A method and system for forming syngas

Abstract

A method of forming a syngas, the method comprising providing a first feed gas stream comprising carbon dioxide; providing a second feed gas stream comprising hydrogen; heating the first and / or second

Description

Field The present specification relates to a method and system for forming a syngas (synthesis gas) comprising carbon monoxide and hydrogen, and also a method and system for producing liquid hydrocarbons from the syngas. Background 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 (CnHjn+z). 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 Syngas generation using a reverse water-gas shift reaction can be beneficial since it makes use of carbon dioxide that may have been destined to be released to the atmosphere. WO2022079408 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 watergas 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. WO2022079408 also discloses that the gas streams fed to the reverse water-gas shift vessel may be preheated. The pre-heat temperature of the feed gases to the reverse water- gas shift vessel may be in the range of 400 to 1000°C or 450 to 800°C to sustain combustion. The hydrogen and carbon dioxide streams may be premixed before preheating or preheated and mixed. Preheating of the feeds to their pre-heat temperatures may be done by interchange with the crude product gas mixture, and / or by steam heating, or by using a fired heater or by electrical heating or by a combination of two or more these. Preferably, the feed gas mixture comprising carbon dioxide and hydrogen is heated by interchange with the crude product gas mixture. The present specification is concerned with providing an improved method and system for producing syngas and particularly focuses on the pre-heating of feed gases to the reverse water-gas shift reactor. Summary As described in the background section, WO2022079408 teaches that it can be advantageous to preheat the carbon dioxide and hydrogen feed gases to a reverse water-gas shift reactor and that the carbon dioxide and hydrogen streams may be premixed before preheating or preheated and mixed. From a production system perspective, the simplest, and seemingly preferable, approach is to mix the hydrogen and carbon dioxide feed gases and then preheat the mixture of gases before passing the mixture of feed gases into the reverse water-gas shift reactor. However, it has surprisingly been found that the mixture of carbon dioxide and hydrogen feed gases may be more reactive than previously believed under certain pre-heating conditions and that a reverse water-gas shift reaction can be initiated, at least to some extent, when pre-heating the mixed feed gas prior to entering the reverse water-gas shift reactor. This can be problematic as such a reaction may generate a CO / H2 / CO2 / H2O bearing gas in the pre-heater equipment and pre-reactor pipe work and such as gas mixture can degrade standard pre-heater equipment and pipework via materials damage mechanisms such as carburisation and metal dusting. This can lead to a reduction in the lifetime of these parts and require such parts to be replaced leading to increased equipment costs and plant downtime. The aforementioned problem can be exacerbated by metallic construction materials catalysing the reverse water-gas shift reaction in the pre-heater(s) and / or pipework prior to the reverse water-gas shift reactor. For example, heating the mixed gas feed over a large metallic heat exchange surface in a pre-heater can catalyse the reverse water-gas shift reaction. Furthermore, pre-heating the mixed gas feed may also lead to an exothermic methanation reaction, emitting heat and resulting in a rise in temperature. Methanation can also be catalysed by metallic construction materials and can lead to a runaway reaction when the heat release is faster than the heat being removed. This can cause the temperature to rise and exceed the design temperature of the pre-heater or pipework causing equipment damage, resulting in downtime. One solution to these problems is to pre-heat the carbon dioxide and hydrogen gas mixture at a lower temperature and / or pressure to avoid the initiation of a reverse water-gas shift (and / or methanation) reaction prior to entering the reverse water-gas shift reactor. However, this approach can reduce the efficiency of the reverse water-gas shift system. Another approach is to use a pre-heater which comprises parts formed of alloys which are more resistant to degradation by a CO / H2 / CO2 / H2O bearing gas. However, parts formed of such alloys tend to be expensive and more difficult to procure leading to longer lead times, more expensive construction, and less availability of spares. Additionally, such parts may still have a limited lifetime due to eventually succumbing to carburisation and / or metal dusting. Yet another approach is to separately pre-heat the carbon dioxide and hydrogen feed gases which was envisaged as an alternative approach in WO2022079408. However, if these separately heated feed gas streams are then mixed prior to entering the reverse water-gas shift reactor then a reverse watergas shift reaction may occur in a mixing zone prior to entering the reverse water-gas shift reactor. To avoid this problem the separately pre-heated carbon dioxide and hydrogen feed streams could be separately fed into the reverse water-gas shift reactor and only mixed within the reactor. However, this would require a modification to the reverse water-gas shift reactor design and may lead to sub-optimal mixing of reactants and / or increased complexity of the reactor. In light of the above, the present specification provides an alternative / improved