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Energy & Fuels 2000, 14, 76-82
Articles Liquefaction Reactivity of Methylated Illinois No. 6 Coal Masahide Sasaki,* Takeshi Kotanigawa, and Tadashi Yoshida Hokkaido National Industrial Research Institute, 2-17 Tsukisamu-Higashi, Toyohira-ku, Sapporo 062-8517, Japan Received February 18, 1998. Revised Manuscript Received October 13, 1999
The reactivity of O-methylated and reductively methylated coals were tested for noncatalytic liquefaction reactions in tetralin solvent at 693 and 723 K to evaluate the effect of methylation on the conversion of Illinois No. 6 coal. For the liquefaction test at 693 K, the oil yield from reductively methylated coal drastically increased from 37 to 60 wt % compared with the raw coal. On the other hand, the oil yield of O-methylated coal was approximately 50 wt %. The improvement of oil yield for methylated coals was related to the number of methyl groups introduced by methylation. The increased oil yield for methylated coals was due to the acceleration of the decomposition reaction for preasphaltene. The difference in oil yield between raw and methylated coals decreased with increasing reaction temperature to 723 K, but an improvement of oil yield was observed. These results indicated that the methylation of coal molecules was effective for the production of the oil fraction, especially under lower severity conditions.
Introduction The physical and chemical structure of coal is one of the most important factors controlling the reaction at the initial stage of coal liquefaction. Optimization of the initial reaction stages of coal liquefaction has been shown to result in improved conversion in some cases. Song et al.1 have reported that the temperatureprogrammed liquefaction (TPL) would efficiently liquefy low-rank coals by controlling the rate of pyrolytic cleavage of weak bonds while minimizing the recombination of fragments and condensation reactions between thermally sensitive groups. It appears reasonable to conclude from comparative examination of the coal conversion data that the TPL may reduce cross-linking reactions of the thermally sensitive groups such as oxygen functional groups at a low temperature in a hydrogen donor solvent. Similar suppression of crosslinking reactions between oxygen functional groups has also been demonstrated by Derbyshire et al.2 The cross-linking reactions have also been observed to be influenced by the presence of alkali metals, removal of which increases pyrolysis tar yields3 and enhances fluidity.4 These results indicated that the role of the carboxyl group in coal, which was present in a form of carboxylate (cation-exchanged carboxyls), played * Corresponding author. (1) Song, C.; Schobert, H. H.; Hatcher, P. G. Energy Fuels 1992, 6, 328-330. (2) Derbyshire, F.; Davis, A.; Epstein, M.; Stansberry, P. Fuel 1986, 65, 1233-1239. (3) Tyler, R. J.; Schafer, H. N. S. Fuel 1980, 59, 487-494. (4) McCollon, D. P.; Sweeny, P. G.; Benson, S. H. Coal/Char Reactivity, 5th Quarterly Technical Progress Report 1987, April to June, UNDERC.
an important role in retrogressive reactions for low-rank coals. The role of calcium in reducing liquefaction yields from low-rank coals has been suggested in previous works by Whitehurst et al.,5 Mochida et al.,6 and Joseph and Forrai.7 The modification of low-rank coals by, for example, catalytic pretreatment and ion exchange techniques was studied systematically by Serio et al.8-10 Low-rank coals including lignite and sub-bituminous coals were chosen as the subject of the above studies. It is clear that for these coals the controlling factors of the initial reaction are the abundance and types of oxygen functional groups as well as the presence of alkali and alkaline earth metals in the coal. To date, there are very few reports about what controlling factors must be considered in the case of high-rank coals such as bituminous coal. It is likely that there exist different interactions in higher-rank coals which must be considered; however, their exact nature is unclear. Therefore, in the present work we have focused on determining the role of intermolecular associations caused by noncovalent interactions in bituminous coal as one of the controlling factors. We recognize three different types of specific noncovalent interactions, that are well(5) Whitehust, D. D.; Mitchell, T. O.; Farcasiu, M. Coal Liquefaction, The Chemistry and Technology of Thermal Processes; Academic Press: New York, 1980. (6) Mochida, I. Fuel 1983, 62, 659-664. (7) Joseph, J. T.; Forrai, T. R. Fuel 1992, 71, 75-80. (8) Serio, M. A.; Solomon, P. R.; Kroo, E.; Bassilakis, R.; Malhotra, R.; McMillen, D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35 (1), 61-69. (9) Serio, M. A.; Solomon, P. R.; Kroo, E.; Bassilakis, R.; Malhotra, R.; McMillen, D. 1991 Proc. Int. Conf. Coal Sci., New Castle, England, 1991, pp 656-659. (10) Serio, M. A.; Kroo, E.; Teng, H.; Solomon, P. R. Prepr. Pap.s Am. Chem. Soc., Div. of Fuel 1993, 38 (2), 577-586.
