In situ x-ray absorption fine structure spectroscopy investigation of

In situ x-ray absorption fine structure spectroscopy investigation of sulfur ... XAFS Spectroscopy Analysis of Selected Elements in Fine Particulate M...
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Energy & Fuels 1992,6, 293-300

to reach the calcium-carbon interface. The consequence of sintering in TPR is a shift of the temperature at which the gasification starts to higher values. The following points seem to be against the mechanism involving a CaOz-CaO cycle: (a) CaC03appears to be the active phase in C02-carbon gasification; (b) from a chemical point of view, CaC03 is stable in the presence of C02 and at the gasification temperatures, while Ca02 is not; (c) the calcium-carbon contact, and not the external surface, is the responsible of the catalytic activity; and (d) when calcium sintering degree increases, the temperature at which gasification starts is shifted to higher values. Finally, our resulta support that a CaC03-Ca0 cycle takes place during catalysis, as originally proposed by McKee. The mechanism involves the following steps:

293

CaO-C + C02 + CaC03-C (7) CaC03-C + CaO-*COZ-C ==CaO-C(0) CO (1) CaO-C(0) CaO-C + CO (2) where (7) is the carbonation of the Ca-carbon interface, (1)is the decomposition of the carbonate at the interface through a CaO-*C024 intermediate which yields CO, and (2) is the decomposition of an oxidized site of carbon that constitutes the determining step of the process. Previous results,14in which two kinds of CO were observed, support steps 1 and 2 of the mechanism.

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Acknowledgment. We thank the DIGCYT (Project PB-880295) and the MEC for Diego Cazorla's thesis grant. Registry No. C, 1440-44-0; COP,124-38-9;Ca, 1440-10-2.

In Situ X-ray Absorption Fine Structure Spectroscopy Investigation of Sulfur Functional Groups in Coal during Pyrolysis and Oxidation M. Mehdi Taghiei, Frank E. Huggins, Naresh Shah, and Gerald P. Huffman* 233 Mining and Mineral Resources Building, University of Kentucky, Lexington, Kentucky 40506 Received January 13, 1992. Revised Manuscript Received March 2, 1992

Sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy has been utilized to conduct the first direct characterization and quantification of sulfur functional groups in coal during in situ high-temperature oxidation and pyrolysis. The behavior of all major sulfur forms during such treatments was derived for two U.S. bituminous coals and a low-rank Australian brown coal from least-squares analysis of sulfur K-edge XANES spectra taken at constant temperatures up to 600 "C. During pyrolysis, pyrite began to convert to pyrrhotite at temperatures above 400 OC. The organic sulfides decreased significantly above 300 "C, while thiophenic sulfur remained nearly constant throughout the measurements. The formation of sulfate during oxidation experiments was observed above 300 "C.

Introduction It is well recognized that a full understanding of the characterization and behavior of all major forms of sulfur, both inorganic and organic, is essential for the solution of many of the significant research problems involving sulfur in coal. ASTM' methods D2492 and D3177 can be used to determine the total, pyritic, sulfatic, and, by difference, organic sulfur concentrations in coal. Scanning and transmission electron microscopies and electron microprobe2+ provide information on the local concentration and physical distribution of both organic and inorganic sulfur. Mossbauer spectro~cop@~ provides direct quantitative analyses of all iron-sulfur compounds (pyrite, iron sulfates, (1) Gaseous Fuels, Coal and Coke Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1986; Vol 05.05. (2) Wert, C. A.; Hsieh, K. C.; Tseng, B. H.; Ge, Y. P. Fuel 1987, 66, 915. (3) Straszheim, W. E.;Greer, R. T.; Markuszewski,R. Fuel 1983,62, 1070. (4) Karner, F. R.; Hoff, J. L.; Huber, T. P.; Schobert, H. H. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1986, 31, 29. (5)Raymond, R.; Gooley, R. Scanning Electron Microsc. 1978, I, 93.

(6) Huffman, G. P.; Lin, M. C.; Huggins, F. E.; Dunmyre, G. R.; Pignocco, A. G. Fuel 1985, 64,849. (7) Huffman, G. P.; Huggins, F. E. Fuel 1978, 57, 592.

