Co-electrolysis methanation apparatus and operation method of co-electrolysis methanation apparatus

Abstract

To provide a co-electrolysis methanation apparatus which maintains the calorie of production gas in a final outlet part at a high level, and reduces a fluctuation range of the calorie, and an operation method of the same.SOLUTION: A co-electrolysis methanation apparatus 1, which includes a co-electrolysis part 5 for electrolyzing water vapor and carbon dioxide, and generating synthesis gas containing hydrogen, carbon monoxide, water vapor and carbon dioxide, flow rate control means for controlling each flow rate of the water vapor and the carbon dioxide supplied to a cathode electrode 53 of the co-electrolysis part 5, a water separation / compression part 6 for cooling the synthesis gas, and separating and compressing water, a methane synthesis part 7 for subjecting the synthesis gas from which the water is separated to methanation through a methanation catalyst, and a separation part for separating methane from gas sent out from the methane synthesis part 7, includes flow rate ratio calculation means 9 for calculating a reference flow rate ratio which is a ratio of a flow rate of water vapor to the flow rate of the carbon dioxide, and controls each of the flow rates of the water vapor and the carbon dioxide supplied to the cathode electrode 53, on the basis of the reference flow rate ratio.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an SOEC co-electromethanation apparatus and method of operation. [Background technology]

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

[0003] At the cathode of co-electrolysis, it is difficult to completely convert the raw materials, carbon dioxide and water vapor, into carbon monoxide and hydrogen (100% utilization rate), and at the cathode, in addition to carbon monoxide and hydrogen, unelectrolyzed carbon dioxide and water vapor are mixed. The water vapor is removed as water in a gas-liquid separator installed before the methanation process. In co-electrolysis methanation systems that produce methane in a methanation unit, if the flow rates of carbon dioxide and water vapor supplied to the cathode of the co-electrolysis unit are not optimally and precisely controlled, the concentrations of carbon dioxide and hydrogen other than methane in the gas composition at the methanation outlet increase, resulting in a problem of reduced methane purity.

[0004] Several methods have been proposed to solve this problem. Patent Document 1 discloses a fuel production method that includes the steps of supplying water vapor and carbon dioxide to the cathode electrode side of an electrolysis unit after adjusting the flow rates to a predetermined molar ratio corresponding to the type of fuel to be synthesized, and sequentially electrolyzing the supplied water vapor and carbon dioxide to produce hydrogen and carbon monoxide, and synthesizing the fuel by cooling and pressurizing the resulting mixture and then passing it through a catalyst in a synthesis section. It also describes that the optimal carbon dioxide / water vapor ratio for methane synthesis is 1:3, and that the amount of water supplied should be in excess of the weight of the carbon monoxide and hydrogen mixed gas, with the excess supply amount being preferably 1% to 10% of the weight of the mixed gas to be produced.

[0005] Patent Document 2 discloses a method in which the molar flow rate (a) of water vapor to be supplied to a solid oxide electrolysis device and the molar flow rate (b) of carbon dioxide gas to be supplied to the solid oxide electrolysis device are first determined, and the rate (e) at which the supplied water vapor and carbon dioxide gas are reduced and converted to hydrogen and carbon monoxide is controlled within a certain range, within a range calculated from this ratio. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-119556 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-152219 Summary of the Invention [Problem to be solved by the invention]

[0007] Through investigations by the inventors, it has been found that the following problems exist in a fuel production apparatus using co-electrolysis.

[0008] In the method of Patent Document 1, the excess water supply amount is too small from the perspective of preventing carbon deposition. Furthermore, when the supply gas utilization rate of water vapor and carbon dioxide is high at 90%, comparing the water supply rate of 1% and 10%, the calorific value increased when the water supply rate was 10%, but the proportion of carbon dioxide was high. Furthermore, it was found that when the supply gas utilization rate was 85% or less, the methane purity did not increase and the calorific value decreased significantly.

[0009] In addition, in the method of Patent Document 2, carbon dioxide was contained at a high concentration of 18 to 24%. In addition, the methane concentration was low at 76 to 82%, and the calorific value of the outlet gas was 30 to 32 MJ / m 3 It was found that the value of the fuel was low.

