Process and plant for producing a synthetic fuel

Abstract

Disclosed herein is a plant and a process for producing a synthetic fuel, preferably methane and / or methanol. The process includes electrolyzing (S10) water, removing (S20) at least some residual water (21) from a hydrogen product stream (11) from the electrolysis, and reacting (S30) a dried hydrogen product stream (22) and a carbon oxide stream (62), thereby providing a first fuel product stream (31). The method also comprises removing (S40) at least some water (41) from the first fuel product stream (31) and supplying (S50) at least a portion of the residual water (21) removed from the hydrogen product stream (11) and at least a portion of the water (41) removed from the first fuel product stream (31) to the electrolyzer (10) for electrolysis.

Description

[0001] P28903PC00 23 October 2025

[0002] 1 / 41

[0003] Process and Plant for Producing a Synthetic Fuel

[0004] FIELD OF THE DISCLOSURE

[0005] The present disclosure relates to the field of synthetic fuel production, in particular to the production of methane and / or methanol. Disclosed herein are a process for producing a synthetic fuel and a plant for producing synthetic fuel.

[0006] BACKGROUND OF THE DISCLOSURE

[0007] Fuels such as methane and / or methanol are important energy sources, including for industrial applications and for private households. Many industries rely on natural gas for different purposes, including heat production. The fuels often derive from natural gas. However, reducing the consumption of natural gas is one of the world population targets to reduce or mitigate the negative impacts of the human made climate change. Therefore, synthetic fuels, in particular methane and / or methanol, are an important component of a modern and sustainable energy ecosystem. Accordingly, there is a need to improve the production of synthetic fuels, in particular with respect to sustainability, environmental impact, energy efficiency and resource efficiency.

[0008] Many approaches and processes for producing synthetic fuels have been developed so far. For the synthesis of methane in particular, many of them rely on the Sabatier process or related processes. Different energy sources may be used in the processes, including renewable energy sources such as solar, nuclear or wind power plants.

[0009] The synthesis of methane or other synthetic fuels is a complex procedure which requires a plurality of components and input products. For example, one challenge is that a constant energy supply and product inflow is often required to achieve efficient fuel P28903PC00 23 October 2025

[0010] 2 / 41 synthesis. A further challenge is that a complex network of input resources are required, including a stable supply of reactants and an efficient energy management system to control the fuel production. Some of the input resources such as water may be limited, which requires an efficient resource management system. In particular, in regions that are scarce of water, an important water management system may be required to minimize waste of water. At the same time, the need to manage water flows efficiently must be balanced with other process requirements, such as heat management, high fuel production rates and potential scarcity of other input resources. A further reason for the desire to improve water management is the general desire to improve sustainability.

[0011] Accordingly, there is a need to improve the known processes and plants for producing synthetic fuels such as methane and / or methanol. In particular, there is a need to improve the management of the resources used in the processes and plants, in particular the management of the reactant and product streams.

[0012] SUMMARY OF THE DISCLOSURE

[0013] It is an object of the present disclosure to provide a plant and a process for producing synthetic fuel, in particular methane and / or methanol, which address at least some of the disadvantages of the known plants and processes for producing synthetic fuel. It is a particular object to improve the overall management of the process, particularly regarding the management of the resources used in the process, such as the management of reactant streams and product streams. In at least some embodiments, it is an object to improve the autarky with respect to the raw materials used, in particular water.

[0014] According to the present disclosure, these objects are addressed by the features of the independent claims. Further preferred embodiments follow from the dependent claims, figures and the description. P28903PC00 23 October 2025

[0015] 3 / 41

[0016] According to a first aspect of the present disclosure, a process for producing a synthetic fuel, preferably methane and / or methanol, is provided. The process comprises the steps of: a) Electrolyzing water using an electrolyzer, thereby providing a hydrogen product stream; b) Removing at least some residual water from the hydrogen product stream, thereby providing a dried hydrogen product stream; c) Reacting the dried hydrogen product stream and a carbon oxide stream, using a first fuel synthesis reactor, thereby providing a first fuel product stream comprising the synthetic fuel; d) Removing, using a second separation unit, at least some water from the first fuel product stream, thereby providing a dried first fuel product stream; and e) Supplying at least a portion of the residual water removed from the hydrogen product stream and at least a portion of the water removed from the first fuel product stream to the electrolyzer for electrolysis.

[0017] By removing at least some water from the first fuel product stream and from the hydrogen product stream and then supplying it to the electrolyzer for electrolysis, the water autarky of the process is improved. The technical effort required to use both of these residual water sources in the electrolysis is significant and typically exceeds the effort required for conventional freshwater treatment. For example, it typically requires removing gaseous components from the water thus removed. However, this technical effort is accepted according to the present disclosure as this creates opportunities for relieving other parts of the process. P28903PC00 23 October 2025

[0018] 4 / 41

[0019] According to the process of present disclosure, at least a portion of the residual water removed from the hydrogen product stream and at least a portion of the water removed from the first fuel product stream are supplied to the electrolyzer for electrolysis. Depending on the application, they may be supplied to the electrolyzer separately or jointly, i.e. e.g. after mixing of the portion of the residual water removed from the hydrogen product stream with the portion of the water removed from the first fuel product stream. In preferred embodiments, at least a portion of the residual water removed from the hydrogen product stream and at least a portion of the water removed from the first fuel product stream are mixed together and then supplied to the electrolyzer for electrolysis. In some embodiments, the mixed portions may optionally be degassed together before they are supplied to the electrolyzer for electrolysis.

[0020] Depending on the application, different amounts of the residual water may be supplied to the electrolyzer for electrolysis. In some embodiments, at least 10%, preferably at least 25%, more preferably at least 50%, even more preferably at least 80%, particularly more than 90%, of the residual water removed from the hydrogen product stream is supplied to the electrolyzer for electrolysis. Alternatively or in combination, in some embodiments, at least 10%, preferably at least 25%, more preferably at least 50%, even more preferably at least 80%, particularly more than 90%, of the water removed from the first fuel product stream may be supplied to the electrolyzer for electrolysis. It is understood that the ranges disclosed in the previous two sentences can be combined freely with each other. As a non-limiting example, in some embodiments, at least 50% of the residual water removed from the hydrogen product and at least 50% of the water removed from the first fuel product stream may be supplied to the electrolyzer for electrolysis. Preferably, in some embodiments, at least 80% of the residual water removed from the hydrogen product and at least 80% of the water removed from the first fuel product stream may be supplied to the electrolyzer for electrolysis. P28903PC00 23 October 2025

[0021] 5 / 41

[0022] Depending on the application, the residual water may be removed from the hydrogen product stream in different ways and, optionally, at more than one locations between the electrolyzer and the first fuel synthesis reactor. For example, at least some of the residual water may be removed as condensate in a compression. Alternatively or in combination, some of the residual water may be removed in a first separation unit, as described hereinbelow. The first separation unit may optionally be arranged downstream of a hydrogen compressor.

[0023] In some embodiments, the process comprises the step of compressing, using a hydrogen compressor arranged downstream of the electrolyzer and upstream of the first fuel synthesis reactor, the hydrogen product stream, thereby providing a water condensate stream and a compressed hydrogen product stream. The process may further comprise supplying at least a portion of the water condensate stream to the electrolyzer for electrolysis. Preferably, the portion of the compressed hydrogen product stream is supplied directly to the electrolyzer for electrolysis. This variant is preferable because it increases the efficiency by obviating the need for any further post-processing steps. Furthermore, the variant is also advantageous because the water condensate stream typically includes further residual components (e.g. KOH) from the electrolysis, which may be incompatible with other post-processing steps.

