Carbon dioxide electrolytic apparatus

Abstract

To provide a carbon dioxide electrolytic apparatus in which carbon deposition can be effectively suppressed in a solid oxide electrolysis cell for electrolyzing a carbon dioxide.SOLUTION: A carbon dioxide electrolytic apparatus relating to the present disclosure includes a solid oxide electrolysis cell for generating a carbon monoxide by electrolyzing a carbon dioxide supplied to a fuel electrode. The apparatus includes a carbon dioxide supply system for supplying the carbon dioxide to the fuel electrode and a hydrogen supply system for supplying hydrogen to the fuel electrode. Thus, the carbon deposition can be effectively suppressed in the solid oxide electrolysis cell without significantly increasing the temperature of the solid oxide electrolysis cell.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a carbon dioxide electrolysis device that electrolyzes carbon dioxide to produce carbon monoxide. [Background technology]

[0002] Conventionally, a method has been known in which a current is applied to a solid oxide electrolysis cell (SOEC), carbon dioxide (CO2) is supplied to the anode of the solid oxide electrolysis cell, and carbon monoxide (CO) is produced by electrolyzing the carbon dioxide at the anode (see, for example, Patent Document 1). In this method, specially treated food-grade carbon dioxide is supplied to the anode of the solid oxide electrolysis cell to obtain ultra-high purity carbon monoxide. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2018-505958 Summary of the Invention [Problem to be solved by the invention]

[0004] When carbon dioxide is electrolyzed using a solid oxide electrolysis cell, the Boudoir reaction (2CO → C + CO2) may occur in the solid oxide electrolysis cell, resulting in the deposition of carbon. If the deposited carbon clogs the gas passages and the like in the solid oxide electrolysis cell, this will lead to deterioration of the device and make it impossible to generate carbon monoxide.

[0005] Therefore, a main object of the present disclosure is to provide a carbon dioxide electrolysis device that can effectively suppress carbon deposition in a solid oxide electrolysis cell that electrolyzes carbon dioxide. [Means for solving the problem]

[0006] The carbon dioxide electrolysis device disclosed herein is a carbon dioxide electrolysis device including a solid oxide electrolysis cell that generates carbon monoxide by electrolyzing carbon dioxide supplied to an anode, and includes a carbon dioxide supply system that supplies carbon dioxide to the anode, and a hydrogen supply system that supplies hydrogen to the anode.

[0007] In the carbon dioxide electrolysis device disclosed herein, hydrogen is supplied to the anode of the solid oxide electrolysis cell in addition to carbon dioxide. Here, the hydrogen does not substantially participate in the electrolysis of carbon dioxide, and essentially most of it flows out from the anode. Therefore, by supplying hydrogen to the anode in addition to carbon dioxide, the composition of the carbon dioxide not electrolyzed at the anode and the carbon monoxide and hydrogen flowing out from the anode can be adjusted to a composition that is not included in the carbon deposition region determined by the temperature of the solid oxide electrolysis cell. As a result, the carbon dioxide electrolysis device disclosed herein can effectively suppress carbon deposition in the solid oxide electrolysis cell that electrolyzes carbon dioxide without significantly increasing the temperature of the solid oxide electrolysis cell. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram illustrating a carbon dioxide electrolysis device according to the present disclosure. [Figure 2] 1 is a flowchart showing a series of processes for producing carbon monoxide by electrolyzing carbon dioxide using a carbon dioxide electrolysis device of the present disclosure. [Figure 3] Carbon-hydrogen-oxygen ternary phase diagram. DETAILED DESCRIPTION OF THE INVENTION

[0009] Next, embodiments of the present disclosure will be described with reference to the drawings.

[0010] Figure 1 is a schematic diagram showing the configuration of a carbon dioxide electrolysis device 1 according to the present disclosure. The carbon dioxide electrolysis device 1 shown in the figure includes a carbon dioxide electrolysis module 3 including a solid oxide electrolysis cell stack 2 that electrolyzes carbon dioxide to produce carbon monoxide, a power supply device 4 that exchanges electric power (DC power) with the electrolysis cell stack 2, a carbon dioxide supply system 10 that supplies gaseous carbon dioxide to the electrolysis cell stack 2, a hydrogen supply system 20 that supplies hydrogen to the electrolysis cell stack 2, a carbon monoxide recovery system 30 that recovers carbon monoxide and other compounds produced in the electrolysis cell stack 2, and a control device 100 that controls the entire system.

