How Carbon Capture Works


Steam and smoke are emitted from a coal-fired power station in the England. See more green science pictures.
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Imagine a scenario where an evil super-genius finds a way to suck all the oxygen out of the air, then buries it in the ground. Sound like the stuff of comic books? Well, yes, if we're talking about oxygen. But scientists are working on a way to do just that with carbon dioxide. Why capture carbon dioxide from the air? To combat global warming.

Carbon dioxide (CO2) is a natural gas that allows sunlight to reach the Earth but also prevents some of the sun's heat from radiating back into space, thus warming the planet. Scientists call this warming the greenhouse effect. When t­his effect occurs naturally, it warms the Earth enough to sustain life. In fact, if we had no greenhouse effect, our planet would be an average temperature of minus 22 degrees Fahrenheit (minus 30 degrees Celsius) [source: UNEP]. Sure, the skiing might be great, but we'd all be too dead to enjoy it.

Yes, carbon dioxide and the greenhouse effect are necessary for Earth to survive. But human inventions like power plants and transportation vehicles, which burn fossil fuels, release extra CO2 into the air. Because we've added (and continue to add) this carbon dioxide to the atmosphere, more heat is stored on Earth, which causes the temperature of the planet to slowly rise, a phenomenon called global warming.

Carbon dioxide isn't the only greenhouse gas (GHG). Others include water vapor, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride. Scientists estimate that global GHG emissions due to human activities increased 70 percent between 1970 and 2004. Carbon dioxide emissions alone grew 80 percent in the same period [source: IPCC]. Many researchers believe that the process of carbon capture and storage can help us to get this number down to a healthy level.

Carbon capture involves trapping the carbon dioxide at its emission source, transporting it to a storage location (usually deep underground) and isolating it. This means we could potentially grab excess CO2 right from the power plant, creating greener energy.

In this article, we'll look at some of the existing and emerging carbon capture and storage methods. How could a device snatch CO2 out of the air? And where in the world is it stored? Keep reading to find out.

Trapping Carbon Dioxide: Carbon Capture Technology

Smoke and steam vapour from electric power station in New York
Smoke and steam vapour from electric power station in New York
Lester Lefkowitz/Getty Images

Carbon capture has actually been in use for years. The oil and gas industries have used carbon capture for decades as a way to enhance oil and gas recovery [source: CSS]. Only recently have we started thinking about capturing carbon for environmental reasons.

Currently, most research focuses on carbon capture at fossil fuel-powered energy plants, the source of the majority of man-made CO2 emissions. Many of these power plants rely on coal to create energy, and the burning of coal emits CO2 into the atmosphere. Some researchers envision a future where all new power plants employ carbon capture.

There are three main steps to carbon capture and storage (CCS) -- trapping and separating the CO2 from other gases, transporting this captured CO2 to a storage location, and storing that CO2 far away from the atmosphere (underground or deep in the ocean). Let's take a more detailed look at the trapping and separation process­:

Carbon is taken from a power plant source in three basic ways -- post-combustion, precombustion and oxy-fuel combustion. A fossil fuel power plant generates power by burning fossil fuel (coal, oil or natural gas), which generates heat that turns into steam. That steam turns a turbine connected to an electricity generator. We call the process that turns the turbine combustion.

With post-combustion ­carbon capture, the CO2 is grabbed after the fossil fuel is burned. The burning of fossil fuels­ produces something called flue gase­s, which include CO2, water vapor, sulfur dioxides and nitrogen oxides. In a post-combustion process, CO2 is separated and captured from the flue gases that result from the combustion of fossil fuel. This process is currently in use to remove CO2 from natural gas. The biggest benefit to using this process is that it allows us to retrofit older power plants, by adding a "filter" that helps trap the CO2 as it travels up a chimney or smokestack. This filter is actually a solvent that absorbs carbon dioxide. The solvent can later be heated, which will release water vapor and leave behind a concentrated stream of CO2.­ Post-combustion carbon capture can prevent 80 to 90 percent of a power plant's carbon emissions from entering the atmosphere [source: GreenFacts]. But the post-combustion process requires a lot of energy to compress the gas enough for transport.

With precombustion carbon capture,­ CO2 is trapped before the fossil fuel is burned. That means the CO2 is trapped before it's diluted by other flue gases. Coal, oil or natural gas is heated in pure oxygen, resulting in a mix of carbon monoxide and hydrogen. This mix is then treated in a catalytic converter with steam, which then produces more hyd­rogen, along with carbon dioxide. These gases are fed into the bottom of a flask. The gases in the flask will naturally begin to rise, so a chemical called amine is poured into the top. The amine binds with the CO2, falling to the bottom of the flask. The hydrogen continues rising, up and out of the flask. Next, the amine/CO2 mixt­ure is heated. The CO2 rises to the top for collection, and the amine drops to the bottom for reuse [source:­ Allen]. The excess hydrogen also can be used for other energy production processes.

