Process and plant for hydrogen production

Abstract

The invention relates to a process for producing hydrogen from water, in which a pumpable suspension of zinc particles in alkaline solution is supplied to an oxidation unit, in the oxidation unit (3) zinc is electrochemically or thermally oxidized to zinc oxide with the release of hydrogen, the suspension leaving the oxidation unit (3) is supplied to a reduction unit (4) and in the reduction unit (4) zinc oxides formed during oxidation are electrochemically reduced to zinc with the release of oxygen, whereupon the suspension leaving the reduction unit (4) is again supplied to the at least one oxidation unit (3).

Description

[0001] The present invention relates to a process and a plant for producing hydrogen from water.

[0002] Green hydrogen is currently produced predominantly electrochemically using water electrolysis processes. Technically, these processes are divided into PEM electrolysis (PEM), alkaline electrolysis (AEL), or solid oxide electrolysis (SOEC). These technologies have in common that they require large quantities of critical materials, such as iridium, ruthenium, platinum, palladium, or perfluorinated hydrocarbons.

[0003] Explosion risks, in particular, have driven the development of decoupled water electrolysis in recent years. In these processes, a mediator is used to spatially and / or temporally decouple the production of hydrogen and oxygen. The mediators used are substances with a suitable redox potential that are either reduced or oxidized to complement the hydrogen and oxygen half-reactions. These can be polyoxometalates, dissolved transition metals such as vanadium or solids such as nickel or manganese (MCHUGH, PJ [et al.]: Decoupled electrochemical water splitting: from fundamentals to applications. In: Advanced Energy Materials, Vol. 10, 2020, No. 44, Article-No.: 2002453, pp. 1-21. - ISSN 1614-6832 (P); 1614-6840 (E). DOI: 10.1002 / aenm.202002453).

[0004] In 2018, Horri presented a zinc-mediated electrolysis system, which, however, can only be operated intermittently (HORRI, BA [et al.]: A highly efficient hydrogen generation electrolysis system using alkaline zinc hydroxide solution. In: International Journal of Hydrogen Energy, Vol. 44, 2019, No. 1, pp. 72-81. - ISSN 0360-3199 (P); 1879-3487 (E). DOI: 10.1016 / j.ijhydene.2018.03.048).

[0005] Zinc-air batteries store energy through the oxidation and reduction of zinc. However, the process parameters are selected to suppress hydrogen evolution. Additionally, zinc-air batteries can be designed as flow batteries to decouple power and storage capacity (APPLEBY, AJ; JACQUIER, M.: The CGE circulating zinc / air battery: A practical vehicle power source. In: Journal of Power Sources, Vol. 1, 1976, No. 1, pp. 17-34. - ISSN 0378-7753 (P); 1873-2755 (E). DOI: 10.1016 / 0378-7753(76)80003-1).

[0006] According to the current state of the art, hydrogen production using electrolysis technologies is primarily increased by producing and operating a large number of electrolysis stacks in parallel. The construction of large electrolysis stacks is less common, as conventional technologies encounter problems with pressure resistance, service life, or membrane stability when scaling up to dimensional scale. The close proximity of hydrogen and oxygen production, in particular, poses design and operational difficulties for existing electrolysis technologies, as even the smallest material defects can greatly increase the risk of explosion.

[0007] Solid-state electrolysis concepts have so far suffered from the problem that the production of hydrogen and oxygen occurs at different times, making them incapable of continuous operation. Furthermore, the metal surfaces of the electrodes can quickly become passivated due to oxide formation, significantly reducing the performance of the electrolysis device. Furthermore, dendrite formation is a problem, particularly in the case of stationary zinc electrodes, which damages the separators used, deforms the electrodes, and can lead to short circuits.

[0008] Weiteren Stand der Technik bilden die Offenbarungen WO 2021 / 099 986 A1; HAHN, R. [et al.]: Lifetime of the gas evolution electrode for the Zn-H2 storage system; Research Article, In: Fuel Cells, special issue EFCF2023, 0:e202300209, 2024, S. 1-10. https: / / doi.org / 10.1002 / fuce.202300209. URL: https: / / onlinelibrary.wiley.com / doi / 10.1002 / fuce.202300209 [abgerufen am 2024-11-13]; HUANG, J. [et al.].: Efficient renewable-to-hydrogen conversion via decoupled electrochemical water splitting. Review. In: Cell Reports Physical Science, Vol. 1, No. 8, 2020, 100138. DOI: 10.1016 / j.xcrp.2020.100138. URL: https: / / www.sciencedirect.com / science / article / pii / S2666386420301429 [abgerufen am 2024-11-13]; US 11 447 879 B2 und US 2008 / 0 190 781 A1.

