Carbon dioxide removal

Carbon dioxide removal (CDR) refers to a number of technologies, the objective of which is the large-scale removal of carbon dioxide from the atmosphere.[1][2] Among such technologies are bio-energy with carbon capture and storage, biochar, ocean fertilization, enhanced weathering, and direct air capture when combined with storage.[1] CDR is a different approach than removing CO2 from the stack emissions of large fossil fuel point sources, such as power stations. The latter reduces emission to the atmosphere but cannot reduce the amount of carbon dioxide already in the atmosphere. As CDR removes carbon dioxide from the atmosphere, it creates negative emissions, offsetting emissions from small and dispersed point sources such as domestic heating systems, airplanes and vehicle exhausts.[3][4] It is regarded by some as a form of climate engineering,[1] while other commentators describe it as a form of carbon capture and storage or extreme mitigation.[5] Whether CDR would satisfy common definitions of "climate engineering" or "geoengineering" usually depends upon the scale on which it would be undertaken.

The likely need for CDR has been publicly expressed by a range of individuals and organizations involved with climate change issues, including IPCC chief Rajendra Pachauri,[6] the UNFCCC executive secretary Christiana Figueres,[7] and the World Watch Institute.[8] Institutions with major programs focusing on CDR include the Lenfest Center for Sustainable Energy at the Earth Institute, Columbia University,[9] and the Climate Decision Making Center,[10] an international collaboration operated out of Carnegie-Mellon University's Department of Engineering and Public Policy.

The mitigation effectiveness of air capture is limited by societal investment, land use, availability of geologic reservoirs, and leakage. The reservoirs are estimated to be sufficient to for storing at least 545 GtC.[11] Storing 771 GtC would cause a 186 ppm atmospheric reduction.[12] In order to return the atmospheric CO2 content to 350 ppm we need atmospheric reduction of 50 ppm plus an additional 2 ppm per year of current emissions.

General

Carbon dioxide removal is different from reducing emissions, as the former produces an outlet of carbon dioxide from Earth's atmosphere, whereas the latter decreases the inlet of carbon dioxide to the atmosphere. Both have the same net effect, but for achieving carbon dioxide concentration levels below present levels, carbon dioxide removal is critical. Also for meeting higher concentration levels, carbon dioxide removal is increasingly considered to be crucial as it provides the only possibility to fill the gap between needed reductions to meet mitigation targets and global emission trends.

In the OECD Environmental Outlook to 2050 released at the 2011 United Nations Climate Change Conference, the authors commented on the need for negative emissions, stating "Achieving lower concentration targets (450 ppm) depends significantly on the use of BECCS".

A carbon dioxide sink such as a concentrated group of plants or any other primary producer that binds carbon dioxide into biomass, such as within forests and kelp beds, is not carbon negative, as sinks are not permanent. A carbon dioxide sink of this type moves carbon, in the form of carbon dioxide, from the atmosphere or hydrosphere to the biosphere. This process could be undone, for example by wildfires or logging.

Carbon dioxide sinks that store carbon dioxide in the Earth's crust by injecting it into the subsurface, or in the form of insoluble carbonate salts (mineral sequestration), are considered carbon negative. This is because they are removing carbon from the atmosphere and sequestering it indefinitely and presumably for a considerable duration (thousands to millions of years). However, Carbon Capture technology remains, at best, theoretical and is yet to reach more than 33% efficiency. Furthermore, this process could be rapidly undone, for example by earthquakes or mining.

Methods

Bio-energy with carbon capture & storage

Bio-energy with carbon capture and storage, or BECCS, uses biomass to extract carbon dioxide from the atmosphere, and carbon capture and storage technologies to concentrate and permanently store it in deep geological formations.

