Energy & Fuels 1996, 10, 141-148
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Quantitative Electron Spectroscopic Analysis of the Surface Chemistry of Bituminous Coal Cara L. Weitzsacker† and Joseph A. Gardella, Jr.* Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000 Received February 1, 1995. Revised Manuscript Received September 19, 1995X
ESCA has been used to analyze coals from four seams: Illinois No. 6, Kentucky No. 9, Upper Freeport, and Pittsburgh. The coal was analyzed in raw, milled, and agglomerated forms. The coal was simultaneously analyzed for the effects of different storage conditions (air, water, or nitrogen gas atmosphere) over a 17 month period. Each coal was sampled seven times over the period. Elemental compositions are reported. Storage conditions had no detectable influence on surface concentrations of the element in any of the coals. Over the 17 month period no significant oxidation of these coals was detected by ESCA.
I. Introduction Beneficiation of coal removes components which do not contribute to fuel content or which pollute. Beneficiation is typically accomplished in three ways: physical removal of mineral matter, chemical removal of compounds such as organic sulfur, and dewatering of coal.1 Removal of mineral matter, water, sulfur, and nitrogen upgrades the fuel content and reduces emissions from burning coal. The results of such beneficiation also enable coal to be transported and utilized more efficiently. Removal of mineral matter by physical means is the most common method of beneficiation. Organic sulfur and nitrogen are much more difficult to remove on a large scale. Sulfur and nitrogen are of concern due to the association of their gaseous oxides with the acid rain phenomena. Dewatering is the removal of water content in freshly mined coal, as well as the removal of water used for transport of coal or used during the removal of mineral matter. The process of coal agglomeration is a method for separating the organic phase of coal from the mineral phase. Mineral matter may consist of clay minerals and species such as SiO2 and Al2O3, which are hydrophilic, while the organic portion of coal is hydrophobic. Oil is added to a coal/water mixture, causing the organic phase to agglomerate. The agglomerates will be large enough to be screened out of the resulting water/mineral matter mixture. Another consideration in the utilization of coal is aging. Aging of coal during storage is also important in evaluating fuel content.1-4 One of the reactions associated with aging is oxidation of coal. Aging occurs * Author to whom correspondence should be addressed. † Present address: Composite Materials and Structures Center, B100 Research Complex, Michigan State University, E. Lansing, MI 48824-1324. X Abstract published in Advance ACS Abstracts, November 1, 1995. (1) Schobert, H. H.; Coal: The Energy Source of the Past and Future; American Chemical Society: Washington, DC, 1987; Chapter 6. (2) Jakab, E.; Yun, Y.; Meuzelaar, H. L. C. In Chemistry of Coal Weathering; Nelson, C. R., Ed.; Coal Science and Technology 14; Elsevier: Amsterdam, 1989; Chapter 3. (3) Huggins, F. E.; Huffman, G. P. In Chemistry of Coal Weathering; Nelson, C. R., Ed.; Coal Science and Technology 14; Elsevier: Amsterdam, 1989; Chapter 2.
