COAL AND CHAR TRANSFORMATION IN HYDROGASIFICATION OF COALS RANGING IN RANK FROM LIGNITE TO LOW-VOLATI LE BI TUMIN0US D A V I D
M .
MASON
Institute of Gas Technology, 3424 South State St., Chicago, Ill. 60616 The effect of rank on the behavior of coal a t different stages in the hydrogasification process was investigated petrographically. Among five high-volatile bituminous coals tested, the yield of coal from the pretreatment process increased slightly with rank. Structure and anisotropy of the hydrogasification residues indicated that the higher the rank of the coal, the greater the fluidity in the interior of the particles when they were rapidly heated in the reactor. Escape of this material from inside the pretreatment skin caused agglomeration when a lowvolatile bituminous coal was processed. Lignite and subbituminous coals run without pretreatment, gave residue chars with a lenticular pore structure. In some regions of these residue chars, the presence of rounded pores indicated the development of a degree of plasticity. HE behavior of a high-volatile A bituminous coal from Tthe Pittsburgh No. 8 seam in the hydrogasification process was discussed previously (Mason and Schora, 1967). This study of the petrographic and physical properties of the coal and chars a t different stages of the process had two objectives: to find out as much as possible about what happens to the coal and how it behaves in the process, and to develop a correlation between petrographic properties and suitability for hydrogasification. I n the present paper the investigation was extended to coal ranging in rank from lignite to low-volatile bituminous. In the hydrogasification process bituminous coal is treated in three successive stages:
Pretreatment with air at 800°F. to destroy the agglomerating power of the coal. First-stage hydrogasification a t 1200" to 1300"F. with pretreated coal as the feed. Second-stage hydrogasification a t 1700" to 2000" F. with first-stage residue as the feed. In the present work the hydrogasification processing method differed from that previously reported. I n the former study, the first and second stages were conducted in different moving-bed runs. I n this work the two stages were combined in a single operation. The first stage was limited to a period of a few seconds while the pretreated coal dropped in free fall through the rising gas in the upper 9 feet of the reactor tube. The second-stage hydrogasification occurred in a fluid bed in the lower 3 to 4 feet of the reactor tube. With this processing regime, it was impossible to obtain char samples between the two stages. Most of the char residues investigated here were from runs in which the hydrogen fed was 25 298
Ind. Eng. Chem. Process Des. Develop., Voi. 9,No. 2, 1970
to 3 0 5 of the stoichiometric amount required for complete gasification, and the steam comprised 50% of the steamhydrogen mixture. The sources, ranks, and petrographic properties of the coals processed are shown in Table I. Work is continuing on two or three other coals. Our petrographic methods have been described (Mason and Schora, 1967). The only change is that a 40X Achromat objective was used for both maceral analysis and reflectance determination. Surface areas were determined by Numec Instruments and Controls Corp. with carbon dioxide adsorption at 198" K. High-Volatile Bituminous Coals
The high-volatile bituminous coals ranged in rank from an Illinois No. 6 high-volatile C coal with a mean maximum vitrinite reflectance of 0.45%; to a West Virginia No. 5 Block high-volatile A coal with a mean maximum vitrinite reflectance of 0.81%. All of these coals required pretreatment of about the same severity. The effect of such pretreatment is not noticeably different from that described previously for the Pittsburgh No. 8 coal-mainly inflation of particles to cenospheres, disappearance of most of the exinite, development of a high-reflectance skin differing in thickness from particle to particle (Figure I), and a varying lesser increase in reflectance (over that of the coal) in the interior of the particles. Particle structure appears similar, although quantitative determinations of cell wall thickness and vesicle size might show differences. Several different types of particle structures are found in the char residues after hydrogasification (Figure 2). A fine-textured foam structure is prevalent in the residue from the highest rank coal (West Virginia No. 5 Block),
Table I. Co
Seam
Pocahontas No. 4
Mine Stoteshury No. 10
Sewell (Lower Kittanning)
Lochgelly
and Petrographic Properties
L"uncy a m
aLme
Ivb
0.3
74.1
25.6
1.78
mvb
1.8
71.2
21.0
1.45
hvAb
10.5
68.2
21.3
0.81
hvAh
2.7
84.3
13.0
0.68
hvRh .I
fi7
__
74 4
14.4
0.53
hvC:b
4.5
89.9
5.6
0.48
hvC:b
2.8
86.4
10.8
0.45
B
2.9
84.5
13.6
0.52
5.6
84.2
10.2
0.31
W. Va. No. 5 Block
Bell Creek
Ohio No. 6
Broken Am
Coshocton County,
Ireland
Rank
M@nl Maximum Reflectance in Oil,X !
