Ethylenediamine-Assisted Solvent Extraction of Coal in N-Methyl-2

194

Energy & Fuels 2002, 16, 194-204

Ethylenediamine-Assisted Solvent Extraction of Coal in N-Methyl-2-pyrrolidone: Synergistic Effect of Ethylenediamine on Extraction of Coal in N-Methyl-2-pyrrolidone Shailaja Pande and D. K. Sharma* Centre for Energy Studies, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi-110016, India Received August 7, 2000. Revised Manuscript Received June 19, 2001

Coals ranging from lignite to high rank bituminous coal were extracted with N-methyl-2pyrrolidone (NMP), ethylenediamine (EDA), and NMP containing a small amount of EDA (e,N) under reflux conditions at atmospheric pressure. The extraction yield was maximum in the third case suggesting a synergistic effect of the system. The system was found to enhance the extraction yield in NMP. Stepwise successive extraction in NMP was also possible using this system. A possible explanation for the results obtained is given.

Introduction As we enter the 21st century need for clean and efficient utilization of coal is felt increasingly. There are potential uses of coal not only as a fuel but as a nonfuel as well. Coal, being carbon-based and predominantly aromatic in nature, can serve as a feedstock for aromatic chemicals and carbon materials such as graphite, carbon fibers, etc.1,2 However, to use coal in an environmentally acceptable way, the cleaning of coal to reduce the mineral matter content of coal is required. Organorefining of coal through solvent extraction techniques is a promising approach in this direction. Solvent extraction of coal helps in recovery of super clean coal (SCC) containing very less ash by using common organic solvents under mild experimental conditions (i.e., under atmospheric pressure conditions),3 unlike the conventional techniques employing the use of high hydrogen pressure at elevated temperatures by using preferably hydrogen donor solvents.4 The SCC obtained may be used as a cleaner burning fuel as well as for the production of industrial carbons, speciality chemicals, etc.1,2,5 Hence, organo-refining is particularly important to utilize the valuable coal reserves and studies may be directed to further develop and understand this process. Coals are cross-linked macromolecules.6-8 The crosslinks of the macromolecular network of coal consist of (1) Song, C.; Schobert, H. H. Fuel Process. Technol. 1993, 34, 157. (2) Song, C.; Schobert, H. H. Fuel 1996, 75, 724. (3) Renganathan, K.; Zondlo, J. W.; Mintz, E. A.; Kneisl, P.; Stiller, A. H. Fuel Process. Technol. 1988, 18, 273. (4) Elliott, M. A., Ed. Chemistry of Coal Utilization; John Wiley: New York, 1981 (Second Supplemental Volume). (5) Zondlo, J. W.; Stansberry, P. G.; Stiller, A. H. Proceedings of the 10th Annual International Pittsburgh Coal Conference, Sept 20-24, 1993, University of Pittsburgh, Pittsburgh, 1993; p 379. (6) van Krevelen, D. W. Coal; Elsevier: Amsterdam, Netherlands, 1993. (7) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729. (8) Hall, P. J.; Thomas, K. M.; Marsh, H. Fuel 1992, 71, 1271.

both covalent bonds and noncovalent interactions.9 The covalent bonds are mainly ethylenic, methylenic, and ether linkages10,11 while the principal associative or noncovalent interactions include H-bonds, van der Waals’ forces and the π-π aromatic interactions.12-14 There may be some coal-coal interactions in the raw coal, which are determined by the geological conditions under which the coal was formed or deposited.15,16 Since coals are cross-linked macromolecules they show a tendency to swell in organic solvents for which they have an affinity, a property characteristic of crosslinked polymers.9,17-20 Swelling results from the disruption of noncovalent interactions in coal such as H-bonds in case of polar solvents and dispersion forces in case of nonpolar solvents.21 Painter et al. have applied theories of swelling to coal using pyridine, the coalcoal interactions are replaced by more favorable coalsolvent interactions.19 This causes the coal to swell to accommodate the solvent.23 Swelling of coal is accompanied by solubilization of coal molecules in the solvent if coal-solvent interactions are stronger than the coal-coal interactions.24-27 (9) Chen, C.; Gao, J.; Yan, Y. Energy Fuels 1998, 12, 1328. (10) Juentgen, H. Fuel 1984, 63, 731. (11) Ndaji, F. E.; Thomas, K. M. Fuel 1993, 72, 1531. (12) Haenel, M. W. Fuel 1992, 71, 1211. (13) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100. (14) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4, 107. (15) Otake, Y.; Suuberg, E. M.Energy Fuels 1997, 11, 1155. (16) Painter, P. C.; Park, Y.; Sobkowiak, M.; Coleman, M. M. Energy Fuels 1990, 4, 384. (17) Schobert, H. H. The Chemistry of Hydrocarbon Fuels; Butterworth: London, 1990. (18) Takanohashi, T.; Iino, M.; Nishioka, M. Energy Fuels 1995, 9, 788. (19) Green, T. K.; Larsen, J. W. Fuel 1984, 63, 1538. (20) Hu, Y.; Coleman, M. M.; Painter, P. C. Macromolecules 1998, 31, 3394. (21) Hall, P. J.; Marsh, H.; Thomas, K. M. Fuel 1988, 67, 863. (22) Painter P. C.; Graf, J.; Coleman, M. M. Energy Fuels 1990, 4, 393. (23) Larsen, J. W.; Lee, D. Fuel 1985, 64, 981. (24) Szeliga, J.; Marzec, A. Fuel 1983, 62, 1229.

