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Greenhouse Gases

Capturing carbon: Can it save us?

We have technologies to remove greenhouse gases from air, but it’s less clear we can scale them fast enough to make a difference

by Jeff Johnson, special to C&EN
February 25, 2019 | APPEARED IN VOLUME 97, ISSUE 8

Credit: Yang H. Ku/C&EN/Shutterstock


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.

50 billion


Metric tons of greenhouse gases currently emitted to the atmosphere annually

Source:United Nations Environment Programme.

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

Credit: Climeworks/Julia Dunlop/Cover Images/Newscom
Climeworks extracts CO2 from the atmosphere and sends it to greenhouses to promote plant growth.

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.

Air capture at a glance

Primary benefits: Mobile, measurable

Primary constraints: Energy intensive

Current cost per metric ton of CO2: $200–$1,000

Estimated removal capacity: High

Research needs: Sorbent development to lower energy

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

Credit: Drax Group
The 3,900 MW Drax power station in North Yorkshire, the UK’s largest power plant, has converted four of six electricity generation units from coal to biomass and is beginning to demonstrate carbon capture and sequestration at the site.

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.

Bioenergy at a glance

Primary benefits: Renewable energy

Primary constraints: Land availability, transportation infrastructure

Current cost per metric ton of CO2: $200–$1,000

Estimated removal capacity: 3.5 billion–5.2 billion t of CO2 annually

Research needs: Increase energy density of crops

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

Credit: US Department of Energy/National Academies of Sciences, Engineering, and Medicine
Map of basalt formations (red) at and near the surface of the US that would be suitable for CO2 sequestration.

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.

Geological sequestration at a glance

Primary benefits: Capacity

Primary constraints: Transportation infrastructure, long-term liability

Current cost per metric ton of CO2: Low, but amount unclear

Estimated total storage capacity: 2 trillion t of CO2

Research needs: Scale up injection

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

Credit: Proc. Natl. Acad. Sci. USA 2008, DOI: 10.1073/pnas.0805794105
Reaction of atmospheric CO2 with alkaline spring water from a peridotite aquifer forms travertine deposits.

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 at a glance

Primary benefits: Capacity

Primary constraints: Speed, water needed, transportation infrastructure

Current cost per metric ton of CO2: $100

Estimated removal capacity: High

Research needs: Fundamental understanding of mineralization chemistry

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

Credit: Tampa Bay Estuary Program
Some 8,000 salt marsh grass plugs were planted by 90 volunteers at Florida’s Perico Preserve during the “Give a Day for the Bay” environmental restoration program and celebration.

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.

Plant growth at a glance

Primary benefits: Environmental cobenefits

Primary constraints: Land availability

Current cost per metric ton of CO2: $20–$50

Estimated removal capacity: Blue carbon, 130 million t of CO2 annually; reforestation and enhanced agriculture, 2.5 billion–3 billion t of CO2 annually

Research needs: Impact of sea-level rise and land-use changes, increasing crop uptake of CO2

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.



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Tore Fossum (February 25, 2019 1:05 PM)
Just a thought that perhaps the anthropogenic warming theory might not be as solid as the IPCC report proclaims. The graph they showed looked much like Jim Hansen's discredited hockey stick temperature plot. The earth's surface temperature has varied up and down for sure as far back as our geologists can see and estimate. The earth has been warming for the past 12,000 years, since the end of the last ice age, but within that warming trend that melted 2 km of ice sheet over the northern hemisphere, and raised the ocean levels by 400 feet, there have been cold times and warm times. The Roman warming and the Medieval warming had temperatures similar to today's, and the little ice age, that was more or less from 1350 to 1850, was pretty darn cold. Carbon dioxide is a very small part of causes of warming. The point is that no one speaks about the variability of the sun's output. We have solid data only since sensors were put in orbit. Many of us suspect that the topic of Global Warming and Global Climate Change is a ploy to get the populace to submit to more taxes for governments to use on their pet projects, and of course, to siphon off much of that to the supporters of growth of government. It is a way for governments and their supporters of communitarian projects to pry more money and power from the citizens. I rather resent it when someone says I can use your money for better things than you can, so give it to me.

The ACS is an organization for the advancement of the science of Chemistry. It does not need to be a social engineering organization, so please stop with the Climate Change agenda.

