석탄화력발전시설의 수은 배출특성과 대기 중 수은 거동 분석

Abstract

Rhok Ho Kim Advisor : Prof. Young Nam Chun, Ph.D. Department of Environment Engineering, Graduate School of Chosun University

Due to concerns about the risks of mercury compounds emitted from major stationary sources to the atmosphere, major countries around the world are making great efforts to identify the current state of their emissions and research on control technologies. In 2009, the UNEP Executive Board promised to establish a mutually binding international mercury management system and adopted the “Minamata Convention on Mercury” through several intergovernmental negotiating committees, and it has recently come into effect. Furthermore, Korea is considered as one of the major mercury -emitting countries in the world. Therefore, it is urgently necessary for Korea to examine the current state of domestic mercury use and emission as well as its environmental pollution, and to carry out a study to identify its characteristics. In recent years, Korean researchers have also focused their attention on the emission characteristics of mercury compounds and their behavior in the atmosphere. However, there is a lack of publicly certified data on emission characteristics other than those obtained through small-scale studies in several domestic organizations. Therefore, there is a great need for the development of emission factors by actual measurement to produce an inventory of domestic mercury compound emission sources and to calculate a reliable emission amount. In this study, the mercury in exhaust gas was measured from three large coal-fired power plants in Korea using the continuous emission monitoring system and the sorbent trap measuring method (EPA Method 30B) in line with the international trend, and a comparative evaluation was also conducted on the total mercury method, a standard method for the measurement of air pollution in Korea. A research was also conducted on the distribution of each chemical speciation. In addition, this study tried to identify the mercury emission influence area by combining the emission source data and the atmospheric mercury measurement data obtained through study of the mercury emission characteristics of coal-fired power plants with an atmospheric model, and to grasp the extent of its long-distance and local contribution. The target emission source of this study was the coal-fired power plant (CPP), which accounts for the largest portion of domestic mercury emission facilities. The CPP is a major mercury emission source pointed out by UNEP and the US EPA. Each domestic CPP site is also making great efforts to reduce the emission of air pollutants through air pollution–prevention technologies. Domestic CPPs are installed adjacent to coastal areas, such as the West Coast, and the total annual power generation from 12 domestic CPPs is estimated at about 210 million MWh. Among them, large-scale CPPs are mostly located in West Coast areas such as Incheon and Chungcheongnam-do. In those areas, air pollution caused by fine dust and harmful air pollutants has recently become a major environmental issue. In this study, a measurement and analysis of mercury samples was carried out by changing the measurement method and period targeting three different power plants of 800 and 500 MW capacities. In those plants, selected catalytic reduction (SCR) facilities to control nitrogen oxides, an electrostatic precipitator (ESP) to control particulate matters, and wet flue gas desulfurization (WFGD) have been installed and operated. Sampling for the study was conducted at the final outlet after passing through all the prevention facilities. As a result of the study on the mercury concentration in flue gas, the concentration of mercury in three CPPs showed a concentration range of 0.05–0.38 µg/Sm3 in CPP-1, and the result of mercury measurement for three months using the continuous monitoring device showed that the mean concentration of total gaseous mercury was 0.217 µg/Sm3. The mean concentration of elemental mercury was 0.161 µg/Sm3 and that of mercury oxide was 0.056 µg/Sm3. The mercury concentration range measured in CPP-2 was 0.8–1.8 µg/Sm3, and the concentration of mercury in CPP-3 measured by the total mercury method was 0.6–2.0 µg/Sm3, similar to that of CPP-2. The relative standard deviation was in the range of 3.4%–13.6%, and the reproducibility and precision were analyzed to be better than those of existing studies. As for the mercury mass balance in CPP-1, the mercury emissions were highest in the form of fly ash at 67.4%, and the mercury emission in the form of fly ash in CPP-2 was 79.5%, accounting for the greatest percentage. This is because the mercury emitted in the form of elemental mercury was converted to mercury oxide after passing through SCR, and some parts of the mercury were adsorbed in the particles and removed through the electrostatic precipitator. In CPP-3, the greatest part of mercury (46.1%) was emitted through gypsum and desulfurization wastewater, which are by-products of WFGD, and it was analyzed that such method of mercury emission accounted for a relatively higher proportion in other facilities as well. These results suggest that the water solubility of mercuric oxide and particulate mercury were removed from wet-corrosion prevention facilities. The range of mercury emission factors obtained from this study was somewhat different for each facility. The research results showed that 1.70 mg of mercury was produced from CPP-1; 13.20 mg from CPP-2; and 7.87 mg from CPP-3 was produced and emitted to the atmosphere. These results are somewhat lower than the emission factor values obtained from existing CPPs. To generalize the measured data in relevant facilities in the future, more measurement and data collection need to be performed. Using the emission factors obtained from this study, the annual mercury emissions from the three power plants were estimated to be 27.74, 195.52, and 103.89 kg, respectively. As a result of computer simulations using atmospheric models, the emission contribution of local pollution sources, such as CPPs, was high in spring, and there were no apparent pollution sources in spring and autumn. In winter, eastern China and domestic emission sources were simulated to have a great effect, and the results of the annual analysis showed that the effect of long-distance movement was somewhat higher. In general, the mercury concentrations in winter were significantly higher than those in other seasons. The computer simulation results suggest that the long-distance movement of pollutants from large-scale emission sources in China had a great effect. In addition, the increase of coal consumption in winter in major domestic emission sources, such as CPPs, is considered to have a great effect on this phenomenon.