0887-0624/92/2506-0293$03.00/0

pyrrhotite, etc.) in coal and coal-derived materials. X-ray photoelectron spectroscopy (XPS)"13 can distinguish between unoxidized and oxidized (divalent, tetravalent, and hexavalent) sulfur concentration on the surface of the coal. Until recently, information about the different organic forms of sulfur in coal could not be obtained directly but was based on indirect methods involving pyrolysis of the coal. Analytical pyrolysis and oxidative techniques emerged as a new discipline from the converging applications of two techniques: pyrolysis-gas chromatography (GC) and pyrolysis-mass spectrometry (MS), to analyze different forms of sulfur present in coal. Pyrolysis-GC/MS techniques eventually led to specific techniques for analysis of sulfur forms in coal such as flash pyrolysis ('pyroprobe") (8) Frost, D. C.; Leeder, W. R.; Tapping, R. L. Fuel 1974, 53, 206. (9) Frost, D.C.;Leeder, W. R.; Tapping, R. L.; Walbank, B. Fuel 1977, 56, 277. (10) Perry, D. L.; Grint, A. Fuel 1983, 62, 1024. (11) Pillai, K. C.: Young, V. Y.; Bockris, J. M. Colloid Interface Sci. 1985, 103, 145.

(12) Kelemen, S.R.;Gorbaty, M. L.; George, G. N.; Kwiatek, P. J. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991, 36, 1213. (13) Kelemen, S.R.;Gorbaty, M. L.; Kwiatek, P. J.; George, G. N. Fuel 1991, 70, 396.

0 1992 American Chemical Society

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Table I. Ultimate (Elemental) and Proximate Analyses for Coal Samples Used in This Study moisture ash volatile total s pyritic S organic S carbon . hydrogen nitrogen

Upper Freeport 1.13 13.03 27.14 2.32 1.77 0.54 74.23 4.08 1.35

Illinois No. 6 7.97 14.25 36.86 4.83 2.81 2.01 65.65 4.23 1.16

Australian 37.02 2.73 33.20 5.73 0.02 5.70 68.80 4.90 0.63

developed by Calkinsl4J5and the temperature-programmed reduction method advanced by Attar.16J7 Conventional pyrolysis and oxidation techniques will certainly make a lasting contribution to coal science. However, these t e c h n i q u e ~ ' ~are J ~ performed not on the bulk coal but on extracts and volatile fractions of the\coal and the results are related to sulfur forms in the whole coal. The analyses obtained, therefore, are based on the assumption that each sulfur functional group is totally pyrolyzed or oxidized, and the distribution and molecular arrangement of the organic sulfur structures remain the same as a function of time and temperature. As a result, the field of analytical pyrolysis has always had some rather interesting that preclude firm conclusions concerning the original sulfur in coal. In the past two years, X-ray absorption fine structure to be a powerful (XAFS)spectroscopy has been method for the direct, nondestructive, and quantitative determination of all major sulfur forms in coal. In particular, it has been demonstrated that analysis of the sulfur K-edge X-ray absorption near-edge structure (XANES) region of the spectra, both by a third derivative method*32 and by direct least-squares de~onvolution,3~-~~ can provide

(14) Calkins, W. H. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1985, 30(4), 450. (15) Calkins, W. H. Energy Fuels 1987, 1, 59. (16) Attar, A. In Analytical Methods for Coal and Coal Products; Karr, C., Ed.; Academic Press: New York, 1979; Vol. 3, Chapter 56. (17) Attar, A.; Dupuis, F. In Coal Structure; Gorbaty, M. L., Ouchi, K., Eds.; Advances in Chemistry 192; American Chemical Society: Washington, DC, 1981; Chapter 16. (18) Bakel, A. J.; Philp, R. P.; Galves-Silibaldi,A. In Geochemistry of Sulfur in Fossil Fuels; Orr,W. L., White, C. M., Eds.; American Chemical Society: Washington, DC, 1989; Vol. 429, p 326. (19) Boudou, J. P. In Geochemistry of Sulfur in Fossil Fuels; Orr, W. L., White, C. M., Eds.; American Chemical Society: Washington, DC, 1989; Vol. 429, p 345. (20) Krouse, H. R. J. Anal. Appl. Pyrolysis 1988, 14, 3. (21) Burnham, A. K. J. Anal. Appl. Pyrolysis 1988, 14, 1. (22) Damste, J. S. J. Anal. Appl. Pyrolysis 1991, 18, 353. (23) Ishwatari, M. J. Anal. Appl. Pyrolysis 1991, 18, 357. (24) Huffman, G. P.; Huggins, F. E.; Francis, H. E.; Mitra, S.; Shah, N. In Processing and Utilization of High-Sulfur Coal III; Elsevier: New York, 1990; p 21. (25) Huffman, G. P.; Huggins, F. E.; Shah, N.; Mitra, S.; Pugmire, R. J.; Davis, B.; Lytle, F. W.; Greegor, R. B. Energy Fuels 1989, 3, 200. (26) George, G. N.; Gorbaty, M. L. J. Am. Chem. SOC.1989,111,3182. (27) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1990, 35(3), 779. (28) Huffman, G. P.; Huggins, F. E.; Mitra, S.; Shah, N. Proceedings of the International Conference on Coal Science; Int. Eng. Agency; Butterworth-Heineman Ltd.: Oxford, 1989; p 47. (29) Huffman, G. P.; Shah, N.; Taghiei, M. M.; Huggins, F. E.; Mitra, S. Proceedings of the International Conference on Coal Science; Int. Eng. Agency; Butterworth-Heineman Ltd.: Oxford, 1991; p 969. (30) Kelemen, S. R.; Gorbaty, M. L.; George, G. N. Fuel 1990,69,939. (31) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990,69,945. (32) Gorbaty, M. L.; George, G. N.; Kelemen, S. R. Fuel 1990,69,1065. (33) Huffman, G. P.; Mitra, S.; Huggins, F. E.; Shah, N.; Vaidya, S.; Lu, F. Energy Fuels 1991, 5, 574.

