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Structure and chemistry of coals: Calorimetric analyses
Structure and Chemistry of Coals: Calorimetric Analyses 1 E. L. F U L L E R , JR. Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee (Received December 21, 1979; accepted January 29, 1980) Heats of immersion (hi) of coals have been shown to be a valuable means of investigating structure and chemistry of coals. This report outlines some of the factors involved. Lower ranked coals imbibe more liquids (i.e., HzO) onto more polar sites (carbonyl, phenolic, etc.) than higher ranked coals. Mineral matter reacts strongly with polar liquids (i.e., H20) giving rise to enhanced hi. Grinding of coals not only decreases particle size but modifies the coal structure to an increasing degree dependent upon the extent and severity of grinding. The magnitude of hi and the rate of reaction are both modified consistent with the existence of a "shrinking core" or unperturbed coal structure serving as substrate to which the modified (less ordered) material is bound. Chemical (alkali) attack seems to loosen the coal structure markedly to allow enhanced access to fluid reagents. These exploratory studies have shown that calorimetric analyses similar to those developed and used by A. C. Zettlemoyer and his co-workers are excellent means for elucidating the structure and chemistry of coals and related materials. The complexity of the problem is quite apparent due to the complexity of coals themselves. Further work is warranted to assure efficient utilization of this exhaustible natural resource.
Coal has become an increasingly important resource for energy and chemical feedstock as our petroleum resources are waning. Most preparative and processing techniques require fluid-phase access into the complex structure of the coal. This study is the result of calorimetric analyses of the thermodynamics and kinetics of fluid-phase access into coals of varying rank, characteristic of economically important reserves in the United States. The effects of processing variables (grinding, thermal treatment, chemical treatment, and mineral separation) have also been studied and the results interpreted in terms of existing and proposed models of coal structure. The techniques and equipment have been described elsewhere (1). Known amounts of pretreated samples are hermetically sealed in Pyrex ampoules, thermally equilibrated in 1 Research sponsored by the Division of Material Sciences, Office of Basic Energy Sciences, U. S. Department of Energy under Contract W-7405-eng-26 with the Union Carbide Corporation.
the calorimeter with the reactant liquid (water in this case), and the rate and amount (2) of heat released are measured when the ampoules are broken. Kinetic analyses (3) aid in interpreting the mechanisms involved in the wetting process. This report is the result of the conscientious and diligent efforts of the members of the Massachusetts Institute of Technology, School of Chemical Engineering Practice (4 - 12). Further information is available (13). VARIATION WITH COAL RANK
The results shown in Fig. 1. show that the heat of immersion, hi, varies markedly with the rank (14) of the coal. The lower ranked coals have a more loosely bound structure (15) and a much higher heteroatom (O, N, etc.) content (16). The heat evolution on wetting is the result of two factors: (A) an intensity factor due to the polarity (polar functional groups, such as carbonyl, phenolic, etc.); and (B) a capacity factor due to the openness and/or structural yielding 577
Journal of Colloid and Interface Science, Vol. 75, No. 2, June 1980
0021-9797/80/060577-07502.00/0 Copyright© 1980by AcademicPress, Inc. All rightsof reproductionin any formreserved.
578
E . L . F U L L E R , JR.
lowed first-order kinetics (3) in all cases: 200 l t80
t60
+ RUN OF !INE m PARAMAGNETIC m DIAMAGNETIC
dh
¢'~
$ 800 °C CHAR
[I
A ALKALIIMPREGNATION
dt
[1]
- k [ h ( t ) - h~],
where h ( t ) = heat of immersion at time t (J/g)
I I ~-
i
_ _ , ~u
,~ = y
~, ,.,,.2o,
JI
~o -I ¥ ,,
v 4o '--
20 $"
2
It 3o;:o.,\ \\2 ~ 30-40
0
~
~ sm~-3o\\ ~ 1 \
]
~'
Y
_z
- .: .
o=
c~
o -
mesh
~
',,
I~ \
Z ~o
,
\1\ \
J
'~\ \
l
]'X \ \
"
o
c
LIGNITE
-
%
~
,
hi = total heat of immersion (J/g) k = first-order rate constant (sec-a).
SUBBITUMINOUS COAL RANK
e
$
Our results for the heat of immersion expressed as the time required for half the heat to evolve, tl/2 = (In 2)/k, for different size fractions are given in Fig. 2 for the Cbituminous coal (Illinois No. 6). This graphically points out that h~ increases monotonically with decreasing particle size (increasing sieve number). However, t,2 does show a marked decrease for the finer sieve fraction. In the intermediate size range,
BITUMINOUS
FIG. 1. Heat of immersion: Variation with rank, mineral content, particle size, thermal effects, and chemical treatment. Significant variations in chemistry and structure are apparent with respect to each of these parameters.
