Method for operating an SOEC-type stack reactor for producing methane in the absence of available electricity

Abstract

A method for the operation of an SOEC stack reactor ( Solid Oxide Electrolyser Cell), according to which, in the absence of electricity, synthesis gas H.sub.2+CO or a mixture H.sub.2+CO.sub.2 is injected at the cathode inlet of the reactor in such a way as to produce methane inside the reactor. Since the catalytic methanation reaction is exothermic, the stack reactor can therefore be held at temperature, without loss of fuel. The fuel used for the methanation (synthesis gas or hydrogen) can advantageously be that which has been previously produced during the operating phases with available electricity.

Claims

1. A process for operating a reactor, termed first reactor, comprising a stack of elemental electrolysis cells of SOEC type, each formed from a cathode, an anode and an electrolyte inserted 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 cathodes being made of methanation reaction catalyst material(s), according to which process the following steps are carried out: a/ the first reactor is supplied with electricity, and either steam H.sub.2O or a mixture of steam and carbon dioxide CO.sub.2 is supplied and distributed to each cathode, or steam is supplied and distributed to the cathode of one of the two adjacent elemental cells and carbon dioxide is supplied and distributed to the cathode of the other of the two elemental cells, so as to carry out, at each cathode, either a high-temperature electrolysis of the steam H.sub.2O, or a high-temperature co-electrolysis of steam and carbon dioxide, b/ after step a/ and when the first reactor is supplied with a level of electric current that is insufficient to carry out an HTE electrolysis or a co-electrolysis of H.sub.2O and CO.sub.2 within the first reactor, either a mixture of hydrogen H.sub.2 and carbon monoxide CO, or a mixture of hydrogen H.sub.2 and carbon dioxide CO.sub.2, is supplied and distributed to each cathode, so as to carry out, at each cathode, a methanation by heterogeneous catalysis.

2. The operating process as claimed in claim 1, wherein the hydrogen H.sub.2 or the mixture of hydrogen H.sub.2 and carbon monoxide CO supplying the cathodes during step b/ is produced beforehand during step a/.

3. The operating process as claimed in claim 1, step a/ being carried out at temperatures of between 600 C. and 1000 C.

4. The process as claimed in claim 1, step a/ being carried out at pressures of between 0 and 100 bar.

5. The process as claimed in claim 4, step a/ being carried out at pressures of between 4 and 80 bar.

6. The process as claimed in claim 1, step b/ being carried out at pressures of between 0 and 100 bar.

7. The process as claimed in claim 6, step b/ being carried out at pressures of between 4 and 80 bar.

8. The process as claimed in claim 1, the cathodes being based on nickel (Ni) supported on zirconia (ZrO.sub.2) or ceria.

9. The process as claimed in claim 1, a draining gas circulating at each anode, during step a/.

10. The process as claimed in claim 1, a draining gas circulating at each anode, during step b/.

11. The process as claimed in claim 10, wherein the flow rate of draining gas at each anode is adjusted to the cathode flow rate for the heat management and the equilibration of the pressures between chambers.

12. A process for producing methane implementing the operating process as claimed in claim 1, comprising the following step: c/ methane produced at the outlet of the first reactor is supplied to a second reactor suitable for carrying out a methanation, when the degree of conversion of the methane at the outlet of the first reactor is below a threshold value, then a storage reservoir or a distribution network is supplied with methane produced at the outlet of the second reactor, or c/ methane produced at the outlet of the first reactor is directly supplied to a storage reservoir or a distribution network.

13. A process for producing methane CH.sub.4 from an intermittent energy source, implementing the operating process as claimed in claim 1, step b/ being carried out when said intermittent source is no longer capable of producing electricity in a sufficient amount to carry out step a/.

14. The process as claimed in claim 9, wherein the draining gas is air.

15. The process as claimed in claim 10, wherein the draining gas is air.

16. The process as claimed in claim 13, further comprising the following step: c/ methane produced at the outlet of the first reactor is supplied to a second reactor suitable for carrying out a methanation, when the degree of conversion of the methane at the outlet of the first reactor is below a threshold value, then a storage reservoir or a distribution network is supplied with methane produced at the outlet of the second reactor, or c/ methane produced at the outlet of the first reactor is directly supplied to a storage reservoir or a distribution network.

