Method and apparatus for pretreatment of a biomass comprising lignocellulosic fibers

The present invention relates to pretreatment of biomass using targeted alkaline hydrolysis with optional alkali recovery, where the biomaterial is steam exploded before an alkaline hydrolysis. The invention in particular relates to a method for pretreatment of a biomass comprising lignocellulosic fibers, comprising the following steps: a) Providing a wetted biomass having a dry matter content of at least 15%, b) Flashing the wetted biomass into a pretreatment reactor at a pressure between 5 and 14 bar, c) Subjecting the biomass to thermal hydrolysis and steam explosion after holding the biomass at a temperature between 150-210°C, at steam saturation pressure between 3,7 – 18,5 bar, for 10-45 minutes, d) Subjecting the steam exploded biomass to an alkaline hydrolysis by adding an alkali component in a concentration of between 0.1 to 10% w/w obtaining an alkali reacted biomass slurry. The invention also relates to an apparatus for pretreatment of a biomass comprising lignocellulosic fibers comprising a conditioning unit (B), a feed sluice (C) configured to receive conditioned biomass and guide the biomass to the inlet of the pretreatment reactor/tank, a pretreatment reactor/tank (D) for thermal hydrolysis and steam explosion of biomass, a flash tank (E), a hydrolysis tank (F) for alkaline hydrolysis takes place, and a separation unit (G) in which the pretreated slurry can be separated into its liquid and solid (fiber) constituents.