approach which avoids at least some of the problems as outlined above. The method comprises: providing a first feed gas stream comprising carbon dioxide; providing a second feed gas stream comprising hydrogen; heating the first and / or second feed gas streams; after heating, mixing the first and second feed gas streams in a mixing zone which is either lined with a refractory ceramic material and / or formed of a nickel-chromium alloy to form a heated mixed feed gas stream comprising carbon dioxide and hydrogen; and feeding the heated mixed feed gas stream from the mixing zone into a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst to form a crude syngas product stream by converting at least a portion of the carbon dioxide to carbon monoxide, wherein the mixing zone and the reverse water-gas shift reactor are connected via a connector tube lined with a refractory ceramic material and / or formed of a nickel-chromium alloy or the mixing zone is formed of a connector tube to the reverse water-gas shift reactor. The present specification also provides a system for performing the aforementioned method, the system comprising: one or more heaters for heating a first feed gas stream comprising carbon dioxide and / or a second feed gas stream comprising hydrogen; a mixing zone lined with a refractory ceramic material and / or formed of a nickel-chromium alloy for mixing the first and second feed gas streams after heating to form a heated mixed feed gas stream comprising carbon dioxide and hydrogen; and a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst, the reverse water-gas shift reactor being configured to receive the heated mixed feed gas stream from the mixing zone and pass the heated mixed feed gas stream over the reverse water-gas shift catalyst within the reverse water-gas shift reactor to form a crude syngas product stream by converting at least a portion of the carbon dioxide to carbon monoxide, wherein the mixing zone and the reverse water-gas shift reactor are connected via a connector tube lined with a refractory ceramic material and / or formed of a nickel-chromium alloy or the mixing zone is formed of a connector tube to the reverse water-gas shift reactor. Such an approach separately heats the carbon dioxide and hydrogen feed streams to avoid reaction of the carbon dioxide and hydrogen in a pre-heater. This approach thus avoids the risk of metal dusting and carburisation of the pre-heater(s) (feed heater) without requiring the pre-heater(s) to be formed of expensive metal dusting resistant alloys. This approach also avoids the risk of an exothermic methanation reaction in the pre-heater(s). Furthermore, the separately heated feed streams are then mixed within a mixing zone which is lined with a refractory ceramic material and / or formed of a nickelchromium alloy such that the mixing zone is resistant to metal dusting and carburisation if any reverse water-gas shift reaction occurs in the mixing zone. This also enables the carbon dioxide and hydrogen feed streams to be mixed prior to entering the reverse water-gas shift reactor such that the reactor doesn't need to be modified to receive separate carbon dioxide and hydrogen feed streams. The mixing zone and the reverse water-gas shift reactor are connected via a connector tube lined with a refractory ceramic material and / or formed of a nickel-chromium alloy or the mixing zone is formed of a connector tube to the reverse water-gas shift reactor. As such, the present method can provide a system which is robust to degradation and can provide optimal levels of feed gas pre-heating at desired gas pressures while avoiding expensive modifications to the pre-heating apparatus and / or the reverse water-gas shift reactor. This can simplify the procurement of capital equipment, increase plant lifetime, and minimise unplanned downtime by elimination of the risk of system materials damage. Brief Description of the Drawings Figure 1 shows a flow sheet for a method of forming crude syngas. Figure 2 shows a flow sheet for a method of forming a hydrocarbon product stream by combining the syngas forming method of Figure 1 with a Fischer-Tropsch unit. Figure 3 shows a flow sheet for a method of forming crude syngas not according to the present specification. Figure 4(a) shows a graph of CO content versus temperature for a heated feed mixture of carbon dioxide and hydrogen. Figure 4(b) shows the graph of Figure 4(a) extended beyond the 500°C test data to show the trend in CO formation up to 650°C. Figure 5 shows a flow sheet for a method of forming crude syngas according to the present specification and illustrates how a flow sheet such as that shown in Figure 3 can be modified to achieve the present invention. A summary of the reference numerals used in the figures is set out in the table below. Reference Item 2 Carbon Dioxide Feed Gas 4 CO2 Feed Heater 6 Heated Carbon Dioxide Feed Gas 8 Hydrogen Feed Gas 10 H2 Feed Heater 12 Heated Hydrogen Feed Gas 14 Mixing Zone with Ceramic Lining 16 Heated Mixed Feed Gas 18 Reverse Water-Gas Shift Reactor 20 Crude Syngas 22 Water Removal Unit 24 Separated Water Stream 26 Water-Depleted Syngas 28 Electrolyser 30 Hydrogen Stream 32 Oxygen Stream 34 Carbon Dioxide Removal Unit 36 Separated Carbon Dioxide Stream 38 Carbon Dioxide-Depleted Syngas 40 Fischer-Tropsch Unit 42 Hydrocarbon Product Stream 44 Tail Gas Stream 46 Derichment Reactor 48 Methane Stream 50 Mixed feed gas 52 Mixed feed gas interchanger 54 Mixed feed gas heater 56 Heated mixed feed gas 58 Tails gas interchanger 60 Oxygen feed 62 Syngas boiler 64 Carbon dioxide feed interchanger 66 Hydrogen feed interchanger 68 Refractory lined pipe / mixed hot feed gas Detailed Description