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 degrees C, 40min). It was followed by (a) mechanical refining (3000rpm, 0.5mm), (b) kraft pulping (155 degrees C, 90-140min, alkali charge 2.1-3.4%), or (c) soda pulping (155 degrees C, 90min, alkali charge 2.4-4.0% NaOH). The remaining solid fractions were enzymatically hydrolyzed using 25FPU/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 48h, 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 250L of ethanol per tonne of sawdust. Under this condition of combined treatments, 300kg lignin/t (sawdust) and 120kg xylose/t (sawdust) can be obtained.