Capturing carbon: Can it save us?

Time is not on our side.

Catastrophic consequences of climate change are just steps away, according to a slew of reports released at the end of 2018. The Intergovernmental Panel on Climate Change (IPCC) says that without swift action, global temperatures will rise by 1.5 °C by 2030 and 2 °C by 2050—and will continue to climb beyond then. Those increases will cause disastrous effects, including record-breaking sea-level rise, flooding, wildfires, extreme weather events, famine, and wildlife habitat destruction, the IPCC says. The impacts will hit the world’s poor particularly hard.

And these effects seem all but certain. Humans are on a path to generate so much carbon dioxide, methane, and other greenhouse gases that it appears nearly impossible to cut emissions enough to avoid the worst.

Enter “negative-emissions technologies,” a term but a few years old. NETs are methods that physically and chemically remove CO₂ or other gases from the atmosphere. Today, a handful of technologies capture emitted CO₂ before it ever reaches the atmosphere. NETs would extract CO₂ or other gases directly from the air, change land-use practices to plant more carbon-sequestering trees and plants, and aggressively use natural systems to remove CO₂ from the environment.

NETs would not relieve the world of the need to cut emissions, but they could ease the path to reach net zero emissions by 2050—the timeline that the United Nations Environment Programme says is necessary to keep the global temperature rise below 2 °C, the original goal of the Paris Agreement on climate change.

Emission cuts and NETs “are two tools in the same toolbox,” says Stephen Pacala, an ecology and environmental biology professor at Princeton University who chaired a 2018 US National Academies of Sciences, Engineering, and Medicine study of NETs. “They both are necessary and are likely to coexist for a long, long time.”

In the following sections, C&EN examines some NET approaches that are just getting underway. Whether they will be able to scale up to meet the need is an open question. The numbers are staggering: globally, nearly 50 billion metric tons (t) of greenhouse gases are emitted to the atmosphere annually, the UN Environment Programme estimates. Of those emissions, about 37 billion t is CO₂ and the rest is mostly methane. And even with various efforts in place to reduce greenhouse gas emissions, global CO₂ emissions increased by nearly 3% in 2018.

Avoiding a climate disaster would require some 10 billion t of CO₂ emissions to be eliminated from the atmosphere each year by midcentury through emission reductions or NETs, the National Academies study estimates on the basis of UN data. By 2100, that number grows to 20 billion t per year.

Scientists estimate that NETs, if scaled up successfully, could address roughly 30% of the needed reductions.

Getting there will require policy changes such as carbon taxes or new economic drivers. A separate National Academies report, also released in late 2018, examined the possible use of CO2 or methane as a feedstock to make chemicals, fuels, or other products. It found potential markets in construction materials, chemicals, and fuels. However, at best the marketplace could use about 10% of greenhouse gas emissions, the report concluded. And if CO₂ is used to produce fuels, little is gained unless CO₂ emissions are again captured when that fuel is burned.

Globally, advocates of the need to address climate change typically point to the US as not doing its share. The country has historically been the global leader in carbon emissions; it is currently second to China for total greenhouse gas emissions worldwide and the world’s largest emitter per capita. President Donald J. Trump has announced plans to withdraw from the Paris Agreement, and his administration is working to reverse tighter emission requirements on coal-fired power plants.

Nevertheless, there are some bright spots for NETs in the US, Pacala says. For example, a 2018 federal law, the FUTURE Act, provides a $50 tax credit for each metric ton of CO₂ that is captured and stored underground. Also, recent changes to the California Low Carbon Fuel Standard program allows greenhouse gas polluters that fail to meet a declining state emission cap to buy emission credits from companies that captured and sequestered CO₂. Those emission credits have been trading at $190 per metric ton. Both programs could generate funds for NET development.

But to develop NETs further, and especially to get them to scale, additional investment is needed. The National Academies report estimates that the US may need to invest up to $900 million annually in NET R&D. In a world increasingly focused on constraining carbon emissions, such investment in NETs could have large economic rewards, with intellectual property rights and economic benefits accruing to nations and companies that develop the best technologies. However, the Department of Energy’s total budget for its Office of Fossil Energy, which covers development of carbon capture and storage and supports certain oil and gas resources, is just $740 million for 2019.

Meanwhile, the European Union plans to spend approximately €15 billion ($17 billion) on what it calls climate, energy, and mobility programs from 2021 to 2027 as part of its Horizon Europe research program. The EU estimates that overall, Europe will need to invest €520 billion to €575 billion annually in its energy systems to reach net zero greenhouse gas emissions by 2050. China’s approach to NETs is unclear.

