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If you’ve ever watched a fishing vessel or container ship sail into port, you may have noticed a trailing plume puffing out of its smokestack. Unlike other vehicles that have gone green in the past few years, many ships still run on diesel or bunker fuel, a byproduct of oil refining. The black smoke is the remaining unburned carbon disgorged by the boats’ engines. In simple terms, it’s soot.
New research by Jun Kang at the Korea Maritime and Ocean University in Busan, South Korea, however, has found a way to convert soot into graphite—a different form of carbon widely used in lithium batteries, which every modern computer relies on as a power source.
Until now, soot has been at best a nuisance and at worst a serious health concern. According to international estimates, ships emit more than a million tonnes of exhaust particles a year. The tiny black soot particles can settle in the lungs of humans and other animals, causing pulmonary and cardiovascular problems.
Soot also clogs up ships’ internal machinery, such as the economizer—a device in diesel engines that cools the exhaust gases before they’re emitted to the atmosphere. All of this soot needs to be periodically removed, and ship engineers typically clean the economizer pipes every three months. The extracted soot is offloaded and disposed of.
The most common way to get rid of soot is to burn it. When heated to 400 °C, soot turns into carbon monoxide and carbon dioxide. But this isn’t good for the environment—the discharged gases contribute to climate change. That’s why Kang and his team of scientists and material engineers have been working to figure out how to put soot to a better use.
Graphite and soot are composed of the same basic material, carbon atoms, but their internal structures are different. Soot also contains impurities—the various compounds left over from burning. But under extreme temperatures, soot can transform into graphite.
In their study, Kang and his team heated soot extracted from a ship’s economizer to 2,700 °C—roughly twice as hot as fresh lava or half as hot as the surface of the sun. The heat treatment burned out the impurities and caused the carbon atoms to yield tiny graphite particles.
Currently, graphite is mined, and unlike many other raw materials for high-tech products, such as rare earth elements, it’s abundant and cheap to acquire, so there wasn’t an economic driver to find an alternative source for it. However, Kang’s method turns a polluting byproduct into a potentially useful substance—the demand for which will only increase in coming years as the number of electronic devices continues to grow.
Yuan Yang, a materials scientist at Columbia University in New York whose research focuses on designing lithium batteries, finds the research promising. “I think this is an interesting effort to recycle a waste product,” he says. He points out the resulting graphite particles are too small to be used in batteries as they are, but thinks further work can solve this issue. He doesn’t think the cost of recycled graphite would be too high, but “a lot of effort will be needed to commercialize it.”
Kang is optimistic and hopes that as more people learn about this work and its possible application, it will gain traction.