WO2023001422A1

WO2023001422A1 - Method for operating an electrolysis plant, and electrolysis plant

Info

Publication number: WO2023001422A1
Application number: PCT/EP2022/062738
Authority: WO
Inventors: Du-Fhan Choi; Markus Ungerer; Dirk Wall
Original assignee: Siemens Energy Global GmbH & Co. KG
Priority date: 2021-07-19
Filing date: 2022-05-11
Publication date: 2023-01-26
Also published as: CA3226820A1; AU2022313398A1; CN117751209A; EP4123053A1; EP4334501A1

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

description

Process for operating an electrolysis plant and electrolytic plant

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

Hydrogen is now 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 large number of electrolytic cells which are arranged adjacent to one another. Using water electrolysis, water is broken down into hydrogen and oxygen in the electrolysis cells. In a PEM electrolyser, distilled water is typically fed in as starting material on the anode side and passed through a proton-permeable membrane (“proton exchange membrane”);

PEM) split into hydrogen and oxygen. The water is oxidized to oxygen at the anode. The protons pass through the proton-permeable membrane. Hydrogen is produced on the cathode side. In this case, the water is generally conveyed from an underside into the anode compartment and/or cathode compartment.

This electrolysis process takes place in what is known as the electrolysis stack, which is made up of several electrolysis cells. In the electrolysis stack, which is under DC voltage, water is introduced as educt, with two fluid streams consisting of water and gas bubbles (oxygen O ₂ or hydrogen H _{2 )} emerging after passing through the electrolysis cells. The respective separation of the water and gas phase in the fluid flows takes place in gas separators or gas separators.

In practice, there are small amounts of hydrogen in the oxygen gas stream and small amounts of oxygen in the hydrogen gas stream. The quantity of the respective foreign gas depends on the electrolysis cell design and also varies under the influence of current density, catalyst composition, aging and, in the case of a PEM electrolysis system, on the membrane material. It is inherent in the system that in the gas stream of one product gas, the other product gas is present in very small amounts. In the further course of the process, even small traces of oxygen are usually removed from the hydrogen in downstream gas cleaning steps, which can sometimes be very complex and cost-intensive cleaning steps, especially if a particularly high product gas quality is required, as is the case when using hydrogen, for example e.g. for fuel cells.

For example, in an electrolysis plant for gas purification of the product gas streams from the electrolyzer, both product gas streams in particular can be fed to a respective catalytically activated recombiner, in which a catalyst allows the hydrogen to recombine with the oxygen to form water (DeOxo unit). To do this, the gas flow must first be heated to at least 80°C so that the conversion rates of the recombiner are sufficiently high and the required gas purity is achieved. However, the process engineering plant used for this is expensive and, due to its energy requirements, reduces the system efficiency of the entire electrolytic plant. For this reason, attention must be paid to the purity and quality of the product gas streams initially produced in the electrolyser and discharged from the electrolyser, also in order to keep operational safety aspects and the costs and effort for the subsequent cleaning steps within reasonable limits. The purity or quality of the two product gas streams of the gases originally produced in the electrolyzer depends on many parameters and can also change during the operation of an electrolysis system. On the one hand, this is problematic and particularly relevant to safety when the concentration of oxygen in hydrogen increases, but on the other hand it also occurs when the concentration of hydrogen in oxygen increases. If a certain concentration limit is exceeded here, especially in the respective gas separator (container) immediately downstream of the electrolysis, the oxygen gas produced can no longer be handed over for other purposes, for example. If the proportion of hydrogen in the oxygen product gas continues to rise, a flammable or explosive mixture can even form. A potentially dangerous operating state then prevails in the gas separator (container), which must be avoided at all costs for safety reasons. This also applies correspondingly to the hydrogen side.

Reliable and continuous monitoring of the gas quality of the product gases during operation of the electrolysis plant is therefore essential. This also applies in particular to the oxygen side of the electrolyser, ie the monitoring of the concentration of hydrogen as a foreign gas in the oxygen produced during the electrolysis. Monitoring and the corresponding operational management represent an important protective measure in order to identify critical operating states and to take safety measures, including temporarily shutting down the electrolysis system.

