Method for operating solid oxide electrolytic cell stack and system for carrying out method

Abstract

The invention relates to a method for operating a solid oxide electrolytic cell (SOEC) stack for hydrogen production, and to a system for carrying out the method; the invention relates to a method for operating a SOEC stack comprising at least one solid oxide electrolytic cell (SOEC), said at least one SOEC comprising an electrolyte layer sandwiched between a fuel side and an oxygen side, the method comprising: transient operation wherein the transient operation comprises: operating the SOEC stack at an open circuit voltage (OCV); -providing a feed gas comprising ammonia; -supplying at least a portion of the feed gas comprising ammonia to a guard bed reactor comprising a catalyst active in the cracking of ammonia into nitrogen and hydrogen; and withdrawing a forming gas comprising nitrogen and hydrogen from the guard bed reactor; -supplying at least a portion of a forming gas comprising nitrogen and hydrogen to the fuel side of at least one solid oxide electrolysis cell (SOEC) of the SOEC stack; and withdrawing a first fuel side outlet gas from at least one SOEC of the SOEC stack.

Description

[0001] This invention relates to a method for operating a solid oxide electrolyzer (SOEC) stack having a fuel side (cathode) and an oxygen side (anode), suitable for steam electrolysis to produce hydrogen, and more specifically to the safe and efficient transient operation of an SOEC stack, such as the start-up operation of an SOEC stack before establishing normal operation. The invention also relates to a system (apparatus) for performing this method.

[0002] Solid oxide electrolyzers (SOECs) can be used to electrochemically reduce water (H₂O) to hydrogen (H₂), carbon dioxide (CO₂) to carbon monoxide (CO), or a combination of H₂O and CO₂ to syngas, i.e., a mixture of H₂ and CO. This conversion occurs on the fuel (cathode) side of the SOEC. On the oxygen (anode) side of the electrolyzer, oxygen is generated electrochemically. As is well known in the art, SOECs are typically stacked to form an SOEC stack. One or more SOEC stacks (e.g., multiple SOEC stacks) constitute an SOEC cell, i.e., an SOEC cell contains one or more SOEC stacks. An SOEC cell may also be referred to as a module.

[0003] Due to the inherent intermittency of electricity / power sources (since electricity is typically generated from renewable energy sources such as wind and solar power), SOEC stacks are frequently in transient operating states, such as startup. Achieving rapid, smooth, and efficient transient operation (e.g., startup, until the SOEC stack reaches the conditions capable of continuously producing the required hydrogen products) presents numerous challenges.

[0004] Patent application IL 257361 discloses a solid oxide fuel cell (SOFC) for generating electricity in electric vehicles, wherein the SOFC includes a catalyst for promoting the cracking of ammonia into hydrogen (2 NH3 = 3 H2 + N2). The SOFC uses H2 as fuel (i.e., feed gas source) to continuously generate electricity.

[0005] Similarly, GB 2393320 discloses a fuel cell that uses ammonia (NH3) as input fuel and employs an iron-based catalyst. A yttrium-stabilized zirconium oxide electrolyte is used as the solid oxide electrolyte. The ammonia fuel is decomposed into hydrogen within the battery, and the hydrogen is used in the electrolyte to generate electricity, thus enabling continuous power generation.

[0006] In contrast, SOEC stacks typically operate by supplying steam (i.e., water, H2O) to the fuel side of the solid oxide electrolyzer during hydrogen production. Before the ability to continuously produce hydrogen from the supplied steam is possible, an efficient, rapid, and safe transient operation method, such as a start-up approach, is required.

[0007] A typical startup process involves heating the SOEC stack in the SOEC cell with a formation gas, which is nitrogen (N2) containing a small amount of H2 (typically 2-10 vol% H2). The H2 ensures that the active material nickel (Ni) on the fuel side remains reduced and active even when oxygen permeates from the oxygen side of the SOEC stack, even when the SOEC cell uses hydrogen as fuel. Once the SOEC stack reaches its normal operating temperature and is ready to begin continuous H2 production, the formation gas is replaced with steam, which should also contain a small amount of H2 (e.g., 2-10 vol% H2). This startup hydrogen is typically prepared by having sufficient H2 cylinders or tanks on-site for the initial startup cycle and establishing local storage facilities to supply subsequent startup cycles. This is a costly solution, requiring significant investment and resources, and necessitates stringent measures for the safe handling and storage of the hydrogen.

[0008] Therefore, it is desirable to provide an improved method for operating an SOEC fuel cell stack for hydrogen production, which includes transient operation, such as a safe startup method for an SOEC fuel cell stack.

[0009] More specifically, it is desirable to provide a safe method for transient operation (e.g., startup) of SOEC fuel cells for hydrogen production, which can protect the fuel side from oxygen migrating from the oxygen side of the SOEC fuel cell.

[0010] It is also expected that a smooth transition from transient operation (e.g., from startup operation) to normal operation of the SOEC stack can be provided.

[0011] It is also desirable to provide a smooth transition from transient operation (e.g., startup operation) to normal operation of the SOEC stack, while being able to utilize existing equipment / units associated with the SOEC stack required for said normal operation.

[0012] It is also desirable to provide an improved method for operating SOEC stacks for hydrogen production, which includes transient operation, which is any one of startup operation, hot standby operation, or shutdown operation.

[0013] Therefore, according to a first general embodiment of the first aspect of the present invention, a method is provided for operating a solid oxide electrolyzer (SOEC) stack for hydrogen production, the SOEC stack including at least one solid oxide electrolyzer (SOEC), the at least one SOEC including an electrolyte layer sandwiched between a fuel side and an oxygen side, the method comprising:

[0014] Transient operation, which includes:

[0015] - Operate the SOEC stack at open-circuit voltage (OCV);

[0016] - Provide feed gas containing ammonia;

[0017] - At least a portion of the feed gas containing ammonia is supplied to a guard bed reactor, the guard bed reactor containing a catalyst active in the cracking of ammonia into nitrogen and hydrogen; and a forming gas containing nitrogen and hydrogen is removed from the guard bed reactor;

[0018] - At least a portion of the forming gas, comprising nitrogen and hydrogen, is supplied to the fuel side of at least one solid oxide electrolyzer (SOEC) of the SOEC stack; and a first fuel side outlet gas is removed from the at least one SOEC of the SOEC stack.

[0019] Therefore, NH3 cracking is performed in the existing, subsequently normal and continuous operating unit (i.e., the guard bed reactor) to provide H2 for the formation gas supplied to the SOEC stack, thereby keeping the nickel on the fuel electrode in its reduced form. The undesirable oxidation of nickel (Ni) to nickel oxide (NiO) caused by oxygen migration from the oxygen side through the electrolyte is minimized or eliminated. This achieves protection of the cathode (i.e., the fuel electrode) while simultaneously enabling (e.g., during start-up operation) the SOEC stack to the temperature required for normal operation, and thereby also heating the SOEC unit containing the SOEC stack.

[0020] In one implementation, transient operation is selected from any one of startup operation, hot standby operation, and shutdown operation.

