Current understanding of cracking of recovery boiler primary air port composite tubes

Co-extruded 304L stainless steel/SA210 carbon steel tubes have found widespread use in the floors and lower walls of black liquor recovery boilers. However, cracking has been found in the stainless steel layer of these tubes, and the cracks in the tubes that form primary air port openings have been found, in some cases, to advance into the carbon steel portion of the tubes. This causes serious concern about the potential for a tube leak with the possibility of an ensuing boiler explosion. This research has been conducted so as to address possible ways to avoid this cracking: changes in operating procedures, use of alternate tube cladding materials, and modification of the airport design. Results have provided guidance on reducing the likelihood of tube cracking through selection of materials and air port design and operational changes.