Monday, May 5, 2008
7-31

Co-Culture Fermentation to Produce Ethanol from Dilute Acid Pretreated Corn Stover

Gary A. McMillen, Andrew C. Lowell, Nancy S. Dowe, and Daniel J. Schell. National Bioenergy Center, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401

Successful implementation of biochemical conversion technology for producing fuels from lignocellulose depends on achieving good conversion of biomass derived sugars to ethanol. It may be possible to reach better overall conversion of these sugars by using more than one fermentative microorganism. We investigated the ability of a co-culture fermentation using a recombinant glucose-xylose fermenting bacteria, Zymomonas mobilis 8b, and a glucose fermenting yeast, Saccharomyces pastorianus, to convert all of the cellulosic and hemicellulosic sugars available in dilute-acid pretreated corn stover slurries. The whole pretreated stover slurry was conditioned by ammonium hydroxide and inoculated with Z. mobilis 8b to ferment glucose and xylose produced during pretreatment, and then later, S. pastorianus and cellulase were added to convert the cellulose by a simultaneous saccharification and fermentation process. We also tested a process, in which, a fraction of the liquor was removed from the slurry, separately conditioned by ammonium hydroxide, and then added back to the concentrated solids, then continued with the procedure just described. Both processes were tested at a 10% and 20% total solids concentration. The difference between the two processes is that in the latter configuration the cellulosic solids were not exposed to a high pH. The whole slurry process achieved a theoretical ethanol yield of 87% at 10% total solids from all fermentable sugars, the yield dropped to 52% at 20% total solids. Ethanol yield was about 30% greater when liquor was separately conditioned. Future work will investigate the cause of the performance difference.

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