| The environmental challenge caused by the emission of SO2, NO and Hg from power plant have been concerned worldwide. However, in the existing gas purification technology, the removal of SO2, NO and Hg were completed separately in flue gas system, therefore, it is highly desired that a multipollutant removal system should be integrated to simplify the cleaning up system. Activated carbon and activated coke were widely noticed because of its high efficiency of adsorbing SO2, NO and Hg, but its high cost restricted the industrial application. So the finding of cheap and efficient adsorbents became the main direction of researchers. In this paper, based on the purpose of biomass and natural mineral utilization, coconut shell and attapulgite were used to simultaneously remove SO2, NO and Hg in place of activated carbon or activated coke.Coconut shell was first chosen to prepare the coconut shell activated carbon which was then impregnated with elemental sulfur at different temperatures for modification. The prepared coconut shell activated carbons were characterized by means of nitrogen (N2) adsorption/desorption, thermogravimetry-mass spectrometry (TG-MS), and X-ray absorption near-edge structure (XANES). Experimental studies on Hg removal were investigated in a fixed bed reactor system. The raw CSAC samples without sulfur impregnation basically had little adsorption capacity, while the Hg adsorption capacities significantly increased for all of the CSACs samples after sulfur modification. It is interesting to note that sample CSAC-500S had the most excellent Hg adsorption performance for all of the samples tested. The factors including porous structure, sulfur content, and sulfur species of adsorbent were important for Hg adsorption. Elevating the impregnation temperature decreased the total sulfur content while optimized the pore structure parameters. Elemental sulfur, thiophene (main form of organic sulfur), and sulfate were the main forms of sulfur deposited on the adsorbent surface, all of them promoted the Hg adsorption capacity. But, elemental sulfur was the most effective, and thiophene second. The conversion from elemental sulfur to organic sulfur happened quite easily at higher modification temperature. Elemental sulfur tended to be short-chain species at higher temperature while the long-chain at lower temperature. Among them, Short-chain elemental sulfur species (S2-S6) were believed to be much more effective in Hg adsorption than long-chain species (S7-S8) for their adverse impact on porous structure.Attapulgite (ATP) was then chosen to prepare the non-carbon adsorbent which was treated by KI solution at different concentrations for modification. The prepared attapulgite adsorbents were characterized by means of nitrogen (N2) adsorption/desorption, X-ray diffractometer (XRD), scanning electron microscopy and X-ray energy dispersive spectroscope (SEM/EDS). Experimental studies on Hg removal were investigated in a fixed bed reactor system. Despite of its high surface area, the raw ATP samples presented a poor adsorption capacity for its scarce micropore. The mesopore area of multimodal particle size distribution (PSD) for all of the samples tested were around 2-10nm. The modifier KI loaded on samples destroyed pore structure, but had less effect on its crystal structure. Apparently, the Hg adsorption capacities significantly increased for all of the ATPs samples after KI modification. The highly reactive I2 oxidized from KI deposited on the adsorbent surface promoted Hg adsorption. It suggests that 2.5% additive of KI into the raw attapulgite (ATP-I-2.5) can remove nearly all mercury with average Hg removal efficiency of 96%.According to the high Hg removal efficiency of CSAC-500S, ATP-I-2.5 tested in a fixed bed reactor system, both adsorbents were tested on an entrained flow reactor system to further evaluate their Hg removal efficiency under flow condition. These results were used to understand the effects of flue gas components, injection quantity, and residence time of adsorbents on Hg removal efficiency. CSAC-500S, ATP-I-2.5 still had excellent adsorption capacity in air atmosphere. The higher Hg removal efficiency could be achieved at longer residence time, however, the increased injection of adsorbent would not result in the corresponding increase of Hg removal efficiency at a short residence time. SO2 presence enhanced mercury adsorption for CSAC-500S, while for ATP-I-2.5, the mercury adsorption was promoted firstly and then inhibited, and the more concentration of SO2 was, the more the inhibition was. NO did have no effect on mercury adsorption for CSAC-500S, while for ATP-I-2.5, the mercury adsorption was enhanced slightly. HCl could strongly promote mercury adsorption of both adsorbents and homogeneous oxidation of Hg.The co-removal capacities of both adsorbents were evaluated under simulated flue gas condition. Components of real flue gas contributed to an enhanced Hg removal efficiency, adsorbents could remove Hg0 in flue gas effectively. Especially, both adsorbents with excellent Hg adsorption capacity could also remove some SO2, NO. The adsorption of SO2 for CSAC-500S was based on Zawadzi mechanism, and NO removal was correlative result of physical and chemical adsorption. K2O formed on ATP-I-2.5 during the sample preparation was critical to remove SO2 and NO. Adsorbent would make it a priority to remove SO2 under the co-existing of SO2 and NO. |
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