“On the distillation of waste tire pyrolysis oil: A structural characterization of the derived fractions” by Campuzano, Jameel, Zhang, Emwas, Agudelo, Martinez and Sarathy (Fuel, Volume 290, Elsevier, 15 April 2021). 

Environmental concerns, global warming, finite raw material resources, political ambitions, finally unstable petroleum prices, have accelerated the development of alternative fuels equivalent to petroleum derivatives. Waste, including end-of-life tires (ELT), as a sustainable source of raw materials has been experiencing a boom, has already achieved a high level of technical maturity in many areas, is increasingly economically viable (with the best future prospects) and is in many places already an essential contribution to the realization of a global circular economy.

Tire derived pyrolysis oil (TDO): Potential and challenges

Untreated raw TDO is a mixture of valuable hydrocarbon fractions, has a wide range of boiling points and is completely miscible with crude oil. Furthermore, because of the natural rubber content in tires, TDO’s renewable characteristic is recognized and fulfills the definition of drop-in biofuels established by Task 39 of the International Energy Agency (IEA): “Drop-in biofuels are liquid hydrocarbons that are functionally equivalent to petroleum fuels and are fully compatible with existing petroleum infrastructure”.

In addition, TDO does not contribute towards global warming and is not exhibiting the drawbacks associated with agriculturally derived biofuels (e.g., food-fuel debate).

Tire derived pyrolysis oil (TDO) therefore undoubtedly has a great potential to substitute petroleum-derived fuels in several industrial processes but is relatively unsuitable for direct use in these applications because of its wide boiling point range (from < 70°C up to 550°C) and its complex composition.

In particular, the significant presence of benzene, toluene, xylene, and limonene, as well as the high aromatic content (up to 65 wt%), sulfur (0.6 - 1 , 4 wt%) and oxygen (0.2 - 3.50 wt%), require supplementary upgrading from a technical and environmental perspective. 

A high fraction of volatile compounds results in a low flash point (< 30°C). This would on one hand ease the ignition of the fuel/air mixture, but on the other hand makes it difficult to handle and store. Else, a high final distillation point (550 °C), is seriously affecting its vaporization during combustion.

Anyway, the use of raw TDO as an alternative fuel in combustion engines is still limited (as high aromatic hydrocarbon content is associated with incomplete combustion) by the current legislation on fuels, i.e., European Air Quality Standard (EU2015/2193). 

The discussion about proposed economically viable upgrading steps to increase the quality and value of TDO therefore makes sense (ecologically, technically, and economically).

Upgrading tire derived pyrolysis oil (TDO) by fractional distillation

A possible upgrading path would be an oxidative desulfurization (ODS) and hydro processing (HP), as usual in conventional refineries. However, these processes are highly effective but in the dimensions of existing or planned ELT pyrolysis activities hardly economically feasible. Perhaps this path will also prove to be realistically feasible in the future, through capacity increases and / or joint ventures by pyrolysis companies, as well as through down-scaling of existing large-scale industrial applications.

As an alternative, fractional distillation offers interesting advantages for upgrading tire derived pyrolysis oil (TDO). 

The chemical complexity, the wide carbon number (C6 – C55) and boiling point range (70 -550°C) of raw TDO indicates the attractiveness of this upgrading method. As usual in the petrochemical industry this separation process converts the tire derived pyrolysis oil into groups with similar properties. These groups could then be further upgraded (by the industrial off-takers) and used in different systems.

In an analogous manner, different fractions of TPO could be separated according to their boiling points to be refined and used individually. Separating the oil into distinct fractions by distillation could expand the range of possibilities, making it more suitable for specific applications, rather than using the raw pyrolysis oil directly. 

In addition, and cited here as an example, it was already described in 2020 by CAMPUZANO and colleagues that the sulfur components in the oil, which have a high molecular weight, can (at least) be concentrated in the heaviest oil fraction by distillation. Although this is not the same result that can be achieved through (complex and costly) oxidative desulfurization (ODS) and hydro processing (HP), it at least reduces the further upgrading efforts and costs of the customers and makes the lighter oil fractions more valuable.

CAMPUZANO et.al. explored and described in detail the characteristic of four distillable volume fractions obtained from raw tire derived pyrolysis oil.

Those fractions are:

1. Light fraction – 40 Vol.% (boiling point range: 70 – 176 ◦C), yellowish color, low viscosity, like gasoline fuel.
2. Low-middle fraction - 20 Vol.% (boiling point range: 176 – 240 ◦C), dark brownish color, slightly more viscous than the light fraction, compatible with diesel.
3. High-middle fraction - 20 Vol.% (boiling point range: 240 – 285 ◦C), brownish color, slightly more viscous than the low-middle fraction, compatible with distillate marine fuels.
4. Heavy fraction - 20 Vol.% (boiling point range: 285 – 550.9 ◦C), black, with high viscosity at room temperature (barely flowing), like bitumen.

The scientists concluded that the viscosity, density, heating value, as well as the physical, chemical, and molecular properties of these four fractions clearly indicates similar applications to those established for the conventional fuels (gasoline, diesel, distillate marine fuels and bitumen). Further the use of distillation to fractionate the tire derived pyrolysis oil allows the concentration of sulfur containing compound and highly aromatic structures in the heaviest fraction, this in turn improves the characteristics of the lightest fraction.

The extended fractionation of the raw TDO by distillation (a two-stage fractionation is already common today), could therefore be considered as a sensible, environmentally friendly, and economically sensible upgrade path, which might be also suitable for medium-sized pyrolysis plants.