Electrolysis system, electrolysis plant and method for producing synthesis gas

Abstract

The present invention relates to an electrolysis system (10), comprising: - an electrolysis cell stack (100) with a cathode portion (110) which has a cathode supply portion (112) and a cathode discharge portion (114), and with an anode portion (120) which has an anode supply portion (122) and an anode discharge portion (124), - an anode gas port (202) fluidically coupled to the anode supply portion (112) by means of an anode supply connection (200) for supplying anode gas to the anode portion (120), - an anode discharge port (308) fluidically coupled to the anode discharge portion (124) by means of an anode discharge connection (300) for discharging anode exhaust gases produced by the electrolysis cell stack (100), - a cathode supply port (502) fluidically coupled to the cathode supply portion (112) by means of a cathode supply connection (500) for supplying cathode gas to the cathode portion (110), and - a cathode discharge port (612) fluidically coupled to the cathode discharge portion (114) by means of a cathode discharge connection (600) for discharging synthesis gas produced by the electrolysis cell stack (100).

Description

[0001] Electrolysis system, electrolysis plant and process for producing synthesis gas

[0002] The present invention relates to an electrolysis system, an electrolysis plant and a method for producing synthesis gas by means of an electrolysis system.

[0003] One way to reduce dependence on fossil fuels and CO2 emissions is to replace crude oil with synthetic hydrocarbons produced from carbon dioxide (CO2) and water (H2O). By applying electrical current, high-temperature electrolysis (SOE for "Solid Oxide Electrolysis") can produce a synthesis gas containing hydrogen (H2) and carbon monoxide (CO). In a subsequent synthesis process, the synthetic hydrocarbons are obtained from the synthesis gas.

[0004] The object of the present invention is to increase the efficiency of the high-temperature electrolysis described above in a cost-effective and simple manner.

[0005] The above object is achieved by an electrolysis system having the features of claim 1, an electrolysis plant having the features of claim 13, and a method having the features of claim 14. Further features and details of the invention emerge from the subclaims, the description, and the drawings. Features and details described in connection with the electrolysis system according to the invention naturally also apply in connection with the electrolysis plant according to the invention and the method according to the invention, and vice versa, so that with regard to the disclosure of the individual aspects of the invention, reference is or can always be made to each other.

[0006] According to the invention, an electrolysis system is provided. The electrolysis system comprises an electrolysis cell stack with a cathode section, which has a cathode supply section and a cathode discharge section, and an anode section, which has an anode supply section and an anode discharge section. Furthermore, the electrolysis system comprises an anode gas connection, fluidly coupled to the anode supply section by means of an anode supply connection, for supplying anode gas to the anode section. Furthermore, the electrolysis system comprises an anode discharge connection, fluidly coupled to the anode discharge section by means of an anode discharge connection, for discharging anode exhaust gases generated by the electrolysis cell stack.The electrolysis system further comprises a cathode supply connection, fluidly coupled to the cathode supply section by means of a cathode supply connection, for supplying cathode gas to the cathode section. Furthermore, the electrolysis system comprises a cathode discharge connection, fluidly coupled to the cathode discharge section by means of a cathode discharge connection, for discharging synthesis gas generated by the electrolysis cell stack. The electrolysis system also comprises a residual gas supply connection for providing residual gas that is separated from the synthesis gas generated by the electrolysis cell stack during a subsequent synthesis process for producing synthetic hydrocarbons. The electrolysis system further comprises at least one catalyst, fluidly coupled to the residual gas supply connection by means of a residual gas supply connection and arranged in the anode discharge connection, for catalytically combusting the residual gas.The electrolysis system also comprises at least one heat exchanger, which is arranged in the residual gas supply connection downstream of the catalyst discharge section in the flow direction and is connected to the anode supply connection and / or the cathode supply connection for heat transfer. Furthermore, the residual gas supply connection is fluidly connected to the anode supply connection and / or the cathode supply connection downstream of the at least one heat exchanger for recirculating the catalytically combusted residual gas into the anode supply connection and / or the cathode supply connection.

[0007] A key advantage of the present invention is the dual utilization of the residual gas, which can also be referred to as tail gas. A first utilization of the residual gas occurs through catalytic combustion and the associated thermal conversion of at least a portion of the chemical energy content of the residual gas. The heated residual gas can then transfer this thermal energy in the at least one heat exchanger to other process gases, in particular an anode feed gas and / or a cathode feed gas, and in this way increase the energetic efficiency during operation of the electrolysis system. The further utilization of the residual gas relates to its chemical composition. Since the residual gas usually contains carbon directly as carbon dioxide and / or as short-chain hydrocarbons, in the present invention this carbon is fed back into the electrolysis cell stack.In other words, the residual gas was converted again using the electrolysis cell stack. In addition to increasing conversion efficiency, this also reduces the need for externally supplied carbon dioxide.

[0008] In summary, an electrolysis system according to the invention thus brings about an increase in both energetic and chemical efficiency. Synthesis gas can be produced with reduced energy consumption and a reduced amount of carbon dioxide. A further side effect of the multiple use of residual gas is the reduction of exhaust gases from the synthesis plant. The amount of residual gas that cannot be used as useful gas from the synthesis plant therefore does not need to be post-treated and released into the environment. Rather, by recirculating the residual gas into the electrolysis system, the amount of exhaust gas is reduced by this recirculation amount. In addition to the increase in efficiency, a reduction in exhaust gases is also achieved.

[0009] According to the invention, the efficiency of an electrolysis system is increased by utilizing residual gas from the synthesis process for producing synthetic hydrocarbons for catalytic combustion, in particular within two catalysts of the electrolysis system. The heat obtained during the catalytic combustion is utilized within the electrolysis system by at least one heat exchanger. The additional heat can be provided at various locations, in particular connections, very special feed connections, such as in particular the anode feed connection and / or cathode feed connection, in the electrolysis system, thus increasing the efficiency of the high-temperature electrolysis, in particular high-temperature co-electrolysis, which is carried out by the electrolysis cell stack to produce the synthetic gas or, in other words, synthesis gas.Compared to the use of a single catalyst and parallel heat exchangers, the use of two catalysts has the particular advantage that a second heat exchanger and a third heat exchanger can be supplied with the same amount of heat at lower catalyst temperatures. Higher temperatures can also be achieved at the second heat exchanger and the third heat exchanger at the same catalyst temperatures. These and other advantages of the invention are explained in more detail below.

