Adsorption-Electrochemical Reclaimation Of Mercury From Flue Gas In Southwest China

Hg is one toxic heavy metal element both for environment and human. In China, coal is major one-off energy. The quantity of Hg discharged by human was about 302.9t from coal combustion in 1995, including 213.18t into air. Power plants are the largest discharging resources. The migration and transformation of Hg in the coal-fired process has become an important way in the ecosphere cycle. Hg pollution from combustion in air is severe environmental issue we confront.Element Hg is the primary form of the gaseous mercury from gas flue, also the primary form in air. It's a common way to treat Hg in the gas flue by commercial activated carbon(AC) impregnation technology. In this research, AC was improved by loading catalysts(Co_xO_y,Mn_xO_y,Co_xCu_yO_z,CoCl₂ and MnCl₂)on them, to intensify AC's adsorption capacity, activation temperature, load percent, reaction temperature were effect factors investigated in the research. Results showed that the breakpoint respectively were 16h, 10h, 7.5h, 135h, 113h; adsorption capacity respectively were 26mgHg/gAC, 18mgHg/gAC, 13mgHg/gAC, 171mgHg/gAC, 151mgHg/gAC, which respectively were 126 times, 87 times, 63 times, 826 times, 729 times than unimproved AC, when adsorption temperature was 150℃, load percent was about 20%. The best adsorbents were CoCl₂/AC, MnCl₂/AC. TGA, XRD indicated that: (1)Reaction between Cl and Hg could effectively enhance AC's capacity. (2)Co_xO_y and Mn_xO_y formed on the CoCl₂/AC, MnCl₂/AC could catalyze element Hg to form Hg oxide, for example HgO, which was removed from gas by adsorption.The adsorbent Co_xCu_yO_z (20%)/AC+Cl(Br) was also studied. The Hg removal rate respectively were 84%, 89%, 10% and 15% percent more than Co_xCu_yO_z(20%)/AC, when adsorption time was 1h, Co:Cu:Cl(Br) =1:1:0.5(mol ratio). The results also manifested the reaction between halogen and Hg could intensify AC's adsorption capacity.Simulation using different kinetic equations,including first-order, Elovich, two-constant and parabola equation, showed that Co_xO_y/AC,Mn_xO_y/AC and Co_xCu_yO_z/AC's Hg adsorption could be well described by two-constant equation, which indicated that adsorption,diffusion and combination all played important roles during the complicated process; CoCl₂/AC and MnCl₂/AC's Hg adsorption could be well described by first-order equation, which indicated that chemical reaction played a major role during adsorption process. Comparison constant b with q_∞showed that the order of maxium capacity and adsorption rate were: CoCl₂/AC>MnCl₂/AC>Co_xO_y/AC>Mn_xO_y/AC >Co_xCu_yO_z/AC.The selected catalysts were applied to the flue gas Hg adsorption. The coal used in the test was from Guizhou province, whose mercury concentration was 1.38mg/kg. The combustion test was carried out on small-scale equipment in lab. The result showed that improved AC adsoebed 97% of the gaseous element Hg, compared with 3% of element Hg adsorbed by solution, which indicated that improved AC was feasible in the actual gas treatment. Under lab conditions, 1kg AC can adsorb element Hg produced by 7472t coal combustion; In large-scale combustion equipment, 1kg AC can adsorb element Hg produced by 790t coal combustion, when Hg release rate is 98% in coal combustion process and the percent of particle Hg and gaseous Hg are 80%, 20% in flue gas, meanwhile percent of element Hg is 80% in gaseous mercury.Electrochemical method was used to reclaim spent AC. The factors, pH, current, electrical conductivity, Eh were considered during process. The results showed that Hg removal rate was about 61%, which indicated electrolysis was a promising approach to reclaim spent AC.