Fischer-tropsch process and system

Abstract

A method for synthesising hydrocarbons, the method comprising: (a) feeding water and carbon dioxide to an electrolyser system to co-electrolyse the water and carbon dioxide producing an electrolyser syngas stream and an oxygen containing stream; (b) passing the electrolyser syngas stream though a Fischer-Tropsch system comprising a Fischer-Tropsch reactor to form a hydrocarbon product stream and a waste stream comprising one or more hydrocarbons; (c) passing the waste stream through a reformer system to generate a reformer syngas stream; and (d) passing the reformer syngas stream though the Fischer-Tropsch system to form further hydrocarbon product, wherein both the electrolyser system and the reformer system provide syngas streams to the Fischer-Tropsch system.

Description

Field The present specification relates to a Fischer-Tropsch process and a system for implementing the process. Background Examples of Fischer-Tropsch (FT) plants and their operation are described in WO2021140227A1, WO2018146276A1, WO2017037175A1, WO2015140100A1, WO2015140099A1, WO2015010939A1 and WO2009128865A1. The Fischer-Tropsch process is a collection of chemical reactions that convert a mixture of carbon monoxide and hydrogen (also known as "synthesis gas" or "syngas") into liquid hydrocarbons. These reactions occur in the presence of metal catalysts, typically at temperatures of 150-300 °C and pressures of one to several tens of atmospheres. The Fischer-Tropsch process involves a series of chemical reactions that produce a variety of hydrocarbons, ideally having the formula (CnH2n+2). The more useful reactions produce alkanes as follows: (2n +1) H2 + n CO -> CnH2n+2 + n H2O where n is typically 1-100 or higher. The formation of methane (n = 1) is unwanted. Most of the alkanes produced tend to be straight-chain and are suitable to be upgraded to produce middle distillate fuels such as diesel and jet fuel. In addition to alkane formation, competing reactions give small amounts of alkenes, as well as alcohols and other oxygenated hydrocarbons. In a Fischer-Tropsch plant, the syngas fed to the Fischer-Tropsch unit can be prepared by subjecting a feed gas comprising hydrogen and carbon dioxide to a reverse-water-gas-shift reaction to convert some of the carbon dioxide and hydrogen to carbon monoxide and water as follows: CO2 + H2 # CO + H2O It is also possible to recycle tail gas from the Fischer-Tropsch unit to a derichment reactor in order to produce a methane containing feed gas for the reverse-water-gas-shift reactor system. This methane containing feed gas can then contribute to syngas production within the reverse-water-gas-shift reactor system for input to the Fischer-Tropsch unit thereby increasing the efficiency of the system while reducing emissions. Methane and steam can produce syngas via a steam methane reforming reaction as follows: CH4 + H2O # CO + 3H2 Such a reaction can occur together with the reverse-water-gas-shift reaction with a reverse-watergas-shift reactor. Alternative, a separate reformer system may be utilized to convert the methane into syngas. Further still, it is also possible to separate unwanted hydrocarbon products, such as naphtha, from the hydrocarbon product stream of the Fischer-Tropsch unit and recycle these to a derichment reactor in order to produce a methane containing feed gas for the reverse-water-gas-shift reactor system (or separate steam-methane reforming system). This methane containing feed gas can also contribute to syngas production for input to the Fischer-Tropsch unit in a similar manner to the recycling of tail gas from the Fischer-Tropsch unit. Accordingly, in certain configurations both unwanted hydrocarbon products and tail gas from the Fischer-Tropsch unit can be recycled, deriched, and used to generate syngas for input to the Fischer-Tropsch unit. Crude syngas from the reverse-water-gas-shift system can be processed to separate carbon dioxide (and also water and other inpurities) in order to produce a purified syngas for the Fischer-Tropsch unit. In this case, the separated carbon dioxide can be recycled back into the reverse-water-gas-shift reactor system to produce more crude syngas. Again, this can increase the efficiency of the system while reducing emissions. Yet another adaptation of the reverse-water-gas-shift system is to use an electrolyser to produce hydrogen by electrolysis of water (which may be water recycled from the crude syngas and / or water produced by the Fischer-Tropsch unit) and feed the hydrogen produced by the electrolyser into the reverse-water-gas-shift system to generate more syngas. This can provide an internal supply of