[0010] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a co-electrolytic methanation apparatus and an operating method of a co-electrolytic methanation apparatus that can maintain the calorific value of the produced gas at a high level and reduce the fluctuation range of the calorific value of the produced gas at the final outlet portion. [Means for solving the problem]

[0011] The characteristic configuration of the co-electrolytic methanation device according to the present invention to achieve the above object is as follows: a co-electrolysis unit that electrolyzes water vapor and carbon dioxide to generate a synthesis gas containing hydrogen, carbon monoxide, water vapor, and carbon dioxide; a flow rate control means for controlling the flow rates of the water vapor and the carbon dioxide supplied to the cathode of the co-electrolysis unit; a water separation and compression unit that cools the synthesis gas to separate water from the synthesis gas and compresses the water; a methane synthesis unit for methanating the synthesis gas from which water has been separated by passing the synthesis gas through a methanation catalyst; a separation unit that separates methane from the gas delivered from the methane synthesis unit; A co-electromethanation apparatus comprising: The present invention is characterized in that it includes a flow rate ratio calculation means for calculating a reference flow rate ratio (Rs), which is the ratio of the flow rate of the water vapor to the flow rate of the carbon dioxide, and controls the flow rates of the water vapor and the carbon dioxide to be supplied to the cathode electrode based on the reference flow rate ratio (Rs).

[0012] According to the above characteristic configuration, the flow rate ratio calculation means calculates the reference flow rate ratio (Rs), which is the ratio of the flow rate of water vapor to the flow rate of carbon dioxide, and sets an optimal ratio of water vapor to carbon dioxide in accordance with the operating state, thereby enabling the co-electrolysis unit to produce hydrogen and CO at an optimal ratio. Therefore, it is possible to maintain the calorific value of the produced gas at a high level and reduce the fluctuation range of the calorific value of the produced gas at the final outlet portion.

[0013] Further characteristic features of the co-electromethanation device according to the present invention include: The flow rate ratio calculation means calculates the total supply gas utilization rate (α) according to the following equation (1), and calculates the reference flow rate ratio (Rs) according to the following equation (2). Total flow rate of hydrogen and carbon monoxide produced by electrolysis / Total flow rate of water vapor and carbon dioxide supplied to the cathode = α (1) 4 / α-1=Rs (2)

[0014] According to the above characteristic configuration, the flow rate ratio calculation means calculates the ratio of hydrogen and carbon monoxide generated by electrolysis in the co-electrolysis unit to the flow rates of water vapor and carbon dioxide supplied to the co-electrolysis unit as the total supply gas utilization rate (α). Then, a reference flow rate ratio (Rs) is calculated based on the total supply gas utilization rate (α). The flow rates of water vapor and carbon dioxide supplied to the co-electrolysis unit are controlled based on the reference flow rate ratio (Rs). In this way, it is possible to produce an appropriate amount of hydrogen necessary for methanating a mixed gas containing carbon monoxide and carbon dioxide in the methane synthesis unit and maintain a high purity of methane. Therefore, it is possible to maintain the calorific value of the produced gas at a high level and reduce the fluctuation range of the calorific value of the produced gas at the final outlet portion.

[0015] Further characteristic features of the co-electromethanation device according to the present invention include: The flow rate ratio calculation means sets the reference flow rate ratio (Rs) to 100%, sets a reference fluctuation range to be between 80% and 110% of the reference flow rate ratio (Rs), and controls the flow rates of the water vapor and the carbon dioxide supplied to the cathode electrode so that they fall within the range of the reference fluctuation range.

[0016] According to the above characteristic configuration, a standard fluctuation range is set based on a standard flow rate ratio (Rs) corresponding to the total supply gas utilization rate (α). The flow rates of steam and carbon dioxide supplied to the co-electrolysis unit are controlled so that they fall within the range. This makes it possible to generate the amount of hydrogen necessary for methanating a mixed gas containing carbon monoxide and carbon dioxide in the methane synthesis unit and maintain a high level of methane purity. Therefore, it is possible to maintain the calorific value of the produced gas at a high level and reduce the fluctuation range of the calorific value of the produced gas at the final outlet portion.

[0017] Further characteristic features of the co-electromethanation device according to the present invention include: The methane synthesis unit includes two or more reactors, and the separation unit includes two or more heat exchangers that cool the heat of reaction and two or more gas-liquid separators that remove condensed water. The heat exchangers and the gas-liquid separators are provided downstream of the reactors, and gas whose temperature has increased due to the heat of reaction in the reactors is cooled by the heat exchangers. The gas from which the condensed water has been removed in the gas-liquid separators is introduced into the reactor in the next stage, where the methanation is further performed.

[0018] Because the methanation reaction generates a large amount of heat, it can rapidly progress locally near the reactor inlet, causing the catalyst to reach a temperature close to its heat resistance limit, resulting in thermal degradation. In addition, the temperature rise can also cause the outlet gas temperature to exceed the desired temperature. According to the above characteristic configuration, the reactors are arranged in two or more stages, and gas whose temperature has risen due to the heat of reaction in the first-stage reactor is cooled by a heat exchanger for cooling provided in the second stage of the reactor, and after cooling, condensed water is removed by a gas-liquid separator before being introduced into the second-stage reactor. The compressed gas is passed through the gas-liquid separator to remove and separate the condensed water, and the gas with a reduced volume flow rate is recompressed, thereby reducing the compression power. Therefore, according to the above-described characteristic configuration, by precisely controlling the reaction temperature in the downstream reactor to a lower temperature, high-purity methane can be obtained, and it is therefore possible to maintain the calorific value of the produced gas at a high level and reduce the fluctuation range of the calorific value of the produced gas at the final outlet portion.

