Performance research and optimization of the production of high-value nitrogen-containing compounds from tobacco stems via two-step hydrothermal liquefaction

In order to obtain nitrogen-containing compounds (NCCs) from tobacco stems(TS) under mild conditions, glycine, urea, and ammonium acetate were added in hydrothermal liquefaction of TS. The oil, gas, and carbon produced by hydrothermal liquefaction was analyzed by GC/MS, GC, and XPS. It was found the yield of NCCs in bio-oil increased significantly after adding nitrogen source coliquefaction, and the two-step hydrothermal liquefaction also promoted the production of NCCs in bio-oil. Among them, urea promoted the production of high-value NCCs most obviously. NCCs account for 57.34% of bio-oil of two-step hydrothermal liquefaction. NCCs in bio-oil were mainly composed of nicotine, pyridines, pyrroles, and pyrazines. The two-step hydrothermal liquefaction promoted the precipitation of nicotine and inhibited the decomposition of nicotine. In the two-step hydrothermal liquefaction without nitrogen source, the relative content of nicotine was the highest, accounting for 21.95% of bio-oil components. The coliquefaction of nitrogen source and TS with two-step hydrothermal liquefaction could significantly increase the content of nitrogen in biochar and provide the possibility for the preparation of activated carbon. Two-step hydrothermal liquefaction and the introduction of urea were the best methods to prepare NCCs.