Additives were tested for their ability to modify the rheology of lignocellulosic biomass. Additive types included water-soluble polymers (WSPs), surfactants, and fine particles. WSPs were the most effective rheological modifiers, reducing yield stresses of concentrated biomass by 60–80% for additive concentrations of 1–2 wt. % (based on mass of dry biomass solids). Yield stress and plastic viscosity of rheologically modified biomass depended on WSP molecular weight and degree of substitution. The apparent shear stress-shear rate data are represented with the Bingham model. In the absence of WSP, the biomass exhibited a positive yield stress and a negative plastic viscosity, which suggests a nonmonotonic dependence of shear stress on shear rate. When WSP was added, the yield stress decreased and the plastic viscosity increased, becoming positive for sufficiently large WSP concentrations.

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