[0019] The characteristic configuration of the operating method of the co-electrolytic methanation apparatus according to the present invention is as follows: A method for operating a co-electrolytic methanation device for producing methane from hydrogen and carbon monoxide obtained by electrolyzing water vapor and carbon dioxide supplied to a cathode, comprising: The flow rates of the water vapor and carbon dioxide are controlled based on a reference flow rate ratio (Rs), which is the ratio of the water vapor flow rate to the carbon dioxide flow rate.

[0020] A further characteristic feature of the method for operating a co-electromethanation apparatus according to the present invention is that a calculation step of a total supply gas utilization rate (α) calculated by the following formula (1); and calculating a reference flow rate ratio (Rs) of the water vapor and carbon dioxide calculated by the following equation (2), and controlling the flow rate of the water vapor relative to the flow rate of the carbon dioxide based on the calculated reference flow rate ratio (Rs): Total flow rate of hydrogen and carbon monoxide produced by electrolysis / Total flow rate of water vapor and carbon dioxide supplied to the cathode = α (1) 4 / α-1=Rs (2)

[0021] A further characteristic feature of the method for operating a co-electromethanation apparatus according to the present invention is that The reference flow rate ratio (Rs) is set to 100%, and a reference fluctuation range is set to 80% or more and 110% or less of the reference flow rate ratio (Rs), and the flow rates of the water vapor and the carbon dioxide supplied to the cathode are controlled so as to be within the range of the reference fluctuation range. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 illustrates a co-electromethanation apparatus according to an embodiment of the present invention. [Figure 2] FIG. 1 illustrates a modified co-electromethanation apparatus according to an embodiment of the present invention. [Figure 3] FIG. 10 illustrates a further modification of a co-electromethanation apparatus according to an embodiment of the present invention. [Figure 4] 10 is a graph showing the change in supply gas utilization rate (α) and H2O / CO2 ratio when the standard fluctuation range is set to 80% or more and 110% or less. [Figure 5] 10 is a graph showing the change in supply gas utilization rate (α) and H2O / CO2 ratio when the standard fluctuation range is set to 88% or more and 106% or less. [Figure 6] 10 is a graph showing the change in supply gas utilization rate (α) and H2O / CO2 ratio when the standard fluctuation range is set to 94% or more and 103% or less. [Figure 7] FIG. 4 is a diagram showing the fluctuation range of the calorific value of the generated gas when the total supply gas utilization rate is 60% in an embodiment of the present invention. [Figure 8] FIG. 4 is a diagram showing the fluctuation range of the calorific value of the generated gas when the total supply gas utilization rate is 70% in an embodiment of the present invention. [Figure 9] FIG. 4 is a diagram showing the fluctuation range of the calorific value of the generated gas when the total supply gas utilization rate is 80% in an embodiment of the present invention. [Figure 10] FIG. 4 is a diagram showing the fluctuation range of the calorific value of the generated gas when the total supply gas utilization rate is 90% in an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, a co-electrolytic methanation apparatus 1 according to an embodiment of the present invention will be described with reference to the drawings.

[0024] As shown in FIG. 1, the co-electrolytic methanation apparatus 1 includes a water vapor supply unit 2, a carbon dioxide supply unit 3, a confluence flow path 4, a co-electrolysis unit 5, a water separation / compression unit 6, a methane synthesis unit 7, a gas-liquid separator 8 (an example of a separation unit), a flow rate ratio calculation means 9, and an operation control unit A capable of controlling the operation of each component.

[0025] [Water vapor supply section] The water vapor supply unit 2 is provided with an evaporator 21 for evaporating water. A flow rate regulator 22 (an example of a flow rate control means) for regulating the flow rate of water vapor is provided downstream of the evaporator 21. Heat generated in the methane synthesis unit 7 can be used as a heat source for evaporating water. Any shortage can be made by heating with an electric heater. The flow rate regulator 22 is designed to constantly control the flow rate of water vapor based on the reference flow rate ratio (Rs) calculated by the flow rate ratio calculation means 9.

[0026] [Carbon dioxide supply unit] The carbon dioxide supply unit 3 is provided with a flow rate regulator 32 (an example of a flow rate control means) that regulates the flow rate of carbon dioxide. The flow rate regulator 32 is designed to constantly control the flow rate of carbon dioxide based on the reference flow rate ratio (Rs) calculated by the flow rate ratio calculation means 9.