Figure 1. Schematic of the high-temperature in situ XAFS reaction furnace.

quantitative concentrations for all major organic and inorganic sulfur functional forms. In this study, results from the first in situ XAFS spectroscopicinvestigations of sulfur in coal under conditions of high-temperature pyrolysis and oxidation are reported. Emphasis has been given to the quantitative analysis of the various forms of sulfur produced during slow pyrolysis and oxidation of coal at temperatures up to 600 "C. In situ sulfur K-edge XAFS measurements have been performed on Illinois No. 6 and Upper Freeport bituminous coals from the Argonne Premium Coal Sample Bank, and a low-rank Australian brown coal. The results show that, under the relatively slow pyrolysis and oxidation conditions employed, pyrrhotite is formed from pyrite during both pyrolysis and oxidation of Illinois No. 6 and Upper Freeport coals above 400 "C. Degradation of organic disulfidic and sulfidic sulfur starts as low as 300 "C, while thiophenic sulfur remains stable up to temperatures above 500 "C. Results obtained during oxidation at temperatures up to 450 "C show gradual formation of small amounts of sulfate in both Upper Freeport and Illinois No. 6 coal samples and of a sulfonic acid species in the Australian brown coal.

Experimental Procedure Sample Description. The coal samples used in this s t u d y were a high-volatile bituminous Illinois No. 6 and a mediumvolatile bituminous Upper Freeport coal obtained from the Argonne Premium Coal Sample B a n k (APCSB), and a low-rank Australian brown coal (Glencoe). These coals cover a range of rank, pyrite, and organic sulfide content. The Argonne coals have been widely studied i n conventional pyrolysis and oxidation research. Table I shows the ultimate and proximate analyses for the three coals used in this study. The Illinois No. 6 coal contains 4.83 wt % sulfur of which 2.81 wt % is pyritic sulfur, while Upper Freeport is a higher rank coal than Illinois No. 6 coal and contains total sulfur of 2.32 wt %, of which 1.77 wt % is pyritic sulfur. The Australian coal contains 5.73% organic sulfur and very little ~~

(34) Huggins, F. E.; Mitra, S.; Vaidya, S.; Taghiei, M. M.; Lu, F.; Shah, N.; Huffman, G. P. In Processing and Utilization of High Sulfur Coal IV; Dugan, P. R., Quigley, D. A., Attia, Y. A., Eds.; Elsevier: New York, 1991; Vol. 4, p 13. (35) Huggins, F. E.; Shah, N.; Huffman, G. P. EPRI Report EAR/ GS-7322, Research Project 8003-20, Electric Power Research Institute, Palo Alto, CA, 1991.

