| Summary: | Spent coffee grounds (SCG) are the main residues of the coffee industry, and a potentially valuable source of lipids for sustainable biodiesel production. However, feedstock properties, such as the high SCG moisture content and the relatively high free fatty acid (FFA) content of recovered oil, can impact on the efficiency of the extraction and the quality of extracted oil and derived biodiesel, thus reducing the possible environmental benefits of producing biodiesel from this waste stream. Therefore, a better understanding of feedstock properties and processing steps is required to improve the efficiency of SCG valorization as a biodiesel feedstock and contribute to its future industrialization. This work presents experimental studies including feedstock characterization of SCG, laboratory and pilot plant scale solvent extraction experiments and utilization of mechanical pressing for processing of coffee residues. The solvent extraction experiments investigated effects of solvent type, SCG moisture content and particle size, SCG-to-solvent ratio, and the duration, temperature and pressure of the extraction process on oil extraction efficiency and composition. Transesterification was performed with SCG oil containing high FFA content, and the combustion of derived biodiesel was investigated in a compression-ignition engine. Instant SCG were found to possess higher lipid and FFA content than retail SCG. Solvent extraction experiments showed that longer durations, higher temperatures, low moisture presence and mixed size SCG particles generally improved extraction efficiency, while the impact of pressure depended on temperature. A correlation was observed between longer extraction durations and lower FFA content, while extraction temperature and solvent selection affected the oil composition. Pilot plant extraction showed reduced sensitivity to moisture, while mechanical pressing was efficient in removing a fraction of residual moisture. A two-step transesterification process achieved a biodiesel conversion yield of 86.7 % relative to initial oil weight. SCG biodiesel showed similar combustion and emissions characteristics to commercial soybean and rapeseed biodiesel.
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