A method for reducing nitrogen oxide emissions in oxyfuel combustion, which method comprises supplying in a furnace (11) of a circulating fluidised bed boiler (10) at least one primary gas flow (15) and at least one secondary gas flow (16), which both have been produced by mixing oxygen and circulated flue gas together. The oxygen content of the primary gas (15) is adjusted such that a reducing zone (I) is formed at the bottom of the furnace, in which zone nitrogen oxides are reduced to nitrogen. The oxygen content of the secondary gas is adjusted such that above the reducing zone (I) is formed an oxidising zone (II), in which zone combustion is completed.