Energy & Fuels 1993, 7, 399-405
399
Liquefaction of Lignite Containing Cation-Exchanged Iron M. Mehdi Taghiei, Frank E. Huggins, Bhaswati Ganguly, and Gerald P. Huffman* 233 Mining and Mineral Resources, University of Kentucky, Lexington, Kentucky 40506 Received November 19,1992. Revised Manuscript Received February 22,1993
A significant enhancement of the liquefaction yields and desirable products from two lignites has been achieved by incorporating iron in the lignites by an ion-exchange process. The total conversion of iron ion-exchanged Hagel and Beulah lignites was found to increase by up to 25 % and the oil yield by 10% relative to the untreated lignites. The ion-exchanged lignites were prepared by stirring a slurry mixture of the lignite and ferric acetate [Fe(OOCCH3)3]in a 10-L fermenter. The ion-exchange process, in which iron was exchanged primarily for calcium, yielded a highly dispersed catalytic iron species for coal liquefaction. 57Fe Mossbauer and X-ray absorption fiie structure (XAFS) spectroscopies were used to characterize this iron species after both the ion-exchange and the liquefaction processes. The results indicate that added iron is initially present in bimodal form. A significant fraction of the iron species is in particles finer than 30A in diameter but with the majority of the iron particles in the form of oxyhydroxide (a-FeOOH) ranging from 30 to 100 A in diameter. The former size category includes molecularly dispersed ferric ions at the ion-exchange (carboxyl) sites. With sufficient sulfur present in the system, the iron is rapidly transformed to pyrrhotite (Fel,S) during liquefaction. Introduction One of the most important factors in achieving optimum conversions during direct coal liquefaction is the use of an effective catalyst that can be highly dispersed in the coal structure. Catalyst particle size also has a strong effect on catalytic reactivity because small particles have a large percentage of surface sites that are accessible for chemisorption and catalysis. have been conducted on the A number of activity of various iron-baaed catalysts during liquefaction of coals of different rank. Recent research4 has shown that sulfated iron catalysts can significantly enhance the conversion yield of high-rank coals. The solid superacid Fe203/S042- is prepared by adding (NH4)2S04 or H2S04 to Fe(OH)3, followed by calcining at 800-900 K. It has been suggestedsthat the sulfate group induces the removal of electrons from the metal ion to enhancethe Lewis acidity of the metal-ion site. In coal liquefaction, however, the main role of the sulfate anion is attributed to stabilization of fine particles by retarding the agglomeration and sintering of iron oxide particles species, thereby maintaining a high catalytic surface area during liquefaction. Nevertheless, for lower rank coals, similar improvement in conversion yield through the addition of similar sulfated catalysts has not yet been achieved. The oxygen contents of low-rank coals such as lignites are significantly higher than those of high-rank coals, and as a result, oil yields from liquefaction of low-rank coals tend to be low due to C02 formation.6 A considerable portion of this oxygen derives from carboxylgroups and other oxygen functional groups. In low-rank coals, most of the exchangeablecations ~~
(1) Herrick,D.E.;Tiemey, J. W.; Wender,L;Huffman,G.P.;Huggins,
F. E. Energy Fuels 1991,4, 231. (2) Tanabe, K.; Yamaguchi, T.;Hattori, H.; Sanada, Y.; Yokoyama, S. Fuel Procesr. Technol. 1984,8, 117. (3) Cook, P. S.; Caehion, J. D. Fuel 1987, 66,661. (4) Pradhan, V. R.; Tierney, J. W.; Wender, I.; Huffman, G. P. Energy Fuels 1991,5,497. (5) Jin, T.; Yamaguchi, T.; Tanabe, K. J. Phys. Chem. 1986,90,4794. (6)Redlich, P.; Jackson, W. R.; Larkins, F. P. Fuel 1985,64, 1383.
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are associated with carboxyl groups, and a number of studies7~8have shown that the behavior of lignites in coal conversion processes is greatly affected by the amount and type of exchangeable cations present. This paper presents the results of an investigation into the role of ion-exchanged iron during direct coal liquefaction (DCL) of two lignites. The results indicate that iron ion-exchanged into the low-rank coals constitutes a catalyst in a state of dispersion ranging Gom molecular ions to particles a few nanometers in diameter. Superparamagnetic modeling of Mossbauer spectrag indicates that the iron particle size in lignite samples has a bimodal character, with the majority of the particle sizes ranging from 30 to 100 A, but with a significant fraction less than 30 A. The latter fraction may derive from ferric cations in ion-exchange sites bound to the oxygen anions of carboxyl groups. Particles larger than 30 A may be due to the hydrolysis of iron acetate or ion-exchanged iron forming iron oxyhydroxide. The DCL total conversion and oil yields of the iron ion-exchanged Beulah and Hagel lignites used in this work were enhanced sigificantly compared to the raw lignites.
Experimental Section The two lignites used in this study are Beulah (obtained from Department of Energy Coal Samples, DECS-11) and Hagel (obtainedfrom Penn. State Office of Coal, PSOC-1482)from the Fort Union region in North Dakota. The proximate and ultimate analyses of these two lignites are shown in Table I. The original iron contents of these lignites are less than 0.5%. The lignite samples were first ground to
