Co-electrolysis methanation apparatus

Abstract

To provide a co-electrolysis methanation apparatus that maintains methane purity and keeps the heat quantity per unit volume of a process outlet gas at a high level.SOLUTION: A co-electrolysis methanation apparatus comprises: a co-electrolysis section that electrolyzes steam and carbon dioxide; flow rate control means that controls a flow rate of the steam and a flow rate of the carbon dioxide; and a methanation section. The co-electrolysis methanation apparatus further comprises electrolytic voltage control means, electrolytic current measurement means, and gas utilization calculation means, wherein the flow rate control means supplies the steam and the carbon dioxide to the co-electrolysis section as a first control flow rate value, and the electrolytic current measurement means measures a current value as a first actual electrolytic current value, the gas utilization calculation means calculates an electrolytic current value as a first theoretical electrolytic current value in a case where the total amount of the steam and the carbon dioxide supplied as the first control flow rate value is completely electrolyzed into hydrogen and carbon monoxide, and when a ratio of the first actual electrolytic current value to the first theoretical electrolytic current value is defined as a first actual gas utilization rate, the flow rate control means controls the flow rate of the steam and the flow rate of the carbon dioxide as a second control flow rate value based on the first actual gas utilization rate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an SOEC co-electromethanation device. [Background technology]

[0002] A known method involves co-electrolyzing the raw materials, carbon dioxide and steam, 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] Furthermore, during long-term operation of several years, which is the required lifespan of a plant, changes in the basic performance of the SOEC cell stack may make it impossible to maintain the target methane purity.

[0005] To solve these problems, a method such as that disclosed in Patent Document 1 has been proposed. Patent Document 1 aims to ensure the quality of the product gas used in methane production when methanation is performed from carbon dioxide and hydrogen, and describes a methane production apparatus equipped with first and second reactors, measuring the CO / H gas in the gas supplied to the first reactor, and supplying additional hydrogen. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2020-158403 Summary of the Invention [Problem to be solved by the invention]

[0007] Co-electrolytic methanation systems require additional feedstock supply units if the stoichiometric ratio of carbon monoxide, hydrogen, and carbon dioxide is not supplied in the optimal ratio, making them costly. In co-electrolytic methanation systems, if the operating temperature of the co-electrolytic stack changes due to fluctuations in the ambient air temperature or temperature control mechanism, or if the cell stack deteriorates over time, the IV (electrolysis current - electrolysis voltage) of the cell stack changes, resulting in a change in the electrolysis current when reaching the target voltage. In this case, the initially expected total feed gas utilization rate (α) cannot be achieved, and the expected carbon monoxide and hydrogen production ratio cannot be realized, resulting in a problem of reduced methane purity.

[0008] 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 that can maintain the purity of methane and keep the calorific value per unit volume of the process outlet gas at a high level even if a change occurs in the electrolysis performance of the co-electrolysis cell stack. [Means for solving the problem]

[0009] 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: an electrolysis voltage control means for supplying electrolysis power to the co-electrolysis unit; an electrolysis current measuring means for measuring an electrolysis current; a gas utilization rate calculation means, the flow rate control means supplies the water vapor and the carbon dioxide to the co-electrolysis unit as first control flow rate values ​​and stabilizes them; the electrolysis current measuring means measures the current value when the electrolysis current is stabilized as a first actual electrolysis current value; the gas utilization rate calculation means calculates, based on the first control flow rate value, an electrolysis current value corresponding to when all of the water vapor and the carbon dioxide supplied are electrolyzed into hydrogen and carbon monoxide, as a first theoretical electrolysis current value, calculates a ratio of the first actual electrolysis current value to the first theoretical electrolysis current value as a first actual gas utilization rate (αr1), and calculates, based on the first actual gas utilization rate (αr1), the flow rates of the water vapor and the carbon dioxide as second control flow rate values; The flow rate control means controls the flow rates of the water vapor and the carbon dioxide based on the second control flow rate value.

[0010] If the operating temperature of the co-electrolysis unit changes due to fluctuations in the outside air temperature or the temperature control mechanism, or if the co-electrolysis unit deteriorates over time, the electrolysis current required to reach the target voltage of the electrolysis voltage control means will change. In this case, the initially expected gas utilization rate cannot be achieved, the initially expected hydrogen / carbon monoxide production ratio cannot be realized, and the methane purity in the outlet gas will decrease.

[0011] According to the above characteristic configuration, the electrolysis current measuring means measures the current value during stabilization as the first actual electrolysis current value. The gas utilization rate calculating means calculates, as the first theoretical electrolysis current value, the electrolysis current value corresponding to when all of the water vapor and carbon dioxide supplied as the first control flow rate value are electrolyzed into hydrogen and carbon monoxide. The gas utilization rate calculating means calculates, as the first actual gas utilization rate, the ratio of the first actual electrolysis current value to the first theoretical electrolysis current value. The supply flow rates of water vapor and carbon dioxide are reset based on the first actual gas utilization rate. Therefore, even if the co-electrolysis unit deteriorates and the electrolysis performance changes, the flow rates of water vapor and carbon dioxide can be changed in accordance with the deterioration and supplied as the second control flow rate value, so the methane concentration in the outlet gas can be maintained and a decrease in the heat quantity per unit volume of the gas at the outlet of the co-electrolytic methanation device can be suppressed.

[0012] A further characteristic feature of the co-electrolytic methanation apparatus according to the present invention is that the first control flow rate value determines the flow rate ratio of the water vapor and the carbon dioxide based on a first target gas utilization rate.