0887-0624/96/2510-0141$12.00/0
when coal is stockpiled. The weight of piled coal causes internal pressure and results in localized heating of stored coal. This, coupled with the presence of oxygen, may result in oxidation (and even self-ignition!). Exposure to air, even without pressure, leads to various degrees of oxidation. Coal which has been partially oxidized prior to burning loses fuel content.5 Physical changes in coal during aging, such as plasticity or fluidity,5,6 may cause difficulties in processing. Electron spectroscopy for chemical analysis (ESCA or XPS) is a nondestructive technique which provides analysis of a sample surface.7,8 One advantage of ESCA is that samples can be analyzed in the powder form. Coal is often utilized in powder form in the fuel industry. Several previous studies have utilized ESCA for the analysis of coal.9-29 In those studies, elemental analysis of the surface of coal and the identification of functional groups of carbon, sulfur, nitrogen, and mineral associated elements were obtained through analysis by ESCA.12-15,21-24,27-29 In addition, many of those studies also investigated coal oxidation.3,9-11,16-20,25,26 ESCA (4) Nelson, C. R. In Chemistry of Coal Weathering; Nelson, C. R., Ed.; Coal Science and Technology 14; Elsevier: Amsterdam, 1989; Chapter 1. (5) Davidson, R. M. Natural Oxidation of Coal; IEA Coal Research, IEACR/29, Sept. 1990. (6) Seki, H.; Ito, O.; Iion, M. Fuel 1990, 69, 317-321. (7) Hercules, D. M.; Hercules, S. H. J. Chem. Educ. 1984, 61 (5), 402-409. (8) Hercules, D. M.; Hercules, S. H. J. Chem. Educ. 1984, 61 (6), 483-489. (9) Weitzsacker, C. L.; Schmidt, J. J.; Gardella, J. A., Jr. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1989, 34 (2), 545-550. (10) Weitzsacker, C. L.; Schmidt, J. J.; Gardella, J. A., Jr. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1990, 35 (2), 344-351. (11) Weitzsacker, C. L.; Gardella, J. A., Jr. Anal. Chem. 1992, 64, 1068-1075. (12) Perry, D. L.; Grint, A. Fuel 1983, 62, 1024-1033. (13) Brown, J. R.; Kronberg, B. I.; Fyfe, W. S. Fuel 1981, 60, 439446. (14) Hirokawa, K.; Danzaki, Y. SIA, Surf. Interface Anal. 1984, 6 (4), 193-195. (15) Frost, D. C.; Leeder, W. R.; Tapping, R. L. Fuel 1974, 53, 206211. (16) Huffman, G. P.; Huggins, F. E.; Dunmyre, G. R.; Pignocco, A. J.; Lin, M-C. Fuel 1985, 64, 849-856. (17) Clark, D. T.; Wilson, R. Fuel, 1983, 62, 1034-1040. (18) Wu, M. M.; Winschel, R. A.; Robbins, G. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1988, 33 (4), 699-705.
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analysis of a powder surface presents results from the topmost 100 Å of the surface, and an underlying assumption is that this region is homogeneous over that depth. In this study, results are presented from the examination of coal through various processing steps: raw, milled, and oil agglomeration. Simultaneously, effects on surface composition of three different storage conditions over 17 months were also studied. The goals were to determine what changes occurred in surface composition throughout processing and to determine if any of the storage atmospheres (air, water, nitrogen) would inhibit or prevent oxidation due to aging. We investigated, in a previous study, two approaches for using ESCA O/C ratios to determine the degree of surface oxidation.11 In the first model, the C 1s envelope curve fit was used to determine the carbon functionality present in the coal sample. These functionalities include carbon-oxygen functional groups such as C-O, CdO, and CO32-. From curve fits we calculated the amount of oxygen bound to carbon (“organic oxygen”) and ratioed this to the concentration of surface carbon. The second method stoichiometrically calculated the quantity of organic oxygen by separating the oxygen signal into inorganic (mineral) and organic compounds.11,12 The result was used to find the surface oxygen to surface carbon ratio. The coals used in this study were chosen based on an assessment of high-sulfur coals from the U.S. Department of Energy, Pittsburgh Energy Technology Center (PETC), in 1987.30 The four coals all make a significant contribution to total U.S. sulfur emissions. The present work was part of the Otisca Industries, Ltd., project, “Sulfur and Mineral Matter Reduction in Coal Using Selective Agglomeration”.30 The goal of that project was to “evaluate ability of selective agglomeration to reject pyrite as a function of raw coal handling, storage and aging”. The present studies evaluate the surface chemistry of coals during selective agglomeration. II. Experimental Section Sample Handling and Treatments. The coals used in this study were collected from four different mine sites as the coal exited the mine. The locations and coal seams are listed in Table 1.30 Fresh coal samples were collected following (19) McIntyre, N. S.; Martin, R. R.; Chauvin, W. J.; Winder, C. G.; Brown, J. R.; MacPhee, J. A. Fuel 1985, 64, 1705-1712. (20) Gonzalez-Elipe, A. R.; Martinez-Alonso, A.; Tascon, J. M. D. SIA, Surf. Interface Anal. 1988, 12 (1-12), 565-571. (21) Lai, R. W.; Diehl, J. R.; Hammack, R. W.; Khan, S. U. M. Miner. Metall. Process. 1990, 7 (1), 43-48. (22) Schultz, H. D.; Proctor, W. G. Appl. Spectrosc. 1976, 27 (5), 347351. (23) Marsh, H.; Sherwood, P. M. A.; Augustyn, D. Fuel 1976, 55, 97-98. (24) Dutta, S. N.; Dowerah, D.; Frost, D. C. Fuel 1983, 62, 840841. (25) Kelemen, S. R.; George, G. N.; Gorbaty, M. L. Fuel 1990, 69, 939-944. (26) George, G. N.; Gorbaty, M. L.; Kelemen, S. R.; Sansone, M. Energy Fuels 1991, 5, 93-97. (27) Bartle, K. D.; Perry, D. L.; Wallace, S. Fuel Process. Technol. 1987, 15, 351-361. (28) Wallace, S.; Bartle, K. D.; Perry, D. L. Fuel 1989, 68, 14501455. (29) Burchill, P.; Welch, L. S. Fuel 1989, 68, 100-104. (30) Simmons, F. J.; Keller, D. V., Jr.; Burry, W. M.; Holloway, B. E.; Keller, D. S. Sulfur and Mineral Matter Reduction in Coal Using Selective Agglomeration: Final Technical Report; Office of Scientific and Technical Information: Oak Ridge, TN, Nov 1989; DOE/PC/79880T8.
