AU2022313398A1

AU2022313398A1 - Method for operating an electrolysis plant, and electrolysis plant

Info

Publication number: AU2022313398A1
Application number: AU2022313398A
Authority: AU
Inventors: Du-Fhan Choi; Markus Ungerer; Dirk Wall
Original assignee: Siemens Energy Global GmbH and Co KG
Current assignee: Siemens Energy Global GmbH and Co KG
Priority date: 2021-07-19
Filing date: 2022-05-11
Publication date: 2024-01-18
Also published as: CA3226820A1; WO2023001422A1; EP4123053A1; EP4334501A1; CN117751209A

Abstract

Disclosed is a method for operating an electrolysis plant (100) for producing hydrogen and oxygen as product gases, wherein the oxygen product gas, which additionally contains hydrogen as a foreign gas, is fed from an electrolyser (1) to a downstream gas separator (11), wherein when a predefined limit value for the hydrogen concentration in the oxygen product gas is exceeded, an inert gas (L) is fed to the gas separator (11) such that the hydrogen concentration in the oxygen product gas is lowered. The invention further relates to a corresponding electrolysis plant (100).

Description

Method for operating an electrolysis plant, and electrolysis plant

The invention relates to a method for operating an electrolysis system comprising an electrolyzer for the generation of hydrogen and oxygen as product gases. The invention also relates to such an electrolysis system.

Nowadays, hydrogen is generated for example by means of proton exchange membrane (PEM) electrolysis or alkaline electrolysis. The electrolyzers use electrical energy to produce hydrogen and oxygen from the water supplied.

An electrolyzer generally has a multiplicity of electrolysis cells arranged adjacent to one another. In the electrolysis cells, water is decomposed into hydrogen and oxygen by means of water electrolysis. In a PEM electrolyzer, distilled water as reactant is typically supplied on the anode side and split into hydrogen and oxygen at a proton-permeable membrane (proton exchange membrane; PEM). This involves the water being oxidized to oxygen at the anode. The protons pass through the proton exchange membrane. Hydrogen is produced on the cathode side. The water is generally conveyed into the anode space and/or the cathode space from a bottom side.

This electrolysis process takes place in what is known as the electrolysis stack, which is composed of a plurality of electrolysis cells. In the electrolysis stack, to which a DC voltage is applied, water is introduced as reactant, with two fluid streams consisting of water and gas bubbles (oxygen 02 and hydrogen H2 , respectively) exiting after passing through the electrolysis cells. The respective water phase and gas phase in the fluid streams are separated in gas separators.

In practice, small amounts of hydrogen are located in the oxygen gas stream and small amounts of oxygen are located in the hydrogen gas stream. The quantity of the respective extraneous gas depends on the design of the electrolysis cells and also varies under the influence of the current density, catalyst composition, ageing and, in the case of a PEM electrolysis system, the membrane material. It is an inherent feature of the system that the gas stream of one product gas contains very small amounts of the respective other product gas. In the further course of the process, in downstream steps of the gas purification even small traces of oxygen are generally removed from the hydrogen using purification steps that are in some cases highly complex and cost-intensive, especially when a particularly high product gas quality is required, as is the case when using the hydrogen for fuel cells, for example.

In an electrolysis system, by way of example, for gas purification of the product gas streams from the electrolyzer, both product gas streams are supplied in particular to a respective, catalytically activated recombiner, in which a catalyst allows the hydrogen to recombine with the oxygen to form water (DeOxo unit). To this end, the gas stream needs to be heated beforehand to at least 800C so that the conversion rates of the recombiner are sufficiently high and hence the required gas purity is achieved. The processing system used for this is expensive, however, and reduces the system efficiency of the entire electrolysis system because of its energy requirements. Accordingly, attention must already be given to the purity and quality of the product gas streams which are initially produced in the electrolyzer and discharged from the electrolyzer, not only for operational safety but also to keep the costs and complexity for the subsequent purification steps within reasonable limits.

The purity or quality of the two product gas steams of the gases originally produced in the electrolyzer is dependent on many parameters and can change in the course of operation of an electrolysis system. It is problematic and particularly safety relevant here on the one hand when the concentration of oxygen in hydrogen increases, but also on the other hand when the concentration of hydrogen in oxygen increases. If a certain concentration limit is exceeded here, especially in the respective gas separator (vessel) immediately downstream of the electrolysis, then the produced oxygen gas can for example no longer be transferred for further purposes. If the proportion of hydrogen in the oxygen product gas increases further, then a flammable or explosive mixture may even form. The gas separator (vessel) is then in a potentially dangerous operating state which for safety reasons absolutely has to be avoided. This also applies correspondingly to the hydrogen side.