[0013] According to the above characteristic configuration, the flow rate ratio of water vapor and carbon dioxide, and the supply flow rates of water vapor and carbon dioxide as the first control flow rate value can be determined from the first target gas utilization rate.

[0014] Further characteristic features of the co-electromethanation device according to the present invention include: The second control flow rate value is determined by changing at least one of the total flow rate of the water vapor and the carbon dioxide and the flow rate ratio of the water vapor and the carbon dioxide from the first control flow rate value.

[0015] According to the above characteristic configuration, by changing and determining at least one of the total flow rate of water vapor and carbon dioxide and the flow rate ratio of water vapor to carbon dioxide from the first control flow rate value, it is possible to supply an optimal raw material gas in accordance with the deterioration of the co-electrolysis unit, thereby maintaining the methane concentration in the outlet gas and suppressing a decrease in the calorific value per unit volume of the gas at the outlet of the co-electrolytic methanation device.

[0016] Further characteristic features of the co-electromethanation device according to the present invention include: The supply flow rate ratio of the water vapor to the carbon dioxide is based on a value calculated from the following formula using the supply gas utilization rate (α). 400 / Supply gas utilization rate (%)-1

[0017] According to the above characteristic configuration, it is possible to always achieve the optimum supply flow rate ratio of steam and carbon dioxide for methanation. Here, the supply gas utilization rate (α) is the ratio of the electrolysis current (Ia) at that time to the theoretical electrolysis current (Ir) required when all of the water vapor and carbon dioxide are electrolyzed at 100%, and can be calculated using the following formula. α=Ia / Ir Here, the theoretical electrolysis current value Ir (kA) is calculated by dividing the total flow rate of water vapor and carbon dioxide by Fs (Nm 3 / h), Ir (kA) can be easily calculated using the following formula: Ir=2.393×Fs

[0018] Further characteristic features of the co-electromethanation device according to the present invention include: The gas utilization rate calculation means compares the first actual gas utilization rate with the first target gas utilization rate, and if they do not match, changes the second control flow rate value so that it becomes the first target gas utilization rate used in determining the first control flow rate value.

[0019] According to the above characteristic configuration, the second control flow rate value is changed so as to achieve the first target gas utilization rate used in determining the first control flow rate value. That is, the flow rate ratio of water vapor to carbon dioxide is not changed, but the total flow rate of water vapor and carbon dioxide, i.e., the individual supply amounts of water vapor and carbon dioxide, are changed. By doing so, even if the co-electrolysis section deteriorates and the electrolysis performance changes, the methane concentration in the outlet gas can be maintained, and the heat quantity per unit volume of the process outlet gas can be kept high.

[0020] Further characteristic features of the co-electromethanation device according to the present invention include: The gas utilization rate calculation means changes the second control flow rate value using the first actual gas utilization rate when the first actual gas utilization rate and the first target gas utilization rate do not match.

[0021] According to the above configuration, the first actual gas utilization rate is calculated using the following formula: In the case of 400 / first actual gas utilization rate (%)-1, the first actual supply gas utilization rate, which is different from the first target gas utilization rate, is used, so the flow rate ratio of water vapor to carbon dioxide changes. On the other hand, the first actual electrolytic current value is maintained, so the total supply flow rate of water vapor and carbon dioxide does not change. By doing so, even if the co-electrolysis section deteriorates and the electrolysis performance changes, the methane concentration in the outlet gas can be maintained, and a decrease in the calorific value per unit volume of the gas at the outlet of the co-electrolytic methanation device can be suppressed.

[0022] Further characteristic features of the co-electromethanation device according to the present invention include: The gas utilization rate calculation means changes the second control flow rate value so that, when the first actual gas utilization rate and the first target gas utilization rate do not match, the second control flow rate value becomes an arbitrarily set target gas utilization rate other than the first actual gas utilization rate and the first target gas utilization rate.

[0023] According to the above characteristic configuration, the second control flow rate value is calculated so as to achieve an arbitrarily set target gas utilization rate other than the first actual gas utilization rate and the first target gas utilization rate. By doing so, even if the co-electrolysis section deteriorates and the electrolysis performance changes, the methane concentration in the outlet gas can be maintained, and the heat quantity per unit volume of the process outlet gas can be kept high.

[0024] Further characteristic features of the co-electromethanation device according to the present invention include: The gas utilization rate calculation means changes the supply flow rate of the carbon dioxide in accordance with deterioration of the co-electrolysis unit, and does not change the supply flow rate of the water vapor from the first control flow rate value in accordance with deterioration of the co-electrolysis unit.

[0025] According to the above characteristic configuration, by changing only the supply flow rate of carbon dioxide while keeping the supply flow rate of water vapor fixed, it is possible to achieve an optimal supply ratio of water vapor and carbon dioxide depending on the deterioration of the co-electrolysis section. Therefore, even if the co-electrolysis section deteriorates and the electrolysis performance changes, the methane concentration in the outlet gas can be maintained, and the heat quantity per unit volume of the process outlet gas can be kept high.

[0026] Further characteristic features of the co-electromethanation device according to the present invention include: the flow rate control means supplies and stabilizes the flow rates of the water vapor and the carbon dioxide based on the second control flow rate value; the electrolysis current measuring means measures a current value during stabilization as a second actual electrolysis current value, the gas utilization rate calculation means calculates, based on the second control flow rate value, an electrolysis current value corresponding to when all of the water vapor and the carbon dioxide supplied are electrolyzed into hydrogen and carbon monoxide, as a second theoretical electrolysis current value, calculates, based on the second theoretical electrolysis current value, a ratio of the second actual electrolysis current value to the second theoretical electrolysis current value, and calculates, based on the second actual gas utilization rate, the flow rates of the water vapor and the carbon dioxide as third control flow rate values; The flow rate control means controls the flow rates of the water vapor and the carbon dioxide based on the third control flow rate value.