[0009] Based on this prior art, it is an object of the present invention to provide an alternative method and an alternative system of the type mentioned at the outset, by which at least some of the disadvantages mentioned are overcome.

[0010] To achieve this object, the present invention provides a process for producing hydrogen from water, in which a pumpable suspension of zinc particles in alkaline solution is fed to an oxidation unit, zinc is electrochemically or thermally oxidized to zinc oxide in the oxidation unit, releasing hydrogen, the suspension leaving the oxidation unit is fed to a reduction unit, and in the reduction unit, zinc oxides formed during the oxidation are electrochemically reduced to zinc, releasing oxygen, whereupon the suspension leaving the reduction unit is fed back to the at least one oxidation unit. According to the invention, hydrogen is thus produced in a continuously operable process in which the oxidation unit and the reduction unit, both of which can be designed as electrochemical electrolysis stacks, are spatially decoupled from one another.This decoupling is advantageous in that it reduces the risk of explosion. Furthermore, only a single gas is released in the oxidation unit and the reduction unit, eliminating the need for gas-tight barrier layers. Only electrical separation is required. The resulting significantly reduced sealing requirements are accompanied by lower costs. Potential leaks are visible from the outside, making them easy to detect and repair. Any passivation of the zinc surface is compensated for by the very high specific surface area of ​​the zinc particles. Dendrites that form on the zinc particles are sheared off by the continuous conveyance of the highly concentrated particle suspension before they reach a critical size. This can be facilitated by the use of a shear-intensive pump.A further advantage is that no critical, rare or expensive materials are used.

[0011] Preferably, a storage unit is provided downstream of the oxidation unit in which the suspension leaving the oxidation unit is temporarily stored before being fed to the reduction unit.

[0012] Preferably, a storage unit is provided downstream of the reduction unit in which the suspension leaving the reduction unit is temporarily stored before being fed to the oxidation unit.

[0013] The memories serve as buffers.

[0014] Advantageously, the volume flows of the suspension through the oxidation unit and through the reduction unit can be adjusted independently of each other.

[0015] Thanks to storage tanks acting as buffers and adjustable flow rates, it is possible to adjust the temporal production output to the available energy quantity, at least for a certain period of time, whereby the period depends on the size of the storage tanks. This allows the applied power to be adjusted at short notice by adjusting the feed rate without damaging the electrodes or risking mixing of hydrogen and oxygen. Likewise, reversing the polarity of the electrodes is not necessary for operation, so this cannot cause any damage.

[0016] According to one embodiment of the method according to the invention, the reduction capacity of the reduction unit is designed to be greater than the oxidation capacity of the oxidation unit. This contributes to the hydrogen-producing oxidation unit being designed for continuous operation, as it operates with very low energy input. This allows downstream process steps to be continuously supplied with hydrogen. The oxygen-producing reduction unit, on the other hand, is scaled larger and can thus reduce the appropriate amount of zinc oxide, albeit in fewer full-load hours. Thanks to the larger scale, the more energy-intensive operation of the reduction unit can be adapted to the expected energy production from locally available volatile energies, such as solar, wind, surplus electricity, or the like.

[0017] The oxidation unit is preferably designed to electrochemically oxidize the zinc particles. Alternatively, it is also possible for zinc to be thermally oxidized to zinc oxide in the oxidation unit. For this purpose, the zinc particles are preferably mechanically separated from the suspension in the oxidation unit, heated to a temperature above 400°C, and brought into contact with steam for oxidation. The oxidized zinc particles are then returned to the suspension. At this temperature, the zinc reacts with the steam, releasing hydrogen.

[0018] If the reduction unit and / or the oxidation unit are electrochemically operated, the flow is meandering. Accordingly, the suspension can pass through the cell perpendicular to the electric field.

[0019] According to one embodiment of the process according to the invention, either only the oxidation unit is operated under pressure or both the oxidation unit and the reduction unit are operated under pressure in order to directly produce compressed hydrogen and / or compressed oxygen.

