Sequential cascade biorefinery for the valorization of spent coffee grounds: Energy, biodiesel, furfural, and biochar production

Abstract

This study presents a sequential biorefinery approach for the integrated valorization of spent coffee grounds (SCG) within a circular economy framework. The process comprises multiple operational units. Initially, SCG are pelletized for direct combustion, demonstrating promising energy potential. In the second unit, coffee oil is extracted via Soxhlet using hexane, isopropanol, and ethanol, and subsequently converted into biodiesel through transesterification, yielding a biofuel with a favorable fatty acid profile. The third unit involves ultrasound-assisted cold alkaline extraction (CAE) of the residual solid to selectively recover hemicelluloses with minimal degradation. Optimal CAE conditions (40 °C, 90 min, 100 g L⁻¹ NaOH) enhanced hemicellulose yield while maintaining high calorific value (21.529 kJ kg⁻¹), comparable to the raw material (21.560 kJ kg⁻¹). The hemicellulose-rich liquor was subjected to acid hydrolysis for furfural production, with maximum yield (590.3 mg L⁻¹) achieved under central experimental conditions (2% acid, 160 °C). The solid residue from hydrolysis underwent pyrolysis to produce biochar. Kinetic analysis of the pyrolysis process using the Flynn–Wall–Ozawa method on TGA data enabled determination of activation energy without assuming a specific reaction model. The results confirm the feasibility of integrating thermochemical and biochemical pathways for SCG valorization, supporting the development of sustainable biorefineries and contributing to renewable energy and chemical production within circular economy strategies.