Weitzsacker and Gardella Table 1. Coal Sample Origin coal seam
mine location and operator
type of mine
Illinois No. 6 Pittsburgh Upper Freeport Kentucky No. 9
Randolph County, IL Belmont County, OH Indiana County, PA Hopkins County, KY
deep mine strip mine deep mine strip mine
procedures outlined in ASTM D2234. Upon collection, the coal samples were divided into three different storage atmospheres at the mine site: ambient air, local mine tap water, and dry nitrogen gas. All coals were maintained in their respective atmospheres while transported to Otisca Industries, Ltd., Syracuse, NY. Collection took place between October 18 and 22, 1987. The coal samples were riffled and size reduced at Otisca; all such manipulations were done under nitrogen atmosphere. During riffling, the water-stored samples were drained. Once completed, all samples were returned to their respective storage atmospheres. The coal size during storage was approximately 3/8 in. diameter. Raw coal was reduced to 250 µm diameter, milled coal was approximately 15 µm, and agglomerated coal was milled to 1-2 µm in size. After riffling, samples were divided and stored in 5 gal polyethylene cans, with approximately 17 lb of coal per sample can. Twelve raw coal samples were used as baseline samples (four seams, three storage atmospheres), and 192 samples were analyzed in aging studies. At six intervals over 17 months raw samples were collected and portions of these samples were also milled and processed by agglomeration. Sets of coal were examined after storage through a cold period (10 °C) of weather and then a warm period (25 °C), and the oldest samples were taken after being stored over a winter, summer, and the following winter. ESCA analysis were performed at each point of processing. The temperatures are averages for the warm and cold seasons at the storage site. Milling further reduced the size of the coal particles. Samples at the milled stage were reduced to 15 µm in size. For the agglomerated samples, each sample was ball milled for 16 h using a ceramic mill and distilled water. The resulting particle size was 1-2 µm. No mineral matter was removed at this point. The third processing step involved agglomeration using pentane. A portion of each milled sample was taken and diluted to 5% solids with distilled water, and 1:1 by volume pentane was added to this solution in a Waring blender. Agglomeration time was recorded as the time from pentane addition to the blender until the color of the solution turned from black to the color of the mineral matter. For the Illinois No. 6 sample, castor oil was also added prior to addition of pentane. None of the other samples had castor oil added during the agglomeration step. The agglomerated coal was drained and then sent to our laboratory for ESCA analysis without further washing. Raw, milled, and processed coals were received in our laboratory under N2 in sealed bottles from Otisca Industries, Ltd. Raw coals were analyzed as received. Water which was used to store coal was drained and the coals were dried (vacuum oven, 70 °C30) and analyzed or went on to be processed without further rinsing. The water was analyzed for pH, conductivity, dissolved solids, and F, Cl, N, P, Na, NH4, Mg, Ca, and Fe.30 Milled and processed coals were vacuum dried to remove water and water/pentane, respectively. Vacuum drying was carried out at room temperature using a glass vacuum line. Samples were manipulated under dry N2 when loaded into vacuum tubes and when the dried coal was removed from the vacuum tube. A liquid nitrogen trap prevented pump oil from backstreaming onto the samples. Samples were evacuated at room temperature for 5 h. After drying, samples were stored in vials under dry N2 until analysis.
