Effects of Temperature and Duration on the Combustion Characteristics of Hydrochar Prepared From Traditional Chinese Medicine Residues Using Hydrothermal Carbonization Process

Abstract

Due to the depleting fossil fuels and increasing environmental issues, the application of biomass-derived energy is becoming significant. The widespread use of traditional Chinese medicine (TCM) in China results in a large amount of TCM residue, which can be used to generate biomass energy and then mitigate environment problems. This paper focuses on the treatment of Glycyrrhiza uralensis (GU) residue, a common TCM waste, using the methods of hydrothermal carbonization (HTC), elemental analysis, proximate analysis, Fourier transform infrared (FTIR) analysis, and thermal analysis, to investigate the fuel properties and combustion performance of hydrochars produced from GU under different process conditions (temperature and duration). The results showed that the raw GU had a higher heating value (HHV) of only 16.27 MJ/kg, whereas the hydrochar achieved yields ranging from 34.11% to 43.74% and HHVs between 24.00 and 29.01 MJ/kg. The HTC temperature significantly influenced the yield, physicochemical properties, and thermal behavior of the hydrochar. In contrast, the duration had a comparatively minor effect on its combustion performance. With increase in temperature, the yield of hydrochar decreases along with the reduction of certain oxygen-containing functional groups, but the fixed carbon (FC), HHV, and stability of the hydrochar increase. Furthermore, the reduction in the combustion index (Z-values) between 200 and 225°C across the three hydrothermal duration groups was significantly greater than that observed between 225 and 250°C. The average reduction for the former reached 39.3%, whereas for the latter, it was merely 9.4%. This variation indicates that 225°C is the optimal temperature for producing fuel from GU via HTC.