Spectra of Sulfur Functional Groups in Coal

Energy & Fuels, Vol. 6, No. 3,1992 295

Table 11. Values for Slope and Intercept Used in Least-Squares Analyses of Sulfur SDectra Sulfur form slope intercept 0.13 0.00 pyrrhotitic sulfur 0.13 0.00 pyritic sulfur 0.92 0.00 sulfidic sulfur thiophenic sulfur 1.00 0.00 sulfoxide 2.20 0.47 2.70 0.18 sulfone 2.20 0.14 sulfate pyritic sulfur (X0.2 wt %). All experiments were carried out on 1.5-2.0 g of dried -100 mesh coal samples. High-Temperature Reaction Furnace. The XAFS reaction furnace used in this work is modified from the design of Sinfelt and LytleP and a schematic drawing of the furnace and sample cell is shown in Figure 1. The sample chamber is fabricated from a stainless steel block with a beryllium window. The beryllium window is sealed to the sample cell by a stainless steel frame using a copper gasket. A chromel-alumel thermocouple is imbedded in the back plate of the sample cell and connected to a temperature controller. The hot cell is inserted into a sealed aluminum box, which is filled with helium gas during the experiment. Water flow through channels in the box dissipates the heat and prevents the Mylar window of the fluoreecent detector from melting. Readant gases are flowed through the sample cell, which is packed with the coal samples. Sulfur K-edge XAFS spectra were obtained from the coal sampleswhile flowing a gas stream of helium or hydrogen during pyrolysis or a mixture of 95% helium and 5 % oxygen during oxidation at specific temperatures up to 500 O C . Since each spectrum took approximately 30 min to complete, these experiments are considered to be slow pyrolysisand oxidationprocesses. Heating rates were on the order of 3 O C / s between spectra. XAFS Analysis. The XAFS measurements were performed at beam line X-19A at the National Synchrotron Light Source (NSLS)in Brookhaven National Laboratory (BNL). The experimental procedures are described in detail elsewhere.% Sulfur K-edge XAFS spectra were taken in the fluorescent mode using a Si(ll1) double crystal monochromatorand a Stem-Heald type dete~tor.~' The energy scale was calibrated with respect to elemental sulfur by assigning 2472.0 eV to the s p peak maximum in the sulfur XAFS spectrum. The least-squares method for analysis of the XANES of the sulfur K-edge spectra is based on the concept that the experimental spectrum can be modeled as the sum of one or two arctangent functions representing the transitions of photoelectron to the continuum and a number of absorption peaks that arise from the 1s 3p electronic transitione of the various functional forms of sulfur in the coaL Each of the individualabsorption peaks is fundamentally Lorentzian shaped, but Gaussian broadened. A L0rentZian:Gaussianfunction was found to fit the XANES experimental data most successfully. To convert the measurementsof each specific sulfur peak area to weight percent sulfur, a scaling factor is required. This can be obtained by observing the correlationbetween area of the sulfur peak and the valence states of different sulfur standard compounds, as described elsewhere.% The accuracy of this method in the resulting sulfur percentages is i5-10%. Calibration Experiments. The eight standard model compounds that were selected for determining calibration constants are pyrrhotite, pyrite, elemental sulfur, dibenzyl sulfide, dibenzothiophene, sulfoxide, butyl sulfone, and ferrous sulfate. These compounds were chosen because they represent the principal sulfur functional forms believed to occur in coal. The values of the slope and intercept obtained for the mixtures of dibenzothiopheneand different standard compounds are tabulated in Table 11. These findings are revised somewhat with respect to the previous results obtained by Mitra,= due to a correction

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(36) Sinfelt, J. H.; Via, G. H.; Lytle, F. W. Chem. Phys. 1982, 76,2779. (37) Lytle, F. W.; Greegor, R. B.; Sandstrom, D. R.; Marquies, E. C.; Wong, J.; Spiro, C. L.; Huffman, G. P.; Huggins, F. E. Nucl. Instrum. Methods, 1986,226, 542. (38) Mitra, S. Ph.D. Dissertation, Department of Physics, University of Kentucky, 1991.

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Figure 2. (a) Sequence of sulfur K-edge XANES spectra from Illinois No. 6 coal at indicated temperatures during pyrolysis in helium. (b and c) Least-squares fits for spectra at 600 "C and room temperature. made in the formula for the peak-shapeanalysis. Note that values for slope are extracted from calibration data for mixture of reference compounds and dibenzothiophene, while the intercepts are derived from Argonne premium coal data, assuming sulfur in oxidized forms is not present in these coals. The procedure for evaluating the slopes and intercepts is discussed in detail Table III summarizes the information obtained for percentages of sulfur in different functional groups present in the Argonne premium coals. These data were obtained based on the leastsquares analysis of sulfur K-edge XANES spectra of each specific coal. It can be seen that, except for the Pittsburgh No. 8 coal, the ratio of thiophenic sulfur to sulfidic sulfur form increases with coal rank, as reported p r e v i o u ~ l y . ~It~is* interesting ~~ to note that Attar and Hendrickson" also observed that Pittsburgh coal was an exception to their trends and that the content of thiophenic sulfur in this coal is less than would be expected on the basis of its rank.

Results and Discussion A. Pyrolysis of Illinois No. 6 Coal. A sequence of in situ sulfur K-edge XANES spectra obtained during pyrolysis of the Illinois No. 6 coal and least-squares fitted spectra at ambient temperature and 600 "C are shown in Figure 2a-c. The appearance of a peak at a negative energy at 400 "C corresponds to the transformation of pyritic sulfur to pyrrhotite form; this transformation continues as the temperature is raised and appears to be more or less completed at 500 "C. Transformation of pyrite to pyrrhotite at elevated temperatures is thermodynamically favorable and has been reported in several other s t ~ d i e s . Reduction ~ ~ ? ~ ~ of pyrite to pyrrhotite produces a nascent sulfur which in turn reacts with H2and f or (39) George, G. N.; Gorbaty, M. L.; Kelemen, S.R.; Sansone, M. Energy Fuels, 1991,5, 93. (40) Attar, A.; Hendrickson, G. G. Coal Structure; Academic Press: New York, 1982; p 131. (41) Reid, W. T. Chemistry of Coal Utilization; Elliott, M. A., Ed.; John Wiley and Sons, Inc.: New York, 1981; Vol. 2, p 1389. (42) Raask, E. Mineral Impurities in Coal Combustion: Hemiwhere Pub. Corp.: New York, 1985

