Co-electrolysis methanation apparatus

Abstract

To provide a co-electrolysis methanation apparatus which achieves a ratio of (carbon monoxide+carbon dioxide) / hydrogen suitable for a methanation reaction, and can improve energy conversion efficiency, in a process from takeout of current at the time of start of co-electrolysis to obtaining of rated-equivalent current.SOLUTION: A co-electrolysis methanation apparatus, which includes a co-electrolysis part 5 for electrolyzing water vapor and carbon dioxide, flow rate control means for controlling flow rates of the water vapor and the carbon dioxide, a methane synthesis part 7, and a separation part, includes electrolysis voltage control means for supplying electrolysis power to the co-electrolysis part 5, and electrolysis current measurement means for measuring electrolysis current, wherein the flow rate control means controls each of the flow rates of the water vapor and the carbon dioxide supplied to a cathode electrode 53, according to increase of the electrolysis current, in a process from take out of the electrolysis current of the co-electrolysis part 5 to reaching rated electrolysis current.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] Patent Document 1 relates to an apparatus that uses carbon dioxide and water vapor as raw materials, synthesizes hydrogen and carbon monoxide using electricity, and then synthesizes fuel. It discloses a method for starting the apparatus by first supplying only carbon dioxide, followed by supplying water vapor. It also discloses that carbon dioxide is supplied, electrolysis is performed in which 90% or more of the carbon dioxide is converted to carbon monoxide, and then the supply of water vapor is started. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-8655 Summary of the Invention [Problem to be solved by the invention]

[0005] In the method of Patent Document 1, it takes a long time to reach a state where high-purity methane can be obtained, and the carbon monoxide converted from the input power is exhausted without being used. Therefore, the energy conversion efficiency, which is the ratio of the calorific value of the gas with the target gas composition to the total input energy, decreases during the series of processes of start-up, electrolysis, and shutdown.

[0006] Furthermore, in conventional start-up methods for co-electrolytic methanation systems, fixed amounts of water vapor and carbon dioxide are supplied before electrolysis to produce the optimal ratio of product gas for methanation when electrolysis is performed at a rated current, and electrolysis current is extracted in a stabilized state. Only when the electrolysis current reaches the rated current does the methane purity at the methanation outlet reach the target concentration. Therefore, during the current extraction process up to the rated current, the target methane purity and calorific value per unit volume are almost always low, and most of the produced gas must be discarded.

[0007] 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-electrolysis methanation device that can achieve an optimal (carbon monoxide + carbon dioxide) / hydrogen ratio for the methanation reaction and improve energy conversion efficiency in the process from current extraction at the start of co-electrolysis to obtaining a current equivalent to the rated current. [Means for solving the problem]

[0008] 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, The flow rate control means controls the flow rates of the water vapor and the carbon dioxide supplied to the cathode electrode in accordance with an increase in the electrolytic current during the process from when the electrolytic current of the co-electrolysis section starts to be extracted until the rated electrolytic current is reached.

[0009] Conventionally, in order to produce the optimal ratio of product gas for methanation when electrolysis is performed at a rated current, fixed amounts of water vapor and carbon dioxide are supplied before electrolysis, and the electrolysis current is extracted in a stabilized state. In this case, the methane purity at the methanation outlet reaches the target concentration only when the electrolysis current reaches the rated current. Therefore, in the process of current extraction up to the rated current, the target methane purity and calorific value per unit volume are almost always low, and most of the produced gas must be discarded.

[0010] However, according to the above-described characteristic configuration, the flow rate control means controls the flow rates of water vapor and carbon dioxide supplied to the cathode in accordance with an increase in the electrolytic current from the start of extraction of the electrolytic current from the co-electrolysis unit until the rated electrolytic current is reached. In this way, water vapor and carbon dioxide are supplied in accordance with the electrolytic current, and hydrogen can be methanated without waste even during start-up of the co-electrolysis unit, making it possible to produce gas containing a high concentration of methane. Therefore, in the process from when the current is drawn at the start of co-electrolysis until a current equivalent to the rated value is obtained, it is possible to achieve the optimal (carbon monoxide + carbon dioxide) / hydrogen ratio for the methanation reaction, thereby improving energy conversion efficiency.