10.1021/ef9800314 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/22/1999
Liquefaction Reactivity of Methylated Coals
known,11 i.e., hydrogen bonds, charge-transfer interactions and aromatic π-π interactions, and note that their relative contributions to intermolecular cohesion changes with coal rank. These inter- and intramolecular associations are expected to contribute significantly to the formation of the secondary and tertiary structure in coal. It is believed that such interactions greatly affect the solubility of coal and coal-derived liquid in various organic solvents and viscosity of coal-derived liquids.11 Numerous studies have shown that application of chemical reaction techniques such as alkylation can greatly increase the solubility of coal at ambient conditions in organic solvents such as toluene and THF. These results suggest that coal is much less condensed and refractory than previously thought. In this study, two kinds of methylation procedures were adopted for the pretreatment for bituminous coal. One is the reductive methylation as described by Sternberg,12,13 and the other is oxygen methylation as proposed by Liotta.14,15 The effect of methylation of bituminous coal molecules on reactivity enhancement has been evaluated under noncatalytic liquefaction using tetralin as a hydrogen donor solvent. Experimental Section Illinois No. 6 coal was chosen for methylation and liquefaction experiments. The coal was pulverized to pass through a 100 mesh screen and then dried in a vacuum at 373 K before use. The coal was methylated according to the method described by Sternberg,12,13 following procedures in the literature. The reagents, potassium, methyl iodide, naphthalene, and tetrahydrofuran (THF), used in this study were of the highest purity available commercially. THF was additionally purified by refluxing over potassium metal for 48 h followed by distillation under a protective cover of helium. In this study, methyl iodide was used as an alkylation reagent. The coal anion was readily formed in the presence of 2.4 g of naphthalene per 6.0 g of Illinois No. 6 coal. The overall time required to complete coal anion formation was 240 h. In contrast, the O-methylation reaction was carried out according to the Liotta method.14,15 The suspension, including 6 g of coal with THF, was stirred mechanically while a 40% aqueous solution of tetrabutylammonium hydroxide (TBAH) was slowly added with a buret to the reaction mixture. Generally, an excessive amount of base, approximately 13 mL of TBAH, was added, and the mixture was stirred for 2 h. At this point a 2-fold amount of methyl iodide as an alkylating reagent was added. The reaction mixture was stirred for 48 h. Then, 1 M hydrochloric acid solution was added to neutralize the unreacted base. Subsequently the volatile substances such as THF, methyl iodide, and water were distilled away under vacuum at about 343 K. Solid material was washed by methanol using the Soxhlet extraction method for 48 h. The O-methylated coal was finally dried under vacuum at 383 K overnight. The extent of methylation was calculated from the elemental analysis data of the raw and methylated coals, assuming a constant weight of the nitrogen atom during the methylation. All liquefaction experiments were conducted in a 55 cm3 magnetically stirred autoclave reactor, which was connected (11) Stenberg, V. I.; Baltisbuger, R. J.; Patal, K. M.; Raman, K.; Woolsey, N. F. Coal Science; Academic Press: New York, 1983; Vol. 2, pp 125-168. (12) Sternberg, H. W.; Delle Donne, C. L.; Pantages, P.; Moroni, E. C.; Markby, R. E. Fuel 1971, 50, 432-442. (13) Sternberg, H. W.; Delle Donne, C. L. Fuel 1974, 53, 172-175. (14) Liotta, R. Fuel 1979, 58, 724-728. (15) Liotta, R. J. Am. Chem. Soc. 1981, 103, 1735-1742.
