Maurice E. Bailey
Pikeville College Pikeville, Kentucky 41501
The Chemistry of Coal and its Constituents
With the developing shortage of petroleum in this countrv. an examination of F i w e 1 leads quickly to the conenergy source cfusion that reliance on coal as a will increase in this country. Future generations of research chemists will be as concerned with the proper manipulation of this source as the past generation was with petroleum. One of the crowning achievements of petroleum chemists in this country in the late 1930's was the development of high octane gasoline which added significantly t o the use efficiency of this natural resource. This was accomplished from perhaps the same level of understanding of the basic constitution of petroleum as now exists in the case of coal. Applying the usual methods of elemental analysis, and discounting ash, a typical low-volatile bituminous coal will he found to have the empirical formula C1x,HwO~NS ( I ) . Such formulas might range from CrsHl40OssNzS for a low made peat to CzroHsoOlNS for a high grade anthracite coal. T o the research chemist this means very little, except that he knows that he is probably dealing with a highly complex set of organic molecules. ~
The Organic Structure of Bituminous Coal is Primarily Aromatic and Cycloaliphatic
In recent years, a number of now more or less standard analytical techniques have been- applied in an effort to further elucidate coal structure (Table 1).Without needing to be specific a t this point with respect to which technique gives which result, estimates can he made (Table 2) of organic structural features of a low-volatile bituminous coal, having the empirical formula noted above, deduced from spectroscopic and chemical evidence (3, 4). Since very little of the oxygen appears as phenolic in the coke distillate (most going out as water and carbon oxides), a reasonable hypothesis is that most of the hydroxyl is nonaromatic, although there is disagreement on this point since about half of the oxygen is said to be acidic (51. Nitrogen is classed as hydroaromatic although an equally good case might be developed for classing it as aromatic. With this background, one might imagine a cluster of condensed aromatic rings with hydroaromatic rings peripherally arranged. However, if all of the 92 aromatic carbons were in a cluster, practically all of the edge sites would be needed for the 24 aromatic hydrogens, leaving no places for attachment of the other atoms. Alternatively, smaller clusters joined with ether oxygen, -CH2-, -CH and hydroaromatic bridges seems more appropriate, keeping in mind that the ratio of hydroaromatic carbon to hydrogen (Table 2) isahout 1:1.5 (6).
Figure 1 . Coal reserves of the world. Source: U.S.Geological Survey.
Counting up atoms in the above formula, one can develop a table of data corresponding closely to the data in Table 2, as seen in Table 3. This is not to say that other arrangements equally in agreement with the data in Table 2, cannot he drawn (7). Likewise, different ranks of coal would be different; hut over a fairly wide range, these differences should he in degree rather than kind. Also, since free radicals are present, an occasional hydrogen would be found missing; one Table 1. Techniques Used in Coal Elucidation Yields information on
Measurement of
size distribution of ammatic ring systems average diameter of s~omatic Ismellae, nuclei, or eluaters (related to Re) mean C-C bond length average thickness of the packets of lsmellae aromaticity (fraction of carbon in ammatic structures) sveragenvmber of rings in aromatic nuclei, Ra . , aromatic surface a(related to Rd optical anisotmpy
X-ray ditrraetion
ultm"i0let m d visible absorption
Reflectance Ogtica1 refractive index (molar refraction) Infrared absorption
characteristic group such as OH, CH.,, CHal, (C=CIar, H d H d
Pmtm *pin =sonan= Electmn epin resonance Electrical conductivity
HdHd ratio
Diamsgnetieauseeptibility (molar diamagnetic susceptibility) Die1ectl.i~comtsnt Sound velocity Density (molar volume)
average "umber of ringa in am-
Table 2.
Free radical content average number of ring* in s m maticnuc1ei. Ro matic nuclei, Ra dipole moment aromaticity aromaticity ring eondensation index. 2(Ro li/C ire1ated to n u )
Element Distribution in a Bituminous Coal Per 135 Carbon Atoms Carbon Hydrogen
How big n is, is hard to say. It is probably a small number. 446
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Ammatie Hydroammatic CHI CH? and CH in side chain Hydroryl Ether and Carhonyl
92 34 6 3
Oxygen Nitrogen Sulfur
23 51
18
5 (6)
6 3
would expect about one unpaired electron per 5000 carhon atoms in the ahove sample. In any case, this "linear structure," containing small aromatic clusters alternating with small, hydroaromatic segments is rational with two properties of hituminous coal Bituminous coal shows a tendency to start t o melt, but then solidify on heating, in this respect not unlike heat reactive, linear polymers, e.g., certain polyurethanes. When viewed under a microscope, bituminous coal is red to yellow, not unlike small polynuclear hydrocarbons and their hydroxyl and sulfur derivatives. X-Ray measurements show that the average size of the aromatic clusters changes little between 80 and 90% carbon (hituminous range), but increases rapidly ahove 90% carbon (anthracite range). Coal Seams Vary with Respect to Rank and Type Coals are ranked upward in quality as lignite, suhbituminous, hituminous, and anthracite with carbon content increasing and oxygen content decreasing with higher type, a very useful classification, particularly in terms of the value of coal as a fuel. A narticular seam will he of one rank. On the other hand, important petrographically different types will he discernable in a particular seam, some visible to the eye unaided, called for example fusain, vitrain, and durain, and others seen with the aid of a microscope called fusinite, vitrinite, exinite, and micrinite. Typically, a hituminous coal might contain 60-80% vitrain and vitrinite with fusain and fusinite often as the next most abundant constituent, with others also present in abundance. These morphologically different types also differ in other physical as well as chemical properties. Densities and wettability are often significantly different and form the basis for fairly good separations. Also, grinding characteristics are different, fusains being most friable. Chemically, exinites, vitrinites, and micrinites are substantially different from each other in hydrogen content and type of hydrogen (Table 4) (8). The size of the aromatic clusters increases from exinite, through vitrinite to micrinite. Fusinites have the highest aromatic character. Fisher found fusain considerably more resistant to hydrogenation than vitrain (9). Much of the research on coal has been performed on whole coal, without regard to these differences. However, this is not the case with the data in Table 2, which refers to a sample of vitrain. Exinites might be less aromatic and fusanites more so.
