A methanation method and system
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- CERES INTELLECTUAL PROPERTY COMPANY LIMITED
- Filing Date
- 2024-07-29
- Publication Date
- 2026-06-10
AI Technical Summary
Current methanation methods and systems are inefficient in converting carbon monoxide and carbon dioxide into methane, particularly in achieving high methane yields and maintaining efficiency across varying temperatures.
A novel methanation method and system that utilizes an electrolyser with a methanation catalyst, operating at temperatures between 150°C to 650°C, to convert carbon dioxide and carbon monoxide into methane through hydrogenation, with a reverse water gas shift catalyst used to enhance carbon monoxide production at higher temperatures.
The system achieves high methane yields, with the potential to produce offgas with up to 100% methane, while optimizing energy efficiency and reducing the risk of carbon formation within the electrolyser.
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Abstract
Description
[0001] A methanation method and system.
[0002] The present invention relates to a methanation method and a system for performing methanation.
[0003] Methanation is the conversion of carbon monoxide and / or carbon dioxide into methane through hydrogenation. The following reactions describe the methanation of carbon monoxide and carbon dioxide respectively:
[0004] CO + 3H2CH4+ H2O
[0005] CO2+ 4H2CH4 + H2O
[0006] The present invention seeks to provide a novel methanation method and a novel electrolyser system that performs methanation.
[0007] According to a first aspect of the present invention there is provided a methanation method comprising providing an electrolyser system, the electrolyser system comprising an electrolyser that has at least one electrolyser cell, at least one fuel input through which fuel enters the electrolyser and at least one offgas output from which offgas exits the electrolyser, the method further comprising: supplying fuel to the at least one fuel inlet, the fuel comprising at least water (H2O) and either or both carbon dioxide (CO2) and carbon monoxide (CO); operating the electrolyser system by powering the electrolyser cell with electricity to electrolyse the fuel in the at least one electrolyser cell such that a part of the water splits into hydrogen (H2) and oxygen (O2); wherein the electrolyser is operated at a temperature at or in excess of 150 degrees C; and methanation occurs to the carbon dioxide and / or carbon monoxide in the electrolyser - i.e. to produce methane (CH4) in the electrolyser.
[0008] Methanation is the conversion of carbon monoxide and / or carbon dioxide to methane through hydrogenation - the hydrogen from the water, and / or from the formed hydrogen from the electrolysis of that water, thus combines with the carbon monoxide and / or the carbon dioxide to form methane (CH4) and water (H2O), along with heat as the hydrogenation / methanation reaction is exothermic. In typical embodiments, the offgas from the at least one offgas output is a gas mixture comprising at least methane and steam. Usually it will also include carbon monoxide, carbon dioxide and hydrogen. In some embodiments, the method comprises recirculating at least a part of any steam, carbon monoxide, carbon dioxide and hydrogen within the gas mixture back through the electrolyser.
[0009] The carbon dioxide may be provided by air. For example, the water may be steam entrained in air.
[0010] In some embodiments, the electrolyzing of the fuel also involves a part of any carbon dioxide splitting into carbon monoxide and oxygen.
[0011] In some embodiments, the electrolyser is operated at a temperature between 450 - 650 degrees C. Such temperatures encourage the methanation process and improve the efficiency of the method. Typically the electrolyser operates at a temperature not exceeding 550 degrees C as beyond that methanation may be reduced.
[0012] In some embodiments, the fuel is a mixture of steam and carbon dioxide.
[0013] In some embodiments, separate streams of steam and carbon dioxide are fed to the fuel inlet.
[0014] In some embodiments the fuel inlet has at least two separate inlets, comprising one for steam and one for carbon dioxide.
[0015] In some embodiments, there are two offgas outputs, comprising a first for the methane and steam and a second for the oxygen.
[0016] In some embodiments, the first offgas output is also for any carbon monoxide, carbon dioxide and hydrogen in the gas mixture.
[0017] In some embodiments, the gas mixture released from the at least one offgas output is passed through a gas separation process to separate at least the methane from the gas mixture.
[0018] In some embodiments, the gas separation process comprises a condensation step to condense a majority of the water out of the gas mixture.
[0019] In some embodiments, the gas separation process comprises a CO2 separation step to separate a majority of the CO2 from the gas mixture.
[0020] In some embodiments, the gas separation process comprises a H2 and / or CO separation process to separate a majority of the H2 and / or CO from the gas mixture. In some embodiments, the H2 and / or CO separation process comprises a methane separation process to separate a majority of the methane from the H2 and / or CO in the gas mixture, a majority of the CO2 and H2O having been previously been removed from the gas mixture.
[0021] The H2, H2O, CO and CO2 in the gas mixture may be referred to as unreacted gases of the methanation process.
[0022] In some embodiments, unreacted gases in the gas mixture are recirculated into the electrolyser via the one or more gas input.
[0023] In some embodiments, the electrolyser is operated at a temperature between 500 and 600 degrees C, or more preferably between 525 and 575 degrees C, inclusive.
[0024] In some embodiments, the temperature is between 500 and 550 degrees C. In some embodiments, the temperature may be up to 700 degrees C or even up to 750 degrees C.
[0025] In some embodiments, a methanation catalyst is used in the electrolyser or the at least one electrolyser cell to improve the efficiency of the methanation occurring within the electrolyser. Preferably the methanation catalyst is located at a lower temperature region of the electrolyser or stack or cells. For example, the methanation catalyst may be located at, near or towards an outlet end of the electrolyser cells. This is helpful when the electrolyser cell operates in an endothermic condition - i.e. at an under-potential or endothermic voltage. This is because methanation (CO+ 2H2 — > CH4 +2 H2O) occurs preferentially at lower temperatures
[0026] In some embodiments, the methanation catalyst is a nickel based catalyst.
