(196c) Properties of Olivine Catalysts for Indirect Gasification of Biomass

To meet the increasing demand for renewable, CO₂-neutral energy, biomass (in particular waste products) can be used for the production of fuels such as hydrogen, synthetic natural gas or Fisher-Tropsch diesel [1]. Conversion of biomass to such fuels can be performed using an indirect gasification process, where gasification and partial combustion (necessary to sustain the endothermic gasification process) take place in separate chambers. Heat generated by combustion of chars and tars derived from the biomass is transferred to the gasification chamber by a bed material, conventionally sand, that is circulated between the two chambers. By exchanging the inert sand with a catalytically active bed material it is possible to improve the efficiency of the gasification process and to reduce the content of polluting tars in the product gas. A widely used and studied catalytic bed material is the naturally occurring mineral olivine ((Mg,Fe)₂SiO₄). Although the exact catalytic function of the olivine is not yet fully understood, it has been observed that pretreatments can improve the performance of this catalyst[2] and that the phase composition of the mineral as well as its activity change over time, under typical process conditions[3].
We have used a flow reactor to mimic the conditions that olivine catalysts are exposed to during gasification of biomass. Oxidation (20% O₂ in Ar) and reduction (CO, H₂ and H₂O mixtures) treatments at 750˚C were applied. The resulting changes in the olivine catalyst’s bulk composition were characterized using X-ray Absorption Spectroscopy (XAS) and X-Ray Diffraction (XRD). Surface chemistry and morphology changes were characterized using X-ray Photoelectron Spectroscopy (XPS) and electron microscopy. The main changes in the catalyst after exposure to high temperatures, necessary for biomass gasification, concern the iron present in the olivine. After exposure to the oxidizing environment, the olivine catalysts exhibit a significant increase of surface Fe and a simultaneous decrease of Si. Furthermore, Fe₂O₃ and Fe₃O₄ phases are observed to form from Fe segregated out of the bulk mineral. On the catalyst surface, formation of crystallites is observed and the amount of Fe, the most dynamic element, at the surface is more than doubled. The catalyst entering the gasification zone is thus best described as a largely Fe-depleted olivine with the inclusion of FeOx phases at the surface.
After exposure to reducing conditions (H₂+CO+H₂O) the FeOx phases are reduced to FeO and Fe⁰, both in the bulk and at the surface. The surface content of Fe is reduced to roughly half the amount present after oxidation and the surface crystallites decrease in size. In CO containing gases significant carbon deposition is observed, some of this in the form of nanofibers.
These results suggest that Fe segregates reversibly to and from the surface under oxidizing and reducing conditions and on time scales which are relevant in commercial plants.
In addition, characterization of the olivine samples using XPS depth profiling and Thermo Gravimetric Analysis (TGA) provided further insight in the dynamic behavior of iron in olivine and its chemical looping abilities under relevant process conditions.

[1] A. V. Bridgwater, A. J. Toft and J. G. Brammer, Renewable & Sustainable Energy Reviews 6, 181-248 (2002)
[2] L. Devi, K. J. Ptasinski and F. Janssen, Industrial & Engineering Chemistry Research 44, 9096-9104 (2005)
[3] J. N. Kuhn, Z. K. Zhao, L. G. Felix, R. B. Slimane, C. W. Choi and U. S. Ozkan, Applied Catalysis B-Environmental 81, 14-26 (2008)