Figure 1 shows a flow sheet for a method of forming crude syngas. The method comprises heating a first feed gas 2 comprising carbon dioxide using a heater 4 to form a heated carbon dioxide feed gas stream 6. A second feed gas 8 comprising hydrogen which is separately heated using a heater 10 to form a heated hydrogen feed gas stream 12. The heated carbon dioxide feed gas stream 6 and the heated hydrogen feed gas stream 12 are then mixed in a mixing zone 14 lined with a refractory ceramic material (and / or formed of a nickel-chromium alloy) to form a heated mixed feed gas stream 16 comprising carbon dioxide and hydrogen. Subsequently, the heated mixed feed gas stream 16 is fed from the mixing zone 14 into a reverse water-gas shift reactor 18 comprising a reverse water-gas shift catalyst and the heated mixed feed gas stream is passed over the reverse water-gas shift catalyst within the reverse water-gas shift reactor 18 to form a crude syngas product stream 20 by converting at least a portion of the carbon dioxide to carbon monoxide. The first and second heaters for separately heating the carbon dioxide and hydrogen feed streams may be provided by separate heaters. This configuration allows the option to heat the carbon dioxide and hydrogen feed streams at different temperatures. Alternatively, the first and second heaters may be provided by a single heater unit. The important feature is that the carbon dioxide and hydrogen feed streams are kept as separate streams during the pre-heating process. This can be achieved by two separate streams routed through the same heating unit or by using two separate heaters. An alternative option is to only heat one of the feed streams. In that case, one of the feed streams would need to be heated to a sufficiently high temperature such that after mixing with the other, unheated, feed stream then the mixed feed stream is sufficiently hot. The mixing zone 14 may be formed by a vessel or pipework which is configured to receive and mix the heated carbon dioxide and hydrogen streams 6, 12. If this vessel or pipework 14 is then coupled to the reverse water-gas shift reactor 18 via a further connector tube 16, then the further connector tube is also be lined with a refractor ceramic material (and / or formed of a nickel-chromium alloy). Alternatively, the mixing zone 14 is formed within the connector tube 16 to the reverse water-gas shift reactor 18 without the need for a separate mixing vessel. As described in the summary section, a key feature of the present specification is that the carbon dioxide and hydrogen feed streams are separately heated and then mixed in a zone which has a refractory ceramic lining (and / or is formed of a nickel-chromium alloy). The heated mixed feed gas within the mixing zone is at a temperature and pressure at which a reverse water-gas shift reaction is initiated to form a mixture of carbon dioxide, hydrogen, carbon monoxide and water. The refractory ceramic lining (and / or the nickel-chromium alloy) at the mixing zone prevents this gas mixture from degrading metal components. Furthermore, to avoid such metal degradation downstream of the mixing zone, downstream system components can also be lined with a refractory ceramic material and / or made of an alloy which is resistant to degradation. For example, one or more components of the reverse water-gas shift reactor can be lined with a refractory ceramic material and / or made of an alloy which is resistant to degradation. In this regard, reverse water-gas shift reactors are already designed to be resistant to damage by the reverse water-gas shift reaction. A key feature of certain configurations of the present specification is that they can be implemented using standard heaters and reactors while avoiding the problems of prior configurations if the pre-heating is sufficient to initiate the reverse water-gas shift reaction prior to the reactants entering the reverse water-gas shift reactor. As described in the summary section, an alternative to the aforementioned approach is to mix the carbon dioxide and hydrogen prior to heating, and then keep the temperature of the gas mixture sufficiently low that the reverse water-gas shift reaction is not initiated prior to entry into the reverse water-gas shift reactor. However, this leads to inefficiencies within the reverse water-gas shift reactor. Another alternative the aforementioned approach is to mix the carbon dioxide and hydrogen prior to heating, and then use a heater which is especially configured to be resistant to degradation by a gas mixture comprising carbon dioxide, hydrogen, carbon monoxide and water which is formed by the onset of the reverse water-gas shift reaction. However, such an approach requires such a heater to be manufactured from expensive metal dusting resistant alloys and / or ceramic coated components. The present approach is advantageous as it enables more standard heating equipment to be used to heat the feed gases while still avoiding degradation issues with the heating equipment. It is much easier to provide a ceramic lined mixing zone / vessel / pipe than to provide a heater with bespoke, corrosion resistant parts. As such, the present methodology can be implemented where the first and second feed gases are heated and fed to the mixing zone using heaters and / or feed tubes which are not lined with refractory ceramic material or formed of an expensive metal dusting resistant alloy (e.g., a nickelchromium alloy such as a nickel-chromium only alloy, a nickel-chromium-aluminium alloy, or a nickelchromium-copper alloy). For configurations in which the mixing zone is lined with a refractory ceramic material, the refractory ceramic material can be