200
Energy & Fuels 2005, 19, 200-207
Comparison of Temperature Conditions in Direct Liquefaction of Selected Low-Rank Coals Lili Huang† and Harold H. Schobert* The Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 Received October 23, 2003
The direct liquefaction of one lignitic and two subbituminous coals was studied in laboratoryscale microautoclave reactors. The principal focus of this work was to investigate the effects of three strategies for employing temperature: single-staged liquefaction at one temperature; temperature-staged liquefaction involving rapid heat-up between low- and high-temperature reaction stages; and temperature-programmed liquefaction, with slow heat-up between stages. Both the temperature-staged and temperature-programmed conditions are advantageous relative to the single-staged reaction, particularly for coals that are less easily converted. The benefits of using temperature-staging or programming appear to derive from both physical and chemical processes related to the catalyst and solvent, such as allowing more time for the catalyst precursor to transform into an active form and enhancing solvent-coal interactions. For a highly reactive coal, single-stage conditions appear to be sufficient. In single-stage reactions without solvent or catalyst, or with nonreactive solvent, the conversion of these three coals is related to their organic sulfur content.
Introduction Direct liquefaction of coals involves two steps: the breakup of the macromolecular structure of the coals into fragmental radicals and the stabilization and hydrogenation of those fragments to produce molecules of lower molecular weight. The eventual outcome of liquefactionsthat is, the conversion of the coal to lowermolecular-weight products and the relative yields of each class of productssreflects the complex interplay of many parameters of the reaction: the specific properties of the coal being used, the reaction temperature, the gaseous atmosphere, the solvent, and the catalyst. This paper is the first in a planned series of papers in which we contribute to sorting out aspects of the effects of temperature, solvent, and catalyst for the liquefaction of low-rank coals. We have chosen three coals, one lignite and two subbituminous, that are representative of the low-rank coals of the United States (to the extent that any coal can be considered representative or typical). Low-rank coals are generally characterized by small aromatic ring clusters, abundant aliphatic and hydroaromatic carbon and hydrogen, and high oxygen contents in a variety of functional groups.1 Because of the diversity of various C-O and C-C bonds in lowrank coals, the bond dissociation energies are likely distributed over a broad range. It is assumed that, for the low-rank coals we have studied, bond-breaking and * Author to whom correspondence should be addressed. Telephone: 814-863-1337. Fax: 814-865-3075. E-mail address: schobert@ ems.psu.edu. † Present address: 2911 Zanker Road, San Jose, CA 95051. E-mail:
[email protected]. (1) Schobert, H. H. Resour., Conserv. Recycl. 1990, 3, 111.
free-radical formation can be done well by thermal energy, based on earlier work of Derbyshire2 and Song.3,4 Some bonds may be readily broken at relatively low temperature. In fact, Song and co-workers demonstrated that more bonds were thermally cleaved in lowrank coals at low temperature than in high-rank coals.3,4 If the radicals generated by bond cleavage can be stabilized by hydrogenation, molecular weight reduction is achieved and the goal of liquefaction is accomplished. For low-rank coals, the question becomes one of how to stabilize effectively free radicals that may be generated in a wide temperature range. Because different bonds may be cleaved at different temperatures, optimum results may not be achieved if only one temperature is used. Temperature-staged liquefaction (TSL) and temperature-programmed liquefaction (TPL) conditions are thus designed to cleave some weak bonds at low temperatures at reasonable rates so as to match the rate of hydrogenation. The structural features of low-rank coals suggest that their macromolecular structures might be broken down at low temperatures. The next step in the liquefaction would be to stabilize the free radicals by hydrogenation. If the rate of hydrogenation cannot catch up with that of the formation of free radicals, undesirable recombination of the radicals, so-called retrogressive reactions, will result. To match the rate of hydrogenation with the rate of free-radical formation, two approaches can be taken. One is to accelerate the rate of hydrogenation by using a good hydrogen-donor solvent or an active hydrogena(2) Derbyshire, F. J.; Davis, A.; Epstein, M.; Stansberry, P. G. Fuel 1986, 65, 1233. (3) Song, C.; Nomura, M.; Miyake, M. Fuel 1986, 65, 933. (4) Song, C.; Schobert, H. H.; Hatcher, P. G. Proc. 6th Int. Conf. Coal Sci. 1991, 664.
