Characterization of Coal Substance by Pyrolysis-Gas Chromatography J i i i Romovitek and Jaroslav Kubat Department of Coke and Gas Technology, University of Chemical Technology, Prague, Czechoslovakia A method is described in which samples of coal are weighed into small tin capsules and, at certain preset times, automatically dropped into a bath of molten tin maintained at a desired temperature. The pyrolysis of the coal sample starts almost as soon as the capsule enters the bath and is complete in less than 50 seconds. As the pyrolysis products are diluted by carrier gas and removed very quickly from the hot medium, secondary degradation is largely eliminated, and the pyrolysis products reflect fairly accurately the chemical nature of the original material. The accurate control and measurement of the bath temperature, which is possible, make the method suitable for kinetic studies on the formation of various compounds produced during the thermal decomposition of complex materials.
THE STUDY of the rapid pyrolysis of coals is one route by which one can obtain a better knowledge of their complex chemical nature. To this end pyrolysis-gas chromatography is a very promising method, as it is possible to arrange the conditions of the thermal degradation in such a way that secondary changes to the primary pyrolysis products are largely eliminated. Many methods have already been described for carrying out pyrolysis-gas chromatography. Among these the most commonly used is the degradation of the substance applied to an electrically-heated spiral wire (1-5). This method cannot be used for solid samples without special precautions. A number of devices have been designed for this purpose (6-10). Most of them suffer from the unsatisfactory heat transfer to the sample and some from the necessity of interrupting the carrier gas flow before each experiment. Karr, Comberiati, and Warner (11) used this technique to compare the chemical natures of the resinous fractions isolated from the low temperature tar formed by brown coals, caking and noncaking coals, and electrode pitch. Griling (12) studied the degradation products formed when different coals were heated in such a manner as to produce a steady rate of temperature rise. Bricteux (13) compared the pyrograms of exinite and vitrinite. Holden and Robb (14) heated coal samples directly in the ionization chamber of a mass spectrometer. (1) R. S. Lehrle and J. C. Robb, Nature, 183, 1671 (1959). (2) J. Jandk, Zbid., 185, 684 (1960). (3) E. C. Jennings and K. P. Dimick, ANAL.CHEM.,34, 1543 (1962). (4) C. E. R. Jones and A. F. Moyles, Nature, 191, 663 (1961). ( 5 ) A. Barlow, R. S. Lehrle, and J. C . Robb, International Symposium on Macromolecular Chemistry, Prague, 1965. (6) F. A. Lehman and G. M. Brauer, ANAL.CHEM., 33, 673 (1961). (7) K. Ettre and P. F. VBradi, Zbid.,34, 753 (1962). (8) Barney Groten, Zbid., 36, 91 (1964). (9) C. Karr, Jr. and J.. R. Comberiati, Fuel, 42,211 (1963). (IO) K. Ettre and P. F. Vdradi, ANAL.CHEM., 35, 69 (1963). (11) C. Karr, Jr., J. R. Comberiati, and W. C. Warner, Zbid., p 1441. (12) G. W. Griling, J . Appl. Clzem., 13, 77 (1963). (13) J. Bricteux, Annales des Mines de Belgique, December 1, 1966. (14) H. W. Holden and J. C. Robb, Nature, 182, 340 (1958).
