Co-electrolysis system and method for operating same

Abstract

The purpose of the present invention is to suppress a methanation reaction of a product gas in a co-electrolysis system and a method for operating same. A co-electrolysis system (120) according to the present disclosure comprises an electrolysis cell stack (101) and a control unit (123). The electrolysis cell stack (101) includes: an electrolysis cell (105) in which a hydrogen electrode, a solid electrolyte, and an oxygen electrode are laminated in this order; a raw material gas flow path (124) through which a raw material gas containing H2O and CO2 supplied to the hydrogen electrode flows; and a product gas flow path (125) through which a product gas produced at the hydrogen electrode flows. The control unit (123) includes a methane concentration suppression unit (135) by which the concentration of methane in the off-gas discharged from the product gas flow path (125) is maintained below a threshold.

Description

Co-electrolytic system and its operating method

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

[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 electrolytic 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. Electrolytic cells using solid electrolytes can also be used for ammonia electrolysis.

[0004] Electrolytic cells use high-temperature water vapor and carbon dioxide (CO2). ２ Co-electrolysis is also possible, in which a mixed gas (raw material gas) of ) is supplied, and the hydrogen and carbon dioxide produced by electrolysis are reacted on the electrolytic cell to directly produce carbon monoxide (CO) or hydrocarbon compounds (see Patent Document 1).

[0005] Electrolytic cells using solid electrolytes use CO as a co-electrolytic agent. ２ / H ２ By simultaneously electrolyzing oxygen, the hydrogen necessary for synthesizing FT (Fischer-Tropsch), which is used in the production of synthetic fuels (e-fuels) such as sustainable aviation fuel (SAF), is produced. ２ This allows for the production of CO synthesis gas. This could potentially simplify the system compared to the production process using a reverse shift reaction from water electrolysis.

[0006] Japanese Patent Publication No. 2023-50701

[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 are hydrogen (H２ It acts as a catalyst (methanation catalyst) for the reaction in which methane is synthesized from water vapor (H₂O) and carbon monoxide (CO).

[0008] CO ２ / H ２ The product gas generated by the co - electrolysis of H₂O / CO contains H₂ ２ and CO. Therefore, when the product gas comes into contact with a configuration containing a methanation catalyst, the methanation reaction proceeds.

[0009] In a pressurized operation where integration with Fischer - Tropsch synthesis is easy, due to the action of the methanation catalyst, a methanation reaction occurs, methane is generated, and there is a problem that the yield of H₂ / CO decreases. ２ / CO decreases.

[0010] To prevent the methanation reaction from occurring, the methanation catalyst may be removed from the hydrogen electrode or the metal support. However, Ni and Fe have purposes other than being a catalyst, such as controlling the linear expansion coefficient and adjusting the porosity, and thus cannot be easily removed.

[0011] The present disclosure has been made in view of such circumstances, and an object thereof is to provide a co - electrolysis system capable of suppressing the methanation reaction of the product gas and a method for operating the same.

[0012] To solve the above problems, the co - electrolysis system and the method for operating the same according to the present disclosure employ the following means.

[0013] The present disclosure provides a co - electrolysis system including an electrolysis cell stack and a control unit. The electrolysis cell stack includes an electrolysis cell in which a hydrogen electrode, a solid electrolyte, and an oxygen electrode are laminated in this order, a raw material gas flow path through which a raw material gas containing H₂O ２ and CO ２ flows, and a product gas flow path through which the product gas generated at the hydrogen electrode flows. The control unit includes a methane concentration suppression unit that makes the methane concentration in the outlet gas of the product gas flow path equal to or less than a threshold value.

[0014] The present disclosure provides an electrolysis cell in which a hydrogen electrode, a solid electrolyte, and an oxygen electrode are laminated in this order, and H₂O ２ and CO ２A method for operating a co-electrolytic system comprising an electrolytic cell stack having a raw material gas flow passage for supplying raw material gas containing and a product gas flow passage for discharging 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.

[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.

