Methods and systems for promoting hydrogen gas distribution within cellulosic biomass solids during hydrothermal digestion. One exemplary method can comprise providing cellulosic biomass solids in a hydrothermal digestion unit in the presence of a digestion solvent and a slurry catalyst capable of activating molecular hydrogen; and heating the cellulosic biomass solids and the digestion solvent in the presence of molecular hydrogen, thereby forming an alcoholic component derived from the cellulosic biomass solids, at least a portion of the molecular hydrogen being introduced to the hydrothermal digestion unit via a plurality of spaced apart fluid inlets vertically disposed about the height of the hydrothermal digestion unit.