10.1021/ef0301710 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/30/2004
Direct Liquefaction of Selected Low-Rank Coals
tion catalyst. The second is to slow the rate of freeradical formation by controlling the reaction temperature appropriately. In single-stage liquefaction (SSL) only one reaction temperature is used. According to the heat-up procedure, single-stage liquefaction can be further classified as slow heat-up SSL, where the reactants are slowly heated from ambient temperature to the desired reaction temperature, and fast heat-up SSL, where the reactants are heated to the reaction temperature in a very short time, usually (in laboratory-scale work) less than 5 min. In temperature-staged and temperatureprogrammed liquefaction, two temperature levels are involved. The low temperature, usually between 200 and 350 °C, is designed for pretreatment, and the high temperature is between 400 and 500 °C for reaction. In temperature-staged liquefaction (TSL), the reactants are immediately heated to the higher temperature. In temperature-programmed liquefaction (TPL), there is a slow heat-up between the two temperature stages. In TPL the heating time between stages is 30 min or longer. The reactions at the lower temperature and during the heat-up period in TPL are designed for transformation of the catalyst precursor into an active catalyst; penetration of the solvent into the coal particles to achieve good solvent-coal contacting; and reduction of the rate of thermal cracking of the coal structure, which might be too fast at high temperature and lead to retrogressive reactions. SSL is the simplest procedure, but its drawback is that it may cause problems, especially for low-rank coals, because different kinds of bonds may be broken at the reaction temperature, some so rapidly that hydrogenation cannot catch up. Other bonds may be broken so slowly that reactions may not occur to a significant extent during the reaction time. Retrogressive reactions from too slow hydrogenation or low conversion from too slow bond breaking are both undesirable. Indeed, numerous publications suggest that conversions and yields of light distillate products can be improved by using TSL rather than SSL.2,5-12 Earlier work at our institute also suggests that it is beneficial to choose TPL or TSL rather than SSL.13-15 Experimental Section Coals and Solvents Used. Three low-rank coals were chosen and were obtained from the Penn State Coal Sample Bank and Data Base. The relevant properties of these coals are provided in Table 1. The coals were stored in multilaminate (5) Pott, A.; Broche, H. Fuel 1934, 13, 91. (6) Derbyshire, F. J.; Varghese, P.; Whitehurst, D. D. Fuel 1983, 62, 491. (7) Narain, N. K. Fuel Process. Technol. 1985, 11, 13. (8) Bockrath, B. C.; Finseth, D. H.; Illig, E. G. Fuel Process. Technol. 1986, 12, 175. (9) Derbyshire, F. J.; Davis, A.; Lin, R.; Stansberry, P. G.; Terrer, M. T. Fuel Process. Technol. 1986, 12, 127. (10) Epstein, M. J. M.S. Thesis, The Pennsylvania State University, University Park, PA, 1987. (11) Tsucarda, Y.; Makabe, M.; Itoh, H.; Ouchi, K. Fuel 1987, 66, 639. (12) Derbyshire, F. J. IEA Coal Res. Rep. 1988, No. IEA CR/08. (13) Song, C.; Schobert, H. H.; Hatcher, P. G. Energy Fuels 1992, 6, 326. (14) Song, C.; Schobert, H. H. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1992, 37, 976. (15) Huang, L.; Song, C.; Schobert, H. H. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1992, 37, 223.
Energy & Fuels, Vol. 19, No. 1, 2005 201 Table 1. Characteristics of the Coals Used in This Work DECS-1 state county seam ASTM rank
DECS-9
Source and Rank Texas Montana Freestone Bighorn Bottom Dietz Subbit C Subbit B
DECS-11 North Dakota Mercer Beulah Lignite
Proximate Analysis, % (as Received) moisture 30.0 24.7 33.4 ash 11.1 4.8 6.4 volatile matter 33.2 33.5 37.4 fixed carbon 25.8 37.1 22.9 Elemental Analysis, % dmmf carbon 76.1 76.1 hydrogen 5.5 5.1 nitrogen 1.5 0.9 organic sulfur 1.1 0.3 oxygen (by difference) 15.8 17.5
74.2 4.4 1.0 0.4 20.0
bags under argon. Laboratory solvents and reagents were obtained from commercial sources and were used without further purification. The sample of Wilsonville middle distillate was kindly provided by Dr. Michael Baird of the U.S. Department of Energy; its composition has been reported by Lai et al.16 Catalyst. Commercially available ammonium tetrathiomolybdate (ATTM) was used as the catalyst precursor. For each experiment a calculated amount of ATTM sufficient to provide a molybdenum loading of 1% of the dry, mineral-matter free (dmmf) coal was used. It was dissolved in a 1:1 H2O:THF (THF ) tetrahydrofuran) mixture in an amount equivalent to ∼3 mL of solvent mixture/(g of coal). The ATTM solution was added to the coal sample to make a slurry. The slurry was stirred by magnetic stirrer for 15 min at room temperature under N2, followed by drying under vacuum at room temperature for 48 h, at 40 °C for another 48 h, and finally at 95 °C for 6 h to remove the THF and H2O. Reaction Procedure and