Co-electrolytic system and its operating method

Abstract

The objective is to suppress the methane reaction of the generated gas in a co-electrolysis system and its operating method. [Solution] The co-electrolysis system according to the present disclosure is a co-electrolysis system comprising an electrolytic cell stack 101 and a control unit 123, wherein the electrolytic cell stack 101 comprises an electrolytic cell 105 in which a hydrogen electrode, a solid electrolyte, and an oxygen electrode are stacked in order, a raw material gas flow passage 124 through which raw material gases containing H2O and CO2 supplied to the hydrogen electrode flow, and a generated gas flow passage 125 through which generated gases produced at the hydrogen electrode flow, and the control unit 123 comprises a methane concentration suppression unit 135 that sets the methane concentration in the outlet gas of the generated gas flow passage below a threshold.

Description

[Technical Field]

[0001] This disclosure relates to a co-electrolysis system and a method for operating the same. [Background technology]

[0002] Water electrolysis, which electrochemically decomposes water to produce hydrogen and oxygen, is a hydrogen production method that does not emit carbon dioxide and has excellent environmental characteristics. There are various types, including alkaline electrolysis and solid polymer electrolysis, which electrolyze liquid water, and steam electrolysis, which electrolyzes water vapor.

[0003] In particular, solid oxide electrolysis cells (SOECs), which electrolyze high-temperature steam, use oxygen ion conductive ceramics such as yttria-stabilized zirconia as a solid electrolyte. Because they can utilize the thermal energy of high-temperature steam as part of the energy required for the electrolytic reaction, they can produce hydrogen with higher efficiency compared to other electrolysis methods. Electrolysis cells using solid electrolytes can also be used for ammonia electrolysis.

[0004] Furthermore, the electrolytic cell can also be supplied with a mixed gas of high-temperature steam and carbon dioxide (CO2) (raw material gas), and the hydrogen produced by electrolysis reacts with carbon dioxide on the electrolytic cell to directly produce carbon monoxide (CO) or hydrocarbon compounds in a co-electrolytic manner (see Patent Document 1).

[0005] Electrolytic cells using solid electrolytes can simultaneously electrolyze CO2 / H2O as a co-electrolysis to produce H2 / CO synthesis gas necessary for Fischer-Tropsch (FT) synthesis, which is used in the production of synthetic fuels (e-fuels) such as sustainable aviation fuel (SAF). This may simplify the system compared to production processes using reverse shift reactions from water electrolysis. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2023-50701 [Overview of the project] [Problems that the invention aims to solve]

[0007] In Patent Document 1, the electrode layer (hydrogen electrode) material contains Ni, and the metal support supporting the hydrogen electrode contains Fe. The Ni and Fe particles contained in the electrode layer and metal support act as catalysts (methanization catalysts) for the reaction in which methane is synthesized from hydrogen (H2) and carbon monoxide (CO).

[0008] The product gas generated in the CO2 / H2O co-electrolysis contains H2 and CO. Therefore, when the product gas comes into contact with a structure containing a methanation catalyst, the methanation reaction proceeds.

[0009] In pressurized operation, which facilitates integration with FT synthesis, a problem arises because the methanation reaction occurs due to the action of the methanation catalyst, producing methane and reducing the H2 / CO yield.

[0010] To prevent the methanation reaction, the methanation catalyst can be removed from the hydrogen electrode and metal support. However, Ni and Fe serve purposes other than catalysis, such as controlling the coefficient of thermal expansion and adjusting porosity, so they cannot be easily removed.

[0011] This disclosure has been made in view of these circumstances and aims to provide a co-electrolysis system and a method for operating the same that can suppress the methane reaction of the generated gas. [Means for solving the problem]

[0012] To solve the above problems, the co-electrolysis system and its operating method described herein employ the following means.

[0013] This disclosure provides a co-electrolytic system comprising an electrolytic cell stack and a control unit, wherein the electrolytic cell stack comprises an electrolytic cell in which a hydrogen electrode, a solid electrolyte, and an oxygen electrode are stacked in order, a raw material gas flow passage through which a raw material gas containing H2O and CO2 supplied to the hydrogen electrode flows, and a product gas flow passage through which a product gas generated at the hydrogen electrode flows, and the control unit comprises a methane concentration suppression unit that keeps the methane concentration in the outlet gas of the product gas flow passage below a threshold.

[0014] This disclosure provides a method for operating a co-electrolytic system comprising an electrolytic cell stack having an electrolytic cell in which a hydrogen electrode, a solid electrolyte, and an oxygen electrode are stacked in order; a raw material gas flow passage for supplying raw material gas containing H2O and CO2 to the hydrogen electrode; and a product gas flow passage for discharging the product gas generated at the hydrogen electrode, wherein the method for operating the co-electrolytic system controls the methane concentration in the outlet gas of the product gas flow passage to a threshold by reducing the gas utilization rate, which is the ratio of the amount of raw material gas used for electrolysis to the amount of raw material gas supplied to the electrolytic cell stack. [Effects of the Invention]

[0015] This disclosure provides a methane concentration suppression unit that can suppress the methane concentration in the generated gas (outlet gas) discharged from the generated gas flow path (electrolytic cell stack) to below a threshold.

[0016] This disclosure shows that even if the member defining the outer casing of the generated gas flow path contains a methanation catalyst, the methane concentration in the generated gas (outlet gas) discharged from the generated gas flow path containing the methanation catalyst can be kept below a threshold by reducing the gas utilization rate. [Brief explanation of the drawing]

[0017] [Figure 1] This is a partial cross-sectional view showing an example of an electrolytic cell stack related to this disclosure. [Figure 2] This is a schematic diagram of the co-electrolysis system according to the first embodiment. [Figure 3]It is a diagram showing the relationship between the gas flow rate per catalyst area ((mol / s) × (1 / catalyst area)) in the first embodiment and the methane concentration in the outlet gas of the cell stack. [Figure 4] It is a diagram showing an example of the setting flow of the gas utilization rate in the first embodiment. [Figure 5] It is a schematic configuration diagram of the co - electrolysis system according to the second embodiment. [Figure 6] It is a diagram showing the relationship between the gas flow rate per catalyst area ((mol / s) × (1 / catalyst area)) in the second embodiment and the methane concentration in the outlet gas of the cell stack. [Figure 7] It is a diagram showing an example of the setting flow of the gas utilization rate in the second embodiment. [Figure 8] It is a schematic configuration diagram showing an example of the raw material gas composition ratio adjustment unit. [Figure 9] It is a schematic configuration diagram of the co - electrolysis system according to the third embodiment. [Figure 10] It is a diagram showing the relationship between the gas flow rate per catalyst area ((mol / s) × (1 / catalyst area)) in the third embodiment and the methane concentration in the outlet gas of the cell stack. [Figure 11] It is a diagram showing an example of the setting flow of the gas utilization rate in the third embodiment. [Figure 12] It is a schematic configuration diagram of the co - electrolysis system according to the fourth embodiment. [Figure 13] It is a diagram showing the relationship between the gas flow rate per catalyst area ((mol / s) × (1 / catalyst area)) and the methane concentration in the outlet gas of the cell stack in the case of the circulating generated gas flow rate (set value 3). [Figure 14] It is a diagram showing the relationship between the gas flow rate per catalyst area ((mol / s) × (1 / catalyst area)) in the fourth embodiment and the methane concentration in the outlet gas of the cell stack in the case of the circulating generated gas flow rate (set value 4). [Figure 15] It is a diagram showing an example of the setting flow of the gas utilization rate in the fourth embodiment. [Figure 16] It is a schematic configuration diagram of the co - electrolysis system according to the fifth embodiment. [Figure 17] This is a schematic diagram of the co-electrolysis system according to the first modified example of the fifth embodiment. [Figure 18] This is a schematic diagram of the co-electrolysis system according to a second modified example of the fifth embodiment. [Figure 19] This is a schematic diagram of the co-electrolysis system according to the third modified example of the fifth embodiment. [Figure 20] This is an illustrative diagram of an electrolytic cell module according to the sixth embodiment. [Figure 21] This is a schematic diagram of a modified electrolytic cell stack. [Figure 22] This is a schematic diagram of a modified electrolytic cell stack. [Figure 23] This is a schematic diagram of a modified electrolytic cell stack. [Modes for carrying out the invention]

[0018] The electrolytic system and its operating method related to this disclosure will be described below with reference to the drawings.

[0019] First, the configuration of the electrolytic cell stack provided in the co-electrolysis system will be explained with reference to Figure 1.

[0020] For the sake of clarity, the positional relationships of each component described using the terms "upper" and "lower" relative to the paper surface refer to the vertically upward and vertically downward directions, respectively. Furthermore, in this embodiment, components that achieve similar effects in both the vertical and horizontal directions do not necessarily have to be limited to the vertically up and down directions on the paper surface; for example, they may correspond to the horizontal direction perpendicular to the vertical direction.

