Method of operating a solid oxide electrolysis cell stack and system for carrying out the method

Abstract

The invention relates to a method of operating a solid oxide electrolysis cell (SOEC) stack for producing hydrogen, and a system for carrying out the method, said SOEC stack comprising at least one solid oxide electrolysis cell (SOEC), said at least one SOEC comprising an electrolyte layer interposed between a fuel-side and an oxy-side, the method comprising transient operation, in which the transient operation comprises: - providing a feed gas comprising ammonia; - supplying at least a portion of said feed gas comprising ammonia to a guard bed reactor, said guard bed reactor comprising a catalyst active in the cracking of ammonia to nitrogen and hydrogen; and withdrawing from said guard bed reactor a forming gas comprising nitrogen and hydrogen; - supplying at least a portion of the intermediate gas comprising nitrogen and hydrogen to the fuel-side of the at least one of the solid oxide electrolysis cells (SOECs) of the SOEC stack; and withdrawing from said at least one of the SOECs of the SOEC stack, a first fuel-side exit gas.

Description

[0001] Method of operating a solid oxide electrolysis cell stack and system for carrying out the method

[0002] The invention relates to a method of operating a solid oxide electrolysis cell (SOEC) stack having a fuel (cathode) side and an oxy (anode) side, the SOEC stack adapted for steam electrolysis to hydrogen, more specifically for the safe and efficient transient operation, such as start-up operation of a SOEC stack as well as for normal operation. The invention relates also to a system (plant) for carrying out the method.

[0003] Solid oxide electrolysis cells (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 gas mixture containing H2 and CO. This conversion occurs on the fuel-(cathode) side of the solid oxide electrolysis cell. On the oxy- (anode) side of the cell, oxygen is electrochemically generated. SOECs as is well- known in the art, are normally stacked to form a SOEC stack. One or more SOEC stacks, such as a plurality of SOEC stacks form a SOEC unit, i.e. a SOEC unit comprises one or more SOEC stacks. A SOEC unit may also be referred to as a module.

[0004] Due to the inherent intermittent nature of the electricity / power sources, as the electricity is often generated from renewable sources such as wind and solar, the SOEC stack is often subject to transient operation, such as start-up or idle. A quick, smooth and efficient transient operation such as start-up until reaching conditions where the SOEC stack continuously produces the desired hydrogen product is posed with a number of challenges.

[0005] Patent application IL 257361 discloses a solid oxide fuel cell (SOFC) for producing electricity in electric vehicles, in which the SOFC includes a catalyst for promoting ammonia cracking to hydrogen (2 NH3 = 3 H2 + N2). SOFC utilizes the H2 as the fuel, i.e. feed gas source, for continuously producing the electricity.

[0006] Similarly, GB 2393320 discloses a fuel cell having ammonia (NH3) as the input fuel and an iron-based catalyst. An yttria stabilized zirconia electrolyte is used as a solid oxide electrolyte. The ammonia fuel is broken down into hydrogen in the cell, the hydrogen being used in the electrolyte to generate electrical power, hence also for continuously producing electricity.

[0007] In contrast thereto, for producing hydrogen, a SOEC stack is typically operated by supplying steam i.e. water (H2O) to the fuel-side of the solid oxide electrolysis cell. An efficient, quick and safe transient operation, such as a starting-up method is required before being able to continuously produce the hydrogen from the supplied steam.

[0008] A typical start-up comprises heating-up a SOEC stack in a SOEC unit while feeding an intermediate gas comprising a reducing gas such as H2, typically 2-10 vol% H2 (the remainder normally being an inert gas such as N2). The intermediate gas may be a forming gas which comprises nitrogen (N2) with some H2, typically 2-10 vol% H2. The H2 enables that nickel (Ni), which is the active material on the fuel-side and with hydrogen being the fuel of the SOEC cell, is kept reduced and active despite any penetration of oxygen taking place from the oxy-side of the SOEC stack. When the normal operation temperature of the SOEC stack is reached and continuous H2 production is going to be started, the forming gas is replaced with steam, which again should have some H2, for instance 2-10 vol% H2. This hydrogen for start-up will typically be arranged by having sufficient H2 bottles or cylinders on site for an initial start-up cycle and a local storage for the following start-ups cycles. This an expensive solution that requires substantial investment and costs for providing, as well as strict measures for safely handling and storing the hydrogen.

[0009] It would therefore be desirable to provide an improved method for operation of a SOEC stack for producing hydrogen, including a method comprising transient operation, such as a safe method of starting-up of the SOEC stack.

[0010] More specifically, it would be desirable to provide a safe method for transient operation, such as start-up of the SOEC stack for producing hydrogen which enables protecting the fuel-side from oxygen that migrates from the oxy-side of the SOEC stack.

[0011] It would also be desirable to be able to provide a smooth transition from transient operation, such as from start-up operation to normal operation of the SOEC stack. It would also be desirable to provide a smooth transition from transient operation, such as start-up operation to normal operation of the SOEC stack, while being able to utilize existing equipment / units associated to the SOEC stack required for said normal operation.

[0012] It would also be desirable to provide an improved method of operating of a SOEC stack for producing hydrogen, the method of operating comprising transient operation, the transient operation being any of start-up operation, hot-idle operation or shut-down operation.

[0013] Hence, in a first general embodiment according to a first aspect of the invention, there is provided a method of operating a solid oxide electrolysis cell (SOEC) stack for producing hydrogen, said SOEC stack comprising at least one solid oxide electrolysis cell (SOEC), said at least one SOEC comprising an electrolyte layer interposed between a fuel-side and an oxy-side, the method comprising: transient operation, in which the transient operation comprises:

[0014] - operating the SOEC stack under open-circuit voltage (OCV);

[0015] - providing a feed gas comprising ammonia;

[0016] - supplying at least a portion of said feed gas comprising ammonia to a guard bed reactor, said guard bed reactor comprising a catalyst active in the cracking of ammonia to nitrogen and hydrogen; and withdrawing from said guard bed reactor a forming gas comprising nitrogen and hydrogen;

[0017] - supplying at least a portion of the forming gas comprising nitrogen and hydrogen to the fuel-side of the at least one of the solid oxide electrolysis cells (SOECs) of the SOEC stack; and withdrawing from said at least one of the SOECs of the SOEC stack, a first fuel-side exit gas.

[0018] Thereby, NH3 cracking via an existing unit required for later normal and continuous operation, namely the guard bed reactor, provides H2 in the forming gas being fed to the SOEC stack for keeping the nickel of the fuel electrode on the fuel-side in its reduced form. The undesirable oxidation of nickel (Ni) to nickel oxide (NiO) by oxygen migrating from the oxy-side through the electrolyte, is minimized or eliminated. Protection of the cathode, i.e. fuel electrode, is thereby achieved while at the same time being able to heat, for instance during start-up operation, the SOEC stack to the required temperature for the normal operation of the SOEC stack and thereby also the SOEC unit comprising the SOEC stacks.

[0019] In an embodiment, the transient operation is selected from any of start-up operation, hot idle operation and shut-down operation.

[0020] The transient operation, such as start-up or hot-idle operation, provides for a dedicated NH3 cracking step which is conducted in the guard bed reactor and enables the storage of liquid ammonia on the plant site rather than the storage of hydrogen. The solution provided by the present invention not only is safer than storing the hydrogen on site, but also fits particularly well in connection with a SOEC-based ammonia plant, i.e. where the end product is ammonia and hence an ammonia converter is arranged downstream the SOEC stack. The solution of the present invention fits also well in other settings such as in a SOEC-based hydrogen plant, e.g. for producing green hydrogen as the end product, as ammonia is significantly easier to have in storage and to transport e.g. to truck than hydrogen.