10.1021/ef0001742 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/04/2001

Solvent Extraction of Coal

There has been a growing interest in the use of NMP as a solvent for extraction of coals.27,28 The interest stems from the fact that it is an industrial solvent used in petroleum and other industries for the separation of olefins and aromatics, for refining oils, etc.28,29 It is convenient and easy to use, and above all, it is an ecofriendly solvent.30 NMP is a polar aprotic selective solvent for coal which is a good H-bond acceptor and has affinity for aromatic nuclei.28,31 Renganathan and Zondlo27 have reported the coal extraction, varying from 0.1% to as high as 50.9% by using NMP under reflux conditions at atmospheric pressure. Cagniant et al.32 have reported NMP extraction yields in the range of 30%-40% depending upon the rank of coal. Klotzkin33 has reported NMP extraction yields of about 25% of the two coals studied by Soxhlet extractions. Studies were carried out presently on the extraction of seventeen Indian coals ranging from lignite to high rank bituminous coals with NMP. However, the extractability of most of the coals studied was found to be only about 10%-15% (on dmmf basis) in NMP under reflux conditions at atmospheric pressure in an extraction time of 2 h. Only Neyveli lignite and Loiyo coal gave better extraction yields in NMP of about 23% and 27% respectively. Moreover, successive extraction in NMP was also not found to be possible. In fact, extractability of coal by a single solvent is rather limited.6 An approach of successive extraction by using solvents in a stepwise process6,34-39 or using a mixture of solvents19,28,33,40-42 tends to enhance extraction yield. Since swelling and extraction have common fundamental causes (i.e. coal-solvent interactions), the two are related, and normally good swelling solvents are also good solvents for extraction.23,24,28,43 In fact, ethylenediamine (EDA) has been identified to be a powerful swelling solvent for coals capable of extracting a considerable quantity of coal.43 Work on the use of EDA as (25) Marzec, A.; Juzwa, M.; Betlej, K.; Sobkowiak, M. Fuel Proc. Technol. 1979, 2, 35. (26) Roy, J.; Lahiri, A. Paper 48. In Proceedings of the Symposium on Chemicals and Oil From Coal, Dec 1969; Central Fuel Research Institute: Dhanbad, India, 1972; p 447. (27) Renganathan, K.; Zondlo, J. W. Fuel Sci. Technol. Int. 1993, 11, 677. (28) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (29) Kirk, R. E., Othmer, D. E., Eds. Encyclopedia of Chemical Technology, 3rd ed.; John Wiley and Sons: New York, 1982; Vol. 19. (30) Indian Chem. Eng., Sect. B: Ind. News 1996, 38. (31) Larsen, J. W.; Shawver, S. Energy Fuels 1990, 4, 74. (32) Cagniant, D.; Gruber, R.; Lacordairc, C.; Jasienko, S.; Machnikowska, H.; Salbut, P. D.; Bimer, J. and Puttmann, W. Fuel 1990, 69, 902. (33) Klotzkin, M. P. Fuel 1985, 64, 1092. (34) Sharma, D. K.; Singh. S. K. Fuel Proc. Technol. 1988, 19, 73. (35) Sharma, D. K.; Mishra, S. Energy Fuels 1989, 3, 641. (36) Giri, C. C. Studies on Development of a Process for Solvent Deashing of Coal to Obtain Environmentally Clean Fuels and Characterization of Products. Ph.D. Thesis, I. I. T. Delhi, New Delhi, India, 1995. (37) Singh, S. K. Extraction Enhancement of Coal through Alkali Treatment, Pyrolysis and Gasification of Residual Coals Obtained after Solvent Extraction. Ph.D. Thesis, I. I. T. Delhi, New Delhi, India, 1989. (38) Sharma, D. K.; Singh, S. K. Energy Sources 1996, 18, 1. (39) Painter, P. C.; Park, Y.; Coleman, M. M. Energy Fuels 1988, 2, 693. (40) Speight, J. G. The Chemistry and Technology of Coal; Marcel Dekker: New York, 1984. (41) Cooke, N. E.; Fuller, O. M.; Gaikwad, R. P. Fuel 1989, 68, 1227. (42) Nishioka, M.; Gebhard, L. A.; Silbernagel, B. G. Fuel 1991, 70, 341. (43) Dryden, I. G. C. Nature 1948, 162, 959.

Energy & Fuels, Vol. 16, No. 1, 2002 195

a solvent for coal has been extensively reviewed.4,6,40,43,44 EDA is also an industrial solvent and has been used by the chemical industries over the years. EDA is considered to be a specific solvent for coal owing to its ability to disrupt specific interactions in coal such as the H-bonds.6,23,40,43,45-47 Therefore, an approach of successive extraction of coals by using both NMP and EDA in a stepwise process was adopted. Chinakuri coal, a low rank bituminous coal and Loiyo coal, a high rank bituminous coal, were selected as candidate coals for study. Different solvent sequences of NMP and EDA were selected on the premise that extraction with a weaker solvent opens the structure for a powerful solvent.48 EDA was found to be the more powerful solvent. Swelling studies using NMP, EDA, and NMP+ EDA (1:1, vol/vol) mixture were also performed. Interestingly, maximum swelling of the coal was obtained with the mixture followed by that in EDA and then next in NMP. Since swelling depends purely on the forces of interaction between solvent molecules and polymer segments and is not influenced by stirring or agitation,49 the above result indicated a synergistic effect of the NMP+EDA solvent system. Synergism of NMP with CS2 has been reported by Iino et al.28 They had performed extractions at room temperature and obtained high extraction yields by using a 1:1 NMP+CS2 mixture as compared to NMP alone. Nishioka et al.42 reported that the extractability of a high volatile bituminous coal with pyridine at room temperature may be enhanced by adding a small amount of triethylamine (a donor solvent) due to the disruption of charge-transfer interactions. In fact, present studies have shown that extractability of coals in NMP may be enhanced by addition of a small amount of EDA to NMP under reflux conditions. The solvent system of NMP containing a small amount of EDA was termed as e,N. No work seems to have been reported on the interesting and promising synergistic effects of EDA on NMP extraction of coals. For all the coals studied, extraction yield was maximum in e,N solvent system compared to that in either NMP or EDA. Stepwise successive extraction was also possible by using e,N solvent system. SCC yields varying from 45% to about 70% (on dmmf basis) have been obtained for the coals studied using this solvent system of NMP containing a small amount of EDA in a stepwise process. The following paper reports the details of the studies carried out on the solvent extraction of coals using NMP containing a small amount of EDA. Experimental Section Neyveli lignite, Assam, Samla, Pasang, Chinakuri, Godavari, Topa, Loiyo, Pindra, Kuzu, Rajrappa cleans, Akash Kinari, Bachra, North Tisra, Karanpura Diwar Khan, Karanpura Diwar Haslong, and Talcher bituminous coals were ground to -60 to +120 BSS (British Sieve Standard) and dried in an oven at 105 °C for 24 h. After drying, the coals were stored in a desiccator. (44) Dryden, I. G. C. Fuel 1950, 29, 197. (45) Shibaoka, M., Stephens, J. F.; Russel, N. J. Fuel 1979, 58, 515. (46) Suuberg, E. M.; Otake, Y.; Langer, M. J.; Lenng, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247. (47) Brenner, D. Fuel 1983, 62, 1347. (48) Rubicka, S. M. Fuel 1959, 38, 45. (49) Gowariker, V. R.; Viswanathan, N. V.; Sreedhar, J. Polymer Science Wiley Eastern Limited: New Delhi, India, 1983

