Energy & Fuels 1994,8, 155-171
155
A Study of Mequinenza Lignite? Curt M. White,* Leo W. Collins, Garret A. Veloski, Gin0 A. Irdi, and Kurt S. Rothenberger Pittsburgh Energy Technology Center, Analytical Research Group, P.O. Box 10940, Pittsburgh, Pennsylvania 15236
Ralph J. Gray Ralph Gray Services, 303 Drerel Drive, Monroeville, Pennsylvania 15146
Robert B. LaCount ViRoLac Industries, 403 Arbor Court, Waynesburg, Pennsylvania 15370
Masoud Kasrai and G. Michael Bancroft Department of Chemistry, University of Western Ontario, London, Canada N 6 A 5B7
James R. Brown Energy Research Laboratories, C A N M E T , Energy Mines and Resources Canada, Ottawa, Ontario, Canada K I A OGl
Frank E. Huggins, Naresh Shah, and Gerald P. Huffman Mining and Mineral Resources, University of Kentucky, Lexington, Kentucky 40506 Received May 24, 1993. Revised Manuscript Received September 7,1993@
The sulfur constituents in a Mequinenza lignite (Spain) were characterized using a variety of analytical techniques. The investigation was performed to provide information to scientists and engineers attempting to design methods to remove sulfur from coal and lignite, test new analytical methods directed at characterization of sulfur constituents in fuel, and provide basic analytical information concerning the sulfur constituents in coal and lignite. Such information is particularly useful during the evaluation of coal desulfurization processes. The nature of the lignite and the sulfur components were explored using standard ASTM methods; petrographic techniques; mineral analysis using microscopy and X-ray diffraction; low-voltage, high-resolution mass spectrometry (LVHRMS); sulfur L-edge X-ray absorption near-edge structure spectroscopy (XANES); sulfur K-edge XANES; electron paramagnetic resonance (EPR); and controlled atmosphere programmed temperature oxidation (CAPTO). The Mequinenza material is a lignite containing 13.6 % sulfur (maf) and 21.6% (dry) ash. The sulfur is primarily organic sulfur (12.80%) and contains smaller amounts of pyrite and very little sulfate. Microscopic analysis showed that clay minerals were the most abundant minerals and that the lignite consisted mainly of the macerals huminite and liptinite. A small amount of elemental sulfur was also detected (1 pg/g) in a tetrachloroethylene extract. LVHRMS analysis of a pyridine extract showed that molecular formulas consistent with thiophenes, aryl sulfides, their corresponding oxidized forms, and disulfides were present. Results from sulfur L-edge and K-edge XANES spectroscopy on the whole lignite were in quantitative and qualitative agreement with LVHRMS results on the pyridine extract with respect to thiophenes. Sulfur L-edge XANES indicated that 40 % of the sulfur was present as thiophenes and 20 % as aryl sulfides. CAPTO reports the sum of these two sulfur forms as 60.4 wt %. The CAPTO and sulfur L-edge XANES results are in excellent agreement. The L-edge XANES indicated that 20% of the sulfur was present as aliphatic sulfides and 20% as disulfides. Results of the K-edge XANES analysis indicated that nonpyritic sulfur was distributed as follows: 10% disulfide, 33 % sulfide, 42 % thiophenic, 9 % sulfoxide, and 5% sulfate. CAPTO results indicated that 34.4 wt % of the sulfur was present as nonaromatic sulfur. The W-band EPR spectrum was unusually rich in features compared to other coals. These signals are attributed to aromatic radicals that are either purely hydrocarbon or heteroatomic in nature.
Introduction Mequinenza lignite is unusual and of interest because it is very high in sulfur (13.1 % maf), most of which is
* Author to whom correspondence should be addressed. t
This article is written in part to commemorate the 500th anniversary
of the discovery of the Americas by the Spanish expedition led by
Christopher Columbus in 1492. 0 Abstract published in Advance ACS Abstracts, December 1, 1993.
0887-0624/94/2508-0155$04.50/0
organic (12.8% maf). It is mined in the Mequinenza area of the Ebro Basin in northeast Spain about 90 km southsouthwest of Zaragoza. The Ebro Basin is surrounded by mountains: the Pyrenees and the Cantabrian mountains in the north, the Iberian System in the southwest, and smaller coastal ranges in the east. ~~~i~~ its formation, shallow lakes and marshes were present from the fluvial 0 1994 American Chemical Society
White et al.
