| Mercury is a highly toxic contaminant to human health and ecosystems. Theemission of mercury from anthropogenic activities is a serious concern in both thedeveloped and developing countries. Coal combustion is generally considered as thedominant mercury emission source for the global mercury emission inventory. China isthe largest mercury emitter in the world and coal combustion is the most importantmercury source in China. Therefore, it is stringent to find any effective ways to reducemercury emission from coal-fired power plants. Activated carbon injection is nowconsidered to be the most robust technology for mercury control at coal-fired powerplants. However, the application of activated carbons as the current mercury sorbentsis limited by following two reasons. On one hand, a huge amount of sorbent demandedby coal-fired power plants result in a high cost for mercury control. On the other hand,the application of carbon-based sorbents exist potential problem of secondary pollution.This is not only a waste of resource but also acause of new environmental problems.Therefore, it is important to develop efficient and no secondary pollution mercurysorbents. In this paper, chemical modified activated carbons were applied asalternative to activated carbons due to their high efficient and mercury adsorptionstability. A series of modified activated carbon were synthesized and their mercuryremoval performances were evaluated. The mercury removal mechanism of modifiedactivated carbons was also deeply discussed.The abilities of different types of activated carbons had been investigatedaccording to adsorption capacity of activated carbons. The effects of the carbonparticle size and adsorption temperature were investigated. Results showed that thewooden activated carbon has a better removal ability of gaseous elemental mercury.Low adsorption temperature could accelerate the rate of Hg0removal. And suitablescale of carbon particle siza (0.15＜d＜0.25) could reach the optimal effect, bigger orsmaller size are both negative.Experiment used Na2S, S and CeCl3to modify the activated carbon. Thecomposite modified processing was first impregnated activated carbon with aqueousphase Na2S or CeCl3and followed with vapor-phase S. The samples were characterizedusing Brunauer-Emmett-Teller (BET), X-ray diffractometer (XRD) analyses, ScanningElectron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Experiment results showed that the optimal loading value of Na2S was5%(mass fraction). Inaddition the impact of temperature and the initial sulfur to carbon ratio during Simpregnation were investigated. The results showed that carbons impregnated withsulfur at600℃exhibited the highest efficiency for mercury removal. When sulfur tocarbon ration was varied from2:1to1:2, the sulfur content decreased only slightly,this resulted in a small decrease in mercury uptake capacity. A maximum saturatedadsorptive capacity of Hg0up to3282μg-Hg/g-C was observed for the compositeNa2S-S/AC, which was approximately1.5times higher than S/AC. The effect ofvarious gas constituents found in a real flue gas on the performance of Na2S-S/ACwere investigated. Fixed-bed experiments showed that the presence of O2(up to9%)increased the removal efficiency up by25%. while H2O (up to8%) decreased the Hg0removal efficiency as much as24%. NO and SO2showed no significant effects. Thecomposite CeCl3-S/AC showed better catalytic oxidation properties and adsorptionproperties than CeCl3/AC. |
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