The IPCC Special Report on Carbon dioxide Capture and Storage published in 2005 [1] explained that, if the world is to achieve net zero carbon dioxide (CO2) emissions by 2050 then it will need not only to replace all its fossil fuel power stations with clean energy [2] but also absorb the CO2 emitted by the remaining sources such as transport which are even more difficult to replace with clean energy.

Carbon dioxide (CO2) capture and storage (CCS) is a process consisting of the capture of CO2 from the atmosphere or separation of CO2 from industrial and energy-related sources, transport to a storage location and long-term isolation from the atmosphere. The process is sometimes known as carbon capture and sequestration or carbon control and sequestration.

Carbon capture and utilization (CCU) also deals with capture of CO2 but then considers how to recycle that material to produce useful products that can be reused.

In this article we deal with both storage and utilization, so we call this topic Carbon Capture and Storage or Utilization (CCSU). We also examine issues of leakage and look briefly at cost.

CCSU is an important way to stabilize atmospheric greenhouse gas concentrations. Other ways include energy efficiency improvements, the switch to less carbon-intensive fuels, nuclear power, renewable energy sources, enhancement of biological sinks, and reduction of non-CO2 greenhouse gas emissions.

CCSU has the potential to reduce overall mitigation costs and increase flexibility in achieving greenhouse gas emission reductions. The widespread application of CCSU would require significant technical improvements and cost reduction.

Capture

Pre-combustion Capture

The technology for pre-combustion is widely applied in producing fertilizer, chemicals and power production. The fossil fuel is partially oxidized to produce carbon monoxide (CO) and hydrogen (H2). Steam (H2O) is added and the CO reacts to produce CO2 and H2. The resulting CO2 can be extracted and the H2 can be used as fuel.

Oxyfuel Combustion Capture

In oxyfuel combustion the fuel is burned in oxygen (O2) instead of air to produce mainly carbon dioxide (CO2) and water vapour (H2O). The vapour is condensed to form water leaving an almost pure carbon dioxide stream to be transported to the sequestration site and stored. Note that some of the water contains some dissolved CO2.

Post Combustion Capture

In post combustion capture the CO2 is removed after combustion of the fossil fuel. This is the procedure that would be applied if capturing CO2 from existing fossil-fuel burning power plants. The technology is currently used in other industrial applications, although not at the same scale as might be required in a commercial scale power station. Post combustion capture is most popular in research because existing fossil fuel power plants can be retrofitted to include CCSU technology in this configuration.

CO2 separation technologies

Carbon dioxide can be separated out of air or flue gas with absorption, adsorption, or membrane gas separation technologies. Absorption, or carbon scrubbing, with amines is the dominant capture technology.

Direct air capture

Direct air capture (DAC) is the process of removing CO2 directly from the air. Work in this area is still in its infancy. A pilot plant owned by Carbon Engineering has operated in British Columbia, Canada since 2015. An economic study of this plant in 2018 estimated the cost at US$94–$232 per tonne of atmospheric CO2 removed. Several companies are now working on this approach.

CO2 transportation

After capture, CO2 has to be transported to storage or utilization sites. This could be done by pipeline or on ships. Pipelines are the cheapest form of transport for large volumes of CO2. For example, a pipeline is used in Norway to transport CO2 to oil production sites where it is then injected into older fields to extract oil. Ships are also used for transporting CO2 for other applications and could be used where pipelines were not feasible.

Storage

Methods of permanent storage of CO2 include gaseous storage in various deep geological formations and solid storage by reaction of CO2 with metal oxides to produce stable carbonates. In the past it was suggested that CO2 could be stored in the oceans, but this would exacerbate ocean acidification and has been made illegal under the London and OSPAR conventions. Ocean storage is no longer considered feasible.

There are several pilot programs in various stages of development to test the long-term storage of CO2 in non-oil producing geologic formations. As the technology develops, costs, benefits and detractions are changing.