[0021] During normal operation, which includes continuous operation of the SOEC stack, i.e. after start-up or after hot-idle, as a fuel-side feed gas comprising steam is fed to the fuel-side of a SOEC or SOEC stack, H2O is electrochemically reduced, thereby forming a fuel-side exit gas that is enriched in hydrogen. Similarly, an oxy-side gas stream may be enriched in O2 upon exiting an electrolysis cell by having a higher concentration of O2 than in the oxy-side feed gas stream upon entering the solid oxide electrolysis cell, such as air.

[0022] Also, during normal operation, the normal stack operation temperature, herein also referred to as normal operating temperature, is suitably 600-1000°C, such as 650-900°C, or 700-800°C, e.g. 700, 750°C or 800°C, which is the temperature used during production of H2 in a stack with a continuously applied electrolysis current. The high temperatures, for instance 700 or 800°C or 900°C, are normally required in order to reach sufficient conductivities in the ceramic membranes that are used as electrolytes.

[0023] For the purposes of the present application: The term “present invention” or simply “invention” may be used interchangeably with the terms “present application” or simply “application”, respectively.

[0024] The term “first aspect of the invention” means the method of operating a SOEC stack. The term “second aspect of the invention” means the system (plant) for operating a SOEC stack. The term system (plant) means that the terms “system” and “plant” may be used interchangeably.

[0025] The method of operation includes transient operation, which in an embodiment is any of start-up operation, normal operation, hot-idle operation, and shut-down operation. The term “and / or” means in connection with a given embodiment any of three options. The term “and / or” may be used interchangeably idwith the term “at least one of’ the three options.

[0026] The term “comprising” includes “comprising only” i.e. “consisting of”.

[0027] The term “suitably” means “optionally”, i.e. an optional embodiment.

[0028] The term “SOEC” or “SOECs” means solid oxide electrolysis cell(s) and may be used interchangeably with the term “SOE cell(s)”. The term “SOEC stack” means a stack assembly of SOECs. A solid oxide electrolysis cell unit, i.e. a SOEC unit, comprises one or more SOEC stacks. A SOEC unit may also be referred to as “module”.

[0029] The terms “fuel-side” and “cathode-side” may be used interchangeably. It would also be understood that cathode means fuel electrode in electrolysis operation mode.

[0030] The terms “oxy-side” and “anode side” may be used interchangeably. It would be understood that anode means oxygen (oxy-) electrode in electrolysis operation mode.

[0031] The term “start-up method” means operation of the SOEC stack from a low starting temperature, such as room temperature, up to the normal temperature and normal operation of the SOEC. It would be understood that “start-up” is a specific form of transient operation.

[0032] The term “transient operation” means non-continuous operation of the SOEC stack, where the stack has not reached a steady state corresponding to normal operation, including the supply of a continuous current. Thus, during the transient operation there is often no hydrogen product gas being generated.

[0033] The term “room temperature” means 15-25°C.

[0034] The term “normal operation”, means continuously applying a current of typically 0.1 or higher A / cm2, such as 0.45 A / cm2or higher, and at normal SOEC stack operation temperature of 600-1000°C, suitably 650-900°C, such as 700-750°C or 700-800°C. The term “operating the SOEC stack under open-circuit voltage (OCV)” means that there is no external load connected and there is no electrical current and thereby also no product gases, e.g. hydrogen, being generated. The potential difference is only due to pC>2 differences at the two electrodes.

[0035] The term “hot-idle operation” is also referred to as “hot-idle” or “hot-idle mode”. “Hot- idle operation” is defined as open-circuit voltage (OCV) operation of the SOEC stack at temperatures lower than normal SOEC stack operation temperature, such as 10- 200°C, e.g. 50-200°C or 100-200°C lower than normal SOEC stack operation temperature. At OCV there is no external load connected and there is no electrical current and thereby also no product gases being generated. The current density is 0 A / cm2. The potential difference is only due to pO2 differences at the two electrodes. It would be understood that hot-idle is another specific form of transient operation.

[0036] The term “shut-down operation” or simply “shut-down” means operation of the SOEC stack away from normal operation by cooling down the SOEC stack, for instance to room temperature, and removing the current applied during normal operation. It would be understood that shut-down is another specific form of transient operation.

[0037] The term “enriched in X”, for instance where X is hydrogen, is understood as “the concentration of X in a stream is increased compared to the concentration of X in the corresponding feed gas”.

[0038] As used herein, the term “electrochemically generated” refers to a process where chemical species are formed via an electrochemical process, i.e. a chemical process involving electron transfer. Such processes include e.g. the oxygen evolution reaction (2O2-= 02 + 4e_) and the water reduction reaction (H2O + 2e_= H2 + O2').

[0039] The term “at least a portion of” a certain gas stream such as the anode-side product gas stream, means that the entire gas stream or a portion (fraction) of the stream is used. The gas stream may simply be split into fractions of identical composition. It is not meant to refer to a separation of the components of the gas.

[0040] The term “steam” and “water” may be used interchangeably.

[0041] The use of the article “a” or “an” means at least one. For instance, the term “a guard bed reactor” means “at least one guard bed reactor”.

[0042] Other definitions are provided in connection with one or more of above or below embodiments. In an embodiment, the transient operation comprises:

[0043] - supplying at least a portion of said first fuel-side exit gas, as a first fuel-side recycle gas, to said guard bed reactor.

[0044] By the invention, the forming gas after the guard bed reactor is suitably supplied to SOEC cells of a SOEC stack. This single SOEC stack being hot, i.e. a “single hot SOEC stack”, not only enables self-protection of this stack against nickel oxidation, but also protection of the other SOEC stacks of the SOEC unit. It turns out, that the transient operation, such as start-up, is quick, simple and efficient, as for instance at first only a single of SOEC stack is heated. Although e.g. only one stack i.e. the single hot SOEC stack” is heated and provided with the electrode-protecting hydrogen, the other SOEC stacks are also protected because of the recycle system being provided. The first fuel-side recycle gas, comprising a minor concentration of H2, for instance 2-10 vol.% H2, is advantageously provided for protecting the single hot SOEC stack and subsequently the other SOEC stacks as the recycle is established. A hydrogen recycle blower is suitably provided for conducting such recycle.

[0045] In an embodiment, the transient operation comprises:

[0046] - combining said first fuel-side recycle gas with an ammonia stream, suitably a pure ammonia stream, into said feed gas comprising ammonia, said feed gas comprising ammonia further comprising hydrogen.

[0047] Hence, now more specifically, the feed gas comprising ammonia also comprises some hydrogen used for protecting the cells and generated in the guard bed reactor upstream the SOEC stack and thus upstream the SOEC unit. Instead of using hydrogen from e.g. bottles or any other storage means for e.g. the start-up, an ammonia stream such as a pure ammonia stream is used for the start-up. The ammonia is much easier and safer to transport and store than the hydrogen. The first fuel-side recycle gas is combined, e.g. admixed, with the ammonia stream and then fed to the guard bed reactor.

[0048] It would be understood that the term “first fuel-side recycle gas” defines the cathode side exit gas being recycled under transient operation, such as start-up. The term “pure ammonia stream” means an ammonia stream from storage, such as a stream at least comprising 99.98% NH3.

[0049] In an embodiment, the transient operation is start-up operation, the start-up operation comprising:

[0050] - heating said feed gas comprising ammonia to a temperature of 400-900°C, such as 450-850°C or 500-800°C, at inlet of the guard bed reactor;

[0051] - upon approaching or reaching normal operation temperature of the SOEC stack, said normal operation temperature being 650-900°C, replacing said feed gas comprising ammonia optionally further comprising hydrogen and / or nitrogen, with a feed gas comprising steam optionally further comprising hydrogen.

[0052] The ammonia cracking produces 75% vol. H2 in N2, with the target being 1-10 vol. % H2. Hence, for instance, the feed gas comprising ammonia optionally hydrogen and / or nitrogen, is a stream comprising 75% vol. H2 with the balance, i.e. 25 vol.% being N2.

