Multiscale characteristics dynamics of hydrochar from hydrothermal conversion of sewage sludge under sub- and near-critical water

Dewatered sewage sludge was upgraded to hydrochar using hydrothermal conversion in sub- and near-critical water. Three characteristic temperature regimes responsible for the upgrading were identified. Drastic hydrolysis of carbohydrates, amide II or secondary amines occurred at 200 °C while noticeable decarboxylation initiated above 260 °C. Elevated temperature improved porosity but did not induce higher surface area. Aliphatic C was mainly transformed to aromatic hydrocarbon rather than aromatic C-O in subcritical water, whereas COO/N-C. O and aromatic C-O were decomposed to carbohydrate C at 380 °C. Below 300 °C, carbon functionalities in hydrochars were thermally stable and faster decomposition of N than C-(C,H) resulted in dramatic decline of N/C. Above 300 °C, C-H was gradually polymerized to aromatic C-(C,H) which was considerably transformed to C-(O,N) and C-H at 380 °C. CaO favored intense destruction of aromatic C-C/C-H, anomeric O-C-O, C-H and C-(O,N) functionalities but introduced more aromatic C-O and O=C-O.