CO2: from a pollutant emission to a trading commodity

16 Jan 2023

The idea of CCUS: Carbon Capture Utilization and Storage

Carbon is responsible for all forms of life on Earth, as it is the principal structural element of their molecules and DNA and it contributes to the regulation of the atmosphere's temperature, creating ideal conditions for the development of life. It is also a principal element of the food chain and energy supply and is assumed as one of the foundations of human society and the global economy. Carbon is present in our atmosphere in the form of carbon dioxide, and it has its own cycle, the "Carbon Cycle". It is so called because, although the total amount of carbon that exists on the planet remains unchanged, the Earth and its atmosphere form a closed loop, in which carbon travels, without interruption.

Atmosphere, rocks, oceans, and living organisms are where carbon is found. On the one hand, it is released in the air by natural sources, such as volcanos, forest fires, respiration and in vast extend by human processes, such as the extraction and burning of fossil fuels, agricultural activities, and deforestation. On the other hand, carbon returns to its origins only through natural processes, mainly photosynthesis, which is carried out by land plants, sea algae and cyanobacteria (microscopic organisms found in all types of water); all of them, absorb carbon dioxide, water, and sunlight energy to synthesize carbohydrates and the most vital by-product, oxygen. Since the beginning of the Industrial Revolution (approx. 1750), human activities interfere with carbon cycle's equilibrium and have contributed substantially to climate change, by releasing CO2 and other greenhouse gases to the planet's atmosphere, in such rate, that the natural processes cannot counterbalance it. As a result, the amount of carbon dioxide in the atmosphere is rapidly rising and it is already considerably greater than at any time in the last 800,000 years (NOAA, 2022).

Therefore, if carbon has its own cycle in Earths' atmosphere and humans are disturbing this cycle by artificially adding more and more CO2 to it, then why not imitate carbon's natural cycle and capture it before or after it is released in the atmosphere and then re-use it or return it to the source, which it came from? And that's how the idea of Carbon Capture, Storage and Utilization was born.

CCUS: Carbon Capture

Today, roughly 80% of all global energy sources are fossil fuel based. Thus, fossil fuels are likely to continue to play an important role in the short and medium term. As many countries benefit from fossil fuel energy and coal, oil and gas remain vital for their energy security and economic well-being. Therefore, although the most effective way to reduce CO2 emissions is to reduce fossil fuel consumption, carbon neutrality will require rapid deployment of carbon capture, usage, and storage (CCUS) technologies, to act as a steppingstone towards decarbonization, until next generation fuels, i.e., zero-carbon fuels, are commercially available, in the required amounts and at a reasonable cost.
Carbon capture can be very efficient and cost-effective when applied at large scale, land-based infrastructures, and facilities, which use fossil fuels for activities, such as energy production, and emit major carbon dioxide quantities. Extracting carbon dioxide from air is possible, although far more complicated and expensive since its concentration in the atmosphere is much lower compared to typical combustion flu gases. Broadly, four different technologies exist: post-combustion, pre-combustion, oxyfuel combustion and direct air capture.

• Post-Combustion Carbon Capture: typically uses chemical solvents to separate carbon dioxide out of the flue gas from fossil fuel combustion at power stations or
  other point sources. The technology is maturing rapidly and is already being used in applications, although they are of a smaller scale. Post-combustion carbon capture occupies a large part of research and development because retrofitting existing power plants with carbon capture technologies, seems like the only path during the transitional period towards decarbonization.
• Pre-Combustion Carbon Capture: Fuel is gasified (rather than combusted) to produce a synthesis gas, or syngas, consisting mainly of carbon monoxide (CO) and hydrogen (H2). A subsequent shift reaction converts the CO to CO2, and then a physical solvent typically separates the CO2 from H2. The remaining CO2 can be captured from a quite purified stream. It is widely applied in the production of fertilizers, chemicals, gaseous fuel (H2, CH4), and power. A characteristic example (Figure 2), where the pre-combustion carbon capture takes place, is the innovative project developed by RINA Ship Classification Society, for the production of hydrogen onboard a vessel, bypassing the need of the extremely difficult and dangerous transportation and storage of liquid hydrogen.

