Stoichiometric Analysis of Oil Shales by Laser-Pyrolysis Gas Chromatography Ray L. Hanson,‘” Douglas Brookins, and N. E. Vanderborgh2 University of New Mexico, Albuquerque, N.M. 87131
Analysls of the hydrogen, carbon monoxide, methane, acetylene, and ethylene produced by laser-pyrolysls of oil shales can be used In stolchlometric analysis of these shales. The carbonate content of the samples correlates wlth the quantlty of carbon monoxide produced, Fischer Assays of the oil content of the shales correlates with the hydrocarbon gases produced, and hydrocarbon gases also correlate with the organlc carbon content of the samples. The hydrogen to carbon monoxide ratio of the products increases with increasing organic hydrogen content of the samples.
Oil shales represent a large fossil fuel resource in the United States. Oil shales are a marlstone-type inorganic rock mixed with kerogen (the organic fraction). Typical inorganic minerals are dolomite, calcite, quartz, illite, albite, trona, nahcolite, microcline, pyrite, and analcite ( I ) . Kerogen is a high molecular weight material, with a macromolecular structure, which is insoluble in common organic solvents (2). Although the structure of kerogen is not known, it is thought to resemble the lignin-derived humic acids. Characteristically, the organic matter is high in hydrogen and low in oxygen, suggesting octadecylalcohol derived from marine and aquatic organisms, as a common precursor of these materials. The hydrogen to carbon (H/C) ratio in kerogen remains relatively uniform over hundreds of square miles or more ( 2 ) .Typical H/C ratios for oil shales are 1.6 f 0.10 ( 3 , 4 ) .Analysis of Colorado shale oil reveals that over 60% of the oil is not hydrocarbons. The oil is composed of 36% nitrogen-bearing compounds, 6% sulfurcontaining compounds, and 18% oxygen-containing compounds (5). The organic content of oil shales is routinely evaluated by modified Fischer Assay (6).Atwood (7) described the TOSCO Modified Fischer Assay in which the product gas was collected and analyzed to obtain a closed material balance. He reported that the standard deviation on oil yield averaged about 0.5% on a volume basis (0.2 gal/ton for a 40 gal/ton shale), although Cook ( 3 )reported that routine Fischer determinations were precise to within 0.5 gal/ton for average shales. Retorting oil shales or application of these modified Fischer Assays leave 20 to 25%of the total organic carbon in the shale residue. Cook ( 3 )also reported on extensive studies of the correlation between oil yield from the total carbon, organic carbon, and total hydrogen content, and Fischer Assay. The 95% confidence limit for these correlations was f2.46 gal/ton for organic carbon, f5.00 gal/ton for total carbon, and f 3 . 7 3 galhon for total hydrogen. Thus, standard methods for oil shale characterization include elemental analysis and the Fischer Assay. A conference-workshop of the National Science Foundation entitled “Analytical Chemistry Pertaining to Oil Shale and Shale Oil” (8) recommended the development of analytical chemical expertise in the characterization of oil shale. Present address, Inhalation Toxicology Research Institute, Lovelace Foundation, P.O. Box 5890, Albuquerque, N.M. 87115. Present address, Los Alamos Scientific Laboratory, University of California, Los Alamos, N.M. 87544. 2210
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Several pyrolysis methods have been reported. Barker (9) used 2- to 3-g samples in a pyrolysis method with heating a t 9 K/min. Products were trapped at liquid nitrogen temperatures and analyzed subsequently by gas chromatography. This method requires 8 h per sample. Giraud (IO) also applied pyrolysis and gas chromatography to the characterization of kerogen in sedimentary rock. He listed two important conditions that are necessary for this type of kerogen analysis: 1) The duration of the evolution of pyrolysis products must be brief. This requires rapid thermal rise times. 2) The cracking of the degradation products must be minimal. These criteria for pyrolysis reactions, which have also been stated by Levy et al. ( I I ) , readily follow from the necessity for forming products a t essentially a single temperature. If the products form over a temperature range, a variety of compounds result. Laser-pyrolysis gas chromatography (LPGC) most closely approaches these experimental conditions. This method provides a rapid means for pyrolysis of solids which reduces the possibility of secondary reactions by the pyrolysis fragments. Biscar (I2,13)used a normal-pulse ruby laser to pyrolyze oil shales. The gaseous products were swept into a gas chromatograph for separation and quantitation. He demonstrated that the acetylene yield from laser pyrolysis of oil shales correlated with the Fischer Assay oil yield of the samples. This LPGC study of oil shales was undertaken to determine additional implications of this method. The LPGC method yields two types of products due to plasma quenching and thermal blow-off. The former result from both kerogen and the inorganic matrix (especially carbonates), and the latter yield a number of higher molecular weight fragments from kerogen. The low molecular weight gaseous products (H2, CO, C2H2, CH4, CzH4, and H20) are used to characterize the oil shales.
