Electrolysis system

Abstract

To produce high-purity, high-pressure hydrogen while aiming to improve energy efficiency.SOLUTION: An electrolysis system includes: an electrolysis module comprising a solid oxide electrolytic cell that produces hydrogen by water vapor electrolysis and a thermal insulation housing container that accommodates the electrolytic cell; a water vapor feed line that supplies water vapor to the electrolytic cell; and an electrochemical pump accommodated in the housing container. The electrochemical pump has a pump cell including a solid electrolyte, an anode, and a cathode, and is connected to the electrolytic cell so as to direct off-gas containing hydrogen produced in the electrolytic cell and unreacted water vapor into the inlet of the anode. The electrochemical pump is connected to a hydrogen recovery line to output pressurized hydrogen from the cathode outlet to the outside of the housing container, and is connected to a hydrogen supply line to output the remaining hydrogen and water vapor from the anode outlet to the water vapor feed line inside the housing container.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This specification discloses an electrolysis system. [Background technology]

[0002] Conventionally, this type of electrolysis system has been proposed to include a hot box housing a solid oxide electrolysis cell (SOEC), a condenser that cools a product stream containing hydrogen and water vapor produced in the SOEC to condense the water vapor, and a hydrogen pump that pumps the product stream that has passed through the condenser and been dehydrated (see, for example, Patent Document 1). The hydrogen pump includes a solid polymer electrochemical hydrogen pump, which pumps pure compressed hydrogen product through a polymer membrane (electrolyte) by applying an electric current or voltage to the polymer membrane. The unpumped discharge of the hydrogen pump is recirculated to the SOEC after being reheated. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-180308 Summary of the Invention [Problem to be solved by the invention]

[0004] In the electrolysis system described above, the hydrogen and water vapor contained in the product stream extracted from the hot box are cooled once outside the hot box, and therefore must be heated again when recycled to the SOEC, resulting in reduced energy efficiency.

[0005] The main object of the present disclosure is to produce high-purity, high-pressure hydrogen while improving energy efficiency. [Means for solving the problem]

[0006] The present disclosure has adopted the following means to achieve the above-mentioned main object.

[0007] The electrolysis system of the present disclosure comprises: an electrolysis module including a solid oxide electrolysis cell that generates hydrogen by steam electrolysis and an insulated container that houses the electrolysis cell; a steam supply line for supplying steam to the electrolysis cell; an electrochemical pump having a pump cell including a solid electrolyte, an anode, and a cathode and housed in the container, connected to the electrolytic cell to introduce an off-gas containing hydrogen produced in the electrolytic cell and unreacted water vapor into an inlet of the anode, connected to a hydrogen recovery line to output pressurized hydrogen from an outlet of the cathode to the outside of the container, and connected to the water vapor supply line to output remaining hydrogen and water vapor from the outlet of the anode to the water vapor supply line within the container; The gist of the project is to provide the following:

[0008] The electrolysis system disclosed herein includes an electrochemical pump that introduces off-gas containing hydrogen produced in the electrolytic cell and unreacted water vapor from an anode inlet, outputs pressurized hydrogen from a cathode outlet to the outside of a storage container, and outputs the remaining hydrogen and water vapor from an anode outlet to a water vapor supply line within the storage container. The electrochemical pump enables the production of high-purity, high-pressure hydrogen. Furthermore, because the remaining hydrogen and unreacted water vapor are returned to the water vapor supply line while still at a high temperature, the amount of heat released outside the storage container can be reduced, thereby further improving energy efficiency. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic configuration diagram of an electrolysis system according to an embodiment of the present invention. [Figure 2] FIG. 1 is a schematic diagram of an electrochemical pump. [Figure 3] FIG. [Figure 4] FIG. 2 is a plan view of the pump cell as seen from direction A. DETAILED DESCRIPTION OF THE INVENTION

[0010] Next, embodiments of the present disclosure will be described with reference to the drawings.