[0020] To achieve the object mentioned above, the present invention further provides a plant designed to produce hydrogen from a pumpable suspension of zinc particles in alkaline solution, comprising a circuit with an oxidation unit designed to electrochemically or thermally oxidize zinc contained in the suspension to zinc oxide with the release of hydrogen, a reduction unit spatially spaced from the oxidation unit and designed to electrochemically reduce zinc oxide contained in the suspension to zinc with the release of oxygen, lines that fluidically connect an outlet of the oxidation unit to an inlet of the reduction unit and an outlet of the reduction unit to an inlet of the oxidation unit, and at least one pump designed to circulate the suspension in the circuit between the oxidation unit and the reduction unit,a water inlet designed to supply water to the circuit, a hydrogen outlet designed to discharge hydrogen released in the oxidation unit from the circuit, and an oxygen outlet designed to discharge oxygen released in the reduction unit from the circuit.

[0021] Preferably, the circuit comprises a reservoir arranged between the outlet of the oxidation unit and the inlet of the reduction unit.

[0022] Additionally or alternatively, the circuit preferably comprises a reservoir arranged between the outlet of the reduction unit and the inlet of the oxidation unit.

[0023] Advantageously, two pumps are provided, the first pump being arranged between the first reservoir and the oxidation unit and the second pump being arranged between the second reservoir and the reduction unit.

[0024] The reduction capacity of the reduction unit is advantageously designed to be larger than the oxidation capacity of the oxidation unit.

[0025] The reduction unit and, if operated electrochemically, the oxidation unit preferably each have a plurality of electrodes arranged one above or next to one another, each separated from one another by a separator and sealed against one another, preferably made of stainless steel, wherein one electrode side is provided with a meandering flow channel through which the suspension is passed from a feed opening to an opposite outlet opening, and the other electrode side has a catalyst, in particular is coated with a catalyst, wherein the catalyst is preferably nickel or tungsten.

[0026] Further features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings. Fig. 1 is a schematic view of a plant according to an embodiment of the present invention, designed to produce hydrogen from a pumpable suspension of zinc particles in alkaline solution, Fig. 2 a schematic plan view of an electrode of an oxidation unit and / or a reduction unit of the Fig. 1 shown system and Fig. 3 is a schematic side exploded view of two of the Fig. 2 shown electrodes with separator arranged in between.

[0027] The same reference numbers refer to the same or similar components below.

[0028] Fig.1 shows a system 1 according to an embodiment of the present invention, which is designed to produce hydrogen from a pumpable suspension of zinc particles in an alkaline solution. The system 1 comprises a circuit 2 having at least one oxidation unit 3, a reduction unit 4, and a pump 5, which are connected to one another via lines 6. In the present embodiment, a first reservoir 7, a second reservoir 8, and a further pump 5 are also provided. Furthermore, a water inlet 9 is designed to supply water to the circuit 2, a hydrogen outlet 10 is designed to discharge hydrogen released in the oxidation unit 3 from the circuit 2, and an oxygen outlet 11 is designed to discharge oxygen released in the reduction unit 4 from the circuit 2.

[0029] The oxidation unit 3 is designed to electrochemically oxidize zinc contained in the suspension to zinc oxide with the release of hydrogen.

[0030] The reduction unit 4, which is arranged at a distance from the oxidation unit 3, is designed to electrochemically reduce zinc oxides contained in the suspension to zinc with the release of oxygen.

[0031] The outlet of the oxidation unit 3 is fluidly connected to the inlet of the second reservoir 8, the outlet of the second reservoir 8 is fluidly connected to the inlet of the reduction unit 4, the outlet of the reduction unit 4 is fluidly connected to the inlet of the first reservoir 7, and the outlet of the first reservoir 7 is fluidly connected to the inlet of the oxidation unit 3, each of the connections being made via one of the lines 6. In this case, the first pump 5 is positioned between the outlet of the first reservoir 7 and the inlet of the oxidation unit 3, and the second pump 5 is arranged between the outlet of the second reservoir 8 and the reduction unit 4. The water inlet 9 is arranged between the second reservoir 8 and the second pump 5, the hydrogen outlet 10 is arranged between the oxidation unit 3 and the second reservoir 8, and the oxygen outlet 11 is arranged between the reduction unit 4 and the first reservoir 7.

[0032] The oxidation unit 3 and the reduction unit 4 each have, as shown in the Fig. 2 and Fig. 3, a plurality of electrodes 12 arranged one above or next to one another, each of which is sealed against one another by seals (not shown in detail) and electrically separated from one another by a separator 13. Each electrode 12 has a meandering flow channel 15 on one electrode side 14, which extends between a supply opening 16 and an oppositely arranged outlet opening 17, wherein the seals in the region of the supply openings 16 and outlet openings 17 are correspondingly recessed. The other electrode side 18 is flat in the present case and coated with a catalyst, for example made of nickel or tungsten.

