Scientists at the Lawrence Livermore National Laboratory (LLNL), Harvard University and the University of Illinois at Urbana-Champaign have created an advance in carbon dioxide (CO₂) capture using baking soda found in most grocery stores.

The team developed a type of carbon capture media composed of core-shell microcapsules. These microcapsules consist of a highly permeable polymer shell and fluid (made up of sodium carbonate solution) that reacts with and absorbs CO₂. Sodium carbonate is the main ingredient in baking soda. The capsules keep the liquid contained inside the core, and allow the CO₂ gas to pass back and forth through the capsule shell.

To date, microcapsules have been used for controlled delivery and release (for example, pharmaceuticals, food flavoring, cosmetics, agriculture, and so on)—but this is the first demonstration of using this approach for controlled CO₂ capture and release.

Researchers Will Smith and John Vericella look at microcapsules that can be used to capture carbon dioxide from coal or natural gas-fired power plants, as well as in industrial processes like steel and cement production. Photo by Julie Russell/LLNL.However, the technology to capture carbon dioxide from power plant flue gas has existed for decades that it is rarely used due to cost and environmental concerns. The newly developed microcapsules offer the promise of a less expensive and more environmentally friendly way of preventing CO₂ from reaching the atmosphere.

Corrosiveness also is improved because the chemical is more benign and always is encapsulated. Putting the carbonate solution inside of the capsules allows it to capture CO₂ without making direct contact with the surface of equipment in the power plant.

New Look at an Old Problem

The state-of-the-art method for removing flue gas is carbon scrubbing. The process involves chilling the post-combustion flue gas and adding a solvent such as monoethanolamine (MEA) to absorb the carbon and form a solid compound. The compound is then separated from the other gases. Reheating it releases the carbon, which can then be used in industry or sequestered. The MEA process is prohibitively expensive and utilities would not invest in a process that they are not mandated to pursue.

Roger Aines, the Carbon Fuel Cycle Program leader at LLNL, had a concern about the MEA process; if some of the chemical escapes into the atmosphere it can turn into nitrosamine, a combination of nitrate and amine that is a potent carcinogen. He says that although relatively little nitrosamine is created in the process, if thousands of power plants are emitting it “you’re putting out pollution that you don’t want to be putting out.”

He knew that using a carbonate process to remove CO₂ was a fundamentally better method than MEA on both an environmental and energy efficiency basis. However, the reaction was too slow and too much calcium carbonate was needed to capture the CO₂. Aines and Chris Spadaccini, engineers in LLNL’s Materials Engineering Division sought to speed up the reaction by increasing the surface area of the calcium carbonate capture process.

Carbon-Eating Capsules

To do so, they turned to 0.5-millimeter-diameter capsules that are made of a polymer shell and that pass CO₂ and a solution of baking soda and water. In a power plant flue, the microcapsules are intended to be placed after the plant’s existing pollution equipment. Aines says that the flue gas lifts the capsules as the stream comes through “and the whole thing kind of bubbles and roils as the gas goes through it [and is captured.”]

He says the “magic” that makes the microcapsules work effectively is that the droplets are so small that they have a large surface area per weight. The researchers made tiny droplets which, if they were not held in the polymer, would form into much bigger droplets.

“It’s all about surface area,” he says. “The capsules force the baking soda to stay in little tiny droplets (an order of magnitude smaller than a drop of amines would take on), and little drops react faster because they contact more of the CO₂.”

One challenge was to find a polymer coating that allowed the CO₂ to pass freely. “You don’t want any resistance to carbon dioxide going across the shell.” It took a while to get the polymer coating right, he says. “It’s WACKER silicone,” which is similar to commercially available thermal set silicone, but in this case set with a UV catalyst.

Microcapsules containing sodium carbonate solution are suspended on a mesh during carbon dioxide absorption testing. The mesh allows many capsules to be tested at one time while keeping them separated, exposing more of their surface area. Photo by John Vericella/LLNL.The microcapsules work well with a polymer thickness of 20 to 30 microns. Aines says that the team would like to make the capsules even thinner to reduce the amount of “wasted space” in the reactor.

When the capsules are full, all of the nitrogen and oxygen will have been replaced by CO₂. Heating the capsules to about 80 degrees Celsius allows the recovery of pure CO₂.

Once the capsules have surrendered their carbon, they are ready to capture more. “Carbonate’s advantage is that it just stays in that form,” he says. “We’ve reused them 100 times.” The researchers hope to have them be reusable a few thousand times.

Flue Gas Placement

He has two ideas about how the balls will be situated in the flue. “One is basically a fluidized bed reactor where you have a great big bed of these things,” he says. The balls would move about freely as the gas passed through them, similar to a child’s ball-filled jumping game.

The second method involves making a fixed bed out of the microcapsules. In this case, the microcapsules would be glued together so they do not move as the gas passes through. However, doing so would entail using possibly thousands of tons of capsules that might fill a structure—the size of a farm silo.

Reasonable Solution

Matthew Realff, a professor of chemical and biomolecular engineering and the associate director of the Strategic Energy Institute at Georgia Tech University, says that the microcapsules “seem like a fairly reasonable solution”, but raises issues that the researchers may need to consider.

The first is the pressure drop that occurs as the gas flows through the microcapsules. Many flue gas systems “don’t have a lot of additional pressure drop to waste on blowing this stuff through fluidized beds,” says Realff. Providing energy to move the gas through the bed could result in a “fairly sizable energy penalty for doing this.”

A second issue is how the research team plans to recapture the heat used to extract the CO₂ from the capsules. Realff says that MEA systems use heat exchangers to remove the energy from the regenerated solvent. He wonders how the process obtains the energy required to do the regeneration. “You don’t want to just cool the beads back down. You have to recover that energy and use it in your regeneration system,” Realff says.

Remaining Challenges

Several challenges remain before the microcapsules can be brought to market, which Aines estimates may be around 2020. The first is to scale up production. Currently, the capsules are produced in “jelly jar quantities,” he says. The goal is to produce them in kilogram quantities as soon as September 2015.

Once sufficient quantities have been produced, the next milestone will be to test them in a pilot project, which the Department of Energy is funding. That project is scheduled to last two years.

A final challenge will be to make the technology affordable. The researchers are working to limit the amount of equipment needed for the process to perform and beat the cost of an MEA system by 50%.

Aines says it is early to be talking about a technology that is still in its infancy.

“If you run a power plant that puts out a gigawatt of electricity and I’m doing tests on a jelly jar worth of stuff,” Aines says, “there’s not a lot of cross talk between those two yet.”