Torrefaction of Tomato Peel Residues in a Fluidized Bed of Inert Particles and a Fixed-Bed Reactor

The potential of torrefaction treatment for upgrading low-value tomato peel residues into high-quality solid energy carriers was investigated by using a new bench-scale batch experimental apparatus based on fluidized-bed technology. In particular, the influence of the main process variables (i.e., temperature and time) on both the key performance parameters (i.e., mass and energy yields) and the main properties of the solid product (i.e., elemental composition, ash content, calorific value and equilibrium moisture content) was studied. Fluidized bed experimental runs were performed at 200, 240, and 285 °C by keeping the torrefaction time at 5, 15, and 30 min. Results suggested tomato peels as a good candidate for the torrefaction treatment. In more detail, it was observed that higher temperatures and longer holding times (with a more marked effect of the torrefaction temperature) led to an increase in the calorific value of the torrefied tomato peels, with respect to the parent ones. More specifically, the calorific value increased by a factor of 1.2 for the biomass treated at 285 °C and 30 min. Under the same experimental conditions, a 40% reduction in the O/C elemental ratio and an improved hydrophobicity of the torrefied tomato peels were also observed. These positive effects of the torrefaction treatment occurred while maintaining the mass yield (approximately between ~75% and ~94%, daf) and energy yield (~90% and ~96%, daf) at satisfactory levels. Fluidized-bed torrefaction experiments were also complemented by TGA-MS investigations and comparative tests carried out in a bench-scale fixed-bed reactor. Outcomes showed that the fluidized-bed technology is more suitable than the fixed bed one to cope with the exothermicity associated with the thermal degradation of nonwoody biomass, which has a tendency to ignite or carbonize easily during torrefaction. Furthermore, the fluidized bed proved to be more effective in ensuring a uniform and consistent quality of the torrefied solids.