[0011] Fig. 1 is a schematic configuration diagram of an electrolysis system 10 of this embodiment. Fig. 2 is a schematic configuration diagram of an electrochemical pump 30, Fig. 3 is a perspective view of a pump cell 32, and Fig. 4 is a plan view of the pump cell 32 as viewed from direction A. The electrolysis system 10 of this embodiment is configured as a solid oxide electrolysis cell system that produces hydrogen by electrolyzing high-temperature water vapor. As shown in Fig. 1 , the electrolysis system 10 includes an electrolysis module 20 including a solid oxide electrolysis cell stack (hereinafter referred to as SOEC stack) 21, a water supply system 40 that supplies water to the electrolysis module 20, an air supply system 45 that supplies air as a sweep gas to the electrolysis module 20, an electrochemical pump 30 that separates and pumps hydrogen from off-gas (anode off-gas) containing hydrogen generated in the SOEC stack 21 and unreacted water vapor, a hydrogen recovery system 50 that recovers hydrogen, an electrolysis power supply 61, and an electrochemical pump power supply 62.

[0012] In addition to the SOEC stack 21, the electrolysis module 20 also includes a heater 22, a water evaporator 23, and a heat exchanger 24, all of which are housed in a module case 25. The module case 25 also houses an electrochemical pump 30. The module case 25 is a box-shaped case made of a heat insulating material, and is maintained at a temperature of approximately 600 to 800°C (e.g., 700°C), which is suitable for the operation of the SOEC stack 21.

[0013] The SOEC stack 21 includes multiple unit cells, each of which includes a solid electrolyte, a fuel electrode (cathode) disposed on one side of the solid electrolyte, and an oxidizer electrode (anode) disposed on the other side of the solid electrolyte. In the SOEC stack 21, when water vapor is introduced into the fuel electrode through the fuel electrode inlet and the electrolysis power supply 61 supplies the power required for electrolysis of the water vapor, the water vapor is decomposed into hydrogen and oxygen ions, generating hydrogen at the fuel electrode. The decomposed oxygen ions then permeate the solid electrolyte and combine with electrons, generating oxygen at the oxidizer electrode. The hydrogen generated at the fuel electrode (produced hydrogen) is discharged from the fuel electrode outlet together with unreacted water vapor as fuel electrode off-gas. The oxygen generated at the oxidizer electrode is discharged from the oxidizer electrode outlet together with the sweep gas (air) introduced into the oxidizer electrode through the oxidizer electrode inlet as oxidizer electrode off-gas. The electrolysis power supply 61 can be a power grid, a renewable energy device such as a solar power generation system, or a storage battery. The same applies to the electrochemical pump power supply 62.

[0014] Because the SOEC stack 21 operates in a high-temperature environment of approximately 600 to 800°C, the solid electrolyte, fuel electrode, and oxidizer electrode are made of ceramic materials. Furthermore, to decompose water vapor into oxygen ions and hydrogen using a catalyst, the fuel electrode is made of a cermet made of ceramic and a metal such as nickel that has catalytic properties. To maintain good catalytic activity of the fuel electrode, it is necessary to keep the fuel electrode in a reducing atmosphere and prevent oxidation of the metal. For this reason, in this embodiment, hydrogen is mixed with the water vapor supplied to the fuel electrode to prevent oxidation.

[0015] One end of a fuel supply pipe L1 is connected to the anode inlet of the SOEC stack 21, and the other end of the fuel supply pipe L1 is connected to a water supply system 40. A heat exchanger 24, a water evaporator 23, and a heater 22 are provided in this order from upstream to downstream in the electrolysis module 20 in the fuel supply pipe L1. One end of anode off-gas piping L3 is connected to the anode outlet of the SOEC stack 21. The other end of the anode off-gas piping L3 is connected to a hydrogen recovery piping L5 and a reflux piping L6 via an electrochemical pump 30. The hydrogen recovery piping L5 is provided with the heat exchanger 24. The reflux piping L6 is connected to the fuel supply pipe L1.

[0016] One end of an oxidant supply pipe L2 is connected to an oxidant electrode inlet of the SOEC stack 21, and the other end of the oxidant supply pipe L2 is connected to an air supply system 45. A heat exchanger 24 and a heater 22 are provided in this order from the upstream side in the oxidant supply pipe L2 within the electrolysis module 20. In addition, an oxidant electrode off-gas pipe L4 is connected to an oxidant electrode outlet of the SOEC stack 21. The heat exchanger 24 is provided in the oxidant electrode off-gas pipe L4.

