8 future technologies for carbon capture

Osamu Terasaki (left) and his team at the University of Stockholm are creating crystals designed to capture carbon in the presence of water. Image: University of Stockholm

Deployment of carbon capture and storage (CSS) technology is "not optional" if the world hopes to meet the targets set out in the Paris climate agreement, the International Energy Agency said recently.

"IEA scenario analysis has consistently highlighted that CCS will be important in limiting future temperature increases to two degrees Celsius, and we anticipate that this role for CCS will become increasingly significant if we are to move towards well below two degrees Celsius,” IEA executive director Fatih Birol wrote in the foreword to 20 Years of Carbon Capture and Storage: Accelerating Future Deployment.

Canada has three large-scale CCS projects in commercial operation, including SaskPower’s CCS facility at the Boundary Dam Power Station near Estevan, Sask., the Weyburn-Midale enhanced oil recovery projects operated by Cenovus Energy and Apache Canada, and the Shell Quest project at the Scotford oilsands upgrader near Edmonton.

While CCS operators in Canada and globally work to improve existing technologies, in laboratories around the world, scientists are working on the next wave of technologies. Here is a look at several of them.

1. Metal-organic frameworks

In recent years, a class of highly absorbent, nanoporous materials called metal-organic frameworks (MOFs) have emerged as a promising material for carbon capture in power plants.

“People are really excited about these materials because we can make a huge variety and really tune them,” says Northwestern University’s Randall Snurr. “But there’s a flip side to that. If you have an application in mind, there are thousands of existing MOFs and millions of potential MOFs you could make. How do you find the best one for a given application?”

Snurr and his group have discovered a way to rapidly identify top candidates for carbon capture—using just one per cent of the computational effort that was previously required. By applying a genetic algorithm, they rapidly searched through a database of 55,000 MOFs.

One of the identified top candidates, a variant of NOTT-101, has a higher capacity for CO2 than any MOF reported in scientific literature for the relevant conditions.

“The percentage of carbon dioxide that the MOF can absorb depends on the process,” Snurr says. “The [United States] Department of Energy target is to remove 90 per cent of carbon dioxide from a power plant; it’s likely that a process using this material could meet that target.”

With their nanoscopic pores and incredibly high surface areas, MOFs are excellent materials for gas storage. MOFs’ vast internal surface areas allow them to hold remarkably high volumes of gas. The volume of some MOF crystals might be the size of a grain of salt, for example, but the internal surface area, if unfolded, could cover an entire football field.

Snurr’s previous work has explored how to use MOFs to capture carbon from existing power plants during the post-combustion process. About 10–15 per cent of power plant exhaust is CO2; the rest is mainly nitrogen and water vapor. Snurr and his team have designed a MOF that can sort these gases to capture CO2 before it enters the atmosphere. Chemically processing the fuel before it enters the power plant can turn it into CO2 and hydrogen. After the MOF captures the CO2, the hydrogen is burned, and the only byproduct is water. This extra chemical processing step would need to be built into new power plants as a pre-combustion process.

“In places like China, where they are still building a lot of power plants,” Snurr says, “this would make a lot of sense.”

2. Nanosponges

Cornell University materials scientists have invented low-toxicity, highly effective carbon-trapping “sponges” that could improve carbon capture economics.

A research team led by Emmanuel Giannelis has invented a powder that performs as well as or better than industry benchmarks for carbon capture.

The most common carbon capture method today is called amine scrubbing, in which post-combustion, CO2-containing flue gas passes through liquid vats of amino compounds, or amines, which absorb most of the CO2. The carbon-rich gas is then pumped away—sequestered—or reused. The amine solution is extremely corrosive and requires capital-intensive containment.

The researchers have been working on a better, safer carbon-capture method since about 2008, and they have gone through several iterations. Their latest consists of a silica scaffold, the sorbent support, with nanoscale pores for maximum surface area. They dip the scaffold into liquid amine, which soaks into the support like a sponge and partially hardens. The finished product is a stable, dry white powder that captures CO2 even in the presence of moisture.

Solid amine sorbents are used in carbon capture, Giannelis says, but the supports are usually only physically impregnated with the amines. Over time, some of the amine is lost, decreasing effectiveness and increasing cost.

The researchers instead grew their amine onto the sorbent surface, which causes the amine to chemically bond to the sorbents, meaning very little amine loss over time.

3. Hybrid membranes

A new, highly permeable carbon capture membrane developed by scientists at the Lawrence Berkeley National Laboratory (Berkeley Lab) could lead to more efficient ways of separating CO2 from power plant exhaust.

The researchers focused on a hybrid membrane that is part polymer and part MOF.

In a first, the scientists engineered the membrane so that CO2 molecules can travel through it via two distinct channels. Molecules can travel through the polymer component of the membrane, like they do in conventional gas-separation membranes, or they can flow through “CO2 highways” created by adjacent MOFs.