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

[0028] When carbon dioxide contains sulfur components, halogen compounds, siloxane compounds, heavy hydrocarbons, and the like, these may cause deterioration of the solid oxide electrolysis cell or the methanation catalyst in the downstream stage, and therefore, it is preferable to remove these, as necessary, before subjecting the carbon dioxide to the reaction.

[0029] The flow rate regulator 22 provided in the water vapor supply unit 2 and the flow rate regulator 32 provided in the carbon dioxide supply unit 3 supply water vapor and carbon dioxide based on predetermined control. The ratio of water vapor to carbon dioxide supplied is always controlled based on the total supply gas utilization rate (α) and the reference flow rate ratio (Rs) calculated by a flow rate ratio calculation means 9 described later.

[0030] The water vapor supply unit 2 and the carbon dioxide supply unit 3 are connected to a junction flow path 4 that joins the water vapor and carbon dioxide. The junction flow path 4 is connected to a cathode 53 of a solid oxide electrolysis cell included in a co-electrolysis unit 5.

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

[0032] The anode 52 and the cathode 53 are connected to a power supply unit (not shown) that supplies power to the anode 52 and the cathode 53. The power supply unit applies a positive potential to the anode 52 and a negative potential to the cathode 53. The voltage applied to the anode 52 and the cathode 53 is preferably about 1.0V to 1.3V.

[0033] The electrolysis reaction is usually carried out at about 600°C to 1000°C, preferably about 650°C to 850°C. When the temperature of the electrolysis reaction is within this range, the oxygen ion conductivity of the electrolyte material is ensured and the electrode reaction occurs quickly, and if the temperature is set to about the theoretical electrolysis voltage, electrolysis can be continued, enabling highly efficient operation.

[0034] The co-electrolysis unit 5 obtains a mixed gas (mixed fluid) containing hydrogen (H) and carbon monoxide (CO) by co-electrolysis of water (H2O) and carbon dioxide (CO2). As shown below, the co-electrolysis proceeds simultaneously at the cathode 53 as shown in formula (I) and formula (II). Oxygen ions generated by the co-electrolysis of water vapor and carbon dioxide at the cathode 53 pass through the solid electrolyte 51 due to the potential gradient and move to the anode 52, where oxygen gas is produced according to formula (III). The combined electrolytic reaction is an endothermic reaction. <Cathode> H2O+2e - → H2+O 2- (I) CO2+2e -→ CO+O 2- (II) <Anode> 2O 2- → O2+4e - (III)

[0035] Nitrogen or air (hereinafter referred to as SWEEP gas) is supplied to the anode inlet to remove the oxygen generated by formula (III) from the anode 52, and the generated oxygen is added to this SWEEP gas and discharged from the anode 52 outlet (hereinafter referred to as oxygen-enriched gas). This oxygen-enriched gas is also at a sufficiently high temperature, so it is desirable to heat it by exchanging heat with the SWEEP gas, but because the oxygen-enriched gas is still at a high temperature, it is desirable to recover it using a heat medium (oil, water) for heat recovery and discharge it.

[0036] [Water separation and compression section] The water separation / compression unit 6 includes a gas-liquid separator 61 that cools the gas containing hydrogen, carbon monoxide, water vapor, and carbon dioxide obtained by the co-electrolysis unit 5 and separates the water vapor into water, and a pressure booster 62 that boosts the pressure to a level suitable for methane synthesis in the subsequent stage. Since the gas after the electrolysis reaction is at a sufficiently high temperature, it is preferable to use it for preheating, for example, a mixed gas of carbon dioxide and hydrogen to be subjected to the electrolysis reaction by heat exchange.

[0037] If the temperature is not sufficiently reduced by heat exchange with the mixed gas of carbon dioxide and hydrogen to be subjected to the electrolysis reaction, a further heat exchanger is connected and the temperature is cooled by means of cooling water or air cooling, preferably to 50°C or less, more preferably to 40°C or less, and the condensed water is separated.

[0038] In the water separation / compression section 6, in addition to separating water in the gas-liquid separator 61, the synthesis gas is compressed in the pressure booster 62 to a pressure suitable for the subsequent methanation reaction step. The higher the pressure after compression, the more desirable it is for increasing the methane purity. However, an optimal pressure range can be selected and configured taking into consideration factors such as equipment costs and the specifications of the destination facility. For example, the reaction can be carried out at about 2.0 MPa, equivalent to a high-pressure gas pipeline for city gas (1.0 MPaG) or higher, or the methanation reaction can be carried out at a pressure range equivalent to a medium-pressure gas pipeline (0.3 to 1.0 MPaG). However, even if the gas-liquid separator 61 is installed midway through the methane synthesis section 7, a pressure of 0.5 MPaG or higher is desirable.