The invention is therefore based on the object of enabling improved operation in terms of safety and plant efficiency in an electrolysis plant.

The object is achieved by a method for operating an electrolysis plant for the production of hydrogen and oxygen as product gases, in which the oxygen product gas from an electrolyzer, which also contains hydrogen contains as foreign gas, is fed to a downstream gas separator, wherein when a predetermined limit value for the hydrogen concentration in the oxygen product gas is exceeded, compressed air is fed to the gas separator, so that in the gas separator the mixing of the gases causes a dilution of the hydrogen in the gas oxygen product gas is effected, whereby the hydrogen concentration in the oxygen product gas is lowered.

The object is also achieved according to the invention by an electrolysis system comprising an electrolyzer for generating hydrogen and oxygen as product gases, in which the oxygen product gas also contains hydrogen as a foreign gas, and a compressed air system with a gas tank for storing compressed air, the electrolyzer is connected to a gas separator via a product flow line for the oxygen product gas, and the compressed air system is connected to the gas separator via a supply line, so that compressed air from the gas tank can be fed to the gas separator as required .

The advantages and preferred configurations listed below in relation to the method can be transferred analogously to the electrolysis system.

The invention is based on the knowledge that previous operating concepts for electrolysis systems with regard to the monitoring and rectification of critical operating states with regard to the quality of the oxygen produced are complex in terms of system technology and therefore have considerable economic disadvantages.

For a quality measurement on the oxygen side of an electrolysis plant, the concentration of hydrogen in the oxygen product gas in the corresponding gas separator is usually measured and monitored in previous operating concepts. If the concentration exceeds a predetermined limit value, the operation of the electrolyzer is stopped. A depressurization is carried out on the gas separator containing the oxygen, ie this gas container is completely vented and depressurized. Disposal of the oxygen gas and complete replacement of the gas in the gas tank is required. Due to the depressurization or complete venting on the oxygen side, the entire valuable hydrogen product gas must also be discarded in the corresponding gas separator on the hydrogen side, in particular to counteract the high differential pressure caused by the venting and to prevent system damage across the PEM membrane avoid. The hydrogen product gas is therefore also completely drained from the container volume of the gas separator and any supply lines of the gas system on the hydrogen side. For this purpose, the gas separator is fully vented. The entire gas system, including the gas separators, is then flushed with nitrogen from a storage tank in the nitrogen system of the electrolysis plant using a complex flushing procedure to make it inert.

The nitrogen system must be designed with a correspondingly large volume for this safety-relevant need for nitrogen in order to provide sufficient nitrogen. After the cause of the critical quality of the oxygen product gas has been remedied, the electrolysis is restarted. Due to the inert gas nitrogen in the gas system, the newly produced hydrogen product gas must first be discarded until the desired gas quality is achieved again. In addition to the provision of a large-volume nitrogen inerting system and nitrogen storage, it is precisely the discarding of the hydrogen product gas produced that is particularly disadvantageous from an economic point of view.

This is where the present invention starts in a targeted manner, in that a critical foreign gas concentration of hydrogen in the oxygen product gas, which is in the corresponding gas separator downstream of the electrolyser, is reduced in that the oxygen product gas is made inert by compressed air of good quality Quality in a targeted and well-dosed way way is supplied. The targeted supply of compressed air from the compressed air system causes the gases to mix in the gas separator, which reduces the hydrogen concentration in the oxygen, with the dilution effect being utilized by the mixing of the gases. The compressed air thus fulfills the task of inerting and diluting, similar to an inert gas, in order to counteract critical conditions in the gas separator. The hydrogen concentration is reduced simply because of the effect of the intimate gas mixture and the dilution of the hydrogen in the oxygen product gas through the supply of compressed air for inerting or dilution, so that, for example, a risk of explosion from an ignitable mixture is avoided. This effect is used particularly advantageously in the invention. Thus, on the one hand, discarding of the oxygen product gas in the gas separator on the oxygen side is avoided, since it remains in this container in the procedure proposed here, with a container pressure being maintained. The gas volume can therefore be used when restarting the electrolyser. The oxygen yield of the electrolysis plant increases, since practically no high-quality oxygen that has already been produced is discarded.