selected from any ceramic material which is resistant to degradation by a mixture of carbon dioxide, hydrogen, carbon monoxide and water at the process temperatures and pressures required for optimal process gas feed to a reverse water-gas shift reactor. For example, the refractory ceramic material can be an oxide, a carbide or a nitride of one or more of the following elements: silicon, aluminium, magnesium, calcium, boron, chromium and zirconium. For configurations in which the mixing zone is formed of a metal dusting resistant alloy, such as a nickelchromium alloy, then the nickel-chromium alloy can be selected from a nickel-chromium only alloy, a nickel-chromium-aluminium alloy, or a nickel-chromium-copper alloy. For example, the nickelchromium alloy can be selected from one or more of alloy 310, 310S, BlOSi, 800 / H / HT, RA 330, alloy 600 / H, 625, 890, RA330, RA230, Alloy X, CT15C, Paralloy CR39W, H39WMR, H39WM+, HK40, HPW, HPNb, HP Microalloy, alloy 601, 602 CA, 603XL, 617, 671, 690, 693, 803, NEXAGE™696, VDM® ALLOY 699 XA, HAYNES® HR-235®, HR214, HR120, HR160, H46M, RA253MA, RA333, RA353MA, 45TM, Paralloy OPTIM-AL, H46M, or H48T. Preferred alloys include alloy 600 / H, 625, 890, RA330, RA230, Alloy X, CT15C, Paralloy CR39W, H39WMR, H39WM+, HK40, HPW, HPNb, and HP Microalloy. Most preferred alloys include alloy 601, 602 CA, 603XL, 617, 671, 690, 693, 803, NEXAGE™696, VDM® ALLOY 699 XA, HAYNES® HR-235®, HR214, HR120, HR160, H46M, RA253MA, RA333, RA353MA, 45TM, Paralloy OPTIM-AL, H46M, and H48T. The alloy may also provide pressure containment in the mixing zone, or the alloy may form a coating with another metal providing mechanical support for pressure containment in the mixing zone. Furthermore, the nickel-chromium alloy may comprise a coating to increase corrosion resistance. Coatings may be selected from an aluminium diffusion coating, a chrome diffusion coating, a silicon coating (e.g., a silicon CVD coating), or a catalytic coating. In order to further decrease corrosion, a corrosion inhibitor can be added to one or more of the feed streams prior to the mixing zone. For example, the corrosion inhibitor may be a sulphur-based corrosion inhibitor such as HjS or a disulphide. Such reagents form a protective sulphur atmosphere on the metallic surface that preferentially prevents carbon from being absorbed onto the surface. Accordingly, for increased corrosion protection, a low level of a reactive sulphur species such as HjS or a disulfide may be provided in the process environment. Sulphur levels less than 10 ppm (e.g., in a range 1 to 10 ppm) are sufficient to mitigate metal dusting. In terms of process gas conditions, one or both of the first and second feed gases can be heated to a temperature of: at least 300°C, 400°C, 450°C, 500°C, 600°C, or 700°C; no more than 1500°C, 1300°C, 1200°C, 1100°C,1000°C, 900°C, or 850°C; or within a range defined by any combination of the aforementioned lower and upper limits. Furthermore, one or more of the first feed gas, the second feed gas, the heated carbon dioxide feed gas stream, the heated hydrogen feed gas stream, and the heated mixed feed gas stream may have a pressure of: at least 5, 10, 15 or 20 Bar abs; no more than 60, 50, 40, or 30 bar abs; or within a range defined by any combination of the aforementioned lower and upper limits. The heaters used to heat the first and second feed gases may be of any suitable type to achieve the required operating temperatures. For example, the first and / or second feed gases can be heated using a heater type selected from: a heat exchange heater; a turbo-machinery heater; an electrical heater; a radiant heater; or a resistive heater. Heat source options for the heaters may include geothermal, nuclear (fusion / fission), rotodynamic heating, concentrated solar, infrared, microwave, radio wave, induction, resistance, electric arc, plasma, microwave, ultrasonic, or industrial waste heat, or any of the above via a thermal storage medium such as molten salt or high temperature thermal store. The reverse water-gas shift reactor can be an autothermal reverse water-gas shift reactor in which further heating to the reaction temperature is provided by combustion of a portion of the heated mixed feed gas with an oxygen stream in a burner in the reverse water-gas shift reactor. In this case, advantageously the burner may comprise a nozzle which is also lined with a refractory ceramic material. Alternatively, the reverse water-gas shift reactor can be an electrically heated reactor or an adiabatic reactor. For example, the feed gas may be pre-heated completely to reaction temperature (e.g., electrically) before being passed over the catalyst in the reverse water-gas shift reactor. Figure 2 shows a flow sheet for a method of forming a hydrocarbon product stream by combining the syngas forming method of Figure 1 with a Fischer-Tropsch unit. The crude syngas product stream 20 is produced in the same manner as described above in relation to Figure 1 and for conciseness these steps / parts will not be repeated. The crude syngas from the reverse water-gas shift reactor is advantageously processed to remove water and carbon dioxide prior to feeding into the Fischer-Tropsch unit. As such, the crude syngas 20 can be passed to a water removal unit 22 to form a separated water stream 24 and a water-depleted syngas 26. The separated water stream 24 can be fed into an electrolyser 28 to produce a hydrogen stream 30 which is recycled into the second feed gas 8 comprising hydrogen. The electrolyser 28 can also produce an oxygen stream 32 which is fed to the burner of the reverse water-gas shift reactor 18. In Figure 2 the hydrogen stream 30 is recycled into the second