Method and device for operating an electrolysis system

Abstract

The invention relates to a method for operating an electrolysis system (10) which has at least one electrolyzer stack (100), with an air side (120) and a reactant side (130), and different operating situations. The method has the steps of detecting the operating situation of the electrolysis system (10) and controlling the electrolysis system (10) on the basis of the detected operating situation. According to the method, residual gas (RG), having a composition such as that resulting from a synthesis process in which synthesis gas (SG) is converted into hydrocarbons in a synthesis installation (900), is guided to a catalyzer (411, 412) in order to catalytically combust the residual gas (RG). In the process, the residual gas (RG) is selectively supplied at least from a residual gas reservoir (443) of the electrolysis system (10) or from the synthesis installation (900) on the basis of the detected operating situation. The invention also relates to a computer program product, to a control device (20) for carrying out the method, and to a correspondingly equipped electrolysis system (10).

Description

[0001] Method and device for operating an electrolysis system

[0002] The present invention relates to a method for operating an electrolysis system, a computer program product, and a control device for carrying out such a method. The invention further relates to an electrolysis system.

[0003] Replacing crude oil with synthetically produced hydrocarbons represents a way to reduce dependence on fossil fuels and emissions in energy production. Synthetic hydrocarbons can be produced from synthesis gas, which is generated in a fuel cell through the electrolysis of carbon dioxide and water with the addition of electricity. The synthesis gas produced in this way is often referred to as "syngas" and contains, in particular, hydrogen and carbon monoxide. The synthetic hydrocarbons are obtained from the synthesis gas in a synthesis process following electrolysis, such as the Fischer-Tropsch process. High-temperature electrolysis is the preferred method for electrolysis, for which solid oxide electrolyzer cells (SOECs) are preferably used.These are solid oxide fuel cells (SOFCs) that operate in reverse mode to electrolyze water and carbon dioxide. Like solid oxide fuel cells, solid oxide electrolyzer cells operate particularly efficiently at temperatures between 500°C and 900°C.

[0004] It is known from the prior art to operate electrolysis systems in different operating states, such as a resting state or an active operating state. Due to the high temperatures required for high-temperature electrolysis, the intermediate states that occur between a resting state and an active operating state pose particular technical challenges for an operating strategy for controlling the electrolysis system. Startup and shutdown processes, in particular, must be designed with consideration for temperature-sensitive components and system safety. Disadvantages of existing operating strategies are that the startup and shutdown processes implemented therein are inflexible, take a relatively long time, and require a lot of energy. The components of the electrolysis system are also subjected to relatively high thermal and mechanical stress. The applicant has investigated solutions for improving high-temperature electrolysis systems.One solution identified here pursues the approach of providing the heat required for start-up and shutdown processes using electric air heaters. In comparison, the heat during active operation of the high-temperature electrolysis system can be obtained by utilizing residual gas, which is produced as a byproduct of synthesis processes. Such residual gas is often referred to as "tail gas."

[0005] This approach poses technical challenges to the operating strategy of the electrolysis system. For example, the air heaters can often only provide a sufficient amount of heat after a prolonged heat-up period. This can lead to delays in operational processes. In particular, the electrolysis system cannot respond flexibly to a sudden request to interrupt or terminate synthesis gas production. Instead, after such a request, the air heaters must wait until they have reached sufficient heat-up temperatures before shutting down operations can be initiated. This not only makes the operation of the electrolysis system inflexible but can also be associated with relatively high energy consumption. Overall, this can lead to a reduction in the efficiency of the electrolysis system. Furthermore, the electrolysis system must be designed for these interruption situations.This increases the complexity and cost of the electrolysis system. Furthermore, the start-up and run-back processes of the fuel cell system can often only be controlled imprecisely. This makes it difficult to reliably maintain temperature gradients of system components within acceptable ranges during warm-up or cool-down.

[0006] It is therefore an object of the present invention to at least partially remedy the disadvantages described above. In particular, the object of the present invention is to improve the control of start-up and run-back processes of an electrolysis system, particularly with regard to permissible temperature gradients.

[0007] The above object is achieved by a method having the features of claim 1, a computer program product having the features of claim 11, an electrolysis system having the features of claim 12 and a control device having the features of claim 15. Further advantages and features of the invention emerge from the subclaims, the description and the drawings. Features and details described in connection with the method according to the invention naturally also apply in connection with the computer program product according to the invention, with the control device according to the invention and the electrolysis system according to the invention and vice versa, so that with regard to the disclosure of individual aspects of the invention, reference is always made or can be made reciprocal.

[0008] One aspect of the invention relates to a method for operating an electrolysis system. The electrolysis system has at least one electrolyzer stack with an air side, to which air can be supplied via an air supply path. The electrolyzer stack further has a reactant side, to which a reactant can be supplied via a reactant supply path. The electrolysis system has various operating situations, which include at least a rest operating situation, a normal operation operating situation, a start-up operating situation, and a return operating situation. In the rest operating situation, the electrolysis system is not operated. The normal operation operating situation corresponds to normal operation, in which the electrolysis system is operated to generate a specified synthesis gas. The start-up operating situation is provided for reaching the normal operation operating situation from the rest operating situation.The return operating situation is intended to achieve the idle operating situation from the normal operating situation. The method records an operating situation of the electrolysis system. Depending on the recorded operating situation, residual gas is fed to a catalyst for catalytic combustion, either from a residual gas reservoir or from a synthesis plant.

[0009] In other words, a method for operating an electrolysis system is provided, with which, for example, an operating strategy for the electrolysis system can be implemented.

[0010] "Operating a system" can preferably be understood as instructing the system to operate. The electrolysis system can be, for example, a fuel cell electrolyzer system, a high-temperature fuel cell electrolyzer system, an SOEC system, or a CO-SOEC system. Furthermore, the electrolysis system can preferably be a reversible fuel cell system, such as an rSOC system. The electrolysis system has at least one electrolyzer stack with an air side and a reactant side. The electrolysis system has operating situations.

[0011] Within the scope of the invention, an "operating situation" can be understood in particular as a situation or circumstances that occur during operation of the electrolysis system. An operating situation can be defined by the current condition and / or a control state of the electrolysis system. An operating situation can also be defined, for example, by physical variables such as actual pressures or actual temperatures that occur in the electrolysis system and / or by a control command. The electrolysis system has a plurality of different operating situations. Preferably, the electrolysis system can only have one operating situation at a time. An operating situation can be defined such that the requirements for the electrolysis system in a specific operating situation are clearly defined and can be clearly distinguished from other operating situations.

[0012] According to the invention, the electrolysis system has at least one idle operating situation, one normal operating situation, one start-up operating situation and one return operating situation.

[0013] In the context of the invention, a “resting operating situation” or a “resting state” can be understood in particular as an operating situation in which the electrolysis system is, for example, in a sleep mode or is switched off, and / or in which no operating materials are pumped through the electrolysis system.