Initial tests show this two-route approach makes the hybrid membrane eight times more CO2-permeable than membranes composed only of the polymer. Boosting CO2 permeability is a big goal in efforts to develop carbon capture materials that are energy efficient and cost competitive.

“In our membrane, some CO2 molecules get an express ride through the highways formed by metal-organic frameworks, while others take the polymer pathway. This new approach will enable the design of higher performing gas separation membranes,” says Norman Su, a graduate student in the chemical and biomolecular engineering department at the University of California, Berkeley and a user at the Molecular Foundry.

Berkeley Lab scientists have developed a hybrid membrane where MOFs account for 50 per cent of its weight, which is about 20 per cent more than other hybrid membranes. Previously, the mechanical stability of a hybrid membrane limited the amount of MOFs that could be packed in it.

“But we got our membrane to 50 weight per cent without compromising its structural integrity,” says Su.

And 50 weight per cent appears to be the magic number. At that threshold, there are so many MOFs in the membrane that they form a continuous network of highways through the membrane. When that happens, the hybrid membrane switches from having a single channel to transport CO2, in which the molecules must go through the polymer, to two channels, in which the molecules can either move through the polymer or through the MOFs.

“This is the first hybrid polymer-MOF membrane to have these dual transport pathways, and it could be a big step toward more competitive carbon capture processes,” says Su.

4. Crystals

Swedish scientists have created crystals that capture CO2 much more efficiently than previously known materials, even in the presence of water.

One way to mitigate climate change could be to capture CO2 from the air. So far, this has been difficult since the presence of water prevents the adsorption of CO2. Complete dehydration is a costly process. Scientists have now created a stable and recyclable material where the micro-pores within the crystal have different adsorption sites for CO2 and water.

“As far as I know, this is the first material that captures CO2 in an efficient way in the presence of humidity. In other cases, there is competition between water and CO2, and water usually wins. This material adsorbs both, but the CO2 uptake is enormous,” says Osamu Terasaki, a professor in the department of materials and environmental chemistry at Stockholm University.

The new material is called SGU-29, named after Sogang University in South Korea, and is the result of international cooperation. It is a copper silicate crystal. The material could be used for capturing CO2 from the atmosphere and especially to clean emissions.

“CO2 is always produced with moisture, and now we can capture CO2 from humid gases. Combined with other systems that are being developed, the waste carbon can be used for new valuable compounds. People are working very hard, and I think we will be able to do this within five years. The most difficult part is to capture CO2, and we have a solution for that now,” says Terasaki.

5. Turning carbon to rock

An international team of scientists report they may have found a potentially permanent way to remove CO2 emissions from the atmosphere—turn it into rock.

The study, published in Science, has shown for the first time that CO2 can be permanently and rapidly locked away from the atmosphere by injecting it into volcanic bedrock. The CO2 reacts with the surrounding rock, forming environmentally benign minerals.

Until now, it was thought that this process would take several hundreds or thousands of years and is therefore not a practical option. But the current study—led by Columbia University, the University of Iceland, the University of Toulouse and Reykjavik Energy—has demonstrated that it can take as little as two years.

Juerg Matter, the lead author and associate professor in geoengineering at the University of Southampton, says: “Our results show that between 95 and 98 per cent of the injected CO2 was mineralized over the period of less than two years, which is amazingly fast.”

The gas was injected into a deep well at the study site in Iceland. As a volcanic island, Iceland is made up of 90 per cent basalt, a rock rich in elements required for carbon mineralization, such as calcium, magnesium and iron. The CO2 is dissolved in water and carried down the well. On contact with the target storage rocks at 400–800 metres under the ground, the solution quickly reacts with the surrounding basaltic rock, forming carbonate minerals.

“Carbonate minerals do not leak out of the ground, thus our newly developed method results in permanent and environmentally friendly storage of CO2 emissions,” says Matter, who is also a member of the University’s Southampton Marine and Maritime Institute and an adjunct senior research scientist at Lamont-Doherty Earth Observatory at Columbia. “On the other hand, basalt is one of the most common rock type on Earth, potentially providing one of the largest CO2 storage capacity.

“The overall scale of our study was relatively small. So, the obvious next step for CarbFix is to upscale CO2 storage in basalt. This is currently happening at Reykjavik Energy’s Hellisheiđi geothermal power plant, where up to 5,000 tonnes of CO2 per year are captured and stored in a basaltic reservoir."

The investigation is part of the CarbFix project, a European Commission– and Department of Energy–funded program to develop ways to store anthropogenic CO2 in basaltic rocks through field, laboratory and modelling studies.

6. Turning carbon into fuel

They’re making fuel from thin air at the University of Southern California’s Loker Hydrocarbon Research Institute.