[0021] Transient operation (e.g., start-up or hot standby operation) provides a dedicated NH3 cracking step in a protective bed reactor, enabling on-site storage of liquid ammonia instead of hydrogen. The solution provided by this invention is not only safer than on-site hydrogen storage but is also particularly suitable for connection to SOEC-based ammonia plants (i.e., where the final product is ammonia), and thus allows for the placement of an ammonia converter downstream of the SOEC stack. The solution of this invention is also suitable for other applications, such as in SOEC-based hydrogen production plants (e.g., for producing green hydrogen as the final product), because ammonia is significantly easier to store and transport (e.g., by truck) compared to hydrogen.

[0022] During normal operation (including continuous operation of the SOEC stack, i.e., after startup or hot standby), H2O is electrochemically reduced as steam-containing fuel-side feed gas is supplied to the fuel side of the SOEC or SOEC stack, resulting in a hydrogen-rich fuel-side outlet gas. Similarly, the oxygen-side gas stream leaving the electrolyzer may have a higher O2 concentration than the O2 concentration in the oxygen-side feed gas stream (e.g., air) entering the solid oxide electrolyzer, thus becoming enriched with O2.

[0023] Furthermore, during normal operation, the operating temperature of the fuel cell stack (also referred to herein as the normal operating temperature) is preferably 600-1000°C, for example 650-900°C, or 700-800°C, for example 700°C, 750°C, or 800°C, which are the temperatures used during hydrogen production in fuel cell stacks using continuously applied electrolysis current. To achieve sufficient conductivity in the ceramic membrane used as the electrolyte, high temperatures, such as 700°C, 800°C, or 900°C, are typically required.

[0024] For the purposes of this application:

[0025] The term “invention” or simply “invention” may be used interchangeably with the term “this application” or simply “application”.

[0026] The term "first aspect of the invention" refers to a method of operating an SOEC fuel cell stack. The term "second aspect of the invention" refers to a system (apparatus) for operating an SOEC fuel cell stack. The term "system (apparatus)" is used interchangeably with "apparatus".

[0027] The operation method includes transient operation, which in one implementation is any one of startup operation, normal operation, hot standby operation, and shutdown operation.

[0028] The term "and / or" in relation to a given implementation means any of the three options. The term "and / or" may be used interchangeably with the term "at least one of the three options".

[0029] The term "contains" includes "contains only", that is, "composes of".

[0030] The term “suitable” means “optional”, that is, an optional implementation method.

[0031] The term "SOEC" or "SOECs" refers to a solid oxide electrolyzer and is used interchangeably with the term "SOE cell." The term "SOEC stack" refers to an SOEC stack assembly. A solid oxide electrolyzer unit, or SOEC unit, contains one or more SOEC stacks. An SOEC unit may also be referred to as a "module."

[0032] The terms "fuel side" and "cathode side" are used interchangeably. It will also be understood that, in electrolysis operation mode, the cathode refers to the fuel electrode.

[0033] The terms "oxygen side" and "anodine side" are used interchangeably. It should be understood that the anode in electrolysis operation mode refers to the oxygen electrode.

[0034] The term "startup method" refers to the operation of an SOEC stack from a low initial temperature (e.g., room temperature) until it reaches the normal operating temperature and normal operation of the SOEC. It should be understood that "startup" is a specific form of transient operation.

[0035] The term "transient operation" refers to the discontinuous operation of an SOEC fuel cell stack, where the stack has not yet reached the steady state corresponding to normal operation (including continuous current supply). Therefore, no hydrogen product gas is typically produced during transient operation.

[0036] The term "room temperature" refers to 15-25°C.

[0037] The term "normal operation" means a continuous application of a temperature typically 0.1 A / cm. 2 Or a higher current, such as 0.45 A / cm 2 Or higher, and at 600-1000°C, appropriately 650-900°C, such as 700-750°C or 700-800°C, which is the normal SOEC stack operating temperature.

[0038] The term "SOEC stack operating at open-circuit voltage (OCV)" means that there is no external load connected and no current flowing, and therefore no product gas (e.g., hydrogen) is produced. The potential difference is solely due to the difference in pO2 at the two electrodes.

[0039] The term "hot standby operation" is also known as "hot standby" or "hot standby mode." "Hot standby operation" is defined as the open-circuit voltage (OCV) operation of an SOEC fuel cell at a temperature lower than the normal SOEC fuel cell operating temperature (e.g., 10-200°C lower, for example, 50-200°C or 100-200°C lower). Under OCV, there is no external load connected and no current flows, thus no product gas is generated. The current density is 0 A / cm². 2 The potential difference is solely due to the difference in pO2 at the two electrodes. It should be understood that thermal standby is another specific form of transient operation.

[0040] The term "shutdown operation," or simply "shutdown," refers to the process of removing an SOEC fuel cell stack from normal operation by cooling the stack (e.g., to room temperature) and removing the current applied during normal operation. It should be understood that shutdown is another specific form of transient operation.

[0041] The term “rich in X” (e.g., where X is hydrogen) is understood to mean “the concentration of X in the stream is increased compared to the concentration of X in the corresponding feed gas”.

[0042] As used herein, the term "electrochemically generated" refers to the process by which chemical species are formed through electrochemical processes (i.e., chemical processes involving electron transfer). Such processes include, for example, the oxygen evolution reaction (2O₂). 2- = O2 + 4e - ) and water reduction reaction (H2O + 2e) - = H2 + O 2- ) .

[0043] The term "at least a portion of a gas flow (such as an anode-side product gas flow)" means using the entire gas flow or a portion (share) of that gas flow. A gas flow can simply be divided into shares with the same composition. This does not refer to the separation of gas components.

[0044] The terms "steam" and "water" are used interchangeably.

[0045] The use of the article "a" or "an" means at least one. For example, the term "a protectant bed reactor" means "at least one protectant bed reactor".

[0046] Other definitions are provided in conjunction with one or more of the above or below implementation schemes.

[0047] In one implementation, transient operation includes:

[0048] - At least a portion of the first fuel-side outlet gas is supplied to the guard bed reactor as first fuel-side recirculated gas.

[0049] According to the invention, the formation gas following the guard bed reactor is suitably supplied to the SOEC cells of the SOEC stack. This single SOEC stack in a hot state (i.e., "single hot SOEC stack") not only protects itself from nickel oxidation but also protects other SOEC stacks in the SOEC cell. Transient operation (e.g., startup) has proven to be fast, simple, and efficient because, for example, only a single SOEC stack is initially heated. Although, for example, only one stack (i.e., "single hot SOEC stack") is heated and supplied with hydrogen to protect the electrodes, other SOEC stacks are also protected due to the provision of a recirculation system. A first fuel-side recirculation gas containing a small amount of H2 (e.g., 2-10 vol.% H2) is advantageously supplied to protect the single hot SOEC stack and subsequently to protect the other SOEC stacks during recirculation. A hydrogen recirculation blower is suitably provided for this recirculation.

