Carbon Sequestration

What is Carbon Sequestration?

Carbon sequestration is the term used to describe the natural or anthropogenic long-term process of storing carbon dioxide primarily into plants, soil, geologic formations, oceans, etc., as a response to climate change. With carbon dioxide being the most abundant greenhouse gas contributing to global warming, it is becoming increasingly necessary to find ways of sequestering carbon to slow down this process. Currently, biological and geological carbon sequestration are the two leading methods of storing carbon, however, they have their limits. As these biological and geological pools begin to reach maximum storage capacity of carbon, the Earth will soon face harsh climate-related consequences [2].  Besides biological and geological carbon sequestration, scientists and engineers are working on new inventions and technological developments that can capture carbon before it enters the atmosphere. These are called carbon capture and storage (CCS) technologies and they are constantly being researched to develop the safest, most effective and inexpensive methods.

Types of Carbon Sequestration

Biological Carbon Sequestration

Biological carbon sequestration is a method of storing atmospheric CO₂ in biological ecosystems such as vegetation, soils, wood products, and aquatic environments. This type of carbon sequestration is a natural occurrence that has been disrupted in recent years due to the excessive emissions of CO₂ from human activities [4].

Carbon in Forests and Grasslands

Landscapes that are rich in plants generally act as carbon sinks as they naturally absorb carbon dioxide through photosynthesis, accounting for the capture of roughly 25% of global carbon emissions. However, when these ecosystems are affected by droughts, wildfires, or deforestation, they become carbon sources as they release significant amounts of carbon into the atmosphere [6].

Carbon in Soils

Carbon sequestered by plants through photosynthesis can be stored as soil organic carbon (SOC) which can last several decades. Carbon in the form of carbonates can be stored for over 70,000 years, however, the formation of carbonate itself takes thousands of years. Scientists have been trying to discover ways to speed up this process to permit longer periods of carbon storage now that SOC levels are deteriorating due to agroecosystems (ecosystems where agricultural practices occur) [6].

Carbon in Oceans

Oceans act like lungs in the sense that they absorb and release carbon dioxide at steady rates. The release of carbon dioxide by the ocean is called a positive atmospheric flux, while the absorption of carbon dioxide is called a negative atmospheric flux. Phytoplankton, a microscopic algae found in the ocean, carries out photosynthesis which allows the ocean to absorb and contain approximately 25% of annual anthropogenic carbon dioxide emissions. Carbon sinks are generally located in the polar regions since cool waters are rich in nutrients that are better at absorbing carbon dioxide [6].

Geological Carbon Sequestration

Geological carbon sequestration — a method of carbon capture and storage — is the process of storing atmospheric carbon dioxide in deep, underground geologic formations by injection of pressurized liquid carbon dioxide [7]. This method is effective for storing copious amounts of carbon dioxide over a long period of time. In order to prevent leakage, the carbon must be injected into porous rock formations [4].

Technologies for Carbon Capture Storage

Since the Industrial Revolution, levels of atmospheric carbon have been increasing at significant rates. Carbon emissions have contributed greatly to climate change and the slow rise of the global mean surface temperature. In 2020, this average annual anomaly saw an increase of 1.02°C, relative to the average annual temperature of 14°C between the years 1951 and 1980 [8, 9]. At this rate, Earth will soon face great changes which will likely be detrimental and unsafe for its inhabitants. For this reason, scientists and engineers around the world have been working hard in creating and improving various CCS technologies that will help reduce amounts of carbon entering the atmosphere [10]. The following segment lists and describes a few of these newer technologies.

Nanosponges

Since 2008, a team of researchers from Cornell University have been investigating safer, non-toxic methods of carbon capture storage. By the year 2014, the researchers invented a white powder consisting of a sorbent silica support scaffold with nanopores to maximize surface area which proved to be as effective, if not, better than industry standards of CCS. The basis of this invention stemmed from the pre-existing common method of CCS called amine scrubbing where liquid amino compounds (amines) were used to absorb carbon dioxide from post-combustion flue gas — gas exiting from a channel directed into the atmosphere — in fossil fuel plants. Amine scrubbing's effectiveness only lasts so long as some amine is lost over time which increases costs.

Following this idea, the researchers developed the powder in such a way that little amine is lost. The white powder is dipped into liquid amine where it proceeds to soak in the amine through its various nanopores like a sponge — hence the name nanosponge. The liquid amine then chemically bonds itself onto the sorbent surface where it is left to grow and partially harden, after which it is ready to capture carbon dioxide [11].

Metal-Organic Frameworks

Metal-organic frameworks (MORs) are highly porous, three-dimensional materials characterized by their crystalline structure which is composed of a hybrid of inorganic and organic components [13]. The inorganic component is a cluster of metal ions that form metal nodes. These metal nodes then bind to surrounding organic linkers — called organic ligands — to create the uniform and porous framework. The crystalline structure of MORs offer an internal surface area that can span an entire soccer field which becomes useful for trapping copious amounts of gas. By selecting different metals and organic linkers, scientists are able to synthesize a variety of MORs and tailor each combination to have a high affinity for specific gases. There are many applications of MOFs which includes its use in CCS [14].

