Co-electrolytic methanation device

Abstract

To provide a co-electrolytic methanation apparatus that suppresses temperature rise near the entrance of the methanation reaction section, thereby extending the lifespan of the catalytic reaction and achieving stable operation. [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, an electrolytic utilization rate detection unit 56 for detecting the electrolytic current or electrolytic voltage, a water separation / compression unit 6, a methanation reaction unit 7, a separation unit for separating methane, and a control means, wherein the apparatus is equipped with a temperature detection means 10 having an exothermic temperature measurement unit 10a disposed in the high-temperature portion of the methanation reaction unit 7, the carbon dioxide supply unit 3 is equipped with a first supply path 3a for supplying carbon dioxide to the co-electrolytic unit 5, a second supply path 3b for supplying carbon dioxide to the methanation reaction unit 7, and a first branching section 3c connecting the first supply path 3a and the second supply path 3b, the second supply path 3b is connected downstream of the exothermic temperature measurement unit 10a.

Description

Technical Field

[0001] The present invention relates to a SOEC co - electrolysis methanation device.

Background Art

[0002] A method is known in which carbon dioxide and steam as raw materials are co - electrolyzed by a solid oxide electrolysis cell (SOEC) stack to produce carbon monoxide and hydrogen, and methane is produced from carbon monoxide and hydrogen in a methanation device provided in the subsequent stage.

[0003] In the SOEC electrolysis section, mainly the following gases are generated by power: H2O → H2+ 1 / 2O2···(1) CO2→ CO + 1 / 2O2···(2) In the methanation device, CO + 3H2→ CH4+ H2O··(3) CO2+ 4H2→ CH4+ 2H2O···(4) By performing the methanation reaction above, methane is generated by power throughout. Since both reactions (3) and (4) in the methanation device are exothermic reactions, it is expected to realize a highly efficient methane production system by using the heat of this reaction as heat for generating steam to be supplied to the SOEC electrolysis section.

[0004] In co - electrolysis, the ratio of the heat required for steam generation in the SOEC electrolysis device compared to steam electrolysis is highly satisfied by the heat recovered by the methanation reaction and can ideally approach zero. Therefore, co - electrolysis methanation has the potential to achieve a higher total energy conversion efficiency than steam electrolysis methanation. On the other hand, in co - electrolysis methanation, the temperature of the methanation reactor tends to rise, and there are problems in temperature control within the reactor.

[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] In the invention disclosed in Patent Document 1, if CO, CO2, and H2 in a chemically hypothesis ratio for methane are supplied at the inlet of the hydrocarbon synthesis reaction section, a vigorous exothermic reaction will proceed at the inlet regardless of whether or not there is a preceding reverse water-gas shift reaction section, potentially exceeding the heat resistance of the catalyst.

[0008] This invention has been made in view of the above-mentioned problems, and its purpose is to provide a co-electrolytic methanation apparatus that suppresses the temperature rise near the entrance of the methanation reaction section, thereby extending the lifespan of the catalytic reaction and achieving stable operation. [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 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, An electrolysis utilization rate detection unit that recognizes the flow rates of water vapor and carbon dioxide supplied to the cathode electrode and can detect the electrolysis current value or electrolysis voltage, A water separation and compression unit that cools the mixed gas to separate the water and compress it, A methanation reaction section comprising multiple stages of a methanation reactor, which performs methanation by passing the synthesis gas from which water has been separated through a methanation catalyst, A separation unit for separating methane from the gas produced in the aforementioned methanation reactor, Control means for controlling operation, A co-electrolytic methanation apparatus comprising, The system includes a temperature detection means for detecting the temperature of the methane reactor, The temperature detection means includes at least one heat-generating temperature measuring unit disposed in the high-temperature portion of the methane reactor, The carbon dioxide supply unit comprises a first supply path for supplying the carbon dioxide to the co-electrolysis unit, a second supply path for supplying the carbon dioxide to the methane reactor, and a first branching section connecting the first supply path and the second supply path. The second supply channel is located downstream of the heat generation temperature measurement unit in the methanation reactor.

