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 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.

Description

[0001]A METHOD AND SYSTEM FOR FORMING SYNGAS FieldThe present specifica^on 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 liquidhydrocarbons from the syngas. BackgroundThe Fischer–Tropsch process converts a mixture of carbon monoxide and hydrogen intoliquid hydrocarbons. These reac^ons occur in the presence of metal catalysts, typically attemperatures of 150–300°C and pressures of one to several tens of atmospheres. The Fischer–Tropsch process involves a series of chemical reac^ons that produce a variety of hydrocarbons, ideally havingthe formula (CnH2n+2). The more useful reac^ons produce alkanes as follows: where n may be 1–100, or higher. The forma^on of methane (n = 1) is unwanted. In addi^on to alkaneforma^on, compe^ng reac^ons give small amounts of alkenes, as well as alcohols and otheroxygenated hydrocarbons. The Fischer–Tropsch reac^on is a highly exothermic reac^on due to astandard reac^on enthalpy (ΔH) 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 gasifica^on or carbon dioxide and hydrogen via a reverse water-gas shi^ reac^on. The reverse water-gas shi^ reac^on may be depicted as follows: H2 + CO2 ⇌ CO + H2OSyngas genera^on using a reverse water-gas shi^ reac^on can be beneficial since it makes use ofcarbon dioxide that may have been des^ned 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 water- gas shi^ vessel and combus^ng 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 shi^ 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 shi^ vessel, and a gas comprising carbon monoxide and hydrogen.WO2022079408 also discloses that the gas streams fed to the reverse water-gas shi^ vessel may bepreheated. The pre-heat temperature of the feed gases to the reverse water- gas shi^ vessel may bein the range of 400 to 1000°C or 450 to 800°C to sustain combus^on. The hydrogen and carbon dioxide streams may be premixed before prehea^ng or preheated and mixed. Prehea^ng of the feeds to their pre-heat temperatures may be done by interchange with the crude product gas mixture, and / or by steam hea^ng, or by using afired heater or by electrical hea^ng or by a combina^on 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 specifica^on is concerned with providing an improved method and system for producingsyngas and par^cularly focuses on the pre-hea^ng of feed gases to the reverse water-gas shi^ reactor.SummaryAs described in the background sec^on, WO2022079408 teaches that it can be advantageous to pre-heat the carbon dioxide and hydrogen feed gases to a reverse water-gas shi^ reactor and that thecarbon dioxide and hydrogen streams may be premixed before prehea^ng or preheated and mixed.From a produc^on system perspec^ve, the simplest, and seemingly preferable, approach is to mix thehydrogen and carbon dioxide feed gases and then preheat the mixture of gases before passing themixture of feed gases into the reverse water-gas shi^ reactor. However, it has surprisingly been foundthat the mixture of carbon dioxide and hydrogen feed gases may be more reac^ve than previouslybelieved under certain pre-hea^ng condi^ons and that a reverse water-gas shi^ reac^on can beini^ated, at least to some extent, when pre-hea^ng the mixed feed gas prior to entering the reversewater-gas shi^ reactor. This can be problema^c as such a reac^on may generate a CO / H2 / CO2 / H2Obearing gas in the pre-heater equipment and pre-reactor pipe work and such as gas mixture candegrade standard pre-heater equipment and pipework via materials damage mechanisms such ascarburisa^on and metal dus^ng. This can lead to a reduc^on in the life^me of these parts and requiresuch parts to be replaced leading to increased equipment costs and plant down^me.The aforemen^oned problem can be exacerbated by metallic construc^on materials catalysing thereverse water-gas shi^ reac^on in the pre-heater(s) and / or pipework prior to the reverse water-gasshi^ reactor. For example, hea^ng the mixed gas feed over a large metallic heat exchange surface in a pre-heater can catalyse the reverse water-gas shi^ reac^on.Furthermore, pre-hea^ng the mixed gas feed may also lead to an exothermic methana^on reac^on,emi^ng heat and resul^ng in a rise in temperature. Methana^on can also be catalysed by metallicconstruc^on materials and can lead to a runaway reac^on when the heat release is faster than theheat being removed. This can cause the temperature to rise and exceed the design temperature of thepre-heater or pipework causing equipment damage, resul^ng in down^me.One solu^on to these problems is to pre-heat the carbon dioxide and hydrogen gas mixture at a lowertemperature and / or pressure to