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

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 H2O or a mixture of steam and carbon dioxide CO2 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 H2O, 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 H2O and CO2 within the first reactor, either a mixture of hydrogen H2 and carbon monoxide CO, or a mixture of hydrogen H2 and carbon dioxide CO2, is supplied and distributed to each cathode, so as to carry out, at each cathode, a methanation by heterogeneous catalysis.