On the other hand, it has proven to be of particular advantage that on the hydrogen side, discarding the valuable hydrogen product gas in the gas separator is also avoided. In the procedure of the invention proposed here, the hydrogen product gas remains in the hydrogen-soapy container of the gas separator, with a container pressure being maintained and complete venting not being carried out. In this way, an appreciable increase in the differential pressure between the oxygen side and the hydrogen side of the electrolyzer, in particular across the membrane of a PEM electrolyzer, can advantageously be avoided in a targeted manner. The gas volume of hydrogen product gas in the gas holder, the hydrogen-soapy gas separator, can now be used more when restarting the electrolyser, which increases the economy. The complex and complete flushing and inerting of the entire gas system, in particular the gas separator, with nitrogen can also be omitted and a nitrogen system that is still required in the electrolysis system can be correspondingly smaller due to the use of the compressed air system.

In a preferred embodiment of the method, air is sucked in at atmospheric pressure and compressed to a working pressure and fed to the gas separator as compressed air. Here, air is advantageously sucked in from the environment, which is at atmospheric pressure. The compression brings the air to a desired pressure level of the working pressure, which can be flexibly adjusted to the container pressure of the oxygen product gas in the gas separator depending on the current operating status and mode of operation of the electrolysis system. For the precise setting of the working pressure to the desired pressure level and the supply of compressed air to the gas separator, flow control elements can be used, such as pressure reducers, pressure regulators, control valves or orifices, which are preferably operated via a measuring and control device. The compressed air fulfills the task of inerting and diluting, similar to the effect of an inert gas, whereby ambient air can simply be used, which is very cost-effective and easier to implement in terms of location.

Preferably the hydrogen concentration in the gas separator is measured. The concentration of hydrogen as a foreign 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 correspondingly sensitive gas sensors, with monitoring and control units for selective gas sensors preferably also being used. men 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 process of lowering the hydrogen concentration in the oxygen product gas below the desired one As a result, critical operating states in the gas separator are reliably detected and dangerous operating states, in particular with regard to the risk of explosion due to ignitable gas mixtures of hydrogen in the oxygen product gas, can be counteracted at an early stage.

Compressed air is preferably taken from a pressurized gas tank and fed to the gas separator. In the pressurized gas tank which is compressed air is therefore introduced as inerting or dilution gas under a pressure, gespei chert and stored and kept available for dilution purposes in suffi sponding volume for the need. The pressurized gas tank therefore acts as a store or storage tank for the inert gas and is dimensioned accordingly.

This ensures that inert gas is only fed in to dilute and lower the hydrogen concentration when required. The gas container is preferably loaded with compressed air of good quality, that is to say of high purity, that is to say the compressed air of inert gas has a low or very low concentration of harmful foreign gases. In particular, water-soluble foreign gas components in the compressed air should be avoided, since when they are fed into the gas separator, they can dissolve in the process water (educt) for further water electrolysis due to the phase mixture and have a disadvantageous effect on the operation and service life of the electrolysis system, at least in the long term. This is because a media exchange takes place at the liquid/gas phase boundary in the gas separator. For the stationary state, one can assume that the gas phase of the product gases is completely saturated with water vapor. A corresponding supply of compressed air is stored and kept available in the gas tank. The gas tank is designed as a pressure tank, which is designed accordingly in terms of volume and adapted to the needs. The gas tank is advantageously charged with compressed air during normal operation of the electrolyser, i.e. during the electrochemical decomposition of water into hydrogen and oxygen, so that a supply of compressed air of good quality is kept in the gas tank, which then acts as a buffer store or storage tank. It is also conceivable that during normal operation of the electrolyser the gas tank is continuously flowed through so that a volume is available at all times should the gas quality deteriorate beyond the critical value of a still tolerable hydrogen concentration in the oxygen product gas.

