The evaluation of spent coffee grounds as feedstock for continuous hydrothermal liquefaction

Abstract

This study looks at converting a low-value waste in a continuous hydrothermal liquefaction reactor to a high-value biocrude and biochar product. The feedstock chosen for this study was spent coffee grounds (SCG) as spent coffee grounds is a readily available, low-value biomass that does not compete as a food source. The global coffee consumption was reported to be 8.5 million tons in 2015 and is expected to reach 10.5 million tons in 2020. This makes SCG a significant waste product that can be used for the production of renewable fuels. SCG was gathered from a local coffee shop, mixed with water and placed in a high-temperature, high-pressure environment inside a continuous hydrothermal liquefaction pilot reactor. For the purpose of this study, the reactor was operated at 305°C and 90-95 bar where the flow rate was varied between 60-120 L/h. All of the products obtained from the continuous HTL reactor were quantitatively analysed to determine the optimal residence time. The maximum biocrude and biochar yields obtained from the HTL of SCG was 302.7 g/kg SCG and 170.7 g/kg SCG respectively, at a biomass loading of 3 vol.%. The average higher heating value (HHV) was relatively high at 36.43 MJ/kg and 30.28 MJ/kg for the biocrude and biochar respectively. The biocrude had a low oxidative stability as it consisted mostly of C16 and C18 fatty acids. Analyses on the gas phase indicated that mainly CO and CO2 were produced during the HTL of SCG