Like any good housekeeper, sweeping up the dirt and sending it to a landfill keeps the house tidy and clean. Researchers interested in tidying up the earth's atmosphere are devising ways to sweep up excess carbon dioxide -- a potentially problematic greenhouse gas -- and store it out of harm's way.
Through the Fossils Energy Program, the Department of Energy is funding research projects that explore unique, cost-effective ways to sequester newly produced carbon dioxide before it hits the atmosphere. Ultimately, the DOE wants to develop methods that cost only $10 per ton of carbon, equivalent to adding a fraction of a cent per kilowatt-hour to the cost of electricity.
Researchers at the Idaho National Engineering and Environmental Laboratory, in collaboration with universities and other national laboratories, have been awarded almost $1.4 million to pursue three projects that pull carbon dioxide out of various gaseous streams such as smokestack exhaust or natural gas production lines, before the greenhouse gas has a chance to mix in with the air.
Sequestering greenhouse gases might reduce their impact on global warming. "If fossil energy is beginning to turn the climate in a different direction," says INEEL's Dick Rice, manager of this lab's fossil energy technologies department, "than we need to start looking at carbon dioxide management."
One way to manage that carbon dioxide is to remove it from sources before it hits the air. "If you can get carbon dioxide out of an exhaust stack," Rice says, "that might be a way to mitigate its effect on the environment. The earth can heal itself -- we're just giving it too much to handle."
While these grants are examining ways to remove the carbon dioxide from sources, other research is examining ways to store the gas after its collection -- such as by injecting it into old mines or into briny aquifers, or by replacing drilled oil with the gas.
These INEEL carbon sequestration projects include developing membranes for filtering carbon dioxide from methane and gas streams, developing a vortex tube to spin out carbon dioxide from exhaust streams, and improving the ability of algae to remove carbon dioxide from power plant exhausts.
Tornado in a Tube
In order to capture carbon dioxide from a power plant's smokestack, the greenhouse gas needs to be separated from the other flue gases. Typically, carbon dioxide makes up anywhere from 2 percent to 70 percent of the flue exhaust. To separate the gas, INEEL researchers are investigating an old technology in a new way -- the vortex tube.
Used for separating gases based on the size of their molecules, a vortex tube spins gas at a high centrifugal force -- this causes gases to move at different speeds through the tube. "The vortex is sort of like a tornado in a tube. It's a unique beast," says chemical engineer Kevin Raterman, one of two INEEL researchers attempting to improve the tube for this application.
Raterman and his colleagues at INEEL, Purdue University and several industrial companies are trying to improve the efficiency of a vortex tube by at least 50 percent and make the process more cost efficient. They'll experiment first with a natural gas stream, which usually contains about 8 percent carbon dioxide, and then adapt the system to the exhaust streams you might find on a power plant.
"We need a pressurized gas stream, so it makes sense to start with natural gas since it's already pressurized," said Raterman. They're hoping to develop a prototype carbon dioxide separator by the end of their three-year, $750,000 grant. By mixing liquid in with the gas in the vortex, they expect a large jump in the efficiency of the separation. Making the gas travel through liquid will separate the gas molecules more efficiently than through the air typically found in a vortex tube.
Their goal is to create a one-tube separating process that can process 30 million cubic feet of gas per day. Although they will initially work with a natural gas stream, the team says the technology should be easily converted to power plant flue streams. "The technology will be strictly applicable," says Raterman, "but practical details will have to be worked out."
The Gas-Eating Mat
Another team of researchers will use nature to pull carbon dioxide out of exhaust streams of power plants and convert it to useful products such as oxygen and hydrocarbons. While humans breathe in oxygen and breathe out carbon dioxide, plants do just the opposite. INEEL chemical engineer Patricia Stoots, in collaboration with the Center for Biofilm Engineering at Montana State University, is working to develop a mat of algae that will use power plant emissions of carbon dioxide for food.
"Most biological processes for carbon sequestration use microorganisms in their planktonic state -- free cells," she said. "We are going to use a biofilm mat." Stoots will optimize how much carbon dioxide the mat of cells takes in by varying conditions such as the other nutrients she feeds it. She also plans on finding ways to increase the efficiency of photosynthesis, the cellular process that transforms carbon dioxide into carbohydrates and releases oxygen at the same time.
In addition to performing her experiments at Montana State, Stoots will be enlisting civil engineering professor Calvin Abernathy at the University of Memphis to help create a durable biofilm mat. "We've worked together in the past," Stoots said. "We make a pretty effective team."
For the two-year study, Stoots and her collaborators were awarded $420,000 from the DOE's Fossil Energy Program. The study will also form the basis of Stoots' doctoral dissertation.
Stop that gas!
A third approach to carbon sequestration will exploit a membrane's ability to selectively block certain molecules. In a collaboration between Los Alamos National Laboratory, the University of Colorado and INEEL, researchers are developing a durable membrane that can separate both carbon dioxide and acid gas from streams such as natural gas.
According to Peterson, using membranes for separating small molecules from each other, such as in industrial exhaust gas, is more difficult than separating molecules of a variety of sizes. And to do this on an industrial scale creates more obstacles, including a very high temperature at which most membranes disintegrate. "It's hard to do," he says. "It's never been done before."
Membranes for use in large-scale applications require structural stability, but they also must be thin to allow molecules to flow through. "They need to be tough," says Peterson. "If you peel a piece of skin after a sunburn, that's a very thick membrane." Membranes for industrial processes must be durable and hold up to a lot of wear and tear. The membranes also need to be affordable. "What's challenging about making membranes on a large scale is that processes that take a lot of surface area are cost-prohibitive," says INEEL chemist Eric Peterson.
The goals are to create a membrane that can effectively separate carbon dioxide from other exhaust gases and to improve membrane durability by developing new molecular composites. The composites, combinations of polymers and ultra-tiny sand particles, will need to be characterized and tested. "Scaling up membranes remains one of the major challenges," Peterson says. "It is one of our hopes that this collaboration will provide a significant step toward large scale membrane applications in carbon dioxide sequestration."
Spearheading the work will be LANL, which has a major synthetic membrane initiative. The University of Colorado will characterize the membrane's transport abilities. INEEL chemists will test and evaluate the composite materials.
The INEEL is a science-based, applied engineering national laboratory dedicated to completing its environmental mission and meeting the nation's environmental, energy, nuclear science and technology needs. The INEEL is operated for the Department of Energy by Bechtel BWXT Idaho, LLC, in partnership with the Inland Northwest Research Alliance.
Media contact: Deborah Hill, 208-526-4723, dahill@inel.gov
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