In an advantageous embodiment, air is sucked in at atmospheric pressure, for example from the environment, compressed and the gas container is loaded with the compressed air, i.e. filled with compressed air. A compressor that is oil-free is preferably used to compress the air, in order to prevent the entry of oil-based foreign gas components avoid in the air. The pressure ratio and the compression capacity are adjusted accordingly. The inert gas is advantageously sucked in by the compressor at atmospheric pressure and compressed to the desired pressure level, in particular for loading the gas container. Advantageously, the integration of the gas tank for compressed air for the required supply of compressed air to the gas separator on the oxygen side takes place in the operating concept of the electrolysis system. This gas container is usually under a working pressure and contains compressed air of good quality or high purity with regard to foreign gas components.

In a particularly advantageous embodiment of the method, the compressed air is freed from water-soluble foreign matter such as carbon dioxide (CO _{2 )} and/or sulfur dioxide (SO _{2 )} in a cleaning step. In the event that the quality fact and purity of the compressed air is not sufficient, a gas cleaning is preferably carried out before the compressed air is fed to the gas separator to dilute its use for reducing the hydrogen concentration in the oxygen product gas.

Advantageously, special attention is paid to the quality of the compressed air used, in particular that there are no longer any foreign gas components that could damage the electrolysis system in a critical concentration in the inert gas. For example, when using air or compressed air as the inert gas, the cleaning step advantageously ensures that no significant components remain in the which are chemically dissolved in water and/or adversely affect the reactions on the oxygen side of the electrolytic cell. An example of this is carbon dioxide. Other components such as sulfur dioxide, depending on the concentration in the intake air, can play a site-specific role that should be avoided in the inert gas. A suitable purification step is therefore provided for these components.

For this purpose, the compressed air is preferably brought into contact with an adsorbent and/or an absorbent in the cleaning step such that the water-soluble foreign gas components are separated and bound from the compressed air, with the inert gas of high purity being obtained. The design of the cleaning step using adsorption or absorption or combinations of both separation methods is particularly effective in order to separate or separate the foreign gas components from the inert gas. Adsorption is the accumulation of substances from gases or liquids on the surface of a solid, more generally on the interface between two phases. This differs from absorption, in which the substances penetrate into the interior of a solid or a liquid. As a result, the inert gas for example based on air, with high purity and quality.

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

With a differential pressure control of a PEM-based electrolysis system, the membrane in particular is protected, since the pressure difference between the oxygen side and the hydrogen soap is run at a permissible target value in order to achieve the highest possible system efficiency and corresponding hydrogen yield while at the same time ensuring operational safety. The differential pressure can advantageously continue to be controlled in the invention via existing control valves and control devices for operational management. The pressure levels can therefore be different on the hydrogen side and the oxygen side, as long as a permissible differential pressure is observed with a view to the membrane, which is used for regulation. In general, electrolysers are designed and well suited for operation in differential pressure mode.

For example, the hydrogen side can be operated at high pressure, while the oxygen side is simultaneously vented to the atmosphere without pressure. However, both the hydrogen side and the oxygen side can also be at a respective higher working pressure than the atmosphere.

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

The normal operation of the electrolysis system is thus advantageously only interrupted until the supply of compressed air of good quality and high purity from the gas tank into the oxygen-side gas separator for dilution and reduction of the hydrogen concentration below the limit value is reached. As a result, the time required for troubleshooting with the accompanying downtime of the electrolyzer for the method according to the invention is advantageously significantly reduced compared to conventional methods. In these conventional methods, both the oxygen product gas and the hydrogen product gas are discharged completely , whereby the respective gas separator is emptied. The gas system is then completely rendered inert with stick material from the nitrogen system of the electrolysis system and finally the electrolysis system is started up again until the electrolyzer reaches or resumes a normal operating state with good product gas quality.

