An Experimental Study On Gaseous Mercury Adsordption By Chemically Modified Mineral Adsorbents

Mercury has been identified as a hazard to human health and the environment. Since coal-fired power plants represent a significant fraction of the anthropogenic emissions of mercury into the atmosphere, the speciation and control of mercury in coal-fired power plants are currently an active topic of research. Adsorbents injection has been shown to have the best potential to remove both elemental and oxidized mercury, but the high cost of the adsorbents prevents its development. To solve this problem, four cheap mineral adsorbents were selected which are impregnated to improve their adsorption efficiency.The method of solid sample mercury content analysis was established by microwave digestion and atomic fluorescence measurement. With HNO3,HC1and HF mixed at a ratio of5:2:1at230℃, the solid samples can be completely digested under microwave heating. With the measurement of coal, fly ash, slag and desulfurization gypsum by atomic fluorescence, mercury content of four samples were detected. Mercury content of four samples combining with coal consumption and output of fly ash,slag and desulfurization gypsum can lead to distribution of mercury in coal-fired power plants, which shows that40%mercury in coal stays in fly ash and desulfurization gypsum,and the other mercury in coal is emitted into atmosphere. The distribution is affected by the chlorinity content of coal.To solve the high cost problem of adsorbent injection, four cheap mineral adsorbents were selected, which are bentonite, kaolin, vermiculite, zeolite. Then they were impregnated by CuCl2solution. Elemental mercury is mainly physically adsorbed into the internal pore of the original mineral adsorbents, and the adsorbent efficiency of the original mineral adsorbents is below20%. With impregnation, the adsorbent efficiency is significantly improved because they are loaded with chlorine functional groups. The efficiency of the impregnated bentonite can reach as high as70.9%which is similar to activated carbon. The influence of physical structure, chlorinity of adsorbents, elemental mercury concentration, temperature, hydrogen chloride concentration, oxygen concentration, category of modification reagents were analyzed. Elemental mercury concentration decreases the adsorbent efficiency, but increases mercury content per unit mass of the adsorbent. There is a peak point between120℃and150℃, above which adsorbent efficiency decreases as the temperature rises. When the temperature goes down below this point, there is a conversed law, because higher temperature improves the chemical adsorption. O2and HCl are important to the adsorbent efficiency. Adsorbent efficiency increases about10%when HCl exists, and the absorbent doesn’t adsorb elemental mercury at all without O2.On the whole, the impregnated mineral adsorbents have high adsorption efficiency, which have the potential for business application because of the low cost.