206
Energy & Fuels 1997, 11, 206-212
Improvement of Coal Direct Liquefaction by Steam Pretreatment Olga Ivanenko, Robert A. Graff,* Valeria Balogh-Nair, and Claude Brathwaite Departments of Chemical Engineering and Chemistry, The City College of New York, New York, New York 10031 Received June 11, 1996. Revised Manuscript Received October 3, 1996X
Pretreatment of coal by reaction with subcritical steam enhances its performance in direct liquefaction. Illinois No. 6 coal, first reacted with 51 atm of steam for 15 min at 340 °C, was liquefied in a coal injection autoclave to provide rapid heating. Liquefactions were carried out with raw and pretreated coal at high-severity (400 °C, 30 min) and low-severity (385 °C, 15 min) conditions under 1500 psia of hydrogen with tetralin as the donor solvent. Substantial improvement in product liquid quality is realized provided the pretreated coal is protected from oxygen and heated rapidly to liquefaction temperature. Under low-severity conditions, the oil yield is more than doubled, going from 12.5 to 29 wt %. Since previous work pointed to the destruction of ether cross-links by water as the dominant depolymerization mechanism during pretreatment, tests were conducted with several aromatic ethers as model compounds. These were exposed to steam and inert gas at pretreatment conditions and in some cases to liquid water at 315 °C. R-Benzylnaphthyl ether and R-naphthylmethyl phenyl ether show little difference in conversion and product distribution when the thermolysis atmosphere is changed from inert gas to steam. Hence, these compounds are poor models for coal in steam pretreatment. The otherwise thermally stable 9-phenoxyphenanthrene, on the other hand, is completely converted in 1 h by liquid water at 315 °C. At pretreatment conditions, however, mostly rearranged starting material is obtained. Therefore, 9-phenoxyphenanthrene, though less reactive, is a model for ether linkages in coal.
Introduction Development of commercially viable and environmentally sound processes for coal liquefaction remains a formidable challenge. Although advances in coal liquefaction technology in recent years have led to the production of liquids of improved quality, this improvement has been achieved mainly by increased consumption of expensive catalyst. Catalyst and capital equipment costs are major factors in the high price of the coalderived liquids. One way to improve the economics of direct liquefaction is to modify the coal feed before solubilization. Treatments such as alkylation,1-4 acylation,5 partial oxidation,6,7 and alkali hydrolysis8,9 weaken and rupture the cross-linked bonds and result in a partially depolymerized coal. Although all of these methods of enhancing coal reactivity somewhat improve liquefaction yield, none has yet been employed as a process step in liquefaction. Chemical treatment methods remain in the laboratory mainly because they employ expensive and sometimes hazardous reagents. Abstract published in Advance ACS Abstracts, November 15, 1996. (1) Sternberg, H.; Delle Donne, C. L. Fuel 1974, 53, 172. (2) Liotta, R. Fuel 1979, 58, 724. (3) Schlosberg, R. H.; Neavel, R. C.; Maa, P. S.; Gorbaty, M. L. Fuel 1980, 59, 45. (4) Baldwin, R. M.; Kennar, D. R.; Nguanprasert, O.; Miller, R. L. Fuel 1991, 70, 432. (5) Hodek, W.; Kolling, G. Fuel 1973, 52, 220. (6) Deno, N. C.; Curry, K. W.; Greigger, B. A.; Jones, A. D.; Rakitsky, W. G.; Smith, K. A.; Wagner, K.; Minard, R. D. Fuel 1980, 59, 694. (7) Deno, N. C.; Jones, A. D.; Koch, C. C.; Minard, R. D.; Potter, T.; Sherrard, R. S.; Stroh, J. G.; Yevak, R. J. Fuel 1982, 61, 490. (8) Mirza, Z. B.; Sarkar, M. K.; Sharma, D. K. Fuel Process. Technol. 1984, 9, 149. (9) Chow, C. K. Fuel 1983, 62, 317. X
S0887-0624(96)00088-6 CCC: $14.00
In this work, steam is the pretreatment agent for noncatalytic donor solvent liquefaction. The advantages of this approach are that steam is among the cheapest of reagents and does not incorporate undesired chemical residues into the coal structure. Previous studies have demonstrated the effectiveness of exposing Illinois No. 6 coal to subcritical steam in the temperature range 320-360 °C at 50 atm as a pretreatment for room temperature pyridine extraction and for flash pyrolysis.10 As pretreatment temperature was increased, the pyridine extraction yield gradually increased, reaching a maximum at a pretreatment temperature of 340-350 °C. When the temperature was raised above 360 °C, the effect was extinguished. At the optimum pretreatment temperature of about 340 °C, the liquid yield in coal pyrolysis was more than doubled and the total volatiles yield was increased by about 20%. When, however, steam-treated coal was exposed to ambient air, both pyrolysis and extraction yields were reduced to the level of raw coal. Analysis of pyrolysis liquids from pretreated Illinois No. 6 coal showed a decrease in molecular weight with a simultaneous increase in oxygen content compared to raw coal.11 On the basis of the results of analyses performed on raw and steam-pretreated coal,12 it was concluded that during steam pretreatment new hydroxyl groups are introduced. As a result, the covalent cross-links in coal are greatly reduced, yielding a partially depoly(10) Graff, R. A.; Brandes, S. D. Energy Fuels 1987, 1, 84. (11) Graff, R. A.; Zhou, P.; Brandes, S. D. Steam Conditioning of Coal for Synfuels Production: Final Report; 1988; DOE Contract DEAC21-87MC23288. (12) Brandes, S. D.; Graff, R. A.; Gorbaty, M. L.; Siskin, M. Energy Fuels 1989, 3, 494.