hydrogen for the reverse-water-gas-shift process to produce syngas and water usage can be reduced if the water is internally recycled. As an alternative to using a reverse-water-gas-shift reactor system, syngas can be produced by steam methane reforming of a methane containing gas. Furthermore, tail gas and / or unwanted hydrocarbon products from the Fischer-Tropsch unit can be recycled through one or more pre-reformer or derichment reactors to generate a methane containing gas which is then subjected to steam-methane reforming to generate syngas. As another alternative to using a reverse-water-gas-shift reactor system as described above for generating syngas for the Fischer-Tropsch unit, it has been proposed that a solid-oxide electrolyser (SOE) can instead be used to produce syngas for a Fischer-Tropsch unit by simultaneous electrolysis (co-electrolysis) of CO2 and H2O. In this process, water is split into hydrogen and oxygen and carbon dioxide is split into carbon monoxide and oxygen. The two reactions are indicated below: H2O + 2e   H2 + O2 CO2 + 2e   CO + The solid-oxide electrolyser can thus generate a stream of hydrogen and carbon monoxide (syngas) which can be fed to the Fischer-Tropsch unit. See, for example, "Integration of Solid Oxide Electrolyzer and Fischer-Tropsch: A sustainable pathway for synthetic fuel" by Giovanni Cinti et al. (Applied Energy, Volume 162, 15 January 2016, Pages 308-320). In that paper, it is described that carbon dioxide for the solid-oxide electrolyser is provided from external processes while water for the solid-oxide electrolyser can be recycled internally. The paper described three configurations: (i) a basic configuration in which a solid oxide electrolyser is used to co-electrolyse carbon dioxide and water to produce syngas for a Fischer-Tropsch process; (ii) a modified version in which tail gas from the Fischer-Tropsch process is subjected to reforming, with the reformed gas recycled into the solid oxide electrolyser; or (iii) a modified version in which tail gas from the Fischer-Tropsch process is recycled into the solid oxide electrolyser without reforming. US2023155150 also discloses the use of a solid-oxide electrolyser to produce syngas for a Fischer-Tropsch process. This document describes a method of operating a solid oxide electrolyser in which water, carbon dioxide, and a hydrocarbon are input to the electrolyzer. It is disclosed that the method can be applied when the solid oxide cell system is connected to a Fischer-Tropsch synthesis process. It is described that a hydrocarbon-rich residual / circulation gas arises which can be converted into H2 and CO directly in a solid oxide electrolyser stack or via a reformer and a solid oxide electrolyser stack in series (similar to the configuration (ii) of the previously discussed paper). Further examples of using a solid oxide electrolyser in combination with a Fischer-Tropsch process are described in "Operating Strategies for Fischer-Tropsch Tail Gas Recirculation on a 100 kW SOEC Reactor" (Diana Maria Amaya Duenas et al., 2023, ECS Trans. Ill, 1941) and "Low carbon fuel production from combined solid oxide CO2 co-electrolysis and Fischer-Tropsch synthesis system: A modelling study" (Applied Energy Volume 242, 15 May 2019, Pages 911-918). It is an aim of the present specification to provide an improved Fischer-Tropsch process and system taking account of reliability, flexibility of operation, efficiency, and emissions. Summary Previous Fischer-Tropsch processes as described in the background section have proposed reversewater-gas-shift, reforming, or co-electrolysis of water and carbon dioxide as alternative ways to produce syngas to feed a Fischer-Tropsch process. Furthermore, certain prior art configurations disclose combinations of certain elements of such systems. For example, documents discussed in the background section disclose the use of a reformer in series with a solid oxide co-electrolyser. In that case, the system is configured to reform tail gas from the Fischer-Tropsch process and feed the reformed gas into the solid oxide electrolyser which then outputs syngas to the Fischer-Tropsch process. In contrast, the present specification provides a process in which both a reformer system and a co-electrolyser system provide syngas (in parallel) to a Fischer-Tropsch system. That is, instead of the reformer system directing a reformed gas into a co-electrolyser system, both the electrolyser system and the reformer system provide syngas feeds to the Fischer-Tropsch system. One aspect of the present specification provides a method for synthesising hydrocarbons, the method comprising: (a) feeding water and carbon dioxide to an