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CAPTURING CARBON: CAN IT SAVE US? We have technologies to remove greenhouse gases from air, but it’s unclear we can scale them fast enough to make a difference
C R E D I T: YA N G H . KU/C & EN / S H U TT E RSTOC K
JEFF JOHNSON, special to C&EN
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T
ime 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. mostly methane. And even with various Humans are on a path to generate so much efforts in place to reduce greenhouse gas carbon dioxide, methane, and other green- emissions, global CO2 emissions increased house gases that it appears nearly imposby nearly 3% in 2018. sible to cut emissions enough to avoid the Avoiding a climate disaster would reworst. quire some 10 billion t of CO2 emissions to Enter “negative-emissions technolobe eliminated from the atmosphere each gies,” a term but a few years old. NETs are year by midcentury through emission remethods that physically and chemically ductions or NETs, the National Academies remove CO2 or other gases from the atmo- study estimates on the basis of UN data. By 2100, that number grows to 20 billion t sphere. Today, a handful of technologies capture emitted CO2 before it ever reaches per year. Scientists estimate that NETs, if scaled the atmosphere. NETs would extract CO2 or other gases directly from the air, change up successfully, could address roughly 30% of the needed reductions. land-use practices to plant more carGetting there will require policy changbon-sequestering trees and plants, and ages such as carbon taxes or new economic gressively use natural systems to remove drivers. A separate National Academies CO2 from the environment. report, also released in late 2018, examNETs would not relieve the world of ined the possible use of CO2 or methane the need to cut emissions, but they could ease the path to reach net zero emissions as a feedstock to make chemicals, fuels, or by 2050—the timeline that the United other products. It found potential markets Nations Environment Programme says is in construction materials, chemicals, and necessary to keep the global temperature fuels. However, at best the marketplace rise below 2 °C, the original goal of the could use about 10% of greenhouse gas Paris Agreement on climate change. emissions, the report concluded. And Emission cuts and NETs “are two tools if CO2 is used to produce fuels, little is in the same toolbox,” says Stephen Pacala, gained unless CO2 emissions are again an ecology and environmental biology pro- captured when that fuel is burned. fessor at Princeton University who chaired Globally, advocates of the need to ada 2018 US National Academies of Sciences, dress climate change typically point to the Engineering, and Medicine study US as not doing its share. The of NETs. “They both are necescountry has historically been the sary and are likely to coexist for a global leader in carbon emissions; long, long time.” it is currently second to China for In the following sections, total greenhouse gas emissions C&EN examines some NET worldwide and the world’s largapproaches that are just getting est emitter per capita. President underway. Whether they will Donald J. Trump has announced be able to scale up to meet the plans to withdraw from the Paris Metric tons of need is an open question. The Agreement, and his administragreenhouse numbers are staggering: globally, gases currently tion is working to reverse tighter nearly 50 billion metric tons (t) of emitted to the emission requirements on coalgreenhouse gases are emitted to fired power plants. atmosphere the atmosphere annually, the UN annually Nevertheless, there are some Environment Programme estibright spots for NETs in the US, Source: United mates. Of those emissions, about Pacala says. For example, a 2018 Nations Environment Programme. 37 billion t is CO2 and the rest is federal law, the FUTURE Act,
Contents ▸ Extracting from air, 40 ▸ Burning new fuels, 41 ▸ Making rocks, 41 ▸ Burying underground, 42 ▸ Growing plants, 43
50 billion
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▹ EXTRACTING FROM AIR Georgia Institute of Technology who is a Pulling CO2 out of thin air and piping it technology development adviser for Globdeep into Earth involves viable technoloal Thermostat and was a member of the gies already in use—but they have yet to National Academies study committee. A be tried on a planetary scale. Carbon capture has long been employed stream of air is sent through a liquid or solid sorbent that collects CO2. The sorbent is to treat air in submarines and spacecraft to keep sailors and astronauts alive. Simthen heated to release CO2 in a concentratilar approaches are used throughout the ed form that can be sequestered or used as world to reduce CO2 emissions from coala feedstock for fuels or other products. Conveniently, direct air