Energy & Fuels 1997, 11, 915-919
915
Secondary Pyrolysis of the Products of the Thermal Destruction of High-Sulfur Oil Shale Valentin Fainberg,* Alina Garbar, and Gad Hetsroni Center for Research in Energy Engineering and Environmental Conservation, Faculty of Mechanical Engineering, TechnionsIsrael Institute Of Technology, Haifa 32000, Israel Received October 8, 1996X
Secondary pyrolysis of products of Israeli oil shale processing was studied in a two-stage benchscale unit. The gas and oil vapors generated from the primary pyrolysis were sent to the converter for the secondary pyrolysis at temperatures of 650-820 °C. The oil yield on kerogen decreased from 35.3% at the pyrolysis temperature of 500 °C to 15.4% at 820 °C. The gas yield increased in the same temperature range from 10.7 to 25.5%. The yields of hydrogen, methane, ethylene, and carbon monoxide increased with temperature, whereas yields of alkanes decreased. The secondary pyrolysis enables us to simplify substantially the composition of the primary shale oil. The higher the severity of the conversion, the higher the yield of the simplest homologuessthiophene, naphthalene, phenanthrene, anthracene, thionaphthene, and dibenzothiophenesand the lower the yield of alkyl derivativesstoluene, methylthiophenes, styrene, etc. Maximal content of methyl and dimethyl derivatives was observed at a temperature of 730 °C. The total thiophenes yield may be as high as 6.4% on oil shale organic matter, and this can be of practical interest because thiophenes are an important source for the production of light- and photoemitting polymers, materials for semiconductors, electrochemical cells, films, sensors, and other high-tech devices.
Introduction Several papers and books1-3 concerning the effect of various parameters (temperature, rate of heating, gas medium, etc.) on oil shale and shale oil pyrolysis have been published. In all these investigations, attention is paid mainly to the conditions of thermal decomposition of oil shale kerogen. However, secondary reactions of the decomposition, which take place in the retorting zone, are no less important from the point of view of the composition and properties of the end products. It is known that as a result of the secondary thermal reactions, oil yield decreases, gas yield increases, aromatics content in the oil increases; but these phenomena were not, as a rule, the object of special research and were practically not investigated. Usually, the secondary pyrolysis is undesirable, since the maximum oil yield is the main goal of commercial retorting. As a result of the overheating of oil vapors, the oil yield in commercial retorts is fairly far from the theoretical. To control and prevent these undesirable reactions, it is important to know more thoroughly the phenomenology of the secondary pyrolysis. Besides, in some cases the secondary reactions may be used and controlled so that products of a desirable quality will be obtained. The primary shale oil is often of a low quality, since it has a high pour point, low content of light fractions, and high content of nitrogen, sulfur, and oxygen. Because of the complexity of the shale oil, Abstract published in Advance ACS Abstracts, May 1, 1997. (1) Shadle, L. J.; Seshadri, K. S.; Webb, D. L. Characterization of shale oils. Fuel Process. Technol. 1994, 37 (2), 101-120. (2) Kavianian, H. R.; et al. Kinetic simulation model for steam pyrolysis of shale oil feedstock. Ind. Eng. Chem. 1990, 29 (4), 1527534. (3) Fainberg, V.; Pokonova, Yu. Oil Shale Chemistry; VINITI: Moscow, 1985; pp 78-87 (in Russian).
extraction of valuable compounds is impractical. An increase in the retorting temperature decreases oil yield but makes it possible to improve oil quality. However, higher retorting temperatures require heating of the ballast inorganic components of the oil shale and promote the decomposition of carbonates contained in the mineral matter of the shale. For this reason, it may be practical to subject only the primary oil and gas products of the retorting to the action of elevated temperatures. According to this two-stage scheme, the oil shale is retorted in the first stage under conditions providing a maximal yield of organic products (i.e., at a temperature of 500-530 °C). In the second stage, the products of the shale decomposition undergo pyrolysis at temperatures of 600-800 °C. By variation of the temperatures, composition of the gas medium, and other conditions of the secondary pyrolysis, it is possible to vary the yield and composition of the products within a wide range. General principles of the pyrolysis of hydrocarbon mixtures are known.4-7 At elevated temperatures, the dominant trend is cracking, with the formation of thermally stable light compounds. However, some unstable products of thermal decomposition, such as dienes, may be polymerized and form one-ring aromatics or compounds of a higher molecular mass. The result is observed to be light products of a simple structure (benzene, methane, naphthalene, etc.) and, on the other hand, high-molecular weight products of the polymerization (heavy aromatic oil, coke).
