Remoyal Of Hg~0 From Coal-fired Flue Gas By Modified Activated Carbon

The air pollution is extremely severe in China. Mercury has become the precedent-controlled harmful pollutants following the dust, SO2 and NOx due to its high volatility, long persistence, and bioaccumulative properties. Coal combustion is one of the major anthropogenic mercury emissions sources. Elemental mercury (Hg0), which is the most hard to be removed, has become research focus of flue gas mercury control. Because of the high specific surface area and abundant pore structure of the activated carbon, it has been applied in actual industrial flue gas mercury control. The cost of activated carbon is high, thus limiting its widely use. Therefore, other modification methods should be explored to prepare the activated carbon of high mercury removal capacity, and the cost of activated carbon could be lowered by reducing the amount of the used activated carbon.The mercury removal efficiencies of the 12 commercial activated carbons were evaluated. The mercury removal difference on the commercial activated carbons was discussed based on the characterization results. Nitric acid impregnation method was adopted to modify the activated carbons to enhance its oxygen and nitrogen groups, and the relationship between the physical, chemical properties and mercury removal performance on the modified activated carbon is established. Combined with the TPD results, the mercury removal mechanism under different flue gases were revealed. The Cu, Mn, Fe, Co and Ce were used as active component to prepare a series of samples using the incipient-wetness impregnation method. The effects of temperature, loading value of active component and component flue gases on Hg0 removal were discussed, and the mercury removal mechanism was discussed initially based on the mercury-containing products. The primary results obtained were as followed:(1) The removal efficiency for Hg0 on HNO3-modified activated carbon was investigated, and the relationship between the physical, chemical properties and mercury removal performance is revealed. The carbonyl, ester or anhydride groups could enhance the Hg0 adsorption capacity. And the pyrrolic groups were catalysis for Hg0 removal.(2) The impacts of flue gases on Hg0 removal on the HNO3-modified activated carbons were evaluated. The mercury removal mechanism under single simulated flue gas, such as SO2, NO and HC1 was revealed, and the binding abilities between the mercury and flue gases production formed on the samples were compared. The physically adsorbed SO2 inhibited mercury removal because of competitive adsorption, while chemically adsorbed sulfate species promoted the Hg0 removal. The NO and HC1 could promote the Hg0 removal by the active species generated on the surface of the samples. In addition, the presence of SO2 could inhibit the active species formed by NO, thus resulting in the decrease in Hg0 removal. The binding abilities between Hg0 and products of acidic gases formed on the samples follow the order:Cl->S->N-containing groups.(3) The mercury removal efficiencies of metal loaded activated carbons were evaluated. The best mercury removal efficiencies over the Cu, Mn, Fe, Co, Ce doped samples were chosen. The results indicated that the Mn/AC catalysts showed highest activity for the mercury removal, and 10% was found to be the best loading value. The content of the Mn3O4 and BET surface areas were beneficial to the mercury removal.(4) The impacts of flue gases on Hg0 removal on the Mn/AC were evaluated, and probable reaction mechanisms were revealed. O2 could oxidize Hg0 to HgO, thus promoting Hg0 removal. SO2 inhibited Hg0 removal by destroying the cation vacancies of the catalysts. NO was adsorbed to the samples to form active NO2 and NO3- species, thus enhancing the Hg0 removal. The HC1 adsorbed to sample surface to form active Cl, which oxidized Hg0 to HgCl2. H2O inhibited the fact of gas O2 regenerate the lattice oxygen and replenish the chemisorbed oxygen, resulting in decrease in Hg0 removal on the samples.