Investigation on the Liquefaction Performances, Mechanism and Kinetics of Wheat Straw in Different Solvents

The effects of three alcoholic solvents, including ethylene glycol (EG), glycerol (G) and polyethylene glycol/glycerol (PEG400/G), on the performances, mechanism and kinetics of wheat straw liquefaction with sulphuric acid as catalyst to produce bio-polyols were systematically studied. The results show that the liquefaction solvents largely influenced on the composition of liquefaction products and liquefaction residue content. The wheat straw shows higher liquefaction efficiency in the PEG400/G than EG and G solvents in which the obtained bio-polyols shows lower change in hydroxyl number compared to the liquefaction solvent. The liquefaction products is mainly composed of liquefaction solvents and derivatives, followed by carbonyl compounds with relative content of 4.4–17.5 area%. Whether liquefaction or pyrolysis, the hemicellulose in wheat straw is the easiest degradation component, and then is acid soluble lignin and cellulose. Liquefaction mechanism shows that polyhydric alcohol can be combined with cellulose and hemicellulose degradations (C-OH) or lignin degradations (L-OH) to increase the molecule weight of bio-polyols. In addition, there are three stages during biomass liquefaction, including biomass dehydration, volatilization of alcoholic solvents and biomass alcoholysis. The liquefaction kinetics show that the alcoholysis process of wheat straw could be divided into rapid liquefaction due to the degradation of hemicellulose, amorphous phase in the cellulose and acid-soluble lignin and slow liquefaction attributed to the decomposition of the crystalline phase in cellulose. The liquefaction is also an endothermic reaction process. Graphical Abstract