[0011] Further characteristic features of the co-electromethanation device according to the present invention include: The electrolysis device includes an electrolysis current ratio calculation means for calculating an electrolysis current ratio, which is the ratio of the electrolysis current to the rated electrolysis current, and controls the flow rates of the water vapor and the carbon dioxide to be supplied to the cathode based on the electrolysis current ratio.

[0012] According to the above-mentioned characteristic configuration, the optimal supply flow rates of water vapor and carbon dioxide can be determined based on the electrolysis current ratio, which is the ratio of the electrolysis current to the rated electrolysis current. This allows for the optimal (carbon monoxide + carbon dioxide) / hydrogen ratio for the methanation reaction to be achieved during the process from the current draw at the start of co-electrolysis to the attainment of a current equivalent to the rated current, enabling the early supply of high-calorie gas. This reduces the generation of wasted low-calorie gas, improving the total energy conversion efficiency during operation, including startup.

[0013] Further characteristic features of the co-electromethanation device according to the present invention include: The flow rate ratio of the water vapor to the carbon dioxide is calculated by the following formula based on the rated electrolysis utilization rate (%), which is the gas utilization rate when the rated electrolysis current is reached. 400 / (Rated electrolysis utilization rate x electrolysis current ratio)-1

[0014] According to the above-described characteristic configuration, the flow rate ratio of water vapor to carbon dioxide can be calculated based on the electrolysis current ratio and the rated electrolysis utilization factor. In this way, it is possible to determine the optimal supply flow rates of water vapor and carbon dioxide based on the electrolysis current value reached at a certain point in time, and it is possible to maintain a high purity of methane even when the co-electrolysis unit is started up. Therefore, in the process from when current is drawn at the start of co-electrolysis until a current equivalent to the rated value is obtained, the optimal (carbon monoxide + carbon dioxide) / hydrogen ratio for the methanation reaction is achieved, which reduces the generation of low-calorie gas that is wasted during the start-up process and improves the total energy conversion efficiency during the operating period, including start-up.

[0015] Further characteristic features of the co-electromethanation device according to the present invention include: Only the water vapor is supplied until the extraction of the electrolytic current from the co-electrolysis section starts, and simultaneously with the start of the extraction of the electrolytic current from the co-electrolysis section, the supply of the carbon dioxide is started in addition to the supply of the water vapor.

[0016] According to the above-described characteristic configuration, the methane concentration in the gas at the outlet of the separation section can be increased from the initial stage of drawing electrolytic current from the co-electrolysis section. Therefore, in the process from when current is drawn at the start of co-electrolysis until a current equivalent to the rated value is obtained, the optimal (carbon monoxide + carbon dioxide) / hydrogen ratio for the methanation reaction is achieved, which reduces the generation of low-calorie gas that is wasted during the start-up process and improves the total energy conversion efficiency during the operating period, including start-up.

[0017] Further characteristic features of the co-electromethanation device according to the present invention include: The feature is that, in the process from the start of extraction of the electrolysis current from the co-electrolysis section until the rated electrolysis current is reached, the supply amount of the water vapor is gradually reduced and the supply amount of the carbon dioxide is gradually increased.

[0018] According to the above-described characteristic configuration, by gradually decreasing the supply amount of water vapor and gradually increasing the supply amount of carbon dioxide, it is possible to increase the methane concentration in the separation section outlet gas from the initial stage of extraction of electrolytic current from the co-electrolysis section. Therefore, in the process from when current is drawn at the start of co-electrolysis until a current equivalent to the rated value is obtained, the optimal (carbon monoxide + carbon dioxide) / hydrogen ratio for the methanation reaction is achieved, which reduces the generation of low-calorie gas that is wasted during the start-up process and improves the total energy conversion efficiency during the operating period, including start-up.

