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

Co-extruded 304L stainless steel/SA210 carbon steel tubes have been widely used in the floors and lower walls of black liquor recovery boilers in order to provide needed corrosion resistance. However, cracking has been found in the stainless steel layer of both floor and wall tubes, and the cracks in the tubes that form primary air port openings have, in some cases, advanced into the carbon steel portion of the tubes. A few tube failures have been reported, and consequently, there has to be serious concern about the potential for a tube leak with the possibility of an ensuing boiler explosion. This research has been conducted to address possible ways to avoid this cracking: changes in operating procedures, use of alternate tube cladding materials, and modification of the air port design. Results have provided guidance on reducing the likelihood of tube cracking through selection of materials, air port design, and operational changes.