Corrosion of Carbon Steel Tubes in the Mid-Furnance of Kraft Recovery Boilers--Environmental Characterization

Abstract

Carbon steel tubes in the mid-furnace of kraft recovery boilers have experienced accelerated corro-sion in recent years. Gaseous environments in the mid-furnace of kraft recovery boilers were generally considered to be oxidizing and not very corrosive to the carbon steel waterwall tubes. However, mid-furnace corrosion has occurred above the cut line where composite tubes that have a stainless steel outer layer used in the lower furnace are welded to the carbon steel waterwall section. This study characterized the local environments in three boilers to find the reasons for accelerated corrosion of waterwalls in the mid-furnace. Gas sampling ports were installed in the mid-furnace with high and low corrosion rates of carbon steel waterwall tubes. During boiler operation, gases were sampled and analyzed using a specially designed gas chromatograph. In each boiler, the areas with higher corrosion rates had higher concentrations of reducing gases and sulfur-containing gases at the waterwall surface than in the low-corrosion areas. Results from this study indicate that accelerated mid-furnace corrosion likely results from changes in the boiler operation, such as decreases in the air supply at the lower air port levels of the furnace. These changes also might result in higher local gas temperatures and local thermal cycling in the mid-furnace of boilers.