Valorization of Orange Peel Residues via Fluidized Bed Torrefaction: Comparison between Different Bed Materials

An experimental study was carried out to evaluate an alternative disposal route for waste orange peels, which includes their upgrade into a solid biofuel and, possibly, the production of valuable liquid bio-chemicals via fluidized bed torrefaction. The influence of the torrefaction temperature (200 and 250°C), the reaction time (5 and 15 min) and the use of different bed materials (Ticino sand and γ-alumina spheres) on the distribution and composition of the main output products (torrefied solids, condensable volatiles and permanent gases) was investigated. Results show that the torrefaction at 250°C transforms orange peels from low-value residue to high-quality energy carrier by increasing their energy content (by a factor of 1.5) and reducing the O/C ratio (up to 67%). The temperature strongly influences the process performance and is the key process variable. Conversely, almost no effect were detected by changing the reaction time and the kind of granular solids on both the distribution and the composition of the main torrefaction products. Although γ-alumina does not exhibit a remarkable catalytic activity in the investigated temperature range, its use instead of sand is recommended as its lower density makes the bed smoother to be fluidized.