EXPERIMENTAL
Apparatus. The apparatus consisted of a laboratoryconstructed gas chromatograph equipped with a flame ionization detector and 0- to 1-mV recorder. Pyrolysis was accomplished by dropping a small sample into a bath of molten tin kept at a constant temperature by an external electric heater. The sample was carefully weighed beforehand into a hollow tin cylinder which was then closed by a small plug of the same metal. The cylinder had a height and diameter of 5 mm, and a wall thickness of 1 mm; the plug had a diameter of 4 mm and a height of 2 mm. Because of the high heat capacity of the bath and the rapid rate of heat transfer, the melting of the sample container was almost instantaneous. ' The pyrolyzer (Figure 1) was made of quartz. The samples fell from the sample magazine down Tube 4 into the bath of molten tin, I , the temperature of which was measured at a point about 5 mm below its surface by a thermocouple situated in the well, 7. The pyrolysis products were taken by the carrier gas from the space above through a capillary, 8, cooled by a stream of air. The upper part of the pyrolyzer was similarly cooled. The air entered at 9 and left through the vent, 10. The level in the bath was kept constant by the overflow, 2. The tin which had accumulated in Vessel 3 was withdrawn from time to time through Tube 5 by means of a small stainless steel scoop. During pyrolysis, this tube was kept sealed by means of a plug of silicone rubber. Coke residues were withdrawn from the pyrolysis vessel in a similar manner, Junction 6 permitted the same pressure in Tubes 4 and 5. The pyrolyzer was heated externally by an electric heater to the desired metal bath temperature. The feeding of the samples in the cylinders to the pyrolyzer was accomplished by means of an automatic magazine (Figure 2 ) which operated at certain preset times. The magazine was made from a block of polymethylmethacrylate, I and 2, with holes drilled in it as shown in Figure 2. The samples in the cylinders were piled one above the other in the tubular hole, 13, which was closed with a metal screw with packing, IO. The carrier gas entered at 12 and purged the magazine containing the samples of coal. The lowest sample cylinder in the pile sat in the hole, 14, of the feeder rod, 4 , and was moved by Solenoid 3 to the upper opening of Tube 7, through which it fell into the metal bath of the pyrolyzer. The rod was adjusted with Screw 6. The next lower sample then moved into the hole, 14, upon the return of the feeder rod activated by Spring 5. The magazine was connected to the pyrolyzer by a piece of vacuum rubber tubing. Tube 7 was fastened to the feeder with a gland nut, 8, and PVC seal, 9. The advantages of this method are: Precise quantities of coal samples are automatically fed to the pyrolyzer at predetermined times. Very accurate control of the pyrolysis temperature is possible. Samples are fed without interruption of the stream of carrier gas. Procedure. The tin cylinders had to be cleaned with chloroform before use to remove any trace of grease, the thermal degradation of which could have interfered with the VOL. 40, NO. 7, JUNE 1968
1 1 19
Figure 2. Automatic feeder peaks produced by the pyrolysis products of the coal sample, The cleaned cylinders were handled by forceps. The column was 12 meters long, with a diameter of 6 mm, and was packed with 235 grams of granular porcelain coated with 1 squalane. Particle size was 0.2 to 0.4 mm. The temperature of the column was 70 "C. The interval between successive samples was set at 150 minutes. The flow of carrier gas was maintained a t its optimum value of 50 ml/ minute (measured at 0 "C and 760 mm Hg). The flow rate of hydrogen was 40 ml/minute, and of air 500 ml/minute. In a typical run, precise amounts of the coal samples (air-dried, particle size less than 200 microns) were used, the weight of the samples chosen to be the same on a dry mineral- and ash-free basis. Some conventional analyses of the coal samples are given in Table I.
Figure 1. Quartz pyrolyzer
Table I. Analyses of the Coal Samples Used in Pyrolysis-Chromatography
Class and group Anthracitic Semianthracite Bituminous Low volatile Medium volatile
Sample A
Urx
B
Odra
C D E F G
Sverma Stachanov Vitgznj. h o r Doubrava Jeremenko blexandr CSA Zdrubek (I) Zdrubek (11) Zollverein exinite vitrinite inertinite FuElk Pionir Viesovd
H I High volatile
Subbituminous Lignitic Brown coal
Seam
K L
M N F
= Volatile matter dry ash-free basis.
1120
ANALYTICAL CHEMISTRY
Ash dry basis, VMa C Moisture, % d.a.f., % d.a.f.,
H d.a.f.,
z
Dilatometer ConExpanSwelling traction, sion, index %
Code
z
1.01 0.88
7.04 5.78
12.92 16.50
90.0 90.6
3.96 4.46
0
0
0
200
8
19
15
332
0.75 1.06
18.68 19.91 22.21 25.97 27.07 29.55 30.27 31.61 32.02
89.9 88.8 91.0 87.8 88.6 86.5 87.0 84.95 78.5
4.70 4.06 4.90 4.87 5.07
4 9 8.5 1 8.5 5.5 6 8 6
23 33
0
15
45 120
0.74 1.65 1.44 1.09 1.13
10.93 6.28 5.32 9.81 5.32 21.62 8.10 3.13 19.75
33 25 20 28 19 20
187 - 18 -4 61 8
321 333 434 411 435 532 532 533 532
2.03 1.90 18.9
0.7 0.9 7.05 6.64 8.84 19.5
59.1 32.5 20.9 34.61 37.18 53.8
86.53 86.27 88.68 83.25 81.25 64.4
7.09 5.25 4.03 5.2 5.35 5.4
1.5 1.5
30 46
0 0
611 611
0.60 1.40
5.00
5.52 4.85 5.15
0
Peak No.
