Kinetics of faecal biomass hydrothermal carbonisation for hydrochar production

Highlights: • Solids decomposition and hydrochar production modelled by first order kinetics. • Arrhenius rate constant characterisation of sewage sludge carbonisation. • The activation energy of synthetic faeces was higher than that of sewage sludge. • Reaction temperature had more effect on solids decomposition than reaction time. • Feedstock moisture content affected hydrochar yield and extent of carbonisation. - Abstract: Decomposition kinetics of primary sewage sludge (PSS) and synthetic faeces (SF), of various moisture contents, were investigated over different reaction times and temperatures using a hydrothermal batch reactor. Solid decomposition of PSS and SF was first-order with activation energies of 70 and 78 kJ/mol, and pre-exponential factors of 4.0 × 106 and 1.5 × 107 min−1, respectively. Solid decomposition was significantly affected by reaction temperature more so than reaction time. Higher temperature resulted in higher solids conversion to hydrochar. Equilibrium solid hydrochar yields (relative to the original dry mass used) were 74%, 66%, 61% and 60% for PSS at 140, 170, 190 and 200 °C respectively, and 85%, 49%, 48% and 47% for SF at 140, 160, 180 and 200 °C respectively. Energy contents of the hydrochars from PSS carbonised at 140–200 °C for 4 h ranged from 21.5 to 23.1 MJ/kg, and increased following carbonisation. Moisture content was found to affect the Hydrothermal Carbonisation (HTC) process; feedstocks with higher initial moisture content resulted in lower hydrochar yield and the extent of carbonisation was more evident in feedstock with lower moisture content. The results of this study provide information useful for the design and optimisation of HTC systems for waste treatment