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Carbon capture and storage
Carbon capture and storage, or CCS, is a non-biological means of mitigating our impact on the earth from carbon dioxide (CO2) emissions. By preventing this greenhouse gas from entering the atmosphere, the climate impact of the power plant or industrial process in question is greatly reduced.
CCS is done by capturing emissions from stationary sources, liquefying it, and sending it into underground storage via pipeline for long periods of time. Early methods of mitigation included carbon sequestration where trees were used as carbon storage. Carbon capture and long term storage is a relatively new science that still has many unanswered questions.
Carbon capture methods
CCS focuses on stationary sources, (i.e. power plants) and how to capture the carbon being emitted before it is released into the atmosphere. There are three main approaches to carbon capture.
Post combustion capture
Post combustion capture is a process that separates the CO2 from the other flue gases (gas that escapes from the pipes during production) after combustion, using a chemical or organic solvent. The image above shows an illustration of how post combustion capture would work.(Allam 2005: 110)
Pre-combustion is a process that starts by processing the primary fuel with steam and air or oxygen. The resulting carbon monoxide then reacts with the steam in a second reactor. This produces hydrogen and CO2. This process would reduce the total amount of CO2 emitted, although CO2 would still be released during combustion of the liquid fuel used for transportation or electricity generation. (Allam 2005: 110)
Oxy-fuel combustion capture
Oxy-fuel combustion capture is a process that uses oxygen instead of air for combustion and produces a flue gas that is mostly CO2 and water. After this, the CO2 can be compressed, transported, and stored. This technique is the most questionable of the three options because temperature needed for pure oxygen combustion (about 3,500o C) is too high for typical power plants. (Allam 2005: 110) Despite the high temperatures involved with this process, much attention has been directed in recent years at determining practical applications for CO2 separation and capture within the combustion process. Chemical looping combustion (CLC) is an emerging technology encapsulated by oxy-fuel combustion that uses redox reactions to first oxidize a metal in air and then reduce it using a particular hydrocarbon fuel. The effect of these separate reactions is the combustion of fuel in an oxygen rich environment caused by release of oxygen by the reduced metal. CLC is accomplished using a dual fluidized bed system to generate turbine power. The beds comprise distinct reactors for both the air and fuel environments . There are many metal oxygen carriers and fuels that could potentially be combined in CLC systems, leading to varying degrees of operational success. Typically used species include oxides of Ni, Fe, Mn, Cu, and Co. The range of operating temperatures for air and fuel reactors functioning in the CLC process is thought to be about 800-1200 degrees Celsius. Temperatures around 1000 degrees Celsius in both reactors have been achieved with a capacity of 100MWh using gaseous fuels such as methane, biogas, and syngas . The reactor temperatures can be moderated by partial recycling of flue gas, leading to combustive environments that are rich in both oxygen and carbon dioxide. The decrease in oxygen yields less intense temperatures within the fuel reaction. Gaseous fuels are the most compatible with current CLC technology, as opposed to solids like coal and biomass. The breakdown of metal carriers, as well as the buildup of ash, can cause serious decreases in long-term viability of these systems. While the technology is still in an early stage, the established literature concerning CLC asserts that some implementation of the process at an industrial scale for gaseous fuels is achievable. Given current technology, CLC and oxy-fuel combustion lend themselves most to indirect heating via steam generation and process heating. Many experimental reactor proposals such as ion transfer membranes have been proposed to assist in efficiency of oxygen transfer rates within the system .
After the carbon is captured, it must be transported. Pipelines are the most common method for transporting CO2 in the U.S. “Currently, over 5,800 kilometers (about 3,600 miles) of pipeline are used to transport CO2 in the United States.” (Stephens,2009) The technology behind the transporting of the carbon is much more widely used than any other aspect of CCS and is used mostly for enhanced oil recovery (EOR). Most of the infrastructure is in place but still requires much attention to; design, monitoring for leaks, and protection against overpressure. The main objective is to capture the carbon from a stationary source, by the means previously discussed, transport the CO2 via pipeline to a storage house, and then distribute the carbon into different CO2 storage options.
Also known as geo-sequestration, this method involves injecting carbon dioxide directly into underground geological formations. There are three main storage options for captured CO2.
Unminable coal seams
The Department of Energy (DOE) states that nearly 90% of U.S. coal resources are not minable with current technology. These coal seams can be used to store CO2 because CO2 binds very tightly to the surface of coal. This method also displaces methane which can be extracted and sold as an energy source- and possible offset the costs of injecting the CO2 into the unminable coal seam. The main issue with the unminable coal seams is that of permeability of the seam is questionable. If the coal seam is thick, it will be difficult to drill into it to store the carbon but once the carbon is in, the chances of it escaping are very minimal. If the seam is too thin, it will be easy to store the carbon but the possibility of it escaping is greater. The whole process is very costly as well. (Stephens 2009: 3)
Oil and gas reservoirs
Pumping carbon into gas and oil reservoirs is already used for enhanced oil recovery (EOR) and the technology needed to store carbon in these reservoirs, as well as the infrastructure, is available. According to the Earth Encyclopedia, “the U.S. is a world leader in this technology and injects approximately 48 metric tons of CO2 underground each year to help recover oil and gas resources.” The main advantage of this is the opportunity to offset the cost of storing carbon due to the revenue that can be gained by the recovered oil and gas. If the CO2 was injected into a depleted reservoir, the offset of the cost would not be present. This process is heavily dependent on the depth of the reservoirs. If the reservoir is less than 800 meters deep, the possibility of the CO2 leaking is much greater. (Stephens 2009: 3)
Deep saline reservoirs
Deep saline reservoirs are commonly occurring and have a large potential for storage capacity. The downside to these saline reservoirs is that little is known about them. This means that more research would have to go into the studying the integrity of these reservoirs and that research will be expensive. In addition to the this, there are no resources that would help offset the costs. In order to keep the costs low, little research would be conducted which would result in larger uncertainty about the aquifer’s structure. This is the least researched option for possible carbon storage. (Stephens 2009: 3)
According to the Global CCS Institute, there are eight large scale integrated CCS projects in operation and 77 large-scale, integrated projects in various phases of development  around the world. Most of the projects are in North America and Europe. More information is available on the current state of CCS projects in the Institute's 2010 status report .
- http://www.globalccsinstitute.com/sites/default/files/Chapter_3_-_CCS_Projects_0.pdf (projects chapter)
- OpenCCS (started 2011). Planned to be a wiki-style open knowledge resource on Carbon Capture and Storage. As of October 2011, still semi-closed, requiring signup to gain access.