Carbon capture and storage
by Chris Woodford. Last updated: December 15, 2019.
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?
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: The world has thousands of electricity power plants like this one, many of them burning coal and belching carbon dioxide into the air.
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.
How can we capture 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).
Artworks: Precombustion carbon capture and storage removes carbon from coal before it gets to a
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.
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.
Artworks: Postcombustion carbon capture and storage removes carbon dioxide after the carbon has been burnt in a power plant.
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.
How can we store 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
Pros and cons of carbon capture and storage
“The worry that CCS is too expensive is beside the point: no matter what it costs, we're almost sure to find some productive uses of that fossil carbon.”
Professor Myles Allan, Oxford University
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
wind turbines, tidal power—and
perhaps even nuclear plants (themselves hugely controversial). Another
drawback is that CCS uses considerable extra energy
(some estimates say 25–40 percent, others
30–60 percent) and could
almost 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: 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. (For example, according to the
US Energy Information Administration,
China made 72 percent of its electricity from coal in 2015 and will still make 47 percent that way in 2040. Its total electricity production will more than double in that period, so it won't actually be burning very much less coal then as it does now.) The hard reality is that much of the world runs on coal and will do so for decades,
and that suggests CCS is much more important than critics suggest.
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
currently the world's largest post-combustion CCS plant, which has been
operating in Houston, Texas since 2016.
But skeptics are pointing to the
failure of Kemper County, a $7.5 billion CCS project in Mississippi, as evidence that the
technology is fundamentally unachievable. In June 2017, after several years of controversy,
Kemper finally abandoned coal altogether and opted to burn gas instead.
In the UK, the government has repeatedly tried and failed to get CCS projects off the ground
with the help of private-sector power plant operators.
In November 2018, it announced new funding for the technology, with more emphasis on reducing carbon emissions from heavy industry. In December 2019, the UK's huge Drax Power plant
announced plans to become a pioneering, carbon-neutral BECCs plant within a decade.
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, concrete, 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.
Closely related to cement is the production of concrete (which uses cement as one of its
ingredients); you can find a brief discussion about lower-carbon, more environmentally friendly ways of making that in our article about concrete.