Cracking and Corrosion of Composite Tubes in Black Liquor Recovery Boilers

Black liquor recovery boilers are an essential part of kraft mills. Their design and operating procedures have changed over time with the goal of providing improved boiler performance. These performance improvements are frequently associated with an increase in heat flux and/or operating temperature with a subsequent increase in the demand on structural materials associated with operation at higher temperatures and/or in more corrosive environments. Improvements in structural materials have therefore been required. In most cases the alternate materials have provided acceptable solutions. However, in some cases the alternate materials have solved the original problem but introduced new issues. This paper addresses the performance of materials in the lower portion of recovery boilers and, particularly, the problems associated with use of co-extruded tubes and the solutions that have been identified.