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Pyrolysis works but there's more involved.

PETER Sunman (WM letters, February 3) is correct in stating that plastic waste may be destroyed by pyrolysis technology - but there are both logistical and contractual issues to overcome before it can be used.

One problem is the contamination of simple hydrocarbon plastics with other plastics and other substances. So, for example, PVC will generate acidic gases and PET will make a lot of gas, but little liquid hydrocarbons - while any associated wood and other biomass will produce resins that will block up the equipment, or solidify in the product stream.

For mixed-plastic pyrolysis to work, the mixture has be cleaned and the various plastics all separated. Although cleaning to the 0.5% contamination level barrier set by the Chinese is not required, the level typically has to be below 5% - only then will the process be able to operate stably with a controllable set of outputs. This requires a major infrastructure and investment (by the local authorities) that will cost a lot of money to build and operate (which is why the waste companies have been shipping unmixed waste to China in the first place).

There are also minor but significant problems with cleaning the products to meet the fuel standards required before putting it in a vehicle, ship, or even burning it to raise heat.

The contractual issues arise from the (short-sighted) decision of most (possibly all) Welsh councils to sign ridiculously long-term agreements with "waste-to-energy" companies to dispose of waste. These effectively prevent new technology companies securing a guaranteed large supply of feedstock (the plastic waste) and so makes it unattractive for new investors.

A number of trial pyrolysis units have been tried in the UK (with the aim of making transport fuels) but all have failed on the altar of the quality of the feedstock. This poor track record, including the failure of two plants in the UK in 2017, acts as a further disincentive to investors.

Despite this I agree with Mr Sunman that, with an appropriate infrastructure to separate the plastic waste into like chemical streams, pyrolysis of plastics into fuels and hydrocarbons is viable. But if the feedstock isn't controlled, "garbage in" will equal "garbage out" - despite the potential merits of the technology.

Dr John Cox, consultant engineer, Pontypool

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Title Annotation:Features
Publication:Western Mail (Cardiff, Wales)
Date:Feb 7, 2018
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