BECCS is currently (as of October 2012) the only CDR technology deployed at full industrial scale, with 550 000 tonnes CO2/year in total capacity operating, divided between three different facilities (as of January 2012).[13][14][15][16][17]

The Imperial College London, the UK Met Office Hadley Centre for Climate Prediction and Research, the Tyndall Centre for Climate Change Research, the Walker Institute for Climate System Research, and the Grantham Institute for Climate Change issued a joint report on carbon dioxide removal technologies as part of the AVOID: Avoiding dangerous climate change research program, stating that "Overall, of the technologies studied in this report, BECCS has the greatest maturity and there are no major practical barriers to its introduction into today’s energy system. The presence of a primary product will support early deployment."[18]

According to the OECD, "Achieving lower concentration targets (450 ppm) depends significantly on the use of BECCS".[19]

Biochar

Biochar is created by the pyrolysis of biomass, and is under investigation as a method of carbon sequestration. Biochar is a charcoal that is used for agricultural purposes which also aids in carbon sequestration, the capture or hold of carbon. It is created using a process called pyrolysis, which is basically the act of high temperature heating biomass in an environment with low oxygen levels. What remains is a material known as char, similar to charcoal but is made through a sustainable process, thus the use of biomass.[20] Biomass is organic matter produced by living organisms or recently living organisms, most commonly plants or plant based material.[21] The offset of GHG emission, if biochar were to be implemented, would be a maximum of 12%. This equates to about 106 metric tons of CO2 equivalents. On a medium conservative level, it would be 23% less than that, at 82 metric tons.[22] A study done by the UK Biochar Research Center has stated that, on a conservative level, biochar can store 1 gigaton of carbon per year. With greater effort in marketing and acceptance of biochar, the benefit could be the storage of 5–9 gigatons per year of carbon in biochar soils.[23]

Enhanced weathering

Enhanced weathering refers to chemical approach to remove carbon dioxide involving land or ocean-based techniques. Examples of land based enhanced weathering techniques are in-situ carbonation of silicates. Ultramafic rock, for example, has the potential to store from hundreds to thousands of years worth of CO2 emissions according to estimates.[24] [25] Ocean-based techniques involve alkalinity enhancement, such as, grinding, dispersing and dissolving olivine, limestone, silicates, or calcium hydroxide to address ocean acidification and CO2 sequestration. Enhanced weathering is considered as one of the least expensive of geoengineering options. One example of a research project on the feasibility of enhanced weathering is the CarbFix project in Iceland.[26][27][28]

Direct air capture (DAC)

Carbon dioxide can be removed from ambient air through chemical processes, sequestered, and stored. Traditional modes of carbon capture such as precombustion and postcombustion CO2 capture from large point sources can help slow the rate of increase of the atmospheric CO2 concentration, but only the direct removal of CO2 from the air, or “direct air capture” (DAC), can actually reduce the global atmospheric CO2 concentration if combined with long-term storage of CO2.

A few engineering proposals have been made for removing CO2 from the atmosphere, but work in this area is still in its infancy. [29] Among the main technologies proposed, three of them stand out: Causticization with alkali and alkali-earth hydroxides,[30] carbonation,[31] and organic−inorganic hybrid sorbents consisting of amines supported in porous adsorbents. A 2016 article reviews the research in these various areas.[29]

One proposed method is by so-called artificial trees.[32][33] This concept, proposed by climate scientist Wallace S. Broecker and science writer Robert Kunzig,[34] imagines huge numbers of artificial trees around the world to remove ambient CO2. The technology is now being pioneered by Klaus Lackner, a researcher at the Earth Institute, Columbia University,[35] whose artificial tree technology can suck up to 1,000 times more CO2 from the air than real trees can, at a rate of about one ton of carbon per day if the artificial tree is approximately the size of an actual tree.[36][37] The CO2 would be captured in a filter and then removed from the filter and stored.

The chemistry used is a variant of that described below, as it is based on sodium hydroxide. However, in a more recent design proposed by Klaus Lackner, the process can be carried out at only 40 °C by using a polymer-based ion exchange resin, which takes advantage of changes in humidity to prompt the release of captured CO2, instead of using a kiln. This reduces the energy required to operate the process.[38]

Another substance which can be used are Metal-organic frameworks (or MOF's).[39] A special MOF has been made specifically for locking CO2 by Joeri Denayer.[40]

In 2008, the Discovery Channel covered[41] the work of David Keith,[42] of University of Calgary, who built a tower, 4 feet wide and 20 feet tall (1.2×6.1 meters), with a fan at the bottom that sucks air in, which comes out again at the top. In the process, about half the CO2 is removed from the air.

This device uses the chemical process described in detail below. The system demonstrated on the Discovery Channel was a 1/90,000th scale test system of the capture section; the reagents are regenerated in a separate facility. The main costs of a full plant will be the cost to build it, and the energy input to regenerate the chemicals and produce a pure stream of CO2.

