Study on mercury emission and migration from large-scale pulverized coal fired boilers

In order to study the characteristics of mercury emission and migration, EPA 30B method has been used to determine the Hg concentrations in flue gases emitted from four typical pulverized coal fired boilers in a large-scale power plant. The Hg contents of related input/output materials in two of these boilers were also analyzed and compared. According to mass balance, the Hg distribution in different materials was obtained, and then effects of the major pollution control facilities including selective catalytic reduction (SCR), electrostatic precipitation (ESP) and wet flue gas desulfurization (WFGD) on Hg emission were clarified. Based on these, the migration pattern of Hg in pulverized coal fired boilers was summarized systematically. Experimental results indicate that Hg emissions of all the four boilers were below 3 μg/m3, which was much lower than the limit value specified by the new Chinese national emission standard. The emission is even significantly lowered when SCR system is located upstream. The reason was that lots of Hg0 in flue gas will be oxidized to Hg2+ with SCR and then adsorbed easily by fly ash. The solubility of Hg2+ is also much higher than that of Hg0 in WFGD system, thus the major Hg speciation in exhaust flue gas is Hg0. Most of the Hg absorbed in WFGD is transferred into desulfurization gypsum, while the enrichment in both desulfurization waste water and bottom ash is limited.