Recent observations of recovery boiler primary air port cracking and characterization of environmental conditions

Cracking of 304L stainless steel/SA210 carbon steel co-extruded (composite) tubes in primary air ports of black liquor recovery boilers is an issue of serious concern for boiler operators and the subject of intensive study by a Department of Energy-funded research team. In two recent publications by this team, cracking of composite tubes in recovery boiler primary air ports and floor tubes has been compared and an overview of primary air port tube cracking has been presented. Since those reports were prepared, a significant number of boilers have been examined and considerable work has been done to further the understanding of air port cracking. This paper provides an update on this team's work during the period from late December, 2000 through May, 2001. These recent boiler inspections have provided additional information on the type and extent of cracking in the tubes that form primary air ports as well as an opportunity to assess the corrosion rates of various alloys that are being used as the cladding on primary air port tubes. During this period, additional temperature data have been collected from the thermocouples on primary air ports in four recovery boilers, and studies have been conducted to investigate the effect of air and liquor distribution and liquor properties on the temperature variations. To better characterize the environment around the air ports, gas samples have been collected from air ports that have experienced cracking and from air ports without a history of cracking. Furthermore, a determined effort has been made to observe and photograph the activities on the combustion and windbox sides of both sets of airports. In addition, laboratory studies are being conducted to further understand corrosion and cracking of various alloys under appropriate environmental conditions. Although no complete explanation for the cause of primary air port tube cracking has been reached, recent studies have added to our knowledge base. This knowledge, when combined with information that will be gained from studies that are currently underway and proposed should lead to an explanation for the cracking as well as provide effective prevention approaches.