Converting Lignocellulosic Biomass to Low-Cost Fermentable Sugars

Concentrated hydrochloric acid-driven hydrolysis provides the most powerful and industrially proven technology for converting all cellulosic wastes-wood, solids from city sewage plants, bagasse, grasses, etc. - to sugars that can be fermented to ethanol or other biofuels as well as a large variety of bio-products and food and feed. Our process begins by steam expansion of debarked chipped wood, which undergoes a pre-extraction stage to remove all extractives, for example, tall oils and ash. The pre-extracted wood continues into hydrolysis stage performed using highly concentrated HCl at low-temperature (10-15 °C), thus affording sugars hydrolyzate (98 % of the theoretically available sugars, composing 65 % of the dry weight of the wood chips for pine wood, are converted into sugars) with minimum degradation products, while simultaneously separating the solid lignin. A key limitation to any concentrated acid hydrolysis is the difficulty in recovering the acid. In particular, HCl solution forms an azeotrope at between 21 and 25 % depending on the pressure; simple distillation cannot concentrate a dilute solution beyond the azeotropic point. The efficiency of acid recovery is a key condition to making acid hydrolysis of lignocellulosic materials an economically viable source of fermentable sugars. Full recovery of HCl at high acid concentration and its reuse yields very minor waste stream, no complicating air emissions, and favorable life cycle analysis. © Springer-Verlag Berlin Heidelberg 2013.