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 chromiaand 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.