CO2 Capture Project Descriptions Utah Center for Ultra Clean Coal Utilization Fossil Energy Technology Strategy Metal Monolitic Amine=Grafted Zeolites for CO2 Capture Fabrication and Scale-up of Polybenzimidazole–Based Membrane System for Pre–Combustion... Thermally Optimized Membranes for Separation and Capture of CO2 CO2 Capture for PC-Boiler Using Flue-Gas Recirc.: Eval of CO2 Capture/Utilization/Disposal Options Greenhouse Gas Emissions Control by Oxygen Firing in Circulating Fluidized-Bed Boilers Carbon Dioxide Capture by Absorption with Potassium Carbonate Advanced Oxyfuel Boilers and Process Heaters for Cost Effective CO2 Capture and Sequestration Conceptual Design of Oxygen-Based PC Boiler Conceptual Design of Optimized Fossil Energy Systems with Capture and Sequestration of CO2 Combined Power Generation and Carbon Sequestration Using a Direct FuelCell® Reference Shelf

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.