Liquid hot water extraction followed by mechanical extrusion as a chemical-free pretreatment approach for cellulosic ethanol production from rigid hardwood

Abstract

To alleviate the heavy dependence on chemical input of chemical pretreatments and make a better use of current idle thermo-mechanical pretreatment plant for mechanical pulp, a sequential chemical-free pretreatment approach, i.e., liquid hot water extraction followed by mechanical extrusion was investigated in this work. Its ability to fractionate hemicellulose while facilitating cellulose hydrolysis of rigid eucalyptus and aspen hardwood was assessed. The structural changes of lignin during liquid hot water extraction were elucidated using spectrum analysis. Cellulose accessibility to hydrolytic enzymes, cellulose hydrolysis yields and fermentation potential were also assessed. Results showed that the mild liquid hot water extraction recovered 54.0% and 38.1% of xylan hemicellulose from eucalyptus and aspen hardwood respectively, while causing limited lignin condensation reactions. Extending liquid hot water extraction time from 45 to 90 min significantly facilitated the subsequent mechanical deconstruction of both wood substrates without compromising their overall carbohydrate recovery. A maximum cellulose hydrolysis yield of 79.6% and nearly 100% (10 w/v solids loading, enzyme dosage, 20 mg g−1 cellulose) was observed on eucalyptus and aspen substrate respectively, which were feasible for further fermentation process. This work showed liquid hot water extraction followed by mechanical extrusion was a feasible pretreatment strategy for bioethanol production even the starting wood chip substrate was highly rigid.