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ScienceFebruary 19, 2019

Single use plastic is piling up. Is pyrolysis the answer?

Photo: Getty Images
Photo: Getty Images

With global angst mounting about the buildup of plastic ending up in landfills and the environment, chemists and materials scientists are considering pyrolysis as an option. But how does it work? And is it really a solution? 

For decades, putting used plastic into recycling bins was considered a convenient way to get rid of rubbish, while doing something worthwhile for the environment. The only problem was, it was almost entirely reliant on China importing it, so the rest of the world could wash their hands of it. Then the Chinese decided they’d had enough.

It left the waste management systems of the world in turmoil, because previously the country had imported two-thirds of the recyclable plastic produced in the world. That astonishing figure shows just how much the rest of the world came to rely on those imports – which in many cases, simply ended up being dumped anyway, rather than recycled. Now, around the world, giant bales of plastic are piling up in warehouses.

What if there was a way to simply get rid of it, without filling up more landfills? Or even generate energy, or turn the plastic back into raw materials? Such technologies would naturally be extremely attractive to a world desperately looking for solutions. 

So it was understandable that there was strong interest in the MacDiarmid Institute AMN9 Conference lecture given by Assistant Professor Grzegorz Lisak, a renowned electro-chemist from Nanyang Technological University in Singapore. He’s currently piloting a reactor for the process of pyrolysis for single use plastics – by which material is broken down into individual components through the use of extremely high temperatures. Almost all of Singapore’s non-recyclable waste is incinerated, because with a tiny land area they simply don’t have the space for endless landfills, and so have had to become a technological leader in the field. But for those hoping incineration and pyrolysis might allow our society to keep bingeing on single use plastic, the message Dr Lisak gave wasn’t supportive of that.

What can be done with pyrolysis?

The first thing to note is that incineration and pyrolysis are not the same things. Incineration involves burning, whereas pyrolysis is more like melting through thermochemical conversion being applied. The high temperatures are produced without exposing the material to oxygen, as opposed to incineration which requires oxygen to work. As such, what goes into the reactor, and what comes out, is quite different. Household or common waste, if it goes to a plant, is more likely to go an incineration plant, which can be used primarily for generating energy.  

The other thing to note is that different types of plastics have different chemical formulas, and a single plastic product might have many different material layers on it. For example, a single use takeaway food container might involve a combination of polyethylene or polypropylene, inks, bits of paper and PVC. They’re complex, and it takes a lot of energy to gather them all up. That makes them a nightmare for recyclers, not just from a technical perspective, but from an economic perspective. Pyrolysis sidesteps some of those problems, as it can also take contaminated plastic – for example the takeaway container that is still covered in butter chicken residue, that would otherwise be unable to be recycled. 

Of significantly more interest to Dr Lisak is the potential for the pyrolysis process to take single use plastic – which is effectively without value as a commodity – and turn it into something far more valuable. A range of materials come out of plastic that has gone through the process. Some of them are chemical elements – carbon and hydrogen, for example – along with other more immediately useful materials, like wax, coke (the fuel, not the drink) or oil. Aluminium is also possible to be recovered, along with gas to power the reactor, depending on which process is used, and what makeup of material is fed in. 

Carbon nanotubes

There’s also in increasing amount of interest in another material that can be produced from Dr Lisak’s pyrolysis pilot – carbon nanotubes. They’re basically very small tubes of carbon, which take the form of a sheet of graphene. That’s basically a single layer of carbon atoms, in the form of a lattice, structurally akin to a relatively long and extremely thin tube. The story of how graphene was first isolated has become legendary in materials science circles. Researchers at the University of Manchester spent their Friday afternoons each week coming up with silly experiments to try out. One of them was to get some scotch tape, and use it to peel off a layer from a block of graphite. They realised that if they kept pulling layers off, they could eventually get it down to a layer one atom thick. They won the Nobel Prize for it. The methods are generally a bit more sophisticated now, though sometimes the scotch tape method – more scientifically referred to as ‘exfoliation’ – is still used.