Description

DETAILED DESCRIPTION

(1) Other advantages and characteristics of the invention will emerge more clearly on reading the detailed description of exemplary embodiments of the invention given by way of nonlimiting illustration with reference to the following figures among which:

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

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

(4) FIG. 3 is a diagrammatic view of a process for producing methane from methanation step b/ of the process of an SOEC reactor according to the invention,

(5) FIG. 4 is a diagrammatic view of an experimental SOEC reactor with a single electrochemical cell making it possible to carry out methanation step b/ of the process according to the invention,

(6) FIGS. 5A to 5C illustrate the production of the various species during methanation step b/ of the process according to the invention, using syngas, and at respective pressures of 1 bar, 5 bar and 30 bar,

(7) FIGS. 6A to 6C illustrate the production of the various species during methanation step b/ of the process according to the invention, using a mixture of H.sub.2 and CO.sub.2, and at respective pressures of 1 bar, 5 bar and 30 bar.

(8) Throughout the present application, the terms vertical, lower, upper, bottom, top, below and above are to be understood with reference relative to an SOEC reactor as it is in the vertical operating configuration.

(9) Likewise, throughout 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 SOEC reactor until their exit therefrom.

(10) It is specified that, in FIGS. 1 to 3, the symbols and the arrows for supply of steam H.sub.2O, distribution and recovery of dihydrogen H.sub.2 and of oxygen O.sub.2, and the current, carbon dioxide CO.sub.2, distribution and recovery of carbon monoxide CO and oxygen O.sub.2 and the current, and the methane CH.sub.4 are shown for the purposes of clarity and precision, to illustrate the operating of an SOEC reactor 1 according to the invention and of a distinct methanation reactor 6.

(11) It is also specified that all the reactors operating according to steps a/ and b/ of the process according to the invention that are described are of solid oxide type (SOEC, acronym for Solid Oxide Electrolyte Cell) operating at high temperature. Thus, all the constituents (anode/electrolyte/cathode) of an electrolysis cell are ceramics.

(12) Such constituents may be those of an SOFC fuel cell. The high operating temperature of the reactor 1 during the electrolysis or the co-electrolysis is typically between 600 C. and 1000 C.

(13) Typically, the characteristics of an SOEC elemental electrolysis cell suitable for the invention, of the cathode-supported cell (CSC) type, 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 .sup.10.sup.5 Porosity 0.37 Permeability m.sup.2 .sup.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 .sup.10.sup.13 Tortuosity 4 Current density A .Math. m.sup.2 2000 Electrolyte 3 Constituent material YSZ Thickness m 5 Resistivity m 0.42

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

(16) In HTE high-temperature electrolyzers, the electrolysis of water at high temperature is carried out 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 carried out electrochemically in the cells of the electrolyzer. As represented diagrammatically 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 electron insulator and an ion conductor. The electrolyte may in particular be an anionic conductor, more specifically 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:
2 H.sub.2O+4e.sup..fwdarw.2 H.sub.2+2O.sup.2.

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

(21) Electrolyte 3 inserted between the two electrodes 2, 4 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 at the cathode inlet may be accompanied by hydrogen H.sub.2 and the hydrogen produced and recovered at the outlet may be accompanied by steam. Likewise, as illustrated with dashed lines, a draining gas, such as air, may also be injected at the inlet in order to remove the oxygen produced. The injection of a draining gas has the additional 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 of 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 one another, separating them with interconnection devices, usually known as interconnectors or bipolar interconnecting plates. The assembly is positioned between two end interconnecting 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 inserted between the anode and the cathode.

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

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

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

(29) Satisfactory operating of an HTE electrolyzer requires: good electrical insulation between two adjacent interconnectors in the stack, otherwise the elemental electrolysis cell inserted 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 distinct compartments, i.e. and cathodic, otherwise the gases produced will undergo recombination resulting in a lowering of yield and especially 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 represents an exploded view of elemental 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 (SOEC) type stacked alternately with interconnectors 5. Each cell C1, C2, etc. consists of a cathode 2.1, 2.2, etc. and of 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 elemental 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 carried out using steam and carbon dioxide CO.sub.2. The function of an SOEC high-temperature co-electrolyzer is to convert the steam and the 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 the carbon dioxide CO.sub.2 are mixed before entry into the co-electrolyzer and injected simultaneously into each cathode compartment 50.

