Energy & Fuels 1995,9, 560-565
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Liquefaction Behavior of the Maceral Groups in Point of Ayr Coal Using Hydrogenated Anthracene Oil in a Tubing Bomb at Varying Temperatures Michael Cloke* Coal Technology Research Group, Department of Chemical Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K.
Changxian Wang College of Resources and Environmental Engineering, Chongqing University, Chongqing, 630044, P. R . China Received January 23, 1995@
Point of Ayr bituminous coal was liquefied in hydrogenated anthracene oil in a tubing bomb over a range of temperatures (300-450 "C, a t 25 "C increments). The residual material was investigated microscopically to determine the volume percentage of various residues and conversion ratio. The hydrogenation behavior of each maceral group was investigated and it was found that the vitroplast formed from vitrinite has different optical features from that formed from liptinite, with the liquefaction of liptinite occurring mainly above 400 "C and that of vitrinites at lower temperatures. Two types of semicoke were recognized. The influence of temperature, residence time, and coalholvent ratio on coal conversion was examined and compared with the above behavior.
Introduction Essentially, coal hydrogenation is a chemical process. However, microscopic techniques have proved to be of great significance for understanding the phenomena that happen during the hydrogenation It is found that each maceral group has its own hydrogenation and the behavior is also influenced by the physical state of the coal particle^.^ Through microscopic investigation the content of various macerals in the feed coal can be determined and their physical features observed. Furthermore, important information can be provided for the determination of optimal hydrogenation conditions by microscopic investigation of the solid residues arising from different liquefaction conditions. Mainly on the basis of microscopic investigation, Shibaoka and Heng advanced a model for the coal hydrogenation process in 1984.8 Classification of solid hydrogenation residues and their optical identifications have been proposed by Mitchell et al. and Ng.3b9 Keogh et al.1° studied the liquefaction behavior of macerals separated from a single coal using density gradient @Abstractuublished in Advance A C S Abstracts. Auril 15. 1995. (l)Wakele$, L. D.; David, A,; Jenkings, R. G:: kitchell, G. D.; Walker, P. L. Fuel 1979,58,379. (2) Shibaoka, M. Fuel 1982,61, 265. (3) Mitchell, G. D.; Davis, A,; Spackman, W. A petrographic classification of solid residues derived from the hydrogenation of bituminous coals. In Liquid Fuel from Coal; Ellington, R. T., Ed.; Academic Press: London, 1977; p 255. (4) Shibaoka, M. Fuel 1981,60, 240. (5) Shibaoka, M.; Russell, N. J.Proc. Int. Conf Coal Sci., Dusseldorf 1981,453. (6) Shibaoka, M.; Heng, S.; Okada, K. Fuel 1985,64, 600. (7) Shibaoka, M.; Thomas, C. G.; Heng, S.; Macay, G. H. Proc. Int. Conf. Coal Sci., Amsterdam 1987,107. ( 8 ) Shibaoka, M.; Heng, S. Fuel 1984,63,174. (9) Ng, N. J . Microsc. 1983,132, 289.
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separation. Their results indicated that synergistic effects may occur between the macerals in the parent coal and that at the lower temperature studied liptinite gave a lower conversion than vitrinite. Maceral effects during liquefaction are, therefore, not clear-cut. In this work, the liquefaction behavior of Point of Ayr coal was investigated in a tubing bomb over a range of temperatures from 300 to 450 "C. Hydrogenated anthracene oil was used as the hydrogen-donor solvent, with no hydrogen overpressure or presence of catalyst. This simulates the conditions used in the British Coal, Liquid Solvent Extraction Process. The volume percentage of each kind of residual product was determined by statistical measurements using reflected-light microscopy, so as t o obtain a quantative understanding. The conversion of dmmf coal to liquid products was also determined.
