Solar-Driven Torrefaction of a Lignin-Rich Biomass Residue in a Directly Irradiated Fluidized Bed Reactor

The share of energy production from renewable sources is continuously increasing and aims at reaching two-thirds of primary energy supply in 2050. In this scenario, the 2009/28/EC Directive requires that at least 10% of the transportation fuel of every EU Country derives from renewable sources by 2020, thus largely encouraging the production of biofuels. The synthesis of second-generation biofuels is characterized by extensive generation of residues with large lignin content. Torrefaction can be applied to sterilize and stabilize this residue, and to upgrade its properties for subsequent utilization in the energy field. In this study, the use of solar energy via concentrating solar power systems is proposed to perform solar-assisted torrefaction of a lignin-rich residue derived from a second-generation bioethanol production plant. The process was carried out in a lab-scale directly irradiated bubbling fluidized bed (BFB) reactor, a system designed so as to simultaneously operate as a solar receiver and a reactor in real applications. Experiments were also performed in a lab-scale conventional (i.e. electrically heated) BFB test reactor. Chemical and physical analyses were performed on the product materials to investigate changes in the elemental composition and volatile matter content as a function of the reaction temperature. Heating values of the torrefied materials were also characterized. Scanning electron microscopy (SEM) analyses were performed on the torrefied residues to highlight possible differences in the structure of the materials recovered after solar and nonsolar experiments. The influence of operating conditions on the quality of the torrefied biomass has been assessed, with a particular emphasis on the possible overheating of fuel particles under the high radiative flux conditions experienced in solar-driven torrefaction.