Fluidized Bed Torrefaction of Commercial Wood Pellets: Process Performance and Solid Product Quality

This paper reports an experimental study aimed at investigating the influence of fluidized bed torrefaction treatment on the quality of commercial wood pellets. In particular, an experimental program was performed, which allowed us to investigate, at a laboratory scale, the impact of the torrefaction temperature (200, 230, 250 °C) and the reaction time (7 and 15 min) on (a) the distribution and the composition of the main output products of the torrefaction process (torrefied solids, condensable volatiles, and permanent gases); (b) the quality of torrefied pellets; and (c) process performance in terms of mass and energy yields of the solid product. In particular, the quality of pellets was characterized in terms of apparent density, bulk density, calorific values, volumetric energy density, moisture uptake, swelling behavior, hardness (Shore D), and nonstandard durability index. Results suggest that light torrefaction (200 °C and 7-15 min) is the most suitable to ensure a sustainable production (84-85% mass yield and 94-95% energy yield) of high quality torrefied wood pellets (no swelling in water, about 27% decrease in the moisture uptake after 7 days of exposure in an environment at a high relative humidity of 80%, hardness and durability comparable to those of untreated wood pellets, only a 2-3% decrease in the volume energy density) in a downstream configuration (torrefaction after pelletization). The torrefaction treatment of wood pellets in a fluidized bed reactor has not been investigated so far; therefore, findings of this work can be useful to highlight potential advantages and drawbacks related to the use of such a technology in this specific application.