Daniel J. Schell,* Jody Farmer, and Mildred Zuccarello

National Bioenergy Center

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401

Phone: (303)384-6869

Fax: (303)384-6227

E-mail: dan_schell@nrel.gov

Technoeconomic analysis of a bioethanol process model indicates that increasing pretreatment solids loading from 20% to 30% reduces the cost of ethanol production by $0.26/gal. While this analysis assumes that no yield losses occur in pretreatment as solids loading is increased, heat and mass transfer limitations are likely to increase at high solids loadings and sugar degradation reactions, which are first order (or higher) with respect to sugar concentration, are also likely to increase. Thus, the goal of this work was to investigate the effect of solids loading during dilute sulfuric acid pretreatment of corn stover in a continuous pilot-scale reactor on xylose yields and on the enzymatic cellulose digestibility of the pretreated solids. Performance was investigated by conducting a response surface experimental design and using the resulting data to develop predictive empirical response surface models describing how pretreatment and overall sugar yields vary as a function of pretreatment conditions. The experimental design explored the following domain of pretreatment operating conditions using a fixed residence time of approximately 1 min: solids loadings of 25-35%; acid loadings of 0.028-0.056 g acid/g dry biomass; and temperatures from 180-200° C. At optimum conditions, monomeric and total xylose yields of 67% and 76%, respectively, and cellulose digestibility of 96% were achieved experimentally, although not at the same pretreatment conditions. The experimental results and empirical response surface models show that higher xylose yields are achieved at lower solids loadings (25%) and higher cellulose digestibilities are achieved at higher solid loadings (35%). The most promising result is that the optimum conversion of cellulose and xylan in corn stover to monomeric and oligomeric sugars by the combination of dilute acid pretreatment and subsequent enzymatic cellulose hydrolysis is achieved at a 30% solids loading.