AN AMERICAN CHEMICAL SOCIETY JOURNAL VOLUME 3, NUMBER 4
JULYIAUGUST 1989
0 Copyright 1989 by the American Chemical Society
Reviews Considerations of Physicochemical Phenomena in Coal Processing+ Frank Derbyshire* Sutcliffe Speakman Carbons Limited, Guest Street, Leigh, Lancashire, U.K.
Alan Davis* and Rui Lin Energy and Fuels Research Center, The Pennsylvania State University, University Park, Pennsylvania 16802 Received October 31, 1988. Revised Manuscript Received March 22, 1989
Certain aspects of the chemical and physical changes that occur during coal processing are reviewed and discussed in relation to the influences of coal oxidation and catalytic hydrogenation under low-severity conditions. The adverse effects of oxidation on thermoplastic properties and on the conversion of coals to liquids are related to the formation of cross-links in the coal structure through the thermally induced reactions of oxygen-containing functional groups. Conversely low-temperature catalytic hydrogenation suppresses this tendency and can promote the cleavage of existing crosslinkages. The breakdown of the coal by hydrogenation is accompanied by the development of fluidity. The structural modifications introduced by low-temperature reaction can have a profound effect upon coal behavior in subsequent reactions a t higher temperatures. Microscopic techniques and, in particular, fluorescence microscopy can be usefully applied to studying the structural changes caused by reaction under hydrogenating and non-hydrogenating conditions.
Introduction Essentially all forms of coal utilization involve decomposition of the coal structure, which is accompanied by important physical as well as chemical changes. No coal structural model has yet been developed that fully accounts for these phenomena or allows the accurate prediction of the behavior of single coals and, still less reliably, of coal blends. In this paper, some aspects of these changes are reviewed and discussed in relation to coal rank and coal pretreatment by reaction with oxygen and with hydrogen in the presence of catalysts at low temperatures. The data that are presented are derived mostly from the literature. Some information that has not been published previously by the authors is also included.
In the generally accepted view, the greater proportion of the coal structure is considered to consist of relatively low molecular weight structural units that are joined together by a mixture of weak and strong linkages to form a three-dimensional macromolecular network. The structural units consist of cyclical carbon structures of one, two, three, or more condensed and hydrogenated rings and are linked by alkyl or etheric (oxygen or sulfur) bridges. With increasing rank, the number of oxygen groups decreases and the ring systems become more condensed. This concept should be modified to take into account evidence which suggests that hydrogen bonds and other secondary interactions play an important role in crosslinking the coal structural units.'V2
'A version of this paper was first presented at the EPRI Thirteenth Annual Conference on Fuel Science and Conversion, Santa Clara, CA, May 18-19, 1988.
(1) Brenner, D. Fuel 1985,65, 167-173. ( 2 ) Larsen, J. W.; Green, T. K.; Kovac, J. J . Org. Chem. 1985, 50, 4729-4735.
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I
I I L
-*’
i
Coal *
9 Char or
Coke Figure 1. Decomposition of coal (modified from ref 5).
Accommodated within this structure are less firmly attached species that are retained by weak bonding or physical entrapment. These constituents have been referred to collectively as the mobile phase, an accurate description of which is l a ~ k i n g . ~They can be partially removed by solvent extraction and are richer in hydrogen than the bulk coal, and their presence is integrally related to the development of plasticity in bituminous coals. Estimates of the proportion of coals that comprises the mobile phase range from 10-20%4 to as much as 45% for some depending upon the way in which mobile phase is defined. During coal decomposition, the cleavage of connecting linkages, by thermal or catalytic action, leads to the liberation of radical fragments with a wide range of molecular weights5 (Figure 1). The fragments can be stabilized by hydrogen addition and by rearrangement, which leads to the formation of stable liquid products, and by adduction, which leads ultimately to the formation of solid products (char) and quantities of hydrocarbon gases. The balance between the various reactions determines the distribution and composition of the decomposition products. When the only source of hydrogen is in the coal, carbonization is the predominant process. The extent of hydrogenative radical stabilization is limited, and the principal product is a char. Introducing external sources of hydrogen allows the capacity for suppressing regressive reactions to be expanded, and liquid yields can be increased. Hydrogenation can be enhanced in the presence of hydrogen (hydropyrolysis), donor solvent, or donor solvent with hydrogen (liquefaction) and by catalysts. However, even if char formation is substantially eliminated, regressive reactions can lead to the formation of strongly bonded high molecular weight liquid products that do not readily lend themselves to upgrading by catalytic hydroprocessing.