[0014] In the context of the invention, “normal operation” can be understood as meaning, in particular, stationary or at least semi-stationary operation of the electrolysis system. During normal operation, the electrolysis system generates a designated synthesis gas from reactant. Normal operation can preferably comprise operation of the electrolysis system under partial load (preferably at least 5%, 10%, 20%, 30%, 40%, 50% or 80% or more of production capacity) and / or full load. A “designated synthesis gas” can preferably be understood as a gas or gas mixture generated from the supplied reactant for further processing. Normal operation can preferably be intended to generate a synthesis gas as the designated synthesis gas. The electrolysis system can be designed to generate this synthesis gas. The reactant can preferably be understood as a gaseous mixture of water and carbon dioxide.This is also preferably referred to as reactant-gas-carbon dioxide mixture or reactant-gas mixture.

[0015] Within the scope of the invention, fuel can also advantageously be understood as a reactant. The synthesis gas is, in particular, an exhaust gas from the electrolysis stack.

[0016] The start-up operating situation and the return operating situation can each be a "special operating situation" that deviates from normal operation of the electrolysis system. These operating situations can therefore preferably be rest and / or transition states of the electrolysis system, which occur, for example, before and / or after normal operation of the electrolysis system.

[0017] According to the invention, an operating situation is detected.

[0018] Within the scope of the invention, "detecting an operating situation" can be understood in particular as registering an operating situation. For example, in addition to recording and / or measuring information relating to operating situations, detecting can preferably also comprise evaluating or classifying this information. For example, an operating situation can be determined from the current and / or historical information recorded relating to an operating situation. Within the scope of the invention, detecting an operating situation can advantageously also be understood as specifying an operating situation to be achieved and / or set. This can, for example, be specified externally by an operator.

[0019] According to the invention, depending on the detected operating situation, residual gas is fed to a catalyst for catalytic combustion, either from a residual gas reservoir of the electrolysis system or from a synthesis plant. The residual gas is preferably generated during a synthesis process in the synthesis plant.

[0020] Residual gas can, of course, also be selectively supplied to the catalyst from other gas sources depending on the detected operating situation. The invention is therefore not limited to the two mentioned sources of residual gas. Thus, residual gas can be selectively supplied to the catalyst from one of three, four, or more different gas sources depending on the detected operating situation. In at least one detected operating situation, the supply of residual gas is provided via the synthesis system, and in at least one further detected operating situation, the supply of residual gas is provided via the residual gas reservoir.

[0021] A "synthesis process" can be understood in the context of the invention, in particular, as a process for producing synthetic hydrocarbons from a synthesis gas. Preferably, a synthesis plant or a synthesis system can be provided to implement the synthesis process. Typically, not all of the synthesis gas can be converted during the synthesis process, and / or short-chain hydrocarbons are formed, which are separated. This separated gas portion can be referred to, for example, as "residual gas" within the context of the invention. Of course, the invention is not limited solely to such gas.

[0022] Thus, in the context of the invention, "residual gas" can be understood in particular as a gas mixture containing carbon dioxide and / or short-chain hydrocarbons. The residual gas can also comprise, for example, methane or natural gas. The residual gas can, for example, be a gas or gas mixture functionally equivalent to a residual gas from a synthesis process in which synthesis gas is converted into hydrocarbons.

[0023] Within the scope of the invention, a "residual gas reservoir" can be understood, in particular, as a device for releasing and / or storing residual gas. For example, the residual gas reservoir can be a container or a tank. Preferably, the residual gas reservoir can have a capacity. Furthermore, it is conceivable for the residual gas reservoir to be an external gas supply source with which a gas or gas mixture functionally equivalent to the residual gas can be provided. For example, the residual gas reservoir can be a gas supply source for providing natural gas or methane.

[0024] With the method according to the invention, operating situations can be identified and thus controlled in a situation-specific manner. One advantage is that in the method, a choice can be made between the synthesis plant itself and another residual gas source for supplying the electrolysis system with residual gas. Accordingly, the supply of residual gas can take place independently of the synthesis process or, conversely, independently of the supply status of the residual gas reservoir. This enables the electrolysis system to be operated flexibly, since heat from catalytic combustion of the residual gas in the catalyst can be provided in the synthesis plant even after the synthesis process has ended. This allows a short-term response to a request to terminate the synthesis process, since heat-up times of electric air heaters can be bridged with the residual gas from the residual gas reservoir. Accordingly, heat can be provided continuously.The energy required to operate the electrolysis system can also be reduced, as electric air heaters do not need to be maintained at a minimum temperature. Furthermore, the process is relatively simple to implement in terms of control technology and design.

[0025] Preferably, the residual gas from the synthesis process, with regard to the piping, can be fed to the catalyst immediately or directly from the synthesis process, a synthesis plant or a synthesis system for implementing the synthesis process.

[0026] This ensures that the supply of residual gas can be achieved through direct connection to the plant implementing the synthesis process. Pressure losses and delays caused by pipeline transport can be avoided.

[0027] According to a preferred embodiment, the residual gas can be supplied to the catalyst depending on the detected operating situation. For example, the supply can be controlled such that the residual gas is at least partially supplied in a normal operating situation and / or in a return operating situation. Alternatively or additionally, the control can be such that no residual gas is supplied to the catalyst in a start-up operating situation.

[0028] This makes it possible to decide whether or not to feed residual gas into the electrolysis system depending on the respective operating situation identified. Especially in start-up operating situations, a sufficient amount of residual gas may not yet be available, or its use would not yet be advantageous for the operation of the electrolysis system. This allows for flexible, situation-adapted control of the electrolysis system. According to a further preferred embodiment, the residual gas can be fed to the catalyst from the residual gas reservoir in a return operating situation.

[0029] This allows residual gas to be supplied to the electrolysis system from the residual gas reservoir during operating situations in which the electrolysis system is shut down, regardless of the activity status of the synthesis process. This increases operational flexibility, as residual gas can be supplied even during interruptions, failures, or terminations of the synthesis process.

[0030] Alternatively or additionally, the residual gas can be fed to the catalyst in a normal operating situation, preferably directly from the synthesis plant.

[0031] Thus, in operating situations where residual gas production is usually sufficient, the residual gas can be fed directly from the synthesis plant.

[0032] According to a preferred embodiment, the temperature of the electrolyzer stack can be controlled in a normal operating situation by means of the catalyst fed by the residual gas. For this purpose, heat from a catalyst exhaust stream resulting from the catalytic combustion of the residual gas can be transferred to the air to be supplied and / or the reactant to be supplied by means of at least one heat exchanger.

[0033] In this context, "controlling" can be understood in particular as the control and / or regulation of input variables, parameters, and / or components of the electrolysis system. The "temperature of the electrolyzer stack" can, in particular, be a temperature measurable locally on the electrolyzer stack or an average temperature calculated from locally different measured temperatures.

[0034] This makes it possible to generate heat in the electrolysis system during normal operation from catalytic combustion of the residual gas, thus advantageously utilizing a byproduct of the synthesis process for energy purposes. This increases the overall efficiency of a plant with such a controlled electrolysis system.