7
9
Height (mm) Sample 1 Sample 2 Sample 3 Sample 4
185 195 183 183
70 70 65 65
Variation for individual peak height in from arithmetic mean
3.06
4.3
Table 11. Precision of Pyrolysis 10 12 13
15
17
27 85 80
25
45 45 48 42
35 35 37 35
27 30 30 28
115 112 112 118
83
3.6
5.3
3
5
2.4
2.5
30 30 30
82
In most experiments the optimum pyrolysis temperature of 720 OC was used. For measuring the velocity of coal decomposition, the flame ionization detector was connected directly to the pyrolyzer. Only a small proportion of the exit gas stream was passed through the detector, the remainder being discharged to the atmosphere. The start of pyrolysis was indicated by the first movement of the recorder pen, the end by the return of the pen to its initial position. The size of the samples was such that they always contained 5 mg of dry and ash-free (d.a.f.) matter, and the pyrolyses were carried out under the same conditions as described above. When a sample was dropped into the bath, degradation could be seen to commence the moment it touched the surface. After a period of between 1 and 22 seconds, the deflection of the recorder reached its maximum and thereafter fell back to zero as the volatile matter was driven off. A plot of the deflection against the time elapsed after the sample touched the surface had a shape reminiscent of an Erlangian distribution curve, with its maximum corresponding to 45 % of the area under the curve. The rate of decrease of deflection beyond the point corresponding to 90% of the area was very slow, the curve approaching the abscissa asymptotically. The time interval between the first movement and the maximum deflection could be measured with satisfactory precision. The heights of the peaks in the pyrograms were compared with those obtained from GLC analyses of calibration mixtures containing known amounts of the constituent under
Table 111. Characterization of Some Peaks in the Pyrogram Peak no. Elution time same as 5 Isopentane 6 2-Methylbutene-1 7 Pentane 8 2-Methylbutene-2 9 Cyclopentadiene 12 4-Methylpentene-1 13 Cyclopentane 14 Hexane 15 Meth ylcyclopentane 17 Benzene 20 Methylpentane 24 Methylcyclohexane 27 Toluene 32 Ethylbenzene 33 m,p-Xylene
b Figure 3. Chromatograms obtained from a bituminous coal at different pyrolyzer bath temperatures (from above 610,700,750,780 "C)
T I M E MINUTES VOL 40, NO. 7, JUNE 1968
0
1121
investigation. These mixtures were prepared by evaporating known quantities of the constituent into a large vessel of known volume (80.5 liters) (15).
RESULTS AND DISCUSSION Precision. Four identical samples of one coal were analyzed and the relative standard deviation for the individual peak heights in the chromatograms was evaluated (Table 11). The results in all but one instance were better than 5x. A greater variation was observed with the smaller peaks because these could not be measured with the same accuracy as the larger ones. Characterization of the Peaks in the Pyrograms. The elution times of the peaks of the pyrograms were compared with the elution times for the various pure hydrocarbons. This was done by adsorbing the vapors of these pure hydrocarbons on the surface of a coal sample. Thus a sample would be kept for a definite time in a stoppered weighing (15) J. RomovekEk and Z. NovotnL, Brennstoff-Chernie, 42, 161 (1961).