[0021] (Electrolytic cell stack) Figure 1 is a partial cross-sectional view showing an example of a cylindrical solid oxide electrolytic cell stack (hereinafter abbreviated as cell stack). The cell stack 101 in Figure 1 comprises a cylindrical base tube 103, multiple electrolytic cells 105 formed on the outer circumferential surface of the base tube 103 and arranged in the axial direction of the base tube 103, and interconnectors 107 formed between adjacent electrolytic cells 105. The electrolytic cell 105 is formed by sequentially stacking a hydrogen electrode 109, a solid electrolyte membrane 111, and an oxygen electrode 113. Furthermore, the cell stack 101 has a lead membrane 115 electrically connected via the interconnector 107 to the oxygen electrode 113 of the electrolytic cell 105 formed at one end of the multiple electrolytic cells 105 formed on the outer circumferential surface of the base tube 103, and a lead membrane 115 electrically connected to the hydrogen electrode 109 of the electrolytic cell 105 formed at the other end of the base tube 105.

[0022] In Figure 1, the electrolytic cell 105 is supported by a base tube, but the use is not limited to a base tube; for example, the hydrogen electrode 109 may be made thicker and serve as the base tube as well. Also, although the base tube 103 in this embodiment is described as being cylindrical, the base tube can be cylindrical, and its cross-section is not necessarily limited to a circle; for example, it may be elliptical. A cell stack such as a flat tubular cylinder, formed by vertically crushing the circumferential surface of a cylinder, may also be used.

[0023] The base tube 103 is made of a porous material, mainly composed of, for example, CaO-stabilized ZrO2 (CSZ), a mixture of CSZ and nickel oxide (NiO) (CSZ+NiO), Y2O3-stabilized ZrO2 (YSZ), or MgAl2O4. This base tube 103 supports the electrolytic cell 105, the interconnector 107, and the lead film 115, and diffuses water vapor supplied to the inner surface of the base tube 103 through the pores of the base tube 103 to the hydrogen electrode 109 formed on the outer surface of the base tube 103. The base tube contains Ni particles that can act as a methanation catalyst. This Ni has methanation catalytic activity, but it was not added with the expectation of methanation catalytic activity.

[0024] The inner surface 104 of the base tube 103 defines the outline of the gas flow passage (the raw material gas flow passage and the generated gas flow passage described later). In a cell stack in which the hydrogen electrode 109 is formed thickly and also serves as the base tube, the inner surface of the hydrogen electrode 109 defines the outline of the raw material gas flow passage and the generated gas flow passage described later.

[0025] The hydrogen electrode 109 is composed of an oxide of a composite material of Ni and a zirconia-based electrolyte material, for example, Ni / YSZ is used. The thickness of the hydrogen electrode 109 is 50 μm to 250 μm, and the hydrogen electrode 109 may be formed by screen printing of a slurry.

[0026] The solid electrolyte membrane 111 is mainly made of YSZ, which has airtightness that prevents gas from passing through and high oxygen ion conductivity at high temperatures. This solid electrolyte membrane 111 is made of oxygen ions (O) generated at the hydrogen electrode 109. 2- This transfers the ) to the oxygen electrode 113. The thickness of the solid electrolyte membrane 111 located on the surface of the hydrogen electrode 109 is 5 μm to 100 μm, and the solid electrolyte membrane 111 may be formed by screen printing of a slurry.

[0027] The oxygen electrode 113 is composed of, for example, a LaSrMnO3-based oxide or a LaCoO3-based oxide, and the oxygen electrode 113 is applied as a slurry using screen printing or a dispenser. The oxygen electrode 113 can also have a two-layer structure. In this case, the oxygen electrode layer (oxygen electrode intermediate layer) on the solid electrolyte membrane 111 side is composed of a material that exhibits high ionic conductivity and excellent catalytic activity. The oxygen electrode intermediate layer may be composed of Sm-doped ceria that exhibits high ionic conductivity, and the oxygen electrode layer (oxygen electrode conductive layer) on the oxygen electrode intermediate layer may be composed of a perovskite-type oxide such as Sr and Ca-doped LaMnO3.

[0028] Interconnector 107 is M for SrTiO3 series, etc. 1-x L xThe interconnector 107 is composed of conductive perovskite-type oxides represented by TiO3 (where M is an alkaline earth metal element and L is a lanthanide element) or lanthanum chromite (LaCrO3), and the slurry is screen printed. The interconnector 107 has a dense film to prevent mixing of the source gas and the oxidizing gas. Furthermore, the interconnector 107 has stable durability and electronic conductivity under both oxidizing and reducing atmospheres. In adjacent electrolytic cells 105, the interconnector 107 electrically connects the oxygen electrode 113 of one electrolytic cell 105 to the hydrogen electrode 109 of the other electrolytic cell 105, thereby connecting adjacent electrolytic cells 105 in series.

[0029] The lead film 115 needs to possess electronic conductivity and have a coefficient of thermal expansion similar to that of the other materials constituting the cell stack 101. Therefore, composite materials of Ni and zirconia-based electrolyte materials such as Ni / YSZ or M such as SrTiO3 are used. 1-x L x It is composed of TiO3 (where M is an alkaline earth metal element and L is a lanthanide element). This lead film 115 applies the DC power necessary for the electrolytic reaction to the ends of the cell stack 101 to multiple electrolytic cells 105 connected in series by an interconnector 107. In addition, the surface on the oxidizing gas side may be protected with an airtight, oxidation-resistant material to prevent oxidation of metal materials such as Ni.

[0030] (Electrolytic reaction in an electrolytic cell stack) A raw material gas is supplied into the base tube 103 (hydrogen electrode 109). The supplied gas (raw material gas) contains water vapor (H2O) and carbon dioxide (CO2). Hydrogen may also be included in the raw material gas.

[0031] An oxidizing gas is supplied to the outer circumference (oxygen electrode 113) of the base tube 103. The oxidizing gas is usually a gas containing approximately 15% to 30% oxygen, and air is typically preferred, but other gases such as a mixture of combustion exhaust gas and air, a mixture of oxygen and air, or an inert gas such as nitrogen can also be used.

[0032] The gas (raw material gas) supplied into the cell stack 101 flows from the left side to the right side of the paper surface and is electrolyzed in the electrolytic cell 105.

[0033] By applying a negative voltage to the hydrogen electrode 109 and a positive voltage to the oxygen electrode 113, at the hydrogen electrode 109, the water vapor contained in the raw material gas receives electrons and is electrolyzed to generate hydrogen molecules and oxygen ions (O 2- ). (Refer to the following reaction formula (1)). Also, the carbon dioxide contained in the raw material gas receives electrons and is electrolyzed to generate carbon monoxide molecules and oxygen ions (O 2- ). (Refer to the following reaction formula (2)). H2O + 2e - →H2 + O 2- ···(1) CO2 + 2e - →CO + O 2- ···(2)

[0034] On the other hand, the oxygen ions pass through the solid electrolyte membrane 111 due to the potential difference, move to the oxygen electrode 113, release electrons, and become oxygen molecules. (Refer to the following reaction formula (3)). The generated oxygen is discharged to the outside together with the oxidizing gas supplied to the oxygen electrode 113. 2O 2- →O2 + 4e - ···(3)

[0035] The oxidizing gas is not directly involved in the electrolysis reaction, but supplies the heat required for the electrolysis reaction (endothermic) and discharges the waste heat of the electrolysis reaction.

[0036] The gas (generated gas) generated at the hydrogen electrode by electrolysis is discharged to the outside of the electrolytic cell stack (gas discharge pipe) through the generated gas flow path together with the un-electrolyzed raw material gas that did not contribute to the electrolysis reaction. Hereafter, the gas flowing through the generated gas flow path is also called "generated gas" even if it contains un-electrolyzed raw material gas.

[0037] Electrolysis is an endothermic reaction, but if current is continuously passed through the electrolytic cell 105 for the electrolytic reaction, heat is generated due to electrical resistance. Therefore, the temperature of the axial central portion of the base tube 103 on which the electrolytic cell 105 is formed becomes higher than that of the cell stack 101 on the raw material gas inlet side. For example, if the temperature of the base tube on the raw material gas inlet side is 550°C, the axial central portion of the base tube 103 will be 900°C to 950°C, and the electrolytic cell portion located downstream will be 850°C. The temperature of the gas that has passed through the portion on which the electrolytic cell 105 is formed will gradually decrease, and for example, the temperature near the generated gas outlet of the cell stack 101 will be around 550°C.

[0038] The product gas (H2,CO) generated at the hydrogen electrode 109 by electrolysis may undergo side reactions as it flows through the substrate tube 103 (see reaction equations (4) and (5) below). CO + H2O → CO2 + H2 ... (4) CO2 + 3H2 → CH4 + H2O...(5)

[0039] According to simulations conducted by the inventors, the methane reaction (reaction equation (5) above) proceeds most readily during the process of temperature decrease from high temperature (850°C) to low temperature (550°C), that is, on the raw material gas outlet side of the cell stack 101.

[0040] [First Embodiment] (Co-electrolysis system) Figure 2 is a schematic diagram of the co-electrolysis system according to this embodiment. The co-electrolysis system 120 includes an electrolytic cell stack (cell stack) 101, a gas utilization rate reduction unit (power supply unit 121, gas flow rate adjustment unit 122), and a control unit 123.

[0041] The cell stack 101 comprises an electrolytic cell 105, a raw material gas flow passage 124 through which raw material gases containing H2O and CO2 supplied to the hydrogen electrode flow, and a product gas flow passage 125 through which product gases generated at the hydrogen electrode flow.

[0042] The outer surface of the cell stack 101 is provided with an oxidizing gas flow passage (not shown) for supplying an oxidizing gas to the oxygen electrode, and an exhaust gas flow passage (not shown) for discharging exhaust gas discharged from the oxygen electrode.