[0035] Furthermore, the temperature of the electrolyzer stack can be at least partially controlled in a recirculation mode by means of the catalyst fed by the residual gas. For this purpose, heat from a catalyst exhaust stream resulting from the catalytic combustion of the residual gas can be transferred to the air and / or reactant to be supplied by means of at least one heat exchanger.

[0036] This makes the electrolysis system at least temporarily independent of the availability of electric heaters, as the residual gas can also be used in a return flow mode to provide heat. This allows permissible temperature gradients to be maintained during cooling and allows cooling processes to be initiated quickly.

[0037] According to a further preferred embodiment, one or at least two electric heaters can be heated to a respective heater temperature in a return flow operating situation. The heaters are provided in the air supply path. In the return flow operating situation, the residual gas from the residual gas reservoir can also be supplied until the heater temperature of at least one or two of the heating electric heaters is reached.

[0038] In principle, the at least one heater within the scope of the invention does not necessarily have to be an electric heater. The heaters can also be formed, for example, by a torch and a heat exchanger. Other possible heaters are also conceivable.

[0039] This makes it possible to bridge the necessary heating times of the electric heaters by accessing residual gas in the residual gas reservoir. At the same time, a heating process for the heaters can be initiated. The two heaters can support each other in heating up and in heating the electrolysis system. The heaters can be used to efficiently return the electrolysis system to a standby state.

[0040] According to a preferred embodiment, the method can comprise heating each of the electric heaters to a respective heater temperature in a return flow operating situation. Furthermore, at least one of the electric heaters can be switched off when each of the heater temperatures is reached. Preferably, the electric heater with the lowest heat output of the electric heaters to be heated can be switched off as soon as the electric heater with the highest heat output reaches the heater temperature. Furthermore, in a return flow operating situation, the residual gas can preferably be supplied from the residual gas reservoir until the heater temperatures of each of the electric heaters to be heated are reached.

[0041] This ensures that only the minimum number of heaters is operated during a return flow operation. This reduces the energy consumption of the electrolysis system during this operating situation.

[0042] According to a further preferred embodiment, the residual gas from the synthesis plant can be fed to the residual gas reservoir in a start-up operating situation and / or in a normal operation operating situation until the residual gas reservoir has reached a minimum fill level of the residual gas. Preferably, a temperature of the electrolyzer stack can be controlled in a start-up operating situation and / or in a normal operation operating situation by controlling at least one electric heater for controlling the air temperature of air that can be supplied to the air side via the air supply path until the residual gas reservoir has reached a minimum fill level of the residual gas. Alternatively or additionally, the residual gas from the synthesis plant can be fed to the catalyst in the normal operation operating situation as soon as the residual gas reservoir has reached a minimum fill level of the residual gas.Preferably, a temperature of the electrolyzer stack can be controlled in the normal operation situation by adjusting a mass flow of the residual gas from the synthesis plant as soon as the residual gas from the synthesis process is fed to the catalyst or as soon as the residual gas reservoir has reached the minimum filling level of the residual gas.

[0043] This makes it possible to store residual gas in the residual gas reservoir immediately before and / or upon commencement of active operation of the electrolysis system under partial or full load. Normal operation with steady-state production of synthesis gas is often only achieved by a synthesis plant after a certain start-up period. While residual gas is available during this start-up period, it is often insufficient to cover the full heat demand of the electrolysis system. Therefore, it can be particularly advantageous to store the residual gas produced during this start-up period in the residual gas reservoir. The residual gas stored in this way, or any residual gas that can be optionally supplied from other gas sources, can then be used during a shutdown of the electrolysis system.According to a preferred embodiment, a temperature of the electrolyzer stack in a start-up operating situation can be controlled by means of a first electric heater and preferably by means of a second electric heater. The heaters can be arranged in two parallel and fluidically connected air heating paths of the air supply path. Preferably, the air can be heated to a first air temperature by the first electric heater, and the air can be heated to a second air temperature by the second electric heater, wherein furthermore, the first air temperature and the second air temperature are preferably different.

[0044] According to a further preferred embodiment, the temperature of the electrolyzer stack in a return flow operating situation can be controlled by means of at least a second electric heater and a third electric heater. The first electric heater is provided in an air heat path of the air supply path, and the third electric heater is provided in an additional air heater heat path parallel to the air heat path, which is fluidly connected to an air exhaust gas discharge path for discharging the air exhaust gas generated on the air side.

[0045] The electric heaters allow the electrolysis system's start-up and shut-down processes to be carried out precisely, efficiently, and flexibly. In particular, the maximum permissible temperature gradients during heating or cooling of system components can be controlled and maintained.

[0046] Preferably, a standby operating situation can transition to a start-up operating situation if, for example, a heat-up command is given by the user. Furthermore, a start-up operating situation can preferably transition to a normal operation operating situation if, for example, a load command is given by the user and, at the same time, the electrolysis system is in an operating state suitable for normal operation (temperatures, pressures, operating materials). Furthermore, a normal operation operating situation can preferably transition to a return operating situation if, for example, a command to return and / or terminate the operation of the electrolyzer stack is given. Furthermore, a return operating situation can preferably transition to a standby operating situation if the electrolysis system is in an operating state suitable for switching off the electrolysis system (temperatures, pressures, operating materials).For this purpose, appropriate steps can be carried out in the procedure to record the operating situation.

[0047] A further aspect of the invention relates to a computer program product which has instructions which, when the program is executed by a computer, cause the computer to carry out the method described above.

[0048] Another aspect of the present invention relates to an electrolysis system. The electrolysis system comprises at least one electrolyzer stack having an air side and a reactant side. Furthermore, the electrolysis system comprises an air supply path for supplying air to an air supply section of the air side. The air supply path comprises a primary air supply path and a secondary air supply path provided in parallel with the primary air supply path. The secondary air supply path comprises two parallel air heating paths in which a first electric heater and a second electric heater are provided. The electrolysis system further comprises an air exhaust discharge path for discharging the generated air exhaust from an air exhaust section of the air side. Furthermore, the electrolysis system comprises a reactant supply path for supplying reactant to a reactant supply section of the reactant side.A synthesis gas discharge path is further provided in the electrolysis system for discharging synthesis gas generated on the reactant side from a synthesis gas section of the reactant side. A residual gas connection section for providing residual gas to the electrolysis system is further provided in the electrolysis system. The residual gas is preferably separated during synthesis of the synthesis gas generated by the electrolyzer stack in a synthesis plant. The residual gas connection section is provided in the electrolysis system in such a way that it can be fluidly coupled optionally to at least one residual gas reservoir or to a synthesis plant. The electrolysis system further comprises a catalyst for catalytic combustion of the residual gas. The catalyst is arranged in the air exhaust gas discharge path and is fluidly coupled to the residual gas connection section upstream of a catalyst inlet section via a residual gas supply line.The electrolysis system further comprises a reactant heat exchanger, which is connected to the reactant supply path for heat transfer and is arranged downstream of the catalyst in the air exhaust gas discharge path. Furthermore, the electrolysis system comprises an air heat exchanger, which is connected to the primary air supply path for heat transfer and is arranged downstream of the catalyst in the air exhaust gas discharge path. The electrolysis system also comprises an auxiliary air heater heat path in which a third electric heater is provided. The third electric heater is fluidly coupled to the air supply path and air exhaust gas discharge line sections of the air exhaust gas discharge path. The air exhaust gas discharge line sections are provided downstream of the reactant heat exchanger and upstream of the air heat exchanger.

