Fractionation of lignocellulosic biomass for cellulosic ethanol and chemical production

A continuous process for fractionation of lignocellulosic biomass into a cellulose-rich solids fraction, a xylo-oligosaccharides-rich liquid fraction and a vapor fraction containing inhibitors to cellulose hydrolysis and/or fermentation, the method comprising the steps of:

• a) obtaining biomass having a lignin content of less than 12% weight/weight in the dry matter and an acetyl group content of 3-6% weight/weight in the dry matter;
• b) autohydrolysis of a hemicellulose fraction in the lignocellulosic biomass by exposing the biomass in a reaction vessel to steam at an elevated temperature and pretreatment pressure and for a preselected retention time to achieve a severity index of about 4 to maximize autohydrolysis of the hemicellulose fraction in the absence of any added acid catalyst, while simultaneously minimizing hemicellulose degradation, the severity index (SI) being calculated as
• SI=Log×Exp[(treatment temperature(° C.)−100° C.)/14.75]×Retention Time(min),
• to obtain a prehydrolyzed lignocellulosic biomass;
• c) during the autohydrolysis step, purging liquid condensate, cooking liquids, and vapor generated by the autohydrolysis to remove and collect a first liquid stream with hemicellulose sugars free of lignin and water soluble compounds, and a first vapor stream with volatile inhibitors;
• d) after the autohydrolysis step, liquid extracting under pretreatment pressure from the prehydrolyzed lignocellulosic biomass a liquid hemicellulose breakdown products stream containing hemicellulose sugars substantially free of lignin and hemicellulose degradation components;
• e) after the liquid extracting step, rapidly releasing the reaction pressure to afford explosive decompression of the extracted, prehydrolyzed lignocellulosic biomass into a cellulose-rich fibrous solids fraction, vapor with hemicellulose degradation components and volatile chemicals inhibitory to fermentation, and a condensate containing mostly hemicellulose sugars free of lignin;
• f) collecting the vapor released during the explosive decompression as a second vapor stream and the condensate formed during the explosive decompression as a second liquid stream;
• g) combining the first and second liquid streams with the liquid hemicellulose breakdown products stream into a combined liquids stream;
• and
• h) evaporating hemicellulose degradation products from the combined liquids stream for recovery of a xylo-oligosaccharides-rich liquid fraction.