[0053] For instance, the feed gas comprising ammonia optionally hydrogen and / or nitrogen, is a stream comprising 90-98 vol.% NH3, and 2-10 vol.% H2.

[0054] For instance, the feed gas comprising steam optionally hydrogen, is a stream comprising 90-98 vol.% H2O, and 2-10 vol.% H2.

[0055] The above inlet temperatures of guard bed reactor are suitable for ammonia cracking. The ammonia cracking into hydrogen is an endothermic reaction, so there is a temperature drop across the bed comprising the catalyst, i.e. catalyst bed, thus providing said forming gas at a temperature of 30-100°C lower than the inlet temperatures to the guard bed reactor. The SOEC stack is then heated up to the normal operation temperature, said normal operation temperature being 650-900°C, such as 700-800°C. At these temperatures the SOEC stack(s) is / are ready for being fully transitioned into operation with the actual feed gas, namely steam, for producing hydrogen as product.

[0056] In an embodiment, the start-up operation comprises: gradually replacing said feed gas comprising ammonia by first combining a feed gas comprising steam, such as a pure steam stream, with the feed gas comprising ammonia, such as the ammonia stream e.g. a pure ammonia stream, and later interrupting the flow of the feed gas comprising ammonia, such as by interrupting the supply of the ammonia stream, e.g. pure ammonia stream.

[0057] Hence, in an embodiment, the feed gas comprising steam is pure steam i.e. a pure steam stream. The term “pure steam” means steam withdrawn from a steam drum. Suitably, the steam drum is comprised in the system of the invention.

[0058] As the SOEC stack approaches or reaches the normal operation temperature range of e.g. 700-800°C, hydrogen is still being generated via the ammonia cracking in the guard bed, thereby even tolerating that some oxygen migrates from the oxy-side of the SOEC stack to the fuel-side. The migrated oxygen splits and converts into H2O with the H2 being produced from NHs-cracking, hence further protecting the fuel-electrode against undesired nickel oxidation.

[0059] Furthermore, since the guard bed reactor is already available for use as a cleaning unit for removing impurities during normal operation of the SOEC stack, as this unit is adapted for steam electrolysis, there is a smooth transition to said normal operation. Safe and smooth transient operation with minimum number of modifications with respect to e.g. the start-up is thereby achieved. In general, a smooth and quick transition from a given transient operation into normal operation is achieved.

[0060] Accordingly, the intermediate feed gas may be provided as a clean intermediate feed gas.

[0061] In an embodiment, the method further comprises: normal operation, which comprises:

[0062] - changing the OCV operation of the SOEC stack into normal operation by continuously applying a load, thus an electricity current, to the SOEC stack for operation in electrolysis mode;

[0063] - supplying at least a portion of said feed gas comprising steam to said guard bed reactor; and withdrawing from said guard bed reactor a clean feed gas comprising steam optionally also hydrogen; - supplying at least a portion of the clean gas comprising steam optionally also hydrogen to the fuel-side of at least one of the SOECs of the SOEC stack; and withdrawing from said at least one of the SOECs of the SOEC stack, a second fuel-side side exit gas.

[0064] It would be understood that the method of operating a SOEC stack comprises transient operation and the transient operation is, in an embodiment, start-up.

[0065] It would be understood that the method of operating a SOEC stack comprises also normal operation.

[0066] It would be understood that the term “second fuel-side exit gas” defines the fuel-side exit gas being withdrawn under normal operation.

[0067] When applying a continuous current and thus establishing normal operation, including not only normal operation temperatures but also continuous operation, hydrogen is produced in the SOECs so the ammonia feed stream may be interrupted, i.e. stopped. Steam is supplied instead of ammonia. A gradual transition in which ammonia and steam are first combined, as recited earlier, is also envisaged. In addition, from the guard bed reactor, a clean feed gas comprising steam is now withdrawn. The guard bed reactor acts therefore in normal operation as a cleaning unit for removing impurities from the feed gas comprising steam, which suitably is a steam stream withdrawn from a steam drum of the system (plant). An example of an impurity requiring removal is silicon, as it will also become apparent from a below embodiment.

[0068] In an embodiment, the normal operation further comprises: supplying a portion of said second fuel-side exit gas, as a second fuel-side recycle gas, to said guard bed reactor; suitably by combining said second cathode recycle gas with said feed gas comprising steam prior to supplying the feed gas to the guard bed reactor.

[0069] It would be understood that the term “second fuel-side recycle” defines the fuel-side exit gas being recycled under normal operation.

[0070] Thereby, a portion of hydrogen being produced in the SOEC stack is still recirculated for protecting the fuel electrode by keeping the nickel therein in its reduced form. The feed gas comprising steam may for instance comprise 2-10 vol. % H2. Hence, as recited above, the feed gas comprising steam is for instance 90-98 vol.% H2O and 2-10 vol. % H2.

[0071] In an embodiment, the normal operation comprises:

[0072] - continuously heating said feed gas comprising steam to a temperature of 650-900°C, such as 700-800°C, at inlet of the guard bed reactor.

[0073] As the feed gas to the SOEC stack is now steam, the guard reactor acts as a cleaning unit for removing undesired impurities, for instance silicon (Si) or silicon-derived species such as silica (SiCh); or volatile species derived therefrom such as Si(OH)4 which may deposit and / or react with metal oxides of downstream units or equipment, including the SOEC stack. The Si content is reduced to ppb level in the guard bed reactor, thereby increasing the lifetime of the SOEC stack and thereby SOEC unit. As it will become apparent from a below embodiment, the guard bed reactor, being necessary for removing such impurities during normal operation where the SOEC stack is producing H2, is thus advantageously arranged to have a dual function: ammonia cracking during transient operation such as start-up up to normal operation temperatures; and cleaning of impurities when approaching or reaching the required temperatures of normal operation and the ammonia feed gas is replaced or gradually replaced by the feed gas comprising steam.

[0074] In an embodiment, during start-up operation and / or normal operation, the heating of any of said feed gas comprising ammonia, said feed gas comprising steam, or a combination thereof, comprises heat exchange with said first or second fuel-side recycle gas.

[0075] Thereby, high heat integration is achieved, as the fuel-side exit gas, which is withdrawn at gradually higher temperatures from the SOEC stack, suitably prior to being recycled and combined with any of the feed gas streams, is used as heat exchanging medium for at least preheating the feed gas stream(s) entering the guard bed reactor. Suitably also, the fuel-side exit gas, prior to being recycled, is advantageously used as heat exchanging medium for preheating a liquid water stream from storage, such as boiler feed water (BFW) or from a water main; this water stream is further transformed, e.g. by further heating, into steam which is stored in a steam drum and which is then suitably used as said feed gas comprising steam, hence as a pure steam stream.

[0076] In an embodiment, during transient operation and / or normal operation, said catalyst active in the cracking of ammonia to nitrogen and hydrogen is any of a catalyst comprising nickel (Ni), a catalyst comprising iron (Fe), or a combination thereof.

[0077] For instance, the catalyst active in the cracking of ammonia to nitrogen and hydrogen is a monometallic nickel catalyst, or a monometallic iron catalyst.

[0078] For instance, the catalyst comprising nickel comprises 20-60 wt% Ni, and 40-80 wt% of any oxides of Al, Ca, Mg or combinations thereof, optionally promoted with a lanthanide group metal oxide, such as La2Os.

[0079] It would be understood that that term “lanthanide” refers to any of the fifteen elements from La to Lu in the periodic table of elements. Lanthanum (La), as used herein, is also a lanthanide.

[0080] For instance, the catalyst is a Fe-based catalyst in the form of a monometallic catalyst system having Fe as the metal.

[0081] For instance, the catalyst is a Ni-based catalyst in the form of a monometallic catalyst system having Ni as the metal.