Energy & Fuels, Vol. 14, No. 1, 2000 77 with a 500 cm3 buffer vessel. Details of the reactor system have been described elsewhere.16 Normally, 3 g of the coal sample and 7 g of tetralin as a solvent were charged into the reactor. The reactor was heated at 90 K/min in an infrared image furnace. The reaction temperature was 693 or 723 K. Hydrogen pressure was maintained at 10.1 MPa for each run. To clearly identify the reactivity of methylated coal, we have conducted only noncatalytic liquefaction. In general, the deviations in conversions were within 2 wt %. After each run the reaction products were subjected to sequential extraction using THF, toluene, and n-hexane. These products divided into four fractions, namely residue (THFI), preasphaltene (THFS-TI), asphaltene (TS-HI), and oil (HS), respectively. The yields of the THFS, TS, and HS fractions were calculated on the basis of the weight of dry ash-free (daf) coal, assuming a constant weight of ash during liquefaction. The gas sample recovered from the reactor and buffer vessel was analyzed for mole percentages of CO, CO2, H2S, and C1C4 hydrocarbons with a gas chromatograph, which was calibrated with standard mixtures of hydrocarbon gases in hydrogen. Raw and methylated coals were prepared for FT-IR analysis by grinding a sample KBr mixture at 1:50 ratio. All absorbance spectra were recorded on a SHIMADZU FT-IR 8100M spectrometer. Gel permeation chromatographic analysis using THF solvent was carried out on solvent-soluble fractions using a TOSOH 8010 system in which two polystyrene gel columns (300 mm × 7.8 mm i.d. and 300 mm × 21.5 mm i.d.) were connected in series. A UV detector operating at 254 nm was used. The column temperature was fixed at 313 K. Energy-dispersive X-ray fluorescence (EDXRF) spectrometry was used in the determination of inorganic elements in coal ash. All coal samples were first ashed at 1088 K. A SEA-2001 EDXRF spectrometer was used to fluorescence coal ash samples powdered to approximately 100 mesh. These powders were pressed into cups made of Mylar film pulled tightly over a Teflon ring with a Teflon collar. The resultant surface is apparently perfectly planar. Standards used in the construction of calibration graphs were prepared by mixing a metalcontaining compound, such as Fe2O3, CaO, K2SO4, V2O5, SiO2, MnO2, Al2O3, and Na2CO3.
Results and Discussion 1. Methylation of Illinois No. 6 Coal. First of all, there is serious problem in evaluating the liquefaction reactivity of reductively methylated coal, namely contamination of potassium in methylated coal. The contamination of potassium in methylated coal has a negative effect on its hydrocracking reaction. Therefore, as much as possible removal of potassium from methylated coal is necessary in order to evaluate its reactivity accurately. The ash content of reductively methylated coal, prepared by the method described by Sternberg, increased from 8.1 to 13.2 wt %. The increase in ash content is thought to be due to contamination of potassium in the sample. Baldwin et al.17 have suggested that reductive methylation is ineffective for reactivity enhancement of Illinois No. 6 coal with a hydrogen donor solvent. Despite the methylation of coal, an increase in liquefaction yield was not observed. One of the reasons for the negative effects is contamination of potassium in methylated coal. In this study, exhaustive washing (16) Nagaishi, H.; Moritomi, H.; Sanada, Y.; Chiba, T. Energy Fuels 1988, 2, 522-528. (17) Baldwin, R. M.; Kennar, D. R.; Nguanprasert, O.; Miller, R. L. Fuel 1991, 70, 429-433.
