The reaction kinetics for carbonization of biogas slurry and the influence of diffusion for high-density feedstocks (pistachio shell) were investigated. The experiments were carried out with a 1.5 L batch reactor for hydrothermal carbonization at reaction temperatures of 190, 200, and 220 degrees C and a residence time of 0-160 min. The obtained hydrochar samples were dried and analyzed via scanning electron microscopy and a C, H, and O elemental analyzer. It was found that the average activation energy of the biogas slurry is 50.7 kJ/mol. In the case of the diffusion experiments, Thiele moduli of 1.21-1.61 were found, which indicates an influence on diffusion for the hydrothermal carbonization of pistachio shells.