Metal chlorides and ammonium persulfate hydrothermal carbonization for enhanced pyrolysis behavior and biochar properties

To elucidate the impact of a modified reaction medium on the hydrothermal carbonization (HTC) coupled pyrolysis process, a two-stage conversion of rice straw (RS) was investigated in the present work. RS was initially treated hydrothermally in a conventional medium (water) and a modified reaction medium (metal chlorides and ammonium persulfate AP), followed by an upgrade of the obtained hydrochar via pyrolysis. The characteristics of the modified hydrochar and the pyrolysis biochar were examined. It was shown that a modified reaction medium promoted a higher mass yield of hydrochar. Compared to conventional hydrochar, modified hydrochar exhibited higher ash and fixed carbon content, and lower carbon and oxygen content. Furthermore, the pyrolysis behavior of modified hydrochar was more complex. Not only the DTG curve entirely different was, but the values of activation energy and pre-exponential factor also significantly increased. Ultimately, the modified medium enhanced the adsorption capacity of the HTC coupled pyrolysis biochar. Among them, B-HTC-AP had the highest SBET (544.83m²/g) and the largest pore volume (0.4644cm³/g). The TC adsorption capacity of B-HTC-AP reached 187.31mg/g. The pseudo-second-order kinetic model and the Freundlich isotherm model provided a better description of the TC adsorption process on B-HTC-AP. Possible adsorption mechanisms of B-HTC-AP could encompass pore filling, electron transfer, hydrogen bonding, and π-π interactions. Therefore, the use of a modified reaction medium in the two-stage process could be an effective strategy for upgrading biomass.