electrolyser system to co-electrolyse the water and carbon dioxide producing an electrolyser syngas stream and an oxygen containing stream; (b) passing the electrolyser syngas stream though a Fischer-Tropsch system comprising a Fischer-Tropsch reactor to form a hydrocarbon product stream and a waste stream comprising one or more hydrocarbons; (c) passing the waste stream through a reformer system to generate a reformer syngas stream; and (d) passing the reformer syngas stream though the Fischer-Tropsch system to form further hydrocarbon product, wherein both the electrolyser system and the reformer system provide syngas streams to the Fischer-Tropsch system. Another aspect of the present specification provides a system for synthesising hydrocarbons according to the method as defined above, the system comprising: (a) an electrolyser system configured to co-electrolyse water and carbon dioxide producing an electrolyser syngas stream and an oxygen containing stream; (b) a Fischer-Tropsch system comprising a Fischer-Tropsch reactor, the Fischer-Tropsch system being configured to receive the electrolyser syngas stream and form a hydrocarbon product stream and a waste stream comprising one or more hydrocarbons; (c) a reformer system configured to receive the waste stream and generate a reformer syngas stream; and (d) wherein the Fischer-Tropsch system is configured to receive the reformer syngas stream to form further hydrocarbon product, the system being configured such that both the electrolyser system and the reformer system provide syngas streams to the Fischer-Tropsch system. Such a configuration can have several advantages. For example, hydrocarbon containing waste streams from the Fischer-Tropsch system can be recycled via the reformer system to produce more syngas without risk of contaminating the electrolyser system and reducing performance and / or operational lifetime of the electrolyser. Input streams to the electrolyser system can therefore be better controlled to optimize the performance and lifetime of the electrolyser. At the same time, relatively high purity carbon dioxide which may, for example, be separated from crude syngas from the reformer system, can be safely recycled back into the electrolyser system without undue risk of contaminating the electrolyser system. Furthermore, since the electrolyser system for co-electrolysis of water and carbon dioxide produces both a syngas stream and an oxygen stream, it has been realized that the oxygen stream from the electrolyser system can be utilized within other parts of the overall system. In particular, at least a portion of the oxygen containing stream from the electrolyser system can be passed to the reformer system and combusted to generate heat within the reformer system to drive generation of the reformer syngas stream. As such, the present configuration can ensure that waste streams from various parts of the process can be efficiently recycled back into the overall system. Further still, it has been found that integrating a co-electrolyser system for generating syngas with a reformer system for generating syngas can lead to an improvement in the overall efficiency of the system. Brief Description of the Drawings Figure 1 shows a flow sheet for a method of synthesising hydrocarbons according to the present specification. Figure 2 shows a more detailed example of a flow sheet for a method of synthesising hydrocarbons according to the present specification with some additional features to those illustrated in Figure 1. A summary of the reference numerals used in the figures is set out in the table below. Reference Item 2 Electrolyser system for co-electrolysis of water and CO2 4 Water (steam) stream 6 Carbon dioxide stream 8 Electrolyser syngas stream 10 Fischer-Tropsch system 12 Reformer system 14 Reformer syngas stream 16 Hydrocarbon product stream 18 Reformer Recycle Loop 20 Electrolyser Recycle loop 22 Oxygen Stream 24 Syngas cooling and CO2 recovery 26 Syngas purification 28 Fischer-Tropsch reactor / synthesis loop 30 Process condensate 32 Medium-Pressure (MP) steam 34 FT produced water 36 Pre-reformer 38 1st stage reformer (gas heated reformer or GHR) 40 2nd stage reformer (autothermal reformer or ATR) 42 MP steam 44 Methane containing gas stream Detailed Description As described in the summary section, the present specification provides a method for synthesising hydrocarbons, the method comprising: (a) feeding water and carbon dioxide to an electrolyser system to co-electrolyse the water and carbon dioxide producing an electrolyser syngas stream and an oxygen containing stream; (b) passing the electrolyser syngas stream though a Fischer-Tropsch system