[0011] The electrolysis cell stack 2 includes multiple solid oxide electrolysis cells (SOECs, hereinafter referred to as "single cells") stacked in a predetermined direction and is disposed inside a module case 3c of a carbon dioxide electrolysis module 3 that is insulated with a thermal insulator. Each single cell includes an electrolyte 2a, a fuel electrode 2b disposed on one side of the electrolyte 2a, and an oxidizer electrode 2c disposed on the other side of the electrolyte 2a. The electrolyte 2a is a solid electrolyte such as stabilized zirconia (e.g., YSZ). The fuel electrode 2b is formed from a composite material of a catalytic metal such as Ni and stabilized zirconia. The oxidizer electrode 2c is formed from a ceramic such as LSCF.

[0012] Each unit cell electrolyzes carbon dioxide in a high-temperature environment of, for example, about 700°C in response to the application of power from the power supply 4, producing carbon monoxide at the fuel electrode 2b and oxygen at the oxidant electrode 2c. In this embodiment, each unit cell is a reversible solid oxide electrolysis cell capable of generating electricity through an electrochemical reaction between hydrogen and oxygen. The power supply 4 may be a system power supply, a renewable energy power generation facility, a storage battery, or a combination of these.

[0013] The carbon dioxide supply system 10 includes a carbon dioxide tank 11, a pressure regulator 12, a pump 15, and a carbon dioxide supply passage (carbon dioxide supply pipe) L1. The carbon dioxide tank 11 stores, for example, gaseous carbon dioxide, and the pressure regulator 12 regulates (depressurizes) the carbon dioxide from the carbon dioxide tank 11 to a constant pressure. The pump 15 draws in the carbon dioxide whose pressure has been adjusted by the pressure regulator 12 and discharges it into the carbon dioxide supply passage L1. The carbon dioxide supply passage L1 is inserted inside the module case 3c and is connected to the anode inlet of the electrolysis cell stack 2, which communicates with the anode 2b of each unit cell. As a result, the carbon dioxide discharged from the pump 15 is supplied to the anode 2b of each unit cell of the electrolysis cell stack 2 via the carbon dioxide supply passage L1.

[0014] The hydrogen supply system 20 includes a hydrogen tank 21, a pump 25, and a hydrogen supply passage (hydrogen supply pipe) L2. The hydrogen tank 21 stores hydrogen, and the pump 25 draws hydrogen from the hydrogen tank 21 and discharges it into the hydrogen supply passage L2. The hydrogen supply passage L2 communicates with the carbon dioxide supply passage L1 of the carbon dioxide supply system 10 outside the module case 3c. As a result, hydrogen discharged from the pump 25 is supplied to the fuel electrode 2b of each unit cell of the electrolysis cell stack 2 via the hydrogen supply passage L2 and the carbon dioxide supply passage L1 of the carbon dioxide supply system 10.

[0015] The carbon monoxide recovery system 30 includes a carbon monoxide tank 31 and a separator 35, each of which is disposed outside the module case 3c. The separator 35 is connected to the anode outlet of the electrolysis cell stack 2, which communicates with the anode 2b of each unit cell, via an anode off-gas passage (anode off-gas pipe) L3. The separator 35 includes, for example, a hydrogen separation membrane and a carbon dioxide separation membrane, and separates hydrogen and carbon dioxide from the anode off-gas containing carbon monoxide from the anode 2b of each unit cell, and allows the remaining carbon monoxide to flow into the carbon monoxide tank 31. The hydrogen separated by the separator 35 may be introduced into the hydrogen tank 21 or may be supplied to equipment for a separate process. The carbon dioxide separated by the separator 35 is introduced into the carbon dioxide tank 11 or another tank, for example.