Raleigh County, w. Va.
Faye.tte County, W . Va. Man;hall County, W . Va. Kanawha County,
Pittsburgh No. 8
Petrographic Analysis, Vol. % Exinite + Resinite Vitrinite Znertinite
~~
w. Va. Ohio
Indiana No. 6
Minnehaha
Illinois No. 6
Crown
Sullivan County, Id. Montgomery County, Ill.
Colorado subbituminous
Eagle
N. Dakota lignite
Glenharold
hut becomes less frequent with decreasing rank until it is virtually absent in the residue from the Illinois No. 6 coal (Figure 3). Otherwise, the residue particles from the different high-volatile bituminous coals appear similar in form. There is also little difference in reflectance; the mean reflectance of the residue from the lowest rank coal (Illinois No. 6) does not differ significantly from that obtained from the Pittsburgh No. 8 (Table 11). The anisotropy of the residue particles increases with rank of the coal. Less than half of the residue particles from the Illinois No. 6 coal show any anisotropy when observed with crossed polars, and these only in small areas (Figure 4, left). Most of the residue particles from the West Virginia No. 5 Block coal show a little anisotropy, while some, especially those with the foam structure, are predomi-
nantly anisotropicexcept for the pretreatment skin (Figure 4, right). The greater degree of anisotropy and prevalence of foam structure in the. residue from the higher rank coals indicate that a greater degree of fluidity is attained in the interior of these particles when the pretreated coal is rapidly heated in the hydrogasification reactor. Surface areas of 503 and 463 sq. meters per gram were obtained on the residues from Pit.tshurgh No. S and Illinois No. 6 coals. The surface area of the original Pittsburgh No. 8 coal was 131 sq. meters per gram; that of the Illinois No. 6 coal may have been higher, hut was not measured. After pretreatment the Pittsburgh No. 8 coal had a surface area of 148 sq. meters per gram. Thus, most of the increase in the surface area occurred during hydrogasification. Very few differences were evident in processing charac-
-
I
-.
Figure l . Pretreated Illinois No. 6 coal
.,,,es
_ , . e -
7
of char structure in residue from W. Va.
No. 5 block cool Ind. Eng. Chem. Process Der. Develop., Vol. 9, No. 2, 1970 299
Figure 3. Residue chars leh.
lllinoir No. 6 c o d
Right.
Table II. Reflectance of Hydrogasification Residues
Coal Colomdo IllinoiS subbiNo. 6 turninma Run No.
W. Vo. No. 5 block c o d
teristics among the high-volatilc? bituminous coals, in either pretreatment or hydrogasific;Ition. However our data indicate that the pretreatment 1iield increases with the . rank of the vitrinite. This is shown m laole 111, where the c()ais are listed in order of increasing vitrinite reflectimce. Yields were calculated by two methods for LL" - _ _ ^ _ I _ _ ^ & ... -:*LA- Lea :..- VI runs l i ~ ___L:^L u WUUL L.LK ~ I U U U ~ C W L I ~ CLULSA MW-UVWUL~ only very lightly caked in a laboratory agglomeration test (Kavlick and Lee, 1967). In one method the yield of moisture- and ash-free coal was calculated from the measured total yield and the feed and product analyses. (Runs in which the material balance recovery was less than 85 or greater than 100% were excluded.) In the
-
N . Dakota lignite
^^
HT-255
HT-184
HT-139
4.45
3.36
2.44
0.71
0.70
0.90
234
115
111
Figure 4. Anisotropy of residue chars observed with crossed polors leh.
