Carbon sequestration

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See carbon cycle for information on natural carbon sequestration processes

Carbon sequestration is a geoengineering technique for the long-term storage of carbon dioxide or other forms of carbon, for the mitigation of global warming. Carbon dioxide is usually captured from the atmosphere through biological, chemical or physical processes.^[1] It has been proposed as a way to mitigate the accumulation of greenhouse gases in the atmosphere released by the burning of fossil fuels.^[2]

CO₂ may be captured as a pure by-product in processes related to petroleum refining or from flue gases from power generation.^[3] CO₂ sequestration can then be seen as being synonymous with the storage part of carbon capture and storage which refers to the large-scale, permanent artificial capture and sequestration of industrially-produced CO₂ using subsurface saline aquifers, reservoirs, ocean water, aging oil fields, or other carbon sinks.

Sequestration techniques are not instantaneous and when considering their efficacy, consideration has to be given to the fact that they will therefore be acting on future (not current) CO₂ levels. These levels are expected by the IPCC to be higher than today's.

Contents 1 Biological processes 1.1 Ocean iron fertilization 1.2 Ocean urea fertilisation 1.3 Forestry 1.4 Agriculture 1.4.1 Improving primary productivity 1.4.2 Reduce residue reconversion 1.4.3 Other 1.5 Peat production 1.6 Ocean mixing 2 Physical processes 2.1 Biochar burial 2.2 BECCS 2.3 Biomass burial 2.4 Biomass ocean storage 2.5 Carbon capture and storage 3 Chemical techniques 3.1 Ocean basalt storage 3.2 Industrial use 3.3 Chemical scrubbers 3.4 Ocean acid neutralisation 3.5 Ocean hydrochloric acid removal 4 References 5 External links

[edit] Biological processes

Biological processes have a huge effect on the Global carbon cycle. Major climatic fluctuations have been driven by these processes in the past, such as at the Azolla event which started the current Arctic climate. Fossil fuel formation is as a result of such processes, as is the formation of clathrate or limestone. By manipulating such techniques, geoengineers seek to enhance sequestration. Methods such as ocean iron fertilization are examples of such geoengineering techniques.^[4]

[edit] Ocean iron fertilization

Iron fertilization^[5] of the ocean to encourage plankton growth which removes carbon from the atmosphere on a temporary, or arguably permanent basis.^[6][7] This technique is controversial due to difficulties of predicting its effect on the marine ecosystem,^[8] and the potential for side effects or large deviations from expected efficacy. Such effects potentially include the release of nitrogen oxides,^[9][9] and disruption to the nutrient balance in the ocean. Iron fertilization is a natural process and it is the enhancement of this process which is the geoengineering technique.^[4]

[edit] Ocean urea fertilisation

Proposed by Ian Jones with the purpose to fertilize the ocean with urea, a nitrogen rich substance, to encourage phytoplankton growth.^{[citation needed]}

Australian company Ocean Nourishment Corporation (ONC) plans to sink hundreds of tonnes of urea into the ocean, in order to boost the growth of CO₂-absorbing phytoplankton, as a way to combat climate change. In 2007, Sydney-based ONC completed an experiment involving 1 tonne of nitrogen in the Sulu Sea off the Philippines.^[10]

[edit] Forestry

Reforestation of marginal crop and pasture lands to transfer CO₂ from the atmosphere to new biomass.^[11] It is essential to ensure that the carbon does not return to the atmosphere from burning or rotting when the trees die. To this end, it would be important to either manage such forests in perpetuity or use the wood from them for biochar, BECS (see below) or landfill. This technique can give 0.27W/m² of globally-averaged negative forcing,^[12] which is sufficient to reverse the warming effect of 1/6 of current levels of anthropogenic CO₂ emissions.

[edit] Agriculture

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Globally, soils are estimated to contain approximately 1,500 gigatons of organic carbon, more than the total of carbon in vegetation and the atmosphere.^[13]

Modification of agricultural practices is a recognized method of carbon sequestration as soil can act as an effective carbon sink offsetting as much as 20% of carbon dioxide emissions annually.^{[citation needed]}

Emissions in agriculture fall under one of three categories: reducing emission, enhancing removal, or avoiding/ displacing emissions. Specific methods include: conservation or riparian buffers, conservation tillage on croplands, grazing and land management, and biofuel substitution.

Conservation or riparian buffers prevents erosion by planting trees and grasses along streams and croplands. Conservation tillage on croplands buries 30% or more of crop residue in the soil after planting.^{[citation needed]} Tillage methods include no till, ridge till, minimum till, and mulch till. Reduced or eliminated tillage — the products of weed control and more efficient machinery — disturb the soil less than plowing, resulting in increased carbon storage.