[0017] This is a partial cross-sectional view showing an example of an electrolytic cell stack according to the present disclosure. This is a schematic configuration diagram of a co-electrolytic system according to the first embodiment. This is a diagram showing 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 in the first embodiment. This is a diagram showing an example of the gas utilization rate setting flow in the first embodiment. This is a schematic configuration diagram of a co-electrolytic system according to the second embodiment. This is a diagram showing 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 in the second embodiment. This is a diagram showing an example of the gas utilization rate setting flow in the second embodiment. This is a schematic configuration diagram of a co-electrolytic system according to the third embodiment. This is a schematic configuration diagram showing an example of a raw material gas composition ratio adjustment unit. This is a diagram showing 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 in the third embodiment. This is a diagram showing an example of the gas utilization rate setting flow in the third embodiment. This is a schematic configuration diagram of a co-electrolytic system according to the fourth embodiment. This figure 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 the circulating generated gas flow rate (set value 3) is set to 3. This figure 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 in the fourth embodiment when the circulating generated gas flow rate (set value 4) is set to 4. This figure shows an example of the setting flow for the gas utilization rate in the fourth embodiment. This is a schematic diagram of the co-electrolysis system according to the fifth embodiment. This is a schematic diagram of the co-electrolysis system according to the first modified example of the fifth embodiment. This is a schematic diagram of the co-electrolysis system according to the second modified example of the fifth embodiment. This is a schematic diagram of the co-electrolysis system according to the third modified example of the fifth embodiment. This is an image diagram of the electrolytic cell module according to the sixth embodiment. This is a schematic diagram of a modified example of the electrolytic cell stack. This is a schematic diagram of a modified example of the electrolytic cell stack. This is a schematic diagram of a modified example of the electrolytic cell stack.

[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. In this embodiment, components that achieve similar effects in both the vertical and horizontal directions may not necessarily be limited to the vertically up and down directions on the paper surface, but may, for example, 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 type 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 an interconnector 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. The cell stack 101 includes 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 of a base tube is not limited to this; for example, the hydrogen electrode 109 may be made thicker and serve as the base tube as well. In this embodiment, the base tube 103 is described as being cylindrical, but 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, for example, CaO-stabilized ZrO ２ (CSZ), a mixture of CSZ and nickel oxide (NiO) (CSZ + NiO), or Y ２ O ３Stabilized ZrO ２ (YSZ), or MgAl ２ O ４ These are the main components. The base tube 103 supports the electrolytic cell 105, the interconnector 107, and the lead film 115, and diffuses the 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 process moves 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 a slurry.

[0027] The oxygen electrode 113 is, for example, LaSrMnO ３ LaCoO oxides, or LaCoO ３Composed of a system oxide, the oxygen electrode 113 is coated using screen printing or a dispenser after being formed into a slurry. 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, which exhibits high ionic conductivity, and the oxygen electrode layer (oxygen electrode conductive layer) on the oxygen electrode intermediate layer may be Sr and Ca-doped LaMnO ３ It may be composed of perovskite-type oxides such as the following.

[0028] Interconnector 107 is SrTiO ３ M systems, etc. １－ｘ L ｘ TiO ３ (M is an alkaline earth metal element, L is a lanthanide element) and lanthanum chromite (LaCrO ３ It is composed of a conductive perovskite-type oxide represented by ), and the slurry is screen printed. The interconnector 107 is a dense film that prevents the raw material gas and the oxidizing gas from mixing. The interconnector 107 has stable durability and electronic conductivity in 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, and connects 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 SrTiO are suitable. ３ M systems, etc. １－ｘ L ｘ TiO ３ (M is an alkaline earth metal element, 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 a plurality of electrolytic cells 105 connected in series by an interconnector 107. 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) is water vapor (H ２ O) and carbon dioxide (CO) ２ ) is included. The raw material gas may contain hydrogen.

[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 left to right on the page 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, the water vapor contained in the raw material gas at the hydrogen electrode 109 receives electrons and undergoes electrolysis, producing hydrogen molecules and oxygen ions (O). ２－ ) is produced (see reaction equation (1) below). The carbon dioxide contained in the raw material gas accepts electrons and is electrolyzed to produce carbon monoxide molecules and oxygen ions (O). ２－ ) is produced (see reaction equation (2) below). H ２ O + 2e － →H ２ +O ２－ ... (1) CO ２ +2e － →CO+O ２－ ... (2)

[0034] Meanwhile, 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 (see reaction equation (3) below). The generated oxygen is discharged to the outside along with the oxidizing gas supplied to the oxygen electrode 113. 2O ２－ →O ２ +4e － 　　　　 ... (3)

[0035] Oxidizing gases do not directly participate in the electrolytic reaction, but they supply the heat necessary for the electrolytic reaction (endothermic reaction) and discharge the waste heat from the electrolytic reaction.