[0017] The water supply system 40 includes a water tank for storing water (raw water), a water pump, a flow rate controller 41, etc. Water pumped from the water tank is introduced into the electrolysis module 20. The water introduced into the electrolysis module 20 is evaporated into water vapor through the heat exchanger 24 and the water evaporator 23, and is heated to a required temperature by the heater 22 before being supplied to the fuel electrode of the SOEC stack 21.

[0018] The air supply system 45 includes a filter, an air blower, a flow rate controller 46, and the like. Air sucked through the filter by the air blower is introduced into the electrolysis module 20. The air introduced into the electrolysis module 20 is preheated in the heat exchanger 24 by heat exchange with the anode off-gas flowing through the anode off-gas piping L3 and the oxidizer electrode off-gas flowing through the oxidizer electrode off-gas piping L4, and is heated to a required temperature by the heater 22 before being supplied to the oxidizer electrode of the SOEC stack 21.

[0019] When the electrolysis power supply 61 supplies the SOEC stack 21 with the power required for electrolyzing the water vapor, hydrogen is generated at the fuel electrode and oxygen is generated at the oxidizer electrode. The hydrogen generated at the fuel electrode (produced hydrogen) is discharged together with unreacted water vapor from the fuel electrode outlet into the fuel electrode off-gas piping L3, and is introduced into the electrochemical pump 30 where it is pressurized. The high-pressure hydrogen pressurized by the electrochemical pump 30 is then recovered by the hydrogen recovery system 50. The oxygen generated at the oxidizer electrode is discharged together with the sweep gas (air) from the oxidizer electrode outlet into the oxidizer electrode off-gas piping L4, and is then discharged outside the electrolysis module 20 through the heat exchanger 24.

[0020] The hydrogen recovery system 50 includes a hydrogen tank 51 connected to one end of a hydrogen recovery pipe L5, the other end of which is connected to the high-pressure hydrogen outlet O2 of the electrochemical pump 30, and a boost pump 52 (mechanical pump) installed in the hydrogen recovery pipe L5. The high-pressure hydrogen that has been pressurized by the electrochemical pump 30 and output from the high-pressure hydrogen outlet O2 is further pressurized by driving the boost pump 52 and then stored in the hydrogen tank 51.

[0021] In this embodiment, the electrochemical pump 30 is configured as a proton-conducting ceramic (PC) electrochemical pump that exhibits good proton conductivity in the high-temperature environment of 600 to 800°C where the SOEC stack 21 operates. As shown in FIG. 2, the electrochemical pump 30 includes a cell stack 31 including multiple pump cells 32 and a substantially rectangular parallelepiped pressure vessel 37 that houses the cell stack 31. As shown in FIGS. 3 and 4, each pump cell 32 is configured as a cylindrical cell having a cylindrical solid electrolyte 33, an anode 34 (low-pressure hydrogen electrode) disposed on the inner circumferential surface of the solid electrolyte 33, and a cathode 35 (high-pressure hydrogen electrode) disposed on the outer circumferential surface of the solid electrolyte 33. In the electrochemical pump 30, high-pressure hydrogen is generated on the cathode 35 side, resulting in a relatively large pressure difference between the anode 34 and the cathode 35. In this embodiment, each pump cell 32 is configured as a cylindrical cell, ensuring sufficient pressure resistance against the pressure difference. The pump cell 32 may be a flat cylindrical cell or a metal-supported cell in which the solid electrolyte 33, anode 34, and cathode 35 are supported by a metal support. These configurations also ensure the pump cell 32's pressure resistance.

[0022] In this embodiment, the pump cell 32 is an anode-supported cylindrical cell and is fabricated as follows: First, various binders and water are added to a mixture of proton-conductive powder and NiO powder as the anode material, and the mixture is kneaded, followed by extrusion molding to fabricate an anode support. Next, BaCe 1-x-y Zr x M y O 3-δ and BaZr 1-x M x O 3-δA material having a composition such as M (where M is a trivalent cation) is mixed with a solvent to form a slurry, which is then coated on an anode support by tip coating to form a solid electrolyte. A cathode material is then mixed with a solvent to form a slurry, which is then coated on the solid electrolyte by dip coating to form a cathode, and the anode support, solid electrolyte, and cathode are then co-fired. In the case of an electrochemical pump, a Ni-based material similar to the anode material is selected for the cathode material of the pump cell 32, as there is no risk of the cathode material being oxidized. An interconnector 36 is then placed in the pump cells 32, and the pump cells 32 are electrically connected to each other to form a cell stack 31.