Given sufficient R&D investment and an appropriate policy framework, NETs could be a “powerful policy or economic lever” to offset future emissions, says Howard Herzog, a Massachusetts Institute of Technology senior research engineer, who for 30 years has specialized in carbon capture technologies. Nevertheless, NETs will come at a price—they will likely always be a more expensive option than approaches that limit emissions in the first place.

“If we as a people are unwilling to use the relatively cheap mitigation technologies to lower carbon emissions available today, such as improved efficiency, increased renewables, or switching from coal to natural gas, what makes anyone think that future generations will use NETs, which are much, much more expensive?” Herzog says. Expecting NETs to save the world on their own is, he says, “more hope than reality.” 

Extracting from air

Pulling CO₂ out of thin air and piping it deep into Earth involves viable technologies already in use—but they have yet to be tried on a planetary scale.

Carbon capture has long been employed to treat air in submarines and spacecraft to keep sailors and astronauts alive. Similar approaches are used throughout the world to reduce CO₂ emissions from coal-fired power plants, natural gas processing plants, fertilizer and biofuel manufacturing sites, and other industrial point sources. And the technology has been successfully coupled with underground injection and sequestration of CO₂.

But the current scale of these approaches is millions—not billions—of metric tons per year. And they’re capturing carbon from relatively concentrated sources, not extracting it after it’s been diluted into the atmosphere.

To capture CO₂ at a power plant or other point source, costs vary from roughly $50 to more than $100 per metric ton. But CO₂ in the atmosphere is orders of magnitude less concentrated than emissions blasting from a power plant’s smokestack. If the same technology is used to extract CO₂ from the atmosphere, estimated costs run from $600 to $1,000 per metric ton.

Several companies are working to increase the capacity and lower costs of atmospheric CO₂ extraction systems, also called direct air capture, to make them commercially viable. Among those companies are Carbon Engineering, Climeworks, and Global Thermostat. Climeworks, based in Switzerland, pipes its captured CO₂ to greenhouses to enhance vegetable and other plant growth.

The basic operating principles of point-source capture and atmospheric extraction of CO₂ are similar, says Christopher Jones, a chemical engineering professor at the Georgia Institute of Technology who is a technology development adviser for Global Thermostat and was a member of the National Academies study committee. A stream of air is sent through a liquid or solid sorbent that collects CO₂. The sorbent is then heated to release CO₂ in a concentrated form that can be sequestered or used as a feedstock for fuels or other products.

Conveniently, direct air capture can be placed near the location of sequestration or where the CO₂ might be used as a feedstock, eliminating the need for complicated piping systems. The biggest cost driver is the energy used to heat the sorbent to release the captured CO₂.

Jones and other researchers are focusing on developing sorbent materials that allow for spontaneous absorption and low-temperature desorption of CO₂. Climeworks and Global Thermostat are working with amine-based solid materials, while Carbon Engineering is experimenting with a potassium hydroxide solution, Jones says. Pilot studies show promise at getting costs below $200 per metric ton of CO₂, where the technology would become commercially viable. Scaling up and lowering costs will be a “significant but not an impossible challenge,” Jones says.

There is currently no marketplace sufficiently large to support enough innovators to explore and develop the chemical processes and physical machinery that might decrease the cost to extract CO₂ from the air, the National Academies report says. Thus, direct-air-capture advocates are hoping for long-term government investments, a carbon tax, other state or federal tax incentives, or some other inducements that would mirror past efforts to develop solar cells or fossil-fuel extraction through hydraulic fracturing—efforts that led to upheavals in energy generation and fossil-fuel production. 

Burning new fuels

Combining energy production with carbon capture and sequestration could prove to be a powerful negative-emissions technology. So-called bioenergy systems use recently grown biomass as a feedstock to create energy in the forms of electricity and heat while permanently storing the resulting carbon dioxide underground, forever, explains Erica Belmont, a University of Wyoming mechanical engineering professor.

Importantly, the feedstocks—wood, energy crops such as elephant grass and switchgrass, agricultural waste, or other biomass sources—have all been grown recently. That means that they took up and concentrated “new” carbon from today’s environment. Burning new biomass, when combined with capturing and sequestering emissions, makes the bioenergy approach a NET because it captures carbon twice: first through photosynthesis and then through carbon-capture technology. Fossil fuels, in contrast, don’t have the benefit of incorporating contemporary carbon even if emissions are captured.

