Coal major elements

Fourier transform infrared (FTIR) spectroscopy of coal low-temperature ashes was applied to the determination of coal mineralogy and the prediction of ash properties during coal combustion. Analytical methods commonly applied to the mineralogy of coal are critically surveyed. Conventional least-squares analysis of spectra was used to determine coal mineralogy on the basis of forty-two reference mineral spectra. The method described showed several limitations. However, partial least-squares and principal component regression calibrations with the FTIR data permitted prediction of all eight ASTM ash fusion temperatures to within 50 to 78 F and four major elemental oxide concentrations to within 0.74 to 1.79 wt % of the ASTM ash (standard errors of prediction). Factor analysis based methods offer considerable potential in mineral-ogical and ash property applications. 

The primary component of coal is carbonaceous material resulting from the accumulation and decay of plant matter in marine or freshwater environments and marshes (Hessley et al. 1986). As plant matter accumulates it becomes humified and may eventually be consolidated into coal through a process called coalification. In the organic matrix, C is the major element by weight, with smaller amounts of H, O, N, and S, and many trace elements. The abundance of these trace elements is highly variable, but based on the reported trends in the affinity of elements for the organic fraction of coal (Table 1), elements such as B, Ge, Be, Ti, and V are expected to exist primarily within the organics in coal. 

The chemical composition of CCPs varies with coal origin and rank however, the major elemental constituents of all coal ash residues are O, Si, Al, Fe, and Ca, along with lesser amounts of Mg, S, and C. The relative abundance of constituents that typically make up more than 1 % of the total mass of fly ash and bottom ash are summarized in Table 4. These elements are found in the ash because of their lower volatility and the short time the particles actually remain in the furnace during combustion (Helmuth 1987). Both crystalline and non-crystalline compounds form on the surface of fly ash particles when elements react with oxygen in the flue gases, and through... 

Inasmuch as mineral matter has been defined broadly to include all inorganic elements in coals, the chemical characterization of mineral matter involves the determination of many elements. In general, chemical analyses of geological materials have progressed from the wet chemical methods to sophisticated instrumental methods. The major elements in the mineral constituents of coal, Si, Al, Ti, Ca, Mg, Fe, P, S, Na, K, are the same as those in silicate rocks and are often determined by x-ray fluorescence spectroscopy and flame photometry. 

The introduction of atomic absorption spectroscopy has resulted in major advances in the rapid analysis of many elements. Initially, atomic absorption was applied only to aqueous systems or to materials that could be readily solubilized. There are methods to analyze major elements in such complex materials as silicates and vitreous siliceous coal ashes (1-5). More recently, lithium metaborate has been reported to be a good fluxing agent (6) and has also been used in conjunction with atomic absorption analysis in silicate analysis (7). This paper describes a lithium tetraborate-atomic absorption analytical technique which is being used to analyze coal ash. 

The use of x-ray fluorescence was originally intended to obtain information about the major element matrix of coal ashes that were to be analyzed for trace elements by optical emission spectroscopy. Both low-temperature (<150°C) and high-temperature (450°C) coal ashes, prepared as described by Ruch et al. (I), were analyzed, and the method of Rose et al. (2) was adapted to determine the major and minor elements (Si, Ti, Al, Fe, Mg, Ca, K, and V). The instrumental parameters used for these elements are given in Table I. 

It is apparent from Table IV that trace elements determined by the x-ray fluorescence method are limited to those occurring in whole coals at concentrations of at least a few parts per million. Elements such as selenium, mercury, and antimony, which are generally present in whole coal at levels below 1 ppm, cannot be determined by this method. The major elements in coal, hydrogen, carbon, oxygen, and nitrogen, cannot be determined by x-ray fluorescence, but this should not inhibit the use of the method for trace and minor element determinations. 

The results of the mass balance calculations for eight major elements and 22 minor elements for run 9 are given in Tables II and III, together with the corresponding concentrations in the coal, precipitator inlet and outlet fly ash, and in the slag tank solids. Complete tabulation of results for all three runs and some data for 57 elements is given in a project progress report (4). 