[0027] In some embodiments, the electrolyser comprises a reverse water gas shift catalyst. The reverse water gas shift catalyst serves to convert carbon dioxide into carbon monoxide by combining carbon dioxide and hydrogen to form carbon monoxide and water. Preferably the reverse water gas shift catalyst is located at a higher temperature region of the electrolyser or stack or cells. For example, the reverse water gas shift catalyst may be located at, near or towards an inlet end of the electrolyser cells. This is helpful when the electrolyser cell operates in an endothermic condition - i.e. at an under-potential or endothermic voltage. This is because the reverse water gas shift reaction (CO2+H2 — > H2O+CO) has a higher yield at higher temperatures.
[0028] In some embodiments, the reverse water gas shift catalyst is made from a different material to the methanation catalyst.
[0029] In some embodiments, the reverse water gas shift catalyst is not a nickel based catalyst. In some embodiments, the methanation catalyst and / or the reverse water gas shift catalyst is / are provided elsewhere within the electrolyser system than the electrolyser cells. For example they may be provided on surfaces of manifolds within the electrolyser over which offgas flows.
[0030] The provision of a catalyst can improve the rate of methanation, and may even maintain methanation at higher or lower temperatures.
[0031] In some embodiments, the ratio of the fuel gases, and typically steam to carbon dioxide as the original source gases, is controlled to target a ratio of methane to unreacted gases of the electrolysis and methanation processes, in the offgas gas mixture, of 5% by volume or more, i.e. such that at least 5% of the total volume of the gas mixture exiting the at least one offgas output is methane. This can be measured at the temperature and pressure of the offgas mixture at the offgas output(s).
[0032] In some embodiments, the target ratio is >1 :10, preferably 10% or better (>1 :9) and more preferably between 3:7 and 1 :1 - i.e. between 30 and 50% inclusive. In some embodiments, for example, it may be at least 10% methane, at least 15% methane, at least 20% methane, at least 30% methane, at least 50% methane, or even at least 80%, 90% or 95% methane. In some fully optimized embodiments it can even produce offgas at close to 100% methane (for example between 95 and 100% methane. In other words the one or more of H2, H2O, CO and CO2 from the fuel outlet could in theory be merely a trace or minimal amount, or absent.
[0033] In some embodiments, the ratio of the fuel gases is controlled to target a ratio of methane to unreacted gases of the electrolysis and methanation processes, in the offgas gas mixture, such that at least 10% of the total volume of the gas mixture exiting the at least one offgas output is methane.
[0034] Methanation of CO with H2 is an exothermic reaction. Therefore, in some embodiments, heat from the methanation reaction is used to heat fluids entering the electrolyser, such as the water (or steam), the CO2, the CO, the recycled gases or a gas mixture).
[0035] In some embodiments, the electrolyser is a solid oxide electrolyser.
[0036] In some embodiments, the at least one electrolyser cell is part of a stack of electrolyser cells.
[0037] In some embodiments, there are one or more stacks of electrolyser cells in the electrolyser.
[0038] The cells and / or the stacks may be connected in series or in parallel. There may be a combination of the two. In some embodiments, the or each electrolyser cell is a solid oxide electrolyser cell. In other words, the electrochemically active region of the electrolyser cell is a solid oxide. A solid oxide electrolyser cell (SOEC) typically operates in the 400-900 degrees C range. These include intermediate and high temperature SOECs. For some chemistries, the cells are just operating between 400 and 700 degrees C, or more particularly in the 450-650 degrees C temperature range. Such electrolyser cells may be referred to as intermediate temperature solid oxide electrolyser cells, or IT-SOECs.
[0039] Typically the present invention uses water in the form of steam, rather than liquid water. An advantage of steam-based electrolysers is that steam electrolysis - particularly intermediate and high temperature steam electrolysis at temperatures above 400 degrees C - efficiently produces hydrogen as the high temperature environment can reduce the electric power requirements for the electrolysis of the water molecules from steam compared to electrolysis of liquid water. Additionally, the higher temperature can relatively increase the reaction activity with the electrolyser versus that of liquid water. The present invention is thus highly suitable for use with solid oxide electrolyser cells (or SOECs) operating at temperatures above 400 degrees C (generally known as intermediate temperature SOECs). It can also be used in high temperature SOECs, where the operating temperature of the cells is above 700 degrees C. Sometimes the cut-off between intermediate temperature and high temperature SOECs is 750 degrees C.
[0040] There are many possible forms of SOEC, using different electrochemically active electrolyte chemistries. For example, three well known electrolyte materials are yttria-stabilized zirconia (YSZ), scandia stabilized zirconia (ScSZ) and gadolinium doped ceria (GDC or CGO). The present invention can be used with any of these, along with many other forms of electrolyser.
[0041] Due to the operating temperature of a SOEC (usually in excess of 400 degrees C), any liquid water passing through the electrolyser cell will be vaporised into high temperature steam. More usually this occurs outside the cells - i.e. in or prior to pipework connecting to the at least one fuel input.
[0042] In some embodiments, the electrolyser cell system instead comprises a high temperature electrolyser cell with an operational stack temperature between 750 degrees C and 1100 degrees C.
[0043] According to a further aspect of the present invention there is provided an electrolyser system configured to operate using the method defined above.
[0044] In some embodiments, the electrolyser comprises a methanation catalyst. In some embodiments, the electrolyser comprises a reverse water gas shift catalyst.
[0045] According to a further aspect of the present invention there is provided an electrolyser system comprising an electrolyser that has at least one electrolyser cell, at least one fuel input and at least one offgas output, wherein the offgas output is connected to a methane separator for separating methane from offgas exiting the offgas output during use of the electrolyser system.
[0046] In some embodiments, the offgas output is directly connected to the methane separator.
[0047] In some embodiments, the offgas output is fluidly connected to the methane separator, the methane separation occurring in a downstream operation.
