Energy & Fuels 1989,3, 662-670
662
Articles Density Gradient Centrifugation Separation and Characterization of Maceral Groups from a Mixed Maceral Bituminous Coal Darrell Taulbee,* Steven H. Poe, Tom Robl, and Bob Keogh Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, Kentucky 40511 Received March 14, 1989. Revised Manuscript Received August 23, 1989
A -100-mesh demineralized sample of high-volatile A bituminous coal from eastern Kentucky has been separated into concentrates of the three major maceral groups by density gradient centrifugation (DGC). Petrographic analyses of a preliminary separation were used to design a “large-scale”separation in which 270 g of sample were split into nine density fractions. Each of these nine fractions was reprocessed by DGC and recovered in nine additional density cuts for a totalof 81 samples. Proximate, ultimate, and petrographic analyses are presented for all samples where quantities permit. Selected density fractions were ultimately composited into three final concentrates comprised of liptinites, 88.6 vol %/21.0 g; vitrinites, 75.4 vol %/37.2 g; and inertinites, 92.0 vol %/31.8 g, respectively. The liptinitic fraction was roughly recovered in the 1.0-1.25 g/mL portion of the density gradient, the vitrinites from 1.28-1.33 g/mL, and the inertinites from 1.34-1.45 g/mL. A major contaminant of the vitrinite concentrate was micrinite which exhibited a significantly lesser density than the other inertinites present in this coal. The average mineral-matter-free densities calculated on a “pure” maceral group basis were 1.20, 1.275, and 1.36 g/mL for the liptinites, vitrinites, and inertinites, respectively. Fourier transform IR and pyrolysis data show no evidence of alteration of the organic matrix during demineralization or residual surfactant from DGC processing. Selected samples were subjected to liquefaction and pyrolysis studies that will be the focus of subsequent papers. Introduction Gaining a basic understanding of coal chemistry and the reactions that transpire during pyrolysis or liquefaction is a difficult task due to the highly complex, heterogeneous nature of coal. One must contend with not only an inorganic matrix that is often comprised of numerous mineral components but also a mixed organic fraction that can usually be divided into several microscopically identifiable maceral types as well. Thus, attempts to explicitly define fundamental chemical or reaction parameters are frequently ineffective due to both the complexity and the potential for interaction between these components. Accordingly, the approach taken in this work was to simplify the problem by chemical removal of the inorganic matrix followed by isolation of relatively homogeneous maceral groups. Further, it is important that this approach not induce significant organic alteration and yield concentrates representative of components found in the starting material. The isolation of maceral types in coal has until recently been only partially successful. Macerals such as vitrinite which often occur in bands large enough to permit handpicking, have been physically and chemically described.l However, isolation of other maceral types that may be small and/or dispersed throughout the coal matrix are not always amenable to this approach. Moreover, handpicking often yields small samples which are not (1) Coal Petrography, 3rd ed.; Stach, E., Ed.; Gebruder Borntraeger: Berlin, 1982.
necessarily representative of analogous maceral types within the coal seam. Density gradient centrifugation (DGC) as first described by Dyrkacz and co-worker~~-~ and recently reviewed by Crelling5provides a more practical approach to the separation of coal macerals. DGC methods have been applied to the study of both oil shale and coal at CAER since about 1983. Our approach has been similar to those cited with certain modifications, the more significant being the use of larger starting particles and the omission of HCl during demineralization.68 The advantages of these changes are suppressed organic alteration, more rapid processing of the density fractions, and more reliable petrographic analyses, particularly with respect to differentiation of maceral types. One disadvantage resulting from these changes is a slight increase in ash content due to additional encapsulation of minerals in the larger feed particles. The most serious effect, however, is the increase in the number of mixed maceral particles and the resultant decrease in maceral purity. This has resulted in a good deal of additional work in this study due to the need to reprocess (2) Dyrkacz, G. R.; Horwitz, E. P. Fuel 1982, 61, 3-12. (3) Dyrkacz, G. R.; Bloomquist, C. A. A.; Horwitz, E. P. Sep. Sci. Technol. 1981, 16, 1571-1588. (4) Dyrkacz, G. R. Fuel 1984,63, 1367-1373. (5) Crelling,J. C. Prepr. Pap. Am. Chem. SOC.,Diu. Fuel Chem. 1989, 34, 249-255. (6) Poe, S.; Taulbee, D. N.; Keogh, R. Org. Geochem., in press. (7) Robl, T. L.; Taulbee, D. N.; Barron, L. S. Energy Fuels 1987, 1 , 507-513. (8)Taulbee, D. N.; Seibert, E. D. Energy Fuels 1987, 1, 514-519.
0 1989 American Chemical Society 088~-0624/~9/2503-0662~0~.50/0
DGC Separation and Characterization of Maceral Groups Table I. Analytical Data for Coal 3761 (As Determined) %C 71.0 % moisture 0.5 % volatile matter 31.9 4.60 %H % fixed carbon 50.5 1.19 % N 5.73 % high-temp ash 17.1 %O % st 0.99 High Temperature Ash Analyses, wt 70 1.26 A1203 26.20 54.37 Ti02 1.30 3.10 0.12 KZO 0.20 11.76 so3 0.60
many of the samples to enhance purity. Nonetheless, the authors feel this approach is warranted and the disadvantages are offset by the enhanced reliability of the petrographic analyses. I t has been our experience that the scatter associated with the petrographic analyses of particles smaller than about 5 pm is considerably larger than that associated with the chemical analyses. This suggests that the error that stems from the subjective element of petrographic analyses of near micrometer size particles is large and in need of further evaluation. The overall goal of this project is to develop and implement techniques for separating and concentrating relatively unaltered macerals in sufficient quantity for subsequent chemical, pyrolytic, and liquefaction studies. Details of the separation with emphasis on the bulk chemical analyses are given here. Later reports will present pyrolysis and liquefaction data and liquefaction kinetics from these same or similar concentrates.
Experimental Section Coal. The sample used in this study was a high-volatile A bituminous coal from the Lower Elkhorn Seam (Pond Creek) of Pike County, Kentucky (KCERL-3761). The selection and sampling criteria focused on procurement of a sample containing near equal amounts of the three major maceral groups. A narrow bench was hand sampled at a working mine face and stored under Ar for transport. The entire sample was composited then sequentially passed through a jaw crusher, a roller mill, and screened to -100 mesh. A 3-kg split of the -100-mesh coal was subsampled for chemical analyses (Table I) and then stored in a sealed container under Ar until it was demineralized and under vacuum or N2 thereafter. Demineralization. Demineralization was based on routinely used HF acid digestion techniques with certain modifications's8 devised to minimize chemical alteration of the organic fraction. HC1 is commonly used during demineralization to remove carbonates and suppress mineral fluoride formation. However, since this sample contained