196 Energy & Fuels, Vol. 16, No. 1, 2002 Chemicals. Commercially available (E (Merck) India Limited) laboratory grade chemicals N-methyl-2-pyrrolidone (NMP) and ethylenediamine (EDA) were used. Solvent Extraction of Coals in NMP and EDA. The coal sample (5 g) was placed in a 250 mL round-bottom flask fitted with a reflux condenser and containing NMP or EDA (85 mL). A coal:solvent ratio of 1:17 (wt/vol, g/mL) was used (coal:solvent ratios are wt/vol and solvent:solvent ratios are vol/vol wherever mentioned). The mixture was refluxed for 2 h and filtered. The residual coal (RC) obtained after NMP extraction was washed with toluene and methanol sequentially in a Soxhlet apparatus. The RC obtained after EDA extraction was washed with 2% aq HCl to remove EDA and then with distilled water to remove excess acid. Final washing was done with a 1:1 methanol-water mixture. The RC obtained after washing was dried overnight in the oven and weighed to record the loss in weight of coal. Recovery of Solid Extracts. The liquid extracts (filtrates) collected after filtration were concentrated by distillation and the super clean coal (SCC) was precipitated out using distilled water as the antisolvent. The SCC was filtered and washed several times with water till the washings were almost colorless. Then it was dried overnight in the oven to constant weight. Stepwise Successive Extraction of Chinakuri and Loiyo Coal in NMP and EDA. The coal sample (5 g) was placed in a 250 mL round-bottom flask fitted with a reflux condenser and containing NMP (85 mL). The mixture was refluxed for 2 h and filtered. The RC obtained after filtration and washing was dried and then subjected to further extraction for 2 h in EDA following the same procedure as before under reflux conditions. The solvent sequence was designated as NMP-EDA. The RC obtained after filtration and washing was dried overnight in the oven and weighed to record the loss in weight of coal. The same procedure was followed for the reverse sequence, i.e., EDA-NMP in which EDA was used as the solvent in the first step and NMP was used as the solvent in the second step. Similarly, Chinakuri coal and Loiyo coal were subjected to stepwise successive extraction in NMP where NMP was used as the solvent in both the steps. The solvent sequence was designated as NMP-NMP. Chinakuri coal was also subjected to stepwise successive extraction in EDA where EDA was used as the solvent in both the steps. The solvent sequence was designated as EDA-EDA. Recovery of Solid Extracts. The liquid extracts (filtrates) collected after filtration at each stage of extraction were concentrated by distillation and the super clean coal (SCC) was precipitated out using distilled water as the antisolvent. The SCC was filtered and washed several times with water till the washings were almost colorless. Then it was dried overnight in the oven to constant weight. Assam, Samla, Pasang, Godavari, Topa, Kuzu, Pindra, and Rajrappa cleans were similarly subjected to stepwise successive extraction using NMP-EDA solvent sequence. Solvent Extraction of Coals in NMP Containing a Small Amount of EDA (Termed as e,N). (e,N refers to the small amount of EDA, wherever mentioned in the present paper, to the use of the EDA:NMP ratio of 1:17 (vol/vol)). The coal sample (5 g) was taken in a 250 mL round-bottom flask containing EDA (5 mL) and NMP (85 mL). A coal:EDA ratio of 1:1 and a coal:NMP ratio of 1:17 was used. The mixture was refluxed for 2 h and filtered. The RC obtained after filtration was washed with 2% aq HCl followed by distilled water. Final washing was done with a 1:1 methanol-water mixture. The RC was dried overnight in the oven to record the loss in weight of coal. Recovery of Solid Extracts. The SCC was precipitated out using 2% aq. HCl as antisolvent. The SCC was filtered and washed with distilled water till the washings were

Pande and Sharma colorless. Then, the washed SCC was dried overnight in the oven to constant weight. Extraction of Chinakuri Coal with NMP after Pretreatment of Coal with a Small Amount of EDA and Removal of EDA. The coal sample (5 g) was kept in contact with EDA (5 mL) at room temperature for wetting the coal with EDA for 2 h. After 2 h, EDA was removed from the coal sample by washing by the method described (for EDA) previously. Coal was dried and extracted with NMP (85 mL) for 2 h and filtered. The RC obtained after filtration was washed with toluene and methanol sequentially in a Soxhlet apparatus. The RC was dried overnight in the oven to record the loss in weight of coal. Recovery of Solid Extracts. The SCC was precipitated out from the liquid extract using distilled water as the antisolvent by the method described before. Stepwise Successive Extraction of Coals in NMP Containing a Small Amount of EDA (termed as e,N-e,N). Chinakuri, Loiyo, Assam, Samla, Pasang, Topa, Godavari, Kuzu, Pindra, and Rajrappa cleans were subjected to stepwise successive extraction in NMP containing a small amount of EDA. The coal sample (5 g) was taken in a 250 mL round-bottom flask containing EDA (5 mL) and NMP (85 mL). The mixture was refluxed for 2 h and filtered. The RC obtained after filtration was washed with 2% aq HCl followed by distilled water. Final washing was done with a 1:1 methanol-water mixture. The RC was dried and then subjected to further extraction in NMP containing a small amount of EDA for 2 h following the same conditions as in the first step. The RC obtained after filtration and washing was dried to record the loss in weight of coal. Chinakuri coal and Loiyo coal were also subjected to a third step of extraction in NMP containing a small amount of EDA, termed as e,N-e,N-e,N. Recovery of Solid Extracts. The SCC was precipitated out from the liquid extract using distilled water as the antisolvent by the method described before. Calculation of Extractability. The extractability of coal was calculated on a dmmf basis on the basis of percent loss in weight of coal after extraction and was compared with the weight of the recovered extract, i.e., SCC. The results were found to be within (1 to 3%. The reproducibility of the extraction experiments was (2 to 3%.