EXPERIMENTAL A Perkin-Elmer 3920 gas chromatograph was used with a series connected beta-induced luminescence detector (BILD) and hydrogen flame ionization detector (FID). A quartz sample tube containing the pelletized oil shale sample was mounted on the injection port of the gas chromatograph. The powdered oil shale samples were pelletized at 20 000 psi in a pellet press. Portions of the pellets were sectioned and placed in the sample tubes. A pulsed ruby laser (694.3 nm) with an output of 2.6 J in the normal mode and a normal-pulsed neodymium laser (1064 nm) with 1.8-5 output were used in these studies. The laser output was focused on the samples in the sample tubes. Helium carrier gas swept the pyrolysis products onto the analytical column.
RESULTS AND DISCUSSION An analysis of oil shale samples obtained by Sandia Laboratories from the Green River Formation, Wyoming is given in Table I. The percent carbonate was determined by warming portions of the samples in sulfuric acid and back-titrating the excess acid with sodium hydroxide. The inorganic carbon is entirely from the carbonate fraction and the organic carbon is the total carbon minus the inorganic carbon. Table I1 contains the comparison of the relative composition of methane, ethylene, and acetylene, produced by pyrolysis with the pulsed ruby laser as measured by the corrected re-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Table I. Characterized Oil Shales
Sample No.
Fischer Assay, gal/ton
Carbon, %
Hydrogen, %
Nitrogen, %
H/C
CO,z-,%
20 21 22 23 24 25 26 27
10.1 15.2 20.1 25.6 36.2 40.0 45.4 49.4
8.50 f 0.09 13.66 f 0.07 15.69 f 0.11 19.01 f 0.04 22.11 f 0.08 24.14 f 0.19 24.93 f 0.23 28.04 f 0.22
0.91 f 0.02 1.43 f 0.01 1.70 f 0.02 2.03 f 0.01 2.36 f 0.04 2.64 f 0.03 2.96 f 0.03 3.41 f 0.03
0.36 0.38 f 0.01 0.53 0.58 0.67 f 0.02 0.73 f 0.02 0.82 f 0.03 1.06 f 0.01
1.28 1.25 1.30 1.28
15.07 15.24 20.14 16.59
1.28
18.85 17.18
1.31 1.43 1.46
8.96 15.22
Inorganic Carbon, %
Organic Carbon, %
3.01 3.05 4.03 3.32 3.77 3.43 1.79 3.04
5.49 10.61 11.66 15.69 18.34 20.71 23.14 25.00
Table 11. Comparison of BILD and FID Response from Oil Shale Pyrolysis
Relative percentage composition Oil yield, gal/ton
FID
CH4 BILD
6.0 18.0 27.4 36.9 39.7
36.5 36.7 32.6 32.3 37.4
36.1 36.6 32.6 32.5 36.9
CZH4 BILD
C2Hz FID BILD
FID
40.3 35.0 34.3 32.8 40.9
23.1 28.3 33.1 34.9 21.7
43.1 37.7 37.1 35.6 43.7
INCREASING TIME (min)
20.8 25.7 30.3 31.9 19.5
I
Figure 1. Separation of low molecular weight products from neodymium laser pyrolysis of oil shale The upper trace is the BILD response and the lower is the FID. The laser is fired at x and time increases from left to right. Peak identifications are: (A) hydrogen: (B) carbon monoxide, oxygen, nitrogen: (C) methane: (D) carbon dioxide: (E) water: (F) acetylene: (G) ethylene: (H) ethane; (I) propyne: (J) cyclopropane: (K) allene: and (L) propene. For the FID, the attenuation is 200 for methane, acetylene, and ethylene, and the attenuation is 10 for the remaining peaks. The helium carrier gas has a flow rate of 40 ml/min. The temperature program is 2 min at 313 K, 16 K/min to 473 K, then isothermal for the remainder of the program. 80-100 Mesh Carbosieve B, 6-ft column
