Corn stover is emerging as a viable feedstock for producing bioethanol from renewable resources. Dilute-acid pretreatment of corn stover can solubilize a significant portion of the hemicellulosic component and enhance the enzymatic digestibility of the remaining cellulose for fermentation into ethanol. In this study, dilute H2SO4 pretreatment of corn stover was performed in a steam explosion reactor at 160 degrees C, 180 degrees C, and 190 degrees C, approx 1 wt % H2SO4, and 70-s to 840-s residence times. The combined severity (Log10 [Ro] - pH), an expression relating pH, temperature, and residence time of pretreatment, ranged from 1.8 to 2.4. Soluble xylose yields varied from 63 to 77% of theoretical from pretreatments of corn stover at 160 and 180 degrees C. However, yields >90% of theoretical were found with dilute-acid pretreatments at 190 degrees C. A narrower range of higher combined severities was required for pretreatment to obtain high soluble xylose yields when the moisture content of the acidimpregnated feedstock was increased from 55 to 63 wt%. Simultaneous saccharification and fermentation (SSF) of washed solids from corn stover pretreated at 190 degrees C, using an enzyme loading of 15 filter paper units (FPU)/ g of cellulose, gave ethanol yields in excess of 85%. Similar SSF ethanol yields were found using washed solid residues from 160 and 180 degrees C pretreatments at similar combined severities but required a higher enzyme loading of approx 25 FPU/g of cellulose.

Nutrient cost is an important aspect in the fermentation of biomass to ethanol. With a goal of developing a cost-effective fermentation medium, several industrially available nutrient sources were evaluated for their effectiveness in the simultaneous saccharification and fermentation of pretreated poplar with Saccharomyces cerevisiae D5A. These studies showed that a low-cost medium containing 0.3% corn steep liquor and 2.5 mM MgSO4 7H2O was similar in performance to a nutrient-rich medium. Besides its low cost, this alternative medium consists of components that are available on a commercial scale, thereby making it industrially relevant.

Simultaneous saccharification and cofermentation (SSCF) was carried out at approximately 15% total solids using conditioned dilute-acid pretreated yellow poplar feedstock, an adapted variant of National Renewable Energy Laboratory (NREL) xylose-fermenting Zymomonas mobilis and either commercial or NREL-produced cellulase enzyme preparations. In 7 d, at a cellulase loading of 12 filter paper units per gram cellulose (FPU/g), the integrated system produced more than 3% w/v ethanol and achieved 54% conversion of all potentially available biomass sugars (total sugars) entering SSCF. A control SSCF employing Sigmacell cellulose and a commercial cellulase at an enzyme loading of 14 FPU/g achieved 65% conversion of total sugars to ethanol.

Cost reductions for pretreatment and bioconversion processes are key objectives necessary to the successful deployment of a bioethanol industry. These unit operations have long been recognized for their impact on the production cost of ethanol. One strategy to achieve this objective is to improve the pretreatment process to produce a pretreated substrate resulting in reduced bioconversion time, lower cellulase enzyme usage, and/or higher ethanol yields. Previous research produced a highly digestible pretreated yellow poplar substrate using a multistage, continuously flowing, very dilute sulfuric acid (0.07% (w/v)) pretreatment. This process reduced the time required for the bioconversion of pretreated yellow poplar sawdust to ethanol. This resulted in a substantially improved yield of ethanol from cellulose. However, the liquid volume requirements, steam demand, and complexity of the flow-through reactor configuration were determined to be serious barriers to commercialization of that process. A reconfigured process to achieve similar performance has been developed using a single-stage batch pretreatment followed by a separation of solids and liquids and washing of the solids at a temperatures between 130 and 150 degrees C. Separation and washing at the elevated temperature is believed to prevent a large fraction of the solubilized lignin and xylan from reprecipitating and/or reassociating with the pretreated solids. This washing of the solids at elevated temperature resulted in both higher recovered yields of soluble xylose sugars and a more digestible pretreated substrate for enzymatic hydrolysis. Key operating variables and process performance indicators included acid concentration, temperature, wash volume, wash temperature, soluble xylose recovery, and performance of the washed, pretreated solids in bioconversion via simultaneous saccharification and fermentation (SSF). Initial results indicated over a 50% increase in ethanol yield at 72 h for the hot washed material as compared to the control (no washing, no separation) and a 43% reduction of in the bioconversion time required for a high ethanol yield from cellulose

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