Method of forming a syngas

Abstract

A method of forming a syngas, the method comprising: providing a reverse water-gas shift reactor 44 comprising a reverse water-gas shift catalyst; providing a feed gas 36 comprising carbon dioxide and

Description

FIELD OF THE INVENTION The invention relates to a method of forming a syngas, and a method of producing liquid hydrocarbons from the syngas. BACKGROUND OF THE INVENTION The Fischer-Tropsch process converts a mixture of carbon monoxide and hydrogen 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 (CJL^v). The more useful reactions produce alkanes as follows: (2n + 1) H2 + n CO —> CnH2n+2 + n H2O where n may be 1-100, or higher. The formation of methane (n = 1) is unwanted. In addition to alkane formation, competing reactions give small amounts of alkenes, as well as alcohols and other oxygenated hydrocarbons. The Fischer-Tropsch reaction is a highly exothermic reaction due to a standard reaction enthalpy (AH) of -165 kJ / mol CO combined. Synthesis gas (syngas) fed to a Fischer-Tropsch process can be derived from a number of feedstocks; for example, natural gas via steam reforming and / or auto-thermal reforming, municipal solid waste and biomass via high-temperature gasification or carbon dioxide and hydrogen via a reverse water-gas shift reaction. The reverse water-gas shift reaction may be depicted as follows: H2 + CO2 CO + H2O The latter source is beneficial since it makes use of carbon dioxide that may have been destined to be released to the atmosphere. However, the reverse water-gas shift reaction is highly endothermic. WO2022079408A1 describes a process for producing a gas stream comprising carbon monoxide by feeding a gas mixture comprising carbon dioxide and hydrogen to a burner disposed in a reverse water-gas shift vessel and combusting it with a sub-stoichiometric amount of an oxygen gas stream to form a combusted gas mixture containing carbon monoxide, carbon dioxide, hydrogen and steam. The mixture is then passed through a reverse water-gas shift catalyst to form a crude product gas comprising carbon monoxide, carbon dioxide, hydrogen and steam. The gas is then cooled so that the water content condenses and can be separated and removed; it then passes to a carbon dioxide removal unit to remove carbon dioxide, which can be recycled to the feed gas mixture to the reverse water-gas shift vessel, and a gas comprising carbon monoxide and hydrogen. This application teaches a practical, effective method to impart heat into a H2 / CO2 mixture before subjecting it to the reverse water-gas shift reaction. Furthermore, as the renewable hydrogen stream may well be the product of water electrolysis, there will be an available coproduct stream of oxygen gas. However, it suffers the disadvantage that part of the valuable hydrogen feed is combusted in the burner. Heat for the reverse water-gas shift reaction could be provided by a resistance electric heater. However, at desirable reverse water-gas shift reaction temperatures, electric heater elements are prone to overheating and / or damage to the electric heater. WO2022079098A1 describes a plant to synthesise hydrocarbons from hydrogen and carbon dioxide, comprising an electrically heated reverse water-gas shift section. This is exemplified by using a. structured catalyst comprising a macroscopic structure of electrically conductive material coated with a catalyst capable of catalysing both a reverse water gas shift reaction and a methanation reaction. However, the conductive material and catalyst has to operate and be stable at a temperature higher than that of the reacting gas and provide effective and stable catalysis for RWGS over an extended period of time. Moreover, the fabrication of such structured catalysts is more complex and expensive than conventional RWGS catalysts and apparatus. Accordingly, there is a need for a practical effective heating method to provide the endotherm for the reverse water-gas shift reaction at high temperatures, which efficiently uses the hydrogen feed. The present invention seeks to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto. SUMMARY OF THE INVENTION The present specification provides a method of forming a syngas, the method comprising: providing a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst; providing a feed gas comprising carbon dioxide and hydrogen; and passing the feed gas over the reverse water-gas shift catalyst to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide, wherein the feed gas is heated using a turbo-machinery heater that generates heat via kinetic energy. The term “syngas” or “synthesis gas” as used herein may encompass any gas mixture containing hydrogen and carbon monoxide. In the method of the present invention, the crude syngas comprises carbon monoxide (i.e., CO) and hydrogen (i.e., molecular hydrogen H2). A “turbo-machinery heater” is an apparatus that imparts kinetic energy to a gas, thereby heating the gas, by means of a rotatable shaft assembly. Such heaters are known. They typically comprise: a gas inlet, a gas outlet, and a gas feed path extending from the gas inlet, via a heating zone to the gas outlet; the heating zone comprising a rotatable shaft assembly comprising a rotatable shaft and one or more sets of rotor blades, and one or more sets of stationary blades; the one or more sets of rotor blades functioning to convert kinetic energy of the rotatable shaft assembly into kinetic energy of a gas passing along the gas feed path through the heating zone; and at least a portion of the one or more sets of stationary blades functioning to convert the kinetic energy of the gas into heat energy of the gas to raise the temperature of the gas. The present specification is concerned with the use of a turbo-machinery heater in a method of forming a syngas using a reverse water-gas shift reactor. It has been identified that the use of a turbo-machinery heater in a method of forming a syngas using a reverse water-gas shift reactor is advantageous as the method is capable of forming syngas without the need to combust hydrogen. As a result, in comparison to conventional syngas formation methods, the method of the present invention is capable of producing the same amount and composition of syngas from less hydrogen feed. In other words, the hydrogen efficiency of the method is improved because the entirety of the hydrogen feed is available for the reverse water-gas shift reaction. This may reduce the overall cost of the method by reducing the size and capital expenditure of an upstream electrolysis unit used to form the hydrogen feed, and the amount of renewable power required to operate it. The method is capable of forming syngas without the need to rely on the use of an electric heater, which may be susceptible to damage at the reverse water-gas shift reaction temperature. It also avoids the need for a complex multi-functional structured catalyst, containing both electrically conductive elements and RWGS catalyst. In the present method, the RWGS catalyst can be provided in conventional pellet form. The turbo-machinery heater can be driven by an electric motor using renewable energy. Accordingly, the heat may be generated at high efficiency, and the environmental impact of the method may be reduced in comparison to conventional methods. According to certain configurations, the turbo-machinery heater directly heats the feed gas or a component thereof. Alternatively, the turbo-machinery heater can be configured to indirectly heat the feed gas or a component thereof via a heat transfer gas, the turbo-machinery heater being used to heat the heat transfer gas and the heat transfer gas being used to heat the feed gas, or a component thereof, via heat exchange. Optionally, the turbo-machinery heater is configured to heat the gas without unduly increasing the pressure of the gas. As such, optionally a ratio of pressure of gas heated by the turbomachinery heater (Pheated) to pressure of unheated gas input to the turbo-machinery heater (Punheated) is no more than 2.2, 2.0, 1.7, 1.5, 1.3, 1.2, or 1.1. The turbo-machinery heater according to such configurations is different to apparatus which is intended to significantly increase the pressure of industrial process gases, such as the supersonic compressor as described in US2022 / 0135414. The use of a turbo-machinery heater, which does not substantially increase gas pressure to high Pheated : Punheated pressure ratios, has advantages in terms of power requirements and heat recovery for reverse water-gas shift systems. The turbo-machinery heater can be used to heat the feed gas prior to the feed gas entering the reverse water-gas shift reactor and passing over the reverse water-gas shift catalyst. Alternatively, or additionally, the turbo-machinery heater can be used to heat the feed gas within the reverse water-gas shift reactor as the feed gas is passing over the reverse water-gas shift catalyst. Furthermore, the feed gas can be heated using two or more turbo-machinery heaters. These may be in parallel or series. Furthermore, these may be of the direct heating type, the indirect heating type, or a mixture of direct and indirect heating types. For example, one turbo-machinery heater may partially heat one or more of the feed gases and then another turbo-machinery heater may further heat the one or more feed gases to a reaction temperature in series. Alternatively, different feed gases may be heated by different turbo-machinery heaters in parallel. According to certain configurations, the or each turbo-machinery heater is used (in combination if more than one such heater is used) to heat the feed gas to a reaction temperature to convert at least a portion of the carbon dioxide to carbon monoxide as the feed gas is passed over the reverse water-gas shift catalyst. In this case, no heating is required by combustion of the feed gas in the reverse water-gas shift reactor or by means of another heating system within the reverse water-gas shift reactor. In these configurations, hydrogen efficiency is increased (as no hydrogen combustion is required) and / or reliability of the reverse water-gas shift apparatus is improved and / or the complexity of the reverse water-gas shift apparatus is reduced (as no complex resistance electrical heating configurations are required). Alternatively, the or each turbo-machmery heater can be used to heat the feed gas to a temperature less than a reaction temperature to convert at least a portion of the carbon dioxide to carbon monoxide as the feed gas is passed over the reverse water-gas shift catalyst. In this case, the water-gas shift reactor can be an autothermal reverse water-gas shift reactor in which heating to the reaction temperature is provided by combustion of a portion of the feed gas with an oxygen stream in a burner in the reverse water-gas shift reactor or alternatively heating to the reaction temperature is provided by another heating system within the reverse water-gas shift reactor. These configurations can still reduce the requirement for hydrogen combustion or electrical (or other) heating by partially heating the feed gas using one or more turbomachinery heaters. Once the crude syngas has been produced by the reverse water-gas shift reactor, the crude syngas can be conditioned by removing carbon dioxide and water from the crude syngas to produce a purified syngas. Such syngas conditioning units are known in the art. The removed carbon dioxide is advantageously then recycled into the feed gas to increase syngas production for a given quantity of feed carbon dioxide and so as to reduce carbon dioxide emissions. The purified syngas formed as described above can be passed through a Fischer-Tropsch reactor to produce hydrocarbon products and co-produced water. As such, the present specification also provides a method of producing hydrocarbon products from carbon dioxide and hydrogen using a combination of a reverse water-gas shift reactor and a Fisher-Tropsch reactor. Such a combined system is suitable for production of sustainable fuels (e.g., sustainable aviation fuel) or other sustainable hydrocarbon chemical products. In such systems, advantageously the hydrocarbon products and the co-produced water are separated from unreacted syngas and the unreacted syngas is returned to the Fischer-Tropsch reactor. Furthermore, the hydrocarbon products are processed into a liquid hydrocarbon product and a hydrocarbon by-product stream comprising one or more of a tail gas, liquefied petroleum gas (LPG), and naphtha. The present specification also provides a system for performing the method as described herein. The system comprises: a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst; and a feed gas system for providing a feed gas comprising carbon dioxide and hydrogen to the reverse water-gas shift reactor; wherein the feed gas system and the reverse water-gas shift reactor are configured to pass the feed gas over the reverse water-gas shift catalyst to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide, and wherein the system further comprises a turbo-machinery heater for heating the feed gas. Furthermore, for systems configured to produce hydrocarbon products, the systems further comprise: a syngas conditioning unit for removing carbon dioxide and water from the crude syngas to produce a purified syngas; and a Fischer-Tropsch reactor configured to convert the purified syngas into hydrocarbon products and co-produced water. Other features of the systems are as described in relation to the method. For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described with reference to the accompanying drawings. It will be understood that various embodiments are envisaged. For example, according to one configuration described in the present specification, a method of forming a syngas is provided, the method comprising: providing a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst; providing a feed gas comprising carbon dioxide and hydrogen at an elevated temperature of at least 200 °C; and passing the feed gas at a reaction temperature of at least 700°C over the reverse water-gas shift catalyst to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide, wherein: providing the feed gas comprises combining a first carbon dioxide stream and a hydrogen stream; and the elevated temperature is achieved by using a turbo-machinery heater that uses kinetic energy to heat one or more of the carbon dioxide stream, the hydrogen stream and the feed gas. According to another configuration described in the present specification, a method of forming a syngas is provided, the method comprising: providing a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst; providing a feed gas comprising carbon dioxide and hydrogen; and passing the feed gas over the reverse water-gas shift catalyst at an elevated temperature to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide, wherein the elevated temperature is achieved by: heating a heat transfer gas to form a heated heat transfer gas using a turbo-machinery heater that uses kinetic energy to heat the heat transfer gas; and heating the reverse water-gas shift reactor via heat exchange with the heated heat transfer gas. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1(a) shows a flow diagram of an example of a method in which feed gas is directly heated by passing through a turbo-machinery heater prior to the feed gas passing through a reverse water-gas shift reactor to produce syngas; Figure 1 (b) shows a flow diagram of an example of a method in which feed gas is indirectly heated by a turbo-machinery heater by circulating a heat transfer gas (e.g., carbon dioxide) through a turbo-machinery heater and through a reverse water-gas shift reactor to heat the feed gas within the reactor by heat exchange; Figure 2 shows a flow diagram of an example of a method according to the present invention in which a hydrogen / carbon dioxide feed stream 36 (in this example also mixed with a methane containing stream 34) is fed through a turbo-machinery heater 40 and then through a reverse water-gas shift reactor 44. Figure 3 shows a flow diagram of another example of a method according to the present invention which is similar to that of Figure 2 but wherein the feed gas is fed to externally -heated catalyst-containing tubes in a heat exchange reactor 70 prior to heating the feed in the turbo-machinery heater 40 and passing the feed gas through a reverse water-gas shift reactor 44. Figure 4 shows a flow diagram of another example of a method according to the present invention which differs in that the hydrogen and carbon dioxide gas mixture 36 is not combined with a methane containing gas upstream of the turbo-machinery heater 40 and instead a mixture of hydrocarbon by-product 20 and steam 22 is fed to a heat exchange reformer (post reformer) 80 in which catalyst-containing tubes are open-ended to discharge the steam reformed gas into a shell-side of a heat exchange reformer and crude syngas 46 from the water-gas shift reactor 44 is fed to the shell side of the reformer 80 thereby mixing the reformed gas and the crude syngas directly. This mixed gas provides the heat for the endothermic steam reforming in the tubes to produce a partially cooled syngas 82 that is fed to the heat recovery unit 48. Figure 5 shows a flow diagram of an example of a method in which feed gas is indirectly heated by a turbo-machinery heater by circulating a heat transfer gas (e.g., carbon dioxide) through a turbo-machinery heater and through a gas heated reverse water-gas shift reactor (GHR) to heat the feed gas within the reactor by heat exchange. Figure 6 shows a flow diagram of another example of a method in which feed gas is indirectly heated by a turbo-machinery heater by circulating a heat transfer gas (e.g., carbon dioxide) through a turbo-machinery heater and through a gas heated reverse water-gas shift reactor (GHR) to heat the feed gas within the reactor by heat exchange, this example also including a heat exchange reformer (post-reformer) downstream of the reverse water-gas shift reactor, which has the same function as the heat exchange reformer in Figure 4. Figure 7 shows some