Catalytic Biomass Gasification Process in Fluidized bed Reactor

Abstract

This thesis focuses on the development of a new catalytic system for conversion of tar produced during biomass gasification in a fluidized bed gasifier in order to overcome the typical drawbacks (low activity, coke deactivation) of conventional catalysts. The innovative catalytic system is a gamma-alumina-supported lanthanum-cobalt perovskite (20 wt %) promoted with small amounts of rhodium (0.1 to 1wt%) which was proposed for the high reforming activity of the noble metal and the good oxygen availability of perovskite, respectively. In addition, the large dispersion of rhodium into the LaCoO3 matrix inhibits its possible sintering at high temperatures, typical of biomass gasification (800-900°C). In order to investigate the catalytic properties by modifying both the catalyst formulation and the operative parameters, an experimental plant at a laboratory scale, which allows the contact between catalyst and a real mixture of biomass devolatilization products, has been set up. It consists of two connected fixed bed micro-reactors, heated independently in two different electric furnaces and it is equipped with an analysis system for detection and characterization of all gaseous and liquid products. This set up allowed an easy and economic catalytic screening, since it did not involve the multiplicity of phenomena (fluidodynamics, segregation and mass transfer) occurring in a real scale fluidized bed gasifier and allowed the use of much smaller amounts of catalytic material. The activity in biomass tar conversion of the novel catalytic formulation has been compared, in pyrolysis conditions, to that of conventional catalysts (olivine, dolomite, Ni/Al2O3). It was found that the novel catalyst was able to completely convert tar and light hydrocarbons contained in the biomass devolatilization products, but also to significantly increase the syngas yield due to prevailing of reforming properties in contrast with more conventional catalysts mainly providing cracking activity. Moreover, the catalyst had a limited sensitivity to coke deactivation. These findings were supported by the study of redox properties of the active phases deposited on the alumina support by TPR analysis. The study of catalytic activity and redox properties also led to define the best catalytic formulation. The best performances were obtained with catalysts containing both rhodium and perovskite due to the synergic effect of the two phases coupling the highest reforming activity with the lowest coke deposition. In addition, the deposition of the perovskite layer prevents the encapsulation of rhodium into the alumina matrix which led to the formation of a less active rhodium aluminate. A very efficient tar conversion activity was maintained also for a rhodium content as low as 0.1 wt% thus strongly limiting the amount of the expensive precious metal. Likewise, the operation temperature can be lowered to 600°C keeping the same performances observed at high temperatures. The set-up of a stainless steel fluidized bed gasifier at pilot-scale (140 mm ID) was also performed. The efficiency of the mixing between biomass, volatiles and catalyst, which is difficult due to the low density of biomass with respect to the catalytic bed particles and to the formation of endogenous volatiles bubbles, was improved by the use of a conical gas distributor at the bottom of the fluidizing column. Sampling of the elutriated solid and unconverted tars was performed isokinetically. Gaseous, liquid products and elutriated solid fines were characterized by suitable analytical systems. An experimental campaign of air/steam gasification, to enhance the production of an hydrogen-rich syngas, using conventional catalyst (olivine, dolomite, Ni/Al2O3) was carried out in the fluidized bed gasifier.