feed gas 8 comprising hydrogen upstream of the heater 10. However, alternatively the hydrogen stream 30 could be recycled into the feed gas downstream of the heater 10. For example, the hydrogen stream 30 could be recycled directly into the mixing zone 14. In that case, the hydrogen stream 30 may optionally be heated separately from the second feed gas 8. The crude syngas, advantageously after removing water, is fed to a carbon dioxide removal unit 34 to form a separated carbon dioxide stream 36 and a carbon dioxide-depleted syngas 38. The separated carbon dioxide stream 36 can be recycled into the first feed gas 2 comprising carbon dioxide. In Figure 2 the carbon dioxide stream 36 is recycled into the first feed gas 2 comprising carbon dioxide upstream of the heater 4. However, alternatively the carbon dioxide stream 36 could be recycled into the feed gas downstream of the heater 4. For example, the carbon dioxide stream 36 could be recycled directly into the mixing zone 14. In that case, the carbon dioxide stream 36 may optionally be heated separately from the first feed gas 2. After removal of water and carbon dioxide, and optional further processing to remove other contaminants, the syngas can then be passed to the Fischer-Tropsch unit 40 to produce a hydrocarbon product stream 42 and a tail gas stream 44. Advantageously, the tail gas stream 44 is recycled into the reverse water-gas shift reactor 18. In this case, the tail gas stream can be passed through a derichment reactor 46 to form a methane containing stream 48 for feeding back into the reverse water-gas shift reactor 18. Both reverse water-gas shift and steam methane reforming reactions can occur within the reverse water-gas shift reactor 18 to produce the crude syngas. In Figure 2, the methane containing stream 48 is illustrated as being recycled directly into the reverse water-gas shift reactor 18. However, as an alternative the methane containing stream 48 can be recycled into one of the first and second feed streams 2, 8 or into the mixing zone 14. To illustrate the benefits of the present approach a counter-example is described below, followed by a summary of some of the experiments which have resulted in modifications according to the present specification. Figure 3 shows an example of a flow sheet for a method of forming crude syngas which is not according to the present specification. A carbon dioxide feed gas 2 is mixed with a hydrogen feed gas 8. The mixed feed gas 50 is passed through a mixed feed gas interchanger 52 to heat up the mixed feed gas using the hot syngas product stream from the reverse water-gas shift reactor. For example, the mixed feed gas at this stage may be at a temperature around 400°C (398°C in the illustrated example). The mixed feed gas is then passed through a mixed feed gas heater 54 to further increase the temperature of the mixed feed gas. For example, in the illustrated example the mixed feed gas is heated to 550°C. The heated mixed feed gas 56 is then passed to the reverse water-gas shift reactor 18. A tails gas stream 44 (e.g., recycled from a down-stream hydrocarbon synthesis unit) is passed through a tails gas interchanger 58 to heat up the tails gas using the hot syngas product stream from the reverse water-gas shift reactor. The tails gas is then passed through a derichment reactor 46 to produce a methane containing gas stream 48 which is mixed with the heated mixed feed gas stream 56. In the illustrated example, the methane containing gas stream from the derichment reactor is at a temperature of 530°C, the heated mixed H2 / CO2 gas stream is at 550°C, and when the two streams are mixed and fed to the reverse water-gas shift reactor the feed gas is at a temperature of 548°C. It may be noted that the heating element of the feed gas heater will be at a higher temperature than the process gas temperature. An oxygen feed 60 is also fed to the reverse water-gas shift reactor 18 and combusted to further increase the temperature of the reactants to drive the reverse water-gas shift reaction and produce a crude syngas product stream. In the illustrated example the crude syngas exiting the reactor is at a temperature of 900°C. The hot crude syngas can be used to raise steam in a syngas boiler 62 and is then passed through the tails gas interchanger 58 and the H2 / CO2 feed gas interchanger 52. The present inventors have identified that the mixed feed gas heater 54 and the gas line 56 from the heater 54 to the reverse water-gas shift reactor 18 may be susceptible to damage under certain operating conditions and over prolonged periods of operation (highlighted region in Figure 3). This is due to the potential for CO formation in this region causing metal degradation. In relation to the above, experiments have been performed to determine how much CO may be formed when pre-heating a mixed H2 and CO2 feed prior to entering the reverse water-gas shift reactor as is the case for the flow sheet of Figure 3. A mixed H2 and CO2 feed was preheated from 300°C to 500°C through an empty reactor. The reactor was preheated with nitrogen before switching over to a H2 and CO2 mixture. Each temperature test was carried out for 1 hour before switching to a higher temperature in a step-wise manner over the test temperature range. At each test temperature the amount of CO was measured. Test conditions are summarized below: • H2 - 70.1 mol%, CO2- 29.9 mol% • Test pressure - 24.7 barg • Reactor tube material - 800 HT aluminised • Reactor ID (inner diameter) - 28 mm • Gas sampled at reactor exit and CO content measured. Results are illustrated in Figure 4(a) showing a graph of CO content (mol%) versus temperature. The amount of CO remained relatively low between 