[0050] In one implementation, transient operation includes:

[0051] - The first fuel-side recirculated gas is combined with the ammonia stream (suitably a pure ammonia stream) to form the ammonia-containing feed gas, which further contains hydrogen.

[0052] Therefore, more specifically, the feed gas containing ammonia also contains some hydrogen used to protect the battery and produced in the guard bed reactor upstream of the SOEC stack (and thus upstream of the SOEC cell). Instead of using hydrogen, for example, from a gas cylinder or any other storage device, for startup, an ammonia stream (e.g., a pure ammonia stream) is used. Ammonia is easier and safer to transport and store than hydrogen. The first fuel-side recirculated gas is combined (e.g., mixed) with the ammonia stream and then supplied to the guard bed reactor.

[0053] It should be understood that the term "first fuel-side recirculated gas" defines the cathode-side outlet gas that is recirculated during transient operation (e.g., startup).

[0054] The term "pure ammonia stream" refers to an ammonia stream from a storage device, such as a stream containing at least 99.98% NH3.

[0055] In one implementation, transient operation is a startup operation, which includes:

[0056] - At the inlet of the protective bed reactor, the feed gas containing ammonia is heated to a temperature of 400-900°C, for example, 450-850°C or 500-800°C;

[0057] - When approaching or reaching the normal operating temperature of the SOEC stack (the normal operating temperature is 650-900°C), the feed gas containing ammonia and optionally further containing hydrogen and / or nitrogen is replaced with a feed gas containing steam and optionally further containing hydrogen.

[0058] Ammonia cracking produces a mixture of N2 and 75 vol.% H2, while the target is 1-10 vol.% H2. Therefore, for example, a feed gas containing ammonia and optionally hydrogen and / or nitrogen is a stream containing 75 vol.% H2 with the balance (i.e., 25 vol.%) N2.

[0059] For example, the feed gas containing ammonia and optionally hydrogen and / or nitrogen is a stream containing 90-98 vol.% NH3 and 2-10 vol.% H2.

[0060] For example, the feed gas, which contains steam and optionally hydrogen, is a stream containing 90-98 vol.% H2O and 2-10 vol.% H2.

[0061] The inlet temperature of the aforementioned guard bed reactor is suitable for ammonia cracking. Ammonia cracking to hydrogen is an endothermic reaction, thus a temperature drop occurs in the catalyst-containing bed (i.e., the catalyst bed), providing a forming gas at a temperature 30-100°C lower than the inlet temperature of the guard bed reactor. The SOEC stack is then heated to its normal operating temperature, which is 650-900°C, for example, 700-800°C. At these temperatures, the SOEC stack is ready for a full transition to operation using the actual feed gas (i.e., steam) to produce hydrogen as a product.

[0062] In one embodiment, startup includes gradually replacing the ammonia-containing feed gas by first combining a steam-containing feed gas (e.g., a pure steam stream) with an ammonia-containing feed gas (e.g., an ammonia stream, such as a pure ammonia stream), and then interrupting the flow of the ammonia-containing feed gas, for example by interrupting the supply of the ammonia stream (e.g., a pure ammonia stream).

[0063] Therefore, in one embodiment, the feed gas containing steam is pure steam, i.e., a pure steam stream. The term "pure steam" refers to steam drawn from a steam drum. Suitably, the system of the present invention includes a steam drum.

[0064] As the SOEC stack approaches or reaches its normal operating temperature range (e.g., 700-800°C), hydrogen is still produced through ammonia cracking in the protective layer, which even allows some oxygen to migrate from the oxygen side of the SOEC stack to the fuel side. The migrating oxygen decomposes and is converted into H2O by the H2 produced by NH3 cracking, further protecting the fuel electrode from adverse nickel oxidation.

[0065] Furthermore, since the guard bed reactor can be used as a cleaning unit for removing impurities during normal operation of the SOEC stack (because this unit is suitable for steam electrolysis), a smooth transition to normal operation is achieved. This results in safe and smooth transient operation with minimal modifications compared to, for example, startup. Typically, a smooth and rapid transition from a given transient operation to normal operation is achieved.

[0066] In one implementation, the method further includes:

[0067] Normal operation includes:

[0068] - By continuously applying a load (i.e. current) to the SOEC stack, the OCV operation of the SOEC stack is changed to normal operation in electrolysis mode;

[0069] - Supply at least a portion of the steam-containing feed gas to the guard bed reactor; and remove a clean feed gas containing steam and optionally also containing hydrogen from the guard bed reactor;

[0070] - At least a portion of a clean gas containing steam and optionally also containing hydrogen is supplied to the fuel side of at least one SOEC of the SOEC stack; and a second fuel side outlet gas is removed from at least one SOEC of the SOEC stack.

[0071] It should be understood that the methods of operating an SOEC stack include transient operation, and in one implementation, transient operation is startup.

[0072] It should be understood that the operation method of SOEC stacks also includes normal operation.

[0073] It should be understood that the term "second fuel-side outlet gas" defines the fuel-side outlet gas extracted under normal operating conditions.

[0074] When a continuous current is applied and normal operation (including not only normal operating temperature but also continuous operation) is established, hydrogen is generated in the SOEC, thus allowing the ammonia feed stream to be interrupted (i.e., stopped). Steam is supplied instead of ammonia. A gradual transition method, as previously described, in which ammonia is initially combined with steam, is also envisioned. Furthermore, a clean feed gas containing steam is now removed from the guard bed reactor. Thus, the guard bed reactor, during normal operation, acts as a cleaning unit for removing impurities from the steam-containing feed gas, which is suitably a steam stream drawn from the steam drum of the system (unit). An example of an impurity that needs to be removed is silicon, as will become apparent from the embodiments described below.

[0075] In one implementation, normal operation further includes:

[0076] A portion of the second fuel-side outlet gas is supplied to the guard bed reactor as a second fuel-side recirculation gas; suitably, the second cathode recirculation gas is combined with the steam-containing feed gas before the feed gas is supplied to the guard bed reactor.

[0077] It should be understood that the term "second fuel-side recirculation" defines the fuel-side outlet gas that is recirculated under normal operating conditions.

[0078] Therefore, a portion of the hydrogen produced in the SOEC stack is still recycled to protect the fuel electrodes by keeping the nickel therein in its reduced form. The feed gas containing vapor may, for example, contain 2-10 vol.% H2. Thus, as described above, the feed gas containing vapor is, for example, 90-98 vol.% H2O and 2-10 vol.% H2.

[0079] In one implementation scheme, normal operation includes:

[0080] - The feed gas containing steam is continuously heated to a temperature of 650-900°C, for example, 700-800°C, at the inlet of the protective bed reactor.

