Co-electrolytic methanation apparatus and method of operating the co-electrolytic methanation apparatus

Abstract

To provide a co-electrolytic methanation apparatus and its operating method that stabilizes the gas composition at the process outlet and maintains a high level of heat per unit volume. [Solution] A co-electrolytic methanation apparatus 100 comprises a co-electrolytic unit 5 for electrolyzing water vapor and carbon dioxide, a water vapor supply unit 2, a carbon dioxide supply unit 3, a flow rate control means for controlling the flow rates of water vapor and carbon dioxide, a flow rate ratio calculation means 9 for calculating the supply gas utilization rate α, a water separation / compression unit 6, a methanation reaction unit 7, a separation unit, and a control means. The carbon dioxide supply unit 3 includes a first supply path 3a for supplying carbon dioxide to the cathode electrode 53, a second supply path 3b for supplying carbon dioxide to the methanation reaction unit 7, and a branching section 3c. The flow rate ratio calculation means 9 calculates the branching ratio β, which is the proportion of carbon dioxide supplied via the first supply path 3a, and the ratio Ri, which is the ratio of the flow rate of water vapor to the flow rate of carbon dioxide. The flow rate control means controls the amount of carbon dioxide supplied via the first supply path 3a or the second supply path 3b to change the branching ratio β.

Description

[Technical Field]

[0001] The present invention relates to an SOEC co-electrolytic methanation apparatus and a method for operating the co-electrolytic methanation apparatus. [Background technology]

[0002] A known method involves using a solid oxide electrolysis cell (SOEC) stack to co-electrolyze carbon dioxide and water vapor to produce carbon monoxide and hydrogen, and then using a methanation device installed downstream to produce methane from the carbon monoxide and hydrogen.

[0003] In the SOEC electrolysis unit, the following gases are mainly generated by electricity. H2O → H2+ 1 / 2O2···(1) CO2 → CO + 1 / 2O2···(2) In a methanation device, CO + 3H2 → CH4 + H2O (3) CO2+ 4H2→ CH4+ 2H2O···(4) By carrying out the methanation reaction, methane is produced using electricity throughout the entire process. Since both reactions (3) and (4) above in the methanation apparatus are exothermic reactions, it is expected that a highly efficient methane production system can be realized by recovering this reaction heat and using it to generate steam supplied to the SOEC electrolysis unit.

[0004] In co-electrolysis, compared to steam electrolysis, the heat required for steam generation within the SOEC electrolytic device is more effectively met by the heat recovered from the methanation reaction, potentially approaching zero in ideal cases. Therefore, co-electrolytic methanation has the potential to achieve higher overall energy conversion efficiency than steam electrolytic methanation. On the other hand, in co-electrolytic methanation, it is difficult to set the optimal supply ratio of carbon monoxide, carbon dioxide, and hydrogen at the methanation reactor inlet for the methanation reaction. Therefore, there are still challenges in controlling the methane concentration at the methanation reactor outlet and suppressing fluctuations in heat.

[0005] Patent Document 1 discloses a gas production system in which an electrolytic reaction section, a reverse water-gas shift reaction section, and a hydrocarbon synthesis reaction section are connected. In this configuration, carbon dioxide is supplied to the upstream side of each of the electrolytic reaction section, the reverse water-gas shift reaction section, and the hydrocarbon synthesis reaction section, and individual control means are provided. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2023-50700 [Overview of the project] [Problems that the invention aims to solve]

[0007] The invention disclosed in Patent Document 1 primarily aims to increase the amount of carbon monoxide in the hydrocarbon synthesis reaction section by supplying carbon dioxide to promote a reverse shift reaction when the amount of carbon monoxide generated in the electrolytic reaction section is less than expected, thereby increasing the amount of carbon monoxide in the gas. Therefore, there are still challenges in maintaining a high methane concentration at the process outlet and stabilizing the methane concentration.

[0008] The present invention has been made in view of the above-mentioned problems, and its objective is to provide a co-electrolytic methanation apparatus and a method for operating a co-electrolytic methanation apparatus that can stabilize the gas composition at the process outlet and maintain a high level of heat per unit volume. [Means for solving the problem]

[0009] The characteristic configuration of the co-electrolytic methanation apparatus according to the present invention for achieving the above objective is: A co-electrolytic unit that electrolyzes water vapor and carbon dioxide to produce a mixed gas consisting of an electrolytic gas mainly composed of hydrogen and carbon monoxide, and unelectrolyzed water vapor and carbon dioxide. A water vapor supply unit that supplies the water vapor to the cathode electrode of the aforementioned electrolytic unit, A carbon dioxide supply unit that supplies the carbon dioxide to the cathode electrode, Flow rate control means for controlling the flow rates of the water vapor and carbon dioxide supplied by the water vapor supply unit and the carbon dioxide supply unit, A flow rate ratio calculation means that calculates the ratio of the total flow rate of hydrogen and carbon monoxide produced by electrolysis in the co-electrolysis unit to the total flow rate of water vapor and carbon dioxide supplied to the cathode electrode as the supply gas utilization rate (α), A water separation and compression unit that cools the aforementioned synthesis gas to separate and compress water, A methanation reaction section where the synthesis gas from which water has been separated is passed through a methanation catalyst to perform methanation, A separation unit for separating methane from the gas produced in the methanation reaction unit, Control means for controlling operation, A co-electrolytic methanation apparatus comprising, The carbon dioxide supply unit further comprises a first supply path for supplying the carbon dioxide to the cathode electrode, a second supply path for supplying the carbon dioxide to the methane reaction unit, and a branching section connecting the first supply path and the second supply path. The flow rate ratio calculation means calculates the proportion of the carbon dioxide supplied through the first supply path out of the total flow rate of the carbon dioxide supplied by the carbon dioxide supply unit as the branch ratio (β), and the flow rate control means changes the amount of carbon dioxide supplied through at least one of the first supply path and the second supply path so as to change the branch ratio (β). The key feature is that the ratio (Ri) of the water vapor flow rate to the carbon dioxide flow rate is calculated using the supply gas utilization rate (α) and the branching ratio (β).