[0048] Byproducts from the separation (for example steam or water, carbon dioxide, carbon monoxide and / or hydrogen) may be recirculated back to the electrolyser. The electrolyser system thus may further comprise one or more connection from the methane separator to the at least one fuel input.
[0049] In some embodiments, the electrolyser is connected to a source of steam and carbon dioxide.
[0050] In some embodiments, the electrolyser is a solid oxide electrolyser.
[0051] In some embodiments, the electrolyser has an operating temperature of between 450 - 650 degrees C, inclusive.
[0052] In some embodiments, the at least one electrolyser cell is part of a stack of electrolyser cells.
[0053] In some embodiments, there are one or more stacks of electrolyser cells in the electrolyser.
[0054] In some embodiments, the methane separator is a methane separator with an operating temperature in excess of 400 degrees C.
[0055] In some embodiments, the methane separator is a methane membrane.
[0056] In some embodiments, the methane separator is a cryogenic separator.
[0057] In some embodiments, heat exchangers are provided and configured to reutilize heat from the offgas output’s gas mixture, or separated components thereof, for preheating the fuel.
[0058] The electrolyser system may be configured to operate using the above-mentioned method.
[0059] According to a further aspect of the present invention there is provided a method for generating methane using at least one electrochemical cell, the method comprising: providing at least water and carbon dioxide as inputs to the electrochemical cell; and powering the electrochemical cell with electrical energy to perform the at least partial conversion of the water into hydrogen and oxygen; the method further comprising the at least partial conversion of the carbon dioxide into carbon monoxide and water; and the at least partial conversion of the carbon monoxide into methane (CH4).
[0060] In some embodiments, the method further comprises taking output fluids from the cell, and extracting at least part of the methane from the output fluids. The methane can then be used as a product, for example for onward use.
[0061] In some embodiments, the electrochemical cell also performs at least part of the at least partial conversion of the carbon dioxide into carbon monoxide and water.
[0062] In some embodiments, a reverse water gas shift catalyst performs at least part of the at least partial conversion of the carbon dioxide into carbon monoxide and water.
[0063] In some embodiments, the electrochemical cell is an electrolyser cell.
[0064] This aspect of the present invention may be carried out using the above-mentioned electrolyser system, the electrolyser cell being the at least one electrolyser cell of the electrolyser.
[0065] This aspect of the present invention may be the methanation method of the first aspect of the present invention, the electrolyser cell being the at least one electrolyser cell of the electrolyser.
[0066] According to a further aspect of the present invention there is provided a method for operating an electrolyser system, the electrolyser system comprising at least one electrochemical cell having a fuel inlet, a fuel outlet and a fuel fluid flow path connecting the fuel inlet and the fuel outlet, the method comprising; i) providing to the fuel inlet a fuel gas containing water and a source of carbon selected from one or more of CO and CO2; ii) operating the electrolyser system by applying a current to the at least one electrochemical cell; iii) at least partially electrolysing the steam into hydrogen and oxygen; iv) reacting the hydrogen with the source of carbon in the fuel fluid flow path to produce methane; and v) exhausting a product gas containing at least 5% methane by volume and one or more of H2, H2O, CO and CO2 from the fuel outlet
[0067] In some embodiments, the target ratio is >1 :10, preferably 10% or better (>1 :9) and more preferably between 3:7 and 1 :1 - i.e. between 30 and 50% inclusive. In some embodiments, for example, it may be at least 10% methane, at least 15% methane, at least 20% methane, at least 30% methane, at least 50% methane, or even at least 80%, 90% or 95% methane. In some fully optimized embodiments it can even produce offgas at close to 100% methane (for example between 95 and 100% methane. In other words the one or more of H2, H2O, CO and CO2 from the fuel outlet could in theory be merely a trace or minimal amount, or absent.
[0068] In some embodiments, the electrochemical cell is a solid oxide electrolyser cell.
[0069] In some embodiments, the electrolyser system - preferably the at least one electrochemical cell or the fuel fluid flow path, further comprises a reverse water gas shift catalyst.
[0070] In preferred embodiments, the reverse water gas shift catalyst is situated in the fuel fluid flow path.
[0071] In preferred embodiments, the method further comprises the following step after step iii) but before step iv); iiia) passing the mixture of fuel gas and hydrogen over the reverse water gas shift catalyst.
[0072] Features of this method may be incorporated with any of the preceding methods. For example, the electrochemical cell can be the at least one electrolyser cell of the electrolyser of such methods.
[0073] This method may be carried out using any one of the previously described electrolyser systems, wherein the electrochemical cell is the at least one electrolyser cell of the electrolyser.
[0074] The present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:
[0075] Figure 1 schematically shows a typical electrolyser cell, multiples of which may be stacked in a stack;
[0076] Figure 2 shows the process carried out in a first embodiment of the present invention; and
[0077] Figure 3 shows a schematic of a possible configuration for the first embodiment. Referring first to Figure 1 , the basic structure and operation of a typical prior art electrolyser cell 11 within an electrolyser 10 of an electrolyser system 20 is shown by reference to one fuel / electrolyser cell 11 of a electrolyser cell stack 12 - hereinafter a stack. It should be noted that other ancillary components related to the electrolyser cell 11 are included in an electrolyser system 20. These usually include heat exchangers 52, 56, 88, 90 (Figure 3), heaters, valves and sensors.
[0078] The electrolyser cell 11 comprises an anode 33, a cathode 34 and an electrolyte 35. Such a structure for an electrolyser cell 11 is well known in the art. In this prior art example, the electrolysis of water will be discussed, although it is also known instead to feed carbon dioxide into the electrolyser cell.
