Valorisation of Spent Coffee Grounds: Production of Biodiesel via Enzymatic Catalysis with Ethanol and a Co-solvent

This work aims to explore the production of biodiesel from oil extracted from spent coffee grounds (SCG) via enzymatic catalysis with the Lipozymes RM IM, TL 100L and CALB L, in the presence of a co-solvent, and to optimize the reaction conditions. A three factors design of experiments was performed to evaluate the influence on the reaction performance of ethanol/oil molar ratio, enzyme concentration and reaction temperature. Oil was extracted from air dried SCG with hexane in a pilot solid/liquid extractor, resulting in 6.4% (m/m oil) in dry SCG. The coffee oil was characterized for its kinematic viscosity (38.04 mm2/s), density (0.903 g/cm3), heating value (38 MJ/kg), iodine value (63 g I2/100 g oil), water content (0.1458%), acid value (44.78 mg KOH/g oil), flash point (>120 °C) and triglyceride content (82.8%). Based on the design of experiments, the best conditions for the transesterification reaction are 5:1 molar ratio of ethanol/oil, 4.5% (m/m oil) of enzyme and 45 °C of temperature, resulting in an experimental yield of 96.7%, with 87.6% (m/m) of esters content. Additionally, it was studied the effect of water in the transesterification reaction, using ethanol at 92, 85 and 75% (v/v). Results show that the best reaction yield (97.2%) and esters content (92.1%) were obtained using ethanol 92%. Although the esters content of the biodiesel obtained from coffee oil does not accomplish the EN 14214:2009 requirements, it is promising as there is the possibility to blend the coffee oil with oil from other sources in order to meet the standard requirements.