capture can fired power plants, natural gas processing be placed near the location plants, fertilizer and biofuel of sequestration or where manufacturing sites, and the CO2 might be used as a other industrial point sources. And the technology has feedstock, eliminating the been successfully coupled need for complicated piping ▸ Primary benefits: with underground injection systems. The biggest cost and sequestration of CO2. Mobile, measurable driver is the energy used to ▸ Primary constraints: heat the sorbent to release But the current scale Energy intensive the captured CO2. of these approaches is ▸ Current cost per millions—not billions—of Jones and other researchmetric ton of CO2: metric tons per year. And ers are focusing on developthey’re capturing carbon $200–$1,000 ing sorbent materials that al▸ Estimated removal from relatively concentratlow for spontaneous absorpcapacity: High ed sources, not extracting it tion and low-temperature ▸ Research needs: after it’s been diluted into desorption of CO2. ClimeSorbent development to the atmosphere. works and Global Thermolower energy To capture CO2 at a stat are working with aminebased solid materials, while power plant or other point Carbon Engineering is experimenting with source, costs vary from roughly $50 to a potassium hydroxide solution, Jones says. more than $100 per metric ton. But CO2 Pilot studies show promise at getting costs in the atmosphere is orders of magnitude below $200 per metric ton of CO2, where less concentrated than emissions blasting from a power plant’s smokestack. If the the technology would become commersame technology is used to extract CO2 cially viable. Scaling up and lowering costs will be a “significant but not an impossible from the atmosphere, estimated costs run challenge,” Jones says. from $600 to $1,000 per metric ton. There is currently no marketplace suffiSeveral companies are working to ciently large to support enough innovators increase the capacity and lower costs of to explore and develop the chemical proatmospheric CO2 extraction systems, also cesses and physical machinery that might called direct air capture, to make them commercially viable. Among those compa- decrease the cost to extract CO2 from the air, nies are Carbon Engineering, Climeworks, the National Academies report says. Thus, and Global Thermostat. Climeworks, direct-air-capture advocates are hoping for based in Switzerland, pipes its captured long-term government investments, a carCO2 to greenhouses to enhance vegetable bon tax, other state or federal tax incentives, or some other inducements that would and other plant growth. mirror past efforts to develop solar cells or The basic operating principles of pointsource capture and atmospheric extraction fossil-fuel extraction through hydraulic fracof CO2 are similar, says Christopher Jones, turing—efforts that led to upheavals in energy generation and fossil-fuel production. a chemical engineering professor at the
Air capture at a glance
C R E D I T: CL I MEWO R KS /JUL IA D U NLO P/COV E R I MAGES / N EWS COM
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.”
Climeworks extracts CO2 from the atmosphere and sends it to greenhouses to promote plant growth.
▹ BURNING NEW FUELS
cesses 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.
The primary challenge is growing Combining energy production with carbon enough biomass to make a dent in capture and sequestration could prove to greenhouse gas emissions without afbe a powerful negative-emissions techfecting food production. Capturing and nology. So-called bioenergy systems use sequestering 10 billion t of CO2 annually recently grown biomass as a feedstock to create energy in the forms of electricity from biomass energy production would and heat while permanently storing the rerequire almost 40% of global cropland, sulting carbon dioxide underground, forev- according to the National Academies er, explains Erica Belreport. Realistically, mont, a University of 3.5 billion–5.2 bilWyoming mechanical lion t of CO2 per ▸ Primary benefits: Renewable energy year globally could engineering professor. ▸ Primary constraints: Land Importantly, the be captured withfeedstocks—wood, availability, transportation infrastructure out causing food ▸ Current cost per metric ton of CO2: shortages. energy crops such as elephant grass and As for tech$200–$1,000 switchgrass, agricultur- ▸ Estimated removal capacity: 3.5 nologies to burn al waste, or other biobillion–5.2 billion t of CO2 annually biomass for energy mass sources—have all ▸ Research needs: Increase energy production while been grown recently. capturing CO2, prodensity of crops 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,” 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 Belmont says. “But still, the problems are beginning to demonstrate carbon capture and sequestration at the site. formidable.”