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296 Energy & Fuels, Vol. 6, No. 3, 1992

Table 111. Percentages of Different Sulfur Forms in the Argonne Premium Coal Samples Argonne Premium Coal Sample Pocahontas No. 3, VA Upper Freeport, PA Pittsburgh No. 8, PA Lewis.-Stock., WV Blind Canyon, U T Illinois No. 6, IL Wyodak-Anders., WY Beulah, ND

total sulfur, wt % 0.66 2.32 2.19 0.71 0.62 4.83 0.63 0.80

pyritic sulfur, % 12 59 43 15 32 37 15 28

sulfidic sulfur, % 0 2 22 18 18 23 33 27

thiophenic sulfur, % 85 39 32 64 50 40 49 33

sulfoxidic sulfur, % 0 0 2 2 0 0 2 4

sulfonic sulfur, % 3 0 1 1 0 0 1 0

~~

sulfatic sulfur, % 0 0 0 0 0 0 0 8

~

Figure 3. Percentwes of different sulfur forms in Illinois No. 6 coal under pyrolysis in helium at t h e indicated temperatures obtained bfleast-squares fit -analysis.

CO evolved during devolatilization by the following endothermic reactions: FeS2 = Fel-,S + S FeS2 + H2 = Fel-,S FeS2

+ CO = Fel-,S

+ H2S + COS

Mossbauer spectroscopy measurement of the Illinois No. 6 char confirmed that pyrrhotite was the only iron sulfide remaining in the sample after pyrolysis at 600 "C in a helium atmosphere. Based on the least-squares fitting results, the effect of pyrolysis on different forms of sulfur in this coal as a function of temperature is illustrated in bar-graph form in Figure 3. It is evident that between 250 and 600 "C the organic sulfide component decreases from 20% of the total sulfur to about 5%. However, the percentage of total sulfur that is thiophenic sulfur remains nearly constant up to 400 "C and decreases slightly above that temperature. No significant differences in pyrolysis trends were observed between the pyrolysis in helium and that in hydrogen atmosphere. However, the amount of total sulfur left in the sample after pyrolysis in helium atmosphere was 3.16 wt %, while it was 2.50 wt % for pyrolysis in hydrogen atmosphere. This indicates the greater loss of organic sulfur in the form of H2S in hydrogen environment than in helium. Winans et al.43conducted a vacuum pyrolysis study of Illinois No. 6 using mass spectrometry at temperatures up to 600 "C. They concluded that aliphatic sulfur compounds evolved at temperatures between 250-350 "C, while aromatic sulfur volatilized at above 425 "C. Flash pyro(43) Winans, R. E.; Scott, R. G.; McBeth, R. L.; Neill, P. H. In Second Int. Conf. Proc. Util. High Sulfur Coals; 1987, 1, 3.

lysis-gas chromatograph studies of Illinois No. 6 coal by Calkins et al.14J5and Chou et al.,44coupled with observations of Coleman and his c o - ~ o r k e r smade ~ ~ during a pyrolysis study on Illinois No. 6 coal, further confirm the results found in this study that volatilization of organosulfur compounds begins at about 250 "C as a consequence of thermal decomposition. Chu et a1.46y47also measured the slow pyrolysis of Illinois No. 6 coal using Fourier transform infrared (FTIR) spectroscopy. They observed the evolution of H2S at temperatures from 200 to 500 "C and evolution of CS2between 800 and 1200 "C. Therefore, they suggested that no clear correlation is present between the weights of H2Sand CS2 evolved and the organic sulfur content of the coal. The evolution of CS2 at higher temperature may be related to the organic sulfur retained in the char in thiophenic form. B. Pyrolysis of Upper Freeport Coal. The relative percentages of the sulfur functional groups in the Upper Freeport coal during pyrolysis are shown in bar-graph form in Figure 4. Least-squares analysis of the sulfur XANES spectra for the Upper Freeport coal at room temperature indicates that this coal contains very little organic sulfide sulfur. The percentage of thiophenic sulfur remains nearly constant during the course of pyrolysis experiment up to 400 "C and decreases slightly thereafter, as was seen for pyrolysis of Illinois No. 6. Therefore, the decrease in total sulfur content as pyrolysis temperature increases is attributed to the loss of pyritic sulfur associated with the conversion of pyrite to pyrrhotite above 400 "C. (44) Chou, M. M.; Loffredo, D. M. Fuel 1985, 65, 731. (45) Coleman, D. D.; Liu, C. L.; Frost, R. R.; Hughes, R. E.; Frost, J. K. Illinois State Geological Survey; Annual Report, 1984. (46) Chu, C.; Fredin, L.; Hauge, R.; Margrave, J. High Temp. Sci. 1985, 20,51. (47) Chu, C. J.; Cannon, S. A.; Hauge, R. H.; Margrave, J. L. J. Anal. Appl. Pyrolysis 1988, 14, 115.