X
S0887-0624(96)00173-9 CCC: $14.00
(4) Allred, V. D. Oil Shale Processing Technology; The Center for Professional Advancement: East Brunswick, NJ, 1982; pp 55-66. (5) Hatch, L. F.; Matar, S. From Hydrocarbons to Petrochemicals; Gulf Publishing: Houston, 1982; pp 74-91. (6) Speight, J. G. Fuel Science and Technology Handbook; Dekker: New York, 1991; pp 864-870. (7) Jouce, W. F.; Uden, P. C. Anal. Chem. 1983, 55, 540.
© 1997 American Chemical Society
916 Energy & Fuels, Vol. 11, No. 4, 1997
Fainberg et al.
Table 1. Oil Shale Characteristics (wt %, Dry Basis) characteristics
value
ash CO2 CaO total S Spyr Fepyr H2Ocryst organic matter Corg Horg Norg Oorg Sorg Fisher assay oil water spent shale gas and losses LHV, kcal/kg
63.0 23.2 32.1 2.47 0.44 0.38 2.3 13.8 9.08 0.93 0.27 1.42 1.46
Figure 1. Unit for secondary pyrolysis of the products of thermal decomposition of the oil shale.
5.0 2.9 90.1 2.0 810
Compounds with heteroatom elements (S, O, N) usually are pyrolyzed with the formation of H2S, H2O, NH3, and heterocyclic compounds. Approximately 80% of the organic sulfur in shale oil occurs as thiophenic compounds with the remainder as thiols, sulfides, and disulfides.8-10 Since thiophenes are the most stable form of organic sulfur compounds11 and the simplest homologues are the most stable form of thiophenes, thermal treating may be used as a method for simplifying the composition of the sulfur-containing compounds of shale oil and for concentrating thiophenes in it. Sulfur compounds are usually pyrolyzed with the formation of H2S and thiophene derivatives or other thermostable heterocyclic compounds. On the other hand, further decomposition of thiophenes into H2S and hydrocarbons can take place at higher temperatures.12 Experimental Section Oil Shale Preparation and Characteristics. The characteristics of the oil shale sample (the Northern Negev deposit, Israel) are presented in Table 1. The oil shale is characterized by a low content of organic matter, low oil yield (both from the shale and from the organic matter), and a high content of sulfur in the oil shale (2.5%) and in the shale oil (up to 7%). Before the experiments, the initial oil shale was ground into 2-8 mm particles and dried at a temperature of 110 °C for 8 h. Experimental Unit. The batch-operated unit (Figure 1) consisted of a retort (first stage), converter (second stage), system for electric heating of both apparatuses, equipment for temperature control, and a system for cooling, condensing, collecting, and measurement of the products of processing. The gas and oil vapors resulting from the oil retorting (primary pyrolysis) were sent to the converter for the secondary pyrolysis at temperatures of 650-820 °C. The temperature of the pyrolysis without the converter (i.e., a usual retorting similar to that of the Fisher Assay) was conventionally taken to be 500 °C. The shale oil was condensed, separated from water and gas, and collected in a receiver. The gas was cleaned of H2S and, partly, of CO2 and directed to a gas holder. The retort was a horizontal cylindrical stainless steel apparatus, which contained about 16 kg of oil shale. The (8) Philp, R. P.; Xavier De Las Heras, F. Chromatography, Part B; Elsevier: Amsterdam, 1992; pp B462-B464. (9) Nishioka, M. Energy Fuels 1988, 2, 214. (10) Rall, H. T., et al. Sulfur compounds in crude oil; U.S. Bureau of Mines: Washington, DC, 1972. (11) Galpern, G. D. Thiophene and Its Derivatives, Part I; Wiley-Interscience: New York, 1985; pp 325-351. (12) Selsbo, P.; Ericsson, E. Polym. Degrad. Stab. 1996, 51, 83.
