Capturing CO₂ to Help the Earth

Fossil fuels supply more than 75% of the world’s energy and more than 70% of our nation’s energy.  However, burning them to generate electricity produces about one-third of our Nation’s emissions of carbon dioxide (CO₂), believed to be a significant contributor to climate change. Since only a small percentage of power plant exhaust gas is CO₂, separation and capture of CO₂ can be very expensive. To curb global warming, researchers at NETL are developing cost-effective technologies to capture the CO₂ and significantly reduce their emission from power plants.

	NETL scientist, Thomas Simonyi, testing CO2 sorbents in a thermo-gravimetric analyzer.

NETL is investigating alternative CO₂ capture and separation technologies, including sorbents, solvents, and membranes. Sorbents are materials that can “adsorb” substances such as CO₂, which means that the CO₂ clings to the surface of the sorbent. One sorbent developed at NETL requires 30-50% less energy to regenerate the sorbent for reuse than a conventional aqueous amine sorbent system. The potential cost savings of this clay-based sorbent garnered it an R&D 100 Award from R&D Magazine, as one of the 100 most technologically significant products introduced into the marketplace during the previous year. Research on these regenerable sorbents and their overall performance in flue gas has already led NETL scientists to investigate other, possibly more cost-effective, sorbent candidates.

Clay-based sorbent.

Another novel approach is to use ionic liquids rather than amine solutions to capture the CO₂. Ionic liquids are odd; they are organic salts that, in many ways, are similar to common inorganic table salt (NaCl).  The reason these ionic salts are liquid at room temperature lies in the shape of their ions.  The sodium (Na) and chlorine (Cl) that make up table salt are small, spherical ions.  Their compact shape allows them to stack very efficiently into cubic crystal structures, so that they only melt at high temperatures. But unlike table salt, ionic liquids contain large organic ions with much more complicated shapes that cannot stack as efficiently, so they become liquid at much lower temperatures.

	Sorbent reactor studies: Ken Champagne is operating the lab-scale pack bed reactor system.

The simplest way to capture CO₂ using an ionic liquid is to use liquid scrubbing, where CO₂ dissolves in the liquid, which is then transferred to another tank and heated. The rising temperature causes CO₂ gas to escape from the liquid, where it can easily be captured. The gas is then compressed and forced into liquid form to be pumped deep underground for long-term storage. Another way to capture CO₂ using ionic liquids is to convert the liquid into a membrane. These ionic membranes act like selective gates, allowing CO₂ to pass through, while keeping out all other gases. When these thin membrane films are formed into thin hollow fibers, as seen below, CO₂ separates from the flue gas, outside of the fibers and moves into the center of the fibers, where it can be siphoned away for compression and sequestration.

For more information on our carbon sequestration work, please visit the following links.

• Carbon Sequestration - CO₂ Capture
• Ten Projects Selected by DOE to Advance State-of-the-Art Carbon Capture from Coal Power Plants
• Technologies - Carbon Sequestration
• CO₂ Reuse Photocatalysts One of Top 10 Most Accessed Papers
• NETL Explores Potential Beneficial Uses of Captured CO₂

Hollow membrane fibers made out of an ionic liquid.		Scanning electron microscopy image of an ionic liquid membrane fiber.

Other Ways to Capture CO₂

Most of our nation’s electricity is created by burning coal. Coal-burning power plants mix pulverized coal with air and burn it in a boiler. The water in tubes in the boiler’s walls boils and becomes steam, which rises and turns turbines that generate electricity. During coal combustion, carbon, the most abundant element in coal, is converted into carbon dioxide (CO₂) and released into the atmosphere—a possible cause of global climate change.

	Casey Carney, an NETL researcher, adjusts a monochromator, which can be focused on very small bands of light wavelengths. Different gases radiate in different wavelengths of visible and infrared light. By measuring the intensity at various wave bands, Casey can determine how the gases in an oxygen and coal flame change the amount of heat getting to the boiler walls.

NETL is researching CO₂ capture methods to generate cleaner electricity while scientists develop more efficient non-carbon-based energy sources. Because separating CO₂ from other exhaust gases is so expensive, one option involves redesigning power plants to burn coal with oxygen rather than air. “Oxy-combustion” or “oxy-fuel” forms a gas of mostly CO₂ and steam during combustion, making CO₂ capture much easier than in current methods. NETL and several partners are developing oxy-combustion coal-burning power plants, with large-scale tests already underway. Because of oxy-combustion’s higher heat output, boilers—designed to burn coal with air—need a different design or operating approach. Researchers study oxy-fuel flame properties and heat transfer rates at a range of temperatures and gas compositions in NETL’s oxy-fuel burner lab to help power plant designers ready these plants for oxy-combustion.

NETL researchers are also collecting flame and heat transfer data to provide information for engineering software packages to help design new oxy-combustion applications and identify research needed to further this particular technology.  Finding an efficient, low-cost way to separate oxygen from air is still an issue, though.  Researchers are looking at alternatives to commercial separation of oxygen from air, which consumes a great deal of energy. One possibility involves transition metal complexes, using reversible binding of oxygen—basically attracting oxygen to a plate and increasing the temperature to loosen the oxygen for collection.

NETL researcher Ranjani Siriwardane analyzes CO2 sorbents and oxygen carriers in an x-ray photo-electron spectrometer.

Another option is chemical looping combustion (CLC), which also burns coal in oxygen. However, CLC supplies the oxygen using a metal oxidized in air to carry oxygen to where the coal is burned. The byproducts of CLC are also CO₂ and water, but in CLC, the metal oxidation/reduction process is repeated in a cyclic loop. CLC does not require air separation. Once the steam is condensed, a relatively pure stream of sequestration-ready CO₂ can be produced, without any additional separation. The technology thus avoids the energy penalty that traditional fossil fuel-fired combustors must pay to produce either nearly pure oxygen or CO₂.