| Coal-fired power stations have become the main source of anthropogenic mercury emissions pollution, and have caused great harm to human health and ecological environment. The technology of using fly ash to the removal mercury in flue gas has become the research hot spot, but the exploration and understanding of the adsorption reaction mechanism is not deep enough. At first, this paper detailed the mercury pollution hazard and the present situation of mercury pollution at home and abroad, the existing mercury pollution control technology and theory of fly ash on the adsorption of mercury in flue gas were summarized, experiments were conducted to study the fly ash on the adsorption of mercury in flue gas, which provided theoretical support to the development of the mercury pollution control technology for coal-fired power stations.Synthesizing the existing research results at home and abroad, this paper decided to modify fly ashes of coal-fired power plants with chemicals, and a lab-scale entrained-flow reactor was used to simulate power station flue to evaluate mercury adsorption characteristics of fly ash before and after modified. The study focused on the effect of the types of fly ash, residence time, particle size, the modification reagent and flue gas composition on the fly ash adsorption characteristics, in order to get a kind of feasible and efficient mercury sorbent for industrial, and to provide experience for the perfection of adsorbent evaluation system. Based on central difference method, this paper calculated the change rate of the mercury concentration in flue gas after the injection of fly ash which could reflect the adsorption rate of fly ash adsorbent for mercury in flue gas, and it could evaluate the effect of fly ash adsorption more effectively,In this paper, A, B two coal-fired power stations of the electrostatic precipitator of fly ash were chosed as experimental objects, after simple sieving of the fly ashes, calcium chloride (CaCl2), calcium bromide (CaBr2) and hydrogen bromide (HBr) were used respectively to modify fly ash with impregnation method. And the modified fly ash samples were characterized by applying different analytical instruments thus different specific surface area, pore diameter and pore volume of the sample, chemical composition, scanning electron microscopy images and TGA thermo-gravimetric curve and other information of the samples can be obtained.Fly ash ejection mercury adsorption experiments were conducted on a Entrained-flow Reactor which was a evaluation platform of mercury adsorption properities in the independent design and development. Results indicated that, the adsorption efficiency of flue gas mercury of the two power plant fly ashes were both below10%, the mercury adsorption efficiency of A fly ash was slightly stronger than that of B fly ash, this was due to the larger the specific surface area, the smaller the aperture, and the greater the microporous,the more the physical adsorption performance of fly ash. Meanwhile,it was also found that, because of the limited contact between the fly ash and flue gas mercury atoms restricted by dynamics, so the adsorption cannot be fully exerted,, total absorption rate increased slightly as fly ash injection volume increased, but the absolute adsorption of unit fly ash decreased; And improve the, the adsorption of unit fly ash almost increased proportionally with the flue gas inlet mercury concentration, fly ash adsorption efficiency at high inlet mercury concentration only decreased by less than10%compared with the original entrance concentrationThe mercury adsorption properities of fly ashes after modification had been significantly improved, and the surface structure of modified fly ash did not change to much degree, while there was a little growth in the specific surface area and pore volume. It was found that the mercury adsorption process of modified fly ash was mainly chemical adsorption, in which due to the co-effect of H+and Br-the character of adsorption of HBr modified fly ash was optimal; on the condition that the specific surface area and pore volume were similar, A modified fly ash adsorption performance was still better than B modified fly ash, this was because, on the one hand.B almost had no microporous on the surface, on the other hand, there was a lot of Ca in B than A, who combined with the Br of HBr in the process of modification to form CaBr2crystals, which greatly reduced the chemical adsorption performance of B fly ash; despite the acceleration of adsorption rate of thetwo kinds of HBr modified fly ashes, they can only reach1/5and1/8of the adsorption rate of the modified activated carbon respectively.Effect of temperature on the mercury adsorption properties of modified fly ash was more complex, the results showed that there were both physical and chemical adsorption in the mercury adsorption process of of fly ash. low temperature could promote the physical adsorption, while under high temperature, chemical adsorption would be effectively improve; the adsorption rate of chemical adsorption process was a little slower than that of the physical adsorption, but the overall adsorption capacity was higher than that of the physical adsorption. It was also found that the temperature effect on the fly ash mercury adsorption properties in general was weak, so it was not the main influencing factor, and residence time had a visual impact on adsorption process. With the increase of residence time, the adsorption effect was better and better, but the increasing proportion of the adsorption efficiency increased with the residence time per unit time tended to be smaller and smallerlt was found, in a series of experiments whose simulated flue gas contained different concentrations of SO2, that SO2reducing atmosphere inhibited of mercury adsorption of HBr modified fly ash in flue gas, the adsorption efficiency decreased constantly with the increase of concentration of SO2, and the SO2block the contact between fly and Hg, which makes the Hg adsorption rate of the modified fly ash fell sharply in the early time ofthe reaction;when mixing NO in the flue gas, at the beginning, the Hg adsorption rate slightly decreased, but on the whole, the adsorption rate and adsorption efficiency of AO-HBr fly ash are on the rise with the increase of NO concentration.fly ash injection experiment of different particle size were conducted, the fly ashes of different particle size of the same injection could be collected in different ash hoppers, through analysis, it could be found that in the experimental condition in the thesis, the fly ash particle size is one of the main factors affecting the adsorption effect, adsorption performance of small particle fly ash was higher than that of the big. This was due to the larger specific surface area, the better modified effect of the small particle fly ash and its greater chance to contact and react with Hg atom of the small particle fly ash. Average pore size of small particle fly ash was small, which made it hard for the adsorped mefcury to be removed.In this paper, besides the study of Hg0which was stable and hard to remove, Hg2+content of three representative experiments were also tested, it was found that the content of Hg2+in the process of all reaction has been very less, which indicated that under the short residence time in the flue, the fly ash has enough adsorption sites to adsorbe mercury oxided by itself. |
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