Rheology of high-solids biomass slurries for biorefinery applications

Biomass slurries, such as dilute-acid pretreated corn stover (PCS), will be a common process stream in biorefineries designed to convert agricultural residues into biofuels such as ethanol. In this work, the advantages and disadvantages of several rheological techniques are evaluated for PCS suspensions. Three flow regimes were evaluated: (i) shear flow using a vane, (ii) torsional flow between parallel plates, and (iii) biaxial extensional flow between plates. The vane provided the simplest methodology and the most reproducible results. Four experiments were conducted using the vane: (i) transient flow, (ii) stress ramps, (iii) creep, and (iv) oscillatory shear. PCS slurries with fractions of insoluble solids (FIS) ranging from 5% to 17% by weight exhibited soft-solid characteristics, including an apparent yield stress. Yield stresses were highly dependent on stover concentration, scaling with FIS to the sixth power, and ranged from 0.2-5000 Pa between 5% and 17% FIS. PCS suspensions were strongly shear thinning, with flow and dynamic viscosities that were highly dependent on FIS. Last, as with many concentrated suspensions, the Cox-Merz rule was not followed, although flow and dynamic viscosities were coincident when plotted versus an effective shear rate.