comprising a Fischer-Tropsch reactor to form a hydrocarbon product stream and a waste stream comprising one or more hydrocarbons; (c) passing the waste stream through a reformer system to generate a reformer syngas stream; and (d) passing the reformer syngas stream though the Fischer-Tropsch system to form further hydrocarbon product, wherein both the electrolyser system and the reformer system provide syngas streams to the Fischer-Tropsch system. The electrolyser system can be a solid oxide electrolyser system. Such systems are known in the art for co-electrolysis of carbon dioxide and water. Alternatively, the electrolyser system can be a more conventional lower temperature liquid electrolyser. However, solid oxide electrolyser systems have been found to have significant power savings over conventional electrolysis in this application. The waste stream from the Fischer-Tropsch system which is recycled to the reformer system can be one or both of: a tail gas stream from the Fischer-Tropsch reactor; and an unwanted portion of the hydrocarbon product stream which is separated from the hydrocarbon product stream (e.g., a naphtha stream). In this method the hydrocarbon containing waste stream(s) from the Fischer-Tropsch system can be recycled via the reformer system to produce more syngas without risk of contaminating the electrolyser system and reducing performance and / or operational lifetime of the electrolyser. Input streams to the electrolyser system can therefore be better controlled to optimize the performance and lifetime of the electrolyser. At least a portion of the oxygen containing stream from the electrolyser system can be passed to the reformer system and combusted to generate heat within the reformer system to drive generation of the reformer syngas stream within the reformer system. As such, while the electrolyser system and the reformer system generate separate syngas streams for the Fischer-Tropsch system, the systems can be linked to make use of the oxygen stream from the electrolyser system to drive reformer syngas generation. Alternatively, or additionally, the heat required for the reformer system can be provided by an alternative heating system (for example, a rotary dynamic heater) in place of burning hydrogen with oxygen. This can lead to further improves in process efficiency. The reformer syngas stream can be treated to remove carbon dioxide prior to passing though the Fischer-Tropsch reactor, and the removed carbon dioxide can be recycled back into the electrolyser system for further generation of the electrolyser syngas stream. While recycling of other waste streams back into the electrolyser system can cause contamination issues and potential loss of performance or lifetime, the carbon dioxide separated from the reformer syngas stream is relatively pure and thus contamination issues can be avoided. Again, this recycle scheme enables the electrolyser, reformer, and FT parts of the system to be linked together in a manner which enables waste streams from various parts of the process to be efficiently recycled back into the overall system while avoiding contamination, performance, and lifetime issues. Further still, it has been found that integrating a co-electrolyser system for generating syngas with a reformer system for generating syngas can lead to an improvement in the overall efficiency of the system. In addition to removal of carbon dioxide from the reformer syngas as described above, the reformer syngas stream can also be treated to remove water prior to passing though the Fischer-Tropsch reactor. Optionally, this removed water can be recycled back into the electrolyser system for further generation of the electrolyser syngas stream. Depending on the purity of the water / steam separated from the reformer syngas, and the robustness of the electrolyser system to impurities, the water may be subjected to purification treatment prior to feeding back into the electrolyser. The reformer syngas stream may also be treated to remove further impurities prior to passing though the Fischer-Tropsch reactor, particularly those which may poison the Fischer-Tropsch catalyst material in the Fischer-Tropsch reactor. In this regard, syngas may typically contain ppm levels of hydrogen cyanide and ammonia, which deactivate the Fischer-Tropsch catalyst, and so ideally the hydrogen cyanide and ammonia are removed down to single-digit ppb levels. Methods of removing water, carbon dioxide, and impurities such as hydrogen cyanide and ammonia are known in the art. The difference here is the integration of these processes into a system which combines production of reformer syngas and electrolyser syngas for a Fischer-Tropsch