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Table 1. Results of Elemental Analysis and Methylation for Illinois No. 6 Coals sample
C
Illinois No. 6 raw for O-Me O-methylated coal Illinois No. 6 raw for Red.-Me reductively methylated coal
74.5 78.3 73.9 80.9
[wt %, daf base] H N O+S(diff.) 5.3 5.9 5.4 7.0
1.3 1.3 0.9 0.8
18.9 14.6 19.9 11.3
ash [wt %] 7.6 6.5 8.1 7.9
yield [wt %] 102.0 73.9
THF solubles [wt %] 4.2 9.0 4.2 43.1
methyl group added [per 100 C atom] 3.9 23.0
Table 2. Element Concentration (wt %) in Coal Asha coal sample Al2O3 SiO2 SO3 K2O CaO MnO Fe2O3 ash Illinois raw O-Meb EtOH/H2Oc Red.-Med
2.2 1.8 2.0 0.7
2.9 2.5 2.9 1.1
0.3 0.0 0.1 0.9
0.5 0.5 0.5 3.9
0.1 0.0 0.0 0.0
0.0 0.0 0.0 0.0
2.1 1.7 1.1 1.3
8.1 6.5 6.6 7.9
a Based on coal weight. b O-Me: O-methylated coal. c EtOH/ H2O: EtOH/H2O extracted residue. d Red.-Me: reductively methylated coal.
Table 3. Summary for Reductive Alkylation of Illinois No. 6 Coal author
carbon wt %
alkyl iodide
reaction time h
alkyl group added per 100 C atoms
present study Stock18 Baldwin17 Larsen20
74 77 78 75
CH3I CH3I CH3I C2H5I
240 120 n.a.a n.a.a
23 10-13 3 8-12
a
n.a.: not available.
of the sample with EtOH/H2O mixtures reduced the ash content of the sample from 13.2 to 7.9 wt % as shown in Table 1. This value was approximately equal to that of raw coal. From the measurement of X-ray fluorescence spectrometry, however, reductively methylated coal after EtOH/H2O washing showed a relatively higher potassium yield, as shown in Table 2. So, such treatment can remove potassium to some extent, but not completely. Some of the lower-molecular-weight materials of reductively methylated coal might be removed by this treatment. Thus, the yield of reductively methylated coal was 73.9% of the total weight of the coal. For comparison, the ash content of raw coal after the treatment is also specified in Table 2. EtOH/ H2O washing of raw coal slightly contributed to reducing ash content. Reducing the ash content of O-methylated coal is believed to be due to the washing treatment after the reaction. The methylated coal contains 23.0 methyl groups per 100 carbon atoms. Table 3 summarized several results of reductive alkylation for Illinois No. 6 coal. Our result was much higher than others in terms of the number of added alkyl groups. The discrepancy between Stock’s data18 and ours is probably due to the difference in reaction time. Our reaction time was just twice that of Stock’s method. On the other hand, steric hindrance between CH3I and C2H5I contributes to penetrating to active sites in coal.19 The lower value of Larsen’s result20 using C2H5I as an alkyl reagent results in the steric hindrance effect of the alkyl reagent. In the case of O-methylation, the absence of TBAH in the methylated sample was confirmed by the measurement of solid state 13C-nmr. The yield of O-methylated coal was 102 wt % based on the total weight of (18) Stock, L. M.; Willis, R. S. J. Org. Chem. 1985, 50, 3566-3573. (19) Wachowska, H. M. Fuel 1979, 58, 99-1041. (20) Larsen, J. W.; Urban, L. O. J. Org. Chem. 1979, 44, 32193222.
Figure 1. GPC chromatograms of THF-soluble fraction. Raw, Illinois No. 6 coal; O-Me, oxygen-methylated coal; red.-Me, reductively methylated coal.
coal as shown in Table 1. The increase in the THFsoluble fraction of O-methylated coal was much less than that of reductive methylation. The increase in THF-soluble yield by methylation corresponds to the number of the introduced methyl group. In this study, O-methylated coal contained about 4 methyl groups per 100 carbons. This value is very similar to Baldwin’s result,17 but slightly smaller than Liotta’s14 (about 5 methyl groups per 100 carbons). The GPC profile for THF-soluble fraction at room temperature is shown in Figure 1. The distribution of the chromatogram for methylated coal shifted to a lower retention time range, especially for reductively methylated coal as compared with that of raw coal. This result suggests that THF-soluble fractions of methylated coals contain higher-molecular-weight materials which are produced by the methylation procedures. By virtue of reductive methylation, the yield of the THFsoluble fraction was drastically increased from 5.2 to 43.1 wt %. The result of FT-IR analysis for methylated coals, which were obtained in the way described above, is shown in Figure 2. For reductively methylated coal, a broad band at around 3500 cm-1, corresponding to OH stretching vibration, nearly disappeared. The band around 1700 cm-1 assigned to the COOH stretching vibration also decreased. These results suggest that most hydrogen bonds were disrupted by reductive methylation. In addition, the absorbance at 1250 cm-1 decreased in methylated coal as a consequence of the ether bond cleavage during treatment with potassium/ naphthalene/THF. On the other hand, O-methylation could not disrupt all of the hydrogen bonds in coal under our experimental conditions. It might be due to the lower amount of methyl groups by O-methylation. Our
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Energy & Fuels, Vol. 14, No. 1, 2000 79
Figure 3. Change with time in THF-soluble fraction. b, raw coal; 4, O-methylated coal; 0, reductively methylated coal. Reaction temperature: 693 K.
