Valorization of orange peel residues via fluidized bed torrefaction: comparison between different bed materials

This paper reports an experimental study aimed at investigating recovery and upgrade of orange peels waste as a solid biofuel and, potentially, as a source of valuable bio-based chemicals via a fluidized bed torrefaction process. In particular, an experimental program was performed to investigate in a lab-scale reactor the influence of the torrefaction temperature (200 and 250 °C), the reaction time (5 and 15 min) and the use of two different beds of granular solids (Ticino sand and γ-alumina spheres) on the distribution and the composition of the main output products (torrefied solids, condensable volatiles and permanent gases) arising from the torrefaction treatment of orange peels (with 6-7 %wt. moisture content). Results show that the torrefaction of orange peels makes them more suitable for energy applications by leading to a remarkable increase in their energy content (by a factor of 1.5 for the feedstock torrefied at 250 °C) and a significant reduction in the O/C ratio (up to approximately 60 % for the biomass torrefied at 250 °C). The temperature was found to be the key process variable affecting the torrefaction performances in terms of both solid product yield and quality, whereas almost no effects were detected by increasing the reaction time in the investigated temperature range. An interesting number of valuable biochemicals was detected in the condensable fractions of the volatiles evolved during the torrefaction treatment of orange peels, which could make the process economically more feasible. The chemisorption of condensable compounds on the surface of γ-alumina spheres was found to occur, which turned out to be completely reversible. This surprisingly suggests a possible application route: commercial γ-alumina might be used as an assisting adsorbent solid material for the recovery of condensable volatiles from fluidized bed torrefaction plant, while eliminating many of the problems associated with traditional condenser-based recovery systems.