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Carbon Sequestration
CO2 Capture
CO2 capture is the separation of CO2 from emissions sources or the atmosphere. From emissions sources, CO2 is recovered in a concentrated stream that is amenable to sequestration or conversion.
Pre-combustion CO2 capture relates to gasification plants, where fuel is converted into gaseous components by applying heat under pressure in the presence of steam. CO2 can be captured from the synthesis gas that emerges from the coal gasification reactor before it is mixed with air in a combustion turbine. Here the CO2 is relatively concentrated and at a high pressure.
NETL's pre-combustion CO2 capture focus area calls for the following R&D goals:
- By 2014, initiate at least two slipstream tests of novel CO2 capture technologies that offer significant cost reductions.
- By 2018, initiate large-scale field testing of promising novel CO2 capture technologies.
Near-term applications of CO2 capture from pre-combustion systems will likely involve physical or chemical absorption processes, with the current state of the art being a glycol-based solvent called Selexol. Analysis conducted at NETL shows that CO2 capture and compression using Selexol raises the cost of electricity from a newly built IGCC power plant by 30 percent, from an average of 7.8 cents per kilowatt-hour to 10.2 cents per kilowatt-hour.
Mid-term to long-term opportunities to reduce capture costs through improved performance could come from membranes and sorbents currently at the laboratory stage of development. Under DOE-funded research, ionic liquid membranes and absorbents are being developed for capture of CO2 from power plants. Ionic liquid membranes have been developed at NETL that surpass polymers in terms of CO2 selectivity and permeability at elevated temperatures for pre-combustion applications.
Membrane separation units that can selectively permeate H2 and retain CO and CO2 are also promising for IGCC power plants. Another application currently being developed is to utilize H2 to power fuel cells with the intent of significantly raising overall plant efficiency. After CO2 removal, the H2-rich syngas also can be used as a fuel in a combustion turbine to produce electrical or thermal power.
In a gasification reactor, the amount of air or oxygen (O2) available inside the gasifier is carefully controlled so that only a portion of the fuel burns completely. This “partial oxidation” process provides the heat necessary to chemically decompose the fuel and produce the synthesis gas (syngas), which is composed of hydrogen (H2), carbon monoxide (CO), and minor amounts of other gaseous constituents. The syngas is then processed in a water-gas-shift reactor, which converts the CO to CO2 and increases the CO2 and H2 molecular concentrations to 40 percent and 55 percent, respectively, in the syngas stream.
Because CO2 is present at much higher concentrations in syngas than in post-combustion flue gas, CO2 capture should be less expensive for pre-combustion capture than for post-combustion capture. Currently, however, there are few gasification plants in full-scale operation, and capital costs are higher than for conventional pulverized coal plants.
Research being conducted by the DOE Gasification Research Program is expected to improve gasification technology such that its costs without capture will be comparable to electricity costs from pulverized coal without capture, potentially reducing further the cost of pre-combustion CO2 capture in the future.
Post-combustion CO2 capture. Pulverized coal plants, which comprise 99 percent of all coal-fired power plants in the United States, burn coal in air to raise steam. CO2 is exhausted in the flue gas at atmospheric pressure and a concentration of 10-15 volume percent. This post-combustion capture of CO2 is a challenging application because:
- The low pressure and dilute concentration dictate a high actual volume of gas to be treated
- Trace impurities in the flue gas tend to reduce the effectiveness of the CO2 adsorbing processes
- Compressing captured CO2 from atmospheric pressure to pipeline pressure (1,200–2,000 pounds per square inch (psi)) represents a large parasitic load.
Oxygen Combustion (oxy-combustion) combusts coal in an enriched oxygen environment using pure oxygen diluted with recycled CO2 or H2O. The CO2 is then captured by condensing the water in the exhaust stream. Oxy-combustion offers several benefits for existing coal-powered plants as determined through large-scale laboratory testing and systems analysis.
In FY08, NETL’s Innovations for Existing Plants (IEP) Program redirected its focus to include CO2 emissions control for existing pulverized coal-fired plants. This new focus on post-combustion and oxy-combustion CO2 emissions control technology and related areas of CO2 compression and CO2 beneficial reuse is in response to the priority for advanced technological options for the existing fleet of coal-fired power plants in order to address climate change. For more information related to post-combustion and oxy-combustion technologies for existing plants, please visit IEP’s CO2 Emissions Control homepage.
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