Coal Science II - ACS Publications - American Chemical Society

Chapter 7

Two-Component Concept of Coal Structure 1

2

3

Frank J . Derbyshire , Alan Davis , and Rui Lin 1

Center for Applied Energy Research, 3572 University of Kentucky, Lexington, KY 40511-8433 Energy and Fuels Research Center, The Pennsylvania State University, University-Park, PA 16802 Unocal Corporation, P.O. Box 76, Brea, CA 92621

Downloaded by UNIV OF PITTSBURGH on March 11, 2016 | http://pubs.acs.org Publication Date: May 6, 1991 | doi: 10.1021/bk-1991-0461.ch007

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The hypothesis t h a t c o a l s can be considered t o c o n s i s t of two component phases has i t s o r i g i n s i n observations of c o a l behaviour as w e l l as d e r i v i n g from the a n a l y s i s of c o a l s and attempts to define t h e i r s t r u c t u r e . The r e s u l t s of extensive s t u d i e s of u n t r e a t e d , preheated and hydrogenated c o a l s , using a n a l y t i c a l and microscopic techniques, have allowed some i n s i g h t i n t o the a s s o c i a t i o n between the so-called mobile phase and macromolecular network, and have provided information upon d i f f e r e n c e s i n t h e i r chemical properties. The yield of c h l o r o f o r m - s o l u b l e e x t r a c t has been used t o i n d i c a t e the extent of the mobile phase. In untreated c o a l s , only a p o r t i o n of the mobile phase i s readily removable. A f t e r m i l d preheating or m i l d hydrogenation, there are sharp increases i n the yield of e x t r a c t and accompanying changes i n p r o p e r t i e s . Under these c o n d i t i o n s , it i s b e l i e v e d t h a t weak bonding between the mobile phase and network is d i s r u p t e d . The solubilisation of the network r e q u i r e s much higher energy input and n e c e s s i t a t e s the consumption of hydrogen. The p r o p e r t i e s of the network components differ s i g n i f i c a n t l y from those of the mobile phase and both are c o a l rank-dependent.

0097-6156/91/0461-0072$06.00/0 © 1991 American Chemical Society

Schobert et al.; Coal Science II ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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The concept t h a t c o a l s can be u s e f u l l y considered t o c o n s i s t of two d i s t i n c t phases of c o n s t i t u e n t s i s not new and has been advanced i n d i f f e r e n t ways s i n c e the e a r l y p a r t of t h i s century. I t s r e v i v a l i n recent times i s l a r g e l y a t t r i b u t a b l e t o the e f f o r t s of the l a t e Peter Given, t o whom i s owed the now wide use of the terms "mobile phase" and "macromolecular network" (1-3). In broad terms, the mobile phase comprises the s m a l l e r molecules i n c o a l s , a p o r t i o n of which i s e x t r a c t a b l e i n solvents. The g r e a t e r p r o p o r t i o n of c o a l s i s s o l v e n t i n s o l u b l e and c o n s i s t s p r i m a r i l y of a three-dimensional c r o s s l i n k e d macromolecular network or matrix. The mobile phase i s attached t o or h e l d w i t h i n the network by p h y s i c a l c o n s t r a i n t s and weak bonds. In 1986, Peter Given organized a "debate i n p r i n t " by s e v e r a l c o n t r i b u t i n g authors on the nature and extent of the mobile phase as i n t e r p r e t e d from H NMR studies (2) . He had planned a sequel which was intended t o update the NMR work and t o introduce f i n d i n g s from other types of i n v e s t i g a t i o n . The second debate was completed a f t e r Peter Given's death and was published i n t r i b u t e t o h i s c o n t r i b u t i o n s t o c o a l science (3.) . Much of the research pursued by the authors of t h i s paper and by t h e i r a s s o c i a t e s has i n v o l v e d s t u d i e s of the c a t a l y t i c hydrogénation of c o a l s i n the absence of solvent. The technique has been used t o e l u c i d a t e the mechanisms of c a t a l y t i c c o a l l i q u e f a c t i o n and t o provide simultaneously some i n s i g h t i n t o the s t r u c t u r e of c o a l s . Peter Given was d i r e c t l y instrumental i n p r o v i d i n g the i n c e n t i v e f o r t h i s research which has extended s i n c e 1983. Previous f i n d i n g s were disseminated through s e v e r a l p u b l i c a t i o n s (4-8). In t h i s paper, some of the e a r l i e r data have been c o l l a t e d with more recent r e s u l t s (9) t o provide an account of the relevance of these s t u d i e s t o the two-component concept. 1

