Influence of Coal Rank

Coal rank has a considerable influence on the nature and amount of extracts obtained by the solvent extraction of coal (Kiebler, 1945). In addition, the soluble products of the extraction, whether they be called extracts or (incorrectly) bitumen, vary according to the means by which they are obtained. 

The solubility of coal in solvents decreases rapidly as the carbon content of the coal increases from 85% to 89% (dry ash-free basis) and is negligible for anthracite (92%-93% w/w carbon). This phenomenon is generally applicable to all temperatures and types of solvents. However, a decrease in coal particle size (thereby increasing the surface if the coal available for solvent contact) or an increase in temperature increases the extent of extraction. 

It is generally recognized that vitrain is the most soluble constituent of any particular coal whereas fusain is the least soluble. Indeed, early work on the liquefaction of coal by dissolution in a solvent (Chapters 18 and 19) showed that even at temperatures of the order of dOO C (TSO F) fusain is, to all intents and purposes, insoluble. Under these aforementioned conditions, durain did show some response to the solvent but still did not match the solubility of vitrain. 

The results of extracting coals with benzene and benzene/ethanol mixtures have been reported (Table 11.1), but only a broad general trend seems to emerge from these observations. Thus, it appears that for coals with more than 88% carbon content and less than 25% volatile matter, the amount of extract obtainable decreases rapidly for coals of carbon content lower than this limit, no definite trend with rank appears to be evident. 

Influence of Coal Rank on the Degree of Extraction by Aromatic and Hydroaromatic Solvents 

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Influence of Coal Rank. The effect of the rank of the coal on the reactions in the plasma jet was studied by carrying out experiments in an argon plasma with the range of coals shown in Table II. The size range used for these measurements was 53-74m. The conversions of the various coals to acetylene, which was taken as a measure of the extent of the reaction, are shown in Figure... 

Figure 6. Highly schematized depiction of the orthogonal influences of coal rank and coal type on coal grade (economic value) as reflected simply by the yield of liquids in a coal conversion (liquefaction) process (( ) reactive ingredients are vitrinite and exinite in higher rank coals, vitrinite and exinite give low yields)...

Rubio, B. and Izquierdo, M.T. (1997). Influence of low-rank coal char properties on their SO2 removal capcity from flue gases I non-activated chars. Carbon, 35, 100-11. 

For example, certain physical properties of coal (which are themselves a function of coal rank) change with the depth of burial (Figure 3.23) (Breger, 1958 Francis, 1961 Schmidt, 1979) but it should be noted here that the temperature gradient is, of course, influenced by the thermal conductivity of the rocks, which essentially makes comparisons of coals from different locales extremely... 

Esterle, J.S., O Brien, G., and Kojovic, T. 1994. Influence of coal texture and rank on breakage energy and resulting size distributions in Australian coals. In Proceedings. 6th Australian Coal Science Conference. Australian Institute of Energy, Newcastle, New South Wales, Australia, pp. 175-181. 

The first part of this paper has shown that Australian black and brown coals differ significantly in a number of respects from coals of similar ranks from North America and elsewhere in the northern hemisphere. The rest of the paper than proceeded to indicate the progress being made to determine how the characteristics of Australian coals influence their conversion to volatile and liquid products during pyrolysis and hydrogenation. 

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 ( ). 

The mobile phase and the nature of H-donor and non-donor solvents all have a profound influence on primary conversions. In contrast, distillate or oil yields often correlate with parameters reflecting the aliphaticity of coals (H/C ratio -decreasing vitrinite reflectance - CHj content - Z), better correlations being achieved for low-rank coals if yields are expressed on a free" basis... 

In this paper, a number of low-severity liquefaction regimes are considered. The influence of different H-donor and non-donor solvents on primary conversions without a hydrogen overpressure is discussed in the light of other recent work (10-131. Also, it is demonstrated that oil yields broadly increase with decreasing coal rank in both H-donor extraction and dry catalytic hydrogenation provided that retrogressive reactions are avoided in the initial stages of coal dissolution. 

Figure 6. Influence of temperature and coal rank on yield of chloroform-soluble liquids in solvent-free hydrogenation with a sulphided Mo catalyst.

Another proof of the importance of temperature is the fact that there is often a strict relationship between the run of isovols and the run of isotherms in deep profiles, both being influenced no doubt by the varying thermal conductivity of the different rocks. The strong influence of temperature on the rank of coal is obvious in the case of contact-metamorphic coals, whose rank increases distinctly when approaching the intrusive body. Apart from these geological observations, all experiments on artificial coalification have shown that temperature is the decisive factor in the coalification process. Thermodynamic and reaction kinetic considerations (9) also support this opinion. 

The proof of the influence of time on the rank of coal can be found in the following comparison Kuyl and Patijn (13) have described subbituminous... 

Geological investigations have shown that undoubtedly temperature has most influence on geochemical coalification. This is proved by such evidence as contact metamorphism of coals, increase of rank with increasing temperature of the earth s crust with depth, increase of rank of the same seam as magma chambers (batholiths) are approached, and relations between the thermal... 

Pressure of the overburden does not cause chemical reactions which lead to a higher rank. Experiments have shown that static pressure even retards coalification processes. By contrast, pressure affects the physical properties, notably the porosity and moisture content in low rank coals. Further, the optical anisotropy of vitrinites (which is a tension anisotropy) is caused by pressure. Shearing movements have influenced the chemical coalification only occasionally and locally in the foredeeps that we have studied (for instance in the immediate vicinity of overthrusts). In such cases the tectonic movements probably were so quick that the friction heat and the shearing could operate. Shearing in no way can account for the gradual increase in coal rank with depth. 

Dr. Teichmuller Very often it is impossible to separate the influence of overburden pressure and the influence of rock temperature on rank of coal in a subsidized formation. 

The dielectric constant varies with coal rank (Chatterjee and Misra, 1989). The theorem that the dielectric constant is equal to the square of the refractive index (which is valid for nonconducting, nonpolar substances) holds only for coal at the minimum dielectric constant. The decreasing value of dielectric constant with rank may be due to the loss of polar functional groups (such as hydroxyl or carboxylic acid functions), but the role of the presence of polarizable electrons (associated with condensed aromatic systems) is not fully known. It also appears that the presence of intrinsic water in coal has a strong influence on the dielectric properties (Chatterjee and Misra, 1989). 

Coal Rank The type of coal strongly influences pyrolysis behavior. Low-rank coals, such as lignite, contain oxygen functional groups that evolve water and carbon oxides upon pyrolysis. Higher-rank bituminous coals contain less oxygen consequently, these coals produce significantly less water and carbon oxides when pyrolized. The nature of the tar produced is also dependent on coal rank. 

The success of any DCL process is highly dependent on the type of coal used. Coal rank influences both overall conversion and product distribution. Lower-rank coals have been reported to give both higher and lower conversions than bituminous coals,42 with most evidence supporting the latter. Furthermore, the liquid products from lower-rank coals are generally more volatile and of lower molecular weight43 than those from bituminous coals. 

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