© 1997 American Chemical Society
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Energy & Fuels, Vol. 11, No. 1, 1997 207
Figure 1. Ether model compounds.
merized coal. Such a depolymerized coal is, potentially, an improved feedstock for any coal conversion process. With this success of steam pretreatment in pyrolysis, it was logical to test its application to direct liquefaction. We are aware of previous unpublished attempts by others to obtain an improvement in liquefaction yield by this method. These attempts had, however, failed. Liquefaction was carried out either in an autoclave or in a tubing bomb. As noted above, temperatures above 360 °C destroy the effect of pretreatment. On the other hand, our previous pyrolysis studies were carried out at about 800 °C, but under heating times of a second or less. It was, therefore, reasoned that the heating of pretreated coal to liquefaction temperatures must likewise be done rapidly to minimize deleterious retrogressive reactions. A tubing bomb or stirred autoclave (employed in the conventional way) would not be adequate to carry out liquefaction under sufficiently rapid heating conditions. One way to achieve high heating rates is to use a method in which a slurry of pretreated coal is injected into a hot liquefaction reaction vessel as in Whitehurst et al.13 A system of this type, with some modifications, was designed, assembled, tested, and used for the liquefaction tests. Furthermore, as it is known that the benefits of pretreatment are observed only if exposure of steam-treated coal to air is rigorously avoided, it is necessary to conduct all operations with pretreated coal under an inert atmosphere. To satisfy this requirement, a nitrogen-filled glovebag was used for sample workup and the oxygen concentration of the bag monitored by mass spectrometer. To verify successful pretreatment, the pyridine extraction yield was determined for each steam-treated coal sample. In this work liquefaction tests of raw and steampretreated Illinois No. 6 coal were made to determine the pretreatment effect on yield of liquids and on quality of product. These tests were conducted using the slurry injection autoclave described above pressurized with hydrogen and using tetralin as the donor solvent. The effect of exposure of treated coal to air and the role of coal slurry heating rate were examined at three different conditions of liquefaction severity. It is also of interest to investigate the chemistry of the steam pretreatment process using model compounds. An analysis of structural changes in the coal macromolecular network resulting from steam treatment showed that covalent bonds, particularly ether linkages, had been cleaved.11-12 For the work reported here, the diaryl ethers R-benzylnaphthyl ether (R-BNE), R-naphthylmethyl phenyl ether (R-NMPE), and 9-phenoxyphenanthrene were chosen as model compounds. (13) Whitehurst, D. D.; Farcasiu, M.; Mitchel, T. O. The Nature and Origin of Asphaltenes in Processed Coals: Annual Report; Feb 1976; EPRI AF-252.