Dr Natalie Plank, a MacDiarmid investigator and senior physics lecturer at Victoria University, says the potential uses of graphene and carbon nanotubes are immense. Depending on what that use will be, they’ll have different quality grades. “People do use them in composite materials, and that’s happening already. So you might have them in your tennis racquet, or on coatings for airplanes, or cars. All of those thing where you need high quality carbon coatings.” Her interest is in using carbon nanotubes for electronics, and she uses nanotubes with a diameter of about 1-1.5 nanometers (a billionth of a meter.) Making them to a high enough level for electronics is extremely difficult, and depending on how they’re rolled up, they can either be metallic in conduction, or semiconducting. Research is going into the latter as to whether they might be able to compete with silicon, currently the primary material for semiconductors in computers. Regardless though, they’re incredibly useful in both formats, and thus the production of them is highly valuable. 

Much of Dr Lisak’s lecture focused on the specific method of making carbon nanotubes. Currently, the standard method by which they’re made is by reforming carbon monoxide, on top of a catalyst. Dr Chris Bumby, an applied materials physicist at the Robinson Research Institute and an investigator with the MacDiarmid Institute, put it in blunt terms. “You put a gas into a big hot furnace, heat it up, the carbon monoxide decomposes to carbon and oxygen and the oxygen disappears. Sometimes you’ll use an alkene, sometimes you’ll use various other precursors like methane, and they grow form these little precursor particles.” The method proposed by Dr Lisak replaced the feed gas, that had previously been an artificial fossil fuel, with the gas from pyrolysing single use plastic.

Dr Natalie Plank, a MacDiarmid investigator and senior physics lecturer at Victoria University of Wellington (Photo supplied)

The circular economy? 

Making use of these products brings single use plastic closer to being part of the circular economy, says Dr Lisek, but it comes with some caveats. “We can circulate the product, but that will end at some point because with any cycle we lose some kind of property, so at the end we will still have to face a disposal issue.” His process might provide useful materials, and might help address a significant ecological crisis being faced by the world. But Dr Lisek would be the first to admit that it’s not perfect. 

Dr Bumby points out that it’s the nature of chemistry and physics that nothing can be 100% efficient, and the inefficiencies will come out in the form of greenhouse gases and solids that need to be disposed of. “You’re seeing a lot of diagrams in the media, who draw these beautiful circles, where everything gets recycled and goes back to the start, and goes round and round. But actually, physics, chemistry and economics all tell you that that is impossible. There will always be a waste stream.” 

There are also social questions to be answered, before any sort of pyrolysis project can be set up. The conditions in Singapore, where there is an extreme shortage of space, and a ruthlessly enforced culture of cleanliness, are very different to those in New Zealand. Attempts were made at setting up a pyrolysis plant near Blenheim last year, using a combustion method to process the thousands of tonnes of timber that go to landfill every year into charcoal. But local residents were furious about the proposal, and managed to block the resource consent on the grounds that timber treated with copper chrome arsenic could send dangerous chemicals into the air around the town. A new application has since been lodged, but the battle is ongoing. 

Dr Lisek says the environmental impact of pyrolysis and incineration can be mitigated, if the plant itself is constructed with state of the art technology. “Currently the plants which are operating are operating under conditions which mean the exhaust that is actually released into the environment, there is nothing to be afraid of. The peak investment of each waste management plant is the air control system, which allows it to release the exhaust as simply CO2 and vapour.” By way of comparison, he says “landfilling is much worse, because you do have a storage of waste, which will slowly decompose, and the with decomposing you have the release of gases in that process as well.” The carbon still ends up in the atmosphere, it just happens slowly, rather than quickly with incineration or pyrolysis.  

But perhaps the most telling detail of Dr Lisek’s presentation came near the start, in which he put up a pyramid showing the heirarchy of waste disposal methods in terms of environmental outcomes. Right up the top was prevention – i.e, not using or creating products in the first place. Below that was minimisation and reusing. Out of the 6 layers in total, the two that fit the bill in some shape or form for pyrolysis, recycling and energy recovery were 4th and 5th respectively. In fact, the only thing they were considered better than is the current status quo – just dumping everything in a landfill. 

This content was created in paid partnership with the MacDiarmid Institute. Learn more about our partnerships here

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