[0036] The gas produced at the hydrogen electrode by electrolysis (the product gas), along with the unelectrolyzed raw material gas that did not contribute to the electrolytic reaction, is discharged to the outside of the electrolytic cell stack (gas discharge pipe) through the product gas flow path. From this point forward, the gas flowing through the product gas flow path is also referred to as the "product gas," even if it contains unelectrolyzed 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 (H) generated at the hydrogen electrode 109 by electrolysis ２ CO may undergo side reactions as it flows through the base tube 103 (see reaction equations (4) and (5) below). CO + H ２ O→CO ２ +H ２ ... (4) CO ２ +3H ２ →CH ４ +H ２ O... (5)

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

[0040] [First Embodiment] (Co-electrolytic System) Figure 2 is a schematic diagram of the co-electrolytic system according to this embodiment. The co-electrolytic 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 consists of an electrolytic cell 105 and H supplied to the hydrogen electrode. ２ O and CO ２ The system includes a raw material gas flow passage 124 through which raw material gas containing the raw material flows, and a product gas flow passage 125 through which product gas generated at the hydrogen electrode flows.

[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 portion 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 a 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 121b and an ammeter 121a. The power supply body 121b 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 121a 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 part 129 and the gas supply line 130. The gas flow rate adjustment unit 122 is H ２ H from gas source 131 ２ O gas and CO ２ CO from gas source 132 ２ The gas can be received and the flow rate of the raw material gas supplied into the cell stack 101 can be adjusted.

[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 receives methane (CH4) from the outlet gas of the production gas flow passage 125. ４ A gas concentration measuring device (not shown) may be provided to measure the concentration of the gas.

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

[0052] The control unit 123 is composed 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 on the ROM or other storage medium, provided in a state where it is stored on 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, semiconductor memory, etc.

[0053] The methane concentration suppression unit 135 is connected to the power supply unit 121b and / or the gas flow rate adjustment unit 122 of the power supply unit 121, either electrically or via wireless communication. The methane concentration suppression unit 135 can send signals to the power supply unit 121b and / or the 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 so as 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 co-electrolytic cell stack) In this embodiment, H ２ O and CO ２ A raw material gas containing H 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. ２ O and CO ２ The ratio of these is kept constant. The raw material gas is electrolyzed in electrolytic cell 105, H ２ And CO is produced. H produced by electrolysis ２ The generated gas, including CO, 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 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) Step 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 121a 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) Step to adjust the 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) Step to confirm methane concentration (S5_Evaluation 3) 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 (S1) to (S5) 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.

[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 ​​instead of actual measurements. Steps (A) through (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-electrolytic System) Figure 5 shows a schematic diagram of the co-electrolytic 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 in 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] (Operation method of the co-electrolysis system) In the operation method of the co-electrolysis system according to this embodiment, H ２ O and CO ２ A raw material gas containing H 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, H ２ And CO is produced. H produced by electrolysis ２ The generated gas, including CO, is discharged through the generated gas flow passage to the gas discharge line 134. A portion of the generated gas (outlet gas) discharged to the gas discharge line 134 is circulated through the generated gas circulation line 141a to the (raw material) gas inlet side of the cell stack 101, 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 set value 1 and set value 2 for the circulating generated gas flow rate. Set value 1 is a different flow rate value from set value 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 concentration (F) required to keep the methane concentration in the outlet gas below the threshold C will decrease. ２ , F ２ The ') will be different.

[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) a step of adjusting the current value, (2B) a step of adjusting the gas flow rate, and (2C) a 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) Step 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 step (2C).

[0078] (2C) Step 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 also 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 is started up, 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] Fig. 8 shows a schematic configuration diagram of the co-electrolysis system according to this embodiment. For ease of viewing, Fig. 8 shows a simplified display of the cell stack and its peripheral parts. It should be understood that even though they are simplified, their configurations are the same as those in the first embodiment.

[0082] The co-electrolysis system 150 according to this embodiment includes a raw material gas composition ratio adjuster 151 in addition to the co-electrolysis system 120 of the first embodiment. The same reference numerals are used for the common configurations as in the first embodiment.

