Spent coffee grounds (SCG) are an abundant and underutilized lignocellulosic waste biomass, composed of approximately 57% carbohydrates, 27% lignin, and 8% lipids. Up to 90 wt% of SCG components can be used as substrates to produce sustainable aviation fuel (SAF)-range hydrocarbons (C8-C16), making SCG a promising feedstock for an integrated biorefinery aimed at SAF production. Based on our preliminary theoretical estimations, complete fractionation of SCG's major components, (hemi)cellulose, glycerides, and lignin, combined with established SAF conversion technologies (e.g., HEFA and ETJ), can yield up to 50% energy recovery and approximately 26% mass recovery as jet-range products. Despite the maturity of SAF conversion technologies, the primary challenge remains in the efficient fractionation of the complex SCG biomass into its major components. This study focuses on alcohol-based solvolysis as a one-pot fractionation approach for the recovery of SAF-relevant substrates (carbohydrates, lignin, and lipids). To improve both economic viability and process circularity, this system integrates process-derived intermediates, glycerol and ethanol, as repurposed endogenous solvents.
Methods
Methanol solvolysis was conducted at 180, 200, and 220 °C for 3 hours using 3 g of SCG and 20 mL of methanol in Parr Series 5000 multiple reactor systems. A reactor vessel was purged then slightly pressurized with nitrogen to 1 bar. The system was then heated to the set temperature and maintained for 3 hours for fractionation. This process produced a (hemi)cellulose-rich solid residue and an oil comprising lignin and fatty acid methyl esters (FAME), formed via methanol-induced transesterification of the extracted lipids. The oil phase was further subjected to solvent extraction to isolate FAME from lignin. Oil-phase products were analyzed using HPLC, GC-FID/MS, NMR, and GPC. Compositional analysis of the solid residue was conducted to quantify glucan, xylan, arabinan, and lignin content. The performance of glycerol and ethanol as alternative endogenous solvents was also investigated.
Results and discussion
Methanol solvolysis effectively separated SCG into its major components. The retention of glucan and xylan in the solid residue was 81% and 71%, respectively, while the recovery of lignin and glycerides in the produced oil reached 63% and 93%, respectively. Fractionation using glycerol and ethanol demonstrated comparable fractionation efficiency to methanol-based fractionation, enabling a 90% reduction in operating pressure. These findings support the development of a fully integrated and scalable SAF production system using abundant biomass waste.