Abstract: Improved methods and apparatus for removing unwanted pollutants from flue gases are disclosed. A sequence of at least three vertical beds of particulates is disposed in the flue gas exhaust duct. A first bed comprises boiler ash or sand. Lime in the fly ash and sulfuric acid and water in the flue gas react with the particulates to form gypsum, removing the fly ash from the flue gas. In the second bed, the remaining sulfuric acid is condensed and removed on a further bed of boiler ash. In the third bed, a catalyst, typically an activated aluminous particulate with an alkali coating thereon, provided by adding ammonia to the flue gas stream, condenses out the remaining sulphur dioxide, which can then be removed from the catalyst particles in a regenerative step. A scrubber embodying these principles is readily retrofittable to pre-existing power plants burning sulphur-containing coal.