Air separation method and plant

Procedure air separation, which separates and purifies a flow of compressed air in a separation apparatus air (10) to produce a gaseous flow (11) rich in nitrogen to between 2 and 7 bar, the gas flow enriched nitrogen is decompressed in a turbine (17), and the decompressed gas flow (19) is sent to a convection zone located downstream of a combustion chamber (15), not having to be mixed gaseous flow decompressed with no flow fuel, without having to be mixed with no air flow after decompression, and the gas flow (11) rich in nitrogen is pre-heated by indirect heat exchange with the gases inside the convection zone of the combustion chamber (15) before being decompressed, wherein the flow enriched in nitrogen (11) is pre-heated by indirect exchange in the combustion chamber in one step up to an intermediate temperature and then in a second stage to inlet temperature of the turbine, and the decompressed gas sent to the combustion chamber (15) gives calories a gas flow that has to decompress during the first stage preheating.