Energy & Fuels 1996, 10, 631-640
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Effect of Different Drying Methods on Coal Structure and Reactivity Toward Liquefaction Francis P. Miknis,* Daniel A. Netzel, Thomas F. Turner, Jefferey C. Wallace, and Clint H. Butcher Western Research Institute, 365 North 9th Street, Laramie, Wyoming 82070-3380 Received December 15, 1995. Revised Manuscript Received February 21, 1996X
Different methods of drying coal were investigated to determine if drying can be accomplished without altering the coal structure and reactivity toward liquefaction. Coal-drying methods included thermal and microwave drying at elevated temperatures and chemical drying at low temperature. Six coals from lignite to high volatile bituminous rank were studied. Laboratoryscale liquefaction experiments were carried out on premoisturized and dried coals to determine the effects of different drying methods on liquefaction yields. Solid-state nuclear magnetic resonance (NMR) and swelling measurements were made to assess any changes in coal structure brought about by the different methods of drying. The NMR measurements showed that, in general, there were no major structural changes in coals dried thermally or with microwaves other than partial decarboxylation. Chemically dried coals exhibited increased resolution in the aliphatic carbon region that was attributed to adsorbed methanol, which was a reaction product as well as solvent for the chemical drying method. The swelling ratios of thermally dried and microwave-dried coals were lower than those of premoisturized coals, indicating a greater degree of cross linking in coals dried using these methods. The swelling ratios of the chemically dried coals were greater than those of the premoisturized coals. Coals that were dried or partially dried thermally and with microwaves had lower liquefaction conversions than coals containing equilibrium moisture contents. However, chemically dried coals had conversions ranging from 11 to 60% greater than the premoisturized coals. The conversion behavior is consistent with changes in the physical structure and cross-linking reactions because of drying. Thermal and microwave drying appeared to cause a collapse in the pore structure, thus preventing donor solvents from contacting reactive sites inside the coals. Chemical dehydration did not appear to collapse the pore structure.
Introduction Although great strides have been made in developing the technology of coal liquefaction processes in recent years, unsolved problems still remain before a viable and economical process can be achieved. The technological problems that still exist can be solved through a more fundamental understanding of the chemistry associated with each stage of the coal liquefaction process, starting with any pretreatment steps that may be carried out on the coal itself.1,2 One pretreatment process that can improve the economics of coal liquefaction is coal drying, particularly for the lower rank coals. There are a number of theoretical advantages to drying coal prior to liquefaction. Because direct liquefaction conversion and kinetics are heavily dependent upon the partial pressure of hydrogen in the liquefaction reactor, the removal of moisture in the coal allows the partial pressure of hydrogen to be increased without increasing the operating pressure. Eliminating the partial pressure of steam in the reactor increases the partial pressures of hydrogen and other gaseous components if the reactor operating pressure is held constant. In addition, removal of Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Rao, S. N.; Schindler, H. D.; McGurl, G. V. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1988, 33 (3), 145-156. (2) Coal Liquefaction: A Research & Development Needs Assessment. DOE/ER-0400; U.S. Department of Energy: Washington, DC, 1989; Vol. 1. X
0887-0624/96/2510-0631$12.00/0
most of the water from the coal prior to liquefaction reduces the cost of both separating water from the coal and waste water treatment. Water removed from the coal at low temperature presently can be vented to the atmosphere as steam without treatment. Finally, moisture in the coal reduces the throughput of coal on a dry basis if the liquefaction reactor has a fixed capacity. However, there is considerable evidence to show that some methods of drying have a detrimental effect on the liquefaction behavior of coals.3-11 Silver and Frazee3 dried a Clovis Point subbituminous coal using a variety of methodssdifferent atmospheres, solvent drying, (3) Silver, H. F.; Frazee, W. S. Integrated Two-Stage Coal Liquefaction Studies. EPRI Report AP-4193; University of Wyoming: Laramie, WY, 1985; p 460. (4) Silver, H.; Hallinan, P. J.; Frazee,W. S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1986, 31 (3), 755. (5) Serio, M. A.; Solomon, P. R.; Kroo, E.; Bassilakis, R.; Malhotra, R.; McMillen, D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35 (1), 61. (6) Serio, M.; Kroo, E.; Teng, H.; Solomon, P. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (2), 577. (7) Saini, A. K.; Song C.; Shobert, H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (2), 593. (8) Saini, A. K.; Song, C.; Shobert, H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (2), 601. (9) Okuma, O.; Masuda, M.; Murakoshi, K.; Yanai, S.; Matsumura, T. Nenryo Kyokaishi 1990, 69, 259. (10) Song, C.; Saini, A. K.; Schobert, H. H. Energy Fuels 1994, 8, 301. (11) Miknis, F. P.; Netzel, D. A.; Turner, T. F. A. C .Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38 (2), 609.