[0027] According to the above characteristic configuration, even if the common electrolysis unit gradually deteriorates due to aging, the second theoretical electrolysis current value is calculated from the amounts of water vapor and carbon dioxide supplied based on the second control flow rate value, the ratio of the second actual electrolysis current value to the second theoretical electrolysis current value is calculated as the second actual gas utilization rate, and the flow rates of water vapor and carbon dioxide are reset as the third control flow rate value based on the second actual gas utilization rate. Therefore, even if the co-electrolysis unit deteriorates and the electrolysis performance changes, the flow rates of water vapor and carbon dioxide can be changed in accordance with the deterioration and supplied as the third control flow rate value, so the methane concentration in the outlet gas can be maintained and a decrease in the heat quantity per unit volume of the gas at the outlet of the co-electrolytic methanation device can be suppressed. [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a diagram showing a schematic configuration of a co-electrolytic methanation device according to an embodiment. FIG. [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] 1 is a flowchart showing the start of operation of a co-electrolytic methanation apparatus according to an embodiment. [Figure 5] 1 is a flowchart showing the continuous operation of the co-electrolytic methanation apparatus according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0029] Hereinafter, a co-electrolytic methanation apparatus 1 according to an embodiment of the present invention will be described with reference to the drawings.

[0030] 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 channel 4, a co-electrolysis unit 5, a water separation and compression unit 6, a methane synthesis unit 7, a gas-liquid separator 8 (an example of a separation unit), a gas utilization rate calculation unit 9 (an example of gas utilization rate calculation means), and an operation control unit A capable of controlling the operation of each component.

[0031] [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.

[0032] [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.

[0033] 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.

[0034] 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.

[0035] 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 that has been set in advance.

[0036] 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.

[0037] 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 are controlled by the operation control unit A so that the flow rates of water vapor and carbon dioxide can be supplied to the co-electrolysis unit 5 as first control flow rate values.

[0038] The flow rate ratio of water vapor to carbon dioxide supplied as the first control flow rate value is calculated using the supply gas utilization rate (%) according to the following formula 1. The supply gas utilization rate (%) refers to the ratio of the electrolysis current (Ia) at that time to the theoretical electrolysis current (Ir) required when all of the water vapor and carbon dioxide are electrolyzed at 100%. 400 / Supply gas utilization rate (%)-1 (Equation 1)

[0039] For example, when the supply gas utilization rate is 80%, the flow rate ratio of water vapor to carbon dioxide supplied as the first control flow rate value is calculated as 4 according to (Equation 1). Therefore, the flow rates of water vapor and carbon dioxide are set as the first control flow rate value so that the flow rate ratio of water vapor to carbon dioxide is 4. The supply gas utilization rate is preferably 60 to 90%, and more preferably 75 to 85%. If the supply gas utilization rate (%) is less than 60%, the water flow rate ratio becomes high, which requires additional input of heat for vaporization, reducing the overall energy conversion efficiency. If the supply gas utilization rate (%) exceeds 90%, precision in the distribution ratio of the supply gas to the cell stack is required, and there is a higher risk of partial supply gas shortage due to poor supply or concerns about carbon deposition.

[0040] [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.

[0041] The anode 52 and the cathode 53 are connected to a power supply unit 54 (an example of electrolysis voltage control means) that supplies electrolysis power to the anode 52 and the 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 the cathode 53 is preferably about 1.3 V. The co-electrolysis unit 5 is also provided with an ammeter 55 (an example of electrolysis current measurement means) that measures the electrolysis current value. The electrolysis current value measured by the ammeter 55 is configured to be able to be transmitted to the gas utilization rate calculation unit 9 as a first actual electrolysis current value.

[0042] When deterioration of the solid oxide electrolysis cell progresses, the supply rates of water vapor and carbon dioxide change based on a second control flow rate value described below. The electrolysis current value measured in this case can be transmitted again to the gas utilization rate calculation unit 9 as a second actual electrolysis current value.

[0043] 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.

[0044] 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)

[0045] 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.

[0046] [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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] [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 metal can be used.

[0051] 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)

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] [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 a gas mainly composed of methane. Separation methods such as liquefaction separation, membrane separation, and adsorption separation can be used as the separation means. Gas-liquid separator 8 may use one or a combination of two or more of these separation means.

[0057] Liquefaction separation involves adjusting the temperature to liquefy and separate components, including water, to obtain a gas primarily composed of methane.

[0058] 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.

[0059] 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.

[0060] [Gas utilization rate calculation section] The gas utilization rate calculation unit 9 calculates a first theoretical electrolytic current value from the first control flow rate value, and then calculates the ratio of the first actual electrolytic current value to the first theoretical electrolytic current value as the first actual gas utilization rate (αr1).

[0061] The gas utilization rate calculation unit 9 calculates, as a first theoretical electrolysis current value, an electrolysis current value corresponding to when the total amount of water vapor and carbon dioxide supplied based on the first control flow rate value is electrolyzed into hydrogen and carbon monoxide. The first theoretical electrolysis current value is Ir (kA), and the total flow rate of water vapor and carbon dioxide is Fs (Nm 3 / h), Ir is calculated by the following (Equation 2). Ir=2.393×Fs (Equation 2)

[0062] The total flow rate Fs of water vapor and carbon dioxide is calculated by the following (Equation 3). Fs = Actual electrolysis current value / 2.393 / Target gas utilization rate (%) × 100 (Equation 3)

[0063] The gas utilization rate calculation unit 9 checks whether the first actual gas utilization rate (αr1) and the target gas utilization rate (%) match. If they match, the supply amounts and flow rate ratios of water vapor and carbon dioxide are not changed.