more detail of an example of a gas heated reverse water-gas shift reactor (GHR) for circulating heat transfer gas which has been heated by a turbo-machinery heater. Figure 8 shows an example of a bayonet gas heated reverse water-gas shift reactor (GHR) configuration comprising a core tube surrounded by catalyst with enhanced cooling of gas flow up the core tube. Reference numerals in the figures are listed in the table below: Reference Item 10 Hydrogen Stream 12 Carbon dioxide stream 14 Carbon dioxide recycle stream 16 Heater / Interchanger 18 Steam stream 20 Hydrocarbon by-product stream 22 Steam stream 24 Heater / Interchanger 26 Derich ment reactor 28 Hydrocarbon containing tailgas 30 Fischer-Tropsch hydrocarbon synthesis unit 32 Naphtha / LPG stream 34 Deriched hydrocarbon stream 36 Heated hydrogen / carbon dioxide stream 38 Line to turbo-machinery heater 40 Turbo-machinery heater 42 Line to reverse water-gas shift reactor 44 Reverse Water-Gas Shift Reactor / Gas Heated Reverse water-gas shift reactor 46 Line to heat recovery / Cooling unit 48 Heat recovery / Cooling unit 50 Process condensate 52 De-watered syngas gas 54 Carbon dioxide removal unit 56 Removed carbon dioxide 58 H2 / CO syngas 60 Hydrocarbon products from Fischer-Tropsch 62 Oxygen gas stream 64 Steam stream 70 Heat exchange reactor 72 Pre-heated reverse water gas shifted gas mixture 74 Crude synthesis gas 76 Preheated mixture of hydrogen and carbon dioxide 80 Heat Exchange Reformer (Post Reformer) 82 Partially cooled crude synthesis gas 90 Electric heater 92 Turbo-machinery heat transfer fluid loop 94 Carbon dioxide make-up to buffer 96 Buffer 98 Heat Exchange Reformer (Post-reformer) 100 Gas heated reverse water-gas shift reactor 102 Sheath tubes or baffles 104 Methanating+RWGS+Reforming Catalyst 106 Labyrinth seal 108 Feed gas 110 Crude Syngas 112 Hot carbon dioxide heat transfer fluid 114 Inleakage of CO2 via seals 116 Cool carbon dioxide heat transfer fluid 120 Feed gas 122 Crude Syngas 124 Refractory lined tubesheet 126 Sheath tubes or baffles 128 Core tube 130 Structured catalyst 132 Porous support grid for structured catalyst sections 134 Hot carbon dioxide heat transfer fluid 136 Cool carbon dioxide heat transfer fluid 138 Radiant cooler 140 Catalyst tube 142 Structured heat transfer medium with support 144 Core tube insert DETAILED DESCRIPTION OF THE INVENTION Reverse water-gas shift reactors and reverse water-gas shift catalysts are known in the art. Under the reverse water-gas shift reaction, carbon dioxide and hydrogen are converted to carbon monoxide and steam according to the following reaction: H2 + CO2 CO + H2O Excess hydrogen may be provided in the feed to produce a crude synthesis gas comprising hydrogen, carbon monoxide, residual carbon dioxide and steam, which after steps of cooling to condense the steam and carbon dioxide removal, is suitable for use in the Fischer-Tropsch synthesis of hydrocarbons. The method comprises passing the feed gas at a reaction temperature of at least 700°C over the reverse water-gas shift catalyst to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide. This is carried out by reacting hydrogen with the carbon dioxide to form carbon monoxide and steam, as per the reaction scheme above. As described in the summary section, the present specification provides a method of forming a syngas, the method comprising: providing a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst; providing a feed gas comprising carbon dioxide and hydrogen; and passing the feed gas over the reverse water-gas shift catalyst to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide, wherein the feed gas is heated using a turbo-machinery heater that generates heat via kinetic energy. While turbo-machinery heaters are known in other applications, it has been identified that such heaters can have specific advantages for using with reverse water-gas shift reactors to produce syngas including the following: the method is capable of forming syngas while negating, or at least reducing, the need to combust hydrogen thus increasing hydrogen efficiency, reducing power requirements, and / or reducing the overall cost of the method; the method is capable of forming syngas while negating, or at least reducing, the need to rely on the use of an electrical resistance (or other) heater, which may be susceptible to damage at the reverse water-gas shift reaction temperature and avoids the need for a complex multi-functional structured catalyst, containing both electrically conductive elements and reverse water-gas shift catalyst; the turbo-machinery heater can be driven by an electric motor using renewable energy and the heat can be generated at high efficiency such that the environmental impact of the method can be reduced in comparison to conventional methods. According to certain configurations, the turbo-machinery heater directly heats the feed gas or a component thereof. Such a configuration is shown in Figure 1(a). A feed stream is fed through a turbo-machinery heater and then passed through a reverse water gas-shift reactor / reformer. In the illustrated configuration, the feed stream comprises a mixture of hydrogen and carbon dioxide for the reverse water-gas shift reaction to produce syngas. The feed gas may also comprise one or both of deriched hydrocarbon tails gas (e.g., from a Fischer-Tropsch reactor) and steam. In the illustrated configuration, the feed gas is heated to a temperature of 390°C prior to passing the feed gas through the turbo-machinery heater. The turbo-machinery heater then increases the temperature of the feed gas to 1100°C. The heated feed gas is then passed through a reverse water-gas shift reactor / reformer to generate a crude syngas with an exit temperature of 866°C. Alternatively, the turbo-machinery heater can be configured to indirectly heat the feed gas or a component thereof via a heat transfer gas, the turbo-machinery heater being used to heat the heat transfer gas and the heat transfer gas being used to heat the feed gas, or a component thereof, via heat exchange. Such a configuration is shown in Figure 1(b). As in the configuration of Figure 1(a), the feed stream comprises a mixture of hydrogen and carbon dioxide for the reverse water-gas shift reaction to produce syngas. Furthermore, the feed gas may also comprise one or both of deriched hydrocarbon tails gas (e.g., from a Fischer-Tropsch reactor) and steam. In this case, the feed gas is passed directly into the reverse water-gas shift reactor (e.g., at 420°C) without first passing through a turbo-machinery heater. A turbomachinery heating loop is provided for heating the reverse water-gas shift reactor. In this configuration, an inert heat exchange gas (e.g., CO2) is passed through a turbo-machinery heater and then passed through a reverse water-gas shift reactor which is configured to heat the feed gas by heat exchange with the inert heat exchange gas. The inert gas is then recirculated through the turbo-machinery heater. In the illustrated example, the turbo-machinery heating loop can be fed (or topped up) with inert CO2 gas via a buffer. In the illustrated configuration, the circulating CO2 is at a temperature of 975°C on exiting the turbo-piachinery heater while the recirculated CO2 is at 825°C on re-entry to the turbo-machinery heater. The exit temperature for the crude syngas product is 870°C. When comparing direct (Figure 1(a)) and indirect (Figure 1(b)) heating configurations using a turbo-machinery heating it may be noted that the direct heating configurations can be made simpler and slightly more efficient. The direct heating configurations may directly heat H2, CO2 and methane containing gas mixtures and tend to require heating to higher temperatures. In contrast, the indirect heating configurations tend to be more complex, slightly less efficient, and require more capital expenditure. The indirect heating configurations heat an inert gas such as pure CO2 (or argon or nitrogen) and tend to require heating to lower temperature. As also described in the summary section, the turbo-machinery heater can heat the gas without unduly increasing the pressure of the gas. That is, a ratio of the pressure of the gas heated by the turbo-machinery heater to the pressure of the unheated gas input to the turbo-machinery heater is no more than 2.2, 2.0, 1.7, 1.5, 1.3, 1.2, or 1.1. This differs from supersonic compressors which are designed to significantly increase both pressure and temperature as described, for example, in US 2022 / 0135414. In the present reverse water-gas shift application, it is advantageous to minimise the power required by the process, particularly because it is desired that this power is sourced renewably. If a supersonic compressor is used, because the gas becomes much hotter and occupies more volume it would use more power during the latter stages of compression. In accordance with certain configurations of the present process, the gas is pre-compressed at low temperature with cooling of the gas between each stage of compression to keep the gas cool during later stages of compression and hence minimise the power needed. Unduly increasing the pressure of the process gas also causes problems with heat recovery. For example, creating hot high pressure process feed gas using a supersonic compressor would mean that either: (i) it would be difficult to recover heat from the cooling crude syngas back to the process feed gas (efficient heat recovery would likely need to be via a third medium, such as steam); or (ii) heat recovery would have to be from a high pressure (HP) gas to a low pressure (LP) gas, which has two disadvantages: (a) heat transfer coefficients would be much lower requiring much more tube heat transfer area or enhancement of heat transfer such as finning; and (b) tube wall thicknesses separating HP and LP gas would need to be greater to withstand the larger pressure differential between HP and LP gas, reducing heat transfer and increasing cost. In addition to the above, if hydrogen is sourced from an electrolyser (e.g., a proton exchange membrane electrolyser), these can be specified to produce hydrogen which is already be at the required reverse water-gas shift pressure so no additional pressure increase is required. Furthermore, in configurations which provide indirect heating via a heat transfer gas, the heat transfer gas pressure only requires boosting to overcome the resistance of the closed circulation system (the resistance of the gas heated reverse water-gas shift reactor)). A pressure ratio of 1.2 can typically achieve this. Turbo-machinery heaters typically comprise a gas inlet, a gas outlet, and a gas feed path extending from the gas inlet, via a heating zone to the gas outlet; the heating zone comprising a rotatable shaft assembly comprising a rotatable shaft and one or more sets of rotor blades, and one or more sets of stationary blades; the one or more sets of rotor blades functioning to convert kinetic energy of the rotatable shaft assembly into kinetic energy of a gas passing along the gas feed path through the heating zone; and at least a portion of the one or more sets of stationary blades functioning to convert the kinetic energy of the gas into heat energy of the gas to raise the temperature of the gas. The one or more sets of rotor blades may function to increase the kinetic energy of the gas passing along the gas feed path through the heating zone by increasing the flow velocity of the gas. The rotatable shaft may be driven by an electric motor. The electric motor may be powered by renewable energy. The one or more sets of rotor blades may be coupled to or integrated with the rotatable shaft. The one or more sets of rotor blades may be provided on one or more rotors that may be coupled to or integrated with the rotatable shaft. The one or more sets of rotor blades may comprise a plurality of sets of rotor blades forming a cascade of rotor blades arranged along the gas feed path. The one or more sets of stationary blades may remain stationary and not rotate with the rotatable shaft. A portion of the one or more sets of stationary blades may comprise one or more sets of guide blades. The guide blades may function to guide the entry of the gas into a set of rotor blades downstream of the set of guide blades Another portion of the one or more sets of stationary blades may comprise one or more sets of diffuser blades. The diffuser blades may function to slow the velocity of the gas ahd convert kinetic energy of the gas into thermal energy of the gas to raise the temperature of the gas. The one or more sets of rotor blades and the one or more sets of stationary blades may define one or more blade sets, each blade set comprising a set of rotor blades, a set of guide blades upstream of the set of rotor blades, and a set of diffuser blades downstream of the set of rotor blades. Consequently, in a flow direction the gas feed path may include one or more guide blade-rotor blade-diffuser blade flow sections. Each set of rotor blades may be interposed between a set of guide blades and a set of diffuser blades. Alternatively, some sets of rotor blades may not be adjacent a set of guide blades and / or diffuser blades. The gas may pass once through the heating zone, or may be circulated through the heating zone multiple times, before exiting via the gas outlet. The rotatable shaft is preferably electrically-driven. The one or more sets of stationary blades preferably comprise one or more sets of diffuser blades and / or one or more sets of guide blades. The one or more sets of rotor blades and the one or more sets of stationary blades preferably define one or more blade sets, each blade set comprising a set of rotor blades, a set of guide blades upstream of the set of rotor blades, and a set of diffuser blades downstream of the set of rotor blades. An example of a turbo-machinery heater suitable for use in the method Of the present invention is described in WO2022106754A1, the contents of which are hereby incorporated by reference. The feed gas is a combination of a carbon dioxide stream and a hydrogen stream. The hydrogen stream is preferably produced at least m part by electrolysis of water. The water may be, for example, demineralised water or purified condensate recovered from the syngas. The electrolysis may be performed using any suitable electrolyser. The hydrogen may if required, be compressed to a pressure in the range 10 to 50 bar abs. Certain electrolysers, such as polymer electrolyte membrane (PEM) electrolysers may be operated at pressure, which advantageously enables hydrogen to be formed at a suitable pressure, e.g. about 25 bar abs, for downstream processing without further compression. Furthermore, an oxygen stream will be formed during the electrolysis, which may be passed to the reverse water-gas shift reactor if combustion of hydrogen is required in a burner. The electrolysis is preferably carried out using renewable energy. This may reduce the environmental impact of the method. The carbon dioxide stream may be a by-product of combustion. The carbon dioxide stream may be derived from flue gas. The carbon dioxide stream may be derived from partial oxidation and / or from a gasifier used to oxidise and / or gasify carbonaceous materials. The carbon dioxide may be recovered from a carbon capture and storage (CCS) unit. The carbon dioxide may be recovered from a direct air CO2 capture unit. The CO2 will typically be compressed to a suitable pressure for downstream processing, e.g. in the range 10 to 50 bar abs. Examples of both direct and indirect heating configurations for a reverse water-gas shift reactor using a turbo-machinery heater are described below. It should be noted that each aspect or embodiment as described below may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any features indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous. Furthermore, features described in relation to direct heating configurations may also be combined with features described m relation to indirect heating configurations and vice versa. DIRECT HEATING In these configurations, a method of forming a syngas is provided, the method comprising: providing a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst; providing a feed gas comprising carbon dioxide and hydrogen at an elevated temperature of at least 200 °C; and passing the feed gas at a reaction temperature of at least 700 °C over the reverse water-gas shift catalyst to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide, wherein: providing the feed gas comprises combining a first carbon dioxide stream and a hydrogen stream; and the elevated temperature is achieved by using a turbo-machinery heater that uses kinetic energy to (directly) heat one or more of the carbon dioxide stream, the hydrogen stream and the feed gas. The elevated temperature is achieved by using a turbo-machinery heater to heat one or more of the first carbon dioxide stream, the hydrogen stream and the feed gas. In one embodiment using a single turbo-machinery heater, the hydrogen and carbon dioxide may be mixed together and then the mixture heated in the turbo-machinery heater. Alternatively, the hydrogen and carbon dioxide feeds may be heated separately using two turbo-machinery heaters in parallel. When only the carbon dioxide stream or the hydrogen stream are heated using the turbomachinery heater, the carbon dioxide stream or hydrogen stream may be heated to a temperature higher than the elevated temperature so that when combined with the unheated carbon dioxide stream or the unheated hydrogen stream, the elevated temperature will be achieved. More than one, for example two or more turbo-machinery heaters operating in parallel may be provided. Thus, in some embodiments the elevated temperature may be achieved by using two or more turbo-machinery heaters operating in parallel. This may be useful where the heating capacity cannot be achieved with one turbo-machinery heater (e.g. because the power required from one electric motor is too large). Alternatively, this may be useful where it is decided to heat one of the hydrogen and carbon dioxide streams to a higher temperature than the other before