The invention relates to a method for producing a gaseous medium comprising steam, said steam being produced from a first fluid medium separated from the exhaust gas from combustion of a fuel in a steam generating step, said fuel being provided for burning for providing energy for heating said first fluid medium, comprising the steps of mixing the steam thus produced with exhaust gas from combustion of said fuel; and supplying at least one chemical substance different from air and water to said steam and exhaust gas mixture. The invention also relates to a device for producing a gaseous medium comprising steam.