avoid the ini^a^on of a reverse water-gas shi^ (and / or methana^on) reac^on prior to entering the reverse water-gas shi^ reactor. However, this approach can reduce the efficiency of the reverse water-gas shi^ system. Another approach is to use a pre-heater which comprises parts formed of alloys which are moreresistant to degrada^on by a CO / H2 / CO2 / H2O bearing gas. However, parts formed of such alloys tendto be expensive and more difficult to procure leading to longer lead ^mes, more expensiveconstruc^on, and less availability of spares. Addi^onally, such parts may s^ll have a limited life^me due to eventually succumbing to carburisa^on and / or metal dus^ng. Yet another approach is to separately pre-heat the carbon dioxide and hydrogen feed gases which was envisaged as an alterna^ve approach in WO2022079408. However, if these separately heated feed gas streams are then mixed prior to entering the reverse water-gas shi^ reactor then a reverse water- gas shi^ reac^on may occur in a mixing zone prior to entering the reverse water-gas shi^ reactor. To avoid this problem the separately pre-heated carbon dioxide and hydrogen feed streams could be separately fed into the reverse water-gas shi^ reactor and only mixed within the reactor. However,this would require a modifica^on to the reverse water-gas shi^ reactor design and may lead to sub-op^mal mixing of reactants and / or increased complexity of the reactor.In light of the above, the present specifica^on provides an alterna^ve / improved approach whichavoids at least some of the problems as outlined above. The method comprises: providing afirst feed gas stream comprising carbon dioxide; providing a second feed gas stream comprising hydrogen; hea^ng thefirst and / or second feed gas streams; a^er hea^ng, mixing thefirst 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 shi^ reactor comprising a reverse water-gas shi^ catalyst to form a crude syngas product stream by conver^ng at least a por^on of the carbon dioxide to carbon monoxide, wherein the mixing zone and the reverse water-gas shi^ 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 shi^ reactor. The present specifica^on also provides a system for performing the aforemen^oned method, the system comprising: one or more heaters for hea^ng afirst 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 thefirst and second feed gas streams a^er hea^ng to form a heated mixed feed gas stream comprising carbon dioxide and hydrogen; and a reverse water-gas shi^ reactor comprising a reverse water-gas shi^ catalyst, the reverse water-gas shi^ 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 shi^ catalyst within the reverse water-gas shi^ reactor to form a crude syngas product stream by conver^ng at least a por^on of the carbon dioxide to carbon monoxide, wherein the mixing zone and the reverse water-gas shi^ 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 shi^ reactor. Such an approach separately heats the carbon dioxide and hydrogen feed streams to avoid reac^on of the carbon dioxide and hydrogen in a pre-heater. This approach thus avoids the risk of metal dus^ngand carburisa^on of the pre-heater(s) (feed heater) without requiring the pre-heater(s) to be formedof expensive metal dus^ng resistant alloys. This approach also avoids the risk of an exothermicmethana^on reac^on in the pre-heater(s). Furthermore, the separately heated feed streams are thenmixed within a mixing zone which is lined with a refractory ceramic material and / or formed of a nickel- chromium alloy such that the mixing zone is resistant to metal dus^ng and carburisa^on if any reverse water-gas shi^ reac^on 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 shi^ 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 shi^ 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 shi^ reactor. As such, the present method can provide asystem which is robust to degrada^on and can provide op^mal levels of feed gas pre-hea^ng at desiredgas pressures while avoiding expensive modifica^ons to the pre-hea^ng apparatus and / or the reversewater-gas shi^ reactor. This can simplify the procurement of capital equipment, increase plantlife^me, and minimise unplanned down^me by elimina^on of the risk of system materials damage.Brief Descrip^on of the DrawingsFigure 1 shows aflow sheet for a method of forming crude syngas.Figure 2 shows aflow 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 aflow sheet for a method of forming crude syngas not according to the present specifica^on.Figure 4(a) shows a graph of CO content versus temperature for a heated feed mixture of carbondioxide and hydrogen. Figure 4(b) shows the graph of Figure 4(a) extended beyond the 500oC test data to show the trend in CO forma^on up