Catalytic syngas purification from model biomass gasification streams

Efficient cleaning and conditioning of biomass derived syngas for use in fuel synthesis continues to be a significant technical barrier to commercialising biofuels. The relative effectiveness of a range of nickel and rhodium containing catalysts for the conversion of model tar compounds (toluene and naphthalene or benzene) in a simulated bio-syngas stream containing up to 100 ppm H2S and up to 5 vol.% CH4 at temperatures from 700 to 900 °C were assessed at JMTC. Though H2S was found to significantly reduce hydrocarbon conversion near complete conversion of methane and tar could be achieved on a Rh based catalyst at 900 °C with a very high GHSV of 150,000 h−1. At more realistic GHSV of 60,000 h−1 and higher methane levels (5 vol.%) the effect of H2S concentration on catalyst activity followed an apparent linear activity decay law. Furthermore, the H2S deactivation was fully reversible and could be attributed to a transient site-blocking mechanism (TSB). Selected catalysts were also evaluated at NREL for tar and methane conversions using oak modelled syngas. The best Ni and best Rh catalyst were then tested under real conditions using oak derived syngas from NRELʹs pilot scale gasification unit. Rh based catalysts gave significantly better hydrocarbon conversion than the Ni catalysts at comparable conditions. The best Rh catalyst demonstrated 200 h of steady state methane conversion of 95% with little apparent loss of activity using oak modelled syngas.