Torrefaction operation and optimization of microalga residue for energy densification and utilization

The torrefaction characteristics of a microalga (Chlorella vulgaris ESP-31) residue in inert (N₂) and non-inert (CO₂) atmospheres at temperatures of 200-300°C with the durations of 15-60min are investigated. A parameter of torrefaction severity index (TSI) is employed to account for the thermal degradation phenomena. The results indicate that the enhancement factor of higher heating value, energy yield, and atomic H/C and O/C ratios versus TSI are strongly characterized by a linear relationship. The solid and energy yields of the residue torrefied in CO₂ are lower than in N₂ inasmuch as the thermal degradation in the former is more active, presumably due to the intensified reaction of CO₂ with volatile matters in the biomass. At a given energy yield, the microalga residue torrefied at a lower temperature accompanied by a longer duration results in a fuel with higher energy densification and lower solid yield, thereby rendering the better torrefaction quality. On the other hand, a higher efficiency of energy utilization for upgrading the biomass can be achieved at a higher temperature along with a shorter duration. It is thus concluded that the optimization of torrefaction operation depends on the requirement of energy densification or energy utilization on fuel.