156 Energy &Fuels, Vol. 8, No. 1, 1994
system marginal to the basin. The Mequinenza lignite deposits date from the late Oligocene and occur as horizontal seams that range in thickness from a few centimeters to 0.9 m and are not laterally persistent.’ The geologic, paleoenvironmental, and general sedimentary conditions that existed during deposition and coalification of Mequinenza lignite are thoroughly described by Cabrera and Saez.1 Vegetable debris deposition occurred under lacustrine conditions in shallow marshswamp-lake complexes that were closed.’ The swamp vegetation that accumulated in the lacustrine and nonmarine evaporitic environment resulted in lignite formation, Most of the coal-forming accumulation occurred in palustrine mud flats. Much of the Mequinenza lignite is closely associated with lake limestones and marly limestones, and less frequently associated with mud and sandstone facies. The climate of coal formation was temperate and arid, and the vegetation mainly consisted of grassy savannah-like plants with some wooded areas and reed swamps. The peat deposits were autochthonous and had a high and constant water table.’ The paleoenvironment that existed during lignite formation favored organic sulfur enrichment. According to Cabrera and Saez, the high ash and sulfur contents of the Mequinenza lignite resulted from clastic influx and an aqueous environment that was sometimes high in sulfate during times when the lakes were drying up. They describe the aqueous environment as one of “low acidity.”’ Sedimentological and paleontological data imply that the shallow marsh-swamp-lake system underwent repeated changes between fresh and slightly saline.’ Information concerning the nature of the organosulfur species in Mequinenza lignite is sparse. Garcia et al. have studied the supercritical methanol extract and residue of the lignite performed at 250 and 350 “C using combined gas chromatography-mass spectrometry (GC-MS) and both ‘Hand l3C nuclear magnetic resonance (NMR).2They concluded, “alkylbenzothiophenes are the predominant class of sulfur heterocycles present but some 3 and 4 ring thiophenes were identified in the asphaltenes”. These same authors reported the presence of mercaptans and low molecular weight sulfides in the supercritical methanol extract but caution that these are formed because of the reaction of methanol with sulfur in the lignite. Torres-Ordonez et al. also investigated the nature of organosulfur components in Mequinenza lignite,3 employing the technique of continuous isothermal flash pyrolysis (IFP) at 750 and 960 OC. By studying the temperatures at which various model sulfur-containing compounds evolve HzS, COS, and CS2 during pyrolysis and making certain assumptions about the origins of these sulfur gases from the Mequinenza lignite, they inferred the distribution of organic sulfur constituents as “67% aliphatic sulfides and/or mercaptan, essentially 0 % aromatic sulfides and 33 ?6 heterocyclics (by differen~e)”.~ When Torres-Ordonez et al. examined the tars produced by pyrolysis of the lignite at 750 and 850 “C using GC-MS, no sulfur-containing compounds were detected. (1)Cabrera, L.;Saez, A. J. Geol Soc., London 1987,144,451-461. (2)Garcia, R.;Moinelo, S. R.; Snape, C. E.; Bernad, P. Fuel Process. Technol. 1990,24,187-193. (3)Torres-Ordonez, R.J.; Calkins,W. H.; Klein,M. T. In Geochemistry of Sulfur i n Fossil Fuels, Orr, W. L.;White, C. M., Eds.; ACS Symposium Series 429;American Chemical Society: Washington, DC, 1990;pp 287295.
Kelemen et aL4 studied the sulfur components in Mequinenza lignite by temperature-programmed decomposition (TPD), sulfur K-edge XANES, and X-ray photoelectron spectroscopy (XPS). They found that the sample they studied contained substantial amounts of aliphatic sulfur, that is, sulfur bound to sp3-hybridized carbons as in dialkylsulfides. XPS determined that 66 mol ?6 of the sulfur present was aliphatic sulfur, and XANES found 48 mol % was aliphatic sulfur. The Kelemen study of Mequinenza lignite was followed up a year later by the same group of workers, only this time using IFP.5 IFP found that 67 mol 9% of the sulfur in Mequinenza lignite was aliphatic sulfur. Sinninghe Damste et aL6 recently reported the results of an investigation of sulfur-rich Spanish coals, including Mequinenza lignite, by flash pyrolysis gas chromatography-mass spectrometry. The only sulfur-containing organic compounds they identified in the pyrolysate from Mequinenza lignite were thiophenes, including alkylthiophenes and alkylbenzothiophenes. Thiophenes constituted the major portion of the organosulfur constituents in the pyrolysate. Lastly, Lafferty et aL7 have investigated the nature of the organosulfur constituents in Mequinenza lignite using high-pressure temperature-programmed reduction. They found that “thiophenic sulfur was the dominant form with sulfides accounting for 20-30 5% of the total organic sulfur.” Because of its high organic sulfur content, Mequinenza lignite has been the focus of several investigations concerning sulfur removal by thermolysis and hydrodesulfurization.a11 Cebolla et al. l 2 recently described the effects of lignitic petrographic composition and sulfur content on liquefaction behavior. Two Mequinenza lignites were among those studied. Development of techniques for the routine determination of the kinds and distribution of sulfur species in coals, including information on various organosulfur compound classes, is among the most challenging problems facing fuel analysts. Information concerning the nature and distribution of organosulfurconstituents in coal is desirable for the design and evaluation of coal desulfurization p r o c e s ~ e s . ~ JAnswers ~ - ~ ~ to such practical problems as sulfur removal from coal are inexorably linked to understanding the nature and distribution of sulfur constituents present in coal. The body of knowledge concerning the nature and distribution of organosulfur components in coal is small, but growing. The current article describes the results of (4)Kelemen, S. R.;Gorbaty, M. L.;George, G. N.; Kwiatek, P. J.; Sansone, M. Fuel 1991,70,396-402. (5)Calkins, W. H.; Torres-Ordonez, R. J.; Jung, B.; Gorbaty, M. L.; George, G. N.;Kelemen, S. R. Energy Fuels 1992,6,411-413. (6)Sinninghe Damste, J. S.; de las Heras, F. X. C.; de Leeuw, J. W. J . Chromatogr. 1992,607,361-375. (7)Lafferty, C. J.; Mitchell, S. C.; Garcia, R.; Snape, C. E. Fuel 1993, 72,367-371. (8)Garcia-Suarez, A. B.; Schobert, H. H. Prepr.-Am. Chem. SOC., Diu. Fuel Chem. 1988,33,241-246. (9)Garcia, A. B.; Schobert, H. H. Fuel 1989,68, 1613-1616. (10)Garcia, A. B.; Schobert, H. H. Coal Preparation 1989,7,47. (11)1. Garcia, A.B.; Schobert, H. H. Fuel Process. Technol. 1990,24, 179-185. . ~ . . (12)Cebolla, V.L.;Martinez, M. T.; Miranda, J. L.;Fernandez, I. Fuel 1992,71,81-85. (13)White, C. M.; Douglas, L. J.; Perry, M. B.; Schmidt, C. E. Energy Fuels 1987,1, 222-226. (14)White, C. M.; Douglas, L. J.; Hackett, M. Energy Fuels 1988,2, 221)-22.1. __ - -__ . (15)Orr, W. L.;White, C. M. Geochemistry of Sulfur in Fossil Fuels; ACS Symposium Series 429;American Chemical Society: Washington, DC, 1990. ~