[0079] The input fluid (i.e. the water / carbon dioxide) is at least partially electrolysed within the electrolyser cell 11 when electricity is passed across the electrolyser cell 11 . This, for example, splits at least a part of the water into its component parts - hydrogen (H2) and oxygen (O2). Carbon dioxide would instead at least in part be split into oxygen (O2) and carbon monoxide (CO).
[0080] The water will usually be in the form of steam 43, and is generally provided from a water source - which may be a steam source if less internal heating of the water is desired in the electrolyser system 20. The water / steam is passed over the cathode 34 of the electrolyser cell 11 via an inlet 41 and hot air 42 may be passed over the anode 33 via another inlet 40. To power the electrolyser cell, electrical power is applied across the electrolyser cell 11 via electric terminals / connections 36, 37 at the anode and cathode sides of the electrolyser cell 11. These terminals may be positioned adjacent to one-another on one side or end of the stack 12 of cells 11 , for example by having stacked cells in parallel and extending one terminal to the other end of the stack 12 using a bus bar, as known in the art. Via the terminals 36, 37, a voltage can be applied across the stack 12 - and thus a current through the cells 11. The application of the electrical power through an electrolyser 10 in this manner is well known in the art.
[0081] As a consequence of the electrical current, an electrolytic reaction occurs across the electrolyte 35, with oxygen ions passing across the electrolyte 35 from the cathode 34 to the anode 33, and some of the steam will brake down into hydrogen on the cathode side of the electrolyser cell 11 and oxygen is produced at the anode side.
[0082] The oxygen can be extracted via an air flow or sweep flow provided by the hot air 42, thus venting it out of an off-gas outlet 38 on the anode side of the electrolyser cell 11 . That output is generally oxygen enriched air (with the oxygen enriching the hot gas flow). The hydrogen can instead be extracted and vented out of another off-gas outlet 39 on the cathode side of the electrolyser cell 11.
[0083] The cathode side off gas typically will be mixed with the remaining steam from the input fluid, as the splitting of the steam into oxygen and hydrogen is usually only in respect of a proportion of the supplied steam. The hydrogen is thus vented as ‘wet’ hydrogen on the cathode side. Thus, the steam exiting the cathode side is hydrogen enriched, and the air exiting the anode side is oxygen enriched.
[0084] Those off-gases will usually be at a similar temperature to the operational temperature of the electrolyser cell 11 . However, the specific delta from the input temperature will depend upon the amount of electrical power supplied to the electrolyser, and the internal resistance of the cells.
[0085] Such operational characteristics of electrolyser cells, including SOECs, are well known in the art, with the input fluid being either steam or carbon dioxide. It is not conventional for both to be fed into the electrolyser cell at the same time as the intention is to generate hydrogen or carbon monoxide as the primary product of the process, with oxygen being a potentially useful byproduct for either of these. The input fluid is thus chosen as appropriate for the target output.
[0086] Due to the high operating temperature of the electrolyser cell 11 (i.e. above 100 degrees C for a steam electrolyser, and in the case of a solid oxide electrolyser cell (SOEC) it is usually in excess of 400 degrees C), the heat of the off-gases from the off-gas outlets 38, 39 is able to be usefully used by the electrolyser system 20, rather than being wasted, for example to provide at least some of the heat for generating the steam on the input side of the stack, and likewise for heating the hot air entering the electrolyser, both in advance of the inlets 40, 41 . Heat exchangers, as previously mentioned, are thus commonly provided within the electrolyser system 20.
[0087] Referring now to Figures 2 and 3, an embodiment of the present invention is shown and is discussed below.
[0088] In the present invention water and either or both carbon dioxide and carbon monoxide, are used as the input fluid(s). Usually this will be supplied at an inlet of the electrolyser as a mixture. With the present invention, instead of hydrogen and / or carbon monoxide being the target product, methane is the target product.
[0089] With the present invention, the water will be at least partially converted into hydrogen and oxygen across the electrolyser, and carbon dioxide within the input fluid may also be at least partially converted, but instead into carbon monoxide and oxygen. However, there will be an additional chemical reaction in the electrolyser - methanation. As discussed previously, methanation is the conversion of carbon monoxide and / or carbon dioxide to methane through hydrogenation. The hydrogen from the water, and / or from the formed hydrogen from the electrolysis of that water, combines with the carbon monoxide and / or the carbon dioxide to form methane (CH4) and water (H2O). In addition the reaction produces heat as the hydrogenation / methanation reaction is exothermic. As discussed previously, the following reactions describe the methanation of carbon monoxide and carbon dioxide respectively:
[0090] CO + 3H2CH4+ H2O
[0091] CO2+ 4H2CH4 + H2O
[0092] With the present invention this methanation occurs within the electrolyser system 20, and ideally within the electrolyser 10, within the stack 12 of the electrolyser 10 or within the electrolyser cells 11 themselves (i.e. within all three).
[0093] Referring first to Figure 2, the basic operation of a preferred embodiment of the present invention is shown.
[0094] As shown in Figure 2, the present invention is an electrolyser system 20 which uses an electrolyser 20 to combine electrolysis and methanation to produce methane by feeding both water and carbon dioxide (and / or carbon monoxide).
[0095] In this embodiment the process operates in a substantially closed loop system, although a steam and carbon dioxide (and / or carbon monoxide) supply line 48 is also provided fortopping up the electrolyser 10 with fresh input fluid or “fuel” (i.e. steam and carbon dioxide and / or carbon monoxide) at a first fuel input 14. This is to keep the process ongoing as the produced methane is extracted 50 from the electrolyser system 20 at the end of the process line 50.
[0096] The electrolyser 11 of this embodiment comprises a single stack 12 of electrolyser fuel cells 11 which function as electrochemical cells for electrolyzing the water. This stack 12 is shown to have seven cells 11 , although generally a stack 12 will have tens or even hundreds of cells 11 - usually all in parallel, although some may use cells 11 in series or combinations of the two. In some electrolysers 10, multiple stacks 12 may be provided, electrically connected in series or in parallel.
