Plastic is everywhere in our lives these days, it is necessary for modern human life. But because of how much plastic we produce and discard we also have to come up with ways of dealing with the plastic we don’t use anymore.
From water bottles and grocery bags to car dashboards, recycling becomes a challenge especially because every kind of plastic type has to be separated beforehand.
Some kinds of plastics are very similar though. By observing this, scientists managed to come up with a new clever laboratory process that can feed 3 of the most common plastics into a single reactor, that then produces hydrogen gas that is more than 90% pure. Not only that: it also prevents nearly all direct carbon dioxide emissions by mixing carbon with solid materials in its process.
Why mixed plastic is so difficult
Only a measly 9 percent of discarded plastic is recycled, while up to 79 percent of it goes to landfills and 12 percent is burned down, which is not only a waste but also a terrible way of dealing with it due to all the harmful gases it produces. But sorting needs trained personnel for now, which raises the costs and slows the recycling down, so the mixed waste barely has any value once it eventually arrives at the disposal facility.
The 3 common plastics that are now being targeted are the PET, the PE and the PP, respectively meaning polyethylene terephthalate, polyethylene, and polypropylene. Those are the most common components of the plastics that we use in our daily lives be it bottles, bags, or car parts, but at the same time their chemistry usually makes them hard to process together.
Turning waste into hydrogen
And why is the purity of hydrogen a matter of importance? A united team of scientists from the UCLA and Ewha Womans University tried a previously known method called “alkaline thermal treatment”. Translating to simpler terms it means that the sodium hydroxide when under heat reacts with the plastic and helps with the hydrogen release.
This method was conceived out of an earlier carbon-neutral process that turned biomass such as seaweed into hydrogen. The new team took advantage of it and adapted it for plastic, finding out that the PETs could produce highly pure hydrogen if the temperatures were around 540 to 720 degrees Fahrenheit lower than the conventional steam gasification.
When consulted the head coauthor Ah-Hyung “Alissa” Park said that “we are solving two urgent global problems at the same time”, “plastic waste is accumulating at an alarming rate, and clean hydrogen is essential for decarbonizing energy.”
Making stubborn plastics react
Stubbornness is a commonly used term used for human behavior but that with a bit of mental gymnastics also fits the behavior of the plastics. While the PETs contain oxygen and respond more readily to the adapted treatment both the PEs and PPs show much more resistance due to their stable chemical structure that does not easily react under the alkaline conditions.
To try and overcome that problem the researchers ingeniously exposed PEs and PPs to a more moderate heat and air before the main reaction took place. That pretreatment added features containing oxygen to the plastic chains which in turn powered the alkaline treatment giving more room for it to break them apart.
Once they were activated all three plastics could then be processed together meaning in practical terms the waste would need no more careful separation before entering the reactor which greatly reduced the costs.
Keeping carbon out of the air
The sodium hydroxide captures released carbon during the reaction and turns it into solid sodium carbonate instead of allowing it to escape as carbon dioxide. The result is that more than 75 percent of the original carbon ends up in stable carbonates or organic liquid residues, and less than 13 percent showed up as pure gas.
The sodium carbonates are not very useful in their original form, but once converted into calcium carbonates they become useful in a wider list of options. Locking the carbon during that recovery step means less carbon thrown into the atmosphere which in turn means a healthier atmosphere.
What still needs to happen
Previous trials with low-temperature approaches only really worked with oxygen-containing plastics such as PETs, and even though the high-temperatures gasifications can handle the mixed waste they resulted in a bigger carbon dioxide release, which left an uncomfortable trade-off between flexibility and emissions.
“By reducing sorting costs and process complexity, this technology has the potential to become a core next-generation technology,” said corresponding coauthor Woo-Jae Kim. The team is optimistic but said more work still needs to be done to optimize the process and determine if it can compete economically outside the laboratory.
The full study was published in the Academy of Sciences.
