Physicochemical characterization of South African coals upon short-term flue gas exposure using conventional and advanced techniques

Abstract

In the view of the current environmental problems, especially in the light of mitigating the effect of global warming due to anthropogenic carbon dioxide (CO2) emissions to the atmosphere, numerous technologies have been proposed like CO2 sequestration in deep and unmineable coal seams. This paper presents advanced characterization of two South African coals upon short-term flue gas exposure. A synthetic industrial flue gas containing 12% CO2, 5.5% O2, 82% N2, 0.38% SO2, and 0.12% NO2 was used in the study. The two coals were exposed to the flue gas at a temperature of 60 °C and pressure of 9.0 MPa for up to 120 h. Subsequently, the coals were analysed using conventional characterization techniques (density Stereopycnometer, proximate and ultimate, textural, and petrographic) and advanced characterization techniques (carbon 13 nuclear magnetic resonance (13C NMR), attenuated total reflectance-Fourier transform infrared (ATR-FTIR), wide-angle X-ray diffraction (WXRD), and field emission gun scanning electron microscopy (FEG-SEM)) to check the physical and chemical structural changes during the CO2/flue gas sorption to simulate sequestration.The conventional analytical techniques have revealed significant observable changes of the physical and chemical structure of the coal upon flue gas exposure. However, clear changes in chemical structure were observed through ATR-FTIR analysis. The observable change in surface chemistry of the coals suggests that there is most certainly a chemical interaction between the coals and the flue gas components at high pressure. Slight structural changes were observed from FEG-SEM and WXRD characterization. Lastly, there were no obvious structural changes from the results obtained from 13C NMR, except for slight changes in the aliphatic carbons bonded to oxygen of 37% for Coal EML and 27% for Coal SML. These changes could be due to the presence of oxygen in the flue gas; thus, substituting to form stronger intensities of the C-O and COO bonds as observed in the ATR-FTIR spectra.