[0024] Alternatively or in combination with the embodiments described in the previous paragraph, step b) of the process may in some embodiments comprise removing, using a first separation unit, at least a first portion of residual water from the hydrogen product stream respectively from the compressed hydrogen product stream, thereby providing the dried hydrogen product stream. In those embodiments in which the process also comprises the step of compressing the hydrogen product stream, as described e.g. in the previous paragraph, step b) may comprise removing at least a first portion of residual P28903PC00 23 October 2025

[0025] 6 / 41 water from the compressed hydrogen product stream. In other words, the first separation unit may optionally be arranged downstream of the hydrogen compressor.

[0026] Depending on the application, the step of e) supplying at least a portion of the residual water removed from the hydrogen product stream and at least a portion of the water removed from the first fuel product stream to the electrolyzer for electrolysis may be performed in different ways. In some embodiments, step e) comprises the sub-step of: e.1) combining at least the first portion of residual water removed from the hydrogen product stream (respectively e.g. from the compressed hydrogen product stream) and at least a portion of the water removed from the first fuel product stream, thereby forming a combined water recyclate.

[0027] Depending on the application, the combining step may be realized in different ways. For example, combining may comprise merging both portions together. As an example, both portions may initially be guided through separate conduits and the conduits may then combine to form a common conduit for guiding the merged fluid. In some embodiments, the portions may be mixed together.

[0028] It is understood that the combined water recyclate comprises at least the first portion of residual water removed from the hydrogen product stream (respectively e.g. from the compressed hydrogen product stream) and at least the portion of the water removed from the first fuel product stream, typically in a merged or mixed fashion.

[0029] It is understood that the ratios mentioned above with respect to the amount of residual water supplied to the electrolysis may optionally also apply to the step of combining the portions. Therefore, as a non-limiting example, in some embodiments, at least 30% of the first portion of residual water removed from the hydrogen product stream (respectively e.g. from the compressed hydrogen product stream) and at least 30% of the P28903PC00 23 October 2025

[0030] 7 / 41 residual water removed from the first fuel product stream are combined, thereby forming the combined water recyclate. In some embodiments, at least 50% of the first portion of residual water removed from the hydrogen product stream (respectively e.g. from the compressed hydrogen product stream) and at least 50% of the water removed from the first fuel product stream are combined, thereby forming the combined water recyclate. In some embodiments, at least 80% of the first portion of residual water removed from the hydrogen product stream (respectively e.g. from the compressed hydrogen product stream) and at least 80% of the residual water removed from the first fuel product stream are combined, thereby forming the combined water recyclate.

[0031] Depending on the application, water recycling may be performed in a continuous or at least partially in a batch-wise fashion. In some embodiments, at least the first portion of residual water removed from the hydrogen product stream (respectively e.g. from the compressed hydrogen product stream) and at least a portion of the residual water removed from the first fuel product stream are combined, thereby forming a combined water recyclate stream. Preferably, the first portion of the residual water removed from the hydrogen product stream (respectively e.g. from the compressed hydrogen product stream) may already be a stream. Similarly, the portion of the water removed from the first fuel product stream may also already be a stream.

[0032] In some embodiments, step e) further comprises the sub-step of: e2) degassing, using a degassing unit, the combined water recyclate, thereby providing a degassed water recyclate.

[0033] It is understood that in the degassing step, the residual gas in the combined water recyclate is removed at least partially, preferably fully. Depending on the application, at least 20%, preferably at least 40%, even more preferably at least 80%, of the residual gas in P28903PC00 23 October 2025

[0034] 8 / 41 the combined water recyclate is removed from the combined water recyclate in the degassing step. The residual gas removed in the degassing step (or at least a portion thereof) may e.g. be sent to a flare. Depending on the application, some entrainment of water into the residual gas being sent to the flare can be tolerated. In some variants, the residual gas removed in the degassing step is at least partially, or even fully, recycled into the gas stream. For example, in some embodiments, at least a portion of the residual gas removed from the combined water recyclate is supplied to the first fuel synthesis reactor. These embodiments may be beneficial to increase product yield and to improve the overall process efficiency. In some embodiments, the removed residual gas is combined with the dried hydrogen product stream and / or with the carbon oxide stream at a position upstream of the first fuel synthesis reactor. In some embodiments, the removed residual gas is fed to a mixing section upstream of the first fuel synthesis reactor, wherein the mixing section is configured for mixing the dried hydrogen product stream and the carbon oxide stream.

[0035] Depending on the application, the degassing may be performed in different ways. For example, it would be conceivable to use a low-pressure separator to separate volatile gaseous components from the combined water recyclate. For example, the combined water recyclate could be expanded from 10 bar to e.g. 1 bar in a low-pressure separator.

[0036] However, it has been found that it is particularly advantageous for the degassing step if the combined water recyclate is heated and then expanded, e.g. using a flash vessel. In some embodiments, the combined water recyclate is even superheated. Heating the combined water recyclate before expansion was found to be particularly advantageous to achieve effective degassing. This is particularly advantageous to ensure that after degassing, the degassed water recyclate that is supplied to the electrolyzer comprises only minimal residual gas, in particular only minimal amounts of carbon dioxide. These embodiments are advantageous to improve the overall efficiency of the process and to P28903PC00 23 October 2025

[0037] 9 / 41 improve the quality and suitability of the water recyclate for resupplying it to the electrolyzer, which ultimately allows for a higher water recycling efficiency and reduces the need for external water supply, e.g. in the form of freshwater. Thus, the described embodiments are particularly advantageous when operating the process of the plant described herein in regions scarce of freshwater.

[0038] In some embodiments, step e2) comprises: e2.i) Heating the combined water recyclate in a heating section of the degassing unit, thereby providing a heated combined water recyclate; and e2.2) Expanding the heated combined water recyclate in a gas separator section of the degassing unit, preferably in a flash vessel.

[0039] Depending on the application, the heating step e2.i) can be performed in different ways. In some variants, in step e2.1), the combined water recyclate is heated to a temperature of at least 60 °C, preferably at least 80 °C. In some embodiments, the combined water recyclate is heated to a temperature of at least 100 °C, such as more than 120 °C. In some embodiments, the combined water recyclate is superheated.

[0040] Similarly, the expansion may also be performed in different ways. In some embodiments, in step e2.2), the heated combined water recyclate is expanded in the gas separator at a pressure lower than 50 barg, preferably lower than 30 barg, more preferably lower than 16 barg.

[0041] In some embodiments, step e) may optionally further comprise the sub-step of: e.3) purifying the degassed water recyclate using a water purification unit, thereby providing a purified degassed water recyclate. P28903PC00 23 October 2025

[0042] 10 / 41

[0043] In some embodiments, step e) may further comprise the sub-step of: e.4) supplying the degassed water recyclate provided in step e.2) and / or the purified degassed water recyclate provided in step e.3) to the electrolyzer for electrolysis.

