Mineral matter classifications

The ASTM Glassification. The ASTM classification system was adopted in 1938 as a standard means of specification. This system is used in the United States and in many other parts of the world, and is designated D388 in the ASTM Standards (18). The higher rank coals are specified by fixed carbon >69%, or for volatile matter <31%, on a dry, mineral-free basis. Lower rank coals are classified by calorific value on the moist, mineral-matter-free... 

As-received is applicable for combustion calculations moisture-free and mineral-matter-free, for classification purposes. 

The mineral matter and the ash in coal have often been informally classified as inherent (stemming from the plant material in the coal swamp) or as adventitious (added after the deposition of the plant material in the swamp). This classification is misleading and difficult to apply, especially for those minerals that are contemporaneous with the peat swamp but were not incorporated by the plants. 

Such data are necessary for calculation of parameters in the classification of coal by rank dry, mineral-matter-free volatile matter (or fixed carbon) as well as moist, mineral-matter-free gross calorific value. For volatile matter and fixed-carbon data, it is also necessary to assume that 50% by weight of the sulfur is volatilized in the volatile matter test and therefore should not be included as part of the organic volatile matter (nor should the loss from clays and carbonate minerals) ... 

This classification does not include a few coals, principally nonbanded varieties, that have unusual physical and chemical properties and that come within the limits of the fixed-carbon or calorific value of the high-volatile bituminous and subbituminous ranks. All of these coals either contain less than 48% dry, mineral-matter-ffee fixed carbon or have more than 15,500 moist, mineral-matter-free British thermal units per pound. 

Inherent or equilibrium moisture is used for calculating moist, mineral-matter-free calorific values for the rank classification of high-volatile bituminous coals. It is also used for estimating free or surface moisture, since total moisture is equal to the sum of the inherent moisture and the free moisture and is considered the inherent moisture of the coal as it occurs in the unexposed seam, where the relative humidity is probably near 100%. However, due to physical limitations, equilibrium moisture determinations are made at 96 to 97% relative humidity and used as inherent moisture values. 

The fixed-carbon value is one of the values used in determining the efficiency of coal-burning equipment. It is a measure of the solid combustible material that remains after the volatile matter in coal has been removed. For this reason, it is also used as an indication of the yield of coke in a coking process. Fixed carbon plus ash essentially represents the yield of coke. Fixed-carbon values, corrected to a dry, mineral-matter-free basis, are used as parameters in the coal classification system (ASTM D-388). 

The classification of coal (ASTM D-388) depends on calculation of the volatile matter yield and fixed carbon values on a dmmf basis. Calorific values are calculated on a moist, mineral-matter-free basis. The Parr formula is used in the classification system to calculate the mineral matter from ash and sulfur data. 

Table VI. SEM-AIA Classification of Mineral Matter in Pittsburgh No. 8 Coal Samples Obtained from TRW. The mineral phases are described by chemistry and area-equivalent diameter (in ym), expressed as weight percent of the total coal.

In terms of coal grade, the grade of a coal establishes its economic value for a specific end use. Grade of coal refers to the amount of mineral matter that is present in the coal and is a measure of coal quality. Sulfur content, ash fusion temperature (i.e., the temperature at which measurement the ash melts and fuses), and quantity of trace elements in coal are also used to grade coal. Although formal classification systems have not been developed around grade of coal, grade is important to the coal user. 

FIGURE 2.5 Comparison of class number (International System) with ASTM classification (1) parameters in the International System are on an ash-free basis, ASTM parameters are mineral matter-free basis and (2) no upper limit of calorific value for class 6 and high-volatile bituminous coals. 

Other bases may be required for the expression of analytical data. Moist, ash-free basis (maf) assumes that the sample is free of ash, but with moisture (ASTM D388, Standard Classification of Coal by Rank, requires calorific value to be expressed on a moist, mineral matter-free basis (mmmf)). The various national standards organizations present different formulae for the calculation of mineral matter (which is not a generally determined value) reference to their publications is necessary to determine which calculation is appropriate in given circumstances. 

The analyses of the air-dried equilibrated sample for moisture, ash, and volatile matter are collectively termed the proximate analysis. Fixed carbon is, by definition, the difference between 100 and the sum of the analytes (moisture, ash, volatile matter). The proximate analysis gives information on the classification of coal by measuring the relative percentage of volatile and nonvolatile organic matter as those of moisture and noncombustible mineral matter. 

One classification of coal is by rank, i.e., according to the degree of metamorphism, or progressive alteration, in the natural series from lignite to anthracite. In the ASTM classification, the basic criteria are the fixed-carbon content and the calorific valnes (in British thermal units) calcnlated on a mineral-matter-free basis. 

The basis for the two ASTM criteria (the fixed-carbon content and the calorific value calculated on a moist, mineral-matter-free basis) are shown in Fig.6.13 for over 300 typical coals of the United States. The classes and groups of Table 6.4 are indicated in Fig. 6.13. For the anthracitic and low- and medium-volatile bituminous coals, the moist, mineral-matter-free calorific value changes very little hence the fixed-carbon criterion is used. Conversely, in the case of the high-volatile bituminous, subbituminous, and lignitic coals, the moist, mineral-matter-free calorific value is used, since the fixed-carbon value is almost the same for aU classifications. 

Dry mineral matter-free (dmmf) Data are expressed in percentage of the coal assumed to be free of both moisture and mineral matter. This theoretical term is mainly used for coal classification and implies that mineral matter has a solid (ash) and gaseous fraction (volatiles originating from clays, carbonates, and oxidation of FeS2, see also Table 3.2). 

The most commonly used system of classification refers to the American standard (ASTM D-388 [13]) using fixed carbon combined with volatile matter on a dmmf basis for higher-rank coals and higher heating value (HHV) on moist mineral matter-free (mmmf) basis for lower-rank coals. The moisture used should be as close as possible to the natural bed moisture content and is obtained by exposing the sample to 30 °C and a relative humidity of 97%. To convert the analysis data to the mentioned reference state, the Parr formulas must be used assuming that a part of the ash sulfur leaves with the volatile matter ... 

From the engineering standpoint, especially, the mechanical properties of soil are emphasized. These properties, which may have important environmental implications in areas such as waste disposal, are largely determined by particle size. According to the United Classification System (UCS), the four major categories of soil particle sizes are the following Gravels (2-60 mm) > sands (0.06-2 mm) > silts (0.06-0.006 mm) > clays (less than 0.002 mm). In the UCS classification scheme clays represent a size fraction rather than a specific class of mineral matter. 

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