To put this into perspective, people in the U.S. emit about 20 tonnes of CO2 per person annually.[43] In other words, each person in the U.S. would require a tower like the one featured by the Discovery Channel to remove this amount of CO2 from the air, requiring an annual 2 megawatt-hours of electricity to operate it. By comparison, a refrigerator consumes about 1.2 megawatt-hours annually (2001 figures).[44] But, by combining many small systems such as this into one large system, the construction costs and energy use can be reduced.

It has been proposed that the Solar updraft tower to generate electricity from thermal air currents also be used at the same time for amine gravity scrubbing of CO2.[45] Some heat would be required to regenerate the amine.

A similar CO2 scrubber has also been built by Carbon Engineering. Besides simply focusing on capturing the CO2, the company also puts emphasis on reuse of the CO2, for example in the production of fuels, which would thus be carbon-neutral.[46][47]

Direct air capture has been proposed as a way of generating carbon-neutral organic chemicals, by harvesting the atmospheric compounds and then using them in the production and synthesis of polymers and fuels.

The Swiss-based Climeworks built the first industrial scale direct air capturing plant in Hinwil, in the canton of Zurich, Switzerland. It started up in May 2017,[48] and could scrub up to 900 metric tons of CO2 per year using heat from a local waste incineration plant. The CO2 was then pumped into local greenhouses, where it was used to help grow vegetables like tomatoes and cucumbers.[49][50]

Climeworks and Reykjavik Energy also started up a small test direct air capturing plant in Hellisheidi, Iceland in October 2017. Using waste heat from a local geothermal power plant, the test plant captured up to 50 tons of CO2 per year which was bound with water and injected over 700 metres underground into basaltic bedrock. The CO2 reacted with the basalt to form solid carbonate minerals.[48][51]

Ocean fertilization

Ocean fertilization or ocean nourishment is a type of climate engineering based on the purposeful introduction of nutrients to the upper ocean to increase marine food production and to remove carbon dioxide from the atmosphere. A number of techniques, including fertilization by iron, urea and phosphorus have been proposed.

Example CO2 scrubbing chemistry

Calcium oxide

Calcium oxide (quicklime) will absorb CO2 from atmospheric air mixed with steam at 400 °C (forming calcium carbonate) and release it at 1,000 °C. This process, proposed by A. Steinfeld, can be performed using renewable energy from thermal concentrated solar power.[52] Quicklime is made by heating limestone to release the CO2 within it. Quicklime is mixed with sand for brick building as mortar, where it hardens by absorption of CO2.

Sodium hydroxide

Zeman and Lackner outlined a specific method of air capture using sodium hydroxide.[53] Carbon Engineering, a Calgary, Alberta firm founded in 2009 and partially funded by Bill Gates, is developing a process to capture carbon dioxide using a solution of potassium hydroxide mixed with some water at their pilot plant. They hope to create and sell synthetic fuels at a cost of $100 a ton.[54]

Direct air capture (DAC) process example using NaOH

Main steps in a DCA process that uses NaOH (caustic soda) as the absorber.[55]

Among the technologies studied for direct air capture (DAC), the use of aqueous hydroxide sorbents is one of the most promising approaches.[29] In this process, CO2 from the air is chemically dissolved into NaOH(aq) solution as Na2CO3; the Na2CO3 is then reacted with solid Ca(OH)2, which regenerates the solvent and produces CaCO3 crystals; lastly, heat is applied to the CaCO3 crystals to produce pure CO2 gas.[55]

Air is pumped through the CO2 absorber as the first step of this process.[55][56] CO2 absorber for DAC are designed either as a counter-current spray tower or as a counter-current thin-falling-film contractor to maximize the contact area between the air and the solvent and thus maximize the absorption driving force.[55][56] The solvent is regenerated in the causticization unit by reacting the Na2CO3 with Ca(OH)2, which also transfers the captured CO2 to the form of CaCO3 solid crystals.[55] A mechanical filter is then used to separate the CaCO3 crystals from the water.[55] Since the crystals come out wet from the filter, they are dried in a steam dryer.[55] Then the dry crystals are heated in a furnace to produce CaO and pure CO2 gas.[55] The CaO is then hydrated to regenerate the Ca(OH)2 used for the causticization reaction.[55] The pure CO2 stream is then compressed and ready to be transported for geologic sequestration, EOR, or other commercial applications.

