Abstract: Process and systems are provided for anaerobic digestion of lignocellulosic feedstock into biogas and co-products, such as fibrous humid cake and a fine-particles slurry. The feedstock (e.g., grass straw) is typically first ground or milled. The feedstock is subjected to pelletizing or extruding to generate pellets (e.g., cubes) or extrudates. The pellets or extrudates may be fed to a hydrolysis unit or a digester, in dry form or wet form. Various chemicals may be added to improve the mechanical disintegration of the lignocellulosic structure of the feedstock, before, during, or after formation of pellets or extrudates. The biogas may be stored, sold, used, or further treated, such as via purification to produce pipeline-quality cellulosic methane. The fibrous humid cake and the fine-solids slurry have many potential uses. Other potential co-products include lignin, biochar, energy (as heat and/or electricity), fertilizers, soil or land conditioners, ammonium sulfate, ammonium hydroxide, and carbon dioxide.