NOVELTY - Digesting cellulosic biomass solids, comprises: (a) providing cellulosic biomass solids in the presence of digestion solvent, molecular hydrogen, and slurry catalyst that activates molecular hydrogen; (b) at least partially converting the cellulosic biomass solids into a phenolics liquid phase comprising lignin, an aqueous phase comprising an alcoholic component derived from the cellulosic biomass solids, and an optional light organics phase, where a portion of the slurry catalyst accumulates in the phenolics liquid phase as it forms; and (c) reducing the viscosity of phenolics liquid phase.    USE - The method is useful for digesting cellulosic biomass solids (claimed) to produce biofuel.    ADVANTAGE - The method: is simple and economical, and eliminates the need for mechanical stirring or mechanical agitation techniques; utilizes the slurry catalyst that distributes uniformly within cellulosic biomass solids and reduces thermal gradients that may occur during hydrothermal digestion; and utilizes the biomass conversion system that is highly energy efficient and cost-effective.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) digesting cellulosic biomass solids, comprising the step (a) as above per se, (b1) heating the cellulosic biomass solids to a first temperature and at least partially converting the cellulosic biomass solids into the phenolics liquid phase, the aqueous phase and the optional light organics phase, (c1) heating the phenolics liquid phase in the presence of molecular hydrogen to a second temperature that is higher than the first temperature, thus reducing the viscosity of the phenolics liquid phase, and (d1) separating the slurry catalyst from the phenolics liquid phase; and    (2) a biomass conversion system (1) comprising a hydrothermal digestion unit (2), a first fluid conduit configured to remove a first fluid from the hydrothermal digestion unit and return the first fluid to it, and a viscosity measurement device (10) within the hydrothermal digestion unit or in flow communication with the hydrothermal digestion unit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the biomass conversion system.    Biomass conversion system (1)    Hydrothermal digestion unit (2)    Solids introduction mechanism (4)    Viscosity measurement device (10)    Polishing reactor (16)