Carbon Capture and Sequestration Research
written by Laurie Jin, edited by Ada Praun-Petrovic
Reducing carbon emissions would slow the progression of global warming, but this task has proven itself easier said than done. Additionally, assuming we did manage to accomplish this feat, what can we do about the harmful carbon dioxide that remains in the ozone layer? Researchers have found a new method to combat climate change, carbon capture and sequestration (CCS).
CCS captures carbon dioxide from the air by dissolving the molecules in water to create carbonic acid, CO32-. A water-soluble magnesium compound is added to the solution to make MgCO3, the mineral magnesite. This product can then be safely stored in Earth’s crust. Currently, there are 65 large scale CCS facilities in the world; 26 of them are operating.
There are three types of CCS: pre-combustion, post-combustion, and oxy-fuel combustion systems. Combustion, the process of burning something, is used to harness energy from fossil fuels. Conveniently, it is also needed in CCS systems in order to provide the necessary heat levels to power the chemical reactions. Pre-combustion CCS systems remove carbon dioxide from fossil fuels before combustion. After the carbon dioxide is removed, further burning of these fuels will not harm the environment. Post-combustion CCS systems capture carbon dioxide from flue gas, which is produced from fossil fuel combustion and is released into the atmosphere as pollution. However, flue gas from energy plants that use both steam and gas only has a 4% concentration of carbon dioxide by volume, making the extraction of carbon dioxide very difficult.
Pre-Combustion and Post-Combustion have three types of systems to capture the carbon dioxide: Solvent based CO2 capture, sorbent based CO2 capture, and membrane based CO2 capture. Solvent based systems use liquid chemical compounds that dissolve or extract other materials, to absorb carbon dioxide from gases. The liquid is regenerated and later reused by increasing its temperature or reducing its pressure in order to break the bond formed between carbon dioxide and the chemical compound. After doing so, the carbon dioxide can go through the rest of the CCS process. Sorbent based systems use solid surfaces coated with a thin film or solute that molecules can adhere onto. Sorbents can be regenerated like the solvents, but less energy is needed to reproduce them as they have a lower heat capacity, defined as the amount of heat (in joules) needed to raise the substance by 1 Kelvin. Membrane-based CCS uses a permeable or semipermeable filter that can separate and transport CO2. The membrane acts like a sifter, only letting in small CO2 particles. This type of CCS is less hazardous as it uses fewer chemicals. However, the low partial pressure of carbon dioxide makes it harder for all the particles to be filtered due to the decreased speed.
Oxy-fuel combustion systems burn fossil fuels with an oxygen-rich gas mixture instead of air. Mixtures ideally have a concentration of oxygen that is 95% by volume. The flue gas produced mainly consists of carbon dioxide (75% by volume) and water. After removing the water vapor, the amount of gas remaining is significantly less than that normally formed through a regular combustion reaction. The small amount of gas has a higher concentration of carbon dioxide (95% by volume), making it easier to extract those harmful particles. Part of the flue gas is recycled and mixed with oxygen to prevent extremely high temperature levels in the furnace.
Carbon capture and sequestration, an expensive yet effective means of cleaning up our environment, is currently our best option to combat global warming. The post-combustion system is currently the most commonly used, but both pre-combustion and oxy-fuel systems are viable as well. While removing fossil fuel emitters will help our environment in the long run, CCS expedites the process and even gives us the option to continue using environmentally toxic machines.
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