process, and the reuse of components separated from the reformer syngas in the electrolyser feed. The electrolyser syngas can be of higher purity than the pre-treated crude syngas produced by the reformer system. As such, the electrolyser syngas stream may be passed through the Fischer-Tropsch reactor without treatment to remove one or more of carbon dioxide, water, and other impurities. For example, the reformer syngas stream and the electrolyser syngas stream can be mixed after exiting the syngas and electrolyser systems respectively and prior to passing though the Fischer-Tropsch reactor and said mixing can occur after cooling, and optional carbon dioxide recovery and purification, of the reformer syngas stream. Alternatively, the electrolyser syngas can also be subjected to purification treatments in a similar manner to the reformer syngas. In this case, removed carbon dioxide and / or water can be recycled back into the electrolyser system in a comparable manner to that described above for the reformer syngas treatments. If both the electrolyser syngas and the reformer syngas are to be treated in the same manner, then the syngas streams can optionally be mixed prior to one or more of said treatments. The reformer system may comprise a pre-reformer which generates a methane containing stream from the waste stream of the Fischer-Tropsch system. The reformer system may further comprise one or both of a gas heated reformer and an autothermal reformer for converting the methane containing stream into the reformer syngas stream. Advantageously, the reformer system comprises both a gas heated reformer and an autothermal reformer and heat is recovered from the autothermal reformer by using outlet gas from the autothermal reformer to heat the gas heated reformer. In this case, the oxygen containing stream from the electrolyser system can be passed to the autothermal reformer of the reformer system to maintain a sufficiently high temperature to drive generation of reformer syngas. As an additional or alterative component, the reformer system may comprise a reverse-water-gasshift reactor. Such a reactor can create syngas from hydrogen and carbon dioxide feeds and can also optionally perform the steam methane reforming reactions for recycling FT waste streams. Providing such a reactor would enable the Fischer-Tropsch process to operate based on syngas generated from either or both of the reformer and electrolyser systems and thus provide a degree of flexibility and reliability, e.g., in the case of an electrolyser fault. In certain configurations such a reverse-water-gasshift reactor does not have a separate external carbon dioxide feed and supports a reverse water-gas shift reaction based on CO2 within the Fischer-Tropsch waste stream (in addition to steam-methane reforming). In other configurations, such a reverse-water-gas-shift reactor may be provided with a separate external carbon dioxide feed (and a hydrogen feed) in addition to the Fischer-Tropsch waste stream. Fischer-Tropsch reactors typically work optimally at a specified CO to H2 ratio (or operating range) of the syngas passing into the Fischer-Tropsch reactor. In the present specification, the syngas fed to the Fischer-Tropsch reactor is provided from two different sources (reformer and electrolyser). However, the CO to H2 ratio of the syngas passing into the Fischer-Tropsch reactor can be adjusted at the electrolyser system by increasing or decreasing the carbon dioxide feed to achieve a desired CO to H2 ratio at the Fischer-Tropsch reactor inlet. The present specification also provides a system for synthesising hydrocarbons according to the method as described above. The system comprises: (a) an electrolyser system configured to co-electrolyse water and carbon dioxide producing an electrolyser syngas stream and an oxygen containing stream; (b) a Fischer-Tropsch system comprising a Fischer-Tropsch reactor, the Fischer-Tropsch system being configured to receive the electrolyser syngas stream and form a hydrocarbon product stream and a waste stream comprising one or more hydrocarbons; (c) a reformer system configured to receive the waste stream and generate a reformer syngas stream; and (d) wherein the Fischer-Tropsch system is configured to receive the reformer syngas stream to form further hydrocarbon product, the system being configured such that both the electrolyser system and the reformer system provide syngas streams to the Fischer-Tropsch system. Additional features of the system are as described in relation to the method and are not repeated here for reasons of conciseness. However, it will be understood that features described in relation to the method can