[0043] As described above, the electrolytic cell 105 has a structure in which a hydrogen electrode 109, a solid electrolyte membrane 111, and an oxygen electrode 113 are stacked in that order from the base tube 103 side. In Figure 2, multiple electrolytic cells 105 are arranged in the axial central portion of the base tube 103.

[0044] In this embodiment, the axial direction of the base tube 103 is divided into three regions. The part where the electrolytic cell 105 is located is called the electrolytic section 126, the part upstream of the electrolytic section 126 from the raw material gas flow is called the upper lead section 127, and the part downstream of the electrolytic section 126 from the raw material gas flow is called the lower lead section 128.

[0045] In Figure 2, the base pipe 103 located at the upper lead section 127 is a component that defines the outer casing of the raw material gas flow passage 124. The upstream end of the raw material gas flow passage 124 becomes the raw material gas inlet (cell stack inlet) of the cell stack 101.

[0046] In Figure 2, the base pipe 103 located at the lower lead section 128 is the component that defines the outer perimeter of the generated gas flow passage. The downstream end of the generated gas flow passage 125 becomes the generated gas outlet (cell stack outlet) of the cell stack 101.

[0047] The power supply unit 121 comprises a power supply body 121a and an ammeter 121b. The power supply body 121a is connected to the cell stack 101 so that a DC voltage can be applied between the hydrogen electrode 109 and the oxygen electrode 113. The ammeter 121b can measure the amount of current applied to the electrolytic cell 105.

[0048] The gas flow rate adjustment unit 122 is connected to one end (raw material gas inlet) of the cell stack 101 via the connection unit 129 and the gas supply line 130. The gas flow rate adjustment unit 122 receives H2O gas and CO2 gas from the H2O gas source 131 and the CO2 gas source 132, and can adjust the flow rate of the raw material gas supplied into the cell stack 101.

[0049] The gas supply line 130 is equipped with a flow meter 136 that can measure the gas flow rate at the cell stack inlet.

[0050] A gas discharge line 134 is connected to the other end (production gas outlet) of the cell stack 101 via a connection part 133. The gas discharge line 134 may be equipped with a gas concentration measuring device (not shown) for measuring the methane (CH4) concentration in the outlet gas of the production gas flow passage 125.

[0051] The control unit 123 includes a methane concentration suppression unit 135 that keeps the methane concentration in the outlet gas of the generated gas flow path below a threshold.

[0052] The control unit consists of, for example, a CPU (Central Processing Unit), RAM (Random Access Memory), ROM (Read Only Memory), and a computer-readable storage medium. A series of processes for realizing various functions are stored in the storage medium in the form of a program, for example. The CPU reads this program into the RAM and performs information processing and calculations to realize the various functions. The program may be pre-installed in ROM or other storage media, provided stored in a computer-readable storage medium, or distributed via wired or wireless communication. Computer-readable storage media include magnetic disks, magneto-optical disks, CD-ROMs, DVD-ROMs, and semiconductor memory.

[0053] The methane concentration suppression unit 135 is connected electrically or wirelessly to the power supply unit 121's power supply body 121a and / or gas flow rate adjustment unit 122. The methane concentration suppression unit 135 can send signals to the power supply body 121a and / or gas flow rate adjustment unit 122 to control the current value applied to the electrolytic cell 105 and / or the gas flow rate at the cell stack inlet to reduce the gas utilization rate.

[0054] "Gas utilization rate" is the ratio of the amount of raw material gas used for electrolysis (theoretical raw material gas consumption) to the amount of raw material gas supplied from the cell stack inlet.

[0055] (Operation method of the electrolytic cell stack) In this embodiment, a raw material gas containing H2O and CO2 is supplied into the raw material gas flow passage 124 at a predetermined flow rate, and a predetermined current is applied to the electrolytic cell 105. The ratio of H2O to CO2 in the raw material gas is kept constant. The raw material gas is electrolyzed in the electrolytic cell 105, and H2 and CO are produced. The generated gas containing H2 and CO produced by electrolysis is discharged to the gas discharge line 134 through the generated gas flow passage 125.

[0056] In this embodiment, the methane concentration in the outlet gas of the generated gas flow path 125 is controlled to be below a threshold by reducing the system gas utilization rate. The gas utilization rate is determined by the (raw material) gas flow rate at the cell stack inlet and the current value applied to the electrolytic cell 105. The gas utilization rate can be reduced by increasing the (raw material) gas flow rate at the cell stack inlet and / or by decreasing the current value applied to the electrolytic cell 105.

[0057] Figure 3 shows the relationship between the gas flow rate per catalyst area ((moles / second) × (1 / catalyst area)) and the methane concentration in the outlet gas of the generated gas flow passage 125. In this figure, the horizontal axis represents the gas flow rate per catalyst area, and the vertical axis represents the methane concentration. "Catalyst area" can be interpreted as the generated gas flow passage area (area of ​​the inner surface of the substrate tube in the lower lead section 128). The gas flow rate in the lower lead section 128 can be calculated from the gas flow rate at the cell stack inlet and the structure of the cell stack 101.

[0058] As shown in Figure 3, it can be confirmed that the methane concentration in the outlet gas decreases as the gas flow rate per catalyst area increases. As shown in Figure 3, to keep the methane concentration in the outlet gas below the threshold C, the gas flow rate at the cell stack inlet should be set to F or higher.

[0059] Figure 4 shows an example of the gas utilization rate setting flow in the operating method according to this embodiment. The setting flow in Figure 4 includes (A) a step of adjusting the current value, (B) a step of adjusting the gas flow rate, and (C) a step of checking the methane concentration.

[0060] (A) Steps to adjust the current value (S1) Input the desired current value into the power supply unit. (S2_Evaluation 1) Next, measure the current value with the ammeter 121b and check if it matches the input value to the power supply unit. If the measured value does not match the input value, return to (S1) and adjust the current value. Repeat (S1) and (S2) until the measured value matches the input value. After it is confirmed that the measured value matches the input value, proceed to the next step (B).

[0061] (B) Process to adjust gas flow rate (S3) First, a predetermined flow rate value is input to the raw material gas flow rate adjustment unit. (S4_Evaluation 2) Next, the gas flow rate at the cell stack inlet is measured with a flow meter, and it is confirmed whether the measured value matches the input value to the raw material gas flow rate adjustment unit. If the measured value does not match the input value, return to (S3) and adjust the flow rate value. Repeat (S3) and (S4) until the measured value matches the input value. After it is confirmed that the measured value matches the input value, proceed to the next step (C).

[0062] (C) Process to check methane concentration (S5_Evaluation 3) The gas concentration measuring device measures the methane concentration of the outlet gas of the cell stack. If the measured value does not match the desired concentration, repeat steps (S1) to (S5) until the measured value matches the desired concentration. Based on the conditions when it is confirmed that the measured value matches the input value, set the gas utilization rate required to keep the methane concentration in the outlet gas below the threshold.

[0063] The setting flow for the gas flow rate at the cell stack inlet and the current value applied to the electrolytic cell in order to achieve the desired gas utilization rate (methane concentration in the outlet gas) is not limited to Figure 4 above.

[0064] The order of (A) and (B) above may be reversed. The evaluations in (S2), (S4), and (S5) above may be confirmed using theoretical values ​​rather than actual measurements. (A) to (C) above may be performed in real time when the co-electrolysis system starts operation, or they may be performed as preliminary tests. In the case of preliminary tests, the information obtained in these preliminary tests may be stored in the control unit (methane concentration suppression unit), and the control unit may control the gas utilization rate reduction unit based on the stored information.

[0065] [Second Embodiment] (Co-electrolysis system) Figure 5 shows a schematic diagram of the co-electrolysis system according to this embodiment. For clarity, Figure 5 shows a simplified representation of the cell stack, its surrounding parts, and the control unit. Although simplified, their configurations should be understood to be the same as those of the first embodiment.

[0066] The co-electrolysis system 140 according to this embodiment includes a generated gas circulation unit 141 in addition to the co-electrolysis system 120 of the first embodiment. Common components are denoted by the same reference numerals as in the first embodiment.

[0067] The generated gas circulation unit 141 is configured to guide the generated gas discharged from the generated gas flow passage to the inlet of the cell stack (raw material gas flow passage). The generated gas circulation unit 141 includes a generated gas circulation line 141a and a circulating generated gas flow rate adjustment unit 141b.

[0068] The generated gas circulation line 141a is connected at one end to the gas discharge line 134 and at the other end to the gas supply line 130. The circulating generated gas flow rate adjustment unit 141b can adjust the flow rate of the circulating generated gas flowing through the generated gas circulation line 141a.

[0069] The methane concentration suppression unit 142 is connected to the power supply unit 121, the gas flow rate adjustment unit 122, and the generated gas circulation unit 141 (circulated generated gas flow rate adjustment unit 141b) electrically or via wireless communication. The methane concentration suppression unit 142 sends a signal to the generated gas circulation unit 141 to control the flow rate of the circulated generated gas, and, similar to the first embodiment, can control the gas flow rate at the cell stack (raw material gas flow path) inlet and the current value applied to the electrolytic cell so as to reduce the gas utilization rate.