R-BNE and R-NMPE had been studied in inert atmosphere under relatively mild conditions (250-350 °C for 30 min) by Chawla et al.14 The major products, ArOH, Ar′CH3, and isomers of the starting ethers Ar′CH2ArOH, were formed by homolysis of the CH2-O bond followed by recombination of the resulting radical. Up to 80% conversion was obtained at these relatively mild conditions. The third ether, 9-phenoxyphenanthrene, along with 1-phenoxynaphthalene, was among the compounds studied by Siskin et al.15-17 in liquid phase aquathermolysis at temperatures from 250 to 350 °C. These compounds are thermally stable at 315 °C. However, in the presence of water they are cleaved to phenol and to 9-hydroxyphenanthrene and 1-naphthol, respectively. The rate of cleavage of diaryl ethers increases with increasing numbers of aromatic rings.17 The reactions are catalyzed by water, which has a dissociation constant 3 orders of magnitude greater at the temperatures employed than at room temperature. Experimental Methods Coal Sample. Tests were conducted on Illinois No. 6 coal (DECS-2) from the Pennsylvania State University Sample Bank, having the following elemental analysis (wt %) on a moisture-free basis: C, 65.49%; H, 4.56%; N, 1.11%; S, 4.52%; O, 8.16%; ash, 16.16%. The coal was refrigerated and stored under an inert atmosphere to avoid oxidation. Samples for pretreatment and liquefaction were ground to pass 200 mesh. Grinding was conducted in a nitrogen-filled glovebag using a Trost air impact pulverizer, operated with inert gas. Ground coal was riffled, placed in a glass jars, flushed with inert gas, sealed, and refrigerated. Model Compounds. Ether model compounds R-NMPE, R-BNE, and 9-phenoxyphenanthrene (Figure 1) were synthesized. The two ethers, R-NMPE and R-BNE, synthesized according to the procedures of Maslak and Guthrie,18 were found to be ca. 95% pure by high-performance liquid chromatography (HPLC). 9-Phenoxyphenanthrene was prepared and purified using the method of Afzali et al.19 R-BNE was further purified by recrystallizations (>99.5% by HPLC), but highly pure R-NMPE could not be obtained by recrystallizations, and flash column chromatography on silica gel led to its extensive decomposition due to acid-catalyzed rearrangement. However, recrystallizations from hexane/ether/ethanol (5/2/2) and final purification by flash column chromatography on basic alumina afforded >99.5% pure (HPLC) R-NMPE. (14) Chawla, B.; Davis, B. H.; Shi, B.; Guthrie, R. D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35 (2), 387. (15) Siskin, M.; Brons, G.; Vaughn, S. N.; Katritzki, A. R.; Balasubramanian, M. Energy Fuels 1990, 4, 488. (16) Siskin, M.; Katritzki, A. R.; Balasubramanian, M. Energy Fuels 1991, 5, 770. (17) Siskin, M.; Katritzki, A. R.; Balasubramanian, M. Fuel 1993, 10, 1435. (18) Maslak, P.; Guthrie, R. D. J. Am. Chem. Soc. 1986, 108, 2637. (19) Afzali, A.; Firouzabadi, H.; Khalafi-Nejad, A. Synth. Commun. 1983, 13, 335.
208 Energy & Fuels, Vol. 11, No. 1, 1997
Figure 2. Stirred autoclave with coal slurry injection. Steam Pretreatment of Coal. Pretreatment of coal was conducted in a continuous flow of 50 atm of steam at 340 °C for 15 min (following the procedure of Graff and Brandes10 and Graff et al.11). After pretreatment, the sealed reactor was transferred to a nitrogen-filled glovebag where all manipulations of the treated sample were performed, including pyridine extraction (using the procedure of Graff and Brandes10) and preparation of the coal slurry for liquefaction. Coal Liquefaction. Liquefaction experiments were conducted with raw coal, steam-treated coal, and steam-treated coal exposed to air under rapid heating and slow heating conditions, using tetralin as the hydrogen donor solvent. The apparatus (Figure 2) consists of a 300 mL stirred autoclave (Autoclave Engineers EZE-Seal, Model P-419) and a coal slurry injection system (loading reservoir and 1000 psia rupture disk assembly which separates the reservoir from the autoclave). Nitrogen and hydrogen are supplied from high-pressure cylinders. Nitrogen is used for pressure testing of the system, and hydrogen is used in the liquefaction process. The liquefaction procedure is as follows: Sixty grams of deoxygenated tetralin is placed into the reaction vessel and the autoclave is sealed. After the system is leak tested with nitrogen at 2000 psia with the bypass line valve open, the reactor is flushed with hydrogen at atmospheric pressure and then heated using an electric furnace, keeping the bypass line valve closed. While the reactor is being heated, slurry is prepared in the nitrogen-filled glovebag from the pretreated coal and solvent. The slurry is made of one part coal to two parts solvent using about 20 g of coal. When the autoclave has reached operating temperature, two syringes are filled, one with about 40 mL of slurry and one with 20 mL of solvent. The syringes are attached to the loading line, the loading valve is opened, and the slurry reservoir is sequentially filled with 10 mL of solvent, coal slurry (full contents of the syringe), and, finally, 10 mL of solvent. The loading valve is then sealed and hydrogen at the desired pressure is applied. This causes the rupture disk to burst, injecting slurry into the reactor and pressurizing it. The valve above the reservoir is then closed. The reaction temperature is maintained with (5 °C accuracy.