Reliable and continuous monitoring of the gas quality of the product gas during the operation of the electrolysis system is therefore indispensable. This applies in particular also to the oxygen side of the electrolyzer, that is to say monitoring of the concentration of hydrogen as extraneous gas in the oxygen produced during the electrolysis. The monitoring and corresponding operational control are an important protective measure for identifying critical operating states and taking safety measures up to and including the temporary shutdown of the electrolysis system.

It is therefore an object of the invention to make possible operation of an electrolysis system that is improved in terms of safety and system efficiency.

The object is achieved according to the invention by a method for operating an electrolysis system for the generation of hydrogen and oxygen as product gases, in which the oxygen product gas from an electrolyzer, which also contains hydrogen as extraneous gas, is supplied to a gas separator connected downstream, wherein, when a predetermined threshold value for the hydrogen concentration in the oxygen product gas is exceeded, compressed air is supplied to the gas separator, so that in the gas separator a dilution of the hydrogen in the oxygen product gas is brought about by the mixing of the gases, lowering the hydrogen concentration in the oxygen product gas.

The object is further achieved according to the invention by an electrolysis system comprising an electrolyzer for the generation of hydrogen and oxygen as product gases, in which the oxygen product gas also contains hydrogen as extraneous gas, and a compressed air system having a gas vessel for the stockpiling of compressed air, wherein the electrolyzer is connected to a gas separator via a product stream conduit for the oxygen product gas, and wherein the compressed air system is connected to the gas separator via a feed conduit, so that compressed air is suppliable from the gas vessel to the gas separator as required.

The advantages and preferred configurations mentioned below in relation to the method can be transferred, mutatis mutandis, to the electrolysis system.

The invention proceeds from just the finding that previous operating concepts for electrolysis systems with respect to the monitoring and remedying of critical operating states in relation to the quality of the oxygen produced are complex in terms of the system and therefore exhibit considerable economic disadvantages.

In previous operating concepts, quality measurement on the oxygen side of an electrolysis system is typically achieved by measuring and monitoring the concentration of hydrogen in the oxygen product gas in the corresponding gas separator. If the concentration exceeds a predetermined threshold value, the operation of the electrolyzer is halted.

The oxygen-containing gas separator is depressurized, that is to say that this gas vessel is completely vented and brought to an unpressurized state. Discarding of the oxygen gas and complete exchange of the gas in the gas vessel is necessary. The depressurization or complete venting on the oxygen side means that on the hydrogen side the entire, valuable, hydrogen product gas in the corresponding gas separator also has to be discarded, in particular in order to counteract the high differential pressure as a result of the venting and in order to avoid damage to the system via the PEM membrane. The hydrogen product gas is therefore also completely discharged from the vessel volume of the gas separator and any supply lines of the hydrogen-side gas system. To this end, the gas separator is completely vented. The entire gas system, including the gas separators, is then purged in a complex purging procedure for inertization with nitrogen from a storage vessel in the nitrogen system of the electrolysis system. For this safety-related need for nitrogen, the nitrogen system must be designed with an appropriately large volume in order to hold sufficient nitrogen available. After the cause of the critical quality of the oxygen product gas has been addressed, the electrolysis is restarted. As a result of the inert gas nitrogen in the gas system, the newly produced hydrogen product gas also has to be discarded at first, and specifically until the desired gas quality has been achieved again. Therefore, in addition to the holding available of a large volume nitrogen inertization system and a nitrogen stockpile, the discarding of the hydrogen product gas generated is specifically also particularly disadvantageous from economic viewpoints.

The present invention proceeds specifically from here, in that a critical extraneous gas concentration of hydrogen in the oxygen product gas, which is located in the corresponding gas separator connected downstream of the electrolyzer, is reduced by supplying compressed air of good quality to the oxygen product gas in a controlled and well-metered manner for inertization.