[0097] During operation of the electrolyser system 20, a reverse water gas shift reaction and the methanation reaction both occur in parallel to steam reforming and electrolysis inside the stack. The reverse water gas shift reaction, as discussed previously, converts carbon dioxide into carbon monoxide by combining carbon dioxide and hydrogen to form carbon monoxide and water. A catalyst may be provided to encourage this reaction - for example as a coating on the electrolyser cells, or elsewhere within the stack, such as on inlet or outlet manifolds.
[0098] Following these reactions, a product gas will be produced, and it will be a reformate mixture consisting of CO, CO2, CH4, H2 and H2O. It will also be mixed with remaining input fluid, as the reactions typically act upon only a part of the fluid passing through the electrolyser cells 11.
[0099] This product gas exits the electrolyser through an offgas outlet 46 before passing through various separation stages 100, 102, 104 - three in this example.
[0100] In this embodiment, the first separation stage is a water condensation stage involving a water condenser 100. This water condensation stage condenses at least part of the water (H2O) out of the product gas by cooling the product gas below 100 degrees C, or by passing the product gas over surfaces within the water condenser 10 that are chilled to below 100 degrees C. That condenses the water in the product gas (or at least a part of the water in the product gas) into liquid form. That liquid water can then be recirculated through the electrolyser 10 or stack 12 or electrolyser cell 11. For that purpose, the liquid water exits the water condenser 100 at a condensate outlet 106 and is channeled through a condensate return line 114 back into the electrolyser 10, the stack 12 or the electrolyser cell 11 via a recycle inlet 110. In some embodiments the water is instead recycled back through the first fuel input 14.
[0101] In this embodiment, the second separation stage is a carbon dioxide separation stage at a carbon dioxide separation unit 102. This may be carried out using known techniques, including membrane separation techniques, air separation unit (ASU) separation techniques and / or pressure swing adsorption (PSA) separation techniques. For example, the method may use cryogenic distillation. That carbon dioxide can then be recirculated through the electrolyser 10 or stack 12 or electrolyser cell 11. Forthat purpose, the carbon dioxide exits the carbon dioxide separation unit 102 at CO2 outlet 108 and is channeled through a p (it can be the same one as for the water, including through the first fuel input 14, or a separate one).
[0102] In this embodiment, the third separation stage is a methane separation stage at a methane separation unit 104. This may likewise be carried out using known techniques, including membrane separation techniques, air separation unit (ASU) separation techniques, and / or pressure swing adsorption (PSA) separation techniques. For example, the method may use cryogenic distillation.
[0103] In some embodiments, the carbon dioxide removal and the methane removal may be carried out in the same process, albeit at different stages / temperatures thereof. However, as will be discussed in relation to the specific embodiment of Figure 3, the separation temperatures for the carbon dioxide and the methane of stages 2 and 3 can be rather different, so separate separation units 102, 104 are preferred in this instance, and are thus so provided in this example.
[0104] That methane can then be collected for distribution at the end of the process 50, whereas the byproducts (i.e. the remaining fractions of the original product gas - hydrogen and carbon monoxide) can be recirculated through the electrolyser 10 or stack 12 or electrolyser cell 11. For that purpose, the byproducts exit the methane separation unit 104 at byproduct outlet 112 and are channeled through a byproduct return line 118 back into the electrolyser 10, the stack 12 or the electrolyser cell 11 via a recycle inlet (it can be the same one 110 as for the water or the carbon dioxide, including through the first fuel input 14, or a separate one).
[0105] It should be appreciated that the order of these three separation stages may be able to be changed, dependent upon the methods used for the separation of the component gases of the product gas, although this discussed order is preferred since it removes potentially the largest volumes of impurities first in each stage making downstream stages, and since when using cooling technologies for the various separations, this is the order by which the respective gases separate, as will be discussed with reference to Figure 3. This thus makes the process more energy efficient.
[0106] In particular, the carbon dioxide and methane separations are best done at below 100 degrees C, even when using membrane separation technologies, as the water is then easy to separate, and below 0 degrees C it is even easier. Note too that when using ASU technologies, these are mostly below 0 degrees C anyway, and thus will already be being done in the absence of liquid or gaseous water. As a result, any remaining water will be frozen and thus simple to extract.
[0107] Referring next to Figure 3, a more specific configuration for the embodiment of Figure 2 is shown. In this example, the product gas again exits the electrolyser 11 at the offgas output 46. That product gas exits the electrolyser 11 at a temperature substantially corresponding to the operating temperature of the electrolyser 11.
[0108] To utilize that heat for heating the fluids that enter the electrolyser 11 at the recycle inlet 110 (or the first fuel input 14), that product gas passes through a first heat exchanger 52, which may be thermally connected to paired heat exchangers 88, 90 on either or both the CO2 return line 116 or the byproduct return line 118. The heat of the product gas thus can heat the recycling carbon dioxide or the recycling carbon monoxide and hydrogen. It may also or instead connect to the infeed fluid via a further heat exchanger - not shown.
[0109] The product gas exiting the first heat exchanger will now be somewhat cooled, and some of the water may condense out of the product gas. That passes via a condensate return line 114 and a recycle inlet 110 back into the electrolyser - for recycling. The product gas then passes through a first compressor 54. In this example, the pressure is raised to 20Barg (i.e. gauge pressure), but it may be higher or lower than that. This heats the product gas, and to utilize that heat the product gas then passes through a second heat exchanger 56. That heat can thus then similarly be reutilized by the electrolyser system for one of the fluid infeeds for the electrolyser system 20.
[0110] This second heat exchanger 56 cools the product gas sufficiently to condense the rest of the water out of the product gas (or at least most of it), and the condensed water thus can exit that second heat exchanger 56 via the condensate outlet 106 for recirculating into the electrolyser 11 via the condensate return line 114 and the recycle inlet 110 - alongside any that was already returned.