Coal Oxidation Mild oxidation can adversely affect the thermoplastic properties of bituminous coals6 (Figure 2). At the same time, the yield of extractable liquids is reduced’ss (Table (3) Given, P. H.; Marzec, A.; Barton, W. A.; Lynch, L. J.; Gerstein, B. C. Fuel 1986,65, 155-164. (4) Derbyshire, F. J.; Terrer, M.-T.; Davis, A.; Lin, R. Fuel 1988, 67,
- -- - -
11329-1 nwi -
(5) Eser, S.College of Earth and Mineral Sciences, The Pennsylvania State University, University Park, PA, personal communication, 1984. (6) Senftle, J. T.; Davis, A. Int. J. Coal Geol. 1984, 3, 375-381.
:
I
i
I
I
I
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I
20 40 60 80 100 120 Time of exposure to air, days.
-
Figure 2. Reduction in fluidity of hvAb coal with progressive oxidation (modifiedfrom ref 6). ddpm = dial divisions per minute.
’O1
1
wt% BORIC ACID
2
3
Figure 3. Suppression of thermoplastic development in bituminous coal by boric acid addition. Table I. Reduction in Yield of Extractable Liquids upon Oxidation‘ mass fraction extracted weathering in tetrahydrofuran time, days (dmmfb), % 0 21.3 5 35 56
19.6 18.3 14.5
“Data from ref 7. bdmmf = dry mineral-matter free.
I). As discussed later, there is a direct relationship between the thermoplastic properties and the content of solvent-extractable liquids. Oxidation can introduce ether, carbonyl, carboxyl, and phenolic groups. The extent of oxygen uptake and the functional group distribution are dependent upon the coal structure and the oxidation conditions. The reduction in plastic properties has been attributed to the formation of ether cross-links?JO While some cross-linking may occur during low-temperatureoxidation, it seems likely that most of the oxygen-related cross-linking results from the condensation of functional groups promoted by thermal en(7) Derbyshire, F.J.;Gray, D. Ullmann’s Encyclopedia of Industrial Chemrstry, VCH Verlagsgesellschaft mbH: Weinheim, FRG, 1986; Vol. A7, pp 197-243. (8) Larsen, J. W.; Lee, D.; Schmidt, T.; Grint, A. Fuel 1986, 65, 595-596. (9) Liotta, R.; Brons, G.; Isaacs, J. Fuel 1983, 62, 781-791. (10) Rhoads, C.; Senftle, J. T.;Coleman, M. M.; Davis, A.; Painter, P. C. Fuel 1983, 62, 1387-1392.
Energy & Fuels, Vol. 3, No. 4, 1989 433
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OH
I
0
CZ.0
.D
-OH
Figure 5. Self-coupling of dihydroxy/aromatic structures
(modified from ref 16).
Tomporrturo ("C)
Figure 4. Effect of temperature and heating rate on tar yield and cross-linking (Zap lignite, slow pyrolysis in He; rapid pyrolysis in COz) (modified from ref 14).
ergy at modest temperatures. At more elevated temperatures, some of the less stable oxygen-containingcross-links may cleave, leading to the formation of direct carboncarbon bonds." Effects similar to those caused by oxygen can be produced by admixing the raw coal with reagents such as boric acid (Figure 3). This technique is used in a process developed by the Sutcliffe Speakman Co. to prevent the development of fluidity and swelling during the carbonization of bituminous coals for the production of active carbons.12 Coal oxidation can be deleterious to the yield of liquids produced during pyrolysis or liquefaction.'l Drying the coal feed can also produce adverse results, especially for low-rank coals, although the available evidence does not allow the changes incurred by drying and oxidation to be clearly distinguished. The high oxygen content of immature coals and lignites creates a potential for cross-linking to occur through reactions involving oxygen functional groups. Suuberg and others13compared the structural changes in a lignite and in a bituminous coal during rapid pyrolysis. The extent of cross-linking in the pyrolysis chars was inferred from the reduction of the swelling ratio in pyridine. The onset of cross-linking in the lignite was found to occur at a somewhat lower temperature than in the bituminous coal and coincided with the release of C02. More recent work by Deshpande and others14has also shown that the temperature at which cross-linking is initiated tends to decrease with decreasing coal rank. Here too, solvent swelling was used as an indicator of the cross-link density. The findings are consistent with the proposal that cross-linking reactions involve the "loss of carboxyl groups and evolution of carbon dioxide and the extent of cross-linking is directly related to the amount of those labile carboxyl groups present in coal".14 The association of C02 evolution with oxidative cross-linking could provide some insight into the mechanisms of these processes. In the same study, a pertinent series of experiments with a North Dakota lignite demonstrated that the processes of cross-linking and volatile release are dependent upon (11)Neavel, R. C. Coal Science; Academic Press Inc.: New York, 1982; VOl. 1, pp 1-19.
