9 Reactivity and Characterization of Coal Macerals RANDALL E. WINANS, RYOICHI HAYATSU, ROBERT G. SCOTT, and ROBERT L. McBETH Chemistry Division, Argonne National Laboratory, Argonne, IL 60439
In a study of the organic s t r u c t u r e s in c o a l s and t h e i r reactivity in chemical and thermal processes, it is d e s i r a b l e to reduce the complexity of the material by some sort of physical separation. One such approach i s the separation of the c o a l s i n t o their maceral groups, which are the m i c r o s c o p i c a l l y identifiable organic p o r t i o n s of coal which may have d i f f e r e n t origins, chemical and physical f e a t u r e s , and reactivity. Two bituminous coals have been separated i n t o t h e i r three main maceral groups: exinite, vitrinite, and inertinite, using a modified float-sink technique which uses a n a l y t i c a l d e n s i t y gradient centrifugation (DGC) to determine the appropriate density ranges. The DGC t e c h n i q u e , which a l s o e x p l o i t s the d i f f e r e n c e s in densities, has j u s t r e c e n t l y been reported ( 1 , 2 ) . A l s o , several maceral samples were prepared by hand p i c k i n g , i n c l u d i n g several vitrinites, a fusinite, and a resinite. An alginite sample was obtained from the DGC separation (1). The maceral p u r i t y of these samples was determined from petrographic examination. The techniques used for c h a r a c t e r i z a t i o n and for studying r e a c t i v i t y have been developed using whole c o a l s . The chemistry of m a c é r a i s separated by DGC, f l o a t - s i n k and hand p i c k i n g has been i n v e s t i g a t e d r e c e n t l y by several t e c h niques i n c l u d i n g : solid C nmr ( 3 , 4 ) , o x i d a t i o n ( 5 , 6 ) , and mass spectrometry (5-12). E a r l y work focused on chemical p r o p e r t i e s (13,14) and esr spectroscopy ( 1 5 ) . The approach used in t h i s study combines pyrolysis-mass spectrometry (Py-MS) f o r c h a r a c t e r i z a t i o n , batch vacuum p y r o l y s i s f o r s t r u c t u r a l and thermal r e a c t i v i t y i n f o r m a t i o n , and a two step degradation to examine chemically r e a c t i v e s i t e s . I 3
The Py-MS technique has been used e x t e n s i v e l y to c h a r a c t e r i z e s y n t h e t i c polymers, biopolymers, and f o s s i l f u e l s ( 5 - 8 , 1 0 , 11 ) . In t h i s work the technique has been modified by using a mass spectrometer which provides p r e c i s e mass measurements d i r e c t l y upon p y r o l y s i s . The advantage of t h i s approach i s two 0097-6156/ 84/ 0252-0137$06.00/ 0 © 1984 American Chemical Society
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fold. F i r s t , i t enables the d e t e c t i o n and i d e n t i f i c a t i o n of p y r o l y s i s products having the same nominal mass but d i f f e r e n t molecular formulae and hence d i f f e r e n t p r e c i s e masses. The results presented here demonstrate that this can be a s i g n i f i c a n t advantage. Second, the technique provides a wealth of d e t a i l e d information which can be conveniently sorted f o r comparisons between d i f f e r e n t samples. High r e s o l u t i o n MS has been shown to be useful i n the a n a l y s i s of v o l a t i l e compounds i n petroleum and coal l i q u i d s ( 1 6 - 2 0 ) . Aczel and co-workers (1820) demonstrated that i t can be a very useful q u a n t i t a t i v e tool. The emphasis in t h i s study has been to compare the v a r i a t i o n s in the d i s t r i b u t i o n of hydrogen d e f i c i e n c y (Z number) and heteroatoms between the d i f f e r e n t coal m a c é r a i s in high v o l a t i l e bituminous c o a l s . In the batch vacuum p y r o l y s i s reaction the maceral concentrates are pyrolyzed i n a vacuum, the products c o l l e c t e d and c h a r a c t e r i z e d by gas chromatography mass s p e c t r o metry and by GC microwave plasma emission spectroscopy (GCMPES). The vacuum technique has been chosen over the t y p i c a l o n - l i n e pyrolysis-GCMS method f o r two reasons. F i r s t , experiments in t h i s laboratory have shown that with the vacuum t e c h n i q u e , secondary reactions such as the aromatization of a l i c y c l i c s are l e s s l i k e l y to o c c u r . Second, b e t t e r p y r o l y s i s y i e l d data can be obtained with a batch type r e a c t i o n scheme. In a d d i t i o n to c h a r a c t e r i z a t i o n of thermal products, the chemical r e a c t i v i t y of these concentrates has been s t u d i e d . Reactive b e n z y l i c s i t e s have been determined from the r e a c t i o n of the m a c é r a i s with p y r i d i n e and iodine to form pyridinium iodides: Coal-ArCHo-R
pyridine I
•Coal-ArCH -R + HI 'Py r
(1)
+
2
It has been shown that the number of pyridinium iodides per 100 carbon atoms in the o r i g i n a l coal decreases with i n c r e a s i n g rank ( 2 1 ) . Further studies have shown that these d e r i v a t i z e d coals are a c t i v a t e d toward o x i d a t i v e s o l u b i l i z a t i o n using a reagent, alkaline silver oxide, which normally is quite i n e f f e c t i v e i n d i s s o l v i n g raw c o a l s . In the r e s u l t s from the thermal and chemical r e a c t i o n s , s i m i l a r i t i e s and d i f f e r e n c e s have been noted which w i l l be discussed l a t e r . A l s o , i t should be emphasized that in t h i s study we are examining maceral concentrates from the three main groups which are derived from the same c o a l . These coals have been chosen to be r e p r e s e n t a t i v e of bituminous c o a l s and not include s a p r o p e l i c coals where the chemistry may be more unusual due to the t y p i c a l l y large e x i n i t e content.
