METHOD AND PLANT FOR THE ELECTROLYTIC PRODUCTION OF HYDROGEN

Description

[0001] The invention relates to a method for the electrolytic production of a liquid hydrogen product and to a system for carrying it out. Background of the invention

[0002] Hydrogen can be obtained using a variety of methods, for example by steam reforming of hydrogen-containing organic compounds such as methane, by co-electrolysis of water and carbon dioxide, or by electrolysis of water.

[0003] The latter can be based in particular on the use of alkaline electrolysis (AEL), proton exchange membranes (PEM), anion exchange membranes (AEM), or solid oxide electrolysis cells (SOEC). It is also possible to use several of these technologies in combination or in parallel within a single system.

[0004] Hydrogen can be liquefied by cooling and subsequent condensation against a suitable cooling medium such as hydrogen or helium. To increase the efficiency of liquefaction, the hydrogen must be pre-cooled before condensation. Vaporizing liquid nitrogen (LIN) is typically used for pre-cooling.

[0005] Industrial plants for the liquefaction of hydrogen are known, for example reference to EP 3 163 236 A1. In these plants, a hydrogen gas stream is typically cooled to a temperature below its condensation point by means of several closed cooling circuits, comprising a pre-cooling circuit and a main cooling circuit, in order to provide a liquid hydrogen stream.

[0006] Conventional cooling circuits typically use nitrogen as a coolant. For example, the evaporation of liquid nitrogen at approximately 78 K is utilized to first cool a hydrogen stream from ambient temperature to about 80 K in pre-cooling circuits. This is achieved by passing the nitrogen and hydrogen streams through a suitably designed heat exchanger. The cooled hydrogen stream is then passed through a purification unit designed as an adsorber to remove residual contaminants.

[0007] Following this purification, an ortho-para conversion of the hydrogen typically takes place in a converter facility, which is typically designed as one or more heat exchangers or containers filled with catalyst material.

[0008] This is usually followed by further cooling within the main cooling cycle until the temperature drops below the critical temperature of hydrogen (approx. 33 K) to a level of about 20-24 K.

[0009] Recently, particular attention has been paid to decoupling energy generation and goods production from carbon dioxide emissions. In this context, water electrolysis is especially suitable for providing hydrogen as a substitute for climate-damaging natural gas and as a feedstock for the chemical industry. Particularly when combined with renewable electrical energy, such as wind power, tidal power, wave power, hydropower, or solar power, electrolysis can provide hydrogen with virtually no emissions. However, it should be noted that the aforementioned renewable energy sources are often subject to significant supply fluctuations, which is why there is a need for flexibly controllable electrolysis processes and plants.

[0010] Patents CN107779906A, US20170321332A1, JP2020024064A and CN112361714A propose to subject the cathode raw gas downstream of electrolysis and upstream of liquefaction at least partially to intermediate storage, thereby balancing a fluctuating supply of renewable energy sources and allowing for flexible control of liquid hydrogen production.

[0011] The object of the invention is to provide an improved concept for the electrolytic production of hydrogen, in particular for more flexible control of its energy requirements. Disclosure of the invention

[0012] This problem is solved by methods and systems according to the respective independent patent claims, in particular by temporarily storing hydrogen downstream of the electrolysis and upstream of the liquefaction in order to compensate for the different dynamics of electrolysis and liquefaction. Advantageous embodiments are the subject of the dependent patent claims and the following description.

[0013] The term "liquid hydrogen product" is used here for liquid media that are produced (exported) from the process according to the invention or from a corresponding plant. These media do not necessarily have to consist exclusively of hydrogen, but they must contain hydrogen in a concentration of more than 80%, 90%, 95% or 99%, particularly on a molar basis.

[0014] The terms "cathode raw gas" and "raw hydrogen," on the other hand, are used for media of the specified state of matter in which the hydrogen content is lower than in the liquid hydrogen product. The liquid hydrogen product is formed by appropriate purification of at least a portion of the raw hydrogen. Raw hydrogen contains, in particular, considerable amounts of oxygen and water as components to be removed.

[0015] In the language used in the present patent application, a gas mixture is rich in one or more components if it has a proportion of more than 50%, 60%, 70%, 80%, 90%, 95%, 98%, 99%, 99.9% or 99.99% of that one or these several components, wherein in the case of several components the proportion is understood as the sum of the individual proportions.

