(14) Korzun, B. P., Dorfman, L., Brody, S. M., Anal. Chem. 35, 950-2 (1963). (15) Kramli, Andras, Arch. Biol. Hung. 1711, 337 (1947). (16) Maas, S. P. J., deHeuss, J. G., Rec. Trau. Chim. 77, 531 (1958). (17) Mauthner, J., Suida, W., Monatsh. 17, 579 (1896). (18) Ruetschi, P., Angstadt, R. T., J. Electrochem. Sod. 111, 1329 (,-I 964). (19) Ryer, A. I., Gebert, W. H., Murill, N. M., J. Am. Chem. SOC. 72, 4247-8 (1950).
(20) Wallis, E. S., Fernholz, E., Ibid., 57, 1379, 1504 (1935). (21) Westphalen, T., Ber. 48, 1064 (1915). (22) Wieland, P., Miescher, K., Helv. Chim. Acta 31,211 (1948). (23) Windaus, A., Be?. 40,257 ( 1 907). (24) Wintersteiner, O., Bergstrom, S., J. Biol. Chem. 137, 785 (1941).
. I .
RECEIVED for review August 16, 1965 ACCEPTED March 11, 1966
EXPERIMENTAL STUDY OF THE EFFECTS OF TEMPERATURE AND ULTRA-HIGH PRESSURE ON THE COALIFICATION OF BITUMINOUS COAL LIN-SEN PAN, TERRELL N. ANDERSEN, AND HENRY E Y R I N G
Rate Processes Institute, University of Utah, Salt Lake City, Utah
Artificial coalification of high volatile bituminous coal was experimentally effected by heating the coal (at temperatures up to 850’ C.) at pressures of 30 kilobars. Such samples were compared with coal heated at atmospheric pressure and also with standard coal samples of various ranks, by means of ultimate analysis, x-ray spectroscopy, IR spectroscopy, and electrical resistivity. The above tests indicate that heat breaks off fragments (“volatiles”) of the coal structure, and drives them off as gases, while high pressure causes retention of many of these products through retardation of bond breakage or through reactions which condense them onto the coal structure. The structure of coal, subjected to elevated temperatures and pressures, tends toward that of higher rank coals, while the product of low pressure heating tends toward coke.
HE ORDINARY PROCESS of coalification of an accumulation of T o r g a n i c debris is generally considered to embrace two stages of prime importance (6, 75, 79, 25): (a) the putrefaction (biochemical) stage, which probably leads no further than the formation of peats, humus, humic concentrates, etc., and ( b ) the alteration or metamorphic (dynamochemical) stage. The latter stage converts the coalifying material into lignite, bituminous (humic) coals, and anthracite coals. Such transformations as ( 6 ) are the direct or indirect result of geodynamic influences ranging from pressure-dehydration to graphitization. The influence of pressure in the last stage ( b ) is not well understood as most evidence for the mechanism of coalification is based upon field observations and distillation studies. Davis and Spackman (8) have made coal-like material from the wood of Taxodium distichurn (bald cypress) in a basic medium with a uniaxial pressure device, at pressure up to 5 kilobars (kb.) and temperatures up to 400’ C., and have shown that those changes in the so-called “biochemical stage” of coal formation could actually have been largely nonbiochemical. Straw has been heated at temperatures of about 300’ C. and pressures of several hundred atmospheres in the presence of limestone to produce a substance which has coal-like properties (26). The purpose of the present study was to investigate the role of pressure in the metamorphic stage ( b ) of coalification. High volatile (hv) bituminous coal was subjected to pressures u p to 30 kb, and temperatures u p to 850’ C., and was then compared with the initial material and with natural coals of various ranks by means of x-ray diffraction, ultimate analysis, infrared spectroscopy, and electrical resistivity. T h e sources of the coal samples studied and those used as references are listed in Table I. Anthracite “A” was used for
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all tests except the x-ray diffraction, in which case A’ was used. This was because sample A’ gave an x-ray pattern much more like those of anthracite’s in thc literature. Sample A, on the other hand, produced other measurements typical of anthracites. The purposes of the extreme pressures and temperatures used were to effect rapid and drastic changes which could easily be studied experimentally in reasonable lengths of time and to provide greater pressures than existent “geostatic” pressures a t coal beds, since the latter may not be those effective in actual coalification (owing to horizontal earth movements) (7, 25). Experimental Procedure
The coal as received was ground (wet) with a 4-inch diameter ball mill to -100 mesh, dried in the atmosphere, and then heated a t 105’ C. for 1 hour. Those samples used as references were ground to -250 mesh in the above manner. High volatile bituminous coal samples were then subjected to various pressures and temperatures in an unsupported pistoncylinder press. The essential features of the sample pressure chamber used are shown in Figure 1. Details have been recorded elsewhere of this ( 7 7) and similar arrangements (2, 77) of the pressure and recording apparatus. Inside the 1-inch
A.
