Article pubs.acs.org/EF
Elucidation of the Structural and Molecular Properties of Typical South African Coals Burgert B. Hattingh,* Raymond C. Everson, Hein W. J. P. Neomagus, John R. Bunt, Daniel van Niekerk, Johan H. L. Jordaan, and Jonathan P. Mathews Research Focus Area for Chemical Resource Beneficiation, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom 2520, South Africa Energy Systems, School of Chemical and Minerals Engineering, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom 2520, South Africa Sasol Technology (Pty) Ltd., Research & Development, Coal & Gas Processing Technology, Box 1, Sasolburg, 1947, South Africa Energy and Mineral Engineering and the EMS Energy Institute, Pennsylvania State University, Hosler Building, University Park, Pennsylvania 16802, United States ABSTRACT: Advanced analytical techniques were performed on four South African coals, three noncoking and one coking, containing different maceral contributions (vitrinite contents ranging between 24 and 66 vol %, m.m.f.b.). Inertinite-rich (INY, UMZ) and vitrinite-rich (G#5 and TSH) coals were investigated to quantify differences and similarities in structural properties. Coals from the Witbank region (INY, UMZ, and G#5) have similar carbon contents (79.2−83.8 wt % d.a.f.), in contrast to a carbon content of 90.8 wt % (d.a.b.) for coal TSH. The free swelling index (FSI) indicated that the Witbank coals were noncoking while TSH was strongly coking. Solid state nuclear magnetic resonance indicated that the vitrinite-rich (65.9 vol % m.m.f.b.), higher rank (1.23 RoV %) coal TSH was more aromatic (81%) and more polycondensed than the other three coals. Coal G#5 was the least aromatic (66%) and was characterized by its larger proportion of protonated aliphatics as compared to the other coals. Coals UMZ, INY, and G#5 had similar average aromatic cluster sizes (ranging between 19 and 21 aromatic carbons) and number of cluster attachments (5 to 6) as estimated from NMR data. Furthermore, the cluster attachments of coal TSH were concentrated more in the side-chains, whereas the attachments of the other three coals were more prominent in bridge-and/or loop structures. XRD carbon crystallite analyses showed that coal G#5 contained the largest amount of amorphous carbon (67%), consistent with a higher volatile matter yield in comparison to the other coals. Laser-desorption ionization mass time-of-flight spectroscopy indicated that all four coals displayed similar molecular weight distributions ranging up to 1800 m/z. Coal TSH showed a maximum abundance at a higher molecular mass (608 m/z) in comparison to the other three coals. HRTEM analyses confirmed the presence of slightly more aromatic fringes in the higher molecular mass range for coal TSH in comparison to the other coals.
1. INTRODUCTION A detailed description and fundamental understanding of the coal structure is one of the most important and difficult problems facing coal scientists and engineers.1 This stems from the fact that in order to use coals effectively, a well based knowledge of the chemical structure of coal should be developed.1−3 In response to understanding the fundamental properties of coal, numerous conventional analytical techniques such as proximate, ultimate, petrographic, ash fusion temperature, etc. analyses have been developed.2 Although these analyses provide valuable insight into elemental- and maceral composition etc. of different coals, they do however only give an overall or bulk description of coal.2 The elucidation of the coal structure has however improved markedly over the past few decades due to advanced analytical strategies such as solid state 13C nuclear magnetic resonance (13C NMR), pyrolysismass spectrometry (py-MS), chromatography, Fourier transform infrared spectroscopy (FTIR), solvent swelling, X-ray fluorescence (XRF), X-ray diffraction (XRD), computer controlled scanning electron microscopy (CCSEM), matrix assisted laser desorption ionization-time-of-flight mass spec© XXXX American Chemical Society
trometry (MALDI-TOF-MS), high-resolution transmission electron microscopy (HRTEM), etc. having emerged.2,4−6 These techniques can all be used in an attempt to more accurately describe both the organic and inorganic counterparts of coal. With the addition of conventional analytical techniques, advanced strategies such as 13C NMR, FTIR, HRTEM, MALDI-TOF-MS, XRD, etc. provide an additional perspective in particular to the molecular constituents of coal. By combining information obtained from different techniques, it is however possible to obtain a comprehensive understanding of the coal structure.7 Solid state 13C nuclear magnetic resonance spectroscopy provides a valuable nondestructive analytical tool for identifying different chemical structural features of the organic matter in coals and coal chars.2,5,6,8,9 Various 13C NMR analyses are available which include cross-polarization with magic angle spinning (CP-MAS), dipolar dephasing (DD), single pulse Received: April 9, 2013 Revised: May 21, 2013
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dx.doi.org/10.1021/ef400633d | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Witbank No. 2 (INY), 4 (UMZ), and 5 (G#5) seams in the Northern Karoo basin and one coking coal (TSH) originating from the VendaPafuri deposit of the Soutpansberg coalfields (seams Nos. 1 and 2).35,36 From a stratographical viewpoint, coal seams in the Witbank region are shallow, with most seams not exceeding a depth of 200 m.35 The thickness of the No. 2 seam ranges between 4.5 and 20 m and is most commonly used as an export steam coal, while the thinner No. 4 seam (2.5−6.5 m) is split into different coal bands (4A, 4 Upper, and 4 Lower) due to the presence of mudstone and/or siltstone partings.36 The shallower No. 5 seam can range in thickness of up to 2 m and is commonly used for metallurgical purposes.36 The difference in depositional depth between seams 2 and 4 can range from 15 to 50 m, while seam 5 has a depositional depth of 20−40 m higher than seam 4. The main seam of the Venda-Pafuri deposit is about 3.5-mthick and constitutes several coal bands interlayered with carbonaceous mudstone.35 Two seams are distinguishable (Nos. 1 and 2 seam), with the lowest seam approximately 72 m from the surface. Seam thickness varies from 0.55-m-thick for the No. 1 seam to 6 m total thickness for the No. 2 seam.36 The three Witbank coals were beneficiated samples from the respective mines, while coal TSH was density separated according to the ISO 7936 (1992) method.37 The choice of the four coals was based on their (1) similarity in bituminous rank (main rank classification), (2) varying vitrinite content (ranging from 24 to 66 vol %, m.m.f.b), (3) relatively low ash yields ( 4), while coals INY, UMZ, and G#5 are all noncoking. All four coals were characterized as low “ash” coals, with ash yields ranging between 13.5 and 18.6 wt % (d.b.). In terms of volatile matter, the volatile matter content decreased in the order of G#5 > INY > UMZ > TSH from 34.1 wt % (d.b.) to 20.5 wt % (d.b.) (Table 2). It is however also of interest to note that coal TSH contained the smallest amount of inherent moisture, with a value of 0.7 wt % (a.d.b.). The higher volatile matter content of coal G#5 (34.1 wt %) could however be attributed to its high liptinite content.58 Although very different in maceral content, coals INY, UMZ, and G#5 displayed quite similar carbon contents (Table 2). The lower H/C atomic ratio for coal TSH indicates that it is more likely to be more aromatic than the other three coals.59−61 The opposite is however true for coal G#5, which displayed the largest H/C atomic ratio. From Table 2, it is evident that the initial ash yields were substantially decreased from values between 13.5 and 18.6 wt % (d.b.) to final ash yield values of