[0070] (Operating method of the electrolytic system) In the operation method of the co-electrolysis system according to this embodiment, a raw material gas containing H2O and CO2 is supplied into the cell stack at a predetermined flow rate, and a predetermined current is applied to the electrolytic cell. The raw material gas is electrolyzed in the electrolytic cell, and H2 and CO are produced. The generated gas containing H2 and CO produced by electrolysis is discharged to the gas discharge line 134 through the generated gas flow passage. A portion of the generated gas (outlet gas) discharged to the gas discharge line 134 is circulated to the (raw material) gas inlet side of the cell stack 101 via the generated gas circulation line 141a, and is introduced into the cell stack together with the raw material gas.

[0071] In this embodiment as well, similar to the first embodiment, the gas utilization rate is set so that the methane concentration in the outlet gas of the cell stack 101 is below a threshold.

[0072] Figure 6 shows the relationship between the gas flow rate per catalyst area ((moles / second) × (1 / catalyst area)) and the methane concentration in the outlet gas of the cell stack when a portion of the generated gas is circulated. In this figure, the horizontal axis represents the gas flow rate per catalyst area, and the vertical axis represents the methane concentration. The figure compares the cases of setpoint 1 and setpoint 2 for the circulating generated gas flow rate. Setpoint 1 is a different flow rate value from setpoint 2.

[0073] As shown in Figure 6, even when the circulating product gas is supplied into the cell stack together with the raw material gas, the methane concentration in the outlet gas decreases as the gas flow rate per catalyst area increases. On the other hand, if the flow rate of the circulating product gas is different, the inlet gas concentrations (F2, F2') required to keep the methane concentration in the outlet gas below the threshold C will differ.

[0074] Therefore, the gas utilization rate needs to be determined by considering the flow rate of the circulating generated gas, and setting the gas flow rate and current value at the cell stack inlet.

[0075] Figure 7 shows an example of the setting flow for the gas utilization rate in the operating method according to this embodiment. The setting flow in Figure 7 includes (2A) the step of adjusting the current value, (2B) the step of adjusting the gas flow rate, and (2C) the step of checking the methane concentration.

[0076] (2A) Step to adjust the current value (S21) Input the desired current value into the power supply unit. (S22_Evaluation 21) Next, measure the current value with an ammeter and check if it matches the input value to the power supply unit. If the measured value does not match the input value, return to (S21) and adjust the current value. Repeat (S21) and (S22) until the measured value matches the input value. After it is confirmed that the measured value matches the input value, proceed to the next step (2B).

[0077] (2B) Process to adjust gas flow rate (S23) First, a predetermined flow rate value is input to the circulating generated gas flow rate adjustment unit. (S24) Next, a predetermined flow rate value is input to the raw material gas flow rate adjustment unit. (S25_Evaluation 22) Next, the flow rate of the circulating generated gas in the generated gas circulation line and the gas flow rate at the cell stack inlet are measured with a flow meter, and it is confirmed whether the measured values ​​match the input values ​​to the circulating generated gas flow rate adjustment unit and the raw material gas flow rate adjustment unit. If the measured values ​​do not match the input values, return to (S23) and adjust the flow rate value of the circulating generated gas. Repeat (S23) to (S25) until the measured values ​​match the input values. After it is confirmed that the measured values ​​match the input values, proceed to the next (2C) step.

[0078] (2C) Steps to confirm methane concentration (S26_Evaluation 23) The methane concentration of the outlet gas of the cell stack is measured using a gas concentration measuring device. If the measured value does not match the desired concentration, steps (S21) to (S26) are repeated until the measured value matches the desired concentration. Based on the conditions when it is confirmed that the measured value matches the input value, the gas utilization rate required to keep the methane concentration in the outlet gas below the threshold is set.

[0079] The setting flow for the flow rate of the circulating generated gas, the gas flow rate at the cell stack inlet, or the current value applied to the electrolytic cell in order to achieve the desired gas utilization rate (methane concentration in the outlet gas) is not limited to Figure 7 above.

[0080] The order of (2A) and (2B) above may be reversed. The order of (S23) and (S24) above may be reversed. The evaluations in (S22) and (S25) above may be omitted. Steps (2A) to (2C) above may be performed in real time when the co-electrolysis system starts operation, or they may be performed as a preliminary test. In the case of a preliminary test, the information obtained in this preliminary test may be stored in the control unit (methane concentration suppression unit), and the control unit may control the gas utilization rate reduction unit based on the stored information.

[0081] [Third Embodiment] Figure 8 shows a schematic diagram of the co-electrolysis system according to this embodiment. For clarity, the cell stack and its surrounding area are simplified in Figure 8. Although simplified, their configuration should be understood to be the same as in the first embodiment.

[0082] The co-electrolysis system 150 according to this embodiment includes a raw material gas composition ratio adjustment unit 151 in addition to the co-electrolysis system 120 of the first embodiment. Common components are denoted by the same reference numerals as in the first embodiment.

[0083] The raw material gas composition ratio adjustment unit 151 is configured to adjust the composition ratio of H2O and CO2 contained in the raw material gas. In Figure 8, the raw material gas composition ratio adjustment unit 151 is connected upstream of the gas flow rate adjustment unit 122.

[0084] Figure 9 shows an example of a raw material gas composition ratio adjustment unit 151. The raw material gas composition ratio adjustment unit 151 in Figure 9 has an H2O line 153 and a CO2 line 154.

[0085] The H2O line 153 is connected at one end to an H2O gas source (not shown) and at the other end to a raw material gas flow rate adjustment unit (not shown). The H2O line 153 is equipped with a flow meter 153a for measuring the flow rate of H2O gas flowing through the H2O line.

[0086] The H2O line 153 is connected to a regulated H2O gas supply line 155, which introduces regulated H2O gas into the H2O line 153, and a regulated H2O gas discharge line 156, which discharges H2O from the H2O line 153. The regulated H2O gas supply line 155 and the regulated H2O gas discharge line 156 are each equipped with valves (155a, 156a) and flow meters (155b, 156b) for measuring the flow rate of the gas flowing through the line.

[0087] One end of the CO2 line 154 is connected to a CO2 gas source (not shown), and the other end is connected to a raw material gas flow rate adjustment unit (not shown). The CO2 line 154 is equipped with a flow meter 154a for measuring the flow rate of H2O gas flowing through the CO2 line.

[0088] CO2 line 154 is connected to a regulated CO2 gas supply line 157 that introduces regulated CO2 gas into CO2 line 154, and a regulated CO2 gas discharge line 158 that discharges CO2 from CO2 line 154. The regulated CO2 gas supply line 157 and the regulated CO2 gas discharge line 158 are equipped with valves (157a, 158a) and flow meters (157b, 158b) for measuring the flow rate of gas flowing through the line, respectively.

[0089] For example, to increase the H2O gas composition in the raw material gas, one can either open valve 155a of the adjustment H2O gas supply line 155 to introduce adjustment H2O gas at a predetermined gas flow rate into the H2O line 153, or open valve of the adjustment CO2 gas discharge line 158 to exhaust adjustment CO2 gas at a predetermined gas flow rate from the CO2 line 154.

[0090] The raw material gas composition ratio adjustment unit 151 is not limited to the one shown in Figure 9; for example, the gas flow rate adjustment unit 122 may be equipped with a function to adjust the raw material gas composition ratio.

[0091] The methane concentration suppression unit 159 is connected to the power supply unit 121, the gas flow rate adjustment unit 122, and the raw material gas composition ratio adjustment unit 151 either electrically or via wireless communication. The methane concentration suppression unit 159 sends a signal to the raw material gas composition ratio adjustment unit 151 to adjust the composition ratio of H2O and CO2 in the raw material gas, and, similar to the first embodiment, can control the gas flow rate at the cell stack inlet and the current value applied to the electrolytic cell so that the gas utilization rate decreases.

[0092] (Operating method of the electrolytic system) In the operation method of the co-electrolysis system according to this embodiment, the composition ratio of H2O and CO2 in the raw material gas is adjusted, and then the raw material gas is supplied into the cell stack at a predetermined flow rate, and a predetermined current is applied to the electrolytic cell. The raw material gas is electrolyzed in the electrolytic cell, and H2 and CO are produced. The product gas containing H2 and CO produced by electrolysis is discharged to the gas discharge line 134 through the product gas flow passage.

[0093] By changing the composition ratio of H2O and CO2, the amount of H2 or CO obtained as a product gas can be adjusted. Furthermore, by adding more H2O, the chemical equilibrium can be shifted to suppress the methane reaction.

[0094] If the system allows for arbitrary pressure settings, the composition ratio of H2O to CO2 in the source gas can be adjusted by changing the pressure.

[0095] In this embodiment as well, similar to the first embodiment, the gas utilization rate is set so that the methane concentration in the outlet gas of the cell stack 101 is below a threshold.

[0096] Figure 10 shows the relationship between the gas flow rate per catalyst area ((moles / second) × (1 / catalyst area)) and the methane concentration in the outlet gas of the cell stack when using raw material gases with different composition ratios of H2O and CO2. In this figure, the horizontal axis represents the gas flow rate per catalyst area, and the vertical axis represents the methane concentration. Cases A to C are raw material gases with different composition ratios of H2O and CO2.

[0097] As shown in Figure 10, even when the composition ratio of the raw material gas is different, the methane concentration in the outlet gas decreases as the gas flow rate per catalyst area increases. On the other hand, when the composition ratio of the raw material gas is different, the inlet gas concentrations (F3, F3', F3'') required to keep the methane concentration in the outlet gas below the threshold C will be different.

