Hydrothermal Carbonization Process of Digestate from Sewage Sludge: Chemical and Physical Properties of Hydrochar in Terms of Energy Application

Abstract

Hydrochars (HTCD) derived from digestates, namely D1 and D2 (from two plants) of sewage sludge, were examined with respect to their fuel properties. The hydrothermal carbonization (HTC) tests were performed at temperatures of 200 and 220 °C, for 2 and 4 h of residence times, and with 1:10 and 1:8 digestate to water ratios (D/W), causing an increase of ash content (max. 55.8%), and a decrease c.a. 20% of the higher heating value except for a slight increase to 15 kJ/kg at 200 °C and 4 h in hydrochars. Conversely, the combustion profiles of hydrochars moved towards higher temperatures (225–257 °C) and finished earlier at lower temperatures (423–438 °C). The HTCD from D1 and D2 showed very similar properties under the same conditions (200 °C, 4 h, 1:8 D/W) for combustion characteristic temperatures, indices and profiles. The best efficiency was found for HTCD2. In addition, the polluted post-processing liquid phase was treated by a distillation process providing 30% higher pH, 50% lower BOD values, up to 15 times lower COD values, and c.a. three times lower conductivity.