Evaluation of alloy components from a biomass gasifier

Commercial alloy tube components from a biomass gasifier were removed and characterized to quantify the extent of corrosion. The Fe- and Ni-base outer bed tubes exposed to the fluidized bed of the gasifier had a metal temperature of ~700°C. Oxalic acid etching suggested that all of the bed tubes were internally carburized. However, evidence of grain boundary carbides or sigma phase was not apparent using electron microprobe. Both chromia- and alumina-forming shield tubes were placed inside the bed tubes to shield the combustion flame from overheating the bed tubes. These tubes were exposed to a much higher temperature oxidizing environment. As expected, the alumina-forming shield tube material showed less degradation. However, the Al depletion profiles were much higher than typically observed in laboratory tests. Compared to laboratory exposures of the chromia-forming shield tube alloy, the Cr depletion profiles measured in the shield tube suggest that the metal temperature was much lower than 1100°C.