Energy & Fuels 1992,6, 614-618
614
Liquefaction Characteristics of the Three Major Maceral Groups Separated from a Single Coal Robert A. Keogh, Darrell N. Taulbee, James C. Hower, Birbal Chawla,+and Burtron H. Davis' Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, Kentucky 4051 1 Received March 20, 1992. Revised Manuscript Received June 15, 1992
The three major maceralgroups, vitrinite, liptinites, and inertinites, were separated from an eastern Kentucky coal by density gradient centrifugation. The liquefaction of these macerals was studied at three reaction temperaturesand asingle residence time. The liquefaction products were separated by solubility classes and each class was further analyzed. The liquefaction data showed the absence of an additive effect, suggesting a synergism among the macerals in the original coal matrix.
Introduction
Experimental Section
A fresh sample of the Lower Elkhom coal (petrographicanalysis (~01% dmmf), 36.7% vitrinites, 5.6% fusinite, 21.0% semifusinite, 5.0% micrinite, 1.3% macrinite, 28.9% exinite, 1.5% resinite) was collected at the working face of the mine and transported to the laboratory for processing. The sample representa the durain at the top of the Pond Cree coal bed in the Jamboree 7 1/2 f t quadrangle, Pike County, KY. The durain collected was about 3.5 cm thick. Vitrinite maximum reflectance is l.O%, indicating high-volatile A bituminous rank. The mineralogy of this coal has been reported in detail elsewhere.12 The primary minerals were silicates. The sample was ground to -100 mesh and stored under argon prior to analysis and separation. The DGC procedure utilized to effect the maceral separations has been described in detail elsewhere.12 The chemical and petrographic analysesof the parent coal and the maceral fractions are given in Table I. The primary minerals remaining detected after demineralization were pyrite, marcasite, and rutile. Small amounts of anatase were also found. All liquefaction experimenta were conducted in a 50-mL microautoclave reactor under conditions of excess hydrogen. The microautoclave was charged with a 5-g sample (dried overnight under a vacuum of ca. 635 mmHg at ca. 363 K),7.5 g of tetralin, and a steel ball (6.4 mm) for mixing. The reactor was pressurized with 5.5 mPa of Hz (ca. 13.8 mPa at 718 K)and immersed in a fluidized sand bath for the desired reaction time. Threereaction temperatures (658,700,718 K) and a 15-minresidence time were used in this study. The sand bath was operated such that it required less than 2 min to reach the desired reaction temperature. To ensure thorough mixing of the reactants, the shaker speed (vertical) was set to 400 cycles/min. At the end of the + Present address: Pauleboro Reeearch Laboratory, MobilR& D Corp., experiment, the reactor was placed into a cold fluidized sand Pauleboro, NJ 08066. bath. Once the reactor had attained room temperature (typically (1) Joseph,J.T.;Fisher,R.B.;Maein,C.A.;Dyrkacz,G.R.;Bloomquiat, C. A.; Winans, R. E. Energy Fuels 1991,5,724-729. within 2 min) a gas sample was obtained for analysis (H2, CI-CI, (2) King, H.-H.; Dyrkacz, G.R.; Winans, R. E. Fuel 1984,63,341-345. CO,). The producta were removed from the reactor and separated Pajak, J. Fuel Process. Technol. 1989,21, 245-252. into their respective solubility classes by Soxhlet extraction. (3) Parkash, S.; Lali, K.; Holuszko, M.; du Plessis, M. P. Li9. Fuels Technol. 1985,3, 345-375. The technique used for the determination of the product (4) Crelling, J. C.; Skompka, N. M.; Marsh, H. Fuel 1988,67,781-785. solubility class distributions is described in detail elsewhere.13 (5) Foster, N. R.; McPheraon, W. P.; Tao, C.-Y.; Collin, P. J. Fuel 1986, The operational definitions of the solubility classes used in this 64,917-920. work are as follows: (a) oils (pentane soluble), (b) asphaltenes (6) Heng, S.; Shibaoka, M. Fuel 1983,62,610-612. (7) Shibaoka, M.; Heng, S.; Keyofumi, 0.Fuel 1985,64,6o(wo5. Sem(benzene soluble, pentane insoluble), (c) preasphaltenes (pyriinas, G. B.; Carlin, D. H.; Shi,X. X.;Albright, L. F. Ind. Eng. Chem. dine soluble,benzene insoluble),and (d) IOM (pyridine insoluble). Process Des. Deu. 1985,24, 1091-1096. The analytical HPLC/FID method has previously been deecribed (8) Neavel, R. C. Fuel 1976,55,237-242. in detail elsewhere." The elemental analyses were performed (9) Dyrkacz, G.R.; Horwitz, E. P. Fuel 1982,61, 3-12. (IO) Dyrkacz, G.R.; Bloomquist, C. A.;Horwitz, E. P. Sep. Sci. Techusing standard techniques.
