NOVELTY - Pretreating biomass, comprises: contacting a biomass stream countercurrently with a pretreatment solution stream; and producing a hydrolyzate stream (24) and a pretreated biomass stream.    USE - The method is useful for pretreating a biomass, where the pretreated biomass is useful for producing biofuels (all claimed) that are useful in power generation, and as transportation fuels in vehicles.    ADVANTAGE - The method utilizes the hydrolyzing diffuser unit, which provides improved breakdown of biomass, as compared to conventional methods.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a hydrolyzing diffuser unit (D1) (10) comprising: a series of stages (12) including stage n to stage n+z, where stage n comprises an inlet (14) for biomass, and stage n+z comprises an inlet (16) for the pretreatment solution; a system that continually moves biomass from stage n to stage n+1; a system that continually moves biomass from stage n+z to stage n+z-1; a system that continually withdraws the pretreatment solution from stage n+y, thus producing a hydrolyzate stream; and a system that continually withdraws pretreated biomass from stage n+m (where the stage n is greater than or equal to 1, z is greater than or equal to n and less than or equal to n+20, y is greater than or equal to 0 to less than or equal to z-1, and m is less than or equal to z), thus producing a pretreated biomass stream;    (2) a hydrolyzing diffuser unit (D2) comprising: a series of stages including a first stage to a final stage with at least one intermediate stage between them, where the first stage comprises an inlet for biomass and the final stage comprises an inlet for the pretreatment solution; and a system that continually withdraws the pretreatment solution from an intermediate stage and feeds the pretreatment solution into the first stage thus producing a hydrolyzate stream, while simultaneously moving biomass from the first stage to an intermediate stage, thus producing a pretreated biomass stream;    (3) a reactor (R1) comprising: a twin-channel screw conveyor having an inlet for biomass mixed with acid at a first end of the screw conveyor; an inlet for steam at a second end of the screw conveyor opposite the first end, where the steam flows countercurrent to the biomass mixture; and an outlet for the steam-heated biomass mixture;    (4) a reactor (R2) comprising: a high-temperature zone having a first inlet for biomass mixed with acid and steam; a low-temperature zone downstream of the high-temperature zone, where the low-temperature zone has a second inlet for cooled aqueous acid; and an outlet for the biomass downstream of the low-temperature zone;    (5) a reactor (R3) comprising: an inlet for biomass at or near a top end of the reactor; an inlet for liquid acid catalyst near a bottom end of the reactor; and a screw conveyor positioned below the inlet for the liquid acid catalyst, where the screw conveyor continuously removes pretreated biomass from the reactor through an outlet;    (6) a reactor (R4) comprising: an inlet for biomass at or near a bottom end of the reactor; and an inlet for liquid acid catalyst near a top end of the reactor;    (7) a reactor (R5) comprising many mixing tanks and many settling tanks, where biomass and an aqueous acid flow counter-currently into the mixing tanks and the settling tanks; and    (8) a biorefinery for producing biofuels, comprising: the hydrolyzer diffuser unit (D1) or (D2) or reactors (R1)-(R5); a saccharification unit for converting the biomass to a sugar; and a conversion unit for producing a product from the sugar.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the hydrolyzing diffuser unit.    Hydrolyzing diffuser unit (10)    Series of stages (12)    Inlet for biomass (14)    Inlet for pretreatment solution (16)    Hydrolyzate stream (24)