bottle under the vapors of the hydrocarbon suspected of producing the particular peak of interest. On pyrolysis of the sample prepared in this way, the adsorbed hydrocarbon caused a marked heightening of the peak, the elution time being the same. This technique was successful with higher boiling hydrocarbons such as benzene and toluene, but could not be used with hydrocarbons of low boiling point such as pentane. In this instance a mixture of the light boiling hydrocarbon with benzene or toluene was examined and its elution time related to theirs. The results of these measurements are summarized in Table I11 Changes of the Pyrograms Due to Rank of Coal Substance. The optimum pyrolysis temperature was selected using the pyrograms obtained at bath temperature of 610, 700, 750, and 780 "C. The optimum temperature was considered to be the one which gave the greatest difference in relative peak heights for coals of different rank. This value corresponds to about 700 "C. At lower temperatures--e.g., 600 "C-the decomposition of coal is too slow, which prevents satisfactory resolution of the peaks. Above 750 "C the degradation is so intense that the initial peaks correspond-
A
C
TIME MINUTES Figure 4, A and C. Chromatograms obtained from coal of different ranks 1 122
ANALYTICAL CHEMISTRY
ing to the nonaromatic hydrocarbons almost completely disappear and only the peaks at the end of the chromatogram (mostly aromatic hydrocarbons) remain. By visual observation one can see that the line connecting the minima of the peaks at the start of a pyrogram is hyperbolic in shape. The exact shape of this line is influenced by
the rate of the thermal degradation, which is a function of the pyrolysis temperature (Figure 3), and the rank of the coal (Figure 4). Thus with the semianthracite A (high rank coal) the highest point on the hyperbola is far above the asymptotic value which the end of the pyrogram approaches. On the other hand, with the bituminous low volatile coals C and E
E
I\
TlME MINUTES Figure 4, E, H, and M. Chromatograms obtained from coals of different ranks VOL. 40, NO. 7 , JUNE 1968
1123
this differencein height is smaller and with the subbituminous low rank coals is very small. Another characteristic feature may be discerned in peaks 15 and 24. The height of peak 15 increases with decreasing rank and peak 24 attains its maximum value with very good coking bituminous coals such as E. In order to evaluate the pyrograms quantitatively, the heights of all peaks have been compared with that of some peak selected as a standard. This quantitative evaluation is shown in Figure 5. Curve 1 represents a plot of the ratio of the height of peak 17 to that of peak 7 against the rank expressed as percentage of volatile matter on a dry and ashfree basis (VM d.a.f.). In curve 2 the ratio of peak 27 to peak 7 is plotted against rank and in curve 3 the ratio of 12 to 13. Curve 4 is a plot of the ratio of the area under the peaks for aromatics (A) to that for nonaromatics (NA). For the accurate characterization of the high rank coals, curves 1
and 2 are suitable because of their steep slopes, whereas for low rank coals, curves 3 and 4 are available. Pyrograms of the Macerals. Because the amount of the sample needed for one analysis is very small ( 5 mg) this method is well suited to the characterization of macerals isolated from coal. Figure 6 shows how differences in the nature of macerals from a bituminous high volatile coal are reflected in the shape of the pyrograms. Compared with vitrinite, exinite shows a greater number of nonaromatic peaks appearing before benzene (peak 17) in the pyrogram. The pyrogram for inertinite is poor in all the characteristic pyrolysis products except for toluene (peak 27) from the residual solvent used for the isolation of the maceral (16). (16) W. Wassenberg, Dissertation der Rheinisch-Westfalischen Technischen Hochschule, Aachen, 1964.
3
4
I
2
IO
20
30
Figure 5. Plot of relative peak heights and nonaromatics/aromatics ratio us. coal rank expressed as percentage volatile matter dry-ash-free basis 11 24
ANALYTICAL CHEMISTRY
-
t+o%VMd*r;f:
_ _ _ ~
Table IV. Artificial Oxidation of Coal Relative peak Time of height 17 over 7 oxidation (hours) 0.74 0 1.21 0.5 1.26 1 1.7 2 1.91 4 3 8 4.2 24 7.1 72
INERTlNlTE
Changes in Pyrograms Caused by Artificial Changes of Coal Substance. Coal samples E (bituminous low volatile) and M (subbituminous) were heated in air to a temperature of 190 "C for periods ranging from 0.5 to 72 hours. Artificially oxidized samples prepared under these conditions were then analyzed by pyrolysis-gas chromatography. The changes of the coal substance were well reflected by changes in the pyrograms. There was a continuous decrease with time of all peak heights observed. This phenomenon mainly affected the nonaromatic peaks, the height of the benzene and toluene peaks changing very little. The change in the ratio of the height of peak 17 to that of peak 7 with the time of oxidation can be seen in Table IV. The devolatilization of coal by carbonization causes changes in the pyrograms sirnilar to induced oxidation. In the pyrogram of the semi-coke only the benzene and toluene peaks are present, besides methane and other low-molecular-weight gases. When the coal sample was heated to 350 "C in an autoclave under hydrogen at a pressure of 100 atmospheres (measured at 20 "C) for 2 hours and then pyrolyzed, there were no significant changes observed in any of the peak heights except for benzene, which increased substantially. The ratio of benzene peak height to pentane peak height is 0.86 before hydrogenation and 3.1 after hydrogenation. Kinetics of Benzene and Toluene Evolution during Coal Pyrolysis. Because the apparatus allowed precise measurements of the benzene and toluene concentrations in the degradation products, as well as accurate control of the pyrolysis temperature, an attempt was made to find out how the evolution of these two main pyrolysis products depended on the temperature. For the evaluation of the kinetic data in Table VI, we used the following values : (a) Time of 45% decomposition (Table V). It was assumed that the evolution of the particular constituent proceeded at a uniform rate during the whole pyrolysis period. (b) The amount of particular hydrocarbon formed at pyrolysis bath temperatures of 500, 600, 700, 800, 900, and 950 "C.