A fundamental approach for the study of a complex and heterogeneous matrix is to separate the matrix into a number of componentswhich comprise the matrix. This analytical approach has been applied to the study of the liquefaction of Most comparisons of the liquefaction characteristics of the major maceral groups of bituminous coals have utilized different coals which are enriched in one or more of the maceral groups. The macerals are either hand picked for enrichment or the coals are used as is for the liquefaction studies. The developmentof the density gradient centrifugation technique (DGC) has provided another method for the separation of the macerals from the coal matrix.+l1 This technique was used with some modifications developed at the Center for Applied Energy Research (CAER)12for the separation of macerals from an eastern Kentucky coal. A bench sample of the Lower Elkhorn seam was found to have approximatelyequal concentrations of the major maceral groups (vitrinites, inertinites, and liptinites) which makes this an ideal coal for the liquefaction study of the individual maceral groups from the same coal. Sufficient quantities and purities of the three maceral groups were separated by DGC for liquefaction in a microautoclave reactor (50 mL) and subsequent product analysis. The results of these experiments are reported in this paper.
nol. 1981,16, 1571-88. (11) Dyrkacz, G.R. Fuel 1984,63, 1367-1373. (12) Taulbee, D.; Poe, S. H.; Robl, T.; Keogh, R. Energy Fuels 1989,
3, 662-670.
0887-0624/92/2506-0614$03.00/0
(13) Keogh, R. A.; Davis, B. H. J. Coal Quality 1988, 7,27-31. (14) Chawla, B.; Davis, B. H. Fuel Sci. Technol. 1989, 7, 1-14.
Q 1992 American Chemical Society
Liquefaction of Maceral Groups from a Single Coal
density range as received basis moisture ash daf basis volatile matter fixed carbon carbon hydrogen nitrogen sulfur oxygen (by diff) H/Cratio (M) petrography ( ~ 0 1 % ,dmmf) vitrinites inertinitea liptinites
Table I. Analysis of Lower Elkhorn Samples (KCERL 3761) parent coal deminerald coal liptinite concentrate vitrinite concentrate 1.00-1.25 1.261.33
inertinite concentrate 1.34-1.45
0.50 17.50
0.70 0.82
2.50 0.20
3.50 0.10
1.26 0.60
38.69 61.31 86.11 5.51 1.29 1.20 5.89 0.77
38.66 63.34 86.29 5.36 1.40 1.07 5.88 0.75
55.34 46.66 88.11 6.52 1.16 0.75 3.46 0.89
33.61 66.39 87.55 5.06 1.45 0.63 5.30 0.69
24.71 75.29 87.80 4.27 1.16 0.79 5.98 0.58
36.7 32.9 30.4
31.9 37.5 30.6
1.4 10.0 88.6
75.4 16.9 7.7
5.4 92.0 2.6
Results and Discussion Liquefaction Data. The conversions obtained from the parent coal and three maceral concentrates are shown in Figures 1-3. A set of simultaneous equations for each reaction temperature studied was used to obtain a calculated conversion. Each set of equations included the weight percent of each maceral group in the maceral concentratesand their respective conversion data obtained at each reaction temperature. By using this method of calculation, conversion data representative of the pure maceral group is obtained. The conversion data were used with the weight percent of the maceral composition in the original coal to obtain the calculated values. The conversion data obtained using the 658 K reactor temperature show that each of the three maceral groups has a lower conversion than the parent coal. Although all the maceral conversions are low, the vitrinite group attained the highest conversion of the three maceral groups. The inertinite group (mostly semifusinite) and the liptinites exhibited similar conversions. The low conversion of the liptinites at this temperature is similar to that observed for a bituminite maceral previously reported.l6 As can be seen in Figure 1,the calculated s u m of the individual macerals is substantially lower than that obtained from the parent coal. The primary liquefaction products of parent coal and maceral samples are the asphaltene plus preasphaltene (A + P)intermediates. The yields of these intermediates vary linearly with conversion. The lower conversions of the maceral concentrates are primarily due to lower yields of these intermediates during liquefaction,relative to the yields obtained from the parent coal at this temperature (Table 11). At this low conversion level, the three maceral groups produced approximately equal oil plus gas (0 + G ) yields. The calculated 0 + G yield is lower than that obtained from the parent coal; however, this difference does not account for a significant portion of the difference in the total conversion data. Similar trends are observed in the 700 K (Figure 2) and 718 K (Figure 3) conversion data and solubility class data (Table11). The difference between the data obtained from the parent coal and the calculated conversion from the individual maceral data decrease with increasing reaction temperature. From the solubility class and conversion data, it appears that one or all of the separated maceral groups are lesa reactive than they are in the original coal matrix. ~
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(15) Keogh, R. A.; Poe, S. H.; Chawla, B.; Davis,B. H. In Coal Science
and Technology II; Moulin, J. A., Nater, K. A., Chermin, H. A. G., Eds.; Elsevier: Amsterdam, 1987.