[0023] The cell stack 31 (plurality of pump cells 32) is housed within the pressure vessel 37, penetrating opposing side walls 37a, 36b of the pressure vessel 37. A gap between the through-hole of the pressure vessel 37 and the periphery of the cell stack 31 is sealed with a sealant 38. This simplifies the structure and assembly of the electrochemical pump 30. A low-pressure hydrogen inlet I1 is formed at one opening of the pump cell 32 (cylindrical cell), and the other end of an anode off-gas pipe L3, one end of which is connected to the anode outlet of the SOEC stack 21, is connected to the low-pressure hydrogen inlet I1. A low-pressure hydrogen outlet O1 is formed at the other opening of the pump cell 32 (cylindrical cell), and one end of a reflux pipe L6 is connected to the low-pressure hydrogen outlet O1. The reflux pipe L6 extends within the module case 25 and is connected to the fuel supply pipe L1. The pressure vessel 37 is formed with a high-pressure hydrogen outlet O2 that communicates with the space facing the cathode 35 of each pump cell 32 housed in the pressure vessel 37. A hydrogen recovery pipe L5 that extends to the outside of the electrolysis module 20 is connected to the high-pressure hydrogen outlet O2.

[0024] In the electrochemical pump 30 configured as described above, anode off-gas containing hydrogen and unreacted water vapor discharged from the anode outlet of the SOEC stack 21 is introduced into the anode 34 via the low-pressure hydrogen electrode inlet I1. At the same time, voltage is applied between the anode 34 and the cathode 35 from the electrochemical pump power supply 62. As shown in the following reaction equations (1) and (2), protons migrate from the anode 34 to the cathode 35 and combine with electrons at the cathode 35, generating hydrogen at the cathode 35. The generated hydrogen is pressurized as the hydrogen content increases and is pumped from the high-pressure hydrogen outlet O2 to the hydrogen recovery pipe L5. In the PC-type electrochemical pump 30, protons hop rapidly, enabling efficient generation of high-pressure hydrogen. Generally, the pressure loss in an electrochemical pump is minimal at relatively low boost pressures and increases sharply as the boost pressure increases. On the other hand, although the pressure loss of a mechanical pump is relatively large even at a relatively small boost pressure, the increase in pressure is relatively gradual even when the boost pressure increases. Therefore, by placing the electrochemical pump 30 upstream of the boost pump 52 (mechanical pump) and boosting the hydrogen pressure in stages, high-pressure hydrogen can be easily produced with little drive energy. Note that multiple boost pumps 52 may be connected in series depending on the required pressure of high-pressure hydrogen. Furthermore, with the PC-type electrochemical pump 30, as shown in equations (1) and (2), water vapor is not generated on the cathode 35 side. Therefore, hydrogen can be purified (dehydrated) from the anode off-gas containing water vapor while the hydrogen is being boosted. Therefore, there is no need to install a separate hydrogen purifier, such as a PSA (Pressure Swing Adsorption) in the hydrogen recovery pipe L5.

[0025] Anode: H2 [low pressure] → 2H + +2e - …(1) Cathode: 2H + +2e - →H2 [high pressure] …(2)

[0026] The reaction equations when a polymer electrolyte membrane (PEM) electrochemical pump is used as a hydrogen pump are shown in equations (3) and (4) below. In a PEM electrochemical pump, low-pressure hydrogen is supplied to the anode side, and when a voltage is applied between the anode and cathode, hydrated protons move from the anode to the cathode side, where they combine with electrons to generate hydrogen. However, as shown in equations (3) and (4), PEM electrochemical pumps require water to operate, and the high-pressure hydrogen generated at the cathode contains water, so the water must be removed separately to achieve high purity.

