Valorisation of hardwood hemicelluloses in the kraft pulping process by using an integrated biorefinery concept

A primary hydrolysis treatment (auto or acid-catalysed) of Eucalyptus globulus wood was performed before the cooking stage to extract part of the hemicelluloses that otherwise would be dissolved in the kraft liquor and burned. As xylose was the main monosaccharide Pichia stipitis was selected to produce bioethanol. Two methods were tested, with different alkalis, to reduce hydrolysates toxicity and adjust pH. A two-step method using Ca(OH)2 leads to better fermentation results. Acid hydrolysates promoted higher ethanol concentrations (12 g L−1) with high productivity and yield values (0.22 geth L−1h−1 and 0.48 geth/gxyleqs), whilst auto-hydrolysates, even after a secondary hydrolysis, gave low ethanol concentrations (2–4 g L−1). The impacts on kraft cooking and pulp quality were also studied in order to fully understand the feasibility of this biorefinery concept (combining ethanol production and hardwood pulping). A decrease of the overall pulp yield (10% for auto- and 15% for acid-hydrolysis) was observed. However, a decrease on bleaching requirements (up to 15%) and on brightness reversion was registered. Moreover, auto-hydrolysis improves pulp viscosity, whilst acid-hydrolysis is more efficient in metals leaching. Overall, from the pulp production point of view, auto-hydrolysis conditions are more attractive than the acid-catalysed ones.