Geological storage

This involves injecting carbon dioxide, generally under enough pressure to convert it into a liquid, directly into underground geological formations. Oil fields, gas fields, saline formations, unmineable coal seams, and saline-filled basalt formations would be possible storage sites. Various physical and geochemical trapping mechanisms would prevent the CO2 from escaping to the surface.

Mineral storage

There are two ways to store CO2 using minerals.

Mineral Carbonation

Mineral carbonation is fixation in the form of inorganic carbonates. It is also known as ‘mineral carbonation’ or ‘mineral sequestration’.

Captured CO2 is reacted with metal-oxide bearing materials, thus forming the corresponding carbonates and a solid byproduct, silica for example. Magnesium and calcium silicate deposits are sufficient to fix the CO2 that could be produced from the combustion of all fossil fuels resources. To fix a tonne of CO2 requires about 1.6 to 3.7 tonnes of rock. From a thermodynamic viewpoint, inorganic carbonates represent a lower energy state than CO2 ; hence the carbonation reaction can theoretically yield energy.

However, the kinetics of natural mineral carbonation is slow. Hence all currently implemented processes require energy-intensive preparation of the solid reactants to achieve affordable conversion rates and/or additives that must be regenerated and recycled using external energy sources. The resulting carbonated solids must be stored at an environmentally suitable location.

The technology is still in the development stage and is not yet ready for implementation.

Carbon Utilization

Carbon utilization is the process of recycling the carbon that has been captured.

Carbon dioxide degrading algae or bacteria

The idea is to feed algae or bacteria with CO2 which would degrade the carbon dioxide. For example, researchers have created a strain of Escherichia coli that grows by consuming carbon dioxide instead of sugars or other organic molecules [3].

Industrial Utilization of CO2 as a Feedstock

Industrial uses provide a carbon sink, as long as the pool size keeps growing and the lifetime of the compounds produced is long. However, neither prerequisite is fulfilled in practice, since the scale of CO2 utilization is small compared to anthropogenic CO2 emissions, and the lifetime of the chemicals produced is too short with respect to the scale of interest in CO2 storage.

Therefore, the contribution of industrial uses of captured CO2 to the mitigation of climate change is expected to be small.

Leakage

Observations from engineered and natural geological storage sites as well as theoretical models suggest that the fraction retained in appropriately selected and managed geological reservoirs is likely to exceed 99% over 1,000 years. For well-selected, designed and managed geological storage sites, the vast majority of the CO2 will gradually be immobilized by various trapping mechanisms and, in that case, could be retained for up to millions of years. Because of these mechanisms, storage could become more secure over longer timeframes.

Costs

About two thirds of the total cost of CCSU is attributed to capture, making it limit the wide-scale deployment of CCSU technologies. Optimizing CO2 capture would significantly increase the feasibility of CCSU since the transport and storage steps of CCSU are rather mature technologies.

Carbon sequestration in a fossil fuel power station adds about $0.18/kWh to the cost of energy, placing it far out of reach of profitability and competitive advantages over renewable power. In one example, sequestration consumed 25% of the plant’s rated 600-megawatt output capacity. After adding CO2 capture and compression, the capacity of the coal-fired power plant was reduced to 457 MW.

Utilization by manufacturing products from CO2 is energy intensive since CO2 is a thermodynamically stable form of carbon. In addition, concerns on availability of carbon dioxide is a major disincentive for investment since alternative raw materials are already available.

One of the drivers for the possible implementation of CCU is a price on carbon. A price on carbon will incentivize the reduction of CO2 being released into the atmosphere. Thus, CCU can be one of the main alternatives for a company to reuse the emitted CO2 for creating useful commercial products.

References

[1] IPCC Special Report on Carbon dioxide Capture and Storage

[2] We discussed the cost of replacing all its fossil fuel power stations with clean energy in this article

[3] Gleizer, S. et al. Cell https://doi.org/10.1016/j.cell.2019.11.009 (2019).

[Passim] Wikipedia article on Carbon Capture and Storage and Carbon Capture and Utilization