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"Cotreatment of Cellulosic Biomass: Rheological Characterization of Fermented Slurries and Exploration using Ferment-Mill-Ferment Experiments with Pilot-Scale Disc Milling"

Abstract

Biological processing of lignocellulose to fuels and chemicals benefits from, and in most cases, requires augmentation of non-biological approaches to increase the rate and extent of biomass deconstruction. Such augmentation is conventionally provided by thermochemical pretreatment. However, there is incentive to look at alternative approaches since thermochemical pretreatment is costly and operationally challenging. One such alternative approach is milling during fermentation, termed "cotreatment." Cotreatment can be combined with lignocellulose fermentation using cellulolytic microorganisms without added enzymes, termed consolidated bioprocessing (CBP). For such C-CBP to be feasible, substantial enhancement of carbohydrate solubilization (i.e., conversion of fermentable carbohydrates to sugars) needs to be achieved at the same time that fermenting microorganisms survive, and milling energy requirements and capital investment are sufficiently low. Earlier work established that high solubilization and active fermentation are achieved with ball milling. This thesis is the first report investigating the feasibility of disc milling for cotreatment aimed at enhancing biological production of liquid fuels from cellulosic biomass.

First, rheological changes in the physical properties of biomass during fermentation by the thermophilic, anaerobic bacterium Clostridium thermocellum were characterized. Biomass slurries showed a 2000-fold viscosity reduction, with the first 8-fold reduction occurring during the first 10% of carbohydrate solubilization. Cotreatment was implemented by sequential fermentation, milling and fermentation to explore milling at industrially representative conditions and to allow for flexible milling conditions. Cotreatment energy requirements were measured for the first time with a pilot-scale disc mill. A techno-economic analysis on C-CBP was performed with updated cotreatment configuration, milling equipment costs and milling energy requirements. Payback period was found to be moderately sensitive to milling energy, and a cost-benefit analysis showed that cotreatment would become progressively more advantageous with larger enhancements in carbohydrate solubilization than is currently achieved with disc milling. Results from this work will inform the overall process feasibility of cotreatment, specifically the potential of using disc milling to augment biological reaction for cellulosic ethanol production.

Thesis Committee

• Lee R. Lynd
• Charles E. Wyman
• Mark S. Laser
• Colin R. Meyer
• Susan M. Hennessey

Contact

For more information, contact Daryl Laware at daryl.a.laware@dartmouth.edu.