Experimental Section The coal used in this work was Point of Ayr coal, a British bituminous coal. The analysis of the coal is given in Table 1. It has a high concentration of vitrinite, which is dominated by collinite. The coal was ground to a size of ~ 2 0 pm 0 prior to digestion in the tubing bomb. The hydrogenated anthracene oil (HAO) was obtained from the British Coal Liquefaction Project. The tubing-bomb used for these studies was constructed from V 4 in. thick-walled stainless steel tubing and had a volume of 21 cm3. 5 g of the sample coal was slurried with the appropriate amount of HA0 and placed carefully into the tubing bomb. The whole of the assembly was then weighed. The bomb was then fixed t o a mechanical shaker and lowered into a preheated, fluidized bed, sand bath a t the selected (10)Keogh, R. A,; Taulbee, D. N.; Hower, J. C.; Chawla, B.; Davis B. H. Energy Fuels 1992,6, 614.
0 1995 American Chemical Society
Energy & Fuels, Vol. 9, No. 3, 1995 561
Maceral Groups in Point of Ayr Coal
Table 1. Chemical and Petrological Analyses of Sample Coal petrographic analysis (vol %) asha 13.1
ma 30.5 a
chemical analysis moisturea Cb Hb 3.5 85.5 5.4
Nb 1.9
Sb 2.8
Ob 7.7
vitrinite 79.5
liptinite 9.5
inertinite 5.9
mineral matter 6.2
vitrinite reflectance Ro (960) 0.77
Weight percent as received. Weight percent, dmmf.
Table 2. Results of Statistical Measurement of Residue and Conversion experimental conditions temperature
("0 300 325 350 375 400 425 450 425 425
solventto-coal ratio 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 3:1
residence time(min) 60 60 60 60 60 60 60 90 60
V 66.8 17.4 19.9 22.0 18.5 17.1 23.5 13.8 11.4
content of various residues (vol %/op L I Vitro Sc Ceno Mix 11.0 4.5 0.0 0.0 8.4 4.6 19.5 17.1 2.5 41.5 0.0 0.0 0.0 25.5 27.8 7.0 12.9 0.0 35.8 0.0 7.9 0.0 22.1 5.0 0.0 50.2 3.3 0.1 8.5 5.8 0.1 64.4 0.1 1.3 5.9 0 0.0 65.6 0.8 3.1 0.1 0.4 0.0 0.0 69.1 1.4 7.6 0.1 79.3 0.1 0.0 0.3 5.6 0.0
Min 3.9 2.0 7.0 7.3 13.6 11.1 6.5 8.0 3.3
vitrinite reflectance6 Ro(%) 0.59 0.73 1.31 1.29 1.73 1.78 2.10 1.60 1.91
conversion (wt%,dmmf) 19.1 26.1 68.0 72.1 87.0 88.6 90.2 91.1 93.4
a Based on 800 measurements. V, vitrinite; L, liptinite; I, inertinite; Vitro, vitroplast, Sc, semicoke; Ceno, cenosphere; Mix, mixture of organic and inorganic materials; Min, mineral particle. Based on 100 measurements.
temperature. The temperature recorded is that of the sand bath. Digestion was carried out for either 60 or 90 min, and the bomb was removed from the sand bath and allowed to cool for about 3 h. The bomb was then weighed to ensure that it had not leaked. The gaseous products were released, and the contents of the bomb were extracted with hot quinoline to give soluble and insoluble fractions which were separated by filtration using a Whatman GF/C filter pad. The quinolineinsoluble material consists of mineral matter and undissolved coal. From this a value of conversion on a dmmf basis was determined through calculation. The mineral matter in the original coal was determined by low-temperature ashing. The experimental conditions, such a s solvent-to-coal ratio, temperature, and residence time are given in Table 2. The solid residues, obtained from the dried quinoline insolubles, were mounted in epoxy resin, polished, and examined under a n optical microscope and a microphotometer by reflected light using oil immersion objectives. The magnifications used were from 320 x to 1000x .
Figure 1. Vitrinite remnants with irregular eroded margin. 300 "C treatment. Magnification 20 pm, 1.41 cm.