It is therefore particularly advantageous to carry out the dilution with compressed air, i.e. compressed air. The availability of air or compressed air is a given in an electrolysis system, since electrolysis systems usually already have a compressed air system. Air or compressed air is thus available, in that the compressed air system can advantageously be used to obtain the dilution gas. This is also very favorable from an economic point of view. Preference is given to cleaning steps for confectioning the inert gas for the intended use, as described above. In comparison to using the nitrogen system, the integration of the compressed air system into the system concept of the electrolysis system is much cheaper.

The volume of air or compressed air that is required to eliminate a potentially dangerous state of high hydrogen concentration in the gas separator for dosing and dilution is significantly lower than in the known methods, since the gas separator is not intended to be completely emptied and flushed . Since, moreover, a multiple of purified compressed air is required for the generation of nitrogen in a nitrogen system, the necessary capacity of the compressed air system according to the present invention falls in the Compared to an electrolysis plant, in which nitrogen is used on site to completely inertize the gas separator on the oxygen side, it is significantly lower.

The electrolysis system according to the invention accordingly comprises an electrolyzer for generating hydrogen and oxygen as product gases, in which the oxygen product gas also contains hydrogen as a foreign gas, and a compressed air system with a gas tank for storing compressed air, the electrolyzer having a Product flow line for the oxygen product gas is connected to a gas separator, and the compressed air system is connected to the gas separator via a supply line, so that compressed air from the gas tank can be fed to the gas separator as required is.

The separation of the water and gas phase takes place in the gas separator or gas separator. The gas separator is preferably constructed as a gravity separator, so that the water phase can be removed from the bottom and the gas phase, in this case the oxygen product gas, can be removed from the top. The water column inside the separator also serves as a buffer storage for changing load specifications. A media exchange takes place at the phase boundary in the gas separator. For the stationary state, it can be assumed that the gas phase of the product gas, in this case oxygen product gas, is completely saturated with water vapor. The same applies to a gas separator on the hydrogen side of the electrolysis plant with the phase separation of hydrogen product gas and process water (educt) for the electrolysis.

In this case, a valve is preferably switched into the supply line, which is designed in particular as a control valve. The design of the valve as a control valve allows a precise dosing 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 adjusted with a hydraulic or electromechanical valve control or valve control device. to be controlled. A corresponding control and regulation device as well as sensor devices are preferably integrated into the system concept of the electrolysis system.

A cleaning device for the compressed air is preferably connected into the supply line, so that foreign matter can be separated from the compressed air.

More preferably, the cleaning device has an adsorption agent and/or an absorption agent, by means of which foreign gas components from the compressed air can be adsorbed and/or absorbed. The materials can be selected accordingly in order to adapt the cleaning device to the requirements. Of particular interest is the deposition or separation of water-soluble foreign gas components in the compressed air or traces of originally present components, which is particularly important when using ambient air as the inert gas. Adsorption or absorption of carbon monoxide or sulfur dioxide from the air is particularly advantageous here. An adsorbent or adsorbent is used to remove trace substances from the air. The same applies to absorbents or absorbents.

In a particularly preferred embodiment of the electrolysis system, it has a compressor to which the gas tank is connected via a connecting line, so that compressed air, ie compressed air, can be fed to the gas tank or metered into the gas tank.

Furthermore, the compressor is preferably designed as an oil-lubricated air compressor, which is followed by an oil filter. The oil filter is designed to be correspondingly efficient in terms of filtering oil components in the compressed air.

When it comes to the quality and purity of the compressed air, particular attention must be paid to ensuring that there is sufficient freedom from oil. is guaranteed. Otherwise there would be a risk of oil residues igniting in the oxygen atmosphere. Oil residues as gaseous foreign matter in the compressed air should also generally be avoided as far as possible in order to ensure reliable operation of the electrolysis system with a high level of availability. This can be done in two ways, for example. When using an oil-lubricated air compressor, an appropriate oil filter can advantageously be installed before the compressed air is used for inerting. With the use of such highly efficient oil filters, a class 2 oil-free status can be achieved with very low residual oil quantities of less than 0.1 mg/m ³ .

