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

Abstract

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).

Keywords: Bioadsorbent; Bioeconomy; Pore volume upgrade; Spent coffee grounds; Torrefaction; Waste reuse.