Methods for producing combustible gas from the electrolysis of water (HTE) or co-electrolysis with H2O/CO2 in the same chamber, and associated catalytic reactor and system

Abstract

The invention relates to a novel reactor design, wherein the pressurized chamber contains both a high-temperature electrolysis (HTE) reactor with elementary electrolysis cell stacking for producing either hydrogen or a synthesis gas (syngas for a H.sub.2+CO mixture) from water vapor H.sub.2O and carbon dioxide CO.sub.2, and at least one catalyst arranged at a distance and downstream of the outlet of the electrolyzer for converting the previously produced synthesis gas into the desired combustible gas, by means of heterogeneous catalysis, the synthesis gas having being produced either directly from the electrolysis reactor or indirectly by mixing the hydrogen produced with carbon dioxide CO.sub.2 injected into the chamber.

Claims

1. A process for obtaining a combustible gas chosen from methane, methanol, dimethyl ether (DME) and diesel by heterogeneous catalysis, comprising the following steps: a/ a step of high-temperature electrolysis of steam H.sub.2O performed in an electrolysis reactor housed in a leaktight chamber maintained at a given pressure, in which step a/ each cathode of the reactor is fed with steam at the given pressure; b/ a step of catalytic conversion performed in at least one reaction zone placed at a distance from and radially to the electrolysis reactor in the same chamber under pressure and containing at least one solid conversion catalyst, step b/ being performed using hydrogen H.sub.2 produced during the electrolysis step a/ and carbon dioxide CO.sub.2 injected into the space between the electrolysis reactor and the radial reaction zone; c/ a step of recovery of the combustible gas produced and of the steam not converted in step a/ and produced in step b/, in the space between said radial reaction zone and the wall(s) delimiting the chamber.

2. The process as claimed in claim 1, wherein step b/ is performed with the radial reaction zone closed on itself, being arranged concentrically around the electrolysis or co-electrolysis reactor, respectively.

3. The process as claimed in claim 1, wherein step a/ is performed at temperatures of between 600 C. and 1000 C.

4. The process as claimed in claim 1, wherein step a/ is performed at pressures of between 0 and 100 bar.

5. The process as claimed in claim 1, wherein the walls delimiting the chamber are cooled to a temperature below the water saturation temperature at the given pressure of the chamber, such that step c/ consists of a separation of the combustible gas from the condensed water in the chamber, followed by recovery of the separated combustible gas and of the condensed water by gravity on the bottom of the chamber.

6. The process as claimed in claim 1, constituting a methanation process.

7. The process as claimed in claim 1, wherein the given pressure of the chamber and of operation of the electrolysis or co-electrolysis reactor is equal to about 30 bar, the temperature at which step a/ is performed being maintained equal to about 800 C., the temperature in the radial reaction zone is maintained equal to about 400 C., the temperature of the walls delimiting the chamber is maintained below 230 C.

8. A process for obtaining a combustible gas chosen from methane, methanol, dimethyl ether (DME) and diesel by heterogeneous catalysis, comprising the following steps: a/ a step of high-temperature co-electrolysis of steam H.sub.2O and carbon dioxide CO.sub.2 performed in a co-electrolysis reactor housed in a leaktight chamber maintained at a given pressure; in which step a/ each cathode of the reactor is fed with steam H.sub.2O and carbon dioxide CO.sub.2 at the given pressure; b/ a step of catalytic conversion being performed in at least one reaction zone placed at a distance from and radially to the co-electrolysis reactor in the same chamber under pressure and containing at least one solid conversion catalyst, step b/ being performed using hydrogen H.sub.2 and carbon monoxide CO produced during the co-electrolysis step a/; c/ a step of recovering the combustible gas produced and the steam not converted in step a/ and produced in step b/, in the space between said radial reaction zone and the wall(s) delimiting the chamber.

9. The process as claimed in claim 8, wherein step b/ is performed with the radial reaction zone closed on itself, being arranged concentrically around the electrolysis or co-electrolysis reactor, respectively.

10. The process as claimed in claim 8, wherein step a/ is performed at temperatures of between 600 C. and 1000 C.

11. The process as claimed in claim 8, wherein step a/ is performed at pressures of between 0 and 100 bar.

12. The process as claimed in claim 8, wherein the walls delimiting the chamber are cooled to a temperature below the water saturation temperature at the given pressure of the chamber, such that c/ consists of a separation of the combustible gas from the condensed water in the chamber, followed by recovery of the separated combustible gas and of the condensed water by gravity on the bottom of the chamber.

13. The process as claimed in claim 8, constituting a methanation process.

14. The process as claimed in claim 8, wherein the given pressure of the chamber and of operation of the electrolysis or co-electrolysis reactor is equal to about 30 bar, the temperature at which step a/ is performed being maintained equal to about 800 C., the temperature in the radial reaction zone is maintained equal to about 400 C., the temperature of the walls delimiting the chamber is maintained below 230 C.

