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Carbon capture and storage

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by Chris Woodford. Last updated: September 23, 2017.

Earth is warming, glaciers are thawing, seas are rising—and so far we're doing little or nothing to stop it. The consequences of climate change could be utterly catastrophic unless we act very soon. What can we do? One option is to switch to renewable energy that produces less carbon dioxide (CO2)—the gas most of the world's scientists consider is responsible for global warming. But that will take decades, at least, and in the meantime we're still locked into using coal, gas, and oil and producing even more CO2. Another option is to keep burning fossil fuels but to prevent the CO2 they release from entering the atmosphere. That idea is called carbon capture and storage (CCS) and it's currently one of the hottest topics in the field of energy technology. What is it and how does it work? Let's take a closer look!

Photo: Engineers are certain we could stop fossil-fueled power plants from releasing carbon dioxide, but is it the quickest and most cost-effective way to tackle climate change? Photo courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

What's the problem with carbon dioxide?

Let's quickly recap on global warming and climate change. The current scientific consensus is that they're "very likely" happening because people are using huge quantities of fossil fuels such as coal in power plants and oil in cars and trucks. Burning carbon-rich fuels releases very handy supplies of energy, but it involves a chemical reaction called combustion that lets carbon dioxide (CO2) gas escape into the atmosphere as a byproduct. This invisible (and mostly harmless) gas builds up and wraps around our planet like a blanket, making it hotter than it would otherwise be. Now a warming planet might sound "cool" if you're currently shivering away in winter, but what global warming actually means in practice is a major change in Earth's climate that will make the planet much harder for most people to live on. Find out more in our main article on global warming and climate change.

Photo of coal-fired electricity generating power plant.

Photo: The world has thousands of power plants like this one, many of them burning coal and belching carbon dioxide into the air. Photo by Warren Gretz courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

What is carbon capture and storage?

If you're got a power plant burning mountains of coal and releasing clouds of carbon dioxide into the air through its smokestacks, solving the problem sounds simple in theory. All you have to do is catch the CO2 as it drifts upward through the pipe and stick it somewhere it won't cause any problems (like underground or deep in the ocean). This idea is called carbon capture and storage (CCS). It's an artificial version of carbon sequestration, which is what plants and trees do naturally during photosynthesis: powered by sunlight, they suck carbon dioxide from the air and use it to build their roots, shoots, and leaves. As the name suggests, CCS involves two separate processes—carbon capture and carbon storage—so let's look at each of those in turn.

Capturing carbon

There are essentially three ways to capture the carbon dioxide from a power plant: before the fuel is burned (precombustion), after the fuel is burned (postcombustion), or by burning the fuel in more oxygen and storing all the gases produced as a result (oxyfuel).

Precombustion

Diagram illustrating the process of precombustion CCS.

Artworks: Precombustion carbon capture and storage removes carbon from coal before it gets to a power plant.

In precombustion, the aim is to remove the carbon from coal fuel before it's burned. The coal is reacted with oxygen (O2) to make syngas (synthesis gas), a mixture of carbon (CO) monoxide and hydrogen (H2) gases. The hydrogen can be removed and either burned directly as fuel or compressed and stored for use in fuel-cell cars. Water is added to the carbon monoxide to make carbon dioxide (which is stored) and additional hydrogen, which is added to the hydrogen previously removed.

Postcombustion

Diagram illustrating how CCS works after the carbon fuel has been burned.

Artworks: Postcombustion carbon capture and storage removes carbon dioxide after the carbon has been burnt in a power plant.

In postcombustion, we're trying to remove carbon dioxide from a power station's output after a fuel has been burned. That means waste gases have to be captured and scrubbed clean of their CO2 before they travel up smokestacks. The scrubbing is done by passing the gases through ammonia, which is then blasted clean with steam, releasing the CO2 for storage.

Oxyfuel (oxycombustion)

CCS would be much easier if power plants produced pure CO2 as their smokestack waste. Then, instead of laboriously separating out the CO2 from other waste gases, we could trap the entire output from the smokestacks and store the lot. The trouble is that power plants don't produce pure CO2: because there's often not enough oxygen for complete combustion they produce other pollutant gases as well. One way to purify the exhaust is to blow extra oxygen into the furnace so the fuel burns completely producing relatively pure steam and CO2. Once the steam is removed (by cooling and condensing it to make water), the CO2 can be stored.