Figure 2. Gas formation rate during retorting. converter was a cylindrical reactor filled with a ceramic packing. The oil shale pyrolysis rate in the retort is shown in Figure 2. About 80% of the oil and gas was formed within the temperature range 300-450 °C. A drawback of the retort used was its batch operation and, as a consequence, various holdup times of the pyrolysis products in the converter during the experiment (about 4-6 s for the major part of the oil vapors). The investigation was not aimed at a maximal oil yield; therefore, relative long mean oil vapor residence time in the converter was selected so that this value was close to the oil vapor residence time in some typical commercial oil shale retorts. Since all the experiments were carried out under the same conditions, variations in the holdup time of the pyrolysis products in the converter did not prevent the comparison of the results of the pyrolysis at various temperatures. Methods of Product Preparation and Analysis. The shale oil was distilled under vacuum into three fractions: light oil (boiling point to 150 °C), middle oil (150-500 °C), and oil residue. The analysis of the liquid products was performed by means of a GC (Varian 3600), employing helium carrier gas, and an injector with a split ratio of 1:100. Most of the analyses of the liquid products were performed using an AT-PETRO column (Heliflex fused silica capillary column, L )100 m, D ) 0.25 mm). After passing the column, the sample stream was divided into two parts by means of a splitter. The first part was sent into a FID detector, and the second one into a FPD (flame photometric detector). The GC temperature program was 3 °C/min from 50 °C (1 min) to 130 °C and 25 °C/min from 130 °C (1 min) to 280 °C. Calibration was made using the internal standard method. N-methyl pyrrolidone (for the light oil) and n-heptane (for the middle oil) were used as standards. The polycyclic sulfur heterocycles were also identified using a Finnigan Model ITS-40 ion trap GC-MS system. The solution for the acid gas absorption was analyzed for the CO2 content according ASTM D 1756 and for the H2S content by iodometric titration. The ash content in the oil shale and spent shale was determined according to ASTM D 3174. CO2 content was determined according to ASTM D 1756.
High-Sulfur Oil Shale
Energy & Fuels, Vol. 11, No. 4, 1997 917
Table 2. Yield of the Products of the Two-Stage Pyrolysis of Oil Shale (on Oil Shale Organic Matter Basis, wt %) secondary pyrolysis temperature, °C product
500
650
730
780
820
gas light oil middle oil oil residue total oil
10.7 5.6 26.0 3.6 35.3
18.4 6.1 19.3 2.0 27.4
25.5 7.3 10.5 2.4 20.3
23.8 5.0 9.0 3.3 17.4
23.5 4.6 8.2 2.6 15.4
Table 3. Composition of the Gas (vol %) secondary pyrolysis temperature, °C compound
500
650
730
780
820
hydrogen 30.6 23.3 23.0 27.5 26.2 carbon monoxide 4.0 3.0 3.9 4.0 4.3 methane 18.3 19.5 25.4 27.5 26.7 ethylene 0.4 3.0 8.4 8.1 8.3 ethane 3.7 9.7 6.0 4.7 2.8 propylene 1.5 4.1 4.9 2.9 2.7 propane 1.8 2.1 0.6 0.2 0.1 C4 1.0 2.6 1.7 0.7 0.5 C5 0.3 0.2 0.1 0.0 0.0 carbon dioxide 22.3 19.2 15.5 14.7 15.5 hydrogen sulfide 16.0 13.0 10.5 9.7 12.9 1.044 1.137 1.051 0.948 0.976 density, kg/m3 5078 7127 7209 6427 6120 LHV, kcal/m3
Figure 3. Yield of gas components vs pyrolysis temperature.