(35) Currently, when it is desired to carry out a methanation by heterogeneous catalysis, two routes are possible. The first route 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.

(36) The second route 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+3H.sub.2.fwdarw.CH.sub.4+H.sub.2O.

(37) The methanation is carried out in a reactor in which the solid catalyst of the reaction is present.

(38) The hydrogen and, where appropriate, the carbon monoxide may be produced beforehand either by HTE electrolysis in an electrolysis reactor 1 described with reference to FIGS. 1 to 3, or by high-temperature co-electrolysis, also in a co-electrolysis reactor 1 described.

(39) It is envisioned to carry out the HTE electrolysis or high-temperature co-electrolysis of the steam H.sub.2O and carbon dioxide CO.sub.2 with decarbonized energies which are by nature intermittent (wind power, solar power).

(40) However, with such energies, electricity production may be interrupted, or at the very least be reduced to a low level, over the course of more or less long periods which may be impromptu.

(41) Under these conditions, the HTE electrolysis or the co-electrolysis may no longer be provided by the SOEC reactor 1 owing to the absence of electric current (voltage).

(42) However, in the event of it being impossible to carry out the electrolysis or the co-electrolysis, it is necessary to be able to maintain the SOEC reactor 1 at temperature in order, on the one hand, to prevent thermal cycling that may damage it and, on the other hand, to make it possible to rapidly start up again as soon as the electric current is again available.

(43) To solve this problem, the inventors of the present invention have thought to supply and distribute to each cathode 2.1, 2.2, etc. either a mixture of hydrogen H.sub.2 and carbon monoxide CO, or a mixture of hydrogen H.sub.2 and carbon dioxide CO.sub.2, so as to carry out, at each cathode, a methanation by heterogeneous catalysis.

(44) In other words, in the absence of available electricity, an additional function of methanation reactor is assigned to the SOEC reactor 1.

(45) Likewise in other words, the operation of the SOEC reactor 1 is bimodal: HTE electrolysis or co-electrolysis of H.sub.2O and CO.sub.2 when the electricity is available in sufficient amount, methanation, in the event of absence of available current, or at the very least a level insufficient to carry out an HTE electrolysis or co-electrolysis reaction.

(46) Judiciously, the inventors started from the observation that certain solid oxide cathode materials, in particular those based on nickel (Ni) supported on zirconia (ZrO.sub.2), used in SOEC reactors, were also proven as methanation reaction solid catalysts.

(47) The methanation operation of an SOEC reactor 1 makes it possible to envision methane production in the absence of electricity: thus, the methane CH.sub.4 produced at the outlet of the reactor 1 supplies: either a second reactor 6, capable of carrying out a methanation, with methane produced at the outlet of the first reactor when the degree of conversion of the latter is below a threshold value, then methane produced at the outlet of the second reactor is supplied to a storage reservoir or a distribution network, as illustrated in FIG. 3; or directly a methane storage reservoir or distribution network.

(48) The inventors of the present invention have produced a solid oxide single-cell C1 reactor 1 in order to experimentally prove the methanation within such a reactor.

(49) The experimental reactor 1 is shown in FIG. 4. It comprises a cell C1 consisting of a cathode 2.1, an anode 4.1 and an electrolyte 3.1 inserted between the cathode 2.1 and the anode 4.1.

(50) The cathode 2.1 concerned that is used is a cermet 50 mm in diameter and consists of a stack of two layers, namely: a support layer made of Ni-3YSZ (ZrO.sub.2+3 mol % Y.sub.2O.sub.3), having a thickness of 500 m, a functional layer made of Ni-8YSZ (ZrO.sub.2+8 mol % Y.sub.2O.sub.3), having a thickness of between 5 and 10 m.

(51) The electrolyte 3.1 is made of 8YSZ (ZrO.sub.2+8 mol % Y.sub.2O.sub.3), having a thickness of 5 m.

(52) The anode 4.1 is made of LSCo (strontium-doped lanthanum cobaltite), having a thickness of 20 m.