EXPERIMENTAL Coals covering a range of rank downwards from lowvolatile bituminous were examined in solvent-free c a t a l y t i c hydrogénation over the temperature range 300400°C and f o r r e a c t i o n times up t o 60 min. The work d i s c u s s e d here s p e c i f i c a l l y Involved four c o a l s which were obtained form the Penn S t a t e Coal Sample Bank. These were a subbituminous c o a l PSOC-1403, and three hvAb bituminous c o a l s , PSOC-1168, PSOC-1266 and PSOC-1510. C h a r a c t e r i s t i c s of the c o a l s are summarised i n Table I. The r e a c t o r s were of the tubing bomb type and the i n i t i a l (cold) hydrogen pressure was 7 MPa. A sulphided Mo c a t a l y s t was introduced t o the c o a l by impregnation from an aqueous s o l u t i o n of ammonium tetrathiomolybdate.



88 2 11

vitrinite exinite inert i n i t e

reported as ASTM ash by difference by low-temperature ashing

72.9 4.5 20.4 1.2 0.9

carbon hydrogen oxygen (b) nitrogen organic sulfur

(a) (b) (c)

-

91 3 6

Pétrographie Composition

83.2 5.0 8.6 2.1 0.5

(% dmmf) 77.4 4.9 15.2 1.7 0.8

-

7.2

1.4

Pennsylvania hvAb 0.89

-

L. Kittanning

PSOC-1510

88 4 7

83 10 7

(Mineral-Free, vol %)

71.8 4.9 18.8 1.6 2.9

Elemental Composition

14.5

-

6.1

11.9

-

2.4

L. Kittanning Lawrence Pennsylvania hvAb 0.83

PSOC-1168

3.4

L. Kittanning Mehining Ohio hvAb 0.83

PSOC-1266

23.3

Anderson Campbell Wyoming sub Β 0.40

PSOC-1403

Bank No. seam county state ASTM rank mean-maximum reflectance of vitrinite moisture (ar wt%) mineral matter (a) (dry, wt%) (c)

Penn State Sample

Table I. Coal Properties


H

5 3 W

ο

g >

2

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Two-Component Concept of Coal Structure

75

The gaseous products, c h l o r o f o r m - s o l u b l e e x t r a c t s and c h l o r o f o r m - i n s o l u b l e r e s i d u e s were c h a r a c t e r i z e d u s i n g a range o f a n a l y t i c a l techniques. A particular advantage o f conducting the hydrogénation r e a c t i o n s i n the absence o f s o l v e n t i s t h a t microscopy can be used t o observe d i r e c t l y the changes wrought i n the whole r e a c t e d c o a l s and i n the e x t r a c t e d r e s i d u e s . Considerable emphasis was placed on t h i s method o f a n a l y s i s u s i n g both v i s i b l e l i g h t microscopy and q u a n t i t a t i v e f l u o r e s c e n c e microphotometry. D e s c r i p t i o n s o f the procedures and techniques have been given e a r l i e r (4-9). Downloaded by UNIV OF PITTSBURGH on March 11, 2016 | http://pubs.acs.org Publication Date: May 6, 1991 | doi: 10.1021/bk-1991-0461.ch007

CATALYTIC HYDROGENATION D e t a i l e d examination o f the composition and d i s t r i b u t i o n of the r e a c t i o n products over a range o f r e a c t i o n c o n d i t i o n s showed t h a t the four c o a l s e x h i b i t e d c e r t a i n common trends. Of p a r t i c u l a r note, there was i n each case, a sharp d i s t i n c t i o n between the events which took p l a c e a t low and high conversions. The d i v i s i o n occurred at a chloroform-solubles y i e l d o f approximately 25% wt dmmf c o a l . Low e x t r a c t y i e l d s were obtained e i t h e r by s h o r t r e a c t i o n times a t high temperatures o r by more extended r e a c t i o n a t low temperatures. The d e s i g n a t i o n s o f high and low temperature depend upon the i n d i v i d u a l c o a l and, more e s p e c i a l l y upon the c o a l rank. The evidence accrued i n t h i s research has shown t h a t there e x i s t s a t h r e s h o l d temperature below which the p o t e n t i a l for liquids formation i s minimal and above which conversion can proceed a t an a p p r e c i a b l e r a t e . I t i s supposed t h a t the t h r e s h o l d temperature i s determined e s s e n t i a l l y by the d e n s i t y and nature o f the c r o s s l i n k s i n the macromolecular network. This i s c o n s i s t e n t with evidence showing t h a t t h e ease o f conversion and the s e l e c t i v i t y t o lower molecular weight l i q u i d s i n c r e a s e s with decreasing c o a l rank (10.11) . For the subbituminous c o a l , the r a t e o f l i q u i d s formation was n e g l i g i b l e a t 300°C and became a p p r e c i a b l e a t 350°C and higher. The bituminous c o a l s began t o respond only a t temperatures around 400°C. S i m i l a r observations have been made i n experiments which compared the s o l v e n t - f r e e hydrogénation o f a l i g n i t e and a bituminous c o a l (12). During the i n i t i a l stages o f conversion, as the l i q u i d y i e l d increased there were r a d i c a l changes i n the p r o p e r t i e s o f the chloroform-soluble e x t r a c t s . The atomic r a t i o , H/C a l i p h a t i c hydrogen content and o i l t o asphaltene (O/A) r a t i o each increased r a p i d l y a t f i r s t , passed through a maximum and decreased e q u a l l y p r e c i p i t a t e l y t o a lower value. I l l u s t r a t i o n s o f these changes are presented i n Figure 1 and Figure 2 f o r the 0/A r a t i o and H/C atomic r a t i o r e s p e c t i v e l y . The p r o g r e s s i o n o f