The vessel has LNG as primary fuel and is equipped with 4- stroke engines capable of burning LNG mixed with a percentage of hydrogen, steam-methane reformer units, fuel cell units, LNG tanks and CO2 tanks. Part of the LNG is directed to the reformer, where hydrogen and CO2 is produced; hydrogen is used for powering the fuel cells and for mixing with LNG for the engines fuel. Accordingly, the CO2 stream, after the reformer, passes through a heat exchanger, where the cooler LNG stream cools it down and then the carbon dioxide is stored in the relevant tanks as a liquid.

• Oxy-fuel combustion: the fuel is combusted in the presence of nearly pure oxygen (approx. 98%) to ensure that the combustion's by-products are solely carbon dioxide and water. In that way, the exhaust gas stream is rich in CO2 and water, resulting in a very efficient carbon capture. However, the process of cryogenic separation, which is used for the separation of oxygen from air, demands high amounts of energy. Notably, latest research has pointed out an alternative technique for oxygen separation, the chemical looping combustion, "CLC". To briefly describe it, the main principal is that the air is not mixed with the fuel, but rather, the oxygen is extracted from the air and then directed via an oxygen carrier, a metal oxide. Laboratory results have shown that even 100% carbon capture is achievable.
• Direct Air Capture (DAC): a process of capturing carbon dioxide directly from the ambient air (as opposed to capturing from point sources) and shaping a pure stream of carbon dioxide, which can be then directed either for sequestration or utilization. It is possibly the least complex method of carbon capture, as the CO2 is absorbed from the atmosphere, when the ambient air is directed through various means of filtration. Usually, aqueous, alkaline solvents or sorbents are utilized for the absorption of CO2, which are later heated and forced to release the captured CO2. As a result, a CO2-rich stream is produced, which can now easily undergo any forms of processing, while the chemical absorbents can be used again. DAC is not an alternative to traditional, point-source carbon capture but can be used to capture carbon dioxide that already exists in the atmosphere.

CCUS: Carbon Utilization and Storage

Once the CO2 has been captured, can either be permanently stored or utilized for various processes. Different approaches have been conceived for permanent storage. One of them is the possibility of injecting the captured carbon dioxide back into geological formations and store it deep underground. Some options for storage are:

• Saline formations
• Oil and natural gas reservoirs (Enhanced Oil Recovery with Carbon Dioxide)
• Coal seams that cannot be mined
• Organic-rich shales
• Basalt formations

At the same time, captured carbon may as well replace the artificially created CO2 for the purpose of many industries such as food, oil, chemical and for other various commercial purposes and activities; the two possible directions, that carbon dioxide may follow after it's captured.

Some significant examples of industries that will rely on carbon capture and utilization technologies are referred below:

• Energy Industry: production of zero carbon fuels (hydrogen, ammonia, methanol) and recycled fuels (e.g., recycled methane).
• Chemical Industry: carbon dioxide is mainly utilized as an ingredient in the production of urea, with a smaller fraction being used to produce methanol and a range of other sub-products.
• Agriculture: as plants require carbon dioxide to conduct photosynthesis the atmospheres of greenhouses are usually enriched with additional CO2 to sustain and increase the rate of plant growth.
• Food: one of its greatest uses as a chemical is in the production of carbonated beverages; it provides the sparkle in carbonated beverages such as soda water, beer, and sparkling wine. It is also used as a propellant and acidity regulator and can be found in candy, baker's yeast, baking powder, baking soda. Additionally, it is used during the cold soak phase in winemaking in the form of dry ice, for the removal of caffeine from coffee and as a refrigerant.
• Inert gas / fire extinguishing gas
• Supercritical CO2 as solvent
• Medical and pharmacological uses
• Other minor uses

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