METHOD FOR OPERATING AN ELECTROLYSIS PLANT AND ELECTROLYSIS PLANT

Description

[0001] The invention relates to a method for operating an electrolysis plant for water electrolysis, as well as to such an electrolysis plant which is used, for example, for the production of hydrogen. State of the art

[0002] To produce hydrogen, a process known as electrolysis can be used. This process involves splitting or converting water into oxygen and hydrogen using electrical energy. This process is also referred to as water electrolysis. Proton exchange membrane electrolysis (PEM electrolysis) is a possible method.

[0003] During PEM electrolysis, a large portion of the water typically remains on the oxygen side of the membrane. While the hydrogen is generated and removed on the other side of the membrane, the oxygen initially remains in the water and is then typically separated from the water in a container.

[0004] However, it can happen that a certain amount of hydrogen is also produced on the oxygen side, or diffuses back there, or gets there in some other way, e.g., due to defects or cracks in the membrane. This can form an explosive mixture that could potentially ignite somewhere and damage the electrolysis system due to the resulting explosion. Against this backdrop, the task of making electrolysis systems and their operation safer arises. Disclosure of the invention

[0005] This object is achieved by a method for operating an electrolysis system and an electrolysis system having the features of the independent patent claims. Further embodiments are the subject of the dependent patent claims and the following description. Advantages of the invention

[0006] The invention relates to water electrolysis and electrolysis systems, or their operation therefor. Such electrolysis systems are typically used to generate or produce hydrogen by electrolysis. In so-called water electrolysis, water is converted (split) into hydrogen and oxygen, i.e., in addition to hydrogen, oxygen is always extracted or produced at the same time. In water electrolysis, there are, for example, so-called alkaline water electrolysis (AEL, "Alkaline Electrolysis") and so-called proton exchange membrane electrolysis (PEM electrolysis, "Proton Exchange Membrane" electrolysis). The principles of this are known per se, e.g., from "Bessarabov et al: PEM electrolysis for Hydrogen production. CRC Press."

[0007] There are also so-called solid oxide electrolysis cells (SOEC) and anion exchange membrane electrolysis (AEM). Electrolysis technologies that operate at low temperatures, such as PEM, AEL, and AEM electrolysis, are particularly suitable for supporting the transition to renewable energy sources due to their flexible operation options.

[0008] In PEM electrolysis, for example, water, particularly demineralized water, is fed as the feed medium to an electrolysis unit with a proton exchange membrane (PEM), in which the feed medium, i.e. the water, is converted (split) into hydrogen and oxygen.

[0009] As mentioned, during PEM electrolysis, a large portion of the water typically remains on the oxygen side of the membrane. While the hydrogen is generated and removed on the other side of the membrane, the oxygen initially remains in the water and is then typically separated from the water in a container (used as a gas or oxygen separator).

[0010] However, it can happen that a certain amount of hydrogen is also produced on the oxygen side, or diffuses back there, or gets there in some other way, e.g. due to defects or cracks in the membrane. The fluid flow to be discharged from the oxygen side therefore contains not only water and oxygen, but possibly also hydrogen, i.e. generally water and gas; the term gas is generally understood here to mean a gaseous medium, not just a single gas, but also any gas mixture that may be present. An explosive mixture can therefore form on the oxygen side of the electrolysis unit or in the fluid flow, which can potentially ignite somewhere in the electrolysis system downstream of the electrolysis unit, be it in the fluid flow to the oxygen or gas separator or subsequently in the gas flow separated and discharged there (in this case, not only the oxygen but also any hydrogen that may be present is separated).The resulting explosion or detonation can damage the electrolysis system.

[0011] Ignition of the mixture can occur particularly when the hydrogen content in the gas (the water content is not relevant here) exceeds a certain predetermined level, the so-called lower explosive limit (LEL). This is typically around 4% during standby operation or when the electrolysis plant is operating at low load. The (lower) explosive limit of a gas indicates the concentration in a gas mixture above which ignition or explosion is possible, even if the oxygen content is sufficient.

[0012] Since an ignition source in the electrolysis unit or somewhere downstream cannot generally be excluded or avoided, potential ignition, explosion, or detonation must always be taken into account during operation of an electrolysis plant. An explosion is the uncontrolled combustion of an ignitable gas mixture with a laminar flame front. An explosion differs from a detonation primarily in its propagation speed.

