WO2025087865A1

WO2025087865A1 - Solid oxide cell system and guard bed reactor for silicon removal therefore

Info

Publication number: WO2025087865A1
Application number: PCT/EP2024/079731
Authority: WO
Inventors: John Bøgild Hansen; Bengt Peter Gustav Blennow; Christian DANNESBOE; Thomas Heiredal-Clausen; Jens Sehested; Elena Marzia SALA
Original assignee: Haldor Topsoe AS
Current assignee: Topsoe AS
Priority date: 2023-10-27
Filing date: 2024-10-21
Publication date: 2025-05-01
Anticipated expiration: 2026-04-27

Abstract

The present invention relates to a guard bed reactor for silicon removal, a solid oxide electrode system for producing hydrogen comprising a guard bed reactor for silicon removal, a method of operating the system to produce hydrogen and a use of the guard bed reactor for silicon removal for depleting a stream of steam from volatile silica species.

Description

Solid Oxide Cell system and guard bed reactor for silicon removal therefore

FIELD OF THE INVENTION

The present invention relates to a guard bed reactor for silicon removal , a solid oxide electrode system comprising a guard bed reactor for silicon removal , a method of operating the system to produce hydrogen and a use of the guard bed reactor for silicon removal for depleting a stream of steam from volatile silica species .

BACKGROUND OF THE INVENTION

Solid oxide electrolysis has the potential to play a signi ficant role in converting renewable energy, such as wind or solar power, into hydrogen, which has the potential to replace fossil fuels in a variety of applications , including transportation and energy storage . Solid oxide electrolysis uses Solid oxide cells ( SOCs ) to convert steam into hydrogen at high temperatures ( ~ 600- 1000 ° C ) .

SOCs are still in the early stages of commerciali zation and research is ongoing to improve their ef ficiency, li fetime , robustness etc . Another focus area is the integration of SOC' s into systems suitable for industrial scale use . For industrial scale use an important aspect is to reduce costs . SOC' s have shown promising results and are considered a key technology in the transition to a low-carbon energy system .

SOCs are generally reversible , meaning that they may be used in both solid oxide electrolysis cell mode and in solid oxide fuel cell mode . A solid oxide cell ( SOC ) is an electrochemical conversion device generally including a fuel electrode , an oxy electrode , and a solid electrolyte separating the fuel electrode and the oxy electrode and allowing oxygen ions , to pass through it . The SOCs generally also include contact layers to increase the in-plane electrical conductivity and provide improved electrical contact between adj acent SOC' s when arranged in stacks . Adj acent SOC' s are generally separated by interconnect layers , also referred to as interconnects . SOC stacks may further be arranged in modules .

Interconnects generally serve as a gas barrier to separate the fuel and oxy sides of adj acent SOC' s , and at the same time they enable current conduction between the adj acent cells , i . e . between a fuel electrode of one cell and an oxy electrode of a neighbouring cell . Further, interconnects are normally provided with a plurality of flow paths for the passage of process gas on both sides of the interconnect .

Silicon ( Si ) is known to cause problems in process equipment handling steam . In the presence of steam volatile silica species are formed and may then later sediment and form ( amorphous ) silica barriers on surfaces in downstream equipment . The volatile silica species present in steam is known to stem from water feed and Si containing balance of plant components ( supporting and auxiliary components making the plant work) . The presence of a high fraction of steam at high temperatures is known to result in a volatilisation of silica species from such components according to the following reaction I :

5iO₂(s) + 2H₂O(g) Si(0tf)₄(,g) ( I ) It is thus known that the vapor pressure of Si ( OH) ₄ from SiO2 increases with temperature and pressure (pH₂O) . The Si ( OH) ₄ thus present in the steam may deposit ( e . g . form inclusions ) and/or react with metal oxides present in downstream units , including in the solid oxide cells .

To solve such problems , it is known to remove silica from water by reverse osmosis or ion exchange before feeding the water into e . g . power plants or to simply avoid Si containing balance of plant components . It is also known from WO 2017 / 042574 that a Chromium getter may be placed before the Cathode of an SOFC ( oxy side ) to clean oxidant flow to the cathode and that this Chromium getter also may catch volatile silica species from the oxidant ( air stream) . It is concluded that "Reducing the concentration of the chromium and silica contaminant species from the air stream by providing sorbent getters within the inlet duct of the fuel cell stack to remove chromium and silica species from the air stream, serves to extend the li fetime of the fuel cells and fuel cell stack by reducing cathode degradation" . WO 2017 / 042574 suggests that the Chromium getter comprises "at least one member group consisting of magnesium oxide , calcium oxide , and manganese oxide is that they are particularly good at extracting volatile chromium and silica species from the air stream by reacting with volatile Cr based species" .