[0081] Since the feed gas to the SOEC stack is now steam, the guard bed reactor acts as a cleaning unit for removing undesirable impurities, such as silicon (Si) or silicon derivatives (e.g., silicon dioxide (SiO2)); or volatile substances derived therefrom, such as (Si(OH)4), which may deposit on and / or react with the metal oxides of downstream units or equipment (including the SOEC stack). The Si content is reduced to ppb levels in the guard bed reactor, thereby increasing the lifespan of the SOEC stack and consequently the SOEC unit. As will become apparent from the embodiments described below, the guard bed reactor necessary for removing such impurities during normal operation of H2 electrolysis is advantageously arranged to have a dual function: ammonia cracking during transient operation (e.g., start-up until normal operating temperature is reached); and impurity removal when approaching or reaching the temperature required for normal operation and when the ammonia feed gas is replaced or gradually replaced by a feed gas containing steam.

[0082] In one embodiment, heating any one of the ammonia-containing feed gas, the steam-containing feed gas, or a combination thereof during start-up and / or normal operation includes heat exchange with the first fuel-side or second fuel-side recirculated gas.

[0083] This achieves a high degree of thermal integration because the fuel-side outlet gas extracted from the SOEC stack at gradually increasing temperatures (suitably before being recycled and merged with any feed gas stream) is used as a heat exchange medium to at least preheat the feed gas stream entering the guard bed reactor. Suitably, the fuel-side outlet gas is also advantageously used as a heat exchange medium before being recycled to preheat a liquid water stream from a storage unit, such as boiler feedwater (BFW) or water from the mains; this water stream is converted, for example, by further heating, into steam stored in the steam drum, which is then suitably used as the steam-containing feed gas, thus as a pure steam stream.

[0084] In one embodiment, during transient operation and / or normal operation, the catalyst active in the cracking of ammonia into nitrogen and hydrogen is any one of a nickel (Ni) catalyst, an iron (Fe) catalyst, or a combination thereof.

[0085] For example, the active catalysts in the cracking of ammonia into nitrogen and hydrogen are single-metal nickel catalysts or single-metal iron catalysts.

[0086] For example, a nickel-containing catalyst contains 20-60 wt% Ni and 40-80 wt% any oxide or combination thereof of Al, Ca, Mg, or other metals, optionally promoted by a lanthanide metal oxide (such as La2O3).

[0087] It should be understood that the term "lanthanide" refers to any one of the fifteen elements in the periodic table, from La to Lu. Lanthanum (La) as used in this article is also a lanthanide element.

[0088] For example, this catalyst is an iron-based catalyst in the form of a single-metal catalyst system with Fe as the metal.

[0089] For example, this catalyst is a nickel-based catalyst in the form of a single-metal catalyst system with Ni as the metal.

[0090] For example, a catalyst active in the cracking of ammonia into nitrogen and hydrogen is a bimetallic catalyst, or a metal alloy selected from any combination of Fe, Co, Ru, Ni, or iron-cobalt based catalyst. Iron-cobalt catalysts can be bimetallic or alloys.

[0091] Therefore, for example, this catalyst is an iron-cobalt based catalyst in the form of a bimetallic catalyst system with Fe and Co as metal alloys.

[0092] For example, this catalyst is a nickel-cobalt based catalyst in the form of a bimetallic catalyst system with Ni and Co as metal alloys.

[0093] Suitable catalysts active in the cracking of ammonia into nitrogen and hydrogen are co-catalysts of any one of K2O, CaO, SiO2, and Al2O3.

[0094] For example, catalysts containing Ni can be supported, such as nickel supported on alumina, i.e., Ni / Al2O3.

[0095] For example, Ni-containing catalysts may also contain active metal (Me) oxides, as described in the embodiments below. These Me oxides can act as co-catalysts for ammonia cracking, but are not active themselves.

[0096] For example, Fe-containing catalysts can be supported, such as Fe-Co / Al2O3, or unsupported, such as Fe fused with any of K2O, CaO, or Al2O3. Due to the lower price of iron, Fe-based or Fe-Co-based catalysts offer a more affordable solution. Furthermore, they may be more active than single-metal Ni catalysts, enabling operation at lower temperatures during start-up, hot standby, or shutdown.

[0097] It should be understood that wt% is relative to the total weight of the catalyst.

[0098] It should be understood that the total wt% sums up to 100%.

[0099] In one embodiment, the protective bed reactor includes:

[0100] A bed comprising the catalyst active in the cracking of ammonia into nitrogen and hydrogen, wherein the catalyst comprises nickel and an active metal (Me) oxide; wherein the active metal (Me) is selected from the group consisting of: alkaline earth metals (including Ca, Mg, Sr); transition metals (including Zr); rare earth metals (including La and Ce); or mixtures thereof.

[0101] Therefore, the catalyst active in ammonia cracking and the active metal (Me) oxide (the latter being a clean material, i.e., an impurity-binding material) are provided as a single material, namely the catalyst.

[0102] While ammonia cracking activity is absolutely necessary during, for example, startup, the presence of Ni is also required during normal operation. Surprisingly, the Ni-containing catalyst bed also acts as a pre-cleaning bed, which helps remove impurities such as Si and / or S contained in the incoming steam. Thus, a single material provides a dual function as both an ammonia cracking catalyst and an impurity binding material, thereby also removing impurities such as Si and / or S. The nickel surface becomes a sulfur-binding material, thus enabling not only ammonia cracking but also, due to the active Me oxide's ability to remove S while simultaneously removing Si, providing a highly synergistic effect.

[0103] A guard bed reactor includes a guard bed, and more specifically a three-dimensional (3D) bed structure.

[0104] The impurity is at least one of Si (silicon), N (nitrogen), S (sulfur), P (phosphorus), As (arsenic), or a compound thereof. For example, the impurity is SiO2(s) or Si(OH)4(g). For example, the impurity is NOx (nitrogen oxides). For example, the impurity can be H2S. For example, the impurity is a combination thereof.

[0105] In addition to the catalyst active in the ammonia cracking to nitrogen and hydrogen, the guard bed reactor may also contain a separate and distinct cleaning material, i.e., an impurity binding material. Accordingly, in another embodiment, the guard bed reactor further comprises an impurity binding material, which is at least any one of silicon-binding materials, sulfur-binding materials, or combinations thereof; wherein the impurity binding material comprises a metal (Me) oxide, wherein the metal (Me) is selected from the group consisting of: alkaline earth metals (including Ca, Mg, Sr); transition metals (including Zr); rare earth metals (including La and Ce); or mixtures thereof.

[0106] For the purposes of this application, the term "binding" should be understood as a general term encompassing several types of interactions between molecules or particles, including adsorption, absorption, and reaction. Although adsorption, absorption, and reaction are distinct processes, they can all be considered as binding interactions of a type between molecules and / or particles.

[0107] Similarly, when the ammonia cracking catalyst and the impurity binding material (e.g., the S-binding material) are provided as separate and distinct materials, the nickel surface can, for example, be used as the sulfur binding material. For instance, the active catalyst for ammonia cracking in a guard bed reactor is an Fe-containing catalyst (e.g., the aforementioned iron-cobalt-based catalyst); this catalyst does not contain Ni. The guard bed reactor may also contain both Si and S-binding materials. The Si-binding material is an active Me oxide, while the S-binding material is, for example, a nickel-based catalyst material.

