Desulfurization and tar removal from gasifier effluents using mixed rare earth oxides

Biomass gasification is a promising source of fuels. However, hydrogen sulphide, tars and other by-products must be removed from the raw gas because they deactivate downstream reforming and water gas shift catalysts. The goal of this project is to find the best REO combination for simultaneous tar cracking and desulfurization of gasifier effluents and to find the sorbents that are stable at high operating temperatures of gasifiers. Simultaneous tar cracking and H2S removal from a simulated gasifier effluent was tested using different rare earth mixed oxide (REO) catalysts/sorbents based on Ce/LaOx, Ce/La/MOx and Ce/La/M2Ox/Al2O3 where M is a transition metal and M2 is a third rare earth metal. These catalysts were prepared using sol gel and incipient wetness impregnation methods. Desulfurization tests were done at 903K using a gas composition of 23.4% H2, 32% CO2, 3.1% H2O, 41.4% N2 and 0.1% H2S. The tar cracking/reforming capability of these materials was tested by adding 0.35 mole% naphthalene as a model compound of tar to the simulated effluent and reacting it during the adsorption cycle. Sorbents containing pure Ce/La oxides have low sulfur capacities and are not very effective in removing H2S from a real gasifier effluent. Supporting the REOs on Al2O3 (20 wt% REO) or ZrO2, and addition of a small amount of a third REO known to enhance the thermal stability of CeO2 (either Tb2O3 or Gd2O3), greatly increased the total sulfur capacities of the REOs. These ternary REOs maintained their capacity over a minimum of four successive runs and were regenerated in air. The tar removal capacity of these sorbents was found to be low in the simultaneous presence of H2S, H2O and CO2 and all the sorbents deactivated in 30 mins. A mixed Ce/La/Mn oxide was found to be the best catalyst for simultaneous desulfurization and tar removal