As mentioned, there is still a need to improve many aspects of operating an industrial scale SOC plant , for example to extend the li fetime of SOCs and to reduce operational costs of an industrial scale SOC plant . In particular, there is great focus on designing improved SOEC systems suitable for producing hydrogen at an industrial scale . SUMMARY OF THE INVENTION

In their ef forts to improve existing SOEC systems to obtain an ef ficiency suitable for industrial applicability, the inventors found that , even though the amount of silica present in a steam feed to an SOEC was low and no adverse ef fect was expected in the solid oxide cell , it turned out that during long term use they did observe a build-up of silica species on the surfaces of the fuel side of the SOEC, even at low silicon concentrations . In systems converting steam into hydrogen, the problem was even more severe , since H₂0 was consumed thus shi fting the equilibrium between solid and gaseous Si towards gaseous Si ( see reaction scheme ( I ) ) .

The inventors surprisingly found that even i f silicon had been removed from a steam feed in conventional manners the li fetime of the solid oxide cell could still be improved signi ficantly by adding a guard bed ( reactor ) for silicon removal upstream of the solid oxide cell inlet to remove volatile silica species ( such as Si ( OH) ₄ ) . The inventors also found it to be advantageous to pass hydrocarbon feeds containing humidity through a guard bed ( reactor ) for silicon removal upstream of the fuel side of an SOEC to remove volatile silica species . In addition the inventors identi fied some metal oxides which turned out to be particularly suitable as active materials in Si guard bed reactors for removing even low concentrations of Si hydroxide ( Si ( OH) ₄ ) from feed gases containing steam . The inventors found that generally a Si concentration below 50ppb in the steam feed to the SOC was required to avoid or at least signi ficantly reduce the adverse ef fects of amorphous silica species depositing on the surfaces within the SOC and/or downstream of the SOC . According to an aspect of the present invention a guard bed reactor for silicon removal is provided having a guard bed comprising an active metal (Me ) oxide as a Si binding material , wherein the active metal (Me ) is selected from the group consisting of alkaline earth metals ( group 2 of the periodic table ) ; and transition metals ( groups 3- 12 of the periodic table ) including rare earth metals ; or mixtures thereof .

Suitable active metals (Me ) are thus e . g . one of the alkaline earth metals Ca, Mg, Sr ; one of the transition metals Zr, the rare earth metals La and Ce ; or mixtures thereof . In an embodiment the guard bed reactor for silicon removal has a guard bed wherein the active metal , Me , is selected from the group consisting of Ca, Mg, Sr, Zr, Ce , and La ; or mixtures thereof .

An advantage of the guard bed reactor for silicon removal according to the invention is that under the operational conditions for operating a solid oxide cell , signi ficantly less adverse ef fects from formation of amorphous silica species on the surfaces are observed within the fuel side of the SOC as well as downstream of the SOC . When using the guard bed reactor according to the invention, silicon ( Si ) concentrations in the feed comprising steam below 50 ppb on a molar basis were observed . For example , when using cerium oxide (Me=Ce ) as Si binding material , a 100- fold reduction of Si in a steam feed to the fuel side of an SOC may be obtained and Si concentrations as low as 15 ppb on a molar basis have been observed . An advantage of lowering the silicon concentration in the feed is that the SOC system may be operated at a higher steam conversion, such as above 70 wt% conversion . According to another aspect of the present invention a solid oxide cell ( SOC ) system for producing hydrogen is provided which comprises : a solid oxide cell comprising an electrolyte layer interposed between a fuel side and an oxy-side ; and

- a guard bed reactor for silicon removal having a guard bed comprising a Si binding material , the guard bed reactor being arranged upstream of the fuel sideof the solid oxide cell .

An advantage of the SOC system for producing hydrogen according to the invention is that the guard bed reactor may be arranged after heating of the feed to the fuel side . The inventors found that after the heating of a feed comprising steam, a larger fraction of the Si present in the steam would be removed within the guard bed reactor, probably since a larger fraction of the Si present in the steam would be in gaseous form . Integrating the guard bed reactor into the solid oxide cell ( SOC ) system for producing hydrogen saves energy since a separate heating of the feed for the guard bed reactor can be dispensed with . Suitable units for heating of the feed are heaters and/or heat exchangers .

According to another aspect of the present invention a method for producing hydrogen is provided, which comprises :

- passing a feed comprising steam through a guard bed reactor for silicon removal at a temperature in the range of from 150 ° C to 1000 ° C to produce a Si depleted stream; and feeding the Si depleted stream to the fuel side of a solid oxide cell comprising an electrolyte layer interposed between a fuel side and an oxy-side , wherein the solid oxide cell is operated at a temperature in the range of from 600 to 1000 ° C to produce a product stream comprising hydrogen .