[0016] Accordingly, a mixture is poor in one or more components if it is not rich in these, i.e., if the proportion of these in the total mixture is below 50%, 40%, 30%, 20%, 10%, 5%, 2%, 1%, 0.1% or 0.01%.

[0017] A gas or mixture enriched with one or more components refers to a gas or mixture that has a higher concentration of one or more components compared to a base gas or mixture. In particular, a gas enriched with one component has at least 1.1, 1.3, 2, 3, 10, 30, 100, 300, or 1000 times the proportion of that component compared to the corresponding base gas.

[0018] Accordingly, a gas depleted of a component has at most 0.001, 0.003, 0.01, 0.03, 0.1, 0.3, 0.5 or 0.9 times the proportion of that component compared to the corresponding original gas.

[0019] When the term "part of a gas or mixture" is used below, this can mean either that a volume fraction of the gas or mixture up to 100% of the total standard volume of the original gas or mixture with the same composition is used, or that a gas or mixture formed solely from certain components of the original gas or mixture is used. The part of the gas or mixture can therefore have the same or a different composition than the original gas or mixture. Furthermore, the term can mean that in a first period the respective gas or mixture is used completely or to a greater extent, but in a second period it is not used or is used to a lesser extent. This results in partial use in the sense that, on average over time, neither 100% nor 0% of the gas or mixture is used.The mixture is used, but rather a proportion that lies between these extremes.

[0020] A process according to the invention for the electrolytic production of a liquid hydrogen product, in which a water-containing feed is subjected to electrolysis to obtain an oxygen-rich and hydrogen-containing anode raw gas and an oxygen-poor and hydrogen-rich cathode raw gas, wherein the cathode raw gas is subjected downstream of the electrolysis to purification, compression, and liquefaction, and wherein the cathode raw gas is at least partially subjected to intermediate storage downstream of the electrolysis and upstream of the liquefaction, is characterized in that the electrolysis is carried out at two different pressure levels and the cathode raw gas of the electrolysis carried out at a higher pressure level is at least partially subjected to intermediate storage, and the cathode raw gas of the electrolysis carried out at a lower pressure level is subjected to intermediate storage.is not subject to intermediate storage. This allows the different dynamics of electrolysis and liquefaction to be harmonized, so that the process can be carried out largely continuously even with strong fluctuations in electrolysis power.

[0021] By carrying out different electrolyses in parallel at different pressure levels, storage can take place without intermediate compression, while for the subsequent liquefaction, which in such cases advantageously operates at an input pressure level that is below the lower electrolysis pressure level, only the temporarily stored cathode raw gas needs to be expanded to the lower pressure level.

[0022] In particular, the electrolysis power is more dynamically variable than the liquefaction power. For example, the electrolysis power can be varied between 10% and 100% of the nominal power, with a variation rate of up to 10 percentage points per second being achievable. In contrast, the liquefaction power is significantly more slow to regulate. Typically, it can be varied between 30% and 100% of the corresponding nominal power, with variation rates of a maximum of 2 percentage points per minute being attainable. The described power ratings are understood here as throughput capacity, for example, in terms of mass per unit time or volume per unit time. In particular, intermediate storage takes place at a temperature level close to ambient temperature (e.g.,from 250 to 330 K, particularly between 273 K and 313 K) and a pressure level in the range of 1 to 20 MPa and / or as cold gas at a temperature level in the range of 50 to 100 K, particularly 75 K to 100 K, and a pressure level of 1 to 20 MPa, particularly 3 to 10 MPa. Suitable materials for such an intermediate storage system are, for example, steel or carbon or glass fiber reinforced plastic (CFRP / GFRP), e.g., type 1-4 high-pressure gas cylinders (so-called tube bundles). This allows the most suitable storage solution to be selected for the respective application. Typically, the hydrogen is produced from electrolysis at a pressure of up to 3 MPa. For intermediate storage at pressures between 1 and 20 MPa, one or more hydrogen compressors can be provided upstream of the storage containers, e.g.,Turbo compressors, positive displacement compressors (reciprocating compressors, diaphragm compressors, ionic compressors), screw compressors, or electrochemical compressors. For example, it can be considered that storage at the lower temperature level, due to the higher density of hydrogen under these conditions, allows for significantly smaller storage volumes with the same storage capacity. Storage at the higher temperature level, on the other hand, has the advantage of less cooling energy loss, since a small amount of heat input cannot be ruled out during the storage of pre-cooled hydrogen, which is significantly less relevant when storing hydrogen at essentially ambient temperature. Storage in the form of a metal hydride or using liquid organic hydrogen carriers (LOHC) may also offer volume advantages, albeit with higher investment costs.