15. A reactor for obtaining a combustible gas chosen from methane, methanol and dimethyl ether (DME) by heterogeneous catalysis, comprising: a leaktight chamber capable of being placed under a given pressure; a reactor either for the high-temperature electrolysis of steam or for the high-temperature co-electrolysis of steam and carbon dioxide, comprising a stack of elemental electrolysis cells of SOEC type each formed from a cathode, an anode and an electrolyte intercalated between the cathode and the anode, and a plurality of electrical and fluid interconnectors each arranged between two adjacent elemental cells with one of its faces in electrical contact with the anode of one of the two elemental cells and the other of its faces in electrical contact with the cathode of the other of the two elemental cells, the electrolysis or co-electrolysis reactor being housed in the chamber and the outlet of the cathodes emerging inside the chamber; at least one porous partition placed at a distance from and radially to the electrolysis or co-electrolysis reactor in the chamber and containing at least one solid catalyst for converting syngas (H.sub.2+CO or H.sub.2+CO.sub.2) into combustible gas; at least one tube for feeding steam under pressure and, where appropriate, carbon dioxide to the cathodes of the electrolysis or co-electrolysis reactor, where appropriate, at least one tube for injecting carbon dioxide of the space between the electrolysis reactor and the porous partition; at least one tube for recovering combustible gas and/or steam, where appropriate, at least one tube for recovering water condensed on the walls delimiting the chamber, each tube passing through a wall delimiting the chamber.

16. The reactor as claimed in claim 15, wherein the porous partition is closed on itself and is placed concentrically around the electrolysis or co-electrolysis reactor.

17. The reactor as claimed in claim 15, wherein the porous partition consists of two porous metal walls, the space separating them being at least partially filled with a conversion catalyst in the form of powder or granulates.

18. The reactor as claimed in claim 17, wherein the two metal walls each consists of a sheet perforated with a plurality of holes regularly spaced both over the height and over the length of the partition.

19. The reactor as claimed in claim 15, wherein the solid conversion catalyst is based on nickel (Ni) supported on a zirconium oxide (ZrO.sub.2), or based on nickel (Ni) supported on an aluminum oxide (Al.sub.2O.sub.3), or bimetallic based on nickel (Ni) and iron (Fe) supported on an aluminum oxide (Al.sub.2O.sub.3), such as NiFe/-Al.sub.2O.sub.3, or based on nickel (Ni) supported on mixed oxides of cerium (Ce) and zirconium, such as Ce.sub.0.72Zr.sub.0.28O.sub.2.

20. The reactor as claimed in claim 15, wherein the porous partition comprises, in the solid catalyst, part of the cooling circuit suitable for cooling the catalytic reaction between the hydrogen and the carbon monoxide produced upstream in the co-electrolysis reactor or between the hydrogen produced upstream in the electrolysis reactor and carbon dioxide injected into the space between the porous partition and the electrolysis reactor.

21. The reactor as claimed in claim 15, wherein the feed tube is partly wound on itself close to the electrolysis or co-electrolysis reactor to heat the steam under pressure and, where appropriate, the carbon dioxide before feeding the cathodes.

22. The reactor as claimed in claim 15, comprising a tube for recovering the hydrogen and, where appropriate, the carbon monoxide produced at the cathodes, the recovery tube being wound on itself forming a circle and being pierced with a plurality of holes regularly distributed along the circle to homogeneously diffuse hydrogen and, where appropriate, carbon monoxide in the space between the electrolysis or co-electrolysis reactor and the porous partition arranged concentrically.

23. The reactor as claimed in claim 15, wherein the carbon dioxide injection tube is wound on itself forming a circle and is pierced with a plurality of holes regularly distributed along the circle to homogeneously diffuse carbon dioxide in the space between the electrolysis or co-electrolysis reactor and the porous partition arranged concentrically.

24. The reactor as claimed in claim 15, wherein the leaktight chamber comprises a side envelope, a lid and a base assembled with the envelope in a leaktight manner, and a first support for supporting both the electrolysis or co-electrolysis reactor and the porous partition so as to arrange them at a distance from the base and from the lid of the chamber.

25. The reactor as claimed in claim 24, comprising a second support, fixed onto the first support, to support only the electrolysis or co-electrolysis reactor so as to arrange it facing the central portion of the porous partition.

26. The reactor as claimed in claim 24, wherein the side envelope comprises a part of a circuit for cooling to a temperature below the water saturation temperature at the given pressure.

27. The reactor as claimed in claim 24, wherein the base constitutes a basin for recovering the water condensed on the lid and/or the side envelope and/or the base.

28. The reactor as claimed in claim 15, being a catalytic reforming reactor and fuel cell, the chamber not being under pressure, the combustible gas recovery tube constituting a combustible gas feed tube and the cell-stack electrolysis or co-electrolysis reactor constituting an SOFC fuel cell.

29. A system comprising: a reactor as claimed in claim 15; a heat exchanger forming a steam generator for vaporizing liquid water at the given pressure, the exchanger being placed outside the chamber.

30. The system as claimed in claim 29, wherein part of the secondary circuit of the exchanger comprises the tube for recovering the condensed water in the base.