% SCC (on dmmf basis) ) weight of SCC × 100 initial weight of sample - (1.1 × ash content) % extraction (on dmmf basis) ) initial weight of sample - final weight after extraction × 100 Initial weight of sample - (1.1 × ash content)

Swelling Measurement. Swelling measurements were carried out using the procedure reported by Green et al.50 The coal (500 mg) was placed in a 15 mL graduated glass tube and centrifuged (dry) and its height measured. A solvent (5 mL) such as NMP or EDA or NMP+EDA (1:1) was added, and the tube was shaken well. After 24 h, the tube was centrifuged and the height of the coal column was measured again. The swelling ratio is the ratio of coal column heights:

Q)

column height after swelling with solvent column height of dried coal without solvent

Infrared (IR) Spectral Studies. The FTIR spectra of original Chinakuri coal and Loiyo coal, SCC and RC, were (50) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935.



Table 1. Proximate Analyses of Indian Coalsa) volatile fixed matter carbon moisture ash (VM) (FC)

coal Neyveli Lignite (N.L.) Assam (A.C.) Samla (S.C.) Pasang (Ps.C.) Chinakuri (C. C.) Topa (T.C.) Godavari (G.C.) Rajrappa Cleans (R.Cl) Kuzu (K.C.) Pindra (P. C.) Loiyo (L.C.) Akash Kinari (A.K.C.) Bachra (B.C.) North Tisra (N.T. C.) Karanpura Diwar Khan (K.P.D.) Karanpura Diwar Haslong (K.D.H.) Talcher (Tl.C.) a

8.2 4.0 6.6 6.3 5.1 0.7 5.3 2.0 2.8 1.8 1.0 0.3 2.8 1.8 3.0 7.3 3.7

2.9 2.2 14.3 13.3 13.6 32.7 17.7 11.1 13.6 16.5 19.4 38.7 40.5 36.2 40.5 31.0 20.7

54.3 42.0 32.6 31.9 32.9 26.1 30.8 31.2 27.8 27.0 26.2 16.1 26.3 20.7 26.2 28.5 36.8

34.6 51.8 46.5 48.5 48.4 40.5 46.2 55.7 55.8 54.7 53.4 44.9 30.4 41.3 30.3 33.2 38.8

On % air-dried basis.

recorded on a Nicolet 460 Prote´ge´ FTIR spectrometer in the absorption range 400-4000 cm-1 by using KBr pellets.

Results and Discussion Table 1 shows the proximate analyses and Table 2 shows the ultimate analyses of the coals used. Extraction of Coals with NMP and EDA. All extractions in individual solvents were performed for 2 h only in order to keep the extraction time reasonably short. Table 3 shows the results of solvent extraction of different coals with NMP and EDA for 2 h. Table 4 shows the swelling ratios of coals in NMP and EDA. Extraction yields in NMP were low except for Loiyo coal and Neyveli lignite where the yields were 27% and 23%, respectively. The functional oxygen groups such as carboxyl and hydroxyl are abundant in lignites, resulting in a polar solvent, such as NMP, giving a high extraction yield in lignite.51 A relatively high NMP extraction yield of 27% obtained for high-rank Loiyo coal (among Indian coals studied presently) can be explained by the ability of NMP to disrupt the π-π interactions, which are dominant in higher rank coals.9,28,52,56 Figure 1 shows the effect of rank of coal on NMP extraction of coal.

EDA was found to be better solvent than NMP for most of the coals studied. Figure 2 shows the effect of rank of coal on EDA extraction of coals. The general trend is as expected.6 EDA is a reactive solvent for brown coals.53 The high extraction yield of Neyveli lignite may be due to the acid-base interaction involved between the strong base EDA and the acidic groups in lignite. However, the dissolution of bituminous coals in EDA involves a physical dissolution process only.54 It is a polar solvent and can interact through H-bonding interactions. Stepwise Successive Extraction of Chinakuri Coal and Loiyo Coal in NMP and EDA. Studies carried out on stepwise successive extraction of Chinakuri coal and Loiyo coal using different solvent sequences of NMP and EDA showed that NMP-EDA was the best solvent sequence to obtain a higher extraction yield (Table 5). Swelling studies were performed in order to study the mechanism of stepwise successive extraction of coal using NMP-EDA and EDA-NMP sequence. Table 6 shows the results. When NMP was used as the first solvent, the RC obtained after NMP extraction was further rendered extractable in EDA in case of both Chinakuri coal and Loiyo coal (Table 5). In fact, stepwise successive extraction in NMP-EDA solvent sequence rendered about 34% and 36% of Chinakuri coal and Loiyo coal extractable compared to 10% and 27% in NMP and 21% and 16% in EDA separate extractions, respectively. High swelling ratios were also obtained for RCI(N) in EDA in case of both Chinakuri coal and Loiyo coal (Table 6). In fact, for Chinakuri coal the RCI(N) showed higher swelling in EDA than the original coal. Extraction of coal with NMP removes some of the extractable material from the coal by disrupting some of the coal-coal interactions such as the H-bonds. After the removal of NMP not all of the original H-bonds reform. When RCI(N) with slightly reduced number of cross-linkages is swelled in EDA, which is a better swelling solvent than NMP, EDA is able to even further expand or swell the already deformed matrix by relaxing it and penetrating into it by disrupting more number of cross-linkages including H-bonds, van der Waals' interactions, and charge-transfer interactions.