[0083] The raw material gas composition ratio adjuster 151 is a configuration for adjusting the composition ratio of H ２ O and CO ２ in the raw material gas. In Fig. 8, the raw material gas composition ratio adjuster 151 is connected upstream of the gas flow of the gas flow adjuster 122.

[0084] Fig. 9 shows an example of the raw material gas composition ratio adjuster 151. The raw material gas composition ratio adjuster 151 in Fig. 9 has an H ２ O line 153 and a CO ２ line 154.

[0085] The H ２ O line 153 is connected at one end to an H ２ O gas source (not shown) and at the other end to a raw material gas flow adjuster (not shown). A flow meter 153a for measuring the flow rate of the H ２ O gas flowing in the H ２ O line 153 is provided. ２

[0086] The H ２ O line 153 is connected to an adjustment H ２ O gas supply line 155 for introducing the adjustment H ２ O gas into the H ２ O line 153 and an adjustment H ２ O gas discharge line 156 for discharging H ２ O from the H ２ O line 153. The adjustment H ２ O gas supply line 155 and the adjustment H ２The O gas discharge line 156 is provided with valves (155a, 156a) and flow meters (155b, 156b) for measuring the flow rate of the gas flowing in the line, respectively.

[0087] CO ２ One end of the line 154 is connected to a CO ２ gas source (not shown), and the other end is connected to a raw material gas flow rate adjustment unit (not shown). CO ２ The line 154 is provided with a flow meter 154a for measuring the flow rate of the CO ２ gas flowing in the line. ２

[0088] CO ２ The line 154 is provided with an adjustment CO ２ gas supply line 157 for introducing the adjustment CO ２ gas into the line 154, and an adjustment CO ２ gas discharge line 158 for discharging the CO ２ from the line 154. The adjustment CO ２ gas supply line 157 and the adjustment CO ２ gas discharge line 158 are connected. The adjustment CO ２ gas supply line 157 and the adjustment CO ２ gas discharge line 158 are provided with valves (157a, 158a) and flow meters (157b, 158b) for measuring the flow rate of the gas flowing in the line, respectively.

[0089] For example, when increasing the H ２ O gas composition in the raw material gas, the valve 155a of the adjustment H ２ O gas supply line 155 is opened to introduce the adjustment H ２ O gas with a predetermined gas flow rate into the H ２ O line 153, or the valve of the adjustment CO ２ gas discharge line 158 is opened to exhaust the adjustment CO ２ gas with a predetermined gas flow rate from the CO ２ line 154. That's all.

[0090] Note that the raw material gas composition ratio adjustment unit 151 is not limited to FIG. 9. For example, the gas flow rate adjustment unit 122 may be provided with a function of adjusting the raw material gas composition ratio. [[ID=I55]]

[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 wirelessly. The methane concentration suppression unit 159 sends a signal to the raw material gas composition ratio adjustment unit 151, which controls the H in the raw material gas. ２ O and CO ２ In addition to adjusting the composition ratio, the gas flow rate at the cell stack inlet and the current value applied to the electrolytic cell can be controlled to reduce the gas utilization rate, similar to the first embodiment.

[0092] (Operation method of the co-electrolysis system) In the operation method of the co-electrolysis system according to this embodiment, H in the raw material gas ２ O and CO ２ After adjusting the composition ratio, 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, H ２ And CO is produced. H produced by electrolysis ２ The generated gas, including CO, is discharged through the generated gas flow passage to the gas discharge line 134.

[0093] H ２ O and CO ２ By changing the composition ratio, H can be obtained as a product gas. ２ Alternatively, the amount of CO can be adjusted. ２ By adding a large amount of oxygen, the chemical equilibrium can be shifted in a way that suppresses the methane reaction.

[0094] If the system allows for arbitrary pressure settings, changing the pressure will allow the H in the source gas to be reduced. ２ O and CO ２ The composition ratio may be adjusted.

[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 H ２ O and CO ２This figure shows the relationship between the gas flow rate per catalyst area ((moles / second) × (1 / catalyst area)) and the methane concentration in the cell stack outlet gas when different composition ratios of raw material gases are used. In the figure, the horizontal axis is the gas flow rate per catalyst area, and the vertical axis is the methane concentration. Cases A to C are H ２ O and CO ２ These are raw material gases with different composition ratios.