It’s “a very appealing approach and carbon negative as long as we keep these CO₂ emissions out of the atmosphere,” Belmont says. “But still, the problems are formidable.”

The primary challenge is growing enough biomass to make a dent in greenhouse gas emissions without affecting food production. Capturing and sequestering 10 billion t of CO₂ annually from biomass energy production would require almost 40% of global cropland, according to the National Academies report. Realistically, 3.5 billion–5.2 billion t of CO₂ per year globally could be captured without causing food shortages.

As for technologies to burn biomass for energy production while capturing CO₂, processes are already used at small scales.

Overall, more research is needed to increase the energy stored in bioenergy crops and subsequently released by burning, improve infrastructure to move the biomass for processing, and scale up processes. Some solutions, such as heating and drying feedstock to make it easier to transport and burn, appear straightforward. Other problems, such as developing a transportation infrastructure for biomass feedstock akin to that already in place for moving coal, are more complex.

On the sequestration side of the bioenergy equation lies biochar, a solid carbon bioproduct of biomass burning. Technology developers hope to use it as a soil amendment to aid plant growth. However, there are outstanding questions around biochar’s long-term stability in soil and whether it would eventually become a carbon source rather than a sink. 

Burying underground

For two potentially powerful NETs—direct air capture and bioenergy with carbon capture—it’s not enough just to capture CO₂. The substance must also be stored. Fortunately, deep geological reservoirs have sufficient space to sequester plenty of CO₂, according to the National Academies report.

Sequestering CO₂ underground involves compressing it to a supercritical fluid and then piping or shipping it to an injection well. Compressing the gas allows more CO₂ to be transferred and sequestered than if it remained in gaseous form. At the well, the CO₂ is injected into a geologic formation that is sufficiently deep—typically 1 km or farther underground—and impenetrable so that the CO₂ stays in a supercritical form, explains Princeton’s Pacala, the National Academies panel chair.

Suitable geological formations for storing CO₂ are porous and permeable reservoir rock such as sandstone, limestone, dolomite, or mixtures of these rock types. Typically, the reservoir rock is overlain by an impermeable rock species such as shale.

The oil industry has used a similar process for years, in which it injects CO₂ into nearly depleted oil fields to drive residual oil and natural gas to the surface for processing. Currently, about 64 million t of CO₂ is injected annually as part of this process, which is called enhanced oil recovery. About one-third of the CO₂ used for injection comes from captured emissions from sources such as power plants and natural gas processing facilities. The rest comes from natural sources.

A portion of the CO₂ used to extract oil remains sequestered in the oil fields; hence, this process is considered a successful means to sequester CO₂. But the demand for CO₂ to enhance oil recovery is far too small to curb global warming.

Separate from oil fields, however, deep geological formations with the necessary rock characteristics are sprinkled around the globe. In total, they could hold more than 2 trillion t of CO₂, enough to substantially contribute to greenhouse gas mitigation strategies. Carbon capture, coupled with underground sequestration, could contribute about 14% of the CO₂ emission reductions needed to stabilize the climate at a 2 °C increase, the National Academies report says.

However, although the capacity for carbon sequestration exists, developing a sequestration infrastructure and addressing liability issues may be challenging. Sequestration sites are unlikely to be near large sources of CO₂ emissions, and sequestration scaled to address global warming will require development of new large-scale and long-distance infrastructure to transport CO₂ by pipelines or ships—with the accompanying risk of leaks or releases from accidents. Financial liability and legal responsibility issues will need to be sorted out as quantities grow to billions of metric tons and storage stretches to hundreds of thousands of years. 

Making rocks

Carbon mineralization is an emerging NET that pulls carbon dioxide from the air and stores it in the permanent form of carbonate minerals, such as calcite or magnesite.

Mineralization occurs naturally during the weathering of silicate materials such as olivine, serpentine, and wollastonite. It also occurs in rocks rich in calcium and magnesium—particularly peridotite, which composes Earth’s upper mantle, and basaltic lava formed by partial melting of the upper mantle.

Mineralization takes advantage of rocks that geological processes have brought from deep within Earth up near or to the surface, where they are far from equilibrium and therefore reactive. Because mineralization uses this naturally available chemical energy, the approach may offer a low-cost means to mitigate greenhouse gas emissions. And because the CO₂ is locked in solid carbonate minerals, storage is potentially permanent and nontoxic.