Advantages of the method according to the present invention are the same as mentioned above for the SOC system for producing hydrogen .

Further aspects of the present invention are set out in the following description text , figures and the appended claims .

DETAILED DESCRIPTION OF THE INVENTION

Def ini tions

Unless otherwise speci fied, any given percentages for gas content are % by volume .

In the present context it is to be understood that a solid oxide cell ( SOC ) generally includes a fuel electrode on the fuel side , an oxy electrode on the oxy-side , and a solid electrolyte layer separating the fuel electrode and the oxy electrode and allowing oxygen ions as well as electrons , to pass through it . The SOCs generally comprises contact layers to increase the in-plane electrical conductivity and provide improved electrical contact .

In the present context it is to be understood that The SOCs are generally arranged in stacks ( SOC stacks ) and adj acent SOC' s are generally separated by interconnect layers , also referred to as interconnects . SOC stacks may further be arranged in modules comprising multiple stacks .

In the present context it is to be understood that the fuel side always comprises a fuel side inlet , a fuel electrode and a fuel side outlet . When arranged in stacks the SOC may have a single fuel side inlet providing feed to multiple SOCs and/or a single fuel side outlet allowing product to exit from multiple SOCs .

When referring to the solid oxide cell it is to be understood that whether the SOC is operated in SOEC mode or in SOFC mode , the term " fuel side" refers to the side of the SOC that comprises the fuel electrode and where a feed gas is converted into a product gas ( in electrolysis mode ) or electrochemically oxidi zed ( fuel cell mode ) and the term "oxy side" refers to the side of the SOC that comprises the oxy electrode where oxygen is consumed or produced depending on whether it is in fuel mode or in electrolysis mode . For the avoidance of doubt , it is noted that whether the SOC is operated in SOEC mode or in SOFC mode , the fuel electrode is always the negative electrode and the oxy electrode is always the positive electrode . However, when the SOC is operated in SOEC mode , the fuel electrode functions as the cathode , whereas in SOFC mode the fuel electrode functions as the anode . Correspondingly, when the SOC is operated in SOEC mode , the oxy electrode functions as the anode , and in SOFC mode the oxy electrode functions as the cathode .

Where nothing else is stated the terms "Si" and " silicon" is meant to refer to the chemical element in group 14 of the periodic table . It may be in its elemental form or bound in a chemical composition . The terms are used interchangeably . In the present context the term "Si 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 or particles which result in the removal of Si from the feed stream. The same applies to "sulfur binding", "nitrogen binding" and "phosphorous binding".

"Directly upstream of the fuel side of the solid oxide cell" and "fed directly to the fuel side of the solid oxide cell" means that there are no unit operations between the two.

Where nothing else is stated, the Si concentration is determined by ICP-OES (Inductively Coupled Plasma-Optical Emission Spectroscopy) .

System

The solid oxide cell (SOC) system for producing hydrogen according to the invention comprises: a solid oxide cell comprising an electrolyte layer interposed between a fuel side and an oxy-side; and - a guard bed reactor for silicon removal having a guard bed comprising a Si binding material, the guard bed reactor being arranged upstream of the fuel side.

The SOC system according to the present invention may be used in solid oxide electrolysis cell mode (SOEC) . Or it may be used in solid oxide fuel cell mode (SOFC) even though for simplicity some parts of the description below relate only to SOEC mode. When operated in SOEC mode, the goal is to produce H2, CO or mixtures of H2 and CO (also referred to as synthesis gas) from steam, CO2, or mixtures thereof. When operated in SOFC mode, the goal is to produce energy from e.g. hydrogen or from low molecular hydrocarbons. When used in SOFC mode, steam is normally present in the feed to the SOC.

A system having a Si guard bed reactor arranged upstream of the fuel side has an advantage that volatile silica species being formed within feeds comprising steam ( for example during unit operations performed on the feed upstream of the solid oxide cell ) may be removed prior to entering the SOC whereby silica deposits within the SOC may be avoided or at least greatly reduced .

In the SOC system, the Si guard bed reactor may be any Si guard bed reactor suitable for use with feeds comprising steam and preferably at Si guard bed reactor temperatures in the range of from 150 to 1000 °C . For this temperature range , the Si guard bed reactor is preferably operated close to ambient pressure . I f higher pressures are used, then the preferred temperature range is somewhat lower .

Common fuel electrode materials are : Ni/YSZ (=nickel on yttria-stabilised zirconia ) , Ni/GDC (=Ni/CGO=nickel on gadolinium-doped ceria ) .

