Combustion trace elements during coal

The Fate of Some Trace Elements During Coal Pretreatment and Combustion... 

As a result of the well-documented environmental concerns posed by coal combustion, and the disposal of CCPs, a large body of research has focused on characterizing the mechanisms of mobilization and attenuation of trace elements in coal and its ash. Based on their reported distribution in the solid phases of both source coals and coal ash, knowledge of the thermal transformations that occur to major mineral constituents during coal combustion, and a limited number of studies that have identified discrete solid phases of trace elements, a conceptual model of the chemical and mineralogical characteristics of trace elements in coal ash has been developed. 

Pets, L., Vaganov, P. Rongsheng, Z. 1995. A comparative study of remobilization of trace elements during combustion of oil shale and coal at power plants. Oil Shale, 12, 129-138. 

Until recently, chemical analyses of coals were done on ash produced from the coal at relatively high temperatures. This was the standard approach for many years, and analyses of trace elements in coals do have a long history. An early article on an element as rare as cadmium in coal was published 125 yrs ago (28). One limitation of high-temperature ash sample is that volatile elements may be lost during combustion and will not be detected. Another problem which applies especially to analyses for trace and minor elements is that there have not been any coal standards available until very recently. 

Combustion of coal and other fossil fuels is a major source in the envi- ronment of trace elements that are hazards to human health. Toxic elements such as Hg, As, Sb, F, Se, and T1 are volatilized during coal combustion and are emitted directly into the atmosphere or concentrated in the fly ash (1, 2, 3). Most elements in coal occur at only parts per million levels, but large tonnages of coal are consumed each year in the United States. In addition, coal conversion processes, which could vastly increase coal use are now being considered seriously. The fate of trace elements during these processes is largely unknown. 

Nodelman, I.G. Pisupati, S.V. Miller, S.F. Scaroni, A.W. Partitioning behavior of trace elements during pilot-scale combustion of pulverized coal and coal-water slurry fuel. J. Hazard. Mater. 2000, 74, 47-59. 

FIGURE 22.10 Behavior of trace elements during combustion. (From Clarke, L.B., Management of byproducts from IGCC power generation, lEA Coal Research, IEACR/38, International Energy Agency, London, U.K.)... 

Group 3 elements which are not mostly vaporized in the boiler (1423 K) V, Cr, Mn, Co, Ni Referring to the classification, we investigated the temperature dependency of release of trace metals in coal combustion. We already reported the behavior of these three types of elements during high temperature coal processing and reported elsewhere . So in this paper, we investigated the effect of atmosphere for the emission behavior of trace elements. 

Mims, C.A. Neville, M. Quann, R.J. Sarofim, A.F. Laboratory Studies of Trace Element Transformations During Coal Combustion, presented at the 87th National AIChE Meeting, Boston, Mass., August 1979. 

As was previously mentioned, trace elements that sublime at temperatures below those attained during coal combustion (e.g., As, Se, Hg, Zn), and are associated with thermally unstable solid phases (in particular organic matter and sulphide minerals), are subject to vaporization into furnace gases. Once these gases, and fly ash particles entrained in the gases, are vented from the combustion furnace they quickly cool, leading to the condensation of volatilized elements onto the... 

Smith, R. D. 1980. The trace element chemistry of coal during combustion and the emissions from coal-fired plants. Progress in Energy and Combustion Science, 6, 53-119. 

Despite its availability and current use, coal is not as widely used today as the other fossil fuels. Coal s major weakness is that it does not burn cleanly. It often contains trace amounts of other elements, including mercury, arsenic, and sulfur, and when it burns, it releases these toxic substances into the air. Over time, coal pollution builds up in the environment. Mercury released during coal combustion, for example, settles in water and builds up in the bodies of fish and shellfish. When these fish and shellfish are eaten by humans and other animals, harmful amounts of mercury can be ingested. In 2008, bluefm tuna served in expensive New York restaurants was found to contain unacceptably high levels of mercury. These fish eat smaller organisms in the ocean, and when these small organisms contain mercury, the toxic element becomes concentrated in the body of the tuna. 

