Much of the approach to combatting climate change has been about reducing emissions. However, this is not always entirely possible, for example in industries like aviation and agriculture we simply will not get emissions to zero in the near future[1]. Here, we’ll examine the much-forgotten approach that is cleaning up what it is already out there: Carbon sequestration. 

How does carbon dioxide sequestration actually work? 

Carbon sequestration is the process of capturing and storing CO2 before it enters the atmosphere. CO2 removal also opens up the possibility of going net negative, required for many of the pathways to achieving the Paris Agreement[2]. Sequestration comprises two main parts: capture and storage. 

  

Carbon Capture 

The most common processes capture the exhaust gases when fossil fuels are burned in power plants, predominantly coal.  Post-combustion carbon capture takes the exhaust flue gases (produced during the combustion of a fossil fuel), cooling and then pumping them into a chamber. In the chambers are chemical “scrubbers”, or air pollution control chemicals that bind to COto extract it from the cooled exhaust[3]

Precombustion carbon capture is less widely used. Fossil fuels are heated in steam and oxygen, which results in production of a synthesis gas, containing a mixture of CO2, H2 and CO (toxic carbon monoxide). After that, while the H2 is reacting to produce water, some of the CO reacts to form CO2. Thus, a mixture of H2 and CO2 is left, from which it is easier to capture, store and sequester the CO2

Oxy-fuel combustion carbon capture occurs when a plant burns fossil fuels not in air but instead a gas mixture, containing large amounts of O2 gas. As a result, the flue gas produced is mainly CO2 and H2O. The CO2 is afterwards separated by compressing and cooling the water. Though promising, this technique still needs to be proven on the large-scale. 

Carbon Storage 

After the CO2 has been extracted, and the carbon-free gas is re-used or released, the CO2 must be stored somewhere where it cannot escape into the atmosphere. Firstly, CO2 is transported through pipelines and then, in some cases, by tanker to finish its journey in storage sites. CO2 is usually transported in its gaseous state, compressed to between 100-150 times atmospheric pressure before its journey[4]. There are a variety of safety issues associated with this transportation including pipes rupturing causing issues for the environment and public health including asphyxiation. 

CO2 is predominantly stored underground, in deep aquifers, permeable rocks and other locations which meet similar criteria. While deep underground, we can keep CO2 at a pressure of over 73 atm and a temperature of above 31 degrees Celsius[5]. Upon meeting those conditions, CO2 becomes supercritical, meaning it exhibits properties of both gases and liquids. For example, it has low viscosity like a gas while having the high density of a liquid. As it can seep into small areas in porous rocks, a large volume of CO2 can be stored in a small area, including oil and gas reservoirs. We inject CO2 into these reservoirs, and it is kept from escaping by overlaying rocks that form a seal. Underground storage does not come without its risks. Basaltic rock formations, of volcanic origin, are also attractive for CO2 storage. It has been discovered that when the magnesium and calcium naturally contained within basalt react with CO2, it is transformed into minerals, particularly dolomite, calcite, and magnesite. 

Ocean carbon storage is another option that is being looked at. While largely untested, theoretically, as CO2 is denser than water at the very depths of the sea, it could be simply dumped very deep down and remain trapped for many years. However, the safety of marine life needs to be considered. 

Carbon capture and storage projects typically aim for 90% efficiency, 90% of the carbon emitted by a large industrial power plant will be captured[6]. Traditionally, this target has been set as a baseline because a system would need to remove at least 90% of emissions for any initial investment to be worth it, as Howard Herzog (Senior Research Engineer at MIT Energy Initiative) explains. However, we must be more ambitious with our targets, even if it means more investment is needed to meet them. Considering the fact that an untreated exhaust from a coal-based power plant can contain 300 times as much carbon dioxide as the entire Earth’s atmosphere[7], the remaining 10% efficiency really does make a difference. 

To exceed the 90% efficiency that has been set for carbon capture and storage’s relatively short lifetime is a problem for both engineers and economists. As efficiency increases it becomes more technically and economically difficult. A gas with higher concentration of CO2 has more CO2 molecules flowing past the scrubbers, and vice versa. 98% efficiency is not too drastic or distant of an idea with some companies already achieving 95%[8]. However, the price per each molecule of CO2 captured increases as you go up, and so the incentive must also increase. Some governments have looked at a carbon pricing system, a policy that causes producers (and consumers) to pay for each ton of CO2 they emit. One type of carbon pricing being suggested is carbon tax: emitters such as factories and plants will change their behaviour and invest in carbon sequestration to avoid paying higher taxes. 


[1] https://www.ox.ac.uk/news/2021-09-23-co2-removal-essential-achieving-net-zero

[2] https://www.lse.ac.uk/granthaminstitute/explainers/what-is-carbon-capture-and-storage-and-what-role-can-it-play-in-tackling-climate-change/

[3] https://science.howstuffworks.com/environmental/green-science/carbon-capture.htm

[4] https://science.howstuffworks.com/environmental/green-science/carbon-capture.htm

[5] https://en.wikipedia.org/wiki/Carbon_capture_and_storage

[6] https://climate.mit.edu/ask-mit/how-efficient-carbon-capture-and-storage#:~:text=CCS%20projects%20typically%20target%2090,will%20be%20captured%20and%20stored

[7] https://climate.mit.edu/ask-mit/how-efficient-carbon-capture-and-storage#:~:text=CCS%20projects%20typically%20target%2090,will%20be%20captured%20and%20stored

[8] https://climate.mit.edu/ask-mit/how-efficient-carbon-capture-and-storage#:~:text=CCS%20projects%20typically%20target%2090,will%20be%20captured%20and%20stored

About the author

Luca Matta