A selective catalytic reduction reactor (8), which comprises at least one catalyst layer (14), is operative for removing NOx from a flow of process gas of a process plant, such as a combustion plant or an incineration plant. A device for controlling the feeding of a reducing agent, such as urea or ammonia, to said at least one catalyst layer (14), which has an attack area (A) facing the flow (P) of process gas, comprises at least one supply nozzle (32) which is operative for supplying reducing agent in at least one supply area (34) which corresponds to a part of the attack area (A), and a moving device (36) which is operative for moving said supply area (34) over the attack area (A).