(12)Schwabe, P. H.Chem. Br. 1970,6,388-393. (13)Suuberg, E.M.; Lee, D.; Larsen, J. W. Fuel 1985,64,1668-1671. (14)Deshpande, G. V.; Solomon, P. R.; Serio, M. A. Prepr. Pap.Am. Chem. SOC.,Diu. Fuel Chem. 1988,33(2),310-321.
Table 11. Carbonization of Asphaltenes' sample % pyridine ratio of FTIRbpeak intens ref insolubles (C=O/Ar C-C) A 70.5 0.61 B 69.9 0.41 C 65.4 0.33 D 60.5 0.32 E 60.0 0.23 F 14.7 0.11 aData from ref 17. Reaction conditions: 30 min, 450 "C, tubing bomb reactor. bC=O stretch in alkyl-aryl ketones (1690 cm-'); aromatic C-C stretch (1600 cm-').
the rate of heating (Figure 4). At a heating rate of 0.5 OC/s, tar evolution commenced after the onset of crosslinking (from the swelling ratio) and only became appreciable when the propensity for swelling was almost completely suppressed. With a much higher rate of heating (20 000 "C/s), tar evolution coincided approximately with the decline in swelling ratio and the tar yield was appreciably higher. Microscopic examination of the chars produced at low (here 600 OC/s) and high heating rates revealed that, in the latter case, there was evidence of the development of thermoplasticity. It is inferred that rapid heating creates conditions where the rate of bond cleavage is competitive with that of cross-linking; at low heating rates, cross-linking appears to be the faster step. Many experiences have shown that low-rank coals are less easily liquefied than bituminous coals, although there is some evidence to the contrary. This apparent discrepancy is thought to relate to the propensity for low-rank coals to form cross-links upon heating, rendering the coal less amenable to liquefaction. In the presence of hydrogen and an active catalyst, this tendency can be reduced, thereby facilitating the conversion to 1 i q ~ i d s . l ~ McMillen and othersl8 have provided some insight into the chemistry of regressive reactions involving polyhydroxy aromatic structures. Dihydroxy aromatics, such as resorcinol, can undergo self-coupling reactions to form refractory dibenzofuran linkages (Figure 5). The self-coupling of 1,3-dihydroxynaphthalenetakes place much more readily than that of resorcinol. Low-rank coals appear to contain structures that are subject to regressive reactions that are still more rapid than the selfcoupling reactions of these model compounds, even under the reducing conditions present during coal liquefaction. The authors suggested that these reactive native coal structures are polyhydroxy aromatics. Other evidence implicating oxygen functional groups in the promotion of regressive reactions has been afforded by studies of the carbonization of petroleum asphaltenes.17 Asphaltenes from six different sources were characterized by Fourier transform infrared spectroscopy (FTIR). The (15)Derbyshire, F. J.; Stansberry, P. G. Fuel 1987,66, 1741-1742. (16)McMillen, D. F.; Chang, S.-J.; Nigenda, S. E.; Malhotra, R. Prepr. Pup.-Am. Chem. SOC.,Diu. Fuel Chem. 1985,30(4),414-426. (17)Eser, S.;Karsner, G.; Derbyshire, F. J. Proceedings of the Fourth International Carbon Conference; Der Deutschen Keramischen Gesellschaft e.V.: Baden-Baden, FRG, 1986;pp 99-100.