[0010] According to the above characteristic configuration, the carbon dioxide supply unit can supply carbon dioxide to the cathode electrode of the co-electrolysis unit via the first supply channel and to the methane reaction unit downstream of the co-electrolysis unit via the second supply channel. Furthermore, the flow rate ratio calculation means can calculate the ratio of the amount of carbon dioxide supplied via the first supply channel to the total flow rate of carbon dioxide supplied by the carbon dioxide supply unit as the branching ratio (β). The flow rate control means changes the amount of carbon dioxide supplied via the first supply channel based on the calculated branching ratio (β). This makes it possible to stabilize the gas composition at the outlet of the methane reaction unit and maintain a high heat content per unit volume.

[0011] Further characteristic configurations of the co-electrolytic methanation apparatus according to the present invention are: The flow rate ratio calculation means calculates the ratio (Ri) of the amount of water vapor supplied to the amount of carbon dioxide supplied based on the following formula using the supply gas utilization rate (α) and the branching ratio (β), and the flow rate control means controls the supply flow rates of the water vapor and carbon dioxide based on the ratio (Ri). Ri=4 / α-β

[0012] The flow rate ratio calculation means can calculate the ratio (Ri) using the supply gas utilization rate (α) and the branching ratio (β). Based on the ratio (Ri), the flow rates of water vapor and carbon dioxide are controlled. This stabilizes the gas composition at the outlet of the methane reaction section and makes it possible to maintain a high level of heat per unit volume.

[0013] The characteristic configuration of the operation method of the co-electrolysis methanation apparatus having the above characteristic configuration is that in the operation steps from the start of electrolysis of the co-electrolysis section to the rated state, the electrolysis current after the rated state, and the adjustment process of the supply gas utilization rate, the target values of the supply gas utilization rate and the branch ratio are set, and the supply flow rates of the water vapor and the carbon dioxide are controlled based on the target values.

[0014] According to the above characteristic configuration, it is possible to stabilize the gas composition at the outlet of the methanation reaction section and maintain a high heat quantity per unit volume.

[0015] A further characteristic configuration of the operation method of the co-electrolysis methanation apparatus according to the present invention is that in the operation steps, the total flow rate of the water vapor and the carbon dioxide is fixed, and the supply flow rates of the water vapor and the carbon dioxide are controlled based on the ratio calculated in the operation steps.

[0016] According to the above characteristic configuration, it is possible to stabilize the gas composition at the outlet of the methanation reaction section and maintain a high heat quantity per unit volume.

[0017] A further characteristic configuration of the operation method of the co-electrolysis methanation apparatus according to the present invention is that in the operation steps, the flow rate of the water vapor is fixed, and the supply flow rates of the water vapor and the carbon dioxide are controlled based on the ratio calculated in the operation steps.

[0018] According to the above characteristic configuration, it is possible to stabilize the gas composition at the outlet of the methanation reaction section and maintain a high heat quantity per unit volume.

[0019] A further characteristic configuration of the operation method of the co-electrolysis methanation apparatus according to the present invention is that the target value of the supply gas utilization rate is set based on a control target map set according to the branch ratio.

[0020] By determining the target value of the supply gas utilization rate (α) based on the control target map, it becomes possible to operate the system without significantly changing the electrolysis current. Using the supply gas utilization rate (α) and branching ratio (β) determined based on the control target map, the ratio (Ri) is calculated to determine the supply amounts of water vapor and carbon dioxide. This stabilizes the gas composition at the outlet of the methane reaction section and makes it possible to maintain a high heat content per unit volume. [Brief explanation of the drawing]

[0021] [Figure 1] This is a diagram showing the schematic configuration of a co-electrolytic methanation apparatus according to an embodiment. [Figure 2] This figure shows the change in the gas composition at the outlet of the methane synthesis section when the co-electrolytic methanation apparatus according to the embodiment is operated with a target value of 80% for the supply gas utilization rate (α) and the ratio of the amount of water vapor supplied to the amount of carbon dioxide supplied (Ri) is varied. [Figure 3] This figure shows the change in the gas composition at the outlet of the methane synthesis section when the co-electrolytic methanation apparatus according to the embodiment is operated with a target value of 80% for the supply gas utilization rate (α) and without changing the ratio of the amount of water vapor supplied to the amount of carbon dioxide supplied (Ri). [Figure 4] This figure shows a control target map that is set according to the branch ratio (β). [Modes for carrying out the invention]

[0022] A co-electrolytic methanation apparatus 100 according to an embodiment of the present invention will be described below with reference to the drawings.

