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Emission Characteristics of Gaseous and Particulate Mercury from a Subcritical Power Plant Co-Firing Coal and Sludge

Field tests were carried out in a subcritical coal-fired power plant co-firing coal and sludge to analyze the emission characteristics of gaseous and particulate mercury. EPA30B method was applied to determine the mercury speciation in different positions of the flue gas, including the inlet and outlet of the selective catalytic reduction DeNOX system (SCR) and electrostatic precipitator (ESP); PM10 (with aerodynamic diameter ≤10 μm) was collected using a cyclone and a Dekati low-pressure impactor (DLPI). Before accessing the SCR, Hg in flue gas from both single coal combustion and co-firing mainly existed as Hg0; the higher content of Hg in sludge than coal led to the much higher Hg0 concentration for co-firing. The total Hg concentration at not only the SCR inlet and outlet but also the ESP inlet did not change obviously. However, Hgp concentration at the ESP inlet increased significantly, accompanied by a decrease in Hg0. The transformation of Hg0 to Hgp appeared to be more distinct for co-firing. The higher HCl concentration of co-firing derived from the much higher Cl content of sludge than coal, and together with the higher ash content of sludge containing more minerals capable of adsorbing Hg0, may lead to the greater transformation from Hg0 to Hg2+ and Hgp when co-firing. After the ESP disposal, nearly all Hgp was removed along with PM10, and most Hg0 was also removed. The removal efficiency of mercury after the ESP was 92.12% under coal firing and 92.83% under co-firing conditions, respectively. The slightly higher mercury removal efficiency under co-firing should be attributed to the complete removal of the higher concentration of Hgp.