A Study of Mequinenza Lignite
an investigation of the organosulfur constituents in Mequinenza lignite. A wide array of analytical techniques has been focused on this lignite to gain information on the nature of the organosulfur compounds. The analytical techniques and methods used to study the Mequinenza lignite include low-voltage, high-resolution mass spectrometry (LVHRMS); electron paramagnetic resonance (EPR)spectroscopy;petrography, proximate and ultimate analysis; X-ray diffraction; controlled-atmosphere programmed-temperature oxidation (CAPTO);sulfur L-edge X-ray absorption near-edge structure (XANES) spectroscopy; sulfur K-edge XANES; and other techniques. When all of this information is collectively interpreted, an image of the nature of the organosulfur constituents present in the lignite begins to emerge. Lastly, application of these new and traditional analytical techniques to the analysis of the same sample allows a direct comparison of the results and allows the analyst to begin to formulate an understanding of the strengths and weaknesses of the techniques, thus defining the state-of-the-art. Experimental Section Sample Origin and Preparation. A 1-kg sample of Mequinenza lignite crushed to a particle size of less than 3 mm was obtained from Dr. Ana Garcia of the Instituto Nacional del Carbon in Oviedo, Spain. The sample, as received, was under argon in a plastic container. No special storage conditions were employed once the sample reached the Pittsburgh Energy Technology Center (PETC). The sample was crushed in air at PETC until it was -20 mesh and stored in a tightly sealed '/2 gal plastic container. Soxhlet Extraction. Extracts were prepared in triplicate. Three 10.0-g portions of Mequinenza lignite (-100 mesh) were each intimately mixed with 30 g of previously extracted Celite and placed in glass thimbles in preparation for Soxhlet extraction. In addition, a layer of Celite was placed in the bottom of the thimbles before adding the mixtures to attempt to prevent any finely divided lignite particles from exiting through the thimble frit. Into each of the three 1000-mL round-bottomed flasks was added 600 mL of freshly distilled pyridine (distilled over electronic grade KOH) along with a Teflon boiling chip. After exhaustive extraction (5 days), the translucent dark brown extract was filtered through a 10-pm Teflon filter. The filtration procedure was assisted by using slight NPhead pressure. The three filtrates were reduced in vacuo to a viscous dark brown oil, which was subsequently dried to constant weight in a vacuum oven at 70 "C. On average, 16.8 wt % of the Mequinenza was extracted. Petrography. Petrographic studies were conducted using airdried pulverized lignite mixed with a nonreactive epoxy binder. Air drying was performed at ambient temperature. The mixture, which contained about 18-25% binder, was pelletized in a 1 in. internal diameter cylindrical mold with top and bottom plugs using pressures up to 5000 psi. The pellet was ground and polished for microscopic examination according to ASTM DZ79785.16 Two pellets of lignite were prepared using a Buehler Automet polishing device. The macerals were defined according to ASTM D2796-88.17 A Leitz Ortholux microscope fitted with an oil-immersion 60X fluoride objective and lox high eyepoint oculars to give an effective magnification of 720 diameters was used to determine the maceral content. A point count system of analysis was used for the maceral determination. Four points (16) Standard Test Method of Preparing Coal Samples for Microscopical Analysis by Reflected Light. In 1987 Annual Book of ASTM Standards, Petroleum Products, Lubricants, and Fossil Fuels; ASTM: Philadelphia, 1987; Sect. 5, Vol. 05.05, D2797-85, pp 361-366. (17) Standard Definitions of Terms Relating to Megascopic Description of Coal and Coal Seams and Microscopical Description and Analysis of Coal. In 1987 Annual Book of ASTM Standards, Petroleum Products, Lubricants, and Fossil Fuels; ASTM: Philadelphia, 1987; Sect. 5, Vol. 05.05, D2796-82, pp 357-360.
Energy &Fuels, Vol. 8, No. 1, 1994 157 were identified per field, and 3500 counts on two pellets were determined for Mequinenza lignite. This system employs a point count stage and an ocular graticule. The volume percent of macerals was calculated according to ASTM D2799-86.18 This ASTM method makes no statements with respect to the precision, reproducibility or repeatability of this method. Other ASTM Standard Analyses. Elemental analysis of -60 mesh lignite was performed using ASTM methods D3177, D3178, and D3179.lSz1 Moisture and ash were determined by ASTM methods D3173 and D3174,22v23 volatile matter by ASTM method D3175,24and forms of sulfur by ASTM method D2492.B Elemental analysis of the ash was performed using ASTM method D3682.26 The reproducibility of method 3177 for total sulfur is 0.20%. The reproducibility of method 2492 for sulfur forms is *0.04% for sulfate, and *0.30% for pyrite. The mean maximum reflectance of huminite was determined inoil (ca. 1.517 indexof refraction) with 546-nm-wavelengthgreen light. A Leitz MPVZ microscope photometer was used to measure huminite reflectance. Two glass standards were used to calibrate the equipment. The reflectance values were determined as designated in ASTM D2798-85.27 Twenty-five huminite reflectance values were determined for the Mequinenza lignite. The ' reproducibility of method D2798-85 for reflectance is 0.02 % Elemental Sulfur. Elemental sulfur was determined by extracting the lignite with tetrachloroethylene at ambient temperature with shaking. The extract was chromatographed on Florisil to remove some hydrocarbon coextractants. The portion that eluted within the known elution volume of elemental sulfur was collected and analyzed by liquid chromatography. The elution volume of elemental sulfur was determined by measuring it with a sample of elemental sulfur. Additional details of the method used are discussed elsewhere.%
.