mixing them together. It also provides additional operational flexibility in case of fluctuating feed availability, which can result, for example, from the use of renewable electricity to generate hydrogen by electrolysis. A single turbo-machinery heater may be provided to heat the first carbon dioxide stream but not heat the hydrogen stream or the feed gas. This avoids heating hydrogen to high temperatures in the heater outside of the reverse water-gas shift reactor. Alternatively, the turbo-machinery heater preferably heats the feed gas mixture of the carbon dioxide stream and the hydrogen stream. This may produce the desired elevated temperature without the need for multiple parallel heaters and without the need to heat one of the carbon dioxide stream or hydrogen stream to a temperature higher than the elevated temperature. In a preferred embodiment, the elevated temperature is substantially equal to the reaction temperature: and the reverse water-gas shift reactor is an adiabatic reverse water-gas shift reactor in which no heating is provided by combustion of the feed gas in the reverse water-gas shift reactor. As discussed above, by not combusting the feed the hydrogen efficiency of the method is improved. In an alternative embodiment, the elevated temperature is less than the reaction temperature; and the water-gas shift reactor is an autothermal reverse water-gas shift reactor in which heating to the reaction temperature is provided by combustion of a portion of the feed gas with an oxygen stream in a burner in the reverse water-gas shift reactor. The use of an autothermal water-gas shift reactor may be useful where, for example, the cost or scale of providing the turbo-machinery heater to achieve the reaction temperature is excessive. In this case, the hydrogen efficiency is still improved in comparison to conventional methods because, since the turbo-machinery heater has heated the feed to the elevated temperature, less hydrogen needs to be combusted in the burner to reach the reaction temperature. The elevated temperature is preferably at least 400 °C, more preferably at least 700 °C, even more preferably at least 900 °C; and / or the reaction temperature is preferably at least 1000 °C, more preferably at least 1050 °C, even more preferably at least 1100 °C. Such elevated temperatures may mean that no, or only low levels of, hydrogen combustion are required to reach the reaction temperature. As a result, the hydrogen efficiency of the method may be improved. Such reaction temperatures may result in the reverse water-gas shift reaction being carried out at high efficiency and / or with high selectivity and / or with high yield and / or at high speed. In embodiments where the turbo-machinery heater is operated such that the elevated temperature is essentially equal to the reaction temperature, the elevated temperature and the reaction temperature are preferably at least 750 °C, more preferably at least 900 °C, even more preferably at least 1000 °C, still even more preferably at least 1050 °C, still even more preferably at least 1100 °C. The reverse water-gas shift reactor preferably operates at a pressure of at least 10 bara, optionally at least 15 bara or 20 bara. Such pressures may be particularly suitable for carrying out a reverse water-gas shift reaction at favourable temperatures and pressures and at high conversion rates. In addition to the first carbon dioxide stream and hydrogen, the reverse water-gas shift reactor may be fed with a hydrocarbon by-product stream and steam. Alternatively, the reverse water-gas shift reactor may be fed with a methane-containing gas formed by subjecting the hydrocarbon by-product stream to a derichment step with steam in a derichment reactor. Where the reverse water gas shift reactor contains a steam reforming catalyst, or a reverse water-gas shift catalyst active for steam reforming reactions, this provides a means for generating additional syngas, thereby improving the carbon efficiency of the process. Preferably, the hydrocarbon by-product stream is subjected to a derichment step with steam in a derichment reactor over a derichment catalyst to form a methane-containing gas, and the methane-containing gas is combined with the first carbon dioxide stream, the hydrogen stream or the feed gas before heating the feed gas to the elevated temperature in the turbo-machinery heater. Alternatively, the methane-containing gas may be combined with the feed gas after heating the feed gas to the elevated temperature. In performing the derichment step, steam may be introduced into the hydrocarbon by-product stream by direct injection of steam and / or by saturation of the hydrocarbon by-product stream by contact with a stream of heated water. The heated water may coinprise condensed water from a downstream process that contains soluble organic compounds, such as co-produced water recovered from the Fischer-Tropsch reactor. Alternatively, the steam used for direct injection may have been used to strip organic compounds from condensed water from a downstream process. In this way, the organic compounds may be converted to hydrogen and carbon oxides in the derichment reactor and the burden of waste water treatment for the downstream process may be reduced. The amount of steam introduced may be such as to give a steam to carbon molar ratio in the feeds to the derichment reactor of 0.1:1 to 10:1. By steam to carbon molar ratio we mean the molar ratio of steam to the sum of the carbon atoms in the hydrocarbon-containing components in the feed. The derichment reactor feed may have an inlet temperature in the range of 250-650°C. The feed may be passed adiabatically through a bed of a derichment catalyst, such as a particulate nickel catalyst having a high nickel content, for example above 40% by weight. Such catalysts are available commercially. During the derichment step, any hydrocarbons higher than methane react with steam to give a mixture of methane, carbon oxides and hydrogen. In some embodiments, the reverse water-gas shift reactor is an adiabatic reverse water-gas shift reactor; the elevated temperature is substantially equal to the reaction temperature; and the method further comprises optionally subjecting the hydrocarbon by-product stream to a derichment step in a derichment reactor to form a methane-containing gas, and passing the hydrocarbon by-product stream, or the methane-containing gas to the adiabatic reverse water-gas shift reactor. The derichment step adds steam and where the reverse water-gas shift reactor contains a catalyst active for steam reforming reactions the hydrocarbon and steam in the feed gas can be steam reformed to generate additional hydrogen and carbon monoxide. Accordingly, the carbon efficiency of the method may be improved. In other embodiments, the reverse water-gas shift reactor is an autothermal reverse water-gas shift reactor comprising a burner; the elevated temperature is less than the reaction temperature; and the method further comprises optionally subjecting the hydrocarbon byproduct stream to a derichment step in a derichment reactor to form a methane-containing gas, and passing the hydrocarbon by-product stream, or the methane-containing gas, and an oxygen stream to the burner to combust hydrogen and the hydrocarbon by-product or methane-containing gas stream to generate heat. If desired, steam may be included with the oxygen stream. The oxygen stream may be produced at least in part by electrolysis of water. Such electrolysis may concurrently produce at least part of the hydrogen stream. Preferably substantially the entirety of the oxygen stream and / or the hydrogen stream is generated by the electrolysis. Such electrolysis is preferably carried out using renewable energy. This may reduce the environmental impact of the method. In a further embodiment, upstream of the turbo-machinery heater, the feed gas comprising hydrogen and carbon dioxide and including hydrocarbon by-product, or a methane-containing gas, may be pre-heated by passing it through one or more externally heated tubes in a heat exchanger, preferably a heat exchange reactor, that are heated by the crude syngas. In the heat exchanger or heat exchange reactor, the feed gas passes through externally heated tubes in order to raise the temperature of the feed gas. The tubes may contain no catalyst or may contain a reverse water gas shift catalyst, preferably with low selectivity to methane formation. The reverse water-gas shift catalyst with low selectivity to methane formation may be any catalyst which can achieve a conversion of at least 80%, preferably at least 90%, more preferably at least 95%, most preferably at least 99% of equilibrium conversion of carbon dioxide to carbon monoxide, while limiting the conversion of carbon dioxide to methane to less than 20%, preferably less than 10%, more preferably less than 5%, most preferably less than 2% of this amount. Suitable catalysts include Ni catalysts surface modified with MoOx as described by Zhang et al in the Journal of CO2 Utilization, Vol 52, 2021, pl01678, or Nickel oxide on ceria catalysts as described in J. Rare Earths 2013, 31, 559-564 and 969-974, and Catalysts 2022, 12, 1101, p!4-15. The reverse water-gas shift catalyst may be particulate, for example in the form of shaped units such as pellets, which may be lobed or fluted. A heat exchange reactor containing a reverse water gas shift catalyst with low selectivity to methane formation may advantageously be used to convert part of the carbon dioxide and hydrogen in the feed gas into carbon monoxide and water vapour. Using a low-methane selectivity catalyst advantageously avoids unwanted methanation reactions that would generate an exotherm and reduce heat exchange effectiveness. The method may therefore comprise, optionally subjecting the hydrocarbon by-product stream to a derichment step in a derichment reactor to form a methane-containing gas, passing the hydrocarbon by-product stream or the methane-containing gas stream, steam and the feed gas comprising the hydrogen stream and the first carbon dioxide stream through externally heated tubes in a heat exchanger, or a heat exchange reactor, to form a pre-heated feed, or pre-heated partially reverse water-gas shifted feed, wherein the externally heated tubes are heated by a hot gas comprising the crude syngas, thereby forming a partially cooled crude syngas that is recovered from the heat exchanger or heat exchange reactor. A preheated feed gas, or pre-heated partially reverse water-gas shifted feed gas is separately recovered from the heat exchanger or heat exchange reactor and fed to the turbo-machinery heater and thence the reverse water-gas shift reactor to generate the crude syngas. In a further embodiment, the hydrocarbon by-product stream or methane containing gas may be pre-heated separately from the feed gas comprising hydrogen and carbon dioxide. In this embodiment, the hydrocarbon by-product stream or methane containing gas may be pre-heated by passing it through one or more externally heated tubes in a heat exchanger, or heat exchange reactor, heated by the crude syngas. In this case the tubes may contain a steam reforming catalyst that converts hydrocarbons with steam to form hydrogen and carbon oxides. The reforming catalyst preferably comprises nickel and in this case the heat exchange reactor may be described as a heat exchange reformer. The feed gas hydrogen and carbon dioxide streams may be combined with the pre-heated reformed gas recovered from the heat exchange reformer before the combined gas goes to the turbo machinery heater. In an alternative embodiment, an autothermal reformer (ATR) or partial oxidation reactor may be included to combust at least a portion of the hydrocarbon by-product, or methane-containing gas, with a stream of oxygen gas in a burner within the ATR to produce a hot product gas stream, that may be used in the heat exchanger, or heat exchange reactor, to provide additional heat to the feed gas upstream of the turbo machinery heater. In a further embodiment, a feed gas consisting of hydrogen and carbon dioxide is heated in the turbo-machinery heater and subjected to reverse water-gas shift in the reverse water-gas shift reactor to generate the crude syngas. In parallel, the hydrocarbon by-product stream or methane-containing gas is subjected to steam reforming in a heat exchange reformer comprising externally heated, catalyst containing tubes of a design that permits mixing of the crude syngas with the reformed gas. In this embodiment, the heating of the catalyst-containing tubes is performed by a hot gas mixture consisting of the crude syngas and the reformed gas that has passed through the tubes. Mixing may be achieved, for example, using an open-ended tube embodiment with mixing of the reformed gas and the crude syngas taking place on the shell side of the heat exchange reformer (post reformer). Such an embodiment is described, for example, in US5011625, US5122299 and US5362454, the disclosures of which are hereby incorporated by reference. In this embodiment, the feed from the heater to the reverse water-gas shift reactor is effectively rnethane free as it just contains hydrogen and carbon dioxide. In embodiments where the feed gas comprising hydrogen and carbon dioxide and including hydrocarbon by-product, or a methane-containing gas passes through a heat exchanger or heat exchange reactor, the externally heated tubes may contain media or devices that enhance heat transfer inside at least part of the externally heated tubes; and / or the externally heated tubes may have external fins and / or have external concentric sheath tubes that enhance heat transfer outside at least part of the tubes; and / or the heat exchanger may be baffled in order to enhance heat transfer outside at least part of the tubes. The method includes passing the pre-heated feed gas, or pre-heated, partially reverse water-gas shifted feed gas to a reverse water-gas shift reactor containing a reverse water-gas shift catalyst. The reverse water-gas shift catalyst may be any suitable transition metal catalyst, for example a catalyst based on nickel, iron or chromium, but other catalysts used as reverse water-gas shift catalysts may be used. The reverse water-gas shift catalyst preferably comprises nickel. Such a catalyst is particularly suitable for carrying out a reverse water-gas shift reaction at favourable temperatures and pressures and at high conversion rates. Such catalysts are active for the reverse water-gas shift catalyst but advantageously will also be active for steam reforming reactions. The catalyst therefore preferably comprises nickel oxide on a suitable refractory metal oxide support. The refractory metal oxide support may comprise zirconia, alumina, calcium aluminate, magnesium aluminate, titania magnesia, or mixtures thereof. More preferably, the catalyst comprises nickel oxide on zirconia, nickel oxide on alphaalumina, nickel oxide on calcium aluminate or nickel oxide on magnesium aluminate. The nickel content may be in the range of from 3 to 20% by weight, expressed as NiO. The reverse water-gas shift catalyst may be particulate, for example in the form of shaped units such as pellets, rings or extrudates, which may be lobed or fluted. The catalytically active metal, e.g. nickel, may be dispersed throughout the particulate catalyst or present only within an eggshell layer of thickness 200 to 1000 micrometres on the surface of the refractory support. Alternatively, catalyst may comprise one or more monolithic supports such as a metal or ceramic foam or honeycomb supporting the catalytically active metal. Preferably, the catalyst is a particulate catalyst, more preferably 4-hole cylinder, particularly one that is a lobed or fluted to provide a higher geornetric surface area (GSA) than a similarly sized solid cylinder. Catalysts having a GSA in the range 400-550 m2 per cubic metre are preferred. The crude synthesis gas mixture from the reverse water-gas shift vessel comprises steam. Water is recovered from the crude synthesis gas mixture by cooling the product gas mixture to below the dew point in a heat recovery unit comprising one or more coolers or heat exchangers, and separating condensate, e.g. using one or more conventional gas-liquid separators. Removing water condensate from the crude synthesis gas mixture produces a dewatered crude syngas. The cooling in the heat recovery unit may be performed by raising steam and / or by preheating one or more of the hydrogen stream, the carbon dioxide stream, the mixed gas stream comprising hydrogen and carbon dioxide, and optionally the derichment reactor’s feed gases. Further cooling with cold water and / or air may also be performed. Process steam generated by the cooling may be used in the derichment step or in downstream processes and / or for power generation. The recovered condensed water may, if desired, be recycled at least in part to the process. The condensate may be used, after treatment, be used as boiler feed water. In addition, or alternatively, the condensate, optionally after treatment to remove contaminants, may be fed to an electrolysis unit used to generate hydrogen for the process. Accordingly, in some embodiments, a water stream recovered from the crude synthesis gas mixture may be fed to an electrolysis unit. Condensate may also be used, again after treatment if desired, as a boiler feed water. The crude synthesis gas mixture contains carbon dioxide, which may be removed from the dewatered syngas using a carbon dioxide removal unit. The majority of the carbon dioxide may be separated by membrane, solid absorbent or, preferably, a wash system, such as a system operating by counter current contact of the crude synthesis gas mixture or dewatered product gas with absorbent liquid over packing in a tower. The absorbent liquid can be a physical solvent such as potassium carbonate (sold as the Benfield® process), methanol (sold as the Rectisol® process) or glycols (sold as the Selexol® process) or chemical solvents such as amines. The carbon dioxide removal unit may therefore include one or more vessels providing a physical wash system or a reactive