NOVELTY - Improving (M1) the pretreatment of biomass involves: (A) mixing biomass with a first inflow which is a liquid; (B) obtaining a first effluent after first inflow passes through biomass; (C) allowing first effluent to pass through a heat exchanger, where a portion of heat from first effluent is transferred to a second inflow which is a liquid; (D) mixing second inflow with biomass which is in contact with first inflow; and (E) allowing a solvent in first effluent to evaporate to obtain a first effluent concentrate and evaporating steam.    USE - For improving the pretreatment of biomass, and to provide digestible biomass for subsequent conversion to biofuels.    ADVANTAGE - The method provides improved separation of biomass components, hemicellulose recovery, and cellulose digestibility. The method enhances the conversion rate and efficiency from subsequent processing of biomass to biofuels. The evaporation considerably increases the amount of washing that can be employed within the energy constraints. Thus, the fermentable sugars from the biomass are obtained with higher concentration through pretreatment processes and the overall energy put into the system is less than using prior art methods. The evaporation strategy decreases the amount of process energy, thus making the benefits of flow-through pretreatment easier and by lowering the cost for a given extent of washing. By removing lignin and hemicellulose derivatives during pretreatment and wash, the method increases access of enzymes to the substrate and decreases non-productive cellulase binding.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) system (S1) for pretreating biomass comprising pretreatment vessel for holding biomass, pretreatment vessel comprising an inlet and outlet, where inlet allows for inflow to enter into vessel, and outlet allows for effluent to exit the vessel; evaporator for holding effluent and connected to pretreatment vessel, and heating effluent and allowing at least one solvent in effluent to evaporate from effluent, to form an effluent concentrate and evaporating steam containing at least one solvent; and    (2) obtaining (M2) ethanol from a cellulosic biomass where the process energy for obtaining ethanol is less than 47% of the heating value of cellulosic biomass, involving pretreatment of cellulosic biomass in reaction vessel with a flow-through input wash, where effluent output from reaction vessel comprises treated cellulosic biomass, the treated cellulosic biomass effluent exchanges heat in heat exchanger with flow-through input wash, the effluent is evaporated to produce steam, the steam is used for counter-current stripping of a fermentation broth containing ethanol.