[0098] Therefore, the gas utilization rate must be determined by considering the composition ratio of H2O and CO2 in the raw gas, and then setting the gas flow rate and current value at the cell stack inlet.

[0099] Figure 11 shows an example of the gas utilization rate setting flow in the operating method according to this embodiment. The setting flow in Figure 11 includes (3A) a step of adjusting the current value, (3B) a step of adjusting the gas composition ratio and gas flow rate, and (3C) a step of confirming the methane concentration.

[0100] (3A) Step to adjust the current value (S31) Input the desired current value into the power supply unit. (S32_Evaluation 31) Next, measure the current value with an ammeter and check if it matches the input value to the power supply unit. If the measured value does not match the input value, return to (S31) and adjust the current value. Repeat (S31) and (S32) until the measured value matches the input value. After it is confirmed that the measured value matches the input value, proceed to the next step (3B).

[0101] (3B) Steps to adjust the gas composition ratio and gas flow rate. (S33) First, input the composition ratio of H2O and CO2 in the raw material gas into the raw material gas composition ratio adjustment unit. (S34) Next, input a predetermined flow rate value into the raw material gas flow rate adjustment unit. (S35_Evaluation 32) Next, measure the gas flow rate at the cell stack inlet with a flow meter and confirm that the measured value matches the input value to the raw material gas flow rate adjustment unit. Also, measure the composition ratio of H2O and CO2 in the raw material gas at the cell stack inlet with a gas concentration measuring device and confirm that the measured value matches the value input to the raw material gas composition ratio adjustment unit. If any of the measured values ​​do not match the input values, return to (S33) and adjust the composition ratio. Repeat (S33) to (S34) until the measured values ​​match the input values. After it is confirmed that the measured values ​​match the input values, proceed to the next (3C) step.

[0102] (3C) Process to confirm methane concentration (S36_Evaluation 33) The methane concentration of the outlet gas of the cell stack is measured using a gas concentration measuring device. If the measured value does not match the desired concentration, steps (S31) to (S36) are repeated until the measured value matches the desired concentration. Based on the conditions when it is confirmed that the measured value matches the input value, the gas utilization rate required to keep the methane concentration in the outlet gas below the threshold is set.

[0103] The setting flow for the composition ratio of H2O and CO2 in the raw material gas, the gas flow rate at the cell stack inlet, or the current value applied to the electrolytic cell in order to achieve the desired gas utilization rate (methane concentration in the outlet gas) is not limited to that shown in Figure 11 above.

[0104] The order of (3A) and (3B) above may be reversed. The order of (S33) and (S34) above may be reversed. The evaluations in (S32) and (S35) above may be omitted. Steps (3A) to (3C) above may be performed in real time when the co-electrolysis system starts operation, or they may be performed as a preliminary test. In the case of a preliminary test, the information obtained in this preliminary test may be stored in the control unit (methane concentration suppression unit), and the control unit may control the gas utilization rate reduction unit based on the stored information.

[0105] [Fourth Embodiment] (Co-electrolysis system) Figure 12 shows a schematic diagram of the co-electrolysis system according to this embodiment. For clarity, the cell stack and its surrounding area are simplified in Figure 12. Although simplified, their configuration should be understood to be the same as in the first embodiment.

[0106] The co-electrolysis system 160 according to this embodiment further includes, in addition to the configuration of the co-electrolysis system 120 of the first embodiment, a generated gas circulation unit 141, a raw material gas composition ratio adjustment unit 151, a CO2 separation unit 161, and a CO2 gas circulation unit 162.

[0107] The generated gas circulation unit 141 and the raw material gas composition ratio adjustment unit 151 have the same configuration as described in the second and third embodiments. Common components are denoted by the same reference numerals as in the first to third embodiments.

[0108] The CO2 separation unit 161 is connected to the gas discharge line 134 and is configured to separate CO2 from the gas discharged from the cell stack outlet.

[0109] The CO2 gas circulation unit 162 is configured to guide the CO2 separated in the CO2 separation unit 161 (separated CO2) to the raw material gas flow passage (cell stack inlet). The CO2 gas circulation unit 162 includes a separated CO2 gas circulation line 162a and a circulating CO2 gas flow rate adjustment unit 162b.

[0110] The separated CO2 gas circulation line 162a is connected at one end to the CO2 separation unit 161 and at the other end to the raw material gas composition ratio adjustment unit 151. The circulating CO2 gas flow rate adjustment unit 162b is located in the middle of the separated CO2 gas circulation line 162a and can adjust the flow rate of the separated CO2 gas flowing through the separated CO2 gas circulation line.

[0111] The methane concentration suppression unit 163 is connected electrically or wirelessly to the power supply unit 121, the gas flow rate adjustment unit 122, the circulating generated gas flow rate adjustment unit 141b, the raw material gas composition ratio adjustment unit 151, and the CO2 gas circulation unit 162 (circulating CO2 gas flow rate adjustment unit 162b). The methane concentration suppression unit 163 sends a signal to the CO2 gas circulation unit 162 to control the flow rate of separated CO2 gas in the separated CO2 gas circulation line 162a, and, similar to the first to third embodiments, can control the flow rate of the circulating generated gas, the composition ratio of H2O and CO2 in the raw material gas, the gas flow rate at the cell stack inlet, and the current value applied to the electrolytic cell so as to reduce the gas utilization rate.

[0112] (Operating method of the electrolytic system) In the operation method of the co-electrolysis system according to this embodiment, after adjusting the composition ratio of H2O and CO2 in the raw material gas, the raw material gas is supplied into the cell stack at a predetermined flow rate, and a predetermined current is applied to the electrolytic cell. The raw material gas is electrolyzed in the electrolytic cell, and H2 and CO are produced. The generated gas containing H2 and CO produced by electrolysis is discharged to the gas discharge line 134 through the generated gas flow passage. A portion of the generated gas (outlet gas) discharged to the gas discharge line 134 is circulated to the inlet side of the cell stack via the generated gas circulation line.

[0113] Furthermore, the remaining portion of the generated gas (outlet gas) is led to the CO2 separation section to separate the CO2 gas, and the separated CO2 gas is circulated to the inlet side of the cell stack 101 via the separated CO2 gas circulation line. The circulated CO2 gas is used as part of the raw material gas. After the circulated CO2 gas, H2O gas from the H2O gas source 131, and CO2 gas from the CO2 gas source 132 are adjusted to a predetermined composition ratio in the gas composition ratio adjustment section, they are introduced into the cell stack at a predetermined flow rate together with the circulated generated gas.

[0114] The circulated CO2 gas can be used, for example, as adjustment CO2 gas in the raw material gas composition ratio adjustment unit 151.

[0115] Although not shown in Figure 12, surplus H2O gas generated in components outside of this co-electrolysis system may be reused in the gas composition ratio adjustment unit, similar to the separated CO2 gas.

[0116] According to this embodiment, the gas flow rate of the circulating CO2 gas, the gas flow rate of the circulating generated gas, the composition ratio of the raw material gas, the gas flow rate at the cell stack inlet, and the current value applied to the electrolytic cell can be adjusted, thus suppressing the methane reaction more effectively than when each of these processes is performed individually.

[0117] Furthermore, by separating and reusing the CO2 contained in the generated gas, the gas components used as raw materials can be effectively utilized.

[0118] In this embodiment, as in the first embodiment, the gas utilization rate is set so that the methane concentration in the outlet gas of the cell stack is below a threshold.

[0119] Figures 13 and 14 show the relationship between the gas flow rate per catalyst area ((moles / second) × (1 / catalyst area)) and the methane concentration in the outlet gas of the cell stack, when using raw material gases with different H2O and CO2 composition ratios and when a portion of the generated gas is circulated. Figure 13 is a graph showing the case when using raw material gases with composition ratios for Cases A to C and circulating a portion of the generated gas at a set flow rate of 3. Figure 14 is a graph showing the case when using raw material gases with composition ratios for Cases D to F and circulating the generated gas at a set flow rate of 4. In Figures 13 and 14, the horizontal axis represents the gas flow rate per catalyst area and the vertical axis represents the methane concentration.

[0120] As shown in Figures 13 and 14, even when the composition ratio of the raw material gas is changed and the generated gas is circulated, the methane concentration in the outlet gas decreases as the gas flow rate per catalyst area increases. As shown in Figures 13 and 14, if the circulation flow rate of the generated gas is constant, the inlet gas concentrations (F4, F4', F4'', F5, F5', F5'') required to keep the methane concentration in the outlet gas below the threshold C will vary depending on the composition ratio of the raw material gas.

[0121] Therefore, the gas utilization rate needs to be determined by considering the flow rate of the circulating generated gas and the composition ratio of H2O and CO2 in the raw material gas, and then setting the gas flow rate and current value at the cell stack inlet.

[0122] Figure 15 shows an example of the gas utilization rate setting flow in the operating method according to this embodiment. The setting flow in Figure 15 includes (4A) a step of adjusting the current value, (4B) a step of adjusting the gas composition ratio and gas flow rate, and (4C) a step of confirming the methane concentration.

[0123] (4A) Step to adjust the current value (S41) Input the desired current value into the power supply unit. (S42_Evaluation 41) Next, measure the current value with an ammeter and check if it matches the input value to the power supply unit. If the measured value does not match the input value, return to (S41) and adjust the current value. Repeat (S41) and (S42) until the measured value matches the input value.

