Mercury in coal

S. E. Long and W. R. Kelly, Determination of Mercury in Coal by Isotope Dilution Cold-Vapor Generation Inductively Coupled Plasma Mass Spectrometry, Anal. Chem. 2002, 74, 1477. 

Procedures for the determination of 11 elements in coal—Sb, As, Br, Cd, Cs, Ga, Hg, Rb, Se, U, and Zn—by neutron activation analysis with radiochemical separation are summarized. Separation techniques include direct combustion, distillation, precipitation, ion exchange, and solvent extraction. The evaluation of the radiochemical neutron activation analysis for the determination of mercury in coal used by the Bureau of Mines in its mercury round-robin program is discussed. Neutron activation analysis has played an important role in recent programs to evaluate and test analysis methods and to develop standards for trace elements in coal carried out by the National Bureau of Standards and the Environmental Protection Agency. 

In November 1971, NBS issued Standard Reference Material (SRM) 1630, Mercury in Coal, with a provisionally certified mercury content of 0.13 ppm. Later the provisional value of 2.1 ppm selenium in SRM 1630 was issued. 

The overall mercury recovery in the process is 67 15%. The precision of the method is about 20%, and the detection limit is about 0.01 ppm mercury for a 1-g sample of coal. Reliability of the method was determined by the accurate analysis of two coal samples used in the Bureau of Mines study of the problems involved in determining mercury in coal (9) and then by the agreement within experimental error of the results from the 11 Bureau of Mines round-robin coal samples and their probable mercury contents (4). 

Weaver and von Lehmden (20), under sponsorship of the EPA, evaluated two instrumental NAA methods and one with radiochemical separation for determining mercury in coal. 

At NBS, the neutron activation with combustion separation method used for determining mercury in coal (12) was further investigated for determining selenium by Rook (13). The same procedure is used except that the sample is heated finally to 1000°C. Mercuric oxide is also used as carrier for the selenium because selenium oxides are difficult to dissolve in mineral acids. The mercuric selenide formed carries the selenium effectively, and, as it is soluble in nitric acid, the dissolution procedure developed for the mercury separation can be used so that mercury and selenium can be determined in the same sample. 

The determination of mercury in coal produced an interference correction problem which was quite complex. We found that counting an... 

Table III. Mercury in Coal from the Upper Freeport Coalbed, Garrett County, Md.

After the firing was completed, the permanganate solution was analyzed by the same procedure used for the volatile mercury samples. Mercury in coal samples from the National Bureau of Standards and the United States Bureau of Mines were analyzed simultaneously as controls. At least duplicate analyses were performed on all samples. Empty crucibles were fired into acidic permanganate solutions by an identical procedure to obtain solutions for blank determinations. 

Solids Sampling and Analysis. The coal consumption data is compiled in Table II. The ash, BTU, and sulfur analyses on the coal consumed during the 1-wk test period is presented in Table III. The data from the mercury in coal determinations are given in Table IV. Based on the authors experience with mercury in coal studies, the resulting concentrations are average for eastern United States coals. The author has found the spread in the individual coal determinations to be common when analyzing 0.5-g samples. The data in Tables III and IV show a relation-... 

Kalb, G. W., The Determination of Mercury in Coal by Flameless Atomic... 

Table IV gives all of the values obtained for mercury in coal, which range from 0.057 to 0.198 ppm with most values in the range of 0.07 ppm. Our attempt at a mercury balance for runs 5 and 9 is shown in Table V. From these numbers it is clear that very little mercury ( — 12% ) remains with the slag and fly ash particles. The cold trap was not effective in trapping mercury vapor (.—11% ). The results are in qualitative agreement with those of Billings and Matson (5), except that these authors were able to collect the mercury in the gas phase. Their data shows that most of the mercury is in the gas phase. This can also be implied from our results.

Table IV. Mercury in Coal as Determined by Atomic Absorption...

ASTM D-6414. Standard Test Method for Total Mercury in Coal and Coal Combustion Residues by Acid Extraction or Wet Oxidation/Cold Vapor Atomic Absorption. 

Mercury has been identified as a very dangerous environmental contaminant, largely by reason of the process of concentration in the food chain. Thus, the presence of mercury in coal is an extremely sensitive issue. The possible emission of mercury that may be found in coal is an environmental concern. 

