NOVELTY - Biomass conversion system (10) comprises: pressure vessel (12) comprising first and second sections, where first section comprises hydrothermal digestion unit (14), second section comprises first catalytic reduction reactor unit that contains first catalyst capable of activating molecular hydrogen, and hydrothermal digestion unit and first catalytic reduction reactor unit (16) are in fluid communication with one another; biomass feed mechanism that is operatively connected to pressure vessel; and hydrogen feed line that is operatively connected to first catalytic reduction reactor unit.    USE - The biomass conversion system is useful for digesting cellulosic biomass solids (claimed).    ADVANTAGE - The biomass conversion system: utilizes less heat input; and provides more stable reaction product at a low cost.    DETAILED DESCRIPTION - Biomass conversion system (10) comprises: a pressure vessel (12) comprising a first and a second sections, where the first section comprises a hydrothermal digestion unit (14), the second section comprises a first catalytic reduction reactor unit (16) that contains a first catalyst capable of activating molecular hydrogen, and the hydrothermal digestion unit and the first catalytic reduction reactor unit are in fluid communication with one another; a biomass feed mechanism that is operatively connected to the pressure vessel, where the biomass feed mechanism is capable of introducing cellulosic biomass solids to the pressure vessel and also capable of withdrawing a reaction product from the first catalytic reduction reactor unit; and a hydrogen feed line that is operatively connected to the first catalytic reduction reactor unit. An INDEPENDENT CLAIM is also included for digesting cellulosic biomass solids, comprising providing the pressure vessel comprising the first and second section, adding cellulosic biomass solids to the pressure vessel, heating the cellulosic biomass solids in the hydrothermal digestion unit of the pressure vessel, thus forming a hydrolysate comprising soluble carbohydrates within a liquor phase, conveying the liquor phase through the first catalytic reduction reactor unit in the presence of molecular hydrogen to at least partially transform the soluble carbohydrates into a reaction product, and conveying at least a portion of the liquor phase from the pressure vessel to the second catalytic reduction reactor unit that contains the second catalyst capable of activating molecular hydrogen, to further transform the soluble carbohydrates into the reaction product.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of biomass conversion system.    Biomass conversion system (10)    Pressure vessel (12)    Hydrothermal digestion unit (14)    First catalytic reduction reactor unit (16)    Outer pressure housing (18)