[0023] (Configuration of the co-electrolytic methanation apparatus according to this embodiment) A co-electrolytic methanation apparatus 100 according to an embodiment will now be described. As shown in Figure 1, the co-electrolytic methanation apparatus 100 includes a water vapor supply unit 2, a carbon dioxide supply unit 3, a confluence channel 4, a co-electrolytic unit 5, a water separation / compression unit 6, a methanation reaction unit 7, a gas-liquid separator 8 (an example of a separation unit), a flow rate ratio calculation means 9, an electrolysis utilization rate detection unit 56, and an operation control unit A (an example of a control means) capable of controlling the operation of each component.

[0024] [Steam supply unit] The steam supply unit 2 is equipped with a flow regulator 20 (an example of a flow rate adjustment means) for adjusting the steam flow rate and an evaporator 21 for evaporating water. The heat generated in the methane reaction unit 7 can be used as the heat source for evaporating water. Any deficiency can be heated with an electric heater. The flow regulator 20 is designed to constantly control the total steam flow rate based on the branch ratio (β), supply gas utilization rate (α), or ratio (Ri) calculated by the flow rate ratio calculation means 9 (described later) as the proportion of total carbon dioxide supplied through the first supply path 3a. It is also possible to determine and control the steam flow rate based on the ratio of the steam flow rate to the carbon dioxide flow rate and the command value of the carbon dioxide flow rate.

[0025] [Carbon Dioxide Supply Department] The carbon dioxide supply unit 3 includes a flow regulator 30 (an example of flow control means) for adjusting the flow rate of carbon dioxide, a first supply channel 3a for supplying carbon dioxide to the co-electrolysis unit 5, a second supply channel 3b for supplying carbon dioxide to the methanation reaction unit 7, and a branching section 3c connecting the first supply channel 3a and the second supply channel 3b. One end of the first supply channel 3a is connected to the confluence channel 4, and the other end is connected to the branching section 3c. One end of the second supply channel 3b is connected between the first methanation reactor 71 and the second methanation reactor 72, and the other end is connected to the branching section 3c. The branching section 3c is also equipped with a flow control valve (not shown) and is configured to control the flow rate of carbon dioxide supplied via the first supply channel 3a and the flow rate of carbon dioxide supplied via the second supply channel 3b based on the supply gas utilization rate (α), branching ratio (β), and ratio (Ri) calculated by the flow rate ratio calculation means 9 described later. Furthermore, the flow rate of carbon dioxide supplied via the first supply path 3a and the flow rate of carbon dioxide supplied via the second supply path 3b can be controlled, with the branch ratio (β) instructed by the operation control unit A as the target value.

[0026] Furthermore, the flow regulator 30 is capable of constantly controlling the carbon dioxide flow rate based on the branch ratio (β), supply gas utilization rate (α), or ratio (Ri), which are the proportion of carbon dioxide supplied through the first supply path 3a, as calculated by the flow ratio calculation means 9 described later. In addition, the carbon dioxide flow rate can also be determined and controlled based on the ratio of water vapor flow rate to carbon dioxide flow rate and the command value of water vapor flow rate.

[0027] The carbon dioxide used as a raw material can be produced by any method, as long as it has a purity and properties that do not hinder the electrolytic reaction in the solid oxide electrolytic cell. It may be carbon dioxide recovered from combustion exhaust gas by known carbon dioxide recovery methods such as the amine absorption method, or it may be carbon dioxide recovered from biogas obtained by methane fermentation of organic matter.

[0028] If carbon dioxide contains sulfur, halogen compounds, siloxane compounds, heavy hydrocarbons, etc., these can cause deterioration of the solid oxide electrolytic cell or the subsequent methane catalyst. Therefore, it is preferable to remove these before the reaction, if necessary.

[0029] The flow regulator 20 in the steam supply unit 2 and the flow regulator 30 in the carbon dioxide supply unit 3 are controlled by the operation control unit A to supply steam and carbon dioxide to the co-electrolysis unit 5. Furthermore, the flow rates and ratios of the supplied steam and carbon dioxide are constantly controlled by the operation control unit A based on the supply gas utilization rate (α), branching ratio (β), or ratio (Ri) calculated by the flow rate ratio calculation means 9 described later.

[0030] The first supply channels 3a of the water vapor supply section 2 and the carbon dioxide supply section 3 are connected to a confluence channel 4 that combines water vapor and carbon dioxide. The confluence channel 4 is connected to the cathode electrode 53 of the solid oxide electrolytic cell provided in the co-electrolysis section 5.

[0031] [Co-electrolytic part] The co-electrolysis unit 5 includes a solid oxide electrolysis cell (SOEC) having an anode electrode 52 and a cathode electrode 53. It comprises a solid electrolyte 51 that conducts oxygen ions, an anode electrode 52 provided on one side of the solid electrolyte 51, and a cathode electrode 53 provided on the other side of the solid electrolyte 51. Examples of solid electrolytes 51 include yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (ScSZ), ytterbia-stabilized zirconia (YbSZ), lanthanum gallate (LaSrGdMgO3, LSGM), etc.

[0032] The anode 52 and cathode 53 are connected to a power supply unit 54 that supplies power to the anode 52 and cathode 53. The power supply unit 54 is configured to apply a positive potential to the anode 52 and a negative potential to the cathode 53. The voltage applied to the anode 52 and cathode 53 is preferably around 1.0V to 1.3V. The co-electrolysis unit 5 is also equipped with an ammeter 55 for measuring the electrolysis current value. The electrolysis current value measured by the ammeter 55 is configured to be transmitted to the electrolysis utilization rate detection unit 56.