[0016] In this embodiment, a return passage (return pipe) L4 branches off from the anode off-gas passage L3 outside the module case 3c. The return passage L4 communicates with a gas passage (piping) connecting the pressure regulator 12 and pump 15 of the carbon dioxide supply system 10 via a flow control valve 40. The gas passage connecting the pressure regulator 12 and pump 15 is connected to an intake port of a pump 45, and the discharge port of the pump 45 is connected to the combustor 5 located inside the module case 3c via a gas passage (piping). The discharge port of a pump 55 located outside the module case 3c is connected to the combustor 5 via an air supply passage (air supply pipe) inserted inside the module case 3c. The pump 55 draws air through an air filter (not shown) and supplies the air as an oxidant to the combustor 5.

[0017] The carbon dioxide electrolysis device 1 also includes a pump (blower) 65 that supplies air as a sweep gas to the electrolysis cell stack 2. The pump 65 draws in air through an air filter (not shown) and discharges it into an air supply passage (air supply pipe) L6. The air supply passage L6 is inserted into the interior of the module case 3c and connected to an oxidizer electrode inlet of the electrolysis cell stack 2 that communicates with the oxidizer electrode 2c of each unit cell. Furthermore, an oxygen tank 71 located outside the module case 3c is connected to an oxidizer electrode outlet of the electrolysis cell stack 2 that communicates with the oxidizer electrode 2c of each unit cell via a gas passage (pipe) L7. This allows oxygen generated at the oxidizer electrode 2c of each unit cell to flow into the oxygen tank 71 via the oxidizer electrode off-gas pipe L7 together with the air from the pump 65, i.e., the sweep gas.

[0018] The control device 100 includes a computer having a CPU, ROM, RAM, input / output devices, etc. The control device 100 acquires signals from a plurality of temperature sensors (not shown) and a plurality of flow meters (not shown) that are installed near the electrolytic cell stack 2, etc. Then, based on the acquired signals, the control device 100 controls objects to be controlled, such as the power supply device 4, pumps 15, 25, 45, 55, 65, and flow control valve 40.

[0019] Next, the operation of the carbon dioxide electrolysis device 1 will be described with reference to FIGS.

[0020] 2 is a flowchart showing a series of processes executed by the control device 100 when carbon dioxide electrolysis is performed to generate carbon monoxide using the carbon dioxide electrolysis device 1. When the control device 100 is instructed to perform carbon dioxide electrolysis, it operates the pump 25 of the hydrogen supply system 20 and the pump 45 connected to the combustor 5, and controls the flow control valve 40 to open (step S100). Furthermore, in step S100, the control device 100 controls the pump 55 to supply air to the combustor 5 and operates an ignition device (not shown) of the combustor 5. As a result, hydrogen (anode off-gas) supplied from the pump 25 to the electrolysis cell stack 2 is supplied to the combustor 5 via the anode off-gas passage L3 and the pump 45, and is combusted together with air supplied from the pump 55 to the combustor 5. As a result, the heat generated in the combustor 5 warms up the electrolysis cell stack 2 in the module case 3c.

[0021] After the process of step S100, the control device 100 acquires the stack temperature Ts of the electrolysis cell stack 2 detected by a temperature sensor (not shown) (step S110), and determines whether the acquired stack temperature Ts is equal to or higher than a predetermined warm-up completion temperature Ts1 (e.g., approximately 600°C) (step S120). If the stack temperature Ts is lower than the warm-up completion temperature Ts1 (step S120: NO), the control device 100 acquires the stack temperature Ts every time a predetermined time elapses (step S110), and determines whether the acquired stack temperature Ts is equal to or higher than the warm-up completion temperature Ts1 (step S120).

