Coal Density

Coal is a porous solid material and therefore several types of density are distinguished [10]  

---

The precise determination of true density requires complete filling of the pore structure with a fluid that has no interaction with the solid. No fluid meets these requirements completely. Helium has traditionally been considered as the best choice since it is not significantly adsorbed by coal at room temperature and that the use of helium gives a more accurate determination of coal density, but there is evidence (Berkowitz, 1979, and references cited therein) that part of the pore system may be inaccessible to the helium. Thus, when helium is used as the agent for determining coal density, the density (helium density) may differ from the true density and may actually be lower than the true density. 

Thus, it is not surprising that coal density is variable and dependent on the coal type. For example, the density of anthracite is on the order of 1.55, whereas bituminous coal has a density on the order of 1.35, and lignite has a density on the order of 1.25. However, such generalizations are to be treated with caution since coal density is usually determined by displacement of a fluid, but because of the porous nature of coal and also because of physicochemical interactions, the density data observed vary with the particular fluids employed, and different fluids may have to be employed for different coal types (Agrawal, 1959 Mahajan and Walker, 1978). 

Coal density is a useful parameter not only for deducing the spatial structure of coal molecules, but the relationship between the density and porosity suggests that emphasis must be given to density and its determination. Porosity measurement, in turn, provides useful information on the technical behavior of coal toward its end use. Particle density is required for calculating the porosity of individual coal particles. 

Methods of measurement of coal density include use of a gas pycnometer and particle density by mercury porosimetry. However, the difference in density values using different gases must be recognized since, for example, density values measured by nitrogen may be greater than those obtained when helium is used. Density measurement depends on adsorption of gas molecules, and differences (between nitrogen and helium) may be due to nitrogen adsorption on the coal surface. 

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. 

Substantial variation in the chemical composition of macerals has been shown for a number of coals. Density gradient... 

Aliphatic stretching, FTIR of vitrinite, 103-12 Alkyl phenols, Py-MS, 153f Analytical analyses of demineralized coals, density fractions, 71t Aromatic adjacent hydrogen in... 

In the case of pulverized coal flow measurement, the concentration of the pulverized coal is measured by low-power, low-frequency microwave sensors. The variation in the microwave transmission characteristic (dielectric load) is caused by the changing coal concentration, which produces shifts in measurement frequency. The resulting quantifiable values indicate the coal density. This concentration measurement is performed by a microwave transmitter and a microwave receiver, as shown in Figure 3.90. The velocity of the pulverized coal is measured by two identical microwave devices by crosscorrelation. Here, the pair of sensors detect the stochastic signals resulting from the charged coal particles, which are nearly identical but shifted by the time the pulverized coal gets from one sensor to the other. 

We calculated the specific surfaces shown in Table 1 by an improvement of the procedure described in Reference 7. In our more recent studies of coals, rather than using the mass absorption coefficient of carbon, we have computed the mass absorption coefficient of each coal from the elemental composition given by the ultimate analysis. These mass absorption coefficients, which depend quite strongly on the composition and concentration of minerals in the coals, varied from about 7 to 12 cm /gm. We also have taken the values of the coal densities from Fig. 2 of Reference (17). This plot shows the coal density as a function of fixed carbon content and thus provides more reliable densities than the approximation we used in Reference (7). The quantity I A was calculated from the scattering data for colloidal silica samples by the procedure outlined in Reference 7. The proximate and ultimate analyses of... 

PSOC 242, which also is a subbiLuminous coal. Since we did not know the fixed carbon content of PSOC coaj s 86, 240, and 240A2, we took their mass density to be 1.35 gm/cm. Uncertainties in the coal densities probably will not appreciably increase the overall uncertainty in the calculated x-ray specific surfaces, because the density of low-rank coals changes by only a few per cent for fixed carbon contents between about 69 and 73% [Ref. 17, Figure 2]. 

Lower moisture is due to higher coal density, which results in significantly harder particles. Such coals must be briquetted with a binder. After adding a thermoplastic... 

The coal particle radius is 0.27 mm and its permeability coefficient is 1 x 10 mV(MP s). Gas adsorption parameters are obtained by analyzing the experimental results, a is 27.32 mVt, b is 0.35 MPa" and the coal density is 1.2 mVt. The gas flow equation is calculated with the program designed by VB based on the experimental parameters and the dimensionless cumulative desorption of gas is obtained. Then the dimensionless time and the dimensionless cumulative desorption are translated into dimension time and dimension cumulative desorption. 

Only the third term, the content of inorganic matter, is related to coal density. Therefore, in general there is no relationship between coal washability and coal floatability such a relationship can be observed only in some particular cases. 

The measmement of coal densities is complicated by the presence of pores in coals. A number of techniques for... 

FIGURE 9.1 Variation of coal density with carbon content. (From Berkowitz, N., An Introduction to Coal Technology, Academic Press, New York, 1979.)... 

The entrainment velocity Ue is calculated fi-om the particle diameter according to Equation (3.70), the apparent coal density p, the gas density p, the gravitational acceleration g, and a drag coefficient Cd- The drag coefficient can be estimated by the correlation from Morsi et al. [135]... 

V volatile matter lost from particle to time t [gm of VM/gm of d.a.f. coal] d.a.f. coal density (gn/as3j... 

---