[0049] In the context of the invention, a "fluidic coupling" can be understood in particular as a fluid-communicating connection. A "path" in the context of the invention can be understood in particular as a flow path from a starting point to one or more end points.

[0050] The electrolysis system can preferably be an SOEC or CO-SOEC system. Furthermore, the air exhaust gas discharge line sections of the air exhaust gas discharge path can preferably have a second catalyst for catalytic combustion of the residual gas. The second catalyst can be provided downstream of the third electric heater. Furthermore, the air exhaust gas discharge path can preferably be fluidly coupled to the residual gas supply line downstream of the air exhaust gas discharge line sections and upstream of the second catalyst. The electric heaters can preferably have at least partially different heating capacities.

[0051] A further aspect of the present invention relates to a control device for operating an electrolysis system. For this purpose, the control device comprises a detection module for detecting an operating situation of the electrolysis system. Furthermore, the control device comprises a situation control module for controlling the electrolysis system depending on the detected operating situation. Depending on the detected operating situation, the situation control module is configured to guide residual gas, optionally at least from a residual gas reservoir or from a synthesis plant, to a catalyst of the electrolysis system for catalytic combustion of the residual gas. The residual gas can preferably be separated during a synthesis process in a synthesis plant in which the synthesis gas is converted into hydrocarbons.

[0052] The control device can, in particular, be configured to implement the steps of the method described above or at least to initiate the implementation of these steps. With the aforementioned computer program product, electrolysis system, and control device, all of the advantages already explained for the method according to the invention can be achieved.

[0053] 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, components or elements of the same kind or type are numbered consecutively and are referred to as first component, second component, third component, etc., for example first heat exchanger, second heat exchanger, etc. This designation based on the numbering serves solely to distinguish components or elements of the same kind or type, but does not represent any restriction with regard to their nature.

[0054] 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. Flow-influencing devices, such as shut-off devices, can be arranged in the connections.

[0055] 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.

[0056] The aforementioned shut-off devices 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. For this purpose, the shut-off devices can have corresponding control electronics and sensors. The shut-off device can be designed in various ways, for example, as a valve, gate valve, shut-off cock, or butterfly valve. Further advantages, features, and details of the invention will become apparent from the following description. In the description, exemplary embodiments of the invention are described with reference to the figures. They schematically show:

[0057] Fig. 1 shows an embodiment of the electrolysis system and the control device according to the invention,

[0058] Fig. 2 shows another embodiment of the electrolysis system according to the invention,

[0059] Fig. 3 the electrolysis system from Figure 2 during operation in a start-up operating situation,

[0060] Fig. 4 shows the electrolysis system of Figure 2 operating in a normal operation situation, and

[0061] Fig. 5 the electrolysis system from Figure 2 during operation in a return flow operating situation.

[0062] One aspect of the invention relates to a method for operating an electrolysis system. The method may, for example, be an operating strategy.

[0063] Figures 1 to 5 show examples of an electrolysis system 10 that can be operated using the method. In particular, a COSOEC electrolysis system can be operated using the method, although the invention is not limited thereto.

[0064] The electrolysis system 10 shown as an example in the figures has at least one electrolyzer stack 100. Figures 2 to 5 are particularly suitable for obtaining details of the design of the electrolysis system 10 shown.

[0065] The electrolyzer stack 100 has an air side 120, at which an electrode can be supplied with air via an air supply section 122. Such an air electrode is not shown in the figures for the sake of clarity. The electrolyzer stack 100 further has a reactant side 130, at which another electrode can be supplied with reactant via a reactant supply section 131. Such a (fuel) electrode is not shown in the figures for the sake of clarity. Depending on the operating mode as an electrolyzer cell or fuel cell, the electrodes of the electrolyzer stack 100 can function as an anode in one case and as a cathode in the other case. In the case of operation as an electrolyzer cell, for example, the air side 120 can have the air electrode as the anode and the reactant side 130 can have the electrode as the cathode.

[0066] Preferably, the electrolysis system 10 can also have more than one electrolyzer stack 100. This is shown by way of example in Figure 1. Preferably, the electrolysis system 10 can have two electrolysis modules, each with two electrolyzer stacks 100.

[0067] The electrolyzer stack 100 can be connected to a power source 510 by an electrical connection 511 via an electrode terminal 512. The electrical connection 511 can include a relay 514. To simplify the illustration, the negative and positive potential connections to the electrical connection 511 are shown in the figures as a common connection.

[0068] The air can be introduced into the electrolysis system 10 through an air inlet section 2201, for example, from the environment. The air is then guided to the air supply section 122 via an air supply path 2200. The air supply path 2200 can have various sections and branches. For example, a first supply air heater 201 and a second supply air heater 202, as well as an air filter 203, can be provided in an air supply line section 2202. The filtered and preheated air can be conveyed along the air supply path 2200 by an air blower 204.

[0069] Furthermore, the air supply path 2200 has a primary air supply path 2210. At least one or preferably two air heat exchangers 421, 422 are arranged in the primary air supply path 2210, which are coupled to this line section of the primary air supply path 2210 in a heat-transfer manner.

[0070] As can also be seen in Figures 2 to 5, the air supply path 2200 has an additional secondary air supply path 2220, which runs parallel to the primary air supply path 2210. The secondary air supply path 2220 itself has two parallel air heat paths 2221, 2222. An electric heater for heating the air is provided in each of the air heat paths 2221, 2222, namely a first heater 221 in the air heat path 2221 and a second heater 222 in the air heat path 2222. The electric heaters 221, 222 can have identical or different heating capacities. The volume flows from the air heat paths 2221, 2222 can be recombined at a connection point 226.

[0071] The volume flows from the primary air supply path 2210 and the secondary air supply path 2220 can be combined at a mixing section 213. In this way, air with a first temperature from the primary air supply path 2210 and air with a second temperature from the secondary air supply path 2220 can be mixed with each other, so that the air can achieve a mixed temperature. The thus tempered air can be supplied to the air supply section 122 via an air supply section 2212.

[0072] The air can be divided into the primary and secondary air supply paths 2210, 2220 at a branching point 207. Downstream of the branching point 207, a shut-off device 215, 224, 225 can be provided in each of the respective branches, for example, to regulate the air flow volumes in the respective branches.

[0073] The air consumed in the electrolyzer stack 100 is discharged as air exhaust gas via an air exhaust section 125 along an air exhaust discharge path 2500 from the air side 120. For this purpose, the air exhaust discharge path 2500 can have an air exhaust discharge line section 2510. The air exhaust gas can be conducted through a further air exhaust discharge line section 2520, in which a catalyst 412, described in more detail later, is arranged for the catalytic combustion of the air exhaust gas.

