The global production of plastic has increased, from 1.5 million tons in 1950 to approximately 359 million tons in 2018. Approximately 4% of crude oil production is directly utilized in plastic production. Plastic materials (usually packaging) are the third largest contributor of municipal solid waste (about 12 %). The continuous disposal of plastic wastes is destructive to both terrestrial and marine ecosystem, as they are not readily biodegraded and can take several years to vanish.
Mechanical recycling of plastics is being considered as the best option to treat growing volume of plastic waste. In practice, the approach is associated with many difficulties, such as problems related to separation, sorting and cleaning operations, high transport and electricity costs, coupled with feed stock selectivity. Because the plastic tends to lose properties (i.e. clarity, strength and flexibility) as they are recycled, the materials obtained via this process plastics are used in producing articles of lower quality and application. Chemical recycling solves this downcycling problem but it does not alleviate the preparation burden. Thus, it will probably have a low impact on the global recycling rate.Obviously more than more than the mechanical recycling techniques will be needed to keep-up with the technological advances brought about by the use of plastics.
Pyrolysis (thermal degradation of plastic waste between 300–900°C in the absence of oxygen) can be another alternative method which is known to produce fuel and several other useful raw materials. There are different pyrolysis types (slow, fast, flash). Depending on the feedstock composition, process condition (temperature, heating rate, reaction gas) and presence of catalyst, the product and yield composition can be varied.
According to a recent review (link), pyrolysis presents a several advantages over other conventional waste plastic management approaches:
- Pyrolysis is simply thermal depolymerization that reverts plastic waste into valuable resource in the form of monomers and other useful resources. For instance it produces a liquid oil (up to 55% yield rate) that can be added to diesel or gasoline and used as fuel.
- Pyrolysis is a rather simple and affordable process that does not require sorting, washing and blending. Therefore, it can be implemented in small installations with limited capital investment needs. This makes it an attractive option for developping countries where open buring often prevails.
- Pyrolysis is able to use various types and class of plastics as a single feedstock without the need for disassembling and sorting. It can also be used to treat waste of many novel materials, such as composites which are replacing traditional materials in most engineered applications.
There are however some drawbacks::
- Liquid fuel from pyrolysis of different plastic types has been reported to have sulphur content significantly higher than conventional gasoline, kerosene and commercial grade diesel.
- The flash point of liquid fuel from plastic waste is <40 degrees, though it may vary depending on the plastic type used as feed. A low flash point indicates the presence of highly volatile materials in the fuel that is a serious safety concern in handling and transporting.
- Plastic wastes are likely to contain additives: plasticizers, fire retardants, antioxidants, light stabilisers, lubricants, pigments, antistatic agents and thermal stabilisers. These additives may have an impact of the quality of the product and prevent some recovery (i.e. chlorine from PVC may lead generate dioxin during combustion).
- The energy balance is very much in dispute as there is a drying phase before pyrolysis takes. It depends on which feedstock is used. In addition, some shredding work may be necessary at an earlier stage.
The interest in pyrolysis, of platic wastes has increased considerably over the last few years, primarily since China stopped accepting post-consumer plastic waste in 2018, after having taken up to 45% of the world’s plastic waste. Many ongoing research studies are trying to gain a better understanding of the pyrolysis technology with the aim of establishing new industrial processes for plastic recycling. According to recent bibliography survey (link), future directions of plastic pyrolysis research will be based on the following hot topics:
- kinetic triplets as a vital component of plastic pyrolysis and scaling up processes,
- catalysts syntheses and performance,
- co-pyrolysis of plastic/biomass mixtures,
- reactor design and reaction parameters.
As a conclusion, pyrolysis appears to be a promising option to recover plastic waste as it is expected to extend the recovery rate of plastic waste and increase the benefits (not just heat as for incineration) . It is certainly worth to be supported but this must be seen just as a additional building block with a comprehensive strategy on plastic as the EU does (link). Its aim is to protect our environment and reduce marine litter, greenhouse gas emissions and our dependence on imported fossil fuels. It will support more sustainable and safer consumption and production patterns for plastics.
Sustainable Synergies 