Carbon Capture & Storage

How it works, different methods, and do we need it?

Nov 27, 2024

Carbon Capture and Storage, CCS, is a technology designed to capture carbon dioxide emissions from sources like power plants, industrial processes, or directly from the air and store it underground, preventing it from contributing to climate change.

Do We Need It?

If we were to permanently stop emitting all CO₂ and other greenhouse gasses right now, the atmosphere and oceans would still have CO₂ levels 50% above pre-industrial levels. These greenhouse gases would still cause the warming of the Earth since they would still trap solar energy from the greenhouse effect.

So even in this ideal case of zero further emissions, CCS is necessary if we don't want global temperature rise to continue. And there are some things that will be extremely challenging to decarbonize, such as cement production and aviation.

Methods of Carbon Capture

Enhanced Oil Recovery (EOR)

Enhanced Oil Recovery (EOR) with CO₂ is a technique where carbon dioxide that's mixed with oil and gas is captured and reinjected into oil wells to increase oil and gas production by pushing out otherwise inaccessible fossil hydrocarbons. The CO₂ mixes with the oil, reducing its viscosity, which helps it flow more easily through the reservoir. While the primary goal is to maximize oil and gas extraction, some of the CO₂ remains trapped underground, providing incidental carbon storage. Currently the vast majority of CO₂ captured and stored today comes from natural gas wells. This is largely because it's easy and cheap, and even provides a positive financial return for oil and gas companies in the form of higher production.

Point Source Carbon Capture

Another form of carbon capture called "point source carbon capture" is where CO₂ is pulled from flue gasses at industrial sites, such as power, ethanol, ammonia, and cement plants. The products of combustion from burning any fossil hydrocarbon are CO₂, water vapor, and air pollutants like particulate matter, SO₂, and nitrogen oxides. The CO₂ is separated, often using chemical solvents or physical filters. Currently, the Boundary Dam coal-fired power plant in Canada is the only facility using CCS at scale, so it's hard to get accurate cost figures.

Direct air capture (DAC)

Direct air capture (DAC) is a carbon removal technology that uses large fans to pull air through filters coated with chemicals that selectively capture carbon dioxide from the atmosphere. Once the CO₂ binds to these chemicals, the filters are heated or chemically processed to release concentrated CO₂, which is then collected. DAC is the newest approach and needs further experimentation, but it promises to be a tool for both emissions reduction and carbon recycling into other products.

There are two main chemistries used in DAC. Solid DAC (S-DAC) uses materials that are solid at room temperature called amines that come from ammonia to capture the CO₂. Climeworks in Switzerland uses S-DAC. Liquid DAC (L-DAC) uses liquid materials like potassium hydroxide to capture CO₂. Carbon Engineering in Canada uses this approach. While L-DAC is more energy efficient, it requires greater capital expenditures up front to build. S-DAC requires 2,630 kWh of energy per tonne of CO₂ in total—2,000 kWh of heat and 630 kWh of electricity—whereas L-DAC requires 1,692 kWh per tonne—1,470 kWh of heat and 222 kWh of electricity.

The more concentrated the CO₂, the lower the cost of capture. DAC, which removes CO₂ from the air at a concentration of 0.04%, is very energy-intensive and expensive—over $600 per tonne. Contrast the 2,630 and 1,692 kWh per tonne of CO2 figures for S-DAC and L-DAC with only 800 kWh for CO₂ capture from flue gasses at industrial facilities.

Other CCS Methods

An Australian company called Calix is pioneering a method in its Leilac kiln that essentially uses a modified cement-making process. Limestone, calcium carbonate (CaCO3), is heated in a kiln to produce carbon dioxide and lime (CaO). The CO₂ is captured and stored underground. The lime is normally used to make cement, but in this case, it's mixed with water to form calcium hydroxide (Ca(OH)₂). The calcium hydroxide reacts with CO₂ in the air and forms limestone. This is a natural process, but it's sped up from years to days. The limestone is then reused to start the cycle again.

Captura in California is experimenting with taking CO₂ out of seawater. The oceans take up about 30% of human-caused CO₂ emissions, which increases ocean acidity. If CO₂ is removed from seawater, making it more alkaline, the oceans will naturally pull more CO₂ from the atmosphere. Concentrations of CO₂ in the shallow ocean can be 150 times greater than in the air, potentially allowing this "Direct Ocean Capture" method to be cost-effective.

Methods of Storage

Geologic Storage

Once CO₂ is captured, it needs to be stored. The most common method is currently storage in geological structures such as depleted oil and gas fields or deep saline aquifers. The CO₂ is compressed and pumped into these formations, where it can be trapped under impermeable rock layers, within the pores of rocks, or dissolved in water. Geologic CO₂ storage is intended to last thousands to millions of years if conditions remain stable, although we can't be sure with our limited experience.