[0044] Accordingly, the purification step is optional, but preferred. Depending on the application, different purification methods may be used. For example, the water purification may involve ion exchange and / or reverse osmosis and / or membrane technology. Preferably, the water purification involves reverse osmosis of the degassed water recyclate and / or subjecting the degassed water recyclate to an ion exchange. In some embodiments, e.g. to remove dissolved metal ions, the degassed water recyclate may optionally be passed through a mixed-bed cartridge.

[0045] Depending on the application, the degassed water recyclate respectively the purified degassed water recyclate may be checked, particularly quality-checked, at a quality check point arranged upstream of the electrolyzer. This may e.g. be used to check the quality of the water recyclate before entering into the electrolyzer to ensure that certain thresholds are met. For example, in some embodiments, the conductivity of the degassed water recyclate respectively the purified degassed water recyclate is measured at the quality check point upstream of the electrolyzer. In some embodiments, the quality check point is configured to allow supply of the water recyclate (i.e. the degassed water recyclate respectively the purified degassed water recyclate) to the electrolyzer only if the water recyclate (i.e. the degassed water recyclate respectively the purified degassed water recyclate) meets a conductivity threshold.

[0046] Depending on the application, the process may involve the use of further plants or facilities. For example, in some embodiments, the process makes use of a biogas fermentation facility, which may e.g. be used as a source of carbon oxide, specifically e.g. as a P28903PC00 23 October 2025

[0047] 11 / 41 source of carbon dioxide. Alternatively or in combination, the biogas fermentation facility may also be used as an additional source of synthetic fuel, e.g. methane. Alternatively or in combination, the biogas fermentation facility mas also be used as an additional source of water, particularly water to be used in the electrolysis. Alternatively or in combination, the biogas fermentation facility may also be used as a heat recipient for excess heat generated in the fuel synthesis, particularly in the methanation.

[0048] In some embodiments, the process comprises the steps of:

[0049] - separating water from a digestate of a biogas fermentation facility;

[0050] - post-processing the water separated from the digestate; and

[0051] - supplying at least a portion of the post- processed separated water to the electrolyzer for electrolysis.

[0052] In some embodiments, at least 20%, preferably at least 50%, more preferably at least 80%, of the post- processed separated water is supplied to the electrolyzer for electrolysis.

[0053] Optionally, the post- processed separated water may be purified before it is supplied to the electrolyzer. Thus, in some embodiments, the process further comprises purifying at least a portion, preferably all, of the post-processed separated water before supplying it to the electrolyzer for electrolysis. Depending on the application, this purification may be performed independently of or jointly with the purification of the degassed water recy- clate. Thus, in some embodiments, the post- processed separated water and the degassed water recyclate provided in step e.1) are combined and then purified jointly in the water purification unit. P28903PC00 23 October 2025

[0054] 12 / 41

[0055] Depending on the application, different carbon oxide (particularly carbon dioxide) sources may be used in the process disclosed herein. For example, in some embodiments, the carbon oxide used in the electrolysis derives from the biogas fermentation facility. An advantage of this is that this allows the by-product stream of one facility to be used in another facility. A further advantage of this is that a high reliably and / or sufficient purity of the carbon oxide can be ensured.

[0056] Depending on the source of carbon oxide, the carbon oxide stream may optionally comprise residual water. For example, when using carbon oxide from a biogas fermentation facility, the resulting carbon oxide typically comprises residual water. Residual water in the carbon oxide stream can optionally be separated before the carbon oxide stream enters into the first fuel synthesis reactor. Depending on the application, removing the residual water at least partially can be advantageous to increase the efficiency of the fuel synthesis.

[0057] Residual water can be removed in different ways. For example, in some embodiments, the process disclosed herein further comprises f) removing, using a third separation unit, at least some residual water from a wet carbon oxide stream, thereby providing a dried carbon oxide stream; supplying the dried carbon oxide stream to the first fuel synthesis reactor for producing the synthetic fuel; and supplying at least a portion of the residual water removed from the wet carbon oxide stream to the electrolyzer for electrolysis.

[0058] Preferably, at least 20%, such as at least 50%, particularly at least 80%, of the residual water in the wet carbon oxide stream is removed using the third separation unit. Alternatively or in combination with these variants, preferably at least 20%, more preferably at P28903PC00 23 October 2025

[0059] 13 / 41 least 50%, even more preferably at least 80% out of the residual water removed from the wet carbon oxide stream is supplied to the electrolyzer for electrolysis.

[0060] Depending on the application, the dried carbon oxide stream may comprise residual water in an amount of less than 80%, preferably less than 50%, more preferably less than 20%, of the saturation concentration of water at the dew point corresponding to the temperature of the dried carbon oxide stream. The embodiments described in the previous sentence apply in particular to the dried carbon oxide stream in the gas form. Alternatively or in combination, in some embodiments, the dried carbon oxide stream may comprise less than 50 ppm, preferably less than 10 ppm, residual water. The embodiments described in the previous sentence apply in particular to the dried carbon oxide stream in liquid form.

[0061] Depending on the application, the residual water removed from the wet carbon oxide stream can be degassed jointly with other residual water streams. In some embodiments, step e.1) further comprises combining at least a portion of the residual water removed from the wet carbon oxide stream, at least the first portion of residual water removed from the hydrogen product stream (respectively e.g. from the compressed hydrogen product stream) and at least a portion of the water removed from the first fuel product stream, thereby forming the combined water recyclate.

[0062] Depending on the application, different sources may be used for the wet or dry carbon oxide stream. In some embodiments, the wet (or dry) carbon oxide stream derives from a biogas fermentation facility and / or from a carbon removal facility, such as a direct air capture facility. Preferably, in some embodiments, the biogas fermentation facility and / or the carbon removal facility are fluidically interconnected with the first fuel synthesis reactor, thereby supplying the carbon oxide stream to the first fuel synthesis reactor. Typically, a wet carbon oxide stream deriving from the biogas fermentation facility and / or P28903PC00 23 October 2025

[0063] 14 / 41 from the carbon removal facility is compressed in a carbon oxide compressor arranged upstream of the first fuel synthesis reactor, thereby supplying a dry carbon oxide stream to the first fuel synthesis reactor.

[0064] In some embodiments in which the wet carbon oxide stream derives from a direct air capture facility, the process may additionally comprise concentrating a direct air capture output stream to provide the wet carbon oxide stream, such that the wet carbon oxide stream has a higher carbon oxide concentration (particularly carbon dioxide concentration) than the direct air capture output stream. Alternatively or in combination, in some embodiments, the process may additionally comprise compressing a direct air capture output stream to provide the wet carbon oxide stream, such that the wet carbon oxide stream has a higher pressure than the direct air capture output stream.

[0065] Irrespective of the source of the wet carbon oxide stream, typically, the wet carbon oxide stream has a pressure of at least 1 mbarg.

[0066] Depending on the application, water may be recycled at different points of the process and of the plant disclosed herein. For example, in some embodiments, the process comprises supplying at least a portion of water generated in a parallel facility to the electrolyzer. For example, the water generated in a parallel facility may comprise water generated in an electricity generation facility, such as water produced as condensate in the electricity generation facility.