hi = 3.89 + 0.121tl/2
quite accurately. An additional correlation (Fig. 3) shows that ti/2 = 0.051s['~1"96, [3] 4O
induced by the imbibed fluid. Both phenomena are in play to yield the 10-fold increase in h~ for the lower ranked coals. Additional studies are in progress to evaluate the relative contribution of these factors. VARIATION WITH P A R T I C L E SIZE
Samples of coal were obtained from freshly opened mine faces and stored in an inert atmosphere (nitrogen or argon). Grinding with a mortar and pestle, sieving, and storage were all carried out in inert atmospheres to avoid air oxidation (weathering) of the samples prior to calorimetric analyses. The amount of heat evolved on immersion is greater for the smaller size fraction of the ground coal as shown for the Illinois No. 6 and Wyodak samples. Furthermore, the rate of heat evolution is dependent on the particle size. The heat liberation folJournal of Colloid and Interface Science, Vol. 75, No. 2, June 1980
[2]
I
I
>6m~.Omesh
-I
• 16 AS RECEIVED + I6 (30-40) DIAMAGNETIC m I6 ( 3 0 - 4 0 ) PARAMAGNETIC
/
8 ~ 20
--
--
1 ~ 0 - 4 0 mesh
10
3 0 - 7
/+~
Z
--
5 m m - 50 mesh 3o-4o
I 50
mesh I tOO tv2 (s)
I 450
FIG. 2. Heat o f immersion and rate of reaction: Variation with respect to particle size and mineral content. A monotonic increase is noted for hi, whereas the rate function tv2 increases and then decreases for very fine particles.
579
STRUCTURE AND CHEMISTRY OF COALS
300 200
I00
S 5O
20
0 I 0
/
I 20
i
I
I
I ,li
50
t00
BOTTOM SCREEN SIZE (mesh units)
FIG. 3. Kinetic variation with particle size: Exponential dependence of the rate upon the particle size is noted in the initial (larger) size range.
where f~ is the bottom sieve number. Similarly it can be shown that hi = 3.89 + 0.00627~ 1'96.
Figures 1 and 2 show that both the enthalpy and kinetic factors are markedly dependent on the mineral content of the coal in proportion to the relative amounts of each fraction present in the coal, i.e., hi (calcd) in Table I. In every case the heat of immersion is greater for the mineral-rich (paramagnetic) fraction due to the greater polarity of the minerals (SIO2, A1203, CaCO3, etc.) and/or the more open structure due to the mineral-organic interface. To some extent the enhanced heat effect for the finer particles may be due to mineral enhancement. The current interest in "catalytic" (14) chemical and structural effects of minerals in coal led us to a study of the effects of alkali salts (20) as added "minerals." Samples were impregnated with sodium hydroxide aqueous solutions and allowed to dry in air at 23 _+ I°C. The results are shown in Figs. 4 and 5. The exothermicity is markedly enhanced due to the structural effects formed in the alkali treatment. More detailed analyses are available (20) to elucidate the chemical and structural changes brought about by the alkali treatment and subsequent thermal treatment.
[4] EFFECT OF CHEMICAL AND THERMAL PRETREATMENT
VARIATION DUE TO MINERAL CONTENT
Given size fractions were subjected to magnetic separation (17) to yield paramagnetic (mineral-rich) and diamagnetic (organic-rich) fractions as noted in Table I. Visual and microscopic examination showed that Kentucky No. 9 coal had considerable amounts of mineral matter admixed with the coal in the form of sandlike particles. The Illinois No. 6 sample was free of admixed minerals but did have a considerable amount of cleat minerals that existed in the original mine bed in addition to the incorporated mineral component (18). Wyodak coal contains only the incorporated minerals. The relative fractions in Table I reflect these observations.
Results for char formed by the 800°C pyrolysis of Texas lignite (21, 22) were TABLE I Ratio Correlation: Linear Correlation of Weight Fractions of Paramagnetic and Diamagnetic Components
Coal
30-40mesh Illinois No. 6 30-40mesh Kentucky No. 9 40-60mesh Wyodak
Paramagnetic fraction
hia (J/g) Calculated
h~ (J/g) Experiment
0.141
-9.5
-8.9
0.413
-19.8
-19.8
0.188
-125.0
-129.6
hi = x(P)hi(P) + x(D)hi(D). Journal of Colloid and Interface Science, Vol. 75, No, 2, June 1980
580
E . L . FULLER, JR. CONCLUSIONS 420
380
" •540 8 E 300 TREATED
26O
/ z~
w 220
18C
--
I WYODAK COAL [
14£ o
_
o
UNTREATED o
10C --
0
i I J I i I i I r F i 60
100
140
180
OUTGASSING TEMPERATURE
220
260
(°C)
FIG. 4. Heat of immersion of Wyodak coal: Variation due to thermal treatment of the original coal and the alkali-treated coal.
