Biorefinery of agricultural residues by fractionation of their components through hydrothermal and organosolv processes

The combined production of the most abundant agricultural residues in Spain (viz, cereal straw, sunflower stalks, vine shoots, cotton stalks, olive, orange and peach tree prunings, and horticultural and related residues) amounts to over 50 million tons per year. Agricultural residues can be valorized by converting their components jointly (combustion, pyrolysis, gasification, liquefaction) or separately (fractionation). The most useful method for exploiting such components separately involves isolating cellulose fibres for papermaking purposes. In recent times, this valorization method has led to the development of the biorefining concept. Biorefining involves the fractionation or separation of the different lignocellulosic components of agricultural residues with a view to their integral exploitation rather than the mere use of cellulose fibre to obtain paper products. Biorefining replaces the classical pulping methods based on Kraft, sulphite and soda reagents with a hydrothermal treatment followed by organosolv pulping. The hydrothermal treatment provides a liquid phase containing hemicellulose decomposition products [both oligomers and monomers (glucose, xylose, and arabinose)] and a solid phase rich in cellulose and lignin. By contrast, the organosolv process gives a solid fraction (pulp) and a residual liquid fraction containing lignin and other useful substances for various purposes.