The combustion characteristics of hydrothermal carbonization products from traditional Chinese medicine residue

The residue from processing glycyrrhiza uralensis, a kind of traditional Chinese medicine, was converted into hydrochar via hydrothermal carbonization, and then chloroform was used to extract the retained oil from the hydrochar. The products were characterized via elemental and proximate analyses, FTIR, GC-MS, N2 adsorption and TG. The ignition index (Di), burnout index (Db), higher heating value (HHV), comprehensive combustion index (CCI), and activation energy (Ea) were then calculated. The values of these indices for hydrochar (before extraction) were compared to examine whether interactions exist between the resident biocrude and solid hydrochar during combustion. The results show that the calorific value of extracted hydrochar and resident biocrude ranged in 20.36–25.12 and 30.91–35.04 MJ/kg, respectively. The underdeveloped pore structure of hydrochar led to a 39.65 % decrease in the average Di, and a 55.30 % increase in the average Ea when the biocrude remained in the hydrochar during combustion. Oxygen-containing groups in the resident biocrude interact with specific components in the solid hydrochar, resulting in increases in HHV and CCI. Additionally, at 225 °C and a 12-h residence time, the synergistic coefficient reached its maximum, indicating that the product obtained under these conditions was most likely to exhibit synergistic effects.