Ivanenko et al. The run duration is measured from coal injection. To terminate the run, the heater is turned off and removed from the autoclave. For the tests when exposure of pretreated coal to air is desired, all manipulations of coal are done under ambient air. If liquefaction of raw coal is to be carried out, slurry can also be prepared under ambient air. In slow heating liquefaction the above procedure is followed except that slurry is injected when the autoclave is at room temperature. The reaction mixture is then heated to operating temperature. The reaction time count starts when the desired operating temperature is reached. When the reactor has cooled to ambient temperature after liquefaction, coal conversion is determined using Soxhlet extraction as described by Joseph.20 The liquid product is classified as hexane solubles (oils + gases), toluene solubles (asphaltenes), and THF solubles (preasphaltenes). Analysis of the liquefaction gas was obtained in later runs and, where available, is reported below. These were obtained as follows: The gas vented from the autoclave to relieve residual pressure after cool down was continuously sampled from the vent line directly into a mass spectrometer (Extranuclear Laboratories, Inc.; Model C50) through a capillary. The mass spectrometer was calibrated for quantitation using a prepared mixture made up of the major components identified in the reaction gas (CO, CO2, CH4, C2H6, C2H4, C3H6, C4H10) and hydrogen based on concentrations reported in the literature.21 In addition, neon, used as an internal standard, was added to both the calibration mixture and the hydrogen used in liquefaction runs. No attempt was made to analyze for hydrogen sulfide as adsorption of this gas on walls of the transfer line and other internal surfaces makes its quantitation unreliable. In the runs for which the liquefaction gas yield was determined, the total gas make in the process was evaluated by combining the liquefaction gas yield and volatiles loss during pretreatment (8% at the pretreatment conditions used). This value was then subtracted from the value of hexane solubles (oil + gas) to obtain the yield of oils alone. Treatment of Model Compounds. Model compounds were reacted in steam, liquid water, or inert gas. Steam and inert gas treatments of model compounds were conducted utilizing the same method as in coal pretreatment. However, because the compounds are volatile, the reactor scheme utilizing continuous flow of steam was replaced by nonflow open operation in which steam at 51 atm is continuously supplied into the reactor equipped with an inlet valve on one end and a cap on the other. This semiflow reactor arrangement was first tested using Illinois No. 6 coal. The pyridine extraction yield obtained from coal pretreated in nonflow open reactor agreed with results obtained in continuous flow operation. Since the reactor walls may affect conversion, model compound tests were carried out in both stainless steal and Pyrex-lined vessels. When the compounds were treated under inert atmosphere, steam was replaced by nitrogen. The steam or inert gas treatments were carried out at 320 °C and 51 atm (750 psia) for 15 min using 100-300 mg of the model compound. Model compounds were also reacted at a high pressure to maintain water in the liquid phase according to a procedure described by Siskin et al.17 The reactions were carried out in a closed 11 mL stainless steel pressure vessel with 0.1-1 g of material. The reactor was heated by immersion in a fluid bed sand bath at 315 °C for 15 min to 1 h. At this temperature, the water vapor pressure reaches 105.1 atm (1545 psia). The products extracted from the reactor with methylene chloride were analyzed initially by gas chromatography/ chemical ionization mass spectra (GC/CI-MS, SPB-5 fused (20) Joseph, J. T. Fuel 1991, 70, 139. (21) Serio, M. A.; Kroo, E.; Teng, H.; Charpenay, S.; Solomon, P. R. The Dual Role of Oxygen Functions in Coal Pretreatment and Liquefaction: Crosslinking and Cleavage Reactions: First Annual Report 523085; 1992; DOE Contract DE-AC22-91-PC91026.