[0108] In another embodiment, where the catalyst and impurity combining material (i.e., the cleaning material) active in ammonia cracking are provided as separate and distinct materials, the protective bed reactor comprises:

[0109] - A single bed, or bed layer, includes the following two:

[0110] The catalyst that is active in the cracking of ammonia into nitrogen and hydrogen, and

[0111] The impurity-binding material is preferably a silicon-binding material;

[0112] and / or

[0113] - Includes a bed of catalyst active in the cracking of ammonia into nitrogen and hydrogen; and a separate bed containing the impurity binding material (suitably a silicon binding material).

[0114] For example, a guard bed reactor comprises a bed (i.e., a single bed) containing a mixture of the following:

[0115] As a catalyst provided as an active catalyst in ammonia cracking, it may be in the form of particles, spheres, or monolithic blocks; and may be individual but located within the same bed.

[0116] Particles, spheres, or monolithic blocks provided as impurity binding materials (suitably as silicon binding materials).

[0117] It should be understood that a guard bed reactor may include multiple such single-bed layers.

[0118] For example, a guard bed reactor comprises separate beds: a bed containing particles, spheres, or monolithic blocks provided as an active catalyst for ammonia cracking; and a separate (other) bed containing particles, spheres, or monolithic blocks provided as an impurity binding material (suitably as a silicon binding material), thus the separate bed being a Si guard bed.

[0119] Any combination of the above items was also envisioned.

[0120] For example, appropriately, in the following order:

[0121] A single bed, comprising the catalyst active in the cracking of ammonia into nitrogen and hydrogen, and the impurity binding material (suitably a silicon-binding material);

[0122] A bed comprising the catalyst active in the cracking of ammonia into nitrogen and hydrogen; and

[0123] A separate bed containing the impurity-binding material (suitably a silicon-binding material).

[0124] Suitable, the Si guard bed is arranged directly upstream of the fuel-side inlet of the SOEC or SOEC stack.

[0125] For the purposes of this application, the term "direct" means that there are no intermediate units or process steps between the Si guard bed and the fuel-side inlet of the SOEC or SOEC stack.

[0126] More generally, the term "direct" means without altering the intermediate units or steps that constitute the process flow.

[0127] Suitably, the Si guard bed contains less than 50,000 ppb, for example less than 1,000 ppb, less than 100 ppb, or less than 10 ppb of Si in molar quantities. Thus, in one embodiment, the guard bed contains less than 10 ppb of Si in molar quantities.

[0128] Another undesirable species in a Si-protected bed reactor is potassium oxide. In one embodiment, the Si-protected bed contains less than 5000 ppm, for example less than 2000 or 1000 ppm of potassium (K) on a molar basis.

[0129] It has been found that a Si guard bed should preferably not contain any significant amount of Si, and the components of a Si guard bed reactor should preferably not contain any significant amount of Si. Any Si present in a Si guard bed reactor poses a risk of reacting with the steam in the feed, thereby adversely affecting the downstream of the Si guard bed reactor.

[0130] Another impurity is sulfur (S). Accordingly, as previously mentioned, the guard bed reactor can be equipped with a sulfur-binding material; for example, in series as two separate beds in a single vessel, or the guard bed may contain a sulfur-binding material in addition to a Si-binding material, or the Si-binding material may also bind sulfur. In one embodiment, the guard bed contains Ni as the sulfur-binding material; therefore, advantageously, the ammonia cracking active catalyst is a Ni-containing catalyst, for example, the catalyst containing nickel and an active metal (Me) oxide; wherein the active metal (Me) is selected from the group consisting of: alkaline earth metals (including Ca, Mg, Sr); transition metals (including Zr); rare earth metals (including La and Ce); or mixtures thereof. Thus, a bed containing this catalyst as a single material is capable of ammonia cracking while removing at least S and Si impurities.

[0131] Sulfur-protected beds work by reacting with sulfur-containing compounds (such as hydrogen sulfide and sulfur dioxide) and binding them to the surface of the bed. The materials used in sulfur-protected bed reactors can vary, but common materials include metal oxides (such as zinc oxide, nickel oxide, and copper oxide) or activated carbon.

[0132] When the Si guard bed is saturated with silica or its efficiency is low, it can be regenerated or replaced. Regeneration is carried out in high-temperature / high-pressure steam.

[0133] In one implementation scheme, normal operation includes:

[0134] - Take out a portion (preferably all) of the second fuel side outlet gas as a separate hydrogen stream (e.g., hydrogen product stream).

[0135] Therefore, the SOEC stack and the SOEC unit can also achieve normal operation (including continuous operation) by continuously extracting the required H2 product. 。 Recirculation to the SOEC stack fuel side can be reduced or eliminated so that the hydrogen produced therefrom (suitably all of it) can be removed as a hydrogen product stream.

[0136] In one implementation, transient operation is hot standby operation. Hot standby operation means, as previously described, the SOEC stack operating at an open-circuit voltage (OCV) below the normal SOEC stack operating temperature (e.g., 10-200°C, for example, 50-200°C or 100-200°C lower).

[0137] Hot standby mode is suitable for situations where power supply or demand for hydrogen products is low.

[0138] Typically, SOEC units in hot standby mode operate by supplying a nitrogen-forming gas containing hydrogen (e.g., 2-10 vol.% H2). This invention provides a feed gas containing ammonia supplied to a guard bed reactor, thereby generating hydrogen and nitrogen as forming gases and thus maintaining the nickel in the fuel electrode (cathode) in its reduced, active state.

[0139] In one implementation, transient operation is shutdown operation. Shutdown operation means, as previously described, removing the SOEC stack from normal operation by cooling the SOEC stack to below 100°C (e.g., to room temperature) and removing the current applied during normal operation.

[0140] According to a second general embodiment of the first aspect (method) of the present invention, a method is provided for operating a solid oxide electrolyzer (SOEC) stack for hydrogen production, the SOEC stack comprising at least one solid oxide electrolyzer (SOEC) including an electrolyte layer sandwiched between a fuel side and an oxygen side, the method comprising:

[0141] Transient operation, such as any one of startup operation, hot standby operation, and shutdown operation, wherein transient operation includes:

[0142] - Operate the SOEC stack at open-circuit voltage (OCV);

[0143] - Provide feed gas containing ammonia;

[0144] - At least a portion of the feed gas containing ammonia is directly supplied to the fuel side of at least one solid oxide electrolyzer (SOEC) of the SOEC stack; the fuel side contains nickel (Ni);

[0145] - Take out the first fuel side outlet gas from at least one SOEC of the SOEC stack.

[0146] Therefore, the protective bed reactor was bypassed during transient operation. This resulted in in-situ H2 generation within the SOEC, as the active Ni surface of the SOEC cell also acted as an NH3 cracker. The environment thus remained reducing, preventing the unfavorable oxidation of Ni to NiO by oxygen migrating from the oxygen side.

[0147] The fuel electrode is preferably a Ni-YSZ (nickel-yttrium-zirconia) electrode or a Ni-GDC (nickel-gadolinium oxide-doped cerium oxide) electrode, which itself provides nickel to facilitate the cracking of ammonia into hydrogen.

