Mercury in Coal
USGS research provides information on the amount and forms of mercury and other elements present in U.S. Coals (Tewalt et al., 2001). Understanding the distribution of mercury in coal is especially important in light of recent U.S. EPA rulings, outlining plans to limit mercury emissions from U.S. coal-fired utilities. Using selective chemical leaching (Figure 1), laser-ablation ICP-MS (Figure 2), and other approaches, USGS results show that pyrite is the primary host of mercury in bituminous coals, whereas the proportion of mercury present in organic parts of coal is generally greater in low-rank (lignite and sub-bituminous) coal. Many Eastern U.S. bituminous coals are “cleaned” prior to use in utility power stations, to reduce sulfur emissions. This coal preparation reduces sulfur content, primarily by removing pyrite from coal. In doing so, a portion of the mercury present may also be removed, as a co-benefit to sulfur reduction. Currently about 35% of the mercury that goes into coal-fired utility power stations in the U.S. is captured by air pollution control devices, whose primary function is to trap particulates from coal combustion, or sulfur emitted from flue gasses. Under the EPA Clean Air Interstate Rule (CAIR), the initial phase of mercury reductions from coal-fired utility power station will primarily result as a co-benefit of reducing other pollutants with existing control technology. Under the second phase of the plan, the EPA Clean Air Mercury Rule (CAMR), greater reductions of mercury emissions are specified, probably requiring new mercury-specific control technology for utilities. By understanding the mode of occurrence of mercury in coal, and the concentration of constituents that affect capture of mercury, such as chlorine, the USGS provides information needed to help predict and control mercury emissions.
Figure 1. Selective leaching results for 15 coal samples (12 from the U.S.). Yellow bars indicate the proportion of mercury leached with nitric acid, thought to be contained in sulfide minerals, such as pyrite. The fraction of mercury not leached (100 percent minus total bar height) represents organic-associated mercury not removed by the leaching process. Arrows indicate minimum values. Figure is from Palmer and others, 1998.
Figure 2. Plot of mercury in pyrite plotted versus arsenic content (log scale) based on reconnaissance laser ablation ICP-MS analysis of 6 U.S. eastern bituminous coal samples. Data points represent individual analysis points ranging in size from 10 to 50 micrometers. Results for pyrite grains analyzed indicate mercury concentrations that are 10’s to 100’s of times higher than mercury concentrations in the whole coal. Samples include an Ohio 5/6/7 blend (Ohio), Illinois #6 (Illinois), Pittsburgh Coal from West Virginia (Pitts), the Warrior Basin of Alabama (AL), the Upper Coalburg Coal from West Virginia (WV), and an eastern Kentucky coal (KY). Data are from Diehl and others, 2005 and Kolker and others, 2002.
References
Diehl, S.F., Goldhaber, M.B., Tuttle, M.L.W., Ruppert, L.F., Hatch, J.R., Koenig, A.E., and Lowers, H.A., 2005, Distribution of arsenic, selenium, and other trace elements in pyrite-filled structures in Appalachian coals of Alabama, Kentucky, and West Virginia: Proceedings of the 22nd International Pittsburgh Coal Conference, Pittsburgh, PA, September, 2005, 24 p., CD-ROM.
Kolker, Allan, Mroczkowski, S. J., Palmer, C. A., Dennen, K. O., Finkelman, R. B., and Bullock, Jr., J. H., 2002, Toxic substances from coal combustion- A comprehensive assessment, Phase II: Element modes of occurrence for the Ohio 5/6/7, Wyodak, and North Dakota coal samples: U.S. Geological Survey, Open File Report 02-224, 79 p.
Palmer, C. A., Mroczkowski, S. J., Finkelman, R. B., and Crowley, S. S., 1998, The use of sequential leaching to quantify the modes of occurrence of elements in coals: Proceedings of the Fifteenth Annual International Pittsburgh Coal Conference, 28 p., CD-ROM.
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