There are three primary methods of capturing CO2. Pre-combustion, or separating CO2 from gasified coal prior to combustion; post-combustion, which captures CO2 from the flue gas stream after the coal is burned; and oxy coal combustion, where the combustion of coal takes place in an oxygen-rich environment, resulting in flue gas more ready for sequestration.
Pre-combustion
In pre-combustion carbon capture, coal is gasified by applying heat and steam in a high-pressure, controlled-oxygen environment. The resulting syngas consists primarily of hydrogen (H2) and carbon monoxide (CO) gases. By processing the CO in a water-gas-shift reactor, the addition of water produces CO2 and additional H2 gases. The highly concentrated CO2 can be separated and stored, while the hydrogen may be cleanly combusted or, as under a project being developed under the Department of Energy (DOE) Fuel Cell Program, used in hydrogen fuel cells. Due to the increased concentration of CO2 in the pre-combustion syngas, pre-combustion carbon capture technologies are extremely efficient compared to in post-combustion flue gas. By using pre-combustion processes, CO2 emissions may be reduced by 90 to 95 percent.
Integrated gasification combined cycle (IGCC) coal-based plants are prime candidates for pre-combustion gasification. In the near-term, CO2 gas will likely be separated from the syngas streams via physical or chemical solvents such as glycol-based Selexol and amine-based gas removal systems. Work is underway on the development of membrane separation units to selectively separate H2 gas from CO and CO2 gases.
Post-combustion
After combustion in a pulverized coal plant, CO2 may be removed from the resulting flue gas. This approach may be retrofitted to existing coal-based power plants without significant modifications to the plant, but is more challenging than pre-combustion methods due to the low pressure and diluted nature of the post-combustion gases. However, properly deployed, post-combustion technologies can capture 80 to 90 percent of CO2 emissions.
Currently in post-combustion capture, CO2 is captured from flue gas which (largely comprised of nitrogen gas and CO2) through the use of chemical solvents such as amines (nitrogen-based organic compounds). These technologies have been deployed in limited slipstream applications. Future opportunities and improved performance may be found in membranes, sorbents or cryogenic isolation, which are currently being researched.
Oxy-coal combustion
The oxy-coal process creates a oxygen-rich environment for coal combustion, resulting in a more complete combustion and a nearly pure CO2 and water vapor exhaust stream. When cooled, CO2 is easily separated from the water in this process. Oxy-coal combustion may be retrofitted to existing coal plants, however the current process of separating oxygen from air cryogenically is energy-intensive and requires an input of approximately 15 percent of a plant’s annual energy output. A new technology, called chemical looping combustion, in which oxygen is separated from the air via the oxidation of a metallic compound, may reduce costs in the future. Oxy-coal combustion can remove 90 percent of CO2 from emissions.
Diagrams on this page adapted from Vattenfall AB diagrams: Source