Abstract: In some variations, the invention provides a process for fractionating biomass, comprising: fractionating the biomass in the presence of a solvent for lignin, sulfur dioxide, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; hydrolyzing the hemicellulose contained in the liquor, to produce hemicellulosic monomers; hydrolyzing the cellulose-rich solids to produce glucose; and recovering the hemicellulosic monomers and the glucose, as fermentable sugars, wherein a metal sulfite or metal bisulfite additive is introduced to react directly or indirectly with lignin to produce sulfonated lignin. The disclosed processes may enhance lignin separations as well as promote the co-product potential of lignin.

Abstract: Variations of this invention reduce or avoid lignin precipitation during acidic hydrolysis of biomass hydrolysates (such as hemicellulose-containing liquid extracts). Net acid usage and byproduct salt formation are significantly reduced. In some embodiments, hemicellulosic oligomers are hydrolyzed, in the presence of sulfur dioxide, to produce fermentable hemicellulosic sugars; the process comprising recovering and recycling at least a portion of the sulfur dioxide, wherein at least a portion of the sulfur dioxide reacts with the lignin to produce hydrophilic sulfonated lignin that has less tendency to precipitate or stick.