Results and Discussion A solvent-to-coal ratio of 2:l and a residence time of 60 min were adopted in most of the experiments. The volume percentages of the various residual materials, determined by the use of point counting, are shown in Table 2. After treatment at 300 "C, most vitrinite remnants have no evident changes in their morphology. Only a small portion of particles were eroded at their margins, giving irregular outlines as shown in Figure 1. In addition, some vitrinite particles became slightly plastic and agglomerated. However, after treatment a t 325 "C it was found that the majority of the vitrinite particles had been transformed to vitroplast. This leads to a sharp decrease in the quantity of vitrinite remnants in the residue. Vitroplast was first used by Mitchell et al. to describe the plastic or once plastic degradation products of vitrinite, which is optically isotropic, and has a spherical morphology, or appears as broad ('100 pm) areas with inclusions of other residue components, and as angular fragment^.^ It could be regarded as the undissolved
form of partly hydrogenated coaly material.8J1 Vitroplast is capable of being hydrogenated under favorable conditions. In these experiments, the vitroplast derived from vitrinite mainly appears as massive particles with inclusions of other residual particles and cracks as shown in Figure 2. It has a lower reflectance (0.450.71%) in comparison with vitrinite remnants, which may be caused by the disorientation of vitrinite lamellae.3 Vitrinite remnants usually appear as small particles and do not become plastic. The different development of plasticity in vitrinite particles may be caused by their chemical heter~geneity.~ Degasification pores can be seen within some vitrinite remnants. With a further increment of digestion temperature, the population of vitroplast decreases rapidly. It has a low percentage in the residues from 350 to 400 "C and often appears as partly dissolved remnants as shown in Figure 3. Following digestions a t 425 and 450 "C vitroplast could only be found occasionally, as small remnants. However, its existence indicates that, even a t the higher temperatures, the transformation of ~~~
(11) Shibaoka, M.Fuel 1986, 64, 606.
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Figure 2. Vitroplast with cracks and inclusions of maceral remnants. 325 "C treatment. Magnification 20 pm, 2.2 cm.
Figure 3. Dissolution of vitroplast particle. Lek undissolved, with many inclusions. Right: partly dissolved, maceral remnants and small vitroplast fragments. 400 "C treatment. Magnification 20 pm, 2.2 cm.
Cloke and Wang
vitrinite to vitroplast was still taking place. Therefore, the main route for hydrogenation of vitrinite is possibly vitrinite-vitroplast-liquid and gaseous products. In combination with the fact that only a small amount of vitroplast formed by vitrinite exists in the 350 "C treatment residues, a conclusion can be obtained that most vitrinite particles can be hydrogenated at a temperature of about 350 "C. There are, however, still some vitrinite remnants in the residues from digestions at 350-450 "C and their reflectance increases gradually with the increase of digestion temperature. Liptinite shows quite different behavior from vitrinite. After the digestion at 300 "C the majority of liptinite remnants show clear signs of reaction. Some particles were eroded at their margin and many particles became plastic. They developed flow structure and were partly transferred to an isotropic material as shown in Figure 4,with reflectance ranging from 0.9 to 1.5%. Here, the isotropic material is also categorized to vitroplast, since vitroplast could have derived not only from vitrinite but also from other coaly material, such as liptinite and semifusinite, provided these are capable of being digested or dissolved by organic solvents and becoming plasticized on heating.g Liptinite remnants appear frequently in the residues from 325 to 375 "C treatments. Compared with those from the 300 "C treatment, many more particles have been partially transformed into vitroplast. Apart from this, no substantial changes were observed. It seems that the increment of temperature from 300 to 375 "C does not have a very significant influence on the hydrogenation reactivity of liptinite. For this reason, the volume percentage of liptinite remnants is close to or even higher than that of vitrinite remnants, since the majority of the vitrinite particles have been transferred to vitroplast and other materials. But at digestions 2400 "C most liptinite particles have reacted, and only a few remnants exist in the residues. These results show that the hydrogenation of liptinite mainly occurs at temperatures 2400 "C. Under 400 "C, in most cases, liptinite only partially
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1
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Figure 4. Liptinite, partly transferred to vitroplast. 300 "C treatment. Magnification 20 pm, 1.41 cm.
Energy & Fuels, Vol. 9,No. 3, 1995 563
Maceral Groups in Point ofAyr Coal
c
!