to 650oC. Figure 5 shows aflow sheet for a method of forming crude syngas according to the presentspecifica^on and illustrates how aflow sheet such as that shown in Figure 3 can be modified to achievethe present inven^on. A summary of the reference numerals used in thefigures is set out in the table below. Reference Item2 Carbon Dioxide Feed Gas4 CO2 Feed Heater6 Heated Carbon Dioxide Feed Gas8 Hydrogen Feed Gas10 H2 Feed Heater12 Heated Hydrogen Feed Gas14 Mixing Zone with Ceramic Lining16 Heated Mixed Feed Gas18 Reverse Water-Gas Shift Reactor20 Crude Syngas22 Water Removal Unit24 Separated Water Stream26 Water-Depleted Syngas28 Electrolyser30 Hydrogen Stream32 Oxygen Stream34 Carbon Dioxide Removal Unit36 Separated Carbon Dioxide Stream38 Carbon Dioxide-Depleted Syngas40 Fischer-Tropsch Unit42 Hydrocarbon Product Stream44 Tail Gas Stream46 Derichment Reactor48 Methane Stream50 Mixed feed gas52 Mixed feed gas interchanger54 Mixed feed gas heater56 Heated mixed feed gas58 Tails gas interchanger60 Oxygen feed62 Syngas boiler64 Carbon dioxide feed interchanger66 Hydrogen feed interchanger68 Refractory lined pipe / mixed hot feed gasDetailed Descrip^onFigure 1 shows aflow sheet for a method of forming crude syngas. The method comprises hea^ng afirst feed gas 2 comprising carbon dioxide using a heater 4 to form a heated carbon dioxide feed gasstream 6. A second feed gas 8 comprising hydrogen which is separately heated using a heater 10 toform a heated hydrogen feed gas stream 12. The heated carbon dioxide feed gas stream 6 and theheated 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 shi^ reactor 18 comprising a reverse water-gas shi^catalyst and the heated mixed feed gas stream is passed over the reverse water-gas shi^ catalyst withinthe reverse water-gas shi^ reactor 18 to form a crude syngas product stream 20 by conver^ng at least a por^on of the carbon dioxide to carbon monoxide.Thefirst and second heaters for separately hea^ng the carbon dioxide and hydrogen feed streams maybe provided by separate heaters. This configura^on allows the op^on to heat the carbon dioxide and hydrogen feed streams at different temperatures. Alterna^vely, thefirst 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-hea^ng process. This can be achieved by twoseparate streams routed through the same hea^ng unit or by using two separate heaters. Analterna^ve op^on 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 a^er 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 theheated carbon dioxide and hydrogen streams 6, 12. If this vessel or pipework 14 is then coupled tothe reverse water-gas shi^ reactor 18 via a further connector tube 16, then the further connector tubeis also be lined with a refractor ceramic material (and / or formed of a nickel-chromium alloy).Alterna^vely, the mixing zone 14 is formed within the connector tube 16 to the reverse water-gas shi^reactor 18 without the need for a separate mixing vessel. As described in the summary sec^on, a key feature of the present specifica^on is that the carbon dioxide and hydrogen feed streams are separately heated and then mixed in a zone which has arefractory ceramic lining (and / or is formed of a nickel-chromium alloy). The heated mixed feed gaswithin the mixing zone is at a temperature and pressure at which a reverse water-gas shi^ reac^on is ini^ated 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 degrada^on 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 degrada^on. For example, one or more components ofthe reverse water-gas shi^ reactor can be lined with a refractory ceramic material and / or made of analloy which is resistant to degrada^on. In this regard, reverse water-gas shi^ reactors are alreadydesigned to be resistant to damage by the reverse water-gas shi^ reac^on. A key feature of certainconfigura^ons of the present specifica^on is that they can be implemented using standard heaters andreactors while avoiding the problems of prior configura^ons if the pre-hea^ng is sufficient to ini^atethe reverse water-gas shi^ reac^on prior to the reactants entering the reverse water-gas shi^ reactor.As described in the summary sec^on, an alterna^ve to the aforemen^oned approach is to mix the carbon dioxide and hydrogen prior to hea^ng, and then keep the temperature of the gas mixture sufficiently low that the reverse water-gas shi^ reac^on is not ini^ated prior to entry into the reverse water-gas shi^ reactor. However, this leads to inefficiencies within the reverse water-gas shi^ reactor. Another alterna^ve the aforemen^oned approach is to mix the carbon dioxide and hydrogen prior to hea^ng, and then use a heater which is especially configured to be resistant to degrada^on by a gas mixture comprising carbon