13 March 2008—Last week, a power plant operated by Milwaukee-based We Energies became the first to begin capturing and sequestering carbon dioxide from its exhaust with the sole purpose of keeping the planet-warming gas out of the atmosphere. It uses a new chilled-ammonia technology developed by French power equipment company Alstom Power. But successor technologies have recently emerged that could make scrubbing carbon dioxide from smokestacks (the most expensive part of the process) much cheaper. In the past few weeks, research groups have reported of materials that can accumulate enormous volumes of carbon dioxide on their surfaces and can also be easily reused.
Carbon capture and sequestration involves absorbing the carbon dioxide in the plant’s exhaust, separating the carbon dioxide from the captured material—so the sorbent can be reused—and finally, compressing the gas and storing it. Right now, the first step, capturing carbon, makes up three-fourths of the total cost.
The current state-of-the-art materials for soaking carbon dioxide, borrowed from the chemical industry, are amine-water solutions. Amines quickly absorb carbon dioxide, but separating the carbon dioxide from the amine requires a great deal of heat. ”That heat comes primarily from steam that the plant would normally use to drive the turbine to produce electricity,” says Thomas Feeley, a technology manager at the Department of Energy’s National Energy Technology Laboratory (NETL), in Pittsburgh.
The final step, compressing the gas after it’s removed, requires electricity. Together, capturing and compressing carbon dioxide using amines can nearly double the price of the electricity a plant produces from 4.9 U.S. cents to 9 cents per kilowatt-hour, according to an NETL study. ”We’ve seen that 30 to 40 percent of plant-generating capacity goes to operating carbon dioxide capture,” Feeley says.
Alstom’s chilled-ammonia process should, by contrast, use about 10 percent of a plant’s output power, according to preliminary studies by the nonprofit Electric Power Research Institute. In the process, the flue gas is first cooled to about 5 C, which increases carbon dioxide concentration and condenses the water out of the flue gas. The water is removed along with other contaminants such as sulfur dioxide. The remaining flue gas is nearly pure CO2, which can be easily absorbed by the ammonia.
But it’s the next step that really saves energy. ”Amines require a lot of high-quality steam to strip [carbon dioxide],” says Alstom’s Robert Hilton. In contrast, ”ammonia doesn’t absorb the carbon dioxide quickly but gives it up easily.” So the Alstom process needs less heat and ”can use waste heat from the power plant,” Hilton says.
The company’s pilot demonstration in Wisconsin is small—the process will capture less than 1 percent of the plant’s carbon dioxide emissions, about 18 000 metric tons a year. By the end of 2008, the company plans to install a larger commercial-scale system that will trap and sequester 100 000 metric tons of carbon dioxide a year at American Electric Power’s 1300-megawatt plant in New Haven, W.Va.
Feeley says that chilled ammonia is among a handful of technologies that ”are some of the more promising approaches to capturing carbon dioxide from coal-fired power plants.” The NETL is studying ammonia capture along with solid adsorbents, which accumulate carbon dioxide on their surfaces. These include solid amine�based adsorbents and porous crystalline materials called metal-organic frameworks (MOFs).
Researchers have recently reported advances in both of these materials. In the 15 February issue of Science, UCLA researchers led by chemist Omar Yaghi described MOF-related materials that can hold 80 times their volume of carbon dioxide. These materials are extremely porous and have large surfaces where carbon dioxide molecules can attach. Moreover, they release carbon dioxide with a small pressure change, a key advantage since it should not require much energy.
The other advance builds on conventional amine technology. Georgia Tech researchers have made solid-amine adsorbents by attaching amine polymers to a silica substrate. The material, presented in an online report in the Journal of the American Chemical Society on 19 February, soaks five times as much carbon dioxide as currently available solid adsorbents. Making it is an easy one-step process—the researchers mix the silica materials and the polymer precursor with a catalyst at room temperature.
Amine solutions are already known to be good carbon dioxide scrubbers, says chemical and biomolecular engineering professor Christopher Jones, who led the Georgia Tech work. But compared to amine solutions, separating the carbon dioxide from a solid material takes less energy. ”Water needs large energy to heat it up through 1 C,” he says. ”Solids have a lower heat capacity, so there is less energy penalty.”
Unlike the chilled-ammonia technology, methods that use these new materials have yet to prove themselves in an actual power plant. Further testing will show just how much energy they save.
The NETL’s goal is to retrofit existing power plants with systems that capture 90 percent of the carbon dioxide without raising the cost of electricity by more than 20 percent. Feeley says the NETL hopes to do this by 2020. But if all goes as planned with Alstom’s technology, that goal could be met as early as 2011.
About the Author
Prachi Patel-Predd, a regular contributor to IEEE Spectrum, is a freelance writer who covers technology, energy, and the environment. She is also frequently heard on Spectrum Radio.