[0148] In a second aspect of the invention, a system (apparatus) is also provided, the system (apparatus) comprising:

[0149] - A protective bed reactor containing an active catalyst in the cracking of ammonia into nitrogen and hydrogen; the protective bed reactor is arranged to receive a feed gas containing ammonia and to provide a forming gas containing nitrogen and hydrogen;

[0150] - A solid oxide electrolyzer (SOEC) stack for hydrogen production, the SOEC stack including at least one solid oxide electrolyzer (SOEC) including an electrolyte layer sandwiched between a fuel side and an oxygen side; the fuel side of the at least one solid oxide electrolyzer (SOEC) of the SOEC stack is arranged to receive at least a portion of a forming gas comprising nitrogen and hydrogen, and to provide a first fuel side outlet gas.

[0151] - A power source suitable for operating the SOEC stack at least at open-circuit voltage (OCV).

[0152] In a second aspect of the invention, a system (apparatus) is also provided for performing a method according to any embodiment of a first general embodiment of the first aspect of the invention, the system comprising:

[0153] - A protective bed reactor containing an active catalyst in the cracking of ammonia into nitrogen and hydrogen; the protective bed reactor is arranged to receive a feed gas containing ammonia and to provide a forming gas containing nitrogen and hydrogen;

[0154] - A solid oxide electrolyzer (SOEC) stack for hydrogen production, the SOEC stack including at least one solid oxide electrolyzer (SOEC) including an electrolyte layer sandwiched between a fuel side and an oxygen side; the fuel side of the at least one solid oxide electrolyzer (SOEC) of the SOEC stack is arranged to receive at least a portion of a forming gas comprising nitrogen and hydrogen, and to provide a first fuel side outlet gas.

[0155] - Power supply suitable for operating the SOEC stack at least at open-circuit voltage (OCV).

[0156] It should be understood that the term "power supply, which is suitable for operating SOEC stacks at least at open-circuit voltage (OCV)" means that the power supply is also suitable for operating SOEC stacks by applying a load, i.e., current, which corresponds to normal operation.

[0157] In one embodiment, the system includes a conduit for supplying at least a portion of the first fuel-side outlet gas as a first fuel-side recirculated gas to the guard bed reactor.

[0158] It should be understood that the term "conduit" refers to a process pipeline (e.g., a pipe) used to transport a given process flow.

[0159] In one embodiment, the system includes a mixing point (e.g., a mixing unit or confluence point) arranged to combine the first fuel-side recirculated gas with an ammonia stream (suitably a pure ammonia stream) into the ammonia-containing feed gas, which further contains hydrogen.

[0160] In one implementation, the system includes:

[0161] - Heating device, such as one or more heat exchangers, for heating the feed gas containing ammonia to a temperature of 400-900°C, for example 450-850°C or 500-800°C at the inlet of the protective bed reactor;

[0162] - A conduit for replacing the feed gas containing ammonia (and optionally further containing hydrogen) with a feed gas containing steam (and optionally further containing hydrogen); the guard bed reactor is arranged to receive the feed gas containing steam (and optionally further containing hydrogen).

[0163] In one implementation, the system includes:

[0164] - A device, such as a valve (e.g., one or more valves), is arranged to gradually replace the ammonia-containing feed gas by first combining a steam-containing feed gas with an ammonia-containing feed gas and then interrupting the flow of the ammonia-containing feed gas, for example by interrupting the supply of an ammonia stream (e.g., a pure ammonia stream).

[0165] In one implementation, the system includes:

[0166] - The power supply is adapted to operate the SOEC stack under continuous load, thereby providing current to enable the normal operation of the SOEC stack;

[0167] - The conduit is used to supply at least a portion of the steam-containing feed gas to the guard bed reactor; and to provide a clean feed gas containing steam (and optionally also hydrogen) from the guard bed reactor;

[0168] - A conduit for supplying at least a portion of a clean gas containing steam and optionally also hydrogen to the fuel side of at least one SOEC of the SOEC stack; and for providing a second fuel side outlet gas from at least one SOEC of the SOEC stack.

[0169] In one embodiment, the system is further arranged to supply a portion of the second fuel-side outlet gas as a second fuel-side recirculated gas to the guard bed reactor; suitably, the system includes a mixing point (e.g., a mixing unit or confluence point) arranged to combine the second fuel-side recirculated gas with the steam-containing feed gas before the feed gas is supplied to the guard bed reactor.

[0170] In one embodiment, the system includes a heat exchanger (e.g., one or more heat exchangers) arranged to heat any one of the ammonia-containing feed gas, the steam-containing feed gas, or a combination thereof by exchanging heat with the first or second fuel-side recirculated gas.

[0171] In one embodiment, in the protective bed reactor of the system, the catalyst active in the cracking of ammonia into nitrogen and hydrogen is any one of a nickel (Ni) catalyst, an iron (Fe) catalyst, or a combination thereof.

[0172] In one embodiment, the protective bed reactor of the system includes:

[0173] A bed comprising the catalyst active in the cracking of ammonia into nitrogen and hydrogen; wherein the catalyst comprises nickel and an active metal (Me) oxide; wherein the active metal (Me) is selected from the group consisting of: alkaline earth metals (including Ca, Mg, Sr); transition metals (including Zr); rare earth metals (including La and Ce) or mixtures thereof.

[0174] Therefore, the catalyst and cleaning material (i.e., impurity binding material) active in ammonia cracking are provided as a single material. Thus, this single material provides the dual function of being both an ammonia cracking catalyst and an impurity binding material, thereby also removing impurities such as Si.

[0175] A guard bed reactor comprises a guard bed, more specifically a three-dimensional (3D) bed structure.

[0176] Impurities can be at least one of Si (silicon), N (nitrogen), S (sulfur), P (phosphorus), As (arsenic), or compounds thereof. For example, an impurity is SiO2(s) or Si(OH)4(g). For example, an impurity is NOx (nitrogen oxides). For example, an impurity is H2S. For example, an impurity is a combination of the above.

[0177] In addition to the catalyst active in the ammonia cracking to nitrogen and hydrogen, the guard bed reactor may also contain a separate and distinct cleaning material, i.e., an impurity binding material. Accordingly, in one embodiment, the guard bed reactor of the system further comprises an impurity binding material, which is at least any one of silicon-binding materials, sulfur-binding materials, or combinations thereof; wherein the impurity binding material comprises a metal (Me) oxide, wherein the metal (Me) is selected from the group consisting of: alkaline earth metals (including Ca, Mg, Sr); transition metals (including Zr); rare earth metals (including La and Ce); or mixtures thereof.

[0178] In another embodiment, the catalyst and cleaning material (i.e., impurity-binding material) active in ammonia cracking are provided as separate and distinct materials, and the protective bed reactor of the system comprises:

[0179] - Bed layer, which includes the following two:

[0180] The catalyst that is active in the cracking of ammonia into nitrogen and hydrogen, and

[0181] The impurity-binding material is preferably a silicon-binding material;

[0182] and / or

[0183] - A bed comprising a catalyst active in the cracking of ammonia into nitrogen and hydrogen; and a separate bed comprising the impurity binding material, suitably a silicon binding material.

