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.