Cynthia Riley

A bioethanol process development unit: initial operating experiences and results with a corn fiber feedstock.

Interest in bioethanol production from lignocellulosic feedstocks for use as an alternative fuel is increasing, but near-term commercialization will require a low cost feedstock. One such feedstock, corn fiber, was tested in the US Department of Energy (DOE)/National Renewable Energy Laboratory (NREL) bioethanol pilot plant for the purpose of testing integrated equipment operation and generating performance data. During initial runs in 1995, the plant was operated for two runs lasting 10 and 15 days each and utilized unit operations for feedstock handling, pretreatment by dilute sulfuric-acid hydrolysis, yeast inoculum production, and simultaneous saccharification and fermentation using a commercially available cellulase enzyme. Although significant operational problems were encountered, as would be expected with the startup of any new plant, operating experience was gained and preliminary data were generated on corn fiber pretreatment and subsequent fermentation of the pretreated material. Bacterial contamination was a significant problem during these fermentations.

Enhanced cofermentation of glucose and xylose by recombinant Saccharomyces yeast strains in batch and continuous operating modes.

Agricultural residues, such as grain by-products, are rich in the hydrolyzable carbohydrate polymers hemicellulose and cellulose; hence, they represent a readily available source of the fermentable sugars xylose and glucose. The biomass-to-ethanol technology is now a step closer to commercialization because a stable recombinant yeast strain has been developed that can efficiently ferment glucose and xylose simultaneously (coferment) to ethanol. This strain, LNH-ST, is a derivative ofSaccharomyces yeast strain 1400 that carries the xylose-catabolism encoding genes ofPichia stipitis in its chromosome. Continuous pure sugar cofermentation studies with this organism resulted in promising steady-state ethanol yields (70.4% of theoretical based on available sugars) at a residence time of 48 h. Further studies with corn biomass pretreated at the pilot scale confirmed the performance characteristics of the organism in a simultaneous saccharification and cofermentation (SSCF) process: LNH-ST converted 78.4% of the available glucose and 56.1% of the available xylose within 4 d, despite the presence of high levels of metabolic inhibitors. These SSCF data were reproducible at the bench scale and verified in a 9000-L pilot scale bioreactor.

Detailed material balance and ethanol yield calculations for the biomass-to-ethanol conversion process

Applying material balance calculations to the evaluation and optimization of lignocellulosic biomass conversion processes is fundamentally important. The lack of a general framework for material balance calculations and inconsistent compositional analysis data have made it difficult to compare results from different research groups. Material balance templates have been developed to follow accurately the distribution of carbon in lignocellulosic substrates through the pretreatment and simultaneous saccharification and fermentation (SSF) processes, and provide information on overall carbon recovery, recovery of individual sugars, and solubilization of biomass components. Based on material balance considerations, we developed equations that allow us to compute overall ethanol yields for biochemical conversion of biomass correctly.

Implementing Systems Engineering in the U. S. Department of Energy Office of the Biomass Program

The DOE Biomass Program is tackling the challenge of advancing biomass energy technologies and systems from concept to commercial adoption with a goal of enabling the production and use of biofuels to help reduce future U.S. oil consumption. The complexity of the biomass-to-biofuels system of systems and the combined dynamics of the existing agriculture, forestry, energy and transportation markets within which it operates pose challenges for reaching consensus on both a concept of operations and preferred strategies for transitioning to a significantly larger biofuels industry that is secure, reliable, environmentally responsible, and supportive of a thriving economy. To ensure that the program is focused on the activities critical to achieving its goal, the program is implementing systems engineering processes, practices, and tools to guide informed decision-making.