NOVELTY - Converting cellulosic biomass into biofuels, comprises providing a biomass conversion system that comprises a fluid circulation loop (6) comprising a hydrothermal digestion unit (2), a solids separation unit comprising filters and a catalytic reduction reactor unit (4), partially digesting a cellulosic biomass in the digestion unit to form a hydrolysate comprising soluble carbohydrates and cellulosic fines within a liquor phase, flowing the liquor phase through filters to sequester the fines, and backflushing the fines to the digestion unit.    USE - The method is useful for converting cellulosic biomass (claimed) into biofuels including synthetic gasoline, diesel fuels and jet fuels, where the biomass includes corn stover, straw, bagasse, Miscanthus, Sorghum residue, switch grass, bamboo, water hyacinth, hardwood, hardwood chips, hardwood pulp, softwood, softwood chips, softwood pulp, leaves, roots, seeds, stalks, agricultural wastes (e.g. corn stalks, straw, seed hulls, sugarcane leavings and nut shells), wood materials (e.g. wood or bark, sawdust, timber slash and mill scrap), municipal waste (e.g. waste paper, yard clippings or debris) and energy crops (e.g. poplars, willows, switch grass, alfalfa, prairie bluestream, corn and soybeans).    ADVANTAGE - The method enables energy-efficient conversion of the cellulosic biomass into biofuel in an economical manner.    DETAILED DESCRIPTION - Converting cellulosic biomass into biofuels, comprises providing a biomass conversion system that comprises a fluid circulation loop (6) comprising a hydrothermal digestion unit (2), a solids separation unit that is present in fluid communication with an outlet of the hydrothermal digestion unit and comprises filters and a catalytic reduction reactor unit (4) that is present in fluid communication with an outlet of the solids separation unit and an inlet of the hydrothermal digestion unit, providing a cellulosic biomass in the hydrothermal digestion unit, partially digesting the cellulosic biomass in the hydrothermal digestion unit to form a hydrolysate comprising soluble carbohydrates and cellulosic fines within a liquor phase, flowing the liquor phase through filters to sequester the cellulosic fines, and backflushing a portion of the cellulosic fines to the hydrothermal digestion unit while the liquor phase continues to flow through the filters to the catalytic reduction reactor unit. An INDEPENDENT CLAIM is included for the biomass conversion system.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a biomass conversion system.    Hydrothermal digestion unit (2)    Catalytic reduction reactor unit (4)    Pressure zone (5)    Fluid circulation loop (6)    Valves. (21)