(231c) Separate Processing of Hemicellulose and Cellulose to Optimize Ethanol Production From Corn Stover

Separate processing of
hemicellulose and cellulose to optimize ethanol production from corn stover

Alexandre Chapeaux, Nancy
Dowe, Daniel J. Schell

Using a series of thermochemical (pretreatment)
and biochemical steps, lignocellulosic biomass is converted to sugars that subsequently
can be made into fuels and chemicals. One such process uses heat and dilute sulfuric acid to hydrolyzes the hemicellulosic fraction
of the biomass to monomeric and oligomeric sugars.
The removal of hemicellulose and to a lesser extent lignin from the biomass significantly
improves the performance of the next step in the process, enzymatic hydrolysis
of cellulose to glucose. However, enzymatic cellulose hydrolysis can be
performed in the presence or absence of the hemicellulosic sugars produced
during pretreatment. It is well known that removing the hemicellulose sugars
and other compounds produced during pretreatment improves enzymatic cellulose
conversion yields. In previous work, we assessed the potential of a process
that removes the hemicellulosic sugars prior to enzymatic cellulose hydrolysis by
exploring the interaction between biomass solids loading, yields, and process
costs. However, this process is more expensive because of the additional cost
for solid-liquid separation equipment required to remove th-rich
sugar liquor from the cellulosic solids.

In this study, the goal was to deepen our
understanding of the advantages and disadvantages of the separate
hemicellulose/cellulose process. The feedstock was dilute-acid-pretreated corn
stover produced in a 200 kg dry biomass/d continuous horizontal reactor. The
pretreated slurry was separated into a liquor stream (hemicellulosic sugars)
and a washed cellulosic solid stream. Enzymatic cellulose hydrolysis was investigated
at four enzyme loadings (10, 20, 30 and 40 mg protein/g cellulose) and at
insoluble solids concentrations from 9% to 21% (w/w). The resulting glucose was
fermented to ethanol using several yeasts and a glucose-xylose co-fermenting
bacterium, Zymomonas mobilis 8b. The sugars in the liquor
stream were fermented to ethanol with three strains of co-fermenting Z. mobilis after dilution of the stream
with water to lower sugar concentrations. Finally, the overall performance of
the process was assessed after partitioning water between the two phases in a
way that optimizes overall ethanol yields. We then compared these results to a
process configuration in which the streams are not separated. In the latter
process, enzymatic cellulose hydrolysis is performed in the presence of the
background hemicellulosic sugars produced during pretreatment of the biomass.