Common oxy electrode materials are : LSCF ( lanthanum strontium cobalt ferrite ) , LSM ( strontium-doped lanthanum manganite ) , LSC ( lanthanum- and strontium-doped cobalt oxide? ) etc . or mixed electrodes ( LSCF/CGO, LSM/YSZ , LSC/CGO) .

According to an embodiment of the system according to the invention, the guard bed reactor for silicon removal is arranged directly upstream of the fuel side of the SOC .

When the guard bed reactor for silicon removal is arranged directly upstream of a fuel side inlet of the fuel side of the SOC, further advantages may be obtained . When no further unit operations are carried out after the removal of volatile Si in the steam, leaching of volatile Si from such units is avoided thus retaining the very low level of volatile Si in the steam obtained with the guard bed reactor . The system may comprise other operational units , such as units performing heat exchange , heating, cooling, oxygen depletion, ion exchange , separation etc .

According to an embodiment of the present invention the guard bed reactor for silicon removal is as defined in any one of claims 1- 8 . The advantages of such a guard bed reactor are as speci fied in the section "Guard Bed Reactor" .

According to an embodiment of the system according to the present invention, the guard bed further comprises a sulphur, a nitrogen and/or a phosphorous binding material . An advantage of having a guard bed within the guard bed reactor further comprising a sulphur, a nitrogen and/or a phosphorous binding material is that in addition to Si , further impurities , such as sul fur, nitrogen and/ phosphorous containing compounds may be removed from the feed .

According to an embodiment of the system according to the present invention, the guard bed reactor for silicon removal is configured to provide a gas flow from the upper part of the guard bed reactor through the guard bed and to the lower part of the guard bed reactor .

In SOC systems , generally a plurality of cells are arranged into stacks and fluidly connected e . g . via mani folds providing a single fuel side inlet to all the cells of the stack . The stacks may be arranged in modules having a single fuel side inlet or multiple fuels side inlets . It is considered known in the art in general to produce and assemble a solid oxide cell , a stack of solid oxide cells and modules of stacks . Guard Bed Reactor for Silicon Removal

The inventors found that metal oxides where the Gibbs free energy of formation of a mixed silicate made from SiO2 and the metal oxide must be negative in order to remove Si from steam (i.e to reduce the equilibrium vapor pressure of Si (OH) 4 (g) over the guard bed) . However, the inventors considered the Gibbs free energy at temperatures in the range of from 150 to 1000 °C for the reaction:

MeO_x + zSiO₂ <8> MeSi_zO_y (ID where x is an integer between 1 and 5, z is an integer between 1 and 4, and y is an integer between 3 and 8.

The inventors found that metal oxides fulfilling gibbs free energy AG<0 for reaction (II) are particularly useful as Si binding materials in guard bed reactors for silicon removal. Generally, this includes basic metal oxides.

Accordingly, the guard bed reactor for silicon removal according to the present invention has a guard bed comprising an active metal (Me) oxide as a Si binding material, wherein the active metal (Me) is selected from the group consisting of alkaline earth metals (group 2 of the periodic table) ; and transition metals (groups 3-12 of the periodic table) including rare earth metals; or mixtures thereof.

Suitable active metals (Me) are thus e.g. one of the alkaline earth metals Ca, Mg, Sr; one of the transition metals Zr, the rare earth metals La and Ge; or mixtures thereof. In an embodiment the guard bed reactor for silicon removal has a guard bed wherein the active metal, Me, is selected from the group consisting of Ca, Mg, Sr, Zr, Ge, and La; or mixtures thereof . The skilled person will know that a guard bed reactor is a vessel configured to accommodate a fluid flow from an inlet , through a guard bed and to an outlet . The guard bed is in the present context to be understood as a porous material ensuring a large contact area between the fluid travelling through the pores and the porous material . The guard bed is arranged within the vessel to constrict the fluid from passing from vessel inlet to vessel outlet without travelling through the porous material . The Si binding material may be integrated into the porous material or it may be distributed across the surface of the porous material . In the latter case , the porous material may be referred to as a carrier . In the present context , the "surface of the carrier" is to be understood as both the outer surface of the carrier and to the inner surface of the carrier accessible through the pores of the carrier . Using a carrier may be preferred e . g . i f the Si binding material is more costly than the carrier material or i f an increased strength is desired .

The guard bed for silicon removal should preferably not contain any signi ficant amount of Si . Further, the components of the Si guard bed reactor as such should preferably not contain any signi ficant amount of Si . Any Si present in the Si guard bed reactor could risk reacting with the steam in the feed and thus have adverse ef fects downstream of the Si guard bed reactor . In an embodiment , the guard bed comprises less than 50000 ppb, such as less than 1000 ppb, 100 ppb, or 10 ppb of Si on a molar basis .