[0010] According to the above configuration, a second supply channel is connected downstream from the heat generation temperature measurement unit located in the high-temperature section of the methanation reaction unit. Furthermore, the carbon dioxide supply unit is equipped with a first branching section, and is configured to supply carbon dioxide to the co-electrolysis unit and to the second and subsequent stages of the multi-stage methanation reactor. This suppresses the temperature rise of the methanation reactor, making it possible to extend the lifespan of the catalytic reaction and achieve stable operation.

[0011] Further characteristic configurations of the co-electrolytic methanation apparatus according to the present invention are: Flow rate ratio calculation means for calculating the ratio (β1) of the first flow rate of carbon dioxide supplied to the co - electrolysis unit via the first supply path to the total flow rate of the first flow rate of carbon dioxide supplied to the co - electrolysis unit via the first supply path and the second flow rate of carbon dioxide supplied to the methanation reactor via the second supply path is provided. The first branch section is configured to change the ratio (β1) of the first flow rate based on the temperature information detected by the temperature detection means.

[0012] The flow rate ratio calculation means calculates the ratio (β1) of the first flow rate of raw material CO2 supplied to the co - electrolysis unit via the first supply path to the total flow rate of the first flow rate of raw material CO2 supplied to the co - electrolysis unit via the first supply path and the second flow rate of raw material CO2 supplied to the methanation reactor via the second supply path. And the first branch section performs control to decrease the value of β1 based on the temperature information of the temperature measurement means. Specifically, by adjusting the decrease of the CO2 supply ratio to the co - electrolysis unit in the previous stage so that the temperature value at the outlet of the methanation reactor becomes equal to or less than the target value, it is possible to suppress the temperature rise of the methanation reactor and avoid the thermal deterioration of the catalyst. [[ID=X]] [[ID=Y]]

[0013] [[ID=Z]] A further characteristic configuration of the co - electrolysis methanation apparatus according to the present invention is The first stage of the methanation reactor is an adiabatic first methanation reactor, The last stage is an isothermal methanation reactor or an adiabatic methanation reactor, A heat exchanger is provided downstream of the first methanation reactor, The exothermic temperature measurement section is arranged between the first methanation reactor and the heat exchanger.

[0014] According to the above - mentioned characteristic configuration, it is possible to suppress the temperature rise of the methanation reactor, and to achieve a longer service life of the catalytic reaction and stable operation.

[0015] A further characteristic configuration of the co - electrolysis methanation apparatus according to the present invention is The flow rate ratio calculating means calculates the flow rate ratio of the water vapor to the flow rate of the carbon dioxide based on the ratio (β1) of the first flow rate, and the flow rate control means controls the respective flow rates of the water vapor and the carbon dioxide supplied to the cathode based on the flow rate ratio.

[0016] Using the flow rate ratio of the water vapor to the flow rate of the carbon dioxide calculated by the flow rate ratio calculating means based on the ratio (β1) of the first flow rate, the flow rate control means controls the respective flow rates of the water vapor and the carbon dioxide. According to the above characteristic configuration, it is possible to control the temperature of the first methanation reactor so that it does not become too high. It is possible to suppress the temperature rise of the methanation reactor and realize a long service life and stable operation of the catalytic reaction.

[0017] A further characteristic configuration of the co-electrolysis methanation apparatus according to the present invention is The second stage of the methanation reactor is an adiabatic second methanation reactor, The carbon dioxide supply section further includes a third supply path connected to the downstream of the second methanation reactor and a second branch section connecting the second supply path and the third supply path, The flow rate ratio calculating means further calculates the ratio (β2) of the second flow rate and the ratio (β3) of the third flow rate of the carbon dioxide supplied to the downstream of the second methanation reactor via the third supply path with respect to the total flow rate of the first flow rate, the second flow rate, and the third flow rate, The first branch section and the second branch section change the ratio (β1) of the first flow rate and the ratio (β2) of the second flow rate based on the temperature information detected by the temperature detection means.