Table 2. Ultimate Analyses of Coals

rank

coal

C

lignite bituminous coal

Neyveli Lignite Assam Samla Pasang Chinakuri Topa Godavari Rajrappa Cleans Kuzu Pindra Loiyo Akash Kinari Bachra North Tisra KPD KDH Talcher

62.0 78.1 78.7 76.5 78.5 82.5 85.8 80.6 74.3 83.0 88.9 86.8 72.8 90.9 72.2 79.8 75.5

a

By difference.

ultimate analyses (on % dmmf basis) H N S 5.0 5.8 5.3 5.0 5.5 5.0 5.0 5.2 4.6 5.9 6.5 4.7 5.2 5.3 4.9 5.5 5.3

2.0 1.5 2.6 1.5 2.4 1.6 2.0 2.1 2.0 1.8 3.2 1.9 1.6 2.4 1.6 1.9 1.6

1.4 0.9 0.6 1.0 0.5 0.4 1.0 0.6 0.9 1.0 0.4 0.8 0.7 0.9 1.3 1.0 1.0

Oa 29.6 13.7 12.8 16.5 13.1 10.5 6.2 11.5 18.2 9.2 1.0 5.8 19.7 0.5 20.0 11.8 16.6

atomic ratio H:C O:C 0.97 0.89 0.81 0.78 0.84 0.73 0.70 0.77 0.74 0.72 0.87 0.65 0.85 0.69 0.81 0.82 0.85

0.36 0.13 0.14 0.17 0.14 0.14 0.07 0.12 0.20 0.10 0.03 0.11 0.28 0.05 0.29 0.16 0.19

petrographic composition (vitrinites %) 70.2 88.0 33.0 73.0 58.2

38.8 61.6

198

Energy & Fuels, Vol. 16, No. 1, 2002

Pande and Sharma

Figure 1. Effect of coal rank on NMP extraction yield of coals. Table 3. Results of Solvent Extraction of Coals in NMP and EDAa

Figure 2. Effect of coal rank on EDA extraction yield of coals. Table 5. Results of Stepwise Successive Extraction of Chinakuri Coal and Loiyo Coal in Different Solvent Sequences of NMP and EDAa

% SCC (on dmmf basis) coal

NMP

EDA


23.0 8.0 6.0 11.0 10.0 9.0 9.0 14.0 13.0 13.0 27.0 14.0 9.0 17.0 7.0 10.0 13.0

23.0 21.0 33.0 12.0 21.0 14.0 12.0 18.0 19.0 26.0 16.0 0.3 18.0 2.0 16.0 18.0 18.0

a Extraction time: 2 h in each solvent. Coal:solvent ) 1:17 (wt/ vol).

coal Chinakuri

Loiyo

QNMP

QEDA

QNMP+EDA


2.7 1.3 2.0 2.0 2.0 1.5 1.5 2.4 2.0 1.9 2.1 1.1 1.7 1.2 1.6 1.7 1.7

2.4 2.8 2.6 3.0 2.8 2.8 2.3 2.8 2.8 2.2 2.6 1.2 1.8 1.4 2.0 2.0 2.0

2.8 4.2 3.1 3.1 3.7 3.5 3.6 4.0 3.3 3.6 4.4 1.5 1.7 2.0 2.0 2.4 2.5

However, preextraction with EDA had an adverse effect on extraction of coal in the succeeding solvent (51) Takanohashi, T.; Yanagida, T.; Iino, M.; Mainwaring, D. E. Energy Fuels 1996, 10, 1128. (52) Nishioka, M. Fuel 1992, 71, 941. (53) van Bodegom, B. B.; van Veen, J. A. R.; van Kessel, G. M. M.; Sinnige-Nijssen, M. W. A.; Stuiver, H. C. M. Fuel 1984, 63, 346.

% SCC (on dmmf basis)b

NMP-EDA EDA-NMP EDA-EDA NMP-NMP NMP-EDA EDA-NMP NMP-NMP

10.0 + 27.0 (34.0) 21.0 + 3.0 (23.0) 21.0 + 9.0 (28.0) 9.0 + 1.0 (10.0) 27.0 + 13.0 (36.0) 16.0 + 1.0 (17.0) 27.0 + 1.0 (28.0)

% total extraction (on dmmf basis)c 36.0 26.0 11.0 36.0 17.0 28.0

a Extraction time: 2 h in each solvent. Coal:solvent ) 1:17 (wt/ vol). b SCC yield from weight of SCC. (Value in brackets is total SCC yield after the two steps of extraction (on dmmf basis).) c Extraction yield from the weight of RC remaining at the end of extraction, i.e., on loss in weight basis of coal.

Table 6. Swelling Ratio Q of Original and Residual Chinakuri and Loiyo Coals inNMP and EDA Solvents at Room Temperature (at 25 °C)a coal Chinakuri coal O.C.C. RCI(N) RCI(E) Loiyo coal O.L.C. RCI(N) RCI(E)

Table 4. Swelling Ratio (Q) of Original Coals in NMP, EDA, and NMP+EDA (1:1, vol/vol) (at 25 °C) coal

solvent sequence

a

QNMP

QEDA

2.0

2.8 3.2

1.2 2.1

2.6 2.5

1.7

Swelling time: 24 h.

NMP. The RC obtained after EDA extraction showed very poor extractability in NMP. When EDA is used as the first solvent, it is able to extract about 17%-20% of coal. Removal of EDA collapses and shrinks the structure of coal and some of the noncovalent cross-linkages, such as H-bonds, again form such that a new stable configuration is obtained. NMP is not strong enough a solvent to penetrate the newly configured coal matrix by disrupting coal-coal interactions. Hence extraction of RCI(E) in NMP remained low. The RCI(E) in case of both Chinakuri and Loiyo coal did not show high swelling in NMP indicating the effect of removal of EDA on swelling and extraction of coal in NMP. EDA is a (54) van Bodegom, B. B.; van Veen, J. A. R.; van Kessel, G. M. M.; Sinnige-Nijssen, M.W. A.; Stuiver, H. C. M. Fuel 1985, 64, 59. (55) Otake, Y.; Suuberg, E. M. Fuel 1998, 77, 901. (56) Chen, C.; Kurose, H.; Iino, M. Energy Fuels 1999, 13, 1180.