Low-Voltage, High-Resolution Mass Spectrometry (LVHRMS). The LVHRMS data were obtained on a Kratos MS-50 high-resolution mass spectrometer interfaced to a Kratos DS-55 data system. The lignite extract was vaporized directly (18)Standard Method for Microscopical Determination of Volume Percent of Physical Components of Coal. In 1987AnnualBook of ASTM Standards, Petroleum Products, Lubricants, and Fossil Fuels; ASTM Philadelphia, 1987; Sect. 5, Vol. 05.05, D2799-86, pp 371-372. (19) Standard Test Methods for Total Sulfur in the Analysis Sample of Coal and Coke. In 1987Annual Book of ASTM Standards, Petroleum Products, Lubricants, and Fossil Fuels; ASTM Philadelphia,1987;Sect. 5, Vol. 05.05, D3177-84, pp 397-401. (20) Standard Test Methods for Carbon and Hydrogen in the Analysis Sample of Coal and Coke. In 1987 Annual Book of ASTM Standards, Petroleum Products, Lubricants, and Fossil Fuels; ASTM: Philadelphia, 1987; Sect. 5, Vol. 05.05, D3178-84, pp 402-407. (21) Standard Test Method for Nitrogen in the Analysis Sample of Coal and Coke. In I987 Annual Book of ASTM Standards, Petroleum Products, Lubricants, and Fossil Fuels; ASTM Philadelphia, 1987;Sect. 5, Vol. 05.05, D3179-84, pp 408-414. (22) Standard Test Method for Moisture in the Analysis Sample of Coal and Coke. In 1987 Annual Book of ASTM Standards, Petroleum Products, Lubricants, and Fossil Fuels; ASTM: Philadelphia,1987;Sect. 5, Vol. 05.05, D3173--87, pp 382-384. (23) Standard Test Method for Ash in the Analysis Sample of Coal and Coke From Coal. In 1987 Annual Book of ASTM Standards, PetroleumProducts,Lubricants, and Fossil Fuels; ASTM Philadelphia, 1987; Sect. 5, Vol. 05.05, D3174-82, pp 385-388. (24) Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke. In 1987Annual Book of ASTMStandards, Petroleum Products,Lubricants, and Fossil Fuels;,ASTM Philadelphia,1987;Sect. 5, Vol. 05.05, D3175-82, pp 389-391. (25) StandardTestMethodforFormsofSulfurinCoal. Inl987Annuul Book of ASTM Standards, Petroleum Products, Lubricants, and Fossil Fuels; ASTM Philadelphia, 1987;Sect. 5, Vol. 05.05, D2492-84,pp 336340. (26) Standard Test Method for Major and Minor Elementa in Coal and Coke Ash by Atomic Absorption. In 1987 Annual Book of ASTM Standards, Petroleum Products, Lubricants, and Fossil Fuels; ASTM Philadelphia, 1987; Sect. 5, Vol. 05.05, D3682-87, pp 443-449. (27) Standard Method for Microscopical Determination of the Reflectance of the Organic Components in a Polished Specimen of Coal. In 1987AnnualBook of ASTMStandards, PetroleumProducts, Lubricants, and Fossil Fuels; ASTM: Philadelphia, 1987;Sect. 5, Vol. 05.05, D279885, pp 367-370. (28) Buchanan, D. H.; Warfel, L. C. Prepr. Pap.-Am. Chem. Soc., Diuision Fuel Chem. 1990,35,516-522.
