Electrolysis system

Abstract

To produce high-purity hydrogen while suppressing the deterioration in heat efficiency when the off-gas discharged from an electrolytic cell is recirculated to the electrolytic cell.SOLUTION: An electrolysis system includes: an electrolysis module comprising a solid oxide electrolytic cell 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 a hydrogen separation unit having a metal hydrogen-permeable membrane and accommodated in the housing container, which receives off-gas containing produced hydrogen and water vapor discharged from the electrolytic cell, outputs the produced hydrogen that has permeated the hydrogen-permeable membrane to a produced hydrogen line outside the housing container, and outputs the produced hydrogen and water vapor that have not permeated the hydrogen-permeable membrane to the water vapor feed line within the housing container. The housing container has a first storage space that accommodates the electrolytic cell, and a second storage space that is maintained at a temperature lower than that of the first storage space. The hydrogen separation unit is accommodated in the second storage space.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] A conventional electrolysis system of this type includes an electrochemical cell (electrolysis cell), a hydrogen electrode inlet supply line that supplies hydrogen electrode inlet supply gas containing water vapor to the electrochemical cell, a heater installed in the hydrogen electrode inlet supply line, a hydrogen electrode outlet line that discharges hydrogen electrode outlet gas containing hydrogen from the electrochemical cell, a circulation gas suction mechanism (or ejector) that uses water vapor (water vapor for driving the circulation gas suction mechanism) branched from the hydrogen electrode inlet supply line to draw in a portion of the hydrogen electrode outlet supply gas as a circulation gas, a cooler that separates excess water from the non-circulating hydrogen electrode outlet gas to produce hydrogen, and a water circulation pump that circulates the excess water (condensed water) separated by the cooler to the upstream side of the heater in the hydrogen electrode inlet line (see, for example, Patent Document 1). In this electrolysis system, a portion of the hydrogen electrode outlet gas is branched without being cooled and drawn in by the circulation gas suction mechanism, and a mixed gas of water vapor and hydrogen is supplied to the hydrogen electrode inlet as a circulation gas. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-115430 Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-mentioned electrolysis system, a portion of the hydrogen electrode outlet gas (off-gas) is branched off without being cooled and supplied to the hydrogen electrode inlet, the remaining hydrogen electrode outlet gas is cooled in a cooler, and the resulting condensed water is returned to the hydrogen electrode inlet line. This requires latent heat to evaporate the returned condensed water, and further requires raising the temperature of the water vapor supplied to the electrolysis cell to the temperature required for electrolysis, resulting in a decrease in energy efficiency.

[0005] The main object of the present disclosure is to produce high-purity 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; a hydrogen separation unit having a metal-based hydrogen-permeable membrane and accommodated in the accommodation container, into which an off-gas containing produced hydrogen and water vapor discharged from the electrolytic cell is introduced, and which outputs the produced hydrogen that has permeated the hydrogen-permeable membrane from the introduced off-gas to a produced hydrogen line outside the accommodation container, and which outputs the produced hydrogen and water vapor that have not permeated the hydrogen-permeable membrane from the introduced off-gas to the water vapor supply line within the accommodation container; Equipped with the storage container has a first storage space that stores the electrolysis cell and a second storage space that is maintained at a temperature lower than that of the first storage space, The hydrogen separation unit is accommodated in the second accommodation space. The gist of this is as follows.

[0008] In the electrolysis system disclosed herein, the electrolysis module's storage container has a first storage space that stores the electrolysis cell and a second storage space that is maintained at a lower temperature than the first storage space, and the hydrogen separation unit is stored in the second storage space. This allows the metal-based hydrogen-permeable membrane of the hydrogen separation unit to be placed in a temperature environment suitable for its operation, allowing the hydrogen-permeable membrane to fully demonstrate its performance and produce high-purity hydrogen. Furthermore, the remaining hydrogen that did not permeate the hydrogen-permeable membrane and unreacted water vapor are returned to the water vapor supply line at a high temperature, thereby reducing the amount of heat released outside the storage container and 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. 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 diagram of an electrolysis system 10 of this embodiment. 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, a hydrogen separation unit 25 that separates hydrogen from an off-gas that contains hydrogen generated in the SOEC stack 21 and unreacted water vapor, a hydrogen recovery system 50 that recovers the hydrogen, and a power supply 60 that supplies power required for electrolyzing water vapor to the SOEC stack 21.

