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Energy & Fuels 1998, 12, 734-739
Desulfurization and Solubilization of High-Sulfur Coal by Superacid HF/BF3 Kiyoyuki Shimizu* and Ikuo Saito Energy Resources Department, National Institute for Resources and Environment, AIST, 16-3 Onogawa, Tsukuba, Ibaraki 305, Japan
Ken Shimamura and Akira Suganuma Department of Industrial Chemistry, Faculty of Science and Technology, Science University of Tokyo, Noda, Chiba 278, Japan Received November 14, 1997
Anhydrous hydrogen fluoride (HF) alone or combined with a small proportion of boron trifluoride (BF3) desulfurized high-sulfur coal in the presence of hydrocarbons without H2 in mild reaction conditions. HF removed mainly sulfidic and oxidized sulfur at temperatures of 75-200 °C, but at 250 °C decreased thiophenic sulfur. HF/BF3 at 200 °C decreased thiophenic sulfur by ionic hydrogenation as well as decreased sulfidic and oxidized sulfur. The extent of desulfurization of coal was roughly correlated with solubilization. Lighter fractions (hexane insoluble-benzene soluble) in the treated coal had lower sulfur content, especially of sulfidic sulfur, indicating that removal of sulfidic sulfur in coal improves its extractability. In contrast, the heavier fraction in the treated coal had higher sulfur content, which consisted of reactive sulfidic and oxidized sulfur as well as thiophenic sulfur.
1. Introduction Organic sulfur in coal is difficult to remove by physical means or conventional chemical processes because it is firmly linked with the organic units of coal. Organic sulfur has been partly released with catalysts such as pyrite-derived pyrrhotite and acidic minerals (silica and alumina) at temperatures higher than 300-350 °C.1,2 Molybdenum metal and molybdate salts are reported to catalyze the desulfurization of high-sulfur coal at 250-350 °C.3 These catalysts are solid and require higher temperatures and high hydrogen pressures. A Lewis acid, ZrCl2, was found to promote the removal of sulfur from sulfides and disulfides (model aliphatic sulfur compounds) at 325 °C under high hydrogen pressure (12-16 MPa), but not from phenyl sulfide, diphenyl disulfide, thiophenic sulfur, or dibenzothiophene.4 Proton transfer from a Bro¨nsted acid, H+(ZnCl2OH)-, which is produced with water present as an impurity, is believed to initiate such a reaction. Strong acids have been reported to depolymerize coal in milder reaction conditions.5-9 The high acidity of superacids can be expected to desulfurize and solubilize coal catalytically and also contribute to deoxygenation (1) Attar, A. Fuel 1978, 57, 201. (2) Robinson, L. Hydrocarbon Process. 1978, 57, 213. (3) Garcia, A. B.; Schobert, H. H. Fuel 1989, 68, 1613. (4) Mobley, D. P.; Bell, A. T. Fuel 1980, 59, 507. (5) Heredy, L. A.; Neuworth, M. B. Fuel 1962, 41, 21. (6) Butler, R.; Snelson, A. Fuel 1980, 59, 93. (7) Olah, G. A.; Bruce, M. R.; Edelson, E. H.; Husain, A. Fuel 1984, 63, 1130. (8) Kumagai, H.; Shimomura, M.; Sanada, Y. Fuel Process. Technol. 1986, 13, 97. (9) Farcasiu, M. Fuel Process. Technol. 1986, 14, 161.
