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.
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