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Carbon Capture and Storage
Carbon capture is the technique of separating carbon dioxide (CO2) from large point sources, especially fossil fuel-based power plants and large, energy-consuming industries such as oil refining and chemical, cement, iron, and steel production. After being captured, the CO2 needs to be transported and stored securely for hundreds and even thousands of years to prevent it from reaching the atmosphere.
Work is currently in progress to develop reservoirs under the Earth's surface and in the oceans that are suitable for storage. CO2 is sometimes stored in saltwater reservoirs (as under the North Sea) or injected into depleting oil or gas fields to enhance recovery. Another potential storage medium is unmineable coal. CO2 can be injected into suitable coal seams where it will be adsorbed (gathered on the surface) onto the coal, locking it up for as long the coal remains unmined.
Because of the long lifetime of CO2 among the greenhouse gases (GHGs) in the atmosphere, stabilizing concentrations of GHGs at any level would require large reductions of global CO2 emissions from current levels. The advantage of carbon-capture technology over other possible options (such as encouraging conservation, augmenting fuel efficiency, or shifting to renewable energy) lies in its ability to enable the continued use of fossil fuels, but with greatly reduced CO2 emissions. The unintended consequences of pushing the technology forward, however, are the reduction in incentives for encouraging innovation and the delay in shifting investment toward renewable energy. Moreover, as the technology is still in its nascent stage and is yet to be demonstrated on a large scale, there are a host of barriers, uncertainties, and knowledge gaps involved in its adoption and deployment. These relate primarily to cost and performance, efficiency loss, lifecycle emissions, and other environmental concerns.
Understanding the Technology
There are three main ways to capture carbon from large point sources: post-combustion capture, pre-combustion capture, and oxy-fuel combustion.
Post-combustion capture involves separating and capturing CO2 from the flue gas (i.e., exhaust gas) produced by fuel combustion at large point sources. CO2 is only a small part of the flue gas stream emitted into the atmosphere by a power station. Other gases include nitrogen, oxygen, and water vapor. Because of this low concentration, a large volume of flue gas must be handled to separate out CO2, which requires large and expensive equipment.
Pre-combustion capture involves partial oxidization by allowing the fuel to react with oxygen and/or steam to produce mainly carbon monoxide and hydrogen. The carbon monoxide is further reacted with steam in a catalytic reactor, called a shift converter, to produce CO2 and more hydrogen. The CO2 is then separated and the hydrogen is used as fuel in a gas-turbine, combined-cycle plant. In principle, the process is the same for coal, oil, or natural gas.
In oxy-fuel combustion, the concentration of CO2 in flue gas can be greatly increased by using concentrated oxygen instead of air for combustion, either in a boiler or gas turbine. If fuel is burned in pure oxygen, the flame temperature is excessively high, so some CO2 rich flue gas would be recycled to the combustor to make the flame temperature similar to that in a normal, air-blown combustor. The advantage of oxygen-blown combustion is that the flue gas has a CO2 concentration of over 80 percent, so only simple CO2 purification is required. Oxy-fuel combustion relies mainly on physical separation processes for O2 production and carbon capture, thereby avoiding the use of any reagents and/or solvents that contribute to operating costs and the environmental disposal of any related solid or liquid wastes. The main disadvantage of oxy-fuel combustion is that a large quantity of oxygen is required, which is expensive, both in terms of capital cost and energy consumption.
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