Process for using methane containing gases

Process for utilizing methane-containing biogas e.g. mine gas, waste gas and biogas produced from fermentation plants and digestion processes in clarifying plants involves feeding the gas to a gas motor of a gas motor/generator arrangement to produce a current. The inert gas content of the combustion air fed to the motor is reduced using a membrane separation device connected to the motor. Preferred Features: Lean gas containing less than 40 vol.% methane is used to operate the motor. Atmospheric air is compressed and fed to a gas permeation module of the membrane separation device, whose membranes have a preferred permeability for O2. The permeate with a reduced inert gas content is withdrawn from the permeation module and fed to the gas motor as combustion air. The oxygen/nitrogen ratio of the combustion air is adjusted by the operation of the membrane separation device so that the gas motor is operated with a specified air ratio, preferably lambda = 1.5-1.8, preferably lambda = 1.6.