Hydrothermal Carbonization of Digestate in the Presence of Zeolite: Process Efficiency and Composite Properties

A systematic experimental study on hydrothermal carbonization (HTC) of digestate was conducted to evaluate the catalytic impact of natural zeolite and the properties of the produced hydrochar–zeolite composites (HZCs). An agricultural digestate and, as reference, microcrystalline cellulose (MCC) were treated at HTC temperatures of 190, 230, and 270 °C. HZCs were analyzed for their elemental composition, NMR structural properties, thermogravimetric behavior, N2 adsorption porosity, and scanning electron microscopy morphology. The results indicate distinct catalytic effects of zeolite on carbonization. For digestate, catalytic effects of zeolite increased the degree of carbonization equally to a 9–29 K higher HTC temperature. Zeolite increased the energy and carbon recovery in solid products for digestate, whereas MCC showed a lower recovery. Interestingly, zeolite preserved the cellulose fraction of digestate. This was attributed to physical and chemical shielding by formation of a visible zeolite layer on organic particles. Compared to pure hydrochar, the HZCs showed less aromatic and thermally stable carbon but higher surface area and pore volume. Potential areas of applications for the HZCs range from energetic use (e.g., gasification) to soil amendment and additive in bioprocesses (e.g., growing media).