The Investigation Of The Stabilization Of Mercury In WFGD Solution And Gypsum

As China is the largest coal consumption country in the world,the mercury pollution problem brought by coal combustion is increasingly severe. The power plants are the largest anthropogenic source regions that release mercury into the atmosphere, so it is necessary to strengthen the control of mercury emission from coal-fired power plants. At present, most of the coal-fired power plants in China use the existing dedusting equipments to remove the mercury. The wet flue gas desulfurization(WFGD) can effectively remove both the SO2 and Hg2+ in the coal-fired flue gas,but the absorbed mercury is not stable, the mercury in the desulfurization solution can be converted back to elemental mercury(Hg0) and re-emitted, and the mercuy in the WFGD gypsum can be transferred to the environment when the WFGD gypsum is utilized. Therefore, it is essential to investigate the stability of mercury in WFGD solution and gypsum.Hg2+ can be reduced to Hg0 by the existing reductive ions in the solution. It was found the Hg2+ could be effectively captured by stabilizers,but the influences of related factors on the the effects of the stabilizer were unclear. In this study, the influences of several factors on the effects of sodium sulfide(Na2S) and dithiocarbamates(DTCR-3) for Hg2+ capture in the simulated wet desulfurization solution were investigated,include additive doses, temperature,initial pH and the concentration of Cl-. The results indicated that both of the two stabilizing agents could effectively capture the Hg2+ in the solution. With the increasing of additive doses, temperature and initial pH, the precipitation efficiency of Hg2+ was increased. But the capture efficiency decreased by the increasing concentration of Cl-.WFGD gypsum is the main byproduct of WFGD system, the mercury will release when the gypsum is heated. In this study, the thermostability of mercury compounds in the WFGD gypsum was studied. The Hg0 release property in different heating rate and atmosphere was investigated by temperature programmed thermal decomposition(TPTD). The results indicated that the release temperature of mercury in WFGD gypsum ranged mainly in 200℃~500℃. The main mercury compounds in the WFGD gypsum were HgS(red) and HgS(black). In the air atmosphere,portion of the released Hg0 was oxidized to Hg2+ and the peak temperature became higher. With the slowdown of heating rate, the concentration of the released Hg0 was obviously decreased. The change of the heating rate did not influence the mercury release temperature range and the peak temperature.The harmful trace elements exist in the desulfurization gypsum can be transferred to the surrounding environment by the infiltrating effect of rain or underground water. In this study, we used the horizontal vibration method to conduct a toxicity identification experiment and a trace elements leaching characteristics experiment of three kinds of desulfurized gypsum. The results indicated that the leaching concentrations of trace elements in the three kinds of gypsum were far below the concentration limited in related rules. The extraction rate of all trace elements reached maximum when the leaching time was 8h~16h. With the increasing of L/S ratio, the extraction rate of Pb、As、Cr、Se raised up, but the extraction rate of Hg decreased. The elements Pb, As, Cr, Se are mainly in the form of acidic oxide which can be easily dissolved under acid condition, so the rise of pH can significantly reduce their extraction rate.