be combined with features of the aforementioned system. Figure 1 shows a flow sheet for a method of synthesising hydrocarbons according to the present specification. The method uses a combination of an electrolyser system 2 for co-electrolysis of water 4 and carbon dioxide 6 to produce syngas 8 for a Fischer-Tropsch system 10 and a reformer system 12 to produce a second stream of syngas 14 for the Fischer-Tropsch system 10. The Fischer-Tropsch system 10 generates a hydrocarbon product stream 16. One or more hydrocarbon waste streams from the Fischer-Tropsch system (for example FT tails gas and / or a portion of unwanted hydrocarbons separated from the product stream, e.g., naphtha) can be recycled via the reformer recycle loop 18 to feed the reformer system 1. Carbon dioxide separated from the crude reformer syngas can be recycled into the electrolyser system 2 via the electrolyser recycle loop 20. Oxygen 22 generated by the electrolyser can be fed into the reformer system 12 to generated heat for driving reformer syngas production. Figure 2 shows a more detailed example of a flow sheet for a method of synthesising hydrocarbons according to the present specification with some additional features to those illustrated in Figure 1. This flow sheet shows that syngas 14 from the reformer system is subjected to cooling and CO2 recovery 24, and further syngas purification 26 prior to entering the Fischer-Tropsch reactor / FT Synthesis loop 28. The recovered CO2 can be recycled via electrolyser recycle loop 20 to the electrolyser 2. Waste streams from the syngas processing and Fischer-Tropsch reactor system include process condensate 30, MP steam 32, and FT produced water 34 (one or more of which may be recycled back into the system, e.g., water / steam streams can be recycled into the electrolyser and / or the reformer systems). The reformer system in this example comprises a pre-reformer 36, a 1st stage gas heated reformer (GHR) 38 and a 2nd stage autothermal reformer (ATR) 40. Hydrocarbon waste streams from the Fischer-Tropsch reactor system are recycled via reformer recycle loop 18, mixed with MP steam 42, and fed to the pre-reformer 36 to generate a methane stream 44 which is fed through the 1st and 2nd stage reformers 38, 40. Oxygen 22 generated by the electrolyser 2 can be fed into the autothermal reformer 40 and heat is recovered from the autothermal reformer 40 by using outlet gas from the autothermal reformer to heat the gas heated reformer 38. Embodiments of the aforementioned systems provide a Fischer-Tropsch process for production of hydrocarbon products which uses a high temperature solid oxide electrolyser fed with steam and CO2 to produce CO and H2 syngas and a pre-reformer followed by a GHR / ATR reactor set for reforming the FT loop tail gas to CO and H2 syngas. The pre-reformer can convert C2+ paraffinic alkanes to methane. The Pre-reformer is followed by a GHR / ATR reactor set that reforms the methane gas to CO and H2. The GHR / ATR combination is advantageous to: (i) recover the heat from ATR by using the outlet gas to drive the endothermic reaction in the GHR; and (ii) minimise the oxygen gas feed to the ATR which reacts exothermically with H2 to produce H2O. By way of example, operating parameters for the pre-reformer may be set as follows: 1. Steam to carbonaceous feed ratio is set to 1.8 wt / wt by adding MP Steam. 2. Feed temperature: 450°C. 3. Pressure: 33.17 bar. The pre-reformer reaction is exothermic. With the aforementioned operating parameters, the gas temperature at the outlet is 567°C. The gas is cooled to 440°C by generating MP Steam (13 bar, 192°C). The GHR / ATR reactor set operating parameters can be set as follows: 1. GHR tube side (catalyst side) feed gas inlet temperature: 440°C. 2. Inlet pressure: 32 bar. 3. Shell side ATR gas inlet temperature: 1000°C. 4. Shell side gas exit temperature: 540°C (100°C above feed gas inlet temperature). 5. Oxygen is added at inlet of ATR to maintain the ATR outlet gas temperature at 1000°C (the oxygen can be supplied from the electrolyser). Reformed gas is mixed with the syngas from the solid oxide electrolyser cell after cooling, CO2 recovery, and syngas purification. The CO to H2 ratio is adjusted at the solid oxide electrolyser cell by increasing or decreasing the CO2 gas feed. In this manner, the required CO to H2 ratio is achieved at the inlet to the FT synthesis loop. The aforementioned configuration has been found to result in significant power savings when compared to prior art configurations. For example, process simulations have found that power savings in the range of approximately 10 to 25% can be achieved when compared to alternative systems.