[0082] For instance, the catalyst active in the cracking of ammonia to nitrogen and hydrogen is a bimetallic catalyst, or a metal alloy selected from any of: Fe, Co, Ru, Ni, or combinations thereof, such as a Fe-Co based catalyst. The Fe-Co catalyst may be bimetallic or an alloy.

[0083] Hence, for instance, the catalyst is a Fe-Co-based catalyst in the form of a bimetallic catalyst system having Fe and Co as a metal alloy.

[0084] For instance, the catalyst is a Ni-Co-based catalyst in the form of a bimetallic catalyst system having Ni and Co as a metal alloy. Suitably the catalyst active in the cracking of ammonia to nitrogen and hydrogen is promoted with any of K2O, CaO, SiC>2, AI2O3.

[0085] For instance, the catalyst comprising Ni may be supported, such as nickel supported on alumina i.e. Ni / AhCh.

[0086] For instance, the catalyst comprising Ni may also comprise an active metal (Me) oxide, as recited in a below embodiment. The Me oxide could act as a promoter for the ammonia cracking but not be active in itself.

[0087] For instance, the catalyst comprising Fe may be supported such as Fe-Co / AhCh, or unsupported such as Fe fused with any of K2O, CaO, AI2O3. The Fe-based catalyst or Fe- Co-based catalyst provides a much more inexpensive solution due to the lower price of iron. Further, it may be more active than a monometallic Ni catalyst, thus enabling operation at the lower temperatures of the start-up, hot-idle or shut-down operation.

[0088] It would be understood that the wt% are with respect to the total weight of catalyst.

[0089] It would be understood that the total wt% sums up to 100%.

[0090] In an embodiment, the guard bed reactor comprises: a bed comprising said catalyst active in the cracking of ammonia to nitrogen and hydrogen, in which said 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.

[0091] Hence, said catalyst active in the cracking of ammonia and the active metal (Me) oxide, the latter being the cleaning material, i.e. an impurity-binding material, are provided as a single material, which is said catalyst.

[0092] While ammonia cracking activity is definitely desirable during e.g. start-up, the presence of Ni is also desirable during normal operation. Surprisingly, the catalyst bed comprising the Ni acts also as a pre-cleaning bed which contributes in the removal of impurities such as Si and / or S in the steam being fed thereto. A single material thus provides the dual-function of being a catalyst for ammonia cracking and as impuritybinding material, thereby also removing impurities such as Si and / or S. The nickel surface becomes a sulphur-binding material thus providing a high synergy in terms of not only enabling ammonia cracking, but also removal of S along with removal of Si by virtue of said active Me oxide.

[0093] The guard bed reactor comprises a guard bed, more specifically a 3-dimensional (3D) bed structure.

[0094] The impurity is at least one of Si (silicon), N (nitrogen), S (sulphur), P (phosphorus), As (arsenic), or compounds thereof. For instance, an impurity is SiC>2(s) or Si(OH)4(g). For instance, an impurity is NOX(nitrogen oxides). For instance, an impurity is H2S. For instance, an impurity is a combination thereof.

[0095] The guard bed reactor, apart from comprising said catalyst active in the cracking of ammonia to nitrogen and hydrogen, may also comprise 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, said impurity-binding material being any of at least a silicon-binding material, a sulphur-binding material, or a combination thereof; wherein the impurity-binding material comprises a metal (Me) oxide, in which 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.

[0096] For the purposes of the present application, the term “binding” is to be understood as a general term that encompasses several types of interactions between molecules or particles, including adsorption, absorption, and reaction. While adsorption, absorption, and reaction are distinct processes, they can all be considered types of binding interactions between molecules and / or particles.

[0097] Again, a nickel surface may be for instance be utilized as a sulphur-binding material when providing the ammonia cracking catalyst and the impurity-binding material, e.g. a S-binding material, as separate and distinct materials. For instance, the catalyst active in cracking of ammonia of the guard bed reactor is a catalyst comprising Fe, such as a Fe-Co-based catalyst as described above; the catalyst does not comprise Ni. The guard bed reactor may further comprise a Si-binding material and a S-binding material. The Si-binding material is an active Me oxide, while the S-binding material is a e.g. a Ni-based catalyst material.

[0098] In another embodiment in which said catalyst active in the cracking of ammonia and the impurity-binding material i.e. a cleaning material, are provided as separate and distinct materials, the guard bed reactor comprises:

[0099] - a bed, i.e. a single bed, comprising both: said catalyst active in the cracking of ammonia to nitrogen and hydrogen, and said impurity-binding material, suitably as silicon-binding material; and / or

[0100] - a bed comprising said catalyst active in the cracking of ammonia to nitrogen and hydrogen; and a separate bed comprising said impurity-binding material, suitably as a silicon-binding material.

[0101] For instance, the guard bed reactor comprises a bed, i.e. a single bed, which comprises a mixture of: pellets, balls or monoliths, provided as the catalyst active in the cracking of ammonia; and separately yet within the same bed, pellets, balls or monoliths, provided as the impurity-binding material, suitably as silicon- binding material.

[0102] It would be understood that the guard bed reactor may comprise a plurality of such single beds.

[0103] For instance, the guard bed reactor comprises separate beds; a bed which comprises pellets, balls or monoliths, provided as the catalyst active in the cracking of ammonia; and a separate (another) bed which comprises pellets, balls or monoliths, provided as the impurity-binding material, suitably as silicon-binding material, hence this separate bed being a Si-guard bed.

[0104] Combinations of any of the above is also envisaged. For instance, suitably in sequential order: a single bed comprising both said catalyst active in the cracking of ammonia to nitrogen and hydrogen, and said impurity-binding material, suitably as silicon-binding material; a bed comprising said catalyst active in the cracking of ammonia to nitrogen and hydrogen; and a separate bed comprising said impurity-binding material, suitably as a silicon-binding material.

[0105] Suitably, the Si-guard bed is arranged directly upstream of the fuel-side inlet of the SOEC or SOEC stack.

[0106] For the purposes of the present application, the term “directly” 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.

[0107] More generally, the term “directly” means that there is no intermediate unit or step changing the composition of a process stream.

[0108] Suitably, the Si-guard bed comprises less than 50000 ppb, such as less than 1000 ppb, less than 100 ppb, or less than 10 ppb of Si on a molar basis. Hence, in an embodiment, the guard bed comprises less than 10 ppb of Si on a molar basis.

[0109] Another undesired species in the Si guard bed reactor is potassium oxide. In an embodiment the Si guard bed comprises less than 5000, such as less than 2000 or 1000 ppm on a molar basis of potassium (K).

[0110] It has been found that the Si-guard bed should preferably not contain any significant amount of Si and that the components of the Si guard bed reactor as such should preferably not contain any significant amount of Si. Any Si present in the Si guard bed reactor would risk reacting with the steam in the feed and thus have adverse effects downstream of the Si-guard bed reactor.

[0111] Another impurity is S (sulphur). Accordingly, as earlier recited, the guard bed reactor may be provided with a sulphur-binding material; for instance, in series as two separate beds in a single vessel or the guard bed may comprise a sulphur binding material in addition to the Si- binding material or the Si-binding material may also bind sulphur. In an embodiment, the guard bed comprises Ni as sulphur binding material, thus advantageously the catalyst active in the cracking of ammonia is a catalyst comprising Ni, such as said catalyst comprising 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. The bed comprising this catalyst as a single material thus enables ammonia cracking, while at the same time removing at least S and Si impurities.

[0112] A sulphur guard bed works by reacting with sulphur-containing compounds, such as hydrogen sulphide and sulphur dioxide, and binding them to the guard bed surface. The material used in a sulphur guard bed reactor can vary, but common materials include metal oxides, such as zinc oxide, nickel oxide, copper oxide, or activated carbon.

[0113] When the Si-guard bed becomes saturated with silica or becomes less efficient, it can be regenerated or replaced. The regeneration is conducted in high temperature / high pressure steam.

