Thermochemical Characteristics of Anaerobic Dairy Digestate and Its Pyrolysis Conversion for Producing Porous Carbon Materials

In the present study, slurry digestate from a centralized anaerobic digestion (AD) plant, designed for dairy manure treatment and biogas-to-power generation, was utilized as a precursor for the preparation of porous biochars at elevated temperatures ranging from 550 to 850 °C. Proximate analysis and thermogravimetric analysis (TGA) were conducted to determine the thermochemical characteristics of the dried digestate and to explain its complex nature in relation to the physicochemical properties of the resulting biochars. Despite the substantial ash content of the precursor biowaste (approximately 30 wt%), primarily composed of inorganic compounds from calcium, the pore properties of the digestate-derived biochars had an overall increasing trend with regard to rising pyrolysis temperature. Nevertheless, some inconsistencies were observed between the samples produced at 550 °C and 850 °C, which highlighted the heterogeneous and complex nature of the precursor digestate. These observations can be attributed to active pyrolysis and the charring of the lignocellulosic components. The maximum Brunauer–Emmett–Teller (BET) surface area exceeded 200 m2/g when pyrolysis was performed at 850 °C. Nitrogen (N2) adsorption–desorption isotherms and scanning electron microscopy (SEM) confirmed that the porous digestate-based biochars predominantly exhibited both type I (microporous) and type IV (mesoporous) characteristics. Furthermore, the analytical results of energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR) indicated that oxygen-containing surface functional groups on the resulting biochars were retained after pyrolysis. The surface of the digestate-based biochar was also confirmed to be negatively charged at pH > 3.2.