wash system, preferably a reactive wash system, especially an amine wash system. The carbon dioxide may be removed by a conventional acid gas recovery unit (AGRU). In a conventional AGRU, a de-watered gas stream is contacted with a stream of a suitable absorbent liquid, such as an amine, for example an aqueous solution comprising monoethanolamine (MEA), methyldiethanolamine (MDEA) or dimethylethanolamine (DMEA), particularly methyl diethanolamine (MDEA), so that the carbon dioxide is absorbed by the liquid to give a laden absorbent liquid and a gas stream having a decreased content of carbon dioxide. The laden absorbent liquid is then regenerated by heating and / or reducing the pressure to desorb the carbon dioxide and to give a regenerated absorbent liquid, which is then recycled to the carbon dioxide absorption stage. Heat from the regeneration of the laden absorbent may be recovered from within the process. For example, a portion of the crude synthesis gas mixture or steam generated by cooling the crude synthesis gas mixture may be used to heat the laden absorbent. The method preferably further comprises: passing the crude syngas to a carbon dioxide removal unit to form a second carbon dioxide stream and a carbon dioxide-depleted syngas; and recycling the second carbon dioxide stream into the first carbon dioxide stream. Such recycling of the carbon dioxide may improve the carbon efficiency of the method, thereby reducing its environmental impact. In addition, in the event that the carbon dioxide-depleted syngas is subsequently passed to a Fischer-Tropsch unit, the reduced levels of carbon dioxide passed through the Fischer-Tropsch unit may improve the efficiency of the Fischer-Tropsch reaction. Carbon dioxide removal units are known in the art. If desired, one or more purification units may be provided downstream of the carbon dioxide removal unit to remove any remaining contaminants, such as sulphur compounds, from the syngas stream. The syngas, after water condensate and carbon dioxide removal may be fed to a Fischer Tropsch unit comprising a Fischer-Tropsch reactor. In a further aspect, the present specification provides4 method of producing a hydrocarbon product from a syngas, the method comprising: forming a syngas from a feed gas according to the method described herein; passing the syngas through a Fischer-Tropsch reactor to produce hydrocarbons and coproduced water; separating the hydrocarbons and the co-produced water from unreacted syngas and returning the unreacted syngas to the Fischer-Tropsch reactor; and processing the hydrocarbons into a liquid hydrocarbon product and a hydrocarbon byproduct stream comprising one or more of a tail gas, LPG and naphtha. The advantages and preferable features of the first aspect apply equally to this aspect. In comparison to conventional methods of producing a hydrocarbon product, the hydrogen efficiency of the method may be improved. The term “hydrocarbons” as used herein may encompass species formed of carbon and hydrogen. The “liquid hydrocarbon product” typically comprise alkanes, and typically comprises a liquid hydrocarbon fuel product such as diesel, gasoline or kerosene (jet fuel). The term “LPG” or “liquid petroleum gas” as used herein may encompass a stream rich in C3 and C4 hydrocarbons. The term “naphtha” as used herein may encompass a stream rich in C5 to C9 hydrocarbons. The Fischer-Tropsch conversion stage can be carried out according to any one of the known processes. The Fischer-Tropsch process involves a series of chemical reactions that produce a variety of hydrocarbons. The more useful reactions produce alkanes as follows: (2n + 1) H2 + n CO CnH2n+2 + n H2O where n may be in the range 1 to 100 with preferred products having n in the range 10-20. The Fischer-Tropsch reaction may be performed using one or more reactors such as fixed-bed reactors, slurry-phase reactors, bubble-column reactors, loop reactors or fluidised bed reactors. The Fischer-Tropsch reactor may be operated at temperatures in the range 150 to 350°C, preferably 150 °C to 300 °C. Lower temperatures may result in unfavourably low levels of hydrocarbons being generated. Higher temperatures may increase the energy cost of the method without a significant increase in the levels of hydrocarbons being produced. The Fischer-Tropsch reactor may be operated at pressures in the range 0.1 to lOMPa. The gas-hourly-space velocity (GHSV) for continuous operation is in the range 1000 to 25000hr 1. The molar ratio of hydrogen to carbon monoxide in the syngas may be in the range 1.0 to 2.5:1, preferably 1.2 to 2.5:1, more preferably 1.6 to 2.2, which is particularly suitable for hydrocarbon synthesis by the Fischer-Tropsch reaction. Preferably the Fischer-Tropsch synthesis is carried out using one or more fixed bed reactors, i.e. a reaction vessel with a bed of catalyst fixed within the vessel through which the purified synthesis gas is passed. Any Fischer-Tropsch catalyst may be used, but cobalt-based Fischer-Tropsch catalysts are preferred over iron-based catalysts due to their lower carbon dioxide selectivity. Suitable cobalt Fischer-Tropsch catalysts are known, but preferred catalysts in the process comprise 9 to 20% wt Co supported on a suitable support material. Suitable catalysts therefore include agglomerates, pellets or extrudates comprising metal oxides such as alumina, zinc oxide, titania or silica, or mixtures thereof, on which the catalytically active metal, preferably cobalt, is deposited. In a particularly preferred embodiment, the Fischer-Tropsch catalyst is used in combination with a catalyst carrier suitable for use in a tubular Fischer-Tropsch reactor where the catalyst carrier containing the catalyst is disposed within one or more tubes that are cooled by circulating coolant, such as water under pressure. By “catalyst carrier” we mean a catalyst container, for example in the form of a cup or can, configured to allow a gas and / or liquid to flow into and out of the carrier and through a bed of the catalyst or catalyst precursor disposed within the carrier. Any suitable catalyst carrier may be used. In one embodiment, the catalyst carrier is that described in WO2011 / 048361, the contents of which are incorporated herein by reference. In an alternative embodiment, the catalyst carrier may include a catalyst monolith as disclosed in WO2012 / 136971, the contents of which are also incorporated herein by reference. In yet another alternative embodiment, the catalyst carrier may be that disclosed in WO2016 / 050520, the contents of which are also incorporated herein by reference. In preferred embodiments, the Fischer-Tropsch reactor comprises a tubular reactof in which catalyst carriers containing a cobalt Fischer-Tropsch catalyst are disposed within one or more tubes cooled by a cooling medium. The Fischer-Tropsch reaction produces hydrocarbons and co-produced water, which have to be separated from unreacted syngas. Separating the hydrocarbons from the co-produced water and unreacted syngas may be carried out, for example, by condensing the co-produced water and using one or more liquid-liquid or gas-liquid separators in known embodiments. Unreacted syngas is returned to the Fischer-Tropsch reactor, typically via a circulating compressor. Thus, the Fischer Tropsch reactor is preferably operated in a loop, which increases the overall efficiency of the process. A second portion of the unreacted gas is separated off from the loop to avoid the build up of inerts in the feed to the Fischer-Tropsch reactor. This second portion may be termed tail gas and typically comprises methane and other normally gaseous hydrocarbons such as ethane. The tail gas therefore is a useful hydrocarbon by-product that may be fed wholly, or partly, upstream to the reverse water-gas shift reactor. The tail gas may be subjected to a derichment step. Accordingly, the tail gas may be usefully recycled to the reverse water-gas shift reactor via a derichment reactor containing a derichment catalyst that converts any C2+ higher hydrocarbons present in the second portion of the tail gas to methane. Steam is added to the tail gas to provide a suitable steam to carbon ratio for the derichment step. The portion of tail gas that is not recycled to the reverse water-gas shift reactor, which may be termed “purge gas”, may be removed from the process to prevent the build-up of inert gases. The purge gas may be exported as fuel or used within the process in a fired heater or thermal oxidiser to heat feed to the reverse water-gas shift vessel or superheat steam. The co-produced water may be subjected to electrolysis to produce at least a portion of the hydrogen stream. This may reduce the amount of water required by the method. Liquid hydrocarbons recovered from the Fischer-Tropsch reactor are subjected to processing, e.g. in an upgrading unit, to provide more valuable hydrocarbon products. The upgrading unit may be fed with one or more liquid hydrocarbon streams produced by the Fischer-Tropsch reactor, including but not limited to a molten hydrocarbon wax and a light hydrocarbon condensate, which is liquid at ambient temperature. Desirably, the Fischer-Tropsch reactor is operated to produce a molten hydrocarbon wax liquid, which is subjected to processing in an upgrading unit comprising a hydrotreating unit to generate liquid fuels. Accordingly, in some embodiments, at least a portion and preferably all of the liquid hydrocarbon mixture resulting from the hydrocarbon synthesis is fed as a feedstock, in the presence of hydrogen, to a processing unit comprising a hydrotreating unit. The hydrotreating unit may perform various conversions such as hydroisomerization, hydrogenation, hydrodeoxygenation, and / or hydrocracking using one or more vessels containing suitable catalysts. Hydrogen is required by the hydrotreating unit. This may be provided by various sources but is desirably provided by an electrolysis unit to minimise carbon dioxide emissions from the process. Accordingly, in some embodiments, a portion of the hydrogen stream from an electrolysis unit is fed to the hydrotreating unit. The hydrotreating unit may be operated at a temperature generally of between 200 and 450°C, preferably from 250 to 450°C, more preferably from 300 to 450°C and most preferably between 320 to 420°C; a pressure of between 0.2 and 15 MPag, preferably between 0.5 and 10 Mpag and more preferably from 1 to 9 Mpag; a liquid hourly space velocity of between 0.1 and 10 h 1, preferably between 0.2 and 7 h^l and more preferably between 0.5 and 5.0 h^l, and the hydrogen content may be between 100 and 2000 litres H2 per litre of feedstock and preferably between 150 and 1500 litres H2 per litre of feedstock. The hydrotreating stage may suitably be carried out under conditions such that the conversion per pass of products with a boiling point of greater than or equal to 370° C into products having boiling points of less than 370° C is greater than 40% by weight and more preferably at least 50% by weight, so as to obtain middle distillates (gas oil and kerosene) having sufficiently good cold properties (pour point, freezing point) to satisfy the specifications in force for this type of fuel. The catalysts used in this stage are known. For example, hydroisomerization and hydrocracking can be carried out according to any one of the known processes, using any one of the known catalysts, and it is not limited to a specific process or catalyst. The majority of the catalysts suitable for hydroisomerization / hydro-cracking are of the bifunctional type combining an acid function with a hydrogenating function. The acid function is generally provided via supports of high specific surface area (150 to 800 m2 / g generally) exhibiting a surface acidity, such as halogenated (in particular chlorinated or fluorinated) aluminas, phosphorated aluminas, combinations of boron and aluminium oxides, or silicas / aluminas. The hydrogenating function is generally provided either by one or more metals from Group VIII of the Periodic Table of the Elements, such as iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, or by a combination of at least one metal from Group VI, such as chromium, molybdenum and tungsten, and at least one metal from Group VIII. Most conventional hydrocracking catalysts are composed of weakly acidic supports, such as silicas / aluminas. These systems are typically used to produce middle distillates of very good quality. Many catalysts of the hydrocracking market are based on silica / alumina in combination with a metal from Group VIII. These systems have a very good selectivity for middle distillates and the products formed are of good quality. According to one preferred embodiment, the hydroisomerization / hydrocracking catalyst comprises at least one hydrodehydrogenating element chosen from the noble metals of Group VIII, preferably platinum and / or palladium, and at least one amorphous refractory oxide support, preferably silica / alumina. The hydrocarbon products recovered from the hydrotreatment unit may be fed to separation apparatus to recover the valuable hydrocarbon products from hydrocarbon by-products. The separation apparatus may comprise one or more atmospheric distillation columns and optionally one or more vacuum distillation columns that separate the upgrader hydrocarbon off-gas, the naphtha fraction, and preferably at least one kerosene and / or gas oil fraction and a heavy fraction. The heavy fraction generally exhibits an initial boiling point of at least 350° C, preferably of greater than 370° C. This fraction is advantageously recycled to hydrotreatment unit. It may also be advantageous to recycle a portion of the kerosene to the hydrotreatment unit. The gas oil and kerosene fractions may or may not be recovered separately and the cut points may be adjusted to produce the desired hydrocarbon product. Hydrocarbon by-products from the processing and upgrading may be returned to the process as described above. The liquid hydrocarbon product preferably comprises aviation fuel (e.g. sustainable aviation fuel, SAF) or diesel (e.g. renewable diesel, RND). These may be particularly commercially valuable products. The invention will now be described in relation to the following non-limiting examples. DIRECT HEATING - EXAMPLES An example of a method according to the present specification is described with reference to Figure 2. The method results in the production of diesel or jet fuel from hydrogen, carbon dioxide and hydrocarbon by-products. A stream of pressurised hydrogen 10 (e.g., renewable hydrogen from an electrolysis unit) at a pressure of 10-50 bar is imported into the plant. A stream of carbon dioxide 12, for example CO2 recovered from a flue gas or CCS unit, is also imported, compressed if required, and optionally purified to remove compounds that may poison downstream catalysts. A carbon dioxide recycle stream 14 may also be combined with the carbon dioxide feed 12. The hydrogen streams 10 and carbon dioxide streams 12, 14 are mixed together then heated, typically to a temperature of 250-500 °C, preferably 350-450 °C. This heating may be conveniently provided using an interchanger 16 in heat exchange with hot syngas as part of a downstream heat recovery unit (described below). A stream of steam 18 is optionally added to the resulting hot H2 / CO2 gas mixture as shown by the dotted line. Optionally (and separately) a hydrocarbon by-product stream 20, is (optionally heated and) mixed with steam 22 then the mixture heated in heater 24 and passed to a derichment reactor 26 at an inlet temperature in the range of 250-650°C. The heater 24 may be an interchanger operating in heat exchange with hot crude syngas as part of a downstream heat recovery unit (described below). The hydrocarbon by-product and steam are passed adiabatically through a bed of a derichment catalyst, such as a particulate nickel catalyst having a high nickel content, in the derichment reactor 26. The function of the derichment reactor 26 is to convert higher (C2+) hydrocarbons present in the hydrocarbon by-product into methane so that they do not crack to form carbon in downstream processes. The hydrocarbon by-product stream 20 desirably comprises a hydrocarbon containing tailgas 28 stream recovered from a downstream Fischer-Tropsch hydrocarbon synthesis unit 30 comprising a Fischer-Tropsch reactor. It may also contain a recovered stream 32 of naphtha or LPG, which may not be of value as a final product. The naphtha / LPG stream 32 and tails gas 28 streams may be processed in a single derichment reactor or in two separate derichment reactors operating at different conditions. By-product hydrocarbon stream 20 may also contain hydrocarbons from other sources. A deriched hydrocarbon stream 34 enriched in methane is combined with the heated hydrogen / carbon dioxide stream 36 and the resulting mixture fed via line 38 to an electrically-powered turbo-machinery heater 40. The physical effect of this unit (as described above) is to convert electrical energy to kinetic energy to heat the gas mixture, thereby increasing its temperature. The gas mixture is heated in turbo-machinery heater 40 to at least 900 °C, more desirably to at least 1050 °C and preferably to at least 1100 °C. The heated gas mixture is then passed from the heater 40 via line 42 to a reverse water-gas shift reactor 44 containing a fixed bed of a supported nickel reverse water-gas shift catalyst that is also active for steam reforming reactions. The catalyst promotes the steam reforming and reverse water-gas shift (RWGS) reactions to maximise methane conversion and CO formation. The resulting crude syngas is passed from the reverse water-gas shift reactor 44 via line 46 to a heat recovery unit 48 where it is cooled to below the dew point of the steam in a number of cooling stages. This cooling can be in one or more boilers / boiler feed water heaters used to heat water and raise steam (as used in the process and optionally exported) and to preheat feed streams 16, 24. Further cooling, e.g. by heat exchange with water, performed to condense steam and form a process condensate, which is separated within the unit 48 by means of one or more