Warm Cleanup of Coal-Derived Syngas: Multicontaminant Removal Process Demonstration

Warm (250-450 degrees 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 degrees C, while subsequent removal of sulfur to parts-per-billion levels can be accomplished at a lower temperature of approximately 300 degrees 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.