[0111] In this embodiment the first and second heat exchangers 52, 56 and the first compressor 54 may form the water condenser 100.
[0112] The now dryer (or dry) product gas exits the second heat exchanger and passes through a distribution line to a first cryogenic heat exchanger 58. This cryogenic heat exchanger is a contraflow arrangement to maximize the heating of the returning - in this case - carbon dioxide (from the carbon dioxide separation unit 102) and to maximize the cooling of the product gas.
[0113] The product gas exits the first cryogenic heat exchanger 58 at approximately -15 degrees C in this embodiment, although this temperature may be higher or lower than this - albeit preferably a temperature below zero degrees C. That cold product gas then flows through a further distribution line to a second cryogenic heat exchanger 60, which cools the gas further - in this embodiment to -25 degrees C, although this temperature may be higher or lower than this, and can depend upon the pressure to which the product was compressed by the first compressor 54, or its pressure at this part of the process - if not held at that pressure.
[0114] This second cryogenic heat exchanger 60 is also a contraflow arrangement to maximize the temperature variation between the product gas’s inflow and outflow. The other side of the second cryogenic heat exchanger 60 received fluid from a refrigerant circuit 120. The refrigerant circuit will be further described below.
[0115] A first valve 70 then controls the distribution of the product gas into a first separation chamber 68, in which the carbon dioxide is separated from the remaining fractions of the product gas, with the carbon dioxide exiting at the CO2 outlet 108 and returning to the electrolyser 11 via the CO2 return line 116, the first paired heat exchanger 90 and the recycle inlet 110. A second valve 72 can be used for controlling this flow. The remaining fractions of the product gas, following the separation of the carbon dioxide therefrom, then passes out of the first separation chamber 68 and through a second compressor 66. This second compressor 66, in this embodiment, increases the pressure of the product gas even further - in this embodiment to 30 Barg, but it may be higher or lower than that. This increases the temperature of the product gas. To utilize that heat, the product gas then passes through a third cryogenic heat exchanger 84. This is again a counterflow heat exchanger to maximize the temperature variation between the product gas’s inflow and outflow, and correspondingly to fluid on the other side of this third cryogenic heat exchanger 84 - which fluid is the byproducts of the third separation stage.
[0116] The remaining fractions of the product gas is thus cooled by the third cryogenic heat exchanger 84. In this embodiment, the fluid is cooled to around -50 degrees C, although this temperature may be higher or lower than this, and can depend upon the pressure to which the product was compressed by the second compressor 66. Those cooled remaining fractions then pass through another distribution line to a fourth cryogenic heat exchanger 74. This allows the remaining fractions to be further cooled - in this embodiment to around -100 degrees C, although this temperature may be higher or lower than this, and can depend upon the pressure to which the product was compressed by the second compressor 66. Providing this cooling - as per the second cryogenic heat exchanger 66, is the refrigerant circuit 120.
[0117] The refrigerant circuit 120 comprises a coolant distribution line between the fourth cryogenic heat exchanger 74 and the second cryogenic heat exchanger 60, a further coolant distribution line with a refrigerant heat exchanger 64 and a compressor or pump 62, and a return loop through the fourth cryogenic heat exchanger 74.
[0118] Fluid pumping or circulating around the refrigerant circuit 120 passes through a first circuit path in the fourth cryogenic heat exchanger 74. The refrigerant circuit 120 then loops back through a second circuit path in the fourth cryogenic heat exchanger 74. A valve 76 controls that flow. These flow paths are arranged in counterflow. The second circuit path is also in counterflow to a third circuit path through the fourth cryogenic heat exchanger 74. This third circuit path is one through which the remaining fractions flow.
[0119] The refrigerant heat exchanger 64 extracts heat from the refrigerant fluid exiting the second cryogenic heat exchanger 60 (i.e. at c. -15 degrees C in this embodiment) to cool it to -100 degrees C - ready for cooling the remaining fractions in the fourth cryogenic heat exchanger 74 to -100 degrees C, as above. That heat can be used to heat the other of the two paired heat exchangers 88, 90, assuming heat exchanger 56 is already thermally connected to one of them. Each of heat exchangers 52, 56 and 64 may thus be utilized to put heat into the fluids passing through heat exchangers 88 and 90. The remaining fractions can then exit the fourth cryogenic heat exchanger 74 (in this embodiment at -100 degrees C and about 30 Barg) and can enter a second separation chamber 80. Here the methane is separated out of the remaining fractions, and the final fractions can be returned to the electrolyser 11 via a further valve 82, the third cryogenic heat exchanger 84, the byproduct return line 118 and the other of the paired heat exchangers 88. The methane on the other hand - in liquid form at this stage - or easily compressible into liquid form - can be collected in a container 86 for subsequent use elsewhere. For example, liquid methane may be separated by this cryogenic cooling to -100 degrees C, and then to about -160 degrees C for liquefying in the container, with the remaining fractions of the original product gas - i.e. the hydrogen and the carbon monoxide, then also being recirculated to the electrolyser 11 .
[0120] With the present invention, a big advantage is a “process-intensified” system in which a methanation reactor is not required. Furthermore, the exothermic nature of the methanation reaction can be used as a direct energy input into the stack, partially replacing the electrical energy requirement for maintaining heat in the cells - particularly when using thermoneutral voltages in the stack.
[0121] Under optimum use, the fuel infeed ratio (H2O:CC>2), and oxidant utilization, along with the stack temperature, may be optimised to minimize, reduce or avoid risk of carbon formation within the electrolyser or stack, and to maximize, increase or improve methane yield.
[0122] Due to the temperatures of the methane separation, the methane may be already in liquid form, or can be simply put into liquid form using pressure. Higher pressure through the whole process can also favour the methanation reaction and improve the yield per pass. It laos reduces the burden for liquefying the methane.
