Experiences with lower furnace tube cracking in a recovery boiler

The recovery boiler at the DMI Peace River Pulp Mill experienced severe cracking of composite tubes made of stainless steel 304L/SA210. Cracking occurred in tubes on the furnace floor, at primary air ports, at smelt spout openings, and at composite-to-composite butt welds in the lower furnace. The problem of cracking in the primary air ports was related to frequent temperature excursions in the tubes. These temperature variations were caused by the location and configuration of the tertiary air system and by the operation of the six-on-six interlaced secondary air system. Modifications made to the boiler have minimized the cracking problem. The elevation, size, and number of tertiary air ducts were changed, and the interlacing of the secondary air was reconfigured. A number of the SS304L/SA210 composite tubes were replaced with rotary-welded and co-extruded Inconel 625 and 825 tubes.The authors wish to acknowledge Blaine Anderson and Keith Rivers of Babcock & Wilcox Canada for their contributions to the boiler trials, Jim Keiser of Oak Ridge National Laboratory, and Damond Tay and Shery Vafa of University of Toronto for their involvement in the investiga- tion. Special appreciation also goes to Dan Lefebvre, Susan Oslie, Greg Sahaydak, Brett Bryant, and the rest of the Peace River Pulp Steam and Recovery Operations