Green synthesis of fuel grade hydrocarbons from spent coffee grounds

Abstract

Spent coffee grounds (SCGs) are the main residue that remains after a coffee beverage has been brewed from coffee beans. They are a potential source of oil that can be used as a sustainable and environmentally friendly feedstock to produce fuel-grade hydrocarbons. However, the use of fossil fuel-based solvents such as hexane for oil extraction and nonreusable catalyst like sodium hydroxide (NaOH) for converting the oil to fuel-grade hydrocarbons render the valorisation of SCGs uneconomical and pose a threat to the environment. Considering these issues, green solvents and green catalysts need to be developed, to improve the process overall efficiency. SCGs high moisture content can have negative impact of the quantity of oil extracted. The high free fatty acid content on the oil extracted from SCGs oil can negatively affect the quality of the fuel-grade hydrocarbons produced from the oil extracted from the SCGs. This reduces the environmental benefit of using the coffee waste for fuel production. Therefore, a better understanding of the nature of the SCGs as a feedstock and processing route is required, to improve the overall process efficiency. The present study was divided into three major sections namely: characterisation of SCGs in terms of moisture content, particles size, and oil content; extraction of lipids from SCGs which includes the use of hexane as a datum solvent, and 2-methyltetrahydrofuran (2- MeTHF) as an understudy green solvent. The oil extraction experiments investigated the effect of the solids-to-solvent ratio and the effect of extraction time on the yield of lipids for both solvents. The final part of the experimental work involved synthesising lithium doped calcium oxide (Li-CaO(s)) as a renewable catalyst for the transesterification of the SCGs oil. Calcium oxide was derived by the calcination of chicken eggshells in a kiln for 7 h at 600 °C and doped with lithium for 4 h at 900 °C. The effect of catalyst loading and reaction period on FAME yield was investigated. Three SCGs samples from three retailers were used. The samples were found to have an average moisture content of 67.89 wt% of the initial SCGs used on a dry basis, a particle size of 500 µm on average and 27.76 wt% oil of the initial SCGs used. It was observed that 2-MeTHF provided a higher oil yield of 27.76 wt% relative to19.03 wt% which was achieved by hexane, using a Soxhlet apparatus. The optimum oil extraction conditions were observed to be 4.50 h extraction time at 1:18 solventto-solid ratio for 2-MeTHF. One-step transesterification reaction was used for production of FAME. The optimum reactor conditions were observed to be 65 ℃ reactor temperature, a catalyst loading of 4 wt% of the oil used, a 2 h reaction time and a methanol-to-oil molar ratio of 12:1. These reactor conditions achieved a maximum oil conversion of 97.81 wt%