[0033] Electrolytic reactions are typically carried out at temperatures of around 600°C to 1000°C, preferably around 650°C to 850°C. When the electrolytic reaction temperature is within this range, the oxygen ion conductivity of the electrolyte material is ensured, and the electrode reaction occurs rapidly. By setting the voltage to approximately the theoretical electrolytic voltage, electrolysis can be continued, enabling highly efficient operation.

[0034] The co-electrolysis unit 5 obtains a mixed gas (mixed fluid) from water (H2O) and carbon dioxide (CO2) through co-electrolysis, consisting of an electrolytic gas containing hydrogen (H2) and carbon monoxide (CO), and unelectrolyzed gas of water vapor and carbon dioxide. In the co-electrolysis, equations (I) and (II) proceed simultaneously at the cathode electrode 53, as shown below. In addition, reactions (IV) and (V), described later, also occur inside the cathode electrode 53, resulting in the production of a small amount of methane. Oxygen ions produced by the co-electrolysis of water vapor and carbon dioxide at the cathode electrode 53 move through the solid electrolyte 51 due to the potential gradient to the anode electrode 52, where oxygen gas is generated according to equation (III). The combined electrolytic reaction is an endothermic reaction. <Cathode pole> H2O + 2e - → H2 + O 2- ...(I) CO2+2e - → CO+O 2- ...(II) <Anode> 20 2- → O2+4e - ...(III)

[0035] To remove the oxygen generated by equation (III) from the anode electrode 52, nitrogen or air (hereinafter referred to as SWEEP gas) is supplied to the anode side inlet, and the generated oxygen is added to this SWEEP gas and discharged from the outlet on the anode electrode 52 side (hereinafter referred to as oxygen-enriched gas). Since this oxygen-enriched gas is also at a sufficiently high temperature, it is desirable to preheat it by heat exchange with the SWEEP gas. However, since the oxygen-enriched gas is still at a high temperature, it is desirable to pass it through a heat exchanger and recover heat using a heat transfer medium (oil, water) for heat recovery, or to use it to heat carbon dioxide, water, or steam used in the electrolytic reaction before discharging it.

[0036] [Water separation and compression section] The water separation and compression unit 6 includes a gas-liquid separator 61 that cools the gas containing hydrogen, carbon monoxide, water vapor, and carbon dioxide obtained from the co-electrolysis unit 5 and separates the water vapor as water, and a pressurizer 62 that increases the pressure to a level suitable for subsequent methane synthesis. Since the gas after the electrolytic reaction is at a sufficiently high temperature, it is preferable to use it for preheating by exchanging heat with, for example, either the carbon dioxide or hydrogen used in the electrolytic reaction, or the mixed gas.

[0037] If the temperature does not drop sufficiently by heat exchange with the mixed gas of carbon dioxide and hydrogen used in the electrolytic reaction, a further heat exchanger is connected and the mixture is cooled by means of cooling water or air cooling, preferably to 50°C or below, more preferably to 40°C or below, to separate the condensed water.

[0038] In the water separation and compression section 6, in addition to separating water in the gas-liquid separator 61, the synthesis gas is compressed in the booster 62 to a pressure suitable for the subsequent methane reaction process. While a higher pressure after compression is desirable to increase methane purity, the optimal pressure range should be selected and configured considering equipment costs and the specifications of the receiving equipment. For example, it is possible to carry out the reaction at around 2.0 MPaG, equivalent to or higher than a high-pressure gas pipeline for city gas (1.0 MPaG), or the methane reaction may be carried out in a pressure range equivalent to a medium-pressure A gas pipeline (0.3 to 1.0 MPaG). However, even if a gas-liquid separator 61 is installed in the middle of the methane reaction section 7, a pressure of 0.5 MPaG or higher is desirable.

[0039] Compression of synthesis gas can be performed using a conventional rotary or reciprocating gas compressor. Compression of synthesis gas causes the process gas to become hot, but from the standpoint of equipment maintenance, it is preferable to compress it in multiple stages, cooling it in between, rather than compressing it to the desired pressure in a single stage. This is cooled by heat exchange with low-temperature nitrogen or water, and the compressed gas is passed through a gas-liquid separator 8 to remove and separate the condensed water, and the gas with reduced volume flow rate is recompressed, which has the advantage of reducing compression power. For example, the synthesis gas is first compressed to 0.20 MPaG and methane is carried out in the first methane reactor 71. The resulting gas is then passed through a group of heat exchangers (not shown), cooled to about 40°C by heat exchange with water in a cooling heat exchanger 81 installed before the gas-liquid separator 8 to separate the condensed water, further compressed to 0.8 MPaG, methane is carried out again in the next stage, the second methane reactor 72, and finally cooled again to below 20°C with cooling water in the cooling heat exchanger 81 to obtain high-purity methane.

[0040] [Methanation reaction section] The methanation reaction section 7 comprises a multi-stage methanation reactor (first methanation reactor 71, second methanation reactor 72, third methanation reactor 73) having a methanation catalyst in contact with the mixed gas. As the methanation catalyst, a known methanation catalyst supported with Ni or Ru-based metals can be used.