9.
WINANS ET A L .
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Characterization
Experimental Samples. Two of the coals used in t h i s study were obtained from the Penn State Coal Sample Bank, an HVA bituminous coal (PSOC 1103) from the Upper Elkhorn #3 seam i n Eastern Kentucky and an HVA bituminous coal (PSOC 828) from the B r a z i l Block seam i n Indiana. A t h i r d coal from which v i t r i n i t e and f u s i n i t e were hand picked was an I l l i n o i s No. 2 seam HVC bituminous coal from Northern I l l i n o i s . A l s o , r e s i n i t e and v i t r i n i t e samples were hand picked from a Hiawatha seam bituminous coal from the King 6 mine i n Utah. F i n a l l y , the a l g i n i t e sample was obtained from an Ohio No. 5 seam (PSOC-297) coal by DGC ( 2 , 5 ) . A l l of the elemental and p é t r o g r a p h i e analyses are presented in Table I. The d e t a i l s of the s i n k - f l o a t technique have been reported p r e v i o u s l y ( 1_). T y p i c a l l y , a 3 micron p a r t i c l e s i z e demineralized coal Ts c e n t r i f u g e d i n aqueous C s C l s o l u t i o n of the appropriate d e n s i t y with a small amount of s u r f a c t a n t added to d i s p e r s e the coal p a r t i c l e s . The e x i n i t e s in the f l o a t are c o l l e c t e d and the sink f r a c t i o n i s f u r t h e r separated i n t o v i t r i n i t e and i n e r t i n i t e by repeating the procedure at a higher density. The process y i e l d e d gram q u a n t i t i e s of coal concentrates. The density c u t o f f points were determined from a n a l y t i cal DGC of coals of s i m i l a r o r i g i n and rank. The technique used f o r p é t r o g r a p h i e a n a l y s i s has a l s o been reported e a r l i e r (2). 2
Pyrolysis. In the p y r o l y s i s experiment, t y p i c a l l y 30 mg of sample was heated in a quartz tube at 400°C f o r 24 hrs at 2 x l 0 " torr. Tars were trapped at room temperature and the more v o l a t i l e products at l i q u i d nitrogen temperature. These two f r a c t i o n s were analyzed by GCMPES (MPD-850) using a 25m χ 0.25 mm i . d . 0V-101 fused s i l i c a column and by GCMS (Kratos MS-25) using a 30m χ 0.25 mm i . d . DB-5 column. b
The Py-MS data was obtained on the MS-25 spectrometer using a d i r e c t heating probe designed in t h i s l a b o r a t o r y . The maceral samples were deposited as a s l u r r y onto a f i n e platinum g r i d which i s heated with a computer c o n t r o l l e d D.C. c u r r e n t . The l i n e a r heating rates were varied between 2 5 - 1 0 0 ° C / m i n . The spectrometer was operated i n the p r e c i s e mass measurement mode using perfluorokerosene as an i n t e r n a l s t a n d a r d . Preliminary data a n a l y s i s and averaging was performed on the Kratos DS-55 data system. These data then were t r a n s f e r r e d to a VAX 11/780 f o r f i n a l s o r t i n g and p l o t t i n g . Chemical R e a c t i o n s . The pyridinium i o d i d e d e r i v a t i z e d samples were prepared by r e f l u x i n g 1 g of coal or maceral concentrate i n 60 ml of p y r i d i n e with 4 g of i o d i n e f o r 70 hrs ( 2 1 ) . The r e a c t i o n mixture was poured i n t o 10% aqueous NaHSOo and the solution f i l t e r e d . The d e r i v a t i z e d coal was washed f r e e of p y r i d i n e , d r i e d , and a n a l y z e d . P o r t i o n s of the a c t i v a t e d coal
140
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