Power Shift is Ellyn Lapointe’s ongoing Gizmodo series that explores advances in green technology with a focus on renewable energy, grid modernization, and emissions reductions.
Plastic recycling and decarbonizing energy are two of the most pressing sustainability challenges of our time. A new study suggests a solution that can solve both problems simultaneously: transformation plastic waste included pure hydrogen.
Although the concept is not new, the approach is described in one paper has been published Proceedings of the National Academy of Sciences earlier this month—significantly improves on traditional methods. The reaction, called “ATT” for alkaline heat treatment, produces high-purity hydrogen at lower temperatures without requiring extensive waste sorting. Moreover, it does not generate direct greenhouse gas emissions.
Two problems, one solution
Because plastic recycling requires expensive sorting and processing, only a small fraction of the world’s discarded plastic is actually recycled.
“In practice, discarded plastics are often mixed, contaminated with food, adhesives, labels, dyes and other additives, or combined in multilayer packaging,” said co-author Woo Jae Kim, a professor of chemical engineering and materials science at Ewha Women’s University in South Korea. “So sorting and cleaning them up is technically difficult and can be more expensive than producing new plastics from fossil resources.”
Previous research showed In 2022, the global recycling rate remained stagnant at just 9%, 40% ended up in landfills and 34% was incinerated. At the same time, plastic use is expected to continue to grow, rising from 464 megatons in 2020 to 884 megatons by 2050. projection.
At the same time, the world urgently needs clean energy sources. Hydrogen is often touted as a promising fuel because it can be burned like oil or gas, but does not emit planet-warming carbon dioxide (CO2). The problem is that there are no readily available sources of clean hydrogen on Earth. If we want to use it, we have to.
To solve both of these problems, chemical engineers are investigating different ways to convert plastic waste into pure hydrogen. Two methods that have gained significant attention in recent years are pyrolysis and gasification.
Pyrolysis heats plastics in an oxygen-free environment, breaking them down into oil, carbon, and gases (including hydrogen). The process produces relatively low carbon emissions, but it only works well with certain types of plastic and therefore requires extensive sorting and processing.
Gasification works differently, partially oxidizing plastics at higher temperatures to create a mixture of hydrogen, carbon monoxide and hydrocarbons. Because gasification can handle mixed plastics without extensive sorting, it is generally considered a more cost-effective approach, but the high pressures and extreme temperatures it requires make it highly energy-intensive, resulting in significant CO2 emissions.
To overcome these drawbacks, this new study suggests using alkaline heat treatment to recycle mixed plastic waste. The authors adapted the ATT process to the method possessed by Kim developed With Ah-Hyung “Alissa” Park, professor of chemical and biomolecular engineering at the University of California, Los Angeles. They designed an original process to convert biomass like seaweed into hydrogen in a carbon-neutral way, but wondered if a similar approach could be useful for mixed plastic recycling.
A cleaner conversion
In the lab, Kim, Park, and their colleagues used modified ATT to convert the three most common plastics—polyethylene terephthalate (PET), polyethylene (PE), and polypropylene (PP)—into high-purity hydrogen. ATT breaks down plastic by mixing it with sodium hydroxide (NaOH) and heating it. Due to the alkaline conditions provided by NaOH, it does not require as much heat as gasification.
Initially, the process yielded significantly more hydrogen from PET than from PE or PP. These two plastics consist entirely of carbon-hydrogen bonds, so they are chemically inert under alkaline conditions. To solve this problem, the researchers briefly exposed PE and PP to mild heat and oxygen before the main reaction. This pretreatment allowed for efficient degradation of all three plastics.
With this approach, the researchers produced 43.7, 51.9, and 30.2 millimoles of hydrogen gas per gram of PET, PE, and PP, respectively—yields comparable to those achieved by pyrolysis and gasification. Moreover, their post-reaction analysis showed that carbon emissions from the reaction were negligible.
Julie Zimmerman, a professor of chemical and environmental engineering and vice provost for planetary solutions at Yale University, said the research presents an “interesting and potentially important reaction concept,” but it’s too early to say whether this is a scalable, sustainable, sustainable way to convert plastic to pure hydrogen.
“The authors demonstrate a reliable chemical route for the production of high-purity hydrogen from PET and pre-oxidized PE and PP, including a controlled blend of the three plastics,” Zimmerman said. “However, milligram-scale experiments, long oxidation pretreatments, significant alkali usage, and high final temperatures determine chemical feasibility more than technical or economic viability.”
The researchers agree that optimizing the process and evaluating its cost-effectiveness will require further research. Although the reaction produces negligible direct CO2 emissions, a full life-cycle analysis will be needed to understand their overall carbon footprint, Kim said. The team also needs to develop an efficient way to recycle the sodium hydroxide reagent and test whether the method works with plastic waste containing food residues, moisture, additives and other contaminants.
Although much work remains to be done, the research marks an important step toward a more efficient and potentially cleaner way to convert plastic into hydrogen. As both waste and carbon emissions continue to accumulate, finding innovative solutions to these problems will become even more important.
