1c. A compressor 51 that compresses the methane gas produced in the methanation reactor 31 is connected to the methane gas pipe 1c.

[0034] The control unit 60 is a computer including a ROM, a RAM, and a CPU. The control unit 60 is electrically connected to the carbon dioxide capture device 10, the carbon dioxide storage device 20, the fuel production device 30, the hydrogen flow control valve 40d of the hydrogen gas pipe 40c, etc. The control unit 60 controls each part of the fuel gas production system 1 and executes the fuel gas production method by expanding a computer program stored in a ROM (Read Only Memory) (not shown) into a RAM (Random Access Memory) and executing the program.

[0035] Next, a description will be given of a fuel gas production method using the fuel gas production system 1. In the fuel gas production method of this embodiment, the combustion exhaust gas discharged from the combustion furnace 5 flows through the exhaust gas pipe 1a and is sent to the carbon dioxide recovery device 10 and the carbon dioxide storage device 20.

[0036] In the carbon dioxide recovery device 10 to which the combustion exhaust gas is sent, the carbon dioxide contained in the combustion exhaust gas is adsorbed by physical adsorption in the adsorbent contained in the recovery device 11 (adsorption process in the carbon dioxide recovery device 10). The combustion exhaust gas is a gas that mainly contains nitrogen and carbon dioxide, and the remainder of the combustion exhaust gas from which carbon dioxide has been removed by the adsorbent contained in the adsorber of the recovery device 11 is discharged outside the fuel gas production system 1 as adsorption off-gas.

[0037] In the carbon dioxide capture system 10, the carbon dioxide adsorbed in the adsorbent of the capture device 11 is desorbed from the adsorbent using the PTSA (pressure temperature swing) method (desorption process in the carbon dioxide capture system 10). Specifically, the adsorbent of the capture device 11 is heated using a heat medium. The heat medium for heating the adsorbent of the capture device 11 can be the reaction heat of the methanation reaction in the methanation reactor 31. When the adsorbent is heated, the carbon dioxide adsorbed in the adsorbent is desorbed from the adsorbent. When desorption of carbon dioxide from the adsorbent begins, the hydrogen flow control valve 40b of the hydrogen gas pipe 40a is controlled to supply hydrogen from the hydrogen gas tank 40 to the adsorber of the capture device 11. In the carbon dioxide capture system 10 of this embodiment, the vacuum pump provided in the capture device 11 is further operated to reduce the pressure inside the adsorber. Through these operations, the carbon dioxide in the capture device 11, mixed with hydrogen, is sent to the surge tank 12 and temporarily stored in the surge tank 12.

[0038] In the carbon dioxide capture apparatus 10, after the desorption process in the carbon dioxide capture apparatus 10, a room temperature refrigerant is passed through the adsorber to cool the adsorbent. In the carbon dioxide capture apparatus 10, the three adsorbers of the capture apparatus 11 perform their respective processes (adsorption process, desorption process, and cooling process) with a phase shift. This allows a mixed gas of carbon dioxide and hydrogen to be continuously supplied to the surge tank 12. Note that, in the carbon dioxide capture apparatus 10 of this embodiment, as described above, a method of switching between the three processes of the adsorption process, desorption process, and cooling process has been described, but the carbon dioxide capture method in the carbon dioxide capture apparatus 10 is not limited to this.

[0039] In the carbon dioxide storage device 20 to which the combustion exhaust gas is sent, the carbon dioxide contained in the combustion exhaust gas is adsorbed by physical adsorption in the adsorbents 21d and 22d of the adsorption towers 21 and 22, respectively (the adsorption step in the carbon dioxide storage device 20). The adsorption towers that adsorb the carbon dioxide contained in the combustion exhaust gas are controlled by the valves 21a, 21b, 21c, 22a, 22b, and 22c. The remainder of the combustion exhaust gas from which carbon dioxide has been removed by the adsorbents 21d and 22d is discharged into the atmosphere as adsorption off-gas via the pipe 20c.

[0040] In the carbon dioxide storage device 20, the carbon dioxide adsorbed in the adsorbents 21d, 22d is desorbed from the adsorbents 21d, 22d using the PSA method (pressure swing method) (desorption step in the carbon dioxide storage device 20). Specifically, the vacuum pump 23 is operated to reduce the pressure inside one of the adsorption towers 21, 22. By this operation, the carbon dioxide in the adsorption towers 21, 22 is temporarily stored in the storage surge tank 26. That is, in the carbon dioxide storage device 20, only carbon dioxide is stored in the storage surge tank 26.

[0041] FIG. 3 is a diagram illustrating switching of processes in the adsorption towers 21 and 22 of the carbon dioxide storage device 20. FIG. 3 shows a schematic diagram of a time series of processes for each of the two adsorption towers 21 and 22 included in the carbon dioxide storage device 20. In FIG. 3, for convenience, the adsorption tower 21 is referred to as the "first storage tower" and the adsorption tower 22 is referred to as the "second storage tower." In the carbon dioxide storage device 20 of this embodiment, as shown in FIG. 3, when the adsorption process is performed in the first storage tower (adsorption tower 21), the desorption process is performed in the second storage tower (adsorption tower 22), and when the desorption process is performed in the first storage tower, the adsorption process is performed in the second storage tower. As shown in FIG. 3, the desorption process of the carbon dioxide storage device 20 includes a time period Top1 in which a desorption operation is performed by a PSA method and a time period Top2 in which the desorption operation is waited for.

[0042] In the carbon dioxide storage device 20 of this embodiment, upper and lower limit values ​​are set for the pressure of the carbon dioxide to be filled in the storage surge tank 26. When the pressure of carbon dioxide in the storage surge tank 26 reaches the lower limit value, the vacuum pump 23 is started, and the carbon dioxide adsorbed in the adsorbents 21d, 22d of the adsorption towers 21, 22 is desorbed. When the pressure of carbon dioxide in the storage surge tank 26 reaches the upper limit value due to desorption of carbon dioxide in the adsorption towers 21, 22, the vacuum pump 23 is stopped. In this way, in the desorption step in the carbon dioxide storage device 20 of this embodiment, the desorption operation and standby are repeated by driving and stopping the vacuum pump 23.

[0043] In the carbon dioxide storage device 20 of this embodiment, the two adsorption towers 21, 22 perform their respective processes (adsorption process and desorption process) with a phase shift. This allows carbon dioxide to be continuously supplied to the storage surge tank 26. The desorption process in the carbon dioxide storage device 20 will be described in detail later.

[0044] The control unit 60 uses the carbon dioxide concentration and hydrogen concentration in the feed gas detected by the concentration detection unit 33 to adjust the ratio of carbon dioxide to hydrogen in the feed gas supplied to the methanation reactor 31 to an optimum value for the methanation reaction. Specifically, the control unit 60 controls the flow rates of the mixed gas of carbon dioxide and hydrogen supplied from the carbon dioxide recovery device 10, the carbon dioxide supplied from the carbon dioxide storage device 20, and the hydrogen supplied from the hydrogen gas tank 40, thereby adjusting the carbon dioxide concentration and hydrogen concentration in the feed gas. The methanation reactor 31 produces methane gas using the feed gas in which the carbon dioxide concentration and hydrogen concentration have been adjusted.

