Nozzles are provided for spraying a wetting medium into the gas flow in an activating reactor. The nozzle comprises a nozzle body having a plurality of nozzles The nozzle body is surrounded by a flexible and/or porous casing and a conduit introduces a gaseous medium into the casing. The gas can be introduced in either pulses or continuously. The gas introduced in pulses causes the casing to change its form and, where a perforated or porous casing is used, the continuous gas flow forms a protective gas film around the casing, thus removing the solid material accumulated on the casing.