PPRP Studies Mercury in Power Plant Emissions
Background
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| PPRP project team implements flue gas sampling. Stack sampling probes are shown in the foreground, with the plume simulation chamber behind. |
Mercury is a naturally occurring element that is found in air, water and soil. Human and wildlife populations may be exposed through inhalation, ingestion and absorption. The toxic effect of mercury has been known since ancient times, and there has been extensive research on exposure and biological activity. Mercury is known to damage the human nervous system: the notion of being "as mad as a hatter" derives from the use in Victorian times of mercury to shine top hats made from beaver pelts.
Mercury occurs in a number of forms in the environment – metallic (chemical symbol Hg(0)), oxidized forms (chemical symbol Hg(1) or Hg(2), depending on the degree of oxidation), and as part of organic compounds (CH
3HgCl – monomethyl mercury, combined with chloride, is the most common organic form). Monomethyl mercury is of special concern as it has been shown to cause birth defects in humans at high concentrations, while lower doses have been linked to poor motor skill development and brain dysfunction in children exposed in utero. Because many industrial processes use or emit mercury, and because mercury’s biochemical behavior and its interactions with other materials are quite complex, there is a growing concern about the ambient levels of mercury throughout the environment.EPA recognized the concern over mercury and addressed it in Title III of the 1990 CAA Amendments, Hazardous Air Pollutants (HAPs). Title III called for an extensive review of the role of mercury. EPA’s ensuing document, Mercury Study Report to Congress, was published in eight volumes in December 1997. Along with the mercury report, EPA published in February 1998 a comprehensive assessment of the toxic emissions from fossil fueled electric power generation facilities. This study found that a large fraction of the mercury in steam generating units is emitted via the stack into the atmosphere. While the levels in the flue gas are low, the very large volumes of flue gas emitted make coal-fired power plants the single largest source category of anthropogenic (man-made) mercury.
There are currently no regulatory limits on the amount of mercury a power plant can emit. However, EPA has initiated a program that will require all coal-fired plants to analyze the mercury level in the fuel. The agency is also selecting a group of coal-fired plants that must quantify the different forms of mercury in their flue gas. This will provide valuable information on emissions, which is necessary for crafting control regulations.
Recent PPRP Activities
PPRP has been proactive in developing an assessment of mercury emissions from generation facilities in the state. Using information from EPA’s utility report, it is possible to accurately estimate stack emissions of total mercury based on the mercury content of the fuel burned. However, for environmental assessments of fate and effects it is important to know the form of the mercury that is being emitted – metallic, oxidized or organic. Different elemental forms will give rise to either more localized or more distant impacts. In 1994, PPRP published a study assessing this local versus regional impact. Mercury concentrations in fish from a number of ponds were analyzed as a function of distance from a power plant source in order to identify a concentration gradient that could be associated with the point source. No such gradient was observed within the experimental accuracy, suggesting that there was no local effect due to the power plant.
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Figure 4. Mercury species behavior in combustion plumes (average of several simulations) |
In the spring of 1997, PPRP conducted a research project analyzing stack emissions from two different generation facilities: a state-of-the-art resource recovery facility that burns municipal waste, and a typical pulverized coal boiler. The objectives of the study were to assess the levels of mercury species released from the facilities, and most importantly, to investigate the changes in the mercury chemistry in the plume after release into the atmosphere.
Because of the difficulties associated with direct plume sampling, the study simulated a plume using a large reaction vessel. A slip stream of flue gas was diverted from the stack into a TeflonŽ-lined chamber where it was diluted with ambient air that had been scrubbed clean of mercury. A sample from the chamber was analyzed at time zero – representing release from the stack – and then at some time later to represent the plume travel time. To mimic environmental conditions, the contents of the tank could be irradiated with simulated sunshine or kept in the dark, exposed to simulated rain or kept dry. In addition to measuring the split between Hg
0 and Hg2+, the experiment also investigated the role that aerosol particles play as a vehicle for transporting the various forms of mercury.Figure 4 shows the change in speciation from time zero in the flue gas to a plume about five miles downwind. Results for the two facilities show a preponderance of Hg
2+ and Hg22+ at the source (these two species are grouped together and designated as Hg(II)). The municipal waste combustor, with a more diverse fuel supply, showed greater variability in level of total mercury. Emissions from both facilities were on the order of 10 micrograms per cubic meter, with about two-thirds in the Hg(II) form and one-third in the Hg(0) form. Only trace amounts of particle bound mercury were observed.After the plume was "aged" in the reaction chamber to simulate about 45 minutes of plume travel, the distribution had changed with Hg(0) now comprising more than half of the total mercury (see Figure 4). Interestingly, the concentrations of more water soluble Hg(II) were generally higher in the aged plume from the waste combustor than from the coal plant. This may be a consequence of the higher chloride levels in a typical incinerator flue gas. Quantifying the form(s) of the Hg(II) in the atmosphere – so-called reactive gaseous mercury – remains an area of active research.
Future work planned at PPRP will study mercury scavenging from plumes during rain events to validate the plume simulation experiments.
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