Processing Lignocellulosic Biomass into Ethanol - Implications of High Solid Loadings

Abstract: Fuel ethanol from lignocellulosic biomass has the potential to provide a sustainable replacement for traditional oil-based fuels. This dissertation assesses the processing of three different lignocellulosic materials – spruce, wheat straw and giant reed – at industrially relevant solid loadings. The work is divided into two main parts. The first part deals with the degradation of biomass to sugars, focusing on the complex rheological behavior of biomass slurries and the connection to mixing during high solids hydrolysis. The second part deals with the process design of combined hydrolysis and fermentation processes, focusing on efficient xylose co-consumption at high solids loadings.
Rheological characterization of steam pretreated spruce revealed strong non-Newtonian flow behavior with rapidly increasing viscosities and yield stress at high solid loadings, for instance the yield stress more than doubled when increasing the WIS content from 10 to 12 % (from 10 Pa to 24.5 Pa). Moreover, a strong effect of particle size distribution was found on both the viscosity and the yield stress. High viscosities create a need for efficient mixing during enzymatic hydrolysis of pretreated spruce. The hydrolysis rate was significantly influenced by both the amount and type of agitation. For pretreated spruce, for example, an increased agitation rate from 75 rpm to 500 rpm doubled the hydrolysis yield after 96 hours (an increase in yield from 35 to 72 %). The positive effect remained during scale-up to cubic meter scale and could be correlated to the flow conditions in the reactor. However, large discrepancies were found between different pretreated materials, and it became evident that the hydrolysis rate of giant reed was not affected by mixing. This was likely due to the much more rapid liquefaction achieved during the hydrolysis of giant reed.
In addition to glucose, many potential raw materials contain considerable amounts of the pentose sugar xylose. Xylose metabolism has today been successfully implemented in Saccharomyces cerevisiae through genetic engineering, although glucose is still the preferred substrate. In this work, xylose co-consumption was significantly enhanced by applying different process design strategies. By using a dual feed strategy, xylose consumption could be increased by 25 %, which resulted in a 10 % increase in final ethanol titer. It was also found that, in the presence of high acetic acid concentrations, xylose uptake could be significantly enhanced by increasing the pH. Whether or not this was beneficial for ethanol production, however, was found to be dependent on the specific process design.