Ray (March 1, 2019 1:29 PM)
Thank you for sharing your paranoid suspicions, now please leave. The ACS is an organization for the advancement of the science of chemistry, so please take your discredited conspiracy theories and ramblings elsewhere. The rest of us are trying to accomplish things.
Larry (March 4, 2019 12:01 PM)
Some facts for your perusal, Ray:
Robert Tulip (March 5, 2019 4:40 PM)
Thanks Jeff for a good overview of the emerging scientific consensus on NETs and climate. There are a few areas where more ambitious thinking can deliver faster results to prevent dangerous warming.
The world ocean is more than double the size of all the continents. The ocean has the area, energy and nutrients that could be used to deliver net zero by 2030, if the political will emerged, like an Apollo Program. Net zero means carbon removal at the same scale as total emissions, and this may only be possible through an ocean focus, which was completely missing from the National Academies report.
A focus on the carbon removal side of the equation is urgent, and could prove easier in both political and economic terms than the current Paris strategy of doubling down on decarbonisation through emission cuts. Net zero global emissions is just a milestone on the road to future climate stability. The amplifying feedback loops of climate change, illustrated by the sawtooth pattern of temperature over ice ages, shows that the 630 billion tonnes of carbon we have already added are enough to commit dangerous warming unless that whole amount is removed.
So there is no such thing as a carbon budget of further emissions allowing stability at 400 ppm etc. All the extra carbon has to be converted into safe form to achieve climate repair, restoration, management, stability and security.
Finally, the risk of dangerous tipping points is high. We are in the canoe and don't know how close we are to the waterfall of a phase shift into a hothouse earth. That means that NETs must be augmented by direct cooling methods, especially marine cloud brightening, iron salt aerosol and stratospheric aerosol injection. Field tests of these proposed technologies are urgent, but this whole area of scientific research is stymied by the prevailing political focus on emission reduction alone.
BRUCE MELTON (March 6, 2019 10:15 AM)
I have a few comments about direct air capture and carbon dioxide removal. And thanks for the article, the more we talk about this, the faster we can get on with it. The IPCC 1.5 C report says we need 12.5 gt carbon dioxide removal (CDR) annually through 2100 to limit warming to 1.5 C – plus – net zero CO2 emissions by 2050. This is of course way too warm considering the weather catastrophes ongoing and collapse initiation of the West Antarctic Ice Sheet and other Earth systems, but for now, this is the target. To get the required CDR, we need more than can possibly be removed by terrestrial enhancement of forests and agriculture as Johnson quoted in this article from the latest National Academies report on Negative Emissions Technologies (NETS) in December 2018 of 3.5 to 5.2 gt plausible CO2 removal without causing food shortages. (

Comment 1: 2.75 gt CDR from forests and agriculture is the plausible amount from the NAS NET report, without causing food shortages. Considering BECCS and blue carbon, this increases to 4.9 gt. Note: the NAS report does not go into ocean fertility restoration for CDR, which has a very high capacity, but significant controversy.

The NAS report "highlights" states that up to 5 gt atmospheric CO2 removal using forests and agriculture enhancements is plausible, "However, attaining these levels would require unprecedented rates of adoption of agricultural soil conservation practices, forestry management practices, and waste biomass capture. Practically achievable limits are likely substantially less, perhaps half the 1 GtCO2/yr in the US and 10 GtCO2/yr globally." When this logic is applied to the quantities in Table 1, the results are that half of 5.5 gt atmospheric removal are available as the description of Table 1 states, "The 'Safe' rate of CO2 removal means that the deployment would not cause large potential adverse societal, economic, and environmental impacts."
Table 1 (Globally)
Afforestation/Reforestation 1 gt/yr
Forest Management 1.5 gt/yr
Agricultural Soils 3 gt/yr
Total 5.5 gt/yr
Additional removal from BECCS and blue carbon (living plants or sediments in tidal marshlands, seagrass beds, and other tidal or salt-water wetlands) of 2.15 Gt CO2 per year is possible globally. The total "safe" CDR from natural sources is then about 4.9 gt CO2. The rest must come from technologies that use chemical and mechanical means, or oceans, otherwise we exceed 1.5 C goals.
Negative Emissions Technologies and Reliable Sequestration, A Research Agenda, Consensus Study Report, Highlights, National Academy of Sciences, October 2018, Summary, page 2, paragraph 2 and 3, and Table 1.