[0018] The flow rate ratio calculating means can further calculate the ratio (β3) of the third flow rate of the carbon dioxide supplied to the downstream of the second methanation reactor via the third supply path. In addition to monitoring the temperature of the first methanation reactor and changing the ratio (β1) of the first flow rate, the ratio (β2) of the second flow rate is changed. By doing so, it becomes possible to appropriately manage all the reaction temperatures of the methanation reactor, and it becomes possible to realize a long service life and stable operation of the catalytic reaction. [Brief explanation of the drawing]

[0019] [Figure 1] This diagram shows a schematic configuration of a conventional co-electrolytic methanation apparatus. [Figure 2] This figure shows the change in the methanation reactor outlet temperature when a conventional co-electrolytic methanation system is in operation. [Figure 3] This figure shows a schematic configuration of a co-electrolytic methanation apparatus according to the first embodiment. [Figure 4] This figure shows the change in the methanation reactor outlet temperature when the co-electrolytic methanation apparatus according to the first embodiment is in operation. [Figure 5] This figure shows a schematic configuration of a co-electrolytic methanation apparatus according to the second embodiment. [Figure 6] This figure shows the change in the methanation reactor outlet temperature when the co-electrolytic methanation apparatus according to the second embodiment is in operation. [Figure 7] This diagram shows the schematic configuration of a co-electrolytic methanation apparatus according to another embodiment 1. [Figure 8] This diagram shows the schematic configuration of a co-electrolytic methanation apparatus according to another embodiment 2. [Modes for carrying out the invention]

[0020] A co-electrolytic methanation apparatus according to an embodiment of the present invention will be described below with reference to the drawings.

[0021] (Configuration of the co-electrolytic methanation apparatus according to the first embodiment) First, the co-electrolytic methanation apparatus 100 according to the first embodiment will be described. As shown in Figure 3, 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, a temperature detection means 10, 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.

[0022] [Steam supply unit] The steam supply unit 2 is equipped with a flow regulator 20 (an example of a flow control means) for adjusting the flow rate of steam 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 flow rate of steam based on the ratio of the flow rate of steam to the flow rate of carbon dioxide, which is calculated by the flow rate ratio calculation means 9 based on the ratio (β1) of the first flow rate. It can also control the flow rate of steam based on the ratio of the flow rate of steam to the flow rate of carbon dioxide.

[0023] [Carbon Dioxide Supply Department] The carbon dioxide supply unit 3 is equipped with a flow regulator 30 (an example of a flow control means) for adjusting the flow rate of carbon dioxide. The flow regulator 30 is designed to constantly control the flow rate of carbon dioxide based on the ratio of the water vapor flow rate to the carbon dioxide flow rate, which is calculated by the flow rate ratio calculation means 9 based on the ratio of the first flow rate (β1). It can also control the flow rate of carbon dioxide based on the ratio of the water vapor flow rate to the carbon dioxide flow rate. Furthermore, the carbon dioxide supply unit 3 includes 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 first 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 first 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 downstream of the heat generation temperature measurement unit 10a, and the other end is connected to the first branching section 3c. The first 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 temperature information from the temperature detection means 10 described later.

[0024] 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.

[0025] 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.

[0026] 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 based on the supply gas utilization rate (α) calculated by the flow rate ratio calculation means 9, which will be described later.

[0027] 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.

[0028] [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.

[0029] 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.

[0030] 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.

[0031] 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)

[0032] 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.

[0033] [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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] [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. Known adiabatic or isothermal methanation reactors can be used as the methanation reactors. Furthermore, known methanation catalysts supported with Ni or Ru-based metals can be used as the methanation catalyst.

[0038] 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)

[0039] 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.

[0040] To suppress local temperature rises during such methanation reactions and increase methane purity, the methanation reactor is configured in multiple stages 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. After cooling, condensate is removed from the gas by the gas-liquid separator 8 and introduced into the next stage, the second methanation reactor 72. In the preceding first methanation reactor 71, while allowing a temperature rise, the condensate 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 of equations (IV) and (V) proceed to the right, increasing the methane concentration.

[0041] [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.

[0042] Liquefaction separation involves liquefying and separating water-containing components by controlling the temperature to obtain a gas primarily composed of methane.