[0184] In one embodiment, the protective bed of the system contains less than 10 ppb of Si on a molar basis.

[0185] In one implementation, the system includes:

[0186] - Power supply, which is suitable for operating SOEC stacks under continuous load, thereby providing current to enable normal operation of SOEC stacks;

[0187] - A conduit for removing a portion (suitably all of it) of the second fuel-side outlet gas as a separate hydrogen stream (suitably a hydrogen product stream).

[0188] Any embodiment related to the first aspect (method) of the invention and its associated benefits may be used in conjunction with the second aspect (system) of the invention, and vice versa.

[0189] The only accompanying drawing shows a schematic diagram of a system for operating an SOEC fuel cell stack according to one embodiment of the present invention.

[0190] Referring to the accompanying drawings, a specific embodiment of a system (apparatus) 100 for operating an SOEC cell comprising an SOEC stack 14 is shown. During transient operation (e.g., startup), the SOEC stack (and thus the SOEC cell) operates at an open-circuit voltage (OCV), thereby producing no hydrogen gas 113. In the following description, operation is illustrated using startup and subsequent normal operation as examples. The SOEC is illustrated herein as an SOEC stack 14, and it should be understood that an SOEC cell comprises one or more SOEC stacks.

[0191] During startup, a pure ammonia stream 101 is provided and combined with a first fuel-side recirculation gas 109' containing some hydrogen to form an ammonia-containing feed gas 103. As the SOEC unit 14 is heated to its normal operating temperature, the first fuel-side recirculation gas 109' provides increasing amounts of hydrogen generated in the guard bed reactor 12, causing the ammonia-containing feed gas 103 to further contain hydrogen and / or nitrogen. This ammonia-containing feed gas 103 is then preheated in a heat exchanger (e.g., preheating unit 10) to a preheated stream 103' by exchanging heat with a first fuel-side outlet gas 107 taken from the SOEC stack 14, and is taken out at a higher temperature with each cycle through the SOEC. The SOEC unit 14 suitably comprises several stacks and is divided into a fuel (cathode) side 14' and an oxygen (anode) side 14''. A power source (not shown) is adapted to operate the SOEC stack at an open-circuit voltage (OCV). Later, when normal operation is reached, this power source is adapted to provide a continuous load (i.e., current).

[0192] During startup, the first fuel-side outlet gas 107 is cooled to stream 107' in preheating unit 10 and further cooled to stream 107'' via water stream 119' in heat exchanger 18. The first fuel-side outlet gas 107'' is passed to separator 22 to remove water 111 and provide a first fuel-side recirculation stream 109 containing a small amount of hydrogen. This hydrogen is generated in guard bed reactor 12, which includes a guard bed 12' having a catalyst 12' active in the cracking of ammonia into nitrogen and hydrogen, and is arranged upstream of SOEC stack 14. Therefore, a formation gas 105 containing nitrogen and hydrogen is supplied to the fuel side 14' of SOEC stack 14. Preheated feed gas 103' containing ammonia is suitably supplied at the inlet of guard bed reactor 12 at a temperature of 400-900°C. As the SOEC stack 14 is heated to its normal operating temperature, at least a portion (suitably all of it) of the first fuel-side recirculation gas 109 is supplied as the first fuel-side recirculation gas 109' via the hydrogen recirculation blower 24 and combined with the pure ammonia stream 101.

[0193] When approaching or reaching the normal operating temperature of SOEC stack 14 (and thus SOEC unit) (the normal operating temperature is 700-800°C), the power supply is adapted to apply a continuous load (i.e., current). Feed gases 101, 103, 103' containing ammonia (and optionally hydrogen) are replaced with feed gases 121, 103, 103' containing steam (and optionally hydrogen). This can be done gradually by first combining the steam-containing feed gas (e.g., a pure steam stream 121 from steam drum 20) with the ammonia-containing feed gas (e.g., a pure ammonia stream 101); and then interrupting the flow of the ammonia-containing feed gas by interrupting the supply of the ammonia stream (e.g., the pure ammonia stream 101). A liquid water stream 119 (e.g., boiler feedwater (BFW)) is provided and combined with water streams 111 and 117 from separator 22 and water 115 discharged from hydrogen compressor 26, and then pumped as water streams 119' and 119'' to steam drum 20 via pump 116. Optionally, an additional preheating unit (not shown) may be arranged. A pure steam stream 121 is taken from steam drum 20.

[0194] During the normal operation of the SOEC stack, the pure steam stream 121 is optionally combined with a portion of the second fuel-side recirculated gas 109, 109' to form a steam-containing feed gas 103. This feed gas is then preheated into a preheated stream 103' in the preheating unit 10 by heat exchange with the second fuel-side outlet gas 107. The pure ammonia 101 supplied during startup has been gradually reduced or interrupted. The preheated feed gas 103' (now containing steam) enters the guard bed reactor 12, which contains a guard bed 12', which is now advantageously used as a guard bed for removing impurities, suitably for removing silicon impurities, such as Si(OH)4. The clean feed gas 105 containing steam is then extracted and supplied to the fuel side 14' of the SOEC stack 14. As described above, under normal operation, the OCV operation of the SOEC stack is changed to electrolysis mode operation by continuously applying a load (i.e., current) to the SOEC stack. The resulting second fuel-side outlet gas 107 is enriched with hydrogen and exits at, for example, 700-800°C, corresponding to the normal operating temperature of the SOEC stack. After heat is supplied to the heat exchanger (preheating unit) 10, the second fuel-side outlet gas 107' is further cooled in the heat exchanger 18 using water stream 119' as the heat exchange medium, as previously described. Optionally, additional heat exchange units (not shown) may be provided, such as coolers using cooling water as the cooling medium. The second fuel-side outlet gas 107'' is passed to the separator 22 to remove water 111 and provide a second fuel-side recirculation stream 109 containing a small amount of hydrogen. Optionally, a portion of the second fuel-side outlet gas is supplied as second fuel-side recirculation gas 109, 109' to the guard bed reactor 12; however, as normal operation is established and hydrogen is continuously produced in the SOEC unit 14, a portion (suitably all) of the second fuel-side outlet gas 109 is removed via the hydrogen compressor 26 as a separate hydrogen product stream 113. The purge water flow 115 is suitably taken from the hydrogen compressor 26 and merged with the water flow 111 from the upstream separator 22, and further merged as water flow 117 with water flow 119 (e.g., BFW 119).

[0195] In the oxygen side 14'' of the SOEC stack 14, air feed streams 123, 125, and 127 are provided and prepared for further use in the guard bed unit 28 and optionally the drying unit 30, as shown in the figure. A tail stream 129 containing nitrogen and oxygen is drawn from the oxygen side 14'' of the SOEC stack 14, optionally after heat (not shown) is supplied to the air feed stream 127.

