Carbon dioxide is one of the most prominent greenhouse gases released into the atmosphere by human activity. This excess CO2 traps heat in the atmosphere and, along with other greenhouse gases, is responsible for the warming of the earth. The most common way CO2 is emitted is through the combustion of fossil fuels for energy. Other sources of CO2 emissions include industrial processes, biomass, as well as changes in land use.
The reduction of CO2 emissions as well as the CO2 content of the atmosphere via carbon sequestration is an important area of study in the fight for environmental conservation. Carbon sequestration is the long term storage of carbon through both natural and anthropogenic activities. There are many ways carbon sequestration can be accomplished, and research is continually being conducted on new ways CO2 can be captured.
Carbon can be sequestered both from the atmosphere as well as directly from the source of emission. Different sequestration methods exist for each location, and occur in varying levels of complexity and effectiveness. The most obvious methods for carbon sequestration that come to mind are typically those that involve removal of CO2 from the atmosphere using biological means.
Changes in land use and forestry have the potential to remove large quantities of CO2 from the atmosphere. Plants extract CO2 from the atmosphere and change it into different forms of carbon through photosynthesis. This carbon remains out of the atmosphere in the form of biomass, however CO2 can get released when these species decompose or are burned. The uptake of CO2 from the atmosphere via plants can be increased with reforestation, improved agriculture, and revegetation practices. Of the CO2 emitted to the atmosphere, 25% is incorporated into terrestrial ecosystems.
Another 30% of carbon emissions are taken up by the oceans. This amount could be increased through ocean fertilization. Phytoplankton are marine algae that also remove CO2 from the atmosphere via photosynthesis. The idea behind ocean fertilization is to increase the uptake of CO2 from the atmosphere with increased phytoplankton populations. When the phytoplankton die, they would take the CO2 they photosynthesized into tissue with them to the bottom of the ocean floor. Although some CO2 would be released from the decomposition of these organisms, a significant portion would remain on the ocean floor and become sedimentary rock.
To accomplish this ocean fertilization, iron and nitrates would need to be dissolved into the surface of water to promote the growth of the phytoplankton. To be successful, this would require an enormous number of vessels covering most of the ocean, which could take years to accomplish. In addition, the impact on the oceanic ecosystem from this increase in phytoplankton must be considered.
There also exists non biological means to sequester CO2 from the atmosphere. Two examples include scrubbing towers and artificial trees. Inside scrubbing towers, air is funneled in by wind turbines and sprayed with either sodium oxide or calcium oxide to form carbon precipitates and water, which can be piped to safe locations for storage. Artificial trees include a series of sticky, resin-covered filters capable of converting CO2 into a carbonate called soda ash, This soda ash could be washed off the filters and collected for storage elsewhere. An enormous amount of scrubbing towers and artificial trees would be needed to counteract the increase in atmospheric CO2, making this solution both extremely expensive and not necessarily feasible.
CO2 removal technologies from emissions are seemingly easier to implement and more developed than removal technologies directly from the atmosphere. With the source of emission being easily identifiable, technology can be implemented in energy plants and other locations of large CO2 generation to collect the CO2 and send it elsewhere for storage or additional use. These technologies fall into the categories of pre combustion capture, oxyfuel combustion, and post combustion capture.
Pre combustion capture of CO2 involves the creation of synthesis gas predominantly composed of CO and H2. This is accomplished through the partial combustion of fossil fuels at elevated pressures. After, steam is added to the gas, which is then passed through a catalyst. There, a water gas shift reaction occurs that converts the CO into CO2 and increases the yield of H2. Subsequently, CO2 and H2 are separated, with the CO2 being sent off for storage and the H2 being used to produce electricity or for use in hydrogen fuel cells. The separation and compression of CO2 require less energy than for other sequestration technologies, but complicated chemical processes can cause extra plant shutdowns and reduce output. In addition, a chemical plant is required to be in front of the turbine, which most established plants do not have.