The results are shown in Tables I, II, and III. The major elements in coal and in the derived products are, in order of decreasing abundance in the materials studied, silicon, aluminum, calcium, iron, and magnesium. With the minor and trace elements, the detection limits vary with the ash content of each type of material about 1-5 ppm for the coals and residues and 1-3 ppb for the oils. The elements thallium, bismuth, germanium, and gallium were sought but not detected. 

It is suggested here that the greater insolubility of the humic fraction of coal may be the result, in part, of polymerization by complexing with metals, particularly aluminum and silicon. These elements are suggested because of their presumed greater availability. The minor elements also play a part, however, only in relation to their availability compared with major elements such as aluminum and silicon. 

Ash analysis is the term used to designate analysis of the major elements commonly found in coal and coke ash. The elements, expressed as oxides, are SiCK AUCb, Fe 0, TICE, CaO. MgO, Na7Ot K20. P Os, and S03. 

X-ray fluorescence analysis (ASTM D-4326) is a rapid, simple, and reasonably accurate method of determining the concentration of many minor and trace elements in whole coal. The method is dependent on the availability of suitable standards. Although the major elements in coal (carbon, hydrogen, oxygen, and nitrogen) cannot be analyzed by x-ray fluorescence, most other elements at levels greater than a few parts per million (ppm) are readily determined. Sulfur should be determined by an alternative method (ASTM D-1757). 

The major elements present in Rasa coal ash were determined. The results are in Table II. The major elements present in the ash, Ca and Mg, are consistent with the X-ray diffraction findings of calcite and dolomite. The weight percent values in Table II do not and should not add to 100%. The sum of all the values reaches 100% when the weight percent of the minor elements are included. 

The analytical results for trace elements appear better than those for the major elements. Table IV is a summary of results for the raw shale (OS-1), and the spent shale from the TOSCO II pilot plant (SS-2), and the spent shale (FS), unacidified water and unfiltered oil from the Fischer assay studies. The spent shale SS-2 is the residue from the retorting of the same raw feedstock from which OS-1 was taken. In the case of the solid samples, the concentrations are from multiple analyses of different splits. In all cases, the relative deviations of multiple analyses lie within 10%. Comparison of these results with those from other laboratories on different splits of the same solid samples show agreement within 2cr SI), The two exceptions are manganese and zinc for which the results reported here are low in comparison with other methods. Analyses of NBS standard coal (SRM 1632) and coal fly ash (SRM 1633) are included also in Table IV. The concentrations in raw and spent shale are similar to those reported by TOSCO except for selenium where their values range from 10-16 ppm (3). 

Mineralogical techniques such as X-ray diffraction, differential thermal analysis, Fourier transform infrared analysis, and Mossbauer spectroscopy may be useful for determining modes of occurrence of major elements in coal, but the ability of these techniques to determine the modes of occurrence of the minor- and trace-elements is quite limited. X-ray absorption-fine structure (XAFS) spectroscopy has been used to determine the modes of occurrence of several important minor and trace elements (e.g., arsenic and... 

Figure 8. Schematic of the major elements of a matrix relating fundamental coal properties and the responses of coals to various processes. Real life matrices would have many dimensions.

ESCA. Electron spectroscopy for chemical analysis (ESCA) has been demonstrated to be effective in analyzing major elements in coal or ash surfaces having different chemical environments (65). Sulfur can be detected as the sulfide or sulfate. Carbon can be detected as graphite, carbonyl, carboxyl, or hydrocarbon. 

This general review of factors influencing major, minor, and trace element variations in U.S. coals provides an interpretation of coal inorganic elemental data found in the literature. Variations due to ash-related, rank-related, geochemical, and geological factors are discussed. 

Much use is made of the ash composition which is normally a compilation of the major elements in coal ash expressed as the oxide form. From this compilation of elements, expressed as oxides, judgements are often made based on the quantity of certain key con-... 

Table 7-lb. Average Absolute and Relative Abundances of Major Elements in Crustal Rock, Soil, and Shale-, Relative Abundances of Elements in Fly Ash from Coal and Fuel-Oil Combustion and Relative Abundances of Major Elements in the Remote Continental Aerosol, with Enrichment Factors (Aerosol) EF= (X)/(AI)aeroso,/(X)/(AI)crusta, rock ... 

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