UC Riverside

Lignocellulosic biomass provides a low cost and abundant resource for production of cellulosic ethanol for use as a fuel octane booster and a low-carbon standalone transportation fuel. However, native plant polysaccharides and lignin are recalcitrant to biological conversion due to plant’s natural resistance to pathogen invasion. Numerous pretreatments have been developed to overcome biomass recalcitrance, but the solids produced require heavy doses of costly enzymes to breakdown polysaccharides into sugars. In addition, reaching industrial relevant ethanol titers in biological conversion processes has been challenging due product yield limitations at the associated high solids concentrations. Co-Solvent Enhanced Lignocellulosic Fractionation (CELF) is a new pretreatment that applies a mixture of tetrahydrofuran (THF) with water and very dilute acid to extract most of the lignin and solubilize a high portion of the hemicellulose to leave a solid concentrated in glucan that is highly susceptible to enzymatic breakdown into fermentable sugars with low enzyme doses. In addition, extracted high quality lignin could be converted into valuable building blocks for biopolymer synthesis.

In this thesis, CELF pretreatment conditions were defined to maximize sugar yields and lignin recovery from hardwood poplar for the combined operations of CELF pretreatment and subsequent fungal enzyme hydrolysis. Fungal enzyme digestion of solids resulting from CELF pretreatment and characterization of the original poplar and those solids by FTIR, XRD, SEM, and Simon Staining revealed that the lignin left in CELF pretreated biomass profoundly impacted yields from enzymatic hydrolysis of CELF substrates. High solids Simultaneous Saccharification and Fermentation (SSF) of CELF pretreated poplar wood was carried out at a 15 mg protein/g glucan enzyme dose coupled with fermentation of the glucose released by S. cerevisiae variant D5A at 20 wt% solids loading to produce 87 g/L of ethanol in 7 days, 79% of the maximum possible theoretical yield. Fractal kinetic modeling of the enzymatic saccharification data revealed that CELF solids did not require an enzyme loading >15 mg protein/g glucan to improve the saccharification efficiency. Simultaneous Saccharification and Co-Fermentation (SSCF) using a recombinant yeast strain was employed at a 1 L volume to produce 72 g/L of ethanol (72% theoretical yield) at a 17 wt% solid loading in a 3 L bioreactor. In addition, polyurethanes were successfully synthesized from the CELF solubilized lignin recovered when THF was removed from the CELF liquid hydolyzate. Finally, a technoeconomic model was developed to estimate process economics and identify opportunities for further improvements. These results indicate that ethanol production can be most cost-effective if all the sugars in the raw biomass are effectively utilized.