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

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

Exemplary embodiments of the invention are explained in more detail with reference to a drawing. Herein show schematically and greatly simplified:

1 shows a known electrolysis system with nitrogen

system for inerting and flushing,

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

The same reference symbols have the same meaning in the figures.

FIG. 1 shows an electrolysis system 100 in a greatly simplified detail of system parts. The electrolysis system 100 has an electrolyzer 1 which designed as a PEM or alkaline electrolyser. The electrolyzer 1 comprises at least one electrolytic cell, not shown in detail here, for the electrochemical decomposition of water. The electrolysis system 100 also has a nitrogen system 3 which includes a nitrogen tank 5 . A compressor 7 is connected to the nitrogen system 3 to supply the nitrogen system 3 . The nitrogen system 3 is connected to a gas separator 11 via a flushing line 9, so that nitrogen for flushing the gas separator 3 can be taken from the nitrogen container 5 if required and fed to the gas separator 3 via the flushing line 9 ( nitrogen inerting). The nitrogen container 3 is dimensioned to have a correspondingly large volume and is pressurized for the nitrogen requirement in the electrolysis system 100 . For an Inerti tion are - among other tasks - required, large amounts of nitrogen in the electrolysis system are required, which are to be held in the nitrogen tank 5.

A reactant flow of water is introduced into the electrolyzer 1 via a reactant flow line 13 . The water is electrochemically broken down in the electrolyzer 1 into the product gases hydrogen and oxygen, and both product streams are passed out of the electrolyzer 1 separately. For the out line of the oxygen product stream, the electrolyzer 1 has a product stream line 15, with the help of which a first product, here oxygen from the electrolysis, is led out. The structure of the electrolysis plant 100 described here considers the oxygen product stream. On the hydrogen side, there is a corresponding plant engineering structure in the electrolysis plant 100, which is not shown in more detail in FIG. 1 for reasons of clarity and is explained in detail. Accordingly, a pro duct stream line 17 is specially provided for the outflow of the hydrogen product stream from the electrolyzer 1, with the help of which a second Product, namely the hydrogen obtained from electrolysis, is carried out. The hydrogen obtained is then treated in further components of the electrolysis system 100--not shown in greater detail in FIG. 1--and processed further in terms of process technology.

The electrolyzer 1 is connected to the gas separator 11 on the oxygen side via the product current line 15 . A vent line 19 is connected to the gas separator 11, via which the gas separator 11 can be completely emptied by depressurizing it if necessary, so that the water is depressurized or under atmospheric pressure. Furthermore, a compressed air system 21 comprising a gas tank 23 and an air compressor 25 is provided, so that the gas tank 23 can be supplied with compressed air from the air compressor 25 via the connecting line 27 . In this way, the gas tank 23 can be loaded with compressed air L (compressed air) and stored for other purposes. To supply the nitrogen system 3 with compressed air L, the compressed air system 21 is connected via a supply line 29a. Another consumption unit 31 is supplied with compressed air L via a supply line 29b.

During operation of the electrolyzer 1 in the system concept shown in FIG. 1, oxygen product gas is fed to the gas separator 11 .

In order to measure the quality of the oxygen product gas, the concentration of hydrogen in the oxygen product gas in the gas separator 11 is continuously measured and monitored in this operating concept. If the concentration exceeds a certain limit value before, the operation of the electrolyzer 1 is stopped and the entire oxygen product gas in the gas separator 11 is discarded. The oxygen product gas is completely drained from the container volume of the gas separator 11 and any supply lines of the oxygen-side gas system. to let. For this purpose, the gas separator 11 is completely vented via the vent line 19 and depressurized. The entire gas system including gas separator 11 is then flushed ßend by a complex and costly flushing procedure for inerting with nitrogen from a nitrogen tank 5 in the nitrogen system 3 of the electrolysis system 100. The nitrogen system 3 must be designed with a correspondingly large volume for this safety-relevant need for nitrogen in order to provide sufficient nitrogen. After the cause of the critical quality of the oxygen product gas has been eliminated, electrolysis is restarted. By flushing the gas system with nitrogen and in particular in the gas separator 11, the newly produced oxygen product gas must first be discarded until the desired gas quality is achieved again.

