Coal mercury

Solely on the basis of volatility profiles, fossil fuel burning is expected preferentially to transfer As, Hg, Cd, Sn, Sb, Pb, Zn, Tl, Ag, and Bi to the atmosphere (1). In a study designed to detect fallout from a major coal burner equipped with a precipitator, Klein and Russell (27) showed that Ag, Cd, Co, Cr, Fe, Hg, Ni, Ti, and Zn were deposited in the surrounding soil (115 sq mi), and with the exception of mercury, enrichment correlated with the respective metal concentrations in the coal. Mercury was more widely disseminated to the environment. Previous work has indicated that mercury exists primarily in the volatile phase of the flue gas and consequently as much as 90% bypasses the electrostatic precipitation control device (2). Bolton and co-workers have evidence that selenium and arsenic may present a similar problem (see Chapter 13). 

The contents of some trace elements in the continental crust, shales, soils, bituminous coals and plankton are given in Table 1.1 to provide some perspective when considering other aspects of these elements. In each of these situations, organic matter is associated with the elements to a greater or a lesser degree. This is not usually very marked with crustal rocks except shales, but may be a major factor for some elements in surface soils and coals. The data in Table 1.1 show that, for some elements, e.g. beryllium, cadmium, cobalt and molybdenum, the contents of the various reservoirs are similar, while for others, there may be enrichments relative to the crust, e.g. boron and sulfur in many shales, soils and coals, mercury, nickel and selenium in many shales, and germanium in some coals. 

Speciation of mercury in CFPP flue gases is of interest because the molecular form of mercury influences the ability of air quality control devices to remove mercury from flue gas streams. The relative amounts of each mercury species strongly depends on the concentration of mercury in coal and conditions during combustion such as gas residence time, temperature, and gas composition [3]. For bituminous coal, mercury concentrations can be less than 0.01 ppmv up to 3.3 ppmv [4]. In the combustion zone (1200-1400°C), mercury is vaporized from the coal and exists as elemental mercury (Hg ). As the flue gas temperature decreases. Kg is partially oxidized to form Hg2+ and partitions between gas, liquid or solid phases... 

If present in the used coal, mercury and arsenic may require special attention. They can be accumulated in certain streams or locations depending on the employed process steps, and parts of them may even be emitted. 

The high temperatures in the MHD combustion system mean that no complex organic compounds should be present in the combustion products. Gas chromatograph/mass spectrometer analysis of radiant furnace slag and ESP/baghouse composite, down to the part per biUion level, confirms this behef (53). With respect to inorganic priority pollutants, except for mercury, concentrations in MHD-derived fly-ash are expected to be lower than from conventional coal-fired plants. More complete discussion of this topic can be found in References 53 and 63. 

Hazardous air pollutants (HAPs) are substances that may cause immediate or long-term adverse effects on human health. HAPs can be gases, particulates, trace metals such as mercui y, and vapors such as benzene. For coal-fired power plants, the HAPs of most concern are metals such as mercury, arsenic, and vanadium. 

Although the petrochemical and metals industries were the primai y focus of the toxic air pollutants legislation, approximately forty of these substances have been detected in fossil power plant flue gas. Mercury, which is found in trace amounts in fossil fuels such as coal and oil, is liberated during the combustion process and these emissions may be regulated in the future. EPA issued an Information Collection Request (ICR) that required all coal-fired plants to analyze their feed coal for mercury and chlorine. Since these data will be used in making a regulatory decision on mercury near the end ot the year 2000, it is critical that the power industry provide the most accurate data possible. 

Reductions in U.S. mercuiy emissions from medical and municipal waste incinerators and other industrial sectors have already occurred. Additional emission reductions from some coal-fired power plants have also already begun as co-benefits from technologies used to control SO2 and NO emissions. These mercury emissions from power plants are, however, expected to be reduced further over the next few decades. Meanwhile, changes in mercuiy emissions in other parts of the world may also affect some U.S. ecosystems. 

Gonska H, Griepink B, Colombo A, Muntau H (1984) The Certification of the Contents of Arsenic, Cadmium, Chromium, Cobalt, Fluorine, Manganese, Mercury, Nickel, Lead and Zinc in a Coal BCR No. 40. European Commission Report EUR 9473 EN, Community Bureau of Reference, Brussels,... 

US Dept, oflnterior, US Geol. Survey. Mercury in U.S. coal - Abundance, distribution, and modes of occurrence, http //pubs.usgs.gov/factsheet/fs095-01/fs095-01html (Verified 28 Aug. 2002), 2002. 

Small amounts of some contaminants can be serious. When we burn coal, not only do we produce carbon dioxide, the major product, but we can liberate small amounts of mercury and larger amounts of sulfur dioxide. The mercury can form toxins that harm fish as well as humans, while sulfur dioxide can produce acid rain that destroys forests and water supplies. How can these contaminants be most effectively removed or dealt with And how can we generate the energy that we need without releasing such by-products ... 

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