One of the ways to address climate change is through the process called carbon capture and storage or CCS. Also known as carbon capture and sequestration or carbon control and sequestration, it generally involves capturing carbon dioxide, transporting it to a storage site for further processing or direct storage, and depositing it to a final location where it will not enter the atmosphere.
There are three major phases or steps in the entire carbon capture and storage process. Take note of the following:
• Capture: Trapping and separating carbon dioxide from other others gasses coming from large point sources such as fossil fuel power plants and other industrial processes.
• Transportation: Processing the captured carbon dioxide for storage, and transporting the captured and processed gas to a storage location.
• Storage: Depositing the captured and processed carbon dioxide far away from the atmosphere, usually underground or deep in the ocean.
Understanding the Carbon Capture and Storage Process
The Three Methods for Capturing Carbon: Pre-Combustion, Post-Combustion, and Oxyfuel Combustion
1. Pre-Combustion Carbon Capture
Pre-combustion capture essentially centers on trapping the CO2 before the fossil fuel is burned. However, to be more specific, it involves heating fossil fuel in pure oxygen to produce a mixture of hydrogen and carbon monoxide.
The hydrogen is captured. On the other hand, carbon monoxide is reacted with water to produce CO2, which in turn, is captured for storage. The captured hydrogen may be used to run electricity generation facilities and fuel hydrogen-based vehicles.
2. Post-Combustion Carbon Capture
Another method of carbon capture is post-combustion capture. Note that burning fossil fuels produce flue glasses that include carbon dioxide, sulfur dioxides, nitrogen oxides, and water vapor. The method includes directing these gasses to an absorber column containing liquid solvents, such as ammonia to separate the CO2.
More specifically, the separated CO2 is contained in the solvent, which is later heated with superheated steam at around 120 Celsius. The steam releases the CO2 from the solvent, which then can be trapped further for transportation and storage.
3. Oxyfuel Combustion Capture
The oxyfuel combustion method centers on burning fossil fuel in oxygen that results in the production of a gas mixture composed primarily of steam and CO2. Cooling and compressing the gas mixture separate the carbon dioxide and the steam.
Note that the method can capture 90 percent of CO2 emitted by a power plant. However, the oxygen required for this method of capturing carbon increases the costs of power plant operations. Researchers are findings ways to bring the cost down.
Major Options for Storing Captured Carbon Dioxide: Underground, Abandoned Fossil Fuel Reservoirs, and Deep Ocean
Remember that storage is the last phase or step in the entire carbon capture and storage process. There have been several options used and further explored under this CCS phase.
1. Underground Geological Storage
The most common storage destination of capture carbon is underground through a process called geological sequestration. It specifically involves injecting CO2 processed into a supercritical fluid phase into underground rock formations. The depth of the storage keeps the CO2 under a supercritical condition, particularly if the temperature remains above 31.1 Celsius at a pressure of above 72.9 atmospheres.
2. Abandoned Fossil Fuel Reservoirs
Abandoned fossil fuel reservoirs are also another option for underground geological storage of captured carbon dioxide. These sites are ideal because they have natural rock formations that can effectively contain CO2. Furthermore, this option is cost-effective because there is no need to conduct a geological survey on a candidate area and bore a deep hole to the specified site.
3. Deep Ocean Storage
Several studies have proposed that it is safe to safely dump carbon dioxide directly into the ocean, particularly at depths more than 11482 feet or 3500 meters. The research suggests that the depths would allow the CO2 to compress to a sludge that would naturally sink and remain at the bottom of the ocean floor because of the pressure.
4. Volcanic Rock or Basalt Formation
Another option for storing captured carbon dioxide takes into account volcanic rock or basalt formations. Researches such as those from the Pacific Northwest National Laboratory in Washington State and the CarbFix project in Iceland and have found that injecting CO2 into basalt results in the formation of carbonate minerals, which make up limestone. Hence, the option essentially converts the captured CO2 into a rock.
Effectiveness of and Concerns about Carbon Capture and Storage
The Intergovernmental Panel on Climate Change noted that carbon capture and storage has the potential to make up between 10 percent and 55 percent of the total carbon mitigation effort until the year 2020. Hence, the process has been deemed as essential in addressing the environmental offshoots from fossil fuel consumption and thus, addressing global warming and climate change.
Several studies have also investigated the effectiveness of the different methods for capturing carbon dioxide. For example, a 2017 study by R. T. J. Porter et al. revealed that power plants using the proprietary process called Selexol in their pre-combustion efforts had been found to effectively trap up to more than 90 percent of CO2 contained in the synthesized gas. Take note that the capture rate varies between 50 percent and 98 percent.
The 2017 review study of Yuan Wang et al. also revealed that the specific monoethanolamine-based process employed in post-combustion capture has a typical capture rate of around 90 percent. The process has also been found more energy and cost efficient than the membrane-based separation process.
Of course, there are issues or concerns about the effectiveness and safety of carbon capture and storage process. For example, underground storage or geological sequestration may be the most widely used and viable storage solution, but there are concerns about possible leakages and contamination due to unwarranted phenomena such as earthquakes or human-made incidents.
Both storage options via deep ocean and basalt formations remain untested. To be specific, the deep ocean option remains an idea, and it is unclear whether the deposited CO2 would remain on the ocean floor. On the other hand, the basalt option requires 25 tons of waters for each ton of carbon dioxide buried. It is also possible that subterranean microorganism might break down carbonates to methane, which is another greenhouse gas.
Cost considerations for the hydrocarbon industry is another issue. Integrating carbon capture and storage systems in power plants would mean additional activity and thus, additional cost to the downstream hydrocarbon industry, especially to power plant operators.
The CO2 captured from point sources such as fossil fuel power plants would not be enough. Note that about 25 percent of global greenhouse gasses come from electricity generation and heat production. Other sources include transportation, other industrial activities, and agriculture that collectively constitute 60 percent of all greenhouse gas emissions. Hence, other carbon capture processes center on capturing CO2 from the atmosphere. Some technologies and solutions include direct air capture and tree planting programs.
FURTHER READINGS AND REFERENCES
- Intergovernmental Panel on Climate Change. 2005. PPCC Special Report on Carbon Capture and Storage. Eds. Metz, B., O. Davidson, H. C. de Coninck, M. Loos, and L. A. Meyer. New York: Cambridge University Press. ISBN: 10-0-521-68551-6. Available via PDF
- Konsyse Staff. 2019. “The Pros and Cons of Carbon Capture and Storage.” Konsyse. Available online
- Matter, J., Stute, M., Snæbjörnsdottir, S. O., Oelkers, E. H., Gislason, S. R., Aradottir, E. S…Broecker, W. S. 2016. “Rapid Carbon Mineralization for Permanent Disposal of Anthropogenic Carbon Dioxide Emissions.” Science. 352(6291): 1312-1314. DOI: 10.1126/science.aad8132
- Porter, R. T. J., Fairweather, M., Kolster, C., Dowell, N. M., Shah, N., and Woolley, R. M. 2017. “Cost and Performance of Some Carbon Capture Technology Options for Producing Different Quality CO2 Product Streams.” International Journal of Greenhouse Gas Control. 55: 185-195. DOI: 10.1016/j.ijggc.2016.11.020
- Wang, Y., Zhao, L., Otto, A., Robinius, M., and Stolten, D. 2017. “A Review of Post-Combustion CO2 Capture Technologies from Coal-Fired Power Plants.” Energy Procedia. 114: 650-665. DOI: 10.1016/j.egypro.2017.03.1209