As each day passes, the worldwide call for sustainable energy seems to get louder. The price of carbon emissions is increasing, funding is more widely available for renewable energy projects, and residential power solutions are on the rise. While strides are being made in sustainable technology and policy, the worldwide CO₂ emissions have never been higher.

The International Energy Agency (IEA) recently released the Net-Zero by 2050 Roadmap to provide a path for the energy industry to follow in achieving sustainable energy production. This report illustrates that switching to renewable energy will likely be the primary source of emissions reduction. However, the report also highlights the need for other methods to be utilized in reducing CO₂ output. These methods include a large increase in the utilization of carbon capture and storage (CCS) technologies.

CCS offers a few distinct advantages that make it a useful technology in the sustainable energy battle.

First – CCS can retrofit existing power plants. This makes it a viable solution to CO₂ emissions reduction while renewable energy capacity is being built.

Second - fossil fuel power plants (such as coal and natural gas plants) are generally more flexible in their output than current renewable energy sources. As such, it may be useful to have fossil-fuel power plants equipped with CCS technology to help maintain a robust and stable electric grid, even after renewable energy becomes the dominant energy source.

Forms of CCS occur naturally. For example, trees remove CO₂ from the air and store it in the form of new trunks, stems, and roots. Unfortunately, the world is producing CO₂ at a much higher rate than natural processes can handle. Man-made CCS processes can help to handle the excess CO₂ emissions.

As the name suggests, CCS can be broken down into two primary steps:

1. Capture: Separating and collecting the CO₂ from the emissions source.
2. Storage: Transporting and storing the CO₂ at a permanent storage location.

More information on the carbon capture process can be found in the earlier published article, Fundamentals of carbon capture.

Carbon storage methods

Geological reservoirs are the most common carbon storage locations utilized today. Source: Sask Power/ CC BY-NC-SA 2.0

The goal of carbon storage is to permanently find a location for captured CO₂ emissions to remain so that they are not released into the atmosphere.

Though the storage step of the CCS process is not typically as complex or costly as the capture step, it is not without challenges.

After being captured, the CO₂ needs to be transported to the storage location. The most common method of CO₂ transportation is through pipelines, however, ships, trains, and trucks may also be used in some cases. There are significant challenges associated with the transportation of CO2. Significant energy is required to compress the CO₂ to manageable volumes, as well as to move the CO₂ through pipelines. Additionally, this pressure requirement means that both CO₂ pipelines and containment vessels are typically highly pressurized, which can lead to explosive failures and high-volume leaks.

There are three primary storage methods for captured CO₂ used today: Geological, Biological, and Mineral.

Ocean storage was also once considered a viable solution, however, multiple regulatory bodies have banned this practice due to the potential acceleration of ocean acidification.

Geological carbon storage

Geological CO₂ storage refers to the storage of CO₂ in underground formations, which may include depleted oil and gas reservoirs, saline formations, and coal seams.

As CO₂ is injected at high pressure into deep reservoirs, the required volume of downhole storage is much less than that at the surface. As such, geological formations have the capacity for vast volumes of CO₂ storage.

CO₂ can also be used to aid in enhanced oil recovery operations. In this process, CO₂ is injected into the oil reservoir, where it helps to push the oil into the producing well. Though this process results in some CO₂ being trapped, some CO₂ is also produced with the oil and thus this is not a carbon-neutral process.

Though geological storage options have huge potential storage volumes, they are not without their potential drawbacks. For example, concerns regarding the potential leakage of CO₂ through the reservoirs have been raised. In a 2018 study, models illustrated that well-regulated wells would maintain minimal leakage for at least 1000 years. There is also the concern of induced seismic activity associated with high-pressure CO₂ injection.

Biological carbon storage

Biological CO₂ storage refers to several storage locations in living/natural materials. This includes oceans, trees, algae, soils, and other plant life.

While planting more trees helps combat the effects of deforestation, this will likely not be fast-acting enough to provide the required CCS capacity.

Technology advancements in the area of microalgae growth show promise in the ability to act as a CO₂ storage option. Certain algae have the ability to absorb CO₂ and use it as a building material for rapid growth.

As a bonus, some microalgae are capable of being used as renewable biofuels. For example, a process known as Bioenergy with Carbon Capture and Storage (BECCS) has been modeled as a potentially negative emissions energy-producing technology.

Mineral carbon storage

Mineral CO₂ storage refers to the process of converting CO₂ into solid carbonates. This isn’t so much a storage location as a storage process. Once the solid carbonates are formed, they can be stored above ground in an environmentally safe location.

CO₂ mineralization is accomplished by reacting the CO₂ with metal-oxides. Typically these include magnesium and calcium silicates, which are abundantly available.

Mineralization has several unique advantages:

1) It has the theoretical highest storage capacity, with the ability to store all current CO₂ emissions.

2) It is the safest storage method, as there is no concern of loss of storage capacity or leakage of emissions.

3) The mineralized CO₂ can be useful. For example, solid carbonates can be used for construction materials, or in the production of Urea and Methanol.

However, mineralization is also a very expensive carbon storage method, even when the cost is offset by industrial uses. As such, it is not currently widely utilized.

Summary

Advances in technology are bringing new and improved methods of clean energy production. However, the rate of emissions worldwide continues to climb. If the world is to make the transition to sustainable energy, the energy industry will need to invoke a broad portfolio of measures and technologies.

Though not yet widely utilized, CCS can aid the transition from fossil fuel to renewable energy sources, as well as supplement a renewable-dominated energy industry.