Reducing Primary Air Port Composite Tube Cracking in a Kraft Recovery Boiler through Changes in Operation, 2005 Engineering, Pulping & Environmental Conference

Systematic changes were made to the operation of a recovery boiler to reduce the likelihood of SS304L/SA210 composite tube cracking in and around the primary air ports. Changes were evaluated in terms of their influence on both the occurrence of cracking (determined through inspections) and thermal fluctuations (determined by surface mounted thermocouples). For the boiler under study, the occurrence of cracking and large thermal fluctuations were significantly reduced through a set of changes to the combustion air system, black liquor nozzle type and the practice of burning soap in combination with black liquor. Analysis of the results provide a strong case for a link between the occurrence of large thermal fluctuations on a port tube and a set of three critical conditions that must be present simultaneously during operation, namely (i) char bed contacting the port tube, (ii) black liquor droplets contacting the port tube while pyrolysis and possibly dehydration is still incomplete, and (iii) low air jet momentum through the primary air ports.