Figure 2. FT-IR spectra of raw and methylated coals.
result of added methyl groups was slightly smaller than that of Liotta, as described before. There was no drastic difference in the absorbance assigned to ether bonds between raw and O-methylated coal. It is satisfactory to consider that ether bond cleavage does not occur under this condition. From these results, the order of disruption of hydrogen bonds is the following: reductive methylation > O-methylation > no treatment; it also corresponds to the solubility in THF at room temperature. These samples were employed for the liquefaction test to evaluate the effect of methylation of bituminous coal molecules on liquefaction reactivity. 2. Liquefaction at 693 K. Figure 3 indicates changes with reaction time in the THF soluble fraction for raw and methylated coals. In this figure, the value at 0 min means including the heat-up time, about 4.4 min. At the initial reaction stage of liquefaction, the yields of the THF solubles from both methylated coals were higher than that from the raw coal. This tendency continued for up to 60 min, and the difference between them was 6.4 and 2.8%, respectively. To clarify the reason for the increment of THF-soluble fractions of methylated coals, we comparatively considered the yield of oil and preasphaltene. Figure 4 illustrates the change in the distribution of oil and preasphaltene with reaction time. The oil yield was free of gaseous products. The oil yield for all samples increased monotonically with time. Even at the initial reaction stage, the oil yield of reductively methylated coal was much higher than that of raw coal. This improvement continued for up to 60 min.
Figure 4. Typical change with time in yields of liquefaction products. (a) Oil fraction, (b) preasphaltene fraction. Reaction temperature: 693 K.
In contrast, the oil yield of O-methylated coal was almost the same as that of raw coal at the initial reaction stage. However, the difference in oil yield between O-methylated and raw coal gradually increased with time. At 60 min, the improvement of oil yield of methylated coal was 12.2 and 22.2%, respectively. From these results, the enhancement of THF solubles in the case of methylated coal as described in Figure 3 is clearly the result of an increased oil yield. The yield of preasphaltene showed a maximum value
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Figure 5. GPC chromatograms of oil fraction from raw and methylated coals. Reaction temperature: 693 K. Specific reaction time, 60 min.
at the initial reaction stage, but the yield for raw coal was much higher than that for the methylated coals. The preasphaltene yield of raw coal was more than four times that of methylated coals. In addition, the position of the maximum shifted to a shorter reaction time region by methylation. The effect of methylation on decomposition of preasphaltene not only improved the reactivity of the sample but also the acceleration of the reaction rate. As described in Table 1, the THF-soluble fraction of coal was drastically increased by methylation, especially reductive methylation. The reductively methylated coal contains 43.1 wt % of intrinsic THF solubles without any heat treatment. The yield of the THF-soluble fraction is not suitable for the evaluation of liquefaction reactivity for methylated coals, because it is derived from reactions which have already occurred prior to liquefaction. Therefore, we must consider the molecular weight distribution of liquefaction products following a reaction time of 60 min. Figure 5 shows GPC chromatograms of oil fraction for raw and methylated coals. As described in