Several forms of cropland and rangeland management can actively sequester carbon. The basic approaches are to improve primary productivity and to reduce reconversion of residues from soil carbon to CO₂.

[edit] Improving primary productivity

• Improved crop varieties
• Extending crop rotations
• Improving the efficiency of fertilizer absorption
• More accurate estimations of crop nitrogen needs, improved timing, and more precise application of fertilizer.^{[citation needed]}
• Better irrigation
• Tree planting sequesters carbon within the trees and reduces wind erosion as long as the planting process avoids disturbing the soil

[edit] Reduce residue reconversion

• Using cover crops such as grasses and weeds as temporary cover between agricultural crops
• Concentrate livestock in small paddocks for short periods (days) so they graze lightly but evenly. This encourages roots to grow deeper into the soil. Stock also till the soil with their hooves, grinding old grass and manures into the soil..^[14]
• Avoid burning grasses, allowing sequestration to occur, instead of transforming the remnants to CO₂.^[14]
• Covering bare paddocks with hay or dead vegetation. This protects soil from the sun and allows the soil to hold more water and be more attractive to carbon-capturing microbes.^[14]
• Restoring degraded land slows carbon release while returning the land to agriculture or other use.

[edit] Other

• Reducing pesticide use by adopting less intensive cropping systems^{[citation needed]}
• Restore crop lands damaged by soil erosion, overgrazing, salinization, acidification, etc., by applying certain manures, reducing tillage, retaining crop residue.
• Biofuel substitution reduces greenhouse gas emissions

Adoption of the above techniques depends on soil texture, rainfall, and temperature among other factors.

Sequestration practices may have positive effects on soil, air, and water quality, be beneficial to wildlife, and expand food production. On degraded croplands, an increase of 1 ton of soil carbon pool may increase crop yield by 20 to 40 kilograms per hectare of wheat, 10 to 20 kg/ ha for maize, and 0.5 to 1 kg/ha for cowpeas.

The effects of soil sequestration, however, can be reversed. If the sequestering soil is disrupted or tillage practices are abandoned, the soil becomes a net source or greenhouse gases as carbon is released back into the atmosphere. Typically after 15 to 30 years of sequestration, soil becomes saturated and ceases to absorb more carbon. This implies that there is a global limit to the amount of carbon that soil can hold. Soil is less at risk of releasing its carbon stock from natural causes than forestry because it is less vulnerable to fire and wildlife.

It is difficult to asses the costs of carbon sequestration in soils as many factors affect costs including soil quality, transaction costs, and various externalities such as leakage, unforeseen environmental damage. (1729). Furthermore, because the atmosphere is a public good, it is difficult to motivate farmers to voluntarily adopt expensive agricultural techniques. Governments such as Australia and New Zealand are considering allowing farmers to sell carbon credits once they document that they have sufficiently increased soil carbon content^[14]

^{[15] [16][17] [18] [19] [20] [21] [22]}

[edit] Peat production

Peat bogs are a very important store of carbon. By creating new bogs, or enhancing existing ones, carbon sequestration can be achieved.^[23]

[edit] Ocean mixing

Encouraging various layers of the ocean to mix can move nutrients and dissolved gases around and thus act as a geoengineering approach.^[24] This mixing may be achieved by placing large vertical pipes in the oceans to pump nutrient rich water to the surface, triggering blooms of algae which also store carbon when they die.^{[25][26][27][28]} This produces results somewhat similar to ocean iron fertilization. This technique may result in a short-term rise in CO₂ in the atmosphere, which limits its attractiveness.^[29]

Forced upwelling can give 0.28W/m² of globally-averaged negative forcing,^[12] which is sufficient to reverse the warming effect of 1/6th of the current levels of anthropogenic CO₂ emissions. An alternative forced downwelling approach can give 0.16W/m² of globally-averaged negative forcing,^[12] which is sufficient to reverse the warming effect of about 1/10th of the current levels of anthropogenic CO₂ emissions.

[edit] Physical processes

[edit] Biochar burial

Biochar is charcoal created by pyrolysis of biomass. The resulting charcoal-like material is landfilled, or used as a soil improver to create terra preta.^[30][31] Biogenic carbon is recycled naturally in the carbon cycle. By pyrolysing it to biochar, it’s rendered inert and sequestered in soil. Further, the soil encourages bulking with new organic matter, which gives additional sequestration benefit.