[0064] If the first actual gas utilization rate (αr1) and the target gas utilization rate (%) do not match, the gas utilization rate calculation unit 9 selects one of the following patterns and changes the supply rates of water vapor and carbon dioxide as the second control flow rate value. The operation control unit A controls the supply rates of water vapor and carbon dioxide by the flow rate regulators 22 and 32 based on the second control flow rate value changed by the gas utilization rate calculation unit 9. Pattern 1: At least one of the total flow rate Fs of water vapor and carbon dioxide and the flow rate ratio of water vapor to carbon dioxide is changed so that the first actual gas utilization rate (αr1) becomes the first target gas utilization rate. In this case, the flow rate ratio of water vapor to carbon dioxide is not changed, and the total flow rate Fs of water vapor and carbon dioxide is changed. Pattern 2: At least one of the total flow rate Fs of water vapor and carbon dioxide and the flow rate ratio of water vapor to carbon dioxide is changed to achieve the first actual gas utilization rate (αr1). In this case, the total flow rate Fs of water vapor and carbon dioxide is not changed, and the flow rate ratio of water vapor to carbon dioxide is changed. Pattern 3: The supply amount of water vapor is fixed based on the target gas utilization rate, and the flow rate of carbon dioxide is changed to achieve the first actual gas utilization rate (αr1). In this case, the total flow rate Fs of water vapor and carbon dioxide and the flow rate ratio of water vapor and carbon dioxide are changed. Pattern 4: An arbitrary target gas utilization rate is set as the second target gas utilization rate, and at least one of the total flow rate Fs of water vapor and carbon dioxide and the flow rate ratio of water vapor to carbon dioxide is changed.

[0065] The method of changing the second control flow rate value according to patterns 1 to 4 will be described based on the following preconditions. (Prerequisite) First target gas utilization rate: 80% First flow control value: (steam: 40 Nm 3 / h, carbon dioxide: 10Nm 3 / h) Theoretical electrolysis current value Ir: 2.393 x (40 + 10) = 119.65 kA First actual electrolysis current value: 83.75kA First actual gas utilization rate: 83.75 / 119.65×100=70%

[0066] (Pattern 1) First, when the first actual gas utilization rate is calculated to be 70% and deviates from the target value of 80%, a method for changing the second control flow rate value so that the gas utilization rate becomes 80% of the target value will be described. To achieve the target gas utilization rate of 80%, the total flow rate Fs of water vapor and carbon dioxide must be changed. From Equation 3, the total flow rate Fs after the change is 83.75 / 2.393 / 80×100=43.75(Nm 3 / h). On the other hand, the flow rate ratio of water vapor to carbon dioxide remains the first target gas utilization rate (80%), and therefore, according to Equation 1, the flow rate ratio is not changed from 400 / 80−1=4. From these calculated values, the second control flow rate value is 43.75 / 5×4=35(Nm 3 / h), the flow rate of carbon dioxide is 43.75 / 5 = 8.75 (Nm 3 / h). In this state, the electrolysis voltage is maintained again while checking for any change in the electrolysis current. If there is no change in the electrolysis current (83.75 kA), the target gas utilization rate of 80% is maintained, and a synthesis gas with a composition favorable for the subsequent methanation stoichiometric reaction can be produced.

[0067] (Pattern 2) Next, when the first actual gas utilization rate is calculated to be 70% and deviates from the target value of 80%, a method for changing the second control flow rate value so that the gas utilization rate becomes 70% of the actual value will be described. It is necessary to change the flow rate ratio of water vapor to carbon dioxide to be optimal for the first actual gas utilization rate (70%). From Equation 1, the flow rate ratio is calculated as 400 / 70-1=4.714. On the other hand, the total flow rate of water vapor and carbon dioxide Fs is 50 (Nm 3 / h) and there is no need to change it. Therefore, the second control flow rate value is 50 / 5.714×4.714=41.25 (Nm 3 / h), the flow rate of carbon dioxide is 50 / 5.714 = 8.75 (Nm 3 / h). In this state, the electrolysis voltage is maintained again while checking for any change in the electrolysis current. If there is no change in the electrolysis current (83.75 kA), the first actual gas utilization rate is maintained at around 70%, and a synthesis gas with a composition favorable for the subsequent methanation stoichiometric reaction can be produced.

[0068] (Pattern 3) Furthermore, when the first actual gas utilization rate is calculated to be 70% and deviates from the target value of 80%, the second target gas utilization rate can be set to a different value that is neither the target value of 80% nor the actual value of 70%, for example, 75%, and the second control flow rate values ​​of carbon dioxide and water vapor can be changed. If the second target gas utilization rate is set to 75%, the flow rate ratio of water vapor to carbon dioxide is calculated from equation 1 as 400 / 75-1=4.333. The total flow rate Fs of water vapor and carbon dioxide after the change is calculated from Equation 3 as follows: 83.75 / 2.393 / 75×100≒46.875(Nm 3 / h). Therefore, the second control flow rate value is 46.875 / 5.333×4.333=38.09 (Nm 3 / h), the flow rate of carbon dioxide is 46.875 / 5.333 = 8.79 (Nm 3 / h). In this state, the electrolysis voltage is again maintained while checking for any change in the electrolysis current. If there is no change in the electrolysis current (83.75 kA), the actual gas utilization rate is maintained at around 75%, and a synthesis gas with a composition favorable for the subsequent methanation stoichiometric reaction can be produced.