Table 7. Results of Stepwise Successive Extraction of Coals Using NMP-EDA Solvent Sequencea

coal

% SCC (on dmmf basis)b

% total extraction (on dmmf basis)c

Assam Samla Pasang Chinakuri Topa Godavari Rajrappa Cleans Kuzu Pindra Loiyo

8.0 + 16.0 (22.0) 6.0 + 12.0 (17.0) 11.0 + 16.0 (25.0) 10 + 27.0 (34.0) 9.0 + 23.0 (28.0) 9.0 + 7.0 (15.0) 14.0 + 16.0 (28.0) 13.0 + 11.0 (23.0) 13.0 + 11.0 (23.0) 27.0 + 13.0 (36.0)

23.0 19.0 25.0 36.0 30.0 19.0 28.0 23.0 23.0 36.0

a Extraction time: 2 h in each solvent. Coal:solvent ) 1:17 (wt/ vol). b SCC yield from weight of SCC. (Value in bracket is total SCC yield after the two steps of extraction (on dmmf basis).) c Extraction yield from the weight of RC remaining at the end of extraction, i.e., on loss in weight basis of coal.

powerful solvent in relaxing and swelling the coal as it is in contracting and shrinking the coal. Successive extraction was possible in EDA as shown for Chinakuri coal. Successive extraction in EDA, quinoline, liquid paraffin, and anthracene oil was also reported by Singh.37 Successive extraction in NMP resulted in very poor extraction yield in NMP (Table 5). This further shows that NMP is a weaker solvent and has inability to penetrate even the matrix of RCI(N). Stepwise Successive Extraction of Coals Using NMP-EDA Solvent Sequence. Table 7 shows the results of stepwise successive extraction of the 10 coals studied using the NMP-EDA solvent sequence. These studies showed the general applicability of the NMPEDA solvent sequence. The highest extraction yield was obtained for Loiyo coal (36%) and Chinakuri coal (34%) followed by that for Topa coal (28%). Swelling of Coals in NMP+EDA (1:1, vol/vol) Mixed Solvent System. To study further enhancement in extraction yield, a second approach was adopted, i.e., the use of a mixture of solvents for extraction. Swelling studies using NMP+EDA (1:1) mixed solvent system were performed for Chinakuri coal. Interestingly, maximum swelling of the coal was obtained with mixture (Q ) 3.7) followed by that in EDA (Q ) 2.8) and then in NMP (Q ) 2.0). The above result indicated a synergistic effect of the mixed solvent system. In fact, swelling studies in NMP+EDA (1:1) mixed solvent system were carried out for all 17 coals. Table 4 shows the results of swelling of coals in NMP, EDA, and NMP+EDA (1:1). The general trend was QNMP+EDA > QEDA > QNMP indicating the effectiveness of the mixed solvent system for swelling the coal. Swelling results also showed that EDA was a better swelling solvent than NMP. Extraction of Chinakuri Coal with NMP Containing Different Amounts of EDA (Optimization of Coal:EDA Ratio). The objective of the present study was to increase extraction yield in NMP by using EDA. Hence, the effect of gradually increasing the amount of EDA in NMP was studied keeping the coal:solvent ratio fixed. If EDA is powerful in swelling the coal, and the synergistic effect was observed in the NMP+EDA (1:1) mixed solvent system, then even addition of reduced amount of EDA to NMP should increase extraction yield in NMP by swelling the coal. Chinakuri coal was

Figure 3. Extraction of Chinakuri coal with NMP containing different amounts of EDA (optimization of coal:EDA ratio).

extracted for 2 h with NMP containing different amounts of EDA. The coal:EDA ratio was increased from 1:0 to 1:17. The coal:NMP ratio used was 1:17 up to a coal:EDA ratio of 1:1. When coal:NMP ratio used was 1:17 and the coal:EDA ratio used was 1:1, i.e., when EDA:NMP ratio used was 1:17, the solvent system was referred as NMP containing a small amount of EDA (e,N). After that, the coal:NMP ratio was decreased accordingly so as to keep the coal:NMP+EDA ratio as 1:17; e.g.; when the coal:EDA ratio used was 1:8.5, the coal:NMP ratio used was also 1:8.5. This was a 1:1 NMP+EDA solvent system. When the coal:EDA ratio used was 1:17, the coal:NMP ratio used was 1:1. This was a solvent system of EDA containing a small amount of NMP (designated as n,E) since the NMP:EDA ratio used was 1:17. Figure 3 shows the results. The first and last points in the figure are the results of extraction yield obtained when Chinakuri coal was extracted with NMP alone and EDA alone, respectively. As is seen from the figure, the extraction yield increases steeply as a small amount of EDA is added to NMP. When the coal:EDA ratio used was 1:1 the amount extracted was about 34%. Though this amount was less than that extracted in NMP+EDA (1:1) solvent system (about 42%) where the coal:EDA ratio was 1:8.5 the difference is not steep considering the increase in the amount of EDA added to NMP in case of the NMP+EDA (1:1) mixed solvent system. The objective was to enhance extraction yield in NMP using a minimum amount of EDA, and reasonably good yield was obtained using NMP containing a small amount of EDA at a coal:EDA ratio of 1:1. Therefore, the coal:EDA ratio of 1:1 and the coal:NMP ratio of 1:17 was taken as the optimum amount of solvents required for extraction. In fact, it was found that the extraction yield of coal in e,N (34%) was more than that in NMP (10%), EDA (21%), and n,E (28%) indicating a synergistic effect of EDA on NMP extraction of coal. This shows that the potency of NMP increases in the presence of EDA. Also, the extraction temperature for e,N extraction was found to be lower (176 °C) than the extraction temperature for NMP extraction (202 °C) under reflux conditions. Thus addition of a small amount of EDA to NMP not only increases potency of NMP but also lowers the extraction temperature there by conserving on heat energy. Other mixed solvent systems were also studied, however, e,N was found to be the best (Table 8).