Product Workup. Liquefaction experiments were carried out in horizontal microautoclave reactors. The reactors were made of type 316 stainless steel, 19 mm outside diameter, with threaded Swagelok weld-on fittings at both ends. In the middle of the reactor tube, a piece of 6 mm stainless steel tubing was welded vertically. On top of this 6 mm tubing, a gauge, needle valve, and quick-connect stem were attached for purging and pressurizing the reactor. Prior to reaction, the reactors were tested for possible leakage by pressurizing to 7 MPa with nitrogen and immersing into water. For each experiment, 4 g of coal and 4 g of solvent (if a solvent was used) were loaded into a reactor, which was then sealed and tested for leakage. Then it was purged three times with hydrogen to remove air. Subsequently, the reactor was pressurized to 7 MPa with hydrogen at room temperature. The loaded, sealed, and pressurized reactor was attached to an oscillator and plunged into a fluidized sand bath preheated to a desired temperature. The reactor usually reached the desired temperature within 3-5 min. The specific temperature conditions are discussed in detail in the next subsection. The reactor was then usually kept in the sand bath for 30 min. (The reason for selecting this reaction time is discussed below.) The reactor was oscillated at 200 cycles/min at an amplitude of 2.5 cm during the course of the reaction. After reaction, the reactor was quenched in a water bath and then allowed to sit in the laboratory for several hours to equilibrate with room temperature before product separation. The volume of the gas was measured by water displacement. The solid and liquid contents of the reactor were transferred to a weighed ceramic thimble. The reactor, including the stem, (16) Lai, W.; Song, C.; Schobert, H. H. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1992, 37, 1671.
202
Energy & Fuels, Vol. 19, No. 1, 2005
Huang and Schobert
Figure 2. Effects of reaction time on the catalytic liquefaction of DECS-1 sub-bituminous coal in the absence of solvent. For ease of manipulation, the temperature-staged reaction requires two sand baths. The first is preheated to a temperature desired for the pretreatment stage, usually 200 °C. The other is heated to the higher temperature, the reaction temperature. The reactor was kept in the first sand bath for 15 min after it had reached the temperature of the bath. It was then rapidly removed and plunged into the second sand bath, where it was held for 30 min after reaching bath temperature. In the temperature-programmed liquefaction experiments, one sand bath is adequate. It is first heated to the lower temperature (e.g., 200 °C). After the pretreatment at the low temperature for 15 min, the temperature controller is immediately set to a higher level. In the specific system in our laboratory, the temperature controller was set to 425 °C; when the bath temperature reached ≈330 °C, the controller was readjusted to 400 °C. This procedure ensured that the heating time between the two desired temperatures was ∼30 min, giving a temperature ramp of ∼7 °C/min. Again in this case, the reactor was kept at the high temperature (e.g., 400 °C) for 30 min. Figure 1. Comparison of temperature profiles for (a) singlestage, (b) temperature-staged, and (c) temperature-programmed liquefaction. was thoroughly washed with hexane; these washings were filtered through the thimble. The product mixture was then separated using sequential Soxhlet extraction using 250 mL each of hexane, toluene, and THF for about 24 h. The hexane solution was concentrated using a rotary evaporator. The concentrate (oil) was stored under nitrogen. The toluene and THF were evaporated by the rotary evaporator, and the remaining products (asphaltenes and preasphaltenes, respectively) were dried in a vacuum oven at 105 °C for 6 h. The solid residue left in the thimble was washed several times first with acetone and then with pentane, followed by drying under vacuum at 105 °C for at least 6 h. After drying, the products were weighed and stored under nitrogen. The conversion and yields of gas, asphaltenes, preasphaltenes, and residue were calculated on the basis of the weight of dmmf coal. The oil yield was obtained as the difference between the dmmf coal and the total of gas, asphaltenes, preasphaltenes, and residue. Temperature Conditions. The temperature profiles for the single-stage, temperature-staged, and temperature-programmed reactions are illustrated in Figure 1. For purposes of illustration, the reaction temperatures in Figure 1 are shown as 200 and 400 °C; the profiles would be similar in shape if other temperatures had been used. In a single-stage reaction, the sand bath was preheated to the desired temperature before the reactor was plunged into it. Usually the reactor reached bath temperature within 5 min. From the moment that the reaction temperature was achieved, the reactor was kept in the sand bath for 30 min. It was then quickly removed and quenched in water.