[0019] Further characteristic features of the co-electromethanation device according to the present invention include: The feature is that, during the process from the start of extraction of the electrolysis current from the co-electrolysis section until the rated electrolysis current is reached, the supply amount of water vapor is fixed and the supply amount of carbon dioxide is gradually increased.

[0020] According to the above-described characteristic configuration, by fixing the supply amount of water vapor and gradually increasing the supply amount of carbon dioxide, it is possible to increase the methane concentration in the separation section outlet gas from the initial stage of extraction of electrolytic current from the co-electrolysis section. Therefore, in the process from when current is drawn at the start of co-electrolysis until a current equivalent to the rated value is obtained, the optimal (carbon monoxide + carbon dioxide) / hydrogen ratio for the methanation reaction is achieved, which reduces the generation of low-calorie gas that is wasted during the start-up process and improves the total energy conversion efficiency during the operating period, including start-up.

[0021] Further characteristic features of the co-electromethanation device according to the present invention include: A vent exhaust line for disposing of the synthesis gas is provided between the co-electrolysis section and the methane synthesis section.

[0022] According to the above characteristic configuration, it is possible to discard spec-out gas generated in the early stage of electrolysis in the co-electrolysis section. Therefore, the optimal ratio of (carbon monoxide + carbon dioxide) / hydrogen for the methanation reaction is achieved, which reduces the generation of low-calorie gases that are wasted during the start-up process and improves the total energy conversion efficiency during the operating period, including start-up. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a diagram showing a schematic configuration of a co-electrolytic methanation device according to an embodiment. FIG. [Figure 2] FIG. 10 is a diagram showing a modified example of a co-electromethanation device according to an embodiment. [Figure 3] FIG. 10 is a diagram illustrating a further modification of the co-electromethanation device according to the embodiment. [Figure 4] FIG. 1 is a diagram showing the transition of the amounts of water vapor and carbon dioxide supplied from the time when an electrolysis current is taken out until the rated electrolysis current is reached in Example 1. [Figure 5] FIG. 10 is a diagram showing the transition of the gas composition at the inlet of the methane synthesis section when the control according to Example 1 is performed. [Figure 6] 10 is a diagram showing the transition of the gas composition at the outlet of the separation section when the control according to Example 1 is performed. FIG. [Figure 7] FIG. 10 is a diagram showing the transition of the supply amounts of water vapor and carbon dioxide from the time when the electrolysis current is taken out until the rated electrolysis current is reached in Example 2. [Figure 8] FIG. 10 is a graph showing the transition of the gas composition at the inlet of the methane synthesis section when the control according to Example 2 is performed. [Figure 9] FIG. 10 is a diagram showing the transition of the gas composition at the outlet of the separation section when the control according to Example 2 is performed. [Figure 10] FIG. 10 is a diagram showing the transition of the supply amounts of water vapor and carbon dioxide from the time when the electrolysis current is taken out until the rated electrolysis current is reached in a comparative example. [Figure 11] FIG. 10 is a diagram showing the transition of the gas composition at the inlet of the methane synthesis section when control is performed according to a comparative example. [Figure 12] FIG. 10 is a diagram showing the transition of the gas composition at the outlet of the separation section when control is performed according to a comparative example. [Figure 13] FIG. 1 is a diagram showing a co-electrolytic methanation apparatus according to an embodiment, in which a vent exhaust pipe is provided. DETAILED DESCRIPTION OF THE INVENTION

[0024] Hereinafter, a co-electrolytic methanation apparatus 1 according to an embodiment of the present invention will be described with reference to the drawings.

[0025] 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), an electrolysis current ratio calculation unit 9 (an example of electrolysis current ratio calculation means), and an operation control unit A capable of controlling the operation of each component.

[0026] [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.

[0027] [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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] [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.

[0034] 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 electrolysis current ratio calculation unit 9.