Due to the depressurization or complete venting on the oxygen side in the gas separator 11 for the flushing procedure with nitrogen, the entire valuable hydrogen product gas must also be removed on the hydrogen side in a corresponding gas separator--not shown in detail in FIG. 1 discarded, in particular to counteract the high differential pressure caused by venting and to avoid system damage over the PEM membrane. It is therefore ne ben the provision of a large-volume nitrogen system 3 for inerting and sufficient nitrogen storage ge straight and the associated discarding of the generated hydrogen product gas on the hydrogen side from an economic point of view very particularly disadvantageous.

2 shows an electrolysis system 1 with a compressed air system according to the invention. The new operating concept of the invention starts with an advantageous integration of the compressed air system 21 and design of the same as an inert gas system, which is connected to the gas separator 11 on the oxygen side. Compared to an embodiment of the electrolysis system 100 according to FIG. 1, the flushing line 9, which connects the nitrogen system 3 to the gas separator 3 for rendering the entire gas system inert with nitrogen, is omitted. According to FIG. 2, the compressed air system 21 is connected to the gas separator 11 via the supply line 37, so that compressed air L can be removed from the gas tank 23 as required. The gas tank 23 is a store for storing and providing compressed air.

The compressed air system 21 has an air compressor 25 and a gas tank 23 which are connected to one another via a connecting line 27 . The air compressor 25 is designed as an oil-free compressor. In the supply line 37, a cleaning device 33 is connected, which has an absorption agent and/or an adsorbent. In this way, harmful foreign gas components can be removed from the compressed air L stored in the gas container 23 and an inert or diluent gas of very high quality and purity is produced. For example, carbon dioxide and/or sulfur dioxide can be removed from the compressed air L by means of the cleaning device 33 . The heat energy released by adsorption or absorption can be used for other purposes by cooling the cleaning device 33, for example in a heat exchange process by coupling a heat exchanger. A control valve 35 is connected in the flow direction of the compressed air L downstream of the cleaning device 33 in the supply line 37, the valve position of which can be regulated by a control device (not shown in detail) to the requirement and the pressure level of compressed air L for the supply to the gas separator 11. Downstream of the control valve 35, the supply line 37 opens into the gas separator 11. During operation of the electrolysis system 100, the foreign gas concentration of hydrogen in the oxygen product gas from the electrolyzer 1 in the oxygen-side downstream gas separator 11 is continuously measured and monitored. If the measured value shows a critical foreign gas concentration of hydrogen above a predetermined limit value for a still permissible hydrogen content in the oxygen product gas in the gas separator 11, the hydrogen concentration is reduced in that the oxygen product gas is treated with cleaned compressed air L of good quality and purity, that is, with all if very small impurities or harmful foreign gas components is supplied in a targeted and well-dosed manner via the control valve 35. This targeted supply then causes an intimate mixture of the gases in the gas separator 11, as a result of which the hydrogen concentration is reduced. The hydrogen concentration is already reduced because of the effect of the gas phase mixture and the dilution of the hydrogen in the oxygen product gas by metering in compressed air L, an effect that is used particularly advantageously and specifically in the invention. Thus, on the one hand, discarding of the oxygen product gas in the gas separator 11 is avoided, 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 restarting the electrolytic system 1. So if a quality measurement in the gas separator 11 indicates poor quality, the electrolysis process is stopped. Instead of now discarding the gas in the gas separator 11 , cleaned compressed air L is removed from the gas tank 23 and fed to the gas separator 11 . This supply takes place by activating and opening a valve control for the control valve 35 until the gas quality meets the requirements again, i.e. the water Substance concentration is less than the predetermined limit 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 the associated pressure relief on the oxygen side in the gas separator 11 are eliminated. the entire valuable hydrogen product gas can advantageously also be used on the hydrogen side in a corresponding gas separator or gas separator on the hydrogen side--not shown in detail in FIG. Up to now, venting was also required on the hydrogen side to counteract the high differential pressure caused by venting and to avoid system damage across the PEM membrane due to excessive differential pressure. The associated use of the generated hydrogen product gas on the hydrogen side is particularly advantageous both from an operational point of view and from an economic point of view.