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Table 8. Results of Extraction of Chinakuri Coal with Different Mixed Solvent Systemsa solvent system (e,Xb,d; xc,NMPe)

% SCC (on dmmf basis)

e,NMP e,morpholine e,dimethylformamide e,sulfolane e,quinoline dioxane,NMP tetrahydrofuran,NMP morpholine,NMP dimethylformamide,NMP sulfolane,NMP quinoline,NMP anthracene oil,NMP

34.0 10.0 15.0 13.0 13.0 8.0 7.0 16.0 9.0 7.0 6.0 10.0

a Extraction time: 2 h. b “X”: Solvent taken in larger amount. “x”: Solvent taken in smaller amount d “X” containing a small amount of EDA (e). Coal:e ) 1:1 (wt/vol). Coal:X ) 1:17 (wt/vol). e NMP containing a small amount of “x”. Coal:x ) 1: 1 (wt/vol). Coal:N ) 1:17 (wt/vol). c

Extraction of Chinakuri Coal and Loiyo Coal Using a Mixture of NMP and EDA. The synergistic effect of EDA on NMP extraction of coal may be explained by the prewelling of coal matrix by EDA, which promoted accessibility of NMP to the extractable sites in coal, which were not accessible by NMP, when used alone. A comparison of e,N with n,E (Figure 3) suggests that the role of the solvent taken in smaller amount is that of swelling rather than extraction. There is a steeper rise in extraction yield from NMP (10%) to e,N (34%) than from EDA (21%) to n,E (28%) since EDA is a better swelling solvent (Q ) 2.8 for Chinakuri coal) than NMP (Q ) 2.0 for Chinakuri coal). According to Otake and Suuberg,55 it is logical that the accommodation of a larger molecule into the coal network is more difficult. Steric factors such as size and shape of diffusing molecule also influence the swelling as well as solubility of coal molecules in a solvent. EDA is a straight chain amine and smaller in size (MWEDA ) 60.1 g/mol) than NMP (MWNMP ) 99.1 g/mol). Moreover, the viscosity of EDA (1.54 N s m-2) is less than that of NMP (1.67 N s m-2). Straight chain nature, smaller size, and lower viscosity of EDA compared to that of NMP allow it easier access to the coal structure compared to the pyrrolidone ring of NMP. As EDA penetrates the coal matrix it cleaves coal interactions including chargetransfer, H-bonding, and van der Waals' interactions being both a H-bond acceptor solvent and a H-bond donor solvent. Such a situation helps in improving the accessibility of NMP to the potentially extractable but originally entrapped extractable components of coal in the coal matrix. As EDA opens the coal structure for NMP penetration, boiling NMP disrupts more noncovalent interactions, which hold soluble material to the network. NMP acts strongly on coal to cleave H-bonding, charge-transfer, or aromatic-aromatic interactions between coal molecules resulting in greater solubilization and an enhanced extraction yield. It has been reported recently56 that enhancement in extractability of coals in NMP+CS2 system in the presence of additives may not be due to disruption of charge-transfer interactions. It may be due to the induced polarizability of aromatic systems of coal, thereby disrupting stable coal-coal interactions including π-π and van der Waals interactions and steric hindrance may prevent the disruption

of charge-transfer interactions.56,57 Painter has suggested that CS2 limits any self-associations of NMP thereby allowing NMP to extract enhanced amount of coal.16 We suggest that the preswelling and the expansion of the matrix by EDA may reduce steric hindrance to the disruption of charge-transfer interactions by NMP under reflux conditions thereby resulting in enhanced extraction yields. About 34% of Chinakuri coal and about 62% of Loiyo coal was rendered extractable in e,N in an extraction time of 2 h, which was higher than that in NMP alone and EDA alone for both the coals. The difference between the extraction yields of Chinakuri coal and Loiyo coal is attributed to the greater ability of NMP to destroy the aromatic-aromatic stacking interactions in the higher ranked Loiyo coal than the coal-coal interactions in Chinakuri coal. The use of NMP in larger excess as an extraction solvent may not permit strong adsorption and trapping of EDA in coal which is otherwise known to be retained physically (adsorption) in the extracts and residue when used alone as an extraction solvent.43 Extraction of Chinakuri Coal with NMP after Pretreatment of Coal with a Small Amount of EDA and Removal of EDA. The objective of this study was to study the effect of EDA removal, after treatment of the coal with EDA at room temperature, on the extraction yield of coal in NMP. After pretreatment with EDA for 2 h, EDA was removed. Synergistic effect of the mixed solvent system of NMP and EDA and the preswelling effect of EDA was supported by the fact that pretreatment of Chinakuri coal with EDA followed by the removal of EDA and then extraction of the coal in NMP rendered only about 4% coal extractable. During the washing of coal to remove EDA, the filtrate was colorless, implying that no extraction had taken place in EDA during that time. Thus removal of EDA after soaking had a negative effect on the extraction of coal in NMP. As long as EDA is there, coal is in expanded state. As EDA is removed, new coal-coal interactions form and coal shrinkage occurs. The coal rearranges to form new coal-coal noncovalent interactions, thereby increasing the noncovalent cross-link density. Such selfassociations are unfavorable for the dissolution of coal in NMP, the self-associations being so strong that NMP cannot dissociate them. EDA swollen coal matrix is penetrable by NMP rather than the collapsed matrix of coal after removal of EDA. Swelling of the Original and Residual Coals with Pyridine. The objective of these studies was to study the irreversibility of coal swelling. Table 9 shows the results of swelling in pyridine of original Chinakuri coal (O.C.C) and original Loiyo coal (O.L.C) and their respective residual coals obtained after NMP, EDA, and e,N extractions. Pyridine has a similar swelling power as NMP for swelling the original Chinakuri and Loiyo coals. In fact, both NMP and pyridine have a similar ability to cleave coal-coal H-bonds. Since both have the same heat of H-bonding to phenol, ∼ -7 kcal/mol.6 It was found that the swelling of residual coals was less than the original coals indicating a change in the structure of coal after extraction. (57) Thompson, R. L.; Rothenberger, K. S.; Retcofsky, H. L. Energy Fuels 1997, 11, 739.