[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 concentration (F) required to keep the methane concentration in the outlet gas below the threshold C is different. ３ , F ３ ', F ３ The '') will be different.

[0098] Therefore, the gas utilization rate is the amount of H in the raw gas. ２ O and CO ２ The gas flow rate and current values ​​at the cell stack inlet must be set while taking the composition ratio into consideration.

[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) Step to adjust gas composition ratio and gas flow rate (S33) First, H in the raw material gas is added to the raw material gas composition ratio adjustment section. ２ O and CO ２Input the composition ratio. (S34) Next, input a predetermined flow rate value to 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 whether the measured value matches the input value to the raw material gas flow rate adjustment unit. The H of the raw material gas at the cell stack inlet ２ O and CO ２ The composition ratio is measured using a gas concentration measuring device, and it is confirmed whether the measured value matches the value entered into the raw gas composition ratio adjustment unit. If each measured value does not match the input value, the process returns to (S33) and the composition ratio is adjusted. Steps (S33) to (S34) are repeated until the measured values ​​match the input values. After it is confirmed that the measured values ​​match the input values, the process moves to the next step (3C).

[0102] (3C) Step 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] To achieve the desired gas utilization rate (methane concentration in the outlet gas), H in the source gas ２ O and CO ２ The setting flow for the composition ratio, the gas flow rate at the cell stack inlet, or the current value applied to the electrolytic cell is not limited to 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-electrolytic System) Figure 12 shows a schematic diagram of the co-electrolytic system according to this embodiment. For clarity, the cell stack and its surrounding area are simplified in Figure 12. Although simplified, their configurations should be understood to be the same as in the first embodiment.

[0106] The co-electrolysis system 160 according to this embodiment 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, and CO ２ Separation section 161 and CO ２ It further includes a 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] CO ２ The separation unit 161 is connected to the gas discharge line 134 and separates CO from the gas discharged from the cell stack outlet. ２ This configuration separates the two.

[0109] CO ２ The gas circulation section 162 is CO ２ CO separated in the separation unit 161 ２ (separated CO ２ This configuration guides the CO2 to the raw material gas flow path (cell stack inlet). ２ The gas circulation section 162 separates CO ２ Gas circulation line 162a and circulating CO ２ It is equipped with a gas flow rate adjustment unit 162b.

[0110] Separated CO ２ The gas circulation line 162a has one end connected to CO ２ The other end of the separation unit 161 is connected to the raw material gas composition ratio adjustment unit 151. Circulating CO ２ The gas flow rate adjustment unit 162b separates CO ２ It is installed in the middle of the gas circulation line 162a, and separates CO ２ Separated CO2 flowing through the gas circulation line ２ The gas flow rate can be adjusted.

[0111] The methane concentration suppression unit 163 controls 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 CO2 via electrical or wireless communication. ２ Gas circulation section 162 (circulating CO ２ It is connected to the gas flow rate adjustment unit 162b). The methane concentration suppression unit 163 is CO ２ A signal is sent to the gas circulation unit 162, separating CO ２ CO2 separation in gas circulation line 162a ２ In addition to controlling the gas flow rate, the flow rate of the circulating generated gas and the H content in the raw material gas are controlled in the same manner as in the first to third embodiments, so as to reduce the gas utilization rate. ２ O and CO ２ The composition ratio, gas flow rate at the cell stack inlet, and current value applied to the electrolytic cell can be controlled.

[0112] (Operation method of the co-electrolysis system) In the operation method of the co-electrolysis system according to this embodiment, H in the raw material gas ２ O and CO ２ After adjusting the composition ratio, 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, H ２ And CO is produced. H produced by electrolysis ２ The generated gas, including CO, is discharged through the generated gas flow passage to the gas discharge line 134. A portion of the generated gas (outlet gas) discharged to the gas discharge line 134 is recirculated to the inlet side of the cell stack 101 via the generated gas circulation line 141a.

