The capture and storage of carbon dioxide (CO2), also called sequestration, is a process that consists of separating the compound from industrial and energy sources, transporting it to a location where it will be stored, and isolating it in the long term. The carbon can either be captured pre-combustion or post-combustion.
Carbon capture as a way to mitigate the effects of global warming is still a technology with many unanswered questions, in order for it to be viable, from the economic point of view as the environmental.
The knowledge of the implications of climate change generated by human activities has grown rapidly during the last years. This knowledge has boosted research into options for mitigating the likely impacts of carbon emissions on the environment.
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The capture and storage of carbon dioxide are presented as one option in the list of actions to reduce or stabilize the increase in atmospheric concentrations of greenhouse gases. Other options include reductions in growth, improvements in energy efficiency, the shift to less carbon-intensive fuels, the use of renewable energy sources, and reducing the emissions of other greenhouse gases, among others.
An example of this research and information gathering activity was the eighth session of the working group III of the Intergovernmental Panel on Climate Change (IPCC), which took place in Montreal in September 2005. This working group, as part of the IPCC, is responsible for evaluating the available information on climate change.
Its activities are focused on the scientific, technical, environmental, and economic and social aspects of climate change mitigation.
Carbon capture comes with its fair share of problems
This article tries to explore some of the problems associated with CO2 capture and storage. Contrary to what industry expects, especially the coal industry, the availability of carbon capture technology cannot be used as an excuse to continue burning fossil fuels.
Drawbacks to the technology include the inability to retrofit older plants to allow precombustion capture, which is far more efficient than post-combustion capture; the possibility of leaks from storage facilities; the high energy costs of the process; and the high costs of using existing carbon capture processes.
However, despite these drawbacks, the capture and storage of CO2 are still presented as an option with the potential to help reduce the total amount of greenhouse gas emissions. It is expected that the feasibility of its use will depend on the technical maturity, the costs, the total potential, the diffusion and the transfer of technology to developing countries, legal aspects as well as environmental conditions.
Its use could also depend significantly on public opinion, and the willingness of people to pay more for their energy.
However, it is still far from being the ultimate solution to climatic problems. One issue with carbon sequestration is with plans to store the CO2 under the ocean. Some experts believe recovered CO2 can be dumped directly into the ocean, at depths greater than 11,482 feet (3500 meters). The theory is that the CO2 will compress and fall to the ocean floor. However, ocean carbon storage is largely untested, and it is difficult to test it without risking the safety of marine life.
Another storage option, called geological sequestration, injects CO2 into underground rock formations below the Earth's surface. These natural reservoirs have overlying rocks that form a seal, keeping the gas contained. Researchers have also found that when CO2 is injected into basalt, it eventually turns into limestone, essentially converting CO2 into rock.
Oil and gas reservoirs are already being used to store CO2, as they consist of layers of porous rock formations that have already trapped oil and gas. However, carbon sequestered in reservoirs, particularly in disused coal mines, can be released through cracks and then re-enter the atmosphere.
Techniques to capture carbon have actually been in use for a long time as a byproduct of industrial processes and to enhance oil and gas recovery.
In post-combustion carbon capture, the CO2 is captured after the fossil fuel is burned. The burning of fuels produces gases, including CO2, water vapor, sulfur dioxides, and nitrogen oxides. The CO2 is separated and captured from these gases. This process is already used to remove CO2 from natural gas and can be done by retrofitting older power plants, using a filter to trap the gases.
This process is already used to remove CO2 from natural gas and can be done by retrofitting older power plants, using membrane filters to trap the gases. These filters act as a solvent that absorbs carbon dioxide from the gases.
The filter is then heated to release water vapor from the mixture, leaving behind CO2. Post-combustion capture can prevent up to 80 to 90 percent of a power plant's carbon emissions but is a very energy-intensive process, requiring as much as 40 percent of the power plants' total energy.
In pre-combustion carbon capture, CO2 is trapped before the fossil fuel is burned. The fuel is first heated in the presence of pure oxygen to form synthesis gas, also known as syngas, which is a mixture of hydrogen, carbon monoxide, CO2 and smaller amounts of methane.
This mix then undergoes a catalytic conversion process using water-gas reaction to produce a gas consisting of hydrogen and carbon dioxide.
A chemical solvent called amine is usually utilized to remove CO2 from the gas. The amine solution binds with the CO2, making the mixture heavier while causing the hydrogen to rise up the container.
The separation process leaves the amine-CO2 mixture at the bottom while the hydrogen gas is collected from the top of the container, and is utilized as a fuel for combustion.
This solvent-CO2 mixture is heated further, causing the CO2 to rise up and is collected for compression and sequestration. The amine solution, being heavier, is collected at the bottom for reuse.
This process captures a much higher concentration of CO2 than post-combustion and is lower in cost, but cannot be used with older power plants.
An expensive sequestration
Using the current technology, the costs of sequestration are estimated to be anywhere between $100 and $300 dollars per ton of carbon emissions that are avoided. In addition to that, according to the estimates, the capture of CO2 accounts for only three-quarters of the total cost of sequestration.
The costs of storage, transportation, and monitoring must be added to the total cost. In addition, adding the existing technologies for the capture of CO2 to an electricity production process could mean an increase in the cost of electricity between 2.5 and 4 cents per kWh, depending on the type of process.
We must bear in mind that the current cost of producing electricity for a new combined-cycle gas plant is generally between 3 and 5 cents per kWh, while it ranges between 4 and 5 cents for a more modern, supercritical coal plant.
Developments in the carbon capture technologies
Other types of carbon capture technology are also being tested. For example, a startup in Huntsville, Alabama is using giant fans perched on shipping containers to draw air over filters which trap carbon dioxide from the outside air.
In initial trials, the fans captured roughly 4000 tons of carbon dioxide every year, about the same as that emitted by 870 vehicles. The project is run by Global Thermostat, which is a startup that specializes in direct air capture.
If scaled up and combined with efficient carbon storage, direct air capture could help reduce the worst effects of climate change. Direct air capture is also being developed by other companies, such as British Columbia-based Carbon Engineering.
However, some climate researchers criticized direct air capture, arguing that it is yet to be proven to work at scale and that it is a distraction from the real solution of limiting growth - giving companies and countries an excuse to avoid cutting emissions directly. Others point out that some type of carbon capture technology will still be needed, even if we do everything else possible to cut and capture emissions.
A Swiss company, Climeworks, has taken a different approach by developing ways to use captured and compressed CO2 as a fertilizer to grow crops in greenhouses.
The company would like to capture 1 percent of the global annual carbon dioxide emissions by 2025.
Statistics show that in 2019, the concentration of CO2 in the atmosphere has been recorded to be 415 parts per million. This is the highest it has ever been, which is a testimony of the urgency of such developments.
Climate change is a problem that is getting bigger by the day. That said, carbon capture and storage may be a promising way to help to mitigate some of these issues in the long run, along with other systemic changes.