[0124] (4B) Steps to adjust the gas composition ratio and gas flow rate. (S43) First, a predetermined flow rate value is input to the circulating generated gas flow rate adjustment unit. (S44) Next, a predetermined flow rate value is input to the circulating CO2 gas flow rate adjustment unit. (S45) Next, the composition ratio of H2O and CO2 in the raw material gas is input to the raw material gas composition ratio adjustment unit. (S46) Next, a predetermined flow rate value is input to the raw material gas flow rate adjustment unit. (S47_Evaluation 42) The flow rate of the circulating generated gas in the generated gas circulation line, the flow rate of the circulating CO2 gas in the separated CO2 gas circulation line, and the gas flow rate at the cell stack inlet are measured with a flow meter, and it is confirmed that the measured values ​​match the input values ​​to the circulating generated gas flow rate adjustment unit, the circulating CO2 gas flow rate adjustment unit, and the raw material gas flow rate adjustment unit. In addition, the composition ratio of H2O and CO2 in the raw material gas at the cell stack inlet is measured with a gas concentration measuring device, and it is confirmed that the measured value matches the value input to the raw material gas composition ratio adjustment unit. If each measured value does not match the input value, return to (S43) and adjust the flow rate value of the circulating generated gas. Repeat steps (S43) to (S47) until the measured value matches the input value. After it is confirmed that the measured value matches the input value, proceed to the next step (4C).

[0125] (4C) Process to confirm methane concentration (S48_Evaluation 43) The methane concentration of the outlet gas of the cell stack is measured using a gas concentration measuring device. If the measured value does not match the desired concentration, steps (S41) to (S48) are repeated until the measured value matches the desired concentration. Based on the conditions when it is confirmed that the measured value matches the input value, the gas utilization rate required to keep the methane concentration in the outlet gas below the threshold is set.

[0126] The setting flow for the circulating product gas flow rate, circulating CO2 gas flow rate, H2O to CO2 composition ratio in the raw material gas, gas flow rate at the cell stack inlet, and current value applied to the electrolytic cell in order to achieve the desired gas utilization rate (methane concentration in the outlet gas) is not limited to Figure 15 above.

[0127] The order of (4A) and (4B) above may be reversed. The order of (S43) to (S46) above may be reversed. The evaluations in (S42) and (S45) above may be omitted. (4A) to (4C) above may be performed in real time when the co-electrolysis system starts operation, or they may be performed as a preliminary test. In the case of a preliminary test, the information obtained in this preliminary test may be stored in the control unit (methane concentration suppression unit), and the control unit may control the gas utilization rate reduction unit based on the stored information.

[0128] [Fifth Embodiment] (Co-electrolysis system) Figure 16 shows a schematic diagram of the co-electrolysis system according to this embodiment. For clarity, the cell stack and its surrounding area are simplified in Figure 16. Although simplified, their configuration should be understood to be the same as in the first embodiment.

[0129] The co-electrolysis system 170 according to this embodiment further includes an outlet gas concentration measuring unit 171 in addition to the configuration of the co-electrolysis system 120 of the first embodiment. Common components are denoted by the same reference numerals as in the first embodiment.

[0130] The outlet gas concentration measuring unit 171 is installed in the gas discharge line 134 and is configured to measure and monitor the methane concentration in the gas discharged from the cell stack outlet. The outlet gas concentration measuring unit 171 may also be configured to measure the concentrations of CO and / or H2 and / or H2O and / or CO2 and monitor the gas utilization rate in conjunction with these measurements.

[0131] The outlet gas concentration measuring unit 171 may be, for example, gas chromatography, semiconductor laser absorption spectroscopy, or Raman scattering. In particular, Raman scattering is suitable for the direct measurement of gases under high temperature and high pressure conditions, and is also suitable for measuring gas composition (%) because it can measure CO, H2, H2O, CO2, and CH4 with a single instrument.

[0132] The methane concentration suppression unit 172 is connected electrically or wirelessly to the outlet gas concentration measuring unit 171 and the power supply unit 121 and / or gas flow rate adjustment unit 122. The methane concentration suppression unit 172 receives a measured value signal from the outlet gas concentration measuring unit 171, and if the measured value exceeds a threshold, it sends a signal to the power supply unit 121 and / or gas flow rate adjustment unit 122, which can control the current value applied to the electrolytic cell and / or the gas flow rate at the cell stack inlet to reduce the gas utilization rate.

[0133] (Operating method of the electrolytic system) In the operation method of the co-electrolysis system according to this embodiment, a raw material gas containing H2O and CO2 is supplied into the cell stack at a predetermined flow rate, and a predetermined current is applied to the electrolytic cell. The raw material gas is electrolyzed in the electrolytic cell, and H2 and CO are produced. The product gas containing H2 and CO produced by electrolysis is discharged to the gas discharge line 134 through the product gas flow passage.

[0134] In this embodiment, the methane concentration of the generated gas (outlet gas) discharged from the cell stack 101 is measured to monitor the status of the methanation reaction. If the measured value exceeds a threshold for methane concentration, the gas flow rate at the cell stack inlet and / or the current value applied to the electrolytic cell are adjusted to control the methane concentration of the outlet gas to below the threshold. The same control is applied when the gas utilization rate is improving.

[0135] By monitoring the methane concentration in the outlet gas, the methane concentration in the outlet gas can be reliably suppressed even if sudden events occur during the operation of the co-electrolysis system (for example, a temperature drop in the lower lead section).

[0136] Furthermore, this embodiment may be combined with any of the second to fourth embodiments.

[0137] Figure 17 illustrates a schematic configuration of a co-electrolysis system 180 in which the outlet gas concentration measurement unit is combined with the second embodiment. The methane concentration suppression unit 181 is connected electrically or wirelessly to the outlet gas concentration measurement unit 171, the circulating generated gas flow rate adjustment unit 141b, and the power supply unit 121 and / or gas flow rate adjustment unit 122.

[0138] The methane concentration suppression unit 181 receives a signal of the measured value from the outlet gas concentration measurement unit 171, and if the measured value exceeds a threshold, it sends a signal to the circulating generated gas flow rate adjustment unit 141b, the power supply unit 121 and / or the gas flow rate adjustment unit 122, which can control the flow rate of the circulating generated gas, the current value applied to the electrolytic cell and / or the gas flow rate at the cell stack inlet so as to reduce the gas utilization rate.

[0139] Figure 18 illustrates a schematic configuration of a co-electrolysis system 182 in which the outlet gas concentration measuring unit is combined with the third embodiment. The methane concentration suppression unit 183 is connected electrically or wirelessly to the outlet gas concentration measuring unit 171, the raw material gas composition ratio adjustment unit 151, and the power supply unit 121 and / or gas flow rate adjustment unit 122.

[0140] The methane concentration suppression unit 183 receives a signal of the measured value from the outlet gas concentration measurement unit 171, and if the measured value exceeds a threshold, it sends a signal to the raw material gas composition ratio adjustment unit 151, the power supply unit 121 and / or the gas flow rate adjustment unit 122, thereby controlling the raw material gas composition ratio, the flow rate of the circulating generated gas, the current value applied to the electrolytic cell and / or the gas flow rate at the cell stack inlet so as to reduce the gas utilization rate.

[0141] Figure 19 illustrates a schematic configuration of a co-electrolysis system 184 in which the outlet gas concentration measurement unit is combined with the fourth embodiment. The methane concentration suppression unit 185 is connected electrically or wirelessly to the outlet gas concentration measurement unit 171, the CO2 gas circulation unit 162 (circulating CO2 gas flow rate adjustment unit 162b), the raw material gas composition ratio adjustment unit 151, the circulating generated gas flow rate adjustment unit 141b, and the power supply unit 121 and / or gas flow rate adjustment unit 122.

[0142] The methane concentration suppression unit 185 receives a signal of the measured value from the outlet gas concentration measurement unit 171. If the measured value exceeds a threshold, it sends a signal to the CO2 gas circulation unit 162, the raw material gas composition ratio adjustment unit 151, the circulating generated gas flow rate adjustment unit 141b, the power supply unit 121 and / or the gas flow rate adjustment unit 122, which can control the flow rate of the generated gas to be circulated, the raw material gas composition ratio, the flow rate of the circulating CO2 gas, the current value applied to the electrolytic cell and / or the gas flow rate at the cell stack inlet so as to reduce the gas utilization rate.

[0143] [Sixth Embodiment] The co-electrolysis system according to this embodiment differs from the first embodiment in that it includes an electrolytic cell module.

[0144] Figure 20 shows an image diagram of an electrolytic cell module. The electrolytic cell module 191 is a modularized configuration in which the cell stack 101 described in the first embodiment is connected by a common gas supply line 192 and a gas discharge line 193.

[0145] By modularizing multiple cell stacks 101, a compact system can be realized at a low cost.

[0146] This embodiment may be combined with any of the second to fifth embodiments described above.

[0147] In the first to sixth embodiments described above, a cylindrical cross-striped cell stack 101 was described, but the shape of the electrolytic cell stack is not necessarily limited to this, and for example, a flat plate-shaped cell stack may also be used. Although the electrolytic cells are formed on a substrate, the electrodes (hydrogen electrodes or oxygen electrodes) may be formed thickly instead of the substrate, and the substrate may also be used in conjunction with the electrodes. Below, flat plate-shaped, cylindrical flat plate-shaped, and cylindrical vertical-striped electrolytic cell stacks will be described.

