Challenges and Opportunities for Plastic Waste Pyrolysis Oils As Steam Cracking Feedstock

Thermochemical recycling of plastic waste to base chemicals via pyrolysis and subsequent steam cracking is expected to be the dominant chemical recycling technology in the coming decade. Steam crackers are reliant on a stable and predictable feedstock quality representing a challenge with plastic waste compositions being largely influenced by consumer behavior, seasonal changes and last but not least local sorting efficiencies. Nevertheless, with standardization of sorting plants and increased automation this is expected to become less problematic in the coming decade as will be illustrated with some case studies. Advanced analytical techniques such as comprehensive 2D GC combined with selective detectors such as MS, SCD and SCD help to gain insight in several compound classes. Furthermore, techniques such as inductively coupled plasma coupled to optical emission spectroscopy (ICP-OES) or to mass spectrometry (ICP-MS) can be used to detect elements such as halogens and metals in ultra-trace levels ^1-5.

Plastic waste pyrolysis oils contain a vast amount of heteroatoms and metals which are the main driver for corrosion, fouling and catalyst poisoning in industrial steam cracking plants. ⁶ Contaminants are therefore the crucial aspect to evaluate the steam cracking feasibility, a knowledge gap still to be closed.

Our results show that plastic waste pyrolysis oils exceed the typical feedstock specifications employed in steam crackers substantially for several contaminants. These are, for instance, sulfur (up to 1700 ppm vs. 500 ppm max.), nitrogen (up to 10000 ppm vs. 200 ppm max.), oxygen (up to 38000 ppm vs. < 1 ppm max.), chlorine (up to 5000 ppm vs. 3 ppm max.), lead (up to 37 ppm vs. 0.1 ppm max.) and iron (up to 99 ppm vs. 0.001 ppm max.). In short, a pyrolysis oil produced from post-consumer plastic waste requires substantial upgrading to meet the current specifications set for naphtha or other liquid steam cracker feedstocks, with hydrogen based technologies being the most effective for the removal of heteroatoms. This will be illustrated with some case studies. Pretreatment of the feedstock and improved sorting of the plastic waste can also help to decrease contaminant levels, in some cases with an order of magnitude.

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