31. The system as claimed in claim 29, wherein for obtaining a combustible gas, the circuit for cooling the porous partition constitutes the primary circuit of the heat exchanger for vaporizing the liquid water at the given pressure.

32. A process for operating a co-electrolysis reactor in accordance with claim 15, wherein steam is fed and distributed to the cathode of one of the two adjacent elemental cells and carbon dioxide is fed and distributed to the cathode of the other of the two elemental cells.

33. The operating process as claimed in claim 32, wherein an operating regime in exothermic mode is defined for the electrolysis of steam at the cathode of one of the two adjacent elemental cells and an operating regime in endothermic mode is simultaneously performed for the electrolysis of carbon dioxide at the cathode of the other of the two adjacent elemental cells, the heat evolved by the electrolysis of steam being capable of at least partly providing the heat required for the electrolysis of the carbon dioxide.

34. The operating process as claimed in claim 32, wherein an operating regime in exothermic mode is defined for the electrolysis of carbon dioxide at the cathode of one of the two adjacent elemental cells and an operating regime in endothermic mode is simultaneously performed for the electrolysis of steam at the other of the two adjacent elemental cells, the heat evolved by the electrolysis of the carbon dioxide being capable of at least partly providing the heat required for the electrolysis of the steam.

Description

DETAILED DESCRIPTION

(1) Other advantages and characteristics of the invention will emerge more clearly on reading the detailed description of examples of implementation of the invention given as nonlimiting illustrations with reference to the following figures, among which:

(2) FIG. 1 is a schematic view showing the operating principle of a high-temperature water electrolyzer;

(3) FIG. 2 is an exploded schematic view of part of a high-temperature steam electrolyzer comprising interconnectors,

(4) FIG. 3 is a view in perspective partially cutaway of a reactor according to the invention performing in the same chamber under pressure either high-temperature electrolysis of steam H.sub.2O or high-temperature co-electrolysis of steam H.sub.2O and of carbon dioxide CO.sub.2 and methanation using the gas(es) produced by the electrolysis or the co-electrolysis,

(5) FIG. 4 is a detailed view in perspective partially cutaway of the reactor according to FIG. 3,

(6) FIG. 5 is another detailed view in perspective partially cutaway of the reactor according to FIG. 3.

(7) Throughout the present application, the terms vertical, lower, upper, bottom, top, below and above are to be taken by reference relative to a reactor for obtaining a combustible gas with its chamber under pressure such that they are in vertical operating configuration. Thus, in an operating configuration, the chamber is arranged vertically with its base at the bottom and the electrolysis or co-electrolysis reactor is arranged with its cells horizontal on its dedicated support.

(8) Similarly, in the assembly of the present application, the terms inlet, outlet, downstream and upstream are to be understood with reference to the direction of circulation of the gases from their entry into the HTE electrolysis or co-electrolysis reactor or into the leaktight chamber under pressure up to their exit therefrom.

(9) It is pointed out that, in all the FIGS. 1 to 5, the symbols and arrows for feeding steam H.sub.2O, for distributing and recovering dihydrogen H.sub.2 and oxygen O.sub.2, and current, carbon dioxide CO.sub.2, for distributing and recovering carbon monoxide CO and oxygen O.sub.2 and current, and methane CH.sub.4 are shown for the purposes of clarity and precision, to illustrate the functioning of a steam electrolysis or simultaneous steam and carbon dioxide co-electrolysis reactor that are known and of a methanation reactor according to the invention.

(10) It is also pointed out that, in FIGS. 3 to 5 relating to a methanation reactor according to the invention, the recovery of oxygen O.sub.2 at the electrolyzer or co-electrolyzer outlet is not shown, for the purposes of clarity.

(11) It is also pointed out that all the electrolyzers or co-electrolyzers described are of the solid oxide type (SOEC, Solid Oxide Electrolyte Cell) operating at high temperature. Thus, all the constituents (anode/electrolyte/cathode) of an electrolysis cell are ceramic.

(12) Such constituents may be those of an SOFC fuel cell. The high operating temperature of an electrolyzer (electrolysis reactor) is typically between 600 C. and 1000 C. Preferably, in the context of the invention, a preferred range between 650 and 850 C. and more preferably between 700 and 800 C. is envisaged.

(13) Typically, the characteristics of an SOEC elemental electrolysis cell in accordance with the invention, of the cathode-supported type (CSC), may be those indicated as follows in table 2 below.

(14) TABLE-US-00002 TABLE 2 Electrolysis cell Unit Value Cathode 2 Constituent material Ni-YSZ Thickness m 315 Thermal conductivity W m.sup.1 K.sup.1 13.1 Electrical conductivity .sup.1 m.sup.1 10.sup.5 Porosity 0.37 Permeability m.sup.2 10.sup.13 Tortuosity 4 Current density A .Math. m.sup.2 5300 Anode 4 Constituent material LSM Thickness m 20 Thermal conductivity W m.sup.1 K.sup.1 9.6 Electrical conductivity .sup.1 m.sup.1 1 10.sup.4 Porosity 0.37 Permeability m.sup.2 10.sup.13 Tortuosity Current density A .Math. m.sup.2 2000 Electrolyte 3 Constituent material YSZ Thickness m Resistivity m 0.42

(15) A water electrolyzer is an electrochemical device for producing hydrogen (and oxygen) under the effect of an electrical current.