[0043] 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.

[0044] 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.

[0045] [Flow rate ratio calculation means] The paths through which water vapor and carbon dioxide are supplied to the co-electrolysis unit 5, and the first supply path 3a and the second supply path 3b, are equipped with gas flow meters (not shown) in both or either of these. Based on the flow rate values ​​obtained from these flow meters, the flow rate ratio calculation means 9 calculates the ratio of the first flow rate (β1) to the total flow rate of the first carbon dioxide supplied to the co-electrolysis unit 5 via the first supply path 3a and the ratio of the second flow rate (β2) to the total flow rate of the second carbon dioxide supplied to the second methane reactor 72 via the second supply path 3b. The flow rate ratio calculation means 9 can calculate the ratio of the water vapor flow rate to the carbon dioxide flow rate based on the ratio of the first flow rate (β1).

[0046] [Electrolysis utilization rate detection unit] The electrolysis utilization rate detection unit 56 calculates the ratio of the electrolysis current to the rated electrolysis current as the electrolysis current ratio (R). However, R is assumed to be less than 1.

[0047] 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 (%) and the electrolysis current ratio R, according to the following equation (Equation 1). Here, the rated electrolysis utilization rate (%) refers to the gas utilization rate set when the electrolysis current equivalent to the rated value is reached. 400 / (Rated electrolysis utilization rate (%) x electrolysis current ratio (R))-1 (Formula 1)

[0048] For example, if the rated electrolysis utilization rate is 80% and the electrolysis current ratio (R) is 0.5, according to (Equation 1), the flow rate ratio of water vapor to carbon dioxide supplied to the cathode electrode 53 is calculated as 9(400 / (80×0.5)-1). Therefore, the flow rates of water vapor and carbon dioxide are set so that the flow rate ratio of water vapor to carbon dioxide is 9. As the electrolysis current ratio (R) approaches 1, the flow rate ratio of water vapor 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 obtained. This control can be performed by reducing the amount of water vapor supplied as the electrolysis current increases and increasing the flow rate of carbon dioxide supplied as the electrolysis current increases, or by fixing the amount of water vapor supplied at a constant value regardless of the increase in electrolysis current and increasing the flow rate of carbon dioxide supplied as the electrolysis current increases.

[0049] Furthermore, the rated electrolysis utilization rate is preferably set to 60-90%, and more preferably to 60-80%. If the rated electrolysis utilization rate (%) 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 (%) 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.

[0050] [Temperature detection means] The temperature detection means 10 is provided at the outlets of the first methanation reactor 71 and the second methanation reactor 72, and detects the exothermic temperature generated by the methanation reaction. The detected temperature information is transmitted to the operation control unit A, which is configured to change the ratio of the first flow rate (β1) and the ratio of the second flow rate (β2) provided by the first branching section 3c of the carbon dioxide supply unit 3. The temperature detection means 10 also includes an exothermic temperature measurement unit 10a located in the high-temperature part of the methanation reaction unit 7 and upstream of the second supply path 3b.

[0051] (Configuration of the co-electrolytic methanation apparatus according to the second embodiment) Next, the co-electrolytic methanation apparatus 101 according to the second embodiment will be described. As shown in Figure 5, the co-electrolytic methanation apparatus 101 differs from the co-electrolytic methanation apparatus 100 according to the first embodiment in that it includes a carbon dioxide supply unit 3 having a second supply path 3d and a second branching section 3f. The co-electrolytic methanation apparatus 101 according to the second embodiment will be described below, but the same configuration as the co-electrolytic methanation apparatus 100 according to the first embodiment will not be described.

[0052] One end of the third supply channel 3e is connected between the second methanation reactor 72 and the third methanation reactor 73, and downstream of the temperature sensing means 10, while the other end is connected to the second branch section 3f. The second branch section 3f connects the second supply channel 3d and the third supply channel 3e. The first branch section 3c and the second branch section 3f are equipped with flow control valves (not shown) and are configured to control the flow rate of carbon dioxide supplied to the co-electrolysis section 5 via the first supply channel 3a, the flow rate of carbon dioxide supplied to the second methanation reactor 72 via the second supply channel 3d, and the flow rate of carbon dioxide supplied to the third methanation reactor 73 via the third supply channel 3e, based on temperature information from the temperature sensing means 10.