[0010] For the sake of simplicity, the present description refers to an electrolysis cell stack. This means at least one electrolysis cell stack. Of course, it is possible for several electrolysis cell stacks to be provided in the electrolysis system, which can be interconnected in any desired manner, e.g., connected in series or in parallel. In this case, each cathode section and each anode section of each electrolysis cell stack is fluidly coupled to the connections mentioned herein in the manner described herein.

[0011] The electrolysis cell stack can, in particular, be a solid oxide electrolysis cell stack. Thus, the electrolysis system can, in particular, be a solid oxide electrolysis system or solid oxide electrolyzer cell system (also known as an SOEC system). The electrolysis cell stack in the electrolysis system, which is preferably designed as a co-electrolysis system, can be operated in such a way that the electrolysis of water (H2O) and carbon dioxide (CO2) is achieved. The electrolytes in the electrolysis cell stack can thus produce hydrogen gas (H2), carbon monoxide (CO), and oxygen (O2). It is advantageous if the electrolysis cell stack is connected to a power source for providing electricity from a renewable energy source to generate the synthesis gas.With such a power source, powered by renewable energy sources, high-temperature electrolysis operations can be made ecologically sustainable. The electrolysis cell stack is particularly advantageous when designed as a co-SOEC stack with a co-SOEC system. Here, water and CO2 are electrolyzed simultaneously to generate syngas. This is then a mixture of CO2 and H2, which can be used in subsequent steps to produce various synthetic fuels and chemicals.

[0012] For the reaction described above, anode gas, in particular air, especially fresh air, or oxygen, is supplied to the anode section via the anode supply connection. Cathode gas, in particular carbon dioxide, is supplied to the cathode section via the cathode supply connection. The cathode supply connection can be connected to various carbon dioxide sources. For example, it is possible to extract carbon dioxide from the air, from biogas processes, from industrial exhaust gases, etc. Water can be supplied to the cathode supply section via a first additional supply connection for supplying water. For this purpose, the first additional supply connection, which can be fluidically coupled to the cathode supply connection or the cathode supply section by means of a first additional supply connection, can supply water, preferably in the form of steam, to the cathode supply section.Alternatively or additionally, the water in the electrolysis system can be evaporated into water vapor. The water vapor can be considered part of the cathode gas because it is supplied to the cathode supply section. Any protective gas supplied to the cathode supply connection can also be considered part of the cathode gas because it is supplied to the cathode supply section. From the anode discharge section, the anode exhaust gases are discharged via the anode discharge connection to the anode discharge connection. The anode exhaust gases discharged in the anode discharge connection include, in particular, exhaust air or oxygen discharged from the electrolysis system, especially oxygen-enriched air, as well as catalyst exhaust gases downstream of the catalysts, i.e., combustion products of the catalytic combustion of the residual gas mixed with the anode exhaust gas. These can, for example, be released into the environment from the anode discharge connection.From the cathode discharge section, the cathode offgas produced, which is synthesis gas containing, in particular, mainly hydrogen gas and carbon monoxide, is fed to the cathode discharge connection. This connection can be connected to a synthesis plant with a corresponding synthesis system in order to provide the synthetic gas for the production of synthetic hydrocarbons. During this synthesis process, typically not all of the synthesis gas can be converted and / or short-chain hydrocarbons are formed, which are separated. This separated gas portion is referred to herein as residual gas (also known as "tail gas"). It has been discovered that this residual gas has a high calorific value and can advantageously be used to provide heat in high-temperature electrolysis, thus increasing the efficiency of the electrolysis system in the manner according to the invention.

[0013] In order to distinguish components or elements of the same kind or type from one another, such as heat exchangers, shut-off devices, partial paths or bypass paths, the components or elements of the same kind or type mentioned in this description are numbered consecutively and are referred to as first component, second component, third component (or elements), etc., for example first heat exchanger, second heat exchanger, etc. This designation based on the numbering serves solely to distinguish the components or elements of the same kind or type mentioned herein and in no way represents a limitation of the scope of protection.For example, if a claim refers to a fourth component of a kind or type, this does not necessarily presuppose a first, second and third component of that kind or type, unless the first, second and third components of that kind or type are mentioned in a claim to which the claim in question refers back.

[0014] The connections mentioned herein are fluid-conducting, particularly gas-conducting, connections. The connections can be established via various paths or lines, such as pipes or hoses, which are each coupled to one another. Various flow-influencing devices, such as those mentioned herein, can be arranged in the connections.

[0015] To the extent that reference is made herein to the arrangement of a heat exchanger in one connection and the thermal coupling of the heat exchanger to another connection, these features are to be understood as synonymous due to the function of the heat exchanger. This is because the heat exchanger exchanges the heat of two flows in the respective connections, for example, in countercurrent. In this respect, the heat exchanger is actually arranged in each of the two connections, and the heat exchanger also thermally couples both connections.

[0016] Whenever reference is made herein to control or monitoring, particularly in connection with a shut-off device, this is understood to mean controlling and / or regulating. Even if this is not explicitly mentioned, corresponding control electronics and control devices beyond shut-off devices, such as flow meters, can be provided for controlling. The shut-off devices mentioned here serve at least to stop or allow the flow of the respective fluid, particularly gas, flowing through the connections. Controlling the flow rate is also possible depending on the design of the shut-off device used. The shut-off device can be designed in a variety of ways, for example as a valve, gate valve, shut-off cock, or butterfly valve.

[0017] It can be advantageous if, in an electrolysis system according to the invention, the residual gas supply connection has a residual gas bypass which, bypassing the at least one heat exchanger, is connected in fluid communication with the anode supply connection and / or the cathode supply connection. It can be advantageous if the heat generated by the catalytic combustion in the residual gas is not supplied separately via heat exchangers to the respective anode supply gas and / or cathode supply gas. Since operating losses are possible with heat exchangers and, in particular, a heating phase must be taken into account, the residual gas bypass can enable maximum heat input by directly mixing the heated residual gas with the anode supply gas or the cathode supply gas, particularly when maximum heat input into one of these supply gases is desired. In this mode of operation, particular attention must be paid to the gas composition of the residual gas.For example, if CO2 is present, such direct introduction into the air path is prevented. Such a residual gas bypass can be configured, for example, with the aid of a bypass valve to transfer a partial and / or the entire amount of heated residual gas into the respective feed gas. In other words, the catalytically combusted residual gas and the residual gas heated in this way are mixed directly with the respective feed gas stream via a residual gas bypass and, together with this anode feed gas or the cathode feed gas, enter the electrolysis cell stack as a mixed gas.

