Local structure, paramagnetic properties, and porosity of natural coals: Spectroscopic studies

Abstract

Using methods of the scanning electron microscopy, Raman scattering of light(RS), and electron paramagnetic resonance (EPR), consistent research of the local structure and magnetic features of different types of raw coal samples from Donetsk basin is carried out. It is established that the ratio of the main peak intensities of RS spectrum D and G is inversely related to the volatile substance amount Vdaf in the coal samples. The study of the kinetic behavior of the EPR line width in hydrogen, oxygen, and methane sorption-desorption processes in each coal sample helped determine that the diffusion coefficient value for hydrogen in coal at room temperature is equal to DН = (2 ÷ 7) × 10−5 cm2/s. It is demonstrated that the oxygen diffusion occurs with time according to two different exponential laws with diffusion coefficients DO,1 = 5 × 10−6 cm2/s and DO,2 = 5.5 × 10−7 cm2/s, respectively. The smaller coefficient corresponds to the diffusion caused by the hopping process. Finally, it is established that the anthracite is a unique type of coal which does not possess the ability “to conserve” the significant EPR line width after oxygen pumping out from the samples.

Origin of electron paramagnetic resonance signal in anthracite

“…The results show that the anthracite conductivity is strongly dependent on temperature and on the material origin [6,8]. The particular one, studied by us, the temperature dependency of which is seen in Fig.…”

“…Various authors [9][10][11][12] pointed out that the conduction electrons should contribute to the EPR signal intensity of electrically conducting coals. Sometimes, the narrow component of the EPR spectra were attributed to these conduction electrons [8,13]. However, the Curie-type temperature dependency of the signal intensity suggested that most of the centers were localized [14].…”

“…As EPR is very sensitive to small variations in the electronic structure, the signals observed for materials from various deposits can significantly differ in shape and linewidth. This problem is illustrated by a work by Konchits et al [8], who studied anthracites and coal samples from various mines from Donetsk basin and observed various signals for bulk anthracite samples. Moreover, samples from one mine can differ, as it was shown by Bratek et al [15].…”

“…The EPR method and its theory were created in the fifties of the twentieth century and the first work concerning an EPR study of anthracite was published in 1956 [5]. In next nearly sixty years, this method has been frequently used for the characterization of various types of anthracites [6][7][8]. Nevertheless, the origin of the EPR signal in anthracites was never unambiguously established.…”

Porous structure of coals from the usinsk deposit in the Pechora Coal Basin

“…The structure of coals is an irregular polymer like matrix, which consists of the structural elements of different nature: aromatic, aliphatic, and heterocyclic fragments and carbon clusters between them [4][5][6][7][8][9]. The chemical composition and rank of coals are responsible for the mutual arrangement of fragments and porous structure peculiarities.…”