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Energy & Fuels, Vol. 11, No. 1, 1997 209
Table 1. High-Severity Liquefaction of Illinois No. 6 Coal (400 °C, 1500 psia, 30 min) conversion (wt %) run no.
conditions
total (liquid + gas)
hexane solubles (oil + gas)
toluene solubles asphaltenese
THF solubles preasphaltenes
1
raw coal slow heating
81.6
37.0
16.9
27.7
2 8
raw coal rapid heating
73.8 80.0
42.2 48.7
10.0 16.7
21.6 14.6
10
steam-treated coal slow heating
73.8
35.2
14.4
24.2
6 12
steam-treated coal rapid heating
85.7 79.3
60.1 55.8
17.2 14.7
8.4 8.8
11
steam-treated coal, exposure to air, rapid heating
66.3
38.6
19.8
7.9
Table 2. Low-Severity A Rapid-Heating Liquefaction of Illinois No. 6 Coal (350 °C, 1500 psia, 30 min) conversion (wt %) run no.
conditions
hexane solubles total gas oils
total (liquid + gas)
toluene solubles asphaltenes
THF solubles preasphaltenes
28
raw coal
73.9
47.4
5.5
41.9
10.9
15.6
29 30
steam-treated coal
71.3 70.9
47.1 42.6
13.5 12.0
33.6 30.6
9.5 11.0
14.7 17.0
Table 3. Low-Severity A Rapid-Heating Liquefaction Gas Composition (350 °C, 1500 psia, 30 min) run no.
conditions
liq gas yield, wt %
CO
CO2
liquefaction gas composition (wt %) CH4 C2H4 C2H6 C3H6
C3H8 + C4H10
28
raw coal
5.5
1.8
74.1
8.3
3.0
0.8
2.8
9.2
29 30
steam-treated coal
5.5 4.0
1.4 1.4
63.4 63.1
6.9 10.0
3.8 4.1
0.8 1.2
6.0 3.4
17.7 16.8
capillary column, 30 m, Supelco, Bellefonte, PA). The mixture was separated by preparative GC on porous polymeric column (Tenax-GC, Alltech, Deerfield, IL) using a linear temperature gradient of 20 °C/min. The purity of the products was further ascertained by HPLC analyses (µ-Porasil column, 4.5 × 250 mm, hexane/ether, 99:1). The structures of the products were determined by a combination of 1H and 13C nuclear magnetic resonance (NMR) and by CI-MS (NH3) analyses.
Results and Discussion Liquefaction Studies. Liquefaction runs were conducted with Illinois No. 6 coal at three sets of conditions: high severity, 400 °C, 1500 psia, 30 min; low severity A, 350 °C, 1500 psia, 30 min; and low severity B, 385 °C, 1500 psia, 15 min. The results are shown in Tables 1-5, classified according to the liquefaction conditions employed. Yields are reported as a fraction of the maf raw coal charge. High-Severity Liquefaction. Raw coal liquefaction was first examined under slow (run 1) and rapid (runs 2 and 8) heating liquefaction conditions to establish baseline yields for these two different procedures. This was followed by a test series in which the coal was first pretreated in steam: Run 10 was conducted with slow heating, runs 6 and 12 were conducted with rapid heating. In run 11, pretreated coal was deliberately exposed to air. These results are shown in Table 1. The total yield of liquid product from raw coal obtained at rapid heating conditions is slightly lower than at slow heating because of the longer total reaction time in the later case. The yield of oils, however, is increased and preasphaltene yield is decreased. No improvement in the yields was observed when steampretreated coal was subjected to the liquefaction under
slow heating. Compared to raw coal, a lower total yield as well as a lower yield of each fraction was obtained. In rapid heating liquefaction, however, the yield of hexane solubles is substantially increased while that of asphaltenes is reduced. When steam-treated coal is deliberately exposed to ambient air for 1 h before slurrying with deoxygenated tetralin, the pretreatment effect is destroyed. The oil yield is decreased even below the value for raw coal with rapid heating. Total yield is also decreased. Low-Severity Liquefaction A. The liquefaction tests with Illinois No. 6 coal were also conducted under conditions of reduced severity. In the first tests (lowseverity A), the reaction temperature was decreased to 350 °C but the pressure and the reaction time were kept unchanged (1500 psia, 30 min). Raw and steampretreated coal were tested under rapid heating conditions. In addition to the liquid product characterization, the gas formed in the process was also analyzed. Liquefaction yields are shown in Table 2 and the gas composition is shown in Table 3. The results, obtained under these reduced-severity conditions, show no improvement in the liquefaction yields between raw and steam-pretreated coal. (That the pretreatments were successful was confirmed by the increased pyridine extraction yields.) Steam pretreatment did, however, increase gas yield at the expense of oil. The liquefaction gas composition shows a decrease in the yield of carbon dioxide and an increase in the yield of propane and butane for steam-treated coal. A possible explanation for the lack of improvement in yields after steam pretreatment may be found in the temperature history in the autoclave. The coal slurry
210 Energy & Fuels, Vol. 11, No. 1, 1997
Ivanenko et al.