[0148] (Flat plate electrolytic cell stack) A planar electrolytic cell stack has multiple planar electrolytic cells. The multiple electrolytic cells are stacked in a direction perpendicular to the surface of the plate with the largest surface area. Separators (interconnectors) are placed between the multiple electrolytic cells.

[0149] Figure 21 is a schematic diagram of the cell stack 200. The electrolytic cell 201 is made up of a flat hydrogen electrode 202, a solid electrolyte membrane 203, and an oxygen electrode 204 stacked in that order. The hydrogen electrode 202 faces the separator 205. The oxygen electrode 204 faces the separator 206. The cell stack 200 may be of the electrolyte-supported, electrode-supported, or metal-supported type.

[0150] In the cell stack 200, the source gas flows between the hydrogen electrode 202 and the separator 205, and the oxidizing gas flows between the oxygen electrode 204 and the separator 206. The source gas and the oxidizing gas are supplied in perpendicular directions. The hydrogen electrode 202 and the separator 205 facing the hydrogen electrode 202 define the outer perimeter of the flow path (product gas flow path) 207 for the product gas generated at the hydrogen electrode 202.

[0151] In the cell stack 200 shown in Figure 21, the hydrogen electrode 202 contains Ni, which has methanation catalytic properties. The separator 205 that defines the product gas flow path 207 generated at the hydrogen electrode 202 may also contain a methanation catalyst such as Ni or Fe. The Ni or Fe included here does not necessarily have to be intended as a methanation catalyst.

[0152] (Cylindrical flat plate electrolytic cell stack) A cylindrical, flat-plate electrolytic cell stack has multiple oval-shaped (rounded rectangle) electrolytic cells. The electrolytic cells are arranged in parallel.

[0153] Figure 22 is a schematic diagram of the cell stack 210. The electrolytic cell 211 includes a hydrogen electrode 212, a solid electrolyte membrane 213, an oxygen electrode 214, an interconnector 215, and a conductive support layer (substrate) 216. The substrate 216 contains a methanation catalyst such as Ni. The Ni included here does not necessarily have to be intended as a methanation catalyst.

[0154] The substrate 216 has an oval shape, and multiple parallel flow passages (raw material gas flow passages) 217 ​​are formed inside through which the raw material gas passes. The product gas generated at the hydrogen electrode 212 by co-electrolysis also passes through these flow passages 217.

[0155] A hydrogen electrode 212, a solid electrolyte membrane 213, and an oxygen electrode 214 are stacked in order on the outer surface of the substrate 216. The interconnector 215 is provided on the opposite side of the substrate 216 from the oxygen electrode 214 so that it can connect to the oxygen electrode 214 located on the outer surface of an adjacent electrolytic cell 211.

[0156] In the electrolytic cell 211, the raw material gas flows inside the substrate 216 (flow passage 217), and the oxidizing gas flows outside the cylinder. The raw material gas and the oxidizing gas flow in parallel in the same direction.

[0157] (Cylindrical striped electrolytic cell stack) A cylindrical striped electrolytic cell stack has multiple cylindrical electrolytic cells. The electrolytic cells are arranged in parallel.

[0158] Figure 23 is a schematic diagram of the cell stack 220. The electrolytic cell 221 includes a cylindrical hydrogen electrode 222, a solid electrolyte membrane 223, an oxygen electrode 224, and an interconnector 225. The hydrogen electrode 222, the solid electrolyte membrane 223, and the oxygen electrode 224 are stacked in order from the outside to the inside. In the circumferential direction of the cylinder, there is a gap between the solid electrolyte membrane 223 and both ends of the hydrogen electrode 222. The interconnector 225 is stacked on the outer circumference of the oxygen electrode 224 to fill this gap.

[0159] In the cylindrical striped cell stack 220, the oxidizing gas flows along the inner circumference of the cylinder, and the raw material gas flows along the outer circumference of the cylinder. The raw material gas and the oxidizing gas flow in parallel in the same direction.

[0160] The hydrogen electrode 222 contains Ni, which has a methanation catalytic effect. In Figure 23, the outer surface of the hydrogen electrode 222 defines a portion of the outer periphery of the flow channel (production gas flow channel) through which the product gas generated at the hydrogen electrode 222 by co-electrolysis flows.

[0161] <Note> The co-electrolytic system and its operating method described in the embodiments above can be understood, for example, as follows.

[0162] A co-electrolysis system (120) according to a first aspect of the present disclosure is a co-electrolysis system comprising an electrolytic cell stack (101) and a control unit (123), wherein the electrolytic cell stack comprises an electrolytic cell (105) in which a hydrogen electrode (109), a solid electrolyte membrane (111), and an oxygen electrode (113) are stacked in order, a raw material gas flow passage (124) through which raw material gases containing H2O and CO2 supplied to the hydrogen electrode flow, and a generated gas flow passage (125) through which generated gases produced at the hydrogen electrode flow, and the control unit comprises a methane concentration suppression unit (135) that reduces the methane concentration in the outlet gas of the generated gas flow passage to below a threshold.

[0163] By incorporating a methane concentration suppression unit, the methane concentration in the generated gas (outlet gas) discharged from the generated gas flow path (electrolytic cell stack) can be suppressed to below a threshold.

[0164] In the co-electrolysis system according to a second aspect of the present disclosure, in the first aspect, the member defining the outer casing of the generated gas flow passage includes a methanation catalyst, and the methane concentration suppression unit controls a gas utilization rate reduction unit (121, 122) that reduces the gas utilization rate, which is the ratio of the amount of raw material gas used for electrolysis to the amount of raw material gas supplied to the raw material gas flow passage.

[0165] By reducing the gas utilization rate using the gas utilization rate reduction unit, the methane concentration in the generated gas (outlet gas) discharged from the generated gas flow path can be kept below a threshold.

[0166] In the third aspect of the present disclosure, the co-electrolysis system is, in the second aspect described above, a gas utilization rate reduction unit which is a gas flow rate adjustment unit (122) that adjusts the gas flow rate supplied to the raw material gas flow passage and / or a power supply unit (121) that applies current to the electrolytic cell.

[0167] The gas utilization rate can be reduced by increasing the gas flow rate supplied to the raw material gas flow path and / or by decreasing the current applied to the electrolytic cell.

[0168] Increasing the gas flow rate at the inlet of the raw material gas flow path increases the speed at which gas molecules pass through the product gas flow path containing the methanation catalyst. This changes the amount of mass transported to the surface of the component containing the methanation catalyst. Specifically, the ratio of the amount of gas that comes into contact with the inner surface of the product gas flow path (the surface of the component defining the outer edge of the product gas flow path) to the total amount of gas supplied to the raw material gas flow path decreases, and the methanation reaction is relatively suppressed.

[0169] By simply setting the current value and / or the raw material gas flow rate per unit area of ​​the inner surface of the generated gas flow path (the surface of the member defining the outer edge of the generated gas flow path), the catalytic methane reaction downstream of the electrolytic cell can be suppressed. Since there is no need to apply additional films to the electrolytic cell stack, defects such as corrosion of the fixture or delamination of layers do not occur.

[0170] A co-electrolysis system (140) according to a fourth aspect of the present disclosure, in the second or third aspect, includes a generated gas circulation path (141a) that guides the generated gas discharged from the generated gas flow path to the inlet side of the raw material gas flow path, and a circulating generated gas flow rate adjustment unit (141b) that adjusts the circulating generated gas flow rate in the generated gas circulation path.

[0171] By circulating the generated gas to the inlet side of the raw material gas flow path (electrolytic cell stack), the methane reaction can be suppressed, similar to increasing the supply gas flow rate of the raw material gas.

[0172] A co-electrolysis system (150) according to a fifth aspect of this disclosure includes a raw material gas composition ratio adjustment unit (151) that sets the composition ratio of H2O and CO2 in the raw material gas supplied to the raw material gas flow passage, in any of the second to fourth aspects described above.

[0173] The amount of H2 and CO in the generated gas can be adjusted by changing the composition ratio of H2O and CO2 in the source gas. This composition ratio can be changed by adjusting the ratio of H2O and CO2 amounts or the inlet pressure, etc.

[0174] By setting the amount of H2O in the raw material gas to a level higher than the stoichiometric ratio, the chemical equilibrium state shifts towards methane suppression, thereby inhibiting the methane reaction.

[0175] A co-electrolysis system (160) according to a sixth aspect of the present disclosure, in any of the second to fifth aspects described above, includes a CO2 separation unit (161) for separating CO2 from the gas discharged from the generated gas flow passage, a CO2 gas circulation path (162a) for guiding the CO2 separated in the CO2 separation unit to the inlet side of the raw material gas flow passage, and a circulating CO2 gas flow rate adjustment unit (162b) for adjusting the gas flow rate in the CO2 gas circulation path.

[0176] Reducing the system gas utilization rate increases the amount of raw material gas components remaining in the gas discharged from the generated gas flow path. By incorporating a CO2 separation unit, unreacted CO2 can be recovered and reused as raw material gas. This increases the CO2 utilization rate. In order to match the amount of CO2 to be reused, it will also be possible to add a larger amount of H2O to the raw gas.

