NOVELTY - Digesting cellulosic biomass solid involves providing hydrothermal digestion unit (2) containing fluid phase digestion medium and slurry catalyst capable of activating molecular hydrogen; introducing and distributing cellulosic biomass solids laterally within hydrothermal digestion unit, where the fluid phase digestion medium at least partially covers the solids after they have been distributed; supplying upwardly directed flow of molecular hydrogen through cellulosic biomass solids and fluid phase digestion medium; and heating cellulosic biomass solids, to form the alcoholic components.    USE - For digesting cellulosic biomass solid to form fuel blends and other materials.    ADVANTAGE - By altering the configuration of a hydrothermal digestion unit that is used to digest cellulosic biomass solids from a vertical configuration into a horizontal configuration in which the length and/or width of the hydrothermal digestion unit is greater than its height, the problems of biomass compaction, slurry catalyst distribution, and hydrogen bubble coalescence are simultaneously addressed. In addition to promoting lateral distribution of the cellulosic biomass solids, lateral circulation of the fluid phase digestion medium allows reduction of thermal variations within the hydrothermal digestion unit.    DETAILED DESCRIPTION - A method (M1) for digesting cellulosic biomass solid involves providing a hydrothermal digestion unit (2) having length or width that is greater than its height, and containing fluid phase digestion medium and slurry catalyst capable of activating molecular hydrogen; introducing cellulosic biomass solids to the hydrothermal digestion unit; distributing the cellulosic biomass solids laterally within the hydrothermal digestion unit, such that the fluid phase digestion medium at least partially covers the cellulosic biomass solids after they have been distributed; supplying upwardly directed flow of molecular hydrogen through the cellulosic biomass solids and the fluid phase digestion medium after or while the cellulosic biomass solids are being distributed and heating the cellulosic biomass solids in the presence of the slurry catalyst and molecular hydrogen, to form alcoholic component derived from the cellulosic biomass solids. INDEPENDENT CLAIMS are included for the following:    (1) biomass conversion system (101) comprising hydrothermal digestion unit having length or width that is greater than its height; biomass feed mechanism (4) that is operatively connected to the hydrothermal digestion unit, where the biomass feed mechanism is configured for addition of cellulosic biomass solids to the hydrothermal digestion unit while the hydrothermal digestion unit is in pressurized state; gas distribution system (12) laterally disposed within the hydrothermal digestion unit along its lower surface; and fluid conduit configured to establish lateral fluid circulation within the hydrothermal digestion unit;    (2) a method (M2) for digesting cellulosic biomass solids in multiple hydrothermal digesting units involving carrying out steps of method (M1), followed by transferring the partially digested cellulosic biomass solids, fluid phase digestion medium and slurry catalyst to a second hydrothermal digestion unit, that is fluidly coupled to the first hydrothermal digestion unit; supplying upwardly directed flow of molecular hydrogen through the partially digested cellulosic biomass solids and the fluid phase digestion medium in the second hydrothermal digestion unit; and heating the partially digested cellulosic biomass solids in the presence of the slurry catalyst and the molecular hydrogen in the second hydrothermal digestion unit, thus further forming the alcoholic component; and    (3) biomass conversion system (S2) for carrying out method (M2) comprising first hydrothermal digestion unit and second hydrothermal digestion unit, where the first and second hydrothermal digestion units are fluidly coupled to one another, and the first hydrothermal digestion unit has a length or width that is greater than its height; a biomass feed mechanism operatively connected to the first hydrothermal digestion unit, configured for addition of cellulosic biomass solids to the first hydrothermal digestion unit while the first hydrothermal digestion unit is in pressurized state; gas distribution system disposed within each hydrothermal digestion unit along its lower surface; and fluid conduit configured to establish lateral fluid circulation within the first hydrothermal digestion unit.    DESCRIPTION OF DRAWING(S) - The figure shows schematic of biomass conversion system containing a horizontally configured hydrothermal digestion unit.    Hydrothermal digestion unit (2)    Biomass feed mechanism (4)    Gas distribution mechanism (12)    Gas introduction line (16)    Fluid removal line (20)    Biomass conversion system (101)