Torrefied Biomass Detoxification: Extended Residence Time Overcomes Heating Rate Effects on PCDD/PCDF Formation during Torrefaction

Abstract

Environmental applications of torrefied biomass may be severely limited by contamination with carcinogenic polychlorinated dibenzo-p-dioxins (PCDD), dibenzofurans (PCDF), and dioxin-like biphenyls (dl-PCB). In this article, the first systematic assessment of the influence of heating rate (HR) and residence time (RT) on the evolution of PCDD/PCDF/dl-PCB loads in bark-derived chars produced via torrefaction is presented. Sixteen torrefied biomass variants were examined, produced at a fixed temperature of 260 °C under different HR and RT conditions. Short processing durations (≤ 60 min) were found to induce a twofold increase in torrefied biomass toxicity (up to 1.314 ± 0.197 ng-TEQ·kg-1 88% DM; DM stands for the dry matter), driven by precursor-mediated formation of PCDD/PCDF/dl-PCB, whereas extended residence times were shown to progressively detoxify the torrefied biomass, ultimately restoring toxicity levels comparable with, or markedly below, those of the raw biomass (0.689 ± 0.103 ng-TEQ·kg-1 88% DM). The formation pathways of PCDD/PCDF/dl-PCB were governed by HR, as evidenced by the distinct kinetic profile observed at 30 °C·min-1, reflecting suppression of the precursor-driven pathway, and by the newly identified “delay effect” of toxicity increase at 50 °C·min-1 conditions. Collectively, these findings demonstrate the critical influence of HR and RT on the optimization of torrefaction conditions to produce torrefied biomass with reduced PCDD/PCDF/dl-PCB contents.