The controlled supply of compressed air from the compressed air system brings about mixing of the gases in the gas separator, which achieves a lowering of the hydrogen concentration in the oxygen, exploiting the dilution effect through mixing of the gases. The compressed air therefore fulfills the task of inertization and dilution, similarly to an inert gas, in order to counter critical states in the gas separator. The hydrogen concentration is already reduced alone due to the effect of the intimate mixing and the dilution of the hydrogen in the oxygen product gas as a result of the supply of compressed air for inertization/dilution, meaning that for example a risk of explosion because of an ignitable mixture is avoided. This effect is exploited to particular advantage in the invention. Therefore, on the one hand, discarding of the oxygen product gas in the gas separator on the oxygen side is avoided, since in the procedure proposed here it remains in this vessel, with a vessel pressure being maintained. The gas volume can therefore be used when starting the electrolyzer back up again. The oxygen yield of the electrolysis system increases since virtually no high value oxygen that has already been generated is discarded.

On the other hand, however, it has proven to be highly advantageous that on the hydrogen side a discarding of the valuable hydrogen product gas in the gas separator is likewise avoided as a result. The hydrogen product gas in the procedure of the invention proposed here remains in the hydrogen-side vessel of the gas separator, with a vessel pressure being maintained and no complete venting being performed. This can advantageously and specifically avoid a noteworthy rise in the differential pressure between oxygen side and hydrogen side of the electrolyzer, especially across the membrane of a PEM electrolyzer. The gas volume of hydrogen product gas in the gas vessel, the hydrogen-side gas vessel, can now be used when starting the electrolyzer up again, which increases the economic viability.

The complex and complete purging and inertization of the entire gas system, in particular the gas separator, with nitrogen can likewise be dispensed with and a nitrogen system still required on the electrolysis system can be dimensioned to be correspondingly smaller on account of the use of the compressed air system.

In a preferred configuration of the method, air is taken in at atmospheric pressure and compressed to a working pressure and is supplied as compressed air to the gas separator. This advantageously involves taking in air from the environment, this being at atmospheric pressure. The compression brings the air to a desired pressure level of the working pressure, which as a result is flexibly adjustable to the vessel pressure of the oxygen product gas in the gas separator depending on the current operating state and mode of operation of the electrolysis system. For the precise adjustment of the working pressure to the desired pressure level and the supply of the compressed air into the gas separator, flow control elements can be used, such as for example pressure reducers, pressure regulators, control valves or orifices, that can preferably be operated via a measurement and control device. The compressed air fulfills the tasks of inertization and dilution, similarly to the action of an inert gas, with it being possible simply to resort to ambient air which is very cheap and readily feasible in terms of the system.

The hydrogen concentration is preferably measured in the gas separator. The concentration of hydrogen as extraneous gas in the oxygen product gas is particularly preferably measured and monitored in-situ in the gas separator.

The measurement and monitoring of the hydrogen concentration is carried out using appropriately sensitive gas sensors, with preference being given also to the use of monitoring and control units for a selective gas sensor system in order to reliably determine and monitor the hydrogen concentration in the oxygen product gas "in situ". This applies on the one hand to the regular operation of the electrolysis system but advantageously also during the method of lowering the hydrogen concentration in the oxygen product gas to below the desired, predetermined critical threshold value. As a result of this, critical operating states in the gas separator are reliably identified and dangerous operating states, in particular with respect to a risk of explosion due to ignitable gas mixtures of hydrogen in the oxygen product gas, can be counteracted early.

Compressed air is preferably taken from a pressurized gas vessel and supplied to the gas separator. In the pressurized gas vessel, compressed air as inertization or dilution gas is therefore introduced under pressure, stored and stockpiled and held available for dilution purposes in a volume sufficient for requirements. The pressurized gas vessel therefore acts as a store or reservoir for the inert gas and is dimensioned correspondingly.

It is thus achieved that inert gas is supplied for dilution and lowering of the hydrogen concentration only when required. The gas vessel is preferably loaded with compressed air of good quality, that is to say high purity, i.e. the compressed air inert gas has a low or very low harmful extraneous gas concentration. In particular, water-soluble extraneous gas constituents in the compressed air should be avoided since when supplied into the gas separator these can dissolve on account of the phase mixture in the process water (reactant) for the further water electrolysis and have an at least long-term disadvantageous effect on the operation and downtimes of the electrolysis system. Namely, media exchange takes place in the gas separator at the liquid/gas phase boundary. For the steady state, it can be assumed that the gas phase of the product gases is present completely saturated with water vapor.