From Illinois No.
6
cool
#e$.Develop.,Vol. 9, No. 2, 1970
Right.
From W. Yo.
No. 5 block c o d
1.
___
.
-"
Toble 111. Yieldsi from Pretreatment of Hiigh-Volatile Bituminous Coals Yield of Pretreated Maceml content, 1h l . %
Illinois No. 6 Indiana No. 6 Ohio No. 6
Pittsburgh No. 8 W. Va. No. 5 Block
0.45
3
11
0.48 0.53 0.68 0.81
4
6 14 13 21
6 3 10
7 6 2 6 3
82.65 80.0 83.4 89.4 84.7
78.8 80.8 78.8 82.9 82.1
'Mean maximum refictance of uitrinite in oil. Ineluding semifminite. ' T w o IUM on Illinois coal and twD rum on Pittsburgh seam coal excluded because of poor material balances. dFixed carbon assumed lo remain unchanged.
other method, the yield was calculated from the proximate analyses of feed and product, with the assumption that the amount of fixed carbon did not change. A similar calculation based on ash content is sometimes used, hut is distrusted here because of the tendency for dense, highash particles to accumulate in the pretreatment fluid bed. The yields from the second method are consistently less than those from the pilot plant data, hut both show a trend of increasing yield with rank. The yield also depends on maceral composition because inertinite is expected to lose less, and exinite more, volatile matter than vitrinite. The two higher rank coals have, on the average, more exinite and inertinite than the two lowrank coals. Because the exinite content counterbalances the inertinite content, it appears that the higher average yield of the two higher rank coals can he attributed entirely to the higher ranks of their vitrinites. High-Rank Bituminous Coals
The pretreated Pocahontas No. 4 coal showed the usual development of oxidized pretreatment skin, but very little or no vesicle formation (Figure 5 ) and no significant loss of volatile matter. In addition to cracks into which the pretreatment skin extends, many additional fine cracks were present in the particles. We believe that the development of these cracks caused the high rate of attrition of this coal that was observed in the pretreatment processing.
Pretreatment of the UC.WC.II D~~~~~ LVLII, w I I I b L I UVIUrlllllr between medium- and low-volatile rank, resulted in vesicle formation (Figure 61, although not to as great an extent as in the high-volatile coals. Fine cracks were also observed in the pretreated particles of this coal, and a high rate of attrition occurred in the pretreatment processing. Hydrogasiticati on of the Sewell and Pocahontas coals was difficult I3ecause the reactor plugged with agglomerating co al particles. When agglomerated residue particles from thme Pocahontas coal were examined petrographically with crossed polars, the pretreatment skin was easily identiitied. Particles without skins and the interiors of parti cles with skin were strongly anisotropic and had a foam structure. Agglutination or cementing of particles was done by this foam material only. The foam material uias supplied not only by the particles that had underg one little or no pretreatment, hut also by particles wit h holes or breaks in the skin (Figure 7). I t appears t ktat the pretreatment skin can he broken and the content3 discharged to the outside as a result of the simultane(IUS development of pressure and fluidity in the interior of the particles. derived from Only a sm:311 fraction-probably inertinite-of the? residue from the Pocahontas coal does not have a foam structure. The Sewell coal also developed a large amount o,f foam structure, hut has an appreciable, perhaps predominant, amount of material with vesicular structure similar to the residue from high-volatile bitumin~~
~~
c
Figure 5 . Pretreated Pocahontas No. 4 coo I Ind. Eng. Chem,.Process Der. Develop., Vol. 9,No. 2, 1970
301
Conclusions
Petrographic study of the feed coals and chars a t different stages of processing has been fruitful in several ways. The detection of contamination of a nonagglomerating coal by a coking coal explained some anomalous processing results, although the examination was not early enough to avoid their occurrence. Agglomeration of pretreated high-rank bituminous coals was shown to result from the discharge of fluid material from the interior of pretreated particles as well as from the fluidity of the few particles that escaped pretreatment. Lack of anisotropy in the residue chars from lignite, subbituminous coal, and pretreated high-volatile C coal indicates that little or no fluidity developed in these coals, and accounts for the successful processing of lignite and subbituminous coal without pretreatment. Increasing amounts of anisotropy, indicating development of fluidity, were observed in chars from the higher rank bituminous coals. Increased anisotropy and a very different char particle structure were found when a high-volatile A bituminous coal was fed without pretreatment (Mason and Schora, 1967). These differences in structure may become important if process development makes it possible to feed the coal without pretreatment. Because graphitization a t higher temperature occurs only when the char has passed through a fluid stage with resulting anisotropy (Kipling and Shooter, 1966), the degree of anisotropy or lack of it may indicate differences in the electrical characteristics of the char; this may affect its behavior in the electrothermal gasification process for the production of hydrogen. Correlating the petrographic properties of a coal and its suitability for hydrogasification, we find: Exinite is largely lost in pretreatment and, therefore, is of value only if the coal can be processed without pretreatment.
