Trace element removal from hot gases: Screening sorbents for performance and product leachability

High-temperature removal of toxic trace elements (TE) from gasification product streams is a requirement for exploiting the thermodynamic advantages of hot gas cleanup over conventional cold gas scrubbing. A novel bench scale reactor simulating hot gas cleanup using dry sorbents has been designed, constructed, and operated. The design enables vaporization of model sample compounds at up to 1000 degreesC and fixed-bed sorption of vapor phase species between 300 and 600 degreesC. Model compounds of As, Cd, Pb, Se, and Hg have been used in experiments examining the capture capabilities of sorbent kaolin and activated carbon; the subsequent leachability of sorbed trace elements has also been studied, to provide guidance regarding disposal as landfill. Kaolin was found to be the better sorbent for Pb and Cd, achieving high retention performance at 600 degreesC and low subsequent leachability. Activated carbon appears to be a better sorbent for Hg and Se. The optimum combination for high As retention and low leachability was found with kaolin at 600 degreesC. A possible explanation for this finding in terms of the preferred speciation at thermodynamic equilibrium has been presented. A TEM-EDXRF study of the two sorbents before and after use has indicated differences in the mode of trace element capture by the two sorbents. On kaolin, the captured species tend to be retained as an even layer of low concentration over most of the surface, while on activated carbon, the captured species appear mostly to be concentrated in discrete locations.