[0113] The remaining generated gas (outlet gas) is CO ２ The CO is guided to the separation section 161. ２ The gas is separated, and the separated CO ２ Separating the gas, CO ２ The circulated CO is returned to the inlet side of the cell stack 101 via the gas circulation line 162a. ２ The gas is used as part of the raw material gas. CO is circulated in the raw material gas composition ratio adjustment unit 151. ２ Gas, H ２ H from gas source 131 ２ O gas and CO２ CO from gas source 132 ２ After adjusting the gas to a predetermined composition ratio, it is introduced into the cell stack at a predetermined flow rate along with the circulating generated gas.

[0114] recycled CO2 ２ For example, the gas is CO2 used for adjustment in the raw material gas composition ratio adjustment unit 151. ２ It can be used as a gas.

[0115] Although not shown in Figure 12, excess H generated from components outside of this co-electrolysis system ２ O gas, separate CO ２ Like the gas, it may be reused in the raw material gas composition ratio adjustment unit 151.

[0116] According to this embodiment, circulating CO ２ Because the gas flow rate of the 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, the methane reaction can be suppressed more effectively than when each of these processes is performed individually.

[0117] The generated gas contains CO ２ By separating and reusing the gaseous components used as raw materials, the gaseous components 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 H ２ O and CO ２ This figure 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 and circulating a portion of the generated gas. Figure 13 is a graph showing the case when raw material gases with composition ratios from Case A to Case C are used and a portion of the generated gas is circulated at a flow rate of set value 3. Figure 14 is a graph showing the case when raw material gases with composition ratios from Case D to Case F are used and the generated gas is circulated at a flow rate of set value 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 concentration (F) required to keep the methane concentration in the outlet gas below the threshold C is ４ , F ４ ', F ４ '', F ５ , F ５ ', F ５ The result will vary depending on the composition ratio of the source gas.

[0121] Therefore, the gas utilization rate is determined by the flow rate of the circulating generated gas and the amount of H in the raw gas. ２ O and CO ２ The gas flow rate and current values ​​at the cell stack inlet must be set while taking the composition ratio into consideration.

[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) Steps 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, the circulating CO ２ A predetermined flow rate value is input to the gas flow rate adjustment unit. (S45) Next, the raw material gas composition ratio adjustment unit is input to the H in the raw material gas. ２ O and CO ２ Input the composition ratio. (S46) Next, input the predetermined flow rate value into the raw material gas flow rate adjustment unit. (S47_Evaluation 42) Flow rate of circulating generated gas in the generated gas circulation line, separated CO ２Circulating CO2 in gas circulation lines ２ The gas flow rate and the gas flow rate at the cell stack inlet are measured with a flow meter, and these measurements are used to adjust the flow rate of the circulating generated gas, and the circulating CO ２ Verify that the input values ​​to the gas flow rate adjustment unit and the raw material gas flow rate adjustment unit are consistent. The H of the raw material gas at the cell stack inlet. ２ O and CO ２ The composition ratio is measured using a gas concentration measuring device, and it is confirmed whether the measured value matches the value input to the raw gas composition ratio adjustment unit. If each measured value does not match the input value, the process returns to (S43) and the flow rate value of the circulating generated gas is adjusted. Steps (S43) to (S47) are repeated until the measured values ​​match the input values. After it is confirmed that the measured values ​​match the input values, the process moves to the next step (4C).

[0125] (4C) Step 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] To achieve the desired gas utilization rate (methane concentration in the outlet gas), the flow rate of the circulating generated gas and the circulating CO2 ２ Gas flow rate, H in the raw gas ２ O and CO ２ The setting flow for the composition ratio, the gas flow rate at the cell stack inlet, and the current value applied to the electrolytic cell 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 is started up, 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-electrolytic System) Figure 16 shows a schematic diagram of the co-electrolytic system according to this embodiment. For clarity, the cell stack and its surrounding area are simplified in Figure 16. Although simplified, their configurations 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 measures CO and / or H ２ and / or H ２ O and / or CO ２ The system may also measure the concentration of the gas and monitor the gas utilization rate accordingly.

[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 CO and H ２ , H ２ O, CO ２ and CH ４ Because it can measure gas composition (%) with a single instrument, it is suitable for measuring gas composition.

[0132] The methane concentration suppression unit 172 is connected electrically or wirelessly to the outlet gas concentration measurement 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 measurement 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] (Operation method of the co-electrolysis system) In the operation method of the co-electrolysis system according to this embodiment, H ２ O and CO２ A raw material gas containing H 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, H ２ And CO is produced. H produced by electrolysis ２ The generated gas, including CO, is discharged through the generated gas flow passage to the gas discharge line 134.

