Technologies

Pyrolysis is a thermochemical conversion process conducted in the absence of oxygen that can be considered a ‘feedstock recycling’ process and that can be employed for the production of liquid transport fuel intermediaries or finished fuel products from plastic waste.
The required feedstock pretreatment lies somewhere between that for mechanical recycling (intensive) and incineration (non- intensive). The liquid energy-carrying medium/output that is produced is easy to store and can be used on demand, and has a higher economic value than electricity produced via energy recovery from waste (waste-to-energy processes). Furthermore, pyrolysis can be employed alongside mechanical recycling and incineration in a cascaded waste management infrastructure, so it should be viewed as a component of a waste management system rather than a competing technology. Pyrolysis is a versatile process that is suitable for both large and small-scale production of several products. For example, pyrolysis of waste polyolefin plastics under different conditions can yield hydrocarbon waxes and oils, BTX aromatics, and olefin gases (ethene, propene, butadiene) (E. Butler, G. Devlin, K. McDonnel, 2011). 

The pyrolysis method for scrap tires recycling involves heating whole or halved or shredded tires (to temperatures >400°C ) in a reactor containing an oxygen free atmosphere and a heat source. In the reactor, the rubber is softened after which the rubber polymers disintegrate into smaller molecules which eventually vaporize and exit from the reactor. These vapors can be burned directly to produce power or condensed into an oily type liquid, called pyrolysis oil. Some molecules are too small to condense and remain as a gas which can be burned as fuel. The minerals that were part of the tire, about 40% by weight, are removed as a solid (Zafar, 2020).Pyrolytic oils from waste tires have been burned successfully in test furnaces and diesel engines. Pyrolysis oils can also yield useful chemicals for the petrochemical industry, including light olefins and aromatics. Solid-phase pyrolysis char can be used to create carbon black or activated carbon for pollutant removal. Gases produced, including hydrogen and various hydrocarbons, can be used as a fuel for the pyrolysis process (Ali Alsaleh, Melanie L. Sattler, 2014).

Hydropyrolysis is the the thermal decomposition which takes place when organic compounds are heated to high temperatures in the presence of hydrogen. Biomass is converted in a catalytic fluidized bed reactor at temperatures of 350–480ºC in a nearly pure hydrogen atmosphere at 20–35 bar. Catalytic hydropyrolysis removes oxygen from the liquid product, while it does not affect char yield and composition. The outlet stream goes into the hydroconversion reactor operating at same pressure and temperature for the stabilization of the liquid product and further reduction of oxygen content. The oxygen in biomass leaves as water mixed with the liquid product and also as CO and CO2 in the off-gas. The off-gas stream is cleaned and reformed for the production of hydrogen for the hydropyrolysis reactor (Pedro Haro et al., 2014) .

The generic framework of plastic recycling by solvent extraction includes the removal of impurities, dissolution (homogeneous or heterogeneous dissolution), and reprecipitation or devolatilization. Specifically, the polymer(s) is dissolved in the solvent(s), and then each polymer is selectively crystallized. Ideally, when a solvent can dissolve either the target polymer or all the other polymers except the target one, it can be used to for selective dissolution. Many factors affect polymer dissolution, such as molecular weight, composition, structure of the polymer and composition and size of the solvent.Despite the environmentally friendly properties including energy saving and less CO2 emission of the current solvent extraction of waste plastics technique, it still faces difficulties and challenges, which hinder its development to some extent. Generally, waste plastics are mixed polymers. Therefore, the primary challenge is the separation and recycling of waste components one by one. Existence of solvents and impurities in the recovered products results in the degeneration of material properties compared with those of virgin materials. Further, atmospheric or vacuum distillation also leads to the thermal degradation of polymer chains and worse plastic quality; therefore, the development of proper solvent removal and purifying techniques (e.g., SFE) is very crucial (Yi-Bo Zhao, Xu-Dong Lv, Hong-Gang Ni, 2018).