[0013] The reaction between hydrogen and oxygen takes place very quickly and produces very high flame speeds, i.e. high speeds at which the flames spread, for example, in corresponding fluid connections or fluid lines. In an explosion, this is below the speed of sound, while in a detonation it is typically well above it. Explosions and detonations of gases or gas mixtures lead to a massive increase in pressure. Typically, an explosion can increase the pressure by a factor of ten. The effects of a detonation are much more serious. Here, the pressure increase factor can be 25 or even 50 or more. An explosion can turn into a detonation after a certain run-up length and a minimum concentration of fuel and oxygen.Pressures can therefore arise that are sometimes 25 or even 50 times the actual operating pressure in the electrolysis plant, particularly on the oxygen side and the downstream, and possibly also upstream, sections. To avoid any damage to the electrolysis plant, the relevant fluid or gas lines can be designed for correspondingly high pressures. Depending on the electrolysis plant and the use of the oxygen, this can affect a large number of lines or even very long lines. This applies not only to the fluid lines from the electrolysis unit to the gas separator, but also to any gas lines in which the separated oxygen (and possibly hydrogen) is conveyed to one or more desired uses or other processing steps. This can lead to particularly high costs. EP3272907 discloses an ignition source downstream of a gas separator, and EP1995352 also proposes an active ignition source.

[0014] Against this background, it is now proposed, in an electrolysis system as described above, to actively generate (or cause) an ignition in the fluid flow from the oxygen side of the electrolysis unit to the gas separator (or the container used as a gas separator) by providing a suitable ignition device. This ignites any hydrogen present or a potentially ignitable gas or gas mixture in a controlled manner at a desired location. This prevents uncontrolled ignition at any location, since no ignitable gas or gas mixture can be present downstream of the active ignition. Accordingly, it is also expedient if the ignition takes place as close to or immediately after the electrolysis unit as possible. It should be mentioned that although ignition of the ignition device or in the ignition device takes place, the gas orHowever, the gas mixture is generally not ignited, except in exceptional cases where, for example, the hydrogen concentration is too high. Preferably, ignition is generated at regular or irregular intervals, particularly at a predetermined frequency, e.g., a value between 5 Hz and 50 Hz. This ensures that only a small amount of new hydrogen is present in the fluid stream to react upon ignition, thus keeping the extent of the explosion to a minimum.

[0015] It is also preferred if it is monitored whether the ignition is generated, and in particular if it is detected that the ignition is not generated - i.e. for example it has not been generated due to an error even though it was actually triggered - an error reaction is initiated, for example an error message is issued or the electrolysis system is switched off. For this purpose, for example an optical monitor can be provided in the ignition device in order to detect whether an ignition spark has been generated. Likewise, for example a voltage or current monitor can be provided which detects whether the ignition spark has been generated. This allows the ignition of the ignition device to be monitored regardless of whether the gas or gas mixture has also been ignited.

[0016] Since high pressures can also arise during actively induced ignition, it is advisable to provide an ignition chamber in which the ignition device is located and which is part of a fluid connection through which the fluid flow is conducted; the fluid flow is thus guided through the ignition chamber. This ignition chamber can, for example, be spherical in shape to withstand the highest possible pressures. Furthermore, this ignition chamber can be designed as small as possible to ensure a small gas space and short combustion times.

[0017] Preferably, the fluid connection has a siphon-shaped or U-shaped course downstream of the ignition chamber, in particular immediately downstream of the ignition chamber, in the direction of flow. In particular, the siphon-shaped or U-shaped course of the fluid connection is designed such that, in the direction of flow, there is first a section running at least substantially vertically downwards (i.e. in the direction of gravity) and then an at least substantially vertically upwards (i.e. against the direction of gravity). Such a course of the fluid line, e.g. in the form of a correspondingly shaped pipe, ensures that an explosion front that occurs does not propagate further downstream, but is prevented from propagating further by the siphon-shaped or U-shaped course.

[0018] The vertically downward section and / or the vertically upward section should have at least a certain length. The diameter and interior, as well as, if necessary, the length of the vertical sections, should be designed in such a way that, for example, a so-called "bubble flow," a so-called "slug flow," or a so-called "churn flow" (in which gas bubbles are contained in the liquid) can be maintained. Annular flow (ring flow or film flow) or "mist flow," on the other hand, should be avoided through appropriate design or shaping.