[0114] In an embodiment, the normal operation comprises:

[0115] - withdrawing a portion of said second fuel-side exit gas, suitably the entire portion thereof, as a separate hydrogen stream, e.g. a hydrogen product stream.

[0116] Thereby, the SOEC stack and thereby the SOEC unit is also allowed to reach normal operation including continuous operation by continuously withdrawing the required H2 product. The recycle to the fuel-side of the SOEC stack may be reduced or eliminated so that the hydrogen produced therefrom, suitably in its entirety, may be withdrawn as the hydrogen product stream.

[0117] In an embodiment, the transient operation is hot-idle operation. The hot-idle operation means, as earlier recited, open-circuit voltage (OCV) operation of the SOEC stack at temperatures lower than normal SOEC stack operation temperature, such as 10- 200°C, e.g. 50-200°C or 100-200°C lower than normal SOEC stack operation temperature. Hot-idle mode is suitably applied where there is low availability of electricity or there is low demand for the hydrogen product.

[0118] Typically, the SOEC unit in hot-idle mode is operated by providing a forming gas of nitrogen comprising hydrogen, such as 2-10 vol.% H2. The present invention provides the feed gas comprising ammonia which is supplied to the guard bed reactor, thus generating hydrogen and nitrogen as the forming gas, and thereby maintaining the nickel of the fuel electrode (cathode) in its reduced, active, state.

[0119] In an embodiment, the transient operation is shut-down operation. The shut-down operation means, as earlier recited, operating of the SOEC stack away from normal operation by cooling down the SOEC stack to below 100°C, such as to room temperature, and removing the current applied during normal operation.

[0120] In a second general embodiment according to the first aspect (method) of the invention, there is provided a method of operating a solid oxide electrolysis cell (SOEC) stack for producing hydrogen, said SOEC stack comprising at least one solid oxide electrolysis cell (SOEC), said at least one SOEC comprising an electrolyte layer interposed between a fuel-side and an oxy-side, the method comprising: transient operation, such as any of start-up operation, hot idle operation and shut-down operation, in which the transient operation comprises:

[0121] - operating the SOEC stack under open-circuit voltage (OCV);

[0122] - providing a feed gas comprising ammonia;

[0123] - supplying at least a portion of said feed gas comprising ammonia directly to the fuelside of at least one of the solid oxide electrolysis cells (SOECs) of the SOEC stack; said fuel-side comprising nickel (Ni);

[0124] - withdrawing from said at least one of the SOECs of the SOEC stack, a first fuel-side exit gas.

[0125] The guard bed reactor is thus by-passed during the transient operation. Thereby, there is in-situ generation of H2 within the SOEC, as it turns out that the active Ni surface of the SOEC cells also acts also as NH3 cracker. The environment is thus kept reduced so that the undesired oxidation of Ni via migrating oxygen from the oxy-side into NiO is avoided.

[0126] The fuel electrode is suitably a Ni-YSZ (nickel-ytria-zirconia) electrode or a Ni-GDC (nickel-gadolinia-doped ceria) electrode, thus inherently providing nickel for promoting therein the ammonia cracking into hydrogen.

[0127] In a second aspect of the invention, there is also provided a system (plant) comprising:

[0128] - a guard bed reactor comprising a catalyst active in the cracking of ammonia to nitrogen and hydrogen; said guard bed reactor arranged to receive a feed gas comprising ammonia and provide a forming gas comprising nitrogen and hydrogen;

[0129] - a solid oxide electrolysis cell (SOEC) stack for producing hydrogen, said SOEC stack comprising at least one solid oxide electrolysis cell (SOEC), said at least one SOEC comprising an electrolyte layer interposed between a fuel-side and an oxy-side; said fuel-side of the at least one of the solid oxide electrolysis cells (SOECs) of the SOEC stack being arranged to receive at least a portion of said forming gas comprising nitrogen and hydrogen, and provide a first fuel-side exit gas;

[0130] - a power source adapted for at least operating the SOEC stack under open-circuit voltage (OCV).

[0131] In a second aspect of the invention, there is also provided a system (plant) for carrying out the method according to any of the embodiments of the first general embodiment according to the first aspect of the invention, the system comprising:

[0132] - a guard bed reactor comprising a catalyst active in the cracking of ammonia to nitrogen and hydrogen; said guard bed reactor arranged to receive a feed gas comprising ammonia and provide a forming gas comprising nitrogen and hydrogen;

[0133] - a solid oxide electrolysis cell (SOEC) stack for producing hydrogen, said SOEC stack comprising at least one solid oxide electrolysis cell (SOEC), said at least one SOEC comprising an electrolyte layer interposed between a fuel-side and an oxy-side; said fuel-side of the at least one of the solid oxide electrolysis cells (SOECs) of the SOEC stack being arranged to receive at least a portion of said forming gas comprising nitrogen and hydrogen, and provide a first fuel-side exit gas; - a power source adapted for at least operating the SOEC stack under open-circuit voltage (OCV).

[0134] It would be understood that the term “a power source adapted for at least operating the SOEC stack under open-circuit voltage (OCV)” means that the power source may also be adapted for operating the SOEC stack by applying a load thus an electricity current, corresponding to normal operation.

[0135] In an embodiment, the system comprises a conduit for supplying at least a portion of said first fuel-side exit gas, as a first fuel-side recycle gas, to said guard bed reactor.

[0136] It would be understood that the term “conduit” means a process line, such as a pipe, carrying a given process stream.

[0137] In an embodiment, the system comprises a mixing point, such as a mixing unit or juncture, arranged to combine said first fuel-side recycle gas with an ammonia stream, suitably a pure ammonia stream, into said feed gas comprising ammonia, said feed gas comprising ammonia further comprising hydrogen.

[0138] In an embodiment, the system comprises:

[0139] - heating means such as one or more heat exchangers for heating said feed gas comprising ammonia to a temperature of 400-900°C, such as 450-850°C or 500-800°C at inlet of the guard bed reactor;

[0140] - a conduit for replacing said feed gas comprising ammonia optionally further comprising hydrogen, with a feed gas comprising steam optionally further comprising hydrogen; said guard bed reactor arranged to receive said feed gas comprising steam optionally further comprising hydrogen.

[0141] In an embodiment, the system comprises:

[0142] - means, such as a valve e.g. one or more valves, arranged for gradually replacing said feed gas comprising ammonia by first combining a feed gas comprising steam with the feed gas comprising ammonia, and later interrupting the flow of the feed gas comprising ammonia, such as by interrupting the supply of the ammonia stream, e.g. pure ammonia stream.

[0143] In an embodiment, the system comprises:

[0144] - said power source adapted for operating the SOEC stack under continuous load, thus for providing an electricity current and thereby provide normal operation of the SOEC stack;

[0145] - said conduit for supplying at least a portion of said feed gas comprising steam to said guard bed reactor; and providing from said guard bed reactor a clean feed gas comprising steam optionally also hydrogen;

[0146] - a conduit for supplying at least a portion of the clean gas comprising steam optionally also hydrogen to the fuel-side of at least one of the SOECs of the SOEC stack; and providing from said at least one of the SOECs of the SOEC stack, a second fuel-side side exit gas.

[0147] In an embodiment, the system is further arranged to supply a portion of said second fuel-side exit gas, as a second fuel-side recycle gas, to said guard bed reactor; suitably, the system comprises a mixing point, such as a mixing unit or juncture, arranged to combine said second fuel-side recycle gas with said feed gas comprising steam prior to supplying the feed gas to the guard bed reactor.

[0148] In an embodiment, the system comprises a heat exchanger, e.g. one or more heat exchangers, arranged to heat any of said feed gas comprising ammonia, said feed gas comprising steam, or a combination thereof, by heat exchange with said first or second fuel-side recycle gas.

[0149] In an embodiment, in the guard bed reactor of the system, said catalyst active in the cracking of ammonia to nitrogen and hydrogen is any of a catalyst comprising nickel (Ni), a catalyst comprising iron (Fe), or a combination thereof.

