Carbon is a very intriguing element, it is somewhat of a necessary evil, while on one hand due to its catenation property it forms the very basis of known life on the face of the earth, while also threatening that very life’s existence because of it being a major greenhouse gas. But it will be grossly unfair to blame carbon for this, since the beginning of the industrial revolution man has incessantly used fossil fuels to drive his energy needs, this in turn has led massive emissions, which have trapped a lion’s share of earth’s emitted heat(which we don’t need) leading to speeding up of the Milankovitch’s cycle and earth’s climate changing. As per data by the United States Environmental Protection Agency, global carbon emissions from fossil fuels have significantly increased since 1900. Since 1970, CO2 emissions have increased by about 90%, with emissions from fossil fuel combustion and industrial processes contributing about 78% of the total greenhouse gas emissions increase from 1970 to 2011. Agriculture, deforestation, and other land-use changes have been the second-largest contributors.
So, we understand that it’s not long that we face a global meltdown crisis, but the question arises is there any way we can stop this problem? Is there some way we can get rid of our CO2 emission without having to dump it in our atmosphere? Well the answer to this question is carbon sequestration, through this piece which is the second part to our series in geoengineering, I aim to investigate the nobility of the idea of carbon sequestration and how it may revolutionize our point of view on climate change mitigation.
As defined by the USGS, carbon sequestration is the process of capturing and storing atmospheric carbon dioxide. It is one method of reducing the amount of carbon dioxide in the atmosphere with the goal of reducing global climate change. Carbon sequestration occurs both naturally and as a result of anthropogenic activities and typically refers to the storage of carbon that has the immediate potential to become carbon dioxide gas. In response to growing concerns about climate change resulting from increased carbon dioxide concentrations in the atmosphere, considerable interest has been drawn to the possibility of increasing the rate of carbon sequestration through changes in land use and forestry and also through geoengineering techniques such as carbon capture and storage.
Reservoirs that retain carbon and keep it from entering Earth’s atmosphere are known as carbon sinks. Geologically, the earth’s sub-surface would act as a very good sink provided the bounding layers are impermeable, this ensures that the stored gases stays in there for hundreds to millions of years. In general terms a carbon capture and sequestration procedure would involve the following 3 step process:
- Capturing the CO2 from coal or natural based powered plants
- Transporting the stored carbon usually through pipelines
- Storage of CO2 into deep non-porous layers of rock that trap it and prevent it from migrating upward.
Once CO2 is captured from a gas or coal-fired power plant, it would be compressed to ≈100 bar so that it would be a supercritical fluid. In this fluid form, the CO2 would be easy to transport via pipeline to the place of storage. The CO2 would then be injected deep underground, typically around 1 km, where it would be stable for hundreds to millions of years. At these storage conditions, the density of supercritical CO2 is 600 to 800 kg / m3. For consumers, the cost of electricity from a coal-fired power plant with carbon sequestration is estimated to be 0.01–0.05 $ / kWh higher than without carbon sequestration. For reference, the average cost of electricity in the US in 2004 was 0.0762 $ / kWh. In other terms, the cost of carbon sequestration would be 20–70 $/ton of CO2 captured.
The important parameters in determining a good site for carbon storage are: rock porosity, rock permeability, absence of faults, and geometry of rock layers. The medium in which the CO2 is to be stored ideally has a high porosity and permeability, such as sandstone or limestone. Sandstone can have a permeability ranging from 1 to 10−5 Darcy, and can have a porosity as high as ≈30%. The porous rock must be capped by a layer of low permeability which acts as a seal and prevents the CO2 from escaping. Shale is an example of a very good cap rock, with a permeability of 10−5 to 10−9 Darcy. Once injected, the CO2 plume will rise via buoyant forces, since it is less dense than its surroundings. Once it encounters a cap rock, it will spread laterally until it encounters a gap.
Another major method of carbon sequestration, which is exclusive to the ocean is called basalt storage, it 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 containing alkaline-rich compounds. This reaction results in the release of Ca2+ and Mg2+ ions forming stable carbonate minerals. 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 “geochemical, sediment, gravitational and hydrate formation.” Because CO2 hydrate is denser than CO2 in seawater, the risk of leakage is minimal. Injecting the CO2 at depths greater than 2,700 meters (8,900 ft) ensures that the CO2 has a greater density than seawater, causing it to sink. One possible injection site is Juan de Fuca plate. Researchers at the Lamont-Doherty Earth Observatory found that this plate at the western coast of the United States has a possible storage capacity of 208 gigatons. This could cover the entire current U.S. carbon emissions for over 100 years.
The Kyoto Protocol under the United Nations Framework Convention on Climate Change allows countries to receive credits for their carbon-sequestration activities in the area of land use, land-use change, and forestry as part of their obligations under the protocol and these new revolutionary technologies in geoengineering will thrive to help in first stopping and then reversing the possible causes of climate change. There is this famous line from the film The Shawshank Redemption, ” Hope is a good thing, maybe the best of things, and no good thing ever dies.” and the hope for a better tomorrow still prevails.
Pritom Sarma
(Twitter: @thepritomsarma)
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