The carbon contained in the soil is therefore unavailable for oxidation to CO₂ and consequential atmospheric release. As a result, the radiative forcing potential of the avoided CO₂ is removed from the planet’s energy balance. This technique is advocated by prominent scientist James Lovelock, creator of the Gaia hypothesis.^[32] It can give 0.52W/m² of globally-averaged negative forcing,^[12] which is sufficient to reverse the warming effect of about 1/3 current levels of anthropogenic CO₂ emissions. According to Simon Shackley, "I would say people are talking more about something in the range of one to two billion tonnes a year."^[33]

The mechanisms related to the carbon sequestration properties of biochar, is referred to as bio-energy with carbon storage, BECS.

[edit] BECCS

The term BECCS refers to Bio-energy with carbon capture and storage ^[34] – Burning biomass in power stations and boilers which utilise carbon capture and storage.^[35] Using this technology with sustainably produced biomass would result in net-negative carbon emissions, as the carbon sequestered during the growth of the biomass would be captured and stored, thus removing carbon dioxide from the atmosphere.^[36]

This technology is sometimes referred to as bio-energy with carbon storage, BECS, though this term can also refer to the carbon sequestration potential in other technologies, such as biochar.

[edit] Biomass burial

Burying biomass (such as trees^[37]) directly, thus sequestering the carbon in the ground rather than allowing it to escape, mimicking the natural processes that created fossil fuels.^[38] Landfill of trash also represents a physical method of sequestration.

[edit] Biomass ocean storage

The production of fossil fuels is a natural process which often involves the ocean burial of biomass in the ocean, often near river mouths which bring large quantities of nutrients and dead material into the ocean. Transporting material, such as crop waste, out to sea and allowing it to sink into deep ocean storage has been proposed as a means of sequestration of carbon.^[39][40] International restrictions on marine dumping may restrict or prevent use of this technique at present.

[edit] Carbon capture and storage

Carbon dioxide can be injected into old oil wells and other geological features, or can be stored in pure form in the deep ocean.^{[citation needed]}

The first large-scale CO₂ sequestration project (1996) is called Sleipner, and is located in the North Sea where Norway's StatoilHydro strips carbon dioxide from natural gas with amine solvents and disposes of this carbon dioxide in a deep saline aquifer. In 2000, a coal-fueled synthetic natural gas plant in Beulah, North Dakota, became the world's first coal using plant to capture and store carbon dioxide.^[41]

CO₂ has been used extensively in enhanced crude oil recovery operations in the United States beginning in 1972.^[42] There are in excess of 10,000 CO₂ wells in the state of Texas alone. The gas comes in part from anthropogenic sources, but principally from large naturally-occurring geologic formations of CO₂. It is transported to the oil-producing fields through a large network of over 5,000 kilometres (3,100 mi) of CO₂ pipelines. The use of CO₂ for enhanced oil recovery (EOR) methods in heavy oil reservoirs in the Western Canadian Sedimentary Basin (WCSB) has also been proposed.^[43] However, cost of transport remains an important hurdle. A similar CO₂ pipeline system to that of Texas does not yet exist in the WCSB that could connect most of the sources for CO₂ in Canada associated with the mining and upgrading operations in the Athabasca oil sands, with the subsurface heavy oil reservoirs that could most benefit from CO₂ injection hundreds of kilometres to the south.

[edit] Chemical techniques

Carbon, in the form of CO₂ can be removed from the atmosphere by chemical processes, and stored in stable carbonate mineral forms. This process is known as carbon sequestration by mineral carbonation or mineral sequestration. The process involves reacting carbon dioxide with abundantly available metal oxides–either magnesium oxide (MgO) or calcium oxide (CaO)–to form stable carbonates. These reactions are exothermic and occur naturally (e.g., the weathering of rock over geologic time periods).^[44][45]

CaO + CO₂ → CaCO₃

MgO + CO₂ → MgCO₃

In nature calcium and magnesium are found typically as calcium and magnesium silicates (such as forsterite and serpentine) and not as binary oxides. For forsterite and serpentine the reactions are:

Mg₂SiO₄ + 2CO₂ = 2MgCO₃ + SiO₂

Mg₃Si₂O₅(OH)₄ + 3CO₂ = 3MgCO₃ + 2SiO₂ + 2H₂O

The following table lists principal metal oxides of Earth's Crust. Theoretically up to 22% of this mineral mass is able to form carbonates.