and desulfurization of coal in the presence of hydrocarbons or hydrogen pressure at 150-300 °C. We previously reported that a liquid or gaseous Bro¨nsted superacid, trifluoromethanesulfonic acid (TFMS), HF, and HF/BF3 depolymerized and desulfurized coal significantly at 100-150 °C without gaseous hydrogen.10-15 The Bro¨nsted acid, HF, is a frequently used alkylation catalyst for converting light olefins into gasoline components. The acidity of the mixture of HF/7 mol % BF3 (Ho ) -16.6) is much higher than that of TFMS (Ho ) -14.6), leading to higher activity of the former as an acid catalyst.16 HF and HF/BF3 are fully recoverable and reusable by distillation only, because their boiling points are very low (HF, 19.9 °C; BF3, -101 °C). HF/ BF3 has been recognized as a Bro¨nsted/Lewis superacid catalyst for Friedel-Crafts reactions, isomerization, and separation of m-xylene and formylation of aromatic compounds on an industrial scale. Consequently, acidcatalyzed reactions with anhydrous HF or HF/BF3 depolymerize coal efficiently in the presence of hydrocarbons at the lower temperatures; cleavage of sulfurcarbon bonds seems to occur, which promotes desulfurization. (10) Shimizu, K.; Karamatus, H.; Inaba, A.; Suganuma, A.; Saito, I. Fuel 1995, 74, 853. (11) Shimizu, K.; Karamatus, H.; Iwami, Y.; Inaba, A.; Suganuma, A.; Saito, I. Fuel Process. Technol. 1995, 45, 85. (12) Shimizu, K.; Miki, K.; Saito, I. Fuel 1997, 76, 23. (13) Shimizu, K.; Iwami, Y.; Suganuma, A.; Saito, I. Fuel 1997, 76, 939. (14) Shimizu, K.; Shimamura, K.; Suganuma, A.; Saito, I. Chem. Lett. 1997, 76, 943. (15) Shimizu, K.; Saito, I. Energy Fuels 1998, 12, 115. (16) Olah, G. A.; Prakash, G. K. S.; Sommer, J. SUPERACIDS; Wiley: New York, 1985; pp 37, 52.
S0887-0624(97)00209-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/16/1998
Desulfurization of High-Sulfur Coal
Energy & Fuels, Vol. 12, No. 4, 1998 735
Table 1. Reaction Conditions, Elemental Analyses, and Degree of Desulfurizationa reaction conditions run orig MQ sample MQ A B C D E F G H I J K L thermal
elemental analyses (wt %, daf)
HF (g) BF3 (g) solvent (reactant) temp (°C) press (MPa)
toluene 5.46 5.46 3.64 5.46 7.74 5.46 5.46 5.46 5.46 5.45
1.22 1.22 2.44
isopentane toluene toluene toluene toluene toluene toluene toluene toluene toluene tetraline
200 200 200 200 200 200 200 200 200 75 150 250 420
0.9 5.1 4.5 2.9 5.5 6.4 3.1 5.7 5.4 0.6 4.6 8.5 7.1
C
H
N
Odiff
S
ash (wt %)
WI (%)
desulfurization (%)
63.2 53.8 61.5 66.4 70.8 72.3 79.5 79.7 78.5 83.5 82.9 66.1 72.0 79.0 71.0
6.1 4.9 5.0 4.4 5.6 5.6 5.7 5.8 5.5 5.9 5.8 5.6 5.7 5.3 5.9
0.9 4.1 3.9 2.8 2.6 1.8 1.7 2.4 2.0 1.0 1.2 2.8 2.1 1.1 2.2
17.8 28.8 20.7 18.2 13.5 14.5 9.9 8.5 11.8 7.8 8.5 19.5 16.5 12.9 16.5
12.0 8.7 8.9 8.2 7.5 5.8 3.2 3.6 4.9 1.8 1.6 6.0 3.7 1.7 4.3
12.5 10.5 11.3 2.9 3.8 9.3 4.5 4.8 6.5 1.6 2.2 10.8 13.5 13.4 10.4
-10 -8 -10 12 31 31 37 110 152 17 47 58 -17
6 15 22 26 52 46 23 57 55 19 38 69 54
a Amount of HF and BF was g/g-coal. Trifluoromethanesulfonic acid (TFMS): 10.18g/g-coal desulfurization is calculated as follows. 3 Desulfurization (%) ) [1 - Sdaf{WI/100}/8.7]100, where Sdaf is the sulfur content of treated coal (%) (daf base), WI is the weight increase defined as {(treated coal - ash in the treated coal)/(sample coal - ash in the sample coal)}100, and 8.7 is the sulfur content of feed coal (%) (daf base).
