Mercury in flue gas

Current methods for sampling and analysis are known to be problematic and prone to error. Several wet chemical impinger-based methods have been approved for the determination of total Hg in flue gas. These include US EPA method 29 and lOlA. More recently, diese methods have been modified to enable the speciation of particulate, oxidized, and elemental mercury. The most commonly used procedure is the ASTM Standard Test Method for Elemental, Oxidized, Particle-Bound and Total Mercury in Flue Gas Generated from Coal-Fired Stationary Sources (D 6784-02) known as the Ontario Hydro Method. This method was developed by public consultation with ASTM membership. 

Mercury in Flue Gas Desulfurization Gypsum FROM Coal Combustion Processes... 

The fraction of oxidized mercuiy in the stack effluent of a particular power plant depends on the coal type, combustion efficiency and the pollution control equipment used [8]. The specific form of mercury in flue gas has a strong impact on the capture of mercury by air pollution control equipment. 

Minor uses of vanadium chemicals are preparation of vanadium metal from refined pentoxide or vanadium tetrachloride Hquid-phase organic oxidation reactions, eg, production of aniline black dyes for textile use and printing inks color modifiers in mercury-vapor lamps vanadyl fatty acids as driers in paints and varnish and ammonium or sodium vanadates as corrosion inhibitors in flue-gas scmbbers. 

Mercury is one of a number of toxic heavy metals that occur in trace amounts in fossil fuels, particularly coal, and are also present in waste materials. During the combustion of fuels or wastes in power plants and utility boilers, these metals can be released to the atmosphere unless remedial action is taken. Emissions from municipal waste incinerators can substantially add to the environmental audit of heavy metals, since domestic and industrial waste often contains many sources of heavy metals. Mercury vapor is particularly difficult to capture from combustion gas streams due to its volatility. Some processes under study for the removal of mercury from flue gas streams are based upon the injection of finely ground activated carbon. The efficiency of mercury sorption depends upon the mercury speciation and the gas temperature. The capture of elemental mercury can be enhanced by impregnating the activated carbon with sulfur, with the formation of less volatile mercuric sulfide [37] this technique has been applied to the removal of mercury from natural gas streams. One of the principal difficulties in removing Hg from flue gas streams is that the extent of adsorption is very low at the temperatures typically encountered, and it is often impractical to consider cooling these large volumes of gas. 

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... 

Figure 1. Dependence of C/Hg ratio on inlet mercury concentration in flue gas.

Mercury is mainly identified in three distinct forms in flue gas of coal power plant such as (Hg ), (Hg ) and (Hg ) (Pavlish et al. 2003). The Hg and Hgp are soluble in water and hence they are deposited on a local and regional scale, whereas H is present in vapour phase what enables its transportation worldwide (Rao 2010). As it settles in the water bodies, it is converted to a highly toxic compound called Methylmercury (MeHg). MeHg bio accumulates in the bodies of aquatic... 

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. 

The C02 stream obtained from oxy-fuel combustion shows high levels of water vapour, sulphur compounds, N 02 and impurities such as mercury in the flue gas. NOx emission is low when compared with air combustion. 

MEDISORBON An adsorptive process for removing mercury and dioxins from flue-gas. The adsorbent is a dealuminated zeolite Y manufactured by Degussa. For mercury removal, the zeolite is impregnated with sulfur. Developed in 1994 by Lurgi Energie und Umwelt and piloted in Germany and The Netherlands. 

Recently, we returned to the Allen plant and sampled the flue gas using a four impinger train with a pre-scrubber of sodium carbonate to remove the acid gases, followed by three impingers charged with iodine monochloride solution. Preliminary results show that mercury quantities detected were of the expected magnitude based on mercury concentrations in the coal which we had measured previously. This technique will be used for the mercury balance at the next in-plant sampling. 

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 most relevant problem originating from MSW incineration is flue gas treatment, since untreated incineration flue gas can contain large amounts of macropollutants (e.g., CO, S02, HC1, NOx, particulates) andmicropollutants (e.g., PAHs, mercury compounds, polychlorinated biphenyls, dioxins, furans). The necessity of reducing polluting emissions to levels compatible with existing regulations dictates the adoption of rather sophisticated- and expensive flue gas treatment sections in incineration plants. 

In addition to this, another, often overlooked issue is the disposal of solid byproducts of the incineration process. MSW incinerators essentially produce two types of solid by-products. The first is the slag, or bottom ash, which is mostly made of the noncombustible fractions of the waste, plus a small fraction of the combustible fraction. The second is fly ash, the particulate material captured in the particulate removal section of the flue gas treatment plant, which is often mixed with various other chemicals used for flue gas treatment. Bottom ash and fly ash are characterized by very high concentrations of PTE, such as lead, mercury, cadmium, and nickel, which can easily leach into the environment. 

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