Another undesired species in the Si guard bed reactor for silicon removal is potassium oxide . In an embodiment the guard bed comprises less than 5000 , such as less than 2000 or 1000 ppm on a molar basis . The guard bed reactor may be combined with a guard bed reactor for removal of sul fur, nitrogen and/or phosphorous from the feed . Alternatively, the guard bed of the guard bed reactor for silicon removal may comprise a sul fur, nitrogen and/or phosphorous binding material in addition to the Si binding material or the Si binding material may also bind sul fur, nitrogen and/or phosphorous . Common sul fur, nitrogen and/or phosphorous binding materials include metal oxides , such as zinc oxide , nickel oxide , copper oxide , or activated carbon . In an embodiment , the guard bed of the guard bed reactor for silicon removal further comprises Ni . In another embodiment , the guard bed of the guard bed reactor for silicon removal further comprises Al . The guard bed of the guard bed reactor for silicon removal may comprise both Ni and Al in addition to the Si binding material .

In an embodiment , the guard bed comprises La, Ni and/or Mg . In another embodiment , the guard bed comprises La, Ni , Mg and Al . In yet another embodiment , the guard bed comprises a spinel phase Mg (A102 ) 2. In yet another embodiment , the guard bed comprises a spinel phase Mg (AlC>2 ) 2 as a carrier and La, Ni and/or Mg within or on the surface of the carrier .

An advantage of using guard beds comprising La, Ni and Mg in SOC systems comprising guard bed reactors for Si removal before the solid oxide cells is that the system is suited for operating the system in normal operation for producing hydrogen from steam and in transient operation where hydrogen is not produced and the system is operated under open-circuit voltage ( OCV) . When the guard bed comprises La, Ni and Mg, ammonia may be fed as primary feed to the guard bed reactor for silicon removal when in transient operation at a temperature in the range of from 400- 900 °C to convert the ammonia into hydrogen and nitrogen which is then fed to the solid oxide cell . When reverting to normal operation a feed comprising steam is fed to the system . Such transient operation is suitable e . g . for starting up the SOC system and for idle operation .

The guard bed of the guard bed reactor for silicon removal may further comprise a carrier . The carrier may serve to improve characteristics of the silicon binding material , such as surface area per amount of metal oxide , strength, durability etc . The carrier may incorporate the active metal oxide into its structure or the active metal oxide may be adsorbed on the surface of the carrier . According to an embodiment of the invention, the carrier comprises a spinel phase Mg (AlC>2 ) 2 - According to an embodiment of the invention, the carrier is Al-based . According to an embodiment of the invention, the carrier is La, Mg, Al-based . It is to understood that the La, Mg, Al-based carrier is an La, Mg, Al-based oxide carrier . An exemplary carrier is a MgA12O4-based carrier comprising minor amounts of La .

According to an embodiment , the guard bed of the guard bed reactor for silicon removal according to the invention is provided in the form of porous pellets , balls or monoliths . According to an embodiment , the guard bed of the guard bed reactor for silicon removal according to the invention has a BET surface area in the range of from 0 . 01 to 100 m²/g as determined by ASTM D3663-20 .

When the Si guard bed reactor becomes saturated with silica or less ef ficient , it can be regenerated or replaced .

Method of use The solid oxide cells are generally operated at "high temperature" , which in the present context means operation temperatures in the range of from 600 to 1000 ° C, preferably 650- 850 ° C .

According to this aspect of the invention, the method for producing hydrogen comprises : passing a feed comprising steam through a guard bed reactor for silicon removal at a temperature in the range of from 150 ° C to 1000 ° C to produce a Si depleted stream; and feeding the Si depleted stream to the fuel side of a solid oxide cell comprising an electrolyte layer interposed between a fuel side and an oxy-side , wherein the solid oxide cell is operated at a temperature in the range of from 600 to 1000 ° C to produce a product stream comprising hydrogen .

As mentioned the vapor pressure of Si ( OH) ₄ ( g) from SiO₂ ( s ) increases with temperature and water pressure (pH₂O) . Therefore removal of Si at an elevated temperature will result in removal of a higher fraction of the silicon present in the steam . In addition, the closer the temperature of the silicon removal step is to the operation temperature of the solid oxide cell , the less Si will evaporate from the Si depleted stream prior to entering the SOC . According to the present invention the di f ference in operation temperature between the guard bed reactor and the solid oxide cell (AT ) is below 200 °C, such as below 100 or below 50 °C .