For the first time, researchers there have directly converted CO2 from the air into methanol at relatively low temperatures.

The work—led by G.K. Surya Prakash and George Olah from the chemistry department at USC Dornsife—is part of a broader effort to stabilize the amount of CO2 in the atmosphere by using renewable energy to transform the greenhouse gas into its combustible cousin, attacking global warming from two angles simultaneously. Methanol is a clean-burning fuel for internal combustion engines, a fuel for fuel cells and a raw material used to produce many petrochemical products.

“We need to learn to manage carbon. That is the future,” says Prakash, the director of the Loker Hydrocarbon Research Institute.

The researchers bubbled air through an aqueous solution of pentaethylenehexamine, adding a catalyst to encourage hydrogen to latch onto the CO2 under pressure. They then heated the solution, converting 79 per cent of the CO2 into methanol. Though mixed with water, the resulting methanol can be easily distilled, Prakash says.

The new process was published in the Journal of the American Chemical Society. Prakash and Olah hope to refine the process to the point that it could be scaled up for industrial use, though that may be five to 10 years away.

“Of course it won’t compete with oil today, at around $30/bbl,” Prakash says, “but right now we burn fossilized sunshine. We will run out of oil and gas, but the sun will be there for another five billion years. So we need to be better at taking advantage of it as a resource.”

Despite its outsized impact on the environment, the actual concentration of CO2 in the atmosphere is relatively small—roughly 400 parts per million or 0.04 per cent of the total volume, according to the National Oceanographic and Atmospheric Administration. (For a comparison, there’s more than 23 times as much argon in the atmosphere, which still makes up less than one per cent of the total volume.)

Previous efforts have required a slower multistage process with the use of high temperatures and high concentrations of CO2, meaning that renewable energy sources would not be able to efficiently power the process, as Olah and Prakash hope.

The new system operates at around 125–165 degrees Celsius, minimizing the decomposition of the catalyst, which occurs at 155 degrees Celsius. It also uses a homogeneous catalyst, making it a quicker “one-pot” process. In a lab, the researchers demonstrated that they were able to run the process five times with only minimal loss of the effectiveness of the catalyst.

7. Turning carbon into fibres

Finding a technology to shift CO2 from a climate change problem to a valuable commodity has long been a dream of many scientists and government officials. Now, a team of chemists says they have developed a technology to economically convert atmospheric CO2 directly into highly valued carbon nanofibres for industrial and consumer products.

“We have found a way to use atmospheric CO2 to produce high-yield carbon nanofibres,” says Stuart Licht, who leads a research team at George Washington University. “Such nanofibres are used to make strong carbon composites, such as those used in the Boeing 787 Dreamliner, as well as in high-end sports equipment, wind turbine blades and a host of other products.”

Previously, the researchers had made fertilizer and cement without emitting CO2, which they reported. Now, the team says their research could shift CO2 from a global-warming problem to a feedstock for the manufacturing of in-demand carbon nanofibres.

Licht calls his approach “diamonds from the sky.” That refers to carbon being the material that diamonds are made of and also hints at the high value of the products, such as the carbon nanofibres, that can be made from atmospheric carbon and oxygen.

Because of its efficiency, this low-energy process can be run using only a few volts of electricity, sunlight and a whole lot of CO2. At its root, the system uses electrolytic syntheses to make the nanofibres. CO2 is broken down in a high-temperature electrolytic bath of molten carbonates at 750 degrees Celsius. Atmospheric air is added to an electrolytic cell. Once there, the CO2 dissolves when subjected to the heat and direct current through electrodes of nickel and steel. The carbon nanofibres build up on the steel electrode, where they can be removed, Licht says.

To power the syntheses, heat and electricity are produced through an extremely efficient hybrid solar-energy system. The system focuses the sun’s rays on a photovoltaic solar cell to generate electricity and on a second system to generate heat and thermal energy, which raises the temperature of the electrolytic cell.

Licht estimates electrical energy costs of this “solar thermal electrochemical process” to be around $1,000/ton of carbon nanofibre product, which means the cost of running the system is hundreds of times less than the value of product output.

“We calculate that, with a physical area less than 10 per cent the size of the Sahara Desert, our process could remove enough CO2 to decrease atmospheric levels to those of the pre-industrial revolution within 10 years,” he says.

At this time, the system is experimental, and Licht’s biggest challenge will be to ramp up the process and gain experience to make consistently sized nanofibres. “We are scaling up quickly,” he adds, “and soon should be in range of making tens of grams of nanofibres an hour.”

Licht explains that one advance the group has recently achieved is the ability to synthesize carbon fibres using even less energy than when the process was initially developed. “Carbon nanofibre growth can occur at less than one volt at 750 degrees Celsius, which for example, is much less than the three to five volts used in the 1,000-degree-Celsius industrial formation of aluminum," he says.

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