A corresponding gas stockpile of compressed air is stored and held available in the gas vessel. The gas vessel is in the form of a pressurized vessel that in terms of volume is designed according to needs and is adapted in terms of construction. The gas vessel is advantageously loaded with compressed air in the normal operation of the electrolyzer, that is to say during the electrochemical decomposition of water into hydrogen and oxygen, so that a gas stockpile of compressed air of good quality is held available in the gas vessel, which is then acting as a buffer store or reservoir tank. It is also conceivable that in the normal operation of the electrolyzer there is a continuous flow through the gas vessel, meaning that at all times there is a volume available should the gas quality deteriorate above the critical value of a still-tolerable hydrogen concentration in the oxygen product gas.

In an advantageous configuration, air is taken in at atmospheric pressure, for example from the environment, and compressed, and the gas vessel is loaded with the compressed air, that is to say filled with compressed air. For the compression of the air, preference is given to using a compressor that is oil-free in order to avoid introducing oil-based extraneous gas constituents into the air. The pressure ratio and the compression performance are adapted correspondingly. The inert gas is advantageously taken in at atmospheric pressure by the compressor and compressed to the desired pressure level, in particular for loading the gas vessel. Advantageously, the gas vessel for compressed air for the supply as required of compressed air to the gas separator on the oxygen-side is integrated into the operational concept of the electrolysis system. This gas vessel is typically under a working pressure and contains compressed air with a good quality or high purity in terms of extraneous gas constituents.

In a particularly advantageous configuration of the method, in a purification step, the compressed air is freed of water- soluble extraneous constituents such as carbon dioxide (C02) and/or sulfur dioxide (SO2 ) . For the case in which the quality and purity of the compressed air is insufficient, gas purification is preferably carried out before the compressed air for dilution is supplied to its use for reducing the hydrogen concentration in the oxygen product gas gas separator.

Very particular attention is advantageously paid to the quality of the compressed air used, in particular that extraneous gas constituents that might damage the electrolysis system are no longer present in a critical concentration in the inert gas. For example, when using air or compressed air as inert gas, the purification step advantageously ensures that no noteworthy constituents remain therein that are chemically dissolved in water and/or that have a disadvantageous influence on the reactions on the oxygen side of the electrolysis cell. Carbon dioxide can be mentioned here by way of example. Further constituents such as sulfur dioxide can, depending on the concentration in the air taken in, in a location-specific manner play a role that should be avoided in the inert gas. A suitable purification step is therefore provided for these constituents.

To this end, in the purification step, the compressed air is preferably brought into contact with an adsorbent and/or an absorbent, in such a manner that the water-soluble extraneous gas constituents are separated from the compressed air and bound, to obtain high-purity compressed air. The configuration of the purification step with utilization of the adsorption or absorption or combinations of the two separation methods is particularly effective for leaching or separating the extraneous gas constituents from the inert gas. Adsorption denotes the accumulation of substances from gases or liquids at the surface of a solid, or more generally at the interface between two phases. This is distinct from absorption, in which the substances penetrate into a solid or a liquid. The inert gas, for example based on air, can be obtained with a high purity and quality as a result.

The electrolyzer used is preferably a PEM electrolyzer, wherein a differential pressure between the hydrogen product gas and the oxygen product gas is regulated in such a way that a maximum pressure difference across the proton exchange membrane is not exceeded.

Using a differential pressure regulation of a PEM-based electrolysis system in particular protects the membrane, since the pressure difference between oxygen side and hydrogen side is run at a permissible setpoint value in order to achieve as high as possible a system efficiency and corresponding hydrogen yield with simultaneous operational safety. Control valves and control devices present for the operational control can advantageously further be used to regulate the differential pressure in the invention as well. The pressure levels can therefore be different on the hydrogen side and on the oxygen side, provided that with respect to the membrane a permissible differential pressure is observed towards which the regulation takes place. Electrolyzers are generally designed and well suited for operation in differential pressure mode. For example, the hydrogen side can be run at a high pressure, while the oxygen side simultaneously vents unpressurized into the atmosphere. However, both the hydrogen side and the oxygen side may also be at a respective higher pressure compared to the atmosphere.

The generation of hydrogen and oxygen in the electrolyzer is preferably halted, in particular only temporarily.

Normal operation of the electrolysis system is thus advantageously interrupted only until has been achieved by the supply of compressed air of good quality and high purity from the gas vessel into the oxygen-side gas separator for dilution and lowering of the hydrogen concentration below the threshold value. As a result, the time required for fixing faults with the accompanying operational downtime of the electrolyzer is advantageously considerably reduced for the method according to the invention compared to conventional methods. In these conventional methods, complete discharge of both the oxygen product gas and the hydrogen product gas is effected, with the respective gas separators being emptied. The gas system is then completely inertized with nitrogen from the nitrogen system of the electrolysis system and the electrolysis system is lastly started back up until achievement or resumption of a normal operating state of the electrolyzer with good quality of the product gases.

