Energy & Fuels 1996, 10, 751-756
751
Quantitative Analysis of Sulfur in Coal by Pyrolysis-Gas Chromatography and Multivariate Data Analysis Pernille Selsbo, Per Alme´n, and Inger Ericsson* Department of Analytical Chemistry, University of Lund, Box 124, S-221 00 Lund, Sweden Received October 9, 1995. Revised Manuscript Received February 1, 1996X
Fifteen standard coal samples from the European Centre for Coal Specimens (SBN) were analyzed by pyrolysis-gas chromatography equipped with a flame ionization detector and a flame photometric detector (Py-GC(FID/FPD)). The yields of sulfur obtained when coal samples were pyrolyzed and combusted were between 56 and 80% of the total amount of sulfur present. The total amount of sulfur in the pyrolysis products was proportional to the sulfur content of the coals. The data obtained were evaluated with principal component analysis (PCA) and partial least-squares (PLS) regression. Predictive models for the content of total sulfur, organic sulfur, pyritic sulfur, and inorganic sulfur (the sum of pyritic and sulfate sulfur) were built. It was also possible to obtain a predictive model for the content of volatile matter.
Introduction Coal is the major fossil energy resource in the world. The sulfur content of coal poses a major problem for its utilization for power generation. It is of interest to increase the knowledge of the sulfur in coal to be able to develop effective and not too costly methods to remove the sulfur before combustion in addition to the flue gas cleaning techniques currently in use. The sulfur in coal can be divided into pyritic, sulfate, and organic sulfur. The pyritic sulfur of the coal is present as crystalline enclosures in the coal and can to a great extent be removed by means of flotation.1 The sulfate sulfur makes up only a small part of the total sulfur. The organic sulfur in contrast to the pyritic sulfur cannot be removed with physical methods since it is chemically incorporated in the coal matrix.1 There are standard methods, the American Society for Testing and Materials (ASTM),2,3 to determine the amounts of the different forms of sulfur in coal. However, the organic sulfur is determined by the difference. A number of destructive chemical4-6 and nondestructive instrumental7,8 techniques have been developed for the characterization of sulfur in coal with special emphasis on the organic sulfur. Davidson reviewed the quantification of organic sulfur forms in coal,9,10 where pyrolysis was one of the techniques used. * Corresponding author. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Vernon, J. L.; Jones, T. Sulphur and coal; IEA Coal Research: London, 1993; Vol. IEACR/57. (2) 1993 Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1993; Vol. 05.05 (D 2492-90), pp 285-289. (3) 1993 Annual Book of ASTM Standards ASTM: Philadelphia, PA, 1993; Vol. 05.05 (D 3177-89), pp 333-336. (4) Attar, A. Fuel 1977, 57, 201-212. (5) Calkins, W. H. Energy Fuels 1987, 1, 59-64. (6) LaCount, R. B.; Kern, D. G.; King, W. P.; LaCount, R. B., Jr.; Miltz, D. J., Jr.; Stewart, A. L.; Trulli, T. K.; Walker, D. K.; Wicker, R. K. Fuel 1993, 72, 1203-1208. (7) Gorbaty, M. L.; George, G. N.; Keleman, S. R. Fuel 1990, 69, 945-947. (8) Brown, J. R.; Kasrai, M.; Bancroft, G. M.; Tan, K. H.; Chen, J. M. Fuel 1992, 71, 649-53. (9) Davidson, R. M. Organic sulphur in coal; IEA Coal Research: London, 1993; Vol. IEACR/60. (10) Davidson, R. M. Fuel 1994, 73, 988-1005.