[0019] In particular, a flow pattern is desired in which the gas phase is reliably interrupted by a liquid phase. This is not the case with "annular flow" (ring flow or film flow) and "mist flow," which should therefore be avoided by design as much as possible. It should be noted that a siphon-shaped or U-shaped fluid connection represents one option for achieving the desired flow pattern, but other shapes and paths of the fluid connection are also possible, since the interruption of the gas phase by the liquid phase is particularly important.

[0020] Curved sections of the siphon or U-shaped course should be shaped accordingly to prevent reflections of the pressure waves, e.g. according to TRGS 407 they should have at least 5 L / D (L / D stands for a ratio of length to diameter).

[0021] This targeted ignition and, in particular, the specially designed ignition chamber means that the remaining lines can be designed for significantly lower pressure than before, resulting in significant cost savings. This is particularly advantageous because it also allows plastic, e.g., fiber-reinforced plastic, to be used in large sections of the lines, thus avoiding the use of metal lines and the associated higher degradation rates in the PEM. The use of plastic for lines would generally not be possible at an operating pressure of, for example, 3.5 barg in the fluid line or a line for discharged oxygen, at least for larger diameters, as these would then not be able to withstand at least 25 times the pressure.Compressing the gas stream upstream to remove any remaining hydrogen would not have been possible so far, as the compressors required for this would generally not be able to withstand these pressures either.

[0022] Although the invention is primarily described with reference to PEM electrolysis, it should be noted that the invention is suitable for all water electrolysis processes where there is a risk of an ignitable mixture of hydrogen and oxygen.

[0023] The invention will be explained in more detail below with reference to the accompanying drawing, which shows a system according to a preferred embodiment of the present invention.

[0024] Short description of the drawing Figure 1 schematically shows an electrolysis plant according to the invention in a preferred embodiment. Figure 2 schematically shows a part of the electrolysis plant from Figure 1Figure 3 shows schematically different flow types. Detailed description of the drawing

[0025] In Figure 1 An electrolysis plant 100 according to the invention is schematically shown in a preferred embodiment, in which a method according to the invention can also be carried out. This is an electrolysis plant for water electrolysis using PEM. In particular, the electrolysis plant shown here and generally described within the scope of the invention is an industrial-scale electrolysis plant, for example, for producing hydrogen on an industrial scale. A typical output of such an electrolysis plant is, for example, more than 10 MW or even more than 20 MW.

[0026] The electrolysis system 100 has an electrolysis unit 110, which here, for example, has two so-called electrolysis cells or stacks 110.1, 110.2, each of which contains a proton exchange membrane (PEM) 112. The PEM 112 separates the electrolysis cells into an oxygen side 114 and a hydrogen side 116. The oxygen sides 114 and the hydrogen sides 116 can be regarded together as the oxygen side and the hydrogen side of the electrolysis unit 110, respectively.

[0027] It should be mentioned that an electrolysis unit 110 may, for example, have only one or more than two electrolysis cells, depending on the size and requirements.

[0028] The electrolysis system 100 further comprises a container 120, which serves as a gas separator, here in particular as an oxygen separator or oxygen-water separator. The container 120 is connected via a fluid connection to the electrolysis unit 110 or, therein, to each of the electrolysis cells 110.1, 110.2. This allows a fluid stream b to be pumped from the container 120 to the electrolysis unit, e.g., by means of a pump 124. The electrolysis unit 110 is also connected to the container 120 via a fluid connection 126, e.g., pipes. Through the fluid connection 126, a fluid stream c can be pumped from the electrolysis unit 110, therein the oxygen side 114 or the oxygen side of each electrolysis cell, to the container 120; the pump 124 is also sufficient for this purpose.

[0029] In addition, the electrolysis plant 100 has a further gas separator 130, here a hydrogen separator or hydrogen-water separator.

[0030] Although only one electrolysis unit 110 is shown here, several of them can also be provided, e.g. depending on the size and performance of the electrolysis system 100. Several electrolysis units can then, for example, still be connected to a common container for gas or oxygen separation and / or a common hydrogen separator.

[0031] During operation of the electrolysis plant 100, the fluid stream b, which comprises water, is pumped from the container 120 to the electrolysis unit 110. There, the water is converted into oxygen and hydrogen. For this purpose, an electrical voltage is applied to the electrolysis unit 110; the hydrogen is electrochemically transported through the PEM 112 to the hydrogen side 116 and from there, optionally mixed with steam and a liquid water phase, can be fed as stream e to the hydrogen separator 130. There, the hydrogen can be separated and discharged as stream f, for example, for further use or stored. The separated water is, for example, fed to a treatment process and then returned to the main water circuit.

