This study evaluated high sulfur dioxide (SO2) loading in applying Sulfite Pretreatment to Overcome the Recalcitrance of Lignocelluloses (SPORL) to Douglas-fir forest residue (FS-10) for ethanol production through yeast fermentation. Three pretreatments were conducted at 140°C with a targeted total SO2 loading of 32 wt% on wood, or 80 g/L in the pretreatment liquor. Magnesium was used as the metal base with a targeted combined SO2 loading of 4.4 wt% on wood. A pretreatment duration of approximately 60 min was found sufficient to effectively remove the strong recalcitrance of FS-10 with ultra-low degradation of sugar to furans. Pretreatment mixing was performed through either a rotating digester or liquor circulation, and was found to have no significant effect on pretreatment performance.The ultra-low furan formation facilitated enzymatic saccharification and fermentation at total solids loading of 20% without detoxification. At cellulase loading of 35 mL/kg FS-10, a maximal ethanol yield of 322 L/t FS-10 at a titer of 57.3 g/L, equivalent to 79.1% theoretical based on FS-10 glucan, mannan, and xylan content, was achieved. Compared with a similar low-temperature pretreatment at minimal total SO2 loading of 6.6 wt% on wood, high SO2 loading of 32 wt% reduced pretreatment time from 240 min to 60 min with lower furan formation, which improved ethanol yield by 13% through yeast fermentation.
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