NOVELTY - The method involves injecting nitrogen into the biomethane network (6) before the injection of the biomethane into the network having a gross calorific value of value X, so as to reduce the calorific value of the biomethane network to a value between X1 and X2, with the nitrogen derived from the retentate (3) of membrane stage (2).    USE - Method for injecting biomethane into network with gross calorific value X, such as type B natural gas network (claimed).    ADVANTAGE - The purity of the nitrogen injected into the biogas network can be controlled reliably by analyzing the oxygen concentration in the nitrogen retentate or by measuring the pressure of the retentate. The injection of oxygen-enriched stream can facilitate desulfurization of biogas within the digester. The oxygen-enriched stream can be injected into biogas purification unit, so as to facilitate the reduction of hydrogen sulfide by activated charcoal. Thus, the biomethane can be injected into the natural gas network reliably in a simple and cost-effective manner.    DETAILED DESCRIPTION - The air stream (1) can be supplied to the membrane stage from air compressor. The amount of nitrogen injected into the biomethane network can be controlled via a control valve (5). The air stream passing through the membrane stage is dried, and the nitrogen retentate from membrane stage is mixed with the biomethane so as to meet the specifications of the network with a gross calorific value X. The purity of the nitrogen injected in the biogas network can be controlled by analyzing the oxygen concentration in the nitrogen retentate or by measuring the pressure of the retentate. The nitrogen-enriched retentate is configured to produce an oxygen-enriched stream which is injected into digester producing biogas or upstream of charcoal filter in a biogas purification unit. An INDEPENDENT CLAIM is included for installation for injecting biomethane into network with gross calorific value X, which includes biomethane production unit, biomethane network, and a network with a gross calorific value X. A first unit injects the retentate from the membrane stage into the biomethane network. A second unit injects the biomethane produced by biomethane network into the network with gross calorific value X. The oxygen concentration analyzer (4) is located on the upstream of the membrane stage. A pressure sensor is located on the retentate upstream of the membrane stage. The compressed air production system comprises an air inlet, air compressor, compressed gas cooling system, condensate separator, charcoal filter for removing residual oil particles, a particulate filter for removing active carbon particles, a dryer, and air storage tank which are arranged successively in the air flow direction.    DESCRIPTION OF DRAWING(S) - The drawing shows a block flow diagram of the installation for injecting biomethane into natural gas network. (Drawing includes non-English language text)    Air stream (1)    Membrane stage (2)    Retentate (3)    Oxygen concentration analyzer (4)    Control valve (5)    Biomethane network (6)