High-solids biomass slurries exhibit non-Newtonian behavior with a yield stress and require high power input for mixing. The goals were to determine the effect of scale and geometry on power number P-0, and estimate the power for mixing a pretreated biomass slurry in a 3.8 million L hydrolysis reactor of conventional design. A lab-scale computational fluid dynamics model was validated against experimental data and then scaled up. A pitched-blade turbine and A310 hydrofoil were tested for various geometric arrangements. Flow was transitional; laminar and turbulence models resulted in equivalent P-0 which increased with scale. The ratio of impeller diameter to tank diameter affected P-0 for both impellers, but impeller clearance to tank diameter affected P-0 only for the A310. At least 2 MW is required to operate at this scale.