Pyrolysis of waste plastics into fuels and chemicals: A review

Pyrolysis is a viable thermochemical conversion (TCC) process to convert waste plastics into useful chemicals and alternative energy. Specifically, co-pyrolysis of plastics with biomass produce gasoline and diesel range hydrocarbons, aromatics, olefins, lubricants, and other valuable chemicals. Lignocellulosic biomass often produces low-quality fuel through pyrolysis, which could be improved by adding plastics as a co-feedstock. Plastics improve the hydrogen-to-carbon effective (H/C_eff) ratio in the feedstock and donate protons (H⁺) in the reaction mechanism. More petrochemicals (aromatics and olefins) and gasoline with less coke could be generated if a higher H/C_eff ratio is obtained in the feedstock. H⁺ proton can reduce oxygenated compounds and produce aromatics. Besides, high temperature (above 600 °C) promotes cyclic hydrocarbons, aromatics, coke formation, and dewaxing mechanism. Cyclic hydrocarbons and aromatics could be useful to improve plastic pyrolysis oil tribology. Washing and sizing waste plastics before pyrolysis is important for the desired yield. Also, the operating temperature, zeolitic and non-zeolitic catalysts and reactor types play important roles in obtaining specific product types. This research summarizes the pyrolysis of individual and mixed plastics using state-of-the-art literature and summarized their dewaxing and pyrolysis mechanisms. Besides, the co-pyrolysis of plastics and biomass along with their reaction mechanism is summarized. The future direction to utilize plastic pyrolysis for space exploration is also highlighted.