used to study structural and chemical changes akin to those encountered in underground gasification (21, 22) and laboratory pyrolysis (23, 24). For comparison, one sample was pyrolyzed with an inert sweep gas (argon) and the other was purged with a reactive sweep gas (hydrogen). Data related directly to pyrolysis (21-24) are given in Fig. 1. Here we note a twofold decrease in hi for the chars formed by pyrolysis of Texas lignite and nearly a fourfold decrease for the hydropyrolyzed char. It seems that the 800°C pyrolysis removes most of the available volatile matter as tars, oils, and gases leaving an open matrix of the inert skeletal material. In contrast, the hydropyrolysis leaves a much more closed matrix possibly due to hydrogenation of some of the matrix to form a nonvolatile contiguous polynuclear aromatic material in the matrix. Gravimetric sorption analyses have shown that the decrease in hi is indeed due to a decrease in the sorption capacity for the pyrolysis products of lignite (12). Journal of Colloid and Interface Science, Vol. 75, N o . 2, J u n e 1980
Coals are known to be very heterogeneous both structurally and chemically (25). In terms of the structural effect, one can envision a "shrinking c o r e " model consistent with the observed enthalpy and kinetic results as depicted in a very idealized form in Fig. 6. Prolonged and/or more stringent grinding seem to accomplish two things: particle size diminution and structural disorder to an increasing depth from the surface. Thus we note that the amount of perturbed (reactive) material increases, explaining the slower rate (increased tl/2) with increasing depth (b) that can be penetrated. The increase in hi is the combined result of the simultaneous increase in disorder (b) and actual decrease in particle size (a + b). This model rationalizes the increase in 11/2 as well as the subsequent decrease for very fine particles. Apparently the somewhat rigid core (a) serves as a fixed substrate and that as the particle size (a + b) decreases sufficiently the core disappears (a = 0) and the resultant ma100
9O
8O
o
70
60 m 50 --
w I
I 'LL'NO'S CO'' 1
40
3o 20
t
[
60
iI
tO0
i
I
140
I
I
180
i
OUTGASSING TEMPERATURE
I
220
i 260
(°C)
FIG. 5. Heat of immersion of Illinois No. 6 coal: Variation due to thermal pretreatment of the original and alkali-treated coal.
STRUCTURE AND CHEMISTRY OF COALS terial flexes much more freely (at a faster rate) to accept the wetting medium. This concept of coal yielding during sorption is entirely consistent with the conclusions based on gravimetric sorption analyses (26) where the amount and energetics of penetration are correlated to the solid-gas interaction potential for various sorbates. This proposed model is quite tenable in terms of the works of others. Grinding of bulk material has long been known to induce structural disorder (27) due to the energy (heat) involved in the process. This effect is usually minimal in " h a r d , " ionic, refractory materials and appreciable in " s o f t , " molecular, nonrefractory materials. Coal most assuredly falls in the latter category, for both the organic and argilliferous components. Furthermore our model is consistent with the general observation, "physical adsorption (sometimes called physisorption) will likely alter the surface structure of a molecular solid adsorbent (such as ice, paraffin, and polymers), but not that of high surface energy, refractory solids (such as the usual metals and metal oxides, and carbon black)" (28). Thus we see that the " s o f t " nature of coal is quite apparent in terms of both internal (sorption) and external (grinding) forces. With respect to the latter, our results correlated well with grinding (29) and related slurry attrition (30) studies. Percussion grinding leads to attrition via fragmentation, spalling, and plastic deformation of the coal itself. Below 24/~m (400 mesh) plastic flow predominates as the battered particles yield without further fracture (29). Consequently we see that grinding processes, designed to increase the reactivity of coal, are extremely effective. The enhancement is brought about by a combined increase in surface area and an increase in accessibility beyond the geometric macroscopic surface of the particles. One should not construe the depiction of Fig. 6 as descriptive, except in the sense of the general concept, since
A
581
B
t,
iiii~i~mb c
FIG. 6. Stylized model for coal grinding: Sequence A to E depicts changes induced by increased severity and/or period of grinding. The geometry is not an accurate depiction of the real case for coal (26). The demarcation between the amorphous layer (shaded, b) and unperturbed substrate (white, a) is probably more diffuse than shown, possibly even a continuous gradation defined by a characteristic depth parameter related to b. the true geometry is much more complex in terms of a leaflet (laminar) structure on the larger framboidal particles (26). This proposed model is a definite oversimplification, for we know that other factors such as mineral content, coal rank (structural order), chemical modifications, thermal modifications, etc., are to be considered in the global analyses. Nonetheless the general features are consistent. Thus we note that " s u r f a c e a r e a " and " p o r o s i t y " evaluations are subject to scrutiny (26) for these calculations invariably assume a rigid matrix and/or adsorption per se (31). Sorption analyses are extremely valuable in evaluating the "accessibility" of various reagents to the internal structure of coals. This report is not the final analysis of the structure and chemistry of coals but merely a preliminary study to show that a straightJournal of Colloid and Interface Science, Vol. 75, No. 2, June 1980
582
E . L . FULLER, JR.
forward systematic application of the proven technique of calorimetric studies can and will aid immensely in our better understanding of the complex structure and chemistry of coals. At this time, when we are turning (or returning) to coal as an important source of energy and chemical feedstocks, such information is of obvious importance. Coal can only be regarded as one more natural resource, following wood, petroleum, etc., which is irreplaceable and required by modern and future societies. Judicious and efficient processes are imperative to assure maximum utilization and conservation of coal. ACKNOWLEDGMENTS First and foremost I must acknowledge the direct and indirect influence of A. C. Zettlemoyer with respect to this work. He, as an educator, researcher, lecturer, administrator, editor, etc., has had a marked influence. He has always made himself available at conferences, symposia, Lehigh University, and even at his kitchen table (in the early morning hours when an unannounced visitor calls). Thank you AI! One cannot overrate the importance of understanding and learned co-workers. R. A. Strehlow and K. Fallen have been patient and helpful in many discussions.