Table I. Sources of Coal Samples and Standards Sample Sources of Coal Buck Mt. Seam, Pa. Anthracite
B.
Pocahontas
C. D.
Low volatile bituminous
A‘. Anthracite
High volatile bituminous
Russia Mercer County, Southern W.
.
V2
I .
Pocahontas Seam No. 4, W. Va. Spring Canyon, Utah
Y\-
STAINLESS STEEL
POWER LEAD
Infrared absorption spectra were obtained from 900 cm.-1 to 3000 cm.-l by the KBr method using a dual beam Model 21 Perkin-Elmer recording IR spectrophotometer. From 2700 to 3600 cm.-1, spectra were obtained of a mull of the coal in hexachlorobutadiene. This latter procedure was used to measure the 3300 to 3400cm.-l band, which is masked in the KBr method by water adsorbed on the pellets.
M,CB
AL ADS THERMOCOUPLE
Experimental Results
STAINLESS STEEL
7 1
MOVING
1
PISTON
SEALING RING
Figure 1 . Sample cell in which coal samples were pressed and heated
diameter chamber is a 0.005-inch thick layer of lead foil for lubricating purposes. 'This encloses a talc insulator, a stainless steel furnzce, another talc insulator, a copper sample holder, and the sample. The temperature was recorded with a chromel-alumel thermocouple embedded in the sample. With the present experimental arrangement, the air available to the sample is essentially negligible. The percentages of carbon, hydrogen, and nitrogen in the samples were determined with an F & M 180 CHN analyzer, for which the average deviation in the case of calibration standards was 3=470b. Oxygen content was determined with a Coleman oxygen analyzer. Sulfur was determined by the ASTM method number D271-58. X-ray diffraction patterns were obtained by the diffractometer method using a GE X R D 5 instrument with CuK, (Ni filtered) radiation. Th.e samples were ground to -300 mesh and smeared on glass slides as an alcohol suspension. For resistance measurements, the sample was formed into pellets which were placed in a Bridgman anvil press (3). Then the d.c. resistance was measured through the anvil faces (with a Keithley Model 61 OA electrometer) a t varying pressures. The pressure was varied in order to minimize the effect of particle contacts upon the over-all resistance of the samples. The resistances of the coal pellets ('/r-inch diameter X 0.010-inch thick) were much larger than that of the anvils, such that the measured resistance waB that of the coal itself.