16 9 8
31 17
5
16
3
8
16 15
3 3
8
TlME MlNUTES Figure 6. Chromatograms of the macerals from the semi-bituminous coal samples By plotting the logarithm of the effective quantity formed against the reciprocal of the absolute temperature, the graphs in Figure 7 for benzene and toluene, respectively, have been obtained. The plot for benzene is linear, which indicates that
Table V. Time of Pyrolysis of Coal Samples Temperature, "C 700 800
600
B E , Time, sec H M N ,
VI T RlNl TE
6
11 9 7 7 6
4 3
3 2 2
900
8 7 6 5 5
1 1 1
1
1
VOL 40, NO. 7, JUNE 1968
3
3 3 2 2
1125
I
12
‘
I1
IO
8
9
11
9
10
,)
a
I/T Figure 7. Arrhenius plot for benzene and toluene formation Table VI. Kinetics of Pyrolysis Temperature of pyrolysis bath ( “ C ) 500 600 700 800 900 Benzene Toluene Benzene Toluene Benzene Toluene Benzene Toluene Benzene Toluene 0.64 0.04 0.152 0.001 0.02 0.028 0.044 0.18 0.02 0.09 0.0071 0.0045 0.025 0.0073 0.00091 0.00127 0.0069 0.116 0.08 0.22 0.008 0.008 0.028 0.04 0.052 0.086 0.38 0.0193 0.0133 0.073 0.029 0.0028 0,00325 0.028 0.38 0.22 0.12 0.36 0.132 0.44 0.008 0.01 0,064 0.088 0.044 0,025 0.12 0,044 0.44 0.007 0,0098 0.256 0.118 0.4 0.126 0.12 0.078 0.58 0.012 0.016 0.13 0.042 0.064 0.0295 0.013 0,0087 0.58 0.168 0.183 0.4 0.074 0.086 0.202 0.88 0.044 0.016 0.018 0.2 0.101 0.056 0.061 0.044 0.0092 0.0108 0.88 0.42 0.204 0.176 0.1975 0.028 0.02 0.028 0.076 0.092 0.92 0.028 0.92 0.0095 0.0115 0.059 0,0625 0.21 0.102 (1) . . Percentage of benzene and toluene, respectively, formed from d.a.f. matter (wt %) yield of benzene (or toluene) wt (2) Effective rate constant, K, for benzene and toluene, respectively; k = 100 . (time of pyrolysis)
z)
the mechanism of benzene formation from coal is simpler than that for toluene, which yields a curve. It is likely that with toluene formation by thermal degradation removal by some secondary change, probably dealkylation, proceeds simultaneously. The apparent activation energies have been calculated from this Arrhenius plot. The calculated values of activation energies for all samples were practically identical, and were about 29 kcal/mol. The integration constant increased with decreasing rank. 1126
ANALYTICAL CHEMISTRY
950
Toluene
0.02 0.02
0,046 0.046
z
ACKNOWLEDGMENT
The authors are indebted t o Carl Kroger, Technical University, Aachen, for the gift of the set of purified macerals, and to Leslie Cox, University of Strathclyde, Glasgow, for help with the English text of the paper.
RECEIVED for review June 5,1967. Accepted March 20, 1968. Presented in part at the 6th International Conference on Coal Science, Miinster, Germany, 1965.