300°C and 400°C, which corresponds to the temperature range of the mixed feed gas between the feed interchanger 52 and the heater 54 in Figure 3. However, CO content increases significantly as the temperature is raised to 500°C or more, which corresponds to the temperature of the mixed feed gas in the heater 54 and gas line 56 to the reactor in Figure 3. Another test at 600°C resulted in a measured CO content of up to 10 mol%. Figure 4(b) shows the graph of Figure 4(a) extended beyond the 500°C test data to show the trend in CO formation up to 650°C. As such, these experiments would suggest that, at least under these operating conditions, CO generation in the heater 54 and gas line 56 could be a potential problem if mixed feed gas conditions and materials of constructions are not correctly managed. Furthermore, there is some evidence in the literature that certain metal alloys utilized to manufacture reactor equipment can affect the amount of CO2 to CO conversion via a reverse water-gas shift reaction. For example, in "High-Temperature Kinetics of the Homogeneous Reverse Water-Gas Shift Reaction" by F. Bustamante and R. M. Enick, AICHE Journal, vol. 50, No. 5. May 2004. pg 1039, reverse water-gas shift reactions in an Inconel 600 (a nickel-chromium alloy) reactor and a quartz reactor were compared. The paper found that Inconel 600 catalyses the reverse water-gas shift reaction compared to an inert quartz reactor and showed some evidence of CO2 conversion at a temperature range from 538°C, which is similar to the temperature of the heated mixed feed gas stream 56 in the flow-sheet of Figure 3. As such, it is considered that alloy materials which may typically be used for the feed heater 54 and gas line 56 could conceivably contribute to driving a reverse water-gas shift reaction in the heater and gas line 56 to produce CO prior to entering the reverse water-gas. Mechanisms for formation of carbon and metal dusting including the following... H2 + CO2 # CO + HZO [CO formation from RWGS Reaction] 2 CO # C + CO2 [CO forms carbon via Boudouard Carbon] CO + H2 # C + H2O [CO forms carbon via CO Reduction] Methane (e.g., in recycled, deriched tail gas) cracking can also form carbon. Formation of carbon can cause metal dusting, which is a form of corrosion that occurs when susceptible materials are exposed to environments with high carbon activities. Metal components can corrode over time, potentially resulting in equipment failure. While not being bound by theory, initial data suggests that in the present case / conditions for the heated mixed feed gas in heater 54 and gas line 56 of Figure 3, carbon formation via CO reduction is more likely to be problematic than carbon formation via the Boudouard route. Thermodynamically, the feed gas composition falls in the carbon forming region via CO reduction. This means that thermodynamically the feed gas composition can form carbon. However, the extent to which this may be problematic in practice will also be dependent on the kinetics of the carbon forming process. Below 400-450°C, the kinetics of the process are sufficiently slow that metal dusting is not normally a concern. However, lower feed heater temperatures for the present syngas production process reduces the process efficiency. As such, it is desirable to pre-heat the feed gas to temperatures exceeding 500°C from a process efficiency perspective. Figure 5 shows a modified flow sheet which addresses this issue. In this flow sheet, the carbon dioxide and hydrogen feed streams 2, 8 are heated separately so that they do not react in the electric preheater. The heated tails gas, hydrogen and CO2 streams are then combined in a refractory lined pipe 14, 68 at the inlet to the reverse water-gas shift reactor 18. There is no CO formation in the individual hydrogen and CO2 streams. As such, feed piping and electric heaters can be design with standard / lower cost metallurgy suitable for the operating conditions and gas compositions. This configuration does require some additional equipment when compared to the configuration of Figure 3. While the configuration of Figure 3 includes a single interchanger and heater for the mixed H2 / CO2 feed, the configuration of Figure 5 requires two interchangers and two heaters for the H2 / CO2 feeds, i.e., an interchanger 64 and electric heater 4 for the CO2 stream and an interchanger 66 and electric heater 10 for the hydrogen stream. The configuration of Figure 5 also requires the pipe 14, 68 to be refractory lined. In other respects, the flow sheet configuration of Figure 5 corresponds to that of Figure 3 and like parts are labelled with like reference numerals. While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method of forming a syngas, the method comprising:providing a first feed gas stream comprising carbon dioxide;providing a second feed gas stream comprising hydrogen;heating the first and / or second feed gas streams;after heating, mixing the first and second feed gas streams in a mixing zone which is either lined with a refractory ceramic material and / or formed of a nickel-chromium alloy to form a heated mixed feed gas stream comprising carbon dioxide and hydrogen; andfeeding the heated mixed feed gas stream from the mixing zone into a reverse water-gas shift reactor comprising a reverse water-gas shift catalyst to form a crude syngas product stream by converting at least a portion of the carbon dioxide to carbon monoxide,wherein the mixing zone and the reverse water-gas shift reactor are connected via a connector tube lined with a refractory ceramic material and / or formed of a nickel-chromium alloy or the mixing zone is formed of a connector tube to the reverse water-gas shift reactor.

