NOVELTY - The method comprises: fermenting a stackable biomass using a first solid fermenter (2, 2', 2'') of a biogas plant (1); connecting a digester gas space (20, 20', 20'') of the solid fermenter with a gas line (3, 3', 3'') for discharging a biogas from the digester gas space; and performing an inertization of the fermenter gas space and/or the gas line. The production of weak gas with a reduced calorific value of under 8.5 mJ/m3 is performed during the inertization process. The fermentation process is carried out after refilling the solids fermenter with stackable biomass.    USE - The method is useful for operating a biogas plant (claimed).    ADVANTAGE - The method is capable of enhancing the safety of the biogas plant.    DETAILED DESCRIPTION - The method comprises: fermenting a stackable biomass using a first solid fermenter (2, 2', 2'') of a biogas plant (1); connecting a digester gas space (20, 20', 20'') of the solid fermenter with a gas line (3, 3', 3'') for discharging a biogas from the digester gas space; and performing an inertization of the fermenter gas space and/or the gas line. The production of weak gas with a reduced calorific value of under 8.5 mJ/m3 is performed during the inertization process. The fermentation process is carried out after refilling the solids fermenter with stackable biomass. The inertization process is carried out during an end of the fermentation process at a predetermined period before opening of the solids fermenter and before the application of the percolate to the stackable biomass. The inertization process is performed by introducing an inert gas including nitrogen, argon and/or carbon dioxide into the fermenter. The oxygen contents in fermenter gas space or in the gas line is smaller than 12% and/or the portion of air in fermenter gas space or in the gas line is below 60%. The inertization of an appropriate inert gas volume into the fermenter gas space is determined based on the filling level of the stackable biomass. The gaseous mixture present in fermenter gas space or in the gas line for deriving the fermentation gas from fermenter gas space is analyzed and the introduction of the inert gas is controlled according to a predetermined oxygen concentration or a predetermined methane gas concentration. A second solid material fermenter is arranged in a block-type thermal power station (4), and the weak gas from the first fermenter is added to flow of gas from the second solid material fermenter and the gas mixture is then burned, where the mixture is a mixture of gases with at least 45% of methane gas contents. The methane gas concentration in the solid material fermenter is below 2% and the methane gas is diluted by introduction of an inert gas. The inertization process of the fermenter gas space of the fermenters is individual controlled. An INDEPENDENT CLAIM is included for a biogas plant.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a biogas plant.    Biogas plant (1)    First solid fermenter (2, 2', 2'')    Gas line (3, 3', 3'')    Block-type thermal power station (4)    Digester gas space. (20, 20', 20'')