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vacuum drying, and microwave drying. In all cases, the reactivities of the completely dried coals were lower than the reactivity of the original coal. Coals have both a physical and chemical structure, so it is conceivable that drying affects one, the other, or both of these structures. Newer instrumental techniques, such as solid-state NMR, have made it possible to systematically study the role of water on coal structure and reactivity, and such studies are beginning to appear.8-11 The problem that needs to be solved is that of drying coal without adversely affecting its liquefaction reactivity. This is particularly true for subbituminous coals and lignites for which the U.S. reserves are huge. These coals contain significant amounts of water so that simply drying these materials before transportation to their final destination can represent a sizeable reduction in cost.12 Therefore, to derive economic benefits from coal drying, the causes of the lower reactivity of dried coals must be understood, or other methods of drying must be developed that circumvent this problem. Thermal Drying. The most common method of coal drying is to heat the coal in air to some elevated temperature to drive off the moisture. Even a simple process such as this can cause changes in coals, particularly low-rank coals. For example, subbituminous coals contract and crack on drying. Furthermore, only about 60% of the moisture removed can be reversibly replaced. This is attributed to collapse in the gel structure of the low-rank coals.13 On the other hand, a dried Illinois bituminous coal can reabsorb about the same amount of water as it loses in drying. However, some coals dried at room temperature reabsorb more water than when they are dried at a higher temperature.13 This suggests that drying does not significantly alter the physical structure of bituminous coals, but because water is an intimate part of the gel structure of low-rank coals, drying might alter the physical structure of these coals. Thermal drying can cause changes in the chemical structure of the coal, which in turn can affect the liquefaction properties. Low-rank coals show a strong tendency to form cross-links even at low temperature because of the high concentration of organic oxygen functionalities such as carboxyl, hydroxyl, and methoxyl. These functional groups are involved in condensation reactions and free-radical reactions stabilized by cross-links. Song et al.10 have shown that drying and oxidation at 100-150 °C affected the structure of a Wyoming subbituminous coal and its reactivity during liquefaction. Measurement of the volumetric swelling (12) Atherton, L. F. Proc. Int. Conf. Coal Sci. 1985, 553-556. (13) Evans, D. G. Fuel 1973, 52, 186-190. (14) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935-938. (15) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985, 64, 16681671. (16) Suuberg, E. M.; Unger, P. E.; Larsen, J. W. Energy Fuels 1987, 1, 305-308. (17) Suuberg, E. M.; Otake, Y.; Yun, Y.; Deevi, S. C. Energy Fuels 1993, 7, 384. (18) Deshpande, G. V.; Solomon, P. R.; Serio, M. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1988, 33 (2), 310-321. (19) Ibarra, J. V.; Cervero, I.; Garcia, M.; Moliner, R. Fuel Process. Technol. 1990, 24, 19-25. (20) Gethner, J. S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1986, 31 (4), 103-110. (21) Chatterjee, I.; Misra, M. Miner. Metall. Process. 1991, 8, 110. (22) Donoghue, J. T.; Drago, R. S. Inorg. Chem. 1962, 1, 866-872. (23) Adams, R. F. J. Chromatogr. 1974, 95, 189-212. (24) Muller, L. L.; Jacks, T. J. J. Histochem. Cytochem. 1975, 23, 107-110. (25) Critchfield, F. E.; Bishop, E. T. Anal. Chem. 1961, 33, 10341033.