[0018] In an electrolysis system according to the preceding paragraph, it may be advantageous if the residual gas bypass has a bypass valve for controlling the direct fluid-communicating connection past the at least one heat exchanger. Preferably, the residual gas bypass is designed to form a bypass past any heat exchangers that may be present. The bypass valve can be designed as a pure control valve that can be switched between an open position and a closed position. However, a variable control valve is preferred in order to provide variable control, in particular in a quantitative manner, for the desired bypass amount through the residual gas bypass.As already explained in the previous paragraph, in this way the desired mixing temperature in the anode feed section, provided the residual gas is free of CO2, and / or cathode feed section and thus the temperature of the resulting mixed gas can be adjusted in a controlled manner.

[0019] Further advantages can be achieved if, in an electrolysis cell stack according to the invention, the residual gas supply connection forms the anode discharge connection at least in sections. While, in principle, the residual gas aftertreatment can be designed completely separately from all other anode exhaust gas and cathode exhaust gas connections, integration is possible in this embodiment. The residual gas supply connection is integrated into the anode discharge connection here, thus also separating this anode discharge connection. This makes it possible to combine anode exhaust gas from the anode section of the electrolysis cell stack with the residual gas. This mixing of anode exhaust gas and residual gas preferably takes place upstream of the at least one catalyst, so that a mixed gas of residual gas and anode exhaust gas is fed to the catalytic combustion here.Bypasses from the supply air, i.e. the anode supply section, which will be explained later, are also conceivable in principle, in particular in order to be able to adjust the stoichiometric conditions for the catalytic combustion in a controlled manner for an optimized catalytic combustion and thus an improved energetic conversion in the catalyst device.

[0020] In an electrolysis system according to the preceding paragraph, it may be advantageous if a branch section is arranged in the anode discharge connection downstream of the at least one heat exchanger and upstream of the fluid-communicating connection to the anode supply connection and / or to the cathode supply connection. This allows, depending on the actual chemical composition in the residual gas supply connection, a portion of the catalytically combusted residual gases or mixed gases to be discharged again via the anode discharge connection via the branch section. Here, too, it is possible to flexibly respond to the different chemical and thermal gas situations depending on the operating situation of the electrolysis system and to variably adjust the volume flow, in particular for the return to the anode supply section and / or the cathode supply section.This branch section is also preferably equipped with a branch valve, which is designed either quantitatively, for example in the form of a shut-off valve, or quantitatively in the form of a variable control valve. This allows the branch section to be easily opened and closed, or to specify a defined control of a volume flow through this branch section using the quantitative control valves.

[0021] It is advantageous if the two catalysts are coupled to different, splitting partial paths of the residual gas supply connection. This allows the residual gas flow to each of the two catalysts to be controlled. In other words, the amount of residual gas supplied to each of the two catalysts can be controlled. Consequently, the heat released to the heat exchanger located downstream of each catalyst can also be controlled.

[0022] For this purpose, a shut-off device can advantageously be arranged in at least one of the two partial paths. It is particularly advantageous to arrange a shut-off device in each of the partial paths. For example, a butterfly valve can be used as a shut-off device. This allows for simple yet precise control of the residual gas flow to the catalysts and thus the amount of heat generated by them through catalytic combustion.

[0023] It is also advantageous if each of the two subpaths is fluidly connected to the anode discharge connection upstream of a catalyst feed section of one of the two catalysts in order to mix the residual gas and the anode exhaust gas to form a residual gas-anode exhaust gas mixture. Consequently, the residual gas-anode exhaust gas mixture is introduced into the catalysts and catalytically combusted. This not only optimizes catalytic combustion but also allows the residual gas to be preheated for catalytic combustion using the warm anode exhaust gas from the electrolysis cell stack.

[0024] It can also advantageously be provided that a first catalyst of the two catalysts is arranged downstream of a second catalyst of the two catalysts in the flow direction of the anode discharge connection. Consequently, the first catalyst provides a first catalyst stage, and the second catalyst provides a second catalyst stage. Particularly in conjunction with the aforementioned subpaths, each catalyst stage can be individually controlled, for example, switched on or off, or the amount of residual gas supplied can be controlled, as required for optimal operation of the electrolysis system.Accordingly, one of the second and third heat exchangers may be arranged downstream of both catalysts in the anode discharge connection, and another of the second and third heat exchangers may be arranged downstream of only the second catalyst in the anode discharge connection.

[0025] It is also advantageous if one of the second and third heat exchangers is thermally coupled to the anode feed connection. This allows heat from the catalyst exhaust gases of one or both catalysts to be transferred to the anode feed connection, thus heating the anode gas arriving at the anode feed section to increase the efficiency of the high-temperature electrolysis.

[0026] Alternatively or additionally, it can be provided, and is advantageous, for one of the second and third heat exchangers to be thermally coupled to the cathode feed connection. This allows heat from the catalyst exhaust gases of one or both catalysts to be dissipated to the cathode feed connection, thus heating the cathode gas arriving at the cathode feed section to increase the efficiency of the high-temperature electrolysis. Preferably, one of the second and third heat exchangers is used for thermal coupling to the anode feed connection, and another for thermal coupling to the cathode feed connection.

[0027] It is also advantageous if a fourth heat exchanger is arranged downstream of the second and third heat exchangers in the anode discharge connection and is thermally coupled to a first auxiliary supply connection, which connects the cathode supply connection or the cathode supply section to a first auxiliary supply connection for supplying water or steam to the cathode supply section. This allows the residual heat still contained in the catalyst exhaust gas in the anode discharge connection after the heat exchange in the second and third heat exchangers to be utilized to heat the water or steam supplied at the first auxiliary supply connection, thereby further increasing the efficiency of the electrolysis system.

[0028] Additionally or alternatively, it can be provided, and is advantageous, if a fifth heat exchanger is arranged in the anode discharge connection downstream of the second and third heat exchangers in the flow direction and is thermally coupled to the anode supply connection. This allows the residual heat still contained in the exhaust gas in the anode discharge connection after the heat exchange in the second and third heat exchangers to be used to heat the air transported in the anode supply connection, thereby further increasing the efficiency of the electrolysis system.