(16) In HTE high-temperature electrolyzers, the electrolysis of water at high temperature is performed using steam. The function of an HTE high-temperature electrolyzer is to convert the steam into hydrogen and oxygen according to the following reaction:
2H.sub.2O.fwdarw.2H.sub.2+O.sub.2.

(17) This reaction is performed electrochemically in the cells of the electrolyzer. As represented schematically in FIG. 1, each elemental electrolysis cell 1 is formed from a cathode 2 and an anode 4, placed on either side of a solid electrolyte 3. The two electrodes (cathode and anode) 2, 4 are electron conductors, made of porous material, and electrolyte 3 is gas-tight, an electronic insulator and an ion conductor. The electrolyte may in particular be an anionic conductor, more precisely an anionic conductor of O.sup.2 ions and the electrolyzer is then referred to as an anionic electrolyzer.

(18) The electrochemical reactions take place at the interface between each of the electron conductors and the ion conductor.

(19) At cathode 2, the half-reaction is as follows:
2H2O+4e.sup..fwdarw.2H2+2O.sup.2.

(20) At anode 4, the half-reaction is as follows:
2O.sup.2.fwdarw.O2+4e.sup..

(21) Electrolyte 3 is intercalated between the two electrodes 2, 4 and is the site of migration of the O.sup.2 ions under the effect of the electrical field created by the potential difference imposed between anode 4 and cathode 2.

(22) As illustrated in parentheses in FIG. 1, the steam entering the cathode may be accompanied by hydrogen H.sub.2 and the hydrogen produced and recovered at the outlet may be accompanied by steam. Similarly, as illustrated with dashed lines, a draining gas, such as air, may also be injected into the inlet to remove the oxygen produced. The injection of a draining gas has the further function of acting as a heat regulator.

(23) An elemental electrolysis reactor consists of an elemental cell as described above, with a cathode 2, an electrolyte 3 and an anode 4 and two monopolar connectors which ensure the electrical, hydraulic and thermal distribution functions.

(24) To increase the flow rates of hydrogen and oxygen produced, it is known practice to stack several elemental electrolysis cells on top of each other, separating them with interconnection devices, usually known as interconnectors or bipolar interconnection plates. The assembly is positioned between two end interconnection plates which support the electrical feeds and gas feeds of the electrolyzer (electrolysis reactor).

(25) A high-temperature water electrolyzer (HTE) thus comprises at least one, generally a plurality of, electrolysis cells stacked on top of each other, each elemental cell being formed from an electrolyte, a cathode and an anode, the electrolyte being intercalated between the anode and the cathode.

(26) The fluid and electrical interconnection devices that are in electrical contact with one or more electrodes generally ensure the functions of conveying and collecting electrical current and delimit one or more gas circulation compartments.

(27) Thus, a cathode compartment has the function of distributing electrical current and steam and also recovering hydrogen at the cathode in contact.

(28) An anode compartment has the function of distributing electrical current and recovering the oxygen produced at the anode in contact, optionally with the aid of a draining gas.

(29) Satisfactory functioning of an HTE electrolyzer requires: good electrical insulation between two adjacent interconnectors in the stack, otherwise the elemental electrolysis cell intercalated between the two interconnectors will be short-circuited, good electrical contact and a sufficient contact surface between each cell and interconnector, so as to obtain the lowest ohmic resistance between cell and interconnectors, good leaktightness between the two separate compartments, i.e. and cathode, otherwise the gases produced will undergo recombination resulting in a lowering of yield and above all the appearance of hot spots that damage the electrolyzer, good distribution of the gases both at the inlet and on recovery of the gases produced, otherwise there will be a loss of yield, non-uniformity of pressure and temperature in the various elemental cells, or even prohibitive degradation of the cells.

(30) FIG. 2 shows an exploded view of elementary units of a high-temperature steam electrolyzer according to the prior art. This HTE electrolyzer comprises a plurality of elemental electrolysis cells C1, C2, of solid oxide type (SOEC) stacked alternately with interconnectors 5. Each cell C1, C2, etc. consists of a cathode 2.1, 2.2, etc. and an anode 4.1, 4.2, between which is placed an electrolyte 3.1, 3.2, etc.

(31) The interconnector 5 is a component made of metal alloy which ensures separation between the cathode compartment 50 and the anode compartment 51, defined by the volumes between the interconnector 5 and the adjacent anode 4.2 and between the interconnector 5 and the adjacent cathode 2.1, respectively. It also ensures the distribution of the gases to the cells. The injection of steam into each elementary unit takes place in the cathode compartment 50. The collection of the hydrogen produced and of the residual steam at the cathode 2.1, 2.2, etc. is performed in the cathode compartment 50 downstream of the cell C1, C2, etc. after dissociation of the steam by the latter. The collection of the oxygen produced at the anode 4.2 is performed in the anode compartment 51 downstream of the cell C1, C2, etc. after dissociation of the steam by the latter.

