NOVELTY - Method for digesting cellulosic biomass solids, involves: optionally distributing slurry catalyst activating molecular hydrogen within the solids using upwardly directed fluid flow in hydrothermal digestion unit; heating the solids in the digestion unit in presence of the catalyst, digestion solvent and molecular hydrogen to form liquor phase comprising soluble carbohydrates; and performing first catalytic reduction reaction on the carbohydrates within the digestion unit to partially form a reaction product comprising a triol, diol and/or monohydric alcohol, in the digestion unit.    USE - For digesting cellulosic biomass solids for producing a biofuel (all claimed) such as bioethanol.    ADVANTAGE - Digesting cellulosic biomass solids in the presence of a well-distributed slurry catalyst capable of activating molecular hydrogen limit the amount of degradation products that form during digestion. The presence of the slurry catalyst within the hydrothermal digestion unit allow at least one in situ catalytic reduction reaction to take place in it, thus intercepting and transforming soluble carbohydrates into a more stable reaction product as soon as feasible after the soluble carbohydrates form. Formation of the reaction product 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. In the methods, slurry catalyst is distributed within cellulosic biomass solids using fluid flow to convey the slurry catalyst in it. Conveying the slurry catalyst into a cellulosic biomass charge from bottom to top using upwardly directed fluid flow present a number of advantages. Specifically, it overcomes settling and gravity-induced compaction that occurs during the addition and digestion of cellulosic biomass solids. Settling and compaction of cellulosic biomass solids impact fluid flow through the hydrothermal digestion unit and particularly reduce one's ability to effectively distribute a slurry catalyst in it. By using upwardly directed fluid flow, settling and compaction issues can be reduced through promoting expansion of the cellulosic biomass charge to allow the slurry catalyst to become distributed in it. In addition, by using upwardly directed fluid flow, there is a reduced need to use mechanical stirring or mechanical agitation that might otherwise be needed to obtain an adequate catalyst distribution. This feature allows high loadings of cellulosic biomass solids relative to digestion solvent to be used, thus improving throughput and process economics. Use of upwardly directed fluid flow to distribute a slurry catalyst within cellulosic biomass solids allow higher loadings of cellulosic biomass solids relative to digestion solvent to be used than would otherwise be possible with other modes of catalyst distribution. The natural bed porosity of cellulosic biomass solids allow to distribute a slurry catalyst in it without the need for mechanical stirring or mixing. Larger amounts of cellulosic biomass solids can be processed per unit size of the hydrothermal digestion unit, thus improving process economics. Further, smaller volume digestion units, which are simpler to construct and maintain, can also be used without sacrificing throughput, thus further aiding process economics. In addition to rapidly stabilizing soluble carbohydrates as a reaction product, conducting in situ catalytic reduction reactions is particularly advantageous from an energy efficiency standpoint. The hydrothermal digestion of cellulosic biomass is an endothermic process, whereas catalytic reduction reactions are exothermic. Thus, the excess heat generated by the in situ catalytic reduction reaction(s) can be utilized to drive the hydrothermal digestion, thus lowering the amount of additional heat energy input needed to conduct digestion. Since digestion and catalytic reduction take place within the same vessel, there is minimal opportunity for heat transfer loss to take place, as would occur if the catalytic reduction reaction(s) were to be conducted in a separate location. In addition, in such a configuration, the in situ catalytic reduction reaction(s) provide a growing supply of the reaction product within the hydrothermal digestion unit, which serve as and/or replenish the digestion solvent. Since the reaction product and the digestion solvent are the same, there is no express need to separate and recycle a majority of the digestion solvent before further processing the reaction product downstream, which is further advantageous from an energy efficiency standpoint.    DETAILED DESCRIPTION - Method for digesting cellulosic biomass solids, involves: providing cellulosic biomass solids and a slurry catalyst in a hydrothermal digestion unit, where the slurry catalyst is capable of activating molecular hydrogen; optionally distributing the slurry catalyst within the cellulosic biomass solids using upwardly directed fluid flow in the hydrothermal digestion unit; heating the solids in the hydrothermal digestion unit in the presence of the slurry catalyst, a digestion solvent, and molecular hydrogen, thus forming a liquor phase comprising soluble carbohydrates; and performing a first catalytic reduction reaction on the soluble carbohydrates within the hydrothermal digestion unit, thus at least partially forming a reaction product comprising a triol, diol, and/or a monohydric alcohol, in the hydrothermal digestion unit.