[0039] Conventional rotary or reciprocating gas compressors can be used to compress synthesis gas. Compressing synthesis gas raises the temperature of the process gas, but rather than compressing it to the desired pressure in a single stage, it is preferable from the viewpoint of equipment maintenance to compress it in multiple stages while cooling it in between. This gas is cooled by heat exchange with low-temperature nitrogen or water, but the compressed gas can be passed through a gas-liquid separator to remove and separate the condensed water, and the gas with a reduced volumetric flow rate can be recompressed, which has the added benefit of reducing the compression power required. For example, synthesis gas is first compressed to 0.20 MPaG and methanated in the methane synthesis section 7, and the gas after the reaction is 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, and then compressed further to 0.8 MPaG and further methanated in the next-stage methane synthesis section 7. Finally, it is again cooled to 20°C or less with cooling water in the cooling heat exchanger 81, thereby making it possible to obtain high-purity methane.

[0040] [Methane synthesis section] The methane synthesis unit 7 includes a methanation catalyst with which the mixed gas comes into contact. As the methanation catalyst, a known methanation catalyst carrying a Ni or Ru-based metal can be used.

[0041] The methane synthesis unit 7 obtains a product gas (product fluid) containing water (H2O) and methane (CH4) through a methanation reaction from hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2). The methanation reaction proceeds according to the following equations (IV) and (V). This reaction is an equilibrium reaction, and both reactions to the right are exothermic. 3H2+CO⇔H2O+CH4 (IV) 4H2+CO2⇔2H2O+CH4 (V)

[0042] Since the reactions represented by formulas (IV) and (V) are exothermic, it is desirable to allow these reactions to proceed at low temperatures. However, the methanation reaction generates a large amount of heat, and so the methanation reaction may proceed rapidly and locally near the reactor inlet, causing the catalyst to reach a temperature close to its heat resistance temperature, resulting in thermal degradation and possibly even causing the outlet gas temperature to exceed the desired temperature.

[0043] One way to suppress local temperature rises during such methanation reactions and increase methane purity is to configure the methane synthesis unit 7 in two or more stages, provide a cooling heat exchanger 81 and a gas-liquid separator 8 at the rear of the methane synthesis unit 7, and connect the units so that gas whose temperature has risen due to the heat of reaction in the methane synthesis unit 7 at the previous stage is cooled by the cooling heat exchanger 81, and condensed water is removed from the cooled gas by the gas-liquid separator 8 before being introduced into the methane synthesis unit 7 at the next stage. While allowing a temperature rise in the methane synthesis unit 7 at the previous stage, condensed water generated along the way is removed from the system, and the reaction temperature is precisely controlled to a lower temperature in the rear stage, thereby causing the reactions of formulas (IV) and (V) to proceed to the right and increasing the methane concentration.

[0044] In order to precisely control the temperature at a lower temperature in the subsequent reactor, it is preferable to install a heat exchange reactor in the final stage. By setting the reactor outlet temperature as low as possible and proceeding with the reaction, the equilibrium conversion rate is also increased, which may allow the number of reactor stages to be reduced compared to a simple multi-stage adiabatic reactor configuration. The inlet temperature when contacting the methanation catalyst is preferably 200°C or higher and 350°C or lower, and more preferably 225°C or higher and 275°C or lower. Setting the temperature lower within the above range suppresses local temperature increases due to the exothermic reaction described below, extending the catalyst life and making it easier to increase the methane concentration in terms of catalyst heat resistance equilibrium theory.

[0045] One method for suppressing local temperature rises near the reactor inlet in a pre-reactor is to return the downstream gas from the reactor outlet to the reactor inlet to dilute the gas used in the reaction. This allows the reaction temperature to be controlled below the heat resistance temperature of the catalyst.

[0046] [Separation part] As shown in formulas (IV) and (V), HO is produced simultaneously with methane production, so components including water are separated from the product gas by gas-liquid separator 8 to obtain high-purity methane. Separation methods such as liquefaction separation, membrane separation, and adsorption separation can be used as separation means. Gas-liquid separator 8 may use one or a combination of two or more of these separation means.

[0047] Liquefaction separation involves adjusting the temperature to liquefy and separate components, including water, to obtain a gas primarily composed of methane.

[0048] Membrane separation is a method of obtaining a gas containing methane as a main component by separating components containing water using a separation membrane that selectively allows water to pass through. The type of separation membrane is not particularly limited as long as it allows small molecular components to pass through and can separate a specific component from other components.