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ratio using the procedure of Green et al.14 can be used to determine the degree of cross-linking that forms during coal processing.15-19 Cross-linking also produces simple molecules such as CO2, H2O, and CH4 so that correlations have been developed to relate the degree of cross-linking to gas evolution.15-18 Drying in air can also produce CO2 and H2O as a result of low-temperature oxidation reactions, which can be very rapid at 1, the parameter represents a cross-linking coal structure less than that of the initial coal. Table 4 lists the parameter 1 - X for the initial coals and dried coals. As shown in the table, the swelling index profile parameters (1 - X) for the thermally dried coals are all less than those for the initial coals, indicating an increase in the cross-linking structure due to thermal heating of the coals. In general, the lower rank coals (Texas Bottom and North Dakota Beulah) demonstrated a greater degree of cross-linking during thermal drying than did the higher rank coals. For the thermally dried coals, some degree of cross-linking had occurred with the loss of moisture and at the relatively low temperature of 105 °C. The change in coal structure as a result of cross-linking is probably due to partial devolatilization and decarboxylation processes. Microwave drying decreased the swelling index profile parameter of the coals relative to the coals before heating (Table 4). The parameter values are less than those measured for the thermally dried coals with the exception of Wyoming Eagle Butte and Texas Bottom coals. Thus, microwave radiation induces still more cross-linking in the coal structure than thermal heating. These changes do not appear to affect the overall carbon distribution significantly, since major changes are not observed in the NMR spectra. It is assumed that microwave radiation produces intense localized heating in the coals, causing some loss of phenolic groups and more devolatilization and decarboxylation than thermally heating the coals at 105 °C, and thus it promotes additional cross-linking. The swelling index profile parameters of the chemically dehydrated coals are given in Table 4. The parameter for Illinois No. 6 and Texas Bottom coals shows a small decrease relative to the swelling index parameter of the initial coals, whereas Utah Blind Canyon, Wyoming Eagle Butte, Wyoming Black Thunder, and North Dakota Beulah coals show a significant
Coal Structure and Reactivity
Energy & Fuels, Vol. 10, No. 3, 1996 637 Table 5. Summary of Coal Liquefaction Results
thermally dried
microwave dried
% converted
% liquid producta
% gas
% moisture removed
0.0 100.0
70.0 69.8
61.0 67.9
9.1 1.9
0.0 100.0
0.0 13.0 89.0 100.0
67.6 74.2 71.3 66.0
64.3 70.6 67.5 61.3
3.3 3.6 3.8 4.7
% moisture removed
0.0 14.0 82.0 100.0 0.0 23.0 70.0 100.0 0.0 38.0 75.0 100.0 0.0 100.0 aExpressed
73.2 72.5 67.6 55.0 79.0 64.9 66.4 67.1 82.4 85.1 70.3 73.3 74.4 58.4
62.3 59.9 56.8 42.1 67.6 54.6 57.0 56.0 70.0 74.1 59.5 62.7 38.8 45.7
10.9 12.7 10.8 12.9 11.4 10.3 9.5 11.1 12.4 11.0 10.8 10.6 35.6 12.6
% liquid producta
chemically dried
% gas
% moisture removed
% converted
% liquid producta
% gas
Utah Blind Canyon 70.0 61.0 74.4 72.6
9.1 1.7
0.0 100.0
70.0 67.7
61.0 63.7
9.1 4.0
0.0 33.0 79.0 100.0
Illinois No. 6 67.6 64.3 70.0 66.5 70.9 67.3 66.3 62.6
3.4 3.6 3.6 3.8
0.0
67.6
64.3
3.4
100.0
79.8
74.4
5.3
0.0 32.0 73.0 100.0
Wyoming Eagle Butte 73.2 62.3 69.2 57.8 62.2 51.3 58.8 46.5
10.9 11.4 10.9 12.3
0.0
73.2
62.3
10.9
100.0
85.1
75.1
10.0
0.0 30.0 73.0 100.0
Wyoming Black Thunder 79.0 67.6 69.3 58.6 63.0 52.9 68.3 58.3
11.4 10.7 10.1 10.0
0.0
79.0
67.6
11.4
100.0
82.1
72.4
9.7
0.0 30.0 78.0 100.0
Texas Bottom 82.4 70.0 79.9 68.1 76.8 66.4 63.5 51.0
12.4 11.8 10.4 12.5
0.0
82.4
70.0
12.4
100.0
87.4
77.2
10.2
0.0 100.0
North Dakota Lignite 74.4 38.8 53.0 38.6
35.6 14.4
0.0 100.0
74.4 74.2
38.8 62.1
35.6 12.1
% converted
as wt %.
