| Coal is the main energy source in our country as its consumption every year.The environmental problem caused by emittion of mercury during coal combustion has drawn great attention from all over the world.More and more experts and scholars devote themselves to the study of mercury removal from flue gas.The key of flue gas mercury removal technology is the preparation and improvement of adsorbents.Preparation of adsorbents with high mercury removal performance therefore becomes a hot area of research.Cation exchange resins were used as precursors to prepare adsorbents with certain mercury removal properties by loading cationic exchange resins with metal elements.Characterizations,including thermogravimetric analysis,elemental analysis,X-ray diffraction analysis,nitrogen adsorption and desorption isotherm analysis,raman spectroscopy analysis and vibration sample magnetometer analysis were carried out to evaluate the weight loss behavior,elemental content and physicochemical properties of adsorbents.The mercury removal performance of adsorbents was evaluated by fixed-bed reactor in simulated flue gases.The mercury species formed on the adsorbent surface were determined by temperature programmed desorption experiment.1.Cationic exchange resin was used as a carbon source,and the precursors were modified through ion-exchange with Fe3+ and/or Fe2+.Three different magnetic adsorbent materials(Fe3+-H,Fe3+/Fe2+-T,Fe3+/Fe2+-H)were prepared by carbonization(T)and/or activation(H).It was found that the Fe3+/Fe2+-H adsorbent had the highest specific surface area,residual magnetization and saturated magnetization.X-ray diffraction showed that FeS and ferric oxide(FexOy)were formed over the Fe3+/Fe2+-H adsorbent.The Fe3+/Fe2+-H adsorbent showed the highest mercury removal performance among all of the prepared adsorbents,which was probably due to its large specific surface area and catalytic oxidation ability;the optimum mercury removal temperature of Fe3+/Fe2+-H adsorbent was found to be 120℃;introduction of SO2 in to flue gases promoted the mercury removal of Fe3+/Fe2+-H.The reason was that SO2 and Hg can be adsorbed by catalytic oxidation to form HgSO4.The stability of mercury species on the surface of Fe3+/Fe2+-H adsorbent was determined by temperature programmed desorption experiments(TPDD).The results showed that the mercury species on the surface of Fe3+/Fe2+-H adsorbent were mainly HgO,HgS and HgSO4.In addition,Fe3+/Fe2+-H adsorbent still maintained high mercury removal performance after five cycle experiments,indicating the high regeneration ablility of Fe3+/Fe2+-H adsorbent.2.Cationic exchange resin was used as carbon source and Ce was used to modify the precursor.Two kinds of adsorbents,namely Ce-T and Ce-H were prepared by carbonization and activation,respectively.The mercury removal performance and mechanism of Ce-T and Ce-H were studied.X-ray diffraction showed that the states of cerium over Ce-H adsorbent were Ce2S3 and CeO2,while no peaks attributed to cerium compounds were detected.Elemental analysis showed that Ce-H adsorbent contained abundant lattice oxygen,which was conducive to the adsorption of Hg.The mercury removal performance of Ce-H adsorbent was studied at 80℃-180℃.The results showed that the mercury removal performance of Ce-H adsorbent was much higher than that of Ce-T adsorbent because of the synergistic effect of Ce2S3 and lattice oxygen[O]in Ce-H adsorbent.Introduction of O2 into flue gases could promote the mercury removal performance of Ce-H adsorbent.There were two main reasons:one is that O2 can directly promote the oxidation of Hg,and the other is that O2 can supplement the lattice oxygen consumed by the adsorbent.What is more,the mercury removal performance of Ce-H adsorbent was higher than that of commercial coconut shell activated carbon.The mercury species generated on the surface of Ce-H adsorbent were determined by TPDD experiments.The mainly states of mercury on the surface of Ce-H adsorbent was HgO,HgS and HgSO4.HgO was mainly formed by the chemical oxidation of Hg by lattice oxygen,whereas HgS was formed by the reaction of Ce2S3 with Hg.HgSO4 was generated by the reactions of lattice oxygen,SO2 and Ce2S3.After five cycles of experiments,Ce-H adsorbent still maintained high mercury removal performance and the mercry species generated over adsorbent had no significant change.The Ce2S3 on the surface of adsorbent was converted to CeO2 after regeneration of Ce-H adsorbent,which was evidenced by X-ray diffraction analysis.With the above excellent properties,Ce-H adsorbent may prove to be an advantageous alternative to commercial activated carbon for mercury removal in power plants. |