(32) The interconnector 5 ensures the passage of the current between the cells C1 and C2 by direct contact with the adjacent electrodes, i.e. between the anode 4.2 and the cathode 2.1.

(33) In the high-temperature co-electrolyzers HTE, the high-temperature co-electrolysis is performed using steam and carbon dioxide CO.sub.2. The function of an SOEC high-temperature co-electrolyzer is to transform steam and CO.sub.2 into hydrogen, carbon monoxide and oxygen according to the following reaction:
CO.sub.2+H.sub.2O.fwdarw.CO+H.sub.2+O.sub.2.

(34) A co-electrolyzer 1 may comprise exactly the same solid oxide constituents (SOEC) as an HTE electrolyzer which has just been described. Usually, the steam and carbon dioxide CO.sub.2 are mixed before entering the co-electrolyzer and injected simultaneously into each cathode compartment 50.

(35) In order to obtain a variable ratio between the outlet gases produced, H.sub.2/CO, irrespective of the exothermic or endothermic mode of operation of a given electrolysis cell, the Applicant proposed in the abovementioned patent application FR 12 62174, a novel process for the simultaneous but separate electrolysis of steam and CO.sub.2.

(36) More precisely, the process for the high-temperature co-electrolysis of steam H.sub.2O and carbon dioxide CO.sub.2 according to patent application FR 12 62174 is performed with the electrolysis reactor comprising a stack of elemental electrolysis cells of SOEC type (C1, C2, C3) each formed from a cathode 2.1, 2.2, 2.3, an anode 4.1, 4.2, 4.3 and an electrolyte 3.1, 3.2, 3.3, intercalated between the cathode and the anode, and a plurality of electrical and fluidic interconnectors 5 each arranged between two adjacent elemental cells with one of its faces in electrical contact with the anode of one of the two elemental cells and the other of its faces in electrical contact with the cathode of the other of the two elemental cells. Steam is fed and distributed to the cathode 2.1, 2.3 of one (C1 or C3) of the two adjacent elemental cells (C1, C2; C2, C3) and carbon dioxide is fed and distributed to the cathode 2.2 of the other (C2) of the two elemental cells (C1, C2; C2, C3).

(37) In the co-electrolysis reactor according to application FR 12 62174, all the cathode compartments 50 in which circulate the steam H.sub.2O fed in and the hydrogen H.sub.2 produced communicate with each other. Similarly, all the cathode compartments 50 in which circulate the carbon dioxide CO.sub.2 injected in and the carbon monoxide CO produced communicate with each other, but are completely isolated from the compartments 50 dedicated to the steam H.sub.2O and to the hydrogen H.sub.2 produced. Finally, the two simultaneous but separate electrolysis reactions both produce oxygen which is collected by all the anode compartments 51 which communicate with each other, irrespective of the reaction concerned.

(38) At the present time, when it is desired to perform a methanation, two routes are possible. The first is the direct route, with a single reaction according to the following equation:
CO.sub.2+4H.sub.2.fwdarw.CH.sub.4+2H.sub.2O.

(39) The second is the indirect route, with a two-step reaction according to the following equations:
CO.sub.2+H.sub.2.fwdarw.CO+H.sub.2O
CO+3H2.fwdarw.CH.sub.4+H.sub.2O.

(40) The methanation is performed in a reactor in which the solid reaction catalyst is present.

(41) Hydrogen and, where appropriate, carbon monoxide may be produced beforehand either by HTE electrolysis in an electrolysis reactor 1 described with reference to FIG. 1 to 3, or by high-temperature co-electrolysis also in a co-electrolysis reactor 1 described or in a simultaneous co-electrolysis reactor according to patent application FR 12 62174.

(42) Thus, the overall process involves the sequential use of two separate reactors, that for electrolysis/co-electrolysis and that for methanation, with, as the major related drawbacks, a heavy investment and a high production cost especially due to the thermal decoupling between the two separate reactors and the need to compress at the outlet of the methanation reactor the methane produced so as to be able to transport it either in dedicated natural gas pipelines at a pressure of 80 bar, or in medium-pressure distribution networks at 4 bar.

(43) To overcome these drawbacks, the inventors of the present invention thought to integrate a methanation reactor with its solid catalyst and a high-temperature steam electrolyzer (SOEC) or a co-electrolyzer of steam and carbon dioxide CO.sub.2 in the same leaktight chamber under pressure, the pressure being that of the steam feed of the electrolyzer/co-electrolyzer, typically at 30 bar. In the context of the invention, if it is desired to have methane at the outlet that is at a higher pressure, the steam feed pressure, and consequently that in the chamber, is at this higher pressure. In particular, it may be desired to have methane at the outlet at a pressure of 80 bar which corresponds to the pressure encountered in methane gas pipelines: the feed pressure of steam and in the chamber is thus, in this case, equal to 80 bar.

