Pore volume upgrade of biochar from spent coffee grounds by sodium bicarbonate during torrefaction

A novel approach for upgrading the pore volume of biochar at low temperatures using a green additive of sodium bicarbonate (NaHCO₃) is developed in this study. The biochar was produced from spent coffee grounds (SCGs) torrefied at different temperatures (200-300 °C) with different residence times (30-60 min) and NaHCO₃ concentrations (0-8.3 wt%). The results reveal that the total pore volume of biochar increases with rising temperature, residence time, or NaHCO₃ aqueous solution concentration, whereas the bulk density has an opposite trend. The specific surface area and total pore volume of pore-forming SCG from 300 °C torrefaction for 60 min with an 8.3 wt% NaHCO₃ solution (300-TP-SCG) are 42.050 m² g^-1 and 0.1389 cm³·g^-1, accounting for the improvements of 141% and 76%, respectively, compared to the parent SCG. The contact angle (126°) and water activity (0.48 aw) of 300-TP-SCG reveal that it has long storage time. The CO₂ uptake capacity of 300-TP-SCG is 0.32 mmol g^-1, rendering a 39% improvement relative to 300-TSCG, namely, SCG torrefied at 300 °C for 60 min. 300-TP-SCG has higher HHV (28.31 MJ·kg^-1) and lower ignition temperature (252 °C). Overall, it indicates 300-TP-SCG is a potential fuel substitute for coal. This study has successfully produced mesoporous biochar at low temperatures to fulfill "3E", namely, energy (biofuel), environment (biowaste reuse solid waste), and circular economy (bioadsorbent).