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Higher temperatures are needed to ensure carbon nanotubes decomposition

Jenny Rissler is a researcher at LTH and RISE, affiliated with NanoLund and active in the Mistra Environmental Nanosafety research program.
Jenny Rissler is a Principal Investigator at NanoLund, and active in the Mistra Environmental Nanosafety research program. Photo: Kennet Ruona

A new study shows that carbon nanotubes used as additives to polymers – to make materials lighter, stronger, and electrically conductive – do not necessarily degrade under the conditions that normally prevail in waste incineration plants. However, if the temperature is increased, or the residence time in the flue gas is extended, they will be destructed.

“Contrary to what has previously been thought, our study shows that carbon nanotubes are not always destructed during incineration under the conditions prevailing in European waste incineration plants. This is an important result as there is an increased interest in the potential of fly ash as a secondary raw material. The carbon nanotubes can also cause problems in the plants' flue gas cleaning systems,” says Jenny Rissler, a Principal Investigator at NanoLund, a researcher at the Faculty of Engineering (LTH) and RISE Research Institutes of Sweden AB, and active in the research program Mistra Environmental Nanosafety.

Even if the flue gas particles are efficiently removed from the flue gas, and most of these particles never reach the atmosphere, it is important to destruct the carbon nanotubes.

Carbon nanotubes are a nanomaterial used in, among other things, as a filler in plastic. The unique properties of carbon nanotubes make the plastics lighter, more resistant, and electrically conductive. The largest use is in the construction, automotive, and aerospace industries, but the material is also used for consumer products such as sports equipment. Today, most plastics that contain carbon nanotubes probably end up in incinerators for energy recovery.

Temperature and type of waste installation play a role

In the study, Jenny Rissler and her colleagues investigated the fate of carbon nanotubes embedded in plastics when burned in a pilot-scale facility for waste incineration at RISE. In the plant, the plastics were incinerated in accordance with EU waste incineration regulations, which stipulate that fly ash must be kept above 850 degrees for at least two seconds. They produced two different types of polymers with carbon nanotubes which were burned at two temperatures. After combustion, the researchers took samples of the bottom ash and fly ash, and measured the content of carbon nanotubes.

The result was that unburned carbon nanotubes were found in the flue gas but not in the bottom ash. However, when the researchers raised the temperature, the presence of carbon nanotubes decreased. The study concluded that the type of polymer also plays a role: the polycarbonate encapsulated the carbon nanotubes efficiently, which meant that more unburned carbon nanotubes were found in the ash.

“The results are interesting from many perspectives”, says Jenny Rissler.

“On the one hand, they show that higher temperatures are needed to convert carbon nanotubes into carbon, and on the other hand, the plastic in which the pipes are included is also important.” 

“Even if the flue gas particles are efficiently removed from the flue gas, and most of these particles never reach the atmosphere, it is important to destruct the carbon nanotubes. Partly due to the increased interest in using fly ash as a secondary raw material, but also because the carbon nanotubes are electrically conductive and can cause a discharge in the particle trap, and thus affecting its function.”

The amount of materials matter

The amount of plastic containing carbon nanotubes is still relatively small on the global market, especially for consumer products. If industrial use increases, however, it may be important to categorize these types of plastics, so that they are handled in a way that ensures destruction by, for example, raising the temperature during combustion slightly or extending the residence time of the flue gas at high temperature, according to Jenny Rissler.

Additional studies are needed

She further points out that their study is just a small piece of the puzzle. More, and more extensive studies, are needed to gain more knowledge about how nanoparticles behave in waste incineration plants.

“Individual research groups cannot address this issue comprehensively. There is a need for interdisciplinary studies from different parts of the world that look at both different types of facilities and the risks related to carbon nanotubes being present in the ash during secondary use. But carbon nanotubes are a material that we, under the right conditions, theoretically can be oxidized to CO2”, she concludes.

Download the article: Release of carbon nanotubes during combustion of polymer nanocomposites in a pilot-scale facility for waste incineration.