[0067] According to the process of the present disclosure, the hydrogen product stream from the electrolysis is used in the first fuel synthesis reactor (after removal of at least some residual water). Depending on the application, the hydrogen product stream may be compressed before entering the first fuel synthesis reactor. In some embodiments, the method comprises the step of: P28903PC00 23 October 2025

[0068] 15 / 41 g) compressing, using a hydrogen compressor arranged downstream of the electrolyzer and upstream of the first fuel synthesis reactor, the hydrogen product stream of the electrolyzer, thereby providing a compressed hydrogen product stream. The compressed hydrogen product stream may then be supplied to the first fuel synthesis reactor.

[0069] Typically, at least some of the residual water is removed from the hydrogen product stream during and / or after compression, e.g. in the form of condensate. For example, the condensate generated during the compression may be removed during the compression and may optionally be supplied directly to the electrolyzer. Alternatively or in combination, at least a portion of the residual water may be removed from the hydrogen product stream, e.g., after the compression. For example, in some embodiments, step b) comprises removing, using the first separation unit, at least a first portion of residual water from the compressed hydrogen product stream, thereby providing a compressed dried hydrogen product stream. The compressed dried hydrogen product stream may then be supplied to the first fuel synthesis reactor.

[0070] The plant and the reactor disclosed herein involve a fuel synthesis reactor in which a first fuel product stream comprising the synthetic fuel is formed. The first fuel product stream may for example comprise methane and water, including water formed during the methanation reaction. If the first fuel synthesis reactor is a reactor configured for the synthesis of methane and / or methanol (e.g. a methanation reactor), the first fuel synthesis may e.g. be performed at a temperature in the range from 200 °C to 400 °C, e.g. from 200 °C to 300 °C. Preferably, the temperature is chosen such that the methanation reaction can still be controlled if a suitable reactor is selected with isothermal reaction control.

[0071] Optionally, the first fuel synthesis reactor may be operated at a pressure in the range from 8 bar to 30 bar (absolute), preferably from 8 bar to 16 bar. P28903PC00 23 October 2025

[0072] 16 / 41

[0073] Depending on the application, different reactor types may be used. In some embodiments, a honeycomb reactor is used, which may optionally be cooled through an oil cooling and / or a water cooling. The first fuel synthesis reactor may comprise different catalysts. In some embodiments, a nickel-based catalyst is used.

[0074] Depending on the application, one fuel synthesis reactor may be used or multiple fuel synthesis reactor may be used. For example, at least two fuel synthesis reactors arranged in series may be used, e.g. a second fuel synthesis reactor arranged downstream of the first fuel synthesis reactor.

[0075] In some embodiments, the process disclosed herein comprises the steps of: h) Reacting the dried first fuel product stream in a second fuel synthesis reactor, thereby providing a second fuel product stream comprising the synthetic fuel; and i) Removing, using a fourth separation unit, at least some residual water from the second fuel product stream, thereby providing a dried second fuel product stream.

[0076] Optionally, the method may further comprise the step of: j) Recycling at least a portion of the residual water removed from the second fuel product stream to the electrolyzer for electrolysis in step a).

[0077] The second fuel synthesis reactor may e.g. be used to increase the yield of the fuel synthesis and to reduce the amount of residual reactants in the product stream.

[0078] Depending on the application, the synthetic fuel produced in the process disclosed herein (respectively the synthetic fuel produced using the plant disclosed herein) may be used P28903PC00 23 October 2025

[0079] 17 / 41 for different purposes or applications. For example, in some embodiments, the synthetic fuel produced may be fed to a fuel grid, such as a methane grid. To allow feeding into the grid, depending on the grid, the synthetic fuel may need to meet certain requirements. For example, for some fuel grids (e.g. methane grids), the synthetic fuel to be fed into the grid needs to comprise less than 2% residual hydrogen. Furthermore, for some fuel grids (e.g. methane grids), the synthetic fuel to be fed into the grid may need to comprise less than 100 ppm residual water, maybe even less than 50 ppm residual water, or even less than 10 ppm residual water. To meet these requirements, the process or plant disclosed herein may optionally comprise a high-pressure absorber arranged upstream of a grid interface, wherein the high-pressure absorber is configured for absorbing residual water in the fuel product stream (e.g. the first fuel product stream or the second fuel product stream, as the case may be). Preferably, the high-pressure absorber is configured such that the water content in the fuel product stream is reduced to less than 100 ppm, preferably less than 50 ppm, even more preferably less than 10 ppm.

[0080] Carbon oxide, as used herein, generally refers to any oxide of carbon. In particular, carbon oxide is understood to be carbon dioxide, carbon monoxide or a mixture thereof. In some embodiments, the carbon oxide consists of carbon dioxide.

[0081] The process and the plant disclosed herein allow to increase water autarky. Depending on the application, the recycled water may be sufficient to require essentially no external water supply, or at least some external water supply may be necessary (e.g. in the form of freshwater) to compensate e.g. any water losses. If freshwater is supplied, the freshwater may optionally be purified along with the other water recyclate streams. In some embodiments, the process further comprises the step of supplying freshwater to the purification unit. The purification unit may be combined for combining the supplied freshwater and the degassed water recyclate and for jointly purifying the supplied freshwater and the degassed water recyclate. P28903PC00 23 October 2025

[0082] 18 / 41

[0083] According to the method disclosed herein, water is electrolyzed in the electrolyzer, thereby providing a hydrogen product stream. The electrolysis of water typically also generates oxygen, typically in the form of an oxygen product stream. Depending on the application, the oxygen product stream can be used for different applications. In some embodiments, at least some of the oxygen generated in the electrolysis is gasified. Alternatively or in combination, in some embodiments, at least a portion of the oxygen product stream is used to generate power and / or heat. For example, in some embodiments, the process further comprises generating power and / or heat by reacting, using a gas combustion facility, the oxygen product stream provided in the water electrolysis and a combustion fuel. The combustion fuel can, for example, include biogas generated in a / the biogas fermentation facility previously described and / or the synthetic fuel produced in step c) of the method disclosed herein.

[0084] According to a second aspect of the present disclosure, a plant is provided, in particular a plant for producing a synthetic fuel, preferably methane and / or methanol. The plant may, in particular, be configured to carry out the process according to any one of the embodiments disclosed herein, particularly in the context of the first aspect of the disclosure.

[0085] The plant comprises an electrolyzer, particularly an electrolyzer configured for electrolyzing water, thereby providing a hydrogen product stream. The plant typically further comprises a first separation unit arranged downstream of the electrolyzer and configured for removing at least at first portion of residual water from a hydrogen product stream of the electrolyzer, thereby providing a dried hydrogen product stream. Typically, the plant further comprises a first recycling line fluidically interconnecting the first separation unit and the electrolyzer, wherein the first recycling line is configured for supplying to the electrolyzer the first portion of residual water removed from a hydrogen product stream. P28903PC00 23 October 2025

[0086] 19 / 41

[0087] The plant further comprises a first fuel synthesis reactor arranged downstream of the first separation unit and configured for reacting the dried hydrogen product stream and a carbon oxide stream, thereby providing a first fuel product stream comprising the synthetic fuel.

[0088] The plant further comprises a second separation unit arranged downstream of the first fuel synthesis reactor and configured for removing at least some residual water from the first fuel product stream. Depending on the application, different second separation units may be used. For example, the second separation unit may in some embodiments comprise a high-pressure separator.

[0089] The plant further comprises a second recycling line fluidically interconnecting the second separation unit and the electrolyzer, wherein the second recycling line is configured for supplying to the electrolyzer at least a portion of the water removed from the first fuel product stream.