[0150] In an embodiment, the guard bed reactor of the system comprises: a bed comprising said catalyst active in the cracking of ammonia to nitrogen and hydrogen, in which said 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.

[0151] Hence, said catalyst active in the cracking of ammonia and the cleaning material, i.e. an impurity-binding material, are provided as a single material. The single material thus provides the dual-function of being a catalyst for ammonia cracking and as impuritybinding material, thereby also removing impurities such as Si.

[0152] The guard bed reactor comprises a guard bed, more specifically a 3-dimensional (3D) bed structure.

[0153] The impurity is at least one of Si (silicon), N (nitrogen), S (sulphur), P (phosphorus), As (arsenic), or compounds thereof. For instance, an impurity is SiC>2(s) or Si(OH)4(g). For instance, an impurity is NOX(nitrogen oxides). For instance, an impurity is H2S. For instance, an impurity is a combination thereof.

[0154] The guard bed reactor, apart from comprising said catalyst active in the cracking of ammonia to nitrogen and hydrogen, the guard bed reactor may also comprise a separate and distinct cleaning material, i.e. an impurity-binding material. Accordingly, in an embodiment, the guard bed reactor of the system further comprises an impurity-binding material, said impurity-binding material being any of at least a silicon-binding material, a sulphur-binding material, or a combination thereof; wherein the impurity-binding material comprises a metal (Me) oxide, in which 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.

[0155] In another embodiment in which said catalyst active in the cracking of ammonia and the cleaning material, i.e. an impurity-binding material, are provided as separate and distinct materials, the guard bed reactor of the system comprises:

[0156] - a bed comprising both: said catalyst active in the cracking of ammonia to nitrogen and hydrogen, and said impurity-binding material, suitably as silicon-binding material; and / or - a bed comprising said catalyst active in the cracking of ammonia to nitrogen and hydrogen; and a separate bed comprising said impurity-binding material, suitably as a silicon-binding material.

[0157] In an embodiment, the guard bed of the system comprises less than 10 ppb of Si on a molar basis.

[0158] In an embodiment, the system comprises:

[0159] - said power source being adapted for operating the SOEC stack under continuous load, thus for providing an electricity current and thereby provide normal operation of the SOEC stack;

[0160] - a conduit for withdrawing a portion of said second fuel-side exit gas, suitably the entire portion thereof, as a separate hydrogen stream, suitably as a hydrogen product stream.

[0161] Any of the embodiments and associated benefits in connection with the first aspect of the invention (method) may be used in connection with the second aspect of the invention (system), or vice versa.

[0162] Even though the invention has been described with regard to transient operation, the system may equally be used under normal operation. In this case, the method may still be conducted as follows:

[0163] According to the invention a method is provided of operating a solid oxide electrolysis cell (SOEC) stack for producing hydrogen, said SOEC stack comprising at least one solid oxide electrolysis cell (SOEC), said at least one SOEC comprising an electrolyte layer interposed between a fuel-side and an oxy-side, the method comprising:

[0164] - providing a feed gas comprising ammonia;

[0165] - supplying at least a portion of said feed gas comprising ammonia to a guard bed reactor, said guard bed reactor comprising a catalyst active in the cracking of ammonia to nitrogen and hydrogen; and withdrawing from said guard bed reactor an intermediate gas; - supplying at least a portion of the intermediate gas comprising nitrogen and hydrogen to the fuel-side of the at least one of the solid oxide electrolysis cells (SOECs) of the SOEC stack; and withdrawing from said at least one of the SOECs of the SOEC stack, a fuel-side exit gas.

[0166] In this case the intermediate gas comprises H2 and when fed together with the steam feed to the at least one of the SOECs of the SOEC stack it serves to protect the nickel. This may be done together with a recycling of a H2 product stream or instead of a recycling of a H2 product stream.

[0167] The sole appended figure shows a schematic layout of a system for operating a SOEC stack according to an embodiment of the invention.

[0168] With reference to the figure, a specific embodiment of the system (plant) 100 for operating a SOEC unit comprising a SOEC stack 14, is shown. In transient operation, such as start-up, the SOEC stack and thus the SOEC unit operates under open-circuit voltage (OCV), so that there is no production of hydrogen 113. In the following, the operation is exemplified in terms of start-up and subsequent normal operation. The SOEC is illustrated here as a SOEC stack 14, and it will be understood that a SOEC unit comprises one or more SOEC stacks.

[0169] During start-up, a pure ammonia stream 101 is provided and combined with a first fuelside recycle gas 109’ comprising some hydrogen into a feed gas comprising ammonia 103. The first fuel-side recycle gas 109’ provides more and more hydrogen generated in guard bed reactor 12, as the heating of the SOEC unit 14 up to normal operation temperature is established, so that the feed gas comprising ammonia 103 further comprises hydrogen and / or nitrogen. This feed stream comprising ammonia 103 is then preheated in a heat exchanger, e.g. preheating unit 10, into preheated stream 103’ by heat exchange with a first fuel-side exit gas 107 being withdrawn from the SOEC stack 14, and which for each cycle through the SOEC is withdrawn at a higher temperature. The SOEC unit 14 suitably comprises a number of stacks and is divided into a fuelcathode) side 14’ and an oxy-(anode) side 14”. A power source (not shown) is adapted for operating the SOEC stack under open-circuit voltage (OCV). Upon later reaching normal operation, the power source is adapted to provide a continuous load, thus an electricity current. During start-up, the first fuel-side exit gas 107 is cooled in preheating unit 10 into stream 107’ and further cooled via water stream 119’ in heat exchanger 18 into stream 107”. A first fuel-side exit gas 107” is passed to a separator 22 for removing water 111 and providing first fuel-side recycle gas stream 109 comprising some hydrogen. This hydrogen is generated in the guard bed reactor 12 comprising guard bed 12’ with a catalyst 12’ active in the cracking of ammonia to nitrogen and hydrogen, and which is arranged upstream the SOEC stack 14. A forming gas 105 comprising the nitrogen and hydrogen is thus supplied to the fuel-side 14’ of the SOEC stack 14. The preheated feed gas comprising ammonia 103’ is suitably supplied at a temperature of 400-900°C at inlet of the guard bed reactor 12. At least a portion of the first fuel-side recycle gas 109, suitably the entire portion thereof, as the SOEC stack 14 is being heated up to normal operation temperatures, is supplied via hydrogen recycle blower 24 as first fuelside recycle gas 109’ being combined with the pure ammonia stream 101.

[0170] Upon approaching or reaching normal operation temperature of the SOEC stack 14 and thereby the SOEC unit, said normal operation temperature being 700-800°C, the power source is adapted to apply a continuous load, thus an electricity current. The feed gas 101 , 103, 103’ comprising ammonia optionally hydrogen, is replaced with a feed gas 121 , 103, 103’ comprising steam optionally hydrogen. This may be conducted gradually by first combining the feed gas comprising steam, such as pure steam stream 121 from steam drum 20, with feed gas comprising ammonia, such as said pure ammonia stream 101 , and later interrupting the flow of the feed gas comprising ammonia by interrupting the supply of the ammonia stream, e.g. the pure ammonia stream 101. A liquid water stream 119 is provided, e.g. boiler feed water (BFW), combined with water stream 111 , 117 withdrawn from separator 22 as well as water 115 purged from hydrogen compressor 26, and then via pump 116 conducted as water stream 119’, 119” to the steam drum 20. Optionally, additional preheating unit(s) (not shown) may be arranged. From the steam drum 20, said pure steam stream 121 is withdrawn.

