Flue gas, in particular from a steam generator, requires cleaning. A catalytic converter for deNOxing the flue gas, which is used, for example, to heat an air preheater, is often employed for this purpose. Residues of a reducing agent react with unconverted sulfur oxide and form a substance which damages the air preheater. To suppress this reaction, there is provided a first mixer for more uniformly mixing the flue gas with the reducing agent downstream of the catalytic converter, as seen in the direction of flow.