[0049] Adsorption separation is a method of separating components containing water by adsorbing them with an adsorbent to obtain a gas containing methane as the main component. Examples of adsorbents that can be used include silica gel, zeolite, and activated carbon. When adsorption separation is used, a heating device is provided. By heating the adsorbent, components containing water can be desorbed from the adsorbent.

[0050] [Flow rate ratio calculation means] Gas analyzers (not shown) are provided in the path through which the steam and carbon dioxide supplied to the co-electrolysis unit 5 flow, and in the flow path through which the synthesis gas containing hydrogen, carbon monoxide, steam, and carbon dioxide after co-electrolysis in the co-electrolysis unit 5 flows. The flow rate ratio calculation means 9 calculates the total supply gas utilization rate (α) from the total flow rate of the generated hydrogen and carbon monoxide relative to the total flow rate of the steam and carbon dioxide supplied to the cathode electrode 53 (total supply gas utilization rate (α) calculation step). Specifically, this 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)

[0051] In a solid oxide electrolysis cell, it is not possible to reduce all of the supplied water vapor and carbon dioxide; typically, approximately 60 to 95% of the supplied water vapor and carbon dioxide is reduced. In this embodiment, the amounts of water vapor and carbon dioxide supplied to the co-electrolysis unit 5 and the current supplied to the co-electrolysis unit 5 are controlled so that the total supply gas utilization rate (α) is 95% (0.95) or less. If the total supply gas utilization rate (α) exceeds 95% (0.95), the overvoltage of the co-electrolysis unit 5 increases, resulting in increased power consumption and cell deterioration, as well as a higher risk of carbon deposition. Furthermore, the total supply gas utilization rate (α) is preferably 60% (0.60) or more. If it is lower than 60% (0.60), less gas is produced, resulting in reduced operating efficiency.

[0052] The flow rate ratio calculation means 9 determines the amounts of water vapor and carbon dioxide to be supplied to the cathode 53 based on the total supply gas utilization rate (α). The flow rate ratio of water vapor to the flow rate of carbon dioxide (HO / CO2) is constantly varied according to the amounts of hydrogen and carbon monoxide produced in the co-electrolysis unit 5. The reference flow rate ratio (Rs) of HO / CO2 is determined by the following equation (VII) (reference flow rate ratio (Rs) calculation step): 4 / α×100-1···(VII)

[0053] As described above, the total supply gas utilization rate (α) is controlled to be within the range of 60% to 95%. Therefore, according to equation (VII), the reference flow rate ratio (Rs) of HO / CO varies within the range of 3.21 to 5.67 depending on the total supply gas utilization rate (α). In this embodiment, the reference fluctuation range of the reference flow rate ratio (Rs) is calculated with the reference flow rate ratio (Rs) as the center value (100%), and the flow rate ratio of water vapor to carbon dioxide is controlled. In this manner, even if the reference flow rate ratio (Rs) of HO / CO varies with fluctuations in the supply gas utilization rate (α), the calorific value of the outlet gas can be maintained at a high level and the fluctuation range of the calorific value can be reduced. In this embodiment, the reference fluctuation range of the reference flow rate ratio (Rs) is preferably 80% to 110%, more preferably 88% to 106%, even more preferably 94% to 103%, and particularly preferably 96% to 102%.

[0054] The relationship between the standard flow rate ratio (Rs) and the standard fluctuation range is shown in Figures 4 to 6. By determining the H2O / CO2 supply ratio according to the standard fluctuation range that matches the supply gas utilization rate (α), it is possible to keep the calorific value of the outlet gas high and reduce the fluctuation range of the calorific value. [Example]

[0055] The co-electrolytic methanation device 1 shown in Figure 1 was fabricated, and the synthesis gas generation status was investigated by changing the total supply gas utilization rate (α).

[0056] Example 1 The total supply gas utilization rate (α) was set to 60%, and the reference flow ratio of H2O to CO2 (Rs) was varied. Figure 7 shows the results for the methane concentration, hydrogen concentration, carbon dioxide concentration, and gas calorific value of the synthesis gas.

[0057] When the total supply gas utilization rate (α) is 60%, the standard flow rate ratio (Rs) of H2O / CO2 is 5.67 (4 / 0.6-1). If the standard fluctuation range is 80-110% as shown in Figure 4, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 60% is 4.53 or more and 6.23 or less. As is clear from Figure 7, the calorific value is 32.6 MJ / Nm 3 More than 7MJ / Nm 3 It was possible to do the following:

[0058] As shown in Figure 5, if the standard fluctuation range is 88 to 106%, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 60% is 4.99 or more and 6.01 or less. As is clear from Figure 7, when the calorific value is 35.5 MJ / Nm 3 More than 4MJ / Nm 3 It was possible to do the following:

[0059] As shown in Figure 6, if the standard fluctuation range is 94 to 103%, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 60% is 5.33 or more and 5.84 or less. As is clear from Figure 7, when the calorific value is 37.5 MJ / Nm 3 More than 2MJ / Nm 3 It was possible to do the following:

[0060] From the above, it was found that it is possible to maintain the calorific value of the produced gas at a high level and to reduce the fluctuation range of the calorific value of the produced gas at the final outlet portion.

