Environmental Effects Of Mecury Emission From Coal-fired Power Plant And Mechanism Study On Mercury Captured By TAC

Mercury pollution is one of the global environmental challenges. Problems arising from the release of mercury are increasing in China because of the huge coal consumption. However, the mercury pollution is less known among the public and subsequent affordable strategies for mercury control are still needed. Therefore, there is an urgent need to carry out studies on mercury emission characteristics, mercury transportation pattern, and mercury control technology. To understand the impacts of mercury emission from coal-fired power plants and the adsorption mechanism of mercury, researches were carried out on the measurement of mercury, environmental impacts, mechanism of mercury adsorption on activated carbon and the cooperative control technology.At first, a set of flue gas mercury speciation system (FMSS) was designed and built to meet the shortage of measurement methods for coal-fired power plants. Field validations and comparison with CEM and OH method were carried out in three different power plants. Results showed that mercury concentrations in the flue gases from coal-fired power plants were in the range of2.8-14.3μg/m3.The deviation of total mercury concentrations measured by three methods in the same flue gas was6.7%. The mercury concentration measured by FMSS still met the70%-130%requirement in the mercury balance of boiler even in the flue gas with1248ppm SO2, and thus suitable for convenient mercury measurement in coal-fired flue gases.Secondly, in combination with the emission source, local meteorological and geographical conditions, model calculations were carried out and samples of soil and air were collected and analyzed to evaluate the environmental effect of mercury emission from a typical coal-fired power plant. Spatial distribution of mercury in soil and air around a2×125MW power plant were investigated using geostatistics techniques. It was found that mercury levels in soil samples were in the range of0.045-0.529mg/kg with an average of0.180mg/kg. In comparison with the local soil background, the average mercury content of nearby soil increased by2.3%. Within the7km radius areas of sample collection, mercury concentrations in air samples varied from4.3to12.4ng/m3with an average of7.0ng/m3, slightly higher than the local background of6.7ng/m3. As the result of Gaussian diffusion together with physical deposition, mercury concentration in air samples decreased in general with the increase of the distance from the power plant, except for a peak value at certain distance downwind the chimney.Furthermore, absorption mechanisms of mercury on activated carbon were studied based on the morphological analysis of adsorbed mercury on activated carbon surface. Surface functional groups (SFGs) on AC sorbents were removed through heat treatment under the protection of argon (Ar). The Hg0adsorption behavior of heat-treated activated carbon (TAC) when subjected to various synthetic gases was studied. The use of TAC was to avoid the complicated interaction between SFGs and flue gas components. The adsorbed residues were then characterized by XPS and XAFS to investigate the existing state of mercury and its coordination properties. We found that the Hg0adsorption on AC was not simply a process of physical adsorption. Acid gas components enhance Hg0capture on TAC mainly through Eley-Rideal reaction mechanism. The adsorption of acid gas component and subsequent formation of acidic reactive groups on TAC provided activated sites for Hg0sorption, while TAC served as a catalyst for the formation of acidic reactive groups and the oxidization of mercury. Under the experimental conditions of our study, the addition of single acid component (HC1, NO2or NO) to the pure N2can generate higher Hg0capture efficiency larger than50%, while it was only20%when SO2was added. O2exhibits a synergistic effect on the enhancement of Hg0oxidation and capture when acid gases are present in baseline gases. When co-existed with other acid gas components, SO2can inhibit the adsorption of mercury through competition for the same active site on TAC. The addition of water vapor promoted the adsorption of SO2on TAC further.Finally, experimental studies were carried out on the cooperative control of NOx and Hg0, which are the two major pollutants from coal burning. Activated carbon can adsorb NOx very well under the temperature range of80-100℃with a space flow rate of8000h-1. After NOx adsorption reached saturation, the activated carbon still can remove mercury efficiently and simultaneously catalyze the oxidation of NO up to55%in the presence of only4%O2. Therefore, an economical approach that used activated carbon for the simultaneous removal of NOx and mercury in flue gas was developed in this paper.