Hydrothermal carbonization of agricultural and municipal solid waste digestates - Structure and energetic properties of the solid products

Digestate is a nutrient-rich substance produced by anaerobic digestion, it contains organic and inorganic compounds as well as micro-organisms. The European Nitrates Directive (91/676/EEC) gives the regulations concerning the wider implementation of digestate. Due to a significant amount of organic matter in the digestate, it can be utilized as a solid biofuel, a soil amendment or a substrate for activated carbon production. HTC is a pretreatment process which can change digestate into potentially more hydrophobic product with higher energy potential. In this study, two kinds of digestate were examined as a potential feedstock for preparing hydrochar applied as a porous material and a solid biofuel. First digestate sample was a clean digestate from a typical biogas plant, performing anaerobic digestion of typical agricultural residues (cow dung). The second one was a digestate from wet fraction of municipal solid waste, after separation at the MSW sorting plant. HTC was performed in the autoclave, at 200 degrees C process temperature, 4.5 h residence time, under the saturation pressure of water. The proximate and ultimate analysis of hydrochars, along with porosimetry and morphology structure were examined. Additionally, the thermal behaviour under pyrolysis and combustion was analysed using TGA. HTC resulted in increased carbon contents for both types of digestate. Results of DSC and TGA analysis suggests improved ignition due to HTC. HTC increased total pore volume in both of the cases. The change was most visible in the macropore range.