Hydrothermal carbonization mechanism of agricultural waste under different conditions: An experimental and ReaxFF molecular dynamics study

Different hydrothermal carbonization (HTC) conditions will affect the yield, physicochemical properties and application of agricultural waste hydrochar. The hydrochar prepared under different HTC conditions was systematically studied through various experimental methods, and the experimental results were further verified and explained by ReaxFF molecular dynamics method. The results show that the higher HTC temperature, higher liquid-solid ratio and longer reaction time can reduce the yield of solid product. The volatile content of rice straw (RS) hydrochar decreased with the progresses of HTC, but the fixed carbon content increased. The HTC process is accompanied by the breakage of C-H bonds, C-O bonds, C-C bonds and O-H bonds, with C-O bond breaks comprising the most. The HTC process is also accompanied by the C-C bonds regeneration, indicating that decomposition and polymerization reactions coexist in the HTC process. At higher HTC temperature and longer reaction time, RS hydrochar showed lower specific surface area and porosity. The effect of the HTC temperature is the largest, while the effect of holding time is the smallest. Therefore, to expand the potential use of RS hydrochar, it is important to thoroughly investigate the influence of HTC conditions on the physicochemical properties of RS.