| Popis: |
The significant increase in the average global temperature has been determined to be the result of anthropogenic release of greenhouse gases, such as carbon dioxide, into the atmosphere. With this in mind, nations have considered a carbon tax for emission of carbon dioxide into the atmosphere from point sources, such as power plants. However, carbon capture from power plants has proven to be expensive both economically and in terms of energy penalty due to endergonic nature of molecular separation. A number of technologies have been developed, such as oxycombustion, and gasification combined cycle that are able to capture carbon. However, these technologies require a means of providing either providing pure oxygen to oxidize the fuel or a molecular means to separate the carbon dioxide from nitrogen, both of which have the potential to be prohibitively expensive, in terms of capital expense and parasitic load. Chemical looping combustion has been considered a transformational technology for simultaneous carbon capture and electricity generation. The technology consists of providing oxygen to the fuel using a metal oxide in one reactor, producing pure carbon dioxde as the product, and combusting the metal oxide in a separate reactor to produce heat for steam and electricity generation. This configuration has been extensively studied by researchers around the world in small-scale pilot units. Furthermore, the iron-based CDCL technology has been demonstrated at Ohio State in a 25-kWth subpilot unit with reasonable success, with high conversions of solid fuels such as sub-bituminous coal and waste products such as metallurgical coke fines. Furthermore, the carbon dioxide purity in the flue gas is around 99%, with low values of carbon monoxide, methane, and hydrogen impurities. As of this writing, over 680 hours of operation have been performed in this unit, with a successful 200 hour continuous campaign. Furthermore, extensive laboratory bench unit studies have been performed that were used to verify the carbon char and volatile kinetics. This work attempts to review state-of-the-art chemical looping combustion systems for power generation. Designs and experimental results from pilot-scale chemical looping combustion demonstration units are discussed in relation to parameters relevant for a practical commercial unit. Furthermore, the work also reviews the design and experimental results from the Coal Direct Chemical Looping Unit developed at Ohio State. An extensive mass and energy balance has been performed to determine the system efficiency. Finally, further analysis shows the effects of sulfur, nitrogen and alkali materials on the oxygen carrier. |
