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 .