Precombustion carbon capture is already in use for natural gas, and provides a much higher concentration of CO2 than post-combustion. The precombustion process is lower in cost, but it's not a retrofit for older power plant generators. As with post-combustion, precombustion carbon capture can prevent 80 to 90 percent of a power plant's emissions from entering the atmosphere [source: GreenFacts].

With oxy-fuel combustion carbon capture, the power plant burns fossil fuel in oxygen. This results in a gas mixture comprising mostly steam and CO2. The steam and carbon dioxide are separated by cooling and compressing the gas stream. The oxygen required for this technique increases costs, but researchers are developing new techniques in hopes of bringing this cost down. Oxy-fuel combustion can prevent 90 percent of a power plant's emissions from entering the atmosphere [source: GreenFacts].

Once the carbon is captured, how is it transported to a storage location? Keep reading to find out.

Transporting Carbon Dioxide

After carbon dioxide (CO2) is captured, the next step is transporting it to a storage site. The current method of transporting CO2 is through a pipeline. Pipelines have been in use for decades, and large volumes of gases, oil and water flow through pipelines every day. Carbon dioxide pipelines are an existing part of the U.S. infrastructure -- in fact, there are more than 1,500 miles (2,414 km) of CO2 pipelines in the U.S. today, mostly for enhancing oil production [source: IPCC]. You can put a pipeline just about anywhere -- underground or underwater -- with depths ranging from a few feet to a mile.

A CO2 pipeline usually begins at the source of capture and travels directly to the storage site -- although, in some cases, it might travel as far as it can in the pipe, then transition to a tanker or ship to finish off its journey. It all depends on where the source, pipeline and storage site are located. Both the public and private sector can own pipelines.

Pipelines can transport CO2 in three states: gaseous, liquid and solid. Solid CO2 is commonly known as dry ice, and it's not cost-effective to transport CO2 as a solid. Pipelines commonly transport carbon dioxide in its gaseous state. A compressor "pushes" the gas through the pipeline. Sometimes a pipeline will have intermittent compressors to keep the gas moving. The CO2 must be clean (free of hydrogen sulfide) and dry. Otherwise, it can corrode a typical pipeline, which is made of carbon manganese steel. As of yet, there are no standards in place for "pipeline quality" carbon dioxide, but experts say that pipelines built from stainless steel would have a lowered risk of corrosion. This, however, may not be economical, since we would have to build brand new pipelines just for CO2.

Accidents with pipelines are rare, as we've found in decades of use. Only 12 CO2 pipeline leaks occurred from 1986 to 2006, with no human injuries reported. Contrast that with natural gas and hazardous liquid pipelines, which had more than 5,000 accidents and 107 fatalities in the same period [source: Parfomak]. Of course, one reason carbon dioxide pipeline accidents are rare is because we don't really have that many CO2 pipelines in use. Accidents will likely increase as the number of pipelines rises. As CO2 is odorless and colorless, though, adding an odor to the gas could help to detect leaks. Regardless, experts recommend construction of pipelines in low-population areas to minimize any impact.

Pipeline costs fluctuate depending on the route of the pipeline (through heavily congested areas, mountains, offshore). It's also possible to transport carbon dioxide as a liquid, using ships or tanker trucks. Liquid CO2 requires low pressure and a constant low temperature, so cargo tanks need to be both pressurized and refrigerated. You might be wondering what happens if a ship or truck carrying a tank of CO2 gets into an accident. Unfortunately, there isn't much data on the subject, but we do know there is an asphyxiation risk if a massive amount of CO2 escapes into the atmosphere. As with tanks that transport natural gas and other hazardous materials, good construction is key. That, and good driving.

Continue reading to learn how carbon dioxide can be stored underground or underwater.

Carbon Storage

An engineer overlooks the operation of Germany's first underground storage plant for CO2
An engineer overlooks the operation of Germany's first underground storage plant for CO2
Michael Urban/Getty Images

After we collect and transport all that carbon dioxide (CO2), we're going to need somewhere to put it. But where? In some sort of giant storage unit? A huge tank out in the desert? Will we need more landfills to hold our CO2 waste?

Don't worry, the answer to all those questions is "no." There are two places we've found to store CO2 -- underground and underwater. In fact, estimates project that the planet can store up to 10 trillion tons of carbon dioxide. This would allow 100 years of storage of all human-created emissions [source: Science Daily]. (Though we'll obviously survive much longer than that.)

First, we'll talk about underground storage. The pressure found deep underground causes CO2 to behave more like a liquid than a gas. Because it can seep into the spaces in porous rocks, a great amount of CO2 can be stored in a relatively small area. Underground storage, also called geological sequestration, is already in use by the oil and gas industries to squeeze out extra oil or gas from depleted reservoirs. Oil and gas reservoirs are well suited to store CO2 as they consist of layers of porous rock formations that have trapped oil and gas for years. Geological sequestration involves injecting CO2 into underground rock formations below the Earth's surface. These natural reservoirs have overlying rocks that form a seal, keeping the gas contained. There can be risks to underground storage, though, and we'll discuss those a bit later.