Energy & Fuels, Vol. 6, No. 3, 1992 297

Spectra of Sulfur Functional Groups in Coal

"

N Pyrrhotitic 0 Thiophenic

Pyritic

Figure 4. Percentages of sulfur in various functional groups at indicated temperatures for Upper Freeport coal under pyrolysis in helium. It is seen that essentially all of the pyrite in Illinois No. 6 coal is reduced to pyrrhotite during high-temperature pyrolysis, while a significant percentage of pyrite remains unreduced for Upper Freeport coal. The ease of reduction of pyrite to pyrrhotite may be related to the size of the pyrite particles in coal and their encapsulation within the coal matrix. Computer-controlled scanning electron microscopy (CCSEM) analysis for pyrite particle size of 11linois No. 6 and Upper Freeport coals indicates that Illinois No. 6 coal contains a larger percentage of large particle size pyrite than Upper Freeport Therefore, in Upper Freeport coal, which is a coal with more fluidity than Illinois No. 6 coal, pyrite particles can be more easily encapsulated in coal matrix and are less readily accessible for pyrolysis. C. Pyrolysis of Australian Brown Coal. Figure 5a,b illustrates the least-squares fitted XANES spectrum and the third derivative spectrum for the Australian brown coal (Glencoe) at ambient temperature. The presence of a disulfide functional group in this coal is evident from these analyses as indicated by arrows in Figure 5b. The behavior of various sulfur functional groups during pyrolysis of the Australian coal under helium is illustrated in Figure 6. It is seen that disulfide compounds start to degrade at lower temperature than sulfidic form, while the percentage of thiophenic sulfur increases as a function of temperature. It has been previously that aliphatic sulfide converts to aromatic thiophenic form of sulfur during the pyrolysis of low-rank coal. However, it should be noted that the histogram (Figure 6) shows the relative values of the sulfur forms and not the absolute amounts. The minor oxidized forms of sulfur, such as sulfoxide, sulfone, and sulfate compounds, become less abundant during the pyrolysis of Australian brown coal starting above 200 "C. D. Oxidation of Australian Brown Coal. The comparison of the least-squares curve fitting of the XANES spectra of Australian brown coal at ambient temperature and a t 450 "C under oxidative atmosphere is shown in Figure 7. The corresponding changes that occur in the sulfur forms of Australian coal during oxidation in a (48) Shah, A.; Shah, N.; Huggins, F. E.; Huffman, G. P. Unpublished Data, University of Kentucky. (49) Stephenson, M. D.; Rostamagadi, M.; Johnson, L. A.; Kruse, C. W. R o c . Util. High Sulfur Coals; 1985, 535. (50)Attar, A. Fuel 1978, 57, 201. (51) Cleyle, P. T.; Caby, W. F.; Stewart, I.; Whiteway, S. G. Fuel 1984, 63,1579.

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Figure 5. (a) Least-squares XANJ3S spectrum fit for Australian brown coal at room temperature; (b) third derivative of the XANES spectrum indicating the presence of disulfide, sulfide, and thiophene in this coal. mixture of 95% He + 5 % O2flowing gas at various temperatures up to 450 "C are illustrated in bar-graph form in Figure 8. Qualitatively similar trends were observed for degradation of organic sulfides during oxidation of the Australian coal as those found during pyrolysis of this coaL The most significant difference noted during the oxidation of this coal compared to pyrolysis is the growth of a peak at higher energy position. This position is intermediate between that determined for sulfonate (8.9-9.1 eV) and sulfate (9.9-10.1 Because the Australian coal contains very little iron or calcium, but contains substantial amounts of disulfide, the origin of this peak may be due to the formation of sulfonic acid. This speculation is consistent with the studies by S ~ t e and r ~ Redi53 ~ on oxidation of standard compounds in which they concluded ~

(52)Suter, C. M.Organic Chemistry of Sulfur; John Wiley and Sons: London, 1944. (53)Reid, E.E.Chemistry of Diualent Sulfur; Chemical Publishing Co.: New York, 1960.