It is therefore particularly advantageous to perform the dilution with compressed air. The availability of air/compressed air is readily provided in an electrolysis system since electrolysis systems typically already have a compressed air system. Therefore, air or compressed air is available in that it is advantageously possible to resort to the compressed air system for provision of the dilution gas. This is also highly favorable from economic points of view. Preference is given here to using purification steps for conditioning the inert gas for the intended use, as described above. Compared to the use of the nitrogen system, the integration of the compressed air system into the system concept of the electrolysis system is much more favorable.

The volume of air or compressed air required for addition and dilution to eliminate a potentially dangerous state of high hydrogen concentration in the gas separator is much lower than in the known methods since complete emptying and purging of the gas separator is not provided for. Since, in addition, the generation of nitrogen in a nitrogen system requires many times as much purified compressed air, the required capacity of the compressed air system according to the present invention turns out to be much smaller compared to an electrolysis system in which nitrogen is used on-site for complete inertization of the gas separator on the oxygen side.

The electrolyzer system according to the invention accordingly comprises an electrolyzer for the generation of hydrogen and oxygen as product gases, in which the oxygen product gas also contains hydrogen as extraneous gas, and a compressed air system having a gas vessel for the stockpiling of compressed air, wherein the electrolyzer is connected to a gas separator via a product stream conduit for the oxygen product gas, and wherein the compressed air system is connected to the gas separator via a feed conduit, so that compressed air is suppliable from the gas vessel to the gas separator as required.

The separation of the water phase and gas phase is effected in the gas separator. The gas separator is preferably constructed as a gravity separator, so that the water phase can be taken off at the bottom and the gas phase, in the present case the oxygen product gas, can be taken off at the top. The water column within the separator additionally serves as a buffer store in the event of fluctuating load demands. Media exchange takes place at the phase boundary in the gas separator. It can be assumed for the steady state that the gas phase of the product gas, in the present case oxygen product gas, is present completely saturated with water vapor. A corresponding situation applies for a gas separator on the hydrogen side of the electrolysis system with the phase separation of hydrogen product gas and process water (reactant) for the electrolysis.

A valve is preferably connected into the feed conduit, this in particular being configured as a control valve. The configuration of the valve as a control valve allows precise metering of the gas supply of inert gas to the gas separator on the oxygen side. The valve position of the control valve can advantageously be controlled with a hydraulic or electromechanical valve control system or valve regulating device. A corresponding control and regulating device as well as sensor devices is preferably integrated into the system concept of the electrolysis system.

A purification device for the compressed air is preferably connected into the feed conduit, so that extraneous constituents can be separated off from the compressed air.

More preferably, the purification device includes an adsorbent and/or an absorbent, by means of which extraneous gas constituents from the compressed air are adsorbable and/or absorbable. The materials can be chosen appropriately in order to adapt the purification device to the requirement. Of particular interest is the separation or removal of water soluble extraneous gas constituents in the compressed air or traces of constituents originally present, which is important in particular when using air from the environment as inert gas. Adsorption or absorption of carbon monoxide or sulfur dioxide from the air is particularly advantageous here. An adsorbent serves to remove trace substances from the air. The same applies, mutatis mutandis, for absorbents.

In a particularly preferred configuration of the electrolysis system, the latter includes a compressor to which the gas vessel is connected via a connection conduit, so that compressed air is suppliable to the gas vessel or meterable into the gas vessel.

The compressor is more preferably configured as an oil lubricated air compressor downstream of which an oil filter is connected. The oil filter is configured to be appropriately efficient in terms of the performance for filtering oil constituents in the compressed air.

Concerning the quality or purity of the compressed air, particular attention should be paid to ensure sufficient freedom from oil. There would otherwise be the risk of oil residues igniting in the oxygen atmosphere. Oil residues as gaseous extraneous constituents in the compressed air should in general also be avoided as far as possible in order to ensure reliable operation of the electrolysis system with a high availability. This can be done in two ways, for example. For instance, when using an oil-lubricated air compressor, an appropriate oil filter can advantageously be installed prior to the use of the compressed air for inertization. With the use of such high efficiency oil filters, a class 2 freedom from oil can be achieved with very low residual oil amounts of less than 0.1 mg/m 3 .