Other unit operations may be performed e . g . heat exchange , heating, cooling, oxygen depletion, ion exchange , separation etc . However, avoiding such unit operations between the Si removal step and the SOC will reduce the risk of balance of plant components releasing volatile silica species to the feed comprising steam . According to an embodiment of the method according to the present invention the Si depleted stream is fed directly to the fuel side inlet of the solid oxide cell .

According to an embodiment of the method according to the present invention the guard bed reactor for silicon removal is as defined herein .

According to an aspect of the present invention a use of the guard bed reactor for silicon removal as disclosed herein is provided to deplete a stream of steam from volatile silica species at a temperature in the range of from 150 to 1000 ° C .

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further illustrated by the accompanying drawings showing examples of embodiments of the invention .

Fig . 1 shows a solid oxide cell system according to the invention with heat exchange and heating before the guard bed reactor .

Fig . 2 shows a solid oxide cell system according to the invention with heating before the guard bed reactor .

Fig . 3 shows the experimental setup used in the examples .

Fig . 4 depicts Si content in condensate samples at di f ferent pressuri zed conditions ( as measured by ICP-OES ) .

POSITION NUMBERS

01 . feed ( fuel side ) 02 . flush ( oxy side )

10 . solid oxide electrolysis cell

11 . fuel side

12 . fuel side inlet

13 . fuel electrode

14 . fuel side outlet

15 . oxy side inlet

16 . oxy side

17 . oxy side outlet

20 . guard bed reactor for silicon removal

21 . flow inlet

22 . guard bed comprising a Si binding material

23 . flow outlet

30 . pre-heater fuel side

35 . heat exchanger fuel side

40 . pre-heater oxy side

45 . heat exchanger oxy side

AN EMBODIMENT OF THE INVENTION

In the context of this invention, the term " fuel side" of an SOC refers to the side of the SOC that comprises the fuel electrode . The fuel electrode is where the H20 reduction reaction (H20 + 2 e- H2 + 02- ) is occurring when operated in SOEC mode and where the H2 oxidation reaction (H2 + 02- H20

+ 2 e- ) is occurring when operated in SOFC mode . The term "oxy side" of an SOC refers to the side of the SOC that comprises the oxy electrode . The oxy electrode is where the 02 evolution reaction ( 2 02- -^ 02 + 4 e- ) is occurring when operated in SOEC mode and where the reverse reaction is taking place in SOFC mode .

Where nothing else is stated, the amount of silica present in vaporous streams such as a " feed comprising steam" or "a Si depleted stream" are determined by inductively coupled plasma optical emission spectroscopy ( ICP-OES ) . It is an analytical technique that utili zes an inductively coupled plasma and optical emission spectroscopy to determine the elemental composition of a sample . I s recogni zed for its great ability to detect a wide range of elements simultaneously and provide accurate results .

Fig . 1 shows a solid oxide cell system equipped with a guard bed reactor for silicon removal according to an embodiment of the invention . Both the feed stream and the flush stream need to be heated to the correct operating temperature for the SOEC to operate correctly . For this purpose , on both the fuel side and the oxy side there are heat exchangers 35 , 45 which preheat the streams by means of the heat energy in the fluids exiting the stacks . To further heat the process fluid and the oxy fluid to the correct operating temperature , there are heaters 30 , 40 located downstream the heat exchangers to heat the feed stream and flush stream . Downstream of the heater 30 , the fuel stream is passed through a guard bed reactor for silicon removal according to the invention directly upstream of the fuel inlet 12 .

Fig . 2 shows a solid oxide cell system equipped with a guard bed reactor for silicon removal according to an embodiment of the invention . Both the feed stream and the flush stream need to be heated to the correct operating temperature for the SOEC to operate correctly . In this embodiment the process fluid and the oxy fluid are heated to the correct operating temperature only by using heaters 30 , 40 located downstream the heat exchangers to heat the feed stream and flush stream . Downstream of the heater 30 , the fuel stream is passed through a guard bed reactor for silicon removal according to the invention directly upstream of the fuel inlet 12 . Example

Experimental description

Example 1 : Experimental set-up for testing Si hydroxide removal from a stream of hydrogen and/or steam by passing the stream through a guard bed reactor for silicon removal In the experimental set-up schemati zed in Figure 3 a cylindrical reactor ( external 0 13 . 7 mm, with inner thermowell 0 2 . 7 mm) representing a guard bed reactor for silicon removal was placed in a 6- zone electrical furnace . The reactor was equipped with two containers : one container in the upper part contained a Si source in the form of a crushed SiO₂ on a A1₂O₃ carrier and the other container in the lower part contained a Si binding material in the form of a crushed Me oxide/La, Mg, Al-based carrier mixture where Me represents the metal oxide to be tested .