[0029] Furthermore, it is advantageous if a first heat exchanger is arranged in the anode supply connection and is thermally coupled to the anode discharge connection upstream of the two catalysts in the direction of flow. This makes it possible, in particular in a first step, to use the heat of the catalyst exhaust gases with the anode exhaust gas, in particular the discharged air, from the anode discharge section to heat the anode gas, in particular the supplied air. In addition to heating the anode gas, this has the advantage that the anode exhaust gases of the anode section are cooled by the heat transfer, whereby the auto-ignition temperature of the residual gas-anode exhaust gas mixture is undershot, which is generated by mixing the residual gas and the anode exhaust gas downstream of the first heat exchanger in the direction of flow. This is because the anode exhaust gas is very oxygen-rich with approx.30% oxygen, because oxygen diffuses from the cathode section to the anode section in the electrolysis cell stack. Reducing the temperature below the autoignition temperature is beneficial in that it prevents excessive thermal stress on the components in the electrolysis system and ensures controlled combustion via the downstream catalyst.

[0030] It is advantageous if a third bypass path connects the anode supply connection upstream of the first heat exchanger in the direction of flow with the anode supply connection downstream of the first heat exchanger in the direction of flow, wherein a third shut-off device is arranged in the third bypass path bypassing the first heat exchanger and / or a fourth shut-off device is arranged in the anode supply connection downstream of a branch from the anode supply connection to the third bypass path and upstream of the first heat exchanger in the direction of flow. This makes it easy for the supplied air in the anode supply connection to bypass the first heat exchanger. Furthermore, this enables easy control of the temperature of the anode gas and the anode exhaust gas in the respective anode connection. A first heating device can advantageously be arranged in the third bypass path.The first heating device can, in particular, be an electric heater. This allows the temperature of the supplied anode gas to be increased even further to optimize the operation of the electrolysis cell stack.

[0031] Furthermore, the anode discharge connection is advantageously connected to the anode supply connection by means of a bypass path upstream of at least one of the two catalysts in the flow direction. In particular, bypass paths, namely a first bypass path and a second bypass path, can be provided upstream of each of the two catalysts in the flow direction, which are connected to the anode supply connection. In this way, in particular in addition to the mixing already taking place upstream of the catalysts, in which the oxygen-rich exhaust air of the anode exhaust gas from the anode discharge section is mixed with the residual gas, additional, cool air for catalytic combustion can be introduced into the catalyst and thus also cool the catalysts. In this case, a shut-off device is advantageously arranged in the bypass path. Particularly advantageously, a first shut-off device is arranged in the first bypass path and a second shut-off device is arranged in the second bypass path.This allows you to control the amount of additional air supplied.

[0032] It is also advantageous if at least one of the two catalysts is designed as an oxidation catalyst. In particular, both catalysts can be designed as oxidation catalysts. An oxidation catalyst can oxidize pollutants such as carbon monoxide and hydrocarbons, but it cannot reduce nitrogen oxides. With the help of an oxidation catalyst, not only can the energy contained in the residual gas be utilized in the form of heat, but the emissions of carbon monoxide, hydrocarbons, and even particulate matter from the residual gas can also be significantly reduced.

[0033] Finally, it is advantageous if the electrolysis system further comprises a first additional supply connection for providing heated steam, which is heated during cooling in the synthesis process during the conversion of the synthesis gas generated by the electrolysis cell stack. Accordingly, to optimize the efficiency of the electrolysis system, not only the residual gas from the synthesis process but also the heated steam generated during cooling during the synthesis process is utilized, thereby achieving a dual and synergistic efficiency optimization of the high-temperature electrolysis.

[0034] The present invention also relates to an electrolysis plant comprising an electrolysis system according to the invention and a synthesis system comprising a synthesis plant. The cathode discharge connection is fluidly coupled to the synthesis plant by means of a synthesis gas supply connection. The synthesis plant is also configured for synthesizing synthetic hydrocarbons from synthesis gas generated by the electrolysis cell stack and supplied via the synthesis gas supply connection. Finally, the synthesis plant is fluidly coupled to the residual gas supply connection by means of a residual gas discharge connection for providing residual gas.

[0035] Thus, an electrolysis plant according to the invention brings with it the same advantages as have been explained in detail with reference to the electrolysis system according to the invention.

[0036] The present invention also relates to a process for producing synthesis gas by means of an electrolysis system, in particular the electrolysis system according to the invention and furthermore very particularly by means of the electrolysis plant according to the invention, comprising the steps:

[0037] - Feeding residual gas separated from a synthesis process in which synthesis gas is converted into hydrocarbons to two catalysts of an electrolysis system,

[0038] - catalytic combustion of the residual gas using the two catalysts of the electrolysis system,

[0039] - transferring heat of a catalyst exhaust gas stream from the catalytic combustion of the two catalysts to an anode gas and / or a cathode gas by means of at least one heat exchanger,

[0040] - Supplying the anode gas, the cathode gas and electric current to an electrolysis cell stack of the electrolysis system,

[0041] - feeding the catalytically combusted residual gas into the anode feed section and / or the cathode feed section, and - generating the synthesis gas by means of the electrolysis cell stack from the supplied anode gas, cathode gas and electric current.

[0042] Thus, a method according to the invention brings with it the same advantages as have been explained in detail with reference to the electrolysis system according to the invention.

[0043] In particular, the electrolysis system according to the invention and / or the electrolysis plant according to the invention can be set up or designed to carry out the method according to the invention.

[0044] The anode gas is understood to mean the gas supplied to the anode section, in particular air or oxygen. This excludes the anode exhaust gas, i.e., the exhaust gas discharged from the anode section, in particular air and / or oxygen. The cathode gas is understood to mean the gas supplied to the cathode section, in particular carbon dioxide, water vapor, and / or a protective gas. This excludes the cathode exhaust gas, i.e., the synthetic gas discharged from the cathode section.

[0045] It has proven advantageous to divide the residual gas flow into two subpaths, and to feed residual gas to each of the two catalysts via one of each of the two subpaths. This allows the aforementioned control of residual gas flows to the catalysts and thus the heat transfer between the anode and cathode feed connections. In particular, essentially the same amount of heat can be advantageously transferred to both heat exchangers, and thus to the anode and cathode feed connections, by controlling the residual gas flow in the two subpaths accordingly.

[0046] For this purpose, it is advantageous if the catalyst exhaust gas flow in the two partial paths is controlled by means of a shut-off device in each of the two partial paths behind the respective heat exchanger of the respective partial path.

[0047] It is also advantageous if the catalyst exhaust stream from one of the two catalysts is fed to the other of the two catalysts via the anode discharge connection. This allows the catalyst exhaust stream from one catalyst to be used in terms of its heat quantity for the catalytic combustion of the other catalyst, thus maintaining high efficiency. Finally, it is preferred that the catalyst exhaust streams flow through a fourth heat exchanger for heating the water or steam supplied to the electrolysis system and / or through a fifth heat exchanger for heating the anode gas for further heat transfer. This makes it possible to utilize any residual heat still remaining in the catalyst exhaust streams from the catalysts to further increase the efficiency of the electrolysis system.

