14.2.1. STEPS

A) Capturing or Scrubbing

Broadly, three different types of technologies for scrubbing of CO2 exist:

1. Post-combustion

2. Pre-combustion

3. Oxy-fuel combustion

4. Chemical looping

5. Post-Combustion: In post combustion capture, the CO2 is removed after combustion of the fossil fuel — this is the scheme that would be applied to fossil-fuel burning power plants. Here, carbon dioxide is captured from flue gases at power stations or other large point sources. The technology is well understood and is currently used in other industrial applications, although not at the same scale as might be required in a commercial scale power station.

6. Pre-Combustion: The technology for pre-combustion is widely applied in fertilizer, chemical, gaseous fuel (H2, CH4), and power production. In these cases, the fossil fuel is partially oxidized, for instance in a gasifier. The resulting syngas (CO and H2) is shifted into CO and more H2. The resulting CO can be captured from a relatively pure exhaust stream. The H can now be used as fuel; the carbon dioxide is removed before combustion takes place.

7. Oxy-Fuel Combustion: In oxy-fuel combustion, the fuel is burned in oxygen instead of air. To limit the resulting flame temperatures to levels common during conventional combustion, cooled flue gas is re-circulated and injected into the combustion chamber. The result is an almost pure carbon dioxide stream that can be transported to the sequestration site and stored.

8. Chemical Looping: Chemical looping uses a metal oxide as a solid oxygen carrier. Metal oxide particles react with a solid, liquid, or gaseous fuel in a fluidized bed combustor, producing solid metal particles and a mixture of carbon dioxide and water vapor. The water vapor is condensed, leaving pure carbon dioxide which can then be sequestered.

This section describes the various technologies used in the process of capturing carbon dioxide (CO2) from the atmosphere and industrial processes. There are four main types of techniques:

1. Post-combustion: This technique captures CO2 after fossil fuels have been burned. It is suitable for existing power plants where CO2 is extracted from the emitted gases.
2. Pre-combustion: This process takes place before the fuel is burned. The fossil fuel is transformed into a cleaner gas mixture, allowing CO2 to be captured easily before it can be emitted.
3. Oxy-fuel combustion: In this method, fuels are burned using pure oxygen, which results in a very high concentration of CO2 in the exhaust, making capture straightforward.
4. Chemical looping combustion: This advanced technique uses metal oxides to facilitate combustion and capture CO2 during the reaction, resulting in pure CO2 that can be stored. Each method serves a specific purpose and has its advantages, focusing on reducing CO2 emissions effectively.

Imagine you are cleaning a kitchen after cooking. You have a choice: you can clean up immediately as you cook (pre-combustion), wait until everything is done and then clean (post-combustion), use a special sponge that absorbs all the mess as you cook (oxy-fuel combustion), or use magical cleaning tools that don't just wipe surfaces but absorb mess into themselves (chemical looping). Each method effectively reduces mess, just like each CO2 capture technology aims to reduce emissions.

B) Transportation

After capture, the CO2 would have to be transported to suitable storage sites. This is done by pipeline, which is generally the cheapest form of transport. In 2008, there were approximately 5,800 km of CO2 pipelines in the United States, used to transport CO2 to oil production fields where it is then injected into older fields to extract oil. The injection of CO2 to produce oil is generally called Enhanced Oil Recovery or EOR. In addition, there are several pilot programs in various stages to test the long-term storage of CO2 in non-oil producing geologic formations. A CO2 conveyor belt system or ship could also be utilized for transport. These methods are currently used for transporting CO2 for other applications.

Once CO2 is captured using the aforementioned technologies, it must be transported to storage sites. The most cost-effective method is by pipeline. In the U.S., extensive pipelines were already established as of 2008, totaling around 5,800 km for transporting CO2, particularly to enhance oil recovery by injecting CO2 into oil fields. Other transport methods, like conveyor systems or ships, may also be utilized when necessary. Additionally, research is ongoing to assess the feasibility of storing CO2 in geological formations that do not produce oil, highlighting the importance of safe and efficient transport methods in the overall carbon capture process.

Think of transporting CO2 like moving items during a house move. You can shove everything into a truck (pipeline) for a quick and cheap journey, or opt for using boxes and multiple trips (conveyor or ship), which may take longer and be more costly. Just as careful planning is needed to ensure nothing is damaged during transport, similar precautions are taken when moving CO2 to ensure it reaches its storage destination safely.

C) Sequestration or Storage

Various forms have been conceived for permanent storage of CO2. These forms include gaseous storage in various deep geological formations (including saline formations and exhausted gas fields), liquid storage in the ocean, and solid storage by reaction of CO2 with metal oxides to produce stable carbonates.

i) GEOLOGICAL STORAGE: Also known as geo-sequestration, this method involves injecting carbon dioxide, generally in supercritical form, directly into underground geological formations. Oil fields, gas fields, saline formations, unmineable coal seams, and saline-filled basalt formations have been suggested as storage sites. Various physical (e.g., highly impermeable cap rock) and geochemical trapping mechanisms would prevent the CO2 from escaping to the surface.

Enhanced oil recovery: CO2 is sometimes injected into declining oil fields to increase oil recovery. This option is attractive because the geology of hydrocarbon reservoirs is generally well understood and storage costs may be partly offset by the sale of additional oil that is recovered.

The final step in the carbon capture process is sequestration or storage of CO2. This can be accomplished through several methods:

1. Gaseous storage involves injecting CO2 into deep geological formations such as depleted oil and gas fields, saline aquifers, or unmineable coal seams to keep it contained.
2. Liquid storage focuses on the potential of storing CO2 in the oceans, though this method requires careful consideration of environmental impacts.
3. Solid storage, on the other hand, represents a more permanent solution where CO2 chemically reacts with materials like metal oxides to form stable solid carbonates. Geological storage is essential as it ensures that captured CO2 does not re-enter the atmosphere, but factors like geology and safety protocols need to be strictly monitored.

Imagine burying a treasure in a secret spot to keep it safe (sequestration). You choose a solid box that protects it (solid storage) and find a deep hole that no one can access (geological storage). You may also think of hiding it underwater (liquid storage). Just like you would want to ensure that nobody can find or dig up the treasure, effective monitoring and safety must also be a priority when storing CO2 to ensure it remains sealed away safely from the atmosphere.

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