[0074] The air exhaust gas can be mixed with various gas components along the air exhaust gas discharge path 2500 in order to further utilize the air exhaust gas, for example, in the electrolysis system 10. For example, a residual gas RG can be supplied to the air exhaust gas, which enables catalytic combustion in one or more catalysts 411, 412 described in more detail below. Preheated air from the air supply path 2200 can also be supplied to the air exhaust gas, for example, to precisely adjust stoichiometric ratios and / or temperatures.

[0075] Thus, the air exhaust gas can preferably be enriched with warm air from the air supply path 2200. For this purpose, an additional air heater heat path 2550 is provided in the electrolysis system 10, in which a third electric heater 241 for heating air is arranged. The additional air heater heat path 2550 is fluidly coupled to the air supply path 2200 and air exhaust gas discharge line sections 2540, 2560 of the air exhaust gas discharge path 2500. For this purpose, the air supply path 2200 can have a branching point 205 upstream of the branching point 207 to guide the air into the additional air heater heat path 2550. A shut-off device 245 can be provided between the branching point 205 and the third heater 241. The third electric heater 241 can also be used, for example, in a warm-up process of the electrolysis system 10 to heat the catalyst

[0076] 411 to heat up.

[0077] Furthermore, the air supply path 2200 can have a branching point 206 upstream of the branching point 207 in order to guide air along a further additional air heater heat path 2530 to a branching point 233 arranged in the air exhaust gas discharge path 2500. This can be provided, for example, to additionally supply the catalyst 412 with heated air for the catalytic combustion of an air exhaust gas enriched with residual gas RG. Accordingly, a further electric heater 231 can also be provided in the further additional air heater heat path 2530. The further electric heater 231 can thus be fluidically coupled to the air supply path 2200 and a section of the air exhaust gas discharge path 2500. A controllable shut-off device 235 can also be arranged between the further electric heater 231 and the branching point 206.

[0078] The air exhaust gas can further be enriched with residual gas RG, which is separated, for example, during the synthesis of a synthesis gas SG in a synthesis plant 900. Alternatively or additionally, the residual gas RG can also be gas, such as natural gas or methane, from another external gas source. The electrolysis system 10 has a residual gas connection section 401 to provide the residual gas RG. The residual gas RG is then guided via a residual gas supply line 4110 in the electrolysis system 10. In particular, the air exhaust gas discharge path 2500 can have a connection point 251, to which the residual gas RG can be guided along the residual gas supply line 4110 and preferably a residual gas line branching section 4112. Preferably, the connection point 251 can be located between the air exhaust gas section 125 and the catalyst

[0079] 412. Furthermore, the connection point 251 can preferably be arranged downstream of the connection point 233. The electrolysis system 10 further comprises a reactant heat exchanger 420, which is connected in a heat-transfer manner to a line section for transporting the reactant to the reactant side 130. The reactant heat exchanger 420 is further arranged downstream of the catalyst 412 in the air exhaust gas discharge path 2500. Thus, the reactant to be supplied can be heated by the reactant heat exchanger 420 with the exhaust gas from the catalyst 412. The air exhaust gas is preferably guided to the further catalyst 411 via the air exhaust gas discharge line sections 2540 and 2560.

[0080] The further catalyst 411 is arranged in the air exhaust gas discharge path 2500 and has a catalyst inlet section 4113. The further catalyst 411 is fluidly coupled upstream of the catalyst inlet section 4113 to the residual gas connection section 401 via the residual gas supply line 4110, 4111. The same can also apply to the catalyst 412, which can also be arranged in the air exhaust gas discharge path 2500 and can have a catalyst inlet section 414. As shown in Figures 2 to 5, the further catalyst 412 is fluidly coupled upstream of the catalyst inlet section 414 to the residual gas connection section 401 via the residual gas supply line 4112.

[0081] The air heat exchanger(s) 421, 422 are arranged downstream of at least one, preferably both, of the catalysts 411, 412 in the air exhaust gas discharge path 2500. Figures 2 to 5 further show that the air exhaust gas discharge line sections 2540, 2560 are provided downstream of the reactant heat exchanger 420 and upstream of the at least one air heat exchanger 421, 422. Accordingly, the exhaust gas from the catalyst 411 can be used to heat the air in the primary air supply path 2210. The air exhaust gas cooled in the reactant heat exchanger 420 can thus have its temperature increased again in the catalyst 411.

[0082] In an adjoining air exhaust line section 2570, two further heat exchangers 423, 424, namely a steam heat exchanger 423 and a carbon dioxide heat exchanger 424, can be provided. The exhaust gas from the catalyst 411 can thus be used again for heat transfer. The heat exchanger 424 can also be used, for example, during commissioning of the electrolysis system 10 to preheat an inert gas instead of carbon dioxide. Downstream of the heat exchangers 423, 424, the air exhaust gas can be guided into the air exhaust line section 2580 and finally discharged to the environment or downstream system components via an air exhaust outlet section 2501. In the event that several electrolysis cells 100 are provided, the air exhaust gas can also be discharged via a common air exhaust outlet 602 of the electrolysis system 10, as shown by way of example in Figure 1.

[0083] The figures also show that natural gas can optionally be added to the air exhaust gas upstream of the catalysts 411, 412. For this purpose, natural gas can be introduced via a natural gas section 402 and guided in an associated natural gas supply line 4210. The natural gas can be added, for example, as a replacement gas or bridging gas for the residual gas RG. Natural gas can also be added, in particular, during partial load operation. Residual gas RG can also be added to the air exhaust gas via residual gas branching sections 4111, 4112 upstream of the catalysts 411, 412.

[0084] The composition of the reactant for the electrolyzer stack 100 can be provided differently depending on the operating situation.

[0085] For this purpose, the electrolysis system 10 generally has a reactant feed path 3100 for supplying reactant to the reactant feed section 131. To generate synthesis gas SG, carbon dioxide (hereinafter referred to as CO2) and water vapor are used as reactants, for example, in the figures. For this purpose, CO2 can be introduced into the electrolysis system 10 via a CO2 feed section 3101.

[0086] Via a CO2 supply line 3110, the CO2 can then be fed via a reactant supply path 3100 to the reactant supply section 131 of the electrolyzer stack 100. Steam can be introduced into the electrolysis system 10 via a steam introduction section 3105. Via steam supply lines 3151, 3152, the steam can be fed via a connection point 307 into the reactant supply path 3100. For this purpose, the steam can be passed through the previously described steam heat exchanger 423 and further heated. The steam supply line 3151 can have a shut-off device 355. Furthermore, protective gas from a first, second, and third protective gas supply section 3102, 3103, 3104 can be fed into the reactant supply path 3100. For this purpose, corresponding supply lines, such as a first protective gas supply line 3112, a second protective gas supply line 3113 and a third protective gas supply line 3114, can be provided, which open into the reactant supply path 3100 at connection points 305, 306.A heat exchanger 320 and an ejector 372 may further be provided in the reactant supply path 3100.

