NOVELTY - A biomass conversion system comprises a hydrothermal digestion unit that contains a slurry catalyst capable of activating molecular hydrogen; an optional hydrogen feed line that is operatively connected to the hydrothermal digestion unit; and a fluid circulation loop comprising the hydrothermal digestion unit and a catalytic reduction reactor unit, the catalytic reduction reactor unit also containing the slurry catalyst.    USE - As a biomass conversion system (claimed) useful for digesting cellulosic biomass solids.    ADVANTAGE - The biomass conversion system effectively promotes the thermal stabilization of soluble carbohydrates produced from it. Formation of the reaction product may reduce the amount of thermal decomposition that occurs during hydrothermal digestion, thus enabling high yield conversion of cellulosic biomass solids into a desired reaction product to take place in a timely manner. The rapidly stabilizing soluble carbohydrates as a reaction product, conducting at least one in situ catalytic reduction reactions may also be particularly advantageous from an energy efficiency standpoint. The biomass conversion system allows a significant quantity of the initially solubilized carbohydrates to be converted into a form that is suitable for subsequent processing into a biofuel, while forming as small as possible an amount of caramelans and other decomposition products in or near the hydrothermal digestion unit. The biomass conversion system reduces capital and operational costs. The biomass conversion systems provide stabilization of soluble carbohydrates by performing an in situ catalytic reduction reaction in the hydrothermal digestion unit.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method involving providing cellulosic biomass solids in a hydrothermal digestion unit that contains a slurry catalyst capable of activating molecular hydrogen; heating the cellulosic biomass solids in the hydrothermal digestion unit in the presence of molecular hydrogen while circulating the slurry catalyst through it, thus forming a hydrolysate comprising soluble carbohydrates within a liquor phase; at least partially transforming the soluble carbohydrates into a reaction product while the soluble carbohydrates are within the hydrothermal digestion unit; and transferring at least a portion of the liquor phase to a catalytic reduction reactor unit that also contains the slurry catalyst, so as to further transform the soluble carbohydrates into the reaction product.