[0035] The electrolytic reaction is usually carried out at about 600° C. to 1000° C., preferably about 650° C. to 850° C. When the temperature of the electrolytic reaction is in this range, a sufficient electrolytic current density can be ensured, which reduces equipment costs and also suppresses deterioration of the electrolytic cell, which reduces operating costs.

[0036] 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)

[0037] 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.

[0038] [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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] [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.

[0043] The inlet temperature when contacting with the methanation catalyst is preferably 200° C. or higher and 350° C. or lower, more preferably 225° C. or higher and 275° C. or lower. When the temperature is within this range, a sufficient reaction rate is likely to be obtained, making it easy to proceed with the methanation reaction without using an excessive amount of catalyst, and the outlet temperature of the methanation reaction is likely to be suppressed, making it easy to ensure the durability of the catalyst.

[0044] 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)

[0045] 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.

[0046] One way to suppress local temperature rises during the methanation reaction and increase the 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 system 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 can be precisely controlled to a lower temperature at the subsequent stage, thereby causing the reactions of formulas (IV) and (V) to proceed to the right and increasing the methane concentration.

[0047] 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.

[0048] 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.

[0049] [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.

[0050] Liquefaction separation involves adjusting the temperature to liquefy and separate components, including water, to obtain a gas primarily composed of methane.

[0051] 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.

[0052] 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.

[0053] [Electrolysis current ratio calculation section] The electrolysis current ratio calculation unit 9 calculates the ratio of the electrolysis current to the rated electrolysis current as the electrolysis current ratio (R), where R is assumed to be less than 1.

[0054] Furthermore, the electrolysis current ratio calculation unit 9 calculates the flow rate ratio of water vapor to carbon dioxide supplied to the cathode 53 using the rated electrolysis utilization rate (%) and the electrolysis current ratio R, according to the following (Equation 1). Here, the rated electrolysis utilization rate (%) refers to the gas utilization rate that is set when the electrolysis current reaches the rated value. 400 / (Rated electrolysis utilization rate (%) x electrolysis current ratio (R))-1 (Formula 1)

[0055] For example, when the rated electrolysis utilization rate is 80% and the electrolysis current ratio (R) is 0.5, the flow rate ratio of water vapor to carbon dioxide supplied to the cathode 53 is calculated as 9 (400 / (80×0.5)−1) according to (Equation 1). Therefore, the flow rates of water vapor and carbon dioxide are set so that the flow rate ratio of water vapor to carbon dioxide is 9. As the electrolysis current ratio (R) approaches 1, the flow rate ratio of water vapor to carbon dioxide supplied to the cathode 53 decreases. In this embodiment, the supply flow rates of water vapor and carbon dioxide are controlled so that the flow rate ratio calculated by the electrolysis current ratio calculation unit 9 is achieved. This control can be performed by decreasing the water vapor supply rate as the electrolysis current increases and increasing the carbon dioxide supply rate as the electrolysis current increases, or by fixing the water vapor supply rate at a constant value regardless of the increase in electrolysis current and increasing the carbon dioxide supply rate as the electrolysis current increases.

[0056] The rated electrolysis utilization rate is preferably 60 to 90%, and more preferably 60 to 80%. If the rated electrolysis utilization rate (%) is less than 60%, the steam flow rate ratio becomes too high, and water must be heated unnecessarily. In addition, the amount of gas produced is small, resulting in poor operating efficiency. If the rated electrolysis utilization rate (%) exceeds 90%, the overvoltage of the co-electrolysis section 5 increases, resulting in increased power consumption and cell deterioration, as well as an increased risk of carbon deposition. [Example]

[0057] The transition of the gas composition at the inlet of the methane synthesis section 7 and the final outlet gas composition from the time when the electrolysis current was taken out by the co-electrolytic methanation apparatus 1 of the present invention until the rated electrolysis current was reached was investigated.

