When it comes to climate change, carbon dioxide (CO2) is enemy number one. So researchers have been racing for years to find a way to limit CO2 emissions from coal plants, a major source of this greenhouse gas. Now, scientists at the University of Wyoming (UW), Laramie, report that they've come up with an approach potentially twice as cost-effective as current strategies. If the technology can be scaled up, it could make it more palatable for governments and electricity providers to sequester CO2.
To capture escaping CO2, coal plants need to separate it from nitrogen. A handful of power plants do this already to create concentrated streams of CO2 that they then pump into underground oil and gas wells to push out more hydrocarbons. To separate CO2, companies bubble flue gas through a solution containing monoethanolamine (MEA), which binds the CO2. Once the MEA solution is saturated with CO2, it is heated to release the CO2 and regenerate the starting material. The energy required to heat the MEA and replace the portion of the volatile compound lost with each cycle drives the cost of recovering CO2 with this process to about $47 a ton.
Maciej Radosz, a chemical engineer at UW, and colleagues decided to look for a cheaper alternative. Past research had shown that activated carbon--a family of porous carbon-based materials--and other carbon-rich compounds were strong adsorbers of CO2. But these studies had looked at separating CO2 from a highly pressurized flue gas. Radosz's team decided to see if these materials would work at low pressures, because that would potentially reduce the energy requirements needed to run the separation. They found that at the lower pressures and temperatures, activated carbons and other carbon-rich materials were also highly selective at filtering out CO2. If scaled up, the Wyoming scientists estimate that their process could drop the price of CO2 separations to $20 a ton.
"It seems like an interesting idea," says Nick Hutson, a chemical engineer and CO2 separations expert at the U.S. Environmental Protection Agency in Research Triangle Park, North Carolina. But he cautions that to be effective, CO2-filtering technologies must operate on a massive scale, and the scale-up process often reveals hidden costs. Radosz says he and his colleagues are working to make the process even cheaper by creating novel activated carbons that are even more selective at separating out CO2 and have an even higher capacity.