(44) Thus, as illustrated in FIGS. 3 to 5, the inventors have designed a novel reactor 6 for obtaining methane by heterogeneous catalysis integrating both the electrolysis/co-electrolysis reactor 1 with a stack of SOEC electrolysis cells and the solid catalyst required for the catalytic conversion remote from the electrolysis/co-electrolysis reactor 1.

(45) The methanation reactor 6 first comprises an electrolysis/co-electrolysis reactor 1 housed in a leaktight chamber 7 which can be placed under the given pressure at which the feed steam H.sub.2O arrives in the reactor 1. As illustrated in FIGS. 3 to 5, the chamber 7 of the methanation reactor is of generally cylindrical shape of longitudinal axis X and the reactor 1 is centered on this axis X, i.e. the center not shown of each cells C1, C2, etc. constituting the stack of the reactor 1 is on the axis X.

(46) As illustrated in FIGS. 3 to 5, the leaktight chamber 7 comprises a lid 70, a base 71, and a side envelope 72 assembled both with the lid 70 and the base 71. The base 71 and the lid 70 may be assembled on the side envelope 72 via a bolted flange system equipped with a seal.

(47) To cool the chamber 7, a cooling circuit is provided consisting of a tube 73 wound in a uniform coil on the outer wall of the side envelope 72. This cooling circuit 73 may advantageously cool the inner walls 74 delimiting the chamber 7 below the water saturation temperature at the pressure prevailing in the chamber, advantageously below 230 C. at 30 bar. Thus, as explained more precisely below, the unconverted steam may advantageously be condensed on the inner walls 74 and it is thus possible independently to recover the methane produced and the steam by gravity.

(48) Inside the leaktight chamber 7 is placed a porous partition 8 containing a solid catalyst 80 for converting syngas into methane or a mixture of carbon dioxide CO.sub.2 and hydrogen into methane. The solid catalyst may advantageously be NiAl.sub.2O.sub.3 or NiZrO.sub.2 or that mentioned in publication [2], namely the bimetallic catalyst NiFe/-Al.sub.2O.sub.3 which has excellent catalytic properties for methanation at a pressure of 30 bar.

(49) As illustrated in FIGS. 3 to 5, the porous partition 8 consists of two metal walls 81, 82 each formed from a sheet pierced with a plurality of holes 83 regularly spaced both over the height and over the length of the partition 8, the height being the dimension of the partition according to the axis X, the length being its circumference around the axis X. In addition to the uniform distribution of the holes 83, a hole 83 of one of the walls 81 is provided facing a hole 83 of the other of the walls 82. Conversely, an offset may also be provided between these holes 83 from one wall 81 to the other 82.

(50) As also illustrated, the partition 8 is closed on itself forming a cylinder arranged concentrically around and at a distance from the reactor 1. Finally, a lid 84 different from that of the chamber 7 closes the inner volume delimited by the porous partition 8. Thus, the presence of the lid 84 makes it possible to force the gas to pass through the catalyst in order to emerge from the chamber. The space separating the two sheets 81, 82 is filled with conversion catalyst 80. This catalyst is advantageously in the form of powder which may be introduced into the space between the two sheets 81, 82 before closure with the lid 84. Closure of the lid on the sheets may advantageously be performed by welding or by any other mechanical fixing means. The mechanical fixing means do not have to be dimensioned to withstand a substantial force, since this (these) means are not stressed by the pressure prevailing in the chamber 7. It may be, for example, an attachment of cleat type, a screw through the lid 84 entering the wall 82.

(51) As illustrated in FIGS. 3 to 5, a first support 9 is placed in the chamber 7 to support both the electrolysis or co-electrolysis reactor 1 and the porous partition 8 so as to place them at a distance from the base 71 and from the lid 70 of the chamber 7. This first support 9 also closes the volume below the partition 8 and of the reactor 1.

(52) As illustrated in FIGS. 3 to 5, a second support 10 is provided, fixed onto the first support 9, to support only the electrolysis or co-electrolysis reactor 1 so as to place it facing the central portion of the porous partition 8.

(53) Preferably, the reactor 1 is halfway up the porous partition 8, i.e. placed facing a portion located halfway up the height of the walls 81, 82. This makes it possible firstly to have a homogeneous thermal gradient in the inner volume delimited by the wall 81 and secondly to have homogeneous distribution of the gases (H.sub.2 and CO or H.sub.2 and CO.sub.2) leaving the reactor 1 in this inner volume and thus homogeneous distribution of the gases to be converted into methane during their entry into the catalyst 80. Needless to say, as explained in detail below, the thermal gradient between the reactor 1 and the porous partition 8 is necessary due to the difference in reaction temperature between, on the one hand, that for the electrolysis of steam or the co-electrolysis of steam and CO.sub.2, advantageously of about 800 C., and, on the other hand, that for methanation, advantageously about 400 C.

