5 Variations in Properties of Coal Macerals Elucidated by Density Gradient Separation GARY R. DYRKACZ, C. A. A. BLOOMQUIST, L. RUSCIC, and E. PHILIP HORWITZ
Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: May 8, 1984 | doi: 10.1021/bk-1984-0252.ch005
Chemistry Division, Argonne National Laboratory, Argonne, IL 60439
Based on a r e c e n t l y developed separation procedure f o r c o a l macerals, c l o s e l y spaced narrow d e n s i t y fractions of macerals have been examined f o r petrographic content and ultimate a n a l y s i s . Each maceral group or maceral type c l e a r l y e x h i b i t s a range of d e n s i t i e s (a density band) and c o r r e s ponding range of atomic r a t i o s . In g e n e r a l , the H/C and S/C r a t i o s decrease monotonically with d e n s i t y , r e f l e c t i n g changes i n the macerals since as d e n s i t y increases the macerals separate i n the order: exinite, vitrinite, inertinite. The overall range of H/C values f o r t y p i c a l high v o l a t i l e bituminous coals can vary from as much as 1.2 to 0.4 w i t h i n the same coal if all three maceral groups are represented. Within each maceral group, less variation i s observed. In most bituminous coals, there i s 15 to 20% t o t a l variation i n H/C r a t i o across the i n d i v i d u a l maceral bands. The O/C and N/C r a t i o s show d i f f e r e n t behavior. R e l a t i v e to the vitrinite O/C and N/C r a t i o s the e x i n i t e r a t i o s are very low, but r i s e s t e a d i l y as density i n c r e a s e s . Within the vitrinite band there i s some additional increase before finally l e v e l i n g o f f . The inertinite r a t i o s have intermediate values between those of vitrinite and e x i n i t e and vary only s l i g h t l y with d e n s i t y .
Our recent development of a new procedure f o r the d e n s i t y separation of macérais o f f e r s a method f o r obtaining high r e s o l u t i o n separation of the three maceral groups: e x i n i t e , v i t r i n i t e , and i n e r t i n i t e , and can f u r t h e r resolve i n d i v i d u a l maceral types w i t h i n these macérais groups, e.g., s p o r i n i t e from a l g i n i t e i n the e x i n i t e group (1,2). The procedure
0097-6156/ 84/ 0252-0065506.00/ 0 © 1984 American Chemical Society
In Chemistry and Characterization of Coal Macerals; Winans, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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COAL MACERALS
utilizes well-known techniques of density gradient c e n t r i f u g a t i o n (DGC) to separate the c o a l i n t o i t s components i n an aqueous CsCl continuous density gradient. Density gradient c e n t r i f u g a t i o n has been used f o r many years i n the biosciences. We have taken that knowledge and technology and a p p l i e d i t to c o a l . The technique of d e n s i t y gradient c e n t r i f u g a t i o n r e s u l t s i n a s e r i e s of bands representing the d e n s i t y v a r i a t i o n s i n maceral groups and maceral types. The informat i o n obtained i s s i m i l a r to a chromatographic t r a c e . At the simplest level this density p r o f i l e represents a density " f i n g e r p r i n t " of a c o a l . When coupled with micropetrographic analyses, the d e n s i t y d i s t r i b u t i o n of each maceral group can be obtained. The observed changes i n d e n s i t y are r e l a t e d i n a complex way to the chemical s t r u c t u r e of the c o a l or maceral ( 3 ) . In order to b e t t e r understand the chemical v a r i a t i o n s o c c u r r i n g i n c o a l macérais we have examined the ultimate a n a l y s i s of s e l e c t e d d e n s i t y f r a c t i o n s separated from the same c o a l . EXPERIMENTAL Separation The basic advantages (1,2).
procedures f o r DGC separation of macérais with and disadvantages has been discussed p r e v i o u s l y
Chemical A n a l y s i s Because of the small samples often generated, ultimate a n a l y s i s of s e l e c t e d d e n s i t y f r a c t i o n s was performed with a modified Perkin-Elmer 240, C, H, N, analyzer ( m o d i f i c a t i o n XA by Control Equipment Corp.) using a modified burn procedure which includes i n c r e a s i n g the time of burn and the amount of 02 used i n the burn. Good c o r r e l a t i o n f o r carbon and hydrogen ( r = 0.98) with ASTM methods was found f o r a range of coals from sub-bituminous to a n t h r a c i t e s . 2
Maceral A n a l y s i s The microscopic analyses of the coals follows standard practices. D e t a i l s of the a n a l y s i s of the f i n e maceral p a r t i c l e s has been documented (1). We use the term maceral type and maceral group i n a s p e c i f i c sense i n t h i s a r t i c l e . Maceral type r e f e r s to the i n d i v i d u a l maceral species which comprise the three maceral groups: e x i n i t e , v i t r i n i t e , and inertinite. Separation Scheme Since very d e t a i l e d explanations f o r most of the procedures have been published, we w i l l only provide a b r i e f summary of the important aspects of the separation process (1,2). The methodology we have developed f o r the separation of the
In Chemistry and Characterization of Coal Macerals; Winans, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: May 8, 1984 | doi: 10.1021/bk-1984-0252.ch005
5.
D Y R K A C Z ET A L .
Density Gradient
Separation
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macérais i s shown i n Figure 1. One of the most important aspects i s the liberation of one maceral from another. Inadequate comminution w i l l severely r e s t r i c t the density r e s o l u t i o n of the various macérais. However, there are p r a c t i c a l l i m i t a t i o n s on the s e v e r i t y of comminution that can be t o l e r a t e d . The most s i g n i f i c a n t l i m i t for general maceral separation and c h a r a c t e r i z a t i o n i s the a b i l i t y to m i c r o s c o p i c a l l y resolve the p a r t i c l e s i n t o the three maceral groups. This places a lower l i m i t on p a r t i c l e s i z e of two to three microns. In our case, g r i n d i n g i s c a r r i e d out under n i t r o g e n i n two stages: F i r s t , to