Results and Discussion Product Yields. The product yields are presented in Table 2. The oil yield (on the oil shale organic matter basis) decreased from 35.3% at the pyrolysis temperature of 500 °C to 15.4% at 820 °C. The gas yield increased with temperature and reached a maximum at 730 °C. Gas Yield and Composition. Gas composition is presented in Table 3. Increasing secondary pyrolysis temperature increased the content of methane and ethylene in the gas. Hydrogen sulfide content was 1016%. The amounts of COS and CS2 in the gas were negligible (less than 1% of H2S content). The changes in H2S and CO2 content are explained mainly by the changes in the total volume of the obtained gas. The maximal heating value of the gas was observed at 730 °C. The yield of the gas components vs pyrolysis temperature is shown in Figure 3. The yield of hydrogen, methane, ethylene, and carbon monoxide increased with the temperature, whereas that of alkanes decreased as a result of the cracking. Yields of some components (propane, propylene) have a maximum at a definite
Figure 4. Chromatograms of the light oil (without the secondary pyrolysis). Part a results from use of an FID: (1) benzene; (2) thiophene; (3) toluene; (4) 2-methylthiophene; (5) 3-methylthiophene, (6) ethylbenzene, (7) m- and p-xylenes; (8) styrene; (9) o-xylene. Part b results from use of an FPD: (1) thiophene; (2) 2-methylthiophene; (3) 3-methylthiophene; (4) dimethylthiophenes.
temperature. The yields of H2S and CO2 did not vary significantly at pyrolysis temperatures up to 730 °C. A high yield of H2S at 820 °C can be explained by the thermal decomposition of a part of the sulfur-containing compounds of the oil. Composition of the Liquid Products. The liquid products of the secondary pyrolysis differ substantially from the usual (primary) shale oil (Figures 4 and 5). The primary oil consists of a very large number (several hundred) of compounds, which almost cannot practically be identified and separated from each other even by means of chromatography. Complexity of the product does not lend itself to economic recovery of individual components. On the contrary, the oil resulting from the secondary pyrolysis consists of a relatively small number of valuable aromatic and individual sulfur-aromatic components that may be more easily separated not only by means of chromatography but also by technological methods. The light fraction (boiling point to 150 °C) of the primary shale oil contained a large quantity of compounds; more than 60% of them have not been identified (Table 4). The same fraction of the shale oil, subjected to the secondary pyrolysis, consisted mainly of only 10 componentssbenzene, toluene, thiophene, xylenes, 2methylthiophene, 3-methylthiophene, and dimethylthiophenes. All these compounds can be easily separated each from other, and every one of them can be used for chemical syntheses. Thus, secondary pyrolysis enables us to simplify substantially the composition of the primary shale oil. The middle oil consists mainly of naphthalene and other aromatic and heterocyclic compounds, such as alkylbenzenes, naphthalenes, flourene, phenanthrene, anthracene, thionaphthenes, etc. (Table 5). On the whole, the oil of the secondary pyrolysis is similar to coal tar.
918 Energy & Fuels, Vol. 11, No. 4, 1997
Fainberg et al. Table 5. Composition of the Middle Oil (wt %) secondary pyrolysis temperature, °C
Figure 5. Chromatograms of the light oil (pyrolysis temperature 730 °C). Part a results from use of an FID: (1) benzene; (2) thiophene; (3) toluene; (4) 2-methylthiophene; (5) 3-methylthiophene; (6) ethylbenzene; (7) m- and p-xylenes; (8) styrene; (9) o-xylene. Part b results from use of an FPD: (1) thiophene; (2) 2-methylthiophene; (3) 3-methylthiophene; (4) dimethylthiophenes. Table 4. Composition of the Light Oil (wt %) secondary pyrolysis temperature, °C compound aliphatics n-pentane cyclopentene cyclopentane n-hexane cyclohexane n-heptane methylcyclohexane n-octane total identified aliphatics aromatics benzene thiophene pyridine toluene 2-methylthiophene 3-methylthiophene ethylbenzene m- and p-xylenes styrene o-xylene dimethylthiophenes total identified aromatics carbon disulfide unidentified peaks