REFERENCES 1. Holmes, H. F., and Secoy, C. H.,J. Phys. Chem. 69, 151 (1965). 2. Abadi, M. J., Geary, C. T., and Sung, W. F., "Characterization of Porous Catalysts and Catalyst-Support Interactions," ORNL/MIT261, November 1977. 3. Fuller, E. L., Jr., Holmes, H. F., Stuckey, C. H., and Secoy, C. H. ,J. Phys. Chem. 72,573 (1968). 4. Sunberg, D. G., Abadi, M. J., and Giroux, M. S., "Surface Properties of Coal," ORNL/MIT264, March 1978. 5. Alger, M. M., Chow, O. K., and Kahn, M. Z., "Surface Properties and Reactions of Coal," ORNL/MIT-266, March 1978. 6. Field, L. A., Papadopoulas, A. J., and Wang, R. D., "Surface Properties and Reactions of Coal, Part 2," ORNL/MIT-270, March 1978.
Journal of Colloid and Interface Science, Vol. 75, No. 2. June 1980
7. Caron, R. N., Fallon, K. J., Orrik, J. F., and Roux-Buisson, J. L., "Surface Properties of Wyodak Coal," ORNL/MIT-273, May 1978. 8. Dillon, J. J., Talbot, C. M., and Zimmer, M. F., "Surface Properties of Alkali:Treated Wyodak Coal," ORNL/MIT-278, September 1978. 9. Zimmer, M. F., Kausch, W. J., and MarreroAldea, R. J., "Surface Properties of NaOH Treated Coals," ORNL/MIT-280, October 1978. 10. Yu, P. P., and Makarewicz, M. A., "Surface Properties of Magnetically Separated Coal Components," ORNL/MIT-289, March 1979. 11. Alexander, G. L., Hu, B. V., and Kwai, A. H., "Coal Block Pyrolysis: Effects of Changing Surface Characteristics," ORNL/MIT-294, May 1979. 12. Metsa, J. C., Hart, R. P., and Mullins, M. E., "Surface Characterization of Texas Lignite," ORNL/MIT, in press. 13. References (2) and (4) through (12) are available: National Technical Information Service, U. S. Department of Commerce, 5285 Port Royal Road, Springfield, Virginia 22161. 14. Veraa, M. J., and Bell, A. T., Fuel 57, 194 (1978). 15. Hirsch, P. B., Proc. Roy. Soc. London Ser. A 226, 143 (1955). 16. Hall, G. R., and Lyon, L. B., Ind. Eng. Chem. 56, 36 (1962); Wiser, W. H., and Anderson, L. L., Annu. Rev. Phys. Chem. 26, 339 (1975). 17. Hise, E. C., "Correlation of Physical Coal Separations: Part 1," ORNL-5570, September 1979; Maxwell, E., and Kelland, D. R., "Magnetite Recovery in Coal Washing by High Gradient Magnetic Separation," FE-8887-1 (EPA-600/778-183), October 1977; Sealy, G. D., and Howell, W. F . , " Magnetic Recovery of Medium from Heavy Media Circuits," World Coal, June 1977; Palazzi, M. R., " H o w Do You Choose a Magnetic Separator?" Coal Mining and Processing, February 1967. 18. Strehlow, R. A., Harris, L. A., and Yust, C. S., Fuel 57, 185 (1978). 19. Cusumano, J. A., Dalla Betta, R. A., and Levy, R. B., in "Catalysis in Coal Conversion." (J. A. Cusumano and A. Farkus, Eds.). Academic Press, New York, 1978. 20. Senkan, S . M., and Fuller, E. L., Jr., Fuel 58, 729 (1979). 21. Westmoreland, P. R., and Dickerson, L. S., "Pyrolysis of Blocks of Texas Lignite," Presented at 86th AIChE National Meeting, Houston, Texas, 1979. 22. Westmoreland, P. R., "proceedings, 4th Underground Coal Conversion Symposium," p. 423, SAND 78-0941, (1978).
STRUCTURE AND CHEMISTRY OF COALS 23. Anthony, D. B., and Howard, J. B., A I C h E J. 22, 625 (1976). 24. Suuberg, E. M., Peters, W. A., and Howard, J. B., "17th International Symposium on Combustion," p. 117 (1979). 25. Gorbaty, M. L., Wright, F. J., Lyon, R. K., Long, R. B., Schlosberg, R. H., Baset, F., Liotta, R., Silbernagel, B. G., and Neskora, D. R., Science, 206, 1029 (1979). 26. Fuller, E. L., Jr., in "Coal Structure" (M. Gorbaty, Ed.), Advances in Chemistry Series, in press (1979).
