Pyrolytic conversion of biowaste-derived hydrochar: Decomposition mechanism of specific components

Hydrothermal carbonization (HTC) coupled with subsequent downstream conversion is a new concept for the utilization of industrial biowastes for energy production. In this work, three representative biowastes (i.e., lignocellulosic, non-lignocellulosic and ash-rich samples) were selected for HTC pretreatment, and the subsequent pyrolysis of the derived hydrochars was evaluated using multiple kinetic models and combined instrumental analyses. Results showed that the thermal weight loss peak for each biowaste was progressively shifted to a higher temperature after HTC pretreatment, while the maximum value of weight loss was reduced compared to that of the original feedstock. This change was shown to be caused by the shift in biowaste compositions to a more stable one in the hydrothermal stage, but the dissimilarity of evolution pathways among specific components affected the rate and degree of pyrolysis of the hydrochars to different extent. In addition, fewer volatile compounds were released during the pyrolysis of the hydrochars, and the composition of pyrolytic volatiles was found to be mainly dependent on the types of biowaste and HTC conditions. These findings not only provide essential information on the pyrolytic capability of HTC-derived hydrochars, but can also help in designing, optimizing and scaling up the thermochemical conversion of industrial biowastes.