[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, which are housed in a module case 28. The module case 28 also houses a hydrogen separation unit 25.

[0013] The module case 28 is a box-shaped case made of a thermal insulating material and has a first storage space S1 and a second storage space S2 separated by a partition wall 29 made of a thermal insulating material. The first storage space S1 accommodates the SOEC stack 21, the heater 22, the water evaporator 23, and the heat exchanger 24, while the second storage space S2 accommodates the hydrogen separation unit 25. The first storage space S1 is maintained at a temperature of approximately 600 to 800°C (e.g., 700°C) suitable for the operation of the SOEC stack 21, and the second storage space S2 is maintained at a temperature of approximately 300 to 400°C (e.g., 350°C) suitable for the operation of the hydrogen separation unit 25 (hydrogen-permeable membrane 27). The temperature of the first storage space S1 is determined by the output of the heater 22 and the specifications of the water evaporator 23 and the heat exchanger 24, while the temperature of the second storage space S2 is determined by the specifications of the partition wall 29 (thermal insulating material).

[0014] The SOEC stack 21 includes a plurality of 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 power required for electrolysis of the water vapor is supplied from the power source 60, 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 a sweep gas (air) introduced into the oxidizer electrode through the oxidizer electrode inlet as oxidizer electrode off-gas. The power source 60 can be a power grid, a renewable energy device such as a solar power generation system, or a storage battery.

[0015] 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.

[0016] 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 water evaporator 23 and a heater 22 are provided in this order from upstream to downstream in the fuel supply pipe L1 within the electrolysis module 20 (first accommodation space S1). One end of a fuel electrode off-gas pipe L3 is connected to the anode outlet of the SOEC stack 21. The other end of the anode off-gas pipe L3 is connected to a hydrogen recovery pipe L5 and a reflux pipe L6 via a hydrogen separation unit 25. A heat exchanger 24 is provided in the anode off-gas pipe L3. A heat exchanger 31 is provided in the hydrogen recovery pipe L5 outside the electrolysis module 20. The reflux pipe L6 is connected to the fuel supply pipe L1.

[0017] 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 (first accommodation space S1). 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.

[0018] The water supply system 40 includes a water tank for storing water (raw water), a water pump, a flow rate controller 41, etc. Water pressure-fed from the water tank by the water pump is preheated by heat exchange with the produced hydrogen flowing through the hydrogen recovery pipe L5 through a heat exchanger 31 installed outside the electrolysis module 20, and then introduced into the electrolysis module 20. The water introduced into the electrolysis module 20 is evaporated into water vapor through a water evaporator 23, and the water is heated to a required temperature by a heater 22 and then supplied to the anode of the SOEC stack 21.

[0019] 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.

[0020] When the power supply 60 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, passes through the heat exchanger 24, and is supplied to the hydrogen separation unit 25. 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, passes through the heat exchanger 24, and is discharged outside the electrolysis module 20.

[0021] The hydrogen separation unit 25 introduces anode off-gas containing produced hydrogen and unreacted water vapor from the anode off-gas pipe L3, separates some of the hydrogen, and outputs it to the hydrogen recovery pipe L5. The hydrogen separation unit 25 also outputs the remaining hydrogen and water vapor from the anode off-gas to the reflux pipe L6 for reflux to the fuel supply pipe L1. The hydrogen separation unit 25 includes a housing 26 and a hydrogen-permeable membrane 27 that separates the interior of the housing 26 into a first gas chamber C1 and a second gas chamber C2. The first gas chamber C1 has an inlet I1 and an outlet O1. The inlet I1 of the first gas chamber C1 is connected to one end of the anode off-gas pipe L3, the other end of which is connected to the anode outlet of the SOEC stack 21. The outlet O1 of the first gas chamber C1 is connected to one end of the reflux pipe L6. The other end of the reflux pipe L6 extends into the first accommodation space S1 in which the SOEC stack 21 is accommodated and is connected to the fuel supply pipe L1 within the first accommodation space S1. An outlet O2 is formed in the second gas chamber C2. One end of the hydrogen recovery pipe L5, which extends to the outside of the electrolysis module 20, is connected to the outlet O2 of the second gas chamber C2.

