In-situ transesterification process for biodiesel production using spent coffee grounds from the instant coffee industry

Abstract Industrial spent coffee grounds (IND-SCG) are a potential non-edible biodiesel feedstock due to their abundant global supply and high oil content. In this study, an in-situ transesterification ( in-situ TE) was developed and scaled up for IND-SCG biodiesel production. Several hurdles must be overcome, including the high acid value, and wide range in particle size of IND-SCG. Washing IND-SCG with methanol reduced its high acid value with negligible loss of oil. Size reduction (0.25–1.68 mm) and an increase of the reaction temperatures (30–60 °C) were found to improve the biodiesel yield significantly. The whole deacidified IND-SCG was processed at 50 °C; and a maximum biodiesel yield of 77% was achieved within 3 h. The process was successfully scaled up for processing 4 kg IND-SCG per batch with a yield comparable to the 30-g scale. The IND-SCG biodiesel met the ASTM biodiesel standard in terms of total glycerin, water content, kinematic viscosity and oxidative stability index (OSI), but its acid value exceeded the standard. A simple process modification using acidic water to neutralize alkaline catalyst during refining step, instead of strong acid, enabled the IND-SCG biodiesel to meet the standard for acid value. The oxidative stability index of the in-situ IND-SCG biodiesel was superior to that of the conventional process, probably due to the co-extraction of natural antioxidants.