In recent decades, the production and consumption of coffee has increased. As a result of the increased consumption of coffee there is a need for waste management of the spent coffee grounds (SCG). Spent coffee grounds have a very complex chemical composition, which makes them useful for a variety of applications. SCG have a 10–20 wt% of oil content, and, therefore, they are viable feedstocks for the production of biofuels. The high amount of sugars in SCG can be used to produce bioethanol through fermentation. In addition, bioethanol can be used along with lipid fraction extracted from SCG to produce biodiesel through transesterification (Caetano, 2011). Caetano et al. aims to investigate the use of SCG for the production of biofuels, as well as characterize SCG, the oil they contain, and the biofuels they produce. They also identify the optimal operating conditions in order to extract oil from SCG, to perform oil transesterification to biodiesel, and to assess the biodiesel quality (Caetano et al. 2012). —Shelby Long
Caetano, N. et al., 2012. Valorization of Coffee Grounds for Biodiesel Production. Chemical Engineering Transactions 26.
Caetano et al. examined the use of SCG for the production of biofuels. They obtained spent coffee grounds from a local coffee shop and allowed them to air dry for several days. The SCG then underwent repeated cycles of oven drying at 105 + 5 °C followed by cooling in a desiccator. The grounds were then weighed, and levels of total carbon (TC), total nitrogen (TN), protein content, ash content, cellulose content, and insoluble and soluble lignin content were taken. In order to characterize and extract oil from the SCG researchers tested different solvents to determine which would be most effective for the extraction process. For each treatment, 10 g of oven dried SCG and 200 mL of solvent were placed in a Soxhlet extractor for 2.5 to 9.5 hours. When three consecutive measurements of the solvent refraction index were constant and close to the pure solvent’s value the extraction process was stopped. The oil extraction rate was determined. The oil extracted from the SCG were measured for solvent recoverability for hexane, isopropanol, heptane, octane, ethanol. In order to recover the oil from the extracting solvent researchers used a rotary evaporator and a vacuum pump. The extracted oil was characterized and assessed based on its quality. The iodine number, acid value, water content, kinematic viscosity, density, and higher heating value (HHV) were measured. In order to produce biodiesel, several esterification steps were performed in an orbital acclimatized shaker. Researchers monitored the acid value of the product throughout the process and at specific steps added 40% methanol and H2SO4to perform the esterification process. 1% NaOH was used as a catalyst. The biodiesel was then separated from the glycerol phase and washed with distilled water to achieve a neutral pH. In order to characterize the biodiesel Caetano et al. accounted for the importance of compliance with standards for the use of biodiesel in vehicles engines. Therefore, they assessed the biofuel density at 15 ˚C, the kinematic viscosity at 40 ˚C, the acid value, the iodine value, and the methyl esters content, which should all be in accordance with EN biofuel standards.
The results of Caetano et al.’s study somewhat differ from what previous studies report. The moisture content is substantially higher, the higher heating value is lower, and the cellulose content differs from what other studies have indicated (Lago et al. 2001; Bizzo 2003; Mussatto et al. 2011b). The present study suggests these differences may be due to different coffee extraction procedures, different storage conditions of the grounds, and the use of different types of coffee for the experiment. The solvent that allowed for the highest oil recovery was octane. Ethanol and hexane allowed for lower oil recovery. Hexane had one of the higher recover rates, while ethanol exhibited a lower recovery rate. Isopropanol had a strong capacity of oil extraction and solvent recovery. The mixture of hexane and isopropanol at a ratio of 50:50 allowed for high oil extraction, but lower solvent recovery.
The extracted coffee oil had iodine values that were too low, too high viscosity to be used in direct combustion engines, and too high acidity to be directly converted into biodiesel without undergoing pre-treatment. However, the high HHV indicates that the oil extracted may be used for direct combustion. The biodiesel produced was also characterized, and researchers found that the acid value and viscosity were too high to comply with biofuel EN standards. They suggest that the high water content in the oil and the high acid value may have hindered the completion of the reaction. Caetano et al. indicate that more improvements in the SCG biofuel production process must be made in order to improve the quality of biodiesel to meet the EN standards. Some of these improvements include obtaining a higher methyl ester content by drying the oil, removing water between esterifications, neutralizing excess acid before transesterification, and using different methanol-to-oil molar ratios.