Experimental And Mechanism Study On Mercury Removal And Transformation By Activated Carbon In Simulated Flue Gas

Coal dominates the energy production and consumption structure, however, a variety of pollutants will be inevitably produced in the process of combustion. Coal-fired mercury pollution control has been widespread concern all over the world. The mercury removal technology by activated carbon is relatively mature and widely used. But the study of mechanism on mercury removal by activated carbon is still not mature enough. The influences of activated carbon surface physical properties, chemical properties and flue gas components on mercury removal were studied, combining with dynamic analysis, aims to provide a theoretical basis for preparing low-cost and high adsorption capacity activated carbon.The coal activated carbon and wood activated carbon were selected as the carbon-based adsorbents. The desorbed activated carbon was prepared using heat treating by furnace. The modified activated carbon was prepared by benzoic acid impregnation. BET surface area and pore size distribution analysis, scanning electron microscopy and energy spectrum analysis(SEM/EDS), Fourier transform infrared spectroscopy(FTIR) and X ray photoelectron spectroscopy analysis(XPS) were used to characterize the different physical and chemical properties of various activated carbon samples, such as surface area, pore structure, surface element types, surface oxygen functional groups and so on. The results indicated that:(1) The desorbed activated carbon still had a huge surface area and micropore structure. The surface area, micropore volume and other physical parameters of modified activated carbon decreased significantly.(2) After desorption, the oxygen element proportion on activated carbon surface decreased significantly, which can be regarded as completely removed. After impregnation, the oxygen element proportion on activated carbon surface obviously increased.(3) The number of most oxygen containing functional groups on desorbed activated carbon surface decreased. Some functional groups almost disappeared. After impregnation, the number of surface oxygen functional groups on activated carbon increased significantly. As the increase of load, the content of carbonyl, carboxyl and lactone increased, while the content of hydroxyl group decreased.The influences of activated carbon surface physical properties, chemical properties on mercury removal were studied, combining with the characterization analysis. The results indicated that:(1) The desorbed activated carbon almost had no mercury removal ability which still had huge surface area and micropore structure that illustrated the physical structure of activated carbon was not a factor in mercury adsorption, while the oxygen-containing functional groups on activated carbon had a great influence on mercury removal.(2) The mercury adsorption ability of modified activated carbon increased significantly, which adsorption mainly performed chemical adsorption. According to the characterization results of XPS, FTIR, EDS etc. combining with the experiment results, carbonyl, carboxyl and lactone promoted mercury adsorption, while the phenolic hydroxyl group had an inhibition.Flue gas components on mercury removal were studied. The results indicated that: The presence of O2 had an improvement on mercury removal efficiency. HCl can significantly increase mercury removal ability of activated carbon. After addition of O2, the mercury removal efficiency further improved. NO also can greatly improve mercury removal efficiency. The presence of SO2 declined mercury adsorption capacity of activated carbon, and in the N2/SO2/O2 condition, the mercury removal rate was even worse. SO2 can easily combine with surface oxygen functional groups on activated carbon, which was competitive with elemental mercury on active sites, which resulted in the decrease of mercury adsorption capacity of activated carbon. In the presence of single gas, the mercury adsorption of activated carbon followed the Eley-Rideal mechanism.Three kinds of kinetic models were used to study the process of mercury adsorption by modified activated carbon, and the corresponding adsorption mechanism was obtained. The results showed that: The pseudo-second-order model fitted the experimental data best, and the pseudo-first-order model worse, while Fick’s intra- particle diffusion model worst, which revealed that the pseudo-second-order model can well describe the mercury adsorption process of modified activated carbon and chemisorption was the control step of mercury adsorption process of modified activated carbon.