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

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 the level of available electricity is insufficient to carry out a high-temperature electrolysis (HTE) or a co-electrolysis of H2O and CO2 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 H2O, or a mixture of steam and carbon dioxide CO2 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 H2O, or a high-temperature co-electrolysis of steam and carbon dioxide,
• b/ at least one part of the gases resulting from the electrolysis (hydrogen H2, steam H2O ) or from the co-electrolysis (H2, steam, carbon monoxide CO, carbon dioxide CO2, methane CH4) 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.