Table 11. Solubility Class Distributions oils + gases asphaltenes preasphaltenes (wt%,dan ( w t % , d a (wt%,daD 658 K,15 min parent coal 8.7 11.9 14.0 liptinites 5.0 5.8 3.0 vitrinites 5.0 8.3 7.3 inertinites 5.9 2.8 3.0 parent coal, calcd 5.3 6.2 5.0 ~~~
~
700 K,15 min parent coal liptinitea vitrinites inertinitea parent coal, calcd
16.5 16.8 14.2 11.4 14.0
31.8 37.2 22.3 13.9 23.7
8.5 11.6 10.1 3.3 8.6
718 K,15 min parent coal liptinitea vitrinitea inertinitea parent coal, calcd
30.4 33.7 23.7 16.9 24.3
23.9 40.9 22.8 14.5 25.1
14.6 9.8 20.7 7.8 14.4
A number of explanations may account for the lower reactivity of the separated maceral concentrates. One explanation of the lower reactivity observed is that each of the maceral concentrate produced in the DGS separation is not representative of the maceral group in the original coal. Although not all of the starting material used in the separations was combined to obtain the final maceral concentrates, it was assumed that the concentrates were representative of the macerals in the original coal for a number of reasons. The liptinite group in the parent coal was composed almost entirely of sporinite, which likewise 40
"
Parent Con1
Liptinitel
vitrinites
lnertinites
Calculated
blue
Figure 1. Conversionsobtained by the parent coal and the maceral concentrates using a reaction temperature of 658 K.
616 Energy &Fuels, Vol. 6, No.5, 1992
"
Parent Coal
Liptintitcs
Vitrinites
Inertiaites
Keogh et al.
Calculated Value
Figure 2. Conversionsobtained by the parent coal and the maceral concentrates using a reaction temperature of 700 K.
Parent
Liptinites
Vitrinitea
Inertinites
Coal
Calculated Value
Figure 3. Conversionsobtained by the parent coal and the maceral concentrates using a reaction temperature of 718 K. Table 111. Conversions of the Parent and Demineralized Parent Coal conversion ( w t % , daf) parent deminerald parent coal 658 K 34.6 35.3 700 K 56.9 59.5 718 K 68.9 71.4
was true for the liptinite concentrate. Furthermore, the liptinite concentrate was recovered between the density of 1.00 and 1.25g/mL whichcoversthe densityrange within which free or unassociated liptinites partion at this rank. The vitrinite concentrate was recovered in the density
range of 1.28-1.33g/mL which is centered in the density range in which pure vitrinite particles are recovered. Previous experience has shown that vitrinites which fall outside this range are normally present as mixed particles or are particles with such a small diameter than they have insufficient time to reach the appropriate density. The ratio of fusinite:semifusinite:micrinite:macrinitin the parent coal is approximately 17:64:15:4 compared to 23: 67:2:0 in the inertinite concentrates. With the exception of micrinite, these ratios are reasonably good. The fact that micrinite accounts for only 4-5 vol % (dmmf)of the parent coal plus the fact that micrinite is present at higher levels in both the vitrinite and liptinite concentrate suggests that it is unlikely that micrinite is a factor in explaining the differences in the observed conversion differences. Thus, every effort was made to combine the DGC fractions to produce a final maceral concentrate that included the entire density range for each maceral group of the parent coal and was representative of the macerals in the original coal. Another explanation is that the removal of the majority of the ash prior to DGC separation was responsiblefor the observed conversions. To investigate this possibility, the demineralized parent coal was liquefied using the same conditions and the products analyzed. The data in Table I11indicate that the parent and demineralizedparent coal have essentially the same conversion for each of the reaction conditions. Therefore, the removal of the ash does not appear to explain the differences in the liquefaction response between isolated individual maceral groups and the parent coal. Another explanation is that the organicmatrix has been altered during the DGC separation. The question of organic matrix alteration in these macerals has been addressed by Taulbee et in a study of these samples using FTIR and pyrolysis. They concludedthat very little, if any, alteration of the organic matrix was apparent due to the demineralization or DGC separation procedures. In addition, the similarity of the liquefaction conversions of the parent coal and demineralized parent coal suggests that there was not significant alteration of the organic matrix during the demineralizationprocedure. Therefore, this explanation also does not account for the observed differences in the calculated and parent coal conversions. From the elimination of the previously discussed possibilities, it appears that there may be a synergistic effect during the liquefaction of the original coal matrix relative to the maceral isolates that is not related to the ash of the parent coal. A series of petrographic analyses
Table IV. Petrographic Analysis of the Liquefaction Residues (vol %, dmmf) altered isotropic anisotropic liptoplast coke coke inertinite inertinite liptinite vitoplast parent coal 658 K 700 K 718 K vitrinite maceral group 658 K 700 K 718 K liptinite maceral group 658 K 700 K 718 K inert maceral group 658 K 700 K 718 K
2 30 23
1
9 6 3 2
12 11 9
1 2
10 16
88 17
10
