NOVELTY - Process (P1) to produce fermentable hemicellulose sugars from lignocellulosic biomass, involves: (A) providing feedstock of lignocellulosic biomass; (B) extracting with steam to form liquor of hemicellulosic oligomers, cellulose-rich solids and lignin; (C) removing cellulose-rich solids from liquor; (D) hydrolyzing hemicellulosic oligomers in presence of sulfur dioxide, to form fermentable hemicellulosic sugars; (E) recovering and recycling at least a portion of sulfur dioxide; and (F) recovering fermentable hemicellulosic sugars from extract liquor, with formation of sulfonated lignin.    USE - For producing fermentable hemicellulose sugars from lignocellulosic biomass (claimed).    ADVANTAGE - The method reduces, avoids, or deals with lignin precipitation during acidic hydrolysis of biomass and/or biomass hydrolysates (such as hemicellulose-containing liquid extracts); reduces net acid usage or reduce byproduct salt formation; and also reduces salt formation. Also sulfur dioxide is more-efficient catalyst for catalyzing hydrolysis reactions to convert hemicellulose oligomers to monomers. Sulfur dioxide at ambient conditions is gas which will have higher mass-transfer rates within hydrolysis reactor, leading to more uniform hydrolysis chemistry; and functions as hydrolysis catalyst by proceeding through reactive intermediate that contains proton, thus after reaction step, proton may be returned to solution and molecular SO2 is regenerated. The acid hydrolysis of hemicellulose with sulfur dioxide leads to dramatically less lignin deposition, compared to acid hydrolysis with sulfuric acid, for same final sugar yield. The SO2 (or HSO3-) can react directly with lignin to produce sulfonated lignin. The reaction of sulfur dioxide or bisulfite ion with lignin involves acidic cleavage of ether bonds, which connect many of constituents of lignin. The electrophilic carbocations produced during ether cleavage react with bisulfite ions to give lignosulfonates. An important site for ether cleavage is alpha -carbon of propyl side chain of lignin. Sulfur dioxide does not tend to catalyze condensation reactions of lignin that increase molecular weight. Mechanistically, acid-catalyzed condensation and sulfonation can involve same carbon atom, the alpha -carbon of propyl group. The implication is that SO2 or HSO3 may directly react with this carbon atom before condensation reactions can be initiated. Also, native (non-sulfonated) lignin is hydrophobic, while lignosulfonates are hydrophilic. Hydrophilic lignosulfonates has lesser propensity to clump, agglomerate, and stick to surfaces. Even lignosulfonates that do undergo condensation and increase of molecular weight; have HSO3 group that will contribute some solubility (hydrophilic). Further SO2 can be recovered easily from solution after hydrolysis. The majority of SO2 from hydrolysate may be stripped and recycled back to reactor. Recovery and recycling require less lime compared to neutralization of comparable sulfuric acid, has less solids to dispose, and thus less separation equipment. The increased efficiency owing to inherent properties of sulfur dioxide requires less acid. This has cost advantages itself, since sulfuric acid can be expensive. Additionally, less acid usage will translate into lower costs for base (e.g. lime) to increase pH following hydrolysis, for downstream operations. Furthermore, less acid and less base will also mean less generation of waste salts (e.g. gypsum) that may otherwise require disposal.    DETAILED DESCRIPTION - Process (P1) for producing fermentable hemicellulose sugars from lignocellulosic biomass, involves:    (A) providing a feedstock comprising lignocellulosic biomass;    (B) extracting the feedstock with steam and/or hot water under extraction conditions to produce an extract liquor containing hemicellulosic oligomers, cellulose-rich solids, and lignin;    (C) removing the cellulose-rich solids from the extract liquor;    (D) hydrolyzing the hemicellulosic oligomers contained in the extract liquor, in the presence of sulfur dioxide, to produce fermentable hemicellulosic sugars;    (E) recovering and recycling at least a portion of the sulfur dioxide from step (D); and    (F) recovering the fermentable hemicellulosic sugars from the extract liquor, where at least a portion of the sulfur dioxide reacts with the lignin to produce sulfonated lignin.    An INDEPENDENT CLAIM is included for process (p1A) for producing fermentable hemicellulose sugars from lignocellulosic biomass, involving: steps (A)-(C); (D1) hydrolyzing the hemicellulosic oligomers contained in the extract liquor, in the presence of a catalyst selected from sulfuric acid, sulfurous acid, and/or sulfur dioxide, and an additive selected from metal sulfites, and/or metal bisulfites to produce fermentable hemicellulosic sugars; and (E1) recovering the fermentable hemicellulosic sugars, where at least a portion of the additive reacts, directly or indirectly, with the lignin to produce sulfonated lignin.    DESCRIPTION OF DRAWING(S) - The figure shows simplified block-flow diagram of process for producing fermentable hemicellulose sugars from lignocellulosic biomass.