Coal rank

Coals (the plural is deliberately used because coal has no defined, uniform nature or structure) are fossil sources with low hydrogen content. The structure of coals means only the structural models depicting major bonding types and components relating changes with coal rank. Coal is classified, or ranked, as lignite, subbituminous, bituminous, and anthracite. This is also the order of increased aromaticity and decreased volatile matter. The H C ratio of bituminous coal is about 0.8, whereas anthracite has H C ratios as low as 0.2. 

Vitrinite Reflectance. The amount of light reflected from a poHshed plane surface of a coal particle under specified illumination conditions increases with the aromaticity of the sample and the rank of the coal or maceral. Precise measurements of reflectance, usually expressed as a percentage, ate used as an indication of coal rank. 

Table 2. Vitrinite Reflectance Limits, Taken in Oil, and ASTM Coal Rank Classes ...

Aromaticity of coal molecules increases with coal rank. Calculations based on several models indicate that the number of aromatic carbons per cluster varies from nine for lignite to 20 for low volatile bituminous coal, and the number of attachments per cluster varies from three for lignite to five for subbituminous through medium bituminous coal. The value is four for low volatile bituminous (21). 

Electron spin resonance (esr) (6,44) has had more limited use in coal studies. A rough estimate of the free-radical concentration or unsatisfied chemical bonds in the coal stmcture has been obtained as a function of coal rank and heat treatment. For example, the concentration increases from 2 X 10 radicals/g at 80 wt % carbon to a sharp peak of about 50 x 10 radicals/g at 95 wt % carbon content and drops almost to zero at 97 wt % carbon. The concentration of these radicals is less than that of the common functional groups such as hydroxyl. However, radical existence seems to be intrinsic to the coal molecule and may affect the reactivity of the coal as well as its absorption of ultraviolet radiation. Measurements from room... 

Properties Involving Utilization. Coal rank is the most important single property for appHcation of coal. Rank sets limits on many properties such as volatile matter, calorific value, and swelling and coking characteristics. Other properties of significance include grindabHity, ash content and composition, and sulfur content. 

Coke Production. Coking coals are mainly selected on the basis of the quaUty and amount of coke that they produce, although gas yield is also considered. About 65—70% of the coal charged is produced as coke. The gas quaUty depends on the coal rank and is a maximum, measured in energy in gas per mass of coal, for coals of about 89 wt % carbon on a dry, mineral matter-free basis, or 30% volatile matter. 

Fig. 11. Effect of coal rank on furnace sizing (constant heat output) (82), where W = width, D = depth, and h and H are the heights indicated. A represents medium volatile bituminous B, high volatile bituminous or subbituminous C, low sodium lignite D, medium sodium lignite and E, high...

Much work has been described in the literature on determining which chemical and physical features distinguish coals in terms of reactivity or hydrogenation potential [21]. Coal rank is certainly one of the featiues as shown by the difference between WVGS 13421, which is a medium-volatile bitummous coal, and the lower rank WVGS 13423, which is a high-volatile A bituminous coal. 

In very broad terms, hydrogenation activity in terms of hydrogen utilization increases as coal rank decreases. Thus, in the absence of any other information, one can expect WVGS 13423 to be the more reactive of the two Obviously, generalities about hydrogenation behavior should not be extended to other coals based solely on a limited data set. Nevertheless, important differences between the pitches derived from WVGS 13421 and WVGS 13423 are apparent. 

Lambert, J. B., J. S. Frye, and A. Jurkiewicz (1992), The provenance and coal rank of jet by carbon-13 nuclear magnetic resonance spectroscopy, Archaeometry 34, 121-128. 

NCB Coal rank code (CRC) General Description 1977/78 Production 106 ton/annum Principle Mining Area... 

Coal NCB coal rank code (CRC) Characterisation of products 2 ... 

A more detailed study has shown that the solubility of vitrinites is dependent upon coal rank (D7). Those from the prime coking coals and mid rank coals (e.g. CRC 301a and... 

CRC 502) are the most soluble. In comparison, the extraction yields of exinites were independent of coal rank. The subdivision of inertinites into fusinite and micrinite showed that fusinite was completely insoluble in anthracene oil, whilst the micrinite was slightly soluble. 

Coal Rank Natural Water (wt%) Volatile Matter (Dry, Ash Free) (wt%) Total Carbon (Dry, Ash Free) (wt%) Heat of Combustion (MJ/kg)... 

It is important to note that coal rank has little to do with quality. As a coal matures its ash content actually increases as a proportion because of the loss of moisture and volatiles. Lower rank coals may have lower energy contents, but they... 

ABSTRACT CO2-ECBM Micro-Pilots have been carried out in Canada and China to assess the response of low- and high-rank coal reservoirs to C02 injection. The selectivity of coals for C02, compared to methane, can vary from 10 to 1 depending on coal rank. Although the C02 is more efficiently stripped by coals of low rank, permeability impairment is greater and the amount of methane recovered is less than for high-rank coals, providing other reservoir properties are similar. If a pure C02 source is not available, N2-C02 mixtures may produce more favourable economics. 

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