[0087] In the electrolyzer stack 100, a synthesis gas can be generated from the reactant in the reactant side 130. The synthesis gas can preferably be a synthesis gas SG. For the removal of the synthesis gas, the electrolysis system 10 has a synthesis gas discharge path 3200, by means of which synthesis gas generated on the reactant side 130 can be discharged from a synthesis gas section 132 of the reactant side 130. The synthesis gas can be guided to a synthesis gas outlet section 3201, which can preferably be fluidly connected to a synthesis system 900. This is shown by way of example in Figure 1. The synthesis system 900 can thus be conveyed, for example, via a synthesis gas outlet 601 from the electrolysis system 10 into a synthesis gas line 901.

[0088] Before leaving the electrolysis system 10, the synthesis gas can be passed through a synthesis gas cooler 321. The synthesis gas cooler 321 can be flowed through by a coolant, which can be introduced via a coolant supply section 801 and discharged again downstream of the synthesis gas cooler 321 via a coolant discharge section 801. The synthesis gas discharge path 3200 can further comprise the heat exchanger 320 to transfer heat from the synthesis gas to the reactant. The electrolysis system 10 can further comprise a recirculation path 3700 to conduct synthesis gas from the synthesis gas section 132 back to the reactant supply section 131 via the ejector 372. Instead of the ejector 372 shown as an example, other correspondingly suitable components can of course also be provided. For example, a gas blower device could be provided. A shut-off device 371 may also be provided in the recirculation path 3700.

[0089] The residual gas connection section 401 is provided in a special way, as can be seen in particular from Figure 1. Thus, the residual gas connection section 401 is provided such that it can be fluidically coupled optionally to at least one residual gas reservoir 443 or to the synthesis system 900. The residual gas reservoir 443 can be, for example, a buffer tank. Alternatively, it is also conceivable that, instead of being designed as a buffer tank, the residual gas reservoir 443 has a line end of an external gas supply source. The electrolysis system 10 can, for example, have a residual gas connection 440, via which the residual gas RG can be supplied to the electrolysis system 10. A controllable branching point 441 can be provided to guide the residual gas RG optionally directly to the residual gas supply line 4110 or, alternatively, to the residual gas reservoir 443.The residual gas RG from the residual gas reservoir 443 can be fed to the residual gas supply line 4110 via a controllable connection point 442. It is also conceivable that one or more additional gas sources, such as a natural gas storage facility or a natural gas supply line, can be fluidly coupled to the residual gas supply line 4110 via the residual gas connection section 401 and in particular the controllable connection point 442. In this way, for example, a supply of the electrolysis system 10 with residual gas RG from more than just the residual gas reservoir 443 and the synthesis system 900 can be selected.

[0090] The electrolysis system 10 has various operating situations. Among other things, the electrolysis system 10 has a rest operating situation in which the electrolysis system is not operating; a normal operating situation corresponding to normal operation in which the electrolysis system is operated to generate a specified synthesis gas; a start-up operating situation for reaching the normal operating situation from the rest operating situation; and a return operating situation for reaching the rest operating situation from the normal operating situation.

[0091] In the method, an operating situation is detected, and the electrolysis system 10 is controlled depending on the detected operating situation such that, depending on the detected operating situation, residual gas RG is optionally fed at least from the residual gas reservoir 443 or from the synthesis plant 900 to at least one of the catalysts 411, 412 in order to catalytically combust it there. It is also conceivable that other gas sources, such as a natural gas pipeline or a methane storage facility, can also provide the residual gas RG, and accordingly, depending on the detected operating situation, the residual gas RG can also be supplied from one of these other gas sources. Furthermore, it is also preferably conceivable that the residual gas reservoir 443 itself is also provided in a controllable manner such that, depending on the operating situation, residual gas RG can be supplied either from the buffer tank or an external gas supply line.

[0092] Figures 3 to 5 show examples of how the electrolysis system 10 is controlled for the operating situations. Non-active components of the electrolysis system are grayed out. Accordingly, the idle operating situation is not shown here, since in this operating situation the electrolysis system 10 is predominantly to completely inactive. Figure 3 shows operation for a start-up operating situation. Figure 4 shows operation for a normal operating situation. Figure 5 shows operation for a return operating situation.

[0093] Figure 3 shows an embodiment of the method according to the invention for a start-up operating situation. This operating situation occurs when, for example, the electrolysis system 10 or a control device 20 provided therein receives a request from the operator to start electrolysis. When the aforementioned condition occurs, CO2 is added to the reactant side 130 in addition to protective gas and water vapor. In order to adjust the amount of CO2 to be added, a feed curve can preferably be used. A feed curve can, for example, be a predetermined concentration profile that depends, for example, on another variable such as time, temperature or concentrations of other substances. The amount of protective gas can be reduced at the same time. Recirculation of the reactant results in a gas mixture with a composition including protective gas, CO2, water vapor, hydrogen and recirculate.Furthermore, electrical current can be supplied to the electrolyzer stack 100. For this purpose, a power curve for controlling the amount of current can be run through. The synthesis gas generated as synthesis gas in the electrolyzer stack 100 during this operation can be discharged to a synthesis plant 900 for further processing. Since no or only insufficient amounts of residual gas RG may be present at this time, catalytic heating with the catalysts 411, 412 does not yet take place in the low-power operating situation. The air side 120 therefore continues to be supplied with electrically heated air via the first and second electric heaters 221, 222. Filling of the residual gas reservoir 443 can preferably begin during the start-up operating situation. Figure 4 shows an embodiment of the method according to the invention for a normal operation operating situation.The normal operation situation is detected, for example, as soon as the electrolyzer stack 100 has a minimum electrical stack current (e.g., 63.5 A) and the synthesis gas has a minimum operating temperature (e.g., 800°C). In the normal operation situation, protective gas is no longer supplied to the reactant side 130. Air, reactant, and electrical current are fed to the electrolyzer stack 100, and the intended synthesis gas is generated therefrom. The reactant comprises carbon dioxide and water vapor, as well as recirculated synthesis gas. Operation can take place under full or partial load. Furthermore, it can be provided that the residual gas RG from the synthesis system 900 is supplied to the residual gas reservoir 443 until the residual gas reservoir 443 has reached a minimum fill level of residual gas RG.During the filling time, the temperature of the electrolyzer stack 100 can be controlled by controlling the first and second electric heaters 221, 222. Once the minimum fill level is reached, the temperature of the electrolyzer stack 100 can be controlled by catalytic heating. In this case, the residual gas RG is fed directly from the synthesis system 900 to the catalysts 411, 412 to enable catalytic heating of the air and / or reactant to be supplied.