[0053] Furthermore, gas flow meters (not shown) are provided in two or more of the second supply channels 3b, second supply channel 3d, and third supply channel 3e within the carbon dioxide supply section 3. The flow rate ratio calculation means 9 calculates the ratio of the first flow rate (β1), the ratio of the second flow rate (β2), and the ratio of the third flow rate (β3) to the total flow rate of the first flow rate of carbon dioxide supplied to the co-electrolysis section 5 via the first supply channel 3a, the second flow rate of carbon dioxide supplied to the second methane reactor 72 via the second supply channel 3d, and the third flow rate of carbon dioxide supplied to the third methane reactor 73 via the third supply channel 3e. [Examples]

[0054] Co-electrolytic methanation apparatuses shown in Figures 1, 3, and 5 were constructed, and the temperature changes of the first methanation reactor 71 and the second methanation reactor 72 were investigated when the ratio of the first flow rate (β1) or the ratio of the second flow rate (β2) was changed.

[0055] (Example 1) The results obtained using the co-electrolytic methanation apparatus 100 shown in Figure 3 are shown in Figure 4.

[0056] A first flow rate ratio (β1) = 1 indicates that all the carbon dioxide supplied by the carbon dioxide supply unit 3 is supplied to the co-electrolysis unit 5, and no carbon dioxide is supplied to the subsequent methanation reaction unit 7. Furthermore, β1 = 0 indicates that all the carbon dioxide supplied by the carbon dioxide supply unit 3 is supplied to the subsequent methanation reaction unit 7, and no carbon dioxide is supplied to the cathode electrode 53 of the co-electrolysis unit 5, meaning that only steam electrolysis is performed in the co-electrolysis unit 5. As shown in Figure 4, when β1 = 1, both the first stage first methanation reactor 71 and the second stage second methanation reactor 72 exceed 600°C. By lowering β1 from a state where β1=1 to 0 and increasing the ratio of the second flow rate (β2), it was possible to lower the temperature of the first methanation reactor 71. Specifically, the temperature of the first methanation reactor 71 began to decrease significantly when β1 was reduced to around 0.4, and when β1 was reduced to 0.35, the temperature of the first methanation reactor 71 fell below 650°C. Further reducing β1 to around 0.25 allowed the temperatures of both the first methanation reactor 71 and the second methanation reactor 72 to fall below 600°C. Therefore, by changing β1 so that the temperature of the first methanation reactor 71 reaches the target value, it becomes possible to avoid thermal degradation of the catalyst in the methanation reaction section 7.

[0057] (Example 2) Next, as shown in Figure 5, Figure 6 shows the results obtained using a co-electrolytic methanation apparatus 101 in which the carbon dioxide supply unit 3 is further branched to provide a second supply path 3d and a third supply path 3e.

[0058] In Example 2, β1 was fixed at 0.25, and β2 + β3 = 0.75. The ratio of the second flow rate (β2) to the sum of the ratio of the second flow rate (β2) and the ratio of the third flow rate (β3), β2 / (β2 + β3), was used to vary the ratio of the second flow rate (β2) within the range of 0 to 1. Furthermore, β2 / (β2 + β3) = 1 indicates that of the carbon dioxide supplied by the carbon dioxide supply unit 3, 0.25 is supplied to the co-electrolysis unit 5, 0.75 is supplied to the second methanation reactor 72, and not to the third methanation reactor 73. Furthermore, β2 / (β2 + β3) = 0 indicates that of the carbon dioxide supplied by the carbon dioxide supply unit 3, 0.25 is supplied to the co-electrolysis unit 5, not to the second methanation reactor 72, and 0.75 is supplied to the third methanation reactor 73.