In the present study, the activity of HF and HF/BF3 in coal desulfurization and solubilization through ionic reaction in milder reaction conditions without gaseous hydrogen was examined. Sulfur forms in the treated coals were characterized by X-ray photoelectron spectroscopy (XPS) in order to describe their behavior in the highly acidic media by identifying the remaining sulfur species. The relationship between the extent of desulfurization and solubilization of coal in this acidcatalyzed reaction process was considered in terms of removal and behavior of organic sulfur and the average molecular weight of the soluble fractions. 2. Experimental Section A lignite, Mequinenza coal (MQ; C 63.2, H 6.1, N 0.9, Odiff 17.8, S 12.0 wt % daf, ash 12.5 wt %) was ground to a diameter of less than 0.25 mm. It was then treated with an aqueous nitric acid solution (25 vol %) for 24 h at room temperature to remove inorganic sulfur (FeS2 and FeSO4). The organic sulfur content in the coal was determined according to the JIS M8813-1976 method in a Heraeus CHN-O-PAPID. The elemental analyses of original and HNO3-treated MQ coal (denoted sample coal) are shown in Table 1. Sample coal (5 g) almost free of inorganic sulfur with toluene or isopentane (10 mL) were placed in a dry ice-methanol cooled hastelloy-C microautoclave of 100 mL capacity. First, the reactor of the autoclave was evacuated by vacuum pump, and then the HF (0-5.82 g/g-coal) and BF3 (0-2.19 g/g-coal) were introduced into the coal-solvent slurry while dry icemethanol cooling continued. The autoclave was then heated to 200 °C at a heating rate of 3 °C/min for 3 h with stirring (1000 rpm) under autogenous pressure. After the reaction, gaseous HF/BF3, solvent, and the volatile fraction from toluene in the autoclave were depressurized and absorbed into icewater at 90-110 °C under flowing nitrogen gas (100-150 mL/ min) for 2 h with stirring (300 rpm). The contents of the autoclave were slowly poured into cool water, and then were gradually neutralized with an cool aqueous solution (5 wt %) of Na2CO3. The products were filtered and washed in water with sonication. Further washing with an aqueous methanol solution (30-50 vol %) was repeated 5 times in order to remove a small amount of neutralized product, NaF, and the dimer of toluene. The solid product obtained was vacuum-dried at 110 °C for 24 h. For comparison, the coal was also heat-treated in tetrahydronaphthalene (tetraline) at 420 °C for 30 min under autogenous pressure plus H2 of 2.1 MPa. The reaction conditions are also shown in Table 1.
Table 2. Distribution of Sulfur Forms and Their Content in the Original MQ Coal and Sample MQ Coal % run
SO42-
SO2
SO
Saliphatic
Sthiophenic
original MQ sample MQ
20 1.6
5 6.4
5 8
30 28
40 36
a
Sulfur content was based on original MQ coal.
The solvent, toluene, is incorporated into the subsequent carbonium ions by cleavage of coal molecules, resulting in a weight increase of the feed coal. Polymerization of toluene itself in the presence of HF and HF/BF3 at 150 °C was negligible, with 0.7-2.3 wt % in the products after the reactions with and subsequent washing and vacuumdrying. Therefore, we can use the following calculations to obtain the yield of solubles and weight increase.
% yield of solubles ) {1 (weight of insoluble fraction weight of ash in the insoluble fraction)/ (weight of product - weight of ash in product)}100 % weight increase ) {(weight of product weight of ash in the product)/(weight of feed coal weight of ash in feed coal) - 1}100 The products were extracted with benzene, tetrahydrofuran (THF), and pyridine in a sequential Soxhlet extractor. The average molecular weights of the soluble fractions (benzene soluble, benzene insoluble-THF soluble, pyridine soluble) were measured by vapor pressure osmometry (KNAUER vapor pressure osmometer). XPS was measured using microfocused monochromatic Al KR radiation from an ESCALAB 220i-XL (VG Scientific). Bonding energy was corrected to the carbon (1s) peak at 284.6 eV. Spectra were obtained at a pass energy of 20 eV. The full width at half-maximum (fwhm) of the silver metal 3d5/2 signal was