Earthen Oxide	Percent of Crust	Carbonate	Enthalpy change (kJ/mol)
SiO2	59.71
Al2O3	15.41
CaO	4.90	CaCO3	-179
MgO	4.36	MgCO3	-117
Na2O	3.55	Na2CO3
FeO	3.52	FeCO3
K2O	2.80	K2CO3
Fe2O3	2.63	FeCO3
	21.76	All Carbonates

These reactions are favored at low temperatures.^[44] This process occurs naturally over geologic time frames and is responsible for much of the surface limestone. The reaction rate can be made faster, for example by reacting at higher temperatures and/or pressures, or by pre-treatment of the minerals, although this method requires additional energy.

CO₂ naturally reacts with peridotite rock in surface exposures of ophiolites, notably in Oman. It has been suggested that this process can be enhanced to carry out natural mineralisation of CO₂.^[46][47]

[edit] Ocean basalt storage

Carbon dioxide sequestration in basalt involves the injecting of CO₂ into deep-sea formations. The CO₂ first mixes with seawater and then reacts with the basalt, both of which are alkaline-rich elements. This reaction results in the release of Ca²⁺ and Mg²⁺ ions forming stable carbonate minerals.^[48]

Underwater basalt offers a good alternative to other forms of oceanic carbon storage because it has a number of trapping measures to ensure added protection against leakage. These measures include “geothermal, sediment, gravitational and hydrate formation.” Because CO₂ hydrate is denser than CO₂ in seawater, the risk of leakage is minimal. And assuming the injected CO₂ is added into the ocean at depths greater than 2,700 meters, the seawater will have a greater density than the carbon dioxide, causing it to sink.^[49]

Possible injection site: Juan de Fuca plate

Researchers at the Lamont-Doherty Earth Observatory found that the Juan de Fuca plate of the western coast of the United States has a possible storage capacity of 208 Gtons of carbon. This could cover the entire U.S. carbon emissions for over 100 years. When speaking of the possibilities for basalt storage, David Goldberg of the Earth Observatory expressed the belief that while years of research are still required, there is no technical reason why this method of storage would not work. ^[50]

[edit] Industrial use

Traditional cement manufacture releases large amounts of carbon dioxide, but newly developed cement types from Novacem^[51] can absorb CO₂ from ambient air during hardening.^[52] A similar technique was pioneered by TecEco, who have been producing EcoCement since 2002.^[53]

In Estonia, oil shale ash, generated by the oil shale-fired power stations could be used as sorbents for CO₂ mineral sequestration. The amount of CO₂ captured averaged 60–65% of the carbonaceous CO₂ and 10–11% of the total CO₂ emissions.^[54]

[edit] Chemical scrubbers

Various carbon dioxide scrubbing processes have been proposed to remove CO₂ from the air, usually using a variant of the Kraft process. Carbon dioxide scrubbing variants exist based on potassium carbonate^[55] which can be used to create liquid fuels, or on sodium hydroxide.^[55][56][57] These notably include the artificial trees proposed by Klaus Lackner with the purpose to remove carbon dioxide from the atmosphere using chemical scrubbers.^[58][59] This technique can give 1.43W/m² of globally-averaged negative forcing,^[12] which is almost sufficient to reverse the warming effect of current levels of anthropogenic CO₂ emissions.

[edit] Ocean acid neutralisation

Adding crushed limestone^[60] or volcanic rock^[61] to oceans to restore the solubility pump, which naturally tends to remove excess CO₂ from the atmosphere. This technique can give 0.46W/m² of globally-averaged negative forcing,^[12] which is sufficient to reverse the warming effect of around a third of current levels of anthropogenic CO₂ emissions. Various other scientists have explored this technique, and suggested a variety of different bases which may be added to the ocean. ^{[62][63][64][65][66][67]}

[edit] Ocean hydrochloric acid removal

Chemically removing hydrochloric acid from the ocean by electrolysis and neutralize the acid through reactions with silicate minerals or rocks.^[68]

[edit] References

[edit] External links

• Carbon Sequestration Leadership Forum International carbon capture and storage initiative.
• Scottish Centre for Carbon Storage Research Carbon Capture and Storage Research in Edinburgh, Scotland.
• UK Carbon Capture and Storage Consortium Overview of the UK academic consortium focused on researching issues related to Carbon Capture and Storage.
• Sieves put a lid on greenhouse gas
• www.osti.gov The capture, utilization and disposal of carbon dioxide from fossil fuel-fired power plants.
• [2] Carbon Capture and Storage Information Center (Chinese + English)
• Carbon Sequestration: Science, Technology, and Policy MIT program covers carbon capture and storage (CCS)