The ultimate analyses, electrical resistivities, x-ray diffraction tracings, and IR absorption spectra for the high volatile bituminous coal, the same coal after heat and pressure treatments, and for several higher rank coals are shown in Table I1 and in Figures 2 to 4. The standard coal samples of various ranks do not necessarily display average properties of coals of that particular rank. They do, however, show the general trend of properties which has often been observed by authors in comprehensive rank correlation studies, and are shown to illustrate better the changes accompanying the heat and pressure treatments. Trends with Rank. ULTIMATE ANALYSIS(9, 24). Table I1 shows the percentages of C, H, N, 0, and S on an ash-free, moisture-f'ee basis. The ash content of the hv bituminous coal was 9%. An increase in carbon content and a decrease in hydrogen and oxygen content with rank increase are observed, the nitrogen and sulfur percentages are small and quite independent of rank. ELECTRICAL RESISTANCE (74, 78). The relative resistance, at any given pressure, is seen from Figure 2(a-d) to decrease with an increase in rank. The resistances of both coke and graphite are less than 0.1 ohm and, hence, lie far below any of the curves of the figure. X-RAYDIFFRACTION (73, 76,22,23). The major change of x-ray pattern with an increase in rank of coal (Figure 3 A ) is the appearance of a 26' (20) peak, which corresponds to the (002) peak of graphite (the distance between layers of carbon atoms). Very low rank coals display only a low, broad scattering curve which increases in size and becomes less broad as the carbon content increases. Some anthracites and graphite also display several other very small peaks at 28 > 26'. Although the present patterns were obtained to 85', no peaks were obtained except the prominent 26' one, and, hence, the patterns are shown only to 35'. As 20 is decreased beyond 12', the background steadily increases owing to incoherent scattering. INFRARED ABSORPTION SPECTRA(4, 70, 23). Figure 4A shows the I R spectra for hv bituminous and anthracite coals from 800 to 3000 cm.-I, obtained by the KBr method. Figure
Table II. Results of Ultimate Analyses of Coal Samples
Anthracite Pocahontas Low volatile bituminous High volatile bituminous, As received 5 hours 30 kb. 10 hours 400' C. 20 hours 5 hours 400' C. 10 hours 20 hours
{ { i
850" C , 30 kb. 30 min. 1 atm. a
c
H
93.66 92.35 91 .12
3.41 3.99 4.65
N 0.27 3.66 0.64
2.66
78.08 85.93 85.99 86.10 84.41 85.02 90.76 98.16 95.84
5.99 4.65 4.73 4.78 3.99 3.98 2.82 0.72
1.62 0.94 1.39 1.56 1.64 1.36 1.28 0.28 1.58
13.63 8.48 7.88 7.56 9.94 9.59 5.14 0.86 1.52
1.05
0
...
3.70
S
CIH
4
4 4
0.69 a L?
Li 4
e (1
(1
4
c/o
27.66 23.15 19.60
35.21
13.03 18.48 18.17 18.01 21.16 21.36 32.18 138.25 91.28
5.72 10.13 10.91 11.39 8.47 10.16 17.66 114.13 63.05
24 .'63
The S content is below the detection level, within experimental deviation.
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243
A
I
GRAPHITE E' L9
C d R O M CURRAN O M N
0 -I
>.
c_ In
Ez
10
20 30 PRESSURE IN KB
40
50
35
31
27
Figure 2. Resistances of coal samples as a function of pressure, P
25 -28
19
15
II
15
II
DEG.
a. High volatile bituminous, as received; b. Low volatile bituminous; C. Pocahontas; d. Anthracite; e. hvb, 400' C. 5 hours; f. hvb, 4 0 0 ' C. 10 hours; g. hvb. 400' C. 20 hours; h. hvb, 4 0 0 ' C. 30 kb. 5 hours; i. hvb, 4 0 0 ' C. 30 kb. 10 hours.; i. hvb, 400' C. 30 kb. 20 hours. The resistances of both coke and graphite are too small to b e detected b y the present measuring technique-i.e., measuring R through the anvils-as are the resistances of coal samples heated to 850' a t both 30 kb. and atmospheric pressure. The limiting R which could b e detected was about 0.1 to 1 ohm
4B shows the spectra for the same samples from 2900 to 3700 cm.? using a mull with hexachlorobutadiene. General absorption band regions with their assignments as to functional group are: 3400 ern.-', hydrogen bonded O H groups; 3030, aromatic CH groups; 2850-2950, aliphatic CH groups; 1600, aromatic C=C and perhaps some oxygen-containing groups; 1450, aromatic C=C, CH2 and CHI groups; 1000 to 1300, various C-0 groups including phenols, alcohols, and ethers; 1030 cm.-l, aromatic ethers of type ph-0-CHzR. Also absorbing near 1000 cm.-1 are mineral impurities which constitute part of the ash; this was shown by gravimetrically separating the coal and the mineral and measuring the spectra of each. Neither the I R bands greater than 1250 cm.-' nor the x-ray patterns were influenced by the mineral matter. The changes in I R spectra accompanying rank increase are seen to be decreases in the 3400, 2900, and 1200 cm-1 bands and an equalization of the 1600 and 1450 cm.-' bands. Interference with the 1000 cm.-' band by the mineral matter makes impossible the determination of changes in the coal, associated with the latter band. Work by others on demineralized coal shows that the 1000 cm.? band decreases with increasing rank. Effects of Temperature a n d Pressure on hv Bituminous Coal. Upon heating the coal samples in the Kiloton press at atmospheric pressure, evolution of gases could be detected by their odors. Also, dismantling of the sample cell showed that tar-like substances had extruded through the thermocouple hole. Samples which had been pressed, both at room and elevated temperatures, did not give either of the above observations. The effects of heating coal a t atmospheric pressure are seen, from the data, to uprank the coal qualitatively. Since more 244
I&EC PROCESS DESIGN A N D DEVELOPMENT
31
27
23 =-~~,DEG.