158 Energy & Fuels, Vol. 8, No. 1, 1994 into the ion source by a direct-insertion probe. Two spectra were recorded at a probe temperature of 350 "C. The magnet was scanned from 700 to 60 amu in 330 s, and was operated at a static resolving power exceeding one part in 30 000 with an average dynamic resolving power (while scanning) of approximately one part in 26 000. A heated (250 "C) 1-L expansionvolume inlet, equipped with a molecular leak, was employed for introduction of the reference compound mixture and was maintained at 250 "C and 1.3 X 104 Pa. Sample preparation involved weighing the extract into glass melting point capillaries and cutting to the proper length to allow sample evaporation in close proximity to the ion source. The ionizing voltage was regulated at 11.5 eV. Details concerning the low-voltage, highresolution mass spectrometric procedure and data reduction routines are presented elsewhere.29,30 A complete description of the direct-insertion probe technique, and an auxiliary computation method to improve the interpolation of sample peak masses, have also been reported.31 Electron Paramagnetic Resonance (EPR). An approximately 30-mm plug (187 mg) of lignite was placed in a tared 4 mm 0.d. quartz tube for X-band electron paramagnetic resonance (EPR) spectroscopy. The tube, which had been fitted with a standard taper joint, was then transferred to a vacuum line and the sample was evacuated for 3 days until an ionization gauge downstream of the sample read less than 1.3 X lo4 Pa. The sample tube was flame sealed with a torch while on the line and components were reweighed. The final mass after evacuation, 173 mg, was used in the spin density calculation. A similar preparation was used for W-band EPR spectroscopy. A Varian Model E-112 spectrometer equipped with a fieldfrequency lock, a Model E-102 (X-band) microwave bridge, and a Model E-232 dual sample cavity was used to make theg-value, line width, and spin density measurements. The procedure, standards, and calibrants used have been described elsewhere.32 All spectra were recorded at room temperature, 1.0-mW microwave power (set at the bridge), and 1.0-Gpeak-to-peak modulation amplitude. Reported measurements were the average from two sets of 10 time-averaged scans. W-band EPR spectra were recorded using a unique highfrequency (95 GHz) spectrometer constructed at the Illinois EPR Research Center in Urbana, IL.33 X-ray Powder Diffraction. The mineralsin the whole lignite were identified by X-ray powder diffraction performed using a Rigaku Geigerflex powder diffractometer and the Joint Committee for Powder Diffraction Standards (JCPDS) powder diffraction file.34
Sulfur L-edge X-ray Absorption Near-Edge Structure Spectroscopy (XANES). Sulfur L-edge XANES spectra were acquired at the Canadian Synchrotron Radiation Facility (CSRF) at the Aladdin storage ring, University of Wisconsin. The X-ray beam is monochromatized using a 1800/mm grating, yielding a photon resolution of better than 0.2 eV at the sulfur L-edge. The L-edge XANES spectra of model sulfur-containing compounds and the Mequinenza lignite were recorded in the region of 150220 eV in the total electron yield mode using a microchannel plate. The energy scale was calibrated with reference to the lowest pre-edge peak of elemental sulfur at 162.7 eV. At least three scans were digitally combined and the background was (29)Schmidt, C. E.; Sprecher, R. F.; Batts, B. D. Anal. Chem. 1987, 59,2027-2033. (30)White, C. M. InHandbook ofPolycyclic Aromatic Hydrocarbons, Bjorseth, A., Ed.; Marcel Dekker, New York, 1983;pp 525-616. (31)Schmidt, C. E.; Sprecher, R. F. In Novel Techniques in Fossil Fuel Mass Spectrometry; Ashe, T. R.; Wood, K. V., Eds.; ASTM: Philadelphia, 1989; ASTM STP 1019;pp 116-132. (32)Rothenberger, K.S.;Sprecher,R. F.; Castellano, S. M.; Retcofsky, H. L. In Techniques in Magnetic Resonance for Carbonaceous Solids; Botto, R.. Ed.; Advancesin Chemistry Series No. 229;American Chemical Society: Washington, DC, 1992;Chapter 31. (33)Clarkson, R. B.; Wang, W.; Nilges, M. J.; Belford, R. L. In Processing and Utilization of High-Sulfur Coals III; Markuszewski, R., Wheelock, T. D., Eds.; Elsevier: New York, 1990;pp 67-77. (34)PowderDiffractionFile, 1986,International Centre for Diffraction Data, 1601 Park Lane, Swarthmore, 1986.
White et al. removed. To apply the XANES technique for quantification of organosulfur species in coal, the spectra of model compounds and the lignite sample were truncated at 160-170 eV and the intensities were normalized with respect to the coal sample. The model sulfur-containing compounds used to calibrate the XANES spectrum were obtained from Aldrich in the highest purity available.
Sulfur K-edge X-ray Absorption Near Edge Structure Spectroscopy (XANES). Sulfur K-edge XANES spectra were acquired at the National Synchrotron Light Source at Brookhaven National laboratory. The X-ray beam was monochromatized using a silicon (111)double crystal monochromator. Spectra were collected over the range from 2400 to 2600 eV. In the vicinity of the sulfur K-edge (2460-2500 eV), data pointa were taken every 0.08/eV with a 1 s counting time. The energy scale was calibrated with respect to a zero point of energy defined as the major peak position in the sulfur K-edge XANES spectrum of elemental sulfur, at approximately 2472.0eV. As the sulfur signal from Mequinenza lignite was so intense, it was diluted by a factor of 5 in boric acid to avoid self-adsorption effects. The spectrum was analyzed according to standard procedures to obtain the normalized sulfur K-edge XANES spectrum from the raw s p e c t r ~ m . The ~ ~ ,XANES ~ region from -10 to 16 eV was then subjected to a least-squares analysis package that was developed specifically for the analysis of sulfur forms in coals and similar materiala.373 This procedure fits the sulfur XANES spectrum as the sum of (1)an arctangent function, representing the continuum edge step for the sulfur K-edge absorption, and (2) a number of LorentzianGaussian shaped peaks that represent white lines (1s-3p local transitions) of different sulfur forms. As discussed in detail elsewhere, the position of the peak is used to identify the form of sulfur and the area under the peak is converted, by means of a calibration procedure, to a quantitative measure of the amount of sulfur present in the sample in a particular form.37,40v41 This method of analysis differs from the L-edge analysis (and other proposed K-edge XANES analysis methods). The method of spectral interpretation and quantitation used here does not compare the sulfur K-edge XANES spectrum of the coal to any specific compound but rather uses computer fitting to estimate sulfur functional forms.