[0048] It is also advantageous if the residual gas upstream of the catalysts is mixed with the anode exhaust gas from the electrolysis cell stack to form a residual gas-anode exhaust gas mixture. The oxygen-rich air from the anode exhaust gas can thus raise the temperature of the residual gas-anode exhaust gas mixture and be used for controlled catalytic combustion.

[0049] It is advantageous if the anode exhaust gas transfers heat to the supplied anode gas via a first heat exchanger before mixing with the residual gas. This allows, on the one hand, the supplied anode gas to be heated with the air, and, on the other hand, the anode exhaust gas to be cooled with the air, in particular below the autoignition temperature of the residual gas-anode exhaust gas mixture.

[0050] It is also advantageous if anode gas is added to the residual gas / anode exhaust gas mixture. This can be achieved through the aforementioned bypass paths, in particular the first and second bypass paths. Thus, the air quantity in the residual gas / anode exhaust gas mixture can be further increased by anode gas containing fresh air.

[0051] It has been found to be advantageous if the residual gas / anode exhaust gas mixture has a temperature in the range of 300 to 550 °C, particularly in the range of 400 to 500 °C. This refers to the temperature at the catalyst feed sections of the catalysts. The greatest increase in efficiency in the production of synthesis gas has been observed in this temperature range.

[0052] It is advantageous if the catalyst exhaust gases from the catalytic combustion have a temperature in the range of 800 to 1,000 °C, particularly in the range of 850 to 950 °C. This refers to the temperature at the catalyst discharge sections of the catalysts. The greatest efficiency increase in the production of synthesis gas has been observed in this temperature range. The produced synthesis gas is advantageously fed into the synthesis process, from which the residual gas is separated and fed to the two catalysts.

[0053] It is also advantageous if the synthesis process is a Fischer-Tropsch process. The coupling of high-temperature electrolysis, in particular high-temperature co-electrolysis, and Fischer-Tropsch synthesis (FTS for short) has proven to be a particularly promising variant for the production of various hydrocarbons. In FTS, synthesis gas produced from high-temperature co-electrolysis is converted into hydrocarbon molecules with various chain lengths at comparatively moderate temperatures, particularly in the temperature range of 200 to 300 °C, and elevated pressures, particularly in the pressure range of 10 to 30 bar, with the help of a catalyst, particularly a Co- or Fe-based one. The FTS process is highly exothermic. To maintain the temperature within the specified temperature range, cooling can be carried out along the length of a reactor in the synthesis plant. Cooling can be achieved by water evaporation at the specified pressure level.The steam can subsequently be used for further process steps and, as previously mentioned, for the high-temperature electrolysis itself by adding the steam to the cathode gas. The hydrocarbon chain length distribution resulting from FTS is described by a chain growth probability (a high chain growth probability results in large molecules and thus a shift towards liquid fuels). However, the synthesis gas is not fully converted. Furthermore, depending on the chain growth probability, short-chain molecules are formed that cannot be used as liquid fuel. The unconverted synthesis gas and the resulting short-chain hydrocarbons can be separated as the residual gas during product processing. While some of the residual gas can be recirculated into the FTS, some must be discharged. In particular, the discharged portion of the residual gas is utilized in the process according to the invention.

[0054] Further advantages, features, and details of the invention will become apparent from the following description, in which exemplary embodiments are described in detail with reference to the drawings. They show schematically:

[0055] Fig. 1 shows a first embodiment of an electrolysis plant according to the invention, Fig. 2 shows a second embodiment of an electrolysis plant according to the invention, and

[0056] Fig. 3 shows a third embodiment of an electrolysis plant according to the invention.

[0057] Fig. 4 shows a further embodiment of an electrolysis plant according to the invention.

[0058] Identical or functionally equivalent elements are designated by the same reference numeral in Figures 1 to 4.

[0059] Figure 1 schematically shows an electrolysis plant 30 comprising an electrolysis system 10 with an electrolysis cell stack 100 and a synthesis system 20 with a synthesis plant 900. The electrolysis system 10 and the synthesis system 20 are fluidically coupled to one another, as will be explained in more detail later.

[0060] By way of example, only one electrolysis cell stack 100 is shown in Fig. 1. However, it is possible to provide multiple electrolysis cell stacks 100. The electrolysis cell stack 100 has a cathode section 110 with a cathode supply section 112 and a cathode discharge section 114. The electrolysis cell stack 100 also has an anode section 120 with an anode supply section 122 and an anode discharge section 124. A power supply source 130, which provides electricity from renewable energies, is connected to the electrolysis cell stack 100. The electrolysis cell stack 100 is presently designed as a solid oxide electrolysis cell stack and is used in reverse mode for high-temperature co-electrolysis.

[0061] Anode gas in the form of fresh air is provided to the electrolysis system 10 by means of an anode gas connection 202. The anode gas is supplied to the electrolysis cell stack 100 for electrolysis via an anode supply connection 200, which is fluidly coupled to the anode gas connection 202 and the anode supply section 122. A filter device 204, in particular in the form of an air filter, for air filtering and a fan 206 for transporting the anode gas are arranged in the anode supply connection 200. A first heat exchanger 224 is arranged in the anode supply connection 200 downstream of the filter device 204 and the fan 206 in the flow direction of the anode gas from the anode gas connection 202 to the anode supply section 122. The first heat exchanger 224 is used for heat exchange with a warm anode exhaust gas, in particular in the form of exhaust air discharged from the anode section 120, from the electrolysis cell stack 100.For this purpose, the first heat exchanger 224 is thermally coupled to an anode discharge connection 300 upstream of a second catalyst 418 in the form of an oxidation catalyst. The anode discharge connection 300 fluidically connects the anode discharge section 124 to an anode discharge connection 308.

[0062] The second catalyst 418 is arranged in the anode discharge connection 300 downstream of the first heat exchanger 224 in the flow direction of the anode exhaust gas and is fluidly coupled to a residual gas connection 402 by means of a residual gas supply connection 400. The residual gas connection 402 receives residual gas from the synthesis plant 900, as will be described in more detail later. The residual gas provided to a second catalyst supply section 420 of the second catalyst 418, which is mixed with the anode exhaust gas in the anode discharge connection 300 at a junction of the residual gas supply connection 400 with the anode discharge connection 300 to form a residual gas-anode exhaust gas mixture, is catalytically combusted by the second catalyst 418. Hot catalyst exhaust gases with a temperature in the range of 800 to 1000 °C, in particular around 950 °C, emerge from the second catalyst discharge section 422.