[0069] As described above, there is a degree of freedom in the method of setting the second target gas utilization rate when comparing the first actual gas utilization rate with the target value, but as one form of the method of setting the second target gas utilization rate, it is also possible to determine the second target gas utilization rate so that the second control flow rate value of either carbon dioxide or water vapor becomes a desired fixed value. Here, a method of determining the set value of the second target gas utilization rate and the second flow rate control value of CO2 so that the supply amount of water vapor becomes a fixed value over time will be described. As described later, a reference pattern of the deterioration rate vs. the target utilization rate is set in advance as the deterioration progresses. In this case, the optimal target utilization rate when the deterioration rate has progressed by 85% is 70.2%, and this value is set. From Equation 1, the flow rate ratio is calculated as 400 / 70.2-1=4.73. On the other hand, the flow rate of water vapor Fs is 40 (Nm 3 / h), and there is no need to change the initial value, while the second control flow rate value is 40 / 4.73 = 8.45 (Nm 3 / h), the flow rate of carbon dioxide is 50 / 5.714 = 8.75 (Nm 3 / h). In this state, the electrolysis voltage is again maintained while checking for any change in the electrolysis current. If there is no change in the electrolysis current (81.4 kA), the target gas utilization rate is maintained at 70.2%, indicating that synthesis gas with a composition favorable for the subsequent methanation stoichiometric reaction can be produced.

[0070] The gas utilization rate calculation unit 9 calculates a second theoretical electrolysis current value from the second control flow rate value. Next, the ratio of the second actual electrolysis current value to the second theoretical electrolysis current value is calculated as the second actual gas utilization rate (αr2). Based on the second actual gas utilization rate (αr2), the supply amounts or flow rate ratios of water vapor and carbon dioxide are changed as the third control flow rate value. Thereafter, the fourth control flow rate value and the fifth control flow rate value are similarly determined in accordance with the deterioration over time of the solid oxide electrolysis cell, thereby enabling always optimal synthesis gas to be produced.

[0071] [Flow of Operation Start-Up Process of Co-electrolytic Methanation Apparatus 1 According to the Embodiment] Next, the process when the co-electrolytic methanation apparatus 1 having the above configuration starts operation will be described with reference to FIGS.

[0072] As shown in FIG. 4, when the co-electrolytic methanation apparatus 1 is started, a first target gas utilization rate (%) is first obtained (step S101). The flow rate ratio of water vapor to carbon dioxide is calculated from the first target gas utilization rate (%), and the flow rates of water vapor and carbon dioxide are set as first flow rate control values ​​and supplied to the co-electrolysis unit 5 (step S102). The gas utilization rate calculation unit 9 calculates a theoretical electrolysis current value Ir equivalent to the case where all of the water vapor and carbon dioxide supplied to the co-electrolysis unit 5 in step S102 are electrolyzed into hydrogen and carbon monoxide (step S103). Next, the power supply unit 54 controls the target electrolysis voltage to start electrolysis (step S104). As the target electrolysis voltage, for example, the electrolysis current is controlled so that the rated electrolysis voltage of the stack is 1.3 V / cell.

[0073] The operation control unit A determines whether the electrolysis current value has remained stable for a certain period of time without any change from a predetermined value. The ammeter 55 measures the electrolysis current value when the electrolysis current has stabilized (step S105). This value is sent to the gas utilization rate calculation unit 9 as the actual electrolysis current value. The gas utilization rate calculation unit 9 calculates the ratio of the actual electrolysis current value to the theoretical electrolysis current value Ir calculated in step S103 as the first actual gas utilization rate (αr1) (step S106).

[0074] Next, it is checked whether the first actual gas utilization rate (αr1) calculated in step S106 matches the target gas utilization rate (%) (step S107). If they match, the flow rates and flow rate ratios of water vapor and carbon dioxide are not changed (step S108). If they do not match, a second control flow rate value is set (step S109).

[0075] The second control flow rate value is adjusted to the first target gas utilization rate (step S109-1). To adjust to the first target gas utilization rate, the flow rates of water vapor and carbon dioxide are changed without changing the flow rate ratio of water vapor to carbon dioxide, and the second control flow rate value is set.

[0076] The second control flow rate value is adjusted to the first actual gas utilization rate (step S109-2). At least one of the flow rate ratio of water vapor to carbon dioxide and the flow rates of water vapor and carbon dioxide is changed to set the second control flow rate value in accordance with the first actual gas utilization rate.

[0077] The second control flow rate value is adjusted to an arbitrary target gas utilization rate (step S109-3). The second control flow rate value is set to an arbitrary target gas utilization rate that is neither the first target gas utilization rate nor the first actual gas utilization rate.

[0078] [Continuous operation flow of the co-electrolytic methanation device 1 according to the embodiment] The process of continuous operation of the co-electrolytic methanation apparatus 1 having the above configuration will be described with reference to FIGS. 1 and 5.

[0079] After the flow rates of water vapor and carbon dioxide are set in step S101 of Fig. 4, the electrolysis current is monitored. This monitoring control should be performed periodically, such as once every few hours or once every few days. If a tendency for changes in the IV (electrolysis current - electrolysis voltage) characteristics of the solid oxide electrolysis cell stack is confirmed in the early stages of start-up, the electrolysis current value will change when the target electrolysis voltage is maintained. In this case, the gas composition generated by methanation in the downstream methane synthesis unit 7 may not be stable. The interval between monitoring control is changed as appropriate depending on the stability of the solid oxide electrolysis cell stack.