[0041] A methanation reactor uses hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2) to produce a product gas (product fluid) containing water (H2O) and methane (CH4) through the methanation reaction. The methanation reaction proceeds according to equations (IV) and (V) shown below. This reaction is an equilibrium reaction, and all reactions moving to the right are exothermic. 3H2 + CO ⇔ H2O + CH4 ... (IV) 4H2 + CO2 ⇔ 2H2O + CH4···(V)

[0042] Since equations (IV) and (V) are exothermic reactions, it is desirable to carry out these reactions at low temperatures. However, the methanation reaction generates a large amount of heat, so the methanation reaction proceeds rapidly and locally near the reactor inlet, raising the temperature close to the catalyst's heat resistance temperature, which can lead to thermal degradation. In addition, the temperature rise may cause the outlet gas temperature to exceed the desired temperature.

[0043] One method to suppress local temperature rises during such methanation reactions and increase methane purity is to use a multi-stage methanation reactor consisting of two or more stages. A cooling heat exchanger 81 and a gas-liquid separator 8 are installed downstream of the first methanation reactor 71. The gas, whose temperature has risen due to the reaction heat in the preceding first methanation reactor 71, is cooled by the cooling heat exchanger 81. The cooled gas has condensed water removed by the gas-liquid separator 8 and is then introduced into the next stage, the second methanation reactor 72. In the preceding first methanation reactor 71, a temperature rise is tolerated, but the condensed water generated along the way is removed from the system. By precisely controlling the reaction temperature to a lower temperature in the subsequent stages, the reactions in equations (IV) and (V) proceed to the right, increasing the methane concentration.

[0044] [Separation part] As shown in equations (IV) and (V), H2O is produced simultaneously with methane production. Therefore, the gas-liquid separator 8 separates the water-containing components from the product gas to obtain high-purity methane. Separation methods such as liquefaction separation, membrane separation, and adsorption separation can be employed. The gas-liquid separator 8 may combine one or more of these separation methods.

[0045] Liquefaction separation involves liquefying and separating water-containing components by controlling the temperature to obtain a gas primarily composed of methane.

[0046] Membrane separation involves using a separation membrane that selectively allows water to pass through, thereby separating water-containing components and obtaining a gas primarily composed of methane. The type of separation membrane is not particularly limited, as long as it can allow small molecular sizes to pass through and separate specific components from other components.

[0047] Adsorption separation involves separating water-containing components using an adsorbent to obtain a gas primarily composed of methane. Adsorbents such as silica gel, zeolite, and activated carbon can be used. When employing adsorption separation, a heating device is provided. Heating the adsorbent allows for the desorption of water-containing components from the adsorbent.

[0048] [Flow rate ratio calculation means] Gas flow meters (not shown) are provided in the paths through which water vapor and carbon dioxide supplied to the co-electrolysis unit 5 flow, in the first supply path 3a and the second supply path 3b, and in the flow path through which the synthesis gas containing hydrogen, carbon monoxide, water vapor, and carbon dioxide after co-electrolysis in the co-electrolysis unit 5 flows. The flow rate ratio calculation means 9 calculates the supply gas utilization rate (α), which is the ratio of the total flow rate of generated hydrogen and carbon monoxide to the total flow rate of water vapor and carbon dioxide supplied to the cathode electrode 53, based on the flow rate values ​​obtained from these flow meters. Specifically, it is expressed by the following equation (VI). (Molar flow rate of hydrogen produced + molar flow rate of carbon monoxide produced) / (Molar flow rate of water vapor supplied + molar flow rate of carbon dioxide supplied) = α...(VI)

[0049] In solid oxide electrolytic cells, it is not possible to reduce all of the supplied water vapor and carbon dioxide; typically, about 60-95% of the supplied water vapor and carbon dioxide are reduced. In this embodiment, the amount of water vapor and carbon dioxide supplied to the co-electrolytic unit 5 and the current supplied to the co-electrolytic unit 5 are controlled so that the supply gas utilization rate (α) in the rated state is 95% (0.95) or less. If the supply gas utilization rate (α) in the rated state exceeds 95% (0.95), the overvoltage of the co-electrolytic unit 5 increases, leading to increased power consumption, cell deterioration, and a higher risk of carbon deposition. Furthermore, it is preferable that the supply gas utilization rate (α) in the rated state be 60% (0.60) or higher. If it is lower than 60% (0.60), less gas is generated, and the operating efficiency decreases.

[0050] Furthermore, the flow rate ratio calculation means 9 calculates the ratio of the amount of carbon dioxide supplied via the first supply path 3a to the total flow rate of carbon dioxide supplied by the carbon dioxide supply unit 3 as the branch ratio (β). A branch ratio (β) of 0 indicates that no carbon dioxide is supplied to the cathode electrode 53 of the co-electrolysis unit 5, and all carbon dioxide is supplied to the methane reaction unit 7, with electrolysis of only water vapor being performed in the co-electrolysis unit 5. A branch ratio (β) of 1 indicates that all carbon dioxide is supplied to the cathode electrode 53 of the co-electrolysis unit 5, no carbon dioxide is supplied to the methane reaction unit 7, and co-electrolysis of water vapor and carbon dioxide is being performed in the co-electrolysis unit 5.