[0059] As is clear from Figure 6, the temperature of the second methanation reactor begins to decrease when β2 / (β2+β3) falls below 0.4. For example, if the goal is to maintain the temperatures of the first methanation reactor 71 and the second methanation reactor 72 at around 550°C, then β2 / (β2+β3) is preferably around 0.34 to 0.36. In other words, it can be seen that the values ​​should be determined so that β1 / β2 / β3 = 0.25 / 0.25 / 0.50. Therefore, by determining β1 so that the temperature of the first methanation reactor 71 reaches the target value, and further determining β2 so that the temperature of the second methanation reactor 72 reaches the target value, it becomes possible to avoid thermal degradation of the catalyst in the methanation reaction section 7.

[0060] (Comparative example) Next, Figure 2 shows the results obtained using the conventional co-electrolytic methanation apparatus 1 shown in Figure 1. In the conventional co-electrolytic methanation apparatus 1, the second supply channel 3b is connected upstream of the first methanation reactor 71, and the supply of carbon dioxide via the second supply channel 3b is carried out to the first methanation reactor 71.

[0061] When the first flow rate ratio (β1) = 1, meaning all the carbon dioxide supplied by the carbon dioxide supply unit 3 is supplied to the co-electrolysis unit 5, the outlet temperature of the first methanation reactor 71 was extremely high at approximately 690°C. By decreasing β1, the temperature of the first methanation reactor 71 can be lowered. However, even when β1 = 0, meaning only steam electrolysis is performed in the co-electrolysis unit 5, the temperature of the first methanation reactor 71 is around 630°C, which still has the potential to exceed the heat resistance of the catalyst. Therefore, if the carbon dioxide supplied via the second supply channel 3b is supplied from the upstream side of the first methanation reactor 71, as in the conventional co-electrolytic methanation apparatus 1 shown in Figure 1, it is not possible to suppress the temperature rise of the methanation reaction section 7. <Another Embodiment>

[0062] (1) In the above embodiment, the second supply channel 3d and the third supply channel 3e of the carbon dioxide supply unit 3 are connected by the second branching section 3f. However, the embodiment is not limited to this, and the first supply channel 3a, the second supply channel 3b, and the third supply channel 3e may be connected to the first branching section 3c. In this case, one end of the second supply channel 3b is connected between the first methanation reactor 71 and the second methanation reactor 72, and the second supply channel 3d is not formed.

[0063] (2) In the above embodiment, a configuration using a three-stage methanation reactor was shown. However, it is not limited to this, and it can also be carried out with a multi-stage reactor. Figure 7 shows an example of a multi-stage reactor. As shown in Figure 7, the co-electrolytic methanation apparatus 102 is equipped with N-stage methanation reactors, and all stages up to the last stage are adiabatic methanation reactors. The carbon dioxide supply unit 3 is equipped with a first supply path 3a, a second supply path 3b and a second supply path 3d, a third supply path 3e and a third supply path 3g, an N-1 supply path 3n-1, and an N supply path 3n. Furthermore, the carbon dioxide supply unit 3 is equipped with an Nth branching section 3N that connects the N-1 supply path 3n-1 and the N supply path 3n. If the outlet temperature of the N-1 methanation reactor exceeds the target temperature, the amount of carbon dioxide supplied via the N-1 supply path 3n-1 is reduced, and the amount of carbon dioxide supplied via the N supply path 3n is increased. This method allows for precise control of the reaction temperature throughout the entire multi-stage reactor.

[0064] (3) In the above embodiment, adiabatic methanation reactors were used for the first methanation reactor 71 and the second methanation reactor 72. However, the first methanation reactor 71 and the second methanation reactor 72 may not be adiabatic methanation reactors. As an example, Figure 8 shows a co-electrolytic methanation apparatus 103 in which a heat recovery reactor 7a is used as the first reactor and an isothermal reactor 7b is used as the second reactor. If heat transfer is insufficient in the heat exchange section necessary for heat and isothermal maintenance in the first-stage reactor, the temperature may rise locally in the gas flow path, which is the catalyst-filled section, especially near the reactor inlet. Therefore, the detection target portion of the temperature detection means 10 may be selected to be the area near the gas inlet, which is likely to reach the highest temperature in the heat recovery reactor 7a. In that case, the temperature of the area most susceptible to thermal degradation of the catalyst is monitored, and if there is a risk of exceeding the target temperature, the ratio β1 of the first flow rate supplied via the first supply path 3a is lowered, and the ratio of carbon dioxide supplied via the path from the second supply path 3b onward is increased. This method makes it possible to extend the lifespan of the methanation reaction unit 7 through SOEC methanation.