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298 Energy & Fuels, Vol. 6, No. 3, 1992

Figure 6. Percentages of sulfur in various functional forms as a function of temperature for Australian brown coal under pyrolysis

in helium atmosphere. formed to pyrrhotite and to an iron sulfate. Mossbauer spectroscopy measurements were performed on the Upper Freeport char left behind after the in situ pyrolysis and oxidation experiments. The results confiied the presence of both pyrite and pyrrhotite in the samples. Thiophenic sulfur content remained essentially constant throughout the experiments.

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Figure 7. Comparison of least-squares curve fitted XANES

spectra of Australian brown coal at room temperature and 450 "C in oxidative condition. that oxidation of sulfide will result in production of sulfoxide or sulfone, whereas sulfonic acid would be produced by oxidation of disulfide. At the conclusion of the experiment, the sample contained 4.2 wt % sulfur, indicating that about 1.5 w t % of the sulfur had left the sample as SO2formed by oxidation of the organic sulfides. E. Oxidation of Illinois No. 6 Coal. Figure 9 shows the percentage of different sulfur forms in Illinois No. 6 coal under oxidative conditions in a mixture of 95% helium + 5% oxygen. The principal changes during oxidation are a conversion of pyritic sulfur into pyrrhotite above 400 "C and oxidation of pyrite to sulfate compounds. Thiophenic sulfur is more or less stable throughout the experiments, while the sulfidic form decreases as a function of temperature. F. Oxidation of Upper Freeport Coal. Least-squares analyses of sulfur K-edge XANES spectra obtained during oxidation of Upper Freeport coal indicate a similar pattern. Figure 10 shows the bar graph for the behavior of different sulfur forms present in Upper Freeport coal under oxidative conditions. As expected, pyrite is partially trans-

Conclusions Based on high-temperature in situ sulfur K-edge XAFS spectroscopy measurements of several coal samples, the following remarks can be made. 1. High-temperature in situ XAFS measurements can determine quantitatively the reactions of both organic and inorganic sulfur functional forms during thermal treatment of coal. 2. During pyrolysis of Illinois No. 6 coal in either helium or hydrogen, degradation of sulfidic sulfur begins above 250 "C, while thiophenic sulfur remains nearly constant up to 400 "C. Conversion of pyritic to pyrrhotitic sulfur starts at 400 "C and is complete at 500 "C. 3. Similar trends were observed for the behavior of organic sulfur functional forms during pyrolysis of Upper Freeport coal as for Illinois No. 6. However, pyritic sulfur is only partially converted to pyrrhotitic form and the presence of pyrite is still evident at 500 "C. 4. During pyrolysis of an Australian brown coal, the degradation of disulfide starts at about 300 "C, while the organic sulfide starts to decompose at temperatures above 425 "C. Reduction of minor oxidized forms of sulfur such as sulfoxide, sulfone, and sulfate starts at temperatures above 200 "C and increases as a function of temperature. 5. Under oxidative conditions, in a mixture of 95% helium + 5% oxygen, sulfur functional groups in Illinois No. 6 coal undergo the following changes: partial conversion of pyritic to pyrrhotitic sulfur f u m at temperatures above 400 "C; oxidation of pyritic sulfur form to sulfate sulfur above 200 "C; and a decrease in organic sulfide, presumably by release of SOz. Thiophenic sulfur remains nearly constant throughout the measurement. 6. Oxidation of Upper Freeport coal results in partial conversion of pyritic sulfur form to pyrrhotitic sulfur form as well as oxidation of pyrite to iron sulfate. No significant changes was observed for thiophenic sulfur at temperatures up to 450 "C. 7. During oxidation of a brown coal at temperatures up to 450 "C, organic disulfide disappeared and organic sulfide

Spectra of Sulfur Functional Groups in Coal

I

Energy & Fuels, Vol. 6, No. 3, 1992 299

Sulfoxide E3 Sulfonic Acid

Sulfone

Disulfidic 0 Sulfidic

Thiophenic

I

U

Figure 8. Percentages of sulfur in various functional forms as a function of temperature for Australian brown coal under oxidative condition in 95% He + 5% 02.

Figure 9. Pe temperatiures

at the indicated

I

I

I

70

60 50 40

30 20 10

I

Sulfatic El Pyrrhotite 0 Thiophenic

Pyritic

I

0

Figure 10. Percentages of sulfur in various functional groups at indicated temperatures for Upper Freeport coal under oxidative condition.

Energy & Fuels 1992,6, 300-307

300

decreased by approximately 25% relative to thiophene. A gradual increase of a peak at a high-energy position was attributed to the formation of sulfonic acid as a result of oxidation of the disulfidic sulfur form. Acknowledgment. Support for this study by the Office

of Exploratory and Applied Fiesearch of the Electric Power Research Institute under EPRI Contract RP 8003-20 is gratefully acknowledged. Registry NO.Pyrite, 1309-36-0;pyrrhotite, 1310-50-5;sulfate,

14808-79-8.