583
27. Fuller, E. L., Jr., J. Colloid Interface Sci. 55, 358 (1976). 28. Adamson, A. W., "Physical Chemistry of Surfaces." Wiley, New York, 1976. 29. Luckie, P. T., and Austin, L. G., Mineral Sci. Eng. 9, 24 (1972). 30. Shook, C. A., Haas, D. B., Husband, W. H. W., and Small, M., Canad. J. Chem. Eng. 56, 448 (1978). 31. Gregg, S. J., and Sing, K. S. W., "Adsorption, Surface Area and Porosity." Academic Press, London/New York, 1967.
Journal of Colloid and Interface Science, Vol. 75, No. 2. June 1980
Coal has become an increasingly important resource for energy and chemical feedstock as our petroleum resources are waning. Most preparative and processing techniques require fluid-phase access into the complex structure of the coal. This study is the result of calorimetric analyses of the thermodynamics and kinetics of fluid-phase access into coals of varying rank, characteristic of economically important reserves in the United States. The effects of processing variables (grinding, thermal treatment, chemical treatment, and mineral separation) have also been studied and the results interpreted in terms of existing and proposed models of coal structure. The techniques and equipment have been described elsewhere (1). Known amounts of pretreated samples are hermetically sealed in Pyrex ampoules, thermally equilibrated in 1 Research sponsored by the Division of Material Sciences, Office of Basic Energy Sciences, U. S. Department of Energy under Contract W-7405-eng-26 with the Union Carbide Corporation.
the calorimeter with the reactant liquid (water in this case), and the rate and amount (2) of heat released are measured when the ampoules are broken. Kinetic analyses (3) aid in interpreting the mechanisms involved in the wetting process. This report is the result of the conscientious and diligent efforts of the members of the Massachusetts Institute of Technology, School of Chemical Engineering Practice (4 - 12). Further information is available (13). VARIATION WITH COAL RANK
The results shown in Fig. 1. show that the heat of immersion, hi, varies markedly with the rank (14) of the coal. The lower ranked coals have a more loosely bound structure (15) and a much higher heteroatom (O, N, etc.) content (16). The heat evolution on wetting is the result of two factors: (A) an intensity factor due to the polarity (polar functional groups, such as carbonyl, phenolic, etc.); and (B) a capacity factor due to the openness and/or structural yielding 577
Journal of Colloid and Interface Science, Vol. 75, No. 2, June 1980
0021-9797/80/060577-07502.00/0 Copyright© 1980by AcademicPress, Inc. All rightsof reproductionin any formreserved.
578
E . L . F U L L E R , JR.
lowed first-order kinetics (3) in all cases: 200 l t80
t60
+ RUN OF !INE m PARAMAGNETIC m DIAMAGNETIC
dh
¢'~
$ 800 °C CHAR
[I
A ALKALIIMPREGNATION
dt
[1]
- k [ h ( t ) - h~],
where h ( t ) = heat of immersion at time t (J/g)
I I ~-
i
_ _ , ~u
,~ = y
~, ,.,,.2o,
JI
~o -I ¥ ,,
v 4o '--
20 $"
2
It 3o;:o.,\ \\2 ~ 30-40
0
~
~ sm~-3o\\ ~ 1 \
]
~'
Y
_z
- .: .
o=
c~
o -
mesh
~
',,
I~ \
Z ~o
,
\1\ \
J
'~\ \
l
]'X \ \
"
o
c
LIGNITE
-
%
~
,
hi = total heat of immersion (J/g) k = first-order rate constant (sec-a).
SUBBITUMINOUS COAL RANK
e
$
Our results for the heat of immersion expressed as the time required for half the heat to evolve, tl/2 = (In 2)/k, for different size fractions are given in Fig. 2 for the Cbituminous coal (Illinois No. 6). This graphically points out that h~ increases monotonically with decreasing particle size (increasing sieve number). However, t,2 does show a marked decrease for the finer sieve fraction. In the intermediate size range,
BITUMINOUS
FIG. 1. Heat of immersion: Variation with rank, mineral content, particle size, thermal effects, and chemical treatment. Significant variations in chemistry and structure are apparent with respect to each of these parameters.
hi = 3.89 + 0.121tl/2
quite accurately. An additional correlation (Fig. 3) shows that ti/2 = 0.051s['~1"96, [3] 4O
induced by the imbibed fluid. Both phenomena are in play to yield the 10-fold increase in h~ for the lower ranked coals. Additional studies are in progress to evaluate the relative contribution of these factors. VARIATION WITH P A R T I C L E SIZE
Samples of coal were obtained from freshly opened mine faces and stored in an inert atmosphere (nitrogen or argon). Grinding with a mortar and pestle, sieving, and storage were all carried out in inert atmospheres to avoid air oxidation (weathering) of the samples prior to calorimetric analyses. The amount of heat evolved on immersion is greater for the smaller size fraction of the ground coal as shown for the Illinois No. 6 and Wyodak samples. Furthermore, the rate of heat evolution is dependent on the particle size. The heat liberation folJournal of Colloid and Interface Science, Vol. 75, No. 2, June 1980
[2]
I
I
>6m~.Omesh
-I
• 16 AS RECEIVED + I6 (30-40) DIAMAGNETIC m I6 ( 3 0 - 4 0 ) PARAMAGNETIC
/
8 ~ 20
--
--
1 ~ 0 - 4 0 mesh
10
3 0 - 7
/+~
Z
--
5 m m - 50 mesh 3o-4o
I 50
mesh I tOO tv2 (s)
I 450
FIG. 2. Heat o f immersion and rate of reaction: Variation with respect to particle size and mineral content. A monotonic increase is noted for hi, whereas the rate function tv2 increases and then decreases for very fine particles.