electromagnetic radiation by unpaired electrons, called electron spin resonance, is a measure of free radicals in a material. The absorption bands may be characterized by their intensity which measures the total number of unpaired electrons, and their half-width which measures interactions with the surroundings; the wider the band, the greater the interaction. In coal of 70% carhon one would find an average of one radical for every 50,000 carbon atoms, at 85% carhon, one for every 5000, and for anthracite with 94% carbon, about one radical for every 1M)O (10). On the other hand, half-widths are essentially the same un to ahout 92% carbon. then decrease sharnlv. . This correlates with a rapid increase in size of the aromatic clusters a t about this carhon level, affording- isolation of the electron from the surroundings. The remarkable fact is the difference between fusain and fusinites and the other types both in concentration of free radicals and half-width. Fusinites contain significantly higher concentrations of free radicals and a narrower half-width a t all carhon levels than the other types (Fig. 2 and 3) (11). With coal seams sometimes containing as much as 20% fusain and fusinite types, one then recognizes that he is working with a t least two very different materials together when coal is studied as i t comes from the seam. With increasing rank, the differences between types becomes progressively smaller. Schopf's coalification sequence (Fig. 4) (12) relates these matters vividly. The early geologic devElopment of fusains, along with other evidence, has led to the theory that in the coalification process fusinization occurred at temperatures in the range of 400-@JOT, in sharp contrast to other metamorphic changes which presumably occurred a t earth temperatures as we know them. Another theory is that fusains developed (at ordinary temperatures) from purely cellulosic materials while the other petrographic types were derived from plant materials high in lignite and waxes.
F U S I N I T E S (D)
Free Radicals
The free radicals seem to he of two different kinds; the petrographic types exhibit these differences. Absorption of Table 3.
Element Distribution in Above Formula Per 136 Carbon Atoms Carbon
cfII CH?and CH in side chain Hydroxyl Ether
Table 4. Type
Hvdroeen
6
18
2
4 6
Oxvaen Nitroaen Sulfur
6 2
6
EXINITES
(6)
Hvdrosen Differences in Three Petrograuhic Tvues 70 Carbon
% Hydmge"
% Ammatic Hydrogen
% Aliphatic Hydmgen
%
CARBON
go.
I., I N ASSOCIATED VITRINITE
Figure 2. Unpaired spin concentration.
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447
Figure 4. M e t a m o r p h i c c h a n g e s of plant r e m a i n s .
EXlNlTES ALMOST I D E N T I C A L W I T H ASSOCIATED VITRINITES
kept within optimum particle size of 3-1% mm. Durain, the hardest component and inferior in coking qualities is kept above 3 mm size. Fusain, havine virtuallv no resistance to crushing, is reduced to The coal researcher can reasonably expect that separation methods can he developed, if he finds the advantage.
a
Literature Cited
%
C A R B O N g. o. f., I N
A S S O C I A T E D VITRINITE
Figure 3. Line widths.
Can the Various Coal Types be Separated Efficiently?
The commercial success of the earlv ~etroleumchemists had its foundation in the constructiin'and demonstration of rnultiplate fractional distillation columns. This made it possible-for the petroleum chemist to work with a reasonably simple set of molecules and expect to have these commercially available. Obviously, fractional distillation is not applicable to coal. Flotation is, on the basis of density and wettahility differences. Friability differences have been utilized on a commercial scale in the so-called Sovacs process (13). By this process, in a controlled grinding-sieving operation, vitrains and vitrinites of a high volatile hituminous coal possessing good coking qualities but marked friability are
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/ Journal of Chemical Education
I11 Fueh's W., and S.. and Gaff,A. G.,lnd. Ew. Chem., 32.567. (19421. 12) Kirk-Othmer, "Eneyelopedis of Chemical Technologq," 2nd Ed.. hfer~eiencePubliphers. New Yo*. I9M.p. 628. I31 Kirk~Ofhmer,op.r i l . p.639. (41 Low, H. H., "Chemistry of Cosl Utilization," John Wiley 8 Sons. Inc., Ne" . L.".m,
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