0887-0624/96/2510-0751$12.00/0
Pyrolysis of coal yields large amounts of data. Multivariate data analysis is an attractive method for reduction of large and complex data sets to a more comprehensible format while still keeping as much as possible of the original information. Principal component analysis (PCA)11 explores similarities and dissimilarities among samples. With the use of partial least-squares (PLS) regression,12,13 quantitative agreement can be obtained from different data sets. In the literature, multivariate data analysis has been used to evaluate data from coal analysis obtained by analytical pyrolysis14-20 and other techniques, like IR,21-23 NMR,24 and elemental analysis.25,26 Most of the papers characterized coal regarding rank and properties. Only a few papers dealt with sulfur.19,22,23 Kerogens, bitumen, and petroleum asphaltenes and coals were analyzed by pyrolysis-gas chromatography (Py-GC) and the distri(11) Wold, S.; Esbensen, K.; Geladi, P. Chemom. Intell. Lab. Syst. 1987, 2, 37-52. (12) Geladi, P.; Kowalski, B. R. Anal. Chim. Acta 1986, 185, 1-17. (13) Martens, H.; Næs, T. Multivariate Calibration; John Wiley & Sons Ltd.: Chichester, U.K., 1991. (14) Metcalf, G. S.; Windig, W.; Hill, G. R.; Meuzelaar, H. L. C. J. Coal Geol. 1987, 7, 245-68. (15) Nip, M.; Genuit, W.; Boon, J. J.; De Leeuw, J. W.; Schenck, P. A.; Blazso´, M.; Szekely, T. J. Anal. Appl. Pyrol. 1987, 11, 125-47. (16) Yeh, G. J. C.; Ward, B.; Quigley, D. R.; Crawford, D. L.; Meuzelaar, H. L. C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1988, 33, 612-22. (17) Øygard, K.; Larter, S.; Senftle, J. Org. Geochem. 1988, 13, 1153-62. (18) Gray, N. R.; Lancaster, C. J.; Gethner, J. J. Anal. Appl. Pyrol. 1991, 20, 87-106. (19) Eglinton, T. I.; Sinninghe Damste´, J. S.; Pool, W.; De Leeuw, J. W.; Eijkel, G.; Boon, J. J. Geochim. Cosmochim. Acta 1992, 56, 154560. (20) Meuzelaar, H. L. C.; Yun, Y.; Chakravarty, T.; Metcalf, G. S. In Advances in Coal Spectroscopy; Meuzelaar, H. L. C., Ed.; Plenum: New York, 1992; pp 275-94. (21) Christy, A. A.; Velapoldi, R. A.; Karstang, T. V.; Kvalheim, O. M.; Sletten, E.; Telnaes, N. Chemom. Intell. Lab. Syst. 1987, 2, 199207. (22) Tesch, S.; Rentrop, K. H.; Otto, M. Fresenius J. Anal. Chem. 1992, 344, 206-8. (23) Hobert, H.; Kempe, J. Erdoel, Erdgas, Kohle 1993, 109, 42630. (24) Axelson, D. E. Adv. Chem. Ser. 1993, 229, 253-67. (25) Neavel, R. C.; Smith, S. E.; Hippo, E. J.; Miller, R. N. Fuel 1986, 65, 312-20. (26) Glick, D. C.; Davis, A. Org. Geochem. 1987, 11, 331-42.
© 1996 American Chemical Society
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Energy & Fuels, Vol. 10, No. 3, 1996
Selsbo et al. Table 1. Apparatus and Conditions apparatus
model
pyrolyzer
PYROLA, Pyrolab, Sweden
gas chromatograph
Vista 6000, Varian, USA
conditions temperature rise time: 8 ms pyrolysis time: 2 s chamber temp: 150 °C carrier gas: He or O2 carrier gas flow rate: 20 mL/min injector temp: 250 °C column: DB-1 (J&W), 3.0 µm film, 30 m × 0.53 mm column temp: -50 °C (1 min), 15 °C/min, 280 °C detectors: FPD and FID detector temp: 250 °C
integrator software
baseline, Waters, USA
Table 2. Characteristics (in wt %) of the Coal Samples, Taken from the SBN Certificates
Figure 1. Schematic of the PYROLA pyrolyzer.
butions of the sulfur-containing products were studied by PCA.19 Infrared spectroscopy and PLS22 and infrared spectroscopy and principal component regression23 have been used to predict several properties of coal, including prediction of total sulfur, but not the different sulfur forms. Techniques are still missing which can give detailed information in a short time on the sulfur content of coal. In this paper, 15 standard coals of different geographical origin and characteristics were studied in order to get as much information as possible using Py-GC(FID/FPD). The objective of this work was to determine the yield of sulfur and to find bivariate and multivariate correlations between the pyrolysis products and the coal content. The final goal in our studies of sulfur in coal is to determine and characterize the organic sulfur content.
coal sample
total sulfur
organic sulfur
pyritic sulfur
sulfate sulfur
volatile matter
carbon
101 126 129 133 136 141 153 162 171 173 177 179 501 511 515
1.82 0.93 2.10 4.41 5.05 2.54 2.31 0.41 0.83 0.59 1.35 0.39 2.46 3.92 1.87
1.01 0.17 1.13 1.85 3.86 0.11 0.53 0.40 0.73 0.27 0.46 0.01 0.77 2.46 1.13
0.77 0.54 0.45 2.48 1.13 1.85 1.64 0.01 0.03 0.15 0.87 0.09 0.76 1.08 0.52
0.04 0.22 0.52 0.08 0.06 0.58 0.14 0.00 0.07 0.17 0.05 0.29 0.93 0.38 0.22
7.33 31.64 20.80 41.77 44.99 9.67 29.30 42.70 30.12 35.10 32.80 30.60 27.11 42.09 36.89
85.23 71.60 75.12 72.74 60.15 72.77 65.40 79.50 72.42 73.80 69.40 71.30 64.89 71.25 75.67
Pyrolysis-Gas Chromatography. Pyrolysis was performed with a PYROLA pyrolyzer connected to a Varian Vista 6000 gas chromatograph. The pyrolysis probe of the pyrolyzer is equipped with a platinum filament (15 × 2.6 × 0.012 mm) where the sample is placed. A photodiode, allowing optical monitoring of the filament’s temperature during pyrolysis, enables the registration of heat losses respectively contributions during the experiments. A glass cell in the pyrolysis chamber surrounded the platinum filament. Further details of the pyrolyzer, shown in Figure 1, can be found elsewhere.27 In order to obtain an inert system, the GC column was placed directly into the glass cell. The GC column was split at the outlet to a flame photometric detector (FPD) in sulfur-selective mode and a flame ionization detector (FID) using a “Y”-press fit.28 Apparatus and conditions are shown in Table 1.