[0171] Upon reaching or during normal operation of the SOEC stack, the pure steam stream 121 is optionally combined with a portion of second fuel-side recycle gas 109, 109’ into feed gas comprising steam 103, which is then preheated into preheated stream 103’ in preheating unit 10 by heat exchange with second fuel-side exit gas 107. The provision of pure ammonia 101 during start-up has been gradually reduced or interrupted. The preheated feed gas 103’, now comprising steam, enters the guard bed reactor 12 comprising guard bed 12’ which now advantageously functions as a cleaning guard bed for removing impurities, suitably for removing silicon impurities, e.g. Si(OH)4. A clean feed gas 105 comprising steam is thus withdrawn 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 by continuously applying a load, thus an electricity current, to the SOEC stack for operation in electrolysis mode. The second fuel-side exit gas 107 being generated is enriched in hydrogen and exits at for instance 700-800°C corresponding to the normal operation temperature of the SOEC stack. After delivering heat to heat exchanger (preheating unit) 10, the second fuel-side exit gas 107’, as already mentioned above, is further cooled in heat exchanger 18 using the water stream 119’ as heat exchanging medium. Optionally additional heat exchanging units (not shown) such as cooler using cooling water as cooling medium may be provided. The second fuel-side exit gas 107” is passed to a separator 22 for removing water 111 and providing second fuel-side recycle stream 109 comprising some hydrogen. Optionally, a portion of said second fuel-side exit gas, as a second fuel-side recycle gas 109, 109’, is supplied to said guard bed reactor 12; yet, as the normal operation is established and there is continuous production of hydrogen in the SOEC unit 14, a portion of said second fuel-side exit gas 109, suitably the entire portion thereof, is withdrawn as a separate hydrogen product stream 113 via hydrogen compressor 26. A purge water stream 115 is suitably withdrawn from the hydrogen compressor 26 and combined with water stream 111 from the upstream separator 22 and further as water stream 117 with the water stream 119, e.g. BFW 119.

[0172] In the oxy-side 14” of the SOEC stack 14, an air feed stream 123, 125, 127 is provided and prepared for further use in guard bed unit 28 and optional drying unit 30, as shown in the figure. Off-gas stream 129 comprising nitrogen and oxygen is withdrawn from the oxy-side 14” of the SOEC stack 14, optionally after providing heat (not shown) to air feed stream 127.

Claims

CLAIMS1 . Method of operating a solid oxide electrolysis cell (SOEC) stack for producing hydrogen, said SOEC stack comprising at least one solid oxide electrolysis cell (SOEC), said at least one SOEC comprising an electrolyte layer interposed between a fuel-side and an oxy-side, the method comprising: transient operation, in which the transient operation comprises:- operating the SOEC stack under open-circuit voltage (OCV);- providing a feed gas comprising ammonia;- supplying at least a portion of said feed gas comprising ammonia to a guard bed reactor, said guard bed reactor comprising a catalyst active in the cracking of ammonia to nitrogen and hydrogen; and withdrawing from said guard bed reactor a forming gas comprising nitrogen and hydrogen;- supplying at least a portion of the forming gas comprising nitrogen and hydrogen to the fuel-side of the at least one of the solid oxide electrolysis cells (SOECs) of the SOEC stack; and withdrawing from said at least one of the SOECs of the SOEC stack, a first fuel-side exit gas.

2. Method according to claim 1 , wherein the transient operation is selected from any of start-up operation, hot-idle operation and shut-down operation.

3. Method according to any of claims 1-2, the transient operation comprises:- supplying at least a portion of said first fuel-side exit gas, as a first fuel-side recycle gas, to said guard bed reactor.

4. Method according to claim 3, the transient operation comprising:- combining said first fuel-side recycle gas with an ammonia stream, suitably a pure ammonia stream, into said feed gas comprising ammonia, said feed gas comprising ammonia further comprising hydrogen.

5. Method according to any of claims 1-4, wherein the transient operation is start-up operation, the start-up operation comprising:- heating said feed gas comprising ammonia to a temperature of 400-900°C, such as 450-850°C or 500-800°C, at inlet of the guard bed reactor;- upon approaching or reaching normal operation temperature of the SOEC stack, said normal operation temperature being 650-900°C, replacing said feed gas comprising ammonia optionally further comprising hydrogen and / or nitrogen, with a feed gas comprising steam optionally further comprising hydrogen.

6. Method according to claim 5, the start-up operation comprising: gradually replacing said feed gas comprising ammonia by first combining a feed gas comprising steam with the feed gas comprising ammonia, and later interrupting the flow of the feed gas comprising ammonia, such as by interrupting the supply of the ammonia stream, e.g. pure ammonia stream.

7. Method according to any of claims 5-6, wherein the method further comprises: normal operation, which comprises:- changing the OCV operation of the SOEC stack into normal operation by continuously applying a load, thus an electricity current, to the SOEC stack for operation in electrolysis mode;- supplying at least a portion of said feed gas comprising steam to said guard bed reactor; and withdrawing from said guard bed reactor a clean feed gas comprising steam optionally also hydrogen;- supplying at least a portion of the clean gas comprising steam optionally also hydrogen to the fuel-side of at least one of the SOECs of the SOEC stack; and withdrawing from said at least one of the SOECs of the SOEC stack, a second fuel-side side exit gas.

8. Method according to claim 7, further comprising: supplying a portion of said second fuel-side exit gas, as a second fuel-side recycle gas, to said guard bed reactor; suitably by combining said second fuel-side recycle gas with said feed gas comprising steam prior to supplying the feed gas to the guard bed reactor.

9. Method according to any of claims 7-8, the normal operation comprising:- continuously heating said feed gas comprising steam to a temperature of 650-900°C at inlet of the guard bed reactor.

10. Method according to any of claims 5-9, wherein during start-up operation and / or normal operation, the heating of any of said feed gas comprising ammonia, said feed gas comprising steam, or a combination thereof, comprises heat exchange with said first or second fuel-side recycle gas.

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

12. Method according to any of claims 1-11 , wherein the guard bed reactor comprises: a bed comprising said catalyst active in the cracking of ammonia to nitrogen and hydrogen, in which said 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.

13. Method according to any of claims 1-11 , wherein the guard bed reactor further comprises an impurity-binding material, said impurity-binding material being any of at least a silicon-binding material, a sulphur-binding material, or a combination thereof; wherein the impurity-binding material comprises a metal (Me) oxide, in which 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.

14. Method according to any of claims 1-11 and 13, in which said catalyst active in the cracking of ammonia and the impurity-binding material, are provided as separate and distinct materials, wherein the guard bed reactor comprises:- a bed comprising both: said catalyst active in the cracking of ammonia to nitrogen and hydrogen, andsaid impurity-binding material, suitably as silicon-binding material; and / or- a bed comprising said catalyst active in the cracking of ammonia to nitrogen and hydrogen; and a separate bed comprising said impurity-binding material, suitably as a silicon-binding material.

15. Method according to any of claims 7-14, the normal operation comprising:- withdrawing a portion of said second fuel-side exit gas, suitably the entire portion thereof, as a separate hydrogen stream, suitably as a hydrogen product stream.

16. Method of operating a solid oxide electrolysis cell (SOEC) stack for producing hydrogen, said SOEC stack comprising at least one solid oxide electrolysis cell (SOEC), said at least one SOEC comprising an electrolyte layer interposed between a fuel-side and an oxy-side, the method comprising: transient operation, such as any of start-up operation, hot idle operation and shut-down operation, in which the transient operation comprises:- operating the SOEC stack under open-circuit voltage (OCV);- providing a feed gas comprising ammonia;- supplying at least a portion of said feed gas comprising ammonia directly to the fuelside of at least one of the solid oxide electrolysis cells (SOECs) of the SOEC stack; said fuel- side comprising nickel (Ni);- withdrawing from said at least one of the SOECs of the SOEC stack, a first fuel-side exit gas.

