2.4 Non-mechanical recycling – part of the future solution for waste plastics?
2.4.1 Chemical Recycling Chemical recycling is seen as a solution for ‘difficult to recycle plastic’ such as multi-layer films, heavily printed and filled plastics or complex components. It has attracted attention as it also has the potential to satisfy food contact regulations where not possible through existing technology. In October 2020 UK Research and Innovation (UKRI) announced that their Industry Strategy
Challenge Fund will be investing in three projects which use chemical recycling, showing that it is seen as a key recycling area. Below explains in more detail how the different technologies work and the opportunities they present. More information is available on Plastipedia: https://www.bpf.co.uk/plastipedia/chemicalrecycling-101.aspx.
Four main types of chemical recycling processes are used to break down plastic materials to varying degrees, as follows: 1. Chemical depolymerisation
2. Pyrolysis
This technique involves a process by which a polymer is converted into a monomer via a chemically aided route.86 A typical use of this method is for PET which is broken down to monoethylene glycol (MEG).87
Also known as feedstock recycling, this technique for recycling is the heating of polymer at elevated temperatures in the absence of oxygen to produce hydrocarbon raw materials which can then be used for polymer manufacture. Pyrolysis is typically used for polymers such as polystyrene (produces styrene as main product) and polyethylene (produces 1-phenylbenzene as main product).88
3. Gasification
4. Hydrothermal treatment
This technique involves the partial oxidation of a polymer to produce ‘Syngas’ (a mixture of CO, CO2, H2 and CH3) through heating with an oxidising agent.89 In most cases the gas is then combusted for energy generation, although ethanol can be made from the gases and used to create polyethylene.
Hydrolysis is a process where a compound is broken down by water molecules in nearcritical conditions, with temperatures of about 160 – 240°C and the pressure needed to keep the water in a liquid state. This causes reactions of hydrolysis, dehydration, decarboxylation and depolymerisation. This process produces synthetic crude oil.
The above processing technologies will either produce a ‘chemical feedstock’ or monomer which can be converted back into the raw material for products. These can therefore be considered as a type of recycling and should be counted towards recycling figures. Although the stage in the process the material is returned varies, all maintain the original material-value and enable a circular flow of plastic to take place. Each process type has a different level of energy demand and output mass yield of useful product; solvolysis and monomer recycling giving 95%+ useful yield, hydrothermal treatment 85% and pyrolysis near 50%.
Non-mechanical recycling can be considered as a complementary solution to mechanical recycling as its feedstock is normally focused on material which cannot be mechanically recycled. There have been a range of life cycle assessments done looking at chemical recycling, and it compares favourably with incineration and landfill, which would be the alternative end of life option. ‘Chemical recycling involves some loss of carbon as C02, but the C02 savings can be as much as 75% with technologies now under development’.90
BPF RECYCLING ROADMAP FUTURE VISION
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