[0045] Methane gas produced in the methanation reactor 31 is pressurized by a compressor 51 and then stored in a methane gas tank 50. In this embodiment, the methane gas stored in the methane gas tank 50 is sent to the combustion furnace 5 via a fuel supply pipe 1d. The combustion furnace 5 generates thermal energy using the supplied methane gas. The combustion exhaust gas discharged from the combustion furnace 5 by burning methane gas in the combustion furnace 5 is sent to the carbon dioxide capture device 10 and the carbon dioxide storage device 20 via an exhaust gas pipe 1a. In this way, the fuel gas production system 1 can realize a carbon circulation cycle.

[0046] Next, a description will be given of a method for capturing carbon dioxide in the carbon dioxide storage device 20 of this embodiment. In order to improve the utilization rate of carbon dioxide contained in the combustion exhaust gas, the carbon dioxide storage device 20 switches between desorption operation and standby in the desorption step of either the adsorption towers 21, 22 using the fuel conversion reaction pressure in the methanation reactor 31 and the pressure of carbon dioxide stored in the storage surge tank 26.

[0047] FIG. 4 is a flowchart of the desorption process in an adsorption tower. The flowchart in FIG. 4 shows the desorption process in one adsorption tower, for example, the content of the portion surrounded by the two-dot chain line Pdes in FIG. 3. When executing the flowchart in FIG. 4, in the fuel gas production system 1 of this embodiment, it is desirable that one of the adsorbents 21d, 22d of the two adsorption towers 21, 22 is in a state in which carbon dioxide is sufficiently adsorbed. In this case, the flowchart shown in FIG. 4 is executed in an adsorption tower equipped with an adsorbent that has sufficiently adsorbed carbon dioxide. Here, the desorption process in the adsorption tower 22 will be described. The flowchart in FIG. 4 is started, for example, simultaneously with the start-up of the fuel gas production system 1.

[0048] In the desorption step of the adsorption tower 22, first, the adsorption tower 22 stands by for a desorption operation (step S11). In step S11, the control unit 60 puts the adsorption tower 22 on standby to desorb the carbon dioxide adsorbed in the adsorbent 22d. Specifically, the control unit 60 closes the valves 22a and 22c to stop the combustion exhaust gas from flowing into the adsorption tower 22. At this time, the valve 22b may be left open to prepare for the operation in the next step.

[0049] Next, the supply of carbon dioxide from the storage surge tank 26 to the methanation reactor 31 is started (step S12). In step S12, the control unit 60 supplies the carbon dioxide in the storage surge tank 26 to the methanation reactor 31 via the raw material gas pipe 1b. In this embodiment, the control unit 60 uses the carbon dioxide concentration and hydrogen concentration detected by the concentration detection unit 33 to supply carbon dioxide from the storage surge tank 26 to the methanation reactor 31 so that the ratio of carbon dioxide to hydrogen supplied to the methanation reactor 31 becomes an optimum value for the methanation reaction.

[0050] Next, the supply differential pressure ΔPs is calculated (step S13). In step S13, the control unit 60 calculates the supply differential pressure ΔPs by subtracting the fuel conversion reaction pressure Pf in the methanation reactor 31 from the carbon dioxide pressure P_st in the storage surge tank 26. The pressure of carbon dioxide in the storage surge tank 26 is detected by the storage pressure detection unit 27 provided in the storage surge tank 26. The fuel conversion reaction pressure Pf is detected by the reaction pressure detection unit 32.

[0051] Next, it is determined whether the supply differential pressure ΔPs is smaller than a differential pressure threshold ΔPth (step S14). In step S14, the control unit 60 determines whether the supply differential pressure ΔPs calculated in step S13 is smaller than a preset differential pressure threshold ΔPth. If the control unit 60 determines that the supply differential pressure ΔPs is smaller than the differential pressure threshold ΔPth (step S14: YES), the process proceeds to step S15. If the control unit 60 determines that the supply differential pressure ΔPs is equal to or greater than the differential pressure threshold ΔPth (step S14: NO), the process returns to step S13, and the supply differential pressure ΔPs is calculated again. In this embodiment, the differential pressure threshold ΔPth is a value that changes depending on the operating state of the fuel gas production system 1, as will be described later, and a differential pressure threshold ΔPth1 is preset as an initial value when performing the desorption process of the adsorption tower.

[0052] If it is determined in step S14 that the supply differential pressure ΔPs is smaller than the differential pressure threshold ΔPth, the desorption operation is started (step S15). In step S15, the control unit 60 starts the vacuum pump 23 to reduce the pressure inside the adsorption tower 22. As a result, the carbon dioxide adsorbed to the adsorbent 22d in the adsorption tower 22 is desorbed from the adsorbent 22d and supplied to the storage surge tank 26 through the pipe 20b.

[0053] Next, it is determined whether the flow rate Q_des of carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26 is greater than a flow rate threshold Qth (step S16). In step S16, the control unit 60 determines whether the flow rate Q_des of carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26 is greater than a preset flow rate threshold Qth of carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26. If the control unit 60 determines that the flow rate Q_des of carbon dioxide is greater than the flow rate threshold Qth (step S16: YES), the process proceeds to step S171. If the control unit 60 determines that the flow rate Q_des of carbon dioxide is equal to or less than the flow rate threshold Qth (step S16: NO), the process proceeds to step S172.

[0054] If it is determined in step S16 that the flow rate Q_des of carbon dioxide supplied to the storage surge tank 26 is greater than the flow rate threshold Qth, the differential pressure threshold ΔPth is maintained (step S171). In step S171, the control unit 60 uses the differential pressure threshold ΔPth (=ΔPth1) used in the comparison with the supply differential pressure ΔPs in the immediately preceding step S14 without changing it, for the comparison with the supply differential pressure ΔPs in the next step S14.

[0055] On the other hand, if it is determined in step S16 that the flow rate Q_des of carbon dioxide supplied to the storage surge tank 26 is equal to or less than the flow rate threshold Qth, the differential pressure threshold ΔPth is changed (step S172). In step S172, the control unit 60 changes the differential pressure threshold ΔPth used in the comparison with the supply differential pressure ΔPs in the immediately preceding step S14 to a differential pressure threshold ΔPth2 that is greater than the differential pressure threshold ΔPth1 for comparison with the supply differential pressure ΔPs in the next step S14.

[0056] After steps S171 and S172, the control unit 60 determines whether the supply differential pressure ΔPs is greater than the stop threshold ΔPth0 (step S18). In step S18, the control unit 60 makes the determination using a preset stop threshold ΔPth0. Here, the stop threshold ΔPth0 is a pressure value below which, if it drops any further, the supply of carbon dioxide to the methanation reactor 31 may be hindered. If the control unit 60 determines that the supply differential pressure ΔPs is greater than the stop threshold ΔPth0 (step S18: YES), the process proceeds to step S19. If the control unit 60 determines that the supply differential pressure ΔPs is equal to or less than the stop threshold ΔPth0 (step S18: NO), the process proceeds to step S202.

