and the University of California at Riverside, leading pretreatment technologies based on ammonia fiber expansion, aqueous ammonia recycle, dilute sulfuric acid, lime, neutral pH, and sulfur dioxide were applied to a single source of poplar wood, and the remaining solids from each technology were hydrolyzed to sugars using the same enzymes. Identical analytical methods and a consistent material balance methodology were employed to develop comparative performance data for each combination of pretreatment and enzymes. Overall, compared to data with corn stover employed previously, the results showed that poplar was more recalcitrant to conversion to sugars and that sugar yields from the combined operations of pretreatment and enzymatic hydrolysis varied more among pretreatments. However, application of more severe pretreatment conditions gave good yields from sulfur dioxide and lime, and a recombinant yeast strain fermented the mixed stream of glucose and xylose sugars released by enzymatic hydrolysis of water washed solids from all pretreatments to ethanol with similarly high yields. An Agricultural and Industrial Advisory Board followed progress and helped steer the research to meet scientific and commercial needs.

Previous kinetic modeling and bench-scale demonstration efforts using batch, percolation, or plug-flow reactors for the dilute sulfuric acid hydrolysis of cellulose have concluded that glucose yields above 70% of theoretical were not possible. This has been explained to be a result of reactions involving glucose or the cellulose itself in a destructive manner, as well as hydrolyzed soluble oligomers which have been modified chemically so as not to release glucose. However, recently, we have demonstrated that near-quantitative yields of glucose from cellulose can indeed be obtained using a bench-scale shrinking-bed percolation reactor in which an internal spring compresses the biomass as the reaction progresses. The present study was initiated to gain a fundamental understanding of the kinetic sequences involved in these high yields. Three reactor configurations (batch, percolation, and shrinking-bed percolation) were studied using similar hydrolysis severities to begin addressing chemical, physical, and hypothesized boundary layer phenomenon governing rate-limiting steps of glucose release from two prehydrolyzed yellow poplar cellulosic substrates. The characteristics of the logarithmic release of glucose as well as the logarithmic disappearance of cellulose as a linear function of time were found to be reactor dependent. Use of a percolation reactor was described where the initial hydrolysis rate constant for cellulose using 0.07% w/w sulfuric acid at 225 °C is enhanced 5-fold compared to a batch reactor. Additionally, when lower hydrolysis severities are used for hydrolyzing yellow poplar cellulose in batch mode, biphasic kinetics were observed. Several hypothesized boundary layer resistances, such as structured water, viscosity, and re-hydrogen bonding of released glucose, will be suggested as diffusion resistances for released glucose to the bulk medium, which would be a function of the reactor configuration and define potential glucose yields.

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Hydrolysis of alpha-cellulose by H2SO4 is a heterogeneous reaction. As such the reaction is influenced by physical factors. The hydrolysis reaction is therefore controlled not only by the reaction conditions (acid concentration and temperature) but also by the physical state of the cellulose. As evidence of this, the reaction rates measured at the high-temperature region (above 200 C) exhibited a sudden change in apparent activation energy at a certain temperature, deviating from Arrhenius law. Furthermore, alpha-cellulose, once it was dissolved into concentrated H2SO4 and reprecipitated, showed a reaction rate two orders of magnitude higher than that of untreated cellulose, about the same magnitude as cornstarch. The alpha-cellulose when treated with a varying level of H2SO4 underwent an abrupt change in physical structure (fibrous form to gelatinous form) at about 65% H2SO4. The sudden shift of physical structure and reaction pattern in response to acid concentration and temperature indicates that the main factor causing the change in cellulose structure is disruption of hydrogen bonding. Finding effective means of disrupting hydrogen bonding before or during the hydrolysis reaction may lead to a novel biomass saccharification process.

The Biomass Refining Consortium for Applied Fundamentals and Innovation, with members from Auburn University, Dartmouth College, Michigan State University, the National Renewable Energy Laboratory, Purdue University, Texas A&M University, the University of British Columbia, and the University of California at Riverside, has developed comparative data on the conversion of corn stover to sugars by several leading pretreatment technologies. These technologies include ammonia fiber expansion pretreatment, ammonia recycle percolation pretreatment, dilute sulfuric acid pretreatment, flowthrough pretreatment (hot water or dilute acid), lime pretreatment, controlled pH hot water pretreatment, and sulfur dioxide steam explosion pretreatment. Over the course of two separate USDAand DOE-funded projects, these pretreatment technologies were applied to two different corn stover batches, followed by enzymatic hydrolysis of the remaining solids from each pretreatment technology using identical enzyme preparations, enzyme loadings, and enzymatic hydrolysis assays. Identical analytical methods and a consistent material balance methodology were employed to develop comparative sugar yield data for each pretreatment and subsequent enzymatic hydrolysis. Although there were differences in the profiles of sugar release, with the more acidic pretreatments releasing more xylose directly in the pretreatment step than the alkaline pretreatments, the overall glucose and xylose yields (monomers ? oligomers) from combined pretreatment and enzymatic hydrolysis process steps were very similar for all of these leading pretreatment technologies. Some of the water-only and alkaline pretreatment technologies resulted in significant amounts of residual xylose oligomers still remaining after enzymatic hydrolysis that may require specialized enzyme preparations to fully convert xylose oligomers to monomers.