Controlled Atmosphere Programmed-Temperature Oxidation (CAPTO). A portion of the crushed and homogenized lignite sample was washed with 0.01 N HC1 for one hour to remove salts that might potentially absorb SOz. Then it was vacuum filtered and washed with distilled water for several hours. The distilled water washing and filtration were repeated twice, after which the sample was dried under vacuum at 105 "C for 2 h, cooled, and ground to pass a 100-mesh sieve. A 50-mg portion of this sample was thoroughly mixed with 1 2 g of tungsten trioxide and subjected to CAPTO analysis using equipment and procedures described elsewhere.424 Briefly, the mixture of lignite and tungsten trioxide was packed into a quartz tube, placed in (35)Lee, P. A.;Citrin, P. H.; Eisenberger, P.; Kincaid, B. M.Reu. Mod. Phys. 1981,53,769-806. (36)Brown,G.S.;Doniach,S. InSynchronRadiationResearch,Winich, H., Doniach, S., Eds.; Plenum Press: New York, 1980; pp 335-385. (37)Huffman, G. P.;Mitra, S.; Huggins, F. E.; Shah, N.; Vaidya, S.; Lu, F. Energy Fuels 1991,5,574-581. (38)Huggins,F. E.; Mitra, S.; Vaidya, S.;Taghiei, M. M.; Lu, F.; Shah, N.; Huffman, G. P. In Processing and Utilization of High Sulfur Coals I V ; Dugan, P. R., Quigley, D. R., Attia, Y. A., Eds.; Elsevier: Amsterdam, The Netherlands, 1991;pp 13-42. (39)Taghiei, M. M.; Huggins, F. E.; Shah, N.; Huffman, G, P. Energy Fuels 1992,6,293-300. (40)Huffman, G.P.; Huggins, F. E.; Mitra, S.; Shah, N.; Pugmire, R.; Davis, J.; Lytle, F. W.; Greegor, R. B. Energy Fuels 1989,3, 200-205. (41)Gorbats, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990,69, 945-952. (42)LaCount, R. B.: Anderson, R. R.; Friedman, S.; Blaustein, B. D. Fuel 1987,66,909-913. (43)LaCount, R. B.; Gapen, D. K.; King, W. P.; Dell, D. A.; Simpson, F. W.; Helms, C. A. In New approaches In Coal Chemistry; Blaustein, B. D., Bockrath, B. C., Friedman, S., Eds.; ACS Symposium Series 169 American Chemical Society: Washington, DC, 1981;pp 415-426.
A Study of Mequinenza Lignite
Energy &Fuels, Vol. 8, No. 1, 1994 159
Table 1. Results of the Ultimate, Proximate, and Sulfur Forms Analyses (in w t %) of Mequinenza Lignite aa recd moisture volatile matter fixed carbon ash hydrogen carbon nitrogen sulfur oxygen heating value, BTUs/lb sulfur forms sulfate pyritic organic
moisture free
Table 2. Major Elements (Expressed as Oxides) in the Mequinenza Lignite Ash*
moisture and ash free
7.4 40.1 32.5 20.0 4.5 48.9 0.7 9.9 16.1 9110
NA
NA
43.3 35.1 21.6 4.0 52.8 0.7 10.7 10.3 9840
55.3 44.7
0.02 0.57 9.30
0.03 0.61 10.04
0.03 0.78 12.80
NA 5.0 67.3 0.9 13.6 13.1 12,550
the furnace, and heated from room temperature to 1050 "C at a rate of 3 OC/min. During this, 100 mL/min of oxygen passed through the sample, causing the lignite to be oxidized to carbon dioxide, sulfur dioxide, water, and nitrogen dioxide. These gases were subsequently passed through an oxidation catalyst to ensure complete oxidation and analyzed by infrared spectroscopy.
Results and Discussion Proximate, Ultimate, and Forms of Sulfur Analyses. The results of the proximate and ultimate analyses of Mequinenza lignite are in Table 1 and are generally consistent with the material being ranked as lignite, although the moisture value is low, and the heating value is high for a lignite. The volatile matter is high at 55.3 % on a moisture- and ash-free (ma0 basis. It has a carbon content of 67.3% (maf). The results of the forms of sulfur analyses in Table 1 indicate that the sulfur is primarily organic sulfur (12.80 5% ) with smaller amounts of pyritic sulfur and very little sulfate. The ASTM value for organic sulfur may be somewhat high due to incomplete removal of pyritic sulfur from the lignite during nitric acid leachingbecause of pyrite encapsulation (vide infra). The results of the proximate and ultimate analyses are quite comparable to those reported by other investigators.gJ2 The sulfur content is extraordinarily high at 13.6% (maf). A major impediment to the characterization of organosulfur constituents in lower organic sulfur coals by LVHRMS and some other spectral techniques is the high hydrocarbon background, which interferes with the spectral observation of organosulfur compounds. Mequinenza lignite does not suffer significantly from this problem, and thus, for the scientist interested in characterizing organic sulfur components in coal, Mequinenza lignite is a good starting point. It should be noted that the XANES technique and XPS do not suffer from interference from a high hydrocarbon background. Mineral Matter: Microscopy. The lignite is high in ash-forming minerals, having a dry ash content of 21.6%. Microscopic examination found clay minerals to be the most abundant minerals present, with quartz, calcite, and pyrite also observed. The quartz is highly rounded, indicating considerable transport prior to deposition. Pyrite is mostly disseminated single euhedral crystals and ~~
(44) LaCount, R. B.; Kern, D. G.; King, W. P.; LaCount, R. B., Jr.; Miltz, D. J.; Stewart, A. L.; Trulli, T. K.; Walker, D. K.; Wider, R. K. Prepr. Pap-Am. Chem. Soc., Diu.Fuel Chem. 1991,36, 1217-1224. (45) LaCount, R. B.; Kern, D. G.; King, W. P.; Truilli, T. K.; Walker, D. K. Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1992,37, 10831086.
element Si02 A1203 Fez03 Ti02 a
wt%
60.20 13.27 6.27 0.89
element CaO MgO NazO KzO
wt%
5.11 1.81 1.15 2.32
SO3 in ash = 9.1 wt %.