sponse of the FID and BILD. The agreement of the detectors for methane was excellent; some difficulty was experienced for acetylene and ethylene because these peaks occurred concurrently with the water peak. Water was only detectable on the BILD. The BILD was necessary here because i t responds to hydrogen, carbon monoxide, carbon dioxide, and water, and these are not detected by the FID. Thermal conductivity detectors, although sensitive to CO, COZ, and HzO, are insensitive to hydrogen when helium is the carrier gas. The average variation between the detectors was about 0.3% for methane, 2.8% for acetylene, and 2.5% for ethylene. The FID,
Figure 2. Upper trace BILD response for neodymium laser pyrolysis of oil shale. Lower trace FID response for neodymium laser pyrolysis of oil shale Porapak Q Column 6 4 100-120 mesh. Recorder attenuation as shown. Helium carrier gas at 30 ml/min. Temperature program: 2 min at 373 K, 8 K/min to 493 K, isothermal at 493 K. Laser fired at A
with greater sensitivity for hydrocarbons, was more reliable for hydrocarbon analysis. Figure 1 contains the pyrograms for the separation of the lower molecular weight gases with the Carbosieve B column. This column was especially suited to separate lower molecular weight gases. The higher molecular weight products were not eluted. The upper trace is that of the BILD response and the lower trace due to the FID. Carbon monoxide, carbon dioxide, hydrogen, the broad tailing water peak, and other compounds are present in the BILD trace. This illustrates the difficulty of comparing the acetylene and ethylene peaks from the BILD and FID. The BILD response to acetylene and ethylene was superimposed on the tailing water peak which made data interpretation difficult. The water content of the oil shales varies, which results in different quantities of water that might have affected the response of Cz hydrocarbons in the BILD. Here the utility of the BILD was clearly shown. Although the FID showed acetylene as the major product, the BILD data
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Table 111. Low Molecular Weight Gases from LPGC of a Typical Oil Shale (Sample 23) Relative volume percentage Power, J/pulse
Laser detector
CH4
C2H2
CZH4
C2H6 CH3--C=CH
2.6
17.65 11.70
46.92 61.31
26.59 20.62
1.27 1.69
Laser detector
Power J/pulse
H2
CO
Neodymium BILD
1.8
41.6
43.3
C02 3.55
HzO 3.94
Neodymium FID Ruby FID
1.8
1.70 1.06
CH2 CHz-CHz 1.02 1.04
CHz=C=CH 4.11 2.53
CHs-CH=CH2 0.75
Table IV. Oil Shale Results
Sample No.
