Upgrading and Pretreatment of Douglas-Fir Forest Residue for Fermentable Sugar Production

Forest harvest residues can be a cost-effective feedstock for a biorefinery, but the high lignin content of forest residues is a major barrier for enzymatic sugar production. Physical fractionation was applied to a Douglas-fir (Pseudotsuga menziesii (Mirb) Franco var. menziesii) forest residue and found to be very effectiev to selectively reduce bark and ash content.  The accepted residue is was used for pretreatment by sulfite pretreatment to overcome strong recalcitrance of lignocelluloses (SPORL) in a range of sulfite and acid loadings at 165 °C for 75 minutes with liquor to wood ratio of 3:1.  Sodium bisulfite and sulfuric acid charge as mass fraction of oven dry biomass of 12 % and 2.21 %, respectively, was optimal in terms of enzymatic cellulose saccharification, sugar yield and formation of Hydroxymethylfurfural (HMF) and Furfural.  Enzymatic glucose yield of a dry biomass was 345 g kg^-1, or equivalent to 82.3 % of theoretical at a cellulase (CTec2) dosage of 15 filter paper unit (FPU) per gram of glucan.  HMF and furfural formation were low at approximately 2.5 g L^-1 each in the pretreatment hydrolysate.  Delignification was important to achieve good cellulose saccharification efficiency, however, and approximately 90 % hemicellulose removal is also required.  A combined hydrolysis factor (CHF) was found to correlate well with the enzymatic hydrolysis glucose yield (EHGY), suggesting that CHF can be used to predict saccharification of forest residue for scale-up studies to reduce numbers of experiments.