Trace element combustion

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. 

L.B.Clarke and L. L.Sloss, Trace Elements-emission from Coal Combustion and Gasification, IEACR/49, lEA Coal Resereh (1992)... 

Decomposition methods are usually classified as melt decompositions, wet decompositions (with liquid decomposing agents) and dry decompositions by combustion. Sample decomposition methods are varied, and involve open and closed systems (at low and high pressure), UV and thermal activation, low or high temperature, and use of conventional convective or microwave heating. Table 8.4 lists the main sample decomposition methods for trace-element determination. 

Clarke L.B., Sloss L.L. Trace Elements - emissions from coal combustion and gasification. London IEA Coal Research, 1992. 

Fine particles from coal combustion are a threat to human health and air quality. Numerous studies have reported higher concentrations of volatile trace elements in small particles... 

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. 

In a number of studies, the elemental composition of aerosol particles has been related to their source. Schroeder and co-workers (1987) review the sources, sinks, analysis, deposition, chemical forms, and global cycles of various trace elements. Because the relative amounts of trace elements vary for coal compared to oil-fueled power plants, for example, it has been suggested that certain elements or ratios of elements may serve as tracers for various sources (e.g., see Gordon, 1988 and Rahn and Lowenthal, 1984). Thus V and Ni are indicative of oil combustion, and elevated concentrations of elements such as As and Se are usually... 

Donahoe, R. J. (2004). Secondary Mineral Formation in Coal Combustion Byproduct Disposal Facilities Implications for Trace Element Sequestration. In Giere, R. Stille, P. (eds) Energy, Waste, and the Environment a Geochemical Perspective. Geological Society, London, Special Publications, 236, 641-658. 

The concern is that the combustion process mobilizes the major/minor and trace element concentrations of combustion product and that fresh water (streams, lakes) as well as ground-water may become contaminated by leachate. In general, sites for ash disposal are physically close to the combustion site. Thus, the location is chosen primarily on the basis of economics, and the disposal sites exhibit a wide range of local hydrological regimes and are, of course, subject to prevailing climatic conditions. 

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. 

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. 

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. 

Overall, the Kentucky and Purdue studies show similar trends. The most important change resulting from the addition of TDF to coal is the increase in the Zn content of the fly ash. In contrast, most other trace elements have lower concentrations in the fly ash resulting from the combustion of the blend. Despite many similarities, two notable differences between the two test bums are observed. S03 and Pb showed opposite trends in the Purdue fly ash compared to the Kentucky fly ash. In both studies, enrichments of many trace elements... 

In Eggborough power plant it was found that Zn, Pb, Mo, Cu and As are concentrated in the fly ash compared with the bottom ash, whereas for the other trace elements studied (Ba, Cr, Nb, Rb, Sr, V, Y, Zr) there was very little fractionation (Martinez-Tarazona Spears 1996, tables 3 and 4). Mass balance calculations show that for the elements studied only S and As are depleted in the combustion ashes and all the other elements analysed appear to have been retained. Not included within the analyses were Hg, Cl, and F, and loss of these elements would be anticipated based on their volatility (Sloss Davidson 2001). 

Unburnt carbon is another minor component of fly ash. Ideally its content should be less than 1 wt%, but concentrations of 3-4 wt% or higher may be recorded if the combustion efficiency falls. Any trace elements contained within the unburnt carbon are likely to have condensed from the gas stream rather than inherited from the original organic matter. Loss of Hg from the gas stream due to unbumt... 

As discussed in the previous section, trace elements are essentially retained in the solid combustion products and, because many are present on the surfaces of the particles, they are potentially leachable. Our data show the elements Mo, As, Cu, Zn, Pb, U, Tl, and Se will be readily accessible for leaching. A significant fraction of the V, Cr, and Ni, and a minor proportion of the Ba and Sr will also be potentially leachable because of the surface association, but most of these elements appear to be located in particles and will be released more slowly as the dissolution of the glass and other phases takes place. Rubidium, Y, Zr, Mn, and Nb are contained almost entirely within the particles and dissolution is potentially slower. The extent to which elements are leached also depends on their speciation and solubility in the porewaters, and the pH exerts a major control. In oxidizing solutions, elements such as, Cd, Cu, Mn, Ni, Pb, and Zn form hydrated cations that adsorb onto mineral surfaces at higher pH values and desorb at lower pH values. In contrast, the elements As, U, Mo, Se, and V, under similar Eh conditions, form oxyanions that adsorb onto mineral surfaces at low pH values and desorb at higher values (Jones 1995). 

CLARKE, L. B. Sloss, L. L. 1992. Trace Elements -Emissions from Coal Combustion and Gasification. IEA Coal Research, London, 111 pp. 

Martinez-Tarazona, M. R. Spears, D. A. 1996. The fate of trace elements and bulk minerals in pulverised coal combustion in a power station. Fuel Processing Technology, 47, 79-92. 

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