[0090] Depending on the application, different first and second recycling lines may be chosen. In some embodiments, the first and second recycling lines merge together. For example, the first recycling line may comprise an upstream section and a downstream section, and the second recycling line may comprise an upstream section and a downstream section. In this embodiments, for example, the upstream section of the first recycling line and the upstream section of the second recycling line may be separate lines, but the first recycling line and the second recycling line may optionally merge, such that the downstream section of the first recycling line and the downstream section of the second recycling line are part of a single recycling line. In a typical embodiment, the first recycling line and the second recycling line merge at a merging point arranged upstream of the electrolyzer. At this merging point, the first portion of residual water removed from the hydrogen product P28903PC00 23 October 2025

[0091] 20 / 41 stream (respectively e.g. from the compressed hydrogen product stream) and the portion of the water removed from the first fuel product stream are combined.

[0092] It is understood that the first recycling line respectively the second recycling line are configured for supplying to the electrolyzer the first portion of residual water removed from the hydrogen product stream respectively a portion of the water removed from the first fuel product stream. Depending on the application, different lines can be used. For example, the lines may each comprise one or more conduits.

[0093] In some embodiments, the plant comprises a hydrogen compressor arranged downstream of the electrolyzer and upstream of the first fuel synthesis reactor. The hydrogen compressor may be provided alternatively to or in additional to the first separation unit. The hydrogen compressor is configured for compressing the hydrogen product stream, thereby providing a water condensate stream and a compressed hydrogen product stream. In some embodiments, the plant further comprises a fourth recycling line fluidi- cally interconnecting the hydrogen compressor and the electrolyzer. The fourth recycling line is configured for supplying to the electrolyzer at least a portion of the water condensate stream. Preferably, the fourth recycling line directly interconnects the hydrogen compressor and the electrolyzer.

[0094] In some embodiments, the plant further comprises a combining unit arranged downstream of the first separation unit and downstream of the second separation unit, wherein the combining unit is configured for combining at least the first portion of residual water removed from the hydrogen product stream (respectively e.g. from the compressed hydrogen product stream) and at least a portion of the residual water removed from the first fuel product stream, thereby forming the combined water recyclate. In some embodiments, the combining unit is configured for combining at least 20%, preferably at least 50%, more preferably at least 80%, of the first portion of residual water removed from a P28903PC00 23 October 2025

[0095] 21 / 41 hydrogen product stream (respectively e.g. from a compressed hydrogen product stream) and at least 20%, preferably at least 50%, more preferably at least 80%, of the water removed from the first fuel product stream. It is understood that the combining unit is arranged upstream of the electrolyzer.

[0096] In some embodiments, the plant further comprises a degassing unit arranged downstream of the combining unit and configured for degassing the combined water recyclate, thereby providing a degassed water recyclate, wherein the degassing unit is fluidically interconnected with the electrolyzer to supply the degassed water recyclate to the electrolyzer for electrolysis. It is understood that the degassing unit is arranged upstream of the electrolyzer.

[0097] Depending on the application, different degassing units may be used. For example, in some embodiments, the degassing unit comprises

[0098] - A heating section configured for heating the combined water recyclate, thereby providing a heated combined water recyclate; and

[0099] - A gas separator section, preferably a flesh vessel, arranged downstream of the heating section and configured for expanding the heated combined water recyclate.

[0100] The heating section of the degassing unit may optionally be configured for heating the combined water recyclate to a temperature of at least 60 °C, preferably at least 80 °C. In some embodiments, the heating section may even be configured for heating the combined water recyclate to a temperature of at least 100 °C, such as more than 120 °C. In some embodiments, the heating section may even be configured for superheating the combined water recyclate. P28903PC00 23 October 2025

[0101] 22 / 41

[0102] In some embodiments, the gas separator section is configured for expanding the heated combined water recyclate at a pressure lower than 50 barg, preferably lower than 30 barg, more preferably lower than 16 barg.

[0103] Depending on the application, the degassed water may either be provided to the electrolyzer directly from the degassing unit, or it may be subjected to additional steps, such as upgrading and / or purification.

[0104] Depending on the application, an internal condensation may be performed during or after step e2.2) to reduce a water content of flash gas. In some embodiments, step e.2) further comprises the step of e2.3) condensing a gas fraction separated in the gas separator during step e2.2), thereby reducing a water content of said gas fraction. The gas fraction may in particular be a flash gas fraction.

[0105] In some embodiments, the plant further comprises a water purification unit arranged downstream of the degassing unit and upstream of the electrolyzer, wherein the water purification unit is configured for purifying the degassed water recyclate, thereby providing a purified degassed water recyclate. The purified degassed water recyclate may then be supplied to the electrolyzer, either directly or indirectly.

[0106] Depending on the application, the water may be purified using different techniques and machines. Water purification may e.g. be necessary to meet the requirements of the respective electrolyzer. For example, the water purification may involve ion exchange and / or reverse osmosis and / or membrane technology. In some embodiments, the water purification unit comprises an ion exchange unit and / or a reverse osmosis unit.

[0107] As outlined above, depending on the application, a biogas fermentation facility may be combined with the process and / or the plant disclosed herein, e.g. to provide carbon oxide (particularly carbon dioxide). Alternatively or in combination, it is possible to separate P28903PC00 23 October 2025

[0108] 23 / 41 some of the residual water from a digestate deriving from the biogas fermentation facility, and to recycle this water as well, preferably using at least a portion of the existing recycling infrastructure of the plant.

[0109] In some embodiments, the plant further comprises a digestate water separator configured for removing water from a digestate from a biogas fermentation facility. It is understood that the digestate is a product of the biogas fermentation facility. The plant may further comprise a digestate post-processing unit configured for post-processing the water removed from the digestate. The digestate post-processing unit may be fluidically interconnected with the electrolyzer to supply the post-processed separated water to the electrolyzer for electrolysis. Thus, it is understood that the digestate post-processing unit is arranged upstream of the electrolyzer.

[0110] One advantage of these embodiments is that water loss is minimized and water autarky is enhanced by utilizing an additional input source of water for the electrolyzer.

[0111] Depending on the application, the biogas fermentation facility itself may or may not be part of the plant of the present disclosure. Thus, in some embodiments, the plant as disclosed herein comprises the biogas fermentation facility. In these embodiments, the biogas fermentation facility is arranged upstream of the digestate water separator.

[0112] Depending on the application, the post- processed separated water may optionally be purified before entering into the electrolyzer for electrolysis. Optionally, the purification may be carried out using the water purification unit described herein, i.e. the water purification unit configured for purifying the degassed water recyclate, thereby providing a purified degassed water recyclate.

[0113] In some embodiments, the water purification unit is arranged downstream of the digestate post-processing unit and is configured for combining and purifying the post- P28903PC00 23 October 2025

[0114] 24 / 41 processed separated water from the digestate post-processing unit and the degassed water recyclate provided from the degassing unit.

[0115] Depending on the application, different carbon oxide streams (e.g. carbon dioxide streams) may be used. If the carbon oxide stream used comprises at least some residual water, it is possible to remove at least a portion of this water and also recycle it to the electrolysis.