434 Energy & Fuels, Vol. 3, No. 4, 1989
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Table 111. Coals Used in Liquefaction Experiments PSOC-1266 PSOC-1296 PSOC-1403 Ohio No. 5 L. Kittannine Anderson Penn Penn Paleocene OH WY PA Mahoning Campbell Armstrong hvAb hvAb subC 0.83 0.40 0.87 91 87 92 10.4 3.9 16.2 1.3 5.2 1.2
coal sample no. seam age state county ASTM rank % reflectance (mean max) % vitrinite (mineral free) % ash (dry) % sulfur (dry)
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230
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0
PSOC-1504
U. Sunnvside U. Cret UT
ND Mercer
Carbon hvAb 0.80 87 7.5 0.8
lig 0.34 83 9.4 1.2
Fluidity C k n p with Progrerrlve Catalytic Hydropnation (400%)
(a1
b)
PSOC-1414 Beulah-Zau Paleocene
290
350 410 T m p n h r e OC C U I ' PSOC-1266, hvAP
410
Fluidity of fubbihfninau Coals Catalytically Hydropmted at W C (Prent c w l s hare no Iluidlty.)
I
I
40 60 Yield of Chloroform-Soluble Extract (96 dmmf)
20
Figure 6. Influence of extract yield on initial softening temperature (bituminouscoal, PSOC-1266, dry hydrogenation at 400 O C , 7 MPa of H2initial pressure, and 1w t % Mo) (modified from
ref 20).
zio
zbo
3io Temperature OC
rbo
480
~
"
Efhctr 01 Mild Oxidation and Subseqwnl Catalytic Hydropnation on Coal Fluidity
250
I
312
375 437 Temperature OC Coal: PSOC-12%. hrkb
I
500
Figure 8. Gieseler plastometry measurements of hydrogenated and parent coals (modified from ref 21).
0
I
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15
30
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45 O h CHC13-Soluble Liquids
1
60
Figure 7. Extension of plastic range with progressive hydrogenation (bituminouscoal, PSOC-1266; dry hydrogenation at 400 "C, 7 MPa of H2initial pressure, and 1 wt % Mo).
rate of asphaltene carbonization was found to correlate with the ratio of the intensity of the absorption band attributed to the C=O stretch in alkyl-aryl ketones (1690 cm-') to that of the aromatic C-C stretch (1600 cm-') (Table 11).
Coal Hydrogenation The reactions of coals in hydrogen can effect structural changes that, in many respects, are the antithesis of those produced by oxidation. A number of the illustrations given
below were obtained in research conducted at The Pennsylvania State University under DOE sponsorship. Those studies investigated the solvent-free catalytic hydrogenation of coals using an impregnated, sulfided molybdenum c a t a l y ~ t . ~Characteristics J~~ of the coals that were used in this research and are cited here are shown in Table 111. The technique of solvent-free liquefaction allows the chemical and physical changes in the coal to be directly (18) Davis, A.; Derbyshire, F. J.; Finseth, D. H.; Lin, R.; Stansberry, P. G.; Terrer, M.-T. Fuel 1986, 65, 5M-506. (19) Derbyshire, F. J.; Davis, A.; Epstein, M.; Stansberry,P. Fuel 1986, 65, 1233-1240. (20) Derbyshire, F. J.; Davis, A.; Lin, R.; Stansberry, P. G.; Terrer, M.-T. Fuel Process. Technol. 1986,12, 127-141. (21) Stansberry,P. G.; Lin, R.; Terrer, M.-T.; Lee, C. W.; Davis, A.; Derbyshire, F. J. Energy Fuels 1987,1, 89-93. (22) Terrer, M.-T.; Derbyshire, F. J. The mobile phase in coals: ita nature and modes of release-final report part 1. DOEPC-60811-F1; The Pennsylvania State University: University Park, PA, 1986; p 113. (23) Stansberry,P. G. Ph.D. Thesis,The PennsylvaniaState University, University Park, PA, 1988.