[0065] 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]

[0066] 100, 101: Co-electrolytic methanation apparatus 2: Steam supply unit 3: Carbon Dioxide Supply Unit 3a: 1st supply route 3b: 2nd supply path 3d: 2nd supply route 3e: 3rd supply route 3g: 3rd supply route 3c: First branching point 3f: Second branching point 5: Co-electrolytic part 6: Water separation and compression section 7: Methanation reactor 71: First Methanation Reactor 72: Second Methanation Reactor 8: Gas-liquid separator (separation section) 9:Flow rate ratio calculation means 10: Temperature detection means 10a: Heat generation temperature measurement unit 20:Flow rate regulator (flow rate control means) 30: Flow rate regulator (flow rate control means) 53: Cathode pole 56: Electrolysis utilization rate detection unit 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, An electrolysis utilization rate detection unit that recognizes the flow rates of water vapor and carbon dioxide supplied to the cathode electrode and can detect the electrolysis current value or electrolysis voltage, A water separation and compression unit that cools the mixed gas to separate the water and compress it, A methanation reaction section comprising multiple stages of a methanation reactor, which performs methanation by passing the synthesis gas from which water has been separated through a methanation catalyst, A separation unit for separating methane from the gas produced in the aforementioned methanation reactor, Control means for controlling operation, A co-electrolytic methanation apparatus comprising, The system includes a temperature detection means for detecting the temperature of the methane reactor, The temperature detection means includes at least one heat-generating temperature measuring unit disposed in the part of the methane reactor that becomes hot, The carbon dioxide supply unit comprises a first supply path for supplying the carbon dioxide to the co-electrolysis unit, a second supply path for supplying the carbon dioxide to the methane reactor, and a first branching section connecting the first supply path and the second supply path. The second supply channel is a co-electrolytic methanation apparatus connected downstream of the heat generation temperature measurement unit in the methanation reactor.

2. The system includes a flow rate ratio calculation means for calculating the ratio (β1) of the first flow rate of carbon dioxide supplied to the co-electrolysis unit via the first supply passage and the second flow rate of carbon dioxide supplied to the methane reactor via the second supply passage to the total flow rate, The co-electrolytic methanation apparatus according to claim 1, wherein the first branch section changes the ratio (β1) of the first flow rate based on temperature information detected by the temperature sensing means.

3. The first stage of the aforementioned methanation reactor is an adiabatic first methanation reactor. The final stage is an isothermal methane reactor or an adiabatic methane reactor. A heat exchanger is provided downstream of the first methanation reactor. The co-electrolytic methanation apparatus according to claim 2, wherein the heat generation temperature measuring unit is disposed between the first methanation reactor and the heat exchanger.

4. The co-electrolytic methanation apparatus according to claim 3, wherein the flow rate ratio calculation means calculates the ratio of the water vapor flow rate to the carbon dioxide flow rate based on the ratio of the first flow rate (β1), and the flow rate control means controls the respective flow rates of the water vapor and carbon dioxide supplied to the cathode electrode based on the flow rate ratio.

5. The second stage of the aforementioned methanation reactor is an adiabatic second methanation reactor. The carbon dioxide supply unit further comprises a third supply channel connected downstream of the second methane reactor, and a second branching section connecting the second supply channel and the third supply channel. The flow rate ratio calculation means further calculates the ratio of the second flow rate (β2) and the ratio of the third flow rate (β3) to the total flow rate of the first flow rate, the second flow rate, and the third flow rate of carbon dioxide supplied downstream of the second methane reactor via the third supply path. The co-electrolytic methanation apparatus according to claim 4, wherein the first branch and the second branch change the ratio of the first flow rate (β1) and the ratio of the second flow rate (β2) based on temperature information detected by the temperature detection means.

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