[0057] If it is determined in step S18 that the supply differential pressure ΔPs is greater than the stop threshold ΔPth0, the control unit 60 determines whether the carbon dioxide pressure P_st in the storage surge tank 26 is greater than or equal to the upper limit Pmax (step S19). In step S19, the control unit 60 determines whether the carbon dioxide pressure P_st in the storage surge tank 26 detected by the storage pressure detection unit 27 is greater than or equal to the upper limit Pmax set for the storage surge tank 26. If the control unit 60 determines that the carbon dioxide pressure P_st in the storage surge tank 26 is greater than or equal to the upper limit Pmax (step S19: YES), the process proceeds to step S201. If the control unit 60 determines that the carbon dioxide pressure P_st in the storage surge tank 26 is less than the upper limit Pmax (step S19: NO), the process returns to step S16, where it determines whether the flow rate Q_des of the carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26 is greater than the flow rate threshold Qth (step S16).

[0058] If it is determined in step S19 that the carbon dioxide pressure P_st in the storage surge tank 26 is equal to or greater than the upper limit Pmax, the desorption operation is terminated (step S201). In step S201, the control unit 60 stops driving the vacuum pump 23 and temporarily terminates the desorption operation in the adsorption tower 22. Thereafter, the adsorption tower 22 waits for the desorption operation (step S11).

[0059] When it is determined in step S18 that the supply differential pressure ΔPs is equal to or less than the stop threshold ΔPth0, the control unit 60 terminates the desorption process in the adsorption tower 22 and switches the process in the carbon dioxide storage device 20 (step S202). Specifically, the control unit 60 closes valves 21a and 21c and opens valve 21b to terminate the adsorption process in the adsorption tower 21 and transition to the desorption process. Furthermore, the control unit 60 opens valves 22a and 22c and closes valve 22b to terminate the desorption process in the adsorption tower 22 and transition to the adsorption process.

[0060] After step S201, the adsorption tower 22 repeats steps S11 to S19, repeating the desorption operation and standby. When the amount of carbon dioxide adsorbed by the adsorbent 22d of the adsorption tower 22 decreases due to the repeated desorption operation and standby, the control unit 60 determines in step S18 that the supply differential pressure ΔPs is equal to or less than the stop threshold ΔPth0. As a result, the desorption process in the adsorption tower 22 ends and the adsorption process starts, while the adsorption process in the adsorption tower 21 ends and the desorption process starts. In this way, in the carbon dioxide capture method for the carbon dioxide storage device 20 of the present embodiment, the desorption process proceeds in one of the two adsorption towers 21, 22 and the adsorption process proceeds in the other, thereby enabling carbon dioxide to be continuously supplied to the storage surge tank 26.

[0061] Fig. 5 is a diagram showing the changes over time in the flow rate and pressure of carbon dioxide during the desorption process in the adsorption tower. From top to bottom, Fig. 5 shows the changes over time in the flow rate Q_des of carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26, the carbon dioxide pressure P_st in the storage surge tank 26, and the carbon dioxide flow rate Q_st supplied from the storage surge tank 26 to the methanation reactor 31 during the desorption process in the adsorption tower 22. The changes over time in the carbon dioxide pressure P_st in the storage surge tank 26 are indicated by the fuel conversion reaction pressure Pf, which is the basis for calculating the supply differential pressure ΔPs. Note that while the changes in these values ​​during the desorption process in the adsorption tower 22 will be described here, the same applies to the desorption process in the adsorption tower 21.

[0062] At time t0, which is the origin of the horizontal axis of the diagram shown in Fig. 5, for example, the maximum amount of carbon dioxide is adsorbed in the adsorbent 22d of the adsorption tower 22, and the carbon dioxide pressure P_st in the storage surge tank 26 is equal to the upper limit Pmax, which is the maximum pressure of carbon dioxide that can be stored in the storage surge tank 26. Assume that the supply of carbon dioxide in the storage surge tank 26 to the methanation reactor 31 starts at time t0 (first step S12). As shown in Fig. 5, after time t0, carbon dioxide is supplied from the storage surge tank 26 to the methanation reactor 31 at a flow rate Q0. As a result, the carbon dioxide pressure P_st in the storage surge tank 26 gradually decreases over time. In this embodiment, the flow rate Q0 of carbon dioxide supplied from the storage surge tank 26 to the methanation reactor 31 is constant.

[0063] 5, at time t1, when the pressure P_st of carbon dioxide in the storage surge tank 26 reaches pressure P_st1, the supply differential pressure ΔPs reaches the differential pressure threshold ΔPth1. Therefore, after time t1, it is determined that the supply differential pressure ΔPs is smaller than the differential pressure threshold ΔPth1 (first step S14). As a result, the desorption operation is started after time t1, and the flow rate Q_des of carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26 becomes greater than 0 (first step S15). In response to this, the pressure P_st of carbon dioxide in the storage surge tank 26 increases, and the supply differential pressure ΔPs becomes greater than the differential pressure threshold ΔPth1. In the desorption operation started after time t1, the flow rate Q_des of carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26 is greater than the preset flow rate threshold Qth, and therefore the differential pressure threshold ΔPth1 is not changed (first step S16 and step S171). Furthermore, in the desorption operation started after time t1, the supply differential pressure ΔPs does not become equal to or lower than the stop threshold value ΔPth0 (first step S18). At time t2 after time t1, when the flow rate Q_st of carbon dioxide supplied from the storage surge tank 26 to the methanation reactor 31 becomes 0, the increase rate per unit time of the carbon dioxide pressure P_st in the storage surge tank 26 becomes large. At time t3, when the upper limit Pmax of the storage surge tank 26 is reached, the desorption operation in the adsorption tower 22 is temporarily terminated, and the desorption of carbon dioxide is stopped (first step S19 and step S201).

[0064] At time t4, the supply of carbon dioxide in the storage surge tank 26 to the methanation reactor 31 is restarted (second step S12). After time t4, carbon dioxide is supplied from the storage surge tank 26 to the methanation reactor 31 at a flow rate Q_st, and the carbon dioxide pressure P_st in the storage surge tank 26 gradually decreases over time. At time t5, when the carbon dioxide pressure P_st in the storage surge tank 26 reaches pressure P_st1, the supply differential pressure ΔPs reaches the differential pressure threshold ΔPth1 (second step S14). Therefore, after time t5, carbon dioxide desorbed from the adsorbent 22d in the adsorption tower 22 is supplied to the storage surge tank 26 (second step S15).

[0065] In the adsorption tower 22, the amount of carbon dioxide desorbed from the adsorbent 22d after time t5 is smaller than the amount of carbon dioxide desorbed from the adsorbent 22d between time t1 and time t3. This is because the amount of carbon dioxide adsorbed on the adsorbent 22d is small when desorption starts. In desorption of carbon dioxide after time t5, as shown in FIG. 5 , when the flow rate Q_des of carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26 becomes equal to or less than the flow rate threshold Qth, the control unit 60 changes the next differential pressure threshold ΔPth to a threshold Pth2 that is greater than the threshold Pth1 (second step S16 and step S172). Thereafter, when the flow rate Q_st of carbon dioxide supplied from the storage surge tank 26 to the methanation reactor 31 becomes 0 at time t6, the carbon dioxide pressure P_st in the storage surge tank 26 becomes the upper limit Pmax of the storage surge tank 26 at time t7 due to the carbon dioxide supplied from the adsorption tower 22. As a result, the desorption operation in the adsorption tower 22 is stopped, and the adsorption tower 22 waits for the desorption operation.