[0023] Intermediate storage can be carried out upstream and / or downstream of the purification process. Upstream storage at the higher temperature level is particularly advantageous, while downstream storage at the lower temperature level is especially beneficial. A combination of both methods can also offer advantages, as this allows the dynamics of the purification process to be taken into account or compensated for.

[0024] The purification process can include at least one of the following methods: catalytic conversion of oxygen to water, adsorption, distillation separation, and washing with an absorption fluid. These are particularly effective methods for removing relevant impurities from a stream consisting predominantly of hydrogen.

[0025] The electrolysis process is advantageously operated in response to an external energy supply, so that a high electrolysis power is set when the supply is high and a low electrolysis power is set when the supply is low. This allows for flexible utilization of fluctuating energy availability. It follows that, due to the intermediate storage of raw anode gas, the liquefaction rate does not need to be throttled, or only to a lesser extent.

[0026] In some configurations, the low electrolysis power and / or low liquefaction power is maintained using hydrogen that has undergone intermediate storage and / or liquefaction. This is particularly advantageous when the external energy supply is insufficient to operate the electrolysis or liquefaction at a minimum level, specifically corresponding to a lower power limit. However, the use of locally produced hydrogen ensures rapid readiness of the electrolysis process, as it eliminates the need for a "cold start," which would have significantly lower dynamics. In particular, this allows the electrolysis process to be operated in such a way that, even during a short-term drop in supply, its power output can be quickly increased again as soon as the external energy supply permits.

[0027] In particular, liquefaction is maintained using the temporarily stored hydrogen (or rather, its load is adjusted slowly). Electrolysis can be varied more quickly in terms of load (for example, with load change rates of over 1% / min, over 0.1% / s, or over 1% / s, each relative to a maximum electrolysis output), while the liquefaction can be varied less effectively or more slowly (e.g., at a rate of less than 5% / min or less than 2% / min, each relative to a maximum liquefaction output). The faster load adjustment of electrolysis works in both directions: When electricity supply is high, electrolysis can be ramped up quickly and hydrogen can be temporarily stored, while liquefaction is adjusted less dynamically to the variable supply.

[0028] When electricity supply is low, electrolysis can be quickly throttled back and temporarily stored hydrogen can be used for liquefaction so that it can be slowed down or maintained (depending on the storage level).

[0029] An inventive plant for the production of a liquid hydrogen product, which benefits mutatis mutandis from the advantages explained with regard to the process, comprises an electrolysis unit with at least one electrolyzer for the electrolysis of a water-containing insert, a purification unit configured to enrich a cathode raw gas produced in the electrolysis unit with hydrogen and to at least partially deplete it of other components, a liquefaction unit configured to liquefy a hydrogen-rich gas stream, and an intermediate storage unit arranged downstream of the electrolysis unit and upstream of the liquefaction unit and configured to store at least a portion of the cathode raw gas produced in the electrolysis unit, characterized in thatthat the electrolysis unit for the production of cathode raw gas is set up at two different pressure levels and connected to the intermediate storage in such a way that only the cathode raw gas available at a higher pressure level can be fed at least partially into the intermediate storage.

[0030] Optionally, the system can also include one or more compressors (especially upstream of the intermediate storage) to increase the density of the hydrogen.

[0031] The facility includes resources that enable the facility to carry out a procedure as described above.

[0032] Further features and advantages of the invention or advantageous embodiments thereof are explained in more detail below with reference to the accompanying drawing, wherein Fig. 1 An advantageous embodiment of the concepts according to the invention is illustrated in a simplified representation.

[0033] As already explained at the outset, the invention relates to both a method and a system for the electrochemical production of a liquid hydrogen product. The schematic representation in Figure 1 This can be interpreted both as a plant diagram and as a process flowchart. Therefore, if a plant component is described below, the statements also apply analogously to a process step carried out in that plant component, and vice versa. Consequently, the reference symbols are used accordingly to denote process steps carried out in corresponding plant components and vice versa.