Figure 5. Fusinite, broken into fragments; degasfication pores, appearing within some fragments. 400 "C treatment. Magnification 20 pm, 1.41 cm.
~i~~~ 7. partly dissolved semifusinite. 450
octreatment.
Magnification 20 pm, 1-41 cm.
w
Figure 6. Semifusinite, partly transferred to vitroplast. 350 "C treatment. Magnification 20 pm, 2.2 cm.
reacts with HAO. The liptinite in Point of Ayr coal is almost entirely sporinite, both mio- and megaspores. Thus, in this work, the liptinite remnants observed are, for all practical purposes, derived from sporinite. It is possible, therefore, that, for coals from a different geological origin containing predominantly other liptinite species, liquefaction behavior may be different. High-reflectance inertinite is essentially inert during the coal hydrogenation process.g In the experiments it was found that, usually, it did not become plasticized and could not be dissolved, with only some physical changes being observed. Most high-reflectance inertinite particles have been broken into small fragments after treatment, as shown in Figure 5, and degasification pores were formed within some particles. Low-reflectanceinertinite has a partial hydrogenation r e a ~ t i v i t y , ~but . ~ Jits ~ hydrogenation occurs at a higher temperature compared with vitrinite and liptinite. At 300 and 325 "C, low-reflectance inertinite particles change little in their morphology. After 350 and 375 "C, some particles became plastic (see Figure 6) and degasification pores appeared. In the residues from the 400-450 "C digestions, they often appear as partly (12)Schulken, H. R.; Marzec, A.; Czajkowka, S. Proc. Znt. Conf Coal Sci. Tokyo 1989, 876.
Figure 8. Cenosphere, with high reflectance. 425 "C treatment, 60 min of residence time, 2:l solvent-to-coal ratio. Magnificdtion 20 pm, 1.41 cm.
dissolved remnants, as shown in Figure 7. This suggests that they have some reactivity in the liquefaction process. Cenospheres could be seen occasionally in the residues derived from 400 to 425 "C digestions. They can possess some reticular texture or just appear as simple hollow spheres. Cenosphere is a morphological term. Mitchell et al. indicated that it was derived from vitrinite and spherical vitroplast, where, in the plastic state, gases formed by thermal cracking exert sufficient pressure to give expansion and lead to the formation of a ~enosphere.~ However, in these experiments, cenospheres appear to have been mainly derived from inertinite particles. This is shown by their high reflectance and relief; see Figure 8. As mentioned above, inertinite particles can develop plasticity and degasification pores, so it is possible that cenospheres can also be formed from them through a process which is similar to that described by Mitchell et al.3 Semicoke appears occasionally in the residues from the experiments carried out at 2400 "C, and its population increases with increments of experimental temperature. There are, mainly, two types of semicoke. One is isotropic, with very high reflectance (75%)and pores, and often appears at the edge of vitrinite remnants, as
564 Energy & Fuels, Vol. 9,No. 3, 1995
Cloke and Wang
Figure 10. Anisotropic semicoke, with higher reflectance than surrounding maceral remnants. 425 "C treatment, 60 min of residence time, 2: 1solvent-to-coal ratio. Magnification 20 pm, 2.2 cm.
Figure 9. Isotropic semicoke with very high reflectance and pores, mainly forming the edge of a vitrinite remnant. 450 "C treatment. Magnification 20 pm, 1.77 cm.
