Energy and Element Fate of Hydrochar from Hydrothermal Carbonization of Dairy Manure Digestate

Hydrothermal carbonization (HTC) of dairy manure digestate (DMD) was explored in this study. Response surface methodology was used to investigate the influence of temperature, time, and DMD pH on hydrochar properties. Hydrochar obtained in the study exhibited higher heating value (HHV) in the range of 17.5–20.6 MJ kg −1 , with energy and hydrochar yields 45.8–72.4% and 57.6–80.1%, respectively. It was demonstrated that temperature was the most significant factor. At higher temperature, the HHV of hydrochar was higher while both energy and hydrochar yields were lower. In particular, hydrochar obtained at temperature ≥ 260 °C had HHV ≥ 20.0 MJ kg −1 , which was close to that of lignite and implied its potential as fuel. To gain insights about the fate of various elements during HTC and facilitate overall evaluation of hydrochar, a wide range of elements (30 in total, including macro- and micro-nutrients, as well as heavy metals) was analyzed. Hydrochar had carbon to nitrogen (C/N) ratio between 10 and 12, which was in the suitable range for nutrient availability as soil amendment. Half of the elements remained in the hydrochar, while most of Na, K, and half of S leached into process water, especially at temperature ≥ 260 °C, thus combusting hydrochar can potentially alleviate the slagging and fouling problems associated with direct combustion of dried DMD. The study provided insights for further rational utilization of DMD hydrochar, and integration of HTC into existing on-farm anaerobic digestion was a promising option for reduction disposal of DMD.