Elucidation of the Structural and Molecular Properties of Typical South African Coals

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.

Chemical–structural properties of South African bituminous coals: Insights from wide angle XRD–carbon fraction analysis, ATR–FTIR, solid state 13 C NMR, and HRTEM techniques

“…710 These results are summarised in Table 5. 711 The aromaticity, f a , ranges from 74% for coal FOZ (0.63 Rr%) to 712 86% for coal TKD (1.2 Rr%), in agreement with published results 713 on South African coals [3,17,19,27]. These values showed a satis-714 factory comparison with the aromaticity of the samples deter-715 mined from WAXRD-CFA (Table 3) found to be higher in coal TKD (45%) than in the other samples (Tables 3 and 5).…”

“…The resulting hetero- 68 geneity complicates the characterisation of coal samples, and thus 69 a wider variety of techniques is necessary to provide the greatest 70 insight into the structure of coal. Conventional analytical methods 71 that give the bulk properties of the samples, often give poor predic- 72 tions of coal behaviour for most of its utilisation processes [1][2][3]. 73 Moreover, outliers that do not conform to the prediction equations 74 are common [4].…”

“…This may also 95 enable the synergy of enhanced coal bed methane production that 96 will add to the energy resource and generate income to offset some 97 of the capital expenditure of the sequestration infrastructure [5]. 98 In coal utilisation processes, the fundamental understanding of 99 the coal's chemical, physical, and structural properties remains 100 paramount and often challenging to coal scientists and engineers 101 [1][2][3]. However, the variety of analytical techniques employed by 102 researchers is greatly dependent on problem description, intended 103 coal use or motive, and availability of the desired technique.…”

“…106 These include: solid state 13 C nuclear magnetic resonance spec-107 troscopy (ss 13 C NMR), attenuated total reflection Fourier transform 108 infra-red spectroscopy (ATR-FTIR), wide angle X-ray diffraction-109 carbon fraction analysis (WAXRD-CFA), high resolution transmis-110 sion electron microscopy (HRTEM), matrix-assisted laser desorption 111 ionization-time-of-flight mass spectrometry (MALDI-TOF) and 112 computer controlled scanning electron microscopy (CCSEM). 113 These techniques have been used to examine coals with varying 114 degrees of success [3,, but, overall, have enabled a better 115 understanding of coal molecular chemical structure. Some of these 116 methods provide average values, while others provide vital distribu-117 tion data.…”

“…HRTEM. The molecular structural features of coals have 440 been studied by the HRTEM image analysis technique, using the 441 acquired high resolution fringe images[3,[9][10][11][12]17,24,27,30]. The442 HRTEM images of the coal samples were acquired from a 200 kV 443 Tecnai F20 FEGTEM and captured using a Gatan Inc. ORIUS™ CCD 444 camera (Model 895).…”