framboidal forms suggestingan authigenic origin;however, some occurs in quartz and/or calcite and some is enveloped in liptinite (spores, pollen and cuticle) or resinite. The encapsulated pyrite (vide supra) would not be soluble in nitric acid and would be reported as organic sulfur.aB The determination of pyrite by microscopic methods yielded a much larger value than with determinations by other methods. For example, microscopic examination determined that the lignite was 1.4 vol 5% pyrite.'a ASTM analysis using nitric acid leaching determined that pyritic sulfur was present at 0.78 (ma0 wt 5%. X-ray diffraction indicated that pyrite was present in an amount less than 1%. CAPTO analysis of the Mequinenza lignite determined that 0.71 wt % pyritic sulfur was present. Cebolla et reported 0.89 and 1.04 (ma0 wt % pyritic sulfur for two Mequinenza lignite samples. Garcia et al." reported a pyritic sulfur concentration of 1.15 (maf) wt % for the Mequinenza lignite sample they investigated. Sinninghe Damste reported 1.2 wt % pyritic sulfur in the Mequinenza lignite they studied.6 Thus, the pyrite determined by ASTM microscopicmethods does not agree well with pyrite determined by any other method. The reasons for this discrepancy are not known. Nevertheless, the microscopic pyrite determination appears to be high relative to the other measurements. Mineral Matter: X-ray Diffraction. The analytical results for mineral matter obtained by microscopic examination of the whole lignite were confirmed by X-ray diffraction analysis. Quartz (SiOz), kaolinite [AlzSizOs(OH)*], illite [(K,H30)A12Si3A101o(OH)21, and pyrite (FeS2) were observed. The pyrite content was estimated using semiquantitative techniques at less than 1% by X-ray diffraction. Ash Analysis. The major elements in Mequinenza lignite ash were determined and the results are in Table 2. The major elements in the ash were Si, Al, Fe, Ca, and K, which are consistent with both the microscopic and X-ray diffraction observation of quartz, kaolinite, illite, pyrite, and calcite. The percent values in Table 2 do not, and should not, add to 100%. Petrographic Analysis. Mequinenza lignite had a huminite (precursor of vitrinite) mean maximum reflectance in green light and oil of 0.35% (Table 31, a value compatible with the lignite classification. Material that resembled the maceral fluorinite B, as described by Kalkreuth et a1.,49 frequently occurred in distinct patches or particle concentrations. Fluorinite is not a common coal maceral and is an outstanding feature of this lignite. Material labeled as fluorinite B is shown on a photomicrograph of the Mequinenza lignite in Figure 1. This (46) Edwards,A. H.; Jones, J. M.; Newcombe,W. Fuel 1964,43,55-62. (47) Straszheim, W. E. The Direct Determination of Organic Sulfur in Coal by Electron-Beam Microanalysis. Masters Thesis, Iowa State University, 1982. (48) Oder, R. R.; Gray, R. J. Proc. Znt. Conf. Coal Sci., Tokyo, Jpn. 1989; 995-998. (49) Kalkreuth, W.; Kotis, T.; Papanicolaou, C.; Kokkinakis, P. International J. Coal Geol. 1991, 17, 51-67.
White et al.
160 Energy &Fuels, Vol. 8, No. I , 1994
material was frequently found on cutinite, the waxy coating of leaves. It is highly fluorescent and is high in sulfur, as determined by energy-dispersive X-ray spot analysis. The petrographic maceral and mineral analysis of the Mequinenza lignite is in Table 3. The various macerals and minerals are illustrated in the photomicrographs shown in Figures 1 and 2. The lignite consists mainly of huminite (67.8%1. Liptinite makes up 8.8% of the sample, of which cutinite is 3.4% and exinite is 5.4%. Currently, the term liptinite includes exinite and cutinite. A large portion of the huminite (telogelinite) appears to be circular cell filings. About half of the semifusinites are fungal spores and/or sclerotia, and about half of the micrinite is inertodetrinite. The lignite contains 20.8 % inert material (inertinite and mineral matter) which is 3.0 % organic and 17.8% (by volume) mineral matter. The mineral matter is approximately 1.4% pyrite, 5.0% quartz, 11.1%clay, and 0.3% calcite by volume of the whole lignite. The ASTM method D2799-8618 makes no statements on the precision, reproducibility, or repeatability of the method. Cebolla et al. reported the reflectance of the two Mequinenza lignite samples they studied as 0.25% and 0.3276, respectively,12while the sample studied here had a reflectance of 0.35 % of the incident light. Although it is tempting to conclude that this sample is more mature than either of Cebolla's samples, the disparity may merely be a consequence of differences in measurement methods and lignites sampled. Cebolla et al. did not report if they used the mean maximum reflectance method or the random reflectance method.12 They also reported the results of petrographic analyses of two different Mequinenza lignites.12 The petrographic results displayed in Table 3 are in some instances quite different from the petrographic results for either of the two samples reported by Cebolla et al., which were also substantially different from one another. The samples reported by Cebolla et al. were from different seams. Apparently, the petrographic composition of Mequinenza lignite varies substantially from seam to seam. Sinninghe Damste et al. reported a reflectance of 0.31% for the Mequinenza lignite they studied.6 Elemental Sulfur. Liquid chromatographic analysis of the tetrachloroethylene extract of Mequinenza lignite showed that elemental sulfur was present in a concentration of about 1 pg/g of lignite. Elemental sulfur has been found in other coals and is thought to arise from oxidation of pyrite in the coal to form elemental sulfur and sulfate l 5 ~ 4 5 0 Low-Voltage,High-ResolutionMass Spectrometry. The pyridine extract was analyzed by LVHRMS. On average, 16.8% of the Mequinenza lignite was extractable with pyridine. An average of 22 % of the extract volatilized when heated in the direct-insertion probe of the mass spectrometer at 350 "C and 1.3 X lo4 Pa. Thus, only 3.7 % of the original lignite was analyzed by LVHRMS. A significant portion of the total volatiles were artifacts from the extracting solvent such as pyridine, hydroxypyridine, and bipyridyl. These were easily identified and excluded from the final report. It is difficult to make generalizations about the organosulfur constituents in the whole lignite based on LVHRMS analysis of about 3.7 % of the sample. This caveat should be kept in mind when analyzing the LVHRMS results presented below.