[0066] At time t8, the supply of carbon dioxide in the storage surge tank 26 to the methanation reactor 31 is started again (third step S12). After time t8, carbon dioxide is supplied from the storage surge tank 26 to the methanation reactor 31 at a flow rate Q_st, and the carbon dioxide pressure P_st in the storage surge tank 26 gradually decreases over time. At time t9, when the carbon dioxide pressure P_st in the storage surge tank 26 reaches pressure P_st2, the supply differential pressure ΔPs reaches the changed differential pressure threshold ΔPth2, and therefore, after time t9, the carbon dioxide desorbed from the adsorbent 22d in the adsorption tower 22 is supplied to the storage surge tank 26 (third steps S14 and S15). Thereafter, at time t10, when the flow rate Q_st of carbon dioxide supplied from the storage surge tank 26 to the methanation reactor 31 becomes 0, the carbon dioxide pressure P_st in the storage surge tank 26 becomes the upper limit Pmax of the storage surge tank 26 at time t11 due to the carbon dioxide supplied from the adsorption tower 22. This causes the desorption operation in the adsorption tower 22 to stop, and the adsorption tower 22 waits for the desorption operation. Comparing the times required for the desorption operation in the adsorption tower 22, it can be seen that the time tqds3 required for the desorption operation in the adsorption tower 22 from time t9 to time t11 is longer than the time tqds1 required for the desorption operation in the adsorption tower 22 from time t1 to time t3 and the time tqds2 required for the desorption operation in the adsorption tower 22 from time t5 to time t7.

[0067] At time t12, the supply of carbon dioxide in the storage surge tank 26 to the methanation reactor 31 is restarted (fourth step S12). After time t12, carbon dioxide is supplied from the storage surge tank 26 to the methanation reactor 31 at a flow rate Q_st, and the carbon dioxide pressure P_st in the storage surge tank 26 gradually decreases over time. At time t13, when the carbon dioxide pressure P_st in the storage surge tank 26 reaches pressure P_st2, the supply differential pressure ΔPs reaches the differential pressure threshold ΔPth2. Therefore, after time t13, the carbon dioxide desorbed from the adsorbent 22d in the adsorption tower 22 is supplied to the storage surge tank 26 (fourth steps S14 and S15). However, the amount of carbon dioxide adsorbed in the adsorbent 22d in the adsorption tower 22 has decreased due to repeated desorption operations. For this reason, if the flow rate Q_des of carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26 is relatively small and the carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26 continues to be supplied to the methanation reactor 31, there is a risk that the carbon dioxide necessary for the methanation reaction in the methanation reactor 31 may not be supplied. Therefore, at time t14 when the supply differential pressure ΔPs reaches the stop threshold ΔPth0, if it is determined that the supply differential pressure ΔPs is equal to or less than the stop threshold ΔPth0, the desorption process in the adsorption tower 22 is terminated, and the carbon dioxide storage device 20 switches between the processes of the adsorption tower 21 and the adsorption tower 22 (fourth step S18 and step S202).

[0068] Next, the effects of the fuel gas production system 1 of this embodiment will be described. The fuel gas production system 1 of this embodiment includes a carbon dioxide storage device 20 that captures and stores carbon dioxide, in addition to a carbon dioxide capture device 10 that supplies carbon dioxide to the methanation reactor 31. The carbon dioxide storage device 20 includes adsorption towers 21 and 22 that house adsorbents 21d and 22d that adsorb carbon dioxide, and a storage surge tank 26 that stores carbon dioxide desorbed from the adsorbents 21d and 22d.

[0069] Fig. 6 is a first graph showing the change over time in the flow rate of carbon dioxide desorbed from the adsorbent. Fig. 7 is a second graph showing the change over time in the flow rate of carbon dioxide desorbed from the adsorbent. The graphs shown in Fig. 6 and Fig. 7 show the change over time in the amount of carbon dioxide desorbed when carbon dioxide adsorbed on the adsorbent is desorbed from the adsorbent in a general adsorber containing an adsorbent having carbon dioxide adsorption capacity.

[0070] FIG. 6 shows the time course of carbon dioxide continuously desorbed from the adsorbent after the desorption operation was started (time ta1). As shown in FIG. 6, the flow rate of desorbed carbon dioxide reaches its maximum immediately after the desorption operation was started (time ta1), and then decreases over time. The horizontal axis of FIG. 6 shows the amount of desorbed carbon dioxide, for example, the lower limit flow rate Qmin, which is the lower limit of the supply amount required for the methanation reaction to proceed in the methanation reactor. If the flow rate of carbon dioxide, which decreases over time, falls below the lower limit flow rate Qmin at time ta2, carbon dioxide desorbed after time ta2 cannot be supplied to the methanation reactor. Therefore, the carbon dioxide in the hatched area R1 in FIG. 6 cannot be used for the methanation reaction.

[0071] Fig. 7 shows the time change when carbon dioxide is desorbed from the adsorbent by repeating the desorption operation and waiting after the desorption operation is started (time tb1). Even with the method shown in Fig. 7, if the flow rate of the desorbed carbon dioxide falls below the lower limit flow rate Qmin, the carbon dioxide desorbed thereafter (the areas corresponding to the hatched regions R21, R22, and R23 in Fig. 7) cannot be used for the methanation reaction.

[0072] In the fuel gas production system 1 of this embodiment, the carbon dioxide storage device 20 includes a storage surge tank 26 that stores carbon dioxide desorbed from the adsorbents 21d and 22d in the adsorption towers 21 and 22. As a result, as shown in Figures 6 and 7, even if the flow rate of carbon dioxide is too low to be utilized in the methanation reaction, the amount of recovered carbon dioxide can be increased by extending the desorption operation time, and therefore the carbon dioxide can be utilized in the methanation reaction (see the times tqds1, tqds2, and tqds3 required for the desorption operation in Figure 5). This allows for an improvement in the utilization rate of carbon dioxide in combustion exhaust gas, etc.

[0073] Furthermore, as shown in Figures 6 and 7, when the desorption operation in the adsorber is controlled using a constant lower limit flow rate Qmin, a relatively large amount of carbon dioxide remains in the adsorbent. If the next adsorption step is performed in this state, the adsorption off-gas will contain a relatively large amount of carbon dioxide, which means that the carbon dioxide contained in the combustion exhaust gas, etc., is not effectively utilized. On the other hand, in the fuel gas production system 1 of this embodiment, the carbon dioxide that is desorbed after the flow rate falls below the lower limit flow rate Qmin is also stored in the storage surge tank 26. If the next adsorption step is performed in this state, the amount of carbon dioxide contained in the adsorption off-gas will be relatively small. This allows the carbon dioxide contained in the combustion exhaust gas, etc., to be effectively utilized.