[0022] Furthermore, if the stack temperature Ts becomes equal to or higher than the warm-up completion temperature Ts1 (step S120: YES), the control device 100 operates the pump 15 of the carbon dioxide supply system 10 and controls the power supply device 4 to apply a current to the electrolysis cell stack 2 (step S130). As a result, carbon dioxide is supplied to the fuel electrode 2b of each unit cell of the electrolysis cell stack 2, and carbon monoxide is produced at each fuel electrode 2b and oxygen is produced at each oxidizer electrode 2c through electrolysis of the carbon dioxide. Furthermore, hydrogen from the pump 25 of the hydrogen supply system 20 continues to be supplied to the fuel electrode 2b of each unit cell via the carbon dioxide supply passage L1.

[0023] Here, for example, when the stack temperature Ts is approximately 700°C and 50% of the moles of carbon dioxide supplied to the anode 2b are electrolyzed, as can be seen from the carbon-hydrogen-oxygen ternary phase diagram shown in Figure 3, the composition of the carbon dioxide and carbon monoxide that are not electrolyzed and flow out of the anode 2b (1:1, C:H:O = 0.4:0:0.6, see the squares in Figure 3) are included in the carbon deposition region above the carbon deposition boundary line (threshold) when the stack temperature Ts is 700°C. In this case, to suppress carbon deposition, it is necessary to increase the temperature of the electrolysis cell stack 2.

[0024] In contrast, when the stack temperature Ts is approximately 700°C and hydrogen is supplied to the anode 2b in addition to carbon dioxide, the hydrogen does not substantially participate in the electrolysis of carbon dioxide and essentially flows out of the anode 2b of each unit cell. Therefore, by supplying hydrogen to the anode 2b in addition to carbon dioxide, the composition of the carbon dioxide, carbon monoxide, and hydrogen that flows out of the anode 2b without being electrolyzed can be adjusted to a composition that is outside the carbon deposition region determined by the temperature of the electrolysis cell stack 2 (e.g., 1:2:17 (C:O:H=0.1:0.15:0.75), see the triangles in Figure 3 ). As a result, the carbon dioxide electrolysis device 1 can effectively suppress carbon deposition (coking) in the electrolysis cell stack 2 without significantly increasing the temperature of the electrolysis cell stack 2 that electrolyzes carbon dioxide.

[0025] In the carbon dioxide electrolysis device 1, while carbon dioxide is being electrolyzed in the electrolysis cell stack 2, the flow control valve 40 is opened as appropriate, and the anode off-gas containing carbon monoxide and hydrogen flowing out from the anode 2b of each unit cell is returned to the carbon dioxide supply system 10 outside the module case 3c. Furthermore, in the carbon dioxide electrolysis device 1, while carbon dioxide is being electrolyzed in the electrolysis cell stack 2, the pumps 45 and 55 are operated as needed, and a portion of the anode off-gas (carbon monoxide and hydrogen) from the return passage L4 is supplied to the combustor 5. As a result, even while carbon dioxide is being electrolyzed in the electrolysis cell stack 2, the temperature of the electrolysis cell stack 2 inside the module case 3c can be adjusted by the heat generated in the combustor 5.

[0026] After the process of step S130, the control device 100 acquires the stack temperature Ts of the electrolysis cell stack 2 detected by a temperature sensor (not shown) (step S140), and calculates the minimum required hydrogen concentration CHmin at the anode 2b of each unit cell required to eliminate carbon deposition based on the acquired stack temperature Ts (step S150). In this embodiment, a map defining the relationship between the stack temperature Ts and the required hydrogen concentration CHmin is created in advance through experiments and analysis based on the carbon-hydrogen-oxygen ternary phase diagram shown in Figure 3, and in step S150, the required hydrogen concentration CHmin corresponding to the stack temperature Ts is derived from the map.

[0027] Furthermore, the control device 100 acquires the current hydrogen concentration CH at the fuel electrode 2b of each unit cell of the electrolysis cell stack 2 (step S160). The current hydrogen concentration CH may be an actual measurement, or may be calculated (estimated) based on the carbon dioxide utilization rate (proportion electrolyzed), the operating state of the pumps 25 and 45, the opening of the flow control valve 40 (hydrogen reflux rate), etc. Next, the control device 100 determines whether the current hydrogen concentration CH is less than the required hydrogen concentration CHmin (step S170). If the current hydrogen concentration CH is equal to or greater than the required hydrogen concentration CHmin (step S170: NO), the control device 100 executes the processes of steps S140-S170 every time a predetermined time elapses.

