Product

Plastics pyrolysis oil is the condensable hydrocarbon fraction that is formed during pyrolysis of plastic waste, commonly denominated PyOil, a form of gasoil (also known as diesel).

Table 1 summarizes the fuel properties of the liquid oil produced in the pyrolysis processes. The experimental calorific value of HDPE, PP and LDPE are all above 40 MJ/kg and are considered high for energy utilization. The calorific value of PyOil from polystyrene is commonly lower than for PyOil from polyolefin plastic due to the presence of the aromatic ring in PS, since this chemical structure has a lower combustion energy compared with aliphatic hydrocarbons. Overall, PyOil from PET and PVC have the lowest calorific values (below 30 MJ/kg). The physical properties of PyOil obtained from polyolefinic plastics are close to the properties of commercial gasoline and diesel.^[1]

Table 1 - Fuel properties of Plastic Pyrolysis Oil^[1]

*n.a., not available, ^a Viscosity at 40°C, ^b Viscosity at 30°C, ^c Viscosity at 25°C, ^d Viscosity at 50°C

The properties of PyOil from experimental pyrolysis of mixed polyolefin waste (MSW, coming from a traditional mechanical recycling company in the state of Guanajuato, Mexico, that processes mainly PP, LDPE, and HDPE) presented in Table 2 make it suitable for use in thermal devices, such as boilers, incinerators, ovens, etc.; however, for it to be used in internal combustion engines, the fuels must meet certain specifications to ensure proper engine operation. Thus, a fractionation is necessary.^[2]

Table 2 - Fuel properties of Plastic Pyrolysis Oil^[2]

*Cetane number (CN)

For the purpose of chemical recycling, the PyOil shall preferably be fractionated in order to isolate the lighter fraction. Fractionnation of the liquid fraction formed during catalytic pyrolysis of mixed polyolefin waste via its plastics-to-liquids (PtL) process is reported by Quantafuel to be according to:^[3]

• 16 wt-% light fraction (C₆-C₁₂)
• 56% diesel fraction (C₁₁-C₂₁)
• 8% heavy fraction (C₂₀-C₂₈)

The 16 wt-% of the light fraction in absolute terms^[3] translates into 20 wt-% relative to the PyOil fraction, which is well aligned with the 21% content of the light fraction reported in Table 3.^[2]

This lighter fraction also needs to undergo hydrogenation before it is returned to the steam cracker since this naphtha-range liquid cut also concentrates the olefinic compounds present in the PyOil at 57% concentration^[2] as described in Table 3. Olefins in cracker feedstock are notoriously nefarious to the cracking process so that they must be removed in order to render the PyOil light fraction suitable for the cracking process for producing circular monomers such as ethylene, propylene and butadiene.

Table 3 - Properties and Composition of Distillate Products from PyOil^[2]

References

1. S D A Sharuddin et al, Pyrolysis of plastic waste for liquid fuel production as prospective energy resource, 2018 IOP Conf. Ser.: Mater. Sci. Eng. 334 012001
2. Riesco-Avila, J.M.; Vera-Rozo, J.R.; Rodríguez-Valderrama, D.A.; Pardo-Cely, D.M.; Ramón-Valencia, B. Effects of Heating Rate and Temperature on the Yield of Thermal Pyrolysis of a Random Waste Plastic Mixture. Sustainability 2022, 14, 9026.
3. Quantafuel's 4Q 2019 report (no longer available online).