increase. The data suggest that only a small increase or no change in the cross-linking internal structure for the Illinois No. 6 and Texas Bottom coals occurred as a result of chemical dehydration. However, a significant decrease in the cross-linking structure (1 - X > 1) is observed for the other four coals because of chemical dehydration. Water in coal can form extensive hydrogen bonding, and its incorporation into the coal structure has been postulated for the strained state of coal.29 The removal of the water reduces the strained state and, in effect, the cross-linking. In addition, methanol occupies the water sites but the extent of hydrogen bonding is less than that of water. Effect of Drying on Liquefaction Yields. The results of the coal liquefaction experiments on the six coals that were dried using various methods are summarized in Table 5. Relative to the premoisturized coals, drying coals thermally and with microwave radiation decreased the percent conversion for all coals except for Utah Blind Canyon and Illinois No. 6 coals. Both these coals are of high rank and have low moisture content, with most of the moisture tightly bound in the internal structure of the coal.27 However, Wyoming Eagle Butte, Wyoming Black Thunder, and Texas Bottom all show a general trend of decreasing percent conversion with increasing percentage of water removed. These results are in agreement with the results obtained by others.3,4,6,7,10 Chemical drying increased the conversion yields by as much as 18% as in the case of Illinois No. 6 coal. The reason for the differences in liquefaction behavior of the chemically dried and the thermally and microwavedried coals appears to be some retention of the reaction products and solvents by the chemically dried coals.
Calorimetric measurements of the heats of vaporization of volatile material by DSC have shown that some of the methanol (5-19%) was incorporated into the coal pore structure in place of the water,28 thus possibly preventing formation of cross-links and collapse of the pore structure during the dehydration reaction. This would allow for greater diffusion of tetralin during liquefaction and, hence, greater conversion. Alternatively, the residual methanol could have acted as a reducing agent and could have enhanced the conversion as a result of H-transfer from the methanol. The chemically dried coals also produced more methane than the premoisturized or the thermally and microwave-dried coals except for the Utah Blind Canyon coal (Table 6 and Figure 5). During chemical drying some methanol is retained by the coal. An increase in the amount of produced methane could have resulted from cross-linking reactions involving reactive methoxy functional groups in coals.32 In the presence of the tetralin donor solvent, the methanol could be converted to methane, although the mechanistic details are not known. However, the swelling experiments indicated little or no cross-linking structural change for the chemically dehydrated coals at room temperature. The percentage of CO2 produced was greater for the lower rank coals (increasing moisture content).28 More CO2 was produced from liquefaction of the coals that were completed dried thermally and by microwave than those that were dried chemically. These data support the coal-swelling data that suggest cross-linking of the coal structure had occurred. However, other studies have shown that CO2 production does not result in crosslinking.33 Also, the addition of water back to a dried coal restores the liquefaction behavior and results in increased CO2 yields.3,10
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Miknis et al.