Basalt formations (volcanic rock) also appear to be suitable for storing CO2. In fact, basalt is one of the most common types of rock in the Earth's crust -- even the ocean floor is made of basalt [source: USGS]. Researchers have found that when they inject CO2 into basalt, it eventually turns into limestone -- essentially converting to rock. The Pacific Northwest National Laboratory in Washington State currently has a team devoted to running a pilot project to test basalt carbon storage [source: MSNBC].

Another project, called CO2 Sink, is testing geological sequestration in a location near Berlin, Germany. The project, started in 2004, aims to create a standard for CO2 injection. After injecting CO2 into a sandstone reservoir, scientists will actively study the area for long-term integrity and safety, leakage concerns, and movement of the CO2 within the reservoir [source: CO2Sink]. Also, the Sleipner gas field offshore in Norway has been injecting carbon dioxide into the sea floor since 1996 [source: Solomon].

In addition to underground storage, we're also looking at the ocean for permanent CO2 storage. Some experts claim that we can safely dump CO2 directly into the ocean -- provided we release it at depths greater than 11,482 feet (3500 meters). At these depths, they think the CO2 will compress to a slushy material that will fall to the ocean's floor. Ocean carbon storage is largely untested, and there are many concerns about the safety of marine life and the possibility that the carbon dioxide would eventually make its way back into the environment. For more information on this topic, read Can we bury our CO2 problem in the ocean?.

Next, we'll look at some of these concerns in more detail and find out if carbon ca­pture and storage is a viable solution for our future.

Carbon Storage Concerns

A danger sign warns visitors of high levels of carbon dioxide near Horseshoe Lake in California. The lethal gas is produced by molten magma, which rises to the earth's surface.
A danger sign warns visitors of high levels of carbon dioxide near Horseshoe Lake in California. The lethal gas is produced by molten magma, which rises to the earth's surface.
David McNew/Getty Images

Although carbon capture and storage may seem like a miracle solution, it's not without concern or controversy. To begin, it's important to remember that carbon capture and storage (CCS) is not a license to continue emitting CO2 into the atmosphere. We need to use CCS in addition to other emission-reduction efforts. However, CCS provides a way to clean up our existing power plants.

Opponents of CCS believe that, while it may be viable, the focus is all wrong. They argue that we should be coming up with ways to wean ourselves off fossil fuels instead of spending time and money on ways to continue using fossil fuels. According to the environmental group Greenpeace, widespread deployment of CCS isn't even possible until at least 2030.

Another drawback? Current CCS technologies actually require a lot of energy to implement and run -- up to 40 percent of a power station's capacity [source: Greenpeace]. Additionally, if we transport that captured CO2 by truck or ship, those vehicles will require fuel. And, the burning of fossil fuels is what got us into this predicament in the first place.

Creating a CCS-enabled power plant also requires a lot of money. For example, the United States has its own CCS project in the works. FutureGen hopes to build the first coal-fueled zero-emissions power plant. Its goal is to create a power plant that runs on coal but stores carbon emissions underground. The plant would power 150,000 homes and generate 275 megawatts of electricity [source: FutureGen]. Private partnerships and federal monies helped support the project. But President Bush pulled support when projected costs topped $1.8 billion. The government had already sunk $50 million into the project when it pulled its backing [source: Wald]. FutureGen continues to seek private and federal funding today.

The biggest concern with CCS, though, is the environmental risk. What happens if the carbon dioxide leaks out underground? We can't really answer this question. Because the process is so new, we don't know its long-term effects. Proponents, however, point to the Sleipner gas field, which has been in operation for more than 10 years without any detectable underground leakage.

What if the carbon dioxide leaks out in the ocean? We do have a little bit of knowledge on this one. In 1986, a natural volcanic eruption of carbon dioxide from a lake in Cameroon killed nearly 2,000 people. They died of asphyxiation from being in close vicinity to the release of CO2. These numbers don't even take into account the death toll of marine life that called the lake home [source: BBC].

Another effect of excess CO2 in the water is increased acidity. The ocean actually absorbs CO2 from the atmosphere -- a phenomenon known as carbon sink. Scientists have recently discovered that some oceans aren't absorbing as much CO2 as they did in the past. The Southern Ocean, in particular, no longer soaks up as much carbon dioxide, a fact that alarms scientists. The excess CO2 from human emissions appears to be staying on the surface of the oceans instead of sinking. And the more CO2 an ocean surface absorbs, the more acidic it becomes [source: Rincon]. Higher water acidity adversely affects marine life. For example, it reduces the amount of vital calcium carbonate marine creatures need to build their shells.

There are still many questions about whether carbon capture and storage will help to alleviate the greenhouse effect and slow climate change. But one thing's for certain: carbon dioxide emissions are a worldwide problem.

For more information about carbon, dig your way into the links on the next page.

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More Great Links

Sources

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