Alternatively, oil-free compressors can also be used when compressing the air to the desired pressure level, meaning that the compressed air supplied to the gas separator is also completely oil free.

In an advantageous configuration of the electrolysis system, the compressor is therefore configured as an oil-free compressor.

Exemplary embodiments of the invention are elucidated in more detail with reference to a drawing. In the drawing, in a schematic and highly simplified manner:

FIG 1 shows a known electrolysis system with nitrogen system for inertizing and purging,

FIG 2 shows an electrolysis system with compressed air system according to the invention.

Like reference signs have the same meaning in the figures.

FIG 1 shows an electrolysis system 100 in a highly simplified detail of system components. The electrolysis system 100 has an electrolyzer 1 that is implemented as a PEM or alkaline electrolyzer. The electrolyzer 1 comprises at least one electrolysis cell (not shown in more detail here) for the electrochemical decomposition of water. The electrolysis system 100 additionally has a nitrogen system 3 comprising a nitrogen vessel 5. A compressor 7 is connected to the nitrogen system 3 in order to supply the nitrogen system 3. The nitrogen system 3 is connected to a gas separator 11 via a purging conduit 9, so that nitrogen for purging the gas separator 3 is taken from the nitrogen vessel 5 and is suppliable to the gas separator 3 via the purging conduit 9 when required (nitrogen inertization). The nitrogen vessel 3 is dimensioned with an appropriately large volume for the nitrogen requirement in the electrolysis system 100, and pressurized. For inertization - in addition to other tasks - large amounts of nitrogen in the electrolysis system are required on demand, which are to be held available in the nitrogen vessel 5.

A reactant stream of water is introduced into the electrolyzer 1 via a reactant stream conduit 13. The water is electrochemically decomposed in the electrolyzer 1 into the product gases hydrogen and oxygen and both product streams are separately conducted out of the electrolyzer 1. To conduct out the oxygen product stream, the electrolyzer 1 has a product stream conduit 15, with the aid of which a first product, in this case oxygen from the electrolysis, is guided out. The presently described structure of the electrolysis system 100 considers the oxygen product stream. On the hydrogen side, a corresponding system structure is present in the electrolysis system 100, which is not further shown and explained in detail in FIG 1 for the sake of clarity. Accordingly, to conduct the hydrogen product stream out of the electrolyzer 1 a product stream conduit 17 is specially provided, with the aid of which a second product, namely the hydrogen obtained from the electrolysis, is guided out. The hydrogen obtained is then treated in further components (not shown in more detail in FIG 1) of the electrolysis system 100 and further processed.

On the oxygen side, the electrolyzer 1 is connected to the gas separator 11 via the product stream conduit 15. Connected to the gas separator 11 is a venting conduit 19, via which the gas separator 11 can be completely emptied as needed by depressurization, so that it is unpressurized or at atmospheric pressure. Further provided is a compressed air system 21 comprising a gas vessel 23 and an air compressor 25, so that compressed air is suppliable from the air compressor 25 to the gas vessel 23 via the connection conduit 27. The gas vessel 23 can in this way be loaded with compressed air L for further purposes stockpiled. To supply the nitrogen system 3 with compressed air L, the compressed air system 21 is thus connected via a supply conduit 29a. The supply of another consumer unit 31 with compressed air L is effected via a supply conduit 29b.

During the operation of the electrolyzer 1 in the system concept of FIG 1, oxygen product gas is supplied to the gas separator 11. For measurement of quality of the oxygen product gas, in this operating concept the concentration of hydrogen in the oxygen product gas in the gas separator 11 is continuously measured and monitored. If the concentration exceeds a predetermined threshold value, the operation of the electrolyzer 1 is halted and the entire oxygen product gas in the gas separator 11 is discarded. The oxygen product gas is discharged completely from the vessel volume of the gas separator 11 and any conduits of the oxygen-side gas system. To this end, the gas separator 11 is completely vented via the venting conduit 19 and put into an unpressurized state. The entire gas system, including gas separator 11, is then purged in a complex and cost-intensive purging procedure for inertization with nitrogen from a nitrogen vessel 5 in the nitrogen system 3 of the electrolysis system 100. For this safety-related need for nitrogen, the nitrogen system 3 must be designed with an appropriately large volume in order to hold sufficient nitrogen available. After the cause of the critical quality of the oxygen product gas has been addressed, the electrolysis is restarted.