(53) The cell C1 is mounted in a housing-support 7 made of alumina (Al.sub.2O.sub.3). The mounting of the cell C1 is leaktight by means of a glass seal 8 at the periphery of the electrolyte 3.1. The mounting of the cell C1 is also under compression from a maximum load of 2 kg/cm.sup.2 as illustrated by the arrow F in FIG. 4.

(54) The housing 7 has, in its lower part, a central opening 70 for supplying gas (syngas or mixture of H.sub.2 and CO.sub.2) and also an opening 71 for recovering gas obtained by means of the reaction within the cathode 2.1.

(55) It also has, in its upper part, a central opening 72 for supplying draining gas (air) at the anode 4.1 and an opening 73 for evacuating the draining gas.

(56) Various contact grids 9, 10, 11 are arranged in contact, respectively, with the anode 4.1 and the cathode 2.1. More specifically, the grid 11 in contact with the cathode 2.1 is made of nickel, having a thickness equal to 0.45 mm, with 100 mesh cells/cm.sup.2 and a unit wire diameter of 0.22 mm. The contact grids 9, 10 are for their part made of gold and platinum.

(57) As illustrated in FIG. 4, the grids 9, 10, 11 allow an electrical supply of current I and/or voltage U to the cell C1 in order for it to carry out a reaction for HTE electrolysis or co-electrolysis of steam H.sub.2O and of CO.sub.2.

(58) Either a syngas (H.sub.2+CO) or a mixture H.sub.2+CO.sub.2 is injected at the inlet 70 of cathode 2.1 and air is simultaneously injected at the inlet 72 of anode 4.1, without polarizing the cell.

(59) The gas analysis was carried out, after condensation of the gases in order to remove H.sub.2O, by micro gas chromatography by means of the apparatus sold by the company SRA under the name MicroGC 3000.

(60) Two series of measurements were carried out with two different types of cell C1 using the same materials but different manufacturers.

(61) The measurement results and also calculations are reproduced hereinafter in tables 3 and 4, each for a type of cell. It is specified that the total value of the composition of the gases is slightly greater than 100% owing to measurement uncertainties.

(62) TABLE-US-00003 TABLE 3 Cell Total in- Inlet gas composition Measured composition of outlet gases Calculated composition of outlet gases with H.sub.2O temper- let flow (vol %) (vol %) (vol %) ature rate % % % % % % % % % % Crude % % % % % % % Crude ( C.) (NL/h) H2 CO CO2 N2 H2 O2 N2 CH4 CO CO2 total H2 O2 N2 CH4 CO CO2 H2O total 600 8.17 75 25 65.1 0.1 2.2 9.5 19.3 5.9 102.2 59.5 0.1 2.0 8.7 17.7 5.4 8.7 102.2 600 8.17 80 20 71.0 0.1 1.8 3.8 11.8 12.8 101.3 66.0 0.1 1.7 3.6 10.9 11.9 7.1 101.3

(63) TABLE-US-00004 TABLE 4 Cell Air temper- Total in- Inlet gas composition Measured composition of outlet gases flow ature let flow (vol %) (vol %) rate ( C.) rate % % % % % % % % % (NL/h) T1 T2 (NL/h) H2 CO CO2 N2 H2 O2 N2 CH4 CO 43.58 616.3 603.2 8.17 75 25 62.1 0.0 3.8 13.2 15.4 30 616.7 604.3 8.17 75 25 62.8 0.0 3.0 32.9 15.7 Measured composition Air of outlet gases Calculated composition of outlet gases with H.sub.2O flow (vol %) (vol %) rate % Crude % % % % % % % Crude (NL/h) CO2 total H2 O2 N2 CH4 CO CO2 H2O total 43.58 6.1 100.7 54.9 0.0 3.4 11.7 13.6 5.4 11.7 100.7 30 6.3 100.6 55.7 0.0 2.7 11.4 13.9 5.6 11.4 100.6

(64) From these tables 3 and 4, it may be concluded first of all that the cell C1 actually carried out a methanation reaction.

(65) It may also be concluded therefrom that the cell C1 and therefore the reactor 1 undergoes significant heating during the methanation, which is all the greater the lower the air flow rate at the anode.

(66) Consequently, it is advantageously possible to adjust the flow rate of draining gas (air) at the anodes of an SOEC reactor in order to control, by forced convection, the rise in temperature due to the methanation and to thus protect the SOEC cells against excessive heating.