579
STRUCTURE AND CHEMISTRY OF COALS
300 200
I00
S 5O
20
0 I 0
/
I 20
i
I
I
I ,li
50
t00
BOTTOM SCREEN SIZE (mesh units)
FIG. 3. Kinetic variation with particle size: Exponential dependence of the rate upon the particle size is noted in the initial (larger) size range.
where f~ is the bottom sieve number. Similarly it can be shown that hi = 3.89 + 0.00627~ 1'96.
Figures 1 and 2 show that both the enthalpy and kinetic factors are markedly dependent on the mineral content of the coal in proportion to the relative amounts of each fraction present in the coal, i.e., hi (calcd) in Table I. In every case the heat of immersion is greater for the mineral-rich (paramagnetic) fraction due to the greater polarity of the minerals (SIO2, A1203, CaCO3, etc.) and/or the more open structure due to the mineral-organic interface. To some extent the enhanced heat effect for the finer particles may be due to mineral enhancement. The current interest in "catalytic" (14) chemical and structural effects of minerals in coal led us to a study of the effects of alkali salts (20) as added "minerals." Samples were impregnated with sodium hydroxide aqueous solutions and allowed to dry in air at 23 _+ I°C. The results are shown in Figs. 4 and 5. The exothermicity is markedly enhanced due to the structural effects formed in the alkali treatment. More detailed analyses are available (20) to elucidate the chemical and structural changes brought about by the alkali treatment and subsequent thermal treatment.
[4] EFFECT OF CHEMICAL AND THERMAL PRETREATMENT
VARIATION DUE TO MINERAL CONTENT
Given size fractions were subjected to magnetic separation (17) to yield paramagnetic (mineral-rich) and diamagnetic (organic-rich) fractions as noted in Table I. Visual and microscopic examination showed that Kentucky No. 9 coal had considerable amounts of mineral matter admixed with the coal in the form of sandlike particles. The Illinois No. 6 sample was free of admixed minerals but did have a considerable amount of cleat minerals that existed in the original mine bed in addition to the incorporated mineral component (18). Wyodak coal contains only the incorporated minerals. The relative fractions in Table I reflect these observations.
Results for char formed by the 800°C pyrolysis of Texas lignite (21, 22) were TABLE I Ratio Correlation: Linear Correlation of Weight Fractions of Paramagnetic and Diamagnetic Components
Coal
30-40mesh Illinois No. 6 30-40mesh Kentucky No. 9 40-60mesh Wyodak
Paramagnetic fraction
hia (J/g) Calculated
h~ (J/g) Experiment
0.141
-9.5
-8.9
0.413
-19.8
-19.8
0.188
-125.0
-129.6
hi = x(P)hi(P) + x(D)hi(D). Journal of Colloid and Interface Science, Vol. 75, No, 2, June 1980
580
E . L . FULLER, JR. CONCLUSIONS 420
380
" •540 8 E 300 TREATED
26O
/ z~
w 220
18C
--
I WYODAK COAL [
14£ o
_
o
UNTREATED o
10C --
0
i I J I i I i I r F i 60
100
140
180
OUTGASSING TEMPERATURE
220
260
(°C)
FIG. 4. Heat of immersion of Wyodak coal: Variation due to thermal treatment of the original coal and the alkali-treated coal.
used to study structural and chemical changes akin to those encountered in underground gasification (21, 22) and laboratory pyrolysis (23, 24). For comparison, one sample was pyrolyzed with an inert sweep gas (argon) and the other was purged with a reactive sweep gas (hydrogen). Data related directly to pyrolysis (21-24) are given in Fig. 1. Here we note a twofold decrease in hi for the chars formed by pyrolysis of Texas lignite and nearly a fourfold decrease for the hydropyrolyzed char. It seems that the 800°C pyrolysis removes most of the available volatile matter as tars, oils, and gases leaving an open matrix of the inert skeletal material. In contrast, the hydropyrolysis leaves a much more closed matrix possibly due to hydrogenation of some of the matrix to form a nonvolatile contiguous polynuclear aromatic material in the matrix. Gravimetric sorption analyses have shown that the decrease in hi is indeed due to a decrease in the sorption capacity for the pyrolysis products of lignite (12). Journal of Colloid and Interface Science, Vol. 75, N o . 2, J u n e 1980
Coals are known to be very heterogeneous both structurally and chemically (25). In terms of the structural effect, one can envision a "shrinking c o r e " model consistent with the observed enthalpy and kinetic results as depicted in a very idealized form in Fig. 6. Prolonged and/or more stringent grinding seem to accomplish two things: particle size diminution and structural disorder to an increasing depth from the surface. Thus we note that the amount of perturbed (reactive) material increases, explaining the slower rate (increased tl/2) with increasing depth (b) that can be penetrated. The increase in hi is the combined result of the simultaneous increase in disorder (b) and actual decrease in particle size (a + b). This model rationalizes the increase in 11/2 as well as the subsequent decrease for very fine particles. Apparently the somewhat rigid core (a) serves as a fixed substrate and that as the particle size (a + b) decreases sufficiently the core disappears (a = 0) and the resultant ma100
9O
8O
o
70
60 m 50 --
w I
I 'LL'NO'S CO'' 1
40
3o 20
t
[
60
iI
tO0
i
I
140
I
I
180
i
OUTGASSING TEMPERATURE
I
220
i 260
(°C)
FIG. 5. Heat of immersion of Illinois No. 6 coal: Variation due to thermal pretreatment of the original and alkali-treated coal.