[0034] In Figure 1 An advantageous embodiment of the invention is shown schematically and is generally designated by 100.

[0035] A system 100 comprises an electrolysis unit E, a cooling and / or compression unit K, a purification unit R, an intermediate storage unit Z and a liquefaction unit L.

[0036] Such a system 100 can also be integrated into one or more containers, such as those commonly used for transport purposes by land and sea, which can thus be transported and set up very quickly and cost-effectively.

[0037] Accordingly, the process includes an electrolysis step E, a cooling and / or compression step K, a purification step R, an intermediate storage step Z and a liquefaction step L.

[0038] In electrolysis E, a water-containing feedstock 1 is converted into a hydrogen-containing cathode raw gas 2 and an oxygen-containing anode raw gas 3 using electrical energy. The feedstock 1 can contain additional components besides water, in particular electrolytes such as alkaline, acidic, or neutral salts or ions. The feedstock 1 can be fed into the electrolysis unit E, especially on the anode side.

[0039] The cathode raw gas 2 is extracted from the electrolysis unit E and fed into a post-treatment process, which, in a suitable sequence, includes the steps of cooling and / or compression, purification, intermediate storage, and liquefaction. Liquefaction always concludes the post-treatment process; the order of the remaining steps can be varied.

[0040] The cooling or compression K of the cathode raw gas 2 can be carried out using a conventional mechanical refrigeration machine, or, particularly advantageously, using waste heat from the electrolysis E by means of an absorption or adsorption chiller, which has a particularly beneficial effect on the overall energy balance of the system 100. Conventional mechanical refrigeration machines can also utilize waste heat from the electrolysis E, which can initially be used to perform work, e.g., by generating steam in combination with a turbine to compress the cathode raw gas 2.

[0041] Various methods are available for the purification of R, for example (especially cryogenic) adsorption, oxidative combustion of oxygen, condensation of components with a comparatively high boiling point, etc.

[0042] The purification requirement can be significantly reduced if the feed 1 is already purified or depleted of impurities upstream of the electrolysis E. Particularly relevant in this context are gases dissolved in the water of feed 1, such as nitrogen, carbon dioxide, and / or noble gases. These can be driven off or otherwise removed from feed 1, for example, by stripping using the raw anode gas 3 generated in the electrolysis or by other degassing strategies such as membrane degassing.

[0043] The intermediate storage Z can, for example, be achieved at constant volume using a pressure accumulator, wherein the pressure accumulator can be operated at a pressure level corresponding to the cathode-side electrolysis pressure level, or it can be filled by means of a compressor with cathode raw gas 2, which can be at a pressure level above the cathode-side pressure level. Units arranged downstream of the accumulator can, in particular, be designed to operate with variable inlet pressures, or a pressure regulator can be provided downstream of the intermediate storage Z to ensure a constant pressure.

[0044] Within the scope of the invention, storage devices with constant pressure can also be used as the intermediate storage device Z. Such storage devices have, for example, a variable volume (e.g., in the form of a piston or plunger in a hollow cylinder or similar), or they can regulate the pressure by appropriately controlling the storage temperature.

[0045] Furthermore, metal hydride storage systems can be used, in which a metal, for example a palladium-containing alloy, is capable of absorbing hydrogen to form a metal hydride. In the case of using such a metal hydride storage system, the release of hydrogen stored in the intermediate storage Z can again be achieved using waste heat from the electrolysis E, with corresponding energy advantages.

[0046] To further increase dynamics, particularly in the downstream processing stage of electrolysis E, the respective components can also be operated in parallel in multiple configurations, thus providing a larger controllable range. For example, several compressors and / or turbines can be provided for compression, allowing the liquefaction capacity to be reduced to below 30% of the nominal capacity by completely switching off at least one of the multiple components.

[0047] As mentioned at the outset, it is advantageous to control the output of plant 100 depending on the availability of external energy. For example, such plant 100 can be operated with renewable electricity, e.g., from a wind farm. During periods of low wind, little or no electricity is available, so the electrolysis process can be significantly throttled back. If no electricity is available, some of the hydrogen produced, for example, from the intermediate storage Z, can be used to generate electricity in order to continue operating the electrolysis process E at a minimum power level, e.g., 10% of its nominal power. Using hydrogen that evaporates in a liquid storage tank downstream of the liquefaction unit L is also a viable option. This allows for faster restart or...An increase in electrolysis output is ensured with increasing external energy supply, such as stronger winds. In comparison, restarting a deactivated electrolysis unit takes considerably longer, especially since the electrolysis unit must be brought up to operating temperature. Such a system can, in principle, be directly integrated into a wind turbine (e.g., an offshore wind farm) to minimize power transmission losses. Liquid hydrogen and / or cold gases produced by the wind turbine can also be partially used to cool the wind turbine (e.g., the generator), for example, to minimize power transmission losses through the use of superconducting materials.

