Carbon capture using plastic waste kills two birds with one stone

A new technique has turned plastic waste into an effective material for absorbing carbon dioxide emissions.

In a potential ‘win-win’ for a pair of environmental challenges, researchers from Rice University in Texas used a newly discovered process to turn the waste material into a CO₂ sorbent for industry.

The team heated plastic waste such as water jugs in the presence of potassium acetate, producing particles with nanometre-scale pores that trap CO₂ molecules. The particles could remove the gas from flue gas streams, the researchers said.

“Point sources of CO₂ emissions like power plant exhaust stacks can be fitted with this waste plastic derived material to remove enormous amounts of CO₂ that would normally fill the atmosphere,” said researcher James Tour. “It is a great way to have one problem, plastic waste, address another problem, CO₂ emissions.”  

A current process to pyrolyse plastic, known as chemical recycling, produces oils, gases and waxes, but the carbon by-product is nearly useless, Tour said. Pyrolysing plastic in the presence of potassium acetate produces porous particles able to hold up to 18% of their own weight in CO₂ at room temperature, however.

While typical chemical recycling doesn’t work for polymer wastes with low fixed carbon content, including polypropylene and high- and low-density polyethylene, those plastics – the main constituents in municipal waste – work especially well for capturing CO₂ when treated with potassium acetate. 

The researchers estimated the cost of CO₂ capture from a point source like post-combustion flue gas would be $19 per tonne, far less expensive than the energy-intensive, amine-based process that is commonly used to extract CO₂ from natural gas feeds, which costs $73-145 per tonne.

Like amine-based materials, the sorbent can be reused. Heating it to about 75ºC released trapped CO₂ from the pores, regenerating about 90% of the material’s binding sites. The team expect the sorbent to have a longer lifetime than liquid amines, cutting downtime due to corrosion and ‘sludge’ formation.

To make the material, waste plastic was turned into powder, mixed with potassium acetate and heated at 600ºC for 45 minutes to optimize the pores, most of which are about 0.7 nanometres wide. The process also produced a wax by-product that can be recycled into detergents or lubricants. 

The work was reported in the American Chemical Society journal ACS Nano.

Want the best engineering stories delivered straight to your inbox? The Professional Engineering newsletter gives you vital updates on the most cutting-edge engineering and exciting new job opportunities. To sign up, click here.

Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.