gas-liquid separators (not shown) and recovered via line 50. The de-watered syngas gas is then fed from the heat recovery unit 48 via line 52 to a CO2 removal unit 54, where unreacted CO2 is separated and recovered via line 56. The CO2 removal unit operates by absorption using an amine wash which is regenerated using steam recovered from the heat recovery unit. At least a portion of the recovered carbon dioxide 56 can be recompressed (compressor not shown) and recycled as the carbon dioxide recycle stream 14. Following CO2 removal, the resulting syngas is substantially a H2 CO syngas having a Ik molar ratio suitable for the Fischer-Tropsch (FT) synthesis of hydrocarbons. The H2 CO syngas is optionally compressed using a compressor (not shown) before being passed via line 58 to the Fischer-Tropsch hydrocarbon synthesis unit 30. The Fischer-Tropsch hydrocarbon synthesis unit 30 comprises a Fischer-Tropsch reactor containing a Fischer-Tropsch catalyst within externally cooled reaction tubes. The catalyst may be disposed within a plurality of catalyst carriers to enhance heat transport arid conversion efficiency. The inlet temperature to the Fischer Tropsch reactor is in the range 150 to 350°C and the pressure is in the range 10 to 100 bar abs (0.1 to lOMpa). The syngas is converted into hydrocarbons in the reactor, which are recovered from the unit by line 60. As well as producing a FT hydrocarbon product, this unit also produces the hydrocarbon containing tail gas stream 28 and a co-produced FT water stream (not shown). The co-produced FT water stream may be (optionally purified and) recycled and provided as feed to an electrolysis unit if one is being used locally to provide renewable hydrogen feed. Furthermore, if the unit produces a stream 32 of naphtha and / or LPG surplus to requirements, this can be recycled as part of the by-product hydrocarbon stream 20. The combination of electrically-powered turbo-machinery heater 40 and reverse water-gas shift reactor 44 can be configured as a single unit. Alternatively, two or more turbo-machinery heater plus reactor units may be provided in series. By this means more heat can be imparted into the reacting gas giving a higher gas temperature leaving the final reactor and hence a greater conversion of CO2 to CO. In one embodiment, it is possible to combust a portion of the heated feed gas from turbomachinery heater 40 with a stream of oxygen gas 62 and steam 64 (shown by the dotted lines) to provide additional heat to the reverse water-gas shift reactor 44. The oxygen 62 can be the co-product from an electrolysis unit producing renewable hydrogen. In this way the turbomachinery heater 40 can provide most of the heat using imported electrical energy, while a small amount of combustion can be used to raise the feed gas to a higher temperature. An example of another method according to the present invention is described with reference to Figure 3. In this example, the feed gases 10, 12, 14, 20, 22, derichment reactor 26, heater 40, heat recovery unit 48, CO2 removal unit 54 and FT and processing unit 30 are the same as in Figure 2. However, in this example, the mixed feed gas 38 of hydrogen, carbon dioxide, deriched by-product hydrocarbon and steam mixture is fed to externally-heated catalystcontaining tubes in a heat exchange reactor 70 prior to heating the feed in the electrically powered turbo-machinery heater 40. In the heat exchange reactor 70, carbon dioxide and hydrogen are reacted over a supported reverse water gas shift catalyst, with low selectivity to methane formation, in the tubes to form a pre-heated reverse water-gas shifted gas mixture, which is recovered from the tubes and fed via line 72 to the electrically powered turbomachinery heater 40. The crude syngas 46 recovered from the downstream reverse water-gas shift reactor 40 is used to heat the tubes in the heat exchange reactor 70 and is partially cooled by the endothermic reverse water-gas shift reaction taking place in the tubes to produce a partially cooled crude synthesis gas 74 that is fed to the heat recovery unit 48. Alternatively, the preheated mixture of hydrogen and carbon dioxide from interchanger 16 is not combined with the deriched by-product hydrocarbon and steam mixture 34 and by-passes the heat exchange reactor 70 via the dotted line 76. In this case, just the deriched hydrocarbon and steam mixture 34 is fed to externally-heated catalyst-containing tubes in the heat exchange reactor 70. In this case, steam is reacted with methane over a supported nickel steam reforming catalyst to form a pre-heated reformed gas mixture. The by-passed feed 76 is combined with the pre-heated reformed gas and fed via line 72 to the turbo machinery heater 40. In one embodiment (not shown in Figure 3) just the heated hydrogen and carbon dioxide feed gas 36 is fed to the heat exchange reactor 70. Additional heat for the heat exchange reactor 70 may be provided by taking all, or part, of the methane-containing gas mixture 34 and passing it through a small autothermal reformer or partial oxidation reactor, operated in parallel to the turbo-machinery heater 40 and reverse water-gas shift reactor 44, to create a hot gas that is fed, along with the crude syngas 46, to the shell side of the heat exchange reactor 70. An example of another method according to the present invention is described with reference to Figure 4. In this example, the feed gases 10, 12,14, 20, 22, heater 40, reverse water-gas shift reactor 44, heat recovery unit 48, CO2 removal unit 54 and FT and processing unit 30 are the same as in Figures 2 and 3. However, in this example, the hydrogen and carbon dioxide gas mixture 36 is not combined with a deriched gas upstream of the electrically powered turbomachinery heater 40. In this example, the mixture of hydrocarbon by-product 20 and steam 22, with or without derichment, is fed to a heat exchange reformer (or post reformer) 80 in which the catalyst-containing tubes are open-ended to discharge the steam reformed gas into the shell-side of the heat exchange reformer. The crude syngas 46 from the water-gas shift reactor 44 is fed to the shell side of the heat exchange reformer 80 thereby combining the reformed gas and the crude syngas directly. The combined gas provides the heat for the reforming reaction to produce a partially cooled syngas 82 that is fed to the heat recovery unit 48. The methods according to Figures 2 and 3 were modelled in the following 2 examples using conventional process modelling software to study the effect of using the turbo machinery heater for direct heating These examples are compared to a baseline process with no turbo-machinery heater but with an electric feed preheater and using oxygen combustion in a burner in the RWGS vessel to provide heat for RWGS. All cases relate to production of FT wax and condensate at approximately 150 ktes / year. Hydrogen feed is supplied by electrolysis of water using power made from renewable resources at an efficiency of 65% (based on HHV). Example 1 Based on the flowsheet depicted in Figure 2. The turbo machinery heater 40 is used to raise the temperature of the feed gas 38 to 1 lOOdeg.C. The process requires an import of hydrogen of 8,430kgs / hr. It would require a power import of 71.7MW for the heater and 511.6MW to produce the electrolytic hydrogen; a total of 583.3MW. If the process according to Figure 2 was configured without a turbo machinery heater 40 and used only an oxygen feed (with steam) combustion in an autothermal reverse water-gas shift reactor to raise the feed gas to RWGS reaction temperature, it would require 10,100kgs / hr hydrogen which would require 612.9MW electrolysis power. Overall use of the method of the invention saves approximately 17% of the hydrogen feed. It also reduces overall power consumption by almost 30MW including power required for the heater and electrolysis. The temperatures, pressures, flows and gas compositions are set out in Table 1. Table 1 Stream 10 12 14 20 22 Temperature °C 40 40 45 30 267 Pressure bara ,23.0 23.0 23.0 35.0 50.7 Flowrate kgmols hr 4016.7 1307.7 1056.2 477.9 1580.7 kgs / hr 8430.0 57375.0 45877.5 8940.0 28485.0 Methane mol fraction 0.0000 0.0005 0.0003 0.5456 0.0000 Ethane mol fraction 0.0000 0.0000 0.0000 0.0156 0.0000 Propane mol fraction 0.0000 0.0000 0.0000 0.0121 0.0000 Butane mol fraction 0.0000 0.0000 0.0000 0.0071 0.0000 Pentane mol fraction 0.0000 0.0000 0.0000 0.0035 0.0000 Hydrogen mol fraction 0.9957 0.0000 0.0040 0.1637 0.0000 CO2 mol fraction 0.0000 0.9925 0.9792 0.0475 0.0000 CO mol fraction 0.0000 0.0060 0.0015 0.1045 0.0000 Nitrogen mol fraction 0.0000 0.0010 0.0000 0.0952 0.0000 H2O mol fraction 0.0033 0.0000 0.0150 0.0017 1.0000 Ethylene mol fraction 0.0000 0.0000 0.0000 0.0001 0.0000 Olefin C3+ mol fraction 0.0000 0.0000 0.0000 0.0010 0.0000 Alkane C6+ mol fraction 0.0000 0.0000 0.0000 0.0022 0.0000 Stream 28 34 42 46 60 Temperature °C 30 368 1100 866 242 Pressure bara 35.0 22.2 22.1 21.1 5 Flowrate kgmols / hr 482.7 2046.5 8425.5 8786.9 kgs / hr 9030.0 37425.0 149100.0 149100.0 18039.0 Methane mol fraction 0.5456 0.1607 0.0391 0.0169 Ethane mol fraction 0.0156 0.0000 0.0000 0.0000 Propane mol fraction 0.0121 0.0000 0.0000 0.0000 Butane mol fraction 0.0071 0.0000 b.oooo 0.0000 Pentane mol fraction 0.0035 0.0000 0.0000 0.0000 Hydrogen mol fraction 0.1637 0.0299 0.4824 0.3717 CO2 mol fraction 0.0475 0.0324 0.2847 0.1210 CO mol fraction 0.1045 0.0001 0.0011 0.1738 Nitrogen mol fraction .0.0952 0.0222 0.0056 0.0053 H2O mol fraction 0.0017 0.7547 0.1866 0.3112 Ethylene mol fraction 0.0001 0.0000 0.0000 0.0000 Olefin C3+ mol fraction 0.0010 0.0000 0.0000 0.0000 Alkane C6+ mol fraction 0.0022 0.0000 0.0000 0.0000 Example 2 Based on the flowsheet depicted in Figure 3. The temperature of the feed gas leaving the Heat Exchanger 70 is approximately 760 deg.C., and the turbo machinery heater 70 raises the temperature to about 1080 deg.C. The process again requires an import of hydrogen of 8,430kgs / hr. It would require a power import of 35.5MW for the heater and 511.6MW to produce the electrolytic hydrogen; a total of 547.1 MW. Overall use of the method of the invention saves approximately 17% hydrogen feed. It reduces overall power consumption by about 66MW including power required for the heater and electrolysis. The temperatures, pressures, flows and gas compositions are set out in Table 2. Table 2 Stream 10 12 14 20 22 Temperature °C 40 40 45 30 267 Pressure bara 25.0 25.0 25.0 35.0 50.7 Flowrate kgmols / hr 4016.7 1316.3 1048.4 477.9 1580.7 kgs / hr 8430.0 57750.0 45600.0 8940.0 28485.0 Methane mol fraction 0.0000 0.0005 0.0003 0.5456 0.0000 Ethane mol fraction 0.0000 0.0000 b.oooo 0.0156 0.0000 Propane mol fraction 0.0000 0.0000 0.0000 0.0121 0.0000 Butane mol fraction 0.0000 0.0000 0.0000 0.0071 0.0000 Pentane mol fraction 0.0000 0.0000 0.0000 0.0035 0.0000 Hydrogen mol fraction 0.9957 0.0000 0.0040 0.1637 0.0000 CO2 mol fraction .0.0000 0.9925 0.9792 0.0475 0.0000 CO mol fraction 0.0000 0.0060 0.0015 0.1045 0.0000 Nitrogen mol fraction 0.0000 0.0010 0.0000 0.0952 0.0000 H2O mol fraction 0.0033 0.0000 0.0150 0.0017 1.0000 Ethylene mol fraction 0.0000 0.0000 0.0000 0.0001 0.0000 Olefin C3+ mol fraction 0.0000 0.0000 0.0000 0.0010 0.0000 Alkane C6+ mol fraction 0.0000 0.0000 0.0000 0.0022 0.0000 Stream 28 34 38 42 46 60 Temperature °C 30 368 388 1078 866 242 Pressure bara 35.0 24.2 24.2 22.1 21.1 5 Flowrate kgmols / hr 482.7 2045.3 8425.5 8425.5 8786.9 kgs / hr 9030.0 37425.0 149205.0 149205.0 149205.0 18039.0 Methane mol fraction 0.5456 0.1610 0.0392 0.0392 0.0168 Ethane mol fraction 0.0156 0.0000 0.0000 0.0000 0.0000 Propane mol fraction 0.0121 0.0000 0.0000 0.0000 0.0000 Butane mol fraction 0.0071 0.0000 0.0000 0.0000 0.0000 Pentane mol fraction 0.0035 0.0000 0.0000 0.0000 0.0000 Hydrogen mol fraction 0.1637 0.0290 0.4821 0.4821 0.3716 CO2 mol fraction 0.0475 0.0322 0.2849 0.2849 0.1211 CO mol fraction 0.1045 0.0001 0.0011 0.0011 0.1739 Nitrogen mol fraction 0.0952 0.0222 0.0056 0.0056 0.0053 H2O mol fraction 0.0017 0.7555 0.1866 0.1866 0.3112 Ethylene mol fraction 0.0001 0.0000 0.0000 0.0000 0.0000 Olefin C3+ mol fraction 0.0010 0.0000 0.0000 0.0000 0.0000 Alkane C6+ mol fraction 0.0022 0.0000 0.0000 0.0000 0.0000 In summary, these examples illustrate that the use of a turbo-machinery heater as described can lead to a significant saving in both hydrogen usage and power usage. INDIRECT HEATING The previous examples describe how a turbo-machinery heater can be used advantageously to heat a hydrogen and carbon dioxide gas mixture (directly) to a very high temperature for feeding to a reverse water-gas shift reactor. In contrast, in the following examples a turbomachinery heater is used to heat a circulating, inert gas to a high temperature (for example, >750°C, >850°C, or >900°C). Such a gas could be CO2, Ar, or N2. This gas is passed to the shell-side of a counter-current gas heated reverse water-gas shift reactor. A feed gas comprising H2 and CO2 is also passed into the reactor. The feed gas can be passed through tubes in the reactor which are at least partly filled with a reverse water-gas shift catalyst as previously described. The heated inert gas is passed along an outer shell-side of the tubes. Heat is exchanged between the heated circulating inert gas outside the reactor tubes and the feed gas within the reactor tubes so that a crude syngas comprising H2 and CO leaves the tubes and a cooled inert gas stream leaves the reactor shell. As such, in these examples the feed gas is indirectly heated by the turbo-machinery heater via an intermediary (inert) heat transfer gas. The cooled inert gas can then be routed to a compressor and a turbo machinery heater to provide the motive pressure for the inert gas to flow through the reactor and the heat for transferring to the process. The compressor stage may be before or after the turbo machinery heater or the turbo machinery heater may be configured to provide the necessary compression. The compressor and turbo-machinery heater may be physically on the same shaft receiving rotational energy from the same driver. The driver can be an electric motor utilising electric power which has been derived from renewable sources. These configurations may have the following advantages compared with more conventional autothermal reverse water-gas shift reactors (noting that several of the advantages are shared with the direct heating examples): (i) No oxygen is combusted, so no valuable hydrogen feed is consumed, and all the hydrogen is available for reverse water-gas shift reaction. This saves money by reducing the size and capital expenditure of the upstream electrolysis unit for hydrogen production and the amount of renewable power required to operate it. (ii) The turbo-machinery heater (and compressor) can be driven by an electric motor, which uses renewable electricity to generate heat within the gas at high efficiency. (iii) No electrical resistance heating in the reverse water-gas shift reactor is required, obviating the need to provide, verify, and operate a complicated catalytic process. (iv) Direct heating of a gas comprising high-pressure hydrogen by the turbo-machinery heater is avoided as an inert gas can be used for the heat transfer gas. This can avoid or negate issues in the turbo-machinery heater caused by contact with high temperature, high pressure hydrogen which may requiring internal cooling and / or selection of more expensive materials of construction for the turbo-machinery heater. Figure 5 shows a flow diagram of an example of a method in which feed gas is indirectly heated by a turbo-machmery heater by circulating a heat transfer gas (e.g., carbon dioxide) through a turbo-machinery heater and through a gas heated reverse water-gas shift reactor to heat the feed gas within the reactor via heat exchange. A stream of pressurised hydrogen (e.g., renewable hydrogen from a pressurised electrolysis unit) is imported into the system. A stream of carbon dioxide is also introduced, compressed if required, and optionally purified to remove compounds that poison catalysts (e.g., sulphur compounds). One part of the CO2 stream is imported into the system. Another part of the CO2 stream may be provided internally within the system by separating CO2 from cooled, dewatered syngas and recycling back to the reverse water-gas shift unit. The CO2 and hydrogen streams are mixed together and then heated, typically to a temperature of 250-500°C, preferably 350-450°C, and certainly hot enough to be at a margin above the dew point if steam is subsequently added. Alternatively, the hydrogen arid CO2 streams may be heated separately, then mixed. Optionally (and separately) a hydrocarbon stream (optionally heated) is mixed with steam then heated and passed to a derichment (pre-reforming) reactor. The function of this reactor is to convert higher (C2+) hydrocarbons into methane so that it won’t crack to form carbon in the reverse water-gas shift reactor downstream. The hydrocarbon stream desirably comprises a hydrocarbon stream purged from the downstream Fischer-Tropsch (FT) synthesis unit. The hydrocarbon stream may also contain naphtha (typically C3-C7 hydrocarbons), which may not be of value as a final product. The naphtha and tailgas streams may be processed in a single derichment reactor or in two separate ones at different conditions. The hydrocarbon stream may also contain hydrocarbons from other sources. Accordingly, in these examples a more conventional oxygen-blown autothermal reactor (ATR) has been replaced in the process by a turbo-machinery heater (TMH) loop. Outside of the TMH loop, an additional electric heater on the feed to the gas heated reverse water-gas shift reactor is shown. Figure 6 shows a flow diagram of another example of a method in which feed gas is indirectly heated by a turbo-machinery heater by circulating a heat transfer gas (e.g., carbon dioxide) through a turbo-machinery heater and through a reverse water-gas shift reactor to heat the feed gas within the reactor by heat exchange, this example also including a heat exchange reformer (post-reformer) downstream of the gas heated reverse water-gas shift reactor. Figure 7 shows some more