Table 6. Relative Amount of Gases Produced during the Coal Liquefaction of Coals Dried Using Thermal, Microwave, and Chemical Methods (wt % maf Basis) % H 2O removed
CO
CO2
methane
ethane
propane
isobutane
n-butane
premoisturized thermal microwave chemical
0 100 100 100
0.67 0.0 0.0 0.15
4.62 0.57 0.58 1.54
Utah Blind Canyon 1.65 0.79 0.78 1.23
0.84 0.19 0.20 0.40
0.74 0.22 0.24 0.40
0.10 0.03 0.03 0.03
0.24 0.06 0.03 0.06
premoisturized thermal microwave chemical
0 100 100 100
0.13 .18 .10 .07
1.87 2.61 1.26 1.34
Illinois No. 6 .72 .87 .83 2.82
.47 .58 .52 .46
.30 .36 .35 .49
.00 .04 .03 .04
.04 .07 .10 .07
premoisturized thermal microwave chemical
0 100 100 100
.61 .79 .84 .58
9.17 9.67 9.10 6.30
Wyoming Eagle Butte .65 1.10 1.13 2.45
.36 .69 .71 .39
.24 .44 .45 .36
.00 .03 .03 .03
.04 .09 .13 .06
premoisturized thermal microwave chemical
0 100 100 100
.61 .82 .71 .61
Wyoming Black Thunder 9.04 .69 8.67 .75 7.62 .74 6.04 2.23
.45 .49 .48 .39
.37 .36 .36 .36
.00 .03 .03 .03
.08 .10 .10 .06
premoisturized thermal microwave chemical
0 100 100 100
.69 .64 .75 .60
9.87 8.22 8.96 5.98
.50 .46 .79 .42
.32 .32 .57 .39
.00 .04 .07 .04
.05 .07 .18 .07
premoisturized thermal microwave chemical
0 100 100 100
1.61 .70 .72 .54
30.18 10.39 10.56 9.27
North Dakota Beulah 1.86 1.06 .70 .37 1.65 .82 1.48 .40
.75 .27 .48 .34
.05 .03 .03 .03
.25 .07 .10 .07
Texas Bottom .73 .78 1.21 2.57
Figure 5. Comparison of amount of methane produced from coals dried by different methods.
Liquid Analyses. The percentages of liquid products produced from liquefaction of the six coals dried by the various methods are given in Table 5. Relative to the premoisturized coal, the thermally and microwave-dried subbituminous Wyoming Eagle Butte, Wyoming Black Thunder, and Texas Bottom coals showed a significant decrease in the amount of liquids produced, whereas Utah Blind Canyon coal gave a slight increase in liquid yield. The liquid yields for the completely dried Illinois No. 6 coal and North Dakota Beulah lignite were slightly less than that of the premoisturized coals. All chemically dried coals showed an increase in the liquid yield. The percent increase was greater for the lower (33) Eskay, T. P.; Britt, P. F.; Buchanan, A. C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41 (2), 739-743. (34) Boysen, J. E.; Cha, C. Y.; Barbour, F. A.; Turner, T. F.; Kwang, T. W.; Berggren, M. H.; Hogsett, R. F.; Jha, M. C. Development of an Advanced Process for Drying Fine Coal in an Inclined Fluidized Bed. WRI Report WRI-90-R031; Laramie, WY, 1990.