Energy & Fuels, Vol. 3, No. 4, 1989 435
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measured and observed and was employed to try to elucidate the mechanisms of hydroliquefaction under mild reaction conditions and to investigate the dependence of these reactions upon coal rank. Catalytic hydrogenation alters the thermoplastic properties of coals, concomitant with an increase in their content of extractable liquids. In the examples shown for a bituminous coal in Figures 6 and 7, the initial softening temperature decreased and the plastic range was extended as the content of chloroformsoluble extracts was increased. Illustrations of the effects of hydrogenation on coal fluidity are shown in Figure 8. The curves in Figure 8a, obtained with a bituminous coal, show a progressive increase in fluid properties with increasing reaction time. For times longer than 15 min, the maximum fluidity was beyond the measurable range of the instrument. Subbituminous coals can also become fluid upon hydrogenation, Figure 8b, even though, characteristically, the raw coals do not exhibit thermoplastic behavior. This, of itself, indicates that hydrogenation inhibits the formation of cross-links that would otherwise occur. Suuberg and others13suggested that the softening of lignites is prevented by low-temperature cross-linking. The results in Figure 8c show, in accord with other published data, that the adverse effects of coal oxidation on thermoplasticity can be more than offset by catalytic hydrogenation. Larsen and others* observed that the ability to restore thermoplasticity to a bituminous coal (oxidized by air exposure) by reduction with lithium aluminium hydride was dependent upon the exposure time (which must reflect the degree of oxidation). Fluidity was completely regained when the exposure times were comparatively short (ca. 28 days). Following more extended exposure, only partial restoration of the coal fluidity was achieved. It was proposed that the fully reversible oxidation involved the loss of easily donated hydrogen. The presence of labile hydrogen in the coal structure appears to play an important role in the development of thermoplasticity and in the conversion of coals to liquids." Prolonged oxidation did not exert any appreciable further change in extract yield or cross-link density. Some of this oxygen must therefore be present in a form that is not readily reduced (see the discussion relating to Figure 5). Studies of the catalytic hydrogenation of a subbituminous coal showed that, at low levels of hydrogenation, there was an increase in the pyridine swelling ratio of the chloroform-insoluble residue above that of the parent (chloroform-extracted) coal4(Figure 9). From this it may be inferred that, under appropriate hydrogenation conditions, not only is thermally induced cross-linking prevented but also some of the preexisting cross-links are removed. The subsequent reduction in swelling ratio at higher extract yields is assumed to be due to the reduced concentration of remaining (nonsolubilized) vitrinite. The resuhx displayed in Figure 9 were obtained by a study of the catalytic hydrogenation of a subbituminous coal in which experiments were conducted over a range of temperatures (300, 350, and 400 "C) and reaction times (up to 60 min). The low extract yields, which are associated with the increase in residue swelling ratio, were obtained either upon reaction for various times at 300 OC or short reaction times (less than 5 min) at the higher temperatures. Similar findings were recently reported for a bituminous coal, where an increase in residue swelling ratio was observed under conditions of limited conversion to chloroform s ~ l u b l e s .It~ is ~ probable that two factors contribute to the phenomenon of increased swelling ratio: the limited degree of conversion and the use of conditions
8
0
10
20
30
40
50
meld of Chlorofonn-.dubler
(Oh
60
70
dmml
Figure 9. Swelling ratio w extracted yield (subbituminous coal, PSOC-1403; dry hydrogenation at 7 MPa of H2initial pressure and 1 w t % Mo) (modified from ref 4).
where hydrogenation processes dominate regressive reactions. Microscopic examination of reacted coals and residues in normal and fluorescent light has been used to assist in understanding the structural changes caused by hydrogenation.1g20i24.25Solvent-free catalytic hydrogenation leads to the formation of a low-reflecting,highly fluorescent solvent-soluble form of vitroplast.26 Such material is isotropic and pitch-like, occurring as a matrix that cements other residual components in the case of lignite and as large uniform particles in the case of bituminous coal. Also, in some of these residues, a high-reflecting, nonfluorescent, insoluble type of vitroplast with a fine granular anisotropy has developed; this type of vitroplast may be equivalent in part to the carboplast described by Shibaoka and Following reaction under a nitrogen atmosphere, the high-reflecting vitroplast is the predominant component. It is speculated that, under hydrogenation conditions, high-reflectingvitroplast is formed by regressive reactions, resulting from localized hydrogen deficiency. A comparison is presented in Figure 10 of the reflectograms of unextracted reacted coals and tetrahydrofuran(THF-) insoluble residues produced by the hydrogenation of a bituminous coal in the presence and absence of a sulfided Mo catalyst. The reflectograms of the whole reaction products, Figure 10a,c shows that the catalyst has had a marked effect upon the reflectance distribution. The whole catalytic reaction products, Figure loa, have a bimodal distribution consisting of a low-reflecting (Rh = 0.6-1.2%) and a high-reflecting (Ro- = 1.2-1.8%) population. The low-reflecting vitroplast was strongly fluorescent under blue-light illumination and was soluble in THF, which explains the absence of this population in the extracted residue (Figure lob). The high-reflecting vitroplast (24) Lin, R.; Davis, A.; Bensley, D. F.; Derbyshire, F. J. Int. J. Coal Geol. 1986, 6, 215-228. (25) Lin, R.; Davis, A.; Bensley, D. F.; Derbyshire, F. J. Org. Geochem. 1987, 11(5), 393-399. (26) Mitchell, G. D.; Davis, A.; Spackman, W. Liquid Fuels from Coal; Ellington, R. T.; Ed.; Academic Press, Inc: New York,1987; pp 255-270. (27) Shibaoka, M.; Russell, N. J. Proceedings of the International Conference on Coal Science; Glueckauf: Duesseldorf, FRG, 1981; pp 453-458.