[0028] If the current hydrogen concentration CH is less than the required hydrogen concentration CHmin (step S170: YES), the control device 100 determines whether or not to stop the electrolysis of carbon dioxide in the electrolysis cell stack 2 (step S180). If the electrolysis of carbon dioxide in the electrolysis cell stack 2 is to be continued (step S180: NO), the control device 100 controls at least one of the pumps 25, 45 and the flow control valve 40 based on the difference between the current hydrogen concentration CH and the required hydrogen concentration CHmin, etc., to increase the amount of hydrogen supplied to the fuel electrode 2b of each unit cell of the electrolysis cell stack 2 (step S185). After processing step S185, the control device 100 executes the processes of steps S140 to S170 again.

[0029] On the other hand, when electrolysis of carbon dioxide by the electrolysis cell stack 2 is to be stopped (step S180: YES), the control device 100 stops the pump 15 of the carbon dioxide supply system 10 and the pumps 25, 45, and 55 of the hydrogen supply system 20, and operates the pump 65 (step S190). This stops the supply of carbon dioxide and hydrogen to the electrolysis cell stack 2 and the operation of the combustor 5, and cools the electrolysis cell stack 2 with air supplied from the pump 65 to each unit cell.

[0030] Furthermore, the control device 100 acquires the stack temperature Ts of the electrolysis cell stack 2 detected by a temperature sensor (not shown) (step S200), and determines whether the acquired stack temperature Ts is equal to or lower than a predetermined cooling completion temperature Ts2 (e.g., approximately 200°C) (step S210). If the stack temperature Ts is higher than the cooling completion temperature Ts2 (step S210: NO), the control device 100 acquires the stack temperature Ts every time a predetermined time elapses (step S200), and determines whether the acquired stack temperature Ts is equal to or lower than the cooling completion temperature Ts2 (step S210). Then, when the stack temperature Ts becomes equal to or lower than the cooling completion temperature Ts2 (step S210: YES), the control device 100 stops the pump 65 and ends the series of processes shown in FIG. 2.

[0031] As described above, the carbon dioxide electrolysis device 1 includes a solid oxide electrolysis cell stack 2 that electrolyzes carbon dioxide supplied to the anode 2b of each unit cell to produce carbon monoxide, a carbon dioxide supply system 10 that supplies carbon dioxide to the anode 2b of each unit cell, and a hydrogen supply system 20 that supplies hydrogen to the anode 2b of each unit cell. This configuration effectively suppresses carbon deposition in the electrolysis cell stack 2, which electrolyzes carbon dioxide, without significantly increasing the temperature of the electrolysis cell stack 2. Furthermore, the hydrogen supplied to the electrolysis cell stack 2 does not substantially participate in the electrolysis of carbon dioxide, and essentially most of it flows out from the anode 2b. Therefore, the hydrogen flowing out from the anode 2b can be recycled to the carbon dioxide supply system 10 or the hydrogen supply system 20, or the hydrogen flowing out from the anode 2b can be used, together with the carbon monoxide produced in the electrolysis cell stack 2, as a raw material for carbon products or synthetic fuels.

[0032] Furthermore, the carbon dioxide electrolysis device 1 includes a module case (housing) 3c that houses the electrolysis cell stack 2, and the hydrogen supply system 20 supplies hydrogen to the carbon dioxide supply passage L1 of the carbon dioxide supply system 10 outside the module case 3c. This makes it possible to effectively suppress carbon deposition in the carbon dioxide supply passage L1 and other parts located inside the module case 3c when carbon dioxide is electrolyzed using the electrolysis cell stack 2.

[0033] The carbon dioxide electrolysis device 1 also includes a return passage L4 that returns the anode off-gas containing carbon monoxide and hydrogen flowing out from the anode 2b of each unit cell to the carbon dioxide supply system 10 outside the module case 3c. This makes it possible to suppress an increase in the amount of hydrogen supplied from the hydrogen supply system 20 to the electrolysis cell stack 2 while maintaining an appropriate hydrogen concentration at the anode 2b of each unit cell.

