A method for treating cellulosic biomass feed stock including: feeding the feed stock to an upper inlet of a vertical reactor vessel, wherein the feed stock is deposited on a pile of feed stock within the vertical reactor vessel; adding heat energy to heat the feed stock by injecting steam to an upper region of the vertical reactor vessel; propelling the feed stock through an outlet in a lower region of the vertical reactor vessel by injecting steam into the biomass at, near or after the bottom outlet of the vertical reactor vessel, and moving the propelled feed stock through an expansion device, such as a steam explosion device, to subject the feed stock to a steam explosion process.