19
7
Figure 3. X-ray patterns for standard samples (A), and for high volatile bituminous coal after subjection to elevated temperatures and pressures (6)
detailed investigations of the same type of studies are available (cf. 5, 23), the present results (of heating coal at 1 atm.) will be used only for the purpose of comparison with the results of applying heat and pressure to coal (see Table 11, Figures 2, 3B, and 4, C-F). From this comparison, the effects of pressure may be discerned. Table I1 shows that pressure retards the escape of hydrogen and oxygen at the coalifying temperature of 400" C. Also, a t 400" C., the rates of decrease of H and 0 contents become much slower after the first five hours if 30 kb. is applied, whereas elimination of H and 0 occurs continuously with time a t 1 atm. The effect of pressure a t 850' C. appears to be that of accelerating the H- and 0-removal reactions; however, a t this temperature the coal has been transformed to a coke- or
D, HIGH VOLATILE BITUMINOUS (AS RECEIVED)
[ 0,400: 30 KB, 5 HRS.
2950
2750
1600
1400
1200
1000
WAVENU'MBER IN CM"
WAVENUMBER
I N CM" D, 400: IO HRS.
w 0,400°, 30KB,IO HRS.
a
isile
I
\I
high bituminous (as received)
I
I
I
I
I
E
\Iv, II , B
-3600
-3400
-3W
WAVENUMBER
I
IN
-3000 cm'l
-2800
,I
D, 40OoC, 5 HRS.
I
W
D, 400: 30KB,20 HRS.
IT
q--r4 I w
D, 4OO0C, POHRS.
3600
3400 WAVE:NUMBER
IN cm-l
graphite-like material so a strict comparison with the 400' C. results cannot be made. Comparison of the electrical resistance (R)of heated coal (Figure 2, e , f,and g) to heated and pressed coal (Figure 2, h, i, a n d j ) shows that pressure acts to diminish the effect of temperature during coalification. These data parallel the ultimate analysis trends in showing that great changes of R occur a t 1 atm. u p to 20 hours, while the R of coal at 30 kb. undergoes most of its change in 5 hours. At 850" C. and at 30 kb. or 1 atm., the resistance of coal is reduced to values comparable with those of graphite and coke.
c
2950
2750
1600
WAVE NUMBER
Figure 4.
