Structural characterization of Nigerian coals by X-ray diffraction, Raman and FTIR spectroscopy

The structural parameters of six Nigerian coals were determined by X-ray diffraction, Raman and FTIR spectroscopy. This study reveals that the coals contain crystalline carbon of turbostratic structure with some amount of highly disordered amorphous carbon. The average lateral sizes (L_a), stacking heights (L_c) and interlayer spacing (d₀₀₂) of the crystallite structures calculated from the X-ray intensities range from 16.47 to 25.70, 8.12 to 13.25 and 3.48 to 3.58Å, respectively. The L_a values derived from the Raman spectra analyses using the classical linear relationship between 1/L_a and the D/G band ratio are higher (27.53-34.67Å) than values obtained from X-ray diffraction. The FTIR spectra of the coals show the presence of stretching vibrations of -OH bonds, aliphatic -CH, -CH₂ and -CH₃ absorptions, CC and -CH of aromatic structure, and CO stretching vibrations of carbonyl groups. A strong linear relationship exists between the coal structural parameters (f_a, d₀₀₂, L_c) and the elemental carbon and volatile contents of the coal which reflects the dependency of the coals structure on their ranks.