NOVELTY - Biomass conversion system (10) comprises: a first fluid circulation loop (10) containing a hydrothermal digestion unit (12), and a first catalytic reduction reactor unit (14); and second fluid circulation loop (30) containing a reaction product take-off line in fluid communication with the first fluid circulation loop, a second catalytic reduction reactor unit in fluid communication with the reaction product take-off line, and a recycle line establishing fluid communication between the first fluid circulation loop and an outlet of the second catalytic reduction reactor unit.    USE - The biomass conversion system is useful for processing biomass solids into biofuels (claimed).    ADVANTAGE - The biomass conversion system: enhances lignin solubility without compromising on heat integration; reduces external heat input to the digestion process; favors higher yields and high conversion of biomass into a hydrolysate in an energy- and cost-efficient manner.    DETAILED DESCRIPTION - Biomass conversion system (10) comprises: a first fluid circulation loop (10) containing a hydrothermal digestion unit (12), and a first catalytic reduction reactor unit (14) in fluid communication with an inlet and outlet of the hydrothermal digestion unit, where the first catalytic reduction reactor unit comprises at least one first catalyst that is capable of activating molecular hydrogen; and second fluid circulation loop (30) containing a reaction product take-off line in fluid communication with the first fluid circulation loop, a second catalytic reduction reactor unit in fluid communication with the reaction product take-off line, and a recycle line establishing fluid communication between the first fluid circulation loop and an outlet of the second catalytic reduction reactor unit, where the second catalytic reduction reactor unit comprises at least one second catalyst that is capable of activating molecular hydrogen. An INDEPENDENT CLAIM is also included for processing biomass solids using integrated heat management for process control, comprising: providing the biomass conversion system; providing a cellulosic biomass in the hydrothermal digestion unit; heating the cellulosic biomass in the hydrothermal digestion unit to digest at least a portion of the cellulosic biomass and form a hydrolysate containing soluble carbohydrates within a liquor phase, where at least 70% of the heat added to the cellulosic biomass in the hydrothermal digestion unit is generated internally in the first catalytic reduction reactor unit and the second catalytic reduction reactor unit; transferring at least a portion of the liquor phase to the first catalytic reduction reactor unit; forming a first reaction product in the first catalytic reduction reactor unit; recirculating at least a portion of the liquor phase to the hydrothermal digestion unit at a first flow rate; and altering the first flow rate to increase or decrease a temperature of the liquor phase in the first fluid circulation loop.    DESCRIPTION OF DRAWING(S) - The figure illustrates a schematic representation of a biomass conversion system.    Biomass conversion system (1)    First fluid circulation loop (10)    Hydrothermal digestion unit (12)    First catalytic reduction reactor unit (14)    Second fluid circulation loop (30)