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F i g u r e 1. R e l a t i o n s h i p o f H consumption and 0/A r a t i o f o r hvAb c o a l , PSOC-1266 ( s o l v e n t - f r e e hydrogénation; 400°C; 1% wt s u l f i d e d Mo; 7MPa H c o l d ) . 2

2


7.

DERBYSHIRE E T A L


77

19 H


1.7 H

0.7 H 0

«

1 20

1

! 40

.

1 60

.

CHLOROFORM - SOLUBLES YIELD (% dmmf coal)

F i g u r e 2. V a r i a t i o n i n atomic H/C r a t i o versus c h l o r o f o r m s o l u b l e y i e l d s from subbituminous (PSOC-1403) and bituminous (PSOC-1266) c o a l s ; ( s o l v e n t - f r e e hydrogénation; 300-400°C; 5-180 min; 1% wt s u l f i d e d Mo; 7 MPa H c o l d ) (data from r e f s 7 and 8 ) . 2


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a l i p h a t i c hydrogen content with c h l o r o f o r m - s o l u b l e s y i e l d followed a s i m i l a r trend t o t h a t shown f o r the atomic r a t i o H/C. Each o f the four c o a l s t h a t were s t u d i e d e x h i b i t e d s i m i l a r behavior, and i n each case the t h r e e p r o p e r t i e s passed through e a r l y maximum v a l u e s . Independent o f the c o a l o r the measured property, the maxima a l l occurred between 5-10% wt e x t r a c t y i e l d and the subsequent decreases were a r r e s t e d between 20-35% e x t r a c t y i e l d . No d i s t i n c t i o n could be made on the b a s i s o f c o a l rank although, admittedly, the number o f c o a l s i s s m a l l . I t i s t o be noted t h a t the magnitude o f the i n crease i n each o f the measured p r o p e r t i e s , between the i n i t i a l and maximum values, was q u i t e d i f f e r e n t f o r t h e subbituminous c o a l and the t h r e e bituminous c o a l s . Examples o f the increases from the i n i t i a l v a l u e (determined f o r the chloroform-soluble e x t r a c t from the parent coal) t o the maximum are: the O/A r a t i o increased from 0.8 i n i t i a l v a l u e t o 8.0 maximum v a l u e f o r the subbituminous c o a l , PSOC-1403, and from 0.25 t o 1.5 f o r the bituminous c o a l , PSOC-1266; the H/C atomic r a t i o i n creased from 1.46 t o 1.60 f o r the subbituminous c o a l , PSOC-1403, and from 1.01 t o 1.06 f o r the bituminous c o a l , PSOC-1266 (see Figure 2 ) . For each c o a l , a t the maximum i n hydrogen content, or H/C atomic r a t i o , the a l i p h a t i c hydrogen content (determined by H NMR a n a l y s i s ) accounted f o r over 90% o f the t o t a l hydrogen. The a l i p h a t i c hydrogen contents were 10.5% f o r the subbituminous coal,PSOC-1403, and 6.9% f o r the bituminous c o a l , PSOC-1266. The h i g h a l i p h a t i c hydrogen content was a s s o c i a t e d with the presence o f polymethylene chains. The e a r l y r e l e a s e o f p a r a f f i n i e m a t e r i a l , as n-alkanes and as long chain s u b s t i t u e n t s t o aromatic s t r u c t u r e s , under c o n d i t i o n s o f m i l d p y r o l y s i s has been observed i n other research (13-15). Over the i n i t i a l p e r i o d o f conversion, i t was a l s o observed t h a t only small q u a n t i t i e s o f l i g h t hydrocarbon gases were produced. The l i b e r a t i o n o f carbon oxides, p r i n c i p a l l y C0 , was more f a c i l e . For both subbituminous and bituminous c o a l s , the y i e l d s o f C0 r e a l i s e d a t 300°C were s i g n i f i c a n t , although much h i g h e r f o r the lower rank c o a l (8.9). The p r o d u c t i o n o f e x t r a c t y i e l d s i n excess o f 2035% wt marked the second stage o f c o n v e r s i o n . Over t h i s p e r i o d , t h e H/C atomic r a t i o and the t o t a l a l i p h a t i c hydrogen content remained approximately constant o r decreased only g r a d u a l l y while the 0/A r a t i o passed through a shallow minimum, Figure 1. The secondary g a i n i n s e l e c t i v i t y t o o i l s a t high conversions may represent the i n t e r c o n v e r s i o n o f asphaltenes t o o i l s a f t e r extended r e a t i o n o f the primary d i s s o l u t i o n products. Other phenomena a s s o c i a t e d with the progress o f the second 1