Thus it becomes apparent that, for the very high projected coal consumption in this region, the discharge of trace constituents is important, and potential environmental interaction and effects warrant immediate attention. There has been some attention given to radioactive elements in coal and their release during combustion (7, 8, 9). However, the stable element trace constituents are permanent pollutants of the environment, and their immediate and potential long-term effects should be investigated. 

Research indicates that a significant fraction (50-90% ) of mercury is volatilized and lost during coal combustion (10, 11, 12) and that many of the potentially hazardous trace elements appear concentrated upon finer particulate emissions (13, 14). Several investigators have observed enrichment of these hazardous elements upon particulates in urban areas... 

The composition of trace element emissions during coal combustion is described by Bolton et al. in Chapter 13. The actual quantities are somewhat variable depending upon the coal source, the combustion process, the pollution abatement equipment, and the assay itself. Much less is known about important local concentrations of emissions in and around the source, their chemical and physical characteristics, and their fate in the environment. 

As discussed above, coal contains many of the elements in the periodic table, at least in trace amounts and, during processing and/or combustion, these elements or their compounds can be released into the environment. Examples of harmful pollutants that can result from coal use are sulfur dioxide, sulfur trioxide, nitrogen oxides, particulates, hydrogen chloride, mercury vapor, and a wide... 

Utilization. For example, Be and the chalcophile elements, As, Cd, Hg, Pb, and Se, which are released during coal combustion or leached from coal waste products, can present significant environmental hazards halogens such as Cl and F can cause severe boiler corrosion and volatilized Ni, Ti, or V can cause corrosion and pitting of metal surfaces. On the positive side, some trace elements (e.g., Ge, Zn, U, and Au) may eventually prove to be economic by-products of coal utilization, while other elements (e.g., B) may be useful in helping to understand depositional environments and to correlate coal seams (7.8). 

The deposition and accumulation of fly ash downwind from coal-combustion sites is a concern because it may be significantly enriched in potentially toxic trace elements, including lead (Pb) and arsenic (As), compared to the burned coal (Coles et al, 1979 Eary et al, 1990 Hower et al, 1999 Kaakinen et al, 1975). Other elements such as zinc (Zn) and germanium (Ge), of less environmental concern, may also be enriched in fly ash. The relatively high concentrations of As in fly ash reflect partly its presence in pyrite in coal from the Appalachian Basin (Goldhaber et al, 2002). More importantly, the concentration of these metals and metalloids occurs during the combustion process itself. A suite of elements including As, Ca, Cr, Cu, Ga, Mo, Ni, Pb, Sb, Se, V, and Zn is enriched in the fine fraction of coal fly ash (Coles et al, 1979), because of vaporization in the furnace and subsequent condensation or absorption onto ash particles (Kaakinen et al, 1975). 

Two additional lines of evidence link trends in the MTR data to coal combustion. The first is a comparison of temporal trends in IRM, Zn, and S with SO2 production in the US. This comparison is shown in Fig. 14. By far the dominant source of SO2 is fuel combustion (EPA, 2000). For the period from 1930 to about 1980, there is a close correspondence between SO2 production/combustion and reservoir geochemistry/mineralogy. In particular, SO2 production maxima during World War II (1945) and the period prior to the Clean Air Act of 1970 are mirrored by peaks in reservoir S and IRM. The correspondence between content of sedimentary trace elements and magnetite related to coal combustion with the source function for these constituents is consistent with a relation between the source and sink. 

The fly ash particles can be a potential source of contamination because of the high concentration and surface associations of some trace elements in their composition. Coal contains various trace elements originating from different minerals and macerals in its body (Pavlish et al., 2003 Xu et al., 2003 Balat, 2008b). These elements show different behavior during coal combustion (Figure 22.10) (Suarez-Ruiz and Ward, 2008). 

Major and trace elements in particulate matter can be classified as natural (Na, Mg, K, Ca, Si, Al, Mn, etc.) or anthropogenic (V, Cr, Mn, Ni, Cu, Zn, Cd, Pb, etc.). The principal anthropogenic emission sources are attributed to fossil fuels Cr, Mn and Sb are good markers for this source as they are present in coal, while V, Ni and Pb are emitted by fuel oil combustion. Industrial processes and non-ferrous mineral extraction are important sources for Cd, Zn, Cu and Hg, while elements like Ni, Zn, Pb and Cu are emitted during industrial processing of iron, cast iron and steel. 

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. 

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