Table 4. Low-Severity B Liquefaction of Illinois No. 6 Coal (385 °C, 1500 psia, 15 min) conversion (wt %) run no.
total (liquid + gas)
conditions
hexane solubles total gas oils
toluene solubles asphaltenes
THF solubles preasphaltenes
32 33
raw coal rapid heating
71.6 71.3
27.7 26.5
13.7 15.5
14.0 11.0
17.9 20.2
26.0 24.6
31 34
steam-treated coal rapid heating
77.5 76.2
41.8 38.9
11.9 10.3
29.9 28.6
16.4 15.8
19.3 21.5
35 36
steam-treated coal exposure to air, rapid heating
72.7 71.4
30.5 31.2
14.9 13.8
15.6 17.4
18.9 17.1
23.3 23.1
37 38
steam-treated coal slow heating
74.9 73.9
35.2 34.1
16.4 19.2
18.8 14.9
20.0 18.6
19.7 21.2
Table 5. Low-Severity B Liquefaction Gas Composition (385 °C, 1500 psia, 15 min) run no.
conditions
liq gas yield, wt %
CO
CO2
liquefaction gas composition (wt %) CH4 C2H4 C2H6 C3H6
13.7 15.5
2.3 1.8
51.8 54.4
14.9 17.2
5.2 5.9
1.3 1.4
3.1 4.0
21.3 15.2
C3H8 + C4H10
32 33
raw coal rapid heating
31 34
steam treatment rapid heating
3.9 2.3
1.5 1.4
40.6 39.4
15.4 19.5
6.5 7.8
2.4 2.3
9.1 5.4
24.5 24.0
35 36
steam treatment exposure to air, rapid heating
6.9 5.8
1.4 1.0
48.2 46.2
21.0 19.2
6.8 5.6
2.3 2.0
5.1 4.7
15.3 21.3
37 38
steam treatment slow heating
8.4 11.2
1.1 1.0
40.3 40.2
24.2 23.1
6.2 6.7
2.3 2.5
5.0 4.8
21.0 21.7
(made of raw or steam-treated coal) was injected into the autoclave preheated to 350 °C. In all cases the injection caused the temperature to drop below 300 °C. This was followed by a recovery to 350 °C in about 1.5 min. Consequently, sample has passed through a region below 360 °C where retrogressive reactions occur, resulting in low yield of the liquid product and in a higher yield of gas. Perhaps the chosen reaction temperature itself, 350 °C, is within the region of retrogressive reactions. Low-Severity Liquefaction B. To keep the reaction temperature higher after slurry injection and during the reaction, the low-severity liquefaction conditions were modified (low-severity liquefaction B). The temperature was increased to 385 °C (which is above the upper limit of pretreatment), the reaction time reduced to 15 min, and the pressure left unchanged at 1500 psia. The procedure was also slightly modified: the autoclave was initially preheated to 400 °C. After the temperature drop caused by the slurry injection, the temperature was raised to 385 °C. The results are shown in Table 4 (yield of liquid product) and Table 5 (gas composition). In rapid heating liquefaction experiments the reaction temperature did not drop below 320 °C. Low-severity liquefaction tests show a strong improvement in the yields of liquids resulting from steam pretreatment under rapid heating conditions (runs 31 and 34 compared to runs 32 and 33). The yields of hexane solubles were increased with a slight reduction in the total gas make. Remarkably, the oil yields are doubled while preasphaltenes are reduced. This improvement in the liquid quality occurred together with an increase in the total yield of liquids. Exposure of steam-treated coal to air (runs 35 and 36) resulted in a drop in the rapid heating liquefaction yield of hexane solubles and oils almost to the level of raw coal. The total yield of liquids also decreased. In
slow heating liquefaction (runs 37 and 38), the total yield of liquids has dropped compared to rapid heating but is still higher than that of raw coal under rapid heating. Of the three liquid fractions, only hexane solubles are distinctly affected by heating rate. Under slow heating conditions the highest total gas make was observed; the total yield of hexane solubles and oils alone have intermediate value between raw and steamtreated coal under rapid heating. The higher total yield under slow heating liquefaction may result from a longer reaction time (it takes up to 30 min to preheat the autoclave to 385 °C), since the liquefaction process starts before the reactor temperature reaches 385 °C, even under low pressure. Comparison of the yields of liquefaction gas (Table 5) shows that the lowest yield among treated samples was observed under rapid heating and the highest under slow heating; the sample exposed to air has an intermediate value. Steam treatment and heating rate also affect the liquefaction gas composition. More C3-C4 hydrocarbon gases were obtained from steam-treated coal. The highest yield of carbon dioxide was produced from raw coal. It is possible that a significant amount of carbon dioxide is formed in the pretreatment process, decreasing its yield in the liquefaction gas from treated coal. Exposure of treated coal to air also causes an increase in the yield of CO2. The heating rate does not show any effect on the production of CO2. More methane was produced under slow heating conditions, compared to rapid heating, contributing to the highest overall gas yield. Increased production of CO2 as well as CH4 may indicate the occurrence of retrogressive reactions.22 Comparison of the gas composition at the two low-severity liquefaction conditions (Tables 3 and 5) demonstrates that the amount of carbon dioxide produced at 350 °C is sub(22) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energy Fuels 1990, 4, 42.