[0027] Anode: H2 [low pressure] + nH2O → 2H + (H2O) n +2e - …(3) Cathode: 2H + (H2O) n +2e - →H2[high pressure]+n(H2O) …(4)

[0028] Furthermore, the remaining hydrogen and water vapor of the anode off-gas introduced into the anode 34 of each pump cell 32 that does not migrate to the cathode 35 side are output from the low-pressure hydrogen outlet O1 to the reflux pipe L6. The remaining hydrogen and water vapor output to the reflux pipe L6 are then refluxed to the fuel supply pipe L1 while maintaining a high temperature. The refluxed remaining hydrogen is used as an antioxidant for the anode (cathode) of the SOEC stack 21, and the refluxed water vapor is reused as a raw material for hydrogen production in the SOEC stack 21. Because the water vapor does not migrate to the cathode 35 side, the utilization rate of the water vapor can be made 100%, and the amount of heat released outside the electrolysis module 20 (module case 25) can be reduced. As a result, the energy efficiency of the electrolysis system 10 can be further improved. The set current of the electrochemical pump power supply 62 is set to a value smaller than that of the electrolysis power supply 61 in order to reflux a portion of the hydrogen generated in the SOEC stack 21. The amount of hydrogen reflux (the ratio of hydrogen refluxed to hydrogen generated in the SOEC stack 21) can be adjusted as desired by setting the set current of the electrochemical pump power supply 62 relative to the set current of the electrolysis power supply 61. The voltage applied to the electrochemical pump power supply 62 is determined by the hydrogen boost pressure provided by the electrochemical pump 30 and the overvoltage required for the electrochemical reaction of the electrochemical pump 30. The overvoltage is consumed as heat, and this heat is used as heat required for the electrolysis operation of the SOEC stack 21.

[0029] It goes without saying that the present disclosure is not limited to the above-described embodiments, and can be embodied in various forms as long as they fall within the technical scope of the present disclosure.

[0030] For example, in the above-described embodiment, the electrolysis module 20 includes the heater 22 that heats the water vapor and sweep gas (air) supplied to the SOEC stack 21. However, the electrolysis module 20 may also include a combustor that burns part of the hydrogen produced at the anode of the SOEC stack 21 and heats the water vapor and sweep gas (air) with the combustion heat.

[0031] In the above-described embodiment, the electrolysis system 10 introduces water from the water supply system 40 into the electrolysis module 20 and generates water vapor by evaporating the introduced water in the water evaporator 23 provided in the electrolysis module 20. However, the electrolysis system 10 may also be configured to install a water evaporator, such as a steam generating heat pump, outside the electrolysis module 20 and introduce the water vapor generated by the water evaporator into the electrolysis module 20. [Industrial Applicability]

[0032] The present disclosure is applicable to the electrolysis system manufacturing industry and the like. [Explanation of symbols]

[0033] 10 Electrolysis system, 20 Electrolysis module, 21 SOEC stack (electrolysis cell), 25 Module case (container), 30 Electrochemical pump, 32 Pump cell, 33 Solid electrolyte, 34 Anode, 35 Cathode, 36 Pressure vessel, L1 Fuel supply pipe (steam supply line), L5 Hydrogen recovery pipe (hydrogen recovery line).

Claims

1. an electrolysis module including a solid oxide electrolysis cell that generates hydrogen by steam electrolysis and an insulated container that houses the electrolysis cell; a steam supply line for supplying steam to the electrolysis cell; an electrochemical pump having a pump cell including a solid electrolyte, an anode, and a cathode and housed in the container, connected to the electrolytic cell to introduce an off-gas containing hydrogen produced in the electrolytic cell and unreacted water vapor into an inlet of the anode, connected to a hydrogen recovery line to output pressurized hydrogen from an outlet of the cathode to the outside of the container, and connected to the water vapor supply line to output remaining hydrogen and water vapor from the outlet of the anode to the water vapor supply line within the container; An electrolysis system comprising:

2. 2. The electrolysis system of claim 1, the pump cell is a proton-conductive cell having proton conductivity in a temperature environment in which the electrolytic cell operates; Electrolysis system.

3. 3. The electrolysis system according to claim 1 or 2, The pump cell is a cylindrical cell. Electrolysis system.

4. 4. The electrolysis system according to claim 3, the electrochemical pump includes the pump cell and a pressure vessel that houses the pump cell; the pump cell is inserted through the pressure vessel; the anode is disposed on the inner circumferential surface side of the solid electrolyte; The cathode is disposed on the outer peripheral surface side of the solid electrolyte. Electrolysis system.

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