A comparison of batch extracted bio-oil and continuous hydrothermal liquefaction bio-oil using spent coffee grounds as biomass feedstock

Abstract

This study focuses on bio-oil obtained from spent coffee grounds, either by reflux extraction or continuous hydrothermal liquefaction. Spent coffee grounds was chosen as feedstock for this study as it is available around the world and considered a second generation feedstock, as it is a food waste. The production of coffee in 2017 was more than 9.5 million tons, which translates into an increase of 2.3% from 2016. A large portion of coffee beans end up as spent coffee grounds during the production of instant coffee, making this waste product an ideal feedstock for the biofuel industry. Spent coffee grounds was collected from a local coffee shop in Potchefstroom and used as feedstock in the production and extraction of bio-oil from the spent coffee grounds. Reflux extraction was done on the dried spent coffee grounds using hexane, ethanol and acetone as solvents. Different retention times were investigated for each solvent and the yield of the oil was reported. The maximum yield 11.7 wt% was obtained when hexane was used as a solvent. Continuous hydrothermal liquefaction was done using spent coffee grounds as a feedstock and a bio-crude yield of 28.5 wt% was obtained. The average higher heating value of the extracted oils was 39 MJ/kg, while the higher heating value for the hydrothermal liquefaction oil was a bit lower at 36 MJ/kg