[0063] A second bypass path 212 with a second shut-off device 214 arranged therein connects the anode supply connection 200 in the flow direction of the anode gas upstream of the first heat exchanger 224 with the anode discharge connection 300 upstream of the second catalyst supply section 420 and thus allows the air content of the residual gas-anode exhaust gas mixture to be further increased before entering the second catalyst 418 and the residual gas-anode exhaust gas mixture to be further cooled.

[0064] In addition, a third bypass path 216 with a third shut-off device 218 is provided, which connects the anode supply connection 200 upstream of the first heat exchanger 224 in the flow direction of the anode gas with the anode supply connection 200 downstream of the first heat exchanger 224 in the flow direction of the anode gas, thereby making it possible to control the temperature of the residual gas-anode exhaust gas mixture upstream of the second catalyst 418 by regulating the amount of anode gas flowing through the first heat exchanger 224. Furthermore, a fourth shut-off device 222 is arranged upstream of the second heat exchanger 224 and downstream of the third bypass path 216 in the flow direction.

[0065] The hot catalyst exhaust gases of the second catalyst 418 flow from the second catalyst exhaust section 422 in the anode exhaust connection 300 through a second heat exchanger 302, which is thermally coupled to the anode supply connection 200. This allows the heat of the catalyst exhaust gas of the second catalyst 418 to be transferred to the anode gas upstream of the anode supply section 122.

[0066] In addition to the second catalyst 418, the electrolysis system 10 also includes a further catalyst 408, referred to herein as the first. The first catalyst 408 is arranged in the anode discharge connection 300 downstream of the second catalyst 418 in the flow direction, i.e., downstream of the second heat exchanger 302 and in the catalyst exhaust stream of the second catalyst 418. For this purpose, the residual gas supply connection 400 is divided into two individual sub-paths 404, 414, namely a first sub-path 404 and a second sub-path 414. A fifth shut-off device 406 is arranged in the first sub-path 404. A sixth shut-off device 416 is arranged in the second sub-path 414. In this respect, the amount of residual gas supplied to each of the two catalysts 408, 418 and thus the amount of heat released by the catalyst exhaust gases through catalytic combustion can be controlled by means of the shut-off devices 406, 416.As already explained above, upstream of the second catalyst feed section 420 of the second catalyst 418, a mixing of residual gas and anode exhaust gas occurs at the aforementioned junction point. Upstream of a first catalyst feed section 410 of the first catalyst, a corresponding mixing of residual gas and catalyst exhaust gases, which are also referred to herein as anode exhaust gas because they flow in the anode exhaust connection 300, also occurs at a junction point where the anode exhaust section 300 and the first partial path 404 meet, to form a mixture, also referred to herein as a residual gas-anode exhaust gas mixture.

[0067] A third heat exchanger 304 is located in the anode discharge connection 300 downstream of the first catalyst 408 and its first catalyst discharge section 412. The third heat exchanger 304 is thermally coupled to a cathode supply connection 500. The cathode supply connection 500 fluidically connects a cathode supply connection 502 to the cathode supply section 112. In the cathode supply connection 500, cathode gas, in particular carbon dioxide, is supplied from the cathode supply connection 502 to the cathode supply section 112. A seventh shut-off device 504 and an ejector 506 are arranged in the cathode supply connection 500 upstream of the cathode section 110 in the flow direction of the cathode gas. Furthermore, a second heating device 508, in the form of an electric heater, is arranged behind the ejector 506 in the flow direction of the cathode gas.The third heat exchanger 304 allows the cathode gas to be heated with the heat from the catalyst exhaust gas of one or both catalysts 408, 418.

[0068] A first bypass path 208 with a first shut-off device 210 arranged therein connects the anode supply connection 200 in the flow direction of the anode gas upstream of the first heat exchanger 224, and in particular upstream of the branch to the second bypass path 212, with the anode discharge connection 300 upstream of the first catalyst supply section 410 and thus allows the air content of the residual gas-anode exhaust gas mixture to be increased before entering the first catalyst 408 and the residual gas-anode exhaust gas mixture to be cooled.

[0069] With the above-described arrangement of catalysts 408, 418 in the electrolysis system 10, it is thus possible to operate only one or both catalysts 408, 418 together, the latter being preferred. Thus, the oxygen-rich exhaust air in the anode exhaust gas can be mixed with a first amount of residual gas, which can be controlled by the sixth shut-off device 416, for controlled catalytic combustion via the second catalyst 418, which in this respect functions as the first oxidation catalyst stage, to provide the temperature at the second heat exchanger 302, which functions as an air superheater for the air in the anode gas.The still oxygen-rich catalyst exhaust gas from the second catalyst 418 can again be mixed with a second amount of residual gas, which can be controlled by the fifth shut-off device 406, for controlled catalytic combustion via the first catalyst 408, which in this respect functions as a second oxidation catalyst stage, to provide the temperature at the third heat exchanger 304, which acts as a reactant superheater for the cathode gas. The two oxidation catalyst stages can ensure the same heat exchange at lower oxidation catalyst target temperatures (meaning the total mass flow through both heat exchangers 302, 304 and the same amount of enthalpy at lower temperatures), or vice versa, such as in a single-stage system with a parallel arrangement of the two heat exchangers 302, 304.Another advantage is that higher air and reactant temperatures can be achieved at the same oxidation catalyst target temperature.

[0070] As shown in Figure 1, this embodiment provides two parallel return options. One is a return to the anode feed section 200 and the other is a return to the cathode feed section 500. Embodiments with only one of these two return options are also conceivable within the scope of the present invention.

[0071] As an alternative to the arrangement shown in Fig. 1, the present interconnection of catalysts 408, 418 in the electrolysis system 10 can also be such that the third heat exchanger 304 is arranged in the catalyst exhaust stream of the second catalyst 418 and the second heat exchanger 302 is arranged in the catalyst exhaust stream of the first catalyst 408. In this respect, it is pointed out again that the designation of components or elements of the same type or kind here merely serves to distinguish them from one another and does not follow a technically necessary sequence or the like.