Comment 2: Johnson does not mention David Keith's Carbon Engineering work from June 2018 in Joule. This work describes the ongoing operation of the 1 kiloton per year Carbon Engineering process in Squamish, British Columbia, at $94 to $232 per ton CO2 based on capital recovery factors (CRF) of 7.5 and 12.5. A 7.5 CRF is typical for a wind energy installation, a 35 CFR is typical for a nuclear energy installation. CRF is only applicable to for profit facilities, different economic rules would apply for government sponsored cleanup of already emitted climate pollution. The typical number used in academia for describing Keith's work is $100 per ton CO2. This is 90 percent efficient, levelized, using $0.03 to $0.06 kWh natural gas energy. Analysis of Keith's work (by me) says that large costs reductions would be realized with near-future renewable energy at $0.01 kWh that coincidentally removes the carbon penalty for using fossil fuel energy, scaling to from the kiloton to hundred megaton, or gigaton level, and process refinements with the nth process.

Keith et al., A Process for Capturing CO2 from the Atmosphere, Joule, June 7, 2018.

Comment 3: The $600 to $1,000 per ton CO2 cost Johnson quotes for air capture has been significantly rebutted. This is how we get $100 per ton CO2 from Carbon Engineering. This $600 to $1,000 cost is based on the dilution principle, where highly dilute substances are more difficult to deal with than more concentrated substances (flue gas CO2 vs. ambient CO2.) The hypothetical analysis done to get these high costs made poor process decisions and used enthalpy backwards. The academic chain of publishing and rebuttal is listed below.

American Physical Society (APS) and MIT say $600 to $1,000 per ton CO2:

American Physical Society Study…
$80 per ton from Flue gas, $600 per ton for DAC using "mature technologies."
Socolow et al., Direct Air Capture of CO2 with Chemicals, The American Physical Society, June 2011.
MIT Study…
House et al., economic and energetic analysis of capturing CO2 from ambient air, PNAS, September 2011.


APS research revealed as significantly incomplete by Nature… Socolow 2011 [APS] evaluated existing WWII Era atmospheric removal technology and not surprisingly found them economically infeasible to address climate pollution.
Van Norden, Sucking carbon dioxide from air too costly, say physicists, Nature, May 11, 2011.

Further rebuttal of APS and MIT (Holmes and Keith 2012)…Holmes and Keith identify short fallings of MIT and APS work calling out different design choices, insufficient optimization, and use of higher cost processes. When new DAC technologies are evaluated, costs are at or below those of mature DAC removal technology.
Holmes and Keith, An air-liquid contactor for large-scale capture of CO2 from air, Philosophical transactions of the Royal Society A, 370, 4380-4403, 2012.

Flawed analysis of the Basic physics of enthalpy (Realff and Eisenberger 2012)… These researchers point out a fundamental flaw in the work of APS and MIT showing direct air capture takes more energy than flue capture because of CO2 concentration: “The notion of minimum work does not apply to the capture of CO2, because the capture process is exothermic.” When CO2 is reacted with something to remove it from air or flue gas, the reaction creates heat, “is exothermic.” So instead of 400 kJ or work per mole CO2 energy required the actual energy required involves moving air over whatever process is used to remove the CO2 from the air. This is 6 kJ per mole CO2. This relationship of the actual costs of removal of CO2 from the atmosphere being 1.5 percent of the costs suggested by APS and MIT corresponds very well to the costs assumed by research evaluating new technologies of +/- $20 per ton. It is important to note that the cost of regenerating the chemicals used to capture the CO2, whether for flue gas or atmospheric capture, is identical.
Realff and Eisenberger, Flawed analysis of the possibility of air capture, June 19, 2012.

Response to Realff and Eisenberg… Herzog et al. (House 2012) responded to Realff and Eisenberger's circular argument by declaring that the excess energy from the exothermic reaction was not used as an input into the process because it was considered an inefficiency. This is not the point. The enthalpy was used backwards, and required heat be delivered to this portion of the process when none was required, significantly biasing the results. See the discussion in the link below—scroll down for the rebuttal rebuttal by Herzog et al.
(Might need to manually enter this link)

Bruce Melton PE

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