A coal sample was first pyrolyzed at 800 °C and then the same sample was pyrolyzed a second time at 1350 °C. This kind of treatment of the sample is defined as fractionated pyrolysis.29,30 The remaining residue was combusted in oxygen at 1100 and 1350 °C. The first combustion temperature was lower than the second, because a too high temperature could cause the platinum filament to break, due to the heat contribution from the burning char. The carrier gas was shifted from He to O2 when the pyrolysis residue was combusted, allowing the sulfur trapped in the residue to be detected as SO2. All pyrolysis analyses were performed in duplicates. When the FPD was calibrated the pyrolysis probe was exchanged with a septum-equipped calibration probe. The response of sulfur varies between different substances. The system was calibrated for H2S, COS, CH3SH, and SO2 by injecting different volumes of standard gases (0.099% H2S in N2, 0.092% COS in N2, 0.112% CH3SH in N2, and 0.105% SO2 in N2) and for CS2 by injecting 1 µL of different concentrations of CS2 in heptane. All the other sulfur-containing pyrolysis products were assumed to have the same response as CS2, as thiophene and dibenzothiophene were found to have almost the same response as CS2. Sample and Sample Handling. Fifteen coal samples with very different characteristics were obtained from the European Centre for Coal Specimens in the Netherlands (Stichting Steenkoolbank Nederland, SBN). Their characteristics are shown in Table 2. The coals were of different ranks with
(27) Tyde´n-Ericsson, I. Chromatographia 1973, 6, 353-358. (28) Alme´n, P.; Ericsson, I.; Selsbo, P. J. Anal. Appl. Pyrol. 1993, 25, 243-254.
(29) Ericsson, I.; Lattimer, R. P. J. Anal. Appl. Pyrol. 1989, 14, 219221. (30) Uden, P. C. Pure Appl. Chem. 1993, 65, 2406.
Experimental Section
Quantitative Analysis of Sulfur in Coal
Energy & Fuels, Vol. 10, No. 3, 1996 753
Figure 3. Yield of sulfur of coals obtained at fractionated pyrolysis at 800 and 1350 °C and combustion at 1100 and 1350 °C.
Figure 2. Pyrograms of coal 511 from two detectors: (a) FPD, (b) FID. Coal 511 was pyrolyzed at 800 °C. Table 3. Major Sulfur-Containing Pyrolysis Products and Amount of Sulfur in the Products When SBN Coals Were Pyrolyzed at 800 °C coal sample
H2S
COS
ng of S/µg of coal SO2 CH3SH CS2
101 126 129 133 136 141 153 162 171 173 177 179 501 511 515
0.89 1.14 2.25 8.14 12.67 1.77 3.00 1.23 1.11 1.11 2.73 1.04 2.57 11.27 4.32
0.02 0.03 0.05 0.25 0.47 0.08 0.10 0.03 0.03 0.03 0.06 0.02 0.08 0.25 0.12
2.59 2.75 7.81 8.02 2.87 7.87 4.55 0.36 0.80 1.13 1.90 0.49 9.40 7.85 4.64
0.00 0.02 0.12 0.19 0.63 0.00 0.06 0.05 0.04 0.00 0.07 0.03 0.06 0.54 0.11
0.20 0.03 0.28 0.43 0.68 0.41 0.20 0.00 0.02 0.01 0.08 0.00 0.17 0.66 0.14
others
total sulfur
0.03 0.00 0.04 0.39 1.18 0.29 0.08 0.00 0.00 0.05 0.00 0.00 0.00 0.14 0.03
3.73 3.97 10.55 17.42 18.49 10.43 7.99 1.65 2.00 2.33 4.83 1.57 12.29 20.71 9.36
carbon content between 60 and 85 wt % and volatile matter between 7 and 45 wt %. The sulfur content of the samples ranged from 0.4 to 5.0 wt % and the ratio of pyritic, sulfate, and organic sulfur differed. The particle size was