rank coals, ranging from 11% for Black Thunder to 60% for the North Dakota lignite. This increase may have been the result of replacement of water by methanol, which might have prevented collapse of the pore structure, allowing for greater diffusion and interaction of the tetralin with the coal components. The results from gas chromatographic analyses of the coal liquids from each liquified coal are reported in Table 7. The results are reported as a percentage of oil below and above a retention time (RT) of 19 min. The amount of oil below an RT of 19 min is defined as a light oil and above an RT of 19 min as a heavy oil. Except for the premoisturized Utah Blind Canyon coal (0% H2O removed), the percentage of light oil produced from the other premoisturized coals decreased slightly with decreasing rank (increasing moisture content) of the coals. Conversely, the percentage of heavy oil increased with increasing moisture content. Relative to the premoisturized coals, complete drying of the coals (100% H2O removed), regardless of the drying method, increased the percentage of light oil and decreased the heavy oil produced during the coal liquefaction, except for the Utah coal for which the percentage of light oil decreased (heavy oil increased). The light oil produced for the completely dried coals varied from 82.1 to 89.7%, and the heavy oil produced varied from 10.3 to 17.9%. For the Utah coal the percentages varied from 66.0-71.6% for the light oil and 28.4-34.0% for the heavy oil. In general, the premoisturized coals and partially dried coals (Illinois No. 6, Wyoming Eagle Butte, Wyoming Black Thunder, and Texas Bottom) produced about the same amount of light oil (∼85%). Overall, it appears that removal of moisture decreases the percent-
Coal Structure and Reactivity
Energy & Fuels, Vol. 10, No. 3, 1996 639
Table 7. Gas Chromatographic Analysesa of Coal Liquids from Liquefaction Coals Dried Using Thermal, Microwave, and Chemical Methods thermal
microwave
RTb below 19 light oil
RTb above 19 heavy oil
% H2O removed
0 100
71.6 67.3
28.4 32.7
0 100
0 13 89 100
84.3 87.2 86.3 88.4
15.7 12.8 13.7 11.6
0 33 79 100
Illinois No. 6 84.3 79.3 85.2 87.5
15.7 20.7 14.8 12.5
0 32 73 100
Wyoming Eagle Butte 83.1 85.5 86.4 88.6
16.9 14.5 13.6 11.4
0 14 82 100 0 23 70 100 0 38 75 100 0 100 aExpressed
83.1 82.8 83.9 89.4 83.3 81.9 82.4 85.0 79.0 81.5 79.0 82.2 78.5 84.5
16.9 17.2 16.1 10.6 16.7 18.1 17.6 15.0 21.0 18.5 21.0 17.8 21.5 15.5
0 30 73 100 0 30 78 100 0 100
RTb below 19 light oil
chemical
% H2O removed
RTb above 19 heavy oil
Utah Blind Canyon 71.6 28.4 66.0 34.0
Wyoming Black Thunder 83.3 16.7 80.9 19.1 80.6 19.4 85.3 14.7 Texas Bottom 79.0 81.0 80.8 84.2
% H2O removed
RTb below 19 heavy oil
RTb above 19 heavy oil
0 100
71.6 79.6
28.4 20.4
0
84.3
15.7
100
89.7
10.3
0
83.1
16.9
100
86.9
13.1
0
83.3
16.7
100
84.5
15.5
79
21
21.0 19.0 19.2 15.8
0 100
84.7
15.3
North Dakota Beulah 78.5 21.5 82.1 17.9
0 100
78.5 84.6
21.5 15.4
as a percentage of the total liquid product. bRT ) retention time in min.
Figure 6. Solid-state NMR spectra of residues from liquefaction of dried coals.
