Carbon capture and storage typically involves three basic steps:
- Capture: The CO2 is separated from other gases that are produced during industrial processes such as those of cement or steel factories or natural gas and coal fired power plants
- Transport: CO2 can be compressed into a liquid form or kept as a gas before being transported to a storage site
- Storage: Finally, the CO2 is injected into underground rock formations or otherwise placed into permanent storage
Here we will investigate these three steps as well as some alternatives:
Carbon dioxide can be captured from an industrial source (such as a power plant) or directly from the air. There are a variety of technologies that can be used for carbon capture, including absorption, adsorption, chemical looping, gas hydrate technologies and membrane gas separation.
CO2 capture is best performed directly at source, such as at biomass or fossil fuel energy plants, natural gas electric power stations, natural gas processing assets, synthetic fuel plants, fossil fuel-based hydrogen production plants and those industries that produce large quantities of CO2 emissions. As mentioned above, CO2 can be captured directly from the air, although this is less effective and more difficult than capture at source.
Other areas where carbon capture can be used include from organisms that produce ethanol during fermentation. This creates pure CO2, in quantities slightly less than ethanol by weight, which can be pumped underground (see ‘transport’ and ‘storage,’ below).
Methods for carbon capture can be broadly split into three technologies; post combustion, pre-combustion and oxyfuel combustion:
Post Combustion Capture
Post combustion capture involves the removal of CO2 following combustion of the fossil fuel. Typically used at power plants, this process involves the capture of carbon dioxide from flue gases released by power stations or other carbon emission producing sites. The technology for this capture method can be retrofitted to existing power stations as well as being built into new build plants.
Pre-combustion capture is typically used in the fertiliser, chemical, gaseous fuel and power generation industries. The method works by partially oxidising the fossil fuel using, for example, a gasifier. This creates a syngas (CO and H2), which reacts when steam (H2O) is added to become CO2 and H2. The CO2 can then be captured from a comparatively pure exhaust stream, while the H2 can be used as fuel without any carbon dioxide (CO2) emissions. This process is best incorporated into new builds.
Oxyfuel Combustion Capture
Oxyfuel combustion requires the fuel to be burnt in oxygen rather than air. Cooled flue gas is recirculated and injected back into the combustion chamber to prevent high flame temperatures. This flue gas is mainly comprised of carbon dioxide and water vapour. Since the water vapour can be condensed by cooling, the process leaves almost pure carbon dioxide steam that can be captured. While this is known as a ‘zero emission’ process due to the high quantity of carbon dioxide that is captured, some of it still gets into the condensed water, which therefore has to be treated or disposed of correctly so as not to get into the environment.
Carbon capture technology comes in a range of forms, including:
- Calcium Looping
- Chemical Looping Combustion
- Multiphase Absorption
- Oxyfuel Combustion
Capture is the most expensive aspect of CCS, accounting for around two thirds of the total cost. This is partially because the transport and storage steps are already well established as technologies while there is scope for further optimisation of capture processes.
Once it has been captured, the CO2 needs to be transported to a storage site. Pipelines are generally seen as the cheapest option for transporting large quantities of CO2, although in some instances – such as for transportation over long distances, ships can be a cheaper option. CO2 can also be transported by rail or tanker trucks, but these options are around twice as expensive as shipping or pipelines.
Both the USA and the UK currently envisage pipelines as the preferable means of transporting CO2.
A number of methods have been explored for the permanent storage of CO2, including geological storage (as a gas or a liquid), mineral-based solid storage through a reaction with metal oxides to produce stable carbonates, the use of carbon dioxide degrading algae or bacteria to break down the CO2, and even via ocean-based storage. However, ocean storage has been made illegal under the London and OSPAR conventions, since this type of storage could greatly increase ocean acidification.
Current methods of handling and storing (or sequestering) CO2 include:
Geological storage, or geo-sequestration, is the process of injecting CO2 directly underground into geological formations. Suggested storage sites include oil and gas fields, unmineable coal seams, and saline formations, including saline filled basalt formations.
There are various techniques to prevent the CO2 from escaping back to the surface and into the atmosphere, including the use of highly impermeable rock and other natural geological properties.
For example, in unmineable coal seams, the CO2 molecules attach to the surface of the coal where it is then absorbed. During this absorption, the coal releases previously absorbed methane, which can then itself be captured, with a process known as ‘enhanced coal bed methane recovery, before being sold. However, burning this methane would offset some of the benefits of the CCS operation. This particular storage method is also dependent on the permeability of the coal bed.
Saline formations contain highly mineralised brines that have so far believed to have no benefit to humans. As a result, saline aquifers have been used for the storage of chemical waste due to their large potential for storing large quantities of waste and common occurrence. However, a lack of knowledge about these formations means that further exploration is necessary while, unlike with other geological storage, there are no known side products to help offset the costs. However, saline aquifer based storage still use trapping mechanisms like residual trapping, solubility trapping, structural trapping and mineral trapping to keep the CO2 underground and reduce the risk of leaking.
Geological formations are currently considered to be the most promising sequestration sites, although there is still the risk that some CO2 may leak into the atmosphere.
Enhanced Oil Recovery
As a subset of geological storage, it is possible to inject CO2 into an oil field where it can be used for enhanced oil recovery. However, since more carbon dioxide is released when this oil is used, it is not a carbon neutral process, making it less appealing than other storage methods.
Bacteria or Algae-Based Carbon Dioxide Degradation
Rather than injecting CO2 into the ground, it can be stored in containers along with algae or bacteria that is able to degrade the carbon dioxide. For example, the carbon dioxide metabolising bacterium, Clostridium Thermocellum could be used to prevent overpressurisation of the containers.