[0074] Furthermore, if the adsorption process is started while the adsorbent still has carbon dioxide adsorbed thereon, the adsorbent with carbon dioxide adsorbed thereon cannot contribute to the adsorption of carbon dioxide, resulting in a decrease in the utilization rate of the adsorbent in the recovery of carbon dioxide from the combustion exhaust gas. On the other hand, by desorbing carbon dioxide even after the flow rate falls below the lower limit flow rate Qmin, the proportion of the adsorbent with carbon dioxide adsorbed thereon becomes relatively small, resulting in a relatively high utilization rate of the adsorbent in the adsorption process. This improves the carbon dioxide recovery rate, allowing the size of the carbon dioxide storage device 20 to be reduced.

[0075] Furthermore, when a fuel gas such as methane gas is produced using a fuel gas production system, it is expected that the fuel reaction pressure in the methanation reactor will fluctuate. In the fuel gas production system 1 of this embodiment, in step S18, the end time of the desorption process in the adsorption towers 21, 22 is determined based on the magnitude relationship between the supply differential pressure ΔPs, which is the value obtained by subtracting the fuel reaction pressure Pf in the methanation reactor 31 from the carbon dioxide pressure P_st in the storage surge tank 26, and the stop threshold value ΔPth0. Therefore, for example, if the fuel reaction pressure Pf decreases unintentionally, the carbon dioxide in the storage surge tank 26 can be used to a lower pressure in accordance with the decrease in the fuel reaction pressure Pf.

[0076] According to the fuel gas production system 1 of the present embodiment described above, the carbon dioxide storage device 20 that supplies carbon dioxide to the methanation reactor 31 includes a storage surge tank 26 that stores carbon dioxide captured by the adsorption towers 21, 22 that capture carbon dioxide contained in the combustion exhaust gas, separate from the carbon dioxide capture device 10. This allows the amount of carbon dioxide that the carbon dioxide storage device 20 can supply to the fuel production device 30 per unit time to be leveled out. Therefore, even if the amount of carbon dioxide that the adsorption towers 21, 22 supply to the storage surge tank 26 is relatively small, for example, the carbon dioxide captured in the carbon dioxide storage device 20 can be used to generate fuel gas in the methanation reactor 31. Furthermore, the carbon dioxide storage device 20 supplies the carbon dioxide captured in the adsorption towers 21, 22 to the storage surge tank 26 when the supply differential pressure ΔPs, which is the value obtained by subtracting the fuel production reaction pressure in the methanation reactor 31 from the pressure of the carbon dioxide stored in the storage surge tank 26, becomes smaller than a predetermined differential pressure threshold ΔPth. This makes it possible to prevent the pressure of the carbon dioxide stored in the storage surge tank 26 from decreasing to a level at which the carbon dioxide cannot be supplied to the methanation reactor 31. In this way, methane gas can be produced using the small amount of carbon dioxide remaining in the adsorption towers 21, 22, thereby improving the utilization rate of carbon dioxide contained in the combustion exhaust gas.

[0077] Furthermore, according to the fuel gas production system 1 of this embodiment, when the amount of carbon dioxide supplied from the adsorption towers 21, 22 to the storage surge tank 26 is relatively small, the control unit 60 increases the differential pressure threshold ΔPth to advance the timing at which the carbon dioxide recovered in the adsorption towers 21, 22 is supplied to the storage surge tank 26. This lengthens the time during which the carbon dioxide recovered in the adsorption towers 21, 22 is supplied to the storage surge tank 26. Therefore, even when the amount of carbon dioxide supplied from the adsorption towers 21, 22 to the storage surge tank 26 is relatively small, the pressure of the carbon dioxide stored in the storage surge tank 26 can be increased. Therefore, even the small amount of carbon dioxide remaining in the adsorption towers 21, 22 can be used to generate methane gas, further improving the carbon dioxide utilization rate.

[0078] Furthermore, according to the fuel gas production system 1 of this embodiment, the carbon dioxide storage device 20 has a plurality of adsorption towers 21, 22, each containing an adsorbent 21d, 22d capable of adsorbing carbon dioxide. The control unit 60 controls, for example, the adsorption tower 21 of the plurality of adsorption towers 21, 22, to adsorb carbon dioxide, while desorbing the adsorbed carbon dioxide in the adsorption tower 22. This allows the carbon dioxide storage device 20 to continuously supply carbon dioxide to the storage surge tank 26. This makes it possible to further equalize the amount of carbon dioxide that the carbon dioxide storage device 20 can supply to the fuel production device 30 per unit time.

[0079] Furthermore, according to the fuel gas production system 1 of this embodiment, when the supply differential pressure ΔPs becomes equal to or less than a preset stop threshold ΔPth0, the control unit 60 switches between the desorption process and the adsorption process, for example, so that the adsorption tower 21 that adsorbed carbon dioxide contained in the combustion exhaust gas desorbs carbon dioxide, and so that the adsorption tower 22 that desorbed the adsorbed carbon dioxide adsorbs the carbon dioxide contained in the gas. As a result, for example, if the pressure of the carbon dioxide in the storage surge tank 26 does not reach a pressure that allows it to be supplied to the methanation reactor 31 even after the carbon dioxide adsorbed in the adsorbents 21d and 22d is desorbed, the respective processes of the adsorption towers 21 and 22 can be switched to prevent a situation in which carbon dioxide at a sufficient pressure cannot be supplied from the storage surge tank 26 to the methanation reactor 31. Therefore, methane gas can be continuously produced.

[0080] Furthermore, according to the fuel gas production method of this embodiment, in addition to the adsorption step in the carbon dioxide recovery device 10 that recovers carbon dioxide from the combustion exhaust gas, a desorption step in the carbon dioxide storage device 20 is provided in which the carbon dioxide contained in the gas is recovered by the adsorption towers 21, 22 and the carbon dioxide recovered by the adsorption towers 21, 22 is stored in the storage surge tank 26. This allows the amount of carbon dioxide that the storage surge tank 26 can supply to the methanation reactor 31 per unit time to be leveled out, so that even if the amount of carbon dioxide recovered in the adsorption towers 21, 22 is relatively small, it can be used to generate methane gas in the methanation reactor 31. Furthermore, in the desorption operation of the desorption step in the carbon dioxide storage device 20, when the supply differential pressure ΔPs becomes smaller than a predetermined differential pressure threshold ΔPth, the carbon dioxide recovered in the adsorption towers 21, 22 is supplied to the storage surge tank 26. This prevents the pressure of the carbon dioxide stored in the storage surge tank 26 from decreasing to a pressure that makes it impossible to supply the carbon dioxide to the methanation reactor 31, thereby improving the utilization rate of the carbon dioxide contained in the combustion exhaust gas.