[0061] The total supply gas utilization rate (α) was set to 70%, and the standard flow rate ratio of H2O / CO2 was varied. Figure 8 shows the results for the methane concentration, hydrogen concentration, carbon dioxide concentration, and gas calorific value of the synthesis gas.

[0062] When the total supply gas utilization rate (α) is 70%, the standard flow rate ratio (Rs) of H2O / CO2 is 4.71 (4 / 0.7-1). If the standard fluctuation range is 80-110% as shown in Figure 4, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 70% is 3.77 or more and 5.18 or less. As is clear from Figure 8, the calorific value is 32.6 MJ / Nm 3 More than 7MJ / Nm 3 It was possible to do the following:

[0063] As shown in Figure 5, if the standard fluctuation range is 88 to 106%, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 70% is 4.14 or more and 4.99 or less. As is clear from Figure 8, when the calorific value is 35.5 MJ / Nm 3 More than 4MJ / Nm 3 It was possible to do the following:

[0064] As shown in Figure 6, if the standard fluctuation range is 94 to 103%, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 70% is 4.42 or more and 4.85 or less. As is clear from Figure 8, when the calorific value is 37.5 MJ / Nm 3 More than 2MJ / Nm 3 It was possible to do the following:

[0065] From the above, it was found that it is possible to maintain the calorific value of the produced gas at a high level and to reduce the fluctuation range of the calorific value of the produced gas at the final outlet portion.

[0066] The total supply gas utilization rate (α) was set to 80%, and the standard flow rate ratio of H2O / CO2 was varied. Figure 9 shows the results for the methane concentration, hydrogen concentration, carbon dioxide concentration, and gas calorific value of the synthesis gas.

[0067] When the total supply gas utilization rate (α) is 80%, the standard flow rate ratio (Rs) of H2O / CO2 is 4.00 (4 / 0.8-1). If the standard fluctuation range is 80-110% as shown in Figure 4, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 80% is 3.20 or more and 4.40 or less. As is clear from Figure 9, the calorific value is 32.6 MJ / Nm 3 More than 7MJ / Nm 3 It was possible to do the following:

[0068] As shown in Figure 5, if the standard fluctuation range is 88 to 106%, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 80% is 3.52 or more and 4.24 or less. As is clear from Figure 9, when the calorific value is 35.5 MJ / Nm 3 More than 4MJ / Nm 3 It was possible to do the following:

[0069] As shown in Figure 6, if the standard fluctuation range is 94 to 103%, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 80% is 3.76 to 4.12. As is clear from Figure 9, when the calorific value is 37.5 MJ / Nm 3 More than 2MJ / Nm 3 It was possible to do the following:

[0070] From the above, it was found that it is possible to maintain the calorific value of the produced gas at a high level and to reduce the fluctuation range of the calorific value of the produced gas at the final outlet portion.

[0071] The total supply gas utilization rate (α) was set to 90%, and the standard flow rate ratio of H2O / CO2 was varied. Figure 10 shows the results for the methane concentration, hydrogen concentration, carbon dioxide concentration, and gas calorific value of the synthesis gas.

[0072] When the total supply gas utilization rate (α) is 90%, the standard flow rate ratio (Rs) of H2O / CO2 is 3.44 (4 / 0.9-1). If the standard fluctuation range is 80-110% as shown in Figure 4, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 80% is 2.76 or more and 3.79 or less. As is clear from Figure 10, the calorific value is 32.6 MJ / Nm 3 More than 7MJ / Nm 3 It was possible to do the following:

[0073] As shown in Figure 5, if the standard fluctuation range is 88 to 106%, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 90% is 3.03 or more and 3.65 or less. As is clear from Figure 10, when the calorific value is 35.5 MJ / Nm 3 More than 4MJ / Nm 3 It was possible to do the following:

[0074] As shown in Figure 6, if the standard fluctuation range is 94 to 103%, the standard fluctuation range of the standard flow rate ratio (Rs) of H2O / CO2 when the total supply gas utilization rate (α) is 90% is 3.23 or more and 3.54 or less. As is clear from Figure 10, when the calorific value is 37.5 MJ / Nm 3 More than 2MJ / Nm 3 It was possible to do the following:

[0075] From the above, it was found that it is possible to maintain the calorific value of the produced gas at a high level and to reduce the fluctuation range of the calorific value of the produced gas at the final outlet portion.

