Two-stage pyrolysis for effective tobacco stem utilization and tar reduction

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Tobacco stem (TS) is a waste product of tobacco industry that cannot be directly disposed of due to regulatory concerns. This study explored a two-stage pyrolysis for modifying the TS structure, rendering it unusable for tobacco products, while preserving potassium and nitrogen in the char for agricultural use as fertilizer. Furthermore, the study aimed to minimize the production of harmful tar by in-situ converting heavy volatiles into gases. The effects of first-stage pyrolysis temperature (400 – 650 °C), second-stage temperature (600 – 800 °C) and various catalysts (β(60 H), HY, HZSM-5 and TiO2) on product distributions and properties were studied. Elemental analysis, FTIR, XRF and ICP-MS were employed to assess the retention of potassium and nitrogen in the char. The results show that under optimal conditions (450 ℃ for pyrolysis and 800 ℃ for cracking), a 40.3 wt% char yield with 93.0 wt% potassium and 50.3 wt% nitrogen retention, 1.8 wt% tar yield and 34.6 wt% gas yield were achieved. The two-stage pyrolysis established a staged temperature profile within the reactor, decoupling the “primary pyrolysis” of TS from the “secondary reaction” of volatiles. In comparison to one-stage pyrolysis, this approach reduced tar production by 89.9% and increased gas production by 51.6%. The use of HZSM-5 and HY catalysts further reduced tar yields by up to 30.9%. This innovative process altered TS structure, retained crucial nutrients in the char, significantly reduced tar content, and mitigated harmful substances such as nicotine and phenols, offering a sustainable solution for tobacco waste management.