Mechanisms and Main Regularities of Biomass Liquefaction with Alcoholic Solvents

The effects of three alcoholic solvents, including monohydric n-octanol, dihydric ethylene glycol, and trihydric glycerol, on the process of biomass liquefaction were studied by thermogravimetry (TG) analysis. The results showed that there were three stages during biomass liquefaction, including biomass dehydration, volatilization of alcoholic solvents, and biomass alcoholysis. At the stage of biomass alcoholysis, alcoholic solvents combined with fragments from biomass cracking, which resulted in exothermic reaction and formed lowly volatile compounds. The reaction temperature T-R of biomass alcoholysis increased in the following order: T-R ((n-octanol + biomass)) < T-R ((ethylene glycol + biomass)) < T-R ((glycerol + biomass)). In addition, the effects of these alcoholic solvents on the produdt distributions and liquefaction yields have also been investigated with a stainless-steel autoclave. The results indicated that n-octanol was the best solvent for biomass liquefaction. The highest liquefaction yield of 83.54 wt % was obtained with n-octanol; meanwhile, the content of net light oil was up to 30 wt %, and the content of unreactive n-octanol reached 55.62 wt %. Moreover, the gas chromatography-mass spectrometry (GC-MS) results suggested that only five organic compounds, including acetic-octyl-ester, hexanoic-4-octyl-ester, 5-methyl-furfural, octyl-ether, and unreactive n-octanol, have been found in the light oil. The simple components of light oil were beneficial to the separation of valuable chemicals and recycle of unreactive n-octanol. Although ethylene glycol and glycerol can also promote the cracking of biomass, a single molecule of polyhydric alcohol can be combined with multimolecular cracking fragments. Consequently, the liquefaction yields of biomass were less than 60 wt %, and the main products were heavy oil and residue.