Pyrolysis kinetics of the hydrothermal carbons derived from microwave-assisted hydrothermal carbonization of food waste digestate

The treatment of food waste digestate (FWD) is a major obstacle to the development of industrial biogas field. To mitigate the drying cost and promote energy recovery, a strategy combining microwave-assisted hydrothermal carbonization (MHTC) pretreatment with subsequent pyrolysis for FWD was investigated in this work. Firstly, MHTC was performed at different reaction variables, including reaction temperature, reaction time and solid loading, to convert FWD to hydrothermal carbons (HCs). The fuel property of raw FWD could be effectively promoted by the MHTC pretreatments and reaction temperature was the most influential factor affecting the properties of FWD-derived HCs. Afterwards, thermogravimetrc experiments were conducted to study the pyrolysis behavior of FWD and three representative FWD-derived HCs. Thermal decomposition of FWD-derived HCs is consistent with that of the platform biomass with cellulose-lignin structures. Finally, the kinetic parameters and probable reaction mechanism for the pyrolysis of FWD and representative FWD-derived HCs were determined. The average values of apparent activation energy for FWD and the three HCs were in the range of 85.181–106.499 kJ/mol. The pyrolysis of FWD and the three HCs can be described by the model of three-dimensional diffusion and reaction order, respectively. • MHTC of FWD for the produce of HCs were performed. • Pyrolysis of FWD and FWD-derived HCs was investigated. • Kinetic analysis of the pyrolysis process was studied. • Fuel properties of FWD could be promoted by MHTC treatment. • Combining MHTC and pyrolysis is feasible for the treatment of FWD.