[0116] In some embodiments, the plant further comprises a third separation unit arranged upstream of the first fuel synthesis reactor and configured for removing at least some residual water from a wet carbon oxide stream, thereby providing a dried carbon oxide stream, wherein the third separation unit is fluidically interconnected with the first fuel synthesis reactor to supply the dried carbon oxide stream to the first fuel synthesis reactor.

[0117] These embodiments may for example be used to optimize the efficiency of synthetic fuel production. In particular, the embodiments allow carefully controlling the water content of the carbon oxide stream, which may facilitate operating the first fuel synthesis reactor efficiently.

[0118] In some embodiments, the water removed from the wet carbon oxide stream is recycled back to the electrolyzer for electrolysis. Thus, in some embodiments, the plant comprises a third recycling line fluidically interconnecting the third separation unit and the electrolyzer, wherein the third recycling line is configured for supplying to the electrolyzer at least a portion of the residual water removed from the wet carbon oxide stream. Preferably, at least 20%, more preferably at least 50%, even more preferably at least 80% of the residual water removed from the wet carbon oxide stream is supplied to the electrolyzer. P28903PC00 23 October 2025

[0119] 25 / 41

[0120] Depending on the application, the wet carbon oxide stream may be generated in different ways. For example, in some embodiments, the plant further comprises a biogas fermentation facility and / or a carbon removal facility, such as a direct air capture facility, for generating the wet carbon oxide stream. It is understood that if present, the biogas fermentation facility would be arranged upstream of the first fuel synthesis reactor. Similarly, it is understood that if present, the carbon removal facility would be arranged upstream of the first fuel synthesis reactor.

[0121] As used herein, the term “supply” or “supplying”, when used in the context of supplying water form a first location, unit or component (e.g. A) to a second location, unit or component (e.g. B), means that the water is supplied (e.g. from A to B) either directly or indirectly. In particular, the term includes embodiments in which the water is supplied indirectly (e.g. from A to B), for example through the intermediacy of one or more further locations, units or components (e.g. C) arranged downstream of A and upstream of B (e.g. A -> C -> B). For example, when water is said to be supplied from the first separation unit to the electrolyzer, this includes embodiments in which the water is supplied directly from the first separation unit to the electrolyzer, as well as embodiments in which the water is initially supplied from the first separation unit to intermediate units (e.g. initially the combining unit, then the degassing unit, then the purification unit), and ultimately to the electrolyzer.

[0122] It is further understood that if a unit, location or component (e.g. A) is said be arranged “upstream” or “downstream” of another unit, location or component (e.g. B), this implies that the two units, locations or components are fluidically interconnected with each other. This fluidic interconnection may either be a direct fluidic interconnection or an indirect fluidic interconnection (e.g. involving one or more intermittent units, locations or components arranged along the fluid flow path between the two units, locations or components A and B). P28903PC00 23 October 2025

[0123] 26 / 41

[0124] The present disclosure (hereinbefore and hereinafter) is discussed in the context of different aspects and embodiments to facilitate understanding of the disclosure. However, the present document is to be understood as a unified disclosure. In particular, although some embodiments are discussed in the context of a particular aspect, they are nevertheless to be understood generally as embodiments of the present disclosure and, as such, generally also extend to and apply to other aspects of the present disclosure, unless it is clearly specified otherwise or unless the context dictates otherwise. For example, embodiments discussed in the context of the process of the first aspect are also embodiments of the plant of the second aspect (among other aspects), unless it is clearly specified otherwise or unless the context dictates otherwise. Similarly, embodiments discussed in the context of the plant of the second aspect are also embodiments of the process of the first aspect, unless it is clearly specified otherwise or unless the context dictates otherwise.

[0125] It is to be understood that both the foregoing general description and the following detailed description present embodiments and are intended to provide an overview or framework for understanding the nature and character of the disclosure. The accompanying drawings are included to provide a further understanding and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments, and together with the description serve to explain the principles and operation of the concepts disclosed.

[0126] BRIEF DESCRIPTION OF THE DRAWINGS

[0127] The disclosure described herein will be more fully understood from the detailed description given herein below and the accompanying drawings which should not be considered limiting to the invention described in the appended claims. The drawings show: P28903PC00 23 October 2025

[0128] 27 / 41

[0129] Fig. 1 show an embodiment of the plant and of the process for producing a synthetic fuel disclosed herein, focusing in particular on the plant;

[0130] Fig. 2 show an embodiment of the plant and of the process for producing a synthetic fuel disclosed herein, focusing in particular on the process.

[0131] DESCRIPTION OF THE EMBODIMENTS

[0132] Reference will now be made in detail to certain embodiments, examples of which are illustrated in the accompanying drawings, in which some, but not all features are shown. Indeed, embodiments disclosed herein may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Whenever possible, like reference numbers will be used to refer to like components or parts.

[0133] Figures 1 and 2 show an embodiment of the plant 1 and of the process for producing a synthetic fuel disclosed herein. Figure 1 focuses on the different components of the plant, while figure 2 focuses on the steps of the process.

[0134] The illustrated plant and process aim to reduce the environmental footprint of the production of synthetic fuels by utilizing residual water from different sections along the reaction path. Integration of the water cycle is crucial for different reasons and applications, in particular when operating the plant in locations where sources are scarce or limited. For example, it may be attractive to operate the plant in regions allowing high electricity generation via photovoltaics (PV) in order to power the process, particularly the the electrolysis. However, electrolysis is a significant water consumer in a lot of renewable processes, while regions with high electricity generation via PV are often scarce of natural P28903PC00 23 October 2025

[0135] 28 / 41 water supplies. The process and the plant described herein make strategic use of various selected water sources located within and outside the process battery limits to minimize freshwater intake from outside the battery limit.

[0136] The plant 1 includes an electrolyzer 10 for electrolyzing S10 water to produce a hydrogen product stream 11 and an oxygen product stream 150. The hydrogen product stream 11 is compressed S70 in a hydrogen compressor 70 to provide a compressed hydrogen product stream 71. Subsequently, water condensed after the hydrogen compression is removed S20 from the compressed hydrogen product stream 71 using a first separation unit 20, thereby providing residual water 21 removed from the hydrogen product stream and a dried hydrogen product stream 22. The residual water 21 thus removed is one of the sources of recycled residual water.

[0137] The dried hydrogen product stream 22 is introduced into a first fuel synthesis reactor 30 in which it is reacted S30 with a carbon oxide stream 62, thereby forming a first fuel product stream 31 , which comprises methane and water. The first fuel product stream 31 is sent to a second separation unit 40 in which the water 41 is removed S40 from the first fuel product stream to provide a dried first fuel product stream 42. Once again, the water 41 thus removed is another source for recycling.

[0138] Specifically, the water 41 removed from the first fuel product stream and the residual water 21 removed from the hydrogen product stream are ultimately recycled back S50 to the electrolyzer 10 through a first recycling line 501 and a second recycling line 502. Similarly, the water condensate stream 72 formed during compression in the hydrogen compressor 70 is also recycled back to the electrolyzer 10 through a fourth recycling line 721. Depending on the application, different recycling paths may be chosen. In the illustrated embodiments, the water condensate stream 72 is recycled directly back to the electrolyzer 10. This may be advantageous, e.g., because the water condensate stream P28903PC00 23 October 2025

[0139] 29 / 41

[0140] 72 may e.g. still contain KOH from the electrolysis. Furthermore, in the illustrated embodiments, the water 41 removed from the first fuel product and the residual water 21 removed from the hydrogen product stream are sent to a combining unit 51 in which the water streams are combined S51 to form a combined water recyclate 52. The combining unit 51 also receives residual water from other sources, which are also combined into the combined water recyclate 52, as described later.

