Coal rank density

S. P. Nandi. Many parameters (e.g., helium density, heat of wetting, butane adsorption, etc.) for correlating coal rank pass through a minimum at ca. 85-90% carbon content. In the present paper another parameter—the average size of the micropores—has been shown to follow the same pattern. 

There have been a large number of electron spin resonance (ESR) studies of coal and coal products,(1J but a microscopic interpretation of the resulting data has been hampered by the chemical heterogeneity of the coal samples examined. While several surveys of specially selected macerals have appeared, 3), the recent evolution of maceral separation techniques - now allows detailed ESR observations to be made on coals systematically fractionated in which coal rank, maceral type, and maceral density are simultaneously distinguished. The present report surveys the behavior of a variety of ESR properties of carbon radicals in exinite, vitrinite, and inertinite macerals in a variety of coals of different rank. These data... 

Some 37 samples from 16 coals of the Pennsylvania State University coal data base (PSOC) were examined. Separate density fractions were obtained by isopycnic density gradient centrifugation of small ( 3iim) coal particles in an aqueous CsCl density gradient. (4J The individual samples are listed by PSOC numbers, coal description, ASTM designation of coal-rank maceral type, and density in Table I. After separation, the samples... 

Petrographic analysis of the separated macerals, density determinations, and elemental analyses were performed at Argonne National Laboratories. The ash content of these samples is less than 1%. The oxygen levels reported here are obtained by difference. Computer correlations of the resulting parameters were done using the Statistical Analysis System on the VS/CMS system at the ER E-Linden site. Linear-regression analyses are also performed with that system. In the correlation plots which follow, samples will be identified by coal rank and maceral group. 

Figure 2.30. The v iation of total porosity Figure 2.31. The variation of helium density (vol%) with coal rank. (gcm ) with coal rank (Toda, 1972).

Figure 2.31 shows the variation of helium density (gcm ) with coal rank for all coals. Helium, as a gas, has the advantage of being able to penetrate into porosities and yet not to be adsorbed (This is not strictly true and adsorption of helium can be detected but this effect does not radically alter the major conclusions). 

For certain gasification processes it may be necessary to limit the ash fraction in the coal to a certain value. Because the mineral matter has in general a higher density than the organic fraction of the coal, a density or float-sink separation can be carried out to remove material of increased mineral matter. This process is suitable for higher-rank coals and is accomplished using a liquid with an adjusted density. Therefore, it is also called washing of coal. 

The bulk density of broken coal varies according to the specific gravity, size distribution, and moisture content of the coal and the amount of settling when the coal is pded. Following are some useful approximations of the bulk density of various ranks of coal. 

Modern charcoal retorts are charged with wood, biowaste (bark, sawdust, etc.), peat, and sometimes low-rank coals. Yield and properties (hardness, density, surface area, etc.) can vary widely so the desired end use must be considered. Charcoal from coniferous trees is soft and porous, while that from hardwoods is dense and strong. For barbecuing, charcoal is usually compressed into briquettes, with binders and additives chosen to improve handling and ease of ignition. 

Low pressure low density polyethylene (LDPE), 70 595 Low pressure tanks, 24 288 Low Q-state, 74 674 Low rank coal, defined, 6 828 Low resistivity joints, 23 847 Low silica zeolites, 76 833... 

Strong evidence of the dominant Influence of molecular conformation on the properties of coals Is Implicit In the several data sets which show an extremum In the measured property when plotted against carbon rank. Examples are the extrema which occur In the solid state properties of mass density (22,23) and proton spin-lattice relaxation rate (24) as well as In solvent swelling and extractablllty ( ). 

Successfully applying the method used by Fenton to prepare his concentrates depends upon two factors. First, there must be adequate density differences between the macerals in the sample, and second, there must be an initially high concentration of the required maceral. In attempting to separate either resinite or cutinite from sporinite of the same coal, neither of these conditions can be fulfilled, at least when the coal is of bituminous rank or higher. If, however, samples on a semi microscale are acceptable, it is possible to prepare concentrates of resinites of satisfactory purity from bituminous coals by simple mechanical means. The method has been described by Murchison and Jones (17) and mainly involves picking with fine probes on differently prepared surfaces of coal under a stereoscopic microscope. Resinites from lignites pose less of a problem because their occurrence in fairly substantial lumps is quite common these and fossil resins such as kauri gum and amber usually can be prepared to a purity of almost 100% with ease. 

The density of coal shows a notable variation with rank for carbon content (Figure 6.1) and, in addition, the methanol density is generally higher than the helium density because of the contraction of adsorbed helium in the coal pores as well as by virtue of interactions between the coal and the methanol, which results in a combined volume that is notably less than the sum of the separate volumes. Similar behavior has been observed for the water density of coals having 80 to 84% w/w carbon. 

Coal with more than 85% w/w carbon have usually been shown to exhibit a greater degree of hydrophobic character than the lower-rank coals, with the additional note that the water density may be substantially lower than the helium density for the 80 to 84% carbon coals, there is generally little, if any, difference between the helium and water densities. However, the hydrophobicity of coal correlates better with the moisture content than with the carbon content and better with the moisture/carbon molar ratio than with the hydrogen/carbon or oxygen/carbon atomic ratios. Thus, it appears that there is a relationship... 

As already noted with respect to coal density (Figure 6.1), the porosity of coal decreases with carbon content (Figure 6.3) (King and Wilkins, 1944 Berkowitz, 1979) and has a minimum at approximately the 89% w/w carbon coals followed by a marked increase in porosity. The nature of the porosity also appears to vary with carbon content (rank) for example, the macropores are usually predominant in the lower carbon (rank) coals whereas higher carbon (rank) coals contain predominantly micropores. Thus, pore volume decreases with carbon content (Figure 6.4) and, in addition, the surface area of coal varies over the range 10 to 200 m2/g and also tends to decrease with the carbon content of the coal. 

Brown (1955)611 has studied the IR spectra of coals with special attention on the optical densities of the two peaks at 3030 and 2920 cm-1. He found that the ratio of aromatic hydrogen to total hydrogen increased with rank. Fujii and his collegues (1970)62) noted that the absorption band at 2920 cm-1 generally increased with rank to 86% carbon but thereafter decreased sharply with higher carbon contents. 

Free Radicals in Macerals. Electron spin resonance (ESR) has been used to study carbon free radicals in coals, and to some extent, separated macerals. The technique provides information on radical density and the environment of the radicals. The resonance position, termed the g-value, is dependent on the structure of the molecule which contains the free electron. The line width is also sensitive to the environment of the unpaired electron. In an early study, Kroger (71) reported that the spin concentration varied between maceral groups with liptinite < vitrinite inertinite. For this limited set of samples the spin concentration increases with rank for liptinites and vitrinites and decreases for the micrinite samples. On the other hand, van Krevelen (72) found the same general results except... 

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