Effects of hydrothermal carbonization on catalytic fast pyrolysis of tobacco stems

This study investigated the effects of hydrothermal temperature (HT, 160–240 °C) and residence time (RT, 30–90 min) of hydrothermal carbonization (HTC) on the properties of tobacco stems and their pyrolysis behaviors with catalysts (CaO, HZSM-5). The characterization of hydrothermal products (hydrochars) was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and 13 C superconducting nuclear magnetic resonance (NMR). The pyrolysis experiments of the samples were conducted by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetric analyzer coupled to Fourier transform infrared spectroscopy (TG-FTIR). The experimental results showed that with the increase of HT and RT, the fixed carbon content, high heating value, and energy densification of hydrochar were increasing and the crystallinity index increased first and then decreased, and the aromaticity increased continuously. HTC reduced the contents of acetic acid and ketones in the bio-oil and increased the contents of hydrocarbons and 1,6-anhydro-β-d-glucopyranose. When the HT was higher than 180 °C, excessive aromatization of hydrochar affected the yield of chemical compounds in bio-oil. When HT was lower than and equal to 200 °C, the catalyst HZSM-5 and HTC exhibited combined effects of promoting an increase in the production of aromatic hydrocarbons. When HT was higher than 200 °C, the catalytic effects of the double-layer catalysts were better than that of the single-layer catalyst, and CaO and HZSM-5 exhibited synergistic effects that promoted an increase in aromatic hydrocarbon content.