Hydrothermal liquefaction of catering wastes towards biofuel intermediates

The potential application of the hydrothermal liquefaction process on catering wastes, spent coffee grounds (SCG) and orange peels, towards reducing municipal wastes was examined in this study, targeting to the production of biofuel intermediates. The study was focused on the two main process parameters, temperature (280–350 °C) and residence time (5–60 min) while the other parameters such as biomass/solvent ratio and applied extractant were selected according to literature. The results indicated that spent coffee grounds are a more promising feedstock, rendering higher bio-crude oil yield than orange peels (34 wt% and 22 wt% respectively). The optimal conditions for each feedstock varied due to their different structural properties, which not only affected the products’ yields but also their quality. To better understand the distribution of the process degradation products and a part of the reaction’s pathway, the gas product was analyzed via GC-FID, and the aqueous phase was studied via HPLC analysis. Finally, the bio-crude oil was subjected to elemental, thermogravimetric, and GC-MS analysis to determine its quality and biofuel potential, based on which the bio-crude oil derived from spent coffee grounds showed promising results while orange peel–derived oil is of lower value. According to the study’s findings, the bio-crude oil can be utilized as a fuel substitute after being upgraded via hydrotreatment enabling the heteroatoms removal and enhancing its fuel properties.