Bioethanol production from Eucalyptus grandis hemicellulose hydrolyzate recovered before Kraft pulping by using an integrated forest biorefinery concept

Increasing attention has been focused on the integration of a pulp mill into a forest biorefinery, where other value-added products could be produced in addition to pulp. Hemicelluloses can be removed from wood in a pretreatment prior to pulping. In this study, hemicellulose was removed from Eucalyptus grandis wood chips as hydrolyzate with green liquor prior to Kraft pulping and fermented by Scheffersomyces stipitis NBRC 10063 to produce bioethanol. Hemicellulose hydrolyzate was obtained by treating eucalyptus chips with 2% of green liquor in a batch rotary reactor at (155-160)°C for 150 min. These conditions were selected to obtain an extract rich in xylose without changing the quality of pulp produced: best xylose extraction yield and lower pulp viscosity degradation. Hemicellulose extracts contained xyloolygosaccharides and acetic acid as major components, and many minor components including soluble lignin, minerals and sugar degradation products. Hydrolyzates were further acidified with 4% sulfuric acid at 121°C during 1 h to complete hydrolysis. The direct fermentation of these hydrolyzates was possible. However, the fermentation of hydrolyzates after vacuum concentration and lime detoxification proved to be difficult or even impossible. Inhibitory compounds such as organic acids and sodium could be involved in the inhibition on the fermentation. Solvent extraction with ethyl acetate, used for removal of inhibitory compounds, improved fermentability. Fermentation of the hydrolyzate concentrated to 19.1 g/L xylose and pretreated with ethyl acetate, showed a final ethanol concentration of 5.0 g/L after 125 h, fermentation efficiency of 42% and sugar conversion of 94%.