June 2009
Potential Flue Gas Impurities in Carbon Dioxide Streams Separated from Coal-Fired Power Plants
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Joo-Youp Lee,
Department of Chemical and Materials Engineering, University of Cincinnati
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Tim C. Keener,
Department of Civil and Environmental Engineering, University of Cincinnati
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Y. Jeffery Yang,
National Risk Management Research Laboratory, U.S. Environmental Protection Agency
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AbstractFor geological sequestration of carbon dioxide (CO2) separated from pulverized coal combustion flue gas, it is necessary to adequately evaluate the potential impacts of flue gas impurities on groundwater aquifers in the case of the CO2 leakage from its storage sites. This study estimated the flue gas impurities to be included in the CO2 stream separated from a CO2 control unit for a different combination of air pollution control devices and different flue gas compositions. Specifically, the levels of acid gases and mercury vapor were estimated for the monoethanolamine (MEA)-based absorption process on the basis of published performance parameters of existing systems. Among the flue gas constituents considered, sulfur dioxide (SO2) is known to have the most adverse impact on MEA absorption. When a flue gas contains 3000 parts per million by volume (ppmv) SO2 and a wet flue gas desulfurization system achieves its 95% removal, approximately 2400 parts per million by weight (ppmw) SO2 could be included in the separated CO2 stream. In addition, the estimated concentration level was reduced to as low as 135 ppmw for the SO2 of less than 10 ppmv in the flue gas entering the MEA unit. Furthermore, heat-stable salt formation could further reduce the SO2 concentration below 40 ppmw in the separated CO2 stream. In this study, it is realized that the formation rates of heat-stable salts in MEA solution are not readily available in the literature and are critical to estimating the levels and compositions of flue gas impurities in sequestered CO2 streams. In addition to SO2, mercury, and other impurities in separated CO2 streams could vary depending on pollutant removal at the power plants and impose potential impacts on groundwater. Such a variation and related process control in the upstream management of carbon separation have implications for groundwater protection at carbon sequestration sites and warrant necessary considerations in overall sequestration planning, engineering, and management.
ImplicationsCarbon separation and geological sequestration is a viable climate mitigation measure to reduce atmospheric carbon concentrations. Its environmental impacts, particularly to groundwater resources at sequestration sites, are receiving considerable attention as evidenced in current regulatory actions in related rule promulgation. This paper is one of the few attempts to analyze the MEA absorption process used for carbon separation at coal-fired power plants and to estimate the compositions of gas impurities including SO2, sulfur trioxide, nitrogen dioxide, hydrogen chloride, and mercury that can potentially impact groundwater quality when liquefied CO2 and impurities escape from a storage site. The upstream analysis underscores the importance of controlling impurities and their compositions at sources to minimize the risk for groundwater at a geological sequestration site.
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