[0033] The electrode stacks of the oxidation unit 3 and the reduction unit 4 can be manufactured, for example, by first assembling them from individual stainless steel electrodes. A nickel electrolyte, such as nickel sulfate, is then pumped through the electrode stacks. Applying an electric current deposits a nickel layer on the electrically accessible part of the electrode. The electrode stack is then rinsed with clean water. This minimizes the amount of nickel used. This approach eliminates the need to produce a stainless steel-nickel composite material, and any potential damage to the nickel layer during assembly of the electrode stack is eliminated. By adding particles such as diamond, corundum, quartz, or titanium carbide to the nickel electrolyte, a rough electrode surface can be created, which later increases the active electrode surface.Should the nickel layer wear out during operation of the oxidation or reduction unit 3, 4, the nickel layer can also be regenerated using the method.

[0034] The reduction capacity of the reduction unit 4 is designed to be greater than the oxidation capacity of the oxidation unit 3.

[0035] During operation of the Fig. In the system 1 shown in Figure 1, a pumpable suspension of zinc particles in alkaline solution contained in the first reservoir 7 is pumped to the oxidation unit 3 using the first pump 5. In the oxidation unit 3, zinc is oxidized to zinc oxide. The following half-reactions occur: reaction Redox potential Zn (s) + 2OH - ⇔ Zn(OH)2 + 2e - E° = -0.7618 V 2H2O + 2e - ⇔ 2OH - + H 2(g) E° = -0.8277 V

[0036] This reaction proceeds with very low energy input and releases hydrogen, which leaves the circuit 2 through the hydrogen outlet 10. The suspension, now containing zinc oxide, is then stored in the second storage 8.

[0037] The suspension is removed from the second reservoir 8 via the second pump 5 and fed to the reduction unit 4, where the zinc oxide particles are reduced again. The following partial reactions take place: reaction Redox potential Zn(OH)2 + 2e - ⇔ Zn (s) + 2OH E° = -1.25 V 4OH - ⇔ 2H2O + 4e - + O2 (g) E° = 0.4 V

[0038] This releases oxygen, which leaves the circuit 2 through the oxygen outlet 11. The suspension, now containing zinc, is then stored in the first reservoir 7.

[0039] In reduction unit 4, the required cell voltage above 1.23 volts is so high that hydrogen could theoretically be produced, which is undesirable. Such hydrogen production can be prevented, for example, by selecting the electrode materials used to provide sufficient overvoltage for the hydrogen formation reaction.

[0040] Thanks to the two storage units 7 and 8, the two pumps 5 and the fact that the reduction capacity of the reduction unit 4 is designed to be greater than the oxidation capacity of the oxidation unit 3, for example, the less energy-intensive oxidation unit 3 can be operated constantly and the more energy-intensive reduction unit 4 can be operated depending on the currently expected energy production of locally available volatile energies, such as solar, wind, surplus electricity or the like.

[0041] The oxidation unit 3 or both the oxidation unit 3 and the reduction unit 4 can be operated under pressure to directly produce pressurized hydrogen and optionally pressurized oxygen.

[0042] A significant advantage associated with the previously described inventive method is the possibility of continuous hydrogen production. The oxidation unit 3 and the reduction unit 4 are spatially separated, thereby reducing the potential for danger. Furthermore, only a single gas is released in the oxidation unit 3 and the reduction unit 4, which is why gas-tight barrier layers are not necessary. Only electrical separation is required. Overall, this results in a simple and cost-effective design. Furthermore, potential leaks are visible or detectable from the outside, making them both easy to identify and relatively simple to repair. Any passivation of the zinc surface that may occur is compensated for by the very high specific surface area of ​​the zinc particles.Dendrites that form on the zinc particles are sheared off by the continuous conveyance of the highly concentrated particle suspension before they reach a critical size. This can be facilitated by the use of shear-intensive pumps. 5 A further advantage is that no critical, rare, or expensive materials are used.