1400
1200
1000
IN CM"
IR absorption spectra of the various coal samples
The backgrounds a t 850 and 2700 tin.-' were arbitrarily placed at the same intensities. All samples were the some size, were of the same concentration ( 1 0 mg. coal per gram KBr), and were run a t the same instrument settings; ( A ) KBr pellet of hv bituminous (as received) and of anthracite; (B) hexachlorobutadiene mull of hv bituminous and anthracite; ( C ) hexachlorobutadiene mull of treated hv bituminous; ( D , E, F ) KBr pellet of treated hv bituminous
Figure 3B shows that the effect of T and P is to increase the size of the 002 graphite peak (26' to 2 7 O ) , while having little effect upon the gamma band (the broad low peak having its maximum near 2 5 O , 28). The 002 graphite peak is larger for samples to which 30 kb. was applied than for samples heated a t 1 atm. for all times and temperatures studied. Pressing the hv bituminous coal at room temperature produced no change in the."as received" x-ray pattern. The IR spectra for the samples after being subjected to 400' C. and 30 kb. for various lengths of time are shown in Figure 4, C-F. T h e significant changes are: the removal of VOL. 5
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JULY 1966
245
H- bonded OH groups (3400 band), with P acting to slightly retard their removal; the removal of aliphatic C-H groups (the two bands near 2900 crn.-l), with applied pressure significantly retarding the group elimination ; the enlargement of the 1000 crn.-l band with P plus T-since it has been shown by us and others (70, 23) that both mineral matter and aromatic ethers contribute to this band, no definite conclusions can be reached regarding the interpretation of the 1000 cm.-‘ band change; the equalization of the 1600 and 1450 cm.-l bands in the case of application of both 400’ C. and 30 kb. Since the background absorption increases with C content (at constant pellet composition) it is difficult to tell whether the 1600 cm.-l band has decreased or whether the 1450 cm.-’ band has increased in size. The fiqdings of other investigators, while not entirely conclusive, indicate that the size of the 1600 cm.-l band does not grossly change with rank. Therefore, the equalization of the two bands in the authors’ experiments is consistent with an increase in concentration of aromatic carbon double bonds (1450 crn.-’) upon subjecting the coal to both T and P. (The CH bonds contributing to the 1450 cm.-’ band would not appear to increase this band while diminishing the 2900 cm.-’ band.) Discussion
Coal consists of repeating structural units, containing both alicyclic and aromatic groups. The latter groups have often been considered to be surrounded by the former in disordered arrangement (7, 28). Hill and Lyon (72) have recently suggested that the coal structure consists of large heterocyclic nuclei monomers with alkyl side chains held together by three dimensional C-C groups, and includes functional oxygen groups, and ether and thioether bonds. Nitrogen occurs mainly in the heterocyclic ring structure. The heterocyclic nuclei packets consist of planes of polyaromatic ring clusters stacked parallel, but nonoriented with respect to one another, and the distance between these planes produces the x-ray peak near 26’ and probably the band near 25’ (73, 76, 27, 22, 23). Since the polyaromatic planes are nonoriented, and the packets are not aligned or oriented for all but very high rank coals (27), the 26’ x-ray peak is much weaker than for graphite, and the entire x-ray pattern shows only two-dimensional structure-i.e., only one or two of the graphite peaks are present. Upon application of heat, some oxygen functional groups and aliphatic and aromatic fragments are removed as volatiles. The present experimental results indicate that pressure acts to retain some of these volatiles in the structure. This may be done either by retarding the bond breakage which heating produces, or by inducing condensation of some of the fragments onto other parts of the structure. The first explanation qualitatively satisfies the differences between hv bituminous coal heated and that heated a t high pressure as measured by ultimate analysis, electrical resistivity, and IR absorption at 3400 and 2900 cm.-’ However, the fact that high pressure with heat (but not without heat) causes an increase in the graphite peak is not explained by bond breakage but is explained by a condensation which produces more layers of atoms separated by the carbon Van der Waals distance. Also, an increase in the relative size of the 1450 cm.-1 band is more indicative of growth of parts of the aromatic structure than of lack of bond breakage