2. A method according to claim 1,wherein the first and second feed gas streams are both heated by separate heaters.

3. A method according to any preceding claim,where the first and / or second feed gases are heated and fed to the mixing zone using heaters and / or feed tubes which are not lined with refractory ceramic material or formed of a nickel-chromium alloy.

4. A method according to any preceding claim,wherein the mixing zone is lined with a refractory ceramic material which is an oxide, a carbide or a nitride of one or more of the following elements: silicon, aluminium, magnesium, calcium, boron, chromium and zirconium.

5. A method according to any one of claims 1 to 3,wherein the mixing zone is formed of a nickel-chromium alloy which is selected from a nickelchromium only alloy, a nickel-chromium-aluminium alloy, or a nickel-chromium-copper alloy.6.A method according to claim 5,wherein the nickel-chromium alloy is selected from one or more of alloy 310, 310S, 310Si, 800 / H / HT, RA 330, alloy 600 / H, 625, 890, RA330, RA230, Alloy X, CT15C, Paralloy CR39W, H39WMR, H39WM+, HK40, HPW, HPNb, HP Microalloy, alloy 601, 602 CA, 603XL, 617, 671, 690, 693, 803, NEXAGE™696, VDM® ALLOY 699 XA, HAYNES® HR-235®, HR214, HR120, HR160, H46M, RA253MA, RA333, RA353MA, 45TM, Paralloy OPTIM-AL, H46M, or H48T.

