Investigation of separate hemicellulosic liquor fermentation and cellulosic solid processing for production of ethanol

Alexandre Chapeaux, Mildred Zuccarello, Nancy Dowe and Daniel J. Schell

Dilute acid pretreatment of lignocellulosic biomass readily converts the hemicellulose into fermentable sugars, while improving the susceptibility of remaining cellulose to be converted by enzymatic hydrolysis. It may be possible to improve conversion of the sugars to ethanol by separate fermentation of the hemicellulosic-sugar rich stream and the glucose stream derived from enzymatic hydrolysis of cellulose. Removing hemicellulosic sugars prior to enzymatic hydrolysis improves glucose yields by relieving sugar inhibition. However, additional cost is incurred because of the requirement for solid-liquid separation equipment and separate fermentation trains. We tested this concept on dilute acid pretreated corn stover produced in a pilot scale reactor. The hemicellulosic liquor stream was diluted with water to various concentrations and then the sugars were fermented using a glucose-xylose fermenting Zymomonas mobilis. The solids were re-suspended in either fresh water or the spent fermentation broth from the liquor stream. The cellulose solids were converted to ethanol using a simultaneous saccharification and fermentation process or separate enzymatic hydrolysis and fermentation. The fermentation was performed using the yeast Saccharomyces pastorianus. Sugar conversion and ethanol yields were measured as a function of the total solids loading (based on untreated biomass) and used to model process economics. The results show how the interaction of solids loading, liquor dilution and recycling of the spent broth affect conversion yields and ethanol production cost.