2

2


7. DERBYSHIRE ET AL.


stage were t h a t the production of l i g h t hydrocarbon gases increased, the a r o m a t i c i t y o f the e x t r a c t p r o g r e s s i v e l y increased and oxygen f u n c t i o n a l i t i e s were e l i m i n a t e d . Analyses o f the chloroform-soluble e x t r a c t s o f t h e subbituminous c o a l by F o u r i e r transform i n f r a r e d spectroscopy (FTIR) showed the presence o f a sharp carbonyl absorption peak (1800-1650 cm" ) i n the e x t r a c t s from the parent c o a l and i n those obtained a t y i e l d s l e s s than about 10% wt dmmf. The peak, which i s a t t r i b u t e d t o ketones and carboxylates, disappeared a t higher convers i o n s (16). Whitehurst and co-workers (17) e s t a b l i s h e d t h a t c a r b o n y l - c o n t a i n i n g compounds, such as e s t e r s and carboxylates, can cleave under thermal treatment t o produce CO, C0 and phenols. They concluded t h a t t h e e v o l u t i o n o f these gases during c o a l l i q u e f a c t i o n could o r i g i n a t e from the decomposition o f s i m i l a r oxygen f u n c t i o n a l i t i e s i n the c o a l . Analyses o f the a c e t y l a t e d chloroform-insoluble r e s i d u e s (the a c e t y l a t i o n r e a c t i o n converts a l i p h a t i c and aromatic -OH groups t o e s t e r s , a l l o w i n g them t o be c l e a r l y d i s t i n g u i s h e d from water i n t h e FTIR spectra) showed a s i m i l a r r a p i d disappearance of the d e r i v a t i z e d OH groups above about 20% chloroform-solubles y i e l d (16) . The consumption o f gaseous hydrogen mirrored t h e changes d e s c r i b e d above, Figure 1. F o r t h e bituminous c o a l s a t 400°C, the i n i t i a l r a t e o f hydrogen uptake was very low. With f u r t h e r r e a c t i o n , i t a c c e l e r a t e d , passed through a maximum, and then f e l l t o a lower, more steady value. A f t e r about 60 minutes r e a c t i o n , t h e t o t a l consumption was about 1.5% wt dmmf c o a l (about 2.0% wt f o r the subbituminous c o a l ) . A t the p o i n t o f t r a n s i t i o n from the slow i n i t i a l r a t e t o the a c c e l e r a t i n g r a t e , t h e l i q u i d y i e l d s were between 5-15% wt dmmf c o a l . The a c c e l e r a t i o n i n hydrogen uptake c o i n c i d e d with the end o f the f i r s t phase o f conversion. At t h i s juncture, the t o t a l hydrogen consumed was 0.1-0.2% wt. The corresponding f i g u r e f o r the subbituminous c o a l was about 0.3% wt. The existence o f two stages o f conversion has been s e p a r a t e l y demonstrated i n s t u d i e s by Mobil R & D Corporation where, i n n o n - c a t a l y t i c s o l v e n t l i q u e f a c t i o n , two k i n e t i c regimes were d i s t i n g u i s h e d . I n i t i a l convers i o n was r a p i d with low attendant hydrogen consumption while subsequent conversion was slower and c r e a t e d a much g r e a t e r demand f o r hydrogen (18). These f i n d i n g s were re-confirmed i n l a t e r s t u d i e s by the same group, when i t was f u r t h e r shown t h a t the products f i r s t formed were o f higher H content and more a l i p h a t i c than those generated i n the second regime. These observations are i l l u s t r a t e d i n Figure 3 which shows the change i n hydrogen type distribution i n t h e heptane i n s o l u b l e l i q u e f a c t i o n products o f a subbituminous c o a l as a f u n c t i o n o f a r e a c t i o n s e v e r i t y parameter (19). The l a t t e r i s a 1