Results and Discussion Evaluation of Reaction Time. To select an appropriate reaction time for the study of the effects of temperature (described below), we examined the behavior of DECS-1 at 400 °C for 10, 30, and 60 min. ATTM was used as catalyst precursor. Experiments were done with tetralin, 1-methylnaphthalene, or no solvent. The conversion and product distribution from reactions without solvent are presented in Figure 2. At 10 min, about 84% (dmmf basis) of the coal converted to THF-solubles, with 45% oil, 23% asphaltenes, 9% preasphaltenes, and 7% gas. When the reaction time was increased to 30 min, an additional 8% of conversion was achieved, with more oil and less asphaltenes. That is, increasing the reaction time from 10 to 30 min favors conversion from heavy to light products. However, further increase to 60 min did not seem beneficial; the total conversion and the oil yield drop, accompanied by an increase in gas yield. Results of the reactions in the presence of tetralin are shown in Figure 3. A reaction of 10 min with the donor solvent does not provide conversion (67%) as high as that without solvent. This suggests that the solvent has not yet fully penetrated the pore system and reached the reaction sites. This agrees with the earlier work of Tye et al. that tetralin is unable to penetrate some, perhaps most, of the pores of coal at ambient temperature.17 Further, the presence of solvent may inhibit interaction of gaseous hydrogen with the coal.18,19 The
Direct Liquefaction of Selected Low-Rank Coals
Figure 3. Effects of reaction time on the catalytic liquefaction of DECS-1 sub-bituminous coal in the presence of tetralin.
Figure 4. Effects of reaction time on the catalytic liquefaction of DECS-1 sub-bituminous coal in the presence of 1-methylnaphthalene.
presence of the solvent also adds to the mass of the system, increasing the time needed to heat the reactants to the desired temperature. In comparison, increasing the reaction time from 10 to 30 min provides significant increases in conversion, oil yield, and asphaltene yield. In contrast to reactions in the absence of solvent, with a further increase in reaction time to 60 min the conversion reaches 95%. In this case, the oil yield continues to increase, at the expense of asphaltenes. Gas and preasphaltene yields remain almost unchanged compared with those observed at 30 min reaction. Reactions with 1-methylnaphthalene (1-MN), considered not to be a donor solvent, show a trend, Figure 4, generally similar to reactions with tetralin. In a 10 min reaction, only 60% conversion is observed. As the reaction time was increased to 30 min, 85% conversion and 40% oil yield were achieved. When the reaction time was increased to 60 min, the oil yield continued to increase at the expense of preasphaltenes. On the basis of these results, it is apparent that conversion of coal to THF-solubles is relatively fast. Within 10 min, fairly high conversion can be achieved. Conversions of asphaltenes or preasphaltenes to oil are slower and appear to require longer reaction time. However, if the reaction time is too long (e.g., 60 min), conversion and oil yield may drop, especially in reactions with no solvent. Therefore it seemed appropriate to select a 30 min reaction as useful standard time for evaluating the temperature effects described in the following sections. Study of Temperature Effect on DECS-9 Coal. It has been shown in previous work that TPL is superior (17) Tye, C.; Neumann, R. M.; Schobert, H. H. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1985, 30, 134. (18) Font, J.; Fabregat, A.; Salvado, J.; Moros, A.; Bengoa, C.; Giralt, F. Fuel 1992, 71, 1169.
Energy & Fuels, Vol. 19, No. 1, 2005 203
Figure 5. Comparison of the effects of the three temperature conditions on the catalytic liquefaction of DECS-9 subbituminous coal in the presence of Wilsonville middle distillate solvent. The reaction temperature was 400 °C; the first-stage temperature in TSL and TPL was 200 °C.
to SSL for this coal.4,13-15 The coal sample was dried, and, for catalytic experiments, it was impregnated with ATTM before reaction. Wilsonville middle distillate (WIMD), a coal-derived process solvent, was used as reaction solvent. A study of the composition of WIMD was carried out by Lai et al.16 WIMD has high contents of aromatics with two or more rings. Pyrene and various partially hydrogenated pyrenes are the most abundant compounds in WIMD. Pyrene and hydropyrenes have been identified as hydrogen shuttler and hydrogen donor compounds.19-24 Figure 5 presents the results of liquefaction under the three temperature conditions. The results shown in Figure 5 are the averages of two or more replications, with experimental errors of