[0080] As shown in FIG. 5, the nth target gas utilization rate (%) is obtained (step S201). The flow rate ratio of water vapor to carbon dioxide is calculated from the nth target gas utilization rate (%), and the flow rates of water vapor and carbon dioxide are set as the nth flow rate control value, and supplied to the co-electrolysis unit 5 (step S202). The gas utilization rate calculation unit 9 calculates a theoretical electrolysis current value Ir corresponding to the case where all of the water vapor and carbon dioxide supplied to the co-electrolysis unit 5 in step S202 are electrolyzed into hydrogen and carbon monoxide (step S203). Next, the power supply unit 54 controls the target electrolysis voltage to start electrolysis (step S204). Monitoring is performed for h hours or d days with the target electrolysis voltage controlled (step S205). The monitoring interval is selected according to the situation, taking into account the stability of the solid oxide electrolysis cell stack.

[0081] The ammeter 55 measures the electrolysis current value when the electrolysis current has stabilized (step S206). This value is sent as the actual electrolysis current value to the gas utilization rate calculation unit 9. The gas utilization rate calculation unit 9 calculates the ratio of the actual electrolysis current value to the theoretical electrolysis current value Ir calculated in step S203 as the n-th actual gas utilization rate (αrn) (step S207).

[0082] Next, it is checked whether the nth actual gas utilization rate (αrn) calculated in step S207 matches the nth target gas utilization rate (%) (step S208). If they match, the flow rates and flow rate ratios of water vapor and carbon dioxide are not changed (step S209). If they do not match, the (n+1)th controlled flow rate value is set (step S210). The (n+1)th controlled flow rate value is determined in the same manner as in steps S109-1, S109-2, and S109-3. [Example]

[0083] The co-electrolytic methanation device 1 according to this embodiment can maintain the methane purity of the final outlet gas even when the solid oxide electrolysis cell in the co-electrolysis unit 5 deteriorates. This will be further explained using control examples from Examples 1 to 4. The deterioration rate is defined as the ratio of the current value reached at the time of deterioration to the current value reached at a constant voltage under constant voltage control, where the current value is 100% when no deterioration has occurred. The initial deterioration rate is 100%, and as deterioration progresses, the deterioration rate progresses to 95% and then 90%. For example, using Table 1 as an example, the actual electrolysis current value at a deterioration rate of 100% is 95.7 kA. If the actual electrolysis current value at a certain deterioration rate is 81.4 kA, the deterioration rate is calculated as 81.4 / 95.7 × 100 = 85%. Example 1 The operating conditions of the co-electrolytic methanation device 1 in Example 1 are shown in Table 1, and the relationship between the deterioration rate of the co-electrolysis unit 5 and the gas concentration at the final outlet is shown in Table 2. In Example 1, control is performed with the target gas utilization rate fixed at 80%. [Table 1] [Table 2]

[0084] As shown in Table 1, the actual electrolysis current value decreases according to the degradation rate of the solid oxide electrolysis cell. When the first actual gas utilization rate is calculated to be 76% at 95% degradation, the second target gas utilization rate is reset to the first target value of 80%. With the target gas utilization rate of 80%, the flow rate ratio of water vapor to carbon dioxide remains unchanged at 4, and the actual electrolysis current value is 90.9 kA, the total flow rate Fs of water vapor and carbon dioxide is 47.5 Nm 3 / h, and the total flow rate Fs is 50Nm 3 / h. As a result, the water vapor flow rate is changed to 38Nm 3 / h, carbon dioxide flow rate 9.5Nm 3 / h, and control is applied as the second control flow rate value to operate the system.

[0085] A similar judgment is made when deterioration progresses further. At that point, the actual gas utilization rate is calculated again. Even if the actual gas utilization rate immediately after the condition change is 80% and then drops to a lower value, causing a deviation, the total flow rate Fs of water vapor and carbon dioxide is recalculated from this target utilization rate value and the actual electrolysis current value so that the target utilization rate is again 80%, the flow rate ratio of water vapor to carbon dioxide calculated from the target gas utilization rate value is calculated, the control flow rate target values ​​for water vapor and carbon dioxide are calculated, and control is applied to operate. In this way, since the target gas utilization rate is fixed at 80%, the set value of the flow rate ratio of water vapor to carbon dioxide is always 4.0, and only the total flow rate Fs of water vapor and carbon dioxide is controlled to change.

[0086] By implementing this type of control, it is possible to maintain the methane concentration in the final outlet gas at around 97%, even if deterioration progresses as shown in Table 2.

[0087] Example 2 The operating conditions of the co-electrolytic methanation device 1 in Example 2 are shown in Table 3, and the relationship between the deterioration rate of the co-electrolysis unit 5 and the gas concentration at the final outlet is shown in Table 4. In Example 2, the total flow rate Fs of water vapor and carbon dioxide is fixed, and control is performed by changing the flow rate ratio. [Table 3] [Table 4]

[0088] As shown in Table 3, the actual electrolysis current value decreases according to the degradation rate of the solid oxide electrolysis cell. When the first actual gas utilization rate is calculated to be 76% at 95% degradation, the first actual gas utilization rate is set as the second target gas utilization rate value. Since this second target gas utilization rate is 76%, which matches the actual value, the total flow rate Fs of water vapor and carbon dioxide is 50.0 Nm, which is the sum of the first flow rate control values. 3 / h. On the other hand, from the second target gas utilization rate of 76%, the flow rate ratio of water vapor to carbon dioxide can be calculated as 4.26, and as a result, the flow rate of water vapor is set to 40.5 Nm 3 / h, carbon dioxide flow rate 9.5Nm 3 / h, and control is applied as the second control flow rate value to operate the system.