[0032] The oxygen remains together with the majority of the water on the oxygen side 114. As mentioned, a certain amount of hydrogen may also be present on the oxygen side 114. The resulting fluid stream c thus comprises water and gas, in particular water, oxygen, and hydrogen. The fluid stream c is then passed through an ignition chamber 150, in which an ignition device 152 is provided. The fluid stream is then passed through a siphon-shaped or U-shaped passage 154 of the fluid connection 126 and then reaches the container or gas separator 120 as fluid stream d. Overall, a fluid stream is circulated between the container 120 and the electrolysis unit 110.

[0033] As mentioned, gas, in particular oxygen (and any remaining hydrogen present), is separated from the water in vessel 120. The separated or separated gas can be discharged or stored as stream g, for example, for further use. A further branching, for example, as stream h, to a purification unit to remove any remaining hydrogen and dry the gas, or to a compressor, is conceivable.

[0034] Since water is converted into oxygen and hydrogen in the electrolysis unit 110 and the oxygen and hydrogen are removed, the amount of water becomes smaller and therefore - in order to maintain continuous operation - new water (so-called make-up water) can be supplied from outside as stream a.

[0035] This water a can, for example, be further treated beforehand, which, however, is not further relevant to the present invention. Likewise, water separated in the hydrogen separator 130 can be returned to the container 120, possibly also after prior treatment.

[0036] In the ignition chamber 150, for example, an ignition device 152 (i.e., an ignition source) is now ignited at a specific frequency in order to ignite any ignitable gas mixture of hydrogen and oxygen present in the fluid stream c. Due to the siphon- or U-shaped course 154 and the flow regime established there, the resulting pressure wave does not propagate further downstream; the resulting fluid stream d thus reliably has a hydrogen content below the lower ignition limit, either because there was only a small amount of hydrogen (below the lower ignition limit) in the stream c or because, at a higher concentration, ignition has already occurred in the ignition chamber 150.

[0037] In addition, a control unit 160 is shown as an example, by means of which, for example, the ignition device 152 can be controlled and, if necessary, also monitored.

[0038] In Figure 2 is a schematic view of a part of the electrolysis plant from Figure 1 shown in more detail, namely the fluid connection 126 from the ignition chamber 150 to the siphon- or U-shaped path 154. The fluid flow c, coming from the oxygen side of the electrolysis unit, enters the ignition chamber 150, and at the end, the fluid flow d exits, which is led to the gas separator. As already mentioned, the ignition chamber can be arranged as close as possible to the electrolysis unit. This allows the area of ​​high design pressures to be kept small. Furthermore, arranging the ignition chamber above the electrolysis unit (seen in the direction of gravity) is particularly useful in order to keep the electrolysis unit safely covered with water. The integration of the fluid flow c into the ignition chamber 150 should, in particular, take place from above to below the liquid level, as in Figure 2 indicated.

[0039] The ignition chamber 150 is, for example, approximately spherical in shape, and the ignition device 152 comprises, for example, two electrical contacts that extend into the interior of the ignition chamber. By applying an electrical voltage to these contacts, ignition can be actively induced or generated in the ignition chamber 150. The ignition is indicated, for example, as a lightning symbol.

[0040] For example, the fluid connection 126, in the direction of flow, has a siphon-shaped or U-shaped course 154 after the ignition chamber 150. Here, the fluid connection 126 - which can basically be a pipe or the like - runs briefly horizontally, then has an arcuate section 154.1 (with an approx. 90° bend) and thus transitions into a vertically downward section 154.2. Then, the section 154.2 transitions with an arcuate section 154.3 (with an approx. 180° bend) into a vertically upward section 154.4. Then, the section 154.4 transitions again with an arcuate section 154.5 (with an approx. 90° bend) into a horizontal section; this can then continue to run horizontally, for example, up to the container.

[0041] The siphon- or U-shaped course of the fluid connection is designed such that, in the direction of flow, there is first a section running at least substantially vertically downwards (i.e., in the direction of gravity) and then an at least substantially vertically upwards (i.e., opposite to the direction of gravity). Such a course of the fluid line, e.g., in the form of a suitably shaped pipe, ensures that an explosion front that occurs does not propagate further downstream, but is prevented from propagating further by the siphon- or U-shaped course.