A feed stream of gaseous H₂ and H₂0 was passed through a heating element and then fed to the inlet at the top of the reactor . The steam was passed through the reactor from top to bottom : first it passed through the Si source to spike Si into the stream then it was passed through the Si binding material to absorb the Si now present in the stream .

The stream was passed from the outlet at the bottom of the reactor was directed through a cooling element and a subsequent separator for separation of the condensed and gas phases . Exhaust gases were directed to the flare , while aqueous condensate samples were collected and analyzed by inductively coupled plasma optical emission spectroscopy ( ICP- OES ) to determine the Si concentration of the samples . The Si present in the condensed water showed the ef ficiency of the Si binding material . For both the Si source and Si binding materials, the amount of material loaded to the reactor would correspond to a bed height of approximately 3.6 cm.

Example 2: Measurement of Si within experimental setup without Si spiking and Si hydroxide removal

In the present experiment, a feed stream of H₂O/H₂ in the molar ratio 9:1 (dosed using mass flow controllers) was fed to the experimental setup of Example 1, except the Si source and the Si binding material had been removed. Steam was obtained by evaporation of externally supplied de-ionized water and further led through a local ion-exchanger and then mixed with hydrogen. The Si content in the de-ionized feed stream was measured with an empty reactor at different locations of the Experimental setup by ICP-OES. The results showed the presence of 0,051 and 0.069 pg/ml of Si before evaporating water at the inlet of the reactor and after condensation at the bottom of the reactor, respectively.

Example 3: preparation of Si source

5 g of the silica on alumina material (19:81 by weight) was crushed to a particle size of approximately 1.5 mm and placed in the upper container of the reactor. Si will mainly be present as SiO₂ and will react with the steam in the gas feed according to reaction (1) :

SiO₂(s) +2H₂O(g) Si (OH)₄ (g) (1)

The temperature at which the Si source was placed was in all experiments 500-550°C. This was due to practical limitations in the in the experimental setup. It is known that the metal oxides ability to bind silica increases with temperature. Example 4: test of cerium oxide as Si binding material for guard bed reactor for silicon removal

To test the Si absorption capability of cerium oxide, a ceria-based material was prepared by impregnating a MgA12O4 carrier with a cerium nitrate precursor solution followed by calcination at 500°C for 2 h. Scanning Electron Microscopy (SEM) combined with an energy-dispersive X-ray spectroscopy (EDS) detector showed 10-15 wt . % Ce homogeneously distributed in the carrier. Chemical analysis using ICP-OES confirmed a concentration of 12.6 wt% Ce in the material.

5 g of the ceria-based material was then placed in the lower part of the reactor of Example 1 at a temperature of 750- 800°C (reproducing the temperatures of a commercial stack) . The experiment was carried out under pressurized conditions, to obtain a measurable amount of Si in the gas phase. The experiment was accordingly carried out at the three different total pressures of 11, 21 and 31 bar abs . The experiment at each pressure step (comprising heating up to the desired temperature in the different reactor zones, pressurization, and stabilization of the gas mixture) was continued for at least 24 hours. The total gas flow was 100 Nl/h. The temperature profile measurements were measured twice for each experiment (each pressure) and two samples of condensate samples were taken with approximately 5 hours in between.

Example 5: test of strontium oxide and lanthanum oxide as Si binding material for guard bed reactor for silicon removal To test the Si absorption capability of strontium oxide, a strontium-based material was prepared by impregnating a La, Mg, Al-based carrier with a strontium nitrate precursor solution, followed by calcination at 500°C for 2 h. SEM combined with an EDS detector showed an average of 5-10 wt% Sr distributed in the carrier. Chemical analysis using ICP-OES confirmed a concentration of 8.5 wt% Sr in the material.

Similarly, to test the Si absorption capability of lanthanum oxide, a lanthanum-based material was prepared by impregnating a La, Mg, Al-based carrier with a lanthanum nitrate precursor solution followed by calcination at 500°C for 2 h. SEM combined with an EDS detector showed 15-20 wt% La distributed in the carrier. Chemical analysis using ICP-OES confirmed a concentration of 16.2 wt% La in the material.

For testing of strontium oxide, 5 g of the strontium-based material was placed in the lower part of the reactor at a temperature of 750-800°C, reproducing the temperatures of a commercial stack.

The experiment was carried out under pressurized conditions (21 bar abs) to obtain a measurable amount of Si in the gas phase, with a total gas flow of 450 Nl/h. The duration of the experiment (comprising heating up to the desired temperature in the different reactor zones, pressurization, and stabilization of the gas mixture) was maintained for at least 10 hours. Two separate temperature measurements done at different reactor points showed that the temperature was stable and close to the desired temperature throughout the duration of the experiment. A liquid sample for analysis was collected approximately 5 hours after starting the experiment. For testing of lanthanum oxide, 5 g of the lanthanum-based material was tested in the exact same conditions.