Which is best?

Each of these techniques has its advantages and disadvantages. In theory, postcombustion CCS can be applied to any power plant burning any carbon-based fuel, so it could be retrofitted (at a price) to the world's thousands of existing power plants. Precombustion and oxyfuel both alter the fuel before it enters the station and are more suitable for newly built plants. Postcombustion is the best option for cleaning up the plants we have already; precombustion and oxyfuel could help us build cleaner plants in future.

Storing carbon

Once you've captured the carbon dioxide, there's the small matter of where you store it. Carbon dioxide is a gas under everyday conditions so it takes up a huge amount of space—and we're producing it in vast quantities too. Stuffing it in a tank somewhere and closing the lid isn't really an option: the tank would probably need to be the size of a country! The best option is to turn the carbon dioxide into a liquid (so it takes up a tiny fraction as much room) and then pump it either deep underground or into the deep ocean where it will remain safely for perhaps 1000 years or more.

Storing carbon dioxide under Earth's surface is called geo-sequestration and uses things like worked out oil fields, aquifers, or other rock formations deep underground. It might sound like hugely impractical science fiction, but oil companies already routinely pump CO2 into underground rocks to help them flush oil to the surface of declining wells, so it's actually a reasonably well-understood process. Less well understood is the idea of storing CO2 in the oceans. The main problem here is that carbon dioxide reacts with water to form acid, so the oceans could become significantly more acidic with potentially devastating consequences for marine ecosystems. But another difficulty is that the CO2 would also eventually return to the atmosphere. A third option is to store CO2 by reacting it with minerals, though that requires a lot more energy.

Pros and cons of carbon capture and storage

Most people are either heavily in favor of CCS technology or heavily against. Environmentalists tend to see CCS as a distraction from the need to convert humankind quickly to renewable energy. They argue that investing in carbon capture is a waste of money when we could be putting the same investment to better use perfecting such things as building insulation, solar energy, wind turbines, tidal power—and perhaps even nuclear plants (themselves hugely controversial). Another drawback is that CCS uses considerable extra energy (increasing the coal needed by as much as 40 percent) and could double the cost of electricity; both are very unwelcome at a time when energy is becoming increasingly expensive and humans are having trouble meeting their energy needs.

Photo of bulldozer moving coal at an electricity generating power plant.

Photo: Problem or solution? "Clean coal" or "dirty pollutant"? Photo by David Parsons courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

Where opponents see coal as a problem—a filthy polluting fuel that should be left underground at all costs—supporters prefer to call it "clean coal" and see it as a part of the solution. Their argument is that the world is hugely dependent on a giant fleet of aging power plants that will continue to be operational for decades to come. According to this view, if humans must make dramatic cuts in their carbon emissions while old power plants are still running, and before other technologies can be rolled out, back-fitting CCS could be a vital way of cutting the world's overall emissions when it matters most. The latest biomass power plants are already carbon neutral, because biomass (such as fast-growing trees and straw) takes in CO2 as it grows and releases it again when it's burned. So if we fitted CCS to them, we'd effectively have a way of pulling net carbon dioxide from the atmosphere —which would be even better.

Unfortunately, despite some very promising small-scale experiments, CCS is still a very expensive and relatively untried technology and the argument is unlikely to be settled one way or the other until more work has been done to demonstrate its real costs and benefits. With that in mind, many people are eagerly watching progress at Petra Nova, currently the world's largest post-combustion CCS plant, which has been operating in Houston, Texas since 2016.

Carbon capture and storage in other industries

It's important to remember that power plants aren't the only source of smokestack carbon dioxide emissions; cement, steel, and fertilizer plants also release large amounts of CO2. Although modern plants are more efficient than older ones, significant cuts in industrial emissions can only really be achieved by applying CCS to these sorts of plants as well. In the case of cement-making, pre-combustion doesn't apply, which leaves post-combustion and oxyfuel. Several pilot schemes are currently in place around the world to see which method works best. A number of fertilizer plants are also capturing their CO2 for reuse. In 2011, for example, Chaparral Energy started piping CO2 captured from a fertilizer plant in Kansas to Oklahoma to increase oil production from its fields there.

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Text copyright © Chris Woodford 2009, 2017. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2009/2017) Carbon Capture and Storage. Retrieved from http://www.explainthatstuff.com/carbon-capture-and-storage.html. [Accessed (Insert date here)]

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