(67) The inventors of the present invention performed thermodynamic calculations as follows in order to corroborate the experimental feasibility of the methanation with the gases CO.sub.2, CO, H.sub.2 and H.sub.2O.

(68) They first considered the three reactions involved during the methanation of CO.sub.2 (and CO), which are:
CO.sub.2+4H.sub.2CH.sub.4+2H.sub.2O methanation of CO.sub.2(1),
CO+3H.sub.2CH.sub.4+H.sub.2O methanation of CO(2),
CO.sub.2+H.sub.2CO+H.sub.2O RWGS (Reverse Water Gas Shift)(3).

(69) The thermodynamic analysis, as done in publication [3], was carried out by resolving the system:

(70) $\begin{array}{cc}\{\begin{array}{c}\frac{P_{{\mathrm{CH}}_4}{P}_{H_{2}O}^2}{P_{H_2}^4{P}_{{\mathrm{CO}}_2}}-K_{{\mathrm{CO}}_2}^p=0\\ \frac{P_{H_4}{P}_{{\mathrm{CO}}_2}}{P_{\mathrm{CO}}{P}_{H_{2}O}}-K_{\mathrm{RWGS}}^p=0\end{array}& \left(4\right)\end{array}$

(71) with the equilibrium constants which are written:

(72) $\begin{array}{cc}{K}_{{\mathrm{CO}}_2}^p=\frac{P_{H2O}^2{P}_{\mathrm{CH}4}}{P_{H2}^4{P}_{\mathrm{CO}2}}={\left[7.24E^{10}\exp \left(\frac{-21646}{T}\right)\right]}^{-1}& \left(5\right)\\ K_{\mathrm{CO}}^p=\frac{P_{H2O}{P}_{\mathrm{CH}4}}{P_{H2}^3{P}_{\mathrm{CO}}}={\left[1.22E^{13}\exp \left(\frac{-26830}{T}\right)\right]}^{-1}& \left(6\right)\\ K_{\mathrm{RWGS}}^p=\frac{P_{H2O}{P}_{\mathrm{CO}}}{P_{H2}{P}_{\mathrm{CO}2}}={\left[1.26E^{-2}\exp \left(\frac{4639}{T}\right)\right]}^{-1}& \left(7\right)\end{array}$

(73) The temperature T is given in degrees Kelvin, and the unit of the equilibrium constants K.sup.p is given in bar.sup.1.

(74) The expressions of the reaction kinetics are the following:

(75) $\begin{array}{cc}{r}_{{\mathrm{CO}}_2}=-\frac{k_{{\mathrm{CO}}_2}}{P_{H_2}^{3.5}}\left(P_{{\mathrm{CH}}_2}{P}_{H_{2}O}^2-\frac{P_{H_2}^4{P}_{{\mathrm{CO}}_2}}{K_{{\mathrm{CO}}_2}^P}\right).Math.\frac{1}{\left(\begin{array}{c}1+K_{\mathrm{CO}}^{\mathrm{ad}}{P}_{\mathrm{CO}}+K_{H_2}^{\mathrm{ad}}{P}_{H_2}+\\ K_{{\mathrm{CH}}_4}^{\mathrm{ad}}{P}_{{\mathrm{CH}}_4}+\frac{K_{H_{2}O}^{\mathrm{ad}}{P}_{H_{2}O}}{P_{H_2}}\end{array}\right)}& \left(8\right)\\ r_{\mathrm{CO}}=-\frac{k_{\mathrm{CO}}}{P_{H_2}^{2.5}}\left(P_{{\mathrm{CH}}_4}{P}_{H_{2}O}-\frac{P_{H_2}^3{P}_{\mathrm{CO}}}{K_{\mathrm{CO}}^P}\right).Math.\frac{1}{\left(\begin{array}{c}1+K_{\mathrm{CO}}^{\mathrm{ad}}{P}_{\mathrm{CO}}+K_{H_2}^{\mathrm{ad}}{P}_{H_2}+\\ K_{{\mathrm{CH}}_4}^{\mathrm{ad}}{P}_{{\mathrm{CH}}_4}+\frac{K_{H_{2}O}^{\mathrm{ad}}{P}_{H_{2}O}}{P_{H_2}}\end{array}\right)}& \left(9\right)\\ r_{\mathrm{RWGS}}=-\frac{k_{\mathrm{RWGS}}}{P_{H_2}}\left(P_{\mathrm{CO}}{P}_{H_{2}O}-\frac{P_{H_2}{P}_{{\mathrm{CO}}_2}}{K_{\mathrm{RWGS}}^P}\right).Math.\frac{1}{\left(\begin{array}{c}1+K_{\mathrm{CO}}^{\mathrm{ad}}{P}_{\mathrm{CO}}+K_{H_2}^{\mathrm{ad}}{P}_{H_2}+\\ K_{{\mathrm{CH}}_4}^{\mathrm{ad}}{P}_{{\mathrm{CH}}_4}+\frac{K_{H_{2}O}^{\mathrm{ad}}{P}_{H_{2}O}}{P_{H_2}}\end{array}\right)}& \left(10\right)\end{array}$