[0022] The hydrogen-permeable membrane 27 is a metal-based hydrogen-permeable membrane made of a Pd (palladium), V (vanadium), or Nb (niobium)-based material. Compared to V- or Nb-based materials, Pd-based materials are more chemically stable but more expensive, while V- or Nb-based materials have a higher hydrogen permeation rate but lower chemical stability than Pd-based materials. For this reason, the hydrogen-permeable membrane 27 of this embodiment is a metal membrane made of a V- or Nb-based material coated with a thin Pd film. Because only hydrogen atoms can move within the metal crystals of the metal-based hydrogen-permeable membrane 27, highly pure hydrogen can be efficiently recovered by supplying the anode off-gas discharged from the anode outlet of the SOEC stack 21 to the hydrogen-permeable membrane 27.

[0023] The metal-based hydrogen-permeable membrane 27 has a higher hydrogen permeation rate at higher temperatures. Operating it at lower temperatures can lead to embrittlement due to hydrogen, so it is desirable to operate it at temperatures of 300°C or higher. However, the hydrogen-permeable membrane 27 of this embodiment, which is coated with a thin Pd film, alloys with the Pd coating at temperatures of 400°C or higher, resulting in a deterioration in its performance. For this reason, a temperature of approximately 300 to 400°C is selected as the operating temperature for the hydrogen-permeable membrane 27 in this embodiment. For this reason, the hydrogen separation unit 25 including the hydrogen-permeable membrane 27 is housed in the second housing space S2, which is maintained at approximately 300 to 400°C (e.g., 350°C). If a Pd-based material is not used for the hydrogen-permeable membrane 27, temperatures of 400°C or higher are also acceptable.

[0024] 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 outlet O2 of the hydrogen separation unit 25; a vacuum pump 52 installed on the hydrogen recovery pipe L5; an adjustment valve 53 connected to the hydrogen recovery pipe L5 so as to be parallel to the vacuum pump 52; and a boost pump 54 installed on the hydrogen recovery pipe L5 downstream of the vacuum pump 52. The vacuum pump 52 applies negative pressure to the outlet O2 side (second gas chamber C2) of the hydrogen separation unit 25. Hydrogen contained in the anode off-gas introduced from the inlet I1 of the hydrogen separation unit 25 to the first gas chamber C1 permeates the hydrogen permeable membrane 27 by driving the vacuum pump 52, passes through the hydrogen recovery pipe L5, and is discharged from the outlet of the vacuum pump 52. The hydrogen discharged from the outlet of the vacuum pump 52 is then pressurized by driving the boost pump 54 and stored in the hydrogen tank 51. In this embodiment, a diaphragm-type vacuum pump is used as the vacuum pump 52. This is because a diaphragm pump can transport a target fluid from an inlet to an outlet without exposing the fluid to the outside air.

