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

In order to study the characteristics of mercury emission and migration, EPA 30B method was used to determine the Hg concentrations in flue gases emitted from four typical pulverized coal-fired boilers in a large-scale power plant, and 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 the effect of major flue gas purification systems including selective catalytic reduction (SCR), electrostatic precipitation (ESP) and wet flue gas desulfurization (WFGD) on Hg emission was clarified. Based on these, the migration pattern of Hg in pulverized coal-fired boilers was summarized systematically. Experimental results indicated that Hg emissions of all the four boilers were below 3 μg／m³, which was much lower than the limit specified by the new Chinese national standard. The emission was even significantly lowered when SCR system was located upstream. The reason was in the fact that, when there was SCR system located, lots of Hg⁰ in flue gas could be catalyzed to Hg²⁺ and then adsorbed by fly ash. The solubility of Hg²⁺ was much higher than that of Hg⁰ in WFGD system, and the major Hg speciation in exhaust flue gas was Hg⁰. Most of the Hg absorbed in WFGD was transferred into desulfurization gypsum, while the enrichment in both desulfurization waste water and bottom ash was limited.