Carbon can be sequestered through mineralization in three main ways, the National Academies report says. One approach, called ex situ carbon mineralization, involves transporting rocks to a site of CO₂ capture, where the reactants are combined with fluid or gas rich in CO₂. Another process involves reacting a CO₂-bearing fluid or gas with mine waste, alkaline industrial wastes, or sedimentary formations rich in reactive rock fragments. A third method, called in situ carbon mineralization, circulates CO₂-bearing fluids through suitably reactive rock formations beneath Earth’s surface.

Carbon mineralization is not a new concept. Researchers have been investigating its potential to capture atmospheric CO₂ for three decades. A recent study found that chemical reactions in common basalt rock can convert CO2 into solid minerals in less than two years—dramatically faster than the hundreds or thousands of years previously estimated (Science 2016, DOI: 10.1126/science.aad8132). However, mineralization could suffer from other resource problems. For example, the process requires 25 t of water for every metric ton of CO₂ stored. And as with underground sequestration, it will require the development of transportation infrastructure. 

Growing plants

Improved coastal zone management, reforestation, and enhanced agricultural practices could increase carbon dioxide sequestration capacity while also benefiting the environment.

Tidal wetlands incorporating salt marshes, mangroves, and seagrass beds thrive in the soft sediment and shallow water of estuaries between high and mean sea level. These so-called coastal-blue-carbon areas also hold large amounts of carbon in their soils and vegetation and could contain more.

The plants take in some 840 million t of CO₂ each year. The National Academies report estimates this level could more than double in the near future with active restoration and wetland creation, reaching additional cumulative storage of 5.4 billion t of CO₂ by 2100.

Coastal wetlands are already targeted for restoration and management efforts because of the broad range of ecosystem services they provide, including coastal storm protection, water-quality improvement, wildlife habitat protection, and fishery support, notes Tiffany Troxler, science director for the Sea Level Solutions Center at Florida International University and one of the National Academies panel members. Enhancing the quantity of coastal plants available to sequester CO₂ would give added weight to these protections, she says, and CO₂ sequestration could be carried out with almost no additional expense.

Another advantage, she notes, is that these carbon benefits can occur right away, unlike other negative-emissions approaches that are still in early development.

However, coastal regions also face constant development pressure. Globally, some 450 million t of CO₂ annually is lost to the atmosphere from excavation and other human activities in coastal areas.

Sea-level rise, Troxler says, could also be a problem, but that could be avoided by allowing sediments to naturally accrue on estuaries and wetlands—letting the soil keep pace with rising seas rather than be blocked by coastal development and artificial construction.

Similar carbon uptake can be achieved inland, through forest and soil amendments. In these areas, however, sequestration efforts quickly run into conflicts over land for food versus land for CO₂ sequestration.

And there are other considerations. For forests, increasing sequestration means not only more trees but more trees that grow quickly and close together, to increase the amount of carbon uptake per unit area. Forests must be maintained over a long time, which necessitates the consideration of disease, fire, and harvesting operations, the National Academies report says.

For soil-based organic carbon, enhancing sequestration means adding organic waste to soil as well as reducing the decomposition rate of organic compounds into CO₂.

Inland CO₂ capture is inexpensive and can be deployed quickly, says Richard A. Birdsey, a forestry expert at Woods Hole Research Center and a National Academies panel member. However, expansion can be difficult. Birdsey notes there are 11 million US forest landowners, each with different objectives for their property. Some landowners want to raise more timber, some want land for hunting, and some just want to be left alone, he says. He estimates that maybe 10% of forest landowners would be willing to change their practices to promote carbon sequestration.

The National Academies report estimates that implementing inland carbon sequestration practices in a way that would not jeopardize food security and biodiversity globally would allow the capture of 2.5 billion to 3 billion t of CO₂ annually from forests and agricultural soils combined.

The CO₂-removal costs would be less than $50 per metric ton. If more aggressive land-management approaches prove to be practical and economical, rates of carbon removal for both forests and agricultural soils could double, the report says.

However, both inland and coastal-blue-carbon gains are reversible if the carbon-sequestering practices are not maintained. For example, forested land could be cleared again, and reverting agricultural soils to intensive farming practices could stir up and release captured carbon back to the atmosphere. And restored coastal wetland could be drained or simply dug up, ending any carbon benefit.

Jeff Johnson is a freelance writer based in Washington, DC.