Characterization and reactivity in toluene reforming of a Fe/olivine catalyst designed for gas cleanup in biomass gasification

Abstract

To improve the gas production and quality of biomass steam gasification by tar reforming, an iron/olivine catalyst was designed for a use in fluidized bed gasifier. Olivine was impregnated by 10 wt% or 20 wt% of iron and calcined at different temperatures (400 °C, 900 °C, 1000 °C, 1100 °C or 1400 °C). Various characterization techniques such as X-ray diffraction (XRD), Mössbauer spectroscopy, Scanning Electron Microscopy combined with Energy Dispersive X-ray (SEM/EDX), X-ray Photoelectron Spectroscopy (XPS) and Temperature Programmed Reduction (TPR) were performed for detailed physicochemical characterization of Fe/olivine catalyst. Although the iron concentration, calcination temperature is of great importance for establishing interactions between olivine and the added iron. XRD and Mössbauer spectroscopy indicate that iron (II) leaves the olivine structure by oxidation leading to catalysts with high iron (III) contents after calcination between 1000 and 1400 °C. The TPR results allow to determine that 10Fe/oliv1000 °C seems to be the best compromise involving significant amount of metallic iron available after reduction and sufficient iron migration through olivine grain. Catalytic reforming of toluene as tar model compound was investigated. Experiments were conducted in a fixed bed reactor at 825 °C in the presence of a gas mixture (H₂, CO, CO₂ and CH₄) representative of the gas present at the biomass steam gasifier outlet. Toluene conversion (91%) and hydrogen production ($0.066{\text{\hspace{0.17em}mol}}_{{\text{H}}_2}{\text{/h/g}}_{\text{cat}}$) were three times higher than with olivine (39% of conversion and $0{\text{.010\hspace{0.17em}mol}}_{{\text{H}}_2}{\text{/h/g}}_{\text{cat}}$ of H₂ production). Characterizations performed after tests indicate that a high percentage of iron in its metallic form is maintained leading to C–C and C–H bonds break and a sufficient percentage of iron (II) is available by simple oxidation in the olivine structure. Carbon formed by toluene decomposition is oxidized due to the presence of high water concentration. These observations can explain the efficiency and stability of the Fe/olivine catalyst. This work illustrates that 10Fe/oliv1000 °C, which is a non toxic and cheap catalyst, has properties for tar steam reforming and for a use in biomass steam gasification in a fluidized bed.