Hydrothermal conversion behavior of Chinese medicine residues and pyrolysis and combustion characteristics of hydrochars

Chinese medicine residues (CMRs), as an organic solid waste with high moisture content, have been considered as a potential energy source. In this study, three types of CMRs (licorice-rhizome, cassia seeds-fruit, and loquat leaves-leaf) were used as raw materials for hydrothermal carbonization (HTC). The total hemicellulose and cellulose content of licorice residue (LC) was 92.37%, cassia seed residue (CS) was 73.11%, and loquat leaf residue (LL) was 63.59%. The hydrothermal conversion characteristics of three kinds of CMRs were explored by studying the distribution characteristics of the solid, liquid and gas products, coalification degree, surface functional groups and carbon structure of hydrochar, and the composition distribution characteristics of the liquid phase extraction products. High hemicellulose content resulted in the highest hydrochar yield at low HTC temperature (160 degrees C). With increasing HTC temperature, high hemicellulose and cellulose content promoted the dehydration and decarboxylation reactions during the hydrothermal process and the C element was enriched on the hydrochar. As the HTC temperature increased, the degree of hemicellulose and cellulose hydrolysis intensified, the degree of de-OH reaction increased, the lignin was slowly hydrolyzed, and the aromatization was enhanced. High lignin content helped to form a graphite layer structure. The higher the HTC temperature, the more difficult it was to preserve the cellulose crystals in the hydrochar. Low-temperature hydrothermal treatment promoted the hydrolysis of hemicellulose and cellulose into furfural, while high-temperature hydrothermal treatment promoted the formation of ketones and phenolic substances. LC-T280, CS-T250 and LL-T250 had the highest utilization value as solid fuels. In this paper, the mechanisms for the hydrothermal conversions of different CMRs under different HTC temperatures were proposed.