Optimizing product yield and composition in chemical recycling of mixed plastics through temperature-staged pyrolysis

Chemical recycling of plastic waste via pyrolysis contributes to higher recycling rates in a circular economy. One important prerequisite is the conformity of pyrolysis products with defined chemical feedstock specifications. In the pyrolysis of mixed plastics, specification compliance is particularly hindered by the presence of aromatic compounds and heteroatom-containing species (e.g., Cl, N, O) in the pyrolysis oils. Temperature-staged pyrolysis facilitates oil decontamination, but scaling up this concept in pyrolysis processes poses challenges. Therefore, investigations in thermogravimetry (TG) and a scalable stirred-tank reactor (STR) system are performed. Under well-defined TG conditions, low temperature stages allow for dehalogenation and the selective decomposition of non-polyolefinic polymers. Polyolefins decompose mainly in a separate stage at higher temperatures above 400 °C. Polymer interaction effects significantly influence the decomposition kinetics and depend strongly on the polymer type in the mixture. The application in the STR system confirms technical feasibility, but also reveals scale-up engineering challenges through heat and mass transfer limitations. The heat transfer limitations reduce the polymer temperature during the two-staged STR process, requiring a higher first-stage set temperature than in TG. Compared to isothermal pyrolysis at 500 °C, the total heteroatom content in the oil produced in the second stage decreases. Toluene, benzoic acid, and ε-caprolactam content are reduced by a factor of 2–3 while aliphatic compounds are enriched. Staging also facilitates the separation of heteroatom-containing gases from the aliphatic-rich gas fraction. In contrast, elevating the first-stage pyrolysis temperature from 370 °C to 430 °C reduces the target product yield from 56 wt% to 23 wt%. However, the temperature elevation also results in a higher quality of the target product because of a reduced amount of incorporated non-aliphatic compounds. These findings highlight the trade-off between maximizing the yield and enhancing the quality of the aliphatic-rich target product fraction.