[0089] The same determination is made when deterioration progresses further. At that point, the actual gas utilization rate is calculated again. The actual gas utilization rate decreases as deterioration progresses. This actual gas utilization rate is used as the target gas utilization rate, and the flow rate ratio of water vapor to carbon dioxide is changed again. The total flow rate Fs of water vapor and carbon dioxide is not changed, and only the flow rate ratio of water vapor to carbon dioxide is changed. Therefore, the supply amount of water vapor increases as deterioration progresses, and the supply amount of carbon dioxide decreases as deterioration progresses.

[0090] By implementing such control, it is possible to maintain the methane concentration in the final outlet gas at around 97%, even if deterioration progresses as shown in Table 4.

[0091] Example 3 The operating conditions of the co-electrolytic methanation device 1 in Example 3 are shown in Table 7, and the relationship between the deterioration rate of the co-electrolysis unit 5 and the gas concentration at the final outlet is shown in Table 8. In Example 3, control is performed so that the target gas utilization rate is set arbitrarily other than the actual gas utilization rate and the initial target gas utilization rate. [Table 5] [Table 6]

[0092] As shown in Table 5, the actual electrolysis current value decreases as the solid oxide electrolysis cell deteriorates. When the first actual gas utilization rate is calculated to be 76% at 95% deterioration, the target gas utilization rate is set to 78%, not the first actual gas utilization rate of 76% at that time or the initial target gas utilization rate of 80%. From this target gas utilization rate of 78% and the actual electrolysis current value of 90.9 kA, the total flow rate Fs of water vapor and carbon dioxide can be calculated from Equation 3 as 48.7 Nm 3 / h. From the target gas utilization rate of 78%, the flow rate ratio of water vapor to carbon dioxide can be calculated as 4.13 using Equation 1, and the flow rate of water vapor is 39.22 Nm 3 / h, carbon dioxide flow rate 9.49 Nm 3 / h, and control is applied as the second control flow rate value to operate the system.

[0093] If the deterioration progresses further, the same determination is made. At that point, the actual gas utilization rate is calculated again. The actual gas utilization rate decreases as the deterioration progresses. In the third embodiment, the supply flow rates of water vapor and carbon dioxide are reviewed based on the target gas utilization rate, which can be set arbitrarily.

[0094] By implementing such control, it is possible to maintain the methane concentration in the final outlet gas at around 97%, even if deterioration progresses as shown in Table 6.

[0095] Example 4 The operating conditions of the co-electrolytic methanation device 1 in Example 4 are shown in Table 7, and the relationship between the deterioration rate of the co-electrolysis unit 5 and the gas concentration at the final outlet is shown in Table 8. In Example 4, control in which the supply amount of water vapor is fixed and the supply amount of carbon dioxide is changed will be described.

[0096] The control is performed according to the deterioration rate of the solid oxide electrolysis cell, using a formula for calculating the deterioration rate calculated in advance vs. the target utilization rate. Target utilization rate (%) = 0.0067 x deterioration rate (%) + 0.1323 (Formula 4) If the initial utilization rate is calculated to be 80%, the above formula is obtained as the optimum value for the utilization rate when the H2O flow rate is fixed. The target utilization rates shown in Table 7 are those calculated using this formula. For example, at 95% degradation, even if the first actual gas utilization rate is calculated to be 76%, the target gas utilization rate is set to 76.8%, not the first actual gas utilization rate of 76% at that time or the initial target gas utilization rate of 80%. From this target gas utilization rate of 76.8% and the actual electrolysis current value of 90.9 kA, the total flow rate Fs of water vapor and carbon dioxide is calculated to be 49.5 Nm3 using formula 3. 3 / h. From the target gas utilization rate of 76.8%, the flow rate ratio of water vapor to carbon dioxide can be calculated as 4.21 using Equation 1, which means the flow rate of water vapor is 40.0 Nm 3 / h, carbon dioxide flow rate 9.5Nm 3 / h, and control is applied as the second control flow rate value. In this way, by sequentially setting and controlling the optimal utilization rate based on a formula that calculates the optimal target utilization rate from the deterioration rate, it is possible to appropriately change only the carbon dioxide flow rate while keeping the water vapor flow rate fixed. [Table 7] [Table 8]

[0097] If deterioration progresses further, a similar judgment is made. By performing this type of control, it is possible to maintain the methane concentration in the final outlet gas at around 96%, even if deterioration progresses as shown in Table 8.

[0098] Comparing the degradation rate and CO2 flow rate shown in Table 7, the degradation rate and CO2 flow rate can be described by a linear correlation equation as shown in Equation 5. CO2 flow rate = 0.1033 x Deterioration rate (%) - 0.3431 (Formula 5) In this way, if the CO2 flow rate is determined using the above optimal correlation equation for the deterioration rate, it is possible to send out an optimal CO2 flow rate while keeping the water vapor concentration fixed.

[0099] That is, by changing the carbon dioxide supply flow rate calculated based on Equation 5 as the second control flow rate value in accordance with the deterioration of the solid oxide electrolysis cell, an optimal supply ratio of water vapor to carbon dioxide in accordance with the deterioration of the solid oxide electrolysis cell can be realized. As a result, it is possible to maintain a high methane concentration in the final outlet gas.

