A Practical Way to Upgrade End-of-Life Tire-Derived Pyrolysis Oil
Upgrading tire pyrolysis oil from end-of-life tires into cleaner, higher-value fuels is critical for scaling sustainable fuel markets and creating accurate circular economy solutions.
“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, and 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 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. It 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 or exhibit 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%), requires supplementary upgrading from a technical and environmental perspective.
A high fraction of volatile compounds produces a low flash point (< 30°C). This would, on one hand, ease the ignition of the fuel/air mixture, but on the other hand, make it challenging to handle and store. Otherwise, a high final distillation point (550 °C) seriously affects 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 financially).
Upgrading tire-derived pyrolysis oil (TDO) by fractional distillation
As usual in conventional refineries, a possible upgrading path would be an oxidative desulfurization (ODS) and hydro processing (HP). Although these processes are highly effective, they are hardly economically feasible in the dimensions of existing or planned ELT pyrolysis activities. This path may also be realistically possible in the future, through capacity increases and/or joint ventures by pyrolysis companies and downscaling existing large-scale industrial applications.
As an alternative, fractional distillation offers interesting advantages for upgrading tire-derived pyrolysis oil (TDO).
The chemical complexity, the vast carbon number (C6–C55), and the boiling point range (70 -550°C) of raw TDO indicate 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.
Analogously, 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 possibilities, making it more suitable for specific applications, rather than directly using the raw pyrolysis oil.
In addition, and cited here as an example, CAMPUZANO and colleagues already described in 2020 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 the characteristics of four distillable volume fractions obtained from raw tire-derived pyrolysis oil.
Those fractions are:
- Light fraction – 40 Vol.% (boiling point range: 70 – 176 ◦C), yellowish color, low viscosity, like gasoline fuel.
- Low-middle fraction - 20 Vol.% (boiling point range: 176 – 240 ◦C), dark brownish color, slightly more viscous than the light fraction, compatible with diesel.
- 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.
- 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, and the physical, chemical, and molecular properties of these four fractions indicate similar applications to those established for the conventional fuels (gasoline, diesel, distillate marine fuels, and bitumen). Further, using distillation to fractionate the tire-derived pyrolysis oil allows the concentration of sulfur-containing compounds and highly aromatic structures in the heaviest fraction, improving the characteristics of the lightest fraction.
The extended fractionation of the raw TDO by distillation (a two-stage fractionation already standard today) could therefore be considered a sensible, environmentally friendly, and economically sensible upgrade path, which might also be suitable for medium-sized pyrolysis plants.
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