This is the process of capturing waste carbon dioxide (CO2) from large point sources, such as fossil fuel power plants, transferring it to a storage site, and dumping it where it will not enter the atmosphere, normally an underground geological formation. The aim is to prevent the release of large quantities of CO2 into the atmosphere (from fossil fuel use in power generation and other industries) that cause environmental pollution. It is a potential means of mitigating the impact of fossil fuel emissions to global warming and ocean acidification. Although CO2 has been injected into geological formations for several decades for various purposes including enhanced oil recovery, the long term storage of CO2 is a relatively new concept. The first commercial example was Weyburn in 2000. Carbon capture and sequestration can also be used to describe the scrubbing of CO2 from ambient air as a geo engineering technique.
CCS applied to a modern conventional power plant could reduce CO2 discharges to the atmosphere by approximately 80-90% compared to a plant without CCS. The IPCC estimates that the economic potential of CCS could be between 10% and 55% of the total carbon mitigation effort until year 2100. Capturing and compressing CO2 may increase the fuel needs of a coal-fired CCS plant by 25-40%.
CARBON CAPTURE TECHNIQUES
Broadly, three different types of technologies for cleaning exist: post-combustion, pre-combustion, and oxyfuel combustion. Concentrated CO2 from the combustion of coal in oxygen is relatively pure, and could be directly processed. Impurities in CO2 streams could have a significant effect on their phase behaviour and could pose a significant threat of increased corrosion of pipeline and well materials. In instances where CO2 impurities exist and especially with air capture, a scrubbing process would be needed.
* Post combustion capture, the CO2 is removed after combustion of the fossil fuel this is the scheme that would be applied to fossil-fuel burning power plants. Here, CO2 is captured from flue gases at power stations or other large point sources. The technology is well assumed and is currently used in other industrial applications, although not at the same scale as might be required in a commercial scale power station.
* Pre combustion is widely applied in fertilizer, chemical, gaseous fuel (H2, CH4), and power production. In these cases, the fossil fuel is partially oxidized, for instance in a gasifier. The resulting syngas (CO and H2) is shifted into CO2 and H2. The resulting CO2 can be captured from a relatively pure exhaust stream. The H2 can now be used as fuel; the carbon dioxide is removed before combustion takes place. There are several advantages and disadvantages when compared to conventional post combustion carbon dioxide capture. The CO2 is removed after combustion of fossil fuels, but before the flue gas is expanded to atmospheric pressure. This scheme is applied to new fossil fuel burning power plants, or to existing plants where re-powering is an option.
* Oxy fuel combustion the fuel is burned in oxygen instead of air. To limit the resulting flame temperatures to levels common during conventional combustion, cooled flue gas is re-circulated and injected into the combustion chamber. The flue gas consists of mainly carbon dioxide and water vapours, the latter of which is condensed through cooling. The result is an almost pure carbon dioxide stream that can be transported to the sequestration site and stored.
TRANSPORT
After capture, the CO2 would have to be transported to suitable storage sites. This is done by pipeline, which is generally the cheapest form of transport. The injection of CO2 to produce oil is generally called Enhanced Oil Recovery or EOR. In addition, there are several pilot programmes in various stages to test the long-term storage of CO2 in non-oil producing geologic formations. Ships could also be utilized for transport where pipelines are not feasible. These methods are currently used for transporting CO2 for other applications.
SEQUESTRATION OR STORAGE
Various forms have been perceived for permanent storage of CO2. These forms contain gaseous storage in several deep geological creations (including saline formations and exhausted gas fields), and solid storage by reaction of CO2 with metal oxides to harvest stable carbonates.
GEOLOGICAL STORAGE
This method involves injecting carbon dioxide, generally in supercritical form, directly into underground geological formations. Oil fields, gas fields, saline formations, unmineable coal seams, and saline-filled basalt formations have been suggested as storage sites. Various physical and geochemical trapping mechanisms would prevent the CO2 from escaping to the surface.
OCEAN STORAGE
Carbon dioxide could be stored in the oceans, but this would only aggravate ocean acidification and has been made illegal under specific regulations. Ocean storage is no longer considered feasible. The ocean contains an estimated 40,000 Gt of carbon, whereas the atmosphere and the terrestrial biosphere contain an estimated 750 and 2200 Gt, correspondingly. A doubling of the carbon concentration levels in the atmosphere, therefore, represents only enough carbon to increase the oceans concentration levels by about 2%. By direct injection into the ocean, this natural process is effectively enhanced, thus reducing peak atmospheric carbon dioxide concentrations and their rate of increase. However, using this method, it is estimated that around 15-20% of the carbon dioxide injected into the ocean will leach back into the atmosphere over hundreds of year.
There have been a number of injection techniques suggested in order to dissolve the carbon
dioxide into the ocean. These are
1. Droplet plume-liquid CO2 injected from a multiple below 1000 m, forming a rising plume.
2. Dense plume-a dense CO2 seawater mix that sinks, injected at a depth between 500 and
1000 m.
3. Dry ice-dropped off a boat and allowed to sink and diffuse.
4. Towed pipe-injected from a boat at a depth of 1000 m, forming a rising plume.
5. CO2 lake-injection at a depth of around 4000 m to form a stable deep lake.
In the short term, the droplet plume and towed pipe methods are probably the most viable due
to technological and economic reasons.
Natural terrestrial sequestration
Terrestrial sequestration involves the capture and storage of carbon dioxide by plants and the storage of carbon in to soil. During photosynthesis, carbon from atmospheric carbon dioxide is transformed into components essential for plants to live and grow. As part of this process, the carbon present in the atmosphere as carbon dioxide becomes part of the planta leaf, stem, root, etc. Long-lived plants like trees might keep the carbon sequestered for a long period of time. Once the tree dies, or as limbs, leaves, seeds, or blossoms drop from the tree, the plant material decomposes and the carbon is released that can be captured with several techniques and stored in to soil.
ENVIRONMENTAL EFFECTS
The notional merit of CCS systems is the decline of CO2 discharges by up to 90%, depending on plant type. Generally, environmental effects from use of CCS arise during power production, CO2 capture, transport, industries and storage.
Additional energy is required for CO2 capture, and this means that substantially more fuel has to be used to produce the same amount of power, depending on the plant type. For new super-critical pulverized coal (PC) plants using current technology, the extra energy requirements range from 24 to 40%, while for natural gas combined cycle (NGCC) plants the range is 11-22% and for coal-based or ignitions gasification combined cycle (IGCC) systems it is 14-25%. Apparently, fuel use and environmental problems arising from mining and withdrawal of coal or gas increase accordingly.
Plants equipped with flue-gas desulfurization (FGD) systems for sulfur dioxide control require proportionally greater amounts of limestone, and systems equipped withselective catalytic reduction systems for nitrogen oxides produced during combustion require proportionally greater amounts of ammonia. Carbon is drastically reduced though never completely captured, emissions of air pollutants increase significantly.
The authors are from the Department of Agronomy University of Agriculture, Faisalabad, Punjab, Pakistan. They can be reached at shakoor2914@gmail.com