1 M NaOH (aq) is a typical solvent concentration because this concentrations is limited by the causticization reaction that regenerates the solvent it is not too far from the practical maximum of 2 M NaOH.[55] The furnace/kiln can be powered renewably or by burning fuel on-site with pure oxygen produces in an on-site air separation unit.

NaOH is economically competitive with other absorbents (e.g. Amines) used for DAC processes.[55] DAC processes are energy intensive.[55][56] Calcination (at the furnace) is the most energy intensive step of this process.[55][56]

Economic issues

A crucial issue for CDR methods is their cost, which differs substantially among the different technologies: some of these are not sufficiently developed to perform cost assessments. In 2011 the American Physical Society estimated the costs for direct air capture to be $600/tonne with optimistic assumptions.[57] A 2018 study found this estimate lowered to between $94 and $232 per tonne.[58][59] The IEA Greenhouse Gas R&D Programme and Ecofys provides an estimate that 3.5 billion tonnes could be removed annually from the atmosphere with BECCS (Bio-Energy with Carbon Capture and Storage) at carbon prices as low as €50 per tonne,[60] while a report from Biorecro and the Global Carbon Capture and Storage Institute estimates costs "below €100" per tonne for large scale BECCS deployment.[5]

Risks, problems and criticisms

CDR is slow to act, and requires a long-term political and engineering program to effect.[61] CDR is even slower to take effect on acidified oceans. In a Business as usual concentration pathway, the deep ocean will remain acidified for centuries, and as a consequence many marine species are in danger of extinction.[62]