[0051] The flow rate ratio calculation means 9 can calculate the ratio of the water vapor flow rate (Ri) to the carbon dioxide flow rate using the following formula (Equation 1) based on the supply gas utilization rate (α) and the ratio of the first flow rate (β). Ri = 4 / α - β (Equation 1)

[0052] (Operation method of the co-electrolytic methanation apparatus according to the embodiment) An example of the operation method of the co-electrolytic methanation apparatus 100 having the above configuration will be explained below using Tables 1 to 4. First, Table 1 shows example of the operation method 1. Table 1 shows how the flow rate of water vapor and carbon dioxide is supplied to the co-electrolytic section 5, from the start of electrolysis (operation step 1) to the stabilization of the methane temperature (operation step 6) and then to the adjustment of the electrolysis current and utilization rate (α) (operation steps 13 and 14), and how the flow rate of water vapor and carbon dioxide is instructed to the flow rate regulators 20 and 30. In operation steps 1 to 6 in Table 1, the supply gas utilization rate (α) is gradually increased from 0.0 (0%) to 0.8 (80%) from the start of electrolysis. Also, in operation steps 1 to 6, the branch ratio (β) is set to 0.0 (0%). That is, in operation steps 1 to 6, carbon dioxide is not supplied to the cathode electrode 53 of the co-electrolytic section 5, and only water vapor electrolysis is performed, and the electrolysis current equivalent to the rated value is taken. In the next operation steps 7 to 12, the supply gas utilization rate (α) is fixed at 0.8 (80%). Furthermore, the branching ratio (β) is gradually increased from 0.2 (20%) to 1.0 (100%). In other words, in operating steps 7 to 12, the process is gradually transitioned from steam electrolysis to co-electrolysis. Furthermore, in operation steps 1 to 14, the supply amounts of water vapor and carbon dioxide are determined based on the ratio (Ri) calculated using Equation 1 with the supply gas utilization rate (α) and branching ratio (β). In Example 1, the supply amount of water vapor is 40 Nm³. 3 The carbon dioxide supply was kept constant at / h and varied. By performing the control shown in Table 1, the methane concentration at the outlet of the methanation reaction section 7 can be maintained at a high level.

[0053] [Table 1]

[0054] Next, Table 2 shows example 2 of the operating method. Table 2 shows how to instruct the co-electrolysis unit 5 to supply water vapor and carbon dioxide at the flow rates from the start of electrolysis (operating step 1) to the adjustment process of the electrolysis current and utilization rate (α) (operating steps 12 and 13), through to the start of electrolysis (operating step 1) and methane temperature monitoring (operating step 7). In operating steps 1 to 6 in Table 2, the supply gas utilization rate (α) is gradually increased from 0.0 (0%) to 0.8 (80%) from the start of electrolysis. Also, in operating steps 1 to 6, the branch ratio (β) is set to 1.0 (100%). That is, in operating steps 1 to 6, water vapor and carbon dioxide are supplied to the cathode electrode 53 of the co-electrolysis unit 5, and an electrolysis current equivalent to the rated value is taken while co-electrolysis is in progress. In the following operating steps 7 to 11, the supply gas utilization rate (α) is fixed at 0.8 (80%). Also, the branch ratio (β) is gradually decreased from 1.0 (100%) to 0.2 (20%). In other words, in operation steps 7 to 11, the process is gradually transitioned from co-electrolysis to electrolysis according to the branching ratio (β). Furthermore, in operation steps 1 to 13, the supply amounts of water vapor and carbon dioxide are determined based on the ratio (Ri) calculated using Equation 1 with the supply gas utilization rate (α) and branching ratio (β). In Example 2, the supply amount of water vapor is 40 Nm³. 3 The carbon dioxide supply was kept constant at / h and varied. By performing the control shown in Table 2, the methane concentration at the outlet of the methanation reaction section 7 can be maintained at a high level.

[0055]

Table 2

[0056] Next, Example 3 of the operation method is shown in Table 3. While fixing the supply gas utilization rate (α) at 0.8 (80%), the ratio (Ri) is calculated during the process of changing the branch ratio (β). There is a certain degree of freedom in the supply flow rates of steam and carbon dioxide based on that ratio (Ri). Therefore, there may be a case of taking a method of not varying the supply gas utilization rate (α) and the electrolysis current value as shown in Table 3. In Table 3, an example is shown when the supply gas utilization rate (α) is 0.80 (80%), the electrolysis current is 95.72 kA, and the total supply amount of steam and carbon dioxide is 50.00 (Nm 3 / h). By performing such control, it becomes possible to stably maintain the methane concentration at the outlet of the methanation reaction section 7 without significantly changing the supply gas utilization rate (α) and the electrolysis current value.

[0057]

Table 3

[0058] Next, Table 4 shows example 3 of the operating method. There is an upper limit to the steam generation capacity of the steam supply unit 2, and it is sometimes desirable to fix the amount of steam supplied. In this case, the amount of steam supplied is fixed and the amount of carbon dioxide supplied is changed, but since a target voltage is set in the co-electrolysis unit 5, it is desirable to keep the electrolysis current value as constant as possible. Therefore, control that fixes the product of the total amount of steam and carbon dioxide supplied and the supply gas utilization rate (α) is effective. Table 4 shows an example of operation in which the product of the total amount of steam and carbon dioxide supplied and the supply gas utilization rate (α) is approximately 40. As the amount of carbon dioxide supplied decreases, the total amount of steam and carbon dioxide supplied also decreases. In this process, by gradually increasing the supply gas utilization rate (α) from 0.80 (80%), it becomes possible to maintain the electrolysis current value at a constant value of 95.72kA. Control that links the supply gas utilization rate (α) and the branch ratio (β) to keep the electrolysis current value at an almost constant value is preferable because it also reduces fluctuations in steam generation and electrolysis voltage current within the process. In this way, by determining and controlling the ratio (Ri) with respect to the supply gas utilization rate (α) and the branching ratio (β), it becomes possible to stably maintain the methane concentration at the outlet of the methane reaction section 7.