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Energy & Fuels, Vol. 11, No. 1, 1997 211
Figure 3. Products obtained from the pretreatment of R-benzylnaphthyl ether.
Figure 4. Products obtained from the pretreatment of R-naphthylmethyl phenyl ether. Table 6. Pretreatment of r-Benzylnaphthyl Ether Product Distribution in Steam and in Inert Atmosphere (wt %) product isomeric starting material recovered starting material 1-naphthol
minor product
conditions steam metal glass inert metal glass
41 37
53 58
0 0
6 5
42 40
49 52
0 0
11 8
stantially higher than the yields of other constituents, while at 385 °C the relative amounts of CO2 are reduced. The behavior of Illinois No. 6 coal in low-severity liquefaction (385 °C, 15 min, 1500 psia) follows the same trends as when higher temperature and longer reaction time are used (400 °C, 30 min, 1500 psia). The best results, highest total yield and yield of hexane solubles, are obtained from steam-pretreated coal under rapid heating conditions. Moreover, the improvement in the hexane solubles is achieved as a result of an increase in the yield of oils and a decrease in gas yield (at least at low severity). The liquefaction yields (total yield and hexane solubles) obtained from steam-treated coal under slow heating conditions have intermediate values be-
tween raw and steam-treated coal under rapid heating. When steam-treated coal is deliberately exposed to air, the total yield drops to the level of raw coal (rapid heating); the yield of hexane solubles is slightly higher than from raw coal under rapid heating but does not exceed the value obtained under slow heating. Model Compound Studies. A series of tests were conducted with R-BNE and R-NMPE using reactors either of stainless steel or lined with glass. Both ethers were treated in steam and in a flow of nitrogen at 51 atm (750 psia) and 322 ( 3 °C. The condensed reaction product was fractionated by preparative GC and analyzed by a combination of 1H and 13C NMR and CI-MS. The major reaction products of R-benzylnaphthyl ether are 1-naphthol, isomeric starting materials 2-benzyl-1-naphthol and 4-benzyl-1-naphthol, and minor products (Figure 3). The presence of toluene in the reaction product was detected, prior to GC separation, by mass spectrometry. The reaction products of R-naphthylmethyl phenyl ether are phenol, 1-methylnaphthalene, 1-naphthaldehyde, and isomeric starting material o-naphthylmethylphenol and p-naphthylmethylphenol (Figure 4). The results are shown in Table 6 for R-BNE and in Table 7 for R-NMPE, where the reaction product distributions are based on the amount of condensed material recovered. About half of the product obtained from R-BNE, as well as from R-NMPE, consists of isomeric starting material both in steam and in inert atmosphere. A slightly larger amount of isomeric starting material and less cleavage product is obtained in a glass-lined reactor from R-BNE. In the case of R-NMPE the difference is more pronounced, especially in steam, possibly due to a catalytic effect of the metal wall surface. Although a significant difference between glass and metal reactors is not observed, the glass-lined reactor is less suitable since it promotes undesired isomerization. The isomeric starting material is produced by recombination of benzyl radicals formed by homolysis. The radical recombination reaction, therefore, competes with the formation of cleavage product. It is possible that the model compound has far more freedom to isomerize than similar structures in a coal matrix. For both R-BNE and R-NMPE only small differences in conversion and product distribution were observed when steam was changed to inert gas. In contrast, the otherwise thermally stable 9-phenoxyphenanthrene, extensively studied by Siskin et al.,16,17 can be cleaved in the presence of liquid water at 315 °C. Hence, it is
Table 7. Pretreatment of r-Naphthylmethyl Phenyl Ether Product Distribution in Steam and in Inert Atmosphere (wt %) product
1-naphthaldehyde
recovered starting material
isomeric starting material
phenol
1-methylnaphthalene
23.1 6.3
22.5 17.1
4.3 6.0
9.0 14.2
41.2 56.4
22.4 24.2
25.4 16.5
9.2 7.3
0 0
43.0 52.0
conditions steam metal glass inert metal glass
212 Energy & Fuels, Vol. 11, No. 1, 1997
Ivanenko et al.