See also

References

  1. 1 2 3 "Geoengineering the climate: science, governance and uncertainty". The Royal Society. 2009. Retrieved 2011-09-10.
  2. ↑ Minx, Jan C; Lamb, William F; Callaghan, Max W; Fuss, Sabine; Hilaire, Jérôme; Creutzig, Felix; Amann, Thorben; Beringer, Tim; De Oliveira Garcia, Wagner; Hartmann, Jens; Khanna, Tarun; Lenzi, Dominic; Luderer, Gunnar; Nemet, Gregory F; Rogelj, Joeri; Smith, Pete; Vicente Vicente, Jose Luis; Wilcox, Jennifer; Del Mar Zamora Dominguez, Maria (2018). "Negative emissions: Part 1 – research landscape and synthesis". Environmental Research Letters. 13 (6): 063001. doi:10.1088/1748-9326/aabf9b. Retrieved 2018-05-31.
  3. ↑ Vergragt, P.J.; Markusson, N.; Karlsson, H. (2011). "Carbon capture and storage, bio-energy with carbon capture and storage, and the escape from the fossil-fuel lock-in". Global Environmental Change. 21 (2): 282–92. doi:10.1016/j.gloenvcha.2011.01.020.
  4. ↑ Azar, C.; Lindgren, K.; Larson, E.; Möllersten, K. (2006). "Carbon Capture and Storage from Fossil Fuels and Biomass – Costs and Potential Role in Stabilizing the Atmosphere". Climatic Change. 74: 47–79. doi:10.1007/s10584-005-3484-7.
  5. 1 2 "Global Status of BECCS Projects 2010". Biorecro and The Global Carbon Capture and Storage institute. 2011. Archived from the original on 2013-09-28. Retrieved 2011-09-10.
  6. ↑ Pagnamenta, Robin (2009-12-01). "Carbon must be sucked from air, says IPCC chief Rajendra Pachauri". Times Online. London. Retrieved 13 December 2009.
  7. ↑ Harvey, Fiona (2011-06-05). "Global warming crisis may mean world has to suck greenhouse gases from air". Guardian Online. Retrieved 10 September 2011.
  8. ↑ Hollo, Tim (2009-01-15). "Negative emissions needed for a safe climate". Retrieved 10 September 2011.
  9. ↑ "National Geographic Magazine - NGM.com". Ngm.nationalgeographic.com. 2013-04-25. Retrieved 2013-09-22.
  10. ↑ "Snatching Carbon Dioxide from the Atmosphere" (PDF). Cdmc.epp.cmu.edu. Retrieved 2013-09-22.
  11. ↑ "Carbon dioxide capture and storage" (PDF). IPCC. 2005. Retrieved 1 September 2016.
  12. ↑ Lenton, TM; NE Vaughan (2009). "The radiative forcing potential of different climate geoengineering options". Atmospheric Chemistry and Physics. 9 (15): 2559–608. doi:10.5194/acp-9-5539-2009.
  13. ↑ "Global Status of BECCS Projects 2010". Biorecro AB, Global CCS Institute. 2010. Retrieved 2012-01-20.
  14. ↑ "Global Technology Roadmap for CCS in Industry Biomass-based industrial CO2 sources: biofuels production with CCS" (PDF). ECN. 2011. Retrieved 2012-01-20.
  15. ↑ "First U.S. large demonstration-scale injection of CO2 from a biofuel production facility begins". Retrieved 20 January 2012.
  16. ↑ "Ethanol plant to sequester CO2 emissions". Archived from the original on 10 March 2011. Retrieved 20 January 2012.
  17. ↑ "Production Begins at Biggest Ethanol Plant in Kansas". Retrieved 20 January 2012.
  18. ↑ "The Potential for the Deployment of Negative Emissions Technologies in the UK" (PDF). Grantham Institute for Climate Change, Imperial College. 2010. Retrieved 2012-01-16.
  19. ↑ Archived May 26, 2013, at the Wayback Machine.
  20. ↑ "What is biochar?". UK Biochar research center. University of Edinburgh Kings Buildings Edinburgh. Retrieved 25 April 2016.
  21. ↑ "What is Biomass?". Biomass Energy Center. Direct.gov.uk. Retrieved 25 April 2016.
  22. ↑ "Climate change and Biochar". International Biochar Initiative. International Biochar Initiative. Retrieved 25 April 2016.
  23. ↑ "Biochar reducing and removing CO2 while improving soils: A significant sustainable response to climate change" (PDF). UKBRC. UK Biochar research Center. Retrieved 25 April 2016.
  24. ↑ "Maps show rocks ideal for sequestering carbon - NYTimes.com". archive.nytimes.com. Retrieved 2018-05-15.
  25. ↑ U.S. Department of the Interior. "Mapping the Mineral Resource Base for Mineral Carbon-Dioxide Sequestration in the Conterminous United States" (PDF). U.S. Geological Survey. Data Series 414. line feed character in |title= at position 38 (help)
  26. ↑ "CarbFix Project | Global Carbon Capture and Storage Institute". www.globalccsinstitute.com. Retrieved 2018-05-15.
  27. ↑ "The CarbFix Project". www.or.is (in Icelandic). 2017-08-22. Retrieved 2018-05-15.