7. A method according to claim 5 or 6,wherein the nickel-chromium alloy comprises a coating.

8. A method according to claim 7,wherein the coating is an aluminium diffusion coating, a chrome diffusion coating, a silicon coating, or a catalytic coating.

9. A method according to any preceding claim,wherein a corrosion inhibitor is added to one or more of the feed streams prior to the mixing zone.

10. A method according to claim 9,wherein the corrosion inhibitor is a sulphur-based corrosion inhibitor.

11. A method according to any preceding claim,wherein one or both of the first and second feed gases are heated to a temperature of: at least 300°C, 400°C, 450°C, 500°C, 600°C, or 700°C; no more than 1500°C, 1300°C, 1200°C, 1100°C,1000°C, 900°C, or 850°C; or within a range defined by any combination of the aforementioned lower and upper limits.

12. A method according to any preceding claim,wherein one or more of the first feed gas, the second feed gas, the heated carbon dioxide feed gas stream, the heated hydrogen feed gas stream, and the heated mixed feed gas stream have a pressure of: at least 5, 10, 15 or 20 Bar abs; no more than 60, 50, 40, or 30 bar abs; or within a range defined by any combination of the aforementioned lower and upper limits.

13. A method according to any preceding claim,wherein the heated mixed feed gas within the mixing zone is at a temperature and pressure at which a reverse water-gas shift reaction is initiated to form a mixture of carbon dioxide, hydrogen, carbon monoxide and water.

14. A method according to any preceding claim,wherein the first and / or second feed gases are heated using a heater type selected from: a heat exchange heater; a turbo-machinery heater; an electrical heater; a resistive heater; and a radiant heater.

15. A method according to any preceding claim,wherein the reverse water-gas shift reactor is an autothermal reverse water-gas shift reactor, an electrically heated reverse water-gas shift reactor, or an adiabatic reverse water-gas shift reactor.

16. A method according claim 15,wherein the reverse 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 heated mixed feed gas with an oxygen stream in a burner in the reverse water-gas shift reactor.

17. A method according to claim 16,wherein the burner comprises a nozzle lined with a refractory ceramic material.

18. A method of any preceding claim, further comprising:passing the crude syngas to a water removal unit to form a separated water stream and a water-depleted syngas; andfeeding the separated water stream into an electrolyser to produce hydrogen which is recycled into the second feed gas comprising hydrogen.

19. A method according to claim 18,wherein the electrolyser produces oxygen which is fed to a burner of the reverse water-gas shift reactor.

20. A method of any preceding claim, further comprising:passing the crude syngas to a carbon dioxide removal unit to form a separated carbon dioxide stream and a carbon dioxide-depleted syngas; andrecycling the separated carbon dioxide stream into the first feed gas comprising carbondioxide.

21. A method according to any preceding claim,wherein the syngas is passed from the reverse water-gas shift reactor to a Fischer-Tropsch unit to produce a hydrocarbon product stream and a tail gas stream, and wherein the tail gas stream is recycled into the reverse water-gas shift reactor.

22. A method according to claim 21,wherein the tail gas stream is mixed with one or more of the first feed gas stream, the second feed gas stream, and the heated mixed feed gas stream.

23. A system for performing the method according to any preceding claim, the system comprising:one or more heaters for heating a first feed gas stream comprising carbon dioxide and / or a second feed gas stream comprising hydrogen;a mixing zone lined with a refractory ceramic material and / or formed of a nickel-chromium alloy for mixing the first and second feed gas streams after heating to form a heated mixed feed gas stream comprising carbon dioxide and hydrogen; anda reverse water-gas shift reactor comprising a reverse water-gas shift catalyst, the reverse water-gas shift reactor being configured to receive the heated mixed feed gas stream from the mixing zone and pass the heated mixed feed gas stream over the reverse water-gas shift catalyst within the reverse water-gas shift reactor to form a crude syngas product stream by converting at least a portion of the carbon dioxide to carbon monoxide,wherein the mixing zone and the reverse water-gas shift reactor are connected via a connector tube lined with a refractory ceramic material and / or formed of a nickel-chromium alloy or the mixing zone is formed of a connector tube to the reverse water-gas shift reactor.

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