Figure 5. Aquathermolysis of 9-phenoxyphenanthrene.
Figure 6. Rearranged 9-phenoxyphenanthrene. Table 8. Conversion of r-Naphthylmethyl Phenyl Ether (wt %) product
conditions inert steam liquid water
isomeric methylnaphth- starting starting phenol naphthalene aldehyde material material 22.4 23.1 20.0
25.4 22.5 8.0
9.2 4.3 7.0
43.0 41.2 65.0
0.0 9.0 0.0
of interest to determine if liquid water is generally required to cleave ethers. Accordingly, aquathermolysis tests in liquid water were carried out using R-NMPE as well as 9-phenoxyphenanthrene. The reaction products obtained from cleavage of 9-phenoxyphenanthrene are shown in Figure 5. The results of Siskin et al.16,17 for aquathermolysis of 9-phenoxyphenanthrene were successfully duplicated giving complete conversion of the diaryl ether to phenol and phenanthrol in 1 h at 315 °C. When the reaction time was reduced from 1 h to 15 min (the reaction time employed in the coal pretreatment by steam), the conversion was incomplete. The starting material (not isomerized) accounted for 83% of the material recovered (recovery was close to 100%). Under coal pretreatment conditions (325 °C, 750 psia, 15 min) 9-phenoxyphenanthrene was unreactive. The product was approximately 80% rearranged starting material (Figure 6). Bond cleavage is believed to occur, followed by recombination of the primary fragments to yield isomeric starting material rather than phenol and phenanthrol, which would require proton abstraction from water. It is possible that the reaction follows a radical mechanism under low pressure, even in the presence of steam, rather than the ionic pathway proposed by Siskin et al.16,17 for aquathermolysis in liquid. Clearly, the ether linkages in coal are more reactive than 9-phenoxyphenanthrene even at the lower water density of steam pretreatment. Liquid phase aquathermolysis tests were then conducted with R-NMPE. The results are shown in Table 8 along with values obtained under steam treatment conditions and under inert atmosphere.
In all cases there is a substantial amount of isomeric starting material present. The major difference between results obtained at high pressure in the liquid phase and those obtained at low pressure (750 psia) is in the yield of methylnaphthalene and isomeric starting material. When water is present in the reactor in the liquid phase, the amount of methylnaphthalene is greatly reduced while the yield of isomeric starting material is increased. R-NMPE is, evidently, a poor model for ether bonds in coal since its course of thermolysis is little affected by the presence of water even when the liquid phase is present. 9-Phenoxyphenanthrene is a better model, at least in liquid phase aquathermolysis, cleaving only in the presence of water, and then almost quantitatively. Cleavage by aquathermolysis appears to have considerable discriminatory power in identifying coal-like ether linkages in model compounds. Conclusions The following conclusions are drawn from this work: 1. Steam pretreatment substantially improves the quality of the liquid product in coal slurry liquefaction. Under low-severity conditions, the oil yield is more than doubled, going from 12.5 to 29 wt %. 2. The benefits of steam pretreatment can be realized only if the pretreated coal is rapidly heated to liquefaction temperature. This is necessary in order that the pretreated coal pass quickly through a region where retrogressive effects occur. This region, in which such processes mitigate or even destroy the effects of pretreatment, extends downward from 360 °C to perhaps as low as 350 °C. The upper limit on heating time cannot yet be specified but appears to be about 1 min (which was the recovery time for the low-severity B series of tests). (Tubing bombs, in which improved yields after pretreatment are not obtained, require about 3 min to reach reaction temperature.) 3. Pretreated coal must be protected from oxygen to preserve the benefits of pretreatment. 4. The compounds R-benzylnaphthyl ether and R-naphthylmethyl phenyl ether show little difference in conversion and product distribution when the thermolysis atmosphere is changed from inert gas to steam. Hence, these compounds are poor models for coal in steam pretreatment. The otherwise thermally stable 9-phenoxyphenanthrene, on the other hand, is completely converted in 1 h by liquid water at 315 °C. At pretreatment conditions, however, mostly rearranged starting material is obtained. 9-Phenoxyphenanthrene, therefore, is a model for ether linkages in coal, although it is less reactive. Acknowledgment. This work was supported by the U.S. Department of Energy under Contract DE-AC2290PC90052. EF960088V