[0094] Figure 5 shows an embodiment of the method according to the invention for a return flow operating situation. The return flow operating situation is detected when the electrolysis system 10 or a control device provided therein receives, for example, a process request to terminate the operation of the electrolyzer stack 100. Area F5 is now increasingly supplied to the reactant as a protective gas in order to use it to cool the electrolyzer stack 100. At the same time, the proportions of carbon dioxide and water vapor are reduced to zero according to corresponding decrease curves. Furthermore, the flow rates of this gas mixture and the air, as well as the amount of electrical current supplied, are reduced. The current reduction continues until no more current is supplied, i.e., the current intensity is zero.In the return flow operating situation, the residual gas RG from the residual gas reservoir 443 can now be supplied to the catalysts 411, 412 in order to enable catalytic heating in this operating situation as well. At the same time, one or more of the electric heaters 221, 222, 231, 241 can be heated to a heater temperature. Figure 5, for example, shows that the first, second, and third electric heaters 221, 222, 241 can be provided for this purpose. The residual gas RG from the residual gas reservoir 443 can be used for heating until the heater temperature of at least one or two of the heating electric heaters 221, 222, 241 is reached. It can further be provided that at least one of the electric heaters 221, 222, 241 is switched off when each of the heater temperatures is reached. Preferably, the electric heater 221, 222, 241 with the lowest heat output is then switched off.Once the heating temperatures are reached, the additional electric heater 241 can be used in addition to the electric heater 221 to control the temperature of the electrolyzer stack 100. The control is preferably carried out in such a way that component-dependent maximum temperature gradients are not exceeded.

[0095] A further aspect of the invention relates to a control device 20 for operating an electrolysis system. Figures 1 to 5 show the control device 20 by way of example. The control device 20 is designed in particular to carry out the method described above. The control device 20 has a detection module 21 for detecting the operating situations. Furthermore, it has a situation control module 22 for monitoring the electrolysis system 10 depending on the detected operating situation. The control device 20 can be connected to components of the electrolysis system 10 via signaling. For this purpose, corresponding control lines 23, 24, 25, 26, 27 can be provided in the electrolysis system 10, for example to enable monitoring of the electrolyzer stack 100. The controllable branching point 441 or the controllable connection point 442 can also be monitored depending on the operating situation.The residual gas reservoir 443 can be queried for its fill level.

[0096] The above explanation of the embodiments describes the present invention exclusively by way of example. Of course, individual features of the embodiments can be freely combined with one another, provided they are technically feasible, without departing from the scope of the present invention.

[0097] 10 Electrolysis system

[0098] 20 Control device

[0099] 21 Recording module

[0100] 22 Situation Control Module

[0101] 23 to 27 control lines

[0102] 100 electrolyzer stacks

[0103] 120 airside

[0104] 122 Air supply section

[0105] 125 Air exhaust section

[0106] 130 Reactant side

[0107] 131 Reactant feed section

[0108] 132 Synthesis gas section

[0109] 201 first flow air heater

[0110] 202 second flow air heater

[0111] 203 Air filter

[0112] 204 air blowers

[0113] 205 branching point

[0114] 206 branching point

[0115] 207 branch point

[0116] 213 mixing section

[0117] 215 shut-off device

[0118] 221 first electric heater

[0119] 222 second electric heater

[0120] 224 shut-off device

[0121] 225 shut-off device

[0122] 226 junction

[0123] 231 additional electric heaters

[0124] 233 liaison office

[0125] 235 Shut-off device

[0126] 241 additional electric heaters

[0127] 245 Shut-off device

[0128] 251 liaison office

[0129] 305 Connection point Connection point

[0130] Connection point heat exchanger synthesis gas cooler connection point

[0131] liaison office

[0132] shut-off device

[0133] shut-off device

[0134] Ejector

[0135] Residual gas connection section

[0136] Natural gas section

[0137] catalyst

[0138] Catalyst, 414 Catalyst inlet section Reactant heat exchanger Air heat exchanger Air heat exchanger

[0139] steam heat exchanger

[0140] CO2 heat exchanger

[0141] Residual gas connection controllable branching point controllable connection point residual gas reservoir

[0142] Residual gas supply line Power supply source Electrical connection Electrode connection

[0143] Relay

[0144] Synthesis gas outlet

[0145] Air exhaust outlet

[0146] Coolant supply section

[0147] Coolant discharge section synthesis plant synthesis gas line 2200 air supply path

[0148] 2201 Air intake section

[0149] 2202 Air supply line section

[0150] 2210 primary air supply path

[0151] 2212 Air supply section

[0152] 2220 secondary air supply path

[0153] 2221, 2222 Air heat path

[0154] 2500 air exhaust gas discharge path

[0155] 2501 Air exhaust outlet section

[0156] 2510 Air exhaust pipe section

[0157] 2520 Air exhaust gas discharge line section

[0158] 2530 additional air heater heat path

[0159] 2540 Air exhaust gas discharge line section

[0160] 2550 Additional air heater heat path

[0161] 2560 Air exhaust gas discharge line section

[0162] 2570 Air exhaust gas discharge line section

[0163] 2580 Air exhaust gas discharge line section

[0164] 3100 Reactant feed path

[0165] 3101 CO2 supply section

[0166] 3102 first shielding gas supply section

[0167] 3103 second shielding gas supply section

[0168] 3104 third shielding gas supply section

[0169] 3105 Steam introduction section

[0170] 3110 CO2 supply line

[0171] 3112 first shielding gas supply line

[0172] 3113 second shielding gas supply line

[0173] 3114 third shielding gas supply line

[0174] 3115 branch line

[0175] 3151 steam supply line

[0176] 3152 steam supply line

[0177] 3200 Synthesis gas discharge path

[0178] 3201 Synthesis gas outlet section

[0179] 3700 Recirculation path

[0180] 4110 Residual gas supply line 4111 Residual gas line branching section

[0181] 4112 residual gas pipeline branching section

[0182] 4210 Natural gas supply line

[0183] SG Synthesis Gas

[0184] RG residual gas

Claims

Patent claims 1. A method for operating an electrolysis system (10), wherein the electrolysis system (10) has at least one electrolyzer stack (100) with an air side (120), to which air can be supplied via an air supply path (2200), and a reactant side (130), to which reactant can be supplied via a reactant supply path (3100), and wherein the electrolysis system (10) has various operating situations, which at least comprise: o a rest operating situation in which the electrolysis system (10) is not operated, o a normal operating situation corresponding to normal operation, in which the electrolysis system (10) is operated to generate a specified synthesis gas (SG), o a start-up operating situation for reaching the normal operating situation from the rest operating situation, and o a return operating situation for reaching the rest operating situation from the normal operating situation, characterized by - detecting an operating situation of the electrolysis system (10), and - feeding residual gas (RG) to a catalyst (411, 412) for catalytic combustion of the residual gas (RG), wherein the residual gas (RG) is fed optionally at least from a residual gas reservoir (443) or from a synthesis plant (900) depending on the detected operating situation.

2. The method according to claim 1, characterized in that the residual gas (RG) is from a synthesis process in which synthesis gas (SG) is converted into hydrocarbons in the synthesis plant (900).

3. Method according to claim 1 or claim 2, characterized in that in a return operating situation, the residual gas (RG) is fed to the catalyst (411, 412) from the residual gas reservoir (443), and / or in a normal operation operating situation, the residual gas (RG) is fed to the catalyst (411, 412) preferably directly from the synthesis plant (900).

4. Method according to one of the preceding claims, characterized by Controlling the supply of the residual gas (RG) to the catalyst (411, 412) as a function of the detected operating situation, so that residual gas (RG) is at least partially supplied to the catalyst (411, 412) in a normal operation operating situation and / or in a return operating situation, and / or so that no residual gas (RG) is supplied to the catalyst (411, 412) in a start-up operating situation.