[0141] After combining, the combined water recyclate 52 is degassed S53 in a degassing unit 53, which involves heating and then expanding in a flash vessel (low or mid pressure), thereby providing a degassed water recyclate 54. The separated gas may e.g. be sent to a flare. Some entrainment of water into the gas stream being sent to the flare or recycled into the gas stream might be tolerated. Between the combining unit 51 and the degassing unit 53, the combined water recyclate 52 may optionally be submitted to a heat exchanger.

[0142] After degassing, the degassed water recyclate 54 is purified S55 in a water purification unit 55, e.g. using reverse osmosis. Optionally, fresh water from a fresh water source 170 may also be supplied to the purification unit 55 for purification S55. The resulting purified degassed water recyclate 56 is then supplied S57 back to the electrolyzer 10.

[0143] The process and the plant may also include further components and steps, as illustrated in figures 1 and 2. For example, the dried first fuel product stream 42 may react further S80 in a second fuel synthesis reactor 80, thereby forming a second fuel product stream 81 . The second fuel synthesis reactor 80 may for example be used to increase the overall yield. The resulting second fuel product stream 81 may then be sent to a fourth separation unit 90 in which water is removed S90 from the second fuel product stream, thereby providing the removed water 91 and a dried second fuel product stream 92. The water thus removed 91 may then also be sent to the combining unit 51 for combining, P28903PC00 23 October 2025

[0144] 30 / 41 degassing, purifying, and ultimately recycling back to the electrolyzer S100, as described hereinbefore. The dried second fuel product stream 92 may optionally be compressed in a fuel compressor 95 before it is fed into a fuel grid 96, e.g. a methane grid.

[0145] The carbon oxide used in the process and in the plant described herein is typically carbon dioxide, although it could also be carbon monoxide or a mixture of carbon dioxide and carbon monoxide. The carbon oxide stream 62 may derive from different sources. In the illustrated variant, the carbon oxide stream derives from a biogas fermentation facility 111. More specifically, the biogas fermentation facility 111 may produce a wet carbon oxide stream 62w, which may be sent to a third separation unit 60 in which residual water 61 is removed S60 from the wet carbon oxide stream to provide a dried carbon oxide stream 62d. The dried carbon oxide stream 62d is then typically compressed in a carbon oxide compressor 140, before it is sent to the first fuel synthesis reactor 30. The residual water 61 removed from the wet carbon oxide stream may be sent to the combining unit 51 where it may be combined with the other water streams to provide the combined water recyclate 52.

[0146] The biogas fermentation facility 111 may also be used as yet another source of water. For example, a digestate 112 formed in the biogas fermentation facility may be sent to a fifth separation unit 110 in which water 113 is separated S110 from the digestate. The water thus separated 113 is then typically post-processed S120 in a post-processing unit 120, before the resulting post-processed separated water 121 may be supplied S130 to the water purification unit 55 and ultimately to the electrolyzer 10. In the water purification unit 55, the post-processed separated water 121 may be purified S55 along with the degassed water recyclate 54. Typically, the post-processed separated water 121 is combined with the degassed water recyclate 54 in the purification unit 55, preferably before purification S55, so that a single purification process can be used for purification S55. Ultimately, the purified water 56 is supplied to the electrolyzer 10 for electrolysis. P28903PC00 23 October 2025

[0147] 31 / 41

[0148] Besides hydrogen, the electrolysis of water also generates an oxygen product stream 150, which may optionally be sent to a compressor 155 before the resulting compressed oxygen may be filled in gas bottles 160.

[0149] P28903PC00 23 October 2025

[0150] 32 / 41

[0151] LIST OF DESIGNATIONS

[0152] I Plant for producing a synthetic 501 First recycling line fuel 502 Second recycling line

[0153] S10 Electrolyzing 30 S51 Combining residual water

[0154] 10 Electrolyzer 51 Combining unit

[0155] I I Hydrogen product stream 52 Combined water recyclate

[0156] S20 Removing residual water from 52h Heated combined water recyhydrogen product stream clate

[0157] 20 First separation unit 35 S53 Degassing combined water re¬

[0158] 21 Residual water removed from cyclate hydrogen product stream 53 Degassing unit

[0159] 22 Dried hydrogen product stream 54 Degassed water recyclate

[0160] S30 Reacting dried hydrogen prodS55 Purifying degassed water recy- uct stream and carbon oxide 40 elate stream 55 Water purification unit

[0161] 30 First fuel synthesis reactor 56 Purified degassed water recy¬

[0162] 31 First fuel product stream clate

[0163] S40 Removing residual water from S57 Supplying water recyclate to hydrogen product stream 45 electrolyzer

[0164] 40 Second separation unit S60 Removing residual water from

[0165] 41 Water removed from first fuel wet carbon oxide stream product stream 60 Third separation unit

[0166] 42 Dried first fuel product stream 61 Residual water removed from

[0167] S50 Supplying residual water from 50 wet carbon oxide stream hydrogen product stream and 62 Carbon oxide stream from first fuel product stream to 62w Wet carbon oxide stream electrolyzer 62d Dried carbon oxide stream P28903PC00 23 October 2025

[0168] 33 / 41

[0169] S64 Supplying dried carbon oxide 96 Fuel grid stream to first fuel synthesis reS100 Recycling residual water to elecactor trolyzer

[0170] S67 Supplying residual water re30 S110 Separating water from a digesmoved from wet carbon oxide tate of a biogas fermentation fastream to electrolyzer cility

[0171] S70 Compressing hydrogen product 110 Fifth separation unit stream S111 Operation of biogas fermenta-

[0172] 70 Hydrogen compressor 35 tion plant

[0173] 71 Compressed hydrogen product 111 Biogas fermentation facility stream 112 Digestate

[0174] 72 Water condensate stream 113 Water separated from digestate

[0175] 721 Fourth recycling line S120 Post-processing the water sepa-

[0176] S80 Reacting dried first fuel product 40 rated from the digestate stream in second fuel synthesis 120 Post-processing unit reactor 121 Post-processed separated water

[0177] 80 Second fuel synthesis reactor S130 Supplying post- processed sepa¬

[0178] 81 Second fuel product stream rated water to electrolyzer

[0179] S90 Removing residual water from 45 S140 Carbon oxide compression second fuel product stream 140 Carbon oxide compressor

[0180] 90 Fourth separation unit 150 Oxygen product stream

[0181] 91 Residual water removed from 155 Oxygen compressor second fuel product stream 160 Gas bottle

[0182] 92 Dried second fuel product 50 170 Fresh water source stream

[0183] 95 Fuel compressor

Claims

P28903PC00 23 October 202534 / 41PATENT CLAIMS1 . Process for producing a synthetic fuel, preferably methane and / or methanol, comprising the steps of: a) Electrolyzing (S10) water using an electrolyzer (10), thereby providing a hydrogen product stream (11); b) Removing at least some residual water (21) from the hydrogen product stream (11), thereby providing a dried hydrogen product stream (22); c) Reacting (S30) the dried hydrogen product stream (22) and a carbon oxide stream (62), using a first fuel synthesis reactor (30), thereby providing a first fuel product stream (31) comprising the synthetic fuel; d) Removing (S40), using a second separation unit (40), at least some water (41) from the first fuel product stream (31), thereby providing a dried first fuel product stream (42); and e) Supplying (S50) at least a portion of the residual water (21) removed from the hydrogen product stream (11) and at least a portion of the water (41) removed from the first fuel product stream (31) to the electrolyzer (10) for electrolysis.