detail of an example of a gas heated reverse water-gas shift reactor for circulating heat transfer gas which has been heated by a turbo-machinery heater. The figure shows how the reverse water-gas shift reaction can be carried out in a gas heated reactor. The gas heated reverse water-gas shift reactor is a counter-current flow tubular heat exchanger with two fixed tube-sheets, cold at the top and hot at the bottom. The heated hydrogen / CO2 stream and (optionally) the deriched hydrocarbon stream are fed to the tube-side of the gas heated reverse water-gas shift reactor, which at least partly contains a reverse water-gas shift catalyst to convert hydrogen and CO2 to a CO rich gas (and water). In certain examples, the reverse water-gas shift catalyst in the tubes also comprisese a reforming catalyst such as nickel. In such cases the steam reforming reaction converts methane in the gas, generating extra hydrogen and carbon monoxide. Separately to the feed gas, a hot, inert gas is passed through the gas heated reverse water-gas shift reactor shell in counter-current flow to the heated feed gas (hydrogen / CO2 and optionally deriched hydrocarbon stream). The inlet temperature of this inert gas is sufficiently hot (for example, >770°C, >870°C, or >970°C) such that it can provide heat to the feed gas and provide the reaction heat to give close to equilibrium reverse water-gas shift and reforming reaction conversion for the feed gas at a temperature of, for example, >700°C, >780°C, or >870°C. The heat transfer coefficient on the shell-side of the gas heated reverse water-gas shift reactor can be enhanced by using one or more of fins, sheath tubes, or baffles. After heat exchange with the feed gas within the reactor, the cooled inert gas leaves the gas heated reverse water-gas shift reactor shell. It flows to a compressor / turbo machinery heater unit, where electrical energy provides the motive energy to overcome the pressure drop across the shell-side of the gas heated reverse water-gas shift reactor, and the heat for the reaction. In this manner the inert gas is boosted to the temperature and pressure required and recycled to the shell-side inlet of the gas heated reverse water-gas shift reactor. The compressor / turbo-machinery heater and pipework connecting the turbo-machinery heater to and from the heat exchange reactor shell are collectively known as the turbo-machinery heater (TMH) loop. Optionally, an empty vessel or enlarged section of pipework can be placed within the TMH loop as a ‘buffer’ volume. This ‘buffer’ volume can assist in the design of stable control dynamics of temperatures and pressures in the TMH loop. The reacted CO rich crude syngas can be cooled in a number of steps. This cooling can be used to raise steam (as used in the process and optionally exported) and to preheat feed streams. Further cooling is needed to condense and separate water content from the gas. The gas is then fed to a CO2 removal unit, where unreacted CO2 is separated and can be recycled upstream of the reverse water-gas shift unit so that the remaining syngas is substantially a H2 / CO syngas. The H2 / CO syngas can be purified (to remove Fischer-Tropsch catalyst poisons) and optionally compressed before passing to a Fischer-Tropsch (or other liquid hydrocarbon) synthesis unit. As well as producing Fischer-Tropsch liquids, the Fischer-Tropsch synthesis unit also produces a hydrocarbon rich tailgas stream and co-produced water. The latter water stream (optionally purified) may be recycled and provided as feed to an electrolysis unit if one is being used locally to provide a renewable hydrogen feed. Furthermore, if the liquid hydrocarbon production produces a naphtha stream surplus to requirements, this can be recycled to the reverse water-gas shift process. Carbon dioxide can be utilised as the inert gas, with the following advantages: it has a relatively high specific heat capacity which keeps the mass flow low; it is already produced at high pressure elsewhere in the process, therefore its use does not require any extra equipment items; and it can be added into the reverse water-gas shift process gas in a controlled manner, without detrimentally affecting the CO rich syngas produced, so that the pressure differential between the gas in the turbo-machinery heater loop and the feed gas in the gas heated reverse water-gas shift reactor is kept low and the tubes can be designed with thin wall thicknesses to minimise cost and maximise heat transfer. As an alternative to carbon dioxide, nitrogen or argon can be used as the inert gas in the turbo-machinery heater loop. According to certain configurations, the turbo-machinery heater loop equipment includes the following features: • Heat transfer Fluid - CO2 • Reactor type - gas heated reverse water-gas shift reactor • Reactor Fluid flows - counter-current • Turbo-machinery heater and compressor, separate or combined • Optional ‘buffer’ volume The gas heated reverse water-gas shift reactor as illustrated in Figure 7 is shown with tubes, which are free to expand, relative to the bottom tube-sheet, when the temperature increases. Labyrinth seals with free vertical movement of tube tail-pieces within housings in the tubesheet are shown. Venturi Seals can also be used. Such an embodiment is described, for example, in WO2022180354A1, the disclosures of which are hereby incorporated by reference. As previously indicated, the advantage of using CO2 within the turbo-machinery heater loop is that CO2 is being added to the process in any case. By designing the turbo-machinery heater loop pressure slightly above (feed gas) process pressure, a small inward leakage (of CO2 into the process gas) through the seals occurs, with very little effect on the process (a ventun seal, which costs extra process pressure drop is not necessarily required). By linking the process side together with the turbo-machinery heater loop it is possible to design the tubes for a low-pressure differential (e.g., 5 bars). This enables the tube thickness to be thin, e.g., <5mm. Sheath tubes are shown to enhance shell-side heat transfer (a baffled design can be used). The design requires the turbo-machinery heater / compressor unit to only impart a modest pressure increase to force the fluid to flow through the gas heated reverse water-gas shift reactor shell and around the loop. Alternative gas heated reverse water-gas shift reactor designs and flowsheets have also been considered. One gas heated reverse water-gas shift reactor design uses a bayonet tube and core tube instead of a lower tube-sheet, shown in Figure 8. CO rich crude syngas flows out of the catalyst at the bottom of the annular tubes, enters the core tubes at the bottom, flows up the core tubes and leaves the reactor at the top. Heat can be recuperated by cooling the gas flowing up the core tubes into the feed gas passing in the annular space through the catalyst. A bayonet tube gas heated reverse water-gas shift reactor also allows the tubes to hang / expand freely downwards from a tube-sheet when the temperature increases, i.e., no lower tube-sheet or labyrinth seal arrangement is required. While the use of core tubes allows different options in addressing mechanical design, it does have two potential disadvantages: (i) It reduces the cross-sectional area (XS A) available in the catalyst tube for catalyst and also tends to increase catalyst pressure drop. (ii) If particulate catalyst is used, when the catalyst &core tubes expand (as they are heated to slightly different temperatures) particulate catalyst pieces tend to drop and settle, and when the tubes cool down again the pieces are ‘stuck’ as the tubes contract and they tend to break - each successive temperature cycle compounds the problem. This both increases pressure drop and can exert very high static loads on the core tubes. Therefore, an attractive option is to use catalyst-coated structured packing to avoid this. The individual packing sections can be designed to use the core tube as a concentric support / guide, with the column of sections being supported on a catalyst support grid installed in the bottom of the bayonet tubes. In this situation, it is desirable if the H2 / CO crude syngas produced is cooled down as it passes up the core tubes by recycling part of its heat back into the RWGS reacting gas in the catalyst. This reduces the amount of heat needed to be supplied by the TMH loop, which reduces the size of the turbo-machinery heater and the amount of power needing to be imported. The downside is that less high-pressure steam is produced in heat recovery from the crude syngas. On the inside of the core tube, pne of many devices can be used to increase the gas velocity or turbulence to increase the inside heat transfer coefficient (e.g., core rods, inserts, turbulators). CO2 from the TMH loop can still be added in a controlled manner into the process, without detrimentally affecting the CO rich syngas produced, so that the pressure differential between the gas in the TMH loop and the feed gas in the heat exchange reactor is kept low and the tubes can be designed with thin wall thicknesses (to minimise cost and maximise heat transfer). INDIRECT HEATING - EXAMPLES Indirect heating configurations are illustrated by reference to the two following examples. These are compared to a baseline process with no turbo-machinery heater but with an electric feed preheater and using oxygen combustion in a burner in the RWGS vessel to provide heat for RWGS. All cases relate to production of FT wax and condensate at approximately 67 ktes / year. Hydrogen feed is supplied by electrolysis of water using power made from renewable resources at an efficiency of 65% (based on HHV). Example 1 This example is illustrated in Figure 5. Carbon dioxide is used as a heat transfer fluid. The turbo machinery heater is used to raise the heat transfer fluid temperature to 975°C. The process requires an import of hydrogen of 3,740kgs / hr. It would require a power import of 31.8MW for the electric pre-heater and turbo-machinery heating / compression and 227.4MW to produce the electrolytic hydrogen, i.e., a total of 259.2MW. If the process according to Figure 5 was configured without indirect heating and using only electricity for pre-heating and an oxygen feed (with steam) combustion instead to raise the feed gas to RWGS reaction temperature it would require ~4MW electrical heating and 4,490kgs / hr hydrogen which would require 272.4MW electrolysis power. Overall use of the invention saves -17% hydrogen feed. It reduces overall power consumption by over 17MW including power required for the electric preheater, turbo-machmery heater / compressor and electrolysis. The temperatures, pressures, flows and gas compositions are set out in the table below. Example 1 10 12 14 20 22 34 42 46 28 50 51 60 H2 feed CO2 feed CO2 recycle FT tailgas recycle Steam feed Deriched gas Feed gas to RWGS Crude syngas ex RWGS Tailgas ex FT CO2 exTMH CO2 toTMH FT wax and condensate Temperature "c 40 40 45 30 267 380 420 870 30 975 825 242 Pressure bara 23.0 23.0 23.0 35.0 50.7 22.0 22.0 20.2 35.0 23.0 22.2 5 Methane mol fr 0.0000 0.0005 0.0003 0.5456 0.0000 0.1564 0.0387 0.0168 0.5456 0.0000 0.0000 Ethane mol fr 0.0000 0.0000 0.0000 0,0156 0.0000 0.0000 0.0000 0.0000 0.0156 0.0000 0,0000 Propane mol fr 0.0000 0.0000 0.0000 0.0121 0.0000 0.0000 0.0000 0.0000 0.0121 0.0000 0.0000 Butane mol fr 0.0000 0.0000 0.0000 0.0071 0.0000 0.0000 0.0000 0.0000 0.0071 0.0000 0,0000 Penlane rnol fr 0.0000 0.0000 0.0000 0.0035 0.0000 0.0000 0.0000 0.0000 0.0035 0.0000 0.0000 Hydrogen mol fr 0.9957 0.0000 0.0041 0.1637 0.0000 0.0347 0.4820 0.3710 0.1637 0.0000 0.0000 CO2 mol fr 0.0000 0.9925 0.9754 0.0475 0.0000 0.0331 0.2831 0.1202 0.0475 1.0000 1.0000 CO mol fr 0.0000 0.0060 0.0015 0.1045 0.0000 0.0001 0.0011 0.1730 0.1045 0.0000 0.0000 Nitrogen mol fr 0.0000 0.0010 0.0000 0.0952 0.0000 0.0218 0.0055 0.0053 0.0952 0.0000 0.0000 H2O mol fr 0.0033 0.0000 0.0188 0.0017 1.0000 0.7537 0.1891 0.3137 0.0017 0.0000 0.0000 Ethylene mol fr 0.0000 0.0000 0.0000 0.0001 0.0000 0.0000 0.0000 0.0000 0.0001 0.0000 0.0000 Olefin C3+ mol fr 0.0000 0.0000 0.0000 0.0010 0.0000 0.0000 0.0000 0.0000 0.0010 0.0000 0.0000 Alkane C6+ mol fr 0.0000 0.0000 0.0000 0.0022 0.0000 0.0000 0.0000 0.0000 0.0022 0.0000 0.0000 Total 1.0000 1.0000 1.0000 1,0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1,0000 FLOW kgmols / hr 1783.4 584.1 464.7 212.2 716.1 925.4 3755.8 3911.8 214.3 12467.8 12467.8 kgs / hr 3742.9 25474.5 20133.2 3969.4 12900.4 16871.8 66315,0 66315.6 4009.3 548650.8 548650.8 8009.3 Example 2 This example is illustrated in Figure 6. The deriched gas is fed to a heat exchange reformer(post-reformer), therefore the mass flow through the RWGS reactor is reduced and so is the heating duty. Carbon dioxide is used as a heat transfer fluid. The turbo machinery heater is used to raise the heat transfer fluid temperature to 1000°C. The process again requires an import of hydrogen of 3,740kgs / hr. It would require a power import of 18.7MW for the electric heater and turbo-machinery heating / compression and 227.4MW to produce the electrolytic hydrogen, i.e., a total of 246.1MW. Overall use of the invention saves -17% hydrogen feed. It reduces overall power consumption by over 26MW including power required for the electric preheater, turbo-machinery heater / compressor and electrolysis. The temperatures, pressures, flows and gas compositions are set out in the table below. Example 2 10 12 14 20 22 34 42 46 47 28 50 51 60 H2 feed CO2 feed CO2 recycle FT tailgas recycle Steam feed Deriched gas Feed gas to RWGS Crude syngas ex RWGS Mix syngas ex fpr Tailgas ex FT CO2 ex TMH CO2 toTMH FT wax and condensate Temperaturf 'c 40 40 45 30 267 426 420 897 582 30 1000 825 242 Pressure bara 24.0 24.0 24.0 350 50.7 23.8 23.3 213 19.8 35.0 23.0 22.2 5 Methane mol fr 0 0000 0.0005 0.0005 0.5456 0.0)00 0.1496 0.0002 00170 0.0164 0.5456 0.0000 0 0000 Ethane mol fr 0.0000 0.0000 0.0000 0.0156 0.0000 0.0000 0.0000 0.0000 0.0000 0.0156 0.0000 0.0000 Propane mol fr 0.0000 0.0000 0.0000 0.0121 0.0)00 0.0000 0.0000 0.0)0) 0.0000 0.0121 0.0000 0.0000 Butane mol fr 0.0000 0.0000 0.0000 0.0071 0.0)00 0.0000 0.0000 0.0)0) 0.0000 0.0071 0.0000 0.0000 Pentane mol fr 0.0000 0.0000 0.0000 0.0035 0.0)00 0.0000 0.0000 0.0)0) 0.0000 0.0035 0.0000 0.0000 Hydrogen mol fr 0.9957 0.0000 0.0018 0.1637 0.0)00 0.0549 0.6427 0.3719 0.3784 0.1637 0.0000 0.0000 CO 2 mol fr 0.0000 0.9925 0.9867 0.0475 0.0)00 0.0378 0.3505 0.1214 0.1049 0.0475 1.0000 1.0000 CO mol fr 0.0000 0.0060 0.0042 0.1045 0.0)00 0.0003 0.0015 0.2258 0.1768 0.1045 0.0000 0.0000 Nitrogen mol fr 0.0000 0.0010 0.0000 0.0952 0.0)00 0.0216 0.0002 0.0002 0.0054 0.0952 0.0000 0.0000 H2O mol fr 0.0033 0.0000 0.0062 0.0017 1.0)00 0.7358 0.0043 0.2637 0.3181 0.0017 0.0000 0.0000 Ethylene mol fr 0.0000 0.0000 0.0000 0.0001 0.0)00 0.0000 0.0000 0.0)0) 0.0000 0.0001 0.0000 0.0000 Olefin C3+ mol fr 0.00)0 0.0000 0.0000 0.0010 0.0000 0.0000 0.0000 0.0)0) 0.0000 0.0010 0.0000 0.0000 Alkane C6+ mol fr 0 0000 0.0000 0.0000 0.0022 0.0)00 0.0000 0.0000 0 0000 0.0000 0.0022 0.0000 0 0000 Total 10000 1.0000 1.0000 1.0)0) 1.0)00 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 10000 FLOW kgmols / hr 1783.4 584.1 393.9 212.2 716.1 935.1 2764.6 2678.3 3852.7 214.3 6272.5 6272.5 kgs / hr 3742.9 25641.0 17212.8 3969.4 12900.4 16871.8 46603.9 46603.9 63475.6 4009.3 276057.0 276057.0 8009.3 In summary, these examples illustrate that the use of a turbo-machinery heater as described can lead to a significant saving in both hydrogen usage and power usage. The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents. For example, the following clauses define certain embodiments: 1. A method of forming a syngas, the method comprising: providing a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst; providing a feed gas comprising carbon dioxide and hydrogen; and passing the feed gas over the reverse water-gas shift catalyst to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide, wherein the feed gas is heated using a turbo-machinery heater that generates heat via kinetic energy, optionally a method of forming a syngas, the method comprising: providing a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst; providing a feed gas comprising carbon dioxide and hydrogen at an elevated temperature of at least 200°C; and passing the feed gas at a reaction temperature of at least 700°C over the reverse water-gas shift catalyst to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide, wherein: providing the feed gas comprises combining a first carbon dioxide stream and a hydrogen stream; and the elevated temperature is achieved by using a turbo-machinery heater that uses kinetic energy to heat (directly or indirectly) one or more of the carbon dioxide stream, the hydrogen stream and the feed gas. 2. The method of clause 1, wherein, the turbo-machinery heater heats the first carbon dioxide stream but does not heat the hydrogen stream or the feed gas. 3. The method of clause 1, wherein the turbo-machinery heater heats the feed gas but does not heat the carbon dioxide stream or the hydrogen stream. 4. A method of any preceding clause, wherein the elevated temperature is achieved by using two or more turbo-machinery heaters operating in parallel. 5. A method of any preceding clause wherein the turbo-machinery heater comprises a gas inlet, a gas outlet, and a gas feed path extending from the gas mlet, via a heating zone to the gas outlet; the heating zone comprising a rotatable shaft assembly comprising a rotatable shaft and one or more sets of rotor blades, and one or more sets of stationary blades; the one or more sets of rotor blades functioning to convert kinetic energy of the rotatable shaft assembly into kinetic energy of a gas passing along the gas feed path through the heating zone; and at least a portion of the one or more sets of stationary blades functioning to convert the kinetic energy of the gas into heat energy of the gas to raise the temperature of the gas. 6. The method of any preceding clause, wherein: the elevated temperature is substantially equal to the reaction temperature: and the water-gas shift reactor is an adiabatic reverse water-gas shift reactor in which no heating is provided by combustion of the feed gas in the reverse water-gas shift reactor. 