Fluidized bed torrefaction of biomass pellets: A comparison between oxidative and inert atmosphere

The paper reports on an experimental study aimed at investigating the potential of performing biomass torrefaction using air instead of nitrogen as a carrier gas, which allows reducing the operating costs of upgrading biomass. Specifically, the performances of oxidative and non-oxidative torrefaction treatments were comparatively investigated by using commercial wood pellets and olive pomace pellets as a feedstock. Batch torrefaction tests at three different temperature levels (200, 230 and 250 °C) were performed in a laboratory-scale fluidized bed reactor by fixing the reaction time equal to 15 min; this allowed to investigate the impact of the torrefaction temperature and atmosphere on: a) the distribution of the main output products of the torrefaction process (torrefied solids, condensable volatiles and permanent gases); b) the properties of torrefied pellets as a fuel (i.e., proximate composition, ultimate composition and calorific values); c) the process performances in terms of mass and energy yields of the solid product; and d) the quality of torrefied pellets in terms of mass and energy densities, hardness (shore D) and non-standard durability index. Fluidized-bed torrefaction tests were also complemented by non-isothermal thermogravimetric measurements (TG/DTG/DSC) performed in both air and nitrogen atmospheres. Results show that, under the same operating conditions, the process performance of oxidative torrefaction (i.e., mass and energy yields of the solid product) are worse than that of the non-oxidative treatment, particularly in the case of olive pomace pellets. This is mostly a consequence of the higher degradation rate triggered by oxidative reactions. On the other hand, the physical properties (i.e., mass density, energy density, hardness and durability) of torrefied olive pomace pellets obtained under oxidative atmosphere are better than those obtained via non-oxidative torrefaction. A similar behavior, however, was not observed in the case of woody fir pellets. This confirms that the fluidized bed torrefaction is feedstock-sensitive process; in particular, non-woody olive pomace pellets can be torrefied in oxidative environments, whereas fir pellets are more suitable for non-oxidative torrefaction. Since the torrefaction treatment of biomass pellets in a fluidized bed reactor has not been investigated so far, findings of this work can be useful to highlight potential advantages related to use of such a technology in this specific application.