Tobacco Biowaste Hydrothermal Carbonization: Physicochemical Properties of Hydrochars and Evolution of Active Compounds

The physicochemical characteristics of hydrochars produced from tobacco stems through hydrothermal carbonization (HTC) at different temperatures were investigated, along with the variation in contents of nicotine, niacin, and chlorogenic acid in both the hydrochars and the liquid phase. The results indicated that dehydration was the predominant reaction during HTC of wet tobacco stems (WTS), leading to a decrease in the H/C and O/C atomic ratios of the hydrochars. As temperature increased, polycondensation and aromatization reactions became more pronounced, which corresponded with a reduction in the intensity of functional group vibrations such as C–N and N–O in FT-IR spectra. XPS analysis revealed a gradual increase in C=O content, whereas the proportions of C–OH and C–O bonds declined from 51.74% and 35.13% to 36.95% and 20.84%, respectively. Furthermore, the content of pyridine-N rose from 31.08% to 41.30%, while pyrrole-N and quaternary-N contents decreased to varying degrees. Both nicotine and niacin levels in the hydrochars and carbonization liquids exhibited an initial increase followed by a decline, whereas chlorogenic acid content consistently decreased. The underlying mechanisms for the observed changes in nicotine, niacin, and chlorogenic acid contents during HTC are discussed in detail.