[0177] A co-electrolysis system according to a seventh aspect of this disclosure (170, 180, 182, 184) includes, in any of the second to sixth aspects above, a gas concentration measuring unit (171) for measuring the concentration of CH4 in the outlet gas of the generated gas flow path, and a methane concentration suppression unit (172, 181, 183, 185) receives a signal of the measured value from the gas concentration measuring unit, and when the measured value exceeds a threshold, sends a signal to the gas utilization rate reduction unit to reduce the gas utilization rate.

[0178] The gas concentration measurement unit can monitor the concentration of gas components contained in the outlet gas. By controlling the gas utilization rate reduction unit based on the measurement values ​​from the gas concentration measurement unit, the methane concentration in the outlet gas can be more reliably kept below a threshold even in the event of an unexpected event.

[0179] The co-electrolysis system according to the eighth aspect of this disclosure comprises an electrolytic cell module (191) in which a plurality of electrolytic cell stacks are modularized, as described in any of the second to seventh aspects above.

[0180] By modularizing multiple electrolytic cell stacks, a low-cost and compact co-electrolysis system can be realized.

[0181] A method for operating a co-electrolytic system according to a ninth aspect of the present disclosure is a method for operating a co-electrolytic system comprising an electrolytic cell stack having an electrolytic cell in which a hydrogen electrode, a solid electrolyte, and an oxygen electrode are stacked in order; a raw material gas flow passage for supplying raw material gas containing H2O and CO2 to the hydrogen electrode; and a product gas flow passage for discharging the product gas generated at the hydrogen electrode, wherein the gas utilization rate, which is the ratio of the amount of raw material gas used for electrolysis to the amount of raw material gas supplied to the electrolytic cell stack, is reduced to control the methane concentration in the outlet gas of the product gas flow passage to below a threshold.

[0182] The operation method of the co-electrolytic system according to the tenth aspect of this disclosure, in the ninth aspect described above, reduces the gas utilization rate by increasing the gas flow rate at the inlet of the raw material gas flow passage, decreasing the current value applied to the electrolytic cell, or a combination thereof.

[0183] The operating method of the co-electrolysis system according to the 11th aspect of this disclosure is, in the 9th or 10th aspect described above, to guide the generated gas discharged from the generated gas flow passage to the inlet side of the raw material gas flow passage and supply it to the raw material gas flow passage together with the raw material gas.

[0184] The operating method of the co-electrolysis system according to the twelfth aspect of this disclosure is to set the composition ratio of H2O and CO2 of the raw material gas supplied to the raw material gas flow path to be different from the stoichiometric ratio, in any of the ninth to eleventh aspects described above.

[0185] The method for operating a co-electrolysis system according to the 13th aspect of this disclosure involves separating CO2 from the gas discharged from the generated gas flow passage in any of the 9th to 12th aspects described above, guiding the separated CO2 to the inlet side of the raw material gas flow passage, and using it as part of the raw material gas.

[0186] The operating method for a co-electrolysis system according to the 14th aspect of this disclosure involves monitoring the concentration of CH4 in the outlet gas of the generated gas flow path in any of the 9th to 13th aspects described above, and reducing the gas utilization rate when the measured value exceeds a threshold. [Explanation of symbols]

[0187] 101,200,210,220 Cell Stack (Electrolytic Cell Stack) 103 Base tube (member that defines the outer casing) 104 Inner surface of the base tube 105,201,211,221 electrolytic cells 107,205,215 Interconnectors (Separators) 109,202,212,222 Hydrogen electrodes 111,203,213,223 Solid electrolyte membrane (solid electrolyte) 113,204,214,224 Oxygen electrodes 115 Lead film 120, 140, 150, 160, 170, 180, 182, 184 Co-electrolysis system 121 Power supply section 121a Power Supply Unit 121b Ammeter 122 Gas flow rate adjustment section 123 Control Unit 135, 142, 159, 163, 172, 181, 183, 185 Methane concentration suppression section 124 Raw material gas flow path 125,207 Gas flow path 126 Electrolytic section 127 Upper lead section 128 Lower lead section 129,133 Connection part 130,192 gas supply lines 131 H2O gas source 132 CO2 gas sources 134,193 gas emission lines 136 Flow meter (for cell stack inlet) 141 Gas Circulation Section 141a Generated gas circulation line 141b Circulating generated gas flow rate adjustment unit 151 Raw material gas composition ratio adjustment unit 153 H2O line 153a, 154a Flow meter (for adjusting the raw gas composition ratio) 154 CO2 line 155 Regulated H2O gas supply line 155a, 156a valves (for H2O adjustment) 155b,156b Flow meter (for H2O adjustment) 156 Regulated H2O gas discharge line 157 Regulated CO2 gas supply line 157a, 158a Valves (for H2O adjustment) 157b,158b Flowmeter (for H2O adjustment) 158 Regulated CO2 gas emission line 161 CO2 separation section 162 CO2 gas circulation section 162a Separation CO2 gas circulation line 162b Circulating CO2 gas flow rate adjustment unit 171 Outlet gas concentration measurement unit 216 Conductive support layer (substrate) 217 Raw material gas flow path (generated gas flow path)

Claims

1. A co-electrolytic system comprising an electrolytic cell stack and a control unit, The aforementioned electrolytic cell stack is An electrolytic cell in which a hydrogen electrode, a solid electrolyte, and an oxygen electrode are stacked in that order, H supplied to the hydrogen electrode ２ O and CO ２ A raw material gas flow path through which raw material gas containing the raw material gas flows, A generated gas flow path through which the generated gas produced at the hydrogen electrode flows, Equipped with, The control unit is a co-electrolysis system that includes a methane concentration suppression unit that keeps the methane concentration in the outlet gas of the generated gas flow passage below a threshold.

2. The member defining the outer casing of the generated gas flow passage includes a methane catalyst. The co-electrolysis system according to claim 1, wherein the methane concentration suppression unit controls a gas utilization rate reduction unit that reduces the gas utilization rate, which is the ratio of the amount of raw material gas used for electrolysis to the amount of raw material gas supplied to the raw material gas flow passage.

3. The co-electrolytic system according to claim 2, wherein the gas utilization rate reduction unit is a gas flow rate adjustment unit that adjusts the gas flow rate supplied to the raw material gas flow passage and / or a power supply unit that applies current to the electrolytic cell.

4. The co-electrolysis system according to claim 3, further comprising: a generated gas circulation path that guides the generated gas discharged from the generated gas flow path to the inlet side of the raw material gas flow path; and a circulating generated gas flow rate adjustment unit that adjusts the circulating generated gas flow rate in the generated gas circulation path.

5. H in the raw material gas supplied to the raw material gas flow passage ２ O and CO ２ The co-electrolysis system according to claim 3, further comprising a raw material gas composition ratio adjustment unit for setting the composition ratio of the gases.

6. CO from the gas discharged from the generated gas flow path ２ separating CO ２ separation unit, and the CO ２ CO separated by the CO separation unit ２ CO guiding the CO separated by the separation unit to the inlet side of the raw material gas flow path ２ gas circulation path, and the CO ２ circulation CO gas flow rate adjustment unit for adjusting the gas flow rate in the gas circulation path ２ The co - electrolysis system according to claim 3, comprising a circulation CO gas flow rate adjustment unit.

7. CH in the outlet gas of the generated gas flow passage ４ It is equipped with a gas concentration measuring unit that measures the concentration of, The co-electrolysis system according to claim 2, wherein the methane concentration suppression unit receives a signal of the measured value from the gas concentration measurement unit, and when the measured value exceeds a threshold, sends a signal to the gas utilization rate reduction unit to reduce the gas utilization rate.

8. The co-electrolytic system according to claim 2, comprising an electrolytic cell module in which a plurality of the electrolytic cell stacks are modularized.

9. An electrolytic cell in which a hydrogen electrode, a solid electrolyte, and an oxygen electrode are stacked in that order, H ２ O and CO ２ A raw material gas flow path that supplies raw material gas containing, A generated gas flow passage for discharging the generated gas produced at the hydrogen electrode, A method for operating a co-electrolytic system equipped with an electrolytic cell stack having, A method for operating a co-electrolysis system, which involves reducing the gas utilization rate, which is the ratio of the amount of raw material gas used for electrolysis to the amount of raw material gas supplied to the electrolytic cell stack, in order to control the methane concentration in the outlet gas of the generated gas flow path to below a threshold.

10. A method for operating a co-electrolytic system according to claim 9, wherein the gas utilization rate is reduced by increasing the gas flow rate at the inlet of the raw material gas flow passage, decreasing the current value applied to the electrolytic cell, or a combination thereof.

11. A method for operating a co-electrolysis system according to claim 10, wherein the generated gas discharged from the generated gas flow passage is guided to the inlet side of the raw material gas flow passage and supplied to the raw material gas flow passage together with the raw material gas.

12. H of the raw material gas supplied to the raw material gas flow passage ２ O and CO ２ A method for operating a co-electrolytic system according to claim 10, wherein the composition ratio is set to deviate from the stoichiometric ratio.

13. CO ２ Separate them, The separated CO ２ Guide it towards the inlet side of the raw material gas flow passage, A method for operating a co-electrolysis system according to claim 10, which is used as part of the raw material gas.

14. CH in the outlet gas of the generated gas flow passage ４ Monitor the concentration, A method for operating a co-electrolysis system according to claim 10, wherein the gas utilization rate is reduced when the measured value exceeds a threshold.

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