500
650
730
780
820
1.90 0.24 0.14 2.53 0.55 2.91 0.28 0 8.6
0.77 0.31 0.12 0.76 0.47 0.77 0.13 0.26 3.6
0.07 0.04 0.00 0.09 0.07 0.06 0.03 0.05 0.4
0 0 0 0 0 0 0.04 0 0.04
0 0 0 0 0 0 0.03 0 0.17
2.0 1.4 0 4.3 5.7 1.4 1.3 2.8 0 0 11.3 30.3
10.5 1.9 0.4 12.2 6.9 1.4 1.9 4.9 3.1 0.5 12.6 56.4
24.4 3.6 0.4 18.4 8.0 2.5 1.0 5.5 2.7 2.5 13.8 82.9
34.2 6.8 0.6 20.2 7.9 2.9 0.6 5.3 2.9 3.0 9.1 93.6
39.4 8.8 0.5 18.7 7.1 2.6 0.5 4.1 2.3 1.8 5.6 91.4
0 61.1
0 40.0
0.7 16.0
1.6 4.8
4.8 3.7
The higher the severity of the conversion, the higher the yield of the simplest homologuessthiophene, naphthalene, fluorene, phenanthrene, anthracene, thionaphthene, and dibenzothiophene (Table 6). Maximal yield of benzene and alkyl derivativesstoluene, methylthiophenes, dimethylbenzenes, etc.swas observed at a temperature of 730 °C (Figures 6 and 7). The light oil and middle oil contained up to 28% and
compound
500
650
730
780
820
alkylbenzenes alkylthiophenes polycyclic hydrocarbons (PCH) naphthalene thionaphthene quinoline indole 2-methylnaphthalene methylnaphthalenes methylthionaphthenes biphenyl 2,6-dimethylnaphthalene dimethylnaphthalenes dimethylthionaphthenes diphenylmethane acenaphthene alkylthionaphthenes fluorene dibenzothiophene phenanthrene anthracene total identified PCH CnH2n (n ) 12-25) CnH2n+2 (n ) 12-26) unidentified peaks
9.6 4.7
10.3 5.2
14.1 8.2
7.6 2.5
5.1 0.9
1.0 1.2 0 0 1.3 1.0 2.3 0 0.1 0.9 2.8 0 1.1 2.2 1.0 0.9 0.9 0.4 17.0 5.8 13.5 49.3
2.0 2.0 0.3 0.5 1.9 2.9 3.7 0.7 1.2 0.9 3.7 0.8 1.1 2.4 1.1 1.2 1.1 0.4 28.0 0 0 56.4
6.1 4.7 0.6 0.9 3.6 2.5 8.3 0.8 0.6 0.4 7.0 1.3 1.4 3.1 2.1 1.9 2.1 0.6 48.0 0 0 29.6
9.4 6.5 0.9 0.8 4.0 2.3 9.1 0.9 0.3 0.1 6.5 1.1 1.8 2.1 1.9 2.6 3.8 1.0 55.0 0 0 35.0
11.5 7.0 1.0 0.8 3.4 1.4 8.2 0.9 1.1 0.3 2.7 1.4 2.0 1.6 1.7 3.2 5.0 1.5 54.7 0 0 39.3
Table 6. Yield of the Oil Components (on the Oil Shale Organic Matter Basis, wt %) secondary pyrolysis temperature, °C compound
500
650
730
780
820
total identified aliphatics benzene thiophene toluene 2-methylthiophene 3-methylthiophene dimethylbenzenes dimethylthiophenes alkylbenzenes alkylthiophenes naphthalene thionaphthene methylnaphthalenes methylthionaphthenes dimethylnaphthalenes dimethylthionaphthenes acenaphthene alkylthionaphthenes fluorene dibenzothiophene phenanthrene
0.55 0.13 0.09 0.28 0.37 0.09 0.26 0.73 2.86 1.40 0.29 0.35 0.69 0.68 0.29 0.84 0.34 0.65 0 0.27 0.27
0.25 0.73 0.13 0.85 0.48 0.10 0.73 0.87 2.26 1.15 0.45 0.44 1.06 0.82 0.46 0.82 0.24 0.53 0.25 0.26 0.25
0.04 2.05 0.30 1.54 0.67 0.21 0.98 1.16 1.70 0.99 0.73 0.57 0.74 1.00 0.12 0.84 0.17 0.37 0.26 0.23 0.25
0.010 1.72 0.34 1.02 0.40 0.15 0.60 0.46 0.68 0.22 0.84 0.58 0.56 0.82 0.04 0.59 0.16 0.19 0.17 0.23 0.35
0.01 1.83 0.41 0.87 0.33 0.12 0.40 0.26 0.42 0.08 0.94 0.58 0.39 0.67 0.11 0.22 0.17 0.13 0.14 0.26 0.41
33% sulfur compounds, respectively. Sulfides, disulfides, and other non-thiophene sulfur-containing compounds were cracked into hydrocarbons and H2S, or are transformed into thiophenes (we have not found any organic compounds of non-thiophene nature in the products of pyrolysis at temperatures higher than 650 °C). Practically all heavy alkylthiophenes were transformed during the secondary pyrolysis into simpler productssmethyl and dimethyl derivativesswhich, in turn, were decomposed into the simplest homologuessthiophene, thionaphthene, and dibenzothiophene. The summing process, depending on the conditions, may result both in an increase and in a decrease in the amounts of methyl- and dimethylthiophenes; the amounts of thiophene and thionaphthene grow steadily with a rise in temperature. Practical Significance of the Process. Owing to the two-stage processing, gas and a mixture of aromatic
High-Sulfur Oil Shale
Figure 6. Yield of light oil components vs secondary pyrolysis temperature.
