(532f) Enhanced Xylan Conversion of Process Relevant Dilute Sulfuric Acid Pretreated Corn Stover

Dilute sulfuric acid pretreatment is well established as an effective pathway for the conversion of lignocellulosic biomass to fermentable sugars using a variety of reactor configurations. However, a high solids continuous reactor is strongly favored economically. While such a reactor configuration poses a number of challenges, the National Renewable Energy Lab's (NREL) horizontal screw pretreatment reactor has historically achieved nearly 80 percent conversion of xylan to monomeric xylose on a normalized basis using corn stover as feedstock. Further enhancement of conversion to monomeric xylose is driven by two approaches: improvement of xylan solubilization with a reduction in degradation products and development of processes to better convert xylo-oligomers to xylose.

A series of statistically designed experiments in a high solids batch reactor, the NREL Steam Digester, attempted to maximize xylose yields (monomeric and oligomeric) and minimize furfural production through optimization of reaction conditions (acid loading, temperature and residence time). Results indicated that improved xylan conversion yields could be achieved at temperatures slightly higher than the conditions used historically for the pilot-scale continuous reactor (158 °C, 5 min, ~1% H2SO4). The findings of these bench-scale experiments were used to refine the reaction space for pilot-scale operation.

Preliminary results indicate that enhanced xylo-oligomer conversion is achieved through a hybrid secondary mild thermochemical-enzymatic process compared to either stage alone. To refine the secondary mild thermochemcial xylo-oligomer conversion process, statistically designed experiments investigating reaction time, temperature, catalyst and co-catalyst addition were executed on the bench scale. Enzymatic conversion of xylo-oligomers achieved monomer yields as high as 90% when preceeded by the mild thermochemical conversion step. This set of enzymatic hydrolysis experiments was performed at dilutions mimicking late stage fermentation. While enzyme activity was low in undiluted samples, likely due to end-product inhibition, further experiments were conducted to evaluate enzyme activity throughout fermentation. Because the amount of solubilized xylo-oligomers is low in comparison to cellulose, the cost contribution of additional enzymes for xylo-oligomer conversion is relatively insignificant to the final fuel selling price.

The xylan conversion results of these integrated strategies using the NREL 200 kg/day process-relevant, pilot scale dilute sulfuric acid continuous pretreatment reactor will be presented.