dioxide, hydrogen, carbon monoxide and water which is formed by the onset of the reverse water-gas shi^ reac^on. However, such an approach requires such a heater to be manufactured from expensive metal dus^ng resistant alloys and / or ceramic coated components. The present approach is advantageous as it enables more standard hea^ng equipment to be used to heatthe feed gases while s^ll avoiding degrada^on issues with the hea^ng equipment. It is much easier toprovide 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 thefirst 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 dus^ng resistant alloy (e.g., a nickel-chromium alloy such as a nickel-chromium only alloy, a nickel-chromium-aluminium alloy, or a nickel-chromium-copper alloy).For configura^ons 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 degrada^on by amixture of carbon dioxide, hydrogen, carbon monoxide and water at the process temperatures andpressures required for op^mal process gas feed to a reverse water-gas shi^ reactor. For example, therefractory ceramic material can be an oxide, a carbide or a nitride of one or more of the followingelements: silicon, aluminium, magnesium, calcium, boron, chromium and zirconium. For configura^ons in which the mixing zone is formed of a metal dus^ng resistant alloy, such as a nickel-chromium alloy, then the nickel-chromium alloy can be selected from a nickel-chromium only alloy, anickel-chromium-aluminium alloy, or a nickel-chromium-copper alloy. For example, the nickel-chromium alloy can be selected from one or more of alloy 310, 310S, 310Si, 800 / H / HT, RA 330, alloy600 / 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 mixingzone, or the alloy may form a coa^ng with another metal providing mechanical support for pressurecontainment in the mixing zone.Furthermore, the nickel-chromium alloy may comprise a coa^ng to increase corrosion resistance.Coa^ngs may be selected from an aluminium diffusion coa^ng, a chrome diffusion coa^ng, a siliconcoa^ng (e.g., a silicon CVD coa^ng), or a cataly^c coa^ng.In order to further decrease corrosion, a corrosion inhibitor can be added to one or more of the feedstreams prior to the mixing zone. For example, the corrosion inhibitor may be a sulphur-basedcorrosion inhibitor such as H2S or a disulphide. Such reagents form a protec^ve sulphur atmosphereon the metallic surface that preferen^ally prevents carbon from being absorbed onto the surface. Accordingly, for increased corrosion protec^on, a low level of a reac^ve sulphur species such as H2S ora disulfide may be provided in the process environment. Sulphur levels less than 10 ppm (e.g., in arange 1 to 10 ppm) are sufficient to mi^gate metal dus^ng.In terms of process gas condi^ons, one or both of thefirst and second feed gases can be heated to atemperature of: at least 300oC, 400oC, 450oC, 500oC, 600oC, or 700oC; no more than 1500oC, 1300oC, 1200oC, 1100oC,1000oC, 900oC, or 850oC; or within a range defined by any combina^on of the aforemen^oned lower and upper limits. Furthermore, one or more of thefirst 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 combina^on of the aforemen^oned lower and upper limits. The heaters used to heat thefirst and second feed gases may be of any suitable type to achieve therequired opera^ng temperatures. For example, thefirst and / or second feed gases can be heated usinga heater type selected from: a heat exchange heater; a turbo-machinery heater; an electrical heater;a radiant heater; or a resis^ve heater. Heat source op^ons for the heaters may include geothermal,nuclear (fusion / fission), rotodynamic hea^ng, concentrated solar, infrared, microwave, radio wave,induc^on, resistance, electric arc, plasma, microwave, ultrasonic, or industrial waste heat, or any ofthe above via a thermal storage medium such as molten salt or high temperature thermal store.The reverse water-gas shi^ reactor can be an autothermal reverse water-gas shi^ reactor in whichfurther hea^ng to the reac^on temperature is provided by combus^on of a por^on of the heated mixed feed gas with an oxygen stream in a burner in the reverse water-gas shi^ reactor. In this case, advantageously the burner may comprise a nozzle which is also lined with a refractory ceramicmaterial. Alterna^vely, the reverse water-gas shi^ reactor can be an electrically heated reactor or anadiaba^c reactor. For example, the feed gas may be pre-heated completely to reac^on temperature(e.g., electrically) before being passed over the catalyst in the reverse water-gas shi^ reactor.Figure 2 shows aflow sheet for a method of forming a hydrocarbon product stream by combining thesyngas forming method of Figure 1 with a Fischer-Tropsch unit. The crude syngas product stream 20is produced in the same manner