| China’s energy structure is mainly contributed by coal. In coal combustion process, mercury, a trace heavy metal, has becoming a concerned chemical contaminant in a global scale. The emission of mercury can cause serious problems to human health and to the environment, not only damaging human’s nervous system, causing respiratory and cardiovascular disease, leading to kidney failure, but also having toxic effects on plants. In 2000, the total amount of mercury emitted into the atmosphere of China’s was 220 t, and coal-fired power plants accounted for 35.3% of it. And in 2010, about 150 t of mercury was emitted from coal-fired power plants in China. Therefore, such a large amount of mercury emission would pose a great threat to the environment of human being. In the Thermal Power Plant Air Pollutant Emission Standards(GB13223-2011) of China, the emission concentration of mercury in the flue gas was limited to 0.03mg/m3 for the first time, which indicates that the problem of mercury emission and pollution have aroused great attention of China’s government. In the flue gas, there are mainly three forms of mercury: gaseous elemental mercury Hg0(g), gaseous mercury oxide Hg2+(g), and solid particles of mercury Hg(p), and in which Hg0(g) is the major form of mercury under the coal-fired flame temperature. Speciation of mercury in the flue gas has a significant impact on the removal of mercury. Wet desulfurization devices can remove oxidized mercury, and dust removal devices are able to capture particulate mercury. Because of its high volatility and low water solubility, it is difficult to capture elemental mercury by both dust and desulfurization devices, which was almost all released into the atmosphere, causing irreparable long-term harm to humans and the environment. Thus how to improve the convert ratio of oxidized mercury and particulate mercury is the main task of mercury removal in coal-fired flue gas.This experiment established a system consisting of the combustion and morphological evaluation of mercury under laboratory conditions. Using an inverse diffusion flame burner as combustion set-up, ethylene as fuel, mercury samples were captured around the outer edge along the height of the flame. The US EPA Ontario-Hydro law was adopted to do the collection, recovery and digestion of mercury samples, followed by the measurements through cold-vapor atomic fluorescence spectrometry. The influence of oxygen, sulfur dioxide and hydrogen chloride on mercury species distribution was also investigated. The detected results indicated that elemental mercury is the main form of Hg in the samples, and the conversion rate of particulate mercury and oxidized mercury could be improved with the increasing concentration of oxygen and sulfur dioxide in different degrees, but could be significantly improved with that of hydrogen chloride. In a combustion system including hydrogen chloride, the intake of sulfur dioxide will inhibit the conversion rate of particulate mercury and oxidized mercury. Hydroxy(-OH), carbonyl(-C=O) and lactone(-COOCH3) and other functional groups on soot particles surface could improve the conversion rate of particulate mercury, as well as sulfur and chlorine elements. Finally, the control technologies of mercury in coal-fired power plants were analyzed. Through analysis of the enrichment regularity of mercury and the effect of its behavior characteristics in the edge of a flame, this paper provides a scientific basis for the emissions of mercury in coal-fired power plants. |
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