Biomass torrefaction in gas atmospheres containing oxygen, such as hot flue gases or air, is an interesting concept with potentially positive implications with respect to processing costs and efficiency. This study aims to investigate the effects of varying concentrations of oxygen in the atmosphere on the torrefaction of sugarcane bagasse in a packed bed at different temperatures, under realistic conditions of direct convective heat transfer. In addition to understanding the development of bed temperature profiles, the study includes an analysis of the torrefied solids (HHV determination using bomb calorimetry) and the torrefaction volatiles (permanent gases: Near-infrared spectroscopy, condensates: HPLC with UV-Vis/RI detection). It was observed that for this specific reactor configuration and heat transfer regime, there are temperature specific limits beyond which an increase in oxygen concentration leads to an oxidative thermal runaway in the packed bed. An increase in torrefaction temperatures from to 290°C, for e.g. leads to a reduction in the tolerable limits of oxygen concentration from 5% to 1%. It was further determined that away from the ignition zone, moderate addition of oxygen does not drastically reduce mass and energy yields of the solids, and in some cases may also lead to more uniform heating of the biomass bed. Increase in oxygen concentration also leads to a shift in the partitioning of the volatiles in favour of gaseous products, with specifically an increase in carbon dioxide resulting in reduction of the HHV of the volatiles.
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