Figure 1, the newly converted THF-soluble fractions resulting from methylation contain highermolecular-weight materials than the intrinsic THFsoluble fraction of raw coal. GPC profiles for oil fractions were essentially the same for both the raw and methylated coals. This result indicates that the extent of the hydrocracking reaction of the methylated coals has proceeded to an extent equal to that of raw coal, because of the equivalency of the GPC profile for both products. The results of these experiments indicate that methylation of bituminous coal molecules is a beneficial pretreatment step for enhancing the reactivity of coal. 3. Liquefaction at 723 K. Results of liquefaction at 723 K for raw and methylated coals are shown in Figure 6. There was no remarkable difference in THF-soluble yield between raw and methylated coals. We could not observe any improvement in the THF-soluble yield of both methylated coals with an increase in reaction temperature. For raw coal, the gas yield at 60 min increased from 4 to 9 wt % with an increase in reaction temperature. These data are summarized in Table 4. At 693 K, both methylated coals showed a much higher
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Figure 6. Change with time in THF-soluble fraction. b, raw coal; 4, O-methylated coal; 0, reductively methylated coal. Reaction temperature: 723 K. Table 4. Gas Yield and Composition for Raw and Methylated Coalsa runb,c 693/0sraw 693/5sraw 693/10sraw 693/15sraw 693/30sraw 693/60sraw 693/0sO-Me 693/5sO-Me 693/10sO-Me 693/15sO-Me 693/30sO-Me 693/60sO-Me 693/0sraw 693/5sred 693/10sred 693/15sred 693/30sred 693/60sred 723/0sraw 723/15sraw 723/60sraw 723/0sO-Me 723/15sO-Me 723/60sO-Me 723/0sRed 723/15sRed 723/60sRed
gas gas composition, mol % yield CO2 CO CH4 C2H4 C2H6 C3H6 C3H8 n-C4 wt % 61.6 52.4 47.5 48.1 29.7 24.7 40.6 14.7 14.5 15.5 12.0 9.2 10.8 10.9 10.0 8.1 7.6 7.1 45.5 27.9 18.4 28.1 9.6 9.1 9.7 7.6 5.7
38.4 47.6 46.5 44.6 57.6 63.9 59.4 80.2 76.9 75.1 77.9 77.1 89.2 86.1 86.0 87.2 85.5 86.4 54.5 60.5 66.2 68.7 77.0 75.1 75.6 84.1 84.9
6.2 5.6 10.9 8.9
1.8 1.9 2.6
3.8 5.3 6.0 6.5 8.7
1.4 1.72 2.0 2.1 3.4
3.0 3.1 3.7 5.9 4.8
0.9 0.9 1.0 1.7
10.0 10.5 2.0 8.9 9.9 14.6 6.5 6.7
1.7 5.0 1.2 3.1 4.0 1.7 2.8
1.2 1.5 1.5 1.6
1.7 1.8
1 1 2 2 3 4 6 6 6 7 8 10 3 4 5 5 6 7 2 5 9 8 8 10 6 8 9
a Based on daf and hydrogen free. b Run #: reaction temp./ holding timessample name. c raw: raw coal; O-Me: O-methylated coal; red: reductively methylated coal.
gas yield compared with raw coal. It is thought that the increment of gas yield results from demethylation of added methyl groups in methylated coals. For example, the gas yield of O-methylated coal reaction at 693 K for 5 min was 6 wt %. Methane gas occupied approximately 4 wt % based on total feed amount. On the other hand, O-methylated coal contained approximately 4 methyl groups per 100 carbons. From these results, it can be seen that most of the added methyl groups in Omethylated coal were eliminated as methane gas at the initial reaction stage. In addition, trace amounts of n-C4 were observed in the gaseous products. This presumably corresponds to the contamination of TBAH as a reaction reagent in the sample. For solid-state 13C-nmr measurement of O-methylated coal, no signal related to TBAH could be observed. This is probably due to a trace amount of TBAH present in the coal sample. As de-
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Energy & Fuels, Vol. 14, No. 1, 2000 81
Figure 8. Typical change with time in yields of asphaltene fraction at 693 K. b, raw coal; 4, O-methylated coal; 0, reductively methylated coal; O, raw coal with catalyst.
