TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The molecular hydrogen is continuously introduced to the hydrothermal digestion unit via the spaced apart fluid inlets while heating takes place. At least a portion of the molecular hydrogen is introduced to the hydrothermal digestion unit via a fluid inlet disposed on a bottom surface of the hydrothermal digestion unit. The method further comprises: removing at least a portion of the molecular hydrogen from the hydrothermal digestion unit; returning the molecular hydrogen to the hydrothermal digestion unit, where the molecular hydrogen is returned to the hydrothermal digestion unit via the fluid inlet disposed on a bottom surface of the hydrothermal digestion unit; and supplying upwardly directed fluid flow from a fluid inlet disposed on a bottom surface of the hydrothermal digestion unit, where the upwardly directed fluid flow comprises a gas flow and/or a liquid flow. The molecular hydrogen is distributed in the cellulosic biomass solids once introduced from the spaced apart fluid inlets. The molecular hydrogen is distributed as bubbles in the cellulosic biomass solids, where the bubbles are less than or equal to 3 mm in size when introduced to the hydrothermal digestion unit from a first spaced apart fluid inlet and reaching a maximum size of 20 mm as the bubbles rise in the cellulosic biomass solids, before the bubbles reach a second spaced apart fluid inlet located above the first spaced apart fluid inlet and additional molecular hydrogen is introduced to the hydrothermal digestion unit from the second spaced apart fluid inlet. At least a portion of the molecular hydrogen being introduced to the hydrothermal digestion unit via the spaced apart fluid inlets comprises recirculated molecular hydrogen. The method further comprises: removing molecular hydrogen from the hydrothermal digestion unit at a location above an uppermost of the spaced apart fluid inlets; returning the molecular hydrogen to the hydrothermal digestion unit via the spaced apart fluid inlets; removing at least a portion of the digestion solvent and the molecular hydrogen from the hydrothermal digestion unit, where the digestion solvent and the molecular hydrogen are removed from the hydrothermal digestion unit as a mixture; and returning the digestion solvent and the molecular hydrogen to the hydrothermal digestion unit, where the molecular hydrogen from the mixture is returned to the hydrothermal digestion unit via many spaced apart locations vertically disposed about the height of the hydrothermal digestion unit. Preferred Components: The biomass conversion system further comprises: a fluid inlet disposed on a bottom surface of the hydrothermal digestion unit; a fluid recirculation line configured to remove a mixture of a gas and a liquid from the hydrothermal digestion unit and return at least a portion of the gas to the hydrothermal digestion unit; and a solids introduction mechanism operatively coupled to the top of the hydrothermal digestion unit. The gas recirculation line is fluidly connected to the hydrothermal digestion unit at a location above an uppermost of the spaced apart fluid inlets. The biomass conversion system (S1) further comprises: a fluid inlet disposed on a bottom surface of the hydrothermal digestion unit, where the fluid recirculation line is fluidly connected to the hydrothermal digestion unit at a location above an uppermost of the spaced apart fluid inlets, or fluidly connected to the hydrothermal digestion unit at multiple spaced apart locations vertically disposed about the height of the hydrothermal digestion unit; and a flow dispersal system within the hydrothermal digestion unit that is operably connected to the spaced apart fluid inlets. EXAMPLE - Two 75 ml reactors were charged with 20 g deionized water solvent, 0.12 g potassium carbonate buffer, and 0.6 g slurry catalyst. The slurry catalyst was a sulfided nickel oxide promoted cobalt molybdate catalyst containing 1-10% cobalt oxide and molybdenum trioxide (up to 30 wt.%) on alumina, and less than 2% nickel. Each reactor was charged with 2.7 g southern pine mini-chips (39% moisture) having nominal dimensions of 3x 4x 5 mm. The first reactor was pressurized with 52 bar of hydrogen while stirring with a stir bar, and the second reactor was only pressurized with 5 bar of hydrogen. The reactors were heated to 190 degrees C for 1 hour, followed by heating to 250 degrees C for 4.5 hours and then to 270 degrees C for 2 hours. Following the completion of a first reaction cycle, the reactors were cooled, depressurized, and then charged with 2.7 g fresh mini-chips. Then, each reactor was pressurized to the same pressure with hydrogen gas as before. A second digestion cycle was then conducted as above. At the completion of the two cycles, virtually all of the wood chips had been digested. The liquid reactor contents were then analyzed using gas chromatography. In the first reactor, the measured yield was 48% of the theoretical amount, based on the amount of carbohydrates present in the wood charge.