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Pilot-Scale Evaluation of Fluidizable Reforming Catalyst for Biomass Syngas Cleanup

Calvin J. Feik, Daniel L. Carpenter, Katherine R. Gaston, Jason A. Hrdlicka, Steven D. Phillips, and Marc D. Pomeroy. National Bioenergy Center, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401

The volatile content of biomass lends itself well to indirect gasification. However, a challenge of indirect biomass gasification is higher tar and methane concentration in the resultant syngas. The National Renewable Energy Laboratory (NREL) has been developing fluidizable reforming catalysts to convert the residual tars and light hydrocarbons present in biomass-derived syngas to additional hydrogen and carbon monoxide. Converting these other compounds to syngas improves overall process yield and decreases production cost. A key step in our research program is evaluation of new catalysts in the Thermochemical Process Development Unit (TCPDU), a 0.5-ton per day pilot plant used for evaluating thermochemical biomass conversion processes. A Full-Stream Reformer (FSR) integrated into the TCPDU has been used over the last few years to evaluate several catalyst formulations for syngas cleanup with both herbaceous and woody biomass. The best performing catalysts have consisted of nickel and magnesium supported on fluidizable alumina. A key finding of the catalyst evaluation results with biomass-derived syngas is the impact of inorganic species on catalyst deactivation. It is well known that sulfur is particularly detrimental to reforming catalyst activity. Although biomass has low levels of sulfur compared to other gasification feedstocks such as coal, the syngas hydrogen sulfide concentration ranges from 20 to 50 ppmv for woody feedstocks to as high as 600 ppmv from corn stover. The concentration of hydrogen sulfide in the raw syngas is inversely correlated to catalyst lifetime. Initial catalytic conversion of tar, ethylene, and acetylene is over 99%, with methane conversion over 93%. The most active catalysts are able to maintain this high tar, ethylene, and acetylene conversion for over 200 minutes, while the methane conversion is observed to decrease linearly from the outset. Breakthrough of the heavier tar species (phenanthrene, pyrene) is not observed until almost 300 minutes on stream. Catalyst regeneration using steam with 10% air for oxidation and 12% hydrogen in nitrogen for reduction has shown promise for fully recovering the initial catalyst activity. These results will be discussed in the context of developing biomass derived fuels from conditioned syngas.