(76) with kmol.kg.sub.cat.sup.1.h.sup.1 as unit. The symbols P.sub.i represent the partial pressures in bar.

(77) The kinetic constants k.sub.i are given by the equations:

(78) $\begin{array}{cc}{k}_{{\mathrm{CO}}_2}=1.02E^{15}\exp \left(\frac{-243.9E^3}{\mathrm{RT}}\right)& \left(11\right)\\ k_{\mathrm{CO}}=4.225E^{15}\exp \left(\frac{-240.9E^3}{\mathrm{RT}}\right)& \left(12\right)\\ k_{\mathrm{RWGS}}=1.955E^6\exp \left(\frac{-67.13E^3}{\mathrm{RT}}\right)& \left(13\right)\end{array}$

(79) and are expressed in kmol.bar.sup.1.kg.sub.cat.sup.1.h.sup.1.

(80) It should be noted that these values were identified for reforming between 400 and 600 C. at 2 bar.

(81) The adsorption constants K.sub.i.sup.ad are for their part given by the equations:

(82) $\begin{array}{cc}{K}_{{\mathrm{CO}}_2}^{\mathrm{ad}}=8.23E^{-5}\exp \left(\frac{-70.65E^3}{\mathrm{RT}}\right)& \left(14\right)\\ K_{H_2}^{\mathrm{ad}}=6.12E^{-9}\exp \left(\frac{82.9E^3}{\mathrm{RT}}\right)& \left(15\right)\\ K_{{\mathrm{CH}}_4}^{\mathrm{ad}}=6.65E^{-4}\exp \left(\frac{38.28E^3}{\mathrm{RT}}\right)& \left(16\right)\\ K_{H_{2}O}^{\mathrm{ad}}=1.77E^5\exp \left(\frac{-88.68E^3}{\mathrm{RT}}\right)& \left(17\right)\end{array}$

(83) with bar.sup.1 as unit.

(84) The curves of the molar fractions of each species obtained, which were calculated as above for the methanation of CO at respective pressures of 1 bar, 5 bar and 30 bar, have been represented in FIGS. 5A to 5C.

(85) The curves of the molar fractions of each species obtained, which were calculated as above for the methanation of CO.sub.2 at respective pressures of 1 bar, 5 bar and 30 bar have been represented in FIGS. 6A to 6C.

(86) It results from these curves that the pressure has a strong beneficial effect on the methanation starting from a temperature of 700K. For an identical degree of conversion, the pressure makes it possible to work at temperatures 250 C. higher compared with atmospheric pressure.

(87) Finally, whatever the pressure, methanation of CO is preferable to that of CO.sub.2.

(88) The invention which has just been described, both by means of tests and by means of a thermodynamic analysis, makes it possible to envision methane production in situ within an SOEC reactor 1.

(89) At 600 C. and at atmospheric pressure, methane production is greater starting from CO than starting from CO.sub.2.

(90) Under the experimental conditions that were retained, the production causes a significant increase in temperature of several degrees, thereby making it possible to envision maintaining an SOEC reactor at temperature.

(91) Finally, a methanation within a reactor under pressure makes it possible to envision a higher degree of conversion to methane and also maintaining at a higher temperature.

(92) The invention is not limited to the examples which have just been described; characteristics of the examples illustrated may in particular be combined with one another in variants which have not been illustrated.

REFERENCES CITED

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