Total H/ Organic C
20 21 22 23 24 25 26 27
1.99 1.62 1.75 1.55 1.54 1.53 1.54 1.64
Assume H/C = 1.55 organic hydrogen percent
Percent water from remaining hydrogen
0.71 1.37
1.8 0.5
1.50
1.8
2.03 2.37 2.67 2.99 3.23
None None None None 1.6
BILD percent HZ 37.5 41.7 41.9 41.6 42.8 45.2 46.9 42.7
BILD percent
co
52.1 47.1 44.7 43.3 40.8 39.2 36.8 38.1
BILD H2/CO 0.658 0.884 0.938 0.959 1.05 1.15
1.27 1.12
Table V. Oil Shale Pyrolysis Results
Sample No. 20 21
22 23 24 25 26 27
Ruby laser, total FID counts x 10-6 1.92 4.00 4.59 6.00 7.86 6.86 9.90 9.66
indicated that CzH2 was only a relatively minor component in the low molecular weight gases. Figure 2, pyrograms for neodymium laser pyrolysis of an oil shale sample, shows some of the higher molecular fragments detected by the BILD and FID, respectively. The peak with attenuation 50 about midway in Figure 2 has a retention time similar to that of benzene. The pyrograms under these conditions show fragments of molecular weight up to 150. These are expected as thermal blow off products from kerogen, since molecules with a t least 80 carbon atoms have been reported ( 1 4 ) .The integrated peak areas from the FID indicate that 67% of the counts resulted from the peaks with retention times of less than 100 s. These are the low molecular weight gases which have a smaller response per molecule than the larger hydrocarbons. Thus, the volume percent of the low molecular weight gases is about 85%. The third and fourth peaks in the BILD trace are only partially resolved. These peaks represent carbon dioxide, water, and possibly other gases that the FID does not detect. The hydrocarbon products from ruby and neodymium laser pyrolysis of a typical oil shale are listed in Table 111.The BILD response for neodymium laser pyrolysis is also listed for this sample. Acetylene, ethylene, and methane were the primary 2212
Neodymium laser, total FID counts x 10-6 1.62 2.88
3.42 4.78 4.90 5.69 5.89 7.43
Neodymium laser FID results C2H2/ CHd CZH4 CzHz 3.78 0.210 3.49 0.235 2.20 0.259 1.76 0.376 1.43 0.366 1.25 0.415 1.69 0.327 1.07 0.454
hydrocarbon products. Hydrogen and carbon monoxide were the primary products from laser pyrolysis of oil shales. Some water is also present in oil shales. Table IV contains the ratio of total hydrogen to organic carbon, the organic hydrogen percentage assuming a constant H/C ratio of 1.55, the percentage of water from the remaining hydrogen, experimentally determined hydrogen and carbon monoxide percentages, and the Hz/CO ratio. Samples which showed no calculated water, such as No. 25, resulted in small experimentally determined water peaks. Table V contains the experimental response showing the total hydrocarbon products from both ruby and neodymium pyrolysis using the Carbosieve B column. The use of ratios of relative products minimized pulse-to-pulse variations from the lasers. Thermodynamic equilibrium calculations predicted that the CH&zHz ratio should increase with increasing organic content in the shales, because if water and carbonate content are assumed constant, the relative hydrogen to carbon stoichiometry should increase with increasing oil content. The values for the CH4/CzHz ratio from Table V show this predicted trend. Sample 26 seems to be anomalous. The acetylene to ethylene ratio also indicates the quantity of kerogen. With more kerogen in the rock, a greater quantity of ethylene was
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I
I
1
1
I
-
L
30
0.1
0.2
0.3
0.4
0.5
00
1.0
0.5
I.4
BILD H 2 K 0
a6
Figure 4. Experimental H&O
INORGANIC CARBON /ORGANIC CARBON
Figure 3. Inorganic carbon/organic carbon ratio vs. the volume percentage of carbon monoxide in the gaseous products from neodymium laser pyrolysis of oil shales Neodymium laser pulse of 1.8 J. Inorganic carbon equals the carbon in the experimentally determined carbonate content. Organic carbon equals the total carbon minus the inorganic carbon. The least squares line has a slope of 32.9, an intercept of 34.7, and a standard error of 1.44