A Comparative Gas Oil Hydroprocessing Study of Alumina, Carbon, and Carbon-Covered Alumina Supported Ni-Mo Catalysts: Effect of Quinoline, Thiophene, and Vanadium Spiking P. M. Boorman* and K. Chong Department of Chemistry, University of Calgary, Calgary, Alberta, Canada, T2N 1N4 Received September 19, 1991. Revised Manuscript Received January 13, 1992

Catalysts with 3-15 w t % NiO-Mo03 supported on alumina, carbon-covered alumina, and carbon were used to hydroprocess gas oil spiked with quinoline, thiophene, and vanadium to give an insight into the relationship between the support and deactivation. Experiments were carried out in a batch reactor at 410 "Cand lo00 psi initial H2 pressure. It was found that N, S, and V compounds in the feed suppress the hydrogen uptake by gas oil hydroprocessed with alumina or carbon-covered alumina supported catalysts but carbon supported catalysts are insensitive to these compounds. Quinoline HDN and thiophene HDS do occur thermally to a small extent but a catalyst is required for efficient HDN and HDS. Under the conditions used for our experiments, quinoline is rapidly hydrogenated to 1,2,3,4-tetrahydroquinolineand it is the subsequent C-N bond hydrogenolysis that is the ratedetermining step. For hydroprocessingthe high nitrogen content quinoline spiked gas oil,the carbon supported catalyst is comparable to the alumina supported catalyst for sulfur removal but is superior for quinoline HDN. Basic nitrogen compounds, such as quinoline, have an affiiity for acidic catalyst surfaces and undergo coking reactions thereby deactivating metal centers and reducing the total surface area. The use of a carbonaceous support decreases the amount of coke deposited on the catalyst surface.

Introduction In addition to high catalytic activity and selectivity, a successful catalyst must also have an acceptable lifetime, resisting deactivation, if it is to be any of any industrial value. This will be the focus and the challenge in hydroprocessing catalyst research and development due to the shift in feedstock quality to heavier feeds' combined with the implementation of stricter environmental regulations.2 With feedstocks becoming heavier, Le., more aromatic content, higher levels of sulfur, nitrogen, and metals such as vanadium, catalysts need to be able to withstand the deleterious effects imposed by these compounds in the feed. There are many causes of dea~tivation.~Generally, the catalyst can be deactivated by the condensation of olefins and aromatics to form heavier polynuclear species (coke) or by the irreversible adsorption of metal species and/or other compounds in the feed. These compounds can deactivate the catalyst by having an affinity for catalyst active sites, e.g., basic nitrogen compounds poisoning acidic sites, a chemical interaction, or by physically plugging the pore structure preventing the reactants from accessing the active sites. Both result in a decrease in the catalyst's active surface area. It has been reported that catalyst

* To whom correspondence should be addressed. 0887-0624/92/2506-0300$03.00/0

deactivation by coke formation is several times greater than that caused by metal deposition' for the conversion of high boiling feedstocks to lower boiling distillates. However, the exact mechanism by which coke attenuates the catalytic activity is unclear and is complicated by the fact that deactivation is dependent on feedstock composition. For example, a naphtha hydrodesulfurization catalyst may remain active for years, whereas a residuum hydroprocessing catalyst may last only a few months.6 Furthermore, it is generally accepted that catalyst deactivation by coke and metals involves acid sites and that the basicity of a compound will strongly influence its propensity to form coke? It is apparent then that catalyst deactivation is not a simple process; rather, it is a complex network of reactions involving physical and chemical processes. ~~

(1) 'Energy Alberta 1990"; Energy Resources Conservation Board Review of Alberta Energy Resources in 1990 (ERCB 91-40). (2) Skubnik, M.; Wilson, M. F.; McCann, T. J. AOSTRA J . Res. 1990, 6, 1. (3) Richardson, J. T. In Principles of Catalyst Deuelopment; Twigg, M. V., Spencer, M. S., Eds.; Plenum Press: New York, 1989; p 185. (4) Ternan, M.; Furimsky, E.; Parsons, B. I. Fuel Process. Technol. 1979, 2, 45. (5) Diez, F.; Gates, B. C.; Miller, J. T.; Sajkowski, D. J.; Kukes, S. G. Ind. Eng. Chem. Res. 1990,29, 1999. (6) Adkins, B. D.; Milburn, D. R.; Goodman, J. P.; Davis, B. H. Appl. Catal. 1988, 44, 199.

0 1992 American Chemical Society