NOVELTY - Promoting hydrogen gas distribution within cellulosic biomass solids during hydrothermal digestion comprises: providing cellulosic biomass solids in a hydrothermal digestion unit (10) in presence of a digestion solvent and a slurry catalyst to activate molecular hydrogen; and heating the solids and solvent in presence of molecular hydrogen, to form an alcoholic component derived from the biomass solids. At least a portion of the molecular hydrogen is introduced to the digestion unit via many spaced apart fluid inlets vertically disposed about the height of the digestion unit.    USE - The method is useful for promoting hydrogen gas distribution within cellulosic biomass solids during hydrothermal digestion.    ADVANTAGE - The method provides: more rapid digestion rate of cellulosic biomass solids under elevated temperature and pressure conditions; and degradation of soluble carbohydrates during hydrothermal digestion under an in situ catalytic reduction reaction process to convert the soluble carbohydrates into more stable compounds as soon as possible after their formation, in a continual manner.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a biomass conversion system comprising a hydrothermal digestion unit, many spaced apart fluid inlets vertically disposed about the height of the hydrothermal digestion unit and a gas recirculation line configured to remove a gas from the hydrothermal digestion unit and return at least a portion of the gas to the hydrothermal digestion unit via the spaced apart fluid inlets, where the gas recirculation line is fluidly connected to the spaced apart fluid inlets; and    (2) a biomass conversion system (S1) comprising a hydrothermal digestion unit, many spaced apart fluid inlets vertically disposed about the height of the hydrothermal digestion unit and a fluid recirculation line configured to remove a mixture of a liquid and a gas from the hydrothermal digestion unit and return at least a portion of the gas to the hydrothermal digestion unit via the spaced apart fluid inlets, where the fluid recirculation line is fluidly connected to the spaced apart fluid inlets.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic biomass conversion system in which cellulosic biomass solids may be digested in the presence of a digestion solvent to produce an alcoholic component.    Hydrothermal digestion unit (10)    Solids introduction mechanism (12)    Loading mechanism (14)    Pressure transition zone (16)    Line (22)