produced by the thermal blow off route and the ethylene formation was far greater than that predicted by the free energy calculations. The CzHz/C2H4 and CH4/CzHz ratios indicate that the quantity of methane and ethylene increases at a greater rate than the quantity of acetylene as the organic content of the shales increases. The hydrocarbon products detected by the FID and BILD increased linearly with increasing Fischer Assay of the oil shale samples. Positive intercepts indicate that the Fischer Assays underestimate the total hydrocarbon content. These results agree with the ruby laser pyrolysis results and indicate that the total hydrocarbon product yield also is representative of the oil content of shales, as suggested by Biscar (13).The total hydrocarbon products from the FID and BILD also increased in a direct linear relationship with the organic carbon content of the oil shale samples. these relationships suggest that, for a simple Fischer Assay correlation, a total hydrocarbon yield is sufficient. This could be rapidly determined using the FID, since no separation of the hydrocarbon compounds is required and thus the gaseous products could go directly to the FID. The laser pyrolysis of calcium carbonate produced a gaseous mixture of 52% carbon monoxide and 48% carbon dioxide. Determination of the carbonate content of the oil shale is necessary for possible correlation with these products. In Figure 3 the inorganic carbon to organic carbon ratio for the samples is plotted vs. the relative volume percentage of carbon monoxide in the gaseous products from neodymium laser pyrolysis. The nonzero intercept indicates that carbon monoxide also results from the organic matter in the oil shales. Chemical analysis of typical kerogen yielded 5.75 f 0.49 weight percent oxygen ( 1 ) .The equilibrium calculations for oxygen-containing organic compounds show that significant amounts of carbon monoxide will result over the temperature range of 1100-2500 K. Therefore, carbon monoxide is expected to be a plasma-quenching product from the kerogen in oil shales. Figure 4 shows the plot of the product gases Hz/CO ratio from LPGC of the oil shale samples vs. the calculated organic hydrogen content. This shows the increase in the relative organic hydrogen to organic carbon ratio as the oil content increases. The highest oil content sample may have a still higher organic hydrogen to organic carbon ratio. These apparent changes in slope may be another indication of the same fac-
ratio for neodymium laser pyrolysis products of oil shales vs. the organic hydrogen content
Neodymium laser pulse of 1.8 J. Organic hydrogen content equals the organic carbon content times 1.55 (which is a typical H/C ratio for Green River kerogen). Line A, the least squares line for ail the experimental points, has a slope of 4.33, an intercept of -2.34, and a standard error of 0.283. Line B, least squares line for the fourth through seventh highest organic hydrogen content points, has a slope of 3.06, an intercept of -0.878, and a standard error of 0.0274. Line C, the least squares line for the three lowest organic hydrogen points, has a slope of 2.85. an intercept of 1,16, and a standard error of 0.0099
-
1.0
2.0
3.0
4.0
CZHZK 2%
Figure 5. Experimental C2H2/C2H4 ratio from neodymium laser pyrolysis of oil shales vs. percentage organic carbon in the samples The neodymium laser pulse is 1.8 J. The organic carbon percentage equals the total carbon minus the inorganic carbon from the experimentally determined carbonate content. The least squares line for ail the points has a slope of -5.94, an intercept of 28.7, and a standard error of 2.76
tor(s) that produced a similar break in the acetylene vs. Fischer Assay relationship (13).The change in slope in Figure 5 is a t the same Fischer Assay content as found previously. Another indication of a change in the H/C ratio of the organic matter in the oil shales is shown in Figure 5 where the experimental CzHz/CzHd ratio is plotted vs. the organic content of the oil shales. The CzHz/CzH4 ratio decreases with increasing carbon content. The four points with organic carbon content between 11and 21% exhibit a linear relationship which may indicate changes in the H/C ratio of the organic matter of the samples, or may result from a combination of analytical procedures used in determining these parameters. The CH4/CzH* ratio from ruby laser pyrolysis is lower than that from neodymium laser pyrolysis. This indicates lower relative thermal blow off products due to more of the sample being pumped into the plume. The ruby laser pulse of 2.6 J has greater energy than the neodymium laser pulse of 1.8 J. The
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