Opportunities and challenges for the application of post-consumer plastic waste pyrolysis oils as steam cracker feedstocks: To decontaminate or not to decontaminate?

Abstract

Thermochemical recycling of plastic waste to base chemicals via pyrolysis followed by a minimal amount of upgrading and steam cracking is expected to be the dominant chemical recycling technology in the coming decade. However, there are substantial safety and operational risks when using plastic waste pyrolysis oils instead of conventional fossil-based feedstocks. This is due to the fact that plastic waste pyrolysis oils contain a vast amount of contaminants which are the main drivers for corrosion, fouling and downstream catalyst poisoning in industrial steam cracking plants. Contaminants are therefore crucial to evaluate the steam cracking feasibility of these alternative feedstocks. Indeed, current plastic waste pyrolysis oils exceed typical feedstock specifications for numerous known contaminants, e.g. nitrogen (∼1650 vs. 100 ppm max.), oxygen (∼1250 vs. 100 ppm max.), chlorine (∼1460vs. 3 ppm max.), iron (∼33 vs. 0.001 ppm max.), sodium (∼0.8 vs. 0.125 ppm max.)and calcium (∼17vs. 0.5 ppm max.). Pyrolysis oils produced from post-consumer plastic waste can only meet the current specifications set for industrial steam cracker feedstocks if they are upgraded, with hydrogen based technologies being the most effective, in combination with an effective pre-treatment of the plastic waste such as dehalogenation. Moreover, steam crackers are reliant on a stable and predictable feedstock quality and quantity representing a challenge with plastic waste being largely influenced by consumer behavior, seasonal changes and local sorting efficiencies. Nevertheless, with standardization of sorting plants this is expected to become less problematic in the coming decade.

Keywords: Characterization; Chemical recycling; Contaminants; Steam cracking; Thermochemical conversion; Upgrading.

Graphical abstract

Fig. 1

Post-consumer plastic packaging waste treatment distribution in Europe in 2018 (adapted from (Plastics Europe, 2020)).

Fig. 2

Schematic overview of the state-of-the-art domestic waste recycling schemes and a potential integration of thermochemical recycling.

Fig. 3

Schematic overview of a GC × GC setup. Re-drawn from (Dijkmans et al., 2014).

Fig. 4

GC × GC-FID chromatogram of a plastic waste pyrolysis oil. Re-drawn from (Toraman et al., 2014).

Fig. 5

Two-dimensional gas chromatography combined with a Polyarc reactor. ().

Fig. 6

Schematic overview of the micro-pyrolysis setup mounted to a 1D-GC–MS. Re-used with permission (Tsuge et al., 2011).

Fig. 7

GC × GC-NCD chromatogram of a shale oil sample with individually marked compound groups. Re-used with permission (Dijkmans et al., 2015).

Fig. 8

Sankey chart depicting the pyrolysis mass balances of the respective pure virgin polymers pyrolyzed in a fixed-bed batch reactor at 700 °C, based on (Williams and Williams, 1997b). For the sake of comparability, the reported mass balances by were normalized to 100 %.

Fig. 9

Thermal decomposition mechanism of PVC (Ye et al., 2019).

Fig. 10

Structural formulas of terephthalic acid, phthalic acid and benzoic acid (from left to right).

Fig. 11

Sankey chart depicting the influence of the feedstock material on the hydrocarbon composition. Feedstock data is based on averaged values from Table 3 (numbers in wt%).

Fig. 12

Schematic overview of the Niigata plastic waste liquefaction plant. Re-used with permission (Okuwaki, 2004).

Fig. 13

Structural formula of a metalloporphyrin.

Fig. 14

Coke layer represented by a polyaromatic structure. Re-drawn from (Wauters and Marin, 2002).

Fig. 15

Overview of the heteroatom content in the light, medium and heavy fractions compared with the threshold value for industrial steam crackers. Values taken from Table 4 and (Baumgartner et al., 2004).

Fig. 16

Decrease of performance of commercial downstream catalysts as a function of the feedstock nitrogen concentration. Re-drawn from (Letzsch and Ashton, 1993).

Fig. 17

Concentrations of (a) Ca and Cu (in ppm) and (b) Fe, Na, Si and Pb (in ppb) in light and medium fractions of plastic waste pyrolysis oils vs. the thresholds for industrial steam crackers.

Fig. 18

Skikda explosion, January 19, 2004 (Mashyanov, 2021).

Fig. 19

Comparison of fossil feedstocks and plastic waste pyrolysis oils.

Fig. 20

Plastic waste recycling scheme of the future.