From plastics to methane and carbon spheres: The evolution of pyrolysis products during pyrolysis under autogenic atmosphere

Pyrolysis of plastics is a promising technique towards high-value utilization and solvent-free treatment of plastic waste. However, low product selectivity and broad product distribution may restrict the application of plastic pyrolysis. In this study, polyolefins and polystyrene (PS) were converted into methane and carbon spheres through pyrolysis under autogenic atmosphere (AAP). To elucidate the product evolution and reaction mechanism of AAP, product distributions, compositions and properties at 450–700 °C were investigated. At 450 °C, polyolefins were converted into 89.3–96.91 wt% pyrolysis oil; PS was converted into 88.4 wt% of pyrolysis oil, 2.3 wt% of gas and 9.3 wt% of char. As the increase of temperature, the yields of methane and pyrolysis char increased significantly. At 700 °C, the yields of pyrolysis chars were as high as ~45 wt% and 74.1 wt% for polyolefins and PS, respectively, and the rest products was mainly pyrolysis gas with less than 1 wt% of pyrolysis oil. Pyrolysis chars were well carbonized spheres with diameters of 2 − 8 µm. Pyrolysis gas contained 93.83–95.74 vol% of methane, and possessed higher heating values of 35.8–36.2 MJ m −3 . The pyrolysis products with high purity and narrow distribution can be attributed to the enhancement of AAP on the secondary reactions of primary decomposition products, such as cracking, hydrogenation, aromatization and condensation. • Polyolefins and PS were converted into methane-rich syngas and carbon spheres. • Enhancing temperature increased methane and carbon yields and reduced oil yields. • AAP enhanced the reactions of hydrogenation, aromatization and condensation. • Polyolefins and PS exhibited different laws of product characters.