Effect of hydrothermal carbonization pretreatment on the pyrolysis behavior of the digestate of agricultural waste: A view on kinetics and thermodynamics

Abstract

Anaerobic digestion is the most promising disposal methods to treat organic waste. Also, a feasible management is necessary for the resulted digestate. Hydrothermal carbonization (HTC) combination with pyrolysis could be a proper solution to use for the treatment of digestate. In this study, the effect of an HTC on the pyrolysis of the digestate of agricultural waste (AWD) was investigated, focusing on the kinetic and thermodynamic aspects. Three model-free methods, including Friedman, KAS, and OFW methods, were used to evaluate the kinetic performance of the total and pseudo pyrolytic reactions of AWD and its hydrochar. Furthermore, kinetic predictions were made to provide more information for further studies. It was found that the HTC treatment decreased the activation energy ranges of the pyrolysis of AWD from 182.9–274.43 kJ/mol to 144.59–205.20 kJ/mol by using the Friedman method. For a more thorough understanding of the effect of HTC treatment on the pyrolysis of AWD, the pyrolysis reactions of AWD and its hydrochar were divided into two pseudoreactions using the Fraser-Suzuki deconvolution method. The mean activation energy of the deduced pseudo 2 pyrolytic reaction of hydrochar was 175.64 kJ/mol, which was 28.11 kJ/mol less than that of AWD. In addition, the ΔH‡ values of the pseudo 2 reactions of AWD and its hydrochar were 197.97 and 169.68 kJ/mol, respectively. The results of kinetic isothermal predictions suggested that the peak temperature for the further research and application of the pyrolysis of AWD and its hydrochar should not be lower than 450 °C.