MOFs can be used in two different ways when it comes to carbon. The first is that it can act as a detector for carbon dioxide in the air. This allows scientists to monitor atmospheric levels of carbon and also check for the presence of carbon dioxide in areas where it may not regularly appear [15]. The second is in carbon capture. An MOF called Mg-MOF-74 has a strong affinity for carbon and serves its use in capturing carbon dioxide emitted from power plants before entering the atmosphere [16, 17].

Hybrid Membranes

As an energy efficient method of separating gases, polymer membranes have been considered for use in CCS, however, their permeability to carbon dioxide is low. For this reason, scientists have been trying to develop a hybrid of a polymer and an MOF that could "harness the carbon dioxide selectivity of metal-organic frameworks while maintaining the processability of polymers" [18]. The purpose of the MOF is to create continuous channels through which carbon dioxide molecules can travel, but until the year 2016, scientists struggled to create a membrane that would maintain their continuity. Another issue with the hybrid membranes was the limitations in MOF that could be added due to its mechanical stability [18].

In early 2016, scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) successfully developed an effective hybrid membrane with an MOF worth 50 percent of the membrane's weight. The scientists found that at a 50 percent weight MOF, carbon dioxide was able to travel through two distinctive channels, either through the polymer or the MOF, without comprising structural integrity [18].

Crystals

Carbon is always released with moisture which made carbon capture a lot more difficult with earlier CCS technologies as the common materials used had a higher affinity for water. The high adsorption (collection of liquid or gas molecules at the surface) of water prevented the adsorption of carbon dioxide, making the process a lot more costly if the carbon was to be completely dehydrated [19, 20].

In late 2015, a group of international scientists based at Sogang University in South Korea created a microporous crystal made of copper silicate, SGU-29. This was believed to be the first CCS technology that was effective even in the presence of water. The materials adsorb both carbon and water with a high uptake of carbon — an improvement from past technologies [19].

Carbon to Rocks (Geological Carbon Sequestration)

Deep under the mountainous landscape in western Iceland lies Hellisheidi, the largest geothermal plant in the world. The power station harnesses heat from surrounding volcanoes to power and provide electricity to all of Iceland, emitting steam in the process. Under a project called CarbFix, the carbon dioxide released in the steam is captured and diluted in large volumes of water, similar to the carbonation of liquids or juices to make fizzy drinks. The carbonated liquid is then transported by pipes to various small injection sites where it is then injected 1,000 metres below the surface into the pores of basalt. Within a few months and through chemical reactions, the liquid carbon calcifies in these pores to be permanently trapped underground with no way of escaping to the atmosphere [22].

Carbon to Fuel

Since 2015, the Canadian clean energy company, Carbon Engineering, has been removing carbon dioxide directly from the atmosphere with their Direct Air Capture system. The system uses only energy and water to purify and compress carbon dioxide so that it is readily accessible through pipelines for use in hydrocarbon fuels or industrial purposes. To suck the carbon dioxide from the atmosphere, the system uses massive air contractors that can capture approximately one million tons of carbon dioxide per year [23]. For the amount of carbon dioxide that the system is able to process, the cost is relatively low at 100 CAD.

In 2017, Carbon Engineering began converting carbon dioxide into a low-carbon fuel by combining it with hydrogen through their system. The fuel can be blended with hydrocarbons such as gasoline, diesel and Jet-A, that are greatly contributing to the amount of carbon dioxide dispelled into the atmosphere. In hopes that by 2050, all modes of transport will run on this newer and less harmful fuel, Carbon Engineering plans to gradually replace fossil fuels with their fuel. The pricing of the fuel will fall at approximately $1/L which is cost-competitive with current biodiesels, but slightly more costly than fossil fuels [23, 24].

Carbon to Fibers

Stuart Licht, a Ph.D chemistry professor at George Washington University, first introduced the concept of transforming atmospheric carbon dioxide into carbon nanofibers through electrochemical processes in August of 2015. Previously, this transformation was achieved through a high-energy process called vapor deposition where carbon is heated to a gas and then cooled, forming fibers. Similar to vapor deposition, carbon dioxide undergoes heating first in the electrochemical process, however, it is heated to a liquid instead of a gas. An electric current with a pair of nickel-and-steel electrodes is then passed through the liquid to extract the carbon, which forms on the steel anode. Unlike vapor deposition, the electrochemical process is run on a highly efficient solar energy system [25]. The carbon nanofibers produced by the end are stronger than steel and have demonstrated superior "properties of conductivity, nanoelectronics, higher capacity batteries and flexibility" [26].