Catalytic solvothermal liquefaction of kitchen waste for the production of high-quality bio-oil: Effects of reusing ethanol phase as the reaction medium.

Abstract

Solvothermal liquefaction (STL) can convert kitchen waste (KW) into bio-oil without pre-drying. This study systematically investigated the effects of Fe-Ni bimetallic catalysts and recycled ethanol on enhancing bio-oil production. It also assessed the stability and reusability of the recycled catalysts, as well as the environmental benefits of STL technology. Compared to non-catalytic liquefaction, the Fe-Ni bimetallic catalysts significantly promoted the hydrodeoxygenation and esterification processes in bio-oil production. The highest yields of hydrocarbons (13.52%) and esters (69.86%) in bio-oil were achieved using the 6Fe-10Ni/Al₂O₃ catalyst. Increasing Fe loading positively facilitated the random scission of long-chain and C-C bonds, resulting in an increase in the fraction of lighter molecules in bio-oil to 74.85%. After the sixth cycle, the 6Fe-10Ni/Al₂O₃ catalyst maintained excellent stability and reusability. In comparison to fresh ethanol, the recycled ethanol significantly reduced the nitrogen and sulfur content of the bio-oil. The nitrogen of 70.45% and the sulfur of 84.67% were primarily dissolved to the aqueous phase due to the higher polarity of recycled ethanol. The results of a techno-economic assessment (TEA) and a life cycle assessment (LCA) confirmed that the STL of KW is both economically viable and environmentally friendly. The experimental findings provide valuable insights for future research on the upgrading, processing, and refining of bio-oil.