Figure 7. Typical change with time in yields of liquefaction products. (a) Preasphaltene fraction, (b) oil fraction. Reaction temperature: 723 K. Table 5. Effect of Reaction Temperature on Oil Yield at 60 min 693 K sample
oil yield [wt %]
Illinois No. 6 raw coal O-methylated coal reductively methylated coal
37.3 49.5 59.5
a
723 K
diff.a [-]
oil yield [wt %]
diff.a [-]
12.2 22.2
52.7 59.3 66.3
6.6 13.6
Diff. means oil yield of methylated coal minus that of raw coal.
scribed before, the gas yield of raw coal at 60 min increased more than 2-fold at 723 K. The gas yields of both methylated coals were the same as that of raw coal. For gas composition of methylated coals, methane gas occupied a higher percentage in gaseous products. Figure 7 indicates the effect of reaction time on the formation of preasphaltene at 723 K. In all samples, the yield of preasphaltene decreased monotonically with increasing reaction time. At 693 K the yield of preasphaltene reached a maximum point at the initial reaction stage as described in Figure 4b. These results suggest that the hydrocracking of coal to preasphaltene has already proceeded appreciably during the heating stage prior to reaching 723 K. Figure 7 also shows a typical change with time in the yield of the oil fraction. Table 5 summarizes the effect of the reaction temperature on the oil yield at 60 min. The effect of methylation on the oil yield decreased with increasing reaction temperature. This must be due to the acceleration of
the decomposition reaction of raw coal by heat. At 693 K, the increment of oil yield for reductively methylated coal was about twice that of O-methylated coal. This tendency can also be observed at 723 K. From these results, it can be seen that the methylation procedures contributed to an increase in oil yield at both reaction conditions. Especially, reductive methylation showed a much higher oil yield than O-methylation. It corresponded to the number of methyl groups added in the treatment. Thus far the effect of methylation of bituminous coal on liquefaction reactivity has been discussed in terms of oil and preasphaltene yields and molecular weight distribution of liquefaction products. The behavior of asphaltene formation during liquefaction has also been examined. Figure 8 shows the change with time in the yield of asphaltene at 693 K. The yields for raw and O-methylated coals showed a maximum at approximately 20 min of reaction time, then gradually decreased. For reductively methylated coal, the value, on the contrary, monotonically decreased with time. However, the final yields of asphaltene for all samples were over 20 wt %. It seems that the asphaltene fraction cannot be decomposed, despite methylation of coal molecules. For comparison, the result of catalytic liquefaction is also indicated as an open circle in Figure 8. Catalytic liquefaction was carried out using FeS2 as a catalyst. Catalyst loading was 5 wt % based on the weight of daf coal. The THF-soluble fraction increased from 85.7 to 90.9 wt % with the addition of catalyst. However, this catalyst did not affect the decomposition of asphaltene, since the difference in asphaltene yield between catalytic and noncatalytic liquefaction was very little as shown in Figure 8. From these results it can be seen that, however coal molecules are methylated or catalyst be added, asphaltene cannot convert to oil fraction under the conditions used. Therefore, to realize effective liquefaction of bituminous coal, we must use a more effective catalyst for conversion of asphaltene to oil21 as a liquefaction catalyst. Conclusion The objective of the present study was to demonstrate the detrimental effects of methylation of the bituminous (21) Song, C.; Nihonmatsu, T.; Momura, M. Ind. Eng. Chem. Res. 1991, 30, 1726-1734.
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coal structure on liquefaction reactivity. The following conclusion can be made from experiments with two kinds of methylated coal samples using hydrogen donor solvent in a batch reactor: (1) Susceptibility of Illinois No. 6 coal to hydrocracking reactions increases with methylation, especially hydrocracking from preasphaltene to oil; (2) A change in oil yield for methylated coals is closely related to the degree of methylation for Illinois No. 6 coal; (3) As reaction temperature increases, the difference in oil yields between raw and methylated coals decreases due to an acceleration of hydrocracking reactions by heat. Methylation may have many effects on coal molecules. It will change molecular mobility, solubility properties,
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and non covalent association such as a hydrogen bond. In this study, we could not determine the dominant effect of methylation on the reactivity of bituminous coal. However, these results suggest the possibility of developing high-conversion and low-severity liquefaction procedures and the potentially reactive nature of bituminous coal. Acknowledgment. We are grateful to Mr. Shoichi Iwai for his efficient technical assistance. We also thank Prof. Chunshan Song of Pennsylvania State University for his useful comments on this paper. EF9800314
