Since the initial use of plastics in the 50s, over 8.3 billion metric tonnes of plastic have been created – equivalent to the weight of 80 million blue whales! Of this vast quantity, it is believed that 60% has been discarded in landfill or elsewhere in the environment. Plastic pollution constitutes 90% of waste on the ocean’s surface and has been documented in the bodies of 44% of seabird species. As none of the mass produced plastics biodegrade readily, we need to consider alternative ways to deal with this waste. Pyrolysis of plastics has been touted as a technology that could solve the problem by breaking down plastic and using it for energy.
The word pyrolysis comes from the Greek ‘pyro’ meaning heat and ‘lysis’ meaning breaking down. Plastic is made up of long chain molecules called polymers. These chains are degraded by heat and pressure in the absence of oxygen. This forms increasingly smaller molecules.
The waste plastic is cleaned then placed in a high pressure reactor and heated up to 400 – 500 °C, causing the atoms within the long polymers chains to vibrate to such an extent that the bonds between them break. The plastic does not burn but is melted to a chewing gum consistency. Further heating vapourises it to form a gaseous mixture of different sized molecules, which are separated by a process called fractional distillation.
There are 3 products of plastic pyrolysis: carbon black, liquid oil and hydrocarbon gas. The carbon black can be used in the place of coal or as raw material for making carbon nanotubes. The oil is used to power electricity generators or as a raw material in making petrochemicals, such as lubricants used in manufacturing. The hydrocarbon gas produced is used in the pyrolysis process itself, in order to create the high temperatures required.
Aside from the reduction in waste going to landfill and plastic pollution, there are some other major benefits to using pyrolysis. Only 80% of plastic produced can be physically recycled (the type of recycling where plastic waste is broken down into small granules which are used to manufacture new materials). In contrast, pyrolysis can be used to break down all plastics.
The process is environmentally friendly: a vacuumed chamber is used, which means toxins are not emitted into the atmosphere, plus the gas is collected and used to power the plant, saving energy for the whole process.
There are also economic benefits to using pyrolysis. Primarily, it is cheaper than disposal in landfill. Implementing the technology is simple and inexpensive and the construction of a pyrolysis plant can be relatively fast. There is the potential that with the introduction of new plants there will be the creation of many new jobs.
A few companies have tried to commercialise pyrolysis in the United Kingdom. One example was Cynar, a plastics-to-fuel company based in London. They constructed their first pyrolysis plant, in Ireland in 2008, with a capacity 20 tonnes per day. The company set about building further facilities, through partnership deals in Spain and the UK, however the company went into liquidation before completion. Another company, called Enval, has utilised microwave pyrolysis to recycle plastic aluminium laminates (which cannot be recycled any other way). They have a plant based in Huntingdon, Cambridgeshire, with the ability to process 2000 tonnes per year. However, no councils are using this type of recycling at the moment. This is because councils have existing contracts with waste management service providers. Hopefully, there will be an increase in the use of pyrolysis as contracts come up for renewal.