(54) Thus, a concentric arrangement of the porous partition 8 containing the conversion catalyst 80 around the reactor 1, a uniform distribution of the holes 83 for passage of the gases (H.sub.2 and CO or H.sub.2 and CO.sub.2) leaving the reactor 1 and an arrangement of the reactor 1 halfway up the partition 8 contribute toward a very homogeneous thermal gradient in the inner volume delimited by the partition 8, its lid 84 and the support 9 and very homogeneous distribution of the gases (H.sub.2 and CO or H.sub.2 and CO.sub.2) in this inner volume. The path of the gases in the catalyst 80 may be relatively short, even for a large amount of catalyst present between the walls 81, 82, which is advantageous for the thermal management of the methanation reaction over the entire circumference of the partition 8. The thickness of the partition 8, i.e. its smallest dimension transversely to the axis X, may thus be relatively small compared to its other dimensions.

(55) As illustrated in FIGS. 3 to 5, the partition 8 comprises, in the solid catalyst 80, a part of the cooling circuit 85 suitable for cooling the catalytic methanation reaction or, in other words, for maintaining a constant temperature, advantageously of 400 C., for said reaction. Specifically, since the methanation reaction is exothermic, the cooling circuit 85 in the catalyst 80 makes it possible to maintain this catalyst at a suitable temperature, preferably close to 400 C. More precisely, the cooling circuit 85 may comprise a tube wound in a regular coil in the space between the inner wall 81 and the outer wall 82, preferably being close to the inner wall 81. The cooling circuit 85 may contain an oil as cooling agent and may be a closed circuit.

(56) As illustrated in FIGS. 3 to 5, a feed tube 11 is provided to feed steam under pressure and, where appropriate, carbon dioxide to the cathodes of the electrolysis or co-electrolysis reactor 1. This tube 11 passes from the outside through the base 71 of the chamber 7 and the first support 9. It is partly wound on itself close to the reactor 1, preferably around the second support 10 to superheat the steam under pressure and, where appropriate, the carbon dioxide before feeding the cathodes, as explained more precisely below.

(57) To form steam under pressure, a heat exchanger 12 is provided, placed outside the chamber 7, and which constitutes a steam production device or steam generator. To do this, liquid water, compressed beforehand to a given pressure, in a tube 13 feeds the steam generator (SG) 12. In the case of co-electrolysis by the reactor 1, carbon dioxide CO.sub.2 is introduced via a tube 14 to be mixed in the SG 12 with the steam formed. It may be envisaged to place the steam generator 12 inside the chamber 7, but, for safety reasons associated with the SG (especially the amount of gas present in the case of depressurization), it is preferable to place it outside as shown.

(58) As a source of heat for the SG 12, use may advantageously be made of the closed cooling circuit 85 of the methanation reaction. Thus, as illustrated in FIGS. 3 to 5, the tube 85 in a regular coil inside the partition 8 and closed on itself passes through the base 71 of the chamber and forms the primary circuit, i.e. that conveying the hottest fluid, of the steam generator-exchanger 12. In other words, the cooling circuit 85 of the catalysis reaction in the partition 8 advantageously constitutes the heat circuit for vaporizing the liquid water under pressure in the SG-exchanger 12.

(59) As illustrated in FIGS. 3 to 5, a tube 15 for injecting carbon dioxide into the inner volume between the electrolysis reactor 1 and the porous partition 8 is provided. This makes it possible to perform a methanation via the direct route between the hydrogen produced by the electrolysis of the steam under pressure in the reactor 1 and the CO.sub.2 injected via the holes 16 emerging from the tube 15. Thus, in this direct route, the H.sub.2+CO.sub.2 mixture passes through the holes 83 of the partition 8 containing the catalyst 80 to be converted into methane. An advantage subsequent to this injection of cold CO.sub.2 via the tube 15 is that of allowing management of the thermal gradient necessary between the electrolysis in the reactor 1 and the catalysis in the catalyst 80 in the partition 8.

(60) As illustrated in FIGS. 3 to 5, a tube 17 is provided for recovering methane produced and a tube 18 is provided for recovering by gravity water condensed on the inner walls 74 delimiting the chamber, each tube 17, 18 passing through the base 71 of the chamber 7. So as not to introduce condensed water into the methane recovery tube 17, this tube protrudes from the base 71. In contrast, the end of the tube 18 for recovering the condensed water by gravity does not protrude from the base 71. It may also be envisaged to place the recovery end of the tube 17 on the lid 70 to definitively ensure that said tube 17 does not recover condensates.

(61) It may be advantageously envisaged to reintroduce the condensed water recovered by the tube 18 into the liquid water inlet 13 at the same pressure, of the SG-heat exchanger 12.

(62) As better illustrated in FIGS. 4 and 5, to achieve uniform diffusion of the carbon dioxide CO.sub.2 injected into the inner volume delimited by the porous partition 8, the injection tube 15 is wound on itself forming a circle and being pierced with a plurality of holes 16 regularly distributed along the circle.

