Summary: There are a number of different ways to capture, transport, store, and even reuse carbon to prevent it from ultimately making it into the atmosphere to contribute to global warming.
Environmental Effects
Carbon Capture and Storage (CCS) systems are designed to try to reduce CO2 emissions levels into the atmosphere. Depending on the plant used, CCS can reduce emissions by up to 90%, although they do require more energy to accomplish this goal. The IPCC has provided estimates of air emissions (in kg/MWh) from different CCS plant designs.
| | Natural gas combined cycle | Pulverized coal | Integrated gasification combined cycle |
| CO2 | 43 (-89%) | 107 (-87%) | 97 (-88%) |
| NOx | 0.11 (+22%) | 0.77 (+31%) | 0.1 (+11%) |
| SOx | | 0.001 (-99.7%) | 0.33 (+17.9%) |
| Ammonia | 0.002 (before:0) | 0.23 (+2200%) | |
Capture Methods
CO2 can be captured from the atmosphere with the use of biomass, or from large point sources, in three different ways:
Post-combustion, in which the CO2 captured in the flue gases at power stations. This is the most well-known and tested method. Pre-combustion, where a fossil fuel is partially oxidized into CO and H2, then turned into CO2 and H2. At this point, the CO2 is removed and H2 combusted. This technology is widely applied in fertilizer, chemical, gaseous fuel, and power production plants. In Oxy-fuel combustion, the fuel is burned in oxygen instead of air. This causes the flue gas to consist mainly of carbon dioxide and water vapor, the water is condensed, and then the nearly pure CO2 is captured. This results in a near zero emission process, however, the process of producing the pure oxygen is very energy-expensive. In the case of ethanol fermentation, nearly pure CO2 is produced that can be captured and stored. There is also another method being developed, which is chemical looping combustion (CLC). This involves using a metal oxide as a solid oxygen carrier. This metal oxide reacts with the fuel in a fluidized bed combustor, producing solid metal particles and a mixture of CO2 and water vapor. The water vapor is condensed, CO2 captured, and the solid metal particles are recirculated to another fluidized bed where they react with air, producing heat, and regenerating the metal oxide particles. There are attempts to research a way to capture CO2 from the air without the use of biomass.
Transport
After being captured, CO2 must be transported to suitable storage sites. This is done by pipeline, which is generally the cheapest form of transport. There is already a large network of CO2 pipelines that are used to transport CO2 to oil production fields for induction in older fields. These are likely to come under jurisdictional problems due to CO2's classification as both a commodity by the Bureau of Land Management and a pollutant by the Environmental Protection Agency. For other purposes, ships can also be used.
Storage
There are a number of different places carbon can be stored once it is sequestered.
Geological Storage
Also known as geo-sequestration, this involves injecting CO2 underground into geological formations. Examples of these formations are spent oil fields, gas fields, saline formations, unminable coal seams, old mines, etc. These formations have to have highly impermeable caprock and geochemical trapping mechanisms that would prevent CO2 from escaping to the surface. The IPCC estimates that CO2 stored in properly selected and managed geological sites could be trapped for millions of years, with the ability to retain over 99% of the injected CO2 over 1000 years.
Ocean Storage
There are a number of ideas for how to store CO2 in ocean. - Dissolution injects CO2 by ship or pipeline into the water column at depths of 1000m or more, where it dissolves.
- Lake systems deposit CO2 directly onto the sea floor at depths greater than 3000m, where CO2 is denser than water and is expected to form a lake that would delay dissolution of CO2 into the environment.
- Convert the CO2 to bicarbonates using limestone
- Store the CO2 in solid clathrate hydrates already existing on the ocean floor or growing more solid clathrate.
The environmental effects of oceanic storage are poorly understood, but are expected to be negative because of its ability to kill organisms and because it is ultimately a temporary solution. CO2 also reacts with water to form carbonic acid, which raises the acidity of the water. The bicarbonate approach would reduce the effects of pH and enhance the retention of CO2 in the ocean.
Mineral Storage
A couple of abundant metal oxides, magnesium and calcium, can be reacted with CO2 to produce carbonates, which not only sequester CO2, but also release energy. The carbonates are inherently stable, but take a long time to form naturally. To speed up the process, the reaction has to occur under high temperatures and/or pressures, which require energy to create.
Terrestrial Sequestration
Terrestrial sequestration involves storing carbon in plants and microorganisms that use CO2 in their natural cycles. The carbon is stored in soils and through increases in biomass, which is essentially the growth of an organism. These means of storage are also typically referred to as carbon sinks. We can help reduce greenhouse gas emissions by avoiding destroying plant biomass, increasing storage capacity by planting more trees and conservatively tilling agricultural lands to maintain soil carbon levels, and by substituting bio-based fuels fossil fuels, because they are a part of a closed loop carbon cycle This last argument may need a bit more explaining . Unfortunately, these sources can also be destroyed, releasing the carbon trapped in them. For example, tropical deforestation is responsible for about 20% of the world's annual CO2 emissions.
Reuse
As with all materials, it is important to look for methods of reusing carbon to reduce emissions from future needs. CO2 can be converted into hydrocarbons to be reused as fuel or to make plastics. There are a few different methods for this: - Single Step: CO2 + H2 to Methanol: This is a proven, easy process to create a hydrocarbon from CO2.
- Single Step: CO2 to Hydrocarbons: There is research into trying to use sunlight to split water into hydrogen and oxygen ions, which then react with CO2 to create hydrocarbons.
- Two Step: CO2 to CO to Hydrocarbons: If CO2 is heated to 2400 degrees C, it splits into carbon monoxide and oxygen. The Fischer-Tropsch process can then be used to convert the CO into hydrocarbons Add a link to this process . It is hypothesized that such a system would use a chamber with a mirror to focus the sunlight on the gas.