The key to developing an economically viable biorefinery is to employ a holis­tic approach that integrates the unit steps, maximizing the yield at each, while minimizing both capital and operating costs. At each step of the process, from pretreatment to fermentation, effort must be made to minimize any loss in potential ethanol production. In the example in Fig. 2, the production of degraded sugars during pretreatment, incomplete cellulose or hemicellulose

conversion to fermentable sugars during hydrolysis, and fermentation losses due to sugar consumption by the yeast all contribute to lost value in the con­version. If biomass feedstock such as corn stover, purchased at $5/ton, could be converted with perfect efficiency to its theoretical potential of 113 gal­lons of ethanol per ton of stover with an ethanol selling price of $2/gallon, the value of the ethanol would be ~$225/ton, creating an “operating cost window” for depreciation of capital, operation, and profit of ~$220/ton [5]. Losses in any unit step that reduces the overall yield will reduce the value per ton, whether the losses result from reduced enzyme hydrolysis, poor fer — mentability of the hydrolyzate sugars, or reduced fermentation yield. It is also important to note that maximizing the conversion of the two most abundant sugars, glucose and xylose, is important to viable economics. If only cellulose is utilized with no conversion of hemicellulose, the theoretical yield drops 39% to 69 gallons/ton, reducing the cost window to ~ $135/ton. Unless the xylose is utilized to produce something of equal or higher value, it is un­likely that such a process could be viable. Similarly, a pretreatment selected on the basis of a reduced capital cost for installed equipment, but increasing the required enzyme dosage, may reduce the operating cost window significantly.

3