7. The method of any of clauses 1 to 5, wherein: the elevated temperature is less than the reaction temperature: and the water-gas shift reactor is an autothermal reverse water-gas shift reactor in which heating to the reaction temperature is provided by combustion of a portion of the feed gas with an oxygen stream in a burner in the reverse water-gas shift reactor. 8. The method according to clause 7, wherein the oxygen stream is produced at least in part by electrolysis of water. 9. The method of any preceding clause, further comprising: passing the crude syngas to a carbon dioxide removal unit to form a second carbon dioxide stream and a carbon dioxide-depleted syngas; and recycling the second carbon dioxide stream into the first carbon dioxide stream. 10. The method of clauses 1 to 5, 7, 8 and 9, wherein: the elevated temperature is at least 400°C, preferably at least 700°C, more preferably at least 900°C; and the reaction temperature is at least 1000°C, preferably at least 1050°C, more preferably at least 1100°C. 11. The method of clause 6, wherein the elevated temperature and the reaction temperature are at least 750°C, preferably at least 900°C, more preferably at least 1000°C, even more preferably at least 1050°C, still even more preferably at least 1100°C. 12. The method of any preceding clause, wherein the reverse water-gas shift catalyst comprises nickel. 13. The method of any preceding clause, wherein the hydrogen stream is produced at least in part by electrolysis of water. 14. The method of any preceding clause, wherein the carbon dioxide is derived from a combustion flue gas, a gasification or partial oxidation unit, a direct air capture unit, or a CCS unit. 15. A method of producing a hydrocarbon product from a syngas, the method comprising: forming a syngas from a feed gas according to the method of any preceding clause; passing the syngas through a Fischer-Tropsch reactor to produce hydrocarbons and coproduced water; separating the hydrocarbons and the co-produced water from unreacted syngas and returning the unreacted syngas to the Fischer-Tropsch reactor; and processing the hydrocarbons into a liquid hydrocarbon product and a hydrocarbon byproduct stream comprising one or more of a tail gas, LPG and naphtha. 16. The method of clause 15 wherein at least a portion of the hydrocarbon by-product stream is combined with the first carbon dioxide stream, the hydrogen stream or the feed gas before heating the feed gas to the elevated temperature in the turbo-machinery heater, or is added to the feed gas after heating the feed gas to the elevated temperature. 17. The method according to clause 16, wherein the hydrocarbon by-product stream is subjected to a derichment step with steam in a derichment reactor to form a methane-containing gas, and the methane-containing gas is combined with the first carbon dioxide stream, the hydrogen stream or the feed gas before heating the feed gas to the elevated temperature in the turbo-machinery heater, or is added to the feed gas after heating the feed gas to the elevated temperature. 18. The method according to clause 17, wherein: the reverse water-gas shift reactor is an adiabatic reverse water-gas shift reactor; the elevated temperature is substantially equal to the reaction temperature; and the method further comprises passing the methane containing gas to the adiabatic reverse water-gas shift reactor. 19. The method of clause 15, wherein: the reverse water-gas shift reactor is an autothermal reverse water-gas shift reactor comprising a burner; the elevated temperature is less than the reaction temperature; and the method further comprises optionally subjecting the hydrocarbon by-product stream to a derichment step in a derichment reactor to form a methane-containing gas, and passing the hydrocarbon by-product stream, or the methane-containing gas, and an oxygen stream to the burner to combust hydrogen and the hydrocarbon by-product or methane-containing gas stream to generate heat. 20. The method of clause 15, wherein: the reverse water-gas shift reactor is an adiabatic reverse water-gas shift reactor; the elevated temperature is substantially equal to the reaction temperature; and the method further comprises: optionally subjecting the hydrocarbon by-product stream to a derichment step in a derichment reactor to form a methane-containing gas, and passing the hydrocarbon by-product stream, or the methane-containing gas, and an oxygen stream to an autothermal steam reformer to form a second syngas; and introducing the second syngas into the first syngas. 21. The method of clause 15, further comprising, optionally subjecting the hydrocarbon by-product stream to a derichment step in a derichment reactor to form a methane-containing gas, passing the hydrocarbon by-product stream, or the methane-containing gas stream, through externally heated tubes in a heat exchanger or heat exchange reformer to form a preheated feed or pre-heated reformed gas, combining the pre-heated feed or pre-heated reformed gas with the first carbon dioxide stream, the hydrogen stream or the feed gas comprising hydrogen and the first carbon dioxide stream to form a combined gas feed, and passing the combined gas feed to the turbo-machinery heater, wherein the externally heated tubes are heated by a hot gas comprising the crude syngas, thereby forming a partially cooled crude syngas that is recovered from the heat exchanger. 22. The method of clause 15, further comprising, optionally subjecting the hydrocarbon by-product stream to a derichment step in a derichment reactor to form a methane-containing gas, passing the hydrocarbon by-product stream, or the methane-containing gas stream, through externally heated tubes in a heat exchanger or heat exchange reformer, which are open-ended at the exit, to form a pre-heated feed or pre-heated reformed gas, combining the preheated feed or pre-heated reformed gas which is leaving the open-ended tubes with the crude syngas to form a combined syngas and cooling the combined syngas in the shell side of the heat exchanger or heat exchange reformer thereby forming a cooled crude syngas that is recovered from the heat exchanger or heat exchange reformer. 23. The method of clause 15, further comprising, optionally subjecting the hydrocarbon by-product stream to a derichment step in a derichment reactor to form a methane-containing gas, combining the hydrocarbon by-product stream, or the methane-containing gas stream with the feed gas, the hydrogen stream and / or the first carbon dioxide stream to form a combined feed gas, passing the combined feed gas through externally heated tubes in a heat exchanger or heat exchange reactor containing a reverse water-gas shift catalyst to form a pre-heated feed or pre-heated reverse water gas shifted gas and feeding the preheated gas or pre-heated reverse water gas shifted gas to the turbo machinery heater, wherein the externally heated tubes are heated by a hot gas comprising the crude syngas, thereby forming a partially cooled crude syngas that is recovered from the heat exchanger. 24. The method of any of clauses 15 to 23, wherein the co-produced water is subjected to electrolysis to produce at least a portion of the hydrogen stream. Furthermore, the following clauses define certain further embodiments: 1. A method of forming a syngas, the method comprising: providing a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst; providing a feed gas comprising carbon dioxide and hydrogen; and passing the feed gas over the reverse water-gas shift catalyst to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide, wherein the feed gas is heated using a turbo-machinery heater that generates heat via kinetic energy, optionally a method of forming a syngas, the method comprising: providing a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst providing a feed gas comprising carbon dioxide and hydrogen; and passing the feed gas over the reverse water-gas shift catalyst at an elevated temperature to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide, wherein the elevated temperature is achieved by: heating a heat transfer gas to form a heated heat transfer gas using a turbo-machinery heater that uses kinetic energy to heat the heat transfer gas; and heating the reverse water-gas shift reactor via heat exchange with the heated heat transfer gas. 2. The method of clause 1, wherein: the reverse water-gas shift reactor comprises a tubular heat exchanger comprising a tube side and a shell side; the reverse water-gas shift catalyst is situated on the tube side; and heating the reverse water-gas shift reactor via heat exchange with the heated heat transfer gas comprises passing the heated heat transfer gas through the shell side. 3. The method of clause 2, wherein the tubular heat exchanger comprises a countercurrent flow tubular heat exchanger. 4. The method of clause 2 or clause 3, wherein the tubular heat exchanger comprises a bayonet heat exchanger which consists of pairs of concentric tubes in the vertical plane, the outer of which have closed end caps that create clearance passes between the inner tubes and annular tubes such that (a) the feed gas enters the annular tubes at the top and flows downwards through the reverse water-gas shift catalyst reacting to produce the crude syngas and (b) the crude syngas passes through the clearance gaps and back up the inner tubes to leave the heat exchanger at the top and (c) the heated heat transfer gas flows outside the tubes from the bottom to the top. 5. The method of clause 4, where the inside or outside surfaces of the inner tubes, or both, are insulated such that heat transfer from the crude syngas passing up the inner tubes to the gas flowing through the catalyst is minimised. 6. The method of clause 4, where heat transfer from the crude syngas passing up the inner tubes to the gas flowing through the catalyst is enhanced, such as by designing the inner tubes’ diameter to be wide, or placing inserts in the inner tubes to block off some of the cross-sectional area available for flow, so the crude syngas velocity is high or installing structures in the tubes to cause turbulence and promote convective heat transfer so that a cooled crude syngas leaves the heat exchanger at the top. 7. The method of any of clauses 2 to 6, wherein: the tubular heat exchanger comprises a heat transfer gas inlet and a heat transfer gas outlet; and the heat transfer gas enters the heat transfer gas inlet at a temperature of from 750 to 1100 °C, preferably from 800 to 1050 °C, more preferably from 850 to 1000 °C and / or a pressure of from 10 to 40 bara, preferably from 15 to 30 bara. 8. The method of any preceding clause, wherein the heat transfer gas comprises one or more of nitrogen, argon and carbon dioxide. 9. The method of clause 8, wherein the heat transfer gas comprises carbon dioxide. 10. The method of any preceding clause, wherein the elevated temperature is achieved by using two or more turbo-machinery heaters operating in parallel or in series 11. The method of any preceding clause, wherein the turbo-machinery heater comprises a gas inlet, a gas outlet, and a gas feed path extending from the gas inlet, via a heating zone to the gas outlet; the heating zone comprising a rotatable shaft assembly comprising a rotatable shaft and one or more sets of rotor blades, and one or more sets 6f stationary blades; the one or more sets of rotor blades functioning to convert kinetic energy of the rotatable shaft assembly into kinetic energy of a gas passing along the gas feed path through the heating zone; and at least a portion of the one or more sets of stationary blades functioning to convert the kinetic energy of the gas into heat energy of the gas to raise the temperature of the gas. 12. The method of any preceding clause, wherein the turbo-machinery heater heats the heat transfer gas to a temperature of from 750 to 1100 °C, preferably from 800 to 1050 °C, more preferably from 850 to 1000 °C. 13. The method of any preceding clause, wherein the heat transfer gas is compressed in a compressor prior to, or after, being heated by the turbo-machinery heater or the compression is achieved within the turbo-machinery heater. 14 The method of clause 13, where the compressor blades are attached to the same rotatable shaft assembly, which forms the turbo-machinery heater. 15. The method of clauses 13 or 14, wherein the compressor compresses the heat transfer gas to a pressure of from 10 to 40 bara, preferably from 15 to 30 bara. 16. The method of any preceding clause, wherein the elevated temperature is at least 700 °C, preferably at least 750 °C, more preferably at least 850 °C, even more preferably at least 900 °C. 17. The method of any preceding clause, wherein passing the feed gas over the reverse water-gas shift catalyst at an elevated temperature occurs at a pressure of from 10 to 40 bara, preferably from 15 to 30 bara. 18. The method of any preceding clause, wherein the pressure of the heat transfer gas during heat exchange in the reverse water-gas shift reactor is substantially similar to the pressure of the feed gas being passed over the reverse water-gas shift catalyst at the elevated temperature, preferably wherein the difference in pressure between the pressure of the heat transfer gas during heat exchange with the reverse water-gas shift reactor and the pressure of the feed gas being passed over the reverse water-gas shift catalyst at the elevated temperature is less than 10 bar, more preferably less than 5 bar, even more preferably less than 3 bar. 19. The method of any preceding clause, wherein the feed gas is pre-heated using a preheater before passing the feed gas over the reverse water-gas shift catalyst. 20. The method of clause 19, wherein the pre-heater heats the feed gas to a temperature of from 350 to 500 °C, preferably from 350 to 450 °C. 21. The method of any preceding clause, further comprising: passing the crude syngas to a carbon dioxide removal unit to form a carbon dioxide stream and a carbon dioxide-depleted syngas; and recycling the carbon dioxide stream into the feed gas. 22. The method of any preceding clause, wherein the reverse water-gas shift catalyst comprises nickel. 23. The method of any preceding clause, wherein the reverse water-gas shift catalyst comprises a metal structure, coated with catalyst 24. The method of any preceding clause, wherein the hydrogen in the feed gas is produced at least in part by electrolysis of water. 25. The method of any preceding clause, wherein the carbon dioxide in the feed gas is derived from a combustion flue gas, a gasification or partial oxidation unit, a direct air capture unit, or a CCS unit. 26 A method of producing a hydrocarbon product from a syngas, the method comprising: forming a syngas from a feed gas according to the method of any preceding clause; passing the syngas through a Fischer-Tropsch reactor to produce hydrocarbons and coproduced water; separating the hydrocarbons and the co-produced water from unreacted syngas and returning the unreacted syngas to the Fischer-Tropsch reactor; and processing the hydrocarbons into a liquid hydrocarbon product and a hydrocarbon byproduct stream comprising one or more of a tail gas, LPG and naphtha. 27. The method of clause 26 wherein at least a portion of the hydrocarbon by-product stream is combined with the feed gas before passing the feed gas over the reverse water-gas shift catalyst. 28. The method according to clause 27, wherein the hydrocarbon by-product stream is subjected to a derichment step with steam in a derichment reactor to form a methane-containing gas, and the methane-containing gas is combined with the feed gas before passing the feed gas over the reverse water-gas shift catalyst. 29. The method of clause 26, wherein the method further comprises optionally subjecting the hydrocarbon by-product stream to a derichment step in a derichment reactor to form a methane-containing gas, arid passing the hydrocarbon by-product stream, or the methane-containing gas, and an oxygen stream to an autothermal steam reformer to form a second syngas; and introducing the second syngas into the crude syngas. 30. The method of clause 26, further comprising, optionally subjecting the hydrocarbon by-product stream to a derichment step in a derichment reactor to form a methane-containing gas, passing the hydrocarbon by-product stream, or the methane-containing gas stream, through externally heated tubes in a heat exchange reformer to produce a steam reformed gas, wherein the externally heated tubes are open-ended so that the steam reformed gas is discharged into the shell-side of the heat exchange reformer where it mixes with the crude syngas from the reverse water-gas shift reactor, thereby forming a mixed crude syngas that flows through the shell-side of the heat exchange reformer, thereby forming a partially cooled mixed crude syngas that is recovered from the heat exchanger. 31. The method of any of clauses 26 to 30, wherein the co-produced water is subjected to electrolysis to produce at least a portion of the hydrogen of the feed gas. The aforementioned sets of clauses define both direct and indirect (i.e., via a heat transfer gas) routes to heating of reverse water-gas shift reactants to perform a reverse water-gas shift reaction in order to generate a syngas using turbo-machinery heating. It will also be appreciated that certain further embodiments may combine features of both direct and indirect heating routes using turbo-machinery heating. For example, one or more of the reactant gases may be heated directly using turbo-machinery heating (e.g., before entering the reverse water-gas shift reactor) and then further heated indirectly via a heat transfer gas (e.g., up to reaction temperature within the reverse water-gas shift reactor). It will also be appreciated that any feature described in relation to one aspect or embodiment of the present specification can be applied to any other aspect or embodiment of the present specification, either alone or in combination.