  28. ↑ "Turning Carbon Dioxide Into Rock, and Burying It". The New York Times. 2015-02-09. ISSN 0362-4331. Retrieved 2018-05-15.
  29. 1 2 3 Sanz-Pérez, E.S.; Murdock, C.R.; Didas, S.A.; Jones, C.W. (2016). "Direct Capture of CO2 from Ambient Air". Chem. Rev. 116 (19): 11840–76. doi:10.1021/acs.chemrev.6b00173. PMID 27560307.
  30. ↑ Lackner, K.S.; Ziock, H.; Grimes, P. (1999). Carbon Dioxide Extraction from Air: Is It an Option?. Proceedings of the 24th Annual Technical Conference on Coal Utilization & Fuel Systems. pp. 885–896.
  31. ↑ Nikulshina, V.; Ayesa, N.; Gálvez, M.E.; Stainfeld, A. (2016). "Feasibility of Na–Based Thermochemical Cycles for the Capture of CO2 from air. Thermodynamic and Thermogravimetric Analyses". Chem. Eng. J. 140 (1–3): 62–70. doi:10.1016/j.cej.2007.09.007.
  32. ↑ "New Device Vacuums Away Carbon Dioxide". LiveScience. 2007-05-01. Retrieved 2009-10-29.
  33. ↑ Adam, David (2008-05-31). "Could US scientist's 'CO2 catcher' help to slow warming? | Environment". London: The Guardian. Retrieved 2009-10-29.
  34. ↑ Artificial trees designed by Wallace Broecker
  35. ↑ The Earth Institute, Columbia University http://www.earth.columbia.edu/sections/view/9
  36. ↑ "Cleaning up the Carbon Mess". Energy Now. July 31, 2011. Retrieved 2013-09-22.
  37. ↑ 'Artificial trees' to cut carbon. Retrieved November 7, 2010.
  38. ↑ "Lenfest Center for Sustainable Energy". Energy.columbia.edu. Retrieved 2013-09-22.
  39. ↑ Scrubbing CO2 with MOFs
  40. ↑ Wetenschap redt de wereld docu on Joeri Denayer's MOFs
  41. ↑ – Discovery Channel, 2008 Archived January 19, 2009, at the Wayback Machine.
  42. ↑ – David Keith Archived February 1, 2009, at the Wayback Machine.
  43. ↑ Vaughan, Adam (2009-09-02). "Carbon emissions per person, by country". the Guardian. Retrieved 2016-12-05.
  44. ↑ "End-Use Consumption of Electricity by End Use and Appliance". Eia.doe.gov. Retrieved 2009-10-29.
  45. ↑ The Methane Economy
  46. ↑ Forbes on Carbon Engineering's carbon scrubber
  47. ↑ More info on the co2 scrubber
  48. 1 2 Doyle, Alister (2017-10-11). "From thin air to stone: greenhouse gas test starts in Iceland". Reuters. Retrieved 2017-12-04.
  49. ↑ "The World's First Commercial Carbon Capture Plant Will Turn Pollution Into Cash". Co.Exist. 2015-10-22. Retrieved 2017-01-05.
  50. ↑ McGrath, Matt (2017-11-15). "Climate's magic rabbit: Pulling CO2 out of thin air". BBC News. Retrieved 2017-12-04.
  51. ↑ Prroctor, Daryll (2017-12-01). "Test of Carbon Capture Technology Underway at Iceland Geothermal Plant". POWER Magazine. Retrieved 2017-12-04.
  52. ↑ "Can technology clear the air?". New Scientist. 2009-01-12. Retrieved 2009-10-29.
  53. ↑ Zeman, F.S.; Lackner, K.S. (2004). "Capturing carbon dioxide directly from the atmosphere". World Resour. Rev. 16: 157–72.
  54. ↑ Anne Eisenberg (January 5, 2013). "Pulling Carbon Dioxide Out of Thin Air". The New York Times. Retrieved January 8, 2013.
  55. 1 2 3 4 5 6 7 8 9 10 11 12 13 Zeman, Frank (2007-11-01). "Energy and Material Balance of CO2 Capture from Ambient Air". Environmental Science & Technology. 41 (21): 7558–63. doi:10.1021/es070874m. ISSN 0013-936X.
  56. 1 2 3 4 Berend Smit, Jeffrey A. Reimer, Curtis M. Oldenburg and Ian C. Bourg (2014). Introduction to Carbon Capture and Sequestration, Vol 1.
  57. ↑ "Direct Air Capture of CO2 with Chemicals". The American Physical Society. 2011-06-01. Retrieved 2011-09-10.
  58. ↑ Keith, David W.; Holmes, Geoffrey; St. Angelo, David; Heidel, Kenton (2018). "A Process for Capturing CO2 from the Atmosphere". Joule. doi:10.1016/j.joule.2018.05.006.
  59. ↑ "Climate Change Breakthrough: Large-scale capture of atmospheric CO₂ shown to be feasible and affordable". 2018-06-07.
  60. ↑ "Potential for Biomass and Carbon Capture and Storage" (PDF). IEA Greenhouse Gas R&D Programme. 2011-07-06. Retrieved 2011-09-10.
  61. ↑ Cao, L.; Caldeira, K. (2010). "Atmospheric carbon dioxide removal: Long-term consequences and commitment". Environmental Research Letters. 5 (2): 024011. doi:10.1088/1748-9326/5/2/024011.
  62. ↑ Mathesius, Sabine; Hofmann, Matthias; Caldeira, Ken; Schellnhuber, Hans Joachim (2015). "Long-term response of oceans to CO2 removal from the atmosphere". Nature Climate Change. 5 (12): 1107–13. doi:10.1038/nclimate2729.
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