As a result of purging the gas system with nitrogen and in particular the gas separator 11, the newly produced oxygen product gas also has to be discarded at first, and specifically until the desired gas quality has been achieved again.

The depressurization or complete venting on the oxygen side in the gas separator 11 for the purging procedure with nitrogen means that on the hydrogen side the entire, valuable, hydrogen product gas in a corresponding gas separator (not shown in more detail in FIG 1) also has to be completely discarded, in particular in order to counteract the high differential pressure as a result of the venting and in order to avoid damage to the system via the PEM membrane. Therefore, in addition to the holding available of a large-volume nitrogen system 3 for inertization and a sufficient nitrogen stockpile, the accompanying discarding of the hydrogen product gas generated on the hydrogen side is specifically also particularly disadvantageous from an economic point of view.

FIG 2 shows an electrolysis system 1 with compressed air system according to the invention. The novel operating concept of the invention proceeds with an advantageous integration of the compressed air system 21 and configuration of the same as an inert gas system that is connected on the oxygen side to the gas separator 11. Compared to a configuration of the electrolysis system 100 according to FIG 1, the purging conduit 9 that connects the nitrogen system 3 with the gas separator 3 for inertization of the entire gas system with nitrogen is dispensed with. According to FIG 2, the compressed air system 21 is connected to the gas separator 11 via the feed conduit 37, so that compressed air L can be taken from the gas vessel 23 as required. The gas vessel 23 is a store for stockpiling and providing compressed air. The compressed air system 21 has an air compressor 25 and a gas vessel 23 which are connected to one another via a connection conduit 27. The air compressor 25 is in this case configured as an oil-free compressor. A purification device 33 having an absorbent and/or adsorbent is connected into the feed conduit 37. Harmful extraneous gas constituents in the compressed air L stored in the gas vessel 23 can thus be removed and an inert or dilution gas is generated with very high quality and purity. By way of example, carbon dioxide and/or sulfur dioxide can be removed from the compressed air L by means of the purification device 33. The heat energy released by an adsorption or absorption can be used for further purposes by means of cooling the purification device 33 for example in a heat exchange process by coupling-in a heat exchanger. Connected into the feed conduit 37 downstream of the purification device 33 in the flow direction of the compressed air L is a control valve 35 the valve position of which is regulatable via a regulating device (not shown in more detail) according to the need and the pressure level for compressed air L for supply to the gas separator 11. Downstream of the control valve 35, the feed conduit 37 opens into the gas separator 11.

During operation of the electrolysis system 100, the extraneous gas concentration of hydrogen in the oxygen product gas from the electrolyzer 1 in the gas separator 11 connected downstream on the oxygen side is continuously measured and monitored. If the measured value indicates a critical extraneous gas concentration of hydrogen above a predetermined threshold value for a still permissible hydrogen proportion in the oxygen product gas in the gas separator 11, the hydrogen concentration is reduced by supplying purified compressed air L with good quality and purity, that is to say with at most very low impurities or harmful extraneous gas constituents, to the oxygen product gas in a controlled and well-metered manner via the control valve 35. This controlled supply then brings about in the gas separator 11 an intimate mixing of the gases, as a result of which a lowering of the hydrogen concentration is achieved. The hydrogen concentration is already reduced alone on account of the effect of the gas phase mixing and dilution of the hydrogen in the oxygen product gas by the addition of compressed air L, an effect that is particularly advantageously exploited in a targeted manner by the invention. This on the one hand avoids discarding the oxygen product gas in the gas separator 11, since this electrolysis product remains under pressure in the gas separator 11 in the procedure proposed here. The gas volume in the oxygen side gas separator 11 can therefore be used when starting the electrolyzer 1 back up again. Therefore, if a quality measurement in the gas separator 11 indicates a poor quality, the electrolysis process is halted. Instead of then discarding the gas in the gas separator 11, purified compressed air L is taken from the gas vessel 23 and supplied to the gas separator 11. This supply is effected by activating and opening a valve control system for the control valve 35 until the gas quality satisfies the requirements again, that is to say the hydrogen concentration is less than the predetermined threshold value for safe normal operation of the electrolyzer 1.