Surface Chemistry of Bituminous Coals Instrumentation. The ESCA spectrometer used in these analyses was a Perkin Elmer Physical Electronics (PHI) 5100 using a hemispherical analyzer and single channel detector. The excitation source was the Mg KR1,2 X-ray source (1253.6 eV) from a dual anode. Experimental conditions of the X-ray were 300 W, 15 kV, and 20 mA. Base pressure of the instrument was 2 × 10-9 Torr and operating pressure ranged from 1 × 10-7 to 2 × 10-8 Torr depending on the sample. A pass energy of 35.5 eV was used for high-resolution experiments. Under these conditions, the Ag 3d 5/2 peak at 367.9 eV had a full width at half-maximum of 1.0 eV and 210 000 counts/s. PHI ESCA Software Version 2.0, running on a Perkin Elmer Model 7700 computer, was used to collect and manipulate the data. Signal was collected from a spot size of 3 × 10mm. All spectra were taken using a 45° take-off angle. Charging of the organic components (C, O, N, S) of coal samples was corrected based on a reference of 285.0 eV for the C 1s CHx. The mineral components were corrected to the Si 2p peak at 103.2eV, corresponding to SiO2. The mineral components detected in the coals were Si, Al, K, Fe, Na, and S where the S 2p peak at higher binding energy corresponded to 168.0 eV after charge correction. This corresponds to sulfate. Once all samples were dry, preparation for ESCA analysis was identical. Air exposure was minimized to less than 10 min for the transfer of samples into instruments. Samples were mounted by pressing coal powder onto double-sided sticky tape on a sample stage. Powder was pressed onto the tape until the tape was no longer visible. This was when no more shine was seen through the coal sample. Excess powder was tapped off, and more pressed on until no tape was visible and no more powder could be tapped off from the stage. The coal was immediately transferred to the introduction chamber of the instrument and pumped down for 1 h before being moved into the main chamber for analysis. Low-resolution survey spectra were taken for 15-20 min, followed by high-resolution scans for 50 min. There was no evidence of sample damage after 65-70 min under the X-ray. A change in the shape of the oxygen or carbon envelopes, and/or an increase in oxygen content, would indicate damage occurred. Select samples of processed Illinois No. 6 coal were oxidized to a high degree by radio-frequency glow discharge (RFGD) before introduction to the ESCA. The RFGD chamber was a Harrick PDC-23G Plasma Cleaner utilizing a 60 W, 13.56 MHz radio-frequency induced plasma.31 The sample was placed into the chamber, pumped down by a rotary mechanical pump to approximately 30 mTorr for 10 min, and the RFGD turned on. The sample was discharged in air for 10 min on the HI setting. This oxidized coal was used to further develop the C 1s curve-fit model.9,10 Radio-frequency glow discharge in air was used to achieve a severe and rapid oxidation of coal which had already been mounted such that the affected area of coal was the same area analyzed by ESCA.
III. Results and Discussion A. Definitions. The Otisca coal consisted of seven sets, each set representing one aging period. These sets are referred to as batches. Batch 1 served as the baseline samples for the study. These samples were collected at the start of the 17 month aging period. The next six batches were collected after being exposed to different seasons. Table 2 lists the storage history of the batches. The samples were stored in an unheated garage. Batches 1 through 7 consisted of raw, milled, and processed (agglomerated) coal samples. For each of these batches, samples were analyzed which had been stored in either air or water. Additionally, samples (31) Cornelio, P. A. Ph.D. Thesis, The State University at Buffalo, 1990.
Energy & Fuels, Vol. 10, No. 1, 1996 143 Table 2. Storage History of Coal Samples batch no.
season
sample
storage
1 2 3 4,5 6 7
baseline first cold first cold warm second cold second cold
R, M, P R, M, P R, M, P R, M, P R, M, P R, M, P
A, W, N A, W A, W, N A, W A, W A, W, N
a R ) raw, M ) milled, P ) agglomerated (processed), A ) air, W ) water, N ) nitrogen.
