NOVELTY - A method of treating biomass feed stock (16) involves feeding the feed stock through pressure isolation device and to upper inlet of vertical reactor system to deposit a pile of feed stock (18), supplying heat energy by injecting steam (22) volume (A) to upper region of vertical reactor vessel system (14), propelling feed stock through an outlet valve in lower region by injecting steam volume (B) into biomass at an elevation corresponding to outlet of reactor vessel and moving the propelled feed stock through expansion device (38) for steam explosion process.    USE - Method of treating biomass feed stock (claimed).    ADVANTAGE - The method of treating biomass feed stock is economical and environmentally-friendly.    DETAILED DESCRIPTION - A method of treating biomass feed stock involves feeding the feed stock through pressure isolation device and to upper inlet of vertical reactor system to deposit a pile of feed stock, adding heat energy by injecting steam volume (A) to upper region of vertical reactor vessel system to heat and pressurize feed stock, propelling feed stock through an outlet valve in lower region of vertical reactor vessel system by injecting steam volume (B) into biomass at an elevation corresponding to outlet of reactor vessel where steam volume (A) and steam volume (B) are vertically separated by portion of biomass column (33) and moving the propelled feed stock through expansion device for steam explosion process. An INDEPENDENT CLAIM is included for assembly for treating biomass feed stock, which comprises vertical reactor vessel system containing upper feed stock inlet and upper steam inlet into reactor, feed stock outlet in lower region of vertical reactor vessel, propelling steam inlet at elevation corresponding to lower region of reactor vessel where upper steam inlet and propelling steam inlet are vertically separated by height of biomass and a conduit extending from feed stock outlet to expansion device for receiving feed stock from vertical reactor vessel.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of flow through a steam explosion process including a vertical reactor vessel.    Vertical reactor vessel (14)    Biomass feed system (16)    Pile of feed stock (18)    Source of steam (22, 28)    Lower steam injection inlet (30)    Biomass movement device (33)    Conduit (36)    Expansion device (38)