High-reflectance inertinite is believed to behave as an unreactive diluent in a t least the first stage of the hydrogasification process. We have no information on the degree of activity of semifusinite. Very little difference was evident in the processing characteristics of the high-volatile bituminous coals in either pretreatment or hydrogasification. The yield of coal from pretreatment increased with the rank of the vitrinite; a 3 to 6 7 increase in yield was noted when the reflectancein-oil of the vitrinite increased from an average of 0.46Lc for two high-volatile C coals to an average of 0.745 for two high-volatile A coals. Acknowledgment
The writer thanks the American Gas Association and the U. S. Department of the Interior, Office of Coal Research, joint sponsors of the project, for permission to publish these results. William Moffett and Ellen Maeda did some of the work. literature Cited
Brown, H . R., Taylor, G. H., Cook, A. C., Fuel 43, 43-54 (1964). Kavlick, V. J., Lee, B. S., Advan. Chem. Ser. No. 69, 12 (1967). Kipling, J. J., Shooter, P. V., Carbon 4, 1-4 (May 1966). Mackowsky, M. T., “Progress in the Field of Coal Petrography,” Paper 1, Petrography of Coal and Coke Session, Symposium on Application of Science Technology to Coal, Ottawa, March 29-31, 1967. Mason, D. M., Schora, F. C., Jr., Advan Chem. Ser. NO. 69, 18-30 (1967). RECEIVED for review June 16, 1969 ACCEPTED December 19, 1969 Division of Fuel Chemistry, 156th Meeting, ACS, Atlantic City, S . J., September 1968.
APPLICATION OF A SUBOPTIMAL DESIGN METHOD TO A DISTRIBUTED-PARAMETER REACTOR PROBLEM S. G . B A N K O F F Department of Chemical Engineering, Northwestern University, Euanston, Ill. 60201
J .
D .
P A Y N T E R ’ ,
THE
OPTIMAL DESIGN of plug-flow tubular reactors having only axial gradients of composition and temperature has been the subject of numerous studies. However, little work has been reported on the more difficult problem which occurs when large heat effects accompany reaction, resulting in both axial and radial variations within the reactor. For such two-dimensional situations, partial differential equations are required to represent the reactor behavior, leading to a much more complex optimization problem than the plug flow case. The only previous study of such a problem was carried
’ Present La.
address, Esso Research Laboratories, Baton Rouge,
J .
S.
D R A N O F F ,
A N D
out byTGray (1965) and reported by Denn, Gray, and Ferron (1966). They considered the case of a fictitious single reaction, which was first-order, reversible, and highly exothermic, and determined the optimal wall heat flux distribution required to maximize the outlet conversion of both the two-dimensional situation and the usual lumped-parameter model involving only axial gradients. They showed that there was a significant degradation from optimality for the one-dimensional approximation compared to the two-dimensional solution. Hence, whenever the dimensionless source term is not small everywhere, the two-dimensional model should be used to obtain accurate results. The computational labor in this case, however, can be very burdensome, since 40 to 200 Ind. Eng. Chem. Process Des. Develop., Vol. 9 , No. 2, 1970 303