Mineral storage uses an exothermic reaction between CO2 and metal oxides to produce stable carbonates like calcite or magnesite. This is a naturally-occurring process that is responsible for creating a large amount of surface limestone. This natural process can be sped up by using a catalyst, by increasing temperatures or pressure, or by pre-treating the minerals. However, the down side is that these processes often require additional energy use. The Intergovernmental Panel on Climate Change (IPCC) estimated that a power plant equipped with mineral storage CCS would use 60-180% more energy than a plant without CCS.
Despite these challenges, mineral storage offers a number of advantages including the abundance of minerals that could be used for the process and the lower energy states of carbonates as compared to CO2, which means this process is thermodynamically favourable. The stable nature of the carbonates prevents the rerelease of CO2 into the atmosphere.
Carbon capture and storage as well as carbon capture, utilisation and storage (CCUS) are both important in the fight against climate change. CCUS differs slightly from CCS in that the captured carbon dioxide is used to create products or services rather than simply being stored.
Reducing emissions is important to the environment to help prevent adverse effects such as global warming. CCS could reduce the CO2 emissions from a modern conventional power plant by 80-90%.
Although power plants can benefit from carbon capture, most CCS projects are industrial, coming from industries such as cement or steel making and fertiliser production. Furthermore, CCS, when combined with biomass, can even result in net negative emissions.
CCS is important to the future of the environment. The Intergovernmental Panel on Climate Change (IPCC) has noted that, in order to meet the ambitions of the Paris Agreement to limit the Earth’s temperature increase to 1.5 degrees, we must do more than simply reduce emissions.
In addition to reducing emissions we need to work to remove carbon from the atmosphere. CCS is an important tool in achieving this goal and tackling global warming.
Carbon capture and storage is being used around the world, with the largest single user of the technology currently being the USA. The United States have been using CCS since 1972, with over 200 million tons of CO2 having been stored underground by natural gas plants in Texas alone.
A Global CCS Institute report from 2019 determined that there were 51 large-scale CCS facilities around the world, of which 19 were in operation, 4 under construction and the rest in various different stages of development. 24 of these plants were in the Americas, 12 in Europe, 12 in the Asia-Pacific region and 2 in the Middle East.
CCS is currently being used at power stations as well as to help decarbonise industrial processes and heating.
As well as the evident positive environmental impact of carbon capture and storage in reducing climate change, CCS also brings a number of advantages that could offer benefits to industry. These include:
1. Creating Additional Power
The process of pressurising CO2 into a fluid form using steam cycles could transfer heat more easily and use less energy, allowing power generation turbines to run more efficiently. In addition, CO2 could be injected into geothermal locations for storage, at the same time helping to extract the geothermal resource to create renewable geothermal energy.
2. Using CO2 as Fuel
CO2 could theoretically be used as a fuel; however, the processes to accomplish this are often difficult and costly, meaning that further development is needed to make this a truly viable option.
3. Concrete Enrichment
Captured CO2 can be used to improve the properties of concrete, strengthening it and thereby offering increased durability.
4. Chemical and Plastic Manufacture
Captured CO2 could be used to manufacture chemicals and plastics like polyurethanes used to create soft foams.
5. Job Creation
Another advantage of CCS lies in job creation, as more technicians are required to manage the growing field of CCS around the world.
6. Saving the Planet
Ultimately, the main advantage of CCS is in reducing toxic carbon emissions and decarbonising the atmosphere to prevent irreparable damage being to the Earth’s ecosystems.
While the overall goal of CCS in helping to save the environment from further damage outweighs the disadvantages of the process, it doesn’t mean that carbon capture and storage is not without its challenges. These include:
As mentioned above, the process of capturing CO2 is expensive as a result of the high deployment and energy costs involved. The extraction, pumping and compression of CO2 means that a plant with CCS uses more fuel than one without, although these costs vary between different processes. For example, processes where CO2 is produced separately, such as with fertiliser manufacture, are less expensive than those where the CO2 needs to be separated, such as in cement production.
Because the implementation of a CCS strategy generally involves cost increases for industry, commodity products become less competitive and businesses may be tempted to avoid transitioning to CCS in order to maintain their margins.
However, progress in research and development should see the cost of CCS reduce and this is already being seen, as the price of CCS for a tonne of CO2 has gone down in recent years.
2. CO2 Leakage
Another genuine concern related to CCS is CO2 leakage, which could lead to environmental damage. To prevent this, storage sites need to be carefully selected, managed and monitored. Because the carbon dioxide is highly toxic, large leakages could render the air at the site largely unbreathable. Despite the lack of conclusive research about the absolute security of underground CO2 storage, a study by Princeton University found the risk of leakage to be low.
This is the big question, yet since CCS is currently the best option for reducing emissions from large industrial applications, it is certainly an essential tool in preventing climate change.
When combined with bioenergy technologies for power generation – bioenergy with carbon capture and storage (BECCS) – CCS is able to generate ‘negative emissions’ and remove CO2 from the atmosphere.
Capturing carbon from the atmosphere is crucial to limiting temperature rises and starting the process of reversing climate change. However, these is a lot of work to be done to reach the capacity required as estimated by the Global CCS Institute, who say that we need 2,500 CCS installations by 2040, with each capturing around 1.5 million tonnes of CO2 per year.
So, considering how much more we need to do to achieve the required levels of CCS to meet the Paris Agreement, what is the future of carbon capture and storage?
Global investment into CCS is required and many governments are beginning to show a renewed commitment to the technology, while initiatives like the UK Government’s goal to reach Net Zero should also help with growth. In addition, pledges like that made by the UK Government to phase out coal burnt in power plants without CCS (‘unabated coal’), should also drive uptake and research into this area.
It seems that the future of CCS is being shaped right now, but the fact remains that it looks set to grow as a viable technology to help address the problem of climate change over the coming years.
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