[0058] Example 1 As shown in Fig. 4, the rated electrolysis utilization rate was set to 80%, and the water vapor supply rate was controlled to decrease with increasing electrolysis current, and the carbon dioxide supply rate was controlled to increase with increasing electrolysis current. Fig. 5 shows the change in the gas composition at the inlet of the methane synthesis unit 7 from the time the electrolysis current was taken out until the rated electrolysis current was reached, and Table 1 and Fig. 6 show the change in the composition of the final outlet gas, under the control of Example 1. [Table 1]

[0059] As shown in Figure 5, the hydrogen concentration of the gas at the inlet of the methane synthesis section 7 remained at 70 to 80%, and the carbon dioxide concentration remained at approximately 10 to 20%, from the time when the electrolysis current extraction started until the rated electrolysis current was reached. This indicates that hydrogen and carbon dioxide were produced in the optimum ratio for the methanation reaction.

[0060] Furthermore, as shown in Figure 6, the methane concentration in the final outlet gas exceeded 96% immediately after the start of electrolysis current extraction. Therefore, the quality of the generated gas does not exceed the specifications during the current extraction process up to the rated value, and it can be effectively utilized without being discarded.

[0061] Example 2 As shown in Fig. 7, the rated electrolysis utilization rate was set at 80%, and the steam supply rate was 40 Nm 3 The change in the gas composition at the inlet of the methane synthesis section 7 from the time the electrolysis current was taken out until the rated electrolysis current was reached, under the control of Example 2, is shown in Figure 8, and the change in the composition of the final outlet gas is shown in Table 2 and Figure 9. [Table 2]

[0062] As shown in Figure 8, the hydrogen concentration of the gas at the inlet of the methane synthesis section 7 remained at 70 to 80%, and the carbon dioxide concentration remained at approximately 10 to 20%, from the time when the electrolysis current extraction started until the rated electrolysis current was reached. This indicates that hydrogen and carbon dioxide were produced in the optimum ratio for the methanation reaction.

[0063] Furthermore, as shown in Figure 9, the methane concentration in the final outlet gas was 85.5% immediately after the start of electrolysis current extraction, but exceeded 97% thereafter, resulting in a high overall methane concentration. Therefore, the quality of the generated gas does not exceed the specifications during the current extraction process up to the rated value, and it can be effectively utilized without being discarded.

[0064] (Comparative Example) In the conventional start-up method for co-electrolytic methanation equipment, fixed amounts of steam and carbon dioxide are supplied before electrolysis so that the optimum ratio of product gases for methanation is obtained when electrolysis is performed at a rated capacity, and electrolytic current is extracted in a stabilized state. As a comparative example, as shown in Figure 10, a system was started with a rated electrolysis utilization rate of 80% and a steam supply rate of 40 Nm 3 / h, carbon dioxide supply of 10Nm 3 The change in the gas composition at the inlet of the methane synthesis section 7 from the time when the electrolysis current was taken out until the rated electrolysis current was reached under the control of the comparative example is shown in Fig. 11, and the change in the composition of the final outlet gas is shown in Table 3 and Fig. 12. [Table 3]

[0065] As shown in Figure 11, the hydrogen concentration of the gas at the inlet of the methane synthesis section 7 immediately after the start of electrolysis current extraction was low at about 5%, gradually increased, and reached 70% when the rated electrolysis current was reached. Also, the carbon dioxide concentration was high at 90% immediately after the start of electrolysis current extraction, and dropped to about 10% when the rated electrolysis current was reached. It was found that in the control according to the comparative example, the carbon dioxide concentration was too high, and hydrogen and carbon dioxide were not produced in the optimal ratio for the methanation reaction.

[0066] As shown in Figure 12, the methane concentration in the final outlet gas remained low during the initial stage of electrolysis current extraction, and only reached approximately 97% when the electrolysis current reached the rated value. The carbon dioxide concentration remained high during the initial stage of electrolysis current extraction, and approached 0% when the electrolysis current reached the rated value. Therefore, during the current extraction process up to the rated value, the methane purity and calorific value per unit volume were mostly low, and most of the generated gas had to be discarded. Therefore, it was found that the energy conversion efficiency of the conventional method was lower than that of Examples 1 and 2.

