Short answer: Kind of.
written by: jessica wang
graphics by: yunyi cui
Rates of global warming and climate change have been rapidly escalating, and one of the driving factors is anthropogenic emission of carbon dioxide, which is a greenhouse gas that traps heat in the atmosphere. The IPCC (Intergovernmental Panel on Climate Change) now estimates that by 2035, global temperatures may rise by 2°C and pass a threshold of no return (1, 2). As such, the United Nations is aiming by 2050 to reach global net zero, which means releasing no new emissions into the atmosphere (1). That being said, Elon Musk’s timely announcement of a $100 million prize competition has skyrocketed carbon capture and storage (CCS) technology into the general view (3). This makes now a great time to discuss the current and future technologies that may help bring the world to net zero.
CCS
Although there is no magic solution, carbon capture and storage technology can aid other efforts, such as switching to renewable energy and increasing electric transport, to curb emissions (1). Fossil fuel and biofuels rely on combustion or burning to release energy, which also emits CO2. As such, the premise of CCS is to capture CO2 prior to entering the atmosphere. Although some CO2 can be recycled into usable polymers or fuels, most would need to be stored or sequestered biotically or abiotically.
Capture
The capture process can occur through either pre-combustion, during combustion through oxy-fuel combustion, or post-combustion. Pre-combustion capture can occur by converting fossil fuels into synthesis gases, which are a mixture of carbon monoxide and hydrogen gas (2). Then we have oxy-fuel combustion. Normally, combustion in air is inefficient and can produce soot that pollutes the environment. As such, oxy-fuel combustion relies on combustion with pure oxygen rather than air to produce a gas composed mainly of CO2 and water. CO2 can be relatively easily compressed, transported and stored, while water can be removed through condensation (4). However, oxy-fuel combustion requires separation of oxygen from air that requires large amounts of energy (2), which requires more fuel burning and release of CO2.
Lastly is post-combustion capture, which can be through the use of absorption, adsorption, membrane separation, cryogenics or usage of microbial or algal systems (5). Chemical absorption is the most conventional method currently in use. It involves producing a counter-current (running in opposite directions, Figure 1) of CO2 gas and a liquid called the solvent. This lets the CO2 absorb into the solvent as the solvent flows vertically from top to bottom (4). Then, heat would be used to release the CO2 from the solvent, which will be condensed then reused (4,5). The most commonly used method is called amine scrubbing that uses amines, a type of nitrogen compound, as solvents (2). However, not only is the heating and separation process very energy intensive and can reduce the efficiency of a modern power plant by 10%, but amine scrubbing can also corrode the machines and release toxic chemicals (2,4). To address these challenges, there are also other solvents currently in development, such as ionic liquids which are liquids made of ions (atoms with charge) (4).
With many challenges in the prior method, we look to the next method: adsorption. Adsorption is different from absorption because rather than dissolving the CO2, a sorbent would capture CO2 from other gases having it “stick” to the sorbent’s surface. This can occur physically by attracting the CO2 and trapping it in pores on its surface or chemically through covalent bonding with CO2 (2). This includes carbonate looping technology, which is a technology that uses solid sorbents to capture CO2 from air of which the most promising sorbent is CaO (calcium oxide) made from limestone (4). Through a reversible reaction (Figure 2), the sorbent CaO would bind to CO2 in a container at 650-850oC, called the carbonator reactor, to form solid CaCO3. Then, in a second container at temperatures higher than 930 oC, called the calciner reactor, the reaction would reverse and release the CO2 and regenerate the sorbent (2).
Storage
Once the CO2 is extracted, it must be stored in carbon sinks. This includes biotic (living) sequestration by storing CO2 using plants and microorganisms, such as phytoplankton in the oceans, as well as abiotic (non-living) sequestration through physical, chemical or engineering means (6). Abiotic sequestration methods include injecting CO2 deep into the ocean at depths from 500-3000m (depending on the method) or the ground in unmineable coal seams, old wells, between rock strata or saline aquifers, or by depositing CO2 in minerals (6).
Glossary
Aquifers: underground layer of water
Anthropogenic: human-made/ human-caused
Carbon sinks: anything or place that stores carbon, such as plants or the ocean
Sequestration: storage of carbon
Sorbent: a material that absorbs or adsorb fluids and gases (think like a sponge!)
References
The race to zero emissions, and why the world depends on it. UN News [Internet]. 02 Desember 2020; Available at: https://news.un.org/en/story/2020/12/1078612
Ahmed R, Liu G, Yousaf B, Abbas Q, Ullah H, Ali MU. Recent advances in carbon-based renewable adsorbent for selective carbon dioxide capture and separation-A review. J Clean Prod. Januarie 2020;242:118409.
The Associated Press. Elon Musk puts up $100M US for global carbon capture competition. CBC [Internet]. 09 Februarie 2021; Available at: https://www.cbc.ca/news/technology/elon-musk-carbon-capture-1.5906788
MacDowell N, Florin N, Buchard A, Hallett J, Galindo A, Jackson G, et al. An overview of CO2 capture technologies. Energy Environ Sci. 2010;3(11):1645.
Wilberforce T, Baroutaji A, Soudan B, Al-Alami AH, Olabi AG. Outlook of carbon capture technology and challenges. Sci Total Environ. Maart 2019;657:56–72.
Lal R. Carbon sequestration. Philos Trans R Soc B Biol Sci. 27 Februarie 2008;363(1492):815–30.
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