[0072] In the embodiment of Fig. 1, a fourth heat exchanger 306 is located in the anode discharge connection 300 downstream of the second heat exchanger 304 in the flow direction of the catalyst exhaust gas. The fourth heat exchanger 306 is thermally coupled to a first auxiliary supply connection 700, which fluidically connects a first auxiliary supply connection 702 to the cathode supply connection 500. Water or steam is provided from the first auxiliary supply connection 702 for the high-temperature co-electrolysis; this water or steam is heated by the fourth heat exchanger 306 and flows to the cathode supply connection 500.

[0073] By means of a cathode discharge connection 600, which fluidically connects the cathode discharge section 114 to a cathode discharge connection 612, cathode exhaust gas in the form of the synthesis gas generated by the high-temperature co-electrolysis, comprising hydrogen and carbon monoxide, is discharged to the synthesis system 20. Two heat exchangers 608, 610, namely a sixth heat exchanger 608 and a seventh heat exchanger 610, are thermally arranged in the cathode discharge connection 600 and thermally coupled to the cathode supply connection 500 in order to transfer heat from the synthesis gas to the cathode gas and thus increase the efficiency of the electrolysis system 10.

[0074] A fourth bypass path 602 leads from the cathode discharge section 600 to the ejector 506. In the fourth bypass path 602, a nozzle 604, in particular a Venturi nozzle, and an eighth shut-off device 606, in particular a valve, are arranged.

[0075] A second additional supply connection 800 fluidically connects a second additional supply connection 802 for supplying a protective gas to the cathode supply connection 500 in the flow direction of the cathode supply connection 500 upstream of the seventh shut-off device 504.

[0076] The electrolysis cell stack 100, supplied with anode gas comprising air, and cathode gas comprising carbon dioxide, water vapor, and protective gas in the manner described above, generates the cathode exhaust gas in the form of synthesis gas, comprising hydrogen and carbon monoxide, and the anode exhaust gas, comprising exhaust air, in reverse mode by high-temperature co-electrolysis. The anode exhaust gas is catalytically combusted by the two catalysts 408, 418 together with residual gas, so that catalyst exhaust gases are separated from the electrolysis system 10 at the anode discharge connection 308.

[0077] The synthesis gas is provided to the synthesis system 900 of the synthesis system 20 via a synthesis gas supply connection 906, which fluidically connects a synthesis supply section 902 of the synthesis system 900 to the cathode discharge connection 612. In a reactor there (not explicitly shown), it undergoes a synthesis process, in particular a Fischer-Tropsch synthesis process, and is converted into synthetic hydrocarbons. The hydrocarbons are discharged via a hydrocarbon discharge connection 908 fluidically connected to a synthesis discharge section 904. However, synthesis gas not converted during the synthesis process and short-chain hydrocarbons remain, some of which can be returned to the synthesis process and some of which can be discharged as residual gases to the residual gas supply connection 402 via a residual gas discharge connection 910, which are fluidically coupled to one another in this respect.Figure 2 shows a modification of the embodiment of the electrolysis system 30 shown in Fig. 1. Specifically, the fourth heat exchanger 306 was omitted in Fig. 2. Instead, a fifth heat exchanger 310 was inserted in the anode discharge connection 300, downstream of the two heat exchangers 302, 304, which is thermally coupled to the anode supply connection 200, particularly downstream of the blower 206 and upstream of the first heat exchanger 224, in the flow direction of the anode gas. This allows the residual heat in the anode exhaust gas or the catalyst exhaust gas to be alternatively provided for the anode gas. However, it is of course also possible to provide both the fourth heat exchanger 306 and the fifth heat exchanger 310, either in series or in parallel with appropriate shut-off devices and bypass paths.

[0078] Furthermore, any configuration of the heat exchangers 224, 302, 304, 306, 310, 608, 610 shown is possible, which means that these heat exchangers can be used individually or in any selection thereof in the electrolysis system 10, so that it is not necessary to equip the electrolysis system 10 with all heat exchangers 224, 302, 304, 306, 310, 608, 610.

[0079] Figure 3 shows a variation of the electrolysis system 30 of the embodiment of Figure 1, in which residual gas bypasses 430 are provided, which introduce the residual gas in the catalytically combusted form past the respective heat exchangers 304, 302 into the anode feed section 200, provided the residual gas is free of CO2, and / or the cathode feed section 500. Furthermore, the variant of Figure 3 shows the possibility of separating the anode exhaust gas and the residual gas. Thus, downstream of the heat exchanger 224, the anode exhaust gas can be fed directly to the anode discharge connection 308. By means of a branch section 440, it is possible to transfer the residual gas to the anode discharge connection 300.

[0080] Figure 4 shows an alternative embodiment. This is a variant with the first heat exchanger 224 in a separate path of the anode supply connection 200. This separate path can be switched qualitatively and / or quantitatively as a bypass using valves (not shown). In this way, the controllability, in particular the adjustability of the operating mode, can be further improved. This embodiment can also be applied to all other variants, in particular those of Figures 1 to 3. The above explanations of the embodiments describe the present invention exclusively within the framework of examples.

[0081] List of reference symbols

[0082] 10 Electrolysis system

[0083] 20 Synthesis system

[0084] 30 electrolysis plant

[0085] 100 electrolysis cell stacks

[0086] 110 Cathode section

[0087] 112 Cathode feed section

[0088] 114 Cathode discharge section

[0089] 120 anode section

[0090] 122 Anode feed section

[0091] 124 Anode discharge section

[0092] 130 Power source

[0093] 200 Anode feed connection

[0094] 202 Anode feed connection

[0095] 204 Filter device

[0096] 206 blowers

[0097] 208 first bypass path

[0098] 210 first shut-off device

[0099] 212 second bypass path

[0100] 214 second shut-off device

[0101] 216 third bypass path

[0102] 218 third shut-off device

[0103] 220 first heating device

[0104] 222 fourth shut-off device

[0105] 224 first heat exchanger

[0106] 300 anode discharge connection

[0107] 302 second heat exchanger

[0108] 304 third heat exchanger

[0109] 306 fourth heat exchanger

[0110] 308 anode discharge connection

[0111] 310 fifth heat exchanger

[0112] 400 residual gas supply connection

[0113] 402 Residual gas supply connection

[0114] 404 first sub-path

[0115] 406 fifth shut-off device first catalyst first catalyst feed section first catalyst discharge section second partial path sixth shut-off device second catalyst second catalyst feed section second catalyst discharge section residual gas bypass

[0116] Branch section Cathode supply connection Cathode supply connection Seventh shut-off device Ejector Second heater Cathode discharge connection Fourth bypass path