as described above in rela^on to Figure 1 and for conciseness thesesteps / parts will not be repeated. The crude syngas from the reverse water-gas shi^ 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-depletedsyngas 26. The separated water stream 24 can be fed into an electrolyser 28 to produce a hydrogenstream 30 which is recycled into the second feed gas 8 comprising hydrogen. The electrolyser 28 canalso produce an oxygen stream 32 which is fed to the burner of the reverse water-gas shi^ 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, alterna^vely 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 op^onally be heated separately from the second feed gas 8.The crude syngas, advantageously a^er removing water, is fed to a carbon dioxide removal unit 34 toform a separated carbon dioxide stream 36 and a carbon dioxide-depleted syngas 38. The separatedcarbon dioxide stream 36 can be recycled into thefirst feed gas 2 comprising carbon dioxide. In Figure 2 the carbon dioxide stream 36 is recycled into thefirst feed gas 2 comprising carbon dioxide upstreamof the heater 4. However, alterna^vely the carbon dioxide stream 36 could be recycled into the feedgas downstream of the heater 4. For example, the carbon dioxide stream 36 could be recycled directlyinto the mixing zone 14. In that case, the carbon dioxide stream 36 may op^onally be heatedseparately from thefirst feed gas 2.A^er removal of water and carbon dioxide, and op^onal further processing to remove othercontaminants, the syngas can then be passed to the Fischer-Tropsch unit 40 to produce a hydrocarbonproduct stream 42 and a tail gas stream 44. Advantageously, the tail gas stream 44 is recycled into thereverse water-gas shi^ reactor 18. In this case, the tail gas stream can be passed through a derichmentreactor 46 to form a methane containing stream 48 for feeding back into the reverse water-gas shi^reactor 18. Both reverse water-gas shi^ and steam methane reforming reac^ons can occur within thereverse water-gas shi^ 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 shi^ reactor 18. However, as an alterna^ve the methane containing stream 48 can be recycled into one of thefirst 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 modifica^ons according to the present specifica^on.Figure 3 shows an example of aflow sheet for a method of forming crude syngas which is not accordingto the present specifica^on. 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 shi^ reactor. For example, the mixedfeed gas at this stage may be at a temperature around 400oC (398oC in the illustrated example). Themixed 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 550oC.The heated mixed feed gas 56 is then passed to the reverse water-gas shi^ reactor 18.A tails gas stream 44 (e.g., recycled from a down-stream hydrocarbon synthesis unit) is passed througha tails gas interchanger 58 to heat up the tails gas using the hot syngas product stream from the reverse water-gas shi^ reactor. The tails gas is then passed through a derichment reactor 46 to produce amethane containing gas stream 48 which is mixed with the heated mixed feed gas stream 56. In theillustrated example, the methane containing gas stream from the derichment reactor is at atemperature of 530oC, the heated mixed H2 / CO2 gas stream is at 550oC, and when the two streams are mixed and fed to the reverse water-gas shi^ reactor the feed gas is at a temperature of 548oC. It may be noted that the hea^ng 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 shi^ reactor 18 and combusted to further increase the temperature of the reactants to drive the reverse water-gas shi^ reac^on and produce acrude syngas product stream. In the illustrated example the crude syngas exi^ng the reactor is at atemperature of 900oC. The hot crude syngas can be used to raise steam in a syngas boiler 62 and isthen passed through the tails gas interchanger 58 and the H2 / CO2 feed gas interchanger 52.The present inventors have iden^fied that the mixed feed gas heater 54 and the gas line 56 from the heater 54 to the reverse water-gas shi^ reactor 18 may be suscep^ble to damage under certainopera^ng condi^ons and over prolonged periods of opera^on (highlighted region in Figure 3). This isdue to the poten^al for CO forma^on in this region causing metal degrada^on.In rela^on to the above, experiments have been performed to determine how much CO may be formedwhen pre-hea^ng a mixed H2 and CO2 feed prior to entering the reverse water-gas shi^ reactor as isthe case for theflow sheet of Figure 3. A mixed H2and CO2feed 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 highertemperature in a step-wise manner over the test temperature range. At each test temperature theamount of CO was