shown in Figure 9, indicating that it was formed from vitrinite. It was also once found forming the edge of vitroplast derived from liptinite. Another one is slightly anisotropic, with a reflectance of 1.2-3.0%. This kind of semicoke usually retains a characteristic morphology, indicating that it might also be directly formed from maceral particles. The first kind of semicoke appears more frequently. The formation of semicoke represents repolymerization and loss of reactive constituent^.^ Only a very small amount of semicoke was formed, probably due to the lack of severity of the digestion conditions. Mineral matter in the residues can appear as single particles. However, in most cases, it was found mixed with fine carbonaceous material, forming dark, massive, agglomerated particles. The quantity of mixed organic and inorganic material increases with increments of conversion ratio, as shown in Table 2. The temperature of digestion has a significant influence on the conversion of coal. As shown in Table 2, with a solvent-to-coal ratio of 2:l and a residence time of 60 min, increasing the digestion temperature from 300 to 450 "C gives a corresponding increase in conversion. The reason for this is that, generally, within the temperature limits of these experiments, coal particles become more reactive as the temperature is increased, and this is beneficial to digestion. However, no simple proportional relationship exists between conversion and temperature. In the experiments, there are two notable increases in conversion. The first occurs when the digestion temperature is increased from 325 to 350 "C and the conversion increases from 26.1 to 68.9%. This corresponds to the reaction of most of the vitrinite particles. The second occurs between 375 and 400 "C when conversion increases from 72.1 to 87.0%, corresponding to the reaction of most of the liptinite particles. Clearly, continued increases of temperature will not improve coal dissolution, since the rate of thermal repolymerization will begin to exceed the rate of hydrogenation8 and as a result large quantities of semicoke will be formed. In these experiments, digestion tem-
perature increases from 400 to 425 "C resulted in only small increases in conversion but the population of semicoke increases gradually. A temperature of 425 "C and 60 min residence time is normally used as the digestion conditions in the British Coal Liquefaction Process using Point of Ayr coal as the feedstock. In order to analyze the influence of residence time and solvent-to-coal ratio on coal liquefaction, two further experiments were performed, both at a temperature of 425 "C. In one case the residence time was increased to 90 min and in the other case the solvent-to-coal ratio was increased to 3:l. These results are also shown in Table 2. It can be seen that the extension of residence time does not significantly improve the coal digestion but a significant increase was obtained when the solvent-to-coal ratio was increased. Microscopic investigation also shows that increasing the residence time reduces the quantity of vitrinite remnants in the residue and increasing the solvent-to-coal ratio reduces the quantity of both vitrinite and liptinite residues. In the latter case this shows that the hydrogenation of vitrinite and liptinite was carried out more completely under conditions of greater hydrogen availability. However, set against this in a commercial process would be the extra costs of the additional recycling solvent. In coal liquefaction it has often been assumed that liptinite liquefies more readily than vitrinite and that inertinite is difficult to digest. The results described demonstrate that the liquefaction behavior of individual macerals is far more complex than this. Some parts of the inertinite fractions are reactive, and liptinites, in this study, required higher digestion temperatures than vitrinites. However, it is probable that there is a synergistic effect involving liptinite donating hydrogen to other coal fractions thus facilitating their digestion.
Conclusions In the experiments the majority of vitrinite particles are transformed to vitroplast at 325 "C and can be further liquified at 350 "C. However, the liquefaction of liptinite mainly occurs at 2400 "C. For this reason,
Energy & Fuels, Vol. 9, No. 3, 1995 565
Maceral Groups in Point of Ayr Coal the volume percentage of liptinite remnants is close to or even higher than that of vitrinite remnants in the residues obtained from the 325-375 "C digestions, although the content of liptinite is much lower than that of vitrinite in the untreated coal. The vitroplast formed by liptinite has a higher reflectance than that formed by vitrinite. Some low-reflectance inertinite particles become plastic at 350-375 "C and can be dissolved at higher temperature. Cenospheres can be formed from inertinite. A very small amount of isotropic semicoke (with very high reflectance) and anisotropic semicoke (with relatively low reflectance) were found in the 0 digestions. Their popuresidues derived from ~ 4 0 "C lation increases with the increase in digestion temperature.
There are two big increases in coal conversion ratio. One happens between 325 and 350 "C, and the other happens between 375 and 400 "C. They respectively correspond to the hydrogenation of most of the vitrinite and liptinite particles. When the solvent-to-coal ratio is increased from 2:l to 3:1, coal hydrogenation was improved significantly.
Acknowledgment. The authors are grateful to A. Pickering and D. Clift for their help during the experimental and microscopic investigation. Both the Point of Ayr coal and hydrogenated anthracene oil were supplied by British Coal. EF9500189