[0100] (Conventional example) Table 9 shows the operating conditions of the co-electrolysis unit 5 under conventional control, and Table 10 shows the relationship between the deterioration rate of the co-electrolysis unit 5 and the gas concentration at the final outlet. [Table 9] [Table 10]

[0101] In the conventional operation method, the supply flow rate of steam and carbon dioxide is fixed (50 Nm 3 As shown in Table 9, the theoretical electrolysis current remains unchanged at 119.7 kA, but as degradation progresses, the actual electrolysis current value decreases, and the actual gas utilization rate at that point also decreases. If operation continues under these conditions, the composition of the gas at the methanation outlet after passing through the co-electrolysis and methanation sections will gradually decrease from an initial methane concentration of over 97% and increase in carbon dioxide concentration, as shown in Table 10. This will result in a decrease in the calorific value per unit volume of the outlet gas, significantly reducing its fuel value.

[0102] From the above results, with conventional control, the calorific value per unit volume of the outlet gas decreased as the deterioration of the solid oxide electrolysis cell progressed, and the value of the fuel decreased significantly. In contrast, with the control of Examples 1 to 4, it was possible to maintain a methane concentration of 96% or more even when the deterioration of the solid oxide electrolysis cell progressed.

[0103] <Another embodiment>

[0104] In this embodiment, the gas utilization rate calculation unit 9 and the operation control unit A are configured separately, but the present invention is not limited to this, and the gas utilization rate calculation unit 9 may be provided in the operation control unit A.

[0105] 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]

[0106] 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: Gas utilization rate calculation unit (gas utilization rate calculation means) 22: Flow rate regulator (flow rate control means) 32: Flow rate regulator (flow rate control means) 53: Cathode 54: Power supply unit (electrolysis voltage control means) 55: Ammeter (electrolysis current measuring means)

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: an electrolysis voltage control means for supplying electrolysis power to the co-electrolysis unit; an electrolysis current measuring means for measuring an electrolysis current; a gas utilization rate calculation means, the flow rate control means supplies the water vapor and the carbon dioxide to the co-electrolysis unit at first control flow rate values ​​and stabilizes them; the electrolysis current measuring means measures the current value when the electrolysis current is stabilized as a first actual electrolysis current value; the gas utilization rate calculation means calculates, based on the first control flow rate value, an electrolysis current value corresponding to when all of the water vapor and the carbon dioxide supplied are electrolyzed into hydrogen and carbon monoxide, as a first theoretical electrolysis current value, calculates a ratio of the first actual electrolysis current value to the first theoretical electrolysis current value as a first actual gas utilization rate (αr1), and calculates, based on the first actual gas utilization rate (αr1), the flow rates of the water vapor and the carbon dioxide as second control flow rate values; The flow rate control means controls the flow rates of the water vapor and the carbon dioxide based on the second control flow rate value.

2. 2. The co-electrolytic methanation apparatus according to claim 1, wherein the first control flow rate value is determined by a flow rate ratio of the water vapor to the carbon dioxide according to a first target gas utilization rate.

3. 3. The co-electrolytic methanation apparatus according to claim 2, wherein the second control flow rate value is determined by changing at least one of the total flow rate of the water vapor and the carbon dioxide and the flow rate ratio of the water vapor to the carbon dioxide from the first control flow rate value.

4. 4. The co-electrolytic methanation apparatus according to claim 3, wherein the flow rate ratio of the water vapor to the carbon dioxide is calculated based on a supply gas utilization rate (α) using the following formula (1): 400 / supply gas utilization rate (%)-1 (1)

5. 4. The co-electrolytic methanation apparatus according to claim 3, wherein the gas utilization rate calculation means compares the first actual gas utilization rate (αr1) with the first target gas utilization rate, and if the first actual gas utilization rate (αr1) and the first target gas utilization rate do not match, changes the second control flow rate value so that the second control flow rate value becomes the first target gas utilization rate used in determining the first control flow rate value.

6. 4. The co-electrolytic methanation apparatus according to claim 3, wherein, when the first actual gas utilization rate (αr1) does not match the first target gas utilization rate, the gas utilization rate calculation means changes the second control flow rate value so that the first actual gas utilization rate (αr1) becomes the first target gas utilization rate (αr1).

7. 5. The co-electrolytic methanation apparatus according to claim 4, wherein, when the first actual gas utilization rate (αr1) does not match the first target gas utilization rate, the gas utilization rate calculation means changes the second control flow rate value so as to achieve an arbitrary target gas utilization rate other than the first actual gas utilization rate (αr1) and the supply gas utilization rate (α).

8. 5. The co-electrolytic methanation apparatus according to claim 4, wherein the gas utilization rate calculation means changes the supply flow rate of the carbon dioxide in accordance with deterioration of the co-electrolysis unit, and does not change the supply flow rate of the water vapor from the first control flow rate value in accordance with deterioration of the co-electrolysis unit.

9. the flow rate control means supplies and stabilizes the flow rates of the water vapor and the carbon dioxide based on the second control flow rate value; the electrolysis current measuring means measures a current value during stabilization as a second actual electrolysis current value, the gas utilization rate calculation means calculates, based on the second control flow rate value, an electrolysis current value corresponding to when all of the water vapor and the carbon dioxide supplied are electrolyzed into hydrogen and carbon monoxide, as a second theoretical electrolysis current value, calculates a ratio of the second actual electrolysis current value to the second theoretical electrolysis current value as a second actual gas utilization rate (αr2), and calculates the flow rates of the water vapor and the carbon dioxide as third control flow rate values ​​based on the second actual gas utilization rate (αr2); The co-electrolytic methanation apparatus according to any one of claims 1 to 8, wherein the flow rate control means controls the flow rates of the water vapor and the carbon dioxide based on the third control flow rate value.

Images