With the electrolysis system 100 with compressed air system 21 according to the invention, complete venting and a hitherto accompanying depressurization on the oxygen side in the gas separator 11 is dispensed with. As a result of the supply, proposed here, of pressurized purified air L, thus compressed air L, it is also possible advantageously on the oxygen side to continue to utilize the entire, valuable, hydrogen product gas in a corresponding hydrogen-side gas separator (not shown in more detail in FIG 2). Until now, venting was also provided on the hydrogen side in order to counteract the high differential pressure as a result of the venting and in order to avoid damage to the system via the PEM membrane due to an excessive differential pressure. The accompanying use of the generated hydrogen product gas on the hydrogen side is very particularly advantageous both in operational aspects and from economic points of view.

It proves to be a further economic advantage that the nitrogen system 3 can, as a result of the proposed system concept according to FIG 2 for the electrolysis system 100, be designed to be much more compact compared to the design according to FIG 1. The corresponding system components, especially the nitrogen vessel 5, can be dimensioned to be smaller. With the operating concept of the invention, it suffices merely to provide the continuous nitrogen consumption for the so-called compressor operation of the system.

Claims

1. A method for operating an electrolysis system (100) for the generation of hydrogen and oxygen as product gases, in which the oxygen product gas from an electrolyzer (1), which also contains hydrogen as extraneous gas, is supplied to a gas separator (11) connected downstream, wherein, when a predetermined threshold value for the hydrogen concentration in the oxygen product gas is exceeded, compressed air (L) is supplied to the gas separator (11), so that in the gas separator (11) a dilution of the hydrogen in the oxygen product gas is brought about by the mixing of the gases, lowering the hydrogen concentration in the oxygen product gas.

2. The method as claimed in claim 1, in which air is taken in at atmospheric pressure and compressed to a working pressure and is supplied as compressed air (L) to the gas separator (11).

3. The method as claimed in claim 1 or 2, in which the hydrogen concentration is measured and monitored in-situ in the gas separator (11).

4. The method as claimed in any of claims 1, 2 and 3, in which compressed air (L) is taken from a pressurized gas vessel (23) and supplied to the gas separator (11).

5. The method as claimed in claim 4, in which air is taken in at atmospheric pressure and compressed to give compressed air (L), and wherein the gas vessel (23) is loaded with compressed air (L).

6. The method as claimed in any of the preceding claims, in which, in a purification step, the compressed air (L) is freed of water-soluble extraneous constituents such as carbon dioxide (C02) and/or sulfur dioxide (SO 2 )

. 7. The method as claimed in claim 6, in which, in the purification step, the compressed air (L) is brought into contact with an adsorbent and/or an absorbent, so that water-soluble extraneous constituents are separated from the compressed air (L) and bound, to obtain high-purity compressed air (L).

8. The method as claimed in any of the preceding claims, in which the electrolyzer (1) used is a PEM electrolyzer, wherein a differential pressure between the hydrogen product gas and the oxygen product gas is regulated in such a way that a maximum pressure difference across the proton exchange membrane is not exceeded.

9. The method as claimed in any of the preceding claims, in which the generation of hydrogen and oxygen in the electrolyzer (1) is halted as required.

10. An electrolysis system (100) comprising an electrolyzer (1) for the generation of hydrogen and oxygen as product gases, in which the oxygen product gas also contains hydrogen as extraneous gas, and a compressed air system (21) having a gas vessel (23) for the stockpiling of compressed air (L), wherein the electrolyzer (1) is connected to a gas separator (11) via a product stream conduit (15) for the oxygen product gas, and wherein the compressed air system (21) is connected to the gas separator (11) via a feed conduit (37), so that compressed air (L) is suppliable from the gas vessel (23) to the gas separator (11) as required.

11. The electrolysis system (100) as claimed in claim 10, comprising a valve (35) connected into the feed conduit (37), in particular a control valve.

12. The electrolysis system (100) as claimed in claim 10 or 11, having a purification device (33) for the compressed air (L) connected into the feed conduit (37), so that extraneous constituents can be separated off from the compressed air (L).

13. The electrolysis system (100) as claimed in claim 12, in which the purification device (33) includes an adsorbent and/or an absorbent, by means of which extraneous constituents from the compressed air (L) are adsorbable and/or absorbable.

14. The electrolysis system (100) as claimed in any of claims 10 to 13, comprising a compressor (25) to which the gas vessel (23) is connected via a connection conduit (27), so that compressed air is suppliable to the gas vessel (23) as compressed air (L).

15. The electrolysis system (100) as claimed in claim 14, in which the compressor (25) is configured as an oil lubricated air compressor downstream of which an oil filter is connected.

16. The electrolysis system (100) as claimed in claim 14, in which the compressor (25) is configured as an oil-free air compressor.