Claims

1. A method for operating a solid oxide electrolyzer (SOEC) stack for hydrogen production, the SOEC stack comprising at least one solid oxide electrolyzer (SOEC), the at least one SOEC comprising an electrolyte layer sandwiched between a fuel side and an oxygen side, the method comprising: Transient operation, wherein the transient operation includes: - Operate the SOEC stack at open-circuit voltage (OCV); - Provide feed gas containing ammonia; - At least a portion of the feed gas containing ammonia is supplied to a guard bed reactor, the guard bed reactor containing a catalyst active in the cracking of ammonia into nitrogen and hydrogen; and a forming gas containing nitrogen and hydrogen is removed from the guard bed reactor; - Supply at least a portion of the forming gas containing nitrogen and hydrogen to the fuel side of at least one solid oxide electrolyzer (SOEC) of the SOEC stack; and remove first fuel side outlet gas from at least one SOEC of the SOEC stack.

2. The method according to claim 1, wherein the transient operation is selected from any one of startup operation, hot standby operation, and shutdown operation.

3. The method according to any one of claims 1-2, wherein the transient operation comprises: - At least a portion of the first fuel-side outlet gas is supplied to the guard bed reactor as first fuel-side recirculated gas.

4. The method according to claim 3, wherein the transient operation comprises: - The first fuel-side recirculated gas and the ammonia stream are combined into the ammonia-containing feed gas, which further contains hydrogen, and the ammonia stream is suitably a pure ammonia stream.

5. The method according to any one of claims 1-4, wherein the transient operation is a startup operation, the startup operation comprising: - At the inlet of the protective bed reactor, the feed gas containing ammonia is heated to a temperature of 400-900°C, for example, 450-850°C or 500-800°C; - When approaching or reaching the normal operating temperature of the SOEC stack, the feed gas containing ammonia and optionally further containing hydrogen and / or nitrogen is replaced with a feed gas containing steam and optionally further containing hydrogen; the normal operating temperature is 650-900°C.

6. The method according to claim 5, wherein the startup includes: The ammonia-containing feed gas is gradually replaced by first combining the steam-containing feed gas with the ammonia-containing feed gas, and then interrupting the flow of the ammonia-containing feed gas, for example, by interrupting the supply of an ammonia stream, such as a pure ammonia stream.

7. The method according to any one of claims 5-6, wherein the method further comprises: Normal operation includes: - By continuously applying a load (i.e. current) to the SOEC stack, the OCV operation of the SOEC stack is changed to normal operation in electrolysis mode; - Supply at least a portion of the steam-containing feed gas to the guard bed reactor; and remove a clean feed gas containing steam and optionally also containing hydrogen from the guard bed reactor; - At least a portion of a clean gas containing steam and optionally also containing hydrogen is supplied to the fuel side of at least one SOEC of the SOEC stack; and a second fuel side outlet gas is removed from at least one SOEC of the SOEC stack.

8. The method of claim 7, further comprising: A portion of the gas exiting the second fuel side is supplied to the protective bed reactor as second fuel side recirculation gas. Suitablely, the second fuel-side recirculated gas is combined with the steam-containing feed gas before the feed gas is supplied to the guard bed reactor.

9. The method according to any one of claims 7-8, wherein normal operation comprises: - The feed gas containing steam is continuously heated to a temperature of 650-900°C at the inlet of the protective bed reactor.

10. The method according to any one of claims 5-9, wherein heating any one of the ammonia-containing feed gas, the steam-containing feed gas, or combinations thereof during start-up and / or normal operation comprises heat exchange with the first fuel-side or second fuel-side recirculated gas.

11. The method according to any one of claims 1-10, wherein during transient operation and / or normal operation, the catalyst active in the cracking of ammonia into nitrogen and hydrogen is any one of a nickel (Ni) catalyst, an iron (Fe) catalyst, or a combination thereof.

12. The method according to any one of claims 1-11, wherein the protective bed reactor comprises: A bed containing the catalyst that is active in the cracking of ammonia into nitrogen and hydrogen; The catalyst comprises nickel and an active metal (Me) oxide; wherein the active metal (Me) is selected from the group consisting of: alkaline earth metals including Ca, Mg, and Sr; transition metals including Zr; rare earth metals including La and Ce; or mixtures thereof.

13. The method according to any one of claims 1-11, wherein the protective bed reactor further comprises an impurity binding material, said impurity binding material being at least any one of silicon binding material, sulfur binding material, or combinations thereof; wherein said impurity binding material comprises a metal (Me) oxide, said metal (Me) being selected from the group consisting of: alkaline earth metals including Ca, Mg, and Sr; transition metals including Zr; rare earth metals including La and Ce; or mixtures thereof.

14. The method according to any one of claims 1-11 and 13, wherein the catalyst active in ammonia cracking and the impurity binding material are provided as separate and distinct materials, wherein the protective bed reactor comprises: - Bed layers including the following two: The catalyst that is active in the cracking of ammonia into nitrogen and hydrogen. and The impurity-binding material is preferably a silicon-binding material; and / or - A bed comprising the catalyst active in the cracking of ammonia into nitrogen and hydrogen; and a separate bed comprising the impurity binding material, suitably a silicon-binding material.

15. The method according to any one of claims 7-14, wherein normal operation comprises: - Take out a portion of the second fuel side outlet gas, suitably all of it, as a separate hydrogen stream, suitably as a hydrogen product stream.

16. A method of operating a solid oxide electrolyzer (SOEC) stack for hydrogen production, the SOEC stack comprising at least one solid oxide electrolyzer (SOEC), the at least one SOEC comprising an electrolyte layer sandwiched between a fuel side and an oxygen side, the method comprising: Transient operation, such as any one of startup operation, hot standby operation, and shutdown operation, wherein transient operation includes: - Operate the SOEC stack at open-circuit voltage (OCV); - Provide feed gas containing ammonia; - At least a portion of the feed gas containing ammonia is directly supplied to the fuel side of at least one solid oxide electrolyzer (SOEC) of the SOEC stack; the fuel side contains nickel (Ni); - Take the first fuel side outlet gas from at least one SOEC of the SOEC stack.

17. A system (100) for performing the method according to any one of claims 1-15, comprising: - Protective bed reactor (12), which contains an active catalyst (12') in the cracking of ammonia into nitrogen and hydrogen; The protective bed reactor (12) is arranged to receive a feed gas (103, 103') containing ammonia and to provide a forming gas (105) containing nitrogen and hydrogen. - A solid oxide electrolyzer (SOEC) stack (14) for hydrogen production, the SOEC stack (14) comprising at least one solid oxide electrolyzer (SOEC), the at least one SOEC comprising an electrolyte layer sandwiched between a fuel side (14') and an oxygen side (14'); the fuel side (14) of at least one solid oxide electrolyzer (SOEC) of the SOEC stack (14) is arranged to receive at least a portion of a forming gas (105) comprising nitrogen and hydrogen, and to provide a first fuel side outlet gas (107, 107', 107''); - A power supply suitable for operating the SOEC(14) stack at least at open-circuit voltage (OCV).

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