Inspired by the Optical Properties of Char and Coke: A Study on Differences between Them from Perspectives of Organic Elemental Contents and the Carbon Nanostructure

Caking coal and noncaking coal show significantly different behaviors during thermal conversion processes. Inspired by the optical properties of chars and cokes derived from noncaking and caking coals, respectively, under four different pyrolysis conditions, elemental analysis (EA), Fourier transform infrared (FTIR), CP/MAS solid-state C-13 nuclear magnetic resonance (CP/MAS C-13 NMR), Raman spectroscopy, X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) were jointly employed to investigate and compare their organic element contents and carbon nanostructures. The FTIR and C-13 NMR spectra demonstrate that the 500 degrees C/30 min pyrolysis condition has transformed the aromatic, aliphatic, and carbonyl/carboxyl structures of various rank coals to varying degrees. Higher pyrolysis temperatures and longer residence times make it difficult to relate which raw coal the pyrolysis products are to the parent coals. Raman parameters, such as AD/AG vs A(G(R) + V-L + V-R)/AD, describe a dynamic process of coal structure evolution, indicating aromatics with 3-5 rings tend to decrease, resulting in the formation of aromatics with not less than 6 rings; moreover, higher pyrolysis temperature and longer residence time enhance the Raman parameters consistency of the chars and cokes. EA shows that during the same pyrolysis process (at least higher than 750 degrees C), the caking coal has a higher potential to be graphited, and the graphitization potential of semianthracite and anthracite is still higher than that of other noncaking coal. Furthermore, XRD indicates the d(002) of pyrolysis products derived from caking coal is about 0.34-0.35 nm, which is generally smaller than that from noncaking coal, larger than 0.35 nm. HRTEM images reveal a direct observation of the carbon nanostructure in chars and cokes, illustrating that the carbon nanostructure derived from caking coal has a flatter arrangement while that from noncaking coal has a curlier arrangement. Oxygen is considered one of the factors that inhibits the alignment of carbon nanostructure evolution.