[0067] From the above results, it was found that the control in Examples 1 and 2 achieved an optimum (carbon monoxide + carbon dioxide) / hydrogen ratio for the methanation reaction from the initial stage of electrolysis current extraction, thereby reducing the generation of low-calorie gas that is wasted during the start-up process and improving the total energy conversion efficiency during the operation period, including the start-up period.

[0068] <Another embodiment>

[0069] In this embodiment, the electrolysis current ratio calculation unit 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 electrolysis current ratio calculation unit 9.

[0070] Furthermore, in the present invention, the generation of out-of-spec gas at the beginning of electrolysis is suppressed, and the ratio of the electrolysis current to the rated electrolysis current is defined as the electrolysis current ratio (R), which is preferably greater than 0 and less than 1, but is not limited thereto and may be, for example, 0.2 or greater and less than 1. In this case, when R is greater than 0 and less than 0.2, a vent pipe for disposing of the produced gas may be provided on the way to the downstream methane synthesis unit 7.

[0071] Figure 13 shows one example. That is, a branch is taken out from the main pipe that sends out to the methanation, and a vent exhaust pipe 10 (an example of a vent exhaust line) is provided. In this example, the branch is provided between the compressor of the booster 62 when sending out from the outlet of the SOEC to the methane side, but the location is not limited to this. Furthermore, this vent exhaust pipe 10 can not only be used to discharge out-of-spec gas at the beginning of electrolysis, but also serve as a pipe for introducing purge gas when other abnormalities occur.

[0072] 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]

[0073] 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: Electrolysis current ratio calculation section (electrolysis current ratio calculation means) 10: Vent exhaust piping (vent exhaust line) 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, the flow rate control means controls the flow rates of the water vapor and the carbon dioxide supplied to the cathode in accordance with an increase in the electrolytic current from the start of extraction of the electrolytic current of the co-electrolysis unit until the rated electrolytic current is reached.

2. 2. The co-electrolytic methanation apparatus according to claim 1, further comprising an electrolysis current ratio calculation means for calculating an electrolysis current ratio, which is a ratio of the electrolysis current to the rated electrolysis current, and controlling the flow rates of the water vapor and the carbon dioxide supplied to the cathode based on the electrolysis current ratio.

3. 3. The co-electrolytic methanation apparatus according to claim 2, wherein the flow rate ratio of the water vapor to the carbon dioxide is calculated by the following formula (1) based on a rated electrolysis utilization rate (%), which is the gas utilization rate when the rated electrolysis current is reached: 400 / (rated electrolysis utilization rate x electrolysis current ratio) -1...(1)

4. 4. The co-electrolytic methanation apparatus according to claim 3, wherein only the water vapor is supplied until extraction of the electrolytic current from the co-electrolysis unit is started, and simultaneously with the start of extraction of the electrolytic current from the co-electrolysis unit, supply of the carbon dioxide is started in addition to the supply of the water vapor.

5. 4. The co-electrolytic methanation apparatus according to claim 3, wherein the supply amount of the water vapor is gradually decreased and the supply amount of the carbon dioxide is gradually increased during a process from when extraction of the electrolytic current from the co-electrolysis section starts until the rated electrolysis current is reached.

6. 4. The co-electrolytic methanation apparatus according to claim 3, wherein the supply amount of the water vapor is fixed and the supply amount of the carbon dioxide is gradually increased during a process from when the electrolytic current of the co-electrolysis unit starts to be extracted until the rated electrolytic current is reached.

7. The co-electrolytic methanation apparatus according to any one of claims 1 to 6, wherein a vent exhaust line for disposing of the synthesis gas is provided between the co-electrolysis section and the methane synthesis section.

Images