| In this thesis, firstly the research background including mercury emission and its speciation transformation during coal-fired process, the mercury emission control standards for coal-fired power plants and mercury removal technologies was reviewed. Then the experimental system and the basic principle behind its design was descibed in detail.Coal-fired derived fly ash, a kind of non-carbon sorbent, was studied in this thesis. Fly ashes were sampled from six typical coal-fired power plants. The fly ash sampled from plant A was screened by physical methods to get different size distribution fly ash particles. These fly ash samples were characterized and analyzed to obtain information such as morphology, chemical composition, surface structure, unburned carbon content and mercury content.The flue gas mercury adsorption by the fly ash was experimentally studied on the sorbent evaluation system developed by our research group.. The results showed that mercury removal efficiency was affected by different factors including the fly ash sampling source, the particle size, unburnt carbon content and mineral composition in the fly ash, and the modification on the fly ash.It was found that the mercury adsorption efficiencies by the fly ash left on the150mesh screen, staying between150mesh and200mesh screen,200mesh-300mesh,300mesh-600mesh and under600mesh screen were67.83%,35.44%,50.05%,19.53%and13.67%respectively. Unburnt carbon contents in the fly ash left on150mesh screen from the six power plants were5.44%,7.68%,6.43%,5.74%,5.22%and3.63%respectively, and their adsorption efficiencies were61.27%,51.32%,30.43%,40.36%,36.58%and33.44%respectively. The mercury adsorption efficiencies did not increase linearly with the increasing unburnt carbon content in fly ash. Chemical compositions of the fly ash also had some impact on mercury adsorption efficiencies. The adsorption efficiencies of SiO2, CaO, Al2O3, MgO and Fe2O3were11.67%,12.38%,8.56%,9.27%and13.22%. Unburnt carbon, among the coal fly ash compositions, still played a major role in the capture of flue gas mercury.Then Ca-based sorbents and attapulgite were modified by different chemical reagents. And the impact of modified sorbents on flue gas mercury adsorption was studied. The result showed that the mercury removal efficiencies of unmodified Ca(OH)2and CaCO3were low, which were14.50%and17.76%. However, the mercury removal efficiencies of Ca(OH)2were greatly improved after its modification by different concentrations of KMnO4, which arrived at80.92%,85.38%,82.35%,83.51%and89.84%respectively. While, with the increasing of KMnO4concentration, the mercury removal efficiencies by the CaCO3modified by KMnO4increased gradually. The mercury removal efficiencies were34.55%,34.34%,43.74%,44.50%and58.35%respectively, corresponding to the modified concentration of1%,2%,3%,4%and5%. It showed that the promotion effect of KMnO4was related to the content of KMnO4. The more addition amount of KMnO4, the more obvious the promotion effect. The mercury removal efficiency of unmodified attapulgite was not high, which was only28.91%. But the mercury removal efficiency was improved after its modification by different concentrations of HNO3, which arrived at6.26%,39.89%and34.17%respectively. While, after modified by different concentrations of HC1, the mercury removal efficiency of attapulgite didn’t improved but dropped, which were7.30%,27.45%and20.66%.In this thesis, mathematical model of the mercury adsorption by the sorbents during its flight process at the flue gas duct was primarily developed based on the experimental data, and the flow field, temperature field, pressure field and particle trajectories inside the multiphase flow reactor were numerically simulated. The homogeneous reaction between the flue gas and mercury was simulated by using CHEMKIN software, which may provide a theoretical basis for further study on the reaction mechanism between the sorbent and flue gas mercury. |
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