age of coal converted to liquids but increases slightly the formation of light oil components relative to coals that have been completely dried thermally and with microwaves. Residue Analyses. Representative 13C CP/MAS NMR spectra of the coal liquefaction residues from the premoisturized coals and the thermally, chemically, and microwave-dried coals are shown in Figure 6. In all cases, relative to the starting coal spectra, the spectra of the residues showed a significant reduction in the aliphatic component relative to the aromatic component, which was expected. The aromaticities of the liquefaction residues from coals that were completely dried were nearly the same (0.82 ( 0.04), being independent of the coal rank and the method of drying. One exception was the residues from the North Dakota lignite after being
completely dried using thermal, microwave, and chemical methods. Those had aromaticities of about 0.88. In addition, there is a substantial narrowing of the aromatic resonance band as carbons substituted on aromatic rings (∼140 ppm), phenolic carbons (∼155 ppm), and carboxylic carbons (∼180 ppm) evolve as liquid products during conversion. There is also a shift in the resonance position from ∼30 ppm to ∼20 ppm for the aliphatic carbons. The residual aliphatic component at ∼20 ppm could be due to methyl groups attached to aromatic rings that would not be cleaved during liquefaction at 425 °C. It is also possible that some of the aromatic carbon in the residues resulted from aromatization of a fraction of the aliphatic carbon. However, without NMR measurements of the liquid fractions and elemental analyses, the extent of aromatization could not be ascertained. The NMR spectra of the residues from the chemically dried coals showed enhanced resolution of the aliphatic carbons (Figure 6). This enhanced resolution of the aliphatic carbons in the 13C NMR spectra was also noted in the NMR spectra of the chemically dried coals before liquefaction (Figure 4). It is unlikely that the carbon resonances observed are due to solvent and reaction products of the chemical drying technique unless the methanol is so strongly adsorbed that, even at the liquefaction temperature of 425 °C, it is not removed. It may be that the methanol solvent removed some of the soluble aliphatic carbon substituents (decreasing the chemical shift dispersion), thus in effect giving higher resolution of the remaining carbon types. Also, incorporation of methyl groups from methanol via reactions with carboxylic acids to form esters is possible. However, the residue spectra show no evidence of additional carboxylate functionality if such reactions had occurred.
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Conclusions Different methods for coal drying have been investigated to determine if drying can be accomplished without destroying coal reactivity toward liquefaction, thereby making coal drying a possible pretreatment method for coal liquefaction. The results of this study have led to a number of conclusions about the effects of drying on the structure and reactivity of coals. During microwave drying of coals, the temperature rapidly rises close to the boiling point of water, followed by a plateau in temperature. This temperature is maintained until about 80% of the coal moisture is removed, after which the temperature rises rapidly during removal of the remaining moisture. The temperature rises because more energy is required to remove water molecules that are more tightly bound or that are located deeper in the pore structure. Solid-state 13C NMR measurements did not show any significant changes in the general chemical structure of the coal, regardless of the drying method save for some possible decarboxylation in the low-rank coals when heated ballistically to a temperature of 250 °C. There were, however, changes in the chemical nature of the coals that were dried chemically using 2,2dimethoxypropane (DMP). In particular, there was enhanced resolution in the aliphatic region of the 13C NMR spectra. This enhanced resolution was attributed to CH3OH, which is a reaction product of DMP with water and imbibed methanol from the solvent. Swelling ratios were determined on the premoisturized and dried coals to obtain an assessment of crosslinking during drying. The results showed that there was less cross-linking in the coals dried chemically than in the coals dried thermally or with microwaves, suggesting that the pore volume increased when the water molecules were removed (less hydrogen bonding to contract the pores). The liquefaction behavior of the microwave-dried and thermally dried coals was similar. That is, the extent
Miknis et al.
of conversion was lower for coals partially and completely dried using microwaves or thermal drying than for the premoisturized coals. For the chemically dried coals, the conversions were the same or up to 18% greater than for the premoisturized coals. This was attributed to the retention of some of the solvent and reaction products by the coal, which would have had the effect of preventing complete or partial collapse of the pore structure, enabling donor solvent penetration into the pores. Relative to the premoisturized coals, the low-rank coals that were thermally and microwave-dried showed a decrease in the amount of liquids produced during liquefaction. For the chemically dried low-rank coals, the liquid yields were between 11 and 60% greater. These results suggest that chemical dehydration can be used as a pretreatment step to coal liquefaction. However, additional research is needed to establish any correlations between increased yields and absorbed methanol and/or incorporated methoxy functionalities. In general, the quality of the liquids produced was better for the coals that were completely dried, even though the overall conversions were lower. Disclaimer Mention of specific brand names or models of equipment is for information only and does not imply endorsement of any particular brand. Acknowledgment. The authors greatly acknowledge support of this study by the U.S. Department of Energy under Contract DE-AC22-91PC91043 and Grant DE-FG22-91PC91310. The authors thank F. A. Barbour, J. Rovani, S. M. Pope, P. A. Holper, B. E. Thomas, and L. G. Nickerson for their assistance. EF950257W
