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Driven by recycling limitations in South Korea, chemists have made hydrogen from plastic waste with less than half the carbon dioxide emissions of current industrial hydrogen production. Though researchers have previously produced hydrogen from plastic, Woo-Jae Kim from Ewha Womans University and colleagues reduce the energy needed by adding a pretreatment step. They also minimised the reaction’s carbon dioxide emissions.
While pretreated polyethylene generated the least emissions, the researchers also show that the technique can work with mixed plastic waste. That’s important because, despite having strict regulations, Korea only recycles around 27% of its waste, explains Kim. The global average is 9%. Poor waste sorting means more plastic is incinerated or landfilled than recycled, he explains. ‘If you can do recycling well, that’s way better,’ Kim says. ‘But if it’s not working that way, then give it to this process, so we can convert it into hydrogen.’
Clean hydrogen is central to decarbonising steel, ammonia and speciality chemical manufacturing. Industry mainly makes hydrogen by reacting methane and steam at high temperatures. Kim notes that this steam reforming process produces around 10kg of carbon dioxide per kg of hydrogen.
Meanwhile, green hydrogen made by electrolysing water using renewable energy is currently expensive. ‘As in most countries, water electrolysis in Korea is very limited, so we have to find another way,’ Kim tells Chemistry World.
Exploring options
Previous efforts to chemically release hydrogen from plastic meant reacting it with steam above 700°C, making it energy hungry and wasteful. Pretreating plastic waste with sodium hydroxide, a process developed by the team of Kim’s colleague Ah-Hyung ‘Alissa’ Park at the University of California, Los Angeles, lowers the temperature needed.
The researchers added sodium hydroxide to the plastic in a quartz tube, adding nitrogen and water and gradually heating it to 350°C. They could do this directly with polyethylene terephthalate (PET) but had to oxidise polypropylene and polyethylene first by heating them at up to 350°C for 50 hours.
The process cuts emissions compared with other methods of making hydrogen from plastic – by as much as half for PET. It cuts emissions by over 98% for polyethylene, bringing it just above the threshold of 4kg of carbon dioxide per kg of hydrogen produced required to qualify as clean hydrogen. This is less than half the emissions released by steam reforming.
Kim acknowledges some limitations. These initial studies used less than 1g of plastic, but the team is now scaling up to 100g. The researchers also want to make the process more efficient, as it currently produces less than half of the theoretical maximum hydrogen possible. Finally, in capturing the carbon dioxide that would otherwise be emitted, the process makes a lot of sodium carbonate, which has limited commercial use. Kim therefore wants to find other uses for the material.
‘In the transition to net zero we will need large quantities of hydrogen and so this approach is welcome,’ says Graham Hutchings from the University of Cardiff. ‘However, if the plastic is fossil derived then this will not be green hydrogen, which is what is really needed.’ He adds that handling the large amounts of sodium carbonate will be challenging, and that there’s a long way to go until adoption.
Nevertheless, Kim says he’s been approached by several companies seeking to investigate the approach. He also wants researchers to develop other chemical processes that convert plastic waste into useful products. ‘It’s good to have a lot of options for people to use,’ he adds.
