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Scientifically Speaking | Can we stop the climate crisis by sucking up CO2?

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To solve the climate crisis, the world will need to reduce the emission of carbon dioxide into the atmosphere. This will require a rethink in all aspects of life because carbon is the backbone of the global economy. 

Last year, Larry Fink, the CEO and chairman of BlackRock, the world’s largest asset management company, predicted that addressing the climate crisis would become the next big business opportunity. “It is my belief that the next 1,000 unicorns — companies that have a market valuation over a billion dollars — won’t be a search engine, won’t be a media company, they’ll be businesses developing green hydrogen, green agriculture, green steel and green cement,” said Fink.

It is cheaper to not add carbon dioxide to the atmosphere than to pull it out, and mitigation will remain essential to any plan to decarbonise the world. The sooner we can decarbonise the economy by shifting to renewable energy, the better. However, since the Industrial Revolution, we have already added a lot of carbon dioxide to the environment. The fact is there’s no one-sized solution or a switch to flip to immediately transition the world: We will need multiple technologies deployed rapidly at scale to address the greatest challenge that we have ever faced. 

One of these ideas is carbon capture in which carbon dioxide released from the burning of fossil fuels is stopped from escaping from smokestacks into the atmosphere.  In contrast to this kind of carbon capture at an industrial point source is direct air capture which is essentially pulling carbon dioxide out from the air. Capturing carbon dioxide is only half the battle since it needs to be concentrated and put to some industrial use or geologically stored somewhere where it can’t escape back into the atmosphere.

Trees are an excellent store of carbon and planting them is an easy thing to do. However, maintaining trees requires care and manpower, and the carbon stored in them, like in many natural reservoirs, is less permanent. Forest ecosystems are dynamic environments and the actual amount of carbon captured depends on the types of trees and the maturity of the forest.

On the other hand, technological solutions that pull carbon dioxide from the air are very costly in terms of the capture of carbon dioxide, but they may be more stable in terms of the geological storage of carbon. Once carbon capture plants are set up and working at scale, they tend to require fewer people for upkeep.

I’ve reviewed some of the most innovative carbon capture pilot plants and the technologies they employ. First, the good news. The chemistry of carbon capture is simple enough to understand.

For example, one plant run by the Canadian company Carbon Engineering uses large fans to draw in air to a wet contactor containing potassium hydroxide. Carbon dioxide reacts with this alkaline solution to yield potassium carbonate and water. In a subsequent reaction, solid pellets of calcium carbonate are formed upon reaction with calcium hydroxide. Carbon dioxide is captured and released for storage by heating up the carbonate in oxygen and natural gas. It’s a closed system that can be explained to a high-school chemistry class.

In fact, carbon capture happens naturally too. The White Cliffs of Dover in England are made up of chalk, which is a store of carbon dioxide that has occurred over a long time. 

But here’s the problem. We do not have millions of years to capture all the carbon dioxide we have already released. Billions of tonnes of carbon dioxide will need to be removed from the atmosphere to prevent a climate catastrophe. 

This brings me to the second issue. The concentration of carbon dioxide is increasing in the atmosphere. However, carbon dioxide still only makes up only a fraction of air. Direct air capture plants run huge fans that suck in a lot of air and force it onto liquid or solid contactors that filter out carbon dioxide. Then, carbon dioxide is removed from the filters so they can be used again. A massive amount of energy is required to run the fans and to regenerate the materials for reuse. Passive flow of air would draw less energy, but it would be too slow to capture sufficient carbon dioxide to make a big dent.

Revealingly, a scientific article by Ryan Long-Innes and Henning Struchtrup of the University of Victoria in Canada that was published in Cell Reports Physical Science on March 16 found that most of the energy supplied to a Carbon Engineering direct air capture plant isn’t used productively. Only about 8% of energy supplied is used for the separation of carbon dioxide from atmospheric air, the rest is lost mainly as heat. It turns out that when trying to suck out carbon dioxide from the air, thermodynamics is not our friend. 

A further challenge is that natural gas is used to run the direct air capture process. The use of renewable energy sources would require technological modifications to the plant but would result in sustainable operations. In an ideal future, we will not need to use fossil fuels to capture carbon dioxide released from burning fossil fuels. 

Then there’s the question of cost. Carbon Engineering’s process costs between $94 and $232 per ton of carbon dioxide captured. Another company, Climeworks has a plant that captures carbon dioxide for around $600 per ton. In the United States, there’s a tax credit for companies at around $50 per ton of carbon dioxide. For comparison, some forests are estimated to capture carbon dioxide at under $50 per ton, but it’s not an apples-to-apples comparison. A tree might live for a hundred years; geologically stored carbon could well stay for thousands.

If rebates are the carrot, then governments also have the stick of carbon taxes on industries that emit greenhouse gases into the atmosphere. In addition, companies may be able to sell carbon dioxide for use in making products in a carbon marketplace. Right now, fresh fossil fuels are the main source for many of these materials, but that could change in the future.   

That said, we will not be able to reuse most of the carbon dioxide we pull from the air.  Instead, we will need to force it underground or under the sea in places where it can’t leak into the atmosphere. There is precedent here and Norway recently greenlit a proposal to inject millions of tonnes of carbon dioxide into the North Sea. 

Overall, while the technology for direct air capture shows that it is possible to pull carbon dioxide from the air, improvements are needed. Some of these improvements might come from scaling up plants. Others will have to come from more efficient design of systems for greater energy conservation.

With the right incentives in place, innovation will spur better and cheaper ways to suck out carbon dioxide from the air and store it. But right now, capturing carbon dioxide costs more than not releasing it in the first place. 

Anirban Mahapatra, a scientist by training, is the author of COVID-19: Separating Fact from Fiction

The views expressed are personal

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