Another test method for the determination of mercury in coal (ASTM D-6414) involves (method A) solubilizing of the mercury in the sample by heating the sample at a specified temperature in a mixture of nitric and hydrochloric acids. The acid solutions produced are transferred into a vessel in which the mercury is reduced to elemental mercury. The mercury vapor is determined by flameless cold-vapor atomic absorption spectroscopy. An alternative method (method B) involved solubilization of the mercury by heating the sample in a mixture of nitric acid and sulfuric acid with vanadium pentoxide. The acid solution is then transferred into a vessel in which the mercury is reduced to elemental mercury. The mercury content is determined by flameless cold-vapor atomic absorption spectroscopy. However, mercury and mercury salts can be volatilized at low temperatures, and precautions against inadvertent mercury loss should be taken when using this method. 

The determination of mercury in coal, and in coal combustion residues, can also be accomplished by controlled heating of the sample in oxygen (ASTM... 

Long, S.E., Kelly, W.R. Determination of mercury in coal by isotope dilution cold-vapor generation inductively coupled plasma mass spectrometry. Anal. Chem. 74, 1477-1483 (2002)... 

Horvat M, Lupsina V. 1991. Determination of total mercury in coal fly ash by gold amalgamation cold vapour atomic-absorption spectrometry. Anal Chim Acta 243(l) 71-79. 

K.C. Galbreath, C.J. Zygarlicke. Environ Sci Technol 1996 30 2421 [mercury in coal combustion gases]. 

NBS-mercury in coal Used for recovery studies C-Hg bond... 

Mercury concentrations were highest in lignite coal (0.12mg/kg DW), lowest in sub-bituminous coal (0.03 mg/kg DW) and intermediate (0.07 mg/kg DW) in bituminous coal samples measured. More recent information indicates that coal contains, on an average 0.2 mg Hg/kg and may contain as much as 1.0 mg/kg. Most of the mercury in coal is associated with arsenic-bearing pyrite other forms include organically bound mercurials, elemental mercury, and mercuric sulfides and selenides. In coal samples with low pyrite, mercury selenides may be the primary form. 

Kolker, A., Senior, C.L., and Quick, J.C. 2006. Mercury in coal and the impact of coal quality on mercury emissions from combustion systems. Applied Geochemistry, 21 1821-1836. 

Yang, H., Xua, Z., Fan, M., Bland, A.E., and Judkins, R.R. 2007. Adsorbents for capturing mercury in coal-fired boiler fine gas. Journal of Hazardous Materials, 146 1-11. 

Additional analysis is necessary when one considers energy-related emissions. First, a description of the energy-related emission factors will be presented. Assuming that coal is burned for electricity generation [75], and using an EPA report [76] for the amount of mercury in coal and emitted after pollution controls (assuming 90% capture), the controlled emission rate is 5.58 x 10 kg mercury/kWh. For the benzene, toluene, formaldehyde, and hexane emissions from coal-fired electricity generation, EPA and... 

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... 

Mercury concentration in coal varies fi om mine to mine but usually is <1.0 mg/kg [43]. During coal combustion mercury can suffer physical and chemical transformations [44]. During this process, mercury in the coal is transformed into three species (i) particle-bound mercuiy (Hg-p) (ii) vapour-phase elemental mercuiy (Hg ), and (iii) vapour-oxidized mercury (Hg ), primarily as HgCl2 (Figure 1). Mercury in coal begins to volatilize at temperatures below 200 C almost regardless of the mode of occun ence of mercuiy in the coal. At temperatures above 600-700°C, Hg is the only stable form [44-45]. At lower temperatures (350-450 C) in the flue gas duct, with the presence of chloride, sulphur and calcium, part of the Hg vapour is oxidized into Hg [43]. 

Gale TK, Lani BW, Offen GR. Mechanisms governing the fate of mercury in coal-fired power systems. Fuel Process Technol, 2008, 89, 139-51. 

Meij R, Vredenbregt LHJ, te Winkel H. The fate of mercury in coal-fired power plants. J Air Waste Manage Assoc 2002, 52, 912-7. 

Acid rain is not the only environmental contaminant fi-om coal burning. The average concentration of mercury in coal is about 0.3 pg/g (i.e., less than 1 ppm). A 755-MW steam turbine-driven power station bums approximately 7,100 t of coal per day. This corresponds to about 2.5 kg/day of mercury being sent up the stack. With present North America estimates of coal consumption at about lO t/year, about 3,0001 of mercury is put into the environment. This is about four times the natural source. ... 

Rao, P.S. 2010. Advanced adsorbents for warm gas capture of mercury in coal gasification. University of Cincinnati. 

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