Maurice
E.
of Coal
The Chemistry
Bailey
PikevilIe College Pikeville, Kentucky 41501
and its Constituents
J. Chem. Educ. 1974.51:446. Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 01/23/19. For personal use only.
With the developing shortage of petroleum in this country, an examination of Figure 1 leads quickly to the conclusion that reliance on coal as a primary energy source 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 to the This was accomuse efficiency of this natural resource. plished 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 be found to have the empirical formula C135H9709NS (l). Such formulas might range from CjsHuoOseNzS for a low grade peat to C240H90O4NS for a high grade anthracite coal. To 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 at this point with respect to which technique gives which result, estimates can be 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 (5). 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 duster, 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, —CHa—, —CH and hydroaromatic bridges seems more appropriate, keeping in mind that the ratio of hydroaromatic carbon to hydrogen (Table 2) is about 1:1.5 (6).
Counting up atoms in the above formula, one can devela 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 be drawn (7), Likewise, different ranks of coal would be different; but over a fairly wide range, these differences should be in degree rather than kind. Also, since free radicals are present, an occasional hydrogen would be found missing; one op
Table
1.
Techniques Used in Coal Elucidation
Measurement of
Yields information
X-ray diffraction
size distribution
diameter of aromatic average lamellae, nuclei, or clusters (related to Ra) mean G—C bond length average thickness of the packets of lamellae aromaticity (fraction of carbon in aromatic structures) average number of rings in aromatic nuclei, Ra aromatic surface area (related to
Ultraviolet and visible absorption
Reflectance
Optical refractive index (molar fraction) Infrared absorption
Ra)
optical anisotropy
re-
characteristic groups such as OH, CHar* CHal, (C==Cjj)ar, Har/Hal
Proton spin resonance Electron spin resonance Electrical conductivity
Har/Hal ratio
Diamagnetic susceptibility (molar diamagnetic susceptibility) Dielectric constant Sound velocity Density (molar volume)
average
Table
2.
free radical Content average number of rings in
CHa CHa and CH in side
number of rings in matic nuclei, Ra
446
j
Journal of Chemical Education
a
small
num-
Hydroxyl
Ether and Carbonyl
aro-
dipole moment
aromaticity aromaticity ring condensation index, 2(Ra 1 )/C (related to Ra)
Carbon
Hydrogen
92 34 6
23 51 18 5
hard to say. It is probably
aro-
matic nuclei, Ra
Element Distribution in a Bituminous Coal Per Carbon Atoms
Hydroaromatic
n is, is
ring
systems
Aromatic
How big ber.
on
of aromatic
(6)
Oxygen
—
135
Nitrogen Sulfur 1
would expect about one unpaired electron per 5000 carbon atoms in the above sample. In any case, this “linear structure,” containing small aromatic clusters alternating with small, hydroaromatic segments is rational with two properties of bituminous coal Bituminous coal shows a tendency to start to 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 tittle between 80 and 90% carbon (bituminous range), but increases rapidly above 90% carbon (anthracite range). Coal Seams Vary with Respect to Rank and Type Coals are ranked upward in quality as lignite, subbituminous, bituminous, 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 particular seam will be of one
rank.
On the other hand, important petrographically different types will be 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 bituminous 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% carbon one would find an average of one radical for every 50,000 carbon atoms, at 85% carbon, one for every 5000, and for anthracite with 94% carbon, about one radical for every 1000 (10). On the other hand, half-widths are essentially the same up to about 92% carbon, then decrease sharply. This correlates with a rapid increase in size of the aromatic clusters at about this carbon 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 at all carbon 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 at least two very different materials together when coal is studied as it 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 400-600°C, in sharp contrast to other raetamorphic changes which presumably occurred at 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.
Free Radicals
The free radicals seem to be of two different kinds; the petrographic types exhibit these differences. Absorption of Table
3.
Element Distribution in Above Formula Per Carbon Atoms Hydrogen
Aromatic CHa
Nitrogen Sulfur
22 50
94 34 6
Hydroaromatic
Oxygen
136
18
CH; and CH in side 4
Hydroxyl
6
Ether
Table
4.
Hydrogen Differences in Three Petrographic Types % Carbon
% Aromatic
% Aliphatic
% Hydrogen
Hydrogen
Hydrogen
Exinite
84.1
1.4
Vitrinite
83.9
7.0 5.5
Micrinite
85.7
3.9
2.0
5.4 3.3 1.7
Type
1.9
°/„ CARBON
g.a.f.,
IN ASSOCIATED
VITRINITE
Figure 2. Unpaired spin concentration.
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7,
July 1974
/
447
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, having virtually no resistance to crushing, is reduced to a powder. The coal researcher can reasonably expect that separation methods can be developed, if he finds the advantage.
Literature Cited
°/0 CARBON
g.
a.
IN
ASSOCIATED VITRINITE
Figure 3. Line widths.
Can the Various Coal Types be Separated Efficiently? The commercial success of the early petroleum chemists had its foundation in the construction and demonstration of multiplate 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 wettability 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 bituminous coal possessing good coking qualities but marked friability are
448
j
Journal ol Chemical Education
(L) Fuch’s W., and S., and Goff, A. G., Ind. Eng. Chem., 32, 567, (1942). (2) Kirk-Othmer, “Encyclopedia of Chemical Technology,” 2nd Ed., Interscience Publishers, New York, 1964, p. 628. (3) Kirk-Ot.hmer, op. cit.. p. 639. (4) Lowry, H. H.( “Chemistry of Coal Utilization," John Wiley & Sons, Inc., New York, 1963, p. 380. (5) Katz, M., Anal. Chem., 24, 1157, (1952). (6) Cartz, R., Diamond, R., and Hirsch, P. B„ Nature, 177, 500, (1956). (7) Given, P. H., Fuel!, 39, 147, (1960). (8) Tschamber, H., and DeRuiter, E., “Coal Science. Advances in Chemistry, Series 55," American Chemical Society, 1966, p. 333. (9) Fisher. C. H., et aL Ind. Eng. Chem.. 31, 1155, (1935). (10) Lowery, H. H., "Chemistry of Coal Utilization,” John Wiley & Sons, Inc., New York, 1963, p. 81. (11) Ausen, et aL, “Coal Science, Advances in Chemistry, Series 55," American Chemical Society, 1966, p. 350. (12) Schopf, J. M., Econ. Gelo., 43,207, (1948). (13) Lowry, op. cit., p. 24.
Suggested for Further Reading Given. P. H., “Coal Science, Advances in Chemistry, Series 55,” American Chemical Society, 1966. Francis, W., "Coal,” 2nd. Ed., Edward Arnold, Ltd., London, 1961. Kreulen, D. J. W., “Elements of Coal Chemistry," Nijgh and Von Ditman, Rotterdam, 1948.
Kroeger, C., Brenstoff-Chem., 37, 183 (1956). Lowry, H. H., “Chemistry of Coal Utilization," John Wiley and Sons, Inc., New York, 1963. Especially Chapter 6 by I, G, C. Dryden. Van Krevelen, D. W., “Coal," 2nd Ed., Elsevier, New York, 1961.