[0059] [Table 4]

[0060] Figure 4 shows a control target map for the operation method example 3 shown in Table 4, where the water vapor supply amount and electrolysis current value are fixed, and the supply gas utilization rate (α) is changed in conjunction with the change in the branching ratio (β). By changing the target value of the supply gas utilization rate (α) based on this control target map as the branching ratio (β) goes from 1.0 (co-electrolysis) to 0.0 (water vapor electrolysis), operation becomes possible without significantly changing the electrolysis current value. The ratio (Ri) is determined based on the supply gas utilization rate (α) and branching ratio (β) determined based on the control target map. The water vapor supply amount and carbon dioxide supply amount are determined based on the ratio (Ri), but since the water vapor supply amount is fixed, the carbon dioxide supply amount is varied. In this way, it becomes possible to stably maintain the methane concentration at the outlet of the methanation reaction section 7. [Examples]

[0061] A co-electrolytic methanation apparatus 100 shown in Figure 1 was constructed and operated according to the following examples, and the change in gas composition at the outlet of the methanation reaction section 7 was investigated.

[0062] (Examples) In the example, the supply gas utilization rate (α) was set to 0.8 (80%), and the branching ratio (β) was varied from 0.0 (0%) to 1.0 (100%). The ratio of water vapor to carbon dioxide (Ri) was changed according to the values ​​of the supply gas utilization rate (α) and the branching ratio (β). Figure 2 shows the results of investigating the change in methane concentration at the outlet of the methanation reaction section 7 when the co-electrolytic methanation apparatus 100 was operated under these conditions.

[0063] As is clear from Figure 2, even when the branching ratio (β) was varied from 0.0 (0%) to 1.0 (100%), the methane concentration at the outlet of the methanation reaction section 7 remained high at over 95%.

[0064] (Comparative example) In the comparative example, the supply gas utilization rate (α) was set to 0.8 (80%), and the branching ratio (β) was varied from 0.0 (0%) to 1.0 (100%). The ratio of water vapor to carbon dioxide (Ri) was fixed at 4. Figure 3 shows the results of investigating the change in methane concentration at the outlet of the methanation reaction section 7 when the co-electrolytic methanation apparatus 100 was operated under these conditions.

[0065] As is clear from Figure 3, when the branching ratio (β) was 1.0 (100%), the methane concentration at the outlet of the methanation reaction section 7 was a high 96.5%. However, as the branching ratio (β) was changed to 0.0 (0%), the methane concentration at the outlet of the methanation reaction section 7 decreased, and when the branching ratio (β) was 0.0 (0%), the methane concentration at the outlet of the methanation reaction section 7 had decreased to 78.2%.

[0066] From the above, it became clear that by changing the ratio of water vapor (Ri) to carbon dioxide based on the supply gas utilization rate (α) and branching ratio (β), the gas composition at the process outlet can be stabilized and high-purity methane can be obtained. <Another Embodiment>

[0067] (1) In the above embodiment, the second supply channel 3b is connected between the first methanation reactor 71 and the second methanation reactor 72. However, it is not limited to this and may be connected between the second methanation reactor 72 and the third methanation reactor 73.

[0068] (2) In the above embodiment, control was performed using a command value of the supply gas utilization rate (α), but the electrolysis utilization rate detection unit 56 may be further provided, and the flow rates of water vapor and carbon dioxide may be calculated based on the electrolysis current ratio (R) calculated by the electrolysis utilization rate detection unit 56. The electrolysis utilization rate detection unit 56 calculates the ratio of the actual electrolysis current to the rated electrolysis current as the electrolysis current ratio (R). However, R is assumed to be less than 1.

[0069] Furthermore, the electrolysis utilization rate detection unit 56 calculates the flow rate ratio of water vapor to carbon dioxide supplied to the cathode electrode 53 using the rated electrolysis utilization rate (αr) and the electrolysis current ratio R, according to the following equation (Equation 2). Here, the rated electrolysis utilization rate (αr) refers to the gas utilization rate set when the electrolysis current equivalent to the rated value is reached. Ri=4 / (Rated electrolysis utilization rate (αr) x electrolysis current ratio (R))-β (Formula 2)

[0070] As the electrolysis current ratio (R) approaches 1, the ratio of water vapor flow rate to carbon dioxide supplied to the cathode electrode 53 decreases. In this embodiment, the supply flow rates of water vapor and carbon dioxide are controlled so that the flow rate ratio calculated by the electrolysis utilization rate detection unit 56 is achieved. This control can be performed by reducing the water vapor supply amount as the electrolysis current increases and increasing the carbon dioxide supply flow rate as the electrolysis current increases, or by fixing the water vapor supply amount at a constant value regardless of the increase in electrolysis current and increasing the carbon dioxide supply flow rate as the electrolysis current increases.

[0071] Furthermore, the rated electrolysis utilization rate (αr) is preferably set to 60-90%, and more preferably to 60-80%. If the rated electrolysis utilization rate (αr) is less than 60%, the steam flow rate ratio becomes too high, requiring unnecessary heating of the water. In addition, the amount of generated gas is small, resulting in poor operating efficiency. If the rated electrolysis utilization rate (αr) exceeds 90%, the overvoltage of the co-electrolysis unit 5 increases, leading to increased power consumption, cell deterioration, and a higher risk of carbon deposition.

