(669a) Mechanistic Models for High-Solids Loading Pretreatment and Enzymatic Hydrolysis of Lignocellulosic Biomass

Griggs, A. J., National Renewable Energy Laboratory
Kuhn, E. M., National Renewable Energy Laboratory
Chen, X., National Renewable Energy Laboratory
Lischeske, J. J., National Renewable Energy Laboratory
Tucker, M., National Renewable Energy Laboratory
Stickel, J. J., National Renewable Energy Laboratory

Conversion of lignocellulosic biomass to fermentable sugars for renewable fuel production remains a formidable challenge for the economically feasible operation of bio-refineries. An improved understanding of the conversion reactions and the development of predictive models will help in overcoming this challenge, especially for high-solids loadings, where mass transfer may limit overall conversion yields. In this work, mechanistically based kinetic models for dilute-acid pretreatment and enzymatic hydrolysis have been developed, which consider the various physical and chemical changes to the feedstock. For dilute-acid pretreatment at high-solids loadings, we have developed a population-balance model, which considers the spatial distribution of reactants and products. The effects of the initial polydisperse particle-size distribution, and its evolution, are captured using a fragmentation-abrasion model. Such a model can aid in interpreting experimental results, determining reaction mechanisms, scale-up of pretreatment processes, and identifying the chemical and structural changes that result in high-xylose yields and enhanced-enzymatic digestibility. For example, when sieved fractions of milled corn stover are subjected to high-solids, dilute-acid pretreatment, the fraction with the largest particle size showed the greatest amount of fragmentation and yielded the most enzymatically digestible substrate, relative to the other sieved fractions. This finding suggests that the amount of fragmentation during dilute-acid pretreatment may be a measure of an effective pretreatment. For enzymatic hydrolysis, the distinct modes of action of the cellulase enzymes on soluble and insoluble substrates are each described by separate rate expressions. Depolymerization of the insoluble substrate by adsorbed enzyme is calculated using distribution kinetics, which tracks the molecular-weight distribution of the cellulose chains, rather than the concentration of individual species, over time. When combined with the method-of-moments, computationally efficient calculations are obtained. Model results throughout enzymatic saccharification are compared to experimental results for cellulose substrates, over a range of solids loadings. These models are capable of examining many of the relevant phenomena occurring during pretreatment and enzymatic saccharification, and aim to guide process development and identify optimal process-operating conditions.