measured. Test condi^ons 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. Theamount of CO remained rela^vely low between 300oC and 400oC, which corresponds to thetemperature range of the mixed feed gas between the feed interchanger 52 and the heater 54 in Figure3. However, CO content increases significantly as the temperature is raised to 500oC or more, whichcorresponds to the temperature of the mixed feed gas in the heater 54 and gas line 56 to the reactorin 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 500oC test data to show the trend in CO forma^onup to 650oC. As such, these experiments would suggest that, at least under these opera^ng condi^ons, CO genera^on in the heater 54 and gas line 56 could be a poten^al problem if mixed feed gas condi^ons and materials of construc^ons are not correctly managed.Furthermore, there is some evidence in the literature that certain metal alloys u^lized to manufacturereactor equipment can affect the amount of CO2to CO conversion via a reverse water-gas shi^ reac^on. For example, in “High-Temperature Kine^cs of the Homogeneous Reverse Water–Gas Shi^Reac^on” by F. Bustamante and R. M. Enick, AICHE Journal, vol.50, No.5. May 2004. pg 1039, reversewater-gas shi^ reac^ons in an Inconel 600 (a nickel-chromium alloy) reactor and a quartz reactor werecompared. The paper found that Inconel 600 catalyses the reverse water-gas shi^ reac^on comparedto an inert quartz reactor and showed some evidence of CO2 conversion at a temperature range from538oC, which is similar to the temperature of the heated mixed feed gas stream 56 in theflow-sheetof Figure 3. As such, it is considered that alloy materials which may typically be used for the feedheater 54 and gas line 56 could conceivably contribute to driving a reverse water-gas shi^ reac^on inthe heater and gas line 56 to produce CO prior to entering the reverse water-gas.Mechanisms for forma^on of carbon and metal dus^ng including the following…H2 + CO2 ⇌ CO + H₂O [CO forma^on from RWGS Reac^on]2 CO ⇌ C + CO₂ [CO forms carbon via Boudouard Carbon]CO + H2 ⇌ C + H2O [CO forms carbon via CO Reduc^on]Methane (e.g., in recycled, deriched tail gas) cracking can also form carbon. Forma^on of carbon cancause metal dus^ng, which is a form of corrosion that occurs when suscep^ble materials are exposedto environments with high carbon ac^vi^es. Metal components can corrode over ^me, poten^allyresul^ng in equipment failure. While not being bound by theory, ini^al data suggests that in thepresent case / condi^ons for the heated mixed feed gas in heater 54 and gas line 56 of Figure 3, carbonforma^on via CO reduc^on is more likely to be problema^c than carbon forma^on via the Boudouardroute. Thermodynamically, the feed gas composi^on falls in the carbon forming region via COreduc^on. This means that thermodynamically the feed gas composi^on can form carbon. However,the extent to which this may be problema^c in prac^ce will also be dependent on the kine^cs of thecarbon forming process. Below 400-450°C, the kine^cs of the process are sufficiently slow that metaldus^ng is not normally a concern. However, lower feed heater temperatures for the present syngasproduc^on process reduces the process efficiency. As such, it is desirable to pre-heat the feed gas totemperatures exceeding 500oC from a process efficiency perspec^ve.Figure 5 shows a modifiedflow sheet which addresses this issue. In thisflow sheet, the carbon dioxideand hydrogen feed streams 2, 8 are heated separately so that they do not react in the electric pre-heater. The heated tails gas, hydrogen and CO2 streams are then combined in a refractory lined pipe14, 68 at the inlet to the reverse water-gas shi^ reactor 18. There is no CO forma^on in the individualhydrogen and CO2 streams. As such, feed piping and electric heaters can be design with standard / lower cost metallurgy suitable for the opera^ng condi^ons and gas composi^ons. This configura^ondoes require some addi^onal equipment when compared to the configura^on of Figure 3. While theconfigura^on of Figure 3 includes a single interchanger and heater for the mixed H2 / CO2 feed, theconfigura^on of Figure 5 requires two interchangers and two heaters for the H2 / CO2 feeds, i.e., aninterchanger 64 and electric heater 4 for the CO2 stream and an interchanger 66 and electric heater 10for the hydrogen stream. The configura^on of Figure 5 also requires the pipe 14, 68 to be refractorylined. In other respects, theflow sheet configura^on of Figure 5 corresponds to that of Figure 3 andlike parts are labelled with like reference numerals. While this inven^on has been par^cularly 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 depar^ng from the scope of the inven^on as defined by the appended claims.