[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 via wireless communication 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, and 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 171 is combined with the third embodiment. The methane concentration suppression unit 183 is connected electrically or via wireless communication 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 measuring unit 171 is combined with the fourth embodiment. The methane concentration suppression unit 185 communicates with the outlet gas concentration measuring unit 171 electrically or wirelessly, and CO ２ Gas circulation section 162 (circulating CO ２ It is connected to the 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 the gas flow rate adjustment unit 122.

[0142] The methane concentration suppression unit 185 receives a measured value signal from the outlet gas concentration measurement unit 171. If the measured value exceeds the threshold, CO ２ Signals are sent to the 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 to reduce the gas utilization rate, the flow rate of the generated gas to be circulated, the raw material gas composition ratio, and the amount of CO to be circulated. ２ The gas flow rate, the current applied to the electrolytic cell, and / or the gas flow rate at the cell stack inlet can be controlled.

[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] Furthermore, this embodiment may be combined with any of the first to fifth embodiments described above.

[0147] In the first to sixth embodiments described above, a cylindrical transverse 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 type 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 type, cylindrical flat plate type, and cylindrical vertical striped type electrolytic cell stacks will be described.

[0148] (Planar 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 activity. 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 flow passages (raw material gas flow passages) 217 ​​through which the raw material gas passes are formed in parallel inside. 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 be connected 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 part of the outer edge 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 above-described embodiment can be understood, for example, as follows.

[0162] A co-electrolytic system (120) according to a first aspect of the present disclosure is a co-electrolytic 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, and H supplied to the hydrogen electrode ２ O and CO ２ The system comprises a raw material gas flow passage (124) through which a raw material gas containing the above flows, and a generated gas flow passage (125) through which the generated gas produced at the hydrogen electrode flows. The control unit includes 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 value.

[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 gas utilization rate reduction units (121, 122) that reduce 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-electrolytic system is, in the second aspect, 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 material 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 jig corrosion or layer delamination 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, in any of the second to fourth aspects, provides H in the raw material gas supplied to the raw material gas flow passage. ２ O and CO ２ It is equipped with a raw material gas composition ratio adjustment unit (151) for setting the composition ratio of the gases.

[0173] H in the source gas ２ O and CO ２ By changing the composition ratio, the H in the generated gas ２ The amount of CO can be adjusted. The composition ratio is H ２ O and CO ２ This can be changed by adjusting the ratio of the amount or the inlet pressure, etc.

[0174] H in the source gas ２ Setting the amount of oxygen to a level greater than the stoichiometric ratio will shift the chemical equilibrium towards suppressing methane production, thereby inhibiting the methane reaction.

[0175] The co-electrolysis system (160) according to the sixth aspect of this disclosure, in any of the second to fifth aspects, generates CO from the gas discharged from the generated gas flow passage. ２ CO2 separation ２ Separation section (161) and the CO ２ CO separated in the separation section ２ CO2 is guided towards the inlet side of the aforementioned raw material gas flow passage. ２ Gas circulation path (162a) and the CO ２ Circulating CO2 to regulate gas flow rate in the gas circulation path ２ It is equipped with a gas flow rate adjustment unit (162b).

[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. ２ By including a separation unit, unreacted CO ２ CO2 can be recovered and reused as a raw material gas. ２ This can increase the utilization rate of CO2. ２ In proportion to the quantity, H is added to the raw material gas. ２ It also becomes possible to include many O's.

[0177] The co-electrolysis system according to the seventh aspect of this disclosure (170, 180, 182, 184) is, in any of the second to sixth aspects, the CH in the outlet gas of the generated gas flow path. ４ The system includes a gas concentration measuring unit (171) for measuring the concentration of methane, and the 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, it 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-electrolytic 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 the ninth aspect of this disclosure comprises an electrolytic cell in which a hydrogen electrode, a solid electrolyte, and an oxygen electrode are stacked in order, and H ２ O and CO ２A method for operating a co-electrolytic system comprising an electrolytic cell stack having a raw material gas flow passage for supplying a raw material gas containing a certain substance, 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 eleventh aspect of this disclosure is, in the ninth or tenth 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, in any of the ninth to eleventh aspects, the H of the raw material gas supplied to the raw material gas flow passage. ２ O and CO ２ The composition ratio is set by deviating from the stoichiometric ratio.