which had been stored in nitrogen were received for the first, third and seventh batches of each seam and processing step. The sample sets were designed in this way to compare the behavior of the coal over time and temperature, storage atmosphere, and compare processing of coal which has been “aged”. Atomic concentration was calculated by dividing the intensity (I) of each element (n), measured as peak area (counts/second), by its sensitivity factor (S).32 Atomic concentration (C) of each of the elements is then expressed as a percentage as calculated using eq 1:
Cx (%) ) {nx/
∑i ni} × 100 ) {[Ix/Sx]/∑i [Ix/Sx]} × 100
(1)
Eight samples from the first batch (air and nitrogen stored raw samples) were run in triplicate. The relative standard deviation (RSD) of the concentrations of major components (C, O) was 1.4-13.0%. The Illinois samples had the largest deviations of 13.0%. In the other three coals, the relative standard deviation in carbon and oxygen concentrations ranged from 1.4 to 1.8%. This demonstrates the reproducibility of the analysis of coal powder by the ESCA technique. Relative standard deviations (RSD) were higher for the components present at smaller percent atomic concentration (% AC). The limits of detection for the ESCA experiment are typically 0.1% AC. The most variability was observed where the percent atomic concentration, % AC, of the elements was 5% AC or less. Here the RSD ranged from 0.27 to 100%. Table 3 shows a statistical analysis of the Illinois No. 6 and Kentucky No. 9 samples run in triplicate, where avg ) average of the three runs, RSD is the relative standard deviation, and STD is the standard deviation. B. Elemental Analysis. i. Overall Results. For each sample carbon, oxygen, nitrogen, sulfur, aluminum, silicon, and iron were analyzed at high resolution. When the low-resolution survey indicated sodium and potassium were present, these were also analyzed at high resolution. Carbon and oxygen were detectable in all coal samples. Carbon was detected in quantities ranging from 23 to 85% atomic concentration (% AC). Oxygen was detected in quantities ranging from 8 to 53% AC. Carbon and oxygen were the major components in all coal samples. The lower concentrations of carbon and higher concentrations of oxygen correspond to higher concentrations of mineral matter. Nitrogen was detected in almost every coal sample in amounts ranging from 0.8 to 1.7% AC. Nitrogen was not detected in single samples of the Illinois No. 6 and Upper Freeport seams. The surface concentration of (32) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-Ray Spectroscopy; Muilenberg, G. E., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN 1979.
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Table 3. Statistical Analysis of Samples Run in Triplicate Illinois No. 6 Raw: Baseline, Air Stored 234P22-1AA C 1s O 1s N 1s S 2p Al 2p Si 2p Fe 2p Na 1s K 2p
avg
STD
RSD
43.2 39.7 1.1 0.3 5.0 9.2 0.0 1.4 0.0
2.8 1.7 0.3 0.2 0.4 0.6 0.0 0.2 0.0
6.5 4.4 23.8 71.0 7.0 6.4 15.6
Kentucky No. 9 Raw: Baseline, Air Stored 235P8-1AA C 1s O 1s N 1s S 2p Al 2p Si 2p Fe 2p Na 1s K 2p
avg
STD
RSD
71.1 20.1 2.0 1.1 2.4 3.2 0.0 0.0 0.0
0.2 0.4 0.3 0.3 0.2 0.2 0.0 0.0 0.0
0.3 2.0 16.1 27.0 9.9 4.5
nitrogen in these samples is below the limits of detection (typically 0.1% AC) for ESCA. Sulfur was detected at levels ranging from 0.2 to 1.7% AC in most samples. Sulfur was not detected in many of the Upper Freeport raw samples but was detected in milled and agglomerated samples. This indicates that in the raw coal, sulfur was not near the surface of the coal powder in detectable amounts. After further grinding, and removal of mineral matter, sulfur was detected in both the milled and agglomerated coal. Aluminum and silicon were detected in all the raw and milled samples and many of the processed samples. The % AC of silicon was 2-5% AC greater than aluminum consistently throughout all samples. Sodium was detected in the raw and milled Illinois No. 6 samples and some of the Kentucky samples. Iron was only detected in a few samples of all of the seams. Iron was only detected in one raw sample from the Illinois No. 6 seam. Iron was detected in a few of the milled samples and one Kentucky No. 9 agglomerated sample. Like sulfur, milling or agglomeration brought detectable amounts of iron to the surface. The iron was present either as iron sulfate or oxide, as assigned from binding energies. Iron was only detectable in very small amounts (