[0081] Furthermore, according to a computer program for causing a computer to execute the production of fuel gas using the fuel gas production system 1 of this embodiment, the computer is caused to execute a storage function of recovering carbon dioxide contained in the gas using the adsorption towers 21 and 22 and storing the carbon dioxide recovered by the adsorption towers 21 and 22 in the storage surge tank 26, in addition to a recovery function of recovering carbon dioxide from the combustion exhaust gas. This allows the storage surge tank 26 to level out the amount of carbon dioxide that can be supplied to the methanation reactor 31 per unit time. Therefore, even if the amount of carbon dioxide recovered by the adsorption towers 21 and 22 is relatively small, it can be used to generate methane gas in the methanation reactor 31. Furthermore, as a control function, when the supply differential pressure ΔPs becomes smaller than a predetermined differential pressure threshold ΔPth, the carbon dioxide recovered in the adsorption towers 21 and 22 is supplied to the storage surge tank 26. This prevents the pressure of the carbon dioxide stored in the storage surge tank 26 from decreasing to a pressure that makes it impossible to supply the carbon dioxide to the methanation reactor 31, thereby improving the utilization rate of the carbon dioxide contained in the combustion exhaust gas.

[0082] Second Embodiment 8 is a schematic diagram showing the general configuration of a fuel gas production system according to a second embodiment. The fuel gas production system according to the second embodiment differs from the fuel gas production system according to the first embodiment (FIG. 1) in that the flow rate of the raw material gas supplied to the methanation reactor can be controlled.

[0083] The fuel gas production system 2 of this embodiment includes a carbon dioxide capture device 10, a carbon dioxide storage device 20, a fuel production device 70, a hydrogen gas tank 40, a methane gas tank 50, and a control unit 60. The fuel gas production system 2 captures carbon dioxide contained in the combustion exhaust gas generated in the combustion furnace 5, and uses the captured carbon dioxide and hydrogen to produce methane gas as "fuel gas."

[0084] The fuel production device 70 includes a methanation reactor 31, a reaction pressure detection unit 32, a concentration detection unit 33, and a flow rate control unit 74. The flow rate control unit 74 is connected to the raw material gas pipe 1b between the connection point P1 of the raw material gas pipe 1b and the concentration detection unit 33. The flow rate control unit 74 controls the flow rate of the raw material gas supplied to the methanation reactor 31 in response to a command from a control unit 60 to which it is electrically connected. The flow rate control unit 74 corresponds to the "fuel production pressure control unit" in the claims.

[0085] 9 is a flowchart of the desorption step of the adsorption tower in this embodiment. Next, a carbon dioxide capture method in the carbon dioxide storage device 20 of this embodiment will be described. In this embodiment, compared to the carbon dioxide capture method in the carbon dioxide storage device 20 of the first embodiment, the content of the step selected based on the determination in step S16 of the magnitude relationship between the flow rate Q_des of carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26 and the flow rate threshold Qth is different. Note that in the carbon dioxide capture method in the carbon dioxide storage device 20 of this embodiment, an initial value Pf0 is set in advance as the fuel conversion reaction pressure Pf in the methanation reactor 31.

[0086] If it is determined in step S16 that the flow rate Q_des of the carbon dioxide supplied to the storage surge tank 26 is greater than the flow rate threshold Qth, the fuel conversion reaction pressure Pf is maintained (step S271). In step S271, the control unit 60 controls the flow rate control unit 74 to control the flow rate of the raw material gas supplied to the methanation reactor 31 so that the value of the fuel conversion reaction pressure Pf remains at the initial value Pf0.

[0087] On the other hand, if it is determined in step S16 that the flow rate Q_des of the carbon dioxide supplied to the storage surge tank 26 is equal to or less than the flow rate threshold Qth, the control unit 60 changes the fuel conversion reaction pressure Pf (step S271). In step S272, the control unit 60 controls the flow rate control unit 74 to control the flow rate of the raw material gas supplied to the methanation reactor 31 so that the value of the fuel conversion reaction pressure Pf becomes a value smaller than the initial value Pf0.

[0088] In the carbon dioxide capture method in the carbon dioxide storage device 20 of this embodiment, when the fuel conversion reaction pressure Pf becomes a relatively small value, the supply differential pressure ΔPs calculated in step S13 becomes relatively large. As a result, in step S18, when compared with the stop threshold value ΔPth0, the time during which the supply differential pressure ΔPs is greater than the stop threshold value ΔPth0 becomes longer, so that the desorption operation in the adsorption tower 22 continues even if the flow rate Q_des of carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26 becomes relatively small. Therefore, in the carbon dioxide capture method in the carbon dioxide storage device 20 of this embodiment, the desorption operation can be performed for a longer time than in the carbon dioxide capture method in the carbon dioxide storage device 20 of the first embodiment.

[0089] According to the fuel gas production system 2 of this embodiment described above, the carbon dioxide storage device 20 has the storage surge tank 26 that stores the carbon dioxide recovered by the adsorption towers 21, 22. This allows the amount of carbon dioxide that the carbon dioxide storage device 20 can supply to the fuel production device 30 per unit time to be leveled out, so that even a small amount of carbon dioxide recovered by the carbon dioxide storage device 20 can be used to generate fuel gas in the methanation reactor 31. Furthermore, in the carbon dioxide storage device 20, when the supply differential pressure ΔPs becomes smaller than a preset differential pressure threshold ΔPth, the carbon dioxide recovered in the adsorption towers 21, 22 is supplied to the storage surge tank 26, so that it is possible to prevent the pressure of the carbon dioxide stored in the storage surge tank 26 from decreasing to a pressure at which the carbon dioxide cannot be supplied to the methanation reactor 31.

[0090] Furthermore, according to the fuel gas production system 2 of this embodiment, when the flow rate of carbon dioxide supplied from the adsorption towers 21, 22 to the storage surge tank 26 is relatively low, the control unit 60 controls the flow rate control unit 74 to reduce the value of the fuel conversion reaction pressure Pf and decrease the amount of carbon dioxide that needs to be supplied from the storage surge tank 26 to the methanation reactor 31. As a result, even when the amount of carbon dioxide supplied from the adsorption towers 21, 22 to the storage surge tank 26 is relatively small, the carbon dioxide recovered in the carbon dioxide storage device 20 can be used to generate methane gas in the methanation reactor 31. Therefore, the utilization rate of carbon dioxide can be further improved.

[0091] <Modification of this embodiment> The present invention is not limited to the above-described embodiment, and can be embodied in various forms without departing from the spirit of the invention. For example, the following modifications are also possible.

[0092] [Variation 1] In the above-described embodiment, the carbon dioxide capture device 10 captures carbon dioxide using three adsorbers containing adsorbents with carbon dioxide adsorption properties, and the carbon dioxide storage device 20 captures carbon dioxide using two adsorption towers 21 and 22, each containing adsorbents 21d and 22d with carbon dioxide adsorption properties. The carbon dioxide capture device 10 and the carbon dioxide storage device 20 may capture carbon dioxide from combustion exhaust gas containing carbon dioxide. The methods for capturing carbon dioxide from combustion exhaust gas containing carbon dioxide in the carbon dioxide capture device 10 and the carbon dioxide storage device 20 are not limited to these. When using an adsorbent with carbon dioxide adsorption properties, it is necessary to alternately perform at least an adsorption process in which carbon dioxide is adsorbed onto the adsorbent and a desorption process in which the carbon dioxide adsorbed on the adsorbent is desorbed. However, the fuel gas production method of this embodiment can temporarily store the carbon dioxide desorbed from the adsorbent in the surge tank 12 and the storage surge tank 26, respectively. This allows the amount of carbon dioxide supplied to the methanation reactor 31 to be leveled and prevents a temporary shortage of carbon dioxide supplied to the methanation reactor 31.