17. System (100) for carrying out the method of any of claims 1-15, comprising:- a guard bed reactor (12) comprising a catalyst (12’) active in the cracking of ammonia to nitrogen and hydrogen; said guard bed reactor (12) arranged to receive a feed gas (103, 103’) comprising ammonia and provide a forming gas (105) comprising nitrogen and hydrogen;- a solid oxide electrolysis cell (SOEC) stack (14) for producing hydrogen, said SOEC stack (14) comprising at least one solid oxide electrolysis cell (SOEC), said at least one SOEC comprising an electrolyte layer interposed between a fuel-side (14’) and an oxy- side (14’); said fuel-side (14) of the at least one of the solid oxide electrolysis cells(SOECs) of the SOEC stack (14) being arranged to receive at least a portion of said forming gas (105) comprising nitrogen and hydrogen, and provide a first fuel-side exit gas (107, 107’, 107”);- a power source adapted for at least operating the SOEC (14) stack under open-circuit voltage (OCV).

18. A method of operating a solid oxide electrolysis cell (SOEC) stack for producing hydrogen, said SOEC stack comprising at least one solid oxide electrolysis cell (SOEC), said at least one SOEC comprising an electrolyte layer interposed between a fuel-side and an oxy-side, the method comprising:- providing a feed gas comprising ammonia;- supplying at least a portion of said feed gas comprising ammonia to a guard bed reactor, said guard bed reactor comprising a catalyst active in the cracking of ammonia to nitrogen and hydrogen; and withdrawing from said guard bed reactor an intermediate gas;- supplying at least a portion of the intermediate gas comprising nitrogen and hydrogen to the fuel-side of the at least one of the solid oxide electrolysis cells (SOECs) of the SOEC stack; and withdrawing from said at least one of the SOECs of the SOEC stack, a fuel-side exit gas.

19. The method according to claim 18, wherein the method comprises transient operation, in which the transient operation comprises operating the SOEC stack under opencircuit voltage (OCV) and the method further comprises withdrawing from said at least one of the SOECs of the SOEC stack, the first fuel-side exit gas.

20. The method according to claim 19, wherein the intermediate gas is a forming gas comprising nitrogen and hydrogen.21 . The method according to any one of claims 19-20, wherein the transient operation is selected from any of start-up operation, hot-idle operation and shut-down operation.

22. The method according to any one of claims 19-21 , the transient operation comprises:- supplying at least a portion of said first fuel-side exit gas, as a first fuel-side recycle gas, to said guard bed reactor, and optionally,- prior to the supplying step, combining said first fuel-side recycle gas with an ammonia stream, suitably a pure ammonia stream, into said feed gas comprising ammonia.

23. The method according to any one of claims 19-22, the transient operation being a start-up operation, wherein the start-up operation comprises:- heating said feed gas comprising ammonia to a temperature of 400-900°C, such as 450-850°C or 500-800°C, measured at the inlet of the guard bed reactor.

24. Method according to any one of claims 18-23, wherein the method comprises normal operation, in which the normal operation comprises- operating the SOEC stack while continuously applying a load, such as an electric current, to the SOEC stack for operation in electrolysis mode,- supplying at least a portion of said feed gas comprising ammonia, said feed gas further comprising steam, to said guard bed reactor; and withdrawing from said guard bed reactor an intermediate feed gas comprising steam and hydrogen;- supplying at least a portion of the intermediate feed gas comprising steam and hydrogen to the fuel-side of at least one of the SOECs of the SOEC stack; and withdrawing from said at least one of the SOECs of the SOEC stack, a second fuel-side side exit gas (comprising hydrogen).

25. Method according to any one of claims 18-24, wherein the method comprises normal operation, in which the normal operation comprises- operating the SOEC stack under continuously applying a load, such as an electric current, to the SOEC stack for operation in electrolysis mode,- supplying a second feed gas comprising steam to said guard bed reactor; and withdrawing from said guard bed reactor an intermediate feed gas comprising steam and hydrogen;- supplying at least a portion of the intermediate feed gas comprising steam and hydrogen to the fuel-side of at least one of the SOECs of the SOEC stack; and withdrawing from said at least one of the SOECs of the SOEC stack, a second fuel-side side exit gas.

26. The method according to any one of claims 24-25, wherein the intermediate feed gas comprising steam and hydrogen is a clean intermediate feed gas.

27. The method according to any one of claims 23-26, wherein the intermediate feed gas comprising steam and hydrogen further comprises nitrogen and preferably comprises 1-10 vol% hydrogen.

28. The method according to any one of claims 23-27, the normal operation comprising- supplying a portion of said second fuel-side exit gas, as a second fuel-side recycle gas, to said guard bed reactor; suitably by combining at least a portion of said second fuel-side recycle gas with said feed gas comprising ammonia, said feed gas further comprising steam, prior to supplying the said feed gas to the guard bed reactor, and / or suitably by combining at least a portion of said second fuel-side recycle gas with said second feed gas comprising steam prior to supplying the second feed gas to the guard bed reactor.

29. The method according to any one of claims 23-28, the normal operation comprising:- continuously heating said feed gas comprising ammonia, said feed gas further comprising steam and / or the second feed gas comprising steam to a temperature of 650- 900°C at the inlet of the guard bed reactor.

30. The method according to any one of claims 23-29, the normal operation comprising:- withdrawing a portion of said second fuel-side exit gas, suitably the entire portion thereof, as a separate hydrogen stream, suitably as a hydrogen product stream.

31. The method according to any one of claims 18-30, wherein said method comprises changing from transient operation to normal operation, said method comprising:- changing the OCV operation of the SOEC stack into normal operation by continuously applying a load, thus an electricity current, to the SOEC stack for operation in electrolysis mode and further withdrawing from said at least one of the SOECs of the SOEC stack, the second fuel-side exit gas.

32. The method according to any of claims 31 , wherein the step of changing from transient operation to normal operation further comprises:- heating said feed gas comprising ammonia to a temperature of 400-900°C, such as 450-850°C or 500-800°C, at inlet of the guard bed reactor;- upon approaching or reaching normal operation temperature of the SOEC stack, said normal operation temperature being 650-900°C, the method further comprises enriching (optionally gradually enriching) the intermediate feed gas in steam such as by increasing the amount of steam arrange to be received by the guard bed reactor.

33. The method according to any one of claims 18-32, wherein the step of providing a feed gas comprising ammonia further comprises that the feed gas comprising ammonia is arranged to receive ammonia from an ammonia storage unit or from an ammonia production unit, preferably, wherein said ammonia production unit is an ammonia synthesis loop.

34. The method according to any of claims 18-33, wherein during transient operation and / or normal operation, the heating of any of said feed gas comprising ammonia, said second feed gas comprising steam, or a combination thereof, comprises heat exchange with one or more fuel-side exit gas(es) such as with said first or second fuelside recycle gas(es).

35. A system (100) for carrying out the method of any of claims 1-34, comprising:- a guard bed reactor (12) comprising a catalyst (12’) active in the cracking of ammonia to nitrogen and hydrogen; said guard bed reactor (12) arranged to receive a feed gas (103, 103’) comprising ammonia and provide an intermediate gas such as a forming gas (105) comprising nitrogen and hydrogen;- a solid oxide electrolysis cell (SOEC) stack (14) for producing hydrogen, said SOEC stack (14) comprising at least one solid oxide electrolysis cell (SOEC), said at least one SOEC comprising an electrolyte layer interposed between a fuel-side (14’) and an oxyside (14’); said fuel-side (14) of the at least one of the solid oxide electrolysis cells (SOECs) of the SOEC stack (14) being arranged to receive at least a portion of said intermediate gas such as a forming gas (105) comprising nitrogen and hydrogen, and provide a fuel-side exit gas such as a first fuel-side exit gas (107, 107’, 107”);- a power source adapted for at least operating the SOEC (14) stack under open-circuit voltage (OCV).

36. An ammonia plant comprising: - the system as of claim 35,- an ammonia loop, wherein the system of claim 35 is arranged to receive an ammonia stream from the ammonia loop such that the feed gas comprising ammonia is arranged to receive an ammonia stream from the ammonia loop.

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