How does carbon capture and storage work?
With only 11 or so years to dramatically cut our emissions and avert the worst impacts of climate change, officials are proposing big goals. California, Hawaii, and New Mexico all aim to switch to carbon-free energy by 2045. Nevada wants to follow in 2050. At the federal level, talk of a Green New Deal to switch American energy entirely to renewable sources in just 10 years continues in Congress.
A lot of the actions proposed in these plans are familiar: scaling up wind and solar power, building more public transit, and restoring natural landscapes to store carbon in trees, plants, and soil. But then there’s talk about “carbon capture and storage,” or CCS, an amorphous, futuristic-sounding technology. Some climate experts think it’s basically impossible to meet goals like keeping warming below 1.5ºC without this tool. But what exactly is it?
How it works
The basic idea is simple. Instead of allowing power or cement plants or other heavy industries to spew CO2-heavy emissions into the atmosphere, CCS would instead extract that carbon before its emitted and stow it away in the earth. But putting these technologies to work is a little more complicated.
Pre-combustion carbon capture separates out the carbon from from fossil fuels before they’re burned, allowing for cleaner energy generation. With natural gas, a chemical reaction separates the carbon from the methane molecule, creating clean-burning hydrogen gas. With coal, you first have to turn it into a gas, called syngas, and then you can filter out the carbon.
Plants with post-combustion CCS burn fossil fuels as they normally would, but then add an extra step that extracts the CO2 before it wafts away into the atmosphere. There are different ways to execute that extra step. The most common is to run the exhaust gas through aqueous amines—liquid chemicals that bind to the carbon. Later, plant operators can heat these amines to release the CO2, which they then pressurize and pipe away.
The advantage with post-combustion is that it’s adaptable, says Niall Mac Dowell, of the Clean Fossil and Bioenergy Research Group at Imperial College in London. “We can take all of the installations we have around the world, and add CCS onto them as they stand.” But post-combustion can be costly, because the carbon in the exhaust gas is dilute so it’s hard to filter it out from the other combustion byproducts. Oxicombustion tries to solve that problem. The process separates nitrogen from air before combustion, so it’s just fuel and oxygen burning. This concentrates the resulting CO2, making it easier to extract.
There are also many emerging carbon capture technologies to consider. One of the most exciting, according to Sally Benson, co-director of the Precourt Institute for Energy at Stanford University, is called the Allam cycle. It basically recycles the CO2 from combustion into a high-pressure fluid that can run a turbine, turning a waste product into power. It’s currently in use at a small demonstration power plant in Texas, which is expected to soon be able to power 5,000 homes on a hot summer day. The company operating the plant, NetPower, estimates that future commercial-scale plants could sell power for $20 per megawatt hour, taking into account tax credits currently available; now, Americans pay an average of $130 per megawatt hour. “You’re recirculating a lot of carbon dioxide and then you just take off a small amount of the carbon dioxide for storage,” says Benson. “And that results in very high efficiency and it's a completely zero emission power plant.”
The last step is stowing the greenhouse gas away. One way or another, the carbon needs to be pressurized and piped away for storage. One promising storage option is deep underground saline reservoirs, areas of porous rock saturated with salty water. These sites can keep CO2 trapped in the pores of the rocks. By putting the carbon from the extracted coal, oil, natural gas back into the ground, it’s kind of like fossil fuel mining in reverse.
But actually extracting carbon that’s already been released into the atmosphere is another story. It’s theoretically possible, but that process—direct carbon capture—isn’t up and running in the real world. One study from 2018 found that a proposed direct air capture system could remove CO2 from the air and recycle it into a fuel for between $94 and $232 a ton, but that’s probably more expensive than just avoiding emitting that ton of carbon using renewables. CO2 in the atmosphere is relatively diffuse compared to oxygen and nitrogen, so pulling it straight from the air is energy intensive.