STRUCTURE AND CHEMISTRY OF COALS terial flexes much more freely (at a faster rate) to accept the wetting medium. This concept of coal yielding during sorption is entirely consistent with the conclusions based on gravimetric sorption analyses (26) where the amount and energetics of penetration are correlated to the solid-gas interaction potential for various sorbates. This proposed model is quite tenable in terms of the works of others. Grinding of bulk material has long been known to induce structural disorder (27) due to the energy (heat) involved in the process. This effect is usually minimal in " h a r d , " ionic, refractory materials and appreciable in " s o f t , " molecular, nonrefractory materials. Coal most assuredly falls in the latter category, for both the organic and argilliferous components. Furthermore our model is consistent with the general observation, "physical adsorption (sometimes called physisorption) will likely alter the surface structure of a molecular solid adsorbent (such as ice, paraffin, and polymers), but not that of high surface energy, refractory solids (such as the usual metals and metal oxides, and carbon black)" (28). Thus we see that the " s o f t " nature of coal is quite apparent in terms of both internal (sorption) and external (grinding) forces. With respect to the latter, our results correlated well with grinding (29) and related slurry attrition (30) studies. Percussion grinding leads to attrition via fragmentation, spalling, and plastic deformation of the coal itself. Below 24/~m (400 mesh) plastic flow predominates as the battered particles yield without further fracture (29). Consequently we see that grinding processes, designed to increase the reactivity of coal, are extremely effective. The enhancement is brought about by a combined increase in surface area and an increase in accessibility beyond the geometric macroscopic surface of the particles. One should not construe the depiction of Fig. 6 as descriptive, except in the sense of the general concept, since
A
581
B
t,
iiii~i~mb c
FIG. 6. Stylized model for coal grinding: Sequence A to E depicts changes induced by increased severity and/or period of grinding. The geometry is not an accurate depiction of the real case for coal (26). The demarcation between the amorphous layer (shaded, b) and unperturbed substrate (white, a) is probably more diffuse than shown, possibly even a continuous gradation defined by a characteristic depth parameter related to b. the true geometry is much more complex in terms of a leaflet (laminar) structure on the larger framboidal particles (26). This proposed model is a definite oversimplification, for we know that other factors such as mineral content, coal rank (structural order), chemical modifications, thermal modifications, etc., are to be considered in the global analyses. Nonetheless the general features are consistent. Thus we note that " s u r f a c e a r e a " and " p o r o s i t y " evaluations are subject to scrutiny (26) for these calculations invariably assume a rigid matrix and/or adsorption per se (31). Sorption analyses are extremely valuable in evaluating the "accessibility" of various reagents to the internal structure of coals. This report is not the final analysis of the structure and chemistry of coals but merely a preliminary study to show that a straightJournal of Colloid and Interface Science, Vol. 75, No. 2, June 1980
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E . L . FULLER, JR.
forward systematic application of the proven technique of calorimetric studies can and will aid immensely in our better understanding of the complex structure and chemistry of coals. At this time, when we are turning (or returning) to coal as an important source of energy and chemical feedstocks, such information is of obvious importance. Coal can only be regarded as one more natural resource, following wood, petroleum, etc., which is irreplaceable and required by modern and future societies. Judicious and efficient processes are imperative to assure maximum utilization and conservation of coal. ACKNOWLEDGMENTS First and foremost I must acknowledge the direct and indirect influence of A. C. Zettlemoyer with respect to this work. He, as an educator, researcher, lecturer, administrator, editor, etc., has had a marked influence. He has always made himself available at conferences, symposia, Lehigh University, and even at his kitchen table (in the early morning hours when an unannounced visitor calls). Thank you AI! One cannot overrate the importance of understanding and learned co-workers. R. A. Strehlow and K. Fallen have been patient and helpful in many discussions.