Carbon Mineralization

Another method of carbon storage that promises to last many millions of years is carbon mineralization or mineral carbonation. CO₂ is chemically reacted with naturally occurring minerals, like magnesium or calcium silicates, to form stable carbonates. In other words, the CO2 is turned into rock. This can be done by dissolving CO₂ in water then injecting it underground to react with minerals naturally. While this is still a newer method of carbon storage that needs more experimentation, companies like Carbfix are seeing early success.

CO₂ + H₂O + Ca/Mg-silicate minerals → CaCO3 or MgCO3 (solid carbonates)

Enhanced Rock Weathering

Enhanced Rock Weathering is a method that accelerates the natural formation of rock from CO₂ in the atmosphere. While this still needs further research, it mimics the natural process of rock weathering and can potentially have additional positive effects beyond atmospheric carbon removal.

Natural rock weathering converts atmospheric CO₂ into rock through a slow chemical reaction with silicate minerals in rocks. CO₂ dissolves in rainwater to form carbonic acid, which then reacts with minerals, releasing ions like calcium and bicarbonate into rivers. These ions eventually reach the ocean, where they combine to form stable carbonate minerals, such as limestone, that settle on the ocean floor. Over millions of years, these carbonates become solid rock, sequestering CO₂ for geological timescales.

Companies are thinking of all sorts of ways to accelerate this process. Undo Carbon, for example, is simply crushing silicate rock (basalt) and spreading it on agricultural land. Other companies like Eion Carbon are crushing and spreading rock rich in the mineral olivine.

A third method involves dissolving CO₂ into ocean water, particularly in the deep ocean, but it's unclear how long this will last. When CO₂ is dissolved in water, it acidifies it, which can have harmful effects on marine ecosystems.

Biological Carbon Capture and Storage

I'll be brief here, saving the details for a future article. For now, we'll mention some rough numbers and state that the balance of biological carbon is critical for ecosystem health. Biological methods of carbon dioxide removal (CDR) may be much cheaper than DAC because of the vast carbon-holding capacity of nature's repositories.

Since pre-industrial times, humans have emitted about 1,300 gigatonnes of carbon dioxide into the atmosphere, which is 350 gigatonnes of carbon, since carbon dioxide is 27% carbon (12/44).

The easiest, cheapest, and healthiest method to store carbon in ecosystems is simply not to destroy those ecosystems. When forests are cleared or soils are depleted, huge amounts of carbon are released into the atmosphere. Other more active methods of biological carbon storage include regenerative agriculture, biochar, peatland restoration, wood burial, reforestation, growing seaweeds, and restoring ecosystems to their prior health.

Duration of storage

The duration of carbon storage is an important variable that determines how much CCS can affect global temperatures in the long run. Projections indicate that carbon storage should last for several thousand years. The Climate Brink has a great article on this. That doesn't mean that biological carbon storage methods such as reforestation are useless. If depleted farmland is restored to forest, those trees and all the other forest life will be pumping carbon back into the soil. So even if decomposition of organic matter occurs from fungi and microbial activity after trees die in decades or hundreds of years, the forest as a whole will continue to store carbon for as long as it exists—which can be tens of thousands of years. Forests live longer than anything in them.

Thermodynamic Alternatives and Scale

I've always thought it's a little silly that there's so much talk of carbon capture and storage when we continue to emit 38 billion tonnes of CO₂ every year. Fossil hydrocarbons (coal, oil, gas) that are in the ground are doing a great job at sequestering carbon at no cost, and have been for hundreds of millions of years. We're trying to capture and store all this carbon we're emitting at a high cost, when we could potentially use those same investment dollars and brilliant engineers to stop the carbon emissions in the first place—at lower cost.

Of course, we are investing in renewable energy systems to stop burning fossil hydrocarbons, but physics and economics favor more of this. Thermodynamic laws tell us that we should first turn off the faucet filling the tub (atmosphere) before we try to drain it (CCS).

Burning natural gas that emits 1 tonne of CO₂ emissions will produce 5,525 kWh of electricity. If that 1 tonne of CO₂ from burning natural gas were captured and put back in the ground using DAC, it would cost $600+ per tonne.

The average LCOE of wind and solar is $0.035/kWh. Using renewables to generate an equivalent 5,525 kWh of electricity and leave that 1 ton of CO₂ underground costs only $193.

It would cost only $99 dollars in renewables to produce the same amount of electricity that would be generated by emitting 1 tonne of CO₂ from a coal power plant.

It's cheaper to leave fossil fuels in the ground than to try to clean up their CO₂ emissions later. Besides that, we'd benefit from better air quality from less particulate matter, sulfur dioxide, and nitrogen oxides.

That said, we will need to capture and store much of the carbon dioxide that's already been emitted, so it's a good thing companies and innovators are working on the problem.

Currently, DAC only stores about 10,000 tonnes of CO₂ annually. The IPCC estimates that in 2050, we will emit 6 gigatonnes of CO₂ annually, which is only 15% of what we emit now. If that's the case, we would need almost a million-fold increase in DAC capture.

CCS is necessary, but it's cheaper and better for the Earth and human health to stop using fossil hydrocarbons first.

What other methods of CCS are promising?
Should we be investing more in CCS or renewable energy systems?