[0025] In the metal-based hydrogen-permeable membrane 27, the greater the hydrogen partial pressure difference between the front and back surfaces of the membrane, the higher the hydrogen permeation rate. Therefore, by using the vacuum pump 52 to reduce the pressure on the outlet O2 side (second gas chamber C2 side) of the hydrogen separation unit 25 and lowering the hydrogen partial pressure there compared to the inlet I1 side (first gas chamber C1 side), the rate of hydrogen purification and separation from the anode off-gas containing hydrogen and water vapor can be improved. The hydrogen permeation rate of the metal-based hydrogen-permeable membrane 27 is determined not only by the hydrogen partial pressure difference between the front and back surfaces of the membrane, but also by the hydrogen permeability coefficient of the permeable material at each temperature, the membrane area, and the membrane thickness. The area and thickness of the hydrogen-permeable membrane 27 are determined based on the hydrogen production rate in the SOEC stack 21 and the hydrogen permeability coefficient of the hydrogen-permeable membrane 27, and a vacuum pump 52 having a pumping speed greater than the hydrogen production rate in the SOEC stack 21 is selected.

[0026] The remaining hydrogen and water vapor that do not permeate the hydrogen-permeable membrane 27 are output from the 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 fuel electrode (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 water vapor does not permeate the hydrogen-permeable membrane 27, the utilization rate of 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.

[0027] In this embodiment, the hydrogen recovery pipe L5 is connected to the adjustment valve 53 in parallel with the vacuum pump 52, allowing the adjustment valve 53 to adjust the amount of pressure reduction by the vacuum pump 52. As described above, the amount of hydrogen permeation through the hydrogen-permeable membrane 27 is determined by the hydrogen partial pressure difference between the front and rear surfaces. Therefore, by adjusting the amount of pressure reduction by the vacuum pump 52 with the adjustment valve 53 to adjust the amount of hydrogen permeation through the hydrogen-permeable membrane 27, the hydrogen reflux amount (the ratio of hydrogen refluxed to hydrogen generated in the SOEC stack 21) can be adjusted as desired. Note that if an ejector is provided in the fuel supply pipe L1 and the reflux pipe L6 is connected to the suction port of the ejector, it is difficult to control the reflux amount because the suction pressure (hydrogen reflux amount) is affected by the flow rate and density of the working fluid flowing through the fuel supply pipe L1. In contrast, in this embodiment, the hydrogen reflux amount can be easily adjusted by simply adjusting the amount of pressure reduction by the vacuum pump 52 with the adjustment valve 53.

[0028] 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.

[0029] For example, in the above-described embodiment, the adjustment valve 53 is connected to the hydrogen recovery pipe L5 in parallel with the vacuum pump 52, but the adjustment valve 53 may be omitted and the vacuum pump 52 may adjust the amount of pressure reduction by controlling the pump alone. Also, the vacuum pump 52 may be omitted.

[0030] 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 Hydrogen separation unit, 27 Hydrogen permeable membrane, 28 Module case (container), 52 Vacuum pump (negative pressure generating part, pump), 53 Adjusting valve (negative pressure generating part), L1 Fuel supply pipe (water vapor supply line), L5 Hydrogen recovery pipe (produced hydrogen line), S1 First accommodation space, S2 Second accommodation space.

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; a hydrogen separation unit having a metal-based hydrogen-permeable membrane and accommodated in the accommodation container, into which an off-gas containing produced hydrogen and water vapor discharged from the electrolytic cell is introduced, and which outputs the produced hydrogen that has permeated the hydrogen-permeable membrane from the introduced off-gas to a produced hydrogen line outside the accommodation container, and which outputs the produced hydrogen and water vapor that have not permeated the hydrogen-permeable membrane from the introduced off-gas to the water vapor supply line within the accommodation container; Equipped with the storage container has a first storage space that stores the electrolysis cell and a second storage space that is maintained at a temperature lower than that of the first storage space, The hydrogen separation unit is accommodated in the second accommodation space. Electrolysis system.

2. 2. The electrolysis system of claim 1, a negative pressure generating unit that generates negative pressure in the produced hydrogen line; Electrolysis system.

3. 3. The electrolysis system according to claim 2, The negative pressure generating unit includes a pump provided in the produced hydrogen line and an adjusting valve connected to the produced hydrogen line in parallel with the pump. Electrolysis system.

4. 4. The electrolysis system according to claim 1, The second storage space is maintained at a temperature of 300°C or higher. Electrolysis system.

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