Relationship of recovery boiler parameters and primary air port cracking

Cracking of 304L stainless steel/carbon steel co-extruded tubes that form primary air ports in most recovery boilers is being observed with increasing frequency in North American boilers. In-situ measurements using thermocouples mounted on the air port tubes have shown that the tubes that experience the most severe temperature fluctuations are the ones most likely to experience cracking. Studies have been conducted to relate these temperature variations to boiler parameters including the pattern of air introduction and the black liquor properties and spray characteristics. The relationship between stresses in the tube wall, operating conditions, and crack advance into the carbon steel is being investigated using X-ray and neutron diffraction and finite element modeling. Other studies have evaluated the cracking resistance of several alloys that are alternatives to the conventionally used 304L stainless steel. These studies have resulted in recommendations on alternative materials and boiler operating conditions.