REFERENCES 1. Holmes, H. F., and Secoy, C. H.,J. Phys. Chem. 69, 151 (1965). 2. Abadi, M. J., Geary, C. T., and Sung, W. F., "Characterization of Porous Catalysts and Catalyst-Support Interactions," ORNL/MIT261, November 1977. 3. Fuller, E. L., Jr., Holmes, H. F., Stuckey, C. H., and Secoy, C. H. ,J. Phys. Chem. 72,573 (1968). 4. Sunberg, D. G., Abadi, M. J., and Giroux, M. S., "Surface Properties of Coal," ORNL/MIT264, March 1978. 5. Alger, M. M., Chow, O. K., and Kahn, M. Z., "Surface Properties and Reactions of Coal," ORNL/MIT-266, March 1978. 6. Field, L. A., Papadopoulas, A. J., and Wang, R. D., "Surface Properties and Reactions of Coal, Part 2," ORNL/MIT-270, March 1978.
Journal of Colloid and Interface Science, Vol. 75, No. 2. June 1980
7. Caron, R. N., Fallon, K. J., Orrik, J. F., and Roux-Buisson, J. L., "Surface Properties of Wyodak Coal," ORNL/MIT-273, May 1978. 8. Dillon, J. J., Talbot, C. M., and Zimmer, M. F., "Surface Properties of Alkali:Treated Wyodak Coal," ORNL/MIT-278, September 1978. 9. Zimmer, M. F., Kausch, W. J., and MarreroAldea, R. J., "Surface Properties of NaOH Treated Coals," ORNL/MIT-280, October 1978. 10. Yu, P. P., and Makarewicz, M. A., "Surface Properties of Magnetically Separated Coal Components," ORNL/MIT-289, March 1979. 11. Alexander, G. L., Hu, B. V., and Kwai, A. H., "Coal Block Pyrolysis: Effects of Changing Surface Characteristics," ORNL/MIT-294, May 1979. 12. Metsa, J. C., Hart, R. P., and Mullins, M. E., "Surface Characterization of Texas Lignite," ORNL/MIT, in press. 13. References (2) and (4) through (12) are available: National Technical Information Service, U. S. Department of Commerce, 5285 Port Royal Road, Springfield, Virginia 22161. 14. Veraa, M. J., and Bell, A. T., Fuel 57, 194 (1978). 15. Hirsch, P. B., Proc. Roy. Soc. London Ser. A 226, 143 (1955). 16. Hall, G. R., and Lyon, L. B., Ind. Eng. Chem. 56, 36 (1962); Wiser, W. H., and Anderson, L. L., Annu. Rev. Phys. Chem. 26, 339 (1975). 17. Hise, E. C., "Correlation of Physical Coal Separations: Part 1," ORNL-5570, September 1979; Maxwell, E., and Kelland, D. R., "Magnetite Recovery in Coal Washing by High Gradient Magnetic Separation," FE-8887-1 (EPA-600/778-183), October 1977; Sealy, G. D., and Howell, W. F . , " Magnetic Recovery of Medium from Heavy Media Circuits," World Coal, June 1977; Palazzi, M. R., " H o w Do You Choose a Magnetic Separator?" Coal Mining and Processing, February 1967. 18. Strehlow, R. A., Harris, L. A., and Yust, C. S., Fuel 57, 185 (1978). 19. Cusumano, J. A., Dalla Betta, R. A., and Levy, R. B., in "Catalysis in Coal Conversion." (J. A. Cusumano and A. Farkus, Eds.). Academic Press, New York, 1978. 20. Senkan, S . M., and Fuller, E. L., Jr., Fuel 58, 729 (1979). 21. Westmoreland, P. R., and Dickerson, L. S., "Pyrolysis of Blocks of Texas Lignite," Presented at 86th AIChE National Meeting, Houston, Texas, 1979. 22. Westmoreland, P. R., "proceedings, 4th Underground Coal Conversion Symposium," p. 423, SAND 78-0941, (1978).
STRUCTURE AND CHEMISTRY OF COALS 23. Anthony, D. B., and Howard, J. B., A I C h E J. 22, 625 (1976). 24. Suuberg, E. M., Peters, W. A., and Howard, J. B., "17th International Symposium on Combustion," p. 117 (1979). 25. Gorbaty, M. L., Wright, F. J., Lyon, R. K., Long, R. B., Schlosberg, R. H., Baset, F., Liotta, R., Silbernagel, B. G., and Neskora, D. R., Science, 206, 1029 (1979). 26. Fuller, E. L., Jr., in "Coal Structure" (M. Gorbaty, Ed.), Advances in Chemistry Series, in press (1979).
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27. Fuller, E. L., Jr., J. Colloid Interface Sci. 55, 358 (1976). 28. Adamson, A. W., "Physical Chemistry of Surfaces." Wiley, New York, 1976. 29. Luckie, P. T., and Austin, L. G., Mineral Sci. Eng. 9, 24 (1972). 30. Shook, C. A., Haas, D. B., Husband, W. H. W., and Small, M., Canad. J. Chem. Eng. 56, 448 (1978). 31. Gregg, S. J., and Sing, K. S. W., "Adsorption, Surface Area and Porosity." Academic Press, London/New York, 1967.
Journal of Colloid and Interface Science, Vol. 75, No. 2. June 1980