79

2


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F i g u r e 3. Change i n H , H and 0 H reaction severity ( r e f . 19: subbituminous c o a l ; ) h e p t a n e - i n s o l u b l e product fraction. W 1 t h

AL

A R

O


7.

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81

composite time-temperature f u n c t i o n which was employed t o smoothe the e f f e c t o f temperature f l u c t u a t i o n s i n s h o r t r e a c t i o n time experiments: maximum d u r a t i o n 6 min. a t temperatures up t o 460°C. The same research a l s o showed t h a t C0 was q u i c k l y evolved during l i q u e f a c t i o n and t h a t the subbituminous c o a l consumed more hydrogen than a bituminous c o a l a t the same l e v e l of conversion. The foregoing observations have d i r e c t i m p l i c a t i o n s to c o a l s t r u c t u r e . In the present work, the production of l i q u i d s i s f a c i l e below about 15% l i q u i d s y i e l d and r e q u i r e s l i t t l e hydrogen consumption. The processes most probably i n v o l v e the r e l e a s e o f s p e c i e s which a r e p h y s i c a l l y trapped o r are weakly bonded t o the i n s o l u b l e matrix. A t high conversions, the products a r e d e r i v e d from the breakdown o f the macromolecular network. T h i s phase o f conversion r e q u i r e s the cleavage and s t a b i l i s a t ion o f strong bonds, thereby c r e a t i n g an a p p r e c i a b l e demand f o r hydrogen. The products formed during the two stages a r e q u i t e d i f f e r e n t i n composition which can account f o r the r a t h e r dramatic swings observed i n the e x t r a c t p r o p e r t i e s with i n c r e a s i n g conversion. The m a t e r i a l s f i r s t l i b e r a t e d a r e hydrogen-rich and a l i p h a t i c which causes the i n i t i a l i n c r e a s e i n 0/A r a t i o , H/C atomic r a t i o and a l i p h a t i c hydrogen content. Later, as these products a r e progress i v e l y d i l u t e d by the a d d i t i o n o f more aromatic s p e c i e s emanating from the network, the values o f these same measured p r o p e r t i e s pass through maximum values and subsequently decrease.


2

FLUORESCENCE MICROSCOPY The phenomenon o f v i t r i n i t e f l u o r e s c e n c e has been d e s c r i b e d f o r 76 humic and v i t r i n i t i e m a t e r i a l s with l e v e l s o f organic maturation ranging from peat t o low v o l a t i l e bituminous c o a l (5,6). Figure 4a shows t h a t f l u o r e s c e n c e i n t e n s i t y decreases s h a r p l y through the s e r i e s peat - p e a t / l i g n i t e t r a n s i t i o n a l m a t e r i a l s l i g n i t e - subbituminous c o a l . The s o - c a l l e d primary f l u o r e s c e n c e w i t h i n t h i s range i s b e l i e v e d t o be due t o the presence o f fluorophors d e r i v e d from biopolymers, especially lignin. A second peak i n f l u o r e s c e n c e i n t e n s i t y ( F i g . 4b) commences a t about subbituminous rank, i s reached i n c o a l s with a v i t r i n i t e maximum r e f l e c t a n c e o f about 0.9%, and i s l a r g e l y l o s t by medium v o l a t i l e bituminous rank. T h i s "secondary f l u o r e s c e n c e " i s a t t r i b u t e d mainly t o the presence o f t h e mobile phase, a c o n c l u s i o n which i s supported by a notable p a r a l l e l i s m among the trends, i n t h i s rank range, o f f l u o r e s c e n c e i n t e n s i t y , chloroform-solubles y i e l d and G i e s e l e r maximum f l u i d i t y (5) . The s p e c t r a l d i s t r i b u t i o n s o f the secondary fluorescence emissions undergo a r e d s h i f t due t o the


82

COAL SCIENCE II

(a)

5.6 5.4 1.8

if ni

Ε

1.6 1.4


1.2 1.0

Coal Science II - ACS Publications - American Chemical Society

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