436 Energy &Fuels, Val. 3, No. 4, 1989
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50
(C)
Whole Product
W h o l e . Product Catalytic
40
5 E A
40
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20
10
10
a,
t U
0
0 0.6
0.8
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1.6
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2.0
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0.8
0.6
0.8
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1.2
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2.0
1.0 1.2 1.4 1.6 Maximum Reflectance (Ro max, %)
1.8
2.0
50 Residue Cataiytlc
40
40
30
20
10
0 0.6
0.8 1.0 1.2 1.4 1.6 Maximum Reflectance (Ro max, %)
1.8
2.0
Figure 10. Reflectograms of liquefaction whole products (a and c) and THF residues (b and d): (a and b) catalytic; (c and d) noncatalytic (bituminouscoal, PSOC-1504; dry temperature-staged liquefaction with first stage 30 min at 275 "C and second stage 30 min at 425 "C; catalyst 1 wt % sulfided Mo). was nonfluorescent and THF insoluble. The catalytic residue comprised only 40.8% of the whole product, and its quite different reflectance distribution was evidently masked (in Figure loa) by the presence of a large quantity of THF-soluble material. It is possible that materials represented by the bimodal populations in Figure 10a are transitional, one to another, and that the intermediates (Row = 1.2-1.4%) are partially soluble in THF. In contrast, the noncatalytic products have a narrow unimodal population (R = 1.2-1.4%) (Figure 10c,d). It can be seen that there is3ose similarity in the reflectance distributions for the whole reacted coals and the insoluble residue, which reflects the fact that the latter constitutes the major proportion (71.7%) of the whole product. The production of solvent-solubleliquids is accompanied by changes in the intensity of vitrinite fluorescence and in the spectral distribution. The maximum intensity of fluorescence has been found to correlate with the yield of chloroform-soluble extract (Figure 11). The higher fluorescence intensity measured for the bituminous coal at yields greater than about 30 wt % is probably related to the more aromatic character of the bituminous coal liquids. The measurement of the spectral distribution of hydrogenated coals is a relatively new application of this microscopic technique. Although the significance of changes in the distribution is difficult to interpret, such studies offer the potential for providing insight into the mechanisms of coal hydrogenation. Coal hydrogenation at temperatures below about 300-350 "C results in few obvious changes in the commonly measured coal properties. Nevertheless, catalytic hydrogenation under these conditions has been found to have
2o 1 hvAb bauminous coal (PSW - 1258)
subbilumincus B (PSOC 1403P)
-
10
20 30 40 50 Chbmlorm - s0lut-k exlnct Iwi%dmml)
60
70
Figure 11. Correlation between maximum intensity of vitrinite fluorescence and chloroform-soluble extract (coals hydrogenated dry: 400 "C; 7 MPa of H2initial pressure; 1 wt % sulfided Mo) (plotted from data in ref 20). Table IV. Influence of Catalytic Hydrogenation on Pyrolysis Yields (wt % daf)" parent coal pretreated coal co + coz 2.1 0.5 1.7 2.5 CH4 tot. CI-C4 gases 4.9 4.3 char 60.3 42.0 tot. liquids 34.8 53.0 a Data from ref 28. Reaction conditions: pyrolysis, fluidized bed, 600 "C, N2 atmospheric pressure; hydrogenation, 400 "C, 7 MPa of H2initial pressure, 60 min, 1 wt % sulfided Mo.