[0093] [Variation 2] In the above embodiment, the fuel conversion reaction pressure in the methanation reactor 31 is detected by the reaction pressure detection unit 32 provided in the methanation reactor 31. The method for detecting the fuel conversion reaction pressure is not limited to this. For example, the fuel conversion reaction pressure may be calculated using the carbon dioxide concentration and hydrogen concentration detected by the concentration detection unit 33, the pressure of the raw material gas flowing through the raw material gas pipe 1b, the amount of methane gas sent from the methanation reactor 31 to the methane gas tank 50, etc.

[0094] [Variation 3] In the first embodiment, the differential pressure threshold ΔPth is changed based on the magnitude relationship between the flow rate Q_des of carbon dioxide supplied from the adsorption towers 22 to the storage surge tank 26 and the flow rate threshold Qth. Although it is not necessary to change the differential pressure threshold ΔPth, the desorption operation time can be extended by changing the differential pressure threshold ΔPth based on the magnitude relationship between the carbon dioxide flow rate Q_des and the flow rate threshold Qth. This makes it possible to store carbon dioxide in the storage surge tank 26 at a pressure that allows it to be supplied to the methanation reactor 31, even if the amount of carbon dioxide desorbed in the adsorption towers 21, 22 is relatively small.

[0095] [Variation 4] In the second embodiment, the fuel conversion reaction pressure Pf is changed based on the magnitude relationship between the flow rate Q_des of carbon dioxide supplied from the adsorption tower 22 to the storage surge tank 26 and the flow rate threshold Qth. Although it is not necessary to change the fuel conversion reaction pressure Pf, by changing the fuel conversion reaction pressure Pf based on the magnitude relationship between the carbon dioxide flow rate Q_des and the flow rate threshold Qth, the desorption operation time can be extended. As a result, even if the amount of carbon dioxide desorbed in the adsorption towers 21, 22 is relatively small, carbon dioxide can be stored in the storage surge tank 26 at a pressure that allows it to be supplied to the methanation reactor 31.

[0096] [Variation 5] In the second embodiment, the fuel gas production system 2 changes the fuel conversion reaction pressure Pf in the methanation reactor 31 by controlling the flow rate of the raw material gas supplied to the methanation reactor. The method for changing the fuel conversion reaction pressure is not limited to this.

[0097] Fig. 10 is a schematic diagram showing the overall configuration of a modified example of the fuel gas production system of the second embodiment. The fuel production device 70 included in the fuel gas production system 2 shown in Fig. 10 includes a methanation reactor 31, a reaction pressure detection unit 32, a concentration detection unit 33, and a pressure control unit 75. The pressure control unit 75 is provided in a methane gas pipe 1c connecting the methanation reactor 31 and a methane gas tank 50. The pressure control unit 75 is, for example, a valve that adjusts the aperture of a flow path in the methane gas pipe 1c, and controls the pressure of methane gas sent from the methanation reactor 31 to the methane gas tank 50. Specifically, the fuel production reaction pressure Pf in the methanation reactor 31 can be reduced by increasing the aperture of the valve in the pressure control unit 75. 10, the fuel gas production system 2 can control the pressure control unit 75 to change the pressure in the methanation reactor 31 in accordance with the determination result of the magnitude relationship between the carbon dioxide flow rate Q_des and the flow rate threshold Qth in step S16 in the flowchart of the adsorption tower desorption step shown in Fig. 9. The pressure control unit 75 corresponds to the "fuel production pressure control unit" in the claims.

[0098] This aspect has been described above based on embodiments and modifications. However, the above-described embodiments are intended to facilitate understanding of this aspect and are not intended to limit this aspect. This aspect may be modified or improved without departing from the spirit and scope of the claims, and equivalents thereof are included in this aspect. Furthermore, if a technical feature is not described as essential in this specification, it may be deleted as appropriate.

[0099] (Application example 1) A fuel gas production system, comprising: a recovery device that recovers carbon dioxide from a gas containing carbon dioxide supplied from a carbon dioxide supply source; a storage device including a recovery unit that recovers carbon dioxide contained in the gas separately from the recovery device, and a storage unit that stores the carbon dioxide recovered by the recovery unit; a fuel production apparatus having a reactor connected to each of the recovery apparatus and the storage apparatus, the fuel production apparatus supplying at least one of the carbon dioxide recovered by the recovery apparatus and the carbon dioxide stored in the storage apparatus, and hydrogen supplied by a hydrogen supply source, to the reactor to produce a fuel gas; a first pressure detection unit that detects a fuel reaction pressure in the reactor; a second pressure detection unit that detects the pressure of the carbon dioxide stored in the storage unit; and a control unit that controls the storage device so that the carbon dioxide recovered in the recovery unit is supplied to the storage unit when a differential pressure, which is a value obtained by subtracting the pressure detected by the first pressure detection unit from the pressure detected by the second pressure detection unit, becomes smaller than a preset differential pressure threshold. Fuel gas production system. (Application example 2) The fuel gas production system according to Application Example 1 further comprises: a flow rate detection unit that detects the flow rate of carbon dioxide supplied from the recovery unit to the storage unit, the control unit increases the differential pressure threshold when the flow rate of carbon dioxide detected by the flow rate detection unit is equal to or less than a preset flow rate threshold. Fuel gas production system. (Application example 3) The fuel gas production system according to Application Example 1 or Application Example 2 further comprises: a fuelization pressure control unit for controlling the fuelization reaction pressure in the reactor; a flow rate detection unit that detects the flow rate of carbon dioxide supplied from the recovery unit to the storage unit, the control unit controls the fuel pressure control unit to reduce the pressure of the carbon dioxide supplied to the reactor when the flow rate of the carbon dioxide detected by the flow rate detection unit is equal to or lower than a preset flow rate threshold. Fuel gas production system. (Application example 4) The fuel gas production system according to any one of Application Examples 1 to 3, the recovery unit has a plurality of adsorption towers that accommodate adsorbents capable of adsorbing carbon dioxide, the control unit controls the storage device so as to desorb carbon dioxide adsorbed by a first adsorbent housed in a first adsorption tower among the plurality of adsorption towers, while causing a second adsorbent housed in a second adsorption tower among the plurality of adsorption towers to adsorb carbon dioxide contained in the gas. Fuel gas production system. (Application example 5) The fuel gas production system according to any one of Application Examples 1 to 4, the control unit controls the storage device so that, when the differential pressure becomes equal to or less than a preset lower limit threshold, the first adsorbent adsorbs carbon dioxide contained in the gas, while the second adsorbent desorbs carbon dioxide. Fuel gas production system. (Application example 6) A fuel gas production method using a fuel gas production system, comprising: a recovery step of recovering carbon dioxide from a gas containing carbon dioxide supplied from a carbon dioxide supply source; a storage step of recovering carbon dioxide contained in the gas by a recovery unit and storing the carbon dioxide recovered by the recovery unit in a storage unit, separately from the recovery step; a fuel production step of supplying at least one of the carbon dioxide recovered in the recovery step and the carbon dioxide stored in the storage step, and hydrogen supplied from a hydrogen supply source, to a reactor to produce a fuel gas; a first pressure detection step of detecting a fuel reaction pressure in the reactor; a second pressure detection step of detecting the pressure of the carbon dioxide stored in the storage unit; a control step of controlling the recovery unit so that the carbon dioxide recovered in the recovery unit is supplied to the storage unit when a differential pressure, which is a value obtained by subtracting the pressure detected in the first pressure detection step from the pressure detected in the second pressure detection step, becomes smaller than a predetermined differential pressure threshold. Fuel gas production method. (Application Example 7) A computer program that causes a computer to execute production of fuel gas using a fuel gas production system, a recovery function of recovering carbon dioxide from a gas containing carbon dioxide supplied from a carbon dioxide supply source; A storage function that recovers carbon dioxide contained in the gas by a recovery unit and stores the carbon dioxide recovered by the recovery unit in a storage unit, separately from the recovery function; a fuel production function that supplies at least one of the carbon dioxide recovered by the recovery function and the carbon dioxide stored by the storage function, and hydrogen supplied by a hydrogen supply source to a reactor to produce a fuel gas; a first pressure detection function for detecting a fuel reaction pressure in the reactor; a second pressure detection function for detecting the pressure of the carbon dioxide stored in the storage unit; a control function of controlling the recovery unit so that the carbon dioxide recovered in the recovery unit is supplied to the storage unit when a differential pressure, which is a value obtained by subtracting the pressure detected by the first pressure detection function from the pressure detected by the second pressure detection function, becomes smaller than a predetermined differential pressure threshold value; Computer program. [Explanation of symbols]

