Torrefaction
Wei-Hsin Chen, in Pretreatment of Biomass, 2015
10.5 Oxidative Torrefaction
Oxidative torrefaction provides a different approach from conventional torrefaction to upgrade solid biomass. Oxidative torrefaction refers to when biomass is torrefied in oxidative environments at temperatures of 200–300 °C, which is at the same temperature range of nonoxidative torrefaction. Although nonoxidative torrefaction is able to upgrade biomass, additional operating costs are incurred due to heating and the supply of an inert gas (usually nitrogen). If biomass torrefaction is carried out in oxidative atmospheres, it would thus be possible to reduce costs, for the following three reasons. To start with, if air or a combustion flue gas is utilized as a carrier gas, the process of gas separation to extract nitrogen is no longer needed. Next, oxidative torrefaction not only involves the devolatilization and thermal degradation of biomass [54], which also occur in nonoxidative torrefaction, but also involves oxidative reactions [55–58]. The oxidative reactions in oxidative torrefaction are usually exothermic, implying that heat is generated from the torrefaction. This also reduces the heating demand for endothermic biomass torrefaction reactions. Finally, the reaction rates of oxidative torrefaction are generally faster than those of thermal pyrolysis, which will shorten the torrefaction duration and thus the operating costs.
In oxidative torrefaction, a higher decomposition rate of biomass can be obtained if a higher O2 concentration is used in a torrefaction environment [58–62], and thus a shorter torrefaction time or a lower torrefaction temperature can be desired to achieve a target weight loss. However, if light torrefaction or lower torrefaction temperatures are performed, the influence of increasing the O2 concentration on reducing the solid yield and changing the composition of the resulting material is not significant [63]. In short, oxidative torrefaction is a feasible way to upgrade ligneous biomass such as eucalyptus, but is not suitable for fibrous biomass, such as oil palm fiber, as a consequence of low solid and energy yields [10]. Overall, an increase in the severity of nonoxidative torrefaction increases the enhancement factor of HHV, but decreases the solid yield, as shown in Figure 10.9. In contrast, increasing the severity of oxidative torrefaction decreases both the enhancement factor and solid yield, due to the oxidative reactions in the biomass.