Plastic Waste to Light Olefins – a detailed analytical study of plastic pyrolysis oils with focus on contaminants

Thermochemical recycling, including the pyrolysis of plastic waste and subsequent steam cracking of the pyrolysis oil to produce light olefins is a sustainable plastic recycling route, assisting in the transition to a circular economy. The presence of olefins, aromatics, heteroatom-containing compounds and metal contaminants in plastic waste pyrolysis oils however induce corrosion issues, fouling of process equipment, downstream catalyst poisoning and enhanced coke formation when used on industrial scale. Via an extensive database handling the composition of plastic waste pyrolysis oils, more light is shed on the correlation between the pyrolyzed feed and pyrolysis conditions on one hand, and the composition of the obtained pyrolysis oil on the other hand. A higher polypropylene (PP) fraction in the waste stream results in more unsaturated and branched compounds in the pyrolysis oil, whereas aromatics generally stem from polyethylene terephthalate (PET) and polystyrene (PS) impurities. Further, the calcium, potassium, iron, sodium,  silicon, nitrogen, chlorine and oxygen concentrations all exceed the industrial threshold concentrations for steam cracker feedstocks. Dilution with fossil naphtha or extra treatments are thus critical to improve the steam cracking suitability of the pyrolysis oils. Via reversed-phase two dimensional gas chromatography with a polar × mid-polar column combination coupled to a quadrupole mass spectrometer (qMS) and flame ionization detector (FID), the oxygenates were successfully separated from the dominant hydrocarbon matrix, identified and quantified. It is found that ketones are the most abundant type of oxygenates in PE- and PP-derived pyrolysis oils. Also substantial concentrations of alcohols and phenols are found, whereas the concentrations of acids, phenyl ethers and aldehydes are minor.