(63) This same homogeneous distribution may advantageously be achieved in the inner volume delimited by the porous partition 8, for the hydrogen H.sub.2 or the syngas CO+H.sub.2 produced in the reactor 1. Thus, as better illustrated in FIGS. 4 and 5, a tube 19 for recovering the hydrogen and, where appropriate, the carbon monoxide produced at the cathodes of the reactor 1 is provided. More precisely, this recovery tube 19 is connected to the outlet of the cathode compartments 50 of the reactor 1 and it is wound on itself forming a circle. It is pierced with a plurality of holes 20 regularly distributed along the circle to homogeneously diffuse the hydrogen and, where appropriate, the carbon monoxide in the inner volume delimited by the porous partition 8.

(64) The functioning of the reactor 6 and methanation system that has just been described will now be indicated more precisely, in reference with a nominal operating point. The operating conditions are as follows: injection of liquid water at 20 C., and compressed to a pressure of 30 bar by the tube 13 into the steam generator 12; leaktight maintenance at a pressure of 30 bar of the chamber 7 and maintenance at a constant temperature below 230 C. walls 74; removal of the steam, where appropriate mixed with CO.sub.2 injected at 14, from the SG 12 by the tube 11 at 300 C., at the same pressure of 30 bar; superheating of the steam to 300 C. and 30 bar, where appropriate mixed with CO.sub.2 injected at 14, in the part wound on itself of the tube 11 close to the reactor 1 to reach a temperature of 800 C. at the inlet of this reactor; when the steam removed in the tube 11 does not contain any CO.sub.2, then injection of CO.sub.2 at room temperature via the tube 15 with holes 16; maintenance at constant temperature at about 400 C. of the partition 8; passage of the H.sub.2+CO+H.sub.2O mixture removed by the tube 19 at the outlet of the co-electrolysis reactor 1, and/or with CO.sub.2 injected via the tube 15, into the porous partition 8; methanation reaction at 400 C. in the partition 8; removal via the holes 83 of the outer wall 82 of the methane CH.sub.4 produced and of the water not converted in the HTE and formed by the methanation in the volume delimited between the partition 8 and the chamber 7; condensation of the water on walls 74 delimiting chamber 7; recovery of the methane produced at a pressure of 30 bar via the tube 17; recovery by gravity via the tube 18 of the liquid water condensed and at a pressure of 30 bar;

(65) reinjection of the liquid water recovered at 30 bar into the steam generator 12.

(66) Under non-nominal operating conditions, it may be envisaged to inject CO.sub.2 both via the tube 15 (direct route) and via the tube 14 (indirect route).

(67) The rise of the steam under pressure from 300 C. to 800 C. close to the electrolyzer (co-electrolyzer) 1 may take place solely by the exothermic evolution of the reaction in this reactor. A heating system not shown may also be used.

(68) The reactor 6 and methanation system that have just been described are simple to produce with a low investment cost. In particular, all the walls 81, 82 and lid 84 of the partition 8, the constituents 70, 71, 72 of the chamber 7, the supports 9, 10, the tubes 11, 13, 14, 15, 17, 18, 19, 73, 85 may be made using a relatively inexpensive metal, such as stainless steel 316L. Needless to say, care will be taken to select a suitable metal for the parts that need to withstand the high temperatures of the electrolysis/co-electrolysis, typically 800 C. Thus, for at least the parts of the tubes 11, 19 inside which circulate gases at 800 C. and 30 bar, a production with nickel-based alloys may be envisaged.

(69) The reactor 6 and methanation system that have just been described allow a lower production cost than those of the prior art, especially due to the optimized thermal coupling between the two reactions (electrolysis/co-electrolysis and methanation) in the same chamber 7 under pressure and due to the absence of methane compression equipment, the absence of a pressure chamber specific to methanation, the absence of a condenser at 30 bar, all these functions being performed de facto in the chamber 7.

(70) The invention is not limited to the examples that have just been described; it is especially possible to combine together features of the illustrated examples within variants not illustrated.

(71) Thus, whereas in the detailed implementation example, the reactor 6 and system are envisaged for performing methanation, they may just as equally be envisaged for obtaining methanol CH.sub.3OH; DME or diesel. Irrespective of the combustible gas that it is sought to obtain, the following preferred parameters may remain identical: liquid water feed pressure equal to the pressure of the chamber 7, of about 30 bar, electrolysis or co-electrolysis temperature of about 800 C. to produce H.sub.2+CO.

(72) On the other hand, depending on the type of combustible (fuel) targeted, the H.sub.2/CO ratio, the choice of the catalyst 80 and the temperature for the catalysis, i.e. in the porous partition 8, are different. For this last parameter, the partition temperature 8 may be about 400 C. for the production of methane CH.sub.4, and about 250 C.-300 C. for methanol CH.sub.3OH and DME.

REFERENCES CITED

(73) [1]: Fabien Ocampo et al., Methanation of carbon dioxide over nickel-based Ce0.72Zr0.28O2 mixed oxide catalysts prepared by sol-gel method, Journal of Applied Catalysis A: General 369 (2009) 90-96; [2]: Dayan Tiang et al., Bimetallic NiFe total-methanation catalyst for the production of substitute natural gas under high pressure, Journal of Fuel 104 (2013) 224-229.