[0048] To further increase energy efficiency, a conventional heat exchanger can be used in which the insert 1 is heated against the raw anode gas 3 and / or raw cathode gas 2 taken from the electrolysis E.

[0049] The raw anode gas 3 can also be utilized, so steps such as purification, drying, compression, liquefaction, and / or storage may be considered. Alternatively, the raw anode gas can be released into the surrounding atmosphere, as it inherently contains no harmful components and is therefore harmless to health and the environment.

Claims

1. A method (100) for the electrolytic production of a liquid hydrogen product (4) in which a water-containing feed is subjected to electrolysis (E) to obtain an oxygen-rich and hydrogen-containing anode raw gas (3) and an oxygen-poor and hydrogen-rich cathode raw gas (2), wherein the cathode raw gas (2) is subjected downstream of the electrolysis (E) to purification (R), compression (K), and liquefaction (L), wherein the cathode raw gas (2) is at least partially subjected to an intermediate storage (Z) downstream of the electrolysis (E) and upstream of the liquefaction (L), characterized in that the electrolysis (E) is carried out at two different pressure levels, and the cathode raw gas (2) of the electrolysis (E) which is carried out at a higher pressure level is at least partially subjected to the intermediate storage (Z), and the cathode raw gas (2) of the electrolysis (E) which is carried out at a lower pressure level is not subjected to the intermediate storage (Z).

2. The method (100) according to claim 1, wherein the intermediate storage (Z) takes place at a temperature level in a range of 250 to 330 K, in particular 273 to 313 K, and a pressure level in a range of 1 to 20 MPa or at a temperature level in a range of 50 to 100 K, in particular 75 to 100 K, and a pressure level of 1 to 20 MPa, in particular 3 to 10 MPa.

3. The method (100) according to claim 1 or 2, wherein the intermediate storage (Z) is carried out upstream of the purification (R).

4. The method (100) according to claim 1 or 2, wherein the intermediate storage (Z) is carried out downstream of the purification (R).

5. The method (100) according to claim 1 or 2, wherein the intermediate storage (Z) is carried out upstream and downstream of the purification (R).

6. The method (100) according to any of the preceding claims, wherein the cathode raw gas (2) is subjected to compression upstream of the intermediate storage (Z).

7. The method (100) according to any of the preceding claims, wherein the purification (R) comprises at least a catalytic conversion of oxygen to water or an adsorption or a distillative separation or a washing with an absorption fluid.

8. The method (100) according to any of the preceding claims, wherein the electrolysis (E) is operated depending upon an external energy supply, such that high electrolysis power is set when the supply is high, and low electrolysis power is set when the supply is low.

9. The method (100) according to claim 8, wherein the electrolysis power is adapted to the external energy supply at a rate of change of more than 1% / min, in particular more than 0.1% / s, particularly preferably more than 1% / s, based upon a maximum electrolysis power, and / or a liquefaction power is adapted to the external energy supply more slowly than the electrolysis power, at a rate of less than 5% / min, in particular less than 2% / min, based upon a maximum liquefaction power.

10. A system for the production of a liquid hydrogen product (4), comprising: an electrolysis unit with at least one electrolyzer for the electrolysis of a water-containing feed, a purification unit that is configured to enrich a cathode raw gas (2), generated in the electrolysis unit, with hydrogen and to at least partially deplete it of other components, a liquefaction unit that is configured to liquefy (L) a hydrogen-rich gas stream, and an intermediate storage means that is arranged downstream of the electrolysis unit and upstream of the liquefaction unit and configured to store at least a part of the cathode raw gas (2) generated in the electrolysis unit, characterized in that the electrolysis unit is configured for the production of cathode raw gas (2) at two different pressure levels and is connected to the intermediate storage means in such a way that only the cathode raw gas (2) available at a higher pressure level can be supplied at least partially to the intermediate storage means.