NOVELTY - Promoting hydrogen gas distribution within cellulosic biomass solids involves: interfacially contacting continuous liquid phase of biomass with continuous gas phase in upwardly directed flow of molecular hydrogen and slurry catalyst; heating the cellulosic biomass solids (S1) and continuous liquid phase in presence of molecular hydrogen; conveying the continuous liquid phase and slurry catalyst to location within the continuous gas phase; and after conveying continuous liquid phase and slurry catalyst, releasing them such that they contact second portion of (S1) in continuous gas phase.    USE - For promoting hydrogen gas distribution within cellulosic biomass solids (claimed).    ADVANTAGE - The present method provides a relatively simple and low cost engineering solution so that a slurry catalyst may be at least partially distributed within cellulosic biomass solids using fluid flow to convey the slurry catalyst particulates into the interstitial spaces within a charge of cellulosic biomass solids. In addition, when upwardly directed fluid flow is present, there may be a reduced need to utilize mechanical stirring or like mechanical agitation techniques that might otherwise be needed to obtain an adequate catalyst distribution in the cellulosic biomass solids.    DETAILED DESCRIPTION - Promoting hydrogen gas distribution within cellulosic biomass solids involves: providing a first portion of cellulosic biomass solids being contacted by a continuous liquid phase and a second portion of cellulosic biomass solids being contacted by a continuous gas phase, where the continuous liquid phase and the continuous gas phase is interfacially contacting one another, where a slurry catalyst capable of activating molecular hydrogen is present in the continuous liquid phase; supplying an upwardly directed flow of molecular hydrogen through the cellulosic biomass solids; heating the cellulosic biomass solids and the continuous liquid phase in the presence of the molecular hydrogen, thereby forming an alcoholic component derived from the cellulosic biomass solids; conveying at least a portion of the continuous liquid phase and at least a portion of the slurry catalyst to a location within the continuous gas phase that is above at least a portion of the cellulosic biomass solids; and after conveying the continuous liquid phase and the slurry catalyst, releasing them such that they contact the second portion of cellulosic biomass solids in the continuous gas phase. An INDEPENDENT CLAIM is included for a biomass conversion system comprising a hydrothermal digestion unit having a first zone and a second zone, where the first zone is located below the second zone, where the first zone is configured to contain a continuous liquid phase and the second zone is configured to contain a continuous gas phase, where the continuous liquid phase contains a slurry catalyst in that; a fluid conduit external to the hydrothermal digestion unit configured to convey the continuous liquid phase from the first zone to the continuous gas phase in the second zone; and an inlet configured to provide upwardly directed gas flow in the first zone.