There is disclosed a method for processing a biomass (for example straw) containing lignocellulose such that cellulose and hemicellulose are made accessible for further processing, typically by decomposition, without needing energy-consuming dissolution of the biomass in water. The method includes repeated compressions of the biomass in a reciprocating piston press, where loose biomass is continuously fed into a piston chamber in front of a piston which moves the loose biomass into a tubular reaction chamber in which the biomass is compressed for producing a vapour explosion and autohydrolysis under simultaneous displacement of compressed biomass through the reaction chamber. After compression, to the biomass can be added fluid livestock manure, fluid waste water sludge etc. in a biogas plant for subsequent biogas processing.
