Mercury Speciation Measurements in the Fluegas and Plume of Two Maryland Combustion Sources

In this study, the Maryland Power Plant Research Program used a state-of-the-art, in-stack mercury (Hg) sampling technique (a static plume dilution chamber, SPDC) to investigate the influence that two known Hg sources—coal-fired power plants and municipal incinerators—have on mercury levels in the environment. The study examines the physical and chemical transformations of Hg in the plumes of the two types of combustion sources to determine the species and volumes of Hg that are being released from combustion source plumes. Two co-located combustion sources in Montgomery County, Maryland were sampled for this project: a coal-fired generating unit at the Dickerson Generating Station and waste-to-energy incinerator unit at the Montgomery County Resource Recovery Facility (RRF). The SPDC was used to simulate a combustion plume under controlled field conditions to measure particulate Hg, gas phase Hg ⁰ , Hg(II), and total Hg; dry deposited Hg; and dissolved and particulate phase Hg in simulated rainwater (SRW). The overall mass balance at both sources was within acceptable limits for detailed interpretation; however, the limited number of SPDC runs make statistically supported conclusions impossible; instead major trends and differences in SPDC variables are discussed.

At the RRF, unexpectedly, there was a significant fraction of Hg(II) in the fluegas, instead of a predominance of Hg ⁰ . In all SPDC runs at the RRF, there was more reactive Hg (RHg) in the SPDC than reactive gaseous Hg (RGHg) injected, and there was less Hg ⁰ measured than Hg ⁰ injected. The trend in the RRF plume was conversion of Hg ⁰ to Hg(II). The RGHg injected into the SPDC, or formed during aging, was quickly deposited to walls or washed out in the SRW. SRW washout indicates most Hg is removed into the dissolved phase, not as particulate Hg, suggesting efficient washout of gaseous Hg, not particulate Hg. At Dickerson, there was less RHg observed than RGHg injected based on fluegas speciation; there was also much more Hg ⁰ measured than Hg ⁰ injected. These results may be partially explained by a bias in the measurement of fluegas speciation; however, it is also likely that there is some conversion of Hg(II) to Hg ⁰ in the SPDC. Gas-phase Hg washout dominated; however, removal of Hg by simulated rainwater was less effective than expected, as there was a significant amount of dry deposited Hg after the SRW input.