[0100] 1, 2...Fuel gas production system 5...Combustion furnace 10...Carbon dioxide capture device 20...Carbon dioxide storage device 21,22…Adsorption tower 21d, 22d...Adsorbent 24...Flow rate detector 26...Storage surge tank 27...Storage pressure detector 30…Fuel conversion device 31...Methanation reactor 32...Reaction pressure detection unit 40...Hydrogen gas tank 74...Flow control section 75...Pressure control section ΔPs: Supply differential pressure ΔPth, ΔPth1, ΔPth2... Differential pressure threshold ΔPth0: Stop threshold Q_des: carbon dioxide flow rate Qth: Flow rate threshold

Claims

1. A fuel gas production system, comprising: a recovery device that recovers carbon dioxide from a gas containing carbon dioxide supplied from a carbon dioxide supply source; a storage device including a recovery unit that recovers carbon dioxide contained in the gas separately from the recovery device, and a storage unit that stores the carbon dioxide recovered by the recovery unit; a fuel production apparatus having a reactor connected to each of the recovery apparatus and the storage apparatus, the fuel production apparatus supplying at least one of the carbon dioxide recovered by the recovery apparatus and the carbon dioxide stored in the storage apparatus, and hydrogen supplied by a hydrogen supply source, to the reactor to produce a fuel gas; a first pressure detection unit that detects a fuel reaction pressure in the reactor; a second pressure detection unit that detects the pressure of the carbon dioxide stored in the storage unit; a control unit that controls the storage device so that the carbon dioxide recovered in the recovery unit is supplied to the storage unit when a differential pressure, which is a value obtained by subtracting the pressure detected by the first pressure detection unit from the pressure detected by the second pressure detection unit, becomes smaller than a predetermined differential pressure threshold value. Fuel gas production system.

2. The fuel gas production system according to claim 1 further comprises: a flow rate detection unit that detects the flow rate of carbon dioxide supplied from the recovery unit to the storage unit, the control unit increases the differential pressure threshold when the flow rate of carbon dioxide detected by the flow rate detection unit is equal to or less than a preset flow rate threshold. Fuel gas production system.

3. The fuel gas production system according to claim 1 further comprises: a fuelization pressure control unit that controls the fuelization pressure in the reactor; a flow rate detection unit that detects the flow rate of carbon dioxide supplied from the recovery unit to the storage unit, the control unit controls the fuel production pressure control unit to reduce the fuel production reaction pressure in the reactor when the flow rate of carbon dioxide detected by the flow rate detection unit is equal to or lower than a preset flow rate threshold value. Fuel gas production system.

4. The fuel gas production system according to any one of claims 1 to 3, the recovery unit has a plurality of adsorption towers that accommodate adsorbents capable of adsorbing carbon dioxide, the control unit controls the storage device so as to desorb carbon dioxide adsorbed by a first adsorbent housed in a first adsorption tower among the plurality of adsorption towers, while causing a second adsorbent housed in a second adsorption tower among the plurality of adsorption towers to adsorb carbon dioxide contained in the gas. Fuel gas production system.

5. The fuel gas production system according to claim 4, the control unit controls the storage device so that, when the differential pressure becomes equal to or less than a preset lower limit threshold, the first adsorbent adsorbs carbon dioxide contained in the gas, while the second adsorbent desorbs carbon dioxide. Fuel gas production system.

6. A fuel gas production method using a fuel gas production system, comprising: a recovery step of recovering carbon dioxide from a gas containing carbon dioxide supplied from a carbon dioxide supply source; a storage step of recovering carbon dioxide contained in the gas by a recovery unit and storing the carbon dioxide recovered by the recovery unit in a storage unit, separately from the recovery step; a fuel production step of supplying at least one of the carbon dioxide recovered in the recovery step and the carbon dioxide stored in the storage step, and hydrogen supplied from a hydrogen supply source, to a reactor to produce a fuel gas; a first pressure detection step of detecting a fuel reaction pressure in the reactor; a second pressure detection step of detecting the pressure of the carbon dioxide stored in the storage unit; a control step of controlling the recovery unit so that the carbon dioxide recovered in the recovery unit is supplied to the storage unit when a differential pressure, which is a value obtained by subtracting the pressure detected in the first pressure detection step from the pressure detected in the second pressure detection step, becomes smaller than a predetermined differential pressure threshold. Fuel gas production method.

7. A computer program that causes a computer to execute production of fuel gas using a fuel gas production system, a recovery function of recovering carbon dioxide from a gas containing carbon dioxide supplied from a carbon dioxide supply source; A storage function that recovers carbon dioxide contained in the gas by a recovery unit and stores the carbon dioxide recovered by the recovery unit in a storage unit, separately from the recovery function. a fuel production function that supplies at least one of the carbon dioxide recovered by the recovery function and the carbon dioxide stored by the storage function, and hydrogen supplied by a hydrogen supply source to a reactor to produce a fuel gas; a first pressure detection function for detecting a fuel reaction pressure in the reactor; a second pressure detection function for detecting the pressure of the carbon dioxide stored in the storage unit; a control function of controlling the recovery unit so as to supply the carbon dioxide recovered in the recovery unit to the storage unit when a differential pressure, which is a value obtained by subtracting the pressure detected by the first pressure detection function from the pressure detected by the second pressure detection function, becomes smaller than a predetermined differential pressure threshold value; Computer program.

Images