On energy consumption for size-reduction and yields from subsequent enzymatic saccharification of pretreated lodgepole pine

This study investigated the effects of chemical pretreatment and disk-milling conditions on energy consumption for size-reduction and the efficiency of enzymatic cellulose saccharification of a softwood. Lodgepole pine wood chips produced from thinnings of a 100-year-old unmanaged forest were pretreated by hot-water, dilute-acid, and two SPORL processes (Sulfite Pretreatment to Overcome Recalcitrance of Lignocellulose) at acid charge on oven dry (od) wood of 0% and 2.21%. The pretreated wood chips were then milled using a laboratory disk mill under various solids-loadings and disk-plate gaps to produce substrates for enzymatic hydrolysis. We found that post-chemical-pretreatment size-reduction of forest biomass can decrease size-reduction energy consumption by 20–80% depending on the pretreatment applied under 20% solids-loading and a disk-plate gap of 0.76 mm in milling. SPORL with a sodium bisulfite charge of 8% and sulfuric acid charge of 2.21% on wood was the most effective in decreasing size-reduction energy consumption. Solids-loading had the most significant effect on disk-milling energy. When solids-loading was reduced from 30% to 3%, disk-milling energy could be decreased by more than a factor of 10 for wood chips pretreated by both SPORL and dilute-acid at an acid charge of 2.21%. The enzymatic hydrolysis glucose yields (EHGY) from the substrates produced by all pretreatments were independent of the solids-loading in milling, indicating that these energy savings in size-reduction can be realized without affecting EHGY. When wood chips were pretreated by SPORL with 2.21% acid charge, size-reduction energy consumption was decreased to less than 50 Wh/kg od wood at a practical solids-loading of approximately 10–20%, equivalent to that used in size-reduction of agriculture biomass, with excellent EHGY of about 370 g per kg od wood. Similar effects on size-reduction energy savings and excellent EHGY were also achieved when large disk-plate gaps (up to 1.52 mm studied) were applied in disk-milling of wood chips pretreated by SPORL with acid.