Another economic advantage is that the nitrogen system 3 can be made significantly more compact for the electrolysis system 100 using the proposed system concept according to FIG. 2 compared to the concept according to FIG. The corresponding parts of the system, in particular the nitrogen tank 5, can be made smaller.

With the operating concept of the invention, it is sufficient merely to provide the continuous nitrogen consumption for the so-called compressor operation of the system.

Claims

patent claims

1. A method for operating an electrolysis system (100) for generating hydrogen and oxygen as product gases, in which the oxygen product gas from an electrolyzer (1), which also contains hydrogen as a foreign gas, is fed to a downstream gas separator (11), wherein when a predetermined limit 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) the mixture of gases dilutes the hydrogen in the oxygen product gas is effected, whereby the hydrogen concentration in the oxygen product gas is lowered.

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

3. The method according to claim 1 or 2, wherein the hydrogen concentration is measured and monitored in situ in the gas separator (11).

4. The method according to any one of claims 1, 2 or 3, wherein the compressed air (L) is taken from a pressurized gas container (23) and fed to the gas separator (11).

5. The method as claimed in claim 4, in which air is sucked in at atmospheric pressure and compressed to form compressed air (L), and the gas container (23) is charged with compressed air (L).

6. The method as claimed in one of the preceding claims, in which the compressed air (L) is freed from water-soluble foreign components such as carbon dioxide (CO2) and/or sulfur dioxide (SO2) in a cleaning step.

7. The method as claimed in claim 6, in which, in the cleaning step, the compressed air (L) is brought into contact with an adsorbent and/or an absorbent, so that the water-soluble foreign matter is separated from the compressed air (L) and bound, with compressed air ( L) high purity is obtained.

8. The method according to any one of the preceding claims, in which a PEM electrolyzer is used as the electrolyzer (1), 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 according to any one of the preceding claims, in which the production of hydrogen and oxygen in the electrolyzer (1) is stopped as required.

10. Electrolysis system (100) comprising an electrolyzer (1) for generating hydrogen and oxygen as product gases, in which the oxygen product gas also contains hydrogen as a foreign gas, and a compressed air system (21) with a gas tank (23) for Storage of compressed air (L), wherein the electrolyzer (1) via a product current line (15) for the oxygen product gas to a gas separator (11) is connected, and wherein the compressed air system (21) via a supply line (37) is connected to the gas separator (11), so that compressed air (L) from the gas container (23) can be supplied to the gas separator (11) as required. a

11. Electrolysis system (100) according to claim 10, comprising a valve (35) connected in the supply line (37), in particular a special control valve.

12. Electrolysis system (100) according to claim 10 or 11, with a cleaning system connected in the supply line (37) device (33) for the compressed air (L), so that foreign components can be separated from the compressed air (L).

13. Electrolysis system (100) according to claim 12, wherein the cleaning device (33) has an adsorbent and/or an absorbent by means of which foreign components from the compressed air (L) can be adsorbed and/or sorbed.

14. electrolysis system (100) according to any one of claims 10 to

13, comprising a compressor (25) to which the gas container (23) is connected via a connecting line (27), so that compressed air as compressed air (L) can be fed to the gas container (23).

15. Electrolysis system (100) according to claim 14, wherein the compressor (25) is designed as an oil-lubricated air compressor, which is followed by an oil filter.

16. electrolysis system (100) according to claim 14, wherein the

Compressor (25) is designed as an oil-free air compressor.