To increase the service time of a flue gas purification device, comprising a heat exchanger and a subsequent raw gas duct, extending from said heat exchanger downwardly to a raw gas entrance at a lower part of a gas scrubber, the invention provides at least one retainer, arranged within the raw gas duct at or beneath a first outlet port of the heat exchanger and at a distance to the raw gas entrance of the gas scrubber, which retainer extends at least partially transverse to said raw gas flow direction and has a plurality of openings each of a size, allowing the raw gas to flow through the retainer and preventing solid particles of a size larger than one opening, such as desintegrated plates, hinges, bolts, screws etc., to pass said retainer.