Claims

1. A method for synthesising hydrocarbons, the method comprising:(a) feeding water and carbon dioxide to an electrolyser system to co-electrolyse the water and carbon dioxide producing an electrolyser syngas stream and an oxygen containing stream;(b) passing the electrolyser syngas stream though a Fischer-Tropsch system comprising a Fischer-Tropsch reactor to form a hydrocarbon product stream and a waste stream comprising one or more hydrocarbons;(c) passing the waste stream through a reformer system to generate a reformer syngas stream; and(d) passing the reformer syngas stream though the Fischer-Tropsch system to form further hydrocarbon product,wherein both the electrolyser system and the reformer system provide syngas streams to the Fischer-Tropsch system.

2. A method according to claim 1,wherein at least a portion of the oxygen containing stream from the electrolyser system is passed to the reformer system and combusted to generate heat within the reformer system to drive generation of the reformer syngas stream within the reformer system.

3. A method according to claim 1 or 2,wherein the reformer syngas stream is treated to remove carbon dioxide prior to passing though the Fischer-Tropsch reactor, and the removed carbon dioxide is recycled back into the electrolyser system for further generation of the electrolyser syngas stream.

4. A method according to any preceding claim,wherein the reformer syngas stream is treated to remove water prior to passing though the Fischer-Tropsch reactor, and the removed water is recycled back into the electrolyser system for further generation of the electrolyser syngas stream.

5. A method according to claim 3 or 4,wherein the reformer syngas stream is treated to remove further impurities prior to passing though the Fischer-Tropsch reactor.

6. A method according to any preceding claim,wherein the electrolyser syngas stream is treated to remove carbon dioxide prior to passing though the Fischer-Tropsch reactor, and the removed carbon dioxide is recycled back into the electrolyser system for further generation of the electrolyser syngas stream.

7. A method according to any preceding claim,wherein the electrolyser syngas stream is treated to remove water prior to passing though the Fischer-Tropsch reactor, and the removed water is recycled back into the electrolyser system for further generation of the electrolyser syngas stream.

8. A method according to claim 6 or 7,wherein the electrolyser syngas stream is treated to remove further impurities prior to passing though the Fischer-Tropsch reactor.

9. A method according to any one of claims 1 to 5,wherein the electrolyser syngas stream is passed through the Fischer-Tropsch reactor without treatment to remove one or more of carbon dioxide, water, and other impurities.

10. A method according to any preceding claim,wherein the reformer system comprises a pre-reformer which generates a methane containing stream from the waste stream of the Fischer-Tropsch system.

11. A method according to claim 10,wherein the reformer system further comprises one or both of a gas heated reformer and an autothermal reformer for converting the methane containing stream into the reformer syngas stream.

12. A method according to claim 11,wherein the reformer system comprises both the gas heated reformer and the autothermal reformer and wherein heat is recovered from the autothermal reformer by using outlet gas from the autothermal reformer to heat the gas heated reformer.

13. A method according to claim 11 or 12,wherein the oxygen containing stream from the electrolyser system is passed to the autothermal reformer of the reformer system.

14. A method according to any preceding claim,wherein the reformer syngas stream and the electrolyser syngas stream are mixed after exiting the syngas and electrolyser systems respectively and prior to passing though the Fischer-Tropsch reactor.

15. A method according to claim 14,wherein said mixing occurs after cooling, and optional carbon dioxide recovery and purification, of the reformer syngas stream.

16. A method according to any preceding claim,wherein a CO to H2 ratio of the syngas passing into the Fischer-Tropsch reactor is adjusted at the electrolyser system by increasing or decreasing the carbon dioxide feed to achieve a desired CO to H2 ratio at the Fischer-Tropsch reactor inlet.

17. A method according to any preceding claim,wherein the waste stream from the Fischer-Tropsch system comprising one or more hydrocarbons is one or both of: a tail gas stream from the Fischer-Tropsch reactor; and an unwanted portion of the hydrocarbon product stream which is separated from the hydrocarbon product stream.

18. A method according to any preceding claim,wherein the electrolyser system is a solid oxide electrolyser system.

19. A system for synthesising hydrocarbons according to the method of any preceding claim, the system comprising:(a) an electrolyser system configured to co-electrolyse water and carbon dioxide producing an electrolyser syngas stream and an oxygen containing stream;(b) a Fischer-Tropsch system comprising a Fischer-Tropsch reactor, the Fischer-Tropsch system being configured to receive the electrolyser syngas stream and form a hydrocarbon product stream and a waste stream comprising one or more hydrocarbons;(c) a reformer system configured to receive the waste stream and generate a reformer syngas stream; and(d) wherein the Fischer-Tropsch system is configured to receive the reformer syngas stream to form further hydrocarbon product, the system being configured such that both the electrolyser system and the reformer system provide syngas streams to the Fischer-Tropsch system.

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