[0123] The present invention has particular application to electrolysers 10 in the intermediate and high temperature electrolyser cell sectors. In some embodiments the at least one electrolyser cell in the stack is a solid oxide electrolyser cell, i.e. the electrochemically active region is a solid oxide. A solid oxide electrolyser cell (SOEC) typically operates in the 400-900 degrees C range, or for some chemistries, 400 to 700 degrees C, or more particularly in the 450-650 degrees C temperature range. Such electrolyser cells may be referred to as an intermediatetemperature solid oxide electrolyser cell, or IT-SOEC.
[0124] The present invention operates on an electrolyser system 20 that is converting any liquid water into steam, and an electrolyser 11 within that system 20 that is converting that steam into hydrogen and oxygen. An advantage of steam-based electrolysers 11 is that steam electrolysis - particularly intermediate and high temperature steam electrolysis at temperatures above 400 degrees C - efficiently produces hydrogen as the high temperature environment can reduce the electric power requirements for the electrolysis of the water molecules from steam compared to electrolysis of liquid water. Furthermore, these temperatures produce low resistance in the cells. Additionally, the higher temperature can relatively increase the reaction activity with the electrolyser versus that of liquid water. The present invention is thus highly suitable for use with solid oxide electrolyser cells (or SOECs) operating at temperatures above 400 degrees C (generally known as intermediate temperature SOECs, or instead high temperature SOECs if above 750 degrees C.
[0125] As previously discussed, there are many possible forms of SOEC, using different electrochemically active electrolyte chemistries. For example, three well known electrolyte materials are yttria-stabilized zirconia (YSZ), scandia stabilized zirconia (ScSZ) and gadolinium doped ceria (GDC or CGO).
[0126] Due to the temperature of a SOEC (usually in excess of 400 degrees C), any liquid water passing through to the electrolyser (e.g. through the condensate return line 114) will be vaporised into high temperature steam upon or prior to entry into the electrolyser or stack thereof, although typically the various heat exchangers, and any other fluid temperature control systems (e.g. trim heaters) for controlling the temperature of the fluids for entering the electrolyser cell stack, will ensure that any liquid water is already converted into superheated steam prior to it entering the stack.
[0127] In some embodiments, the electrolyser cell system instead comprises a high temperature electrolyser cell with an operational stack temperature between 750 degrees C and 1100 degrees C.
[0128] Another beneficial aspect of the present invention in so far as it is occurring in an electrolyser cell - particularly electrolyser cell operating in an endothermic condition. The electrolysis reaction that produces the hydrogen can result in a temperature gradient across the cell (hot at the start and becoming colder at the exit when at an endothermic voltage. Thermoneutral voltages can be used to control or eliminate this, but allowing the temperature gradient can be beneficial in optimizing the yield of the product gas.
[0129] In order to convert carbon dioxide and water to methane, carbon monoxide is an important intermediate step. It is release by the reverse water gas shift reaction (CO2+H2 — > H2O+CO) and this equilibrium reaction has a higher yield at higher temperatures. Once CO is created, a second step, methanation (CO+ 2H2 — > CH4 +2 H2O), occurs preferentially at lower temperatures. Optimising catalysts coating in the high temperature region (i.e. near the inlet end of the cells) for enhancing the reverse water gas shift reaction, and methanation catalysts at the lower temperature region (i.e. near the outlet end of the cells) can utilize these characteristics. Not only can this mean that the yield of the total system can “beat” the combined thermodynamic equilibrium, but it can also represent an optimised chemical recuperation of heat, thus maximising both reactor energy efficiency and yield.
[0130] The present invention has therefore been described above purely by way of example with reference to the accompanying drawings. Modifications in detail may be made to the invention within the scope of the claims as appended hereto.
Claims
CLAIMS1. A methanation method comprising providing an electrolyser system, the electrolyser system comprising an electrolyser that has at least one electrolyser cell, at least one fuel input through which fuel enters the electrolyser and at least one offgas output from which offgas exits the electrolyser, the method further comprising: supplying fuel to the at least one fuel inlet, the fuel comprising at least water and either or both carbon dioxide and carbon monoxide; operating the electrolyser system by powering the electrolyser cell with electricity to electrolyse the fuel in the at least one electrolyser cell such that a part of the water splits into hydrogen and oxygen; wherein the electrolyser is operated at a temperature at or in excess of 150 degrees C; and methanation occurs to the carbon dioxide and / or carbon monoxide in the electrolyser.
2. The method of claim 1 , wherein the fuel is a mixture of steam and carbon dioxide.
3. The method of any one of the preceding claims, wherein separate streams of steam and carbon dioxide are fed to the fuel inlet.
4. The method of any one of the preceding claims, wherein there are two offgas outputs, comprising a first for the methane and steam and a second for the oxygen.
5. The method of any one of the preceding claims, wherein the gas mixture released from the at least one offgas output is passed through a gas separation process to separate at least the methane from the gas mixture.
6. The method of claim 5, wherein the gas separation process comprises a condensation step to condense a majority of the water out of the gas mixture.
7. The method of claim 5 or claim 6, wherein the gas separation process comprises a CO2 separation step to separate a majority of the CO2 from the gas mixture.
8. The method of any one of claims 5 to 7, wherein the gas separation process comprises a H2 and / or CO separation process to separate a majority of the H2 and / or CO from the gas mixture.
9. The method of claim 8, wherein the H2 and / or CO separation process comprises a methane separation process to separate a majority of the methane from the H2 and / or CO inthe gas mixture, a majority of the CO2 and H2O having been previously been removed from the gas mixture.
