Final Upcycling of Post-treatment Agri-food Residues by Torrefaction

Various agri-food residues were taken into consideration in this work in a dedicated experimental program centered on lab-scale fluidized bed torrefaction. They include some “difficult” feedstocks for fluidization (e.g., Industrial Cherry Cake and Coffee silverskin) or unusual ones (e.g., spent Lemon peels and Tomato seeds), for their upcycling to solid biofuels. Three different temperature levels (200, 250 and 300°C) were considered under a constant residence time (5 min) under actual torrefaction conditions of a biomass batch in the bed. A drawback induced by the biomass torrefaction mechanism, i.e., the definitive "capture" of the small bed inert solids on the surface of the particles undergoing torrefaction, was overcome by a bed switch from fine but rather heavy solids (100-250 µm quartz sand) to coarser and lighter ones (expanded clay 1000-1400 μm). This yielded torrefied products of a better quality as solid biofuels being free of “extraneous” inert particles. As far as the ranking of the feedstocks, Industrial Cherry Cake and Grape Marc performed better in terms of Energy Densification Index (IED even larger than 1) and Energy Yield (EY close to 100%).