Açaí seed valorization through torrefaction and pyrolysis: Pathways toward high-energy and low-carbon biocoal

The sustainable use of Amazonian residues plays a key role in Brazil's decarbonization strategy and circular bioeconomy. Among these residues, açaí seed ( Euterpe sp. ) accounts for 70–80% of the fruit mass and remains largely underutilized, despite its high lignin and low ash contents. This study systematically investigated the torrefaction (200–320 °C) and pyrolysis (350–700 °C) of açaí seed in a fixed-bed reactor to define process boundaries and optimal operating conditions for high-energy solid fuel production. The reactor was operated at a heating rate of 1.67 °C min −1 with a residence time of 30 min. Progressive carbonization increased the fixed carbon from 18.70% to 91.67%, while the higher heating value (HHV) rose from 19.43 to 32.97 MJ kg −1 . The transition from fuel-grade biocoal to reductant-grade biochar occurred between 450 and 500 °C, with the EMCI peaking (23.02) at 500 °C, where the optimal balance between solid retention and energy densification was achieved. The volumetric energy density reached 15.99 GJ m −3 , and the resulting biochar exhibited H/C and O/C ratios as low as 0.03 and 0.10, respectively, satisfying the European Biochar Certificate criteria for highly carbonized materials. These properties indicate that açaí seed can yield carbon-rich biocoal comparable to petroleum coke, suitable for metallurgical and decentralized energy applications. Overall, this work provides an integrated assessment of açaí seed torrefaction and pyrolysis, establishing quantitative benchmarks for the valorization of this abundant agro-industrial residue and contributing to the United Nations Sustainable Development Goals (SDG 7, 12, and 13) through low-carbon bioenergy and waste-to-resource strategies. • Acai seed torrefaction tailors biochar properties for energy or metallurgical use. • Biochar quality transitions from fuel grade to reductant grade with temperature. • Mild torrefaction enhances acai seed valorization within Amazon bioeconomy. • Energy–mass conversion index identifies optimal torrefaction window. • Torrefied acai biochar supports circular valorization of Amazon agro-residues.