[0117] Nozzle eighth shut-off device sixth heat exchanger seventh heat exchanger cathode discharge connection first additional feed connection first additional feed connection second additional feed connection second additional feed connection synthesis plant

[0118] Synthesis feed section Synthesis discharge section Synthesis gas feed connection Hydrocarbon discharge connection Residual gas discharge connection

Claims

Patent claims 1. Electrolysis system (10), comprising: - an electrolytic cell stack (100) with a cathode section (110) having a cathode supply section (112) and a cathode discharge section (114), and an anode section (120) having an anode supply section (122) and an anode discharge section (124), - an anode gas connection (202) fluidly coupled to the anode supply section (112) by means of an anode supply connection (200) for supplying anode gas to the anode section (120), - an anode discharge connection (308) fluidly coupled to the anode discharge section (124) by means of an anode discharge connection (300) for discharging anode exhaust gases generated by the electrolysis cell stack (100), - a cathode supply connection (502) fluidly coupled to the cathode supply section (112) by means of a cathode supply connection (500) for supplying cathode gas to the cathode section (110), and - a cathode discharge connection (612) fluidly coupled to the cathode discharge section (114) by means of a cathode discharge connection (600) for discharging synthesis gas generated by the electrolysis cell stack (100), characterized in that the electrolysis system (10) further comprises: - a residual gas supply connection (402) for providing residual gas which is separated from synthesis gas generated in the electrolysis cell stack (100) in a subsequent synthesis process for producing synthetic hydrocarbons, - at least one catalyst (408, 418) fluidically coupled to the residual gas supply connection (402) by means of a residual gas supply connection (400) for the catalytic combustion of the residual gas, - at least one heat exchanger (302, 304, 306, 310) which is arranged in the residual gas supply connection (400) downstream of the catalyst discharge section (412, 422) in the flow direction and is connected to the anode supply connection (200) and / or the cathode supply connection (500) in a heat-transfer manner, - wherein the residual gas supply connection (400) downstream of the at least one heat exchanger (302, 304, 306, 310) is fluidly connected to the anode supply connection (200) and / or the cathode supply connection (500) for returning the catalytically combusted residual gas to the anode supply connection (200) and / or the cathode supply connection (500).

2. Electrolysis system (10) according to claim 1, characterized in that the residual gas supply connection (400) has a residual gas bypass (430) which is connected to the anode supply connection (200) and / or the cathode supply connection (500) in fluid communication past the at least one heat exchanger (302, 304, 306, 310, 314, 320).

3. Electrolysis system (10) according to claim 2, characterized in that the residual gas bypass (430) has a bypass valve for controlling the direct fluid-communicating connection past the at least one heat exchanger (302, 304, 306, 310).

4. Electrolysis system (10) according to one of the preceding claims, characterized in that the residual gas supply connection (400) forms the anode discharge connection (300) at least in sections.

5. Electrolysis system (10) according to claim 4, characterized in that downstream of the at least one heat exchanger (302, 304, 306, 310) and upstream of the fluid-communicating connection to the anode supply connection (200) and / or to the cathode supply connection (500), a branch section (440) is arranged in the anode discharge connection (300), wherein the branch section (440) branches off from the anode discharge connection (300) downstream of the first heat exchanger (224).

6. Electrolysis system (10) according to one of the preceding claims 4 or 5, characterized in that it further comprises: - two catalysts (408, 418) for the catalytic combustion of the residual gas, which are fluidically coupled to the residual gas supply connection (402) by means of a residual gas supply connection (400) and arranged in the anode discharge connection (300), and - a second heat exchanger (302) and a third heat exchanger (304) which are arranged in the anode discharge connection (300) in the flow direction behind at least one of the two catalysts (408, 418).

7. Electrolysis system (10) according to claim 6, characterized in that the two catalysts (408, 418) are coupled to different, dividing partial paths (404, 414) of the residual gas supply connection (400).

8. Electrolysis system (10) according to claim 6 or 7, characterized in that a first catalyst (408) of the two catalysts (408, 418) is arranged in the flow direction of the anode discharge connection (300) behind a second catalyst (418) of the two catalysts (408, 418).

9. Electrolysis system (10) according to one of claims 6 to 8, characterized in that one of the second and third heat exchangers (302, 304) is thermally coupled to the anode supply connection (200).

10. Electrolysis system (10) according to one of claims 6 to 9, characterized in that one of the second and third heat exchangers (302, 304) is thermally coupled to the cathode supply connection (500).

11. Electrolysis system (10) according to one of claims 6 to 10, characterized in that a fourth heat exchanger (306) is arranged in the flow direction in the anode discharge connection (300) behind the second and third heat exchangers (302, 304) and is thermally coupled to a first additional feed connection (700) which connects the cathode feed connection (500) or the cathode feed section (112) with a first additional feed connection (702) for supplying water or steam to the cathode supply section (112).

12. Electrolysis system (10) according to one of claims 6 to 11, characterized in that a fifth heat exchanger (310) is arranged in the flow direction in the anode discharge connection (300) behind the second and third heat exchangers (302, 304) and is thermally coupled to the anode supply connection (200).

13. Electrolysis plant (30) with an electrolysis system (10) according to one of the preceding claims and a synthesis system (20) with a synthesis plant (900), wherein - the cathode discharge connection (612) is fluidically coupled to the synthesis plant (900) by means of a synthesis gas supply connection (906), - the synthesis plant (900) is designed to synthesize synthetic hydrocarbons from synthesis gas generated by the electrolysis cell stack (100) and supplied by means of the synthesis gas supply connection (906), and - the synthesis plant (900) is fluidically coupled to a residual gas supply connection (402) by means of a residual gas discharge connection (910) for providing residual gas.

14. A method for producing synthesis gas by means of an electrolysis system (10) comprising the steps: - feeding residual gas separated from a synthesis process in which synthesis gas is converted into hydrocarbons to a catalyst (404) of an electrolysis system (10), - catalytic combustion of the residual gas by means of a catalyst (408, 418) of the electrolysis system (10), - transferring heat of a catalyst exhaust gas stream of the catalytic combustion by means of at least one heat exchanger (302, 304, 306, 310) to an anode gas and / or a cathode gas, - supplying the anode gas, the cathode gas and electric current to an electrolysis cell stack (100) of the electrolysis system (10), - feeding the catalytically combusted residual gas into the anode feed section (200) and / or the cathode feed section (500), and - generating the synthesis gas by means of the electrolysis cell stack (100) from the supplied anode gas, cathode gas and electric current.

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