Experimental results.

Experimental results from example 4 are plotted in Figure 4, where the concentration of Si measured in the aqueous condensate samples by ICP-OES is reported at the different pressurized conditions investigated. In the figure, the black filled squares show the concentration of Si measured in the condensate sample when only the Si source was loaded, i.e. without the presence of the Si guard. It can be observed that the Si content in the gas feed increases linearly with the total pressure, in line with the linear dependence expected from in reaction (1) . The empty square symbols show the Si content measured under the same conditions when the ceriabased Si guard material was introduced close to the reactor outlet: it was observed that the amount of Si measured in the condensate samples was reduced approximately by a factor of 10, with 0.06, 0.09 and 0.21 pg/ml Si measured at 11, 21 and 31 bar abs, respectively.

Experimental results from example 5 are shown in Table 1, where the concentration of Si measured in the aqueous condensate samples by ICP-OES is reported for the two materials investigated together with with the concentration of Si measured in the condensate sample when only the Si source was loaded, i.e. without the presence of any guard. It can be observed that the Si content in the gas was reduced significantly by the presence of the two guard materials, with the concentration of Si dropping from 1650 pg/ml to 970 and 250 pg/ml for the La-based and Sr-based guards, respectively.

Table 1: Si content measured by ICP-OES in condensate samples for different guard materials, compared to the Si content in absence of any guard.

Claims

1. A guard bed reactor for silicon removal having a guard bed comprising an active metal (Me) oxide as a Si binding material, wherein the active metal (Me) is selected from the group consisting of alkaline earth metals including Ca, Mg, Sr; transition metals including Zr and the rare earth metals including La and Ce; or mixtures thereof.

2. The guard bed reactor for silicon removal according to claim 1, wherein the active metal, Me, is selected from the group consisting of Ca, Mg, Sr, Zr, Ce, and La; or mixtures thereof.

3. The guard bed reactor for silicon removal according to claim 1 or 2, wherein the guard bed further comprises Ni .

4. The guard bed reactor for silicon removal according to any one of the preceding claims, wherein the guard bed further comprises Al.

5. The guard bed reactor for silicon removal according to any one of the preceding claims, wherein the guard bed comprises less than 10 ppb of Si on a molar basis.

6. The guard bed reactor for silicon removal according to any one of the preceding claims, wherein the guard bed further comprises a carrier.

7. The guard bed reactor for silicon removal according to any one of the preceding claims, wherein the guard bed is provided in the form of porous pellets, balls or monoliths .

8. The guard bed reactor for silicon removal according to any one of the preceding claims, wherein the guard bed has a BET surface area in the range of from 0.01 to 100 m²/g as determined by ASTM D3663-20.

9. A solid oxide cell (SOC) system for producing hydrogen comprising : a . a solid oxide cell comprising an electrolyte layer interposed between a fuel side and an oxy-side ; and b . A guard bed reactor for silicon removal having a guard bed comprising a Si binding material , the guard bed reactor being arranged upstream of the fuel side of the solid oxide cell .

10 . The system according to claim 9 , wherein the guard bed reactor for silicon removal is as defined in any one of claims 1- 8 .

11 . The system according to claim 9 or 10 , wherein the guard bed reactor for silicon removal is arranged directly upstream of the fuel side of the solid oxide cell .

12 . The system according to any one of claims 9 to 11 , wherein the guard bed further comprises a sulphur, a nitrogen and/or a phosphorous binding material .

13 . The system according to any one of claims 9 to 12 , wherein the guard bed reactor for silicon removal is configured to provide a gas flow from the upper part of the guard bed reactor through the guard bed and to the lower part of the guard bed reactor .

14 . A method for producing hydrogen comprising i . Passing a feed comprising steam through a guard bed reactor for silicon removal at a temperature in the range of from 150 ° C to 1000 ° C to produce a Si depleted stream; and ii . feeding the si depleted stream to the fuel side of a solid oxide cell comprising an electrolyte layer interposed between a fuel side and an oxy-side , wherein the solid oxide cell is operated at a temperature in the range of from 600 to 1000 ° C to produce a product stream comprising hydrogen . 1

15 . The method according to claim 14 , wherein the guard bed reactor for silicon removal is as defined in any one of claims 1- 8 .

16 . The method according to one of claims 14 to 15 , wherein the Si depleted stream is fed directly to the fuel side of the solid oxide cell .

17 . A use of the guard bed reactor for silicon removal according to any one of claims 1 to 8 to deplete a stream of steam from volatile silica species at a temperature in the range of from 150 to 1000 °C .