[0076] From the above results, it was found that by taking the standard fluctuation range based on the standard flow rate ratio (Rs) of H2O / CO2, which differs for each utilization rate, the fuel fluctuation range can be kept constant regardless of the utilization rate, and stable operation of the co-electromethanation system is possible.

[0077] <Another embodiment>

[0078] In this embodiment, the flow rate ratio calculation means 9 and the operation control unit A are configured separately, but this is not limiting, and the operation control unit A may be configured to include the flow rate ratio calculation means 9.

[0079] Furthermore, the configurations disclosed in the above embodiments (including other embodiments, the same applies below) can be applied in combination with configurations disclosed in other embodiments, as long as no contradictions arise. Furthermore, the embodiments disclosed in this specification are examples, and the embodiments of the present invention are not limited to these, and can be modified as appropriate within the scope that does not deviate from the purpose of the present invention. [Explanation of symbols]

[0080] 1: Co-electrolytic methanation device 5: Co-electrolytic part 6: Water separation and compression section 7: Methane synthesis section 8: Gas-liquid separator (separation section) 9:Flow rate ratio calculation means 22: Flow rate regulator (flow rate control means) 32: Flow rate regulator (flow rate control means) 53: Cathode

Claims

1. a co-electrolysis unit that electrolyzes water vapor and carbon dioxide to generate a synthesis gas containing hydrogen, carbon monoxide, water vapor, and carbon dioxide; a flow rate control means for controlling the flow rates of the water vapor and the carbon dioxide supplied to the cathode of the co-electrolysis unit; a water separation and compression unit that cools the synthesis gas to separate water and compresses the synthesis gas; a methane synthesis unit for methanating the synthesis gas from which water has been separated by passing the synthesis gas through a methanation catalyst; a separation unit that separates methane from the gas delivered from the methane synthesis unit; A co-electromethanation apparatus comprising: a flow rate ratio calculation means for calculating a reference flow rate ratio (Rs) that is a ratio of a flow rate of the water vapor to a flow rate of the carbon dioxide, and controlling the flow rates of the water vapor and the carbon dioxide to be supplied to the cathode based on the reference flow rate ratio (Rs).

2. 2. The co-electrolytic methanation apparatus according to claim 1, wherein the flow rate ratio calculation means calculates a total supply gas utilization rate (α) by the following formula (1), and calculates the reference flow rate ratio (Rs) by the following formula (2): Total flow rate of hydrogen and carbon monoxide produced by electrolysis / Total flow rate of water vapor and carbon dioxide supplied to the cathode = α (1) 4 / α-1=Rs (2)

3. 2. The co-electrolytic methanation apparatus according to claim 1, wherein the flow rate ratio calculation means sets the reference flow rate ratio (Rs) to 100%, a reference fluctuation range of 80% to 110% of the reference flow rate ratio (Rs), and controls the flow rates of the water vapor and the carbon dioxide supplied to the cathode so that the flow rates are within the reference fluctuation range.

4. 4. The co-electrolytic methanation apparatus according to claim 1, wherein the methane synthesis unit includes two or more reactors, the separation unit includes two or more heat exchangers that cool reaction heat and two or more gas-liquid separators that remove condensed water, the heat exchangers and the gas-liquid separators are provided in stages subsequent to the reactors, and gas whose temperature has been increased by the reaction heat in the reactors is cooled by the heat exchangers, and the gas from which the condensed water has been removed in the gas-liquid separators is introduced into the reactors in the subsequent stage, and the methanation is further performed.

5. A method for operating a co-electrolytic methanation device for producing methane from hydrogen and carbon monoxide obtained by electrolyzing water vapor and carbon dioxide supplied to a cathode, comprising: a reference flow rate ratio (Rs) that is a ratio of the flow rate of the water vapor to the flow rate of the carbon dioxide;

6. a calculation step of a total supply gas utilization rate (α) calculated by the following formula (1); and controlling the flow rate of the water vapor relative to the flow rate of the carbon dioxide based on a calculation step of a reference flow rate ratio (Rs) of the water vapor and the carbon dioxide calculated by the following equation (2): Total flow rate of hydrogen and carbon monoxide produced by electrolysis / Total flow rate of water vapor and carbon dioxide supplied to the cathode = α (1) 4 / α-1=Rs (2)

7. 6. The method for operating a co-electrolytic methanation apparatus according to claim 5, wherein the reference flow rate ratio (Rs) is set to 100%, a reference fluctuation range is set to be 80% or more and 110% or less of the reference flow rate ratio (Rs), and the flow rates of the water vapor and the carbon dioxide supplied to the cathode are controlled so as to be within the reference fluctuation range.

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