2. Process according to claim 1 , further comprising compressing, using a hydrogen compressor (70) arranged downstream of the electrolyzer (10) and upstream of the first fuel synthesis reactor (30), the hydrogen product stream (11), thereby providing a water condensate stream (72) and a compressed hydrogen productP28903PC00 23 October 202535 / 41 stream (71), wherein the process further comprises supplying at least a portion of the water condensate stream (72) to the electrolyzer (10) for electrolysis.

3. Process according to claim 2, wherein the portion of the water condensate stream (72) is supplied directly to the electrolyzer (10) for electrolysis.

4. Process according to any one of the previous claims, wherein step b) comprises removing, using a first separation unit (20), at least a first portion of residual water (21) from the hydrogen product stream (11) respectively from the compressed hydrogen product stream (71), thereby providing the dried hydrogen product stream (22).

5. Process according to claim 4, wherein step e) comprises: e.1) combining (S51) at least a portion of the water (41) removed from the first fuel product stream and at least the first portion of residual water removed from the hydrogen product stream respectively from the compressed hydrogen product stream, thereby forming a combined water recyclate (52); e.2) degassing (S53), using a degassing unit (53), the combined water recyclate (52), thereby providing a degassed water recyclate (54); e.3) optionally purifying (S55) the degassed water recyclate (54) using a water purification unit (55), thereby providing a purified degassed water recyclate (56); e.4) supplying (S57) the degassed water recyclate (54) provided in step e.2) respectively the purified degassed water recyclate (56) provided in step e.3) to the electrolyzer (10) for electrolysis.P28903PC00 23 October 202536 / 416. Process according to claim 5, wherein step e.2) comprises: e2.1) Heating the combined water recyclate (52) in a heating section of the degassing unit (53), thereby providing a heated combined water recyclate; and e2.2) Expanding the heated combined water recyclate in a gas separator section of the degassing unit (53), preferably in a flash vessel.

7. Process according to claim 6, wherein in step e2.1) the combined water recyclate (52) is heated to a temperature of at least 60 °C, preferably at least 80 °C.

8. Process according to claim 6 or 7, wherein in step e2.2) the heated combined water recyclate (52h) is expanded in the gas separator at a pressure lower than 50 barg, preferably lower than 30 barg, more preferably lower than 16 barg.

9. Process according to any one of the previous claims, further comprising separating (S110), using a fifth separation unit (110), water from a digestate (112) of a biogas fermentation facility (111), post-processing (S120), using a post-pro- cessing unit (120), the water separated from the digestate (113) and supplying (S130) at least a portion of the post-processed separated water (121) to the electrolyzer (10) for electrolysis.

10. Process according to claim 9, further comprising purifying at least a portion of the post- processed separated water (121) before supplying it to the electrolyzer (10) for electrolysis.1 1 . Process according to claim 10 and 5, wherein the post- processed separated water (121) and the degassed water recyclate (54) provided in step e.1) are combined and then purified jointly in the water purification unit (55).P28903PC00 23 October 202537 / 4112. Process according to any one of the previous claims, further comprising the step of: f) removing (S60), using a third separation unit (60), at least some residual water (61) from a wet carbon oxide stream (62w), thereby providing a dried carbon oxide stream (62d); supplying (S64) the dried carbon oxide stream (62d) to the first fuel synthesis reactor (30) for producing the synthetic fuel; and supplying (S67) at least a portion of the residual water removed (61) from the wet carbon oxide stream to the electrolyzer (10) for electrolysis.

13. Process according to claim 12 and 5, wherein step e.1) further comprises combining at least a portion of the residual water (61) removed from the wet carbon oxide stream, at least a portion of the water (41) removed from the first fuel product stream and at least the first portion of residual water removed from the hydrogen product stream respectively from the compressed hydrogen product stream, thereby forming the combined water recyclate (52).

14. Process according to claim 12 or 13, wherein the wet carbon oxide stream (62w) derives from a / the biogas fermentation facility (111) and / or from a carbon removal facility, such as a direct air capture facility.

15. Process according to any one of the previous claims, further comprising the steps of: h) Reacting (S80) the dried first fuel product stream (42) in a second fuel synthesis reactor (80), thereby providing a second fuel product stream (81) comprising the synthetic fuel;P28903PC00 23 October 202538 / 41 i) Removing (S90), using a fourth separation unit (90), at least some residual water from the second fuel product stream (91), thereby providing a dried second fuel product stream (92); and j) Optionally recycling (S100) at least a portion of the residual water removed (91) from the second fuel product stream to the electrolyzer (10) for electrolysis in step a).

16. Process according to any one of the previous claims, wherein the carbon oxide stream (62) comprises carbon dioxide, carbon monoxide or a mixture thereof.

17. Plant (1) for producing a synthetic fuel, preferably methane and / or methanol, comprising: a. An electrolyzer (10); b. A first separation unit (20) arranged downstream of the electrolyzer (10) and configured for removing at least a first portion of residual water (21) from a hydrogen product stream (11) of the electrolyzer (10), thereby providing a dried hydrogen product stream (22); c. A first fuel synthesis reactor (30) arranged downstream of the first separation unit (20) and configured for reacting the dried hydrogen product stream (22) and a carbon oxide stream (62), thereby providing a first fuel product stream (31) comprising the synthetic fuel; d. A second separation unit (40) arranged downstream of the first fuel synthesis reactor (30) and configured for removing at least some residual water (41) from the first fuel product stream (31);P28903PC00 23 October 202539 / 41 e. A first recycling line (501) fluidically interconnecting the first separation unit (20) and the electrolyzer (10), wherein the first recycling line (501) is configured for supplying to the electrolyzer (10) the first portion of residual water (21) removed from a hydrogen product stream; f. A second recycling line (502) fluidically interconnecting the second separation unit (40) and the electrolyzer (10), wherein the second recycling line (502) is configured for supplying to the electrolyzer (10) at least a portion of the water (41) removed from the first fuel product stream.

18. Plant (1) according to claim 17, further comprising: a. A combining unit (51) arranged downstream of the first separation unit (20) and downstream of the second separation unit (40), wherein the combining unit (51) is configured for combining at least the first portion of residual water (21) removed from the hydrogen product stream and at least a portion of the water (41) removed from the first fuel product stream, thereby forming a combined water recyclate (52); b. A degassing unit (53) arranged downstream of the combining unit (51) and configured for degassing the combined water recyclate (52), thereby providing a degassed water recyclate (54), wherein the degassing unit (53) is fluidically interconnected with the electrolyzer (10) to supply the degassed water recyclate (54) to the electrolyzer (10) for electrolysis.

19. Plant (1) according to claim 18, further comprising a water purification unit (55) arranged downstream of the degassing unit (53) and upstream of the electrolyzerP28903PC00 23 October 202540 / 41(10), wherein the water purification unit (55) is configured for purifying the degassed water recyclate (54), thereby providing a purified degassed water recy- clate (56).

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