[0034] Furthermore, the carbon dioxide electrolysis device 1 includes a combustor 5 that is disposed inside the module case 3c and connected to the reflux passage L4, and that combusts the anode off-gas (carbon monoxide and hydrogen) from the reflux passage L4. This allows the combustor 5 to combust part of the anode off-gas, thereby making it possible to appropriately adjust the stack temperature Ts of the electrolysis cell stack 2.

[0035] Each unit cell of the electrolysis cell stack 2 is a reversible solid oxide electrolysis cell capable of generating electricity through an electrochemical reaction between hydrogen and oxygen. In other words, in the carbon dioxide electrolysis device 1, hydrogen is supplied from the hydrogen supply system 20 to the anode 2b of each unit cell of the electrolysis cell stack 2, and air is supplied from the pump 65 to the oxidizer electrode 2c of each unit cell of the electrolysis cell stack 2, thereby enabling the electrolysis cell stack 2 to operate as a fuel cell. When the electrolysis cell stack 2 generates electricity, water (HO) is generated at the anode 2b, and the generated water can react with carbon deposited during the electrolysis of carbon dioxide. As a result, even if carbon deposits on the anode 2b of each unit cell of the carbon dioxide electrolysis device 1, the carbon dioxide electrolysis device 1 can obtain the carbon deposition level based on the power supplied to the pump 15 of the carbon dioxide supply system 10 and the voltage of the power supply device 4, and then operate the electrolysis cell stack 2 as a fuel cell to remove the deposited carbon when the obtained carbon deposition level exceeds an allowable value.

[0036] In the carbon dioxide electrolysis device 1, the hydrogen supply passage L2 communicates with the carbon dioxide supply passage L1 of the carbon dioxide supply system 10 outside the module case 3c, but this is not limiting. That is, the hydrogen supply passage L2 may be inserted into the module case 3c and connected to the anode inlet of the electrolysis cell stack 2, which communicates with the anode 2b of each unit cell. Furthermore, the unit cells of the electrolysis cell stack 2 may be co-electrolysis cells that simultaneously electrolyze carbon dioxide and water vapor. Furthermore, heat generated by equipment other than the carbon dioxide electrolysis device 1 may be introduced into the module case 3c of the carbon dioxide electrolysis module 3.

[0037] Furthermore, the invention of the present disclosure is not limited to the above-described embodiment, and various modifications can be made within the scope of the present disclosure. Furthermore, the above-described embodiment is merely one specific form of the invention described in the Summary of the Invention, and does not limit the elements of the invention described in the Summary of the Invention. [Industrial Applicability]

[0038] The presently disclosed invention is applicable to industries such as the manufacturing of carbon dioxide electrolysis devices. [Explanation of symbols]

[0039] 1 carbon dioxide electrolysis device, 2 electrolysis cell stack, 2b fuel electrode, 5 combustor, 10 carbon dioxide supply system, 20 hydrogen supply system, L4 reflux passage.

Claims

1. A carbon dioxide electrolysis device including a solid oxide electrolysis cell that electrolyzes carbon dioxide supplied to an anode to generate carbon monoxide, a carbon dioxide supply system that supplies carbon dioxide to the anode; a hydrogen supply system for supplying hydrogen to the fuel electrode;

2. The carbon dioxide electrolysis device according to claim 1, a housing that houses the solid oxide electrolysis cell; The carbon dioxide electrolysis device is configured such that the hydrogen supply system supplies hydrogen to the carbon dioxide supply system outside the housing.

3. The carbon dioxide electrolysis device according to claim 2, The carbon dioxide electrolysis device further comprises a return passage that returns the anode off-gas flowing out from the anode to the carbon dioxide supply system outside the casing.

4. The carbon dioxide electrolysis device according to claim 3, The carbon dioxide electrolysis device further includes a combustor that is disposed inside the housing and connected to the return passage, and that combusts the anode off-gas from the return passage.

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