246
I h E C PROCESS D E S I G N A N D D E V E L O P M E N T
(within the limits of uncertainty of the assignment of the 1450 cm.’ band). The experimental findings, other than the last two named facts, d o not preclude some of the broken-off fragments condensing onto the system as well as some volatilizing. This conclusion has been given by other investigators as a basic concept of coalification (4,20, 27) based on changes with rank similar to those found after applying heat and pressure in this study. By comparing the results of heating with those of heating and pressing the coal for 5, 10, and 20 hours, one might suggest that the role of pressure is to retard coalification and thus to result in coal rather than coke formation. Ac knowledgrnenl
The authors thank the Army Research Office (Durham) for financial support of this work under contract number DAARO(D)-31-124-G18. We also thank L. Sutton and J. F. Betts for experimentally aiding us in obtaining IR spectra and oxygen analyses patterns. To R. R. Dutcher of The Pennsylvania State University, we extend our appreciation for his donation of three of the coal standards. literature Cited ( 1 ) Bh;wmik, J. N., Mukherjee, P. N., Mukherjee, A. K., Lahiri, A,, Symposium on the Nature of Coal,” p. 242, Jealgora, India, 1959. ( 2 ) Boyd, F. R., England, J. R., J . Geojhys. Rcs. 65,741 (1960). ( 3 ) Bridgman. P. W.. Proc. Am. Acad. ArtsSci. 81.165 ,~ (1952). ( 4 ) BroGn, J.’K., J.’Chern. Soc. 1955, p. 744. ( 5 ) Ibid., p. 752. ( 6 ) Cady, G. H., Econ. Geol. 44, 1 (1949). ( 7 ) Cook, M. A., Department of Metallurgy, University of Utah, Salt Lake City, Utah, private communication (1965); (8) Davis, A., Spackman, W., Fuel 43, 215 (1964). ( 9 ) Francis, W., “Coal, Its Formation and Composition,” Chap. VlII, Edward Arnold, Ltd., London, 1954. (10) Friedel, R. A., Queiser, J. A., Anal. Chem. 28, 22 (1956). ( 1 1 ) Gabrysh, A. F., Eyring, H.,Vanhook, A., J. Phys. Chcm. Solids 25, 129 (1964). (12) Hill, G. R., Lyon, L. B., Ind. Eng. Chem. 54, 37 (1962). (13) Hirsch, P. B., Proc. Roy. SOC.A226, 143 (1954). Japan 24,222 (1951). (14) Inokuchi, H., Bull. Chem. SOC. (15) Karavayer, N. M., “Symposium on the Nature of Coal,” p. 27, Jealgora, India, 1959. (16) Kassatochhin, V. I., Larina, N. K., Proc. Acad. Sci. USSR, Phys. Chem. Sect. 114 ( 6 ) , 273 (1957). (17) Kennedy, G . C., Newton, R. C., “Solids Under Pressure,” W. Paul and D. M. Warschauer, eds., Chap. 7, McGraw-Hill, New York, 1962. ( 1 8 ) Kroger, C., Dobmaier, N., “Symposium on the Nature of Coal,” p. 267, Jealgora, India, 1959. (13) Lahira, A., Ibid., p. 7. ( 2 0 ) Mazumdar, B. K., Chakrabartty, S. K., Lahiri, A., Fuel 41, 129 (1962). (21) Mentser, M., O’Donnell, H. J., Ergun, S.,Ibid., 41, 153 (1962). (22) Sever, R., “Second Conference on the Origin and Constitution of Coal,” p. 341, Crystal vpffs, Nova Scotia, 1952. (23) Tschamler, H., DeRuiter, E., Chemistry of Coal Utilization,” H. H. Lowry, ed., Suppl. Vol., Chap. 2, Wiley, New York, 1963. (24) Van Krevelen, D. W., “Coal,” Chap. VI, Elsevier, New York, 1961. (25) White, D., Econ. Geol. 3,292 (1908). ( 2 6 ) Wilson, P. J., Jr., Wells, J. H., “Coal, Coke and Coal Chemicals,” p. 71, McGraw-Hill, New York, 1950. (27) Yokokawa, C., Kajiyama, S., Watanabe, Y . ,Takegami, H., “Symposium on the Nature of Coal,” p. 194, Jealgora, India, i1727. nco
(28) Yokokawa, C., Watanabe, J., Kajiyama, S., Takegami, Y . , Fuel 41, 209 (1962). RECEIVED for review August 16, 1965 ACCEPTEDDecember 20, 1965
