NOVELTY - Lignocellulosic biomass conversion involves heating solid biomass feedstock in hydrothermal digestion unit (2) in presence of liquor for forming hydrolysate comprising partially digested solids and soluble carbohydrates within liquor phase; contacting hydrolysate with molecular hydrogen in first fixed bed catalytic reduction reactor unit (6) for transforming hydrolysate into reaction product comprising alcohol in liquor phase; recirculating liquor phase and partially digested solids; and transferring reaction product to second catalytic reduction reactor unit (12).    USE - For production of higher hydrocarbons useful in forming industrial chemicals and transportation fuels e.g. biofuel, synthetic gasoline, diesel fuels, and jet fuels.    ADVANTAGE - The method allows lignocellulosic biomass solid that contains cellulose and lignin to be efficiently digested to form soluble carbohydrates, which may subsequently be converted through catalytic reduction reactions e.g. hydrogenolysis and/or hydrogenation, into more stable reaction products comprising oxygenated intermediates that may be further processed into higher hydrocarbons. The digestion rate of cellulosic biomass may be accelerated in the presence of liquor (digestive solvent) at elevated temperatures and pressures that maintain the liquor in a liquid state above its normal boiling point. The supported metal catalyst is capable of activating molecular hydrogen having an average void fraction of at least 40% (high void fraction) and a surface area in the range of 10-1000 m2/g allows high permeability and extends reaction run time in the first fixed bed catalytic reduction reactor unit. The biomass conversion system features allow a significant quantity of the initially solubilized carbohydrates to be converted into a form that is suitable for subsequent processing into a biofuel. The biomass conversion system and method is configured to rapidly stabilize a significant fraction of the hydrolysate produced. By fluidly coupling the first catalytic reduction reactor unit containing the high void fraction catalyst to the hydrothermal digestion unit, excellent heat integration efficiency may be realized with extended run life. The hydrothermal digestion unit in the systems can be continuously operated at elevated temperatures and pressures. The fuel yield of the present method is greater than other bio-based feedstock conversion processes with substantially less loss of oxygenated product yield. The high void fraction supported metal catalyst substantially increases permeability (non-plugging) thus extends catalyst run life.    DETAILED DESCRIPTION - Lignocellulosic biomass conversion method involves providing a solid biomass feedstock containing cellulose, lignin and water in a hydrothermal digestion unit (2); heating the solid biomass feedstock in the hydrothermal digestion unit in the presence of a liquor to digest portion of the solid biomass feedstock, thereby forming a hydrolysate comprising partially digested solids and soluble carbohydrates within a liquor phase; contacting portion of the hydrolysate, in a first fixed bed catalytic reduction reactor unit (6), with molecular hydrogen at 180-300 degrees C in presence of a supported metal catalyst (7) (C1) capable of activating molecular hydrogen having an average void fraction of at least 40% and a surface area of 10-1000 m2/g, thereby transforming the hydrolysate into a reaction product comprising at least one alcohol in the liquor phase in the first fixed bed catalytic reduction reactor unit; recirculating at least a portion of the liquor phase and portion of the partially digested solids from the first fixed bed catalytic reduction reactor unit to the hydrothermal digestion unit; and transferring portion of the reaction product to a second catalytic reduction reactor unit (12) so as to further transform the soluble carbohydrates into a reaction product. INDEPENDENT CLAIMS are included for the following:    (1) lignocellulosic biomass conversion system (1) comprising hydrothermal digestion unit (2); first catalytic reduction reactor unit (6) coupled to the hydrothermal digestion unit along its height by at least two fluid inlet lines (8) and at least two fluid return lines (8'), the first catalytic reduction reactor unit containing the supported metal catalyst (7) (C1); and a fluid circulation loop (10) comprising the hydrothermal digestion unit and second catalytic reduction reactor unit (12) that contains catalyst capable of activating molecular hydrogen, where the second fluid inlet line is after the first fluid outlet line; and    (2) composition comprising lignocellulosic biomass; supported metal catalyst having a void fraction of 45-95% and surface area of 10-1000 m2/g containing Mo, W, Co, and/or Ni; water; and liquor phase.    DESCRIPTION OF DRAWING(S) - The figure shows block flow diagram of biomass conversion process.    Lignocellulosic biomass conversion system (1)    Hydrothermal digestion unit (2)    First catalytic reduction reactor unit (6)    Fluid inlet lines (8)    Fluid return lines (8')    Supported metal catalyst (7)    Fluid circulation loop (10)    Second catalytic reduction reactor unit (12)