5. Method according to one of the preceding claims, characterized by - Controlling a temperature of the electrolyzer stack (100) by means of the catalyst (411, 412) fed by the residual gas (RG) in a normal operation operating situation and at least partially in a return operating situation by transferring heat of a catalyst exhaust gas stream resulting from the catalytic combustion of the residual gas (RG) to the air to be supplied and / or to the reactant to be supplied by means of at least one heat exchanger (420, 421, 422, 423, 424).

6. Method according to one of the preceding claims, characterized by - heating at least one electric heater (221, 222, 231, 241) to a respective heater temperature in a return operating situation, wherein the heaters (221, 222, 231, 241) are provided in the air supply path (2200); and - Supplying the residual gas (RG) from the residual gas reservoir (443) in the return flow operating situation until the heater temperature of at least one of the heating electric heaters (221, 222, 231, 241) is reached.

7. Method according to claim 6, characterized by - preferably heating each of the electric heaters (221, 222, 231, 241) to a respective heater temperature in a return operating situation, - Switching off at least one of the electric heaters (221, 222, 231, 241) when each of the heater temperatures is reached, wherein preferably from the electric heaters (221, 222, 231, 241) to be heated, the electric heater (221, 222, 231, 241) with the lowest heat output is switched off as soon as the electric heater (221, 222, 231, 241) with the highest heat output reaches its associated heater temperature, and - preferentially supplying the residual gas (RG) from the residual gas reservoir (443) in a return flow operating situation until the heater temperatures of each of the electric heaters (221, 222, 231, 241) to be heated are reached.

8. Method according to one of the preceding claims, characterized by - supplying the residual gas (RG) from the synthesis plant (900) to the residual gas reservoir (443) in a start-up operating situation and / or in a normal operation operating situation until the residual gas reservoir (443) has reached a minimum filling level of the residual gas (RG); - controlling a temperature of the electrolyzer stack (100) in a start-up operating situation and / or in a normal operation operating situation by controlling at least one electric heater (221, 222, 231, 241) for controlling the air temperature of air that can be supplied to the air side (120) via the air supply path (2200) until the residual gas reservoir (443) has reached a minimum filling level of the residual gas (RG); - feeding the residual gas (RG) from the synthesis plant (900) to the catalyst (411, 412) in the normal operation situation as soon as the residual gas reservoir (443) has reached the minimum filling level of the residual gas (RG); and - Controlling a temperature of the electrolyzer stack (100) in the normal operation situation by adjusting a mass flow of the residual gas (RG) from the synthesis plant (900) as soon as the Residual gas reservoir (443) has reached the minimum filling level at the residual gas (RG).

9. Method according to one of the preceding claims, characterized by - Controlling a temperature of the electrolyzer stack (100) in a start-up operating situation by means of a first electric heater (221) and preferably by means of a second electric heater (222), which are arranged in two mutually parallel and fluidically connected air heat paths (2221, 2222) of the air supply path (2200).

10. Method according to one of the preceding claims, characterized by - Controlling a temperature of the electrolyzer stack (100) in a return operating situation by means of at least a second electric heater (222) and a third electric heater (241), wherein the second electric heater (222) is provided in an air heat path (2222) of the air supply path (2200) and the third electric heater (241) is provided in an additional air heater heat path (2550) parallel to the air heat path (2221), which additional air heater heat path is fluidically connected to an air exhaust gas discharge path (2500) for discharging the air exhaust gas (SG) generated in the air side (120).

11. A computer program product comprising instructions which, when executed by a computer, cause the computer to carry out a method according to any one of the preceding claims 1 to 10.

12. Electrolysis system (10), preferably a SOEC or CO-SOEC system, comprising: - at least one electrolyzer stack (100) with an air side (120) and a reactant side (130), - an air supply path (2200) for supplying air to an air supply section (122) of the air side (120), comprising a primary air supply path (2210), and o a secondary air supply path (2220) parallel to the primary air supply path (2210) with two parallel air heat paths (2221, 2222), wherein a first electric heater (221) is provided in one of the air heat paths (2221) and a second electric heater (222) is provided in the other air heat path (2222), - an air exhaust discharge path (2500) for discharging the generated air exhaust from an air exhaust section (125) of the air side (120), - a reactant supply path (3100) for supplying reactant to a reactant supply section (131) of the reactant side (130), - a synthesis gas discharge path (3200) for discharging synthesis gas (SG) generated on the reactant side (130) from a synthesis gas section (132) of the reactant side (130), - a residual gas connection section (401) for providing residual gas (RG) to the electrolysis system (10), - a catalyst (411, 412) for the catalytic combustion of the residual gas (RG), wherein the catalyst (411, 412) is arranged in the air exhaust gas discharge path (2500) and is fluidically coupled to the residual gas connection section (401) via a residual gas supply line (4110) upstream of a catalyst inlet section (4111, 4121), - a reactant heat exchanger (420, 423, 424) which is connected to the reactant supply path (3100) for heat transfer and is arranged downstream of the catalyst (411, 412) in the air exhaust gas discharge path (2500), - an air heat exchanger (421, 422) which is connected to the primary air supply path (2210) for heat transfer and is arranged downstream of the catalyst (411, 412) in the air exhaust gas discharge path (2500), - an additional air heater heat path (2550) in which a third electric heater (241) is provided, which is fluidically coupled to the air supply path (2200) and air exhaust gas discharge line sections (2540, 2560) of the air exhaust gas discharge path (2500), wherein the air exhaust gas discharge line sections (2540, 2560) are provided downstream of the reactant heat exchanger (420, 423, 424) and upstream of the air heat exchanger (421, 422), characterized in that - the residual gas connection section (401) is provided so as to be fluidically coupled optionally to at least one residual gas reservoir (443) or to a synthesis plant (900) in which the residual gas (RG) can be separated in a synthesis process in which synthesis gas (SG) is converted into hydrocarbons.

13. Electrolysis system (10) according to claim 12, characterized in that the air exhaust gas discharge line sections (2540, 2560) have a second catalyst (411, 412) for the catalytic combustion of the residual gas (RG), which is provided downstream of the third electric heater (241), wherein preferably the air exhaust gas discharge path (2500) downstream of the air exhaust gas discharge line sections (2540, 2560) and upstream of the second catalyst (411, 412) is fluidically coupled to the residual gas supply line (4110).

14. Electrolysis system (10) according to claim 12 or claim 13, characterized in that the electric heaters (221, 222, 231, 241) have at least partially different heating capacities.

15. Control device (20) for operating an electrolysis system (10), comprising - a detection module (21) for detecting an operating situation of the electrolysis system (10), - a situation control module (22) for controlling the electrolysis system (10) as a function of the detected operating situation, characterized in that the situation control module (22) is designed to guide residual gas (RG) as a function of the detected operating situation, optionally at least from a residual gas reservoir (443) or from a synthesis plant (900) to a catalyst (411, 412) of the electrolysis system (10) for the catalytic combustion of the residual gas (RG).

Images