Cladding/overlay thickness measurements were made on several primary air ports fabricated from alternative composite tubes installed in a kraft recovery boiler to document the fireside corrosion. Laboratory corrosions tests were then conducted to reproduce the relative corrosion rates determined by the field thickness measurments. It was found that all of the major available composite tube systems are suceptible to corrosion. Hydrated sodium sulphide and oxygen in combination with sodium hydroxide are implicated as major components in the liquid environment that causes the corrosion. Prevenative measures discussed include the need for a well-sealed port, and the likely need to avoid having black liquor droplets contacting the port tubes while dehydration is incomplete.
Composite 304L stainless steel/SA-210 carbon steel tubes have replaced conventional carbon steel boiler tubes as the construction material for the lower-furnace of kraft recovery boilers to resolve the general fireside corrosion problems experienced with conventional carbon steel boiler tubes. While composite tubes have been very successful in resolving that concern, they have introduced other unanticipated problems. One such problem involves corrosion or balding, which has been observed predominantly on primary air port opening tubes. Corrosion of the 304L stainless steel cladding has occurred on both the cold-side surface and the fireside surface of primary air port opening tubes. Corrosion is a concern because of the potential for a water leak and subsequent explosion resulting from a smelt-water reaction.
To resolve the more serious composite tube cracking problem, boiler tube suppliers have promoted the use of alternative co-extruded tubes, weld-overlaid tubes, and chromized tubes. Several North American mills have installed primary air ports fabricated from those alternatives. Based on reported inspection results, those alternatives are susceptible to corrosion, some more so than others. Several mechanisms have been proposed to account for the corrosion, which include corrosion by molten hydroxide, molten smelt and molten pyrosulphate. However, until a consensus on the true mechanism is attained, a resolution to this problem may not be achieved.
Paprican has been involved in a collaborative United States Department of Energy research program with Oak Ridge National Laboratory to address the composite tube cracking problem in kraft recovery boilers. One task of the multi-disciplinary research program has been to identify the most likely corrosive environment that causes corrosion of primary air port composite tubes. This was done by conducting careful corrosion surveys within a single North American recovery boiler over an extended time frame to determine relative corrosion resistance of the various composite tubes installed, and by conducting lab-based corrosion testing to reproduce that relative corrosion resistance. This report documents the results of those efforts.
RECOVERY BOILER INSPECTION OBSERVATIONS
The relative corrosion resistance of composite tubes fabricated into primary air ports was determined from the analysis of inspection data, and from measuring the cladding/overlay thickness as a function of time. Essential design and operation details of the recovery boiler, within which observations and measurements were made, are provided below.
The recovery boiler under study is a 1997 Babcock and Wilcox (B&W) single-drum cogeneration unit constructed using 2½ in. (63.5 mm) diameter tubes on 3 in. (76.2 mm) centers in a membrane-type design with a sloped floor. The unit typically burns 3.6 million lbs (1.63 million kg) of black liquor dry solids