Process for starting mode or stand-by mode operation of a power-to-gas unit comprising a plurality of high-temperature electrolysis (SOEC) or co-electrolysis reactors

1. A process for operating in starting mode or in stand-by mode a unit, termed power-to-gas unit, comprising a number N of reactors with a stack of solid oxide (SOEC) type elemental electrolysis cells, the cathodes of which are made of methanation reaction catalyst material(s),

wherein, when it is desired to carry out an increase in temperature of the N reactors or of a fraction thereof, or when a level of available electricity is insufficient to carry out a high-temperature electrolysis (HTE) or a co-electrolysis of H 2 O and CO 2 within all of the N reactors, the process comprises the following steps:

a/ a number P of reactors are supplied with electricity and, if required, with heat, and either steam H 2 O, or a mixture of steam and carbon dioxide CO 2 is supplied and distributed to the cathodes of the P reactors so as to carry out, at each cathode of the P reactors, either a high-temperature electrolysis (HTE) of the steam H 2 O, or a high-temperature co-electrolysis of steam and carbon dioxide,

b/ at least one part of the gases resulting from the electrolysis (hydrogen H 2 , steam H 2 O) or from the co-electrolysis (H 2 , steam, carbon monoxide CO, carbon dioxide CO 2 , methane CH 4 ) is recovered and is supplied and distributed to each cathode of a number X of reactors not supplied with electricity, the number X being less than or equal to N-P, so as to carry out, at each cathode of the X reactors, a methanation by heterogeneous catalysis.

2. The operating process according to claim 1 , wherein the carbon dioxide is emitted by a production source from a production site, the production site optionally comprising at least one from energy production sites optionally comprising nuclear power stations for waste heat and/or biomass methanization or gasification power stations for CO 2 -emission and/or waste heat, industrial production sites optionally comprising cement works and/or steel industries for CO 2 emission and/or waste heat, service industry buildings optionally comprising hospitals, closed-site transportation networks, elimination sites optionally comprising waste heat treatment units.

3. The operating process according to claim 1 , wherein, in stand-by mode, the heat requirements of the hot utilities of the unit, including the reactors (SOEC), are supplied by a waste heat of a production source, of a production site.

4. The operating process according to claim 3 , wherein the production site is chosen from energy production sites optionally comprising nuclear power stations for the waste heat and/or biomass methanization or gasification power stations for CO 2 -emission and/or waste heat, industrial production sites optionally comprising cement works and/or steel industries for CO 2 emission and/or waste heat, service industry buildings optionally comprising hospitals and/or closed-site transportation networks, elimination sites.

5. The operating process according to claim 1 , comprising a step c/ wherein the gases produced by methanation produced by step b/ are introduced into the N dedicated methanation reactors of the power-to-gas unit.

6. The operating process according to claim 1 , wherein step a/ is carried out at temperatures of between 800° C. and 1000° C.

7. The operating process according to claim 1 , wherein step b/ is carried out at temperatures of between 600° C. and 800° C.

8. The operating process according to claim 1 , wherein steps a/ and b/ are carried out at pressures of between 0 and 100 bar, preferably of between 4 and 80 bar.

9. The operating process according to claim 1 , the N reactors all being reactors with 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.

10. The operating process according to claim 9 , wherein step a/ is carried out by supplying and distributing to each cathode of the P reactors either steam H 2 O, or a mixture of steam and carbon dioxide CO 2 , or by supplying and distributing steam to each cathode of one of the two adjacent elemental cells of the P reactors and carbon dioxide is fed and distributed to the cathode of the other of the two elemental cells of the P reactors, so as to carry out, at each cathode of the P reactors, either a high-temperature electrolysis of the steam H 2 O, or a high-temperature co-electrolysis of steam and of carbon dioxide.

11. The process for producing methane CH 4 from an “intermittent” energy source, implementing the process for operating in stand-by mode a power-to-gas unit according to claim 1 , wherein step b/ is carried out when said intermittent source is no longer capable of producing the level of electricity sufficient for all the N reactors.