NOVELTY - A fluid (A) comprising carbon dioxide and nitrogen dioxide and oxygen, argon and/or nitrogen is fed to a rectifying distillation column. Gaseous stream (I) and liquid stream are withdrawn from the rectifying column. The liquid stream is fed to a fluid separation membrane. Gaseous stream (II) comprising a permeate gas is withdrawn from the membrane. A non-permeate liquid stream is withdrawn from the membrane and recycled to the rectifying column, to separate the nitrogen dioxide from carbon dioxide and nitrogen dioxide containing fluid.    USE - Separation of nitrogen dioxide from fluid generated from combustion of carbon containing fuel e.g. oxycombustion fossil fuel and biomass power plant.    ADVANTAGE - The method enables efficient separation of nitrogen dioxide and purification of flue gas and provides carbon dioxide product from flue gas with less amount of nitrogen oxide.    DETAILED DESCRIPTION - A fluid (A) comprising carbon dioxide and nitrogen dioxide and oxygen, argon and/or nitrogen is fed to a rectifying distillation column. Gaseous stream (I) which is enriched in carbon dioxide and deficient in nitrogen dioxide relative to fluid (A) is withdrawn from the rectifying column. A liquid stream which is enriched in nitrogen dioxide relative to the fluid (A) is withdrawn from the rectifying column and fed to a fluid separation membrane. Gaseous stream (II) comprising a permeate gas which is enriched in nitrogen dioxide and deficient in carbon dioxide relative to the liquid stream is withdrawn from the membrane. A non-permeate liquid stream which is deficient in nitrogen dioxide and enriched in carbon dioxide relative to the liquid stream fed to the membrane is withdrawn from the membrane and recycled to the rectifying column, to separate the nitrogen dioxide from carbon dioxide and nitrogen dioxide containing fluid. An INDEPENDENT CLAIM is included for apparatus for separating nitrogen dioxide from fluid, which has a source of fluid (A), rectifying distillation column, fluid separation membrane and a pump for feeding the non-permeate liquid from the membrane to the rectifying column.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of an oxycombustion plant.    Boiler (1)    Air separation unit (2)    Coal mill (3)    Steam turbine (8)    Oxygen stream (10)    Coal (14)    Primary flue gas recycle stream (15)