Claims

1. A method of forming a syngas, the method comprising:providing a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst;providing a feed gas comprising carbon dioxide and hydrogen; andpassing the feed gas over the reverse water-gas shift catalyst to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide,wherein the feed gas is heated using a turbo-machinery heater that generates heat via kinetic energy.

2. A method according to claim 1, wherein the turbo-machinery heater directly heats the feed gas or a component thereof.

3. A method according to claim 1,wherein the turbo-machinery heater indirectly heats the feed gas or a component thereof via a heat transfer gas, the turbo-machinery heater being used to heat the heat transfer gas and the heat transfer gas being used to heat the feed gas, or a component thereof, via heat exchange.

4. A method according to any preceding claim,wherein a ratio of pressure of gas heated by the turbo-machinery heater to pressure of unheated gas input to the turbo-machinery heater is no more than 2.2, 2.0, 1.7, 1.5, 1.3, 1.2, or 1.1.

5. A method according to any preceding claim,wherein the turbo-machinery heater is used to heat the feed gas prior to the feed gas entering the reverse water-gas shift reactor and passing over the reverse water-gas shift catalyst.

6. A method according to any preceding claim,wherein the turbo-machinery heater is used to heat the feed gas within the reverse water-gas shift reactor as the feed gas is passing over the reverse water-gas shift catalyst.

7. A method of any preceding claim, wherein the feed gas is heated using two or more turbo-machinery heaters.

8. A method of any preceding claim, wherein:the or each turbo-machinery heater is used to heat the feed gas to a reaction temperature to convert at least a portion of the carbon dioxide to carbon monoxide as the feed gas is passed over the reverse water-gas shift catalyst; andno heating is provided by combustion of the feed gas in the reverse water-gas shift reactor or by means of another heating system within the reverse water-gas shift reactor.

9. A method of any of claims 1 to 7, wherein:the or each turbo-machinery heater is used to heat the feed gas to a temperature less than a reaction temperature to convert at least a portion of the carbon dioxide to carbon monoxide as the feed gas is passed over the reverse water-gas shift catalyst; andthe water-gas shift reactor is an autothermal reverse water-gas shift reactor in which heating to the reaction temperature is provided by combustion of a portion of the feed gas with an oxygen stream in a burner in the reverse water-gas shift reactor or alternatively heating to the reaction temperature is provided by another heating system within the reverse water-gas shift reactor.

10. A method according to any preceding claim, further comprising:removing carbon dioxide and water from the crude syngas to produce a purified syngas.

11. A method according to claim 10,wherein removed carbon dioxide is recycled into the feed gas.

12. A method of producing hydrocarbon products, the method comprising:forming a purified syngas according to claim 10 or 11; andpassing the purified syngas through a Fischer-Tropsch reactor to produce hydrocarbon products and co-produced water.

13. A method according to claim 12, further comprising:separating the hydrocarbon products and the co-produced water from unreacted syngas and returning the unreacted syngas to the Fischer-Tropsch reactor; andprocessing the hydrocarbon products into a liquid hydrocarbon product and a hydrocarbon by-product stream comprising one or more of a tail gas, liquefied petroleum gas (LPG), and naphtha.

14. A method according to any preceding claim,wherein the feed gas is heated using the turbo-machinery heater by separately heating one or both of the carbon dioxide and hydrogen prior to mixing.

15. A method according to any preceding claim,wherein the crude syngas formed by passing the feed gas over the reverse water-gas shift catalyst is fed to a heat exchange reformer which is configured to receive a mixture of methane and steam, and heat from the crude syngas drives steam methane reforming to produce more syngas.

16. A system for performing the method according to any preceding claim, the system comprising:a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst;a feed gas system for providing a feed gas comprising carbon dioxide and hydrogen to the reverse water-gas shift reactor;wherein the feed gas system and the reverse water-gas shift reactor are configured to pass the feed gas over the reverse water-gas shift catalyst to form a crude syngas by converting at least a portion of the carbon dioxide to carbon monoxide, andwherein the system further comprises a turbo-machinery heater for heating the feed gas.

17. A system according to claim 16, further comprising:a syngas conditioning unit for removing carbon dioxide and water from the crude syngas to produce a purified syngas; anda Fischer-Tropsch reactor configured to convert the purified syngas into hydrocarbon products and co-produced water.

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