[0042] As mentioned, the ignition in the ignition chamber 150 causes any ignitable mixture present to explode, the pressure wave of which, however, only extends to the very end (in the flow direction) of section 154.4 due to the siphon- or U-shaped configuration. Therefore, the fluid connection 126 further downstream—and likewise the container 120 and any other gas connections (e.g., for streams g and h)—do not need to be designed to be explosion- or detonation-proof. Rather, it is sufficient if the ignition chamber 150, the fluid connection 126 up to and including section 154.4 (or possibly slightly further) and upstream of the ignition chamber 150 are designed to be explosion- and detonation-proof.

[0043] As already mentioned, the siphon- or U-shaped course 154, particularly with regard to the diameter and length of the individual sections, should be designed in such a way that no "annular flow" (ring flow or film flow) and no "mist flow" (fog flow) occur.

[0044] For this purpose, Figure 3 different flow types are shown. In each view (A) to (F) a part of the fluid connection 126 is shown, in particular in the area of ​​the siphon- or U-shaped course 154, there again in particular where in Figure 2 Light hatching is shown inside. i denotes gas bubbles, k denotes liquid.

[0045] View (A) illustrates a bubble flow, in which medium-sized gas bubbles disperse in the liquid. View (B) illustrates a slug flow, while views (C) and (D) each illustrate a churn flow, albeit in different forms. These three or four flow types are permitted or desired to safely interrupt the gas phase between the ignition chamber 150 and the fluid connection 126 and prevent the pressure wave from propagating downstream.

[0046] View (E) illustrates a so-called "annular flow" (ring flow or film flow), in which pure liquid collects on the wall of the fluid connection (or pipe), while further inside, a fine mixture of gas and liquid is present. View (F) illustrates a so-called "mist flow," in which a fine mixture of gas and liquid is present throughout. Both of these flow types should be prevented by appropriately shaping the siphon- or U-shaped flow path 154. The flow pattern can be influenced by the flow velocity, i.e., in particular, by the diameter of the pipe, especially in the vertically rising section.

Claims

1. A method for operating an electrolysis system (100) in which water is converted into oxygen and hydrogen in an electrolysis unit (110), wherein a fluid stream (c, d) that comprises water and gas is conveyed from an oxygen side (116) of the electrolysis unit to a gas separator (120), wherein an ignition of an ignition device (152) is actively generated in the fluid stream (c).

2. The method according to claim 1, wherein the ignition is generated at regular or irregular time intervals, in particular at a prespecified frequency.

3. The method according to claim 1 or 2, wherein it is monitored whether the ignition has been generated, and wherein in particular if it is detected that the ignition has not been generated, an error reaction is initiated.

4. The method according to any of the preceding claims, wherein in the fluid stream (c) between the ignition device (152) and the gas separator (120), a gas phase is interrupted by a liquid phase, in particular by means of an adaptation of a flow type of a two-phase flow.

5. An electrolysis system (100) having an electrolysis unit (110) in which water can be converted into oxygen and hydrogen, and having a gas separator (120) and having a fluid connection (126) via which an oxygen side (114) of the electrolysis unit (110) is connected to the gas separator (120), wherein the electrolysis system (100) is configured to generate a fluid stream (c, d) in the fluid connection (126) from the oxygen side (114) of the electrolysis unit to the gas separator (120), wherein the fluid stream comprises water and gas, wherein the electrolysis system (100) has an ignition device (152) in the fluid connection (126) and is configured to actively generate an ignition of the ignition device (152) in the fluid connection.

6. The electrolysis system (100) according to claim 5, which has an ignition chamber (150) as part of the fluid connection (126), wherein the ignition device (152) is arranged in the ignition chamber (150).

7. The electrolysis system (100) according to claim 6, wherein the fluid connection (126) has a siphon-shaped or U-shaped course (154) after the ignition chamber (150), in particular immediately after the ignition chamber, in the direction of flow.

8. The electrolysis system (100) according to claim 7, wherein the siphon-shaped or U-shaped course (154) of the fluid connection is designed such that there are, in the direction of flow, first an at least substantially vertically downward portion (154.2) and then an at least substantially vertically upward portion (154.4).

9. The electrolysis system (100) according to any of claims 6 to 8, wherein the ignition chamber (150) is arranged above the electrolysis unit (110) and / or, in the direction of flow, immediately after the electrolysis unit (110).

10. The electrolysis system (100) according to any of claims 5 to 9, which is configured to carry out a method according to any of claims 1 to 4.