Energy & Fuels 2007, 21, 1655-1662
1655
Comparative Analysis of Structural Transformations of Two Bituminous Coals with Different Maximum Fluidity during Carbonization Valentina Zubkova,* Victor Prezhdo, and Andrzej Strojwas Institute of Chemistry, Jan Kochanowski UniVersity, Checinska Str. 5, 25-020 Kielce, Poland ReceiVed NoVember 9, 2006. ReVised Manuscript ReceiVed February 2, 2007
The variation of the volume of two bituminous coals with different maximum fluidity (MF) values has been determined using carbonization tests, and the quality of coke obtained has been examined using scanning electron microscopy (SEM) micrographs. The structural and chemical changes in bituminous coals at the preplastic stage during carbonization were studied using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) techniques and compared to the changes in their electric and dielectric parameters. It was observed that the structural and chemical transformations occurred in the disordered phase of both coals in different ways. These differences are attributed to the different redistributions of hydrogen between the radicals generated in the aliphatic and aromatic parts of the macromolecule fragments.
Introduction The processes that occur at the pre-plastic and plastic stages during the carbonization of coals predetermine their caking capacity and the quality of coke. The transition of coals to the plastic state is accompanied by a variety of different chemical reactions and physical-chemical processes, which can be investigated by various methods.1-3 Coals have a network structure where aromatic and hydroaromatic clusters can be connected by crosslinks of different natures (covalent links of ether, methylene, sulfide types);4,5 however, it is not excluded that entanglements and noncovalent interactions including hydrogen bonds have a more important role in the coal structure.6-8 The thermoplastic state that is developed during the heat treatment of coals is attributed to a decrease in the density of crosslinks, as monitored by the coal solvent swelling technique.9-11 During pyrolysis, changes occur in coal ordering,12-15 * Author to whom correspondence should be addressed. Fax: +48 41 349 70 62. E-mail address:
[email protected]. (1) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energy Fuels 1990, 4, 42-54. (2) Wornat, M. J.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1987, 1, 431-437. (3) Marzec, A.; Czajkowska, S.; Schulten, H.-R. Energy Fuels 1994, 8, 360-368. (4) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 47294735. (5) Larsen, J. W.; Gurevich, I.; Glass, A. S.; Stevenson, D. S. Energy Fuels 1996, 10, 1269-1273. (6) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100-106. (7) Iino, M. Fuel Process. Technol. 2000, 62, 89-101. (8) Painter, P.; Shenoy, S. Energy Fuels 1995, 9, 364-371. (9) Ndaji, F. E.; Butterfield, I. M.; Thomas, K. M. Fuel 1997, 90, 169177. (10) Nomura, S.; Thomas, K. M. Fuel 1998, 77, 829-836. (11) Butterfield, I. M.; Thomas, K. M. Fuel 1995, 74, 1780-1785. (12) Diamond, R. Philos. Trans. 1960, A252, 193-223. (13) Marsh, H. Fuel 1973, 52, 205-211. (14) Zubkova, V. Fuel 2005, 84, 741-754. (15) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10, 672678.
as well as in their dielectric and electric properties.16,17 Differential scanning calorimetry (DSC) established that a glassrubber transition occurred at a temperature of ∼300 °C.18-20 During heat treatment, structural chemical changes occur in the macromolecular structure of coals and macerals, and, as a result, the ratio of the intensity of the aromatic C-H stretching mode to that of the aliphatic C-H stretching mode increases above the thermoplastic range.10,21 The cause of this may be the growth in aromaticity, which is due to the cleavage of weak alkyl chains and their removal from the carbon skeleton, together with volatile matter.21 At the thermoplastic stage (