[0072] Furthermore, the ratio of water vapor flow rate to carbon dioxide flow rate (Ri) calculated by equations (1) and (2) above can also be applied if it deviates slightly numerically from the calculated value. For example, the Faraday efficiency in electrolysis of a co-electrolytic stack is ideally 100% (the state in which CO and hydrogen corresponding to the electrolysis current value are produced), but it is conceivable that a stack with less than 100% efficiency may be used. In that case, if the Faraday efficiency is γ (0 < γ < 1), a correction using this can be applied, i.e. Ri = 4 / α × γ - β (Equation 1') or, Ri = 4 / αr × R × γ - β (Equation 2') You can make the decision like this. Stacks with low Faraday efficiency are undesirable for electrolysis due to concerns about heat loss from electron and hole conduction (other than ion conduction) or combustion loss due to cross-leakage of generated gases. However, if a cell stack with an efficiency of around 95% is to be used, the optimal ratio can be calculated and determined by multiplying it by the Faraday efficiency (γ=0.95) using the above formula.

[0073] Furthermore, the configurations disclosed in the above embodiments (including other embodiments, the same applies hereinafter) can be applied in combination with configurations disclosed in other embodiments, provided that no inconsistencies arise. Moreover, the embodiments disclosed herein are illustrative, and the embodiments of the present invention are not limited thereto, and can be modified as appropriate without departing from the object of the present invention. [Explanation of Symbols]

[0074] 100: Co-electrolytic methanation apparatus 2: Steam supply unit 3: Carbon Dioxide Supply Unit 3a: 1st supply route 3b: 2nd supply path 3c: Branching point 5: Co-electrolytic part 6: Water separation and compression section 7: Methanation reactor 8: Gas-liquid separator (separation section) 9:Flow rate ratio calculation means 20:Flow rate regulator (flow rate control means) 30: Flow rate regulator (flow rate control means) 53: Cathode pole A: Operation control unit (control means)

Claims

1. A co-electrolytic unit that electrolyzes water vapor and carbon dioxide to produce a mixed gas consisting of an electrolytic gas mainly composed of hydrogen and carbon monoxide, and unelectrolyzed water vapor and carbon dioxide. A water vapor supply unit that supplies the water vapor to the cathode electrode of the aforementioned electrolytic unit, A carbon dioxide supply unit that supplies the carbon dioxide to the cathode electrode, Flow rate control means for controlling the flow rates of the water vapor and carbon dioxide supplied by the water vapor supply unit and the carbon dioxide supply unit, A flow rate ratio calculation means that calculates the ratio of the total flow rate of hydrogen and carbon monoxide produced by electrolysis in the co-electrolysis unit to the total flow rate of water vapor and carbon dioxide supplied to the cathode electrode as the supply gas utilization rate (α), A water separation and compression unit that cools the mixed gas to separate the water and compress it, A methanation reaction section where the synthesis gas from which water has been separated is passed through a methanation catalyst to perform methanation, A separation unit for separating methane from the gas produced in the methanation reaction unit, Control means for controlling operation, A co-electrolytic methanation apparatus comprising, The carbon dioxide supply unit further comprises a first supply path for supplying carbon dioxide to the cathode electrode, a second supply path for supplying carbon dioxide to the methane reaction unit, and a branching section connecting the first supply path and the second supply path. The flow rate ratio calculation means calculates the proportion of the carbon dioxide supplied through the first supply path out of the total flow rate of the carbon dioxide supplied by the carbon dioxide supply unit as the branch ratio (β), and the flow rate control means changes the amount of carbon dioxide supplied through at least one of the first supply path and the second supply path so as to change the branch ratio (β). A co-electrolytic methanation apparatus that calculates the ratio (Ri) of the water vapor flow rate to the carbon dioxide flow rate using the supply gas utilization rate (α) and the branching ratio (β).

2. The co-electrolytic methanation apparatus according to claim 1, wherein the flow rate ratio calculation means calculates the ratio (Ri) based on the following formula using the supply gas utilization rate (α) and the branching ratio (β), and the flow rate control means controls the supply flow rates of the water vapor and carbon dioxide based on the ratio (Ri). Ri = 4 / α - β (However, assume 0 < α < 1 and 0 ≤ β ≤ 1)

3. A method for operating a co-electrolytic methanation apparatus according to claim 2, A method for operating a co-electrolytic methanation apparatus, in which, during the operating steps from the start of electrolysis in the co-electrolytic section to the rated state, and the adjustment process of the electrolysis current and the supply gas utilization rate (α) after the rated state, target values ​​for the supply gas utilization rate (α) and the branch ratio (β) are set, and the supply flow rates of the water vapor and carbon dioxide are controlled based on the target values.

4. A method for operating a co-electrolytic methanation apparatus according to claim 3, wherein the total flow rate of the water vapor and the carbon dioxide is fixed in the operation step, and the supply flow rates of the water vapor and the carbon dioxide are controlled based on the ratio (Ri) calculated in the operation step.

5. A method for operating a co-electrolytic methanation apparatus according to claim 3, wherein the flow rate of the water vapor is fixed in the operation step, and the supply flow rates of the water vapor and the carbon dioxide are controlled based on the ratio (Ri) calculated in the operation step.

6. The method for operating a co-electrolytic methanation apparatus according to claim 3, wherein the target value of the supply gas utilization rate (α) is set based on a control target map set according to the branching ratio (β).

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