Bioenergy at a glance
C R E D I T: D RAX G RO UP
▹ 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
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
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. FEBRUARY 25, 2019 | CEN.ACS.ORG | C&EN
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▹ BURYING UNDERGROUND and natural gas processing facilities. The For two potentially powerful NETs—direct air capture and bioenergy with carbon rest comes from natural sources. A portion of the CO2 used to extract capture—it’s not enough just to capture CO2. The substance must also be stored. oil remains sequestered in the oil fields; hence, this process is considered a sucFortunately, deep geological reservoirs cessful means to sequester CO2. But the have sufficient space to sequester plenty of CO2, according to the National Acadedemand for CO2 to enhance oil recovery is far too small to curb global warming. mies report. Separate from oil fields, however, deep Sequestering CO2 underground involves geological formations compressing it to a suwith the necessary rock percritical fluid and then characteristics are sprinpiping or shipping it to an kled around the globe. injection well. CompressIn total, they could hold ing the gas allows more more than 2 trillion t CO2 to be transferred and ▸ Primary benefits: Capacity of CO2, enough to subsequestered than if it re▸ Primary constraints: mained in gaseous form. stantially contribute to Transportation infrastructure, At the well, the CO2 is ingreenhouse gas mitigalong-term liability tion strategies. Carbon jected into a geologic for▸ Current cost per metric capture, coupled with unmation that is sufficiently ton of CO2: Low, but amount derground sequestration, deep—typically 1 km or could contribute about farther underground—and unclear ▸ Estimated total storage 14% of the CO2 emission impenetrable so that the CO2 stays in a supercritical capacity: 2 trillion t of CO2 reductions needed to ▸ Research needs: Scale up stabilize the climate at a form, explains Princeton’s injection 2 °C increase, the NationPacala, the National Acadal Academies report says. emies panel chair. Suitable geological formations for storHowever, although the capacity for caring CO2 are porous and permeable reserbon sequestration exists, developing a sevoir rock such as sandstone, limestone, questration infrastructure and addressing dolomite, or mixtures of these rock types. liability issues may be challenging. SequesTypically, the reservoir rock is overlain by tration sites are unlikely to be near large an impermeable rock species such as shale. sources of CO2 emissions, and sequestraThe oil industry has used a similar tion scaled to address global warming will process for years, in which it injects CO2 require development of new large-scale and long-distance infrastructure to transinto nearly depleted oil fields to drive residual oil and natural gas to the surface for port CO2 by pipelines or ships—with the processing. Currently, about 64 million t accompanying risk of leaks or releases of CO2 is injected annually as part of this from accidents. Financial liability and legal responsibility issues will need to be sorted process, which is called enhanced oil reout as quantities grow to billions of metric covery. About one-third of the CO2 used tons and storage stretches to hundreds of for injection comes from captured emisthousands of years. sions from sources such as power plants
Map of basalt formations (red) at and near the surface of the US that would be suitable for CO2 sequestration.
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C R E D I T: US D EPA RTM E NT O F E N E RGY/N AT I O N A L ACA D EM I ES O F S C I E N C ES, E N G I N E E R I N G , A N D M E DI CI N E
Geological sequestration at a glance
▹ GROWING PLANTS
Similar carbon uptake can be achieved Improved coastal zone management, inland, through forest and soil amendreforestation, and enhanced agricultural ments. In these areas, however, sequespractices could increase carbon dioxide tration efforts quickly run into conflicts sequestration capacity while also benefitover land for food versus land for CO2 ing the environment. Tidal wetlands incorsequestration. porating salt marshes, And there are other mangroves, and seagrass considerations. For beds thrive in the soft forests, increasing sesediment and shallow questration means not ▸ Primary benefits: water of estuaries beonly more trees but more tween high and mean Environmental cobenefits trees that grow quickly ▸ Primary constraints: Land sea level. These so-called and close together, to coastal-blue-carbon areas availability increase the amount of ▸ Current cost per metric ton also hold large amounts carbon uptake per unit of CO2: $20–$50 of carbon in their soils area. Forests must be and vegetation and could ▸ Estimated removal maintained over a long contain more. time, which necessitates capacity: Blue carbon, 130 The plants take in the consideration of dismillion t of CO2 annually; some 840 million t of ease, fire, and harvesting reforestation and enhanced CO2 each year. The Naagriculture, 2.5 billion–3 billion t operations, the National Academies report says. of CO2 annually tional Academies report For soil-based organic estimates this level could ▸ Research needs: Impact carbon, enhancing seof sea-level rise and land-use more than double in the changes, increasing crop uptake questration means adding near future with active organic waste to soil as of CO2 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 Some 8,000 salt marsh grass plugs were planted by 90 volunteers at Florida’s constant development pressure. Globally, Perico Preserve during the “Give a Day for the Bay” environmental restoration some 450 million t of CO2 annually is lost program and celebration. to the atmosphere from excavation and well as reducing the decomposition rate of practices could stir up and release capother human activities in coastal areas. organic compounds into CO2. tured carbon back to the atmosphere. Sea-level rise, Troxler says, could also And restored coastal wetland could be be a problem, but that could be avoided Inland CO2 capture is inexpensive and drained or simply dug up, ending any carby allowing sediments to naturally accrue can be deployed quickly, says Richard bon benefit. on estuaries and wetlands—letting the A. Birdsey, a forestry expert at Woods soil keep pace with rising seas rather than Hole Research Center and a National be blocked by coastal development and Academies panel member. However, exJeff Johnson is a freelance writer based in artificial construction. pansion can be difficult. Birdsey notes Washington, DC.
Plant growth at a glance
C R E D I T: TA MPA BAY EST UA RY P ROG RA M
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
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