Fe-catalyzed low-temperature hydrothermal liquefaction of coffee shells: Influence of operating parameters on bio-oil yield, distribution of bio-oil products

Coffee shells were hydrothermally liquefied under FeCl3 catalysis to produce crude bio-oils at temperatures ranging from 190 to 300 C-degrees. This study investigated the impact of operational parameters (hydrothermal temperature, residence time, Fe3+ catalyst concentration, extraction solvents) on the bio-oil yield. Optimized hydrothermal conditions resulted in a conversion rate of 68.47 % for coffee shells, leading to an 11.51 % yield of light bio-oil extracted from the aqueous phase and a 35.99 % yield of heavy bio-oil from the solid phase. The chemical composition and distillate distribution of the bio-oil were determined through organic element analysis, GC-MS analysis, and TG/DTG analysis. The light bio-oil primarily consisted of furfural derivatives and phenolic derivatives; however, it only accounted for approximately 14.50 % of the total energy recovered from the feedstock. The components present in heavy bio-oils varied depending on the extraction solvents used but mainly comprised fatty acids and their derivatives. Acetone extraction yielded the highest amount of bio-oil; however, its high O/C ratio reduced its higher heating value (HHV). Ethanol extraction also resulted in higher yields but TG/DTG analysis indicated that these fractions contained more refractory volatiles and solid residues compared to other solvents used for extraction. In comparison, n-hexane extraction had a lower yield than both previous methods but offered a lower O/C ratio and higher HHV instead. Our study demonstrated that Fe-catalyzed hydrothermal liquefaction is an efficient method for obtaining value-added compounds and hydrocarbon fuels from coffee shells.