Claims

Claims1. A method of forming a syngas, the method comprising:providing afirst feed gas stream comprising carbon dioxide; providing a second feed gas stream comprising hydrogen; hea^ng thefirst and / or second feed gas streams; a^er hea^ng, mixing thefirst 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 shi^ reactor comprising a reverse water-gas shi^ catalyst to form a crude syngas product stream by conver^ng at least a por^on of the carbon dioxide to carbon monoxide, wherein the mixing zone and the reverse water-gas shi^ 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 shi^ reactor.

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

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

4. A method according to any preceding claim,wherein the mixing zone is lined with a refractory ceramic material which is an oxide, a carbideor 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 nickel-chromium 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 coa^ng.

8. A method according to claim 7,wherein the coa^ng is an aluminium diffusion coa^ng, a chrome diffusion coa^ng, a siliconcoa^ng, or a cataly^c coa^ng.

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 thefirst and second feed gases are heated to a temperature of: at least300oC, 400oC, 450oC, 500oC, 600oC, or 700oC; no more than 1500oC, 1300oC, 1200oC, 1100oC,1000oC, 900oC, or 850oC; or within a range defined by any combina^on of the aforemen^oned lower and upper limits.

12. A method according to any preceding claim,wherein one or more of thefirst 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 combina^on of the aforemen^oned 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 shi^ reac^on is ini^ated to form a mixture of carbon dioxide, hydrogen, carbon monoxide and water.

14. A method according to any preceding claim,wherein thefirst and / or second feed gases are heated using a heater type selected from: aheat exchange heater; a turbo-machinery heater; an electrical heater; a resis^ve heater; and a radiant heater.

15. A method according to any preceding claim,wherein the reverse water-gas shi^ reactor is an autothermal reverse water-gas shi^ reactor, an electrically heated reverse water-gas shi^ reactor, or an adiaba^c reverse water-gas shi^ reactor.

16. A method according claim 15,wherein the reverse water-gas shi^ reactor is an autothermal reverse water-gas shi^ reactor in which hea^ng to the reac^on temperature is provided by combus^on of a por^on of the heated mixed feed gas with an oxygen stream in a burner in the reverse water-gas shi^ 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; and feeding the separated water stream into an electrolyser to produce hydrogen which is recycledinto 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-gasshi^ 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; and recycling the separated carbon dioxide stream into thefirst feed gas comprising carbondioxide.

21. A method according to any preceding claim,wherein the syngas is passed from the reverse water-gas shi^ 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 shi^ reactor.

22. A method according to claim 21,wherein the tail gas stream is mixed with one or more of thefirst feed gas stream, the secondfeed 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 hea^ng afirst 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 thefirst and second feed gas streams a^er hea^ng to form a heated mixed feed gas stream comprising carbon dioxide and hydrogen; and a reverse water-gas shi^ reactor comprising a reverse water-gas shi^ catalyst, the reverse water-gas shi^ 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 shi^ catalyst within the reverse water-gas shi^ reactor to form a crude syngas product stream by conver^ng at least a por^on of the carbon dioxide to carbon monoxide, wherein the mixing zone and the reverse water-gas shi^ 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 shi^ reactor.