Table 9. Swelling Ratio Q of Original and Residual Chinakuri and Loiyo Coals in Pyridine (at 25 °C) coal Chinakuri O.C.C. RC(N) RC(E) RC(e,N) Loiyo O.L.C. RC(N) RC(E) RC(e,N)

Q 2.00 1.83 1.44 1.73 2.18 1.87 1.64 1.53

The decrease in swelling of the residual coals in pyridine suggests that there is an increase in noncovalent cross-link density after extractions in NMP, EDA, and e,N solvent systems. Nishioka58 and Larsen and Mohammadi14 observed a decrease in pyridine swelling ratios of pyridine-extracted coals compared with the raw coals. This irreversibility or decrease in pyridine swelling was explained as due to the formation of new relatively strong secondary associations. In the present studies, in the case of Chinakuri coal, RC(E) swells the least in pyridine which shows the effect EDA removal has in reconfiguration of the coal matrix after the collapsing of the coal matrix on removal of EDA. The result confirms the studies that EDA-NMP solvent system is less effective for successive extraction and also the presence of EDA is desirable for NMP extraction. The swelling of RC(e,N) is also less than that of the original coal which is also due to the formation of secondary interactions in coal following the removal of solvents. In case of Loiyo coal, RC(e,N) shows the minimum swelling in pyridine which indicates the formation of strong solvent-induced associative interactions in coal after the removal of more than 60% of the coal molecules followed by removal of the solvents from coal. Stepwise Successive Extraction of Chinakuri Coal and Loiyo Coal in e,N (e,N-e,N-e,N). Chinakuri coal and Loiyo coal were extracted successively in three steps using the above-mentioned solvent system. Table 10a shows the results along with a comparison of the extraction yields obtained when Chinakuri coal and Loiyo coal were extracted with NMP (for 2 h); EDA (for 2 h); e,N (for 2 and 24 h); and with e,N-e,Ne,N, successively (for 6 h). Whereas the pyridine swelling of RC(e,N) was reduced, the matrix was, however penetrable by e,N further establishing the potency of the mixed solvent system in cleaving the noncovalent interactions formed following the first extraction step. Significant extraction was achieved in the first step itself for both the coals. About 62% of Loiyo coal and 34% of Chinakuri coal was rendered extractable in the first step. RCII(e,N) was also further rendered extractable in e,N. Extraction yield was considerably reduced in the third step suggesting that the limiting value for extraction had been reached after the third step and the major extraction had taken place in the first two steps, i.e., in 4 h. The total amount of SCC obtained after 6h was 51% in case of Chinakuri coal and 75% in case of Loiyo coal which was only little less than the exhaustive yields obtained after 24 h (58) Nishioka, M. Fuel 1993, 72, 1001.

Table 10. (a) Comparison among the Results of Extraction of Chinakuri and Loiyo coals in NMP, EDA, and NMP Containing a Small Amount of EDA and Successive Extraction in NMP Containing a Small Amount of EDAa Along with (b) Swelling Ratio of Original, Extracted, and Residual Chinakuri and Loiyo Coals in Quinoline at 25 °C (a) Comparison among the Results of Extraction of Chinakuri and Loiyo Coal SCC (on % dmmf basis) NMP EDA Total e,N e,N (2 h) (2 h) (4 h) (2 h) (24 h) e,N-e,N-e,N (6 h)

coal Chinakuri Loiyo

10 27

21 16

31 43

34 62

60 77

34 + 18 + 9 (51) 62 + 20 + 15 (75)

(b) Swelling Ratio of Original, Extracted, and Residual Chinakuri and Loiyo Coals in Quinoline at 25 °C coal

Q

Chinakuri O.C.C. SCC(e,N) RC(e,N) Loiyo O.L.C. SCC(e,N) RC(e,N) a

1.27 1.33 1.10 1.44 1.76 1.60

Extraction time: 2 h in each step.

Table 11. Results of Ash Content of SCC and RC Obtained after Solvent Extraction of Chinakuri and Loiyo Coal with NMP and e,N coal Chinakuri Coal O.C.C. SCC(N) SCC(e,N) RC (e,N) Loiyo Coal O.L.C. SCC(N) SCC(e,N) RC(e,N)

ash (% db) (theoret.)

ash (% db) (exptl)

14.3 0.0 0.0 20.0

14.30 0.50 0.40 17.00

19.6 0.0 0.0 38.1

19.60 0.60 0.30 35.80

extraction in e,N. Since EDA and NMP interact with coal through noncovalent specific interactions only, it suggests that 50%-75% of the extractable, solvent soluble substances exist originally in the coal. Such a high percentage of the solvent soluble substances cannot be present as trapped molecules in the 50%-55% of cross-linked networks. Recently, extracts have also been reported to have a cross-linking network.59 Swelling studies of SCC and RC obtained after solvent extraction of Chinakuri and Loiyo coals with e,N solvent system were carried out in quinoline. Table 10b shows the results. Swelling of coal involves disruption of noncovalent cross-links in coal by coal-solvent interactions.19 The SCC in case of both Chinakuri coal and Loiyo coal was found to show greater swelling than original coal and RC. This may be due to the presence of noncovalent physical interactions, such as H-bonding interactions in the extracts, which are cleaved by the basic solvent quinoline. Table 11 shows the ash content in SCC obtained from Chinakuri coal and Loiyo coal after NMP extraction and after extraction with NMP containing a small amount (59) Iino, M.; Takanohashi, T.; Ohkawa, T.; Yanagida, T. Fuel 1991, 70, 1236.

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Table 12. Results of Solvent Extraction of Coals in NMP, EDA, and e,Na % SCC (on dmmf basis) coal Neyveli Lignite Assam Samla Pasang Chinakuri Topa Godavari Rajrappa Cleans Kuzu Pindra Loiyo Akash Kinari Bachra North Tisra KPD KDH Talcher

NMP EDA total of NMP + e,N (2 h) (2 h) EDA (2 + 2 ) 4 h) (2 h) b 23.0 8.0 6.0 11.0 10.0 9.0 9.0 14.0 13.0 13.0 27.0 14.0 9.0 17.0 7.0 10.0 13.0

23.0 21.0 33.0 12.0 21.0 14.0 12.0 18.0 19.0 26.0 16.0 0.3 18.0 2.0 16.0 18.0 18.0

23 + 23 ) 46 8 + 21 ) 29 6 + 33 ) 39 11 + 12 ) 23 10 + 21 ) 31 9 + 14 ) 23 9 + 12 ) 21 14 + 18 ) 32 13 + 19 ) 32 13 + 26 ) 39 27 + 16 ) 43 14 + 0.3 ) 14.3 9 + 18 ) 27 17 + 2 ) 19 7 + 16 ) 23 10 + 18 ) 28 13 + 18 ) 31

34.0 33.0 31.0 30.0 34.0 33.0 27.0 45.0 40.0 40.0 62.0 23.0 38.0 28.0 40.0 32.0 40.0

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