Warm (250–450 °C) cleanup
of coal- or biomass-derived
syngas requires sorbents and catalysts to protect downstream conversions.
We report first a sequential ZnO bed operation in which the capacity
is optimized for bulk desulfurization at 450 °C, while subsequent
removal of sulfur to parts-per-billion levels can be accomplished
at a lower temperature of approximately 300 °C. At this temperature,
gaseous sulfur (H2S and COS) could be adsorbed equally
well using ZnO, both with and without the presence of H2O in the feed, suggesting direct absorption of COS can occur. Following
five sulfidation and regeneration cycles, the bulk desulfurization
bed lost about a third of its initial sulfur capacity; however, sorbent
capacity stabilized. A bench-scale process consisting of five unit
operations is described for the cleanup of a several contaminants
in addition to sulfur. Syngas cleanup was demonstrated through successful
long-term performance of a poison-sensitive Cu-based water-gas shift
catalyst placed downstream of the cleanup process train. The process
removed 99+% of the sulfur; however, improvements can be made toward
full regenerability of the ZnO bed and with complete elimination of
sulfur slip through the guard beds. The use of a tar reformer was
found to be an important and necessary operation with this particular
gasification system; its inclusion provided the difference between
deactivating the water-gas catalyst through carbon deposition and
having a largely successful 100 h test using 1 LPM of coal-derived
syngas.