Pyrolysis behavior of hydrochar from hydrothermal carbonization of pinewood sawdust

This work intends to reveal the pyrolysis behavior of hydrochar from hydrothermal carbonization (HTC) of pinewood sawdust. The elemental analysis and pyrolysis test were performed to study the effect of HTC on hydrochar performance. The elemental analysis indicates that the fuel properties could be highly upgraded by the HTC process. Compared with raw biomass (48.10 % and 19.12 MJ/kg), the carbon content and heating value was improved to 69.05 % and 26.66 MJ/kg for HTC-260. The atomic ratios H/C and O/C were decreased from 1.31 to 0.90 and 0.72 to 0.26, respectively, by means of dehydration and decarboxylation in the HTC process. The pyrolysis test implies that the thermal stability of hydrochars has been improved by the HTC. The hydrochars decomposition temperature and the pyrochar yields became higher and higher with the HTC temperature. For the HTC-260, the pyrochar yield was increased from 24.4%–53.02% at a pyrolysis temperature of 800 ºC. According to the TGA-FTIR, the main gas products were CO2, CO, CH4, and H2O. Higher HTC temperature promoted more CH4 production due to the devolatilization, decarboxylation, dehydration and polymerization occurred in the HTC process. 240–260 °C could be considered as a good option to maximize the energy efficiency when taking the HTC as a pretreatment procedure over thermal drying or torrefaction. All these results indicate the HTC is promising to upgrade high-moist biomass for multi-value-added products.