a profound influence upon the yield and composition of the products generated by subsequent reaction at higher temperatures. The effect of prehydrogenation on liquefaction at temperatures above about 400 "C is to increase the conversion to liquids and the selectivity to oils over a~pha1tenes.l~Similarly for pyrolysis, hydrogenative pretreatment has been found to substantially enhance the
Energy & Fuels 1989, 3, 437-443 liquids yieldB (Table IV). In neither case is there any significant change in the yield of hydrocarbon gases. A possible explanation of these phenomena is that the addition of small quantities of hydrogen by low-temperature reaction serves to cleave some connecting linkages, to weaken others, and to create a pool of labile hydrogen within the coal. At higher temperatures, this hydrogen will facilitate the further cleavage and stabilization of connecting linkages. Vitrinite reflectance measurement or even qualitative microscopic observations can provide an indication of whether the coal has been altered by hydrogenation (reduced reflectance) or by condensation reactions (increased reflectance). However, this measurement does not appear (28) Bolton, C.;Riemer, C.; Snape, C. E.; Derbyshire, F. J.; Terrer, M.-T. Fuel 1988,67,901-906.
437
to be particularly sensitive to following the subtle structural modifications that are brought about by low-temperature catalytic hydrogenation. Recent research has provided some suggestion that the changes in the fluorescence spectral characteristics might be a more fruitful area for study. In summary, different coal utilization technologies attempt to exploit the effects of oxidation or hydrogenation, or at least to achieve a practical compromise between these opposing factors. There is a need to develop a better understanding of the attendant changes in coal structure and, underlying this, to develop a structural model that is consistent with the experimentally observed phenomena.
Acknowledgment. We wish to acknowledge the contributions of our colleagues who were involved in these studies, Peter Stansberry and Maite Terrer, and the U.S. DOE,which provided most of the financial support.
Articles Recycle Oils from Fluid Coking of Coal Liquefaction Bottomst R.A. Winschel* and F. P. Burke Research & Development, Consolidation Coal Company, 4000 Brownsville Road, Library, Pennsylvania 15129 Received October 20, 1988. Revised Manuscript Received March 17, 1989 Ten fluid-coker tars, produced by Lummus-Crest, Inc., from coal liquefaction vacuum bottoms, were characterized to evaluate their use as liquefaction recycle oils. The primary variables in the coking tests were temperature (1000-1200 O F ) and coker feedstock source. Most of the properties of the tars are principally influenced by the coking temperature. Those produced at higher temperature are more aromatic, they contain more carbon and less hydrogen, and they are principally unsubstituted and methyl-substituted condensed aromatic compounds. The heteroatom contents of the tars appear to be mainly determined by the coker feedstock; however, heteroatom contents are quite low for these tars. The tars produced at 1000 O F are expected to be poor hydrogen donor solvents, whereas those produced at 1200 O F can donate virtually no hydrogen. However, a 1200 O F tar was readily hydrotreated to produce an excellent donor solvent. None of the tars produced at lo00 or 1100 OF were hydrotreated. Since the properties of those tars are different than those produced at 1200 O F , they may respond differently to hydrotreating. However, the difference is such that the tars produced at lower temperature are more similar to their feedstock. Therefore, it is expected that their susceptibility to hydrotreating should be intermediate between that of their feedstocks and that of the high-temperature coker tars. On the basis of these results, it would appear that tars produced from fluid coking of liquefaction vacuum bottoms can be recycled to a catalytic liquefaction reactor to produce additional liquids without adversely affecting process performance. The impact of the coker tars on long-term catalyst activity remains to be addressed.
Introduction In the development of processes for the direct liquefaction of coal, the efficient removal of solids from the product has proven to be particularly difficult. Many techniques have been tested and used, including filtration, 'Presented at the Symposium on Coal Liquefaction, 196th National Meeting of the American Chemical Society, Loa Angeles, CA, Sept 25-30,1988.
hydrocyclones, vacuum distillation, critical solvent deashing, and antisolvent deashing; however, no truly satisfactory means has been developed. All suffer from high product rejection, high cost, or serious engineering difficulties. A potential alternate method of removing solids in liquefaction is the coking of the liquefaction vacuum bottoms product. In the conceptual integration of liquefaction and coking, the coker tars would be processed in the liquefaction plant, ultimately to produce
0887-0624/89/2503-0437$01.50/00 1989 American Chemical Society
