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Plastic production

  

Plastic consumption has quadrupled over the past 30 years, driven by growth in emerging markets.  Global plastics production doubled from 2000 to 2019 to reach 460 million tons [OECD, 2022].

  

Plastic waste

  

Globally, 9% of plastic waste is recycled, 19% is incinerated, 50% ends up in landfill and 22% is mismanaged (evades waste management systems and goes into uncontrolled dumpsites: is burned in open pits or ends up in terrestrial or aquatic environments) [OECD, 2022].

  

As a result, global plastic waste generation more than doubled from 2000 to 2019 to 353 million tons [OECD, 2022], and the amount of plastic waste generated worldwide is projected to triple by 2060, to surpass one billion ton [STAT, 2023].  There is now (2022) an estimated 30 million tons of plastic waste in seas & oceans, and a further 109 million tons has accumulated in rivers [OECD, 2022].  According to specialists, it would take more than 450 years (HDPE plastic-type) to biodegrade.

  

Figure 1 shows plastic waste generation by industry sector, highlighting the "packaging sector".  Since packaging tends to have a much lower product lifetime than other products (such as construction or textiles), it is also dominant in terms of annual waste generation [OWID, 2018].

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Plastic waste valorization

  

Figure 2 shows primary chemical routes for plastic waste valorization.

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Plastic recycling

  

There are two types of plastic recycling: mechanical & chemical; mechanical recycling involves sorting, cleaning, and shredding them to make pellets, which can then be fashioned into other products.  This approach works very well if plastic wastes are sorted according to their chemical composition.

  

Chemical recycling, in contrast, turns the plastic into an energy carrier or feedstock for fuels.  There are two main processes by which this can be done: pyrolysis & gasification.  In pyrolysis, plastic waste is heated at temperature around 300–650 °C in the absence of O2 and then, oil fuel can be provided.  In gasification, plastic waste is reacted with gasifying agent (e.g., steam, oxygen, and air) at high temperature around 500–1300 °C, which can produce syngas.  It can be observed that the main difference of these methods is the obtained product (oil or syngas).  Syngas can be further used to produce many products and fuel for fuel cell to generate electricity [SD, 2019].

  

One of the advantages of plastic waste-to-fuel is that plastic doesn’t have to be separated into different types [WR, 2023].  A modern office can produce a considerable amount of waste, ranging from plastic packaging to small or medium-sized electronic devices: these products are sometimes difficult to separate & recycle properly. 

  

Pyrolysis & gasification are alternatives to incineration.  The main goal of incineration is to destroy the waste, thus keeping it out of landfill: the heat released from incineration might be used to produce steam to drive a turbine and generate electricity.  But, energy from incineration is not possible as burning plastics releases harmful gases: burning plastics is actually one of the highest GHG emitting processes.

  

Which is the most recyclable?

 

The general rule is, the lower the resin identification code (RIC), the more likely the plastic type is to be easily recyclable.  Many plastic types can be recycled, even if the process is not widespread, however they aren’t recycled simply because they aren’t "easily" recyclable [SL, 2023].

  

Most commonly recycled plastics:

  

• 1 – Polyethylene Terephthalate (PET) – water bottles & plastic trays

• 2 – High Density Polyethylene (HDPE) – milk cartoons & shampoo bottles

• 5 – Polypropylene (PP) – margarine tubs & ready-meal trays

  

Somewhat recyclable plastics (at specialist facilities):

  

• 3 – Polyvinyl Chloride (PVC) – piping

• 4 – Low Density Polyethylene (LDPE) – food bags

• 6 – Polystyrene (PS) – plastic cutlery

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Plastic to fuel

 

Gasification involves heating the waste plastic with air or steam, to produce syngas.  This can then be used to produce diesel & petrol, or burned directly in boilers to generate electricity [WR, 2023].  Plastic pyrolysis plants have already been built in the UK, Japan, and the U.S.  Various catalysts, such as fly ash synthesized natural catalyst, zeolites (ZSM-5), and pillared clay have been developed and used to enhance the pyrolysis oil yield by reducing wax formation and obtain a potentially valuable fuels or chemicals [SD, 2022]. Advantages of converting plastic waste into fuel include:

    

• Relatively low-cost.

• The plants that convert waste to fuel are producing fuels from combustible materials, which are either hard to recycle or non-recyclable, preventing those materials from ending up in a landfill.

• The produced fuels can be tailored to a certain need, such as transportation, where heat is required. This makes them suitable alternatives to fossil fuels.

• It can be burned with a lower carbon footprint than fossil fuels.

• There is potential to expand the materials used to metal waste and others that may not be easily recyclable.

  

Pyrolysis is one of the most popular processes in converting plastic waste into fuel & chemicals and widely recognized as the most efficient method for producing fuels & chemicals from plastic waste [SD, 2019].  The global waste-derived pyrolysis oil market is segmented by raw material (waste plastic, waste rubber, wood, oil sludge, and other raw materials), and application (fuels & chemicals) [MI, 2023].  Figure 3 shows the pyrolysis process, with plastics as input and petrol & diesel as output.