Abstract: A process is provided for preparing a low density, fibrous primary lignocellulose biomass, particularly straw for anaerobic digestion at large scale, i.e. >2 MWth, which comprises a step that greatly increasing the density of the straw through size reduction, moisture adjustment and compression ahead of loading and subsequent agitation of the resultant biomass in a pressure vessel through an atmosphere of saturated steam providing heat for thermal-pressure hydrolysis and recovering treated biomass from the vessel. The primary lignocellulose biomass may be prepared in admixture with secondary biomass which may be manure-based.