Combined pretreatments of eucalyptus sawdust for ethanol production within a biorefinery approach

Eucalyptus sawdust is a residue from the pulp and timber industries which can be used as a raw material in a biorefinery. In this work, two consecutive treatment steps were applied to eucalyptus sawdust from a pulp mill, as a fractionation strategy, to recover and preserve lignocellulosic components while enhancing enzyme accessibility to cellulose. The first treatment step assayed was autohydrolysis (170 °C, 40 min). It was followed by (a) mechanical refining (3000 rpm, 0.5 mm), (b) kraft pulping (155 °C, 90–140 min, alkali charge 2.1–3.4%), or (c) soda pulping (155 °C, 90 min, alkali charge 2.4–4.0% NaOH). The remaining solid fractions were enzymatically hydrolyzed using 25 FPU/g of Cellic CTec 2 from Novozymes and a solid content of 13%. The efficiency of the enzymatic hydrolysis was higher than 70% in the case of an additional kraft or soda pulping while only autohydrolysis led to efficiencies lower than 60%. The best hydrolysis parameters and lignin and xylose recovery yields were obtained for autohydrolysis followed for a kraft pulping (cellulose conversion up to 71%, cellulose hydrolysis 95% at 48 h, lignin and xylose recovery 99 and 85%, respectively). The treated solid that reached the highest enzymatic yields was fermented using Saccharomyces cerevisiae in a 3.5-L reactor. The highest bioethanol yield was found for the autohydrolysis-treated solids followed by soda pulping, reaching a value of 250 L of ethanol per tonne of sawdust. Under this condition of combined treatments, 300 kg lignin/t _sawdust and 120 kg xylose/t _sawdust can be obtained.