Figure 7. Yield of middle oil components vs secondary pyrolysis temperature.
liquids are produced. The gas can be used as an excellent clean fuel for the combined cycle power unit (instead of the ash-rich and sulfur-rich oil shale or shale oil in the usual processes). Aromatic products can be a good prime matter for the production of chemicals. This scheme is more effective from a technical and economical point of view than direct combustion of oil shale. At the same time, it is less harmful to the environment. The preservation of the natural environment will be provided by the use of the high-quality ashless, moistureless, and sulfurless gas fuel instead of the oil shale (or sulfur-rich shale oil), lowering CO2 content in the flue gases and eliminating other wastes. The economic data for a commercial plant having a capacity of 202 tons of the dried oil shale per hour have been evaluated. The process of combined production of energy and chemicals has better economical characteristics than the direct combustion of shale or coal. The
Energy & Fuels, Vol. 11, No. 4, 1997 919
cost of the power production in the integrated process is 2.53 cents kW-1 h-1, compared to 4.9 cents kW-1 h-1 for power production by the direct combustion of the oil shale. The integrated process requires a lower investment and has a lower operating cost. The total thiophene yield in the products of secondary pyrolysis can be as high as 6.4% on oil shale organic matter (730 °C), and this can be of practical interest because thiophenes are now being widely investigated as a source for the production of light- and photoemitting polymers, materials for semiconductors, electrochemical cells, films, sensors, and other high-tech devices.13-15 The data obtained can be used for the prediction of the results of, and the control of, commercial retorting processes. Conclusions Secondary pyrolysis of products of Israeli oil shale processing was studied in a two-stage bench-scale unit. The gas and oil vapors resulting from the oil shale retorting (primary pyrolysis) were sent to the converter for the secondary pyrolysis at temperatures of 650-820 °C. The oil yield of kerogen decreased from 35.3% at the pyrolysis temperature of 500 °C to 15.4% at 820 °C. The gas yield increased from 10.7% at 500 °C to 25.5% at 730 °C. The yield of hydrogen, methane, ethylene, and carbon monoxide increased with the temperature, while the yield of alkanes decreased. The secondary pyrolysis enables us to simplify substantially the composition of the primary shale oil. The higher the severity of the conversion, the higher the yield of the simplest homologuessbenzene, thiophene, naphthalene, phenanthrene, anthracene, thionaphthene, and dibenzothiophenesand the lower the yield of alkyl derivativesstoluene, methylthiophenes, styrene, etc. Maximal content of methyl and dimethyl derivatives was observed at a temperature of 730 °C. The total yield of thiophenes is 6.4% on oil shale organic matter (730 °C). Acknowledgment. This research was supported by PAMA (Energy Resources Development) Ltd. It was also supported by the Belfer Center for Energy Research and by the Center for Absorption in Science, Ministry of Immigrants Absorption, State of Israel. Support was also received from the Technion Vice-President for Research. EF9601733 (13) Proceedings of the 2nd International Conference on Optical Probes of Conjugated Polymers and Fullerenes Salt Lake City. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1994, 256. (14) Proceedings of the International Conference on Science and Technology of Synthetic Metals (ICSM’94). Synth. Met. 1995, 69. (15) Proceedings of Symposium D on Organic Materials for Electronics. Synth. Met. 1994, 67.