Oxy-fuel combustion involves the modification of the combustion process to produce a flue gas predominantly made of CO2 for easy separation. In this process, fuel is burned in a combustion chamber of pure O2. Because combustion with pure oxygen produces extremely high temperatures and normal equipment cannot handle these temperatures, some flue gas is recycled to produce normal air combustion conditions. The resulting flue gas contains predominantly CO2 and water vapor, which can be easily purified at low costs. This method for CO2 removal has the advantage of preventing NOx from forming and reducing CO2 capture costs. However, the cost of producing a pure O2 stream as well as recycling the flue gas offset this benefit.
Post combustion capture involves the removal of CO2 from the flue gas of combustion chambers. Equipment can be added to the end of a process to collect CO2 and send it to be stored. Typically, the CO2 concentration for flue gas ranges from 8–15%. Because of these low concentrations, it is more difficult to capture CO2, requires large equipment, and can become quite expensive. However, compared to the other options for carbon capture, post combustion capture has the lowest total electricity costs and is the most widely used.
Absorption is the most common means for post combustion capture. Flue gas is bubbled through a solvent and CO2 is selectively absorbed. The CO2 rich stream is then sent to a regenerator, where it is heated, and the desorbed CO2 can be compressed and sent off. Absorption has a fairly high removal efficiency of around 90%, however it does consume a large quantity of energy and there are often problems with solvent degradation and corrosion.
Adsorption, another popular removal technology, is based on the intermolecular forces between the process gas and a solid material, the adsorbent. The process gas is passed through an adsorption column over the solid adsorbent, and CO2 is selectively adsorbed by the solid. The CO2 can then be desorbed via heat and collected. Adsorption is fairly easy to implement, has no byproducts and requires little energy. However, adsorption has low selectivity, lower removal efficiency, and there are also problems with regeneration of the adsorbent. All of these qualities depend on the specific adsorbent used, and thus can be modified.
Membrane technology uses semi-permeable barriers to separate substances via diffusion, adsorption, or ionic transport. Such technology can be incorporated into other removal methods, such as carbon absorption, to increase efficiency. On its own, membrane technology yields low removal efficiency and low purity of CO2, and is not feasible for streams with low concentrations of CO2.
Once the CO2 has been collected, it must be stored or fixed for long term storage. The first step is to find appropriate storage locations. Then, leak testing must be performed to ensure CO2 will not leak back into the atmosphere. Both geological injection and oceanic injection are popular disposal options.
In geological injection, CO2 is injected into some sort of sedimentary rock where it reacts to form carbonate minerals. Geological injection has less capacity for CO2 storage, but does have longer CO2 residence times. In oceanic injection, CO2 is injected at various depths of the ocean, with increasing injection depths leading to decreased leakage. Concerns with decreasing ocean pH from CO2 injection force scientists to develop proper dispersion techniques.
Collected CO2 can also be used to aid in recovery of natural gas and oil. CO2 can be stored in old oil seams, where it diffuses through the pore structure and is adsorbed. In doing so, it helps recover trapped methane for use. In addition, CO2 can be injected into oil reservoirs to promote the movement of oil out of wells. The economic viability of both solutions depend on the prices of oil and natural gas, as well as their proximity to CO2 sources.
An option for fixing CO2 into another form, rather than storing is made possible by a bioreactor. A photo-bioreactor uses the natural photosynthetic processes where microorganisms fix CO2 into biomass, O2 and H2. Key parameters for the success of such a device on an industrial scale include the collection, transmission and delivery of collected light. In addition, the distribution of nutrients and the health of the microorganism colony are other concerns. Bioreactors are a promising solution to control CO2 emissions without some of the drawbacks associated with other CO2 removal technologies. However, bioreactor technology has not yet been utilized on large scale processes and more research needs to be done to develop such a device.
As the cost and demand for carbon increases, these carbon capture technologies will become more economically feasible. 75% of the cost of carbon sequestration is related to the capture and compression processes. Thus, as more methods for the utilization of CO2 are developed, and as more sequestration research is conducted, capturing and storing CO2 can become more economic and popular practices.
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