10. The method of any one of the preceding claims, wherein unreacted gasses in the gas mixture are recirculated into the electrolyser via the one or more gas input.11 . The method of any one of the preceding claims, wherein the electrolyser is operated at a temperature between 500 and 600 degrees C, or more preferably between 525 and 575 degrees C, inclusive.
12. The method of any one of the preceding claims, wherein a methanation catalyst is used in the electrolyser or the at least one electrolyser cell to improve the efficiency of the methanation occurring within the electrolyser.
13. The method of claim 12, wherein the methanation catalyst is a nickel based catalyst.
14. The method of any one of the preceding claims, wherein the ratio of the fuel gases is controlled to target a ratio of methane to unreacted gases of the electrolysis and methanation processes, in the offgas gas mixture, such that at least 10% of the total volume of the gas mixture exiting the at least one offgas output is methane.
15. The method of any one of the proceeding claims, wherein heat from the methanation reaction is used to heat fluids entering the electrolyser.
16. The method of any one of the preceding claims, wherein the electrolyser is a solid oxide electrolyser.
17. The method of any one of the preceding claims, wherein the at least one electrolyser cell is part of a stack of electrolyser cells.
18. The method of any one of the preceding claims, wherein there are one or more stacks of electrolyser cells in the electrolyser.
19. An electrolyser system configured to operate using the method according to any one of the preceding claims.
20. The electrolyser system of claim 19, wherein the electrolyser comprises a methanation catalyst.
21. The electrolyser system of claim 20, wherein the methanation catalyst is located at a lower temperature region of the electrolyser or stack or cells.
22. The electrolyser system of claim 20 or claim 21 , wherein the methanation catalyst is located at, near or towards an outlet end of the electrolyser cells.
23. The electrolyser system of any one of claims 19 to 22, wherein the electrolyser comprises a reverse water gas shift catalyst.
24. The electrolyser system of claim 23, wherein the methanation catalyst is located at a lower temperature region of the electrolyser or stack or cells.
25. The electrolyser system of claim 23 or claim 24, wherein the methanation catalyst is located at, near or towards an outlet end of the electrolyser cells.
26. An electrolyser system comprising an electrolyser that has at least one electrolyser cell, at least one fuel input and at least one offgas output, wherein the offgas output is connected to a methane separator for separating methane from offgas exiting the offgas output during use of the electrolyser system.
27. The electrolyser system of claim 26, wherein the electrolyser is connected to a source of steam and carbon dioxide.
28. The electrolyser system of claim 26 or claim 26, wherein the electrolyser is a solid oxide electrolyser.
29. The electrolyser system of any one of claims 26 to 28, wherein the electrolyser has an operating temperature of between 450 - 650 degrees C, inclusive.
30. The electrolyser system of any one of claims 26 to 29, wherein the at least one electrolyser cell is part of a stack of electrolyser cells.
31. The electrolyser system of any one of claims 26 to 30, wherein there are one or more stacks of electrolyser cells in the electrolyser.
32. The electrolyser system of any one of claims 26 to 31 , wherein the methane separator is a methane separator with an operating temperature in excess of 400 degrees C.
33. The electrolyser system of any one of claims 26 to 32, wherein the methane separator is a methane membrane.
34. The electrolyser system of any one of claims 26 to 31 , wherein the methane separator is a cryogenic separator.
35. The electrolyser system of any one of claims 26 to 31 , wherein heat exchangers are provided and configured to reutilize heat from the offgas output’s gas mixture, or separated components thereof, for preheating the fuel.
36. The electrolyser system of any one of claims 26 to 35, configured to operate the method according to any one of claims 1 to 19.
37. A method for generating methane using at least one electrochemical cell, the method comprising: providing at least water and carbon dioxide as inputs to the electrochemical cell; and powering the electrochemical cell with electrical energy to perform the at least partial conversion of the water into hydrogen and oxygen; the method further comprising the at least partial conversion of the carbon dioxide into carbon monoxide and water; and the at least partial conversion of the carbon monoxide into methane.
38. The method of claim 37, further comprising taking output fluids from the cell, and extracting at least part of the methane from the output fluids.
39. The method of claim 37 or claim 38, wherein the electrochemical cell also performs at least part of the at least partial conversion of the carbon dioxide into carbon monoxide and water.
40. The method of any one of claims 37 to 39, wherein a reverse water gas shift catalyst performs at least part of the at least partial conversion of the carbon dioxide into carbon monoxide and water.
41. The method of any one of claims 37 to claim 40, wherein the electrochemical cell is an electrolyser cell.
42. The method of claim 41 , carried out using the electrolyser system of any one of claims 20 to 35, the electrolyser cell being the at least one electrolyser cell of the electrolyser of the electrolyser system.
43. The method of claim 41, the method being the methanation method of any one of claims 1 to 19, the electrolyser cell being the at least one electrolyser cell of the electrolyser of the electrolyser system.
44. A method for operating an electrolyser system, the electrolyser system comprising at least one electrochemical cell having a fuel inlet, a fuel outlet and a fuel fluid flow path connecting the fuel inlet and the fuel outlet, the method comprising; i) providing to the fuel inlet a fuel gas containing water and a source of carbon selected from one or more of CO and CO2;SUBSTITUTE SHEET (RULE 26)ii) operating the electrolyser system by applying a current to the at least one electrochemical cell; iii) at least partially electrolysing the steam into hydrogen and oxygen; iv) reacting the hydrogen with the source of carbon in the fuel fluid flow path to produce methane; and v) exhausting a product gas containing at least 5% methane by volume and one or more of H2, H2O, CO and CO2 from the fuel outlet.
45. The method of claim 44, carried out using the electrolyser system of any one of claims20 to 31, the at least one electrochemical cell being the at least one electrolyser cell of the electrolyser.
46. The method of claim 44, the method being the methanation method of any one of claims 1 to 19, the at least one electrochemical cell being the at least one electrolyser cell of the electrolyser.