[0043] Even if the Fig. While the oxidation unit 3 shown in Figure 1 is one that oxidizes the zinc particles electrochemically, the oxidation can alternatively be carried out thermally. In this case, the zinc particles are preferably mechanically separated from the suspension in the oxidation unit 3, heated to a temperature above 400°C, and brought into contact with steam for oxidation. The oxidized zinc particles are then returned to the suspension before it leaves the oxidation unit 3 again. REFERENCE NUMBER LIST 1 system 2nd circuit 3 Oxidation unit 4 Reduction unit 5 Pump 6 Line 7 storage 8 storage 9 Water inlet 10 Hydrogen discharge 11 Oxygen drain 12 Electrode 13 Separator 14 Electrode side 15 flow channel 16 Feed opening 17 Outlet opening 18 Electrode side

Claims

[1] A process for producing hydrogen from water, in which a pumpable suspension of zinc particles in alkaline solution is fed to an oxidation unit (3), zinc is electrochemically or thermally oxidized to zinc oxide in the oxidation unit (3) with the release of hydrogen, the suspension leaving the oxidation unit (3) is fed to a reduction unit (4) and in the reduction unit (4) zinc oxides formed during the oxidation are electrochemically reduced to zinc with the release of oxygen, whereupon the suspension leaving the reduction unit (4) is fed back to the at least one oxidation unit (3). [2] Method according to claim 1, characterized by that downstream of the oxidation unit (3) a storage (8) is provided in which the suspension leaving the oxidation unit (3) is temporarily stored before it is fed to the reduction unit (4). [3] Method according to claim 1 or 2, characterized by that downstream of the reduction unit (4) a storage (7) is provided in which the suspension leaving the reduction unit (4) is temporarily stored before it is fed to the oxidation unit (3). [4] Method according to claim 2 or 3, characterized by that the volume flows of the suspension through the oxidation unit (3) and through the reduction unit (4) can be adjusted independently of each other. [5] Method according to claim 4, characterized by that the reduction capacity of the reduction unit (4) is designed to be greater than the oxidation capacity of the oxidation unit (3). [6] Method according to one of the preceding claims, characterized bythat zinc is thermally oxidized to zinc oxide in the oxidation unit (3), wherein the zinc particles in the oxidation unit (3) are mechanically separated from the suspension, heated to a temperature above 400°C and brought into contact with steam for the purpose of oxidation, whereupon the oxidized zinc particles are returned to the suspension. [7] Method according to one of the preceding claims, characterized by that the reduction unit (4) and / or the oxidation unit (3), when operating electrochemically, is / are flowed through in a meandering manner. [8] Method according to one of the preceding claims, characterized by that either only the oxidation unit (3) is operated under pressure or both the oxidation unit (3) and the reduction unit (4) are operated under pressure. [9] Plant (1) designed to obtain hydrogen from a pumpable suspension of zinc particles in alkaline solution, comprising a circuit (2) with an oxidation unit (3) designed to electrochemically or thermally oxidise zinc contained in the suspension to zinc oxide with the release of hydrogen, a reduction unit (4) spatially spaced from the oxidation unit (3) and designed to electrochemically reduce zinc oxide contained in the suspension to zinc with the release of oxygen, lines (6) fluidly connecting an outlet of the oxidation unit (3) to an inlet of the reduction unit (4) and an outlet of the reduction unit (4) to an inlet of the oxidation unit (3), and at least one pump (5) designed to circulate the suspension in the circuit (2) between the oxidation unit (3) and the reduction unit (4), a water inlet (9),which is designed to supply water to the circuit (2), a hydrogen outlet (10) which is designed to discharge hydrogen released in the oxidation unit (3) from the circuit (2), and an oxygen outlet (11) which is designed to discharge oxygen released in the reduction unit (4) from the circuit (2). [10] Annex (1) according to claim 9, characterized by that the circuit (2) has a reservoir (8) arranged between the outlet of the oxidation unit (3) and the inlet of the reduction unit (4). [11] Annex (1) according to claim 9 or 10, characterized by that the circuit (2) has a reservoir (7) arranged between the outlet of the reduction unit (4) and the inlet of the oxidation unit (3). [12] Annex (1) according to claim 11, characterized bythat two pumps (5) are provided, the first pump (5) being arranged between the first reservoir (7) and the oxidation unit (3) and the second pump (5) being arranged between the second reservoir (8) and the reduction unit (4). [13] Plant (1) according to one of claims 9 to 12, characterized bythat the reduction unit (4) and, if operated electrochemically, the oxidation unit (3) each have a plurality of electrodes (12) arranged one above the other or next to the other, each separated from one another by a separator and sealed against one another, preferably made of stainless steel, wherein one electrode side (14) is provided with a meandering flow channel (15) through which the suspension is guided from a feed opening (16) to an opposite outlet opening (17), and the other electrode side (18) has a catalyst, in particular is coated with a catalyst, wherein the catalyst is preferably nickel or tungsten.

Images