Flue gas is purified by a process in which the gas stream is forced by a finned tube header (3) through a neutralising bath to wet exhaust air or flue gases. The combination of gas in neutralising bath pref. leads to substantial foam generation in a washer (19). Foam bubbles of less than 2 mm dia. are formed by means of a suction-pressure fan, resistance 300 mm water gauge. Also claimed is an arrangement of a coke filter (12) which can be operated by a dry process with back-flushing (16) or alternatively with the foam gas washer.