.
(50) White,C. M.; Lee, M. L. Geochim. Cosmochim. Acta 1980, 44, 1825-1832.
Table 3. Petrographic Maceral and Mineral Composition of Mequinenza Lignite. macerals Huminite Liptinite Fluorinite B Resinite
vol % 67.8 8.8 1.4 1.2
macerals Semifusinite Micrinite Fusinite Mineral Matter
vol % 1.0 1.5 0.5 17.8
Huminite reflectance, mean maximum = 0.35% in oil.
Low ionizing voltages were used to minimize fragmentation and favor the production of molecular ions. High resolution was used to separate ions having the same nominal mass. Table 4 displays a septet of peaks detected at m/z 268. Under low-resolution conditions, these ions would merge into one unresolved peak, providing little or no information. Also in Table 4 are calculated molecular masses for the formulas which are within f4.0 milli-amu of the measured mass, and the relative abundance of each peak. Those formulas that do not obey the nitrogen rule, or which contain 13C,are not included in Table 4. These formulas were generated using an atom combinatorial analysis routine with the heteroatom selection limited to two nitrogen, three oxygen, four sulfur atoms, and one 34s atom. Thus, they represent all logical combinations of those elements that could produce a molecular ion within k4.0 milli-amu. The possible assignments were then reviewed by the analyst and each measured mass was assigned a molecular formula as described below. The assigned formula is presented in bold type in Table 4. Traditionally, one persistent and seemingly unapproachable problem in high-resolution mass spectrometry has been the high-resolution requirements for the identification of organosulfur species in the medium to high mass range. The Kratos MS-50 was operated at a static resolving power of 1part in 30 000 and a dynamic resolution of 1part in 26 000, which is insufficient to baseline resolve the C + % H 4 doublet beyond a nominal mass of 100 amu. Table 5 provides an example of an unresolved C3-32SH4 doublet along with resolved 32S-02doublets and their corresponding baseline resolution requirements. In the absence of baseline resolution, some ambiguities can be overcome by interpretation. The peak at 268.1291 could be assigned toeither Cl&I2&, C15H220zMS,C15H24S2,or C21H16. C15Hzz02% can be ruled out because there was no peak corresponding to the parent isotope 2 amu lower at mlz 266 for C15H2202S. Of the remaining three possibilities, the peak at mlz 268.1291 is assigned to Cl&oS because its precise mass is closer to the measured mass than the other possibilitiesand because there were other members of the Cl&& homologous series present of lower and higher carbon number. The peak at mlz 268.0541 could be assigned to either C1gH802Or C16H1202S. The resolution required to baseline separate these two ions, if both are present in the sample, is 78 824 (Table 5). Here it is not possible to clearly assign one molecular formula based on measured mass alone, because the measured mass is precisely equidistant from either calculated mass by 0.0017 amu. Therefore, other conditions are relied upon for selection of the most likely molecular formula, namely, by providing limitations on hydrogen deficiency. C19H802 is very hydrogen deficient and less likely to exist in a low-rank coal or lignite extract than ClsH1202S. The peak at mlz 268.0362 could be assigned to C ~ ~ H ~ OClgH&, O ~ ~ ~C16H12S2, S, or C13H140234SS. The C16Hl002~S and C13H140PSS formulas can be eliminated
A Study of Mequinenza Lignite
Energy & Fuels, Vol. 8, No. 1, 1994 161
Figure 1. Photomicrograph of Mequinenza lignite showing pyrite framboids and quartz in the top photo and fluoronite B (white area) in the bottom photo. Reflected light in oil X450.
because there was no corresponding peak 2 amu lower. Even though the precise mass of C1&S is slightly closer, the peak at m/z 268.0362 is best assigned to C16H12S2, because C1&$ is also too hydrogen deficient. Using a dynamic resolution of 1part in 26 000, these two ions are not baseline separated,and thus it is difficult to distinguish between these two possibilities. The peak at m/z 268.0718 was not assigned a molecular formula because, in our epinion, none of the possible ~S combinations of CHONS were logical. C 1 ~ H 1 4 0 ~was rejected because there was no corresponding peak 2 amu ~OS~, lower, and the only other choice, C ~ ~ H I ~ N contained too many heteroatoms to be a realistic possibility. Table 6 contains part of the LVHRMS information obtained during analysis of the Mequinenza lignite extract
(85 of the 410 peaks observed). The structures provided in Table 6 are included only to facilitate understanding of the LVHRMS group analysis technique. Although the structures drawn are consistent with the observed molecular formulas, it must be strongly emphasized that the structures have not been identified in the lignite extract and that other isomers are possible. Few compounds can be identified based on LVHRMS results alone. The experimentally determined mass measurements compare very closely with the calculated precise masses of the assigned molecular formulas. On average, the deviation between the measured and calculated masses was