[0185] The operating method of the co-electrolysis system according to the 13th aspect of this disclosure is, in any of the 9th to 12th aspects above, CO2 is generated from the gas discharged from the generated gas flow path. ２ Separate the CO2 ２ It is guided to the inlet side of the raw material gas flow passage and used as part of the raw material gas.

[0186] The operating method of the co-electrolysis system according to the 14th aspect of this disclosure is, in any of the 9th to 13th aspects, CH in the outlet gas of the generated gas flow path. ４ The concentration is monitored, and if the measured value exceeds a threshold, the gas utilization rate is reduced.

[0187] 101, 200, 210, 220 Cell stack (electrolytic cell stack) 103 Base tube (member defining the outer casing) 104 Inner surface of base tube 105, 201, 211, 221 Electrolytic cell 107, 205, 215 Interconnector (separator) 109, 202, 212, 222 Hydrogen electrode 111, 203, 213, 223 Solid electrolyte membrane (solid electrolyte) 113, 204, 214, 224 Oxygen electrode 115 Lead membrane 120, 140, 150, 160, 170, 180, 182, 184 Co-electrolytic system 121 Power supply unit 121a Ammeter 121b Power supply unit 122 Gas flow rate adjustment unit 123 Control unit 135, 142, 159, 163, 172, 181, 183, 185 Methane concentration suppression section 124 Raw material gas flow passage 125, 207 Generated gas flow passage 126 Electrolysis section 127 Upper lead section 128 Lower lead section 129, 133 Connection section 130, 192 Gas supply line 131 H ２ O gas source 132 CO ２ Gas source 134, 193 Gas discharge line 136 Flow meter (for cell stack inlet) 141 Generated gas circulation section 141a Generated gas circulation line 141b Circulated generated gas flow rate adjustment section 151 Raw material gas composition ratio adjustment section 153 H ２ O-line 153a, 154a Flow meter (for adjusting raw gas composition ratio) 154 CO ２ Line 155 Adjustment H ２ O gas supply line 155a, 156a valve (H ２ (for O adjustment) 155b, 156b Flowmeter (H ２ (for O adjustment) 156 Adjustment H ２ O gas discharge line 157 Adjusted CO ２ Gas supply line 157a, 158a Valve (H ２ (for O adjustment) 157b, 158b Flow meter (H ２ (for O adjustment) 158 Adjustment CO ２ Gas discharge line 161 CO ２ Separation section 162 CO ２ Gas circulation section 162a Separation CO ２ Gas circulation line 162b Circulating CO ２Gas flow rate adjustment unit 171 Outlet gas concentration measurement unit 191 Electrolytic cell module 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, 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, and H supplied to the hydrogen electrode ２ O and CO ２ A co-electrolysis system comprising: a raw material gas flow passage through which a raw material gas containing a substance flows; and a product gas flow passage through which a product gas generated at the hydrogen electrode flows, wherein the control unit comprises a methane concentration suppression unit that reduces the methane concentration in the outlet gas of the product gas flow passage to below a threshold.

2. The co-electrolysis system according to claim 1, wherein 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 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 path. 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 separation unit for separating ２ CO, and the CO ２ separated by the CO separation unit ２ is led to the inlet side of the raw material gas flow path by the CO 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:

7. CH in the outlet gas of the generated gas flow path ４ The co-electrolysis system according to claim 2, comprising a gas concentration measuring unit for measuring the concentration of methane, wherein the methane concentration suppression unit 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.

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 order, and H ２ O and CO ２ A method for operating a co-electrolytic system comprising an electrolytic cell stack having a raw material gas flow passage for supplying a raw material gas containing and a product gas flow passage for discharging the product gas generated at the hydrogen electrode, wherein the method for operating a 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.

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. The H of the raw material gas supplied to the raw material gas flow path. 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. CO2 from the gas discharged from the generated gas flow path. ２ Separate the CO2 ２ A method for operating a co-electrolysis system according to claim 10, wherein the gas is guided to the inlet side of the raw material gas flow passage and used as part of the raw material gas.

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

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