(599a) Controlling the Rheology of Biomass

Rising global demand for energy combined with an increasing appreciation for the environmental consequences of non-renewable, petroleum based fuel use is lending urgency to the need for a carbon-neutral, widely accessible source of renewable energy. Particularly critical to the transportation industry is an abundant source of renewable liquid fuels such as ethanol to counter chronically rising petroleum costs. A likely feedstock in the production of fuel ethanol is lignocellulosic biomass. The greatest advantage of biomass as a feedstock is that it is the most abundant renewable carbon resource on the planet. The National Renewable Energy Laboratory (NREL) has already proposed a process for converting lignocellulosic biomass to liquid fuel. The process has tremendous promise but the economics of it are inhibitory to large scale implementation. Currently, NREL's process calls for using roughly 10 wt% biomass at any step in the conversion. The utility and equipment costs associated with heating and pumping of 90 wt% water slurries can be dramatically reduced by increasing the solids fraction. NREL's plans call for an increase in solids content from 10 to 40 wt% resulting in a 4-fold decrease in heating energy cost per pound of dry biomass and a 6-fold decrease in the water requirement.

Processing of high solids fraction biomass leads to a number of difficulties that are not present in more dilute systems. Concentrated biomass acts as a Bingham fluid with a yield stress that increases rapidly with solids concentration. This behavior makes pumping and transporting the process slurry difficult if not impossible. In addition, the mass transfer required in order to mix reactants, enzymes and reagents into the slurry is severely inhibited. Controlling the rheology of the biomass feed is critical for the success of high solids fraction biomass processing.

In this presentation, we describe the use of rheological modifiers to control the rheology of biomass slurries. We have found that by using high molecular weight water-soluble polymers, we can significantly reduce the yield stress of wood fiber and corn stover slurries. Using only a few wt% (based on the dry solid biomass) of these rheological modifiers, yield stresses can be reduced several-fold, permitting the flow of biomass at high concentrations. We will describe the effects of properties of the rheological modifiers (e.g., type, concentration and molecular weight) on rheological behavior, show evidence of the microscopic mechanisms producing the observed behavior, and examine the effects of the modifiers on the downstream biomass processing (e.g., acid hydrolysis, enzymatic hydrolysis and fermentation).