Effect of hydrotreating on the pyrolysis oil composition: GC-MS studies

The processing of waste tires is a significant challenge. The pyrolysis of waste tires offers a sustainable pathway to produce fuels, addressing both waste management and energy recovery challenges. However, due to the presence of a large amount of unsaturated, aromatic hydrocarbons and sulfur compounds, their direct use as fuel is not possible. Catalysts based on Pd, Pt, and other metals are commonly used for the hydrotreating of pyrolysis oil for producing alternative transportation fuels. The effect of hydrotreating with the commercial Pt/Al₂O₃ and NiMo/Al₂O₃ catalysts on the chemical composition of tire pyrolysis oil was analyzed by GC-MS following a developed extraction sample preparation procedure. The pyrolysis of tires was conducted at 500 °C in a nitrogen atmosphere. The catalytic hydrotreating was performed using a 250 mL reactor at 250-350 °C and 6.5 MPa hydrogen pressure. The hexane solution of pyrolysis oil was sequentially extracted with water and dimethyl sulfoxide and then treated with oleum to separate complex pyrolysis oil into water-soluble compounds, polycyclic aromatic hydrocarbons, and saturated hydrocarbons (naphthalenes and alkanes). It was found that unsaturated hydrocarbons and water-soluble compounds were effectively removed from pyrolysis oil by hydrotreating with the NiMo/Al₂O₃ catalyst, which demonstrated superior performance compared to the Pt/Al₂O₃ catalyst that showed only partial removal. Moreover, it was demonstrated that NiMo/Al₂O₃ was a more efficient catalyst for the hydrotreating of pyrolysis oil to reduce the contents of toxic PAHs, sulfur, nitrogen, and oxygen in pyrolysis oil. Compared with the conventional Urals crude oil, TPOs possessed higher proportions of valuable gasoline and diesel but contained significantly more sulfur, nitrogen, and aromatic compounds as contaminants. Although hydrotreating produced a diesel fraction meeting key fuel specifications (e.g., heating value, flash point, and density), its residual PAH (0.200 wt%) and sulfur (0.0432 wt%) content still exceeded commercial diesel standards.